Telomere protection by TPP1/POT1 requires tethering to TIN2

Telomere Length and Oxidative Stress and Its Relation with Metabolic Syndrome Components in the Aging

A great amount of scientific evidence supports that Oxidative Stress (OxS) can contribute to telomeric attrition and also plays an important role in the development of certain age-related diseases, among them the metabolic syndrome (MetS), which is characterised by clinical and biochemical alterations such as obesity, dyslipidaemia, arterial hypertension, hyperglycaemia, and insulin resistance, all of which are considered as risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases, which are associated in turn with an increase of OxS. In this sense, we review scientific evidence that supports the association between OxS with telomere length (TL) dynamics and the relationship with MetS components in aging. It was analysed whether each MetS component affects the telomere length separately or if they all affect it together. Likewise, this review provides a summary of the structure and function of telomeres and telomerase, the mechanisms of telomeric DNA repair, how telomere length may influence the fate of cells or be linked to inflammation and the development of age-related diseases, and finally, how the lifestyles can affect telomere length.

Telomeres and Cancer: Resolving the Paradox

Decades of study on cell cycle regulation have provided great insight into human cellular life span barriers, as well as their dysregulation during tumorigenesis. Telomeres, the extremities of linear chromosomes, perform an essential role in implementing these proliferative boundaries and preventing the propagation of potentially cancerous cells. The tumor-suppressive function of telomeres relies on their ability to initiate DNA damage signaling pathways and downstream cellular events, ranging from cell cycle perturbation to inflammation and cell death. While the tumor-suppressor role of telomeres is undoubtable, recent advances have pointed to telomeres as a major source of many of the genomic aberrations found in both early- and late-stage cancers, including the most recently discovered mutational phenomenon of chromothripsis. Telomere shortening appears as a double-edged sword that can function in opposing directions in carcinogenesis. This review focuses on the current knowledge of the dual role of telomeres in cancer and suggests a new perspective to reconcile the paradox of telomeres and their implications in cancer etiology.

ER-directed TREX1 limits cGAS activation at micronuclei

Micronuclei are aberrant nuclear compartments that can form as a result of chromosome mis-segregation. Frequent loss of micronuclear envelope integrity exposes DNA to the cytoplasm, leading to chromosome fragmentation and immune activation. Here, we use micronuclei purification to show that the endoplasmic reticulum (ER)-associated nuclease TREX1 inhibits cGAS activation at micronuclei by degrading micronuclear DNA upon micronuclear envelope rupture. We demonstrate that the ER accesses ruptured micronuclei and plays a critical role in enabling TREX1 nucleolytic attack. TREX1 mutations, previously implicated in immune disease, untether TREX1 from the ER, disrupt TREX1 localization to micronuclei, diminish micronuclear DNA damage, and enhance cGAS activation. These results establish ER-directed resection of micronuclear DNA by TREX1 as a critical regulator of cytosolic DNA sensing in chromosomally unstable cells and provide a mechanistic basis for the importance of TREX1 ER tethering in preventing autoimmunity.

Molecular mechanisms of telomere biology disorders

Genetic mutations that affect telomerase function or telomere maintenance result in a variety of diseases collectively called telomeropathies. This wide spectrum of disorders, which include dyskeratosis congenita, pulmonary fibrosis, and aplastic anemia, is characterized by severely short telomeres, often resulting in hematopoietic stem cell failure in the most severe cases. Recent work has focused on understanding the molecular basis of these diseases. Mutations in the catalytic TERT and TR subunits of telomerase compromise activity, while others, such as those found in the telomeric protein TPP1, reduce the recruitment of telomerase to the telomere. Mutant telomerase-associated proteins TCAB1 and dyskerin and the telomerase RNA maturation component poly(A)-specific ribonuclease affect the maturation and stability of telomerase. In contrast, disease-associated mutations in either CTC1 or RTEL1 are more broadly associated with telomere replication defects. Yet even with the recent surge in studies decoding the mechanisms underlying these diseases, a significant proportion of dyskeratosis congenita mutations remain uncharacterized or poorly understood. Here we review the current understanding of the molecular basis of telomeropathies and highlight experimental data that illustrate how genetic mutations drive telomere shortening and dysfunction in these patients. This review connects insights from both clinical and molecular studies to create a comprehensive view of the underlying mechanisms that drive these diseases. Through this, we emphasize recent advances in therapeutics and pinpoint disease-associated variants that remain poorly defined in their mechanism of action. Finally, we suggest future avenues of research that will deepen our understanding of telomere biology and telomere-related disease.

TRF2-independent chromosome end protection during pluripotency

Mammalian telomeres protect chromosome ends from aberrant DNA repair1. TRF2, a component of the telomere-specific shelterin protein complex, facilitates end protection through sequestration of the terminal telomere repeat sequence within a lariat T-loop structure2,3. Deleting TRF2 (also known as TERF2) in somatic cells abolishes T-loop formation, which coincides with telomere deprotection, chromosome end-to-end fusions and inviability3-9. Here we establish that, by contrast, TRF2 is largely dispensable for telomere protection in mouse pluripotent embryonic stem (ES) and epiblast stem cells. ES cell telomeres devoid of TRF2 instead activate an attenuated telomeric DNA damage response that lacks accompanying telomere fusions, and propagate for multiple generations. The induction of telomere dysfunction in ES cells, consistent with somatic deletion of Trf2 (also known as Terf2), occurs only following the removal of the entire shelterin complex. Consistent with TRF2 being largely dispensable for telomere protection specifically during early embryonic development, cells exiting pluripotency rapidly switch to TRF2-dependent end protection. In addition, Trf2-null embryos arrest before implantation, with evidence of strong DNA damage response signalling and apoptosis specifically in the non-pluripotent compartment. Finally, we show that ES cells form T-loops independently of TRF2, which reveals why TRF2 is dispensable for end protection during pluripotency. Collectively, these data establish that telomere protection is solved by distinct mechanisms in pluripotent and somatic tissues.

The end protection problem-an unexpected twist in the tail

In this perspective, we introduce shelterin and the mechanisms of ATM activation and NHEJ at telomeres, before discussing the following questions: How are t-loops proposed to protect chromosome ends and what is the evidence for this model? Can other models explain how TRF2 mediates end protection? Could t-loops be pathological structures? How is end protection achieved in pluripotent cells? What do the insights into telomere end protection in pluripotent cells mean for the t-loop model of end protection? Why might different cell states have evolved different mechanisms of end protection? Finally, we offer support for an updated t-loop model of end protection, suggesting that the data is supportive of a critical role for t-loops in protecting chromosome ends from NHEJ and ATM activation, but that other mechanisms are involved. Finally, we propose that t-loops are likely dynamic, rather than static, structures.

