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Mutational Analysis of the H-ras Oncogene in Spontaneous C57BL/6 x C3H/He Mouse Liver Tumors and Tumors Induced with Genotoxic and Nongenotoxic Hepatocarcinogens¹

Toxicology Research Laboratory, The Dow Chemical Company, Midland, Michigan 48674 [T. R. F., A. M. S., P. G. W., B. L. Y.], and Carcinogenesis Laboratory, Michigan State University, East Lansing, Michigan 48824-1316 [T. R. F., V. M. M., J. J. M.]

The frequency and mutational profile of H-ras gene activation were determined in spontaneous liver tumors of male C57BL/6 x C3H/He mice and in tumors induced with the genotoxic hepatocarcinogen benzidine · 2 HCl or the nongenotoxic hepatocarcinogens phenobarbital, chloroform, and ciprofibrate. DNA sequence analysis of the H-ras gene from representative tumors revealed that 32 of 50 (64%) spontaneous tumors and 13 of 22 (59%) benzidine·2 HCl-induced tumors contained a point mutation in codon 61. Tumors induced with the nongenotoxic agents had a much lower frequency of codon 61 mutations, i.e., phenobarbital, 1 of 15 (7%); chloroform, 5 of 24 (21%), and ciprofibrate, 8 of 39 (21%). No mutations were observed at codons 12, 13, and 117 in tumors from any of the groups. Only three base pair substitutions within codon 61 were found. The one most frequently detected in all of the groups was a C · G to A · T transversion at the first nucleotide position, occurring at a 59%, 85%, 100%, 80%, and 88% frequency in the spontaneous tumors and in the tumors induced with benzidine 2 · HCl, phenobarbital, chloroform, and ciprofibrate, respectively. In these same groups an A · T to G · C transition or an A · T to T · A transversion at the second nucleotide position occurred at a frequency of 34%, 8%, 0%, 0%, and 12%, and 6%, 8%, 0%, 20%, and 0%, respectively. The number of tumors carrying an activated H-ras gene in the nongenotoxic treatment groups is within the range that would be expected if those animals had not received any treatment. This indicates that the activation of the H-ras gene in those tumors is probably the result of a spontaneous event. The data suggest that these toxicologically and pharmacologically diverse nongenotoxic hepatocarcinogens increase the frequency of liver tumors but do not induce mutations in the H-ras gene. Instead these agents appear to interact with a population of cells that do not contain an activated H-ras gene. This suggests that the mechanisms of tumor development by these nongenotoxic carcinogens differ at least partially from the mechanisms responsible for the development of spontaneous tumors or those induced by a typical genotoxic agent.

¹ This work was supported in part by USPHS Grant CA-21289, National Cancer Institute, Department of Health and Human Services, and by Department of Energy Grant ER-60524.

² To whom requests for reprints should be addressed, at The Toxicology Research Laboratory, Building 1803, The Dow Chemical Company, Midland, MI 48674.

Received 10/12/89. Revised 2/ 9/90.

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