FAP Syndrome

Familial adenomatous polyposis (FAP) is an inherited disorder sometimes found in people with colon or rectal cancer. People with the classic type of FAP may develop noncancerous (benign) colon growths (polyps) as early as their teenage years (screening usually begins at 8 to 10 years old). The type of polyp most often seen in FAP syndrome, called an adenoma, is precancerous and has the potential to develop into cancer. Unless the growths are removed, these polyps may become malignant (cancerous). The average age at which an individual with class FAP develops colorectal cancer is 39 years old. Some people have a variation of the disorder, called attenuated familial adenomatous polyposis (AFAP), with fewer polyps. The average age of colon cancer onset for those with AFAP is 55 years.

In people with classic FAP, the number of polyps increases with age, and hundreds to thousands of polyps can develop in the colon. They could also develop noncancerous growths called desmoid tumors. These fibrous tumors usually occur in the tissue covering the intestines and may be provoked by colon removal surgery. Desmoid tumors tend to recur after they are surgically removed. In both classic FAP and AFAP, benign and malignant tumors are sometimes found in other places in the body, including the duodenum (a section of the small intestine), stomach, bones, skin, and other tissues. People who have colon polyps as well as growths outside the colon are sometimes described as having Gardner syndrome.

Approximately 2% of all colon cancer could be caused by a hereditary adenomatous polyposis condition. They can be categorized into three conditions:

• Familial adenomatous polyposis (FAP)

• Attenuated familial adenomatous polyposis (AFAP)

• MYH-associated polyposis (MAP)

Classic familial adenomatous polyposis (FAP) and attenuated familial adenomatous polyposis (AFAP) are due to mutations in the adenomatous polyposis coli (APC) gene. MYH-associated polyposis (MAP) is caused by mutations in the mutY homolog (MYH) gene. Individuals with MAP have mutations in both of their MYH genes (one from each parent, often referred to as “biallelic MYH mutations”). Patients often have no family history of colon cancer or polyps in parents (although siblings may be affected).

When assessing hereditary cancer risk, a patient’s personal and family history is collected to understand risk for a polyposis syndrome. Once it is thought that a patient might have one of these syndromes, genetic test results are the most accurate way to assess risk of cancer. Approximately 20% to 30% of FAP cases are caused by new mutations, meaning that an individual could have an APC mutation even if both parents don’t.

Also, because MAP has an autosomal recessive inheritance pattern,many affected patients have no relatives with polyps or cancer. Genetic testing is the only way to identify at-risk family members.

Finding out if you are at risk for adenomatous polyposis syndromes and following up is the most critical step.