Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Pathways for genome integrity in G2 phase of the cell cycle

The maintenance of genome integrity is important for normal cellular functions, organism development and the prevention of diseases, such as cancer. Cellular pathways respond immediately to DNA breaks leading to the initiation of a multi-facetted DNA damage response, which leads to DNA repair and cell cycle arrest. Cell cycle checkpoints provide the cell time to complete replication and repair the DNA damage before it can continue to the next cell cycle phase. The G2/M checkpoint plays an especially important role in ensuring the propagation of error-free copies of the genome to each daughter cell. Here, we review recent progress in our understanding of DNA repair and checkpoint pathways in late S and G2 phases. This review will first describe the current understanding of normal cell cycle progression through G2 phase to mitosis. It will also discuss the DNA damage response including cell cycle checkpoint control and DNA double-strand break repair. Finally, we discuss the emerging concept that DNA repair pathways play a major role in the G2/M checkpoint pathway thereby blocking cell division as long as DNA lesions are present.

Mre11 ATLD17/18 mutation retains Tel1/ATM activity but blocks DNA double-strand break repair

The Mre11 complex (Mre11-Rad50-Nbs1 or MRN) binds double-strand breaks where it interacts with CtIP/Ctp1/Sae2 and ATM/Tel1 to preserve genome stability through its functions in homology-directed repair, checkpoint signaling and telomere maintenance. Here, we combine biochemical, structural and in vivo functional studies to uncover key properties of Mre11-W243R, a mutation identified in two pediatric cancer patients with enhanced ataxia telangiectasia-like disorder. Purified human Mre11-W243R retains nuclease and DNA binding activities in vitro. X-ray crystallography of Pyrococcus furiosus Mre11 indicates that an analogous mutation leaves the overall Mre11 three-dimensional structure and nuclease sites intact but disorders surface loops expected to regulate DNA and Rad50 interactions. The equivalent W248R allele in fission yeast allows Mre11 to form an MRN complex that efficiently binds double-strand breaks, activates Tel1/ATM and maintains telomeres; yet, it causes hypersensitivity to ionizing radiation and collapsed replication forks, increased Rad52 foci, defective Chk1 signaling and meiotic failure. W248R differs from other ataxia telangiectasia-like disorder analog alleles by the reduced stability of its interaction with Rad50 in cell lysates. Collective results suggest a separation-of-function mutation that disturbs interactions amongst the MRN subunits and Ctp1 required for DNA end processing in vivo but maintains interactions sufficient for Tel1/ATM checkpoint and telomere maintenance functions.

A review of the inherited ataxias: recent advances in genetic, clinical and neuropathologic aspects

Inherited ataxias are a heterogeneous group of disorders characterized by autosomal dominant and recessive inheritance. Recent advances in genetic research have resulted in an improved comprehension of their clinical presentation. Autosomal dominant cerebellar ataxias (ADCAs) include spinocerebellar ataxias (SCAs) and dentatorubral-pallidoluysian atrophy (DRPLA); six of these have been found to be trinucleotide repeat disorders. Episodic ataxia, types 1 and 2, are at present recognized to be channelopathies, caused by point mutations. Friedreich's ataxia (FA) which is an autosomal recessive disorder, resulting from a a unique trinucleotide repeat, is now recognized to have a wide age of onset and clinical spectrum. Ataxia-telangiectasia (AT), also an autosomal recessive cerebellar ataxia, is characterized by immunodeficiency. In this article, the genetic and clinical characteristics of these diseases are reviewed in detail.

Sp1 facilitates DNA double-strand break repair through a nontranscriptional mechanism

Sp1 is a ubiquitously expressed transcription factor that is phosphorylated by ataxia telangiectasia mutated kinase (ATM) in response to ionizing radiation and H(2)O(2). Here, we show by indirect immunofluorescence that Sp1 phosphorylated on serine 101 (pSp1) localizes to ionizing radiation-induced foci with phosphorylated histone variant γH2Ax and members of the MRN (Mre11, Rad50, and Nbs1) complex. More precise analysis of occupancy of DNA double-strand breaks (DSBs) by chromatin immunoprecipitation (ChIP) shows that Sp1, like Nbs1, resides within 200 bp of DSBs. Using laser microirradiation of cells, we demonstrate that pSp1 is present at DNA DSBs by 7.5 min after induction of damage and remains at the break site for at least 8 h. Depletion of Sp1 inhibits repair of site-specific DNA breaks, and the N-terminal 182-amino-acid peptide, which contains targets of ATM kinase but lacks the zinc finger DNA binding domain, is phosphorylated, localizes to DSBs, and rescues the repair defect resulting from Sp1 depletion. Together, these data demonstrate that Sp1 is rapidly recruited to the region immediately adjacent to sites of DNA DSBs and is required for DSB repair, through a mechanism independent of its sequence-directed transcriptional effects.

SPARC overexpression inhibits cell proliferation in neuroblastoma and is partly mediated by tumor suppressor protein PTEN and AKT

Secreted protein acidic and rich in cysteine (SPARC) is also known as BM-40 or Osteonectin, a multi-functional protein modulating cell–cell and cell–matrix interactions. In cancer, SPARC is not only linked with a highly aggressive phenotype, but it also acts as a tumor suppressor. In the present study, we sought to characterize the function of SPARC and its role in sensitizing neuroblastoma cells to radio-therapy. SPARC overexpression in neuroblastoma cells inhibited cell proliferation in vitro. Additionally, SPARC overexpression significantly suppressed the activity of AKT and this suppression was accompanied by an increase in the tumor suppressor protein PTEN both in vitro and in vivo. Restoration of neuroblastoma cell radio-sensitivity was achieved by overexpression of SPARC in neuroblastoma cells in vitro and in vivo. To confirm the role of the AKT in proliferation inhibited by SPARC overexpression, we transfected neuroblastoma cells with a plasmid vector carrying myr-AKT. Myr-AKT overexpression reversed SPARC-mediated PTEN and increased proliferation of neuroblastoma cells in vitro. PTEN overexpression in parallel with SPARC siRNA resulted in decreased AKT phosphorylation and proliferation in vitro. Taken together, these results establish SPARC as an effector of AKT-PTEN-mediated inhibition of proliferation in neuroblastoma in vitro and in vivo.

