MUTYH-associated polyposis (MAP): inherited bowel polyps and cancer risk

Bowel polyps are small growths on the inner lining of the large bowel. Many adults develop a few, and most are harmless, but some types can turn cancerous if left to grow. An NHS genetics service leaflet explains that MAP is an inherited condition associated with an increased risk of developing multiple bowel polyps and bowel cancer, and that adenomas are the type of polyp commonly associated with it.

The cause is a change, or variant, in the MUTYH gene. The leaflet explains that MUTYH has a key role in repairing damage to our bodies that occurs during normal daily life and that variants stop it working properly, making a person more susceptible to bowel polyps and bowel cancer. A person who has a variant in both copies of the gene has MAP.

MAP is distinct from other inherited bowel conditions such as familial adenomatous polyposis (FAP), which is dominant and caused by a different gene, and from Lynch syndrome, which carries a high risk of bowel cancer without large numbers of polyps. Distinguishing between them matters because the surveillance and family implications differ.

We all have two copies of the MUTYH gene, one from each parent. The leaflet explains that people with a variant in one copy are carriers, and are typically healthy with no signs or symptoms because they still have one working copy, and that 1 in 70 people in the UK population are carriers. If both parents are carriers, each child has a 1 in 4 (25%) chance of inheriting two variants and having MAP.

This is why MAP often seems to appear without a family history. The leaflet notes that although MAP is inherited, a person may be the first in their family to develop the condition. The education fact sheet says suspicion is raised when someone has developed multiple adenomas below the age of 40, has 10 to hundreds of polyps, particularly where FAP has been excluded, or when the family history suggests a recessive pattern, for example where affected relatives are all brothers and sisters of one generation.

For relatives, the fact sheet states that siblings of an affected person have a 1-in-4 chance of also having MAP, whereas children and parents are usually at low risk. It adds that partners of someone with MAP have a 1% to 2% chance of carrying a variant, so children are usually at low risk, though testing of partners or offspring may be offered in some circumstances. An NHS Lothian leaflet for carriers likewise says that carriers will not develop MAP themselves and that their close blood relatives can be offered a genetic test.

MAP often causes no symptoms until polyps or a cancer have developed. The fact sheet describes the key clinical feature as the development of polyps in the bowel, often between 10 and a few hundred by the age of 50, with adenomas, hyperplastic polyps and serrated polyps possible. It also says the condition is very variable, with some people developing few polyps and others hundreds. Duodenal and gastric polyps have been reported in some individuals.

When symptoms do occur they are those of bowel polyps or bowel cancer, such as blood in the stool, a persistent change in bowel habit, unexplained weight loss or tiredness from anaemia. These are not specific, so see a GP if you notice them, whatever your genetics. For anyone with a known family history, tell your GP, because it changes how seriously symptoms are taken.

Diagnosis is confirmed with a genetic test showing two changes in the MUTYH gene. The fact sheet says suspicion is based on a detailed family history and on pathology details of polyp type and number, and that diagnosis is confirmed through genetic testing that identifies two alterations in the MUTYH gene. The leaflet says the test uses DNA from a blood sample and is offered first to the person in the family with the suspected diagnosis before other relatives.

You may be offered testing if you have multiple polyps, have bowel cancer and a family history of bowel cancer or multiple polyps, or develop bowel cancer before the age of 40, according to the NHS genetics service leaflet. An appointment with a genetic counsellor or doctor discusses the risks and benefits of testing, what outcomes mean, and the chances of you and your partner passing MAP on to children. You can decline testing, but the leaflet advises you should still have regular bowel screening as recommended.

Regular colonoscopy is the cornerstone of management. The fact sheet says it allows the removal of polyps and consequently reduces the risk of later cancer. A colonoscopy uses a long, flexible tube with a camera passed through the back passage into the large bowel, and polyps found are usually removed during the procedure and sent to the laboratory.

The recommended intervals differ between sources and services, so follow the plan from your own genetics team. An NHS genetics service leaflet from 2025 recommends a colonoscopy every year from the age of 18 and regular gastroscopies from the age of 35 to look at the stomach and upper small intestine. A Scottish NHS leaflet advises colonoscopy every 2 years from 18 years of age, and a 2015 education fact sheet describes regular colonoscopy as usually twice a year from age 18 to 20 onwards.

If the number and density of polyps prohibits removal, the fact sheet says colectomy, surgery to remove part or all of the large bowel, may be considered. For anyone who develops colorectal cancer, continued surveillance of the remaining bowel is important because of the risk of further cancers. The leaflet adds that if cancer is detected at an early stage, treatment is more likely to be successful, and that not everyone with MAP will develop bowel cancer.

Being a carrier of a single MUTYH variant is common and does not cause MAP. The NHS Lothian leaflet states that carriers will not develop MAP and that there are no implications for their own health, and that a person with this result should be bowel aware and participate in the National Bowel Screening Programme. The leaflet advises that your partner could have the gene test and that children would only need to consider testing if the partner were also a carrier.

Carrier status is mainly of importance for family planning and for relatives. Your genetics service can provide specific guidance about who in the family could be offered testing, and relatives outside the service area can ask their GP to refer them to local genetics.

A diagnosis can feel alarming, but MAP is manageable with regular surveillance, and the risk of cancer can be reduced by finding and removing polyps. Keep a record of your colonoscopy dates and results, attend appointments, and report new bowel symptoms promptly rather than waiting for the next scheduled test.

Ask your genetics team who coordinates your surveillance, who to call with symptoms, and how your relatives can be offered testing. You may find it helpful to speak with others living with inherited bowel conditions through cancer and bowel charities.