DNA double-strand breaks: signaling, repair and the cancer connection

Differential regulation of DNA repair protein Rad51 in human tumour cell lines exposed to doxorubicin

Radiotherapy and chemotherapy often induce DNA double-strand breaks in both normal and malignant cells. The proteins involved in the repair of such lesions are central to cancer prognosis and treatment, as they can be overexpressed in many cancers, accelerating malignant transformation and increasing repair capacity, potentially leading to cellular resistance. If malignant cells can be selectively targeted repair proteins could also be candidates for targeted therapy. In this study, two keyplayers in eukaryotic DNA double-strand break repair, Rad51 and DNA-dependent protein kinase catalytic subunit, were analysed in noncancerous human breast cells (MCF12A) and the breast cancer cell lines (MDA MB 231 and MCF7) in response to treatment with doxorubicin. A cell cycle-independent increase in Rad51 protein levels (a recombinase involved in homologous recombination repair) was observed 24 and 48 h after treatment in MDA MB 231 and MCF12A when exposed to low levels of doxorubicin, whereas MCF7 cells displayed a continuous decrease in Rad51 protein with increasing drug concentration. DNA-dependent protein kinase catalytic subunit, which is involved in nonhomologous end joining of DNA lesions, remained unaltered under all conditions tested. Topoisomerase II-alpha protein, the primary target of doxorubicin, was upregulated at low concentrations of doxorubicin in all cell lines tested. Here we show that Rad51 protein levels can be differentially regulated in normal and malignant breast cell lines in response to doxorubicin, independent of cell cycle state. These observations have direct relevance to chemosensitivity and add an additional prognostic factor that could be taken into account when designing targeted therapeutic regimes.

Ku70/80 Modulates ATM and ATR Signaling Pathways in Response to DNA Double Strand Breaks

Double strand break (DSB) recognition is the first step in the DSB damage response and involves activation of ataxia telangiectasia-mutated (ATM) and phosphorylation of targets such as p53 to trigger cell cycle arrest, DNA repair, or apoptosis. It was reported that activation of ATM- and Rad3-related (ATR) kinase by DSBs also occurs in an ATM-dependent manner. On the other hand, Ku70/80 is known to participate at a later time point in the DSB response, recruiting DNA-PKcs to facilitate non-homologous end joining. Because Ku70/80 has a high affinity for broken DNA ends and is abundant in nuclei, we examined their possible involvement in other aspects of the DSB damage response, particularly in modulating the activity of ATM and other phosphatidylinositol (PI) 3-related kinases during DSB recognition. We thus analyzed p53(Ser18) phosphorylation in irradiated Ku-deficient cells and observed persistent phosphorylation in these cells relative to wild type cells. ATM or ATR inhibition revealed that this phosphorylation is mainly mediated by ATM-dependent ATR activity at 2 h post-ionizing radiation in wild type cells, whereas in Ku-deficient cells, this occurs mainly through direct ATM activity, with a secondary contribution from ATR via a novel ATM-independent mechanism. Using ATM/Ku70 double-null cell lines, which we generated, we confirmed that ATM-independent ATR activity contributed to persistent phosphorylation of p53(Ser18) in Ku-deficient cells at 12 h post-ionizing radiation. In summary, we discovered a novel role for Ku70/80 in modulating ATM-dependent ATR activation during DSB damage response and demonstrated that these proteins confer a protective effect against ATM-independent ATR activation at later stages of the DSB damage response.

3'-azido-3'-deoxythymidine induces deletions in L5178Y mouse lymphoma cells

3'-Azido-3'-deoxythymidine (AZT), a nucleoside analogue used for the treatment of acquired immunodeficiency syndrome (AIDS), induced a significant dose-related increase in the thymidine kinase (Tk) mutant frequency (MF) in L5178Y/Tk(+/-) 3.7.2C mouse lymphoma cells. Treatment with 1 mg/ml (3,742 muM) AZT for 24 hr resulted in a MF of 407 x 10(-6) compared to a control MF of 84 x 10(-6). The MFs of the large and small colony mutants resulting from AZT exposure were 142 x 10(-6) and 265 x 10(-6), respectively. One hundred and fifty mutants from the 1 mg/ml (3,742 muM) AZT-treated culture and sixty-nine mutants from independent untreated cultures were isolated and analyzed. LOH analysis using a heteromorphic microsatellite locus located in the Tk gene was performed to determine the presence or absence of the Tk(+) allele. Eight other microsatellite markers spanning the entire mouse chromosome 11 also were examined for heterozygosity to determine the extent of LOH. In addition, Tk gene dosage analysis was conducted using Real-Time PCR in those mutants showing LOH at the Tk locus. The presence of only one Tk allele based on Real-Time PCR indicated that the mutant resulted from deletion while the presence of two alleles was consistent with a recombination event. More mutants from the AZT-treated culture showed Tk LOH than did independent mutants from the untreated cultures (91% vs. 64%) and the induced mutants also showed distinct chromosome 11 LOH patterns. The mutation spectrum of mutants from AZT-treated cells was also significantly different from that of spontaneous mutants. More deletions and fewer intragenic mutations were observed in the mutants from the AZT-treated culture than independent mutants from the untreated control. Our data indicate that AZT primarily induced LOH mutations in L5178Y mouse lymphoma cells and a large number of LOH mutations resulted from deletions.

Spermatocyte responses in vitro to induced DNA damage

Spermatocytes normally sustain many meiotically induced double-strand DNA breaks (DSBs) early in meiotic prophase; in autosomal chromatin, these are repaired by initiation of meiotic homologous-recombination processes. Little is known about how spermatocytes respond to environmentally induced DNA damage after recombination-related DSBs have been repaired. The experiments described here tested the hypothesis that, even though actively completing meiotic recombination, pachytene spermatocytes cultured in the absence of testicular somatic cells initiate appropriate chromatin remodeling and cell-cycle responses to environmentally induced DNA damage. Two DNA-damaging agents were employed for in vitro treatment of pachytene spermatocytes: gamma-irradiation and etoposide, a topoisomerase II (TOP2) inhibitor that results in persistent unligated DSBs. Chromatin modifications associated with DSBs were monitored after exposure by labeling surface-spread chromatin with antibodies against RAD51 (which recognizes DSBs) and the phosphorylated variant of histone H2AFX (herein designated by its commonly used symbol, H2AX), gammaH2AX (which modifies chromatin associated with DSBs). Both gammaH2AX and RAD51 were rapidly recruited to irradiation- or etoposide-damaged chromatin. These chromatin modifications imply that spermatocytes recruit active DNA damage responses, even after recombination is substantially completed. Furthermore, irradiation-induced DNA damage inhibited okadaic acid-induced progression of spermatocytes from meiotic prophase to metaphase I (MI), implying efficacy of DNA damage checkpoint mechanisms. Apoptotic responses of spermatocytes with DNA damage differed, with an increase in frequency of early apoptotic spermatocytes after etoposide treatment, but not following irradiation. Taken together, these results demonstrate modification of pachytene spermatocyte chromatin and inhibition of meiotic progress after DNA damage by mechanisms that may ensure gametic genetic integrity.

The mutagenic potential of non-homologous end joining in the absence of the NHEJ core factors Ku70/80, DNA-PKcs and XRCC4-LigIV

Non-homologous end joining (NHEJ), the major pathway of double-strand break (DSB) repair in mammalian cells, comprises two subpathways: one that requires the three core factors Ku70/80, DNA-PKcs and XRCC4/LigIV (DNA-PK-dependent NHEJ) and the other that is independent of these factors. Using a cell-free NHEJ assay, we have investigated the ability of three Chinese hamster ovary (CHO) mutants deficient in Ku80 (xrs6), DNA-PKcs (XR-C1) and XRCC4 (XR-1) in comparison with CHO-K1 wild-type cells to rejoin non-compatible DSB ends. Both NHEJ efficiency and fidelity are strongly reduced in the mutants with xrs6 and XR-1 exhibiting the strongest reduction and XR-C1 displaying a phenotype intermediate between the wild-type and the other two mutants indicating a non-essential but facilitating role of DNA-PKcs in NHEJ. The decrease in fidelity in the mutants is expressed by an increase of deletion junctions formed at microhomologies (microhom) near the DSB (microhomology-mediated non-homologous end joining: microhomNHEJ). Using a novel microhomNHEJ assay, we show that microhom regions of 6-10 bp that are located directly at the DSB termini strongly enhance the mutagenic microhomNHEJ reaction even in the wild type. Due to its error proneness, DNA-PK-independent microhomNHEJ may actively promote genome instability. It will, therefore, be of increasing importance to examine NHEJ fidelity in the context with tumorigenesis and cellular senescence for which we here provide two efficient and reliable tools.

