Inhibition of DNA replication fork progression and mutagenic potential of 1, N6-ethenoadenine and 8-oxoguanine in human cell extracts

8-Oxoadenine: A «New» Player of the Oxidative Stress in Mammals?

Numerous studies have shown that oxidative modifications of guanine (7,8-dihydro-8-oxoguanine, 8-oxoG) can affect cellular functions. 7,8-Dihydro-8-oxoadenine (8-oxoA) is another abundant paradigmatic ambiguous nucleobase but findings reported on the mutagenicity of 8-oxoA in bacterial and eukaryotic cells are incomplete and contradictory. Although several genotoxic studies have demonstrated the mutagenic potential of 8-oxoA in eukaryotic cells, very little biochemical and bioinformatics data about the mechanism of 8-oxoA-induced mutagenesis are available. In this review, we discuss dual coding properties of 8-oxoA, summarize historical and recent genotoxicity and biochemical studies, and address the main protective cellular mechanisms of response to 8-oxoA. We also discuss the available structural data for 8-oxoA bypass by different DNA polymerases as well as the mechanisms of 8-oxoA recognition by DNA repair enzymes.

Tracking of progressing human DNA polymerase δ holoenzymes reveals distributions of DNA lesion bypass activities

During DNA replication, DNA lesions in lagging strand templates are initially encountered by DNA polymerase δ (pol δ) holoenzymes comprised of pol δ and the PCNA processivity sliding clamp. These encounters are thought to stall replication of an afflicted template before the lesion, activating DNA damage tolerance (DDT) pathways that replicate the lesion and adjacent DNA sequence, allowing pol δ to resume. However, qualitative studies observed that human pol δ can replicate various DNA lesions, albeit with unknown proficiencies, which raises issues regarding the role of DDT in replicating DNA lesions. To address these issues, we re-constituted human lagging strand replication to quantitatively characterize initial encounters of pol δ holoenzymes with DNA lesions. The results indicate pol δ holoenzymes support dNTP incorporation opposite and beyond multiple lesions and the extent of these activities depends on the lesion and pol δ proofreading. Furthermore, after encountering a given DNA lesion, subsequent dissociation of pol δ is distributed around the lesion and a portion does not dissociate. The distributions of these events are dependent on the lesion and pol δ proofreading. Collectively, these results reveal complexity and heterogeneity in the replication of lagging strand DNA lesions, significantly advancing our understanding of human DDT.

Knockout and Inhibition of Ape1: Roles of Ape1 in Base Excision DNA Repair and Modulation of Gene Expression

Apurinic/apyrimidinic endonuclease 1/redox effector-1 (Ape1/Ref-1) is the major apurinic/apyrimidinic (AP) endonuclease in mammalian cells. It functions mainly in the base excision repair pathway to create a suitable substrate for DNA polymerases. Human Ape1 protein can activate some transcription factors to varying degrees, dependent on its N-terminal, unstructured domain, and some of the cysteines within it, apparently via a redox mechanism in some cases. Many cancer studies also suggest that Ape1 has potential for prognosis in terms of the protein level or intracellular localization. While homozygous disruption of the Ape1 structural gene APEX1 in mice causes embryonic lethality, and most studies in cell culture indicate that the expression of Ape1 is essential, some recent studies reported the isolation of viable APEX1 knockout cells with only mild phenotypes. It has not been established by what mechanism the Ape1-null cell lines cope with the endogenous DNA damage that the enzyme normally handles. We review the enzymatic and other activities of Ape1 and the recent studies of the properties of the APEX1 knockout lines. The APEX1 deletions in CH12F3 and HEK293 FT provide an opportunity to test for possible off-target effects of Ape1 inhibition. For this work, we tested the Ape1 endonuclease inhibitor Compound 3 and the redox inhibitor APX2009. Our results confirmed that both APEX1 knockout cell lines are modestly more sensitive to killing by an alkylating agent than their Ape1-proficient cells. Surprisingly, the knockout lines showed equal sensitivity to direct killing by either inhibitor, despite the lack of the target protein. Moreover, the CH12F3 APEX1 knockout was even more sensitive to Compound 3 than its APEX1⁺ counterpart. Thus, it appears that both Compound 3 and APX2009 have off-target effects. In cases where this issue may be important, it is advisable that more specific endpoints than cell survival be tested for establishing mechanism.

