p53-mediated apoptosis is attenuated in Werner syndrome cells

d-galactose induces premature senescence of lens epithelial cells by disturbing autophagy flux and mitochondrial functions

Cataract is the leading cause of blindness with an estimated 16 million people affected worldwide. d-galactose (d-gal) is a reducing sugar that widely distributed in foodstuffs, and studies show that d-gal could promote cataract formation by damaging nature lens epithelial cells (LECs). However, the underlying mechanism is unclear. In our present study, d-gal resulted in premature senescence of LECs, which was confirmed by determining the β-galactosidase activity, cell proliferative potential and cell cycle distribution, though apoptosis of LECs was not observed. We also verified that d-gal induced the impairment of autophagy flux by measuring the expression of LC3II and P62. Meanwhile, we found that d-gal induced mitochondrial dysfunctions of LECs through increasing reactive oxygen species (ROS), reducing ATP synthesis and mitochondrial potential (MMP), enhancing the concentration of cytoplasm Ca²⁺ and permeability transition pore (mPTP) opening. Metformin, as a potential anti-aging agent, suppressed the senescence of LECs by restoring autophagy flux and mitochondria functions. Nevertheless, the antioxidant N-acetylcysteine (NAC) scavenged ROS significantly but was not efficient in preventing LECs from premature senescence. Our data suggests that restoring autophagy activity and improving mitochondrial functions may be a potential strategy for the prevention of LECs senescence-related cataract.

Vascular senescence and ageing: a role for the MEOX proteins in promoting endothelial dysfunction

In the vascular system, ageing is accompanied by the accrual of senescent cells and is associated with an increased risk of vascular disease. Endothelial cell (EC) dysfunction is a hallmark of vascular disease and is characterized by decreased angiogenic potential, reduced nitric oxide bioavailability, impaired vasodilation, increased production of ROS, and enhanced inflammation. In ECs, the major producer of nitric oxide is the endothelial nitric oxide synthase (eNOS) enzyme that is encoded by the NOS3 gene. NOS3/eNOS function is tightly regulated at both the transcriptional and post-transcriptional levels to maintain normal vascular function. A key transcriptional regulator of eNOS expression is p53, which has been shown to play a central role in mediating cellular senescence and thereby vascular dysfunction. Herein, we show that, in ECs, the MEOX homeodomain transcription factors decrease the expression of genes involved in angiogenesis, repress eNOS expression at the mRNA and protein levels, and increase the expression of p53. These findings support a role for the MEOX proteins in promoting endothelial dysfunction.

Interactive Roles of DNA Helicases and Translocases with the Single-Stranded DNA Binding Protein RPA in Nucleic Acid Metabolism

Helicases and translocases use the energy of nucleoside triphosphate binding and hydrolysis to unwind/resolve structured nucleic acids or move along a single-stranded or double-stranded polynucleotide chain, respectively. These molecular motors facilitate a variety of transactions including replication, DNA repair, recombination, and transcription. A key partner of eukaryotic DNA helicases/translocases is the single-stranded DNA binding protein Replication Protein A (RPA). Biochemical, genetic, and cell biological assays have demonstrated that RPA interacts with these human molecular motors physically and functionally, and their association is enriched in cells undergoing replication stress. The roles of DNA helicases/translocases are orchestrated with RPA in pathways of nucleic acid metabolism. RPA stimulates helicase-catalyzed DNA unwinding, enlists translocases to sites of action, and modulates their activities in DNA repair, fork remodeling, checkpoint activation, and telomere maintenance. The dynamic interplay between DNA helicases/translocases and RPA is just beginning to be understood at the molecular and cellular levels, and there is still much to be learned, which may inform potential therapeutic strategies.

p53 in the DNA-Damage-Repair Process

The cells in the human body are continuously challenged by a variety of genotoxic attacks. Erroneous repair of the DNA can lead to mutations and chromosomal aberrations that can alter the functions of tumor suppressor genes or oncogenes, thus causing cancer development. As a central tumor suppressor, p53 guards the genome by orchestrating a variety of DNA-damage-response (DDR) mechanisms. Already early in metazoan evolution, p53 started controlling the apoptotic demise of genomically compromised cells. p53 plays a prominent role as a facilitator of DNA repair by halting the cell cycle to allow time for the repair machineries to restore genome stability. In addition, p53 took on diverse roles to also directly impact the activity of various DNA-repair systems. It thus appears as if p53 is multitasking in providing protection from cancer development by maintaining genome stability.

A novel Werner Syndrome mutation: pharmacological treatment by read-through of nonsense mutations and epigenetic therapies

Werner Syndrome (WS) is a rare inherited disease characterized by premature aging and increased propensity for cancer. Mutations in the WRN gene can be of several types, including nonsense mutations, leading to a truncated protein form. WRN is a RecQ family member with both helicase and exonuclease activities, and it participates in several cell metabolic pathways, including DNA replication, DNA repair, and telomere maintenance. Here, we reported a novel homozygous WS mutation (c.3767 C > G) in 2 Argentinian brothers, which resulted in a stop codon and a truncated protein (p.S1256X). We also observed increased WRN promoter methylation in the cells of patients and decreased messenger WRN RNA (WRN mRNA) expression. Finally, we showed that the read-through of nonsense mutation pharmacologic treatment with both aminoglycosides (AGs) and ataluren (PTC-124) in these cells restores full-length protein expression and WRN functionality.

