Effect of single mutations in the OGG1 gene found in human tumors on the substrate specificity of the Ogg1 protein

OGG1: An emerging multifunctional therapeutic target for the treatment of diseases caused by oxidative DNA damage

Oxidative DNA damage-related diseases, such as incurable inflammation, malignant tumors, and age-related disorders, present significant challenges in modern medicine due to their complex molecular mechanisms and limitations in identifying effective treatment targets. Recently, 8-oxoguanine DNA glycosylase 1 (OGG1) has emerged as a promising multifunctional therapeutic target for the treatment of these challenging diseases. In this review, we systematically summarize the multiple functions and mechanisms of OGG1, including pro-inflammatory, tumorigenic, and aging regulatory mechanisms. We also highlight the potential of OGG1 inhibitors and activators as potent therapeutic agents for the aforementioned life-limiting diseases. We conclude that OGG1 serves as a multifunctional hub; the inhibition of OGG1 may provide a novel approach for preventing and treating inflammation and cancer, and the activation of OGG1 could be a strategy for preventing age-related disorders. Furthermore, we provide an extensive overview of successful applications of OGG1 regulation in treating inflammatory, cancerous, and aging-related diseases. Finally, we discuss the current challenges and future directions of OGG1 as an emerging multifunctional therapeutic marker for the aforementioned challenging diseases. The aim of this review is to provide a robust reference for scientific researchers and clinical drug developers in the development of novel clinical targeted drugs for life-limiting diseases, especially for incurable inflammation, malignant tumors, and age-related disorders.

The potential for OGG1 inhibition to be a therapeutic strategy for pulmonary diseases

INTRODUCTION

Pulmonary diseases impose a daunting burden on healthcare systems and societies. Current treatment approaches primarily address symptoms, underscoring the urgency for the development of innovative pharmaceutical solutions. A noteworthy focus lies in targeting enzymes recognizing oxidatively modified DNA bases within gene regulatory elements, given their pivotal role in governing gene expression.

AREAS COVERED

This review delves into the intricate interplay between the substrate-specific binding of 8-oxoguanine DNA glycosylase 1 (OGG1) and epigenetic regulation, with a focal point on elucidating the molecular underpinnings and their biological implications. The absence of OGG1 distinctly attenuates the binding of transcription factors to cis elements, thereby modulating pro-inflammatory or pro-fibrotic transcriptional activity. Through a synergy of experimental insights gained from cell culture studies and murine models, utilizing prototype OGG1 inhibitors (O8, TH5487, and SU0268), a promising panorama emerges. These investigations underscore the absence of cytotoxicity and the establishment of a favorable tolerance profile for these OGG1 inhibitors.

EXPERT OPINION

Thus, the strategic targeting of the active site pocket of OGG1 through the application of small molecules introduces an innovative trajectory for advancing redox medicine. This approach holds particular significance in the context of pulmonary diseases, offering a refined avenue for their management.

Strong Cumulative Evidence of Associations of 6 Single Nucleotide Polymorphisms with Ovarian Cancer Risk: An Umbrella Review

Background: An increasing number of studies have reported associations between single nucleotide polymorphisms (SNPs) and ovarian cancer (OC) risk. However, some of the findings were inconsistent. The objective of this umbrella review was to evaluate the associations comprehensively and quantitatively. Methods: The protocol of this review was registered in PROSPERO (No. CRD42022332222). We searched the PubMed, Web of Science, and Embase databases to identify related systematic reviews and meta-analyses from inception to 15 October 2021. In addition to estimating the summary effect size by using fixed and random effects models and calculating the 95% prediction interval, we evaluated the cumulative evidence for associations with nominally statistical significance based on the Venice criteria and false positive report probability (FPRP). Results: Forty articles were included in this umbrella review, which referred to a total of 54 SNPs. The median number of original studies per meta-analysis was four, while the median number of total subjects was 3455. All included articles had greater than moderate methodological quality. A total of 18 SNPs were nominally statistically associated with OC risk; 6 SNPs (8 genetic models), 5 SNPs (7 genetic models), and 16 SNPs (25 genetic models) were identified as strong, moderate, and weak cumulative evidence, respectively. Conclusion: This umbrella review revealed associations between SNPs and OC risk and suggested strong cumulative evidence of associations of six SNPs (eight genetic models) with OC risk.

Inherited MUTYH mutations cause elevated somatic mutation rates and distinctive mutational signatures in normal human cells

Cellular DNA damage caused by reactive oxygen species is repaired by the base excision repair (BER) pathway which includes the DNA glycosylase MUTYH. Inherited biallelic MUTYH mutations cause predisposition to colorectal adenomas and carcinoma. However, the mechanistic progression from germline MUTYH mutations to MUTYH-Associated Polyposis (MAP) is incompletely understood. Here, we sequence normal tissue DNAs from 10 individuals with MAP. Somatic base substitution mutation rates in intestinal epithelial cells were elevated 2 to 4-fold in all individuals, except for one showing a 31-fold increase, and were also increased in other tissues. The increased mutation burdens were of multiple mutational signatures characterised by C > A changes. Different mutation rates and signatures between individuals are likely due to different MUTYH mutations or additional inherited mutations in other BER pathway genes. The elevated base substitution rate in normal cells likely accounts for the predisposition to neoplasia in MAP. Despite ubiquitously elevated mutation rates, individuals with MAP do not display overt evidence of premature ageing. Thus, accumulation of somatic mutations may not be sufficient to cause the global organismal functional decline of ageing.

Protocols to Measure Oxidative Stress and DNA Damage in Asthma

Asthma is associated with oxidative stress and oxidative damage of biomolecules, including DNA. Here, we describe the protocols to quantify reactive oxygen species (ROS) and oxidative stress markers in a mouse model of allergic airway inflammation. We also provide detailed methods to measure DNA damage by long-run real-time PCR for DNA-damage quantification (LORD-Q) assay and gene-specific DNA damage analyses by long amplicon (LA)-qPCR. Additionally, we describe methods to quantify oxidized DNA base lesions in lung genomic DNA by mass spectrometry, and to measure enzymatic activity of 8-oxoguanine DNA glycosylase (OGG1). Using these methods, the levels of oxidative stress and DNA damage in allergic inflammation and asthma can be elucidated.

Significance of base excision repair to human health

Oxidative and alkylating DNA damage occurs under normal physiological conditions and exogenous exposure to DNA damaging agents. To counteract DNA base damage, cells have evolved several defense mechanisms that act at different levels to prevent or repair DNA base damage. Cells combat genomic lesions like these including base modifications, abasic sites, as well as single-strand breaks, via the base excision repair (BER) pathway. In general, the core BER process involves well-coordinated five-step reactions to correct DNA base damage. In this review, we will uncover the current understanding of BER mechanisms to maintain genomic stability and the biological consequences of its failure due to repair gene mutations. The malfunction of BER can often lead to BER intermediate accumulation, which is genotoxic and can lead to different types of human disease. Finally, we will address the use of BER intermediates for targeted cancer therapy.

Chromatin and other obstacles to base excision repair: potential roles in carcinogenesis

DNA is comprised of chemically reactive nucleobases that exist under a constant barrage from damaging agents. Failure to repair chemical modifications to these nucleobases can result in mutations that can cause various diseases, including cancer. Fortunately, the base excision repair (BER) pathway can repair modified nucleobases and prevent these deleterious mutations. However, this pathway can be hindered through several mechanisms. For instance, mutations to the enzymes in the BER pathway have been identified in cancers. Biochemical characterisation of these mutants has elucidated various mechanisms that inhibit their activity. Furthermore, the packaging of DNA into chromatin poses another obstacle to the ability of BER enzymes to function properly. Investigations of BER in the base unit of chromatin, the nucleosome core particle (NCP), have revealed that the NCP acts as a complex substrate for BER enzymes. The constituent proteins of the NCP, the histones, also have variants that can further impact the structure of the NCP and may modulate access of enzymes to the packaged DNA. These histone variants have also displayed significant clinical effects both in carcinogenesis and patient prognosis. This review focuses on the underlying molecular mechanisms that present obstacles to BER and the relationship of these obstacles to cancer. In addition, several chemotherapeutics induce DNA damage that can be repaired by the BER pathway and understanding obstacles to BER can inform how resistance and/or sensitivity to these therapies may occur. With the understanding of these molecular mechanisms, current chemotherapeutic treatment regiments may be improved, and future therapies developed.

