Familial adenomatous polyposis
Familial adenomatous polyposis (FAP) is an inherited condition in which numerous polyps form mainly in the epithelium of the large intestine. While these polyps are benign, they may become malignant, predisposing patients to colorectal cancer. more...
Signs and symptoms
From the age of 16 onward, patients develop hundreds to thousands of polyps. These may bleed, leading to admixture of blood in the stool. If the blood is not visible, it is still possible for the patient to develop anemia due to gradually developing iron deficiency. If malignancy develops, this may present with weight loss, altered bowel habit, or even with metastasis in the liver or elsewhere.
The genetic determinant in familial polyposis may also predispose carriers to other malignancies, e.g. of the duodenum and stomach. Other signs that may point at FAP are pigmented lesions of the retina ("congenital hypertrophy of the retinal pigment"), jaw cysts, sebaceous cysts, and osteomata (benign bone tumors). The combination of polyposis, osteomas, fibromas and sebaceous cysts is termed Gardner syndrome (with or without abnormal scarring).
Diagnosis and treatment
In patients with a strong family of colorectal cancer and symptoms suggestive of polyposis, colonoscopy is indicated, with biopsy of a number of polyps (especially of those that appear dysplastic). In severe cases, a full or partial colectomy is required.
Blood tests (liver enzymes) and ultrasound of the abdomen are often performed to rule out metastasis to the liver.
Genetic testing provides the ultimate diagnosis in 95%; genetic counseling is usually needed in families where FAP has been diagnosed. Testing may also aid in the diagnosis of borderline cases in families that are otherwise known to have the FAP mutation.
FAP is due to mutations in the APC gene, which is located on the fifth chromosome (5q21-q22), or in the MUTYH gene located on chromosome 1 (p34.3-p32.1).
APC is a tumour suppressor gene, acting as a "gatekeeper" to prevent development of tumours. Mutation of APC also occurs commonly in incident cases of colorectal carcinoma, emphasizing its importance in this form of cancer.
Although the polyps are inherently benign, the first step of the two-hit hypothesis has already taken place: the inherited APC mutation. Often, the remaining "normal" allele is mutated or deleted, accelerating generation of polyps. Further mutations (e.g. in p53 or KRAS) to APC-mutated cells are much more likely to lead to cancer than they would in non-mutated epithelial cells.
The normal function of the APC gene product is still being investigated; it is present both the cell nucleus and the membrane. The canonical tumor-suppressor function of Apc is suppression of the oncogenic protein beta-catenin. However, other tumor-suppressor functions of Apc may be related to cell adherence and cytoskeleton organization.
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