MUT-7 Provides Molecular Insight into the Werner Syndrome Exonuclease

Werner syndrome (WS) is a rare recessive genetic disease characterized by premature aging. Individuals with this disorder develop normally during childhood, but their physiological conditions exacerbate the aging process in late adolescence. WS is caused by mutation of the human WS gene (WRN), which encodes two main domains, a 3′-5′ exonuclease and a 3′-5′ helicase. Caenorhabditis elegans expresses human WRN orthologs as two different proteins: MUT-7, which has a 3′-5′ exonuclease domain, and C. elegans WRN-1 (CeWRN-1), which has only helicase domains. These unique proteins dynamically regulate olfactory memory in C. elegans, providing insight into the molecular roles of WRN domains in humans. In this review, we specifically focus on characterizing the function of MUT-7 in small interfering RNA (siRNA) synthesis in the cytoplasm and the roles of siRNA in directing nuclear CeWRN-1 loading onto a heterochromatin complex to induce negative feedback regulation. Further studies on the different contributions of the 3′-5′ exonuclease and helicase domains in the molecular mechanism will provide clues to the accelerated aging processes in WS.

Human WRN is an intrinsic inhibitor of progerin, abnormal splicing product of lamin A

Werner syndrome (WRN) is a rare progressive genetic disorder, caused by functional defects in WRN protein and RecQ4L DNA helicase. Acceleration of the aging process is initiated at puberty and the expected life span is approximately the late 50 s. However, a Wrn-deficient mouse model does not show premature aging phenotypes or a short life span, implying that aging processes differ greatly between humans and mice. Gene expression analysis of WRN cells reveals very similar results to gene expression analysis of Hutchinson Gilford progeria syndrome (HGPS) cells, suggesting that these human progeroid syndromes share a common pathological mechanism. Here we show that WRN cells also express progerin, an abnormal variant of the lamin A protein. In addition, we reveal that duplicated sequences of human WRN (hWRN) from exon 9 to exon 10, which differ from the sequence of mouse WRN (mWRN), are a natural inhibitor of progerin. Overexpression of hWRN reduced progerin expression and aging features in HGPS cells. Furthermore, the elimination of progerin by siRNA or a progerin-inhibitor (SLC-D011 also called progerinin) can ameliorate senescence phenotypes in WRN fibroblasts and cardiomyocytes, derived from WRN-iPSCs. These results suggest that progerin, which easily accumulates under WRN-deficient conditions, can lead to premature aging in WRN and that this effect can be prevented by SLC-D011.

Review of How Genetic Research on Segmental Progeroid Syndromes Has Documented Genomic Instability as a Hallmark of Aging But Let Us Now Pursue Antigeroid Syndromes!

The purpose of this early contribution to the new Fellows Forum of this pioneering journal for what is now called Geroscience is to provide an example of how the author's interest in using the emerging tools of human genetics has led to strong support for one of the hallmarks of aging-Genomic Instability. We shall also briefly review our emerging interests in the genetic analysis of what we have called Antigeroid Syndromes. While there has been significant progress in that direction via genetic studies of centenarians, the search for genetic pathways that make individuals unusually resistant or resilient to the ravages of specific geriatric disorders has been comparatively neglected. We refer to these disorders as Unimodal Antigeroid Syndromes. It is our hope that our young colleagues will consider research efforts in that direction.

Accelerated aging and aging process in the brain

One of the approaches to the research of the problem of aging is the study of genetic pathologies leading to accelerated aging, such as the Hutchinson-Gilford progeria syndrome, Werner syndrome, and Down syndrome. Probably, this approach can be used in an attempt to understand the neuronal mechanisms underlying normal and pathological brain aging. The analysis of the current state of scientific knowledge about these pathologies shows that in the Hutchinson-Gilford progeria and Werner syndrome, the rate of brain aging is significantly lower than the rate of whole body aging, whereas in Down syndrome, the brain ages faster than other organs due to amyloid-beta accumulation and chronic oxidative stress in the brain tissue. The main point of a previously proposed hypothesis is that the aging of higher animals and humans is associated with an increased level of reactive oxygen species in mitochondria with age, which activates apoptosis, thus reducing the number of functioning cells.