Inhibition by Tetrahydroquinoline Sulfonamide Derivatives of the Activity of Human 8-Oxoguanine DNA Glycosylase (OGG1) for Several Products of Oxidatively induced DNA Base Lesions

DNA glycosylases involved in the first step of the DNA base excision repair pathway are promising targets in cancer therapy. There is evidence that reduction of their activities may enhance cell killing in malignant tumors. Recently, two tetrahydroquinoline compounds named SU0268 and SU0383 were reported to inhibit OGG1 for the excision of 8-hydroxyguanine. This DNA repair protein is one of the major cellular enzymes responsible for excision of a number of oxidatively induced lesions from DNA. In this work, we used gas chromatography-tandem mass spectrometry with isotope-dilution to measure the excision of not only 8-hydroxyguanine but also that of the other major substrate of OGG1, i.e., 2,6-diamino-4-hydroxy-5-formamidopyrimidine, using genomic DNA with multiple purine- and pyrimidine-derived lesions. The excision of a minor substrate 4,6-diamino-5-formamidopyrimidine was also measured. Both SU0268 and SU0383 efficiently inhibited OGG1 activity for these three lesions, with the former being more potent than the latter. Dependence of inhibition on concentrations of SU0268 and SU0383 from 0.05 μmol/L to 10 μmol/L was also demonstrated. The approach used in this work may be applied to the investigation of OGG1 inhibition by SU0268 and SU0383 and other small molecule inhibitors in further studies including cellular and animal models of disease.

Association of the hOGG1 Ser326Cys polymorphism with gynecologic cancer susceptibility: a meta-analysis

The association between the hOGG1 Ser326Cys polymorphism and gynecologic cancer susceptibility is inconclusive. We performed a comprehensive meta-analysis to precisely estimate of the impact of the hOGG1 Ser326Cys polymorphism on gynecologic cancer susceptibility. Electronic databases including PubMed, Embase, WanFang, and the China National Knowledge Infrastructure were searched for relevant studies. Odds ratios (ORs) with corresponding 95% confidence intervals (CIs) were determined to assess the strength of the association. Fourteen studies with 2712 cases and 3638 controls were included in the final meta-analysis. The pooled analysis yielded a significant association between the hOGG1 Ser326Cys polymorphism and overall gynecologic cancer susceptibility (dominant model: OR = 1.16, 95% CI = 1.03–1.30, P=0.017). A significantly higher gynecologic cancer risk was found for the European population (homozygous model: OR = 2.17, 95% CI = 1.80–2.61, P<0.001; recessive model: OR = 2.11, 95% CI = 1.41–3.17, P<0.001; dominant model: OR = 1.29, 95% CI = 1.12–1.48, P<0.001; and allele model: OR = 1.40, 95% CI = 1.13–1.74, P=0.002), but not in the Asian population. The stratified analysis by cancer type revealed endometrial cancer was significantly associated with the hOGG1 Ser326Cys polymorphism (dominant model: OR = 1.29, 95% CI = 1.09–1.54, P=0.003; and allele model: OR = 1.28, 95% CI = 1.02–1.60, P=0.031). In conclusion, the hOGG1 Ser326Cys polymorphism was associated with higher overall gynecologic cancer susceptibility, especially for endometrial cancer in the European population.

The correlation of combined OGG1, CYP1A1 and GSTP1 gene variants and risk of lung cancer of male Iraqi waterpipe tobacco smokers

Genetic polymorphisms of genes whose products are responsible for activities, such as xenobiotic metabolism, mutagen detoxification and DNA-repair, have been predicted to be associated with the risk of developing lung cancer (LC). The association of LC with tobacco smoking has been extensively investigated, but no studies have focused on the Arab ethnicity. Previously, we examined the association between genetic polymorphisms among Phase I and Phase II metabolism genes and the risk of LC. Here, we extend the data by examining the correlation of OGG1 Ser326Cys combined with CYP1A1 (Ile462Val and MspI) and GSTP1 (Ile105Val and Ala103Val) polymorphisms with the risk of LC. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and gene sequencing were carried out for genotyping the OGG1 polymorphisms of 123 LC patients and 129 controls. No significant differences in the frequencies of the OGG1 mutant allele between patients and controls were found. The distributions of heterozygous Ser/Cys or Cys/Cys genotypes of OGG1 were not associated with the risk of LC either according to the histological types of LC or based on waterpipe tobacco (WP) smoking status. In contrast, the combined effect of OGG1 variants with CYP1A1 and GSTP1 variants revealed a significant correlation with the OGG1 Ser326Cys-CYP1A1 MspI variants pairing. This association was significant (p = 0.001) in individuals who carried homozygous or heterozygous variant type genotypes of both genes in a reference with carriers of both wild-type genotypes (wt/wt - wt/wt). The odds ratios were 2.99 (95% CI 1.67-5.36), 2.68 (95% CI 1.08-6.62), and 2.80 (95% CI 1.18-6.69) for those who carried (wt/wt - wt/vt + vt/vt), (wt/vt + vt/vt - wt/wt), and (wt/vt + vt/vt - wt/vt + vt/vt), respectively. The study suggests a limited correlation is present between carrying OGG1 Ser326Cys polymorphism and the risk of developing LC in Arab populations.

Enzymatically inactive OGG1 binds to DNA and steers base excision repair toward gene transcription

8‐Oxoguanine DNA glycosylase1 (OGG1)‐initiated base excision repair (BER) is the primary pathway to remove the pre‐mutagenic 8‐oxo‐7,8‐dihydroguanine (8‐oxoG) from DNA. Recent studies documented 8‐oxoG serves as an epigenetic‐like mark and OGG1 modulates gene expression in oxidatively stressed cells. For this new role of OGG1, two distinct mechanisms have been proposed: one is coupled to base excision, while the other only requires substrate binding of OGG1––both resulting in conformational adjustment in the adjacent DNA sequences providing access for transcription factors to their cis‐elements. The present study aimed to examine if BER activity of OGG1 is required for pro‐inflammatory gene expression. To this end, Ogg1/OGG1 knockout/depleted cells were transfected with constructs expressing wild‐type (wt) and repair‐deficient mutants of OGG1. OGG1's promoter enrichment, oxidative state, and gene expression were examined. Results showed that TNFα exposure increased levels of oxidatively modified cysteine(s) of wt OGG1 without impairing its association with promoter and facilitated gene expression. The excision deficient K249Q mutant was even a more potent activator of gene expression; whereas, mutant OGG1 with impaired substrate recognition/binding was not. These data suggested the interaction of OGG1 with its substrate at regulatory regions followed by conformational adjustment in the adjacent DNA is the primary mode to modulate inflammatory gene expression.

OGG1 deficiency alters the intestinal microbiome and increases intestinal inflammation in a mouse model

OGG1-deficient (Ogg1^-/-) animals display increased propensity to age-induced and diet-induced metabolic diseases, including insulin resistance and fatty liver. Since the intestinal microbiome is increasingly understood to play a role in modulating host metabolic responses, we examined gut microbial composition in Ogg1^-/- mice subjected to different nutritional challenges. Interestingly, Ogg1^-/- mice had a markedly altered intestinal microbiome under both control-fed and hypercaloric diet conditions. Several microbial species that were increased in Ogg1^-/- animals were associated with increased energy harvest, consistent with their propensity to high-fat diet induced weight gain. In addition, several pro-inflammatory microbes were increased in Ogg1^-/- mice. Consistent with this observation, Ogg1^-/- mice were significantly more sensitive to intestinal inflammation induced by acute exposure to dextran sulfate sodium. Taken together, these data indicate that in addition to their proclivity to obesity and metabolic disease, Ogg1^-/- mice are prone to colonic inflammation. Further, these data point to alterations in the intestinal microbiome as potential mediators of the metabolic and intestinal inflammatory response in Ogg1^-/- mice.

