MYH-Associated Polyposis

Overview

What is MAP?

MAP is a newly identified hereditary colorectal cancer condition. MAP is caused by genetic changes, called mutations, in the MYH gene.
Genes are found in our DNA and they carry the instructions that tell our bodies how to grow, develop and function properly. Genes are found on chromosomes and we typically have 23 pairs of chromosomes, for a total of 46. In each pair, we inherit one copy from our mother and one from our father, thus we have two copies of every gene, one from each parent. Occasionally, a gene contains a change, called a mutation, which prevents it from working properly. These gene mutations can cause certain hereditary conditions, such as MAP.
Individuals with MAP often have many polyps in their colon and rectum; this is called “polyposis”. Individuals who have MAP also have an increased chance of developing colorectal cancer.
Polyposis is a condition where 100 or more polyps can form in the colon. This type of polyposis is often called “classic” polyposis. Individuals with MAP may have the classic form of polyposis, but more often have less than 100 polyps — this is called “attenuated” polyposis.

Who is at risk?

MAP is an autosomal recessive condition. This means that for individuals to have MAP they must have two MYH gene mutations, one mutation in each of their MYH genes. Typically each one of these mutations is inherited from each one of the parents.
Siblings (brothers and sisters) of individuals with MAP have a 25 per cent chance of having also inherited both MYH gene mutations from their parents, and thus having MAP. Siblings also have a 50 per cent chance of having inherited just one MYH gene mutation; this is referred to as a “carrier”. Siblings also have a 25 per cent chance of not inheriting either MYH gene mutation.

Are there risks to children of parents with MAP?

Children of an individual with MAP will always be at least a carrier of an MYH gene mutation. If their other parent has a MYH mutation, then each child will have a 50% chance of having MAP. If the other parent does not have a MYH mutation, it is very unlikely that their children will have MAP.

How common is MAP?

Because MAP is a newly described condition, we don’t know exactly how common it is. There are two common mutations in the Caucasian (white) population and some research studies have found that approximately 1 in 100 (1 per cent) Caucasians will have one of these MYH mutations.
Currently, we don’t know how common MYH mutations are in other ethnic populations.

Where do polyps grow in MAP?

People who have MAP have polyps that develop mainly in the large bowel (colon). People with MAP may also develop polyps in the stomach and the first part of the small intestine (duodenum). Our understanding of MAP is increasing as more people with MAP are followed and monitored over time.

How will I know if I have MAP?

Most people with MAP have no symptoms. People may undergo a colonoscopy for rectal bleeding or anemia (low red blood cell count) and be found to have colonic polyps. Other people are evaluated because of a family history of polyps or early colon cancer. If a person is found to have many polyps the doctor needs to determine the type of polyp. If the type of polyp and other clinical features (such as the number of polyps) suggests a possible inherited form of polyps, genetic testing can be completed.

What is the treatment for MAP?

The management of people with MAP is changing because MAP is a recently described condition. Our understanding of the natural history of MAP is increasing rapidly. People with two MYH mutations are best monitored with screening colonoscopy. Current recommendations include colonoscopy every 2-3 years beginning at age 20 years. An upper endoscopy is recommended at 25 years of age. The frequency of the upper endoscopy depends on whether polyps are found in the stomach or duodenum and the characteristics of the polyps.