What is FAP?
FAP is a hereditary syndrome that makes a person more likely to get polyps and cancer. FAP is caused by mutations (changes) in the APC gene.
There are 2 types of FAP: classic FAP and attenuated FAP (AFAP).
People with classic FAP have hundreds to thousands of polyps (tumor-like growths) in their colon (large intestine). Some colon polyps, called adenomatous polyps, can develop into colorectal cancer over time. People with classic FAP are at a very high risk of developing colorectal cancer. People with FAP can also have conditions outside of the colon, such as:
- Bony growths called osteomas
- Extra or missing teeth
- A rare eye condition called congenital hypertrophy of the retinal pigment epithelium (CHRPE)
- Benign (noncancerous) skin cysts
- Benign abdominal tumors
- Polyps or cancer of other parts of the gastrointestinal system, such as the stomach and small intestine
- A small increased risk of developing other types of cancer
People with attenuated FAP (AFAP) have less than 100 adenomatous polyps in their colon and are usually diagnosed at a later age than those with classic FAP. However, they still have an increased risk of developing colorectal cancer. People with AFAP are less likely to develop conditions outside of the colon than people who have classic FAP.Back to top
What is MAP?
MAP is a hereditary syndrome that makes a person more likely to get gastrointestinal polyps and cancer. It is caused by mutations in both copies of the MYH gene. Like people with AFAP, the number of adenomatous polyps in people with MAP is usually less than 100. There have also been a few reports of people with MAP getting colorectal cancer at a young age without other polyps being present. The risk of people with MAP developing conditions outside of the colon has not been well-studied.Back to top
How are FAP/AFAP and MAP diagnosed?
FAP/AFAP and MAP are usually diagnosed when a doctor finds many polyps but can also be diagnosed through genetic testing.Back to top
How common are FAP/AFAP and MAP?
About 1% of people with colorectal cancer have FAP/AFAP or MAP.Back to top
What are the chances of inheriting FAP/AFAP?
Both men and women can pass down an APC gene mutation to their children. Every person has 2 copies of the APC gene (1 from each parent), and it only takes a mutation in 1 copy to cause FAP/AFAP. If a person has a parent with FAP/AFAP, he or she has a 50% chance of inheriting the APC gene mutation. The person would also have a 50% chance of not inheriting the mutation.
A person can have FAP/AFAP even if it doesn’t run in his or her family. This happens if the APC gene mutates during fetal development. If this person has children, each child will have a 50% chance of inheriting the APC mutation.Back to top
What are the chances of inheriting MAP?
Unlike most hereditary cancer syndromes, MAP is a recessively inherited syndrome. This means a person must inherit 2 MYH gene mutations (1 from each parent) in order to develop MAP.
If both parents carry MYH mutations, there is a 25% chance that their child will inherit 2 MYH gene mutations and therefore develop MAP.
If only 1 parent has MAP, the chances of having a child with MAP are low because the chance that the other parent has a MYH mutation is low.
When a person has only 1 MYH mutation, he or she is said to be a “carrier.” At this time, people who have only 1 MYH gene mutation are thought to have a slight increased risk of developing colorectal polyps or cancer, particularly if there is a family history of colon cancer.Back to top
What does a positive APC or MYH test result mean?
A positive test result means that an APC or MYH mutation was found. This person has an increased risk for developing polyps and colorectal cancer. The risk for other cancers and features varies.Back to top
What does a “variant of uncertain significance” (or “ambiguous”) test result mean?
Genes are written in letters called bases. We call the wrong spelling of a gene a “mutation” and a mutation can cause an increased risk of cancer. However, some spelling changes in genes do not cause disease. We are still learning which spellings are normal (harmless) and which may be disease-causing. These kinds of uncertain changes are called variants of uncertain significance.
If you have a variant in the APC or MYH gene, we may not be able to tell you if it is a normal change or a disease-causing change. In some situations, we may be able to test your relatives for the variant or we may be able to do lab tests, but we still may not be able to tell if the variant will increase your risk for polyps or cancer. If you have a variant, we will make recommendations for cancer screening based on your family history.Back to top
What does a negative APC or MYH test result mean?
A negative test result means that no mutation was found in the APC or MYH gene. This could mean any of the following:
- You could have a mutation in the APC or MYH gene that cannot be detected with the testing method that was used. No one genetic testing method can find all mutations.
