How telomerase reaches its end: mechanism of telomerase regulation by the telomeric complex

Ustilago maydis Trf2 ensures genome stability by antagonizing Blm-mediated telomere recombination: Fine-tuning DNA repair factor activity at telomeres through opposing regulations

TRF2 is an essential and conserved double-strand telomere binding protein that stabilizes chromosome ends by suppressing DNA damage response and aberrant DNA repair. Herein we investigated the mechanisms and functions of the Trf2 ortholog in the basidiomycete fungus Ustilago maydis, which manifests strong resemblances to metazoans with regards to the telomere and DNA repair machinery. We showed that UmTrf2 binds to Blm in vitro and inhibits Blm-mediated unwinding of telomeric DNA substrates. Consistent with a similar inhibitory activity in vivo, over-expression of Trf2 induces telomere shortening, just like deletion of blm, which is required for efficient telomere replication. While the loss of Trf2 engenders growth arrest and multiple telomere aberrations, these defects are fully suppressed by the concurrent deletion of blm or mre11 (but not other DNA repair factors). Over-expression of Blm alone triggers aberrant telomere recombination and the accumulation of aberrant telomere structures, which are blocked by concurrent Trf2 over-expression. Together, these findings highlight the suppression of Blm as a key protective mechanism of Trf2. Notably, U. maydis harbors another double-strand telomere-binding protein (Tay1), which promotes Blm activity to ensure efficient replication. We found that deletion of tay1 partially suppresses the telomere aberration of Trf2-depleted cells. Our results thus point to opposing regulation of Blm helicase by telomere proteins as a strategy for optimizing both telomere maintenance and protection. We also show that aberrant transcription of both telomere G- and C-strand is a recurrent phenotype of telomere mutants, underscoring another potential similarity between double strand breaks and de-protected telomeres.

Structural and functional insights into yeast Tbf1 as an atypical telomeric repeat-binding factor

Telomeric repeat-binding factor 1 (Tbf1) has a similar architecture as the TRF family of telomeric proteins and plays important roles in both telomere homeostasis and ribosome regulation. However, the molecular basis of why Tbf1 has such different functions compared to other TRFs remains unclear. Here, we present the crystal structures of the TRF homology (TRFH) and Myb-L domains from Schizosaccharomyces pombe Tbf1 (spTbf1). TRFH-mediated homodimerization is essential for spTbf1 stability. Importantly, spTbf1TRFH lacks the conserved docking motif for interactions with telomeric proteins, explaining why spTbf1 does not participate in the assembly of the shelterin complex. Finally, structural and biochemical analyses demonstrate that TRFH and Myb-L domains as well as the loop region of spTbf1 coordinate to recognize S. pombe telomeric double-stranded DNA. Overall, our findings provide structural and functional insights into how fungi Tbf1 acts as an atypical telomeric repeat-binding factor, which helps to understand the evolution of TRFH-containing telomeric proteins.

The cell cycle revisited: DNA replication past S phase preserves genome integrity

Accurate and complete DNA duplication is critical for maintaining genome integrity. Multiple mechanisms regulate when and where DNA replication takes place, to ensure that the entire genome is duplicated once and only once per cell cycle. Although the bulk of the genome is copied during the S phase of the cell cycle, increasing evidence suggests that parts of the genome are replicated in G2 or mitosis, in a last attempt to secure that daughter cells inherit an accurate copy of parental DNA. Remaining unreplicated gaps may be passed down to progeny and replicated in the next G1 or S phase. These findings challenge the long-established view that genome duplication occurs strictly during the S phase, bridging DNA replication to DNA repair and providing novel therapeutic strategies for cancer treatment.

Chromosome territory reorganization through artificial chromosome fusion is compatible with cell fate determination and mouse development

Chromosomes occupy discrete spaces in the interphase cell nucleus, called chromosome territory. The structural and functional relevance of chromosome territory remains elusive. We fused chromosome 15 and 17 in mouse haploid embryonic stem cells (haESCs), resulting in distinct changes of territories in the cognate chromosomes, but with little effect on gene expression, pluripotency and gamete functions of haESCs. The karyotype-engineered haESCs were successfully implemented in generating heterozygous (2n = 39) and homozygous (2n = 38) mouse models. Mice containing the fusion chromosome are fertile, and their representative tissues and organs display no phenotypic abnormalities, suggesting unscathed development. These results indicate that the mammalian chromosome architectures are highly resilient, and reorganization of chromosome territories can be readily tolerated during cell differentiation and mouse development.

Single-chromosome fission yeast models reveal the configuration robustness of a functional genome

In eukaryotic organisms, genetic information is usually carried on multiple chromosomes. Whether and how the number and configuration of chromosomes affect organismal fitness and speciation remain unclear. Here, we have successfully established several single-chromosome fission yeast Schizosaccharomyces pombe strains, in which the three natural chromosomes have been fused into one giant chromosome in different orders. Chromosome fusions accompanied by the deletions of telomeres and centromeres result in the loss of chromosomal interactions and a drastic change of global chromosome organization, but alter gene expression marginally. The single-chromosome strains display little defects in cell morphology, mitosis, genotoxin sensitivity, and meiosis. Crosses between a wild-type strain and a single-chromosome strain or between two single-chromosome strains with different fusion orders suffer defective meiosis and poor spore viability. We conclude that eukaryotic genomes are robust against dramatic chromosomal reconfiguration, and stochastic changes in chromosome number and genome organization during evolution underlie reproductive isolation and speciation.

Structural insights into Pot1-ssDNA, Pot1-Tpz1 and Tpz1-Ccq1 Interactions within fission yeast shelterin complex

The conserved shelterin complex caps chromosome ends to protect telomeres and regulate telomere replication. In fission yeast Schizosaccharomyces pombe, shelterin consists of telomeric single- and double-stranded DNA-binding modules Pot1-Tpz1 and Taz1-Rap1 connected by Poz1, and a specific component Ccq1. While individual structures of the two DNA-binding OB folds of Pot1 (Pot1_OB1-GGTTAC and Pot1_OB2-GGTTACGGT) are available, structural insight into recognition of telomeric repeats with spacers by the complete DNA-binding domain (Pot1_DBD) remains an open question. Moreover, structural information about the Tpz1-Ccq1 interaction requires to be revealed for understanding how the specific component Ccq1 of S. pombe shelterin is recruited to telomeres to function as an interacting hub. Here, we report the crystal structures of Pot1_DBD-single-stranded-DNA, Pot1_372-555-Tpz1_185-212 and Tpz1_425-470-Ccq1_123-439 complexes and propose an integrated model depicting the assembly mechanism of the shelterin complex at telomeres. The structure of Pot1_DBD-DNA unveils how Pot1 recognizes S. pombe degenerate telomeric sequences. Our analyses of Tpz1-Ccq1 reveal structural basis for the essential role of the Tpz1-Ccq1 interaction in telomere recruitment of Ccq1 that is required for telomere maintenance and telomeric heterochromatin formation. Overall, our findings provide valuable structural information regarding interactions within fission yeast shelterin complex at 3’ ss telomeric overhang.

Telomeres, composed of repetitive DNA sequences and specialized proteins, are protective structures at the ends of linear chromosomes. The telomere structure is essential for the maintenance of genome integrity and stability, and telomere dysfunction has been linked to human development, aging, cancer and a variety of degenerative diseases. An evolutionarily conserved multiple-protein complex called shelterin plays versatile roles in telomere homeostasis regulation, end protection and heterochromatin establishment. However, the highly flexible nature of shelterin complex has greatly impeded our structural and functional understanding for this important complex. In fission yeast, structures of the shelterin dsDNA-binding protein subcomplex Taz1-Rap1 and the bridge subcomplex Tpz1-Poz1-Rap1 are available. Although individual OB-fold subdomains structures have been characterized, structural information about the complete Pot1_DBD bound to telomeric repeats with spacers remains to be revealed. Here, by determining the crystal structures of the telomeric overhang binding Pot1_DBD-ssDNA, Pot1_372-555-Tpz1_185-212 and Tpz1_425-470-Ccq1_123-439 subcomplexes, we provide structural basis not only for the recognition of S. pombe degenerate telomeric sequences by Pot1, but also for the essential function of the Tpz1-Ccq1 interaction in Ccq1 recruitment to telomeres for telomere maintenance and telomeric heterochromatin formation. These findings provide an integrated model depicting the assembly mechanism of the shelterin complex at telomeres and its multiple roles in telomere biology.

