Interaction with OGG1 is required for efficient recruitment of XRCC1 to base excision repair and maintenance of genetic stability after exposure to oxidative stress

Dapagliflozin suppresses diabetes-induced oxidative DNA damage and hypermethylation in mouse somatic cells

Diabetes mellitus is a complex metabolic disorder resulting from the interplay of environmental, genetic, and epigenetic factors that increase the risk of cancer development. However, it is unclear whether the increased cancer risk is due to poor glycemic control or the use of some antidiabetic medications. Therefore, we investigated the genetic and epigenetic changes in somatic cells in a mouse model of diabetes and studied whether multiple exposures to the antidiabetic medication dapagliflozin influence these changes. We also elucidated the mechanism(s) of these ameliorations. The micronucleus test and modified comet assay were used to investigate bone marrow DNA damage and methylation changes. These assays revealed that dapagliflozin is non-genotoxic in the tested regimen, and oxidative DNA damage and hypermethylation were significantly higher in diabetic mice. Spectrophotometry also evaluated oxidative DNA damage and global DNA methylation, revealing similar significant alterations induced by diabetes. Conversely, the dapagliflozin-treated diabetic animals significantly reduced these changes. The expression of some genes involved in DNA repair and DNA methylation was disrupted considerably in the somatic cells of diabetic animals. In contrast, dapagliflozin treatment significantly restored these disruptions and enhanced DNA repair. The simultaneous effects of decreased oxidative DNA damage and hypermethylation levels suggest that dapagliflozin can be used as a safe antidiabetic drug to reduce DNA damage and hypermethylation in diabetes, demonstrating its usefulness in patients with diabetes to control hyperglycemia and decrease the development of its subsequent complications.

XRCC1 Polymorphisms p.Arg194Trp, p.Arg280His, and p.Arg399Gln, Polycyclic Aromatic Hydrocarbons, and Infertility: A Case-Control and In Silico Study

XRCC1 is involved in repair of single-strand breaks generated by mutagenic exposure. Polymorphisms within XRCC1 affect its ability to efficiently repair DNA damage. Polycyclic aromatic hydrocarbons or PAHs are genotoxic compounds which form bulky DNA adducts that are linked with infertility. Few reports suggest combined role of XRCC1 polymorphisms and PAHs in infertility. Present study investigates association of three XRCC1 polymorphisms (p.Arg194Trp, p.Arg280His, p.Arg399Gln) with male and female infertility in a North-West Indian population using case-control approach. Additionally, in silico approach has been used to predict whether XRCC1 polymorphisms effect interaction of XRCC1 with different PAHs. For case-control study, XRCC1 polymorphisms were screened in peripheral blood samples of age- and gender-matched 201 infertile cases (♂-100, ♀-101) and 201 fertile controls (♂-100, ♀-101) using PCR-RFLP method. For in silico study, AutoDock v4.2.6 was used for molecular docking of B[a]P, BPDE-I, ( ±)-anti-BPDE, DB[a,l]P, 1-N, 2-N, 1-OHP, 2-OHF with XRCC1 and assess effect of XRCC1 polymorphisms on their interaction. In case-control study, statistical analysis showed association of XRCC1 p.Arg280His GA genotype (p = 0.027), A allele (p = 0.019) with reduced risk of male infertility. XRCC1 p.Arg399Gln AA genotype (p = 0.021), A allele (p = 0.014) were associated with reduced risk for female primary infertility. XRCC1 p.Arg194Trp T allele was associated with increased risk for female infertility (p = 0.035). In silico analysis showed XRCC1-PAH interaction with non-significant effect of XRCC1 polymorphisms on predicted binding. Therefore, present study concludes that XRCC1 polymorphism-modified risk for male and female infertility in North-West Indians without significant effect on predicted XRCC1-PAH interactions. This is the first report on XRCC1 in female infertility.

Dapagliflozin Mitigated Elevated Disomic and Diploid Sperm in a Mouse Model of Diabetes and Recover the Disrupted Ogg1, Parp1, and P53 Gene Expression

Increases in numerical chromosomal syndromes were observed in children of diabetic mothers. However, the effects of diabetes on male reproduction, specifically numerical chromosomal aberrations (aneuploidy), have not been studied. Furthermore, despite the increasing use of dapagliflozin for diabetes treatment, no data exists on its ability to affect aneuploidy levels in germ cells. Thus, our investigation aimed to evaluate the effects of diabetes on spontaneous sperm aneuploidy and whether treatment with dapagliflozin influences the frequency of aneuploidy in the sperm of an experimental diabetic animal model. Our findings show that dapagliflozin has no aneugenic effects on the meiotic stages of spermatogenesis. In contrast, diabetes raised the frequency of aneuploidy, and dapagliflozin administration decreased the elevated levels of disomic and diploid sperm. The level of oxidative stress was markedly increased in diabetic mice, but were reduced by dapagliflozin treatment. Furthermore, the expression of some of DNA repair genes was disrupted in diabetic animals, whereas dapagliflozin therapy restored these disruptions and significantly enhanced DNA repair. Thus, dapagliflozin may effectively ameliorate diabetes-induced aneugenic effects on male meiosis and treating diabetic patients with dapagliflozin may effectively mitigate the transmission of diabetes-induced chromosomal defects to offspring.

Spatio-temporal dynamics of the DNA glycosylase OGG1 in finding and processing 8-oxoguanine

OGG1 is the DNA glycosylase responsible for the removal of the oxidative lesion 8-oxoguanine (8-oxoG) from DNA. The recognition of this lesion by OGG1 is a complex process that involves scanning the DNA for the presence of 8-oxoG, followed by recognition and lesion removal. Structural data have shown that OGG1 evolves through different stages of conformation onto the DNA, corresponding to elementary steps of the 8-oxoG recognition and extrusion from the double helix. Single-molecule studies of OGG1 on naked DNA have shown that OGG1 slides in persistent contact with the DNA, displaying different binding states probably corresponding to the different conformation stages. However, in cells, the DNA is not naked and OGG1 has to navigate into a complex and highly crowded environment within the nucleus. To ensure rapid detection of 8-oxoG, OGG1 alternates between 3D diffusion and sliding along the DNA. This process is regulated by the local chromatin state but also by protein co-factors that could facilitate the detection of oxidized lesions. We will review here the different methods that have been used over the last years to better understand how OGG1 detects and process 8-oxoG lesions.

