Nibrin forkhead-associated domain and breast cancer C-terminal domain are both required for nuclear focus formation and phosphorylation

A novel role for the peptidyl-prolyl cis-trans isomerase Cyclophilin A in DNA-repair following replication fork stalling via the MRE11-RAD50-NBS1 complex

Cyclosporin A (CsA) induces DNA double-strand breaks in LIG4 syndrome fibroblasts, specifically upon transit through S-phase. The basis underlying this has not been described. CsA-induced genomic instability may reflect a direct role of Cyclophilin A (CYPA) in DNA repair. CYPA is a peptidyl-prolyl cis-trans isomerase (PPI). CsA inhibits the PPI activity of CYPA. Using an integrated approach involving CRISPR/Cas9-engineering, siRNA, BioID, co-immunoprecipitation, pathway-specific DNA repair investigations as well as protein expression interaction analysis, we describe novel impacts of CYPA loss and inhibition on DNA repair. We characterise a direct CYPA interaction with the NBS1 component of the MRE11-RAD50-NBS1 complex, providing evidence that CYPA influences DNA repair at the level of DNA end resection. We define a set of genetic vulnerabilities associated with CYPA loss and inhibition, identifying DNA replication fork protection as an important determinant of viability. We explore examples of how CYPA inhibition may be exploited to selectively kill cancers sharing characteristic genomic instability profiles, including MYCN-driven Neuroblastoma, Multiple Myeloma and Chronic Myelogenous Leukaemia. These findings propose a repurposing strategy for Cyclophilin inhibitors.

NBS1 promotes the endonuclease activity of the MRE11-RAD50 complex by sensing CtIP phosphorylation

DNA end resection initiates DNA double-strand break repair by homologous recombination. MRE11-RAD50-NBS1 and phosphorylated CtIP perform the first resection step via MRE11-catalyzed endonucleolytic DNA cleavage. Human NBS1, more than its homologue Xrs2 in Saccharomyces cerevisiae, is crucial for this process, highlighting complex mechanisms that regulate the MRE11 nuclease in higher eukaryotes. Using a reconstituted system, we show here that NBS1, through its FHA and BRCT domains, functions as a sensor of CtIP phosphorylation. NBS1 then activates the MRE11-RAD50 nuclease through direct physical interactions with MRE11. In the absence of NBS1, MRE11-RAD50 exhibits a weaker nuclease activity, which requires CtIP but not strictly its phosphorylation. This identifies at least two mechanisms by which CtIP augments MRE11: a phosphorylation-dependent mode through NBS1 and a phosphorylation-independent mode without NBS1. In support, we show that limited DNA end resection occurs in vivo in the absence of the FHA and BRCT domains of NBS1. Collectively, our data suggest that NBS1 restricts the MRE11-RAD50 nuclease to S-G2 phase when CtIP is extensively phosphorylated. This defines mechanisms that regulate the MRE11 nuclease in DNA metabolism.

The Mre11-Nbs1 Interface Is Essential for Viability and Tumor Suppression

The Mre11 complex (Mre11, Rad50, and Nbs1) is integral to both DNA repair and ataxia telangiectasia mutated (ATM)-dependent DNA damage signaling. All three Mre11 complex components are essential for viability at the cellular and organismal levels. To delineate essential and non-essential Mre11 complex functions that are mediated by Nbs1, we used TALEN-based genome editing to derive Nbs1 mutant mice (Nbs1^mid mice), which harbor mutations in the Mre11 interaction domain of Nbs1. Nbs1^mid alleles that abolished interaction were incompatible with viability. Conversely, a 108-amino-acid Nbs1 fragment comprising the Mre11 interface was sufficient to rescue viability and ATM activation in cultured cells and support differentiation of hematopoietic cells in vivo. These data indicate that the essential role of Nbs1 is via its interaction with Mre11 and that most of the Nbs1 protein is dispensable for Mre11 complex functions and suggest that Mre11 and Rad50 directly activate ATM.

Heterozygous p.I171V mutation of the NBN gene as a risk factor for lung cancer development

The NBN gene, also known as NBS1, is located on the chromosome band 8q21.3, and encodes a 754-amino acid-long protein named nibrin. This protein is a member of the MRE1-RAD50-NBN nuclear complex, and is involved in numerous cell processes essential for maintaining genomic stability. Heterozygous variants in the NBN gene, including p.I171V, c.657del5 and p.R215W, have been described as risk factors for the development of several malignancies. However, there is no report regarding the association of these mutations with lung cancer thus far. Therefore, the present study aimed to evaluate whether there is an association between the heterozygous p.I171V, c.657del5 and p.R215W variants of the NBN gene and the risk of developing lung cancer. The frequency of these variants was estimated in a group of 453 adults diagnosed with non-small cell lung cancer (NSCLC) and in healthy controls (2,400 for p.I171V, 2,090 for c.657del5 and 498 for p.R215W). The p.I171V variant was assessed by restriction fragment length polymorphism analysis of polymerase chain reaction (PCR) products, using MunI (MfeI) restriction enzyme, whereas the c.657del5 and p.R215W variants were assessed by the PCR single-strand conformation polymorphism method. A significantly increased risk of developing lung cancer was observed for the p.I171V variant, which was present in 17 (3.75%) of the 453 cases of lung cancer and in 12 (0.5%) of the 2,400 healthy individuals (odds ratio, 7.759; P<0.0001). The results obtained indicated an association between the p.I171V mutation and the development of lung cancer. Therefore, this variant may be considered a risk factor for NSCLC. Prospective studies with larger groups of patients may reveal the potential impact of the p.I171V variant in the occurrence of lung cancer.

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

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

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

Background

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

Methods

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

Results

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

Conclusion

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

Productive replication of human papillomavirus 31 requires DNA repair factor Nbs1

Activation of the ATM (ataxia telangiectasia-mutated kinase)-dependent DNA damage response (DDR) is necessary for productive replication of human papillomavirus 31 (HPV31). We previously found that DNA repair and homologous recombination (HR) factors localize to sites of HPV replication, suggesting that ATM activity is required to recruit factors to viral genomes that can productively replicate viral DNA in a recombination-dependent manner. The Mre11-Rad50-Nbs1 (MRN) complex is an essential component of the DDR that is necessary for ATM-mediated HR repair and localizes to HPV DNA foci. In this study, we demonstrate that the HPV E7 protein is sufficient to increase levels of the MRN complex and also interacts with MRN components. We have found that Nbs1 depletion blocks productive viral replication and results in decreased localization of Mre11, Rad50, and the principal HR factor Rad51 to HPV DNA foci upon differentiation. Nbs1 contributes to the DDR by acting as an upstream activator of ATM in response to double-strand DNA breaks (DSBs) and as a downstream effector of ATM activity in the intra-S-phase checkpoint. We have found that phosphorylation of ATM and its downstream target Chk2, as well as SMC1 (structural maintenance of chromosome 1), is maintained upon Nbs1 knockdown in differentiating cells. Given that ATM and Chk2 are required for productive replication, our results suggest that Nbs1 contributes to viral replication outside its role as an ATM activator, potentially through ensuring localization of DNA repair factors to viral genomes that are necessary for efficient productive replication.