Human shelterin protein POT1 prevents severe telomere instability induced by homology-directed DNA repair

The evolutionarily conserved POT1 protein binds single-stranded G-rich telomeric DNA and has been implicated in contributing to telomeric DNA maintenance and the suppression of DNA damage checkpoint signaling. Here, we explore human POT1 function through genetics and proteomics, discovering that a complete absence of POT1 leads to severe telomere maintenance defects that had not been anticipated from previous depletion studies in human cells. Conditional deletion of POT1 in HEK293E cells gives rise to rapid telomere elongation and length heterogeneity, branched telomeric DNA structures, telomeric R-loops, and telomere fragility. We determine the telomeric proteome upon POT1-loss, implementing an improved telomeric chromatin isolation protocol. We identify a large set of proteins involved in nucleic acid metabolism that engage with telomeres upon POT1-loss. Inactivation of the homology-directed repair machinery suppresses POT1-loss-mediated telomeric DNA defects. Our results unravel as major function of human POT1 the suppression of telomere instability induced by homology-directed repair.

Characterization of t-loop formation by TRF2

T-loops are thought to hide telomeres from DNA damage signaling and DSB repair pathways. T-loop formation requires the shelterin component TRF2, which represses ATM signaling and NHEJ. Here we establish that TRF2 alone, in the absence of other shelterin proteins can form t-loops. Mouse and human cells contain two isoforms of TRF2, one of which is uncharacterized. We show that both isoforms protect telomeres and form t-loops. The isoforms are not cell cycle regulated and t-loops are present in G1, S, and G2.  Using the DNA wrapping deficient TRF2 Topless mutant, we confirm its inability to form t-loops and repress ATM. However, since the mutant is also defective in repression of NHEJ and telomeric localization, the role of topological changes in telomere protection remains unclear.  Finally, we show that Rad51 does not affect t-loop frequencies or telomere protection. Therefore, alternative models for how TRF2 forms t-loops should be explored.

Replication stress conferred by POT1 dysfunction promotes telomere relocalization to the nuclear pore

In this study, Pinzaru et al. set out to uncover the pathways that enable the proliferation of cells expressing cancer-associated POT1 mutations. Using complementary genetic and proteomic approaches, the authors identify a conserved function for the NPC in resolving replication defects at telomere loci.

Mutations in the telomere-binding protein POT1 are associated with solid tumors and leukemias. POT1 alterations cause rapid telomere elongation, ATR kinase activation, telomere fragility, and accelerated tumor development. Here, we define the impact of mutant POT1 alleles through complementary genetic and proteomic approaches based on CRISPR interference and biotin-based proximity labeling, respectively. These screens reveal that replication stress is a major vulnerability in cells expressing mutant POT1, which manifests as increased telomere mitotic DNA synthesis at telomeres. Our study also unveils a role for the nuclear pore complex in resolving replication defects at telomeres. Depletion of nuclear pore complex subunits in the context of POT1 dysfunction increases DNA damage signaling, telomere fragility and sister chromatid exchanges. Furthermore, we observed telomere repositioning to the nuclear periphery driven by nuclear F-actin polymerization in cells with POT1 mutations. In conclusion, our study establishes that relocalization of dysfunctional telomeres to the nuclear periphery is critical to preserve telomere repeat integrity.

Cancer-associated mutations in the condensin II subunit CAPH2 cause genomic instability through telomere dysfunction and anaphase chromosome bridges

Genome instability in cancer drives tumor heterogeneity, undermines the success of therapies, and leads to metastasis and recurrence. Condensins are conserved chromatin‐binding proteins that promote genomic stability, mainly by ensuring proper condensation of chromatin and mitotic chromosome segregation. Condensin mutations are found in human tumors, but it is not known how or even if such mutations promote cancer progression. In this study, we focus on condensin II subunit CAPH2 and specific CAPH2 mutations reported to be enriched in human cancer patients, and we test how CAPH2 cancer‐specific mutations may lead to condensin II complex dysfunction and contribute to genome instability. We find that R551P, R551S, and S556F mutations in CAPH2 cause genomic instability by causing DNA damage, anaphase defects, micronuclei, and chromosomal instability. DNA damage and anaphase defects are caused primarily by ataxia telangiectasia and Rad3‐related‐dependent telomere dysfunction, as anaphase bridges are enriched for telomeric repeat sequences. We also show that these mutations decrease the binding of CAPH2 to the ATPase subunit SMC4 as well as the rest of the condensin II complex, and decrease the amount of CAPH2 protein bound to chromatin. Thus, in vivo the R551P, R551S, and S556F cancer‐specific CAPH2 mutant proteins are likely to impair condensin II complex formation, impede condensin II activity during mitosis and interphase, and promote genetic heterogeneity in cell populations that can lead to clonal outgrowth of cancer cells with highly diverse genotypes.

Role of POT1 in Human Cancer

Simple Summary

The segmentation of eukaryotic genomes into discrete linear chromosomes requires processes to solve several major biological problems, including prevention of the chromosome ends being recognized as DNA breaks and compensation for the shortening that occurs when linear DNA is replicated. A specialized set of six proteins, collectively referred to as shelterin, is involved in both of these processes, and mutations in several of these are now known to be involved in cancer. Here, we focus on Protection of Telomeres 1 (POT1), the shelterin protein that appears to be most commonly involved in cancer, and consider the clinical significance of findings about its biological functions and the prevalence of inherited and acquired mutations in the POT1 gene.

Abstract

Telomere abnormalities facilitate cancer development by contributing to genomic instability and cellular immortalization. The Protection of Telomeres 1 (POT1) protein is an essential subunit of the shelterin telomere binding complex. It directly binds to single-stranded telomeric DNA, protecting chromosomal ends from an inappropriate DNA damage response, and plays a role in telomere length regulation. Alterations of POT1 have been detected in a range of cancers. Here, we review the biological functions of POT1, the prevalence of POT1 germline and somatic mutations across cancer predisposition syndromes and tumor types, and the dysregulation of POT1 expression in cancers. We propose a framework for understanding how POT1 abnormalities may contribute to oncogenesis in different cell types. Finally, we summarize the clinical implications of POT1 alterations in the germline and in cancer, and possible approaches for the development of targeted cancer therapies.

