A retarded rate of DNA replication and normal level of DNA repair in Werner's syndrome fibroblasts in culture

Plugged into the Ku-DNA hub: The NHEJ network

In vertebrates, double-strand breaks in DNA are primarily repaired by Non-Homologous End-Joining (NHEJ). The ring-shaped Ku heterodimer rapidly senses and threads onto broken DNA ends forming a recruiting hub. Through protein-protein contacts eventually reinforced by protein-DNA interactions, the Ku-DNA hub attracts a series of specialized proteins with scaffolding and/or enzymatic properties. To shed light on these dynamic interplays, we review here current knowledge on proteins directly interacting with Ku and on the contact points involved, with a particular accent on the different classes of Ku-binding motifs identified in several Ku partners. An integrated structural model of the core NHEJ network at the synapsis step is proposed.

The Werner Syndrome Helicase Coordinates Sequential Strand Displacement and FEN1-Mediated Flap Cleavage during Polymerase δ Elongation

The Werner syndrome protein (WRN) suppresses the loss of telomeres replicated by lagging-strand synthesis by a yet to be defined mechanism. Here, we show that whereas either WRN or the Bloom syndrome helicase (BLM) stimulates DNA polymerase δ progression across telomeric G-rich repeats, only WRN promotes sequential strand displacement synthesis and FEN1 cleavage, a critical step in Okazaki fragment maturation, at these sequences. Helicase activity, as well as the conserved winged-helix (WH) motif and the helicase and RNase D C-terminal (HRDC) domain play important but distinct roles in this process. Remarkably, WRN also influences the formation of FEN1 cleavage products during strand displacement on a nontelomeric substrate, suggesting that WRN recruitment and cooperative interaction with FEN1 during lagging-strand synthesis may serve to regulate sequential strand displacement and flap cleavage at other genomic sites. These findings define a biochemical context for the physiological role of WRN in maintaining genetic stability.

A cascade leading to premature aging phenotypes including abnormal tumor profiles in Werner syndrome (review)

This perspective review focused on the Werner syndrome (WS) by addressing the issue of how a single mutation in a WRN gene encoding WRN DNA helicase induces a wide range of premature aging phenotypes accompanied by an abnormal pattern of tumors. The key event caused by WRN gene mutation is the dysfunction of telomeres. Studies on normal aging have identified a molecular circuit in which the dysfunction of telomeres caused by cellular aging activates the TP53 gene. The resultant p53 suppresses cell growth and induces a shorter cellular lifespan, and also compromises mitochondrial biogenesis leading to the overproduction of reactive oxygen species (ROS) causing multiple aging phenotypes. As an analogy of the mechanism in natural aging, we described a hypothetical mechanism of premature aging in WS: telomere dysfunction induced by WRN mutation causes multiple premature aging phenotypes of WS, including shortened cellular lifespan and inflammation induced by ROS, such as diabetes mellitus. This model also explains the relatively late onset of the disorder, at approximately age 20. Telomere dysfunction in WS is closely correlated with abnormality in tumorigenesis. Thus, the majority of wide and complex pathological phenotypes of WS may be explained in a unified manner by the cascade beginning with telomere dysfunction initiated by WRN gene mutation.

Werner interacting protein 1 promotes binding of Werner protein to template-primer DNA

Werner interacting protein 1 (WRNIP1) that is highly conserved from Escherichia coli to human was originally identified as a protein that interacts with the Werner syndrome responsible gene product (WRN). Here, human WRNIP1 and WRN are shown to bind to template-primer DNA, and WRNIP1, but not WRN, requires ATP for DNA binding. Under conditions of a limiting amount of WRN, WRNIP1 facilitated binding of WRN to DNA in a dose-dependent manner. However, WRNIP1 did not stimulate the DNA helicase activity of WRN, and WRN displaced pre-bound WRNIP1 from DNA. Functional relationships between WRNIP1 and WRN will be discussed.

A complex forefoot malunion in a patient with Werner's syndrome: a case report

Lower extremity surgical procedures in patients with Werner's syndrome are uncommon, and for this reason they are rarely reported in the literature. In this report, we present the case of a 39-year-old man with a history of Werner's syndrome, and a malunited right hallux varus and first ray insufficiency after previous bunionectomy performed 4 years earlier. Further reconstructive surgery involved forefoot osteotomy and arthrodesis with internal fixation, and the patient ultimately healed the soft tissues and bones, despite delayed wound healing.

The Werner's Syndrome protein collaborates with REV1 to promote replication fork progression on damaged DNA

DNA damage tolerance pathways facilitate the bypass of DNA lesions encountered during replication. These pathways can be mechanistically divided into recombinational damage avoidance and translesion synthesis, in which the lesion is directly bypassed by specialised DNA polymerases. We have recently shown distinct genetic dependencies for lesion bypass at and behind the replication fork in the avian cell line DT40, bypass at the fork requiring REV1 and bypass at post-replicative gaps requiring PCNA ubiquitination by RAD18. The WRN helicase/exonuclease, which is mutated in the progeroid and cancer predisposition disorder Werner's Syndrome, has previously been implicated in a RAD18-dependent DNA damage tolerance pathway. However, WRN has also been shown to be required to maintain normal replication fork progression on a damaged DNA template, a defect reminiscent of REV1-deficient cells. Here we use the avian cell line DT40 to demonstrate that WRN assists REV1-dependent translesion synthesis at the replication fork and that PCNA ubiquitination-dependent post-replicative lesion bypass provides an important backup mechanism for damage tolerance in the absence of WRN protein.

Use of p38 MAPK Inhibitors for the Treatment of Werner Syndrome

Werner syndrome provides a convincing model for aspects of the normal ageing phenotype and may provide a suitable model for therapeutic interventions designed to combat the ageing process. Cultured primary fibroblast cells from Werner syndrome patients provide a powerful model system to study the link between replicative senescence in vitro and in vivo pathophysiology. Genome instability, together with an increased pro-oxidant state, and frequent replication fork stalling, all provide plausible triggers for intracellular stress in Werner syndrome cells, and implicates p38 MAPK signaling in their shortened replicative lifespan. A number of different p38 MAPK inhibitor chemotypes have been prepared rapidly and efficiently using microwave heating techniques for biological study in Werner syndrome cells, including SB203580, VX-745, RO3201195, UR-13756 and BIRB 796, and their selectivity and potency evaluated in this cellular context. Werner syndrome fibroblasts treated with a p38 MAPK inhibitor reveal an unexpected reversal of the accelerated ageing phenotype. Thus the study of p38 inhibition and its effect upon Werner pathophysiology is likely to provide new revelations into the biological mechanisms operating in cellular senescence and human ageing in the future.