IMPORTANCE

The mechanisms that regulate human papillomavirus (HPV) replication during the viral life cycle are not well understood. Our finding that Nbs1 is necessary for productive replication even in the presence of ATM (ataxia telangiectasia-mutated kinase) and Chk2 phosphorylation offers evidence that Nbs1 contributes to viral replication downstream of facilitating ATM activation. Nbs1 is required for the recruitment of Mre11 and Rad50 to viral genomes, suggesting that the MRN complex plays a direct role in facilitating productive viral replication, potentially through the processing of substrates that are recognized by the key homologous recombination (HR) factor Rad51. The discovery that E7 increases levels of MRN components, and MRN complex formation, identifies a novel role for E7 in facilitating productive replication. Our study not only identifies DNA repair factors necessary for HPV replication but also provides a deeper understanding of how HPV utilizes the DNA damage response to regulate viral replication.

The interaction of CtIP and Nbs1 connects CDK and ATM to regulate HR-mediated double-strand break repair

CtIP plays an important role in homologous recombination (HR)-mediated DNA double-stranded break (DSB) repair and interacts with Nbs1 and BRCA1, which are linked to Nijmegen breakage syndrome (NBS) and familial breast cancer, respectively. We identified new CDK phosphorylation sites on CtIP and found that phosphorylation of these newly identified CDK sites induces association of CtIP with the N-terminus FHA and BRCT domains of Nbs1. We further showed that these CDK-dependent phosphorylation events are a prerequisite for ATM to phosphorylate CtIP upon DNA damage, which is important for end resection to activate HR by promoting recruitment of BLM and Exo1 to DSBs. Most notably, this CDK-dependent CtIP and Nbs1 interaction facilitates ATM to phosphorylate CtIP in a substrate-specific manner. These studies reveal one important mechanism to regulate cell-cycle-dependent activation of HR upon DNA damage by coupling CDK- and ATM-mediated phosphorylation of CtIP through modulating the interaction of CtIP with Nbs1, which significantly helps to understand how DSB repair is regulated in mammalian cells to maintain genome stability.

E2F1 enhances 8-chloro-adenosine-induced G2/M arrest and apoptosis in A549 and H1299 lung cancer cells

The E2F1 transcription factor is a well known regulator of cell proliferation and apoptosis, but its role in response to DNA damage is less clear. 8-Chloro-adenosine (8-Cl-Ado), a nucleoside analog, can inhibit proliferation in a variety of human tumor cells. However, it is still elusive how the agent acts on tumors. Here we show that A549 and H1299 cells formed DNA double-strand breaks after 8-Cl-Ado exposure, accompanied by E2F1 upregulation at protein level. Overexpressed wild-type (E2F1-wt) colocalized with double-strand break marker γ-H2AX and promoted G2/M arrest in 8-Cl-Ado-exposed A549 and H1299, while expressed S31A mutant of E2F1 (E2F1-mu) significantly reduced ability to accumulate at sites of DNA damage and G2/M arrest, suggesting that E2F1 is required for activating G2/M checkpoint pathway upon DNA damage. Transfection of either E2F1-wt or E2F1-mu plasmid promoted apoptosis in 8-Cl-Ado-exposed cells, indicating that 8-Cl-Ado may induce apoptosis in E2F1-dependent and E2F1-independent ways. These findings demonstrate that E2F1 plays a crucial role in 8-Cl-Ado-induced G2/M arrest but is dispensable for 8-Cl-Ado-induced apoptosis. These data also suggest that the mechanism of 8-Cl-Ado action is complicated.

Cleavage of the BRCT tandem domains of nibrin by the 657del5 mutation affects the DNA damage response less than the Arg215Trp mutation

The Nijmegen breakage syndrome (NBS) is a genetic disorder caused by mutations in NBN gene and characterized by chromosomal instability and hypersensitivity to ionizing radiations (IR). The N-terminus of nibrin (NBN) contains a tandem breast cancer 1 (BRCA1) carboxy-terminal (BRCT) domain that represents one of the major mediators of phosphorylation-dependent protein-protein interactions in processes related to cell cycle checkpoint and DNA repair functions. Patients with NBS compound heterozygous for the 657del5 hypomorphic mutation and for the Arg215Trp missense mutation (corresponding to the 643C>T gene mutation) display a clinical phenotype more severe than that of patients homozygous for the 657del5 mutation. Here, we show that both the 657del5 and Arg215Trp mutations, occurring within the tandem BRCT domains of NBN, although not altering the assembly of the MRE11/RAD50/NBN (MRN) complex, affect the MRE11 IR-induced nuclear foci (IRIF) formation and the DNA double-strand break (DSB) signaling via the phosphorylation of both ataxia-telangiectasia-mutated (ATM) kinase and ATM downstream targets (e.g., SMC1 and p53). Remarkably, data obtained indicate that the cleavage of the BRCT tandem domains of NBN by the 657del5 mutation affects the DNA damage response less than the Arg215Trp mutation. Indeed, the 70-kDa NBN fragment, arising from the 657del5 mutation, maintains the capability to interact with MRE11 and γ-H2AX and to form IRIF. Altogether, the role of the tandem BRCT domains of NBN in the localization of the MRN complex at the DNA DSB and in the activation of the damage response is highlighted.

A divalent FHA/BRCT-binding mechanism couples the MRE11-RAD50-NBS1 complex to damaged chromatin

The MRE11-RAD50-NBS1 (MRN) complex accumulates at sites of DNA double-strand breaks in large chromatin domains flanking the lesion site. The mechanism of MRN accumulation involves direct binding of the Nijmegen breakage syndrome 1 (NBS1) subunit to phosphorylated mediator of the DNA damage checkpoint 1 (MDC1), a large nuclear adaptor protein that interacts directly with phosphorylated H2AX. NBS1 contains an FHA domain and two BRCT domains at its amino terminus. Here, we show that both of these domains participate in the interaction with phosphorylated MDC1. Point mutations in key amino acid residues of either the FHA or the BRCT domains compromise the interaction with MDC1 and lead to defects in MRN accumulation at sites of DNA damage. Surprisingly, only mutation in the FHA domain, but not in the BRCT domains, yields a G2/M checkpoint defect, indicating that MDC1-dependent chromatin accumulation of the MRN complex at sites of DNA breaks is not required for G2/M checkpoint activation.

