Oxygen as a friend and enemy: How to combat the mutational potential of 8-oxo-guanine

Genome sequencing of normal cells reveals developmental lineages and mutational processes

The somatic mutations present in the genome of a cell accumulate over the lifetime of a multicellular organism. These mutations can provide insights into the developmental lineage tree, the number of divisions that each cell has undergone and the mutational processes that have been operative. Here we describe whole genomes of clonal lines derived from multiple tissues of healthy mice. Using somatic base substitutions, we reconstructed the early cell divisions of each animal, demonstrating the contributions of embryonic cells to adult tissues. Differences were observed between tissues in the numbers and types of mutations accumulated by each cell, which likely reflect differences in the number of cell divisions they have undergone and varying contributions of different mutational processes. If somatic mutation rates are similar to those in mice, the results indicate that precise insights into development and mutagenesis of normal human cells will be possible.

How DNA polymerase X preferentially accommodates incoming dATP opposite 8-oxoguanine on the template

The modified base 8-oxo-7,8-dihydro-2'-deoxyguanosine (oxoG) is a common DNA adduct produced by the oxidation of DNA by reactive oxygen species. Kinetic data reveal that DNA polymerase X (pol X) from the African swine fever virus incorporates adenine (dATP) opposite to oxoG with higher efficiency than the non-damaged G:C basepair. To help interpret the kinetic data, we perform molecular dynamics simulations of pol X/DNA complexes, in which the template base opposite to the incoming dNTP (dCTP, dATP, dGTP) is oxoG. Our results suggest that pol X accommodates the oxoGsyn:A mispair by sampling closed active conformations that mirror those observed in traditional Watson-Crick complexes. Moreover, for both the oxoGsyn:A and oxoG:C ternary complexes, conformational sampling of the polymerase follows previously described large subdomain movements, local residue motions, and active site reorganization. Interestingly, the oxoGsyn:A system exhibits superior active site geometry in comparison to the oxoG:C system. Simulations for the other mismatch basepair complexes reveal large protein subdomain movement for all systems, except for oxoG:G, which samples conformations close to the open state. In addition, active site geometry and basepairing of the template base with the incoming nucleotide, reveal distortions and misalignments that range from moderate (i.e., oxoG:Asyn) to extreme (i.e., oxoGanti/syn:G). These results agree with the available kinetic data for pol X and provide structural insights regarding the mechanism by which this polymerase can accommodate incoming nucleotides opposite oxoG. Our simulations also support the notion that α-helix E is involved both in DNA binding and active site stabilization. Our proposed mechanism by which pol X can preferentially accommodate dATP opposite template oxoG further underscores the role that enzyme dynamics and conformational sampling operate in polymerase fidelity and function.

Pyrosequencing for the quantitative assessment of 8-oxodG bypass DNA synthesis

Translesion synthesis (TLS) with specialized DNA polymerases allows dealing with a base lesion on the template strand during DNA replication; a base is inserted opposite the lesion, correctly or incorrectly, depending on the lesion, the involved DNA polymerase(s) and the sequence context. The major oxidized DNA base 8-oxo-7, 8-dihydro-2'-deoxyguanosine (8-oxodG) is highly mutagenic due to its ability to pair with either cytosine or adenine during DNA synthesis, depending on its conformation and involved DNA polymerases. To measure the correct or mutagenic outcome of lesion bypass, an original quantitative pyrosequencing method was developed and analytically validated. The method was applied to the study of DNA synthesis fidelity through an 8-oxodG or an undamaged guanine. After an in vitro primer-extension through 8-oxodG in the presence of the four deoxynucleotides triphosphates and a total nuclear protein extract, obtained from normal human intestinal epithelial cells (FHs 74 Int cell line), the reaction products were amplified by polymerase chain reaction and analyzed by pyrosequencing to measure nucleotides inserted opposite the lesion. The 8-oxodG bypass fidelity of FHs 74 Int cells nuclear extract is about 85.3%. We calculated within-day and total precisions for both 8-oxodG (2.8% and 2.8%, respectively) and undamaged templates (1.0% and 1.1%, respectively). We also demonstrated that only cytosine is incorporated opposite a normal guanine and that both cytosine and adenine can be incorporated opposite an 8-oxodG lesion. The proposed method is straightforward, fast, reproducible and easily adaptable to other sequences and lesions. It thus has a wide range of applications in the biological field, notably to elucidate TLS mechanisms and modulators.

One-week intervention period led to improvements in glycemic control and reduction in DNA damage levels in patients with type 2 diabetes mellitus

AIMS

Hyperglycemia leads to increased production of reactive oxygen species (ROS), which reduces cellular antioxidant defenses and induces several DNA lesions. We investigated the effects on DNA damage of a seven-day hospitalization period in patients with type 2 diabetes mellitus (T2DM) to achieve adequate blood glucose levels through dietary intervention and medication treatment, compared with non-diabetic individuals.

METHODS

DNA damage levels were evaluated by the alkaline comet assay (with modified and without conventional use of hOGG1 enzyme, which detects oxidized DNA bases) for 10 patients and 16 controls. Real time PCR array method was performed to analyze the transcriptional expression of a set of 84 genes implicated in antioxidant defense and response to oxidative stress in blood samples from T2DM patients (n=6) collected before and after the hospitalization period.

RESULTS

The seven-day period was sufficient to improve glycemic control and to significantly decrease (p<0.05) DNA damage levels in T2DM patients, although those levels were slightly higher than those in control subjects. We also found a tendency towards a decrease in the levels of oxidative DNA damage in T2DM patients after the hospitalization period. However, for all genes analyzed, a statistically significant difference in the transcriptional expression levels was not observed.

CONCLUSIONS

The study demonstrated that although the transcriptional expression of the genes studied did not show significant alterations, one-week of glycemic control in hospital resulted in a significant reduction in DNA damage levels detected in T2DM patients, highlighting the importance of an adequate glycemic control.

