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

Abstract

The process of carcinogenesis is initiated by mutagenesis, which often involves replication past damaged DNA. One question — what exactly is a DNA polymerase seeing when it incorrectly copies a damaged DNA base (e.g., inserting dATP opposite a dG adduct)? — has not been answered in any case. Herein, we reflect on this question, principally by considering the mutagenicity of one activated form of benzo[ a]pyrene, (+)- anti-B[ a]PDE, and its major adduct [+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 C), and ∼95% G→A mutations in another (5′-A G A). This raises the question — how can a single chemical entity induce different mutations depending upon DNA sequence context? Our current working hypothesis is that adduct conformational complexity causes adduct mutational complexity, where DNA sequence context can affect the former, thereby influencing the latter. Evidence supporting this hypothesis was discussed recently (Seo et al., Mutation Res. [in press]). Assuming this hypothesis is correct (at least in some cases), one goal is to consider what these mutagenic conformations might be. Based on molecular modeling studies, 16 possible conformations for [+ta]-B[ a]P-N 2-dG are proposed. A correlation between molecular modeling and mutagenesis work suggests a hypothesis (Hypothesis 3): a base displaced conformation with the dG moiety of the adduct in the major vs. minor groove gives G→T vs. G→A mutations, respectively. (Hypothesis 4, which is a generalized version of Hypothesis 3, is also 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 trapped in different conformations, and why we think it makes sense to interpret 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.