How does MAP differ from FAP?
MAP is similar to FAP in that patients can grow large numbers of adenomatous polyps in the colon and rectum and will usually need to undergo surgery to prevent bowel cancer. Polyps caused by MAP may grow slightly later than with FAP, however and usually appear in adulthood. They also tend to be less numerous than those found in patients with FAP. Most people with MAP have between 10 and 500 polyps.
The main difference between MAP and FAP is the way it is inherited from one family member to another. MAP is inherited in a recessive way. This means that both copies of the MYH gene have to have a genetic alteration in order for the person to grow polyps. People can also have one altered copy of the MYH gene. These people are called carriers. It is estimated that around one in every 100 people carry a single mutation in the MYH gene. They do not have polyposis themselves but they have a 50% chance of passing on their altered gene to each individual child they have. When both parents are carriers of MAP, there is a 25% chance (one in four), in each pregnancy, of a child inheriting two altered copies of the MYH gene. If a person is diagnosed with MAP, then we offer testing to their partner as this helps us to provide advice regarding risk to any children.