Abasic site-peptide cross-links are blocking lesions repaired by AP endonucleases.

Apurinic/apyrimidinic (AP) sites are abundant DNA lesions arising from spontaneous hydrolysis of the N-glycosidic bond and as base excision repair (BER) intermediates. AP sites and their derivatives readily trap DNA-bound proteins, resulting in DNA-protein cross-links. Those are subject to proteolysis but the fate of the resulting AP-peptide cross-links (APPXLs) is unclear. Here, we report two in vitro models of APPXLs synthesized by cross-linking of DNA glycosylases Fpg and OGG1 to DNA followed by trypsinolysis. The reaction with Fpg produces a 10-mer peptide cross-linked through its N-terminus, while OGG1 yields a 23-mer peptide attached through an internal lysine. Both adducts strongly blocked Klenow fragment, phage RB69 polymerase, Saccharolobus solfataricus Dpo4, and African swine fever virus PolX. In the residual lesion bypass, mostly dAMP and dGMP were incorporated by Klenow and RB69 polymerases, while Dpo4 and PolX used primer/template misalignment. Of AP endonucleases involved in BER, Escherichia coli endonuclease IV and its yeast homolog Apn1p efficiently hydrolyzed both adducts. In contrast, E. coli exonuclease III and human APE1 showed little activity on APPXL substrates. Our data suggest that APPXLs produced by proteolysis of AP site-trapped proteins may be removed by the BER pathway, at least in bacterial and yeast cells.

The Impact of Human DNA Glycosylases on the Activity of DNA Polymerase ß toward Various Base Excision Repair Intermediates.

Base excision repair (BER) is one of the important systems for the maintenance of genome stability via repair of DNA lesions. BER is a multistep process involving a number of enzymes, including damage-specific DNA glycosylases, apurinic/apyrimidinic (AP) endonuclease 1, DNA polymerase ß, and DNA ligase. Coordination of BER is implemented by multiple protein-protein interactions between BER participants. Nonetheless, mechanisms of these interactions and their roles in the BER coordination are poorly understood. Here, we report a study on Polß's nucleotidyl transferase activity toward different DNA substrates (that mimic DNA intermediates arising during BER) in the presence of various DNA glycosylases (AAG, OGG1, NTHL1, MBD4, UNG, or SMUG1) using rapid-quench-flow and stopped-flow fluorescence approaches. It was shown that Polß efficiently adds a single nucleotide into different types of single-strand breaks either with or without a 5'-dRP-mimicking group. The obtained data indicate that DNA glycosylases AAG, OGG1, NTHL1, MBD4, UNG, and SMUG1, but not NEIL1, enhance Polß's activity toward the model DNA intermediates.

Kinetic Features of 3'-5'-Exonuclease Activity of Apurinic/Apyrimidinic Endonuclease Apn2 from Saccharomyces cerevisiae.

In yeast Saccharomyces cerevisiae cells, apurinic/apyrimidinic (AP) sites are primarily repaired by base excision repair. Base excision repair is initiated by one of two AP endonucleases: Apn1 or Apn2. AP endonucleases catalyze hydrolytic cleavage of the phosphodiester backbone on the 5' side of an AP site, thereby forming a single-strand break containing 3'-OH and 5'-dRP ends. In addition, Apn2 has 3'-phosphodiesterase activity (removing 3'-blocking groups) and 3' → 5' exonuclease activity (both much stronger than its AP endonuclease activity). Nonetheless, the role of the 3'-5'-exonuclease activity of Apn2 remains unclear and presumably is involved in the repair of damage containing single-strand breaks. In this work, by separating reaction products in a polyacrylamide gel and by a stopped-flow assay, we performed a kinetic analysis of the interaction of Apn2 with various model DNA substrates containing a 5' overhang. The results allowed us to propose a mechanism for the cleaving off of nucleotides and to determine the rate of the catalytic stage of the process. It was found that dissociation of a reaction product from the enzyme active site is not a rate-limiting step in the enzymatic reaction. We determined an influence of the nature of the 3'-terminal nucleotide that can be cleaved off on the course of the enzymatic reaction. Finally, it was found that the efficiency of the enzymatic reaction is context-specific.

Comparative Analysis of Exo- and Endonuclease Activities of APE1-like Enzymes.

