DNA damage in Nijmegen Breakage Syndrome cells leads to PARP hyperactivation and increased oxidative stress

The Effect of Inactivating Heterozygous Mutation in NBS1 Gene on DNA Damage and Repair Markers and Apoptosis Markers in Mice

We studied the state of the DNA repair system and apoptosis in young mice carrying heterozygous inactivating mutation in the NBS1 gene (c.1971insT, p.Arg658Stop). In the peripheral blood cells of 4-month-old NBS1insT males, the %DNA in the comet tail was higher by 10% than in wild-type mice (wt) (p<0.05). In hepatocytes of NBS1insT mice, the proportion of γH2AX+ nuclear regions marking DNA double-strand breaks was lower by 2 times than in wt mice (p<0.05), which can be an indicator of less efficient DNA repair. In the kidney tissue of NBS1insT mice, a tendency towards the proapoptotic ratio of Bax and Bcl-2 protein markers was revealed against the background of their reduced expression. Thus, the disturbances detected NBS1insT mice in young age suggest that this model is promising for further studies of carcinogenesis.

DNA damage and mitochondria in cancer and aging

Age and DNA repair deficiencies are strong risk factors for developing cancer. This is reflected in the comorbidity of cancer with premature aging diseases associated with DNA damage repair deficiencies. Recent research has suggested that DNA damage accumulation, telomere dysfunction and the accompanying mitochondrial dysfunction exacerbate the aging process and may increase the risk of cancer development. Thus, an area of interest in both cancer and aging research is the elucidation of the dynamic crosstalk between the nucleus and the mitochondria. In this review, we discuss current research on aging and cancer with specific focus on the role of mitochondrial dysfunction in cancer and aging as well as how nuclear to mitochondrial DNA damage signaling may be a driving factor in the increased cancer incidence with aging. We suggest that therapeutic interventions aimed at the induction of autophagy and mediation of nuclear to mitochondrial signaling may provide a mechanism for healthier aging and reduced tumorigenesis.

Telomere attrition and dysfunction: a potential trigger of the progeroid phenotype in nijmegen breakage syndrome

Background: Nibrin, as part of the NBN/MRE11/RAD50 complex, is mutated in Nijmegen breakage syndrome (NBS), which leads to impaired DNA damage response and lymphoid malignancy.

Results: Telomere length (TL) was markedly reduced in homozygous patients (and comparably so in all chromosomes) by ~40% (qPCR) and was slightly reduced in NBS heterozygotes older than 30 years (~25% in qPCR), in accordance with the respective cancer rates. Humanized cancer-free NBS mice had normal TL. Telomere elongation was inducible by telomerase and/or alternative telomere lengthening but was associated with abnormal expression of telomeric genes involved in aging and/or cell growth. Lymphoblastoid cells from NBS patients with long survival times (>12 years) displayed the shortest telomeres and low caspase 7 activity.

Conclusions: NBS is a secondary telomeropathy. The two-edged sword of telomere attrition enhances the cancer-prone situation in NBS but can also lead to a relatively stable cellular phenotype in tumor survivors. Results suggest a modular model for progeroid syndromes with abnormal expression of telomeric genes as a molecular basis.

Methods: We studied TL and function in 38 homozygous individuals, 27 heterozygotes, one homozygous fetus, six NBS lymphoblastoid cell lines, and humanized NBS mice, all with the same founder NBN mutation: c.657_661del5.

Perturbed autophagy and DNA repair converge to promote neurodegeneration in amyotrophic lateral sclerosis and dementia

Maintaining genomic stability constitutes a major challenge facing cells. DNA breaks can arise from direct oxidative damage to the DNA backbone, the inappropriate activities of endogenous enzymes such as DNA topoisomerases, or due to transcriptionally-derived RNA/DNA hybrids (R-loops). The progressive accumulation of DNA breaks has been linked to several neurological disorders. Recently, however, several independent studies have implicated nuclear and mitochondrial genomic instability, perturbed co-transcriptional processing, and impaired cellular clearance pathways as causal and intertwined mechanisms underpinning neurodegeneration. Here, we discuss this emerging paradigm in the context of amyotrophic lateral sclerosis and frontotemporal dementia, and outline how this knowledge paves the way to novel therapeutic interventions.

