Dog Cancer
As in humans, cancer is a major killer of dogs. According to research published in UC Davis School of Veterinary Medicine Book of Dogs, by Bruce R. Madewell, D.V.M., one out of four dogs will develop cancer and nearly half of the dogs over the age of 10 years will die from the disease. There are many kinds of cancers, but malignant melanoma is a deadly form often seen in the Doberman. Melanomas are tumors of pigmented skin cells — the cells responsible for skin coloring. Melanomas that occur in the skin generally exhibit a more benign behavior; malignant melanomas, which are dangerous and fast-spreading, occur in an affected dog’s mouth, toenail bed and inside of the eye, where they are not easily detected until the tumor is well advanced.
Usually, malignant melanomas are discovered during a routine physical exam when the mouth is inspected or during dental procedures. Some dogs display eating or breathing problems if the tumor obstructs the oral cavity or the larynx. Less commonly, these tumors grow in the Doberman’s toenail bed and appear as darkly pigmented, often black, masses. Malignant melanomas usually occur in dogs over the age of 6 years old.
“Definitive diagnosis for melanoma is made through biopsy of the affected tissues,” says Michelle Ritt, D.V.M., of Animal Hospital Center in Highlands Ranch, Colo. Chest X-rays and sampling regional lymph nodes for evidence of cancer cells can help determine if the cancer has spread to other areas.
Malignant melanomas are almost always incurable; treatment is geared toward extending the dog’s quality of life via surgical removal of the bulk of the mass and follow-up radiation therapy. “Even when treated aggressively with surgery and radiation therapy, malignant melanomas still usually end the dog’s life within 6 to 24 months, depending on how big the tumor is when it is first diagnosed and whether there is evidence of metastasis — the spreading of cancer cells to other locations such as the lungs. Metastatic cancer has a worse prognosis,” Ritt says.
Dog Breed predisposition for melanomas suggests a genetic basis. “This cancer is suspected to be caused, at least in part, by defective tumor suppressor genes due to inherited mutations,” Ritt says. “We suspect this because certain dog breeds have a very high prevalence of certain cancers.”
Tumor suppressor genes are the genes that tell cells when to stop dividing or growing, and constrain renegade cells; mutations can disable these genes in cancer cells, contributing to the development and progression of cancer. “Cells divide in order to maintain our normal state of being,” explains researcher Jaime Modiano, V.M.D., Ph.D. “We need to replace blood cells, cells from the intestine, cells from the skin, and so forth on a continuous basis. In the normal state, this is tightly controlled, so you get signals to induce cells to undergo that division, and then other signals that make cells stop growing at the appropriate time. The signals that make them stop are largely mediated by tumor suppressor genes precisely for the reason that they make cells stop growing.
“In the last 15 years or so, we’ve learned that these particular genes are somehow disabled in many tumors. This is one of the reasons —not the only reason, but one important reason — that cells become cancerous. The mechanisms that normally tell them to stop growing are somehow functionally disabled, and so they start dividing and nothing tells them to stop.”
Modiano is studying the frequency of the important tumor suppressor genes in canine melanoma with partial funding provided by the DPFA. “We’re trying to determine what their precise roles are,” he says. “Then, we want to find out whether there are certain groups or families or pedigrees that have a predisposition to cancer due to mutations in these particular genes. If we find a correlation between mutations of these genes and the types of tumors we’re looking at, we can then develop a test to identify potential carriers.”
