Photos of a bay Thoroughbred stallion with white face and legs, and his all-white offspring with blue eyes

VGL researchers identify a novel Splashed White variant (SW8) in a Thoroughbred horse

A VGL research study lead by Dr. Rebecca Bellone identified a de novo genetic variant in the microphthalmia-associated transcription factor (MITF) gene of a Thoroughbred stallion with splashed white coat pattern. Splashed white got its name because horses with this coat pattern look like they splashed in white paint, as they typically have high amounts of white markings on the legs, face, and abdomen.

The change in the DNA of the Thoroughbred stallion, recently discovered by our team, is a 2.3 thousand base pair deletion in the MITF gene and has been designated SW8, since it is the 8th variant named to date describing a splashed white pattern.

The study determined that the change was a de novo mutation, that happened on the maternal chromosome, meaning it occurred either in the formation of the egg or early in embryogenesis on the chromosome that was inherited from the mother.

The study also identified that the SW8 variant was passed on from the investigated stallion to his nearly all-white offspring that has blue eyes and was clinically confirmed to be completely deaf. The offspring was also compound heterozygote for two dominant white variants in the KIT gene (W22/W20). To date, deafness has not been reported in horses that only have the dominant white phenotype. At this time, it is unknown whether SW8 alone can cause deafness or if the combination of the different white patterning alleles in addition to SW8 is what caused deafness in this offspring.

The present study is the first to identify the parental origin of one of the four known de novo MITF variants reported in the horse and clinically confirm deafness in one horse with the variant.

The study highlights the importance of investigating clinical phenotypes associated with novel pigmentation variants and indicates that additional studies are needed to determine the risk of deafness associated with the MITF variants, whether alone or in combination with other white pattern alleles.

As a result of this study, the VGL has added SW8 to the Splashed White test. Visit our test page for more information about Splashed White and the SW8 allele, or click here to review the latest study.