FIELD OF THE INVENTION

The present invention relates to methods and nucleic acid fragments for detecting a genetic deletion at the SHOX locus of a horse, where the presence of such a genetic deletion indicates that the horse is a carrier of disease-causing mutation that can lead to skeletal atavism.

BACKGROUND OF THE INVENTION

Speed [1] and Hermans [2] published the first scientific reports on fully developed ulna and fibula in horses, and they both described this defect in the Shetland pony breed. Speed described this anatomic variation as a type of atavism in certain families of Shetland pony foals in the United Kingdom. The affected foals had complete ulnas and fibulas with associated angular limb deformities and developed progressive interrupted movements and lameness. The recurrence in certain families suggested a hereditary basis for the anomaly. Speed described that the parents were phenotypically normal and the anomaly was noted to skip generations. Hermans (1970) reported that fifty Shetland ponies with atavism were recorded in Utrecht from year 1961 to 1967. Hermans performed test matings to elucidate the inheritance pattern of the defect and the results suggested that skeletal atavism in Shetland ponies follow an autosomal recessive mode of inheritance.

In addition to Shetland ponies rare occurrences of the defect have been described in Welsh pony [3] and in Miniature horse [4].

There is a strong need in the horse breeding industry to develop a diagnostic test to identify animals that are carriers of the recessive allele causing skeletal atavism so that such carriers can be eliminated from the breeding population which means that the disorder will be eradicated.

DESCRIPTION OF THE INVENTION

The present inventors have identified mutations that can lead to skeletal atavism in the horse. The mutations are demonstrated to be two different deletion alleles, Dl and D2, associated with the Short Stature Homeobox (SHOX) locus.

Accordingly, the present invention provides methods for detecting disease- causing genetic deletions at the SHOX locus of a horse, where the presence of such a genetic deletion indicates that the horse is a carrier of a mutation that can lead to skeletal atavism.

Put another way, the present invention provides methods for determining if a horse is a carrier of a mutation that can lead to skeletal atavism. The method can comprise the steps,

i) extracting DNA from a sample obtained from a horse,

ii) determining in said DNA the presence of a genetic deletion at the SHOX locus,

where the presence of said genetic deletion indicates that the horse is a carrier of a mutation that can lead to skeletal atavism.

The genetic deletion can be the Dl and/or the D2 deletion according to the present invention. The horse carrying a disease-causing mutation can be heterozygous for the Dl allele, the D2 allele, or both the Dl and D2 alleles, or homozygous for the Dl allele or the D2 allele.

The Dl allele is the allele with the Dl genetic deletion, the D2 allele is the allele with the D2 deletion.

In one aspect, the present invention provides methods for detecting the presence of a nucleic acid sequence in the genome of a horse, said nucleic acid sequence being a part of the Dl deletion. Preferably, the nucleic acid sequence to be detected can be selected from a nucleic acid sequence present in the nucleic acid sequences SEQ ID NOs: 10 to 117.

The presence of one copy only of said nucleic acid in the genome of said horse, being indicative of said horse being heterozygous for the Dl deletion, and hence a carrier of a disease-causing mutation than can lead to skeletal atavism. The absence of said nucleic acid in the genome of said horse, being indicative of said horse being homozygous for the Dl deletion, and hence a carrier of a disease-causing mutation that can lead to skeletal atavism.

The Dl deletion according to the invention comprises the contigs indicated in Table 2 of the horse reference genome assembly EquCab2 corresponding to SEQ ID NOs: 10 to 110.

The Dl deletion according to the invention comprises the nucleic acid sequences present in the BAC derived contigs indicated in Table 5 corresponding to SEQ ID NOs: 111 to 117. In another aspect, the present invention provides methods for detecting the presence of a nucleic acid sequence in the genome of a horse, said nucleic acid sequence being a part of the D2 deletion. Preferably, the nucleic acid sequence to be detected can be selected from a nucleic acid sequence present in the nucleic acid sequences SEQ ID NOs: 10 to 58, 91 to 95, 97 to 110, and 115 to 117.

