Summary

# Editorial Summary Whilst over 40 genetic variants are known to influence white spotting in horses, many white phenotypes remain genetically unexplained, particularly where multiple pigmentation loci interact. Rosa et al. (2022) sequenced the KIT and MITF genes in two stallions with uncharacterised white spotting and genotyped 147 related individuals, using three independent observers to score white coat extent via an Average Grade of White (AGW) phenotype. They identified a novel missense variant in KIT (designated W34) that showed highly significant association with AGW score (p = 3.3 × 10⁻¹²), with chestnut horses (MC1R e/e) displaying substantially higher white extent than Extension carriers (p = 3.09 × 10⁻¹⁷), demonstrating epistatic interaction between these two loci. The W34 variant is predicted to be deleterious and shows complete linkage with the previously documented W19 allele, establishing a molecular basis for a previously unidentified white spotting pattern. For equine professionals involved in breeding selection, health screening or coat colour prediction, this work clarifies the genetic architecture underlying white spotting and explains why certain families express markedly more extensive white than would be predicted from single-locus models alone.

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Key Findings

• •A novel missense variant in KIT (rs1140732842, p.T391A) designated W34 shows significant association with white spotting phenotype (p = 3.3 × 10⁻¹²)
• •The W34 variant demonstrates complete linkage to the previously reported KIT W19 allele
• •MC1R Extension locus shows epistatic interaction with white spotting, with chestnut (e/e) individuals having significantly higher white spotting scores than Extension carriers (p = 3.09 × 10⁻¹⁷)
• •The KIT W34 variant is predicted to be deleterious by both SIFT and PROVEAN computational tools

Conditions Studied

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