- You could have a mutation in a different gene. Your genetic counselor may recommend testing for mutations in other genes that are also associated with polyps and certain cancers.
- Your personal or family history of FAP/AFAP/MAP-like features could be related to other factors such as age, environment, and chance.
- If you had genetic tests to look for a mutation that was already found in a family member who has FAP/AFAP or MAP, then a negative test result means that your risk of developing FAP/AFAP/MAP-like features (such as polyps and cancer) is the same as that of the general population.
How should people with FAP/AFAP or MAP manage their risks for related cancers?
The first goal for people with APC or MYH gene mutations is to prevent cancers. The second goal is to find cancers at early enough stages so they can be treated.
People with FAP should have a flexible sigmoidoscopy every year starting at 10 to 15 years of age, or younger if symptoms develop. Once colorectal polyps are found, a colonoscopy should be done every year until prophylactic (risk-reducing) surgery is done. Even if no polyps have been found, people with FAP should have a colonoscopy every year starting at the age of 20. This is because polyps are often located in the right side of the colon, and a colonoscopy is the best way to find them.
People with AFAP should have their first colonoscopy in their late-teenage years. As long as no polyps are found, a colonoscopy should be repeated every 2 to 3 years. Once polyps are found, a complete colonoscopy should be done every 1 or 2 years, or as directed by a doctor.
People with MAP should have their first colonoscopy at 25 to 30 years of age. If no polyps are found, a colonoscopy should be repeated every 2 to 3 years. Once polyps are found, a colonoscopy should be done every 1 to 2 years, or as directed by a doctor.
The preferred way to prevent colorectal cancers in people with classic FAP is to remove the colon before cancer is found, which is called a prophylactic colectomy. This is because the risk of colorectal cancer is almost 100%, and removing hundreds to thousands of polyps through a colonoscopy is not possible. The right age for someone to have a prophylactic colectomy depends on the results of past colonoscopies and symptoms, as well as on psychological and social factors. If the rectum (the end part of the colon) is not removed, an endoscopy of the remaining tissue should be done every year to prevent rectal cancer.
For people with AFAP or MAP, the need for surgery depends on the number and location of polyps. For people with few polyps, a polypectomy (the removal of polyps during a colonoscopy) may be enough. People with a large number of polyps and people who are not able to get a thorough colonoscopy may be recommended to have a prophylactic colectomy.
People with FAP or AFAP should have an upper gastrointestinal (stomach and part of the small bowel) endoscopy starting at 25 to 30 years of age, or younger if symptoms develop. An upper gastrointestinal endoscopy should be repeated at least every 4 years, depending on how many polyps are found in the upper gastrointestinal tract.
People with MAP should have an upper gastrointestinal endoscopy starting at 30 to 35 years of age, or younger if symptoms develop. An upper gastrointestinal endoscopy should be repeated at least every 4 years, depending on how many polyps are found in the upper gastrointestinal tract.
People with FAP, AFAP, or MAP should have a yearly physical exam. For people with FAP and AFAP specifically, the yearly physical exam should include an exam of the thyroid gland. In people with FAP, an eye exam may be considered to check for congenital hypertrophy of the retinal pigment epithelium (CHRPE), although no treatment is necessary for this finding. Some researchers have recommended an annual serum alpha-fetoprotein and hepatic ultrasound in children with classic FAP to screen for hepatoblastoma. However, we don’t yet know how effective this screening is.
It is important to know that data regarding risks for other types of cancer in the setting of the FAP, AFAP, and MAP is not clear at this time. We ask that you and your family members keep in contact with our office every few years to learn about potentially important updates.Back to top
If I have an APC or MYH mutation, what are my options for having children?
Through in vitro fertilization (IVF), embryos can be tested for the family’s genetic mutation. This process is called preimplantation genetic diagnosis (PGD). PGD may be an option for families who wish to have children without the familial mutation. To learn more, ask for a referral to an IVF-PGD specialist before trying to get pregnant.
Other possible family planning options include having children with the assistance of donor eggs or sperm or through adoption.
People with MYH mutations may also want their partner to have genetic testing to help determine risks to future children.Back to top
What if I have questions?
If you have any questions or concerns, contact the MSK Clinical Genetics Service at 646-888-4050.Back to top