Temporal recruitment of base excision DNA repair factors in living cells in response to different micro-irradiation DNA damage protocols

Laser micro-irradiation across the nucleus rapidly generates localized chromatin-associated DNA lesions permitting analysis of repair protein recruitment in living cells. Recruitment of three fluorescently-tagged base excision repair factors [DNA polymerase β (pol β), XRCC1 and PARP1], known to interact with one another, was compared in gene-deleted mouse embryonic fibroblasts and in those expressing the endogenous factor. A low energy micro-irradiation (LEMI) forming direct single-strand breaks and a moderate energy (MEMI) protocol that additionally creates oxidized bases were compared. Quantitative characterization of repair factor recruitment and sensitivity to clinical PARP inhibitors (PARPi) was dependent on the micro-irradiation protocol. PARP1 recruitment was biphasic and generally occurred prior to pol β and XRCC1. After LEMI, but not after MEMI, pol β and XRCC1 recruitment was abolished by the PARPi veliparib. Consistent with this, pol β and XRCC1 recruitment following LEMI was considerably slower in PARP1-deficient cells. Surprisingly, the recruitment half-times and amplitudes for pol β were less affected by PARPi than were XRCC1 after MEMI suggesting there is a XRCC1-independent component for pol β recruitment. After LEMI, but not MEMI, pol β dissociation was more rapid than that of XRCC1. Unexpectedly, PARP1 dissociation was slowed in the absence of XRCC1 as well with a PARPi after LEMI but not MEMI, suggesting that XRCC1 facilitates PARP1 dissociation from specific DNA lesions. XRCC1-deficient cells showed pronounced hypersensitivity to the PARPi talazoparib correlating with its known cytotoxic PARP1 trapping activity. In contrast to DNA methylating agents, PARPi only minimally sensitized pol β and XRCC1-deficient cells to oxidative DNA damage suggesting differential binding of PARP1 to alternate repair intermediates. In summary, pol β, XRCC1, and PARP1 display recruitment kinetics that exhibit correlated and unique properties that depend on the DNA lesion and PARP activity revealing that there are multiple avenues utilized in the repair of chromatin-associated DNA.

Identification of crucial genes involved in thyroid cancer development

BACKGROUND

A malignancy of the endocrine system, one of the most common types, is thyroid cancer. It is proven that children who receive radiation treatment for leukemia or lymphoma are at a heightened risk of thyroid cancer due to low-dose radiation exposure throughout childhood. Several factors can increase the risk of thyroid cancer (ThyCa), such as chromosomal and genetic mutations, iodine intake, TSH levels, autoimmune thyroid disorders, estrogen, obesity, lifestyle changes, and environmental contaminants.

OBJECTIVES

The study aimed to identify a specific gene as an essential candidate for thyroid cancer progression. We might be able to focus on developing a better understanding of how thyroid cancer is inherited.

METHODS

The review article uses electronic databases such as PubMed, Google Scholar, Ovid MEDLINE, Embase, and Cochrane Central. The most frequently associated genes with thyroid cancer found on PubMed were BAX, XRCC1, XRCC3, XPO5, IL-10, BRAF, RET, and K-RAS. To perform an electronic literature search, genes derived from DisGeNET: a database of gene-disease associations, including PRKAR1A, BRAF, RET, NRAS, and KRAS, are used.

CONCLUSION

Examining the genetics of thyroid cancer explicitly emphasizes the primary genes associated with the pathophysiology of young and older people with thyroid cancer. Developing such gene investigations at the beginning of the thyroid cancer development process can identify better outcomes and the most aggressive thyroid cancers.

Fluorescently labeled human apurinic/apyrimidinic endonuclease APE1 reveals effects of DNA polymerase β on the APE1-DNA interaction

The base excision repair (BER) pathway involves sequential action of DNA glycosylases and apurinic/apyrimidinic (AP) endonucleases to incise damaged DNA and prepare DNA termini for incorporation of a correct nucleotide by DNA polymerases. It has been suggested that the enzymatic steps in BER include recognition of a product-enzyme complex by the next enzyme in the pathway, resulting in the "passing-the-baton" model of transfer of DNA intermediates between enzymes. To verify this model, in this work, we aimed to create a suitable experimental system. We prepared APE1 site-specifically labeled with a fluorescent reporter that is sensitive to stages of APE1-DNA binding, of formation of the catalytic complex, and of subsequent dissociation of the enzyme-product complex. Interactions of the labeled APE1 with various model DNA substrates (containing an abasic site) of varied lengths revealed that the enzyme remains mostly in complex with the DNA product. By means of the fluorescently labeled APE1 in combination with a stopped-flow fluorescence assay, it was found that Polβ stimulates both i) APE1 binding to an abasic-site-containing DNA duplex with the formation of a catalytically competent complex and ii) the dissociation of APE1 from its product. These findings confirm DNA-mediated coordination of APE1 and Polβ activities and suggest that Polβ is the key trigger of the DNA transfer between the enzymes participating in initial steps of BER.

High Glucose Increases DNA Damage and Elevates the Expression of Multiple DDR Genes

The DNA Damage Response (DDR) pathways sense DNA damage and coordinate robust DNA repair and bypass mechanisms. A series of repair proteins are recruited depending on the type of breaks and lesions to ensure overall survival. An increase in glucose levels was shown to induce genome instability, yet the links between DDR and glucose are still not well investigated. In this study, we aimed to identify dysregulation in the transcriptome of normal and cancerous breast cell lines upon changing glucose levels. We first performed bioinformatics analysis using a microarray dataset containing the triple-negative breast cancer (TNBC) MDA-MB-231 and the normal human mammary epithelium MCF10A cell lines grown in high glucose (HG) or in the presence of the glycolysis inhibitor 2-deoxyglucose (2DG). Interestingly, multiple DDR genes were significantly upregulated in both cell lines grown in HG. In the wet lab, we remarkably found that HG results in severe DNA damage to TNBC cells as observed using the comet assay. In addition, several DDR genes were confirmed to be upregulated using qPCR analysis in the same cell line. Our results propose a strong need for DDR pathways in the presence of HG to oppose the severe DNA damage induced in cells.

