RAS ONCOGENE ACTIVATION DURING HEPATOCARCINOGENESIS IN B6C3F1 MALE-MICE BY DICHLOROACETIC AND TRICHLOROACETIC ACIDS

Dichloroacetic (DCA) and trichloroacetic (TCA) acids, two major by-pro ducts formed during chlorine disinfection of drinking water, increase the incidence of tumors in B6C3F1 mice by 6- and 3-fold respectively, In order to understand better the mechanism by which these two compoun ds induce liver tumors, the incidence and spectrum of mutations in the K- and H-ras proto-oncogenes in these tumors were analyzed, DNA from spontaneous, DCA- and TCA-induced liver tumors from B6C3F1 male mice w as evaluated for point mutations in exons 1, 2 and 3 of the two genes by single-stranded conformation polymorphism, Results demonstrated a s imilar incidence of mutations for exon 2 of H-ras in spontaneous carci nomas (58%), and in carcinomas induced by DCA 3.5 g/l (50%), 1.0 g/l ( 48%) and TCA 4.5 g/l (45%), Only four samples showed mutations in the other exons of H-ras or in K-ras, Sequence analysis of spontaneous tum or samples with second exon H-ras mutations revealed a change in codon 61 from CAA to AAA in 80% and CAA to CGA in 20% of tumors, In contras t, tumors with H-ras mutations from DCA-treated mice revealed a H-61 c hange from CAA to AAA in 21% at 3.5 g/l and 16% at 1.0 g/l, CAA to CGA was observed in 50% of tumors from mice given DCA 3.5 or 1.0 gn, and CAA to CTA was present in 29% and 34% of the two dosage groups respect ively, Interestingly, TCA showed the same mutational spectrum as the s pontaneous liver tumors, The data indicates that induction of liver ca rcinoma by DCA and TCA involves activation of the H-ras protooncogene at a frequency similar to that observed in spontaneous tumors. However , the mechanism(s) for inducing hepatocellular carcinoma does not appe ar to be identical for DCA and TCA.