Werner's syndrome cell lines are hypersensitive to camptothecin-induced chromosomal damage

KBM-mediated interactions with KU80 promote cellular resistance to DNA replication stress in CHO cells

The KU heterodimer (KU70/80) is rapidly recruited to DNA double-strand breaks (DSBs) to regulate their processing and repair. Previous work has revealed that the amino-terminal von Willebrand-like (vWA-like) domain in KU80 harbours a conserved hydrophobic pocket that interacts with a short peptide motif known as the Ku-binding motif (KBM). The KBM is present in a variety of DNA repair proteins such as APLF, CYREN, and Werner protein (WRN). Here, to investigate the importance of KBM-mediated protein-protein interactions for KU80 function, we employed KU80-deficient Chinese Hamster Ovary (Xrs-6) cells transfected with RFP-tagged wild-type human KU80 or KU80 harbouring a mutant vWA-like domain (KU80L68R). Surprisingly, while mutant RFP-KU80L68R largely or entirely restored NHEJ efficiency and radiation resistance in KU80-deficient Xrs-6 cells, it failed to restore cellular resistance to DNA replication stress induced by camptothecin (CPT) or hydroxyurea (HU). Moreover, KU80-deficient Xrs-6 cells expressing RFP-KU80L68R accumulated pan-nuclear γH2AX in an S/G2-phase-dependent manner following treatment with CPT or HU, suggesting that the binding of KU80 to one or more KBM-containing proteins is required for the processing and/or repair of DNA ends that arise during DNA replication stress. Consistent with this idea, depletion of WRN helicase/exonuclease recapitulated the CPT-induced γH2AX phenotype, and did so epistatically with mutation of the KU80 vWA-like domain. These data identify a role for the KBM-binding by KU80 in the response and resistance of CHO cells to arrested and/or collapsed DNA replication forks, and implicate the KBM-mediated interaction of KU80 with WRN as a critical effector of this role.

R-Loop-Associated Genomic Instability and Implication of WRN and WRNIP1

Maintenance of genome stability is crucial for cell survival and relies on accurate DNA replication. However, replication fork progression is under constant attack from different exogenous and endogenous factors that can give rise to replication stress, a source of genomic instability and a notable hallmark of pre-cancerous and cancerous cells. Notably, one of the major natural threats for DNA replication is transcription. Encounters or conflicts between replication and transcription are unavoidable, as they compete for the same DNA template, so that collisions occur quite frequently. The main harmful transcription-associated structures are R-loops. These are DNA structures consisting of a DNA–RNA hybrid and a displaced single-stranded DNA, which play important physiological roles. However, if their homeostasis is altered, they become a potent source of replication stress and genome instability giving rise to several human diseases, including cancer. To combat the deleterious consequences of pathological R-loop persistence, cells have evolved multiple mechanisms, and an ever growing number of replication fork protection factors have been implicated in preventing/removing these harmful structures; however, many others are perhaps still unknown. In this review, we report the current knowledge on how aberrant R-loops affect genome integrity and how they are handled, and we discuss our recent findings on the role played by two fork protection factors, the Werner syndrome protein (WRN) and the Werner helicase-interacting protein 1 (WRNIP1) in response to R-loop-induced genome instability.

SUMO orchestrates multiple alternative DNA-protein crosslink repair pathways

Endogenous metabolites, environmental agents, and therapeutic drugs promote formation of covalent DNA-protein crosslinks (DPCs). Persistent DPCs compromise genome integrity and are eliminated by multiple repair pathways. Aberrant Top1-DNA crosslinks, or Top1ccs, are processed by Tdp1 and Wss1 functioning in parallel pathways in Saccharomyces cerevisiae. It remains obscure how cells choose between diverse mechanisms of DPC repair. Here, we show that several SUMO biogenesis factors (Ulp1, Siz2, Slx5, and Slx8) control repair of Top1cc or an analogous DPC lesion. Genetic analysis reveals that SUMO promotes Top1cc processing in the absence of Tdp1 but has an inhibitory role if cells additionally lack Wss1. In the tdp1Δ wss1Δ mutant, the E3 SUMO ligase Siz2 stimulates sumoylation in the vicinity of the DPC, but not SUMO conjugation to Top1. This Siz2-dependent sumoylation inhibits alternative DPC repair mechanisms, including Ddi1. Our findings suggest that SUMO tunes available repair pathways to facilitate faithful DPC repair.

Pharmacological targeting of differential DNA repair, radio-sensitizes WRN-deficient cancer cells in vitro and in vivo

Werner (WRN) expression is epigenetically downregulated in various tumors. It is imperative to understand differential repair process in WRN-proficient and WRN-deficient cancers to find pharmacological targets for radio-sensitization of WRN-deficient cancer. In the current investigation, we showed that pharmacological inhibition of CHK1 mediated homologous recombination repair (HRR), but not non-homologous end joining (NHEJ) repair, can causes hyper-radiosensitization of WRN-deficient cancers. This was confirmed in cancer cell lines of different tissue origin (osteosarcoma, colon adenocarcinoma and melanoma) with WRN silencing and overexpression. We established that WRN-depleted cells are dependent on a critical but compromised CHK1-mediated HRR-pathway for repairing ionizing radiation (IR) induced DSBs for their survival. Mechanistically, we unraveled a new finding that the MRE11, CTIP and WRN proteins are largely responsible for resections of late and persistent DSBs. In response to IR-treatment, MRE11 and CTIP-positively and WRN-negatively regulate p38-MAPK reactivation in a CHK1-dependent manner. A degradation resistant WRN protein, mutated at serine 1141, abrogates p38-MAPK activation. We also showed that CHK1-p38-MAPK axis plays important role in RAD51 mediated HRR in WRN-silenced cells. Like CHK1 inhibition, pharmacological-inhibition of p38-MAPK also hyper-radiosensitizes WRN-depleted cells by targeting HR-pathway. Combination treatment of CHK1-inhibitor (currently under various clinical trials) and IR exhibited a strong synergy against WRN-deficient melanoma tumor in vivo. Taken together, our findings suggest that pharmacological targeting of CHK1-p38-MAPK mediated HRR is an attractive strategy for enhancing therapeutic response of radiation treatment of cancer.

Way out/way in: How the relationship between WRN and CDK1 may change the fate of collapsed replication forks

Replication-dependent double-strand breaks (DSBs) are the main source of genomic instability as their inaccurate repair stimulates chromosomal rearrangements. In a recent work, we uncover a novel regulatory circuit that involves the Werner's syndrome helicase and CDK1, and that is essential for repair pathway choice at replication-dependent DSBs.

Lurbinectedin Inactivates the Ewing Sarcoma Oncoprotein EWS-FLI1 by Redistributing It within the Nucleus

There is a great need to develop novel approaches to target oncogenic transcription factors with small molecules. Ewing sarcoma is emblematic of this need, as it depends on the continued activity of the EWS-FLI1 transcription factor to maintain the malignant phenotype. We have previously shown that the small molecule trabectedin interferes with EWS-FLI1. Here, we report important mechanistic advances and a second-generation inhibitor to provide insight into the therapeutic targeting of EWS-FLI1. We discovered that trabectedin functionally inactivated EWS-FLI1 by redistributing the protein within the nucleus to the nucleolus. This effect was rooted in the wild-type functions of the EWSR1, compromising the N-terminal half of the chimeric oncoprotein, which is known to be similarly redistributed within the nucleus in the presence of UV light damage. A second-generation trabectedin analogue lurbinectedin (PM01183) caused the same nuclear redistribution of EWS-FLI1, leading to a loss of activity at the promoter, mRNA, and protein levels of expression. Tumor xenograft studies confirmed this effect, and it was increased in combination with irinotecan, leading to tumor regression and replacement of Ewing sarcoma cells with benign fat cells. The net result of combined lurbinectedin and irinotecan treatment was a complete reversal of EWS-FLI1 activity and elimination of established tumors in 30% to 70% of mice after only 11 days of therapy. Our results illustrate the preclinical safety and efficacy of a disease-specific therapy targeting the central oncogenic driver in Ewing sarcoma. Cancer Res; 76(22); 6657-68. ©2016 AACR.