The scaffold protein XRCC1 stabilizes the formation of polβ/gap DNA and ligase IIIα/nick DNA complexes in base excision repair

The base excision repair (BER) pathway involves gap filling by DNA polymerase (pol) β and subsequent nick sealing by ligase IIIα. X-ray cross-complementing protein 1 (XRCC1), a nonenzymatic scaffold protein, assembles multiprotein complexes, although the mechanism by which XRCC1 orchestrates the final steps of coordinated BER remains incompletely defined. Here, using a combination of biochemical and biophysical approaches, we revealed that the polβ/XRCC1 complex increases the processivity of BER reactions after correct nucleotide insertion into gaps in DNA and enhances the handoff of nicked repair products to the final ligation step. Moreover, the mutagenic ligation of nicked repair intermediate following polβ 8-oxodGTP insertion is enhanced in the presence of XRCC1. Our results demonstrated a stabilizing effect of XRCC1 on the formation of polβ/dNTP/gap DNA and ligase IIIα/ATP/nick DNA catalytic ternary complexes. Real-time monitoring of protein–protein interactions and DNA-binding kinetics showed stronger binding of XRCC1 to polβ than to ligase IIIα or aprataxin, and higher affinity for nick DNA with undamaged or damaged ends than for one nucleotide gap repair intermediate. Finally, we demonstrated slight differences in stable polβ/XRCC1 complex formation, polβ and ligase IIIα protein interaction kinetics, and handoff process as a result of cancer-associated (P161L, R194W, R280H, R399Q, Y576S) and cerebellar ataxia-related (K431N) XRCC1 variants. Overall, our findings provide novel insights into the coordinating role of XRCC1 and the effect of its disease-associated variants on substrate-product channeling in multiprotein/DNA complexes for efficient BER.

Spinocerebellar Ataxia Type 1 protein Ataxin-1 is signaled to DNA damage by ataxia-telangiectasia mutated kinase

Spinocerebellar Ataxia Type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the ataxin-1 protein. Recent genetic correlational studies have implicated DNA damage repair pathways in modifying the age at onset of disease symptoms in SCA1 and Huntington's Disease, another polyglutamine expansion disease. We demonstrate that both endogenous and transfected ataxin-1 localizes to sites of DNA damage, which is impaired by polyglutamine expansion. This response is dependent on ataxia-telangiectasia mutated (ATM) kinase activity. Further, we characterize an ATM phosphorylation motif within ataxin-1 at serine 188. We show reduction of the Drosophila ATM homolog levels in a ATXN1[82Q] Drosophila model through shRNA or genetic cross ameliorates motor symptoms. These findings offer a possible explanation as to why DNA repair was implicated in SCA1 pathogenesis by past studies. The similarities between the ataxin-1 and the huntingtin responses to DNA damage provide further support for a shared pathogenic mechanism for polyglutamine expansion diseases.

Small molecule inhibitor of OGG1 blocks oxidative DNA damage repair at telomeres and potentiates methotrexate anticancer effects

The most common oxidative DNA lesion is 8-oxoguanine which is mainly recognized and excised by the 8-oxoG DNA glycosylase 1 (OGG1), initiating the base excision repair (BER) pathway. Telomeres are particularly sensitive to oxidative stress (OS) which disrupts telomere homeostasis triggering genome instability. In the present study, we have investigated the effects of inactivating BER in OS conditions, by using a specific inhibitor of OGG1 (TH5487). We have found that in OS conditions, TH5487 blocks BER initiation at telomeres causing an accumulation of oxidized bases, that is correlated with telomere losses, micronuclei formation and mild proliferation defects. Moreover, the antimetabolite methotrexate synergizes with TH5487 through induction of intracellular reactive oxygen species (ROS) formation, which potentiates TH5487-mediated telomere and genome instability. Our findings demonstrate that OGG1 is required to protect telomeres from OS and present OGG1 inhibitors as a tool to induce oxidative DNA damage at telomeres, with the potential for developing new combination therapies for cancer treatment.

Xeroderma Pigmentosum C (XPC) Mutations in Primary Fibroblasts Impair Base Excision Repair Pathway and Increase Oxidative DNA Damage

Xeroderma Pigmentosum C (XPC) is a multi-functional protein that is involved not only in the repair of bulky lesions, post-irradiation, via nucleotide excision repair (NER) per se but also in oxidative DNA damage mending. Since base excision repair (BER) is the primary regulator of oxidative DNA damage, we characterized, post-Ultraviolet B-rays (UVB)-irradiation, the detailed effect of three different XPC mutations in primary fibroblasts derived from XP-C patients on mRNA, protein expression and activity of different BER factors. We found that XP-C fibroblasts are characterized by downregulated expression of different BER factors including OGG1, MYH, APE1, LIG3, XRCC1, and Polβ. Such a downregulation was also observed at OGG1, MYH, and APE1 protein levels. This was accompanied with an increase in DNA oxidative lesions, as evidenced by 8-oxoguanine levels, immediately post-UVB-irradiation. Unlike in normal control cells, these oxidative lesions persisted over time in XP-C cells having lower excision repair capacities. Taken together, our results indicated that an impaired BER pathway in XP-C fibroblasts leads to longer persistence and delayed repair of oxidative DNA damage. This might explain the diverse clinical phenotypes in XP-C patients suffering from cancer in both photo-protected and photo-exposed areas. Therapeutic strategies based on reinforcement of BER pathway might therefore represent an innovative path for limiting the drawbacks of NER-based diseases, as in XP-C case.

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

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

Chromatin recruitment of OGG1 requires cohesin and mediator and is essential for efficient 8-oxoG removal

One of the most abundant DNA lesions induced by oxidative stress is the highly mutagenic 8-oxoguanine (8-oxoG), which is specifically recognized by 8-oxoguanine DNA glycosylase 1 (OGG1) to initiate its repair. How DNA glycosylases find small non-helix-distorting DNA lesions amongst millions of bases packaged in the chromatin-based architecture of the genome remains an open question. Here, we used a high-throughput siRNA screening to identify factors involved in the recognition of 8-oxoG by OGG1. We show that cohesin and mediator subunits are required for re-localization of OGG1 and other base excision repair factors to chromatin upon oxidative stress. The association of OGG1 with euchromatin is necessary for the removal of 8-oxoG. Mediator subunits CDK8 and MED12 bind to chromatin and interact with OGG1 in response to oxidative stress, suggesting they participate in the recruitment of the DNA glycosylase. The oxidative stress-induced association between the cohesin and mediator complexes and OGG1 reveals an unsuspected function of those complexes in the maintenance of genomic stability.

