Mitochondrial maintenance under oxidative stress depends on mitochondrially localised α-OGG1

Redox shuttle of cytosolic Thioredoxin to mitochondria protects against hyperoxia-mediated alteration of mitochondrial structure and dysfunction

Cytosolic thioredoxin (Trx) is a critical redox protein that converts protein disulfides to thiols via catalytic activity of thioredoxin reductase-1 (TrxR1) and NADPH. Thioredoxin-2 (Trx2) is a mitochondria-localized isoform. It is generally believed that Trx and Trx2 perform similar functions within the cytosol and mitochondria respectively. Here, we demonstrate that cytosolic Trx shuttles into mitochondria in the presence of normal levels of Trx2 in physiological state and higher levels of Trx translocate to mitochondria in oxidative stress conditions such as exposure to high concentrations of oxygen. This shuttle is required to maintain mitochondrial structure and function during physiological and oxidative stress conditions. Further, reduced Trx (Trx-SH) shuttle into mitochondria to protect against the downregulation of several mitochondrially coded genes and proteins of respiratory chain complexes in oxidative stress. Translocation of Trx occurs only in the reduced state as oxidized or cysteine mutant Trx is unable to translocate to the mitochondria. Accumulation of mitochondrial DNA damage product 8-Oxo-dG in hyperoxia is decreased in the presence of higher levels of cytosolic Trx within the mitochondrion. Collectively, our data demonstrate that shuttling of reduced cytosolic Trx into mitochondria protects against mitochondrial DNA damage, decreased gene and protein expression of respiratory chain complexes and mitochondrial dysfunction resulting in restoration of their native function and cell survival in physiological and oxidative stress conditions.

Modulation of OGG1 enzymatic activities by small molecules, promising tools and current challenges

Oxidative DNA damage, resulting from endogenous cellular processes and external sources plays a significant role in mutagenesis, cancer progression, and the pathogenesis of neurological disorders. Base Excision Repair (BER) is involved in the repair of base modifications such as oxidations or alkylations as well as single strand breaks. The DNA glycosylase OGG1, initiates the BER pathway by the recognition and excision of 8oxoG, the most common oxidative DNA lesion, in both nuclear and mitochondrial DNA. Beyond DNA repair, OGG1 modulates transcription, particularly pro-inflammatory genes, linking oxidative DNA damage to broader biological processes like inflammation and aging. In cancer therapy, BER inhibition has emerged as a promising strategy to enhance treatment efficacy. Targeting OGG1 sensitizes cells to chemotherapies, radiotherapies, and PARP inhibitors, presenting opportunities to overcome therapy resistance. Additionally, OGG1 activators hold potential in mitigating oxidative damage associated with aging and neurological disorders. This review presents the development of several inhibitors and activators of OGG1 and how they have contributed to advance our knowledge in the fundamental functions of OGG1. We also discuss the new opportunities they provide for clinical applications in treating cancer, inflammation and neurological disorders. Finally, we also highlight the challenges in targeting OGG1, particularly regarding the off-target effects recently reported for some inhibitors and how we can overcome these limitations.

Decoding mitochondrial DNA damage and repair associated with H. pylori infection

Mitochondrial genomic stability is critical to prevent various human inflammatory diseases. Bacterial infection significantly increases oxidative stress, driving mitochondrial genomic instability and initiating inflammatory human disease. Oxidative DNA base damage is predominantly repaired by base excision repair (BER) in the nucleus (nBER) as well as in the mitochondria (mtBER). In this review, we summarize the molecular mechanisms of spontaneous and H. pylori infection-associated oxidative mtDNA damage, mtDNA replication stress, and its impact on innate immune signaling. Additionally, we discuss how mutations located on mitochondria targeting sequence (MTS) of BER genes may contribute to mtDNA genome instability and innate immune signaling activation. Overall, the review summarizes evidence to understand the dynamics of mitochondria genome and the impact of mtBER in innate immune response during H. pylori-associated pathological outcomes.