The hOGG1 Ser326Cys polymorphism and esophageal cancer risk: a meta-analysis of 1,875 cancer cases and 3,041 controls

Background

Recently, there have been several studies that have looked at the association between hOGG1 Ser326Cys polymorphism and esophageal cancer (EC) risk. However, the results of previous reports remain controversial and ambiguous. Thus, we performed a meta-analysis to explore more precisely the association between hOGG1 Ser326Cys polymorphism and the risk of EC.

Methods

A meta-analysis was performed to examine the association between hOGG1 Ser326Cys polymorphism and EC risk. Odds ratio (OR) and its 95% confidence interval (CI) were used for statistical analysis.

Results

Our publication search identified a total of 9 studies with 1,875 cases and 3,041 controls. There was no significant associations in all genetic models between hOGG1 Ser326Cys polymorphism and EC observed (OR =1.024, 95% CI: 0.932-1.125 for Cys vs. Ser, P=0.624; OR =1.126, 95% CI: 0.901-1.408 for Cys/Cys vs. Ser/Ser, P=0.296; OR = 0.961, 95% CI: 0.844-1.093 for Ser/Cys vs. Ser/Ser, P =0.540; OR =0.989, 95% CI: 0.874-1.118 for Cys/Cys + Ser/Cys vs. Ser/Ser, P=0.855; OR =1.165, 95% CI: 0.945-1.436 for Cys/Cys vs. Ser/Cys + Ser/Ser, P=0.153). Also, in the stratified analyses by ethnicity and cancer type, no significant association was observed.

Conclusions

This meta-analysis on hOGG1 Ser326Cys polymorphism and the risk of EC suggests there is no statistically significant association between the two. Additional primary studies may be necessary to provide evidence of any significant association between this specific polymorphism and EC.

Thermostability and excision activity of polymorphic forms of hOGG1

Objectives

Reactive oxygen species (ROS) oxidize guanine residues in DNA to form 7,8-dihydro-oxo-2′-deoxyguanosine (8oxoG) lesions in the genome. Human 8-oxoguanine glycosylase-1 (hOGG1) recognizes and excises this highly mutagenic species when it is base-paired opposite a cytosine. We sought to characterize biochemically several hOGG1 variants that have been found in cancer tissues and cell lines, reasoning that if these variants have reduced repair capabilities, they could lead to an increased chance of mutagenesis and carcinogenesis.

Results

We have over-expressed and purified the R46Q, A85S, R154H, and S232T hOGG1 variants and have investigated their repair efficiency and thermostability. The hOGG1 variants showed only minor perturbations in the kinetics of 8oxoG excision relative to wild-type hOGG1. Thermal denaturation monitored by circular dichroism revealed that R46Q hOGG1 had a significantly lower T_m (36.6 °C) compared to the other hOGG1 variants (40.9 °C to 43.2 °C). Prolonged pre-incubation at 37 °C prior to the glycosylase assay dramatically reduces the excision activity of R46Q hOGG1, has a modest effect on wild-type hOGG1, and a negligible effect on A85S, R154H, and S232T hOGG1. The observed thermolability of hOGG1 variants was mostly alleviated by co-incubation with stoichiometric amounts of competitor DNA.

Electronic supplementary material

The online version of this article (10.1186/s13104-019-4111-9) contains supplementary material, which is available to authorized users.

Abasic and oxidized ribonucleotides embedded in DNA are processed by human APE1 and not by RNase H2

Ribonucleoside 5′-monophosphates (rNMPs) are the most common non-standard nucleotides found in DNA of eukaryotic cells, with over 100 million rNMPs transiently incorporated in the mammalian genome per cell cycle. Human ribonuclease (RNase) H2 is the principal enzyme able to cleave rNMPs in DNA. Whether RNase H2 may process abasic or oxidized rNMPs incorporated in DNA is unknown. The base excision repair (BER) pathway is mainly responsible for repairing oxidized and abasic sites into DNA. Here we show that human RNase H2 is unable to process an abasic rNMP (rAP site) or a ribose 8oxoG (r8oxoG) site embedded in DNA. On the contrary, we found that recombinant purified human apurinic/apyrimidinic endonuclease-1 (APE1) and APE1 from human cell extracts efficiently process an rAP site in DNA and have weak endoribonuclease and 3′-exonuclease activities on r8oxoG substrate. Using biochemical assays, our results provide evidence of a human enzyme able to recognize and process abasic and oxidized ribonucleotides embedded in DNA.

Mutation-promoting molecular networks of uncontrolled inflammation

More and more studies show that chronic inflammation can lead to tumor formation. The complex interactions of inflammatory cells, stroma and tumor parenchymal cell are closely related to tumor formation. Under the state of chronic inflammatory microenvironment, long-term interaction of inflammatory cells and stromal cells as well as the parenchymal cells makes signaling pathway in parenchyma cells disordered. A series of gene level editor modification, epigenetic changes, and the regulation of transcription and translation changes will happen based on signaling pathway disorder. The changes ultimately lead to cell mutations and phenotypic transformation occurred. Recent findings provide an objective basis for cancer treatment and prevention. However, further discusses at the core of the possible molecular in tumor formation provide a theoretical foundation for future study of the pathogenesis and molecular targeted therapy of cancer. This review summarizes the research in the field of chronic inflammation and cancer in recent years, and analyze the molecules network in the process of the carcinogenic inflammation comprehensively. Beyond that, this review intends to describe possible carcinogenic inflammation core molecular and provides a theoretical basis for future study of the pathogenesis, chemoprevention and molecular targeted therapy of cancer.

Single nucleotide polymorphisms in DNA glycosylases: From function to disease

Oxidative stress is associated with a growing number of diseases that span from cancer to neurodegeneration. Most oxidatively induced DNA base lesions are repaired by the base excision repair (BER) pathway which involves the action of various DNA glycosylases. There are numerous genome wide studies attempting to associate single-nucleotide polymorphisms (SNPs) with predispositions to various types of disease; often, these common variants do not have significant alterations in their biochemical function and do not exhibit a convincing phenotype. Nevertheless several lines of evidence indicate that SNPs in DNA repair genes may modulate DNA repair capacity and contribute to risk of disease. This overview provides a convincing picture that SNPs of DNA glycosylases that remove oxidatively generated DNA lesions are susceptibility factors for a wide disease spectrum that includes besides cancer (particularly lung, breast and gastrointestinal tract), cochlear/ocular disorders, myocardial infarction and neurodegenerative disorders which can be all grouped under the umbrella of oxidative stress-related pathologies.

Repair of 8-oxo-7,8-dihydroguanine in prokaryotic and eukaryotic cells: Properties and biological roles of the Fpg and OGG1 DNA N-glycosylases

Oxidatively damaged DNA results from the attack of sugar and base moieties by reactive oxygen species (ROS), which are formed as byproducts of normal cell metabolism and during exposure to endogenous or exogenous chemical or physical agents. Guanine, having the lowest redox potential, is the DNA base the most susceptible to oxidation, yielding products such as 8-oxo-7,8-dihydroguanine (8-oxoG) and 2-6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG). In DNA, 8-oxoG was shown to be mutagenic yielding GC to TA transversions upon incorporation of dAMP opposite this lesion by replicative DNA polymerases. In prokaryotic and eukaryotic cells, 8-oxoG is primarily repaired by the base excision repair pathway (BER) initiated by a DNA N-glycosylase, Fpg and OGG1, respectively. In Escherichia coli, Fpg cooperates with MutY and MutT to prevent 8-oxoG-induced mutations, the "GO-repair system". In Saccharomyces cerevisiae, OGG1 cooperates with nucleotide excision repair (NER), mismatch repair (MMR), post-replication repair (PRR) and DNA polymerase η to prevent mutagenesis. Human and mouse cells mobilize all these pathways using OGG1, MUTYH (MutY-homolog also known as MYH), MTH1 (MutT-homolog also known as NUDT1), NER, MMR, NEILs and DNA polymerases η and λ, to prevent 8-oxoG-induced mutations. In fact, mice deficient in both OGG1 and MUTYH develop cancer in different organs at adult age, which points to the critical impact of 8-oxoG repair on genetic stability in mammals. In this review, we will focus on Fpg and OGG1 proteins, their biochemical and structural properties as well as their biological roles. Other DNA N-glycosylases able to release 8-oxoG from damaged DNA in various organisms will be discussed. Finally, we will report on the role of OGG1 in human disease and the possible use of 8-oxoG DNA N-glycosylases as therapeutic targets.

