Overview
Familial adenomatous polyposis (FAP) is an inherited condition. This means it is passed from parents to their children through genes. FAP is rare, affecting about 1 in 10,000 people.
FAP leads to the development of hundreds to thousands of growths, called polyps, in the colon and rectum. Polyps typically begin to develop during adolescence and increase in number over time. Without treatment, the lifetime likelihood of colorectal cancer approaches 100%, often by age 40.
FAP is caused by a change in the APC gene. This change is called a mutation or a variant. The APC gene typically helps regulate cell growth in the intestinal lining. When this gene is altered, cells can grow out of control. This leads to the formation of many polyps.
These polyps usually grow throughout the colon rather than being limited to one part. In addition to having colorectal polyps, people with FAP may have polyps in other parts of the digestive tract, including the stomach and small intestine. There also is a higher risk of tumors and cancers in other parts of the body.
Variants and related conditions of FAP
Familial adenomatous polyposis (FAP) can vary in how it appears. Differences may include the number of polyps, when they develop and whether the disease affects other parts of the body.
Milder form of FAP
Attenuated familial adenomatous polyposis (AFAP) is a milder form of FAP. It is caused by changes in the APC gene but usually leads to fewer polyps, often fewer than 100. Polyps tend to develop later in life. The likelihood of colorectal cancer remains high, but it is lower than in classic FAP.
Features outside the colon
Some people with FAP have findings outside the colon. In the past, terms such as Gardner syndrome and Turcot syndrome were used to describe these features. Today, they are generally considered part of the same condition rather than separate syndromes. These features may include bone growths, dental changes, soft tissue tumors or tumors of the central nervous system.
Other inherited colorectal cancer conditions
Other genetic conditions can increase the risk of colorectal cancer or cause multiple polyps, but they develop in different ways:
- Lynch syndrome. This condition is caused by changes in a different group of genes. It typically leads to few or no polyps but still increases cancer risk.
- MUTYH-associated polyposis (MAP). Similar to FAP, this condition can cause multiple colon polyps. It is inherited in an autosomal recessive pattern, meaning both copies of the gene must be altered. It may not appear across multiple generations of a family.
- Juvenile polyposis syndrome, Peutz-Jeghers syndrome and Cowden syndrome. These conditions also can cause polyps in the digestive tract. However, the polyps have different features under the microscope and are linked to different genes.
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Symptoms
Most people with familial adenomatous polyposis (FAP) do not have symptoms early on. Polyps can begin to happen in childhood or adolescence, but they often do not cause noticeable changes at first.
As the number and size of polyps increase, symptoms may develop. These can include:
Some people with FAP also have physical features outside of the colon. These are more common in certain forms of the disease and may appear before colon polyps are found. These features may include skin changes, bony growths, dental changes and eye findings.
Rarely, symptoms related to other tumors may occur. For example, headache, neurological symptoms or other changes may be linked to tumors of the central nervous system.
When to see a doctor
Make an appointment with your healthcare professional if you or your child has any of the following:
- Blood in the stool.
- Ongoing diarrhea or a change in bowel habits.
- Unexplained abdominal pain.
- Unexplained weight loss.
- Persistent fatigue or weakness.
You also should seek care if you have a family history of FAP or early-onset colorectal cancer.
If a parent, sibling or child has FAP, talk with your healthcare team about genetic testing and screening. Early testing can help guide care and reduce the risk of cancer.
Children and teens who may be at risk of FAP should be evaluated early. Screening and testing often begin in late childhood or the early teen years.
Causes
Familial adenomatous polyposis (FAP) is caused by a change in the APC gene. This change is called a mutation or variant. The gene helps control how cells grow and divide in the lining of the colon.
When the APC gene is working as expected, it helps prevent cells from growing too quickly. When a mutation is present, this control is lost. Cells can grow and divide out of control. This leads to the formation of many polyps.
Over time, some of these polyps can become cancer. The high number of polyps increases the chance that cancer will develop.
FAP is inherited, meaning it is passed from parents to children through genes. A person only needs to inherit one copy of the altered APC gene to develop the condition. Each child of a parent with FAP has a 50% chance of inheriting the condition.
In about 20% of people with FAP, the condition develops without a family history. In these situations, the variant in the APC gene appears for the first time in that individual. This is known as a de novo variant.
Risk factors
The main risk factor for familial adenomatous polyposis (FAP) is having a family history of the condition.
If a parent has FAP, each child has a 50% chance of inheriting the gene change that causes it. Sometimes, FAP develops in people with no family history. This happens when a new gene change occurs for the first time in that person.
Because FAP is caused by a genetic change, lifestyle or environmental factors do not cause the condition. However, early identification of people at risk is important so that screening and treatment can begin at the right time.
Complications
In addition to colon cancer, familial adenomatous polyposis (FAP) can lead to several complications, most of which are related to polyp growth and increased cancer risk in other areas of the body.
Complications may include:
- Small intestine cancer. Polyps can develop in the upper part of the small intestine. This includes the duodenum and the area where the bile and pancreatic ducts enter, called the ampulla. These polyps can become cancerous over time.
- Stomach polyps. Polyps may form in the stomach. Most do not become cancer, but some types may carry a small risk.
- Desmoid tumors. These are noncancerous growths that can develop in the abdomen or other areas. They can grow into nearby tissues and sometimes cause serious issues.
- Other cancers. FAP increases the likelihood of cancers such as thyroid cancer, adrenal gland tumors and cancer of the central nervous system. Less commonly, cancers of the liver or bile duct may occur.
Other features of FAP
FAP also can cause changes in other parts of the body. They include:
- Bone growths, called osteomas.
- Dental changes, which may include extra teeth, called supernumerary teeth; teeth that do not come in as expected, known as impacted teeth; missing teeth; or other changes in tooth development.
- Skin cysts or other soft tissue growths, such as fibromas.
- Changes in the back of the eye, called congenital hypertrophy of the retinal pigment epithelium (CHRPE).