Biallelic MUTYH - MYH-Associated Polyposis (MAP) (also called Multiple Adnomatous Polyposis)
MAP is very similar to attenuated familial adenomatous polyposis (AFAP) with a greatly increased risk for precancerous colorectal polyps and cancer. Duodenal polyps and other extracolonic features of FAP have been reported in patients with MAP, such as thyroid tumors, gastric fundic gland polyps, and other soft tissue tumors. For this reason, patients should follow screening and management guidelines for attenuated FAP.
MAP is an autosomal recessive condition, requiring mutations in both copies of the MYH gene to cause the disease. Siblings of an individual with MAP are therefore at 25% risk for also having MAP. The risk to children will depend on the genetic status of both parents. At a minimum, all of the children of an individual with MAP will be carriers for a single MYH gene mutation. Of note, carriers of a single MYH gene mutation are believed to have a 2-fold increased risk for colorectal cancer in the absence of polyposis. The probability of children having two MYH gene mutations causing MAP is the same as the carrier frequency in the population, which is believed to be 1-2%.
Those with a mutation in one out of two of their MUTYH genes do not have MAP, as discussed in Biallelic MUTYH. However, some precautions should still be taken. It is recommended that individuals who do not have colorectal cancer but have a first-degree relative that does get a colonoscopy every five years starting at age 40 or 10 years prior to the earliest diagnosis in the family. For those who do not have a first-degree relative with colorectal cancer, no additional screening is necessary.
American Society of Clinical Oncology http://www.cancer.net/cancer-types/myh-associated-polyposis
Mount Sinai Hospital http://www.zanecohencentre.com/gi-cancers/diseases/map
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