Familial adenomatous polyposis: Description, Causes and Risk Factors:
Familial adenomatous polyposis is characterized by the development of many tens to thousands of adenomas (a benign epithelial tumor of glandular origin) in the rectum and colon during the second decade of life. FAP occurs in approximately 1 in 10,000 people and affects both males and females equally and accounts for less than 1% of colorectal cancer (CRC) cases.
The syndrome is inherited as an autosomal dominant trait and caused by mutations in the APC gene. APC is a tumor suppressor gene located on the long arm of chromosome 5 in band q21 (5q21). The coding region is divided into 15 exons and encodes a large protein (309 kDa). The APC protein has multiple domains that mediate oligomerization as well as binding to a variety of intracellular proteins, which have an important role in cell adhesion, signal transduction and transcriptional activation.
Recently, a second gene has been identified that also gives rise to colonic adenomatous polyposis, although the phenotype is less severe than typical Familial adenomatous polyposis. The gene is the MUTYH gene. MUTYH mutation causes the polyposis condition known as MUTYH attenuated FAP (MAP). It is recessively inherited and patients have either a homozygous or compound heterozygous germline mutations of the MUTYH gene. In one series of cases biallelic germ-line MUTYH mutations were found in 18% of APC gene mutation-negative patients with attenuated phenotype. The MUTYH gene contains 16 exons spanning a region of 11147 bp, but only 15 are coding exons [2-16], and it maps to chromosome 1p32-34.
Most of the time, Familial adenomatous polyposis is passed on to a child from the parent who has the condition. If the APC gene mutation is passed on to a child, he or she will inherit FAP. It is important to remember that parents do not have control over which genes are passed on to their children, and that passing on the APC gene mutation occurs by chance. In about one-third (33%) of all cases, people develop FAP even though their parents do not have FAP. When this occurs, it is due to a new gene alteration, or mutation, and it occurs by chance.
Signs and symptoms:
- Rectal pain.
- Bright red blood in the stool.
- Thin stools.
- Diarrhea and/or constipation that cannot be explained by diet or illness.
- Abdominal pain, cramping, or bloating.
- Continued weight loss.
- Continued lack of energy.
FAP may also present with extraintestinal manifestationssuch as osteomas, dental abnormalities (unerupted teeth,congenital absence of one or more teeth, supernumeraryteeth, dentigerous cysts, and odontomas), congenitalhypertrophy of the retinal pigment epithelium (CHRPE),desmoid tumors, or extracolonic cancers (thyroid, liver,bile ducts, central nervous system). Somelesions (skull and mandible osteomas, dental abnormalities, fibromas on the scalp, shoulders, arms, and back) areindicative of the Gardner variant of Familial adenomatous polyposis. Today the condition should rarely present as a colonic or even as an extracolonic malignancy.
Differential diagnoses may include other disorderscausing multiple polyps such as Peutz-Jeghers syndrome, familial juvenile polyps or hyperplastic polyposis, hereditary mixedpolyposis syndromes, and Lynch syndrome.
Diagnosis is based on a suggestive family history, clinicalfindings, and large bowel endoscopy or full colonoscopy. Whenever possible, the clinical diagnosis should be confirmed bygenetic testing.With each test, the nurse or doctor will instruct thepatient about what to do to prepare for the test.
When the APC mutation in the family has been identified, genetic testing of all first-degree relatives should beperformed. Presymptomatic and prenatal (amniocentesis and chorionic villous sampling), and even preimplantation genetictesting is possible. Referral to a geneticist or genetic counselor is mandatory.
There is no cure for FAP. Cancer prevention and maintaining good quality of life are the main goals in management of patients with clinical or genetic evidence of FAP. Treatment is directed to reduce complications, such as abdominal pain, bowel obstruction, GI bleeding, and cancer.
Large bowel endoscopy is the most important clinical examination since there is almost a 100% chance of CRC. However, as discussed previously, the disease is systemic with extracolonic manifestations and should be looked for by systematic reexaminations. CRC is rare in the asymptomatic youth, so after their genetic diagnosis and baseline sigmoidoscopy, they are systematically followed clinically until completing schooling and growth and maturing.
Around ages 16-18 y patients with FAP should be followed by annual or less frequent colonoscopic examinations (depending on the polyp burden at last colonoscopy) and all significant sized adenomas should be removed if surgery is not contemplated at that time. In addition, both forward-viewing and side-viewing upper tract endoscopies should be performed prior to surgery or every 1 to 5 years depending on the polyp burden and Spigelman stage to detect gastric but mainly duodenal and periampullary adenomas, respectively.
Other surgical options may include subtotal colectomy with ileorectal anastomosis (IRA), total proctocolectomy with ileostomy, and proctocolectomy with or without mucosectomy and ileal pouch anal anastomosis (IPAA).
The timing of surgery is based on the patient's age or other high-risk factors, including family history, polyp number, and pre-malignant changes in the polyps.
NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.