Structural Characterization of Clostridium acetobutylicum 8-Oxoguanine DNA Glycosylase in Its Apo Form and in Complex with 8-Oxodeoxyguanosine

DNA is subject to a multitude of oxidative damages generated by oxidizing agents from metabolism and exogenous sources and by ionizing radiation. Guanine is particularly vulnerable to oxidation, and the most common oxidative product 8-oxoguanine (8-oxoG) is the most prevalent lesion observed in DNA molecules. 8-OxoG can form a normal Watson-Crick pair with cytosine (8-oxoG:C), but it can also form a stable Hoogsteen pair with adenine (8-oxoG:A), leading to a G:C → T:A transversion after replication. Fortunately, 8-oxoG is recognized and excised by either of two DNA glycosylases of the base excision repair pathway: formamidopyrimidine-DNA glycosylase and 8-oxoguanine DNA glycosylase (Ogg). While Clostridium acetobutylicum Ogg (CacOgg) DNA glycosylase can specifically recognize and remove 8-oxoG, it displays little preference for the base opposite the lesion, which is unusual for a member of the Ogg1 family. This work describes the crystal structures of CacOgg in its apo form and in complex with 8-oxo-2′-deoxyguanosine. A structural comparison between the apo form and the liganded form of the enzyme reveals a structural reorganization of the C-terminal domain upon binding of 8-oxoG, similar to that reported for human OGG1. A structural comparison of CacOgg with human OGG1, in complex with 8-oxoG containing DNA, provides a structural rationale for the lack of opposite base specificity displayed by CacOgg.     

Structural basis for the lack of opposite base specificity of Clostridium acetobutylicum 8-oxoguanine DNA glycosylase

7,8-Dihydro-8-oxoguanine (8-oxoG) is the major oxidative product of guanine and the most prevalent base lesion observed in DNA molecules. Because 8-oxoG has the capability to form a Hoogsteen pair with adenine (8-oxoG:A) in addition to a normal Watson–Crick pair with cytosine (8-oxoG:C), this lesion can lead to a G:C → T:A transversion after replication. However, 8-oxoG is recognized and excised by the 8-oxoguanine DNA glycosylase (Ogg) of the base excision repair pathway. Members of the Ogg1 family usually display a strong preference for a C opposite the lesion. In contrast, the atypical Ogg1 from Clostridium actetobutylicum (CacOgg) can excise 8-oxoG when paired with either one of the four bases, albeit with a preference for C and A. Here we describe the first high-resolution crystal structures of CacOgg in complex with duplex DNA containing the 8-oxoG lesion paired to cytosine and to adenine. A structural comparison with human OGG1 provides a rationale for the lack of opposite base specificity displayed by the bacterial Ogg.     

The type of DNA glycosylase determines the base excision repair pathway in mammalian cells.

The base excision repair (BER) of modified nucleotides is initiated by damage-specific DNA glycosylases. The repair of the resulting apurinic/apyrimidinic site involves the replacement of either a single nucleotide (short patch BER) or of several nucleotides (long patch BER). The mechanism that controls the selection of either BER pathway is unknown. We tested the hypothesis that the type of base damage present on DNA, by determining the specific DNA glycosylase in charge of its excision, drives the repair of the resulting abasic site intermediate to either BER branch. In mammalian cells hypoxanthine (HX) and 1,N6-ethenoadenine (epsilonA) are both substrates for the monofunctional 3-methyladenine DNA glycosylase, the ANPG protein, whereas 7,8-dihydro-8-oxoguanine (8-oxoG) is removed by the bifunctional DNA glycosylase/beta-lyase 8-oxoG-DNA gly- cosylase (OGG1). Circular plasmid molecules containing a single HX, epsilonA, or 8-oxoG were constructed. In vitro repair assays with HeLa cell extracts revealed that HX and epsilonA are repaired via both short and long patch BER, whereas 8-oxoG is repaired mainly via the short patch pathway. The preferential repair of 8-oxoG by short patch BER was confirmed by the low efficiency of repair of this lesion by DNA polymerase beta-deficient mouse cells as compared with their wild-type counterpart. These data fit into a model where the intrinsic properties of the DNA glycosylase that recognizes the lesion selects the branch of BER that will restore the intact DNA template.     