Programmed cell death in aging

Programmed cell death (PCD) pathways, including apoptosis and regulated necrosis, are required for normal cell turnover and tissue homeostasis. Mis-regulation of PCD is increasingly implicated in aging and aging-related disease. During aging the cell turnover rate declines for several highly-mitotic tissues. Aging-associated disruptions in systemic and inter-cell signaling combined with cell-autonomous damage and mitochondrial malfunction result in increased PCD in some cell types, and decreased PCD in other cell types. Increased PCD during aging is implicated in immune system decline, skeletal muscle wasting (sarcopenia), loss of cells in the heart, and neurodegenerative disease. In contrast, cancer cells and senescent cells are resistant to PCD, enabling them to increase in abundance during aging. PCD pathways limit life span in fungi, but whether PCD pathways normally limit adult metazoan life span is not yet clear. PCD is regulated by a balance of negative and positive factors, including the mitochondria, which are particularly subject to aging-associated malfunction.

RECQL4 is essential for the transport of p53 to mitochondria in normal human cells in the absence of exogenous stress

Mutations in RECQL4 helicase are associated with Rothmund-Thomson syndrome (RTS). A subset of RTS patients is predisposed to cancer and is sensitive to DNA damaging agents. The enhanced sensitivity of cells from RTS patients correlates with the accumulation of transcriptionally active nuclear p53. We found that in untreated normal human cells these two nuclear proteins, p53 and RECQL4, instead colocalize in the mitochondrial nucleoids. RECQL4 accumulates in mitochondria in all phases of the cell cycle except S phase and physically interacts with p53 only in the absence of DNA damage. p53-RECQL4 binding leads to the masking of the nuclear localization signal of p53. The N-terminal 84 amino acids of RECQL4 contain a mitochondrial localization signal, which causes the localization of RECQL4-p53 complex to the mitochondria. RECQL4-p53 interaction is disrupted after stress, allowing p53 translocation to the nucleus. In untreated normal cells RECQL4 optimizes de novo replication of mtDNA, which is consequently decreased in fibroblasts from RTS patients. Wild-type RECQL4-complemented RTS cells show relocalization of both RECQL4 and p53 to the mitochondria, loss of p53 activation, restoration of de novo mtDNA replication and resistance to different types of DNA damage. In cells expressing Δ84 RECQL4, which cannot translocate to mitochondria, all the above functions are compromised. The recruitment of p53 to the sites of de novo mtDNA replication is also regulated by RECQL4. Thus these findings elucidate the mechanism by which p53 is regulated by RECQL4 in unstressed normal cells and also delineates the mitochondrial functions of the helicase.

Apoptosis and aging: increased resistance to apoptosis enhances the aging process

Apoptosis is a vital component in the evolutionarily conserved host defense system. Apoptosis is the guardian of tissue integrity by removing unfit and injured cells without evoking inflammation. However, apoptosis seems to be a double-edged sword since during low-level chronic stress, such as in aging, increased resistance to apoptosis can lead to the survival of functionally deficient, post-mitotic cells with damaged housekeeping functions. Senescent cells are remarkably resistant to apoptosis, and several studies indicate that host defense mechanisms can enhance anti-apoptotic signaling, which subsequently induces a senescent, pro-inflammatory phenotype during the aging process. At the molecular level, age-related resistance to apoptosis involves (1) functional deficiency in p53 network, (2) increased activity in the NF-κB-IAP/JNK axis, and (3) changes in molecular chaperones, microRNAs, and epigenetic regulation. We will discuss the molecular basis of age-related resistance to apoptosis and emphasize that increased resistance could enhance the aging process.

The RecQ DNA helicases in DNA repair

The RecQ helicases are conserved from bacteria to humans and play a critical role in genome stability. In humans, loss of RecQ gene function is associated with cancer predisposition and/or premature aging. Recent experiments have shown that the RecQ helicases function during distinct steps during DNA repair; DNA end resection, displacement-loop (D-loop) processing, branch migration, and resolution of double Holliday junctions (dHJs). RecQ function in these different processing steps has important implications for its role in repair of double-strand breaks (DSBs) that occur during DNA replication and meiosis, as well as at specific genomic loci such as telomeres.

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.

Regulation of p53--insights into a complex process

The p53 protein is one of the most important tumor suppressor proteins. Normally, the p53 protein is in a latent state. However, when its activity is required, e.g. upon DNA damage, nucleotide depletion or hypoxia, p53 becomes rapidly activated and initiates transcription of pro-apoptotic and cell cycle arrest-inducing target genes. The activity of p53 is regulated both by protein abundance and by post-translational modifications of pre-existing p53 molecules. In the 30 years of p53 research, a plethora of modifications and interaction partners that modulate p53's abundance and activity have been identified and new ones are continuously discovered. This review will summarize our current knowledge on the regulation of p53 abundance and activity.