Biological effects of field emission-type X-rays generated by nanotechnology

Thermionic emission (TE)-type X-ray generators have been exclusively used in medicine, but there are many difficulties in making these X-ray sources compact. A field emission (FE)-type X-ray generator using carbon nanotubes is a newly-developed compact system that can be as small as several cm in length. Considering the compactness of the equipment, the FE-type X-ray generator may become a useful tool for endoscopic, intracavitary or intraoperative radiotherapy in the future. The aim of this study was to investigate the biological effects of X-rays generated by the FE-type X-ray source in comparison with those of conventional TE-type X-rays. Mouse thymic lymphoma 3SB cells were irradiated by an FE-type X-ray generator developed by our group and a conventional TE-type X-ray source under identical conditions. DNA damage after radiation was detected by foci formation of phospho-H2AX (gamma-H2AX). Effect on the cell cycle was analyzed by flow cytometry. Activation of the DNA damage checkpoint was analyzed by immunoblotting. Induction of apoptosis was studied using the TUNEL assay. In terms of induction of DNA damage (DNA double-strand breaks), activation of cell cycle checkpoints (p53 stabilization, p21 induction, Chk1 and Chk2 phosphorylations), and induction of apoptotic cell death, FE-type X-rays were as effective as TE-type X-rays, and FE-type X-rays appeared to be applicable to radiation therapy.

Darwinian medicine: a case for cancer

Epidemiological, genetic and molecular biological studies have collectively provided us with a rich source of data that underpins our current understanding of the aetiology and molecular pathogenesis of cancer. But this perspective focuses on proximate mechanisms, and does not provide an adequate explanation for the prevalence of tumours and cancer in animal species or what seems to be the striking vulnerability of Homo sapiens. The central precept of Darwinian medicine is that vulnerability to cancer, and other major diseases, arises at least in part as a consequence of the 'design' limitations, compromises and trade-offs that characterize evolutionary processes.

Chronic Suppression of Angiogenesis following Radiation Exposure Is Independent of Hematopoietic Reconstitution

Radiation can potentially suppress neovascularization by inhibiting the incorporation of hematopoietic precursors as well as damaging mature endothelial cells. The purpose of these studies was to quantify the effect of radiation on angiogenesis and to examine the relationship between bone marrow reconstitution and neovascularization. Immune competent, severe combined immunodeficient, RAG1-deficient, and green fluorescence protein transgenic mice in the C57 genetic background, as well as the highly angiogenic 129S1/SvlmJ strain of mice, underwent whole-body or localized exposure to radiation. The hematopoietic systems in the irradiated recipients were restored by bone marrow transfer. Hematopoietic reconstitution was assessed by doing complete blood counts. Angiogenesis was induced in the mouse cornea using 80 ng of purified basic fibroblast growth factor, and the neovascular response was quantified using a slit lamp biomicroscope. Following whole-body exposure and bone marrow transplantation, the hematopoietic system was successfully reconstituted over time, but the corneal angiogenic response was permanently and significantly blunted up to 66%. Localized exposure of the eyes to radiation suppressed corneal angiogenesis comparably to whole-body exposure. Whole-body irradiation with ocular shielding induced bone marrow suppression but did not inhibit corneal neovascularization. In mice exposed to radiation before tumor implantation, the reduced local angiogenic response correlated with significantly reduced growth of tumor cells in vivo. These results indicate that bone marrow suppression does not suppress neovascularization in the mouse cornea and that although hematopoietic stem cells can readily reconstitute peripheral blood, they do not restore a local radiation-induced deficit in neovascular response.

Benzo[a]pyrene-7,8-dihydrodiol promotes checkpoint activation and G2/M arrest in human bronchoalveolar carcinoma H358 cells

Polycyclic aromatic hydrocarbons (PAHs) are potent carcinogens that require metabolic activation inside cells. The proximate carcinogens PAH-diols can be converted to o-quinones by aldo-keto reductases (AKRs) or to diol-epoxides by cytochrome P450 (P450) enzymes. We assessed the effect of benzo[a]pyrene-7,8-dihydrodiol (BPD) on proliferation in p53-null bronchoalveolar carcinoma H358 cells. BPD treatment led to a significant inhibition of proliferation and arrest in G2/M in H358 cells. The relative contribution of the AKR and P450 pathways to cell cycle arrest was assessed. Overexpression of AKR1A1 did not affect cell proliferation or cell cycle progression, and benzo[a]pyrene-7,8-dione did not cause any noticeable effect on cell growth, suggesting that AKR1A1 metabolic products were not involved in the antiproliferative effect of BPD. On the other hand, blockade of P450 induction or inhibition of P450 activity greatly impaired the effect of BPD. Moreover, P450 induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin significantly enhanced the antiproliferative effect of BPD. Mechanistic studies revealed that BPD caused a DNA damage response, Chk1 activation, and accumulation of phospho-Cdc2 (Tyr15) in H358 cells, effects that were impaired by an ataxia-telangectasia mutated (ATM)/ATM-related (ATR) inhibitor. Similar results were observed in human bronchoepithelial BEAS-2B cells, arguing for analogous mechanisms in tumorigenic and immortalized nontumorigenic cells lacking functional p53. Our data suggest that a p53-independent pathway operates in lung epithelial cells in response to BPD that involves P450 induction and subsequent activation of the ATR/ATM/Chk1 damage check-point pathway and cell cycle arrest in G2/M.

Sporadic colorectal cancer and individual susceptibility: a review of the association studies investigating the role of DNA repair genetic polymorphisms

Mutations in one of the DNA repair genes are one of the most common reasons for cancer, and it may be assumed that the individual genetic background modulating the DNA repair capacity may affect the susceptibility to cancer. Numerous polymorphisms (mainly SNPs) have been identified for DNA repair genes, although their functional outcome and phenotypic effect is often unknown. The aim of the present review is to evaluate the studies investigating a possible influence of DNA repair polymorphisms in the risk of sporadic colorectal cancer and/or adenoma. Overall, no relevant common findings emerge among the studies, except for some statistically significant associations between polymorphisms in the XRCC1 and XPD genes, mainly for colorectal adenoma risk. Other individual associations remain to be confirmed. This inconclusive data may suggest that the modulation of cancer risk depends not only on a single gene/SNP, but also on a joint effect of multiple polymorphisms (or haplotypes) within different genes or pathways, in close interaction with environmental factors. The relevance of many low-penetrance genes in cancer susceptibility is supposed to be very subtle. Several reviewed association studies revealed weaknesses in their design. However, there has been a progressive improvement over the years in aspects such as simultaneous genotyping and combined analyses of different polymorphisms in larger numbers of patients and controls, as well as stratification of results by ethnicity, gender, and tumor localization. This gained experience shows that only carefully designed studies of a sufficient statistical power may resolve the relationships between polymorphisms and colorectal cancer risk.

