DNA damage-dependent nuclear dynamics of the Mre11 complex

The Mre11 complex mediates the S-phase checkpoint through an interaction with replication protein A

The Mre11/Rad50/Nbs1 complex (MRN) plays an essential role in the S-phase checkpoint. Cells derived from patients with Nijmegen breakage syndrome and ataxia telangiectasia-like disorder undergo radioresistant DNA synthesis (RDS), failing to suppress DNA replication in response to ionizing radiation (IR). How MRN affects DNA replication to control the S-phase checkpoint, however, remains unclear. We demonstrate that MRN directly interacts with replication protein A (RPA) in unperturbed cells and that the interaction is regulated by cyclin-dependent kinases. We also show that this interaction is needed for MRN to correctly localize to replication centers. Abolishing the interaction of Mre11 with RPA leads to pronounced RDS without affecting phosphorylation of Nbs1 or SMC1 following IR. Moreover, MRN is recruited to sites at or adjacent to replication origins by RPA and acts there to inhibit new origin firing upon IR. These studies suggest a direct role of MRN at origin-proximal sites to control DNA replication initiation in response to DNA damage, thereby providing an important mechanism underlying the intra-S-phase checkpoint in mammalian cells.

Lead contamination results in late and slowly repairable DNA double-strand breaks and impacts upon the ATM-dependent signaling pathways

Despite a considerable amount of data, evaluation of the potential genotoxicity and cancer proneness of lead compounds remains unclear, probably due to the plethora of experimental procedures, biological endpoints and cellular models used. In parallel, the understanding in DNA damage formation, repair and signaling has considerably progressed all along these last years, notably for DNA double-strand breaks (DSBs). Here, were examined DNA damage formation and repair in human cells exposed to lead nitrate (Pb(NO(3))(2)) and their consequences upon the ATM-dependent stress signaling, cell cycle progression and cell death. As observed with anti-pH2AX immunofluorescence, exposure to Pb(NO(3))(2) results in formation of late DSBs, that would not originate from conversion of nucleotide damage but likely by a direct production of single-strand breaks. Lead contamination inhibits non-homologous end-joining repair process by preventing the DNA-PK kinase activity whereas the MRE11-dependent repair pathway is exacerbated. Lead contamination triggers successive synchronization of cells in G2/M phase in which the RAD51-dependent homologous recombination was found to be activated. Altogether, our findings support that lead contamination generates late unrepairable DSBs that impact upon the ATM-dependent stress signaling pathway by favoring propagation of errors. Such findings should help to consider more carefully the biological action of lead compounds in the frame of public and occupational exposures.

ATR pathway is the primary pathway for activating G2/M checkpoint induction after re-replication

DNA replication is tightly controlled to ensure accurate chromosome duplication and segregation in each cell cycle. Inactivation of Geminin, an inhibitor of origin licensing, leads to re-replication in human tumor cells within the same cell cycle and triggers a G(2)/M checkpoint. We find that the primary pathway to signal that re-replication has been detected is the ATR kinase and the Rad9-Rad1-Hus1 (9-1-1) clamp complex together with Rad17-RFC clamp loader. ATM kinase and the Mre11-Rad50-Nbs1 complex do not appear to play significant roles in the checkpoint. Chk1 activation occurs at early stages, whereas Chk2 activation occurs much later. Overall we conclude that ATR/Chk1 pathway is activated at an early time point after the loss of Geminin and contributes to checkpoint arrest essential for the accumulation of re-replicated cells, whereas activation of the ATM/Chk2 pathway is a by-product of DNA re-replication at a later period.

The Mre11-Rad50-Nbs1 complex acts both upstream and downstream of ataxia telangiectasia mutated and Rad3-related protein (ATR) to regulate the S-phase checkpoint following UV treatment

The Mre11-Rad50-Nbs1 (MRN) complex is required for mediating the S-phase checkpoint following UV treatment, but the underlying mechanism is not clear. Here we demonstrate that at least two mechanisms are involved in regulating the S-phase checkpoint in an MRN-dependent manner following UV treatment. First, when replication forks are stalled, MRN is required upstream of ataxia telangiectasia mutated and Rad3-related protein (ATR) to facilitate ATR activation in a substrate and dosage-dependent manner. In particular, MRN is required for ATR-directed phosphorylation of RPA2, a critical event in mediating the S-phase checkpoint following UV treatment. Second, MRN is a downstream substrate of ATR. Nbs1 is phosphorylated by ATR at Ser-343 when replication forks are stalled, and this phosphorylation event is also important for down-regulating DNA replication following UV treatment. Moreover, we demonstrate that MRN and ATR/ATR-interacting protein (TRIP) interact with each other, and the forkhead-associated/breast cancer C-terminal domains (FHA/BRCT) of Nbs1 play a significant role in mediating this interaction. Mutations in the FHA/BRCT domains do not prevent ATR activation but specifically impair ATR-mediated Nbs1 phosphorylation at Ser-343, which results in a defect in the S-phase checkpoint. These data suggest that MRN plays critical roles both upstream and downstream of ATR to regulate the S-phase checkpoint when replication forks are stalled.

SIRT1 regulates the function of the Nijmegen breakage syndrome protein

MRE11-RAD50-NBS1 (MRN) is a conserved nuclease complex that exhibits properties of a DNA damage sensor and is critical in regulating cellular responses to DNA double-strand breaks. NBS1, which is mutated in the human genetic disease Nijmegen breakage syndrome, serves as the regulatory subunit of MRN. Phosphorylation of NBS1 by the ATM kinase is necessary for both activation of the S phase checkpoint and for efficient DNA damage repair response. Here, we report that NBS1 is an acetylated protein and that the acetylation level is tightly regulated by the SIRT1 deacetylase. SIRT1 associates with the MRN complex and, importantly, maintains NBS1 in a hypoacetylated state, which is required for ionizing radiation-induced NBS1 Ser343 phosphorylation. Our results demonstrate the presence of crosstalk between two different posttranslational modifications in NBS1 and strongly suggest that deacetylation of NBS1 by SIRT1 plays a key role in the dynamic regulation of the DNA damage response and in the maintenance of genomic stability.

The characterization of Saccharomyces cerevisiae Mre11/Rad50/Xrs2 complex reveals that Rad50 negatively regulates Mre11 endonucleolytic but not the exonucleolytic activity

The evolutionarily conserved heterotrimeric Mre11/Rad50/Xrs2 (Nbs1) (MRX/N) complex plays a central role in an array of cellular responses involving DNA damage, telomere length homeostasis, cell-cycle checkpoint control and meiotic recombination. The underlying biochemical functions of MRX/N complex, or each of its individual subunits, at telomeres and the importance of complex formation are poorly understood. Here, we show that the Saccharomyces cerevisiae MRX complex, or its subunits, display an overwhelming preference for G-quadruplex DNA than for telomeric single-stranded or double-stranded DNA implicating the possible existence of this DNA structure in vivo. Although these alternative DNA substrates failed to affect Rad50 ATPase activity, kinetic analyses revealed that interaction of Rad50 with Xrs2 and/or Mre11 led to a twofold increase in the rates of ATP hydrolysis. Significantly, we show that Mre11 displays sequence-specific double-stranded DNA endonuclease activity, and Rad50, but not Xrs2, abrogated endonucleolytic but not the exonucleolytic activity. This repression was alleviated upon ATP hydrolysis by Rad50, suggesting that complex formation between Rad50 and Mre11 might be important for blocking the inappropriate cleavage of genomic DNA. Mre11 alone, or in the presence of ATP, MRX, MR or MX sub-complexes cleaved at the 5' end of an array of G residues in single-stranded DNA, at G quartets in G4 DNA, and at the center of TGTG repeats in duplex DNA. We propose that negative regulation of Mre11 endonuclease activity by Rad50 might be important for native as well as de novo telomere length homeostasis.

