method for producing the same

The present invention relates to a method for producing a site-specifically serine ADP- ribosylated protein or peptide, comprising: (a) subjecting a protein or peptide comprising or consisting of an amino acid sequence comprising two or more serines, wherein at least one serine is phosphorylated and at least one serine is non-phosphorylated, to a serine ADP- ribosylation reaction. The present invention also relates to a site-specifically serine ADP- ribosylated protein or peptide produced by the method of the invention.

In this specification, a number of documents including patent applications and manufacturer’s manuals are cited. The disclosure of these documents, while not considered relevant for the patentability of this invention, is herewith incorporated by reference in its entirety. More specifically, all referenced documents are incorporated by reference to the same extent as if each individual document was specifically and individually indicated to be incorporated by reference.

The present invention is in the technical field of a chemically complex protein post- translational modification (PTM) called ADP-ribosylation (ADPr). The biological and clinical importance of this modification is well established: ADPr and the enzymes responsible therefore (known as poly (ADP-ribose) polymerase (PARPs)) have been found to play a role in many key cellular processes, including maintenance of genomic stability, cell differentiation and proliferation, cytoplasmic stress responses, and microbial virulence. Moreover, dysregulation of ADPr has been linked to diseases including cancer, diabetes, neurodegenerative disorders, and heart failure, leading to the development of therapeutic PARP inhibitors, many of which are currently in clinical trials. In particular, the PARP-1 inhibitors Olaparib (brand name Lynparza), Rucaparib (brand name Rubraca) and Niraparib (brand name Zejula) are EU- and FDA-approved therapeutic agents for cancer therapy.

Despite the clear biological and clinical importance of ADPr, further progress in the field is limited by the difficulties in elucidating the underlying molecular mechanisms. This is due to a lack of essential tools and reagents, ranging from the modified proteins and peptides themselves up to site-specific antibodies, tools which are commercially available for other biologically important protein modifications.

Recently it has been reported that ADP-ribosyl can be attached to serine, a previously unknown and unexpected amino acid target residue for this PTM (Leidecker et al., 2016, Nat Chem Biol 12, 998-1000). Shortly afterwards it was shown that this new form of ADPr, which is called serine-ADP-ribosylation (Ser-ADPr), is a widespread PTM that targets hundreds of different substrates, including PARP-1 itself (Bonfiglio et al., 2017, Mol Cell 65, 932-940 e936). The required elements for the in vitro modification of a target substrate with ADP- ribose on serine are PARP-1 or -2 (including variants, such as PARP-1 E988Q), Histone PARylation factor 1 (HPF1 ), NAD ⁺ , activated (i.e. sonicated) DNA and the substrate itself, all of them contained in a reactor (e.g. a tube) in which an uncontrolled ADP-ribosylation of both the substrate and the modifying enzyme occurs (Bonfiglio et al., 2017, Mol Cell 65, 932-940 e936). The inventors have also recently filed the EP application 18 15 4508.8 which describes an ADP-ribosylation method resulting in essentially pure serine ADP-ribosylated forms of proteins or peptides in a cost-effective manner on a scale varying from small (a few pg) to large (several milligrams).

Hence, at present peptides and proteins with one or more serines that can be enzymatically ADP-ribosylated can be produced. However when peptides and proteins comprise two or more serines, this either creates peptides and proteins, wherein all serines that can be enzymatically ADP-ribosylated are in fact ADP-ribosylated or - in case the conditions are selected, so that not essentially pure serine ADP-ribosylated forms are obtained - it creates a mixture of different partially ADP-ribosylated species.

However, in these cases where a protein or peptide comprises two or more serines, it is currently not possible to generate a protein or peptide wherein one or more selected serines are enzymatically site-specifically ADP-ribosylated. This is because no enzymatic method is available for site-specifically adding an ADP-ribosyl group to a selected serine within a protein or peptide. Site-specific serine ADP-ribosylated proteins or peptides are important, for example, because they allow for the generation of antibodies which are specific for the ADP- ribosylation of a serine within a given protein or peptide. Since serine is the major residue for ADP-ribosylation upon DNA damage (Palazzo et al. (2018), eLife; 7:e34334) such antibodies have the potential of becoming a sensitive and specific marker of DNA single-strand break repair. A standard approach to prevent ADPr of selected serines is converting these serines into alanines (Figure 3). However, the final ADP-ribosylated product is not identical to naturally-occurring amino acid sequences with two or more serines, which is a drawback for downstream applications such as the generation of antibodies. This drawback is overcome by the present invention.

The technical problem underlying the present invention is therefore the provision of a method which allows the production of a protein or peptide that comprises two or more serines, wherein one or more selected serines are site-specifically ADP-ribosylated.

The solution to this technical problem is achieved by providing the embodiments characterized in the claims.

The present invention thus relates in a first aspect to a method for producing a site- specifically serine ADP-ribosylated protein or peptide, comprising: (a) subjecting a protein or peptide comprising or consisting of an amino acid sequence comprising two or more serines, wherein at least one serine is phosphorylated and at least one serine is non-phosphorylated, to a serine ADP-ribosylation reaction.

With respect to step (a) is to be understood that in this step the at least one non- phosphorylated serine becomes ADP-ribosylated whereas the at least one phosphorylated serine is protected from serine ADP-ribosylation. Thus, this step results in a site-specifically serine ADP-ribosylated protein or peptide. It is therefore preferred that the at least one serine being non-phosphorylated and thus to be site-specifically serine ADP-ribosylated has a free side chain (-CH ₂ -OH) carrying no chemical modification. It is more preferred that also the amino acids, in particular the side chains of the amino acids neighbouring the at least one serine being non-phosphorylated and thus to be site-specifically serine ADP-ribosylated carry no chemical modification, such as a post-translational modification. The neighbouring amino acids designate with increased preference one, two, three and four amino acids on each side of said at least one serine.

A site-specifically serine ADP-ribosylated protein or peptide shall designate a protein or peptide having an amino acid sequence comprising two or more serines, wherein at least one serine is ADP-ribosylated and at least one serine is not ADP-ribosylated.

The term“two or more serines” means, for example, 2, 3, 4 or 5. Similarly, the term“at least one” means, for example, 2, 3, 4 or 5.

The term“comprising” preferably means“consisting of.

The method of the invention generally does not result in the production of a single site- specifically serine ADP-ribosylated protein or peptide but in a plurality of site-specifically serine ADP-ribosylated proteins or peptides that are all charachterized by the same site- specific serine ADP-ribosylation pattern. The term“plurality” will be further defined herein below.

The terms “protein” (wherein “protein” is interchangeably used with “polypeptide”) and “peptide” as used herein describe a group of molecules consisting of amino acids. Whereas peptides consist of up to 30 amino acids,“proteins” consist of more than 30 amino acids. Peptides and proteins may further form dimers, trimers and higher oligomers, i.e. consisting of more than one molecule which may be identical or non-identical. The corresponding higher order structures are, consequently, termed homo- or heterodimers, homo- or heterotrimers etc. Peptides and proteins are preferably composed of the 20 naturally- occurring amino acids being encoded by the genetic code optionally plus selenocysteine. However, the peptides and proteins may also comprise one or more non-natural amino acids, noting that about 500 amino acids are known in the art. Any non-natural amino acid is preferably an a-amino acids (generic formula H ₂ NCHRCOOH, where R is an organic substituent known as a "side chain" of the amino acid).

Serine (2-Amino-3-hydroxypropanoic acid) is an a-amino acid that is used in the biosynthesis of proteins. The side chain of serine comprises a hydroxymethyl group, and serine is therefore a polar amino acid.

The term“phosphorylation” as used herein refers to the attachment of a phosphoryl group (- P0 ₃ ^2~ ) (also designated phosphate group herein) to a molecule. Protein and peptide phosphorylation is considered the most abundant post-translational modification in eukaryotes (Vlastaridis et al. (2017), GigaScience, 6:1-1 1 ). Phosphorylation can occur on serine, threonine and tyrosine side chains through phosphoester bond formation, on histidine, lysine and arginine through phosphoramidate bonds, and on aspartic acid and glutamic acid through mixed anhydride linkages. In accordance with the method of the first aspect of the invention phosphorylation occurs at least on one serine. Phosphorylation is reversible. Kinases phosphorylate proteins and peptides whereas phosphatases dephosphorylate proteins and peptides. Since phosphorylated serine is commercially available, proteins and peptides with two or more serines can also be synthesized with at least one phosphorylated serine.

Adenosine diphosphate (ADP; also known as adenosine pyrophosphate (APP)) is an important organic compound in cellular metabolism and is inter alia essential for the flow of energy in living cells. ADP consists of three structural components: a sugar backbone attached to a molecule of adenine and two phosphate groups bonded to the 5’-carbon atom of ribose. ADP-ribose is a related molecular species in which a second ribose is bound via its 5’-carbon to the second phosphate of ADP.

ADP-ribosylation (ADPr) is the addition of one or more ADP-ribose moieties to one or more amino acids of a protein or peptide. Originally, acidic amino acids (glutamate and aspartate) were described as the main sites of ADP-ribosylation. However, many other ADP-ribose acceptor sites such as serine, arginine, cysteine, lysine, diphthamide, phosphoserine, and asparagine have been identified in subsequent works. In accordance with the present invention the one or more amino acids to which one or more ADP-ribose moieties are added is/are serine(s). Said, ADP-ribosylation of serine is referred to herein as serine-ADP- ribosylation (Ser-ADPr). ADPr may be mono-ADP-ribosylation or poly-ADP-ribosylation. Serine-mono-ADP ribosylation is the addition of only one ADP-ribose to a serine side chain. Serine-poly-ADP ribosylation is the addition of two or more ADP-riboses to the side chain of a serine (O-linkage). The method of the present invention may employ wild-type poly-(ADP- ribose) polymerases (PARPs) that catalyse the transfer of a single or multiple ADP-ribose molecules to target proteins. Hence, the use of wild-type PARPs results in a combination of Ser-mono-ADPr and Ser-poly-ADPr. Means for obtaining exclusively Ser-mono-ADP- ribosylated proteins and peptides will be discussed herein below.

A serine ADP-ribosylation reaction therefore refers to a chemical reaction occurring in a reaction mixture which comprises the protein or peptide to be ADP-ribosylated and the elements required for the in vitro modification of serines by ADP-ribose. Suitable reaction mixtures and the required elements are known from the prior art referred to herein above. Further details will be provided herein below.

In accordance with the present invention the amino acid sequence of the protein or peptide to be modified comprises at least two serines, so that by the method of the present invention a site-specifically ADP-ribosylated protein or peptide is obtained. A serine can be at the C- terminus and/or the N-terminus of the protein or peptide. Preferably the serines are surrounded by other amino acids.

