Werner helicase is localized to transcriptionally active nucleoli of cycling cells

G-Quadruplex Assembly by Ribosomal DNA: Emerging Roles in Disease Pathogenesis and Cancer Biology

Unique repetitive elements of the eukaryotic genome can be problematic for cellular DNA replication and transcription and pose a source of genomic instability. Human ribosomal DNA (rDNA) exists as repeating units clustered together on several chromosomes. Understanding the molecular mechanisms whereby rDNA interferes with normal genome homeostasis is the subject of this review. We discuss the instability of rDNA as a driver of senescence and the important roles of helicases to suppress its deleterious effects. The propensity of rDNA that is rich in guanine bases to form G-quadruplexes (G4) is discussed and evaluated in disease pathogenesis. Targeting G4 in the ribosomes and other chromosomal loci may represent a useful synthetic lethal approach to combating cancer.

Human RecQ helicases in transcription-associated stress management: bridging the gap between DNA and RNA metabolism

RecQ helicases are a highly conserved class of DNA helicases that play crucial role in almost all DNA metabolic processes including replication, repair and recombination. They are able to unwind a wide variety of complex intermediate DNA structures that may result from cellular DNA transactions and hence assist in maintaining genome integrity. Interestingly, a huge number of recent reports suggest that many of the RecQ family helicases are directly or indirectly involved in regulating transcription and gene expression. On one hand, they can remove complex structures like R-loops, G-quadruplexes or RNA:DNA hybrids formed at the intersection of transcription and replication. On the other hand, emerging evidence suggests that they can also regulate transcription by directly interacting with RNA polymerase or recruiting other protein factors that may regulate transcription. This review summarizes the up to date knowledge on the involvement of three human RecQ family proteins BLM, WRN and RECQL5 in transcription regulation and management of transcription associated stress.

DNA helicases and their roles in cancer

DNA helicases, known for their fundamentally important roles in genomic stability, are high profile players in cancer. Not only are there monogenic helicase disorders with a strong disposition to cancer, it is well appreciated that helicase variants are associated with specific cancers (e.g., breast cancer). Flipping the coin, DNA helicases are frequently overexpressed in cancerous tissues and reduction in helicase gene expression results in reduced proliferation and growth capacity, as well as DNA damage induction and apoptosis of cancer cells. The seminal roles of helicases in the DNA damage and replication stress responses, as well as DNA repair pathways, validate their vital importance in cancer biology and suggest their potential values as targets in anti-cancer therapy. In recent years, many laboratories have characterized the specialized roles of helicase to resolve transcription-replication conflicts, maintain telomeres, mediate cell cycle checkpoints, remodel stalled replication forks, and regulate transcription. In vivo models, particularly mice, have been used to interrogate helicase function and serve as a bridge for preclinical studies that may lead to novel therapeutic approaches. In this review, we will summarize our current knowledge of DNA helicases and their roles in cancer, emphasizing the latest developments.

The Nucleolus and PARP1 in Cancer Biology

The nucleolus has been known for a long time to fulfill crucial functions in ribosome biogenesis, of which cancer cells can become addicted to in order to produce sufficient amounts of proteins for cell proliferation. Recently, the nucleolus has emerged as a central regulatory hub in many other cancer-relevant processes, including stress sensing, DNA damage response, cell cycle control, and proteostasis. This fostered the idea that nucleolar processes can be exploited in cancer therapy. Interestingly, a significant proportion of poly(ADP-ribose) polymerase 1 (PARP1) molecules are localized in the nucleolus and PARP1 also plays crucial roles in many processes that are important in cancer biology, including genome maintenance, replication, transcription, and chromatin remodeling. Furthermore, during the last years, PARP1 came into focus in oncology since it represents a promising target of pharmacological PARP inhibitors in various types of cancers. Here, we provide an overview of our current understanding on the role of PARP1 in nucleolar functions and discuss potential implications in cancer biology and therapy.

ATM pathway activation limits R-loop-associated genomic instability in Werner syndrome cells

Werner syndrome (WS) is a cancer-prone disease caused by deficiency of Werner protein (WRN). WRN maintains genome integrity by promoting replication-fork stability after various forms of replication stress. Under mild replication stress, WS cells show impaired ATR-mediated CHK1 activation. However, it remains unclear if WS cells elicit other repair pathway. We demonstrate that loss of WRN leads to enhanced ATM phosphorylation upon prolonged exposure to aphidicolin, a specific inhibitor of DNA polymerases, resulting in CHK1 activation. Moreover, we find that loss of WRN sensitises cells to replication-transcription collisions and promotes accumulation of R-loops, which undergo XPG-dependent cleavage responsible for ATM signalling activation. Importantly, we observe that ATM pathway limits chromosomal instability in WS cells. Finally, we prove that, in WS cells, genomic instability enhanced upon chemical inhibition of ATM kinase activity is counteracted by direct or indirect suppression of R-loop formation or by XPG abrogation. Together, these findings suggest a potential role of WRN as regulator of R-loop-associated genomic instability, strengthening the notion that conflicts between replication and transcription can affect DNA replication, leading to human disease and cancer.

Studying Werner syndrome to elucidate mechanisms and therapeutics of human aging and age-related diseases

Aging is a natural and unavoidable part of life. However, aging is also the primary driver of the dominant human diseases, such as cardiovascular disease, cancer, and neurodegenerative diseases, including Alzheimer's disease. Unraveling the sophisticated molecular mechanisms of the human aging process may provide novel strategies to extend 'healthy aging' and the cure of human aging-related diseases. Werner syndrome (WS), is a heritable human premature aging disease caused by mutations in the gene encoding the Werner (WRN) DNA helicase. As a classical premature aging disease, etiological exploration of WS can shed light on the mechanisms of normal human aging and facilitate the development of interventional strategies to improve healthspan. Here, we summarize the latest progress of the molecular understandings of WRN protein, highlight the advantages of using different WS model systems, including Caenorhabditis elegans, Drosophila melanogaster and induced pluripotent stem cell (iPSC) systems. Further studies on WS will propel drug development for WS patients, and possibly also for normal age-related diseases.