7,8-Dihydro-8-oxo-1,N6-ethenoadenine: an exclusively Hoogsteen-paired thymine mimic in DNA that induces A→T transversions in Escherichia coli

This work investigated the structural and biological properties of DNA containing 7,8-dihydro-8-oxo-1,N⁶-ethenoadenine (oxo-ϵA), a non-natural synthetic base that combines structural features of two naturally occurring DNA lesions (7,8-dihydro-8-oxoadenine and 1,N⁶-ethenoadenine). UV-, CD-, NMR spectroscopies and molecular modeling of DNA duplexes revealed that oxo-ϵA adopts the non-canonical syn conformation (χ = 65º) and fits very well among surrounding residues without inducing major distortions in local helical architecture. The adduct remarkably mimics the natural base thymine. When considered as an adenine-derived DNA lesion, oxo-ϵA was >99% mutagenic in living cells, causing predominantly A→T transversion mutations in Escherichia coli. The adduct in a single-stranded vector was not repaired by base excision repair enzymes (MutM and MutY glycosylases) or the AlkB dioxygenase and did not detectably affect the efficacy of DNA replication in vivo. When the biological and structural data are viewed together, it is likely that the nearly exclusive syn conformation and thymine mimicry of oxo-ϵA defines the selectivity of base pairing in vitro and in vivo, resulting in lesion pairing with A during replication. The base pairing properties of oxo-ϵA, its strong fluorescence and its invisibility to enzymatic repair systems in vivo are features that are sought in novel DNA-based probes and modulators of gene expression.

Etheno adducts: from tRNA modifications to DNA adducts and back to miscoding ribonucleotides

Etheno (and ethano) derivatives of nucleic acid bases have an extra 5-membered ring attached. These were first noted as wyosine bases in tRNAs. Some were fluorescent, and the development of etheno derivatives of adenosine, cytosine, and guanosine led to the synthesis of fluorescent analogs of ATP, NAD+, and other cofactors for use in biochemical studies. Early studies with the carcinogen vinyl chloride revealed that these modified bases were being formed in DNA and RNA and might be responsible for mutations and cancer. The etheno bases are also derived from other carcinogenic vinyl monomers. Further work showed that endogenous etheno DNA adducts were present in animals and humans and are derived from lipid peroxidation. The chemical mechanisms of etheno adduct formation involve reactions with bis-electrophiles generated by cytochrome P450 enzymes or lipid peroxidation, which have been established in isotopic labeling studies. The mechanisms by which etheno DNA adducts miscode have been studied with several DNA polymerases, aided by the X-ray crystal structures of these polymerases in mispairing situations and in extension beyond mispairs. Repair of etheno DNA adduct damage is done primarily by glycosylases and also by the direct action of dioxygenases. Some human DNA polymerases (η, κ) can insert bases opposite etheno adducts in DNA and RNA, and the reverse transcriptase activity may be of relevance with the RNA etheno adducts. Further questions involve the extent that the etheno adducts contribute to human cancer.

Deletion of the GAPDH gene contributes to genome stability in Saccharomyces cerevisiae

Cellular metabolism is directly or indirectly associated with various cellular processes by producing a variety of metabolites. Metabolic alterations may cause adverse effects on cell viability. However, some alterations potentiate the rescue of the malfunction of the cell system. Here, we found that the alteration of glucose metabolism suppressed genome instability caused by the impairment of chromatin structure. Deletion of the TDH2 gene, which encodes glyceraldehyde 3-phospho dehydrogenase and is essential for glycolysis/gluconeogenesis, partially suppressed DNA damage sensitivity due to chromatin structure, which was persistently acetylated histone H3 on lysine 56 in cells with deletions of both HST3 and HST4, encoding NAD⁺-dependent deacetylases. tdh2 deletion also restored the short replicative lifespan of cells with deletion of sir2, another NAD⁺-dependent deacetylase, by suppressing intrachromosomal recombination in rDNA repeats increased by the unacetylated histone H4 on lysine 16. tdh2 deletion also suppressed recombination between direct repeats in hst3∆ hst4∆ cells by suppressing the replication fork instability that leads to both DNA deletions among repeats. We focused on quinolinic acid (QUIN), a metabolic intermediate in the de novo nicotinamide adenine dinucleotide (NAD⁺) synthesis pathway, which accumulated in the tdh2 deletion cells and was a candidate metabolite to suppress DNA replication fork instability. Deletion of QPT1, quinolinate phosphoribosyl transferase, elevated intracellular QUIN levels and partially suppressed the DNA damage sensitivity of hst3∆ hst4∆ cells as well as tdh2∆ cells. qpt1 deletion restored the short replicative lifespan of sir2∆ cells by suppressing intrachromosomal recombination among rDNA repeats. In addition, qpt1 deletion could suppress replication fork slippage between direct repeats. These findings suggest a connection between glucose metabolism and genomic stability.