Non-homologous end-joining for repairing I-SceI-induced DNA double strand breaks in human cells

DNA double strand breaks (DSBs) are usually repaired through either non-homologous end-joining (NHEJ) or homologous recombination (HR). While HR is basically error-free repair, NHEJ is a mutagenic pathway that leads to deletion. NHEJ must be precisely regulated to maintain genomic integrity. To clarify the role of NHEJ, we investigated the genetic consequences of NHEJ repair of DSBs in human cells. Human lymphoblastoid cell lines TSCE5 and TSCE105 have, respectively, single and double I-SceI endonuclease sites in the endogenous thymidine kinase gene (TK) located on chromosome 17q. I-SceI expression generated DSBs at the TK gene. We used the novel transfection system (Amaxa Nucleofector) to introduce an I-SceI expression vector into the cells and randomly isolated clones. We found mutations involved in the DSBs in the TK gene in 3% of TSCE5 cells and 30% of TSCE105 cell clones. Most of the mutations in TSCE5 were small (1-30bp) deletions with a 0-4bp microhomology at the junction. The others consisted of large (>60) bp deletions, an insertion, and a rearrangement. Mutants resulting from interallelic HR also occurred, but infrequently. Most of the mutations in TSCE105, on the other hand, were deletions that encompassed the two I-SceI sites generated by NHEJ at DSBs. The sequence joint was similar to that found in TSCE5 mutants. Interestingly, some mutants formed a new I-SceI site by perfectly joining the two original I-SceI sites without deletion of the broken-ends. These results support the idea that NHEJ for repairing I-SceI-induced DSBs mainly results in small or no deletions. Thus, NHEJ must help maintain genomic integrity in mammalian cells by repairing DSBs as well as by preventing many deleterious alterations.

Ataxia-telangiectasia mutated expression is associated with tobacco smoke exposure in esophageal cancer tissues and benzo[a]pyrene diol epoxide in cell lines

Esophageal cancer is a substantial health problem because of its usually late stage at diagnosis and poor prognosis. Tobacco smoking and alcohol use are the most important risk factors in the development of esophageal squamous cell carcinoma (SCC). Our previous study demonstrated the binding of benzo[a]pyrene diol epoxide (BPDE), a carcinogen present in tobacco smoke and environmental pollution, to the ataxia-telangiectasia mutated (ATM) gene. To understand how this binding affects the alteration of ATM expression and to identify biomarkers for the detection of esophageal cancer, we analyzed ATM mRNA expression in tissue specimens from patients with esophageal SCC and premalignant lesions using in situ hybridization. We then performed in vitro experiments to verify and extend our ex vivo observations. We found that ATM expression was increased in esophageal SCC and its premalignant lesions when compared with normal tissues and that increased ATM expression was associated with tobacco smoke exposure and tumor de-differentiation. Moreover, BPDE induced ATM expression in esophageal SCC cell lines in a time-dependent manner. In summary, the BPDE in tobacco smoke may be responsible for increased ATM expression in premalignant and malignant esophageal tissues. Our findings suggest that the ATM gene should be further evaluated as a biomarker for the early detection of esophageal cancer and tobacco use in patients.

Mechanisms leading to nonrandom, nonhomologous chromosomal translocations in leukemia

Nonrandom, reciprocal translocations between nonhomologous chromosomes are critical cellular events that lead to malignant transformation. Therefore, understanding the mechanisms involved in these chromosomal rearrangements is essential for understanding the process of carcinogenesis. There has been substantial discussion in the literature over the past 10 years about mechanisms involved in constitutional chromosomal rearrangements, including deletions, duplications, and translocations. Yet our understanding of the mechanisms of chromosomal rearrangements in cancer is still developing. This review presents what is known about the mechanisms involved in selected nonrandom chromosomal translocations in leukemia.

Topological locking restrains replication fork reversal

Two-dimensional agarose gel electrophoresis, psoralen cross-linking, and electron microscopy were used to study the effects of positive supercoiling on fork reversal in isolated replication intermediates of bacterial DNA plasmids. The results obtained demonstrate that the formation of Holliday-like junctions at both forks of a replication bubble creates a topological constraint that prevents further regression of the forks. We propose that this topological locking of replication intermediates provides a biological safety mechanism that protects DNA molecules against extensive fork reversals.

Involvement of Ku80 in microhomology-mediated end joining for DNA double-strand breaks in vivo

Mammalian cells have an activity of mutagenic repair for DNA double-strand breaks (DSBs), microhomology-mediated end joining (MMEJ), in which DNA ends are joined via microhomologous sequences flanking the breakpoint. MMEJ has been indicated to be undertaken without Ku proteins, which are essential factors for non-homologous end joining (NHEJ). On the other hand, recent studies with cell-free (in vitro) systems indicated the involvement of Ku proteins in MMEJ, suggesting that MMEJ could be also undertaken by a Ku-dependent pathway. To clarify whether Ku proteins are essential in MMEJ in vivo, linearized plasmid DNAs with microhomologous sequences of 10bp at both ends were introduced as repair substrates into Ku80-proficient and Ku80-deficient CHO cells, and were subjected to MMEJ and NHEJ. Activities of MMEJ and NHEJ, respectively, of the cells were evaluated by mathematical modeling for the increase in fluorescence of GFP proteins produced from repaired products. The Ku80 deficiency caused approximately 75% reduction of the MMEJ activity in CHO cells, while it caused is > or =90% reduction of the NHEJ activity. Therefore, it was indicated that there is a Ku-dependent pathway for MMEJ; however, MMEJ is less dependent on Ku80 protein than NHEJ. The fraction of MMEJ products increased in proportion to the increase in the amounts of substrates. The results suggest that the increase in DSBs makes the cell more predominant for MMEJ. MMEJ might function as a salvage pathway for DSBs that cannot be repaired by NHEJ.

New insight into intrachromosomal deletions induced by chrysotile in the gpt delta transgenic mutation assay

BACKGROUND

Genotoxicity is often a prerequisite to the development of malignancy. Considerable evidence has shown that exposure to asbestos fibers results in the generation of chromosomal aberrations and multilocus mutations using various in vitro approaches. However, there is less evidence to demonstrate the contribution of deletions to the mutagenicity of asbestos fibers in vivo.

OBJECTIVES

In the present study, we investigated the mutant fractions and the patterns induced by chrysotile fibers in gpt delta transgenic mouse primary embryo fibroblasts (MEFs) and compared the results obtained with hydrogen peroxide (H2O2) in an attempt to illustrate the role of oxyradicals in fiber mutagenesis.

RESULTS

Chrysotile fibers induced a dose-dependent increase in mutation yield at the redBA/gam loci in transgenic MEF cells. The number of lambda mutants losing both redBA and gam loci induced by chrysotiles at a dose of 1 microg/cm(2) increased by > 5-fold relative to nontreated controls (p < 0.005). Mutation spectra analyses showed that the ratio of lambda mutants losing the redBA/gam region induced by chrysotiles was similar to those induced by equitoxic doses of H2O2. Moreover, treatment with catalase abrogated the accumulation of y-H2AX, a biomarker of DNA double-strand breaks, induced by chrysotile fibers.

CONCLUSIONS

Our results provide novel information on the frequencies and types of mutations induced by asbestos fibers in the gpt delta transgenic mouse mutagenic assay, which shows great promise for evaluating fiber/particle mutagenicity in vivo.

Carcinogen-induced early molecular events and its implication in the initiation of chemical hepatocarcinogenesis in rats: Chemopreventive role of vanadium on this process

Carcinogen-induced formation of DNA adducts and other types of DNA lesions are the critical molecular events in the initiation of chemical carcinogenesis and modulation of such events by chemopreventive agents could be an important step in limiting neoplastic transformation in vivo. Vanadium, a dietary micronutrient has been found to be effective in several types of cancers both in vivo and in vitro and also possesses profound anticarcinogenicity against rat models of mammary, colon and hepatocarcinogenesis. Presently, we report the chemopreventive potential of vanadium on diethylnitrosamine (DEN)-induced early DNA damages in rat liver. Hepatocarcinogenesis was induced in male Sprague-Dawley rats with a single, necrogenic, intraperitoneal (i.p.) injection of DEN (200 mg/kg body weight) at week 4. There was a significant induction of tissue-specific ethylguanines, steady elevation of modified DNA bases 8-hydroxy-2'-deoxyguanosines (8-OHdGs) (P<0.0001; 89.93%) along with substantial increment of the extent of single-strand breaks (SSBs) (P<0.0001) following DEN exposure. Supplementation of 0.5 ppm of vanadium throughout the experiment abated the formations of O(6)-ethylguanines and 7-ethylguanines (P<0.0001; 48.71% and 67.54% respectively), 8-OHdGs (P<0.0001; 81.37%), length:width (L:W) of DNA mass (P<0.01; 62.12%) and the mean frequency of tailed DNA (P<0.001; 53.58%), and hepatic nodulogenesis in preneoplastic rat liver. The study indicates that 0.5 ppm vanadium is potentially and optimally effective, as derived from dose-response studies, in limiting early molecular events and preneoplastic lesions, thereby modulating the initiation stage of hepatocarcinogenesis. Vanadium is chemopreventive against DEN-induced genotoxicity and resulting hepatocellular transformation in rats.