The Nucleolus: In Genome Maintenance and Repair

The nucleolus is the subnuclear membrane-less organelle where rRNA is transcribed and processed and ribosomal assembly occurs. During the last 20 years, however, the nucleolus has emerged as a multifunctional organelle, regulating processes that go well beyond its traditional role. Moreover, the unique organization of rDNA in tandem arrays and its unusually high transcription rates make it prone to unscheduled DNA recombination events and frequent RNA:DNA hybrids leading to DNA double strand breaks (DSBs). If not properly repaired, rDNA damage may contribute to premature disease onset and aging. Deregulation of ribosomal synthesis at any level from transcription and processing to ribosomal subunit assembly elicits a stress response and is also associated with disease onset. Here, we discuss how genome integrity is maintained within nucleoli and how such structures are functionally linked to nuclear DNA damage response and repair giving an emphasis on the newly emerging roles of the nucleolus in mammalian physiology and disease.

Crosstalk between the nucleolus and the DNA damage response

Nucleolar function and the cellular response to DNA damage have long been studied as distinct disciplines. New research and a new appreciation for proteins holding multiple functional roles, however, is beginning to change the way we think about the crosstalk among distinct cellular processes. Here, we focus on the crosstalk between the DNA damage response and the nucleolus, including a comprehensive review of the literature that reveals a role for conventional DNA repair proteins in ribosome biogenesis, and conversely, ribosome biogenesis proteins in DNA repair. Furthermore, with recent advances in nucleolar proteomics and a growing list of proteins that localize to the nucleolus, it is likely that we will continue to identify new DNA repair proteins with a nucleolar-specific role. Given the importance of ribosome biogenesis and DNA repair in essential cellular processes and the role that they play in diverse pathologies, continued elucidation of the overlap between these two disciplines will be essential to the advancement of both fields and to the development of novel therapeutics.

Regulation of BLM Nucleolar Localization

Defects in coordinated ribosomal RNA (rRNA) transcription in the nucleolus cause cellular and organismal growth deficiencies. Bloom's syndrome, an autosomal recessive human disorder caused by mutated recQ-like helicase BLM, presents with growth defects suggestive of underlying defects in rRNA transcription. Our previous studies showed that BLM facilitates rRNA transcription and interacts with RNA polymerase I and topoisomerase I (TOP1) in the nucleolus. The mechanisms regulating localization of BLM to the nucleolus are unknown. In this study, we identify the TOP1-interaction region of BLM by co-immunoprecipitation of in vitro transcribed and translated BLM segments and show that this region includes the highly conserved nuclear localization sequence (NLS) of BLM. Biochemical and nucleolar co-localization studies using site-specific mutants show that two serines within the NLS (S1342 and S1345) are critical for nucleolar localization of BLM but do not affect the functional interaction of BLM with TOP1. Mutagenesis of both serines to aspartic acid (phospho-mimetic), but not alanine (phospho-dead), results in approximately 80% reduction in nucleolar localization of BLM while retaining the biochemical functions and nuclear localization of BLM. Our studies suggest a role for this region in regulating nucleolar localization of BLM via modification of the two serines within the NLS.

A New Nanobody-Based Biosensor to Study Endogenous PARP1 In Vitro and in Live Human Cells

Poly(ADP-ribose) polymerase 1 (PARP1) is a key player in DNA repair, genomic stability and cell survival and it emerges as a highly relevant target for cancer therapies. To deepen our understanding of PARP biology and mechanisms of action of PARP1-targeting anti-cancer compounds, we generated a novel PARP1-affinity reagent, active both in vitro and in live cells. This PARP1-biosensor is based on a PARP1-specific single-domain antibody fragment (~ 15 kDa), termed nanobody, which recognizes the N-terminus of human PARP1 with nanomolar affinity. In proteomic approaches, immobilized PARP1 nanobody facilitates quantitative immunoprecipitation of functional, endogenous PARP1 from cellular lysates. For cellular studies, we engineered an intracellularly functional PARP1 chromobody by combining the nanobody coding sequence with a fluorescent protein sequence. By following the chromobody signal, we were for the first time able to monitor the recruitment of endogenous PARP1 to DNA damage sites in live cells. Moreover, tracing of the sub-nuclear translocation of the chromobody signal upon treatment of human cells with chemical substances enables real-time profiling of active compounds in high content imaging. Due to its ability to perform as a biosensor at the endogenous level of the PARP1 enzyme, the novel PARP1 nanobody is a unique and versatile tool for basic and applied studies of PARP1 biology and DNA repair.

Plasmodium falciparum Werner homologue is a nuclear protein and its biochemical activities reside in the N-terminal region

RecQ helicases, also addressed as a gatekeeper of genome, are an inevitable family of genome scrutiny proteins conserved from prokaryotes to eukaryotes and play a vital role in DNA metabolism. The deficiencies of three RecQ proteins out of five are involved in genetic abnormalities like Bloom syndrome (BS), Werner syndrome (WS), and Rothmund-Thomson syndrome (RTS). It is noteworthy that Plasmodium falciparum contains only two members of the RecQ family as opposed to five members present in the host Homo sapiens. In the present study, we report the biochemical characterization of the homologue of Werner (Wrn) helicase from P. falciparum 3D7 strain. Although there are significant sequence conservations between Wrn helicases of both H. sapiens and P. falciparum as well as among all the other Plasmodium species, they contain some peculiar differences also. In silico studies reveal that PfWrn is evolutionarily close to the bacterial RecQ protein. The N-terminal fragment (PfWrnN) contains all the helicase motifs along with all the functional domains and the predicted structure resembles with the human RecQ1 protein, whereas the C-terminal fragment (PfWrnC) contains no significant domain. Biochemical characterization further revealed that purified recombinant PfWrnN shows ATPase and DNA helicase activity in 3' to 5' direction, but PfWrnC lacks the ATPase and helicase activities. Immunofluorescence study shows that PfWrn is expressed in all the stages of intraerythrocytic development of the P. falciparum 3D7 strain and localizes distinctly in the nucleus. This study can be used for further characterization of RecQ helicases that will aid in understanding the physiological significance of these helicases in the malaria parasite.