DNA repair: the link between primary immunodeficiency and cancer

The adaptive component of the immune system depends greatly on the generation of genetic diversity provided by lymphocyte-specific genomic rearrangements. V(D)J recombination, class switch recombination (CSR), and somatic hypermutation (SHM) constitute complex and vulnerable processes that are orchestrated by a multitude of DNA repair pathways. When inherited defects in certain DNA repair proteins are present, lymphocyte development can be compromised and, consequently, patients can develop primary immunodeficiencies (PIDs). PID patients often have a strong predisposition for cancer development as a result of genomic instability generated from defective DNA repair mechanisms. Tumors of lymphoid origin are one of the most common PID-associated cancers, likely due to DNA lesions resulting from defective V(D)J, CSR, and SHM. In this review, we describe PID syndromes that confer an increased risk for cancer development. Furthermore, we discuss the role of the affected proteins in tumorigenesis/lymphomagenesis.

ATM protein-dependent phosphorylation of Rad50 protein regulates DNA repair and cell cycle control

The Mre11/Rad50/NBN complex plays a central role in coordinating the cellular response to DNA double-strand breaks. The importance of Rad50 in that response is evident from the recent description of a patient with Rad50 deficiency characterized by chromosomal instability and defective ATM-dependent signaling. We report here that ATM (defective in ataxia-telangiectasia) phosphorylates Rad50 at a single site (Ser-635) that plays an important adaptor role in signaling for cell cycle control and DNA repair. Although a Rad50 phosphosite-specific mutant (S635G) supported normal activation of ATM in Rad50-deficient cells, it was defective in correcting DNA damage-induced signaling through the ATM-dependent substrate SMC1. This mutant also failed to correct radiosensitivity, DNA double-strand break repair, and an S-phase checkpoint defect in Rad50-deficient cells. This was not due to disruption of the Mre11/Rad50/NBN complex revealing for the first time that phosphorylation of Rad50 plays a key regulatory role as an adaptor for specific ATM-dependent downstream signaling through SMC1 for DNA repair and cell cycle checkpoint control in the maintenance of genome integrity.

Neurologic presentation in children with ataxia-telangiectasia: is small head circumference a hallmark of the disease?

OBJECTIVE

To define the neurologic characteristics and course of ataxia-telangiectasia (A-T).

STUDY DESIGN

Retrospective cross-sectional chart study of 57 children (ages 2 to 19 years) followed at an A-T clinic. Cerebellar and extracerebellar symptoms were graded according to degree of functional impairment. Head circumferences were plotted from the charts and z-scores were calculated and compared with that of family members.

RESULTS

Ataxia was present in 87.7%, followed by dysarthria (82.1%), dysmetria (75.4%), bradykinesia (69.2%), hyperkinetic movements (58.9%), and dystonia (15.8%). All features aggravated with age. The most striking clinical observation in our patients was low head circumference (z-score below 1), which was present in 60.9%; 17% had true microcephaly (z-score below 2). Microcephaly appeared postnatally, was proportionate to height and weight, and did not correlate with severity of ataxia or genotype.

CONCLUSIONS

In addition to cerebellar ataxia, extrapyramidal symptoms, especially bradykinesia, were frequent and disabling. Microcephaly is an integral part of A-T; understanding its pathogenesis may shed light on the mechanism by which ATM mutation causes dysfunction in the nervous system.

ATM protein kinase: the linchpin of cellular defenses to stress

ATM is the most significant molecule involved in monitoring the genomic integrity of the cell. Any damage done to DNA relentlessly challenges the cellular machinery involved in recognition, processing and repair of these insults. ATM kinase is activated early to detect and signal lesions in DNA, arrest the cell cycle, establish DNA repair signaling and faithfully restore the damaged chromatin. ATM activation plays an important role as a barrier to tumorigenesis, metabolic syndrome and neurodegeneration. Therefore, studies of ATM-dependent DNA damage signaling pathways hold promise for treatment of a variety of debilitating diseases through the development of new therapeutics capable of modulating cellular responses to stress. In this review, we have tried to untangle the complex web of ATM signaling pathways with the purpose of pinpointing multiple roles of ATM underlying the complex phenotypes observed in AT patients.

Oxidative stress resistance in Deinococcus radiodurans

Deinococcus radiodurans is a robust bacterium best known for its capacity to repair massive DNA damage efficiently and accurately. It is extremely resistant to many DNA-damaging agents, including ionizing radiation and UV radiation (100 to 295 nm), desiccation, and mitomycin C, which induce oxidative damage not only to DNA but also to all cellular macromolecules via the production of reactive oxygen species. The extreme resilience of D. radiodurans to oxidative stress is imparted synergistically by an efficient protection of proteins against oxidative stress and an efficient DNA repair mechanism, enhanced by functional redundancies in both systems. D. radiodurans assets for the prevention of and recovery from oxidative stress are extensively reviewed here. Radiation- and desiccation-resistant bacteria such as D. radiodurans have substantially lower protein oxidation levels than do sensitive bacteria but have similar yields of DNA double-strand breaks. These findings challenge the concept of DNA as the primary target of radiation toxicity while advancing protein damage, and the protection of proteins against oxidative damage, as a new paradigm of radiation toxicity and survival. The protection of DNA repair and other proteins against oxidative damage is imparted by enzymatic and nonenzymatic antioxidant defense systems dominated by divalent manganese complexes. Given that oxidative stress caused by the accumulation of reactive oxygen species is associated with aging and cancer, a comprehensive outlook on D. radiodurans strategies of combating oxidative stress may open new avenues for antiaging and anticancer treatments. The study of the antioxidation protection in D. radiodurans is therefore of considerable potential interest for medicine and public health.

Mre11 is expressed in mammalian mitochondria where it binds to mitochondrial DNA

Mre11 is a critical participant in upkeep of nuclear DNA, its repair, replication, meiosis, and maintenance of telomeres. The upkeep of mitochondrial DNA (mtDNA) is less well characterized, and whether Mre11 participates has been unknown. We previously found that high NaCl causes some of the Mre11 to leave the nucleus, but we did not then attempt to localize it within the cytoplasm. In the present studies, we find Mre11 in mitochondria isolated from primary renal cells and show that the amount of Mre11 in mitochondria increases with elevation of extracellular NaCl. We confirm the presence of Mre11 in the mitochondria of cells by confocal microscopy and show that some of the Mre11 colocalizes with mtDNA. Furthermore, crosslinking of Mre11 to DNA followed by Mre11 immunoprecipitation directly demonstrates that some Mre11 binds to mtDNA. Abundant Mre11 is also present in tissue sections from normal mouse kidneys, colocalized with mitochondria of proximal tubule and thick ascending limb cells. To explore whether distribution of Mre11 changes with cell differentiation, we used an experimental model of tubule formation by culturing primary kidney cells in Matrigel matrix. In nondifferentiated cells, Mre11 is mostly in the nucleus, but it becomes mostly cytoplasmic upon cell differentiation. We conclude that Mre11 is present in mitochondria where it binds to mtDNA and that the amount in mitochondria varies depending on cellular stress and differentiation. Our results suggest a role for Mre11 in the maintenance of genome integrity in mitochondria in addition to its previously known role in maintenance of nuclear DNA.