WRN helicase unwinds Okazaki fragment-like hybrids in a reaction stimulated by the human DHX9 helicase

Mutations in the Werner gene promote the segmental progeroid Werner syndrome (WS) with increased genomic instability and cancer. The Werner gene encodes a DNA helicase (WRN) that can engage in direct protein-protein interactions with DHX9, also known as RNA helicase A or nuclear DNA helicase II, which represents an essential enzyme involved in transcription and DNA repair. By using several synthetic nucleic acid substrates we demonstrate that WRN preferably unwinds RNA-containing Okazaki fragment-like substrates suggesting a role in lagging strand maturation of DNA replication. In contrast, DHX9 preferably unwinds RNA-RNA and RNA-DNA substrates, but fails to unwind Okazaki fragment-like hybrids. We further show that the preferential unwinding of RNA-containing substrates by WRN is stimulated by DHX9 in vitro, both on Okazaki fragment-like hybrids and on RNA-containing 'chicken-foot' structures. Collectively, our results suggest that WRN and DHX9 may also cooperate in vivo, e.g. at ongoing and stalled replication forks. In the latter case, the cooperation between both helicases may serve to form and to dissolve Holliday junction-like intermediates of regressed replication forks.

Review of Two Siblings with Werner's Syndrome: A Case Report

We report the clinical course of two siblings with Werner's syndrome (WS) who were diagnosed and followed at our clinics for 12 years. Initial diagnosis of the first sibling (sister) was at age 20, the second (brother) at 16. At the initial diagnosis, the sister had amenorrhea, muscle atrophy at arms and legs, diabetes mellitus (DM), short stature, bilateral cataracts, genital hypoplasia, osteoporosis, and gray hair. During 12 years follow-up period, high-pitched voice, hepatosteatosis, renal parenchymal disease, and urethral obstruction developed. Regarding the brother, DM, cataracts and genital hypoplasia were observed at the initial diagnosis. During the 12 years follow-up period, gray hair, high-pitched voice, steatohepatosis, and osteoporosis developed.

Roles of Werner syndrome protein in protection of genome integrity

Werner syndrome protein (WRN) is one of a family of five human RecQ helicases implicated in the maintenance of genome stability. The conserved RecQ family also includes RecQ1, Bloom syndrome protein (BLM), RecQ4, and RecQ5 in humans, as well as Sgs1 in Saccharomyces cerevisiae, Rqh1 in Schizosaccharomyces pombe, and homologs in Caenorhabditis elegans, Xenopus laevis, and Drosophila melanogaster. Defects in three of the RecQ helicases, RecQ4, BLM, and WRN, cause human pathologies linked with cancer predisposition and premature aging. Mutations in the WRN gene are the causative factor of Werner syndrome (WS). WRN is one of the best characterized of the RecQ helicases and is known to have roles in DNA replication and repair, transcription, and telomere maintenance. Studies both in vitro and in vivo indicate that the roles of WRN in a variety of DNA processes are mediated by post-translational modifications, as well as several important protein-protein interactions. In this work, we will summarize some of the early studies on the cellular roles of WRN and highlight the recent findings that shed some light on the link between the protein with its cellular functions and the disease pathology.

Physical and functional interaction between WRNIP1 and RAD18

WRN interacting protein 1 (WRNIP1) was originally identified as a protein that interacts with the Werner syndrome responsible gene product (WRN). WRNIP1 is a highly conserved protein from E. coli to humans. Genetic studies in budding yeast suggested that the yeast orthlog of WRNIP1, Mgs1, may function in a DNA damage tolerance pathway that is similar to, but distinct from, the template-switch damage avoidance pathway involving Rad6, Rad18, Rad5, Mms2, and Ubc13. Here we report that human WRNIP1 binds in an ATP dependent manner to both forked DNA that mimics stalled replication forks and to template/primer DNA. We found that WRNIP1 interacts physically with RAD18 and interferes with the binding of RAD18 to forked DNA and to template/primer DNA. In contrast, RAD18 enhances the binding of WRNIP1 to these DNAs, suggesting that WRNIP1 targets DNA bound by RAD18.

Roles of the Werner syndrome RecQ helicase in DNA replication

Congenital deficiency in the WRN protein, a member of the human RecQ helicase family, gives rise to Werner syndrome, a genetic instability and cancer predisposition disorder with features of premature aging. Cellular roles of WRN are not fully elucidated. WRN has been implicated in telomere maintenance, homologous recombination, DNA repair, and other processes. Here I review the available data that directly address the role of WRN in preserving DNA integrity during replication and propose that WRN can function in coordinating replication fork progression with replication stress-induced fork remodeling. I further discuss this role of WRN within the contexts of damage tolerance group of regulatory pathways, and redundancy and cooperation with other RecQ helicases.

Vertebrate WRNIP1 and BLM are required for efficient maintenance of genome stability

Bloom syndrome (BS) is rare autosomal recessive disorder associated with chromosomal instability. The gene responsible for BS, BLM, encodes a protein belonging to the RecQ helicase family. Disruptions of the SGS1 gene of Saccharomyces cerevisiae, which encodes the RecQ helicase homologue in the budding yeast, causes accelerated aging, and this phenotype is enhanced by the disruption of MGS1, the budding yeast homologue for WRNIP1. To examine the functional relationship between RecQ and WRNIP1 in vertebrate cells, we generated and characterized wrnip1/blm cells derived from the chicken B-lymphocyte line DT40. wrnip1/blm cells showed an additive elevation of sister chromatid exchange (SCE), suggesting that both genes independently contribute to the suppression of excess SCE formation. The double mutants were more sensitive to DNA damage from camptothecin (CPT), but not to damage from methyl methanesulfonate, than either single mutant. This result suggests that WRNIP1 and BLM function independently to repair DNA or induce tolerance to the lesions induced by CPT.

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.