Increased oxidative stress mediates the antitumor effect of PARP inhibition in ovarian cancer

PARP inhibitors have been widely tested in clinical trials, especially for the treatment of breast cancer and ovarian cancer, and were shown to be highly successful. Because PARP primarily functions in sensing and repairing DNA strand breaks, the therapeutic effect of PARP inhibition is generally believed to be attributed to impaired DNA repair. We here report that oxidative stress is also increased by PARP inhibition and mediates the antitumor effect. We showed that PARP1 is highly expressed in specimens of high grade serous ovarian carcinoma and its activity is required for unperturbed proliferation of ovarian cancer cells. Inhibition or depletion of PARP leads to not only an increase in DNA damage, but also an elevation in the levels of reactive oxygen species (ROS). Importantly, antioxidant N-acetylcysteine (NAC) significantly attenuated the induction of DNA damage and the perturbation of proliferation by PARP inhibition or depletion. We further showed that NADPH oxidases 1 and 4 were significantly upregulated by PARP inhibition and were partially responsible for the induction of oxidative stress. Depletion of NOX1 and NOX4 partially rescued the growth inhibition of PARP1-deficient tumor xenografts. Our findings suggest that in addition to compromising the repair of DNA damage, PARP inhibition or depletion may exert extra antitumor effect by elevating oxidative stress in ovarian cancer cells.

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PARP1 is overexpressed in ovarian cancer.

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PARP inhibition increases oxidative stress and oxidative DNA damage.

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PARP inhibition increases ROS by upregulating NOX1 and NOX4.

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Oxidative stress mediates the antitumor effect of PARP inhibition.

Comparison of Selected Parameters of Redox Homeostasis in Patients with Ataxia-Telangiectasia and Nijmegen Breakage Syndrome

This study compared the antioxidant status and major lipophilic antioxidants in patients with ataxia-telangiectasia (AT) and Nijmegen breakage syndrome (NBS). Total antioxidant status (TAS), total oxidant status (TOS), oxidative stress index (OSI), and concentrations of coenzyme Q10 (CoQ10) and vitamins A and E were estimated in the plasma of 22 patients with AT, 12 children with NBS, and the healthy controls. In AT patients, TAS (median 261.7 μmol/L) was statistically lower but TOS (496.8 μmol/L) was significantly elevated in comparison with the healthy group (312.7 μmol/L and 311.2 μmol/L, resp.). Tocopherol (0.8 μg/mL) and CoQ10 (0.1 μg/mL) were reduced in AT patients versus control (1.4 μg/mL and 0.3 μg/mL, resp.). NBS patients also displayed statistically lower TAS levels (290.3 μmol/L), while TOS (404.8 μmol/L) was comparable to the controls. We found that in NBS patients retinol concentration (0.1 μg/mL) was highly elevated and CoQ10 (0.1 μg/mL) was significantly lower in comparison with those in the healthy group. Our study confirms disturbances in redox homeostasis in AT and NBS patients and indicates a need for diagnosing oxidative stress in those cases as a potential disease biomarker. Decreased CoQ10 concentration found in NBS and AT indicates a need for possible supplementation.

Oxidative stress-driven pulmonary inflammation and fibrosis in a mouse model of human ataxia-telangiectasia

Lung failure is responsible for significant morbidity and is a frequent cause of death in ataxia-telangiectasia (A-T). Disturbance in the redox balance of alveolar epithelial cells must be considered as a causal factor for respiratory disease in A-T. To investigate bronchoalveolar sensitivity to reactive oxygen species (ROS) and ROS-induced DNA damage, we used bleomycin (BLM) to induce experimental inflammation and fibrotic changes in the Atm-deficient mouse model.

BLM or saline was administered by oropharyngeal instillation into the lung of Atm-deficient mice and wild-type mice. Mice underwent pulmonary function testing at days 0, 9, and 28, and bronchoalveolar lavage (BAL) was analysed for cell distribution and cytokines. Lung tissue was analysed by histochemistry.

BLM administration resulted in a tremendous increase in lung inflammation and fibrotic changes in the lung tissue of Atm-deficient mice and was accompanied by irreversible deterioration of lung function. ATM (ataxia telangiectasia mutated) deficiency resulted in reduced cell viability, a delay in the resolution of γH2AX expression and a significant increase in intracellular ROS in pulmonary epithelial cells after BLM treatment. This was confirmed in the human epithelial cell line A549 treated with the ATM-kinase inhibitor KU55933.

