Polymerase Bypass of N⁶-Deoxyadenosine Adducts Derived from Epoxide Metabolites of 1,3-Butadiene

N⁶-(2-Hydroxy-3-buten-1-yl)-2′-deoxyadenosine (N⁶-HB-dA I) and N⁶,N⁶-(2,3-dihydroxybutan-1,4-diyl)-2′-deoxyadenosine (N⁶,N⁶-DHB-dA) are exocyclic DNA adducts formed upon alkylation of the N⁶ position of adenine in DNA by epoxide metabolites of 1,3-butadiene (BD), a common industrial and environmental chemical classified as a human and animal carcinogen. Since the N⁶-H atom of adenine is required for Watson-Crick hydrogen bonding with thymine, N⁶-alkylation can prevent adenine from normal pairing with thymine, potentially compromising the accuracy of DNA replication. To evaluate the ability of BD-derived N⁶-alkyladenine lesions to induce mutations, synthetic oligodeoxynucleotides containing site-specific (S)-N⁶-HB-dA I and (R,R)-N⁶,N⁶-DHB-dA adducts were subjected to in vitro translesion synthesis in the presence of human DNA polymerases β, η, ι, and κ. While (S)-N⁶-HB-dA I was readily bypassed by all four enzymes, only polymerases η and κ were able to carry out DNA synthesis past (R,R)-N⁶,N⁶-DHB-dA. Steady-state kinetic analyses indicated that all four DNA polymerases preferentially incorporated the correct base (T) opposite (S)-N⁶-HB-dA I. In contrast, hPol β was completely blocked by (R,R)-N⁶,N⁶-DHB-dA, while hPol η and κ inserted A, G, C, or T opposite the adduct with similar frequency. HPLC-ESI-MS/MS analysis of primer extension products confirmed that while translesion synthesis past (S)-N⁶-HB-dA I was mostly error-free, replication of DNA containing (R,R)-N⁶,N⁶-DHB-dA induced significant numbers of A, C, and G insertions and small deletions. These results indicate that singly substituted (S)-N⁶-HB-dA I lesions are not miscoding, but that exocyclic (R,R)-N⁶,N⁶-DHB-dA adducts are strongly mispairing, probably due to their inability to form stable Watson-Crick pairs with dT. (Chemical Equation Presented).