From old organisms to new molecules: integrative biology and therapeutic targets in accelerated human ageing

Understanding the basic biology of human ageing is a key milestone in attempting to ameliorate the deleterious consequences of old age. This is an urgent research priority given the global demographic shift towards an ageing population. Although some molecular pathways that have been proposed to contribute to ageing have been discovered using classical biochemistry and genetics, the complex, polygenic and stochastic nature of ageing is such that the process as a whole is not immediately amenable to biochemical analysis. Thus, attempts have been made to elucidate the causes of monogenic progeroid disorders that recapitulate some, if not all, features of normal ageing in the hope that this may contribute to our understanding of normal human ageing. Two canonical progeroid disorders are Werner's syndrome and Hutchinson-Gilford progeroid syndrome (also known as progeria). Because such disorders are essentially phenocopies of ageing, rather than ageing itself, advances made in understanding their pathogenesis must always be contextualised within theories proposed to help explain how the normal process operates. One such possible ageing mechanism is described by the cell senescence hypothesis of ageing. Here, we discuss this hypothesis and demonstrate that it provides a plausible explanation for many of the ageing phenotypes seen in Werner's syndrome and Hutchinson-Gilford progeriod syndrome. The recent exciting advances made in potential therapies for these two syndromes are also reviewed.

Physical and functional interactions between Werner syndrome helicase and mismatch-repair initiation factors

Werner syndrome (WS) is a severe recessive disorder characterized by premature aging, cancer predisposition and genomic instability. The gene mutated in WS encodes a bi-functional enzyme called WRN that acts as a RecQ-type DNA helicase and a 3′-5′ exonuclease, but its exact role in DNA metabolism is poorly understood. Here we show that WRN physically interacts with the MSH2/MSH6 (MutSα), MSH2/MSH3 (MutSβ) and MLH1/PMS2 (MutLα) heterodimers that are involved in the initiation of mismatch repair (MMR) and the rejection of homeologous recombination. MutSα and MutSβ can strongly stimulate the helicase activity of WRN specifically on forked DNA structures with a 3′-single-stranded arm. The stimulatory effect of MutSα on WRN-mediated unwinding is enhanced by a G/T mismatch in the DNA duplex ahead of the fork. The MutLα protein known to bind to the MutS α–heteroduplex complexes has no effect on WRN-mediated DNA unwinding stimulated by MutSα, nor does it affect DNA unwinding by WRN alone. Our data are consistent with results of genetic experiments in yeast suggesting that MMR factors act in conjunction with a RecQ-type helicase to reject recombination between divergent sequences.

Werner syndrome protein interacts functionally with translesion DNA polymerases

Werner syndrome (WS) is characterized by premature onset of age-associated disorders and predisposition to cancer. The WS protein, WRN, encodes 3' --> 5' DNA helicase and 3' --> 5' DNA exonuclease activities, and is implicated in the maintenance of genomic stability. Translesion (TLS) DNA polymerases (Pols) insert nucleotides opposite replication-blocking DNA lesions and presumably prevent replication fork stalling/collapse. Here, we present in vitro and in vivo data that demonstrate functional interaction between WRN and the TLS Pols, Poleta, Polkappa, and Poliota. In vitro, WRN stimulates the extension activity of TLS Pols on lesion-free and lesion-containing DNA templates, and alleviates pausing at stalling lesions. Stimulation is mediated through an increase in the apparent V(max) of the polymerization reaction. Notably, by accelerating the rate of nucleotide incorporation, WRN increases mutagenesis by Poleta. In vivo, WRN and Poleta colocalize at replication-dependent foci in response to UVC irradiation. The functional interaction between WRN and TLS Pols may promote replication fork progression, at the expense of increased mutagenesis, and obviate the need to resolve stalled/collapsed forks by processes involving chromosomal rearrangements.

The role of cellular senescence in Werner syndrome: toward therapeutic intervention in human premature aging

Werner syndrome (WS) is a premature aging disorder used as a model of normal human aging. WS individuals have several characteristics of normal aging, such as cataracts, hair graying, and skin aging, but manifest these at an early age. Additionally, WS individuals have high levels of inflammatory diseases, such as atherosclerosis and type 2 diabetes. The in vivo aging in WS is associated with accelerated aging of fibroblasts in culture. The cause of the accelerated senescence is not understood, but may be due to the genomic instability that is a hallmark of WS. Genome instability results in activation of stress kinases, such as p38, and the p38-specific inhibitor SB203580, prevents the accelerated senescence seen in WS fibroblasts. However, oxidative damage plays a role, as low oxygen conditions and antioxidant treatment revert some of the accelerated senescence phenotype. The effects of oxidative stress appear to be suppressible by SB203580; however, it does not appear to be transduced by p38. As SB203580 is known to inhibit other kinases in addition to p38, this suggests that more than one kinase pathway is involved. The recent development of p38 inhibitors with different binding properties, specificities, and oral bioavailability, and of new potent and selective inhibitors of JNK and MK2, will make it possible to dissect the roles of various kinase pathways in the accelerated senescence of WS cells. If this accelerated senescence is reflective of WS aging in vivo, these kinase inhibitors may well form the basis of antiaging therapies for individuals with WS.

Analyses of the interaction of WRNIP1 with Werner syndrome protein (WRN) in vitro and in the cell

Werner was originally identified as a protein that interacts with the product of the Werner syndrome (WS) gene, WRN. To examine the function of the WRNIP1/WRN complex in cells, we generated knock-out cell lines that were deficient in either WRN (WRN(-/-)), WRNIP1 (WRNIP10(-/-/-)), or both (WRNIP1(-/-/-)/WRN(-/-)), using a chicken B lymphocyte cell line, DT40. WRNIP1(-/-/-)/WRN(-/-) DT40 cells grew at a similar rate as wild-type cells, but the rate of spontaneous sister-chromatid exchange was augmented compared to that of either of the single mutant cell lines. Moreover, while WRNIP1(-/-/-) and WRN(-/-) cells were moderately sensitive to camptothecin (CPT), double mutant cells showed a synergistic increase in CPT sensitivity. This suggested that WRNIP1 and WRN do not always function cooperatively to repair DNA lesions. The lack of a discernable functional interaction between WRNIP1 and WRN prompted us to reevaluate the nature of the physical interaction between these proteins. We found that MBP-tagged WRNIP1 interacted directly with WRN, and that the interaction was enhanced by the addition of ATP. Mutations in the Walker A motifs of the two proteins revealed that WRNIP1, but not WRN, must bind ATP before an efficient interaction can occur.