Our results demonstrate high bronchoalveolar sensitivity to ROS and ROS-induced DNA damage in the Atm-deficient mouse model and support the hypothesis that ATM plays a pivotal role in the control of oxidative stress-driven lung inflammation and fibrosis.

Effect of irradiation on DNA synthesis, NBN gene expression and chromosomal stability in cells with NBN mutations

INTRODUCTION

The NBN gene product is part of the MRE11/RAD50/NBN complex, which plays an essential role in genomic stability. In the study we try to answer the question what is the effect of irradiation on DNA synthesis, NBN gene expression and chromosomal stability in cells with homozygous c.657-661del, and heterozygous c.657-661del, p.I171V and p.R215W NBN gene mutations.

MATERIAL AND METHODS

Immortalized B-lymphocytes with NBN gene mutations were X-ray irradiated at doses of 1, 2, 5 and 8 Gy/min. Radioresistant DNA synthesis rate and the percentage of cells in phase S was analyzed by ³H thymidine and BrdU incorporation assays. NBN gene expression was quantified by real-time PCR with TaqMan fluorescent probe.

RESULTS

Increasing the irradiation dose resulted in gradual decrease of ³H thymidine incorporation in all cells, but significantly only in homo- and heterozygous c.657-661del cells (p-values < 0.0001). After irradiation the relative expression of NBN was significantly higher in homozygous c.657-661del and heterozygous p.R215W cells as compared to heterozygous c.657-661del, p.I171V and control cells (p < 0.01). All cells with NBN gene mutations showed significantly higher total number of chromosomal aberrations per metaphase as compared to control cells, with the highest number of aberrations in homozygous c.657-661del cells (p < 0.001).

CONCLUSIONS

The results obtained indicate that homozygous c.657-661del mutation affects cell sensitivity to irradiation. Moreover, homozygous variant is associated with disturbance in the activation of cell cycle checkpoints and with defects in DNA repair. In turn, heterozygous c.657-661del, p.R215W and p.I171V mutations do not substantially alter the radiosensitivity.

Oxidative stress, mitochondrial abnormalities and antioxidant defense in Ataxia-telangiectasia, Bloom syndrome and Nijmegen breakage syndrome

Rare pleiotropic genetic disorders, Ataxia-telangiectasia (A-T), Bloom syndrome (BS) and Nijmegen breakage syndrome (NBS) are characterised by immunodeficiency, extreme radiosensitivity, higher cancer susceptibility, premature aging, neurodegeneration and insulin resistance. Some of these functional abnormalities can be explained by aberrant DNA damage response and chromosomal instability. It has been suggested that one possible common denominator of these conditions could be chronic oxidative stress caused by endogenous ROS overproduction and impairment of mitochondrial homeostasis. Recent studies indicate new, alternative sources of oxidative stress in A-T, BS and NBS cells, including NADPH oxidase 4 (NOX4), oxidised low-density lipoprotein (ox-LDL) or Poly (ADP-ribose) polymerases (PARP). Mitochondrial abnormalities such as changes in the ultrastructure and function of mitochondria, excess mROS production as well as mitochondrial damage have also been reported in A-T, BS and NBS cells. A-T, BS and NBS cells are inextricably linked to high levels of reactive oxygen species (ROS), and thereby, chronic oxidative stress may be a major phenotypic hallmark in these diseases. Due to the presence of mitochondrial disturbances, A-T, BS and NBS may be considered mitochondrial diseases. Excess activity of antioxidant enzymes and an insufficient amount of low molecular weight antioxidants indicate new pharmacological strategies for patients suffering from the aforementioned diseases. However, at the current stage of research we are unable to ascertain if antioxidants and free radical scavengers can improve the condition or prolong the survival time of A-T, BS and NBS patients. Therefore, it is necessary to conduct experimental studies in a human model.