Ataxia telangiectasia mutated (ATM) is essential for DNA-PKcs phosphorylations at the Thr-2609 cluster upon DNA double strand break

The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is rapidly phosphorylated at the Thr-2609 cluster and Ser-2056 upon ionizing radiation (IR). Furthermore, DNA-PKcs phosphorylation at both regions is critical for its role in DNA double strand break (DSB) repair as well as cellular resistance to radiation. IR-induced DNA-PKcs phosphorylation at Thr-2609 and Ser-2056, however, exhibits distinct kinetics indicating that they are differentially regulated. Although DNA-PKcs autophosphorylates itself at Ser-2056 after IR, we have reported here that ATM mediates DNA-PKcs phosphorylation at Thr-2609 as well as at the adjacent (S/T)Q motifs within the Thr-2609 cluster. In addition, our data suggest that DNA-PKcs- and ATM-mediated DNA-PKcs phosphorylations are cooperative and required for the full activation of DNA-PKcs and the subsequent DSB repair. Elimination of DNA-PKcs phosphorylation at both regions severely compromises radioresistance and DSB repair. Finally, our result provides a possible mechanism for the direct involvement of ATM in non-homologous end joining-mediated DSB repair.

RSC mobilizes nucleosomes to improve accessibility of repair machinery to the damaged chromatin

Repair of DNA double-strand breaks (DSBs) protects cells and organisms, as well as their genome integrity. Since DSB repair occurs in the context of chromatin, chromatin must be modified to prevent it from inhibiting DSB repair. Evidence supports the role of histone modifications and ATP-dependent chromatin remodeling in repair and signaling of chromosome DSBs. The key questions are, then, what the nature of chromatin altered by DSBs is and how remodeling of chromatin facilitates DSB repair. Here we report a chromatin alteration caused by a single HO endonuclease-generated DSB at the Saccharomyces cerevisiae MAT locus. The break induces rapid nucleosome migration to form histone-free DNA of a few hundred base pairs immediately adjacent to the break. The DSB-induced nucleosome repositioning appears independent of end processing, since it still occurs when the 5'-to-3' degradation of the DNA end is markedly reduced. The tetracycline-controlled depletion of Sth1, the ATPase of RSC, or deletion of RSC2 severely reduces chromatin remodeling and loading of Mre11 and Yku proteins at the DSB. Depletion of Sth1 also reduces phosphorylation of H2A, processing, and joining of DSBs. We propose that RSC-mediated chromatin remodeling at the DSB prepares chromatin to allow repair machinery to access the break and is vital for efficient DSB repair.

Developing master keys to brain pathology, cancer and aging from the structural biology of proteins controlling reactive oxygen species and DNA repair

This review is focused on proteins with key roles in pathways controlling either reactive oxygen species or DNA damage responses, both of which are essential for preserving the nervous system. An imbalance of reactive oxygen species or inappropriate DNA damage response likely causes mutational or cytotoxic outcomes, which may lead to cancer and/or aging phenotypes. Moreover, individuals with hereditary disorders in proteins of these cellular pathways have significant neurological abnormalities. Mutations in a superoxide dismutase, which removes oxygen free radicals, may cause the neurodegenerative disease amyotrophic lateral sclerosis. Additionally, DNA repair disorders that affect the brain to various extents include ataxia-telangiectasia-like disorder, Cockayne syndrome or Werner syndrome. Here, we highlight recent advances gained through structural biochemistry studies on enzymes linked to these disorders and other related enzymes acting within the same cellular pathways. We describe the current understanding of how these vital proteins coordinate chemical steps and integrate cellular signaling and response events. Significantly, these structural studies may provide a set of master keys to developing a unified understanding of the survival mechanisms utilized after insults by reactive oxygen species and genotoxic agents, and also provide a basis for developing an informed intervention in brain tumor and neurodegenerative disease progression.

Gallic acid causes inactivating phosphorylation of cdc25A/cdc25C-cdc2 via ATM-Chk2 activation, leading to cell cycle arrest, and induces apoptosis in human prostate carcinoma DU145 cells

We recently reported that gallic acid is a major active agent responsible for grape seed extract activity in DU145 human prostate carcinoma cells. The present study was conducted to examine its efficacy and associated mechanism. Gallic acid treatment of DU145 cells resulted in a strong cell growth inhibition, cell cycle arrest, and apoptotic death in a dose- and time-dependent manner, together with a decrease in cyclin-dependent kinases and cyclins but strong induction in Cip1/p21. Additional mechanistic studies showed that gallic acid induces an early Tyr(15) phosphorylation of cell division cycle 2 (cdc2). Further upstream, gallic acid also induced phosphorylation of both cdc25A and cdc25C via ataxia telangiectasia mutated (ATM)-checkpoint kinase 2 (Chk2) activation as a DNA damage response evidenced by increased phospho-histone 2AX (H2A.X) that is phosphorylated by ATM in response to DNA damage. Time kinetics of ATM phosphorylation, together with those of H2A.X and Chk2, was in accordance with an inactivating phosphorylation of cdc25A and cdc25C phosphatases and cdc2 kinase, suggesting that gallic acid increases cdc25A/C-cdc2 phosphorylation and thereby inactivation via ATM-Chk2 pathway following DNA damage that induces cell cycle arrest. Caffeine, an ATM/ataxia telangiectasia-rad3-related inhibitor, reversed gallic acid-caused ATM and H2A.X phosphorylation and cell cycle arrest, supporting the role of ATM pathway in gallic acid-induced cell cycle arrest. Additionally, gallic acid caused caspase-9, caspase-3, and poly(ADP)ribose polymerase cleavage, but pan-caspase inhibitor did not reverse apoptosis, suggesting an additional caspase-independent apoptotic mechanism. Together, this is the first report identifying gallic acid efficacy and associated mechanisms in an advanced and androgen-independent human prostate carcinoma DU145 cells, suggesting future in vivo efficacy studies with this agent in preclinical prostate cancer models.

Chromatin modifications and DNA double-strand breaks: the current state of play

The packaging and compaction of DNA into chromatin is important for all DNA-metabolism processes such as transcription, replication and repair. The involvement of chromatin modifications in transcriptional regulation is relatively well characterized, and the distinct patterns of chromatin transitions that guide the process are thought to be the result of a code on the histone proteins (histone code). In contrast to transcription, the intricate link between chromatin and responses to DNA damage has been given attention only recently. It is now emerging that specific ATP-dependent chromatin remodeling complexes (including the Ino80, Swi/Snf and RSC remodelers) and certain constitutive (methylation of lysine 79 of histone H3) and DNA damage-induced covalent histone modifications (the most well characterized being the rapid phosphorylation of histone H2A) facilitate responses to double-strand breaks. Indeed, evidence is already accumulating for a DNA repair-specific histone code. In this review, the recent advances in our understanding of the relationship between chromatin modifications and double-strand break signaling and repair is discussed.

Host cell DNA repair pathways in adeno-associated viral genome processing

Recentstudies have shown that wild-type and recombinant adeno-associated virus (AAV and rAAV) genomes persist in human tissue predominantly as double-stranded (ds) circular episomes derived from input linear single-stranded virion DNA. Using self-complementary recombinant AAV (scAAV) vectors, we generated intermediates that directly transition to ds circular episomes. The scAAV genome ends are palindromic hairpin-structured terminal repeats, resembling a double-stranded break repair intermediate. Utilizing this substrate, we found cellular DNA recombination and repair factors to be essential for generating circular episomal products. To identify the specific cellular proteins involved, the scAAV circularization-dependent vector was used as a reporter in 19 mammalian DNA repair-deficient cell lines. The results show that RecQ helicase family members (BLM and WRN), Mre11 and NBS1 of the Mre11-Rad50-Nbs1 (MRN) complex, and ATM are required for efficient scAAV genome circularization. We further demonstrated that the scAAV genome requires ATM and DNA-PK(CS), but not NBS1, to efficiently convert to a circular form in nondividing cells in vivo using transgenic mice. These studies identify specific pathways involved for further elucidating viral and cellular mechanisms of DNA maintenance important to the viral life cycle and vector utilizations.

