MYH-Associated Polyposis

Is Testing Right for Me? I Have My Test Results

MYH-Associated Polyposis

What Is MYH-Associated Polyposis?

MYH-associated polyposis (MAP) is an inherited risk to develop many growths in the colon, called polyps. These growths can lead to cancer. Until recently, people with MAP were incorrectly diagnosed with another condition that causes multiple colon polyps – called familial adenomatous polyposis (FAP).


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MAP usually causes 10 to 100 polyps, which is most like the milder kind of FAP called attenuated FAP. But, some people with MAP can have more than 1000 colon polyps and some develop colon cancer without any polyps.


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The biggest difference between the two conditions is how they are inherited. MAP usually only affects brothers and sisters, while FAP may be seen in many generations passed from parent to child. It isn't always possible to tell the difference between MAP and FAP without genetic testing.

If MAP isn't treated, there is a very high risk for colon cancer. The cancer often starts at a younger age than usual. People with MAP need extra cancer screening starting at an earlier age than usual. See Preventing Colon Cancer for more information.

Next: Genetics of MYH-Associated Polyposis

Why are you considering genetic testing?

I have had more than 10 colon polyps. No one in other generations of my family has had multiple polyps.

I have a brother or sister (same mother and father as me) with MAP diagnosed by genetic testing.

I have a relative, other than a brother or sister, who has MAP or is a carrier based on genetic testing.

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

None of these

MAP genetic testing is recommended for anyone with more than 10 adenomatous colon polyps when there are no signs that multiple polyps are passed from parent to child in the family. Usually this means that only one person in the family has multiple polyps, or only brothers and sisters with both of the same parents do.

  • If you are the only person in the family with polyps, your doctor may recommend testing for familial adenomatous polyposis (FAP) first. About 25% of people with FAP have no known family history of polyps. See Familial Adenomatous Polyposis testing for more information. If FAP testing is normal, then MAP testing should be done.
  • If you have a brother or sister only with multiple polyps, your doctor may start with MAP testing first. If MAP testing is normal, then your doctor may recommend FAP genetic testing.

If you choose MAP genetic testing, this is what your results are likely to mean for you:

  • When testing finds two mutations, the result confirms that you have MAP. This information can help you and your doctor agree on the best cancer screening and treatment plan for you. This is also very important information for figuring out your family members' risk for multiple polyps and what kind of colon cancer screening they need. Your family members can now choose to have genetic testing for only the mutations that causes your MAP. This is usually less expensive and more accurate than complete MAP testing.
  • When testing finds one mutation, the results mean that you are at least a carrier of MAP. Most people who have MAP, have two mutations. Because testing may not find all of the mutations that cause MAP, this result doesn't rule out that you have MAP. Your brothers and sisters may still be at risk for colon cancer and other relatives may be carriers of MAP.
  • When testing doesn't find any mutation, MAP is unlikely. Your doctor will probably recommend FAP genetic testing – if it hasn't already been done. Genetic testing cannot find all of the mutations that cause multiple polyps and inherited colon cancer. You and your family will still need extra colon cancer screening.

Because you have a brother or sister with MAP, you also have a 25% chance to have MAP. 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 mutations that cause MAP in your family.

MAP is caused by mutations in both copies of a person's MYH gene. Usually, genetic testing will find both of the mutations, but sometimes only one is found.

  • If genetic testing found both mutations that cause MAP in your sibling, you can choose to have genetic testing for only these mutations (both mutations can be called the same thing, like Y165C). If you have both mutations, you have MAP and are at increased risk for colon cancer. If you have only one of these mutations, you are a carrier but you don't have MAP. Your children only have a risk for MAP if their other parent is also a carrier (this isn't common).
  • If genetic testing found only one mutation in your sibling, it isn't clear if your sibling has MAP. By chance, someone could be a carrier of MAP, but have a different inherited tendency to develop colon polyps. You can choose to have genetic testing for just that one mutation. If you don't have that mutation, you can be sure you didn't inherit MAP. If you do have that mutation, it won't be clear if you have MAP or not. You probably will still need extra colon cancer screening since your sibling's diagnosis isn't certain.

Testing for specific mutations that run in the family is usually less expensive than complete MAP genetic testing and the results are more reliable. The laboratory will need to know the exact mutation names. If possible, it is best to give the lab a copy of your sibling's genetic test results.

Because you have a family member who has or is a carrier of MAP, you have a higher chance to be a carrier. See the Family History section for information that can help you figure out your carrier risk if you don't already know. Your doctor or genetic counselor can also tell you this based on your family history.

You are unlikely to have MAP and a high risk for colon cancer unless your parents are biologically related to each other or happen to both be MAP carriers. About 1 to 2 out of every 100 people without a family history of MAP is a carrier. Most don't know they are MAP carriers. If you have children, they are also only at risk if both you and your partner are MAP carriers.

Medical guidelines focus on MAP genetic testing to find people who actually have MAP, not those who are carriers. So, your doctor or insurance company may not see testing as medically necessary.

If you decide to have carrier testing, find out the names of the mutations that run in your family. If possible, it is best to give the lab a copy of your relative's genetic test results. Testing for specific mutations that run in the family is usually less expensive than complete MAP genetic testing and the results are more reliable.

If you have a family history of multiple polyps in just one generation (only one person or only brothers and sisters), MAP is a possible explanation.

If you have relatives with polyps in more than one generation (like parents and children), MAP is very unlikely. Other inherited conditions also cause multiple polyps or colon cancer at a young age.

Your doctor or genetic counselor can take a detailed family history to figure out if MAP or another condition is likely in your family. If MAP is likely, the first person tested in your family should be someone who actually has multiple polyps. If testing finds two mutations, you know MAP caused the polyps and that genetic testing can find the mutations that cause it. If testing doesn't find the mutations, testing won't be useful for any other family members. It may be possible to do genetic testing on cells from someone who has passed away.

If it isn't possible to test someone with multiple polyps first, your doctor or genetic counselor can help figure out the next best option for you.

MAP is caused by two mutations – one inherited from the mother and the other one inherited from the father. If you have a brother or sister (with both of the same parents as you) with MAP, you have a 25% chance to have MAP. If you are related to someone with MAP in another way, you have an increased chance to be a carrier. Carriers don't have MAP, but they can have children with MAP if their partner is also a carrier. About 1 to 2 out of 100 people (1% to 2%) is a carrier of MAP.

MAP genetic testing probably won't be useful for you if you don't have signs of MAP, and no one in your family does either. If you have a personal and/or family history of multiple polyps in more than one generation, familial adenomatous polyposis (FAP) is more likely. Other inherited causes of multiple polyps are also possible.

If you had polyps, colon cancer, or a family history that concerns you, talk to your doctor or a genetic counselor. These professionals can take a detailed family history for cancer and other related signs. This is usually the best tool for deciding if you have an inherited risk for colon cancer, and what kind of screening you might need.