Promotes Telomere Maintenance Research Articles

Silencing PinX1 enhances radiosensitivity and antitumor-immunity of radiotherapy in non-small cell lung cancer

BackgroundWe aimed to investigate the effects of PinX1 on non-small cell lung cancer(NSCLC) radiosensitivity and radiotherapy-associated tumor immune microenvironment and its mechanisms.MethodsThe effect of PinX1 silencing on radiosensitivity in NSCLC was assessed by colony formation and CCK8 assay, immunofluorescence detection of γ- H2AX and micronucleus assay. Western blot was used to assess the effect of PinX1 silencing on DNA damage repair pathway and cGAS-STING pathway. The nude mouse and Lewis lung cancer mouse model were used to assess the combined efficacy of PinX1 silencing and radiotherapy in vivo. Changes in the tumor immune microenvironment were assessed by flow cytometry for different treatment modalities in the Lewis luuse model. The interaction protein RBM10 was screened by immunoprecipitation-mass spectrometry.ResultsSilencing PinX1 enhanced radiosensitivity and activation of the cGAS-STING pathway while attenuating the DNA damage repair pathway. Silencing PinX1 further increases radiotherapy-stimulated CD8+ T cell infiltration and activation, enhances tumor control and improves survival in vivo; Moreover, PinX1 downregulation improves the anti-tumor efficacy of radioimmunotherapy, increases radioimmune-stimulated CD8+ T cell infiltration, and reprograms M2-type macrophages into M1-type macrophages in tumor tissues. The interaction of PinX1 and RBM10 may promote telomere maintenance by assisting telomerase localization to telomeres, thereby inhibiting the immunostimulatory effects of IR.ConclusionsIn NSCLC, silencing PinX1 significantly contributed to the radiosensitivity and promoted the efficacy of radioimmunotherapy. Mechanistically, PinX1 may regulate the transport of telomerase to telomeres through interacting with RBM10, which promotes telomere maintenance and DNA stabilization. Our findings reveal that PinX1 is a potential target to enhance the efficacy of radioimmunotherapy in NSCLC patients.

Genomic and transcriptomic somatic alterations of hepatocellular carcinoma in non-cirrhotic livers

Genomic and transcriptomic somatic alterations of hepatocellular carcinoma in non-cirrhotic livers

Alternative splicing regulates telomerase repression.

Human telomerase reverse transcriptase (hTERT) activity is tightly controlled. This report shows that hTERT is primarily regulated by alternative splicing; skipping of exon 2 triggers decay of TERT mRNA in differentiated cells and its retention promotes telomere maintenance in pluripotent cells.

Duplex Telomere-Binding Proteins in Fungi With Canonical Telomere Repeats: New Lessons in the Rapid Evolution of Telomere Proteins.

The telomere protein assemblies in different fungal lineages manifest quite profound structural and functional divergence, implying a high degree of flexibility and adaptability. Previous comparative analyses of fungal telomeres have focused on the role of telomere sequence alterations in promoting the evolution of corresponding proteins, particularly in budding and fission yeast. However, emerging evidence suggests that even in fungi with the canonical 6-bp telomere repeat unit, there are significant remodeling of the telomere assembly. Indeed, a new protein family can be recruited to serve dedicated telomere functions, and then experience subsequent loss in sub-branches of the clade. An especially interesting example is the Tay1 family of proteins, which emerged in fungi prior to the divergence of basidiomycetes from ascomycetes. This relatively recent protein family appears to have acquired its telomere DNA-binding activity through the modification of another Myb-containing protein. Members of the Tay1 family evidently underwent rather dramatic functional diversification, serving, e.g., as transcription factors in fission yeast while acting to promote telomere maintenance in basidiomycetes and some hemi-ascomycetes. Remarkably, despite its distinct structural organization and evolutionary origin, a basidiomycete Tay1 appears to promote telomere replication using the same mechanism as mammalian TRF1, i.e., by recruiting and regulating Blm helicase activity. This apparent example of convergent evolution at the molecular level highlight the ability of telomere proteins to acquire new interaction targets. The remarkable evolutionary history of Tay1 illustrates the power of protein modularity and the facile acquisition of nucleic acid/protein-binding activity to promote telomere flexibility.