MDC1: The art of keeping things in focus

The chromatin structure is important for recognition and repair of DNA damage. Many DNA damage response proteins accumulate in large chromatin domains flanking sites of DNA double-strand breaks. The assembly of these structures-usually termed DNA damage foci-is primarily regulated by MDC1, a large nuclear mediator/adaptor protein that is composed of several distinct structural and functional domains. Here, we are summarizing the latest discoveries about the mechanisms by which MDC1 mediates DNA damage foci formation, and we are reviewing the considerable efforts taken to understand the functional implication of these structures.

A supramodular FHA/BRCT-repeat architecture mediates Nbs1 adaptor function in response to DNA damage

The Mre11/Rad50/Nbs1 protein complex plays central enzymatic and signaling roles in the DNA-damage response. Nuclease (Mre11) and scaffolding (Rad50) components of MRN have been extensively characterized, but the molecular basis of Nbs1 function has remained elusive. Here, we present a 2.3A crystal structure of the N-terminal region of fission yeast Nbs1, revealing an unusual but conserved architecture in which the FHA- and BRCT-repeat domains structurally coalesce. We demonstrate that diphosphorylated pSer-Asp-pThr-Asp motifs, recently identified as multicopy docking sites within Mdc1, are evolutionarily conserved Nbs1 binding targets. Furthermore, we show that similar phosphomotifs within Ctp1, the fission yeast ortholog of human CtIP, promote interactions with the Nbs1 FHA domain that are necessary for Ctp1-dependent resistance to DNA damage. Finally, we establish that human Nbs1 interactions with Mdc1 occur through both its FHA- and BRCT-repeat domains, suggesting how their structural and functional interdependence underpins Nbs1 adaptor functions in the DNA-damage response.

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.

Early events in the mammalian response to DNA double-strand breaks

Physical and chemical agents that induce DNA double-strand breaks (DSBs) are among the most potent mutagens. The mammalian cell response to DSB comprises a highly co-ordinated, yet complex network of proteins that have been categorized as sensors, signal transducers, mediators and effectors of damage and repair. While this provides an accessible classification system, review of the literature indicates that many proteins satisfy the criteria of more than one category, pointing towards a series of highly co-operative pathways with overlapping function. In summary, the MRE11-NBS1-RAD50 complex is necessary for achieving optimal activation of ataxia-telangiectasia-mutated (ATM) kinase, which catalyses a phosphorylation-mediated signal transduction cascade. Among the subset of proteins phosphorylated by ATM are histone H2AX (H2AX), mediator of damage checkpoint protein 1, nibrin (NBS1), P53-binding protein 1 and breast cancer protein 1, all of which subsequently redistribute into DSB-containing sub-nuclear compartments. Post-translational modification of DSB responding proteins achieves a rapid and reversible change in protein behaviour and mediates damage-specific interactions, hence imparting a high degree of vigilance to the cell. This review highlights events fundamental in maintaining genetic integrity with emphasis on early stages of the DSB response.

Polymorphisms and haplotypes of the NBS1 gene in childhood acute leukaemia

DNA repair gene polymorphisms and mutations may influence cancer risk. The product of the NBS1 gene, nibrin, is functionally involved in the double-strand DNA break repair system. Heterozygous, germline mutations of the NBS1 gene are associated with an increased risk of tumours. Thus, common polymorphism and haplotypes of NBS1 may contribute to the risk of cancer. This study verified whether polymorphisms of the NBS1 gene may influence susceptibility to the development of childhood acute leukaemia. We genotyped six polymorphisms of the NBS1 gene in 157 children with acute leukaemia and 275 controls. The TT genotype of c.2071-30A>T polymorphism was higher in leukaemia patients than in controls. Genotyping data from the six polymorphic loci in NBS1 in leukaemia patients and controls were used to impute haplotypes. Two of the evaluated haplotypes were associated with significantly increased leukaemia risk (P=0.0038 and P<0.0001). Our results suggest that some specific haplotypes of the NBS1 gene may be associated with childhood leukaemia.

Differential requirements of the C terminus of Nbs1 in suppressing adenovirus DNA replication and promoting concatemer formation

Adenoviruses (Ad) with the early region E4 deleted (E4-deleted virus) are defective for DNA replication and late protein synthesis. Infection with E4-deleted viruses results in activation of a DNA damage response, accumulation of cellular repair factors in foci at viral replication centers, and joining together of viral genomes into concatemers. The cellular DNA repair complex composed of Mre11, Rad50, and Nbs1 (MRN) is required for concatemer formation and full activation of damage signaling through the protein kinases Ataxia-telangiectasia mutated (ATM) and ATM-Rad3-related (ATR). The E4orf3 and E4orf6 proteins expressed from the E4 region of Ad type 5 (Ad5) inactivate the MRN complex by degradation and mislocalization, and prevent the DNA damage response. Here we investigated individual contributions of the MRN complex, concatemer formation, and damage signaling to viral DNA replication during infection with E4-deleted virus. Using virus mutants, short hairpin RNA knockdown and hypomorphic cell lines, we show that inactivation of MRN results in increased viral replication. We demonstrate that defective replication in the absence of E4 is not due to concatemer formation or DNA damage signaling. The C terminus of Nbs1 is required for the inhibition of Ad DNA replication and recruitment of MRN to viral replication centers. We identified regions of Nbs1 that are differentially required for concatemer formation and inhibition of Ad DNA replication. These results demonstrate that targeting of the MRN complex explains the redundant functions of E4orf3 and E4orf6 in promoting Ad DNA replication. Understanding how MRN impacts the adenoviral life cycle will provide insights into the functions of this DNA damage sensor.

Mdm2 promotes genetic instability and transformation independent of p53

Mdm2, a regulator of the tumor suppressor p53, is frequently overexpressed in human malignancies. Mdm2 also has unresolved, p53-independent functions that contribute to tumorigenesis. Here, we show that increased Mdm2 expression induced chromosome/chromatid breaks and delayed DNA double-strand break repair in cells lacking p53 but not in cells with a mutant form of Nbs1, a component of the Mre11/Rad50/Nbs1 DNA repair complex. A 31-amino-acid region of Mdm2 was necessary for binding to Nbs1. Mutation of conserved amino acids in the Nbs1 binding domain of Mdm2 inhibited Mdm2-Nbs1 association and prevented Mdm2 from delaying phosphorylation of H2AX and ATM-S/TQ sites, repair of DNA breaks, and resolution of DNA damage foci. Similarly, the mutation of eight amino acids in the Mdm2 binding domain of Nbs1 inhibited Mdm2-Nbs1 interaction and blocked the ability of Mdm2 to delay DNA break repair. Both Nbs1 and ATM, but not the ubiquitin ligase activity of Mdm2, were necessary to inhibit DNA break repair. Only Mdm2 with an intact Nbs1 binding domain was able to increase the frequency of chromosome/chromatid breaks and the transformation efficiency of cells lacking p53. Therefore, the interaction of Mdm2 with Nbs1 inhibited DNA break repair, leading to chromosome instability and subsequent transformation that was independent of p53.

