Translesion Synthesis past 2′-Deoxyxanthosine, a Nitric Oxide-derived DNA Adduct, by Mammalian DNA Polymerases

Cellular DNA is damaged by nitric oxide (NO), a multifunctional bioregulator and an environmental pollutant that has been implicated in diseases associated with cancer and chronic inflammation. 2′-Deoxyxanthosine (dX) is a major NO-derived DNA lesion. To explore the mutagenic potential of dX, a 38-mer oligodeoxynucleotide ( 5′CATGCTGATGAATTCCTTCXCTTCTTTCCTCTCCCTTT) modified site-specifically with dX at the X position was prepared post-synthetically and used as a DNA template in primer extension reactions catalyzed by calf thymus DNA polymerase (pol) α and human DNA pol β, η, and κ. Primer extension reactions catalyzed by pol α or β in the presence of four dNTPs were retarded at the dX lesion while pol η and κ readily bypassed the lesion. The fully extended products were analyzed to quantify the miscoding specificity and frequency of dX using two-phase polyacrylamide gel electrophoresis (PAGE). With pol α, η and κ, incorrect dTMP was preferentially incorporated opposite the lesion, along with lesser amounts of dCMP, the correct base. When pol β was used, direct incorporation of correct dCMP was primarily observed, accompanied by small amounts of misincorporation of dTMP, dAMP and dGMP. Steady-state kinetic analyses supported the results obtained from the two-phase PAGE assay. dX is a miscoding lesion capable of preferentially generating G→A mutations. The miscoding frequency varied depending on DNA polymerase used.