By Miriam Komaromy, MD

Reviewed by Peggy Conrad, MS, CGC and Jonathan Terdiman, MD
Last updated August 4, 2000

Familial adenomatous polyposis, or FAP, is the second most common hereditary medical condition that puts you at serious risk for developing colon cancer.

Because FAP causes hundreds or even thousands of polyps (small mushroom-like growths) to appear on the inside walls of the colon, FAP is fairly easy to diagnose in a physical exam. In addition, most patients come from families in which the syndrome has already been identified. To help in further identifying at-risk individuals within FAP families, scientists have identified a specific gene that, if altered or mutated, causes FAP.

The APC Gene

When functioning normally, the APC gene produces a protein that suppresses tumors.

click image for larger representation

Genes of this type (which are often referred to as tumor suppressor genes) inhibit cells from growing and dividing. However, if you have a mutation in one of these genes, this braking system is compromised, resulting in a higher rate of uncontrolled growth, and thereby cancer.

Inheritance

Most people acquire FAP by inheriting a mutated copy of the APC gene. Since you only need to inherit one abnormal copy of the gene to develop FAP, this means that if either of your parents has FAP, you have a 50 percent chance of inheriting the altered gene, and thus the syndrome. This pattern of inheritance is called autosomal dominant inheritance.

If you are concerned that FAP runs in your family, you can use our TreeBuilder tool to assess your risk.

Most people with the APC gene mutation that causes FAP inherit it from an affected parent. However, about one-third of people with FAP get the mutated gene as a result of a spontaneous mutation in the sperm or egg of one of their parents. This means that the parent does not have FAP, but the child has the mutated gene in every cell of their body. Thus, people who inherit FAP in this way do not have a family history of the disease.

References

Terdiman, J. et al. (1999). Genetic testing in hereditary colorectal cancer: indications and procedures. The American Journal of Gastroenterology 94(9): 2344-2356.

Kinzler, K. et al. (1991). Identification of FAP locus genes from chromosome 5q21. Science 253: 661-5.

Giardello, F et al. (1997). APC gene mutations and extraintestinal phenotype of familial adenomatous polyposis. Gut 40(4): 521-5.