METHODS AND COMPOSITIONS FOR EXPANSION OF CELL POPULATION

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is being filed as a PCT application, and claims the benefit of priority to U.S. Patent Application No. 62/570,076 filed on October 9, 2017, as well as U.S. Patent Application No. 62/695,820 filed on July 9, 2018, both of which are hereby incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

[0002] The present invention relates to methods and compositions for expanding a cell population, and particularly stem cell populations such as a hematopoietic stem cell population.

BACKGROUND

[0003] Hematopoietic stem cells (HSCs) are clonogenic cells, which possess the properties of both self-renewal (expansion) and multilineage potential giving rise to all types of mature blood cells. HSCs are responsible for hematopoiesis and undergo proliferation and differentiation to produce mature blood cells of various lineages while still maintaining their capacity for self-renewal. The ability to self- renew maintains the HSC population for the lifespan of an animal and also allows HSCs to repopulate the bone marrow of lethally irradiated congenic hosts.

[0004] Early HSC development displays a hierarchical arrangement, starting from long-term (LT-) HSCs, which have extensive self-renewal capability, followed by the expansion state, which corresponds to short-term (ST-) HSCs (having limited self-renewal ability) and proliferative multipotent progenitors (MPPs) (having multipotent potential but no self-renewal capability). MPP is also a stage of priming or preparation for differentiation. An MPP differentiates and commits to become either a common lymphoid progenitor (CLP), which gives rise to all the lymphoid lineages, or a common myeloid progenitor (CMP), which produces all the myeloid lineages. During this process, the more primitive population gives rise to a less primitive population of cells, which is unable to give rise to a more primitive population of cells. The intrinsic genetic programs that control these processes including the multipotential, self-renewal, and activation (or transient amplification) of HSCs, and lineage commitment from MPP to CLP or CMP, remain largely unknown.

[0005] To sustain constant generation of blood cells for the lifetime of an individual, HSCs located in bone marrow niches (Zhang, J. et al. Nature 425, 836- 841 , 2003; Calvi, L. M. et al. Nature 425, 841 -846, 2003; Kiel, M. J., et al. Cell 121, 1109-1121 , 2005; Arai, F. et al. Cell 118, 149-161 , 2004) must achieve a balance between quiescence and activation so that immediate demands for hematopoiesis are fulfilled, while long-term stem cell maintenance is also assured. In adults, homeostasis between the quiescent and activated states of stem cells is important to protect HSCs from losing their potential for self-renewal and, at the same time, support ongoing tissue regeneration (Li, L. and Xie, T. Annu. Rev. Cell. Dev. Biol. 21, 605-631 , 2005). Over-activation and expansion of stem cells risks both eventual depletion of the stem cell population and a predisposition to tumorigenesis. Although some factors important for stem cell activation have been identified (Heissig, B. et al. Cell 109, 625-637, 2002), the molecular events governing the transition between quiescence and activation are poorly understood.

[0006] HSCs are responsible for life-long hematopoiesis under homeostatic and stress conditions, which relies on an exquisite balance between stem cell self- renewal and differentiation (Li et al. Science, 327: 542-545, 2010; Weissman et al. Cell, 100: 157-168, 2000). Thus, HSC transplantation is a life-saving therapy for a broad spectrum of disorders, including hematologic, immune, and genetic diseases, as well as cancers (Walasek et al. Annals of the New York Academy of Sciences 1266: 138-150, 2012). However, HSC-based treatment can be limited primarily by the lack of HLA-matched donor bone marrow (BM). Allogeneic transplantation offers an alternative approach, but graft vs host disease (GvHD) remains a life-time challenge, since taking immune suppression medicine has numerous side effects, such as delayed immunological recovery, thrombotic microangiopathy (Sung et al. Stem Cells Translational Medicine, 2: 25-32 (2013) ;Shlomchik et al. Nature Reviews. Immunology, 7: 340-352, 2007). Transplantation of HSCs from hUCB reduces the risk of GvHD; however, the lower number of HSCs in hUCB than in BM or mobilized peripheral blood limits its application (Walasek et al. Annals of the New York Academy of Sciences, 1266: 138-150, 2012). Targeting single molecules or pathways has been studied for hUCB HSC expansion (Huang et al. Leukemia, 30: 144-153, 2016; Boitano et al. Science, 329: 1345-1348, 2010; Fares et al. Science, 345; 1509-1512, 2014; Ansellem et al. Nature Medicine, 9: 1423-1427, 2003; Antonchuk et al. Cell, 109: 39-45, 2002; Rentas et al. Nature, 532: 508-511, 2016; Himburg et al. Nature Medicine, 16: 475-482, 2010; North et al. Nature, 447: 1007- 1011, 2007; Guo et al. Nature Medicine, 2018); Varnum-Finney et al. Nature Medicine, 6: 1278-1281 , 2000; and Chou et al. Experimental Hematology, 41 : 479- 490 e474, 2013). However, other approaches are sought in order to relatively favor stem cell self-renewal versus differentiation (Zhao et al. Molecular Cell Biology, 18: 31 -42, 2017). [0007] m ⁶ A is a prevalent internal modification in mRNAs that regulates the outcome of gene expression by modulating RNA processing, localization, translation, and eventual decay, which is modulated by "writers," "erasers" and "readers" of the mark (Roundtree et al. Cell, 169: 1187-1200, 2017; Li et al. Annual Review of Genomics and Human Genetics, 15: 127-150, 2014). Recent studies have elucidated the roles of m ⁶ A modification in stem cell fate determination and endothelial-to-hematopoietic transition during embryogenesis (Batista et al. Cell Stem Cell, 15: 707-719, 2014; Geula et al. Science, 347: 1002-1006, 2015; Yoon et al. Cell, 2017; Zhang et al. Nature, 549: 273-276, 2017; Zhao et al. Nature, 542: 475-478, 2017) as well as in leukemia development (Li et al. Cancer Cell, 31 : 127- 141 , 2017; Vu et al. Nature Medicine, 2017; Barbieri et al. Nature, 2017; Weng et al. Cell Stem Cell, 22: 191 -205 e199, 2018). Interestingly, deficiency in m ⁶ A writer complex, Mettl3 and Mettl14, leads to distinct outcomes in different types of stem cells. For example, Mettl3 or Mettl14 KO promoted differentiation in HSCs (Vu et al. Nature Medicine, 2017; Weng et al. Cell Stem Cell, 22: 191 -205 e199, 2018; Barbieri et al. Nature, 2017) while resulting in enhanced stem cell self-renewal and maintenance in mouse embryonic stem cells (mESCs) and embryonic neuronal stem cells (NSCs) (Batista et al. Cell Stem Cell, 15: 707-719, 2014; Yoon et al. Cell, 2017). Besides, the physiological function of m ⁶ A in stem cells and leukemia are mediated through different mechanisms. In stem cells, m ⁶ A modifications regulate stem cell fate determination by m ⁶ A-mediated decay of mRNAs encoding stem cell fate determinant (Batista et al. Cell Stem Cell, 15: 707-719, 2014; Yoon et al. Cell, 2017) while in acute myeloid leukemia (AML), Mettl3 and Mettl14 promote leukemogenesis as m ⁶ A modifications stabilize the mRNAs of oncogenes and/or increase their translation (Vu et al. Nature Medicine, 2017; Barbieri et al. Nature, 2017; Weng et al. Cell Stem Cell, 22: 191 -205 e199, 2018). Furthermore, previous studies have reported that the leukemogenic functions of FTO and Mettl14 are independent of YTHDF reader proteins (Li et al. Cancer Cell, 31 : 127-141 , 2017; Weng et al. Cell Stem Cell, 22: 191 -205 e199, 2018).

[0008] As the m ⁶ A RNA modification is modulated by "writers," "erasers" and "readers" of the mark (Wang et al. Nature, 505(7481 ): 117-120, 2014), processes that install, recognize and remove this and other marks may have various implications for cellular, developmental, and disease processes. For example, studies have shown that the m ⁶ A mark may act as a key post-transcriptional modification to promote initiation of microRNA (miRNA) biogenesis (Alarcon et al. Nature, 519(7544): 482-485, 2015). Evidence also points to m ⁶ A RNA modifications possibly being involved in the differentiation of stem cells to specific lineages (Batista, Cell stem Cell, 15(6): 707-719, 2014; Zhang et al. Nature, 549(7671 ): 273- 276, 2017), and in regulating gene expression (Dominissini et al., Nature 485(7397):201 -206, 2012; Haussmann et al, Nature 540(7632): 301 -304, 2016). A m ⁶ A transferase METTL3 has been identified as a regulator for terminating murine naive pluripotency (Geula et al. Science, 347(6225): 1002-1006, 2015). The m ⁶ A "writer" protein METTL3 has also been demonstrated in mouse T cells to disrupt T cell homeostasis and differentiation (Li et al. Nature, 548(7667): 338-342, 2017), and m ⁶ A RNA methylation has been found to promote XIST-mediated transcriptional repression (Patil et al. Nature, 537(7620): 369-373, 2016). M ⁶ A RNA modifications have also been shown to regulate the ultraviolet-induced DNA damage response (Xiang et al. Nature, 543(7646): 573-576, 2017). Study of the maternal-to-zygotic transition (MZT) as in zebrafish also indicated a role for m ⁶ A mRNA methylation in transcriptome switching and animal development (Zhao et al. Nature, 542(7642): 475-478, 2017). Accordingly, while accumulative evidence has brought insights into the biological functions of m ⁶ A (Lence et al. Nature, 540 (7632); 242-247, 2016), the function of m ⁶ A in adult stems cells are largely unknown.

[0009] Hematopoietic stem cells (HSCs) in bone marrow (BM) maintain homeostasis hematopoiesis throughout life and also support regeneration after myeloablation (Weissman, 2000). Quiescent HSCs perform superiorly to proliferative HSCs in lethally irradiated mice, which largely attributes to the quiescent state that protects HSCs from DNA damage (Arai et al., 2004; Fleming et al., 1993; Wilson et al., 2008) (Walter et al., 2015). However, a recent study showed that DNA damage accumulation in HSCs was associated with broad attenuation of DNA repair and response pathways that were dependent upon HSC quiescence (Beerman et al., 2014). In fact, the majority of HSCs, despite their quiescence, are sensitive to DNA damage from chemotherapeutic drugs, such as 5-Fluorouracil (5FU) (Lerner and Harrison, 1990). The unresolved issue is how the hematopoietic system overcomes the consequence of myeloablation. In respect to the remarkable heterogeneity of HSCs during development and in adult (Benveniste et al., 2010; Benz et al., 2012; Fleming et al., 1993; Morita et al., 2010; Zhou et al., 2016), the existence of a reserve HSC (rHSC) subpopulation was proposed, with the features of drug- resistance and capacity to regenerate the bulk of HSCs to overcome stress-caused myeloablation (Haug et al., 2008; Li and Clevers, 2010; Wilson et al., 2008). Thus far, however, no functional evidence has been provided in supporting existence of rHSCs in the blood system.

[0010] HSCs are preserved in complex BM niches for their maintenance and regeneration (Li and Clevers, 2010; Mendelson and Frenette, 2014; Morrison and Scadden, 2014; Scadden, 2014; Schofield, 1978). In the past decades, multiple studies have uncovered the complexity of HSC bone marrow niche components, including: endosteal (inner bone surface) cells fCalvi et al., 2003; Zhang et al., 2003J, sinusoidal endothelial cells (Hooper et al., 2009; Kiel et al., 2005), Cxcl12 abundant reticular (CAR) cells (Sugiyama et al., 2006), Nestin ⁺ and NG2 ⁺ perivascular cells (Kunisaki et al., 2013; Mendez-Ferrer et al., 2010), LepR ⁺ and Prx- 1 ⁺ mesenchymal stem and progenitor cells (Ding and Morrison, 2013; Ding et al., 2012; Greenbaum et al., 2013), non-myelinating Schwann cells (Yamazaki et al., 2011), and megakaryocytes (Bruns et al., 2014; Zhao et al., 2014). However, whether and how the BM niche complexity contributes to HSC heterogeneity regulation remain largely unclear (Itkin et al., 2016). Furthermore, the first HSC niche was initially identified as the spindle shaped N-Cadherin ⁺ (N-cad ⁺ ) pre-osteoblastic cells in the endosteum of the trabecular bone region (Calvi et al., 2003; Xie et al., 2009; Zhang et al., 2003), but the nature and function of N-cad ⁺ niche cells in BM remain unclear.

[0011] Accordingly, there remains a need for elucidation and understanding of the role of m ⁶ A and m ⁶ A mRNA pathways to provide insight into molecular regulation of stem cell proliferation and differentiation. There remains a further need for methods of expanding populations of stem cells, both in vivo and ex vivo, and methods of providing treatment with such expanded stem cell populations, such as via transplant into a suitable subject.

SUMMARY

[0012] In one embodiment of the present disclosure, a method for expanding a population of stem cells is provided, the population of stem cells being obtained from a tissue selected from the group consisting of peripheral blood, cord blood and bone marrow. The method includes modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the population of stem cells, to expand the number of stem cells.

[0013] In yet another embodiment, a method for ex vivo expansion of a substantially undifferentiated stem cell population is provided, comprising modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the undifferentiated stem cell population to expand the number of undifferentiated stem cells without significant differentiation of the stem cell population.

[0014] According to yet another embodiment, a method for ex vivo expansion of an hematopoietic stem cell (HSC) population is provided, the HSC population being obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow, the method comprising modulating a Λ/ ⁶ - Methyladenosine (m ⁶ A) mRNA modification pathway in the HSC population to expand the HSC population to a sufficient quantity while maintaining a multilineage differentiation potential in the HSC population, which is sufficient for subsequent transplantation into a subject in need thereof.

[0015] According to yet another embodiment, a method for ex vivo expansion of hematopoietic stem cells (HSCs) by at least 2-fold is provided, the expanded HSCs being competent to reconstitute an HSC lineage upon transplantation into a mammal in need thereof, the method comprising introducing a mutation into the stem cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of HSCs in a suitable culture medium.

[0016] According to a further embodiment, a kit for expanding an hematopoietic stem cell population (HSC) population for subsequent transplantation into a subject in need thereof is provided, the kit comprising a system for introducing a mutation into the HSC population that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0017] According to yet another embodiment, a kit for expanding an hematopoietic stem cell population (HSC) population for subsequent transplantation into a subject in need thereof is provided, the kit comprising an inhibitor of a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0018] In yet a further embodiment, a method for administering an hematopoietic stem cell (HSC) to a subject in need thereof is provided, the method comprising: (a) introducing, into a sample containing an HSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the HSCs to the subject.

[0019] In yet a further embodiment, a method for administering an hematopoietic stem cell (HSC) to a subject in need thereof is provided, the method comprising: (a) culturing, in a suitable culture media, a sample containing an HSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the HSCs to the subject.

[0020] In another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided, comprising: (a) introducing, into a sample containing an HSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the HSCs to the subject.

[0021] In another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided, comprising: (a) culturing, in a suitable culture media, a sample containing an HSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the HSCs to the subject.

[0022] In one embodiment, a method for expanding a population of hematopoietic cells (HSCs) comprising culturing the population of HSCs under conditions sufficient to result in an expansion of the HSC population by at least 2-fold is provided, wherein the expanded population of HSCs is suitable for transplantation into a mammal in need thereof.

[0023] In a further embodiment, a method for expanding a population of hematopoietic stem cells (HSCs) is provided, comprising: (a) obtaining from a mammal a tissue sample comprising an HSC population; (b) expanding, in vitro, the HSC population from the sample, wherein: (i) the HSC population expands by at least 2-fold; and (ii) the expanded HSC population has at least a 5-fold increase in total colony-forming units.

[0024] In a further embodiment, a method for reconstituting a hematopoietic stem cell lineage in a subject in need thereof is provided, the method comprising: (a) obtaining from a mammal a tissue sample comprising an HSC population; (b) expanding, in vitro, the HSC population from the sample, wherein: (i) the HSC population expands by at least 2-fold; and (ii) the expanded HSC population has at least a 5-fold increase in total colony-forming units; and (c) transplanting the expanded HSC population into a subject in need thereof.

[0025] In yet another embodiment, a method for expanding a hematopoietic stem cell population in a mammal in need of such expansion is provided, comprising administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway for a period of time sufficient to expand the HSC population by at least 2-fold with HSCs that possess the ability to reconstitute a hematopoietic lineage in the mammal.

[0026] According to yet another embodiment, a method for ex vivo expansion of a mesenchymal stem cell (MSC) population is provided, the MSC population being obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow, the method comprising modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the MSC population to expand the MSC population to a sufficient quantity while maintaining a multilineage differentiation potential in the MSC population, which is sufficient for subsequent transplantation into a subject in need thereof.

[0027] According to yet another embodiment, a method for ex vivo expansion of mesenchymal stem cells (MSCs) by at least 2-fold is provided, the expanded MSCs being competent to reconstitute a MSC lineage upon transplantation into a mammal in need thereof, the method comprising introducing a mutation into the stem cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of MSCs in a suitable culture medium.

[0028] According to a further embodiment, a kit for expanding a mesenchymal stem cell population (MSC) population for subsequent transplantation into a subject in need thereof is provided, the kit comprising a system for introducing a mutation into the MSC population that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0029] According to yet another embodiment, a kit for expanding an mesenchymal stem cell population (MSC) population for subsequent transplantation into a subject in need thereof is provided, the kit comprising an inhibitor of a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0030] In yet a further embodiment, a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof is provided, the method comprising: (a) introducing, into a sample containing a MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the MSCs to the subject.

[0031] In yet a further embodiment, a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof is provided, the method comprising: (a) culturing, in a suitable culture media, a sample containing a MSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the MSCs to the subject.

[0032] In another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided, comprising: (a) introducing, into a sample containing a MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the MSCs to the subject.

[0033] In another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided, comprising: (a) culturing, in a suitable culture media, a sample containing a MSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the MSCs to the subject.

[0034] In one embodiment, a method for expanding a population of mesenchymal cells (MSCs) comprising culturing the population of MSCs under conditions sufficient to result in an expansion of the MSC population by at least 2- fold is provided, wherein the expanded population of MSCs is suitable for transplantation into a mammal in need thereof.

[0035] In a further embodiment, a method for expanding a population of mesenchymal stem cells (MSCs) is provided, comprising: (a) obtaining from a mammal a tissue sample comprising a MSC population; (b) expanding, in vitro, the MSC population from the sample, wherein: (i) the MSC population expands by at least 2-fold; and (ii) the expanded MSC population has at least a 5-fold increase in total colony-forming units.

[0036] In a further embodiment, a method for reconstituting a mesenchymal stem cell lineage in a subject in need thereof is provided, the method comprising: (a) obtaining from a mammal a tissue sample comprising a MSC population; (b) expanding, in vitro, the MSC population from the sample, wherein: (i) the MSC population expands by at least 2-fold; and (ii) the expanded MSC population has at least a 5-fold increase in total colony-forming units; and (c) transplanting the expanded MSC population into a subject in need thereof.

[0037] In yet another embodiment, a method for expanding a mesenchymal stem cell population in a mammal in need of such expansion is provided, comprising administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway for a period of time sufficient to expand the MSC population by at least 2-fold with HSCs that possess the ability to reconstitute a mesenchymal lineage in the mammal.

[0038] According to year another embodiment, a method for ex vivo expansion of a mesenchymal stem cell (MSC) population is provided, the MSC population being obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow, the method comprising modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the MSC population to expand the MSC population to a sufficient quantity while maintaining a multilineage differentiation potential in the MSC population, which is sufficient for subsequent transplantation into a subject in need thereof.

[0039] According to yet another embodiment, a method for ex vivo expansion of mesenchymal stem cells (MSCs) by at least 2-fold is provided, the expanded MSCs being competent to reconstitute a MSC lineage upon transplantation into a mammal in need thereof, the method comprising introducing a mutation into the stem cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of MSCs in a suitable culture medium.

[0040] According to a further embodiment, a kit for expanding a mesenchymal stem cell population (MSC) population for subsequent transplantation into a subject in need thereof is provided, the kit comprising a system for introducing a mutation into the MSC population that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0041] According to yet another embodiment, a kit for expanding an mesenchymal stem cell population (MSC) population for subsequent transplantation into a subject in need thereof is provided, the kit comprising an inhibitor of a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0042] In yet a further embodiment, a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof is provided, the method comprising: (a) introducing, into a sample containing a MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the MSCs to the subject.

[0043] In yet a further embodiment, a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof is provided, the method comprising: (a) culturing, in a suitable culture media, a sample containing a MSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the MSCs to the subject.

[0044] In another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided, comprising: (a) introducing, into a sample containing a MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the MSCs to the subject.

[0045] In another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided, comprising: (a) culturing, in a suitable culture media, a sample containing a MSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the MSCs to the subject.

[0046] In one embodiment, a method for expanding a population of mesenchymal cells (MSCs) comprising culturing the population of MSCs under conditions sufficient to result in an expansion of the HSC population by at least 2-fold is provided, wherein the expanded population of MSCs is suitable for transplantation into a mammal in need thereof. [0047] In a further embodiment, a method for expanding a population of mesenchymal stem cells (MSCs) is provided, comprising: (a) obtaining from a mammal a tissue sample comprising a MSC population; (b) expanding, in vitro, the MSC population from the sample, wherein: (i) the MSC population expands by at least 2-fold.

[0048] In a further embodiment, a method for reconstituting a mesenchymal stem cell lineage in a subject in need thereof is provided, the method comprising: (a) obtaining from a mammal a tissue sample comprising a MSC population; (b) expanding, in vitro, the MSC population from the sample, wherein: (i) the MSC population expands by at least 2-fold; and (c) transplanting the expanded MSC population into a subject in need thereof.

[0049] In yet another embodiment, a method for expanding a mesenchymal stem cell population in a mammal in need of such expansion is provided, comprising administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway for a period of time sufficient to expand the MSC population by at least 2-fold with MSCs that possess the ability to reconstitute a mesenchymal lineage in the mammal.

[0050] In a further embodiment, a method of isolating mesenchymal stem cells (MSCs) from a biological sample is provided, the method comprising contacting the biological sample having a population of MSCs with one or more N-cadherin antibodies.

[0051] In a further embodiment, an isolated population of mesenchymal stem cells is provided, as made by any of the processes described herein. According to yet another embodiment, an expanded, isolated population of mesenchymal stem cells is provided, as made by any of the processes described herein. [0052] According to yet another embodiment, a kit for isolating a mesenchymal stem cell (MSC) population for subsequent transplantation into a subject in need thereof is provided. The kit comprises a system for contacting a biological sample comprising MSCs with one or more N-cadherin antibodies, and instructions for use thereof.

[0053] According to yet another embodiment, a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof is provided. The method comprises: (a) isolating MSCs from a biological sample comprising a population of MSCs, by contacting the biological sample with one or more N-cadherin antibodies, and (b) administering the isolated MSCs to the subject.

[0054] According to yet another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided. The method comprises: (a) isolating mesenchymal stem cells (MSCs) from a biological sample comprising a population of MSCs, by contacting the biological sample with one or more N-cadherin antibodies; and (b) administering the isolated MSCs to the subject.

[0055] According to yet another embodiment, a method for treating a subject in need of a transplant, selected from the group consisting of a bone marrow transplant, a peripheral blood transplant and an umbilical cord blood transplant, is provided. The method comprises administering to the subject a population of isolated MSCs obtained by any of the methods described herein.

[0056] In yet another embodiment, a method for expanding a population of chimeric antigen receptor (CAR) T-cells prepared by modifying T-cells obtained from a tissue selected from the group consisting of peripheral blood, cord blood and bone marrow, is provided. The method comprises modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the population of CAR T-cells, to expand the number of CAR-T cells.

[0057] In a further embodiment a method for ex vivo expansion of a chimeric antigen receptor (CAR) T-cell population is provided. The method comprises modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the CAR T- cell population to expand the number of CAR T-cells.

[0058] According to yet another embodiment, a method for ex vivo expansion of a chimeric antigen receptor (CAR) T-cell population prepared by modifying T-cells obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow, is provided. The method comprises modulating a Λ/ ⁶ - Methyladenosine (m ⁶ A) mRNA modification pathway in the CAR T-cell population to expand the CAR T-cell population to a sufficient quantity which is sufficient for subsequent transplantation into a subject in need thereof.

[0059] In yet another embodiment, a method for ex vivo expansion of chimeric antigen receptor (CAR) T-cells, the expanded CAR T-cells being competent to treat a cancer and/or blood disorder upon transplantation into a mammal in need thereof, is provided. The method comprises introducing a mutation into the CAR T-cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of CAR T-cells in a suitable culture medium.

[0060] In a further embodiment, a kit for expanding a chimeric antigen receptor (CAR) T-cell (HSC) population for subsequent transplantation into a subject in need thereof, is provided. The kit comprises a system for introducing a mutation into the CAR T-cell population that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and instructions for use thereof.

[0061] In yet a further embodiment, a method for administering chimeric antigen receptor (CAR) T-cell to a subject in need thereof is provided. The method comprises: (a) introducing, into a sample containing CAR T-cell population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of CAR T-cells in the sample to a number sufficient to transplant into the subject; and (c) administering the CAR T-cells to the subject.

[0062] In another embodiment, a method for administering a CAR T-cell to a subject in need thereof, is provided. The method comprises: (a) culturing, in a suitable culture media, a sample containing a CAR T-cell population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of CAR T-cells in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the CAR T-cells to the subject.

[0063] In a further embodiment, a method for treating cancer and/or a blood disorder in a subject in need thereof is provided. The method comprises: (a) introducing, into a sample containing a CAR T-cell population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of CAR T-cells in the sample to a number sufficient to transplant into the subject; and (c) administering the CAR T-cells to the subject. [0064] In yet a further embodiment, a method for treating cancer and/or a blood disorder in a subject in need thereof, is provided. The method comprises: (a) culturing, in a suitable culture media, a sample containing a CAR T-cell population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of CAR T-cells in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the CAR T-cells to the subject.

[0065] In one embodiment, a method for expanding a population of chimeric antigen receptor (CAR) T-cells is provided. The method comprises culturing the population of CAR T-cells under conditions sufficient to result in an expansion of the CAR T-cell population by at least 2-fold, wherein the expanded population of CAR T- cells is suitable for transplantation into a mammal in need thereof.

[0066] In yet another embodiment, a method for expanding a population of chimeric antigen receptor (CAR) T-cells is provided. The method comprises: (a) obtaining from a mammal a tissue sample comprising a T-cell population; (b) modifying the T-cell population with chimeric antigen receptors to provide CAR T- cell population; and (c) expanding, in vitro, the CAR T-cell population from the sample, wherein: (i) the CAR T-cell population expands by at least 2-fold.

[0067] In a further embodiment, a method for treating a subject suffering from cancer and/or a blood disorder is provided. The method comprises: (a) obtaining from a mammal a tissue sample comprising a T-cell population; (b) modifying the T- cell population with a chimeric antigen receptor (CAR) to form a CAR T-cell population; (c) expanding, in vitro, the CAR T-cell population from the sample, wherein: (i) the CAR T-cell population expands by at least 2-fold; and (d) transplanting the expanded CAR T-cell population into the subject. [0068] In yet a further embodiment, a method for expanding a chimeric antigen receptor (CAR) T-cell population in a mammal in need of such expansion is provided. The method comprises administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway for a period of time sufficient to expand the CAR T-cell population by at least 2-fold with CAR T-cells that possess the ability to treat cancer and/or a blood disorder in the mammal.

[0069] In yet another embodiment, a method of treating a subject suffering from a blood disorder is provided. The method comprises (a) obtaining a population of cells selected from the group consisting of stem cells and T-cells, from a tissue selected from the group consisting of peripheral blood, cord blood and bone marrow; (b) optionally, in a case where the population of cells comprises T-cells, modifying the T-cells with a chimeric antigen receptor (CAR) to provide CAR T-cells; (c) expanding the population of cells by modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the cells, to expand the number of cells; and (d) transplanting the expanded cells to the subject to treat the blood disorder.

[0070] These and other aspects of the invention are further disclosed in the detailed description and examples which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein. [0072] Figs 1A-1 H: Ythdf2 KO leads to increase in phenotypic HSCs in mice. Fig. 1A is a shematic showing a deletion of Ythdf2 in the HSPCs of Mx1- cre;Ythdf2 ^f/f conditional KO (cKO) mice; Fig. 1 B shows a Western blot (left) and histogram (right) showing intracellular flow validation of knockout Ythdf2 in mouse HSPCs; Fig. 1 C are representative flow cytometric analysis plots of HSPCs in BM from wt and Ythdf2 KO mice (n = 5 for each group); Figs. 1 D and 1 E are bar graphs showing the frequency in total nucleated cells (TNC) (1 D) and absolute cell number (1 E) of HSPCs in BM from wt and Ythdf2 KO mice (n = 5 for each group); Fig. 1 F is a bar graph showing the absolute number of BM TNC from wt and Ythdf2 KO mice (n= 5 for each group); and Figs. l G and 1 H are bar graphs showing the absolute number of committed progenitors (1 G) and lineage cells (1 H) in BM of wt and Ythdf2 KO mice (n = 5 for each group). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant

[0073] Figs. 2A-2J: Ythdf2 KO results in expansion of functional HSCs in mice. Fig. 2A depicts an experimental scheme for limiting dilution transplantation assay (LDA) to determine the frequency of functional HSCs; Fig. 2B is a graph showing Primary LDA to determine the CRU frequency by ELDA (Extreme Limiting Dilution Analysis) (n = 10 per group) at 16 weeks post transplantation; Fig. 2C is a graph showing a competitive reconstitution assay by transplanting 200K whole bone marrow (WBM) cells with 200K rescue cells into irradiated recipients, (n = 10 for each group); Figs 2D and 2E are bar graphs showing the frequency in TNC (2D) and absolute cell number (2E) of donor derived HSPCs in BM from transplantation recipient mice as (2C) (n = 10 for each group); Figs. 2F and 2G are bar graphs showing absolute cell number of donor derived (CD45.2 ⁺ ) committed progenitors (2F) and lineage cells (2G) in BM from primary 200K BM transplantation recipient mice (n= 9-10 for each group); and Fig. 2H shows a plot with secondary LDA to determine the long-term CRU frequency by ELDA at 16 weeks after secondary transplantation (n = 10); Figs. 2I and 2J are, respectively, a bar graph showing quantification of functional mouse hematopoietic stem cells (HSCs) by transplantation assay with peripheral blood analysis for total engrafted donor cells at 4 weeks after transplantation (2I), and a bar graph showing quantification of functional mouse HSCs by transplantation assay with the percentage of B, T and myeloid lineage cells at 4 weeks after transplantation (2J). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0074] Figs. 3A-3I: Ythdf2 functions as an m ⁶ A reader and regulates HSC gene expression by mediating mRNA decay. Fig. 3A is a schematic of irCLIP-seq workflow; Fig. 3B is a schematic showing Ythdf2 binding motif identified by MEME with all irCLIP peaks found in all three replicates; Fig. 3C is a pie chart depicting the fraction of Ythdf2 binding peaks in each of five transcript segments; Fig. 3D is a chart showing a GO enrichment analysis of Ythdf2 targets from intersect genes of three Ythdf2 irCLIP-seq replicates; Fig. 3E shows representative tracks of Tall harboring m ⁶ A peaks and Ythdf2 irCLIP peaks, with coverage of m ⁶ A immunoprecipitation and input fragments indicated in red and grey, respectively, and Ythdf2 irCLIP reads highlighted in yellow; Fig. 3F is a chart showing qPCR analysis of total mRNA of sorted LSK cells from wt and Ythdf2 KO mice. All Ct values were first normalized to Actb control (not m ⁶ A-tagged). Then the ratio (Ythdf2 KO over wt) was calculated, (n = 3); Fig. 3G shows representative images (left) and quantification via bar graph (right) of staining intensity of wt (n = 65) and Ythdf2 KO (n = 54) HSPCs for TAL1 (green); Fig. 3H shows images depicting fluorescence in situ hybridization of Tall mRNA (red) and fluorescence immunostaining of Dcpl a (P-body marker) (magenta), Ythdf2 (green) in wt and Ythdf2 KO HSPCs, where arrows indicate co-localized staining. Scale bars, 5 μm; Fig. 31 shows Quantification of Tall mRNA and DCP1 a co-localization in sorted LSK cells from wf and Ythdf2 KO mice. Percentage indicates the average frequency of the Tall mRNA that co-localized with DCP1 a over total Tall mRNA level in each LSK cells (n = 9-17). Data shown as mean ± s.e.m. Unpaired t-test.

[0075] Figs. 4A-4I: Role of YTHDF2 in human cord blood HSCs by m ⁶ A- seq and RNA-seq analysis. Fig. 4A shows metagene profiles depicting sequence coverage in windows surrounding the TSS (left) and stop codon (right), where coverage of m ⁶ A IP and control (input) fragments indicated in red and grey, respectively; Fig. 4B depicts pie charts presenting the fraction of m ⁶ A peaks in each of five non-overlapping transcript segments; Fig. 4C depicts a Venn diagram showing shared and unique m ⁶ A-tagged genes in mouse and hUCB HSPCs; Fig. 4D depicts a chart with a GO enrichment analysis of m ⁶ A-tagged transcripts shared in both mouse and hUCB HSPCs; Fig. 4E shows representative tracks of HOXB4 harboring m ⁶ A peaks, where color codes are the same as in Fig 4A; Fig. 4F depicts a schematic of lentivirus mediated YTHDF2 KD in hUCB CD34 ⁺ HSPCs for RNA-seq; Fig. 4G is a plot of a cumulative distribution of log2 (fold change) for m ⁶ A-marked genes (purple line) and non-m ⁶ A-marked genes (black line), with control and YTHDF2 KD hUCB CD34 ⁺ cells; Fig. 4H shows representative coverage plots from the RNA-seq analysis, showing increased reads of m ⁶ A-tagged gene HOXB4 but not a non-m ⁶ A-tagged gene ACTB in YTHDF2 KD compared to control hUCB CD34 ⁺ cells; and Fig. 4I is a bar graph showing relative mRNA expression levels of non-m ⁶ A labeled ACTB (as control) and m ⁶ A-marked transcription factor related to stem cell self-renewal in control and YTHDF2 KD hUCB CD34 ⁺ cells. RPKM from RNA-seq analysis were normalized to controls. Adjusted P value were indicated, n.s., nonsignificant.

[0076] Figs 5A-5F: YTHDF2 KD facilitates expansion of human cord blood HSCs ex vivo. Figs. 5A-5B are bar graphs depicting the fold change of frequency (5A) and absolute number (5B) of indicated cells in YTHDF2 KD over control cells after 7 days culture; Fig. 5C shows a bar graph depicting CFU output from transduced CD34 ⁺ CD38 ^" hUCB cells and images of CFU-granulocyte erythrocyte monocyte megakaryocyte (GEMMs) (scale bar, 200μm); Fig. 5D includes images of burst forming unit-erythroid (BFU-E) (left) and colony-forming unit- granulocyte/macrophage (CFU-GM) (right) from 7-days cultured control or YTHDF2 KD hUCB cells (scale bar, 200μm), where independent cord blood samples were used and repeated twice for the panels; Fig. 5E is a bar graph showing apoptosis analysis of CD34 ⁺ CD38 ^" cells in 7-day cultures of transduced CD34 ⁺ hUCB cells by Annexin V staining (n = 3 independent CB samples); Fig. 5F is a bar graph showing the fold change of Tthdf2 knockdown (KD) to control in indicated human cord blood HSCs (n=3 individual human samples), where lentivirus was used to deliver control shRNA or human Ythdf2 shRNA into sorted CD34 ⁺ CD38 ^" blood cord HSCs. Dashed lines indicate 95% confidence intervals. Data shown as mean ± s.e.m. Unpaired t- test. n.s., nonsignificant.

[0077] Figs. 6A-6G: YTHDF2 KD facilitates expansion of human cord blood functional long-term HSCs. Fig. 6A is an image showing an experimental scheme for measuring frequency of HSCs after in vivo expansion; Fig. 6B includes representative flow plots of hCD45 ⁺ GFP ⁺ reconstitution from primary recipient mice receiving the highest two cell doses. hCD45 = human CD45; Fig. 6C is a plot showing hCD45 ⁺ GFP ⁺ engraftment in BM from the primary recipient mice that received the highest two doses (n = 8); Fig. 6D is a plot showing HSC frequency determined by primary LDA. Dashed lines indicate 95% confidence intervals; Fig. 6E includes representative flow plots of hCD45 ⁺ GFP ⁺ reconstitution from secondary recipient mice receiving the highest two cell doses; Fig. 6F is a plot showing hCD45 ⁺ GFP ⁺ engraftment in BM from the secondary recipient mice that received the highest two doses (n = 6); Fig. 6D is a graph showing HSC frequency determined by secondary LDA. Dashed lines indicate 95% confidence intervals. Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0078] Figs. 7A-7I: Ythd†2 KO HSCs show no signs of lineage bias or differences in quiescence and homing ability but exhibit lower apoptotic rate.

