Rapid inhibition of cancer cell growth induced by lentiviral delivery and expression of mutant-template telomerase RNA and anti-telomerase short-interfering RNA

A persistent variant telomere sequence in a human pedigree

The telomere sequence, TTAGGG, is conserved across all vertebrates and plays an essential role in suppressing the DNA damage response by binding a set of proteins termed shelterin. Changes in the telomere sequence impair shelterin binding, initiate a DNA damage response, and are toxic to cells. Here we identify a family with a variant in the telomere template sequence of telomerase, the enzyme responsible for telomere elongation, that led to a non-canonical telomere sequence. The variant is inherited across at least one generation and one family member reports no significant medical concerns despite ~9% of their telomeres converting to the novel sequence. The variant template disrupts telomerase repeat addition processivity and decreased the binding of the telomere-binding protein POT1. Despite these disruptions, the sequence is readily incorporated into cellular chromosomes. Incorporation of a variant sequence prevents POT1-mediated inhibition of telomerase suggesting that incorporation of a variant sequence may influence telomere addition. These findings demonstrate that telomeres can tolerate substantial degeneracy while remaining functional and provide insights as to how incorporation of a non-canonical telomere sequence might alter telomere length dynamics.

Irreversible inhibition of TRF2TRFH recruiting functions by a covalent cyclic peptide induces telomeric replication stress in cancer cells

The TRF2 shelterin component is an essential regulator of telomere homeostasis and genomic stability. Mutations in the TRF2TRFH domain physically impair t-loop formation and prevent the recruitment of several factors that promote efficient telomere replication, causing telomeric DNA damage. Here, we design, synthesize, and biologically test covalent cyclic peptides that irreversibly target the TRF2TRFH domain. We identify APOD53 as our most promising compound, as it consistently induces a telomeric DNA damage response in cancer cell lines. APOD53 forms a covalent adduct with a reactive cysteine residue present in the TRF2TRFH domain and induces phenotypes consistent with TRF2TRFH domain mutants. These include induction of a telomeric DNA damage response, increased telomeric replication stress, and impaired recruitment of RTEL1 and SLX4 to telomeres. We demonstrate that APOD53 impairs cancer cell growth and find that co-treatment with APOD53 can exacerbate telomere replication stress caused by the G4 stabilizer RHPS4 and low dose aphidicolin (APH).

Dysregulated Stem Cell Markers Musashi-1 and Musashi-2 are Associated with Therapy Resistance in Inflammatory Breast Cancer

BACKGROUND AND AIM

While preliminary evidence points to pro-tumorigenic roles for the Musashi (MSI) RNA-binding proteins Musashi-1 (MSI1) and Musashi-2 (MSI2) in some breast cancer subtypes, no data exist for inflammatory breast cancer (IBC).

METHODS

MSI gene expression was quantified in IBC SUM149PT cells. We then used small interfering RNA-based MSI1 and MSI2 double knockdown (DKD) to understand gene expression and functional changes upon MSI depletion. We characterized cancer stem cell characteristics, cell apoptosis and cell cycle progression via flow cytometry, mammospheres via spheroid assays, migration and proliferation via digital holographic microscopy, and cell viability using BrdU assays. Chemoresistance was determined for paclitaxel and cisplatin with MTT assays and radioresistance was assessed with clonogenic analyses. In parallel, we supported our in vitro data by analyzing publicly available patient IBC gene expression datasets.

RESULTS

MSI1 and MSI2 are upregulated in breast cancer generally and IBC specifically. MSI2 is more commonly expressed compared to MSI1. MSI DKD attenuated proliferation, cell cycle progression, migration, and cell viability while increasing apoptosis. Stem cell characteristics CD44(+)/CD24(-), TERT and Oct4 were associated with MSI expression in vivo and were decreased in vitro after MSI DKD as was ALDH expression and mammosphere formation. In vivo, chemoresistant tumors were characterized by MSI upregulation upon chemotherapy application. In vitro, MSI DKD was able to alleviate chemo- and radioresistance.

CONCLUSIONS

The Musashi RNA binding proteins are dysregulated in IBC and associated with tumor proliferation, cancer stem cell phenotype, chemo- and radioresistance. MSI downregulation alleviates therapy resistance and attenuates tumor proliferation in vitro.