The presence of one copy only of said nucleic acid in the genome of said horse, being indicative of said horse being heterozygous for the D2 deletion, and hence a carrier of a disease-causing mutation that can lead to skeletal atavism. The absence of said nucleic acid in the genome of said horse, being indicative of said horse being homozygous for the D2 deletion, and hence a carrier of a disease-causing mutation that can lead to skeletal atavism.

The D2 deletion according to the invention comprises the nucleic acid sequences present in the contigs indicated in Table 2 of the horse reference genome assembly EquCab2 corresponding to SEQ ID NOs: 10 to 58, 91 to 95, and 97 to 110.

The D2 deletion according to the invention comprises the nucleic acid sequences present in the BAC derived contigs indicated in Table 5 corresponding to SEQ ID NOs: 115 to 117.

The term "SHOX locus" is intended to include all DNA sequences of the BAC derived contigs of the horse genome listed in Table 5, i.e. the DNA sequences corresponding to SEQ NOs: 111 to 117, and any intervening DNA sequences, including 100 kB upstream and downstream of the SHOX gene.

"Sketetal Atavism" refers to the disorder involving ulna and fibula development in the horse, as the condition to some extent mimics the skeletal development present in ancestors of horses with full development of the ulna and tibia.

According to one aspect of the invention the methods according to the present invention can be used for selecting horses for breeding.

According to one aspect of the invention the methods according to the present invention can be used for parental testing of horses.

Preferably, the methods according to the invention comprise extraction of

DNA from a biological sample obtained from a horse.

The term "sample" or "biological sample" according to the present invention refers to any material containing nucleated cells from said horse to be tested. In a preferred embodiment the biological sample to be used in the methods of the present invention is selected from the group consisting of blood, sperm, hair roots, milk, body fluids as well as tissues including nucleated cells.

DNA extraction, isolation and purification methods are well-known in the art and can be applied in the present invention. Standard protocols for the isolation of genomic DNA are inter alia referred to in Sambrook, J., Russell. D. W.. Molecular Cloning: A Laboratory Manual, the third edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor. New York, 1.31-1.38, 2001 and Sharma. R.C., et al. "A rapid procedure for isolation of RNA-free genomic DNA from mammalian cells", BioTechniques, 14. 176-178, 1993.

There are several methods known by those skilled in the art for determining whether a particular nucleotide sequence is present in a DNA sample and for identifying the nucleotide in a specific position in a DNA sequence. These include the amplification of a DNA segment encompassing the genetic marker by means of the polymerase chain reaction (PCR) or any other amplification method, interrogate the genetic marker by means of allele specific hybridization, the 3'exonuclease assay (Taqman assay), fluorescent dye and quenching agent-based PCR assay, the use of allele-specific restriction enzymes (RFLP-based techniques), direct sequencing, the oligonucleotide ligation assay (OLA), pyrosequencing, the invader assay, minisequencing, DHPLC-based techniques, single strand conformational polymorphism (SSCP), allele-specific PCR, denaturating gradient gel electrophoresis (DGGE), temperature gradient gel electrophoresis (TGGE), chemical mismatch cleavage (CMC), heteroduplex analysis based system, techniques based on mass spectroscopy, invasive cleavage assay, polymorphism ratio sequencing (PRS), microarrays, a rolling circle extension assay, HPLC-based techniques, extension based assays, ARMS (Amplification Refractory Mutation System), ALEX