Apurinic/apyrimidinic (AP)-endonucleases are multifunctional enzymes that are required for cell viability. AP-endonucleases incise DNA 5' to an AP-site; can recognize and process some damaged nucleosides; and possess 3'-phosphodiesterase, 3'-phosphatase, and endoribonuclease activities. To elucidate the mechanism of substrate cleavage in detail, we analyzed the effect of mono- and divalent metal ions on the exo- and endonuclease activities of four homologous APE1-like endonucleases (from an insect (Rrp1), amphibian (xAPE1), fish (zAPE1), and from humans (hAPE1)). It was found that the enzymes had similar patterns of dependence on metal ions' concentrations in terms of AP-endonuclease activity, suggesting that the main biological function (AP-site cleavage) was highly conserved among evolutionarily distant species. The efficiency of the 3'-5' exonuclease activity was the highest in hAPE1 among these enzymes. In contrast, the endoribonuclease activity of the enzymes could be ranked as hAPE1 ≈ zAPE1 ≤ xAPE1 ≤ Rrp1. Taken together, the results revealed that the tested enzymes differed significantly in their capacity for substrate cleavage, even though the most important catalytic and substrate-binding amino acid residues were conserved. It can be concluded that substrate specificity and cleavage efficiency were controlled by factors external to the catalytic site, e.g., the N-terminal domain of these enzymes.

Conformational Dynamics of Human ALKBH2 Dioxygenase in the Course of DNA Repair as Revealed by Stopped-Flow Fluorescence Spectroscopy.

Elucidation of physicochemical mechanisms of enzymatic processes is one of the main tasks of modern biology. High efficiency and selectivity of enzymatic catalysis are mostly ensured by conformational dynamics of enzymes and substrates. Here, we applied a stopped-flow kinetic analysis based on fluorescent spectroscopy to investigate mechanisms of conformational transformations during the removal of alkylated bases from DNA by ALKBH2, a human homolog of Escherichia coli AlkB dioxygenase. This enzyme protects genomic DNA against various alkyl lesions through a sophisticated catalytic mechanism supported by a cofactor (Fe(II)), a cosubstrate (2-oxoglutarate), and O2. We present here a comparative study of conformational dynamics in complexes of the ALKBH2 protein with double-stranded DNA substrates containing N1-methyladenine, N3-methylcytosine, or 1,N6-ethenoadenine. By means of fluorescent labels of different types, simultaneous detection of conformational transitions in the protein globule and DNA substrate molecule was performed. Fitting of the kinetic curves by a nonlinear-regression method yielded a molecular mechanism and rate constants of its individual steps. The results shed light on overall conformational dynamics of ALKBH2 and damaged DNA during the catalytic cycle.

Common Kinetic Mechanism of Abasic Site Recognition by Structurally Different Apurinic/Apyrimidinic Endonucleases.

Apurinic/apyrimidinic (AP) endonucleases Nfo (Escherichia coli) and APE1 (human) represent two conserved structural families of enzymes that cleave AP-site-containing DNA in base excision repair. Nfo and APE1 have completely different structures of the DNA-binding site, catalytically active amino acid residues and catalytic metal ions. Nonetheless, both enzymes induce DNA bending, AP-site backbone eversion into the active-site pocket and extrusion of the nucleotide located opposite the damage. All these stages may depend on local stability of the DNA duplex near the lesion. Here, we analysed effects of natural nucleotides located opposite a lesion on catalytic-complex formation stages and DNA cleavage efficacy. Several model DNA substrates that contain an AP-site analogue [F-site, i.e., (2R,3S)-2-(hydroxymethyl)-3-hydroxytetrahydrofuran] opposite G, A, T or C were used to monitor real-time conformational changes of the tested enzymes during interaction with DNA using changes in the enzymes' intrinsic fluorescence intensity mainly caused by Trp fluorescence. The extrusion of the nucleotide located opposite F-site was recorded via fluorescence intensity changes of two base analogues. The catalytic rate constant slightly depended on the opposite-nucleotide nature. Thus, structurally different AP endonucleases Nfo and APE1 utilise a common strategy of damage recognition controlled by enzyme conformational transitions after initial DNA binding.

Mechanistic insight into the role of Poly(ADP-ribosyl)ation in DNA topology modulation and response to DNA damage.

