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.     

Intrinsic ssDNA annealing activity in the C-terminal region of WRN

Werner syndrome (WS) is a rare autosomal recessive disorder in humans characterized by premature aging and genetic instability. WS is caused by mutations in the WRN gene, which encodes a member of the RecQ family of DNA helicases. Cellular and biochemical studies suggest that WRN plays roles in DNA replication, DNA repair, telomere maintenance, and homologous recombination and that WRN has multiple enzymatic activities including 3' to 5' exonuclease, 3' to 5' helicase, and ssDNA annealing. The goal of this study was to map and further characterize the ssDNA annealing activity of WRN. Enzymatic studies using truncated forms of WRN identified a C-terminal 79 amino acid region between the RQC and the HRDC domains (aa1072-1150) that is required for ssDNA annealing activity. Deletion of the region reduced or eliminated ssDNA annealing activity of the WRN protein. Furthermore, the activity appears to correlate with DNA binding and oligomerization status of the protein.     

The enzymatic activities of the Werner syndrome protein are disabled by the amino acid polymorphism R834C

The Werner syndrome protein, WRN, is a member of the RecQ family of DNA helicases. It possesses both 3'-->5' DNA helicase and 3'-->5' DNA exonuclease activities. Mutations in WRN are causally associated with a rare, recessive disorder, Werner syndrome (WS), distinguished by premature aging and genomic instability; all are reported to result in loss of protein expression. In addition to WS-linked mutations, single nucleotide polymorphisms, with frequencies that exceed those of WS-associated mutations, are also present in WRN. We have initiated studies to determine if six of these polymorphisms affect the enzymatic activities of WRN. We show that two common polymorphisms, F1074L and C1367R, and two infrequent polymorphisms, Q724L and S1079L, exhibit little change in activity relative to wild-type WRN; the polymorphism, T172P, shows a small but consistent reduction of activity. However, an infrequent polymorphism, R834C, located in the helicase domain dramatically reduces WRN helicase and helicase-coupled exonuclease activity. The structure of the E. coli helicase core suggests that R834 may be involved in interactions with ATP. As predicted, substitution of Arg with Cys interferes with ATP hydrolysis that is absolutely required for unwinding DNA. R834C thus represents the first missense amino acid polymorphism in WRN that nearly abolishes enzymatic activity while leaving expression largely unaffected.     

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.     

p53 Modulates the exonuclease activity of Werner syndrome protein.

Werner syndrome (WS) is characterized by the early onset of symptoms of premature aging, cancer, and genomic instability. The molecular basis of the defects is not understood but presumably relates to the DNA helicase and exonuclease activities of the protein encoded by the WRN gene that is mutated in the disease. The attenuation of p53-mediated apoptosis in WS cells and reported physical interaction between WRN and the tumor suppressor p53 suggest that p53 and WRN functionally interact in a pathway necessary for the normal cellular response. In this study, we have demonstrated that p53 inhibits the exonuclease activity of the purified full-length recombinant WRN protein. p53 did not have an effect on a truncated amino-terminal WRN fragment that retains exonuclease activity but lacks the physical interaction domain for p53 located in the carboxyl terminus. Two naturally occurring p53 mutants found in human cancer displayed a reduced ability to inhibit WRN exonuclease activity. In cells arrested in S phase with hydroxyurea, WRN exits the nucleolus and colocalizes with p53 in the nucleoplasm. The regulation of WRN function by p53 is likely to play an important role in the maintenance of genomic integrity and prevention of cancer and other clinical symptoms associated with WS.     

Functional deficit associated with a missense Werner syndrome mutation

Werner syndrome (WS) is a rare autosomal recessive disorder caused by mutations in the WRN gene. WRN helicase, a member of the RecQ helicase family, is involved in various DNA metabolic pathways including DNA replication, recombination, DNA repair and telomere maintenance. In this study, we have characterized the G574R missense mutation, which was recently identified in a WS patient. Our biochemical experiments with purified mutant recombinant WRN protein showed that the G574R mutation inhibits ATP binding, and thereby leads to significant decrease in helicase activity. Exonuclease activity of the mutant protein was not significantly affected, whereas its single strand DNA annealing activity was higher than that of wild type. Deficiency in the helicase activity of the mutant may cause defects in replication and other DNA metabolic processes, which in turn could be responsible for the Werner syndrome phenotype in the patient. In contrast to the usual appearance of WS, the G574R patient has normal stature. Thus the short stature normally associated with WS may not be due to helicase deficiency.     

