Grey Horses and the Price of Beauty
• Did you know that the coat color in grey horses (six Lipizzaners in the image above) is linked to a mutation in the STX17 gene? Grey horses are born with their base colour (e.g. black, bay, chestnut), which then greys early in life due to loss of pigment cells (melanocytes), a process similar to the greying of hair in humans, which typically occurs much later in life.
• The mode of inheritance of the greying phenotype is autosomal dominant. So a grey horse will be either GG or Gg: non-grey horses carry the gg genotype. This trait has been selected over decades of breeding, because it brings beauty.
• The STX17 gene product is Syntaxin17, a protein found on the cell membrane involved in vesicle transport. It is possible that the disruption in vesicle fusion causes a defect in melanocyte development and pigment production.
• The mutation is a 4.6 kb duplication in the intron of the gene (a region that does not code for any protein). It is thought that the duplication region contains a melanocyte-specific enhancer that becomes much stronger when duplicated. This presumably increases the activity of STX17 in melanocytes, which has been linked to tumor formation.
• Unfortunately beauty comes at a price: melanoma occurs frequently in older grey horses. A fraction of them also develop vitiligo, a condition in which there is a loss of pigment from areas of skin. It is possible that this condition may be linked with melanoma, since the STX17 gene is activated in melanoma tumors. Understanding the complex interplay between coat color, melanoma incidence and vitiligo could yeild clues to human disease.
Thanks +Rajini Rao for the suggested edits 🙂