SLX4IP Promotes Telomere Maintenance in Androgen Receptor-Independent Castration-Resistant Prostate Cancer through ALT-like Telomeric PML Localization.

In advanced prostate cancer, resistance to androgen deprivation therapy is achieved through numerous mechanisms, including loss of the androgen receptor (AR) allowing for AR-independent growth. Therapeutic options are limited for AR-independent castration-resistant prostate cancer (CRPC), and defining mechanisms critical for survival is of utmost importance for targeting this lethal disease. Our studies focus on identifying telomere maintenance mechanism (TMM) hallmarks adopted by CRPC to promote survival. TMMs are responsible for telomere elongation to instill replicative immortality and prevent senescence, with the two TMM pathways available being telomerase and alternative lengthening of telomeres (ALT). Here, we show that AR-independent CRPC demonstrates an atypical ALT-like phenotype with variable telomerase expression and activity, whereas AR-dependent models lack discernible ALT hallmarks. In addition, AR-independent CRPC cells exhibited elevated levels of SLX4IP, a protein implicated in promoting ALT. SLX4IP overexpression in AR-dependent C4-2B cells promoted an ALT-like phenotype and telomere maintenance. SLX4IP knockdown in AR-independent DU145 and PC-3 cells led to ALT-like hallmark reduction, telomere shortening, and induction of senescence. In PC-3 xenografts, this effect translated to reduced tumor volume. Using an in vitro model of AR-independent progression, loss of AR in AR-dependent C4-2B cells promoted an atypical ALT-like phenotype in an SLX4IP-dependent manner. Insufficient SLX4IP expression diminished ALT-like hallmarks and resulted in accelerated telomere loss and senescence. IMPLICATIONS: This study demonstrates a unique reliance of AR-independent CRPC on SLX4IP-mediated ALT-like hallmarks and loss of these hallmarks induces telomere shortening and senescence, thereby impairing replicative immortality.

TRNA ADENOSINE DEAMINASE 3 is required for telomere maintenance in Arabidopsis thaliana.

KEY MESSAGE: tRNA Adenosine Deaminase 3 helps to sustain telomere tracts in a telomerase-independent fashion, likely through regulating cellular metabolism. Telomere length maintenance is influenced by a complex web of chromatin and metabolism-related factors. We previously reported that a lncRNA termed AtTER2 regulates telomerase activity in Arabidopsis thaliana in response to DNA damage. AtTER2 was initially shown to partially overlap with the 5' UTR of the tRNA ADENOSINE DEAMINASE 3 (TAD3) gene. However, updated genome annotation showed that AtTER2 was completely embedded in TAD3, raising the possibility that phenotypes ascribed to AtTER2 could be derived from TAD3. Here we show through strand-specific RNA-Seq, strand-specific qRT-PCR and bioinformatic analyses that AtTER2 does not encode a stable lncRNA. Further examination of the original tad3 (ter2-1/tad3-1) mutant revealed expression of an antisense transcript driven by a cryptic promoter in the T-DNA. Hence, a new hypomorphic allele of TAD3 (tad3-2) was examined. tad3-2 mutants showed hypersensitivity to DNA damage, but no deregulation of telomerase, suggesting that the telomerase phenotype of tad3-1 mutants reflects an off-target effect. Unexpectedly, however, tad3-2 plants displayed progressive loss of telomeric DNA over successive generations that was not accompanied by alteration of terminal architecture or end protection. The phenotype was exacerbated in plants lacking the telomerase processivity factor POT1a, indicating that TAD3 promotes telomere maintenance througha non-canonical, telomerase-independent pathway. The transcriptome of tad3-2 mutants revealed significant dysregulation of genes involved in auxin signaling and glucosinolate biosynthesis, pathways that intersect the stress response, cell cycle regulation and DNA metabolism. These findings indicate that the TAD3 locus indirectly contributes to telomere length homeostasis by altering the metabolic profile in Arabidopsis.

