Is It a Problem?
by C.A. Sharp
Rev. April 2013
We’ve all had special dogs in our lives. Moby was one of mine. She was bright, personable and, even though she had severe handicaps from birth, I’ve known few dogs with as much zest and joy in life. Yet there I was, that sad July afternoon in 1994, hoisting Moby to my vet’s examining table and steeling myself to say good-by.
Moby had mammary cancer. After surgery it had metastasized to internal organs. Despite her stoicism in the face of what must have been considerable pain, I knew it was time to let her go. She was only nine years old.
Cancer is a common canine ailment. It is the most common natural cause of death in the species as a whole, usually striking elderly dogs. But sometimes the victims are not elderly. They may even be young. Even though cancer is a species-wide canine problem, some breeds are prone to particular types indicating an inherited predisposition for those cancers.
Cancer is not one disease, but many. Any disease causing an uncontrolled proliferation of abnormal cells is classified as cancer. Those abnormal cells may form a tumor, as in mammary cancer, or arise in the bone marrow causing lymphoma or leukemia and sending vast numbers of diseased cells into the lymph or circulatory systems. A cancer may start at one site then metastasize, sending cells racing through the bloodstream to wreck havoc elsewhere.
All cancers stem from genetic mutations. Sometimes genes that are supposed to signal cell death at the appropriate time fail to turn on. Or genes important to cell division get stuck in the “on” position causing rapid production of abnormal cells. Often it isn’t a matter of a mutation within a single gene: Entire chunks of chromosomes can become detached and fasten themselves to other chromosomes, disrupting the proper function of numerous genes.
Most cancerous mutations are not inherited; they accumulate throughout the body over the course of a lifetime. Exposure to certain chemicals, radioactive substances or too much sunlight might scramble part of your dog’s DNA. As the DNA is copied over and over again throughout a dog’s life, DNA transcription errors can accumulate. Most of the body’s cells are autosomal. Only the germ cells produce the sperm and eggs necessary to reproduction. If a cancer-causing mutation occurs in germ cells, it can be passed from one generation to the next. Moby’s dam also had mammary cancer, a disease that has been demonstrated to be heritable in humans.
Cancer inheritance is rarely as clear-cut as that of traits like coat color. Mutations linked to cancer often only predispose a dog to develop the disease. Whether it remains healthy or not will depend on what other risky mutations, or sets of mutations, it has or will acquire as it goes through life. Or the balance can be tipped by something in the environment.
A dog breeder can only do a limited amount to control exposure of her puppies to environmental carcinogens and nothing at all to prevent DNA copying errors. She can do something about inherited predisposition, but only if she knows what cancers her breed and bloodlines are prone to.
I first became concerned about the heritability of cancer in Australian Shepherds in the late 1990s. At that time I was assisting the late Dr. John Armstrong, a geneticist at the University of Ottawa, with his canine longevity study. He gathered data on several breeds, including the Aussie. He told me that cancer was the most common cause of non-accidental death, which is a normal finding in dogs. But he went on to say than among more recent dogs cancer appeared to be happening with increasing frequency in dogs that were not old.
Our breed developed mostly in the early-to-mid-20th century from indigenous ranch dogs of the American West. Since that time, breed type has been refined and standardized through linebreeding. This is a normal and necessary step to breed development. However, it is not without risks. In the process certain individuals, usually males, will become disproportionately represented in the breed gene pool. These will be dogs that not only exhibited highly desired traits, but reproduced them consistently. But along with all those desirable genes, some that are far less desirable will have hitched a ride into subsequent generations.
This process has actually made the dog the premier research animal for cancer. It is no surprise that the key researchers on the Canine Genome Project, completed in 2005, were also studying human cancer. The roughly 400 modern dog breeds represent distinct subsets of canine gene variants. A gene version important to a particular cancer may be found in only a few of those 400 breeds, and may be common in only one or two. Familial patterns of cancer are very difficult to trace in human populations, but dog breeds with their closed gene pools and studbook records are ideal for the purpose.
The apparent increase in cancers among younger Aussies in conjunction with increased coefficients of inbreeding concerned me. I knew the longevity study data was not sufficient to puzzle out what was happening. I told Dr. Armstrong I felt our breed ought to be studied more closely for cancer, but his untimely death brought an end to his work.
The ASHGI Cancer Survey, conducted in 2006-7 made it clear that the Australian Shepherd has two inherited cancers: Hemangiosarcoma and lymphoma. In addition, both cancers tend to occur in the same families, indicating that they may have something in common genetically. This knowledge means that breeders can minimize risk of producing dogs that will develop these cancers by making note of cases and avoiding breedings where both parents have the same cancer in their recent family background. ASHGI has included these cancers in its pedigree analysis service since 2008.
Not long after the cancer survey was complete ASHGI contacted Dr. Matthew Breen at North Carolina State University who agreed to include Aussies in his work. He not only studies our two inherited cancers in Aussies, he has compiled a repository of blood and tissue samples that he willingly shares with other researchers. In addition to developing our relationship with Breen, ASHGI has actively supported hemangiosarcoma and lymphoma research by sponsoring grants administered by the AKC Canine Health Foundation and Morris Animal Foundation.
The decision to let Moby go was one of the hardest I’ve had to make. All of us who have had a Moby in our lives hold he key that may save others from experiencing the same anguish. By sharing what happened to our dogs through the ASHGI/IDASH health database and with each other we may ultimately provide breeders with the tools they need to avoid producing dogs that are genetically predisposed to our two inherited cancers. By supporting ASHGI financially you can aide researchers as they work to gain vital insights which could lead to DNA screening, improved diagnosis, better treatments, and even cures for those dogs who do develop these dread diseases.