Recent advances in molecular genetics of colorectal cancer.

Advances in molecular biology have identified two important genes responsible for the hereditary colorectal cancers familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer. They are the APC gene and mismatch repair genes. The role of these genes in colorectal carcinogenesis has been studied intensively. The adenoma-carcinoma sequence was initially proposed by Vogelstein, and the multistep carcinogenesis theory is now well accepted. The various functions of the APC gene have been elucidated. APC genes are considered to play a role in shedding of the epithelial cells into the lumen. The mechanism behind formation of a unicryptal adenoma is now better understood. Adenoma formation is a monoclonal event with two hits of the APCgene. There is no zonal extension of the proliferative zone in the background colonic mucosa of FAP patients. In addition to the adenoma-carcinoma sequence, there seem to be various carcinogenetic pathways in the development of colorectal cancer. A depressed type of early cancer was recently found by the use of magnifying endoscopy. The incidence of K-ras mutation was extremely low in this group of early cancers. Some of the minute cancers show the p53mutation before the occurrence of APC mutation. Cancers of microsatellite mutator phenotype show exaggerated genomic instability at simple repeat sequences, such as TGFbetaRII. These genes may play a suppressor role in a p53 independent pathway of colorectal carcinogenesis. We are now in an exciting era of this progressing field of science. This genetic information may be more widely applicable clinically in the near future (e.g., for presymptomatic diagnosis, selection of patients for the most appropriate treatments, and assessment of malignant potential).