The oxidized deoxynucleoside triphosphate pool is a significant contributor to genetic instability in mismatch repair-deficient cells

Structural and biochemical characterization of LIG1 during mutagenic nick sealing of oxidatively damaged ends at the final step of DNA repair

DNA ligase 1 (LIG1) joins broken strand-breaks in the phosphodiester backbone to finalize DNA repair pathways. We previously reported that LIG1 fails on nick repair intermediate with 3'-oxidative damage incorporated by DNA polymerase (pol) β at the downstream steps of base excision repair (BER) pathway. Here, we determined X-ray structures of LIG1/nick DNA complexes containing 3'-8oxodG and 3'-8oxorG opposite either a templating Cytosine or Adenine and demonstrated that the ligase active site engages with mutagenic repair intermediates during steps 2 and 3 of the ligation reaction referring to the formation of DNA-AMP intermediate and a final phosphodiester bond, respectively. Furthermore, we showed the mutagenic nick sealing of DNA substrates with 3'-8oxodG:A and 3'-8oxorG:A by LIG1 wild-type, immunodeficiency disease-associated variants, and DNA ligase 3α (LIG3α) in vitro . Finally, we observed that LIG1 and LIG3α seal resulting nick after an incorporation of 8oxorGTP:A by polβ and AP-Endonuclease 1 (APE1) can clean oxidatively damaged ends at the final steps. Overall, our findings uncover a mechanistic insight into how LIG1 discriminates DNA or DNA/RNA junctions including oxidative damage and a functional coordination between the downstream enzymes, polβ, APE1, and BER ligases, to process mutagenic repair intermediates to maintain repair efficiency.

Sanitation enzymes: Exquisite surveillance of the noncanonical nucleotide pool to safeguard the genetic blueprint

Reactive oxygen species (ROS) are common products of normal cellular metabolism, but their elevated levels can result in nucleotide modifications. These modified or noncanonical nucleotides often integrate into nascent DNA during replication, causing lesions that trigger DNA repair mechanisms such as the mismatch repair machinery and base excision repair. Four superfamilies of sanitization enzymes can effectively hydrolyze noncanonical nucleotides from the precursor pool and eliminate their unintended incorporation into DNA. Notably, we focus on the representative MTH1 NUDIX hydrolase, whose enzymatic activity is ostensibly nonessential under normal physiological conditions. Yet, the sanitization attributes of MTH1 are more prevalent when ROS levels are abnormally high in cancer cells, rendering MTH1 an interesting target for developing anticancer treatments. We discuss multiple MTH1 inhibitory strategies that have emerged in recent years, and the potential of NUDIX hydrolases as plausible targets for the development of anticancer therapeutics.

8-oxo-dGTP curbs tumor development via S phase arrest and AIF-mediated apoptosis

Oxidative stress can attack precursor nucleotides, resulting in nucleic acid damage in cells. It remains unclear how 8-oxo-dGTP and 8-oxoGTP, oxidized forms of dGTP and GTP, respectively, could affect DNA or RNA oxidation levels and tumor development. To address this, we intravenously administered 8-oxo-dGTP and 8-oxoGTP to wild-type and MTH1-knockout mice. 8-oxoGTP administration increased frequency of tumor incidence, which is more prominent in MTH1-knockout mice. However, 8-oxo-dGTP treatment rather reduced tumor development regardless of the mouse genotype. The tumor suppressive effects of 8-oxo-dGTP were further confirmed using xenograft and C57/6J-Apc^Min/Nju mouse models. Mechanistically, 8-oxo-dGTP increased the 8-oxo-dG contents in DNA and DNA strand breakage, induced cell cycle arrest in S phase and apoptosis mediated by AIF, eventually leading to reduced tumor incidence. These results suggest distinct roles of 8-oxo-dGTP and 8-oxoGTP in tumor development.

Targeting the DNA damage response and repair in cancer through nucleotide metabolism

The exploitation of the DNA damage response and DNA repair proficiency of cancer cells is an important anticancer strategy. The replication and repair of DNA are dependent upon the supply of deoxynucleoside triphosphate (dNTP) building blocks, which are produced and maintained by nucleotide metabolic pathways. Enzymes within these pathways can be promising targets to selectively induce toxic DNA lesions in cancer cells. These same pathways also activate antimetabolites, an important group of chemotherapies that disrupt both nucleotide and DNA metabolism to induce DNA damage in cancer cells. Thus, dNTP metabolic enzymes can also be targeted to refine the use of these chemotherapeutics, many of which remain standard of care in common cancers. In this review article, we will discuss both these approaches exemplified by the enzymes MTH1, MTHFD2 and SAMHD1. © 2022 The Authors. Molecular Oncology published by John Wiley & Sons Ltd on behalf of Federation of European Biochemical Societies.

Enhancing Repair of Oxidative DNA Damage with Small-Molecule Activators of MTH1

Impaired DNA repair activity has been shown to greatly increase rates of cancer clinically. It has been hypothesized that upregulating repair activity in susceptible individuals may be a useful strategy for inhibiting tumorigenesis. Here, we report that selected tyrosine kinase (TK) inhibitors including nilotinib, employed clinically in the treatment of chronic myeloid leukemia, are activators of the repair enzyme Human MutT Homolog 1 (MTH1). MTH1 cleanses the oxidatively damaged cellular nucleotide pool by hydrolyzing the oxidized nucleotide 8-oxo-2'-deoxyguanosine (8-oxo-dG)TP, which is a highly mutagenic lesion when incorporated into DNA. Structural optimization of analogues of TK inhibitors resulted in compounds such as SU0448, which induces 1000 ± 100% activation of MTH1 at 10 μM and 410 ± 60% at 5 μM. The compounds are found to increase the activity of the endogenous enzyme, and at least one (SU0448) decreases levels of 8-oxo-dG in cellular DNA. The results suggest the possibility of using MTH1 activators to decrease the frequency of mutagenic nucleotides entering DNA, which may be a promising strategy to suppress tumorigenesis in individuals with elevated cancer risks.

Interplay between H3K36me3, methyltransferase SETD2, and mismatch recognition protein MutSα facilitates processing of oxidative DNA damage in human cells

Oxidative DNA damage contributes to aging and the pathogenesis of numerous human diseases including cancer. 8-hydroxyguanine (8-oxoG) is the major product of oxidative DNA lesions. Although OGG1-mediated base excision repair is the primary mechanism for 8-oxoG removal, DNA mismatch repair has also been implicated in processing oxidative DNA damage. However, the mechanism of the latter is not fully understood. Here, we treated human cells defective in various 8-oxoG repair factors with H₂O₂ and performed biochemical, live cell imaging, and chromatin immunoprecipitation sequencing analyses to determine their response to the treatment. We show that the mismatch repair processing of oxidative DNA damage involves cohesive interactions between mismatch recognition protein MutSα, histone mark H3K36me3, and H3K36 trimethyltransferase SETD2, which activates the ATM DNA damage signaling pathway. We found that cells depleted of MutSα or SETD2 accumulate 8-oxoG adducts and fail to trigger H₂O₂-induced ATM activation. Furthermore, we show that SETD2 physically interacts with both MutSα and ATM, which suggests a role for SETD2 in transducing DNA damage signals from lesion-bound MutSα to ATM. Consistently, MutSα and SETD2 are highly coenriched at oxidative damage sites. The data presented here support a model wherein MutSα, SETD2, ATM, and H3K36me3 constitute a positive feedback loop to help cells cope with oxidative DNA damage.

