Mutations in <em>MITF</em> and <em>PAX3</em> Cause “Splashed White” and Other White Spotting Phenotypes in Horses

<div><p>During fetal development neural-crest-derived melanoblasts migrate across the entire body surface and differentiate into melanocytes, the pigment-producing cells. Alterations in this precisely regulated process can lead to white spotting patterns. White spotting patterns in horses are a complex trait with a large phenotypic variance ranging from minimal white markings up to completely white horses. The “splashed white” pattern is primarily characterized by an extremely large blaze, often accompanied by extended white markings at the distal limbs and blue eyes. Some, but not all, splashed white horses are deaf. We analyzed a Quarter Horse family segregating for the splashed white coat color. Genome-wide linkage analysis in 31 horses gave a positive LOD score of 1.6 in a region on chromosome 6 containing the <em>PAX3</em> gene. However, the linkage data were not in agreement with a monogenic inheritance of a single fully penetrant mutation. We sequenced the <em>PAX3</em> gene and identified a missense mutation in some, but not all, splashed white Quarter Horses. Genome-wide association analysis indicated a potential second signal near <em>MITF</em>. We therefore sequenced the <em>MITF</em> gene and found a 10 bp insertion in the melanocyte-specific promoter. The <em>MITF</em> promoter variant was present in some splashed white Quarter Horses from the studied family, but also in splashed white horses from other horse breeds. Finally, we identified two additional non-synonymous mutations in the <em>MITF</em> gene in unrelated horses with white spotting phenotypes. Thus, several independent mutations in <em>MITF</em> and <em>PAX3</em> together with known variants in the <em>EDNRB</em> and <em>KIT</em> genes explain a large proportion of horses with the more extreme white spotting phenotypes.</p>