Telomere length set point regulation in human pluripotent stem cells critically depends on the shelterin protein TPP1

Telomere maintenance is essential for the long-term proliferation of human pluripotent stem cells, while their telomere length set point determines the proliferative capacity of their differentiated progeny. The shelterin protein TPP1 is required for telomere stability and elongation, but its role in establishing a telomere length set point remains elusive. Here, we characterize the contribution of the shorter isoform of TPP1 (TPP1S) and the amino acid L104 outside the TEL patch, TPP1’s telomerase interaction domain, to telomere length control. We demonstrate that cells deficient for TPP1S (TPP1S knockout [KO]), as well as the complete TPP1 KO cell lines, undergo telomere shortening. However, TPP1S KO cells are able to stabilize short telomeres, while TPP1 KO cells die. We compare these phenotypes with those of TPP1^L104A/L104A mutant cells, which have short and stable telomeres similar to the TPP1S KO. In contrast to TPP1S KO cells, TPP1^L104A/L104A cells respond to increased telomerase levels and maintain protected telomeres. However, TPP1^L104A/L104A shows altered sensitivity to expression changes of shelterin proteins suggesting the mutation causes a defect in telomere length feedback regulation. Together this highlights TPP1^L104A/L104A as the first shelterin mutant engineered at the endogenous locus of human stem cells with an altered telomere length set point.

Role of Telomeres and Telomeric Proteins in Human Malignancies and Their Therapeutic Potential

Telomeres are the ends of linear chromosomes comprised of repetitive nucleotide sequences in humans. Telomeres preserve chromosomal stability and genomic integrity. Telomere length shortens with every cell division in somatic cells, eventually resulting in replicative senescence once telomere length becomes critically short. Telomere shortening can be overcome by telomerase enzyme activity that is undetectable in somatic cells, while being active in germline cells, stem cells, and immune cells. Telomeres are bound by a shelterin complex that regulates telomere lengthening as well as protects them from being identified as DNA damage sites. Telomeres are transcribed by RNA polymerase II, and generate a long noncoding RNA called telomeric repeat-containing RNA (TERRA), which plays a key role in regulating subtelomeric gene expression. Replicative immortality and genome instability are hallmarks of cancer and to attain them cancer cells exploit telomere maintenance and telomere protection mechanisms. Thus, understanding the role of telomeres and their associated proteins in cancer initiation, progression and treatment is very important. The present review highlights the critical role of various telomeric components with recently established functions in cancer. Further, current strategies to target various telomeric components including human telomerase reverse transcriptase (hTERT) as a therapeutic approach in human malignancies are discussed.

Regulation of human telomerase in homeostasis and disease

Telomerase is a ribonucleoprotein complex, the catalytic core of which includes the telomerase reverse transcriptase (TERT) and the non-coding human telomerase RNA (hTR), which serves as a template for the addition of telomeric repeats to chromosome ends. Telomerase expression is restricted in humans to certain cell types, and telomerase levels are tightly controlled in normal conditions. Increased levels of telomerase are found in the vast majority of human cancers, and we have recently begun to understand the mechanisms by which cancer cells increase telomerase activity. Conversely, germline mutations in telomerase-relevant genes that decrease telomerase function cause a range of genetic disorders, including dyskeratosis congenita, idiopathic pulmonary fibrosis and bone marrow failure. In this Review, we discuss the transcriptional regulation of human TERT, hTR processing, assembly of the telomerase complex, the cellular localization of telomerase and its recruitment to telomeres, and the regulation of telomerase activity. We also discuss the disease relevance of each of these steps of telomerase biogenesis.

The use of non-traditional models in the study of cancer resistance-the case of the naked mole rat

Non-traditional model organisms are typically defined as any model the deviates from the typical laboratory animals, such as mouse, rat, and worm. These models are becoming increasingly important in human disease research, such as cancer, as they often display unusual biological features. Naked mole rats (NMRs) are currently one of the most popular non-traditional model, particularly in the longevity and cancer research fields. NMRs display an exceptionally long lifespan (~30 years), yet have been observed to display a low incidence of cancer, making them excellent candidates for understanding endogenous cancer resistance mechanisms. Over the past decade, many potential resistance mechanisms have been characterized. These include unique biological mechanisms involved in genome stability, protein stability, oxidative metabolism, and other cellular mechanisms such as cell cycle regulation and senescence. This review aims to summarize the many identified cancer resistance mechanisms to understand some of the main hypotheses that have thus far been generated. Many of these proposed mechanisms remain to be fully characterized or confirmed in vivo, giving the field a direction to grow and further understand the complex biology displayed by the NMR.

Catastrophic Endgames: Emerging Mechanisms of Telomere-Driven Genomic Instability

When cells progress to malignancy, they must overcome a final telomere-mediated proliferative lifespan barrier called replicative crisis. Crisis is characterized by extensive telomere fusion that drives widespread genomic instability, mitotic arrest, hyperactivation of autophagy, and cell death. Recently, it has become apparent that that the resolution of dicentric chromosomes, which arise from telomere fusions during crisis, can initiate a sequence of events that leads to chromothripsis, a form of extreme genomic catastrophe. Chromothripsis is characterized by localized genomic regions containing tens to thousands of rearrangements and it is becoming increasingly apparent that chromothripsis occurs widely across tumor types and has a clinical impact. Here we discuss how telomere dysfunction can initiate genomic complexity and the emerging mechanisms of chromothripsis.

Chromosome extremities under the microscopy lens: molecular cytogenetics in telomere research

At the crossroads of DNA damage repair and genomic instability, telomere research significantly expands our knowledge on fundamental mechanisms involved in cancer initiation and progression, pledging novel tools for targeted and universal onco-therapies. Molecular cytogenetics through the application of a battery of fluorescent hybridization technologies plays an important role toward understanding telomere homeostasis. Herein, we review distinct molecular cytogenetic phenotypes associated with telomere repair, functionality, and elongation. We discuss the underlying mechanisms responsible for their formation or repair, focusing on Break-induced-Replication (BIR)-mediated conservative telomeric neo-synthesis, recently shown to drive the enigmatic Alternative Lengthening of Telomeres in neoplasia.