Cumulative Evidence for Relationships Between Multiple Variants in the TERT and CLPTM1L Region and Risk of Cancer and Non-Cancer Disease

BACKGROUND

Genetic studies previously reported that variants in TERT-CLPTM1L genes were related to susceptibility of cancer and non-cancer diseases. However, conclusions were not always concordant.

METHODS

We performed meta-analyses to assess correlations between 23 variants within TERT-CLPTM1L region and susceptibility to 12 cancers and 1 non-cancer disease based on data in 109 papers (involving 139,510 cases and 208,530 controls). Two approaches (false-positive report probability test and Venice criteria) were adopted for assessing the cumulative evidence of significant associations. Current study evaluated the potential role of these variants based on data in Encyclopedia of DNA Elements (ENCODE) Project.

RESULTS

Thirteen variants were statistically associated with susceptibility to 11 cancers and 1 non-cancer disease (p < 0.05). Besides, 12 variants with eight cancers and one non-cancer disease were rated as strong evidence (rs2736098, rs401681, and rs402710 in bladder cancer; rs2736100, rs2853691, and rs401681 in esophageal cancer; rs10069690 in gastric cancer; rs2736100 and rs2853676 in glioma; rs2242652, rs2736098, rs2736100, rs2853677, rs31489, rs401681, rs402710, rs465498, and rs4975616 in lung cancer; rs2736100 in idiopathic pulmonary fibrosis and myeloproliferative neoplasms; and rs401681 in pancreatic and skin cancer). According to data from ENCODE and other public databases, 12 variants with strong evidence might fall within putative functional regions.

CONCLUSIONS

This paper demonstrated that common variants of TERT-CLPTM1L genes were related to susceptibility to bladder, esophageal, gastric, lung, pancreatic, and skin cancer, as well as to glioma, myeloproliferative neoplasms, and idiopathic pulmonary fibrosis, and, besides, the crucial function of the TERT-CLPTM1L region in the genetic predisposition to human diseases is elucidated.

Chromosome-specific telomere lengths and the minimal functional telomere revealed by nanopore sequencing

We developed a method to tag telomeres and measure telomere length by nanopore sequencing in the yeast S. cerevisiae Nanopore allows long-read sequencing through the telomere, through the subtelomere, and into unique chromosomal sequence, enabling assignment of telomere length to a specific chromosome end. We observed chromosome end-specific telomere lengths that were stable over 120 cell divisions. These stable chromosome-specific telomere lengths may be explained by slow clonal variation or may represent a new biological mechanism that maintains equilibrium unique to each chromosome end. We examined the role of RIF1 and TEL1 in telomere length regulation and found that TEL1 is epistatic to RIF1 at most telomeres, consistent with the literature. However, at telomeres that lack subtelomeric Y' sequences, tel1Δ rif1Δ double mutants had a very small, but significant, increase in telomere length compared with the tel1Δ single mutant, suggesting an influence of Y' elements on telomere length regulation. We sequenced telomeres in a telomerase-null mutant (est2Δ) and found the minimal telomere length to be ∼75 bp. In these est2Δ mutants, there were apparent telomere recombination events at individual telomeres before the generation of survivors, and these events were significantly reduced in est2Δ rad52Δ double mutants. The rate of telomere shortening in the absence of telomerase was similar across all chromosome ends at ∼5 bp per generation. This new method gives quantitative, high-resolution telomere length measurement at each individual chromosome end and suggests possible new biological mechanisms regulating telomere length.

Understanding the Impact of Industrial Stress Conditions on Replicative Aging in Saccharomyces cerevisiae

In yeast, aging is widely understood as the decline of physiological function and the decreasing ability to adapt to environmental changes. Saccharomyces cerevisiae has become an important model organism for the investigation of these processes. Yeast is used in industrial processes (beer and wine production), and several stress conditions can influence its intracellular aging processes. The aim of this review is to summarize the current knowledge on applied stress conditions, such as osmotic pressure, primary metabolites (e.g., ethanol), low pH, oxidative stress, heat on aging indicators, age-related physiological changes, and yeast longevity. There is clear evidence that yeast cells are exposed to many stressors influencing viability and vitality, leading to an age-related shift in age distribution. Currently, there is a lack of rapid, non-invasive methods allowing the investigation of aspects of yeast aging in real time on a single-cell basis using the high-throughput approach. Methods such as micromanipulation, centrifugal elutriator, or biotinylation do not provide real-time information on age distributions in industrial processes. In contrast, innovative approaches, such as non-invasive fluorescence coupled flow cytometry intended for high-throughput measurements, could be promising for determining the replicative age of yeast cells in fermentation and its impact on industrial stress conditions.

Cdc13 is predominant over Stn1 and Ten1 in preventing chromosome end fusions

Telomeres define the natural ends of eukaryotic chromosomes and are crucial for chromosomal stability. The budding yeast Cdc13, Stn1 and Ten1 proteins form a heterotrimeric complex, and the inactivation of any of its subunits leads to a uniformly lethal phenotype due to telomere deprotection. Although Cdc13, Stn1 and Ten1 seem to belong to an epistasis group, it remains unclear whether they function differently in telomere protection. Here, we employed the single-linear-chromosome yeast SY14, and surprisingly found that the deletion of CDC13 leads to telomere erosion and intrachromosome end-to-end fusion, which depends on Rad52 but not Yku. Interestingly, the emergence frequency of survivors in the SY14 cdc13Δ mutant was ~29 fold higher than that in either the stn1Δ or ten1Δ mutant, demonstrating a predominant role of Cdc13 in inhibiting telomere fusion. Chromosomal fusion readily occurred in the telomerase-null SY14 strain, further verifying the default role of intact telomeres in inhibiting chromosome fusion.

Comparative Cytogenetic Mapping and Telomere Analysis Provide Evolutionary Predictions for Devil Facial Tumour 2

The emergence of a second transmissible tumour in the Tasmanian devil population, devil facial tumour 2 (DFT2), has prompted questions on the origin and evolution of these transmissible tumours. We used a combination of cytogenetic mapping and telomere length measurements to predict the evolutionary trajectory of chromosome rearrangements in DFT2. Gene mapping by fluorescence in situ hybridization (FISH) provided insight into the chromosome rearrangements in DFT2 and identified the evolution of two distinct DFT2 lineages. A comparison of devil facial tumour 1 (DFT1) and DFT2 chromosome rearrangements indicated that both started with the fusion of a chromosome, with potentially critically short telomeres, to chromosome 1 to form dicentric chromosomes. In DFT1, the dicentric chromosome resulted in breakage–fusion–bridge cycles leading to highly rearranged chromosomes. In contrast, the silencing of a centromere on the dicentric chromosome in DFT2 stabilized the chromosome, resulting in a less rearranged karyotype than DFT1. DFT2 retains a bimodal distribution of telomere length dimorphism observed on Tasmanian devil chromosomes, a feature lost in DFT1. Using long term cell culture, we observed homogenization of telomere length over time. We predict a similar homogenization of telomere lengths occurred in DFT1, and that DFT2 is unlikely to undergo further substantial rearrangements due to maintained telomere length.

Establishment and characterization of an immortalized human chondrocyte cell line

OBJECTIVES

Following a specific number of mitotic divisions, primary chondrocytes undergo proliferative senescence, thwarting efforts to expand sufficient populations in vitro suitable to meet the needs of scientific research or medical therapies. Therefore, the human telomerase reverse transcriptase (TERT) was used to immortalize human chondrocyte and establish a cell line that escape from cellular senescence.

