Hereditary Colorectal Cancer Syndromes: Familial Adenomatous Polyposis (FAP) 

Familial adenomatous polyposis (FAP) is also known as familial polyposis coli, adenomatous polyposis coli, or Gardner syndrome. It affects males and females equally. The term Gardner syndrome has sometimes been used to refer to people who also have tumors outside the colon, such as osteomas (benign bony growths) and soft tissue tumors.

In its classic form, FAP is characterized by the following:

  • The development of more than 100 noncancerous (benign) polyps in the colon and rectum (polyposis). These are described as having a dense carpet-like appearance when seen on colonoscopy or sigmoidoscopy. These polyps can turn cancerous.

  • Early age of onset. Polyps start to develop about age16 (range of 7 to 36 years).

  • If left untreated, a nearly 100% risk of developing colorectal cancer.

  • Inherited from a mother or father with the disorder.

  • An increased risk of other health problems, such as polyps in the upper gastrointestinal tract, osteomas, epidermoid cysts (skin lesions). Also, desmoid tumors (locally invasive tumors that grow aggressively and can be life-threatening), congenital hypertrophy of retinal pigment (CHRPE), and dental abnormalities

  • An increased risk of thyroid, small bowel, pancreatic, and stomach cancers, brain tumors, and hepatoblastoma (a childhood liver tumor)

Mutations in a gene called APC causes most cases of FAP. The APC gene usually controls cell growth and cell death. Everyone has 2 APC genes. When a person has a mutated APC gene, his or her risk of developing polyps and risk of cancer increases. 

Because FAP starts at an early age, cancer screening often starts in childhood. Genetic testing of children at risk is also something to consider. Usually, genetic tests are not an option choice for children unless there is some type of medical benefit to justify testing. In the case of FAP, a child whose parent has the condition has a 50/50 chance of inheriting it. There is equally as likely a chance the child will not inherit it. Knowing the child doesn't have the defective gene could spare him or her from having tests such as sigmoidoscopy or colonoscopy. Since genetic testing can affect medical management, testing children at risk should be considered.

What are the variations in the APC gene?

Hundreds of mutations have been found throughout the APC gene. Some families with APC mutations have different symptoms than others. The location of a mutation in the gene provides some information about the types of FAP health problems a person will have. For instance, mutations in certain parts of the gene are associated with an increased rate of desmoid tumors, osteomas, and epidermoid cysts. Where in the gene a mutation lies also provides some information about the number of polyps a person will develop.

What is attenuated FAP?

People with attenuated familial adenomatous polyposis (AFAP) develop fewer than 100 polyps (average of 30 polyps). The risk of developing colon cancer is still higher, but the average age of diagnosis is older (about 55 years of age) than in the classic form of FAP. Some of the other health problems associated with classic FAP also happen in the attenuated form. However, cases of congenital hypertrophy of the retinal pigment (CHRPE) are rarely seen.

Mutations in 3 specific areas of the APC gene have been associated with AFAP. The number of polyps developed and the risk of other ways FAP shows up varies depending on which area the mutation is located.

The incidence of AFAP is unknown, but thought to be about the same or less than classic FAP.

What is a I1307K mutation?

One APC mutation in particular, called I1307K, is present in about 6% of the American Ashkenazi Jewish population. This mutation is associated with a 10% to 20% risk of colorectal cancer (slightly more than double the risk of someone else in the general population). However, people with this mutation do not present with the classic carpet of polyps in the colon seen in FAP.