Collaboration of Werner syndrome protein and BRCA1 in cellular responses to DNA interstrand cross-links

Cells deficient in the Werner syndrome protein (WRN) or BRCA1 are hypersensitive to DNA interstrand cross-links (ICLs), whose repair requires nucleotide excision repair (NER) and homologous recombination (HR). However, the roles of WRN and BRCA1 in the repair of DNA ICLs are not understood and the molecular mechanisms of ICL repair at the processing stage have not yet been established. This study demonstrates that WRN helicase activity, but not exonuclease activity, is required to process DNA ICLs in cells and that WRN cooperates with BRCA1 in the cellular response to DNA ICLs. BRCA1 interacts directly with WRN and stimulates WRN helicase and exonuclease activities in vitro. The interaction between WRN and BRCA1 increases in cells treated with DNA cross-linking agents. WRN binding to BRCA1 was mapped to BRCA1 452–1079 amino acids. The BRCA1/BARD1 complex also associates with WRN in vivo and stimulates WRN helicase activity on forked and Holliday junction substrates. These findings suggest that WRN and BRCA1 act in a coordinated manner to facilitate repair of DNA ICLs.

Analysis of the Xenopus Werner syndrome protein in DNA double-strand break repair

Werner syndrome is associated with premature aging and increased risk of cancer. Werner syndrome protein (WRN) is a RecQ-type DNA helicase, which seems to participate in DNA replication, double-strand break (DSB) repair, and telomere maintenance; however, its exact function remains elusive. Using Xenopus egg extracts as the model system, we found that Xenopus WRN (xWRN) is recruited to discrete foci upon induction of DSBs. Depletion of xWRN has no significant effect on nonhomologous end-joining of DSB ends, but it causes a significant reduction in the homology-dependent single-strand annealing DSB repair pathway. These results provide the first direct biochemical evidence that links WRN to a specific DSB repair pathway. The assay for single-strand annealing that was developed in this study also provides a powerful biochemical system for mechanistic analysis of homology-dependent DSB repair.

Prevention of Accelerated Cell Aging in Werner Syndrome Using a p38 Mitogen-Activated Protein Kinase Inhibitor

We investigated the role of p38 mitogen-activated protein kinase (MAPK) signalling in the accelerated aging of Werner Syndrome (WS) fibroblasts by use of SB203580, a cytokine-suppressive anti-inflammatory drug that targets p38 activity. SB203580 treatment reverts the aged morphology of young WS fibroblasts to that seen in young normal fibroblasts. In addition, SB203580 increases the life span and growth rate of WS fibroblasts to within the normal range. In young WS cells, p38 is activated coincident with an up-regulation of p21(WAF1), and a reduction in the levels of both activated p38 and p21(WAF1) are seen following treatment with SB203580. As these effects are not seen in young normal cells, our data suggest that the abbreviated replicative life span of WS cells is due to a stress-induced, p38-mediated growth arrest that is independent of telomere erosion. With some p38 inhibitors already in clinical trials, our data suggest a potential route to drug intervention in a premature aging syndrome.

Genetic alterations in accelerated ageing syndromes

The molecular mechanisms leading to human senescence are still not known mostly because of the complexity of the process. Different research approaches are used to study ageing including studies of monogenic segmental progeroid syndromes. None of the known progerias represents true precocious ageing. Some of them, including Werner (WS), Bloom (BS), and Rothmund-Thomson syndromes (RTS) as well as combined xeroderma pigmentosa-Cockayne syndrome (XP-CS) are characterised by features resembling precocious ageing and the increased risk of malignant disease. Such phenotypes result from the mutations of the genes encoding proteins involved in the maintenance of genomic integrity, in most cases DNA helicases. Defective functioning of these proteins affects DNA repair, recombination, replication and transcription. Other segmental progeroid syndromes, such as Hutchinson-Gilford progeria (HGPS) and Cockayne syndrome are not associated with an increased risk of cancer. In this paper we present the clinical and molecular features of selected progeroid syndromes and describe the potential implications of these data for studies of ageing and cancer development.

The Werner syndrome protein confers resistance to the DNA lesions N3-methyladenine and O6-methylguanine: implications for WRN function

The Werner syndrome (WS) protein (WRN), a DNA helicase/exonuclease, is required for genomic stability and avoidance of cancer. Current evidence suggests that WRN is involved in the resolution of stalled and/or collapsed replication forks. This function is indicated, in part, by replication defects in WS cells and by hypersensitivity to agents causing major structural aberrations in DNA that block replication. We show here that antisense suppression of WRN in two human glioma cell lines reproduces hallmarks of the drug cytotoxicity profile of WS cells, namely, hypersensitivity to 4-nitroquinoline 1-oxide, camptothecin and hydroxyurea. We also show that antisense-treated cells are hypersensitive to methyl-lexitropsin, a site-specific alkylating agent that produces mainly N3-methyladenine, a cytotoxic and replication-blocking lesion. Antisense-treated cells are hypersensitive to O(6)-methylguanine adducts as well, but only when repair by O(6)-methylguanine-DNA methyltransferase is lacking. Our results illustrate the drug sensitivity caused by deficiency of WRN in a uniform genetic background. They extend the WRN DNA damage sensitivity spectrum to methyl base adducts that can result in blocked replication, and suggest that WRN may be required for resumption of processive replication when incomplete repair of DNA damage leaves blocking lesions at forks. The evidence that highly disparate lesions fall within the purview of WRN, and that abrogating DNA repair can reveal dependence on WRN, suggests that WRN may protect the genome from the lethal, mutagenic and carcinogenic effects of widely diverse DNA damage arising from endogenous processes and environmental agents.

A Werner syndrome protein homolog affectsC. elegansdevelopment, growth rate, life span and sensitivity to DNA damage by acting at a DNA damage checkpoint

A Werner syndrome protein homolog in C. elegans (WRN-1) was immunolocalized to the nuclei of germ cells, embryonic cells, and many other cells of larval and adult worms. When wrn-1 expression was inhibited by RNA interference (RNAi), a slight reduction in C. elegans life span was observed, with accompanying signs of premature aging, such as earlier accumulation of lipofuscin and tissue deterioration in the head. In addition,various developmental defects, including small, dumpy, ruptured, transparent body, growth arrest and bag of worms, were induced by RNAi. The frequency of these defects was accentuated by γ-irradiation, implying that they were derived from spontaneous or induced DNA damage. wrn-1(RNAi) worms showed accelerated larval growth irrespective of γ-irradiation, and pre-meiotic germ cells had an abnormal checkpoint response to DNA replication blockage. These observations suggest that WRN-1 acts as a checkpoint protein for DNA damage and replication blockage. This idea is also supported by an accelerated S phase in wrn-1(RNAi) embryonic cells. wrn-1(RNAi) phenotypes similar to those of Werner syndrome, such as premature aging and short stature, suggest wrn-1-deficient C. elegans as a useful model organism for Werner syndrome.