Structural Features of Nucleoprotein CST/Shelterin Complex Involved in the Telomere Maintenance and Its Association with Disease Mutations

Telomere comprises the ends of eukaryotic linear chromosomes and is composed of G-rich (TTAGGG) tandem repeats which play an important role in maintaining genome stability, premature aging and onsets of many diseases. Majority of the telomere are replicated by conventional DNA replication, and only the last bit of the lagging strand is synthesized by telomerase (a reverse transcriptase). In addition to replication, telomere maintenance is principally carried out by two key complexes known as shelterin (TRF1, TRF2, TIN2, RAP1, POT1, and TPP1) and CST (CDC13/CTC1, STN1, and TEN1). Shelterin protects the telomere from DNA damage response (DDR) and regulates telomere length by telomerase; while, CST govern the extension of telomere by telomerase and C strand fill-in synthesis. We have investigated both structural and biochemical features of shelterin and CST complexes to get a clear understanding of their importance in the telomere maintenance. Further, we have analyzed ~115 clinically important mutations in both of the complexes. Association of such mutations with specific cellular fault unveils the importance of shelterin and CST complexes in the maintenance of genome stability. A possibility of targeting shelterin and CST by small molecule inhibitors is further investigated towards the therapeutic management of associated diseases. Overall, this review provides a possible direction to understand the mechanisms of telomere borne diseases, and their therapeutic intervention.

A Truncating Germline Mutation of TINF2 in Individuals with Thyroid Cancer or Melanoma Results in Longer Telomeres

Background: Our genome sequencing analysis revealed a frameshift mutation in the shelterin gene TINF2 in a large family with individuals affected with papillary thyroid carcinoma (PTC) and melanoma. Here, we further characterized the mutation and screened for coding variants in the 6 shelterin genes in 24 families. Methods: Sanger sequencing was performed to screen for the TINF2 mutation in the key family. Quantitative reverse transcription-polymerase chain reaction (PCR) was used for TINF2 gene expression analysis. Exogenous expression and co-immunoprecipitation techniques were used for assessing TINF2 binding to TERF1. Relative telomere length (RTL) was quantified in DNAs from lymphocytes by using quantitative real-time PCR. Whole exome sequencing (WES) was performed in seven families with individuals affected with PTC and other cancer types. Screening for DNA variants in shelterin genes was performed by using whole genome sequencing data from 17 families and WES data from 7 further families. Results: The TINF2 mutation (TINF2 p.Trp198fs) showed complete co-segregation with PTC and melanoma in the key family. The mutation is not reported in databases and not identified in 23 other families we screened. The expression of TINF2 was borderline reduced in individuals with the mutation. The truncated TINF2 protein showed abolished binding to TERF1. The RTL in the individuals with the mutation was significantly longer when compared with those without the mutation from the same family as well as compared with 62 healthy controls. Among the 24 families, we identified 3 missense and 1 synonymous variant(s) in 2 shelterin genes (TINF2 and ACD). Conclusions: The rare frameshift mutation in the TINF2 gene and the associated longer telomere length suggest that dysregulated telomeres could be a mechanism predisposing to PTC and melanoma. DNA coding variants in shelterin genes are rare. Further studies are required to evaluate the roles of variants in shelterin genes in thyroid cancer and melanoma.

Structural biology of telomeres and telomerase

Telomeres are protein-DNA complexes that protect chromosome ends from illicit ligation and resection. Telomerase is a ribonucleoprotein enzyme that synthesizes telomeric DNA to counter telomere shortening. Human telomeres are composed of complexes between telomeric DNA and a six-protein complex known as shelterin. The shelterin proteins TRF1 and TRF2 provide the binding affinity and specificity for double-stranded telomeric DNA, while the POT1-TPP1 shelterin subcomplex coats the single-stranded telomeric G-rich overhang that is characteristic of all our chromosome ends. By capping chromosome ends, shelterin protects telomeric DNA from unwanted degradation and end-to-end fusion events. Structures of the human shelterin proteins reveal a network of constitutive and context-specific interactions. The shelterin protein-DNA structures reveal the basis for both the high affinity and DNA sequence specificity of these interactions, and explain how shelterin efficiently protects chromosome ends from genome instability. Several protein-protein interactions, many provided by the shelterin component TIN2, are critical for upholding the end-protection function of shelterin. A survey of these protein-protein interfaces within shelterin reveals a series of "domain-peptide" interactions that allow for efficient binding and adaptability towards new functions. While the modular nature of shelterin has facilitated its part-by-part structural characterization, the interdependence of subunits within telomerase has made its structural solution more challenging. However, the exploitation of several homologs in combination with recent advancements in cryo-EM capabilities has led to an exponential increase in our knowledge of the structural biology underlying telomerase function. Telomerase homologs from a wide range of eukaryotes show a typical retroviral reverse transcriptase-like protein core reinforced with elements that deliver telomerase-specific functions including recruitment to telomeres and high telomere-repeat addition processivity. In addition to providing the template for reverse transcription, the RNA component of telomerase provides a scaffold for the catalytic and accessory protein subunits, defines the limits of the telomeric repeat sequence, and plays a critical role in RNP assembly, stability, and trafficking. While a high-resolution definition of the human telomerase structure is only beginning to emerge, the quick pace of technical progress forecasts imminent breakthroughs in this area. Here, we review the structural biology surrounding telomeres and telomerase to provide a molecular description of mammalian chromosome end protection and end replication.

The structural biology of the shelterin complex

The shelterin complex protects telomeric DNA and plays critical roles in maintaining chromosome stability. The structures and functions of the shelterin complex have been extensively explored in the past decades. This review summarizes the current progress on structural studies of shelterin complexes from different species. It focuses on the structural features and assembly of common structural domains, highlighting the evolutionary plasticity and conserved roles of shelterin proteins in telomere homeostasis and protection.

Determination of the number of RAD51 molecules in different human cell lines

Knowledge about precise numbers of specific molecules is necessary for understanding and verification of biological pathways. The RAD51 protein is central in the repair of DNA double-strand breaks (DSBs) by homologous recombination repair and understanding its role in cellular pathways is crucial to design mechanistic DNA repair models. Here, we determined the number of RAD51 molecules in several human cell lines including primary fibroblasts. We showed that between 20000 to 100000 of RAD51 molecules are available per human cell that theoretically can be used for simultaneously loading at least 7 DSBs. Interestingly, the amount of RAD51 molecules does not significantly change after the induction of DNA damage using bleomycin or γ-irradiation in cells but an accumulation of RAD51 on the chromatin occurs. Furthermore, we generated an EGFP-RAD51 fusion under the control of HSV thymidine kinase promoter sequences yielding moderate protein expression levels comparable to endogenously expressed RAD51. Initial characterizations suggest that these low levels of ectopically expressed RAD51 are compatible with cell cycle progression of human cells. Hence, we provide parameters for the quantitative understanding and modeling of RAD51-involving processes.