(Amplification Refractory Mutation Linear Extension), SBCE (Single base chain extension), molecular beacon assays, invader (Third wave technologies), ligase chain reaction assays, 5'-nuclease assay-based techniques, hybridization capillary array electrophoresis (CAE), single molecule sequencing such as nanopore sequencing, protein truncation assays (PTT), immunoassays, and solid phase hybridization (dot blot, reverse dot blot, chips). This list of methods is not meant to be exclusive, but just to illustrate the diversity of available methods. Some of these methods can be performed in accordance with the methods of the present invention in microarray format (microchips) or on beads. The invention thus also relates to primers, primer pairs or probes, hybridizing under stringent conditions to a nucleic acid sequence present in or flanking the deleted regions Dl and D2 according to the invention, or to the complementary strand thereof, and their use in the methods according to the invention. By a nucleic acid sequence flanking the deleted regions Dl and D2 is meant nucleic acid sequences 10 kB upstream or downstream of Dl or D2, such as 1 kB, or 100 bases upstream or downstream of Dior D2.

Primers hybridizing to these flanking regions are particularly useful for PCR amplification in the methods according to the invention. Use of one primer hybridizing to a nucleic acid sequence in Dl and one primer hybridizing to a nucleic acid sequence flanking the Dl will only lead to amplification when Dl is present. Use of one primer hybridizing to a nucleic acid sequence in D2 and one primer hybridizing to a nucleic acid sequence flanking the D2 will only lead to amplification when D2 is present. Use of one primer hybridizing to a nucleic acid sequence flanking Dl upstream and one primer hybridizing to a nucleic acid sequence flanking Dl downstream will only lead to amplification when Dl is absent. Use of one primer hybridizing to a nucleic acid sequence flanking D2 upstream and one primer hybridizing to a nucleic acid sequence flanking D2 downstream will only lead to amplification when D2 is absent.

Primers hybridizing to nucleic acid sequences introduced by a mutational event leading to and creating Dl and/or D2 are particularly useful for PCR amplification in the methods according to the invention.

Preferably the primers or primer pairs hybridize(s) under stringent conditions to the sequences SEQ ID NOs: 10 to 110, or to the complementary strand thereof.

Preferably, the primers of the invention have a length of at least 14 nucleotides such as 17 or 21 nucleotides.

More specifically the primers can be selected from SEQ NO:l, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, and SEQ ID NO:6.

Hybridization is preferably performed under stringent or highly stringent conditions. "Stringent or highly stringent conditions" of hybridization are well known to or can be established by the person skilled in the art according to conventional protocols. Appropriate stringent conditions for each sequence may be established on the basis of well-known parameters such as temperature, composition of the nucleic acid molecules, salt conditions etc.: see, for example, Sambrook et al. "Molecular Cloning, A Laboratory Manual", CSH Press, Cold Spring Harbor, 1989 or Higgins and Hames (eds.), "Nucleic acid hybridization, a practical approach", IRL Press, Oxford 1985, see in particular the chapter "Hybridization Strategy" by Britten & Davidson. Typical (highly stringent) conditions comprise hybridization at 65°C in 0.5xSSC and 0.1 % SDS or hybridization at 42°C in 50% formamide, 4xSSC and 0.1 % SDS. Hybridization is usually followed by washing to remove unspecific signals. Washing conditions include conditions such as 65°C, 0.2xSSC and 0.1% SDS or 2xSSC and 0.1% SDS or 0.3xSSC and 0.1 % SDS at 25°C - 65°C. LEGEND TO FIGURES

Figure 1 : Depth of sequence read coverage observed in whole genome resequencing. Shown is the depth at the SHOX locus for six Skeletal Atavism cases (CGI -6) and for the pool of healthy control stallions. Boxes have been inserted to visualize approximate locations of the two deletions in the EquCab2 assembly context.