Everolimus rescues multiple cellular defects in laminopathy-patient fibroblasts

LMNA encodes the A-type lamins that are part of the nuclear scaffold. Mutations in LMNA can cause a variety of disorders called laminopathies, including Hutchinson-Gilford progeria syndrome (HGPS), atypical Werner syndrome, and Emery-Dreifuss muscular dystrophy. Previous work has shown that treatment of HGPS cells with the mTOR inhibitor rapamycin or with the rapamycin analog everolimus corrects several of the phenotypes seen at the cellular level-at least in part by increasing autophagy and reducing the amount of progerin, the toxic form of lamin A that is overproduced in HGPS patients. Since other laminopathies also result in production of abnormal and potentially toxic lamin proteins, we hypothesized that everolimus would also be beneficial in those disorders. To test this, we applied everolimus to fibroblast cell lines from six laminopathy patients, each with a different mutation in LMNA Everolimus treatment increased proliferative ability and delayed senescence in all cell lines. In several cell lines, we observed that with treatment, there is a significant improvement in nuclear blebbing, which is a cellular hallmark of HGPS and other lamin disorders. These preclinical results suggest that everolimus might have clinical benefit for multiple laminopathy syndromes.

Homozygosity for the WRN Helicase-Inactivating Variant, R834C, does not confer a Werner syndrome clinical phenotype

Loss-of-function mutations in the WRN helicase gene cause Werner syndrome- a progeroid syndrome with an elevated risk of cancer and other age-associated diseases. Large numbers of single nucleotide polymorphisms have been identified in WRN. We report here the organismal, cellular, and molecular phenotypes of variant rs3087425 (c. 2500C > T) that results in an arginine to cysteine substitution at residue 834 (R834C) and up to 90% reduction of WRN helicase activity. This variant is present at a high (5%) frequency in Mexico, where we identified 153 heterozygous and three homozygous individuals among 3,130 genotyped subjects. Family studies of probands identified ten additional TT homozygotes. Biochemical analysis of WRN protein purified from TT lymphoblast cell lines confirmed that the R834C substitution strongly and selectively reduces WRN helicase, but not exonuclease activity. Replication track analyses showed reduced replication fork progression in some homozygous cells following DNA replication stress. Among the thirteen TT homozygotes, we identified a previously unreported and statistically significant gender bias in favor of males (p = 0.0016), but none of the clinical findings associated with Werner syndrome. Our results indicate that WRN helicase activity alone is not rate-limiting for the development of clinical WS.

Werner syndrome through the lens of tissue and tumour genomics

Werner syndrome (WS) is the canonical adult human progeroid ('premature aging') syndrome. Patients with this autosomal recessive Mendelian disorder display constitutional genomic instability and an elevated risk of important age-associated diseases including cancer. Remarkably few analyses of WS patient tissue and tumors have been performed to provide insight into WS disease pathogenesis or the high risk of neoplasia. We used autopsy tissue from four mutation-typed WS patients to characterize pathologic and genomic features of WS, and to determine genomic features of three neoplasms arising in two of these patients. The results of these analyses provide new information on WS pathology and genomics; provide a first genomic characterization of neoplasms arising in WS; and provide new histopathologic and genomic data to test several popular models of WS disease pathogenesis.

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.

Werner syndrome: clinical evaluation of two cases and a novel mutation

Werner syndrome (WS) is a premature aging disorder, inherited in an autosomal recessive pattern and caused by the mutation in the WRN gene. In this report we describe two male patients with negative family history who demonstrate characteristic findings of WS, with different mutations, including one novel mutation. The first case was a 47-year-old man who had been suffering from large, ischemic ulcers on both legs for 7 years. Physical examination revealed a thin and short man with severe wasting of all extremities. He had a high-pitched voice, hoarseness, a characteristic bird-like facies, bilateral cataracts, generalized osteoporosis, hypotrichosis, atrophic and poikilodermic skin, flexion contractures of hands, feet and knees, and soft tissue calcifications. Laboratory investigations revealed anemia, high erythrocyte sedimentation rate, low creatinine clearance, and high liver enzymes. Genetic analysis showed a homozygous novel 1bp-deletion in exon 19 of WRN, 2426/27delG, causing frameshift and protein truncation R809SfsX2, which has not been described before. The second case was a 23-year-old man who was referred for large callosities on both feet, present for 7 years. He complained of weakness, weight loss, wasting of muscles, and early graying of hair. The entire skin was thin, wrinkled and dry. Generalized hypotrichosis, scattered ephelid-like macules, sclerotic fingers, calcinosis cutis on ears, hyperpigmentation on elbows were the other alterations of skin. Skeletal survey revealed osteoporosis. Genetic analysis showed a homozygous known pathogenic splice site mutation c.3460-2A>G, causing skipping of Exon 30 in WRN.

