What Is Familial Adenomatous Polyposis (FAP)?
Familial adenomatous polyposis (FAP) is a genetic syndrome that causes colon cancer. The syndrome is also known as adenomatous polyposis coli, or Gardner syndrome. FAP is caused by a mutated adenomatous polyposis coli (APC) gene. Two-thirds of all cases are inherited from a parent with a mutant APC gene, and the remaining one-third of cases arise from a spontaneous APC gene mutation. FAP is a relatively rare cancer, afflicting about one in 10,000 people.
Patients with FAP can develop 100s of polyps in the colon and rectum at a young age. On average, polyps appear by age 16, and if the colon is not surgically removed, cancer will occur on average by age 39. The polyps eventually become so dense that they appear as a carpet of polyps upon visualization by colonoscopy or sigmoidoscopy.
Patient’s afflicted with FAP have an increased risk for other health problems that include: polyps in the upper gastrointestinal tract, osteomas, epidermoid cysts, desmoid tumors, hypertrophy of retinal pigment, dental abnormalities, thyroid cancers, small bowel cancers, pancreatic cancers, stomach cancers, brain cancers, and hepatoblastomas.
Patients with familial adenomatous polyposis can develop hundreds of polyps in the colon and rectum at a young age. On average, polyps appear by age 16, and if the colon is not surgically removed, cancer will occur on average by age 39. The polyps eventually become so dense that they appear as a carpet of polyps upon visualization by colonoscopy or sigmoidoscopy.
Characteristics of FAP
In its classic form, FAP is characterized by the following:
- Polyposis: the development of multiple benign (noncancerous) adenomatous polyps (>100) in the colon and rectum, which are described as having a “dense carpet-like appearance” on colonoscopy or sigmoidoscopy.
- Early age of onset: Polypsbegin to develop at an average age of 16 years (range of seven to 36 years).
- A nearly 100% risk of colorectal cancer in the absence of treatment for polyposis (colectomy, or surgery to remove the colon)
- An autosomal dominant pattern of inheritance (vertical transmission through either the maternal and paternal line)
- An increased risk of other health problems such as:
- Polyps in the upper gastrointestinal tract
- Osteomas (benign bony growths)
- Epidermoid cysts (skin lesions)
- Desmoid tumors (locally invasive tumors that grow aggressively and can be life threatening), congenital hypertrophy of retinal pigment (CHRPE)
- Dental abnormalities
- An increased risk of thyroid, small bowel, pancreatic, and stomach cancers, brain tumors and hepatoblastoma (a childhood liver tumor)
Diagnosing Familial Adenomatous Polyposis (FAP)
Genetic testing will indicate whether or not you will develop familial adenomatous polyposis(FAP). If you do not wish to undergo genetic testing or have learned through testing that you have the mutation, then yearly colorectal cancer screening is necessary.
The two screening examinations for colorectal cancer and FAP are sigmoidoscopy and colonoscopy.
A sigmoidoscopy is a diagnostic procedure that allows the doctor to examine the lower one-third of the large intestine. It’s helpful in identifying the causes of diarrhea, abdominal pain, constipation, abnormal growths, and bleeding. A short, flexible, lighted tube, called a sigmoidoscope, is inserted into the intestine through the rectum into the lower part of the large intestine. Air is injected into the intestine through the sigmoidoscope to inflate it for better viewing.
A sigmoidscopy can also be used to look inside the rectum and the lower colon for polyps, abnormal areas or cancer. It may also have a tool to remove polyps or tissue samples, which are checked under a microscope for signs of cancer. A sigmoidoscopy and a digital rectal exam (DRE) may be used together to screen for colorectal cancer.
Prevention of FAP
There is no way to prevent FAP. However, if you are at risk, you should start colorectal cancer screenings at an early age.
Genetic testing is also available to see if you will develop FAP. If one of your parents has or had FAP, your risk is 50%. If you find through genetic testing that you do not have a FAP-linked mutation, then you will avoid unnecessarly yearly colorectal cancer screenings.