Two Variants in SLC24A5 Are Associated with "Tiger-Eye" Iris Pigmentation in Puerto Rican Paso Fino Horses

A unique eye color, called tiger-eye, segregates in the Puerto Rican Paso Fino Horse breed and is characterized by a bright yellow, amber, or orange iris. Pedigree analysis identified a simple autosomal recessive mode of inheritance for this trait. A genome-wide association study with 24 individuals identified a locus on ECA 1 reaching genome wide significance (P_corrected =1.32 x 10^-5). This ECA1 locus harbors the candidate gene, Solute Carrier Family 24 (Sodium/Potassium /Calcium Exchanger), Member 5 (SLC24A5), with known roles in pigmentation in humans, mice, and zebrafish. Humans with compound heterozygous mutations in SLC24A5 have oculocutaneous albinism type 6 (OCA6) which is characterized by dilute skin, hair, and eye pigmentation as well as ocular anomalies. Twenty tiger-eye horses were homozygous for a non-synonymous mutation in Exon 2 (p.Phe91Tyr) of SLC24A5 (called here Tiger-eye 1), which is predicted to be deleterious to protein function. Additionally, eight of the remaining 12 tiger-eye horses heterozygous for the p.Phe91Tyr variant were also heterozygous for a 628bp deletion encompassing all of Exon 7 of SLC24A5 (c.875-340_1081+82del) which we will call here the Tiger-eye 2 allele. None of the 122 brown-eyed horses were homozygous for either tiger-eye associated allele or were compound heterozygotes. Further, neither variant was detected in 196 horses from four related breeds, not known to have the tiger-eye phenotype. Here we propose that two mutations in SLC24A5 affect iris pigmentation in tiger-eye Puerto Rican Paso Fino horses. Further, unlike OCA6 in humans, the Tiger-eye 1 mutation in its homozygous state or as a compound heterozygote (Tiger-eye 1/ Tiger-eye 2), do not appear to cause ocular anomalies or change in coat color in the Puerto Rican Paso Fino horse.

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