The Caenorhabditis elegans Werner syndrome protein participates in DNA damage checkpoint and DNA repair in response to CPT-induced double-strand breaks

The RecQ helicases play roles in maintenance of genomic stability in species ranging from Escherichia coli to humans and interact with proteins involved in DNA metabolic pathways such as DNA repair, recombination, and replication. Our previous studies found that the Caenorhabditis elegans WRN-1 RecQ protein (a human WRN ortholog) exhibits ATP-dependent 3'-5' helicase activity and that the WRN-1 helicase is stimulated by RPA-1 on a long forked DNA duplex. However, the role of WRN-1 in response to S-phase associated with DSBs is unclear. We found that WRN-1 is involved in the checkpoint response to DSBs after CPT, inducing cell cycle arrest, is recruited to DSBs by RPA-1 and functions upstream of ATL-1 and ATM-1 for CHK-1 phosphorylation in the S-phase checkpoint. In addition, WRN-1 and RPA-1 recruitments to the DSBs require MRE-11, suggesting that DSB processing controlled by MRE-11 is important for WRN-1 at DSBs. The repair of CPT-induced DSBs is greatly reduced in the absence of WRN-1. These observations suggest that WRN-1 functions downstream of RPA-1 and upstream of CHK-1 in the DSB checkpoint pathway and is also required for the repair of DSB.

Dual targeting of EWS-FLI1 activity and the associated DNA damage response with trabectedin and SN38 synergistically inhibits Ewing sarcoma cell growth

PURPOSE

The goal of this study is to optimize the activity of trabectedin for Ewing sarcoma by developing a molecularly targeted combination therapy.

EXPERIMENTAL DESIGN

We have recently shown that trabectedin interferes with the activity of EWS-FLI1 in Ewing sarcoma cells. In this report, we build on this work to develop a trabectedin-based combination therapy with improved EWS-FLI1 suppression that also targets the drug-associated DNA damage to Ewing sarcoma cells.

RESULTS

We demonstrate by siRNA experiments that EWS-FLI1 drives the expression of the Werner syndrome protein (WRN) in Ewing sarcoma cells. Because WRN-deficient cells are known to be hypersensitive to camptothecins, we utilize trabectedin to block EWS-FLI1 activity, suppress WRN expression, and selectively sensitize Ewing sarcoma cells to the DNA-damaging effects of SN38. We show that trabectedin and SN38 are synergistic, demonstrate an increase in DNA double-strand breaks, an accumulation of cells in S-phase and a low picomolar IC50. In addition, SN38 cooperates with trabectedin to augment the suppression of EWS-FLI1 downstream targets, leading to an improved therapeutic index in vivo. These effects translate into the marked regression of two Ewing sarcoma xenografts at a fraction of the dose of camptothecin used in other xenograft studies.

CONCLUSIONS

These results provide the basis and rationale for translating this drug combination to the clinic. In addition, the study highlights an approach that utilizes a targeted agent to interfere with an oncogenic transcription factor and then exploits the resulting changes in gene expression to develop a molecularly targeted combination therapy.

The Drosophila orthologue of progeroid human WRN exonuclease, DmWRNexo, cleaves replication substrates but is inhibited by uracil or abasic sites : analysis of DmWRNexo activity in vitro

Werner syndrome (WS) is a rare late-onset premature ageing disease showing many of the phenotypes associated with normal ageing, and provides one of the best models for investigating cellular pathways that lead to normal ageing. WS is caused by mutation of WRN, which encodes a multifunctional DNA replication and repair helicase/exonuclease. To investigate the role of WRN protein's unique exonuclease domain, we have recently identified DmWRNexo, the fly orthologue of the exonuclease domain of human WRN. Here, we fully characterise DmWRNexo exonuclease activity in vitro, confirming 3'-5' polarity, demonstrating a requirement for Mg(2+), inhibition by ATP, and an ability to degrade both single-stranded DNA and duplex DNA substrates with 3' or 5' overhangs, or bubble structures, but with no activity on blunt ended DNA duplexes. We report a novel active site mutation that ablates enzyme activity. Lesional substrates containing uracil are partially cleaved by DmWRNexo, but the enzyme pauses on such substrates and is inhibited by abasic sites. These strong biochemical similarities to human WRN suggest that Drosophila can provide a valuable experimental system for analysing the importance of WRN exonuclease in cell and organismal ageing.

Topoisomerase degradation, DSB repair, p53 and IAPs in cancer cell resistance to camptothecin-like topoisomerase I inhibitors

Topoisomerase I (TOP1) inhibitors applied in cancer therapy such as topotecan and irinotecan are derivatives of the natural alkaloid camptothecin (CPT). The mechanism of CPT poisoning of TOP1 rests on inhibition of the re-ligation function of the enzyme resulting in the stabilization of the TOP1-cleavable complex. In the presence of CPTs this enzyme-DNA complex impairs transcription and DNA replication, resulting in fork stalling and the formation of DNA double-strand breaks (DSB) in proliferating cells. As with most chemotherapeutics, intrinsic and acquired drug resistance represents a hurdle that limits the success of CPT therapy. Preclinical data indicate that resistance to CPT-based drugs might be caused by factors such as (a) poor drug accumulation in the tumor, (b) high rate of drug efflux, (c) mutations in TOP1 leading to failure in CPT docking, or (d) altered signaling triggered by the drug-TOP1-DNA complex, (e) expression of DNA repair proteins, and (f) failure to activate cell death pathways. This review will focus on the issues (d-f). We discuss degradation of TOP1 as part of the repair pathway in the processing of TOP1 associated DNA damage, give a summary of proteins involved in repair of CPT-induced replication mediated DSB, and highlight the role of p53 and inhibitors of apoptosis proteins (IAPs), particularly XIAP and survivin, in cancer cell resistance to CPT-like chemotherapeutics.

Perturbed replication induced genome wide or at common fragile sites is differently managed in the absence of WRN

The Werner syndrome protein (WRN) is a member of the RecQ helicase family. Loss of WRN results in a human disease, the Werner syndrome (WS), characterized by high genomic instability, elevated cancer risk and premature aging. WRN is crucial for the recovery of stalled replication forks and possesses both helicase and exonuclease enzymatic activities of uncertain biological significance. Previous work revealed that WRN promotes formation of MUS81-dependent double strand breaks (DSBs) at HU-induced stalled forks, allowing replication restart at the expense of chromosome stability. Here, using cells expressing the helicase- or exonuclease-dead WRN mutant, we show that both activities of WRN are required to prevent MUS81-dependent breakage after HU-induced replication arrest. Moreover, we provide evidence that, in WS cells, DSBs generated by MUS81 do not require RAD51 activity for their formation. Surprisingly, when replication is specifically perturbed at common fragile sites (CFS) by aphidicolin, WRN limits accumulation of ssDNA gaps and no MUS81-dependent DSBs are detected. However, in both cases, RAD51 is essential to ensure viability of WS cells, although by different mechanisms. Thus, the role of WRN in response to perturbation of replication along CFS is functionally distinct from that carried out at stalled forks genome wide. Our results contribute to unveil two different mechanisms used by the cell to overcome the absence of WRN.