Ibrutinib synergizes with poly(ADP-ribose) glycohydrolase inhibitors to induce cell death in AML cells via a BTK-independent mechanism

Targeting Bruton's tyrosine kinase (BTK) with the small molecule BTK inhibitor ibrutinib has significantly improved patient outcomes in several B-cell malignancies, with minimal toxicity. Given the reported expression and constitutive activation of BTK in acute myeloid leukemia (AML) cells, there has been recent interest in investigating the anti-AML activity of ibrutinib. We noted that ibrutinib had limited single-agent toxicity in a panel of AML cell lines and primary AML samples, and therefore sought to identify ibrutinib-sensitizing drugs. Using a high-throughput combination chemical screen, we identified that the poly(ADP-ribose) glycohydrolase (PARG) inhibitor ethacridine lactate synergized with ibrutinib in TEX and OCI-AML2 leukemia cell lines. The combination of ibrutinib and ethacridine induced a synergistic increase in reactive oxygen species that was functionally important to explain the observed cell death. Interestingly, synergistic cytotoxicity of ibrutinib and ethacridine was independent of the inhibitory effect of ibrutinib against BTK, as knockdown of BTK did not sensitize TEX and OCI-AML2 cells to ethacridine treatment. Thus, our findings indicate that ibrutinib may have a BTK-independent role in AML and that PARG inhibitors may have utility as part of a combination therapy for this disease.

Prevalence of Germline Mutations in Genes Engaged in DNA Damage Repair by Homologous Recombination in Patients with Triple-Negative and Hereditary Non-Triple-Negative Breast Cancers

Purpose

This study sought to assess the prevalence of common germline mutations in several genes engaged in the repair of DNA double-strand break by homologous recombination in patients with triple-negative breast cancers and hereditary non-triple-negative breast cancers. Tumors deficient in this type of DNA damage repair are known to be especially sensitive to DNA cross-linking agents (e.g., platinum drugs) and to poly(ADP-ribose) polymerase (PARP) inhibitors.

Methods

Genetic testing was performed for 36 common germline mutations in genes engaged in the repair of DNA by homologous recombination, i.e., BRCA1, BRCA2, CHEK2, NBN, ATM, PALB2, BARD1, and RAD51D, in 202 consecutive patients with triple-negative breast cancers and hereditary non-triple-negative breast cancers.

Results

Thirty five (22.2%) of 158 patients in the triple-negative group carried mutations in genes involved in DNA repair by homologous recombination, while 10 (22.7%) of the 44 patients in the hereditary non-triple-negative group carried such mutations. Mutations in BRCA1 were most frequent in patients with triple-negative breast cancer (18.4%), and mutations in CHEK2 were most frequent in patients with hereditary non-triple-negative breast cancers (15.9%). In addition, in the triple-negative group, mutations in CHEK2, NBN, and ATM (3.8% combined) were found, while mutations in BRCA1, NBN, and PALB2 (6.8% combined) were identified in the hereditary non-triple-negative group.

Conclusions

Identifying mutations in genes engaged in DNA damage repair by homologous recombination other than BRCA1/2 can substantially increase the proportion of patients with triple-negative breast cancer and hereditary non-triple-negative breast cancer who may be eligible for therapy using PARP inhibitors and platinum drugs.

Regulation of NAD biosynthetic enzymes modulates NAD-sensing processes to shape mammalian cell physiology under varying biological cues

In addition to its role as a redox coenzyme, NAD is a substrate of various enzymes that split the molecule to either catalyze covalent modifications of target proteins or convert NAD into biologically active metabolites. The coenzyme bioavailability may be significantly affected by these reactions, with ensuing major impact on energy metabolism, cell survival, and aging. Moreover, through the activity of the NAD-dependent deacetylating sirtuins, NAD behaves as a beacon molecule that reports the cell metabolic state, and accordingly modulates transcriptional responses and metabolic adaptations. In this view, NAD biosynthesis emerges as a highly regulated process: it enables cells to preserve NAD homeostasis in response to significant NAD-consuming events and it can be modulated by various stimuli to induce, via NAD level changes, suitable NAD-mediated metabolic responses. Here we review the current knowledge on the regulation of mammalian NAD biosynthesis, with focus on the relevant rate-limiting enzymes. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

Identification of the interactors of human nibrin (NBN) and of its 26 kDa and 70 kDa fragments arising from the NBN 657del5 founder mutation