Werner syndrome: a changing pattern of clinical manifestations in Japan (1917~2008)

As ~75% of the Werner syndrome (WS) patients recognized between 1904 and 2008 all over the world are of Japanese origin, the most case reports and clinical studies on WS has been published in Japanese journals. Thus, the detailed English-written clinical review on the recent WS case reports has been warranted. Although WS has been characterized by a variety of clinical manifestations mimicking premature aging, the recent longevity and delayed age-associated manifestations observed both from Japanese WS and general population may suggest a common environmental effect on some gene(s) other than WRN and may give us a newer pathophysiological look at WS and also natural aging through the molecular dysfunction of WRN.

Werner's syndrome may be lost in the shadow of the scleroderma

We describe three patients with Werner's syndrome (WS), two of whom had been mistakenly diagnosed as having scleroderma. We would like to discuss briefly the importance of differentiation of these two disorders from each other.

The heat shock transcription factor Hsf1 is downregulated in DNA damage-associated senescence, contributing to the maintenance of senescence phenotype

Heat shock response (HSR) that protects cells from proteotoxic stresses is downregulated in aging, as well as upon replicative senescence of cells in culture. Here we demonstrate that HSR is suppressed in fibroblasts from the patients with segmental progerioid Werner Syndrome, which undergo premature senescence. Similar suppression of HSR was seen in normal fibroblasts, which underwent senescence in response to DNA damaging treatments. The major DNA-damage-induced signaling (DDS) pathways p53-p21 and p38-NF-kB-SASP contributed to the HSR suppression. The HSR suppression was associated with inhibition of both activity and transcription of the heat shock transcription factor Hsf1. This inhibition in large part resulted from the downregulation of SIRT1, which in turn was because of decrease in the expression of the translation regulator HuR. Importantly, we uncovered a positive feedback regulation, where suppression of Hsf1 further activates the p38-NF-κB-SASP pathway, which in turn promotes senescence. Overexpression of Hsf1 inhibited the p38-NFκB-SASP pathway and partially relieved senescence. Therefore, downregulation of Hsf1 plays an important role in the development or in the maintenance of DNA damage signaling-induced cell senescence.

Fibroblasts from Werner syndrome patients: cancer cells derived by experimental introduction of oncogenes maintain malignant properties despite entering crisis

Werner syndrome (WS) results from defects in the gene encoding WRN RecQ helicase. WS fibroblasts undergo premature senescence in culture. Because cellular senescence is a tumor suppressor mechanism, we examined whether WS fibroblasts exhibited reduced tumorigenicity, in comparison to control cells, in a model of experimental conversion of normal human cells to cancer cells. The combination of oncogenic Ras (Ha-Ras(V12G)) and SV40 large T antigen (SV40 LT) causes human cells to acquire neoplastic properties in the absence of telomerase. We found that WS cells could also be converted to a tumorigenic state by these oncogenes, as evidenced by invasion and metastasis of cells implanted in immunodeficient mice. Ras/SV40 LT-expressing cells retained invasiveness and malignant properties even when cells reached crisis in tumors in vivo. High levels of gelatinase were found by an in situ assay in Ras/SV40 LT-expressing cells undergoing crisis. We conclude that, despite evidence of accelerated senescence in WS cells, there is no evidence that the absence of active WRN acts as a barrier to neoplastic transformation. Moreover, we find that tumorigenic human cells retain malignant properties of the cells as they approach and reach crisis.

Sequence-specific processing of telomeric 3' overhangs by the Werner syndrome protein exonuclease activity

Werner syndrome is a premature aging disease caused by loss of function mutations in the Werner syndrome protein (WRN) gene. WRN is a RecQ helicase that in contrast to every other member of this family of proteins possesses an exonuclease activity. The findings that cells lacking WRN activity display accelerated telomere shortening and WRN can be detected at chromosome ends suggest that this protein participates in some aspects of telomere metabolism. In this study we examined the impact of WRN on telomeric substrates with a 3' single-stranded overhang in vitro and show that WRN has sequence-specific exonuclease activity that removes several nucleotides inward with a periodical pattern from the 3' end of the telomeric overhang. This activity is strictly dependent on the presence of telomeric sequences in both the duplex DNA and 3' overhang DNA segment and is strongly inhibited by the telomeric factor POT1 but not TRF2. These data demonstrate that WRN processes telomeric DNA substrates with a 3' single-stranded overhang with high specificity and suggest that this protein could influence the configuration of telomere ends prior to the formation of a protective t-loop structure.