Thiol peroxidase deficiency leads to increased mutational load and decreased fitness in Saccharomyces cerevisiae

Thiol peroxidases are critical enzymes in the redox control of cellular processes that function by reducing low levels of hydroperoxides and regulating redox signaling. These proteins were also shown to regulate genome stability, but how their dysfunction affects the actual mutations in the genome is not known. Saccharomyces cerevisiae has eight thiol peroxidases of glutathione peroxidase and peroxiredoxin families, and the mutant lacking all these genes (∆8) is viable. In this study, we employed two independent ∆8 isolates to analyze the genome-wide mutation spectrum that results from deficiency in these enzymes. Deletion of these genes was accompanied by a dramatic increase in point mutations, many of which clustered in close proximity and scattered throughout the genome, suggesting strong mutational bias. We further subjected multiple lines of wild-type and ∆8 cells to long-term mutation accumulation, followed by genome sequencing and phenotypic characterization. ∆8 lines showed a significant increase in nonrecurrent point mutations and indels. The original ∆8 cells exhibited reduced growth rate and decreased life span, which were further reduced in all ∆8 mutation accumulation lines. Although the mutation spectrum of the two independent isolates was different, similar patterns of gene expression were observed, suggesting the direct contribution of thiol peroxidases to the observed phenotypes. Expression of a single thiol peroxidase could partially restore the growth phenotype of ∆8 cells. This study shows how deficiency in nonessential, yet critical and conserved oxidoreductase function, leads to increased mutational load and decreased fitness.

DNA repair pathways in trypanosomatids: from DNA repair to drug resistance

All living organisms are continuously faced with endogenous or exogenous stress conditions affecting genome stability. DNA repair pathways act as a defense mechanism, which is essential to maintain DNA integrity. There is much to learn about the regulation and functions of these mechanisms, not only in human cells but also equally in divergent organisms. In trypanosomatids, DNA repair pathways protect the genome against mutations but also act as an adaptive mechanism to promote drug resistance. In this review, we scrutinize the molecular mechanisms and DNA repair pathways which are conserved in trypanosomatids. The recent advances made by the genome consortiums reveal the complete genomic sequences of several pathogens. Therefore, using bioinformatics and genomic sequences, we analyze the conservation of DNA repair proteins and their key protein motifs in trypanosomatids. We thus present a comprehensive view of DNA repair processes in trypanosomatids at the crossroads of DNA repair and drug resistance.

Genome-wide profiling of 8-oxoguanine reveals its association with spatial positioning in nucleus

8-Oxoguanine (8-oxoG) is one of the most common DNA lesions generated by reactive oxygen species. In this study, we analysed the genome-wide distribution profile of 8-oxoG by combining immunoprecipitation by antibodies specific for the DNA fragments containing 8-oxoG with a microarray that covers rat genome. Genome-wide mapping of 8-oxoG in normal rat kidney revealed that 8-oxoG is preferentially located at gene deserts. We did not observe differences in 8-oxoG levels between groups of genes with high and low expression, possibly because of the generally low 8-oxoG levels in genic regions compared with gene deserts. The distribution of 8-oxoG and lamina-associated domains (LADs) were strongly correlated, suggesting that the spatial location of genomic DNA in the nucleus determines the susceptibility to oxidative modifications. One possible explanation for high 8-oxoG levels in LADs is that the nuclear periphery is more susceptible to the oxidative damage caused by the extra-nuclear factors. Moreover, LADs have a rather compact conformation, which may limit the recruitment of repair components to the modified bases.

Kinetics, structure, and mechanism of 8-Oxo-7,8-dihydro-2'-deoxyguanosine bypass by human DNA polymerase η

DNA damage incurred by a multitude of endogenous and exogenous factors constitutes an inevitable challenge for the replication machinery. Cells rely on various mechanisms to either remove lesions or bypass them in a more or less error-prone fashion. The latter pathway involves the Y-family polymerases that catalyze trans-lesion synthesis across sites of damaged DNA. 7,8-Dihydro-8-oxo-2'-deoxyguanosine (8-oxoG) is a major lesion that is a consequence of oxidative stress and is associated with cancer, aging, hepatitis, and infertility. We have used steady-state and transient-state kinetics in conjunction with mass spectrometry to analyze in vitro bypass of 8-oxoG by human DNA polymerase η (hpol η). Unlike the high fidelity polymerases that show preferential insertion of A opposite 8-oxoG, hpol η is capable of bypassing 8-oxoG in a mostly error-free fashion, thus preventing GC→AT transversion mutations. Crystal structures of ternary hpol η-DNA complexes and incoming dCTP, dATP, or dGTP opposite 8-oxoG reveal that an arginine from the finger domain assumes a key role in avoiding formation of the nascent 8-oxoG:A pair. That hpol η discriminates against dATP exclusively at the insertion stage is confirmed by structures of ternary complexes that allow visualization of the extension step. These structures with G:dCTP following either 8-oxoG:C or 8-oxoG:A pairs exhibit virtually identical active site conformations. Our combined data provide a detailed understanding of hpol η bypass of the most common oxidative DNA lesion.

NADPH oxidases: a perspective on reactive oxygen species production in tumor biology

SIGNIFICANCE

Reactive oxygen species (ROS) promote genomic instability, altered signal transduction, and an environment that can sustain tumor formation and growth. The NOX family of NADPH oxidases, membrane-bound epithelial superoxide and hydrogen peroxide producers, plays a critical role in the maintenance of immune function, cell growth, and apoptosis. The impact of NOX enzymes in carcinogenesis is currently being defined and may directly link chronic inflammation and NOX ROS-mediated tumor formation.

RECENT ADVANCES

Increased interest in the function of NOX enzymes in tumor biology has spurred a surge of investigative effort to understand the variability of NOX expression levels in tumors and the effect of NOX activity on tumor cell proliferation. These initial efforts have demonstrated a wide variance in NOX distribution and expression levels across numerous cancers as well as in common tumor cell lines, suggesting that much remains to be discovered about the unique role of NOX-related ROS production within each system. Progression from in vitro cell line studies toward in vivo tumor tissue screening and xenograft models has begun to provide evidence supporting the importance of NOX expression in carcinogenesis.

CRITICAL ISSUES

A lack of universally available, isoform-specific antibodies and animal tumor models of inducible knockout or over-expression of NOX isoforms has hindered progress toward the completion of in vivo studies.

FUTURE DIRECTIONS

In vivo validation experiments and the use of large, existing gene expression data sets should help define the best model systems for studying the NOX homologues in the context of cancer.

