Two Mutations in Solute Carrier Family 24 Member 5 Are Associated with "Tiger Eye" Iris Pigmentation in Puerto Rican Paso Fino Horses

A unique iris pigmentation known as “tiger eye” segregates in the Puerto Rican Paso Fino breed and is characterized by a yellow, amber, or bright orange coloration. Pedigree analysis of 154 individuals identified a simple autosomal recessive mode of inheritance. A genome-wide association study (N=24) using the Illumina Equine SNP70 BeadChip and a mixed linear model analysis (EMMAX) to correct for relatedness identified a locus on ECA 1 for further investigation (p_corrected = 1.3 x 10^-5). The positional and functional candidate gene Solute Carrier Family 24 Member 5 (SLC24A5) encodes a potassium-dependent sodium-calcium ion exchanger important in melanogenesis. Mutations in SLC24A5 cause human, mouse, and zebrafish pigmentation phenotypes. Sanger sequencing of this gene revealed two coding variants associated with the “tiger eye” phenotype. Specifically, twenty “tiger eye” horses were homozygous for a non-synonymous mutation in Exon 2 (p.Phe91Tyr) predicted to be deleterious. Additionally, eight “tiger eye” horses with one copy of the Exon 2 variant were also heterozygous for a 628bp deletion encompassing all of Exon 7 (ECA1:141,660,611 – 141,661,239). Of the 122 brown-eyed horses none were homozygous for the exon 2 variant or were compound heterozygotes. Tiger eye has not been reported in any other breed of horse and in testing 188 horses of related breeds, neither allele was found. We propose that in the horse these two SLC24A5 mutations affect iris pigmentation while leaving the coat unaffected. Unlike mutations in this gene that cause oculocutanenous albinism in humans, the horse SLC24A5 variants do not result in other ocular anomalies.