IKKα affects the susceptibility of primary human osteoarthritis chondrocytes to oxidative stress-induced DNA damage by tuning autophagy

The functional derangement affecting human chondrocytes during osteoarthritis (OA) onset and progression is sustained by the failure of major homeostatic mechanisms. This makes them more susceptible to oxidative stress (OS), which can induce DNA damage responses and exacerbate stress-induced senescence. The knockdown (KD) of IκB kinase α (IKKα), a dispensable protein in healthy articular cartilage physiology, was shown to increase the survival and replication potential of human primary OA chondrocytes. Our recent findings showed that the DNA Mismatch Repair pathway only partially accounts for the reduced susceptibility to OS of IKKαKD cells. Here we therefore investigated other ROS-mediated DNA damage and repair mechanisms. We exposed IKKαWT and IKKαKD chondrocytes to sub-cytotoxic hydrogen peroxide and evaluated the occurrence of double-strand breaks (DSB), 8-oxo-2'-deoxyguanosine (8-oxo-dG) and telomere shortening. ROS exposure was able to significantly increase the number of γH2AX foci (directly related to the number of DSB) in both cell types, but IKKα deficient cells undergoing cell division were able to better recover compared to their IKKα proficient counterpart. 8-oxo-dG signal proved to be the highest DNA damage signal among those investigated, located in the mitochondria and with a slightly higher intensity in IKKα proficient cells immediately after OS exposure. Furthermore, ROS significantly reduced telomere length both in IKKαWT and IKKαKD, with the former showing more pervasive effects, especially in dividing cells. Assessment of the HIF-1α>Beclin-1>LC3B axis after recovery from OS showed that IKKα deficient cells exhibited a more efficient autophagic machinery that allowed them to better cope with oxidative stress, possibly through the turnover of damaged mitochondria. Higher Beclin-1 levels likely helped in rescuing dividing cells (identified by coupled cell cycle analysis) because of Beclin-1's involvement in both autophagy and mitotic spindle organization. Therefore, our data further confirm the higher capacity of IKKαKD chondrocytes to cope with oxidative stress-induced DNA damage.

The role of DNA and RNA guanosine oxidation in cardiovascular diseases

Cardiovascular diseases (CVD) persist as a prominent cause of mortality worldwide, with oxidative stress constituting a pivotal contributory element. The oxidative modification of guanosine, specifically 8-oxoguanine, has emerged as a crucial biomarker for oxidative stress, providing novel insights into the molecular underpinnings of CVD. 8-Oxoguanine can be directly generated at the DNA (8-oxo-dG) and RNA (8-oxo-G) levels, as well as at the free nucleotide level (8-oxo-dGTP or 8-oxo-GTP), which are produced and can be integrated through DNA replication or RNA transcription. When exposed to oxidative stress, guanine is more readily produced in RNA than in DNA. A burgeoning body of research surrounds 8-oxoguanine, exhibits its accumulation playing a pivotal role in the development of CVD. Therapeutic approaches targeting oxidative 8-Oxoguanine damage to DNA and RNA, encompassing the modulation of repair enzymes and the development of small molecule inhibitors, are anticipated to enhance CVD management. In conclusion, we explore the noteworthy elevation of 8-oxoguanine levels in patients with various cardiac conditions and deliberate upon the formation and regulation of 8-oxo-dG and 8-oxo-G under oxidative stress, as well as their function in CVD.

Functional Characterization of DNA N-Glycosylase Ogg1 and Ntg1 in DNA Damage Stress of Cryptococcus neoformans

