Genetic variant behind heightened risk of ocular cancer identified in three further horse breeds

Squamous cell carcinoma is the most common cancer in the horse.
Squamous cell carcinoma is the most common cancer in the horse. © HorseDVM

A genetic risk factor for the most common form of cancer affecting the eye in horses has been identified in three additional horse breeds.

The work by Margo Crausaz and her fellow researchers focused on a variant of the damage-specific DNA binding protein 2 (DDB2) gene.

It was previously identified as a causal recessive genetic risk factor for the development of ocular squamous cell carcinoma within Haflingers, Belgian Draft horses, and Rocky Mountain Horses, but not in the Appaloosa or Arabian breeds.

The study team aimed to evaluate three cases of the cancer in additional breeds and determine if DNA testing for the DDB2 variant in warmblood horses and Connemara ponies was warranted.

Microscopic examination confirmed the presence of the cancer in all three cases, and DNA testing confirmed each horse was homozygous for the DDB2 risk factor — that is, they had inherited a copy of the variant gene from each parent.

The researchers, in a paper to be published in the journal Genes, estimated the risk allele frequency for Holsteiners, Belgian Warmbloods and Connemara Ponies. It was low in Holsteiners and Belgian Warmbloods, but considerably higher in the Connemara pony.

Taken together, the findings support using DNA testing for DDB2 in Connemara ponies to assist in mate selection and clinical management, they said.

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, the authors said evaluating additional warmbloods affected by the cancer is warranted to fully determine the importance of DDB2 genotyping as a risk factor in warmblood breeds.

The study team comprised Crausaz, Kelly Knickelbein and Rebecca Bellone, all with the University of California-Davis; Thomas Launois, with Bailly Vétérinaires Clinique du Lys in France; and Kathryn Smith-Fleming and Annette McCoy, with the University of Illinois.

Crausaz, M.; Launois, T.; Smith-Fleming, K.; McCoy, A.M.; Knickelbein, K.E.; Bellone, R.R. DDB2 Genetic Risk Factor for Ocular Squamous Cell Carcinoma Identified in Three Additional Horse Breeds. Genes 2020, 11, 1460.

The abstract, published under a Creative Commons License, can be read here

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