Cockayne syndrome protein B interacts with and is phosphorylated by c-Abl tyrosine kinase

The Cockayne Syndrome group B (CSB) protein plays important roles in transcription, transcription-coupled nucleotide excision repair and base excision DNA repair. c-Abl kinase also plays a role in DNA repair as a regulator/coordinator of the DNA damage response. This study presents evidence that the N-terminal region of CSB interacts with the SH3 domain of c-Abl in vitro and in vivo. In addition, c-Abl kinase phosphorylates CSB at Tyr932. The subcellular localization of CSB to the nucleus and nucleolus is altered after phosphorylation by c-Abl. c-Abl-dependent phosphorylation of CSB increased in cells treated with hydrogen peroxide and decreased in cells pre-treated with STI-571, a c-Abl-specific protein kinase inhibitor. Activation of the c-Abl kinase in response to oxidative damage is not observed in CSB null cells. These results suggest that c-Abl and CSB may regulate each other in a reciprocal manner in response to oxidative stress.

Mre11 and Ku regulation of double-strand break repair by gene conversion and break-induced replication

The yeast Mre11-Rad50-Xrs2 (MRX) and Ku complexes regulate single-strand resection at DNA double-strand breaks (DSB), a key early step in homologous recombination (HR). A prior plasmid gap repair study showed that mre11 mutations, which slow single-strand resection, reduce gene conversion tract lengths and the frequency of associated crossovers. Here we tested whether mre11Delta or nuclease-defective mre11 mutations reduced gene conversion tract lengths during HR between homologous chromosomes in diploid yeast. We found that mre11 mutations reduced the efficiency of HR but did not reduce tract lengths or crossovers, despite substantially reduced end-resection at the test (ura3) locus. End-resection is increased in yku70Delta, but this change also had no effect on tract lengths. Thus, heteroduplex formation and tract lengths are not regulated by the extent of end-resection during DSB repair in a chromosomal context. In a plasmid-chromosome DSB repair assay, tract lengths were again similar in wild-type and mre11Delta, but they were reduced in mre11Delta in a gap repair assay. These results indicate that tract lengths are not affected by the extent of end processing when broken ends can invade nearby sites, perhaps because MRX coordination of the two broken ends is dispensable when ends invade nearby sites. Although HR outcome was largely unaffected in mre11 mutants, break-induced replication (BIR) and chromosome loss increased, suggesting that Mre11 function in mitotic HR is limited to early HR stages. Interestingly, yku70Delta suppressed BIR in mre11 mutants. BIR is also elevated in rad51 mutants, but yku70Delta did not suppress BIR in a rad51 background. These results indicate that Mre11 functions in Rad51-independent BIR, and that Ku functions in Rad51-dependent BIR.

DNA damage-induced signalling in ataxia-telangiectasia and related syndromes

ATM, the protein mutated in the human genetic disorder ataxia-telangiectasia, functions by responding to radiation damage to DNA, primarily DNA double strand breaks (dsb), to reduce the risk of genome instability, cancer and neurodegeneration. ATM is rapidly activated as an existing protein to phosphorylate a number of downstream proteins that are involved in DNA repair and cell cycle checkpoint activation. While the exact mechanism of activation of ATM has not been determined, it is now evident that it relies heavily on the Mre11 complex (Mre11/Rad50/Nbs1) and a series of post-translational events for this activation. The Mre11 complex acts as a sensor for the break, recruits ATM to this site where it is autophosphorylated and then is capable of phosphorylating substrates that participate in DNA repair and cell cycle control. A greater understanding of how ATM is activated and functions through different signalling pathways is paramount to devising therapeutic strategies for the treatment of A-T patients. This knowledge can also be used to advantage in sensitizing cells to radiation and ultimately deriving novel therapeutic approaches for the treatment of cancer.

The cellular Mre11 protein interferes with adenovirus E4 mutant DNA replication

Adenovirus type 5 (Ad5) relocalizes and degrades the host DNA repair protein Mre11, and efficiently initiates viral DNA replication. Mre11 associates with Ad E4 mutant DNA replication centers and is important for concatenating viral genomes. We have investigated the role of Mre11 in the E4 mutant DNA replication defect. RNAi-mediated knockdown of Mre11 dramatically rescues E4 mutant DNA replication in cells that do or do not concatenate viral genomes, suggesting that Mre11 inhibits DNA replication independent of genome concatenation. The mediator of DNA damage checkpoint 1 (Mdc1) protein is involved in recruiting and sustaining Mre11 at sites of DNA damage following ionizing radiation. We observe foci formation by Mdc1 in response to viral infection, indicating that this damage response protein is activated. However, knockdown of Mdc1 does not prevent Mre11 from localizing at viral DNA replication foci or rescue E4 mutant DNA replication. Our results are consistent with a model in which Mre11 interferes with DNA replication when it is localized at viral DNA replication foci.

Radiosensibilité intrinsèque et cassures double–brin de l'ADN dans les cellules humaines

Among the large spectrum of DNA damage induced by radiation, DNA double-strand breaks (DSBs) are considered, to date, as the key-lesions responsible for the cell killing. However, although it was always intuitive to radiobiologists, such a conclusion has only been reached after technical developments and conceptual advances and remains consensual rather than demonstrated formally. In this article, we have reviewed the results that have lead to the conclusion that the assessment of successful DSB repair can be the basis of reliable assays predictive of the clinical response to radiotherapy and some chemotherapeutic treatments. We have discussed a number of technical artifacts, the biases due to the extrapolation of data obtained in yeast and rodent model systems to the human situation and the variety of phenotypes observed in human cells and in particular: 1) the most recent techniques developed, based on immunofluorescence, which have revolutionized our understanding of the molecular events occurring early after irradiation but have also raised the crucial questions about the choice of techniques to assess DSB repair and their specificity for different steps of the repair process; 2) While the homologous recombination repair pathway is predominant in yeasts, its importance in human cells appears less obvious, and raises the problem that the existence of randomized repair events may produce many more errors in human cells than in small genome organisms; 3) the impairment of DSB repair is observed in a plethora of genetic diseases, leading to radiosensitivity, immunodeficiency and sometimes cancer-proneness, but the low frequency and the pleiotropism of such diseases makes difficult the development of a single predictive assay. Therefore, although complete DSB repair appears to be crucial for cell survival, further research is still needed to provide innovative techniques fro measuring repair which can be successfully transferred to the clinic and used to ensure the avoidance of deleterious side-effects to cancer therapies.