Inactivation of ataxia telangiectasia mutated gene can increase intracellular reactive oxygen species levels and alter radiation-induced cell death pathways in human glioma cells

PURPOSE

To investigate the effects of ataxia telangiectasia mutated (ATM)-regulated reactive oxygen species (ROS) and cell death pathways on the response of U87MG glioma cells to ionising radiation (IR) and oxidative stress.

MATERIAL AND METHODS

ATM expression was blocked in U87MG glioma cells using a small interfering RNA (siRNA) technique. Cell survival, sub-lethal damage (SLD), and potential lethal damage (PLD) repair following IR were assessed by clonogenic assay while changes in intracellular ROS, the apoptosis, and autophagy were followed by flow cytometry and Western blotting.

RESULTS

Blocking ATM expression in U87MG cells increased intracellular ROS levels and sensitivity to the cytotoxic effects of IR and oxygen stress; effects that could be partly counteracted by the antioxidant N-acetylcysteine (NAC). Knock down of ATM rendered cells unable to repair sub-lethal or potentially lethal damage and DNA double strand breaks (DSB) after IR exposure; something that NAC could not counteract. ATM did control the pathways a cell used to die following IR and this did seem to be ROS-dependent.

CONCLUSION

ATM is involved in redox control but ROS elevations following ATM knock down seem more involved in the decision as to what cell death pathway is utilised after IR than DSB repair and radiosensitivity.

Introduction to telomeres and telomerase

Telomeres are ends of chromosomes that play an important part in the biology of eukaryotic cells. Through the coordinated action of the telomerase and networks of other proteins and factors, the length and integrity of telomeres are maintained to prevent telomere dysfunction that has been linked to senescence, aging, diseases, and cancer. The tools and assays being used to study telomeres are being broadened, which has allowed us to derive a more detailed, high-resolution picture of the various players and pathways at work at the telomeres.

Requirement for NBS1 in the S phase checkpoint response to DNA methylation combined with PARP inhibition

Treatment of PARP-1-expressing cells with the combination of a DNA methylating agent (MMS) and the PARP inhibitor 4-amino-1,8-naphthalimide (4-AN) leads to an ATR/Chk1-dependent S phase checkpoint and cell death by apoptosis. Activation of ATM/Chk2 is involved in sustaining the S phase checkpoint, and double strand break (DSB) accumulation was demonstrated. NBS1, part of the MRN complex that responds to DSBs, is known to modulate ATR- and ATM-dependent checkpoint responses to UV and IR, but a role in the response to PARP inhibition has not been addressed. Here we show that the S phase checkpoint observed 4-8h after MMS+4-AN treatment was absent in cells deficient in NBS1, but was present in NBS1-complemented (i.e., functionally wild-type) cells, indicating a critical role for NBS1 in this checkpoint response. NBS1 was phosphorylated in response to MMS+4-AN treatment, and this was partially ATR- and ATM-dependent, suggesting involvement of both upstream kinases. NBS1 expression had little effect on ATR-mediated phosphorylation of Chk1 and ATM-mediated phosphorylation of Chk2 in response to MMS+4-AN. Phosphorylation of SMC1 was also observed in response to MMS+4-AN treatment. In the absence of ATM and NBS1, phosphorylation of SMC1 was weak, especially at early times after MMS+4-AN treatment. In the absence of ATR activation, reduced SMC1 phosphorylation was seen over a 24h time course. These results suggested that both ATR and ATM phosphorylate SMC1 in response to MMS+4-AN and that this phosphorylation is enhanced by phospho-NBS1. The loss of the MMS+4-AN-induced S phase checkpoint in NBS1-deficient cells may be due to a reduced cellular level of the critical downstream effector, phospho-SMC1.

IL-2-inducible T-cell kinase deficiency: clinical presentation and therapeutic approach

Mutations in the IL-2-inducible T-cell kinase gene have recently been shown to cause an autosomal recessive fatal Epstein Barr virus (EBV) associated lymphoproliferation. We report 3 cases from a single family who presented with EBV-positive B-cell proliferation diagnosed as Hodgkin's lymphoma. Single nucleotide polymorphism array-based genome-wide linkage analysis revealed IL-2-inducible T-cell kinase as a candidate gene for this disorder. All 3 patients harbored the same novel homozygous nonsense mutation C1764G which causes a premature stop-codon in the kinase domain. All cases were initially treated with chemotherapy. One patient remains in durable remission, the second patient subsequently developed severe hemophagocytic lymphohistiocytosis with multi-organ failure and died, and the third patient underwent a successful allogeneic bone marrow transplantation. IL-2-inducible T-cell kinase deficiency underlies a new primary immune deficiency which may account for part of the spectrum of Epstein Barr virus related lymphoproliferative disorders which can be successfully corrected by bone marrow transplantation.

Telomerase-dependent oncolytic adenovirus sensitizes human cancer cells to ionizing radiation via inhibition of DNA repair machinery

The inability to repair DNA double-strand breaks (DSB) leads to radiosensitization, such that ionizing radiation combined with molecular inhibition of cellular DSB processing may greatly affect treatment of human cancer. As a variety of viral products interact with the DNA repair machinery, oncolytic virotherapy may improve the therapeutic window of conventional radiotherapy. Here, we describe the mechanistic basis for synergy of irradiation and OBP-301 (Telomelysin), an attenuated type-5 adenovirus with oncolytic potency that contains the human telomerase reverse transcriptase promoter to regulate viral replication. OBP-301 infection led to E1B55kDa viral protein expression that degraded the complex formed by Mre11, Rad50, and NBS1, which senses DSBs. Subsequently, the phosphorylation of cellular ataxia-telangiectasia mutated protein was inhibited, disrupting the signaling pathway controlling DNA repair. Thus, tumor cells infected with OBP-301 could be rendered sensitive to ionizing radiation. Moreover, by using noninvasive whole-body imaging, we showed that intratumoral injection of OBP-301 followed by regional irradiation induces a substantial antitumor effect, resulting from tumor cell-specific radiosensitization, in an orthotopic human esophageal cancer xenograft model. These results illustrate the potential of combining oncolytic virotherapy and ionizing radiation as a promising strategy in the management of human cancer.