RESULTS

The human chondrocytes were successfully immortalized by ectopic stable expression of TERT. The established TERT-Chondrocyte cell line showed robust proliferation capacity, even in late passages up to P20, and displayed little cellular senescence. Moreover, TERT-Chondrocyte cells at 20th passage showed similar chondrocyte properties to normal chondrocytes at early passages.

CONCLUSIONS

Ectopic stable expression of TERT is an effective way to immortalized human chondrocyte. The immortalized chondrocytes displayed little cellular senescence, showed promise as an in vitro model to investigate osteoarthritis, and may be a promising resource for cell-based therapy for damaged cartilage.

Casein kinase 2 regulates telomere protein complex formation through Rap1 phosphorylation

Telomeres located at the ends of linear chromosomes play important roles in the maintenance of life. Rap1, a component of the shelterin telomere protein complex, interacts with multiple proteins to perform various functions; further, formation of shelterin requires Rap1 binding to other components such as Taz1 and Poz1, and telomere tethering to the nuclear envelope (NE) involves interactions between Rap1 and Bqt4, a nuclear membrane protein. Although Rap1 is a hub for telomere protein complexes, the regulatory mechanisms of its interactions with partner proteins are not fully understood. Here, we show that Rap1 is phosphorylated by casein kinase 2 (CK2) at multiple sites, which promotes interactions with Bqt4 and Poz1. Among the multiple CK2-mediated phosphorylation sites of Rap1, phosphorylation at Ser496 was found to be crucial for both Rap1–Bqt4 and Rap1–Poz1 interactions. These mechanisms mediate proper telomere tethering to the NE and the formation of the silenced chromatin structure at chromosome ends.

The telomere-binding protein Rif2 and ATP-bound Rad50 have opposing roles in the activation of yeast Tel1ATM kinase

Saccharomyces cerevisiae Tel1 is the ortholog of human ATM kinase and initiates a cell cycle checkpoint in response to dsDNA breaks (DSBs). Tel1^ATM kinase is activated synergistically by naked dsDNA and the Mre11-Rad50-Xrs2^NBS1 complex (MRX). A multisubunit protein complex, which is related to human shelterin, protects telomeres from being recognized as DSBs, thereby preventing a Tel1^ATM checkpoint response. However, at very short telomeres, Tel1^ATM can be recruited and activated by the MRX complex, resulting in telomere elongation. Conversely, at long telomeres, Rap1-interacting-factor 2 (Rif2) is instrumental in suppressing Tel1 activity. Here, using an in vitro reconstituted Tel1 kinase activation assay, we show that Rif2 inhibits MRX-dependent Tel1 kinase activity. Rif2 discharges the ATP-bound form of Rad50, which is essential for all MRX-dependent activities. This conclusion is further strengthened by experiments with a Rad50 allosteric ATPase mutant that maps outside the conserved ATP binding pocket. We propose a model in which Rif2 attenuates Tel1 activity at telomeres by acting directly on Rad50 and discharging its activated ATP-bound state, thereby rendering the MRX complex incompetent for Tel1 activation. These findings expand our understanding of the mechanism by which Rif2 controls telomere length.

Catalysis-dependent inactivation of human telomerase and its reactivation by intracellular telomerase-activating factors (iTAFs)

Human telomerase maintains genome stability by adding telomeric repeats to the ends of linear chromosomes. Although previous studies have revealed profound insights into telomerase functions, the low cellular abundance of functional telomerase and the difficulties in quantifying its activity leave its thermodynamic and kinetic properties only partially characterized. Employing a stable cell line overexpressing both the human telomerase RNA component and the N-terminally biotinylated human telomerase reverse transcriptase and using a newly developed method to count individual extension products, we demonstrate here that human telomerase holoenzymes contain fast- and slow-acting catalytic sites. Surprisingly, both active sites became inactive after two consecutive rounds of catalysis, named single-run catalysis. The fast active sites turned off ∼40-fold quicker than the slow ones and exhibited higher affinities to DNA substrates. In a dimeric enzyme, the two active sites work in tandem, with the faster site functioning before the slower one, and in the monomeric enzyme, the active sites also perform single-run catalysis. Interestingly, inactive enzymes could be reactivated by intracellular telomerase-activating factors (iTAFs) from multiple cell types. We conclude that the single-run catalysis and the iTAF-triggered reactivation serve as an unprecedented control circuit for dynamic regulation of telomerase. They endow native telomerase holoenzymes with the ability to match their total number of active sites to the number of telomeres they extend. We propose that the exquisite kinetic control of telomerase activity may play important roles in both cell division and cell aging.

Rad6-Bre1 mediated histone H2Bub1 protects uncapped telomeres from exonuclease Exo1 in Saccharomyces cerevisiae

Histone H2B lysine 123 mono-ubiquitination (H2Bub1), catalyzed by Rad6 and Bre1 in Saccharomyces cerevisiae, modulates chromatin structure and affects diverse cellular functions. H2Bub1 plays roles in telomeric silencing and telomere replication. Here, we have explored a novel role of H2Bub1 in telomere protection at uncapped telomeres in yku70Δ and cdc13-1 cells. Deletion of RAD6 or BRE1, or mutation of H2BK123R enhances the temperature sensitivity of both yku70Δ and cdc13-1 telomere capping mutants. Consistently, BRE1 deletion increases accumulation of telomeric single-stranded DNA (ssDNA) in yku70Δ and cdc13-1 cells, and EXO1 deletion improves the growth of yku70Δ bre1Δ and cdc13-1 bre1Δ cells and decreases ssDNA accumulation. Additionally, deletion of BRE1 exacerbates the rate of entry into senescence of yku70Δ mre11Δ cells with telomere defects, and increases the recombination of subtelomeric Y' element that is required for telomere maintenance and survivor generation. Furthermore, Exo1 contributes to the abrupt senescence of yku70Δ mre11Δ bre1Δ cells, and Rad51 is essential for Y' recombination to generate survivors. Finally, deletion of BRE1 or mutation of H2BK123R results in nucleosome instability at subtelomeric regions. Collectively, this study provides a mechanistic link between H2Bub1-mediated chromatin structure and telomere protection after telomere uncapping.

Association between TERT rs2853669 polymorphism and cancer risk: A meta-analysis of 9,157 cases and 11,073 controls

Background

It has been reported that the functional telomerase reverse transcriptase (TERT) rs2853669 polymorphism might contribute to different types of human cancer. However, the association of this mutation with cancer remains controversial. Here, we conducted a meta-analysis to characterize this relationship.

Materials and methods/Main results

A systematic search of studies on the association of TERT rs2853669 polymorphism with all types of cancer was conducted in PubMed, Embase and Cochrane Library. The summary odds ratios (ORs) and corresponding 95% confidence intervals (95% CIs) were used to pool the effect size in a fixed-effects model or a random-effects model where appropriate. A total of 13 articles and 15 case-control studies, including 9,157 cases and 11,073 controls, were included in this meta-analysis. Overall, the pooled results indicated that the rs2853669 polymorphism was significantly associated with increased cancer risk in a homozygote comparison model (CT vs. TT: OR = 1.085, 95% CI: 1.015–1.159, P = 0.016). In the stratified analyses, a significant increased cancer risk was observed in Asian, but not Caucasian patients. A subgroup analysis by cancer type also revealed a significant increase in the risk of lung cancer, but not breast cancer.

Conclusions

The results of this meta-analysis suggest that the TERT rs2853669 polymorphism is associated with a significantly increased risk of cancer, particularly lung cancer, in Asian populations.

