Other genes

MYH-associated polyposis (MAP) 

Not all polyposis (when multiple polyps develop) syndromes are associated with the APC gene as it happens with FAP  and  AFAP  .

Mutations in the MYH gene (also called MUTYH gene) also increase the chances of developing multiple adenomas and colorectal cancer.

This syndrome is called MYH-associated polyposis (MAP).

Contrary to other polyposis syndromes, to develop MAP a person needs two abnormal copies of the MYH gene (so both the one inherited from the mother and the one from the father). 

Unlike FAP and AFAP, though, MAP is an autosomal recessive disorder. This means that the individual only has the disease when both copies of the gene are abnormal. To understand better why we have two copies for every gene, one inherited from the mother another from the father see .

As a result, contrary to dominant syndromes - where one abnormal copy of the gene is enough to have the disease – in recessive syndromes the disease can jump generations, even appear in one and never again, or on one sibling but not other.

Genetic analysis

Like attenuated FAP (AFAP), MAP is also a weaken syndrome, with patients developing a reduced number of polyps – typically "only" between 10 to a few hundred. Colorectal cancer also tends to appear almost always after the age of 30.

The genetic test for MAP should be done on all individuals suspected to have FAP or AFAP, but without identified mutations in the APC gene. This is particularly recommended if there is a family history of multiple adenomas during one single generation.

There is no evidence that people with only one mutated copy of the MYH gene show increased risk of adenomas or colorectal cancer.