Ataxia-Telangiectasia-Mutated-Dependent Activation of Ku in Human Fibroblasts Exposed to Hydrogen Peroxide

DNA is damaged in cells during cell replication, by infection, or by various environmental stresses. The damaged cells stop cell cycle, repair damaged DNA, and when repaired progress into the next cell cycle stage. But when the attempt to repair the damage fails, the cells undergo apoptosis. The most deleterious damage of all is double-strand DNA breaks (DSBs), where ATM (ataxia-telangiectasia-mutated) serves as a sensor. The ATM pathway culminates in DNA repair through nonhomologous end-joining or through homologous recombination. Upon DNA damage, the DNA repair protein Ku70/80 translocates into the nucleus, which may be mediated by ATM. Previously, we found that pancreatic acinar cells undergo apoptosis upon oxidative stress, and the cell death stems from nuclear loss of Ku70/80. This study aims to investigate whether ATM has a role in Ku activation and prevention of cell death induced by oxidative stress (hydrogen peroxide) using A-T fibroblasts stably transfected with human full-length ATM cDNA or empty vector. As a result, hydrogen peroxide-induced cell death was augmented in A-T cells transfected with empty vector while cell death was prevented in A-T fibroblasts stably transfected with human full-length ATM cDNA. Ku DNA-binding activity induced by hydrogen peroxide treatment was increased in the A-T fibroblasts stably transfected with human full-length ATM cDNA compared to that in A-T cells transfected with empty vector. The results suggest that ATM may be essential for Ku activation to repair DNA damage from oxidative stress and prevent cell death caused by oxidative stress.

Effect of sesamol on radiation-induced cytotoxicity in Swiss albino mice

The radio-protective ability of sesamol (SM) at various doses viz., 0, 10, 25, 40, 50, 70 and 100mg/kg bw, administered intraperitoneally 30min prior to 9.5Gy whole-body gamma-irradiation was studied in Swiss albino mice. Radiation toxicity and mortality were observed during a period of 30 days and the percentage mortality was calculated. SM pretreatment with 50mg/kg bw was found to be the most effective dose in maintaining body weight and in reducing the percentage mortality, while 100mg/kg bw was found to be more effective in maintaining the spleen index and in stimulation of endogenous spleen colony-forming units. Pretreatment with SM (50mg/kg bw) in mice irradiated with 15Gy significantly reduced dead, inflammatory, mitotic and goblet cells in irradiated jejunum. SM at 50mg/kg bw also increased crypt cells, maintained villus height, and prevented mucosal erosion. Nuclear enlargement in epithelial cells was found less in SM-treated mice compared with the irradiated control. The radiation-induced decrease in endogenous antioxidant enzymes (GSH, GST, catalase) and the increase in lipid peroxidation were also reduced by pretreatment with SM [50 and 100mg/kg bw] at all monitored post-irradiation intervals. There was no protection at a dose less than 25mg/kg bw.

Che-1 phosphorylation by ATM/ATR and Chk2 kinases activates p53 transcription and the G2/M checkpoint

Che-1 is a RNA polymerase II-binding protein involved in the transcription of E2F target genes and induction of cell proliferation. Here we show that Che-1 contributes to DNA damage response and that its depletion sensitizes cells to anticancer agents. The checkpoint kinases ATM/ATR and Chk2 interact with Che-1 and promote its phosphorylation and accumulation in response to DNA damage. These Che-1 modifications induce a specific recruitment of Che-1 on the TP53 and p21 promoters. Interestingly, it has a profound effect on the basal expression of p53, which is preserved following DNA damage. Notably, Che-1 contributes to the maintenance of the G2/M checkpoint induced by DNA damage. These findings identify a mechanism by which checkpoint kinases regulate responses to DNA damage.

The corepressor silencing mediator for retinoid and thyroid hormone receptor facilitates cellular recovery from DNA double-strand breaks

Cells are frequently challenged by DNA double-strand breaks (DSB) that threaten their normal function and survival. In mammalian cells, the repair of DSBs is predominantly mediated by the DNA-dependent protein kinase (DNA-PK) complex. We unexpectedly found that the corepressor silencing mediator for retinoid and thyroid hormone receptor (SMRT) associates with the DNA-PK repair complex. The SMRT/histone deacetylase 3 complex is required for the transcriptional repressive property of the Ku70 subunit of the repair complex. Moreover, SMRT, but not the related Nuclear Receptor Corepressor, is required for cellular recovery from DNA DSBs induced by ionizing radiation or DNA damage-inducing drugs. Thus, the corepressor SMRT plays a novel and critical role in the cellular response to DSBs.

Characterization of the breast cancer associated ATM 7271T>G (V2424G) mutation by gene expression profiling

Mutations in ATM are responsible for the autosomal recessive disorder ataxia telangiectasia. Heterozygous mutations in ATM have been associated with an elevated risk of breast cancer. We previously reported one breast cancer family in which ATM 7271T>G (V2424G) segregated with disease, and apparently acted in a dominant negative manner. We now report the screening of 782 multiple-case breast cancer families that identified two additional index cases with ATM 7271T>G. Phylogenetic sequence analysis showed that V2424 is a highly conserved residue, and that the 2424G variant is likely to interfere with function. To elucidate the consequences of this mutation, we expression profiled wild-type, heterozygous, and homozygous lymphoblastoid cell lines (LCLs) from Scottish and Australian families using an oligonucleotide microarray. Cluster analysis revealed 77 genes that were differentially expressed in homozygous and heterozygous V2424G cells (compared to wild-type) and 11 genes differentially expressed in the homozygous cells. We also evaluated the profiles of LCLs after exposure to ionizing radiation (IR) and identified 77 genes that were differentially expressed in wild-type cells, but not in homozygous or heterozygous V2424G cells. We validated the expression differences by RT-PCR in additional heterozygous V2424G LCLs from another breast cancer family. We found no consistent cytotoxicity or abrogation of ATM kinase activity after IR in seven heterozygous V2424G LCLs, compared to wild-type LCLs, but did find an increase in the number of chromosomal aberrations. These data suggest that the V2424G missense mutation acts largely as a dominant negative in terms of the associated expression profiles.

PARP-1 and Ku compete for repair of DNA double strand breaks by distinct NHEJ pathways

Poly(ADP-ribose)polymerase 1 (PARP-1) recognizes DNA strand interruptions in vivo and triggers its own modification as well as that of other proteins by the sequential addition of ADP-ribose to form polymers. This modification causes a release of PARP-1 from DNA ends and initiates a variety of responses including DNA repair. While PARP-1 has been firmly implicated in base excision and single strand break repair, its role in the repair of DNA double strand breaks (DSBs) remains unclear. Here, we show that PARP-1, probably together with DNA ligase III, operates in an alternative pathway of non-homologous end joining (NHEJ) that functions as backup to the classical pathway of NHEJ that utilizes DNA-PKcs, Ku, DNA ligase IV, XRCC4, XLF/Cernunnos and Artemis. PARP-1 binds to DNA ends in direct competition with Ku. However, in irradiated cells the higher affinity of Ku for DSBs and an excessive number of other forms of competing DNA lesions limit its contribution to DSB repair. When essential components of the classical pathway of NHEJ are absent, PARP-1 is recruited for DSB repair, particularly in the absence of Ku and non-DSB lesions. This form of DSB repair is sensitive to PARP-1 inhibitors. The results define the function of PARP-1 in DSB repair and characterize a candidate pathway responsible for joining errors causing genomic instability and cancer.

