A functional and quantitative mutational analysis of p53 mutations in yeast indicates strand biases and different roles of mutations in DMBA- and BBN-induced tumors in rats

In order to analyze the mutational events and to understand the biological significance of the p53 gene in chemical carcinogenesis, we applied a new y east-based p53 functional assay to ovarian tumors induced by 7, 12-dimethyl benz[a]:anthracene (DMBA), as well as to transitional cell carcinomas of th e urinary bladder induced by N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN) i n rats, The assay demonstrated that 15 of 19 DMBA induced rumors harbored c lonal p53 mutations, which is consistent with the expectations of the "clon al expansion" hypothesis. The majority of the mutations were purine (AG) to pyrimidine (CT) transversions (12/19) on the non-transcribed (sense) stran d (NTS), which is likely to be due to depurination created by DMBA adduct f ormation on the NTS. In contrast, we found no purine to pyrimidine transver sion on the NTS, After cessation of BBN treatment, BBN-induced multifocal l esions in the bladder contained heterogeneous p53 mutations at an early sta ge. In the later stage, however, clonal p53 mutations were identified in 4 out of 7 bladders analyzed, conforming with the concept of "field canceriza tion". The observed base substitutions were G-->A (1/6) or C-->T transition s (2/6), and mutations at T (3/6) on the NTS in clonal mutations, together with non-clonal mutations, showing a preference of C-->T to G-->A (17 vs. O ). Thus, preferential repair was found in the transcribed strand of the p53 gene, whether modified by DMBA or by BBN carcinogens. Very similar mutatio n patterns were observed between clonal and non-clonal mutations in the DMB A- and BBN-induced tumors, indicating that the rat: yeast p53 functional as say can be a potential tool for the characterization of in vivo mutation pa tterns of p53, when modified by chemical carcinogens. (C) 1999 Wiley-Liss, Inc.