NEW YORK (GenomeWeb) – Researchers at a canine genetic testing firm have linked a genetic duplication to blue eyes in Siberian Huskies and other dogs.
An increasing number of firms offer breeders, owners, and veterinarians genetic testing, and a few, like Embark Veterinary, focus on marketing their tests to consumers. In 2016, Embark launched a $199 test to give dog owners a glimpse of their pooch's breed, ancestry, and health risks. Like its counterpart 23andMe in human direct-to-consumer genetic testing, Embark has used the data it collected for research.
As in humans, eye color among dogs varies, and while two genetic loci have been linked previously to canine eye color — as well as to coat color — they do not explain all instances of blue eyes in dogs.
Researchers from Embark conducted a genome-wide association study of more than 3,000 dogs that linked a stretch of chromosome 18 to eye color. As they reported today in PLOS Genetics, they then homed in on a duplication in this region that was linked to blue eyes among Siberian Huskies.
"Using genetic data from the pets of our customers, combined with eye colors reported by customers for those same animals, we have discovered a genetic duplication that is strongly associated with blue eye color," senior author Aaron Sams, a senior scientist at Embark, said in a statement.
In their GWAS, using a discovery panel of 3,180 dogs that had undergone genotyping by Embark's service and phenotyping by owner survey and picture uploads, the researchers uncovered two loci associated with blue eyes: one on chromosome 10 at the gene PMEL17 and another on chromosome 18, CFA18, near the gene ALX4.
Previously, variants at PMEL17 and MITF have been linked to eye color among dogs. They also influence coat coloration: PMEL17 is linked to merle coats, while MITF is linked to piebald coats. The researchers noted that MITF likely didn't reach significance in their analysis, as piebald coat was associated with blue eyes in only a small number of cases.
The researchers then traced the association at CFA18 to a 98.6-kilobase tandem duplication found among non-merle, blue-eyed dogs, including a set of Siberian Huskies.
This duplication lies in an intergenic region upstream of ALX4, which plays a role in mammalian eye development and pigmentation. In some rodents, a paralog of ALX4, ALX3, represses MITF, and when ALX3 is upregulated, the rodents have reduced melanin content and lighter coats. Meanwhile, in zebrafish, ALX4 orthologs are expressed in iridophores — pigment-containing iridescent cells — and in humans, ALX4 is expressed in the retinal pigment epithelium. This led the researchers to hypothesize that this duplication increases the expression of ALX4 in the retinal pigment epithelium to repress MITF and reduce melanin in the iris.
In customer data from a separate cohort of 2,890 dogs, the researchers found that this blue-eyed-linked haplotype was present at low frequency. Most dogs that carried it were Siberian Huskies or Klee Kais — a breed derived from Huskies — but it was also present in Australian Shepherd and Australian Cattle Dogs. All but one of the 46 dogs with the haplotype for which a picture was also available had blue eyes. The exception, the researchers noted, was a brown-eyed Siberian Husky, whose owners told the researchers that the dog had sired all blue-eyed or heterochromatic litters.
As 53 percent of their blue-eyed dogs were heterozygotes, the researchers noted that if this duplication were causal, it would likely be dominant. Still, they added, it did not appear to be fully penetrant and other factors might modify its effect.