Deciphering the Functions of Telomerase Reverse Transcriptase in Head and Neck Cancer

Head and neck cancers (HNCs) are among the ten leading malignancies worldwide. Despite significant progress in all therapeutic modalities, predictive biomarkers, and targeted therapies for HNCs are limited and the survival rate is unsatisfactory. The importance of telomere maintenance via telomerase reactivation in carcinogenesis has been demonstrated in recent decades. Several mechanisms could activate telomerase reverse transcriptase (TERT), the most common of which is promoter alternation. Two major hotspot TERT promoter mutations (C228T and C250T) have been reported in different malignancies such as melanoma, genitourinary cancers, CNS tumors, hepatocellular carcinoma, thyroid cancers, sarcomas, and HNCs. The frequencies of TERT promoter mutations vary widely across tumors and is quite high in HNCs (11.9-64.7%). These mutations have been reported to be more enriched in oral cavity SCCs and HPV-negative tumors. The association between TERT promoter mutations and poor survival has also been demonstrated. Till now, several therapeutic strategies targeting telomerase have been developed although only a few drugs have been used in clinical trials. Here, we briefly review and summarize our current understanding and evidence of TERT promoter mutations in HNC patients.

Mutations in the TERC template sequence can be incorporated into the telomeres of human tumors

Telomerase-mediated lengthening is a mechanism by which some cancer cells avoid senescence-mediated cell cycle arrest due to shortened telomeres. By reverse transcribing an RNA template, encoded by TERC, the enzyme telomerase synthesizes the elongation of telomeric DNA using the 3’ end of the chromosome as a primer. TERC harbors a highly conserved template region consisting of 11 nucleotides spanning hg19 coordinates chr3:169482793–169482803. In human cell lines, when TERC was mutated to alter its template region, telomerase generated the predicted mutant telomeric repeats. However, it is unknown if this can occur in human clinical samples. Here, we report on the rare occurrence of two tumor samples where TERC template mutations were reflected in telomeric repeats.

TERT Promoter Mutations and Telomerase in Melanoma

Malignant melanoma is an extremely malignant tumor with a high mortality rate and an increasing incidence with a high mutation load. The frequency of mutations in the TERT promoter exceeds the frequency of any known noncoding mutations in melanoma. A growing number of recent studies suggest that the most common mutations in the TERT promoter (ATG start site −124C>T and −146C>T) are associated with increased TERT mRNA expression, telomerase activity, telomere length, and poor prognosis. Recently, it has been shown that TERT promoter mutations are more correlated with the occurrence, development, invasion, and metastasis of melanoma, as well as emerging approaches such as the therapeutic potential of chemical inhibition of TERT promoter mutations, direct telomerase inhibitors, combined targeted therapy, and immunotherapies. In this review, we describe the latest advances in the role of TERT promoter mutations and telomerase in promoting the occurrence, development, and poor prognosis of melanoma and discuss the clinical significance of the TERT promoter and telomerase in the treatment of melanoma.

Telomerase in Cancer: Function, Regulation, and Clinical Translation

Simple Summary

Cells undergoing malignant transformation must circumvent replicative senescence and eventual cell death associated with progressive telomere shortening that occurs through successive cell division. To do so, malignant cells reactivate telomerase to extend their telomeres and achieve cellular immortality, which is a “Hallmark of Cancer”. Here we review the telomere-dependent and -independent functions of telomerase in cancer, as well as its potential as a biomarker and therapeutic target to diagnose and treat cancer patients.

Abstract

During the process of malignant transformation, cells undergo a series of genetic, epigenetic, and phenotypic alterations, including the acquisition and propagation of genomic aberrations that impart survival and proliferative advantages. These changes are mediated in part by the induction of replicative immortality that is accompanied by active telomere elongation. Indeed, telomeres undergo dynamic changes to their lengths and higher-order structures throughout tumor formation and progression, processes overseen in most cancers by telomerase. Telomerase is a multimeric enzyme whose function is exquisitely regulated through diverse transcriptional, post-transcriptional, and post-translational mechanisms to facilitate telomere extension. In turn, telomerase function depends not only on its core components, but also on a suite of binding partners, transcription factors, and intra- and extracellular signaling effectors. Additionally, telomerase exhibits telomere-independent regulation of cancer cell growth by participating directly in cellular metabolism, signal transduction, and the regulation of gene expression in ways that are critical for tumorigenesis. In this review, we summarize the complex mechanisms underlying telomere maintenance, with a particular focus on both the telomeric and extratelomeric functions of telomerase. We also explore the clinical utility of telomeres and telomerase in the diagnosis, prognosis, and development of targeted therapies for primary, metastatic, and recurrent cancers.

