Role In Telomere Length Maintenance Research Articles

MicroRNAs role in telomere length maintenance and telomerase activity in tumor cells.

MiRNAs, a class of non-coding RNA molecules, have emerged as critical modulators of telomere length and telomerase activity by finely tuning the expression of target genes (and not gene targets) within signaling pathways involved in telomere homeostasis. The primary objective of this systematic review was to compile and synthesize the existing body of knowledge on the role, association, and involvement of miRNAs in telomere length. Additionally, the review explored the regulation, function, and activation mechanism of thehuman telomerase reverse transcriptase(hTERT) gene and telomerase activity in tumor cells. A comprehensive analysis of 47 selected articles revealed 40 distinct miRNAs involved in these processes. These miRNAs were shown to exert their function, in both clinical cases and cell line models, either directly or indirectly, regulating hTERT and telomerase activity through distinct molecular mechanisms. The regulatory roles of these miRNAs significantly affected major cancer phenotypes, with outcomes largely dependent on the tissue type and the cellular actions within the tumor cells, whereby they functioned as oncogenes or tumor suppressors. These findings strongly support the pivotal role of miRNAs in modulating telomere length and telomerase activity, thereby contributing to the intricate and complex regulation of telomere homeostasis in tumor cells.Moreover, they emphasize the potential of targeting miRNAs and key regulatory genes as therapeutic strategies to disrupt cancer cell growth and promote senescence, offering promising avenues for novel cancer treatments.

Association between modifiable lifestyle factors and telomere length: a univariable and multivariable Mendelian randomization study

BackgroundTelomere length has long been recognized as a valuable biomarker of aging and is inversely correlated with chronological age. Various lifestyle factors have been implicated in telomere shortening or preservation; however, the association between lifestyle factors and telomere length remains controversial. To address this issue, we conducted a Mendelian randomization (MR) analysis to investigate the potential causal associations between multiple lifestyle factors and telomere length.MethodsIndependent genetic variants strongly associated with lifestyle factors (tobacco smoking, sleep duration, insomnia, and physical activity) were selected as instrumental variables from corresponding genome-wide association studies (GWASs). Summary-level data for telomere length was obtained from a GWAS comprising 472,174 European ancestries. Univariable and multivariable MR analyses were performed to assess the relationships.ResultsThe genetic liability to lifetime smoking was robustly associated with shorter telomere length (odd ratio [OR]: 0.882; 95% confidence interval [CI]: 0.847–0.918). Genetically predicted insomnia was also linked to shorter telomere length (OR: 0.972; 95% CI: 0.959–0.985), while no significant association was observed between sleep duration and telomere length. Furthermore, a suggestive association was found between moderate-to-vigorous physical activity and longer telomere length (OR: 1.680; 95% CI: 1.115–2.531). In multivariable MR analyses, adjusting for potential mediators such as body mass index, type 2 diabetes, alcohol consumption, and alcohol use disorder, the associations of lifetime smoking and insomnia with telomere length remained robust.ConclusionOur findings suggest that smoking and insomnia may contribute to telomere shortening, while physical activity may play a role in telomere length maintenance. These findings underscore the importance of managing positive risk factors and adopting a healthy lifestyle to promote telomere health.

UBQLN1 deficiency mediates telomere shortening and IPF through interacting with RPA1.

Premature telomere shortening is a known factor correlated to idiopathic pulmonary fibrosis (IPF) occurrence, which is a chronic, progressive, age-related disease with high mortality. The etiology of IPF is still unknown. Here, we found that UBQLN1 plays a key role in telomere length maintenance and is potentially relevant to IPF. UBQLN1 involves in DNA replication by interacting with RPA1 and shuttling it off from the replication fork. The deficiency of UBQLN1 retains RPA1 at replication fork, hinders replication and thus causes cell cycle arrest and genome instability. Especially at telomere regions of the genome, where more endogenous replication stress exists because of G rich sequences, UBQLN1 depletion leads to rapid telomere shortening in HeLa cells. It revealed that UBQLN1 depletion also shortens telomere length at mouse lung and accelerates mouse lung fibrosis. In addition, the UBQLN1 expression level in IPF patients is downregulated and correlated to poor prognosis. Altogether, these results uncover a new role of UBQLN1 in ensuring DNA replication and maintaining telomere stability, which may shed light on IPF pathogenesis and prevention.