A report of two cases of Werner's syndrome and review of the literature

Two cases of Werner's syndrome are reported. The first case is that of a man with grey hair since his 20s, and alopecia since aged about 50 years. At the age of 53 years, Werner's syndrome was diagnosed, along with a malignant soft tissue tumour of the hand. The patient underwent ray amputation for the tumour. The subsequent histopathological diagnosis was synovial cell sarcoma, and the patient died of lung metastasis at 15 weeks postsurgery. The second case is that of a woman diagnosed with diabetes mellitus when aged 34 years. At 39 years, a bilateral cataract was diagnosed and at 40 years, diabetic gangrene of the left calcaneal region and calcaneal osteomyelitis necessitated left below-knee amputation. The incidence of Werner's syndrome in Japan is extremely high (1000 of the around 1300 cases reported worldwide) compared to other countries. Most patients develop malignant tumour or arteriosclerosis, the most important complications of this syndrome. The average life expectancy for patients with Werner's syndrome is 46 years. The incidence of epithelial cancer and mesenchymal sarcoma is 10 times that of the general population. The onset of symptoms of Werner's syndrome generally precedes any later symptoms of associated conditions, such as malignant tumour. Therefore, early recognition of Werner's syndrome is important to assist identification of malignant tumours at an early stage in this patient group.

Slx1-Slx4 is a second structure-specific endonuclease functionally redundant with Sgs1-Top3

The RecQ DNA helicases human BLM and yeast Sgs1 interact with DNA topoisomerase III and are thought to act on stalled replication forks to maintain genome stability. To gain insight into this mechanism, we previously identified SLX1 and SLX4 as genes that are required for viability and for completion of rDNA replication in the absence of SGS1-TOP3. Here we show that SLX1 and SLX4 encode a heteromeric structure-specific endonuclease. The Slx1-Slx4 nuclease is active on branched DNA substrates, particularly simple-Y, 5'-flap, or replication fork structures. It cleaves the strand bearing the 5' nonhomologous arm at the branch junction and generates ligatable nicked products from 5'-flap or replication fork substrates. Slx1 is the founding member of a family of proteins with a predicted URI nuclease domain and PHD-type zinc finger. This subunit displays weak structure-specific endonuclease activity on its own, is stimulated 500-fold by Slx4, and requires the PHD finger for activity in vitro and in vivo. Both subunits are required in vivo for resistance to DNA damage by methylmethane sulfonate (MMS). We propose that Sgs1-Top3 acts at the termination of rDNA replication to decatenate stalled forks, and, in its absence, Slx1-Slx4 cleaves these stalled forks.

Asymmetry of DNA replication fork progression in Werner's syndrome

Human aging is associated with accumulation of cells that have undergone replicative senescence. The rare premature aging Werner's syndrome (WS) provides a phenocopy of normal human aging and WS patient cells recapitulate the aging phenotype in culture as they rapidly lose the ability to proliferate or replicate their DNA. WS is associated with loss of functional WRN protein. Although the biochemical properties of WRN protein, which possesses both helicase and exonuclease activities, suggest an involvement in DNA metabolism, its action in cells is not clear. Here, we provide experimental evidence for a role of the WRN protein in DNA replication in normally proliferating cells. Most importantly, we demonstrate that in the absence of functional WRN protein, replication forks from origins of bidirectional replication fail to progress normally, resulting in marked asymmetry of bidirectional forks. We propose that WRN acts in normal DNA replication to prevent collapse of replication forks or to resolve DNA junctions at stalled replication forks, and that loss of this capacity may be a contributory factor in premature aging.

A minimal exonuclease domain of WRN forms a hexamer on DNA and possesses both 3'- 5' exonuclease and 5'-protruding strand endonuclease activities

Werner syndrome is a rare autosomal recessive disease characterized by a premature aging phenotype, genomic instability, and a dramatically increased incidence of cancer and heart disease. Mutations in a single gene encoding a 1432-amino acid helicase/exonuclease (hWRN) have been shown to be responsible for the development of this disease. We have cloned, overexpressed, and purified a minimal, 171-amino acid fragment of hWRN that functions as an exonuclease. This fragment, encompassing residues 70-240 of hWRN (hWRN-N(70-240)), exhibits the same level of 3'-5' exonuclease activity as the previously described exonuclease fragment encompassing residues 1-333 of the full-length protein. The fragment also contains a 5'-protruding DNA strand endonuclease activity at a single-strand-double-strand DNA junction and within single-stranded DNA, as well as a 3'-5' exonuclease activity on single-stranded DNA. We find hWRN-N(70-240) is in a trimer-hexamer equilibrium in the absence of DNA when examined by gel filtration chromatography and atomic force microscopy. Upon addition of DNA substrate, hWRN-N(70-240) forms a hexamer and interacts with the recessed 3'-end of the DNA. Moreover, we find that the interaction of hWRN-N(70-240) with the replication protein PCNA also causes this minimal, 171-amino acid exonuclease region to form a hexamer. Thus, the active form of this minimal exonuclease fragment of human WRN appears to be a hexamer. The implications these results have on our understanding of hWRN's roles in DNA replication and repair are discussed.

