Talk about a missing link: Researchers in Sweden have discovered that gray horses carry an identical mutation that can be traced back to a common ancestor that lived thousands of years ago. An international team led by researchers at Uppsala University say this mutation also enhances the risk for melanoma. The paper is being published in the July issue of Nature Genetics.

About 75% of grey horses older than 15 years of age have a benign form of melanoma that in some cases develops into a malignant melanoma. As a result, the study has also given new insight in a molecular pathway that may lead to tumor development. The gray mutation does not change any protein structure in the horse but it affects the genetic regulation of two genes. The researchers found that the white horses carry an extra copy of a DNA segment located in one of these genes.

Another aspect of the mutation is that it appears to predispose gray horses to the development of melanomas.Researchers believe that the gray mutation stimulates the growth of melancocytes, and that this leads to a premature loss of the melancocyte stem cells needed for hair pigmentation. Instead, the mutation promotes an expansion of melanocytes that causes skin pigmentation. Learning more about the action of this mutation may well help us better understand the development of melanomas in humans, too.

The “graying out” of a gray horse resembles the graying experienced by humans, but it occurs on a much faster time frame. Gray horses are born black, bay, brown or chestnut, but begin to gray within their first year. Normally, they are completely white by the age of six or eight. About one horse in ten carries the mutation for graying with age.