Splashed White Mutations – SW1, SW2, SW3, SW4, SW5, and SW6

Splashed white is a variable white spotting pattern characterized primarily by a large, broad blaze, extensive white markings on legs, variable white spotting on belly, and often blue eyes. Some, but not all, splashed white horses are also deaf. Six mutations have been identified and have been named in the order they were discovered: SW1, SW2, SW3, SW4, SW5, and SW6. All of these mutations cause a similar splashed white phenotype in horses although the amount of white patterning is variable and thought to be controlled by other genes. Some are known, but mostly these additional genes, contributing to the variation in splash white phenotype, are yet to be identified.

MITF - SW1, SW3, SW5, and SW6

SW1, SW3, SW5, SW6 are all variants in the MITF (microphthalmia associated transcription factor) gene. MITF is an important protein for normal pigment cell function. It is a transcription factor that activates other genes in the pigment process including those known to regulate melanocyte proliferation and migration.

SW1 is found in several breeds – Quarter Horse, Appaloosa, Paint Horse, Morgan Horse, Trakehner, Miniature Horse, Shetland Pony and Icelandic Horse – and may be present in other breeds as well. The SW1 variant is a 10 bp insertion in the MITF promotor (chr16:g.20,117,302Tdelins11). The promoter is the region of the gene that regulates gene expression. SW1 is thought to disrupt the normal gene production thus impacting pigment cell division. Horses homozygous for SW1 (SW1/SW1) have been identified, which suggests that this mutation is not homozygous lethal.

SW3 occurs exclusively in certain lines of Quarter Horses and Paint Horses and is caused by a frameshift mutation MITF (p.C280Sfs*20). SW3 is thought to be lethal in the homozygous state (SW3/SW3) as no individuals with this genotype have yet been identified. The current recommendation is that mating of two horses that carry SW3 should be avoided.

SW5 is a large deletion of 63 thousand nucleotides in the MITF gene (chr16:21,503,211–21,566,617). This variant removes several important regions of the gene and likely causes the gene to have a reduced or complete loss of function. This mutation was identified in an American Paint Horse family. Subsequent research at the VGL, suggest the frequency of this mutation is low in the Paint Horse breed. It is unknown if SW5 is homozygous lethal and no homozygotes have been reported.

SW6 was recently identified by research at the Veterinary Genetics Laboratory and like SW5 is a large deletion of the MITF gene, that likely inhibits proper function of the gene product. This deletion removes 8710 nucleotides of the gene. It is predicted that this change in the DNA results in a shortened MITF protein (the product of the MITF gene). The region that is removed normally binds to the DNA of other pigmentation genes to activate them. The end result is white areas lacking pigment cells. At this time, it is not known whether homozygosity for SW6 (two copies of this mutation) is viable. Based on the type of mutation and the role MITF plays in development, we suspect that SW6/SW6 may be embryonic lethal. However one SW1/SW6 compound heterozygote was identified as part of the VGL study.

SW6 was identified in a single family. An exciting finding to our study was identifying SW6 as a de novo (new/recent) mutation. More specifically, a de novo mutation means that the change in the DNA occurred either during the cellular processes that gave rise to the egg or sperm or during early embryonic development of the stallion investigated in the study. Being a de novo mutation also means only this individual, his offspring, and resulting future generations of horses originating with this stallion can have this mutation. Identification of a de novo mutation was an exciting finding because we know precisely the individual and generation in which the mutation occurred; this is not something we often get to determine.

PAX3 - SW2 and SW4

SW2 and SW4 are mutations in the PAX3 (paired box gene 3) gene. SW2 is found in certain lines of Quarter Horses and Paint Horses and is caused by a missense mutation in PAX3 (p.Cys70Tyr). PAX3 is essential for proper development of some nerve tissue and pigment cells. Based on testing results from the VGL, SW2 is currently considered not homozygous lethal, though horses that have SW2 may be deaf. The rare SW4 mutation has been identified in family of Appaloosa horses and may cause a splashed white or a broad blaze. SW4 is caused by a missense mutation in PAX3 (p.Pro32Arg), and is also thought to be lethal in the homozygous state, though this remains to be determined.

Splashed white mutations are inherited as dominant traits with variable expression, which means that one copy of a SW mutation will produce a white spotting phenotype with variable amount of white. Horses that carry combinations of the splashed white mutations, tobiano, or lethal white overo can display extensive white patterning or their coats maybe entirely white.

Testing for the splashed white pattern allows breeders to identify which splashed white mutation is present in their horse and can aid in breeding decisions and clinical decisions if deafness is suspected.