Conditional deletion of Nbs1 in murine cells reveals its role in branching repair pathways of DNA double-strand breaks

NBS1 forms a complex with MRE11 and RAD50 (MRN) that is proposed to act on the upstream of two repair pathways of DNA double-strand break (DSB), homologous repair (HR) and non-homologous end joining (NHEJ). However, the function of Nbs1 in these processes has not fully been elucidated in mammals due to the lethal phenotype of cells and mice lacking Nbs1. Here, we have constructed mouse Nbs1-null embryonic fibroblasts and embryonic stem cells, through the Cre-loxP and sequential gene targeting techniques. We show that cells lacking Nbs1 display reduced HR of the single DSB in chromosomally integrated substrate, affecting both homology-directed repair (HDR) and single-stranded annealing pathways, and, surprisingly, increased NHEJ-mediated sequence deletion. Moreover, focus formation at DSBs and chromatin recruitment of the Nbs1 partners Rad50 and Mre11 as well as Rad51 and Brca1 are attenuated in these cells, whereas the NHEJ molecule Ku70 binding to chromatin is not affected. These data provide a novel insight into the function of MRN in the branching of DSB repair pathways.

Developmental cell death in the liver and newborn lethality of Ku86 deficient mice suppressed by antioxidant N-acetyl-cysteine

Repair of DNA double-strand breaks (DSBs) is essential for genome integrity and cell survival. Ku86 is involved in the repair of DNA DSBs by non-homologous end joining (NHEJ). Mice deficient in Ku86 show growth retardation, dwarfism, premature aging, and immunodeficiency. In this study, we observed severely compromised survival of Ku86(-/-) mice, such that most Ku86(-/-) mice died within the first postnatal weeks and only 1.5% of the expected 25% from heterozygous crosses survived for 1 month. Since post-mortem analysis was not possible due to parental cannibalism, histopathological examination was performed on Ku86(-/-) fetuses to assess possible causes of newborn death. Eighty percent and 75% of Ku86(-/-) fetuses exhibited apoptosis and necrosis in the liver, while only 20% and 10% of Ku86(+/+) littermates had apoptosis and necrosis, respectively. In addition, the severity of liver damage was significantly higher in Ku86(-/-) fetuses. Developmental liver damage may have led to postnatal lethality because the fetal liver with pre-existing injury may not be able to undergo transformation from a lymphohematopoietic to an indispensable metabolic organ. Free radicals can cause chromosomal breaks and lead to cell death. We postulated that endogenous oxidative stress might be involved in the resulting liver damage and animal lethality in Ku86(-/-) mice deficient in DNA DSB repair. This hypothesis was tested by treating Ku86(-/-) mice with the well known free radical scavenger, thiol antioxidant N-acetyl-cysteine (NAC), during embryonic development. We found that a significantly higher percentage, 7.7% of NAC treated Ku86(-/-) offspring versus 1.5% untreated Ku86(-/-) mice were alive at 1 month of age. In addition, the incidence of liver necrosis decreased by 21% and the severity of necrosis significantly reduced. Thus, Ku86 deficiency results in severe developmental liver damage and newborn lethality associated with oxidative stress.

Telomeres: Cancer to Human Aging

The cell phenotypes of senescence and crisis operate to circumscribe the proliferative potential of mammalian cells, suggesting that both are capable of operating in vivo to suppress the formation of tumors. The key regulators of these phenotypes are the telomeres, which are located at the ends of chromosomes and operate to protect the chromosomes from end-to-end fusions. Telomere erosion below a certain length can trigger crisis. The relationship between senescence and telomere function is more complex, however: Cell-physiological stresses as well as dysfunction of the complex molecular structures at the ends of telomeric DNA can trigger senescence. Cells can escape senescence by inactivating the Rb and p53 tumor suppressor proteins and can surmount crisis by activating a telomere maintenance mechanism. The resulting cell immortalization is an essential component of the tumorigenic phenotype of human cancer cells. Here we discuss how telomeres are monitored and maintained and how loss of a functional telomere influences biological functions as diverse as aging and carcinogenesis.

Genomic mechanisms and measurement of structural and numerical instability in cancer cells

The progression to cancer is often associated with instability and the acquisition of genomic heterogeneity, generating both clonal and non-clonal populations. Chromosomal instability (CIN) describes the excessive rate of numerical and structural genomic change in tumors. Mitotic segregation errors strongly influences copy number, while structural aberrations can occur at unstable genomic regions, or through aberrant DNA repair or methylation. Combined molecular cytogenetic analyses can evaluate cell-to-cell variation, and define the complexity of numerical and structural alterations. Because structural change may occur independently of numerical alteration, we propose the term structural chromosomal instability [(S)-CIN] to distinguish numerical from structural CIN.

DNA repair polymorphisms might contribute differentially on familial and sporadic breast cancer susceptibility: a study on a Portuguese population

The purpose of this study was to evaluate the role of polymorphisms in DNA repair genes as genetic indicators of susceptibility to familial and sporadic breast cancer. We analysed DNA samples from 285 breast cancer patients and 442 control subjects, for XRCC1 Arg399Gln, XPD Lys751Gln, RAD51 G135C and XRCC3 Thr241Met polymorphisms using PCR-RFLP. We observed that women carriers of XRCC1 399Gln genotypes and without family history of breast cancer have a protective effect concerning this disease (OR = 0.54 95% CI 0.35-0.84; p = 0.006). Furthermore, we found that carriers of XRCC3 241Met genotypes without FH have an increased susceptibility of breast cancer (OR = 2.21 95% CI 1.42-3.44; p < 0.001). Additionally, we verified an increased risk of breast cancer in women with FH and carrying RAD51 135C genotypes (OR = 2.17 95% CI 1.19-3.98; p = 0.012). Our results suggest XRCC1 Arg399Gln and XRCC3 Thr241Met DNA repair polymorphisms as important biomarkers to sporadic breast cancer susceptibility, as well as, RAD51 G135C polymorphism as a real risk modifier in familial breast cancer cases.

Syndromes with genetic instability: model diseases for (skin) cancerogenesis

Tumorigenesis is a multi-step process in which exogenous and endogenous mutational events lead to accumulation of DNA mutations resulting in loss of cellular growth control. During the last decades knowledge about the molecular mechanisms of DNA mutations and the cellular systems that counteract the degeneration of the genome has increased rapidly. This expansion was driven in part by clinical and molecular investigations of syndromes with genetic in-stability. These syndromes are typically associated with an increased predisposition for tumor formation. They include nucleotide excision repair defective syndromes, mismatch repair defective syndromes, and the chromosome break-age syndromes. These diseases mirror vividly the phenotypic consequences of the failure of important cellular systems for the maintenance of the genomic integrity. They significantly contribute to our understanding of the molecular mechanisms of (skin) carcinogenesis and for the development of preventive and therapeutic anticancer strategies.

Interplay between Ino80 and Swr1 chromatin remodeling enzymes regulates cell cycle checkpoint adaptation in response to DNA damage

Ino80 and Swr1 are ATP-dependent chromatin remodeling enzymes that have been implicated in DNA repair. Here we show that Ino80 is required for cell cycle checkpoint adaptation in response to a persistent DNA double-strand break (DSB). The failure of cells lacking Ino80 to escape checkpoint arrest correlates with an inability to maintain high levels of histone H2AX phosphorylation and an increased incorporation of the Htz1p histone variant into chromatin surrounding the DSB. Inactivation of Swr1 eliminates this DNA damage-induced Htz1p incorporation and restores H2AX phosphorylation and checkpoint adaptation. We propose that Ino80 and Swr1 function antagonistically at chromatin surrounding a DSB, and that they regulate the incorporation of different histone H2A variants that can either promote or block cell cycle checkpoint adaptation.

Deficiency of cell cycle checkpoints and DNA repair system predispose individuals to esophageal cancer

Cell cycle checkpoints and DNA repair capacity are critical for the maintenance of genome integrity. We hypothesized that, in comparison to healthy controls, esophageal cancer patients might have a higher frequency of deficiencies in cell cycle checkpoints and/or DNA repair system. Using flow cytometry and comet assay, we assessed the gamma-radiation-induced S phase and G2-M phase accumulation, and benzo(a)pyrene-diol-epoxide (BPDE)- and gamma-radiation-induced DNA damage, in peripheral blood lymphocytes of 99 newly diagnosed esophageal cancer patients and 112 age-, gender-, and ethnicity-matched healthy controls. The mean gamma-radiation-induced cell accumulation at G2-M phase was significantly lower in esophageal cancer patients than the control subjects (case versus control: 5.27%+/-5.11% versus. 7.06%+/-5.04%, P = 0.013). The less G2-M phase cell accumulation resulted in a significant increased risk for esophageal cancer with an odds ratio of 2.08 (95% confidence interval 1.15-3.77). After normalization to baseline S fraction, the radiation-induced increment in the 4N/2N ratio was also significantly lower in esophageal cancer patients than in controls (case versus control: 0.76% versus 1.04%, P = 0.0039). The less increment in the radiation-induced 4N/2N ratio was associated with 2.24(95% confidence interval 1.22-4.11)-fold increase of esophageal cancer risk. We also compared the mutagen-induced DNA damage level among individuals with different S or G2-M phase cell accumulation. We found that the less G2-M phase accumulation was associated with both high BPDE induced and gamma-radiation-induced DNA damage in the healthy controls (P for trend = 0.023 and 0.015, respectively). Similar pattern was observed for S phase accumulation (P for trend = 0.033 and 0.022, respectively). However, such association was not seen in esophageal cancer patients. This study provides the first molecular epidemiologic evidence linking increased esophageal cancer risk with defects in cell-cycle checkpoints and DNA repair capacity.