WRN,the protein deficient in Werner syndrome,plays a critical structural role in optimizing DNA repair

Werner syndrome (WS) predisposes patients to cancer and premature aging, owing to mutations in WRN. The WRN protein is a RECQ-like helicase and is thought to participate in DNA double-strand break (DSB) repair by non-homologous end joining (NHEJ) or homologous recombination (HR). It has been previously shown that non-homologous DNA ends develop extensive deletions during repair in WS cells, and that this WS phenotype was complemented by wild-type (wt) WRN. WRN possesses both 3' --> 5' exonuclease and 3' --> 5' helicase activities. To determine the relative contributions of each of these distinct enzymatic activities to DSB repair, we examined NHEJ and HR in WS cells (WRN-/-) complemented with either wtWRN, exonuclease-defective WRN (E-), helicase-defective WRN (H-) or exonuclease/helicase-defective WRN (E-H-). The single E-and H- mutants each partially complemented the NHEJ abnormality of WRN-/- cells. Strikingly, the E-H- double mutant complemented the WS deficiency nearly as efficiently as did wtWRN. Similarly, the double mutant complemented the moderate HR deficiency of WS cells nearly as well as did wtWRN, whereas the E- and H- single mutants increased HR to levels higher than those restored by either E-H- or wtWRN. These results suggest that balanced exonuclease and helicase activities of WRN are required for optimal HR. Moreover, WRN appears to play a structural role, independent of its enzymatic activities, in optimizing HR and efficient NHEJ repair. Another human RECQ helicase, BLM, suppressed HR but had little or no effect on NHEJ, suggesting that mammalian RECQ helicases have distinct functions that can finely regulate recombination events.     

Modulation of Werner syndrome protein function by a single mutation in the conserved RecQ domain

Naturally occurring mutations in the human RECQ3 gene result in truncated Werner protein (WRN) and manifest as a rare premature aging disorder, Werner syndrome. Cellular and biochemical studies suggest a multifaceted role of WRN in DNA replication, DNA repair, recombination, and telomere maintenance. The RecQ C-terminal (RQC) domain of WRN was determined previously to be the major site of interaction for DNA and proteins. By using site-directed mutagenesis in the WRN RQC domain, we determined which amino acids might be playing a critical role in WRN function. A site-directed mutation at Lys-1016 significantly decreased WRN binding to fork or bubble DNA substrates. Moreover, the Lys-1016 mutation markedly reduced WRN helicase activity on fork, D-loop, and Holliday junction substrates in addition to reducing significantly the ability of WRN to stimulate FEN-1 incision activities. Thus, DNA binding mediated by the RQC domain is crucial for WRN helicase and its coordinated functions. Our nuclear magnetic resonance data on the three-dimensional structure of the wild-type RQC and Lys-1016 mutant proteins display a remarkable similarity in their structures.     

Mutations in the WRN gene in mice accelerate mortality in a p53-null background

Werner's syndrome (WS) is a human disease with manifestations resembling premature aging. The gene defective in WS, WRN, encodes a DNA helicase. Here, we describe the generation of mice bearing a mutation that eliminates expression of the C terminus of the helicase domain of the WRN protein. Mutant mice are born at the expected Mendelian frequency and do not show any overt histological signs of accelerated senescence. These mice are capable of living beyond 2 years of age. Cells from these animals do not show elevated susceptibility to the genotoxins camptothecin or 4-NQO. However, mutant fibroblasts senesce approximately one passage earlier than controls. Importantly, WRN(-/-);p53(-/-) mice show an increased mortality rate relative to WRN(+/-);p53(-/-) animals. We consider possible models for the synergy between p53 and WRN mutations for the determination of life span.     

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.     

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.     

WRN helicase expression in Werner syndrome cell lines

Mutations in the chromosome 8p WRN gene cause Werner syndrome (WRN), a human autosomal recessive disease that mimics premature aging and is associated with genetic instability and an increased risk of cancer. All of the WRN mutations identified in WRN patients are predicted to truncate the WRN protein with loss of a C-terminal nuclear localization signal. However, many of these truncated proteins would retain WRN helicase and/or nuclease functional domains. We have used a combination of immune blot and immune precipitation assays to quantify WRN protein and its associated 3′→5′ helicase activity in genetically characterized WRN patient cell lines. None of the cell lines from patients harboring four different WRN mutations contained detectable WRN protein or immune-precipitable WRN helicase activity. Cell lines from WRN heterozygous individuals contained reduced amounts of both WRN protein and helicase activity. Quantitative immune blot analyses indicate that both lymphoblastoid cell lines and fibroblasts contain ~6 × 10⁴ WRN molecules/cell. Our results indicate that most WRN mutations result in functionally equivalent null alleles, that WRN heterozygote effects may result from haploinsufficiency and that successful modeling of WRN pathogenesis in the mouse or in other model systems will require the use of WRN mutations that eliminate WRN protein expression.     