TIN2 Functions with TPP1/POT1 To Stimulate Telomerase Processivity

TIN2 is an important regulator of telomere length, and mutations in TINF2, the gene encoding TIN2, cause short-telomere syndromes. While the genetics underscore the importance of TIN2, the mechanism through which TIN2 regulates telomere length remains unclear. Here, we tested the effects of human TIN2 on telomerase activity. We identified a new isoform in human cells, TIN2M, that is expressed at levels similar to those of previously studied TIN2 isoforms. All three TIN2 isoforms localized to and maintained telomere integrity in vivo, and localization was not disrupted by telomere syndrome mutations. Using direct telomerase activity assays, we discovered that TIN2 stimulated telomerase processivity in vitro All of the TIN2 isoforms stimulated telomerase to similar extents. Mutations in the TPP1 TEL patch abrogated this stimulation, suggesting that TIN2 functions with TPP1/POT1 to stimulate telomerase processivity. We conclude from our data and previously published work that TIN2/TPP1/POT1 is a functional shelterin subcomplex.

DNA damage response manages cell cycle restriction of senile multipotent mesenchymal stromal cells

Multipotent mesenchymal stromal cells (MMSCs) are promising to treat a variety of traumatic and degenerative diseases. However, in vitro-passage aging induces cell cycle arrest and a series of genetic and biological changes, which greatly limits ex vivo cell number expansion and further clinical application of MMSCs. In most cases, DNA damage and DNA damage response (DDR) act as the main cause and executor of cellular senescence respectively. Mechanistically, DNA damage signals induce cell cycle arrest and DNA damage repair via DDR. If the DNA damage is indelible, MMSCs would entry into a permanent cell cycle arrest. It should be noted that apart from DDR signaling, certain proliferation or metabolism pathways are also occupied in DNA damage related cell cycle arrest. New findings of these aspects will also be summarized in this study. In summary, we aim to provide a comprehensive review of DDR associated cell cycle regulation and other major molecular signaling in the senescence of MMSCs. Above knowledge could contribute to improve the limited capacity of in vitro expansion of MMSCs, and then promote their clinical applications.

Heart-Breaking Telomeres

Telomeres, the protective ends of linear chromosomes, shorten throughout an individual's lifetime. Accumulation of critically short telomeres is proposed to be a primary molecular cause of aging and age-associated diseases. Mutations in telomere maintenance genes are associated with pathologies referred to as or telomeropathies. The rate of telomere shortening throughout life is determined by endogenous (genetic) and external (nongenetic) factors. Therapeutic strategies based on telomerase activation are being developed to treat and prevent telomere-associated diseases, namely aging-related diseases and telomeropathies. Here, we review the molecular mechanisms underlying telomere driven diseases with particular emphasis on cardiovascular diseases.

Loss of RNA-binding protein HuR facilitates cellular senescence through posttranscriptional regulation of TIN2 mRNA

Cellular senescence can be induced by high levels of reactive oxygen species (ROS) produced by mitochondria. However, the mechanism by which elevated mitochondrial ROS levels are produced during replicative senescence is not yet fully understood. Here, we report that loss of the RNA-binding protein, human antigen R (HuR), during replicative senescence leads to an increase in ROS levels through enhanced mitochondrial localization of the telomeric protein TIN2. HuR binds to the 3′ untranslated region of TIN2 mRNA. This association decreases TIN2 protein levels by both destabilizing TIN2 mRNA and reducing its translation. Conversely, depletion of HuR levels enhances TIN2 expression, leading to increased mitochondrial targeting of TIN2. Mitochondrial localization of TIN2 increases ROS levels, which contributes to induction and maintenance of cellular senescence. Our findings provide compelling evidence for a novel role of HuR in controlling the process of cellular senescence by regulating TIN2-mediated mitochondrial ROS production, and for a useful therapeutic route for modulating intracellular ROS levels in treating both aging-related complications and cancer.

Therapeutic strategies for targeting telomerase in cancer

Telomere and telomerase play important roles in abnormal cell proliferation, metastasis, stem cell maintenance, and immortalization in various cancers. Therefore, designing of drugs targeting telomerase and telomere is of great significance. Over the past two decades, considerable knowledge regarding telomere and telomerase has been accumulated, which provides theoretical support for the design of therapeutic strategies such as telomere elongation. Therefore, the development of telomere-based therapies such as nucleoside analogs, non-nucleoside small molecules, antisense technology, ribozymes, and dominant negative human telomerase reverse transcriptase are being prioritized for eradicating a majority of tumors. While the benefits of telomere-based therapies are obvious, there is a need to address the limitations of various therapeutic strategies to improve the possibility of clinical applications. In this study, current knowledge of telomere and telomerase is discussed, and therapeutic strategies based on recent research are reviewed.

The role of telomere-binding modulators in pluripotent stem cells

Pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs), ESCs derived by somatic cell nuclear transfer (ntESCs), and induced pluripotent stem cells (iPSCs) have unlimited capacity for self-renewal and pluripotency and can give rise to all types of somatic cells. In order to maintain their self-renewal and pluripotency, PSCs need to preserve their telomere length and homeostasis. In recent years, increasing studies have shown that telomere reprogramming is essential for stem cell pluripotency maintenance and its induced pluripotency process. Telomere-associated proteins are not only required for telomere maintenance in both stem cells, their extra-telomeric functions have also been found to be critical as well. Here, we will discuss how telomeres and telomere-associated factors participate and regulate the maintenance of stem cell pluripotency.

