Familial Adenomatous Polyposis

Is Testing Right for Me? I Have My Test Results

Familial Adenomatous Polyposis

What Is Familial Adenomatous Polyposis?

Familial adenomatous polyposis (FAP) is an inherited risk to develop many growths in the colon, called polyps. These growths can lead to cancer. People with classic FAP may have hundreds to thousands of colon polyps. Attenuated FAP is a milder form that usually causes more than 10 polyps, but less than 100. FAP can also cause polyps to grow in the small intestine and stomach lining.


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If FAP isn't treated, almost all people with it will get colon cancer. The cancer often starts at a young age. Colon cancer may be prevented by removing the colon after many polyps have formed. Occasionally, people with attenuated FAP can just have the polyps removed as they form. People with FAP need extra cancer screening starting at an early age.

Colon cancer is the most important risk, but FAP may also cause a small increased risk of other cancers. Non-cancer physical signs of FAP may help diagnose the condition. Read more about the Other Signs of FAP.

Next: Who Is At Risk for Familial Adenomatous Polyposis?

Why are you considering genetic testing?

I have signs of FAP (100 or more colon polyps) or attenuated FAP (10-99 colon polyps). No one in my family has had FAP testing before.

I have a family member who had genetic testing that found an FAP gene mutation.

I have a family history with signs of FAP, but no one has had genetic testing yet.

I am interested in the Ashkenazi Jewish I1307K variant testing. No one in my family has FAP.

None of these.

Anyone with signs of FAP should think about genetic testing. The first person tested usually has full gene sequencing, which looks at the whole gene, "letter-by-letter," to find any change from normal. This kind of testing is expensive and time-consuming, but necessary to find as many different mutations as possible.

Gene sequencing will find up to 90% of the mutations that cause classic FAP. Gene sequencing finds a smaller percentage of the mutations that cause attenuated FAP, but the exact percentage isn't known. Some people will also have deletion analysis that looks for large parts of the gene that are missing (deleted). About 8% to 12% of people with FAP have this kind of mutation.

  • When testing finds a mutation, the result confirms that you have FAP. This information can help you and your doctor agree on the best cancer screening and treatment plan for you. Also, your family members can now have genetic testing for the mutation that causes your FAP. This is less expensive and more accurate than complete FAP testing.
  • When testing doesn't find a mutation, the results don't rule out FAP or another inherited risk to get cancer. Genetic testing cannot find all of the mutations that cause FAP or other inherited cancers. You and your family may still need to have earlier and more frequent colon cancer screening, and sometimes other tests. If your test doesn't find a mutation, the same kind of test probably won't be useful for your family members. Mutations in other genes can also cause symptoms that look like FAP. Your doctor or genetic counselor may recommend other testing if your FAP gene test is normal or if your family history suggests a different gene could be involved.

If you have only brothers or sisters with multiple polyps or no family history, you could also have MYH-associated polyposis. Your doctor or genetic counselor can help you decide which test is most appropriate for you.

Because you have a family member with an FAP mutation, you have a higher chance for FAP and colon cancer. Your risk depends on how closely related you are to someone with FAP. If you aren't already having extra colon cancer screening based on your family history, talk to your doctor about a screening plan. The only people who can safely avoid early and frequent screening are those who don't have the family mutation. Once a mutation that causes FAP has been found in one family member, you and other relatives can have testing for that one mutation. Testing for one mutation is usually much less expensive than complete FAP testing, and the results are more accurate. The laboratory will need to know the exact name of the mutation that runs in the family. If possible, it is best to give the lab a copy of a relative's genetic test results. Parents should consider testing for children by the time they are 10 years old. This is the earliest age most experts recommend that colon screening start.

  • When testing finds the mutation that runs in your family, you can be certain that you have inherited FAP and a risk for colon polyps. These results mean that you need early and regular screening to prevent cancer.
  • When testing doesn't find a mutation, you can be sure that you did not inherit the mutation that runs in your family. Unless you have other cancer risk factors, you won't need the extra cancer screening normally recommended for people who might have FAP.

You can consider FAP genetic testing, but you may not be the best person in your family to start genetic testing. The first person in the family tested should be someone most likely to have FAP. Usually this means someone with 100 polyps or more (or at least 10 polyps if you are testing for attenuated FAP). Most people with FAP will have polyps by the time they are in their thirties.

Genetic testing cannot find all of the mutations that cause FAP. Testing someone who has FAP first helps find out if genetic testing will be able to find the mutation that causes FAP in your family. It may be possible to do FAP genetic testing on DNA after a person has passed away. If a mutation is found, then all other family members can have testing for the one mutation that runs in the family, which is much less expensive and more accurate than complete gene testing. If a mutation isn't found in someone with signs of FAP, then testing won't be useful for anyone else in the family.

If it isn't possible to test someone with signs of FAP first, you can have testing but the meaning of your results may not be clear.

  • If testing finds a mutation, then it is likely that you have inherited FAP and a risk for colon polyps. These results mean you need extra cancer screening starting at a young age to prevent cancer or detect it early.
  • When testing doesn't find a mutation, there are a few possible explanations.
    • You may not have inherited a mutation that causes FAP in your family
    • The mutation that causes FAP in your family can't be found with the test that was done.
    • Your family has a reason for multiple colon polyps or early colon cancer that isn't FAP.
    This test result won't change your medical care. You may still have a high risk for cancer, so you need extra cancer screening starting at a young age.

About 6% of Ashkenazi Jewish people have a variation in the FAP gene called I1307K. This is not a true mutation. It does not cause FAP. But, it does seem to increase the risk for colorectal cancer up to twice the average risk.

Experts don't agree on who should be tested for I1307K, or if testing is useful at all, since there is only a small increased risk for colorectal cancer. No research has shown that people with an I1307K variation need earlier or more frequent colon cancer screening. If you are considering testing for I1307K, talk to your doctor or genetic counselor about what those results might mean for you.

FAP genetic testing probably won't be useful for you if you don't have signs of FAP, and no one in your family does either.

There are many different causes of colon cancer. Some are genetic and most aren't. If you had colon cancer or you have a family history that concerns you, talk to your doctor or a genetic counselor. You may also be interested in learning about hereditary nonpolyposis colorectal cancer — a more common kind of inherited colon cancer that doesn't involve multiple colon polyps.