Association Between Riboflavin Intake and Telomere Length: A Cross-Sectional Study From National Health and Nutrition Examination Survey 1999-2002.

BackgroundDietary habits and dietary intake affect telomere length, a reliable marker of biological aging and a predictor of chronic disease. Riboflavin (RF) is known as a water-soluble antioxidant vitamin, but its role in telomere length maintenance has yet to be elucidated.ObjectiveThe purpose of this study was to examine the relationship between dietary RF intake and telomere length in a nationally representative sample of adults.MethodsUsing the NHANES (1999–2002), telomere data of 4,298 participants aged ≥45 years were analyzed in a cross-sectional manner. Leukocyte telomere length was measured using the quantitative real-time polymerase chain reaction (qPCR). Dietary RF intake was assessed by a trained interviewer using 24-h dietary recall method. Generalized linear regressions were performed to evaluate the association between dietary RF intake and telomere length. Subgroup analyses were performed to further explore this relationship in sex and body mass index (BMI) subgroups.ResultsAmong the 3,788 participants included, the average telomere length was longer in females (P = 0.014), while they had a lower average RF intake compared to males (P < 0.001). There was a weak positive correlation between RF intake and telomere length both when unadjusted (β = 0.011; P = 0.037) and adjusted for age, sex, and ethnicity (β = 0.013; P = 0.033). Subgroup analyses showed a positive association between RF intake and the telomere length in female after adjusting for confounding factors (β = 0.029; P = 0.046). In the female subgroup, there were significant positive relationships between telomere length and RF intake in the obese group (β = 0.086, P = 0.022).ConclusionIncreased dietary RF intake was significantly associated with longer telomere length in middle-aged and older American females, especially in low RF intake obese female.

Telomerase Regulation: A Role for Epigenetics.

Simple SummaryMaintenance of telomeres is a fundamental step in human carcinogenesis and is primarily regulated by telomerase and the human telomerase reverse transcriptase gene (TERT). Improved understanding of the transcriptional control of this gene may provide potential therapeutic targets. Epigenetic modifications are a prominent mechanism to control telomerase activity and regulation of the TERT gene. TERT-targeting miRNAs have been widely studied and their function explained through pre-clinical in vivo model-based validation studies. Further, histone deacetylase inhibitors are now in pre and early clinical trials with significant clinical success. Importantly, TERT downregulation through epigenetic modifications including TERT promoter methylation, histone deacetylase inhibitors, and miRNA activity might contribute to clinical study design. This review provides an overview of the epigenetic mechanisms involved in the regulation of TERT expression and telomerase activity.Telomerase was first described by Greider and Blackburn in 1984, a discovery ultimately recognized by the Nobel Prize committee in 2009. The three decades following on from its discovery have been accompanied by an increased understanding of the fundamental mechanisms of telomerase activity, and its role in telomere biology. Telomerase has a clearly defined role in telomere length maintenance and an established influence on DNA replication, differentiation, survival, development, apoptosis, tumorigenesis, and a further role in therapeutic resistance in human stem and cancer cells including those of breast and cervical origin. TERT encodes the catalytic subunit and rate-limiting factor for telomerase enzyme activity. The mechanisms of activation or silencing of TERT remain open to debate across somatic, cancer, and stem cells. Promoter mutations upstream of TERT may promote dysregulated telomerase activation in tumour cells but additional factors including epigenetic, transcriptional and posttranscriptional modifications also have a role to play. Previous systematic analysis indicated methylation and mutation of the TERT promoter in 53% and 31%, respectively, of TERT expressing cancer cell lines supporting the concept of a key role for epigenetic alteration associated with TERT dysregulation and cellular transformation. Epigenetic regulators including DNA methylation, histone modification, and non-coding RNAs are now emerging as drivers in the regulation of telomeres and telomerase activity. Epigenetic regulation may be responsible for reversible silencing of TERT in several biological processes including development and differentiation, and increased TERT expression in cancers. Understanding the epigenetic mechanisms behind telomerase regulation holds important prospects for cancer treatment, diagnosis and prognosis. This review will focus on the role of epigenetics in telomerase regulation.