RecQ helicases and PARP1 team up in maintaining genome integrity

Genome instability represents a primary hallmark of aging and cancer. RecQL helicases (i.e., RECQL1, WRN, BLM, RECQL4, RECQL5) as well as poly(ADP-ribose) polymerases (PARPs, in particular PARP1) represent two central quality control systems to preserve genome integrity in mammalian cells. Consistently, both enzymatic families have been linked to mechanisms of aging and carcinogenesis in mice and humans. This is in accordance with clinical and epidemiological findings demonstrating that defects in three RecQL helicases, i.e., WRN, BLM, RECQL4, are related to human progeroid and cancer predisposition syndromes, i.e., Werner, Bloom, and Rothmund Thomson syndrome, respectively. Moreover, PARP1 hypomorphy is associated with a higher risk for certain types of cancer. On a molecular level, RecQL helicases and PARP1 are involved in the control of DNA repair, telomere maintenance, and replicative stress. Notably, over the last decade, it became apparent that all five RecQL helicases physically or functionally interact with PARP1 and/or its enzymatic product poly(ADP-ribose) (PAR). Furthermore, a profound body of evidence revealed that the cooperative function of RECQLs and PARP1 represents an important factor for maintaining genome integrity. In this review, we summarize the status quo of this molecular cooperation and discuss open questions that provide a basis for future studies to dissect the cooperative functions of RecQL helicases and PARP1 in aging and carcinogenesis.

WRN protein as a novel erythroblast immunohistochemical marker with applications for the diagnosis of Werner syndrome

Genetic testing for mutations in the WRN gene is critical for the diagnosis of Werner syndrome (WS); however, these tests cannot be performed in a clinical setting. Nearly all of the WRN mutations result in expression of truncated WRN proteins that are missing the C-terminal nuclear localization signal. We evaluated the use of WRN protein immunohistochemistry for diagnosing WS using paraffin-embedded bone marrow sections. Using a well-defined commercially available polyclonal antibody against the C terminus of WRN, we found that of all the cell types tested, bone marrow erythroid precursors showed the strongest nuclear expression of WRN. Immunohistochemical analysis of bone marrow samples from 120 patients with non-WS hematological disorders (age range, 7 days-90 years) revealed WRN staining of the nuclei of CD71-positive early and late erythroid precursors. Erythroblasts negative for WRN immunostaining were only observed in two patients, both of whom were diagnosed with WS: one with concomitant myelodysplastic syndrome and the other with erythroleukemia with overexpression of TP53. Western blot analysis and immunocytochemistry indicated WRN was localized in the nuclei of the four positive control cell lines from non-WS patients but not in the five cell lines from WS patients, who had three different types of WRN mutations. Thus, immunohistochemical detection of WRN in erythroblasts from bone marrow paraffin sections could be useful in screening of WS cases and worthy of further molecular confirmation.

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.

RECQ1 plays a distinct role in cellular response to oxidative DNA damage

RECQ1 is the most abundant RecQ homolog in humans but its functions have remained mostly elusive. Biochemically, RECQ1 displays distinct substrate specificities from WRN and BLM, indicating that these RecQ helicases likely perform non-overlapping functions. Our earlier work demonstrated that RECQ1-deficient cells display spontaneous genomic instability. We have obtained key evidence suggesting a unique role of RECQ1 in repair of oxidative DNA damage. We show that similar to WRN, RECQ1 associates with PARP-1 in nuclear extracts and exhibits direct protein interaction in vitro. Deficiency in WRN or BLM helicases have been shown to result in reduced homologous recombination and hyperactivation of PARP under basal condition. However, RECQ1-deficiency did not lead to PARP activation in undamaged cells and nor did it result in reduction in homologous recombination repair. In stark contrast to what is seen in WRN-deficiency, RECQ1-deficient cells hyperactivate PARP in a specific response to H₂O₂treatment. RECQ1-deficient cells are more sensitive to oxidative DNA damage and exposure to oxidative stress results in a rapid and reversible recruitment of RECQ1 to chromatin. Chromatin localization of RECQ1 precedes WRN helicase, which has been shown to function in oxidative DNA damage repair. However, oxidative DNA damage-induced chromatin recruitment of these RecQ helicases is independent of PARP activity. As other RecQ helicases are known to interact with PARP-1, this study provides a paradigm to delineate specialized and redundant functions of RecQ homologs in repair of oxidative DNA damage.

BLM helicase facilitates RNA polymerase I-mediated ribosomal RNA transcription

Bloom's syndrome (BS) is an autosomal recessive disorder that is invariably characterized by severe growth retardation and cancer predisposition. The Bloom's syndrome helicase (BLM), mutations of which lead to BS, localizes to promyelocytic leukemia protein bodies and to the nucleolus of the cell, the site of RNA polymerase I-mediated ribosomal RNA (rRNA) transcription. rRNA transcription is fundamental for ribosome biogenesis and therefore protein synthesis, cellular growth and proliferation; its inhibition limits cellular growth and proliferation as well as bodily growth. We report that nucleolar BLM facilitates RNA polymerase I-mediated rRNA transcription. Immunofluorescence studies demonstrate the dependance of BLM nucleolar localization upon ongoing RNA polymerase I-mediated rRNA transcription. In vivo protein co-immunoprecipitation demonstrates that BLM interacts with RPA194, a subunit of RNA polymerase I. (3)H-uridine pulse-chase assays demonstrate that BLM expression is required for efficient rRNA transcription. In vitro helicase assays demonstrate that BLM unwinds GC-rich rDNA-like substrates that form in the nucleolus and normally inhibit progression of the RNA polymerase I transcription complex. These studies suggest that nucleolar BLM modulates rDNA structures in association with RNA polymerase I to facilitate RNA polymerase I-mediated rRNA transcription. Given the intricate relationship between rDNA metabolism and growth, our data may help in understanding the etiology of proportional dwarfism in BS.

The Werner syndrome protein: linking the replication checkpoint response to genome stability

The Werner syndrome protein (WRN) is a member of the human RecQ family DNA helicases implicated in the maintenance of genome stability. Loss of WRN gives rise to the Werner syndrome, a genetic disease characterised by premature aging and cancer predisposition. WRN plays a crucial role in the response to replication stress and significantly contributes to the recovery of stalled replication forks, although how this function is regulated is not fully appreciated. There is a growing body of evidence that WRN accomplishes its task in close connection with the replication checkpoint. In eukaryotic cells, the replication checkpoint response, which involves both the ATR and ATM kinase activities, is deputed to the maintenance of fork integrity and re-establishment of fork progression. Our recent findings indicate that ATR and ATM modulate WRN function at defined steps of the response to replication fork arrest. This review focuses on the novel evidence of a functional relationship between WRN and the replication checkpoint and how this cross-talk might contribute to prevent genome instability, a common feature of senescent and cancer cells.