1,N6-Ethenoadenine: From Molecular to Biological Consequences†

Genomic DNA is chemically reactive and therefore susceptible to damage by many exogenous and endogenous sources. Lesions produced from these damaging events can have various mutagenic and genotoxic consequences. This Perspective follows the journey of one particular lesion, 1,N⁶-ethenoadenine (εA), from its formation to replication and repair, and its role in cancerous tissues and inflammatory diseases. εA is generated by the reaction of adenine (A) with vinyl chloride or lipid peroxidation products. We present the miscoding properties of εA with an emphasis on how bacterial and mammalian cells can process lesions differently, leading to varied mutational spectra. But with information from these assays, we can better understand how the miscoding properties of εA lead to biological consequences and how genomic stability can be maintained via DNA repair mechanisms. We discuss how base excision repair (BER) and direct reversal repair (DRR) can minimize the biological consequences of εA lesions. Kinetic parameters of glycosylases and AlkB family enzymes are described, along with a discussion of the relative contributions of the BER and DRR pathways in the repair of εA. Because eukaryotic DNA is packaged in chromatin, we also discuss the impact of this packaging on BER and DRR, specifically in regards to repair of εA. Studying DNA lesions like εA in this context, from origin to biological implications, can provide crucial information to better understand prevention of mutagenesis and cancer.

Molecular characterization of Plasmodium falciparum DNA-3-methyladenine glycosylase

Background

The emergence of artemisinin-resistant malaria parasites highlights the need for novel drugs and their targets. Alkylation of purine bases can hinder DNA replication and if unresolved would eventually result in cell death. DNA-3-methyladenine glycosylase (MAG) is responsible for the repair of those alkylated bases. Plasmodium falciparum (Pf) MAG was characterized for its potential for development as an anti-malarial candidate.

Methods

Native PfMAG from crude extract of chloroquine- and pyrimethamine-resistant P. falciparum K1 strain was partially purified using three chromatographic procedures. From bio-informatics analysis, primers were designed for amplification, insertion into pBAD202/D-TOPO and heterologous expression in Escherichia coli of recombinant PfMAG. Functional and biochemical properties of the recombinant enzyme were characterized.

Results

PfMAG activity was most prominent in parasite schizont stages, with a specific activity of 147 U/mg (partially purified) protein. K1 PfMAG contained an insertion of AAT (coding for asparagine) compared to 3D7 strain and 16% similarity to the human enzyme. Recombinant PfMAG (74 kDa) was twice as large as the human enzyme, preferred double-stranded DNA substrate, and demonstrated glycosylase activity over a pH range of 4–9, optimal salt concentration of 100–200 mM NaCl but reduced activity at 250 mM NaCl, no requirement for divalent cations, which were inhibitory in a dose-dependent manner.

Conclusion

PfMAG activity increased with parasite development being highest in the schizont stages. K1 PfMAG contained an indel AAT (asparagine) not present in 3D7 strain and the recombinant enzyme was twice as large as the human enzyme. Recombinant PfMAG had a wide range of optimal pH activity, and was inhibited at high (250 mM) NaCl concentration as well as by divalent cations. The properties of PfMAG provide basic data that should be of assistance in developing anti-malarials against this potential parasite target.