Endogenous hSNM1B/Apollo interacts with TRF2 and stimulates ATM in response to ionizing radiation

Human SNM1B/Apollo is involved in the cellular response to DNA-damage, however, its precise role is unknown. Recent reports have implicated hSNM1B in the protection of telomeres. We have found hSNM1B to interact with TRF2, a protein which functions in telomere protection and in an early response to ionizing radiation. Here we show that endogenous hSNM1B forms foci which colocalize at telomeres with TRF1 and TRF2. However, we observed that additional hSNM1B foci could be induced upon exposure to ionizing radiation (IR). In live-cell-imaging experiments, hSNM1B localized to photo-induced double-strand breaks (DSBs) within 10s post-induction. Further supporting a role for hSNM1B in the early stages of the cellular response to DSBs, we observed that autophosphorylation of ATM, as well as the phosphorylation of ATM target proteins in response to IR, was attenuated in cells depleted of hSNM1B. These observations suggest an important role for hSNM1B in the response to IR damage, a role that may be, in part, upstream of the central player in maintenance of genome integrity, ATM.

Phosphorylation of SDT repeats in the MDC1 N terminus triggers retention of NBS1 at the DNA damage-modified chromatin

DNA double-strand breaks (DSBs) trigger accumulation of the MRE11–RAD50–Nijmegen breakage syndrome 1 (NBS1 [MRN]) complex, whose retention on the DSB-flanking chromatin facilitates survival. Chromatin retention of MRN requires the MDC1 adaptor protein, but the mechanism behind the MRN–MDC1 interaction is unknown. We show that the NBS1 subunit of MRN interacts with the MDC1 N terminus enriched in Ser-Asp-Thr (SDT) repeats. This interaction was constitutive and mediated by binding between the phosphorylated SDT repeats of MDC1 and the phosphate-binding forkhead-associated domain of NBS1. Phosphorylation of the SDT repeats by casein kinase 2 (CK2) was sufficient to trigger MDC1–NBS1 interaction in vitro, and MDC1 associated with CK2 activity in cells. Inhibition of CK2 reduced SDT phosphorylation in vivo, and disruption of the SDT-associated phosphoacceptor sites prevented the retention of NBS1 at DSBs. Together, these data suggest that phosphorylation of the SDT repeats in the MDC1 N terminus functions to recruit NBS1 and, thereby, increases the local concentration of MRN at the sites of chromosomal breakage.

Constitutive phosphorylation of MDC1 physically links the MRE11-RAD50-NBS1 complex to damaged chromatin

The MRE11-RAD50-Nijmegen breakage syndrome 1 (NBS1 [MRN]) complex accumulates at sites of DNA double-strand breaks (DSBs) in microscopically discernible nuclear foci. Focus formation by the MRN complex is dependent on MDC1, a large nuclear protein that directly interacts with phosphorylated H2AX. In this study, we identified a region in MDC1 that is essential for the focal accumulation of the MRN complex at sites of DNA damage. This region contains multiple conserved acidic sequence motifs that are constitutively phosphorylated in vivo. We show that these motifs are efficiently phosphorylated by caseine kinase 2 (CK2) in vitro and directly interact with the N-terminal forkhead-associated domain of NBS1 in a phosphorylation-dependent manner. Mutation of these conserved motifs in MDC1 or depletion of CK2 by small interfering RNA disrupts the interaction between MDC1 and NBS1 and abrogates accumulation of the MRN complex at sites of DNA DSBs in vivo. Thus, our data reveal the mechanism by which MDC1 physically couples the MRN complex to damaged chromatin.

The R215W mutation in NBS1 impairs gamma-H2AX binding and affects DNA repair: molecular bases for the severe phenotype of 657del5/R215W Nijmegen breakage syndrome patients

Nijmegen breakage syndrome (NBS) is a genetic disorder characterized by chromosomal instability and hypersensitivity to ionising radiation. Compound heterozygous 657del5/R215W NBS patients display a clinical phenotype more severe than the majority of NBS patients homozygous for the 657del5 mutation. The NBS1 protein, mutated in NBS patients, contains a FHA/BRCT domain necessary for the DNA-double strand break (DSB) damage response. Recently, a second BRCT domain has been identified, however, its role is still unknown. Here, we demonstrate that the R215W mutation in NBS1 impairs histone gamma-H2AX binding after induction of DNA damage, leading to a delay in DNA-DSB rejoining. Molecular modelling reveals that the 215 residue of NBS1 is located between the two BRCT domains, affecting their relative orientation that appears critical for gamma-H2AX binding. Present data represent the first evidence for the role of NBS1 tandem BRCT domains in gamma-H2AX recognition, and could explain the severe phenotype observed in 657del5/R215W NBS patients.

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.

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.

Structural requirements for the BARD1 tumor suppressor in chromosomal stability and homology-directed DNA repair

The BRCA1 tumor suppressor exists as a heterodimeric complex with BARD1, and this complex is thought to mediate many of the functions ascribed to BRCA1, including its role in tumor suppression. The two proteins share a common structural organization that features an N-terminal RING domain and two C-terminal BRCT motifs, whereas BARD1 alone also contains three tandem ankyrin repeats. In normal cells, the BRCA1/BARD1 heterodimer is believed to enhance chromosome stability by promoting homology-directed repair (HDR) of double strand DNA breaks. Here we have investigated the structural requirements for BARD1 in this process by complementation of Bard1-null mouse mammary carcinoma cells. Our results demonstrate that the ankyrin and BRCT motifs of BARD1 are each essential for both chromosome stability and HDR. Tandem BRCT motifs, including those found at the C terminus of BARD1, are known to form a phosphoprotein recognition module. Nonetheless, the HDR function of BARD1 was not perturbed by synthetic mutations predicted to ablate the phospho-recognition activity of its BRCT sequences, suggesting that some functions of the BRCT domains are not dependent on their ability to bind phosphorylated ligands. Also, cancer-associated missense mutations in the BRCT domains of BARD1 (e.g. C557S, Q564H, V695L, and S761N) have been observed in patients with breast, ovarian, and endometrial tumors. However, none of these was found to affect the HDR activity of BARD1, suggesting that any increased cancer risk conferred by these mutations is not because of defects in this repair mechanism.

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.