Long non-coding RNAs at work on telomeres: Functions and implications in cancer therapy.

Long non-coding RNAs (lncRNAs) are known to regulate various biological processes including cancer. Cancer cells possess limitless replicative potential which is attained by telomere length maintenance while normal somatic cells have a limited lifespan because their telomeres shorten with every cell division ultimately triggering replicative senescence. Two lncRNAs have been observed to play a key role in telomere length maintenance. First is the lncRNA TERC (telomerase RNA component) which functions as a template for telomeric DNA synthesis in association with telomerase reverse transcriptase (TERT) which serves as the catalytic component. Together they constitute the telomerase complex which functions as a reverse transcriptase to elongate telomeres. Second lncRNA that helps in regulating telomere length is the telomeric repeat-containing RNA (TERRA) which is transcribed from the subtelomeric region and extends to the telomeric region. TERC and TERRA exhibit important functions in cancer with implications in precision oncology. In this review, we discuss various aspects of these important lncRNAs in humans and their role in cancer along with recent advancements in their anticancer therapeutic application.

Non-canonical Functions of Telomerase Reverse Transcriptase: Emerging Roles and Biological Relevance.

Increasing evidence from research on telomerase suggests that in addition to its catalytic telomere repeat synthesis activity, telomerase may have other biologically important functions. The canonical roles of telomerase are at the telomere ends where they elongate telomeres and maintain genomic stability and cellular lifespan. The catalytic protein component Telomerase Reverse Transcriptase (TERT) is preferentially expressed at high levels in cancer cells despite the existence of an alternative mechanism for telomere maintenance (alternative lengthening of telomeres or ALT). TERT is also expressed at higher levels than necessary for maintaining functional telomere length, suggesting other possible adaptive functions. Emerging non-canonical roles of TERT include regulation of non-telomeric DNA damage responses, promotion of cell growth and proliferation, acceleration of cell cycle kinetics, and control of mitochondrial integrity following oxidative stress. Non-canonical activities of TERT primarily show cellular protective effects, and nuclear TERT has been shown to protect against cell death following double-stranded DNA damage, independent of its role in telomere length maintenance. TERT has been suggested to act as a chromatin modulator and participate in the transcriptional regulation of gene expression. TERT has also been reported to regulate transcript levels through an RNA-dependent RNA Polymerase (RdRP) activity and produce siRNAs in a Dicer-dependent manner. At the mitochondria, TERT is suggested to protect against oxidative stress-induced mtDNA damage and promote mitochondrial integrity. These extra-telomeric functions of TERT may be advantageous in the context of increased proliferation and metabolic stress often found in rapidly-dividing cancer cells. Understanding the spectrum of non-canonical functions of telomerase may have important implications for the rational design of anti-cancer chemotherapeutic drugs.

Yeast Genome Maintenance by the Multifunctional PIF1 DNA Helicase Family.

The two PIF1 family helicases in Saccharomyces cerevisiae, Rrm3, and ScPif1, associate with thousands of sites throughout the genome where they perform overlapping and distinct roles in telomere length maintenance, replication through non-histone proteins and G4 structures, lagging strand replication, replication fork convergence, the repair of DNA double-strand break ends, and transposable element mobility. ScPif1 and its fission yeast homolog Pfh1 also localize to mitochondria where they protect mitochondrial genome integrity. In addition to yeast serving as a model system for the rapid functional evaluation of human Pif1 variants, yeast cells lacking Rrm3 have proven useful for elucidating the cellular response to replication fork pausing at endogenous sites. Here, we review the increasingly important cellular functions of the yeast PIF1 helicases in maintaining genome integrity, and highlight recent advances in our understanding of their roles in facilitating fork progression through replisome barriers, their functional interactions with DNA repair, and replication stress response pathways.