Altered nuclear functions in progeroid syndromes: a paradigm for aging research

Syndromes of accelerated aging could provide an entry point for identifying and dissecting the cellular pathways that are involved in the development of age-related pathologies in the general population. However, their usefulness for aging research has been controversial, as it has been argued that these diseases do not faithfully reflect the process of natural aging. Here we review recent findings on the molecular basis of two progeroid diseases, Werner syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), and highlight functional connections to cellular processes that may contribute to normal aging.

Telomeric DNA induces p53-dependent reactive oxygen species and protects against oxidative damage

BACKGROUND

Reactive oxygen species (ROS) are generated by cellular metabolism as well as by exogenous agents. While ROS can promote cellular senescence, they can also act as signaling molecules for processes that do not lead to senescence. Telomere homolog oligonucleotides (T-oligos) induce adaptive DNA damage responses including increased DNA repair capacity and these effects are mediated, at least in part, through p53.

OBJECTIVE

Studies were undertaken to determine whether such p53-mediated protective responses include enhanced antioxidant defenses.

METHODS

Normal human fibroblasts as well as R2F fibroblasts expressing wild type or dominant negative p53 were treated with an 11-base T-oligo, a complementary control oligo or diluents alone and then examined by western blot analysis, immunofluorescence microscopy and various biochemical assays.

RESULTS

We now report that T-oligo increases the level of the antioxidant enzymes superoxide dismutase 1 and 2 and protects cells from oxidative damage; and that telomere-based gammaH2AX (DNA damage) foci that form in response to T-oligos contain phosphorylated ATM and Chk2, proteins known to activate p53 and to mediate cell cycle arrest in response to oxidative stress. Further, T-oligo increases cellular ROS levels via a p53-dependent pathway, and these increases are abrogated by the NAD(P)H oxidase inhibitor diphenyliodonium chloride.

CONCLUSION

These results suggest the existence of innate telomere-based protective responses that act to reduce oxidative damage to cells. T-oligo treatment induces the same responses and offers a new model for studying intracellular ROS signaling and the relationships between DNA damage, ROS, oxidative stress, and cellular defense mechanisms.

Human papilloma virus type16 E6 deregulates CHK1 and sensitizes human fibroblasts to environmental carcinogens independently of its effect on p53

After treatment with ultraviolet radiation (UV), human fibroblasts that express the HPV type 16 E6 oncoprotein display defects in repair of cyclobutane pyrimidine dimers, hypersensitivity to inactivation of clonogenic survival and an inability to sustain DNA replication. To determine whether these effects are specific to depletion of p53 or inactivation of its function, fibroblast lines were constructed with ectopic expression of a dominant-negative p53 allele (p53-H179Q) to inactivate function or a short-hairpin RNA (p53-RNAi) to deplete expression of p53. Only the expression of HPV16E6 sensitized fibroblasts to UV or the chemical carcinogen, benzo[a]pyrene diolepoxide I (BPDE). Carcinogen-treated cells expressing p53-H179Q or p53-RNAi were resistant to inactivation of colony formation and did not suffer replication arrest. CHK1 is a key checkpoint kinase in the response to carcinogen-induced DNA damage. Control and p53-RNAi-expressing fibroblasts displayed phosphorylation of Ser345 on CHK1 45-120 min after carcinogen treatment with a return to near baseline phosphorylation by 6 h after treatment. HPV16E6-expressing fibroblasts displayed enhanced and sustained phosphorylation of CHK1. This was associated with enhanced phosphorylation of Thr68 on CHK2 and Ser139 on H2AX, both markers of severe replication stress and DNA double strand breaks. Incubation with the phosphatase inhibitor okadaic acid produced more phosphorylation of CHK1 in UV-treated HPV16E6-expressing cells than in p53-H179Q-expressing cells suggesting that HPV16E6 may interfere with the recovery of coupled DNA replication at replication forks that are stalled at [6-4]pyrimidine-pyrimidone photoproducts and BPDE-DNA adducts. The results indicate that HPV16E6 targets a protein or proteins other than p53 to deregulate the activity of CHK1 in carcinogen-damaged cells.

Model of human aging: recent findings on Werner's and Hutchinson-Gilford progeria syndromes

The molecular mechanisms involved in human aging are complicated. Two progeria syndromes, Werner's syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), characterized by clinical features mimicking physiological aging at an early age, provide insights into the mechanisms of natural aging. Based on recent findings on WS and HGPS, we suggest a model of human aging. Human aging can be triggered by two main mechanisms, telomere shortening and DNA damage. In telomere-dependent aging, telomere shortening and dysfunction may lead to DNA damage responses which induce cellular senescence. In DNA damage-initiated aging, DNA damage accumulates, along with DNA repair deficiencies, resulting in genomic instability and accelerated cellular senescence. In addition, aging due to both mechanisms (DNA damage and telomere shortening) is strongly dependent on p53 status. These two mechanisms can also act cooperatively to increase the overall level ofgenomic instability, triggering the onset of human aging phenotypes.

