Ocular squamous cell carcinoma


Good news!   

Your horse is clear of ocular squamous cell carcinoma and does not have this gene mutation.   

Your horse cannot pass on this gene mutation. There are no breeding or performance implications. 

What is ocular squamous cell carcinoma?

Ocular squamous cell carcinoma (OSCC) is the second most common tumor in horses, commonly affecting the eyelids. 

Scientific studies have revealed up to 46% of horses affected by this type of eye cancer were of a chestnut base coat color, indicating that coat color pigmentation put horses at risk for this type of tumor. Horses with no pigment or lightly pigmented eyelids are most affected, and they are usually diagnosed around 8–10 years of age.  

Exposure to sunlight may amplify the chances of developing this type of cancer. This is because the gene affects the horse’s ability to repair ultraviolet damage to the eye. In severe cases, ocular squamous cell carcinoma can lead to loss of the eye. Sometimes horses are humanely euthanized.

Genomic Details

Ocular squamous cell carcinoma is strongly associated with a mutation in the DDB2 gene on chromosome 12. Horses with this mutation are unable to repair DNA damage caused by sunlight. This results in eye cancer that can spread to the brain and sinuses.

The mutation that causes ocular squamous cell carcinoma is a switch in one of the four base pairs that make up a DNA molecule. Cytosine (C) is replaced by Thymine (T) changing the DDB2 gene. Therefore, a horse’s genetic result for ocular squamous cell carcinoma is one of three possibilities:

  • C/C: Normal. Does not have the disease. Does not have the mutation.

  • C/T: Carrier. Does not have the disease. Carries one gene mutation from one parent. This might be passed to their foal.

  • T/T: Affected. Has the genetic disease. The horse has inherited a gene mutation from each parent. One mutation will be passed to their foal.