Fig. 7A is bar graph showing absolute cell number of HSPCs in BM from Mx1-cre ^~ ;Ythdf2 ^f/f and Mx1 -cre ⁺ ;Ythdf2 ^f/f m\ce without pl:pC injection (n=3 per group); Fig. 7B is a bar graph showing cell cycle analysis of HSPCs in wt (n = 3) and Ythdf2 KO (n = 4) mice; Fig. 7C is a bar graph showing apoptosis analysis of BM HSPCs in wt and Ythdf2 KO mice (n = 5 for each group); Fig. 7D shows images and the weight of spleens from wt and Ythdf2 KO mice; Figs. 7E-7H are bar graphs showing absolute number of TNC (7E), LSK CD48 ^" CD150 ⁺ HSCs (7F), committed progenitors (7G) and lineage cells (7H) in the spleen of wt (n = 3) and Ythdf2 KO (n = 4) mice; Fig. 7I is a bar graph showing Homing ability of wt and Ythdf2 KO cells was determined by transplanting 1 x10 ⁶ CFDA SE-labelled BM cells into lethally irradiated mice. 18 hours later, BM was analyzed for homed events (n = 6 mice per group). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0079] Figs. 8A-8F: Transplantation recipient mice of Ythd†2 KO BM display no lineage changes or defects 16 weeks post transplantation. Figs. 8A to 8C are bar graphs depicting absolute cell number of donor derived (CD45.2 ⁺ ) TNC (8A), committed progenitors (8B) and lineage cells (8C) in the BM from secondary 200K transplantation recipient mice at 16 weeks after secondary transplantation (n= 7-10 for each group); Figs. 8D to 8F are bar graphs depicting absolute cell number of donor derived (CD45.2 ⁺ ) TNC (8D), committed progenitors (8E), and lineage cells (8F) in the spleen from secondary 200K transplantation recipient mice at 16 weeks after secondary transplantation (n= 7-10 for each group). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0080] Figs. 9A-9K: Ythdf2 KO has long-term effect on mouse HSC expansion in vivo without inducing lineage bias. Fig 9A is a schematic showing BM and spleen collected from wt and Ythdf2 KO mice were analyzed by flow cytometry at 5-7 months post pl:pC inductions; Figs. 9B to 9E are bar graphs showing: absolute cell number of TNC (Fig. 9B), HSPCs (Fig. 9C), committed progenitors (Fig. 9D) and lineage cells (Fig. 9E) in the BM of wf and Ythdf2 KO mice at 5-7 months post induction, (n = 4-7 mice per group); Fig. 9F is a bar graph showing the weight of spleens from wt and Ythdf2 KO mice at 5-7 months post induction, (n = 4-7 mice per group); Figs. 9G to 9J are bar graphs showing: absolute cell number of TNC (Fig. 9G), LSK CD48 ^" CD150 ⁺ HSCs (Fig. 9H), committed progenitors (Fig. 9I) and lineage cells (Fig. 9J) in the spleen of wt and Ythdf2 KO mice at 5-7 months post induction, (n = 4-7 mice per group); Fig. 9K is a schematic and graph showing 5 months post pl:pC injection, 75k WBM from wt and Ythdf2 KO mice were transplanted with 200K rescue cells into lethally irradiated recipients. Peripheral blood from transplantation recipients were analyzed every 4 weeks post transplantation to determine the donor derived engraftment (n = 10 for each group). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant. [0081] Figs. 10A-10D: Molecular characterization of m ⁶ A modification in mouse HSPCs. Fig. 10A shows plots of metagene profiles depicting sequence coverage in windows surrounding the TSS (up) and stop codon (down), where coverage of m ⁶ A IP and control (input) fragments indicated in red and grey, respectively; Fig. 10B shows pie charts presenting the fraction of m ⁶ A peaks in each of five transcript segments; Fig. 10C shows plots of the fraction of genes in mouse HPSCs with m ⁶ A peaks in each of the segments as a function of expression level; Fig. 10D shows graphs of m ⁶ A-tagged and non-m ⁶ A-tagged mRNA degradation rates as determined by analysis of the expression level at 0 hour and 4 hours post actinomycin D treatment in HSPCs.

[0082] Figs 11A-11 E: Define Ythdf2 functionality in mouse HSPCs by irCLIP-seq. Fig. 11A is an image showing immunoprecipitation of Ythdf2 in control or Flag-Ythdf2 overexpressed HPC7 cells; Fig. 11 B is an irCLIP membrane image showing IR800 labeled RNA-Ythdf2 complex, where red box indicate the RNA- Ythdf2 complex collected for library construction, and with samples without UV crosslinking serve as controls; Fig. 11 C is a Venn diagram showing intersection genes identified in three independent Ythdf2 irCLIP-seq experiments; Fig. 11 D is a Venn diagram showing overlap of Ythdf2 binding targets and m ⁶ A labeled mRNAs; Fig. 11 E shows representative tracks of Gata2 harboring m ⁶ A peaks and Ythdf2 irCLIP peaks, where coverage of m ⁶ A immunoprecipitation and input fragments are indicated in red and grey, respectively, and Ythdf2 irCLIP reads are highlighted in yellow.

[0083] Figures 12A-12F: Ythd†2 KO increased m ⁶ A-tagged mRNA expression, contributing to HSC expansion. Fig. 12A is a bar graph showing total RNA was extracted from 15,000 sorted BM LSK Flk2 ^" cells; Fig. 12B shows quantification of m ⁶ A RNA methylation in wt and Ythdf2 KO Lin ^" cells (n = 6); Fig. 12C shows quantification (right) and histogram (left) showing intracellular flow validation of increased expression of TAL1 , GATA2, RUNX1 and STAT5 in Ythdf2 KO LT-HSCs comparing to wt LT-HSCs (n = 3 mice per group); Fig. 12D shows fluorescence in situ hybridization of Gata2 mRNA (red) and fluorescence immunostaining of Dcpl a (P-body marker) (magenta), Ythdf2 (green) in wt and Ythdf2 KO HSPCs. Arrows indicate co-localized staining. Scale bars, 5 μm; Fig. 12E shows quantification of Gata2 mRNA and DCP1 a co-localization in sorted LSK cells from wt and Ythdf2 KO mice. Percentage indicates the average frequency of the Gata2 mRNA that co-localized with DCP1 a over total Gata2 mRNA level in each LSK cells (n = 12-20); Fig. 12F shows percentage of GFP ⁺ cells in the CD45 ⁺ population at 4 weeks post transplantation (n = 10). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0084] Figures 13A-13G: YTHDF2 regulates expression of transcription factors related to stem cell self-renewal in human cord blood stem cells. Fig. 13A is a Venn diagram showing intersection genes identified in three independent m ⁶ A-seq experiments, using three independent cord blood samples; Fig. 13B shows pie charts with the percentage of mRNAs and non-coding RNAs containing m ⁶ A peaks; Fig. 13C is a bar graph showing GO enrichment analysis of the transcription factors harboring m ⁶ A modifications in hUCB CD34 ⁺ cells; Fig. 13D shows images of Western blotting of YTHDF2 (up) and β-Actin (down) in sorted GFP ⁺ control and YTHDF2 KD hUCB cells, showing knockdown efficiency of YTHDF2; Fig. 13E shows a bar graph with expression level (left) and representative track plots (right) of YTHDF2 from RNA-seq analysis of control and YTHDF2 KD hUCB CD34 ⁺ cells, showing knockdown efficiency of YTHDF2. Figs. 13F through 13G are representative track plots of indicated transcription factors harboring m ⁶ A peaks (up) and their representative coverage plots from the RNA-seq analysis (bottom). Adjusted P values are indicated.

[0085] Figs. 14A-14C: YTHDF2 KD in hUCB cells resulted in HSC expansion without changing lineage output. Fig. 14A depicts representative flow plots of GFP ⁺ CD34 ⁺ CD38 ^" CD45RA ^" EPCR ⁺ HSCs in control and YTHDF2 KD hUCB cells post 7 days culture; Fig. 14B depicts confirmation of YTHDF2 protein knockdown and overexpression in transduced Hela cells; Fig. 14C depicts CFU production by YTHDF2 OE and control transduced CD34 ⁺ CD38 ^" CB from day 10 cultures (n = 3 independent human samples). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0086] Figs. 15A-15D: YTHDF2 KD in hUCB cells resulted in HSC expansion without changing lineage output. Fig. 15A includes representative flow plots of hCD45 ⁺ GFP ⁺ monocyte, megakaryocyte (MK cell), B cell and erythrocyte in primary NSG recipient BM.; Figs. 15B and 15C are bar graphs depicting the percentage of lineage cells in hCD45 ⁺ GFP ⁺ (Fig. 15B) and in total CD45 ⁺ (Fig. 15C) BM cells from primary NSG recipients at 10 weeks post transplantation (n = 13-15); Fig. 15D is a bar graph depicting a summary of human donor derived lineage chimerisms in total CD45 ⁺ BM cells from secondary NSG recipients at 12 weeks post transplantation (n = 6). Data shown as mean ± s.e.m. Unpaired t-test. n.s., nonsignificant.

[0087] FIG. 16: expansion of mesenchymal stem cells in vivo. Fig. 16 is a bar graph showing the frequency of N-Cad+ CD105+ in TNC with both wt Ythdf2 and a Ythdf2 knockout (KO). [0088] Figs. 17A-17J: functional identification of rHSC population. Fig. 17A is a schematic representation of FACS sorting for rHSCs, pHSCs, ST-HSCs and MPPs; Fig. 17B shows a quantification of rHSCs and pHSCs function by transplantation assay. PB analysis for total engrafted donor cells at the indicated number of weeks post transplantation and the percentage of donor-derived B, T and myeloid lineage cells at 20 weeks post transplantation (n = 10 mice per group); Fig. 17C shows donor derived cells from rHSCs or pHSCs transplanted mice at 40 weeks post transplantation; Fig. 17D H2B-GFP label retaining cells in rHSCs and pHSCs at 130 days post chasing (n=4 mice per group); Fig. 17E shows cell cycle gene expression in rHSCs and pHSCs (n=3 replicates from 20 mice); Fig. 17F shows rHSCs and pHSCs transplanted recipients received 5FU injection at 4 weeks post transplantation as indicated. PB analysis for donor engraft cells at indicated weeks post transplantation. The percentage of donor-derived B, T and myeloid lineage cells were shown at 20 weeks post transplantation (n=10 mice per group); Fig. 17G shows rHSCs and pHSCs at day 3 post 5FU treatment (pool from 15 mice); Fig. 17H shows DNA damage gene expression in rHSCs and pHSCs (n=3 replicated from 20 mice); Fig. 171 shows DNA damage genes in rHSCs from control mice and mice at day 3 post 5FU (n=20 in control mouse group, n=40 in D3 5FU mouse group). ^* P<0.05, ^** P<0.01 , ^*** P<0.001 . Error bars, s.e.m; Fig. 17J shows heat map of stress response genes in rHSCs, pHSCs and 5FU rHSCs.

[0089] Figs 18A-18I: rHSCs located by endosteal region in BM niche. Fig. 18A shows representative whole-mount images of mouse sternal bone marrow (BM), with bone (white, generated by second harmonic generation, SHG), MKs (yellow, distinguished by size, morphology and CD41 expression). Green arrowheads denoted phenotypic Lin ^" CD48 ^~ CD41 ^~ CD150 ⁺ CD49b ^~ rHSCs, White arrowheads denoted phenotypic Lin ^" CD48 ^" CD41 ^" CD150 ⁺ CD49b ⁺ pHSCs; Fig. 18B shows representative image of rHSCs, pHSCs and 5FU rHSCs. White arrows denoted phenotypic Lin ^" CD48 ^" CD4rCD150 ⁺ CD49b ^" rHSCs, phenotypic Lin ^~ CD48 ^~ CD41 ^~ CD150 ⁺ CD49b ⁺ pHSCs and phenotypic Lin ^_ CD48 ^_ CD4r CD150 ⁺ CD49b ^~ 5FU rHSCs; Figs. 18C-E show relative distance between rHSCs, pHSCs and 5FU rHSCs to vessels, MKs or bones (n=144 rHSCs, n=685 pHSCs, n=707 5FU rHSCs); Fig. 18F shows absolute number of healthy, apoptotic and dead VE-Cad ⁺ CD31 ⁺ vessel cells in control mice and mice at 1 day post 5FU treatment; Figs. 18F-G show absolute number of AnnexinV ^4" SytoxG- apoptotic Ve-Cad ⁺ CD31 ⁺ endothelial cells in central marrow (CM) or N-cad- tomato ^4" cells in both CM and bone from control mice and mice 1 day post 5FU treatment. N=3 in each group. ^* P<0.05, ^** P<0.01 , ^*** P<0.001 . Error bars, s.e.m; Figs. 18H-I show representative image of N-cad- tomato ⁺ and Ve-Cad ⁺ CD31 ⁺ vessels in control mice and in mice at 3 days post 5FU treatment.

[0090] Figs. 19A-19J: N-cad ⁺ cells maintain functional HSCs in BM niche.

Fig. 19A shows a scheme for DT administration to N-cad-CreER ^T ;iDTR mice used for the experiments shown in B-G. D indicated day (e.g., DO indicates day 0); Fig. 19B shows N-cad ⁺ cell ablation efficiency as indicated by Tomato ^4" cells in N-cad- CreER ^T ;iDTR mice; Figs. 19C-D show flow cytometric analyses to determine the absolute numbers of total nucleated cells (TNC) and HSPCs in the bone marrow (BM) from N-cad-CreER ^T ;iDTR mice post TMX and DT injections (n=5 mice per group); Figs. 19E-H show quantification of functional HSCs by transplantation assay in primary 1 ^° and secondary 2 ^° transplantation. Total BM cells from PB analysis for total engrafted donor cells at the indicated number of weeks post transplantation and the percentage of donor-derived B, T and myeloid lineage cells at 16 weeks post transplantation (n = 10 mice per group); Figs. 191-J show flow cytometric analyses to determine the absolute numbers of HSPCs in the bone marrow (BM) from N-cad- CreER ^T ;SCFf/f (N-cad-CreER ^T _;SCFf/f, n=4 mice, N-cad-CreER ^T+ ;SCFf/f, n=6 mice) and N-cad-CreER ^T+ ; Cxcl12f/f mice (N-cad-CreER ^T _; Cxcl12f/f, n=3 mice, N-cad- CreER ^T+ ; Cxcl12f/f, n=6 mice) post TMX and DT injections. ( ^* P<0.05, ^** P<0.01 , ^*** P<0.001 . Error bars, s.e.m.).

[0091] Figs 20A-20K: transcriptome analysis for hematopoietic cells and niche cells. Figs. 20A-B shows pearson distance tree and PCA analysis for hematopoietic stem and progenitor cells (HSPCs); Figs. 20C-D show pearson distance tree and PCA analysis for BM niche cells; Fig. 20E shows HSC signature gene expression in hematopoietic stem and progenitor cells; Fig. 20F shows BM niche signature gene expression in niche cells; Fig. 20G shows term analysis for BM niche cells; Fig. 20H shows stromal cell development gene expression in niche cells from endosteal and perivascular zones; Fig. 20I shows lineage tracing for N-cad- CreER ^T ; R26-tdT; Nestin-GFP mice after 3 TMX injections, and N-cad-CreER ^T ; R26- ZsG; Cxcl12-DsR mice after 3 TMX injections; Fig. 20J shows enzymatically digested bone marrow cells from N-cad-CreERT; R26-tdT mice post TMX injections. LepR and Pdfgra stained by antibodies shown as red peak. Isotype control shown as gray peak; Fig. 20K shows CFU-F activity in niche cells from endosteal and perivascular zones.

[0092] Fig. 21A-21G: in vitro differentiation potential and localization of N-cad ⁺ derived cells. Fig. 21 A shows experimental design; Fig. 21 B shows live CFU-F colonies cultured from enzymatically digested bone marrow, stained with Cell Trace™ and Tomato ^4" cells in one colony in high magnification; Figs. 21 C-E show in vitro differentiation of stromal cells derived from N-cad-CreER ^T ; R26-tdT mice: live tdTomato ^4" cells and Alkaline Phosphatase staining of culture 21 days after osteo- differentiation (21 C); live Tomato ^4" cells and Oil Red O lipid staining 21 days after adipo-differentiation (21 D); aggrecan antibody stained and toluidine blue stained chondrocytes at 21 days after chondro-differentiation of Tomato ^4" cells (21 E); Figs. 21 F-G show(F-G) experimental design; Fig. 21 H shows localization of cells derived from N-cad-CreERT; R26-tdT mice post TMX injection at 6 hours, 14 hours, 24 hours, 1 week and 4 weeks; Fig. 21 I shows percentage of Tomato ^4" cells post TMX injection at 1 week, 2 weeks, 4 weeks and 6 weeks in trabecular, cortical bone and central marrow (n=2 mice in 1 and 2 weeks group, n=3 mice in 4 and 6 weeks group. ^* P<0.05, ^** P<0.01 , ^*** P<0.001 . Error bars, s.e.m.).

[0093] Figs. 22A-22K: N-cad ⁺ stromal cells give rise to osteoblasts and adipocytes in adult mice. Fig. 22A shows representative femur sections from N- cad-CreER ^T ; R26-tdT; Col2.3-GFP mice of different time points post TMX injection showing the anatomical distribution and increasing generation of Tomato ⁺ Col2.3- GFP ^4" osteoblasts. Scale bar, 100μm; Figs. 22B-C show higher-power images of N- cad-CreER ^T ; R26-tdT; Col2.3-GFP mice at early time points post TMX in region i, ii, iii, iv, v and vi shown in 6hr (22B) and 14hr (22C). Hollow arrowheads show Tomato ⁺ Col2.3GFP ⁺ osteoblasts (yellow cells) at 6 hours and 14 hours post TMX, whereas solid arrowheads indicate the N-cad recombined both in the trabecular (ii, iv, v) and cortical (iii, vi) region (green). These cells are potentially undifferentiated as shown by the absence of Col2.3-GFP. Scale bar, 50 μm; Figs. 22D-E show image quantification showing percentage of Tomato ^4' Col2.3-GFP ^4' and Tomato ⁺ Col2.3- GFP ^~ cells in trabecular (22D) and compact bone (22E) at 6 hours, 14 hours, 24 hours, 2 weeks and 4 weeks post TMX injection; Fig. 22F shows image quantification comparing percentage of potential undifferentiated Tomato ⁺ Col2.3- GFP ^~ between trabecular and compact bone. ^* P<0.05, ^** P<0.01 , ^*** P<0.001 . Error bars, s.e.m; Fig. 22G shows representative image of trabecular bone (TB) and cortical bone (CB) in N-cad-CreER ^T ; R26-tdT; Col2.3-GFP mice at 4 weeks post TMX injection. Scale bar, 20 μm; Fig. 22H shows representative low and high-power image of femur section from N-cad-CreER ^T ; R26-tdT mice with Perilipin staining at 4 weeks post TMX injection. At periosteal region (i), bone marrow near trabecular region (ii) and central marrow(iii), solid arrowheads showed Tomato ⁺ Perilipin ⁺ derived from N-cad ⁺ MSCs; Fig. 22I shows BODIPY staining showed the lipid droplet (green) inside the Tomato ^4" Perilipin ^4" adipocyte (arrowheads). Scale bar, 20 μm; Figs. 22J-K show image quantification of Tomato ^" " ^" Perilipin ^4" adipocytes in trabecular bone (22J) and periosteal region (22K) at 6 hours, 14 hours, 24 hours, 2 weeks and 4 weeks post TMX injection. ( ^* P<0.05, ^** P<0.01 , ^*** P<0.001. Error bars, s.e.m.).

[0094] Figs. 23A-23J: N-cad ⁺ MSCs give rise to chondrocytes during development and post injury. Fig. 23A shows experimental design; Fig. 23B shows tomato ^4" partially colocalized with Aggrecan ^4" chondrocytes in rib at E14.5(2 days post TMX induction); Fig. 23C shows an Illustration of chondrocyte development in femur; Fig. 23D shows a representative femur section from 2-day-old N-cad-CreER ^T ; R26-tdT mice with TMX induction at E12.5. Note that Tomato ^4" cells gave rise to Aggrecan ^4" cartilage cells in articular surface (i) and the developing secondary ossification center (ii). Arrowheads indicated Tomato ^4" Aggrecan ^4" cells; Fig. 23E shows a representative femur section from 10-month-old N-cad-CreER ^T ; R26-tdT mice with TMX induction at E12.5. Tomato ^4" cells from early embryonic stage differentiated to Perilipin ^4" adipocytes; Fig. 23F shows a representative femur section from 2-month-old N-cad-CreER ^T ; R26-tdT m\ce with TMX induction at E12.5. Tomato ^4" cells from early embryonic stage differentiated to Osteopontin+ hypertrophic chondrocytes (i and ii) and osteoblasts (iii, iv, v and vi); Fig. 23G shows an experimental design for femoral groove injury to N-cad-CreER ^T ; R26-tdT mice with TMX induction at E12.5; Fig. 23H shows quantification of Tomato ^4" chondrocytes in control mice and in mice 3 weeks after knee cartilage injury. (I) Representative section of distal femur from N-cad-CreER ^T ; R26-tdT mice without cartilage injury or 3 weeks after cartilage injury. Note that the clustered Tomato ^4" Aggrecan ^4" cells at the knee surface of control mice (i) significantly increased at the correspondent region in mice 3 weeks after knee cartilage injury (ii); Fig. 23J shows AHO staining of the sections in Figure 6H showing the Alcian blue positive chondrocytes in control (i) and in the location of cartilage injury(ii).

[0095] Figs. 24A-24K: BrdU assay and N-cad reporter mouse line post 5FU. Figs. 24A-F show representative images showing BrdU ^4" cells in femur of mice at day 0, 2, 3, 4 and 6 post 5FU treatment. Note that BrdU ^4" cells reduced at day 2 post 5FU, and reactivated from day 3 to day 6 post 5FU near bone (red dotted line) or vessel/adipose structure (green dotted line); Fig. 24G shows a model of dynamic of BrdU ^4" cells; Fig. 24H shows a ratio of BrdU ^4" cells in bone marrow to bone surface from day 3 to 6 post 5FU treatment; Fig. 24I shows a percentage of BrdU ^4" cells near bone surface and near vessel/adipose structures from day 3 to 6 post 5FU treatment; Fig. 24J shows representative whole bone section showing N-cad driven tomato ^4" cells in both bone surface and central marrow; Fig. 24K shows absolute number of N-cad driven tomato ^4" cells in central marrow (CM) and in bone of N-cad- TdT mice of control group and mice 3 days post 5FU treatment. N=3 in each group. ^* P<0.05, ^** P<0.01 , ^*** P<0.001. Error bars, s.e.m.

[0096] Figs. 25A-25B: generation of N-cad-CreER ^T mouse strain. Fig 25A shows a generation of N-cad-CreER ^T mouse strain; Fig. 25B shows lineage tracing for N-cad-CreER ^T , R26-tdT; Col2.3-GFP mice after three TMX injections; blood vessels stained by CD31 and Ve-cadherin antibodies.

[0097] Figs. 26A-26E: sorting strategy and signature gene expression of niche cells. Fig. 26A shows niche cells in endosteal zone harvested from digested bone cells; Fig. 26 B shows niche cells from Perivascular and sinusoid zones harvested from digested bone marrow cells; Fig. 26C shows localization of NG2- RFP ⁺ cells in endosteal and peri-arterial regions; Figs 26D-E show heatmaps of osteo-chondrogenic progenitor gene and adipogenic progenitor gene expression in niche cells.

[0098] Figs. 27A-27E: lineage tracing in N-cad-CreERT; R26-tdT mice at early time point post TMX induction. Figs. 27A-B show representative images of trabecular bone (TB) and cortical bone(CB) in N-cad-CreERT; R26-tdT; Col2.3-GFP mice at 24 hours and 2 weeks post TMX induction. Scale bar, 20μΓη; Figs. 27C-E show representative femur section with high power images of N-cad-CreERT; R26- tdT mice at 6 hours, 14 hours and 24 hours post TMX induction. Adipocytes shown with perilipin antibody staining.

[0099] Figs. 28A-28D: characterization of N-cad ⁺ MSCs from early development and in injury repair. Fig. 28A shows representative images of femur section showing N-cad ⁺ MSCs with Aggrecan staining in N-cad-CreER ^T ; R26-tdT 24 hours post TMX induction at postnatal D2; Fig. 28B shows a representative femur section from 2-month-old N-cad-CreER ^T ; R26-tdT m ce with TMX induction at E12.5. Adipocytes shown with Perilipin antibody staining; Fig. 28C shows a representative femur section from 2-day-old N-cad-CreER ^T ; R26-tdT mice with TMX induction at E12.5. Developing bone cells shown with Osteopontin antibody staining; Fig. 28D shows a representative sagittal knee sections in E12.5 TMX induced N-cad-CreER ^T ; R26-tdT mice of control group and 2 weeks after knee cartilage injury. Scale bar, 1 mm.

[00100] Fig. 29: Image of surviving rHSCs (green, CD150 ⁺ Lin- CD49b ^~ ) at day 3 post 5FU treatment were often detected as single cells adjacent to the bone surface (white, SHG), and proliferating HSCs were often associated with MKs (CD150 ⁺ Lin ⁺ ) or near the vessels (red, CD31 +).

[00101] Fig. 30: Plot showing production of human Thpo by 10K ncad+ hMSCs and 1000k total hMSCs in vivo.

DETAILED DESCRIPTION OF THE INVENTION

[00102] One embodiment of the invention is a method for expanding a population of stem cells obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow. This method comprises modulating a N6-methyladenosine (m ⁶ A) mRNA modification pathway in the population of stem cells to expand the number of stem cells.

[00103] Another embodiment of the invention is a method for expanding a population of chimeric antigen receptor (CAR) T-cells cells obtained by modifying T- cells obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow. This method comprises modulating a N6- methyladenosine (m ⁶ A) mRNA modification pathway in the population of CAR T-cells to expand the number of stem cells.

[00104] In the present invention the population of stem cells and/or T-cells may be obtained from any mammal, such as, e.g., a human, and from any tissue that contains stem cells and/or progenitor cells and/or T-cells. As noted above, in a preferred embodiment the tissue may be peripheral blood, cord blood or bone marrow.

[00105] As used herein, "expand", "expanding" and like terms means to increase the number of stem cells and/or CAR-T cells in the population relative to the number of stem cells and/or CAR T-cells in the original population either in vivo or ex vivo using any of the methods disclosed herein. The expansion may be at least 40- fold compared to the original number of stem cells and/or CAR T-cells in the population. More preferably, the expansion is at least 2-fold, 4-fold, 5-fold, 8-fold, 10-fold, 15-fold, 20-fold or more compared to the original number of stem cells.

[00106] In the present invention "a population of stem cells" means a group of substantially undifferentiated cells that possess the ability to give rise to many different types of cells and which have the ability to self-renew. Representative, non- limiting examples of stem cells according to the present invention include bronchioalveolar stem cells (BASCs), bulge epithelial stem cells (bESCs), corneal epithelial stem cells (CESCs), cardiac stem cells (CSCs), epidermal neural crest stem cells (eNCSCs), embryonic stem cells (ESCs), endothelial progenitor cells (EPCs), hepatic oval cells (HOCs), hematopoetic stem cells (HSCs), hematopoietic stem and progenitor cells (HSPCs), keratinocyte stem cells (KSCs), mesenchymal stem cells (MSCs), neuronal stem cells (NSCs), pancreatic stem cells (PSCs), retinal stem cells (RSCs), and skin-derived precursors (SKPs).

[00107] Hematopoietic stem cells, for example, have the ability to self-renew (i.e., expand) and can give rise to all the types of progenitor cells (such as, e.g., CMP, GMP, MEP and CLP) and ultimately all the types of blood cells (such as e.g., red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, macrophages, and platelets) in the hematopoietic system. Mesenchymal stem cells, as another example, are multipotent stromal cells that can differentiate into a variety of cell types (such as, e.g., osteoblasts, chondrocytes, myocytes and adipocytes).

[00108] In the present invention "a population of chimeric antigen receptor (CAR) T-cells" means a group of T-cells that have been modified with chimeric antigen receptors capable of binding specific antigens, such as antigens on the surface of cancer cells, and may possess the ability to target and kill such cancer cells. The CAR T-cells can be prepared by modifying T-cells with the chimeric antigen receptors, such as by introducing DNA coding for the chimeric antigen receptors into the T-cells, to express the chimeric antigen receptors on the surface of the T-cells (Davila et al. Sci TransI Med 6(224), 224ra25 (2014); Tasian et al. Ther Adv Hematol 6(5), 228-241 (2015).

[00109] In the present invention, "modulating", "modulation" and like terms mean altering the signal transduction pathway, e.g., a protein in the m ⁶ A mRNA modification pathway, including but not limited to lowering or increasing the expression level of a protein, altering the sequence of such a protein (by mutation, pre-translational or post-translational modification or otherwise), or inhibiting or activating such a protein (whether by binding, phosphorylation, glycosylation, translocation or otherwise). Such modulation may be achieved genetically or pharmacologically.

[00110] In one aspect of the present invention, modulating the m ⁶ A mRNA modification pathway comprises introducing a mutation into a population of stem cells and/or CAR-T cells, which mutation results in modulation of a molecule in the m ⁶ A mRNA modification pathway. In another aspect present invention, modulation of the m ⁶ A mRNA modification pathway also includes contacting the stem cells and/or CAR T-cells with a modulator of a molecule in the m ⁶ A mRNA pathway. Representative, non-limiting examples of such modulators include a small molecule, a biologic, an antisense RNA, a small interfering RNA (siRNA), and combinations thereof.

[00111] In the present invention, the phrase "modulation of a molecule in the m ⁶ A mRNA modification pathway" means altering the function of a member of the m ⁶ A mRNA modification pathway, which altered function may have an effect similar to inhibiting or decreasing the function of a molecule involved in a process upstream and/or downstream of m ⁶ A modification of mRNA. Non-limiting examples of such "modulation" include increasing or decreasing the expression or function of proteins involved in any of the incorporation of N ⁶ -methyladenosine modifications in mRNA, removal of N ⁶ -methyladenosine mRNA modifications to mRNA, and/or the recognizing and processing of /V ⁶ -methyladenosine modified mRNA. For example, the modulation may include increasing or decreasing N ⁶ -methyladenosine modifications in mRNA, and/or that affecting the type and/or distributions of such modifications in mRNA, such as by modulating the activity of one or more of a m ⁶ A writer (e.g. methyltransferase) and m ⁶ A eraser (e.g. demethylase). As another example, the modulation may increase or decreases expression or function of proteins that recognize N ⁶ -methyladenosine modifications to mRNA to mediate m ⁶ A- dependent functions, such as by modulating the activity of a m ⁶ A reader (e.g., an RNA binding protein that recognizes methylated adenosine). Thus, modulation of the molecule in the m ⁶ A mRNA modification pathway modulator may result in modulation of the activity and/or expression of a molecule upstream or downstream of an m ⁶ A mRNA modification process. [00112] In one aspect, the modulation of the m ⁶ A mRNA modification pathway involves modulation of a molecule selected from the group consisting of m ⁶ A mRNA modification readers, m ⁶ A mRNA modification writers, m ⁶ A mRNA modification erasers and combinations thereof. Non-limiting examples of m ⁶ A modification writers include methyltransferases that are capable of post-transcriptionally installing the m ⁶ A modification in messenger RNA, and can include any selected from the group consisting of METTL3, METTL14, WTAP, KIAA1429 and combinations thereof. Non-limiting examples of m ⁶ A modification erasers include demethylases that are capable of reversing the methylation, and can include any selected from the group consisting of FTI, ALKBH5 and combinations thereof. M ⁶ A modification readers include proteins that are capable of selectively binding m ⁶ A-methylated mRNA to exert regulatory functions through selective recognition of methylated mRNA. Suitable m ⁶ A modification readers can include any selected from the group consisting of Ythdfl , Ythdf2, Ythdf3, Ythdc1, Ythdc2, HNRNPC, HNRNPA2B1 , elF3 and combinations thereof. According to one aspect, the m ⁶ A modification readers comprise proteins of the YTH domain family of proteins, which includes Ythdfl , Ythdf2, Ythdf3, Ythdc1, Ythdc2 and combinations thereof, {see, e.g., Wang et al. Nature, 505(7481 ): 117-120, 2014; Frayling et al. Science, 316: 889-894, 2007; Zheng et al. Mol. Cell., 49: 18-29, 2012; Cao et al. Open Biol., 6(4): 160003, 2016; Maity et al. The FEBS Journal, 283(9): 1607-1630, 2016).

[00113] As used herein, "introducing a mutation" means any conventional method for producing an alteration in the genetic makeup of the stem cell population and/or CAR T-cell population. Non-limiting examples for introducing a mutation into a stem cell population and/or CAR T-cell population include mutagenesis via ultraviolet light irradiation, chemical mutagenesis, targeted mutagenesis such as site directed mutagenesis of a stem cell and/or CAR T-cell, and creation of a transgenic mouse. According to one aspect, a mutation may be introduced into the stem cell and/or CAR T-cell to delete, replace or reduce expression of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway, such as a molecule selected from the group consisting of a m ⁶ A mRNA modification reader, a m ⁶ A mRNA modification writer, a m ⁶ A mRNA modification eraser and combinations thereof. In one aspect, the mutation is introduced to delete, replace or reduce expression of a gene that expresses a m ⁶ A mRNA modification reader, such as any selected from the group consisting of Ythdf 1 , Ythdf2, Ythdf3, Ythdc1 , Ythdc2, HNRNPC, HNRNPA2B1 , elF3 and combinations thereof. In a preferred aspect, a mutation is introduced to delete, replace or reduce expression of a gene that expresses Ythdf2. In yet another aspect, the mutation is introduced to delete, replace or reduce expression of a gene that expresses a m ⁶ A mRNA modification writer, such as any selected from the group consisting of METTL3, METTL14, WTAP, KIAA1429 and combinations thereof. In yet another aspect, the mutation is introduced to delete, replace or reduce expression of a gene that expresses a m ⁶ A mRNA modification eraser, such as any selected from the group consisting of FTI, ALKBH5 and combinations thereof.

[00114] In one aspect, the mutation can be introduced by exposing the stem cells and/or CAR T-cells to a Mx1 -Cre targeting system (see, e.g., Kuhn et al. Science, 269(5229): 1427-1429, 1995) that inactivates or deletes at least a portion of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. In yet another aspect, a mutation is introduced that incorporates short hairpin RNA (shRNA) into the stem cells and/or CAR T-cells to reduce expression of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. For example, the shRNA may be introduced by exposing the stem cells and/or CAR T-cells to a vector to deliver shRNA, which may be a viral vector such as lentivirus (see, e.g., Chira et al. Oncotarget, 6(31 ): 30675-30703, 2015). The shRNA may be capable of triggering gene silencing to regulate gene expression (see, e.g., Paddison et al. Genes Dev. , 16(8): 948-958, 2002).

[00115] As used herein, "a modulator of a N ⁶ -Methyladenosine mRNA modification pathway" (or "m ⁶ A mRNA modification pathway modulator") is any agent that regulates the activity of any member of the m ⁶ A mRNA modification pathway, which results in, e.g., an increase or decrease in N ⁶ -methyladenosine modifications in mRNA, and/or a change in the types and/or distributions of such modifications in mRNA, such as by modulating the activity of one or more of a m ⁶ A writer (e.g. methyltransferase) and m ⁶ A eraser (e.g. demethylase). As another example, the agent may be one that increases or decreases activity of proteins that recognize Λ/ ⁶ - methyladenosine modifications to mRNA to mediate m ⁶ A-dependent functions, such as by modulating the activity of a m ⁶ A reader (e.g., an RNA binding protein that recognizes methylated adenosine). Thus, the m ⁶ A mRNA modification pathway modulator may act on, or upstream of, or downstream of, an agent that affects the m ⁶ A modification to mRNA.