Genotoxic stress generates single- and double-strand DNA breaks either through direct damage by reactive oxygen species or as intermediates of DNA repair. Failure to detect and repair DNA strand breaks leads to deleterious consequences such as chromosomal aberrations, genomic instability and cell death. DNA strand breaks disrupt the superhelical state of cellular DNA, which further disturbs the chromatin architecture and gene activity regulation. Proteins from the poly(ADP-ribose) polymerase (PARP) family, such as PARP1 and PARP2, use NAD+ as a substrate to catalyse the synthesis of polymeric chains consisting of ADP-ribose units covalently attached to an acceptor molecule. PARP1 and PARP2 are regarded as DNA damage sensors that, upon activation by strand breaks, poly(ADP-ribosyl)ate themselves and nuclear acceptor proteins. Noteworthy, the regularly branched structure of poly(ADP-ribose) polymer suggests that the mechanism of its synthesis may involve circular movement of PARP1 around the DNA helix, with a branching point in PAR corresponding to one complete 360° turn. We propose that PARP1 stays bound to a DNA strand break end, but rotates around the helix displaced by the growing poly(ADP-ribose) chain, and that this rotation could introduce positive supercoils into damaged chromosomal DNA. This topology modulation would enable nucleosome displacement and chromatin decondensation around the lesion site, facilitating the access of DNA repair proteins or transcription factors. PARP1-mediated DNA supercoiling can be transmitted over long distances, resulting in changes in the high-order chromatin structures. The available structures of PARP1 are consistent with the strand break-induced PAR synthesis as a driving force for PARP1 rotation around the DNA axis.

DNA Repair and Mutagenesis in Vertebrate Mitochondria: Evidence for Asymmetric DNA Strand Inheritance.

A variety of endogenous and exogenous factors induce chemical and structural alterations in cellular DNA in addition to the errors occurring throughout DNA synthesis. These types of DNA damage are cytotoxic, miscoding or both and are believed to be at the origin of cancer and other age-related diseases. A human cell, aside from nuclear DNA, contains thousands of copies of mitochondrial DNA (mtDNA), a double-stranded, circular molecule of 16,569 bp. It has been proposed that mtDNA is a critical target of reactive oxygen species: by-products of oxidative phosphorylation that are generated in the organelle during aerobic respiration. Indeed, oxidative damage to mtDNA is more extensive and persistent as compared to that to nuclear DNA. Although transversions are the hallmark of mutations induced by reactive oxygen species, paradoxically, the majority of mtDNA mutations that occur during ageing and cancer are transitions. Furthermore, these mutations show a striking strand orientation bias: T→C/G→A transitions preferentially occur on the light strand, whereas C→T/A→G on the heavy strand of mtDNA. Here, we propose that the majority of mtDNA progenies, created after multiple rounds of DNA replication, are derived from the heavy strand only, owing to asymmetric replication of the DNA strand anchored to the inner membrane via the D-loop structure.

Modulation of the Apurinic/Apyrimidinic Endonuclease Activity of Human APE1 and of Its Natural Polymorphic Variants by Base Excision Repair Proteins.

Human apurinic/apyrimidinic endonuclease 1 (APE1) is known to be a critical player of the base excision repair (BER) pathway. In general, BER involves consecutive actions of DNA glycosylases, AP endonucleases, DNA polymerases, and DNA ligases. It is known that these proteins interact with APE1 either at upstream or downstream steps of BER. Therefore, we may propose that even a minor disturbance of protein-protein interactions on the DNA template reduces coordination and repair efficiency. Here, the ability of various human DNA repair enzymes (such as DNA glycosylases OGG1, UNG2, and AAG; DNA polymerase Polß; or accessory proteins XRCC1 and PCNA) to influence the activity of wild-type (WT) APE1 and its seven natural polymorphic variants (R221C, N222H, R237A, G241R, M270T, R274Q, and P311S) was tested. Förster resonance energy transfer-based kinetic analysis of abasic site cleavage in a model DNA substrate was conducted to detect the effects of interacting proteins on the activity of WT APE1 and its single-nucleotide polymorphism (SNP) variants. The results revealed that WT APE1 activity was stimulated by almost all tested DNA repair proteins. For the SNP variants, the matters were more complicated. Analysis of two SNP variants, R237A and G241R, suggested that a positive charge in this area of the APE1 surface impairs the protein-protein interactions. In contrast, variant R221C (where the affected residue is located near the DNA-binding site) showed permanently lower activation relative to WT APE1, whereas neighboring SNP N222H did not cause a noticeable difference as compared to WT APE1. Buried substitution P311S had an inconsistent effect, whereas each substitution at the DNA-binding site, M270T and R274Q, resulted in the lowest stimulation by BER proteins. Protein-protein molecular docking was performed between repair proteins to identify amino acid residues involved in their interactions. The data uncovered differences in the effects of BER proteins on APE1, indicating an important role of protein-protein interactions in the coordination of the repair pathway.

