Limbal squamous cell carcinoma in a Rocky Mountain Horse: Case report and investigation of genetic contribution

Objective To document a case of limbal squamous cell carcinoma (SCC) in a Rocky Mountain Horse stallion determined to be homozygous for the genetic risk factor (DDB2 c.1013C>T) strongly associated with the disease in Haflinger and Belgian horses, and to determine the frequency of this allele in a larger population of Rocky Mountain Horses. Animals studied One privately owned Rocky Mountain Horse and 84 Rocky Mountain Horses screened for allelic frequency. Procedures A complete ophthalmic examination was performed on a Rocky Mountain Horse stallion for assessment of a mass affecting the right eye. A clinical diagnosis of suspected limbal SCC was made, and routine keratoconjunctivectomy and adjunctive strontium irradiation were performed. Genotyping for the DDB2 c.1013C > T (rs1139682898) risk variant was performed utilizing an allele-specific PCR assay on DNA isolated from whole blood and hair follicles. Results Histopathology confirmed the limbal mass to be consistent with SCC. The horse was genotyped as homozygous for the DDB2 c.1013C >T risk variant. The frequency of the variant allele among a population of 84 Rocky Mountain Horses was found to be 0.20. Conclusion The Rocky Mountain Horse breed possesses the DDB2 variant allele determined to be a significant risk factor for ocular SCC in the Haflinger and Belgian breeds. Genotyping additional Rocky Mountain Horses diagnosed with ocular SCC as well as confirmed healthy controls for this variant should be undertaken to determine whether a significant association exists between ocular SCC and the variant in the Rocky Mountain Horse breed.