As is demonstrated in the examples herein below it has surprisingly been found by the inventors that the post-translational modification of serine by a phosphoryl group and an ADP-ribose are mutually exclusive. If a serine is phosphorylated it cannot be ADP- ribosylated and vice versa. For this reason a phosphoryl group on a serine serves as a protection group against the addition of an ADP-ribose in an ADP-ribosylation reaction. The phosphoryl group at the serine is stable throughout the ADP-ribosylation reaction and cannot be removed. This in turn allows the site-specific addition of ADP-ribose to selected serines within the amino acid sequence of a protein or peptide in an ADP-ribosylation reaction, wherein the one or more serines that shall not become ADP-ribosylated carry a phosphoryl group. For example, in the case of an amino acid sequence with three serines two serines may carry a phosphoryl group, so that only one selected serine becomes ADP-ribosylated, or only one serine may carry a phosphoryl group, so that two selected serines become ADP- ribosylated. The use of the phosphoryl group as a protection group for serines offers several advantages. Phosphorylated serine is commercially available, so that proteins and peptides can be synthesized by standard procedures and then directly subjected to the ADP- ribosylation reaction. Moreover, the phosphoryl group can be easily removed from the serines after the ADP-ribosylation reaction by dephosphorylation. Dephosphorylation is the removal of a phosphate group from an organic compound by hydrolysis. By dephosphorylation proteins and peptides may be obtained having an amino acid sequence identical to the respective natural-occurring proteins or peptides, expect that one or more selected serines thereof are serine ADP-ribosylated.

It can be expected that what is demonstrated herein below in the examples for the phosphoryl group on serines as protection group against ADP-ribosylation is transferable to other post-translational modifications, as well.

Hence, the present invention relates in a broader context also to a method for producing a site-specifically serine ADP-ribosylated protein or peptide, comprising: (a) subjecting a protein or peptide comprising or consisting of an amino acid sequence comprising two or more serines, wherein at least one serine contains a post-translational modification (PTM) different to ADPr and at least one serine does not contain any PTM, to a serine ADP- ribosylation reaction.

With respect to this step (a) is to be understood that the at least one serine not containing a PTM becomes ADP-ribosylated whereas the at least one serine with the PTM is protected from serine ADP-ribosylation. Thus, this step results in a site-specifically serine ADP- ribosylated protein or peptide. It is therefore preferred that the at least one serine without a PTM and thus to be site-specifically serine ADP-ribosylated has a free side chain (-CH ₂ -OH) carrying no chemical modification. It is more preferred that also the amino acids, in particular the side chains of the amino acids neighbouring the at least one serine without a PTM and thus to be site-specifically serine ADP-ribosylated carry no chemical modification, such as a PTM. The neighbouring amino acids designate with increased preference one, two, three and four amino acids on each side of said at least one serine.

The at least one serine containing a PTM preferably contains an O-linked PTM; i.e. a PTM which is covalently linked to the oxygen of the hydroxyl group within the side chain of serine. Also ADP-ribose and the phosphoryl group are linked to said oxygen, so that also other PTMs than a phosphoryl group being linked linked to said oxygen might serve as protection group against ADP-ribosylation in a serine ADP-ribosylation reaction.

Non-limiting and preferred examples of such O-linked PTMs are O-linked glycosylation and O-acylation.

The O-linked glycosylation is preferably selected from O-N-acetylgalactosamine, O-fucose, O-glucose, O-N-acetylglucosamine, and O-mannose.

O-linked glycosylation occurs in the Golgi apparatus in eukaryotes, and also in archaea and bacteria. The enzymes (glycosyltransferases) which mediate O-linked glycosylation in the Golgi apparatus are known and may be employed to generate a peptide comprising or consisting of an amino acid sequence comprising two or more serines which is to be site- specifically serine ADP-ribosylated, wherein at least one serine contains an O-linked glycosylation as PTM. However, it is preferred to synthesize the protein or peptide to be site- specifically serine ADP-ribosylated. Means and methods for the synthesis of a protein or peptide will be discussed in more detail herein below. During the synthesis one or more serines which contain a PTM can be used, so that a protein or peptide comprising or consisting of an amino acid sequence comprising two or more serines is obtained, wherein at least one serine contains a post-translational modification (PTM) different to ADPr and at least one serine does not contain any PTM. For instance, methods for the site-specific incorporation of glycosylated serine into proteins and peptides as well as for the generation of glycosylated serine-analogs are available (see, for example, Fahmi et al. (2007). Am. Chem. Soc., 129(12):3586-3597 and Quast et al. (2015), FEBS Letters, 589(15): 1703-1712).

Moreover, O-linked glycosylation is reversible, so that it is possible to remove O-linked glycosylation after the ADP-ribosylation reaction. Also the enzymes (e.g. b-N- acetylglucosaminidase) involved in the removal of O-linked glycosylation are known to the skilled person. Commercial kits for the removal of O-linked glycosylation are also available, for example, from prozyme. Hence, in case the PTM is O-linked glycosylation the above- method may further comprise as step (b) (after step (a)) removing the O-linked glycosylation from the at least one O-glycosylated serine.

In the following preferred embodiments of the first aspect of the invention and further aspects of the invention will be described. These preferred embodiments and the further aspects apply to the above broader method, as well. For instance, also step (a) of the braoder method may comprises the synthesis of the protein or peptide to be site-specifically serine ADP-ribosylated, wherein the synthesis preferably comprises a solid-phase peptide synthesis. The ADP-ribosylation reaction may be carried out as described herein below for the first aspect of the invention. Likewise the site-specifically serine ADP-ribosylated protein or peptide produced by the broader method may be purified as described herein below in connection with the first aspect of the invention. The broader method may additional comprise the formulation of site-specifically serine ADP-ribosylated protein or peptide as a composition (e.g. a pharmaceutical, diagnostic or cosmetic composition), as well. The site- specifically serine ADP-ribosylated protein or peptide produced by the broader method may comprise further PTMs as detailed herein below in connection with the first aspect of the invention.

Moreover, the invention as described herein also relates to a site-specifically serine ADP- ribosylated protein or peptide produced by the above broader method; a binding molecule (preferably an antibody) specifically binding to the a site-specifically serine ADP-ribosylated protein or peptide produced by the above broader method; and a composition (e.g. a pharmaceutical, diagnostic or cosmetic composition) comprising a site-specifically serine ADP-ribosylated protein or peptide produced by the above broader method.

The broader method may also be implemented into the method of the sixth aspect of the invention, wherein the above-described post-translational modification (PTM) different to ADPr may also be used instead of the phosphoryl group in order to protect a serine form ADP-ribosylation in a serine ADP-ribosylation reaction.

Turning now back to the first aspect of the invention, it is in accordance with a preferred embodiment of the first aspect of the invention that the method further comprises (b) removing the phosphate group(s) from the at least one phosphorylated serine.

As mentioned, by dephosphorylation proteins and peptides may be obtained having an amino acid sequence identical to the respective natural-occurring proteins or peptides, except that one or more selected serines thereof are serine ADP-ribosylated. In the context of the claimed method, step (b) follows step (a).

In accordance with a more preferred embodiment of the first aspect of the invention the phosphate group(s) is/are removed by an enzyme, preferably a phosphatase, more preferably an enzyme selected from the group consisting of protein serine/threonine phosphatases, and most preferably selected from the group consisting of the lambda protein phosphatase and alkaline phosphatase, wherein the alkaline phosphatase is preferably selected from calf intestinal phosphatase (CIP), antarctic phosphatase (AnP), and shrimp alkaline phosphatase (SAP).

As discussed above, phosphate groups are removed from serines by a hydrolysis reaction. Hydrolysis is generally the cleavage of biomolecules where a water molecule is consumed to effect the separation of a larger molecule into component parts. The hydrolysis reaction is often catalyzed by enzymes, so called hydrolases.

A phosphatase is an enzyme that uses water to cleave a phosphoric acid monoester into a phosphate ion and an alcohol. Because a phosphatase enzyme catalyzes the hydrolysis of its substrate, it is a subcategory of hydrolases. The phosphatase is a protein phosphatase and able to remove a phosphate group from the phosphorylated amino acid residue of its substrate protein.

Serine/threonine phosphatases are protein phosphatases that can dephosphorylate serines and threonines. This is because serines and threonines have similar side-chain compositions that contain a hydroxyl group and thus may be dephosphorylated by the same enzymes.

The lambda protein phosphatase is a Mn ^z+ -dependent protein phosphatase with activity towards phosphorylated serine, threonine and tyrosine residues. It can be used to release phosphate groups from phosphorylated serine, threonine and tyrosine residues in proteins and peptides.

Antarctic phosphatase (AnP) is a heat labile alkaline phosphatase purified from a recombinant source. AnP inter alia hydrolyses ribo-, as well as deoxyribonucleoside triphosphates (NTPs and dNTPs).

Akaline phosphatase (ALP) is found across a multitude of organisms, including prokaryotes and eukaryotes, with the same general function but in different structural forms suitable to the environment they function. The akaline phosphatase inter alia hydrolyses ribo-, as well as deoxyribonucleoside triphosphates (NTPs and dNTPs).

Shrimp alkaline phosphatase (SAP) is a heat labile alkaline phosphatase that ca be purified from its natural source or also a recombinant source (rSAP). Rapid and irreversible heat inactivation eliminates unwanted activity.

The akaline phosphatase is preferably calf intestinal phosphatase (CIP), antarctic phosphatase (AnP), and shrimp alkaline phosphatase (SAP).

It is also possible to combine enzymes, for example, CIP and antarctic phosphatase. Antarctic phosphatase has a very high specific activity comparable to CIP, but CIP cannot be completely heat inactivated. Antarctic phosphatase is heat inactivated in minutes, but requires added zinc as co-factor. Hence, the exact properties of the enzymes may be used to adjust the dephosphorylation reaction.

In accordance with a further preferred embodiment of the first aspect of the invention step (a) comprises the synthesis of the protein or peptide as defined in the method of the invention, wherein the synthesis preferably comprises a solid-phase peptide synthesis.

As mentioned, phosphorylated serine is commercially available, so that phosphorylated proteins and peptides to be site-specifically serine ADP-ribosylated can be synthesized by standard procedures and then directly subjected to the ADP-ribosylation reaction. Accordingly, this synthesis may form part of the method of the invention, e.g., as step (a)(i) prior to the ADP-ribosylation, which is then step (a)(ii).

The synthesis of proteins and peptides including proteins and peptides, wherein one or more selected serines are phosphorylated are art-established methods. In these methods multiple amino acids are linked via amide bonds, also known as peptide bonds, in order to generate the amino acid sequence forming the desired protein or peptide. The synthesis generally comprises the condensation reaction of the carboxyl group of one amino acid to the amino group of another. Protecting group strategies are usually necessary to prevent undesirable side reactions with the various amino acid side chains.

The chemical synthesis of peptides can be carried out using classical solution-phase techniques, although these have been replaced in most research and development settings by solid-phase methods. The currently most used method for the production of synthetic proteins and peptides in the lab is solid-phase peptide synthesis. The basic concept in solid phase peptide synthesis is the step-wise construction of a peptide chain attached to an insoluble polymeric support. This approach permits unreacted reagents to be removed by washing without loss of product. Synthesis of the peptide chain proceeds from the carboxyl end to the amino terminus of the peptide. The carboxyl moiety of each incoming amino acid is activated by one of several strategies and couples with the a-amino group of the preceding amino acid. The a-amino group of the incoming residue is temporarily blocked in order to prevent peptide bond formation at this site. The residue is de-blocked at the beginning of the next synthesis cycle. In addition, reactive side chains on the amino acids are modified with appropriate protecting groups. The peptide chain is extended by reiteration of the synthesis cycle. Excess reagents are used to drive reactions as close to completion as possible. This generates the maximum possible yield of the final product. After fully assembling the peptide the side-chain protecting groups are removed, and the peptide is cleaved from the solid support, using conditions that inflict minimal damage on labile residues.