The capacity to remove 8-oxoG is enhanced in newborn neural stem/progenitor cells and decreases in juvenile mice and upon cell differentiation

In mammalian cells, 8-oxoguanine DNA glycosylase-1 (OGG1) is the main DNA glycosylase for the removal of 8-oxoguanine (8-oxoG). 8-oxoG, one of the most common products of the oxidative attack of DNA, is a premutagenic lesion that accumulates spontaneously at high frequencies in the genome. In this study, Ogg1 mRNA expression was detected throughout embryonic development in mice. In situ hybridization showed that in the neonatal brain, Ogg1 expression was detected in a distinct layer of cells in the medial wall of the lateral ventricle, which may correspond to ependymal cells, and in some scattered cells in the subventricular zone (SVZ), a brain region rich in neural stem/progenitor cells. Using neurospheres as a model for the study of neural stem/progenitor cells, we found that both the expression and activity of Ogg1 were high in neurospheres derived from newborn mice and decreased in adults and upon induction of cell differentiation. Furthermore, Ogg1 was shown to be the major DNA glycosylase initiating 8-oxoG repair in neurospheres. Our results strongly indicate that enhanced DNA repair capacity is an important mechanism by which neural stem/progenitor cells maintain their genome.     

The C-terminal Lysine of Ogg2 DNA Glycosylases is a Major Molecular Determinant for Guanine/8-Oxoguanine Distinction

7,8-Dihydro-8-oxoguanine (8-oxoG) is a major oxidative lesion found in DNA. The 8-oxoguanine DNA glycosylases (Ogg) responsible for the removal of 8-oxoG are divided into three families Ogg1, Ogg2 and AGOG. The Ogg2 members are devoid of the recognition loop used by Ogg1 to discriminate between 8-oxoG and guanine and it was unclear until recently how Ogg2 enzymes recognize the oxidized base. We present here the first crystallographic structure of an Ogg2 member, Methanocaldococcus janischii Ogg, in complex with a DNA duplex containing the 8-oxoG lesion. This structure highlights the crucial role of the C-terminal lysine, strictly conserved in Ogg2, in the recognition of 8-oxoG. The structure also reveals that Ogg2 undergoes a conformational change upon DNA binding similar to that observed in Ogg1 glycosylases. Furthermore, this work provides a structural rationale for the lack of opposite base specificity in this family of enzymes.     

Clostridium acetobutylicum 8-oxoguanine DNA glycosylase (Ogg) differs from eukaryotic Oggs with respect to opposite base discrimination

During repair of damaged DNA, the oxidized base 8-oxoguanine (8-oxoG) is removed by 8-oxoguanine-DNA glycosylase (Ogg) in eukaryotes and most archaea, whereas in most bacteria it is removed by formamidopyrimidine-DNA glycosylase (Fpg). We report the first characterization of a bacterial Ogg, Clostridium acetobutylicum Ogg (CacOgg). Like human OGG1 and Escherichia coli Fpg (EcoFpg), CacOgg excised 8-oxoguanine. However, unlike hOGG1 and EcoFpg, CacOgg showed little preference for the base opposite the damage during base excision and removed 8-oxoguanine from single-stranded DNA. Thus, our results showed unambiguous qualitative functional differences in vitro between CacOgg and both hOGG1 and EcoFpg. CacOgg differs in sequence from the eukaryotic enzymes at two sequence positions, M132 and F179, which align with amino acids (R154 and Y203) in human OGG1 (hOGG1) found to be involved in opposite base interaction. To address the sequence basis for functional differences with respect to opposite base interactions, we prepared three CacOgg variants, M132R, F179Y, and M132R/F179Y. All three variants showed a substantial increase in specificity for 8-oxoG.C relative to 8-oxoG.A. While we were unable to definitively associate these qualitative functional differences with differences in selective pressure between eukaryotes, Clostridia, and other bacteria, our results are consistent with the idea that evolution of Ogg function is based on kinetic control of repair.     