Molecular dynamics study on conformational differences between dGMP and 8-oxo-dGMP: Effects of metal ions

The modified nucleotide base 7,8-dihydro-8-oxo-guanine (8-oxo-G) is one of the major sources of spontaneous mutagenesis. Nucleotide-sanitizing enzymes, such as the MutT homolog-1 (MTH1) and nudix-type motif 5 (NUDT5), selectively remove 8-oxo-G from the cellular pool of nucleotides. Previous studies showed that, although the syn conformation generally predominates in purine nucleotides with a bulky substituent at the 8-position, 8-oxo-dGMP binds to both MTH1 and NUDT5 in the anti conformation. This study was initiated to investigate the possibility that 8-oxo-dGMP itself may adopt the anti conformation. Molecular dynamics simulations of mononucleotides (dGMP, 8-oxo-dGMP) in aqueous solution were performed. 8-oxo-dGMP adopted the anti conformation as well as the syn conformation, and the proportion of adopting the anti conformation increased in the presence of metal ions. When 8-oxo-dGMP was in the anti conformation, a metal ion was located between the oxygen atom of phosphate and the oxygen atom at the 8-position of 8-oxo-G. The types of stable anti conformations of 8-oxo-dGMP differed, depending on the ionic radii and charges of coexisting ions. These data suggested a role for metal ions, other than as cofactors for the hydrolysis of the di- and tri-phosphate forms of mononucleotides; that the metal ions help retain the anti conformation of the N-glycosidic torsion angle of 8-oxo-dGMP to promote the binding between the 8-oxo-G deoxynucleotide and the nucleotide-sanitizing enzymes.

Helicobacter pylori and the molecular pathogenesis of intestinal-type gastric carcinoma

Gastric carcinoma is an inflammation-related cancer caused by long-term infection with the human bacterial pathogen, Helicobacter pylori. The pattern of acute-on-chronic inflammation causes progressive mucosal damage which may result in atrophy with metaplastic epithelia and eventually gastric cancer. Recently, it has been recognized that H. pylori can also cause genetic instability such as double-stranded DNA breaks and can produce gene activation and silencing via epigenetic pathways. As genetic instability is the hallmark of cancer, we highlight recent progress in understanding the gastric carcinogenesis in relation to H. pylori-related inflammation, H. pylori-induced double-stranded DNA breakage and aberrant gene expression as well as the mechanisms and role of H. pylori-associated epigenetic change in gene expression.

Base excision repair: a critical player in many games

This perspective reviews the many dimensions of base excision repair from a 10,000 foot vantage point and provides one person's view on where the field is headed. Enzyme function is considered under the lens of X-ray diffraction and single molecule studies. Base excision repair in chromatin and telomeres, regulation of expression and the role of posttranslational modifications are also discussed in the context of enzyme activities, cellular localization and interacting partners. The specialized roles that base excision repair play in transcriptional activation by active demethylation and targeted oxidation as well as how base excision repair functions in the immune processes of somatic hypermutation and class switch recombination and its possible involvement in retroviral infection are also discussed. Finally the complexities of oxidative damage and its repair and its link to neurodegenerative disorders, as well as the role of base excision repair as a tumor suppressor are examined in the context of damage, repair and aging. By outlining the many base excision repair-related mysteries that have yet to be unraveled, hopefully this perspective will stimulate further interest in the field.

An overview of Y-Family DNA polymerases and a case study of human DNA polymerase η

Y-Family DNA polymerases specialize in translesion synthesis, bypassing damaged bases that would otherwise block the normal progression of replication forks. Y-Family polymerases have unique structural features that allow them to bind damaged DNA and use a modified template base to direct nucleotide incorporation. Each Y-Family polymerase is unique and has different preferences for lesions to bypass and for dNTPs to incorporate. Y-Family polymerases are also characterized by a low catalytic efficiency, a low processivity, and a low fidelity on normal DNA. Recruitment of these specialized polymerases to replication forks is therefore regulated. The catalytic center of the Y-Family polymerases is highly conserved and homologous to that of high-fidelity and high-processivity DNA replicases. In this review, structural differences between Y-Family and A- and B-Family polymerases are compared and correlated with their functional differences. A time-resolved X-ray crystallographic study of the DNA synthesis reaction catalyzed by the Y-Family DNA polymerase human polymerase η revealed transient elements that led to the nucleotidyl-transfer reaction.

DNA repair pathways and their therapeutic potential in lung cancer

SUMMARY: 

Lung cancer is the leading cause of cancer-related mortality. According to WHO, 1.37 million deaths occur globally each year as a result of this disease. More than 70% of these cases are associated with prior tobacco consumption and/or cigarette smoking, suggesting a direct causal relationship. The development and progression of lung cancer and other malignancies involves the loss of genetic stability, resulting in acquisition of cumulative genetic changes; this affords the cell increased malignant potential. As such, an understanding of the mechanisms through which these events may occur will potentially allow for development of new anticancer therapies. This review will address the association between lung cancer and genetic instability, with a central focus on genetic mutations in the DNA damage repair pathways. In addition, we will discuss the potential clinical exploitation of these pathways, both in terms of biomarker staging, as well as through direct therapeutic targeting.

Oxidatively modified proteins in the serous subtype of ovarian carcinoma

Serous subtype of ovarian cancer is considered to originate from fallopian epithelium mucosa that has been exposed to physiological changes resulting from ovulation. Ovulation influences an increased in inflammation of epithelial ovarian cells as results of constant exposure of cells to ROS. The imbalance between ROS and antioxidant capacities, as well as a disruption of redox signaling, causes a wide range of damage to DNA, proteins, and lipids. This study applied spectrophotometric, dinitrophenylhydrazone (DNPH) assay, two-dimensional gel electrophoresis, and Western blot analyses to assess the levels of oxidatively modified proteins in 100 primary serous epithelial ovarian carcinoma and normal/surrounding tissues. These samples were obtained from 56 Caucasian and 44 African-American patients within the age range of 61 ± 10 years. Analyses showed that the levels of reactive protein carbonyl groups increased as stages progressed to malignancy. Additionally, the levels of protein carbonyls in serous ovarian carcinoma among African Americans are 40% (P < 0.05) higher relative to Caucasian at similar advanced stages. Results suggest that oxidative stress is involved in the modification of carbonyl protein groups, leading to increased aggressiveness of epithelial ovarian tumors and may contribute to the disease's invasiveness among African Americans.

Molecular dissection of the domain architecture and catalytic activities of human PrimPol

PrimPol is a primase–polymerase involved in nuclear and mitochondrial DNA replication in eukaryotic cells. Although PrimPol is predicted to possess an archaeo-eukaryotic primase and a UL52-like zinc finger domain, the role of these domains has not been established. Here, we report that the proposed zinc finger domain of human PrimPol binds zinc ions and is essential for maintaining primase activity. Although apparently dispensable for its polymerase activity, the zinc finger also regulates the processivity and fidelity of PrimPol's extension activities. When the zinc finger is disrupted, PrimPol becomes more promutagenic, has an altered translesion synthesis spectrum and is capable of faithfully bypassing cyclobutane pyrimidine dimer photolesions. PrimPol's polymerase domain binds to both single- and double-stranded DNA, whilst the zinc finger domain binds only to single-stranded DNA. We additionally report that although PrimPol's primase activity is required to restore wild-type replication fork rates in irradiated PrimPol^−/− cells, polymerase activity is sufficient to maintain regular replisome progression in unperturbed cells. Together, these findings provide the first analysis of the molecular architecture of PrimPol, describing the activities associated with, and interplay between, its functional domains and defining the requirement for its primase and polymerase activities during nuclear DNA replication.