Reactive oxygen species induce DNA strand breaks and DNA oxidation. DNA oxidation leads to DNA mismatches, resulting in mutations in the genome if not properly repaired. Homologous recombination (HR) and non-homologous end-joining (NHEJ) are required for DNA strand breaks, whereas the base excision repair system mainly repairs oxidized DNAs, such as 8-oxoguanine and thymine glycol, by cleaving the glycosidic bond, inserting correct nucleotides, and sealing the gap. Our previous studies revealed that the Rad53-Bdr1 pathway mainly controls DNA strand breaks through the regulation of HR- and NHEJ-related genes. However, the functional roles of genes involved in the base excision repair system remain elusive in Cryptococcus neoformans. In the present study, we identified OGG1 and NTG1 genes in the base excision repair system of C. neoformans, which are involved in DNA oxidation repair. The expression of OGG1 was induced in a Hog1-dependent manner under oxidative stress. On the other hand, the expression of NTG1 was strongly induced by DNA damage stress in a Rad53-independent manner. We demonstrated that the deletion of NTG1, but not OGG1, resulted in elevated susceptibility to DNA damage agents and oxidative stress inducers. Notably, the ntg1Δ mutant showed growth defects upon antifungal drug treatment. Although deletion of OGG1 or NTG1 did not increase mutation rates, the mutation profile of each ogg1Δ and ntg1Δ mutant was different from that of the wild-type strain. Taken together, we found that DNA N-glycosylase Ntg1 is required for oxidative DNA damage stress and antifungal drug resistance in C. neoformans.

OGG1 competitive inhibitors show important off-target effects by directly inhibiting efflux pumps and disturbing mitotic progression

One of the most abundant DNA lesions induced by Reactive oxygen species (ROS) is 8-oxoG, a highly mutagenic lesion that compromises genetic instability when not efficiently repaired. 8-oxoG is specifically recognized by the DNA-glycosylase OGG1 that excises the base and initiates the Base Excision Repair pathway (BER). Furthermore, OGG1 has not only a major role in DNA repair but it is also involved in transcriptional regulation. Cancer cells are particularly exposed to ROS, thus challenging their capacity to process oxidative DNA damage has been proposed as a promising therapeutic strategy for cancer treatment. Two competitive inhibitors of OGG1 (OGG1i) have been identified, TH5487 and SU0268, which bind to the OGG1 catalytic pocket preventing its fixation to the DNA. Early studies with these inhibitors show an enhanced cellular sensitivity to cytotoxic drugs and a reduction in the inflammatory response. Our study uncovers two unreported off-targets effects of these OGG1i that are independent of OGG1. In vitro and in cellulo approaches have unveiled that OGG1i TH5487 and SU0268, despite an unrelated molecular structure, are able to inhibit some members of the ABC family transporters, in particular ABC B1 (MDR1) and ABC G2 (BCRP). The inhibition of these efflux pumps by OGG1 inhibitors results in a higher intra-cellular accumulation of various fluorescent probes and drugs, and largely contributes to the enhanced cytotoxicity observed when the inhibitors are combined with cytotoxic agents. Furthermore, we found that SU0268 has an OGG1-independent anti-mitotic activity-by interfering with metaphase completion-resulting in a high cellular toxicity. These two off-target activities are observed at concentrations of OGG1i that are normally used for in vivo studies. It is thus critical to consider these previously unreported non-specific effects when interpreting studies using TH5487 and SU0268 in the context of OGG1 inhibition. Additionally, our work highlights the persistent need for new specific inhibitors of the enzymatic activity of OGG1.

The Role of 8-oxoG Repair Systems in Tumorigenesis and Cancer Therapy

Tumorigenesis is highly correlated with the accumulation of mutations. The abundant and extensive DNA oxidation product, 8-Oxoguanine (8-oxoG), can cause mutations if it is not repaired by 8-oxoG repair systems. Therefore, the accumulation of 8-oxoG plays an essential role in tumorigenesis. To avoid the accumulation of 8-oxoG in the genome, base excision repair (BER), initiated by 8-oxoguanine DNA glycosylase1 (OGG1), is responsible for the removal of genomic 8-oxoG. It has been proven that 8-oxoG levels are significantly elevated in cancer cells compared with cells of normal tissues, and the induction of DNA damage by some antitumor drugs involves direct or indirect interference with BER, especially through inducing the production and accumulation of reactive oxygen species (ROS), which can lead to tumor cell death. In addition, the absence of the core components of BER can result in embryonic or early post-natal lethality in mice. Therefore, targeting 8-oxoG repair systems with inhibitors is a promising avenue for tumor therapy. In this study, we summarize the impact of 8-oxoG accumulation on tumorigenesis and the current status of cancer therapy approaches exploiting 8-oxoG repair enzyme targeting, as well as possible synergistic lethality strategies involving exogenous ROS-inducing agents.