Mechanisms of mutagenesis induced by DNA lesions: multiple factors affect mutations in translesion DNA synthesis

Environmental mutagens lead to mutagenesis. However, the mechanisms are very complicated and not fully understood. Environmental mutagens produce various DNA lesions, including base-damaged or sugar-modified DNA lesions, as well as epigenetically modified DNA. DNA polymerases produce mutation spectra in translesion DNA synthesis (TLS) through misincorporation of incorrect nucleotides, frameshift deletions, blockage of DNA replication, imbalance of leading- and lagging-strand DNA synthesis, and genome instability. Motif or subunit in DNA polymerases further affects the mutations in TLS. Moreover, protein interactions and accessory proteins in DNA replisome also alter mutations in TLS, demonstrated by several representative DNA replisomes. Finally, in cells, multiple DNA polymerases or cellular proteins collaborate in TLS and reduce in vivo mutagenesis. Summaries and perspectives were listed. This review shows mechanisms of mutagenesis induced by DNA lesions and the effects of multiple factors on mutations in TLS in vitro and in vivo.

Inflammation-induced DNA damage, mutations and cancer

The relationships between inflammation and cancer are varied and complex. An important connection linking inflammation to cancer development is DNA damage. During inflammation reactive oxygen and nitrogen species (RONS) are created to combat pathogens and to stimulate tissue repair and regeneration, but these chemicals can also damage DNA, which in turn can promote mutations that initiate and promote cancer. DNA repair pathways are essential for preventing DNA damage from causing mutations and cytotoxicity, but RONS can interfere with repair mechanisms, reducing their efficacy. Further, cellular responses to DNA damage, such as damage signaling and cytotoxicity, can promote inflammation, creating a positive feedback loop. Despite coordination of DNA repair and oxidative stress responses, there are nevertheless examples whereby inflammation has been shown to promote mutagenesis, tissue damage, and ultimately carcinogenesis. Here, we discuss the DNA damage-mediated associations between inflammation, mutagenesis and cancer.

DNA polymerase ι is acetylated in response to SN2 alkylating agents

DNA polymerase iota (Polι) belongs to the Y-family of DNA polymerases that are involved in DNA damage tolerance through their role in translesion DNA synthesis. Like all other Y-family polymerases, Polι interacts with proliferating cell nuclear antigen (PCNA), Rev1, ubiquitin and ubiquitinated-PCNA and is also ubiquitinated itself. Here, we report that Polι also interacts with the p300 acetyltransferase and is acetylated. The primary acetylation site is K550, located in the Rev1-interacting region. However, K550 amino acid substitutions have no effect on Polι's ability to interact with Rev1. Interestingly, we find that acetylation of Polι significantly and specifically increases in response to SN2 alkylating agents and to a lower extent to SN1 alkylating and oxidative agents. As we have not observed acetylation of Polι's closest paralogue, DNA polymerase eta (Polη), with which Polι shares many functional similarities, we believe that this modification might exclusively regulate yet to be determined, and separate function(s) of Polι.

Mutagenic potential of hypoxanthine in live human cells

Hypoxanthine (Hx) is a major DNA lesion generated by deamination of adenine during chronic inflammatory conditions, which is an underlying cause of various diseases including cancer of colon, liver, pancreas, bladder and stomach. There is evidence that deamination of DNA bases induces mutations, but no study has directly linked Hx accumulation to mutagenesis and strand-specific mutations yet in human cells. Using a site-specific mutagenesis approach, we report the first direct evidence of mutation potential and pattern of Hx in live human cells. We investigated Hx-induced mutations in human nonmalignant HEK293 and cancer HCT116 cell lines and found that Hx is mutagenic in both HEK293 and HCT116 cell lines. There is a strand bias for Hx-mediated mutations in both the cell lines; the Hx in lagging strand is more mutagenic than in leading strand. There is also some difference in cell types regarding the strand bias for mutation types; HEK293 cells showed largely deletion (>80%) mutations in both leading and lagging strand and the rest were insertions and A:T→G:C transition mutations in leading and lagging strands, respectively, whereas in HCT116 cells we observed 60% A:T→G:C transition mutations in the leading strand and 100% deletions in the lagging strand. Overall, Hx is a highly mutagenic lesion capable of generating A:T→G:C transitions and large deletions with a significant variation in leading and lagging strands in human cells. In recent meta-analysis study A→G (T→C) mutations were found to be a prominent signature in a variety of cancers, including a majority types that are induced by inflammation. The deletions are known to be a major cause of copy-number variations or CNVs, which is a major underlying cause of many human diseases including mental illness, developmental disorders and cancer. Thus, Hx, a major DNA lesion induced by different deamination mechanisms, has potential to initiate inflammation-driven carcinogenesis in addition to various human pathophysiological consequences.

