What is MUTYH-associated polyposis (MAP)?
MUTYH-associated polyposis (MAP) is a rare, hereditary (inherited) condition in which a person has numerous adenomatous polyps (abnormal tissue growths) in the colon and rectum.
People with harmful mutations in the MUTYH (formerly called MYH) gene can develop several different types of polyps in the large intestine, including adenomas, sessile serrated polyps and hyperplasic polyps. Most people who have MAP usually develop between 10 and 100 polyps. In rare cases, some people who have MAP can have colorectal cancer without any polyps, while others have more than 1,000 polyps. The colorectal polyps are often found in people in their 40s.
What are the cancer risks associated with MUTYH-associated polyposis (MAP)?
People who have MAP are at an increased risk of developing colon and rectal cancer. Most colorectal cancers will occur between the ages of 40 and 60. Approximately half of all people with MAP will have colorectal cancer at the time of the MAP diagnosis. (Patients who have a single MUTYH gene mutation have a slightly increased risk of colorectal cancer.) Patients who have MAP are also at an increased risk of duodenal and thyroid cancer.
Are there any other risks associated with MUTYH-associated polyposis (MAP)?
Although MAP mainly affects the large intestine (colon) and rectum, the gene mutations are in every cell in the body, and other organs can be affected. For instance, there is a 4% to 25% risk of developing stomach polyps and duodenal adenomas (polyps in the duodenum, the first part the small intestine).
How is MUTYH-associated polyposis (MAP) inherited?
MAP is different from other hereditary colorectal cancer syndromes because it is inherited in an autosomal recessive pattern (mutations in both copies of the gene (one from father and one from mother) must be inherited for the disease to appear).
Everyone has two copies of the MUTYH gene. People with MAP have a mutation in each copy of this gene. This means the mother and father of a person with MAP each have one copy of the MUTYH gene with a mutation; they are known as “carriers.” Carriers may have a slightly increased risk of colorectal cancer or polyps. Approximately 1% to 2% of the population carries a mutation in one of their copies of the MUTYH gene:
- The children of two mutation carriers have a 1 in 4 (25%) chance of having no MUTYH mutations. This person would be completely unaffected and could not pass any MUTYH mutations on to his or her children.
- There is a 2 in 4 (50%) chance of being a carrier. These children would not have MAP, but would have a chance of passing the one MUTYH mutation they have onto their own children.
- Finally, there is a 1 in 4 (25%) chance that both MUTYH mutations will be inherited, in which case this person would have MAP. All children of someone with MAP will have at least one MUTYH mutation.
People diagnosed with MAP should tell their family members about their diagnosis and encourage them to undergo genetic counseling. This includes an evaluation of their personal history, exploration of the family history, and genetic testing for the MUTYH gene mutations identified in the family. Recommendations to keep the patient and his or her family healthy and to prevent cancer will also be provided.
What causes MUTYH-associated polyposis (MAP)?
The genetic cause of MAP are mutations (changes) in the MUTYH gene. MUTYH is a gene involved in the repair of oxidative damage to the DNA. Oxidation causes changes in the DNA molecule that affect many genes, including some genes that are responsible for regulation of cellular growth (such as the APC and KRAS genes).