Researchers discover "graying gene" in horses

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Researchers from Texas A&M’s College of Veterinary Medicine and Biomedical Sciences (VMBS) and Uppsala University in Sweden have discovered a gene that determines how horses change colors as they age.¹ The discovery explains why some horses turn completely white as they grow older, while others remain a “dappled” gray color.¹ The difference between fast- and slow-graying horses is the number of copies of the 4.6 kb intronic sequence within the gene Syntaxin 17 (STX 17).² Slow-graying horses have a gene variant with 2 copies of the duplication (G2), while fast-graying horses have a gene variant with 3 (G3), along with a predisposition to melanoma.²

“There are horses that are born white because they carry a gene variant that is causing white coat color, but the great majority of white horses don’t have this gene variant,” Leif Andersson, PhD, a professor in the VMBS’ Department of Veterinary Integrative Biosciences, and at Uppsala University, said in a news release.¹ “Instead, gray horses are born with normal coat pigment – like black or brown – that gradually lightens as they age and eventually becomes gray or white.”

Published in the journal Nature Communications, the study proved the presence of 2 variant alleles at the Gray locus in horses, in addition to the G1 wild-type allele.² Of the variants, the G3 allele is more common and carries with it 3 copies of the 4.6 kb sequence.² The G3 allele is associated with fast graying, eventually resulting in a white colored coat. Horses with the G2 allele carry 2 copies of the 4.6 kb sequence, gray slowly, and never become pure white.²

“Some horses, the ones that will eventually become white, begin to grow gray eyelashes and hairs at the base of the tail within the first week after birth,” Andersson explained.¹ “A horse that is ‘slow graying’ will typically not show signs of gray until 5 to 7 years old.”

The significance of these findings goes beyond coat color. Horses with the G3 allele have a predisposition to melanoma, whereas horses with the G2 allele have a significantly lower incidence of melanoma, possibly as low as horses with neither variant.^1,2,3 Andersson originally established the connection between duplicated STX17 and fast graying horses in 2008 study.³ At that time, they established that the duplicated sequence within STX17 played a key role in the development of melanocytes—cells that create pigmentation for skin, and host melanoma development.³

“Horses that are ‘fast graying’ are more likely to develop melanomas, whereas we don’t see an elevated risk in horses that are ‘slow graying,’” Andersson said.¹ Researchers hope that this new understanding of coat colors and equine genetics will lead the way for future research on the prevention, and treatment, of melanomas in graying horses.¹

References

1. Texas A&M Researchers Discover Gene Variants That Determine Speed of Graying in Horses. News release. Texas A&M University College of Veterinary Medicine and Biomedical Sciences. September 19, 2024. Accessed September 20, 2024. https://vetmed.tamu.edu/news/press-releases/genes-behind-horse-graying/
2. Rubin, CJ., Hodge, M., Naboulsi, R. et al. An intronic copy number variation in Syntaxin 17 determines speed of greying and melanoma incidence in Grey horses. Nat Commun 15, 7510 (2024). https://doi.org/10.1038/s41467-024-51898-2
3. Good Karma: Aggie Veterinarians Save Horse From Life-Threatening Melanoma. News release. Texas A&M University College of Veterinary Medicine and Biomedical Sciences. January 10, 2024. Accessed September 23, 2024. https://vetmed.tamu.edu/news/press-releases/karma/