Off-pump coronary artery bypass grafting in a patient with Werner's syndrome

Werner's syndrome is a rare hereditary disorder that is characterized by premature aging. We report a case of off-pump coronary artery bypass grafting (OPCAB) in a 56-year-old man with Werner's syndrome. We used an endoscopic vessel-harvesting system to harvest great saphenous vein grafts (SVGs) because this system helps minimize surgical wounds. This is important because poor wound healing is a prominent feature of Werner's syndrome. Revascularization of the coronary arteries in this case was thought to improve his prognosis, although he had already outlived the average life-span of Werner's syndrome. A detailed examination of the cardiovascular system should be performed in patients with this disorder.

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

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

[The atypical of Werner syndrome: effect of laminopathy]

A case of adult progeria has been described. During detailed studies of the cells from this patient the nuclear lamina and cytoskeleton aberrations were detected. It has been suggested that this case is an atypical form of Werner syndrome with laminopathy--not the WRN helicase-nuclease defect.

[An atypical case of Werner syndrome: epigenetic control and DNA damage response alterations]

A case of adult progeria has been described. It has been suggested that this case is an atypical form of Werner syndrome with laminopathy--not WRN helicase-nuclease defect. During detailed studies of the patient's cells, epigenetic control and DNA damage response alterations were detected.

Delayed kinetics of DNA double-strand break processing in normal and pathological aging

Accumulation of DNA damage may play an essential role in both cellular senescence and organismal aging. The ability of cells to sense and repair DNA damage declines with age. However, the underlying molecular mechanism for this age-dependent decline is still elusive. To understand quantitative and qualitative changes in the DNA damage response during human aging, DNA damage-induced foci of phosphorylated histone H2AX (gamma-H2AX), which occurs specifically at sites of DNA double-strand breaks (DSBs) and eroded telomeres, were examined in human young and senescing fibroblasts, and in lymphocytes of peripheral blood. Here, we show that the incidence of endogenous gamma-H2AX foci increases with age. Fibroblasts taken from patients with Werner syndrome, a disorder associated with premature aging, genomic instability and increased incidence of cancer, exhibited considerably higher incidence of gamma-H2AX foci than those taken from normal donors of comparable age. Further increases in gamma-H2AX focal incidence occurred in culture as both normal and Werner syndrome fibroblasts progressed toward senescence. The rates of recruitment of DSB repair proteins to gamma-H2AX foci correlated inversely with age for both normal and Werner syndrome donors, perhaps due in part to the slower growth of gamma-H2AX foci in older donors. Because genomic stability may depend on the efficient processing of DSBs, and hence the rapid formation of gamma-H2AX foci and the rapid accumulation of DSB repair proteins on these foci at sites of nascent DSBs, our findings suggest that decreasing efficiency in these processes may contribute to genome instability associated with normal and pathological aging.

Rapid synthesis of VX-745: p38 MAP kinase inhibition in Werner syndrome cells

The p38 mitogen-activated protein kinase inhibitor VX-745 is prepared rapidly and efficiently in a four-step sequence using a combination of conductive heating and microwave-mediated steps. Its inhibitory activity was confirmed in hTERT immortalized HCA2 and WS dermal fibroblasts at 0.5-1.0 microM concentration by ELISA and immunoblot assay, and displays excellent kinase selectivity over the related stress-activated kinase JNK.

WRN at telomeres: implications for aging and cancer

Werner Syndrome (WS) is a premature aging syndrome characterized by early onset of age-related pathologies and cancer. Since WS is due to a single gene defect, it has attracted much interest from researchers seeking to understand pathways that contribute to cancer and aging at cellular and molecular levels. The protein mutated in WS, WRN, appears to play a major role in genome stability, particularly during DNA replication and telomere metabolism. Much of the pathophysiology associated with WS, including the rapid onset of cellular senescence, early cancer onset and premature aging, can be attributed to a defect in telomere maintenance. Recent genetic evidence from the mTerc(-/-) Wrn(-/-) mouse demonstrates that mice with critically shortened telomeres display aging phenotypes reminiscent of human WS, further reinforcing the notion that telomere dysfunction is required for the manifestation of aging pathophysiologies in the setting of WRN deficiency.

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

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