Replication Protein A-1 Has a Preference for the Telomeric G-rich Sequence in Trypanosoma cruzi

Replication protein A (RPA), the major eukaryotic single-stranded binding protein, is a heterotrimeric complex formed by RPA-1, RPA-2, and RPA-3. RPA is a fundamental player in replication, repair, recombination, and checkpoint signaling. In addition, increasing evidences have been adding functions to RPA in telomere maintenance, such as interaction with telomerase to facilitate its activity and also involvement in telomere capping in some conditions. Trypanosoma cruzi, the etiological agent of Chagas disease is a protozoa parasite that appears early in the evolution of eukaryotes. Recently, we have showed that T. cruziRPA presents canonical functions being involved with DNA replication and DNA damage response. Here, we found by FISH/IF assays that T. cruziRPA localizes at telomeres even outside replication (S) phase. In vitro analysis showed that one telomeric repeat is sufficient to bind RPA-1. Telomeric DNA induces different secondary structural modifications on RPA-1 in comparison with other types of DNA. In addition, RPA-1 presents a higher affinity for telomeric sequence compared to randomic sequence, suggesting that RPA may play specific roles in T. cruzi telomeric region.

Moderate-to-severe obstructive sleep apnea is associated with telomere lengthening

Obstructive sleep apnea (OSA) is associated with cardiometabolic diseases. Telomere shortening is linked to hypertension, diabetes mellitus, and cardiovascular diseases. Because these conditions are highly prevalent in OSA, we hypothesized that telomere length (TL) would be reduced in OSA patients. We identified 106 OSA and 104 non-OSA subjects who underwent polysomnography evaluation. Quantitative PCR was used to measure telomere length in genomic DNA isolated from peripheral blood samples. The association between OSA and TL was determined using unadjusted and adjusted linear models. There was no difference in TL between the OSA and non-OSA (control) group. However, we observed a J-shaped relationship between TL and OSA severity: the longest TL in moderate-to-severe OSA [4,918 ± 230 (SD) bp] and the shortest TL in mild OSA (4,735 ± 145 bp). Mean TL in moderate-to-severe OSA was significantly longer than in the control group after adjustment for age, sex, body mass index, hypertension, dyslipidemia, and depression (β = 96.0, 95% confidence interval: 15.4-176.6, P = 0.020). In conclusion, moderate-to-severe OSA is associated with telomere lengthening. Our findings support the idea that changes in TL are not unidirectional processes, such that telomere shortening occurs with age and disease but may be prolonged in moderate-to-severe OSA.NEW & NOTEWORTHY Here, we show that moderate-to-severe obstructive sleep apnea is associated with longer telomeres, independent of age and cardiovascular risk factors, challenging the hypothesis that telomere shortening is a unidirectional process related to age/disease. A better understanding of the mechanisms underlying telomere dynamics may identify targets for therapeutic intervention in cardiovascular aging/other chronic diseases.

Rad6-Bre1-mediated H2B ubiquitination regulates telomere replication by promoting telomere-end resection

Rad6 and Bre1, ubiquitin-conjugating E2 and E3 enzymes respectively, are responsible for histone H2B lysine 123 mono-ubiquitination (H2Bub1) in Saccharomyces cerevisiae. Previous studies have shown that Rad6 and Bre1 regulate telomere length and recombination. However, the underlying molecular mechanism remains largely unknown. Here we report that H2BK123 mutation results in telomere shortening, while inactivation of Ubp8 and/or Ubp10, deubiquitinases of H2Bub1, leads to telomere lengthening in Rad6–Bre1-dependent manner. In telomerase-deficient cells, inactivation of Rad6–Bre1 pathway retards telomere shortening rate and the onset of senescence, while deletion of UBP8 and/or UBP10 accelerates senescence. Thus, Rad6–Bre1 pathway regulates both telomere length and recombination through its role in H2Bub1. Additionally, inactivation of both Rad6–Bre1–H2Bub1 and Mre11–Rad50–Xrs2 (MRX) pathways causes synthetic growth defects and telomere shortening in telomerase-proficient cells, and significantly accelerates senescence and eliminates type II telomere recombination in telomerase-deficient cells. Furthermore, RAD6 or BRE1 deletion, or H2BK123R mutation decreases the accumulation of ssDNA at telomere ends. These results support the model that Rad6–Bre1–H2Bub1 cooperates with MRX to promote telomere-end resection and thus positively regulates both telomerase- and recombination-dependent telomere replication. This study provides a mechanistic link between histone H2B ubiquitination and telomere replication.

Cell cycle-dependent spatial segregation of telomerase from sites of DNA damage

Telomerase can generate a novel telomere at a DNA break, with potentially lethal consequences for the cell. Ouenzar et al. reveal novel roles for Pif1, Rad52, and Siz1-dependent sumoylation in the spatial exclusion of telomerase from sites of DNA repair during the cell cycle.

Telomerase can generate a novel telomere at DNA double-strand breaks (DSBs), an event called de novo telomere addition. How this activity is suppressed remains unclear. Combining single-molecule imaging and deep sequencing, we show that the budding yeast telomerase RNA (TLC1 RNA) is spatially segregated to the nucleolus and excluded from sites of DNA repair in a cell cycle–dependent manner. Although TLC1 RNA accumulates in the nucleoplasm in G1/S, Pif1 activity promotes TLC1 RNA localization in the nucleolus in G2/M. In the presence of DSBs, TLC1 RNA remains nucleolar in most G2/M cells but accumulates in the nucleoplasm and colocalizes with DSBs in rad52Δ cells, leading to de novo telomere additions. Nucleoplasmic accumulation of TLC1 RNA depends on Cdc13 localization at DSBs and on the SUMO ligase Siz1, which is required for de novo telomere addition in rad52Δ cells. This study reveals novel roles for Pif1, Rad52, and Siz1-dependent sumoylation in the spatial exclusion of telomerase from sites of DNA repair.

Repair of Oxidative DNA Damage in Saccharomyces cerevisiae

Malfunction of enzymes that detoxify reactive oxygen species leads to oxidative attack on biomolecules including DNA and consequently activates various DNA repair pathways. The nature of DNA damage and the cell cycle stage at which DNA damage occurs determine the appropriate repair pathway to rectify the damage. Oxidized DNA bases are primarily repaired by base excision repair and nucleotide incision repair. Nucleotide excision repair acts on lesions that distort DNA helix, mismatch repair on mispaired bases, and homologous recombination and non-homologous end joining on double stranded breaks. Post-replication repair that overcomes replication blocks caused by DNA damage also plays a crucial role in protecting the cell from the deleterious effects of oxidative DNA damage. Mitochondrial DNA is also prone to oxidative damage and is efficiently repaired by the cellular DNA repair machinery. In this review, we discuss the DNA repair pathways in relation to the nature of oxidative DNA damage in Saccharomyces cerevisiae.

Hsp90-binding immunophilin FKBP52 modulates telomerase activity by promoting the cytoplasmic retrotransport of hTERT

Telomerase is a unique ribonucleoprotein enzyme that is required for continued cell proliferation. To generate catalytically active telomerase, human telomerase reverse transcriptase (hTERT) must translocate to the nucleus and assemble with the RNA component of telomerase. The molecular chaperones heat shock protein 90 (Hsp90) and p23 maintain hTERT in a conformation that enables nuclear translocation. However, the regulatory role of chaperones in nuclear transport of hTERT remains unclear. In this work, we demonstrate that immunophilin FK506-binding protein (FKBP)52 linked the hTERT–Hsp90 complex to the dynein–dynactin motor, thereby promoting the transport of hTERT to the nucleus along microtubules. FKBP52 interacted with the hTERT–Hsp90 complex through binding of the tetratricopeptide repeat domain to Hsp90 and binding of the dynamitin (Dyt) component of the dynein-associated dynactin complex to the peptidyl prolyl isomerase domain. The depletion of FKBP52 inhibited nuclear transport of hTERT, resulting in cytoplasmic accumulation. Cytoplasmic hTERT was rapidly degraded through ubiquitin (Ub)-dependent proteolysis, thereby abrogating telomerase activity. In addition, overexpression of dynamitin, which is known to dissociate the dynein–dynactin motor from its cargoes, reduced telomerase activity. Collectively, these results provide a molecular mechanism by which FKBP52 modulates telomerase activity by promoting dynein–dynactin-dependent nuclear import of hTERT.