Occurrence, Biological Consequences, and Human Health Relevance of Oxidative Stress-Induced DNA Damage

A variety of endogenous and exogenous agents can induce DNA damage and lead to genomic instability. Reactive oxygen species (ROS), an important class of DNA damaging agents, are constantly generated in cells as a consequence of endogenous metabolism, infection/inflammation, and/or exposure to environmental toxicants. A wide array of DNA lesions can be induced by ROS directly, including single-nucleobase lesions, tandem lesions, and hypochlorous acid (HOCl)/hypobromous acid (HOBr)-derived DNA adducts. ROS can also lead to lipid peroxidation, whose byproducts can also react with DNA to produce exocyclic DNA lesions. A combination of bioanalytical chemistry, synthetic organic chemistry, and molecular biology approaches have provided significant insights into the occurrence, repair, and biological consequences of oxidatively induced DNA lesions. The involvement of these lesions in the etiology of human diseases and aging was also investigated in the past several decades, suggesting that the oxidatively induced DNA adducts, especially bulky DNA lesions, may serve as biomarkers for exploring the role of oxidative stress in human diseases. The continuing development and improvement of LC-MS/MS coupled with the stable isotope-dilution method for DNA adduct quantification will further promote research about the clinical implications and diagnostic applications of oxidatively induced DNA adducts.

Structural and Kinetic Analysis of Miscoding Opposite the DNA Adduct 1,N6-Ethenodeoxyadenosine by Human Translesion DNA Polymerase η

1,N(6)-Ethenodeoxyadenosine (1,N(6)-ϵdA) is the major etheno lesion formed in the reaction of DNA with epoxides substituted with good leaving groups (e.g. vinyl chloride epoxide). This lesion is also formed endogenously in DNA from lipid oxidation. Recombinant human DNA polymerase η (hpol η) can replicate oligonucleotide templates containing 1,N(6)-ϵdA. In steady-state kinetic analysis, hpol η preferred to incorporate dATP and dGTP, compared with dTTP. Mass spectral analysis of incorporation products also showed preferred purine (A, G) incorporation and extensive -1 frameshifts, suggesting pairing of the inserted purine and slippage before further replication. Five x-ray crystal structures of hpol η ternary complexes were determined, three at the insertion and two at the extension stage. Two insertion complexes revealed incoming non-hydrolyzable dATP or dGTP analogs not pairing with but instead in a staggered configuration relative to 1,N(6)-ϵdA in the anti conformation, thus opposite the 5'-T in the template, explaining the proclivity for frameshift misincorporation. In another insertion complex, dTTP was positioned opposite 1,N(6)-ϵdA, and the adduct base was in the syn conformation, with formation of two hydrogen bonds. At the extension stage, with either an incorporated dA or dT opposite 1,N(6)-ϵdA and 2'-deoxythymidine-5'-[(α,β)-imido]triphosphate opposite the 5'-A, the 3'-terminal nucleoside of the primer was disordered, consistent with the tendency not to incorporate dTTP opposite 1,N(6)-ϵdA. Collectively, the results show a preference for purine pairing opposite 1,N(6)-ϵdA and for -1 frameshifts.

Error-Prone and Error-Free Translesion DNA Synthesis over Site-Specifically Created DNA Adducts of Aryl Hydrocarbons (3-Nitrobenzanthrone and 4-Aminobiphenyl)

