Additional Evidence for DDB2 T338M as a Genetic Risk Factor for Ocular Squamous Cell Carcinoma in Horses

Squamous Cell Carcinomas in Horses: An Update of the Aetiopathogenesis and Treatment Options

Squamous cell carcinomas are a very common tumor type in horses, and are found in a variety of dermatologic and non-dermatologic locations. Metastasis is common, even at the first presentation of the disease, and a full staging workup is therefore strongly recommended to direct treatment and assist with prognostication. Wide surgical excision remains the treatment of choice in most cases, but recurrence is common, and adjunctive therapy may be indicated to improve the long-term prognosis.

Genomics and transcriptomics in veterinary oncology

The sequencing of the canine genome, combined with additional genomic technologies, has created opportunities for research linking veterinary genomics with naturally occurring cancer in dogs. Also, as numerous canine cancers have features in common with human cancers, comparative studies can be performed to evaluate the use of cancers in dogs as models for human cancer. There have been several reviews of veterinary genomics but, to the best of our knowledge, there has been no comprehensive review of the literature of canine cancer genomics. PubMed and CAB Abstracts databases were searched to retrieve relevant literature using the search terms ‘veterinary’, ‘cancer’ or ‘oncology’, and ‘genomics’ or ‘transcriptomics’. Results were manually assessed and grouped based on the techniques used, the cancer type investigated and genomic lesions targeted. The search resulted in the retrieval of 44 genomic and transcriptomic studies, with the most common technique employed being comparative genomic hybridization. Across both fields, the most commonly studied cancer type was canine osteosarcoma. Genomic and transcriptomic aberrations in canine cancer often reflected those reported in the corresponding human cancers. Analysis of the literature indicated that employing genomic and transcriptomic technologies has been instrumental in developing the understanding of the origin, development and pathogenesis of several canine cancers. However, their use in canine oncology is at an early phase, and there appears to be comparatively little understanding of certain canine cancer types in contrast to their human forms. Aberrations detected in all tumors were tabulated, and the results for osteosarcoma, lymphoma and leukemia, mast cell tumor, transmissible venereal tumor and urothelial carcinoma discussed in detail.

DDB2 Genetic Risk Factor for Ocular Squamous Cell Carcinoma Identified in Three Additional Horse Breeds

Squamous cell carcinoma (SCC) is the most common cancer affecting the equine eye. A missense variant within the gene damage-specific DNA binding protein 2 (DDB2 c.1013C>T, p.Thr338Met) was previously identified as a causal recessive genetic risk factor for the development of ocular SCC within Haflingers, Belgian Draft horses, and Rocky Mountain Horses, but not in the Appaloosa or Arabian breeds. This study aimed to evaluate three cases of ocular SCC in additional breeds and determine if DNA testing for the DDB2 variant in warmblood horses and Connemara ponies is warranted. Histopathology confirmed ocular SCC in all three cases and DNA testing confirmed each horse was homozygous for the DDB2 risk factor. The DDB2 risk allele frequency was estimated to be 0.0043 for Holsteiners (N = 115), 0.014 for Belgian Warmbloods (N = 71), and 0.22 for Connemara Ponies (N = 86). Taken together these data support using DNA testing for DDB2 in Connemara Ponies to assist in mate selection and clinical management. Given the low observed allele frequencies in both the Holsteiner and Belgian Warmblood breeds and that the case under investigation was a warmblood cross-bred, evaluating additional SCC affected warmbloods is warranted to fully determine the importance of DDB2 genotyping as a risk factor in warmblood breeds.

A novel DDB2 mutation causes defective recognition of UV-induced DNA damages and prevalent equine squamous cell carcinoma

Squamous cell carcinoma (SCC) occurs frequently in the human Xeroderma Pigmentosum (XP) syndrome and is characterized by deficient UV-damage repair. SCC is the most common equine ocular cancer and the only associated genetic risk factor is a UV-damage repair protein. Specifically, a missense mutation in horse DDB2 (T338M) was strongly associated with both limbal SCC and third eyelid SCC in three breeds of horses (Halflinger, Belgian, and Rocky Mountain Horses) and was hypothesized to impair binding to UV-damaged DNA. Here, we investigate DDB2-T338M mutant's capacity to recognize UV lesions in vitro and in vivo, together with human XP mutants DDB2-R273H and -K244E. We show that the recombinant DDB2-T338M assembles with DDB1, but fails to show any detectable binding to DNA substrates with or without UV lesions, due to a potential structural disruption of the rigid DNA recognition β-loop. Consistently, we demonstrate that the cellular DDB2-T338M is defective in its recruitment to focally radiated DNA damages, and in its access to chromatin. Thus, we provide direct functional evidence indicating the DDB2-T338M recapitulates molecular defects of human XP mutants, and is the causal loss-of-function allele that gives rise to equine ocular SCCs. Our findings shed new light on the mechanism of DNA recognition by UV-DDB and on the initiation of ocular malignancy.

Genetics of Equine Ocular Disease

Horses perform in a variety of disciplines that are visually demanding, and any disease impacting the eye has the potential to threaten vision and thus the utility of the horse. Advances in equine genetics have enabled the understanding of some inherited ocular disorders and ocular manifestations and are enabling cross-species comparisons. Genetic testing for multiple congenital ocular anomalies, congenital stationary night blindness, equine recurrent uveitis, and squamous cell carcinoma can identify horses with or at risk for disease and thus can assist in clinical management and breeding decisions. This article describes the current knowledge of inherited ocular disorders.

Genetic Testing in the Horse

Genetic testing in horses began in the 1960s, when parentage testing using blood group markers became the standard. In the 1990s, parentage testing shifted from evaluating blood groups to DNA testing. The development of genetics and genomics in both human and veterinarian medicine, along with continued technological advances in the last 2 decades, has helped unravel the causal variants for many horse traits. Genetic testing is also now possible for a variety of phenotypic and disease traits and is used to assist in breeding and clinical management decisions. This article describes the genetic tests that are currently available for horses.