Effects on DNA Damage and/or Repair Processes as Biological Mechanisms Linking Psychological Stress to Cancer Risk

Considerable research effort in the past several decades has focused on the impact of psychological stress, and stress hormones, on cancer progression. Numerous studies have reported that stress hormone treatment or in vivo stress exposure can enhance the growth of tumor cell lines in vitro, as well as tumors in animal models, and have begun to explore molecular mechanisms. Comparatively little research has focused on the impact of psychological stress and stress hormones on cancer initiation, in part due to inherent methodological challenges, but also because potential underlying biological mechanisms have remained obscure. In this review, we present a testable theoretical model of pathways by which stress may result in cellular transformation and tumorigenesis. This model supports our overarching hypothesis that psychological stress, acting through increased levels of catecholamines and/or cortisol, can increase DNA damage and/or reduce repair mechanisms, resulting in increased risk of DNA mutations leading to carcinogenesis. A better understanding of molecular pathways by which psychological stress can increase the risk of cancer initiation would open new avenues of translational research, bringing together psychologists, neuroscientists, and molecular biologists, potentially resulting in the development of novel approaches for cancer risk reduction at the population level.

Genetic instability in lymphoblastoid cell lines expressing biallelic and monoallelic variants in the human MUTYH gene

The MUTYH DNA glycosylase counteracts mutagenesis by removing adenine misincorporated opposite DNA 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG). Biallelic germline mutations in MUTYH cause the autosomal recessive MUTYH-associated polyposis (MAP). The impact on genetic instability of the p.Tyr179Cys and p.Arg245His MUTYH variants was evaluated in lymphoblastoid cell lines (LCLs) derived from MAP patients and their relatives in comparison to wild-type LCLs. No difference in MUTYH expression was identified between wild type and LCLs with the p.Tyr179Cys, while the p.Arg245His mutation was associated with an unstable MUTYH protein. LCLs homozygous for the p.Tyr179Cys or the p.Arg245His variant contained increased DNA 8-oxodG levels and exhibited a mutator phenotype at the PIG-A gene. The extent of the increased spontaneous mutation frequency was 3-fold (range 1.6- to 4.6-fold) in four independent LCLs carrying the p.Tyr179Cys variant, while a larger increase (6-fold) was observed in two p.Arg245His LCLs. A similar hypermutability and S-phase delay following treatment with KBrO3 was observed in LCLs homozygous for either variant. When genetic instability was investigated in monoallelic p.Arg245His carriers, mutant frequencies showed an increase which is intermediate between wild-type and homozygous cells, whereas the mutator effect in heterozygous p.Tyr179Cys LCLs was similar to that in homozygotes. These findings indicate that the type of MUTYH mutation can affect the extent of genome instability associated with MUTYH inactivation. In addition, the mild spontaneous mutator phenotype observed in monoallelic carriers highlights the biological importance of this gene in the protection of the genome against endogenous DNA damage.

Plant mitochondrial genome evolution can be explained by DNA repair mechanisms

Plant mitochondrial genomes are notorious for their large and variable size, nonconserved open reading frames of unknown function, and high rates of rearrangement. Paradoxically, the mutation rates are very low. However, mutation rates can only be measured in sequences that can be aligned—a very small part of plant mitochondrial genomes. Comparison of the complete mitochondrial genome sequences of two ecotypes of Arabidopsis thaliana allows the alignment of noncoding as well as coding DNA and estimation of the mutation rates in both. A recent chimeric duplication is also analyzed. A hypothesis is proposed that the mechanisms of plant mitochondrial DNA repair account for these features and includes different mechanisms in transcribed and nontranscribed regions. Within genes, a bias toward gene conversion would keep measured mutation rates low, whereas in noncoding regions, break-induced replication (BIR) explains the expansion and rearrangements. Both processes are types of double-strand break repair, but enhanced second-strand capture in transcribed regions versus BIR in nontranscribed regions can explain the two seemingly contradictory features of plant mitochondrial genome evolution—the low mutation rates in genes and the striking expansions of noncoding sequences.

Chemistry meets biology in colitis-associated carcinogenesis

The intestine comprises an exceptional venue for a dynamic and complex interplay of numerous chemical and biological processes. Here, multiple chemical and biological systems, including the intestinal tissue itself, its associated immune system, the gut microbiota, xenobiotics, and metabolites meet and interact to form a sophisticated and tightly regulated state of tissue homoeostasis. Disturbance of this homeostasis can cause inflammatory bowel disease (IBD)-a chronic disease of multifactorial etiology that is strongly associated with increased risk for cancer development. This review addresses recent developments in research into chemical and biological mechanisms underlying the etiology of inflammation-induced colon cancer. Beginning with a general overview of reactive chemical species generated during colonic inflammation, the mechanistic interplay between chemical and biological mediators of inflammation, the role of genetic toxicology, and microbial pathogenesis in disease development are discussed. When possible, we systematically compare evidence from studies utilizing human IBD patients with experimental investigations in mice. The comparison reveals that many strong pathological and mechanistic correlates exist between mouse models of colitis-associated cancer, and the clinically relevant situation in humans. We also summarize several emerging issues in the field, such as the carcinogenic potential of novel inflammation-related DNA adducts and genotoxic microbial factors, the systemic dimension of inflammation-induced genotoxicity, and the complex role of genome maintenance mechanisms during these processes. Taken together, current evidence points to the induction of genetic and epigenetic alterations by chemical and biological inflammatory stimuli ultimately leading to cancer formation.

Autophagy and genomic integrity

DNA lesions, constantly produced by endogenous and exogenous sources, activate the DNA damage response (DDR), which involves detection, signaling and repair of the damage. Autophagy, a lysosome-dependent degradation pathway that is activated by stressful situations such as starvation and oxidative stress, regulates cell fate after DNA damage and also has a pivotal role in the maintenance of nuclear and mitochondrial genomic integrity. Here, we review important evidence regarding the role played by autophagy in preventing genomic instability and tumorigenesis, as well as in micronuclei degradation. Several pathways governing autophagy activation after DNA injury and the influence of autophagy upon the processing of genomic lesions are also discussed herein. In this line, the mechanisms by which several proteins participate in both DDR and autophagy, and the importance of this crosstalk in cancer and neurodegeneration will be presented in an integrated fashion. At last, we present a hypothetical model of the role played by autophagy in dictating cell fate after genotoxic stress.