Homologous recombination repair is regulated by domains at the N- and C-terminus of NBS1 and is dissociated with ATM functions

The proteins responsible for radiation sensitive disorders, NBS1, kinase ataxia-telangiectasia-(A-T)-mutated (ATM) and MRE11, interact through the C-terminus of NBS1 in response to the generation of DNA double-strand breaks (DSBs) and are all implicated in checkpoint regulation and DSB repair, such as homologous recombination (HR). We measured the ability of several NBS1 mutant clones and A-T cells to regulate HR repair using the DR-GFP or SCneo systems. ATM deficiency did not reduce the HR repair frequency of an induced DSB, and it was confirmed by findings that HR frequencies are only slightly affected by deletion of ATM-binding site at the extreme C-terminus of NBS1. In contrast, The HR-regulating ability is dramatically reduced by deletion of the MRE11-binding domain at the C-terminus of NBS1 and markedly inhibited by mutations in the FHA/BRCT domains at the N-terminus. This impaired capability in HR is consistent with a failure to observe MRE11 foci formation. Furthermore, normal HR using sister chromatid was completely inhibited by the absence of FHA/BRCT domains. These results suggested that the N- and C-terminal domains of NBS1 are the major regulatory domains for HR pathways, very likely through the recruitment and retention of the MRE11 nuclease to DSB sites in an ATM-independent fashion.

The contribution of cytolethal distending toxin to bacterial pathogenesis

Cytolethal distending toxin (CDT) is a bacterial toxin that initiates a eukaryotic cell cycle block at the G2 stage prior to mitosis. CDT is produced by a number of bacterial pathogens including: Campylobacter species, Escherichia coli, Salmonella enterica serovar Typhi, Shigella dystenteriae, enterohepatic Helicobacter species, Actinobacillus actinomycetemcomitans (the cause of aggressive periodontitis), and Haemophilus ducreyi (the cause of chancroid). The functional toxin is composed of three proteins; CdtB potentiates a cascade leading to cell cycle block, and CdtA and CdtC function as dimeric subunits, which bind CdtB and delivers it to the mammalian cell interior. Once inside the cell, CdtB enters the nucleus and exhibits a DNase I-like activity that results in DNA double-strand breaks. The eukaryotic cell responds to the DNA double-strand breaks by initiating a regulatory cascade that results in cell cycle arrest, cellular distension, and cell death. Mutations in CdtABC that cause any of the three subunits to lose function prevent the bacterial cell from inducing cytotoxicity. The result of CDT activity can differ somewhat depending on the eukaryotic cell types affected. Epithelial cells, endothelial cells, and keratinocytes undergo G2 cell cycle arrest, cellular distension, and death; fibroblasts undergo G1 and G2 arrest, cellular distension, and death; and immune cells undergo G2 arrest followed by apoptosis. CDT contributes to pathogenesis by inhibiting both cellular and humoral immunity via apoptosis of immune response cells, and by generating necrosis of epithelial-type cells and fibroblasts involved in the repair of lesions produced by pathogens resulting in slow healing and production of disease symptoms. Thus, CDT may function as a virulence factor in pathogens that produce the toxin.

Heat-induced phosphorylation of NBS1 in human skin fibroblast cells

NBS1 is known to be involved in DNA damage-induced cellular responses after exposure to ionizing radiation (IR). Phosphorylation of NBS1 contributes to cell-cycle checkpoints. The aim of this study was to determine whether heat exposure induces or stimulates cellular responses mediated by the phosphorylation of NBS1 in human skin fibroblast cell lines. The results of immunofluorescent staining and Western blot analysis showed that NBS1 proteins are phosphorylated after exposure to heat in the nucleus of a normal skin fibroblast cell line (82-6 cells). This suggests that the NBS1-mediated signal transduction could be induced by heat. We further examined whether a deficiency in the NBS1 protein modifies heat sensitivity in human skin fibroblast cell lines. A skin fibroblast cell line (Gmtert), derived from a Nijmegen breakage syndrome (NBS) patient containing mutant NBS1, showed higher sensitivity to heat than the same cell line transfected with the wild-type copy of the NBS1 gene. We also showed that transfection of a DNA cassette expressing small interference RNA (siRNA) targeted to NBS1 into 82-6 cells enhanced cell sensitivity to heat. These results suggest that NBS1 is involved in cellular responses to DNA damage which is induced by heat exposure as well as by radiation exposure in human skin fibroblast cells.

Oncogene-induced senescence is a DNA damage response triggered by DNA hyper-replication

Early tumorigenesis is associated with the engagement of the DNA-damage checkpoint response (DDR). Cell proliferation and transformation induced by oncogene activation are restrained by cellular senescence. It is unclear whether DDR activation and oncogene-induced senescence (OIS) are causally linked. Here we show that senescence, triggered by the expression of an activated oncogene (H-RasV12) in normal human cells, is a consequence of the activation of a robust DDR. Experimental inactivation of DDR abrogates OIS and promotes cell transformation. DDR and OIS are established after a hyper-replicative phase occurring immediately after oncogene expression. Senescent cells arrest with partly replicated DNA and with DNA replication origins having fired multiple times. In vivo DNA labelling and molecular DNA combing reveal that oncogene activation leads to augmented numbers of active replicons and to alterations in DNA replication fork progression. We also show that oncogene expression does not trigger a DDR in the absence of DNA replication. Last, we show that oncogene activation is associated with DDR activation in a mouse model in vivo. We propose that OIS results from the enforcement of a DDR triggered by oncogene-induced DNA hyper-replication.

In situ analysis of DNA damage response and repair using laser microirradiation

A proper response to DNA damage is critical for the maintenance of genome integrity. However, it is difficult to study the in vivo kinetics and factor requirements of the damage recognition process in mammalian cells. In order to address how the cell reacts to DNA damage, we utilized a second harmonic (532 nm) pulsed Nd:YAG laser to induce highly concentrated damage in a small area in interphase cell nuclei and cytologically analyzed both protein recruitment and modification. Our results revealed for the first time the sequential recruitment of factors involved in two major DNA double-strand break (DSB) repair pathways, non-homologous end-joining (NHEJ) and homologous recombination (HR), and the cell cycle-specific recruitment of the sister chromatid cohesion complex cohesin to the damage site. In this chapter, the strategy developed to study the DNA damage response using the 532-nm Nd:YAG laser will be summarized.

Nijmegen Breakage Syndrome

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disease, characterized by microcephaly, growth retardation, immunodeficiency, chromosome instability, radiation sensitivity, and a strong predisposition to lymphoid malignancy. The gene responsible for the development of this syndrome (NBS1) was mapped on chromosome 8q21. The product of this gene--nibrin--is a protein with 95 kDa molecular weight (p95). The same mutation in the NBS1 gene (deletion 657del5) was detected in most of the evaluated patients. In this chapter, we describe the analysis of the literature and our results on clinical and immunological features and genetic evaluation of 21 NBS patients.