Also longer peptide chains and proteins may be generated by chemical ligating smaller peptides. Moreover, split inteins, spontaneous isopeptide bond formation and sortase ligation may be used to generate longer peptide chains and proteins (Banerjee and Howarth (2018), Current Opinion in Biotechnology, 51(16-23)).

In accordance with another preferred embodiment of the first aspect of the invention the ADP-ribosylation reaction is carried out in an ADP-ribosylation reaction mixture, said mixture comprising (i) a buffered solution, (ii) NAD+, (iii) PARP-1 , PARP-2 or the PARP-1 variant E988Q, (iv) HPF1 , (v) sonicated DNA, said sonicated DNA preferably comprising DNA fragments of 10 to 330 bp, and (vi) the protein or peptide as defined in the method of the invention.

As discussed above, the required reaction mixture and elements for the in vitro modification of serines by ADP-ribose are known from the prior art (Bonfiglio et al., 2017, Mol Cell 65, 932-940 e936). Accordingly, for producing serine ADP-ribosylated proteins or peptides a buffered solution, NAD ⁺ , one of PARP-1 , PARP-2 and PARP-1 E988Q, HPF1 , sonicated DNA, and the substrate proteins or peptides to be serine ADP-ribosylated are required.

The buffered solution comprises preferably 5 to 60 mM, more preferably 10 to 60 mM of a buffer. The nature of the buffer will be further defined herein below. Moreover, the buffered solution preferably has a pH between 5.0 and 9.0, more preferably between 5.5 to 8.5, and most preferably between 6.1 to 8.3. Preferably 0.2 to 2.5 mM NAD ⁺ , at least 50 nM (more preferably 50 to 3000 nM) PARP-1 , PARP-2 or the PARP-1 variant E988Q, at least 100 nM (more preferably 100 to 5000 nM) HPF1 , at least 10 pg/mL (more preferably 10 pg/mL to 200 pg/mL) sonicated DNA, said sonicated DNA preferably comprising DNA fragments of 10 to 330 bp, and up to 600 pM protein or peptide are comprised in the reaction mixture. Using such a buffered solution and the reactants in the indicated concentrations results in the very efficient ADP-ribosylation of serines in proteins or peptides, unless protected by phosphorylation.

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells. The compound is a dinucleotide consisting of an adenine base and nicotinamide. NAD exists in two forms, an oxidized and reduced form abbreviated as NAD ⁺ and NADH respectively. The NAD ⁺ used in the method of the invention can be radioactive NAD ⁺ . The use of radioactive NAD ⁺ results in the radioactive labelling of serine ADP-ribosylated peptides and proteins. Herein, the concentration of 0.2 to 2.5 mM NAD ⁺ is preferably a concentration of 1.5 to 2.5 mM NAD ⁺ and more preferably a concentration of 1.8 to 2.2 mM NAD ⁺

Poly (ADP-ribose) polymerase (PARP) is a family of proteins involved in a number of cellular processes, such as DNA repair, genomic stability, and programmed cell death. Currently the PARP family comprises 17 members (10 putative). The method of the invention employs PARP-1 , PARP-2 or the PARP-1 variant E988Q. PARP-1 (also known as NAD ⁺ ADP- ribosyltransferase 1 or poly[ADP-ribose] synthase 1 ) is an enzyme that in humans is encoded by the PARP-1 gene and PARP-2, an enzyme that in humans is encoded by the PARP-2 gene. PARP-1 and PARP-2 contain a catalytic domain and are capable of catalyzing a poly(ADP-ribosyl)ation reaction. PARP-2 has a catalytic domain being homologous to that of PARP-1 , but PARP-2 lacks the N-terminal DNA binding domain of PARP-1 which activates the C-terminal catalytic domain of PARP-1. For the action of PARP-1 and PARP-2, NAD ⁺ is required as substrate for generating ADP-ribose monomers, as is sonicated DNA, said DNA mimicking DNA with breaks. The PARP-1 E988Q mutant is incapable of poly(ADP- ribosyl)ation activity but instead mono(ADP-ribosyl)ates (Sharifti et al., EMBO J. 2013 May 2; 32(9): 1225-1237). Since the PARP enzyme is the most sensitive compound of the method it is preferably added last to the aqueous buffered solution.

The amino acid sequence of human PARP-1 is shown in SEQ ID NO: 1 , of human PARP-1 E988Q in SEQ ID NO: 2, of mouse PARP-1 , isoforms 1 and 2 in SEQ ID NOs 3 and 4 and of rat PARP-1 in SEQ ID NO: 5. The sequence identity of human PARP-1 with mouse and rat PARP-1 is 92%. Accordingly, PARP-1 is preferably a sequence being at least 90% identical to SEQ ID NO: 1. PARP-1 is more preferably a sequence being with increasing preference at least 95%, at least 98% or at least 99% identical to any one of SEQ ID NOs 1 to 5.

The amino acid sequences of human PARP-2, isoforms 1 and 2 are shown in SEQ ID NOs 6 and 7, and of mouse PARP-2 in SEQ ID NO: 8. The sequence identity of human PARP-2 with mouse PARP-2 is 84%. Accordingly, PARP-2 is preferably a sequence being at least 80%, preferably at least 84% identical to SEQ ID NO: 6 or 7. PARP-2 is more preferably a sequence being with increasing preference at least 95%, at least 98% or at least 99% identical to any one of SEQ ID NOs 6 to 8.

In accordance with the present invention, the term“percent (%) sequence identity” describes the number of matches (“hits”) of identical amino acids of two or more aligned amino acid sequences as compared to the number of amino acid residues making up the overall length of the template amino acid sequences. In other terms, using an alignment, for two or more sequences or subsequences the percentage of amino acid residues that are the same (e.g. 80%, 85%, 90% or 95% identity) may be determined, when the (sub)sequences are compared and aligned for maximum correspondence over a window of comparison, or over a designated region as measured using a sequence comparison algorithm as known in the art, or when manually aligned and visually inspected. The sequences which are compared to determine sequence identity may thus differ by substitution(s), addition(s) or deletion(s) of amino acids.

The skilled person is also aware of suitable programs to align amino acid sequences. The percentage sequence identity of amino acid sequences can, for example, be determined with programmes such as CLUSTLAW, FASTA and BLAST. Preferably the BLAST programme is used, namely the NCBI BLAST algorithm (Stephen F. Altschul, Thomas L. Madden, Alejandro A. Schaffer, Jinghui Zhang, Zheng Zhang, Webb Miller, and David J. Lipman (1997), "Gapped BLAST and PSI-BLAST: a new generation of protein database search programs", Nucleic Acids Res. 25:3389-3402).

PARP-1 and -2 are activated by DNA breaks and cleave NAD ⁺ thereby generating nicotinamide and ADP-ribose. In higher eukaryotes, PARP-1 and -2 translate the occurrence of DNA breaks detected by its zinc-finger domain into a signal (poly ADP-ribose) which is synthesized and amplified by its DNA-damage dependent catalytic domain. Hence, the ADP- ribosylation activity of PARP-1 and -2 requires the presence of DNA breaks. DNA breaks are present in sonicated DNA. Sonicated DNA is DNA that has been fragmented by sonication. Sonication is the act of applying brief periods of ultrasound energy to DNA, so that the DNA is sheared into smaller fragments, preferably fragments of 10 to 330 bp.

Histone PARylation factor 1 (HPF1 ) acts as a cofactor for serine ADP-ribosylation by conferring serine specificity on PARP-1 and PARP-2. Quantitative proteomics revealed that histone serine ADPr does not occur in cells lacking HPF1. Moreover, adding HPF1 to in vitro PARP-1 /PARP-2 reactions is necessary and sufficient for serine-specific ADPr of histones and PARP-1 itself (Bonfiglio et al., 2017, Mol Cell 65, 932-940 e936).

The amino acid sequence of human HPF1 is shown in SEQ ID NO: 9 and of mouse HPF1 in SEQ ID NO: 10. The sequence identity of human HPF1 with mouse HPF1 is 89%. Accordingly, HPF1 is preferably a sequence being at least 85%, preferably at least 89% identical to SEQ ID NO: 9. HPF1 is more preferably a sequence being with increasing preference at least 95%, at least 98% or at least 99% identical to SEQ ID NO: 9 or 10.

The up to 600 mM protein or peptide, said protein or peptide comprising at least two serines, is preferably between 60 and 600 mM protein or peptide, said protein or peptide comprising at least two serines, and is more preferably between 100 and 600 pM protein or peptide, said protein or peptide comprising at least two serines.

In accordance with a more preferred embodiment of the first aspect of the invention, the reaction mixture further comprises at least 1 pM, preferably 1 to 10 pM PARG and/or the ADP-ribosylated protein or peptide is incubated with at least 1 pM, preferably 1 to 10 pM PARG.

In accordance with this preferred embodiment a site-specifically serine mono-ADP- ribosylated protein or peptide is obtained.

Poly(ADP-ribose) glycohydrolase (PARG) is an enzyme which generates free ADP-ribose from the poly-ADP-ribose chain but cannot cleave the ADP-ribose-protein bond, therefore leading to a serine mono-ADP-ribosylated protein or peptide. Hence, the simultaneous addition of PARG to the reaction mixture provides a means for reducing Ser-poly-ADPr into single Ser-mono-ADPr. The amino acid sequences of human PARG, isoforms 1 , 2 and 3 are shown in SEQ ID NOs 11 , 12 and 13, of mouse PARG, isoforms 1 and 2 in SEQ ID NOs 14 and 15 and of rat PARG in SEQ ID NO: 16. The sequence identity of human PARG with mouse and rat PARG is 87% and 85%, respectively. Accordingly, PARG is preferably a sequence being at least 85% identical to one or more of SEQ ID NOs 11 , 12 and 13. PARG is more preferably a sequence being with increasing preference at least 95%, at least 98% or at least 99% identical to any one of SEQ ID NOs 1 1 to 16.

An alternative strategy the PARP-1 mutant E988Q could be employed by using, which only mono-ADP-ribosylates and, therefore, produces Ser-mono-ADPr. Hence, in case PARG is present it is preferred to use PARP-1 or PARP-2 instead of the PARP-1 mutant E988Q.

In accordance with another more preferred embodiment of the first aspect of the invention, the ADP-ribosylation reaction is carried out at 15-35°C and preferably at room temperature.

The ADP-ribosylation reaction is very robust and works at temperatures that might be found in a laboratory, i.e. 15-35°C. Room temperature preferably refers to a temperature range of 18-27°C and more preferably 20-25°C.

In accordance with a further more preferred embodiment of the first aspect of the invention, the ADP-ribosylation reaction is carried out for at least 90 min, preferably at least 120 min, more preferably at least 240 min, and most preferably at least 320 min.

Carrying out the reaction for at least 90 min ensures the effective ADP-ribosylation of the unprotected target serine/s of the protein or peptide. In particular for longer proteins/peptides carrying many serines that are to be ADP-ribosylation it may be advantageous to prolong the reaction time to at least 120 min or at least 240 min or at least 320 min.

In accordance with a more preferred embodiment of the first aspect of the invention, a fresh pool of 0.2 to 2.5 mM NAD ⁺ is added to the reaction mixture at least every 90 minutes, preferably at least every 60 minutes and more preferably at least every 45 minutes.