Repair of oxidatively induced DNA damage by DNA glycosylases: Mechanisms of action, substrate specificities and excision kinetics

Endogenous and exogenous reactive species cause oxidatively induced DNA damage in living organisms by a variety of mechanisms. As a result, a plethora of mutagenic and/or cytotoxic products are formed in cellular DNA. This type of DNA damage is repaired by base excision repair, although nucleotide excision repair also plays a limited role. DNA glycosylases remove modified DNA bases from DNA by hydrolyzing the glycosidic bond leaving behind an apurinic/apyrimidinic (AP) site. Some of them also possess an accompanying AP-lyase activity that cleaves the sugar-phosphate chain of DNA. Since the first discovery of a DNA glycosylase, many studies have elucidated the mechanisms of action, substrate specificities and excision kinetics of these enzymes present in all living organisms. For this purpose, most studies used single- or double-stranded oligodeoxynucleotides with a single DNA lesion embedded at a defined position. High-molecular weight DNA with multiple base lesions has been used in other studies with the advantage of the simultaneous investigation of many DNA base lesions as substrates. Differences between the substrate specificities and excision kinetics of DNA glycosylases have been found when these two different substrates were used. Some DNA glycosylases possess varying substrate specificities for either purine-derived lesions or pyrimidine-derived lesions, whereas others exhibit cross-activity for both types of lesions. Laboratory animals with knockouts of the genes of DNA glycosylases have also been used to provide unequivocal evidence for the substrates, which had previously been found in in vitro studies, to be the actual substrates in vivo as well. On the basis of the knowledge gained from the past studies, efforts are being made to discover small molecule inhibitors of DNA glycosylases that may be used as potential drugs in cancer therapy.

Drosophila Ogg1 is required to suppress 8-oxo-guanine accumulation following oxidative stress

Reactive oxygen species (ROS) generated during energy production processes are a major cause of oxidative DNA damage. A DNA glycosylase encoded by the Ogg1 gene removes oxidized guanine bases and is widely conserved. However, the biological role of the gene in individual organisms has not yet been characterized in Drosophila, which is a suitable model to study the influence of oxidative damage on senescence. Here, we performed a genetic analysis to confirm that Ogg1 plays an essential role in the removal of 8-oxo-guanines from nuclei. We first confirmed by quantitative real-time PCR that Ogg1 mRNA expression was reduced by 30-55% in Ogg1 mutants and in flies expressing inducible Ogg1 dsRNA compared to control flies. We then showed that additional accumulation of 8-oxo-guanines occurred in the nuclei of epithelial midgut cells after paraquat feeding in flies with downregulated Ogg1 expression. We confirmed that a transposon possessing the UAS sequence was integrated in the 5'-UTR of the Ogg1 alleles and that it is oriented in the same transcriptional direction as the gene. Using the Gal4/UAS system, which enables us to induce ectopic expression in Drosophila, we induced overexpression of Ogg1 by 40-fold. We observed a lower amount of 8-oxo-guanine in the midgut epithelial cells of adults overexpressing Ogg1. These genetic data strongly suggest that the Drosophila Ogg1 ortholog CG1795 plays an essential role in the suppression of 8-oxo-guanines, consistent with its role in other organisms. Although adult flies with reduced Ogg1 expression failed to show elevated sensitivity to paraquat, those with Ogg1 overexpression showed resistance to oxidative stress by paraquat feeding and had a significantly longer lifespan in normal feeding conditions. These observations are consistent with the hypothesis that oxidative DNA damage by ROS accumulation is a major contributor to senescence.

In Vitro Fluorogenic Real-Time Assay of the Repair of Oxidative DNA Damage

The repair of oxidative damage to DNA is essential to avoid mutations that lead to cancer. Oxidized DNA bases, such as 8-oxoguanine, are a main source of these mutations, and the enzyme 8-oxoguanine glycosylase 1 (OGG1) is the chief human enzyme that excises 8-oxoguanine from DNA. The activity of OGG1 has been linked to human inflammation responses and to cancer, and researchers are beginning to search for inhibitors of the enzyme. However, measuring the activity of the enzyme typically requires laborious gel-based measurements of radiolabeled DNAs. Here we report the design and properties of fluorogenic probes that directly report on the activity of OGG1 (and its bacterial homologue Fpg) in real time as the oxidized base is excised. The probes are short, modified DNA oligomers containing fluorescent DNA bases and are designed to utilize 8-oxoguanine itself as a fluorescence quencher. Screening of combinations of fluorophores and 8-oxoguanine revealed two fluorophores, pyrene and tCo, that are strongly quenched by the damaged base. We tested 42 potential probes containing these fluorophores: the optimum probe, OGR1, yields a 60-fold light-up signal in vitro with OGG1 and Fpg. It can report on oxidative repair activity in mammalian cell lysate and with bacterial cells overexpressing a repair enzyme. Such probes might prove useful in quantifying enzyme activity and performing competitive inhibition assays.

Production, Purification, and Characterization of ¹⁵N-Labeled DNA Repair Proteins as Internal Standards for Mass Spectrometric Measurements

Oxidatively induced DNA damage is caused in living organisms by a variety of damaging agents, resulting in the formation of a multiplicity of lesions, which are mutagenic and cytotoxic. Unless repaired by DNA repair mechanisms before DNA replication, DNA lesions can lead to genomic instability, which is one of the hallmarks of cancer. Oxidatively induced DNA damage is mainly repaired by base excision repair pathway with the involvement of a plethora of proteins. Cancer tissues develop greater DNA repair capacity than normal tissues by overexpressing DNA repair proteins. Increased DNA repair in tumors that removes DNA lesions generated by therapeutic agents before they became toxic is a major mechanism in the development of therapy resistance. Evidence suggests that DNA repair capacity may be a predictive biomarker of patient response. Thus, knowledge of DNA-protein expressions in disease-free and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. Our laboratory has developed methodologies that use mass spectrometry with isotope dilution for the measurement of expression of DNA repair proteins in human tissues and cultured cells. For this purpose, full-length (15)N-labeled analogs of a number of human DNA repair proteins have been produced and purified to be used as internal standards for positive identification and accurate quantification. This chapter describes in detail the protocols of this work. The use of (15)N-labeled proteins as internal standards for the measurement of several DNA repair proteins in vivo is also presented.