EXAMPLES

Methods

Illumina sequencing and sequence analysis

DNA samples from six Shetland ponies diagnosed by veterinarians with

Skeletal Atavism (SA), individuals CGI, CG2, CG3, CG4, CG5 and CG6, as well as a control pool consisting of equimolar amounts of DNA from 22 stallions (who had never fathered Atavistic foals despite having fathered many foals) were prepared for sequencing. Illumina paired-end libraries were generated from these DNA samples with mean insert sizes of approximately 220 bp. The two libraries were sequenced using an Illumina HiSeq instrument as paired-end reads (2 x 100 bp). The reads were mapped to the horse reference genome assembly [5] using the software BWA [6], and PCR-duplicates were removed using the software Picard

(http://picard.sourceforge.net). The average read depth obtained was approximately 7x for each SA individual and approximately 55x average depth for the control pool. SNPs and small insertions/deletions were called from the mapping data after subjecting the alignments to realignment around indels and then variant calling using the Genome Analysis Toolkit (GATK) [8]. The variant calls were subjected to recommended VariantFiltrationWalker filters for SNPs listed in the GATK wiki page (http://www.broadinstitute.org/gsa/wiki/index.php/The_Genome _Analysis_Toolkit).

The software SAMtools [7] was used to determine sequence read depths observed in windows of one kilobase over the whole genome and candidate deletions and duplications were called using these depths. Furthermore the paired read mapping distances as well as strands were used to detect structural variations in relation to the reference assembly and in relation to the control pool.

Digital droplet PCR (ddPCR)

The ddPCR reaction mixtures consisted of 11 μΐ 2X ddPCR Supermix for probes (Bio-Rad), 1.1 μΐ of the primer/probe mix for one of the deletion assays and 1.1 ul of the RNAse P reference gene primer/probe mix (900 nM final concentration of each primer, 250 nM of probe) and 1 μΐ of sample DNA (concentration 20 ng/μΐ) in a final volume of 22 μΐ (see Table 1 for primer and probe sequences). 20 μΐ of reaction mixture was loaded into a disposable plastic cartridge (DG8, Bio-Rad) together with 70 μΐ of droplet generation oil (DG oil, Bio-Rad) and placed in the QX100 droplet generator (Bio-Rad). The droplets generated from each sample were then transferred to a 96-well Twin Tec semi-skirted PCR plate (Eppendorf, Germany) which was heat-sealed with Easy Pierce Foil (Thermo).

Table 1: Sequences of primers and probes used in digital droplet PCR to genotype deletions at the Equine SHOX locus

*Non-standard bases correspond to nucleic acid ambiguity codes and indicate positions where mixed bases have been incorporated in primers PCR amplification was carried out on a T1000 Touch thermal cycler (Bio- Rad) using a thermal profile beginning at 95°C for 10 min, followed by 40 cycles of 94°C for 30 s and 60°C for 60 s, 1 cycle of 98°C for 10 min, and ending at 4°C. After amplification, the plate was loaded on the droplet reader (Bio-Rad) and the droplets from each well of the plate were read automatically. ddPCR data were analyzed using the QuantaSoft analysis software (Bio-Rad).

Identification of the mutations causing Skeletal Atavism

To identify the mutation(s) causing skeletal atavism in Shetland ponnies whole genome resequencing of affected horses and controls were performed. DNA samples from six individual Swedish Shetland ponies diagnosed with skeletal atavism, and a pool of 22 unaffected control stallions were sequenced using Illumina Hi-seq technology with resulting average sequence depths of 7X (each affected individual) and 55X (control pool). After aligning the reads to the reference genome assembly EquCab2 [5] (http://www.ncbi.nlm.nih.gOv/assembly/GCA_000002305.l/) using the software BWA [6]. SAMtools [7] was used to determine genome wide depth of coverage for each sequenced sample and GATK [8] to call polymorphisms and to determine genotypes in the samples and to estimate the allele frequencies in the pool.

Next single nucleotide polymorphisms (SNPs) were screened for where each affected horse was homozygous for the variant allele and where the reference allele frequency was 100% in the control pool. In total, 25 SNPs fulfilled this criterion, and 17 of these were located to a very fragmented unplaced scaffold that contains the short stature homeobox (SHOX) gene.

An analysis of the depth of sequence read coverage revealed two separate partially overlapping deletions estimated to be at least 116 Kb and 34 Kb, respectively

(Figure 1) and involved the genome assembly contigs listed in Table 2.