Down-regulation of 8-oxoguanine DNA glycosylase 1 expression in the airway epithelium ameliorates allergic lung inflammation

Allergic airway inflammation is characterized by increased expression of pro-inflammatory mediators, inflammatory cell infiltration, mucus hypersecretion, and airway hyperresponsiveness, in parallel with oxidative DNA base and strand damage, whose etiological role is not understood. Our goal was to establish the role of 8-oxoguanine (8-oxoG), a common oxidatively damaged base, and its repair by 8-oxoguanine DNA glycosylase 1 (Ogg1) in allergic airway inflammatory processes. Airway inflammation was induced by intranasally administered ragweed (Ambrosia artemisiifolia) pollen grain extract (RWPE) in sensitized BALB/c mice. We utilized siRNA technology to deplete Ogg1 from airway epithelium; 8-oxoG and DNA strand break levels were quantified by Comet assays. Inflammatory cell infiltration and epithelial methaplasia were determined histologically, mucus and cytokines levels biochemically and enhanced pause was used as the main index of airway hyperresponsiveness. Decreased Ogg1 expression and thereby 8-oxoG repair in the airway epithelium conveyed a lower inflammatory response after RWPE challenge of sensitized mice, as determined by expression of Th2 cytokines, eosinophilia, epithelial methaplasia, and airway hyperresponsiveness. In contrast, 8-oxoG repair in Ogg1-proficient airway epithelium was coupled to an increase in DNA single-strand break (SSB) levels and exacerbation of allergen challenge-dependent inflammation. Decreased expression of the Nei-like glycosylases Neil1 and Neil2 that preferentially excise ring-opened purines and 5-hydroxyuracil, respectively, did not alter the above parameters of allergic immune responses to RWPE. These results show that DNA SSBs formed during Ogg1-mediated repair of 8-oxoG augment antigen-driven allergic immune responses. A transient modulation of OGG1 expression/activity in airway epithelial cells could have clinical benefits.     

The mammalian mismatch repair pathway removes DNA 8-oxodGMP incorporated from the oxidized dNTP pool

Mismatch repair (MMR) corrects replication errors. It requires the MSH2, MSH6, MLH1, and PMS2 proteins which comprise the MutSalpha and MutLalpha heterodimers. Inactivation of MSH2 or MLH1 in human tumors greatly increases spontaneous mutation rates. Oxidation produces many detrimental DNA alterations against which cells deploy multiple protective strategies. The Ogg-1 DNA glycosylase initiates base excision repair (BER) of 8-oxoguanine (8-oxoG) from 8-oxoG:C pairs. The Myh DNA glycosylase removes mismatched adenines incorporated opposite 8-oxoG during replication. Subsequent BER generates 8-oxoG:C pairs, a substrate for excision by Ogg-1. MTH1-an 8-oxodGTPase which eliminates 8-oxodGTP from the dNTP pool-affords additional protection by minimizing 8-oxodGMP incorporation during replication. Here we show that the dNTP pool is, nevertheless, an important source of DNA 8-oxoG and that MMR provides supplementary protection by excising incorporated 8-oxodGMP. Incorporated 8-oxodGMP contributes significantly to the mutator phenotype of MMR-deficient cells. Thus, although BER of 8-oxoG is independent of Msh2, both steady-state and H(2)O(2)-induced DNA 8-oxoG levels are higher in Msh2-defective cells than in their repair-proficient counterparts. Increased expression of MTH1 in MMR-defective cells significantly reduces steady-state and H(2)O(2)-induced DNA 8-oxoG levels. This reduction dramatically diminishes the spontaneous mutation rate of Msh2(-/-) MEFs.     

Substrate specificity and reaction mechanism of murine 8-oxoguanine-DNA glycosylase

Genomic DNA is prone to oxidation by reactive oxygen species. A major product of DNA oxidation is the miscoding base 8-oxoguanine (8-oxoG). The mutagenic effects of 8-oxoG in mammalian cells are prevented by a DNA repair system consisting of 8-oxoguanine-DNA glycosylase (Ogg1), adenine-DNA glycosylase, and 8-oxo-dGTPase. We have cloned, overexpressed, and characterized mOgg1, the product of the murine ogg1 gene. mOgg1 is a DNA glycosylase/AP lyase belonging to the endonuclease III family of DNA repair enzymes. The AP lyase activity of mOgg1 is significantly lower than its glycosylase activity. mOgg1 releases 8-oxoG from DNA when paired with C, T, or G, but efficient DNA strand nicking is observed only with 8-oxoG:C. Binding of mOgg1 to oligonucleotides containing 8-oxoG:C is strong (K(D) = 51.5 nm), unlike other mispairs. The average residence time for mOgg1 bound to substrate containing 8-oxoG:C is 18.3 min; the time course for accumulation of the NaBH(4)-sensitive intermediate suggests a two-step reaction mechanism. Various analogs of 8-oxoG were tested as substrates for mOgg1. An electron-withdrawing or hydrogen bond acceptor moiety at C8 is required for efficient binding of mOgg1. A substituent at C6 and a keto group at C8 are required for cleavage. The proposed mechanism of 8-oxoG excision involves protonation of O(8) or the deoxyribose oxygen moiety.     

