Toward an understanding of the role of DNA adduct conformation in definingmutagenic mechanism based on studies of the major adduct (formed at N-2-dG) of the potent environmental carcinogen, benzo[a]pyrene

The process of carcinogenesis is initiated by mutagenesis, which often invo lves replication past damaged DNA. One question - what exactly is a DNA pol ymerase seeing when it incorrectly copies a damaged DNA base (e.g., inserti ng dATP opposite a dG adduct)? - has not been answered in any case. Herein, we reflect on this question, principally by considering the mutagenicity o f one activated form of benzo[a]pyrene, (+)-anti-B[a]PDE, and its major add uct [+ ta]-B[a]P-N-2-dG. In previous work, [+ ta]-B[a]P-N-2-dG was shown to be capable of inducing > 95% G --> T mutations in one sequence context (5' -T (G) under bar C), and similar to 95% G --> A mutations in another (5'-A (G) under bar A). This raises the question - how can a single chemical enti ty induce different mutations depending upon DNA sequence context? Our curr ent working hypothesis is that adduct conformational complexity causes addu ct mutational complexity, where DNA sequence context can affect the former, thereby influencing the latter. Evidence supporting this hypothesis was di scussed recently (Seo et al., Mutation Res. [in press]). Assuming this hypo thesis is correct (at least in some cases), one goal is to consider what th ese mutagenic conformations might be. Based on molecular modeling studies, 16 possible conformations for [+ ta]-B[a]P-N-2-dG are proposed. A correlati on between molecular modeling and mutagenesis work suggests a hypothesis (H ypothesis 3): a base displaced conformation with the dG moiety of the adduc t in the major vs. minor groove gives G --> T vs. G --> A mutations, respec tively. (Hypothesis 4, which is a generalized version of Hypothesis 3, is a lso proposed, and can potentially rationalize aspects of both [+ ta]-B[a]P- N-2-dG and AP-site mutagenesis, as well as the so-called "A-rule".) Finally , there is a discussion of how conformational complexity might explain some unusual mutagenesis results that suggest [+ ta]-B[a]P-N-2 dG can become tr apped in different conformations, and why we think it makes sense to interp ret adduct mutagenesis results by modeling ds-DNA (at least in some cases), even though the mutagenic event must occur at a ss/ds-DNA junction in the presence of a DNA polymerase. (C) 2000 Elsevier Science B.V. All rights res erved.