Inactivation of a common OGG1 variant by TNF-alpha in mammalian cells

Reactive oxygen species threaten genomic integrity by inducing oxidative DNA damage. One common form of oxidative DNA damage is the mutagenic lesion 8-oxoguanine (8-oxodG). One driver of oxidative stress that can induce 8-oxodG is inflammation, which can be initiated by the cytokine tumor necrosis factor alpha (TNF-α). Oxidative DNA damage is primarily repaired by the base excision repair pathway, initiated by glycosylases targeting specific DNA lesions. 8-oxodG is excised by 8-oxoguanine glycosylase 1 (OGG1). A common Ogg1 allelic variant is S326C-Ogg1, prevalent in Asian and Caucasian populations. S326C-Ogg1 is associated with various forms of cancer, and is inactivated by oxidation. However, whether oxidative stress caused by inflammatory cytokines compromises OGG1 variant repair activity remains unknown. We addressed whether TNF-α causes oxidative stress that both induces DNA damage and inactivates S326C-OGG1 via cysteine 326 oxidation. In mouse embryonic fibroblasts, we found that S326C-OGG1 was inactivated only after exposure to H2O2 or TNF-α. Treatment with the antioxidant N-acetylcysteine prior to oxidative stress rescued S326C-OGG1 activity, demonstrated by in vitro and cellular repair assays. In contrast, S326C-OGG1 activity was unaffected by potassium bromate, which induces oxidative DNA damage without causing oxidative stress, and presumably cysteine oxidation. This study reveals that Cys326 is vulnerable to oxidation that inactivates S326C-OGG1. Physiologically relevant levels of TNF-α simultaneously induce 8-oxodG and inactivate S326C-OGG1. These results suggest a mechanism that could contribute to increased risk of cancer among S326C-Ogg1 homozygous individuals.

Oxidatively induced DNA damage and its repair in cancer

Oxidatively induced DNA damage is caused in living organisms by endogenous and exogenous reactive species. DNA lesions resulting from this type of damage are mutagenic and cytotoxic and, if not repaired, can cause genetic instability that may lead to disease processes including carcinogenesis. Living organisms possess DNA repair mechanisms that include a variety of pathways to repair multiple DNA lesions. Mutations and polymorphisms also occur in DNA repair genes adversely affecting DNA repair systems. Cancer tissues overexpress DNA repair proteins and thus develop greater DNA repair capacity than normal tissues. Increased DNA repair in tumors that removes DNA lesions before they become toxic is a major mechanism for development of resistance to therapy, affecting patient survival. Accumulated evidence suggests that DNA repair capacity may be a predictive biomarker for patient response to therapy. Thus, knowledge of DNA protein expressions in normal and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. DNA repair proteins constitute targets for inhibitors to overcome the resistance of tumors to therapy. Inhibitors of DNA repair for combination therapy or as single agents for monotherapy may help selectively kill tumors, potentially leading to personalized therapy. Numerous inhibitors have been developed and are being tested in clinical trials. The efficacy of some inhibitors in therapy has been demonstrated in patients. Further development of inhibitors of DNA repair proteins is globally underway to help eradicate cancer.

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.

A meta-analysis of the association between the hOGG1 Ser326Cys polymorphism and the risk of esophageal squamous cell carcinoma

BACKGROUND

Genetic polymorphism of human 8-oxoguanine glycosylase 1 (hOGG1) Ser326Cys (rs1052133) has been implicated in the risk of Esophageal Squamous Cell Carcinoma (ESCC). However, the published findings are inconsistent. We therefore performed a meta-analysis to derive a more precise estimation of the association between the hOGG1 Ser326Cys polymorphism and ESCC risk.

METHODOLOGY/PRINCIPAL FINDINGS

A comprehensive search was conducted to identify eligible studies of hOGG1 Ser326Cys polymorphism and the risk of the ESCC. Three English and two Chinese databases were used, and ten published case-control studies, including 1987 cases and 2926 controls were identified. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to assess the strength of the association in the dominant and recessive model. Pearson correlation coefficient (PCC) and standard error (SE) were used to assess the number of Cys allele and ESCC risk in the additive model. Overall, significant associations between the hOGG1 Ser326Cys polymorphism and ESCC risk were found in the recessive model: OR = 1.37 (95% CI: 1.06-1.76, p = 0.02). We also observed significant associations in the Caucasian, Chinese language, population based control and tissue subgroups. In the additive model, positive correlation was found between the number of Cys allele and the risk of ESCC in overall studies (PCC = 0.109, SE = 0.046, p = 0.02), Caucasian subgroup and population subgroup. Funnel plot and Egger's test indicate there was no publication bias in this meta-analysis.

CONCLUSION

Under the published data, the hOGG1 Ser326Cys polymorphism is associated with ESCC risk in the recessive and additive model. Compared with the Ser/Ser and Ser/Cys genotype, Cys/Cys genotype might contribute to increased risk of ESCC. And the risk of ESCC is positively correlated with the number of Cys allele. A better case-control matched study should be designed in order to provide a more precise estimation.

hOGG1 Ser326Cys polymorphism and risk of hepatocellular carcinoma among East Asians: a meta-analysis

Background

The hOGG1 gene encodes a DNA glycosylase enzyme responsible for DNA repair. The Ser326Cys polymorphism in this gene may influence its repair ability and thus plays a role in carcinogenesis. Several case-control studies have been conducted on this polymorphism and its relationship with the risk of hepatocellular carcinoma (HCC) among East Asians. However, their results are inconsistent.

Methods

We performed a meta-analysis of published case-control studies assessing the association of the hOGG1 Ser326Cys polymorphism with HCC risk among East Asians. PubMed, EMBASE, SCI, BIOSIS, CNKI and WanFang databases were searched. A random-effect model was used to calculate odds ratios (ORs) and 95% confidence intervals (95% CIs). Analyses were conducted for additive, dominant and recessive genetic models.

Results

Eight studies were identified involving 2369 cases and 2442 controls assessing the association of the hOGG1 Ser326Cys polymorphism with HCC risk among East Asians. Applying a dominant genetic model, only in the Chinese population, the Cys allele was significantly associated with increased risk of HCC (OR 1.56, 95% CI 1.12–2.17). However, two studies influenced this finding according to sensitivity analysis. Furthermore, considerable heterogeneity and bias existed among Chinese studies.

Conclusion

There is limited evidence to support that the hOGG1 Ser326Cys polymorphism is associated with HCC risk among East Asians. Well-designed and large-sized studies are required to determine this relationship.

Base excision repair in physiology and pathology of the central nervous system

Relatively low levels of antioxidant enzymes and high oxygen metabolism result in formation of numerous oxidized DNA lesions in the tissues of the central nervous system. Accumulation of damage in the DNA, due to continuous genotoxic stress, has been linked to both aging and the development of various neurodegenerative disorders. Different DNA repair pathways have evolved to successfully act on damaged DNA and prevent genomic instability. The predominant and essential DNA repair pathway for the removal of small DNA base lesions is base excision repair (BER). In this review we will discuss the current knowledge on the involvement of BER proteins in the maintenance of genetic stability in different brain regions and how changes in the levels of these proteins contribute to aging and the onset of neurodegenerative disorders.

Could 8-oxoguanine DNA glycosylase 1 Ser326Cys polymorphism be a biomarker of susceptibility in cancer?

Biomarkers are key molecular or cellular events that give an indication whether there is a threat for disease, whether a disease already exists, or how such disease may develop in an individual case. The discovery of polymorphisms in genes that function in the metabolism of chemicals and in DNA repair has demonstrated the importance of understanding the phenomenon of genetic susceptibility in a population. Polymorphisms in DNA repair genes as an important component of the individual susceptibility to the development of cancer and various hereditary diseases have been commonly studied, since these genes have critical roles in maintaining genome integrity. Furthermore, the evaluation of cancer risk depends on the level of exposure to carcinogenic factors as well as on the genetic codes of the individual. This approach is supported by studies that present positive association between these polymorphic genes and cancers. Although 8-oxoguanine DNA glycosylase 1 (OGG1) is one of the promising biomarker candidates of cancer susceptibility, there are also some controversial results. Epidemiological studies show that the OGG1 might be a biomarker of susceptibility for various cancers; however, the small sample size and difference in the eligibility criteria for inclusion of subjects and sources might limit the studies to demonstrate the association between the OGG1 Ser326Cys polymorphism and the risk of cancer. Thus, meta-analyses may provide more valuable and reliable data to demonstrate the potential of OGG1 Ser326Cys DNA repair enzyme polymorphisms that could be the biomarkers of susceptibility of cancer. Our aim in this review is to compile published studies, including some controversial results on the association between the OGG1 Ser326Cys polymorphism and the risk of cancer.

