Overview (ACVS)

Mast cell tumors are the most common type of skin tumor found in dogs and the second most common skin tumor in cats. These represent 14–21% of all skin tumors diagnosed in dogs. They are usually noticed in middle aged patients but can occur in patients of any age. Boxers and Boston terriers make up ~ 50% of all cases.

Most tumors are solitary although boxers and pugs have an increased predilection for multiple skin tumors. These tumors most often are noticed in the skin and subcutaneous tissue. (ACVS)

Breeds prone to mast cell tumors (Better Pet)

While any breed of dog can get mast cell tumors, certain breeds, especially brachycephalic breeds, are more susceptible. Mast cell tumors are particularly common in:

• Boxers. No one knows exactly why boxers are more prone to mast cell tumors, but they do tend to get them more frequently than other breeds. The good news is that their tumors are often (but not always) less severe.
• Bull terriers. The bull terrier is another breed of dog that is predisposed to mast cell tumors. If you have a bull terrier, make sure they are screened for mast cell tumors regularly and that you know the signs of complications from mast cell tumors.
• Boston terriers. Similar to boxers, this is a breed that is prone to getting mast cell tumors. The reason for this is not known.
• Labrador retrievers. Labrador retrievers are another breed that is prone to mast cell tumors and particularly aggressive cases at that. Early detection is essential. (Better Pet)

What causes this cancer? (VCA Animal Hospital)

Why a particular dog develops MCTs or any cancer is not straightforward.

• Very few cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary.
• Several genetic mutations are known to be involved in the development of MCTs.
• One well-known mutation affects a protein, called KIT, that is involved in the replication and division of cells. (VCA Animal Hospital)

What are the Causes of Mast Cell Tumors? (National Canine Cancer Foundation)

Like most cancers, the etiology of mast cell tumors is also not known. 

• On rare occasions, they have been associated with chronic inflammation or the application of skin irritants. 
• Studies have indicated that chromosomal fragile site expression (unstable genomic loci susceptible to breakage, rearrangement), a phenomenon that is thought to genetically predispose humans to the development of certain tumors, has been found to be increased in Boxer dogs with MCTs.
  • However, this study was conducted on young, non-tumor-bearing boxers, so it was believed that the increased expression was likely due to the age difference.

The genetic alterations that increase the likelihood of tumors in humans are not absolutely understood in dogs.

• Changes in the pathway of p53 tumor suppressor have been indicated in some canines.
• Disturbances in the expression of proteins like p21 and p27, cyclin-dependent kinase (A protein kinase inhibitor is a type of enzyme inhibitor that specifically blocks the action of one or more protein kinases) inhibitors that regulate the cell cycle, have been identified in several dogs.
• Recently an expression of c-Kit, a tyrosine kinase receptor for the growth of hematopoietic stem cells (multipotent stem cells that give rise to all types of blood cells) has been identified in canine MCTs.
• Several studies have also suggested the presence of mutations in the juxtamembrane (adjacent to a membrane on one side of it) region of c-Kit leading to constitutive activation (synthesis of a protein or an enzyme at a constant rate) in the absence of hematopoietic growth stem cell factor. 
• Although it is not clearly understood, studies indicate that estrogen and progesterone may also influence mast cell tumors. (National Canine Cancer Foundation)