DNA helicases, genomic instability, and human genetic disease

DNA helicases are a highly conserved group of enzymes that unwind DNA. They function in all processes in which access to single-stranded DNA is required, including DNA replication, DNA repair and recombination, and transcription of RNA. Defects in helicases functioning in one or more of these processes can result in characteristic human genetic disorders in which genomic instability and predisposition to cancer are common features. So far, different helicase genes have been found mutated in six such disorders. Mutations in XPB and XPD can result in xeroderma pigmentosum, Cockayne syndrome, or trichothiodystrophy. Mutations in the RecQ-like genes BLM, WRN, and RECQL4 can result in Bloom syndrome, Werner syndrome, and Rothmund-Thomson syndrome, respectively. Because XPB and XPD function in both nucleotide excision repair and transcription initiation, the cellular phenotypes associated with a deficiency of each one of them include failure to repair mutagenic DNA lesions and defects in the recovery of RNA transcription after UV irradiation. The functions of the RecQ-like genes are unknown; however, a growing body of evidence points to a function in restarting DNA replication after the replication fork has become stalled. The genomic instability associated with mutations in the RecQ-like genes includes spontaneous chromosome instability and elevated mutation rates. Mouse models for nearly all of these entities have been developed, and these should help explain the widely different clinical features that are associated with helicase mutations.

DNase I footprinting and enhanced exonuclease function of the bipartite Werner syndrome protein (WRN) bound to partially melted duplex DNA

Werner syndrome is a premature aging and cancer-prone hereditary disorder caused by deficiency of the WRN protein that harbors 3' -->5' exonuclease and RecQ-type 3' --> 5' helicase activities. To assess the possibility that WRN acts on partially melted DNA intermediates, we constructed a substrate containing a 21-nucleotide noncomplementary region asymmetrically positioned within a duplex DNA fragment. Purified WRN shows an extremely efficient exonuclease activity directed at both blunt ends of this substrate, whereas no activity is observed on a fully duplex substrate. High affinity binding of full-length WRN protects an area surrounding the melted region of the substrate from DNase I digestion. ATP binding stimulates but is not required for WRN binding to this region. Thus, binding of WRN to the melted region underlies the efficient exonuclease activity directed at the nearby ends. In contrast, a WRN deletion mutant containing only the functional exonuclease domain does not detectably bind or degrade this substrate. These experiments indicate a bipartite structure and function for WRN, and we propose a model by which its DNA binding, helicase, and exonuclease activities function coordinately in DNA metabolism. These studies also suggest that partially unwound or noncomplementary regions of DNA could be physiological targets for WRN.

Werner and Bloom helicases are involved in DNA repair in a complementary fashion

Werner syndrome (WS) is a recessive disorder characterized by premature senescence. Bloom syndrome (BS) is a recessive disorder characterized by short stature and immunodeficiency. A common characteristic of both syndromes is genomic instability leading to tumorigenesis. WRN and BLM genes causing WS and BS, encode proteins that are closely related to the RecQ helicase. We produced WRN-/-, BLM-/- and WRN(-/-)/BLM(-/-) mutants in the chicken B-cell line DT40. WRN-/- cells showed hypersensitivities to genotoxic agents, such as 4-nitroquinoline 1-oxide, camptothecin and methyl methanesulfonate. They also showed a threefold increase in targeted integration rate of exogenous DNAs, but not in sister chromatid exchange (SCE) frequency. BLM-/- cells showed hypersensitivities to the genotoxic agents as well as ultraviolet (UV) light, in addition to a 10-fold increase in targeted integration rate and an 11-fold increase in SCE frequency. In WRN(-/-)/BLM(-/-) cells, synergistically increased hypersensitivities to the genotoxic agents were observed whereas both SCE frequencies and targeted integration rates were partially diminished compared to the single mutants. Chromosomal aberrations were also synergistically increased in WRN(-/-)/BLM(-/-) cells when irradiated with UV light in late S to G(2) phases. These results suggest that both WRN and BLM may be involved in DNA repair in a complementary fashion.

A novel protein interacts with the Werner's syndrome gene product physically and functionally

Werner's syndrome (WS) is a rare autosomal recessive disorder characterized by premature aging. The gene responsible for WS encodes a protein homologous to Escherichia coli RecQ. Here we describe a novel Werner helicase interacting protein (WHIP), which interacts with the N-terminal portion of Werner protein (WRN), containing the exonuclease domain. WHIP, which shows homology to replication factor C family proteins, is conserved from E. coli to human. Ectopically expressed WHIP and WRN co-localized in granular structures in the nucleus. The functional relationship between WHIP and WRN was indicated by genetic analysis of yeast cells. Disruptants of the SGS1 gene of Saccharomyces cerevisiae, which is the WRN homologue in yeast, show an accelerated aging phenotype and high sensitivity to methyl methanesulfonate as compared with wild-type cells. Disruption of the yeast WHIP (yWHIP) gene in wild-type cells and sgs1 disruptants resulted in slightly accelerated aging and enhancement of the premature aging phenotype of sgs1 disruptants, respectively. In contrast, disruption of the yWHIP gene partially alleviated the sensitivity to methyl methanesulfonate of sgs1 disruptants.

Interactions between the Werner syndrome helicase and DNA polymerase delta specifically facilitate copying of tetraplex and hairpin structures of the d(CGG)n trinucleotide repeat sequence

Werner syndrome (WS) is an inherited disorder characterized by premature aging and genomic instability. The protein encoded by the WS gene, WRN, possesses intrinsic 3' --> 5' DNA helicase and 3' --> 5' DNA exonuclease activities. WRN helicase resolves alternate DNA structures including tetraplex and triplex DNA, and Holliday junctions. Thus, one function of WRN may be to unwind secondary structures that impede cellular DNA transactions. We report here that hairpin and G'2 bimolecular tetraplex structures of the fragile X expanded sequence, d(CGG)(n), effectively impede synthesis by three eukaryotic replicative DNA polymerases (pol): pol alpha, pol delta, and pol epsilon. The constraints imposed on pol delta-catalyzed synthesis are relieved, however, by WRN; WRN facilitates pol delta to traverse these template secondary structures to synthesize full-length DNA products. The alleviatory effect of WRN is limited to pol delta; neither pol alpha nor pol epsilon can traverse template d(CGG)(n) hairpin and tetraplex structures in the presence of WRN. Alleviation of pausing by pol delta is observed with Escherichia coli RecQ but not with UvrD helicase, suggesting a concerted action of RecQ helicases and pol delta. Our findings suggest a possible role of WRN in rescuing pol delta-mediated replication at forks stalled by unusual DNA secondary structures.