[00116] In one embodiment, the m ⁶ A mRNA modification pathway may be modulated by down-regulating and/or inhibiting a member of the m ⁶ A mRNA modification pathway, such as down-regulating and/or inhibiting a m ⁶ A mRNA modification reader. As used herein, "down-regulating" means inhibiting or reducing the amount of or inhibiting or decreasing the activity of a member of the m ⁶ A mRNA modification pathway. Such down-regulation may be accomplished using, e.g. antisense RNA, siRNA, antibodies, or small molecules. As another example, the m ⁶ A mRNA modification reader may be down-regulated by contacting the stem cells and/or CAR T-cells with an inhibitor of an m ⁶ A mRNA reader, to inhibit binding and/or recognizing of the m ⁶ A modified mRNA by the m ⁶ A mRNA reader. In one aspect, the m ⁶ A mRNA modification reader that is down-regulated is selected from the group consisting of Ythdf 1 , Ythdf2, Ythdf3, Ythdc1 , Ythdc2, HNRNPC, HNRNPA2B1 , elF3 and combinations thereof. In a preferred aspect, the m ⁶ A mRNA modification reader that is down-regulated is Ythdf2. Inhibitors of the m ⁶ A mRNA modification reader may be any selected from the group consisting of: (inhibitors of HNRNPC) hsa-let-7e-5p (MIRT051596), hsa-mir-455-3p (MIRT037890), hsa-mir-30c-5p (MIRT047904), hsa-mir-186-5p (MIRT045150), hsa- mir-744-5p (MIRT037494), hsa-mir-18a-3p (MIRT040851 ), hsa-mir-484 (MIRT042196), hsa-mir-505-5p (MIRT037959), hsa-mir-615-3p (MIRT039991 ), hsa- mir-342-3p (MIRT043694), hsa-miR-3607-3p, hsa-miR-30d, hsa-miR-3916, hsa-miR- 3162-5p, hsa-miR-1273d, hsa-miR-3161 , hsa-miR-30a, hsa-miR-629, hsa-miR-208b, hsa-miR-489, hsa-miR-3148, hsa-miR-2113, hsa-miR-877, hsa-miR-455-5p, hsa- miR-186, hsa-miR-548o, hsa-miR-3139, hsa-miR-320a, hsa-miR-4311 , hsa-miR- 555, hsa-miR-3605-5p, hsa-miR-515-5p, hsa-miR-144, hsa-miR-499-5p, hsa-miR- 1323, hsa-miR-548x, hsa-miR-299-5p, hsa-miR-653, hsa-miR-576-5p, hsa-miR- 548p, hsa-miR-586, hsa-miR-888, hsa-miR-3647-3p, hsa-miR-484, hsa-miR-320b, hsa-miR-620, hsa-miR-30b, hsa-miR-548q, hsa-miR-29b-1 , hsa-miR-570, hsa-miR- 183, hsa-miR-1276, hsa-miR-208a, hsa-miR-186, hsa-miR-28-5p, hsa-miR-330-3p, hsa-miR-548am, hsa-miR-320d, hsa-miR-3175, hsa-miR-3155, hsa-miR-548aa, hsa- miR-519e, hsa-miR-1270, hsa-miR-513b, hsa-miR-599, hsa-miR-518f, hsa-miR- 4301 , hsa-miR-30c, hsa-miR-3135, hsa-miR-4286, hsa-miR-202, hsa-miR-4263, hsa-miR-4299, hsa-miR-606, hsa-miR-3133, hsa-miR-583, hsa-miR-3125, hsa-miR- 501 -5p, hsa-miR-7-1 , hsa-miR-514b-3p, hsa-miR-3155b, hsa-miR-548d-3p, hsa- miR-224, hsa-miR-7-2, hsa-miR-708, hsa-miR-3199, hsa-miR-514, hsa-miR-30e (see, e.g. Helwak et al. Cell, 153(3): 654-655, 2013; Whisnant et al., MBio 4(2), 2013:e000193); (inhibitors of HNRNPA2B1 ) hsa-mir-92a-3p (MIRT049721 ), hsa-mir- 30c-5p (MIRT048009), hsa-mir-191 -5p (MIRT045809), hsa-let-7f-5p (MIRT051404), hsa-mir-27b-3p (MIRT046213), hsa-mir-877-3p (MIRT037116), hsa-mir-615-3p (MIRT040278), hsa-mir-1260b (MIRT052680), hsa-mir-103a-3p (MIRT027027), hsa- mir-16-5p (MIRT031508), hsa-mir-1296-5p (MIRT036075), hsa-mir-197-3p (MIRT048098), hsa-miR-548j, hsa-miR-3678-3p, hsa-miR-607, hsa-miR-188-5p, hsa-miR-15a, hsa-miR-3653, hsa-miR-371 -5p, hsa-miR-550a, hsa-miR-3622b-3p, hsa-miR-548a-5p, hsa-miR-3170, hsa-miR-3148, hsa-miR-556-3p, hsa-miR-490-3p, hsa-miR-559, hsa-miR-200c, hsa-miR-130a, hsa-miR-548y, hsa-miR-548o, hsa-miR- 23c, hsa-miR-491 -3p, hsa-miR-335, hsa-miR-3667-3p, hsa-miR-466, hsa-miR-23b, hsa-miR-4310, hsa-miR-127-5p, hsa-miR-548b-5p, hsa-miR-616, hsa-miR-16, hsa- miR-338-3p, hsa-miR-3200-5p, hsa-miR-362-3p, hsa-miR-448, hsa-miR-1306, hsa- miR-944, hsa-miR-3684, hsa-miR-373, hsa-miR-103a, hsa-miR-380, hsa-miR-499- 5p, hsa-miR-1323, hsa-miR-323-5p, hsa-miR-3674, hsa-miR-1252, hsa-miR-33b, hsa-miR-580, hsa-miR-548c-3p, hsa-miR-103a-2, hsa-miR-548w, hsa-miR-600, hsa- miR-634, hsa-miR-586, hsa-miR-497, hsa-miR-720, hsa-miR-654-3p, hsa-miR-524- 5p, hsa-miR-543, hsa-miR-548q, hsa-let-7f-2, hsa-miR-330-5p, hsa-miR-500a, hsa- miR-548l, hsa-miR-570, hsa-miR-374a, hsa-miR-1184, hsa-miR-649, hsa-miR-424, hsa-miR-3658, hsa-miR-186, hsa-miR-326, hsa-miR-548d-5p, hsa-miR-23a, hsa- miR-15b, hsa-miR-190, hsa-miR-203, hsa-miR-548h, hsa-miR-3136-5p, hsa-miR- 618, hsa-miR-551 b, hsa-miR-211, hsa-miR-1305, hsa-miR-513b, hsa-miR-96, hsa- miR-2117, hsa-miR-548n, hsa-miR-3910, hsa-miR-217, hsa-miR-892b, hsa-miR- 502-5p, hsa-miR-548i, hsa-miR-520d-5p, hsa-miR-4299, hsa-miR-1285, hsa-miR- 3133, hsa-miR-483-3p (see, e.g., Hafner et al. Cell, 141 (1 ): 129-141 , 2010; Helwak et al. Cell, 153(3): 654-655, 2013); (inhibitors of Ythdfl ) hsa-miR-548g, hsa-miR-204, hsa-miR-3143, hsa-miR-521 , hsa-miR-195, hsa-miR-3182, hsa-miR-3941 , hsa-miR- 34c-3p, hsa-miR-767-3p, hsa-miR-563, hsa-miR-548c-5p, hsa-miR-1911 , hsa-miR- 26b, hsa-miR-190b, hsa-miR-33a, hsa-miR-329, hsa-miR-221 , hsa-miR-612, hsa- miR-3185, hsa-miR-3156-5p, hsa-miR-107, hsa-miR-664, hsa-miR-3657; (inhibitors of Ythdf2) hsa-mir-615-3p (MIRT040054), hsa-mir-106b-5p (MIRT044257), hsa-mir- 1 (MIRT023842), miR-145, hsa-miR-3607-3p, hsa-miR-200a, hsa-miR-301 a, hsa- miR-519a, hsa-miR-141 , hsa-miR-130b, hsa-miR-181 b, hsa-miR-301 b, hsa-miR- 3117-3p, hsa-miR-1236, hsa-miR-181 a, hsa-miR-519c-3p, hsa-miR-551 b, hsa-miR- 519e, hsa-miR-519b-3p, hsa-miR-19b, hsa-miR-1303, hsa-miR-608, hsa-miR-145, hsa-miR-130a, hsa-miR-181 c, hsa-miR-323b-3p, hsa-miR-421 , hsa-miR-515-5p, hsa-miR-3666, hsa-miR-181 d, hsa-miR-146a, hsa-miR-4295, hsa-miR-454, hsa- miR-3919, hsa-miR-19a, hsa-miR-543, hsa-miR-4262 {see, e.g. Helwak et al. Cell, 153(3): 654-655, 2013; Selbach et al. Nature, 455(7209): 58-63, 2008; Yang et al. J Biol Chem., 292(9): 3614-3623, 2017); (inhibitors of Ythdf3) hsa-miR-582-3p, hsa- miR-579, hsa-miR-520e, hsa-miR-520f, hsa-miR-3152-3p, hsa-miR-106a, hsa-miR- 30d, hsa-miR-30a, hsa-miR-93, hsa-miR-508-5p, hsa-miR-29a, hsa-miR-3148, hsa- miR-490-5p, hsa-miR-520b, hsa-miR-20a, hsa-miR-409-3p, hsa-miR-4255, hsa-let- 7i, hsa-miR-373, hsa-let-7e, hsa-miR-520c-3p, hsa-miR-3920, hsa-miR-127-5p, hsa- miR-380, hsa-miR-616, hsa-miR-4277, hsa-miR-448, hsa-miR-16-2, hsa-let-7c, hsa- miR-340, hsa-miR-373, hsa-miR-520a-3p, hsa-miR-144, hsa-miR-1265, hsa-miR- 548x, hsa-miR-362-5p, hsa-miR-33b, hsa-miR-26b, hsa-miR-17, hsa-miR-569, hsa- miR-3618, hsa-miR-576-5p, hsa-miR-922, hsa-miR-302a, hsa-miR-106b, hsa-miR- 888, hsa-miR-484, hsa-let-7b, hsa-miR-582-5p, hsa-let-7f, hsa-miR-30b, hsa-miR- 524-5p, hsa-miR-302d, hsa-let-7d, hsa-miR-513a-5p, hsa-miR-500a, hsa-miR-570, hsa-miR-548l, hsa-miR-105, hsa-miR-374c, hsa-let-7g hsa-miR-372, hsa-miR-3658, hsa-let-7a, hsa-miR-3908, hsa-miR-302b, hsa-miR-526b, hsa-miR-190, hsa-miR- 181 b, hsa-miR-433, hsa-miR-98, hsa-miR-3606, hsa-miR-595, hsa-miR-548am, hsa- miR-187, hsa-miR-561 , hsa-miR-181 a, hsa-miR-3155, hsa-miR-655, hsa-miR-302c, hsa-miR-195, hsa-miR-26a, hsa-miR-590-3p, hsa-miR-30c, hsa-miR-502-5p, hsa- miR-495, hsa-miR-137, hsa-miR-181 c, hsa-miR-520d-5p, hsa-miR-3942-5p, hsa- miR-202, hsa-miR-302e, hsa-miR-513c, hsa-miR-885-5p, hsa-miR-520a-5p, hsa- miR-583, hsa-miR-1297, hsa-miR-7-1 , hsa-miR-520d-3p, hsa-miR-3155b, hsa-miR- 3182, hsa-miR-519d, hsa-miR-550a, hsa-miR-7-2, hsa-miR-181 d, hsa-miR-190b, hsa-miR-1912, hsa-miR-151 -3p, hsa-miR-33a, hsa-miR-525-5p, hsa-miR-20b, hsa- miR-514b-5p, hsa-miR-30e, hsa-miR-4262, hsa-miR-636; (inhibitor of elF3) hsa-mir- 92b-3p (MIRT040734), hsa-mir-16-5p (MIRT031705), hsa-mir-18a-3p

(MIRT040974), hsa-mir-155-5p (MIRT020771 ), hsa-mir-484 (MIRT042324), hsa-let- 7c-5p (MIRT051776), hsa-miR-3910, hsa-miR-148b, hsa-miR-136, hsa-miR-15a, hsa-miR-488, hsa-miR-500a, hsa-miR-1297, hsa-miR-3159, hsa-miR-374c, hsa- miR-424, hsa-miR-7-1 , hsa-miR-186, hsa-miR-195, hsa-miR-15b, hsa-miR-26b, hsa- miR-505, hsa-miR-1206, hsa-miR-653, hsa-miR-1283, hsa-miR-7-2, hsa-miR-196a, hsa-miR-497, hsa-miR-33a, hsa-miR-655, hsa-miR-26a hsa-miR-16, hsa-mir-151 a- 3p (MIRT043600), hsa-mir-92a-3p (MIRT049064), hsa-mir-615-3p (MIRT039779), hsa-mir-877-3p (MIRT036964), hsa-mir-222-3p (MIRT046746), hsa-mir-423- 3p (MIRT042468), hsa-mir-324-3p (MIRT042887), hsa-mir-124-3p (MIRT022932), hsa-miR-3140-3p, hsa-miR-124, hsa-miR-198, hsa-miR-525-5p, hsa-miR-506, hsa- miR-520a-5p, hsa-miR-196a ^* hsa-miR-3117-3p, hsa-mir-342-5p (MIRT038210), hsa- mir-378a-5p (MIRT043981 ), hsa-mir-615-3p (MIRT040086), hsa-let-7b-

5p (MIRT052211), hsa-mir-455-3p (MIRT037879), hsa-miR-4267, hsa-miR-590-3p, hsa-mir-106b-5p (MIRT044355), hsa-mir-320a (MIRT044466), hsa-mir-16- 5p (MIRT032018), hsa-mir-155-5p (MIRT021009), hsa-miR-4302, hsa-mir-191 -5p (MIRT045793), hsa-mir-1303 (MIRT035890), hsa-mir-193b-3p (MIRT016316), hsa- mir-222-3p (MIRT046640), hsa-mir-532-3p (MIRT037924), hsa-mir-18a-3p (MIRT040929), hsa-mir-92a-3p (MIRT049001 ), hsa-miR-582-3p, hsa-miR-4265, hsa- miR-218-2, hsa-miR-1271 , hsa-miR-340, hsa-miR-221 , hsa-miR-20b, hsa-miR-508- 3p, hsa-miR-141 , hsa-miR-4325, hsa-miR-889, hsa-miR-29a, hsa-miR-129-3p, hsa- miR-129, hsa-miR-96, hsa-miR-3163, hsa-miR-187, hsa-miR-196a, hsa-miR-222, hsa-miR-1179, hsa-miR-182, hsa-miR-9 ^* hsa-miR-32, hsa-miR-143, hsa-miR-4296 (see, e.g., Helwak et al. Cell, 153(3): 654-656, 2013; Selbach et al. Nature, 455 (7209):58-63, 2008; Baek et al, Nature, 455(7209):64-71 , 2008; Leivonen et al. Mol Cell Proteomics, 10(7), 2011: M110.005322): (inhibitors of YTHDC1 ) hsa-mir-20a-3p (MIRT038967), hsa-mir-103a-3p (MIRT027037), hsa-mir-1 (MIRT023492), hsa-mir- 19b-3p (MIRT031105), hsa-mir-100-5p (MIRT048400), hsa-mir-93-5p (MIRT027989), hsa-mir-16-5p (MIRT031379), hsa-let-7b-5p (MIRT052150), hsa- miR-520f, hsa-miR-300, hsa-miR-15a, hsa-miR-200a, hsa-miR-605, hsa-miR-30d, hsa-miR-30a, hsa-miR-3613-3p, hsa-miR-509-3-5p, hsa-miR-34c-5p, hsa-miR-324- 3p, hsa-miR-1248, hsa-miR-152, hsa-miR-548t, hsa-miR-4310, hsa-miR-145, hsa- miR-516a-3p, hsa-miR-16, hsa-miR-3668, hsa-miR-4277, hsa-miR-448, hsa-miR-16- 2, hsa-miR-148b, hsa-miR-509-5p, hsa-miR-103a, hsa-miR-1265, hsa-miR-2115, hsa-miR-548c-3p, hsa-miR-148a, hsa-miR-548p, hsa-miR-513a-3p, hsa-miR-497, hsa-miR-3647-3p, hsa-miR-382, hsa-miR-30b, hsa-miR-543, hsa-let-7f-2, hsa-miR- 1269, hsa-miR-3164, hsa-miR-503, hsa-miR-500a, hsa-miR-449a, hsa-miR-141 , hsa-miR-424, hsa-miR-3908, hsa-miR-889, hsa-miR-2116, hsa-miR-330-3p, hsa- miR-15b, hsa-miR-181 b, hsa-miR-187, hsa-miR-1237, hsa-miR-449b, hsa-miR-101 , hsa-miR-381 , hsa-miR-618, hsa-miR-222, hsa-miR-181 a, hsa-miR-432, hsa-miR-96, hsa-miR-19b, hsa-miR-195, hsa-miR-548n, hsa-miR-485-5p, hsa-miR-217, hsa-miR- 30c, hsa-miR-495, hsa-miR-137, hsa-miR-1288, hsa-miR-181 c, hsa-miR-3942-5p, hsa-miR-548v, hsa-miR-487a, hsa-miR-221 , hsa-miR-891 b, hsa-miR-205, hsa-miR- 195, hsa-miR-4271 , hsa-miR-3611 , hsa-miR-516b, hsa-miR-181 d, hsa-miR-154, hsa-miR-646, hsa-miR-153, hsa-miR-34a, hsa-miR-19a, hsa-miR-107, hsa-miR-30e and hsa-miR-4262 {see, e.g. Helwak et al. Cell, 153(3): 654-655, 2013; Hafner et al. Cell, 141 (1 ): 129-141 , 2010; Kishore et al, Nat Methods, 8(7):559-64, 2011; Memczak et al. Nature, 495(7441 ):333-8, 2013; Selbach et al. Nature, 455(7209):58-63, 2008; Chi et al. Nature. 460(7254):479-86, 2009).

[00117] In the present invention, the term "small molecule" includes any chemical or other moiety, other than polypeptides and nucleic acids, that can act to affect biological processes, particularly to modulate members of the m ⁶ A mRNA modification pathway. Small molecules can include any number of therapeutic agents presently known and used, or that can be synthesized in a library of such molecules for the purpose of screening for biological function(s). Small molecules are distinguished from macromolecules by size. The small molecules of the present invention usually have a molecular weight less than about 5,000 daltons (Da), preferably less than about 2,500 Da, more preferably less than 1 ,000 Da, most preferably less than about 500 Da.

[00118] Small molecules include without limitation organic compounds, peptidomimetics and conjugates thereof. As used herein, the term "organic compound" refers to any carbon-based compound other than macromolecules such as nucleic acids and polypeptides. In addition to carbon, organic compounds may contain calcium, chlorine, fluorine, copper, hydrogen, iron, potassium, nitrogen, oxygen, sulfur and other elements. An organic compound may be in an aromatic or aliphatic form. Non-limiting examples of organic compounds include acetones, alcohols, anilines, carbohydrates, monosaccharides, oligosaccharides, polysaccharides, amino acids, nucleosides, nucleotides, lipids, retinoids, steroids, proteoglycans, ketones, aldehydes, saturated, unsaturated and polyunsaturated fats, oils and waxes, alkenes, esters, ethers, thiols, sulfides, cyclic compounds, heterocyclic compounds, imidizoles, and phenols. An organic compound as used herein also includes nitrated organic compounds and halogenated (e.g., chlorinated) organic compounds.

[00119] Preferred small molecules are relatively easier and less expensively manufactured, formulated or otherwise prepared. Preferred small molecules are stable under a variety of storage conditions. Preferred small molecules may be placed in tight association with macromolecules to form molecules that are biologically active and that have improved pharmaceutical properties. Improved pharmaceutical properties include changes in circulation time, distribution, metabolism, modification, excretion, secretion, elimination, and stability that are favorable to the desired biological activity. Improved pharmaceutical properties include changes in the toxicological and efficacy characteristics of the chemical entity.

[00120] In general, a polypeptide mimetic ("peptidomimetic") is a molecule that mimics the biological activity of a polypeptide, but that is not peptidic in chemical nature. While, in certain embodiments, a peptidomimetic is a molecule that contains no peptide bonds (that is, amide bonds between amino acids), the term peptidomimetic may include molecules that are not completely peptidic in character, such as pseudo-peptides, semi-peptides, and peptoids.

[00121] As used herein, the term "biologic" means products derived from living sources as opposed to a chemical process. Non-limiting examples of a "biologic" include proteins, conditioned media, and partially purified products from tissues.

[00122] The terms "peptide," "polypeptide," and "protein" are used interchangeably herein. In the present invention, these terms mean a linked sequence of amino acids, which may be natural, synthetic, or a modification or combination of natural and synthetic. The term includes antibodies, antibody mimetics, domain antibodies, lipocalins, and targeted proteases. The term also includes vaccines containing a peptide or peptide fragment intended to raise antibodies against the peptide or peptide fragment.

[00123] "Antibody" as used herein includes an antibody of classes IgG, IgM, IgA, IgD, or IgE, or fragments or derivatives thereof, including Fab, F(ab')2, Fd, and single chain antibodies, diabodies, bispecific antibodies, and bifunctional antibodies. The antibody may be a monoclonal antibody, polyclonal antibody, affinity purified antibody, or mixtures thereof, which exhibits sufficient binding specificity to a desired epitope or a sequence derived therefrom. The antibody may also be a chimeric antibody. The antibody may be derivatized by the attachment of one or more chemical, peptide, or polypeptide moieties known in the art. The antibody may be conjugated with a chemical moiety. The antibody may be a human or humanized antibody. These and other antibodies are disclosed in U.S. Published Patent Application No. 20070065447.

[00124] Other antibody-like molecules are also within the scope of the present invention. Such antibody-like molecules include, e.g., receptor traps (such as entanercept), antibody mimetics (such as adnectins, fibronectin based "addressable" therapeutic binding molecules from, e.g., Compound Therapeutics, Inc.), domain antibodies (the smallest functional fragment of a naturally occurring single-domain antibody (such as, e.g., nanobodies; see, e.g., Cortez-Retamozo et al., Cancer Res. 2004 Apr 15;64(8):2853-7)).

[00125] Suitable antibody mimetics generally can be used as surrogates for the antibodies and antibody fragments described herein. Such antibody mimetics may be associated with advantageous properties (e.g., they may be water soluble, resistant to proteolysis, and/or be nonimmunogenic). For example, peptides comprising a synthetic beta-loop structure that mimics the second complementarity- determining region (CDR) of monoclonal antibodies have been proposed and generated. See, e.g., Saragovi et al., Science. Aug. 16, 1991 ;253(5021 ):792-5. Peptide antibody mimetics also have been generated by use of peptide mapping to determine "active" antigen recognition residues, molecular modeling, and a molecular dynamics trajectory analysis, so as to design a peptide mimic containing antigen contact residues from multiple CDRs. See, e.g., Cassett et al., Biochem Biophys Res Commun. Jul. 18, 2003;307(1 ): 198-205. Additional discussion of related principles, methods, etc., that may be applicable in the context of this invention are provided in, e.g., Fassina, Immunomethods. October 1994;5(2): 121 -9.

[00126] As used herein, "peptide" includes targeted proteases, which are capable of, e.g., substrate-targeted inhibition of post-translational modification such as disclosed in, e.g., U.S. Patent Application Publication No. 20060275823.

[00127] "Antisense" molecules as used herein include antisense or sense oligonucleotides comprising a single-stranded nucleic acid sequence (either RNA or DNA) capable of binding to target mRNA (sense) or DNA (antisense) sequences. The ability to derive an antisense or a sense oligonucleotide, based upon a cDNA sequence encoding a given protein is described in, for example, Stein and Cohen, Cancer Res. 48:2659, (1988) and van der Krol et al., BioTechniques 6:958, (1988).

[00128] Antisense molecules can be modified or unmodified RNA, DNA, or mixed polymer oligonucleotides. These molecules function by specifically binding to matching sequences resulting in inhibition of peptide synthesis (Wu-Pong, November 1994, BioPharm, 20-33) either by steric blocking or by activating an RNase H enzyme. Antisense molecules can also alter protein synthesis by interfering with RNA processing or transport from the nucleus into the cytoplasm (Mukhopadhyay & Roth, 1996, Crit. Rev. in Oncogenesis 7, 151 -190). In addition, binding of single stranded DNA to RNA can result in nuclease-mediated degradation of the heteroduplex (Wu-Pong, supra). Backbone modified DNA chemistry, which have thus far been shown to act as substrates for RNase H are phosphorothioates, phosphorodithioates, borontrifluoridates, and 2'-arabino and 2'-fluoro arabino- containing oligonucleotides.

[00129] Antisense molecules may be introduced into a cell containing the target nucleotide sequence by formation of a conjugate with a ligand binding molecule, as described, e.g., in WO 91/04753. Suitable ligand binding molecules include, but are not limited to, cell surface receptors, growth factors, other cytokines, or other ligands that bind to cell surface receptors. Preferably, conjugation of the ligand binding molecule does not substantially interfere with the ability of the ligand binding molecule to bind to its corresponding molecule or receptor, or block entry of the sense or antisense oligonucleotide or its conjugated version into the cell. Alternatively, a sense or an antisense oligonucleotide may be introduced into a cell containing the target nucleic acid sequence by formation of an oligonucleotide-lipid complex, as described, e.g., in WO 90/10448.

[00130] The term small interfering RNA ("siRNA") refers to small inhibitory RNA duplexes that induce the RNA interference (RNAi) pathway. (Elbashir, S. M. et al. Nature 411:494-498 (2001 ); Caplen, N. J. et al. Proc. Natl. Acad. Sci. USA 98:9742- 9747 (2001 ); Harborth, J. et al. J Cell Sci. 114:4557-4565 (2001 ).) These molecules can vary in length (generally 18-30 base pairs) and contain varying degrees of complementarity to their target mRNA in the antisense strand. Some, but not all, siRNA have unpaired overhanging bases on the 5' or 3' end of the sense strand and/or the antisense strand. The term "siRNA" includes duplexes of two separate strands, as well as single strands that can form hairpin structures comprising a duplex region. As used herein, siRNA molecules are not limited to RNA molecules but further encompass chemically modified nucleotides and non-nucleotides. siRNA gene-targeting may be carried out by transient siRNA transfer into cells (achieved by such classic methods as liposome-mediated transfection, electroporation, or microinjection).

[00131] In an additional aspect of the present invention, the number of stem cells and/or CAR T-cells is increased by a factor of at least 2-fold. Preferably, the number of stem cells and/or CAR T-cells is increased by a factor of at least 2.5 fold, at least 3 fold, at least 3.5 fold, at least 4-fold, such as at least 5-fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold, or more. Surprisingly and unexpectedly such levels of stem cell and/or CAR T-cell expansion are achieved using the methods of the present invention.

[00132] As noted above, the methods of the present invention may be used to expand any population of stem cells. Preferably, the stem cells that may be expanded according to the methods of the present invention may selected from hematopoietic stem cells (HSCs), hematopoietic stem and progenitor cells (HSPCs), endothelial progenitor cells (EPCs), mesenchymal stem cells (MSCs), cardiac stem cells (CSCs), neuronal stem cells (NSCs), and combinations thereof. According to one aspect, the stem cells are HSCs. According to yet another aspect, the stem cells are MSCs.

[00133] The methods of the present invention may also be capable of expanding stem cells such that the expanded cells have at least a 5-fold increase in total colony-forming units (CFU), such as an 8-fold, 10-fold, and even 15-fold or more increase. Further, the methods may be capable of providing an increase in CFU-granulocyte erythrocyte monocyte megakaryocyte (GEMM) colonies of at least 3.8-fold, such as at least 4-fold, 5-fold, and even at least 8-fold or more.

[00134] Another embodiment of the invention is a method for ex vivo expansion of a substantially undifferentiated stem cell population. This method comprises modulating a /V ⁶ -methyladenosine mRNA modification pathway in the undifferentiated stem cell population to expand the number of undifferentiated stem cells without significant differentiation of the stem cell population.

[00135] In this embodiment, a stem cell population is "substantially undifferentiated" if a sufficient number of cells in that population retain the ability to self-renew and can give rise to various differentiated cell types when transplanted into a recipient, for example, in the case of an HSC population, repopulating the HSC lineage when transplanted (or in the case of an MSC population, repopulating the MSC lineage when transplanted). As used herein, "without significant differentiation" means the expanded stem cell population has a sufficient number of cells that maintain a multi-lineage differentiation potential so that the full scope of a target stem lineage may be regenerated upon transplantation of the expanded stem cell population into a recipient. Thus, e.g., in the case of an HSC population, the expanded HSC population, when transplanted into a recipient, is capable of regenerating the entire hematopoietic cell lineage. In the case of a MSC population, the expanded MSC population, when transplanted into a recipient, is capable of regenerating the entire mesenchymal cell lineage.

[00136] Another embodiment is a method for ex vivo expansion of a chimeric antigen receptor (CAR) T-cell population. This method comprises modulating a Λ/ ⁶ - Methyladenosine (m ⁶ A) mRNA modification pathway in the CAR T-cell population to expand the number of CAR T-cells.

[00137] A further embodiment of the invention is a method for ex vivo expansion of an hematopoietic stem cell (HSC) population obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow. This method comprises modulating a N ⁶ -methyladenosine mRNA modification pathway in the HSC population to expand the HSC population to a sufficient quantity while maintaining a multilineage differentiation potential in the HSC population, which is sufficient for subsequent transplantation into a subject in need thereof.

[00138] A further embodiment of the invention is a method for ex vivo expansion of a mesenchymal stem cell (MSC) population obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow. This method comprises modulating a N ⁶ -methyladenosine mRNA modification pathway in the MSC population to expand the MSC population to a sufficient quantity while maintaining a multilineage differentiation potential in the MSC population, which is sufficient for subsequent transplantation into a subject in need thereof.

[00139] A further embodiment of the invention is a method for ex vivo expansion of a chimeric antigen receptor (CAR) T-cell population prepared by modifying T-cells obtained from a tissue selected from the group consisting of peripheral blood, cord blood, and bone marrow. The method comprising modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the CAR T-cell population to expand the CAR T-cell population to a sufficient quantity which is sufficient for subsequent transplantation into a subject in need thereof.

[00140] As used herein, "obtained" from a tissue means any conventional method of harvesting or partitioning tissue from a donor. As noted previously, the tissue may be any tissue that contains a stem cell such as an HSC and/or MSC, and/or a T-cell that is capable of being modified with chimeric antigen receptors. Thus, for example, the tissue may be obtained from a blood sample, such as a peripheral or cord blood sample, or harvested from bone marrow. Methods for obtaining such samples are well known to the artisan. In the present invention, the samples may be fresh, i.e., obtained from the donor without freezing. Moreover, the samples may be further manipulated to remove extraneous or unwanted components prior to expansion. The samples may also be obtained from a preserved stock. For example, in the case of peripheral or cord blood, the samples may be withdrawn from a cryogenically or otherwise preserved bank of such blood. Such samples may be obtained from any suitable donor. Preferably, the donor is a mammal, for example, a primate, such as a human. Furthermore, the sample may be obtained from an autologous or allogeneic donor or source. Preferably, the sample is obtained from an autologous source. [00141] In this method, "maintaining a multilineage differentiation potential" means that the expanded HSC and/or MSC population has the ability, when transplanted into a subject in need of such a transplant, to regenerate all the types of progenitor cells e.g., CMP, GMP, MEP, and CLP, and ultimately all the types of blood cells including, e.g., red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, macrophages, and platelets in the hematopoietic system.

[00142] In the present invention, that quantity of expanded HSCs and/or MSCs and/or CAR T-cells, which is "sufficient for subsequent transplantation" generally corresponds to that number of HSCs and/or MSCs and/or CAR T-cells, which would result in greater than about 1 % engraftment after transplantation. This is one accepted measure of a successful transplant. In the present invention, any conventional method may be used to determine the % engraftment, including the one set forth in the Examples. Such a measure may be carried out with or without competitor cells, typically and preferably, without competitor cells. (Zhang, C.C., et a/., Nat Med, 12(2): 240-5, 2006. Zhang, C.C. and H.F. Lodish, Blood, 105(11): 4314-20, 2005).

[00143] In the above described ex vivo expansion methods, modulating the Λ/ ⁶ - methyladenosine mRNA modification pathway may be achieved as previously set forth. Modulating the N ⁶ -methyladenosine mRNA modification pathway may include introducing a mutation into the stem cells and/or CAR T-cells that results in modulation of a molecule in the m ⁶ A mRNA modification pathway, or contacting the stem cells and/or CAR T-cells with a modulator of a molecule in the m ⁶ A mRNA modification pathway selected from the group consisting of a small molecule, a biologic, an antisense RNA, a small interfering RNA (siRNA), and combinations thereof.

[00144] In one aspect of the ex vivo expansion methods, the modulation of the m ⁶ A mRNA modification pathway involves modulation of a molecule selected from the group consisting of m ⁶ A mRNA modification readers, m ⁶ A mRNA modification writers, m ⁶ A mRNA modification erasers, and combinations thereof. Non-limiting examples of m ⁶ A modification writers include methyltransferases that are capable of post-transcriptionally installing the m ⁶ A modification in messenger RNA, and can include any selected from the group consisting of METTL3, METTL14, WTAP, KIAA1429 and combinations thereof. Non-limiting examples of m ⁶ A modification erasers include demethylases that are capable of reversing the methylation, and can include any selected from the group consisting of FTI, ALKBH5 and combinations thereof. M ⁶ A modification readers include proteins that are capable of selectively binding m ⁶ A-methylated mRNA to exert regulatory functions through selective recognition of methylated mRNA. Suitable m ⁶ A modification readers can include any selected from the group consisting of Ythdfl , Ythdf2, Ythdf3, Ythdc1, Ythdc2, HNRNPC, HNRNPA2B1 , elF3 and combinations thereof. According to one aspect, the m ⁶ A modification readers comprise proteins of the YTH domain family of proteins, which includes Ythdfl , Ythdf2, Ythdf3, Ythdc1, Ythdc2 and combinations thereof, {see, e.g., Wang et al. Nature, 505(7481 ): 117-120, 2014; Frayling et al. Science, 316: 889-894, 2007; Zheng et al. Mol. Cell., 49: 18-29, 2012; Cao et al. Open Biol., 6(4): 160003, 2016; Maity et al. The FEBS Journal, 283(9): 1607-1630, 2016).

[00145] In one aspect of the ex vivo methods, modulating the m ⁶ A mRNA modification pathway comprises introducing a mutation into the stem cells and/or CAR-T cells to delete, replace, or reduce expression of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. For example, the mutation deletes, replaces or reduces expression of a gene that expresses a molecule selected from the group consisting of a m ⁶ A mRNA modification reader, a m ⁶ A mRNA modification writer, a m ⁶ A mRNA eraser, and combinations thereof. In one aspect, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification reader, such as a m ⁶ A mRNA modification reader selected from the group consisting of Ythdfl , Ythdf2, Ythdf3, Ythdc1, HNRNPC, HNRNPA2B1 , elF3, and combinations thereof. In a preferred aspect, the mutation deletes, replaces or reduces expression of a gene that expresses Ythdf2. In yet another aspect, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification eraser, such as a m ⁶ A mRNA modification eraser selected from the group consisting of FTO, ALKBH5 and combinations thereof. In yet another aspect, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification writer, such as a m ⁶ A mRNA modification writer selected from the group consisting of METTL3, METTL14, WTAP, KIAA1429 and combinations thereof.

[00146] In one aspect of the ex vivo expansion methods, the mutation can be introduced by any of the methods previously disclosed herein. For example, the mutation can be introduced by exposing the stem cells and/or CAR T-cells to a Mx1 - Cre targeting system {see, e.g., Kuhn et al. Science, 269(5229): 1427-1429, 1995) that inactivates or deletes at least a portion of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. In yet another aspect, a mutation is introduced that incorporates short hairpin RNA (shRNA) into the stem cells and/or CAR T-cells to reduce expression of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. For example, the shRNA may be introduced by exposing the stem cells and/or CAR-T cells to a vector to deliver shRNA, which may be a viral vector such as lentivirus (see, e.g., Chira et al. Oncotarget, 6(31 ): 30675- 30703, 2015). The shRNA may be capable of triggering gene silencing to regulate gene expression (see, e.g., Paddison et al. Genes Dev., 16(8): 948-958, 2002).

