MYH - Associated Polyposis - MAP
All individuals have a chance of developing cancer throughout their lifetime. Colorectal cancer refers to cancer that starts in either the colon or the rectum (also known as the large bowel or large intestine). Colorectal cancer is common in Canada. The chance of developing colorectal cancer is about 1 in 16, or 6 per cent.
As we age, the cells that line the colon and rectum are damaged by environmental factors, such as diet and as a normal part of the aging process. Over a lifetime this damage accumulates and can lead to colorectal cancer. This is thought to explain the majority of cases of colorectal cancer.
In a smaller number of cases, colorectal cancer may be related to hereditary factors. Hereditary forms of colorectal cancer account for at least 5 per cent of all colorectal cancers. A family history that is suggestive of hereditary factors may involve individuals who are diagnosed at an early age, several individuals in a family with the same or associated types of cancer, as well as individuals who have been diagnosed with more than one primary cancer. Hereditary colorectal cancer can also be associated with having numerous polyps, called adenomas, in the colon and rectum.
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.
The word polyp refers to a lump of cells which form a mass. There are many different types of polyps and they must be checked by a specialist, a pathologist, to determine whether the polyps are benign (harmless) or pre-malignant (pre-cancerous). In MAP, the type of polyps that are typically found are called adenomas.
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.
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.
Individuals who have been found to have multiple adenomas in their colon and rectum may be eligible for genetic testing. Some of these individuals may have also developed colorectal cancer.
In the past, some people may have been told they had another hereditary polyposis condition called Familial Adenomatous Polyposis (FAP) or attenuated FAP (AFAP), but their genetic testing wasn’t able to find a genetic mutation. In some of these families, MYH mutations have now been found and these people actually have MAP.
It is important for families to know if they have MAP or FAP because there are different cancer screening recommendations and risks for family members based on which condition they have.
We are still learning about MAP and the risks associated with this condition. Currently, we do not think that there is a significantly increased chance of colorectal adenomas or colorectal cancer in MYH carriers.
Although there may be a slight increased chance of colorectal cancer, we do not recommend increased colorectal cancer screening for MYH carriers at this time.
However, if there is a family history of colorectal cancer that would not be explained by the MYH mutations, we would recommend appropriate colorectal cancer surveillance based on that family history..
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.
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.
Our understanding of when the polyps develop in people with MAP is just developing since MAP is a newly appreciated genetic condition. It is difficult to say exactly when polyps develop in people with two MYH mutations. There is likely a spectrum of ages. Polyps in MAP likely develop during the later teenage years or during the twenties. Severe MAP polyposis or colorectal cancer is very rare in children.
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.
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.
The Familial Gastrointestinal Cancer Registry is currently accepting patients for genetic counselling and testing for individuals who may have MAP. If you have had numerous colorectal adenomas, early-onset (under 50 years of age) colorectal cancer or were previously diagnosed with Familial Adenomatous Polyposis (FAP) and genetic testing could not find a gene mutation then you may be eligible for genetic testing for MAP. Please ask your physician to refer you to the Familial Gastrointestinal Cancer Registry.