Canine Mammary Cancer Study Modeled After Advances in Human Stem-Cell Research

Cancer

Mammary cancer is as familiar to dog breeders and owners as breast cancer is to people whose loved ones have been diagnosed with the devastating disease. Research of human mammary stem cells provides a model for research in dogs that may help advance the use of drugs to inhibit the growth potential of cancer cells.

A study underway at the Baker Institute for Animal Health at Cornell University is evaluating the role of citrullination, a cellular process mediated by peptidylarginine deminase (PAD) enzymes, in canine mammary cancer. PAD activity, which is usually low in healthy tissues, often increases during mammary cancer development, says lead investigator Gerlinde Van de Walle, DVM, PhD, assistant professor.

The collaborative study includes Scott Coonrod, PhD, the Judy Wilpon associate professor of cancer biology at Cornell University Laboratory of Epigenetics and Cancer Biology. His research in human breast cancer cell lines provides the basis for the canine study.

“We are basically studying whether the cancer stem-cell lines in dogs, as in people, express higher levels of PAD enzymes and whether drugs that inhibit the function of these enzymes can reduce the tumorigenicity of these cancer cells,” Van de Walle says. “If this is indeed the case, it is possible in the future that we could evaluate in clinical trials whether PAD inhibitor drugs can provide a novel treatment option for mammary cancer in dogs.”

The Most Common Tumor

Mammary cancer is the most common type of tumor in intact female dogs. Studies showing that females almost never get this form of cancer if they are spayed before their first estrous cycle, which occurs around 6 months of age,  indicate that the disease is closely associated with the production of reproductive hormones, principally estrogen. These studies found that the risk of mammary cancer is reduced 0.5 percent if spaying occurs before 6 months of age, to 8 percent if spayed after the first cycle, and to 26 percent if spayed after the second cycle.

Neutering greatly reduces the risk of mammary cancer, but this isn’t a possibility for breeders and exhibitors who show dogs in conformation or compete with them in field trials. Breeders often plan to include these females with quality traits in their breeding programs to perpetuate their bloodlines. 

According to the American College of Veterinary Surgeons, one-quarter of unspayed female dogs will develop a mammary tumor during their lifetime. Although rare, mammary cancer also can occur in male dogs, often affecting them with an aggressive type of cancer that has a poor prognosis.

Early recognition of the signs of canine mammary cancer offers hope that treatment might provide a cure. Signs include large, firm lumps and swelling in mammary tissue; discharge from the mammary gland; ulceration of the skin over a gland; and lack of appetite, weight loss and general weakness. Experts advise owners to consult a veterinarian if a dog has signs of mammary cancer.

Although the cause of mammary cancer is not known, the disease likely involves genetic and environmental influences. About half of mammary tumors in dogs are benign, and half are malignant. The median age of canine mammary cancer is 10 to 11 years old.  

Treatment for mammary cancer is based on the stage of the tumor. Factors that determine the stage are tumor size and whether the tumor has metastasized to regional lymph nodes or the lungs. Lung metastasis may involve multiple tumors and is often resistant to treatment. 

Surgery and chemotherapy can be successful in dogs if the tumor is caught early and has not metastasized. First, it is important to definitively determine whether a dog has cancer. For example, benign fatty tumors, called lipomas, are common and can resemble cancer. 

Oncology specialists base a diagnosis of mammary cancer on blood work; an abdominal ultrasound and chest radio­graphs to check for tumor metastasis; fine-needle aspiration of lymph nodes or mammary mass to identify cancer cells and distinguish them from skin tumors; and possibly a biopsy to rule out inflammatory mammary carcinoma, a type of tumor that does not respond to surgery.

Surgery for dogs is generally conservative and involves removal of either the mass and/or the affected mammary gland. Some dogs can live several years after the complete removal of malignant mammary tumors.

Chemotherapy is used to help reduce the recurrence of a tumor following surgery. Most dogs tolerate chemotherapy well with few side effects. Although the number of treatments varies, dogs may receive four to six doses two to three weeks apart.

The Role of PAD Enzymes

In canine mammary cancer, certain cells in a dog’s mammary chain become abnormal and multiply without control to form a tumor. The cancer is believed to originate in stem cells, which are cells that have the potential to develop into many different cell types in the body.    

In her study at the Baker Institute, Van de Walle focuses on understanding the activity of PAD enzymes in stem cells. PAD enzymes are highly expressed in mammary gland tissue in dogs with cancer. The research is funded by the Morris Animal Foundation with support from the American German Shepherd Dog Charitable Foundation and the Bichon Frise Club of America.

“Mammary stem cells are a small population of cells present in the mammary gland, yet they are the driving force behind regeneration of the gland,” explains Van de Walle. “These cells remain in the tissue for a long time, thus there is a greater chance of them becoming tumors when compared to normal adult cells that die and are replaced by new cells.” 

In his work in human breast cancer, Coonrod showed that the binding of estrogen to the estrogen receptor appears to attract PAD2 enzymes to genes that promote tumor growth. As a result, the PAD2 enzymes citrullinate genes that increase transcriptional activity and promote expression of proteins involved in tumor progression.

Citrullination is a process in which positively charged arginine residues are changed to neutral citrulline residues, thus changing the charge of proteins. “This process changes a protein’s three-dimensional structure and affects how it interacts with other proteins on other cells,” Van de Walle says. “PAD2 enzymes catalyze the conversion of protein-bound arginine to citrulline.”

PAD inhibitors are drugs that inhibit PAD enzymes and thus may have an anti-tumor effect. “Our preliminary studies indicate that PAD inhibitors reduce the tumorigenic potential of canine mammary cancer cells, and they also do not harm healthy mammary cells. We are working on determining the relation between PAD inhibitors and PAD expression and function in dogs with mammary cancer.”

The goal is to study the efficacy of PAD inhibitors in a clinical trial of dogs with mammary cancer. “Before we can evaluate these drugs in dogs with mammary tumors, we need to be sure the drugs are safe at a therapeutic dose,” says Van de Walle.

The future holds promise that PAD inhibitors may one day provide an effective treatment for canine mammary cancer. In the meantime, owners should be diligent about checking their dogs for signs of cancer before it spreads. The earlier a mass is found and surgery is performed to remove it, the better chance there is for a successful outcome. 

Did You Know?

  • Mammary cancer is the most common tumor in intact female dogs
  • Although rare, mammary cancer also can occur in male dogs
  • Spaying before the first estrous cycle (around 6 months of age) reduces the risk of mammary cancer to 0.5 percent
  • Early recognition of signs of mammary cancer offers hope that treatment may provide a cure
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