Characterization of an NBS1 C-terminal peptide that can inhibit ataxia telangiectasia mutated (ATM)-mediated DNA damage responses and enhance radiosensitivity

ATM and NBS1, mutation of which lead to the human autosomal recessive diseases ataxia telangiectasia and Nijmegen breakage syndrome (NBS), respectively, are essential elements in the cellular response to DNA damage induced by ionizing radiation (IR). ATM is a member of the phosphatidylinositol 3-kinase family and is activated by IR in an NBS1-dependent manner. The extreme C terminus of NBS1 contains an evolutionarily conserved sequence motif that is critical for binding to and activation of ATM after IR. ATM phosphorylates a series of targets to initiate cell cycle arrest and promote cell survival in response to DNA damage. Therefore, targeting the NBS1-ATM interaction may lead to a novel approach for specific ATM inhibition and radiosensitization. We developed small peptides containing the conserved C-terminal sequence of NBS1 to investigate whether these peptides can interfere with the DNA damage pathway. We found that wild-type NBS1 inhibitory peptides (wtNIP) can abrogate NBS1-ATM association in the presence or absence of IR. We also found that cells exposed to wtNIP displayed a significant reduction in radiation-induced gamma-H2AX and NBS1 focus formation compared with cells treated with control peptides, demonstrating that wtNIP possesses a strong inhibitory effect on ATM. The inhibitory effect of wtNIP also leads to a significant decrease in clonogenic survival in response to IR. Furthermore, wtNIP does not radiosensitize cells with defective ATM, suggesting a specific inhibition of ATM. Together, these data provide a proof of principle for the use of NBS1 C-terminal small peptides as specific ATM inhibitors and radiosensitizers.

Distinct domains in Nbs1 regulate irradiation-induced checkpoints and apoptosis

The chromosomal instability syndromes Nijmegen breakage syndrome (NBS) and ataxia telangiectasia (AT) share many overlapping phenotypes, including cancer predisposition, radiation sensitivity, cell-cycle checkpoint defects, immunodeficiency, and gonadal dysfunction. The NBS protein Nbs1 is not only a downstream target of AT mutated (ATM) kinase but also acts upstream, promoting optimal ATM activation, ATM recruitment to breaks, and ATM accessibility to substrates. By reconstituting Nbs1 knockout mice with bacterial artificial chromosomes, we have assessed the contribution of distinct regions of Nbs1 to the ATM-dependent DNA damage response. We find that T cell and oocyte development, as well as DNA damage-induced G2/M and S phase checkpoint arrest and radiation survival are dependent on the N-terminal forkhead-associated domain, but not on the principal residues phosphorylated by ATM (S278 and S343) or on the evolutionarily conserved C-terminal region of Nbs1. However, the C-terminal region regulates irradiation-induced apoptosis. These studies provide insight into the complex interplay between Nbs1 and ATM in the DNA damage response.

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.

Impaired elimination of DNA double-strand break-containing lymphocytes in ataxia telangiectasia and Nijmegen breakage syndrome

The repair of DNA double-strand breaks is critical for genome integrity and tumor suppression. Here we show that following treatment with the DNA-intercalating agent actinomycin D (ActD), normal quiescent T cells accumulate double-strand breaks and die, whereas T cells from ataxia telangiectasia (AT) and Nijmegen breakage syndrome (NBS) patients are resistant to this death pathway despite a comparable amount of DNA damage. We demonstrate that the ActD-induced death pathway in quiescent T lymphocytes follows DNA damage and H2AX phosphorylation, is ATM- and NBS1-dependent and due to p53-mediated cellular apoptosis. In response to genotoxic 2-Gy gamma-irradiation, on the other hand, quiescent T cells from normal donors survive following complete resolution of the damage thus induced. T cells from AT and NBS patients also survive, but retain foci of phosphorylated H2AX due to a subtle double-strand break (DSB) repair defect. A common consequence of these two genetic defects in the DSB response is the apparent tolerance of cells containing DNA breaks. We suggest that this tolerance makes a major contribution to the oncogenic risk of patients with chromosome instability syndromes.

RAD50 and NBS1 are breast cancer susceptibility genes associated with genomic instability

The Mre11 complex, composed of RAD50, NBS1 and MRE11, has an essential role in the maintenance of genomic integrity and preventing cells from malignancy. Here we report the association of three Mre11 complex mutations with hereditary breast cancer susceptibility, studied by using a case-control design with 317 consecutive, newly diagnosed Northern Finnish breast cancer patients and 1000 geographically matched healthy controls (P = 0.0004). RAD50 687delT displayed significantly elevated frequency in the studied patients (8 out of 317, OR 4.3, 95% CI 1.5-12.5, P= 0.008), which indicates that it is a relatively common low-penetrance risk allele in this cohort. Haplotype analysis and the screening of altogether 512 additional breast cancer cases from Sweden, Norway and Iceland suggest that RAD50 687delT is a Finnish founder mutation, not present in the other Nordic cohorts. The RAD50 IVS3-1G>A splicing mutation leading to translational frameshift was observed in one patient, and the NBS1 Leu150Phe missense mutation affecting a conserved residue in the functionally important BRCA1 carboxy-terminal (BRCT) domain in two patients, both being absent from 1000 controls. Microsatellite marker analysis showed that loss of the wild-type allele was not involved in the tumorigenesis in any of the studied mutation carriers, but they all showed increased genomic instability assessed by cytogenetic analysis of peripheral blood T-lymphocytes (P = 0.006). In particular, the total number of chromosomal rearrangements was significantly increased (P = 0.002). These findings suggest an effect for RAD50 and NBS1 haploinsufficiency on genomic integrity and susceptibility to cancer.

Importin KPNA2, NBS1, DNA Repair and Tumorigenesis

During the past 20 years, the MRE11-RAD50-NBS1 complex has become an increasingly important focus in basic and clinical cancer research. One main conceptual step forward was made with the discovery of NBS1 and the understanding of its critical pathophysiological role in Nijmegen breakage syndrome. Major efforts were carried out to define the role in DNA repair of this complex. Recently, basic research has continuously extended our understanding of the complexity of the NBS1 complex. MRE11-RAD50-NBS1 complex can no longer be viewed as having a single role in DNA damage repair since it also serves as a sensor and a mediator in cell cycle checkpoint signaling. Meanwhile, studies have challenged the concept that NBS1 only functions as a tumor suppressor in preserving genome integrity in the nucleus. It may also provide an oncogenic role in the cytoplasm which is associated with the PI3-kinase/AKT-activation pathway. Consistent with this aspect, a growing body of clinical evidence suggests that NBS1 contains a deleterious character that depends on its subcellular localization. This review focuses on recent experimental evidences demonstrating how NBS1 is translocated into the nucleus by an importin KPNA2 which mediates NBS1 subcellular localization and the functions of the NBS1 complex in tumorigenesis.