Meta-analysis demonstrates lack of association of the hOGG1 Ser326Cys polymorphism with bladder cancer risk

The functional polymorphism Ser326Cys (rs1052133) in the human 8-oxoguanine DNA glycosylase (hOGG1) gene has been implicated in bladder cancer risk. However, reports of this association between the Ser326Cys polymorphism and bladder cancer risk are conflicting. In order to help clarify this relationship, we made a meta-analysis of seven case-control studies, summing 2521 cases and 2408 controls. We used odds ratios (ORs) with 95% confidence intervals (95%CIs) to assess the strength of the association. Overall, no significant association between the hOGG1 Ser326Cys polymorphism and bladder cancer risk was found for Cys/Cys vs Ser/Ser (OR = 1.10, 95%CI = 0.74-1.65), Ser/Cys vs Ser/Ser (OR = 1.07, 95%CI = 0.81-1.42), Cys/Cys + Ser/Cys vs Ser/Ser (OR = 1.08, 95%CI = 0.87-1.33), and Cys/Cys vs Ser/Cys + Ser/Ser (OR = 1.04, 95%CI = 0.65-1.69). Even when stratified by ethnicity, no significant association was observed. We concluded that the hOGG1 Ser326Cys polymorphism does not contribute to susceptibility to bladder cancer.

8-oxoguanine DNA glycosylases: one lesion, three subfamilies

Amongst the four bases that form DNA, guanine is the most susceptible to oxidation, and its oxidation product, 7,8-dihydro-8-oxoguanine (8-oxoG) is the most prevalent base lesion found in DNA. Fortunately, throughout evolution cells have developed repair mechanisms, such as the 8-oxoguanine DNA glycosylases (OGG), which recognize and excise 8-oxoG from DNA thereby preventing the accumulation of deleterious mutations. OGG are divided into three subfamilies, OGG1, OGG2 and AGOG, which are all involved in the base excision repair (BER) pathway. The published structures of OGG1 and AGOG, as well as the recent availability of OGG2 structures in both apo- and liganded forms, provide an excellent opportunity to compare the structural and functional properties of the three OGG subfamilies. Among the observed differences, the three-dimensional fold varies considerably between OGG1 and OGG2 members, as the latter lack the A-domain involved in 8-oxoG binding. In addition, all three OGG subfamilies bind 8-oxoG in a different manner even though the crucial interaction between the enzyme and the protonated N7 of 8-oxoG is conserved. Finally, the three OGG subfamilies differ with respect to DNA binding properties, helix-hairpin-helix motifs, and specificity for the opposite base.

Conformational dynamics of abasic DNA upon interactions with AP endonuclease 1 revealed by stopped-flow fluorescence analysis

Apurinic/apyrimidinic (AP) sites are abundant DNA lesions arising from exposure to UV light, ionizing radiation, alkylating agents, and oxygen radicals. In human cells, AP endonuclease 1 (APE1) recognizes this mutagenic lesion and initiates its repair via a specific incision of the phosphodiester backbone 5' to the AP site. We have investigated a detailed mechanism of APE1 functioning using fluorescently labeled DNA substrates. A fluorescent adenine analogue, 2-aminopurine, was introduced into DNA substrates adjacent to the abasic site to serve as an on-site reporter of conformational transitions in DNA during the catalytic cycle. Application of a pre-steady-state stopped-flow technique allows us to observe changes in the fluorescence intensity corresponding to different stages of the process in real time. We also detected an intrinsic Trp fluorescence of the enzyme during interactions with 2-aPu-containing substrates. Our data have revealed a conformational flexibility of the abasic DNA being processed by APE1. Quantitative analysis of fluorescent traces has yielded a minimal kinetic scheme and appropriate rate constants consisting of four steps. The results obtained from stopped-flow data have shown a substantial influence of the 2-aPu base location on completion of certain reaction steps. Using detailed molecular dynamics simulations of the DNA substrates, we have attributed structural distortions of AP-DNA to realization of specific binding, effective locking, and incision of the damaged DNA. The findings allowed us to accurately discern the step that corresponds to insertion of specific APE1 amino acid residues into the abasic DNA void in the course of stabilization of the precatalytic complex.

Oxidatively induced DNA damage: mechanisms, repair and disease

Endogenous and exogenous sources cause oxidatively induced DNA damage in living organisms by a variety of mechanisms. The resulting DNA lesions are mutagenic and, unless repaired, lead to a variety of mutations and consequently to genetic instability, which is a hallmark of cancer. Oxidatively induced DNA damage is repaired in living cells by different pathways that involve a large number of proteins. Unrepaired and accumulated DNA lesions may lead to disease processes including carcinogenesis. Mutations also occur in DNA repair genes, destabilizing the DNA repair system. A majority of cancer cell lines have somatic mutations in their DNA repair genes. In addition, polymorphisms in these genes constitute a risk factor for cancer. In general, defects in DNA repair are associated with cancer. Numerous DNA repair enzymes exist that possess different, but sometimes overlapping substrate specificities for removal of oxidatively induced DNA lesions. In addition to the role of DNA repair in carcinogenesis, recent evidence suggests that some types of tumors possess increased DNA repair capacity that may lead to therapy resistance. DNA repair pathways are drug targets to develop DNA repair inhibitors to increase the efficacy of cancer therapy. Oxidatively induced DNA lesions and DNA repair proteins may serve as potential biomarkers for early detection, cancer risk assessment, prognosis and for monitoring therapy. Taken together, a large body of accumulated evidence suggests that oxidatively induced DNA damage and its repair are important factors in the development of human cancers. Thus this field deserves more research to contribute to the development of cancer biomarkers, DNA repair inhibitors and treatment approaches to better understand and fight cancer.

Base excision repair and cancer

Base excision repair is the system used from bacteria to man to remove the tens of thousands of endogenous DNA damages produced daily in each human cell. Base excision repair is required for normal mammalian development and defects have been associated with neurological disorders and cancer. In this paper we provide an overview of short patch base excision repair in humans and summarize current knowledge of defects in base excision repair in mouse models and functional studies on short patch base excision repair germ line polymorphisms and their relationship to cancer. The biallelic germ line mutations that result in MUTYH-associated colon cancer are also discussed.

Association of OGG1 Ser326Cys polymorphism with colorectal cancer risk: a meta-analysis

INTRODUCTION

8-Oxoguanine DNA glycosylase 1 (OGG1), a key protein involved in the base excision repair pathway, can recognize and excise several lesions from oligodeoxynucleotides with single DNA damage. A C/G polymorphism at 1,245 bp (C1245G) in exon 7 of the OGG1 (Ser326Cys, rs1052133) is found to have a lower enzymatic activity. A variety of case-control studies have been published evaluating the association between OGG1 Ser326Cys polymorphism and colorectal cancer (CRC), though their conclusions were always contradictory.

MATERIALS AND METHODS

This meta-analysis enrolled 12 studies to estimate the overall risk of OGG1 Ser326Cys polymorphism associated with CRC. The pooled odds ratios (ORs) were performed for codominant model (Cys/Cys versus Ser/Ser; Ser/Cys versus Ser/Ser), dominant model (Ser/Cys + Cys/Cys versus Ser/Ser) and recessive model (Cys/Cys versus Ser/Cys + Ser/Ser).

RESULTS

No significant associations were found for Cys/Cys versus Ser/Ser (OR = 1.19, 95% confidence interval (CI) 0.92-1.53), Ser/Cys versus Ser/Ser (OR = 1.04, 95% CI 0.95-1.13), Ser/Cys + Cys/Cys versus Ser/Ser (OR = 1.06, 95% CI 0.98-1.16) and Cys/Cys versus Ser/Cys + Ser/Ser (OR = 1.11, 95% CI 0.90-1.38); moreover, in the stratified analyses, no significantly increased risk was found for all genetic models.