AOP report: Development of an adverse outcome pathway for oxidative DNA damage leading to mutations and chromosomal aberrations

The Genetic Toxicology Technical Committee (GTTC) of the Health and Environmental Sciences Institute (HESI) is developing adverse outcome pathways (AOPs) that describe modes of action leading to potentially heritable genomic damage. The goal was to enhance the use of mechanistic information in genotoxicity assessment by building empirical support for the relationships between relevant molecular initiating events (MIEs) and regulatory endpoints in genetic toxicology. Herein, we present an AOP network that links oxidative DNA damage to two adverse outcomes (AOs): mutations and chromosomal aberrations. We collected empirical evidence from the literature to evaluate the key event relationships between the MIE and the AOs, and assessed the weight of evidence using the modified Bradford-Hill criteria for causality. Oxidative DNA damage is constantly induced and repaired in cells given the ubiquitous presence of reactive oxygen species and free radicals. However, xenobiotic exposures may increase damage above baseline levels through a variety of mechanisms and overwhelm DNA repair and endogenous antioxidant capacity. Unrepaired oxidative DNA base damage can lead to base substitutions during replication and, along with repair intermediates, can also cause DNA strand breaks that can lead to mutations and chromosomal aberrations if not repaired adequately. This AOP network identifies knowledge gaps that could be filled by targeted studies designed to better define the quantitative relationships between key events, which could be leveraged for quantitative chemical safety assessment. We anticipate that this AOP network will provide the building blocks for additional genotoxicity-associated AOPs and aid in designing novel integrated testing approaches for genotoxicity.

Roles for the 8-Oxoguanine DNA Repair System in Protecting Telomeres From Oxidative Stress

Telomeres are protective nucleoprotein structures that cap linear chromosome ends and safeguard genome stability. Progressive telomere shortening at each somatic cell division eventually leads to critically short and dysfunctional telomeres, which can contribute to either cellular senescence and aging, or tumorigenesis. Human reproductive cells, some stem cells, and most cancer cells, express the enzyme telomerase to restore telomeric DNA. Numerous studies have shown that oxidative stress caused by excess reactive oxygen species is associated with accelerated telomere shortening and dysfunction. Telomeric repeat sequences are remarkably susceptible to oxidative damage and are preferred sites for the production of the mutagenic base lesion 8-oxoguanine, which can alter telomere length homeostasis and integrity. Therefore, knowledge of the repair pathways involved in the processing of 8-oxoguanine at telomeres is important for advancing understanding of the pathogenesis of degenerative diseases and cancer associated with telomere instability. The highly conserved guanine oxidation (GO) system involves three specialized enzymes that initiate distinct pathways to specifically mitigate the adverse effects of 8-oxoguanine. Here we introduce the GO system and review the studies focused on investigating how telomeric 8-oxoguanine processing affects telomere integrity and overall genome stability. We also discuss newly developed technologies that target oxidative damage selectively to telomeres to investigate roles for the GO system in telomere stability.

OGG1 co-inhibition antagonizes the tumor-inhibitory effects of targeting MTH1

Cancer cells develop protective adaptations against oxidative DNA damage, providing a strong rationale for targeting DNA repair proteins. There has been a high degree of recent interest in inhibiting the mammalian Nudix pyrophosphatase MutT Homolog 1 (MTH1). MTH1 degrades 8-oxo-dGTP, thus limiting its incorporation into genomic DNA. MTH1 inhibition has variously been shown to induce genomic 8-oxo-dG elevation, genotoxic strand breaks in p53-functional cells, and tumor-inhibitory outcomes. Genomically incorporated 8-oxo-dG is excised by the base excision repair enzyme, 8-oxo-dG glycosylase 1 (OGG1). Thus, OGG1 inhibitors have been developed with the idea that their combination with MTH1 inhibitors will have anti-tumor effects by increasing genomic oxidative DNA damage. However, contradictory to this idea, we found that human lung adenocarcinoma with low OGG1 and MTH1 were robustly represented in patient datasets. Furthermore, OGG1 co-depletion mitigated the extent of DNA strand breaks and cellular senescence in MTH1-depleted p53-wildtype lung adenocarcinoma cells. Similarly, shMTH1-transduced cells were less sensitive to the OGG1 inhibitor, SU0268, than shGFP-transduced counterparts. Although the dual OGG1/MTH1 inhibitor, SU0383, induced greater cytotoxicity than equivalent combined or single doses of its parent scaffold MTH1 and OGG1 inhibitors, IACS-4759 and SU0268, this effect was only observed at the highest concentration assessed. Collectively, using both genetic depletion as well as small molecule inhibitors, our findings suggest that OGG1/MTH1 co-inhibition is unlikely to yield significant tumor-suppressive benefit. Instead such co-inhibition may exert tumor-protective effects by preventing base excision repair-induced DNA nicks and p53 induction, thus potentially conferring a survival advantage to the treated tumors.

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Low MTH1/low OGG1 tumors are robustly represented in patient lung adenocarcinoma datasets but low MTH1/high OGG1 are not.

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Co-depletion of OGG1 in lung adenocarcinoma cells mitigates shMTH1-induced DNA strand breaks and p53-induced senescence.

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p53-null tumor cells have lower OGG1 vs. wt p53 counterparts and are more resistant to MTH1 loss-induced anti-tumor effects.

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Pharmacologic co-inhibition of OGG1 and MTH1 does not enhance cytotoxicity over the respective single inhibitors.

Relationships among smoking, oxidative stress, inflammation, macromolecular damage, and cancer

Smoking is a major risk factor for a variety of diseases, including cancer and immune-mediated inflammatory diseases. Tobacco smoke contains a mixture of chemicals, including a host of reactive oxygen- and nitrogen species (ROS and RNS), among others, that can damage cellular and sub-cellular targets, such as lipids, proteins, and nucleic acids. A growing body of evidence supports a key role for smoking-induced ROS and the resulting oxidative stress in inflammation and carcinogenesis. This comprehensive and up-to-date review covers four interrelated topics, including 'smoking', 'oxidative stress', 'inflammation', and 'cancer'. The review discusses each of the four topics, while exploring the intersections among the topics by highlighting the macromolecular damage attributable to ROS. Specifically, oxidative damage to macromolecular targets, such as lipid peroxidation, post-translational modification of proteins, and DNA adduction, as well as enzymatic and non-enzymatic antioxidant defense mechanisms, and the multi-faceted repair pathways of oxidized lesions are described. Also discussed are the biological consequences of oxidative damage to macromolecules if they evade the defense mechanisms and/or are not repaired properly or in time. Emphasis is placed on the genetic- and epigenetic alterations that may lead to transcriptional deregulation of functionally-important genes and disruption of regulatory elements. Smoking-associated oxidative stress also activates the inflammatory response pathway, which triggers a cascade of events of which ROS production is an initial yet indispensable step. The release of ROS at the site of damage and inflammation helps combat foreign pathogens and restores the injured tissue, while simultaneously increasing the burden of oxidative stress. This creates a vicious cycle in which smoking-related oxidative stress causes inflammation, which in turn, results in further generation of ROS, and potentially increased oxidative damage to macromolecular targets that may lead to cancer initiation and/or progression.