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.

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.

WRN protein and Werner syndrome

Werner syndrome is an autosomal recessive disorder associated with premature aging and cancer predisposition. Cells from Werner syndrome patients show increased genomic instability and are hypersensitive to DNA damage agents. Werner syndrome is caused by mutations of the WRN gene. WRN protein is a member of RecQ DNA helicase family. It not only contains a conserved 3'-5' helicase domain as other members of the RecQ family but also contains a unique 3'-5' exonuclease domain. WRN recognizes specific DNA structures as substrates which are intermediates of DNA metabolism. WRN interacts with many other proteins, which function in telomere maintenance, DNA replication, and DNA repair through different pathways.

c-Myc accelerates S-phase and requires WRN to avoid replication stress

c-Myc interacts with components of the pre-replication complex and directly regulates DNA replication [1]. However the consequences of this novel c-Myc function on cell cycle dynamics and replication-associated damage are unknown. Here, we show that c-Myc overexpression in primary human fibroblasts markedly accelerates S-phase while c-Myc deficient fibroblasts exhibit a prolonged S-phase. We also show that the Werner DNA helicase protein (WRN) plays a critical role in supporting c-Myc-driven S-phase, as depletion of WRN in c-Myc overexpressing cells increases DNA damage specifically at sites of DNA synthesis. This excess DNA damage activates a “replication stress” pathway involving ATR, CHK1, CHK2, and p53, leading to rapid senescence of WRN deficient c-Myc overexpressing cells. Indeed, depletion of p53 rescues this senescence response. We propose that WRN functions to repair abnormal replication structures caused by the acceleration of DNA replication by c-Myc. This work provides an additional mechanistic explanation for c-Myc-induced DNA damage and senescence, and reveals a vulnerability of c-Myc overexpressing cells that could potentially be exploited in cancer therapy.

Human Wrnip1 is localized in replication factories in a ubiquitin-binding zinc finger-dependent manner

Wrnip1 (Werner helicase-interacting protein 1) has been implicated in the bypass of stalled replication forks in bakers' yeast. However, the function(s) of human Wrnip1 has remained elusive so far. Here we report that Wrnip1 is distributed inside heterogeneous structures detectable in nondamaged cells throughout the cell cycle. In an attempt to characterize these structures, we found that Wrnip1 resides in DNA replication factories. Upon treatments that stall replication forks, such as UVC light, the amount of chromatin-bound Wrnip1 and the number of foci significantly increase, further implicating Wrnip1 in DNA replication. Interestingly, the nuclear pattern of Wrnip1 appears to extend to a broader landscape, as it can be detected in promyelocytic leukemia nuclear bodies. The presence of Wrnip1 into these heterogeneous subnuclear structures requires its ubiquitin-binding zinc finger (UBZ) domain, which is able to interact with different ubiquitin (Ub) signals, including mono-Ub and chains linked via lysine 48 and 63. Moreover, the oligomerization of Wrnip1 mediated by its C terminus is also important for proper subnuclear localization. Our study is the first to reveal the composite and regulated topography of Wrnip1 in the human nucleus, highlighting its potential role in replication and other nuclear transactions.

The clinical characteristics of Werner syndrome: molecular and biochemical diagnosis

Werner syndrome (WS) is an adult onset segmental progeroid syndrome caused by mutations in the WRN gene. The WRN gene encodes a 180 kDa nuclear protein that possesses helicase and exonuclease activities. The absence of WRN protein leads to abnormalities in various DNA metabolic pathways such as DNA repair, replication and telomere maintenance. Individuals with WS generally develop normally until the third decade of life, when premature aging phenotypes and a series of age-related disorders begin to manifest. In Japan, where a founder effect has been described, the frequency of Werner heterozygotes appears to be as high as 1/180 in the general population. Due to the relatively non-specific nature of the symptoms and the lack of awareness of the condition, this disease may be under-diagnosed in other parts of the world. Genetic counseling of WS patients follows the path of other autosomal recessive disorders, with special attention needed for cancer surveillance in relatives. Molecular diagnosis of WS is made by nucleotide sequencing and, in some cases, protein analysis. It is also of potential interest to measure WRN activities in WS patients. More than 50 different disease-causing mutations in the WRN gene have been identified in WS patients from all over the world. All but one of these cases has mutations that result in the premature termination of the protein. Here we describe the clinical, molecular and biochemical characteristics of WS for use by medical professionals in a health care setting. Additional information is available through the International Registry of WS (http://www.wernersyndrome.org).

The transcriptional response to distinct growth factors is impaired in Werner syndrome cells

The Werner syndrome protein (WRN) is mutated in Werner syndrome (WS) and plays a role in telomere maintenance, DNA repair and transcription. WS represents a premature aging syndrome with severe growth retardation. Here we show that WRN is critically required to mediate the stimulatory effect of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (FGF-b) and epidermal growth factor (EGF) on the activity of RNA polymerase I (Pol I). Recombinant WRN specifically reconstitutes RNA polymerase I transcription in extracts from Werner syndrome fibroblasts in vitro. In addition, we identified a critical role for WRN during promoter clearance of Pol I transcription, but not in elongation. Notably, WRN was isolated in a complex with Pol I and was crosslinked to the unmethylated, active proportion of rDNA genes in quiescent cells suggesting a so far unknown role for WRN in epigenetic regulation. This together with alterations in Pol I transcription provide a novel mechanism possibly underlying at least in part the severe growth retardation and premature aging in Werner syndrome patients.

Hyperinsulinemia and insulin resistance in Wrn null mice fed a diabetogenic diet

Werner syndrome (WS) is an autosomal recessive progeroid syndrome caused by mutations in the Werner (Wrn) gene. WS patients have increased incidence of a number of chronic conditions including insulin resistance and type 2 diabetes. Since ingestion of foods that are high in fat and sugar is associated with increased incidence of diabetes, we examined if Wrn mutations might affect metabolic response to a diabetogenic diet. Four-month-old mice with a null mutation for the Wrn gene were fed a diet consisting of 36% fat (lard), 33% table sugar, and 20% protein plus balanced vitamins and minerals. Wrn null mice had significantly increased body weights, increased serum insulin levels, impaired glucose tolerance, and insulin resistance during 4 months of eating the diabetogenic diet. Diffuse fatty infiltration of the liver and pancreatic islet hyperplasia was characteristic morphological features. These observations suggest that Wrn null mice have impaired glucose homeostasis and fat metabolism, and may be a useful model to investigate metabolic conditions associated with aging.

