|American Miniature Schnauzer Club, Inc.|
By Jaime F. Modiano, Ph.D.
Melanoma is a type of cancer that occurs commonly in dogs with pigmented (dark) skin. Melanomas can occur in areas of haired skin, where they usually form small, dark (brown to black) lumps, but can also appear as large, flat, wrinkled masses. Melanomas also can occur in the mouth, toes, or behind the eye. In general, skin melanomas tend to be benign, and those in the mouth, toes, or eyes tend to be malignant. However, there are many exceptions to this rule.
The term cancer refers to a large number of diseases whose only common feature is uncontrolled cell growth and proliferation (multiplication). This loss of cell growth control results from an accumulation of mutations (errors introduced into the DNA code) in genes that control cell division and cell survival.
The most common mechanism that introduces mutations into the DNA of somatic cells (non-reproductive cells) is the inherent error that occurs during normal cell division. In mammalian cells, there is an error rate of about 1 in 1,000,000 to 1 in 10,000,000 bases during each round of replication. The genome consists of many millions of base pairs, so each daughter cell is likely to carry at least a few mutations in its DNA. Most of these mutations are silent; that is, they do not present any problems to the cell’s ability to function. However, others can disable tumor suppressor genes or activate oncogenes that respectively inhibit or promote cell division and survival. Given the fact that cell division is responsible for most mutations, it is not surprising that the most common cancers arise from cells that divide frequently in the performance of their function.
Fortunately, the body has many built-in safeguards that mandate that abnormal cells be destroyed. For that reason, before cancer can take hold, a malignant cell must eliminate or evade these safeguards. The reliability of these systems is evident in the fact that our pets are not “walking tumors”. Still, cancer is the most frequent cause of death in dogs.
Melanoma arises from melanocytes, cells that impart pigment or coloration to the skin. In humans, melanoma arises due to mutations induced by repeated, intense exposure to ultraviolet light (for example, frequent tanning or working outdoors). This does not seem to be a major factor in dogs, as in most breeds the hair coat affords them protection from sunlight. However, pigment cells divide every time there is injury to the skin, or if there is constant trauma (for example, areas where dogs constantly scratch or lick). Nevertheless, risk factors for canine melanoma are not well established.
Mutations that contribute to cancer can also be inherited. An inherited mutation in a single gene that is important in cell growth control will increase the risk of that individual to develop cancer. This can be due to reducing the overall number of acquired mutations that must accumulate before a cell becomes cancerous, or it can be due to disabling a critical safeguard gene that normally prevents cells from becoming tumors. Specific genes that are responsible for familial melanoma have been identified in humans and in mice. In dogs, there appears to be a predisposition among certain breeds or families to develop specific types of cancer, suggesting that a hereditary component may be important in the development or progression of the disease.
The laboratory of Dr. Jaime Modiano at the AMC Cancer Center has an ongoing program to define the role of genetics in canine cancer. With support from the Canine Health Foundation of the American Kennel Club and specific breed clubs, Dr. Modiano is evaluating the role of various target genes in canine melanoma. For additional information on this project, or to inquire if your breed can be included in Dr. Modiano’s study we encourage you to visit the CHF web site at www.akcchf.org, or contact your breed club’s health representative.
The foregoing article is reprinted with the kind permission of Dr. Modiano. Dr. Modiano completed his veterinary training and PhD in Immunology at the University of Pennsylvania, a residency in Veterinary Clinical Pathology at Colorado State University, and a post-doctoral fellowship at the National Jewish Center for Immunology and Respiratory Medicine. He was appointed to the faculty in the Department of Veterinary Pathobiology at Texas A&M University as Assistant Professor between 1995 and 1999. Currently, Dr. Modiano is a scientist in the Center for Cancer Causation and Prevention at the AMC Cancer Research Center in Denver, Colorado, and a Full Member of the Comprehensive Cancer Center of the University of Colorado Health Sciences Center. His work is supported through federal and private sources, including the following grant from the AKC Canine Health Foundation:
No. 1626: Significance of Tumor Suppressor Genes in Canine Cancer
Last Revised: April 8, 2002