The multifaceted hTR telomerase RNA from a structural perspective: Distinct domains of hTR differentially interact with protein partners to orchestrate its telomerase-independent functions

Human telomerase progressively emerged as a multifaceted ribonucleoprotein complex with additional functions beyond telomeric repeat synthesis. Both the hTERT catalytic subunit and the hTR long non-coding RNA (lncRNA) subunit are engaged in highly regulated cellular pathways that, together, contribute to cell fitness and protection against apoptosis. We recently described a new role for hTR in regulating the abundance of replication protein A at telomeres, adding to the growing repertoire of hTR's functions. Here, we focus on the non-canonical roles of hTR and discuss them in the context of the structural elements of the lncRNA. We propose that some functions of hTR may compete amongst each other through distinct interactions with its partners, proteins or mRNAs. We postulate that hTR's non-canonical functions may be highly relevant in the context of normal somatic cells that naturally silence hTERT gene, while keeping hTR expression.

The structurally conserved TELR region on shelterin protein TPP1 is essential for telomerase processivity but not recruitment

The shelterin protein TPP1 is involved in both recruiting telomerase and stimulating telomerase processivity in human cells. Assessing the in vivo significance of the latter role of TPP1 has been difficult, because TPP1 mutations that perturb telomerase function tend to abolish both telomerase recruitment and processivity. The Saccharomyces cerevisiae telomerase-associated Est3 protein adopts a protein fold similar to the N-terminal region of TPP1. Interestingly, a previous structure-guided mutagenesis study of Est3 revealed a TELR surface region that regulates telomerase function via an unknown mechanism without affecting the interaction between Est3 and telomerase [T. Rao et al., Proc. Natl. Acad. Sci. U.S.A. 111, 214-218 (2014)]. Here, we show that mutations within the structurally conserved TELR region on human TPP1 impaired telomerase processivity while leaving telomerase recruitment unperturbed, hence uncoupling the two roles of TPP1 in regulating telomerase. Telomeres in cell lines containing homozygous TELR mutations progressively shortened to a critical length that caused cellular senescence, despite the presence of abundant telomerase in these cells. Our findings not only demonstrate that telomerase processivity can be regulated by TPP1 in a process separable from its role in recruiting telomerase, but also establish that the in vivo stimulation of telomerase processivity by TPP1 is critical for telomere length homeostasis and long-term viability of human cells.

Recent Advances in the Development of Exogenous dsRNA for the Induction of RNA Interference in Cancer Therapy

Selective regulation of gene expression by means of RNA interference has revolutionized molecular biology. This approach is not only used in fundamental studies on the roles of particular genes in the functioning of various organisms, but also possesses practical applications. A variety of methods are being developed based on gene silencing using dsRNA-for protecting agricultural plants from various pathogens, controlling insect reproduction, and therapeutic techniques related to the oncological disease treatment. One of the main problems in this research area is the successful delivery of exogenous dsRNA into cells, as this can be greatly affected by the localization or origin of tumor. This overview is dedicated to describing the latest advances in the development of various transport agents for the delivery of dsRNA fragments for gene silencing, with an emphasis on cancer treatment.

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.