Nucleolin inhibits G4 oligonucleotide unwinding by Werner helicase

BACKGROUND

The Werner protein (WRNp), a member of the RecQ helicase family, is strongly associated with the nucleolus, as is nucleolin (NCL), an important nucleolar constituent protein. Both WRNp and NCL respond to the effects of DNA damaging agents. Therefore, we have investigated if these nuclear proteins interact and if this interaction has a possible functional significance in DNA damage repair.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that WRNp interacts with the RNA-binding protein, NCL, based on immunoprecipitation, immunofluorescent co-localization in live and fixed cells, and direct binding of purified WRNp to nucleolin. We also map the binding region to the C-terminal domains of both proteins. Furthermore, treatment of U2OS cells with 15 µM of the Topoisomerase I inhibitor, camptothecin, causes the dissociation of the nucleolin-Werner complex in the nucleolus, followed by partial re-association in the nucleoplasm. Other DNA damaging agents, such as hydroxyurea, Mitomycin C, and aphidicolin do not have these effects. Nucleolin or its C-terminal fragment affected the helicase, but not the exonuclease activity of WRNp, by inhibiting WRN unwinding of G4 tetraplex DNA structures, as seen in activity assays and electrophoretic mobility shift assays (EMSA).

CONCLUSIONS/SIGNIFICANCE

These data suggest that nucleolin may regulate G4 DNA unwinding by WRNp, possibly in response to certain DNA damaging agents. We postulate that the NCL-WRNp complex may contain an inactive form of WRNp, which is released from the nucleolus upon DNA damage. Then, when required, WRNp is released from inhibition and can participate in the DNA repair processes.

DNA secondary structure of the released strand stimulates WRN helicase action on forked duplexes without coordinate action of WRN exonuclease

Werner syndrome (WS) is an autosomal recessive premature aging disorder characterized by aging-related phenotypes and genomic instability. WS is caused by mutations in a gene encoding a nuclear protein, Werner syndrome protein (WRN), a member of the RecQ helicase family, that interestingly possesses both helicase and exonuclease activities. Previous studies have shown that the two activities act in concert on a single substrate. We investigated the effect of a DNA secondary structure on the two WRN activities and found that a DNA secondary structure of the displaced strand during unwinding stimulates WRN helicase without coordinate action of WRN exonuclease. These results imply that WRN helicase and exonuclease activities can act independently, and we propose that the uncoordinated action may be relevant to the in vivo activity of WRN.

Recapitulation of Werner syndrome sensitivity to camptothecin by limited knockdown of the WRN helicase/exonuclease

WRN is a RecQ helicase with an associated exonuclease activity important in DNA metabolism, including DNA replication, repair and recombination. In humans, deficiencies in WRN function cause the segmental progeroid Werner syndrome (WS), in which patients show premature onset of many hallmarks of normal human ageing. At the cellular level, WRN loss results in rapid replicative senescence, chromosomal instability and sensitivity to various DNA damaging agents including the topoisomerase inhibitor, camptothecin (CPT). Here, we investigate the potential of using either transient or stable WRN knockdown as a means of sensitising cells to CPT. We show that targeting WRN mRNA for degradation by either RNAi or hammerhead ribozyme catalysis renders human fibroblasts as sensitive to CPT as fibroblasts derived from WS patients, and furthermore, we find altered cell cycle transit and nucleolar destabilisation in these cells following CPT treatment. Such WS-like phenotypes are observed despite very limited decreases in total WRN protein, suggesting that levels of WRN protein are rate-limiting for the cellular response to camptothecin. These findings have major implications for development of anti-WRN agents that may be useful in sensitising tumour cells to clinically relevant topoisomerase inhibitors.

Identification of a coiled coil in werner syndrome protein that facilitates multimerization and promotes exonuclease processivity

Werner syndrome (WS) is a rare progeroid disorder characterized by genomic instability, increased cancer incidence, and early onset of a variety of aging pathologies. WS is unique among early aging syndromes in that affected individuals are developmentally normal, and phenotypic onset is in early adulthood. The protein defective in WS (WRN) is a member of the large RecQ family of helicases but is unique among this family in having an exonuclease. RecQ helicases form multimers, but the mechanism and consequence of multimerization remain incompletely defined. Here, we identify a novel heptad repeat coiled coil region between the WRN nuclease and helicase domains that facilitates multimerization of WRN. We mapped a novel and unique DNA-dependent protein kinase phosphorylation site proximal to the WRN multimerization region. However, phosphorylation at this site affected neither exonuclease activity nor multimeric state. We found that WRN nuclease is stimulated by DNA-dependent protein kinase independently of kinase activity or WRN nuclease multimeric status. In addition, WRN nuclease multimerization significantly increased nuclease processivity. We found that the novel WRN coiled coil domain is necessary for multimerization of the nuclease domain and sufficient to multimerize with full-length WRN in human cells. Importantly, correct homomultimerization is required for WRN function in vivo as overexpression of this multimerization domain caused increased sensitivity to camptothecin and 4-nitroquinoline 1-oxide similar to that in cells lacking functional WRN protein.

Divergent cellular phenotypes of human and mouse cells lacking the Werner syndrome RecQ helicase

Werner syndrome (WS) is a human autosomal recessive genetic instability and cancer predisposition syndrome with features of premature aging. Several genetically determined mouse models of WS have been generated, however, none develops features of premature aging or an elevated risk of neoplasia unless additional genetic perturbations are introduced. In order to determine whether differences in cellular phenotype could explain the discrepant phenotypes of Wrn-/- mice and WRN-deficient humans, we compared the cellular phenotype of newly derived Wrn-/- mouse primary fibroblasts with previous analyses of primary and transformed fibroblasts from WS patients and with newly derived, WRN-depleted human primary fibroblasts. These analyses confirmed previously reported cellular phenotypes of WRN-mutant and WRN-deficient human fibroblasts, and demonstrated that the human WRN-deficient cellular phenotype can be detected in cells grown in 5% or in 20% oxygen. In contrast, we did not identify prominent cellular phenotypes present in WRN-deficient human cells in Wrn-/- mouse fibroblasts. Our results indicate that human and mouse fibroblasts have different functional requirements for WRN protein, and that the absence of a strong cellular phenotype may in part explain the failure of Wrn-/- mice to develop an organismal phenotype resembling Werner syndrome.

Severe toxicity following induction chemotherapy for acute myelogenous leukemia in a patient with Werner's syndrome

Werner's syndrome is an autosomal recessive disorder resulting in premature aging. Most patients die in their fifth decade from malignancies or heart disease. The gene for Werner's syndrome (WRN) encodes a recQ helicase. Cells from patients with Werner's syndrome have increased sensitivity to DNA-damaging drugs in vitro. Here we present a patient with Werner's syndrome who developed severe chemotherapy-induced toxicity during treatment for acute myelogenous leukemia. We propose that lack of WRN resulted in increased sensitivity of the patient's cells to the toxicity of chemotherapy.