MasterPATH: network analysis of functional genomics screening data

Background

Functional genomics employs several experimental approaches to investigate gene functions. High-throughput techniques, such as loss-of-function screening and transcriptome profiling, allow to identify lists of genes potentially involved in biological processes of interest (so called hit list). Several computational methods exist to analyze and interpret such lists, the most widespread of which aim either at investigating of significantly enriched biological processes, or at extracting significantly represented subnetworks.

Results

Here we propose a novel network analysis method and corresponding computational software that employs the shortest path approach and centrality measure to discover members of molecular pathways leading to the studied phenotype, based on functional genomics screening data. The method works on integrated interactomes that consist of both directed and undirected networks – HIPPIE, SIGNOR, SignaLink, TFactS, KEGG, TransmiR, miRTarBase. The method finds nodes and short simple paths with significant high centrality in subnetworks induced by the hit genes and by so-called final implementers – the genes that are involved in molecular events responsible for final phenotypic realization of the biological processes of interest. We present the application of the method to the data from miRNA loss-of-function screen and transcriptome profiling of terminal human muscle differentiation process and to the gene loss-of-function screen exploring the genes that regulates human oxidative DNA damage recognition. The analysis highlighted the possible role of several known myogenesis regulatory miRNAs (miR-1, miR-125b, miR-216a) and their targets (AR, NR3C1, ARRB1, ITSN1, VAV3, TDGF1), as well as linked two major regulatory molecules of skeletal myogenesis, MYOD and SMAD3, to their previously known muscle-related targets (TGFB1, CDC42, CTCF) and also to a number of proteins such as C-KIT that have not been previously studied in the context of muscle differentiation. The analysis also showed the role of the interaction between H3 and SETDB1 proteins for oxidative DNA damage recognition.

Conclusion

The current work provides a systematic methodology to discover members of molecular pathways in integrated networks using functional genomics screening data. It also offers a valuable instrument to explain the appearance of a set of genes, previously not associated with the process of interest, in the hit list of each particular functional genomics screening.

Direct and Oxidative DNA Damage in a Group of Painters Exposed to VOCs: Dose - Response Relationship

Volatile organic compounds (VOCs) are present in several working activities. This work is aimed at comparing oxidative stress and DNA damage biomarkers to specific VOCs in the occupational exposure of painters. Dose-response relationships between biomarkers of oxidative stress and of dose were studied. Unmetabolized VOCs and their urinary metabolites were analyzed. Urinary Methylhyppuric acids (MHIPPs, xylenes metabolite), Phenylglyoxylic and Mandelic acid (PGA, MA ethylbenzene metabolites), S-Benzylmercapturic acid (SBMA, toluene metabolite), and S-Phenylmercapturic acid (SPMA, benzene metabolite) were quantified at the end of work-shift. Oxidative stress was determined by: urinary excretion of 8-oxodGuo, 8-oxoGua and 8-oxoGuo and direct/oxidative DNA damage in blood by Fpg-Comet assay. Multivariate linear regression models were used to assess statistical significance of the association between dose and effect biomarkers. The regressions were studied with and without the effect of hOGG1 and XRCC1 gene polymorphisms. Statistically significant associations were found between MHIPPs and both 8-oxoGuo and oxidative DNA damage effect biomarkers measured with the Comet assay. Oxidative DNA damage results significantly associated with airborne xylenes and toluene, whilst 8-oxodGuo was significantly related to urinary xylenes and toluene. Direct DNA damage was significantly associated to SBMA. XRCC1 wild-type gene polymorphism was significantly associated with lower oxidative and total DNA damage with respect to heterozygous and mutant genotypes. The interpretation of the results requires some caution, as the different VOCs are all simultaneously present in the mixture and correlated among them.

Kinetics of DNA damage repair response accompanying initial hepadnavirus-host genomic integration in woodchuck hepatitis virus infection of hepatocyte

Mechanism of initial hepatitis B virus (HBV) integrations and kinetics of DNA repair immediately after infection remain essentially unknown impairing understanding of hepadnaviral oncogenesis. WCM260 hepatocytes susceptible to HBV-compatible woodchuck hepatitis virus (WHV) were examined from 15 min to 72 h post-infection (p.i.). WHV strongly induced reactive oxygen species (ROS), transiently inducible nitric oxide (iNOS) and DNA damage from 15 min p.i. All initial WHV-host fusions had the head-to-tail format indicating their formation by non-homologous end joining (NHEJ). Transcription of poly(ADP-ribose) polymerase 1 (PARP1) and X-ray repair cross-complementing protein 1 (XRCC1), the PARP1 binding partner, were induced in 30 min p.i. and that of 8-oxyguanine DNA glycosylse (OGG1) responding to oxidative DNA damage at 12 h p.i. Nicotinamide adenine dinucleotide (NAD⁺), a marker of PARP1 activation, and heme oxygenase-1 (HO1), an indicator of pro-oxidative stress, were significantly augmented from 15-30 min p.i. Additionally, PARP1 cleavage activity was evident from 30 min p.i. confirming that PARP1-mediated DNA repair became operational almost instatly after hepatocyte contact with virus. By applying complementary approaches, the study showed that initial WHV integration was due to virus-induced oxidative DNA damage and implied that the NHEJ PARP1-dependent repair pathway determined format of the first virus-host DNA junctions.

Shining light on the response to repair intermediates in DNA of living cells

Fluorescently-tagged repair proteins have been widely used to probe recruitment to micro-irradiation-induced nuclear DNA damage in living cells. Here, we quantify APE1 dynamics after micro-irradiation. Markers of DNA damage are characterized and UV-A laser micro-irradiation energy conditions are selected for formation of oxidatively-induced DNA base damage and single strand breaks, but without detectable double strand breaks. Increased energy of laser micro-irradiation, compared with that used previously in our work, enables study of APE1 dynamics at the lesion site. APE1 shows rapid transient kinetics, with recruitment half-time of less than 1 s and dissociation half-time of less than 15 s. In cells co-transfected with APE1 and PARP1, the recruitment half-time of PARP1 was slower than that of APE1, indicating APE1 is a rapid responder to the damage site. While recruitment of APE1 is unchanged in the presence of co-transfected PARP1, APE1 dissociation is 3-fold slower, revealing PARP1 involvement in APE1 dynamics. Further, we find that APE1 dissociation kinetics are strongly modified in the absence of DNA polymerase β (pol β). After unchanged recruitment to the damage site, dissociation of APE1 became undetectable. This indicates a necessary role for pol β in APE1 release after its recruitment to the damage site. These observations represent an advance in our understanding of in vivo dynamics of base excision repair factors APE1, PARP1 and pol β.