[00147] In these ex vivo expansion methods, according to one aspect, modulating of the m ⁶ A mRNA modification pathway comprises down-regulating and/or inhibiting a member of the m ⁶ A mRNA modification pathway, such as down- regulating and/or inhibiting a m ⁶ A mRNA modification reader. As used herein, "down-regulating" means inhibiting or reducing the amount of or inhibiting or decreasing the activity of a member of the m ⁶ A mRNA modification pathway. Such down-regulation may be accomplished using, e.g. antisense RNA, siRNA, antibodies, or small molecules. As another example, the m ⁶ A mRNA modification reader may be down-regulated by contacting the stem cells and/or CAR T-cells with an inhibitor of an m ⁶ A mRNA reader, to inhibit binding and/or recognizing of the m ⁶ A modified mRNA by the m ⁶ A mRNA reader. In one aspect, the m ⁶ A mRNA modification reader that is down-regulated is selected from the group consisting of Ythdf 1 , Ythdf2, Ythdf3, Ythdc1 , Ythdc2, HNRNPC, HNRNPA2B1 , elF3 and combinations thereof. In a preferred aspect, the m ⁶ A mRNA modification reader that is down-regulated is Ythdf2. The RNA decay role of Ythdf2 has been previously elucidated (see, e.g., Wang et al. Nature 505(7481 ): 117-120, 2014). Inhibitors of the m ⁶ A mRNA modification reader may be any selected from the group consisting of: (inhibitors of HNRNPC) hsa-let-7e-5p (MIRT051596), hsa-mir-455-3p (MIRT037890), hsa-mir-30c-5p (MIRT047904), hsa-mir-186-5p (MIRT045150), hsa- mir-744-5p (MIRT037494), hsa-mir-18a-3p (MIRT040851 ), hsa-mir-484 (MIRT042196), hsa-mir-505-5p (MIRT037959), hsa-mir-615-3p (MIRT039991 ), hsa- mir-342-3p (MIRT043694), hsa-miR-3607-3p, hsa-miR-30d, hsa-miR-3916, hsa-miR- 3162-5p, hsa-miR-1273d, hsa-miR-3161 , hsa-miR-30a, hsa-miR-629, hsa-miR-208b, hsa-miR-489, hsa-miR-3148, hsa-miR-2113, hsa-miR-877, hsa-miR-455-5p, hsa- miR-186, hsa-miR-548o, hsa-miR-3139, hsa-miR-320a, hsa-miR-4311 , hsa-miR- 555, hsa-miR-3605-5p, hsa-miR-515-5p, hsa-miR-144, hsa-miR-499-5p, hsa-miR- 1323, hsa-miR-548x, hsa-miR-299-5p, hsa-miR-653, hsa-miR-576-5p, hsa-miR- 548p, hsa-miR-586, hsa-miR-888, hsa-miR-3647-3p, hsa-miR-484, hsa-miR-320b, hsa-miR-620, hsa-miR-30b, hsa-miR-548q, hsa-miR-29b-1 , hsa-miR-570, hsa-miR- 183, hsa-miR-1276, hsa-miR-208a, hsa-miR-186, hsa-miR-28-5p, hsa-miR-330-3p, hsa-miR-548am, hsa-miR-320d, hsa-miR-3175, hsa-miR-3155, hsa-miR-548aa, hsa- miR-519e, hsa-miR-1270, hsa-miR-513b, hsa-miR-599, hsa-miR-518f, hsa-miR- 4301 , hsa-miR-30c, hsa-miR-3135, hsa-miR-4286, hsa-miR-202, hsa-miR-4263, hsa-miR-4299, hsa-miR-606, hsa-miR-3133, hsa-miR-583, hsa-miR-3125, hsa-miR- 501 -5p, hsa-miR-7-1 , hsa-miR-514b-3p, hsa-miR-3155b, hsa-miR-548d-3p, hsa- miR-224, hsa-miR-7-2, hsa-miR-708, hsa-miR-3199, hsa-miR-514, hsa-miR-30e (see, e.g. Helwak et al. Cell, 153(3): 654-655, 2013; Whisnant et al., MBio 4(2), 2013:e000193); (inhibitors of HNRNPA2B1 ) hsa-mir-92a-3p (MIRT049721 ), hsa-mir- 30c-5p (MIRT048009), hsa-mir-191 -5p (MIRT045809), hsa-let-7f-5p (MIRT051404), hsa-mir-27b-3p (MIRT046213), hsa-mir-877-3p (MIRT037116), hsa-mir-615-3p (MIRT040278), hsa-mir-1260b (MIRT052680), hsa-mir-103a-3p (MIRT027027), hsa- mir-16-5p (MIRT031508), hsa-mir-1296-5p (MIRT036075), hsa-mir-197-3p (MIRT048098), hsa-miR-548j, hsa-miR-3678-3p, hsa-miR-607, hsa-miR-188-5p, hsa-miR-15a, hsa-miR-3653, hsa-miR-371 -5p, hsa-miR-550a, hsa-miR-3622b-3p, hsa-miR-548a-5p, hsa-miR-3170, hsa-miR-3148, hsa-miR-556-3p, hsa-miR-490-3p, hsa-miR-559, hsa-miR-200c, hsa-miR-130a, hsa-miR-548y, hsa-miR-548o, hsa-miR- 23c, hsa-miR-491 -3p, hsa-miR-335, hsa-miR-3667-3p, hsa-miR-466, hsa-miR-23b, hsa-miR-4310, hsa-miR-127-5p, hsa-miR-548b-5p, hsa-miR-616, hsa-miR-16, hsa- miR-338-3p, hsa-miR-3200-5p, hsa-miR-362-3p, hsa-miR-448, hsa-miR-1306, hsa- miR-944, hsa-miR-3684, hsa-miR-373, hsa-miR-103a, hsa-miR-380, hsa-miR-499- 5p, hsa-miR-1323, hsa-miR-323-5p, hsa-miR-3674, hsa-miR-1252, hsa-miR-33b, hsa-miR-580, hsa-miR-548c-3p, hsa-miR-103a-2, hsa-miR-548w, hsa-miR-600, hsa- miR-634, hsa-miR-586, hsa-miR-497, hsa-miR-720, hsa-miR-654-3p, hsa-miR-524- 5p, hsa-miR-543, hsa-miR-548q, hsa-let-7f-2, hsa-miR-330-5p, hsa-miR-500a, hsa- miR-548l, hsa-miR-570, hsa-miR-374a, hsa-miR-1184, hsa-miR-649, hsa-miR-424, hsa-miR-3658, hsa-miR-186, hsa-miR-326, hsa-miR-548d-5p, hsa-miR-23a, hsa- miR-15b, hsa-miR-190, hsa-miR-203, hsa-miR-548h, hsa-miR-3136-5p, hsa-miR- 618, hsa-miR-551 b, hsa-miR-211, hsa-miR-1305, hsa-miR-513b, hsa-miR-96, hsa- miR-2117, hsa-miR-548n, hsa-miR-3910, hsa-miR-217, hsa-miR-892b, hsa-miR- 502-5p, hsa-miR-548i, hsa-miR-520d-5p, hsa-miR-4299, hsa-miR-1285, hsa-miR- 3133, hsa-miR-483-3p (see, e.g., Hafner et al. Cell, 141 (1 ): 129-141 , 2010; Helwak et al. Cell, 153(3): 654-655, 2013); (inhibitors of Ythdfl ) hsa-miR-548g, hsa-miR-204, hsa-miR-3143, hsa-miR-521 , hsa-miR-195, hsa-miR-3182, hsa-miR-3941 , hsa-miR- 34c-3p, hsa-miR-767-3p, hsa-miR-563, hsa-miR-548c-5p, hsa-miR-1911 , hsa-miR- 26b, hsa-miR-190b, hsa-miR-33a, hsa-miR-329, hsa-miR-221 , hsa-miR-612, hsa- miR-3185, hsa-miR-3156-5p, hsa-miR-107, hsa-miR-664, hsa-miR-3657; (inhibitors of Ythdf2) hsa-mir-615-3p (MIRT040054), hsa-mir-106b-5p (MIRT044257), hsa-mir- 1 (MIRT023842), miR-145, hsa-miR-3607-3p, hsa-miR-200a, hsa-miR-301 a, hsa- miR-519a, hsa-miR-141 , hsa-miR-130b, hsa-miR-181 b, hsa-miR-301 b, hsa-miR- 3117-3p, hsa-miR-1236, hsa-miR-181 a, hsa-miR-519c-3p, hsa-miR-551 b, hsa-miR- 519e, hsa-miR-519b-3p, hsa-miR-19b, hsa-miR-1303, hsa-miR-608, hsa-miR-145, hsa-miR-130a, hsa-miR-181 c, hsa-miR-323b-3p, hsa-miR-421 , hsa-miR-515-5p, hsa-miR-3666, hsa-miR-181 d, hsa-miR-146a, hsa-miR-4295, hsa-miR-454, hsa- miR-3919, hsa-miR-19a, hsa-miR-543, hsa-miR-4262 {see, e.g. Helwak et al. Cell, 153(3): 654-655, 2013; Selbach et al. Nature, 455(7209): 58-63, 2008; Yang et al. J Biol Chem., 292(9): 3614-3623, 2017); (inhibitors of Ythdf3) hsa-miR-582-3p, hsa- miR-579, hsa-miR-520e, hsa-miR-520f, hsa-miR-3152-3p, hsa-miR-106a, hsa-miR- 30d, hsa-miR-30a, hsa-miR-93, hsa-miR-508-5p, hsa-miR-29a, hsa-miR-3148, hsa- miR-490-5p, hsa-miR-520b, hsa-miR-20a, hsa-miR-409-3p, hsa-miR-4255, hsa-let- 7i, hsa-miR-373, hsa-let-7e, hsa-miR-520c-3p, hsa-miR-3920, hsa-miR-127-5p, hsa- miR-380, hsa-miR-616, hsa-miR-4277, hsa-miR-448, hsa-miR-16-2, hsa-let-7c, hsa- miR-340, hsa-miR-373, hsa-miR-520a-3p, hsa-miR-144, hsa-miR-1265, hsa-miR- 548x, hsa-miR-362-5p, hsa-miR-33b, hsa-miR-26b, hsa-miR-17, hsa-miR-569, hsa- miR-3618, hsa-miR-576-5p, hsa-miR-922, hsa-miR-302a, hsa-miR-106b, hsa-miR- 888, hsa-miR-484, hsa-let-7b, hsa-miR-582-5p, hsa-let-7f, hsa-miR-30b, hsa-miR- 524-5p, hsa-miR-302d, hsa-let-7d, hsa-miR-513a-5p, hsa-miR-500a, hsa-miR-570, hsa-miR-548l, hsa-miR-105, hsa-miR-374c, hsa-let-7g hsa-miR-372, hsa-miR-3658, hsa-let-7a, hsa-miR-3908, hsa-miR-302b, hsa-miR-526b, hsa-miR-190, hsa-miR- 181 b, hsa-miR-433, hsa-miR-98, hsa-miR-3606, hsa-miR-595, hsa-miR-548am, hsa- miR-187, hsa-miR-561 , hsa-miR-181 a, hsa-miR-3155, hsa-miR-655, hsa-miR-302c, hsa-miR-195, hsa-miR-26a, hsa-miR-590-3p, hsa-miR-30c, hsa-miR-502-5p, hsa- miR-495, hsa-miR-137, hsa-miR-181 c, hsa-miR-520d-5p, hsa-miR-3942-5p, hsa- miR-202, hsa-miR-302e, hsa-miR-513c, hsa-miR-885-5p, hsa-miR-520a-5p, hsa- miR-583, hsa-miR-1297, hsa-miR-7-1 , hsa-miR-520d-3p, hsa-miR-3155b, hsa-miR- 3182, hsa-miR-519d, hsa-miR-550a, hsa-miR-7-2, hsa-miR-181 d, hsa-miR-190b, hsa-miR-1912, hsa-miR-151 -3p, hsa-miR-33a, hsa-miR-525-5p, hsa-miR-20b, hsa- miR-514b-5p, hsa-miR-30e, hsa-miR-4262, hsa-miR-636; (inhibitor of elF3) hsa-mir- 92b-3p (MIRT040734), hsa-mir-16-5p (MIRT031705), hsa-mir-18a-3p

(MIRT040974), hsa-mir-155-5p (MIRT020771 ), hsa-mir-484 (MIRT042324), hsa-let- 7c-5p (MIRT051776), hsa-miR-3910, hsa-miR-148b, hsa-miR-136, hsa-miR-15a, hsa-miR-488, hsa-miR-500a, hsa-miR-1297, hsa-miR-3159, hsa-miR-374c, hsa- miR-424, hsa-miR-7-1 , hsa-miR-186, hsa-miR-195, hsa-miR-15b, hsa-miR-26b, hsa- miR-505, hsa-miR-1206, hsa-miR-653, hsa-miR-1283, hsa-miR-7-2, hsa-miR-196a, hsa-miR-497, hsa-miR-33a, hsa-miR-655, hsa-miR-26a hsa-miR-16, hsa-mir-151 a- 3p (MIRT043600), hsa-mir-92a-3p (MIRT049064), hsa-mir-615-3p (MIRT039779), hsa-mir-877-3p (MIRT036964), hsa-mir-222-3p (MIRT046746), hsa-mir-423- 3p (MIRT042468), hsa-mir-324-3p (MIRT042887), hsa-mir-124-3p (MIRT022932), hsa-miR-3140-3p, hsa-miR-124, hsa-miR-198, hsa-miR-525-5p, hsa-miR-506, hsa- miR-520a-5p, hsa-miR-196a ^* hsa-miR-3117-3p, hsa-mir-342-5p (MIRT038210), hsa- mir-378a-5p (MIRT043981 ), hsa-mir-615-3p (MIRT040086), hsa-let-7b-

5p (MIRT052211), hsa-mir-455-3p (MIRT037879), hsa-miR-4267, hsa-miR-590-3p, hsa-mir-106b-5p (MIRT044355), hsa-mir-320a (MIRT044466), hsa-mir-16- 5p (MIRT032018), hsa-mir-155-5p (MIRT021009), hsa-miR-4302, hsa-mir-191 -5p (MIRT045793), hsa-mir-1303 (MIRT035890), hsa-mir-193b-3p (MIRT016316), hsa- mir-222-3p (MIRT046640), hsa-mir-532-3p (MIRT037924), hsa-mir-18a-3p (MIRT040929), hsa-mir-92a-3p (MIRT049001 ), hsa-miR-582-3p, hsa-miR-4265, hsa- miR-218-2, hsa-miR-1271 , hsa-miR-340, hsa-miR-221 , hsa-miR-20b, hsa-miR-508- 3p, hsa-miR-141 , hsa-miR-4325, hsa-miR-889, hsa-miR-29a, hsa-miR-129-3p, hsa- miR-129, hsa-miR-96, hsa-miR-3163, hsa-miR-187, hsa-miR-196a, hsa-miR-222, hsa-miR-1179, hsa-miR-182, hsa-miR-9 ^* hsa-miR-32, hsa-miR-143, hsa-miR-4296 (see, e.g., Helwak et al. Cell, 153(3): 654-656, 2013; Selbach et al. Nature, 455 (7209):58-63, 2008; Baek et al, Nature, 455(7209):64-71 , 2008; Leivonen et al. Mol Cell Proteomics, 10(7), 2011: M110.005322): (inhibitors of YTHDC1 ) hsa-mir-20a-3p (MIRT038967), hsa-mir-103a-3p (MIRT027037), hsa-mir-1 (MIRT023492), hsa-mir- 19b-3p (MIRT031105), hsa-mir-100-5p (MIRT048400), hsa-mir-93-5p (MIRT027989), hsa-mir-16-5p (MIRT031379), hsa-let-7b-5p (MIRT052150), hsa- miR-520f, hsa-miR-300, hsa-miR-15a, hsa-miR-200a, hsa-miR-605, hsa-miR-30d, hsa-miR-30a, hsa-miR-3613-3p, hsa-miR-509-3-5p, hsa-miR-34c-5p, hsa-miR-324- 3p, hsa-miR-1248, hsa-miR-152, hsa-miR-548t, hsa-miR-4310, hsa-miR-145, hsa- miR-516a-3p, hsa-miR-16, hsa-miR-3668, hsa-miR-4277, hsa-miR-448, hsa-miR-16- 2, hsa-miR-148b, hsa-miR-509-5p, hsa-miR-103a, hsa-miR-1265, hsa-miR-2115, hsa-miR-548c-3p, hsa-miR-148a, hsa-miR-548p, hsa-miR-513a-3p, hsa-miR-497, hsa-miR-3647-3p, hsa-miR-382, hsa-miR-30b, hsa-miR-543, hsa-let-7f-2, hsa-miR- 1269, hsa-miR-3164, hsa-miR-503, hsa-miR-500a, hsa-miR-449a, hsa-miR-141 , hsa-miR-424, hsa-miR-3908, hsa-miR-889, hsa-miR-2116, hsa-miR-330-3p, hsa- miR-15b, hsa-miR-181 b, hsa-miR-187, hsa-miR-1237, hsa-miR-449b, hsa-miR-101 , hsa-miR-381 , hsa-miR-618, hsa-miR-222, hsa-miR-181 a, hsa-miR-432, hsa-miR-96, hsa-miR-19b, hsa-miR-195, hsa-miR-548n, hsa-miR-485-5p, hsa-miR-217, hsa-miR- 30c, hsa-miR-495, hsa-miR-137, hsa-miR-1288, hsa-miR-181 c, hsa-miR-3942-5p, hsa-miR-548v, hsa-miR-487a, hsa-miR-221 , hsa-miR-891 b, hsa-miR-205, hsa-miR- 195, hsa-miR-4271 , hsa-miR-3611 , hsa-miR-516b, hsa-miR-181 d, hsa-miR-154, hsa-miR-646, hsa-miR-153, hsa-miR-34a, hsa-miR-19a, hsa-miR-107, hsa-miR-30e and hsa-miR-4262 {see, e.g. Helwak et al. Cell, 153(3): 654-655, 2013; Hafner et al. Cell, 141 (1 ): 129-141 , 2010; Kishore et al, Nat Methods, 8(7):559-64, 2011; Memczak et al. Nature, 495(7441 ):333-8, 2013; Selbach et al. Nature, 455(7209):58-63, 2008; Chi et al. Nature. 460(7254):479-86, 2009).

[00148] In these ex vivo expansion methods, it is preferred that the stem cell is selected from HSCs, hematopoietic stem and progenitor cells (HSPCs), endothelial progenitor cells, (EPCs), mesenchymal stem cells (MSCs), cardiac stem cells (CSCs), neuronal stem cells (NSCs), and combinations thereof. According to certain aspects, the stem cell is an HSC. According to other aspects, the stem cell is a MSC. The ex vivo expansion methods can also use a population of cells comprising CAR T-cells. In these methods, the HSC and/or MSC is obtained from a mammalian, e.g., primate or human, tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, although any HSC and/or MSC- containing tissue may be used.

[00149] In another aspect of the method for ex vivo expansion of an hematopoietic stem cell (HSC) population, the expansion of the number of stem cells is by at least 2-fold, such as e.g., by at least 2.5-fold, at least 3-fold, at least 3.5-fold, at least 4-fold, and including at least 5-fold, at least 8-fold, at least 10-fold, at least 15-fold, or at least 20-fold or more.

[00150] In another aspect of the method for ex vivo expansion of a mesenchymal stem cell (MSC) population, the expansion of the number of stem cells is by at least 2-fold, such as e.g., by at least 2.5-fold, at least 3-fold, at least 3.5 fold, at least 4-fold, and including at least 5-fold, at least 8-fold, at least 10-fold, at least 15-fold, or at least 20-fold or more.

[00151] In another aspect of the method for ex vivo expansion of CAR T-cell population, the expansion of the number of CAR T-cells is by at least 2-fold, such as e.g., by at least 2.5-fold, at least 3-fold, at least 3.5 fold, at least 4-fold, and including at least 5-fold, at least 8-fold, at least 10-fold, at least 15-fold, or at least 20-fold or more.

[00152] Yet another embodiment of the present invention is an expanded, substantially undifferentiated stem cell population made by a method of the present invention, such as, e.g., the method for ex vivo expansion of a substantially undifferentiated stem cell population or the method for ex vivo expansion of an hematopoietic stem cell (HSC) population.

[00153] Yet another embodiment of the present invention is an expanded, substantially undifferentiated stem cell population made by a method of the present invention, such as, e.g., the method for ex vivo expansion of a substantially undifferentiated stem cell population or the method for ex vivo expansion of an mesenchymal stem cell (MSC) population.

[00154] Yet another embodiment of the present invention is an expanded, CAR T-cell population made by a method of the present invention, such as, e.g., the method for ex vivo expansion of a CAR T-cell population.

[00155] An additional embodiment of the present invention is a method for ex vivo expansion of hematopoietic stem cells (HSCs) by at least 2-fold, wherein the expanded HSCs, are competent to reconstitute an HSC lineage upon transplantation into a mammalian subject in need thereof. This method comprises introducing a mutation into the stem cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of HSCs in a suitable culture medium.

[00156] An additional embodiment of the present invention is a method for ex vivo expansion of mesenchymal stem cells (MSCs) by at least 2-fold, wherein the expanded MSCs, are competent to reconstitute a MSC lineage upon transplantation into a mammalian subject in need thereof. This method comprises introducing a mutation into the stem cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of HSCs in a suitable culture medium.

[00157] An additional embodiment of the present invention is a method for ex vivo expansion of chimeric antigen receptor (CAR) T-cells by at least 2-fold, wherein the expanded CAR T-cells are competent to treat a cancer and/or blood disorder upon transplantation into a mammalian subject in need thereof. This method comprises introducing a mutation into the CAR T-cells that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader and culturing the population of CAR T-cells in a suitable culture medium.

[00158] In this aspect of the invention, "competent to reconstitute an HSC lineage" means that the expanded HSCs, when transplanted into a suitable mammalian subject, result in greater than 1 % engraftment in the recipient, which engrafted cells are able to differentiate into the cell lineages necessary to have a normal functioning hematopoietic system. In this aspect of the invention, "competent to reconstitute a MSC lineage" means that the expanded MSCs, when transplanted into a suitable mammalian subject, result in greater than 1 % engraftment in the recipient, which engrafted cells are able to differentiate into the cell lineages necessary to have a normal functioning hematopoietic system. In this aspect of the invention "competent to treat a cancer and/or blood disorder" means that the expanded CAR T-cells, when transplanted into a suitable mammalian subject, are capable of providing treatment of a cancer and/or blood disorder from which the mammalian subject is suffering, such as for example at least one of leukemia and lymphoma. In this method, a "suitable culture medium", "fluid media" and "media" which are used interchangeably herein, mean physiologically balanced salt solutions that can maintain a stem cell population and/or CAR T-cell population for a required period of time, which solution may optionally be supplemented with suitable m ⁶ A mRNA modification pathway modulators of the present invention. Such base culture media are well known in the arts. A non-limiting example of a suitable base culture medium for HSCs is StemSpan Media (Stem Cell Technologies; Cat. No. 09600), which is supplemented with 10ug/ml Heparin, 5X Penicillin/Streptomycin, 10 ng/ml recombinant mouse (rm) Stem Cell Factor, and 20 ng/ml rm-Thrombopoietin.

[00159] In one aspect of the invention, the ex vivo expansion of HSCs and/or MSCs and/or CAR T-cells by at least 2-fold can be performed by any of the methods that have been described herein. For example, the method may involve introducing a mutation that deletes, replaces or reduces expression of a gene expressing a m ⁶ A mRNA modification reader, such as Ythdf2. Further, the mutation may be introduced by any of the methods described herein, such as by exposing the stem cells and/or CAR T-cells to a Mx1 -Cre targeting system that inactivates or deletes at least a portion of a gene that expresses a m ⁶ A mRNA modification reader. The mutation may also be introduced by incorporating shRNA into the stem cells and/or CAR-T cells to reduce expression of a gene that expresses a m ⁶ A mRNA modification reader. Other methods of introducing a mutation, and mutations that target other m ⁶ A mRNA modification readers that are described herein may also be provided.

[00160] In one aspect of this embodiment, the HSCs and/or MSCs and/or CAR T-cells are obtained from a mammalian tissue, preferably primate or human tissue, which is selected from cord blood, peripheral blood, and bone marrow. In this embodiment, the number of HSCs and/or MSCs and/or CAR T-cells is expanded by a factor of at least 2-fold, such as at least 2.5-fold, at least 3-fold, at least 3.5-fold, at least 4-fold, and including at least 5-fold, at least 8-fold, at least 10-fold, at least 15- fold, at least 20-fold or more.

[00161] Yet another embodiment of the present invention is a kit for expanding an hematopoietic stem cell (HSC) population, mesenchymal stem cell (MSC) population, and/or CAR T-cell population for subsequent transplantation into a subject in need thereof. The kit comprises a system for introducing a mutation into the HSC, MSC and/or CAR T-cell population that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and instructions for use thereof. Preferably, in the kit, the system for introducing a mutation into the HSC, MSC and/or CAR-T cell population includes one or more reagents capable of introducing a mutation into the HSC, MSC and/or CAR-T cell population that results in deletion, replacement or reduced expression of a gene expressing Ythdf2. For example, the kit can include a system for introducing a mutation into the HSC, MSC and/or CAR-T cell population that comprises a Mx1 -Cre targeting system that inactivates or deletes at least a portion of a gene that expresses a m ⁶ A mRNA modification reader. The kit can also include a system for introducing a mutation into the HSC, MSC and/or CAR T-cell population that comprises reagents for delivering a lentivirus that incorporates shRNA into the HSC, MSC and/or CAR T-cell population to reduce expression of a gene that expresses a m ⁶ A mRNA modification reader. The kit may further comprise other systems/methods set forth herein for introducing the mutation to modulate a m ⁶ A mRNA modification pathway, such as by deleting, replacing, or reducing expression of a gene expressing a m ⁶ A mRNA modification reader, including Ythdf2. The kit and the components therein may be packaged in any suitable manner for distribution and/or storage.

[00162] In yet another embodiment, a kit for expanding an hematopoietic stem cell population (HSC) population and/or mesenchymal stem cell (MSC) population and/or CAR-T cell population for subsequent transplantation into a subject in need thereof is provided. The kit comprises an inhibitor of a m ⁶ A mRNA modification reader, and instructions for use thereof, where the inhibitor may be any of the inhibitors disclosed herein, such as, e.g., an inhibitor of Ythdf2.

[00163] In one aspect of this embodiment, the kits may be able to provide an expansion of the number of stem cells and/or CAR T-cells by a factor selected from the group consisting of at least 2-fold, at least 2.5 fold, at least 3-fold, at least 3.5- fold, at least 4-fold, at least 5-fold, at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more.

[00164] A further embodiment of the present invention is a method for administering an hematopoietic stem cell (HSC) to a subject in need thereof. The method comprises (a) introducing, into a sample containing an HSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the HSCs to the subject. In this embodiment, the method of introducing the mutation, and the mutation targeted at the m ⁶ A mRNA modification reader, are as previously disclosed, such as, for example, a mutation that results in deletion of a gene expressing Ythdf2, or a mutation that results in incorporation of shRNA into the HSC population that reduces expression of a gene expressing Ythdf2. Furthermore, the HSCs may be obtained from any appropriate tissue such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00165] A further embodiment of the present invention is a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof. The method comprises (a) introducing, into a sample containing a MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the MSCs to the subject. In this embodiment, the method of introducing the mutation, and the mutation targeted at the m ⁶ A mRNA modification reader, are as previously disclosed, such as, for example, a mutation that results in deletion of a gene expressing Ythdf2, or a mutation that results in incorporation of shRNA into the MSC population that reduces expression of a gene expressing Ythdf2. Furthermore, the MSCs may be obtained from any appropriate tissue such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00166] A further embodiment of the present invention is a method for administering a chimeric antigen receptor (CAR) T-cell to a subject in need thereof. The method comprises (a) introducing, into a sample containing a CAR T-cell population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of CAR T- cells in the sample to a number sufficient to transplant into the subject; and (c) administering the CAR T-cells to the subject. In this embodiment, the method of introducing the mutation, and the mutation targeted at the m ⁶ A mRNA modification reader, are as previously disclosed, such as, for example, a mutation that results in deletion of a gene expressing Ythdf2, or a mutation that results in incorporation of shRNA into the CAR T-cell population that reduces expression of a gene expressing Ythdf2. Furthermore, the CAR T-cells may be obtained from any appropriate tissue such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00167] A further embodiment of the present invention is a method for administering an hematopoietic stem cell (HSC) to a subject in need thereof. The method comprises: (a) culturing, in a suitable culture media, a sample containing an HSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the HSCs to the subject. In this method, the inhibitor is as previously disclosed, such as, e.g., an inhibitor of Ythdf2. Furthermore, the HSCs may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00168] A further embodiment of the present invention is a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof. The method comprises: (a) culturing, in a suitable culture media, a sample containing an MSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the MSCs to the subject. In this method, the inhibitor is as previously disclosed, such as, e.g., an inhibitor of Ythdf2. Furthermore, the MSCs may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00169] A further embodiment of the present invention is a method for administering a chimeric antigen receptor (CAR) T-cell to a subject in need thereof. The method comprises: (a) culturing, in a suitable culture media, a sample containing an CAR T-cell population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of CAR T- cells in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the CAR T-cells to the subject. In this method, the inhibitor is as previously disclosed, such as, e.g., an inhibitor of Ythdf2. Furthermore, the CAR T- cells may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00170] An additional embodiment of the present invention is a method for reconstituting bone marrow in a subject in need thereof. The method comprises (a) introducing, into a sample containing an HSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the HSCs to the subject. In this embodiment, the method of introducing the mutation, and the mutation targeted at the m ⁶ A mRNA modification reader, are as previously disclosed, such as, for example, a mutation that results in deletion of a gene expressing Ythdf2, or a mutation that results in incorporation of shRNA into the HSC population that reduces expression of a gene expressing Ythdf2. Furthermore, the HSCs may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00171] An additional embodiment of the present invention is a method for reconstituting bone marrow in a subject in need thereof. The method comprises (a) introducing, into a sample containing a MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; and (c) administering the MSCs to the subject. In this embodiment, the method of introducing the mutation, and the mutation targeted at the m ⁶ A mRNA modification reader, are as previously disclosed, such as, for example, a mutation that results in deletion of a gene expressing Ythdf2, or a mutation that results in incorporation of shRNA into the MSC population that reduces expression of a gene expressing Ythdf2. Furthermore, the MSCs may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human. [00172] An additional embodiment of the present invention is a method for treating cancer and/or a blood disorder in a subject in need thereof. The method comprises (a) introducing, into a sample containing a CAR T-cell population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader; (b) culturing the sample in a suitable culture media for a period of time sufficient to expand the number of CAR T-cells in the sample to a number sufficient to transplant into the subject; and (c) administering the CAR T-cells to the subject. In this embodiment, the method of introducing the mutation, and the mutation targeted at the m ⁶ A mRNA modification reader, are as previously disclosed, such as, for example, a mutation that results in deletion of a gene expressing Ythdf2, or a mutation that results in incorporation of shRNA into the CAR T-cell population that reduces expression of a gene expressing Ythdf2. Furthermore, the CAR T-cells may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00173] In yet another embodiment, there is provided a method for reconstituting bone marrow in a subject in need thereof. This method comprises (a) culturing, in a suitable culture media, a sample containing an HSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of HSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the HSCs to the subject. In this method, the inhibitor is as previously disclosed, such as an inhibitor of Ythdf2. Furthermore, the HSCs may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00174] In yet another embodiment, there is provided another method for reconstituting bone marrow in a subject in need thereof. This method comprises (a) culturing, in a suitable culture media, a sample containing an MSC population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the MSCs to the subject. In this method, the inhibitor is as previously disclosed, such as an inhibitor of Ythdf2. Furthermore, the MSCs may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00175] In these methods, "reconstituting bone marrow" means restoration of all or a portion of the bone marrow in a subject suffering from a disease in which normal bone marrow function has been compromised. Non-limiting examples of such diseases include blood disorders such as aplastic anemia, myelodysplastic syndromes (MDS), paroxysmal nocturnal hemoglobinuria (PNH), and blood cancers, such as leukemia. Thus, as used herein, "reconstituted" means that the transplanted HSCs and/or MSCs are able to successfully engraft in the host and differentiate into all the cell lineages typically found in or derived from bone marrow. Aspects of the methods herein may involve transplantation of HSCs and/or MSCs obtained from tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, to the subject, for the treatment of blood disorders such as leukemia and lymphoma. [00176] In yet another embodiment, there is provided another method for treating cancer and/or a blood disorder in a subject in need thereof. This method comprises (a) culturing, in a suitable culture media, a sample containing a chimeric antigen receptor (CAR) T-cell population in the presence of an inhibitor of a m ⁶ A mRNA modification reader, for a period of time sufficient to expand the number of CAR T-cells in the sample to a number sufficient to transplant into the subject; (b) removing from the culture the inhibitor of the m ⁶ A mRNA modification reader; and (c) administering the CAR T-cells to the subject. In this method, the inhibitor is as previously disclosed, such as an inhibitor of Ythdf2. Furthermore, the T-cells that are modified to prepare the CAR T-cells may be obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow, and the subject may be a mammal, such as a human.

[00177] In these methods, "treating cancer and/or a blood disorder in a subject" means eradicating cancer cells, alleviating symptoms, or otherwise reducing a disease state in a subject suffering from the cancer and/or blood disorder. Non- limiting examples of cancers and/or blood disorders include blood disorders such as aplastic anemia, myelodysplastic syndromes (MDS), paroxysmal nocturnal hemoglobinuria (PNH), and blood cancers, such as leukemia and lymphoma.

[00178] In these methods, "a period of time sufficient to expand the number of HSCs" means the minimum amount of time to expand the HSCs in culture to a point where there is a sufficient number of HSCs for one or more transplantations, and "a period of time sufficient to expand the number of MSCs" means the minimum amount of time to expand the MSCs in culture to a point where there is a sufficient number of MSCs for one or more transplantations. Similarly, "a period of time sufficient to expand the number of CAR T-cells" means the minimum amount of time to expand the CAR T-cells in culture to a point where there is a sufficient number of CAR T-cells for one or more transplantations. Typically, such a period of time may be at least about 10 days in culture. Under certain circumstances, it may be desirable to expand the stem cell and/or CAR T-cell, e.g., HSC and/or MSC, population beyond what is required for a single transplantation. For example, it may be desirable to expand the stem cell and/or CAR T-cell, e.g., HSC and/or MSC, population to a number sufficient for multiple transplantations, such as e.g., from about 2 to about 100 transplantations. In these circumstances, the excess cells may be preserved for later use by any conventional method, such as e.g., by cryo- preservation.

[00179] As indicated previously, "a number sufficient to transplant" means the minimum number of stem cells, e.g., HSCs and/or MSCs and/or CAR T-cells, necessary to achieve greater than 1 % engraftment in a recipient. "Administering the HSCs to the subject" means conventional methods for delivering HSCs to the subject, including but not limited to, delivering the HSCs surgically and/or intravenously. "Administering the MSCs to the subject" means conventional methods for delivering MSCs to the subject, including but not limited to, delivering the MSCs surgically and/or intravenously. "Administering the CAR T-cells to the subject" means conventional methods for delivering CAR T-cells to the subject, including but not limited to, delivering the MSCs surgically and/or intravenously. In these embodiments, the tissue the HSCs and/or MSCs and/or T-cells are obtained from, and the m ⁶ A mRNA modification reader inhibitors are as previously disclosed.

[00180] An additional embodiment of the present invention is a method for expanding a population of hematopoietic stem cells (HSCs). This method comprises culturing a population of HSCs under conditions sufficient to result in an expansion of the HSC population by at least 2-fold, wherein the expanded population of HSCs is suitable for transplantation into a mammal in need thereof. In this embodiment the "conditions sufficient to result in an expansion of the HSC population" are those conditions that can result in expansion of HSCs in culture by, e.g., at least 2-fold, such as, e.g., by at least 2.5-fold, at least 3-fold, at least 3.5 fold, at least 4-fold, at least 5-fold, at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more. "Suitable for transplantation into a mammal" means that the number and quality of HSCs is sufficient to support greater than 1 % engraftment in a mammalian recipient, such as, e.g., a primate recipient, including an human recipient, in need thereof.

[00181] An additional embodiment of the present invention is a method for expanding a population of mesenchymal stem cells (MSCs). This method comprises culturing a population of MSCs under conditions sufficient to result in an expansion of the MSC population by at least 2-fold, wherein the expanded population of MSCs is suitable for transplantation into a mammal in need thereof. In this embodiment the "conditions sufficient to result in an expansion of the MSC population" are those conditions that can result in expansion of MSCs in culture by, e.g., at least 2-fold, such as, e.g., by at least 2.5-fold, at least 3-fold, at least 3.5 fold, at least 4-fold, at least 5-fold, at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more. "Suitable for transplantation into a mammal" means that the number and quality of MSCs is sufficient to support greater than 1 % engraftment in a mammalian recipient, such as, e.g., a primate recipient, including an human recipient, in need thereof.

[00182] An additional embodiment of the present invention is a method for expanding a population of chimeric antigen receptor (CAR) T-cells. This method comprises culturing a population of CAR T-cells under conditions sufficient to result in an expansion of the CAR T-cell population by at least 2-fold, wherein the expanded population of CAR T-cells is suitable for transplantation into a mammal in need thereof. In this embodiment the "conditions sufficient to result in an expansion of the CAR T-cell population" are those conditions that can result in expansion of CAR T-cells in culture by, e.g., at least 2-fold, such as, e.g., by at least 2.5-fold, at least 3-fold, at least 3.5 fold, at least 4-fold, at least 5-fold, at least 8-fold, at least 10- fold, at least 15-fold, at least 20-fold or more. "Suitable for transplantation into a mammal" means that the number and quality of CAR T-cells is sufficient to support greater than 1 % engraftment in a mammalian recipient, such as, e.g., a primate recipient, including an human recipient, in need thereof.

[00183] Yet another embodiment of the present invention is a method for treating a subject in need of a bone marrow transplant, a peripheral blood transplant, or a cord blood transplant comprising administering to the subject a population of HSCs obtained by a method disclosed herein, particularly the methods for expanding a population of hematopoietic stem cells (HSCs). The subject may be a mammal, such as a human.

[00184] Yet another embodiment of the present invention is a method for treating a subject in need of a bone marrow transplant, a peripheral blood transplant, or a cord blood transplant comprising administering to the subject a population of MSCs obtained by a method disclosed herein, particularly the methods for expanding a population of mesenchymal stem cells (MSCs). The subject may be a mammal, such as a human.

[00185] Yet another embodiment of the present invention is a method for treating a subject suffering from cancer and/or a blood disorder, comprising administering to the subject a population of CAR T-cells obtained by a method disclosed herein, particularly the methods for expanding a population of CAR T-cells. The subject may be a mammal, such as a human.

