Affected gene: DDB2
 Inheritance: Autosomal recessive
 Mutation: EquCab3.0 Chr12 g.41395458G>A (c.1013G>A)
 OMIA link: 002056-9796


Appropriate breeds for testing:
Haflinger, Belgian Draft, Rocky Mountain Horse, Quarter Horse, American Paint Horse, Appaloosa
Symptoms:
Ocular Squamous Cell Carcinoma (OSCC) is a malignant tumor primarily affecting periocular tissues, including the eyelids, conjunctiva, and cornea.
Clinical signs may include:
 
 
Development of masses or proliferative lesions on or around the eye
 
 
Ulceration, chronic inflammation, or persistent ocular discharge
 
 
Sensitivity to light (photophobia)
 
 
Progressive involvement of ocular tissues that may lead to vision loss

Horses carrying the mutation have an increased genetic predisposition, especially when combined with environmental risk factors such as prolonged UV exposure and lack of periocular pigmentation.
Disease severity can range from localized, treatable lesions to invasive tumors requiring extensive surgical intervention, including enucleation.
References:
Bellone, R.R., Holl, H., Setaluri, V., et al. (2017) Evidence for a shared genetic basis for squamous cell carcinoma and ocular pigmentation in horses. PLoS Genetics 13(10): e1006845.
PubMed reference: 29023422. DOI: 10.1371/journal.pgen.1006845.