Aryl hydrocarbons such as 3-nitrobenzanthrone (NBA), 4-aminobiphenyl (ABP), acetylaminofluorene (AAF), benzo(a)pyrene (BaP), and 1-nitropyrene (NP) form bulky DNA adducts when absorbed by mammalian cells. These chemicals are metabolically activated to reactive forms in mammalian cells and preferentially get attached covalently to the N² or C8 positions of guanine or the N⁶ position of adenine. The proportion of N² and C8 guanine adducts in DNA differs among chemicals. Although these adducts block DNA replication, cells have a mechanism allowing to continue replication by bypassing these adducts: translesion DNA synthesis (TLS). TLS is performed by translesion DNA polymerases—Pol η, κ, ι, and ζ and Rev1—in an error-free or error-prone manner. Regarding the NBA adducts, namely, 2-(2′-deoxyguanosin-N²-yl)-3-aminobenzanthrone (dG-N²-ABA) and N-(2′-deoxyguanosin-8-yl)-3-aminobenzanthrone (dG-C8-ABA), dG-N²-ABA is produced more often than dG-C8-ABA, whereas dG-C8-ABA blocks DNA replication more strongly than dG-N²-ABA. dG-N²-ABA allows for a less error-prone bypass than dG-C8-ABA does. Pol η and κ are stronger contributors to TLS over dG-C8-ABA, and Pol κ bypasses dG-C8-ABA in an error-prone manner. TLS efficiency and error-proneness are affected by the sequences surrounding the adduct, as demonstrated in our previous study on an ABP adduct, N-(2′-deoxyguanosine-8-yl)-4-aminobiphenyl (dG-C8-ABP). Elucidation of the general mechanisms determining efficiency, error-proneness, and the polymerases involved in TLS over various adducts is the next step in the research on TLS. These TLS studies will clarify the mechanisms underlying aryl hydrocarbon mutagenesis and carcinogenesis in more detail.

Germ line variants of human N-methylpurine DNA glycosylase show impaired DNA repair activity and facilitate 1,N6-ethenoadenine-induced mutations

Human N-methylpurine DNA glycosylase (hMPG) initiates base excision repair of a number of structurally diverse purine bases including 1,N(6)-ethenoadenine, hypoxanthine, and alkylation adducts in DNA. Genetic studies discovered at least eight validated non-synonymous single nucleotide polymorphisms (nsSNPs) of the hMPG gene in human populations that result in specific single amino acid substitutions. In this study, we tested the functional consequences of these nsSNPs of hMPG. Our results showed that two specific arginine residues, Arg-141 and Arg-120, are important for the activity of hMPG as the germ line variants R120C and R141Q had reduced enzymatic activity in vitro as well as in mammalian cells. Expression of these two variants in mammalian cells lacking endogenous MPG also showed an increase in mutations and sensitivity to an alkylating agent compared with the WT hMPG. Real time binding experiments by surface plasmon resonance spectroscopy suggested that these variants have substantial reduction in the equilibrium dissociation constant of binding (KD) of hMPG toward 1,N(6)-ethenoadenine-containing oligonucleotide (ϵA-DNA). Pre-steady-state kinetic studies showed that the substitutions at arginine residues affected the turnover of the enzyme significantly under multiple turnover condition. Surface plasmon resonance spectroscopy further showed that both variants had significantly decreased nonspecific (undamaged) DNA binding. Molecular modeling suggested that R141Q substitution may have resulted in a direct loss of the salt bridge between ϵA-DNA and hMPG, whereas R120C substitution redistributed, at a distance, the interactions among residues in the catalytic pocket. Together our results suggest that individuals carrying R120C and R141Q MPG variants may be at risk for genomic instability and associated diseases as a consequence.

Minimal detection of nuclear mutations in XP-V and normal cells treated with oxidative stress inducing agents

Elevated levels of reactive oxygen species (ROS) can be induced by exposure to various chemicals and radiation. One type of damage in DNA produced by ROS is modification of guanine to 7,8-dihydro-8-oxoguanine (8-oxoG). This particular alteration to the chemistry of the base can inhibit the replication fork and has been linked to mutagenesis, cancer, and aging. In vitro studies have shown that the translesion synthesis polymerase, DNA polymerase η (pol η), is able to efficiently bypass 8-oxoG in DNA. In this study, we wanted to investigate the mutagenic effects of oxidative stress, and in particular 8-oxoG, in the presence and absence of pol η. We quantified levels of oxidative stress, 8-oxoG levels in DNA, and nuclear mutation rates. We found that most of the 8-oxoG detected were localized to the mitochondrial DNA, opposed to the nuclear DNA. We also saw a corresponding lack of mutations in a nuclear-encoded gene. This suggests that oxidative stress' primary mutagenic effects are not predominantly on genomic DNA.