A Role for N6-Methyladenine in DNA Damage Repair

The leading cause of mutation due to oxidative damage is 8-oxo-2'-deoxyguanosine (8-oxoG) mispairing with adenine (Ade), which can occur in two ways. First, guanine of a G:C DNA base pair can be oxidized. If not repaired in time, DNA polymerases can mispair Ade with 8-oxoG in the template. This 8-oxoG:A can be repaired by enzymes that remove Ade opposite to template 8-oxoG, or 8-oxoG opposite to Cyt. Second, free 8-oxo-dGTP can be misincorporated by DNA polymerases into DNA opposite template Ade. However, there is no known repair activity that removes 8-oxoG opposite to template Ade. We suggest that a major role of N6-methyladenine in mammalian DNA is minimizing incorporation of 8-oxoG opposite to Ade by DNA polymerases following adduct formation.

The Therapeutic Potential of DNA Damage Repair Pathways and Genomic Stability in Lung Cancer

Despite advances in our understanding of the molecular biology of the disease and improved therapeutics, lung cancer remains the most common cause of cancer-related deaths worldwide. Therefore, an unmet need remains for improved treatments, especially in advanced stage disease. Genomic instability is a universal hallmark of all cancers. Many of the most commonly prescribed chemotherapeutics, including platinum-based compounds such as cisplatin, target the characteristic genomic instability of tumors by directly damaging the DNA. Chemotherapies are designed to selectively target rapidly dividing cells, where they cause critical DNA damage and subsequent cell death (1, 2). Despite the initial efficacy of these drugs, the development of chemotherapy resistant tumors remains the primary concern for treatment of all lung cancer patients. The correct functioning of the DNA damage repair machinery is essential to ensure the maintenance of normal cycling cells. Dysregulation of these pathways promotes the accumulation of mutations which increase the potential of malignancy. Following the development of the initial malignancy, the continued disruption of the DNA repair machinery may result in the further progression of metastatic disease. Lung cancer is recognized as one of the most genomically unstable cancers (3). In this review, we present an overview of the DNA damage repair pathways and their contributions to lung cancer disease occurrence and progression. We conclude with an overview of current targeted lung cancer treatments and their evolution toward combination therapies, including chemotherapy with immunotherapies and antibody-drug conjugates and the mechanisms by which they target DNA damage repair pathways.

Spontaneous and photosensitization-induced mutations in primary mouse cells transitioning through senescence and immortalization

To investigate the role of oxidative stress-induced DNA damage and mutagenesis in cellular senescence and immortalization, here we profiled spontaneous and methylene blue plus light-induced mutations in the cII gene from λ phage in transgenic mouse embryonic fibroblasts during the transition from primary culture through senescence and immortalization. Consistent with detection of characteristic oxidized guanine lesions (8-oxodG) in the treated cells, we observed significantly increased relative cII mutant frequency in the treated pre-senescent cells which was augmented in their immortalized counterparts. The predominant mutation type in the treated pre-senescent cells was G:C→T:A transversion, whose frequency was intensified in the treated immortalized cells. Conversely, the prevailing mutation type in the treated immortalized cells was A:T→C:G transversion, with a unique sequence-context specificity, i.e. flanking purines at the 5' end of the mutated nucleotide. This mutation type was also enriched in the treated pre-senescent cells, although to a lower extent. The signature mutation of G:C→T:A transversions in the treated cells accorded with the well-established translesion synthesis bypass caused by 8-oxodG, and the hallmark A:T→C:G transversions conformed to the known replication errors because of oxidized guanine nucleosides (8-OHdGTPs). The distinctive features of photosensitization-induced mutagenesis in the immortalized cells, which were present at attenuated levels, in spontaneously immortalized cells provide insights into the role of oxidative stress in senescence bypass and immortalization. Our results have important implications for cancer biology because oxidized purines in the nucleoside pool can significantly contribute to genetic instability in DNA mismatch repair-defective human tumors.

Purine-Metabolising Enzymes and Apoptosis in Cancer

The enzymes of both de novo and salvage pathways for purine nucleotide synthesis are regulated to meet the demand of nucleic acid precursors during proliferation. Among them, the salvage pathway enzymes seem to play the key role in replenishing the purine pool in dividing and tumour cells that require a greater amount of nucleotides. An imbalance in the purine pools is fundamental not only for preventing cell proliferation, but also, in many cases, to promote apoptosis. It is known that tumour cells harbour several mutations that might lead to defective apoptosis-inducing pathways, and this is probably at the basis of the initial expansion of the population of neoplastic cells. Therefore, knowledge of the molecular mechanisms that lead to apoptosis of tumoural cells is key to predicting the possible success of a drug treatment and planning more effective and focused therapies. In this review, we describe how the modulation of enzymes involved in purine metabolism in tumour cells may affect the apoptotic programme. The enzymes discussed are: ectosolic and cytosolic 5'-nucleotidases, purine nucleoside phosphorylase, adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase, and inosine-5'-monophosphate dehydrogenase, as well as recently described enzymes particularly expressed in tumour cells, such as deoxynucleoside triphosphate triphosphohydrolase and 7,8-dihydro-8-oxoguanine triphosphatase.

Obesity and cancer: A mechanistic overview of metabolic changes in obesity that impact genetic instability

Obesity, defined as a state of positive energy balance with a body mass index exceeding 30 kg/m2 in adults and 95th percentile in children, is an increasing global concern. Approximately one-third of the world's population is overweight or obese, and in the United States alone, obesity affects one in six children. Meta-analysis studies suggest that obesity increases the likelihood of developing several types of cancer, and with poorer outcomes, especially in children. The contribution of obesity to cancer risk requires a better understanding of the association between obesity-induced metabolic changes and its impact on genomic instability, which is a major driving force of tumorigenesis. In this review, we discuss how molecular changes during adipose tissue dysregulation can result in oxidative stress and subsequent DNA damage. This represents one of the many critical steps connecting obesity and cancer since oxidative DNA lesions can result in cancer-associated genetic instability. In addition, the by-products of the oxidative degradation of lipids (e.g., malondialdehyde, 4-hydroxynonenal, and acrolein), and gut microbiota-mediated secondary bile acid metabolites (e.g., deoxycholic acid and lithocholic acid), can function as genotoxic agents and tumor promoters. We also discuss how obesity can impact DNA repair efficiency, potentially contributing to cancer initiation and progression. Finally, we outline obesity-related epigenetic changes and identify the gaps in knowledge to be addressed for the development of better therapeutic strategies for the prevention and treatment of obesity-related cancers.

