Uncovering the Polymerase-induced Cytotoxicty of an Oxidized Nucleotide

Oxidative stress promotes genomic instability and human diseases(1). A common oxidized nucleoside is 8-oxo-7,8-dihydro-2’-deoxyguanosine found both in DNA (8-oxo-G) and as a free nucleotide (8-oxo-dGTP)(2,3). Nucleotide pools are especially vulnerable to oxidative damage(4). Therefore cells encode an enzyme (MutT/MTH1) that removes free oxidized nucleotides. This cleansing function is required for cancer cell survival(5,6) and to modulate E. coli antibiotic sensitivity in a DNA polymerase (pol)-dependent manner(7). How polymerase discriminates between damaged and non-damaged nucleotides is not well understood. This analysis is essential given the role of oxidized nucleotides in mutagenesis, cancer therapeutics, and bacterial antibiotics(8). Even with cellular sanitizing activities, nucleotide pools contain enough 8-oxo-dGTP to promote mutagenesis(9,10). This arises from the dual coding potential where 8-oxo-dGTP(anti) base pairs with cytosine (Cy) and 8-oxodGTP(syn) utilizes its Hoogsteen edge to base pair with adenine (Ad)(11). Here we utilized time-lapse crystallography to follow 8-oxo-dGTP insertion opposite Ad or Cy with human DNA pol β, to reveal that insertion is accommodated in either the syn- or anti-conformation, respectively. For 8-oxo-dGTP(anti) insertion, a novel divalent metal relieves repulsive interactions between the adducted guanine base and the triphosphate of the oxidized nucleotide. With either templating base, hydrogen bonding interactions between the bases are lost as the enzyme reopens after catalysis, leading to a cytotoxic nicked DNA repair intermediate. Combining structural snapshots with kinetic and computational analysis reveals how 8-oxodGTP utilizes charge modulation during insertion that can lead to a blocked DNA repair intermediate.