Poly(ADP-ribose) polymerases covalently modify strand break termini in DNA fragments in vitro.

Poly(ADP-ribose) polymerases (PARPs/ARTDs) use nicotinamide adenine dinucleotide (NAD+) to catalyse the synthesis of a long branched poly(ADP-ribose) polymer (PAR) attached to the acceptor amino acid residues of nuclear proteins. PARPs act on single- and double-stranded DNA breaks by recruiting DNA repair factors. Here, in in vitro biochemical experiments, we found that the mammalian PARP1 and PARP2 proteins can directly ADP-ribosylate the termini of DNA oligonucleotides. PARP1 preferentially catalysed covalent attachment of ADP-ribose units to the ends of recessed DNA duplexes containing 3'-cordycepin, 5'- and 3'-phosphate and also to 5'-phosphate of a single-stranded oligonucleotide. PARP2 preferentially ADP-ribosylated the nicked/gapped DNA duplexes containing 5'-phosphate at the double-stranded termini. PAR glycohydrolase (PARG) restored native DNA structure by hydrolysing PAR-DNA adducts generated by PARP1 and PARP2. Biochemical and mass spectrometry analyses of the adducts suggested that PARPs utilise DNA termini as an alternative to 2'-hydroxyl of ADP-ribose and protein acceptor residues to catalyse PAR chain initiation either via the 2',1″-O-glycosidic ribose-ribose bond or via phosphodiester bond formation between C1' of ADP-ribose and the phosphate of a terminal deoxyribonucleotide. This new type of post-replicative modification of DNA provides novel insights into the molecular mechanisms underlying biological phenomena of ADP-ribosylation mediated by PARPs.

Oxidatively Generated Guanine(C8)-Thymine(N3) Intrastrand Cross-links in Double-stranded DNA Are Repaired by Base Excision Repair Pathways.

Oxidatively generated guanine radical cations in DNA can undergo various nucleophilic reactions including the formation of C8-guanine cross-links with adjacent or nearby N3-thymines in DNA in the presence of O2. The G*[C8-N3]T* lesions have been identified in the DNA of human cells exposed to oxidative stress, and are most likely genotoxic if not removed by cellular defense mechanisms. It has been shown that the G*[C8-N3]T* lesions are substrates of nucleotide excision repair in human cell extracts. Cleavage at the sites of the lesions was also observed but not further investigated (Ding et al. (2012) Nucleic Acids Res. 40, 2506-2517). Using a panel of eukaryotic and prokaryotic bifunctional DNA glycosylases/lyases (NEIL1, Nei, Fpg, Nth, and NTH1) and apurinic/apyrimidinic (AP) endonucleases (Apn1, APE1, and Nfo), the analysis of cleavage fragments by PAGE and MALDI-TOF/MS show that the G*[C8-N3]T* lesions in 17-mer duplexes are incised on either side of G*, that none of the recovered cleavage fragments contain G*, and that T* is converted to a normal T in the 3'-fragment cleavage products. The abilities of the DNA glycosylases to incise the DNA strand adjacent to G*, while this base is initially cross-linked with T*, is a surprising observation and an indication of the versatility of these base excision repair proteins.

Conformational Dynamics of DNA Repair by Escherichia coli Endonuclease III.

