Expression of the myc gene family in different stages of human colorectal cancer.

Colorectal carcinoma serves as a model system for the study of changes in gene expression and structure relating to tumorigenesis. In this study, the levels of c-myc, N-myc and L-myc mRNAs were assessed in normal human colonic mucosa in 33 cases representing different stages of adenocarcinoma and in 36 adenomatous polyps, the presumed premalignant stage. Consistent with the findings of Erisman et al. (1985), we found that the c-myc gene was overexpressed (3-24-fold) in approximately two-thirds of the tumors examined. Amplification of the gene (3-4-fold) was observed in 2 of 12 tumors examined and did not correlate with the level of expression. Greater amounts of c-myc-specific mRNA than occur in normal tissue was also found in about two-thirds of the polyps examined demonstrating that premalignant lesions also overexpress the gene. N-myc and L-myc specific transcripts can be detected at low abundance in normal colonic mucosa. These genes were found to be frequently overexpressed in tumors and polyps, but in most cases the level of overexpression was modest. A single case of adenocarcinoma showed an approximately 30-fold increase in the level of N-myc mRNA without gene amplification. Adenomatous polyps more frequently overexpressed the L-myc gene than tumors.