Also here the concentration of 0.2 to 2.5 mM NAD ⁺ is preferably a concentration of 1.5 to 2.5 mM NAD ⁺ and more preferably a concentration of 1.8 to 2.2 mM NAD ⁺ . Also described herein is a concentration of 1 to 2 mM NAD ⁺ . Connected with the reaction time, the addition of a fresh pool of NAD ⁺ was tested during the optimization of the conditions of the Ser-ADP- ribosylation of proteins or peptides. It was found that it is preferred to add fresh NAD ⁺ to the solution constituting the reaction mixture at least every 90 minutes in order to effectively ADP-ribosylate the target serine/s of the protein or peptide. In particular for longer substrates, it may be advantageous to add fresh NAD ⁺ to the reaction mixture at least every 60 minutes or 45 minutes. After the reaction has been carried out it can be stopped by the addition of a PARP inhibitor. The PARP inhibitor is preferably Oiaparib, and Olaparib is preferably used at a concentration of about 2pm. In this connection the term“about” is preferably ±20% and more preferably ±10%. Another example of a PARP inhibitor is Rucaparib.

In accordance with another more preferred embodiment of the first aspect of the invention, the at least two serines are each neighboured by at least one basic amino acid.

The at least one basic amino acid is with increasing preference at least two, at least three and at least four basic amino acids. The basic amino acids are preferably Lys and/or Arg. The term“neighboured” means that at least one basic amino acid can be found up to three amino acids adjacent to the serine to be ADP-ribosylated, either N-terminally or C-terminally (i.e. at amino acid positions -3 to +3). Non-limiting examples of such a motif are“KAASAAA” and“AAASARA”. The at least one basic amino acid is preferably up to two amino acids adjacent to the serine to be ADP-ribosylated (i.e. at amino acid positions -2 to +2) and more preferably directly adjacent to the serine (i.e. at amino acid positions -1 and/or +1 ). The motif “KS” (i.e. Lysine at the -1 amino acid position) is most preferred since it can be frequently found in nature. Among all these options it is preferred that the serine is neighboured on both sides by at least one basic amino acid.

Related to the above more preferred embodiment of the first aspect of the invention, it is preferred that the protein or peptide to be ADP-ribosylated has an isoelectric point of at least 8.0.

The isoelectric point is the pH at which a particular molecule carries no net electrical charge in the statistical mean. Among the naturally-occurring amino acids the basic amino acids Lys and Arg have the highest isoelectric point. The isoelectric point of Lys is 9.74 and the isoelectric point of Arg is 10.76. Hence, a protein or peptide having an isoelectric point of at least 8.0 is a protein or peptide that carries no net electrical charge at a basic pH of 8.0 or higher.

Within the proteins or peptides the serines being neighboured by at least one basic amino acid are particularly well recognized by the Ser-ADP-ribosylation machinery. Also particularly well recognized by the Ser-ADP-ribosylation machinery are proteins and peptides having an overall basic isoelectric point of at least 8.0. The same holds true for the following preferred embodiment. In accordance with a yet further preferred embodiment of the first aspect of the invention, the peptide or protein to be site-specifically ADP-ribosylated contains a positively charged tail, preferably a poly-arginine and/or poly-lysine tail.

The term“positively charged tail” refers to a C- or N-terminal tail of the peptide or protein to be site-specifically Ser-ADPr, which comprises or consists of at least 3 basic amino acids, preferably Lys and/or Arg residues. The positively charged tail preferably comprises or consists of at least 4 basic amino acids, preferably Lys and/or Arg residues. The addition of a positively charged tail may further boost the reaction efficiently.

In accordance with a yet further more preferred embodiment, the reaction mixture further comprises (e) 10 to 80 mM NaCI or KCI, and/or (f) 0.5 to 2 mM MgCI ₂ .

The activity of enzymes may be affected by the addition of inorganic salts, e.g., during in vitro assays. The concentration of salts, the identity of the ions, and the ionic strength of the solution can affect the activity of an enzyme. The enzymatic activity of PARP does not require the presence of inorganic salts, so that peptides and proteins can be highly efficiently Ser-ADP-ribosylated even in the absence of salts in the aqueous buffered solution. However, it is also possible to use PARP in connection with inorganic salts. A non-limiting example of inorganic salt conditions is the use of 10 to 80 mM NaCI or KCI, and/or 0.5 to 2 mM MgCI ₂ in the aqueous buffered solution.

In accordance with a more preferred embodiment, the buffer is a Tris-HCI buffer and is preferably used at a final concentration of 40-60 mM, a Hepes buffer and is preferably used at a final concentration of 40-60 mM, more preferably about 50 mM, or a phosphate buffer and is preferably used at a final concentration of 5-20 mM, more preferably about 10 mM.

Tris has a pKa of 8.07 at 25 °C, which means that Tris-HCI buffer has an effective pH range between 7.5 and 9.0. The useful buffer range for Tris-HCI (7.5 to 9) coincides with the physiological pH typical of most living organisms. This and its low cost make Tris-HCI one of the most common buffers in the biology/biochemistry laboratory. In this respect it is noted that the pH of a solution is defined as the negative logarithm to the base 10 of the hydrogen ion concentration in g ions L ¹ (pH = - Iog10 [H ⁺ ]). Thus pH can also be defined as the logarithm to the base 10 of the reciprocal of H ⁺ ion concentration. During the ionization of weak acid HA, where the ionization is not complete, the dissociation constant is the ratio of the dissociated and undissociated components (Ka = [H ⁺ ]x[A ^~ ]/[HA]). Hence, pKa is defined as the negative logarithm to the base 10 of the Ka in g ions L ¹ or as logarithm to the base 10 of the reciprocal of Ka. The relationship between pH and pKa is given by Henderson- Hasselbach equation (pH = pKa + log [A ]/[HA]).

Hepes (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) is a zwitterionic organic chemical buffering agent. Hepes has a p _a1 (25 C) = 3 and a p K _a2 (25 C) = 7.5 and accordingly has buffering capacity at pH ranges between 2.5 to 3.5 and 6.8 to 8.2. The latter pH range coincides with the physiological pH typical of most living organisms.

Phosphates have a very high buffering capacity and are highly soluble in water. Gomori buffers, the most commonly used phosphate buffers, consist of a mixture of monobasic dihydrogen phosphate and dibasic monohydrogen phosphate. By varying the amount of each salt, a range of buffers can be prepared that buffer well between pH 5.8 and 8.0.

In accordance with another preferred embodiment, the reaction mixture is an aqueous buffered solution, wherein the solvent of the aqueous buffered solution is preferably water.

An aqueous buffered solution is a solution in which one of the solvents is water. In this respect it is preferred that water is the only solvent so that the aqueous solution is water.

In accordance with a preferred embodiment of the first aspect of the invention, the method further comprises purifying the site-specifically serine ADP-ribosylated protein or peptide.

Once the method of the present invention has been carried out, the reaction mixture comprises the site-specifically serine ADP-ribosylated proteins or peptides and in addition the ingredients needed for the ADP-ribosylation reaction. In accordance with the above preferred embodiment the site-specifically Ser-ADP-ribosylated proteins or peptides are separated from these ingredients and thereby purified. Means and methods for purifying the site-specifically serine ADP-ribosylated proteins or peptides from the reaction mixture are known in the art and preferred examples will be further detailed herein below.

In accordance with a preferred embodiment of the first aspect of the invention, the site- specifically serine ADP-ribosylated protein or peptide is purified by StageTip fractionation employing C8, C18, SCX, SAX or SDB-RPS chromatography media, cation or anion exchange chromatography, hydrophilic interaction chromatography, phosphopeptide enrichment, enrichment with an ADP-ribose-binding protein domain, boronate affinity chromatography, filtering the reaction with an ultrafiltration device, a spin column or a combination thereof. Stage tips can be used to rapidly purify proteins and peptides from a reaction mix. C8, C18, SAX and SCX stage tips are commercially available. C8, C18, SAX, SCX and SDB-RPS are chromatography materials that are loaded into pipet tips. C8 and C18 are silica-based materials. SCX and SAX refer to strong cation exchange and strong anion exchange resins and membranes, respectively. SDB-RPS is a styrenedivinylbenzene resin that has been modified with sulfonic acid groups to make it hydrophilic. Because SDB-RPS displays both reversed phase and cation exchange interactions, both affinities can be considered to design selective extractions.

Cation and anion exchange chromatography are forms of ion exchange chromatography, which can be used to separate molecules based on their net surface charge. Cation exchange chromatography uses a negatively charged ion exchange resin with an affinity for molecules having net positive surface charges. Anion-exchange chromatography is a process that separates substances based on their charges using an ion-exchange resin containing positively charged groups, such as diethyl-aminoethyl groups (DEAE). Cation and anion exchange chromatography are used both for preparative and analytical purposes and can separate a large range of molecules, including serine ADP-ribosylated proteins or peptides. Depending on the overall charge of the proteins or peptides cation and anion exchange chromatography is used.

Hydrophilic interaction chromatography (or hydrophilic interaction liquid chromatography, H!LIC) is a variant of normal-phase liquid chromatography that partly overlaps with other chromatographic applications such as ion chromatography and reversed phase liquid chromatography. HILIC uses hydrophilic stationary phases with reversed-phase type eluents.

Phosphopeptide enrichment enables efficient enrichment of phosphorylated peptides and proteins from complex and fractionated protein digests. Since the ADP-ribosylated protein or peptide contains phosphate groups, phosphopeptide enrichment can be used for its purification. For example, Ti0 ₂ and Fe-NTA phosphopeptide enrichment kits are commercially available.

ADP-ribose-binding domains are domains of proteins that bind to ADP-ribose moieties (Vivelo and Leung (2015), Proteomics. 2015 Jan; 15(0): 203-217). Thus, these domains can be used to purify ADP-ribosylated proteins or peptides. Several protein domains that bind mono- and poly(ADP)-ribose are known, such as the WWE domain, the PBZ (PAR-binding zinc finger) domain, the PBM (PAR-binding motif), the Af1521 macrodomain, and the catalytically inactive E756D mutant of PARG (PARG-DEAD approach).

Boronate affinity chromatography is a unique means for selective separation and enrichment of cis-diol-containing compounds. Cis-diol-containing biomolecules are an important class of compounds, including glycoproteins, glycopeptides, ribonucleosides, ribonucleotides, saccharides, and catecholamines.

Ultrafiltration devices are commercially available for the concentration of biological samples. They can be used, for example, in either a swing bucket or fixed angle rotors which accept 2.0 ml. centrifuge tubes at maximum speed 10,000 x g. During centrifugation the serine ADP-ribosylated peptides flow through the filter whereas other ingredients of the reaction mixture, such as PARP and HPF are retained on the filter.

Spin columns, such as the G25 spin columns are microspin columns that were initially designed for the rapid purification of DNA for use in a wide range of applications, including desalting, buffer exchange, and removal of unincorporated nucleotides from end-labelled oligonucleotides. When used in connection with serine ADP-ribosylated proteins or peptides as comprised in the reaction mixture other ingredients of the reaction mixture, such as NAD ⁺ , can be removed.

In accordance with a further preferred embodiment of the first aspect of the invention, the method further comprises formulating the produced site-specifically serine ADP-ribosylated protein or peptide into a composition, preferably a pharmaceutical, diagnostic or cosmetic composition.

A composition comprises the produced site-specifically serine ADP-ribosylated proteins or peptides formulated together with at least one further compound. Accordingly,“formulating” means bringing together, preferably mixing the produced site-specifically serine ADP- ribosylated proteins or peptides and at least one further compound.