[00186] A further embodiment of the present invention is a method for expanding a population of hematopoietic stem cells (HSCs). The method comprises (a) obtaining from a mammal a tissue sample comprising an HSC population; (b) expanding, in vitro, the HSC population from the sample, wherein (i) the HSC population expands by at least 2-fold; and (ii) the expanded HSC population has at least a 5-fold increase in total colony forming units. In one aspect of this embodiment, the HSC population expands by at least 4-fold, such as e.g., at least 5- fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more. In another aspect of this embodiment, the mammal is a primate, including a human. Preferably, the human requires a peripheral blood transplant, a cord blood transplant, or a bone marrow transplant. In a further aspect, the tissue sample is obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow.

[00187] A further embodiment of the present invention is a method for expanding a population of mesenchylam stem cells (MSCs). The method comprises (a) obtaining from a mammal a tissue sample comprising a MSC population; (b) expanding, in vitro, the MSC population from the sample, wherein (i) the MSC population expands by at least 2-fold. In one aspect of this embodiment, the MSC population expands by at least 2.5-fold, such as at least 3-fold, at least 3.5 fold, at least 4-fold, such as e.g., at least 5-fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more. In another aspect of this embodiment, the mammal is a primate, including a human. Preferably, the human requires a peripheral blood transplant, a cord blood transplant, or a bone marrow transplant. In a further aspect, the tissue sample is obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow.

[00188] A further embodiment of the present invention is a method for expanding a population of chimeric antigen receptor (CAR) T-cells. The method comprises (a) obtaining from a mammal a tissue sample comprising a T-cell population; (b) modifying the T-cell population with chimeric antigen receptors to provide CAR T-cell population; (c) expanding, in vitro, the CAR T-cell population from the sample, wherein (i) the CAR T-cell population expands by at least 2-fold. In one aspect of this embodiment, the CAR T-cell population expands by at least 2.5- fold, such as at least 3-fold, at least 3.5 fold, at least 4-fold, such as e.g., at least 5- fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more. In another aspect of this embodiment, the mammal is a primate, including a human. Preferably, the human is suffering from a cancer and/or a blood disorder. In a further aspect, the tissue sample is obtained from any appropriate tissue, such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow.

[00189] An additional embodiment of the present invention is a method for reconstituting an hematopoietic stem cell lineage in a recipient in need thereof. The method comprises (a) obtaining from a mammal a tissue sample comprising an HSC population; (b) expanding, in vitro, the HSC population from the sample, wherein: (i) the HSC population expands by at least 2-fold, such as for example, by at least 4- fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more, and (ii) the expanded HSC population has at least at 5-fold increase in total colony forming units; and (c) transplanting the expanded HSC population into a subject in need thereof, such as a mammal, including a primate or human. In this embodiment, the human recipient requires a peripheral blood transplant, a cord blood transplant or a bone marrow transplant. Thus, in a further aspect, the tissue sample is obtained from any appropriate tissue such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow. The sample may be obtained from an autologous or allogeneic source. Preferably, the sample is obtained from an autologous source.

[00190] An additional embodiment of the present invention is a method for reconstituting a mesenchymal stem cell (MSC) lineage in a recipient in need thereof. The method comprises (a) obtaining from a mammal a tissue sample comprising a MSC population; (b) expanding, in vitro, the MSC population from the sample, wherein: (i) the MSC population expands by at least 2-fold, such as for example, by at least 4-fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20- fold or more; and (c) transplanting the expanded MSC population into a subject in need thereof, such as a mammal, including a primate or human. In this embodiment, the human recipient requires a peripheral blood transplant, a cord blood transplant or a bone marrow transplant. Thus, in a further aspect, the tissue sample is obtained from any appropriate tissue such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow. The sample may be obtained from an autologous or allogeneic source. Preferably, the sample is obtained from an autologous source.

[00191] An additional embodiment of the present invention is a method for treating a subject suffering from cancer and/or a blood disorder. The method comprises (a) obtaining from a mammal a tissue sample comprising a T-cell population; (b) modifying the T-cell population with a chimeric antigen receptor (CAR) to form a CAR T-cell population; (c) expanding, in vitro, the CAR T-cell population from the sample, wherein: (i) the CAR T-cell population expands by at least 2-fold, such as for example, by at least 4-fold, including at least 8-fold, at least 10-fold, at least 15-fold, at least 20-fold or more; and (c) transplanting the expanded CAR-T cell population into a subject in need thereof, such as a mammal, including a primate or human. In this embodiment, the human recipient may be suffering from, e.g., leukemia and/or lymphoma. Thus, in a further aspect, the tissue sample is obtained from any appropriate tissue such as, e.g., a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow. The sample may be obtained from an autologous or allogeneic source. Preferably, the sample is obtained from an autologous source.

[00192] In aspects of the present invention, it is preferred that the expanded HSC population comprises HSCs that have a phenotype selected from the group consisting of CD34 ^" or CD34 ⁺ /CD38 ^"/low /Thy-1 ⁺ /CD90 ⁺ /Kit ^"/lo /Lin7CD133 ⁺ VEGFR2 ⁺ , which are markers for the most primitive and long-term undifferentiated human HSCs; CD150 ⁺ /CD487CD244 ^" , which is a marker for human HSCs and their progenitors; and/or CD1507CD487CD244 ⁺ and CD1507CD48 ⁺ /CD244 ⁺ , which are markers for non-self-renewing multipotent hematopoietic progenitors, and combinations thereof. (See, e.g., Mimeault, M., et al., Stem Cells: A Revolution in Therapeutics - Recent Advances in Stem Cell Biology and Their Therapeutic Applications in Regenerative Medicine and Cancer Therapies. Clin Pharmacol Then, 82(3):252-64 (2007)). In aspects of the present invention, it is preferred that the expanded MSC population and/or MSC population subject to expansion, comprises MSCs that have a phenotype selected from the group consisting of N- cadherin+ and CD105+, and combinations thereof. That is, the MSC population of any of the embodiments described herein can comprise at least one MSC selected from the group consisting of N-cadherin+ MSCs and CD105+ MSCs. In one embodiment, the MSC population comprises N-cadherin+ MSCs.

[00193] The exact proportions of HSCs and/or MSCs having these markers in the population is not critical, so long as the expanded HSC and/or MSC population as a whole is sufficient to result in at least 1 % engraftment in a recipient.

[00194] In another embodiment, the invention is a method for expanding a hematopoietic stem cell population in a mammal in need of such expansion. This method comprises administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -methyladenosine (m ⁶ A) mRNA modification pathway, for a period of time sufficient to expand the HSC population by at least 2-fold with HSCs that possess the ability to reconstitute an hematopoietic lineage in the mammal.

[00195] In another embodiment, the invention is a method for expanding a mesenchymal stem cell population in a mammal in need of such expansion. This method comprises administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -methyladenosine (m ⁶ A) mRNA modification pathway, for a period of time sufficient to expand the MSC population by at least 2-fold with MSCs that possess the ability to reconstitute a mesenchymal lineage in the mammal.

[00196] In another embodiment, the invention is a method for expanding a chimeric antigen receptor (CAR) T-cell population in a mammal in need of such expansion. This method comprises administering to the mammal a therapeutically effective amount of a modulator of a N ⁶ -methyladenosine (m ⁶ A) mRNA modification pathway, for a period of time sufficient to expand the CAR T-cell population by at least 2-fold with CAR T-cells that possess the ability to treat cancer and/or a blood disorder in the mammal. [00197] In these methods, the modulators may be as previously disclosed herein, and/or modulation may be performed by any method disclosed herein. For example, the modulator may comprise a system for introducing a mutation into the HSC and/or MSC and/or CAR T-cell population that deletes, replaces or reduces expression of a gene expressing a N ⁶ -methyladenosine (m ⁶ A) mRNA modification reader, such as Ythdf2. As yet another example, the modulator may comprise an inhibitor of a N6-methyladenosine (m6A) mRNA modification reader, such as an inhibitor of Ythdf2. The mammal in need of expansion may be a human.

[00198] In another embodiment, the invention includes a method of isolating mesenchymal stem cells (MSCs) from a biological sample, the method comprising contacting the biological sample having a population of MSCs with one or more N- cadherin antibodies. For example, according to certain aspects, the biological sample comprises a tissue selected from the group consisting of peripheral blood, cord blood and bone marrow. Furthermore, in certain embodiments, the method of isolating the MSCs can further comprise one or more steps of expanding the population of MSCs from the biological sample, by modulating a N ⁶ -Methyladenosine (m ⁶ A) mRNA modification pathway in the population of MSCs, to expand the number of mesenchymal stem cells, such as by any of the methods described herein. In one embodiment, the population of MSCs is expanded after isolating from the biological sample. In another embodiment, the population of MSCs in the biological sample is expanded before isolation of the MSCs from the biological sample. In one embodiment, the MSC population is expanded to a sufficient quantity while maintaining a multilineage differentiation potential in the MSC population, which is sufficient for subsequent transplantation into a subject in need thereof, such that the MSCs isolated by the method can be used for such transplantation. For example, the isolated MSCs may be transplanted into a human subject. According to yet another embodiment, the MSCs may be further isolated from the biological sample by contacting with CD105 antibodies, either in addition to or as an alternative to the N-cadherin antibodies. In yet a further embodiment, the N-cadherin antibodies may be used to identify the MSCs in the biological sample, by contacting the biological sample with the N-cadherin antibodies and detecting those cells that bind to the N- cadherin antibodies.

[00199] In one embodiment, the method of isolating the MSCs comprises expanding the MSC population, by modulating the m ⁶ A mRNA modification pathway by introducing a mutation into the stem cells that results in modulation of a molecule in the m ⁶ A mRNA modification pathway or contacting the stem cell with a modulator of a molecule in the m ⁶ A mRNA modification pathway selected from the group consisting of a small molecule, a biologic, an antisense RNA, a small interfering RNA (siRNA), and combinations thereof, such as by any of the modulation methods described herein. For example, in one embodiment, modulating the m ⁶ A mRNA modification pathway comprises introducing a mutation into the stem cells to delete, replace, or reduce expression of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. In one embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a molecule selected from the group consisting of a m ⁶ A mRNA modification reader, a m ⁶ A mRNA modification writer, and a m ⁶ A mRNA eraser. In yet another embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification reader. In one embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification reader selected from the group consisting of Ythdfl , Ythdf2, Ythdf3, Ythdc1, Ythdc2, HNRNPC, HNRNPA2B1 , and elF3. In another embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses Ythdf2. In one embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification eraser. In one embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification eraser selected from the group consisting of FTO and ALKBH5. In another embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification writer. In one embodiment, the mutation deletes, replaces or reduces expression of a gene that expresses a m ⁶ A mRNA modification writer selected from the group consisting of METTL3, METTL14, WTAP and KIAA1429.

[00200] In another embodiment, the method of isolating MSCs comprises expanding the population of MSCS by modulating the m ⁶ A mRNA modification pathway, by exposing the stem cells to a Mx1 -Cre targeting system that inactivates or deletes at least a portion of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. In one embodiment, modulating the m ⁶ A mRNA modification pathway comprises introducing a mutation that incorporates shRNA into the stem cells to reduce expression of a gene that expresses a molecule in the m ⁶ A mRNA modification pathway. For example, in one embodiment, the shRNA is introduced by exposing the stem cells to a lentivirus to deliver the shRNA. According to yet another embodiment, modulating the m ⁶ A mRNA modification pathway comprises down-regulating a m ⁶ A mRNA modification reader. In one embodiment, the m ⁶ A mRNA modification reader is selected from the group consisting of Ythdfl , Ythdf2, Ythdf3, Ythdc1, Ythdc2, HNRNPC, HNRNPA2B1 , and elF3. For example, in one embodiment, the m ⁶ A mRNA modification reader comprises Ythdf2. [00201] In one embodiment, the method of isolating MSCs further comprise expanding the population of MSCs by down-regulating the m ⁶ A mRNA modification reader, by contacting the stem cells with an inhibitor of the m ⁶ A mRNA modification reader that is any of those described herein, such as any of those selected from the group consisting of: (inhibitors of HNRNPC) hsa-let-7e-5p (MIRT051596), hsa-mir- 455-3p (MIRT037890), hsa-mir-30c-5p (MIRT047904), hsa-mir-186-5p (MIRT045150), hsa-mir-744-5p (MIRT037494), hsa-mir-18a-3p (MIRT040851 ), hsa- mir-484 (MIRT042196), hsa-mir-505-5p (MIRT037959), hsa-mir-615-3p (MIRT039991 ), hsa-mir-342-3p (MIRT043694), hsa-miR-3607-3p, hsa-miR-30d, hsa- miR-3916, hsa-miR-3162-5p, hsa-miR-1273d, hsa-miR-3161 , hsa-miR-30a, hsa- miR-629, hsa-miR-208b, hsa-miR-489, hsa-miR-3148, hsa-miR-2113, hsa-miR-877, hsa-miR-455-5p, hsa-miR-186, hsa-miR-548o, hsa-miR-3139, hsa-miR-320a, hsa- miR-4311, hsa-miR-555, hsa-miR-3605-5p, hsa-miR-515-5p, hsa-miR-144, hsa-miR- 499-5p, hsa-miR-1323, hsa-miR-548x, hsa-miR-299-5p, hsa-miR-653, hsa-miR-576- 5p, hsa-miR-548p, hsa-miR-586, hsa-miR-888, hsa-miR-3647-3p, hsa-miR-484, hsa- miR-320b, hsa-miR-620, hsa-miR-30b, hsa-miR-548q, hsa-miR-29b-1 , hsa-miR-570, hsa-miR-183, hsa-miR-1276, hsa-miR-208a, hsa-miR-186, hsa-miR-28-5p, hsa-miR- 330-3p, hsa-miR-548am, hsa-miR-320d, hsa-miR-3175, hsa-miR-3155, hsa-miR- 548aa, hsa-miR-519e, hsa-miR-1270, hsa-miR-513b, hsa-miR-599, hsa-miR-518f, hsa-miR-4301 , hsa-miR-30c, hsa-miR-3135, hsa-miR-4286, hsa-miR-202, hsa-miR- 4263, hsa-miR-4299, hsa-miR-606, hsa-miR-3133, hsa-miR-583, hsa-miR-3125, hsa-miR-501 -5p, hsa-miR-7-1 , hsa-miR-514b-3p, hsa-miR-3155b, hsa-miR-548d-3p, hsa-miR-224, hsa-miR-7-2, hsa-miR-708, hsa-miR-3199, hsa-miR-514, hsa-miR- 30e; (inhibitors of HNRNPA2B1 ) hsa-mir-92a-3p (MIRT049721 ), hsa-mir-30c-5p (MIRT048009), hsa-mir-191 -5p (MIRT045809), hsa-let-7f-5p (MIRT051404), hsa- mir-27b-3p (MIRT046213), hsa-mir-877-3p (MIRT037116), hsa-mir-615-3p (MIRT040278), hsa-mir-1260b (MIRT052680), hsa-mir-103a-3p (MIRT027027), hsa- mir-16-5p (MIRT031508), hsa-mir-1296-5p (MIRT036075), hsa-mir-197-3p (MIRT048098), hsa-miR-548j, hsa-miR-3678-3p, hsa-miR-607, hsa-miR-188-5p, hsa-miR-15a, hsa-miR-3653, hsa-miR-371 -5p, hsa-miR-550a, hsa-miR-3622b-3p, hsa-miR-548a-5p, hsa-miR-3170, hsa-miR-3148, hsa-miR-556-3p, hsa-miR-490-3p, hsa-miR-559, hsa-miR-200c, hsa-miR-130a, hsa-miR-548y, hsa-miR-548o, hsa-miR- 23c, hsa-miR-491 -3p, hsa-miR-335, hsa-miR-3667-3p, hsa-miR-466, hsa-miR-23b, hsa-miR-4310, hsa-miR-127-5p, hsa-miR-548b-5p, hsa-miR-616, hsa-miR-16, hsa- miR-338-3p, hsa-miR-3200-5p, hsa-miR-362-3p, hsa-miR-448, hsa-miR-1306, hsa- miR-944, hsa-miR-3684, hsa-miR-373, hsa-miR-103a, hsa-miR-380, hsa-miR-499- 5p, hsa-miR-1323, hsa-miR-323-5p, hsa-miR-3674, hsa-miR-1252, hsa-miR-33b, hsa-miR-580, hsa-miR-548c-3p, hsa-miR-103a-2, hsa-miR-548w, hsa-miR-600, hsa- miR-634, hsa-miR-586, hsa-miR-497, hsa-miR-720, hsa-miR-654-3p, hsa-miR-524- 5p, hsa-miR-543, hsa-miR-548q, hsa-let-7f-2, hsa-miR-330-5p, hsa-miR-500a, hsa- miR-548l, hsa-miR-570, hsa-miR-374a, hsa-miR-1184, hsa-miR-649, hsa-miR-424, hsa-miR-3658, hsa-miR-186, hsa-miR-326, hsa-miR-548d-5p, hsa-miR-23a, hsa- miR-15b, hsa-miR-190, hsa-miR-203, hsa-miR-548h, hsa-miR-3136-5p, hsa-miR- 618, hsa-miR-551 b, hsa-miR-211, hsa-miR-1305, hsa-miR-513b, hsa-miR-96, hsa- miR-2117, hsa-miR-548n, hsa-miR-3910, hsa-miR-217, hsa-miR-892b, hsa-miR- 502-5p, hsa-miR-548i, hsa-miR-520d-5p, hsa-miR-4299, hsa-miR-1285, hsa-miR- 3133, hsa-miR-483-3p; (inhibitors of Ythdfl ) hsa-miR-548g, hsa-miR-204, hsa-miR- 3143, hsa-miR-521 , hsa-miR-195, hsa-miR-3182, hsa-miR-3941 , hsa-miR-34c-3p, hsa-miR-767-3p, hsa-miR-563, hsa-miR-548c-5p, hsa-miR-1911 , hsa-miR-26b, hsa- miR-190b, hsa-miR-33a, hsa-miR-329, hsa-miR-221 , hsa-miR-612, hsa-miR-3185, hsa-miR-3156-5p, hsa-miR-107, hsa-miR-664, hsa-miR-3657; (inhibitors of Ythdf2) hsa-mir-615-3p (MIRT040054), hsa-mir-106b-5p (MIRT044257), hsa-mir- 1 (MIRT023842), miR-145, hsa-miR-3607-3p, hsa-miR-200a, hsa-miR-301 a, hsa- miR-519a, hsa-miR-141 , hsa-miR-130b, hsa-miR-181 b, hsa-miR-301 b, hsa-miR- 3117-3p, hsa-miR-1236, hsa-miR-181 a, hsa-miR-519c-3p, hsa-miR-551 b, hsa-miR- 519e, hsa-miR-519b-3p, hsa-miR-19b, hsa-miR-1303, hsa-miR-608, hsa-miR-145, hsa-miR-130a, hsa-miR-181 c, hsa-miR-323b-3p, hsa-miR-421 , hsa-miR-515-5p, hsa-miR-3666, hsa-miR-181 d, hsa-miR-146a, hsa-miR-4295, hsa-miR-454, hsa- miR-3919, hsa-miR-19a, hsa-miR-543, hsa-miR-4262; (inhibitors of Ythdf3) hsa-miR- 582-3p, hsa-miR-579, hsa-miR-520e, hsa-miR-520f, hsa-miR-3152-3p, hsa-miR- 106a, hsa-miR-30d, hsa-miR-30a, hsa-miR-93, hsa-miR-508-5p, hsa-miR-29a, hsa- miR-3148, hsa-miR-490-5p, hsa-miR-520b, hsa-miR-20a, hsa-miR-409-3p, hsa-miR- 4255, hsa-let-7i, hsa-miR-373, hsa-let-7e, hsa-miR-520c-3p, hsa-miR-3920, hsa- miR-127-5p, hsa-miR-380, hsa-miR-616, hsa-miR-4277, hsa-miR-448, hsa-miR-16- 2, hsa-let-7c, hsa-miR-340, hsa-miR-373, hsa-miR-520a-3p, hsa-miR-144, hsa-miR- 1265, hsa-miR-548x, hsa-miR-362-5p, hsa-miR-33b, hsa-miR-26b, hsa-miR-17, hsa- miR-569, hsa-miR-3618, hsa-miR-576-5p, hsa-miR-922, hsa-miR-302a, hsa-miR- 106b, hsa-miR-888, hsa-miR-484, hsa-let-7b, hsa-miR-582-5p, hsa-let-7f, hsa-miR- 30b, hsa-miR-524-5p, hsa-miR-302d, hsa-let-7d, hsa-miR-513a-5p, hsa-miR-500a, hsa-miR-570, hsa-miR-548l, hsa-miR-105, hsa-miR-374c, hsa-let-7g hsa-miR-372, hsa-miR-3658, hsa-let-7a, hsa-miR-3908, hsa-miR-302b, hsa-miR-526b, hsa-miR- 190, hsa-miR-181 b, hsa-miR-433, hsa-miR-98, hsa-miR-3606, hsa-miR-595, hsa- miR-548am, hsa-miR-187, hsa-miR-561 , hsa-miR-181 a, hsa-miR-3155, hsa-miR- 655, hsa-miR-302c, hsa-miR-195, hsa-miR-26a, hsa-miR-590-3p, hsa-miR-30c, hsa- miR-502-5p, hsa-miR-495, hsa-miR-137, hsa-miR-181 c, hsa-miR-520d-5p, hsa-miR- 3942-5p, hsa-miR-202, hsa-miR-302e, hsa-miR-513c, hsa-miR-885-5p, hsa-miR- 520a-5p, hsa-miR-583, hsa-miR-1297, hsa-miR-7-1 , hsa-miR-520d-3p, hsa-miR- 3155b, hsa-miR-3182, hsa-miR-519d, hsa-miR-550a, hsa-miR-7-2, hsa-miR-181 d, hsa-miR-190b, hsa-miR-1912, hsa-miR-151 -3p, hsa-miR-33a, hsa-miR-525-5p, hsa- miR-20b, hsa-miR-514b-5p, hsa-miR-30e, hsa-miR-4262, hsa-miR-636; (inhibitor of elF3) hsa-mir-92b-3p (MIRT040734), hsa-mir-16-5p (MIRT031705), hsa-mir-18a-3p (MIRT040974), hsa-mir-155-5p (MIRT020771 ), hsa-mir-484 (MIRT042324), hsa-let- 7c-5p (MIRT051776), hsa-miR-3910, hsa-miR-148b, hsa-miR-136, hsa-miR-15a, hsa-miR-488, hsa-miR-500a, hsa-miR-1297, hsa-miR-3159, hsa-miR-374c, hsa- miR-424, hsa-miR-7-1 , hsa-miR-186, hsa-miR-195, hsa-miR-15b, hsa-miR-26b, hsa- miR-505, hsa-miR-1206, hsa-miR-653, hsa-miR-1283, hsa-miR-7-2, hsa-miR-196a, hsa-miR-497, hsa-miR-33a, hsa-miR-655, hsa-miR-26a hsa-miR-16, hsa-mir-151 a- 3p (MIRT043600), hsa-mir-92a-3p (MIRT049064), hsa-mir-615-3p (MIRT039779), hsa-mir-877-3p (MIRT036964), hsa-mir-222-3p (MIRT046746), hsa-mir-423- 3p (MIRT042468), hsa-mir-324-3p (MIRT042887), hsa-mir-124-3p (MIRT022932), hsa-miR-3140-3p, hsa-miR-124, hsa-miR-198, hsa-miR-525-5p, hsa-miR-506, hsa- miR-520a-5p, hsa-miR-196a ^* hsa-miR-3117-3p, hsa-mir-342-5p (MIRT038210), hsa- mir-378a-5p (MIRT043981 ), hsa-mir-615-3p (MIRT040086), hsa-let-7b-

5p (MIRT052211), hsa-mir-455-3p (MIRT037879), hsa-miR-4267, hsa-miR-590-3p, hsa-mir-106b-5p (MIRT044355), hsa-mir-320a (MIRT044466), hsa-mir-16- 5p (MIRT032018), hsa-mir-155-5p (MIRT021009), hsa-miR-4302, hsa-mir-191 -5p (MIRT045793), hsa-mir-1303 (MIRT035890), hsa-mir-193b-3p (MIRT016316), hsa- mir-222-3p (MIRT046640), hsa-mir-532-3p (MIRT037924), hsa-mir-18a-3p (MIRT040929), hsa-mir-92a-3p (MIRT049001 ), hsa-miR-582-3p, hsa-miR-4265, hsa- miR-218-2, hsa-miR-1271 , hsa-miR-340, hsa-miR-221 , hsa-miR-20b, hsa-miR-508- 3p, hsa-miR-141 , hsa-miR-4325, hsa-miR-889, hsa-miR-29a, hsa-miR-129-3p, hsa- miR-129, hsa-miR-96, hsa-miR-3163, hsa-miR-187, hsa-miR-196a, hsa-miR-222, hsa-miR-1179, hsa-miR-182, hsa-miR-9 ^* hsa-miR-32, hsa-miR-143, hsa-miR-4296: (inhibitors of YTHDC1 ) hsa-mir-20a-3p (MIRT038967), hsa-mir-103a-3p (MIRT027037), hsa-mir-1 (MIRT023492), hsa-mir-19b-3p (MIRT031105), hsa-mir- 100-5p (MIRT048400), hsa-mir-93-5p (MIRT027989), hsa-mir-16-5p (MIRT031379), hsa-let-7b-5p (MIRT052150), hsa-miR-520f, hsa-miR-300, hsa-miR-15a, hsa-miR- 200a, hsa-miR-605, hsa-miR-30d, hsa-miR-30a, hsa-miR-3613-3p, hsa-miR-509-3- 5p, hsa-miR-34c-5p, hsa-miR-324-3p, hsa-miR-1248, hsa-miR-152, hsa-miR-548t, hsa-miR-4310, hsa-miR-145, hsa-miR-516a-3p, hsa-miR-16, hsa-miR-3668, hsa- miR-4277, hsa-miR-448, hsa-miR-16-2, hsa-miR-148b, hsa-miR-509-5p, hsa-miR- 103a, hsa-miR-1265, hsa-miR-2115, hsa-miR-548c-3p, hsa-miR-148a, hsa-miR- 548p, hsa-miR-513a-3p, hsa-miR-497, hsa-miR-3647-3p, hsa-miR-382, hsa-miR- 30b, hsa-miR-543, hsa-let-7f-2, hsa-miR-1269, hsa-miR-3164, hsa-miR-503, hsa- miR-500a, hsa-miR-449a, hsa-miR-141 , hsa-miR-424, hsa-miR-3908, hsa-miR-889, hsa-miR-2116, hsa-miR-330-3p, hsa-miR-15b, hsa-miR-181 b, hsa-miR-187, hsa- miR-1237, hsa-miR-449b, hsa-miR-101 , hsa-miR-381 , hsa-miR-618, hsa-miR-222, hsa-miR-181 a, hsa-miR-432, hsa-miR-96, hsa-miR-19b, hsa-miR-195, hsa-miR- 548n, hsa-miR-485-5p, hsa-miR-217, hsa-miR-30c, hsa-miR-495, hsa-miR-137, hsa- miR-1288, hsa-miR-181 c, hsa-miR-3942-5p, hsa-miR-548v, hsa-miR-487a, hsa- miR-221 , hsa-miR-891 b, hsa-miR-205, hsa-miR-195, hsa-miR-4271 , hsa-miR-3611, hsa-miR-516b, hsa-miR-181 d, hsa-miR-154, hsa-miR-646, hsa-miR-153, hsa-miR- 34a, hsa-miR-19a, hsa-miR-107, hsa-miR-30e and hsa-miR-4262.

[00202] According to one embodiment, the invention comprises an isolated population of mesenchymal stem cells made by any of the processes described herein. For example, the invention in certain embodiments can comprise an expanded, isolated population of mesenchymal stem cells made by any of the expansion and/or isolation processes described herein.

[00203] In one embodiment, a kit for isolating a mesenchymal stem cell (MSC) population for subsequent transplantation into a subject in need thereof is provided. The kit comprises a system for contacting a biological sample comprising MSCs with one or more N-cadherin antibodies, and instructions for use thereof. In one embodiment, the kit further comprises a system for expanding the population of MSCs by introducing a mutation into the MSC population that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and instructions for use thereof. In yet another embodiment, the system for introducing a mutation into the MSC population includes one or more reagents capable of introducing a mutation into the MSC population that results in deletion, replacement or reduced expression of a gene expressing Ythdf2. In yet another embodiment, the system for introducing a mutation into the MSC population comprises a Mx1 -Cre targeting system that inactivates or deletes at least a portion of a gene that expresses a m ⁶ A mRNA modification reader. In one embodiment, the system for introducing a mutation into the MSC population comprises reagents for delivering a lentivirus that incorporates shRNA into the MSC population to reduce expression of a gene that expresses a m ⁶ A mRNA modification reader.

[00204] According to another embodiment of the invention, a method for administering a mesenchymal stem cell (MSC) to a subject in need thereof is provided. The method comprises isolating MSCs from a biological sample comprising a population of MSCs, by contacting the biological sample with one or more N-cadherin antibodies, and administering the isolated MSCs to the subject. Furthermore, in one embodiment, the method further comprises introducing, into the biological sample containing the MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and culturing the biological sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject. In one embodiment, wherein the mutation results in deletion a gene expressing Ythdf2. In one embodiment, the mutation results in incorporation of shRNA into the HSC population that reduces expression of a gene expressing Ythdf2. According to yet another aspect, the MSCs are obtained from a biological sample comprising a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow.

[00205] According to yet another embodiment, a method for reconstituting bone marrow in a subject in need thereof is provided. The method comprises isolating mesenchymal stem cells (MSCs) from a biological sample comprising a population of MSCs, by contacting the biological sample with one or more N-cadherin antibodies, and administering the isolated MSCs to the subject. Furthermore, according to one aspect, the method further comprises introducing, into the biological sample containing the MSC population, a mutation that results in deletion, replacement or reduced expression of a gene expressing a m ⁶ A mRNA modification reader, and culturing the sample in a suitable culture media for a period of time sufficient to expand the number of MSCs in the sample to a number sufficient to transplant into the subject. In one embodiment, the MSCs are obtained from a biological sample comprising a tissue selected from the group consisting of cord blood, peripheral blood, and bone marrow. [00206] According to yet another embodiment, a method for treating a subject in need of a transplant, selected from the group consisting of a bone marrow transplant, a peripheral blood transplant and an umbilical cord blood transplant comprising administering to the subject a population of isolated MSCs obtained by any of the methods described herein. According to embodiment, the sample is from an autologous or allogeneic source. According to yet another embodiment, the sample is from an autologous source.

[00207] In the present invention, a "therapeutically effective amount" is an amount sufficient to effect beneficial or desired results. In terms of treatment of a mammal, a "therapeutically effective amount" of a modulator and/or expanded cells is an amount sufficient to treat, manage, palliate, ameliorate, or stabilize a condition, such as a bone marrow disease, in the mammal. A therapeutically effective amount can be administered in one or more doses.

[00208] The therapeutically effective amount is generally determined by a physician on a case-by-case basis and is within the skill of one in the art. Several factors are typically taken into account when determining an appropriate dosage. These factors include age, sex and weight of the subject, the condition being treated, the severity of the condition and the form of the drug being administered.

[00209] Effective dosage forms, modes of administration, and dosage amounts may be determined empirically, and making such determinations is within the skill of the art. It is understood by those skilled in the art that the dosage amount will vary with the route of administration, the rate of excretion, the duration of the treatment, the identity of any other drugs being administered, the age, size, and species of animal, and like factors well known in the arts of medicine and veterinary medicine. In general, a suitable dose of a modulator according to the invention will be that amount of the modulator, which is the lowest dose effective to produce the desired effect. The effective dose of a modulator maybe administered as two, three, four, five, six or more sub-doses, administered separately at appropriate intervals throughout the day.

[00210] A modulator may be administered in any desired and effective manner: as pharmaceutical compositions for oral ingestion, or for parenteral or other administration in any appropriate manner such as intraperitoneal, subcutaneous, topical, intradermal, inhalation, intrapulmonary, rectal, vaginal, sublingual, intramuscular, intravenous, intraarterial, intrathecal, or intralymphatic. Further, a modulator of the present invention may be administered in conjunction with other treatments. A modulator of the present invention maybe encapsulated or otherwise protected against gastric or other secretions, if desired.

[00211] While it is possible for a modulator of the invention to be administered alone, it is preferable to administer the modulator as a pharmaceutical formulation (composition). Such pharmaceutical formulations typically comprise one or more modulators as an active ingredient in admixture with one or more pharmaceutically- acceptable carriers and, optionally, one or more other compounds, drugs, ingredients and/or materials. Regardless of the route of administration selected, the modulator of the present invention may be formulated into pharmaceutically- acceptable dosage forms by conventional methods known to those of skill in the art. See, e.g., Remington's Pharmaceutical Sciences (Mack Publishing Co., Easton, Pa.).

[00212] Pharmaceutically acceptable carriers are well known in the art (see, e.g., Remington's Pharmaceutical Sciences (Mack Publishing Co., Easton, Pa.) and The National Formulary (American Pharmaceutical Association, Washington, D.C.)) and include sugars {e.g., lactose, sucrose, mannitol, and sorbitol), starches, cellulose preparations, calcium phosphates (e.g. , dicalcium phosphate, tricalcium phosphate and calcium hydrogen phosphate), sodium citrate, water, aqueous solutions (e.g. , saline, sodium chloride injection, Ringer's injection, dextrose injection, dextrose and sodium chloride injection, lactated Ringer's injection), alcohols (e.g. , ethyl alcohol, propyl alcohol, and benzyl alcohol), polyols (e.g., glycerol, propylene glycol, and polyethylene glycol), organic esters (e.g. , ethyl oleate and triglycerides), biodegradable polymers (e.g. , polylactide-polyglycolide, poly(orthoesters), and poly(anhydrides)), elastomeric matrices, liposomes, microspheres, oils (e.g. , corn, germ, olive, castor, sesame, cottonseed, and groundnut), cocoa butter, waxes (e.g. , suppository waxes), paraffins, silicones, talc, silicylate, etc. Each pharmaceutically acceptable carrier used in a pharmaceutical composition comprising a modulator of the invention must be "acceptable" in the sense of being compatible with the other ingredients of the formulation and not injurious to the subject. Carriers suitable for a selected dosage form and intended route of administration are well known in the art, and acceptable carriers for a chosen dosage form and method of administration can be determined using ordinary skill in the art.

[00213] Pharmaceutical compositions comprising a modulator of the invention may, optionally, contain additional ingredients and/or materials commonly used in pharmaceutical compositions. These ingredients and materials are well known in the art and include (1 ) fillers or extenders, such as starches, lactose, sucrose, glucose, mannitol, and silicic acid; (2) binders, such as carboxymethylcellulose, alginates, gelatin, polyvinyl pyrrolidone, hydroxypropylm ethyl cellulose, sucrose and acacia; (3) humectants, such as glycerol; (4) disintegrating agents, such as agar-agar, calcium carbonate, potato or tapioca starch, alginic acid, certain silicates, sodium starch glycolate, cross-linked sodium carboxymethyl cellulose and sodium carbonate; (5) solution retarding agents, such as paraffin; (6) absorption accelerators, such as quaternary ammonium compounds; (7) wetting agents, such as cetyl alcohol and glycerol monosterate; (8) absorbents, such as kaolin and bentonite clay; (9) lubricants, such as talc, calcium stearate, magnesium stearate, solid polyethylene glycols, and sodium lauryl sulfate; (10) suspending agents, such as ethoxylated isostearyl alcohols, polyoxyethylene sorbitol and sorbitan esters, microcrystalline cellulose, aluminum metahydroxide, bentonite, agar-agar and tragacanth; (11) buffering agents; (12) excipients, such as lactose, milk sugars, polyethylene glycols, animal and vegetable fats, oils, waxes, paraffins, cocoa butter, starches, tragacanth, cellulose derivatives, polyethylene glycol, silicones, bentonites, silicic acid, talc, salicylate, zinc oxide, aluminum hydroxide, calcium silicates, and polyamide powder; (13) inert diluents, such as water or other solvents; (14) preservatives; (15) surface- active agents; (16) dispersing agents; (17) control-release or absorption-delaying agents, such as hydroxypropylmethyl cellulose, other polymer matrices, biodegradable polymers, liposomes, microspheres, aluminum monosterate, gelatin, and waxes; (18) opacifying agents; (19) adjuvants; (20) wetting agents; (21 ) emulsifying and suspending agents; (22), solubilizing agents and emulsifiers, such as ethyl alcohol, isopropyl alcohol, ethyl carbonate, ethyl acetate, benzyl alcohol, benzyl benzoate, propylene glycol, 1 ,3-butylene glycol, oils (in particular, cottonseed, groundnut, corn, germ, olive, castor and sesame oils), glycerol, tetrahydrofuryl alcohol, polyethylene glycols and fatty acid esters of sorbitan; (23) propellants, such as chlorofluorohydrocarbons and volatile unsubstituted hydrocarbons, such as butane and propane; (24) antioxidants; (25) agents which render the formulation isotonic with the blood of the intended recipient, such as sugars and sodium chloride; (26) thickening agents; (27) coating materials, such as lecithin; and (28) sweetening, flavoring, coloring, perfuming and preservative agents. Each such ingredient or material must be "acceptable" in the sense of being compatible with the other ingredients of the formulation and not injurious to the subject. Ingredients and materials suitable for a selected dosage form and intended route of administration are well known in the art, and acceptable ingredients and materials for a chosen dosage form and method of administration may be determined using ordinary skill in the art.