SMUG1 Promotes Telomere Maintenance through Telomerase RNA Processing

Telomerase biogenesis is a complex process where several steps remain poorly understood. Single-strand-selective uracil-DNA glycosylase (SMUG1) associates with the DKC1-containing H/ACA ribonucleoprotein complex, which is essential for telomerase biogenesis. Herein, we show that SMUG1 interacts with the telomeric RNA component (hTERC) and is required for co-transcriptional processing of the nascent transcript into mature hTERC. We demonstrate that SMUG1 regulates the presence of base modifications in hTERC, in a region between the CR4/CR5 domain and the H box. Increased levels of hTERC base modifications are accompanied by reduced DKC1 binding. Loss of SMUG1 leads to an imbalance between mature hTERC and its processing intermediates, leading to the accumulation of 3'-polyadenylated and 3'-extended intermediates that are degraded in an EXOSC10-independent RNA degradation pathway. Consequently, SMUG1-deprived cells exhibit telomerase deficiency, leading to impaired bone marrow proliferation in Smug1-knockout mice.

NOVA1 directs PTBP1 to hTERT pre-mRNA and promotes telomerase activity in cancer cells

Alternative splicing is dysregulated in cancer cells, driving the production of isoforms that allow tumor cells to survive and continuously proliferate. Part of the reactivation of telomerase involves the splicing of hTERT transcripts to produce full-length (FL) TERT. Very few splicing factors to date have been described to interact with hTERT and promote the production of FL TERT. We recently described one such splicing factor, NOVA1, that acts as an enhancer of FL hTERT splicing, increases telomerase activity, and promotes telomere maintenance in cancer cells. NOVA1 is expressed primarily in neurons and is involved in neurogenesis. In the present studies, we describe that polypyrimidine-tract binding proteins (PTBPs), which are also typically involved in neurogenesis, are also participating in the splicing of hTERT to FL in cancer. Knockdown experiments of PTBP1 in cancer cells indicate that PTBP1 reduces hTERT FL splicing and telomerase activity. Stable knockdown of PTBP1 results in progressively shortened telomere length in H1299 and H920 lung cancer cells. RNA pulldown experiments reveal that PTBP1 interacts with hTERT pre-mRNA in a NOVA1 dependent fashion. Knockdown of PTBP1 increases the expression of PTBP2 which also interacts with NOVA1, potentially preventing the association of NOVA1 with hTERT pre-mRNA. These new data highlight that splicing in cancer cells is regulated by competition for splice sites and that combinations of splicing factors interact at cis regulatory sites on pre-mRNA transcripts. By employing hTERT as a model gene, we show the coordination of the splicing factors NOVA1 and PTBP1 in cancer by regulating telomerase that is expressed in the vast majority of cancer cell types.

Alternative Lengthening of Telomeres Requires a Specialized Replisome

Abstract A specialized replisome mediates break-induced telomere synthesis to promote telomere maintenance.

Baseline biopsychosocial determinants of telomere length and 6-year attrition rate

BackgroundShort leukocyte telomere length (TL) and accelerated telomere attrition have been associated with various deleterious health outcomes, although their determinants have not been explored collectively in a large-scale study. Material and methodsLeukocyte TL was measured (baseline N=2936; 6-year follow-up N=1860) in participants (18–65 years) from the NESDA study. Baseline determinants of TL included sociodemographics, lifestyle, chronic diseases, psychosocial stressors, and metabolic and physiological stress markers. Multivariate linear regression models were used to examine the associations between these determinants and (1) baseline TL, and (2) 6-year TL change. Multinomial logistic regression analyses were used to examine the predictors of telomere attrition and lengthening, as compared to stable TL. ResultsShort baseline TL was associated with older age, male sex, non-European ethnicity, cigarette smoking, recent life events, and higher triglycerides, glucose and pre-ejection period (R2=11.3%). The 6-year telomere attrition was inversely associated with baseline TL (R2=51.6%); also older age, long sleep, not having a partner, high childhood trauma index, and gastrointestinal disease were associated with 6-year TL attrition (additional R2=3.7%). Telomere attrition seemed to have slightly more predictors than lengthening. ConclusionsSociodemographic, lifestyle, psychosocial stress and metabolic and physiological stress factors are cross-sectionally linked with TL. Telomere attrition over six years was strongly associated with baseline TL, suggesting an internal homeostatic influence. Modulation of the identified determinants may become target of future studies to promote telomere maintenance and healthy aging.