Escherichia coli endonuclease III (Endo III or Nth) is a DNA glycosylase with a broad substrate specificity for oxidized or reduced pyrimidine bases. Endo III possesses two types of activities: N-glycosylase (hydrolysis of the N-glycosidic bond) and AP lyase (elimination of the 3'-phosphate of the AP-site). We report a pre-steady-state kinetic analysis of structural rearrangements of the DNA substrates and uncleavable ligands during their interaction with Endo III. Oligonucleotide duplexes containing 5,6-dihydrouracil, a natural abasic site, its tetrahydrofuran analog, and undamaged duplexes carried fluorescent DNA base analogs 2-aminopurine and 1,3-diaza-2-oxophenoxazine as environment-sensitive reporter groups. The results suggest that Endo III induces several fast sequential conformational changes in DNA during binding, lesion recognition, and adjustment to a catalytically competent conformation. A comparison of two fluorophores allowed us to distinguish between the events occurring in the damaged and undamaged DNA strand. Combining our data with the available structures of Endo III, we conclude that this glycosylase uses a multistep mechanism of damage recognition, which likely involves Gln(41) and Leu(81) as DNA lesion sensors.

Aberrant repair initiated by mismatch-specific thymine-DNA glycosylases provides a mechanism for the mutational bias observed in CpG islands.

The human thymine-DNA glycosylase (TDG) initiates the base excision repair (BER) pathway to remove spontaneous and induced DNA base damage. It was first biochemically characterized for its ability to remove T mispaired with G in CpG context. TDG is involved in the epigenetic regulation of gene expressions by protecting CpG-rich promoters from de novo DNA methylation. Here we demonstrate that TDG initiates aberrant repair by excising T when it is paired with a damaged adenine residue in DNA duplex. TDG targets the non-damaged DNA strand and efficiently excises T opposite of hypoxanthine (Hx), 1,N(6)-ethenoadenine, 7,8-dihydro-8-oxoadenine and abasic site in TpG/CpX context, where X is a modified residue. In vitro reconstitution of BER with duplex DNA containing Hx•T pair and TDG results in incorporation of cytosine across Hx. Furthermore, analysis of the mutation spectra inferred from single nucleotide polymorphisms in human population revealed a highly biased mutation pattern within CpG islands (CGIs), with enhanced mutation rate at CpA and TpG sites. These findings demonstrate that under experimental conditions used TDG catalyzes sequence context-dependent aberrant removal of thymine, which results in TpG, CpA→CpG mutations, thus providing a plausible mechanism for the putative evolutionary origin of the CGIs in mammalian genomes.

Insight into mechanisms of 3'-5' exonuclease activity and removal of bulky 8,5'-cyclopurine adducts by apurinic/apyrimidinic endonucleases.

8,5'-cyclo-2'-deoxyadenosine (cdA) and 8,5'-cyclo-2'-deoxyguanosine generated in DNA by both endogenous oxidative stress and ionizing radiation are helix-distorting lesions and strong blocks for DNA replication and transcription. In duplex DNA, these lesions are repaired in the nucleotide excision repair (NER) pathway. However, lesions at DNA strand breaks are most likely poor substrates for NER. Here we report that the apurinic/apyrimidinic (AP) endonucleases--Escherichia coli Xth and human APE1--can remove 5'S cdA (S-cdA) at 3' termini of duplex DNA. In contrast, E. coli Nfo and yeast Apn1 are unable to carry out this reaction. None of these enzymes can remove S-cdA adduct located at 1 or more nt away from the 3' end. To understand the structural basis of 3' repair activity, we determined a high-resolution crystal structure of E. coli Nfo-H69A mutant bound to a duplex DNA containing an α-anomeric 2'-deoxyadenosine:T base pair. Surprisingly, the structure reveals a bound nucleotide incision repair (NIR) product with an abortive 3'-terminal dC close to the scissile position in the enzyme active site, providing insight into the mechanism for Nfo-catalyzed 3'→5' exonuclease function and its inhibition by 3'-terminal S-cdA residue. This structure was used as a template to model 3'-terminal residues in the APE1 active site and to explain biochemical data on APE1-catalyzed 3' repair activities. We propose that Xth and APE1 may act as a complementary repair pathway to NER to remove S-cdA adducts from 3' DNA termini in E. coli and human cells, respectively.

Uracil in duplex DNA is a substrate for the nucleotide incision repair pathway in human cells.