Slow repair of lipid peroxidation-induced DNA damage at p53 mutation hotspots in human cells caused by low turnover of a DNA glycosylase

Repair of oxidative stress- and inflammation-induced DNA lesions by the base excision repair (BER) pathway prevents mutation, a form of genomic instability which is often observed in cancer as 'mutation hotspots'. This suggests that some sequences have inherent mutability, possibly due to sequence-related differences in repair. This study has explored intrinsic mutability as a consequence of sequence-specific repair of lipid peroxidation-induced DNA adduct, 1, N(6)-ethenoadenine (εA). For the first time, we observed significant delay in repair of ϵA at mutation hotspots in the tumor suppressor gene p53 compared to non-hotspots in live human hepatocytes and endothelial cells using an in-cell real time PCR-based method. In-cell and in vitro mechanism studies revealed that this delay in repair was due to inefficient turnover of N-methylpurine-DNA glycosylase (MPG), which initiates BER of εA. We determined that the product dissociation rate of MPG at the hotspot codons was ≈5-12-fold lower than the non-hotspots, suggesting a previously unknown mechanism for slower repair at mutation hotspots and implicating sequence-related variability of DNA repair efficiency to be responsible for mutation hotspot signatures.

Base pairing and miscoding properties of 1,N(6)-ethenoadenine- and 3,N(4)-ethenocytosine-containing RNA oligonucleotides

Two RNA phosphoramidites containing the bases 1,N(6)-ethenoadenine (εA) and 3,N(4)-ethenocytosine (εC) were synthesized. These building blocks were incorporated into two 12-mer oligoribonucleotides for evaluation of the base pairing properties of these base lesions by UV melting curve (Tm) and circular dichroism measurements. The Tm data of the resulting duplexes with the etheno modifications opposing all natural bases showed a substantial destabilization compared to the corresponding natural duplexes, confirming their inability to form base pairs. The coding properties of these lesions were further investigated by introducing them into 31-mer oligonucleotides and assessing their ability to serve as templates in primer extension reactions with HIV, AMV, and MMLV reverse transcriptases (RT). Primer extension reactions showed complete arrest of the incorporation process using MMLV RT and AMV RT, while HIV RT preferentially incorporates dAMP opposite εA and dAMP as well as dTMP opposite εC. The properties of these RNA lesions are discussed in the context of its putative biological role.

XRCC1 and base excision repair balance in response to nitric oxide

Inflammation associated reactive oxygen and nitrogen species (RONs), including peroxynitrite (ONOO(-)) and nitric oxide (NO), create base lesions that potentially play a role in the toxicity and large genomic rearrangements associated with many malignancies. Little is known about the role of base excision repair (BER) in removing these endogenous DNA lesions. Here, we explore the role of X-ray repair cross-complementing group 1 (XRCC1) in attenuating RONs-induced genotoxicity. XRCC1 is a scaffold protein critical for BER for which polymorphisms modulate the risk of cancer. We exploited CHO and human glioblastoma cell lines engineered to express varied levels of BER proteins to study XRCC1. Cytotoxicity and the levels of DNA repair intermediates (single-strand breaks; SSB) were evaluated following exposure of the cells to the ONOO(-) donor, SIN-1, and to gaseous NO. XRCC1 null cells were slightly more sensitive to SIN-1 than wild-type cells. We used small-scale bioreactors to expose cells to NO and found that XRCC1-deficient CHO cells were not sensitive. However, using a molecular beacon assay to test lesion removal in vitro, we found that XRCC1 facilitates AAG-initiated excision of two key NO-induced DNA lesions: 1,N(6)-ethenoadenine and hypoxanthine. Furthermore, overexpression of AAG rendered XRCC1-deficient cells sensitive to NO-induced DNA damage. These results show that AAG is a key glycosylase for BER of NO-induced DNA damage and that XRCC1's role in modulating sensitivity to RONs is dependent upon the cellular level of AAG. This demonstrates the importance of considering the expression of other components of the BER pathway when evaluating the impact of XRCC1 polymorphisms on cancer risk.

Chemistry and structural biology of DNA damage and biological consequences

The formation of adducts by the reaction of chemicals with DNA is a critical step for the initiation of carcinogenesis. The structural analysis of various DNA adducts reveals that conformational and chemical rearrangements and interconversions are a common theme. Conformational changes are modulated both by the nature of adduct and the base sequences neighboring the lesion sites. Equilibria between conformational states may modulate both DNA repair and error-prone replication past these adducts. Likewise, chemical rearrangements of initially formed DNA adducts are also modulated both by the nature of adducts and the base sequences neighboring the lesion sites. In this review, we focus on DNA damage caused by a number of environmental and endogenous agents, and biological consequences.

