Familial adenomatous polyposis (FAP) increases risk of colon and other cancers, but proper medical care and a comprehensive cancer screening program can manage these risks.
Familial Adenomatous Polyposis (FAP), Genetics, and Familial Colon Cancers
Most colon cancers, about 75% of them, are sporadic cases. There is no known genetic cause or documented family history for the individual who is diagnosed with the colon cancer. Among the non-sporadic colon cancer causes is Familial adenomatous polyposis (FAP).
FAP accounts for approximately 1% of colorectal cancer cases in the US each year. Given that about 160,000 people are diagnosed with colon cancer per year, about 1,600 of those colon cancer cases are caused by FAP.
Along with an increased risk of colon cancer, people with FAP can have an increased risk of developing other cancers, including cancers of the small intestine, stomach, bones, skin, thyroid, and bile ducts. Families in which FAP is present typically have a strong family history of colon and possibly other cancers. With genetic testing becoming more widely available, many of these families are aware that they have the genetic changes that cause FAP.
More than half of people with FAP will develop polyps in their colon by their teens and 90% develop them by the age of 30. Many patients have hundreds to thousands of these polyps in their colon by early adulthood. Polyps are small growths in the colon that if left untreated, can develop into colon cancer. In people with FAP, at least some of these polyps will develop into cancer, 100% of the time, if untreated. Polyps are considered pre-cancers, so the more polyps, the greater the chance that one or more will develop into cancer.
If the colon is not removed, people with FAP have a 100% chance of developing colon cancer at some point in their life. This is why the mainstay of treatment for FAP is surgery to remove the colon and possibly the rectum.
How is FAP Inherited?
FAP is passed from parents to children due to changes in one of two genes: APC and MUTYH. Genes are the instruction manual for building and running the human body. Nearly every cell in our bodies contains two copies of each gene. One copy comes from your mother and one copy comes from your father. In this way, if a mother or a father has the changes in one or more of the genes that cause FAP, they may pass these changes onto their children, when they pass along their genes.
For some diseases, if one of the two gene copies is damaged or missing, the other, good copy, will function normally and there will be no disease or increased risk of disease. This is called an autosomal recessive pattern. For other diseases, if even one copy of a pair of genes is damaged or missing, this is enough to increase disease risk or cause disease. This is called an autosomal dominant pattern.
What are the Different Types of FAP?
There are three main types of FAP. Two types result from damage in the APC gene and are called classical FAP and attenuated FAP. The third type occurs when the MUTYH gene is damaged and is called MYH-associated polyposis.
Classical FAP is passed through families in an autosomal dominant way. This means if even one copy of a person's two copies of the APC gene is damaged, he or she will have FAP. Attenuated FAP also is caused by damage in the APC gene, but it is inherited through an autosomal recessive pattern. This means that both copies of a person's APC gene need to be damaged to cause FAP. MYH-associated polyposis also is autosomal recessive, meaning both copies of the MUTYH gene must be damaged to cause FAP.
Your Best Defense
If you find out that you have FAP, talk to your doctor about getting a referral to see a genetic counselor. Your doctor can help you find a qualified genetic counselor in your area or you can search the National Society of Genetic Counselors website to locate genetic counseling resources.
Seeing a genetic counselor is important because there are many different changes to affected genes that cause FAP. Depending on which changes are present, you will have differing levels of risk for colon cancer occurring very early, such as in your 20s, vs. occurring much later, such as in your 50s. It is important to know your own personal risk so you can develop a plan to manage it. For example, if you have a form of FAP that causes thousands of polyps to develop very early, it is important that you go through surgery to remove your colon as soon as possible to prevent cancer.
The second important step you can take is to talk to your doctor about a cancer screening plan. For people with FAP, the recommended way to manage colon cancer risk is to remove the colon. However, cancer can occur in other tissues too. You will need to develop a screening plan to address these risks.
You will likely need to be screened starting at a younger age and more frequently than people without FAP. While this may seem like a very serious inconvenience, it is one of the best ways to manage your increased risk of the other cancers associated with FAP.
Sources
American Society of Colon & Rectal Surgeons. Family History Registries. Accessed: March 16, 2009.
http://www.fascrs.org/patients/family_history_registries/
Gene Reviews: APC-Associated Polyposis Conditions. Accessed: March 16, 2009.
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=fap
Genetic and Rare Diseases Information Center. Accessed: March 16, 2009.
http://rarediseases.info.nih.gov/GARD/
Genetics Home Reference. Familial adenomatous polyposis. Accessed: Mar 16, 2009.
http://ghr.nlm.nih.gov/condition=familialadenomatouspolyposis
OncoLink. Familial Colorectal Cancers - Familial Adenomatous Polyposis (FAP). Accessed: March 16, 2009.
http://www.oncolink.com/types/article.cfm?c=5&s=11&ss=82&id=7017