The N-terminal region of Sgs1, which interacts with Top3, is required for complementation of MMS sensitivity and suppression of hyper-recombination in sgs1 disruptants

The SGS1 gene of Saccharomyces (cerevisiae is a homologue of the genes affected in Bloom's syndrome, Werner's syndrome, and Rothmund-Thomson's syndrome. Disruption of the SGS1 gene is associated with high sensitivity to methyl methanesulfonate (MMS) and hydroxyurea (HU), and with hyper-recombination phenotypes, including interchromosomal recombination between heteroalleles. SGS1 encodes a protein which has a helicase domain similar to that of Escherichia coli RecQ. A comparison of amino acid sequences among helicases of the RecQ family reveals that Sgs1,WRN, and BLM share a conserved region adjacent to the C-terminal part of the helicase domain (C-terminal conserved region). In addition, Sgs1 contains two highly charged acidic regions in its N-terminal region and the HRDC (helicase and RNaseD C-terminal) domain at its C-terminal end. These regions were also found in BLM and WRN, and in Rqh1 from Schizosaccharomyces pombe. In this study, we demonstrate that the C-terminal conserved region, as well as the helicase motifs, of Sgs1 are essential for complementation of MMS sensitivity and suppression of hyper-recombination in sgs1 mutants. In contrast, the highly charged acidic regions, the HRDC domain, and the C-terminal 252 amino acids were dispensable for the complementation of these phenotypes. Surprisingly, the N-terminal 45 amino acids of Sgs1 were absolutely required for the suppression of the above phenotypes. Introduction of missense mutations into the region encoding amino acids 4-13 abolished the ability of Sgsl to complement MMS sensitivity and suppress hyper-recombination in sgs1 mutants, and also prevented its interaction with Top3, indicating that interaction with Top3 via the N-terminal region of Sgs1 is involved in the complementation of MMS sensitivity and the suppression of hyper-recombination.     

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.     

Differential stem cell aging kinetics in Hutchinson-Gilford progeria syndrome and Werner syndrome

Hutchinson-Gilford progeria syndrome (HGPS) and Werner syndrome (WS) are two of the best characterized human progeroid syndromes. HGPS is caused by a point mutation in lamin A (LMNA) gene, resulting in the production of a truncated protein product—progerin. WS is caused by mutations in WRN gene, encoding a loss-of-function RecQ DNA helicase. Here, by gene editing we created isogenic human embryonic stem cells (ESCs) with heterozygous (G608G/+) or homozygous (G608G/G608G) LMNAmutation and biallelic WRN knockout, for modeling HGPS and WS pathogenesis, respectively. While ESCs and endothelial cells (ECs) did not present any features of premature senescence, HGPS- and WS-mesenchymal stem cells (MSCs) showed aging-associated phenotypes with different kinetics. WS-MSCs had early-onset mild premature aging phenotypes while HGPS-MSCs exhibited late-onset acute premature aging characterisitcs. Taken together, our study compares and contrasts the distinct pathologies underpinning the two premature aging disorders, and provides reliable stem-cell based models to identify new therapeutic strategies for pathological and physiological aging.     

Ku heterodimer binds to both ends of the Werner protein and functional interaction occurs at the Werner N-terminus

The human Werner syndrome protein, WRN, is a member of the RecQ helicase family and contains 3′→5′ helicase and 3′→5′ exonuclease activities. Recently, we showed that the exonuclease activity of WRN is greatly stimulated by the human Ku heterodimer protein. We have now mapped this interaction physically and functionally. The Ku70 subunit specifically interacts with the N-terminus (amino acids 1–368) of WRN, while the Ku80 subunit interacts with its C-terminus (amino acids 940– 1432). Binding between Ku70 and the N-terminus of WRN (amino acids 1–368) is sufficient for stimulation of WRN exonuclease activity. A mutant Ku heterodimer of full-length Ku80 and truncated Ku70 (amino acids 430–542) interacts with C-WRN but not with N-WRN and cannot stimulate WRN exonuclease activity. This emphasizes the functional significance of the interaction between the N-terminus of WRN and Ku70. The interaction between Ku80 and the C-terminus of WRN may modulate some other, as yet unknown, function. The strong interaction between Ku and WRN suggests that these two proteins function together in one or more pathways of DNA metabolism.