Roles of OGG1 in transcriptional regulation and maintenance of metabolic homeostasis

Cellular damage produced by conditions generating oxidative stress have far-reaching implications in human disease that encompass, but are not restricted to aging, cardiovascular disease, type 2 diabetes, airway inflammation/asthma, cancer, and metabolic syndrome including visceral obesity, insulin resistance, fatty liver disease, and dyslipidemia. Although there are numerous sources and cellular targets of oxidative stress, this review will highlight literature that has investigated downstream consequences of oxidatively-induced DNA damage in both nuclear and mitochondrial genomes. The presence of such damage can in turn, directly and indirectly modulate cellular transcriptional and repair responses to such stressors. As such, the persistence of base damage can serve as a key regulator in coordinated gene-response cascades. Conversely, repair of these DNA lesions serves as both a suppressor of mutagenesis and by inference carcinogenesis, and as a signal for the cessation of ongoing oxidative stress. A key enzyme in all these processes is 8-oxoguanine DNA glycosylase (OGG1), which, via non-catalytic binding to oxidatively-induced DNA damage in promoter regions, serves as a nucleation site around which changes in large-scale regulation of inflammation-associated gene expression can occur. Further, the catalytic function of OGG1 can alter the three-dimensional structure of specialized DNA sequences, leading to changes in transcriptional profiles. This review will concentrate on adverse deleterious health effects that are associated with both the diminution of OGG1 activity via population-specific polymorphic variants and the complete loss of OGG1 in murine models. This mouse model displays diet- and age-related induction of metabolic syndrome, highlighting a key role for OGG1 in protecting against these phenotypes. Conversely, recent investigations using murine models having enhanced global expression of a mitochondrial-targeted OGG1 demonstrate that they are highly resistant to diet-induced disease. These data suggest strategies through which therapeutic interventions could be designed for reducing or limiting adverse human health consequences to these ubiquitous stressors.

The transcription-coupled DNA repair-initiating protein CSB promotes XRCC1 recruitment to oxidative DNA damage

Transcription-coupled nucleotide excision repair factor Cockayne syndrome protein B (CSB) was suggested to function in the repair of oxidative DNA damage. However thus far, no clear role for CSB in base excision repair (BER), the dedicated pathway to remove abundant oxidative DNA damage, could be established. Using live cell imaging with a laser-assisted procedure to locally induce 8-oxo-7,8-dihydroguanine (8-oxoG) lesions, we previously showed that CSB is recruited to these lesions in a transcription-dependent but NER-independent fashion. Here we showed that recruitment of the preferred 8-oxoG-glycosylase 1 (OGG1) is independent of CSB or active transcription. In contrast, recruitment of the BER-scaffolding protein, X-ray repair cross-complementing protein 1 (XRCC1), to 8-oxoG lesions is stimulated by CSB and transcription. Remarkably, recruitment of XRCC1 to BER-unrelated single strand breaks (SSBs) does not require CSB or transcription. Together, our results suggest a specific transcription-dependent role for CSB in recruiting XRCC1 to BER-generated SSBs, whereas XRCC1 recruitment to SSBs generated independently of BER relies predominantly on PARP activation. Based on our results, we propose a model in which CSB plays a role in facilitating BER progression at transcribed genes, probably to allow XRCC1 recruitment to BER-intermediates masked by RNA polymerase II complexes stalled at these intermediates.

Genetic identification and characterization of three genes that prevent accumulation of oxidative DNA damage in Drosophila adult tissues

Reactive oxygen species generated in the process of energy production represent a major cause of oxidative DNA damage. Production of the oxidized guanine base, 8-oxo-guanine (8-oxoG), results in mismatched pairing with adenine and subsequently leads to G:C to T:A transversions after DNA replication. Our previous study demonstrated that Drosophila CG1795 encodes an ortholog of Ogg1, which is essential for the elimination of 8-oxoG. Moreover, the Drosophila ribosomal protein S3 (RpS3) possesses N-glycosylase activity that eliminates 8-oxoG in vitro. In this study, we show that RpS3 heterozygotes hyper-accumulate 8-oxoG in midgut cell nuclei after oxidant feeding, suggesting thatRpS3 is required for the elimination of 8-oxoG in Drosophila adults. We further showed that several muscle-aging phenotypes were significantly accelerated in RpS3 heterozygotes. Ogg1 is localized in the nucleus, while RpS3 is in the cytoplasm, closely associated with endoplasmic reticulum networks. Results of genetic analyses also suggest that these two proteins operate similarly but independently in the elimination of oxidized guanine bases from genomic DNA. Next, we obtained genetic evidence suggesting that CG42813 functions as the Drosophila ortholog of mammalian Mth1 in the elimination of oxidized dGTP (8-oxo-dGTP) from the nucleotide pool. Depletion of this gene significantly increased the number of DNA damage foci in the nuclei of Drosophila midgut cells. Furthermore, several aging-related phenotypes such as age-dependent loss of adult locomotor activities and accumulation of polyubiquitylated proteins in adult muscles were also significantly accelerated in CG42813-depleted flies. Lastly, we investigated the phenotype of adults depleted of CG9272, which encodes a protein with homology to mammalian Nth1 that is essential for the elimination of oxidized thymine. Excessive accumulation of oxidized bases was observed in the epithelial cell nuclei after oxidant feeding. In conclusion, three genes that prevent accumulation of oxidative DNA damage were identified in Drosophila.

Identification of an XRCC1 DNA binding activity essential for retention at sites of DNA damage

Repair of two major forms of DNA damage, single strand breaks and base modifications, are dependent on XRCC1. XRCC1 orchestrates these repair processes by temporally and spatially coordinating interactions between several other repair proteins. Here we show that XRCC1 contains a central DNA binding domain (CDB, residues 219–415) encompassing its first BRCT domain. In contrast to the N-terminal domain of XRCC1, which has been reported to mediate damage sensing in vitro, we demonstrate that the DNA binding module identified here lacks binding specificity towards DNA containing nicks or gaps. Alanine substitution of residues within the CDB of XRCC1 disrupt DNA binding in vitro and lead to a significant reduction in XRCC1 retention at DNA damage sites without affecting initial recruitment. Interestingly, reduced retention at sites of DNA damage is associated with an increased rate of repair. These findings suggest that DNA binding activity of XRCC1 plays a significant role in retention at sites of damage and the rate at which damage is repaired.