A novel defect in mitochondrial p53 accumulation following DNA damage confers apoptosis resistance in Ataxia Telangiectasia and Nijmegen Breakage Syndrome T-cells

We have previously shown that whereas T-cells from normal individuals undergo accumulation of p53 and apoptosis when treated with the genotoxic agent Actinomycin D (ActD), those from Ataxia Telangiectasia (AT) and Nijmegen Breakage Syndrome (NBS) patients resist ActD-induced apoptosis [1]. We have now found similar resistance by the p53-null Jurkat T-cell line and by siRNA p53-knockdown normal T-cells. This evidence that ActD initiates a p53-dependent apoptotic responce prompted us to look for defective p53 accumulation by AT and NBS T-cells. Surprisingly the total p53 level was only slightly reduced compared to normal T cells but its intracellular localization was highly defective: p53 was poorly accumulated in the cytosol and nearly undetectable in mitochondria. In accordance with the dependence of ActD-induced apoptosis on a mitochondrial p53 function, in control T-cells specific inhibition of mitochondrial p53 translocation with μ pifithrin reduced apoptosis by 86%, whereas treatment with α pifithrin, which blocks p53-mediated transcription, had no effect. We also showed that nuclear export is not required for mitochondrial p53 translocation. Observation of an altered p53 ubiquitination pattern and Mdm2 accumulation in ActD-treated AT and NBS T-cells provided a mechanistic link to their defective extranuclear p53 localization. Our results disclose an undescribed defect in mitochondrial p53 accumulation in AT and NBS T-cells that makes them resistant to apoptosis following unrepairable DNA damage.

Chk1-cyclin A/Cdk1 axis regulates origin firing programs in mammals

DNA replication is key to ensuring the complete duplication of genomic DNA prior to mitosis and is tightly regulated by both cell cycle machinery and checkpoint signals. Regulation of the S phase program occurs at several stages, affecting origin firing, replication fork elongation, fork velocity, and fork stability, all of which are dependent on S-phase-promoting kinase activity. Somatic mammalian cells use well-established origin programs by which specific regions of the genome are replicated at precise times. However, the mechanisms by which S phase kinases regulate origin firing in mammals are largely unknown. Here, we discuss recent advances in the understanding of how S phase programs are regulated in mammals at the correct regions and at the appropriate times.

[Nijmegen Breakage syndrome]

Nijmegen Breakage syndrome is a rare, autosomal recessive disorder characterized by severe, combined immunodeficiency, recurrent sinopulmonary infections, chromosomal instability, radiosensitivity, predisposition to malignancy, a "bird-like" facial appearance, progressive microcephaly, short stature, and mental retardation. The syndrome is caused by mutations in the NBS1 gene, which encodes a DNA-repair protein, named nibrin. The authors summarize current knowledge on molecular genetics, diagnostic characteristics and therapeutic options of this inborn error of innate immunity.

p38(MAPK) in the senescence of human and murine fibroblasts

Oncogenic and environmental stresses, such as reactive oxygen species, UV radiation etc, can induce premature cellular senescence without critical telomere shortening. The role of the Ras/Raf/ERK signal transduction cascade in this process has been previously established, but recent evidence also indicates a critical role of the p38 MAP kinases pathway. Oncogenic and environmental stresses impinge upon the p38(MAPK) pathway, suggesting a major role of this pathway in senescence induced by stresses. Prematurely senescent cells are most likely to appear in several age-relatedpathologies associated with a stressful environment and/or the release of pro-inflammatory cytokines.

Tumor suppressor protein C53 antagonizes checkpoint kinases to promote cyclin-dependent kinase 1 activation

Cyclin-dependent kinase 1 (Cdk1)/cyclin B1 complex is the driving force for mitotic entry, and its activation is tightly regulated by the G2/M checkpoint. We originally reported that a novel protein C53 (also known as Cdk5rap3 and LZAP) potentiates DNA damage-induced cell death by modulating the G2/M checkpoint. More recently, Wang et al. (2007) found that C53/LZAP may function as a tumor suppressor by way of inhibiting NF-kappaB signaling. We report here the identification of C53 protein as a novel regulator of Cdk1 activation. We found that knockdown of C53 protein causes delayed Cdk1 activation and mitotic entry. During DNA damage response, activation of checkpoint kinase 1 and 2 (Chk1 and Chk2) is partially inhibited by C53 overexpression. Intriguingly, we found that C53 interacts with Chk1 and antagonizes its function. Moreover, a portion of C53 protein is localized at the centrosome, and centrosome-targeting C53 potently promotes local Cdk1 activation. Taken together, our results strongly suggest that C53 is a novel negative regulator of checkpoint response. By counteracting Chk1, C53 promotes Cdk1 activation and mitotic entry in both unperturbed cell-cycle progression and DNA damage response.

Deficiency of the DNA repair enzyme ATM in rheumatoid arthritis

In rheumatoid arthritis (RA), dysfunctional T cells sustain chronic inflammatory immune responses in the synovium. Even unprimed T cells are under excessive replication pressure, suggesting an intrinsic defect in T cell regeneration. In naive CD4 CD45RA⁺ T cells from RA patients, DNA damage load and apoptosis rates were markedly higher than in controls; repair of radiation-induced DNA breaks was blunted and delayed. DNA damage was highest in newly diagnosed untreated patients. RA T cells failed to produce sufficient transcripts and protein of the DNA repair kinase ataxia telangiectasia (AT) mutated (ATM). NBS1, RAD50, MRE11, and p53 were also repressed. ATM knockdown mimicked the biological effects characteristic for RA T cells. Conversely, ATM overexpression reconstituted DNA repair capabilities, response patterns to genotoxic stress, and production of MRE11 complex components and rescued RA T cells from apoptotic death. In conclusion, ATM deficiency in RA disrupts DNA repair and renders T cells sensitive to apoptosis. Apoptotic attrition of naive T cells imposes lymphopenia-induced proliferation, leading to premature immunosenescence and an autoimmune-biased T cell repertoire. Restoration of DNA repair mechanisms emerges as an important therapeutic target in RA.

Rituximab as monotherapy and in addition to reduced CHOP in children with primary immunodeficiency and non-Hodgkin lymphoma

Children with primary immunodeficiency or chromosomal breakage syndromes are at increased risk of developing non-Hodgkin lymphomas; they cannot tolerate standard chemotherapy regimens. We report two children with diffuse, large, B-cell lymphoma; one had ataxia telangiectasia and one had common variable immunodeficiency. Both were given rituximab, 1 as monotherapy and 1 in combination with a reduced CHOP regimen. Complete remission was obtained in each patient. Use of rituximab as a first-line monotherapy or in conjunction with reduced chemotherapy should be considered to reduce cytotoxic effects.