Spontaneous hydrolytic deamination of cytosine to uracil (U) in DNA is a constant source of genome instability in cells. This mutagenic process is greatly enhanced at high temperatures and in single-stranded DNA. If not repaired, these uracil residues give rise to C → T transitions, which are the most common spontaneous mutations occurring in living organisms and are frequently found in human tumors. In the majority of species, uracil residues are removed from DNA by specific uracil-DNA glycosylases in the base excision repair pathway. Alternatively, in certain archaeal organisms, uracil residues are eliminated by apurinic/apyrimidinic (AP) endonucleases in the nucleotide incision repair pathway. Here, we characterized the substrate specificity of the major human AP endonuclease 1, APE1, toward U in duplex DNA. APE1 cleaves oligonucleotide duplexes containing a single U⋅G base pair; this activity depends strongly on the sequence context and the base opposite to U. The apparent kinetic parameters of the reactions show that APE1 has high affinity for DNA containing U but cleaves the DNA duplex at an extremely low rate. MALDI-TOF MS analysis of the reaction products demonstrated that APE1-catalyzed cleavage of a U⋅G duplex generates the expected DNA fragments containing a 5'-terminal deoxyuridine monophosphate. The fact that U in duplex DNA is recognized and cleaved by APE1 in vitro suggests that this property of the exonuclease III family of AP endonucleases is remarkably conserved from Archaea to humans. We propose that nucleotide incision repair may act as a backup pathway to base excision repair to remove uracils arising from cytosine deamination.

Step-by-step mechanism of DNA damage recognition by human 8-oxoguanine DNA glycosylase.

BACKGROUND: Extensive structural studies of human DNA glycosylase hOGG1 have revealed essential conformational changes of the enzyme. However, at present there is little information about the time scale of the rearrangements of the protein structure as well as the dynamic behavior of individual amino acids. METHODS: Using pre-steady-state kinetic analysis with Trp and 2-aminopurine fluorescence detection the conformational dynamics of hOGG1 wild-type (WT) and mutants Y203W, Y203A, H270W, F45W, F319W and K249Q as well as DNA-substrates was examined. RESULTS: The roles of catalytically important amino acids F45, Y203, K249, H270, and F319 in the hOGG1 enzymatic pathway and their involvement in the step-by-step mechanism of oxidative DNA lesion recognition and catalysis were elucidated. CONCLUSIONS: The results show that Tyr-203 participates in the initial steps of the lesion site recognition. The interaction of the His-270 residue with the oxoG base plays a key role in the insertion of the damaged base into the active site. Lys-249 participates not only in the catalytic stages but also in the processes of local duplex distortion and flipping out of the oxoG residue. Non-damaged DNA does not form a stable complex with hOGG1, although a complex with a flipped out guanine base can be formed transiently. GENERAL SIGNIFICANCE: The kinetic data obtained in this study significantly improves our understanding of the molecular mechanism of lesion recognition by hOGG1.

Pre-steady-state fluorescence analysis of damaged DNA transfer from human DNA glycosylases to AP endonuclease APE1.

BACKGROUND: DNA glycosylases remove the modified, damaged or mismatched bases from the DNA by hydrolyzing the N-glycosidic bonds. Some enzymes can further catalyze the incision of a resulting abasic (apurinic/apyrimidinic, AP) site through ß- or ß,δ-elimination mechanisms. In most cases, the incision reaction of the AP-site is catalyzed by special enzymes called AP-endonucleases. METHODS: Here, we report the kinetic analysis of the mechanisms of modified DNA transfer from some DNA glycosylases to the AP endonuclease, APE1. The modified DNA contained the tetrahydrofurane residue (F), the analogue of the AP-site. DNA glycosylases AAG, OGG1, NEIL1, MBD4(cat) and UNG from different structural superfamilies were used. RESULTS: We found that all DNA glycosylases may utilise direct protein-protein interactions in the transient ternary complex for the transfer of the AP-containing DNA strand to APE1. CONCLUSIONS: We hypothesize a fast "flip-flop" exchange mechanism of damaged and undamaged DNA strands within this complex for monofunctional DNA glycosylases like MBD4(cat), AAG and UNG. Bifunctional DNA glycosylase NEIL1 creates tightly specific complex with DNA containing F-site thereby efficiently competing with APE1. Whereas APE1 fast displaces other bifunctional DNA glycosylase OGG1 on F-site thereby induces its shifts to undamaged DNA regions. GENERAL SIGNIFICANCE: Kinetic analysis of the transfer of DNA between human DNA glycosylases and APE1 allows us to elucidate the critical step in the base excision repair pathway.

7,8-Dihydro-8-oxoadenine, a highly mutagenic adduct, is repaired by Escherichia coli and human mismatch-specific uracil/thymine-DNA glycosylases.