LSD1 mediated changes in the local redox environment during the DNA damage response

The redox state of the cell can be affected by many cellular conditions. In this study we show that detectable reactive oxygen species (ROS) are also generated in response to DNA damage by the chromatin remodeling factor and monoamine oxidase LSD1/KDM1A. This raised the possibility that the localized generation of hydrogen peroxide produced by LSD1 may affect the function of proximally located DNA repair proteins. The two major pathways for repair of DNA double-strand breaks (DSBs) are homologous recombination (HR) and non-homologous end joining (NHEJ). Cells were exposed to low levels of ectopic H₂O₂, DNA breaks generated by laser light, and recruitment kinetics of NHEJ protein Ku80 to DNA damage sites determined. Ku80 recruitment to damage sites was significantly decreased in cells pretreated with H₂O₂ while HR end binding protein Nbs1 was increased. This suggests that the DNA repair pathway choice has the potential to be modulated by the local redox state. This has implications for chemotherapeutic approaches involving generating DNA damage to target actively dividing cancer cells, which may be more or less effective dependent on the redox state of the targeted cells and the predominant repair pathway required to repair the type of DNA damage generated.

Broad spectrum detection of DNA damage by Repair Assisted Damage Detection (RADD)

Environmental exposures, reactive by-products of cellular metabolism, and spontaneous deamination events result in a spectrum of DNA adducts that if un-repaired threaten genomic integrity by inducing mutations, increasing instability, and contributing to the initiation and progression of cancer. Assessment of DNA adducts in cells and tissues is critical for genotoxic and carcinogenic evaluation of chemical exposure and may provide insight into the etiology of cancer. Numerous methods to characterize the formation of DNA adducts and their retention for risk assessment have been developed. However, there are still significant drawbacks to the implementation and wide-spread use of these methods, because they often require a substantial amount of biological sample, highly specialized expertise and equipment, and depending on technique, may be limited to the detection and quantification of only a handful of DNA adducts at a time. There is a pressing need for high throughput, easy to implement assays that can assess a broad spectrum of DNA lesions, allowing for faster evaluation of chemical exposures and assessment of the retention of adducts in biological samples. Here, we describe a new methodology, Repair Assisted Damage Detection (RADD), which utilizes a DNA damage processing repair enzyme cocktail to detect and modify sites of DNA damage for a subsequent gap filling reaction that labels the DNA damage sites. This ability to detect and label a broad spectrum of DNA lesions within cells, offers a novel and easy to use tool for assessing levels of DNA damage in cells that have been exposed to environmental agents or have natural variations in DNA repair capacity.

8-Oxoguanine DNA glycosylase 1: Beyond repair of the oxidatively modified base lesions

Oxidative stress and the resulting damage to genomic DNA are inevitable consequences of endogenous physiological processes, and they are amplified by cellular responses to environmental exposures. One of the most frequent reactions of reactive oxygen species with DNA is the oxidation of guanine to pre-mutagenic 8-oxo-7,8-dihydroguanine (8-oxoG). Despite the vulnerability of guanine to oxidation, vertebrate genes are primarily embedded in GC-rich genomic regions, and over 72% of the promoters of human genes belong to a class with a high GC content. In the promoter, 8-oxoG may serve as an epigenetic mark, and when complexed with the oxidatively inactivated repair enzyme 8-oxoguanine DNA glycosylase 1, provide a platform for the coordination of the initial steps of DNA repair and the assembly of the transcriptional machinery to launch the prompt and preferential expression of redox-regulated genes. Deviations/variations from this artful coordination may be the etiological links between guanine oxidation and various cellular pathologies and diseases during ageing processes.

The Relationship Between Single-Nucleotide Polymorphisms, the Expression of DNA Damage Response Genes, and Hepatocellular Carcinoma in a Polish Population

The molecular mechanism of hepatocellular carcinoma (HCC) is related to DNA damage caused by oxidative stress products induced by hepatitis B virus (HBV) or C (HCV) infection and exposure to environmental pollutants. Single-nucleotide polymorphisms (SNPs) of DNA damage response (DDR) genes may influence individual susceptibility to environmental risk factors and affect DNA repair efficacy, which, in turn, can influence the risk of HCC. The study evaluates a panel of 15 SNPs in 11 DDR genes (XRCC1, XRCC3, XPD, MUTYH, LIG1, LIG3, hOGG1, PARP1, NFIL1, FEN1, and APEX1) in 65 HCC patients, 50 HBV- and 50 HCV-infected non-cancerous patients, and 50 healthy controls. It also estimates the mRNA expression of nine DDR genes in cancerous and adjacent healthy liver tissues. Two of the investigated polymorphisms (rs1052133 and rs13181) were associated with HCC risk. For all investigated genes, the level of mRNA was significantly lower in HCC cancer tissue than in non-cancerous liver tissue. Seven of the investigated polymorphisms were statistically related to gene expression in cancer tissues. The disruption of DDR genes may be responsible for hepatocellular transformation in HCV-infected patients.

Coordination of DNA single strand break repair

The genetic material of all organisms is susceptible to modification. In some instances, these changes are programmed, such as the formation of DNA double strand breaks during meiotic recombination to generate gamete variety or class switch recombination to create antibody diversity. However, in most cases, genomic damage is potentially harmful to the health of the organism, contributing to disease and aging by promoting deleterious cellular outcomes. A proportion of DNA modifications are caused by exogenous agents, both physical (namely ultraviolet sunlight and ionizing radiation) and chemical (such as benzopyrene, alkylating agents, platinum compounds and psoralens), which can produce numerous forms of DNA damage, including a range of "simple" and helix-distorting base lesions, abasic sites, crosslinks and various types of phosphodiester strand breaks. More significant in terms of frequency are endogenous mechanisms of modification, which include hydrolytic disintegration of DNA chemical bonds, attack by reactive oxygen species and other byproducts of normal cellular metabolism, or incomplete or necessary enzymatic reactions (such as topoisomerases or repair nucleases). Both exogenous and endogenous mechanisms are associated with a high risk of single strand breakage, either produced directly or generated as intermediates of DNA repair. This review will focus upon the creation, consequences and resolution of single strand breaks, with a particular focus on two major coordinating repair proteins: poly(ADP-ribose) polymerase 1 (PARP1) and X-ray repair cross-complementing protein 1 (XRCC1).