The evolutionarily conserved factor Sus1/ENY2 plays a role in telomere length maintenance.

Sus1 is a conserved protein involved in histone H2B de-ubiquitination and mRNA export from the nucleus in eukaryotes. Previous studies implicated Sus1 partners in genome integrity including telomere homeostasis. However, the implication of Sus1 in telomere maintenance remains largely unknown. In this study, we found that yeast Sus1 interacts physically and genetically with factors involved in telomere maintenance and its absence leads to elongated telomeres. Deletion of several of Sus1's partners also leads to longer telomeres. Our results rule out a direct role for Sus1 in recruiting telomerase subunits to telomeres. However, we observe that deletion of SUS1 leads to elongated telomeres even in the presence of mutations like sem1Δ, esc2Δ and rsc2Δ, which cause telomere shortening. We find that rsc2Δ (short telomeres) have reduced levels of mono-ubiquitinated histone H2B at lysine 123 (H2BK123ub1), whereas sus1Δ mutants or double-mutants sus1Δ rsc2Δ exhibit longer telomeres and higher H2BK123ub1 levels. These results suggest that Sus1 activity as a H2B de-ubiquitination modulator plays a role in negatively regulating telomere length. Our results provide solid evidence for a role of Sus1 in negatively regulating telomere length through the modulation of H2BK123 mono-ubiquitination and its interaction with the nuclear pore complex.

A genetic interaction between RAP1 and telomerase reveals an unanticipated role for RAP1 in telomere maintenance.

Summary RAP1 is one of the components of shelterin, the capping complex at chromosome ends or telomeres, although its role in telomere length maintenance and protection has remained elusive. RAP1 also binds subtelomeric repeats and along chromosome arms, where it regulates gene expression and has been shown to function in metabolism control. Telomerase is the enzyme that elongates telomeres, and its deficiency causes a premature aging in humans and mice. We describe an unanticipated genetic interaction between RAP1 and telomerase. While RAP1 deficiency alone does not impact on mouse survival, mice lacking both RAP1 and telomerase show a progressively decreased survival with increasing mouse generations compared to telomerase single mutants. Telomere shortening is more pronounced in Rap1 −/− Terc −/− doubly deficient mice than in the single‐mutant Terc −/− counterparts, leading to an earlier onset of telomere‐induced DNA damage and degenerative pathologies. Telomerase deficiency abolishes obesity and liver steatohepatitis provoked by RAP1 deficiency. Using genomewide ChIP sequencing, we find that progressive telomere shortening owing to telomerase deficiency leads to re‐localization of RAP1 from telomeres and subtelomeric regions to extratelomeric sites in a genomewide manner. These findings suggest that although in the presence of sufficient telomere reserve RAP1 is not a key factor for telomere maintenance and protection, it plays a crucial role in the context of telomerase deficiency, thus in agreement with its evolutionary conservation as a telomere component from yeast to humans.

The relationship between telomere length and beekeeping among Malaysians.

The belief that beekeepers live longer than anyone else is present since ages. However, no research has been done to explore the longevity of life in beekeepers. Here, we investigated the telomere length in 30 male beekeepers and 30 male non-beekeepers and associated them with the longevity of life using Southern analysis of terminal restriction fragments (TRFs) generated by Hinf I/Rsa I digestion of human genomic DNA using TeloTAGGG Telomere Length Assay. Interestingly, we found that the telomere length of male beekeepers was significantly longer than those of male non-beekeepers with a p value of less than 0.05, suggesting that beekeepers may have longer life compared to non-beekeepers. We further found that the consumption of bee products for a long period and frequent consumption of bee products per day are associated with telomere length. An increase of year in consuming bee products is associated with a mean increase in telomere length of 0.258 kbp. In addition, an increase in frequency of eating bee products per day was also associated with a mean increase of 2.66 kbp in telomere length. These results suggested that bee products might play some roles in telomere length maintenance.