Nibrin (also named NBN or NBS1) is a component of the MRE11/RAD50/NBN complex, which is involved in early steps of DNA double strand breaks sensing and repair. Mutations within the NBN gene are responsible for the Nijmegen breakage syndrome (NBS). The 90% of NBS patients are homozygous for the 657del5 mutation, which determines the synthesis of two truncated proteins of 26 kDa (p26) and 70 kDa (p70). Here, HEK293 cells have been exploited to transiently express either the full-length NBN protein or the p26 or p70 fragments, followed by affinity chromatography enrichment of the eluates. The application of an unsupervised proteomics approach, based upon SDS-PAGE separation and shotgun digestion of protein bands followed by MS/MS protein identification, indicates the occurrence of previously unreported protein interacting partners of the full-length NBN protein and the p26 fragment containing the FHA/BRCT1 domains, especially after cell irradiation. In particular, results obtained shed light on new possible roles of NBN and of the p26 fragment in ROS scavenging, in the DNA damage response, and in protein folding and degradation. In particular, here we show that p26 interacts with PARP1 after irradiation, and this interaction exerts an inhibitory effect on PARP1 activity as measured by NAD⁺ levels. Furthermore, the p26-PARP1 interaction seems to be responsible for the persistence of ROS, and in turn of DSBs, at 24 h from IR. Since some of the newly identified interactors of the p26 and p70 fragments have not been found to interact with the full-length NBN, these interactions may somehow contribute to the key biological phenomena underpinning NBS.

Functional deficiency of NBN, the Nijmegen breakage syndrome protein, in a p.R215W mutant breast cancer cell line

Background

Mutations in NBN, the gene for Nijmegen Breakage Syndrome (NBS), are thought to predispose women to developing breast cancer, but a breast cancer cell line containing mutations in NBN has not yet been described. The p.R215W missense mutation occurs at sub-polymorphic frequencies in several populations. We aimed to investigate its functional impact in breast cancer cells from a carrier of this NBN mutation.

Methods

Breast cancer cell lines were screened by immunoblotting for NBN protein levels, and the NBN coding region was sequenced for mutation analysis. Radiosensitivity assays and functional studies were performed through immunocytochemistry and immunoblotting, and flow cytometry was employed to assess cell cycle progression. Impedance measurements were used to study the consequences of PARP1 inhibition. Statistical comparisons between cell lines were performed using t-tests.

Results

HCC1395 breast cancer cells exhibited reduced NBN protein levels. Direct sequencing identified the NBN p.R215W mutation in the hemizygous state, in addition to a truncation in BRCA1. Mutations in both genes were already present in the heterozygous state in the patient’s germline. HCC1395 cells were highly radiosensitive, susceptible to apoptosis and were deficient in the formation of NBN foci. There was also evidence for some impairment in the formation of γH2AX, MDC1, and 53BP1 foci after irradiation; these foci appeared smaller and irregular compared with repair foci in wild-type cells, although ATM signalling was largely unaffected. In line with their deficiency in NBN and BRCA1, HCC1395 cells were particularly sensitive to PARP1 inhibition.

Conclusion

Our results indicate that the p.R215W mutation in the HCC1395 breast cancer cell line impairs NBN function, making this cell line a potentially useful cellular model for studying defective NBN protein within a mutant BRCA1 background.

Oxidative stress and mitochondrial dysfunction across broad-ranging pathologies: toward mitochondria-targeted clinical strategies

Beyond the disorders recognized as mitochondrial diseases, abnormalities in function and/or ultrastructure of mitochondria have been reported in several unrelated pathologies. These encompass ageing, malformations, and a number of genetic or acquired diseases, as diabetes and cardiologic, haematologic, organ-specific (e.g., eye or liver), neurologic and psychiatric, autoimmune, and dermatologic disorders. The mechanistic grounds for mitochondrial dysfunction (MDF) along with the occurrence of oxidative stress (OS) have been investigated within the pathogenesis of individual disorders or in groups of interrelated disorders. We attempt to review broad-ranging pathologies that involve mitochondrial-specific deficiencies or rely on cytosol-derived prooxidant states or on autoimmune-induced mitochondrial damage. The established knowledge in these subjects warrants studies aimed at elucidating several open questions that are highlighted in the present review. The relevance of OS and MDF in different pathologies may establish the grounds for chemoprevention trials aimed at compensating OS/MDF by means of antioxidants and mitochondrial nutrients.