Roles of RECQ helicases in recombination based DNA repair, genomic stability and aging

The maintenance of the stability of genetic material is an essential feature of every living organism. Organisms across all kingdoms have evolved diverse and highly efficient repair mechanisms to protect the genome from deleterious consequences of various genotoxic factors that might tend to destabilize the integrity of the genome in each generation. One such group of proteins that is actively involved in genome surveillance is the RecQ helicase family. These proteins are highly conserved DNA helicases, which have diverse roles in multiple DNA metabolic processes such as DNA replication, recombination and DNA repair. In humans, five RecQ helicases have been identified and three of them namely, WRN, BLM and RecQL4 have been linked to genetic diseases characterized by genome instability, premature aging and cancer predisposition. This helicase family plays important roles in various DNA repair pathways including protecting the genome from illegitimate recombination during chromosome segregation in mitosis and assuring genome stability. This review mainly focuses on various roles of human RecQ helicases in the process of recombination-based DNA repair to maintain genome stability and physiological consequences of their defects in the development of cancer and premature aging.

The Werner's syndrome 4330T>C (Cys1367Arg) gene variant does not affect the in vitro cytotoxicity of topoisomerase inhibitors and platinum compounds

PURPOSE

Werner's syndrome (WS) is a recessive disorder of premature onset of processes associated with aging. Defective DNA repair has been reported after exposure of cells isolated from WS patients to DNA-damaging agents. The germline 4330T>C (Cys1367Arg) variant in the WS gene (WRN) has been associated with protection from age-related diseases, suggesting it has a functional role. We studied whether the 4330T>C variant confers altered drug sensitivity in vitro.

METHODS

4330T>C was genotyped in 372 human lymphoblastoid cell lines (LCLs) from unrelated healthy Caucasian individuals using a TaqMan-based method. The study was powered to detect the effect of the 4330T>C genotypes after exposure to camptothecin (based upon preliminary data). The effect of the 4330T>C variant on the cytotoxicity of etoposide, carboplatin, cisplatin and daunorubicin was also tested. WRN expression in 57 LCLs was measured by microarray.

RESULTS

No significant difference between the IC50 of the cells was observed among genotypes (P = 0.46) after exposure to camptothecin. No association was also observed for etoposide, carboplatin, cisplatin, and daunorubicin (ANOVA, P > 0.05). WRN expression also did not vary across genotypes (ANOVA, P = 0.37).

CONCLUSION

These results suggest that this nonsynonymous variant has relatively normal function at the cellular level.

WRN protects against topo I but not topo II inhibitors by preventing DNA break formation

The Werner syndrome helicase/3'-exonuclease (WRN) is a major component of the DNA repair and replication machinery. To analyze whether WRN is involved in the repair of topoisomerase-induced DNA damage we utilized U2-OS cells, in which WRN is stably down-regulated (wrn-kd), and the corresponding wild-type cells (wrn-wt). We show that cells not expressing WRN are hypersensitive to the toxic effect of the topoisomerase I inhibitor topotecan, but not to the topoisomerase II inhibitor etoposide. This was shown by mass survival assays, colony formation and induction of apoptosis. Upon topotecan treatment WRN deficient cells showed enhanced DNA replication inhibition and S-phase arrest, whereas after treatment with etoposide they showed the same cell cycle response as the wild-type. A considerable difference between WRN and wild-type cells was observed for DNA single- and double-strand break formation in response to topotecan. Topotecan induced DNA single-strand breaks 6h after treatment. In both wrn-wt and wrn-kd cells these breaks were repaired at similar kinetics. However, in wrn-kd but not wrn-wt cells they were converted into DNA double-strand breaks (DSBs) at high frequency, as shown by neutral comet assay and phosphorylation of H2AX. Our data provide evidence that WRN is involved in the repair of topoisomerase I, but not topoisomerase II-induced DNA damage, most likely via preventing the conversion of DNA single-strand breaks into DSBs during the resolution of stalled replication forks at topo I-DNA complexes. We suggest that the WRN status of tumor cells impacts anticancer therapy with topoisomerase I, but not topoisomerase II inhibitors.

Identification and characterization of a Drosophila ortholog of WRN exonuclease that is required to maintain genome integrity

The premature human aging Werner syndrome (WS) is caused by mutation of the RecQ-family WRN helicase, which is unique in possessing also 3'-5' exonuclease activity. WS patients show significant genomic instability with elevated cancer incidence. WRN is implicated in restraining illegitimate recombination, especially during DNA replication. Here we identify a Drosophila ortholog of the WRN exonuclease encoded by the CG7670 locus. The predicted DmWRNexo protein shows conservation of structural motifs and key catalytic residues with human WRN exonuclease, but entirely lacks a helicase domain. Insertion of a piggyBac element into the 5' UTR of CG7670 severely reduces gene expression. DmWRNexo mutant flies homozygous for this insertional allele of CG7670 are thus severely hypomorphic; although adults show no gross morphological abnormalities, females are sterile. Like human WS cells, we show that the DmWRNexo mutant flies are hypersensitive to the topoisomerase I inhibitor camptothecin. Furthermore, these mutant flies show highly elevated rates of mitotic DNA recombination resulting from excessive reciprocal exchange. This study identifies a novel WRN ortholog in flies and demonstrates an important role for WRN exonuclease in maintaining genome stability.

Werner syndrome helicase activity is essential in maintaining fragile site stability

WRN is a member of the RecQ family of DNA helicases implicated in the resolution of DNA structures leading to the stall of replication forks. Fragile sites have been proposed to be DNA regions particularly sensitive to replicative stress. Here, we establish that WRN is a key regulator of fragile site stability. We demonstrate that in response to mild doses of aphidicolin, WRN is efficiently relocalized in nuclear foci in replicating cells and that WRN deficiency is associated with accumulation of gaps and breaks at common fragile sites even under unperturbed conditions. By expressing WRN isoforms impaired in either helicase or exonuclease activity in defective cells, we identified WRN helicase activity as the function required for maintaining the stability of fragile sites. Finally, we find that WRN stabilizes fragile sites acting in a common pathway with the ataxia telangiectasia and Rad3 related replication checkpoint. These findings provide the first evidence of a crucial role for a helicase in protecting cells against chromosome breakage at normally occurring replication fork stalling sites.

WRN controls formation of extrachromosomal telomeric circles and is required for TRF2DeltaB-mediated telomere shortening

Telomere dysfunction has been proposed to contribute to the pathogenesis of Werner syndrome (WS), a premature-aging disorder. The WS protein WRN binds TRF2, a telomere-specific factor that protects chromosome ends. TRF2 possesses an amino-terminal domain that plays an essential role in preventing telomere shortening, as expression of TRF2(DeltaB), which lacks this domain, leads to the formation of telomeric circles, telomere shortening, and cell senescence. Our data show that the TRF2(DeltaB)-induced telomeric-loop homologous-recombination pathway requires WRN helicase. In addition, we show that WRN represses the formation of spontaneous telomeric circles, as demonstrated by the increased levels of telomeric circles observed in telomerase-positive WS fibroblasts. The mechanism of circle formation in WS cells does not involve XRCC3 function. Circle formation in WS cells is reduced by reconstitution with wild-type WRN but not mutant forms lacking either exonuclease or helicase activity, demonstrating that both enzymatic activities of WRN are required to suppress telomeric-circle formation in normal cells expressing telomerase reverse transcriptase. Thus, WRN has a key protective function at telomeres which influences telomere topology and inhibits accelerated attrition of telomeres.