Human RECQ1 is a DNA damage responsive protein required for genotoxic stress resistance and suppression of sister chromatid exchanges

BACKGROUND

DNA helicases are ubiquitous enzymes that unwind DNA in an ATP-dependent and directionally specific manner. Unwinding of double-stranded DNA is essential for the processes of DNA repair, recombination, transcription, and DNA replication. Five human DNA helicases sharing sequence similarity with the E. coli RecQ helicase have been identified. Three of the human RecQ helicases are implicated in hereditary diseases (Bloom syndrome, Werner syndrome, and Rothmund-Thomson syndrome) which display clinical symptoms of premature aging and cancer. RECQ1 helicase is the most highly expressed of the human RecQ helicases; however, a genetic disease has yet not been linked to mutations in the RECQ1 gene, and the biological functions of human RECQ1 in cellular DNA metabolism are not known.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we report that RECQ1 becomes phosphorylated upon DNA damage and forms irradiation-induced nuclear foci that associate with chromatin in human cells. Depletion of RECQ1 renders human cells sensitive to DNA damage induced by ionizing radiation or the topoisomerase inhibitor camptothecin, and results in spontaneous gamma-H2AX foci and elevated sister chromatid exchanges, indicating aberrant repair of DNA breaks. Consistent with a role in homologous recombinational repair, endogenous RECQ1 is associated with the strand exchange protein Rad51 and the two proteins directly interact with high affinity.

CONCLUSION/SIGNIFICANCE

Collectively, these results provide the first evidence for a role of human RECQ1 in the response to DNA damage and chromosomal stability maintenance and point to the vital importance of RECQ1 in genome homeostasis.

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.

Activation of Holliday junction recognizing protein involved in the chromosomal stability and immortality of cancer cells

We identified a novel gene HJURP (Holliday junction-recognizing protein) whose activation seemed to play a pivotal role in the immortality of cancer cells. HJURP was considered a possible downstream target for ataxia telangiectasia mutated signaling, and its expression was increased by DNA double-strand breaks (DSB). HJURP was involved in the homologous recombination pathway in the DSB repair process through interaction with hMSH5 and NBS1, which is a part of the MRN protein complex. HJURP formed nuclear foci in cells at S phase and those subjected to DNA damage. In vitro assays implied that HJURP bound directly to the Holliday junction and rDNA arrays. Treatment of cancer cells with small interfering RNA (siRNA) against HJURP caused abnormal chromosomal fusions and led to genomic instability and senescence. In addition, HJURP overexpression was observed in a majority of lung cancers and was associated with poor prognosis as well. We suggest that HJURP is an indispensable factor for chromosomal stability in immortalized cancer cells and is a potential novel therapeutic target for the development of anticancer drugs.

The human Werner syndrome protein stimulates repair of oxidative DNA base damage by the DNA glycosylase NEIL1

The mammalian DNA glycosylase, NEIL1, specific for repair of oxidatively damaged bases in the genome via the base excision repair pathway, is activated by reactive oxygen species and prevents toxicity due to radiation. We show here that the Werner syndrome protein (WRN), a member of the RecQ family of DNA helicases, associates with NEIL1 in the early damage-sensing step of base excision repair. WRN stimulates NEIL1 in excision of oxidative lesions from bubble DNA substrates. The binary interaction between NEIL1 and WRN (K(D) = 60 nM) involves C-terminal residues 288-349 of NEIL1 and the RecQ C-terminal (RQC) region of WRN, and is independent of the helicase activity WRN. Exposure to oxidative stress enhances the NEIL-WRN association concomitant with their strong nuclear co-localization. WRN-depleted cells accumulate some prototypical oxidized bases (e.g. 8-oxoguanine, FapyG, and FapyA) indicating a physiological function of WRN in oxidative damage repair in mammalian genomes. Interestingly, WRN deficiency does not have an additive effect on in vivo damage accumulation in NEIL1 knockdown cells suggesting that WRN participates in the same repair pathway as NEIL1.

The Werner Syndrome Helicase Is a Cofactor for HIV-1 Long Terminal Repeat Transactivation and Retroviral Replication

The Werner syndrome helicase (WRN) participates in DNA replication, double strand break repair, telomere maintenance, and p53 activation. Mutations of wrn cause Werner syndrome (WS), an autosomal recessive premature aging disorder associated with cancer predisposition, atherosclerosis, and other aging related symptoms. Here, we report that WRN is a novel cofactor for HIV-1 replication. Immortalized human WRN(-/-) WS fibroblasts, lacking a functional wrn gene, are impaired for basal and Tat-activated HIV-1 transcription. Overexpression of wild-type WRN transactivates the HIV-1 long terminal repeat (LTR) in the absence of Tat, and WRN cooperates with Tat to promote high-level LTR transactivation. Ectopic WRN induces HIV-1 p24(Gag) production and retroviral replication in HIV-1-infected H9(HIV-1IIIB) lymphocytes. A dominant-negative helicase-minus mutant, WRN(K577M), inhibits LTR transactivation and HIV-1 replication. Inhibition of endogenous WRN, through co-expression of WRN(K577M), diminishes recruitment of p300/CREB-binding protein-associated factor (PCAF) and positive transcription elongation factor b (P-TEFb) to Tat/transactivation response-RNA complexes, and immortalized WRN(-/-) WS fibroblasts exhibit comparable defects in recruitment of PCAF and P-TEFb to the HIV-1 LTR. Our results demonstrate that WRN is a novel cellular cofactor for HIV-1 replication and suggest that the WRN helicase participates in the recruitment of PCAF/P-TEFb-containing transcription complexes. WRN may be a plausible target for antiretroviral therapy.

