MYH-Associated Polyposis

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What is MYH-associated polyposis?

MYH-associated polyposis (MAP) is a hereditary condition. People with MAP tend to develop multiple adenomatous colon polyps during their lifetime and will have an increased risk of colorectal cancer. An adenomatous polyp is an area where the normal cells that line the inside of the colon begin to make mucous and form a mass. A polyp is benign (noncancerous) but can eventually turn malignant (cancerous, meaning it can spread to other parts of the body). It is also likely that people with MAP will develop polyps and colorectal cancer at a relatively young age, in their 20s to 50s.

MAP has only recently been described, and there is much to be learned about the condition. MAP appears to be similar to other hereditary conditions of familial adenomatous polyposis (FAP) and attenuated familial adenomatous polyposis (AFAP). It is currently unknown if people with MAP have an increased risk of polyps or cancer in other parts of the digestive tract, such as the stomach or small intestine.

What causes MAP?

MAP is a genetic condition. This means that the risk of colon polyps and colorectal cancer can be passed from generation to generation in a family. Mutations (alterations) in the MYH gene are known to cause MAP.

How is MAP inherited?

Normally, every cell has two copies of each gene: one inherited from the mother and one inherited from the father. MAP follows an autosomal recessive inheritance pattern, in which a mutation needs to be present in both copies of the gene in order for a person to have an increased risk of getting that disease. This means that each parent must pass on a gene mutation for a child to be affected. A person who has only one copy of the gene mutation is called a carrier. When both parents are carriers of a recessive gene mutation, there is a 25% chance that a child will inherit two mutations and be affected. 

How common is MAP?

Most colorectal cancer is sporadic (occurs by chance with no known cause). The percentage of colorectal cancer that can be attributed to MAP is unknown. It is estimated that as many as one in every 100 people may carry a single mutation in the MYH gene.

How is MAP diagnosed?

MAP is considered as a possible diagnosis when a person has multiple adenomatous colon polyps but does not have a mutation in the APCgene associated with FAP and AFAP. It may also be considered if someone has brothers or sisters with multiple colon polyps, but there is no history of colon problems in previous generations. MAP is diagnosed when a person is found to have two mutations in the MYH gene. There are two common mutations in MYH called Y165C and G382D.

What are the estimated cancer risks associated with MAP?

The specific cancer risks associated with MAP have not been determined. The risk of colorectal cancer is considered to be significantly increased, and there may be an increased risk of other cancers of the digestive tract as well.

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