Coat Color Genotypes and Risk and Severity of Melanoma in Gray Quarter Horses

Background Both graying and melanoma formation in horses have recently been linked to a duplication in the STX 17 gene. This duplication, as well as a mutation in the ASIP gene that increases MC 1R pathway signaling, affects melanoma risk and severity in gray horses. Objective To determine if melanoma susceptibility in gray Quarter Horses ( QH ) is lower than gray horses from other breeds because of decreased MC 1R signaling resulting from a high incidence of the MC 1R chestnut coat color allele in the QH population. Animals A total of 335 gray QH with and without dermal melanomas. Methods Blood or hair root samples were collected from all horses for DNA extraction and genotyping for STX 17 , ASIP , and MC 1R genotypes. Age, sex, and external melanoma presence and grade were recorded. The effect of age and genotype on melanoma presence and severity was evaluated by candidate gene association. Results Melanoma prevalence (16%) and grade (0.35) in this QH cohort was lower than that reported in other breeds. Age was significantly associated with melanoma prevalence ( P  = 5.28 × 10 −11 ) and severity ( P  = 2.2 × 10 −13 ). No significant effect of MC 1R genotype on melanoma prevalence or severity was identified. An effect of ASIP genotype on melanoma risk was not detected. Low STX 17 homozygosity precluded evaluation of the gray allele effect. Conclusion and clinical importance Melanoma prevalence and severity is lower in this population of gray QH than what is reported in other breeds. This could be because of the infrequent STX 17 homozygosity, a mitigating effect of the MC 1R mutation on ASIP potentiation of melanoma, other genes in the MC 1R signaling pathway, or differences in breed genetic background.