Cytotoxic and mutagenic properties of minor-groove -alkylthymidine lesions in human cells.

Endogenous metabolism, environmental exposure, and cancer chemotherapy can lead to alkylation of DNA. It has been well documented that, among the different DNA alkylation products, minor-groove -alkylthymidine (-alkyldT) lesions are inefficiently repaired. In the present study, we examined how seven -alkyldT lesions, with the alkyl group being a Me, Et, Pr, Pr, Bu, Bu, or Bu, are recognized by the DNA replication machinery in human cells. We found that the replication bypass efficiencies of these lesions decrease with increasing length of the alkyl chain, and that these lesions induce substantial frequencies of T→A and T→G mutations. Replication experiments using isogenic cells deficient in specific translesion synthesis (TLS) DNA polymerases revealed that the absence of polymerase η or polymerase ζ, but not polymerase κ or polymerase ι, significantly decreased both the bypass efficiencies and the mutation frequencies for those -alkyldT lesions carrying a straight-chain alkyl group. Moreover, the mutagenic properties of the -alkyldT lesions were influenced by the length and topology of the alkyl chain and by TLS polymerases. Together, our results provide important new knowledge about the cytotoxic and mutagenic properties of -alkyldT lesions, and illustrate the roles of TLS polymerases in replicative bypass of these lesions in human cells.