Hydroxyl radicals predominantly react with the C(8) of purines forming 7,8-dihydro-8-oxoguanine (8oxoG) and 7,8-dihydro-8-oxoadenine (8oxoA) adducts, which are highly mutagenic in mammalian cells. The majority of oxidized DNA bases are removed by DNA glycosylases in the base excision repair pathway. Here, we report for the first time that human thymine-DNA glycosylase (hTDG) and Escherichia coli mismatch-specific uracil-DNA glycosylase (MUG) can remove 8oxoA from 8oxoA•T, 8oxoA•G and 8oxoA•C pairs. Comparison of the kinetic parameters of the reaction indicates that full-length hTDG excises 8oxoA, 3,N(4)-ethenocytosine (εC) and T with similar efficiency (k(max) = 0.35, 0.36 and 0.16 min(-1), respectively) and is more proficient as compared with its bacterial homologue MUG. The N-terminal domain of the hTDG protein is essential for 8oxoA-DNA glycosylase activity, but not for εC repair. Interestingly, the TDG status had little or no effect on the proliferation rate of mouse embryonic fibroblasts after exposure to γ-irradiation. Nevertheless, using whole cell-free extracts from the DNA glycosylase-deficient murine embryonic fibroblasts and E. coli, we demonstrate that the excision of 8oxoA from 8oxoA•T and 8oxoA•G has an absolute requirement for TDG and MUG, respectively. The data establish that MUG and TDG can counteract the genotoxic effects of 8oxoA residues in vivo.

Biochemical and structural characterization of the glycosylase domain of MBD4 bound to thymine and 5-hydroxymethyuracil-containing DNA.

Active DNA demethylation in mammals occurs via hydroxylation of 5-methylcytosine to 5-hydroxymethylcytosine (5hmC) by the ten-eleven translocation family of proteins (TETs). 5hmC residues in DNA can be further oxidized by TETs to 5-carboxylcytosines and/or deaminated by the Activation Induced Deaminase/Apolipoprotein B mRNA-editing enzyme complex family proteins to 5-hydromethyluracil (5hmU). Excision and replacement of these intermediates is initiated by DNA glycosylases such as thymine-DNA glycosylase (TDG), methyl-binding domain protein 4 (MBD4) and single-strand specific monofunctional uracil-DNA glycosylase 1 in the base excision repair pathway. Here, we report detailed biochemical and structural characterization of human MBD4 which contains mismatch-specific TDG activity. Full-length as well as catalytic domain (residues 426-580) of human MBD4 (MBD4(cat)) can remove 5hmU when opposite to G with good efficiency. Here, we also report six crystal structures of human MBD4(cat): an unliganded form and five binary complexes with duplex DNA containing a T•G, 5hmU•G or AP•G (apurinic/apyrimidinic) mismatch at the target base pair. These structures reveal that MBD4(cat) uses a base flipping mechanism to specifically recognize thymine and 5hmU. The recognition mechanism of flipped-out 5hmU bases in MBD4(cat) active site supports the potential role of MBD4, together with TDG, in maintenance of genome stability and active DNA demethylation in mammals.

Coordination between human DNA polymerase ß and apurinic/apyrimidinic endonuclease 1 in the course of DNA repair.

Coordination of enzymatic activities in the course of base excision repair (BER) is essential to ensure complete repair of damaged bases. Two major mechanisms underlying the coordination of BER are known today: the "passing the baton" model and a model of preassembled stable multiprotein repair complexes called "repairosomes." In this work, we aimed to elucidate the coordination between human apurinic/apyrimidinic (AP) endonuclease APE1 and DNA polymerase Polß in BER through studying an impact of APE1 on Polß-catalyzed nucleotide incorporation into different model substrates that mimic different single-strand break (SSB) intermediates arising along the BER pathway. It was found that APE1's impact on separate stages of Polß's catalysis depends on the nature of a DNA substrate. In this complex, APE1 removed 3' blocking groups and corrected Polß-catalyzed DNA synthesis in a coordinated manner. Our findings support the hypothesis that Polß not only can displace APE1 from damaged DNA within the "passing the baton" model but also performs the gap-filling reaction in the ternary complex with APE1 according to the "repairosome" model. Taken together, our results provide new insights into coordination between APE1 and Polß during the BER process.