Familial adenomatous polyposis (FAP) is a genetic disorder mainly affecting the colon and rectum. People with FAP develop growths called polyps on the colon, which become cancerous over time. Other parts of the body can become affected by tumors, which can also be cancerous.
And estimated 1 in 7000 to 1 in 22,000 people are affected with FAP. Currently, the disorder cannot be reversed, and treatment sometimes includes removal of the colon.
The Types of Familial Adenomatous Polyposis
The three types of FAP are as follows:
- Classic FAP — people affected develop hundreds or thousands of polyps during adolescence. These polyps lead to colorectal cancer in adulthood, usually when the affected person is in his or her thirties.
- Attenuated FAP — people affected have the same number of polyps as with the classic type. These lead to colorectal cancer later in life, usually when the affected person is in his or her fifties.
- Autosomal recessive FAP — this is a milder type of FAP, with a different gene and inheritance pattern to the other types. People affected usually have fewer than a hundred polyps.
The Genes Involved in Familial Adenomatous Polyposis
The classic and attenuated types are caused by mutations in a gene called adenomatous polyposis coli (APC), located on chromosome 5. It codes for a protein of the same name, and is involved in cell division. When this protein is working properly, it prevents cell overgrowth. If it isn't working, the signs and symptoms of FAP can manifest.
The autosomal recessive type causes mutations in a gene called mutY homolog (E. coli), and is located on chromosome 1. The gene’s name is shortened to MUTHY, and codes for an enzyme called MYH glycosylase. This plays a role in DNA repair within cells. When not working properly, mistakes accumulate and the signs and symptoms of FAP result.
Inheritance of Familial Adenomatous Polyposis
Both classic and attenuated FAP are inherited in an autosomal dominant manner. This means the disorder will manifest when there is only one mutated gene copy in every somatic (body) cell. When one parent is affected by FAP, the chances of having a child affected with FAP are the following for each pregnancy:
- A 1 in 2 (50%) chance of having a child who is affected with FAP.
- A 1 in 2 (50%) chance of having a child who completely unaffected.
Autosomal recessive FAP is named for its inheritance pattern. In autosomal recessive disorders, both parents need to contribute a mutated gene copy in order for a person to be affected. Those with one mutated gene copy carry the disease, and typically do not show signs and symptoms of FAP.
When both parents are carriers of FAP, the chances of having a child affected are the following for each pregnancy:
- A 1 in 4 (25%) chance of having a child affected with FAP.
- A 1 in 2 (50%) chance of having a child who is a carrier of FAP
- A 1 in 4 (25%) chance of having a child who is completely unaffected.
Genetic Testing and Genetic Disorder
Genetic testing is possible for FAP, including carrier and prenatal testing. Genetic counseling may be available for individuals and families seeking support. These professionals are trained to offer information on inheritance patterns and testing options without leading people’s decisions.
For more details on genetic testing and consultation, read Understanding Genetic Testing in Healthcare and Understanding Genetic Counseling. Visit the National Cancer Institute website for more information on the genetics of colorectal cancer.
Sources
“APC-Associated Polyposis Conditions”, updated: 24th July 2008, GeneReviews (NCBI Bookshelf) website, accessed: 1st December 2010, authors: Burt, Randall. W, Jasperson, Kory. W.
“Familial Adenomatous Polyposis”, updated: 7th December 2009, eMedicine website, accessed: 1st December 2010, authors: Wehbi, Mohammad et al.
“Familial adenomatous polyposis”, reviewed: April 2008, Genetics Home Reference website, accessed: 1st December 2010, no author specified.
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