Mitigation of gamma-radiation induced abasic sites in genomic DNA by dietary nicotinamide supplementation: metabolic up-regulation of NAD(+) biosynthesis

The search for non-toxic radio-protective drugs has yielded many potential agents but most of these compounds have certain amount of toxicity. The objective of the present study was to investigate dietary nicotinamide enrichment dependent adaptive response to potential cytotoxic effect of (60)Co γ-radiation. To elucidate the possible underlying mechanism(s), male Swiss mice were maintained on control diet (CD) and nicotinamide supplemented diet (NSD). After 6 weeks of CD and NSD dietary regimen, we exposed the animals to γ-radiation (2, 4 and 6Gy) and investigated the profile of downstream metabolites and activities of enzymes involved in NAD(+) biosynthesis. Increased activities of nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide mononucleotide adenylyltransferase (NMNAT) were observed up to 48h post-irradiation in NSD fed irradiated mice. Concomitant with increase in liver NAMPT and NMNAT activities, NAD(+) levels were replenished in NSD fed and irradiated animals. However, NAMPT and NMNAT-mediated NAD(+) biosynthesis and ATP levels were severely compromised in liver of CD fed irradiated mice. Another major finding of these studies revealed that under γ-radiation stress, dietary nicotinamide supplementation might induce higher and long-lasting poly(ADP)-ribose polymerase 1 (PARP1) and poly(ADP-ribose) glycohydrolase (PARG) activities in NSD fed animals compared to CD fed animals. To investigate liver DNA damage, number of apurinic/apyrimidinic sites (AP sites) and level of 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) residues were quantified. A significant increase in liver DNA AP sites and 8-oxo-dG levels with concomitant increase in caspase-3 was observed in CD fed and irradiated animals compared to NSD fed and irradiated mice. In conclusion present studies show that under γ-radiation stress conditions, dietary nicotinamide supplementation restores DNA excision repair activity via prolonged activation of PARP1 and PARG activities. Present results clearly indicated that hepatic NAD(+) replenishment might be a novel and potent approach to reduce radiation injury.

Susceptibility to DNA damage as a molecular mechanism for non-syndromic cleft lip and palate

Non-syndromic cleft lip/palate (NSCL/P) is a complex, frequent congenital malformation, determined by the interplay between genetic and environmental factors during embryonic development. Previous findings have appointed an aetiological overlap between NSCL/P and cancer, and alterations in similar biological pathways may underpin both conditions. Here, using a combination of transcriptomic profiling and functional approaches, we report that NSCL/P dental pulp stem cells exhibit dysregulation of a co-expressed gene network mainly associated with DNA double-strand break repair and cell cycle control (p = 2.88×10(-2)-5.02×10(-9)). This network included important genes for these cellular processes, such as BRCA1, RAD51, and MSH2, which are predicted to be regulated by transcription factor E2F1. Functional assays support these findings, revealing that NSCL/P cells accumulate DNA double-strand breaks upon exposure to H2O2. Furthermore, we show that E2f1, Brca1 and Rad51 are co-expressed in the developing embryonic orofacial primordia, and may act as a molecular hub playing a role in lip and palate morphogenesis. In conclusion, we show for the first time that cellular defences against DNA damage may take part in determining the susceptibility to NSCL/P. These results are in accordance with the hypothesis of aetiological overlap between this malformation and cancer, and suggest a new pathogenic mechanism for the disease.

Poly(ADP-ribose): PARadigms and PARadoxes

Poly(ADP-ribosyl)ation (PARylation) is a posttranslational protein modification (PTM) catalyzed by members of the poly(ADP-ribose) polymerase (PARP) enzyme family. PARPs use NAD(+) as substrate and upon cleaving off nicotinamide they transfer the ADP-ribosyl moiety covalently to suitable acceptor proteins and elongate the chain by adding further ADP-ribose units to create a branched polymer, termed poly(ADP-ribose) (PAR), which is rapidly degraded by poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3). In recent years several key discoveries changed the way we look at the biological roles and mode of operation of PARylation. These paradigm shifts include but are not limited to (1) a single PARP enzyme expanding to a PARP family; (2) DNA-break dependent activation extended to several other DNA dependent and independent PARP-activation mechanisms; (3) one molecular mechanism (covalent PARylation of target proteins) underlying the biological effect of PARPs is now complemented by several other mechanisms such as protein-protein interactions, PAR signaling, modulation of NAD(+) pools and (4) one principal biological role in DNA damage sensing expanded to numerous, diverse biological functions identifying PARP-1 as a real moonlighting protein. Here we review the most important paradigm shifts in PARylation research and also highlight some of the many controversial issues (or paradoxes) of the field such as (1) the mostly synergistic and not antagonistic biological effects of PARP-1 and PARG; (2) mitochondrial PARylation and PAR decomposition, (3) the cross-talk between PARylation and signaling pathways (protein kinases, phosphatases, calcium) and the (4) divergent roles of PARP/PARylation in longevity and in age-related diseases.

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.