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 ("CHRPE - congenital hypertrophy of the retinal pigment epithelium"), 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).
Once the diagnosis of FAP is made, close colonoscopic surveillance with polypectomy is required. Prophylactic colectomy is indicated if more than a hundred polyps are present, there are severely dysplastic polyps, or multiple polyps larger than 1 cm are present. When partial colectomy is performed, colonoscopic surveillance of the remaining colon is necessary as the individual still carries significant risk of developing colon cancer.
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.
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.
MUTYH encodes DNA repair enzyme MYH glycosylase. During normal cellular activities, guanine sometimes becomes altered by oxygen, which causes it to pair with adenine instead of cytosine. MYH glycosylase fixes these mistakes by base excision repair, such that mutations do not accumulate in the DNA and lead to tumor formation. When MYH glycosylase does not function correctly, DNA errors may accrue to initiate tumorigenesis with a clinical presentation similar to that in patients with Apc mutations.
Mutations in the MUTYH gene are inherited in an autosomal recessive pattern, which means two copies of the gene must be altered for a person to be affected by the disorder. Most often, the parents of a child with an autosomal recessive disorder are not affected but are carriers of one copy of the altered gene.
Prenatal testing is possible if a disease-causing mutation is identified in an affected family member; however, prenatal testing for typically adult-onset disorders is uncommon and requires careful genetic counseling.
Because of the genetic nature of FAP, polyposis registries have been developed around the world. The purpose of these registries is to increase knowledge about the transmissibility of FAP, but also to document, track, and notify family members of affected individuals. One study has shown that the use of a registry to notify family members (call-ups) significantly reduced mortality when compared with probands. The St. Mark's polyposis registry is the oldest in the world, started in 1924, and many other polyposis registries now exist.
In 2007, the "ApcPirc" rat model was isolated with a stop codon at position 1137 . In contrast to the mouse models where >90% of tumors form in the small intestine, the Pirc rat forms tumors preferentially (>60%) in the large intestine, similar to the human clinical presentation.
Various medications are being investigated for slowing malignant degeneration of polyps, most prominently the non-steroidal anti-inflammatory drugs (NSAIDs). The NSAIDS have been shown to significantly decrease the number of polyps but do not usually alter management since there are still too many polyps to be followed and treated endoscopically.
Attenuated Familial Adenomatous Polyposis: A Case Report With Mixed Features and Review of Genotype-Phenotype Correlation
Nov 01, 2005; Familial adenomatous polyposis represents approximately 1% of all colorectal cancers and is caused by germline mutations in the...