The RecQ helicase WRN is required for normal replication fork progression after DNA damage or replication fork arrest

Werner syndrome is an autosomal recessive genetic instability and cancer predisposition syndrome with features of premature aging. Several lines of evidence have suggested that the Werner syndrome protein WRN plays a role in DNA replication and S-phase progression. In order to define the exact role of WRN in genomic replication we examined cell cycle kinetics during normal cell division and after methyl-methane-sulfonate (MMS) DNA damage or hydroxyurea (HU)-mediated replication arrest following acute depletion of WRN from human fibroblasts. Loss of WRN markedly extended the time cells needed to complete the cell cycle after either of these genotoxic treatments. Moreover, replication track analysis of individual, stretched DNA fibers showed that WRN depletion significantly reduced the speed at which replication forks elongated in vivo after MMS or HU treatment. These results establish the importance of WRN during genomic replication and indicate that WRN acts to facilitate fork progression after DNA damage or replication arrest. The data provide a mechanistic basis for a better understanding of WRN-mediated maintenance of genomic stability and for predicting the outcomes of DNA-targeting chemotherapy in several adult cancers that silence WRN expression.

Modeling Werner Syndrome in Drosophila melanogaster: hyper-recombination in flies lacking WRN-like exonuclease

Human progeroid Werner syndrome provides the current best model for analysis of human aging, recapitulating many aspects of normal aging as a result of mutation of the WRN gene. This gene encodes a RecQ-type helicase with additional exonuclease activity. While biochemical studies in vitro have proven invaluable in determining substrate specificities of the WRN exonuclease and helicase, it has been difficult to dissociate the two key enzyme activities in vivo. We are developing Drosophila as a model system for analysis of WRN function; the suitability of Drosophila for extensive and sophisticated genetic manipulation permits us to investigate regulatory pathways and the impact of WRN loss at organismal, cellular, and molecular levels. BLASTP screening of the Drosophila genome with human WRN sequence allowed us to identify three RecQ helicases with strong homology to human WRN, a presumed helicase component of the spliceosome, and two DEAH-box putative RNA helicases with weaker WRN homology. None of these helicases contain a WRN-like exonuclease domain, but two potential WRN-like exonucleases in flies encoded by the loci CG7670 and CG6744 were also identified in the BLAST search. CG6744 and CG7670 are more closely related to human WRN than to each other. We have obtained a fly strain with a piggyBac insertional mutation within the CG6744 locus, which decreases expression of the encoded mRNA. Such flies show elevated levels of somatic recombination. We suggest that WRN-like exonuclease activity is critical in maintaining genomic integrity in flies.

Human polynucleotide kinase participates in repair of DNA double-strand breaks by nonhomologous end joining but not homologous recombination

Human polynucleotide kinase (hPNK) is a bifunctional enzyme possessing a 5'-DNA kinase activity and a 3'-phosphatase activity. Studies based on cell extracts and purified proteins have indicated that hPNK can act on single-strand breaks and double-strand breaks (DSB) to restore the termini to the chemical form required for further action by DNA repair polymerases and ligases (i.e., 5'-phosphate and 3'-hydroxyl termini). These studies have revealed that hPNK can bind to XRCC4, and as a result, hPNK has been implicated as a participant in the nonhomologous end joining (NHEJ) pathway for DSB repair. We sought to confirm the role of hPNK in NHEJ in the cellular setting using a genetic approach. hPNK was stably down-regulated by RNA interference expression in M059K glioblastoma cells, which are NHEJ positive, and M059J cells, which are NHEJ deficient due to a lack of DNA-PK catalytic subunit (DNA-PKcs). Whereas depletion of hPNK significantly sensitized M059K cells to ionizing radiation, no additional sensitization was conferred to M059J cells, clearly implying that hPNK operates in the same DNA repair pathway as DNA-PKcs. On the other hand, depletion of hPNK did not increase the level of sister chromatid exchanges, indicating that hPNK is not involved in the homologous recombination DSB repair pathway. We also provide evidence that the action of hPNK in the repair of camptothecin-induced topoisomerase 1 "dead-end" complexes is independent of DNA-PKcs and that hPNK is not involved in the nucleotide excision repair pathway.

Werner syndrome protein prevents DNA breaks upon chromatin structure alteration

Werner syndrome is a rare disorder characterized by genome instability and the premature onset of several pathologies associated with aging. The gene responsible for Werner syndrome codes for a RecQ-type DNA helicase and is believed to be involved in different aspects of DNA repair, replication, and transcription. The human Werner protein (WRN) translocates from nucleoli to the nucleoplasm upon DNA damage. Here, for the first time we show WRN translocation following treatment with chloroquine (CHL) or trichostatin A (TSA), agents that alter chromatin structure without producing DNA breaks. In contrast to normal cells, WRN deficient human and murine cells incurred extensive DNA breaks upon CHL or TSA treatment, indicating a functional role for WRN in the proper response to these agents. Cells deficient for another RecQ-type helicase, Bloom syndrome, were not sensitive to these agents. WRN is known from in vitro studies to bind and stimulate the activity of topoisomerase I (Topol). CHL enhanced the association between WRN and Topol, suggesting that topological stress elicits a requirement for the stimulation of Topol by WRN. Supporting this idea, overexpression of Topol reduced CHL and TSA-induced DNA breaks in WRN null cells. We thus describe a novel function for WRN in ensuring genome stability to act in concert with Topol to prevent DNA breaks, following alterations in chromatin topology.

WRN exonuclease activity is blocked by DNA termini harboring 3' obstructive groups

Reactive oxygen species, generated either by cellular respiration or upon exposure to environmental agents such as ionizing radiation (IR), attack DNA to form a variety of oxidized base and sugar modifications. Accumulation of oxidative DNA damage has been associated with age-related disease as well as the aging process. Single-strand breaks harboring oxidative 3' obstructive termini, e.g. 3' phosphates and 3' phosphoglycolates, must be removed prior to DNA repair synthesis or ligation. In addition, 3' tyrosyl-linked protein damage, resulting from therapeutic agents such as camptothecin (CPT), must be processed to initiate repair. Several nucleases participate in DNA repair and the excision of 3' obstructive ends. As the protein defective in the segmental progeroid Werner syndrome (WRN) possesses 3'-5' exonuclease activity, and Werner syndrome cells are hypersensitive to IR and CPT, we examined for WRN exonuclease activity on 3' blocking lesions. Moreover, we compared side-by-side the activity of four prominent human 3'-5' exonucleases (WRN, APE1, TREX1, and p53) on substrates containing 3' phosphates, phosphoglycolates, and tyrosyl residues. Our studies reveal that while WRN degrades 3' hydroxyl containing substrates in a non-processive manner, it does not excise 3' phosphate, phosphoglycolate, or tyrosyl groups. In addition, we found that APE1 was most active at excising 3' blocking termini in comparison to the disease-related exonucleases TREX1, WRN, and p53 under identical physiological reaction conditions, and that TREX1 was the most powerful 3'-5' exonuclease on undamaged oligonucleotide substrates.