The role of WRN in DNA repair is affected by post-translational modifications

Werner syndrome (WS) is an autosomal recessive progeroid disease characterized by genomic instability. WRN gene encodes one of the RecQ helicase family proteins, WRN, which has ATPase, helicase, exonuclease and single stranded DNA annealing activities. There is accumulating evidence suggesting that WRN contributes to the maintenance of genomic integrity through its involvement in DNA repair, replication and recombination. The role of WRN in these pathways can be modulated by its post-translational modifications in response to DNA damage. Here, we review the functional consequences of post-translational modifications on WRN as well as specific DNA repair pathways where WRN is involved and discuss how these modifications affect DNA repair pathways.

SUMO, the three Rs and cancer

SUMO modification (sumoylation) plays important roles in nucleo-cytoplasmic transport, maintenance of sub-nuclear architecture, the regulation of gene expression and in DNA replication, repair and recombination. Here we review recent evidence for SUMO's role in protecting genomic integrity at both the chromosomal and the DNA level. Furthermore, the involvement of sumoylation and of specific SUMO targets in cancer is discussed.

Identification and characterization of the nuclear localization/retention signal in the EWS proto-oncoprotein

Ewing sarcoma (EWS) protein, a member of a large family of RNA-binding proteins, contains an N-terminal transcriptional activation domain (EAD) and a C-terminal RNA-binding domain (RBD). Due to its multifunctional properties EWS protein is involved in processes such as gene expression, RNA processing and transport, and cell signaling. Chimeric EWS proteins generated by chromosomal translocations cause malignant tumors. EWS protein is located predominantly in the nucleus, but was found also in the cytosol and associated with the cell membrane. The determinants responsible for the nuclear localization of the protein were as yet unknown. We identified the nuclear localization signal of EWS protein at its C terminus (C-NLS), which is required for the nuclear import and retention of the protein. The C-NLS sequence is conserved in related proto-oncoproteins suggesting an NLS function also in these proteins. Two arginine residues, due to their positive charge, a proline residue and a tyrosine residue are essential for C-NLS function. The nuclear localization of EWS protein is independent of the regions in RBD containing numerous arginine methylation sites, RNA-recognition and zinc finger motifs. Regions in EAD guide the subnuclear partition of EWS protein and contain another but different NLS that allows nucleocytoplasmic shuttling of the N-terminal domain.

The Rothmund-Thomson gene product RECQL4 localizes to the nucleolus in response to oxidative stress

Mutations in the RECQL4 helicase gene have been linked to Rothmund-Thomson syndrome (RTS), which is characterized by poikiloderma, growth deficiency, and a predisposition to cancer. Examination of RECQL4 subcellular localization in live cells demonstrated a nucleoplasmic pattern and, to a lesser degree, staining in nucleoli. Analysis of RECQL4-GFP deletion mutants revealed two nuclear localization regions in the N-terminal region of RECQL4 and a nucleolar localization signal at amino acids 376-386. RECQL4 localization did not change after treatment with the DNA-damaging agents bleomycin, etoposide, UV irradiation and gamma irradiation, in contrast to the Bloom and Werner syndrome helicases that relocate to distinct nuclear foci after damage. However, in a significant number of cells exposed to hydrogen peroxide or streptonigrin, RECQL4 accumulated in nucleoli. Using a T7 phage display screen, we determined that RECQL4 interacts with poly(ADP-ribose) polymerase-1 (PARP-1), a nuclear enzyme that promotes genomic integrity through its involvement in DNA repair and signaling pathways. The RECQL4 nucleolar localization was inhibited by pretreatment with a PARP-1 inhibitor. The C-terminal portion of RECQL4 was found to be an in vitro substrate for PARP-1. These results demonstrate changes in the intracellular localization of RECQL4 in response to oxidative stress and identify an interaction between RECQL4 and PARP-1.

Accumulation of FFA-1, the Xenopus homolog of Werner helicase, and DNA polymerase delta on chromatin in response to replication fork arrest

Werner syndrome is a genetic disorder characterized by premature aging and cancer-prone symptoms, and is caused by mutation of the WRN gene. WRN is a member of the RecQ helicase family and is thought to function in processes implicated in DNA replication and repair to maintain genome stability; however, its precise function is still unclear. We found that replication fork arrest markedly enhances chromatin binding of focus-forming activity 1 (FFA-1), a Xenopus WRN homolog, in Xenopus egg extracts. In addition to FFA-1, DNA polymerase delta (Poldelta) and replication protein A, but not DNA polymerase epsilon and proliferating cell nuclear antigen, accumulated increasingly on replication-arrested chromatin. Elevated accumulation of these proteins was dependent on formation of pre-replicative complexes (pre-RCs). Double-strand break (DSB) formation also enhanced chromatin binding of FFA-1, but not Poldelta, independently of pre-RC formation. In contrast to FFA-1, chromatin binding of Xenopus Bloom syndrome helicase (xBLM) only slightly increased after replication arrest or DSB formation. Thus, WRN-specific, distinct processes can be reproduced in the in vitro system in egg extracts, and this system is useful for biochemical analysis of WRN functions during DNA metabolism.

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.

Evaluation of the nucleolar organizer regions in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder in middle and late age. Ribosomal RNA (rRNA) genes are located in the nucleolus (nucleolar organizer regions = NORs). There are increased deposits of beta-amyloid protein in the brains of the patients with AD and aged individuals with Down's syndrome (DS). The beta-amyloid gene is located in the acrocentric chromosome 21 that is responsible for rRNA synthesis. Therefore, it is possible that there is a relationship between ribosomal genes and AD.

OBJECTIVE

To investigate the activities of ribosomal genes of AD patients by comparing the activities of NORs in AD patients and healthy controls with the silver-staining method.

METHODS

NOR surface/the total nucleus surface proportions in interphase nuclei, and silver stainability and satellite association (SA) of acrocentric chromosomes in the metaphases of cultivated lymphocytes of 20 AD patients and 20 healthy controls (10 elderly and 10 young) were evaluated.

RESULTS

A decrease in NOR surface/total nucleus surface proportions has been observed in the interphase nucleus of AD patients when compared with elderly controls (p = 0.035). When compared with the sizes of Ag+ segments of acrocentric chromosomes of AD patients and control groups, the Ag-staining size 1 of the chromosome 22 of AD patients was found to be more increased than that of the young controls (p = 0.018). There was no statistically significant difference between AD patients and control groups regarding the number of Ag+ acrocentric chromosomes, Ag+ chromosome 21 and SA frequency (p > 0.05). It has been found that there is only a slight increase in the total number of chromosomes in SA in AD patients when compared with elderly controls (p = 0.05).