Base excision repair of oxidative DNA damage: from mechanism to disease

Reactive oxygen species continuously assault the structure of DNA resulting in oxidation and fragmentation of the nucleobases. Both oxidative DNA damage itself and its repair mediate the progression of many prevalent human maladies. The major pathway tasked with removal of oxidative DNA damage, and hence maintaining genomic integrity, is base excision repair (BER). The aphorism that structure often dictates function has proven true, as numerous recent structural biology studies have aided in clarifying the molecular mechanisms used by key BER enzymes during the repair of damaged DNA. This review focuses on the mechanistic details of the individual BER enzymes and the association of these enzymes during the development and progression of human diseases, including cancer and neurological diseases. Expanding on these structural and biochemical studies to further clarify still elusive BER mechanisms, and focusing our efforts toward gaining an improved appreciation of how these enzymes form co-complexes to facilitate DNA repair is a crucial next step toward understanding how BER contributes to human maladies and how it can be manipulated to alter patient outcomes.

Oxidized nucleotide insertion by pol β confounds ligation during base excision repair

Oxidative stress in cells can lead to accumulation of reactive oxygen species and oxidation of DNA precursors. Oxidized purine nucleotides can be inserted into DNA during replication and repair. The main pathway for correcting oxidized bases in DNA is base excision repair (BER), and in vertebrates DNA polymerase β (pol β) provides gap filling and tailoring functions. Here we report that the DNA ligation step of BER is compromised after pol β insertion of oxidized purine nucleotides into the BER intermediate in vitro. These results suggest the possibility that BER mediated toxic strand breaks are produced in cells under oxidative stress conditions. We observe enhanced cytotoxicity in oxidizing-agent treated pol β expressing mouse fibroblasts, suggesting formation of DNA strand breaks under these treatment conditions. Increased cytotoxicity following MTH1 knockout or treatment with MTH1 inhibitor suggests the oxidation of precursor nucleotides.

Oxidative stress in cells leads to the oxidations of DNA precursors. Here the authors show that these oxidized precursors can be incorporated in vivo during base excision repair, leading to DNA breaks and cytotoxicity.

Modulation of Radiation-induced Base Excision Repair Pathway Gene Expression by Melatonin

Objective:

Approximately 70% of all cancer patients receive radiotherapy. Although radiotherapy is effective in killing cancer cells, it has adverse effects on normal cells as well. Melatonin (MLT) as a potent antioxidant and anti-inflammatory agent has been proposed to stimulate DNA repair capacity. We investigated the capability of MLT in the modification of radiation-induced DNA damage in rat peripheral blood cells.

Materials and Methods:

In this experimental study, male rats (n = 162) were divided into 27 groups (n = 6 in each group) including: irradiation only, vehicle only, vehicle with irradiation, 100 mg/kg MLT alone, 100 mg/kg MLT plus irradiation in 3 different time points, and control. Subsequently, they were irradiated with a single whole-body X-ray radiation dose of 2 and 8 Gy at a dose rate of 200 MU/min. Rats were given an intraperitoneal injection of MLT or the same volume of vehicle alone 1 h prior to irradiation. Blood samples were also taken 8, 24, and 48 h postirradiation, in order to measure the 8-oxoguanine glycosylase1 (Ogg1), Apex1, and Xrcc1 expression using quantitative real-time-polymerase chain reaction.

Results:

Exposing to the ionizing radiation resulted in downregulation of Ogg1, Apex1, and Xrcc1 gene expression. The most obvious suppression was observed in 8 h after exposure. Pretreatments with MLT were able to upregulate these genes when compared to the irradiation-only and vehicle plus irradiation groups (P < 0.05) in all time points.

Conclusion:

Our results suggested that MLT in mentioned dose may result in modulation of Ogg1, Apex1, and Xrcc1 gene expression in peripheral blood cells to reduce X-ray irradiation-induced DNA damage. Therefore, administration of MLT may increase the normal tissue tolerance to radiation through enhancing the cell DNA repair capacity. We believed that MLT could play a radiation toxicity reduction role in patients who have undergone radiation treatment as a part of cancer radiotherapy.

FOXO3 Transcription Factor Is Essential for Protecting Hematopoietic Stem and Progenitor Cells from Oxidative DNA Damage

Accumulation of damaged DNA in hematopoietic stem cells (HSC) is associated with chromosomal abnormalities, genomic instability, and HSC aging and might promote hematological malignancies with age. Despite this, the regulatory pathways implicated in the HSC DNA damage response have not been fully elucidated. One of the sources of DNA damage is reactive oxygen species (ROS) generated by both exogenous and endogenous insults. Balancing ROS levels in HSC requires FOXO3, which is an essential transcription factor for HSC maintenance implicated in HSC aging. Elevated ROS levels result in defective Foxo3^-/- HSC cycling, among many other deficiencies. Here, we show that loss of FOXO3 leads to the accumulation of DNA damage in primitive hematopoietic stem and progenitor cells (HSPC), associated specifically with reduced expression of genes implicated in the repair of oxidative DNA damage. We provide further evidence that Foxo3^-/- HSPC are defective in DNA damage repair. Specifically, we show that the base excision repair pathway, the main pathway utilized for the repair of oxidative DNA damage, is compromised in Foxo3^-/- primitive hematopoietic cells. Treating mice in vivo with N-acetylcysteine reduces ROS levels, rescues HSC cycling defects, and partially mitigates HSPC DNA damage. These results indicate that DNA damage accrued as a result of elevated ROS in Foxo3^-/- mutant HSPC is at least partially reversible. Collectively, our findings suggest that FOXO3 serves as a protector of HSC genomic stability and health.

Characterization of a Thermostable 8-Oxoguanine DNA Glycosylase Specific for GO/N Mismatches from the Thermoacidophilic Archaeon Thermoplasma volcanium

The oxidation of guanine (G) to 7,8-dihydro-8-oxoguanine (GO) forms one of the major DNA lesions generated by reactive oxygen species (ROS). The GO can be corrected by GO DNA glycosylases (Ogg), enzymes involved in base excision repair (BER). Unrepaired GO induces mismatched base pairing with adenine (A); as a result, the mismatch causes a point mutation, from G paired with cytosine (C) to thymine (T) paired with adenine (A), during DNA replication. Here, we report the characterization of a putative Ogg from the thermoacidophilic archaeon Thermoplasma volcanium. The 204-amino acid sequence of the putative Ogg (TVG_RS00315) shares significant sequence homology with the DNA glycosylases of Methanocaldococcus jannaschii (MjaOgg) and Sulfolobus solfataricus (SsoOgg). The six histidine-tagged recombinant TVG_RS00315 protein gene was expressed in Escherichia coli and purified. The Ogg protein is thermostable, with optimal activity near a pH of 7.5 and a temperature of 60°C. The enzyme displays DNA glycosylase, and apurinic/apyrimidinic (AP) lyase activities on GO/N (where N is A, T, G, or C) mismatch; yet it cannot eliminate U from U/G or T from T/G, as mismatch glycosylase (MIG) can. These results indicate that TvoOgg-encoding TVG_RS00315 is a member of the Ogg2 family of T. volcanium.

