Focus on Equine Genetics: the Ol' Gray Mare

For more than 100 years, equine researchers have been examining why gray horses that are losing hair pigmentation are often concurrently affected by melanomas—tumors characterized by a massive production of the pigment melanin.

According to European researchers, humans cherry-picking gray coat color milleniums ago--due to the social prestige of riding a white horse—also inadvertently selected a gene mutation that predisposes horses to melanomas.

Professor Leif Andersson, PhD, from Uppsala University and the Swedish University of Agricultural Sciences, explained this selection for melanomas was likely not noticed as the gray mutation has no negative effects on the function of the horse throughout most of its life.

"A gray horse is as strong and runs as fast as any other horse and is probably better adapted to an environment with strong sunlight because the white coat reflects sunlight and its black skin protects from UV-damage," said Andersson. "Identifying the genetic cause of the gray coat color in horses is important because 70-80% of gray horses more than 15 years of age have melanomas and reduced lifespans," due to these growths.

Based on previous research, Andersson and colleagues knew that the mutant gray gene was found on a specific region of chromosome 9. In this study they identified and evaluated four candidate genes located in the same region as the gray-causing mutation.

The researchers identified the relevant gene, based on a duplication they discovered.

"We hypothesize that this genetic duplication results in the proliferation of melanocytes, melanin-producing cells, in certain areas of the skin which predisposes gray horses to the development in melanoma," Andersson explained.

At the same time, there is a hyperproliferation (an abnormally high rate of cell division) of  melanocytes in the hair follicles elsewhere on the body that depletes stem cells necessary for  replenishing  future stores of melanocytes. Eventually, horses with the gray mutation effectively run out of melanocytes and exhibit premature graying.

The research group is now breeding transgenic mice (in which one or more genes of another species—in this case, the horse—have been incorporated) that mimic the mutation in gray horses so that scientists can study the graying process and the development of tumors in great detail.

The study, "A cis-acting regulatory mechanism causes premature hair graying and susceptibility to melanoma in the horse," was published in the August edition of the journal Nature Genetics.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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