Oxidized Cell-Free DNA Role in the Antioxidant Defense Mechanisms under Stress

The present study focuses on the investigation of the oxidized cell-free DNA (cfDNA) properties in several experimental models, including cultured cerebellum cells, peripheral blood lymphocytes (PBL), plasma, and hippocampus under an acute and chronic unpredictable stress model in rats. Firstly, our study shows that Spectrum Green fluorescence-labeled oxidized cfDNA fragments were transferred into the cytoplasm of 80% of the cerebellum culture cells; meanwhile, the nonoxidized cfDNA fragments do not pass into the cells. Oxidized cfDNA stimulates the antioxidant mechanisms and induction of transcription factor NRF2 expression, followed by an activation of NRF2 signaling pathway genes-rise of Nrf2 and Hmox1 gene expression and consequently NRF2 protein synthesis. Secondly, we showed that stress increases plasma cfDNA concentration in rats corresponding with the duration of the stress exposure. At the same time, our study did not reveal any significant changes of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) level in PBL of rats under acute or chronic stress, probably due to the significantly increased Nrf2 expression, that we found in such conditions. 8-oxodG is one of the most reliable markers of DNA oxidation. We also found an increased level of 8-oxodG in the hippocampal homogenates and hippocampal dentate gyrus in rats subjected to acute and chronic stress. Taken together, our data shows that oxidized cfDNA may play a significant role in systemic and neuronal physiological mechanisms of stress and adaptation.

The interplay at the replisome mitigates the impact of oxidative damage on the genetic integrity of hyperthermophilic Archaea

8-oxodeoxyguanosine (8-oxodG), a major oxidised base modification, has been investigated to study its impact on DNA replication in hyperthermophilic Archaea. Here we show that 8-oxodG is formed in the genome of growing cells, with elevated levels following exposure to oxidative stress. Functional characterisation of cell-free extracts and the DNA polymerisation enzymes, PolB, PolD, and the p41/p46 complex, alone or in the presence of accessory factors (PCNA and RPA) indicates that translesion synthesis occurs under replicative conditions. One of the major polymerisation effects was stalling, but each of the individual proteins could insert and extend past 8-oxodG with differing efficiencies. The introduction of RPA and PCNA influenced PolB and PolD in similar ways, yet provided a cumulative enhancement to the polymerisation performance of p41/p46. Overall, 8-oxodG translesion synthesis was seen to be potentially mutagenic leading to errors that are reminiscent of dA:8-oxodG base pairing.

The mismatch repair-dependent DNA damage response: Mechanisms and implications

An important role for the DNA mismatch repair (MMR) pathway in maintaining genomic stability is embodied in its conservation through evolution and the link between loss of MMR function and tumorigenesis. The latter is evident as inheritance of mutations within the major MMR genes give rise to the cancer predisposition condition, Lynch syndrome. Nonetheless, how MMR loss contributes to tumorigenesis is not completely understood. In addition to preventing the accumulation of mutations, MMR also directs cellular responses, such as cell cycle checkpoint or apoptosis activation, to different forms of DNA damage. Understanding this MMR-dependent DNA damage response may provide insight into the full tumor suppressing capabilities of the MMR pathway. Here, we delve into the proposed mechanisms for the MMR-dependent response to DNA damaging agents. We discuss how these pre-clinical findings extend to the clinical treatment of cancers, emphasizing MMR status as a crucial variable in selection of chemotherapeutic regimens. Also, we discuss how loss of the MMR-dependent damage response could promote tumorigenesis via the establishment of a survival advantage to endogenous levels of stress in MMR-deficient cells.

Mechanisms of MTH1 inhibition-induced DNA strand breaks: The slippery slope from the oxidized nucleotide pool to genotoxic damage

Unlike normal tissues, tumor cells possess a propensity for genomic instability, resulting from elevated oxidant levels produced by oncogenic signaling and aberrant cellular metabolism. Thus, targeting mechanisms that protect cancer cells from the tumor-inhibitory consequences of their redox imbalance and spontaneous DNA-damaging events is expected to have broad-spectrum efficacy and a high therapeutic index. One critical mechanism for tumor cell protection from oxidant stress is the hydrolysis of oxidized nucleotides. Human MutT homolog 1 (MTH1), the mammalian nudix (nucleoside diphosphate X) pyrophosphatase (NUDT1), protects tumor cells from oxidative stress-induced genomic DNA damage by cleansing the nucleotide pool of oxidized purine nucleotides. Depletion or pharmacologic inhibition of MTH1 results in genomic DNA strand breaks in many cancer cells. However, the mechanisms underlying how oxidized nucleotides, thought mainly to be mutagenic rather than genotoxic, induce DNA strand breaks are largely unknown. Given the recent therapeutic interest in targeting MTH1, a better understanding of such mechanisms is crucial to its successful translation into the clinic and in identifying the molecular contexts under which its inhibition is likely to be beneficial. Here we provide a comprehensive perspective on MTH1 function and its importance in protecting genome integrity, in the context of tumor-associated oxidative stress and the mechanisms that likely lead to irreparable DNA strand breaks as a result of MTH1 inhibition.

Label-free target identification reveals oxidative DNA damage as the mechanism of a selective cytotoxic agent

Phenotypic screening can not only identify promising first-in-class drug candidates, but can also reveal potential therapeutic targets or neomorphic functions of known proteins. In this study, we identified target proteins of SB2001, a cytotoxic agent that acts specifically against HeLa human cervical cancer cells. Because SB2001 lacks chemical modification sites, label-free target identification methods including thermal stability shift-based fluorescence difference in two-dimensional gel electrophoresis (TS-FITGE) and thermal proteome profiling (TPP) were applied to characterize its mechanism of action. Owing to their differences, the two label-free target identification methods uncovered complementary target candidates. Candidates from both methods were prioritized according to their selective lethality upon the knockdown of those genes in HeLa cells, compared to CaSki cells which were used as a negative control cell line from the human cervix. LTA4H was identified only by TS-FITGE, but not by TPP, because only one isoform was stabilized by SB2001. Furthermore, it was implied that a non-canonical function of LTA4H was involved in the SB2001 activity. MTH1 was identified by both TS-FITGE and TPP, and SB2001 inhibited the function of MTH1 in hydrolyzing oxidized nucleotides. Compared to CaSki cells, HeLa cells displayed downregulated DNA mismatch repair pathways, which made HeLa cells more susceptible to the oxidative stress caused by SB2001, resulting in increased 8-oxoG concentrations, DNA damage, and subsequent cell death.

hMTH1 is required for maintaining migration and invasion potential of human thyroid cancer cells

Cancer cells, including thyroid cancer cells, suffer from oxidative stress damaging multiple cellular targets, such as DNA and the nucleotide pool. The human MutT homologue 1 (hMTH1) controls the oxidative DNA damage load by sanitizing the nucleotide pool from the oxidized DNA precursor, 8-oxodGTP. It has previously been shown that hMTH1 is essential for cancer cell proliferation and survival, therefore hMTH1 inhibition has been proposed as a novel anticancer therapeutic strategy. Here we show that thyroid cancer cells respond to siRNA mediated hMTH1 depletion with increased DNA damage load and moderately reduced proliferation rates, but without detectable apoptosis, cell-cycle arrest or senescence. Importantly, however, hMTH1 depletion significantly reduced migration and invasion potential of the thyroid cancer cells. Accordingly, our results allow us to propose that hMTH1 may be a therapeutic target in thyroid malignancy, especially for controlling metastasis.

MTH1 deficiency selectively increases non-cytotoxic oxidative DNA damage in lung cancer cells: more bad news than good?