Telomerase and Telomeres in Endometrial Cancer

Telomeres at the termini of human chromosomes are shortened with each round of cell division due to the “end replication problem” as well as oxidative stress. During carcinogenesis, cells acquire or retain mechanisms to maintain telomeres to avoid initiation of cellular senescence or apoptosis and halting cell division by critically short telomeres. The unique reverse transcriptase enzyme complex, telomerase, catalyzes the maintenance of telomeres but most human somatic cells do not have sufficient telomerase activity to prevent telomere shortening. Tissues with high and prolonged replicative potential demonstrate adequate cellular telomerase activity to prevent telomere erosion, and high telomerase activity appears to be a critical feature of most (80–90%) epithelial cancers, including endometrial cancer. Endometrial cancers regress in response to progesterone which is frequently used to treat advanced endometrial cancer. Endometrial telomerase is inhibited by progestogens and deciphering telomere and telomerase biology in endometrial cancer is therefore important, as targeting telomerase (a downstream target of progestogens) in endometrial cancer may provide novel and more effective therapeutic avenues. This review aims to examine the available evidence for the role and importance of telomere and telomerase biology in endometrial cancer.

Relative telomere length and gene expression of shelterin complex proteins among vinyl chloride monomer-exposed workers in China

Vinyl chloride monomer (VCM) is a confirmed carcinogen. The effects of VCM on telomeres and the gene expression of telomere complex proteins, shelterin, have not been well studied but could be of potential relevance to the carcinogenic mechanism of VCM and the health surveillance of VCM-exposed workers. A group of 241 VCM-exposed workers and 101 internal controls from the same plant in Shandong, China were recruited and quantitative polymerase chain reaction was preformed to measure relative telomere length (RTL) and gene expression of shelterin proteins. VCM cumulative exposure dose (CED) was estimated for the exposed workers. The differences in RTL and gene expression between groups were compared by Wald test fitted with robust regression. Shorter RTL was observed in VCM-exposed workers than in the controls (P < 0.001) and was related to CED of VCM. Shortened RTL was also significantly related to increasing age (P = 0.012) and high blood pressure (P = 0.056). Levels of gene expression of shelterin components in exposed workers were all lower than in controls except increased TIN2 expression, and the gene expression differences in TIN2 and POT1 among exposed and control groups were significant (P = 0.014 for TIN2 and P < 0.001 for POT1, respectively). VCM exposure is found associated with altered telomere length and gene expression of shelterin components. This provides new insights into the potential carcinogenic mechanisms of VCM and could be helpful for the health surveillance for VCM-exposed workers. Environ. Mol. Mutagen. 60:361-367, 2019. © 2018 Wiley Periodicals, Inc.

Condensin II subunit NCAPH2 associates with shelterin protein TRF1 and is required for telomere stability

Condensin II subunits are known to be expressed and localized to interphase nuclei of eukaryotic cells. Although some studies have shown that condensin II likely exerts axial compaction forces, organizes chromosome territories, and has possible transcriptional modulatory functions, the full range of condensin II interphase activities are not known. In particular, it is not known if condensin II interphase activities are generally genome‐wide or if they have additional local activities unique to specific chromosomal structures such as telomeres. Here, we find that NCAPH2 interacts with TRF1 and these two proteins co‐localize at telomeres. Depletion of NCAPH2 leads to ATR‐dependent accumulation of 53BP1 and γH2AX DNA damage foci, including damage specific to telomeres. Furthermore, depletion of NCAPH2 results in a fragile telomere phenotype and apparent sister‐telomere fusions only days after NCAPH2 depletion. Taken together these observations suggest that NCAPH2 promotes telomere stability, possibly through a direct interaction with the TRF1 shelterin component, and prevents telomere dysfunction resulting from impaired DNA replication. Because proper telomere function is essential for chromosome integrity these observations reveal a previously unappreciated function for NCAPH2 in ensuring genome and telomere stability.

The Response to DNA Damage at Telomeric Repeats and Its Consequences for Telomere Function

Telomeric repeats, coated by the shelterin complex, prevent inappropriate activation of the DNA damage response at the ends of linear chromosomes. Shelterin has evolved distinct solutions to protect telomeres from different aspects of the DNA damage response. These solutions include formation of t-loops, which can sequester the chromosome terminus from DNA-end sensors and inhibition of key steps in the DNA damage response. While blocking the DNA damage response at chromosome ends, telomeres make wide use of many of its players to deal with exogenous damage and replication stress. This review focuses on the interplay between the end-protection functions and the response to DNA damage occurring inside the telomeric repeats, as well as on the consequences that telomere damage has on telomere structure and function.

An N-terminal Flag-tag impairs TPP1 regulation of telomerase function

The shelterin protein complex protects natural chromosome ends from being recognized as DNA damage sites and also regulates the synthesis of telomeric repeats by telomerase. TPP1, a shelterin subunit that is essential for telomerase extension of telomeres, has been studied intensively in recent years. Many such studies utilize epitope tagged TPP1, but it is unclear how the tags may affect the multiple cellular functions of TPP1. Here we analyzed the effect of adding a 3x Flag epitope tag to the N- or C-terminus of TPP1. While the position of the tag did not affect TPP1's interaction within the shelterin complex or its localization to telomeres, the N-terminal Flag tag on TPP1 impaired telomerase function, resulting in reduced telomerase processivity in vitro and a failure to stimulate telomere elongation in vivo. The C-terminally Flag-tagged TPP1, in contrast, behaved similarly to untagged TPP1 in all functional aspects examined. These findings suggest that caution is required when utilizing epitope tagged TPP1 to study its regulation of telomerase function.

High-throughput gene expression analysis identifies p53-dependent and -independent pathways contributing to the adrenocortical dysplasia (acd) phenotype

In mammalian cells TPP1, encoded by the Acd gene, is a key component of the shelterin complex, which is required for telomere length maintenance and telomere protection. In mice, a hypomorphic mutation in Acd causes the adrenocortical dysplasia (acd) phenotype, which includes limb and body axis anomalies, and perinatal lethality. p53 deficiency partially rescues limb and body axis anomalies in acd mutant embryos, but not perinatal lethality, implicating p53-independent mechanisms in the acd phenotype. Loss of function of most shelterin proteins results in early embryonic lethality. Thus, study of the hypomorphic acd allele provides a unique opportunity to understand telomere dysfunction at an organismal level. The aim of this study was to identify transcriptome alterations in acd mutant and acd, p53 double mutant embryos to understand the p53-dependent and -independent factors that contribute to the mutant phenotypes in the context of the whole organism. Genes involved in developmental processes, cell cycle, metabolic pathways, tight junctions, axon guidance and signaling pathways were regulated by p53-driven mechanisms in acd mutant embryos, while genes functioning in immune response, and RNA processing were altered independently of p53 in acd, p53 double mutant embryos. To our best of knowledge, this is the first study revealing detailed transcriptomic alterations, reflecting novel p53-dependent and -independent pathways contributing to the acd phenotype. Our data confirm the importance of cell cycle and DNA repair pathways, and suggest novel links between telomere dysfunction and immune system regulation and the splicing machinery. Given the broad applicability of telomere maintenance in growth, development, and genome stability, our data will also provide a rich resource for others studying telomere maintenance and DNA damage responses in mammalian model systems.