Mutagen Sensitivity and Neoplastic Progression in Patients with Barrett's Esophagus: A Prospective Analysis

BACKGROUND

Defects in DNA damage recognition and repair have been associated with a wide variety of cancers. We conducted a prospective study to determine whether mutagen sensitivity, as determined by an in vitro assay, was associated with the future development of cancer in patients with Barrett's esophagus, which is associated with increased risk of progression to esophageal adenocarcinoma.

METHODS

We measured sensitivity to bleomycin in peripheral blood lymphocytes in a cohort of 220 patients with Barrett's esophagus. We followed these patients for 1,230 person-years (range, 3 months to 10.1 years; median, 6.4 years), using development of cancer and aneuploidy as end points. A subset of these patients was evaluated for inactivation of tumor-suppressor genes CDKN2A/p16 and TP53 [by mutation and loss of heterozygosity (LOH)] in their Barrett's segments at the time of, or before, the bleomycin test, and the patients were stratified by CDKN2A/p16 and TP53 status in an analysis of mutagen sensitivity and progression.

RESULTS

Bleomycin-sensitive patients were found to be at significantly greater risk of developing aneuploidy (adjusted hazard ratio, 3.71; 95% confidence interval, 1.44-9.53) and nonsignificantly greater risk of cancer (adjusted hazard ratio, 1.63; 95% confidence interval, 0.71-3.75). Among patients with detectable LOH at the TP53 locus (on chromosome 17p), increasing bleomycin sensitivity was associated with increased risk of developing cancer (P(trend) < 0.001) and aneuploidy (P(trend) = 0.005).

CONCLUSIONS

This study supports the hypothesis that sensitivity to mutagens increases the risk of neoplastic progression in persons with Barrett's esophagus, particularly those with 17p LOH including TP53.

Vanadium limits the expression of proliferating cell nuclear antigen and inhibits early DNA damage during diethylnitrosamine-induced hepatocellular preneoplasia in rats

Previous studies from our laboratory have shown that vanadium stabilizes xenobiotic metabolizing enzymes and antioxidant status and suppresses DNA-protein crosslinks during chemically-induced hepatocarcinogenesis in rats. In the present study, we have further investigated the in vivo antitumor potential of this micronutrient by determining the effect of 0.5 ppm vanadium in drinking water on biomarkers for the early stages of hepatocarcinogenesis; the biomarkers included gamma-glutamyl transpeptidase (GGT)-positive foci and glycogen-storage foci, in situ expression of proliferating cell nuclear antigen (PCNA), and genotoxic DNA damage assessed by the alkaline Comet assay. Histomorphometry also was assessed during the study. Hepatocarcinogenesis was induced by treating 4-week-old male Sprague-Dawley rats with a single, necrogenic, intraperitoneal (i.p.) injection of 200 mg/kg body weight diethylnitrosamine (DEN). Compared to the carcinogen control, vanadium administration over the 32 weeks of the experiment reduced the relative liver weight by 30%, the incidence of nodules by 69.34%, the total number and multiplicity of nodules by 80.77%, and remodeled the hepatocellular premalignant architecture towards a normal phenotype. Moreover, long-term vanadium treatment reduced the development of GGT foci by 76.2% (P < 0.001), decreased periodic acid-Schiff's reactivity by 59.49% (P < 0.01), and decreased PCNA expression, with the concomitant reduction in PCNA immunolabeling index by 93.36% (P < 0.001). Finally, vanadium inhibited early DNA damage (DNA strand-breaks) in DEN-treated rat hepatocytes as expressed in the Comet assay by a 60.04% reduction in the length:width value of DNA mass (P < 0.01) and a 51.54% reduction in the tail length of the DNA comets (P < 0.001). Our results indicate that continuous supplementation with 0.5 ppm vanadium suppresses hepatocellular neoplastic transformation in rats.

Cell cycle control and beyond: emerging roles for the retinoblastoma gene family

Rb family proteins (pRb/p105, Rb2/p130 and p107) play a key role in cell cycle control and are worthily involved in transcription repression and tumor suppression. The mechanisms of transcriptional activation and repression by the Rb gene family has been extensively investigated: pRb, pRb2/p130 and p107 interact with different E2F family factors and can inhibit E2F responsive promoters, interfering with progression of cell cycle, gene transcription, initiation of apoptotic process and cell differentiation. Recent studies have indicated that Rb and Rb2/p130 may be involved in cellular response to DNA damage events, by influencing the transcription of factors involved in DNA repair pathways. In particular, evidences suggest that Rb loss and target gene deregulation impacts on the repair of UV-induced pyrimidine pyrimidone photoproducts (6-4 PP) by regulating the expression of several DNA damage factors involved in UV DNA damage repair processes, including proliferating cell nuclear antigen. Ongoing studies are focused on the mechanisms by which Rb family genes drive cell cycle exit following DNA damage induction, and how Rb gene family's interaction with chromatin remodeling factors can influence DNA repair dynamics.

Evidence that hydrogen sulfide is a genotoxic agent

Hydrogen sulfide (H2S) produced by commensal sulfate-reducing bacteria, which are often members of normal colonic microbiota, represents an environmental insult to the intestinal epithelium potentially contributing to chronic intestinal disorders that are dependent on gene-environment interactions. For example, epidemiologic studies reveal either persistent sulfate-reducing bacteria colonization or H2S in the gut or feces of patients suffering from ulcerative colitis and colorectal cancer. However, a mechanistic model that explains the connection between H2S and ulcerative colitis or colorectal cancer development has not been completely formulated. In this study, we examined the chronic cytotoxicity of sulfide using a microplate assay and genotoxicity using the single-cell gel electrophoresis (SCGE; comet assay) in Chinese hamster ovary (CHO) and HT29-Cl.16E cells. Sulfide showed chronic cytotoxicity in CHO cells with a %C1/2 of 368.57 micromol/L. Sulfide was not genotoxic in the standard SCGE assay. However, in a modified SCGE assay in which DNA repair was inhibited, a marked genotoxic effect was observed. A sulfide concentration as low as 250 micromol/L (similar to that found in human colon) caused significant genomic DNA damage. The HT29-Cl.16E colonocyte cell line also exhibited increased genomic DNA damage as a function of Na2S concentration when DNA repair was inhibited, although these cells were less sensitive to sulfide than CHO cells. These data indicate that given a predisposing genetic background that compromises DNA repair, H2S may lead to genomic instability or the cumulative mutations found in adenomatous polyps leading to colorectal cancer.