PML induces compaction, TRF2 depletion and DNA damage signaling at telomeres and promotes their alternative lengthening.

The alternative lengthening of telomeres (ALT) mechanism allows cancer cells to escape senescence and apoptosis in the absence of active telomerase. A characteristic feature of this pathway is the assembly of ALT-associated promyelocytic leukemia (PML) nuclear bodies (APBs) at telomeres. Here, we dissected the role of APBs in a human ALT cell line by performing an RNA interference screen using an automated 3D fluorescence microscopy platform and advanced 3D image analysis. We identified 29 proteins that affected APB formation, which included proteins involved in telomere and chromatin organization, protein sumoylation and DNA repair. By integrating and extending these findings, we found that APB formation induced clustering of telomere repeats, telomere compaction and concomitant depletion of the shelterin protein TRF2 (also known as TERF2). These APB-dependent changes correlated with the induction of a DNA damage response at telomeres in APBs as evident by a strong enrichment of the phosphorylated form of the ataxia telangiectasia mutated (ATM) kinase. Accordingly, we propose that APBs promote telomere maintenance by inducing a DNA damage response in ALT-positive tumor cells through changing the telomeric chromatin state to trigger ATM phosphorylation.

Brh2 and Rad51 promote telomere maintenance in Ustilago maydis, a new model system of DNA repair proteins at telomeres

Recent studies implicate a number of DNA repair proteins in mammalian telomere maintenance. However, because several key repair proteins in mammals are missing from the well-studied budding and fission yeast, their roles at telomeres cannot be modeled in standard fungi. In this report, we explored the dimorphic fungus Ustilago maydis as an alternative model for telomere research. This fungus, which belongs to the phylum Basidiomycota, has a telomere repeat unit that is identical to the mammalian repeat, as well as a constellation of DNA repair proteins that more closely mimic the mammalian collection. We showed that the two core components of homology-directed repair (HDR) in U. maydis, namely Brh2 and Rad51, both promote telomere maintenance in telomerase positive cells, just like in mammals. In addition, we found that Brh2 is localized to telomeres in vivo, suggesting that it acts directly at chromosome ends. We surveyed a series of mutants with DNA repair defects, and found many of them to have short telomeres. Our results indicate that factors involved in DNA repair are probably also needed for optimal telomere maintenance in U. maydis, and that this fungus is a useful alternative model system for telomere research.

T.P.40 Implication of SIRT1 and its downstream pathways in dystrophic process

Abstract Muscle necrosis and exhaustible muscle regeneration are key elements in Duchenne muscular dystrophy (DMD) pathogenesis. Our group has previously highlighted the detrimental effects of the cross-talk between oxidative stress, lipid peroxidation and NF-kappaB/TNF-α activation in the necrotic process in DMD and its murine model. The positive effects of their pharmacological inhibition on muscle regeneration were also demonstrated. Moreover the contribution of telomere shortening to the depletion of regenerative spurt is an emerging field of investigation. SIRT1, an enzyme belonging to the deacetylase family of sirtuins, inhibits NF-kappaB/TNF-α inflammatory pathway. SIRT 1 role in muscle regeneration is still controversial: it promotes telomere maintenance and muscle cell proliferation mediating IGF1 effect. On the other hand, it inhibits mTOR pathway, known to mediate skeletal muscle regeneration and remodeling. This evidence has been provided only by in vitro and in vivo studies and administration of resveratrol, a SIRT 1 activator, ameliorated muscle pathology in mdx mice. This study aimed to investigate the expression of SIRT1, NF-kappaB, IGF1-Akt/PKB and mTOR pathways in dystrophic patient muscles. Muscles of DMD, facioscapulohumeral (FSH) and limb-girdle muscular dystrophies (LGMD) and controls were examined by immunohistochemistry Western blot and real-time PCR. The above mentioned factors have been found overexpressed in DMD and LGMD, at variance with FSH. Moreover SIRT 1 was specifically expressed at immunohistochemistry by miogenin-positive satellite cells. The understanding of the mechanisms controlling SIRT1 and downstream pathways in muscle dystrophies could provide clues in identifying novel therapeutic targets.