Table 2: Horse sequence contigs predicted to be part of the two identified deletions based on depth of sequence coverage from the six cases and the control

NW 001867655.1 AAWR02042952.1 D1 AN D D2 17

NW 001867655.1 AAWR02042953.1 D1 AN D D2 18

NW 001867655.1 AAWR02042954.1 D1 AN D D2 19

NW 001867655.1 AAWR02042955.1 D1 AN D D2 20

NW 001867655.1 AAWR02042956.1 D1 AN D D2 21

NW 001867655.1 AAWR02042957.1 D1 AN D D2 22

NW 001867655.1 AAWR02042958.1 D1 AN D D2 23

NW 001867655.1 AAWR02042959.1 D1 AN D D2 24

NW 001867655.1 AAWR02042960.1 D1 AN D D2 25

NW 001867655.1 AAWR02042961 .1 D1 AN D D2 26

NW 001867655.1 AAWR02042962.1 D1 AN D D2 27

NW 001867655.1 AAWR02042963.1 D1 AN D D2 28

NW 001867655.1 AAWR02042964.1 D1 AN D D2 29

NW 001867655.1 AAWR02042965.1 D1 AN D D2 30

NW 001867655.1 AAWR02042966.1 D1 AN D D2 31

NW 001867655.1 AAWR02042967.1 D1 AN D D2 32

NW 001867655.1 AAWR02042968.1 D1 AN D D2 33

NW 001867655.1 AAWR02042969.1 D1 AN D D2 34

NW 001867655.1 AAWR02042970.1 D1 AN D D2 35

NW 001867655.1 AAWR02042971 .1 D1 AN D D2 36

NW 001867655.1 AAWR02042972.1 D1 AN D D2 37

NW 001867655.1 AAWR02042973.1 D1 AN D D2 38

NW 001867655.1 AAWR02042974.1 D1 AN D D2 39

NW 001867655.1 AAWR02042975.1 D1 AN D D2 40

NW 001867655.1 AAWR02042976.1 D1 AN D D2 41

NW 001867655.1 AAWR02042977.1 D1 AN D D2 42

NW 001867655.1 AAWR02042978.1 D1 AN D D2 43

NW 001867655.1 AAWR02042979.1 D1 AN D D2 44

NW 001867655.1 AAWR02042980.1 D1 AN D D2 45

NW 001867655.1 AAWR02042981 .1 D1 AN D D2 46

NW 001867655.1 AAWR02042982.1 D1 AN D D2 47

NW 001867655.1 AAWR02042983.1 D1 AN D D2 48

NW 001867655.1 AAWR02042984.1 D1 AN D D2 49

NW 001867655.1 AAWR02042985.1 D1 AN D D2 50

NW 001867655.1 AAWR02042986.1 D1 AN D D2 51

NW 001867655.1 AAWR02042987.1 D1 AN D D2 52

NW 001867655.1 AAWR02042988.1 D1 AN D D2 53

NW 001867655.1 AAWR02042989.1 D1 AN D D2 54

NW 001867655.1 AAWR02042990.1 D1 AN D D2 55

NW 001867655.1 AAWR02042991 .1 D1 AN D D2 56

NW 001867655.1 AAWR02042992.1 D1 AN D D2 57

NW 001867655.1 AAWR02042993.1 D1 AN D D2 58

NW 001867809.1 AAWR02043090.1 D1 59

NW 001867809.1 AAWR02043091 .1 D1 60

NW 001867809.1 AAWR02043092.1 D1 61

NW 001867809.1 AAWR02043093.1 D1 62

NW 001867809.1 AAWR02043094.1 D1 63

NW 001867809.1 AAWR02043095.1 D1 64

NW 001867809.1 AAWR02043096.1 D1 65

NW 001867809.1 AAWR02043097.1 D1 66

NW 001867809.1 AAWR02043098.1 D1 67

NW 001867809.1 AAWR02043099.1 D1 68

NW 001867809.1 AAWR02043100.1 D1 69

NW 001867809.1 AAWR02043101 .1 D1 70

NW 001867809.1 AAWR02043102.1 D1 71

NW 001867809.1 AAWR02043103.1 D1 72

NW 001867809.1 AAWR02043104.1 D1 73

NW 001869437.1 AAWR02043982.1 D1 74 NW 001869437.1 AAWR02043983.1 D1 75

NW 001869437.1 AAWR02043984.1 D1 76

NW 001869437.1 AAWR02043985.1 D1 77

NW 001869437.1 AAWR02043986.1 D1 78

