Thanks to a genetic study led by UC Davis research scientists, horse owners can now identify horses at risk for ocular SCC and make informed breeding decisions.

Squamous cell carcinoma on 3rd eyelid of horse

A genetic mutation in horses has been identified that should help identify horses that are at risk for squamous cell carcinoma of the eye and enable horse owners to make informed breeding decisions.
© 2019 by Animal Eye Institute New window.

Ocular squamous cell carcinoma (SCC) is the most common form of cancer to affect the eyes and eyelids of horses, and the second most common cause of cancer in horses overall (after equine sarcoid).

The tumor arises in the outermost layer of skin, conjunctival, or corneal cells, with UV light (sunlight) exposure being a known risk factor. Horses that lack pigment in the skin surrounding their eyes (have white or pink eyelids) are at greater risk of developing SCC on their eyelids than horses with pigment around their eyes. Additionally, horses with a chestnut-based coat color appear to be at risk.

The UC Davis scientists announced the discovery of a genetic mutation in horses that is hypothesized to impact the ability of damage specific DNA binding protein 2 (DDB2) to carry out its standard role. Normally, the protein conducts DNA surveillance, looking for UV damage and then calling in other proteins to help repair the harm.

"The mutation is predicted to alter the shape of the protein so it can't recognize UV-damaged DNA," said Dr. Rebecca Bellone, an equine geneticist at the Veterinary Genetics Laboratory and associate adjunct professor at UC Davis School of Veterinary Medicine. "We believe this is a risk factor because cells can't repair the damage and accumulate mutations in the DNA that lead to cancer."

The researchers at UC Davis discovered a variant in the damage-specific DNA binding protein 2 gene (DDB2 for short) that has been identified as a risk factor for ocular SCC in the Haflinger, Belgian, and potentially the Rocky Mountain Horse breeds. The function of this gene is to repair DNA that has been damaged by UV light.

Horses with two copies of the variant (homozygous) are at increased risk of developing ocular SCC compared to horses with only one copy (heterozygous) or no copies of the variant. Approximately 80% of the cases in these breeds were homozygous for this risk variant.  Therefore, this risk factor does not explain all cases of ocular SCC and it is likely that additional genetic risk factors are contributing to cancer in these and other breeds.

Owners can use the DNA test available through the UC Davis Veterinary Genetics Laboratory  to identify horses that are at higher risk for ocular SCC development and take precautions to protect them.

It is advised that homozygous horses have routine eye exams performed so that tumors are caught early. Additionally, these high-risk horses should wear UV-protective fly masks and be stabled during peak sunlight hours.

The test can also be used to help breeders of Haflingers, Belgians, and Rocky Mountain Horses make informed decisions when selecting breeding pairs and avoid producing horses that are at risk of developing ocular SCC.

Press release UC Davis