Sequential phosphorylation and multisite interactions characterize specific target recognition by the FHA domain of Ki67

The forkhead-associated (FHA) domain of human Ki67 interacts with the human nucleolar protein hNIFK, recognizing a 44-residue fragment, hNIFK226-269, phosphorylated at Thr234. Here we show that high-affinity binding requires sequential phosphorylation by two kinases, CDK1 and GSK3, yielding pThr238, pThr234 and pSer230. We have determined the solution structure of Ki67FHA in complex with the triply phosphorylated peptide hNIFK226-269(3P), revealing not only local recognition of pThr234 but also the extension of the beta-sheet of the FHA domain by the addition of a beta-strand of hNIFK. The structure of an FHA domain in complex with a biologically relevant binding partner provides insights into ligand specificity and potentially links the cancer marker protein Ki67 to a signaling pathway associated with cell fate specification.

Mild Nijmegen breakage syndrome phenotype due to alternative splicing

Hypomorphic mutations of the NBS1 gene are responsible for Nijmegen breakage syndrome (NBS), characterized by microcephaly, chromosomal instability, radiosensitivity, immunodeficiency and high cancer predisposition. Over 90% of NBS patients are homozygous for the 657Delta5 mutation and are of Slavic origin; however, 10 further truncating mutations have been identified in patients of other ethnic origin. Partially functional proteins produced by alternative initiation of translation, and possibly diminishing the severity of the NBS phenotype, have been described for several NBS1 mutations. Here, we report a 53-year-old NBS patient, homozygous for the NBS1 mutation, 742insGG, in exon 7 and who presents with a particularly mild phenotype. In an attempt to find a potential molecular explanation for the mild phenotype observed, we carried out a conventional semi-quantitative and quantitative RT-PCR analyses which revealed two transcripts of almost equal amounts in the patient and her parents--the expected full-length transcript carrying the 742insGG mutation and a second transcript with deleted exons 6 and 7. The transcript was also observed in controls and other NBS patients, however, at quantities more than 100-fold lower than that in the patient described here. Because the skipping of exons 6 and 7 results in an internal in-frame deletion, which eliminates the truncating GG-insertion, we propose that this transcript may code for a partially functional protein of approximately 70 kDa that could be responsible for the unusually mild NBS phenotype observed in this patient. Indeed, complementation analysis of null-mutant mouse cells indicates that the alternatively spliced mRNA codes for a protein with significant functional capacity.

Active role for nibrin in the kinetics of atm activation

The Atm protein kinase is central to the DNA double-strand break response in mammalian cells. After irradiation, dimeric Atm undergoes autophosphorylation at Ser 1981 and dissociates into active monomers. Atm activation is stimulated by expression of the Mre11/Rad50/nibrin complex. Previously, we showed that a C-terminal fragment of nibrin, containing binding sites for both Mre11 and Atm, was sufficient to provide this stimulatory effect in Nijmegen breakage syndrome (NBS) cells. To discriminate whether nibrin's role in Atm activation is to bind and translocate Mre11/Rad50 to the nucleus or to interact directly with Atm, we expressed an Mre11 transgene with a C-terminal NLS sequence in NBS fibroblasts. The Mre11-NLS protein complexed with Rad50, localized to the nucleus in NBS fibroblasts, and associated with chromatin. However, Atm autophosphorylation was not stimulated in cells expressing Mre11-NLS, nor were downstream Atm targets phosphorylated. To determine whether nibrin-Atm interaction is necessary to stimulate Atm activation, we expressed nibrin transgenes lacking the Atm binding domain in NBS fibroblasts. The nibrin DeltaAtm protein interacted with Mre11/Rad50; however, Atm autophosphorylation was dramatically reduced after irradiation in NBS cells expressing the nibrin DeltaAtm transgenes relative to wild-type nibrin. These results indicate that nibrin plays an active role in Atm activation beyond translocating Mre11/Rad50 to the nucleus and that this function requires nibrin-Atm interaction.

Hypoxia-Induced Phosphorylation of Chk2 in an Ataxia Telangiectasia Mutated–Dependent Manner

Chk2 is a serine/threonine kinase that signals to cell cycle arrest, DNA repair, and apoptotic pathways following DNA damage. It is activated by phosphorylation in response to ionizing radiation, UV light, stalled replication forks, and other types of DNA damage. Hypoxia is a common feature of solid tumors and has been shown to affect the regulation of many genes, including several DNA repair factors. We show here that Chk2 is phosphorylated on Thr68 and thereby activated in cells in response to hypoxia, and that this phosphorylation is dependent on the damage response kinase ataxia telangiectasia mutated (ATM) but not on the related kinase ATM and Rad3-related. Moreover, phosphorylation of Chk2 under hypoxia was attenuated in cells deficient in the repair factors MLH1 or NBS1. Finally, Chk2 serves to protect cells from apoptosis under hypoxic growth conditions. These results identify hypoxia as a new stimulus for Chk2 activation in an ATM-, MLH1-, and NBS1-dependent manner, and they suggest a novel pathway by which tumor hypoxia may influence cell survival and DNA repair.

Importin KPNA2 Is Required for Proper Nuclear Localization and Multiple Functions of NBS1

Nijmegen breakage syndrome (NBS) is a chromosomal-instability syndrome associated with cancer predisposition, radiosensitivity, microcephaly, and growth retardation. The NBS gene product, NBS1, is a component of the MRE11-RAD50-NBS1 (MRN) complex, a central player associated with double strand break (DSB) repair. In response to radiation, NBS1 is phosphorylated by ATM, and the MRN complex relocalizes to form punctate nuclear foci for DNA repair. NBS1 controls both the nuclear localization of the MRN complexes and radiation-induced focus formation. We report here that the KPNA2 (importin alpha1) is important for the normal nuclear localization of the MRN complex and its proper formation of the nuclear foci. KPNA2 is the only member of the importin alpha family that physically interacts with NBS1, and the KPNA2-mediated nucleus localization sequence (NLS) is mapped to amino acid residues 461-467 of NBS1 that is sufficient for both the interaction with KPNA2 and the proper nuclear localization. Inhibition of KPNA2 or blockage of the KPNA2 interaction with NBS1 results in a reduction of radiation-induced nuclear focus accumulation, DSB repair, and cell cycle checkpoint signaling of NBS1. Collectively, our results strongly suggest that an interaction with KPNA2 contributes to nuclear localization and multiple tumor suppression functions of the NBS1 complex.

