Familial adenomatous polyposis is caused by a defect in the adenomatous polyposis coli (APC) gene. Most people inherit the genetic abnormality from a parent. But in about 30 percent of cases, the genetic mutation occurs spontaneously at conception.
The abnormal gene causes hundreds or even thousands of polyps to grow in your colon and rectum, usually starting by your midteens. The polyps are nearly 100 percent certain to develop into colon cancer or rectal cancer by the time you are 40.
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 arise in the duodenum, close to where the bile and pancreas ducts enter it (ampulla). Periampullary polyps also may become cancerous but can often be detected and removed before cancer develops.
- 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, familial adenomatous polyposis can cause cancer to develop in your thyroid gland, central nervous system, adrenal glands, liver or other organs.
- Noncancerous skin tumors.
- Noncancerous bone tumors.
- Pigment changes in the retina of your eye.
- Dental abnormalities.