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 their colon and rectum.
People with harmful mutations in the MUTYH (formerly called MYH) gene can develop several different types of polyps in their 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.
Symptoms and Causes
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).
Diagnosis and Tests
How is MUTYH-associated polyposis (MAP) diagnosed?
MAP is diagnosed when a person is found to have two MUTYH gene mutations. Once mutations are identified in a person, his or her family members can also be tested for those mutations.
People who have MAP usually have fewer polyps than people with the condition known as familial adenomatous polyposis (FAP). Because the presence of numerous adenomatous colorectal polyps may be due to FAP, MAP, or other, newer syndromes, a detailed family history can help determine which condition it is.
MAP is suspected when a person has more than 10 colon or rectal adenomas and does not have a mutation in the APC gene associated with FAP. A diagnosis of MAP might also be considered if a person has brothers or sisters who have several colorectal adenomas, but the parents have no history of multiple colon polyps or cancer.
Management and Treatment
How is MUTYH-associated polyposis (MAP) treated?
MAP cannot be cured. The aim of treatment therefore is to prevent cancer and preserve a healthy, unaffected lifestyle for patients. If fewer than 20 polyps are found, they can be removed during a colonoscopy. (This is called a polypectomy.)
If there are too many polyps or if they are growing too quickly to be controlled by colonoscopy, it might be necessary to remove the colon and/or rectum with surgery. This is called a colectomy. The bowel is reconstructed by joining the small intestine to the rectum (an ileo-rectal anastomosis) or making a J pouch out of the small intestine to replace the rectum, so that the patient can avoid having a permanent ostomy. While the prospect of surgery may be upsetting, it is important to realize that without it, the risk of colorectal cancer is very high.
The timing and choice of colon surgery depend on several factors, but in particular the number and size of the polyps. Laparoscopic surgery has made removal of the colon less painful and less disabling. Laparoscopic surgery is performed through very small "keyhole" incisions in the abdomen. A laparoscope--a small, telescope-like instrument containing a camera--is placed through an incision near the bellybutton in order to see the inside of the abdomen. The surgery is done using instruments placed via these small incisions. Even after surgery, the remaining bowel is checked every year.
Certain non-steroidal, anti-inflammatory medications, such as sulindac (Clinoril®), can reduce, and may prevent, adenomatous colorectal polyps in FAP. These medications may be considered (in addition to colonoscopy) for patients with MAP. The use of medications to prevent disease is known as chemoprevention.
Can MUTYH-associated polyposis (MAP) be prevented?
Unfortunately no. MAP is an inherited condition.
How often should a person with MUTYH-associated polyposis (MAP) get a colonoscopy?
People with MAP should have regularly scheduled colonoscopies, starting at age 20 or 10 years before the youngest diagnosis in the family (whichever is earlier). It is important that people with MAP have colonoscopies throughout their lives, because new polyps can form that may turn into a cancer.
An upper endoscopy (esophogastroduodenoscopy) is also recommended beginning at age 20 to 25 and continuing every 1 to 3 years, depending on the size, number and microscopic analysis of the duodenal polyps. In an upper endoscopy, a physician uses an endoscope (a long, thin, flexible instrument about 1/2 inch in diameter) to examine the inside of the upper digestive system (esophagus, stomach and duodenum).
What other monitoring should a person with MUTYH-associated polyposis (MAP) undergo?
All MAP patients should have a baseline thyroid ultrasound starting at the age of 20. This should be repeated every year, or more frequently if abnormalities are detected.
Genetic counseling and genetic testing should be offered to anyone with MAP. Genetic testing, which involves drawing a patient’s blood or obtaining a brushing from the inside of the mouth (buccal swab), can be done to determine if MUTYH mutations are present.
How often does a person with MUTYH-associated polyposis (MAP) need to be seen by their doctors?
People who have MAP will need gastrointestinal examinations and medical care for the rest of their lives by a healthcare team knowledgeable about MAP. This team may include gastroenterologists (specialists in diseases of the digestive system), surgeons, endocrinologists (specialists in metabolism and diseases of the endocrine system), primary care physicians, geneticists, genetic counselors, and oncologists (cancer specialists). People and families who are affected by MAP benefit from the care of a hereditary colon cancer registry.
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