In accordance with the present invention, the term“pharmaceutical composition” relates to a composition for administration to a patient, preferably a human patient. As mentioned, the pharmaceutical composition of the invention comprises the produced site-specifically serine ADP-ribosylated proteins or peptides. It may, optionally, comprise further molecules capable of altering the characteristics of said proteins or peptides thereby, for example, stabilizing, modulating and/or activating their function. The composition may be in solid, liquid or gaseous form and may be, inter alia, in the form of (a) powder(s), (a) tablet(s), (a) solution(s) or (an) aerosol(s). The pharmaceutical composition of the present invention may, optionally and additionally, comprise a pharmaceutically acceptable carrier. Examples of suitable pharmaceutical carriers are well known in the art and include phosphate buffered saline solutions, water, emulsions, such as oil/water emulsions, various types of wetting agents, sterile solutions, organic solvents including DMSO etc. Compositions comprising such carriers can be formulated by well known conventional methods. These pharmaceutical compositions can be administered to the subject at a suitable dose. The dosage regimen will be determined by the attending physician and clinical factors. As is well known in the medical arts, dosages for any one patient depends upon many factors, including the patient's size, body surface area, age, the particular compound to be administered, sex, time and route of administration, general health, and other drugs being administered concurrently. The therapeutically effective amount for a given situation will readily be determined by routine experimentation and is within the skills and judgement of the ordinary clinician or physician.

The diagnostic composition of the invention is useful in the detection of an undesired physiological condition or a disease. Accordingly, the diagnostic composition of the invention may be used for assessing the onset or the disease status and in particular a disease state. The diagnostic composition of the invention can be administered as sole active agent or can be administered in combination with other agents, if the diagnostic composition is, for example, used to identify sites of undesired physiological condition or a disease.

A cosmetic composition according to the invention is for use in non-therapeutic applications. Cosmetic compositions may also be defined by their intended use, as compositions intended to be rubbed, poured, sprinkled, or sprayed on, or otherwise applied to the human body for cleansing, beautifying, promoting attractiveness, or altering the appearance. The particular formulation of the cosmetic composition according to the invention is not limited. Envisaged formulations include rinse solutions, emulsions, creams, milks, gels such as hydrogels, ointments, suspensions, dispersions, powders, solid sticks, foams, sprays and shampoos. For this purpose, the cosmetic composition according to the invention may further comprise cosmetically acceptable diluents and/or carriers. Choosing appropriate carriers and diluents in dependency of the desired formulation is within the skills of the skilled person. Suitable cosmetically acceptable diluents and carriers are well known in the art and include agents referred to in WO 2006/053613. Preferred formulations for said cosmetic composition are rinse solutions and creams.

In accordance with another preferred embodiment of the first aspect of the invention, the protein or peptide further comprises at least one post-translational modification other than serine ADP-ribosylation and serine phosphorylation.

A post-translational modification (PTM) refers to a naturally-occurring covalent and generally enzymatic modification of proteins following protein biosynthesis. Post-translational modifications can occur on the amino acid side chains or at the protein's C- or N- termini of a protein or peptide. They can extend the chemical repertoire of the 20 naturally-ocurring amino acids by modifying an existing functional group or introducing a new one, such as phosphate. A post-translational modification may be introduced into the protein or peptide as defined in the method of the invention by the use of an enzyme but also synthetically during protein synthesis as describe herein above for phosphorylated serine.

The at least one post-translational modification may be added to the protein or peptide as defined in the method of the invention before or after the serine ADP-ribosylation reaction. In case the at least one post-translational modification is added before it can be added, for example, during the synthesis of the protein or peptide as described herein above in connection with the addition of the at least one phosphorylated serine.

The post-translational modification is not particularly limited, unless it may reduce or even prevent the serine ADP-ribosylation of the at least one serine which is to be serine ADP- ribosylated in the method of the invention. For this reason it is preferred that said at least one serine to be serine ADP-ribosylated does not comprise any additional post-translational modification. As mentioned above, it is in particular preferred that the at least one serine to be serine ADP-ribosylated has a free side chain (-CH ₂ -OH) carrying no chemical modification.

In accordance with a more preferred embodiment of the first aspect of the invention, the post-translational modification is selected from lipidation, N- or O-linked glycosylation, phosphorylation of an amino acid other than serine, acetylation, amidation, hydroxylation, mono- or di- or tri-methylation, ubiquitylation, SUMOylation, neddylation, butyrylation, propionylation, crotonylation, 2-hydroxyisobutyrylation, malonylation, succinylation, citrullination and pyrrolidone carboxylic acid and sulfation.

Within the above list of post-translational modification a selection from the subgroup consisting of N- or O-linked glycosylation, phosphorylation of an amino acid other than serine, acetylation, mono- or di- or tri-methylation, ubiquitylation, SUMOylation, neddylation, butyrylation, propionylation, crotonylation, 2-hydroxyisobutyrylation, malonylation, succinylation, and citrullination (R) is preferred. These post-translational modifications are the most frequent post-translational modifications as were reported in the prior art (see http://selene.princeton.edu/PTMCuration/).

In accordance with a preferred embodiment of the first aspect of the invention, the method is carried out ex vivo or in vitro.

Ex vivo and in vitro are carried out outside the human oir animal body and do not comprise methods for treatment of the human or animal body by surgery or therapy and diagnostic methods practised on the human or animal body.

The present invention relates in a second aspect to a site-specifically serine ADP-ribosylated protein or peptide produced by the method of the first aspect of the invention.

As discussed herein above, the method of the first aspect of the invention allows for the first time to control which serine(s) in a protein and peptide with two or more serines become(s) ADP-ribosylated and which not by enzymatic means.

For this reason the method of the first aspect of the invention allows for the first time to enzymatically generate site-specifically serine ADP-ribosylated proteins or peptides. Hence, the present invention is directed in a second aspect to these site-specifically serine ADP- ribosylated protein or peptide.

The site-specifically serine ADP-ribosylated protein or peptide is preferably not present as a single, isolated site-specifically serine ADP-ribosylated protein or peptide but is a plurality of site-specifically serine ADP-ribosylated proteins or peptides, wherein each copy of the protein or peptide within the plurality has the site-specific serine ADP-ribosylation pattern as generated by the method of the first aspect of the invention. In this respect the term plurality means with increasing preference at least 10 copies, at least 100 copies, at least 1000 copies, at least 10000 copies, and at least 100000 copies of the discussed protein or peptide.

The present invention relates in a third aspect to a site-specifically serine ADP-ribosylated protein or peptide comprising or consisting of (i) the amino acid sequence of any one of SEQ ID NOs 17 to 309, wherein within the amino acid sequence at least one serine is ADP- ribosylated and at least one serine is not ADP-ribosylated, and wherein the at least one serine not being ADP-ribosylated is optionally phosphorylated, or (ii) an amino acid sequence being at least 80%, preferably at least 85%, more preferably at least 90% and most preferably at least 95% identical to the amino acid sequence of (i), provided that at least two, preferably all the serines within the amino acid sequence of (i) are conserved.

Also described herein is a site-specifically serine ADP-ribosylated protein or peptide comprising or consisting of an amino acid sequence which differs by the amino acid sequence of (i) by up to three, preferably up two and most preferably by one amino acid substitution, deletion or insertion, provided that at least two, preferably all the serines within the amino acid sequence of (i) are conserved. In this context a substitution is preferred over a deletion or insertion. The substitution is preferably a conservative substitution, noting that a conservative substitution is an amino acid replacement that changes a given amino acid to a different amino acid with similar biochemical properties (e.g. charge, hydrophobicity and size).

For example, with respect to a protein or peptide comprising or consisting of the amino acid sequence of SEQ ID NO: 17, the serine in amino acid position 6 of SEQ ID NO: 17 is ADP- ribosylated whereas the serine in amino acid in position 14 of SEQ ID NO: 17 is not ADP- ribosylated or vice versa, and the serine not being ADP-ribosylated can optionally be phosphorylated. The production of a peptide consisting of SEQ ID NO: 17, wherein the serine in amino acid position 6 of SEQ ID NO: 17 is ADP-ribosylated whereas the serine in amino acid position 14 of SEQ ID NO: 1 is not ADP-ribosylated due to the protection by a phosphate group is illustrated herein below in Example 1.

SEQ ID NOs 17 to 309 are all amino acid sequences being a part of a naturally-occurring protein. All amino acid sequences comprise at least two serines.

For example, SEQ ID NO: 17 is a fragment of the enzyme PARP-1 , i.e. amino acids positions 494 to 514 of the human PARP-1 amino acid sequence. Ser499 of PARP-1 is the main endogenous ADPr site upon DNA damage. Hence, a protein or peptide of SEQ ID NO: 17 with a serine ADPr in amino acid position 6 and no serine ADPr in amino acid position 14 may be used to generate antibodies or other binding molecules specifically recognizing this protein or peptide carrying this modification. Such an antibody or other binding molecule may then be used to detect DNA damage. SEQ ID NOs 18 (amino acid position 505-523), 19 (amino acid position 103-125) and 20 (amino acid positions 200-234) are different fragments of the enzyme PARP-1.

SEQ ID NOs 21 to 25 are all amino acid sequences of 25 amino acids derived from histone 3 variants. The histone 3 peptides are highly conserved and amino acid differences can only be found in positions 18 (A or V), 23 (A or T) and 25 (A or S). Hence, SEQ ID NOs 21 to 25 share among each other a sequence identity of above 80%. SEQ ID NOs 26 and 27 are amino acidc sequences being derived from histone 2B and histone 4, respectively. SEQ ID NOs 28 to 30 are peptides of 21 amino acids and are derived from histone 2A variants. The three histone 2A peptides are fully conserved. The production of a site-specific ADP- ribosylated form of a H2B peptide corresponding to the human amino acid sequence (1-32) is illustrated in Example 1. The production of a site-specific ADP-ribosylated form of a H2A peptide corresponding to the human amino acid sequence (1-19) is illustrated in Example 2. Within SEQ ID NOs 28 to 30 it is preferred that the serine at amino acid at position 1 is ADP- ribosylated while the amino acids at positions 18 and 19 are not ADP-ribosylated. SEQ ID NOs 31 to 35 are derived from histone H1 variants but share no significant sequence similarity. Histone ADP-ribosylation plays a role in DNA repair, replication and transcription; see for review Messner and Hottiger (2011 ), Trends in Cell Biology, 21 (9):534-542. Moreover, Histones are a substrate of the enzyme PARP-1.

The proteins and peptides comprsing or consisting of SEQ ID NOs 36 to 309 are all endogenous targets of serine ADP-ribosylation upon DNA damage in vivo.

The at least one serine not being ADP-ribosylated is optionally phosphorylated since in the method of the invention the at least one serine not to be ADP-ribosylated is protected from ADPr by a phosphate group. As also detailed herein above, after ADPr has been added to the desired target serine(s), the phosphate group may be removed.

The proviso that the serines within the amino acid sequence of (i) are conserved means that the serines may not be replaced by another amino acid or deleted from the amino acid sequence. The proviso preferably also requires that no additional serines are added to the amino acid sequence.

The present invention relates in a fourth aspect to a binding molecule, preferably an antibody, specifically binding to the site-specifically serine ADP-ribosylated protein or peptide of the second or third aspect of the invention.

A binding molecule of the fourth aspect is a compound being capable of specifically binding to the site-specifically serine ADP-ribosylated protein or peptide of the second or third aspect of the invention. Specific binding designates that the binding molecule does not or essentially does not bind to other proteins or peptides than the site-specifically serine ADP-ribosylated protein or peptide of the second or third aspect of the invention.