Human cytomegalovirus disrupts both ataxia telangiectasia mutated protein (ATM)- and ATM-Rad3-related kinase-mediated DNA damage responses during lytic infection

Many viruses (herpes simplex virus type 1, polyomavirus, and human immunodeficiency virus type 1) require the activation of ataxia telangiectasia mutated protein (ATM) and/or Mre11 for a fully permissive infection. However, the longer life cycle of human cytomegalovirus (HCMV) may require more specific interactions with the DNA repair machinery to maximize viral replication. A prototypical damage response to the double-stranded ends of the incoming linear viral DNA was not observed in fibroblasts at early times postinfection (p.i.). Apparently, a constant low level of phosphorylated ATM was enough to phosphorylate its downstream targets, p53 and Nbs1. p53 was the only cellular protein observed to relocate at early times, forming foci in infected cell nuclei between 3.5 and 5.5 h p.i. Approximately half of these foci localized with input viral DNA, and all localized with viral UL112/113 prereplication site foci. No other DNA repair proteins localized with the virus or prereplication foci in the first 24 h p.i. When viral replication began in earnest, between 24 and 48 h p.i., there were large increases in steady-state levels and phosphorylation of many proteins involved in the damage response, presumably triggered by ATM-Rad3-related kinase activation. However, a sieving process occurred in which only certain proteins were specifically sequestered into viral replication centers and others were particularly excluded. In contrast to other viruses, activation of a damage response is neither necessary nor detrimental to infection, as neither ATM nor Mre11 was required for full virus replication and production. Thus, by preventing simultaneous relocalization of all the necessary repair components to the replication centers, HCMV subverts full activation and completion of both double-stranded break and S-phase checkpoints that should arrest all replication within the cell and likely lead to apoptosis.

Mre11-Rad50-Nbs1 complex is activated by hypertonicity

When exposed to hypertonic conditions, cells accumulate double-strand DNA breaks (DSBs) like they are exposed to ionizing radiation. It has been proposed that inactivation of the Mre11-Rad50-Nbs1 (MRN) complex due to nuclear exit is responsible for the accumulation of DSBs as cells fail to repair DSBs produced during normal cellular activity. In this study, we examined the MRN complex in cells switched to hypertonicity. Surprisingly, we found that the MRN complex stayed in the nucleus and remained intact in response to hypertonicity. In fact, the MRN complex was dramatically activated after 4 h of switch to hypertonicity in a dose-dependent manner as shown by formation of foci. Activation of ATM and the MRN complex by hypertonicity and bleomycin was additive as was activation of their downstream targets including gammaH2AX and Chk2 indicating that the cellular response to DSB was intact in hypertonic conditions. Activation of Chk2 in response to hypertonicity was not observed in mutant cells with functionally impaired MRN complex confirming that they are in the same pathway. After 20 h of a switch to hypertonicity, MRN foci and gammaH2AX returned to a control level, suggesting that cells adapted to hypertonicity by repairing DNA. We conclude that cells respond normally to DSB and repair the DNA damages induced by hypertonicity.

The Mre11/Rad50/Nbs1 complex interacts with the mismatch repair system and contributes to temozolomide-induced G2 arrest and cytotoxicity

The chemotherapeutic agent temozolomide produces O(6)-methylguanine (O6MG) in DNA, which triggers futile DNA mismatch repair, DNA double-strand breaks (DSB), G(2) arrest, and ultimately cell death. Because the protein complex consisting of Mre11/Rad50/Nbs1 (MRN complex) plays a key role in DNA damage detection and signaling, we asked if this complex also played a role in the cellular response to temozolomide. Temozolomide exposure triggered the assembly of MRN complex into chromatin-associated nuclear foci. MRN foci formed significantly earlier than gamma-H2AX and 53BP1 foci that assembled in response to temozolomide-induced DNA DSBs. MRN foci formation was suppressed in cells that incurred lower levels of temozolomide-induced O6MG lesions and/or had decreased mismatch repair capabilities, suggesting that the MRN foci formed not in response to temozolomide-induced DSB but rather in response to mismatch repair processing of mispaired temozolomide-induced O6MG lesions. Consistent with this idea, the MRN foci colocalized with those of proliferating cell nuclear antigen (a component of the mismatch repair complex), and the MRN complex component Nbs1 coimmunoprecipitated with the mismatch repair protein Mlh1 specifically in response to temozolomide treatment. Furthermore, small inhibitory RNA-mediated suppression of Mre11 levels decreased temozolomide-induced G(2) arrest and cytotoxicity in a manner comparable to that achieved by suppression of mismatch repair. These data show that temozolomide-induced O6MG lesions, acted upon by the mismatch repair system, drive formation of the MRN complex foci and the interaction of this complex with the mismatch repair machinery. The MRN complex in turn contributes to the control of temozolomide-induced G(2) arrest and cytotoxicity, and as such is an additional determining factor in glioma sensitivity to DNA methylating chemotherapeutic drugs such as temozolomide.

SUMO-1-dependent allosteric regulation of thymine DNA glycosylase alters subnuclear localization and CBP/p300 recruitment

Previous studies have demonstrated that the base excision repair enzyme thymine DNA glycosylase (TDG) mediates recruitment of histone acetyltransferases CREB-binding protein (CBP) and p300 to DNA, suggesting a plausible role for these factors in TDG-mediated repair. Furthermore, TDG was found to potentiate CBP/p300-dependent transcription and serve as a substrate for CBP/p300 acetylation. Here, we show that the small ubiquitin-like modifier 1 (SUMO-1) protein binding activity of TDG is essential for activation of CBP and localization to promyelocytic leukemia protein oncogenic domains (PODs). SUMO-1 binding is mediated by two distinct amino- and carboxy-terminal motifs (residues 144 to 148 and 319 to 322) that are negatively regulated by DNA binding via an amino-terminal hydrophilic region (residues 1 to 121). TDG is also posttranslationally modified by covalent conjugation of SUMO-1 (sumoylation) to lysine 341. Interestingly, we found that sumoylation of TDG blocks interaction with CBP and prevents TDG acetylation in vitro. Furthermore, sumoylation effectively abrogates intermolecular SUMO-1 binding and a sumoylation-deficient mutant accumulates in PODs, suggesting that sumoylation negatively regulates translocation to these nuclear structures. These findings suggest that TDG sumoylation promotes intramolecular interactions with amino- and carboxy-terminal SUMO-1 binding motifs that dramatically alter the biochemical properties and subcellular localization of TDG.

Rapid phosphorylation of histone H2A.X following ionotropic glutamate receptor activation

Excessive activation of ionotropic glutamate receptors increases oxidative stress, contributing to the neuronal death observed following neurological insults such as ischemia and seizures. Post-translational histone modifications may be key mediators in the detection and repair of damage resulting from oxidative stress, including DNA damage, and may thus affect neuronal survival in the aftermath of insults characterized by excessive glutamate release. In non-neuronal cells, phosphorylation of histone variant H2A.X (termed gamma-H2AX) occurs rapidly following DNA double-strand breaks. We investigated gamma-H2AX formation in rat cortical neurons (days in vitro 14) following activation of N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate receptors using fluorescent immunohistochemical techniques. Moreover, we evaluated the co-localization of gamma-H2AX 'foci' with Mre11, a double-strand break repair protein, to provide further evidence for the activation of this DNA damage response pathway. Here we show that minimally cytotoxic stimulation of ionotropic glutamate receptors was sufficient to evoke gamma-H2AX in neurons, and that NMDA-induced gamma-H2AX foci formation was attenuated by pretreatment with the antioxidant, Vitamin E, and the intracellular calcium chelator, BAPTA-AM. Moreover, a subset of gamma-H2AX foci co-localized with Mre11, indicating that at least a portion of gamma-H2AX foci is damage dependent. The extent of gamma-H2AX induction following glutamate receptor activation corresponded to the increases we observed following conventional DNA damaging agents [i.e. non-lethal doses of gamma-radiation (1 Gy) and hydrogen peroxide (10 microm)]. These data suggest that insults not necessarily resulting in neuronal death induce the DNA damage-evoked chromatin modification, gamma-H2AX, and implicate a role for histone alterations in determining neuronal vulnerability following neurological insults.

