Multistep hepatocarcinogenesis in transgenic mice harboring SV40 T-antigen gene

We have developed transgenic mice that inherit albumin promoter-regulated simian virus 40 (SV40) large T-antigen gene, expressed specifically in hepatocytes. These mice all develop multifocal hepatocellular carcinomas (HCCs) at around 5 months and die of liver insufficiency by 7 months in remarkable synchrony. The liver tissue appears normal in the initial 3 weeks, and thereafter rapid cytomegalic degeneration of original hepatocytes, proliferation of quasi-regenerative hepatocytes, neoplastic cell foci, nodules and finally HCCs develop in sequence. Considerable variation existed both in morphological and enzymatic features and in T-antigen expression among neoplastic lesions, including carcinomas. Oligonucleotide hybridization studies revealed activating point mutations of the c-H-ras oncogene in 40% (10/25) of tumors obtained at around 6 months. The positive signals were, however, considerably weaker in half of them suggesting that such tumors comprised cells both with and without ras mutation. Sequential observation of culture cell lines established from tumors also revealed the appearance of activated c-H-ras with time, which was associated with biological progression. Thus, in this model system, ras activation may be an event occurring in a relatively late phase of carcinogenesis associated with progression of tumors. Karyotype analysis of cell lines revealed remarkable chromosomal instability. In studies of sister chromatid exchange in hepatocytes, twice as frequent occurrence was demonstrated in transgenic mice as in their counterpart hepatocytes. Thus T-antigen may be contributing as a mutagen to the hepatocarcinogenesis, in addition to its cytotoxic and growth modifying activities.