Small molecule-mediated allosteric activation of the base excision repair enzyme 8-oxoguanine DNA glycosylase and its impact on mitochondrial function

8-Oxoguanine DNA glycosylase (OGG1) initiates base excision repair of the oxidative DNA damage product 8-oxoguanine. OGG1 is bifunctional; catalyzing glycosyl bond cleavage, followed by phosphodiester backbone incision via a β-elimination apurinic lyase reaction. The product from the glycosylase reaction, 8-oxoguanine, and its analogues, 8-bromoguanine and 8-aminoguanine, trigger the rate-limiting AP lyase reaction. The precise activation mechanism remains unclear. The product-assisted catalysis hypothesis suggests that 8-oxoguanine and analogues bind at the product recognition (PR) pocket to enhance strand cleavage as catalytic bases. Alternatively, they may allosterically activate OGG1 by binding outside of the PR pocket to induce an active-site conformational change to accelerate apurinic lyase. Herein, steady-state kinetic analyses demonstrated random binding of substrate and activator. 9-Deazaguanine, which can't function as a substrate-competent base, activated OGG1, albeit with a lower E_max value than 8-bromoguanine and 8-aminoguanine. Random compound screening identified small molecules with E_max values similar to 8-bromoguanine. Paraquat-induced mitochondrial dysfunction was attenuated by several small molecule OGG1 activators; benefits included enhanced mitochondrial membrane and DNA integrity, less cytochrome c translocation, ATP preservation, and mitochondrial membrane dynamics. Our results support an allosteric mechanism of OGG1 and not product-assisted catalysis. OGG1 small molecule activators may improve mitochondrial function in oxidative stress-related diseases.

RELA∙8-Oxoguanine DNA Glycosylase1 Is an Epigenetic Regulatory Complex Coordinating the Hexosamine Biosynthetic Pathway in RSV Infection

Respiratory syncytial virus (RSV), or human orthopneumovirus, is a negative-sense RNA virus that is the causative agent of severe lower respiratory tract infections in children and is associated with exacerbations of adult lung disease. The mechanisms how severe and/or repetitive virus infections cause declines in pulmonary capacity are not fully understood. We have recently discovered that viral replication triggers epithelial plasticity and metabolic reprogramming involving the hexosamine biosynthetic pathway (HBP). In this study, we examine the relationship between viral induced innate inflammation and the activation of hexosamine biosynthesis in small airway epithelial cells. We observe that RSV induces ~2-fold accumulation of intracellular UDP-GlcNAc, the end-product of the HBP and the obligate substrate of N glycosylation. Using two different silencing approaches, we observe that RSV replication activates the HBP pathway in a manner dependent on the RELA proto-oncogene (65 kDa subunit). To better understand the effect of RSV on the cellular N glycoproteome, and its RELA dependence, we conduct affinity enriched LC-MS profiling in wild-type and RELA-silenced cells. We find that RSV induces the accumulation of 171 N glycosylated peptides in a RELA-dependent manner; these proteins are functionally enriched in integrins and basal lamina formation. To elaborate this mechanism of HBP expression, we demonstrate that RSV infection coordinately induces the HBP pathway enzymes in a manner requiring RELA; these genes include Glutamine-Fructose-6-Phosphate Transaminase 1 (GFPT)-1/2, Glucosamine-Phosphate N-Acetyltransferase (GNPNAT)-1, phosphoglucomutase (PGM)-3 and UDP-N-Acetylglucosamine Pyrophosphorylase (UAP)-1. Using small-molecule inhibitor(s) of 8-oxoguanine DNA glycosylase1 (OGG1), we observe that OGG1 is also required for the expression of HBP pathway. In proximity ligation assays, RSV induces the formation of a nuclear and mitochondrial RELA∙OGG1 complex. In co-immunoprecipitaton (IP) experiments, we discover that RSV induces Ser 536-phosphorylated RELA to complex with OGG1. Chromatin IP experiments demonstrate a major role of OGG1 in supporting the recruitment of RELA and phosphorylated RNA Pol II to the HBP pathway genes. We conclude that the RELA∙OGG1 complex is an epigenetic regulator mediating metabolic reprogramming and N glycoprotein modifications of integrins in response to RSV. These findings have implications for viral-induced adaptive epithelial responses.