ATP-dependent chromatin remodeling is required for base excision repair in conventional but not in variant H2A.Bbd nucleosomes

In eukaryotes, base excision repair (BER) is responsible for the repair of oxidatively generated lesions. The mechanism of BER on naked DNA substrates has been studied in detail, but how it operates on chromatin remains unclear. Here we have studied the mechanism of BER by introducing a single 8-oxo-7,8-dihydroguanine (8-oxoG) lesion in the DNA of reconstituted positioned conventional and histone variant H2A.Bbd nucleosomes. We found that 8-oxoguanine DNA glycosylase, apurinic/apyrimidinic endonuclease, and polymerase beta activities were strongly reduced in both types of nucleosomes. In conventional nucleosomes SWI/SNF stimulated the processing of 8-oxoG by each one of the three BER repair factors to efficiencies similar to those for naked DNA. Interestingly, SWI/SNF-induced remodeling, but not mobilization of conventional nucleosomes, was required to achieve this effect. A very weak effect of SWI/SNF on the 8-oxoG BER removal in H2A.Bbd histone variant nucleosomes was observed. The possible implications of our data for the understanding of in vivo mechanisms of BER are discussed.     

Dynamic Processing of a Common Oxidative DNA Lesion by the First Two Enzymes of the Base Excision Repair Pathway

Base excision repair (BER) is the primary pathway by which eukaryotic cells resolve single base damage. One common example of single base damage is 8-oxo-7,8-dihydro-2ʹ-deoxoguanine (8-oxoG). High incidence and mutagenic potential of 8-oxoG necessitate rapid and efficient DNA repair. How BER enzymes coordinate their activities to resolve 8-oxoG damage while limiting cytotoxic BER intermediates from propagating genomic instability remains unclear. Here we use single-molecule Förster resonance energy transfer (smFRET) and ensemble-level techniques to characterize the activities and interactions of consecutive BER enzymes important for repair of 8-oxoG. In addition to characterizing the damage searching and processing mechanisms of human 8-oxoguanine glycosylase 1 (hOGG1), our data support the existence of a ternary complex between hOGG1, the damaged DNA substrate, and human AP endonuclease 1 (APE1). Our results indicate that hOGG1 is actively displaced from its abasic site containing product by protein–protein interactions with APE1 to ensure timely repair of damaged DNA.     

DNA Sequence Context Effects on the Glycosylase Activity of Human 8-Oxoguanine DNA Glycosylase

Human 8-oxoguanine DNA glycosylase (OGG1) is a key enzyme involved in removing 7,8-dihydro-8-oxoguanine (8-oxoG), a highly mutagenic DNA lesion generated by oxidative stress. The removal of 8-oxoG by OGG1 is affected by the local DNA sequence, and this feature most likely contributes to observed mutational hot spots in genomic DNA. To elucidate the influence of local DNA sequence on 8-oxoG excision activity of OGG1, we conducted steady-state, pre-steady-state, and single turnover kinetic evaluation of OGG1 in alternate DNA sequence contexts. The sequence context effect was studied for a mutational hot spot at a CpG dinucleotide. Altering either the global DNA sequence or the 5′-flanking unmodified base pair failed to influence the excision of 8-oxoG. Methylation of the cytosine 5′ to 8-oxoG also did not affect 8-oxoG excision. In contrast, a 5′-neighboring mismatch strongly decreased the rate of 8-oxoG base removal. Substituting the 5′-C in the CpG dinucleotide with T, A, or tetrahydrofuran (i.e. T:G, A:G, and tetrahydrofuran:G mispairs) resulted in a 10-, 13-, and 4-fold decrease in the rate constant for 8-oxoG excision, respectively. A greater loss in activity was observed when T:C or A:C was positioned 5′ of 8-oxoG (59- and 108-fold, respectively). These results indicate that neighboring structural abnormalities 5′ to 8-oxoG deter its repair thereby enhancing its mutagenic potential.     