Structural basis of meiotic telomere attachment to the nuclear envelope by MAJIN-TERB2-TERB1

Meiotic chromosomes undergo rapid prophase movements, which are thought to facilitate the formation of inter-homologue recombination intermediates that underlie synapsis, crossing over and segregation. The meiotic telomere complex (MAJIN, TERB1, TERB2) tethers telomere ends to the nuclear envelope and transmits cytoskeletal forces via the LINC complex to drive these rapid movements. Here, we report the molecular architecture of the meiotic telomere complex through the crystal structure of MAJIN-TERB2, together with light and X-ray scattering studies of wider complexes. The MAJIN-TERB2 2:2 hetero-tetramer binds strongly to DNA and is tethered through long flexible linkers to the inner nuclear membrane and two TRF1-binding 1:1 TERB2-TERB1 complexes. Our complementary structured illumination microscopy studies and biochemical findings reveal a telomere attachment mechanism in which MAJIN-TERB2-TERB1 recruits telomere-bound TRF1, which is then displaced during pachytene, allowing MAJIN-TERB2-TERB1 to bind telomeric DNA and form a mature attachment plate.

The meiotic telomere complex (MAJIN, TERB1, TERB2) tethers telomere ends to the nuclear envelope. Here the authors present the crystal structure of human MAJIN-TERB2 and combine biophysical approaches and structured illumination microscopy analysis of mouse meiotic chromosomes to characterize the molecular architecture of the wider MAJIN-TERB2-TERB1 complex and its interactions with TRF1.

Age-related response to an acute innate immune challenge in mice: proteomics reveals a telomere maintenance-related cost

Ageing is characterized by the impairment of the acute innate immune response and the upregulation of low-grade inflammation, i.e. inflammaging. At the cellular level, telomeres are considered as a marker of biological ageing as their length is progressively eroded in the absence of repair mechanisms. However, the link between telomeres and inflammaging remains underexplored. We aimed to identify proteins that are differentially expressed between age classes in response to an acute inflammatory challenge. We challenged young (two months) and old (12 months) C57BL/6 mice using bacterial lipopolysaccharide (LPS) and measured telomere length and proteomic profiles in splenocytes. In total, 233 out of the 1966 proteins we quantified differed among experimental groups. A hierarchical clustering analysis revealed that nine of those 233 proteins were differently expressed among the experimental groups. Young mice responded to LPS by increasing the expression of proteins involved in the innate immune response, and interestingly, in telomere length maintenance. However, this regulation was impaired at older ages. These results are in agreement with the assumption that the strength of selection declines with age, potentially explaining the maintenance of costly, dysregulated, immune responses at old age. We suggest that the immune response is competing with the telomere maintenance process, highlighting how telomeres reflect the ageing trade-off even in a species where telomere length is not related to lifespan.

Telomere length measurement as a clinical biomarker of aging and disease

Telomere length measurement is increasingly recognized as a clinical gauge for age-related disease risk. There are several methods for studying blood telomere length (BTL) as a clinical biomarker. The first is an observational study approach, which directly measures telomere lengths using either cross-sectional or longitudinal patient cohorts and compares them to a population of age- and sex-matched individuals. These direct traceable measurements can be considered reflective of an individual's current health or disease state. Escalating interest in personalized medicine, access to high-throughput genotyping and resulting acquisition of large volumes of genetic data corroborates the second method, Mendelian randomization (MR). MR employs telomere length-associated genetic variants to indicate predisposition to disease risk based on the genomic composition of the individual. When assessed from cells in the bloodstream, telomeres can show variation from their genetically predisposed lengths due to environmental-induced changes. These alterations in telomere length act as an indicator of cellular health, which, in turn, can provide disease risk status. Overall, BTL measurement is a dynamic marker of biological health and well-being that together with genetically defined telomere lengths can provide insights into improved healthcare for the individual.

Telomere and its role in the aging pathways: telomere shortening, cell senescence and mitochondria dysfunction

Aging is a biological process characterized by a progressive functional decline in tissues and organs, which eventually leads to mortality. Telomeres, the repetitive DNA repeat sequences at the end of linear eukaryotic chromosomes protecting chromosome ends from degradation and illegitimate recombination, play a crucial role in cell fate and aging. Due to the mechanism of replication, telomeres shorten as cells proliferate, which consequently contributes to cellular senescence and mitochondrial dysfunction. Cells are the basic unit of organismal structure and function, and mitochondria are the powerhouse and metabolic center of cells. Therefore, cellular senescence and mitochondrial dysfunction would result in tissue or organ degeneration and dysfunction followed by somatic aging through multiple pathways. In this review, we summarized the main mechanisms of cellular senescence, mitochondrial malfunction and aging triggered by telomere attrition. Understanding the molecular mechanisms involved in the aging process may elicit new strategies for improving health and extending lifespan.

Shelterin-Mediated Telomere Protection

For more than a decade, it has been known that mammalian cells use shelterin to protect chromosome ends. Much progress has been made on the mechanism by which shelterin prevents telomeres from inadvertently activating DNA damage signaling and double-strand break (DSB) repair pathways. Shelterin averts activation of three DNA damage response enzymes [the ataxia-telangiectasia-mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases and poly(ADP-ribose) polymerase 1 (PARP1)], blocks three DSB repair pathways [classical nonhomologous end joining (c-NHEJ), alternative (alt)-NHEJ, and homology-directed repair (HDR)], and prevents hyper-resection at telomeres. For several of these functions, mechanistic insights have emerged. In addition, much has been learned about how shelterin maintains the telomeric 3' overhang, forms and protects the t-loop structure, and promotes replication through telomeres. These studies revealed that shelterin is compartmentalized, with individual subunits dedicated to distinct aspects of the end-protection problem. This review focuses on the current knowledge of shelterin-mediated telomere protection, highlights differences between human and mouse shelterin, and discusses some of the questions that remain.