Werner syndrome protein, WRN, protects cells from DNA damage induced by the benzene metabolite hydroquinone

Werner syndrome (WS) is a rare autosomal progeroid disorder caused by a mutation in the gene encoding the WRN (Werner syndrome protein), a member of the RecQ family of helicases with a role in maintaining genomic stability. Genetic association studies have previously suggested a link between WRN and susceptibility to benzene-induced hematotoxicity. To further explore the role of WRN in benzene-induced hematotoxicity, we used short hairpin RNA to silence endogenous levels of WRN in the human HL60 acute promyelocytic cell line and subsequently exposed the cells to hydroquinone (HQ). Suppression of WRN led to an accelerated cell growth rate, increased susceptibility to hydroquinone-induced cytotoxicity and genotoxicity as measured by the single-cell gel electrophoresis assay, and an enhanced DNA damage response. More specifically, loss of WRN resulted in higher levels of early apoptosis, marked by increases in relative levels of cleaved caspase-7 and cleaved poly (ADP-ribose) polymerase 1, in cells treated with HQ compared with control cells. Our data suggests that WRN plays an important role in the surveillance of and protection against DNA damage induced by HQ. This provides mechanistic support for the link between WRN and benzene-induced hematotoxicity.

Large-scale evaluation of candidate genes identifies associations between DNA repair and genomic maintenance and development of benzene hematotoxicity

Benzene is an established human hematotoxicant and leukemogen but its mechanism of action is unclear. To investigate the role of single-nucleotide polymorphisms (SNPs) on benzene-induced hematotoxicity, we analyzed 1395 SNPs in 411 genes using an Illumina GoldenGate assay in 250 benzene-exposed workers and 140 unexposed controls. Highly significant findings clustered in five genes (BLM, TP53, RAD51, WDR79 and WRN) that play a critical role in DNA repair and genomic maintenance, and these regions were then further investigated with tagSNPs. One or more SNPs in each gene were associated with highly significant 10-20% reductions (P values ranged from 0.0011 to 0.0002) in the white blood cell (WBC) count among benzene-exposed workers but not controls, with evidence for gene-environment interactions for SNPs in BLM, WRN and RAD51. Further, among workers exposed to benzene, the genotype-associated risk of having a WBC count <4000 cells/microl increased when using individuals with progressively higher WBC counts as the comparison group, with some odds ratios >8-fold. In vitro functional studies revealed that deletion of SGS1 in yeast, equivalent to lacking BLM and WRN function in humans, caused reduced cellular growth in the presence of the toxic benzene metabolite hydroquinone, and knockdown of WRN using specific short hairpin RNA increased susceptibility of human TK6 cells to hydroquinone toxicity. Our findings suggest that SNPs involved in DNA repair and genomic maintenance, with particular clustering in the homologous DNA recombination pathway, play an important role in benzene-induced hematotoxicity.

Human premature aging, DNA repair and RecQ helicases

Genomic instability leads to mutations, cellular dysfunction and aberrant phenotypes at the tissue and organism levels. A number of mechanisms have evolved to cope with endogenous or exogenous stress to prevent chromosomal instability and maintain cellular homeostasis. DNA helicases play important roles in the DNA damage response. The RecQ family of DNA helicases is of particular interest since several human RecQ helicases are defective in diseases associated with premature aging and cancer. In this review, we will provide an update on our understanding of the specific roles of human RecQ helicases in the maintenance of genomic stability through their catalytic activities and protein interactions in various pathways of cellular nucleic acid metabolism with an emphasis on DNA replication and repair. We will also discuss the clinical features of the premature aging disorders associated with RecQ helicase deficiencies and how they relate to the molecular defects.

Role for the Werner syndrome protein in the promotion of tumor cell growth

Werner syndrome (WS) is a premature aging and cancer-prone disease caused by loss of the RecQ helicase WRN protein. Cultured WS fibroblasts display high genomic instability and senesce prematurely. Epigenetic inactivation of the WRN gene occurs in numerous tumor types, in which WRN demonstrates tumor suppressor-like activity (Agrelo et al., 2006). However, the role of WRN in tumors that express WRN protein is unknown. Here we report that the inhibition of WRN expression strongly impairs growth of 12 out of 15 cancer cell lines tested. For those cell lines in which WRN depletion induced high cell death, the majority of the surviving proliferative clones exhibited WRN expression. Growth arrest induced by WRN depletion was characterized by an accumulation of cells in the G2/M cell cycle phases and an increase in DNA damage. Importantly, WRN depletion inhibited tumor growth in vivo in SCID mouse xenograft models. Altogether, these findings support a dual role for WRN in tumorigenesis; tumor suppressor-like activity in tumors with WRN inactivation and the promotion of proliferation and survival in tumors that express WRN. These findings suggest a possible therapeutic role for WRN as an anti-cancer target, and highlight the importance of WRN protein status for tumorigenesis and clinical treatments of patients.

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.