In vivo bypass of 8-oxodG

8-oxoG is one of the most common and mutagenic DNA base lesions caused by oxidative damage. However, it has not been possible to study the replication of a known 8-oxoG base in vivo in order to determine the accuracy of its replication, the influence of various components on that accuracy, and the extent to which an 8-oxoG might present a barrier to replication. We have been able to place a single 8-oxoG into the Saccharomyces cerevisiae chromosome in a defined location using single-strand oligonucleotide transformation and to study its replication in a fully normal chromosome context. During replication, 8-oxoG is recognized as a lesion and triggers a switch to translesion synthesis by Pol η, which replicates 8-oxoG with an accuracy (insertion of a C opposite the 8-oxoG) of approximately 94%. In the absence of Pol η, template switching to the newly synthesized sister chromatid is observed at least one third of the time; replication of the 8-oxoG in the absence of Pol η is less than 40% accurate. The mismatch repair (MMR) system plays an important role in 8-oxoG replication. Template switching is blocked by MMR and replication accuracy even in the absence of Pol η is approximately 95% when MMR is active. These findings indicate that in light of the overlapping mechanisms by which errors in 8-oxoG replication can be avoided in the cell, the mutagenic threat of 8-oxoG is due more to its abundance than the effect of a single lesion. In addition, the methods used here should be applicable to the study of any lesion that can be stably incorporated into synthetic oligonucleotides.

Oxidative damage and mutagenesis in Saccharomyces cerevisiae: genetic studies of pathways affecting replication fidelity of 8-oxoguanine

Oxidative damage to DNA constitutes a major threat to the faithful replication of DNA in all organisms and it is therefore important to understand the various mechanisms that are responsible for repair of such damage and the consequences of unrepaired damage. In these experiments, we make use of a reporter system in Saccharomyces cerevisiae that can measure the specific increase of each type of base pair mutation by measuring reversion to a Trp+ phenotype. We demonstrate that increased oxidative damage due to the absence of the superoxide dismutase gene, SOD1, increases all types of base pair mutations and that mismatch repair (MMR) reduces some, but not all, types of mutations. By analyzing various strains that can revert only via a specific CG → AT transversion in backgrounds deficient in Ogg1 (encoding an 8-oxoG glycosylase), we can study mutagenesis due to a known 8-oxoG base. We show as expected that MMR helps prevent mutagenesis due to this damaged base and that Pol η is important for its accurate replication. In addition we find that its accurate replication is facilitated by template switching, as loss of either RAD5 or MMS2 leads to a significant decrease in accurate replication. We observe that these ogg1 strains accumulate revertants during prolonged incubation on plates, in a process most likely due to retromutagenesis.

Base excision repair facilitates a functional relationship between Guanine oxidation and histone demethylation

SIGNIFICANCE

Appropriately controlled epigenetic regulation is critical for the normal development and health of an organism. Misregulation of epigenetic control via deoxyribonucleic acid (DNA) methylation or histone methylation has been associated with cancer and chromosomal instability syndromes.

RECENT ADVANCES

The main function of the proteins in the base excision repair (BER) pathway is to repair DNA single-strand breaks and deamination, oxidation, and alkylation-induced DNA base damage that may result from chemotherapy, environmental exposure, or byproducts of cellular metabolism. Recent studies have suggested that one or more BER proteins may also participate in epigenetic regulation to facilitate gene expression modulation via alteration of the state of DNA methylation or via a reaction coupled to histone modification. BER proteins have also been reported to play an essential role in pluripotent stem cell reprogramming.

CRITICAL ISSUES

One emerging function for BER in epigenetic regulation is the repair of base lesions induced by hydrogen peroxide as a byproduct of lysine-specific demethylase 1 (LSD1) enzymatic activity (LSD1/LSD2-coupled BER) for transcriptional regulation.

FUTURE DIRECTIONS

To shed light on this novel role of BER, this review focuses on the repair of oxidative lesions in nuclear DNA that are induced during LSD1-mediated histone demethylation. Further, we highlight current studies suggesting a role for BER proteins in transcriptional regulation of gene expression via BER-coupled active DNA demethylation in mammalian cells. Such efforts to address the role of BER proteins in epigenetic regulation could broaden cancer therapeutic strategies to include epigenetic modifiers combined with BER inhibitors.

Oxidative stress and DNA lesions: the role of 8-oxoguanine lesions in Trypanosoma cruzi cell viability

The main consequence of oxidative stress is the formation of DNA lesions, which can result in genomic instability and lead to cell death. Guanine is the base that is most susceptible to oxidation, due to its low redox potential, and 8-oxoguanine (8-oxoG) is the most common lesion. These characteristics make 8-oxoG a good cellular biomarker to indicate the extent of oxidative stress. If not repaired, 8-oxoG can pair with adenine and cause a G:C to T:A transversion. When 8-oxoG is inserted during DNA replication, it could generate double-strand breaks, which makes this lesion particularly deleterious. Trypanosoma cruzi needs to address various oxidative stress situations, such as the mammalian intracellular environment and the triatomine insect gut where it replicates. We focused on the MutT enzyme, which is responsible for removing 8-oxoG from the nucleotide pool. To investigate the importance of 8-oxoG during parasite infection of mammalian cells, we characterized the MutT gene in T. cruzi (TcMTH) and generated T. cruzi parasites heterologously expressing Escherichia coli MutT or overexpressing the TcMTH enzyme. In the epimastigote form, the recombinant and wild-type parasites displayed similar growth in normal conditions, but the MutT-expressing cells were more resistant to hydrogen peroxide treatment. The recombinant parasite also displayed significantly increased growth after 48 hours of infection in fibroblasts and macrophages when compared to wild-type cells, as well as increased parasitemia in Swiss mice. In addition, we demonstrated, using western blotting experiments, that MutT heterologous expression can influence the parasite antioxidant enzyme protein levels. These results indicate the importance of the 8-oxoG repair system for cell viability.