The sticky business of histone H2AX in V(D)J recombination, maintenance of genomic stability, and suppression of lymphoma

DNA double strand breaks (DSBs) induced during cellular metabolism, DNA replication, and genomic rearrangement events lead to phosphorylation of the H2AX core histone variant in surrounding chromatin. H2AX is essential for normal DSB repair, maintenance of genomic stability, and suppression of lymphomas with clonal translocations and intra-chromosomal deletions. One current focus of our lab is to elucidate mechanisms through which H2AX functions in the cellular DNA damage response using V(D)J recombination as a model system. A number of potential H2AX functions can be readily tested using novel experimental approaches developed in our lab. These putative functions include: (1) modulation of chromatin accessibility to facilitate kinetics of DSB repair, (2) stabilization of broken DNA strands to maintain ends in close proximity, and (3) amplification of DNA damage signals. Here, we summarize our recent efforts in elucidating mechanisms by which H2AX functions during V(D)J recombination to coordinate DSB repair with cellular proliferation and survival to prevent translocations and suppress lymphomagenesis.

Molecular allelokaryotyping of T-cell prolymphocytic leukemia cells with high density single nucleotide polymorphism arrays identifies novel common genomic lesions and acquired uniparental disomy

BACKGROUND

T-cell prolymphocytic leukemia is a rare aggressive lymphoproliferative disease with a mature T-cell phenotype and characteristic genomic lesions such as inv(14)(q11q34), t(14;14)(q11;q32) or t(X;14)(q28;q11), mutation of the ATM gene on chromosome 11 and secondary alterations such as deletions of chromosome 8p and duplications of 8q.

DESIGN AND METHODS

We analyzed malignant cells from 18 patients with T-cell prolymphocytic leukemia using high density 250K single nucleotide polymorphism arrays and molecular allelokaryotyping to refine understanding of known alterations and identify new target genes.

RESULTS

Our analyses revealed that characteristic disruptions of chromosome 14 are frequently unbalanced. In the commonly deleted region on chromosome 11, we found recurrent microdeletions targeting the microRNA 34b/c and the transcription factors ETS1 and FLI1. On chromosome 8, we identified genes such as PLEKHA2, NBS1, NOV and MYST3 to be involved in breakpoints. New recurrent alterations were identified on chromosomes 5p, 12p, 13q, 17 and 22 with a common region of acquired uniparental disomy in four samples on chromosome 17q. Single nucleotide polymorphism array results were confirmed by direct sequencing and quantitative real-time polymerase chain reaction.

CONCLUSIONS

The first high density single nucleotide polymorphism array allelokaryotyping of T-cell prolymphocytic leukemia genomes added substantial new details about established alterations in this disease and moreover identified numerous new potential target genes in common breakpoints, deletions and regions of acquired uniparental disomy.

RAD50, an SMC family member with multiple roles in DNA break repair: how does ATP affect function?

The protein complex including Mre11, Rad50, and Nbs1 (MRN) functions in DNA double-strand break repair to recognize and process DNA ends as well as signal for cell cycle arrest. Amino acid sequence similarity and overall architecture make Rad50 a member of the structural maintenance of chromosome (SMC) protein family. Like SMC proteins, Rad50 function depends on ATP binding and hydrolysis. All current evidence indicates that ATP binding and hydrolysis cause architectural rearrangements in SMC protein complexes that are important for their functions in organizing DNA. In the case of the MRN complex, the functional significance of ATP binding and hydrolysis are not yet defined. Here we review the data on the ATP-dependent activities of MRN and their possible mechanistic significance. We present some speculation on the role of ATP for function of the MRN complex based on the similarities and differences in the molecular architecture of the Rad50-containing complexes and the SMC complexes condensin and cohesin.

NBS1 prevents chromatid-type aberrations through ATM-dependent interactions with SMC1

Nijmegen breakage syndrome shares several common cellular features with ataxia telangiectasia, including chromosomal instability and aberrant S- and G2-phase checkpoint regulation. We show here that after irradiation, NBS1 interacts physically with both BRCA1 and SMC1, a component of the cohesin complex, and that their interactions are completely abolished in AT cells. It is noted that BRCA1 is required for the interaction of NBS1 with SMC1, whereas the reverse is not the case, since BRCA1 is able to bind to NBS1 in the absence of an NBS1/SMC1 interaction as observed in MRE11- or RAD50-deficient cells. This indicates that ATM and BRCA1 are upstream of the NBS1/SMC1 interaction. Furthermore, the interaction of NBS1 with SMC1 requires both conserved domains of NBS in the N-terminus and the C-terminus, since they are indispensable for binding of NBS1 to BRCA1 and to MRE11/ATM, respectively. The interaction of NBS1 with SMC1 and the resulting phosphorylation are compromised in the clones lacking either the N- or C-terminus of NBS1, and as a consequence, chromatid-type aberrations are enhanced after irradiation. Our results reveal that ATM plays a fundamental role in promoting the radiation-induced interaction of NBS1 with SMC1 in the presence of BRCA1, leading to the maintenance of chromosomal integrity.

Transient inhibition of ATM kinase is sufficient to enhance cellular sensitivity to ionizing radiation

In response to DNA damage, the ATM protein kinase activates signal transduction pathways essential for coordinating cell cycle progression with DNA repair. In the human disease ataxia-telangiectasia, mutation of the ATM gene results in multiple cellular defects, including enhanced sensitivity to ionizing radiation (IR). This phenotype highlights ATM as a potential target for novel inhibitors that could be used to enhance tumor cell sensitivity to radiotherapy. A targeted compound library was screened for potential inhibitors of the ATM kinase, and CP466722 was identified. The compound is nontoxic and does not inhibit phosphatidylinositol 3-kinase (PI3K) or PI3K-like protein kinase family members in cells. CP466722 inhibited cellular ATM-dependent phosphorylation events and disruption of ATM function resulted in characteristic cell cycle checkpoint defects. Inhibition of cellular ATM kinase activity was rapidly and completely reversed by removing CP466722. Interestingly, clonogenic survival assays showed that transient inhibition of ATM is sufficient to sensitize cells to IR and suggests that therapeutic radiosensitization may only require ATM inhibition for short periods of time. The ability of CP466722 to rapidly and reversibly regulate ATM activity provides a new tool to ask questions about ATM function that could not easily be addressed using genetic models or RNA interference technologies.