A Novel Role for the DNA Repair Enzyme 8-Oxoguanine DNA Glycosylase in Adipogenesis

Cells sustain constant oxidative stress from both exogenous and endogenous sources. When unmitigated by antioxidant defenses, reactive oxygen species damage cellular macromolecules, including DNA. Oxidative lesions in both nuclear and mitochondrial DNA are repaired via the base excision repair (BER) pathway, initiated by DNA glycosylases. We have previously demonstrated that the BER glycosylase 8-oxoguanine DNA glycosylase (OGG1) plays a novel role in body weight maintenance and regulation of adiposity. Specifically, mice lacking OGG1 (Ogg1^−/−) are prone to increased fat accumulation with age and consumption of hypercaloric diets. Conversely, transgenic animals with mitochondrially-targeted overexpression of OGG1 (Ogg1^Tg) are resistant to age- and diet-induced obesity. Given these phenotypes of altered adiposity in the context of OGG1 genotype, we sought to determine if OGG1 plays a cell-intrinsic role in adipocyte maturation and lipid accumulation. Here, we report that preadipocytes from Ogg1^−/− mice differentiate more efficiently and accumulate more lipids than those from wild-type animals. Conversely, OGG1 overexpression significantly blunts adipogenic differentiation and lipid accretion in both pre-adipocytes from Ogg1^Tg mice, as well as in 3T3-L1 cells with adenovirus-mediated OGG1 overexpression. Mechanistically, changes in adipogenesis are accompanied by significant alterations in cellular PARylation, corresponding with OGG1 genotype. Specifically, deletion of OGG1 reduces protein PARylation, concomitant with increased adipogenic differentiation, while OGG1 overexpression significantly increases PARylation and blunts adipogenesis. Collectively, these data indicate a novel role for OGG1 in modulating adipocyte differentiation and lipid accretion. These findings have important implications to our knowledge of the fundamental process of adipocyte differentiation, as well as to our understanding of lipid-related diseases such as obesity.

Mechanisms of replication and repair in mitochondrial DNA deletion formation

Deletions in mitochondrial DNA (mtDNA) are associated with diverse human pathologies including cancer, aging and mitochondrial disorders. Large-scale deletions span kilobases in length and the loss of these associated genes contributes to crippled oxidative phosphorylation and overall decline in mitochondrial fitness. There is not a united view for how mtDNA deletions are generated and the molecular mechanisms underlying this process are poorly understood. This review discusses the role of replication and repair in mtDNA deletion formation as well as nucleic acid motifs such as repeats, secondary structures, and DNA damage associated with deletion formation in the mitochondrial genome. We propose that while erroneous replication and repair can separately contribute to deletion formation, crosstalk between these pathways is also involved in generating deletions.

Photon versus proton neurotoxicity: Impact on mitochondrial function and 8-OHdG base-excision repair mechanism in human astrocytes