Special AT-rich Sequence-binding Protein 1 (SATB1) Functions as an Accessory Factor in Base Excision Repair

Base excision repair is initiated by DNA glycosylases that recognize specific altered bases. DNA glycosylases for oxidized bases carry both a glycosylase activity that removes the faulty base and an apyrimidinic/apurinic lyase activity that introduces a single-strand DNA incision. In particular, the CUT domains within the CUX1 and CUX2 proteins were recently shown to interact with the 8-oxoguanine (8-oxoG) DNA glycosylase and stimulate its enzymatic activities. SATB1, which contains two CUT domains, was originally characterized as a T cell-specific genome organizer whose aberrant overexpression in breast cancer can promote tumor progression. Here we investigated the involvement of SATB1 in DNA repair. SATB1 knockdown caused a delay in DNA repair following exposure to H₂O₂, an increase in OGG1-sensitive oxidized bases within genomic DNA, and a decrease in 8-oxoG cleavage activity in cell extracts. In parallel, we observed an increase in phospho-CHK1 and γ-H2AX levels and a decrease in DNA synthesis. Conversely, ectopic expression of SATB1 accelerated DNA repair and reduced the levels of oxidized bases in genomic DNA. Moreover, an enhanced GFP-SATB1 fusion protein was rapidly recruited to laser microirradiation-induced DNA damage. Using purified proteins, we showed that SATB1 interacts directly with OGG1, increases its binding to 8-oxoG-containing DNA, promotes Schiff base formation, and stimulates its glycosylase and apyrimidinic/apurinic lyase enzymatic activities. Structure/function analysis demonstrated that CUT domains, but not the homeodomain, are responsible for the stimulation of OGG1. Together, these results identify another CUT domain protein that functions both as a transcription factor and an accessory factor in base excision repair.

The Response to Oxidative DNA Damage in Neurons: Mechanisms and Disease

There is a growing body of evidence indicating that the mechanisms that control genome stability are of key importance in the development and function of the nervous system. The major threat for neurons is oxidative DNA damage, which is repaired by the base excision repair (BER) pathway. Functional mutations of enzymes that are involved in the processing of single-strand breaks (SSB) that are generated during BER have been causally associated with syndromes that present important neurological alterations and cognitive decline. In this review, the plasticity of BER during neurogenesis and the importance of an efficient BER for correct brain function will be specifically addressed paying particular attention to the brain region and neuron-selectivity in SSB repair-associated neurological syndromes and age-related neurodegenerative diseases.

Targeting Mitochondria and Reactive Oxygen Species-Driven Pathogenesis in Diabetic Nephropathy

Diabetic kidney disease is one of the major microvascular complications of both type 1 and type 2 diabetes mellitus. Approximately 30% of patients with diabetes experience renal complications. Current clinical therapies can only mitigate the symptoms and delay the progression to end-stage renal disease, but not prevent or reverse it. Oxidative stress is an important player in the pathogenesis of diabetic nephropathy. The activity of reactive oxygen and nitrogen species (ROS/NS), which are by-products of the diabetic milieu, has been found to correlate with pathological changes observed in the diabetic kidney. However, many clinical studies have failed to establish that antioxidant therapy is renoprotective. The discovery that increased ROS/NS activity is linked to mitochondrial dysfunction, endoplasmic reticulum stress, inflammation, cellular senescence, and cell death calls for a refined approach to antioxidant therapy. It is becoming clear that mitochondria play a key role in the generation of ROS/NS and their consequences on the cellular pathways involved in apoptotic cell death in the diabetic kidney. Oxidative stress has also been associated with necrosis via induction of mitochondrial permeability transition. This review highlights the importance of mitochondria in regulating redox balance, modulating cellular responses to oxidative stress, and influencing cell death pathways in diabetic kidney disease. ROS/NS-mediated cellular dysfunction corresponds with progressive disease in the diabetic kidney, and consequently represents an important clinical target. Based on this consideration, this review also examines current therapeutic interventions to prevent ROS/NS-derived injury in the diabetic kidney. These interventions, mainly aimed at reducing or preventing mitochondrial-generated oxidative stress, improving mitochondrial antioxidant defense, and maintaining mitochondrial integrity, may deliver alternative approaches to halt or prevent diabetic kidney disease.

CUX2 protein functions as an accessory factor in the repair of oxidative DNA damage

CUX1 and CUX2 proteins are characterized by the presence of three highly similar regions called Cut repeats 1, 2, and 3. Although CUX1 is ubiquitously expressed, CUX2 plays an important role in the specification of neuronal cells and continues to be expressed in postmitotic neurons. Cut repeats from the CUX1 protein were recently shown to stimulate 8-oxoguanine DNA glycosylase 1 (OGG1), an enzyme that removes oxidized purines from DNA and introduces a single strand break through its apurinic/apyrimidinic lyase activity to initiate base excision repair. Here, we investigated whether CUX2 plays a similar role in the repair of oxidative DNA damage. Cux2 knockdown in embryonic cortical neurons increased levels of oxidative DNA damage. In vitro, Cut repeats from CUX2 increased the binding of OGG1 to 7,8-dihydro-8-oxoguanine-containing DNA and stimulated both the glycosylase and apurinic/apyrimidinic lyase activities of OGG1. Genetic inactivation in mouse embryo fibroblasts or CUX2 knockdown in HCC38 cells delayed DNA repair and increased DNA damage. Conversely, ectopic expression of Cut repeats from CUX2 accelerated DNA repair and reduced levels of oxidative DNA damage. These results demonstrate that CUX2 functions as an accessory factor that stimulates the repair of oxidative DNA damage. Neurons produce a high level of reactive oxygen species because of their dependence on aerobic oxidation of glucose as their source of energy. Our results suggest that the persistent expression of CUX2 in postmitotic neurons contributes to the maintenance of genome integrity through its stimulation of oxidative DNA damage repair.