Expression of c-myc and other cell cycle-dependent genes in human colon neoplasia.

We have investigated the expression of certain cell cycle-dependent genes in total RNA isolated from normal and neoplastic cells of human epithelial colon tissue. The genes studied had been previously identified as cell cycle dependent in rodent and human fibroblasts. Levels of expression of G1 genes were compared to the level of expression of the S-phase-specific gene H3 in normal and adjacent neoplastic epithelial cells of six different individuals. We have found that the increase in the expression of c-myc gene detected in colon tumor cells is accompanied by a parallel increase in the expression of two G1-specific genes (p2A9 and ornithine decarboxylase) and the S-phase-specific gene histone H3. An important conclusion that can be drawn from these findings is that the increased level of a cell cycle-specific RNA in a tumor may not indicate overexpression of that gene but simply reflect the increased fraction of cycling cells, unless the ratio of expression between G1 genes and G1-S-phase genes is altered.