In particular, it is preferred that the binding molecule is not capable to bind the same protein or peptide with a different serine ADP-ribosylion pattern and also with no serine ADP- ribosylation. A binding molecule of the fourth aspect is, for example, suitable for research purposes. For example, an antibody binding to the protein of the fourth aspect can be used in immuonassays, such as an ELISA or Western Blot. Immunoassays are biochemical tests that can measure the presence or concentration the protein of the fourth aspect in a sample (e.g. a solution). The binding molecule of the fourth aspect is preferably capable of inhibiting the protein or peptide of the second or third aspect. In this case the binding molecule is designated inhibitor.

Preferably, the binding molecule is an antibody.

The term “antibody” as used in accordance with the present invention comprises, for example, polyclonal or monoclonal antibodies. Furthermore, also derivatives or fragments thereof, which still retain the binding specificity to the target, e.g. the site-specifically ADP- ribosylated protein or peptide, are comprised in the term "antibody". Antibody fragments or derivatives comprise, inter alia, Fab or Fab’ fragments, Fd, F(ab')2, Fv or scFv fragments, single domain VH or V-like domains, such as VhH or V-NAR-domains, as well as multimeric formats such as minibodies, diabodies, tribodies or triplebodies, tetrabodies or chemically conjugated Fab’-multimers (see, for example, Harlow and Lane "Antibodies, A Laboratory Manual", Cold Spring Harbor Laboratory Press, 198; Harlow and Lane“Using Antibodies: A Laboratory Manual” Cold Spring Harbor Laboratory Press, 1999; Altshuler EP, Serebryanaya DV, Katrukha AG. 2010, Biochemistry (Mosc)., vol. 75(13), 1584; Holliger P, Hudson PJ. 2005, Nat Biotechnol., vol. 23(9), 1 126). The multimeric formats in particular comprise bispecific antibodies that can simultaneously bind to two different types of antigen. The first antigen can be found on the protein of the invention. The second antigen may, for example, be a tumor marker that is specifically expressed on cancer cells or a certain type of cancer cells. Non-limting examples of bispecific antibodies formats are Biclonics (bispecific, full length human IgG antibodies), DART (Dual-affinity Re-targeting Antibody) and BiTE (consisting of two single-chain variable fragments (scFvs) of different antibodies) molecules (Kontermann and Brinkmann (2015), Drug Discovery Today, 20(7):838-847).

The term "antibody" also includes embodiments such as chimeric (human constant domain, non-human variable domain), single chain and humanised (human antibody with the exception of non-human CDRs) antibodies. Various techniques for the production of antibodies are well known in the art and described, e.g. in Harlow and Lane (1988) and (1999) and Altshuler et al., 2010, loc. cit. Thus, polyclonal antibodies can be obtained from the blood of an animal following immunisation with an antigen in mixture with additives and adjuvants and monoclonal antibodies can be produced by any technique which provides antibodies produced by continuous cell line cultures. Examples for such techniques are described, e.g. in Harlow E and Lane D, Cold Spring Harbor Laboratory Press, 1988; Harlow E and Lane D, Using Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory Press, 1999 and include the hybridoma technique originally described by Kohler and Milstein, 1975, the trioma technique, the human B-cell hybridoma technique (see e.g. Kozbor D, 1983, Immunology Today, vol.4, 7; Li J, et al. 2006, PNAS, vol. 103(10), 3557) and the EBV-hybridoma technique to produce human monoclonal antibodies (Cole et al., 1985, Alan R. Liss, Inc, 77-96). Furthermore, recombinant antibodies may be obtained from monoclonal antibodies or can be prepared de novo using various display methods such as phage, ribosomal, mRNA, or cell display. A suitable system for the expression of the recombinant (humanised) antibodies may be selected from, for example, bacteria, yeast, insects, mammalian cell lines or transgenic animals or plants (see, e.g., US patent 6,080,560; Holliger P, Hudson PJ. 2005, Nat Biotechnol., vol. 23(9), 1 1265). Further, techniques described for the production of single chain antibodies (see, inter alia, US Patent 4,946,778) can be adapted to produce single chain antibodies specific for an epitope of the site-specifically ADP-ribosylated protein or peptide. Surface plasmon resonance as employed in the BIAcore system can be used to increase the efficiency of phage antibodies.

The binding molecule may also be a small molecule, an antibody mimetic or an aptamer.

The "small molecule" as used herein is preferably an organic molecule. Organic molecules relate or belong to the class of chemical compounds having a carbon basis, the carbon atoms linked together by carbon-carbon bonds. The original definition of the term organic related to the source of chemical compounds, with organic compounds being those carbon- containing compounds obtained from plant or animal or microbial sources, whereas inorganic compounds were obtained from mineral sources. Organic compounds can be natural or synthetic. The organic molecule is preferably an aromatic molecule and more preferably a heteroaromatic molecule. In organic chemistry, the term aromaticity is used to describe a cyclic (ring-shaped), planar (flat) molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. Aromatic molecules are very stable, and do not break apart easily to react with other substances. In a heteroaromatic molecule at least one of the atoms in the aromatic ring is an atom other than carbon, e.g. N, S, or O. For all above-described organic molecules the molecular weight is preferably in the range of 200 Da to 1500 Da and more preferably in the range of 300 Da to 1000 Da.

Alternatively, the "small molecule" in accordance with the present invention may be an inorganic compound. Inorganic compounds are derived from mineral sources and include all compounds without carbon atoms (except carbon dioxide, carbon monoxide and carbonates). Preferably, the small molecule has a molecular weight of less than about 2000 Da, or less than about 1000 Da such as less than about 500 Da, and even more preferably less than about Da amu. The size of a small molecule can be determined by methods well- known in the art, e.g., mass spectrometry. The small molecules may be designed, for example, based on the crystal structure of the target molecule, where sites presumably responsible for the biological activity can be identified and verified in in vivo assays such as in vivo high-throughput screening (HTS) assays.

As used herein, the term“antibody mimetics” refers to compounds which, like antibodies, can specifically bind antigens, such the site-specifically serine ADP-ribosylated protein or peptide in the present case, but which are not structurally related to antibodies. Antibody mimetics are usually artificial peptides or proteins with a molar mass of about 3 to 20 kDa. For example, an antibody mimetic may be selected from the group consisting of affibodies, adnectins, anticalins, DARPins, avimers, nanofitins, affilins, Kunitz domain peptides and Fynomers®. These polypeptides are well known in the art and are described in further detail herein below.

The term“affibody”, as used herein, refers to a family of antibody mimetics which is derived from the Z-domain of staphylococcal protein A. Structurally, affibody molecules are based on a three-helix bundle domain which can also be incorporated into fusion proteins. In itself, an affibody has a molecular mass of around 6kDa and is stable at high temperatures and under acidic or alkaline conditions. Target specificity is obtained by randomisation of 13 amino acids located in two alpha-helices involved in the binding activity of the parent protein domain (Feldwisch J, Tolmachev V.; (2012) Methods Mol Biol. 899:103-26).

The term "adnectin" (also referred to as“monobody”), as used herein, relates to a molecule based on the 10th extracellular domain of human fibronectin III (10Fn3), which adopts an Ig- like b-sandwich fold of 94 residues with 2 to 3 exposed loops, but lacks the central disulphide bridge (Gebauer and Skerra (2009) Curr Opinion in Chemical Biology 13:245-255). Adnectins with the desired target specificity, i.e. against the site-specifically ADP-ribosylated protein or peptide, can be genetically engineered by introducing modifications in specific loops of the protein.

The term "anticalin", as used herein, refers to an engineered protein derived from a lipocalin (Beste G, Schmidt FS, Stibora T, Skerra A. (1999) Proc Natl Acad Sci U S A. 96(5): 1898- 903; Gebauer and Skerra (2009) Curr Opinion in Chemical Biology 13:245-255). Anticalins possess an eight-stranded ⁽ i-barrel which forms a highly conserved core unit among the lipocalins and naturally forms binding sites for ligands by means of four structurally variable loops at the open end. Anticalins, although not homologous to the IgG superfamily, show features that so far have been considered typical for the binding sites of antibodies: (i) high structural plasticity as a consequence of sequence variation and (ii) elevated conformational flexibility, allowing induced fit to targets with differing shape.

As used herein, the term "DARPin" refers to a designed ankyrin repeat domain (166 residues), which provides a rigid interface arising from typically three repeated b-turns. DARPins usually carry three repeats corresponding to an artificial consensus sequence, wherein six positions per repeat are randomised. Consequently, DARPins lack structural flexibility (Gebauer and Skerra, 2009).

The term“avimer”, as used herein, refers to a class of antibody mimetics which consist of two or more peptide sequences of 30 to 35 amino acids each, which are derived from A- domains of various membrane receptors and which are connected by linker peptides. Binding of target molecules occurs via the A-domain and domains with the desired binding specificity, i.e. for the site-specifically ADP-ribosylated protein or peptide, can be selected, for example, by phage display techniques. The binding specificity of the different A-domains contained in an avimer may, but does not have to be identical (Weidle UH, et al., (2013), Cancer Genomics Proteomics; 10(4): 155-68).

A“nanofitin” (also known as affitin) is an antibody mimetic protein that is derived from the DNA binding protein Sac7d of Sulfolobus acidocaldarius. Nanofitins usually have a molecular weight of around 7kDa and are designed to specifically bind a target molecule, such as e.g. the site-specifically ADP-ribosylated protein or peptide, by randomising the amino acids on the binding surface (Mouratou B, Behar G, Paillard-Laurance L, Colinet S, Pecorari F., (2012) Methods Mol Biol.; 805:315-31 ).

The term“affilin”, as used herein, refers to antibody mimetics that are developed by using either gamma-B crystalline or ubiquitin as a scaffold and modifying amino-acids on the surface of these proteins by random mutagenesis. Selection of affilins with the desired target specificity, i.e. against the site-specifically ADP-ribosylated protein or peptide, is effected, for example, by phage display or ribosome display techniques. Depending on the scaffold, affilins have a molecular weight of approximately 10 or 20kDa. As used herein, the term affilin also refers to di- or multimerised forms of affilins (Weidle UH, et al., (2013), Cancer Genomics Proteomics; 10(4): 155-68).

A“Kunitz domain peptide” is derived from the Kunitz domain of a Kunitz-type protease inhibitor such as bovine pancreatic trypsin inhibitor (BPTI), amyloid precursor protein (APP) or tissue factor pathway inhibitor (TFPI). Kunitz domains have a molecular weight of approximately 6kDA and domains with the required target specificity, i.e. against the site- specifically ADP-ribosylated protein or peptide, can be selected by display techniques such as phage display (Weidle et al., (2013), Cancer Genomics Proteomics; 10(4): 155-68).

As used herein, the term "Fynomer®" refers to a non-immunoglobulin-derived binding polypeptide derived from the human Fyn SH3 domain. Fyn SH3-derived polypeptides are well-known in the art and have been described e.g. in Grabulovski et al. (2007) JBC, 282, p. 3196-3204, WO 2008/022759, Bertschinger et al (2007) Protein Eng Des Sel 20(2):57-68, Gebauer and Skerra (2009) Curr Opinion in Chemical Biology 13:245-255, or Schlatter et al. (2012), MAbs 4:4, 1-12).