Werner syndrome protein: functions in the response to DNA damage and replication stress in S-phase

Werner syndrome (WS) is an excellent model system for the study of human aging. WRN, a nuclear protein mutated in WS, plays multiple roles in DNA metabolism. Our understanding about the metabolic regulation and function of this RecQ helicase has advanced greatly during the past decade, largely due to the availability of purified WRN protein, WRN knockdown cells, and WRN knockout mice. Recent biochemical and genetic studies indicate that WRN plays significant roles in DNA replication, DNA repair, and telomere maintenance. Interestingly, many WRN functions require handling of DNA ends during S-phase, and evidence suggests that WRN plays both upstream and downstream roles in the response to DNA damage. Future research should focus on the mechanism(s) of WRN in the regulation of the various DNA metabolism pathways and development of therapeutic approaches to treat premature aging syndromes such as WS.

The Werner syndrome protein is required for recruitment of chromatin assembly factor 1 following DNA damage

The Werner syndrome protein (WRN) and chromatin assembly factor 1 (CAF-1) are both involved in the maintenance of genome stability. In response to DNA-damaging signals, both of these proteins relocate to sites where DNA synthesis occurs. However, the interaction between WRN and CAF-1 has not yet been investigated. In this report, we show that WRN interacts physically with the largest subunit of CAF-1, hp150, in vitro and in vivo. Although hp150 does not alter WRN catalytic activities in vitro, and the chromatin assembly activity of CAF-1 is not affected in the absence of WRN in vivo, this interaction may have an important role during the cellular response to DNA replication fork blockage and/or DNA damage signals. In hp150 RNA-mediated interference (RNAi) knockdown cells, WRN partially formed foci following hydroxyurea (HU) treatment. However, in the absence of WRN, hp150 did not relocate to form foci following exposure to HU and ultraviolet light. Thus, our results demonstrate that WRN responds to DNA damage before CAF-1 and suggest that WRN may recruit CAF-1, via interaction with hp150, to DNA damage sites during DNA synthesis.

Crystal structure of the HRDC domain of human Werner syndrome protein, WRN

Werner syndrome is a human premature aging disorder characterized by chromosomal instability. The disease is caused by the functional loss of WRN, a member of the RecQ-helicase family that plays an important role in DNA metabolic pathways. WRN contains four structurally folded domains comprising an exonuclease, a helicase, a winged-helix, and a helicase-and-ribonuclease D/C-terminal (HRDC) domain. In contrast to the accumulated knowledge pertaining to the biochemical functions of the three N-terminal domains, the function of C-terminal HRDC remains unknown. In this study, the crystal structure of the human WRN HRDC domain has been determined. The domain forms a bundle of alpha-helices similar to those of Saccharomyces cerevisiae Sgs1 and Escherichia coli RecQ. Surprisingly, the extra ten residues at each of the N and C termini of the domain were found to participate in the domain architecture by forming an extended portion of the first helix alpha1, and a novel looping motif that traverses straight along the domain surface, respectively. The motifs combine to increase the domain surface of WRN HRDC, which is larger than that of Sgs1 and E. coli. In WRN HRDC, neither of the proposed DNA-binding surfaces in Sgs1 or E. coli is conserved, and the domain was shown to lack DNA-binding ability in vitro. Moreover, the domain was shown to be thermostable and resistant to protease digestion, implying independent domain evolution in WRN. Coupled with the unique long linker region in WRN, the WRN HRDC may be adapted to play a distinct function in WRN that involves protein-protein interactions.

Molecular bases of progeroid syndromes

Progeroid syndromes (PSs) constitute a group of disorders characterized by clinical features mimicking physiological aging at an early age. In some of these syndromes, biological hallmarks of aging are also present, whereas in others, a link with physiological aging, if any, remains to be elucidated. These syndromes are clinically and genetically heterogeneous and most of them, including Werner syndrome and Hutchinson-Gilford progeria, are known as 'segmental aging syndromes', as they do not feature all aspects usually associated to physiological aging. However, all the characterized PSs enter in the field of rare monogenic disorders and several causative genes have been identified. These can be separated in subcategories corresponding to (i) genes encoding DNA repair factors, in particular, DNA helicases, and (ii) genes affecting the structure or post-translational maturation of lamin A, a major nuclear component. In addition, several animal models featuring premature aging have abnormal mitochondrial function or signal transduction between membrane receptors, nuclear regulatory proteins and mitochondria: no human pathological counterpart of these alterations has been found to date. In recent years, identification of mutations and their functional characterization have helped to unravel the cellular processes associated to segmental PSs. Recently, several studies allowed to establish a functional link between DNA repair and A-type lamins-associated syndromes, evidencing a relation between these syndromes, physiological aging and cancer. Here, we review recent data on molecular and cellular bases of PSs and discuss the mechanisms involved, with a special emphasis on lamin A-associated progeria and related disorders, for which therapeutic approaches have started to be developed.