NW 001869437.1 AAWR02043987.1 D1 79

NW 001870009.1 AAWR02044192.1 D1 80

NW 001870009.1 AAWR02044193.1 D1 81

NW 001870009.1 AAWR02044194.1 D1 82

NW 001870009.1 AAWR02044195.1 D1 83

NW 001870009.1 AAWR02044196.1 D1 84

NW 001870009.1 AAWR02044197.1 D1 85

NW 001873507.1 AAWR02044981 .1 D1 86

NW 001873507.1 AAWR02044982.1 D1 87

NW 001873507.1 AAWR02044983.1 D1 88

NW 001875146.1 AAWR02045249.1 D1 89

NW 001875146.1 AAWR02045250.1 D1 90

NW 001876884.1 AAWR02045517.1 D1 AN D D2 91

NW 001876884.1 AAWR02045518.1 D1 AN D D2 92

NW 001876884.1 AAWR02045519.1 D1 AN D D2 93

NW 001876884.1 AAWR02045520.1 D1 AN D D2 94

NW 001876884.1 AAWR02045521 .1 D1 AN D D2 95

NW 001871 185.1 AAWR02049699.1 D1 96

NW 001869338.1 AAWR02043946.1 D1 AN D D2 97

NW 001869338.1 AAWR02043947.1 D1 AN D D2 98

NW 001869338.1 AAWR02043948.1 D1 AN D D2 99

NW 001869338.1 AAWR02043949.1 D1 AN D D2 100

NW 001869338.1 AAWR02043950.1 D1 AN D D2 101

NW 001869338.1 AAWR02043951 .1 D1 AN D D2 102

NW 001869338.1 AAWR02043952.1 D1 AN D D2 103

NW 001869338.1 AAWR02043953.1 D1 AN D D2 104

NW 001867532.1 AAWR02045716.1 D1 AN D D2 105

NW 001867532.1 AAWR02045717.1 D1 AN D D2 106

NW 001867532.1 AAWR02045718.1 D1 AN D D2 107

NW 001867532.1 AAWR02045719.1 D1 AN D D2 108

NW 001873348.1 AAWR02051849.1 D1 AN D D2 109

NW 001873348.1 AAWR02051850.1 D1 AN D D2 1 10

^Λ Scaffold and Contig accessions: Genbank accession numbers of the reference genome assembly contigs and Scaffolds.

* Deletion overlap: Presumed deletion(s) involving the contig. It was not possible to determine the exact size of the two deletions with confidence using this approach due to the poor assembly of this region. The largest deletion (Dl) spans over the entire coding sequence of SHOX while the other (D2) involves the region immediately downstream of the SHOX coding sequence (Figure 1). SHOX has been mapped to the pseudo-autosomal region (PAR) of the X and Y- chromosomes in other mammals and it is very likely that it is located in the PAR region in horses as well. In humans, mutation and haploinsufficiency of SHOX are associated with idiopathic growth retardation [9,10]. Sequencing of bacterial artificial chromosomes (BACs)

In order to obtain additional sequence information BACs whose ends (BAC- ends) had been previously sequenced as a part of the generation of the horse genome assembly (EquCab2) and that were predicted to reside in the Pseudo-autosomal region close to the SHOX gene were identified. 13 such BACs from the CHORI-241 BAC library (http:/ bacpac.chori.org librar»'.php?id=41 ) made from a Thoroughbred male horse (not carrying the deletions Dell or Del2), available from the BACPAC resource at the Childrens Hospital Oakland Research Institute

(http://bacpac.chori.org/equine24Lhtm) were identified.