Role of MUTYH in human cancer

MUTYH, a human ortholog of MutY, is a post-replicative DNA glycosylase, highly conserved throughout evolution, involved in the correction of mismatches resulting from a faulty replication of the oxidized base 8-hydroxyguanine (8-oxodG). In particular removal of adenine from A:8-oxodG mispairs by MUTYH activity is followed by error-free base excision repair (BER) events, leading to the formation of C:8-oxodG base pairs. These are the substrate of another BER enzyme, the OGG1 DNA glycosylase, which then removes 8-oxodG from DNA. Thus the combined action of OGG1 and MUTYH prevents oxidative damage-induced mutations, i.e. GC->TA transversions. Germline mutations in MUTYH are associated with a recessively heritable colorectal polyposis, now referred to as MUTYH-associated polyposis (MAP). Here we will review the phenotype(s) associated with MUTYH inactivation from bacteria to mammals, the structure of the MUTYH protein, the molecular mechanisms of its enzymatic activity and the functional characterization of MUTYH variants. The relevance of these results will be discussed to define the role of specific human mutations in colorectal cancer risk together with the possible role of MUTYH inactivation in sporadic cancer.

MUTYH DNA glycosylase: the rationale for removing undamaged bases from the DNA

Maintenance of genetic stability is crucial for all organisms in order to avoid the onset of deleterious diseases such as cancer. One of the many proveniences of DNA base damage in mammalian cells is oxidative stress, arising from a variety of endogenous and exogenous sources, generating highly mutagenic oxidative DNA lesions. One of the best characterized oxidative DNA lesion is 7,8-dihydro-8-oxoguanine (8-oxo-G), which can give rise to base substitution mutations (also known as point mutations). This mutagenicity is due to the miscoding potential of 8-oxo-G that instructs most DNA polymerases (pols) to preferentially insert an Adenine (A) opposite 8-oxo-G instead of the appropriate Cytosine (C). If left unrepaired, such A:8-oxo-G mispairs can give rise to CG→AT transversion mutations. A:8-oxo-G mispairs are proficiently recognized by the MutY glycosylase homologue (MUTYH). MUTYH can remove the mispaired A from an A:8-oxo-G, giving way to the canonical base-excision repair (BER) that ultimately restores undamaged Guanine (G). The importance of this MUTYH-initiated pathway is illustrated by the fact that biallelic mutations in the MUTYH gene are associated with a hereditary colorectal cancer syndrome termed MUTYH-associated polyposis (MAP). In this review, we will focus on MUTYH, from its discovery to the most recent data regarding its cellular roles and interaction partners. We discuss the involvement of the MUTYH protein in the A:8-oxo-G BER pathway acting together with pol λ, the pol that can faithfully incorporate C opposite 8-oxo-G and thus bypass this lesion in a correct manner. We also outline the current knowledge about the regulation of MUTYH itself and the A:8-oxo-G repair pathway by posttranslational modifications (PTM). Finally, to achieve a clearer overview of the literature, we will briefly touch on the rather confusing MUTYH nomenclature. In short, MUTYH is a unique DNA glycosylase that catalyzes the excision of an undamaged base from DNA.

Annexin A2: the importance of being redox sensitive

Hydrogen peroxide (H₂O₂) is an important second messenger in cellular signal transduction. H₂O₂-dependent signalling regulates many cellular processes, such as proliferation, differentiation, migration and apoptosis. Nevertheless, H₂O₂ is an oxidant and a major contributor to DNA damage, protein oxidation and lipid peroxidation, which can ultimately result in cell death and/or tumourigenesis. For this reason, cells have developed complex antioxidant systems to scavenge ROS. Recently, our laboratory identified the protein, annexin A2, as a novel cellular redox regulatory protein. Annexin A2 possesses a reactive cysteine residue (Cys-8) that is readily oxidized by H₂O₂ and subsequently reduced by the thioredoxin system, thereby enabling annexin A2 to participate in multiple redox cycles. Thus, a single molecule of annexin A2 can inactivate several molecules of H₂O₂. In this report, we will review the studies detailing the reactivity of annexin A2 thiols and the importance of these reactive cysteine(s) in regulating annexin A2 structure and function. We will also focus on the recent reports that establish novel functions for annexin A2, namely as a protein reductase and as a cellular redox regulatory protein. We will further discuss the importance of annexin A2 redox regulatory function in disease, with a particular focus on tumour progression.

Plant mitochondrial genome evolution can be explained by DNA repair mechanisms

Plant mitochondrial genomes are notorious for their large and variable size, nonconserved open reading frames of unknown function, and high rates of rearrangement. Paradoxically, the mutation rates are very low. However, mutation rates can only be measured in sequences that can be aligned--a very small part of plant mitochondrial genomes. Comparison of the complete mitochondrial genome sequences of two ecotypes of Arabidopsis thaliana allows the alignment of noncoding as well as coding DNA and estimation of the mutation rates in both. A recent chimeric duplication is also analyzed. A hypothesis is proposed that the mechanisms of plant mitochondrial DNA repair account for these features and includes different mechanisms in transcribed and nontranscribed regions. Within genes, a bias toward gene conversion would keep measured mutation rates low, whereas in noncoding regions, break-induced replication (BIR) explains the expansion and rearrangements. Both processes are types of double-strand break repair, but enhanced second-strand capture in transcribed regions versus BIR in nontranscribed regions can explain the two seemingly contradictory features of plant mitochondrial genome evolution--the low mutation rates in genes and the striking expansions of noncoding sequences.

DNA repair dysregulation from cancer driver to therapeutic target

Dysregulation of DNA damage repair and signalling to cell cycle checkpoints, known as the DNA damage response (DDR), is associated with a predisposition to cancer and affects responses to DNA-damaging anticancer therapy. Dysfunction of one DNA repair pathway may be compensated for by the function of another compensatory DDR pathway, which may be increased and contribute to resistance to DNA-damaging chemotherapy and radiotherapy. Therefore, DDR pathways make an ideal target for therapeutic intervention; first, to prevent or reverse therapy resistance; and second, using a synthetic lethal approach to specifically kill cancer cells that are dependent on a compensatory DNA repair pathway for survival in the context of cancer-associated oxidative and replicative stress. These hypotheses are currently being tested in the laboratory and are being translated into clinical studies.