Assessing the role of stress signalling via p38 MAP kinase in the premature senescence of ataxia telangiectasia and Werner syndrome fibroblasts

The premature ageing ataxia telangiectasia (AT) and Werner syndromes (WS) are associated with accelerated cellular ageing. Young WS fibroblasts have an aged appearance and activated p38 MAP kinase, and treatment with the p38 inhibitor SB230580 extends their lifespan to within the normal range. SB203580 also extends the replicative lifespan of normal adult dermal fibroblasts, however, the effect is much reduced when compared to WS cells, suggesting that WS fibroblasts undergo a form of stress-induced premature senescence (SIPS). A small lifespan extension is seen in AT cells, which is not significant compared to normal fibroblasts, and the majority of young AT cells do not have an aged appearance and lack p38 activation, suggesting that the premature ageing does not result from SIPS. The lack of p38 activation is supported by the clinical manifestation, since AT is not associated with inflammatory disease, whereas WS individuals are predisposed to atherosclerosis, type II diabetes and osteoporosis, conditions known to be associated with p38 activation.

Telomeres and aging

Telomeres play a central role in cell fate and aging by adjusting the cellular response to stress and growth stimulation on the basis of previous cell divisions and DNA damage. At least a few hundred nucleotides of telomere repeats must "cap" each chromosome end to avoid activation of DNA repair pathways. Repair of critically short or "uncapped" telomeres by telomerase or recombination is limited in most somatic cells and apoptosis or cellular senescence is triggered when too many "uncapped" telomeres accumulate. The chance of the latter increases as the average telomere length decreases. The average telomere length is set and maintained in cells of the germline which typically express high levels of telomerase. In somatic cells, telomere length is very heterogeneous but typically declines with age, posing a barrier to tumor growth but also contributing to loss of cells with age. Loss of (stem) cells via telomere attrition provides strong selection for abnormal and malignant cells, a process facilitated by the genome instability and aneuploidy triggered by dysfunctional telomeres. The crucial role of telomeres in cell turnover and aging is highlighted by patients with 50% of normal telomerase levels resulting from a mutation in one of the telomerase genes. Short telomeres in such patients are implicated in a variety of disorders including dyskeratosis congenita, aplastic anemia, pulmonary fibrosis, and cancer. Here the role of telomeres and telomerase in human aging and aging-associated diseases is reviewed.

Archaeal DNA helicase HerA interacts with Mre11 homologue and unwinds blunt-ended double-stranded DNA and recombination intermediates

HerA is a novel family DNA helicases that exist ubiquitously in thermophilic archaea. The genes are linked to homologues of eukaryotic recombination and repair proteins Mre11 and Rad50 in some of the genomes. However, the relationship between HerA and the related proteins is unclear. In this study, a homologue from the hyperthermophilic archaeon Sulfolobus tokodaii (StoHerA) was characterized and physical and functional interactions between StoHerA and StoMre11 (Mre11 from S. tokodaii) were studied. It was found that StoHerA was able to unwind blunt-ended double-stranded DNA (dsDNA), although with lower efficiency. StoHerA was also able to unwind Holliday junction, splayed-arm DNA, as well as 5'- or 3'-overhang with high efficiency. Pull-down and yeast two-hybrid analyses revealed that StoHerA interacted with StoMre11 physically. The helicase activity of StoHerA was stimulated by StoMre11, indicating a functional role of this interaction. In addition, site-directed mutagenesis of StoHerA was performed to analyze functions of conserved residues of StoHerA. Interestingly, mutation of E355 to alanine in Walker B resulted in not only loss of ATPase and DNA helicase activities, but also dsDNA-binding ability, indicating that this residue is involved in the coupling of ATP hydrolysis, dsDNA-binding, and helicase activities.

Structure of a second BRCT domain identified in the nijmegen breakage syndrome protein Nbs1 and its function in an MDC1-dependent localization of Nbs1 to DNA damage sites

The Nijmegen breakage syndrome protein Nbs1 is a component of the MRN (Mre11-Rad50-Nbs1) complex, central to the DNA damage response. While Nbs1 is generally believed to encompass a forkhead-associated domain linked to a breast cancer C-terminal (BRCT) domain, to date there is no experimental information on its three-dimensional structure. Through nuclear magnetic resonance (NMR) three-dimensional structure determination, we demonstrate that there is a second BRCT domain (BRCT2) in Nbs1. The domain has the characteristic BRCT topology, but with a long insertion shown to be flexible by NMR relaxation measurements. In the absence of sequence similarity to other proteins, a search for structural analogs of BRCT2 returned the second BRCT domain of the tandem BRCT repeats of cell cycle checkpoint proteins MDC1 (mediator of DNA damage checkpoint protein 1) and BRCA1 (breast cancer protein 1), suggesting that like MDC1 and BRCA1, Nbs1 also possesses tandem BRCT domains with phosphoprotein binding ability. Structure-based single point mutations in human Nbs1 were evaluated in vivo and revealed that BRCT2 is essential for an MDC1-dependent relocalization of Nbs1 to DNA damage sites, most likely through a direct interaction of Nbs1 tandem BRCT domains with phosphorylated MDC1.

Defective responses to DNA single- and double-strand breaks in spinocerebellar ataxia

Failure to maintain the integrity of DNA/chromatin can result in genome instability and an increased risk of cancer. The description of a number of human genetic disorders characterised not only by cancer predisposition but by a broader phenotype including neurodegeneration suggests that maintaining genome stability is also important for preserving post-mitotic neurons. The identification of genes associated with other neurodegenerative disorders provides further evidence for the importance of DNA damage response and DNA repair genes in protecting against neurodegeneration. This theme is further developed in this review.

Etoposide induces ATM-dependent mitochondrial biogenesis through AMPK activation

Background

DNA damage such as double-stranded DNA breaks (DSBs) has been reported to stimulate mitochondrial biogenesis. However, the underlying mechanism is poorly understood. The major player in response to DSBs is ATM (ataxia telangiectasia mutated). Upon sensing DSBs, ATM is activated through autophosphorylation and phosphorylates a number of substrates for DNA repair, cell cycle regulation and apoptosis. ATM has been reported to phosphorylate the α subunit of AMP-activated protein kinase (AMPK), which senses AMP/ATP ratio in cells, and can be activated by upstream kinases. Here we provide evidence for a novel role of ATM in mitochondrial biogenesis through AMPK activation in response to etoposide-induced DNA damage.

Methodology/Principal Findings

Three pairs of human ATM+ and ATM- cells were employed. Cells treated with etoposide exhibited an ATM-dependent increase in mitochondrial mass as measured by 10-N-Nonyl-Acridine Orange and MitoTracker Green FM staining, as well as an increase in mitochondrial DNA content. In addition, the expression of several known mitochondrial biogenesis regulators such as the major mitochondrial transcription factor NRF-1, PGC-1α and TFAM was also elevated in response to etoposide treatment as monitored by RT-PCR. Three pieces of evidence suggest that etoposide-induced mitochondrial biogenesis is due to ATM-dependent activation of AMPK. First, etoposide induced ATM-dependent phosphorylation of AMPK α subunit at Thr172, indicative of AMPK activation. Second, inhibition of AMPK blocked etoposide-induced mitochondrial biogenesis. Third, activation of AMPK by AICAR (an AMP analogue) stimulated mitochondrial biogenesis in an ATM-dependent manner, suggesting that ATM may be an upstream kinase of AMPK in the mitochondrial biogenesis pathway.

