Colorectal carcinogenesis based on molecular biology of early colorectal cancer, with special reference to nonpolypoid (superficial) lesions.

The multistep genetic model of colorectal carcinogenesis is based on the concept of the adenoma-carcinoma sequence. The adenoma-carcinoma sequence theory has been generally accepted for polypoid early colorectal cancers (ECCs). On the other hand, an increasing number of nonpolypoid (superficial) ECC have been reported. Nonpolypoid (superficial) ECCs show distinct characteristics histologically and genetically, and some claim these lesions may develop by de novo type carcinogenesis. In fact, clinicopathologic studies have shown that most nonpolypoid (superficial) cancers have no adenomatous lesions in the surrounding area. Genetic analyses have also revealed that nonpolypoid (superficial) ECCs show a pattern of genetic alterations different from that of polypoid ECCs. The K-ras mutation rate is lower in nonpolypoid (superficial) ECCs than in polypoid ECCs, but there is no significant difference in the p53 mutation rate between two types of tumor. During the development of ECCs, the K-ras gene seems to determine the macroscopic configuration: whether polypoid or nonpolypoid (superficial). These results suggest that nonpolypoid (superficial) ECCs originate from a pathway different from the conventional genetic pathway that follows the adenoma-carcinoma sequence. However, this does not mean that this new pathway is following de novo type carcinogenesis, because there is a possibility that nonpolypoid (superficial) adenomas, or so-called flat adenomas, develop into nonpolypoid (superficial) ECCs following the adenoma-carcinoma sequence. At the present time, there is still not enough evidence to conclude whether nonpolypoid (superficial) ECC is derived from de novo carcinogenesis or the conventional adenoma-carcinoma sequence. Further analysis, especially concerning APC gene mutation in ECCs, is essential to elucidate the carcinogenesis of nonpolypoid (superficial) ECCs.