Mechanisms of RecQ helicases in pathways of DNA metabolism and maintenance of genomic stability

Helicases are molecular motor proteins that couple the hydrolysis of NTP to nucleic acid unwinding. The growing number of DNA helicases implicated in human disease suggests that their vital specialized roles in cellular pathways are important for the maintenance of genome stability. In particular, mutations in genes of the RecQ family of DNA helicases result in chromosomal instability diseases of premature aging and/or cancer predisposition. We will discuss the mechanisms of RecQ helicases in pathways of DNA metabolism. A review of RecQ helicases from bacteria to human reveals their importance in genomic stability by their participation with other proteins to resolve DNA replication and recombination intermediates. In the light of their known catalytic activities and protein interactions, proposed models for RecQ function will be summarized with an emphasis on how this distinct class of enzymes functions in chromosomal stability maintenance and prevention of human disease and cancer.

Redundancy of DNA helicases in p53-mediated apoptosis

A subset of DNA helicases, the RecQ family, has been found to be associated with the p53-mediated apoptotic pathway and is involved in maintaining genomic integrity. This family contains the BLM and WRN helicases, in which germline mutations are responsible for Bloom and Werner syndromes, respectively. TFIIH DNA helicases, XPB and XPD, are also components in this apoptotic pathway. We hypothesized that there may be some redundancy between helicases in their ability to complement the attenuated p53-mediated apoptotic levels seen in cells from individuals with diseases associated with these defective helicase genes. The attenuated apoptotic phenotype in Bloom syndrome cells was rescued not only by ectopic expression of BLM, but also by WRN or XPB, both 3' --> 5' helicases, but not expression of the 5' --> 3' helicase XPD. Overexpression of Sgs1, a WRN/BLM yeast homolog, corrected the reduction in BS cells only, which is consistent with Sgs1 being evolutionarily most homologous to BLM. A restoration of apoptotic levels in cells from WS, XPB or XPD patients was attained only by overexpression of the specific helicase. Our data suggest a limited redundancy in the pathways of these RecQ helicases in p53-induced apoptosis.

The spectrum of WRN mutations in Werner syndrome patients

The International Registry of Werner syndrome (www.wernersyndrome.org) has been providing molecular diagnosis of the Werner syndrome (WS) for the past decade. The present communication summarizes, from among 99 WS subjects, the spectrum of 50 distinct mutations discovered by our group and by others since the WRN gene (also called RECQL2 or REQ3) was first cloned in 1996; 25 of these have not previously been published. All WRN mutations reported thus far have resulted in the elimination of the nuclear localization signal at the C-terminus of the protein, precluding functional interactions in the nucleus; thus, all could be classified as null mutations. We now report two new mutations in the N-terminus that result in instability of the WRN protein. Clinical data confirm that the most penetrant phenotype is bilateral ocular cataracts. Other cardinal signs were seen in more than 95% of the cases. The median age of death, previously reported to be in the range of 46-48 years, is 54 years. Lymphoblastoid cell lines (LCLs) have been cryopreserved from the majority of our index cases, including material from nuclear pedigrees. These, as well as inducible and complemented hTERT (catalytic subunit of human telomerase) immortalized skin fibroblast cell lines are available to qualified investigators.

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

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

Formation of a nuclear complex containing the p53 tumor suppressor, YB-1, and the Werner syndrome gene product in cells treated with UV light

YB-1 is a multifunctional protein involved in the regulation of transcription, translation, and mRNA splicing. In recent years, several laboratories have demonstrated that YB-1 is also directly involved in the cellular response to genotoxic stress. Accordingly, one report has indicated that the Werner syndrome gene product (WRN) is eluted from an YB-1 affinity chromatography column. Werner syndrome is a rare disorder characterized by the premature onset of a number of age-related diseases, including cancer. The gene responsible for Werner syndrome encodes a DNA helicase/exonuclease protein believed to be involved in some aspect of DNA repair with p53. In this study, we demonstrate that the tumor suppressor, p53, bridges the WRN and YB-1 proteins in vitro. Microscopic analyses of fluorescent-tagged proteins and co-immunoprecipitation experiments confirmed the formation of an YB-1/p53/WRN complex in human cells, but only after treatment with UV light. We also confirmed that p53 is a major player in the translocation of GFP-YB-1 fusion proteins from the cytoplasm to several nuclear foci containing WRN proteins upon UV irradiation. Such translocation did not occur in cells treated with the topoisomerase inhibitor, etoposide, or the radiomimetic drug, bleomycin. Such results suggest that an YB-1/p53/WRN complex is formed in response to the emergence of specific DNA lesions in cells.