Immunofluorescent localization of the murine 8-oxoguanine DNA glycosylase (mOGG1) in cells growing under normal and nutrient deprivation conditions

OGG1 is a major DNA glycosylase in mammalian cells, participating in the repair of 7,8-dihydro-8-oxoguanine (8-oxoguanine, 8-oxoG), the most abundant known DNA lesion induced by endogenous reactive oxygen species in aerobic organisms. 8-oxoG is therefore often used as a marker for oxidative DNA damage. In this study, polyclonal and monoclonal antibodies were raised against the purified wild-type recombinant murine 8-oxoG DNA glycosylase (mOGG1) protein and their specificity against the native enzyme and the SDS-denatured mOGG1 polypeptide were characterized. Specific antibodies directed against the purified wild-type recombinant mOGG1 were used to localize in situ this DNA repair enzyme in established cell lines (HeLa cells, NIH3T3 fibroblasts) as well as in primary culture mouse embryo fibroblasts growing under either normal or oxidative stress conditions. Results from these studies showed that mOGG1 is localized to the nucleus and the cytoplasm of mammalian cells in culture. However, mOGG1 levels increase and primarily redistribute to the nucleus and its peripheral cytoplasm in cells exposed to oxidative stress conditions. Immunofluorescent localization results reported in this study suggest that susceptibility to oxidative DNA damage varies among mammalian tissue culture cells and that mOGG1 appears to redistribute once mOGG1 cell copy number increases in response to oxidative DNA damage.     

Stimulation of DNA glycosylase activity of OGG1 by NEIL1: functional collaboration between two human DNA glycosylases

The eukaryotic 8-oxoguanine-DNA glycosylase 1 (OGG1) provides the major activity for repairing mutagenic 7,8-dihydro-8-oxoguanine (8-oxoG) induced in the genome due to oxidative stress. Earlier in vitro studies showed that, after excising the base lesion, the human OGG1 remains bound to the resulting abasic (AP) site in DNA and does not turn over efficiently. The human AP-endonuclease (APE1), which cleaves the phosphodiester bond 5' to the AP site, in the next step of repair, displaces the bound OGG1 and thus increases its turnover. Here we show that NEIL1, a DNA glycosylase/AP lyase specific for many oxidized bases but with weak 8-oxoG excision activity, stimulates turnover of OGG1 in a fashion similar to that of APE1 and carries out betadelta-elimination at the AP site. This novel collaboration of two DNA glycosylases, which do not stably interact with each other, in stimulating 8-oxoguanine repair is possible because of higher AP site affinity and stronger AP lyase activity of NEIL1 relative to OGG1. Comparable levels of NEIL1 and OGG1 in some human cells raise the possibility that NEIL1 serves as a backup enzyme to APE1 in stimulating 8-oxoG repair in vivo.     

Marathon running alters the DNA base excision repair in human skeletal muscle

Reactive oxygen and nitrogen species generated either as products of aerobic metabolism or as a consequence of environmental mutagens, oxidatively modify DNA. Formamidopyrimidine-DNA glycosylase (Fpg) and endonuclease III (endo III) or their functional mammalian homologues repair 7,8-dihydro-8-oxoguanine (8-oxoG) and damaged pyrimidines, respectively, to curb the deleterious effects of oxidative DNA alterations. A single bout of physical exercise can induce oxidative DNA damage. However, its effect on the activity of repair enzymes is not known. Here we report that the activity of a functional homolog of Fpg, human 8-oxoG DNA glycosylase (hOGG1), is increased significantly, as measured by the excision of 32P labeled damaged oligonucleotide, in human skeletal muscle after a marathon race. The AP site repair enzyme did not change significantly. Despite the large individual differences among the six subjects measured, data suggest that a single-bout of aerobic exercise increases the activity of hOGG1 which is responsible for the excision of 8-oxoG. The up-regulation of DNA repair enzymes might be an important part of the regular exercise induced adaptation process.     