Telomere recombination pathways: tales of several unhappy marriages

All happy families are alike; each unhappy family is unhappy in its own way.-Leo Tolstoy, Anna Karenina.

Replication Timing of Human Telomeres is Conserved during Immortalization and Influenced by Respective Subtelomeres

Telomeres are specific structures that protect chromosome ends and act as a biological clock, preventing normal cells from replicating indefinitely. Mammalian telomeres are replicated throughout S-phase in a predetermined order. However, the mechanism of this regulation is still unknown. We wished to investigate this phenomenon under physiological conditions in a changing environment, such as the immortalization process to better understand the mechanism for its control. We thus examined the timing of human telomere replication in normal and SV40 immortalized cells, which are cytogenetically very similar to cancer cells. We found that the timing of telomere replication was globally conserved under different conditions during the immortalization process. The timing of telomere replication was conserved despite changes in telomere length due to endogenous telomerase reactivation, in duplicated homologous chromosomes, and in rearranged chromosomes. Importantly, translocated telomeres, possessing their initial subtelomere, retained the replication timing of their homolog, independently of the proportion of the translocated arm, even when the remaining flanking DNA is restricted to its subtelomere, the closest chromosome-specific sequences (inferior to 500 kb). Our observations support the notion that subtelomere regions strongly influence the replication timing of the associated telomere.

Multi-step coordination of telomerase recruitment in fission yeast through two coupled telomere-telomerase interfaces

Tightly controlled recruitment of telomerase, a low-abundance enzyme, to telomeres is essential for regulated telomere synthesis. Recent studies in human cells revealed that a patch of amino acids in the shelterin component TPP1, called the TEL-patch, is essential for recruiting telomerase to telomeres. However, how TEL-patch—telomerase interaction integrates into the overall orchestration of telomerase regulation at telomeres is unclear. In fission yeast, Tel1^ATM/Rad3^ATR-mediated phosphorylation of shelterin component Ccq1 during late S phase is involved in telomerase recruitment through promoting the binding of Ccq1 to a telomerase accessory protein Est1. Here, we identify the TEL-patch in Tpz1^TPP1, mutations of which lead to decreased telomeric association of telomerase, similar to the phosphorylation-defective Ccq1. Furthermore, we find that telomerase action at telomeres requires formation and resolution of an intermediate state, in which the cell cycle-dependent Ccq1-Est1 interaction is coupled to the TEL-patch—Trt1 interaction, to achieve temporally regulated telomerase elongation of telomeres.

DOI:http://dx.doi.org/10.7554/eLife.15470.001

The genetic blueprints for animals, plants and fungi are mostly contained within long strands of DNA and packaged into more compact thread-like structures called chromosomes. As such, most cells need to duplicate their chromosomes before they divide so that the two new cells each get a complete set of genetic instructions.

The machinery that copies DNA is unable to make it to the very ends of the chromosomes. Instead, an enzyme called telomerase adds new DNA to the chromosome ends to prevent them becoming too short. Problems with this process can cause serious issues, such as cell death or cancer, and so the activity of telomerase is carefully controlled. Other proteins guide telomerase to the ends of the chromosome only after the rest of the DNA has been copied. However, scientists do not know exactly how cells correctly time the arrival of telomerase.

A group of proteins called shelterin protects the chromosome ends, and studies with human cells have shown that telomerase attaches to a specific patch on one of shelterin proteins, called the TEL-patch, to begin its work. Now, Hu, Liu et al. have identified a similar TEL-patch in a shelterin protein from a type of yeast called fission yeast; this patch is also needed to attach telomerase to the chromosome ends. Further experiments with this yeast then showed that telomerase only arrives at the ends of the chromosomes after two parallel interaction interfaces have formed. Importantly, one of these interactions only takes place after most of the chromosomes are have been copied. As such, this “two-pronged interaction” mechanism ensures that the telomerase enzyme arrives at the end of the chromosomes at the right time.

Other similarities between human and fission yeast chromosome ends make it plausible that a comparable process controls the timing of telomerase attachment in human cells. However, more studies will be needed to confirm if this is the case.

DOI:http://dx.doi.org/10.7554/eLife.15470.002

Preferential extension of short telomeres induced by low extracellular pH

The majority of tumor cells overcome proliferative limit by expressing telomerase. Whether or not telomerase preferentially extends the shortest telomeres is still under debate. When human cancer cells are cultured at neutral pH, telomerase extends telomeres in telomere length-independent manner. However, the microenvironment of tumor is slightly acidic, and it is not yet known how this influences telomerase action. Here, we examine telomere length homeostasis in tumor cells cultured at pHe 6.8. The results indicate that telomerase preferentially extends short telomeres, such that telomere length distribution narrows and telomeres become nearly uniform in size. After growth at pHe 6.8, the expression of telomerase, TRF1, TRF2 and TIN2 decreases, and the abundance of Cajal bodies decreases. Therefore, telomerase are insufficient for extending every telomere and shorter telomeres bearing less shelterin proteins are more accessible for telomerase recruitment. The findings support the ‘protein-counting mechanism’ in which extended and unextended state of telomere is determined by the number of associated shelterin proteins and the abundance of telomerase. Decreased expression of telomerase and preferential extension of short telomeres have important implications for tumor cell viability, and generate a strong rationale for research on telomerase-targeted anti-cancer therapeutics.

A novel method for banking stem cells from human exfoliated deciduous teeth: lentiviral TERT immortalization and phenotypical analysis

Background

Stem cells from human exfoliated deciduous teeth (SHED) have recently attracted attention as novel multipotential stem cell sources. However, their application is limited due to in vitro replicative senescence. Ectopic expression of telomerase reverse transcriptase (TERT) is a promising strategy for overcoming this replicative senescence. Nevertheless, its potential application and the phenotype as well as tumorigenicity have never been assessed in SHED.

Methods

TERT expression was stably restored in SHED (TERT-SHED) isolated from healthy children aged 6–8 years using lentiviral transduction with a puromycin selection marker. The expression of TERT was detected using reverse transcription polymerase chain reaction, Western blot and immunofluorescence. Surface markers of SHED were detected by flow cytometry. Enzyme-linked immunosorbent assay was used to assess senescence-associated β-galactosidase, while CCK-8 methods were used to examine the proliferation capacity of SHED and TERT-SHED at different passages. Moreover, multilineage differentiation, karyotype, colony formation in soft agar, and tumor formation in nude mice of SHED and TERT-SHED were also examined.

Results

Lentiviral transduction induced stable TERT expression even in SHED at the 40th passage. TERT-SHED showed robust proliferation capacity and low concentration of β-galactosidase. Although they had some different biomarkers than early passage SHED, TERT-SHED at late passage showed similar mutilineage differentiation as TERT at early passage. Moreover, TERT-SHED at late passage showed normal karyotype, no soft agar colony formation, and no tumor formation in nude mice.

Conclusions

TERT-immortalized SHED may be a promising resource for stem-cell therapy, although attention should be paid to the biological behavior of the cells.

Rif1: A Conserved Regulator of DNA Replication and Repair Hijacked by Telomeres in Yeasts

Rap1-interacting factor 1 (Rif1) was originally identified in the budding yeast Saccharomyces cerevisiae as a telomere-binding protein that negatively regulates telomerase-mediated telomere elongation. Although this function is conserved in the distantly related fission yeast Schizosaccharomyces pombe, recent studies, both in yeasts and in metazoans, reveal that Rif1 also functions more globally, both in the temporal control of DNA replication and in DNA repair. Rif1 proteins are large and characterized by N-terminal HEAT repeats, predicted to form an elongated alpha-helical structure. In addition, all Rif1 homologs contain two short motifs, abbreviated RVxF/SILK, that are implicated in recruitment of the PP1 (yeast Glc7) phosphatase. In yeasts the RVxF/SILK domains have been shown to play a role in control of DNA replication initiation, at least in part through targeted de-phosphorylation of proteins in the pre-Replication Complex. In human cells Rif1 is recruited to DNA double-strand breaks through an interaction with 53BP1 where it counteracts DNA resection, thus promoting repair by non-homologous end-joining. This function requires the N-terminal HEAT repeat-containing domain. Interestingly, this domain is also implicated in DNA end protection at un-capped telomeres in yeast. We conclude by discussing the deployment of Rif1 at telomeres in yeasts from both an evolutionary perspective and in light of its recently discovered global functions.