Background

Targeted therapies are based on exploiting cancer-cell-specific genetic features or phenotypic traits to selectively kill cancer cells while leaving normal cells unaffected. Oxidative stress is a cancer hallmark phenotype. Given that free nucleotide pools are particularly vulnerable to oxidation, the nucleotide pool sanitising enzyme, MTH1, is potentially conditionally essential in cancer cells. However, findings from previous MTH1 studies have been contradictory, meaning the relevance of MTH1 in cancer is still to be determined. Here we ascertained the role of MTH1 specifically in lung cancer cell maintenance, and the potential of MTH1 inhibition as a targeted therapy strategy to improve lung cancer treatments.

Methods

Using siRNA-mediated knockdown or small-molecule inhibition, we tested the genotoxic and cytotoxic effects of MTH1 deficiency on H23 (p53-mutated), H522 (p53-mutated) and A549 (wildtype p53) non-small cell lung cancer cell lines relative to normal MRC-5 lung fibroblasts. We also assessed if MTH1 inhibition augments current therapies.

Results

MTH1 knockdown increased levels of oxidatively damaged DNA and DNA damage signaling alterations in all lung cancer cell lines but not normal fibroblasts, despite no detectable differences in reactive oxygen species levels between any cell lines. Furthermore, MTH1 knockdown reduced H23 cell proliferation. However, unexpectedly, it did not induce apoptosis in any cell line or enhance the effects of gemcitabine, cisplatin or radiation in combination treatments. Contrastingly, TH287 and TH588 MTH1 inhibitors induced apoptosis in H23 and H522 cells, but only increased oxidative DNA damage levels in H23, indicating that they kill cells independently of DNA oxidation and seemingly via MTH1-distinct mechanisms.

Conclusions

MTH1 has a NSCLC-specific p53-independent role for suppressing DNA oxidation and genomic instability, though surprisingly the basis of this may not be reactive-oxygen-species-associated oxidative stress. Despite this, overall our cell viability data indicates that targeting MTH1 will likely not be an across-the-board effective NSCLC therapeutic strategy; rather it induces non-cytotoxic DNA damage that could promote cancer heterogeneity and evolution.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4332-7) contains supplementary material, which is available to authorized users.

Modified nucleoside triphosphates exist in mammals

By establishing a chemical labeling method in combination with liquid chromatography-mass spectrometry analysis, we reported the widespread existence of various modified nucleoside triphosphates in eukaryotes.

DNA and RNA contain diverse chemical modifications that exert important influences in a variety of cellular processes. In addition to enzyme-mediated modifications of DNA and RNA, previous in vitro studies showed that pre-modified nucleoside triphosphates (NTPs) can be incorporated into DNA and RNA during replication and transcription. Herein, we established a chemical labeling method in combination with liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) analysis for the determination of endogenous NTPs in the mammalian cells and tissues. We synthesized 8-(diazomethyl)quinoline (8-DMQ) that could efficiently react with the phosphate group under mild condition to label NTPs. The developed method allowed sensitive detection of NTPs, with the detection limits improved by 56–137 folds. The results showed that 12 types of endogenous modified NTPs were distinctly determined in the mammalian cells and tissues. In addition, the majority of these modified NTPs exhibited significantly decreased contents in human hepatocellular carcinoma (HCC) tissues compared to tumor-adjacent normal tissues. Taken together, our study revealed the widespread existence of various modified NTPs in eukaryotes.

Crystal Structures and Inhibitor Interactions of Mouse and Dog MTH1 Reveal Species-Specific Differences in Affinity

MTH1 hydrolyzes oxidized nucleoside triphosphates, thereby sanitizing the nucleotide pool from oxidative damage. This prevents incorporation of damaged nucleotides into DNA, which otherwise would lead to mutations and cell death. The high level of reactive oxygen species in cancer cells leads to a higher level of oxidized nucleotides in cancer cells compared to that in nonmalignant cells, making cancer cells more dependent on MTH1 for survival. The possibility of specifically targeting cancer cells by inhibiting MTH1 has highlighted MTH1 as a promising cancer target. The progression of MTH1 inhibitors into the clinic requires animal studies, and knowledge of species differences in the potency of inhibitors is vitally important. We here show that the human MTH1 inhibitor TH588 is approximately 20-fold less potent with respect to inhibition of mouse MTH1 than the human, rat, pig, and dog MTH1 proteins are. We present the crystal structures of mouse MTH1 in complex with TH588 and dog MTH1 and elucidate the structural and sequence basis for the observed difference in affinity for TH588. We identify amino acid residue 116 in MTH1 as an important determinant of TH588 affinity. Furthermore, we present the structure of mouse MTH1 in complex with the substrate 8-oxo-dGTP. The crystal structures provide insight into the high degree of structural conservation between MTH1 proteins from different organisms and provide a detailed view of interactions between MTH1 and the inhibitor, revealing that minute structural differences can have a large impact on affinity and specificity.

Linking arsenite- and cadmium-generated oxidative stress to microsatellite instability in vitro and in vivo

Mismatch repair (MMR) corrects replicative errors and minimizes DNA damage that occurs frequently in microsatellites. MMR deficiency is manifested as microsatellite instability (MSI), which contributes to hypermutability and cancer pathogenesis. Genomic instability, including MSI and chromosomal instability, appears to be responsible for the carcinogenesis of arsenic and cadmium, common contaminants in our environment. However, few studies have addressed arsenic- or cadmium-induced MSI, especially its potential link with arsenic- or cadmium-generated oxidative stress, due to the lack of quantifiable MSI assays and cost-effective animal models. Here, using a dual-fluorescent reporter, we demonstrate that sub-lethal doses of cadmium or arsenite, but not arsenate, increased the MSI frequency in human colorectal cancer cells. Arsenite- and cadmium-induced MSI occurred concomitantly with increased levels of reactive species and oxidative DNA damage, and with decreased levels of MMR proteins. However, N-acetyl-l-cysteine (NAC) suppressed arsenite- and cadmium-induced MSI and oxidative stress while restoring the levels of MMR proteins in the cells. Similarly, MSI was induced separately by arsenite and cadmium, and suppressed by NAC, in zebrafish in a fluorescinated PCR-based assay with newly-developed microsatellite markers and inter-segmental comparisons. Of five selected antioxidants examined, differential effects were exerted on the MSI induction and cytotoxicity of both arsenite and cadmium. Compared to MMR-proficient cells, MMR-deficient cells were more resistant to arsenic-mediated and cadmium-mediated cytotoxicity. Our findings demonstrate a novel linkage between arsenite-generated and cadmium-generated oxidative stress and MSI induction. Our findings also caution that antioxidants must be individually validated before being used for preventing arsenite- and cadmium-induced MSI that is associated with cancer development.

