Investigating a single nucleotide polymorphism in DDB2 as a risk factor for squamous cell carcinomas of the nictitans in the Haflinger and Belgian horse breeds

Squamous cell carcinoma (SCC) is the most common cancer of the equine eye. and the second most common tumor of the horse overall. Ocular SCC can originate on the limbus. the nictitans, and upper and lower lids. Several risk factors have been implicated for SCC. including exposure to ultraviolet (UV) radiation and genetics. The Haflinger breed is overrepre-sented in terms of incidence for SCC of the limbus. A genetic investigation in this breed identified a polymorphism in damage-specific DNA binding protein 2 (DDB2, p.Thr338Met) as the potential link between UV damage and the genetic risk for this cancer. DDB2 recognizes and binds to UV damaged DNA to begin the nucleotide excision repair process. The Haflinger and Belgian breeds, which recently were shown to be genetically closely related, also have documented cases of SCC of the nictitans and have similar DDB2 allelic frequencies. Therefore, we propose that the identified DDB2 imitation also increases risk of SCC of the nictitans in these closely related breeds. Genotyping for this polymorphism in SCC affected horses with confirmed pathology (N = 11) and clinically confirmed unaffected horses (N= 47) showed a strong but not perfect association in these two breeds (Fisher's Exact Test, P = 4.98 x 108). Therefore, further work is needed to determine if this is a causative variant for SCC of the nictitans. Specifically, genotyping additional horses will determine the relative risk associated with this variant in each breed. Furthermore, since threonine at position 338 is conserved across 95 vertebrates and nearby highly conserved residues Phe334, Gln335, and His336 make contact with the bases on the opposite strand of the U\-caused lesion, we predict that this substitution (p.Thr338Met) affects proper formation of the protein and thus interferes with DDB2 binding to damaged DNA: this remains to be investigated.