Downregulation of Dyskerin Impairs Expansion of Normal Hematopoietic Progenitors and Erythroid Colony Formation in the Absence of Apparent Telomere Shortening

Dyskerin is an RNA binding protein that functions as a core component of the telomerase holoenzyme as well as in ribonuclear protein complexes involved in ribosome biogenesis and RNA processing. Within the telomerase holoenzyme, dyskerin stabilizes the telomerase RNA component (hTR) that functions as a template for synthesis of telomeric DNA at chromosome ends. Telomerase is active in malignant cells as well as normal hematopoietic stem and progenitor cells where it enables long-term cell replication.Mutation of the gene encoding dyskerin (DKC1) is the underlying cause of the inherited disease dyskeratosis congenita (DC), which is characterized by low levels of telomerase activity and shortened telomeres. Bone marrow failure is the most common cause of mortality in DC, however since dyskerin has telomerase-independent roles in fundamental cellular processes, it is not clear whether telomerase insufficiency is the sole cause of hematopoietic dysfunction in this disorder.To gain further insight to the consequences of deregulated expression of dyskerin in haematopoietic cells, we transduced normal human cord blood CD34+ progenitor cells with retroviral vectors encoding two different shRNAs targeting dyskerin. Transduced cells from seven independent experiments were FACS sorted then assayed for expansion and differentiation in vitro, clonogenic potential, telomerase activity and telomere length maintenance. Results from the functional assays showed that depletion of dyskerin impaired cell expansion and compromised the formation of BFU-E colonies. In contrast to BFU-E, CFU-GM colonies did not appear to be altered by dyskerin depletion. Analysis of non-transuduced cells subject to ex vivo expansion along erythroid and myelomonocytic lineages revealed that erythroid precursor cells expressed disproportionately high levels of dyskerin, and correspondingly high telomerase enzyme activity. shRNA-mediated downregulation of dyskerin lowered hTR levels and quenched telomerase activity in the erythroid precursors without altering expression of the telomerase catalytic component, hTERT. These results suggest that upregulation of dyskerin bolsters telomerase activity and renders erythroid precursors vulnerable to dyskerin depletion. However in spite of the reduction in telomerase activity induced by dyskerin shRNA, Southern blot analysis showed that the rate of telomere length shortening was not significantly altered by dyskerin depletion over the time course of these experiments. Together the data show that dyskerin is upregulated during erythroid differentiation, and has an essential function in the erythroid lineage that appears to be independent of its role in telomere length maintenance. DisclosuresNo relevant conflicts of interest to declare.

Differential regulation of MRN (Mre11–Rad50–Nbs1) complex subunits and telomerase activity in cancer cells

Several lines of evidence suggest that cancer progression is associated with up-regulation or reactivation of telomerase and the underlying mechanism remains an active area of research. The heterotrimeric MRN complex, consisting of Mre11, Rad50 and Nbs1, which is required for the repair of double-strand breaks, plays a key role in telomere length maintenance. In this study, we show significant differences in the levels of expression of MRN complex subunits among various cancer cells and somatic cells. Notably, siRNA-mediated depletion of any of the subunits of MRN complex led to complete ablation of other subunits of the complex. Treatment of leukemia and prostate cancer cells with etoposide lead to increased expression of MRN complex subunits, with concomitant decrease in the levels of telomerase activity, compared to breast cancer cells. These studies raise the possibility of developing anti-cancer drugs targeting MRN complex subunits to sensitize a subset of cancer cells to radio- and/or chemotherapy.