CONCLUSIONS

Our meta-analysis suggests that the OGG1 Ser326Cys polymorphism is not associated with CRC risk.

Parameters of oxidative stress, DNA damage and DNA repair in type 1 and type 2 diabetes mellitus

OBJECTIVES

(i) to determine the extent of oxidative stress and DNA damage and repair using a panel of selected markers in patients with type 1 and type 2 diabetes mellitus (T1DM, T2DM), (ii) to find their possible relationships with diabetes compensation and duration, and finally (iii) to test for the effect of functional polymorphisms in the 8-oxoguanin DNA glycosylase (rs1052133), catalase (rs1001179) and superoxide dismutase (rs4880) genes on respective intermediate phenotypes.

METHODS

A total of 207 subjects (23 children and 44 adults with T1DM, 52 adult patients with T2DM and 88 healthy adult control subjects) were enrolled in the study. The following markers of redox state were determined in participants: erythrocyte superoxide dismutase (Ery-SOD), whole blood glutathione peroxidase (WB-GPx), erythrocyte glutathione (Ery-GSH), plasma total antioxidant capacity (P-tAOC) and plasma malondialdehyde (P-MDA). Furthermore, the extent of DNA damage and repair was ascertained using the following parameters: DNA single strand breaks (DNAssb), DNA repair capacity (DNArc) and DNA repair index (DNRI).

RESULTS

Comparison of T1DM vs. T2DM patients revealed significantly higher Ery-GSH content (P < 0.0001) and significantly lower Ery-SOD activity (P = 0.0006) and P-tAOC level (P < 0.0001) in T1DM subjects. T2DM diabetics exhibited a significant increase in DNAssb (P < 0.0001) and significant decrease in both DNArc (P < 0.0001) and DNRI (P <  .0001) compared with T1DM patients. Patient's age (irrespective of DM type) significantly correlated with DNAssb (r = 0.48, P < 0.0001), DNArc (r = -0.67, P < 0.0001) and DNRI (r = -0.7, P < 0.0001). Allele frequencies of all studied polymorphisms did not exhibit any significant association with the investigated parameters.

CONCLUSION

We demonstrated significant age- and DM type-related changes of oxidative DNA modification and capacity for its repair in subjects with T1DM and T2DM.

Evidence for upregulated repair of oxidatively induced DNA damage in human colorectal cancer

Carcinogenesis may involve overproduction of oxygen-derived species including free radicals, which are capable of damaging DNA and other biomolecules in vivo. Increased DNA damage contributes to genetic instability and promote the development of malignancy. We hypothesized that the repair of oxidatively induced DNA base damage may be modulated in colorectal malignant tumors, resulting in lower levels of DNA base lesions than in surrounding pathologically normal tissues. To test this hypothesis, we investigated oxidatively induced DNA damage in cancerous tissues and their surrounding normal tissues of patients with colorectal cancer. The levels of oxidatively induced DNA lesions such as 4,6-diamino-5-formamidopyrimidine, 2,6-diamino-4-hydroxy-5-formamidopyrimidine, 8-hydroxyguanine and (5'S)-8,5'-cyclo-2'-deoxyadenosine were measured by gas chromatography/isotope-dilution mass spectrometry and liquid chromatography/isotope-dilution tandem mass spectrometry. We found that the levels of these DNA lesions were significantly lower in cancerous colorectal tissues than those in surrounding non-cancerous tissues. In addition, the level of DNA lesions varied between colon and rectum tissues, being lower in the former than in the latter. The results strongly suggest upregulation of DNA repair in malignant colorectal tumors that may contribute to the resistance to therapeutic agents affecting the disease outcome and patient survival. The type of DNA base lesions identified in this work suggests the upregulation of both base excision and nucleotide excision pathways. Development of DNA repair inhibitors targeting both repair pathways may be considered for selective killing of malignant tumors in colorectal cancer.

8-Oxoguanine incision activity is impaired in lung tissues of NSCLC patients with the polymorphism of OGG1 and XRCC1 genes

Decreased repair of oxidative DNA damage is a risk factor for developing certain human malignancies. We have previously found that the capacity of 8-oxo-7,8-dihydroguanine repair was lower in leukocytes of NSCLC patients than in controls. To explain these observations, we searched for mutations and polymorphisms in the OGG1 gene among 88 NSCLC patients and 79 controls. One patient exhibited a heterozygous mutation in exon 1, which resulted in Arg46Gln substitution. Normal lung and tumor tissue carrying this mutation showed markedly lower 8-oxoG incision activity than the mean for all patients. The predominant polymorphism of OGG1 was Ser326Cys. A significant difference was observed in the frequencies of the OGG1 variants between populations of NSCLC patients and controls. The frequency of the Cys326 allele and the number of Cys326Cys homozygotes was higher among patients than controls. In individuals with either Ser326Cys or Cys326Cys genotype 8-oxoG incision rate was lower than in those with both Ser326 alleles, either in lung or leukocytes. Moreover, 8-oxodG level was higher in lung tissue and leukocytes of patients carrying two Cys326 alleles and in leukocytes of patients with the Ser326Cys genotype. We also screened for polymorphisms of the XRCC1 gene. Only heterozygotes of the XRCC1 variants Arg194Trp, Arg280His and Arg399Gln were found among patients and controls, with the frequency of Arg280His being significantly higher among patients. NSCLC patients with Arg280His or Arg399Gln polymorphism revealed lower 8-oxoG incision activity in their lung tissues, but not in leukocytes. We can conclude that the OGG1 Ser326Cys polymorphisms may have an impact on the efficiency of 8-oxoG incision in humans and the XRCC1 His280 and Gln399 may influence the OGG1 activity in tissues exposed to chronic oxidative/inflammatory stress. Higher frequency of the OGG1 Cys326 allele among NSCLC patients may partially explain the impairment of the 8-oxoG repair observed in their leukocytes.

Analysis of the oxidative damage repair genes NUDT1, OGG1, and MUTYH in patients from mismatch repair proficient HNPCC families (MSS-HNPCC)

PURPOSE

Several studies have described molecular differences between microsatellite stable hereditary nonpolyposis colorectal cancer (MSS-HNPCC) and microsatellite unstable Lynch syndrome tumors (MSI-HNPCC). These differences highlight the possibility that other instability forms could explain cancer susceptibility in this group of families. The base excision repair (BER) pathway is the major DNA repair pathway for oxidative DNA damage. A defect in this pathway can result in DNA transversion mutations and a subsequent increased cancer risk. Mutations in MUTYH have been associated with increased colorectal cancer (CRC) risk while no association has been described for OGG1 or NUDT1.

EXPERIMENTAL DESIGN

We performed mutational screening of the three genes involved in defense against oxidative DNA damage in a set of 42 MSS-HNPCC families.

RESULTS

Eight rare variants and 5 frequent variants were found in MSS-HNPCC patients. All variants were previously described by other authors except variant c.285C>T in OGG1. Segregation studies were done and in silico programs were used to estimate the level of amino acid conservation, protein damage prediction, and possible splicing alterations. Variants OGG1 c.137G>A; MUTYH c.1187G>A were detected in Amsterdam I families and cosegregate with cancer. Analysis of OGG1 c.137G>A transcripts showed an inactivation of the splicing donor of exon 1.

CONCLUSIONS

Two rare variants (OGG1 c.137G>A; MUTYH c.1187G>A) and one common polymorphism (NUDT1 c.426C>T) were associated with CRC risk. We show that the BER pathway can play a significant role in a number of MSS-HNPCC colorectal cancers. More studies could be of interest in order to gain further understanding of yet unexplained CRC susceptibility cases.