Will you let me use your nucleus? How Agrobacterium gets its T-DNA expressed in the host plant cell

Agrobacterium is the only known bacterium capable of natural DNA transfer into a eukaryotic host. The genes transferred to host plants are contained on a T-DNA (transferred DNA) molecule, the transfer of which begins with its translocation, along with several effector proteins, from the bacterial cell to the host-cell cytoplasm. In the host cytoplasm, the T-complex is formed from a single-stranded copy of the T-DNA (T-strand) associated with several bacterial and host proteins and it is imported into the host nucleus via interactions with the host nuclear import machinery. Once inside the nucleus, the T-complex is most likely directed to the host genome by associating with histones. Finally, the chromatin-associated T-complex is uncoated from its escorting proteins prior to the conversion of the T-strand to a double-stranded form and its integration into the host genome.

Cellular response to DNA damage

Eukaryotic cells, from yeast to man, possess evolutionarily conserved mechanisms to accurately and efficiently repair the overwhelming majority of DNA damage, thereby ensuring genomic integrity. Important repair pathways include base excision repair, nucleotide excision repair, mismatch repair, non-homologous end-joining, and homologous recombination. Defects in DNA repair processes generally result in susceptibility to cancer and, often, abnormalities in multiple organ systems. While signal transduction pathways have been intensely studied, epigenetic changes occurring in response to DNA damage are rapidly increasing in importance. Effective radiation and chemotherapy sensitization could result from selective inhibition of DNA repair in tumor cells. DNA damage repair is a dynamic field of research where the fruits of basic research often have important clinical implications.

DDB1 maintains genome integrity through regulation of Cdt1

DDB1, a component of a Cul4A ubiquitin ligase complex, promotes nucleotide excision repair (NER) and regulates DNA replication. We have investigated the role of human DDB1 in maintaining genome stability. DDB1-depleted cells accumulate DNA double-strand breaks in widely dispersed regions throughout the genome and have activated ATM and ATR cell cycle checkpoints. Depletion of Cul4A yields similar phenotypes, indicating that an E3 ligase function of DDB1 is important for genome maintenance. In contrast, depletion of DDB2, XPA, or XPC does not cause activation of DNA damage checkpoints, indicating that defects in NER are not involved. One substrate of DDB1-Cul4A that is crucial for preventing genome instability is Cdt1. DDB1-depleted cells exhibit increased levels of Cdt1 protein and rereplication, despite containing other Cdt1 regulatory mechanisms. The rereplication, accumulation of DNA damage, and activation of checkpoint responses in DDB1-depleted cells require entry into S phase and are partially, but not completely, suppressed by codepletion of Cdt1. Therefore, DDB1 prevents DNA lesions from accumulating in replicating human cells, in part by regulating Cdt1 degradation.

The Rothmund-Thomson gene product RECQL4 localizes to the nucleolus in response to oxidative stress

Mutations in the RECQL4 helicase gene have been linked to Rothmund-Thomson syndrome (RTS), which is characterized by poikiloderma, growth deficiency, and a predisposition to cancer. Examination of RECQL4 subcellular localization in live cells demonstrated a nucleoplasmic pattern and, to a lesser degree, staining in nucleoli. Analysis of RECQL4-GFP deletion mutants revealed two nuclear localization regions in the N-terminal region of RECQL4 and a nucleolar localization signal at amino acids 376-386. RECQL4 localization did not change after treatment with the DNA-damaging agents bleomycin, etoposide, UV irradiation and gamma irradiation, in contrast to the Bloom and Werner syndrome helicases that relocate to distinct nuclear foci after damage. However, in a significant number of cells exposed to hydrogen peroxide or streptonigrin, RECQL4 accumulated in nucleoli. Using a T7 phage display screen, we determined that RECQL4 interacts with poly(ADP-ribose) polymerase-1 (PARP-1), a nuclear enzyme that promotes genomic integrity through its involvement in DNA repair and signaling pathways. The RECQL4 nucleolar localization was inhibited by pretreatment with a PARP-1 inhibitor. The C-terminal portion of RECQL4 was found to be an in vitro substrate for PARP-1. These results demonstrate changes in the intracellular localization of RECQL4 in response to oxidative stress and identify an interaction between RECQL4 and PARP-1.

Promyelocytic leukemia nuclear bodies are predetermined processing sites for damaged DNA

The promyelocytic leukemia protein (PML) participates in several cellular functions, including transcriptional regulation, apoptosis and maintenance of genomic stability. A key feature of this protein is its ability to induce the assembly of nuclear compartments termed PML-nuclear bodies (PML-NBs). Here we show that these nuclear structures recruit single-stranded DNA (ssDNA) molecules in response to exogenous DNA damage. ssDNA was readily detected in PML-NBs within 1 hour following exposure of cells to UV light. Confocal real-time imaging of cells expressing YFP-tagged PML did not reveal de novo formation of new PML-NBs following UV-irradiation, which shows that ssDNA focus formation occurred within pre-existing PML-NBs. Moreover, siRNA-mediated depletion of PML prevented ssDNA focus formation and sensitized cells to UV-induced apoptosis. PML-dependent ssDNA focus formation was found to be particularly efficient during S-phase of the cell cycle, and PML-depleted cells became retarded in S-phase upon growth in the presence of etoposide. In addition, we found that caffeine and the poly(ADP-ribose) polymerase (PARP) inhibitor NU1027 enhanced UV-induced recruitment of ssDNA to PML-NBs. Together, our results show that PML-NBs have the capacity to accommodate DNA metabolic activities that are associated with processing of damaged DNA.

Promyelocytic leukemia activates Chk2 by mediating Chk2 autophosphorylation

Chk2 is a kinase critical for DNA damage-induced apoptosis and is considered a tumor suppressor. Chk2 is essential for p53 transcriptional and apoptotic activities. Although mutations of p53 are present in more than half of all tumors, mutations of Chk2 in cancers are rare, suggesting that Chk2 may be inactivated by unknown alternative mechanisms. Here we elucidate one such alternative mechanism regulated by PML (promyelocytic leukemia) that is involved in acute promyelocytic leukemia (APL). Although p53-inactivating mutations are extremely rare in APL, t(15;17) chromosomal translocation which fuses retinoic acid receptor (RARalpha) to PML is almost always present in APL, while the other PML allele is intact. We demonstrate that PML interacts with Chk2 and activates Chk2 by mediating its autophosphorylation step, an essential step for Chk2 activity that occurs after phosphorylation by the upstream kinase ATM (ataxia telangiectasia-mutated). PML/RARalpha in APL suppresses Chk2 by dominantly inhibiting the auto-phosphorylation step, but inactivation of PML/RARalpha with alltrans retinoic acid (ATRA) restores Chk2 autophosphorylation and activity. Thus, by fusing PML with RARalpha, the APL cells appear to have achieved functional suppression of Chk2 compromising the Chk2-p53 apoptotic pathway.