Correction of proliferation and drug sensitivity defects in the progeroid Werner's Syndrome by Holliday junction resolution

The progeroid Werner's syndrome (WS) represents the best current model of human aging. It is caused by loss of the WRN helicase/exonuclease, resulting in high levels of replication fork stalling and genomic instability. Current models suggest that characteristic WS phenotypes of poor S phase progression, low proliferative capacity, and drug hypersensitivity are the result of accumulation of alternative DNA structures at stalled or collapsed forks during DNA replication, and Holliday junction resolution has been shown to enhance survival of cis-platin-treated WS cells. Here, we present a direct test of the hypothesis that the replication/repair defect in unstressed WS cells is the result of an inability to resolve recombination intermediates. We have created isogenic WS cell lines expressing a nuclear-targeted bacterial Holliday junction endonuclease, RusA, and show that Holliday junction resolution by RusA restores DNA replication capacity in primary WS fibroblasts and enhances their proliferation. Furthermore, RusA expression rescues WS fibroblast hypersensitivity to replication fork blocking agents camptothecin and 4NQO, suggesting that the hypersensitivity is caused by inappropriate recombination at DNA structures formed when the replication fork arrests or collapses at 4NQO- or camptothecin-induced lesions. This work is the first to demonstrate that Holliday junction accumulation in primary Werner syndrome fibroblasts results in their poor proliferative capacity, and to rescue WS hypersensitivity to camptothecin and 4NQO by Holliday junction resolution.

Epigenetic inactivation of the premature aging Werner syndrome gene in human cancer

Werner syndrome (WS) is an inherited disorder characterized by premature onset of aging, genomic instability, and increased cancer incidence. The disease is caused by loss of function mutations of the WRN gene, a RecQ family member with both helicase and exonuclease activities. However, despite its putative tumor-suppressor function, little is known about the contribution of WRN to human sporadic malignancies. Here, we report that WRN function is abrogated in human cancer cells by transcriptional silencing associated with CpG island-promoter hypermethylation. We also show that, at the biochemical and cellular levels, the epigenetic inactivation of WRN leads to the loss of WRN-associated exonuclease activity and increased chromosomal instability and apoptosis induced by topoisomerase inhibitors. The described phenotype is reversed by the use of a DNA-demethylating agent or by the reintroduction of WRN into cancer cells displaying methylation-dependent silencing of WRN. Furthermore, the restoration of WRN expression induces tumor-suppressor-like features, such as reduced colony formation density and inhibition of tumor growth in nude mouse xenograft models. Screening a large collection of human primary tumors (n = 630) from different cell types revealed that WRN CpG island hypermethylation was a common event in epithelial and mesenchymal tumorigenesis. Most importantly, WRN hypermethylation in colorectal tumors was a predictor of good clinical response to the camptothecin analogue irinotecan, a topoisomerase inhibitor commonly used in the clinical setting for the treatment of this tumor type. These findings highlight the importance of WRN epigenetic inactivation in human cancer, leading to enhanced chromosomal instability and hypersensitivity to chemotherapeutic drugs.

A cDNA-microarray analysis of camptothecin resistance in glioblastoma cell lines

Chemotherapy, as generally available, is of a limited value in curing malignant brain tumors (gliomas), which often develop resistance to drugs, becoming completely unresponsive to any standard therapeutic approach. Camptothecins, a family of topoisomerase I inhibitor drugs, represent a new promising treatment strategy and are currently under evaluation for testing the clinical efficacy. We selected a CPT-resistant sub-line (U87CPT-R) from U87-MG grade III-IV astrocytoma cells, and compared the expression profile of the two cell lines by cDNA-microarray, as a preliminary screening of the molecular mechanisms involved in the acquisition of CPT resistance in glioma cells. The relevant role of IL-1 beta overproduction as well as a generalised up-regulation of genes implicated in angiogenesis and inflammatory response are discussed in details.

Repair of topoisomerase I-mediated DNA damage

Topoisomerase I (Top1) is an abundant and essential enzyme. Top1 is the selective target of camptothecins, which are effective anticancer agents. Top1-DNA cleavage complexes can also be trapped by various endogenous and exogenous DNA lesions including mismatches, abasic sites and carcinogenic adducts. Tyrosyl-DNA phosphodiesterase (Tdp1) is one of the repair enzymes for Top1-DNA covalent complexes. Tdp1 forms a multiprotein complex that includes poly(ADP) ribose polymerase (PARP). PARP-deficient cells are hypersensitive to camptothecins and functionally deficient for Tdp1. We will review recent developments in several pathways involved in the repair of Top1 cleavage complexes and the role of Chk1 and Chk2 checkpoint kinases in the cellular responses to Top1 inhibitors. The genes conferring camptothecin hypersensitivity are compiled for humans, budding yeast and fission yeast.

Gene expression and DNA repair in progeroid syndromes and human aging

Human progeroid syndromes are caused by mutations in single genes accelerating some but not all features of normal aging. Most progeroid disorders are linked to defects in genome maintenance, and while it remains unknown if similar processes underlie normal and premature aging, they provide useful models for the study of aging. Altered transcription is speculated to play a causative role in aging, and is involved in the pathology of most if not all progeroid syndromes. Previous studies demonstrate that there is a similar pattern of gene expression changes in primary cells from old and Werner syndrome compared to young suggesting a presence of common cellular aging mechanisms in old and progeria. Here we review the role of transcription in progeroid syndromes and discuss the implications of similar transcription aberrations in normal and premature aging.

Current advances in unraveling the function of the Werner syndrome protein

Werner syndrome (WS) is an autosomal recessive premature aging disease manifested by the mimicry of age-related phenotypes such as atherosclerosis, arteriosclerosis, cataracts, osteoporosis, soft tissue calcification, premature thinning, graying, and loss of hair, as well as a high incidence of some types of cancers. The gene product defective in WS, WRN, is a member of the RecQ family of DNA helicases that are widely distributed in nature and believed to play central roles in genomic stability of organisms ranging from prokaryotes to mammals. Interestingly, WRN is a bifunctional protein that is exceptional among RecQ helicases in that it also harbors an exonuclease activity. Furthermore, it preferentially operates on aberrant DNA structures believed to exist in vivo as intermediates in specific DNA transactions such as replication (forked DNA), recombination (Holliday junction, triplex and tetraplex DNA), and repair (partial duplex with single stranded bubble). In addition, WRN has been shown to physically and functionally interact with a variety of DNA-processing proteins, including those that are involved in resolving alternative DNA structures, repair DNA damage, and provide checkpoints for genomic stability. Despite significant research activity and considerable progress in understanding the biochemical and molecular genetic function of WRN, the in vivo molecular pathway(s) of WRN remain elusive. The following review focuses on the recent advances in the biochemistry of WRN and considers the putative in vivo functions of WRN in light of its many protein partners.

Open-minded scepticism: inferring the causal mechanisms of human ageing from genetic perturbations

Given the myriad of age-related changes and the many proposed mechanistic theories of ageing, a major problem in gerontology is distinguishing causes from effects. This review aims to identify and evaluate those mechanisms which have gathered experimental support in favour of seeing them as a cause rather than an effect of ageing. Recent results related to energy metabolism and ageing, the free radical and the DNA damage theories of ageing are reviewed and their predictions evaluated through a systems biology rationale. Crucial in this approach are genetic manipulations in animal models that enable researchers to discriminate causes from effects of ageing and focus on the causal structure of human ageing. Based on a system-level interpretation, the GH/IGF-1 axis appears the most likely explanation for caloric restriction and a possible causal mechanism of human ageing. Although much work remains to fully understand the human ageing process, there is little evidence that free radicals are a causal factor in mammalian ageing, though they may be involved in signalling pathways related to ageing. On the other hand, studying how the DNA machinery affects ageing appears a promising avenue for disclosing the human ageing process.