Targeting the NCOA3-SP1-TERT axis for tumor growth in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) has a high mortality rate and lacks an effective therapeutic target. Elevated expression of human telomerase reverse transcriptase (TERT) is an important hallmark in cancers, but the mechanism by which TERT is activated differentially in cancers is poorly understood. Here, we have identified nuclear receptor coactivator-3 (NCOA3) as a new modulator of TERT expression and tumor growth in HCC. NACO3 specifically binds to the TERT promoter at the -234 to -144 region and transcriptionally activates TERT expression. NCOA3 promotes HCC cell growth and tumor progression in vitro and in vivo through upregulating the TERT signaling. Knockdown of NACO3 suppresses HCC cell viability and colony formation, whereas TERT overexpression rescues this suppression. NCOA3 interacts with and recruits SP1 binding on the TERT promoter. Knockdown of NCOA3 also inhibits the expression of the Wnt signaling-related genes but has no effect on the Notch signaling-targeting genes. Moreover, NCOA3 is positively correlated with TERT expression in HCC tumor tissues, and high expression of both NCOA3 and TERT predicts a poor prognosis in HCC patients. Our findings indicate that targeting the NCOA3-SP1-TERT signaling axis may benefit HCC patients.

TERT-Regulation and Roles in Cancer Formation

Telomerase reverse transcriptase (TERT) is a catalytic subunit of telomerase. Telomerase complex plays a key role in cancer formation by telomere dependent or independent mechanisms. Telomere maintenance mechanisms include complex TERT changes such as gene amplifications, TERT structural variants, TERT promoter germline and somatic mutations, TERT epigenetic changes, and alternative lengthening of telomere. All of them are cancer specific at tissue histotype and at single cell level. TERT expression is regulated in tumors via multiple genetic and epigenetic alterations which affect telomerase activity. Telomerase activity via TERT expression has an impact on telomere length and can be a useful marker in diagnosis and prognosis of various cancers and a new therapy approach. In this review we want to highlight the main roles of TERT in different mechanisms of cancer development and regulation.

Targeting telomerase for its advent in cancer therapeutics

Telomerase has emerged as an important primary target in anticancer therapy. It is a distinctive reverse transcriptase enzyme, which extends the length of telomere at the 3' chromosomal end, and uses telomerase reverse transcriptase (TERT) and telomerase RNA template-containing domains. Telomerase has a vital role and is a contributing factor in human health, mainly affecting cell aging and cell proliferation. Due to its unique feature, it ensures unrestricted cell proliferation in malignancy and plays a major role in cancer disease. The development of telomerase inhibitors with increased specificity and better pharmacokinetics is being considered to design and develop newer potent anticancer agents. Use of natural and synthetic compounds for the inhibition of telomerase activity can lead to an opening of new vistas in cancer treatment. This review details about the telomerase biochemistry, use of natural and synthetic compounds; vaccines and oncolytic virus in therapy that suppress the telomerase activity. We have discussed structure-activity relationships of various natural and synthetic telomerase inhibitors to help medicinal chemists and chemical biology researchers with a ready reference and updated status of their clinical trials. Suppression of human TERT (hTERT) activity through inhibition of hTERT promoter is an important approach for telomerase inhibition.

In silico design of telomerase inhibitors

Telomerase is a reverse transcriptase enzyme involved in DNA synthesis at the end of linear chromosomes. Unlike in most other cells, telomerase is reactivated most cancerous cells and, therefore, has become a promising new anticancer target. Despite extensive research, direct telomerase inhibitors have yet not been introduced to the clinics because of the complexity of this enzyme. Structures of this protein from simple organisms and human homology models are currently available and have been used in structure-based drug design efforts to find potential inhibitors. Different is silico strategies have been applied and different chemical groups have been explored. Here, we provide an overview of recent discoveries.

Targeted Covalent Inhibition of Telomerase

Telomerase is a ribonuceloprotein complex responsible for maintaining telomeres and protecting chromosomal integrity. The human telomerase reverse transcriptase (hTERT) is expressed in ∼90% of cancer cells where it confers the capacity for limitless proliferation. Along with its established role in telomere lengthening, telomerase also serves noncanonical extra-telomeric roles in oncogenic signaling, resistance to apoptosis, and enhanced DNA damage response. We report a new class of natural-product-inspired covalent inhibitors of telomerase that target the catalytic active site.