Requirements for the nucleolytic processing of DNA ends by the Werner syndrome protein-Ku70/80 complex

Werner syndrome (WS) is an inherited disease characterized by premature onset of aging, increased cancer incidence, and genomic instability. The WS gene encodes a protein with helicase and exonuclease activities. Our previous studies indicated that the Werner syndrome protein (WRN) interacts with Ku, a heterodimeric factor of 70- and 80-kDa subunits implicated in the repair of double strand DNA breaks. Moreover, we demonstrated that Ku70/80 strongly stimulates and alters WRN exonuclease activity. In this report, we investigate further the association between WRN and Ku70/80. First, using various WRN deletion mutants we show that 50 amino acids at the amino terminus are required and sufficient to interact with Ku70/80. In addition, our data indicate that the region of Ku80 between amino acids 215 and 276 is necessary for binding to WRN. Then, we show that the amino-terminal region of WRN from amino acid 1 to 388, which comprise the exonuclease domain, can be efficiently stimulated by Ku to degrade DNA substrates, indicating that the helicase domain and the carboxyl-terminal tail are not required for the stimulatory process. Finally, using gel shift assays, we demonstrate that Ku recruits WRN to DNA. Taken together, these results suggest that Ku-mediated activation of WRN exonuclease activity may play an important role in a cellular pathway that requires processing of DNA ends.

Evidence for a replication function of FFA-1, the Xenopus orthologue of Werner syndrome protein

DNA replication in higher eukaryotic cells occurs at a large number of discrete sites called replication foci. We have previously purified a protein, focus-forming activity 1 (FFA-1), which is involved in the assembly of putative prereplication foci in Xenopus egg extracts. FFA-1 is the orthologue of the Werner syndrome gene product (WRN), a member of the RecQ helicase family. In this paper we show that FFA-1 colocalizes with sites of DNA synthesis and the single-stranded DNA binding protein, replication protein A (RPA), in nuclei reconstituted in the egg extract. In addition, we show that two glutathione S-transferase FFA-1 fusion proteins can inhibit DNA replication in a dominant negative manner. The dominant negative effect correlates with the incorporation of the fusion proteins into replication foci to form "hybrid foci," which are unable to engage in DNA replication. At the biochemical level, RPA can interact with FFA-1 and specifically stimulates its DNA helicase activity. However, in the presence of the dominant negative mutant proteins, the stimulation is prevented. These results provide the first direct biochemical evidence of an important role for FFA-1 in DNA replication.

Requirement for three novel protein complexes in the absence of the Sgs1 DNA helicase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Sgs1 protein is a member of the RecQ family of DNA helicases and is required for genome stability, but not cell viability. To identify proteins that function in the absence of Sgs1, a synthetic-lethal screen was performed. We obtained mutations in six complementation groups that we refer to as SLX genes. Most of the SLX genes encode uncharacterized open reading frames that are conserved in other species. None of these genes is required for viability and all SLX null mutations are synthetically lethal with mutations in TOP3, encoding the SGS1-interacting DNA topoisomerase. Analysis of the null mutants identified a pair of genes in each of three phenotypic classes. Mutations in MMS4 (SLX2) and SLX3 generate identical phenotypes, including weak UV and strong MMS hypersensitivity, complete loss of sporulation, and synthetic growth defects with mutations in TOP1. Mms4 and Slx3 proteins coimmunoprecipitate from cell extracts, suggesting that they function in a complex. Mutations in SLX5 and SLX8 generate hydroxyurea sensitivity, reduced sporulation efficiency, and a slow-growth phenotype characterized by heterogeneous colony morphology. The Slx5 and Slx8 proteins contain RING finger domains and coimmunoprecipitate from cell extracts. The SLX1 and SLX4 genes are required for viability in the presence of an sgs1 temperature-sensitive allele at the restrictive temperature and Slx1 and Slx4 proteins are similarly associated in cell extracts. We propose that the MMS4/SLX3, SLX5/8, and SLX1/4 gene pairs encode heterodimeric complexes and speculate that these complexes are required to resolve recombination intermediates that arise in response to DNA damage, during meiosis, and in the absence of SGS1/TOP3.

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

Werner's syndrome (WS) is a recessive human genetic disorder associated with an elevated incidence of many types of cancer. The WS gene product, WRNp, belongs to the RecQ family of DNA helicases and is required for the maintenance of genomic stability in human cells. A possible interaction between helicases and topoisomerases that could co-operate in many aspects of DNA metabolism such as progression of the replication forks, recombination and repair has been recently suggested. In addition, sgs1 gene product in yeast, homologous to WS gene, has been shown to physically interact with topoisomerase types I and II. Earlier data from our laboratory suggested that WRN helicase might play a role in a G2 recombinational pathway of double strand breaks (DSBs) repair, co-operating with topoisomerase II. In this work, the effect of the topoisomerase I inhibitor camptothecin in WS cells has been investigated at the chromosomal level. The data from the present work suggest that the inhibition of topoisomerase I activity by camptothecin results in a higher induction of chromosomal damage in WS cell lines in the G2-phase and in the S-phase of the cell cycle compared to normal cells, perhaps associated with the defects in DNA replication synthesis.

The Werner syndrome protein contributes to induction of p53 by DNA damage

Mutations in the p53 tumor-suppressor gene promote increased genomic instability and cancer. Mutations in the WRN gene, encoding a DNA helicase, underlie the segmental progeroid Werner syndrome (WS). WS is also associated with increased genomic instability and elevated cancer risk. The p53 and WRN proteins can engage in direct protein-protein interactions. We report that excess WRN elicits increased cellular p53 levels and potentiates p53-mediated apoptosis. Importantly, cells derived from WS patients exhibit an attenuated and delayed induction of p53 by UV or by the topoisomerase I inhibitor camptothecin. These results suggest that WRN may participate in the activation of p53 in response to certain types of DNA damage. Furthermore, the failure to induce p53 effectively may contribute to enhanced genomic instability and elevated cancer risk in WS patients.

The Werner syndrome protein: an update

Progeria and progeroid syndromes are characterized by the earlier onset of complex senescent phenotypes. WRN was originally identified as a gene responsible for Werner syndrome (WS; "Progeria of Adults"). The WRN gene product has RecQ-type helicase domains in the central region of the protein. Subsequent studies also revealed that the WRN protein displays exonuclease activity and acts as a transcriptional activation factor. These biochemical studies, combined with cell biological studies, suggested that this protein is likely to be involved in the response to DNA damage during replication, as well as recombination and transcription processes. However, the precise molecular mechanisms by which mutations in WRN cause the WS phenotype remain unknown. Recent progress in the understanding of the WRN protein and its implication in the normal aging process are discussed.

