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Most colorectal cancers occur randomly or by chance (sporadically). Although 20%–30% of people with colorectal cancers have a family history of this cancer, less than 10% of all colorectal cancers are due to a hereditary or inherited gene mutation. Familial adenomatous polyposis (FAP) is the second most common hereditary colorectal cancer syndrome, but it is rare. FAP occurs in only about 1 in 10,000 people and accounts for less than 1% of all colorectal cancers.
People with FAP develop large numbers (hundreds to thousands) of polyps (called adenomasadenomasA non-cancerous (benign) tumour that starts in glandular cells (cells that release substances such as mucus, hormones and lubricating fluids).). Most polyps develop on the lining of the colon and rectum, but they can also develop in the stomach and small intestine. People with FAP usually develop polyps before age 40. Most people with FAP have a history of colorectal polyps and cancer. If the polyps aren’t treated, almost all people with FAP will develop colorectal cancer.
Other, rarer hereditary gene mutations are associated with an increased risk of colorectal cancer, but these are not discussed here.
FAP mainly occurs because of a mutation in the adenomatous polyposis coli (APC) gene. The APC gene is a tumour suppressor gene. These genes normally prevent cancer from developing. They help control cell growth and division, as well as repair damage to deoxyribonucleic acid (DNA)deoxyribonucleic acid (DNA)The molecules inside the cell that program genetic information. DNA determines the structure, function and behaviour of a cell..
Researchers have identified different mutations in the APC gene that can lead to FAP. Studies have shown that there is a link between specific gene mutations and the number and location of polyps a person develops.
Men and women can inherit a mutated APC gene from either their mother or father. People with this mutation can also pass it on to their children. If one parent has the mutation in 1 of the 2 copies of the APC gene, a child has a 50% chance of inheriting the gene mutation. This also means there is a 50% chance that a child will not inherit the gene mutation.
About 25% of FAP cases are due to a new mutation in a germ cell (de novo mutation).
FAP may be suspected if a person has a personal or family history, especially in one or more first-degree relatives (parents, siblings or children), of multiple adenomatous polyps or colorectal cancer that developed at a younger age than usually expected.
People with FAP have almost a 100% chance of developing colorectal cancer in their lifetime (lifetime risklifetime riskThe chance or probability that a person will develop a particular disease over his or her entire life.). Unless the colon is removed, the polyps will become cancerous. If left untreated, the average age at which a person with FAP develops colon cancer is 39 years.
FAP polyps can develop as early as childhood or adolescence. About 50% of people with FAP develop adenomas by age 15. By themselves, the polyps are not any more likely to develop into cancer than polyps found in a person without FAP. However, because there are so many polyps, there is a greater risk that one or more of them could become cancerous.
In addition to colorectal cancer, people with FAP have an increased risk of developing other types of cancer, such as small intestine, thyroid, pancreatic and stomach cancer. Children with FAP have a greater risk of developing hepatoblastomahepatoblastomaA rare cancerous (malignant) tumour that starts in the liver..
Similar to FAP, other conditions cause numerous polyps to grow in the colon and rectum. These are also linked to an APC gene mutation and increase a person’s chances of developing colorectal cancer.
Researchers recently identified MYH-associated polyposis (MAP) as a hereditary colorectal cancer condition. MAP is caused by a mutation in the MUTYH (also called MYH) gene, rather than the APC gene. People with the MYH gene mutation often have many polyps in their colon or rectum, but fewer polyps than with FAP. They also have an increased risk of developing colorectal cancer and polyps and cancer of the small intestine (duodenumduodenumThe first part of the small intestine that receives partially digested food from the stomach, absorbs nutrients and passes digested food to the jejunum. The duodenum also receives digestive juices from the pancreas and bile (a yellow-green fluid that helps digest fat) from the liver and gallbladder.).
For MAP to develop, both parents must pass on a MYH gene mutation to a child. There are no specific screening guidelines for MAP, but researchers have suggested that people with MAP should have similar screening for polyps and colorectal cancer as people with FAP.
Genetic testing is available for specific types of hereditary colorectal cancer syndromes. It can be used to confirm a suspected diagnosis, clarify the risk of cancer in affected people and identify relatives who are also at risk.
Genetic testing for the APC gene may be done if a person is diagnosed with FAP or has a family history of FAP. A healthcare professional will determine if a person is eligible for genetic screening. If they qualify for testing, they will likely be referred to a genetic program where geneticists or genetic counsellors do more detailed assessments. In Canada, genetic testing to see if a person carries an APC gene mutation is done through hereditary (familial) cancer programs. People have to meet certain criteria before they have genetic testing for hereditary colorectal cancer. Genetic testing can also be done for MYH gene mutations.
Genetic counselling is recommended before and after testing. A blood sample is required for genetic testing. Other family members may then wish to have genetic counselling and genetic testing to see if they also carry the mutation.
People who know they have an APC gene mutation (they tested positive for an APC mutation) should talk to their doctor about taking steps to help reduce their risk of cancer and to find it early.