Telomerase activates transcription of cyclin D1 gene through an interaction with NOL1

Telomerase is a ribonucleoprotein enzyme that is required for the maintenance of telomere repeats. Although overexpression of telomerase in normal human somatic cells is sufficient to overcome replicative senescence, the ability of telomerase to promote tumorigenesis requires additional activities that are independent of its role in telomere extension. Here, we identify proliferation-associated nucleolar antigen 120 (NOL1, also known as NOP2) as a telomerase RNA component (TERC)-binding protein that is found in association with catalytically active telomerase. Although NOL1 is highly expressed in the majority of human tumor cells, the molecular mechanism by which NOL1 contributes to tumorigenesis remained unclear. We show that NOL1 binds to the T-cell factor (TCF)-binding element of the cyclin D1 promoter and activates its transcription. Interestingly, telomerase is also recruited to the cyclin D1 promoter in a TERC-dependent manner through the interaction with NOL1, further enhancing transcription of the cyclin D1 gene. Depletion of NOL1 suppresses cyclin D1 promoter activity, thereby leading to induction of growth arrest and altered cell cycle distributions. Collectively, our findings suggest that NOL1 represents a new route by which telomerase activates transcription of cyclin D1 gene, thus maintaining cell proliferation capacity.

M. PfGBP: una proteína de unión al telómero de Plasmodium falciparum

Los telómeros son estructuras complejas de ADN y proteína localizadas en el extremo de los cromosomas eucariotes. Su principal función es proteger el extremo cromosomal de ser reconocido y procesado como ADNs fracturado, evitando así eventos de recombinación y fusión que conducen a inestabilidad cromosomal. El ADN telomérico consta de secuencias cortas, repetidas una tras otra, ricas en guanina; la cadena rica en guanina se extiende formando una región de cadena sencilla denominada extremo 3´ protuberante. Las proteínas por su parte, se pueden clasificar en: dsBPs, o proteínas de unión a la cadena doble, GBPs aquellas que reconocen específicamente el extremo protuberante y, proteínas que las interconectan mediante interacciones proteína-proteína. El gen PF3D7_1006800 de <em>Plasmodium falciparum</em> codifica para una proteína putativa similar a una GBP de <em>Criptosporidium parvum</em>, con el fin de establecer si esta proteína de <em>P. falciparum</em> presenta la capacidad de unión al ADN telomérico del parásito, se produjo una proteína recombinante a partir de la región codificante del gen, se purificó y se utilizó en ensayos de unión a ADN, y en la generación de anticuerpos policlonales específicos contra PfGBP. Nuestros resultados indican que la proteína de <em>P. falciparum</em> es una proteína nuclear con capacidad de unión al ADN telomérico <em>in vitro, </em>por lo<em> </em>que podría ser<em> </em>parte del complejo proteico encargado de proteger y/o mantener el telómero <em>in vivo</em>.

Telomere- and Telomerase-Associated Proteins and Their Functions in the Plant Cell

Telomeres, as physical ends of linear chromosomes, are targets of a number of specific proteins, including primarily telomerase reverse transcriptase. Access of proteins to the telomere may be affected by a number of diverse factors, e.g., protein interaction partners, local DNA or chromatin structures, subcellular localization/trafficking, or simply protein modification. Knowledge of composition of the functional nucleoprotein complex of plant telomeres is only fragmentary. Moreover, the plant telomeric repeat binding proteins that were characterized recently appear to also be involved in non-telomeric processes, e.g., ribosome biogenesis. This interesting finding was not totally unexpected since non-telomeric functions of yeast or animal telomeric proteins, as well as of telomerase subunits, have been reported for almost a decade. Here we summarize known facts about the architecture of plant telomeres and compare them with the well-described composition of telomeres in other organisms.

Expansion of Interstitial Telomeric Sequences in Yeast

Telomeric repeats located within chromosomes are called interstitial telomeric sequences (ITSs). They are polymorphic in length and are likely hotspots for initiation of chromosomal rearrangements that have been linked to human disease. Using our S. cerevisiae system to study repeat-mediated genome instability, we have previously shown that yeast telomeric (Ytel) repeats induce various gross chromosomal rearrangements (GCR) when their G-rich strands serve as the lagging strand template for replication (G orientation). Here, we show that interstitial Ytel repeats in the opposite C orientation prefer to expand rather than cause GCR. A tract of eight Ytel repeats expands at a rate of 4 × 10(-4) per replication, ranking them among the most expansion-prone DNA microsatellites. A candidate-based genetic analysis implicates both post-replication repair and homologous recombination pathways in the expansion process. We propose a model for Ytel repeat expansions and discuss its applications for genome instability and alternative telomere lengthening (ALT).

Human CST abundance determines recovery from diverse forms of DNA damage and replication stress

Mammalian CST (CTC1-STN1-TEN1) is a telomere-associated complex that functions in telomere duplex replication and fill-in synthesis of the telomeric C-strand following telomerase action. CST also facilitates genome-wide replication recovery after HU-induced fork stalling by increasing origin firing. CTC1 and STN1 were originally isolated as a DNA polymerase α stimulatory factor. Here we explore how CST abundance affects recovery from drugs that cause different types of DNA damage and replication stress. We show that recovery from HU and aphidicolin induced replication stress is increased by CST over-expression. Elevated CST increases dNTP incorporation and origin firing after HU release and decreases the incidence of anaphase bridges and micronuclei after aphidicolin removal. While the frequency of origin firing after HU release is proportional to CST abundance, the number of cells entering S-phase to initiate replication is unchanged by CST overexpression or STN1 depletion. Instead the CST-related changes in origin firing take place in cells that were already in S-phase at the time of HU addition, indicating that CST modulates firing of late or dormant origins. CST abundance also influences cell viability after treatment with HU, aphidicolin, MMS and camptothecin. Viability is increased by elevated CST and decreased by STN1 depletion, indicating that endogenous CST levels are limiting. However, CST abundance does not affect viability after MMC treatment. Thus, CST facilitates recovery from many, but not all, forms of exogenous DNA damage. Overall our results suggest that CST is needed in stoichiometric amounts to facilitate re-initiation of DNA replication at repaired forks and/or dormant origins.

Regulation of Telomere Length Requires a Conserved N-Terminal Domain of Rif2 in Saccharomyces cerevisiae

The regulation of telomere length equilibrium is essential for cell growth and survival since critically short telomeres signal DNA damage and cell cycle arrest. While the broad principles of length regulation are well established, the molecular mechanism of how these steps occur is not fully understood. We mutagenized the RIF2 gene in Saccharomyces cerevisiae to understand how this protein blocks excess telomere elongation. We identified an N-terminal domain in Rif2 that is essential for length regulation, which we have termed BAT domain for Blocks Addition of Telomeres. Tethering this BAT domain to Rap1 blocked telomere elongation not only in rif2Δ mutants but also in rif1Δ and rap1C-terminal deletion mutants. Mutation of a single amino acid in the BAT domain, phenylalanine at position 8 to alanine, recapitulated the rif2Δ mutant phenotype. Substitution of F8 with tryptophan mimicked the wild-type phenylalanine, suggesting the aromatic amino acid represents a protein interaction site that is essential for telomere length regulation.