Covalent modification of the Werner's syndrome gene product with the ubiquitin-related protein, SUMO-1

Werner's syndrome is a potential model of accelerated human aging. The gene responsible for Werner's syndrome encodes a protein that has a helicase domain homologous to Escherichia coli RecQ. To identify binding partners that regulate the function in concert with Wrn, we screened for proteins using the yeast two-hybrid system with mouse Wrn as bait and found three. One was a novel protein, and the other two were mouse Ubc9 and SUMO-1. Ubc9 also interacted with the mouse homologue of the Bloom's syndrome gene product, another eukaryotic RecQ-type helicase, but not mouse DNA helicase Q1/RecQL (RecQL1). Deletion experiments indicated that both proteins interacted with the N-terminal segment of Wrn (amino acid 272-514). The interaction between Wrn and SUMO-1 was weaker than that between Wrn and Ubc9. Positive interaction was observed in the heterogeneous combination of Wrn and yeast Ubc9 (yUbc9), as well as yUbc9 and SUMO-1, in the two-hybrid system. The interaction between yUbc9 and SUMO-1 was abolished by deleting the C-terminal Gly residue of SUMO-1, which is reportedly required for the formation of Ubc9-SUMO-1 thioester linkage. The interaction of Wrn and SUMO-1 was also abolished by deleting the Gly residue, indicating that the interaction of Wrn and SUMO-1 is mediated by yUbc9 in the two-hybrid system. Finally, we confirmed by immunoblotting with an anti-SUMO-1 antibody that Wrn was covalently attached with SUMO-1.

MPH1, a yeast gene encoding a DEAH protein, plays a role in protection of the genome from spontaneous and chemically induced damage

We have characterized the MPH1 gene from Saccharomyces cerevisiae. mph1 mutants display a spontaneous mutator phenotype. Homologs were found in archaea and in the EST libraries of Drosophila, mouse, and man. Mph1 carries the signature motifs of the DEAH family of helicases. Selected motifs were shown to be necessary for MPH1 function by introducing missense mutations. Possible indirect effects on translation and splicing were excluded by demonstrating nuclear localization of the protein and splicing proficiency of the mutant. A mutation spectrum did not show any conspicuous deviations from wild type except for an underrepresentation of frameshift mutations. The mutator phenotype was dependent on REV3 and RAD6. The mutant was sensitive to MMS, EMS, 4-NQO, and camptothecin, but not to UV light and X rays. Epistasis analyses were carried out with representative mutants from various repair pathways (msh6, mag1, apn1, rad14, rad52, rad6, mms2, and rev3). No epistatic interactions were found, either for the spontaneous mutator phenotype or for MMS, EMS, and 4-NQO sensitivity. mph1 slightly increased the UV sensitivity of mms2, rad6, and rad14 mutants, but no effect on X-ray sensitivity was observed. These data suggest that MPH1 is not part of a hitherto known repair pathway. Possible functions are discussed.

The Werner syndrome. A model for the study of human aging

Human aging is a complex process that leads to the gradual deterioration of body functions with time. Various models to approach the study of aging have been launched over the years such as the genetic analysis of life span in the yeast S. cerevisiae, the worm C. elegans, the fruitfly, and mouse, among others. In human models, there have been extensive efforts using replicative senescence, the study of centenerians, comparisons of young versus old at the organismal, cellular, and molecular levels, and the study of premature aging syndromes to understand the mechanisms leading to aging. One good model for studying human aging is a rare autosomal recessive disorder known as the Werner syndrome (WS), which is characterized by accelerated aging in vivo and in vitro. A genetic defect implicated in WS was mapped to the WRN locus. Mutations in this gene are believed to be associated, early in adulthood, with clinical symptoms normally found in old individuals. WRN functions as a DNA helicase, and recent evidence, summarized in this review, suggests specific biochemical roles for this multifaceted protein. The interaction of WRN protein with RPA (replication protein A) and p53 will undoubtedly direct efforts to further dissect the genetic pathway(s) in which WRN protein functions in DNA metabolism and will help to unravel its contribution to the human aging process.

The Werner syndrome gene: the molecular basis of RecQ helicase-deficiency diseases

Werner syndrome (WS) is an autosomal recessive genetic disorder that is manifested by genetic instability and premature onset of age-related diseases, including atherosclerosis and cancer. The gene that is mutated in WS cells (WRN) has been identified recently. Characterizations of the WRN gene product indicate that WRN encodes both a 3'-->5' DNA helicase, belonging to the Escherichiacoli RecQ helicase family, and a 3'-->5' DNA exonuclease. Studies to define the molecular mechanism of WRN-DNA transactions are currently underway in many laboratories. Preliminary results indicate that WRN functions as a key factor in resolving aberrant DNA structures that arise from DNA metabolic processes such as replication, recombination and/or repair, to preserve the genetic integrity in cells.

Functional interaction between the Werner Syndrome protein and DNA polymerase delta

Werner Syndrome (WS) is an inherited disease characterized by premature onset of aging, increased cancer incidence, and genomic instability. The WS gene encodes a 1,432-amino acid polypeptide (WRN) with a central domain homologous to the RecQ family of DNA helicases. Purified WRN unwinds DNA with 3'-->5' polarity, and also possesses 3'-->5' exonuclease activity. Elucidation of the physiologic function(s) of WRN may be aided by the identification of WRN-interacting proteins. We show here that WRN functionally interacts with DNA polymerase delta (pol delta), a eukaryotic polymerase required for DNA replication and DNA repair. WRN increases the rate of nucleotide incorporation by pol delta in the absence of proliferating cell nuclear antigen (PCNA) but does not stimulate the activity of eukaryotic DNA polymerases alpha or epsilon, or a variety of other DNA polymerases. Moreover, we show that functional interaction with WRN is mediated through the third subunit of pol delta: i.e., Pol32p of Saccharomyces cerevisae, corresponding to the recently identified p66 subunit of human pol delta. Absence of the third subunit abrogates stimulation by WRN, and stimulation is restored by reconstituting the three-subunit enzyme. Our findings suggest that WRN may facilitate pol delta-mediated DNA replication and/or DNA repair and that disruption of WRN-pol delta interaction in WS cells may contribute to the previously observed S-phase defects and/or the unusual sensitivity to a limited number of DNA damaging agents.