In the previous post, I wrote about mutations in the PMEL17 gene that affect the structure of melanosomes in the pigment cells of zebrafish, chickens, and mice. Subsequent research has shown that similar “silvering” mutations are also responsible for merle coloring in dogs, and for silver dappling in horses. These pigmentation patterns may have relevance for some human disorders, including Waardenburg syndrome, as they include not only changes in coat color, but hearing deficits and eye abnormalities as well. For example, over 50% of Dachshunds homozygous for merle (MM) exhibit mild to severe hearing problems, and individuals with Waardenburg Syndrome may also be congenitally deaf.
Figure 1. SINE insertion in the SILV/PMEL17 gene correlates with merle phenotype (Clark et al., 2006): A. Tricolored nonmerle (mm) Shetland Sheepdog B. Blue merle (Mm) C. Double merle (MM)
Clarke and colleagues performed linkage analysis with genotype data from 16 merle, 2 double merle, and 43 nonmerle Shetland Sheepdogs. Shelties are a charming and intelligent dog breed; I’ve had two tricolored Shelties in the past, and they were both every bit as obsessive-compulsive as is their mistress. SILV/PMEL17 was the candidate gene obtained, and PCR analysis revealed a larger product from exon 11 in both blue merle and double merle dogs. The larger product arises from a canine short interspersed element (SINE), inserted at the intron 10/exon 11 boundary of SILV/PMEL17, and the insert was present in the homozygous state in two double merle Shetland Sheepdogs, and in the heterozygous state in twelve merles. To determine whether this tRNA-derived SINE insertion was specific to Shetland Sheepdogs, the researchers looked for the larger SILV PCR product in nonmerle and merle dogs of six other breeds. Merle dogs of the Collie, Border Collie, Australian Shepherd, Cardigan Welsh Corgi, Dachshund, and Great Dane breeds were all heterozygous for the SINE insertion in the SILV gene. Moreover, 12 harlequin Great Danes were determined to be either heterozygous or homozygous for the insertion, and this retrotransposon insertion also segregated with the merle pattern in 5 additional breeds (Pyrenean Shepherd, Chihuahua, Miniature Poodle, American Pit Bull Terrier, Catahoula Leopard Dog).
Figure 2. Silver colored horses (Brunberg et al., 2006) a. Black Silver Icelandic horse b. Black Silver Rocky Mountain Horses c. Brown Silver Morgan horse d. The legs of a Brown Silver horse, showing dilution from black to greyish
The Silver (Z) coat color in horses is also characterized by autosomal dominant inheritance and a dilution of black pigment (eumelanin) in the hair. In both black and bay horses, the Silver mutation effect is most striking in the flaxen mane and tail; Silver colored foals have a pale coat with white mane, tail, and eyelashes, and striped hooves, which disappear by a year of age. The Silver mutation has very little effect on a chestnut or sorrel horse, consistent with the minimal consequences (if any), of PMEL17 mutations on pheomelanogenesis in chickens. In Icelandic horses, American Miniature horses, and Rocky Mountain horses, the Silver mutation is fairly common, and it has also been observed in other horse breeds, including Morgan horses, Swedish Warmbloods, and American Saddlebreds.
Figure 3. Silver colored foals (Brunberg et al., 2006) a. Silver colored Icelandic horse foal b. Striped hoof of a Silver foal c. White eyelashes on a young Silver Rocky Mountain horse
Linkage analyses for the progeny of a Silver stallion were consistent with PMEL17 as a candidate gene, and therefore the entire gene was sequenced in genomic DNA samples from Silver and non-Silver horses of the Icelandic breed. Two single nucleotide polymorphisms (SNPs) in PMEL17, associated with the Silver coloration, were identified by Brunberg and colleagues (2006); these sequence variants are located in intron 9 and exon 11 of the gene. The researchers expanded their SNP analyses to encompass 14 different horse breeds, 6 of which included Silver individuals. Both of the mutations were associated with Silver coloration, and were not found in non-Silver individuals, with the exception of one chestnut Morgan horse. The locations and types of PMEL17 mutations in other species are summarized in the figure below. As with double merle Australian Shepherd dogs, Silver individuals of some horse breeds (Rocky Mountain horse, Kentucky Saddle horse) may have eye abnormalities. The investigators also point out that their genetic testing method may be useful for identifying carriers of the Silver mutation, which is a highly sought after coat color pattern in horses.
Figure 4. PMEL17 protein with known mutations. (Brunberg et al., 2006)
Brunberg, E., Andersson, L., Cothran, G., Sandberg, K., Mikko, S., and Lindgren, G. (2006). A missense mutation in PMEL17 is associated with the Silver coat color in the horse. BMC Genetics 7:46, DOI: 10.1186/1471-2156-7-46
Clark, L.A. (2006). From The Cover: Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. Proceedings of the National Academy of Sciences, 103(5), 1376-1381. DOI: 10.1073/pnas.0506940103