Conclusions/Significance

These results suggest that activation of ATM by etoposide can lead to mitochondrial biogenesis through AMPK activation. We propose that ATM-dependent mitochondrial biogenesis may play a role in DNA damage response and ROS regulation, and that defect in ATM-dependent mitochondrial biogenesis could contribute to the manifestations of A-T disease.

DNA strand breaks, neurodegeneration and aging in the brain

Defective responses to DNA single- or double-strand breaks can result in neurological disease, underscoring the critical importance of DNA repair for neural homeostasis. Human DNA repair-deficient syndromes are generally congenital, in which brain pathology reflects the consequences of developmentally incurred DNA damage. Although, it is unclear to what degree DNA strand-break repair defects in mature neural cells contributes to disease pathology. However, DNA single-strand breaks are a relatively common lesion which if not repaired can impact cells via interference with transcription. Thus, this lesion, and probably to a lesser extent DNA double-strand breaks, may be particularly relevant to aging in the neural cell population. In this review we will examine the consequences of defective DNA strand-break repair towards homeostasis in the brain. Further, we also consider the utility of mouse models as reagents to understand the connection between DNA strand breaks and aging in the brain.

Phosphorylation of Sp1 in response to DNA damage by ataxia telangiectasia-mutated kinase

Sp1, a transcription factor that regulates expression of a wide array of essential genes, contains two SQ/TQ cluster domains, which are characteristic of ATM kinase substrates. ATM substrates are transducers and effectors of the DNA damage response, which involves sensing damage, checkpoint activation, DNA repair, and/or apoptosis. A role for Sp1 in the DNA damage response is supported by our findings: Activation of ATM induces Sp1 phosphorylation with kinetics similar to H2AX; inhibition of ATM activity blocks Sp1 phosphorylation; depletion of Sp1 sensitizes cells to DNA damage and increases the frequency of double strand breaks. We have identified serine 101 as a critical site phosphorylated by ATM; Sp1 with serine 101 mutated to alanine (S101A) is not significantly phosphorylated in response to damage and cannot restore increased sensitivity to DNA damage of cells depleted of Sp1. Together, these data show that Sp1 is a novel ATM substrate that plays a role in the cellular response to DNA damage.

Is defect in phosphorylation of Nbs1 responsible for high radiosensitivity of T-lymphocyte leukemia cells MOLT-4?

Mutations in NBS1 gene are related to higher occurrence of malignancies. In this work we studied response of T-lymphocyte leukemia cells MOLT-4 to ionizing radiation. We detected IRIF (ionizing radiation forming foci) containing histone gammaH2A.X, protein 53BP1, and Nbs1, which were formed around double-strand breaks of DNA. We found dose-dependent increase in foci number (colocalization of gammaH2A.X and 53BP1) and gammaH2A.X amount (integral optical density) 1h after irradiation. After the dose of 1.5 Gy the number of foci decreases with time, but 72 h after irradiation 9% of live cells still contained big foci around unrepaired DNA damage. Western blot method revealed massive phosphorylation of H2A.X during apoptosis induction, 6-24 h after irradiation by the doses 1.5 and 3 Gy. Cells with apoptotic morphology showed strong phosphorylation of H2A.X, but it was not accompanied by 53BP1. 1h after irradiation by the lethal doses 5 and 10 Gy we detected by Western blot a decrease in repair proteins Mre11, Rad50, and Nbs1. While phosphorylation of H2A.X 1h after irradiation was detected by both confocal microscopy and Western blot, phosphorylation of Nbs1 on serine 343 was not detectable in MOLT-4 cells. Despite functional ATM and p53 the phosphorylation of Nbs1 on serine 343 was impaired in these cells, and might be responsible for high radiosensitivity of MOLT-4 cells.

Enhanced CD95-mediated apoptosis contributes to radiation hypersensitivity of NBS lymphoblasts

The molecular causes for enhanced radiosensitivity of Nijmegen Breakage Syndrome cells are unclear, especially as repair of DNA damage is hardly impeded in these cells. We clearly demonstrate that radiation hypersensitivity is accompanied by enhanced gamma-radiation-induced apoptosis in NBS1 deficient lymphoblastoid cell lines. Differences in the apoptotic behavior of NBS1 (-/-) and NBS1 (+/-) cells are not due to an altered p53 stabilization or phosphorylation in NBS1 (-/-) cells. gamma-radiation-induced caspase-8 activity is increased and visualization of CD95 clustering by laser scanning microscopy shows a significant higher activation of the death receptor in NBS1 (-/-) cells. Further investigation of the molecular mechanisms reveals a role for reactive oxygen species-triggered activation of CD95. These results demonstrate that NBS1 suppresses the CD95 death receptor-dependent apoptotic pathway after gamma-irradiation and evidence is given that this is achieved by regulation of the PI3-K/AKT survival pathway.

Age to survive: DNA damage and aging

Aging represents the progressive functional decline and increased mortality risk common to nearly all metazoans. Recent findings experimentally link DNA damage and organismal aging: longevity-regulating genetic pathways respond to the accumulation of DNA damage and other stress conditions and conversely influence the rate of damage accumulation and its impact for cancer and aging. This novel insight has emerged from studies on human progeroid diseases and mouse models that have deficient DNA repair pathways. Here we discuss a unified concept of an evolutionarily conserved 'survival' response that shifts the organism's resources from growth to maintenance as an adaptation to stresses, such as starvation and DNA damage. This shift protects the organism from cancer and promotes healthy aging.

Ataxia-telangiectasia and related diseases

Appropriate cellular signaling responses to DNA damage and the ability to repair DNA are fundamental processes that are required for organismal survival. Ataxia-telangiectasia (A-T) is a rare neurodegenerative disease that results from defective DNA damage signaling. Understanding the molecular basis of A-T has provided many critical insights into the cellular response to DNA double-strand breaks (DSBs). A-T is a syndrome that shows pronounced neurodegeneration of the nervous system coincident with immune deficiency, radiosensitivity, and cancer proneness. A-T results from inactivation of the A-T mutated (ATM) kinase, a critical protein kinase that regulates the response to DNA-DSBs by selective phosphorylation of a variety of substrates. Therefore, understanding the ATM signaling program has important biological ramifications for nervous system homeostasis. Underscoring the importance of the DNA-DSBs response in the nervous system are other diseases related to A-T that also result from defects in this signaling pathway. In particular, defects in the DNA damage sensor, the Mre11-RAD50-NBS1 complex, also lead to syndromes with neurological deficits and overlapping phenotypes to A-T. Collectively, these diseases highlight the critical importance of appropriate responses to DNA-DSBs to maintain homeostasis in the nervous system.