Not breathing is not an option: How to deal with oxidative DNA damage

Oxidative DNA damage constitutes a major threat to genetic integrity, and has thus been implicated in the pathogenesis of a wide variety of diseases, including cancer and neurodegeneration. 7,8-dihydro-8oxo-deoxyGuanine (8-oxo-G) is one of the best characterised oxidative DNA lesions, and it can give rise to point mutations due to its miscoding potential that instructs most DNA polymerases (Pols) to preferentially insert Adenine (A) opposite 8-oxo-G instead of the correct Cytosine (C). If uncorrected, A:8-oxo-G mispairs can give rise to C:G→A:T transversion mutations. Cells have evolved a variety of pathways to mitigate the mutational potential of 8-oxo-G that include i) mechanisms to avoid incorporation of oxidized nucleotides into DNA through nucleotide pool sanitisation enzymes (by MTH1, MTH2, MTH3 and NUDT5), ii) base excision repair (BER) of 8-oxo-G in DNA (involving MUTYH, OGG1, Pol λ, and other components of the BER machinery), and iii) faithful bypass of 8-oxo-G lesions during replication (using a switch between replicative Pols and Pol λ). In the following, the fate of 8-oxo-G in mammalian cells is reviewed in detail. The differential origins of 8-oxo-G in DNA and its consequences for genetic stability will be covered. This will be followed by a thorough discussion of the different mechanisms in place to cope with 8-oxo-G with an emphasis on Pol λ-mediated correct bypass of 8-oxo-G during MUTYH-initiated BER as well as replication across 8-oxo-G. Furthermore, the multitude of mechanisms in place to regulate key proteins involved in 8-oxo-G repair will be reviewed. Novel functions of 8-oxo-G as an epigenetic-like regulator and insights into the repair of 8-oxo-G within the cellular context will be touched upon. Finally, a discussion will outline the relevance of 8-oxo-G and the proteins involved in dealing with 8-oxo-G to human diseases with a special emphasis on cancer.

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.

hMYH and hMTH1 cooperate for survival in mismatch repair defective T-cell acute lymphoblastic leukemia

hMTH1 is an 8-oxodGTPase that prevents mis-incorporation of free oxidized nucleotides into genomic DNA. Base excision and mismatch repair pathways also restrict the accumulation of oxidized lesions in DNA by removing the mis-inserted 8-oxo-7,8-dihydro-2'-deoxyguanosines (8-oxodGs). In this study, we aimed to investigate the interplay between hMYH DNA glycosylase and hMTH1 for cancer cell survival by using mismatch repair defective T-cell acute lymphoblastic leukemia (T-ALL) cells. To this end, MYH and MTH1 were silenced individually or simultaneously using small hairpin RNAs. Increased sub-G1 population and apoptotic cells were observed upon concurrent depletion of both enzymes. Elevated cell death was consistent with cleaved caspase 3 accumulation in double knockdown cells. Importantly, overexpression of the nuclear isoform of hMYH could remove the G1 arrest and partially rescue the toxicity observed in hMTH1-depleted cells. In addition, expression profiles of human DNA glycosylases were generated using quantitative reverse transcriptase-PCR in MTH1 and/or MYH knockdown cells. NEIL1 DNA glycosylase, involved in repair of oxidized nucleosides, was found to be significantly downregulated as a cellular response to MTH1-MYH co-suppression. Overall, the results suggest that hMYH and hMTH1 functionally cooperate for effective repair and survival in mismatch repair defective T-ALL Jurkat A3 cells.

The Major Replicative Histone Chaperone CAF-1 Suppresses the Activity of the DNA Mismatch Repair System in the Cytotoxic Response to a DNA-methylating Agent

The DNA mismatch repair (MMR) system corrects DNA mismatches in the genome. It is also required for the cytotoxic response of O⁶-methylguanine-DNA methyltransferase (MGMT)-deficient mammalian cells and yeast mgt1Δ rad52Δ cells to treatment with S_n1-type methylating agents, which produce cytotoxic O⁶-methylguanine (O⁶-mG) DNA lesions. Specifically, an activity of the MMR system causes degradation of irreparable O⁶-mG-T mispair-containing DNA, triggering cell death; this process forms the basis of treatments of MGMT-deficient cancers with S_n1-type methylating drugs. Recent research supports the view that degradation of irreparable O⁶-mG-T mispair-containing DNA by the MMR system and CAF-1-dependent packaging of the newly replicated DNA into nucleosomes are two concomitant processes that interact with each other. Here, we studied whether CAF-1 modulates the activity of the MMR system in the cytotoxic response to S_n1-type methylating agents. We found that CAF-1 suppresses the activity of the MMR system in the cytotoxic response of yeast mgt1Δ rad52Δ cells to the prototypic S_n1-type methylating agent N-methyl-N'-nitro-N-nitrosoguanidine. We also report evidence that in human MGMT-deficient cell-free extracts, CAF-1-dependent packaging of irreparable O⁶-mG-T mispair-containing DNA into nucleosomes suppresses its degradation by the MMR system. Taken together, these findings suggest that CAF-1-dependent incorporation of irreparable O⁶-mG-T mispair-containing DNA into nucleosomes suppresses its degradation by the MMR system, thereby defending the cell against killing by the S_n1-type methylating agent.

The Hallmarks of Cancer from a Redox Perspective

SIGNIFICANCE

For a healthy cell to turn into a cancer cell and grow out to become a tumor, it needs to undergo a series of complex changes and acquire certain traits, summarized as "The Hallmarks of Cancer." These hallmarks can all be regarded as the result of altered signal transduction cascades and an understanding of these cascades is essential for cancer treatment.

RECENT ADVANCES

Redox signaling is a long overlooked form of signal transduction that proceeds through the reversible oxidation of cysteines in proteins and that uses hydrogen peroxide as a second messenger.

CRITICAL ISSUES

In this article, we provide examples that show that redox signaling is involved in the regulation of proteins and signaling cascades that play roles in every hallmark of cancer.

FUTURE DIRECTIONS

An understanding of how redox signaling and "classical" signal transduction are intertwined could hold promising strategies for cancer therapy in the future. Antioxid. Redox Signal. 25, 300-325.

Pathways controlling dNTP pools to maintain genome stability

Artificially modified nucleotides, in the form of nucleoside analogues, are widely used in the treatment of cancers and various other diseases, and have become important tools in the laboratory to characterise DNA repair pathways. In contrast, the role of endogenously occurring nucleotide modifications in genome stability is little understood. This is despite the demonstration over three decades ago that the cellular DNA precursor pool is orders of magnitude more susceptible to modification than the DNA molecule itself. More recently, underscoring the importance of this topic, oxidation of the cellular nucleotide pool achieved through targeting the sanitation enzyme MTH1, appears to be a promising anti-cancer strategy. This article reviews our current understanding of modified DNA precursors in genome stability, with a particular focus upon oxidised nucleotides, and outlines some important outstanding questions.

DNA Mismatch Repair Interacts with CAF-1- and ASF1A-H3-H4-dependent Histone (H3-H4)2 Tetramer Deposition

DNA mismatch repair (MMR) is required for the maintenance of genome stability and protection of humans from several types of cancer. Human MMR occurs in the chromatin environment, but little is known about the interactions between MMR and the chromatin environment. Previous research has suggested that MMR coincides with replication-coupled assembly of the newly synthesized DNA into nucleosomes. The first step in replication-coupled nucleosome assembly is CAF-1-dependent histone (H3-H4)2 tetramer deposition, a process that involves ASF1A-H3-H4 complex. In this work we used reconstituted human systems to investigate interactions between MMR and CAF-1- and ASF1A-H3-H4-dependent histone (H3-H4)2 tetramer deposition. We have found that MutSα inhibits CAF-1- and ASF1A-H3-H4-dependent packaging of a DNA mismatch into a tetrasome. This finding supports the idea that MMR occurs before the DNA mismatch is packaged into the tetrasome. Our experiments have also revealed that CAF-1- and ASF1A-H3-H4-dependent deposition of the histone (H3-H4)2 tetramers does not interfere with MMR reactions. In addition, we have established that unnecessary degradation of the discontinuous strand that takes place in both DNA polymerase δ (Pol δ)- and DNA polymerase ϵ (Pol ϵ)-dependent MMR reactions is suppressed by CAF-1- and ASF1A-H3-H4-dependent deposition of the histone (H3-H4)2 tetramers. These data suggest that CAF-1- and ASF1A-H3-H4-dependent deposition of the histone (H3-H4)2 tetramers is compatible with MMR and protects the discontinuous daughter strand from unnecessary degradation by MMR machinery.