Rad50S alleles of the Mre11 complex: questions answered and questions raised

We find that Rad50S mutations in yeast and mammals exhibit constitutive PIKK (PI3-kinase like kinase)-dependent signaling [T. Usui, H. Ogawa, J.H. Petrini, A DNA damage response pathway controlled by Tel1 and the Mre11 complex. Mol. Cell 7 (2001) 1255-1266.; M. Morales, J.W. Theunissen, C.F. Kim, R. Kitagawa, M.B. Kastan, J.H. Petrini, The Rad50S allele promotes ATM-dependent DNA damage responses and suppresses ATM deficiency: implications for the Mre11 complex as a DNA damage sensor. Genes Dev. 19 (2005) 3043-4354.]. The signaling depends on Mre11 complex functions, consistent with its role as a DNA damage sensor. Rad50S is distinct from hypomorphic mutations of Mre11 and Nbs1 in mammals [M. Morales, J.W. Theunissen, C.F. Kim, R. Kitagawa, M.B. Kastan, J.H. Petrini, The Rad50S allele promotes ATM-dependent DNA damage responses and suppresses ATM deficiency: implications for the Mre11 complex as a DNA damage sensor. Genes Dev. 19 (2005) 3043-3054.; J.P. Carney, R.S. Maser, H. Olivares, E.M. Davis, Le M. Beau, J.R. Yates, III, L. Hays, W.F. Morgan, J.H. Petrini, The hMre11/hRad50 protein complex and Nijmegen breakage syndrome: linkage of double-strand break repair to the cellular DNA damage response. Cell 93 (1998) 477-486.; G.S. Stewart, R.S. Maser, T. Stankovic, D.A. Bressan, M.I. Kaplan, N.G. Jaspers, A. Raams, P.J. Byrd, J.H. Petrini, A.M. Taylor, The DNA double-strand break repair gene hMRE11 is mutated in individuals with an ataxia-telangiectasia-like disorder. Cell 99 (1999) 577-587.; B.R. Williams, O.K. Mirzoeva, W.F. Morgan, J. Lin, W. Dunnick, J.H. Petrini, A murine model of nijmegen breakage syndrome. Curr. Biol. 12 (2002) 648-653.; J.W. Theunissen, M.I. Kaplan, P.A. Hunt, B.R. Williams, D.O. Ferguson, F.W. Alt, J.H. Petrini, Checkpoint failure and chromosomal instability without lymphomagenesis in Mre11(ATLD1/ATLD1) mice. Mol. Cell 12 (2003) 1511-1523.] and the Mre11 complex deficiency in yeast [T. Usui, H. Ogawa, J.H. Petrini, A DNA damage response pathway controlled by Tel1 and the Mre11 complex. Mol. Cell 7 (2001) 1255-1266.; D'D. Amours, S.P. Jackson, The yeast Xrs2 complex functions in S phase checkpoint regulation. Genes Dev. 15 (2001) 2238-49. ; M. Grenon, C. Gilbert, N.F. Lowndes, Checkpoint activation in response to double-strand breaks requires the Mre11/Rad50/Xrs2 complex. Nat. Cell Biol. 3 (2001) 844-847. ] where the signaling is compromised. Herein, we describe evidence for chronic signaling by Rad50S and discuss possible mechanisms.

Herpes simplex virus type I disrupts the ATR-dependent DNA-damage response during lytic infection

Like other DNA viruses, herpes simplex virus type 1 (HSV-1) interacts with components of the cellular response to DNA damage. For example, HSV-1 sequesters endogenous, uninduced, hyperphosphorylated RPA (replication protein A) away from viral replication compartments. RPA is a ssDNA-binding protein that signals genotoxic stress through the ATR (ataxia telangiectasia-mutated and Rad3-related) pathway. The sequestration of endogenous hyperphosphorylated RPA away from replicating viral DNA suggests that HSV-1 prevents the normal ATR-signaling response. In this study we examine the spatial distribution of endogenous hyperphosphorylated RPA with respect to ATR, its recruitment factor, ATRIP, and the cellular dsDNA break marker, gammaH2AX, during HSV-1 infection. The accumulation of these repair factors at DNA lesions has previously been identified as an early event in signaling genotoxic stress. We show that HSV-1 infection disrupts the ATR pathway by a mechanism that prevents the recruitment of repair factors, spatially uncouples ATRIP from ATR and sequesters ATRIP and endogenous hyperphosphorylated RPA within virus-induced nuclear domains containing molecular chaperones and components of the ubiquitin proteasome. The HSV-1 immediate early protein ICP0 is sufficient to induce the redistribution of ATRIP. This is the first report that a virus can disrupt the usually tight colocalization of ATR and ATRIP.

Dual role of Nbs1 in the ataxia telangiectasia mutated-dependent DNA damage response

The Nbs1 protein associates with Mre11 and Rad50 proteins to form the Mre11-Rad50-Nbs1 complex, which plays an important role in the intracellular signaling pathway activated in response to DNA damage. Mutations in the genes for each of these three components of the Mre11-Rad50-Nbs1 complex result in human diseases characterized by genomic instability. Insight into the functions of Nbs1 in the DNA damage response mediated by the protein kinase, ataxia telangiectasia mutated, has been provided by recent studies. Nbs1 acts both as a downstream target of ataxia telangiectasia mutated in the S-phase checkpoint of the cell cycle as well as an upstream modulator or activator of ataxia telangiectasia mutated in the DNA damage response.

E2F1 induces MRN foci formation and a cell cycle checkpoint response in human fibroblasts

Deregulation of the Rb/E2F pathway in human fibroblasts results in an E2F1-mediated apoptosis dependent on Atm, Nbs1, Chk2 and p53. Here, we show that E2F1 expression results in MRN foci formation, which is independent of the Nbs1 interacting region and the DNA-binding domain of E2F1. E2F1-induced MRN foci are similar to irradiation-induced foci (IRIF) that result from double-strand DNA breaks because they correlate with 53BP1 and gammaH2AX foci, do not form in NBS cells, do form in AT cells and do not correlate with cell cycle entry. In fact, we find that in human fibroblasts deregulated E2F1 causes a G1 arrest, blocking serum-induced cell cycle progression, in part through an Nbs1/53BP1/p53/p21(WAF1/CIP1) checkpoint pathway. This checkpoint protects against apoptosis because depletion of 53BP1 or p21(WAF1/CIP1) increases both the rate and extent of apoptosis. Nbs1 and p53 contribute to both checkpoint and apoptosis pathways. These results suggest that E2F1-induced foci generate a cell cycle checkpoint that, with sustained E2F1 activity, eventually yields to apoptosis. Uncontrolled proliferation due to Rb/E2F deregulation as well as inactivation of both checkpoint and apoptosis programs would then be required for transformation of normal cells to tumor cells.

c-Myc, genome instability, and tumorigenesis: the devil is in the details

The c-myc oncogene acts as a pluripotent modulator of transcription during normal cell growth and proliferation. Deregulated c-myc activity in cancer can lead to excessive activation of its downstream pathways, and may also stimulate changes in gene expression and cellular signaling that are not observed under non-pathological conditions. Under certain conditions, aberrant c-myc activity is associated with the appearance of DNA damage-associated markers and karyotypic abnormalities. In this chapter, we discuss mechanisms by which c-myc may be directly or indirectly associated with the induction of genomic instability. The degree to which c-myc-induced genomic instability influences the initiation or progression of cancer is likely to depend on other factors, which are discussed herein.