Aptamers are nucleic acid molecules or peptide molecules that bind a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool, but natural aptamers also exist in riboswitches. Aptamers can be used for both basic research and clinical purposes as macromolecular drugs. Aptamers can be combined with ribozymes to self-cleave in the presence of their target molecule. These compound molecules have additional research, industrial and clinical applications (Osborne et. al. (1997), Current Opinion in Chemical Biology, 1 :5-9; Stull & Szoka (1995), Pharmaceutical Research, 12, 4:465-483).

Nucleic acid aptamers are nucleic acid species that normally consist of (usually short) strands of oligonucleotides. Typically, they have been engineered through repeated rounds of in vitro selection or equivalently, SELEX (systematic evolution of ligands by exponential enrichment) to bind to various molecular targets such as small molecules, proteins, nucleic acids, and even cells, tissues and organisms. Peptide aptamers are usually peptides or proteins that are designed to interfere with other protein interactions inside cells. They consist of a variable peptide loop attached at both ends to a protein scaffold. This double structural constraint greatly increases the binding affinity of the peptide aptamer to levels comparable to an antibody's (nanomolar range). The variable peptide loop typically comprises 10 to 20 amino acids, and the scaffold may be any protein having good solubility properties. Currently, the bacterial protein Thioredoxin-A is the most commonly used scaffold protein, the variable peptide loop being inserted within the redox- active site. Peptide aptamer selection can be made using different systems, but the most widely used is currently the yeast two-hybrid system.

Aptamers offer the utility for biotechnological and therapeutic applications as they offer molecular recognition properties that rival those of the commonly used biomolecules, in particular antibodies. In addition to their discriminatory recognition, aptamers offer advantages over antibodies as they can be engineered completely in a test tube, are readily produced by chemical synthesis, possess desirable storage properties, and elicit little or no immunogenicity in therapeutic applications. Non-modified aptamers are cleared rapidly from the bloodstream, with a half-life of minutes to hours, mainly due to nuclease degradation and clearance from the body by the kidneys, a result of the aptamers' inherently low molecular weight. Unmodified aptamer applications currently focus on treating transient conditions such as blood clotting, or treating organs such as the eye where local delivery is possible. This rapid clearance can be an advantage in applications such as in vivo diagnostic imaging. Several modifications, such as 2'-fluorine-substituted pyrimidines, polyethylene glycol (PEG) linkage, fusion to albumin or other half life extending proteins etc. are available to scientists such that the half-life of aptamers can be increased for several days or even weeks.

The present invention relates in a fifth aspect to a composition, preferably a pharmaceutical, diagnostic or cosmetic composition, comprising the site-specifically ADP-ribosylated protein or peptide of the second or third aspect of the invention or the antibody of the fourth aspect of the invention.

The terms “composition”, “pharmaceutical composition”, “diagnostic composition” and “cosmetic composition” as well as their ingredients have been discussed herein above in connection with the first aspect of the invention. This disclosure is equally applicable to the fourth aspect of the invention.

The present invention relates in a sixth aspect to a method for producing a site-specifically serine ADP-ribosylated protein or peptide, comprising: (a) subjecting a protein or peptide comprising or consisting of an amino acid sequence comprising two or more serines to a serine ADP-ribosylation reaction, wherein (i) at least one serine is phosphorylated and at least one serine is non-phosphorylated, and/or (ii) for at least one serine a lysine can be found up to four amino acids N- or C-terminally thereof, preferably can be found directly N- terminally thereof, wherein said lysine is acetylated.

With respect to the method of the sixth aspect of the invention it is to be understood that in step (a) (i) the at least one non-phosphorylated serine becomes ADP-ribosylated whereas the at least one phosphorylated serine is protected from serine ADP-ribosylation, and/or (ii) at least one serine becomes ADP-ribosylated whereas the at least one serine, wherein a lysine can be found up to four amino acids N- or C-terminally thereof, preferably can be found directly N-terminally thereof, wherein said lysine is acetylated, is partially protected from serine ADP-ribosylation. In the above method for the at least one serine to be site- specifically serine ADP-ribosylated no lysine can be found up to four amino acids N- or C- terminally thereof. It is furthermore preferred that the at least one serine being non- phosphorylate has a free side chain (-CH ₂ -OH), for example, carrying no chemical modification.

The definitions and preferred embodiments of the first to fifth aspect of the invention apply mutatis mutandis to the sixth aspect of the invention as far as being also applicable to the sixth aspect. For instance, the serine ADP-ribosylation reaction in the context of the sixth aspect of the invention may be done in the same way as in the context of the first aspect of the invention. Moreover, also in accordance with a sixth aspect of the invention step (a) may comprise the synthesis of the protein or peptide as defined in the method of the invention, wherein the synthesis preferably comprises a solid-phase peptide synthesis.

The lysine that can be found up to four amino acids N- or C-terminally of the serine can be found with increasing preference up to three amino acids, up to two or next to the serine, N- or C-terminally. Most preferred is a“KS” motive.

Acetylation (or in IUPAC nomenclature ethanoylation) describes a reaction that introduces an acetyl functional group into a chemical compound. Acetylation of proteins and peptides naturally occurs as a co-translational or a post-translational modification, for example, of histones, p53, and tubulins. Acetylated lysine residues were first discovered in histones regulating gene transcription, which is the reason why the enzymes catalyzing lysine (K) acetylation were termed histone acetyltransferases (HATs). As discussed herein above, it has surprisingly been found that a phosphate group on a serine serves as a protection group against the addition of an ADP-ribose in an ADP- ribosylation reaction. It has been reported that the acetylation of a lysine which can be found up to four amino acids N- or C-terminally of the serine, partially protects such a serine from serine ADP-ribosylation (Liszczak et al, Nat Chem Biol 2018). Hence, unlike phosphorylated serine no complete protection from serine ADPr is achieved but instead the efficacy of the ADPr of that serine is reduced while the at least one other“unprotected” serine as comprised in the protein or peptide as described in accordance with sixth aspect of the invention is efficiently ADPr. Hence, the discussed lysine acetylation alone or together with serine phosphorylation further increase the possibilities to generate site-specifically serine ADP- ribosylated protein or peptide having a selected ADPr modification pattern.

In accordance with a preferred embodiment of the sixth aspect of the invention, the method further comprises (c) removing the phosphate group(s) from the at least one phosphorylated serine and/or removing the acetyl group(S) from the at least one acetylated lysine.

The phosphate group(s) may be removed as is detailed herein above in connection with the first aspect of the invention. The acetylation status of a lysine is reversible. Several families of enzymes that can deacetylase lysines are known.

Accordingly, in accordance with a more preferred embodiment of the sixth aspect of the invention, the acetyl group(s) is/are removed by an enzyme, preferably a deacetylase, more preferably an enzyme selected from the group consisting of histone deacetylases (HDACs) and sirtuins.

Histone deacetylases (HDACs) (EC 3.5.1.98, HDAC) are a class of enzymes that remove acetyl groups (0=C-CH3) from an e-N-acetyl lysine amino acid on a histone, allowing the histones to wrap the DNA more tightly, and other non-histone proteins. HDACs are now also called lysine deacetylases (KDACs), to describe their function rather than their target, which also includes non-histone proteins. The HDAC is preferably selected from human HDACs 1 to 11.

Some, but not all, sirtuins are protein deacetylases. The sirtuin to be used in accordance with the present invention is a protein deacetylase. Unlike other known protein deacetylases, which simply hydrolyze acetyl-lysine residues, the sirtuin-mediated deacetylation reaction couples lysine deacetylation to NAD hydrolysis. This hydrolysis yields O-acetyl-ADP-ribose, the deacetylated substrate and nicotinamide, which is an inhibitor of sirtuin activity itself. In mammals, including human, seven sirtuins (SIRT1-7) are known. The sirtuin is preferably selected from human SIRT1 , 2, 3, 5, 6 and 7 which all have deacetylase activity.

As regards the embodiments characterized in this specification, in particular in the claims, it is intended that each embodiment mentioned in a dependent claim is combined with each embodiment of each claim (independent or dependent) said dependent claim depends from. For example, in case of an independent claim 1 reciting 3 alternatives A, B and C, a dependent claim 2 reciting 3 alternatives D, E and F and a claim 3 depending from claims 1 and 2 and reciting 3 alternatives G, H and I, it is to be understood that the specification unambiguously discloses embodiments corresponding to combinations A, D, G; A, D, H; A, D, I; A, E, G; A, E, H; A, E, I; A, F, G; A, F, H; A, F, I; B, D, G; B, D, H; B, D, I; B, E, G; B, E, H; B, E, I; B, F, G; B, F, H; B, F, I; C, D, G; C, D, H; C, D, I; C, E, G; C, E, H; C, E, I; C, F, G; C, F, H; C, F, I, unless specifically mentioned otherwise.

Similarly, and also in those cases where independent and/or dependent claims do not recite alternatives, it is understood that if dependent claims refer back to a plurality of preceding claims, any combination of subject-matter covered thereby is considered to be explicitly disclosed. For example, in case of an independent claim 1 , a dependent claim 2 referring back to claim 1 , and a dependent claim 3 referring back to both claims 2 and 1 , it follows that the combination of the subject-matter of claims 3 and 1 is clearly and unambiguously disclosed as is the combination of the subject-matter of claims 3, 2 and 1. In case a further dependent claim 4 is present which refers to any one of claims 1 to 3, it follows that the combination of the subject-matter of claims 4 and 1 , of claims 4, 2 and 1 , of claims 4, 3 and 1 , as well as of claims 4, 3, 2 and 1 is clearly and unambiguously disclosed.

The figures show.

Figure 1. A. Schematic representation of the products of an ADP-ribosylation reaction when using a substrate that comprises at least two target serine. To note, the state-of-the-art methodology to enzymatically generate ADP-ribosylated substrates with more than one target serine cannot ensure site-specificity, creating a mixture of different ADPr species (same amino acid sequence, but modification on different residues). B. Schematic representation of a synthetic peptide corresponding to the amino acids 494-524 of human PARP-1. To note, this amino acid sequence comprises three target serines (underlined), which means that state-of-the-art enzymatic ADP-ribosylation reactions will produce a mixture of different ADPr species. C. Analysis of the ADP-ribosylation of four different variants of synthetic peptides corresponding to the amino acids 494-524 of human PARP-1 (2 pg). In vitro ADP-ribosylation assays were performed as previously described (Bonfiglio et al, Mol Cell, 2017) with 5 mM NAD ⁺ and 63nM (0.05 pCi/pl) ³² P-NAD\ Activated DNA, recombinant PARP-1 (0.1 mM) in the presence or absence of recombinant HPF1 WT (1 pM) for 20 minutes at 25°C. Samples were resolved by SDS-PAGE and analysed by autoradiography. D. Analysis of the ADP-ribosylation of two different variants of synthetic peptides corresponding to the amino acids 494-524 of human PARP-1. In vitro ADP- ribosylation assays were performed as described in panel C, in the presence or absence of recombinant HPF1 WT or Y238A/R239A mutant (1 pM). Samples were resolved by SDS- PAGE and analysed by autoradiography. Figure 1 D is taken from in Bonfiglio et al., 2017, loc. lit.. E. Representative chromatogram illustrating the non-selective Ser-ADPr on substrates with more than one target serine. Biotinylated PARP-1 (494-51 1 ) was subjected to incomplete in vitro enzymatic ADPr and analysed by LC-MS. As depicted, the presence of two target serine (Ser499 and S507) results in non-selective reactions generating heterogenous mixtures of modified species that differ in the positions at which ADP-ribose is attached. For simplicity, the chromatogram shows only the m/z ranges in which mono- and di-ADP-ribosylated PARP- 1 peptides were detected (m/z= 624.63-624.97 and m/z= 759.91- 760.13).