This study assesses and compares the neurotoxic effects of proton and photon radiation on mitochondrial function and DNA repair capabilities of human astrocytes. Human astrocytes received either proton (0.5 Gy and 3 Gy), photon (0.5 Gy and 3 Gy), or sham-radiation treatment. The mRNA expression level of the DNA repair protein OGG1 was determined via RT-qPCR. The levels of 8-OHdG in the cell media were measured via ELISA. Real-time kinetic analysis of extracellular oxygen consumption rates was performed to assess mitochondrial function. Radiation-induced changes in mitochondrial mass and oxidative activity were assessed using fluorescent imaging with MitoTracker™ Green FM and MitoTracker™ Orange CM-H₂TMRos dyes respectively. PCR was used to quantify the alteration in the mitochondrial DNA content, measured as the mitochondrial to nuclear DNA ratio. A significant increase in mitochondrial mass and levels of reactive oxygen species was observed after radiation treatment. Additionally, real-time PCR analysis indicated a significant depletion of mitochondrial DNA content in the irradiated cells when compared to the control. This was accompanied by a decreased gene expression of the DNA base-excision repair protein OGG1 and reduced clearance of 8-OHdG adducts from the genome. Photon radiation treatment was associated with a more detrimental cellular impact when compared to the same dose of proton radiation. These results are indicative of a radiation-induced dose-dependent decrease in mitochondrial function, an increase in senescence and astrogliosis, and impairment of the DNA repair capabilities in healthy glial cells. Photon irradiation was associated with a more significant disruption in mitochondrial function and base-excision repair mechanisms in vitro in comparison to proton treatment.

ATAD5 deficiency alters DNA damage metabolism and sensitizes cells to PARP inhibition

Replication factor C (RFC), a heteropentamer of RFC1-5, loads PCNA onto DNA during replication and repair. Once DNA synthesis has ceased, PCNA must be unloaded. Recent findings assign the uloader role primarily to an RFC-like (RLC) complex, in which the largest RFC subunit, RFC1, has been replaced with ATAD5 (ELG1 in Saccharomyces cerevisiae). ATAD5-RLC appears to be indispensable, given that Atad5 knock-out leads to embryonic lethality. In order to learn how the retention of PCNA on DNA might interfere with normal DNA metabolism, we studied the response of ATAD5-depleted cells to several genotoxic agents. We show that ATAD5 deficiency leads to hypersensitivity to methyl methanesulphonate (MMS), camptothecin (CPT) and mitomycin C (MMC), agents that hinder the progression of replication forks. We further show that ATAD5-depleted cells are sensitive to poly(ADP)ribose polymerase (PARP) inhibitors and that the processing of spontaneous oxidative DNA damage contributes towards this sensitivity. We posit that PCNA molecules trapped on DNA interfere with the correct metabolism of arrested replication forks, phenotype reminiscent of defective homologous recombination (HR). As Atad5 heterozygous mice are cancer-prone and as ATAD5 mutations have been identified in breast and endometrial cancers, our finding may open a path towards the therapy of these tumours.

Mitochondrial 8-oxoguanine DNA glycosylase mitigates alveolar epithelial cell PINK1 deficiency, mitochondrial DNA damage, apoptosis, and lung fibrosis

Alveolar epithelial cell (AEC) apoptosis, arising from mitochondrial dysfunction and mitophagy defects, is important in mediating idiopathic pulmonary fibrosis (IPF). Our group established a role for the mitochondrial (mt) DNA base excision repair enzyme, 8-oxoguanine-DNA glycosylase 1 (mtOGG1), in preventing oxidant-induced AEC mtDNA damage and apoptosis and showed that OGG1-deficient mice have increased lung fibrosis. Herein, we determined whether mice overexpressing the mtOGG1 transgene (mtOgg1^tg) are protected against lung fibrosis and whether AEC mtOGG1 preservation of mtDNA integrity mitigates phosphatase and tensin homolog-induced putative kinase 1 (PINK1) deficiency and apoptosis. Compared with wild type (WT), mtOgg1^tg mice have diminished asbestos- and bleomycin-induced pulmonary fibrosis that was accompanied by reduced lung and AEC mtDNA damage and apoptosis. Asbestos and H₂O₂ promote the MLE-12 cell PINK1 deficiency, as assessed by reductions in the expression of PINK1 mRNA and mitochondrial protein expression. Compared with WT, Pink1-knockout (Pink1-KO) mice are more susceptible to asbestos-induced lung fibrosis and have increased lung and alveolar type II (AT2) cell mtDNA damage and apoptosis. AT2 cells from Pink1-KO mice and PINK1-silenced (siRNA) MLE-12 cells have increased mtDNA damage that is augmented by oxidative stress. Interestingly, mtOGG1 overexpression attenuates oxidant-induced MLE-12 cell mtDNA damage and apoptosis despite PINK1 silencing. mtDNA damage is increased in the lungs of patients with IPF as compared with controls. Collectively, these findings suggest that mtOGG1 maintenance of AEC mtDNA is crucial for preventing PINK1 deficiency that promotes apoptosis and lung fibrosis. Given the key role of AEC apoptosis in pulmonary fibrosis, strategies aimed at preserving AT2 cell mtDNA integrity may be an innovative target.