Identification of potent and selective MTH1 inhibitors

Structure based design of a novel class of aminopyrimidine MTH1 (MutT homolog 1) inhibitors is described. Optimization led to identification of IACS-4759 (compound 5), a sub-nanomolar inhibitor of MTH1 with excellent cell permeability and good metabolic stability in microsomes. This compound robustly inhibited MTH1 activity in cells and proved to be an excellent tool for interrogation of the utility of MTH1 inhibition in the context of oncology.

Endonuclease activities of MutLα and its homologs in DNA mismatch repair

MutLα is a key component of the DNA mismatch repair system in eukaryotes. The DNA mismatch repair system has several genetic stabilization functions. Of these functions, DNA mismatch repair is the major one. The loss of MutLα abolishes DNA mismatch repair, thereby predisposing humans to cancer. MutLα has an endonuclease activity that is required for DNA mismatch repair. The endonuclease activity of MutLα depends on the DQHA(X)2E(X)4E motif which is a part of the active site of the nuclease. This motif is also present in many bacterial MutL and eukaryotic MutLγ proteins, DNA mismatch repair system factors that are homologous to MutLα. Recent studies have shown that yeast MutLγ and several MutL proteins containing the DQHA(X)2E(X)4E motif possess endonuclease activities. Here, we review the endonuclease activities of MutLα and its homologs in the context of DNA mismatch repair.

Echinacoside induces apoptotic cancer cell death by inhibiting the nucleotide pool sanitizing enzyme MTH1

Inhibition of the nucleotide pool sanitizing enzyme MTH1 causes extensive oxidative DNA damages and apoptosis in cancer cells and hence may be used as an anticancer strategy. As natural products have been a rich source of medicinal chemicals, in the present study, we used the MTH1-catalyzed enzymatic reaction as a high-throughput in vitro screening assay to search for natural compounds capable of inhibiting MTH1. Echinacoside, a compound derived from the medicinal plants Cistanche and Echinacea, effectively inhibited the catalytic activity of MTH1 in an in vitro assay. Treatment of various human cancer cell lines with Echinacoside resulted in a significant increase in the cellular level of oxidized guanine (8-oxoguanine), while cellular reactive oxygen species level remained unchanged, indicating that Echinacoside also inhibited the activity of cellular MTH1. Consequently, Echinacoside treatment induced an immediate and dramatic increase in DNA damage markers and upregulation of the G1/S-CDK inhibitor p21, which were followed by marked apoptotic cell death and cell cycle arrest in cancer but not in noncancer cells. Taken together, these studies identified a natural compound as an MTH1 inhibitor and suggest that natural products can be an important source of anticancer agents.

Echinacoside Induces Apoptosis in Human SW480 Colorectal Cancer Cells by Induction of Oxidative DNA Damages

Echinacoside is a natural compound with potent reactive oxygen species (ROS)-scavenging and anti-oxidative bioactivities, which protect cells from oxidative damages. As cancer cells are often under intense oxidative stress, we therefore tested if Echinacoside treatment would promote cancer development. Surprisingly, we found that Echinacoside significantly inhibited the growth and proliferation of a panel of cancer cell lines. Treatment of the human SW480 cancer cells with Echinacoside resulted in marked apoptosis and cell cycle arrest, together with a significant increase in active caspase 3 and cleaved PARP, and upregulation of the G1/S-CDK blocker CDKN1B (p21). Interestingly, immunocytochemistry examination of drug-treated cancer cells revealed that Echinacoside caused a significant increase of intracellular oxidized guanine, 8-oxoG, and dramatic upregulation of the double-strand DNA break (DSB)-binding protein 53BP1, suggesting that Echinacoside induced cell cycle arrest and apoptosis in SW480 cancer cells via induction of oxidative DNA damages. These results establish Echinacoside as a novel chemical scaffold for development of anticancer drugs.

Influence of oxidized purine processing on strand directionality of mismatch repair

Replicative DNA polymerases are high fidelity enzymes that misincorporate nucleotides into nascent DNA with a frequency lower than [1/10(5)], and this precision is improved to about [1/10(7)] by their proofreading activity. Because this fidelity is insufficient to replicate most genomes without error, nature evolved postreplicative mismatch repair (MMR), which improves the fidelity of DNA replication by up to 3 orders of magnitude through correcting biosynthetic errors that escaped proofreading. MMR must be able to recognize non-Watson-Crick base pairs and excise the misincorporated nucleotides from the nascent DNA strand, which carries by definition the erroneous genetic information. In eukaryotes, MMR is believed to be directed to the nascent strand by preexisting discontinuities such as gaps between Okazaki fragments in the lagging strand or breaks in the leading strand generated by the mismatch-activated endonuclease of the MutL homologs PMS1 in yeast and PMS2 in vertebrates. We recently demonstrated that the eukaryotic MMR machinery can make use also of strand breaks arising during excision of uracils or ribonucleotides from DNA. We now show that intermediates of MutY homolog-dependent excision of adenines mispaired with 8-oxoguanine (G(O)) also act as MMR initiation sites in extracts of human cells or Xenopus laevis eggs. Unexpectedly, G(O)/C pairs were not processed in these extracts and failed to affect MMR directionality, but extracts supplemented with exogenous 8-oxoguanine DNA glycosylase (OGG1) did so. Because OGG1-mediated excision of G(O) might misdirect MMR to the template strand, our findings suggest that OGG1 activity might be inhibited during MMR.

Eukaryotic Mismatch Repair in Relation to DNA Replication

Three processes act in series to accurately replicate the eukaryotic nuclear genome. The major replicative DNA polymerases strongly prevent mismatch formation, occasional mismatches that do form are proofread during replication, and rare mismatches that escape proofreading are corrected by mismatch repair (MMR). This review focuses on MMR in light of increasing knowledge about nuclear DNA replication enzymology and the rate and specificity with which mismatches are generated during leading- and lagging-strand replication. We consider differences in MMR efficiency in relation to mismatch recognition, signaling to direct MMR to the nascent strand, mismatch removal, and the timing of MMR. These studies are refining our understanding of relationships between generating and repairing replication errors to achieve accurate replication of both DNA strands of the nuclear genome.

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.

