Familial adenomatous polyposis

FAP is an autosomal dominant cancer genetic syndrome, which means that a child whose parent has the condition has a 50/50 chance of inheriting the familial APC gene mutation. There is equally as likely a chance the child will not inherit the familial APC mutation.

Both copies of a tumor suppressor gene must be altered, or mutated, before a person will develop polyps or cancer. In FAP, the first mutation is usually inherited from either the mother or the father and is therefore present in all cells of the body. This is called a germline mutation. It is not until the second copy of the gene is mutated in, for instance, a colon cell, that a polyp develops.

In order for a benign polyp to become malignant (cancerous), the polyp must acquire mutations in several additional growth control genes. Loss of both copies of APC is just the first step in the process of cancer development. What causes these additional mutations to be acquired is unknown. Possible causes include chemical, physical or biological environmental exposures or chance errors in cell replication.

The incidence varies from 1 in 7,000 to 1 in 22,000 live births, with males and females equally affected.

Hundreds of mutations have been found throughout the APC gene. It has long been recognized that some families with APC mutations have different symptoms than others. Studies comparing symptoms in patients with different and similar APC mutations have been done to see if there are any correlations. A correlation between specific mutations and symptoms is called a genotype-phenotype correlation. Genotype-phenotype correlation studies for the APC gene have shown that the location of a mutation in the gene provides some information about the types of FAP health problems a person will have. For instance, mutations in certain parts of the gene are associated with an increased rate of desmoid tumors (locally invasive tumors that grow aggressively and can be life-threatening), osteomas (benign bony growths), and epidermoid cysts (skin lesions). Where in the gene a mutation lies also provides some information about polyp burden (the number of polyps a person will develop). Even though some correlations exist, there is often variability of symptoms between people who have the same mutation. This is because factors other than the APC mutation (environmental factors, other genetic factors) contribute to the development of polyps and cancer.

Attenuated FAP

An attenuated form of familial adenomatous polyposis (AFAP) has been identified. Individuals with AFAP develop fewer than 100 adenomatous polyps (average of 30 polyps) and the polyps are located more proximally than in classic FAP. The risk of developing colon cancer is increased, but the average age of diagnosis is older (about 55 years of age) than in the classic form. Some of the other health problems associated with classic FAP also occur in the attenuated form, however, cases of congenital hypertrophy of the retinal pigment (CHRPE) are rarely seen.

Mutations in three specific areas of the APC gene have been associated with the AFAP phenotype. The polyp burden (number of polyps developed) and the risk of other FAP manifestations varies depending on which area the mutation is located.

The incidence of AFAP is unknown, but thought to be about the same or less than classic FAP.

I1307K mutation

One APC mutation in particular, called I1307K, is present in 6 percent of the American Ashkenazi Jewish population. This mutation is associated with a 10 to 20 percent risk of colorectal cancer (slightly more than double the risk of someone else in the general population). However, people with this mutation do not present with the classic carpet of polyps in the colorectum seen in FAP.

MYH-associated polyposis (MAP)

Mutations in the MYH gene increase the chance for an individual to develop multiple colorectal polyps or colorectal cancer. Unlike most polyposis and colorectal cancer disorders, MAP is considered to be an autosomal recessive disorder. This means that individuals with MAP have mutations in both copies of their MYH gene.

The number of polyps in individuals with MAP can range from a few to hundreds. The symptoms of MAP overlap with both familial adenomatous polyposis (FAP) and attenuated FAP. Genetic testing for mutations in the MYH gene is available and should be considered in individuals suspected of having FAP or attenuated FAP who do not have mutation in the APC gene. It is not currently known if individuals with only one MYH mutation have a higher than average risk for colorectal polyps and cancer.

Since we do not know how to prevent mutations from occurring, the treatment for classic FAP is a colectomy—a removal of the colon once polyps develop, but before they become cancerous.

Because FAP has such an early age of onset, cancer screening often begins in childhood. In addition, genetic testing of children at risk is a special consideration.