XLF interacts with the XRCC4-DNA ligase IV complex to promote DNA nonhomologous end-joining

DNA nonhomologous end-joining (NHEJ) is a predominant pathway of DNA double-strand break repair in mammalian cells, and defects in it cause radiosensitivity at the cellular and whole-organism levels. Central to NHEJ is the protein complex containing DNA Ligase IV and XRCC4. By searching for additional XRCC4-interacting factors, we identified a previously uncharacterized 33 kDa protein, XRCC4-like factor (XLF, also named Cernunnos), that has weak sequence homology with XRCC4 and is predicted to display structural similarity to XRCC4. We show that XLF directly interacts with the XRCC4-Ligase IV complex in vitro and in vivo and that siRNA-mediated downregulation of XLF in human cell lines leads to radiosensitivity and impaired NHEJ. Furthermore, we establish that NHEJ-deficient 2BN cells derived from a radiosensitive and immune-deficient patient lack XLF due to an inactivating frameshift mutation in its gene, and that reintroduction of wild-type XLF into such cells corrects their radiosensitivity and NHEJ defects. XLF thus constitutes a novel core component of the mammalian NHEJ apparatus.

Recruitment of ATR to sites of ionising radiation-induced DNA damage requires ATM and components of the MRN protein complex

ATM and ATR are two related kinases essential for signalling DNA damage. Although ATM is thought to be the principle kinase responsible for signalling ionising radiation (IR)-induced DNA damage, ATR also contributes to signalling this form of genotoxic stress. However, the molecular basis of differential ATM and ATR activation in response to IR remains unclear. Here, we report that ATR is recruited to sites of IR-induced DNA damage significantly later than activation of ATM. We show that ATR is recruited to IR-induced nuclear foci in G(1) and S phase of the cell cycle, supporting a role for ATR in detecting DNA damage outside of S phase. In addition, we report that recruitment of ATR to sites of IR-induced DNA damage is concomitant with appearance of large tracts of single-stranded DNA (ssDNA) and that this event is dependent on ATM and components of the Mre11/Rad50/Nbs1 (MRN) protein complex.

ATM regulates ATR chromatin loading in response to DNA double-strand breaks

DNA double-strand breaks (DSBs) are among the most deleterious lesions that can challenge genomic integrity. Concomitant to the repair of the breaks, a rapid signaling cascade must be coordinated at the lesion site that leads to the activation of cell cycle checkpoints and/or apoptosis. In this context, ataxia telangiectasia mutated (ATM) and ATM and Rad-3-related (ATR) protein kinases are the earliest signaling molecules that are known to initiate the transduction cascade at damage sites. The current model places ATM and ATR in separate molecular routes that orchestrate distinct pathways of the checkpoint responses. Whereas ATM signals DSBs arising from ionizing radiation (IR) through a Chk2-dependent pathway, ATR is activated in a variety of replication-linked DSBs and leads to activation of the checkpoints in a Chk1 kinase-dependent manner. However, activation of the G2/M checkpoint in response to IR escapes this accepted paradigm because it is dependent on both ATM and ATR but independent of Chk2. Our data provides an explanation for this observation and places ATM activity upstream of ATR recruitment to IR-damaged chromatin. These data provide experimental evidence of an active cross talk between ATM and ATR signaling pathways in response to DNA damage.

Methods for studying the cellular response to DNA damage: influence of the Mre11 complex on chromosome metabolism

Dramatic progress in understanding the mediators and mechanisms of chromosome break metabolism has been made in recent years. As a result, the links between disease and defects in chromosome dynamics have become clearer. In this chapter, we discuss techniques employed in our laboratory to study chromosome break metabolism, which include assessments at the molecular and cellular level. In our laboratory, we use the Mre11 complex as a tool to study this process, but the techniques discussed are of general relevance.

Rapid PIKK-dependent release of Chk1 from chromatin promotes the DNA-damage checkpoint response

BACKGROUND

Checkpoint signaling pathways are of crucial importance for the maintenance of genomic integrity. Within these pathways, the effector kinase Chk1 plays a central role in mediating cell-cycle arrest in response to DNA damage, and it does so by phosphorylating key cell-cycle regulators.

RESULTS

By investigating the subcellular distribution of Chk1 by cell fractionation, we observed that around 20% of it localizes to chromatin during all phases of the cell cycle. Furthermore, we found that in response to DNA damage, Chk1 rapidly dissociates from the chromatin. Significantly, we observed a tight correlation between DNA-damage-induced Chk1 phosphorylation and chromatin dissociation, suggesting that phosphorylated Chk1 does not stably associate with chromatin. Consistent with these events being triggered by active checkpoint signaling, inhibition of the DNA-damage-activated kinases ATR and ATM, or siRNA-mediated downregulation of the DNA-damage mediator proteins Claspin and TopBP1, impaired DNA-damage-induced dissociation of Chk1 from chromatin. Finally, we established that Chk1 phosphorylation occurs at localized sites of DNA damage and that constitutive immobilization of Chk1 on chromatin results in a defective DNA-damage-induced checkpoint arrest.

CONCLUSIONS

Chromatin association and dissociation appears to be important for proper Chk1 regulation. We propose that in response to DNA damage, PIKK-dependent checkpoint signaling leads to phosphorylation of chromatin-bound Chk1, resulting in its rapid release from chromatin and facilitating the transmission of DNA-damage signals to downstream targets, thereby promoting efficient cell-cycle arrest.

Disruption of murine Mus81 increases genomic instability and DNA damage sensitivity but does not promote tumorigenesis

The Mus81-Eme1 endonuclease is implicated in the efficient rescue of broken replication forks in Saccharomyces cerevisiae and Schizosaccharomyces pombe. We have used gene targeting to study the function of the Mus81-Eme1 endonuclease in mammalian cells. Mus81-deficient mice develop normally and are fertile. Surprisingly, embryonic fibroblasts from Mus81(-/-) animals fail to proliferate in vitro. This proliferation defect can be rescued by expression of the papillomavirus E6 protein that promotes degradation of p53. When grown in culture, Mus81(-/-) cells have elevated levels of DNA damage, acquire chromosomal aberrations, and are hypersensitive to agents that generate DNA cross-links. In contrast to the situation in yeast, murine Mus81 is not required for replication restart following camptothecin treatment. Mus81(-/-) mice and cells are hypersensitive to DNA cross-linking agents. Cross-link-induced double-strand break formation is normal in Mus81(-/-) cells, but the resolution of repair intermediates is not. The persistence of Rad51 foci in Mus81(-/-) cells suggests that Mus81 acts at a late step in the repair of cross-link-induced lesions. Despite these defects, Mus81(-/-) mice do not show increased predisposition to lymphoma or any other malignancy in the first year of life.