Telomere dysfunction in genome instability syndromes

Telomeres are nucleoprotein complexes located at the end of eukaryotic chromosomes. They have essential roles in preventing terminal fusions, protecting chromosome ends from degradation, and in chromosome positioning in the nucleus. These terminal structures consist of a tandemly repeated DNA sequence (TTAGGG in vertebrates) that varies in length from 5 to 15 kb in humans. Several proteins are attached to this telomeric DNA, some of which are also involved in different DNA damage response pathways, including Ku80, Mre11, NBS and BLM, among others. Mutations in the genes encoding these proteins cause a number of rare genetic syndromes characterized by chromosome and/or genetic instability and cancer predisposition. Deletions or mutations in any of these genes may also cause a telomere defect resulting in accelerated telomere shortening, lack of end-capping function, and/or end-to-end chromosome fusions. This telomere phenotype is also known to promote chromosomal instability and carcinogenesis. Therefore, it is essential to understand the interplay between telomere biology and genome stability. This review is focused in the dual role of chromosome fragility proteins in telomere maintenance.

Werner syndrome protein, the MRE11 complex and ATR: menage-à-trois in guarding genome stability during DNA replication?

The correct execution of the DNA replication process is crucially import for the maintenance of genome integrity of the cell. Several types of sources, both endogenous and exogenous, can give rise to DNA damage leading to the DNA replication fork arrest. The processes by which replication blockage is sensed by checkpoint sensors and how the pathway leading to resolution of stalled forks is activated are still not completely understood. However, recent emerging evidence suggests that one candidate for a sensor of replication stress is ATR and that, together with a member of RecQ family helicases, Werner syndrome protein (WRN) and MRE11 complex, can collaborate to promote the restarting of DNA synthesis through the resolution of stalled replication forks. Here, we discuss how WRN, the MRE11 complex and the ATR kinase could work together in response to replication blockage to avoid DNA replication fork collapse and genome instability.

Central role for the Werner syndrome protein/poly(ADP-ribose) polymerase 1 complex in the poly(ADP-ribosyl)ation pathway after DNA damage

A defect in the Werner syndrome protein (WRN) leads to the premature aging disease Werner syndrome (WS). Hallmark features of cells derived from WS patients include genomic instability and hypersensitivity to certain DNA-damaging agents. WRN contains a highly conserved region, the RecQ conserved domain, that plays a central role in protein interactions. We searched for proteins that bound to this region, and the most prominent direct interaction was with poly(ADP-ribose) polymerase 1 (PARP-1), a nuclear enzyme that protects the genome by responding to DNA damage and facilitating DNA repair. In pursuit of a functional interaction between WRN and PARP-1, we found that WS cells are deficient in the poly(ADP-ribosyl)ation pathway after they are treated with the DNA-damaging agents H2O2 and methyl methanesulfonate. After cellular stress, PARP-1 itself becomes activated, but the poly(ADP-ribosyl)ation of other cellular proteins is severely impaired in WS cells. Overexpression of the PARP-1 binding domain of WRN strongly inhibits the poly(ADP-ribosyl)ation activity in H2O2-treated control cell lines. These results indicate that the WRN/PARP-1 complex plays a key role in the cellular response to oxidative stress and alkylating agents, suggesting a role for these proteins in the base excision DNA repair pathway.

WRN helicase and FEN-1 form a complex upon replication arrest and together process branchmigrating DNA structures associated with the replication fork

Werner Syndrome is a premature aging disorder characterized by genomic instability, elevated recombination, and replication defects. It has been hypothesized that defective processing of certain replication fork structures by WRN may contribute to genomic instability. Fluorescence resonance energy transfer (FRET) analyses show that WRN and Flap Endonuclease-1 (FEN-1) form a complex in vivo that colocalizes in foci associated with arrested replication forks. WRN effectively stimulates FEN-1 cleavage of branch-migrating double-flap structures that are the physiological substrates of FEN-1 during replication. Biochemical analyses demonstrate that WRN helicase unwinds the chicken-foot HJ intermediate associated with a regressed replication fork and stimulates FEN-1 to cleave the unwound product in a structure-dependent manner. These results provide evidence for an interaction between WRN and FEN-1 in vivo and suggest that these proteins function together to process DNA structures associated with the replication fork.

DNA damage modulates nucleolar interaction of the Werner protein with the AAA ATPase p97/VCP

We report a novel nucleolar interaction between the AAA ATPase p97/VCP and the Werner protein (WRNp), a member of the RecQ helicase family. p97/VCP mediates several important cellular functions in eucaryotic cells, including membrane fusion of the endoplasmic reticulum and Golgi and ubiquitin-dependent protein degradation. Mutations in the WRN gene cause Werner syndrome, a genetic disorder characterized by premature onset of aging symptoms, a higher incidence of cancer, and a high susceptibility to DNA damage caused by topoisomerase inhibitors. We observed that both WRNp and valosin-containing protein (VCP) were present in the nucleoplasm and in nucleolar foci in mammalian cells and that WRNp and p97/VCP physically interacted in the nucleoli. Importantly, the nucleolar WRNp/VCP complex was dissociated by treatment with camptothecin, an inhibitor of topoisomerase I, whereas other WRNp-associated protein complexes, such as WRNp/Ku 80, were not dissociated by this drug. Because WRN syndrome cells are sensitive to topoisomerase inhibitors, these observations suggest that the VCP/WRNp interaction plays an important role in WRN biology. We propose a novel role for VCP in the DNA damage response pathway through modulation of WRNp availability.

The exonucleolytic and endonucleolytic cleavage activities of human exonuclease 1 are stimulated by an interaction with the carboxyl-terminal region of the Werner syndrome protein

Exonuclease 1 (EXO-1), a member of the RAD2 family of nucleases, has recently been proposed to function in the genetic pathways of DNA recombination, repair, and replication which are important for genome integrity. Although the role of EXO-1 is not well understood, its 5' to 3'-exonuclease and flap endonuclease activities may cleave intermediates that arise during DNA metabolism. In this study, we provide evidence that the Werner syndrome protein (WRN) physically interacts with human EXO-1 and dramatically stimulates both the exonucleolytic and endonucleolytic incision functions of EXO-1. The functional interaction between WRN and EXO-1 is mediated by a protein domain of WRN which interacts with flap endonuclease 1 (FEN-1). Thus, the genomic instability observed in WRN-/- cells may be at least partially attributed to the lack of interactions between the WRN protein and human nucleases including EXO-1.

Werner's syndrome protein is phosphorylated in an ATR/ATM-dependent manner following replication arrest and DNA damage induced during the S phase of the cell cycle

Werner's syndrome (WS) is an autosomal recessive disorder, characterized at the cellular level by genomic instability in the form of variegated translocation mosaicism and extensive deletions. Individuals with WS prematurely develop multiple age-related pathologies and exhibit increased incidence of cancer. WRN, the gene defective in WS, encodes a 160-kDa protein (WRN), which has 3'-5'exonuclease, DNA helicase and DNA-dependent ATPase activities. WRN-defective cells are hypersensitive to certain genotoxic agents that cause replication arrest and/or double-strand breaks at the replication fork, suggesting a pivotal role for WRN in the protection of the integrity of the genoma during the DNA replication process. Here, we show that WRN is phosphorylated through an ATR/ATM dependent pathway in response to replication blockage. However, we provide evidence that WRN phosphorylation is not essential for its subnuclear relocalization after replication arrest. Finally, we show that WRN and ATR colocalize after replication fork arrest, suggesting that WRN and the ATR kinase collaborate to prevent genome instability during the S phase.

