Investigation of parameters associated with activity of the Kirsten-ras, Harvey-ras, and myc oncogenes in normal rat liver.

Oncogenes have been implicated in the mechanism(s) involved in the different stages of carcinogenesis. Point mutations have been found to activate the ras family of oncogenes. However, these genes must also be expressed in order for phenotypic alterations to be seen. In addition, overproduction of the normal product of some oncogenes may be a factor in the development of cancer. This study was therefore designed to investigate the potential for activity that the Kirsten (Ki)-ras, Harvey (Ha)-ras, and myc oncogenes possess in normal rat liver; activation of these genes has previously been observed in hepatocellular carcinomas. The serum albumin gene was used as a positive control. Two possible control points of gene expression were examined: (1) gene methylation state, as there is an established positive correlation between hypomethylation and gene activity; and (2) DNase I sensitivity because transcriptionally active genes are preferentially sensitive to digestion by this enzyme. It was found that the serum albumin gene is hypomethylated and preferentially sensitive to DNase I, both states being consistent with gene activity. In contrast, Ki-ras is highly methylated and relatively resistant to DNase I digestion. The Ha-ras and myc oncogens exhibit a degree of methylation and DNase I sensitivity intermediate between the serum albumin gene and the Ki-ras oncogene. It thus appears that Ki-ras is not expressed in normal rat liver, whereas Ha-ras and myc may have the potential for activity. In addition, a correlation was found between the methylation state of a gene and its sensitivity to DNase I. These findings are compatible with the view that carcinogenesis is a multistage process. Our results indicate that possible control points for oncogene expression, as well as mutation, should be considered in investigations of the role these genes play in carcinogenesis.