Melanin photosensitization and the effect of visible light on epithelial cells

Protecting human skin from sun exposure is a complex issue that involves unclear aspects of the interaction between light and tissue. A persistent misconception is that visible light is safe for the skin, although several lines of evidence suggest otherwise. Here, we show that visible light can damage melanocytes through melanin photosensitization and singlet oxygen (¹O₂) generation, thus decreasing cell viability, increasing membrane permeability, and causing both DNA photo-oxidation and necro-apoptotic cell death. UVA (355 nm) and visible (532 nm) light photosensitize ¹O₂ with similar yields, and pheomelanin is more efficient than eumelanin at generating ¹O₂ and resisting photobleaching. Although melanin can protect against the cellular damage induced by UVB, exposure to visible light leads to pre-mutagenic DNA lesions (i.e., Fpg- and Endo III-sensitive modifications); these DNA lesions may be mutagenic and may cause photoaging, as well as other health problems, such as skin cancer.

Cellular levels of 8-oxoguanine in either DNA or the nucleotide pool play pivotal roles in carcinogenesis and survival of cancer cells

8-Oxoguanine, a major oxidized base lesion formed by reactive oxygen species, causes G to T transversion mutations or leads to cell death in mammals if it accumulates in DNA. 8-Oxoguanine can originate as 8-oxo-dGTP, formed in the nucleotide pool, or by direct oxidation of the DNA guanine base. MTH1, also known as NUDT1, with 8-oxo-dGTP hydrolyzing activity, 8-oxoguanine DNA glycosylase (OGG1) an 8-oxoG DNA glycosylase, and MutY homolog (MUTYH) with adenine DNA glycosylase activity, minimize the accumulation of 8-oxoG in DNA; deficiencies in these enzymes increase spontaneous and induced tumorigenesis susceptibility. However, different tissue types have different tumorigenesis susceptibilities. These can be reversed by combined deficiencies in the defense systems, because cell death induced by accumulation of 8-oxoG in DNA is dependent on MUTYH, which can be suppressed by MTH1 and OGG1. In cancer cells encountering high oxidative stress levels, a high level of 8-oxo-dGTP accumulates in the nucleotide pool, and cells therefore express increased levels of MTH1 in order to eliminate 8-oxo-dGTP. Suppression of MTH1 may be an efficient strategy for killing cancer cells; however, because MTH1 and OGG1 protect normal tissues from oxidative-stress-induced cell death, it is important that MTH1 inhibition does not increase the risk of healthy tissue degeneration.

8-oxoguanine causes spontaneous de novo germline mutations in mice

Spontaneous germline mutations generate genetic diversity in populations of sexually reproductive organisms, and are thus regarded as a driving force of evolution. However, the cause and mechanism remain unclear. 8-oxoguanine (8-oxoG) is a candidate molecule that causes germline mutations, because it makes DNA more prone to mutation and is constantly generated by reactive oxygen species in vivo. We show here that endogenous 8-oxoG caused de novo spontaneous and heritable G to T mutations in mice, which occurred at different stages in the germ cell lineage and were distributed throughout the chromosomes. Using exome analyses covering 40.9 Mb of mouse transcribed regions, we found increased frequencies of G to T mutations at a rate of 2 × 10⁻⁷ mutations/base/generation in offspring of Mth1/Ogg1/Mutyh triple knockout (TOY-KO) mice, which accumulate 8-oxoG in the nuclear DNA of gonadal cells. The roles of MTH1, OGG1, and MUTYH are specific for the prevention of 8-oxoG-induced mutation, and 99% of the mutations observed in TOY-KO mice were G to T transversions caused by 8-oxoG; therefore, we concluded that 8-oxoG is a causative molecule for spontaneous and inheritable mutations of the germ lineage cells.

MTH1 inhibition eradicates cancer by preventing sanitation of the dNTP pool

Cancers have dysfunctional redox regulation resulting in reactive oxygen species production, damaging both DNA and free dNTPs. The MTH1 protein sanitizes oxidized dNTP pools to prevent incorporation of damaged bases during DNA replication. Although MTH1 is non-essential in normal cells, we show that cancer cells require MTH1 activity to avoid incorporation of oxidized dNTPs, resulting in DNA damage and cell death. We validate MTH1 as an anticancer target in vivo and describe small molecules TH287 and TH588 as first-in-class nudix hydrolase family inhibitors that potently and selectively engage and inhibit the MTH1 protein in cells. Protein co-crystal structures demonstrate that the inhibitors bind in the active site of MTH1. The inhibitors cause incorporation of oxidized dNTPs in cancer cells, leading to DNA damage, cytotoxicity and therapeutic responses in patient-derived mouse xenografts. This study exemplifies the non-oncogene addiction concept for anticancer treatment and validates MTH1 as being cancer phenotypic lethal.

A dual-fluorescent reporter facilitates identification of thiol compounds that suppress microsatellite instability induced by oxidative stress

The DNA mismatch-repair (MMR) system corrects replicative errors and minimizes mutations that occur at a high rate in microsatellites. Patients with chronic inflammation or inflammation-associated cancer display microsatellite instability (MSI), indicating a possible MMR inactivation. In fact, H2O2-generated oxidative stress inactivates the MMR function and increases mutation accumulation in a reporter microsatellite. However, it remains unclear whether MSI induced by oxidative stress is preventable because of the lack of a sufficiently sensitive detection assay. Here, we developed and characterized a dual-fluorescent system, utilizing DsRed harboring the (CA)13 microsatellite as a reporter and GFP for normalization, in near-isogenic human colorectal cancer cell lines. Via flow cytometry, this reporter sensitively detected H2O2-generated oxidative microsatellite mutations in a dose-dependent manner. The reporter further revealed that glutathione or N-acetylcysteine was better than aspirin and ascorbic acid for suppressing oxidative microsatellite mutations. These two thiol compounds also partially suppressed oxidative frameshift mutations in the coding microsatellites of the hMSH6 and CHK1 genes based on a fluoresceinated PCR-based assay. MSI suppression by N-acetylcysteine appears to be mediated through reduction of oxidative frameshift mutations in the coding microsatellite of hMSH6 and protection of hMSH6 and other MMR protein levels from being decreased by H2O2. Our findings suggest a linkage between oxidative damage, MMR deficiency, and MSI. The two thiol compounds are potentially valuable for preventing inflammation-associated MSI. The dual-fluorescent reporter with improved features will facilitate identification of additional compounds that modulate MSI, which is relevant to cancer initiation and progression.

Mismatch repair deficient mice show susceptibility to oxidative stress-induced intestinal carcinogenesis

We have previously established an experimental system for oxidative DNA damage-induced tumorigenesis in the small intestine of mice. To elucidate the roles of mismatch repair genes in the tumor suppression, we performed oxidative DNA damage-induced tumorigenesis experiments using Msh2-deficient mice. Oral administration of 0.2% Potassium Bromate, KBrO₃, effectively induced epithelial tumors in the small intestines of Msh2-deficient mice. We observed a 22.5-fold increase in tumor formation in the small intestines of Msh2-deficient mice compared with the wild type mice. These results indicate that mismatch repair is involved in the suppression of oxidative stress-induced intestinal tumorigenesis in mice. A mutation analysis of the Ctnnb1 gene of the tumors revealed predominant occurrences of G:C to A:T transitions. The TUNEL analysis showed a decreased number of TUNEL-positive cells in the crypts of small intestines from the Msh2-deficient mice compared with the wild type mice after treatment of KBrO₃. These results suggest that the mismatch repair system may simultaneously function in both avoiding mutagenesis and inducing cell death to suppress the tumorigenesis induced by oxidative stress in the small intestine of mice.