Characterization of rare p53 mutants from carcinogen-treated albumin-simian virus 40 T-antigen transgenic rats.

The p53 gene has been either mutated or deleted in most human tumors examined to date. Mutations in the specific DNA-binding domain are the most common p53 mutations and are of interest because they may produce p53 molecules with transcriptional capabilities unlike those of the wild-type (WT) p53 protein. Mutations in the rat p53 gene were found in hepatic neoplasms of carcinogen-treated transgenic rats that express simian virus 40 (SV40) large T-antigen (TAg). Because this result was unexpected, we examined some of the biochemical and biological properties of the mutant proteins. Corresponding nucleotide changes were made by site-directed mutagenesis of the rat p53 cDNA, which was then inserted into a eukaryotic expression vector and transfected into the human hepatocyte cell line Hep 3B. Four of the mutant p53 molecules from rat hepatomas retained a strict WT conformation. Two others existed in both WT and mutant conformations. All of the mutant proteins were able to bind TAg as well as WT p53 did. Whereas the WT p53 protein was able to repress expression of a reporter gene containing a p53-response element (pSV2CAT), the missense-mutant p53 proteins induced transcription of the reporter to an extent equivalent to that of TAg. The mutant proteins also allowed TAg to induce the pSV2CAT reporter gene. The mutant molecules were able to enhance survival of Hep 3B cells, perhaps by preventing cell death, whereas expression of the WT p53 protein caused a reduction in cell number to nearly 10% of control levels. The results of these experiments suggest that the mutant p53 molecules observed in the carcinogen-treated transgenic rats may have unique properties that are important in carcinogenesis.