Activation of ataxia telangiectasia-mutated DNA damage checkpoint signal transduction elicited by herpes simplex virus infection

Eukaryotic cells are equipped with machinery to monitor and repair damaged DNA. Herpes simplex virus (HSV) DNA replication occurs at discrete sites in nuclei, the replication compartment, where viral replication proteins cluster and synthesize a large amount of viral DNA. In the present study, HSV infection was found to elicit a cellular DNA damage response, with activation of the ataxia-telangiectasia-mutated (ATM) signal transduction pathway, as observed by autophosphorylation of ATM and phosphorylation of multiple downstream targets including Nbs1, Chk2, and p53, while infection with a UV-inactivated virus or with a replication-defective virus did not. Activated ATM and the DNA damage sensor MRN complex composed of Mre11, Rad50, and Nbs1 were recruited and retained at sites of viral DNA replication, probably recognizing newly synthesized viral DNAs as abnormal DNA structures. These events were not observed in ATM-deficient cells, indicating ATM dependence. In Nbs1-deficient cells, HSV infection induced an ATM DNA damage response that was delayed, suggesting a functional MRN complex requirement for efficient ATM activation. However, ATM silencing had no effect on viral replication in 293T cells. Our data open up an interesting question of how the virus is able to complete its replication, although host cells activate ATM checkpoint signaling in response to the HSV infection.

Srs2 and Sgs1 DNA helicases associate with Mre11 in different subcomplexes following checkpoint activation and CDK1-mediated Srs2 phosphorylation

Mutations in the genes encoding the BLM and WRN RecQ DNA helicases and the MRE11-RAD50-NBS1 complex lead to genome instability and cancer predisposition syndromes. The Saccharomyces cerevisiae Sgs1 RecQ helicase and the Mre11 protein, together with the Srs2 DNA helicase, prevent chromosome rearrangements and are implicated in the DNA damage checkpoint response and in DNA recombination. By searching for Srs2 physical interactors, we have identified Sgs1 and Mre11. We show that Srs2, Sgs1, and Mre11 form a large complex, likely together with yet unidentified proteins. This complex reorganizes into Srs2-Mre11 and Sgs1-Mre11 subcomplexes following DNA damage-induced activation of the Mec1 and Tel1 checkpoint kinases. The defects in subcomplex formation observed in mec1 and tel1 cells can be recapitulated in srs2-7AV mutants that are hypersensitive to intra-S DNA damage and are altered in the DNA damage-induced and Cdk1-dependent phosphorylation of Srs2. Altogether our observations indicate that Mec1- and Tel1-dependent checkpoint pathways modulate the functional interactions between Srs2, Sgs1, and Mre11 and that the Srs2 DNA helicase represents an important target of the Cdk1-mediated cellular response induced by DNA damage.

Akt activation suppresses Chk2-mediated, methylating agent-induced G2 arrest and protects from temozolomide-induced mitotic catastrophe and cellular senescence

Pharmacologic inhibition of the DNA signal transducers Chk1 and p38 blocks G2 arrest and sensitizes glioblastoma cells to chemotherapeutic methylating agent-induced cytotoxicity. Because Akt pathway activation has been suggested to also block G2 arrest induced by DNA-damaging agents and because glioma cells frequently have high levels of Akt activation, we examined the contribution of the Akt pathway to methylating agent-induced G2 arrest and toxicity. U87MG human glioma cells containing an inducible Akt expression construct were incubated with inducing agent or vehicle, after which the cells were exposed to temozolomide and assayed for activation of the components of the G2 arrest pathway and survival. Temozolomide-treated control cells activated the DNA damage signal transducers Chk1, Chk2, and p38, leading to Cdc25C and Cdc2 inactivation, prolonged G2 arrest, and loss of clonagenicity by a combination of senescence and mitotic catastrophe. Temozolomide-treated cells induced to overexpress Akt, however, exhibited significantly less drug-induced Cdc25C/Cdc2 inactivation and less G2 arrest. Akt-mediated suppression of G2 arrest was associated not with alterations in Chk1 or p38 activation but rather with suppression of Chk2 activation and reduced recruitment of Chk2 to sites of damage in chromatin. Unlike bypass of the G2 checkpoint induced by pharmacologic inhibitors of Chk1 or p38, however, Akt-induced bypass of G2 arrest suppressed, rather than enhanced, temozolomide-induced senescence and mitotic catastrophe. These results show that whereas Akt activation suppresses temozolomide-induced Chk2 activation and G2 arrest, the overriding effect is protection from temozolomide-induced cytotoxicity. The Akt pathway therefore represents a new target for the sensitization of gliomas to chemotherapeutic methylating agents such as temozolomide.

Differences in the association of p53 phosphorylated on serine 15 and key enzymes of homologous recombination

Phosphorylation of p53 on serine 15 by ATM or ATR is a frequent modification and initiates a cascade of post-translational modifications. To identify possible mechanisms that modulate p53 functions in recombination surveillance, we compared the nuclear localization of p53 phosphorylated on serine 15 (p53pSer15) and the key enzymes of homologous recombination (HR) after replication fork stalling. We demonstrate an almost mutually exclusive subcompartmentalization with Rad52, while p53pSer15 was colocalizing with 40-60% of the Rad51 and Mre11 foci. Therefore, possible sites of p53pSer15-dependent regulation seem to be sites of Rad51- rather than Rad52-dependent HR processes. Remarkably, the association of p53pSer15 with repair complexes containing Rad51 or Mre11 was transient, because less than 20% of the Rad51 and Mre11 foci overlapped with p53pSer15 after 6 h. When we examined colocalization and co-immunoprecipitation of p53pSer15 and the RecQ helicase BLM with recombination surveillance and proapoptotic functions, we observed colocalization within a fraction of approximately 70% of the BLM foci and stable physical interactions until 6 h after replication arrest. Our data suggest that p53pSer15 plays a dual role in the functional interactions with early complexes of Rad51-dependent recombination and with BLM-associated surveillance and signalling complexes within distinct nuclear subcompartments.

Analysis of ionizing radiation-induced foci of DNA damage repair proteins

Repair of DNA double-strand breaks by homologous recombination requires an extensive set of proteins. Among these proteins are Rad51 and Mre11, which are known to re-localize to sites of DNA damage into nuclear foci. Ionizing radiation-induced foci can be visualized by immuno-staining. Published data show a large variation in the number of foci-positive cells and number of foci per nucleus for specific DNA repair proteins. The experiments described here demonstrate that the time after induction of DNA damage influenced not only the number of foci-positive cells, but also the size of the individual foci. The dose of ionizing radiation influenced both the number of foci-positive cells and the number of foci per nucleus. Furthermore, ionizing radiation-induced foci formation depended on the cell cycle stage of the cells and the protein of interest that was investigated. Rad51 and Mre11 foci seemed to be mutually exclusive, though a small subset of cells did show co-localization of these proteins, which suggests a possible cooperation between the proteins at a specific moment during DNA repair.

Imaging of protein movement induced by chromosomal breakage: tiny 'local' lesions pose great 'global' challenges

Interruption of chromosomal integrity by DNA double-strand breaks (DSBs) causes a major threat to genomic stability. Despite tremendous progress in understanding the genetic and biochemical aspects of DSB-induced genome surveillance and repair mechanisms, little is known about organization of these molecular pathways in space and time. Here, we outline the key spatio-temporal problems associated with DSBs and focus on the imaging approaches to visualize the dynamics of DSB-induced responses in mammalian cells. We delineate benefits and limitations of these assays and highlight the key recent discoveries where live microscopy provided unprecedented insights into how cells defend themselves against genome-destabilizing effects of DNA damage.