Recombinational DNA repair and human disease

We review the genes and proteins related to the homologous recombinational repair (HRR) pathway that are implicated in cancer through either genetic disorders that predispose to cancer through chromosome instability or the occurrence of somatic mutations that contribute to carcinogenesis. Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and an ataxia-like disorder (ATLD), are chromosome instability disorders that are defective in the ataxia telangiectasia mutated (ATM), NBS, and Mre11 genes, respectively. These genes are critical in maintaining cellular resistance to ionizing radiation (IR), which kills largely by the production of double-strand breaks (DSBs). Bloom syndrome involves a defect in the BLM helicase, which seems to play a role in restarting DNA replication forks that are blocked at lesions, thereby promoting chromosome stability. The Werner syndrome gene (WRN) helicase, another member of the RecQ family like BLM, has very recently been found to help mediate homologous recombination. Fanconi anemia (FA) is a genetically complex chromosomal instability disorder involving seven or more genes, one of which is BRCA2. FA may be at least partially caused by the aberrant production of reactive oxidative species. The breast cancer-associated BRCA1 and BRCA2 proteins are strongly implicated in HRR; BRCA2 associates with Rad51 and appears to regulate its activity. We discuss in detail the phenotypes of the various mutant cell lines and the signaling pathways mediated by the ATM kinase. ATM's phosphorylation targets can be grouped into oxidative stress-mediated transcriptional changes, cell cycle checkpoints, and recombinational repair. We present the DNA damage response pathways by using the DSB as the prototype lesion, whose incorrect repair can initiate and augment karyotypic abnormalities.

Biochemical characterization of the WRN-FEN-1 functional interaction

Werner Syndrome is a premature aging disorder characterized by chromosomal instability. Recently we reported a novel interaction of the WRN gene product with human 5' flap endonuclease/5'-3' exonuclease (FEN-1), a DNA structure-specific nuclease implicated in pathways of DNA metabolism that are important for genomic stability. To characterize the mechanism for WRN stimulation of FEN-1 cleavage, we have determined the effect of WRN on the kinetic parameters of the FEN-1 cleavage reaction. WRN enhanced the efficiency of FEN-1 cleavage rather than DNA substrate binding. WRN effectively stimulated FEN-1 cleavage on a flap DNA substrate with streptavidin bound to the terminal 3' nucleotide at the end of the upstream duplex, indicating that WRN does not require a free upstream end to stimulate FEN-1 cleavage of the 5' flap substrate. These results indicate that the mechanism whereby WRN stimulates FEN-1 cleavage is distinct from that proposed for the functional interaction between proliferating cell nuclear antigen and FEN-1. To understand the potential importance of the WRN-FEN-1(1) interaction in DNA replication, we have tested the effect of WRN on FEN-1 cleavage of several DNA substrate intermediates that may arise during Okazaki fragment processing. WRN stimulated FEN-1 cleavage of flap substrates with a terminal monoribonucleotide, a long 5' ssDNA tract, and a pseudo-Y structure. The ability of WRN to facilitate FEN-1 cleavage of DNA replication/repair intermediates may be important for the role of WRN in the maintenance of genomic stability.

Chromosomal instability in B-lymphoblasotoid cell lines from Werner and Bloom syndrome patients

Werner's syndrome (WS) and Bloom's syndrome (BS) are rare autosomal genetic diseases that predispose to cancer and are associated with genomic instability. To characterize the genomic instability of WS and BS, we analyzed and compared the cytogenetics of B-lymphoblastoid cell lines (LCLs) from WS and BS patients and healthy donors. Although, similar spontaneous frequencies of micronuclei (MN) and sister chromatid exchanges (SCE) were observed in LCLs from WS patients and healthy donors, they were much higher in BS-LCLs. We also examined the cells' cytotoxic and cytogenetic formation (MN) response to camptothecin (CAM), etoposide (ETO), 4-nitroquinoline 1-oxide (4NQO), and mitomycin C (MMC). Compared to healthy donor LCLs, BS-LCLs but not WS-LCLs tended to be resistant to cytotoxicity and sensitive to MN induction by 4NQO and MMC. Spectrum karyotyping analysis revealed that most WS- and BS-LCLs generated "variegated translocation mosaicism" at high frequencies during cell culture. These findings support the idea that the basis of genomic instability in WS is different from that in BS.

Biochemical characterization of the DNA substrate specificity of Werner syndrome helicase

Werner syndrome is a hereditary premature aging disorder characterized by genome instability. The product of the gene defective in WS, WRN, is a helicase/exonuclease that presumably functions in DNA metabolism. To understand the DNA structures WRN acts upon in vivo, we examined its substrate preferences for unwinding. WRN unwound a 3'-single-stranded (ss)DNA-tailed duplex substrate with streptavidin bound to the end of the 3'-ssDNA tail, suggesting that WRN does not require a free DNA end to unwind the duplex; however, WRN was completely blocked by streptavidin bound to the 3'-ssDNA tail 6 nucleotides upstream of the single-stranded/double-stranded DNA junction. WRN efficiently unwound the forked duplex with streptavidin bound just upstream of the junction, suggesting that WRN recognizes elements of the fork structure to initiate unwinding. WRN unwound two important intermediates of replication/repair, a 5'-ssDNA flap substrate and a synthetic replication fork. WRN was able to translocate on the lagging strand of the synthetic replication fork to unwind duplex ahead of the fork. For the 5'-flap structure, WRN specifically displaced the 5'-flap oligonucleotide, suggesting a role of WRN in Okazaki fragment processing. The ability of WRN to target DNA replication/repair intermediates may be relevant to its role in genome stability maintenance.

The Werner syndrome helicase-nuclease--one protein, many mysteries

Werner syndrome (WS) is an autosomal recessive condition characterized by an early onset of age-related symptoms that include ocular cataracts, premature graying and loss of hair, arteriosclerosis and atherosclerosis, diabetes mellitus, osteoporosis, and a high incidence of some types of cancers. A major motivation for the study of WS is the expectation that elucidation of its underlying mechanisms will illuminate the basis for "normal" aging. In 1996, the gene responsible for the syndrome was positionally cloned. This advance launched an explosion of experiments aimed at unraveling the molecular mechanisms that lead to the WS phenotype. Soon thereafter, its protein product, WRN, was expressed, purified, and identified as a DNA helicase-exonuclease, a bifunctional enzyme that both unwinds DNA helices and cleaves nucleotides one at a time from the end of the DNA. WRN was shown to interact physically and functionally with several DNA-processing proteins, and WRN transgenic and null mutant mouse strains were generated and described. The substantial number of excellent reviews on WRN and WS that were published in the past 2 years (1-7) reflects the rapid pace of advances made in the field. Unlike those comprehensive articles, this review focuses on the biochemistry of the WRN protein and some aspects of its cell biology. Also considered are the putative functions of WRN in normal cells and the consequences of the loss of these functions in WS.

Roles of the Werner syndrome protein in pathways required for maintenance of genome stability

Werners syndrome is a disease of premature aging where the patients appear much older than their chronological age. The gene codes for a protein that is a helicase and an exonuclease, and recently we have learned about some of its protein interactions. These interactions are being discussed as they shed light on the molecular pathways in which Werner protein participates. Insight into these pathways brings insight into the aging process.