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

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

Genotoxic agents promote the nuclear accumulation of annexin A2: role of annexin A2 in mitigating DNA damage

Annexin A2 is an abundant cellular protein that is mainly localized in the cytoplasm and plasma membrane, however a small population has been found in the nucleus, suggesting a nuclear function for the protein. Annexin A2 possesses a nuclear export sequence (NES) and inhibition of the NES is sufficient to cause nuclear accumulation. Here we show that annexin A2 accumulates in the nucleus in response to genotoxic agents including gamma-radiation, UV radiation, etoposide and chromium VI and that this event is mediated by the nuclear export sequence of annexin A2. Nuclear accumulation of annexin A2 is blocked by the antioxidant agent N-acetyl cysteine (NAC) and stimulated by hydrogen peroxide (H₂O₂), suggesting that this is a reactive oxygen species dependent event. In response to genotoxic agents, cells depleted of annexin A2 show enhanced phospho-histone H2AX and p53 levels, increased numbers of p53-binding protein 1 nuclear foci and increased levels of nuclear 8-oxo-2′-deoxyguanine, suggesting that annexin A2 plays a role in protecting DNA from damage. This is the first report showing the nuclear translocation of annexin A2 in response to genotoxic agents and its role in mitigating DNA damage.

A switch between DNA polymerases δ and λ promotes error-free bypass of 8-oxo-G lesions

7,8-Dihydro-8-oxoguanine (8-oxo-G) is a highly abundant and mutagenic lesion. Replicative DNA polymerases (pols) are slowed down at 8-oxo-G and insert both correct cytosine (C) and incorrect adenine (A) opposite 8-oxo-G, but they preferentially extend A:8-oxo-G mispairs. Nevertheless, 8-oxo-G bypass is fairly accurate in vivo. Thus, the question how correct bypass of 8-oxo-G lesions is accomplished despite the poor extension of C:8-oxo-G base pairs by replicative pols remains unanswered. Here we show that replicative pol δ pauses in front of 8-oxo-G and displays difficulties extending from correct C:8-oxo-G in contrast to extension from incorrect A:8-oxo-G. This leads to stalling of pol δ at 8-oxo-G after incorporation of correct C. This stalling at C:8-oxo-G can be overcome by a switch from pol δ to pols λ, β, or η, all of which are able to assist pol δ in 8-oxo-G bypass by translesion synthesis (TLS). Importantly, however, only pol λ selectively catalyzes the correct TLS past 8-oxo-G, whereas pols β and η show no selectivity and even preferentially enhance incorrect TLS. The selectivity of pol λ to promote the correct bypass depends on its N-terminal domain. Furthermore, pol λ(-/-) mouse embryonic fibroblast extracts display reduced 8-oxo-G TLS. Finally, the correct bypass of 8-oxo-G in gapped plasmids in mouse embryonic fibroblasts and HeLa cells is promoted in the presence of pol λ. Our findings suggest that even though 8-oxo-G is not a blocking lesion per se, correct replication over 8-oxo-G is promoted by a pol switch between pols δ and λ.

Loss of MUTYH function in human cells leads to accumulation of oxidative damage and genetic instability

The DNA glycosylase MUTYH (mutY homolog (Escherichia coli)) counteracts the mutagenic effects of 8-oxo-7,8-dihydroguanine (8-oxodG) by removing adenine (A) misincorporated opposite the oxidized purine. Biallelic germline mutations in MUTYH cause the autosomal recessive MUTYH-associated adenomatous polyposis (MAP). Here we designed new tools to investigate the biochemical defects and biological consequences associated with different MUTYH mutations in human cells. To identify phenotype(s) associated with MUTYH mutations, lymphoblastoid cell lines (LCLs) were derived from seven MAP patients harboring missense as well as truncating mutations in MUTYH. These included homozygous p.Arg245His, p.Gly264TrpfsX7 or compound heterozygous variants (p.Gly396Asp/Arg245Cys, p.Gly396Asp/Tyr179Cys, p.Gly396Asp/Glu410GlyfsX43, p.Gly264TrpfsX7/Ala385ProfsX23 and p.Gly264TrpfsX7/Glu480del). DNA glycosylase assays of MAP LCL extracts confirmed that all these variants were defective in removing A from an 8-oxoG:A DNA substrate, but retained wild-type OGG1 activity. As a consequence of this defect, MAP LCLs accumulated DNA 8-oxodG in their genome and exhibited a fourfold increase in spontaneous mutagenesis at the PIG-A gene compared with LCLs from healthy donors. They were also hypermutable by KBrO3--a source of DNA 8-oxodG--indicating that the relatively modest spontaneous mutator phenotype associated with MUTYH loss can be significantly enhanced by conditions of oxidative stress. These observations identify accumulation of DNA 8-oxodG and a mutator phenotype as likely contributors to the pathogenesis of MUTYH variants.

Recent advances in the structural mechanisms of DNA glycosylases

DNA glycosylases safeguard the genome by locating and excising a diverse array of aberrant nucleobases created from oxidation, alkylation, and deamination of DNA. Since the discovery 28years ago that these enzymes employ a base flipping mechanism to trap their substrates, six different protein architectures have been identified to perform the same basic task. Work over the past several years has unraveled details for how the various DNA glycosylases survey DNA, detect damage within the duplex, select for the correct modification, and catalyze base excision. Here, we provide a broad overview of these latest advances in glycosylase mechanisms gleaned from structural enzymology, highlighting features common to all glycosylases as well as key differences that define their particular substrate specificities.

Chaperones, but not oxidized proteins, are ubiquitinated after oxidative stress

After oxidative stress, proteins that are oxidatively modified are degraded by the 20S proteasome. However, several studies have documented an enhanced ubiquitination of yet unknown proteins. Because ubiquitination is a prerequisite for degradation by the 26S proteasome in an ATP-dependent manner this raises the question whether these proteins are also oxidized and, if not, what proteins need to be ubiquitinated and degraded after oxidative conditions. By determination of oxidized and ubiquitinated proteins we demonstrate here that most oxidized proteins are not preferentially ubiquitinated. However, we were able to confirm an increase in ubiquitinated proteins 16 h after oxidative stress. Therefore, we isolated ubiquitinated proteins from hydrogen peroxide-treated cells, as well as from control cells and cells treated with lactacystin, an irreversible proteasome inhibitor, and identified some of these proteins by MALDI tandem mass spectrometry. As a result we obtained 24 different proteins that can be categorized into the following groups: chaperones, energy metabolism, cytoskeleton/intermediate filaments, and protein translation/ribosome biogenesis. The special set of identified, ubiquitinated proteins confirms the thesis that ubiquitination upon oxidative stress is not a random process to degrade the mass of oxidized proteins, but concerns a special group of functional proteins.