Figure 2. A. Schematic representation of the protein or peptide with naturally-occurring amino acid sequence that is intended to be site-specifically ADP-ribosylated. To note, the amino acid sequence of the protein or peptide to be modified comprises at least two target serines. B. Schematic workflow for the means and methods for production of site-specific serine ADP-ribosylated protein or peptide with naturally-occurring amino acid sequences with two or more serines. The strategy consists of (i) providing a protein or peptide comprising or consisting of a naturally-occurring amino acid sequence comprising two or more serines, wherein at least one serine is phosphorylated and at least one serine is non-phosphorylated; (ii) subjecting such protein or protein to a serine ADP-ribosylation reaction; (iii) removing phosphorylation from the phosphorylated serine residues of (ii), thereby obtaining a site- specific Ser-ADPr protein or peptide with naturally-occurring amino acid sequence.

Figure 3. A. Production of a site-specific ADP-ribosylated form of a PARP-1 peptide corresponding to the human amino acid sequence (494-514). 2 pg of Biotinylated PARP-1 (494-517) S507ph peptide (lane 2) were subjected to phosphatase treatment (400U of Lambda Phosphatase for 1 h at 30°C) alone (lane 4), or to in vitro ADP-ribosylation (lane 6, 2 mM NAD ⁺ , activated DNA, recombinant PARP-1 (0.1 pM) and recombinant HPF1 WT (1 pM) for 6h at 25°C) followed by phosphatase treatment (lane 8, 400U of Lambda Phosphatase for 1 h at 30°C). 2 pg of a PARP-1 (494-514) S507A peptide were used as a control (lanes 1 , 3, 5 and 7). Using this system along the lines described herein, a single species site-specific ADP-ribosylated peptide PARP-1 at Ser-499 was obtained. (Lane 1 ) PARP-1 (494-514) S507A; (Lane 2) PARP-1 (494-514) S507ph; (Lane 3) PARP-1 (494-514) S507A treated with phosphatase; (Lane 4) PARP-1 (494-514) S507ph treated with phosphatase; (Lane 5) PARP-1 (494-514) S507A subjected to in vitro ADPr; (Lane 6) PARP- 1 (494-514) S507ph subjected to in vitro ADPr, (Lane 7) PARP-1 (494-514) S507A subjected to in vitro ADPr followed by phosphatase treatment; and (Lane 8) PARP-1 (494-514) S507ph subjected to in vitro ADPr followed by phosphatase treatment. B. Deconvoluted MS spectra displaying the different peptide species described in Fig. 2B. Unmodified biotinylated PARP-1 (494-511 ) peptide with two target serine (Ser499 and Ser507) was synthesized with phosphorylation in Ser507 and subjected to enzymatic HPF1/PARP-1 ADPr, generating an intermediate species comprising Ser499-ADPr and Ser507- phosphorylation. After ADPr reaction, phosphate from Ser507 was removed by Lambda phosphatase, producing biotinylated PARP-1 (494-511 ) Ser499-ADPr peptide. As depicted, chemically synthesized peptide containing S507ph is shifted 79.96 Da with respect to the unmodified counterpart (mass of phosphorylation = 79.96 Da). Next, in vitro reaction produces a shift of 541.06 Da (mass of ADPr = 541.06 Da) and finally, treatment with lambda phosphatase produces a negative shift of 79.96 Da, indicating the removal of phosphorylation. The final product is pure PARP-1 (494-511 ) Ser499 ADP-ribosylated peptide. C. Production of a site-specific ADP-ribosylated form of a Histone H2B peptide corresponding to the human amino acid sequence (1-32). 2 pg of Biotinylated Histone H2B (1-32) S14ph peptide (lane 2) were subjected to phosphatase treatment (400U of Lambda Phosphatase for 1 h at 30°C) alone (lane 1 ), or to in vitro ADP-ribosylation (lane 3, 2 mM NAD ⁺ , activated DNA, recombinant PARP-1 (0.1 mM) and recombinant HPF1 WT (1 pM) for 6h at 25°C) followed by phosphatase treatment (lane 4, 400U of . Lambda Phosphatase for 1 h at 30°C). Using this system along the lines described herein, a single species site-specific ADP-ribosylated peptide Histone H2B at Ser-6 was obtained. (Lane 1 ) Histone H2B (1-32); (Lane 2) Histone H2B (1-32) S14ph; (Lane 3) Histone H2B (1-32) S14ph subjected to in vitro ADPr; (Lane 4) Histone H2B (1-32) S14ph subjected to in vitro ADPr followed by phosphatase treatment.

Figure 4. A. A peptide with more than one target serine residue is still ADP-ribosylated after mutation of the main target serine residue. Representative stained gels (Imperial™ Protein Stain) illustrating the non-selective Ser-ADPr on substrates with more than one target serine. Histone H2A peptides in which the major target serine has been substituted by alanine (Histone H2A (1-19) S1A) are still ADP-ribosylated due to the presence of other target serines. Biotinylated Histone H2A (1-19) peptide (Lane 1 ) was reacted with 2 mM NAD ⁺ and 0.12 pM PARP-1 in the absence (Lane 2) or presence (Lane 3) of 1.5 pM HPF1 for 75 minutes at RT. Similarly, Biotinylated Histone H2A (1-19) S1A peptide (Lane 4) was reacted with 2 mM NAD ⁺ and 0.12 mM PARP-1 in the absence (Lane 5) or presence (Lane 6) of 1.5 mM HPF1 for 75 minutes at RT. B. 2 pg of Biotinylated H2A (1-19) S18,19ph peptide (lane 2) was subjected to phosphatase treatment (400U of Lambda Phosphatase for 1 h at 30°C) alone (lane 4), or to in vitro ADP-ribosylation (lane 5, 2 mM NAD ⁺ , Activated DNA, recombinant PARP-1 (0.1 pM) and recombinant HPF1 WT (1 pM) for 6h at 25°C) followed by phosphatase treatment (lane 6, 400U of Lambda Phosphatase for 1 h at 30°C). 2 pg of a H2A (1-19) peptide were used as a control (lanes 1 and 3). Following the strategy described in this application, a single species site-specific ADP-ribosylated peptide H2A at Ser-1 was obtained. (Lane 1 ) H2A (1-19) (Lane 2) H2A (1-19) S18,19ph (Lane 3) H2A (1-19) treated with phosphatase (Lane 4) H2A (1-19) S18,19ph treated with phosphatase (Lane 5) H2A (1- 19) S18,19ph subjected to in vitro ADPr (Lane 6) H2A (1-19) S18,19ph subjected to in vitro ADPr followed by phosphatase treatment.

The examples illustrate the invention.

Example 1 - Production of a site-specific ADP-ribosylated form of a PARP-1 peptide corresponding to the human amino acid sequence (494-514).

Unlike the commonly-used approach of replacing the undesired target serine with another residue, the methodology described in the present application does not require the replacement of serine residue(s) and the consequent use of a non-naturally-occurring sequence to prepare a protein or peptide being site-specifically serine ADP-ribosylated (Figure 2). By these means, a site-specific -Ser499-ADP-ribosylated peptide corresponding to the naturally-occurring human amino acid sequence (494-514) of PARP-1 (Figure 3A and B) was obtained. Importantly, the PARP-1 Ser499-ADPr site is the main endogenous ADPr site upon DNA damage. A standard approach to prevent ADPr of Ser507 would be replacing this residue with alanine (Figure 3A). However, the final ADP-ribosylated product will not carry the natural sequence, which might represent a drawback for downstream applications (e.g. generation of antibodies). To overcome this limitation, a novel strategy was designed that consists of obtaining by standard chemical synthesis, a phosphorylated Ser507 peptide that prevents ADPr of Ser507. After subjecting this peptide to ADPr reactions, the unmodified Ser-499 becomes ADP-ribosylated. As the resulting peptide is both phosphorylated and ADP-ribosylated, the phosphate is afterwards removed from the peptide without affecting ADP-ribosylation (e.g. a second enzymatic reaction with a recombinant commercially- available phosphatase). The resulting peptide is ADP-ribosylated on a specific serine and comprises a naturally-occurring sequence (Figure 2 and 3A and B). By the above-described means also a Histone H2B (1 -32) which is site-specifically ADPr at Ser-6 was obtained (Figure 3C).

Example 2 - Production of a site-specific ADP-ribosylated form of a H2A peptide corresponding to the human amino acid sequence (1-19).

When a protein or peptide comprising at least two target serines is subjected to an ADP- ribosylation reaction, a mixture of different ADP-ribosylated species (same amino acid sequence, but modification on different residues) is generated (Figures 1A and 4). For example, the peptide corresponding to the human amino acid sequence (1 -19) of H2A contains, in addition to the Ser-1 , two additional Ser residues (Ser-18 and Ser-19) that have never been described in the literature as targets of ADPr. However, when mutating the only reported target serine (Ser-1 ), the peptide is still ADP-ribosylated (Figure 4A, lane 6), demonstrating the limitations of using a peptide with at least two target serine to obtain site- specific ADP-ribosylated species. The methodology described in the present application solves this problem by preventing the ADP-ribosylation of undesired Ser residues and therefore allowing the preparation of a protein or peptide being site-specifically serine ADP- ribosylated. The novel strategy consists of using a phosphorylated Ser peptide (Ser18 and Ser19 of this example, Figure 4B) that prevents ADPr of those residues. Similarly to what has been described in Example 1 , after subjecting the phosphorylated peptide to ADPr reactions, the unmodified Ser-1 becomes ADP-ribosylated and to remove the phosphates of Ser18 and Ser19, the peptide is subjected to a phosphate treatment (Figure 4B).

Example 3 - Experimental conditions under which considerable amounts of pure PARP-1 Ser-499 mono-ADP-ribosylated peptide were obtained

Given that some important applications require considerable amounts of serine ADP- ribosylated peptides (e.g. generation of antibodies), the inventors scaled up the reactions and under the following tested conditions have been able to produce ~ 500pg of pure PARP- 1 Ser-499 mono-ADP-ribosylated peptide:

Solvent: Water

Buffer: 50 mM Tris-HCI, pH = 7.5;

Salts :50 mM NaCI and 1 mM MgCI ₂ ;

NAD ⁺ : 2 mM;

PARP-1 : 100 nM;

HPF1 : 1.5 mM; Substrate peptide: 700 g (Biotinylated PARP-1 (494-517) S507ph peptide)

The reaction mix was incubated for 360 minutes at RT, adding 2 mM NAD ⁺ every 120 minutes and stopped by adding 1 mM Olaparib Afterwards, 1 mM MnCI ₂ , 1X PMP buffer (New England Biolabs), 8000 U of Lambda Protein Phosphatase (Lambda PP, New England Biolabs) and 1 mM PARG were added to the reaction mix and it was incubated for 300 minutes at 30°C. Afterwards, the serine mono-ADP-ribosylated peptides were separated from the other constituents of the reaction mix by using reverse chromatography (C18 cartridge). Pure mono-Ser-499 ADP-ribosylated Biotinylated PARP-1 (494-517) peptide was eluted with 30% Acetonitrile.