[00214] Pharmaceutical compositions suitable for oral administration may be in the form of capsules, cachets, pills, tablets, powders, granules, a solution or a suspension in an aqueous or non-aqueous liquid, an oil-in-water or water-in-oil liquid emulsion, an elixir or syrup, a pastille, a bolus, an electuary or a paste. These formulations may be prepared by methods known in the art, e.g., by means of conventional pan-coating, mixing, granulation or lyophilization processes.

[00215] Solid dosage forms for oral administration (capsules, tablets, pills, dragees, powders, granules and the like) may be prepared by mixing the active ingredient(s) with one or more pharmaceutically-acceptable carriers and, optionally, one or more fillers, extenders, binders, humectants, disintegrating agents, solution retarding agents, absorption accelerators, wetting agents, absorbents, lubricants, and/or coloring agents. Solid compositions of a similar type maybe employed as fillers in soft and hard-filled gelatin capsules using a suitable excipient. A tablet may be made by compression or molding, optionally with one or more accessory ingredients. Compressed tablets may be prepared using a suitable binder, lubricant, inert diluent, preservative, disintegrant, surface-active or dispersing agent. Molded tablets may be made by molding in a suitable machine. The tablets, and other solid dosage forms, such as dragees, capsules, pills and granules, may optionally be scored or prepared with coatings and shells, such as enteric coatings and other coatings well known in the pharmaceutical-formulating art. They may also be formulated so as to provide slow or controlled release of the active ingredient therein. They may be sterilized by, for example, filtration through a bacteria-retaining filter. These compositions may also optionally contain opacifying agents and may be of a composition such that they release the active ingredient only, or preferentially, in a certain portion of the gastrointestinal tract, optionally, in a delayed manner. The active ingredient can also be in microencapsulated form.

[00216] Liquid dosage forms for oral administration include pharmaceutically- acceptable emulsions, microemulsions, solutions, suspensions, syrups and elixirs. The liquid dosage forms may contain suitable inert diluents commonly used in the art. Besides inert diluents, the oral compositions may also include adjuvants, such as wetting agents, emulsifying and suspending agents, sweetening, flavoring, coloring, perfuming and preservative agents. Suspensions may contain suspending agents.

[00217] Pharmaceutical compositions for rectal or vaginal administration may be presented as a suppository, which may be prepared by mixing one or more active ingredient(s) with one or more suitable nonirritating carriers which are solid at room temperature, but liquid at body temperature and, therefore, will melt in the rectum or vaginal cavity and release the active compound. Pharmaceutical compositions which are suitable for vaginal administration also include pessaries, tampons, creams, gels, pastes, foams or spray formulations containing such pharmaceutically-acceptable carriers as are known in the art to be appropriate. [00218] Dosage forms for the topical or transdermal administration include powders, sprays, ointments, pastes, creams, lotions, gels, solutions, patches, drops and inhalants. The active compound may be mixed under sterile conditions with a suitable pharmaceutically-acceptable carrier. The ointments, pastes, creams and gels may contain excipients. Powders and sprays may contain excipients and propellants.

[00219] Pharmaceutical compositions suitable for parenteral administrations comprise one or more modulator in combination with one or more pharmaceutically- acceptable sterile isotonic aqueous or non-aqueous solutions, dispersions, suspensions or emulsions, or sterile powders which may be reconstituted into sterile injectable solutions or dispersions just prior to use, which may contain suitable antioxidants, buffers, solutes which render the formulation isotonic with the blood of the intended recipient, or suspending or thickening agents. Proper fluidity can be maintained, for example, by the use of coating materials, by the maintenance of the required particle size in the case of dispersions, and by the use of surfactants. These compositions may also contain suitable adjuvants, such as wetting agents, emulsifying agents and dispersing agents. It may also be desirable to include isotonic agents. In addition, prolonged absorption of the injectable pharmaceutical form may be brought about by the inclusion of agents which delay absorption.

[00220] In some cases, in order to prolong the effect of a drug containing a modulator of the present invention, it is desirable to slow its absorption from subcutaneous or intramuscular injection. This may be accomplished by the use of a liquid suspension of crystalline or amorphous material having poor water solubility.

[00221] The rate of absorption of the drug then depends upon its rate of dissolution which, in turn, may depend upon crystal size and crystalline form. Alternatively, delayed absorption of a parenterally-administered drug may be accomplished by dissolving or suspending the drug in an oil vehicle. Injectable depot forms may be made by forming microencapsule matrices of the active ingredient in biodegradable polymers. Depending on the ratio of the active ingredient to polymer, and the nature of the particular polymer employed, the rate of active ingredient release can be controlled. Depot injectable formulations are also prepared by entrapping the drug in liposomes or microemulsions which are compatible with body tissue. The injectable materials can be sterilized for example, by filtration through a bacterial-retaining filter.

[00222] The formulations may be presented in unit-dose or multi-dose sealed containers, for example, ampules and vials, and may be stored in a lyophilized condition requiring only the addition of the sterile liquid carrier, for example water for injection, immediately prior to use. Extemporaneous injection solutions and suspensions may be prepared from sterile powders, granules and tablets of the type described above.

[00223] The following examples are provided to further illustrate the methods and compositions of the present invention. These examples are illustrative only and are not intended to limit the scope of the invention in any way.

Examples

[00224] The examples herein examine Ythdf2, a well-recognized m ⁶ A reader promoting targeted mRNA decay (Wang et al. Blood, 505: 17-120, 2014), at least partly with the purpose of investigating its role in the context of HSC maintenance. Without being limited to any particular theory, it is believed that manipulation of Ythdf2 might potentially influence the life span of a great number m ⁶ A-marked mRNAs, thus impacting adult HSC self-renewal versus differentiation and facilitating HSC expansion. As shown in the examples below, Ythdf2 depletion specifically expands mouse and human HSCs without skewing lineage fate. Accordingly, it is believed that Ythdf2 may play an essential role in regulating HSC self-renewal, and provide a novel approach to enhance hUCB HSCs ex vivo expansion, including in clinical applications.

[00225] The exmaples also show functional definition of the drug-resistant rHSC population, and a finding that rHSCs are maintained in the endosteal niche largely by N-cad ⁺ cells during homeostasis and under chemotherapeutic stress. It is further shown that N-cad ⁺ cells in endosteal zone are mesenchymal stem cells and contribute to rHSC maintenance.

EXAMPLE A

[00226] The following protocols were used in the "A" Examples below. [00227] Mice. Ythdf2 conditional KO mice were generated by Chuan He and Bin Shen group. Mice were housed in the animal facility at Stowers Institute for Medical Research (SIMR) and handled according to Institute and NIH guidelines. All procedures were approved by the IACUC of SIMR.

[00228] Flow cytometry and HSPC sorting. Mouse HSPCs, progenitors, and lineage cells were harvested from BM (femur and tibia) and spleen. Red blood cells were lysed using a 0.16 M ammonium chloride solution, and the cells were filtered with 70μm strainers to generate single cell suspensions. For mouse HSC identification, cells were stained with antibodies against Sca-1 (D7), c-Kit (2B8), CD34 (RAM34), Flk2 (A2F10), CD48 (HM48-1 ), CD150 (TC15-12F12.2), together with lineage cocktail including CD3 (145-2C11), CD4 (RM4-5), CD8 (53-6.7), Mac-1 (M1/70), Gr1 (RB6-8C5), CD45R (B220, RA3-6B2), IgM (11-41 ) and Ter119 (TER- 119). For progenitors and lineage cells, cells were stained with antibodies as previously described (Qian, P. et al. Cell Stem Cell, 18:214-228, 2016, doi: 10.1016/j. stem.2015.11.001 ). 7-aminoactinomycin D (7-AAD) (A1310, Life technologies) was used to exclude dead cells. Human cord blood samples were acquired from the St. Louis Cord Blood Bank. Mononuclear cells were isolated with Lymphoprep™ (StemCell technologies), followed by isolation of human CD34 ⁺ cord blood cells by human CD34 MicroBead Kit UltraPure (Miltenyi Biotec). To quantify human HSPCs, cells were stained with antibodies against CD34 (581 ), CD38 (HIT2), CD45RA (HI100), CD90 (5E10), CD49f (GoH3), EPCR/CD201 (RCR-401 ). Cell sorting and analyses were performed on MoFlo (Dako), InFlux Cell Sorter (BD Biosciences), and/or MACSQuant (Miltenyi Biotec). Data analysis was performed using FlowJo software.

[00229] Homing assay. In vivo homing assays were performed as previously described (He et al. Methods in Molecular Biology, 1185: 279-284, 2014). Basically, whole bone marrow (WBM) cells from CD45.2 mice were labelled with 5 μΜ 5-(and - 6)-carboxyfluorescein diacetate succinimidyl ester (CFDA SE) (Molecular Probes) at 37 C for 10 mins, washed three times, and 1 ^χ 10 ⁶ WBM were transplanted into lethally irradiated ptprc mice. After 18 hours, femurs and tibias were flushed, and CFDA SE+ cells were determined.

[00230] Competitive reconstitution assay. Competitive reconstitution assays were performed by intravenous transplantation of 2 *10 ⁵ , 7.5 *10 ⁴ or 2.5 *10 ⁴ donor- derived WBM cells from wt or Ythdf2 KO mice (CD45.2), together with 2x10 ⁵ rescue cells (CD45.1 ) into groups of ten lethally irradiated (10 Gy) ptprc recipient mice. For secondary transplantation, primary transplant recipients were sacrificed. BM cells were dissected from femur and tibia, and then transplanted mouse-to-mouse at a dosage of 1 ^χ 10 ⁶ cells into irradiated secondary recipient mice. Baytril water was given to recipient mice three days before irradiation and continued for another two weeks after irradiation. Primary and secondary CRU frequencies were measured using ELDA software (Hu et al. Journal of Immunological Methods, 347: 70-78, 2009), in which successful engraftment was defined as the presence of a distinct CD45.2 ⁺ CD45.1 ^" population >5% and >1 % of total hematopoietic cells in peripheral blood, respectively (Purton et al. Cell Stem Cell, 1 : 263-270, 2007). Also, the secondary transplantation recipient mice that died before 16 weeks post transplantation were counted for failed engraftment.

[00231] Cell cycle and apoptosis assays. Cell cycle analysis was performed with FITC mouse anti-human Ki67 set (BD Pharmingen) according to the manufacturer's instructions. Briefly, 5x 10 ⁶ BM cells were isolated and stained with HSC antibodies as described above. Cells were fixed by 4% paraformaldehyde at 4 ^° C overnight or room temperature (RT) for 1 hour, and then permeabilized with 0.2% triton X-100 on ice for 1 5 mins. Cells were washed with PBS containing 2% FBS, and then were incubated with Ki-67 antibody at RT for 1 hour in the dark, and SYTOX Red (Invitrogen) at RT for another 5 mins, followed by flow cytometric analysis with InFlux Cell Sorter (BD Biosciences). For apoptosis analysis, Annexin V (Invitrogen) and SYTOX Red staining of 5x 10 ⁶ BM cells was performed according to the manufacturer's protocol.

[00232] m ⁶ A RNA-IP-seq. Two replicates of 10 ⁵ LT-HSC, ST-HSC (LSK CD34 ⁺ FLK2 ^" ) and MPP (LSK CD34 ⁺ FLK2 ⁺ ) from C57BL/6J mouse were sorted into TRIzol (Invitrogen), and total RNA was isolated according to the manufacturer's instructions. RNA was fragmented to ~100 nucleotide fragments with Ambion fragmentation reagent (2 mins incubation at 70 C). The samples were then subjected to Turbo DNase treatment (Ambion), followed by a phenol-chloroform extraction, and resuspension in 85μΙ of nuclease-free water, and 5μΙ was saved as input. Then, the remaining 80μΙ RNA fragments were diluted into IPP buffer (150 mM NaCI, 0.1 % NP- 40, 10 mM Tris-HCI, pH 7.5). RNA was incubated with 25μΙ of protein-G magnetic beads, previously bound to 3 g of anti-m ⁶ A plyclonal antibody (Synaptic Systems), for 3 hours at 4 ^° C in IPP buffer. Beads were washed twice with 200μΙ IPP buffer, twice with 200μΙ low-salt buffer (50 mM NaCI, 0.1 % NP-40, 10 mM Tris-HCI, pH 7.5) and twice with 200μΙ high-salt buffer (500 mM NaCI, 0.1 % NP-40, 10 mM Tris-HCI, pH 7.5). Beads were then treated with 300μΙ Elution Buffer (5 mM Tris-HCL pH 7.5, 1 mM EDTA pH 8.0, 0.05% SDS, 4.2μΙ Proteinase K (20 mg/ml)) for 1 .5 hours at 50 ^° C, and RNA was recovered with phenol:chloroform extraction followed by ethanol precipitation. Three human CD34 ⁺ umbilical cord blood cells were isolated as described above and isolated total RNA with TRIzol. RNA was fragmented to ~100 nucleotide fragments with Ambion fragmentation reagent (2 mins 50 sees incubation at 70 C). The samples were then subjected to Turbo DNase treatment (Ambion), followed by a phenol:chloroform extraction, and resuspension in 18μΙ of nuclease-free water, and 1 μΙ was saved as input. m ⁶ A RNA IP was performed with EpiMark® N6- Methyladenosine Enrichment Kit following manufacturer's instructions.

[00233] Following m ⁶ A preparation of RNA, quality was assessed on Agilent 2100 Bioanalyzer, and 1 ng (mouse) or 10 ng (human) RNA was used to generate RNAseq libraries according to the manufacturer's directions for the SMARTer Stranded Total RNA-Seq Kit - Pico Input Mammalian (Takara Bio Inc) using 16 cycles (mouse) or 13 cycles (human) PCR2 amplification. The method uses random priming and a template switching oligo to generate complimentary DNA, followed by the ligation of barcoded adapters; ribosomal-derived cDNA is then removed through probe-directed enzyme cleavage and subsequent enrichment of un-cleaved fragments.

Ill [00234] The protocol was modified to retain lower molecular weight sample fragments by using a 1 .2x SPRI bead concentration for PCR1 cleanup. To remove dimerized adapters, libraries underwent 160-600 bp size selection with a Pippin Prep (Sage Science) 2% gel. The resulting libraries were checked for quality and quantity using the Bioanalyzer and Qubit Fluorometer (Life Technologies). Then equal molar libraries were pooled and requantified. For mouse m ⁶ A-seq, libraries were sequenced as 50 bp single read on the lllumina HiSeq 2500 instrument using HiSeq Control Software 2.2.58. Following sequencing, lllumina Primary Analysis version RTA 1 .18.64 and Secondary Analysis version bcl2fastq2 v2.18 were run to demultiplex reads for all libraries and generate FASTQ files. For human m ⁶ A-seq, libraries were sequenced as 75 bp single read on the lllumina NextSeq instrument using NextSeq Control Software 2.1 .2. Following sequencing, lllumina Primary Analysis version NextSeq RTA 2.4.11 and Secondary Analysis version bcl2fastq2 v2.18 were run to demultiplex reads for all libraries and generate FASTQ files.

[00235] Plasmid construction and stable cell line generation. Mouse Ythdf2 (mYthdf2) was cloned from commercial cDNA clone (ORIGENE # MC200730) into vector pcDNA5/FRT/Flag plasmid using primers listed: mYthdf2 ORF Clone Bamhl F: 5'-CGC GGA TCC TCG GCC AGC AGC CTC TTG GA-3' and mYthdf2 ORF Clone Notl R: 5'-ATA AGA ATG CGG CCG CCT ATT TCC CAC GAC CTT GAC GT-3'. Then Flag-mYthdf2 was subcloned under EF1 a promoter in pSicoR-EF1 a-IRES-EGFP lentiviral construct (Gibson Assembly ^® , forward primer: 5'- GTC GAC GGT ACC GCG GGC CCA TGG ATT ACA AGG ATG ACG ACG-3' and reverse primer: 5'-GAG GGA GAG GGG CGG ATC CCC TAT TTC CCA CGA CCT TGA CGT-3'). Human Ythdf2 (hYthdf2) was cloned from plasmid provided by the Chuan He lab using primers indicated: Forward 5'-CGT TCG AAA TGT CGG CCA GCA GCC TCT-3'; Reverse 5'-TCC CCC GGG TTA TTT CCC ACG ACC TT-3'. Then hYthdf2 was cloned into pSicoR-EF1 a-IRES-EGFP constructs under EF1 a promoter by BstBI and Xmal restriction digestions and ligation. To generate Flag- mYthdf2 HPC7 stable cell line, lentiviruses were generated by transfection of pSicoR-EF1 a-Flag-mYthdf2-IRES-EGFP constructs together with the psPAX2 and pMD2.G plasmids at a ratio of 10:7.5:2.5 into 293T cells using calcium phosphate transfection. The virus particles were harvested 48, 72, and 96 hours post transfection, filtered by 0.45 micrometers filter unit (Millipore), and then centrifuged at 18,000 RPM, 4 C for 2 hours. HPC7 cells were infected with recombinant lentivirus- transducing units in the presence of 4pg/ml_ polybrene (Sigma). 48 hours after infection, GFP ⁺ cells were sorted and cultured for experiments.

[00236] irCLIP-seq and data analysis. For irCLIP-seq, the procedure was modified from the previously reported methods (Zarneger et al. Nature Methods, 13: 489-492, 2016); Simsek et al. Cell, 169: 1051 -1065 e1018, 2017). In brief, irCLIP was performed on -3*10 ⁸ Flag-Ythdf2 HPC7 cells by UV crosslinking cells at 0.4J/cm ² for 3 times. Whole-cell lysates were generated in lysis buffer (150mM KCI, 10mM HEPES pH 7.6, 2mM EDTA, 0.5% NP-40, 0.5mM DTT, 1 : 100 protease inhibitor cocktail, 400U/ml RNase inhibitor; 1 ml cell pellet and 2ml lysis buffer). Pipetted up and down several times, and then the mRNP lysate was incubated on ice for 5 mins and shock-frozen at -80 C with liquid nitrogen. The mRNP lysate was thawed on ice and centrifuged at 15,000g for 15 mins to clear the lysate. Flag- Ythdf2 was isolated with 30μΙ of protein-G magnetic beads per 1 ml lysate, previously bound to 2pg of anti-Flag monoclonal antibody (Sigma) for 2 hours at 4 ^° C on rotation. The beads were collected, washed eight times with 1 ml ice-cold NT2 buffer (200mM NaCI, 50mM HEPES pH 7.6, 2mM EDTA, 0.05% NP-40, 0.5mM DTT, 200U/ml RNase inhibitor) and one time with 200μΙ irCLIP NT2 buffer (50mM Tris, pH 7.5; 150mM NaCI; 1 mM MgCI ₂ ; 0.0005% NP-40). mRNP complex was digested with RNase 1 (Thermo Fisher #AM2294) at 0.41Ι/μΙ in irCLIP NT2 buffer (aqueous volume of 30μΙ and supplemented with 6 μΙ of PEG400 (16.7% final)). The nuclease reaction was incubated at 30 C for 15 mins in an Eppendorf Thermomixer, 15 s 1 ,400 r.p.m., 90 s rest. Nuclease digestions were stopped by addition of 0.5mL of ice-cold high- stringency buffer (20mM Tris, pH 7.5; 120mM NaCI; 25mM KCI; 5mM EDTA; 1 % Trition-X100; 1 % Na-deoxycholate). Immunoprecipitates were then quickly rinsed with 0.25m L then with 0.05mL of ice-cold irCLIP NT2 buffer. The irCLIP adaptor ligation and library construction followed previously reported protocol (Zarneger et al. Nature Methods, 13: 489-492, 2016).

[00237] Data were demultiplexed using FAST-iCLIP version 0.9.3 and aligned to mouse genome mm 10 from UCSC using STAR (2.4.2a) with parameters "- outFilterScoreMinOverLread 0 -outFilterMatchNminOverLread 0 outFilterMatchNmin 0". RPM-normalized genome browser tracks were created in R (3.4.1 ) and plotted using the Gviz package (1 .20.0). Enriched motifs were identified by taking midpoints of each binding site found in all three replicates, adding 20 bases up and downstream, and running MEME (4.11.1 ) with parameters "-dna -mod zoops -revcomp -minw 5 -maxw 10 -nmotifs 10 -maxsize 1000000". After motifs were identified, we ran tomtom (4.11.1 ) against transfac (1 -2017) to identify known binding sites. GO enrichment analysis was performed using a hypergeometric test in R. GO terms were considered enriched if they had a BH-adjusted p-value less than 0.05. Selected terms of interest are shown in the bar plot. Bars in the bar plot indicate percentage of genes in the list being tested having the term divided by the percentage of genes in the genome having the term. Peaks found by FAST-iCLIP in all three replicates were assigned to various features in the genome. Promoters were defined as upstream 150 bases from the TSS. "trans_stop" was defined as upstream and downstream 200 bases from the transcript start site.

[00238] Cord blood transduction. Cord blood transduction was conducted as described previously (Rentas, S. et al. Nature, 532:508-511, 2016, doi: 10.1038/nature17665). Briefly, fresh CD34 ⁺ cord blood cells or flow-sorted CD34 ⁺ CD38 ^" cells were prestimulated for 12-18h in StemSpan medium (StemCell Technologies) supplemented with growth factors interleukin 6(IL-6; 20ng/ml, Peprotech), stem cell factor (SCF; l OOng/ml, Peprotech), Flt3 ligand (FLT3-L; 100ng/ml, Peprotech) and thrombopoietin (TPO; 20ng/ml, Peprotech). Lentiviruses were then added in the same medium at a multiplicity of infection (MOI) of 50-200 for 24 hours. Cells were then given 2 days after transduction before in vitro or in vivo assays. Human YTHDF2 was targeted for knockdown by shRNA targeting 5'- AAG G AC GTTC C C AATAG C C AA-3' near the N terminus of CDS, as used in a previous report (Wang, X. et al. Nature 505:117-120, 2014, doi: 10.1038/nature12730). Scramble shRNA (seed sequence 5'- G C G C G ATAG C G CTAATAAT-3' ) were used as control.

[00239] Clonogenic progenitor assays. Flow-sorted GFP ⁺ cord blood cells from day 10 cultured transduced cells (12,000 per ml) were resuspended in semisolid methylcellulose medium (Methocult H4034; StemCell Technologies). Colony counts were carried out after 14 days of incubation.

[00240] Human umbilical cord blood HSPC culture. 2 days after transduction, human cord blood CD34 ⁺ or CD34 ⁺ CD38 ^" cells were collected and the GFP ⁺ percentage was determined by flow cytometry. To ensure that equal numbers of GFP ⁺ cells were cultured before expansion, identically cultured GFP ^" cells were added to the one with higher GFP ⁺ percentage to match the % GFP ⁺ between control and hYthdf2 KD. Then cells were seeded at a density of 10 ⁵ per ml in StemSpan medium (StemCell Technologies) supplemented with growth factors IL-6 (20ng/ml), SCF (l OOng/ml), FLT3-L (l OOng/ml), TPO (20ng/ml) and CHIR99021 (250 nM) (Stemgent) (Perry et al. Genes and Development, 25: 1928-1942, 2011 ).

[00241] Human HSC xenotransplantation. For human cord blood HSC ex vivo expansion analysis, 10 ⁵ sorted CD34 ⁺ CD38 ^" cells were transduced with human YTHDF2 shRNA or control shRNA for 3 days and then analyzed for transduction efficiency (%GFP ^"/+ ) and stem cell markers. On day 10, cultured cells were collected for stem cell marker analysis. For hUBC HSC primary LDA assay, CD34 ⁺ cells were enriched as described above and transduced with human YTHDF2 shRNA or control shRNA at 50 MOI. Media were changed at 24 hours post infection. Equal number of GFP ⁺ cells were sorted out from control or YTHDF2 KD cells on 3 days post infection and cultured overnight. Three doses, 50K, 20K and 10K, of sorted GFP ⁺ cells were transplanted into sublethally irradiated (3.25Gy) NSG mice, respectively. The cut-off for HSC engraftment was an exhibition of more than 1 % human CD45 ⁺ GFP ⁺ cells out of total CD45 ⁺ cells in BM of primary transplantation recipients. For hUCB HSC secondary LDA assay, BM cells from highest two doses primary recipients were collected and mixed together at 10 weeks post transplantation. Three doses, 1.2 χ 10 ⁷ , 8 x 10 ⁶ , 4 x 10 ⁶ , of BM cells were transplanted into sublethally irradiated (3.25Gy) NSG mice, respectively. The cut-off for HSC engraftment was an exhibition of more than 0.2% human CD45 ⁺ GFP ⁺ cells out of total CD45 ⁺ cells in BM of secondary transplantation recipients. HSC frequency was assessed using ELDA software (Hu et al. Journal of Immunological Methods, 347: 70-78, 2009). For all human cord blood xenotransplantation experiments, female NSG mice aged 6-8 weeks were used.

[00242] m ⁶ A-seq data analysis. Human and mouse m ⁶ A-seq data were aligned to the transcriptome of hg19 and mm 10. In order to identify m ⁶ A peaks, hg19 and mm 10 transcriptome was divided into 25 nucleotide-wide tiles. The number of reads in the m ⁶ A IP and non-IP (control) sample was counted in each tile, and p value was calculated with Fisher exact test and adjusted for multiple testing. Tiles with significant m ⁶ A signal enrichment (adjust-Pval <= 0.05) were merged into bigger regions. Regions smaller than 100 bp were discarded, and regions over 200 bp were divided into 100 to 200 bp sub-regions; m ⁶ A signal over control was calculated at each region; and regions with at least 2-fold enrichment in all replicates were identified as m ⁶ A peaks. m ⁶ A peaks distribution and m ⁶ A marked genes were determined by overlapping all m ⁶ A peaks with hg19 and mm10 RefGene annotation. m ⁶ A marked genes were identified by overlapping m ⁶ A peaks with hg19 RefGene. To filter for transcription factors, genes marked by m ⁶ A in all three samples were compared against human transcription factor database http://fantom.gsc.riken.jP/5/sstar/Browse_Transcription_Fact ors_hg19. GO term analysis was then performed using R package enrich GO. m ⁶ A marked human transcription factors were used as searching list, and all the expressed genes were used as background. Hemopoiesis related BP terms with significant enrichment were used to generate Figure 3C.

[00243] RNA-seq. Human cord blood CD34 ⁺ cells were transduced with control or human YTHDF2 KD lentivirus and sorted out for GFP ⁺ CD34 ⁺ 10 days later. Three replicates of 12,000 GFP ⁺ CD34 ⁺ cells were sorted for each group and were used to extract total RNA. Four nanograms of high quality total RNA was used for cDNA synthesis and library preparation according to the manufacturer's directions with the SMART-Seq v4 Ultra Low Input RNA Kit for Sequencing (Takara, 634891 ) and Nextera XT (lllumina, FC-131 -1096). Resulting short fragment libraries were checked for quality and quantity using an Agilent 2100 Bioanalyzer and Invitrogen Qubit Fluorometer. Equal molar libraries were pooled, requantified, and sequenced as 75 base pair single reads on a High Output flow cell on the lllumina NextSeq 500 instrument. Following sequencing, lllumina Primary Analysis version NextSeq RTA 2.4.11 and Secondary Analysis version bcl2fastq2 2.18 were run to demultiplex reads for all libraries and generate FASTQ files.

[00244] For RNA-seq analysis, reads were aligned to UCSC genome hg38 with Tophat version 2.0.13 with default parameters, using Ensembl 87 gene models. Read counts were generated using HTSeq-count with -m intersection-nonempty. Reads were also aligned to ERCC control sequences and counts tabulated. A scaling factor was calculated based on the median of the ERCC counts for each sample and used for normalization. Differentially expressed genes were found using the edgeR package (3.18.1 ) in R (3.4.1 ). Differentially expressed genes were required to have a BH-adjusted p-value < .05 and a 2-fold change in expression.

[00245] RNA stability assay. 15,000 sorted LT-, ST-HSCs and MPPs were cultured in StemSpan SFEM medium (Stem Cell Technologies) supplemented with 10pg/mL heparin (Sigma), 0.5* penicillin/streptomycin (Sigma), 10ng/ml_ recombinant mouse (rm) SCF (Biovision, Inc.), and 20ng/ml_ Tpo (Cell Sciences, Inc.) (Perry et al. Genes and Development, 25: 1928-1942 (2011)) at 37°C 5% CO2 5% O2. Sorted cells were treated with 5μΜ actinomycin D (Sigma) for inhibition of mRNA transcription. Cells were harvested at 0 hour or 4 hours post treatment, and total RNA was extracted and used for RNA-seq. [00246] m ⁶ A RNA methylation quantification

[00247] Mouse BM Lineage negative cells from wt and Ythdf2 KO mice were enriched with mouse Lineage Cell Depletion Kit (Miltenyi Biotec), followed by total RNA extraction with TRIzol (Invitrogen). The quantification of m ⁶ A RNA methylation in Lin ^" cells were performed with m ⁶ A RNA Methylation Quantification Kit (Abeam ab185912) following manufacturer's protocol. 200ng total RNA were used per replicates for either group.

[00248] qPCR analysis.

[00249] 10 ⁵ LSK cells were sorted from wt and Ythdf2 KO mice. Total RNA were extracted with TRIzol (Invitrogen). cDNA synthesis was conducted with High- Capacity RNA-to-cDNA™ Kit (Thermo) following manufacturer's protocol. qPCR primers used are listed in Table S5, qPCR primers used to verify the expressional levels of transcription factors in wt and Ythdf2 KO HSPCs.

[00250] Western blot and intracellular staining. To validate the KO or KD efficiency in Ythdf2 KO mouse model or hUCB, 33,000 cKit ⁺ cells or 120,000 GFP ⁺ cells were sorted from BM or transfected hUCB samples, respectively. Hela cells transduced to overexpress human YTHDF2 were used validate overexpression efficiency as shown in Fig. 14B. Immunoblotting was performed with anti-YTHDF2 rabbit polyclonal antibody (MBL, RN123PW) and β-actin mouse monoclonal antibody (NOVUS, NB600-501 ). Secondary antibodies used were IRDye 800CW Goat anti- Mouse IgG and IRDye 800CW Goat anti-Rabbit IgG antibodies (LI-COR). For intracellular staining, BM cells from wt and Ythdf2 KO mice were stained with HSC markers as above, then fixed with the Cytofix/Cytoperm kit (BD Biosciences) according to the manufacturer's instructions. Fixed and permeabilized cells were immunostained with anti YTHDF2 antibody (MBL RN123PW), anti TAL1 antibody (Santacruz sc-393287), anti GATA2 antibody (Santacruz sc-267), anti RUNX1 antibody (Santacruz sc-365644), anti STAT5 antibody(Santacruz sc-74442) and detected by Alexa-488 donkey anti-rabbit IgG antibody (Invitrogen).

[00251] Single cell immunostaining. 10,000 LSKs from wt and Ythdf2 KO mice were sorted onto Poly-L-lysine coating slides, which were placed in a moisture chamber and incubated at 4°C for 30 mins to allow cells settling onto the slides. Cells were fixed with chilled methanol at RT for 10 mins, blocked with universal blocking reagent (BioGenex) at RT for 30 mins, and stained with mouse TAL1 antibody (Santa Cruz, SC393287) or mouse IgG control (Abeam) at 4 ^° C overnight. Cells were then stained with Alexa Fluor 488 donkey anti-mouse IgG (Thermo Fisher Scientific) at 4°C for 30 mins. Images were taken on a PerkinElmer Ultraview spinning disk system with Yokagawa CS-X1 disk. All emission was collected onto a C9100-23 Hamamatsu EM-CCD using Velocity software (PerkinElmer). For Z-stacks, the step size was set at 400nm. Staining intensity per image was quantified by ImageJ program.

[00252] FISH in conjugation with fluorescent immunostaining. Sorted LSKs were spun onto microscope glass slide (Fisher Scientific Cat. No. 12-544-4) using a Cytospin™ 4 Cytocentrifuge at 800 rpm for 1 min with medium acceleration (Thermo Scientific, cat. no. A78300003), followed by an immediate immersion into 4% PFA (diluted from 16% (wt/vol) aqueous solution, Electron Microscopy Sciences, cat. no. 15710). Cells were fixed at RT (25 ± 2 °C) for 30 mins. RNA in situ hybridization was performed using RNAscope multiplex fluorescent detection kit according to the manufacturer's instructions (Advanced Cell Diagnostics) with a couple of modifications: Antigen retrieval was unnecessary, and digestion was performed with 1 :15 diluted proteinase III solution for 10 mins at RT. RNAscope probes targeting mouse Tall and Gata2 were designed and produced by ACDbio. After the in situ hybridization was completed, slides were rinsed twice with PBST and directly processed with background blocking (Background buster solution, Innovex, cat. no. NB306) and primary antibody incubation. Anti-YTHDF2 (MBL, 1 : 500) and anti-Dcpl a (Santa Cruz, SC100706, 1 :200) antibodies were diluted with antibody diluent reagent buffer (Life technologies, cat. no. 003118) and incubated at 4°C overnight. Donkey anti- rabbit Alexa Fluor 488 (Invitrogen, 1 :500) and donkey anti- mouse Alexa Fluor 633 (Invitrogen, 1 :500) were used for protein target multiplexing.

EXAMPLE A-1

[00253] Ythdf2 KO leads to increase in phenotypic HSCs in primary mice. [00254] To investigate the effects of Ythdf2 on phenotypic HSCs, Crispr-Cas9 technology was utilized to generate Ythdf2 ^f/f conditional knockout mice, and then crossed with Mx1-Cre mice to specifically reduce Ythdf2 expression in hematopoietic cells (hereafter Ythdf2 KO mice) (Fig. 1A and B). BM HSPCs showed no discernible difference at the absence of pl:pC (Figure 7A). Four weeks after pl:pC injections, a significant increase was observed in both frequency and absolute number of long- term HSCs (Lin Sca1 ⁺ cKit ⁺ (LSK) CD34 ^" Flk2 ^" ; LT-HSCs) and short-term HSCs (LSK CD34 ⁺ Flk2 ^" ; ST-HSCs), but not multipotent progenitors (LSK CD34 ⁺ Flk2 ⁺ ; MPPs) in Ythdf2 KO mice compared to littermate wild type (wt) mice (Fig. 1 C to 1 E). It was found that the frequency and absolute cell number of Long Term HSCs (LT- HSCs) and ST-HSCs increased by over 2-fold while MPP exhibited milder response (Figure 1 C and 1 E). Although Ythdf2 KO led to increased BM cellularity, the absolute number of committed progenitors, including common myeloid progenitors (CMPs), granulocyte-macrophage progenitors (GMPs), megakaryocyte-erythrocyte progenitors (MEPs) and common lymphoid progenitors (CLPs), as well as mature lineage cells, erythrocytes, myeloid cells, B cells and T cells, showed no significant difference between Ythdf2 KO and wt mice (Fig. 1 F to H). Cell cycle analysis revealed no discernible change of quiescence in HSCs or MPPs after Ythdf2 KO (Fig. 7B). Notably, the percentage of apoptotic cells in Ythdf2 KO LT-, ST-HSCs and MPPs significantly reduced compared to wt controls (Fig. 7C). To further identify any potential HSC defects in Ythdf2 KO mice, the number of HSCs, committed progenitors, and mature lineages in the spleen were examined, and no significant differences were found between wt and Ythdf2 KO mice (Figs. 7D to 7H). In summary, Ythdf2 KO in primary mice specifically increases HSC numbers with no bias or defects in either progenitor or lineage cells.

EXAMPLE A-2

[00255] Ythd†2 KO expands functional HSCs in mice.

[00256] To determine whether Ythdf2 KO expands functional HSCs, a short- term homing assay was initially perfomed by transplanting 1 ^χ 10 ⁶ carboxyfluorescein diacetate succinimidyl ester (CFDA SE)-labelled BM cells from KO mice or their control littermates into lethally irradiated recipient mice, and no significant difference was found in their homing capacity between mutant and wt controls (Fig. 7I). Limited dilution, competitive repopulation unit assay (LDA) was then executed by transplanting 2 ^χ 10 ⁵ , 7.5 ^χ 10 ⁴ or 2.5 ^χ 10 ⁴ donor BM cells (CD45.2), together with 2 x 10 ⁵ recipient BM cells derived from the ptprc mutant strain (CD45.1 ), into lethally irradiated recipient mice (Fig. 2A). Consistent with an increased number of phenotypic HSCs in Ythdf2 KO mice, it was found that competitive repopulating units (CRUs) increased 2.2-fold in Ythdf2 KO HSCs compared to controls (Fig. 2B). In the 2 x 10 ⁵ group, compared to controls, and a significant increase was observed in the overall repopulation rate from Ythdf2 KO donor cells at 16 weeks post transplantation (Fig. 2C). Moreover, recipients of Ythdf2 KO BM cells, compared to that of controls, exhibited markedly higher frequency and absolute number of donor derived LT-HSCs and ST-HSCs, but not MPPs in BM (Fig. 2D and 2E). Furthermore, it was found that donor derived committed progenitors and mature lineages in BM from transplantation recipients of mutant and wt cells showed no significant changes (Fig. 2F and 2G). To determine the long-term repopulation ability of HSCs from Ythdf2 KO mice, the secondary transplantation with BM cells derived from primary recipients was conducted. Notably, it was found that compared to controls, CRUs from Ythdf2 KO cells revealed a 3.5-fold increase (Fig. 2H) and exhibited no signs of leukemia in both BM and spleen at 16 weeks after secondary transplantation (Figs. 9A to 9F). Furthermore, in Figs. 2I and 2J, a limiting dilution assay was conducted by transplanting total bone marrow (BM) from WT or Ythdf2 KO mice with 3 different dosages. Four weeks post transplantation, it was observed that the engraftment of donor cells increased in the mice transplanted with 200,000 Ythdf2 KO BM cells, as compared to that with WT BM cells. Furthermore, this increase did not generate lineage bias in the transplantation recipients.