Human XPF controls TRF2 and telomere length maintenance through distinctive mechanisms

XPF–ERCC1, a structure-specific endonuclease, is involved in nucleotide excision repair, crosslink repair and homologous recombination. XPF–ERCC1 is also found to interact with TRF2, a duplex telomeric DNA binding protein. We have previously shown that XPF–ERCC1 is required for TRF2-promoted telomere shortening. However, whether XPF–ERCC1 by itself has a role in telomere length maintenance has not been determined. Here we report that overexpression of XPF induces telomere shortening in XPF-proficient cells whereas XPF complementation suppresses telomere lengthening in XPF-deficient cells. These results suggest that XPF–ERCC1 can function as a negative mediator of telomere length maintenance. In addition, we find that introduction of wild type XPF into XPF-deficient cells leads to over 40% reduction in TRF2 association with telomeric DNA, indicating that XPF–ERCC1 negatively regulates TRF2 binding to telomeric DNA. Furthermore, we show that XPF carrying mutations in the conserved nuclease domain fails to control TRF2 association with telomeric DNA but it is competent for modulating telomere length maintenance. These results imply that XPF–ERCC1 controls TRF2 and telomere length maintenance through two distinctive mechanisms, with the former requiring its nuclease activity. Our results further imply that TRF2 association with telomeres may be deregulated in cells derived from XPF patients.

Telomerase core components protect Candida telomeres from aberrant overhang accumulation.

Telomerase is a cellular reverse transcriptase that extends one strand (the G-strand) of the telomere terminal repeats. Aside from this role in telomere length maintenance, telomerase has been proposed to serve a protective function at chromosome ends, although this is not well understood mechanistically. Earlier analysis suggests that, in the pathogenic yeast Candida albicans, the catalytic reverse transcriptase subunit of telomerase (TERT/EST2) can protect telomeres against nucleolytic degradation. In this report we demonstrate that the RNA component (TER1) has a similar function; in addition to complete loss of telomerase activity and progressive telomere attrition, the ter1-DeltaDelta strains manifested a dramatic increase in the amount of G-strand overhangs, consistent with aberrant degradation of the complementary C-strand. We also demonstrate that a catalytically incompetent EST2 protein can suppress such overhang accumulation in the est2-DeltaDelta mutant to the same extent as the wild-type protein. Altogether, our data support the notion that the Candida telomerase core components mediate a protective function through a mechanism that is independent of its catalytic activity.

Generation and Characterization of Telomere Length Maintenance in Tankyrase 2-Deficient Mice

Telomere length and function are crucial factors that determine the capacity for cell proliferation and survival, mediate cellular senescence, and play a role in malignant transformation in eukaryotic systems. The telomere length of a specific mammalian species is maintained within a given range by the action of telomerase and telomere-associated proteins. TRF1 is a telomere-associated protein that inhibits telomere elongation by its binding to telomere repeats, preventing access to telomerase. Human TRF1 interacts with tankyrase 1 and tankyrase 2 proteins, two related members of the tankyrase family shown to have poly(ADP-ribose) polymerase activity. Human tankyrase 1 is reported to ADP-ribosylate TRF1 and to down-regulate the telomeric repeat binding activity of TRF1, resulting in telomerase-dependent telomere elongation. Human tankyrase 2 is proposed to have activity similar to that of tankyrase 1, although tankyrase 2 function has been less extensively characterized. In the present study, we have assessed the in vivo function of mouse tankyrase 2 by germ line gene inactivation and show that inactivation of tankyrase 2 does not result in detectable alteration in telomere length when monitored through multiple generations of breeding. This finding suggests that either mouse tankyrases 1 and 2 have redundant functions in telomere length maintenance or that mouse tankyrase 2 differs from human tankyrase 2 in its role in telomere length maintenance. Tankyrase 2 deficiency did result in a significant decrease in body weight sustained through at least the first year of life, most marked in male mice, suggesting that tankyrase 2 functions in potentially telomerase-independent pathways to affect overall development and/or metabolism.

