Three HPS5 mutations associated with depigmentation in diverse horse breeds

Depigmentation is a common phenotype in Equus caballus (horse); over 50 identified variants cause white spotting on the skin and coat. HPS5 may influence melanocyte proliferation and mutations in this gene cause depigmentation in humans, mice, and zebrafish. Here we report three equine HPS5 mutations, found in most major breed groups and associated with white spotting. We used a standardized method to quantitatively grade depigmentation, creating a White Score for each horse. We identified associations to each of three variants in HPS5 by comparing the average white score for carriers to that of 41 control horses possessing none of the studied variants. We estimated the functional impact of each mutation using SIFT, GenScan, I-TASSER and cross-species conservation. The first mutation, S1013C termed Eden White 1 (EDXW1), is a missense mutation found to increase average depigmentation by 5.56 points (p = 6.47E-6, n = 70, t-test). Eden White 2 (EDXW2), a missense mutation L560F, increased depigmentation by 9.5 points for individuals with one copy of the variant, but we were unable to phenotype homozygotes (p = 5.55E-16, n = 47, t-test). Eden White 3 (EDXW3), a putative donor splice site mutation at exon 4 + 4, increased white by 5.59 points on average (p = 9.56E-6, n = 101). EDXW1 and EDXW3 are epistatically influenced by MC1R, the gene that controls black vs red coat color in horses. Individuals possessing the dominant E allele produce black pigment, and this allele was associated with higher levels of depigmentation than individuals with the recessive chestnut genotype (e/e) (p = 1.50E-5, p = 1.26E-5 respectively). We tested for epistatic interactions between EDXW1 and Dominant White 20 and found that horses with one copy of each allele had increased average white spotting by 8 points with respect to EDXW1/n or W20/n horses. Compound heterozygotes were significantly more white than horses with one copy of either mutation (p = 7.33E-5), but interactions between other white spotting alleles and EDXW alleles remain to be tested. We propose the convention Eden White (EDXW) for HPS5 mutations in the domestic horse.