[00257] We also investigated the long-term effect of Ythdf2 KO on hematopoiesis under homeostasis condition by examining the stem cells, progenitor cells, and lineages in both BM and spleen at over 5 months post pl:pC injections (Fig. 9A). Although we observed a modest increase in LT-HSCs in BM from Ythdf2 KO mice compared to that of controls (Fig. 9C), there were no discernible differences between Ythdf2 KO and control mice in progenitors and lineage cells from either BM or spleen (Figs. 9D to 9J). These observations indicate that long-term effect of Ythdf2 KO in vivo neither skews lineage differentiation nor facilitates aberrant proliferation, which is in line with previous reports that Ythdf2 is not required for leukemogenesis. To verify the frequency of functional HSCs in the BM at 5 moths post pl:pC induction, we transplanted 7.5 ^χ 10 ⁴ BM cells from wt and Ythdf2 KO mice with competent cells into lethally irradiated recipients. We found that Ythdf2 KO led to significantly higher engraftment in recipients comparing to wt controls, suggesting that Ythdf2 KO has long-term capability on mouse HSC expansion in vivo (Fig. 9K). Taken together, these data reveal that Ythdf2 KO results in specific and significant mouse HSC expansion in vivo without affecting lineage commitment.

EXAMPLE A-3

[00258] Ythdf2 regulates HSC self-renewal gene expression by m ⁶ A- mediated mRNA decay.

[00259] To explore the underlying mechanisms of how Ythdf2 KO expands HSCs, mapping was performed of the m ⁶ A methylome by methylated RNA immunoprecipitation combined with high-throughput sequencing (MeRIP-seq or m ⁶ A- seq) in LT-HSCs, ST-HSCs, and MPPs sorted from adult C57BL/6J mice (Meyer et al. Cell, 149: 1635-1646, 2012; Schwartz et al. Cell, 155: 1409-1421 , 2013; Dominissini et al. Nature, 485: 201 -206, 2012). m ⁶ A peaks were selected by identifying significantly enriched overlapping peaks from two independent replicates. Consistent with previous studies (Meyer et al. Cell, 149: 1635-1646, 2012); Dominissini et al. Nature, 485: 201 -206, 2012), it was found that m ⁶ A peaks were abundant in mRNA open reading frame (ORF), in 3' untranslated regions (UTRs), and around the stop codon in all three HSPC populations. Transcripts of moderately expressed genes were more likely to be methylated (Figs. 10A to 10C). Intriguingly, it was found that m ⁶ A modifications were enriched in the mRNAs of transcription factors, such as Gata2, Etv6, Stat5 and Tall, which have been documented to be critical for HSC self-renewal and stem cell state maintenance (Wang et al. Blood, 113: 4856-5865, 2009; Ebina et al. The EMBO Journal, 34: 694-709, 2015; Orkin et al. Cell, 132: 631 -644, 2008: de Pater et al. The Journal of Experimental Medicine, 210: 2843-2850, 2013; Hock et al. Genes and Development, 18: 2336-2341 , 2004; Lim et al. The Journal of Clinical Investigation 122: 3705-3717, 2012; Reynaud et al. Blood, 106: 2318-2328, 2005; and Kato et al. The Journal of Experimental Medicine, 202: 169-179, 2005), suggesting the m ⁶ A modification could play critical roles in the regulation of HSCs (Table S1 , Key transcription factors critical for HSC self-renewal and maintenance are labeled by m ⁶ A in HSPCs). Given the accumulating evidence that m ⁶ A mRNA methylation regulates stem cell fate determination by facilitating the decay of mRNAs coding for transcription factors and genes in key signaling pathways involved in self-renewal and differentiation (Batista et al. Cell Stem Cell, 15: 707-719, 2014; (Geula et al. Science 347: 1001 -1006, 2015; Yoon et al. Cell, 2017; Zhang et al. Nature, 549: 273-276, 2017; Zhao et al. Nature, 542: 475-478, 2017; Li et al. Cancer Cell, 31 : 127-141 , 2017; Li et al. Nature, 548: 338-342, 2017) the mRNA degradation rates were next measured in LT-, ST-HSCs, and MPPs by monitoring mRNA levels after transcription inhibition with actinomycin D. It was found that degradation rates of methylated mRNAs were significantly faster than unmethylated mRNAs in ST-HSCs, and MPPs (Fig. 10D). As Ythdf2 is a well- recognized m ⁶ A "reader" that mediates mRNA decay (Wang et al. Nature, 505: 117- 120, 2014), the targets of Ythdf2 were further determined by performing infrared UV- crosslinking immunoprecipitation sequencing (irCLIP-seq) in the mouse multipotent hematopoietic precursor cell line HPC-7 (Pinto do et al. The EMBO Journal, 17: 5744-5756, 1998; Zarnegar et al. Nature Methods, 13: 489-492, 2016) (Fig. 3A; Figs. 11 A to 11 C). The results showed that 57.8% of Ythdf2 target mRNAs contained m ⁶ A peaks (Fig. 11 D). Ythdf2 binding sites were enriched with the conserved m ⁶ A motif and exhibited the characteristic of m ⁶ A distribution features (Fig. 3B and 3C). Gene ontology (GO) analysis of Ythdf2 target transcripts revealed enrichment of genes related to hematopoietic or lymphoid organ development, suggesting the involvement of Ythdf2 in the regulation of hematopoiesis (Fig. 3D). Notably, it was found that Ythdf2 bound to transcription factor mRNAs, such as that of Tall and Gata2, on sites largely overlapping with m ⁶ A peaks (Fig. 3E; Fig. 5E and Table S2, Ythdf2 targeted mRNAs from three irCLIP-seq replicates). Significant change was not observed in the total RNA mass in LSK Flk2 ^" cells from wt and Ythdf2 KO mice. Though m ⁶ A modification only constitute 0.1 -0.4% of adenosine nucleotide in mammal cells, it was found that Ythdf2 KO led to increased level of m ⁶ A content in total RNA from BM Lin ^" cells, suggesting that Ythdf2 specifically regulates the stability of m ⁶ A-marked mRNAs (Figs. 12A and 12B). Consistently, qPCR analysis of total mRNA revealed increased levels of Tall, Gata2, Runxl and Stat5a, whose mRNAs have shown to be modified by m ⁶ A, in Ythdf2 KO LSK cells compared to wt controls (Fig. 3F). Single cell immunofluorescence staining and intracellular flow cytometry further revealed that Ythdf2 KO HSPCs exhibited significant increases in the intensities of rri6A-labeled transcription factors involved in stem cell self-renewal, such as TAL1 , GATA2, RUNX1 and STAT5, indicative of a suppressive role of Ythdf2 in HSC self-renewal (Fig. 3G; Fig. 12C). A previous study has shown that Ythdf2 regulates RNA metabolism through localizing the bound mRNAs to mRNA decay sites (Wang et al. Nature, 505: 117-120, 2014). To further explore the mechanism of Ythdf2 in regulating HSC self-renewal, fluorescence in situ hybridization (FISH) was performed of Tall mRNA and fluorescence immunostaining of Ythdf2 and Dcpl a, a marker of mRNA decay sites (Sheth et al. Science, 30:805- 808, 2003; Kedersha et al. Methods in Enzymology, 431 : 61 -81 , 2007), and their relative spatial distribution was analyzed in sorted wt and Ythdf2 KO HSPCs. Co- localization of Tall mRNA, Dcpl a and Ythdf2 was observed in wt cells while substantially reduced in Ythdf2 KO controls (Figs. 3H and 3I). Furthermore, similar observation was confirmed by co-staining Gata2 mRNA FISH with Ythdf2 and Dcpl a in wt or Ythdf2 KO HSPCs (Figs 12D and 12E). To determine whether the increased transcription factors, such as Tall, expression accounts for the HSC expansion in Ythdf2 KO mice, rescue experiments were performed using short hairpin (sh) RNA- mediated Tall knock down (KD) in wt and Ythdf2 KO LSK cells, followed by transplanting into lethally irradiated recipients. Depletion of Tall in HSPCs significantly impaired the reconstitution capacity of wt cells as reported previously and also rescued the increased engraftment of Ythdf2 KO cells (Fig. 12F). Overall, these data indicate that Ythdf2 regulates HSC self-renewal by enabling the degradation of mRNAs encoding transcription factors essential for stem-cell renewal.

EXAMPLE A-4

[00260] Dissecting the role of Ythdf2 in human UCB HSPCs by m ⁶ A-seq and RNA-seq.

[00261] The limited number of HSCs in a single human umbilical cord blood unit has been an obstacle for clinical applications, such as HSC transplantation (Walasek et al. Annals of the New York Academy of Sciences, 1266: 138-150, 2012). The observation that Ythdf2 KO resulted in an increase of phenotypic and functional mouse HSCs prompted a test whether YTHDF2 knockdown (KD) could facilitate human UCB HSC expansion. First, m ⁶ A-seq with CD34 ⁺ cells isolated from 3 individual hUCB samples was performed (Fig. 13A). m ⁶ A modifications predominantly occurred in mRNAs (~95%), preferential in mRNA ORF regions, 3'UTRs, and near the stop codon, as expected (-90%) (Figs. 4A and 4B; Fig. 13B). m ⁶ A landscapes in mouse and hUCB HSPCs were compared, and it was found that 2,239 genes were commonly m ⁶ A tagged (Fig. 4C). These commonly m ⁶ A-tagged transcripts were enriched for genes related to hematopoiesis and stem cell maintenance (Fig. 4D). Due to the enrichment of m ⁶ A labeling in the mRNAs of transcription factors responsible for mouse HSC self-renewal, the m ⁶ A-marked transcription factor transcripts in hUCB CD34 ⁺ cells were next characterized by performing GO term analysis. Among the 722 identified m ⁶ A-labeled transcription factor mRNAs, major GO terms were related to cell fate commitment and stem cell maintenance (Fig. 13C). For example, HOXB4, overexpression of which has been reported to expand human and mouse HSCs ex vivo (Amsellem et al. Nature Medicine, 9: 1423-1427, 2003; Antonchuck et al. Cell, 109: 39-45, 2002), was marked by m ⁶ A in hUCB CD34 ⁺ cells (Fig. 4E). Other transcription factors required for HSC self-renewal and critical to induce HSCs from other cell types (Ebina et al. The EMBO Journal, 34: 694-709, 2015; Galan-Caridad et al. Cell, 129: 345-357, 2007, such as Zfx, RUNX1 and FOSB, were also m ⁶ A-tagged in hUCB CD34 ⁺ cells (Figs. 13F and 13G and see also Supplementary Table S3 of article "Supression of m6A Reader Ythdf2 Prmotd Hematopoeitic Stem Cell Expansion" by Li et al, Cell Research 28, 904-917 (2018), which article (and including the Supplementary Tables thereof) is hereby incorporated by reference herein in its entirety, Genes marked by m ⁶ A in human UCB CD34 ⁺ HSPCs from individual samples). To further dissect the role of YTHDF2 in hUCB HSPCs, RNA-seq was performed with control or YTHDF2 KD hUCB CD34 ⁺ cells (Fig. 4F; Figs. 13D and 13E). Remarkably, transcripts marked by m ⁶ A, including HOXB4 and other HSC self-renewal related transcription factors, showed significant increases of input mRNA reads in the YTHDF2 KD cells compared to the control, without noticeable changes for non-m ⁶ A labelled genes (Figs. 4G to 41; Figs. 13F and 13G). These results support the role of YTHDF2 in regulating hUCB HSC self-renewal through RNA degradation.

EXAMPLE A-5

[00262] Expansion of hUCB HSCs by YTHDF2 KD.

[00263] To further explore whether suppression of YTHDF2 can expand human HSCs, short hairpin (sh) RNA-induced YTHDF2 KD in hUCB HSPCs was performed as above (Fig. 4F). After 7 days ex vivo culture, lentiviral knockdown of YTHDF2 resulted in an average 14.3-fold and 13.6-fold increase, respectively, in the frequency and absolute number of CD34 ⁺ CD38 ^" CD45RA ^" EPCR ⁺ phenotypic HSCs and a 5.1 -fold increase in CFUs relative to control cells, especially the most primitive CFU-granulocyte erythrocyte monocyte megakaryocyte (GEMM) colony type and burst forming unit-erythroid (BFU-E), reflecting higher expression level of key transcription factors for hematopoiesis, such as TAL1, in YTHDF2 KD hUCB cells (Figs. 5A to 5D; Figure 14A). Interestingly, the apoptotic rate was significantly reduced in YTHDF2 KD hUCB HSPCs compared to control cells, similar to the trend of HSCs in Ythdf2 KO mouse (Fig. 5E). Also, Fig 5F shows that the Ythdf2 knockdown (KD) HSCs exhibited up to a 10-fold increase compared to control HSCs 10 days post transduction. Next, the effect of overexpression (OE) YTHDF2 on HSPC function was explored. Overexpression of YTHDF2 reduced clonogenic potential of hUCB HSPCs by 2.2 fold, suggesting YTHDF2 negatively regulates HSC maintenance ex vivo (Figs. 14B and 14C).

[00264] To determine whether YTHDF2 KD can expand human HSCs in vivo, LDA was performed by transplanting GFP ⁺ cells sorted from hUCB CD34 ⁺ cells infected with control and YTHDF2 shRNA at day 4 post infection (Fig. 6A). At 10 weeks post transplantation, we analyzed BM cells were analyzed from recipient NOD/SCID //2rg ^nu " (NSG) mice and measured the functional HSC frequency after in vivo expansion. Notably, compared to control group, recipients of YTHDF2 KD cells displayed a 9-fold increase in human hematopoietic cell (hCD45 ⁺ GFP ⁺ ) engraftment in BM without changes in the proportions of each lineage (Figs. 6B and 6CH; Figs. 15A and 15B). YTHDF2 KD significantly increased the percentage of myeloid, megakaryocyte and erythrocyte in the BM of primary recipients (Figs 15C). Accordingly, it was found that the HSC frequency in YTHDF2 KD cells was increased 4.4-fold relative to that in control cells (Fig. 6D). It was confirmed the long-term capability of YTHDF2 KD hUCB cells to be reconstituted and undergo self-renewal; 12 weeks after transplantation of BM from primary recipients into sublethally irradiated secondary NSG recipient mice, human hematopoietic cell chimerism in BM were higher in the YTHDF2 KD group, as compared to that in the control group (Figs. 6E, 6F; Fig. 15D). CRUs from YTHDF2 KD cells revealed an 8-fold increase relative to that in control cells (Fig. 6G). These data demonstrate that YTHDF2 KD remarkably facilitates the expansion of both phenotypic and functional hUCB HSCs ex vivo.

EXAMPLE A-6

[00265] The examples herein demonstrate that conditional deletion of Ythdf2, m6A reader, lead to expansion of phenotypic and functional HSCs without lineage bias. To investigate if there is concomitant increase in mesenchymal stem cells in vivo, Ytdhf2 ^f/f mice were crossed with Mx1 -Cre mice to conditionally delete Ythdf2 expression from mesenchymal cells. Nine-months post pl:pc injection bone marrow cells were isolated from Ythdf2 ^KO and their wild type littermates. Single-cell suspension of total bone marrow cells were immuno-stained with flow antibodies. Hematopoietic (CD45, Ter119) and endothelial cells (CD31 ) were excluded using specific markers. Mesenchymal stem cell present within the total bone marrow stromal cells were further purified by inclusion of N-Cadherin and CD105 antibodies. Mesenchymal stem cells marking both N-Cadherin ⁺ and CD105 ⁺ were quantitated. Conditional deletion of Ythdf2 led to 3.6-fold expansion in frequency of mesenchymal stem cells (Fig. 16). Our study clearly demonstrates that loss of Ythdf2 can expand bone marrow mesenchymal stem cells in vivo.

[00266] Discussion

[00267] Although recent studies explore the biological functions of mRNA m ⁶ A modifications (Zheng et al. Molecular Cell, 49: 18-29, 2013; Zhou et al. Nature, 526: 591 -594, 2015; Alarcon et al. Cell, 162: 1299-1308, 2015; Zhang et al. Cancer Cell, 31 : 591 -606 e596, 2017; Lence et al. Nature, 540: 242-247, 2016; Haussmann et al. Nature, 540: 301 -302, 2016; Chen et al. Cell Stem Cell, 16: 289-301 , 2015; Alarcon et al. Nature, 519: 482-485, 2015; Xiao et al. Molecular Cell, 61 : 507-519, 2016; Wojtas et al. Molecular Cell, 68: 374-387 e312, 2017; Ivanovna et al. Molecular Cell, 67: 1059-1067 e1054, 2017; Fustin et al. Cell, 155: 793-806, 2013; Slobodin et al. Cell, 169: 326-337 e312, 2017; Schwartz et al. Cell, 159: 148-162, 2014; Pendleton et al. Cell, 169: 824-835 e814, 2017; Shi et al. Cell Research, 27: 315-328, 2017; Huang et al. Nature Cell Biology, 20: 285-295, 2018; Bertero et al. Nature, 2018; Liu et al. Nature, 518: 560-564, 2015), embodiments herein identify Ythdf2 as an important regulator of human and mouse HSC self-renewal by coupling the post- transcriptional m ⁶ A modification to the degradation of mRNAs encoding key transcription factors for self-renewal. Repression of Ythdf2 in mouse HSPCs and hUCB HSCs can lead to increased expression of multiple key TFs critical for self- renewal, thereby facilitating ex vivo expansion of both phenotypic and functional HSCs without noticeable lineage bias and leukemia potential. In addition, stem cell niches, to some extent, may contribute to Ythdf2 suppression-mediated mouse HSC expansion as Mx1-cre can be activated in mesenchyme stromal cells. It would be intriguing to study the function of Ythdf2 on mesenchymal stem cells (MSCs) and how repression of Ythdf2 in both HSCs and MSCs may synergistically expand HSCs in vivo.

[00268] Given the broad and complicated impact of m ⁶ A writer complex Mettl3 and Mettl14 on mRNA splicing, translation, and pri-miRNA processing (Barbieri et al. Nature,, 2017; Alarcon et al. Nature, 519: 482-485, 2015; Liu et al. Nature, 518: 560- 564, 2015), Mettl3 or Mettl14 depletion results in distinct outcomes in normal stem cells and leukemia. Recent studies have demonstrated that Mettl3 and Mettl14 play essential roles in leukemia development and leukemia stem cell maintenance (Vu et al. Nature Medicine, 2017; Barbieri et al. Nature, 2017; Weng et al. Cell Stem Cell, 22: 191 -205 e199, 2018). In contrast, Ythdf2 is believed to be mainly involved in m ⁶ A-mediated mRNA decay (Batista et al. Cell Stem Cell, 15: 707-719, 2014; Yoon et al. Cell, 2017; Zhang et al. Nature, 549: 273-276, 2017; Wang et al. Nature, 505: 117-120, 2014). According to certain embodiments herein, it is believed that manipulating Ythdf2 may extend the half-life of specific m ⁶ A-marked mRNAs encoding TFs critical for stem cell self-renewal without affecting other aspects of mRNA processing. The Examples herein show that Ythdf2 depletion in HSCs neither skews the lineage commitment nor induces hematological malignancies, reducing the risk of leukemogenesis with expanded HSCs. Furthermore, stem cell self- renewal is a complexed process comprised of cell division, survival, prevention of differentiation and sternness retention. The observation that Yf/7c#2-deficient HSCs exhibited lower apoptotic rate indicates embodiments of methods herein also benefit another feature of stem cell self-renewal.

[00269] A major limitation in using hUCB HSC transplantation is the insufficient number of HSCs in one hUCB unit. Albeit previous studies have revealed that Dlk1 , SR1 , Musashi2 and UM171 can expand hUCB HSCs by targeting Notch, AHR signaling or other unknown pathway (Boitano et al. Science, 329: 1345-1348, 2010; Fares et al. Science, 345: 1509-1512, 2014; Rentas et al. Nature, 532: 508-511, 2016; Chou et al. Experimental Hematology!, 41 : 479-490 e474, 2013). Accordingly, embodiments herein provide a novel and potent way to target multiple key TFs critical for HSC self-renewal and to enhance the expansion of HSCs. For example, reducing Ythdf2 level and function during in vitro culture via small chemicals or AAV- mediated KD may allow the Ythdf2 level and function to restore after transplantation in vivo, and thus not affect normal HSC maintenance and function in human patients. Furthermore, in certain embodiments, methods described herein may be combined with other methods to facilitate the expansion of not only human HSCs, but also other stem cells, rendering an approach for stem cell-based therapies.

EXAMPLE B

[00270] The following protocols were used in the "B" Examples below.

[00271] Animals. C57BL/6-Gt(ROSA)26Sortm1 (HBEGF)Awai/J(iDTR), B6.Cg- Gt(ROSA)26Sortm14(CAG-tdTomato)Hze/J(R26RtdT), Tg(Cspg4- DsRed.T1 )1Akik/J, Cxcl12tm2.1 Sjm/J, Kitltm2.1 Sjm/J (SCF ^f/f ), Cxcl12tm 1 .1 Sjm/J (CXCL12 ^f/f ) mice were obtained from the Jackson Laboratory. N-cad-CreER ^T , and N- cad-TdT mice were generated by Applied StemCell, Inc. To induce N-cad-CreER ^T ; R26-tdT mouse, tamoxifen (Sigma) was injected intraperitoneally at 2mg per injection for 3 days. To induce N-cad-CreER ^T ; R26-tdT at embryonic stage, a single dose of 1 .5mg of TMX was injected intraperitoneally (IP) into the E12.5 pregnant dam. Cesarean section was performed at E19.5 and the neonatal mice were transferred to foster mice. To induce N-Cad ⁺ cells ablation in N-cad-CreER ^T ;iDTR mice, DT (Sigma) was injected intraperitoneally every other day at a dose of 50ng per g body as indicated. 5FU (Sigma-Aldrich) was injected once in the tail vein at 150pg per g body weight. After 5FU injection, mice were analyzed as described in the text. All mouse strains used in this study had a C57BL/6J genetic background. Animals were randomly included in the experiments according to genotyping results. Animal experiments were conducted in a blinded fashion with respect to the investigator. The numbers of animals used per experiment are stated in the figure legends. All mice used in this study were housed in the animal facility at the Stowers Institute for Medical Research (SIMR) and were handled according to SIMR and National Institutes of Health (NIH) guidelines. All procedures were approved by the Institutional Animal Care and Use Committee (IACUC) of the SIMR.

[00272] Flow cytometry. For phenotype analysis, hematopoietic cells were harvested from bone marrow (femur and tibia). Red blood cells were lysed using a 0.16 M ammonium chloride solution. For cell surface phenotyping, a lineage cocktail (Lin, phycoerythrin (PE)-Cy5) was used, including anti-CD3 (145-2C11), anti-CD4 (RM4-5), anti-CD8 (53-6.7), anti-Mac-1 (M1/70), anti-Gr1 (RB6-8C5), anti-B220 (RA3-6B2), anti-lgM (11/41 ) and anti-TER119 (TER-119) (100 ng antibody cocktail per million bone marrow cells, eBioscience). Monoclonal antibodies to SCA1 (D7, eBioscience), c-KIT (2B8, eBioscience), FLK2 (A2F10, eBioscience), CD34 (RAM34, eBioscience), CD48 (HM48-1 , eBioscience), CD150 (TC15-12F12.2, BioLegend) and CD49b (HMa2, Biolegend) (all used as 50 ng per million bone marrow cells) were also used where indicated. For lineage analysis of peripheral blood, monoclonal antibodies to CD45.1 (A20, eBioscience), CD45.2 (104, eBioscience), CD3, B220, Mac-1 and Gr1 were used. 7-aminoactinomycin D (7-AAD) (A1310, Life technologies) was used to exclude dead cells. For stromal niche cell analysis, CD45 (30-F11, eBioscience), CD31 (390, eBioscience), PDGFRa-biotin (APB5, eBioscience), LepR-Bio (R&D), CD51 (clone RM7-V, Biolegend). Samples stained with biotin conjugated antibodies were washed with staining medium, then incubated with streptavidin brilliant violet 421 TM (Biolegend, 1 :500). Cell sorting and analysis were performed using MoFlo (Dako), InFlux Cell Sorter (BD Biosciences), MACSQuant (Miltenyi Biotec) or CyAn ADP (Dako) instruments. Data analysis was performed using FlowJo software.

[00273] Whole-mount sternum HSC immunostaining. Sternal bones were collected and transected with a surgical blade into 3-4 fragments. The fragments were bisected sagittally to expose bone marrow cavity, fixed in 4% PFA. blocked/permeabilized in PBS containing 20% normal goat serum and 0.5% Triton X100, and stained with primary antibodies for 3 days. The tissues were incubated with secondary antibodies for 2 hours (Bruns et al., 2014; Kunisaki et al., 2013). Fluorescence imaging was performed on a spinning-disk confocal microscope (UltraVIEW; PerkinElmer), including an inverted microscope (Axiovert 200 M; Carl Zeiss Microimaging, Jena, Germany), attached to a spinning-disk confocal system (CSU-X1 ; Yokogawa Corporation of America) and Orca-R2 camera (Hamamatsu) with Volocity acquisition software (PerkinElmer) and a 20x/0.8 Plan-Apochromat objective (Carl Zeiss). Images were collected as a series of optical sections, with a step size of 4 μm. Images were collected in a tile pattern (overlap 10%) sufficient to cover the entire sample. Channels were collected sequentially. (Blue dye) was excited using 405 nm light (50 mW diode laser, OEM) and (red dye) was excited using 561 nm light (50 mW solid state laser, OEM), and each was collected using a multibandpass emission filter with 415nm-775nm 580nm-650nm bands. (Green dye) was excited using 488 nm light (50 mW solid state laser, OEM) and collected using a multibandpass emission filter of 500nm-550nm, and (far red dye) was excited using 640 nm light (50 mW solid state laser, OEM) and collected using a multibandpass emission filter with 455nm-515nm and 660nm-750nm. Exposure times and laser powers were adjusted to compensate for variations in staining.

[00274] Second Harmonic Generation (SHG) imaging was performed immediately following fluorescence imaging. SHG images were collected on a LSM- 780 laser scanning confocal microscope (Carl Zeiss) equipped with a QUASAR detection unit, a 10x 0.45 Plan-Apochromat objective (Carl Zeiss), and Zen 2012 acquisition software (v8.1 .3, Carl Zeiss). Images were collected as a series of optical sections, with a step size of 8 μm and a pixel size (1 .32 μm/pixel) an integer multiple of four times the fluorescence pixel size. SHG images were taken with a laser light wavelength of 900 nm and collected at 371 -420 nm. As a reference for image alignment, images of Cd150-PE were taken using the 561 nm line of a DPSS laser (Melles Griot) and collected at 566-735 nm concurrently with the SHG images. Images were collected in a tile pattern (no overlap) sufficient to cover the entire sample. Tiles were stitched into a complete 3D image using Zen software.

[00275] Images were analyzed using Fiji software (1 .51 g National Institutes of Health). To align SHG and fluorescence images, fluorescence images were first background subtracted, and image tiles were stitched into a complete 3D image using the Grid/Collection stitching plugin (reference http://bioinformatics.oxfordjournals.org/content/25/11/1463. abstract). Given that the transfer of the sample from the fluorescence microscope to the SHG microscope involved a small amount of sample rotation, it was necessary to realign the SHG and fluorescence images in 3 dimensions. Alignment of SHG and fluorescence image sets was carried out using a custom plugin available at http://research.stowers.org/imagejplugins. Firstly, a minimum of 8 common landmarks were identified by visual inspection of both the SHG and fluorescence data sets. Next the Kabsch algorithm was used to find the best scaled rotation to transform the fluorescence coordinates to the SHG image coordinates. Finally, each 3D voxel in the fluorescence image was transformed to the corresponding SHG position and trilinear interpolation was used the find the SHG intensity at that position to create the realigned composite image.

[00276] Image analysis was conducted by researchers unfamiliar with the hypotheses of the study. HSCs were identified by eye. Cells were considered negative for staining if the shape of the cell could not be discerned by eye, or if the shape of the cell formed a dark region in a field of positive signal. Distance measurements to niche components were made using Fiji and Microsoft Excel software. The locations of the HSC and nearest point of each of the three niche components were marked using point ROIs in Fiji and locations were transferred to excel, where distances in 3D were then calculated using the 3D Pythagorean Theorem. To calculate distances for randomly distributed HSCs, random point ROIs were generated using a custom plugin in FIJI in the same images analyzed for observed HSCs. The randomly generated points were considered simulated HSCs if they appeared in a reasonable HSC location (as assessed by the presence of Lin+ cells in the surrounding regions). Niche distance measurements for simulated HSCs were then made in the same manner as for observed HSCs. [00277] The statistical significance of differences in the distribution of distances was assessed by Kolmogorov-Smirnov analysis using Origin software. Statistical significance of changes in percentages of HSCs at 5 μm were assessed using a Student's T test in Microsoft Excel. Changes were considered significant if P < 0.05.

[00278] Final images shown in figures are maximum projections that have been background subtracted and contrast adjusted for clarity.

[00279] Transplantation and Repopulation assay. 100 sorted pHSCs or rHSC cells together with 1 .0 χ 10 ⁵ CD45.1 rescue bone marrow cells were transplanted into lethally irradiated (10 Gy) CD45.1 recipients. 2.0 χ 10 ⁵ CD45.2 BM cells from N-cad-CreER ^T ;iDTR and control mice together with 2.0 χ 10 ⁵ CD45.1 rescue bone marrow cells were transplanted into lethally irradiated (10 Gy) CD45.1 recipients. Every 4 weeks post transplantation, peripheral blood was collected from the submandibular vein. Hematopoietic repopulation was measured from donor- derived blood cells (CD45.2).

[00280] RNA sequencing and analysis. cDNA was generated from 1000 purified cells using SMARTer Ultra Low Input RNA kit (Clonetech) and library was generated by the Nextera XT DNA Library Preparation Kit (lllumina), followed by sequencing on an lllumina HiSeq2500 for 50bp single reads. Raw reads were demultiplexed into Fastq format allowing up to one mismatch using lllumina bcl2fastq2 v2.18. Reads were aligned to UCSC genome mm 10 with TopHat v2.0.13, default parameters. FPKM values were generated using Cufflinks v2.2.1 with "-u - max-bundle-frags 100000000". Read counts were generated using HTSeq-count with "-m intersection-nonempty". Three or four replicates were sequenced for each population. Data are accessible at NCBI GEO: GSE104887 [00281] CFU-F assay and in vitro differentiation. Cells were sorted directly into culture at a 96-well plate. The cultures were incubated at 37°c in a humidified atmosphere with 5% O2 and 10% CO2 for 7-10 days. Colonies were stained by CellTracker™ Green CMFDA (Life technologies) and image was acquired by Celigo Imaging Cytometer (Nexcelom). For in vitro differentiation, clonally expanded Ncad- CreER ^T driven Tomato ^4" BM/Bone stromal cells were isolated from CFU-F cultures by digesting with 0.25% Trypsin/EDTA, split into 3 aliquots, and seeded into separate cultures permissive for differentiation: StemPro Osteogenesis kit Gibco A10072-01 , Adipogenesis kit A10070-01 and Chondrogenesis differentiation kit A10071 -01 . The osteoblastic differentiation was assessed by VECTOR Red Alkaline Phosphatase; the adipogenic differentiation was detected by Oil Red O (Sigma); and the chondrogenic differentiation was detected by Toluidine blue (Sigma, 0.1 g T Blue/1 OOmL distilled water).

[00282] Bone sectioning, immunostaining and imaging.

[00283] Freshly isolated femurs were fixed in 4% paraformaldehyde overnight, followed by 1 to 3 days decalcification in 10% EDTA. For paraffin section, bone samples were processed with Sakura Tissue Tek VIP 5 Tissue Processor (Sakura America, Torrance, CA), and paraffin sections were cut in 5 um thickness. Sections were deparaffinized with xylene, followed by Alcian Blue/Hematoxylin/Orange G staining. For frozen section, bone samples were processed with the CryoJane tape- transfer system. Sections were blocked with Power Block™ Universal Blocking Reagent for 30 minutes to 1 hour and then stained overnight with rabbit-anti- Aggrecan (Millipore, 1 :300), rabbit-anti-Perilipin (Cell Signaling, 1 :300) and goat-anti- Osteopontin (R&D, 1 :300). Donkey-anti-goat Alexa Fluor 488 and Donkey-anti-goat Alexa Fluor 647 were used as secondary antibodies (all from Invitrogen, 1 :300). Antibodies were diluted with Antibody Diluent Solution (Invitrogen 00-3218). Slides were mounted with FLUORO-GEL (Electron Microscopy Science 1798510), and images were acquired with an Olympus slide scanner.

[00284] Femoral groove surgery. Mice were anesthetized with 2.5% isoflurane, and buprenorphine was administered for analgesia. The skin of the right leg was shaved and scrubbed with alcohol and iodine. A small incision was made in the skin, lateral to the knee joint. After sliding the skin medially for visualization, an internal incision was made medial to the patella, extending into the quadriceps muscle and along the patella tendon to release the tissue. The patella was subluxated laterally, and the distal femur was exposed. The subchondral bone was perforated using a microsaw to penetrate the articular cartilage at the knee joint. The extensor mechanism (quadriceps, patella tendon and patella) was returned to its original anatomical location. The internal incision was sutured with absorbable suture, and the skin sutured with non-absorbable suture.

[00285] Statistics.

[00286] Values are shown as the mean ± s.e.m. All statistical analyses were generated using GraphPad Prism 5 (GraphPad Software). Student's t test was used for comparisons between two groups. Statistical significance was defined as p < 0.05.

EXAMPLE B-1

[00287] Functionally distinguished reserve and primed HSCs in mouse bone marrow. To explore the reserve HSC (hereafter rHSC(s)) subpopulation, a cell surface marker was adapted, CD49b (Integrin a2), which can distinguish intermediate-term from permanently long-term HSCs (LT-HSCs) (Benveniste et al., 2010; Qian et al., 2015; Wagers and Weissman, 2005; Yang et al., 2005). Intriguingly, it was found a CD48 ^" CD49b ^~ subpopulation which exists only in conventional LT-HSCs (CD34 ^~ Flk2 ^~ Lineage ^~ Sca-1 ⁺ c-Kit ⁺ (LSK) cells) but not in short-term HSCs (ST-HSCs; CD34 ⁺ FLK2 ^" LSK) or multipotent progenitor cells (MPPs; CD34 ⁺ FLK2 ⁺ LSK). It was proposed that the CD48- CD49b ^~ LT-HSCs subpopulation enriches rHSCs and that the CD48 ^~ CD49b ⁺ LT-HSCs subpopulation enriches primed HSCs (hereafter pHSC(s)) (Fig. 17A) and tested with a repopulation assay. It was found that both rHSCs and pHSCs supported hematopoiesis in lethally irradiated mice for up to 40 weeks after transplantation without significant difference (Fig. 17B), consistent with a previous report (Benveniste et al., 2010). However, transplanted pHSCs had very low efficiency in generating rHSCs (95.4% reduction) as well as ST-HSCs and MPPs (-78% reduction for both populations) in recipients compared to transplanted rHSCs, suggesting that rHSCs hierarchically precede pHSCs (Fig. 17C). Using the Scl-tTA- induced H2B-GFP label-retaining model, it was found that rHSCs significantly enriched more H2B-GFP ^high cells compared to pHSCs (P=0.0039), indicating that rHSCs have slower cell cycle compared to pHSCs (Fig. 17D). Molecularly, it was found that rHSCs had higher expression of Gadd45g, Cdknic (encoding p57), and Foxo1 that are all involved in maintaining Go phase of HSCs, and lower expression of cell cycle activators such as Myc, Pcna, Ccng2, and Cdk4. There was no difference in Ki67 expression between rHSCs and pHSCs, indicating that Ki67 expression alone is insufficient to distinguish the two HSC subpopulations (Fig.17E).

