This is an autosomal dominant disorder, characterised by the appearance of hundreds of polyps in the rectum and colon, by age 20-30. There is a 100% incidence of malignancy if the patient lives long enough.
The main gene affected in this syndrome is the tumor suppressor gene adenomatous polyposis coli (APC ). This gene is located on the long arm of chromosome 5 (5q). The gene is a tumor suppressor type gene, so function is thought to be preserved by the presence of one functioning copy. Thus the polyp phenotype only becomes manifest when the second copy of the gene is `knocked out'.
Mutations in APC also play a role in sporadic colo-rectal cancer, it may be found in adenomas. This is good molecular evidence for the adenoma-carcinoma sequence.
Different mutations of the APC gene have been described and these are linked to different patterns of disease. Particular mutations are associated with attenuated and dense types of FAP , and with the development of desmoid tumours.
The function of the APC gene is believed to be in cell to cell adhesion.