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Molecular biology of colon polyps and colon cancer.

From a histologic and endoscopic standpoint, colon and rectal cancer (CRC) begins as a small neoplastic polyp which progressively enlarges and transforms through a dysplasia stage into invasive cancer. Recently, molecular abnormalities underlying the adenomacarcinoma progression have been defined. The adenomatous polyposis coli (APC) gene and mismatch repair genes are found to be dysfunctional early in the neoplastic process; either as inherited or somatic mutations. Subsequently, polyps progress to cancer along one of two paths depending on which gene is abnormal. When the APC gene is the initial mutation tumor development follows the "loss of heterozygocity" (LOH) pathway. If mismatch repair genes are altered, the "replication error" (RER) pathway is followed. Somatic mutations of the K-ras oncogene and the MCC, DCC, and p53 tumor suppressor genes accumulate in the LOH pathway and mark the progression through polyp stages. Microsatellite instability is a characteristic of the RER pathway but the precise genes involved in this pathway currently are not known. Defining these pathways has led to a new classification scheme for CRC with resultant changes in our clinical approach to screening, surveillance, and treatment.