The tumour promoters dieldrin and phenobarbital increase the frequency of c-Ha-ras wild-type, but not of c-Ha-ras mutated focal liver lesions in male C3H/He mice

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Abstract
The frequency and pattern of mutations at codon 61 of the c-Ha-ras protooncogene were analysed in glucose 6-phosphatase-deficient hepatic lesions of male C3H/He mice occurring either spontaneously or after continuous treatment with 10 p.p.m. dieldrin or 500 p.p.m. phenobarbital (PB) in their diet. At 52 weeks after start of promoter administration, enzyme-altered liver lesions had developed in 41% (15/37) of untreated control mice and in 67% (10/15) and 63% (10/16) of mice treated with dieldrin or PB respectively. The average numbers of focal lesions per mouse were 0.57 in the control, 1.5 in the dieldrin and 1.0 in the PB group. Lesions were punched out from frozen liver sections and used for mutation analysis by allele-specific oligonucleotide hybridization following in vitro amplification of DNA via polymerase chain reaction. In the control group, 12 out of 21 liver lesions (57%) showed c-Ha-ras mutations, while five out of 23 (22%) and four out of 16(25%) lesions were mutated in the dieldrin and PB groups. Taking the different numbers of animals in the three experimental groups into account, our data indicate that the tumour promoters increased the frequency of c-Ha-ras wild-type but not of c-Ha-ras mutated focal liver lesions, suggesting that the mutations had occurred spontaneously and were not related to treatment. Since c-Ha-ras mutations were found to be frequent in large but infrequent in small hepatocellular lesions, these mutations may represent in livers of C3H/He mice an endogenous promoting principle that provides a selective growth advantage to the mutated progenitor cells.

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