Werner protein protects nonproliferating cells from oxidative DNA damage

Werner syndrome, caused by mutations of the WRN gene, mimics many changes of normal aging. Although roles for WRN protein in DNA replication, recombination, and telomere maintenance have been suggested, the pathology of rapidly dividing cells is not a feature of Werner syndrome. To identify cellular events that are specifically vulnerable to WRN deficiency, we used RNA interference (RNAi) to knockdown WRN or BLM (the RecQ helicase mutated in Bloom syndrome) expression in primary human fibroblasts. Withdrawal of WRN or BLM produced accelerated cellular senescence phenotype and DNA damage response in normal fibroblasts, as evidenced by induction of gammaH2AX and 53BP1 nuclear foci. After WRN depletion, the induction of these foci was seen most prominently in nondividing cells. Growth in physiological (3%) oxygen or in the presence of an antioxidant prevented the development of the DNA damage foci in WRN-depleted cells, whereas acute oxidative stress led to inefficient repair of the lesions. Furthermore, WRN RNAi-induced DNA damage was suppressed by overexpression of the telomere-binding protein TRF2. These conditions, however, did not prevent the DNA damage response in BLM-ablated cells, suggesting a distinct role for WRN in DNA homeostasis in vivo. Thus, manifestations of Werner syndrome may reflect an impaired ability of slowly dividing cells to limit oxidative DNA damage.

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.

Current advances in unraveling the function of the Werner syndrome protein

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

Biology of Osteogenic Sarcoma

Osteosarcoma is the most common primary malignant bone tumor in children and adolescents. Despite significant clinical improvements over the past several decades through the use of combination chemotherapy and surgery, patients with metastatic or recurrent disease continue to have a very poor prognosis. Therefore, there is a continued need to study and understand the basic biology of osteosarcoma in order to devise more targeted and rational therapeutic strategies and ultimately to improve survival for these patients. This article reviews several aspects of osteosarcoma biology where data exist to suggest that specific pathways may play a role in the pathogenesis of this tumor. These areas include host genetic predispositions, tumor cytogenetics, molecular genetics (including the Rb, p53, RECQ helicase, and telomere pathways), and metastatic factors (ezrin, annexin 2, chemokine receptor 4, Fas/FasL pathways) that may contribute to both the initiation and the progression of tumor formation. Understanding the mechanisms of and interactions between the various molecular pathways that play a role in osteosarcoma pathogenesis may eventually lead to a more rational strategy for devising therapies targeted specifically toward these pathways.

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.

p53 modulates RPA-dependent and RPA-independent WRN helicase activity

Werner syndrome is a hereditary disorder characterized by the early onset of age-related symptoms, including cancer. The absence of a p53-WRN helicase interaction may disrupt the signal to direct S-phase cells into apoptosis for programmed cell death and contribute to the pronounced genomic instability and cancer predisposition in Werner syndrome cells. Results from coimmunoprecipitation studies indicate that WRN is associated with replication protein A (RPA) and p53 in vivo before and after treatment with the replication inhibitor hydroxyurea or gamma-irradiation that introduces DNA strand breaks. Analysis of the protein interactions among purified recombinant WRN, RPA, and p53 proteins indicate that all three protein pairs bind with similar affinity in the low nanomolar range. In vitro studies show that p53 inhibits RPA-stimulated WRN helicase activity on an 849-bp M13 partial duplex substrate. p53 also inhibited WRN unwinding of a short (19-bp) forked duplex substrate in the absence of RPA. WRN unwinding of the forked duplex substrate was specific, because helicase inhibition mediated by p53 was retained in the presence of excess competitor DNA and was significantly reduced or absent in helicase reactions catalyzed by a WRN helicase domain fragment lacking the p53 binding site or the human RECQ1 DNA helicase, respectively. p53 effectively inhibited WRN helicase activity on model DNA substrate intermediates of replication/repair, a 5' ssDNA flap structure and a synthetic replication fork. Regulation of WRN helicase activity by p53 is likely to play an important role in genomic integrity surveillance, a vital function in the prevention of tumor progression.

Monogenic syndromes of abnormal glucose homeostasis: clinical review and relevance to the understanding of the pathology of insulin resistance and beta cell failure

Type 2 diabetes mellitus is caused by a combination of insulin resistance and beta cell failure. The polygenic nature of type 2 diabetes has made it difficult to study. Although many candidate genes for this condition have been suggested, in most cases association studies have been equivocal. Monogenic forms of diabetes have now been studied extensively, and the genetic basis of many of these syndromes has been elucidated, leading to greater understanding of the functions of the genes involved. Common variations in the genes causing monogenic disorders have been associated with susceptibility to type 2 diabetes in several populations and explain some of the linkage seen in genome-wide scans. Monogenic disorders are also helpful in understanding both normal and disordered glucose and insulin metabolism. Three main areas of defect contribute to diabetes: defects in insulin signalling leading to insulin resistance; defects of insulin secretion leading to hypoinsulinaemia; and apoptosis leading to decreased beta cell mass. These three pathological pathways are reviewed, focusing on rare genetic syndromes which have diabetes as a prominent feature. Apoptosis seems to be a final common pathway in both type 1 and type 2 diabetes. Study of rare forms of diabetes may help ion determining new therapeutic targets to preserve or increase beta cell mass and function.