The catalytic function of the Rev1 dCMP transferase is required in a lesion-specific manner for translesion synthesis and base damage-induced mutagenesis

The Rev1-Polzeta pathway is believed to be the major mechanism of translesion DNA synthesis and base damage-induced mutagenesis in eukaryotes. While it is widely believed that Rev1 plays a non-catalytic function in translesion synthesis, the role of its dCMP transferase activity remains uncertain. To determine the relevance of its catalytic function in translesion synthesis, we separated the Rev1 dCMP transferase activity from its non-catalytic function in yeast. This was achieved by mutating two conserved amino acid residues in the catalytic domain of Rev1, i.e. D467A/E468A, where its catalytic function was abolished but its non-catalytic function remained intact. In this mutant strain, whereas translesion synthesis and mutagenesis of UV radiation were fully functional, those of a site-specific 1,N(6)-ethenoadenine were severely deficient. Specifically, the predominant A-->G mutations resulting from C insertion opposite the lesion were abolished. Therefore, translesion synthesis and mutagenesis of 1,N(6)-ethenoadenine require the catalytic function of the Rev1 dCMP transferase, in contrast to those of UV lesions, which only require the non-catalytic function of Rev1. These results show that the catalytic function of the Rev1 dCMP transferase is required in a lesion-specific manner for translesion synthesis and base damage-induced mutagenesis.

Impact of conformational heterogeneity of OxoG lesions and their pairing partners on bypass fidelity by Y family polymerases

7,8-Dihydro-8-oxoguanine (oxoG), the predominant oxidative DNA damage lesion, is processed differently by high-fidelity and Y-family lesion bypass polymerases. Although high-fidelity polymerases extend predominantly from an A base opposite an oxoG, the Y-family polymerases Dpo4 and human Pol eta preferentially extend from the oxoG*C base pair. We have determined crystal structures of extension Dpo4 ternary complexes with oxoG opposite C, A, G, or T and the next nascent base pair. We demonstrate that neither template backbone nor the architecture of the active site is perturbed by the oxoG(anti)*C and oxoG*A pairs. However, the latter manifest conformational heterogeneity, adopting both oxoG(syn)*A(anti) and oxoG(anti)*A(syn) alignment. Hence, the observed reduced primer extension from the dynamically flexible 3'-terminal primer base A is explained. Because of homology between Dpo4 and Pol eta, such a dynamic screening mechanism might be utilized by Dpo4 and Pol eta to regulate error-free versus error-prone bypass of oxoG and other lesions.

Mitochondrial DNA damage in iron overload

Chronic iron overload has slow and insidious effects on heart, liver, and other organs. Because iron-driven oxidation of most biologic materials (such as lipids and proteins) is readily repaired, this slow progression of organ damage implies some kind of biological "memory." We hypothesized that cumulative iron-catalyzed oxidant damage to mtDNA might occur in iron overload, perhaps explaining the often lethal cardiac dysfunction. Real time PCR was used to examine the "intactness" of mttDNA in cultured H9c2 rat cardiac myocytes. After 3-5 days exposure to high iron, these cells exhibited damage to mtDNA reflected by diminished amounts of near full-length 15.9-kb PCR product with no change in the amounts of a 16.1-kb product from a nuclear gene. With the loss of intact mtDNA, cellular respiration declined and mRNAs for three electron transport chain subunits and 16 S rRNA encoded by mtDNA decreased, whereas no decrements were found in four subunits encoded by nuclear DNA. To examine the importance of the interactions of iron with metabolically generated reactive oxygen species, we compared the toxic effects of iron in wild-type and rho(o) cells. In wild-type cells, elevated iron caused increased production of reactive oxygen species, cytostasis, and cell death, whereas the rho(o) cells were unaffected. We conclude that long-term damage to cells and organs in iron-overload disorders involves interactions between iron and mitochondrial reactive oxygen species resulting in cumulative damage to mtDNA, impaired synthesis of respiratory chain subunits, and respiratory dysfunction.