Involvement of SRSF11 in cell cycle-specific recruitment of telomerase to telomeres at nuclear speckles

Telomerase, a unique ribonucleoprotein complex that contains the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC) and the TERC-binding protein dyskerin, is required for continued cell proliferation in stem cells and cancer cells. Here we identify SRSF11 as a novel TERC-binding protein that localizes to nuclear speckles, subnuclear structures that are enriched in pre-messenger RNA splicing factors. SRSF11 associates with active telomerase enzyme through an interaction with TERC and directs it to nuclear speckles specifically during S phase of the cell cycle. On the other hand, a subset of telomeres is shown to be constitutively present at nuclear speckles irrespective of cell cycle phase, suggesting that nuclear speckles could be the nuclear sites for telomerase recruitment to telomeres. SRSF11 also associates with telomeres through an interaction with TRF2, which facilitates translocation of telomerase to telomeres. Depletion of SRSF11 prevents telomerase from associating with nuclear speckles and disrupts telomerase recruitment to telomeres, thereby abrogating telomere elongation by telomerase. These findings suggest that SRSF11 acts as a nuclear speckle-targeting factor that is essential for telomerase association with telomeres through the interactions with TERC and TRF2, and provides a potential target for modulating telomerase activity in cancer.

Replicating through telomeres: a means to an end

Proper replication of the telomeric DNA at chromosome ends is critical for preserving genome integrity. Yet, telomeres present challenges for the replication machinery, such as their repetitive and heterochromatic nature and their potential to form non-Watson-Crick structures as well as the fact that they are transcribed. Numerous telomere-bound proteins are required to facilitate progression of the replication fork throughout telomeric DNA. In particular, shelterin plays crucial functions in telomere length regulation, protection of telomeres from nuclease degradation, control of DNA damage response at telomeres, and the recruitment of associated factors required for telomere DNA processing and replication. In this review we discuss the recently uncovered functions of mammalian telomere-specific and telomere-associated proteins that facilitate proper telomere replication.

Akt-mediated phosphorylation increases the binding affinity of hTERT for importin α to promote nuclear translocation

Telomeres are essential for chromosome integrity and protection, and their maintenance requires the ribonucleoprotein enzyme telomerase. Previously, we have shown that human telomerase reverse transcriptase (hTERT) contains a bipartite nuclear localization signal (NLS; residues 222–240) that is responsible for nuclear import, and that Akt-mediated phosphorylation of residue S227 is important for efficient nuclear import of hTERT. Here, we show that hTERT binds to importin-α proteins through the bipartite NLS and that this heterodimer then forms a complex with importin-β proteins to interact with the nuclear pore complex. Depletion of individual importin-α proteins results in a failure of hTERT nuclear import, and the resulting cytoplasmic hTERT is degraded by ubiquitin-dependent proteolysis. Crystallographic analysis reveals that the bipartite NLS interacts with both the major and minor sites of importin-α proteins. We also show that Akt-mediated phosphorylation of S227 increases the binding affinity for importin-α proteins and promotes nuclear import of hTERT, thereby resulting in increased telomerase activity. These data provide details of a binding mechanism that enables hTERT to interact with the nuclear import receptors and of the control of the dynamic nuclear transport of hTERT through phosphorylation.

The putative Leishmania telomerase RNA (LeishTER) undergoes trans-splicing and contains a conserved template sequence

Telomerase RNAs (TERs) are highly divergent between species, varying in size and sequence composition. Here, we identify a candidate for the telomerase RNA component of Leishmania genus, which includes species that cause leishmaniasis, a neglected tropical disease. Merging a thorough computational screening combined with RNA-seq evidence, we mapped a non-coding RNA gene localized in a syntenic locus on chromosome 25 of five Leishmania species that shares partial synteny with both Trypanosoma brucei TER locus and a putative TER candidate-containing locus of Crithidia fasciculata. Using target-driven molecular biology approaches, we detected a ∼2,100 nt transcript (LeishTER) that contains a 5′ spliced leader (SL) cap, a putative 3′ polyA tail and a predicted C/D box snoRNA domain. LeishTER is expressed at similar levels in the logarithmic and stationary growth phases of promastigote forms. A 5′SL capped LeishTER co-immunoprecipitated and co-localized with the telomerase protein component (TERT) in a cell cycle-dependent manner. Prediction of its secondary structure strongly suggests the existence of a bona fide single-stranded template sequence and a conserved C[U/C]GUCA motif-containing helix II, representing the template boundary element. This study paves the way for further investigations on the biogenesis of parasite TERT ribonucleoproteins (RNPs) and its role in parasite telomere biology.

TERT genetic polymorphism rs2736100 was associated with lung cancer: a meta-analysis based on 14,492 subjects

BACKGROUND

Previous studies focused on the association of the telomerase reverse transcriptase (TERT) gene polymorphism rs2736100 with lung cancer did not reach the same conclusion. In the present study, we performed a meta-analysis to systematically summarize the possible association between TERT polymorphism rs2736100 and the risk for lung cancer.

METHOD

We conducted a search of case-control studies on the association of TERT with susceptibility to lung cancer in PubMed, EMBASE, ISI Web of Science, Wanfang database in China, and Chinese National Knowledge Infrastructure (CNKI) databases. Data from eligible studies were extracted for meta-analysis. Lung cancer risk associated with rs2736100 was estimated by pooled odds ratios (ORs) and 95% confidence intervals (95% CIs).

RESULTS

Six independent case-control studies on rs2736100 were included in our meta-analysis. Our results showed that rs2736100 was associated with the risk of lung cancer not only in an additive model (OR=1.19, 95% CI: 1.04-1.35; p=0.01), but also in a dominant model (OR=1.14, 95% CI: 1.01-1.28; p=0.03).

CONCLUSIONS

This meta-analysis suggests that rs2736100 is associated with the risk of lung cancer.

Recognition and binding of human telomeric G-quadruplex DNA by unfolding protein 1

The specific recognition by proteins of G-quadruplex structures provides evidence of a functional role for in vivo G-quadruplex structures. As previously reported, the ribonucleoprotein, hnRNP Al, and it is proteolytic derivative, unwinding protein 1 (UP1), bind to and destabilize G-quadruplex structures formed by the human telomeric repeat d(TTAGGG)_n. UP1 has been proposed to be involved in the recruitment of telomerase to telomeres for chain extension. In this study, a detailed thermodynamic characterization of the binding of UP1 to a human telomeric repeat sequence, the d[AGGG(TTAGGG)₃] G-quadruplex, is presented and reveals key insights into the UP1-induced unfolding of the G-quadruplex structure. The UP1–G-quadruplex interactions are shown to be enthalpically driven, exhibiting large negative enthalpy changes for the formation of both the Na⁺ and K⁺ G-quadruplex–UP1 complexes (ΔH values of −43 and −19 kcal/mol, respectively). These data reveal three distinct enthalpic contributions from the interactions of UP1 with the Na⁺ form of G-quadruplex DNA. The initial interaction is characterized by a binding affinity of 8.5 × 10⁸ M^–1 (strand), 200 times stronger than the binding of UP1 to a single-stranded DNA with a comparable but non-quadruplex-forming sequence [4.1 × 10⁶ M^–1 (strand)]. Circular dichroism spectroscopy reveals the Na⁺ form of the G-quadruplex to be completely unfolded by UP1 at a binding ratio of 2:1 (UP1:G-quadruplex DNA). The data presented here demonstrate that the favorable energetics of the initial binding event are closely coupled with and drive the unfolding of the G-quadruplex structure.

