The SCC DNA mutation confers a high risk of squamous cell carcinoma, the most frequently occurring type of ocular cancer in horses
Ocular Squamous Cell Carcinoma
Breeds known to be affected
This mutation is most commonly recognised in Haflingers, but also occurs in Connemara ponies, Belgian Drafts and Belgian Warmbloods, Rocky Mountain Horses and Holsteiners.
Squamous cell carcinoma is the most common ocular tumour in the horse. Haflinger horses appear to be most susceptible to SCC, with tumours developing on the limbus and third eyelid, leading to blindness and sometimes necessitating removal of the eye all together.
The SCC mutation is recessive and has incomplete penetrance. This means that carrying two copies of the mutation will significantly increase the risk of the horse developing SCC. However, there are other factors involved, with UV exposure identified as also contributing to the chance that a horse develops SCC. There are also cases of SCC that have occurred in horses without the SCC mutation so there may be other genetic risk factors that have not yet been identified.
Interpretation of results
n/n: Horse does not carry the recessive mutation that is associated with onset of SCC.
n/SCC: Horse has one copy of the recessive mutation that is associated with onset of SCC.
SCC/SCC: Horse has two copies of the recessive mutation that is associated with onset of SCC and if a Haflinger, this horse has approximately 5 times the normal chance of developing SCC. This horse will also pass the mutation to all its offspring.
Bellone et al. A Missense Mutation in Damage-specific DNA Binding Protein 2 Is a Genetic Risk Factor for Limbal Squamous Cell Carcinoma in Horses. International Journal of Cancer 2017; 141:342-353. doi: 10.1002/ijc.30744