Telomeres and DNA damage checkpoints

In all eukaryotic organisms, interruptions in duplex DNA molecules elicit a DNA damage response, which includes activation of DNA repair machineries and surveillance mechanisms, known as DNA damage checkpoints. Telomeres and double-strand breaks (DSBs) share the common feature of being physical ends of chromosomes. However, unlike DSBs, telomeres do not activate the DNA damage checkpoints and are usually protected from end-to-end fusions and other processing events that normally promote repair of DNA breaks. This indicates that they are shielded from being recognized and processed as DSBs. On the other hand, chromosome ends resemble damaged DNA, as several factors required for DNA repair and checkpoint networks play important roles in telomere length maintenance. Due to the critical role of both DNA damage checkpoints and telomere homeostasis in maintaining genetic stability and in counteracting cancer development, the knowledge of their interconnections is essential for our understanding of these key cellular controls.

DNA-Protein Kinase Catalytic Subunit-interacting Protein KIP Binds Telomerase by Interacting with Human Telomerase Reverse Transcriptase

Telomere homeostasis, a process that is essential for continued cell proliferation and genomic stability, is regulated by endogenous telomerase and a collection of associated proteins. In this paper, a protein called KIP (previously reported as a protein that binds specifically to DNA-dependent protein kinase), has been identified as a telomerase-regulating activity based on the following pieces of evidence. First, complexes between KIP and the catalytic subunit of telomerase (hTERT) were identified using the yeast two-hybrid technique. Second, antibodies specific to KIP immunoprecipitate human telomerase in cell-free extracts. Third, immunolocalization experiments demonstrate that KIP is a nuclear protein that co-localizes with hTERT in cells. Fourth, KIP binds to hTERT both in vitro and in vivo in the absence of human telomerase RNA or telomeric DNA, thus defining the catalytic subunit of telomerase as the site of physical interaction. Fifth, co-immunoprecipitation experiments suggest that KIP-hTERT complexes form readily in cells and that overexpression of KIP in telomerase-positive cells increases endogenous telomerase activity. Finally, continued overexpression of KIP (60 population doublings) resulted in cells with elongated telomeres; thus, KIP directly or indirectly stimulates telomerase activity through hTERT and contributes to telomere lengthening. The collective data in this paper suggest that KIP plays a positive role in telomere length maintenance and/or regulation and may represent a novel target for anti-cancer drug development.

Role of mammalian Rad54 in telomere length maintenance.

The homologous recombination (HR) DNA repair pathway participates in telomere length maintenance in yeast but its putative role at mammalian telomeres is unknown. Mammalian Rad54 is part of the HR machinery, and Rad54-deficient mice show a reduced HR capability. Here, we show that Rad54-deficient mice also show significantly shorter telomeres than wild-type controls, indicating that Rad54 activity plays an essential role in telomere length maintenance in mammals. Rad54 deficiency also resulted in an increased frequency of end-to-end chromosome fusions involving telomeres compared to the controls, suggesting a putative role of Rad54 in telomere capping. Finally, the study of mice doubly deficient for Rad54 and DNA-PKcs showed that telomere fusions due to DNA-PKcs deficiency were not rescued in the absence of Rad54, suggesting that they are not mediated by Rad54 activity.

Functional interaction between DNA-PKcs and telomerase in telomere length maintenance.

DNA-PKcs is the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) complex that functions in the non-homologous end-joining of double-strand breaks, and it has been shown previously to have a role in telomere capping. In particular, DNA-PKcs deficiency leads to chromosome fusions involving telomeres produced by leading-strand synthesis. Here, by generating mice doubly deficient in DNA-PKcs and telomerase (Terc(-/-)/DNA-PKcs(-/-)), we demonstrate that DNA-PKcs also has a fundamental role in telomere length maintenance. In particular, Terc(-/-)/DNA-PKcs(-/-) mice displayed an accelerated rate of telomere shortening when compared with Terc(-/-) controls, suggesting a functional interaction between both activities in maintaining telomere length. In addition, we also provide direct demonstration that DNA-PKcs is essential for both end-to-end fusions and apoptosis triggered by critically short telomeres. Our data predict that, in telomerase-deficient cells, i.e. human somatic cells, DNA-PKcs abrogation may lead to a faster rate of telomere degradation and cell cycle arrest in the absence of increased apoptosis and/or fusion of telomere-exhausted chromosomes. These results suggest a critical role of DNA-PKcs in both cancer and aging.