CONCLUSION

The decrease in NOR surface/total nucleus surface proportions of AD patients may indicate a reduction in the activity of the ribosomal genes of these patients.

Cellular dynamics and modulation of WRN protein is DNA damage specific

The human premature aging protein Werner (WRN), deficient in Werner syndrome (WS), is localized mainly to the nucleolus in many cell types. DNA damage or replication arrest causes WRN to redistribute from the nucleolus to the nucleoplasm into discrete foci. In this study, we have investigated DNA damage specific cellular redistribution of WRN. In response to agents causing DNA double strand breaks or DNA base damage, WRN is re-distributed from the nucleolus to the nucleoplasm in a reversible manner. However, after ultraviolet (UV) irradiation such redistribution of WRN is largely absent. We also show that WRN is associated with the insoluble protein fraction of cells after exposure to various kinds of DNA damage but not after UV irradiation. Further, we have studied the DNA damage specific post-translational modulation of WRN. Our results show that WRN is acetylated after mytomycin C or methyl methane-sulfonate treatment, but not after UV irradiation. Also, DNA damage specific phosphorylation of WRN is absent in UV irradiated cells. Inhibition of phosphorylation fails to restore WRN localization. Thus, our results suggest that the dynamics of WRN protein trafficking is DNA damage specific and is related to its post-translational modulation. The results also indicate a preferred role of WRN in recombination and base excision repair rather than nucleotide excision repair.

Accumulation of Werner protein at DNA double-strand breaks in human cells

Werner syndrome is an autosomal recessive accelerated-aging disorder caused by a defect in the WRN gene, which encodes a member of the RecQ family of DNA helicases with an exonuclease activity. In vitro experiments have suggested that WRN functions in several DNA repair processes, but the actual functions of WRN in living cells remain unknown. Here, we analyzed the kinetics of the intranuclear mobilization of WRN protein in response to a variety of types of DNA damage produced locally in the nucleus of human cells. A striking accumulation of WRN was observed at laser-induced double-strand breaks, but not at single-strand breaks or oxidative base damage. The accumulation of WRN at double-strand breaks was rapid, persisted for many hours, and occurred in the absence of several known interacting proteins including polymerase β, poly(ADP-ribose) polymerase 1 (PARP1), Ku80, DNA-dependent protein kinase (DNA-PKcs), NBS1 and histone H2AX. Abolition of helicase activity or deletion of the exonuclease domain had no effect on accumulation, whereas the presence of the HRDC (helicase and RNaseD C-terminal) domain was necessary and sufficient for the accumulation. Our data suggest that WRN functions mainly at DNA double-strand breaks and structures resembling double-strand breaks in living cells, and that an autonomous accumulation through the HRDC domain is the initial response of WRN to the double-strand breaks.

The human Rothmund-Thomson syndrome gene product, RECQL4, localizes to distinct nuclear foci that coincide with proteins involved in the maintenance of genome stability

Rothmund-Thomson syndrome (RTS) is a human genetic disorder characterized by genome instability, cancer susceptibility and premature aging. The gene defective in a subset of RTS cases, RECQL4, encodes a member of the RecQ family of DNA helicases. To better define the function of the RECQL4 protein, we have determined its subcellular localization. We have raised antibodies against the N- and C-terminal parts of RECQL4 and could show that in various human cells endogenous RECQL4 forms discrete nuclear foci that colocalize with promyelotic leukaemia protein (PML). The number of foci and their colocalization with PML does not significantly change after induction of different types of DNA damages. Silencing of RECQL4 expression by siRNA causes a significant reduction in RECQL4 nuclear foci formation. Furthermore, we demonstrate that RECQL4 foci coincide with foci formed by human Rad51 and regions of single-stranded DNA after induction of DNA double-strand breaks. In agreement with this, we also show that RECQL4 and Rad51 form a complex in human cells. Our findings suggest a role for RECQL4 in the repair of DNA double-strand breaks by homologous recombination and shed new light onto RECQL4's function in human cells.

Human ribosomal RNA gene arrays display a broad range of palindromic structures

The standard model of eukaryotic ribosomal RNA (rRNA) genes involves tandem arrays with hundreds of units in clusters, the nucleolus organizer regions (NORs). A first genomic overview for human cells is reported here for these regions, which have never been sequenced in their totality, by using molecular combing. The rRNA-coding regions are examined by fluorescence on single molecules of DNA with two specific probes that cover their entire length. The standard organization assumed for rDNA units is a transcribed region followed by a nontranscribed spacer. While we confirmed this arrangement in many cases, unorthodox patterns were also observed in normal individuals, with one-third of the rDNA units rearranged to form apparently palindromic structures (noncanonical units) independent of the age of the donors. In cells from individuals with a deficiency in the WRN RecQ helicase (Werner syndrome), the proportion of palindromes increased to one-half. These findings, supported by Southern blot analyses, show that rRNA genes are a mosaic of canonical and (presumably nonfunctional) palindromic units that may be altered by factors associated with genomic instability and pathology.

PARP-1 and PARP-2 interact with nucleophosmin/B23 and accumulate in transcriptionally active nucleoli

The DNA damage-dependent poly(ADP-ribose) polymerases-1 and -2 (PARP-1 and PARP-2) are survival factors that share overlapping functions in the detection, signaling and repair of DNA strand breaks resulting from genotoxic lesions in mammalian cells. Here we show that PARP-1 and PARP-2 subnuclear distributions partially overlap, with both proteins accumulating within the nucleolus independently of each other. PARP-2 is enriched within the whole nucleolus and partially colocalizes with the nucleolar factor nucleophosmin/B23. We have identified a nuclear localization signal and a nucleolar localization signal within the N-terminal domain of PARP-2. PARP-2, like PARP-1, interacts with B23 through its N-terminal DNA binding domain. This association is constitutive and does not depend on either PARP activity or ribosomal transcription, but is prevented by mutation of the nucleolar localization signal of PARP-2. PARP-1 and PARP-2, together with B23, are delocalized from the nucleolus upon RNA polymerase I inhibition whereas the nucleolar accumulation of all three proteins is only moderately affected upon oxidative or alkylated DNA damage. Finally, we show that murine fibroblasts deficient in PARP-1 or PARP-2 are not affected in the transcription of ribosomal RNAs. Taken together, these results suggest that the biological role of PARP-1 and PARP-2 within the nucleolus relies on functional nucleolar transcription, without any obvious implication of either PARP on this major nucleolar process.