TERC promotes cellular inflammatory response independent of telomerase

TERC is an RNA component of telomerase. However, TERC is also ubiquitously expressed in most human terminally differentiated cells, which don't have telomerase activity. The function of TERC in these cells is largely unknown. Here, we report that TERC enhances the expression and secretion of inflammatory cytokines by stimulating NK-κB pathway in a telomerase-independent manner. The ectopic expression of TERC in telomerase-negative cells alters the expression of 431 genes with high enrichment of those involved in cellular immunity. We perform genome-wide screening using a previously identified 'binding motif' of TERC and identify 14 genes that are transcriptionally regulated by TERC. Among them, four genes (LIN37, TPRG1L, TYROBP and USP16) are demonstrated to stimulate the activation of NK-κB pathway. Mechanistically, TERC associates with the promoter of these genes through forming RNA-DNA triplexes, thereby enhancing their transcription. In vivo, expression levels of TERC and TERC target genes (TYROBP, TPRG1L and USP16) are upregulated in patients with inflammation-related diseases such as type II diabetes and multiple sclerosis. Collectively, these results reveal an unknown function of TERC on stimulating inflammatory response and highlight a new mechanism by which TERC modulates gene transcription. TERC may be a new target for the development of anti-inflammation therapeutics.

In Situ Visualization of Telomere Length, Telomere Elongation, and TERT Expression in Single Cells

Telomerase plays a critical role in cancer and aging by adding hexa-nucleotide repeats to the ends of telomeres and extending the cellular proliferative lifespan. The very low level of telomerase expression in most cell populations and the difficulty of detecting telomere elongation in single cells have limited the study of telomerase expression and function in individual cells of a heterogeneous population. The method described in this article combines single-molecule detection (RNAscope) of telomerase reverse transcriptase (TERT) with our previously described TSQ1 assay for in situ monitoring of telomere extension, thereby enabling detection of TERT expression, telomere length, and telomere elongation in single cells and providing a unique approach for studying the factors that regulate telomere elongation by telomerase. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: TSQ1 lentivirus production Basic Protocol 2: TSQ1 lentiviral infection and plating Basic Protocol 3: RNAscope analysis Basic Protocol 4: TSQ1 PNA-FISH detection.

In perspective: An update on telomere targeting in cancer

Engaging a telomere maintenance mechanism during DNA replication is essential for almost all advanced cancers. The conversion from normal and premalignant somatic cells to advanced malignant cells often results (85%-90%) from the reactivation of the functional ribonucleoprotein holoenzyme complex, referred to as telomerase. Modulation of the human telomerase reverse transcriptase (hTERT) appears to be rate limiting to produce functional telomerase and engage a telomere maintenance mechanism. The remaining 10% to 15% of cancers overcome progressively shortened telomeres by activating an alternative lengthening of telomeres (ALT) maintenance mechanism, through a DNA recombination pathway. Exploration into the specific mechanisms of telomere maintenance in cancer have led to the development of drugs such as Imetelstat (GRN163L), BIBR1532, 6-thio-dG, VE-822, and NVP-BEZ235 being investigated as therapeutic approaches for treating telomerase and ALT tumors. The successful use of 6-thio-dG (a nucleoside preferentially recognized by telomerase) that targets and uncaps telomeres in telomerase positive but not normal telomerase silent cells has recently shown impressive effects on multiple types of cancer. For example, 6-thio-dG overcomes therapy-resistant cancers in a fast-acting mechanism potentially providing an alternative or additional route of treatment for patients with cancer. In this perspective, we provide a synopsis of the current landscape of telomeres and telomerase processing in cancer development and how this new knowledge may improve outcomes for patients with cancer.

Both the classical and alternative non-homologous end joining pathways contribute to the fusion of drastically shortened telomeres induced by TRF2 overexpression

The double-stranded telomeric binding protein TRF2 is expressed in many human cancers at elevated levels. Moreover, experimental overexpression of TRF2 in human cells causes replication stalling in telomeric tracts, which leads to drastic telomere shortening and fusion of deprotected chromosome ends. To understand which end joining pathway is involved in mediating these chromosome fusions, we overexpressed TRF2 in human HCT116 cell lines that were deficient for the DNA Ligase 4 (Lig4)-dependent classical non-homologous end joining (C-NHEJ) or the DNA Ligase 3 (Lig3)-dependent alternative non-homologous end joining (A-NHEJ) pathway. Surprisingly, abrogation of either Lig4 or nuclear Lig3 significantly reduced inter-chromosomal fusion of drastically shortened telomeres, suggesting that both the C-NHEJ and A-NHEJ pathways are involved in mediating this type of fusion. Fusion between deprotected sister chromatids, however, only required the Lig3-dependent A-NHEJ pathway. Interestingly, a previous study reported similar end joining pathway requirements for the fusion of critically shortened telomeres during a telomere attrition-based cellular crisis. We speculate that, as in cellular crisis, the same repair pathway(s) may drive clonal and genomic evolution in human cancers containing elevated TRF2 levels.