Role of Nbs1 in the activation of the Atm kinase revealed in humanized mouse models

Nijmegen breakage syndrome (NBS) is a chromosomal fragility disorder that shares clinical and cellular features with ataxia telangiectasia. Here we demonstrate that Nbs1-null B cells are defective in the activation of ataxia-telangiectasia-mutated (Atm) in response to ionizing radiation, whereas ataxia-telangiectasia- and Rad3-related (Atr)-dependent signalling and Atm activation in response to ultraviolet light, inhibitors of DNA replication, or hypotonic stress are intact. Expression of the main human NBS allele rescues the lethality of Nbs1-/- mice, but leads to immunodeficiency, cancer predisposition, a defect in meiotic progression in females and cell-cycle checkpoint defects that are associated with a partial reduction in Atm activity. The Mre11 interaction domain of Nbs1 is essential for viability, whereas the Forkhead-associated (FHA) domain is required for T-cell and oocyte development and efficient DNA damage signalling. Reconstitution of Nbs1 knockout mice with various mutant isoforms demonstrates the biological impact of impaired Nbs1 function at the cellular and organismal level.

Ataxia-telangiectasia-like disorder (ATLD)-its clinical presentation and molecular basis

Comparison of the clinical and cellular phenotypes of different genomic instability syndromes provides new insights into functional links in the complex network of the DNA damage response. A prominent example of this principle is provided by examination of three such disorders: ataxia-telangiectasia (A-T) caused by lack or inactivation of the ATM protein kinase, which mobilises the cellular response to double strand breaks in the DNA; ataxia-telangiectasia-like disease (ATLD), a result of deficiency of the human Mre11 protein; and the Nijmegen breakage syndrome (NBS), which represents defective Nbs1 protein. Mre11 and Nbs1 are members of the Mre11/Rad50/Nbs1 (MRN) protein complex. MRN and its individual components are involved in different responses to cellular damage induced by ionising radiation and radiomimetic chemicals, including complexing with chromatin and with other damage response proteins, formation of radiation-induced foci, and the induction of different cell cycle checkpoints. The phosphorylation of Nbs1 by ATM would indicate that ATM acts upstream of the MRN complex. Consistent with this were the suggestions that ATM could be activated in the absence of fully functional Nbs1 protein. In contrast, the regulation of some ATM target proteins, e.g. Smc1 requires the MRN complex as well as ATM. Nbs1 may, therefore, be both a substrate for ATM and a mediator of ATM function. Recent studies that indicate a requirement of the MRN complex for proper ATM activation suggest that the relationship between ATM and the MRN complex in the DNA damage response is yet to be fully determined. Despite the fact that both Mre11 and Nbs1 are part of the same MRN complex, deficiency in either protein in humans does not lead to the same clinical picture. This suggests that components of the complex may also act separately.

A patient with mutations in DNA Ligase IV: Clinical features and overlap with Nijmegen breakage syndrome

The clinical phenotype of Ligase IV syndrome (LIG4 syndrome), an extremely rare autosomal recessive condition caused by mutations in the LIG4 gene, closely resembles that of Nijmegen breakage syndrome (NBS), and is characterized by microcephaly, characteristic facial features, growth retardation, developmental delay, and immunodeficiency. We report a 4(1/2)-year-old boy who presented with acute T-cell leukemia. The facial gestalt was strongly reminiscent of NBS. The patient died shortly after the onset of treatment for his T-cell leukemia. Subsequent chromosome breakage studies showed a high rate of breakage in a fibroblast culture. Radiosensitivity was assessed by a colony survival assay; the results showed radiosensitivity greater than is typically seen in NBS. Mutation screening of the NBS1 gene was negative. Sequencing of the LIG4 gene revealed a homozygous truncating mutation 2440 C>T (R814X). Although this mutation has been previously noted in LIG4 syndrome, this patient is the first reported homozygote for the mutation. In this study, we review the clinical features of this rare syndrome and provide suggestions for differential diagnosis.

Molecular mechanism of the recruitment of NBS1/hMRE11/hRAD50 complex to DNA double-strand breaks: NBS1 binds to gamma-H2AX through FHA/BRCT domain

DNA double-strand breaks represent the most potentially serious damage to a genome, and hence, many repair proteins are recruited to DNA damage sites by as yet poorly characterized sensor mechanisms. We clarified that NBS1 physically interacts with gamma-H2AX to form nuclear foci at DNA damage sites. The fork-head associated (FHA) and the BRCA1 C-terminal domains (BRCT) of NBS1 are essential for this physical interaction and focus formation of NBS1 in response to DNA damage. The inhibition of this interaction by introduction of anti-gamma-H2AX antibody into cells abolishes NBS1 foci formation in response to DNA damage. Consequently, the FHA/BRCT domain is likely to have a crucial role for both binding to histone and for re-localization of the NBS1/hMRE11/hRAD50 complex to the vicinity of DNA damage. Moreover, the foci formation of DNA repair-related proteins containing BRCT domain, such as BRCA1, requires the interaction with gamma-H2AX in response to DNA damage. These findings indicate that the physical interaction between gamma-H2AX and DNA repair-related proteins is indispensable for the recruitment of these proteins. Further, it was recently reported that the NBS1/hMRE11/hRAD50 complex has a crucial role for both the recruitment of ATM to DNA damage sites and the subsequent activation of ATM. Therefore, both gamma-H2AX and the NBS1/hMRE11/hRAD50 complex might function for the initial recognition of DNA damage.

The BRCA1 RING and BRCT domains cooperate in targeting BRCA1 to ionizing radiation-induced nuclear foci

BRCA1 accumulates in nuclear foci during S-phase and reassembles into DNA repair-associated foci after DNA damage, reflecting its role in genome maintenance. BRCA1 comprises a RING domain at the N terminus and a BRCT domain at the C terminus, through which it associates with DNA repair proteins. The key sequences that target BRCA1 to DNA damage-induced foci have not been identified. Here, we mapped the BRCA1 foci-targeting domains of yellow fluorescence protein (YFP)-tagged BRCA1 in MCF-7 breast cancer cells exposed to ionizing radiation (IR). Cancer mutations specific to the BRCT domain, but not the RING domain, abolished BRCA1 recruitment to IR-induced foci. The YFP-BRCT domain itself, however, localized poorly at IR-induced foci, and the RING domain and other sequences were negative. We discovered that only when the RING and BRCT domains were combined was foci targeting restored to levels observed for wild-type BRCA1. The RING-BRCT fusion co-localized at foci with the MDC1 DNA damage response factor and inhibited entry of endogenous BRCA1 into nuclear foci. Our results explain why exon 11-deficient BRCA1 splice variants are targeted to IR-induced foci even though they are incapable of repairing DNA damage. We propose that both RING and BRCT domains together target BRCA1 to large focal assemblies at DNA double-stranded breaks.