The DNA glycosylase Ogg1 defends against oxidant-induced mtDNA damage and apoptosis in pulmonary artery endothelial cells

Emerging evidence suggests that mitochondrial (mt) DNA damage may be a trigger for apoptosis in oxidant-challenged pulmonary artery endothelial cells (PAECs). Understanding the rate-limiting determinants of mtDNA repair may point to new targets for intervention in acute lung injury. The base excision repair (BER) pathway is the only pathway for oxidative damage repair in mtDNA. One of the key BER enzymes is Ogg1, which excises the base oxidation product 8-oxoguanine. Previously we demonstrated that overexpression of mitochondrially targeted Ogg1 in PAECs attenuated apoptosis induced by xanthine oxidase (XO) treatment. To test the idea that Ogg1 is a potentially rate-limiting BER determinant protecting cells from oxidant-mediated death, PAECs transfected with siRNA to Ogg1 were challenged with XO and the extent of mitochondrial and nuclear DNA damage was determined along with indices of apoptosis. Transfected cells demonstrated significantly reduced Ogg1 activity, which was accompanied by delayed repair of XO-induced mtDNA damage and linked to increased XO-mediated apoptosis. The nuclear genome was undamaged by XO in either control PAECs or cells depleted of Ogg1. These observations suggest that Ogg1 plays a critical and possibly rate-limiting role in defending PAECs from oxidant-induced apoptosis by limiting the persistence of oxidative damage in the mitochondrial genome.     

Role of mitochondrial hOGG1 and aconitase in oxidant-induced lung epithelial cell apoptosis

8-Oxoguanine DNA glycosylase (Ogg1) repairs 8-oxo-7,8-dihydroxyguanine (8-oxoG), one of the most abundant DNA adducts caused by oxidative stress. In the mitochondria, Ogg1 is thought to prevent activation of the intrinsic apoptotic pathway in response to oxidative stress by augmenting DNA repair. However, the predominance of the β-Ogg1 isoform, which lacks 8-oxoG DNA glycosylase activity, suggests that mitochondrial Ogg1 functions in a role independent of DNA repair. We report here that overexpression of mitochondria-targeted human α-hOgg1 (mt-hOgg1) in human lung adenocarcinoma cells with some alveolar epithelial cell characteristics (A549 cells) prevents oxidant-induced mitochondrial dysfunction and apoptosis by preserving mitochondrial aconitase. Importantly, mitochondrial α-hOgg1 mutants lacking 8-oxoG DNA repair activity were as effective as wild-type mt-hOgg1 in preventing oxidant-induced caspase-9 activation, reductions in mitochondrial aconitase, and apoptosis, suggesting that the protective effects of mt-hOgg1 occur independent of DNA repair. Notably, wild-type and mutant mt-hOgg1 coprecipitate with mitochondrial aconitase. Furthermore, overexpression of mitochondrial aconitase abolishes oxidant-induced apoptosis whereas hOgg1 silencing using shRNA reduces mitochondrial aconitase and augments apoptosis. These findings suggest a novel mechanism that mt-hOgg1 acts as a mitochondrial aconitase chaperone protein to prevent oxidant-mediated mitochondrial dysfunction and apoptosis that might be important in the molecular events underlying oxidant-induced toxicity.     

Single molecule analysis indicates stimulation of MUTYH by UV-DDB through enzyme turnover

The oxidative base damage, 8-oxo-7,8-dihydroguanine (8-oxoG) is a highly mutagenic lesion because replicative DNA polymerases insert adenine (A) opposite 8-oxoG. In mammalian cells, the removal of A incorporated across from 8-oxoG is mediated by the glycosylase MUTYH during base excision repair (BER). After A excision, MUTYH binds avidly to the abasic site and is thus product inhibited. We have previously reported that UV-DDB plays a non-canonical role in BER during the removal of 8-oxoG by 8-oxoG glycosylase, OGG1 and presented preliminary data that UV-DDB can also increase MUTYH activity. In this present study we examine the mechanism of how UV-DDB stimulates MUTYH. Bulk kinetic assays show that UV-DDB can stimulate the turnover rate of MUTYH excision of A across from 8-oxoG by 4–5-fold. Electrophoretic mobility shift assays and atomic force microscopy suggest transient complex formation between MUTYH and UV-DDB, which displaces MUTYH from abasic sites. Using single molecule fluorescence analysis of MUTYH bound to abasic sites, we show that UV-DDB interacts directly with MUTYH and increases the mobility and dissociation rate of MUTYH. UV-DDB decreases MUTYH half-life on abasic sites in DNA from 8800 to 590 seconds. Together these data suggest that UV-DDB facilitates productive turnover of MUTYH at abasic sites during 8-oxoG:A repair.