Telomerase-Independent Paths to Immortality in Predictable Cancer Subtypes

The vast majority of cancers commandeer the activity of telomerase - the remarkable enzyme responsible for prolonging cellular lifespan by maintaining the length of telomeres at the ends of chromosomes. Telomerase is only normally active in embryonic and highly proliferative somatic cells. Thus, targeting telomerase is an attractive anti-cancer therapeutic rationale currently under investigation in various phases of clinical development. However, previous reports suggest that an average of 10-15% of all cancers lose the functional activity of telomerase and most of these turn to an Alternative Lengthening of Telomeres pathway (ALT). ALT-positive tumours will therefore not respond to anti-telomerase therapies and there is a real possibility that such drugs would be toxic to normal telomerase-utilising cells and ultimately select for resistant cells that activate an ALT mechanism. ALT exploits certain DNA damage response (DDR) components to counteract telomere shortening and rapid trimming. ALT has been reported in many cancer subtypes including sarcoma, gastric carcinoma, central nervous system malignancies, subtypes of kidney (Wilm's Tumour) and bladder carcinoma, mesothelioma, malignant melanoma and germ cell testicular cancers to name but a few. A recent heroic study that analysed ALT in over six thousand tumour samples supports this historical spread, although only reporting an approximate 4% prevalence. This review highlights the various methods of ALT detection, unravels several molecular ALT models thought to promote telomere maintenance and elongation, spotlights the DDR components known to facilitate these and explores why certain tissues are more likely to subvert DDR away from its usually protective functions, resulting in a predictive pattern of prevalence in specific cancer subsets.

Tel1ATM and Rad3ATR kinases promote Ccq1-Est1 interaction to maintain telomeres in fission yeast

SUMMARYThe shelterin complex plays both positive and negative roles in telomerase regulation. While shelterin prevents the checkpoint kinases ATM and ATR from fully activating DNA damage responses at telomeres, those kinases are also required for telomere maintenance. In fission yeast, cells lacking both Tel1 (ATM ortholog) and Rad3 (ATR ortholog) fail to recruit telomerase to telomeres, and survive by circularizing chromosomes. However, the critical telomere substrate(s) of Tel1ATM/Rad3ATR remained unknown. Here, we show that Tel1ATM/Rad3ATR-dependent phosphorylation of the shelterin subunit Ccq1 on Thr93 is essential for telomerase association with telomeres. In addition, we show that the telomerase subunit Est1 interacts directly with the phosphorylated Thr93 of Ccq1 to ensure telomere maintenance. The shelterin subunits Taz1, Rap1 and Poz1 (previously established inhibitors of telomerase) were also found to negatively regulate Ccq1 phosphorylation. These findings establish Tel1ATM/Rad3ATR-dependent Ccq1 Thr93 phosphorylation as a critical regulator of telomere maintenance in fission yeast.

