Mutagenesis studies of the major benzo[a]pyrene N2-dG adduct in a 5′-T(G)under-bar versus a 5′-U(G)under-bar sequence:: removal of the methyl group causes a modest decrease in the [G→T/G→A] mutational ratio

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations primarily at the G:C base pairs (e.g. GC -> TA, GC -> AT, etc.). Each of these mutations can be induced by its major adduct [+ta]-B[a]P-N-2-dG, where DNA sequence context appears to influence both the quantitative and qualitative pattern of mutagenesis. We noted previously that 5'-T (G) under bar sequences tend to have a higher fraction of G -> T mutations for both [+ta]-B[a]P-N-2-dG and (+)-anti-B[a]PDE in comparison with 5'-C (G) under bar, or 5'-A (G) under bar sequences. To investigate a possible structural element for this trend, the role (if any) of the methyl group on the 5'-T is considered. Using adduct site-specific means, the [G -> T/G -> A] mutational ratio for [+ta]-B[a]P-N-2-dG is determined to be similar to 1.08 in a 5'-T (G) under barT sequence, and similar to 0.60 in a 5'-U (G) under barT sequence. (G -> C mutations are minor.) Although this modest similar to 1.8-fold decrease in [G -> T/G -> A] ratio is statistically significant (P = 0.03), it suggests that the methyl group on the 5'-T is not the main reason why a 5'-T tends to enhance G -> T mutations. This study was prompted by an adduct conformational hypothesis, which predicted that the removal of the methyl group in a 5'-T (G) under bar sequence would lower the fraction of G -> T mutations; however, the similar to 1.8-fold decrease is too small to do additional experiments to assess whether this conformational hypothesis, or other hypotheses, are the true cause of the decrease, which is discussed in this paper.