C8-linked bulky guanosine DNA adducts: experimental and computational insights into adduct conformational preferences and resulting mutagenicity

Bulky DNA adducts are formed through the covalent attachment of aryl groups to the DNA nucleobases. Many of these adducts are known to possess conformational heterogeneity, which is responsible for the variety of mutagenic outcomes associated with these lesions. The present contribution reviews several conformational and mutagenic themes that are prevalent among the DNA adducts formed at the C8-site of the guanine nucleobase. The most important conclusions obtained (to date) from experiments are summarized including the anti/syn conformational preference of the adducts, their potential to inflict DNA mutations and mismatch stabilization, and their interactions with DNA polymerases and repair enzymes. Additionally, the unique role that computer calculations can play in understanding the structural properties of these adducts are highlighted.

Mechanistic and conformational flexibility of the covalent linkage formed during β-lyase activity on an AP-site: application to hOgg1

The β/δ-lyase activity of bifunctional glycosylases on damaged nucleotides in DNA involves the formation of a covalent linkage between the protein (lysine or N-terminal proline) and DNA (C1' of the damaged nucleotide). In the present study, the conformational and mechanistic flexibility of the cross-link is examined. Repair of 8-oxoguanine damage by hOgg1 is considered as a representative system, and the glycosylase through β-lyase steps are investigated using density functional theory. (PCM/SMD)-M06-2X/6-311+G(2df,2p)//PCM-B3LYP/6-31G(d) energetics were determined for eight unique mechanisms differing in the conformation of the imine linkage (E/Z), the proton (pro-S/R) abstracted during elimination, and whether the ring-opening step is base catalyzed. This initial study used a model system limited to the damaged nucleoside 3'-monophosphate and a model nucleophile to investigate this series of complex reaction steps. The great flexibility exhibited by the linkage and clustered β-elimination energetics indicate sterics will play a large role in predicting the preferred lyase mechanism for a given enzyme. The stationary points identified herein can be overlaid into a protein structure to assist in generating initial guesses for large model systems. By comparing the characterized geometries and enzyme active sites, methods for catalysis of the various chemical steps can be identified, and these possibilities are discussed in detail for hOgg1. Interestingly, the most stable structure on the potential energy surface occurs before elimination of the 3'-phosphate. Hydrolysis of the protein-DNA cross-link at this point would yield an AP-site, which provides support for the recently observed monofunctional activity of hOgg1.

Liver tumor promoting effect of etofenprox in rats and its possible mechanism of action

To investigate the liver tumor-promoting effects of etofenprox (ETF), a pyrethroid-like insecticide, 6 week-old male F344 rats were given an intraperitoneal injection of N-diethylnitrosamine (DEN). After 2 weeks from the DEN treatment, 12 rats per group received a powdered diet containing 0, 0.25, 0.50, or 1.0% ETF for 8 weeks. At the time of 2nd week of ETF administration, all animals were subjected to two-thirds partial hepatectomy (PH). One rat per group except for the 0.25% ETF group died due to surgical operation of PH. The number and area of glutathione S-transferase placental form (GST-P) positive foci significantly increased in the livers of DEN-initiated rats given 0.50% and 1.0% ETF compared with the DEN-alone group. Quantitative real-time RT-PCR analysis revealed that the mRNA expression of phase I enzymes Cyp2b1/2, phase II enzymes such as Akr7a3, Gsta5, Ugt1a6, Nqo1 significantly increased in the DEN+ETF groups. The immunohistochemistry showed the translocation of CAR from the cytoplasm to the nuclei of hepatocytes in the ETF-treated groups. Reactive oxygen species (ROS) production increased in microsomes isolated from the livers of ETF-treated rats, and thiobarbituric acid-reactive substances (TBARS) levels and 8- hydroxy-2-deoxyguanosine (8-OHdG) content significantly increased in all of the ETF-treated groups and DEN+1.0% ETF group, respectively. The results of the present study indicate that ETF has a liver tumor-promoting activity in rats, and suggest that ETF activates the constitutive active/androstane receptor (CAR) and enhances microsomal ROS production, resulting in the upregulation of Nrf2 gene batteries; such an oxidative stress subsequently induces liver tumor-promoting effects by increased cellular proliferation.

MUTYH-associated polyposis (MAP), the syndrome implicating base excision repair in inherited predisposition to colorectal tumors

In 2002, Al-Tassan and co-workers described for the first time a recessive form of inherited polyposis associated with germline mutations of MUTYH, a gene encoding a base excision repair (BER) protein that counteracts the DNA damage induced by the oxidative stress. MUTYH-associated polyposis (MAP) is now a well-defined cancer susceptibility syndrome, showing peculiar molecular features that characterize disease progression. However, some aspects of MAP, including diagnostic criteria, genotype-phenotype correlations, pathogenicity of variants, as well as relationships between BER and other DNA repair pathways, are still poorly understood. A deeper knowledge of the MUTYH expression pattern is likely to refine our understanding of the protein role and, finally, to improve guidances for identifying and handling MAP patients.

Functional characterization of 8-oxoguanine DNA glycosylase of Trypanosoma cruzi

The oxidative lesion 8-oxoguanine (8-oxoG) is removed during base excision repair by the 8-oxoguanine DNA glycosylase 1 (Ogg1). This lesion can erroneously pair with adenine, and the excision of this damaged base by Ogg1 enables the insertion of a guanine and prevents DNA mutation. In this report, we identified and characterized Ogg1 from the protozoan parasite Trypanosoma cruzi (TcOgg1), the causative agent of Chagas disease. Like most living organisms, T. cruzi is susceptible to oxidative stress, hence DNA repair is essential for its survival and improvement of infection. We verified that the TcOGG1 gene encodes an 8-oxoG DNA glycosylase by complementing an Ogg1-defective Saccharomyces cerevisiae strain. Heterologous expression of TcOGG1 reestablished the mutation frequency of the yeast mutant ogg1(-/-) (CD138) to wild type levels. We also demonstrate that the overexpression of TcOGG1 increases T. cruzi sensitivity to hydrogen peroxide (H(2)O(2)). Analysis of DNA lesions using quantitative PCR suggests that the increased susceptibility to H(2)O(2) of TcOGG1-overexpressor could be a consequence of uncoupled BER in abasic sites and/or strand breaks generated after TcOgg1 removes 8-oxoG, which are not rapidly repaired by the subsequent BER enzymes. This hypothesis is supported by the observation that TcOGG1-overexpressors have reduced levels of 8-oxoG both in the nucleus and in the parasite mitochondrion. The localization of TcOgg1 was examined in parasite transfected with a TcOgg1-GFP fusion, which confirmed that this enzyme is in both organelles. Taken together, our data indicate that T. cruzi has a functional Ogg1 ortholog that participates in nuclear and mitochondrial BER.