Differential C3NET reveals disease networks of direct physical interactions

BackgroundGenes might have different gene interactions in different cell conditions, which might be mapped into different networks. Differential analysis of gene networks allows spotting condition-specific interactions that, for instance, form disease networks if the conditions are a disease, such as cancer, and normal. This could potentially allow developing better and subtly targeted drugs to cure cancer. Differential network analysis with direct physical gene interactions needs to be explored in this endeavour.ResultsC3NET is a recently introduced information theory based gene network inference algorithm that infers direct physical gene interactions from expression data, which was shown to give consistently higher inference performances over various networks than its competitors. In this paper, we present, DC3net, an approach to employ C3NET in inferring disease networks. We apply DC3net on a synthetic and real prostate cancer datasets, which show promising results. With loose cutoffs, we predicted 18583 interactions from tumor and normal samples in total. Although there are no reference interactions databases for the specific conditions of our samples in the literature, we found verifications for 54 of our predicted direct physical interactions from only four of the biological interaction databases. As an example, we predicted that RAD50 with TRF2 have prostate cancer specific interaction that turned out to be having validation from the literature. It is known that RAD50 complex associates with TRF2 in the S phase of cell cycle, which suggests that this predicted interaction may promote telomere maintenance in tumor cells in order to allow tumor cells to divide indefinitely. Our enrichment analysis suggests that the identified tumor specific gene interactions may be potentially important in driving the growth in prostate cancer. Additionally, we found that the highest connected subnetwork of our predicted tumor specific network is enriched for all proliferation genes, which further suggests that the genes in this network may serve in the process of oncogenesis.ConclusionsOur approach reveals disease specific interactions. It may help to make experimental follow-up studies more cost and time efficient by prioritizing disease relevant parts of the global gene network.

The 5′ Guanosine Tracts of Human Telomerase RNA Are Recognized by the G-Quadruplex Binding Domain of the RNA Helicase DHX36 and Function To Increase RNA Accumulation

Telomerase promotes telomere maintenance by copying a template within its integral RNA subunit to elongate chromosome ends with new telomeric repeats. Motifs have been defined within the telomerase RNA that contribute to mature RNA accumulation, holoenzyme catalytic activity, or enzyme recruitment to telomeres. Here, we describe a motif of human telomerase RNA (hTR), not previously characterized in a cellular context, comprised of several guanosine tracts near the RNA 5' end. These guanosine tracts together are recognized by the DEXH box RNA helicase DHX36. The helicase domain of DHX36 does not mediate hTR binding; instead, hTR interacts with the N-terminal accessory domain of DHX36 known to bind specifically to the parallel-strand G-quadruplex substrates resolved by the helicase domain. The steady-state level of DHX36-hTR interaction is low, but hTR guanosine tract substitutions substantially reduce mature hTR accumulation and thereby reduce telomere maintenance. These findings suggest that G-quadruplex formation in the hTR precursor improves the escape of immature RNP from degradation, but subsequently the G-quadruplex may be resolved in favor of a longer terminal stem. We conclude that G-quadruplex formation within hTR can stimulate telomerase-mediated telomere maintenance.

Can Meditation Slow Rate of Cellular Aging? Cognitive Stress, Mindfulness, and Telomeres

Understanding the malleable determinants of cellular aging is critical to understanding human longevity. Telomeres may provide a pathway for exploring this question. Telomeres are the protective caps at the ends of chromosomes. The length of telomeres offers insight into mitotic cell and possibly organismal longevity. Telomere length has now been linked to chronic stress exposure and depression. This raises the question of mechanism: How might cellular aging be modulated by psychological functioning? We consider two psychological processes or states that are in opposition to one another-threat cognition and mindfulness-and their effects on cellular aging. Psychological stress cognitions, particularly appraisals of threat and ruminative thoughts, can lead to prolonged states of reactivity. In contrast, mindfulness meditation techniques appear to shift cognitive appraisals from threat to challenge, decrease ruminative thought, and reduce stress arousal. Mindfulness may also directly increase positive arousal states. We review data linking telomere length to cognitive stress and stress arousal and present new data linking cognitive appraisal to telomere length. Given the pattern of associations revealed so far, we propose that some forms of meditation may have salutary effects on telomere length by reducing cognitive stress and stress arousal and increasing positive states of mind and hormonal factors that may promote telomere maintenance. Aspects of this model are currently being tested in ongoing trials of mindfulness meditation.