Table 4. Sequenced BACs from SHOX region

DNA was prepared from the purchased BACs (Table 4) according to standard laboratory procedures and, for each BAC, purified BAC DNA was subjected to sequencing using the Pacific Biosciences DNA sequencing methodology which is capable of generating long sequencing reads. Following sequencing, generated sequencing reads were subjected to de-novo assembly whereby individual reads from each BAC were assembled together into one or more contigs. The resulting assembled contigs were subsequently used as templates for alignment of the short Illumina sequencing reads from Atavism individuals CGI, CG2, CG3, CG4, CG5 and CG6 as well as the DNA pool comprising normal horses. This alignment information was used to determine sequencing read depth in windows to identify BAC -contigs or parts thereof where depth of coverage was consistent with the genotyped of the Atavistic horses, ie. CGI , CG5 and CG6 are of genotype Dell/Dell and will therefore entirely lack high confidence read alignments for BAC contig parts corresponding to Deletion 1. Individuals CG2, CG3 and CG4 (Genotype=Dell/Del2) will have approximately half the depth of coverage compared to the pool of DNA from normal horses in the BAC contig parts unique to Deletion 1 and entirely lack coverage in the parts shared between Deletion 1 and Deletion 2.

Table 5. BAC sequences identified to contain Dell and/or Del2 sequences

^The Dell/Del2 breakpoint was found in a region rich in repeats not making it possible to exactly define the position of the breakpoint.

Genotvping using ddPCR

Among the six affected horses, three were homozygous Dl/Dl and three were D1/D2 composite heterozygotes. We used digital droplet PCR (ddPCR) (Biorad) to genotype 39 Swedish Shetland ponies, 18 known carriers, 6 affected horses and 15 unaffected horses, for the two deletions (Dl and D2). The six affected horses were the same as used for sequencing and we confirmed that three of them were homozygous Dl/Dl and three were heterozygous D1/D2 (Table 3).

Table 3. Results of digital PCR analysis of the SHOX locus in horses with or without skeletal atavism. Three alleles occur at this locus: WT=wild type, Dl=Deletion 1,

D2=Deletion 2

Genotype

Failed

Horse ³ WT/WT WT/D1 WT/D2 D1 /D1 D1 /D2 D2/D2 Total genotyping

Affected 0 0 0 0 3 3 0 6

Carrier 4 2 8 3 0 0 1 18

Unaffected 1 12 2 0 0 0 0 15

Affected horses show skeletal atavism, Carriers are heterozygous for a disease causing mutation while Unaffected may be homozygous wild type or heterozygous for a disease causing mutation.

It was possible to trace the inheritance of these alleles from known carriers to affected offspring. All but two known carriers for which genotypes could be determined were heterozygous for one of the deletions (WT/D1 or WT/D2). All unaffected horses carried at least one copy of the WT allele. Thus, it was concluded that skeletal atavism is caused by two different deletion alleles associated with the SHOX locus. Affected horses may be homozygous Dl/Dl, heterozygous D1/D2 or possibly homozygous D2/D2. We have observed one carrier with genotype D2/D2 and this individual is not reported as affected suggesting that the D2/D2 genotype may not be associated with skeletal atavism at least not in all individuals with this genotype.

Conclusion

In conclusion, two deletions in the SHOX gene causing skeletal atavism in horses have been identified. Methods for detecting the presence of these deletions can now be used to identify unaffected carriers of these mutations and use this information to avoid the risk that a mating will produce an affected offspring. In matings between two carriers 25% of the progeny are expected to show skeletal atavism. The deletions can be detected using digital PCR or quantitative PCR. REFERENCES

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