The role of polymorphisms of the MSTN, GRIN2B and DOCK8 genes in the performance of pace-racing Icelandic horses

Abstract: This study investigated the association of single-nucleotide polymorphisms (SNPs) of three different genes with pace racing performance in the Icelandic horse. The SNPs chosen were PR3737, PR8604 and PR9482 of the Myostatin gene; SNPs g.41206762 C>T and g.41218272 T>C of the GRIN2B gene and SNP g.22496787 T>C of the DOCK8 gene. Icelandic horses that compete frequently in any of the three pace race disciplines (P1 over 250m, P2 over 100m and P3 over 150m) were genotyped for all SNPs mentioned above. The horses in the sample (n=131) were divided into “Speed groups” based on consistency of racing times and training schemes. Three groups were created: Fast (n=37), Average (n=75) and Slow (n=19). Differences in genotype distribution among the different Speed groups were evaluated using Fisher’s Exact Test. Significant differences were found for all SNPs except PR9482 when comparing only Fast versus Slow horses. Significant differences were also found between all three groups when comparing allele frequencies. Moreover, a drastic difference in allele frequency of the mutant allele for the DOCK8 SNP was found between horses of the Fast group (frequency of the mutant allele was of 0.70) and 280 Icelandic horses genotyped for other studies (mutant allele frequency ranging between 0.33 (n=95) and 0.43 (n=192)). This implies an important role of this SNP and its mutant allele in pace-racing performance. Additionally, horses were evaluated by owners in terms of three temperament traits: Nervousness, Focus and Motivation. Significant differences were found in the distribution of scores for these temperament traits between horses with the different genotypes of the GRIN2B SNPs, suggesting a beneficial effect of the mutant alleles on the horse’s temperament. An association analysis was also performed, where the genotypes were evaluated for association with life-time racing performance results for race times (P1, P2 and/or P3), Breeding Field Test (BFT) assessment scores for pace and estimated breeding values (EBV) for pace at BFTs. Additionally, the genotypes were evaluated for association with performance results obtained when the horses competed at the age of 13 years or under (n=117) and over 13 years of age (n=64). Significant associations were found for PR9482, where the C allele (stamina-related in other breeds) was beneficial to performance in the longer P1 races and the T allele (speed-related in other breeds) was beneficial to performance in P3 for horses of ages 13 and under. Associations were also found for the GRIN2B SNP, with the C (mutant) allele having a beneficial effect in the shorter P3 races for both lifetime career and performance in the age group of 13 years and under. As for DOCK8, associations were also found in relation to P2, with the mutant (T) allele having a positive effect on the performance. No associations were found with any of the studied polymorphisms with BFT assessment scores for pace or with estimated breeding values. This study concludes that a SNP at the DOCK8 gene may well be beneficial to performance in pace-racing Icelandic horses due to the associations found with racing times and, more interestingly, the difference found in mutant allele frequencies between an elite group of racers and the rest of the population. Additionally, the study showcases the importance of temperament traits as factors that affect performance in pace-racing, as evidenced by the associations found between the mutant allele of the GRIN2B polymorphism and temperament scores, along with racing times. Furthermore, the study concludes that Myostatin SNPs have little effect in pace-racing performance, seeing as only one polymorphism affected performance and only in one of the age groups. Moreover, comparing mutant allele frequencies between the elite racers and a large number of Icelandic horses that were not selected for pace racing showed no variation at all. This further indicates these mutations may not play a major role in performance as they do in other breeds, such as the Thoroughbreds.