Recently I wrote about the random bad luck behind the development of many canine cancers. While it’s true research has shown that a roll of the dice determines most cases of cancer, there is an area where the incidence of cancer is not a function of bad luck but of something more concrete: genetics. As a follow-up to my earlier article, I thought it would be useful to look at why this is so and to also consider that, if there is a silver lining here, it is that the predisposition for developing cancer among certain breeds may provide researchers with tools to better study cancer in dogs and ultimately in people.
A Closed Gene Pool
From a genetic standpoint, each breed of purebred dog is a closed, isolated population. Because a registered dog must have ancestors who were registered as well, no new genes enter a purebred dog population, except in extraordinary situations sanctioned by the breed registry. Every purebred dog is a relative, albeit a distant one, of the other dogs within that breed. Since most dogs are never bred but instead live out their lives as pets, the “doggie gene pool” remains relatively small. Selective breeding for each registry maintains the puppy face of the Bernese Mountain Dog, the mahogany coat color of the Hungarian Vizsla and the fluffy black fur of the Flat-Coated Retriever. However, genes that increase a dog’s risk of developing cancer also seem to have tagged along with the genes that control things like facial features, coat color and fur fluffiness. Those genes place these three breeds at the top of the list of dogs with an increased risk of developing certain types of cancer.
A Genetic Treasure Trove
If there is an upside to the limited genetic diversity of purebred dogs, it is their unsurpassed ability to elucidate the genetics behind various cancers and other hereditary diseases. Using the map of the canine genome and the extensive family trees from purebred dogs, the DNA of dogs with a high risk of developing a certain kind of cancer can be compared to dogs with a low risk of developing that type of cancer. The genetic differences identified are likely areas of the canine genome where the genes for increased cancer risk lie. Once the genes are identified, tests can be developed and used to help avoid breeding individual dogs with the “bad genes.” Right now, scientists are just at the point of identifying these genes.
In part, the common ancestry of dogs has perpetuated mutations that increase the risk of or directly cause cancer. Genetic analysis of wolves and dogs shows divergence of dogs from ancestral wolves around 11,000 to 16,000 years ago. Ancient Australian and African breeds, such as the Dingo and Basenji, became distinct about 2,000 to 3,000 years ago, but most of the modern breeds of dogs, like Mastiffs and Herding dogs, are quite recent innovations in dog breeding, stemming only from Victorian times.
The Mastiff Group, which for genetic purposes here is different from the traditional Sporting, Working and Hound Groups we see breeds organized into at dog shows, is a genetically determined grouping of related dogs. This Mastiff Group includes several genetically related dog breeds that have an increased risk of cancer. For example, Boxers are prone to mast cell tumors, Bernese Mountain Dogs to histiocytic sarcoma, Golden Retrievers to lymphoma and hemangiosarcoma, and Rottweilers to osteosarcoma. The exact genetic abnormality resulting in an increased risk of cancer in these and other predisposed breeds is still under intense investigation and supported by the American Kennel Club’s (AKC) Canine Health Foundation and the Morris Animal Foundation, to name two of the major funding agencies behind this groundbreaking research.