Lithium and the Interplay Between Telomeres and Mitochondria in Bipolar Disorder

Bipolar disorder is a severe psychiatric disorder which affects more than 1% of the world's population and is a leading cause of disability among young people. For the past 50 years, lithium has been the drug of choice for maintenance treatment of bipolar disorder due to its potent ability to prevent both manic and depressive episodes as well as suicide. However, though lithium has been associated with a multitude of effects within different cellular pathways and biological systems, its specific mechanism of action in stabilizing mood remains largely elusive. Mitochondrial dysfunction and telomere shortening have been implicated in both the pathophysiology of bipolar disorder and as targets of lithium treatment. Interestingly, it has in recent years become clear that these phenomena are intimately linked, partly through reactive oxygen species signaling and the subcellular translocation and non-canonical actions of telomerase reverse transcriptase. In this review, we integrate the current understanding of mitochondrial dysfunction, oxidative stress and telomere shortening in bipolar disorder with documented effects of lithium. Moreover, we propose that lithium's mechanism of action is intimately connected with the interdependent regulation of mitochondrial bioenergetics and telomere maintenance.

Isoforms of Base Excision Repair Enzymes Produced by Alternative Splicing

Transcripts of many enzymes involved in base excision repair (BER) undergo extensive alternative splicing, but functions of the corresponding alternative splice variants remain largely unexplored. In this review, we cover the studies describing the common alternatively spliced isoforms and disease-associated variants of DNA glycosylases, AP-endonuclease 1, and DNA polymerase beta. We also discuss the roles of alternative splicing in the regulation of their expression, catalytic activities, and intracellular transport.

Mitochondrial DNA Integrity: Role in Health and Disease

As the primary cellular location for respiration and energy production, mitochondria serve in a critical capacity to the cell. Yet, by virtue of this very function of respiration, mitochondria are subject to constant oxidative stress that can damage one of the unique features of this organelle, its distinct genome. Damage to mitochondrial DNA (mtDNA) and loss of mitochondrial genome integrity is increasingly understood to play a role in the development of both severe early-onset maladies and chronic age-related diseases. In this article, we review the processes by which mtDNA integrity is maintained, with an emphasis on the repair of oxidative DNA lesions, and the cellular consequences of diminished mitochondrial genome stability.

The DNA Repair Protein OGG1 Protects Against Obesity by Altering Mitochondrial Energetics in White Adipose Tissue

Obesity and related metabolic pathologies represent a significant public health concern. Obesity is associated with increased oxidative stress that damages genomic and mitochondrial DNA. Oxidatively-induced lesions in both DNA pools are repaired via the base-excision repair pathway, initiated by DNA glycosylases such as 8-oxoguanine DNA glycosylase (OGG1). Global deletion of OGG1 and common OGG1 polymorphisms render mice and humans susceptible to metabolic disease. However, the relative contribution of mitochondrial OGG1 to this metabolic phenotype is unknown. Here, we demonstrate that transgenic targeting of OGG1 to mitochondria confers significant protection from diet-induced obesity, insulin resistance, and adipose tissue inflammation. These favorable metabolic phenotypes are mediated by an increase in whole body energy expenditure driven by specific metabolic adaptations, including increased mitochondrial respiration in white adipose tissue of OGG1 transgenic (Ogg1^Tg) animals. These data demonstrate a critical role for a DNA repair protein in modulating mitochondrial energetics and whole-body energy balance.