Variation in base excision repair capacity

The major DNA repair pathway for coping with spontaneous forms of DNA damage, such as natural hydrolytic products or oxidative lesions, is base excision repair (BER). In particular, BER processes mutagenic and cytotoxic DNA lesions such as non-bulky base modifications, abasic sites, and a range of chemically distinct single-strand breaks. Defects in BER have been linked to cancer predisposition, neurodegenerative disorders, and immunodeficiency. Recent data indicate a large degree of sequence variability in DNA repair genes and several studies have associated BER gene polymorphisms with disease risk, including cancer of several sites. The intent of this review is to describe the range of BER capacity among individuals and the functional consequences of BER genetic variants. We also discuss studies that associate BER deficiency with disease risk and the current state of BER capacity measurement assays.

Oxidative stress, Helicobacter pylori, and OGG1 Ser326Cys, XPC Lys939Gln, and XPD Lys751Gln polymorphisms in a Turkish population with colorectal carcinoma

The contribution of polymorphisms of DNA repair genes OGG1 Ser326Cys, XPC Lys939Gln, and XPD Lys751Gln in developing colorectal carcinoma is controversial. Whether the group 1A carcinogen Helicobacter pylori is a risk factor or not in these patients could not be clearly elucidated. One hundred ten colorectal cancer patients and 116 cancer-free individuals constituted the test and control groups, respectively. The association of OGG1 Ser326Cys, XPC Lys939Gln, and XPD Lys751Gln polymorphisms and the susceptibility to colorectal carcinoma with or without oxidative stress were evaluated. DNA was extracted from peripheral blood cells and genotypes were determined using polymerase chain reaction-restriction fragment length polymorphism. For serum nitric oxide and total antioxidant status assay, spectrophotometric analyses were used. Serum albumin measurements were performed using an autoanalyzer. H. pylori IgG was measured by ELISA. The serum albumin concentrations of cancer patients were significantly lower than those of the controls (p < 0.05). The carriers of the variant genotype of OGG1 (odds ratio: 0.963; 95% confidence interval: 0.446-2.079), XPC (0.789, 0.366-1.700), or XPD (0.532, 0.259-1.094) did not associate with the increased risk of cancer progression, despite the increased oxidative stress in cancer patients. Seropositivity of H. pylori IgG has been found to increase the risk of colorectal carcinoma by 2.2-fold.

8-Oxo-7,8-dihydroguanine: links to gene expression, aging, and defense against oxidative stress

The one-electron oxidation product of guanine, 8-oxo-7,8-dihydroguanine (8-oxoG), is an abundant lesion in genomic, mitochondrial, and telomeric DNA and RNA. It is considered to be a marker of oxidative stress that preferentially accumulates at the 5' end of guanine strings in the DNA helix, in guanine quadruplexes, and in RNA molecules. 8-OxoG has a lower oxidation potential compared to guanine; thus it is susceptible to oxidation/reduction and, along with its redox products, is traditionally considered to be a major mutagenic DNA base lesion. It does not change the architecture of the DNA double helix and it is specifically recognized and excised by 8-oxoguanine DNA glycosylase (OGG1) during the DNA base excision repair pathway. OGG1 null animals accumulate excess levels of 8-oxoG in their genome, yet they do not have shorter life span nor do they exhibit severe pathological symptoms including tumor formation. In fact they are increasingly resistant to inflammation. Here we address the rarely considered significance of 8-oxoG, such as its optimal levels in DNA and RNA under a given condition, essentiality for normal cellular physiology, evolutionary role, and ability to soften the effects of oxidative stress in DNA, and the harmful consequences of its repair, as well as its importance in transcriptional initiation and chromatin relaxation.

Oxidative stress and 8-oxoguanine repair are enhanced in colon adenoma and carcinoma patients

Oxidative stress is involved in the pathogenesis of colon cancer. We wanted to elucidate at which stage of the disease this phenomenon occurs. In the examined groups of patients with colorectal cancer (CRC, n = 89), benign adenoma (AD, n = 77) and healthy volunteers (controls, n = 99), we measured: vitamins A, C and E in blood plasma, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydroguanine (8-oxoGua) in leukocytes and urine, leukocyte 8-oxoGua excision activity, mRNA levels of APE1, OGG1, 8-oxo-7,8-dihydrodeoxyguanosine 5'-triphosphate pyrophosphohydrolase (MTH1) and OGG1 polymorphism. The vitamin levels decreased gradually in AD and CRC patients. 8-OxodG increased in leukocytes and urine of CRC and AD patients. 8-OxoGua was higher only in the urine of CRC patients. 8-OxoGua excision was higher in CRC patients than in controls, in spite of higher frequency of the OGG1 Cys326Cys genotype, encoding a glycosylase with decreased activity. mRNA levels of OGG1 and APE1 increased in CRC and AD patients, which could explain increased 8-oxoGua excision rate in CRC patients. MTH1 mRNA was also higher in CRC patients. The results suggest that oxidative stress occurs in CRC and AD individuals. This is accompanied by increased transcription of DNA repair genes, and increased 8-oxoGua excision rate in CRC patients, which is, however, insufficient to counteract the increased DNA damage.

HO* radicals induce an unexpected high proportion of tandem base lesions refractory to repair by DNA glycosylases

Reaction of HO(*) radicals with double-stranded calf thymus DNA produces high levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) and, to a minor extent, 8-oxo-7,8-dihydro-2'-deoxyadenosine (8-oxodAdo). Formation of the hydroxylated purine lesions is explained by addition of HO(*) to the C8 position of the purine moiety. It has been reported that tandem lesions containing a formylamine residue neighboring 8-oxodGuo could be produced through addition of a transiently generated pyrimidine peroxyl radical onto the C8 of an adjacent purine base. Formation of such tandem lesions accounted for approximately 10% of the total 8-oxodGuo. In the present work we show that addition of HO(*) onto the C8 of purine accounts for only approximately 5% of the generated 8-oxodGuo. About 50% of the 8-hydroxylated purine lesions, including 8-oxodGuo and 8-oxodAdo, are involved in tandem damage and are produced by peroxyl addition onto the C8 of a vicinal purine base. In addition, the remaining 45% of the 8-oxodGuo are produced by an electron transfer reaction, providing an explanation for the higher yield of formation of 8-oxodGuo compared to 8-oxodAdo. Interestingly, we show that >40% of the 8-oxodGuo involved in tandem lesions is refractory to excision by DNA glycosylases. Altogether our results demonstrate that, subsequently to a single oxidation event, peroxidation reactions significantly increase the yield of formation of hydroxylated purine modifications, generating a high proportion of tandem lesions partly refractory to base excision repair.

Evidence for the involvement of DNA repair enzyme NEIL1 in nucleotide excision repair of (5'R)- and (5'S)-8,5'-cyclo-2'-deoxyadenosines

The DNA repair enzyme NEIL1 is a DNA glycosylase that is involved in the first step of base excision repair (BER) of oxidatively induced DNA damage. NEIL1 exhibits a strong preference for excision of 4,6-diamino-5-formamidopyrimidine (FapyAde) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) from DNA with no specificity for 8-hydroxyguanine (8-OH-Gua). In this study, we report on the significant accumulation of (5'R)-8,5'-cyclo-2'-deoxyadenosine (R-cdA) and (5'S)-8,5'-cyclo-2'-deoxyadenosine (S-cdA) in liver DNA of neil1(-/-) mice that were not exposed to exogenous oxidative stress, while no accumulation of these lesions was observed in liver DNA from control or ogg1(-/-) mice. Significant accumulation of FapyGua was detected in liver DNA of both neil1(-/-) and ogg1(-/-) mice, while 8-OH-Gua accumulated in ogg1(-/-) only. Since R-cdA and S-cdA contain an 8,5'-covalent bond between the base and sugar moieties, they cannot be repaired by BER. There is evidence that these lesions are repaired by nucleotide excision repair (NER). Since the accumulation of R-cdA and S-cdA in neil1(-/-) mice strongly points to the failure of their repair, these data suggest that NEIL1 is involved in NER of R-cdA and S-cdA. Further studies aimed at elucidating the mechanism of action of NEIL1 in NER are warranted.