Familial Adenomatous Polyposis

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What is familial adenomatous polyposis?

Classic familial adenomatous polyposis (called FAP or classic FAP) is a genetic condition. It is diagnosed when a person develops more than 100 adenomatous colon polyps. An adenomatous polyp is an area where normal cells that line the inside of a person's colon begin to make mucous and form a mass on the inside of the intestinal tract. The average age for polyps to develop in people with FAP is in the mid-teens. More than 95% of people with FAP will have multiple colon polyps by age 35. If FAP is not recognized and treated, there is almost a 100% chance that a person will develop colorectal cancer. The risk of colon cancer is 87% by age 45.

There is also an increased chance of developing cancer in the stomach and/or small intestines. Other types of cancer found in families with FAP include hepatoblastoma (a type of liver cancer seen in young children);papillary thyroid cancer; pancreatic, adrenal, and bile duct cancers; and a low risk of a type of brain cancer called medulloblastoma.

Not all symptoms of FAP are cancer-related. Some additional features of FAP may include:

Osteomas (noncancerous bony growths, usually found on the jaw)

Extra, missing, or unerupted teeth

Congenital (present at birth) hypertrophy of the retinal pigment epithelium (CHRPE), an eye condition that does not affect vision but can be seen by looking at the retina using a special instrument called an ophthalmoscope.

Benign (noncancerous) skin changes, such as epidermoid cysts and fibromas

Desmoid tumors (noncancerous fibrous tumors that can develop anywhere in the body)

Adrenal masses

There are three subtypes of classic FAP called attenuated FAP (AFAP), Gardner syndrome, and Turcot syndrome. This section addresses classic FAP.

What causes FAP?

FAP is passed from generation to generation in a family. The APC gene is linked to FAP; APC stands for adenomatous polyposis coli. A mutation (alteration) in the APC gene gives a person an increased lifetime risk of developing colorectal cancer or other cancers of the digestive tract.

How is FAP inherited?

Normally, every cell has two copies of each gene: one inherited from the mother and one inherited from the father. FAP follows an autosomal dominant inheritance pattern. In autosomal dominant inheritance, a mutation happens in only one copy of the gene. This means that a parent with a gene mutation may pass along a copy of their normal gene or a copy of the gene with the mutation. Therefore, a child who has a parent with a mutation has a 50% chance of inheriting that mutation. A brother, sister, or parent of a person who has a mutation also has a 50% chance of having the same mutation.

How common is FAP?

FAP is uncommon; specific estimates on how many people have FAP vary from one in 22,000 up to one in 7,000. About 30% of people with FAP do not have any family history of the condition; they have a de novo (new) mutation in the APC gene.

Most colorectal cancer is sporadic (occurs by chance) and is not related to FAP or other known inherited genetic changes. Less than 1% of all colorectal cancer is thought to be due to FAP.

How is FAP diagnosed?

Classic FAP is a clinical diagnosis. This means that it is typically diagnosed by its symptoms rather than a laboratory test. A person with more than 100 adenomatous colon polyps is considered to have FAP. People with FAP can also have a blood test to look for a mutation in the APC gene. If an APC gene mutation is found, other family members may be diagnosed with FAP if they are tested and have the same gene mutation.

What are the estimated cancer risks associated with FAP?

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