Telomere length kinetics assay (TELKA) sorts the telomere length maintenance (tlm) mutants into functional groups

Genome-wide systematic screens in yeast have uncovered a large gene network (the telomere length maintenance network or TLM), encompassing more than 400 genes, which acts coordinatively to maintain telomere length. Identifying the genes was an important first stage; the next challenge is to decipher their mechanism of action and to organize then into functional groups or pathways. Here we present a new telomere-length measuring program, TelQuant, and a novel assay, telomere length kinetics assay, and use them to organize tlm mutants into functional classes. Our results show that a mutant defective for the relatively unknown MET7 gene has the same telomeric kinetics as mutants defective for the ribonucleotide reductase subunit Rnr1, in charge of the limiting step in dNTP synthesis, or for the Ku heterodimer, a well-established telomere complex. We confirm the epistatic relationship between the mutants and show that physical interactions exist between Rnr1 and Met7. We also show that Met7 and the Ku heterodimer affect dNTP formation, and play a role in non-homologous end joining. Thus, our telomere kinetics assay uncovers new functional groups, as well as complex genetic interactions between tlm mutants.

Yeast telomere maintenance is globally controlled by programmed ribosomal frameshifting and the nonsense-mediated mRNA decay pathway

We have previously shown that ~10% of all eukaryotic mRNAs contain potential programmed -1 ribosomal frameshifting (-1 PRF) signals and that some function as mRNA destabilizing elements through the Nonsense-Mediated mRNA Decay (NMD) pathway by directing translating ribosomes to premature termination codons. Here, the connection between -1 PRF, NMD and telomere end maintenance are explored. Functional -1 PRF signals were identified in the mRNAs encoding two components of yeast telomerase, EST1 and EST2, and in mRNAs encoding proteins involved in recruiting telomerase to chromosome ends, STN1 and CDC13. All of these elements responded to mutants and drugs previously known to stimulate or inhibit -1 PRF, further supporting the hypothesis that they promote -1 PRF through the canonical mechanism. All affected the steady-state abundance of a reporter mRNA and the wide range of -1 PRF efficiencies promoted by these elements enabled the determination of an inverse logarithmic relationship between -1 PRF efficiency and mRNA accumulation. Steady-state abundances of the endogenous EST1, EST2, STN1 and CDC13 mRNAs were similarly inversely proportional to changes in -1 PRF efficiency promoted by mutants and drugs, supporting the hypothesis that expression of these genes is post-transcriptionally controlled by -1 PRF under native conditions. Overexpression of EST2 by ablation of -1 PRF signals or inhibition of NMD promoted formation of shorter telomeres and accumulation of large budded cells at the G2/M boundary. A model  is presented describing how limitation and maintenance of correct stoichiometries of telomerase components by -1 PRF is used to maintain yeast telomere length.

Catalytically active telomerase holoenzyme is assembled in the dense fibrillar component of the nucleolus during S phase

The maintenance of human telomeres requires the ribonucleoprotein enzyme telomerase, which is composed of telomerase reverse transcriptase (TERT), telomerase RNA component, and several additional proteins for assembly and activity. Telomere elongation by telomerase in human cancer cells involves multiple steps including telomerase RNA biogenesis, holoenzyme assembly, intranuclear trafficking, and telomerase recruitment to telomeres. Although telomerase has been shown to accumulate in Cajal bodies for association with telomeric chromatin, it is unclear where and how the assembly and trafficking of catalytically active telomerase is regulated in the context of nuclear architecture. Here, we show that the catalytically active holoenzyme is initially assembled in the dense fibrillar component of the nucleolus during S phase. The telomerase RNP is retained in nucleoli through the interaction of hTERT with nucleolin, a major nucleolar phosphoprotein. Upon association with TCAB1 in S phase, the telomerase RNP is transported from nucleoli to Cajal bodies, suggesting that TCAB1 acts as an S-phase-specific holoenzyme component. Furthermore, depletion of TCAB1 caused an increase in the amount of telomerase RNP associated with nucleolin. These results suggest that the TCAB1-dependent trafficking of telomerase to Cajal bodies occurs in a step separate from the holoenzyme assembly in nucleoli. Thus, we propose that the dense fibrillar component is the provider of active telomerase RNP for supporting the continued proliferation of cancer and stem cells.

Tpz1 controls a telomerase-nonextendible telomeric state and coordinates switching to an extendible state via Ccq1

A binary switch between telomerase-extendible and telomerase-nonextendible states determines telomere length homeostasis. Here, Qiao and coworkers address how shelterin complex component Tpz1 regulates telomere length in fission yeast. Separation-of-function mutant analyses indicate that Tpz1-mediated linkage within the shelterin complex defines the telomerase-nonextendible state. Interestingly, the authors show that Tpz1 also plays a role in the activation of telomeres to the extendible state via its interaction with shelterin component Ccq1. Thus, this study suggests that Tpz1 coordinates both positive and negative regulators of telomere length homeostasis.

Telomeres are nucleoprotein complexes comprising telomeric DNA repeats bound by the multiprotein shelterin complex. A dynamic binary switch between telomerase-extendible and telomerase-nonextendible telomeric states determines telomere length homeostasis. However, the molecular nature of the nonextendible state is largely unknown. Here, we show that, in fission yeast, Tpz1 (the ortholog of human TPP1)-mediated complete linkage within the shelterin complex, bridging telomeric dsDNA to ssDNA, controls the telomerase-nonextendible state. Disruption of this linkage leads to unregulated telomere elongation while still retaining the shelterin components on telomeres. Therefore, the linkage within the shelterin components, rather than the individual shelterin components per se, defines the telomerase-nonextendible state. Furthermore, epistasis analyses reveal that Tpz1 also participates in the activation of telomeres to the extendible state via its interaction with Ccq1. Our results suggest critical regulatory roles of Tpz1 in the telomere binary switch.

G-quadruplexes as potential therapeutic targets for embryonal tumors

Embryonal tumors include a heterogeneous group of highly malignant neoplasms that primarily affect infants and children and are characterized by a high rate of mortality and treatment-related morbidity, hence improved therapies are clearly needed. G-quadruplexes are special secondary structures adopted in guanine (G)-rich DNA sequences that are often present in biologically important regions, e.g. at the end of telomeres and in the regulatory regions of oncogenes such as MYC. Owing to the significant roles that both telomeres and MYC play in cancer cell biology, G-quadruplexes have been viewed as emerging therapeutic targets in oncology and as tools for novel anticancer drug design. Several compounds that target these structures have shown promising anticancer activity in tumor xenograft models and some of them have entered Phase II clinical trials. In this review we examine approaches to DNA targeted cancer therapy, summarize the recent developments of G-quadruplex ligands as anticancer drugs and speculate on the future direction of such structures as a potential novel therapeutic strategy for embryonal tumors of the nervous system.

Human TEN1 maintains telomere integrity and functions in genome-wide replication restart

TEN1 is a component of the mammalian CTC1-STN1-TEN1 complex. CTC1 and/or STN1 functions in telomere duplex replication, C-strand fill-in, and genome-wide restart of replication following fork stalling. Here we examine the role of human TEN1 and ask whether it also functions as a specialized replication factor. TEN1 depletion causes an increase in multitelomere fluorescent in situ hybridization (FISH) signals similar to that observed after CTC1 or STN1 depletion. However, TEN1 depletion also results in increased telomere loss. This loss is not accompanied by increased telomere deprotection, recombination, or T-circle release. Thus, it appears that both the multiple telomere signals and telomere loss stem from problems in telomere duplex replication. TEN1 depletion can also affect telomere length, but whether telomeres lengthen or shorten is cell line-dependent. Like CTC1 and STN1, TEN1 is needed for G-overhang processing. Depletion of TEN1 does not effect overhang elongation in mid-S phase, but it delays overhang shortening in late S/G2. These results indicate a role for TEN1 in C-strand fill-in but do not support a direct role in telomerase regulation. Finally, TEN1 depletion causes a decrease in genome-wide replication restart following fork stalling similar to that observed after STN1 depletion. However, anaphase bridge formation is more severe than with CTC1 or STN1 depletion. Our findings indicate that TEN1 likely functions in conjunction with CTC1 and STN1 at the telomere and elsewhere in the genome. They also raise the possibility that TEN1 has additional roles and indicate that TEN1/CTC1-STN1-TEN1 helps solve a wide range of challenges to the replication machinery.