Xrs2p regulates Mre11p translocation to the nucleus and plays a role in telomere elongation and meiotic recombination

The Mre11-Rad50-Xrs2 (MRX) protein complex plays pivotal roles in meiotic recombination, repair of damaged DNA, telomere elongation, and cell cycle checkpoint control. Xrs2p is known to be essential for all the functions of the complex, but its role in the complex has not been clearly elucidated. A 32-amino acid region near the C terminus of Xrs2p was identified as an Mre11p-binding site. No more function of Xrs2p than translocation of Mre11p from the cytoplasm to the nucleus is necessary for response to DNA damage. However, domains in Xrs2p located both 49 amino acids upstream and 104 amino acids downstream of the Mre11p binding site are required for meiotic recombination and telomere elongation, respectively, in addition to the 32-amino acid region. These findings demonstrate that Xrs2p acts as a specificity factor that allows the MRX complex to function in meiotic recombination and in telomere elongation.

Checking on DNA damage in S phase

The precise replication of the genome and the continuous surveillance of its integrity are essential for survival and the avoidance of various diseases. Cells respond to DNA damage by activating a complex network of the so-called checkpoint pathways to delay their cell-cycle progression and repair the defects. In this review we integrate findings on the emerging mechanisms of activation, the signalling pathways and the spatio-temporal organization of the intra-S-phase DNA-damage checkpoint and its impact on the cell-cycle machinery, and discuss its biological significance.

Human SNM1B is required for normal cellular response to both DNA interstrand crosslink-inducing agents and ionizing radiation

DNA interstrand crosslinks (ICLs) are critical lesions for the mammalian cell since they affect both DNA strands and block transcription and replication. The repair of ICLs in the mammalian cell involves components of different repair pathways such as nucleotide-excision repair and the double-strand break/homologous recombination repair pathways. However, the mechanistic details of mammalian ICL repair have not been fully delineated. We describe here the complete coding sequence and the genomic organization of hSNM1B, one of at least three human homologs of the Saccharomyces cerevisiae PSO2 gene. Depletion of hSNM1B by RNA interference rendered cells hypersensitive to ICL-inducing agents. This requirement for hSNM1B in the cellular response to ICL has been hypothesized before but never experimentally verified. In addition, siRNA knockdown of hSNM1B rendered cells sensitive to ionizing radiation, suggesting the possibility of hSNM1B involvement in homologous recombination repair of double-strand breaks arising as intermediates of ICL repair. Monoubiquitination of FANCD2, a key step in the FANC/BRCA pathway, is not affected in hSNM1B-depleted HeLa cells, indicating that hSNM1B is probably not a part of the Fanconi anemia core complex. Nonetheless, similarities in the phenotype of hSNM1B-depleted cells and cultured cells from patients suffering from Fanconi anemia make hSNM1B a candidate for one of the as yet unidentified Fanconi anemia genes not involved in monoubiquitination of FANCD2.

Independent roles for nibrin and Mre11-Rad50 in the activation and function of Atm

The Atm protein kinase and Mre11-Rad50-nibrin (MRN) complex play an integral role in the cellular response to DNA double-strand breaks. Mutations in Mre11 and nibrin result in the radiosensitivity disorders ataxia-telangiectasia-like disorder (ATLD) and Nijmegen breakage syndrome (NBS), respectively. Cells from ATLD and NBS patients are deficient in activation of the Atm protein kinase and phosphorylation of downstream Atm targets following irradiation. However, the roles of individual MRN complex proteins in Atm function are not clear, because the mutations in NBS and ATLD cells result in global effects on the MRN complex. Previously we showed that the C-terminal 100 amino acids of nibrin were necessary and sufficient to translocate the MRN complex to the nucleus. Here we have taken advantage of this feature of nibrin to create isogenic cell lines lacking either nibrin or Mre11-Rad50 in the nucleus. We found that nuclear expression of Mre11-Rad50, but not nibrin, stimulated Atm activation at early times after low doses of radiation. At later times or higher doses of irradiation, Atm activation was independent of Mre11-Rad50 or nibrin. The requirement of MRN complex proteins for downstream Atm phosphorylation events following irradiation was more complex. Phosphorylation of nibrin and Chk2 by Atm required Mre11-Rad50 expression in the nucleus at early times after irradiation, reflecting the stimulation of Atm activation by Mre11-Rad50. By contrast, autophosphorylation of Chk2 and phosphorylation of Smc1 at Ser-957 was dependent on the MRN complex 60 min after irradiation, even though Atm was activated at that time point. These results indicate an independent role for Mre11-Rad50 in the activation of Atm and suggest nibrin and/or Mre11-Rad50 also act as adaptors for some downstream Atm phosphorylation events.

An inducible null mutant murine model of Nijmegen breakage syndrome proves the essential function of NBS1 in chromosomal stability and cell viability

The human genetic disorder, Nijmegen breakage syndrome, is characterized by radiosensitivity, immunodeficiency, chromosomal instability and an increased risk for cancer of the lymphatic system. The NBS1 gene codes for a protein, nibrin, involved in the processing/repair of DNA double strand breaks and in cell cycle checkpoints. Most patients are homozygous for a founder mutation, a 5 bp deletion, which might not be a null mutation, as functionally relevant truncated nibrin proteins are observed, at least in vitro. In agreement with this hypothesis, null mutation of the homologous gene, Nbn, is lethal in mice. Here, we have used Cre recombinase/loxP technology to generate an inducible Nbn null mutation allowing the examination of DNA-repair and cell cycle-checkpoints in the complete absence of nibrin. Induction of Nbn null mutation leads to the loss of the G2/M checkpoint, increased chromosome damage, radiomimetic-sensitivity and cell death. In vivo, this particularly affects the lymphatic tissues, bone marrow, thymus and spleen, whereas liver, kidney and muscle are hardly affected. In vitro, null mutant murine fibroblasts can be rescued from cell death by transfer of human nibrin cDNA and, more significantly, by a cDNA carrying the 5 bp deletion. This demonstrates, for the first time, that the common human mutation is hypomorphic and that the expression of a truncated protein is sufficient to restore nibrin's vital cellular functions.

Nijmegen breakage syndrome: clinical manifestation of defective response to DNA double-strand breaks

Nijmegen breakage syndrome is a rare autosomal recessive genetic disease belonging to a group of disorders often called chromosome instability syndromes. In addition to a characteristic facial appearance and microcephaly, patients suffering from Nijmegen breakage syndrome have a range of symptoms including radiosensitivity, immunodeficiency, increased cancer risk and growth retardation. The underlying gene, NBS1, is located on human chromosome 8q21 and codes for a protein product termed nibrin, Nbs1 or p95. Over 90% of patients are homozygous for a founder mutation: a deletion of five base pairs which leads to a framehift and protein truncation. The protein nibrin/Nbs1 is suspected to be involved in the cellular response to DNA damage caused by ionising irradiation, thus accounting for the radiosensitivity of Nijmegen breakage syndrome. We review here some of the more recent findings on the NBS1 gene and discuss how they impinge on the clinical manifestation of the disease.