[00288] Because the functional definition for rHSC is drug-resistance, rHSCs or pHSCs were transplanted into recipient mice and the mice were challenged with 5FU at 4 weeks post transplantation. As shown in Figure 17F, rHSCs were insensitive to 5FU treatment; however, pHSCs dramatically reduced their reconstitution ability (44% reduction at 20 weeks post transplantation). Taken together, the data indicated that CD48 ^" CD49b ^" LT-HSC indeed enriched rHSCs that were resistant to chemotherapeutic treatment, whereas CD48 ^~ CD49b ⁺ LT-HSC enriched pHSCs that were sensitive to chemotherapy; furthermore, the former gave rise to the latter but not vice versa in the transplantation assay.

[00289] The direct consequences of acute 5FU challenge on rHSCs and pHSCs were further analyzed. As shown in Fig. 17G, at 3 days post 5FU, around 92% of pHSCs were eliminated and only rHSCs survived, suggesting that rHSCs must have specifically turned on their DNA repair system to overcome the chemotherapeutic stress. To test this hypothesis, the transcriptome profiling of DNA damage response genes in rHSCs, pHSCs and rHSCs post 5FU treatment was analyzed. It was observed that rHSCs maintained a lower expression of genes involving the DNA damage repair system compared to pHSCs during homeostasis (Fig. 17H), but that most DNA repair pathways, such as DNA mismatch repair (MMR), nucleotide excision repair (NER), base excision repair (BER), and homologous recombination, (HR) were significantly activated in rHSCs under 5FU challenge (Fig. 171). Furthermore, in parallel, a great number of stress response genes (Rodina et al., 2016), which primarily belong to Hsp90 and Hsp70 family, were also upregulated (1 .8 ± 0.17-fold and 1.4 ± 0.1 -fold respectively) (Fig. 17 J), which partially explained how rHSCs survived and reconstituted the hematopoietic system under chemotherapeutic stress.

[00290] Taken together, the coexistence of pHSCs and rHSCs were functionally demonstrated in BM. Even with their quiescent state and active DNA- repairing pathways, pHSCs were still sensitive to chemotherapy, whereas rHSCs activated their DNA damage repair and stress response genes to survive chemotherapeutic stress and give rise to pHSCs; thus, rHSCs play a critical role in supporting hematopoietic regeneration under severe stress.

Example B-2

[00291] Drug-resistant rHSCs predominantly localize in the endosteal region of bone marrow

[00292] Whether extrinsic mechanisms from the BM niche contributed to rHSC maintenance during hemostasis and under chemotherapeutic stress was further examined. To this end, whole mount HSC staining was performed, which simultaneously detected the relative distribution of rHSCs and pHSCs to bones (achieved by second-harmonic generation, SHG), megakaryocytes (MKs) or vessels within ~75μm thickness bone cavity (Fig. 18A-B). Our quantification data showed that 43.4% rHSCs and 31 .0 % pHSCs were located within 10μm distance from vessels, and that 22.6% rHSCs and 24.6% pHSCs were located within 10μm from MKs (Fig. 18C-D), consistent with previous reports that the bulk of the HSC population resides in perivascular and sinusoid zones (Acar et al., 2015; Bruns et al., 2014; Chen et al., 2016; Zhao et al., 2014). Interestingly, it was noticed that 16.4% rHSCs compared to only 3.69% pHSCs located within 10μm from the bone surface (Fig. 2E). These data showed that both rHSCs and pHSCs were unbiasedly distributed to vessels and MKs, but that rHSCs were located significantly closer to endosteal bone surface compared to pHSCs (P=0.00182).

[00293] To test whether the endosteal region preserved rHSCs during chemotherapeutic stress, the distribution of rHSCs was examined at day 3 post 5FU treatment when pHSCs were eliminated (Fig. 18B). Interestingly, we found that ~55% of surviving rHSCs were preserved by the endosteal niche upon acute 5FU stress, which is a ~3.5-fold enrichment compared to homeostasis (Fig. 18E). However, there was no significant difference in frequency of surviving rHSCs observed near vessels or MKs (Fig. 18C-D). Consistently, it was observed that only 4.24% of pHSCs survived acute 5FU stress as shown in Figure 17G. Next, the dynamic process of BM damage and the subsequent recovery after 5FU stress was studied by examining the BrdU-labeled surviving and proliferation cells. It was noticed that at day 2 post 5FU treatment, there was a large loss of the BrdU-labeled cells, indicating an active apoptosis induced by 5FU. At day 3 post 5FU treatment, it was observed that surviving BrdlT cells (mostly single cells) were mainly detected adjacent to bone lining cells in the endosteal region (Fig. 24A-D). At day 3.5, it was observed that pairs of BrdlT cells appeared at the endosteal surface, indicating activation and division of surviving cells. Starting and continuing at days 4 and 5, the number of proliferating cells gradually increased, and these cells were very often detected as clusters close to either vessels or potential adipocyte structures (~55% at day 4 and ~65% at day 6) (Fig. 24E-I). This observation suggested that bone surface was the niche where cells post 5FU initially survived. These cells were then activated and gave rise to daughter cells, the latter of which underwent expansion mainly in vessels or adjacent to megakaryocytes (Zhao et al., 2014). It was further confirmed that surviving 5FU- rHSCs were indeed detected often as single cells adjacent to the bone surface, and proliferating HSCs often associated with MKs or near the vessels. Surviving rHSCs (green, CD150+ Lin- CD49b-) at day 3 post 5FU treatment were often detected as single cells adjacent to the bone surface (white, SHG), and proliferating HSCs were often associated with MKs (CD150+ Lin+) or near the vessels (red, CD31 +) (Fig. 29).

[00294] N-cad ⁺ pre-osteoblastic cells in bone surface have been found resistant, whereas Osx ⁺ osteoblasts have been found to be sensitive to 5FU treatment and N-cad ⁺ stromal cells have been found to give rise to Osx ⁺ osteoblasts during recovery post 5FU treatment (Sugimura et al., 2012). A recent study showed dramatic depletion of LepR ⁺ stromal cells in central marrow due to cell death 1 day following irradiation (Zhou et al., 2017). To track the early changes in vascular and endosteal niches, an apoptotic assay was performed at day 1 post 5FU treatment. It was found that the apoptotic CD31 ⁺ VE-cadherin ⁺ cells greatly increased 1 day post 5FU (Fig. 18F), consistent with previous reports of disrupted blood vessel structure post 5FU (Dominici et al., 2009; Sugimura et al., 2012). To study the endosteal niche, the N-cad-tdTomato {N-cad-TdT) mouse line was established in which the Tomato ^4" cells report N-cad expression in both central marrow and bone surface (Fig. 24J). A marked increase of apoptotic N-Cad driven Tomato ^4" cells in the central marrow was noticed, whereas the Tomato ^4" cells in endosteal zone remained stable 1 day post 5FU (Fig. 18G). At day 3 post 5FU treatment when hematopoietic regeneration started, N-Cad driven Tomato ^4" cells in central marrow as well as in bone surface increased by 2.2-fold and 1 .7-fold respectively (Fig. 18H, Fig. 24K). The number of CD31 ^4" vessel cells also increased but showed a dilated and damaged architecture (Fig. 181). Taken together, previous reports and this data showed that vessels and associated stromal cells in the perivascular niche, including LepR ^4" and N-cad ⁺ cells, suffered immediate damage and were sensitive to 5FU stress, whereas N-cad ⁺ endosteal stromal cells remained stable.

[00295] The data partially explain previous findings that most HSCs are predominantly distributed in perivascular and sinusoidal zones (Acar et al., 2015; Chen et al., 2016; Kunisaki et al., 2013). pHSCs which account for 51 .3% of the quiescent HSC population are near perivascular and sinusoid zones in homeostasis. Under stress, however, rHSCs reside closer to the bone surface survive chemotherapeutic stress. Collectively, the data indicate that the endosteal niche plays a critical role in protecting rHSCs from chemotherapy.

Example B-3

[00296] N-Cad ⁺ niche cells maintain functional HSCs including rHSCs in bone marrow

[00297] Though N-cad ⁺ stromal cells were the first identified HSC niche cells, and though N-cad ⁺ stromal cells at the endosteal niche were resistant while Osx ⁺ osteoblasts were sensitive to chemotherapy, a direct evidence for N-cad ⁺ functionally supporting HSCs was still missing due to lack of proper genetic mouse lines. According to aspects herein, the N-cad-CreER ^T line was generated (Fig. 25A) and showed that N-cad ⁺ stromal cells gave rise to both Col2.3-GFP ⁺ osteoblastic cells and perivascular cells (Fig. 25B). A previous study showed that depleting the mature Col2.3-GFP ⁺ osteoblasts did not affect overall engraftment of BM cells (Ding et al., 2012; Greenbaum et al., 2013), but only caused impaired regenerative capacity of a subset of LT-HSCs (Bowers et al., 2015). Though N-cad ⁺ stromal cells developmentally proceed Osx ⁺ osteoprogenitor cells, which further give rise to mature Col2.3 ⁺ osteoblasts, a functional contribution to HSC maintenance by N-cad ⁺ stromal cells is unknown. To investigate the HSC niche role of N-cad ⁺ cells, N-cad-CreER ^T induced DTR (encoding diphtheria toxin receptor) line (N-cad-CreER ^T ;iDTR) was generated, in which N-cad ⁺ niche cells were rendered sensitive to diphtheria toxin (DT). Three tamoxifen (TMX) injections were administered and followed with intraperitoneal injections of DT (one injection every other day) to the N-cad- CreER ^T ;iDTR mice and analyzed them on the first day after the last injection (Fig. 19A). The efficient ablation was observed of N-cad ⁺ stromal cells in N-cad- CreER ^T ;iDTR; R26-tdT compared to N-cad-CreER ^T ; R26-tdT mice treated concurrently with DT (Fig. 19B). Whether N-cad ⁺ cell ablation would affect HSCs in vivo was further examined. After ablating N-cad ⁺ cells, no significant change was observed of cellularity in BM compared to controls (Fig. 19C). However, the numbers of HSCs were dramatically reduced: rHSCs (65.0% reduction), pHSCs (60.0% reduction), ST-HSCs (59.6% reduction), and MPPs (29.3% reduction) (Fig. 19D). This indicated that N-cad ⁺ niche cells contributed to the most primitive, including reserve, HSC maintenance.

[00298] A transplantation assay was also performed to test the functional HSC numbers in N-cad ⁺ stromal cell ablated mice. It was found that bone marrow cells from N-cad ⁺ stromal cell ablated mice gave significantly lower levels of donor cell reconstitution (28.3% reduction at 20 weeks) (Fig. 19E) with reduced myeloid cell production (24.5% to 17.1 %) (Fig. 19F). To further investigate whether N-cad ⁺ niche cells contributed to maintaining the long-term self-renewal of HSCs, a secondary transplantation was conducted at 20 weeks post the primary transplantation. It was observed that BM cells from the N-cad ⁺ stromal cell ablated mice had deeper reduction of donor cell reconstitution capacity in the secondary transplantation (40.5% reduction at 16 weeks) (Fig. 19G), although they were capable of multilineage reconstitution (Fig. 19H).

[00299] Furthermore, we found that conditional knockout of Cxcl12 from N- cad ⁺ stromal cells significantly reduced pHSCs (48% reduction) and ST-HSCs (28.8% reduction), but no significant reduction was observed in rHSCs. There was a slight albeit insignificant increase of MPPs. (Figure 191). Conditional deletion of SCF from N-cad ⁺ stromal cells significantly reduced rHSCs (60% reduction), pHSCs (38.6% reduction), and ST-HSCs (53.7% reduction), but with a slight increase in MPPs (Figure 19J). Overall, we provided the first functional evidence that N-cad ⁺ niche cells contributed to HSC maintenance, including rHSCs, via producing maintenance factors.

Example B-4

[00300] Transcriptome analysis for hematopoietic cells and their BM niche cells

[00301] To understand the molecular mechanisms governing how different niche cells contribute to HSC subpopulation regulation, a transcriptome profiling analysis was performed on 4 types of hematopoietic stem and progenitor cells (HSPCs) during homeostasis and on rHSCs at day 3 post 5FU, as well as on 10 types of BM niche cells. The BM niche cells were harvested from different niche zones of endosteum (B) and central bone marrow (M) (Fig. 26A-B). The pearson distance tree and principal component analysis (PCA) data showed that rHSCs and pHSCs shared a unique transcriptome profiling compared to ST-HSCs and MPPs (Fig. 20A). Interestingly, rHSCs post 5FU treatment appeared to be closer to pHSCs (Fig. 20B), suggesting that surviving rHSCs were primed for activation to support subsequent hematopoietic regeneration post chemotherapy.

[00302] It was found that the rHSCs enriched most of the published HSC specific markers, such as Slamfl {CD150), H19, CtnnaH {a-Catulin), Fgd5, vWF, Tek{Tie2), Procr{Epcr), Hoxb5 and Meg3 (or Gtl2) (Acar et al., 2015; Chen et al., 2016; Qian et al., 2015; Sanjuan-Pla et al., 2013; Venkatraman et al., 2013). Particularly, vWF in rHSCs were 6.1 -fold higher than in pHSCs, suggesting that rHSCs enriched most of the vWF ⁺ HSCs which reside at the apex of the HSC hierarchy (Sanjuan-Pla et al., 2013). Consistently, progenitor signature genes such as CD34, CD48, and Flt3(Flk2) and Itga2 (CD49b) had low expression in rHSCs (Acar et al., 2015; Chen et al., 2016; Gazit et al., 2014). Interestingly, rHSCs post 5FU had high expression levels of CXCR4, Ctnnall (a-Catulin)(Park et al., 2002), Esam (Endothelial cell-selective adhesion molecule) and CD150 (encoding Slamfl, signaling lymphocytic activated molecule), consistent with their functions associated with the converted primed state (Fig. 20E).

[00303] In niche cell analysis, it was found the N-cad driven tdTomato (N-cad- TdT) (M) had a very similar transcriptome profile compared to other niche cells with mesenchymal stem cell (MSC) potential such as LepR, Cxcl12-RFP, and Nestin- GFP cells in both pearson distance tree (Fig. 20C) and PCA analysis (Fig. 20D), suggesting that N-cad ⁺ cells might have MSC potential and similar function in regulating HSCs. Interestingly, NG2-RFP cells had a very similar transcriptome profile to Col2.3-GFP ⁺ osteoblasts (Fig. 20C-D). It was also found that NG2-RFP cells were predominantly restricted to endosteum in the epiphysis or diaphysis, though they were also detected in the peri-arterial region (Fig. 26C).

[00304] It was found that Pecam (CD31 ), CDH5 (VE-cadherin) were enriched in Pecam-GFP ⁺ endothelial cells. PF4 and Adgrel were enriched in MKs and macrophages (Macs). Cspg4 (NG2) was enriched in NG2-RFP cells. Nestin-GFP cells did not have endogenous Nestin (Nes) expression, consistent with previous reports (Ding et al.,2012; Greenbaum et al., 2013). Interestingly, it was found that several marker genes such as Kitl (SCF), LepR, Cdh2 (N-cadherin, N-CAD), Cxcl12, Pdgfra were broadly expressed in perivascular niche cells (Fig. 20F). N-cad-TdT(B) cells isolated from the endosteal zone also had high levels of SCF, Cxcl12, LepR and Pdgfra expression compared to other cells from the endosteal zone, such as Col2.3-GFP and NG2-RFP cells (Fig. 20F). Our GO term analysis showed that Col2.3-GFP, Pecam-GFP, MK and Mac cells highly enriched RNA metabolism, protein metabolism, other metabolism pathways and translation activities, indicating that these cells were in a relatively active functional state compared to N-cad-TdT cells and other perivascular zone cells which were in a relatively lower metabolic state. Endothelial cells and perivascular cells had immune system process and stress response activities (Fig. 20G). Because N-cad ⁺ cells had transcriptome profiles similar to other MSCs such as LepR ⁺ , Cxcl12-RFP, and Nestin-GFP cells, we next analyzed and compared stromal development related genes from different niche cells (Fig. 20H). It was found that Col2.3-GFP cells enriched osteoblast gene Col1 a and progenitor cell gene Ly6a {SCA1) (Yang et al., 2014). N-Cad-TdT(B) cells from the endosteal region enriched chondrocyte genes Spockl, Col2a1 and Col1a2 and bone development genes such as Tnc. Interestingly both N-Cad-TdT(B) and N-Cad- TdT(M) enriched most of the mesenchymal stem and progenitor cell (MSPC) genes such as Prrxl, Pdgfre, Pdgfra, Sp7, Sox9 and Greml. Marrow harvested NG2-RFP cells also had higher levels of MSPC gene expression, such as Alcam, Sp7, Mcam, Sox9 and GH1, but with high level osteoblast and chondrocyte related genes such as Col1a1, Col2a1, and Col1a2. It was also found that among all perivascular niche cells, LepR ⁺ cells exclusively enriched Gla as well as several osteoblast and chondrocyte markers such as Atf4 and Tnc. Cxcl12-RFP cells significantly enriched MSPC gene Itgav. Furthermore, Greml was enriched in N-Cad-TdT(B) and in all perivascular niche cells but not in NG2-RFP cells.

[00305] Consistent with earlier data, Col2.3-GFP cells were enriched for more mature osteo-lineage genes (Dmp1, Col1a1, Spp1, Bglap). Nestin-GFP, Cxcl12- RFP and LepR were consistent with their known role in enriching MSC genes (Prrxl, Pdgfra, Itgb l, Greml (Fig. 20H). Interestingly, apart from expressing MSC genes, N-Cad-TdT(M) and N-Cad-TdT(B) were enriched for chondrogenic (Sox9, Col2a1 and Tnc), adipogenic (Cebpct, Ppary, Adipoq) and osteogenic (Col1a1, Runx2) genes, suggesting a tri-potential nature of N-Cad ⁺ stromal cells. Surprisingly, NG2-RFP was enriched for both bone development genes (Dmp1, Bglap, Sp7, Runx2, Col1a1) and chondrogenic genes (Sox9, Col2a1 and Tnc), suggesting an osteo-chondrogenic role (Fig. 26D-E). These data strongly indicated the heterogeneity of the MSPCs in BM.

Example B-5

[00306] N-cad-CreER ^T induced reporter cells largely overlap with LepR ⁺ and Cxcl12 ⁺ stem/stromal cells

[00307] To confirm the transcriptional analysis, N-cad in vivo lineage tracing was performed using the TdT or ZsG reporter, and found that, at day 3 post induction, N-cad-CreER ^T lineage traced cells partially overlapped with Cxcl12-RFP and Nestin-GFP cells (Fig. 20I). Moreover, 98.3% and 97.9% N-cad-CreER ^T derived cells were positive for LepR and Pdgfra expression respectively (Fig. 20J). This result further supported that N-cad ⁺ stromal cells have MSC potential as suggested by the transcriptome analysis. To analyze the lineage potential of N-cad ⁺ stromal cells, colony-forming unit-fibroblasts (CFU-F) assay was performed to test their proliferating capacity. It was found that the majority of reported niche cells, whether from endosteal zone or from perivascular zone, had one out of fewer than 10 cells with CFU-F activity, apart from Nestin-GFP ⁺ cells, which had only one out of 17.6 cells with CFU-F activity. N-cad ⁺ cells from bone had one cell with CFU-F activity out of 8.79 cells. N-cad-CreER ^T derived bone cells had 1 in 10.7 cells with CFU-F activity, and N-cad-CreER ^T derived marrow cells had 1 in 7.37 cells. (Fig. 20K). Most CFU-F colonies maintained tdTomato signals (Fig. 21A-B), suggesting that N- cad ⁺ cells were the main source of MSPCs with CFU-F activity.

Example B-6 [00308] N-cad ⁺ stromal cells give rise to osteoblasts, chondrocytes, and adipocytes in vitro and in vivo

[00309] To test the MSC potential of N-cad ⁺ cells, in vitro differentiation assay was performed by splitting cells obtained from individual CFU-F colonies formed by N-cad ⁺ cells into three aliquots and sub-cloned them into cultures permissive for bone, fat, or cartilage cell differentiation. It was found that Tomato ^4" cells underwent multilineage differentiation, giving rising to Alkaline phosphate-positive osteoblastic cells, Oil-red-O-positive adipocytes, and Aggrecan-stained and Toluidine-blue- positive chondrocytes (Fig. 21 C-E).

[00310] To characterize in vivo function of N-cad ⁺ stromal cells, the dynamic anatomical distribution of N-cad derived cells was analyzed after one dose of TMX treatment (Fig. 21 F, G). Interestingly, detected N-cad derived cells were detected in metaphysis of trabecular as early as 6 hours post TMX, but only very few cells were seen in central marrow, suggesting that a large portion of N-cad ⁺ cells originated from the endosteal region (Fig. 21 H). It was also observed that the number of N-cad derived cells increased by 2.2 ± 0.2-fold in trabecular bone region from 1 to 2 weeks post TMX treatment and remained stable afterwards; in cortical bone region, the cell number increased by 2.95 ± 0.2-fold at 2 weeks post TMX, but then declined at 6 weeks post TMX, suggesting that N-cad ⁺ cells in trabecular bone region were more quiescent compared to cortical bone region. Moreover, N-cad derived cells continuously increased in central marrow up to 6 weeks, suggesting that N-cad derived cells might be proliferating and differentiating within this area (Fig. 211).

[00311] Next, an in vivo lineage tracing assay was performed and it was observed that N-cad ⁺ cells generated Col2.3-GFP ⁺ osteoblasts (Dacic et al., 2001 ) in a time dependent manner (Fig. 22A, Fig. 27A-B). N-cad-derived osteoblasts were detected in the peri-trabecular region as early as 6 hours post TMX induction (Fig. 22B) and in the compact bone region at 14 hours post TMX induction (Fig. 22C). Further analysis showed that at 6 hours post TMX, immature N-cad ⁺ Col2.3 ^~ cells (1.1 % ± 0.2%) were 10-times more enriched in trabecular bone region compared to cortical bone region (0.11% ± 0.04%) (Fig. 22D, E). Consistently, more immature N- cad ⁺ Col2.3 ^~ cells were enriched in trabecular bone region compared to compact bone region at 24 hours and 2 weeks post TMX induction (Fig. 22F). At 4 weeks post TMX induction, a large portion of Col2.3-GFP cells (72% ± 6%) were derived from N-cad ⁺ cells (Fig. 22G).

[00312] It was further observed that at 4 weeks post TMX injection, N-cad ⁺ stromal cells generated adipocytes in the trabecular bone region, particularly the endosteal cells, (Fig. 22H), and this was further confirmed by BODIPY lipid probe staining (Fig. 22I). Interestingly, the frequency of N-cad ⁺ derived adipocytes increased quickly initially and declined later on, as evidenced by the quantification that 39.3% ± 4.5%, 77.1 % ± 6.7% and 17.2% ± 3% of N-cad ⁺ derived cells were observed at 6, 14 and 24 hours respectively after TMX induction, suggesting that adipocytes might have a frequent turnover rate (Fig. 22J-K, Fig. 27C-E). Collectively, the data demonstrated that N-cad ⁺ stromal cells generated both osteoblast and adipocyte lineages in vivo during homeostasis in adult mice.

[00313] Furthermore, it was found that Ncad+ MSCs were enriched > 10-fold as compared to other markers. For example, as shown in Fig. 30, just 10K Ncad+ hMSCs produced even higher human thrombopoietin (THPO or TPO) than total hMSCs in vivo. Similarly, Ncad- hMSCs exhibited a function similar to the total hMSCs. Accordingly, isolating MSC with N-cadherin antibodies can provide for enrichment of the MSC population. Example B-7

[00314] N-cad ⁺ stromal cells give rise to chondrocytes during development and post injury

[00315] Chondrogenesis is active in fetal development and rarely active in adulthood (Raghunath et al., 2005; Sophia Fox et al., 2009). In mice with TMX induction at postnatal day 2 (P2), no Tomato expression was detected among Aggrecan ^4" chondrocytes in the femur of N-cad-CreER ^T ; R26-tdT mice (Fig. 28A) despite Tomato ^4" osteoblasts and perichondrocytes being adjacent to the growth plate. After N-cad-CreER ^T ; R26-tdT mice were induced with TMX at 1 or 2 weeks when the growth plate develops, Tomato ^4" chondrocytes in cartilages of femur or tibia remained undetectable (data not shown). TMX induction was started to N-cad- CreER ^T ; R26-tdT mice at embryonic stage E12.5, when MSCs were undergoing condensation and chondrocyte differentiation in the fetal bone (Fig. 23A). It was found that the majority of undifferentiated N-cad derived tdT ⁺ cells were located peripherally, with differentiated N-cad derived Tomato ^4" chondrocytes located in the central region of the rib at E14.5 embryos (Fig. 23B). Importantly, in the trabecular region of femur at P2, N-cad derived chondrocytes were detected in both the columnar zone where the secondary ossification center forms, and in the superficial zone, which is imperative to resist the sheer force for deeper layer protection (Fig. 23C, D). Consistently, N-cad derived adipocytes were detected at 2 months and up until 10 months after birth (Fig. 23E, Fig. 28B), suggesting that N-cad + cells contributed to early and long-term adipogenesis during development. However, N- cad ^4" cells derived Osteopontin ^4" osteoblast and osteocyte were detected at 2 months after birth but not at early P2 (Fig. 23F, Fig. 28C). These data demonstrated that embryonic N-cad-derived cells gave rise to all three osteo-adipogenic-chondrogenic lineages, and only N-cad ⁺ at embryonic stage could efficiently form chondrocytes.

[00316] Whether N-cad ⁺ cell could generate chondrocytes in adult mice post injury was next investigated. Cartilage perforation was performed in N-cad-CreER ^T ;

R26-tdT mice that received TMX induction at E12.5 (Fig. 23G). At 2 to 3 weeks post cartilage damage, we found that N-cad derived chondrocytes were clustered in the callus at the damage site with a 2-fold increase compared to undamaged control mice (Fig. 23H-I). The femorotibial joint was swollen due to articular cartilage damage in mice post-surgery (Fig. 28D). Alcian blue positive cells with a typical chondrocyte feature were dramatically increased in the post injury region (Fig. 23J), indicating that N-cad ⁺ MSCs quickly regenerated chondrocytes in response to injury.

[00317] Taken together, the data proved that N-cad-derived cells induced at the fetal stage (E12.5) could give rise to chondrocyte progenitor in adults and support chondrocyte regeneration in response to injury.

[00318] Discussion

[00319] rHSCs vs. pHSCs

[00320] The heterogeneity of HSCs has been widely studied. HSCs can be maintained in either active, quiescent or deeply quiescent states. It is well known that the quiescence characterization of HSCs is functionally related to their long-term self-renewal potential (Foudi et al., 2008; Wilson et al., 2008). However, how the quiescent HSC population overcomes the consequence of myeloablation in vivo is an unanswered question. In spite of their quiescence, the majority of HSCs cannot survive chemotherapeutic stress such as 5FU (Longley et al., 2003). According to aspects herein, it was found that a small portion of the HSC subpopulation could survive 5FU treatment in primary mice and were resistant to 5FU treatment in transplantation model. Thus, this HSC subpopulation is defined as rHSCs, while other quiescent but chemotherapeutic sensitive HSC subpopulations are defined as pHSCs (Li and Clevers, 2010). Although both HSC subpopulations supported long- term hematopoiesis in transplantation experiments, pHSCs rarely gave rise to rHSCs, whereas the latter were able to give rise to pHSCs. Mechanistically, it was found that rHSCs have an attenuated DNA repair system compared to pHSCs during homeostasis, but rHSCs can quickly activate their DNA repair pathways and the stress-response program to survive chemotherapeutic stress, and support subsequent hematopoiesis to overcome the consequence of myeloablation.

[00321] Niche matters in term of conferring resistance to chemotherapy

[00322] To explore an extrinsic mechanism underlying the chemo-resistance of rHSCs, an idea was tested that they were preserved in a specific microenvironment in BM. Using whole mount HSC staining (Kunisaki et al., 2013), it was first observed that the bulk of HSCs were associated with vessels and MKs as previously reported. However, it was surprisingly found that the rHSCs were predominantly associated with the endosteal niche compared to pHSCs during homeostasis and post 5FU treatment. This indicated that the endosteal niche could form a distinct BM microenvironment to protect rHSCs from chemotherapeutic stress. Consistently, upon chemotherapeutic stress, most rHSCs were protected in the endosteal niche that enriches chemo-resistant N-cad ⁺ stromal cells, whereas the vessel and perivascular cells were sensitive to 5FU treatment, accounting for a large loss of pHSCs induced by chemotherapy. The whole mount HSC staining was done in sternum which has abundant bone branches and extensions inside the marrow. This feature makes it very similar to the trabecular bone region in the femur. The transplantation assay showing that depletion of N-cad ⁺ niche cells affected HSC maintenance, including rHSCs, supports this notion. HSC quiescence also correlated with their low metabolic state, which may be considered analogous to 'sleeping', and was termed as HSC dormancy or hibernation (Takubo et al., 2013; Wilson et al., 2008; Yamazaki et al., 2011). However, it was further showed according to aspects herein that quiescence was not the only mechanism underlying drug resistance; instead, both the intrinsic stress-response program and the extrinsic niche protection contributed to drug resistance.

[00323] Identity and function of N-cad stromal cell in HSC maintenance in the BM niche

[00324] N-cad ⁺ stromal cells were the first identified HSC niche cells (Zhang et al., 2003) and confirmed by subsequent studies (Arai et al., 2004; Sugiyama et al., 2006). N-cad ⁺ cells were initially proposed as osteoblastic progenitor cells based on their endosteal location. According to apects herein, by using two reporter lines, it was found that N-cad ⁺ stromal cells were distributed in both endosteal region and perivascular sites. More intriguingly, it was found that the majority of N-cad ⁺ cells overlapped with LepR ⁺ cells and Pdgfra ^4" cells. Transcriptional analysis showed that N-cad ⁺ cells, LepR ⁺ cells, Cxcl12-RFP (CAR) cells and Nestin-GFP cells had a very similar gene expression pattern. The data strongly indicated that the long-standing controversy of HSC niche concepts might very likely be due to different cell markers being used rather than to their cellular identities.

[00325] To determine the identity of N-Cad ⁺ stromal cells, their regional distribution was visualized relative to other known niche cells in the bone marrow using different niche reporter mice. N-Cad ⁺ stromal cells generated 72% ± 6% Col2.3-GFP ⁺ cells; however, 9.3% ± 4.7% N-cad ⁺ Col2.3GFP ^" cells were also detected in the endosteal region. These immature cells accounted for the primitive MSCs, which could explain the insufficient efficiency of Col2.3-Cre genetic model in HSC niche function studies (Ding and Morrison, 2013; Ding et al., 2012; Greenbaum et al., 2013). Though both N-Cad-TdT ⁺ and Nestin-GFP ⁺ were enriched in the trabecular region, N-Cad- TdT ⁺ was concentrated in the trabecular region where engraftment of transplanted HSCs was detected as previously reported (Nilsson et al., 2001 ; Xie et al., 2009) and survived after stress as observed here. All these data indicated that although N-cad ⁺ cells share a similar transcriptome profile with other MSCs, their anatomic distribution may indicate their unique HSC niche function; indeed, it was shown that the N-cad ⁺ endosteal niche cell plays a critical role in preserving rHSCs.

[00326] By using an inducible DTR system, it was found that ablation of N-cad ⁺ cells eliminated both pHSCs and rHSCs in BM. This could be explained by the anatomical distribution of N-cad ⁺ niche cell in both endosteal and perivascular zones. Furthermore, it was shown that N-cad expression could be detected in a subset of HSCs; however, the N-cad-TdT reporter mouse lines did not support this observation (data not shown). This could be partially explained by the inconsistency between their protein and transcription levels, because another mouse line of N-cad-mCherry (fusion at protein level) indeed had a small subset of HSCs (CD49b ^" CD34 ^" Flt2 ^" LSK) showing a low level of N-cad expression (primary observation). The functional transplantation data showed that ablation of N-cad ⁺ niche cells resulted in the reduced HSCs including rHSCs. By deletion of Cxcl12 and SCF from N-cad ⁺ cells, it was found N-cad ⁺ stromal cells contributed to HSC maintenance and regulation by producing these two factors. Overall, aspects herein demonstrate that N-cad ⁺ stromal cells function as MSCs and support primitive HSC maintenance, especially under stress. [00327] Transplantation of hematopoietic stem cells (HSCs) from human umbilical cord blood (hUCB) holds great promise for treating a broad spectrum of hematological disorders including cancer, but the limited number of HSCs in a single hUCB unit can restrict its widespread use. Although extensive efforts have developed multiple methods for ex vivo expansion of human HSCs by targeting single molecules or pathways, it has been unknown whether simultaneously manipulating a large number of targets essential for stem cell self-renewal could be achievable. Recent studies have emerged that N ⁶ -methyladenosine (m ⁶ A) modulates expression of a group of mRNAs critical for stem cell fate determination by influencing their stability. Among several m ⁶ A readers, Ythdf2 is well recognized to promote the targeted mRNA decay. However, the physiological functions of Ythdf2 on adult stem cells are still elusive. Embodiments herein demonstrate that conditional knockout (KO) mouse Ythdf2 increased phenotypic and functional HSC numbers, but neither skewed lineage differentiation nor led to hematopoietic malignancies. Furthermore, knockdown (KD) of human YTHDF2 led to over 10-fold increase in ex vivo expansion of hUCB HSCs, 5-fold increase in colony-forming units (CFUs), and more than 8-fold increase in functional hUCB HSCs in the 2 rounds of limiting dilution transplantation assay. Mechanistically, m ⁶ A mapping of RNAs from mouse hematopoietic stem and progenitor cells (HSPCs) as well as from hUCB HSCs revealed m ⁶ A enrichment on mRNAs encoding transcription factors critical for stem cell self-renewal. These m ⁶ A-marked mRNAs were recognized by Ythdf2 and underwent mRNA decay. In Ythdf2 KO HSPCs and YTHDF2 KD hUCB HSCs, these mRNAs were stabilized, leading to an increase in protein levels and facilitating HSC expansion which can be rescued by knockdown the mRNA, such as Tall mRNA. Therefore, embodiments show the function of Ythdf2 in adult stem cells maintenance and identify an important role of Ythdf2 in regulating HSC ex vivo expansion via the mechanism of controlling the stability of multiple mRNAs critical for HSC self- renewal, thus having a strong potential for future clinical applications.

[00328] Furthermore, regulation of hematopoietic stem cells (HSCs) by the bone marrow (BM) niches has been substantially studied; however, whether and how HSC subpopulations are distinctively regulated by different BM niches remain largely unclear. Here, reserve HSCs (rHSCs) have been functionally distinguished from primed HSCs (pHSCs) and their respective BM niches have been further examined. It has been found that both pHSCs and rHSCs could support long-term hematopoiesis under homeostasis; however, pHSCs were sensitive to chemotherapy, whereas rHSCs survived chemotherapy and supported subsequent regeneration after myeloablation. The whole-mount HSC distribution study revealed that rHSCs were preferentially maintained in the endosteal region that enriches N- cadherin* bone-lining cells during homeostasis and post-chemotherapy. pHSCs were predominantly associated with blood vessels which were vulnerable to chemotherapy compared to bone. Transcriptome profiling and in vivo lineage tracing results showed N-cadherin ⁺ stromal cells to be functional mesenchymal stem cells, which gave rise to osteoblasts, adipocytes, and chondrocytes during development and regeneration. Finally, it was demonstrated that ablation of N-cadherin ⁺ niche cells or deletion of either Scf or Cxcl12 from N-cadherin ⁺ niche cells affected HSC number and maintenance.

Example C

[00329] Expansion of CAR-T cells using shRNA

[00330] The effect of manipulating Ythdf2 on the expansion of CAR-T cells is being assessed using lentivirus driven human Ythdf2 shRNAs. Successful cloning of YTHDF2 shRNA in a CAR-T lentivector has occurred. The lentivirus has been used to infect human CAR-T cells, and the expansion of the human CAR-T cells is in progress. The expansion is expected to take days to weeks for results. It is believed that significantly enhanced expansion will be demonstrated in the lentivirus-infected CAR-T cell population as compared to a CAR-T control population.

[00331] All sequencing data, including the m ⁶ A-seq, irCLIP-seq and RNA-seq datasets, are available through the Gene Expression Ombibus (GEO) under accession GSE107957.

[00332] Original Data Repository at http://www.stowers.org/research/publications/LIBPB-1248.

[00333]

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TABLE S5

Table S5. qPCR prim ers used to verify the expressional levels of transcription factors in wt and Ythdf2 KO HSPCs. mouse Tall qPCR primer F CGCTGCTCTATAGCCTTAGCC

mouse Tall qPCR primer R CTCTTCACCCGGTTGTTGTT

mouse Gata2 qPCR primer F CAACCCTTACTACGCCAACC

mouse Gata2 qPCR primer R GCTGTGCAACMGTG TGGTC

mouse Runxl qPCR primer F CCAGCCTC TCTGCAGAACTT

mouse Runxl qPCR primer R GGAGATGGACGGCAGAGTAG

mouse Stat5a qPCR primer F AGAAGCAAGTGTCCC TGGAG

mouse Stat5a qPCR primer R GTCGTCCAGGATGATGGTCT

mouse Actb qPCR primer F TGTCACCAACTGGGACGATA

mouse Actb qPCR primer R ACCCTCATAGATGGGCACAG