Familial adenomatous polyposis (FAP) is a rare, inherited condition caused by a defect in the adenomatous polyposis coli (APC) gene. Most people inherit the gene from a parent. But for 25 to 30 percent of people, the genetic mutation occurs spontaneously.
FAP causes extra tissue (polyps) to form in your large intestine (colon) and rectum. Polyps can also occur in the upper gastrointestinal tract, especially the upper part of your small intestine (duodenum). If untreated, the polyps in the colon and rectum are likely to become cancerous when you are in your 40s.
Most people with familial adenomatous polyposis eventually need surgery to remove the large intestine to prevent cancer. The polyps in the duodenum also can develop cancer, but they can usually be managed by careful monitoring and by removing polyps regularly.
Some people have a milder form of the condition, called attenuated familial adenomatous polyposis (AFAP). People with AFAP usually have fewer colon polyps (an average of 30) and develop cancer later in life.
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The main sign of FAP is hundreds or even thousands of polyps growing in your colon and rectum, usually starting by your mid-teens. The polyps are nearly 100 percent certain to develop into colon cancer or rectal cancer by the time you're in your 40s.
Familial adenomatous polyposis is caused by a defect in a gene that's usually inherited from a parent. But some people develop the abnormal gene that causes the condition.
Your risk of familial adenomatous polyposis is higher if you have a parent, child, brother, or sister with the condition.
In addition to colon cancer, familial adenomatous polyposis can cause other complications:
- Duodenal polyps. These polyps grow in the upper part of your small intestine and may become cancerous. But with careful monitoring, duodenal polyps can often be detected and removed before cancer develops.
- Periampullary polyps. These polyps occur where the bile and pancreas ducts enter the duodenum (ampulla). Periampullary polyps might become cancerous but can often be detected and removed before cancer develops.
- Gastric fundic polyps. These polyps grow in the lining of your stomach.
- Desmoids. These noncancerous masses can arise anywhere in the body but often develop in the stomach area (abdomen). Desmoids can cause serious problems if they grow into nerves or blood vessels or exert pressure on other organs in your body.
- Other cancers. Rarely, FAP can cause cancer to develop in your thyroid gland, central nervous system, adrenal glands, liver or other organs.
- Noncancerous (benign) skin tumors.
- Benign bone growths (osteomas).
- Congenital hypertrophy of the retinal pigment epithelium (CHRPE). These are benign pigment changes in the retina of your eye.
- Dental abnormalities. These include extra teeth or teeth that don't come in.
- Low numbers of red blood cells (anemia).
Preventing FAP is not possible, since it is an inherited genetic condition. However, if you or your child is at risk of FAP because of a family member with the condition, you will need genetic testing and counseling.
If you have FAP, you will need regular screening, followed by surgery if needed. Surgery can help prevent the development of colorectal cancer or other complications.