A Simple Repeat Polymorphism in the MITF-M Promoter Is a Key Regulator of White Spotting in Dogs

Abstract

The white spotting locus (S) in dogs is colocalized with the MITF (microphtalmia-associated transcription factor) gene. The phenotypic effects of the four S alleles range from solid colour (S) to extreme white spotting (sw). We have investigated four candidate mutations associated with the sw allele, a SINE insertion, a SNP at a conserved site and a simple repeat polymorphism all associated with the MITF-M promoter as well as a 12 base pair deletion in exon 1B. The variants associated with white spotting at all four loci were also found among wolves and we conclude that none of these could be a sole causal mutation, at least not for extreme white spotting. We propose that the three canine white spotting alleles are not caused by three independent mutations but represent haplotype effects due to different combinations of causal polymorphisms. The simple repeat polymorphism showed extensive diversity both in dogs and wolves, and allele-sharing was common between wolves and white spotted dogs but was non-existent between solid and spotted dogs as well as between wolves and solid dogs. This finding was unexpected as Solid is assumed to be the wild-type allele. The data indicate that the simple repeat polymorphism has been a target for selection during dog domestication and breed formation. We also evaluated the significance of the three MITF-M associated polymorphisms with a Luciferase assay, and found conclusive evidence that the simple repeat polymorphism affects promoter activity. Three alleles associated with white spotting gave consistently lower promoter activity compared with the allele associated with solid colour. We propose that the simple repeat polymorphism affects cooperativity between transcription factors binding on either flanking sides of the repeat. Thus, both genetic and functional evidence show that the simple repeat polymorphism is a key regulator of white spotting in dogs.

Microphthalmia‐associated transcription factor mutations are associated with white‐spotted coat color in swamp buffalo

“…Subsequent functional studies revealed that regulatory mutations located in the MITF‐ M promoter influenced transcription where a simple repeat polymorphism was shown to be a key factor underlying white spotting (Baranowska Körberg et al . 2014). …”

MITF—the first 25 years

“…Given that Mitf is evolutionarily conserved, it is not surprising that its roles in pigmentation can be seen in other species. In domestic dogs (Baranowska Körberg et al 2014) and horses (Hauswirth et al 2012), for instance, alterations in the promoter giving rise to the major melanocyte isoform of MITF are responsible for white spotting. Nevertheless, pigmentation need not always be the major target for Mitf homologs.…”

Demographic assessment of the Dalmatian dog – effective population size, linkage disequilibrium and inbreeding coefficients

“…The Dalmatians spots are another trait fixed in the breed. One gene responsible for this phenotype is MITF, which causes the extreme white spotting [24,25]. The colored spots result from the influence of a ticking locus [25].…”