In Vitro Replication Studies of Carboxymethylated DNA Lesions with Saccharomyces cerevisiae Polymerase η

Humans are exposed to N-nitroso compounds (NOCs) both endogenously and exogenously from a number of environmental sources, and NOCs are both mutagenic and carcinogenic. After metabolic activation, some NOCs can induce carboxymethylation of nucleobases through a diazoacetate intermediate, which could give rise to p53 mutations similar to those seen in human gastrointestinal cancers. It was previously found that the growth of polymerase η-deficient human cells was inhibited by treatment with azaserine, a DNA carboxymethylation agent, suggesting the importance of this polymerase in bypassing the azaserine-induced carboxymethylated DNA lesions. In this study, we examined how carboxymethylated DNA lesions, which included N⁶-carboxymethyl-2′-deoxyadenosine (N⁶-CMdA), N⁴-carboxymethyl-2′-deoxycytidine (N⁴-CMdC), N3-carboxymethylthymidine (N3-CMdT), and O⁴-carboxymethylthymidine (O⁴-CMdT), perturbed the efficiency and fidelity of DNA replication mediated by Saccharomyces cerevisiae polymerase η (pol η). Our results from steady-state kinetic assay showed that pol η could readily bypass and extend past N⁶-CMdA and incorporated the correct nucleotides opposite the lesion and its neighboring 5′-nucleoside with high efficiency. By contrast, the polymerase could bypass N⁴-CMdC inefficiently, with substantial misincorporation of dCMP followed by dAMP, though pol η could extend past the lesion with high fidelity and efficiency when dGMP was incorporated opposite the lesion. On the other hand, yeast pol η experienced great difficulty in bypassing O⁴-CMdT and N3-CMdT, and the polymerase inserted preferentially the incorrect dGMP opposite these two DNA lesions; the extension step, nevertheless, occurred with high fidelity and efficiency when the correct dAMP was opposite the lesion, as opposed to the preferentially incorporated incorrect dGMP. These results suggest that these lesions may contribute significantly to diazoacetate-induced mutations and those in the p53 gene observed in human gastrointestinal tumors.