Saluki Health

Breeder's Notebook: Cancer and Genetics

D. Caroline Coile, Ph.D.

This article appears with permission of the author

 

            Cancer: the word strikes fear in the heart of any dog owner. It’s devastating enough to have your dog diagnosed; but what does it mean for the future health of its offspring?

            Cancers are uncontrolled growth and proliferation of somatic (non-reproductive) cells, usually because of mutations that either disable tumor suppressor genes, which normally inhibit cell division, or enable oncogenes, which promote cell division. Every time a cell replicates there’s a chance of a mutation; in fact, as many as 1 mutation per 1,000,000 bases (the A, T, C, and G nucleotides that make up DNA). If the mutations occur in the nucleotides that code for tumor suppressor cells or oncogenes, then those replicating cells may give rise to a population that grows unchecked   in other words, a tumor.

            Most cancers occur in later age; about 80% of all older dogs will develop cancer. This may be because each time a cell divides there is a risk of mutation, a risk that may be increased by exposure to carcinogens. In humans, about 80% of all cancers are related to what we inhale, eat, or drink, or to a lesser degree, our exposure to radiation (usually sunlight) or environmental carcinogens, such as chemicals, metals, or pesticides. Well-known examples include asbestos, benzene, cadmium, nickel, radon, and vinyl chloride. Our dogs tend to be exposed to the same or more carcinogens, including second hand smoke.

            It’s important to remember that these mutations occur in somatic cells, which are the cells that make up the body, rather than germ cells, which are the cells that pass on genes to the next generation. So the fact that a dog has a cell mutation that leads to cancer doesn’t mean his offspring are any more likely to develop cancer - or does it?

            Heredity can influence cancer in several ways. Some genes have an overall effect on controlling cell growth, and an inherited mutation could disable normal braking mechanisms or amplify somatic cell mutations. Cancers directly caused by genes are called hereditary cancers. Some cancers seem to occur more often in certain families or breeds, without having a known genetic cause or pattern of heredity. These are called familial cancers. Finally, individual differences caused by normal genetic variation can make some individuals better able to detoxify carcinogens, repair damage that would otherwise lead to cancer, or respond to certain drugs for treatment.

            The inheritance of cancer has been a hotly researched issue for years. In order to estimate the role of genetic factors, a recent study of 44,000 pairs of human identical twins compared how often both members of a pair developed the same type of cancer. In most cases, if one member of the pair developed cancer, the odds of the other twin developing the same cancer were less than 15%, meaning that environmental or random factors were more influential. Those that occurred most often in both twins were prostate, colorectal, stomach, breast, and lung cancers.

            A very small percentage of human cancers have been linked to heritable causes. About 5% of human breast cancer is caused by specific genes, and another 15% is thought to be familial. About 15% of all colorectal cancers are thought to be hereditary. Some less common inherited cancers are retinoblastoma, endocrine tumors, and certain skin cancers. Familial cancers include about 5% of ovarian cancers and 2% of endometrial cancer. Lung cancer, melanoma, and prostate cancer also have familial components.

            That’s in humans. In dogs, there’s far less data. A hereditary form of kidney cancer called multifocal renal cystadenocarcinoma and nodular dermatofibrosis (RCND) has been identified in a family of German Shepherds. It’s inherited as an autosomal dominant with high penetrance but late onset; the gene responsible appears to be on chromosome 5.

            More often, certain types of cancer seem to run in breeds. In a survey of cancer rates in various breeds, the Boxer was found to be 35 times more likely to develop cancer than the least likely breed, the Dunker. Other breeds are more susceptible to specific forms of cancer. Histiocytosis is found much more commonly in Bernese Mountain Dogs. Two cancers of the white blood cells, lymphoma and leukemia, seem overrepresented in Golden Retrievers and Boxers. Melanoma is seen more often in Irish Setters, Gordon Setters, Standard Schnauzers, Miniature Schnauzers, Doberman Pinschers, and Scottish Terriers.  Hemangiosarcoma is overrepresented in German Shepherds, Golden Retrievers, Great Danes, Boxers, English Setters, and Pointers.

            Some cancers that occur more often in certain breeds may be influenced by other physical factors that make the breed at risk. For example, osteosarcoma (bone cancer) is more common in large and giant breeds; this may be related to the greater cell growth necessary in the growth plates of long-legged dogs, rather than “cancer genes.” Similarly, squamous cell carcinomas are more common in certain white breeds of dogs, but lightly pigmented or thinly furred dogs in general, especially those exposed to lots of sunlight, are more susceptible to this form of skin cancer.

            Some canine tumors, such as oral papillomas and tranmissable venereal tumors, are spread from dog to dog. In several species, a virus has been shown to cause lymphoma, although no such virus has been found in the dog.

            So what do you do if cancer strikes? From a breeding viewpoint you find out exactly what kind of cancer it is, then you find out if related dogs have had the same kind and if that cancer is more common in your breed. If there is evidence that it may run in the family, then you must make future breeding decisions involving that family with extra care to select mates that do not share the same background.

            You can take the opportunity to help your breed. Find out if research is being done on that particular cancer, and if researchers need DNA or tissue samples. You can also donate to the American Kennel Club Canine Health Foundation’s Cancer Initiative, which supports cancer research projects that will identify markers of heritable cancer risk, identify markers that predict response to treatment, and develop new therapies based on tumor genetic anomalies. It also funds the annual Canine Cancer Conferences, where top scientists from around the world meet to present the latest findings. With your help, breeders won’t be faced with such difficult choices in the future.

Copyright © 2007 Saluki Club of America.