DNA polymerase θ protects against genomic instability via an alternative end-joining repair pathway for DNA double-strand breaks. Breast, lung and oral cancers over-express polymerase θ, and reduction of its activity in mammalian cells increases sensitivity to double-strand break inducing agents, including ionizing radiation. Reported here are crystal structures of the C-terminal polymerase domain from human polymerase θ, illustrating two potential modes of dimerization. One structure depicts insertion of ddATP opposite an abasic site analog during translesion DNA synthesis. The second structure describes a cognate ddGTP complex. Polymerase θ employs a specialized thumb subdomain to establish unique upstream contacts to the primer DNA strand, including an interaction to the 3’-terminal phosphate from one of five distinctive insertion loops. These observations demonstrate how polymerase θ grasps the primer to bypass DNA lesions, or extend poorly annealed DNA termini to mediate end-joining.

G-quadruplex is a four-stranded G-rich DNA structure that is highly susceptible to oxidation. Despite the important roles that G-quadruplexes play in telomere biology and gene transcription, neither the impact of guanine lesions on the stability of quadruplexes nor their repair are well understood. Here, we show that the oxidized guanine lesions 8-oxo-7,8-dihydroguanine (8-oxoG), guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) reduce the thermostability and alter the folding of telomeric quadruplexes in a location-dependent manner. Also, the NEIL1 and NEIL3 DNA glycosylases can remove hydantoin lesions but none of the glycosylases, including OGG1, are able to remove 8-oxoG from telomeric quadruplexes. Interestingly, a hydantoin lesion at the site most prone to oxidation in quadruplex DNA is not efficiently removed by NEIL1 or NEIL3. However, NEIL1, NEIL2 and NEIL3 remove hydantoins from telomeric quadruplexes formed by five TTAGGG repeats much more rapidly than the commonly studied four-repeat quadruplex structures. We also show that APE1 cleaves furan in selected positions in Na+-coordinated telomeric quadruplexes. In promoter G-quadruplex DNA, the NEIL glycosylases primarily remove Gh from Na+-coordinated antiparallel quadruplexes but not K+-coordinated parallel quadruplexes containing VEGF or c-MYC promoter sequences. Thus, the NEIL DNA glycosylases may be involved in both telomere maintenance and in gene regulation.

Background: Nei is a DNA glycosylase of the base excision repair pathway. Results: We present two crystal structures of an Nei bound to thymine glycol or 5-hydroxyuracil. Conclusion: Mutational analysis of active site residues suggests that lesion recognition happens before the damaged base is everted into the active site. Significance: These are the first structures of any Nei in complex with a damaged base.

Summary Endonuclease VIII-like 3 (Neil3) is a DNA glycosylase of the base excision repair pathway which protects cells from oxidative DNA damage by excising a broad spectrum of cytotoxic and mutagenic base lesions. Interestingly Neil3 exhibits an unusual preference for DNA with single-stranded regions. Here we report the first crystal structure of a Neil3 enzyme. Although the glycosylase region of mouse Neil3 (MmuNeil3Δ324) exhibits the same overall fold as that of other Fpg/Nei proteins, it presents distinct structural features. First, MmuNeil3Δ324 lacks the “αF-β9/10 loop” that caps the flipped-out 8-oxoG in bacterial Fpg, which is consistent with its inability to cleave 8-oxoguanine. Secondly, Neil3 not only lacks two of the three void-filling residues that stabilize the opposite strand but it also harbors negatively-charged residues which create an unfavorable electrostatic environment for the phosphate backbone of that strand. These structural features provide insight into Neil3's substrate specificity and marked preference for ssDNA.

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