Plasmodium telomeres and telomerase: the usual actors in an unusual scenario

In the last decade, telomeres of malaria parasites have been in the spotlight. A number of different host-parasite interactions involve genes that are regulated through processes unique to telomeres. In the highly proliferative human pathogen, Plasmodium falciparum, telomerase appears to not only promote telomere maintenance but also to repair broken chromosome ends. The characterization of the plasmodial gene that encodes the telomerase reverse transcriptase protein has revealed several unusual features. For example, the predicted protein is approximately three times larger than the yeast homologue, due to many insertions of stretches of basic amino acids. Other telomere-associated proteins have also been identified. One of them, the P. falciparum gene homologous to yeast Sir2, seems to be required for the establishment of a heterochromatin-like structure at the telomeres, which leads to the silencing of subtelomeric genes. It has been shown that PfSir2 associates with promoter regions of silenced genes involved in antigenic variation.

VRrm Rrm: Rrm protein jump-starts idle DNA-copying equipment (Genome instability; DNA replication)

When a job becomes boring, a worker sometimes needs a kick in the pants. Inside yeast cells, proteins that copy DNA slack off occasionally, especially when the sequences get repetitive. New research identifies a protein that rouses these enzymes when they stall on monotonous stretches, such as those at the ends of chromosomes. By nudging the copying machinery through tedious DNA sections, the protein ensures chromosomal integrity. Because unstable genomes promote cancer and possibly aging, this protein might occupy a novel cranny in the cupboard of long life. Repetitive DNA sequences--in which the same short bit is slapped down over and over again--called telomeres protect genes from frittering away when the ends of chromosomes shrink as cells copy their DNA. Chromosomes that lose too much telomeric DNA can stick together and tear during cell division. The resulting genetic chaos leads to genomic instability, which triggers senescence, uncontrolled growth, or cell death (see "Dangerous Liaisons" ). Furthermore, these types of chromosomal rearrangements typify the premature aging condition Werner syndrome (see "Of Hyperaging and Methuselah Genes" and Fry Review ). Other recurring sequences seem to spur yeast aging. Previously, scientists found a DNA-unraveling protein, Rrm3p, that accelerates replication of ribosomal DNA (rDNA) and limits the production of rDNA circles, molecular loops known to reduce the number of times a yeast cell can reproduce (see Kaeberlein Perspective ). Because telomeres and rDNA are highly repetitive, Ivessa and colleagues investigated whether Rrm3p promotes telomere maintenance, just as it helps keep rDNA intact. To probe Rrm3p's role at chromosome ends, the researchers tested whether the protein influences the speed of DNA replication. They isolated forked DNA molecules that arise as cells duplicate chromosomes and separated them according to size. The bigger the forked DNA, the farther the copying enzymes have advanced along the chromosome. In cells without Rrm3p, the replication machinery moved through the telomeric DNA at about 1/10 the speed as when the protein was present, indicating that Rrm3p hurries the copying equipment along. Rrm3p probably protects the chromosome from the many consequences of stalled replication machinery, such as double-stranded breaks, says Virginia Zakian, a molecular biologist at Princeton University who led the team. Despite Rrm3p's significance for DNA replication, cells that lack it don't suffer chromosome loss or scrambling, hinting at the existence of a robust backup system. If Rrm3p's pushiness does guard telomeres, it could preserve the yeast genome in ways other than by preventing DNA breaks. Molecular biologist Bradley Johnson of the University of Pennsylvania Medical School in Philadelphia speculates that Rrm3p's ability to help keep chromosomes clear of stalled replication machinery could discourage DNA from hooking up in inappropriate ways. The protein's capacity to curb rDNA circle formation suggests that it combats yeast decrepitude; its potential role in maintaining genome stability raises the possibility that a similar protein might operate in human aging as well. If so, then picking up the pace of DNA copying might hustle creatures to longer life. --Mary Beckman A. S. Ivessa, J.-Q. Zhou, V. P. Schulz, E. K. Monson, V. A. Zakian, Saccharomyces Rrm3p, a 5′ to 3′ DNA helicase that promotes replication fork progression through telomeric and subtelomeric DNA. Genes Dev. 16 , 1383-1396 (2002). [Abstract] [Full Text]