Signaling of DNA damage is not sufficient to induce p53 response: (Re)Activation of wt p53 protein strongly depends on cellular context

It is generally accepted that exposure of cells to a variety of DNA-damaging agents leads to up-regulation and activation of wild-type (wt) p53 protein. We investigated the (re)-activation of p53 protein in two human cancer cell lines in which the gene for this tumor suppressor is not mutated: HeLaS(3) cervix carcinoma and MCF-7 breast cancer cells, by induction via different genotoxic and cytotoxic stimuli. Treatment of human cells with the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or different anti-cancer drugs resulted in a strong DNA damage as evidenced by Comet assay and a marked increase in site-specific phosphorylation of H2AX. Unlike in MCF-7 cells, in HeLaS(3) cells the expression of p53 protein did not increase after MNNG treatment despite a strong DNA damage. However, other agents for example doxorubicin markedly induced p53 response in HeLaS(3) cells. After exposure of these cells to MNNG, the ATM-dependent effector proteins Chk2 and NBS1 were phosphorylated, thereby evidencing that MNNG-induced DNA breakage was recognized and properly signaled. In HeLaS(3) cells wt p53 protein is not functional due to E6-mediated targeting for accelerated ubiquitylation and degradation. Therefore, the activation of a p53 response to genotoxic stress in HeLaS(3) cells seems to depend on the status of E6 oncoprotein. Indeed, the induction of p53 protein in HeLaS(3) cells in response to distinct agents inversely correlates with the cellular level of E6 oncoprotein. This implicates that the capability of different agents to activate p53 in HeLaS(3) cells primarily depends on their inhibitory effect on expression of E6 oncoprotein.

DNA damage and cellular differentiation: more questions than responses

Studies on DNA damage responses in proliferating cells have revealed the relationship between sensing and repair of the DNA lesions and the regulation of the cell cycle, leading to the discovery and molecular characterization of the DNA damage-activated cell cycle checkpoints. Much less is known about the DNA damage response in progenitors of differentiated cells, in which cell cycle arrest is a critical signal to trigger the differentiation program, and in terminally differentiated cells, which are typically post-mitotic. How DNA lesions are detected, processed and repaired in these cells, the functional impact of DNA damage on transcription of differentiation-specific genes, how these events are coordinated at the molecular level, the consequence of defective DNA damage response on tissue-specific functions and its potential relationship with age-related diseases are currently open questions. In particular the biological complexity inherent to the global genome reprogramming of tissue progenitors, such as embryonic or adult stem cells, suggests the importance of an accurate DNA damage response at the transcription level in these cells to ensure the genomic integrity of regenerating tissues.

Activation and regulation of ATM kinase activity in response to DNA double-strand breaks

The ataxia-telangiectasia-mutated (ATM) protein kinase is rapidly and specifically activated in response to DNA double-strand breaks in eukaryotic cells. In this review, we summarize recent insights into the mechanism of ATM activation, focusing on the role of the Mre11/Rad50/Nbs1 (MRN) complex in this process. We also compare observations of the ATM activation process in different biological systems and highlight potential candidates for cellular factors that may participate in regulating ATM activity in human cells.

Ionizing radiation can induce GSK-3beta phosphorylation and NF-kappaB transcriptional transactivation in ATM-deficient fibroblasts

DNA damage by ionizing radiation (IR) can induce activations of both NF-kappaB and p53 through the upstream kinase ataxia telangiectasia mutated (ATM). NF-kappaB activation could also be signaled through two distinct or overlapped pathways; IkappaB kinases (IKKs)-IkappaBalpha and Akt-glycogen synthase kinase-3 (GSK-3). In the present study, however, we show that activation of Akt1 and the subsequent phosphorylation and inactivation of GSK-3beta by IR could also occur in ATM-deficient AT5BIVA cells as well as in normal MRC5CV1 fibroblasts. Similarly, lithium chloride (LiCl) was found to increase the phosphorylation of GSK-3beta independently of ATM. Transfection with either wild-type or kinase dead mutant GSK-3beta to the cells further indicated that phosphorylations of Akt1 and GSK-3beta were closely associated with the transcriptional transactivation of NF-kappaB in response to ionizing radiation. On the other hand, LiCl, having no effect on caspase-3 activation, significantly increased p53 phosphorylation and apoptotic death of the normal MRC5CV1 cells while IR, activating both caspase-3 and p53, profoundly affected AT5BIVA cell death. Hence, our data suggest that although ATM-mediated IKK-IkappaBalpha pathway might be a typical pathway for IR-induced NF-kappaB activation, GSK-3beta phosphorylation could also partially contribute to the transcriptional transactivation of NF-kappaB in an ATM-independent manner and that GSK-3beta phosphorylation could induce ATM-mediated cell apoptosis through the activation of p53.

The Mre11/Rad50/Nbs1 complex plays an important role in the prevention of DNA rereplication in mammalian cells

The Mre11/Nbs1/Rad50 complex (MRN) plays multiple roles in the maintenance of genome stability, including repair of double-stranded breaks (DSBs) and activation of the S-phase checkpoint. Here we demonstrate that MRN is required for the prevention of DNA rereplication in mammalian cells. DNA replication is strictly regulated by licensing control so that the genome is replicated once and only once per cell cycle. Inactivation of Nbs1 or Mre11 leads to a substantial increase of DNA rereplication induced by overexpression of the licensing factor Cdt1. Our studies reveal that multiple mechanisms are likely involved in the MRN-mediated suppression of rereplication. First, both Mre11 and Nbs1 are required for facilitating ATR activation when Cdt1 is overexpressed, which in turn suppresses rereplication. Second, Cdt1 overexpression induces ATR-mediated phosphorylation of Nbs1 at Ser343 and this phosphorylation depends on the FHA and BRCT domains of Nbs1. Mutations at Ser343 or in the FHA and BRCT domains lead to more severe rereplication when Cdt1 is overexpressed. Third, the interaction of the Mre11 complex with RPA is important for the suppression of rereplication. This suggests that modulating RPA activity via a direct interaction of MRN is likely one of the effector mechanisms to suppress rereplication. Moreover, we demonstrate that MRN is also required for preventing the accumulation of DSBs when rereplication is induced. Therefore, our studies suggest new roles of MRN in the maintenance of genome stability through preventing rereplication and rereplication-associated DSBs when licensing control is compromised.