POT1 and TRF2 cooperate to maintain telomeric integrity

Mammalian telomeric DNA contains duplex TTAGGG repeats and single-stranded overhangs. POT1 (protection of telomeres 1) is a telomere-specific single-stranded DNA-binding protein, highly conserved in eukaryotes. The biological function of human POT1 is not well understood. In the present study, we demonstrate that POT1 plays a key role in telomeric end protection. The reduction of POT1 by RNA interference led to the loss of telomeric single-stranded overhangs and induced apoptosis, chromosomal instability, and senescence in cells. POT1 and TRF2 interacted with each other to form a complex with telomeric DNA. A dominant negative TRF2, TRF2(DeltaBDeltaM), bound to POT1 and prevented it from binding to telomeres. POT1 overexpression protected against TRF2(DeltaBDeltaM)-induced loss of telomeric single-stranded overhangs, chromosomal instability, and senescence. These results demonstrate that POT1 and TRF2 share in part in the same pathway for telomere capping and suggest that POT1 binds to the telomeric single-stranded DNA in the D-loop and cooperates with TRF2 in t-loop maintenance.

p53: traffic cop at the crossroads of DNA repair and recombination

p53 mutants that lack DNA-binding activities, and therefore, transcriptional activities, are among the most common mutations in human cancer. Recently, a new role for p53 has come to light, as the tumour suppressor also functions in DNA repair and recombination. In cooperation with its function in transcription, the transcription-independent roles of p53 contribute to the control and efficiency of DNA repair and recombination.

The Werner syndrome protein at the crossroads of DNA repair and apoptosis

Werner syndrome (WS) is a premature aging disease characterized by genetic instability. WS is caused by mutations in a gene encoding for a 160 kDa nuclear protein, the Werner syndrome protein (WRN), which has exonuclease and helicase activities. The mechanism whereby WRN controls genome stability and life span is not known. Over the last few years, WRN has become the focus of intense investigation by a growing number of scientists. The studies carried out by many laboratories have provided a wealth of new information about the functional properties of WRN and its cellular partners. This review focuses on recent findings that demonstrate a functional interaction between WRN and two factors that bind to DNA breaks, Ku and poly(ADP-ribose) polymerase 1, and discuss how these interactions can influence fundamental cellular processes such as DNA repair, apoptosis and possibly regulate cell senescence and organismal aging.

Human MUC1 oncoprotein regulates p53-responsive gene transcription in the genotoxic stress response

The MUC1 oncoprotein is aberrantly overexpressed by most human carcinomas. The present work demonstrates that MUC1 associates with the p53 tumor suppressor, and that this interaction is increased by genotoxic stress. The MUC1 cytoplasmic domain binds directly to p53 regulatory domain. Chromatin immunoprecipitation assays demonstrate that MUC1 coprecipitates with p53 on the p53-responsive elements of the p21 gene promoter and coactivates p21 gene transcription. Conversely, MUC1 attenuates activation of Bax transcription. In concert with these results, MUC1 promotes selection of the p53-dependent growth arrest response and suppresses the p53-dependent apoptotic response to DNA damage. These findings indicate that MUC1 regulates p53-responsive genes and thereby cell fate in the genotoxic stress response.

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.

PML nuclear bodies: dynamic sensors of DNA damage and cellular stress

Promyelocytic leukaemia nuclear bodies (PML NBs) are generally present in all mammalian cells, and their integrity correlates with normal differentiation of promyelocytes. Mice that lack PML NBs have impaired immune function, exhibit chromosome instability and are sensitive to carcinogens. Although their direct role in nuclear activity is unclear, PML NBs are implicated in the regulation of transcription, apoptosis, tumour suppression and the anti-viral response. An emerging view is that they represent sites where multi-subunit complexes form and where post-translational modification of regulatory factors, such as p53, occurs in response to cellular stress. Following DNA damage, several repair factors transit through PML NBs in a temporally regulated manner implicating these bodies in DNA repair. We propose that PML NBs are dynamic sensors of cellular stress, which rapidly disassemble following DNA damage into large supramolecular complexes, dispersing associated repair factors to sites of damage. The dramatically increased total surface area available would enhance interactions between PML-associated factors regulating DNA repair and apoptosis.

Werner syndrome cells escape hydrogen peroxide-induced cell proliferation arrest

Werner syndrome (WS) is a rare disease caused by the lack of a functional nuclear WS protein (WRN). WS is characterized by the early onset of premature aging signs and a high incidence of sarcomas. WS diploid fibroblasts have a short life span and extensive genomic instability. Mammalian cells are continuously exposed to reactive oxygen species (ROS), which represent human mutagens and are thought to be a major contributor to the aging process. Hydrogen peroxide (H2O2) is a common ROS intermediate generated by various forms of oxidative stress. In response to H2O2-induced DNA damage, normal human diploid fibroblasts follow a pathway leading to irreversible proliferation arrest and premature senescence. Here we show that in contrast to normal human fibroblasts, WS diploid fibroblasts continue proliferating after extensive H2O2-induced DNA damage and accumulate oxidative DNA lesions. A direct role of WRN in this abnormal cellular response to H2O2 is demonstrated by interfering with WRN expression in normal human fibroblasts. We propose a role for WRN in the detection and/or processing of oxidative DNA lesions and in cellular responses to H2O2 as they relate to some of the phenotypical aspects of WS cells.