Protection of telomeres 1 proteins POT1a and POT1b can repress ATR signaling by RPA exclusion, but binding to CST limits ATR repression by POT1b

Comprised of telomeric TTAGGG repeats and shelterin, telomeres ensure that the natural ends of chromosomes remain impervious to the DNA damage response. Telomeres carry a long constitutive 3' overhang that can bind replication protein A (RPA) and activate the ATR Ser/Thr kinase (ATR), which induces cell cycle arrest. A single-stranded (ss) TTAGGG repeat-binding protein in mouse shelterin, POT1a, has been proposed to repress ATR signaling by preventing RPA binding. Repression of ATR at telomeres requires tethering of POT1a to the other shelterin subunits situated on the double-stranded (ds) telomeric DNA. The simplest model of ATR repression, the "tethered exclusion model," suggests that the only critical features of POT1a are its connection to shelterin and its binding to ss telomeric DNA. In agreement with the model, we show here that a shelterin-tethered variant of RPA70 (lacking the ATR recruitment domain) can repress ATR signaling at telomeres that lack POT1a. However, arguing against the tethered exclusion model, the nearly identical POT1b subunit of shelterin has been shown to be much less proficient than POT1a in repression of ATR. We now show that POT1b has the intrinsic ability to fully repress ATR but is prevented from doing so when bound to Ctc1, Stn1, Ten1 (CST), the complex needed for telomere end processing. These results establish that shelterin represses ATR with a tethered ssDNA-binding domain that excludes RPA from the 3' overhang and also reveal an unexpected effect of CST on the ability of POT1b to repress ATR.

RPA and RAD51: fork reversal, fork protection, and genome stability

Replication protein A (RPA) and RAD51 are DNA-binding proteins that help maintain genome stability during DNA replication. These proteins regulate nucleases, helicases, DNA translocases, and signaling proteins to control replication, repair, recombination, and the DNA damage response. Their different DNA-binding mechanisms, enzymatic activities, and binding partners provide unique functionalities that cooperate to ensure that the appropriate activities are deployed at the right time to overcome replication challenges. Here we review and discuss the latest discoveries of the mechanisms by which these proteins work to preserve genome stability, with a focus on their actions in fork reversal and fork protection.

Telomerase activity and telomere on stem progeny senescence

The end of linear chromosomes is formed of a special nucleoprotein heterochromatin structure with repetitive TTAGGG sequences called telomere. Telomere length is regulated by a special enzyme called telomerase, a specific DNA polymerase that adds new telomeric sequences to the chromosome ends. Telomerase consists of two parts; the central protein part and the accessory part which is a RNA component transported by the central part. Regulation of telomere length by this enzyme is a multi-stage process. Telomere length elongation is strongly influenced by the level of telomerase and has a strong correlation with the activity of telomerase enzyme. Human Telomerase Reverse Transcriptase (hTERT) gene expression plays an important role in maintaining telomere length and high proliferative property of cells. Except a low activity of telomerase enzyme in hematopoietic and few types of stem cells, most of somatic cells didn't showed telomerase activity. Moreover, cytokines are secretory proteins that control many aspects of hematopoiesis, especially immune responses and inflammation. Also, the induction of hTERT gene expression by cytokines is organized through the PI3K/AKT and NF/kB signaling pathways. In this review we have tried to talk about effects of immune cell cytokines on telomerase expression/telomere length and the induction of telomerase expression by cytokines.

Short telomeres - A hallmark of heritable cardiomyopathies

Cardiovascular diseases are the leading cause of death worldwide and the incidence increases with age. Genetic testing has taught us much about the pathogenic pathways that drive heritable cardiomyopathies. Here we discuss an unexpected link between shortened telomeres, a molecular marker of aging, and genetic cardiomyopathy. Positioned at the ends of chromosomes, telomeres are DNA repeats which serve as protective caps that shorten with each cell division in proliferative tissues. Cardiomyocytes are an anomaly, as they are largely non-proliferative post-birth and retain relatively stable telomere lengths throughout life in healthy individuals. However, there is mounting evidence that in disease states, cardiomyocyte telomeres significantly shorten. Moreover, this shortening may play an active role in the development of mitochondrial dysfunction central to the etiology of dilated and hypertrophic cardiomyopathies. Elucidation of the mechanisms that underlie the telomere-mitochondrial signaling axis in the heart will provide fresh insights into our understanding of genetic cardiomyopathies, and could lead to the identification of previously uncharacterized modes of therapeutic intervention.

Stabilization of Reversed Replication Forks by Telomerase Drives Telomere Catastrophe

Telomere maintenance critically depends on the distinct activities of telomerase, which adds telomeric repeats to solve the end replication problem, and RTEL1, which dismantles DNA secondary structures at telomeres to facilitate replisome progression. Here, we establish that reversed replication forks are a pathological substrate for telomerase and the source of telomere catastrophe in Rtel1-/- cells. Inhibiting telomerase recruitment to telomeres, but not its activity, or blocking replication fork reversal through PARP1 inhibition or depleting UBC13 or ZRANB3 prevents the rapid accumulation of dysfunctional telomeres in RTEL1-deficient cells. In this context, we establish that telomerase binding to reversed replication forks inhibits telomere replication, which can be mimicked by preventing replication fork restart through depletion of RECQ1 or PARG. Our results lead us to propose that telomerase inappropriately binds to and inhibits restart of reversed replication forks within telomeres, which compromises replication and leads to critically short telomeres.

Telomeres: Implications for Cancer Development

Telomeres facilitate the protection of natural ends of chromosomes from constitutive exposure to the DNA damage response (DDR). This is most likely achieved by a lariat structure that hides the linear telomeric DNA through protein-protein and protein-DNA interactions. The telomere shortening associated with DNA replication in the absence of a compensatory mechanism culminates in unmasked telomeres. Then, the subsequent activation of the DDR will define the fate of cells according to the functionality of cell cycle checkpoints. Dysfunctional telomeres can suppress cancer development by engaging replicative senescence or apoptotic pathways, but they can also promote tumour initiation. Studies in telomere dynamics and karyotype analysis underpin telomere crisis as a key event driving genomic instability. Significant attainment of telomerase or alternative lengthening of telomeres (ALT)-pathway to maintain telomere length may be permissive and required for clonal evolution of genomically-unstable cells during progression to malignancy. We summarise current knowledge of the role of telomeres in the maintenance of chromosomal stability and carcinogenesis.