What is familial adenomatous polypo sis?
Familial adenomatous polyposis (FAP) is an inherited disorder characterized by cancer of the large intestine (colon) and rectum. People with the class ic type of familial adenomatous polyposis may begin to develop multiple non cancerous (benign) growths (polyps) in the colon as early as their teenage years. Unless the colon is removed, these polyps will become malignant (can cerous). The average age at which an individual develops colon cancer in cl assic familial adenomatous polyposis is 39 years. Some people have a varian t of the disorder, called attenuated familial adenomatous polyposis, in whi ch polyp growth is delayed. The average age of colorectal cancer onset for attenuated familial adenomatous polyposis is 55 years.
In people with classic familial ad enomatous polyposis, the number of polyps increases with age, and hundreds to thousands of polyps can develop in the colon. Also of particular signifi cance are noncancerous growths called desmoid tumors. These fibrous tumors usually occur in the tissue covering the intestines and may be provoked by surgery to remove the colon. Desmoid tumors tend to recur after they are su rgically removed. In both classic familial adenomatous polyposis and its at tenuated variant, benign and malignant tumors are sometimes found in other places in the body, including the duodenum (a section of the small intestin e), stomach, bones, skin, and other tissues. People who have colon polyps a s well as growths outside the colon are sometimes described as having Gardn er syndrome.
A milder type of familial adenomatous polyposis, called autosomal recessive familial adenomatous polyposis, has also been identified. People with the a utosomal recessive type of this disorder have fewer polyps than those with the classic type. Fewer than 100 polyps typically develop, rather than hund reds or thousands. The autosomal recessive type of this disorder is caused by mutations in a different gene than the classic and attenuated types of f amilial adenomatous polyposis.
How common is familial adenomatous poly posis?
The reported incidence of familial adenomatous polyposis varies from 1 in 7,000 to 1 in 22,000 individuals.
What genes are related to familial adeno matous polyposis?
Mutations in the APC gene cause both classic and attenuated familial adenomato us polyposis. These mutations affect the ability of the cell to maintain no rmal growth and function. Cell overgrowth resulting from mutations in the < span class="geneSymbol" style="FONT-STYLE: italic">APC gene leads to the colon polyps seen in familial adenomatous polyposis. Although most peo ple with mutations in the APC gene will develop colorectal cancer, the number of polyps and the time frame in which they become malignant depend on the location of th e mutation in the gene.
Mutations in the MUTYH gene cause autosomal recessive familial adenomatous polypo sis (also called MYH-associated polyposis). Mutations in this gene prevent cells from correcting mistakes that are made when DNA is copied (DNA replic ation) in preparation for cell division. As these mistakes build up in a pe rson's DNA, the likelihood of cell overgrowth increases, leading to colon p olyps and the possibility of colon cancer.
Ho w do people inherit familial adenomatous polyposis?
Familial adenomatous polypos is can have different inheritance patterns.
When familial adenomatous polyposis results f rom mutations in the AP C gene, it is inherited in an autosomal dominant pattern, which mean s one copy of the altered gene in each cell is sufficient to cause the diso rder. In most cases, an affected person has one parent with the condition.< /p>
When familial adenomatous polyposis results from mutations in the MUTYH gene, it is inherited in an autos omal recessive pattern, which means both copies of the gene in each cell ha ve mutations. Most often, the parents of an individual with an autosomal re cessive condition each carry one copy of the mutated gene, but do not show signs and symptoms of the condition.