HIV-1 Vpr induces ATM-dependent cellular signal with enhanced homologous recombination

An ATM-dependent cellular signal, a DNA-damage response, has been shown to be involved during infection of human immunodeficiency virus type-1 (HIV-1), and a high incidence of malignant tumor development has been observed in HIV-1-positive patients. Vpr, an accessory gene product of HIV-1, delays the progression of the cell cycle at the G2/M phase, and ATR-Chk1-Wee-1, another DNA-damage signal, is a proposed cellular pathway responsible for the Vpr-induced cell cycle arrest. In this study, we present evidence that Vpr also activates ATM, and induces expression of gamma-H2AX and phosphorylation of Chk2. Strikingly, Vpr was found to stimulate the focus formation of Rad51 and BRCA1, which are involved in repair of DNA double-strand breaks (DSBs) by homologous recombination (HR), and biochemical analysis revealed that Vpr dissociates the interaction of p53 and Rad51 in the chromatin fraction, as observed under irradiation-induced DSBs. Vpr was consistently found to increase the rate of HR in the locus of I-SceI, a rare cutting-enzyme site that had been introduced into the genome. An increase of the HR rate enhanced by Vpr was attenuated by an ATM inhibitor, KU55933, suggesting that Vpr-induced DSBs activate ATM-dependent cellular signal that enhances the intracellular recombination potential. In context with a recent report that KU55933 attenuated the integration of HIV-1 into host genomes, we discuss the possible role of Vpr-induced DSBs in viral integration and also in HIV-1 associated malignancy.

Development of a Screening Assay for Surrogate Markers of Chk1 Inhibitor-Induced Cell Cycle Release

Chk1 is a key regulator of the S and G2/M checkpoints and is activated following DNA damage by agents such as the topoisomerase I inhibitor camptothecin (CPT). It has been proposed that Chk1 inhibitors used in combination with such a DNA damaging agent to treat tumors would potentiate cytotoxicity and increase the therapeutic index, particularly in tumors lacking functional p53. The aim of this study was to determine whether gene expression analysis could be used to inform lead optimization of a novel series of Chk1 inhibitors. The candidate small-molecule Chk1 inhibitors were used in combination with CPT to identify potential markers of functional Chk1 inhibition, as well as resulting cell cycle progression, using cDNA-based microarrays. Differential expression of several of these putative marker genes was further validated by RT-PCR for use as a medium-throughput assay. In the presence of DNA damage, Chk1 inhibitors altered CPT-dependent effects on the expression of cell cycle and DNA repair genes in a manner consistent with a Chk1-specific mechanism of action. Furthermore, differential expression of selected marker genes, cyclin E2, EGR1, and DDIT3, was dose dependent for Chk1 inhibition. RT-PCR results for these genes following treatment with a panel of Chk1 inhibitors showed a strong correlation between marker gene response and the ability of each compound to abrogate cell cycle arrest in situ following CPT-induced DNA damage. These results demonstrate the utility of global expression analysis to identify surrogate markers, providing an alternative method for rapid compound characterization to support advancement decisions in early drug discovery.

MYC can induce DNA breaks in vivo and in vitro independent of reactive oxygen species

MYC overexpression is thought to initiate tumorigenesis by inducing cellular proliferation and growth and to be restrained from causing tumorigenesis by inducing cell cycle arrest, cellular senescence, and/or apoptosis. Here we show that MYC can induce DNA breaks both in vitro and in vivo independent of increased production of reactive oxygen species (ROS). We provide an insight into the specific circumstances under which MYC generates ROS in vitro and propose a possible mechanism. We found that MYC induces DNA double-strand breaks (DSBs) independent of ROS production in murine lymphocytes in vivo as well as in normal human foreskin fibroblasts (NHFs) in vitro in normal (10%) serum, as measured by gammaH2AX staining. However, NHFs cultured in vitro in low serum (0.05%) and/or ambient oxygen saturation resulted in ROS-associated oxidative damage and DNA single-strand breaks (SSBs), as measured by Ape-1 staining. In NHFs cultured in low versus normal serum, MYC induced increased expression of CYP2C9, a gene product well known to be associated with ROS production. Specific inhibition of CYP2C9 by small interfering RNA was shown to partially inhibit MYC-induced ROS production. Hence, MYC overexpression can induce ROS and SSBs under some conditions, but generally induces widespread DSBs in vivo and in vitro independent of ROS production.

Characterization of gene rearrangements leading to activation of MDR-1

Expression of the MDR-1/P-glycoprotein gene confers drug resistance both in vitro and in vivo. We previously reported that gene rearrangements resulting in a hybrid MDR-1 transcript represent a common mechanism for acquired activation of MDR-1/P-glycoprotein. We have identified hybrid MDR-1 transcripts in nine MDR-1-overexpressing cell lines and two patients with relapsed ALL. We characterize these rearrangements as follows. 1) Non-MDR-1 sequences in the hybrid MDR-1 transcripts are expressed in unselected cell lines, showing that these sequences are constitutively expressed. 2) The rearrangements occur randomly and involve partner genes (sequences) on chromosome 7 and on chromosomes other than 7. Breakpoints have been characterized in six cell lines. In one, the rearrangement occurred within intron 2 of MDR-1; in the other five, the rearrangement occurred 24 to >96 kb 5' of the normal start of transcription of MDR-1. In one cell line, homologous recombination involving an Alu repeat was observed. However, in the remaining five cell lines, nonhomologous recombination was observed. 3) The rearrangements arise during drug selection. The acquired rearrangements are not detected in parental cells. 4) Five of the six active promoters that captured MDR-1 controlled MDR-1 from a distance of 29 to more than 110 kb 5' to MDR-1. Transcription was initiated in an antegrade or retrograde direction. We conclude that drug selection with natural products targeting DNA or microtubules leads to DNA damage, nonhomologous recombination, and acquired drug resistance, wherein MDR-1 expression is driven by a random but constitutively active promoter.

Escherichia coli induces DNA double-strand breaks in eukaryotic cells

Transient infection of eukaryotic cells with commensal and extraintestinal pathogenic Escherichia coli of phylogenetic group B2 blocks mitosis and induces megalocytosis. This trait is linked to a widely spread genomic island that encodes giant modular nonribosomal peptide and polyketide synthases. Contact with E. coli expressing this gene cluster causes DNA double-strand breaks and activation of the DNA damage checkpoint pathway, leading to cell cycle arrest and eventually to cell death. Discovery of hybrid peptide-polyketide genotoxins in E. coli will change our view on pathogenesis and commensalism and open new biotechnological applications.

DNA damage-induced cell cycle regulation and function of novel Chk2 phosphoresidues

Chk2 kinase is activated by DNA damage to regulate cell cycle arrest, DNA repair, and apoptosis. Phosphorylation of Chk2 in vivo by ataxia telangiectasia-mutated (ATM) on threonine 68 (T68) initiates a phosphorylation cascade that promotes the full activity of Chk2. We identified three serine residues (S19, S33, and S35) on Chk2 that became phosphorylated in vivo rapidly and exclusively in response to ionizing radiation (IR)-induced DNA double-strand breaks in an ATM- and Nbs1-dependent but ataxia telangiectasia- and Rad3-related-independent manner. Phosphorylation of these residues, restricted to the G(1) phase of the cell cycle, was induced by a higher dose of IR (>1 Gy) than that required for phosphorylation of T68 (0.25 Gy) and declined by 45 to 90 min, concomitant with a rise in Chk2 autophosphorylation. Compared to the wild-type form, Chk2 with alanine substitutions at S19, S33, and S35 (Chk2(S3A)) showed impaired dimerization, defective auto- and trans-phosphorylation activities, and reduced ability to promote degradation of Hdmx, a phosphorylation target of Chk2 and regulator of p53 activity. Besides, Chk2(S3A) failed to inhibit cell growth and, in response to IR, to arrest G(1)/S progression. These findings underscore the critical roles of S19, S33, and S35 and argue that these phosphoresidues may serve to fine-tune the ATM-dependent response of Chk2 to increasing amounts of DNA damage.

DNA repair pathways in clinical practice: lessons from pediatric cancer susceptibility syndromes

Human cancers exhibit genomic instability and an increased mutation rate due to underlying defects in DNA repair. Cancer cells are often defective in one of six major DNA repair pathways, namely: mismatch repair, base excision repair, nucleotide excision repair, homologous recombination, nonhomologous endjoining and translesion synthesis. The specific DNA repair pathway affected is predictive of the kinds of mutations, the tumor drug sensitivity, and the treatment outcome. The study of rare inherited DNA repair disorders, such as Fanconi anemia, has yielded new insights to drug sensitivity and treatment of sporadic cancers, such as breast or ovarian epithelial tumors, in the general population. The Fanconi anemia pathway is an example of how DNA repair pathways can be deregulated in cancer cells and how biomarkers of the integrity of these pathways could be useful as a guide to cancer management and may be used in the development of novel therapeutic agents.