A mouse model of Werner Syndrome: what can it tell us about aging and cancer?

The molecular mechanisms involved in mammalian aging and the consequent organ dysfunction/degeneration pathologies are not well understood. Studies of progeroid syndromes such as Werner Syndrome have advanced our understanding of how certain genetic pathways can influence the aging process on both cellular and molecular levels. In addition, improper maintenance of telomere length and the consequent cellular responses to dysfunctional telomeres have been proposed to promote replicative senescence that impact upon the onset of premature aging and cancer. Recent studies of the telomerase-Werner double null mouse link telomere dysfunction to accelerated aging and tumorigenesis in the setting of Werner deficiency. This mouse model thus provides a unique genetic platform to explore molecular mechanisms by which telomere dysfunction and loss of WRN gene function leads to the onset of premature aging and cancer.

Rtg2 protein links metabolism and genome stability in yeast longevity

Mitochondrial dysfunction induces a signaling pathway, which culminates in changes in the expression of many nuclear genes. This retrograde response, as it is called, extends yeast replicative life span. It also results in a marked increase in the cellular content of extrachromosomal ribosomal DNA circles (ERCs), which can cause the demise of the cell. We have resolved the conundrum of how these two molecular mechanisms of yeast longevity operate in tandem. About 50% of the life-span extension elicited by the retrograde response involves processes other than those that counteract the deleterious effects of ERCs. Deletion of RTG2, a gene that plays a central role in relaying the retrograde response signal to the nucleus, enhances the generation of ERCs in cells with (grande) or in cells without (petite) fully functional mitochondria, and it curtails the life span of each. In contrast, overexpression of RTG2 diminishes ERC formation in both grandes and petites. The excess Rtg2p did not augment the retrograde response, indicating that it was not engaged in retrograde signaling. FOB1, which is known to be required for ERC formation, and RTG2 were found to be in converging pathways for ERC production. RTG2 did not affect silencing of ribosomal DNA in either grandes or petites, which were similar to each other in the extent of silencing at this locus. Silencing of ribosomal DNA increased with replicative age in either the presence or the absence of Rtg2p, distinguishing silencing and ERC accumulation. Our results indicate that the suppression of ERC production by Rtg2p requires that it not be in the process of transducing the retrograde signal from the mitochondrion. Thus, RTG2 lies at the nexus of cellular metabolism and genome stability, coordinating two pathways that have opposite effects on yeast longevity.

p14 Arf promotes small ubiquitin-like modifier conjugation of Werners helicase

Here we demonstrate a novel p53-independent interaction between the nucleolar tumor suppressors, p14 Arf and Werners helicase (WRN). Binding of p14 Arf to WRN is multivalent and resembles the binding of p14 Arf to Mdm2. Residues 2-14 and 82-101 of p14 Arf and residues in the central region and C terminus of WRN have particular importance for binding. p14 Arf promotes small ubiquitin-like modifier (SUMO) modification of WRN in a synergistic manner with the SUMO-conjugating enzyme, UBCH9. p14 Arf causes redistribution of WRN within the nucleus, and this effect is reversed by expression of a SUMO-specific protease, thus implicating the SUMO conjugation pathway in WRN re-localization. We establish that the ability to promote SUMO conjugation is a general property of the p14 Arf tumor suppressor.

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.

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.

Mus81 endonuclease localizes to nucleoli and to regions of DNA damage in human S-phase cells

Mus81 is a highly conserved substrate specific endonuclease. Human Mus81 cleaves Holliday junctions, replication forks, and 3' flap substrates in vitro, suggesting a number of possible in vivo functions. We show here that the abundance of human Mus81 peaks in S-phase and remains high in cells that have completed DNA replication and that Mus81 is a predominantly nuclear protein, with super accumulation in nucleoli. Two RecQ related DNA helicases BLM and WRN that are required for recombination repair in human cells colocalize with Mus81 in nucleoli. However, the nucleolar retention of Mus81 is not dependent on the presence of BLM or WRN, or on ongoing transcription. Mus81 is recruited to localized regions of UV damage in S-phase cells, but not in cells that are blocked from replicating DNA or that have completed replication. The retention of human Mus81 at regions of UV-induced damage specifically in S-phase cells suggest that the enzyme is recruited to the sites at which replication forks encounter damaged DNA. The nucleolar concentration of Mus81 suggests that it is required to repair problems that arise most frequently in the highly repetitive nucleolar DNA. Together these data support a role for Mus81 in recombination repair in higher eukaryotes.

Rtg2 Protein Links Metabolism and Genome Stability in Yeast Longevity

Mitochondrial dysfunction induces a signaling pathway, which culminates in changes in the expression of many nuclear genes. This retrograde response, as it is called, extends yeast replicative life span. It also results in a marked increase in the cellular content of extrachromsomal ribosomal DNA circles (ERCs), which can cause the demise of the cell. We have resolved the conundrum of how these two molecular mechanisms of yeast longevity operate in tandem. About 50% of the life-span extension elicited by the retrograde response involves processes other than those that counteract the deleterious effects of ERCs. Deletion of RTG2, a gene that plays a central role in relaying the retrograde response signal to the nucleus, enhances the generation of ERCs in cells with (grande) or in cells without (petite) fully functional mitochondria, and it curtails the life span of each. In contrast, overexpression of RTG2 diminishes ERC formation in both grandes and petites. The excess Rtg2p did not augment the retrograde response, indicating that it was not engaged in retrograde signaling. FOB1, which is known to be required for ERC formation, and RTG2 were found to be in converging pathways for ERC production. RTG2 did not affect silencing of ribosomal DNA in either grandes or petites, which were similar to each other in the extent of silencing at this locus. Silencing of ribosomal DNA increased with replicative age in either the presence or the absence of Rtg2p, distinguishing silencing and ERC accumulation. Our results indicate that the suppression of ERC production by Rtg2p requires that it not be in the process of transducing the retrograde signal from the mitochondrion. Thus, RTG2 lies at the nexus of cellular metabolism and genome stability, coordinating two pathways that have opposite effects on yeast longevity.