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

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

Preliminary development of an assay for detection of TERT expression, telomere length, and telomere elongation in single cells

The telomerase enzyme enables unlimited proliferation of most human cancer cells by elongating telomeres and preventing replicative senescence. Despite the critical importance of telomerase in cancer biology, challenges detecting telomerase activity and expression in individual cells have hindered the ability to study patterns of telomerase expression and function across heterogeneous cell populations. While sensitive assays to ascertain telomerase expression and function exist, these approaches have proven difficult to implement at the single cell level. Here, we validate in situ RNAscope detection of the telomerase TERT mRNA and couple this assay with our recently described TSQ1 method for in situ detection of telomere elongation. This approach enables detection of TERT expression, telomere length, and telomere elongation within individual cells of the population. Using this assay, we show that the heterogeneous telomere elongation observed across a HeLa cell population is in part driven by variable expression of the TERT gene. Furthermore, we show that the absence of detectable telomere elongation in some TERT-positive cells is the result of inhibition by the telomeric shelterin complex. This combined assay provides a new approach for understanding the integrated expression, function, and regulation of telomerase at the single cell level.

The RNA interactome of human telomerase RNA reveals a coding-independent role for a histone mRNA in telomere homeostasis

Telomerase RNA (TR) provides the template for DNA repeat synthesis at telomeres and is essential for genome stability in continuously dividing cells. We mapped the RNA interactome of human TR (hTR) and identified a set of non-coding and coding hTR-interacting RNAs, including the histone 1C mRNA (HIST1H1C). Disruption of the hTR-HIST1H1C RNA association resulted in markedly increased telomere elongation without affecting telomerase enzymatic activity. Conversely, over-expression of HIST1H1C led to telomere attrition. By using a combination of mutations to disentangle the effects of histone 1 RNA synthesis, protein expression, and hTR interaction, we show that HIST1H1C RNA negatively regulates telomere length independently of its protein coding potential. Taken together, our data provide important insights into a surprisingly complex hTR-RNA interaction network and define an unexpected non-coding RNA role for HIST1H1C in regulating telomere length homeostasis, thus offering a glimpse into the mostly uncharted, vast space of non-canonical messenger RNA functions.

Cell-based chemical fingerprinting identifies telomeres and lamin A as modifiers of DNA damage response in cancer cells

Telomere maintenance by telomerase activity supports the infinite growth of cancer cells. MST-312, a synthetic telomerase inhibitor, gradually shortens telomeres at non-acute lethal doses and eventually induces senescence and apoptosis of telomerase-positive cancer cells. Here we report that MST-312 at higher doses works as a dual inhibitor of telomerase and DNA topoisomerase II and exhibits acute anti-proliferative effects on cancer cells and xenografted tumours in vivo. Our cell-based chemical fingerprinting approach revealed that cancer cells with shorter telomeres and lower expression of lamin A, a nuclear architectural protein, exhibited higher sensitivity to the acute deleterious effects of MST-312, accompanied by formation of telomere dysfunction-induced foci and DNA double-strand breaks. Telomere elongation and lamin A overexpression attenuated telomeric and non-telomeric DNA damage, respectively, and both conferred resistance to apoptosis induced by MST-312 and other DNA damaging anticancer agents. These observations suggest that sufficient pools of telomeres and a nuclear lamina component contribute to the cellular robustness against DNA damage induced by therapeutic treatment in human cancer cells.

Mechanisms of template handling and pseudoknot folding in human telomerase and their manipulation to expand the sequence repertoire of processive repeat synthesis

Telomerase adds telomeric repeats to chromosome ends by processive copying of a template within the telomerase RNA bound to telomerase reverse transcriptase. Telomerase RNAs have single-stranded regions that separate the template from a 5' stem and 3' pseudoknot, and mammals gained additional stem P2a.1 separating the template from the pseudoknot. Using human telomerase, we show that the length of template 3'-flanking single-stranded RNA is a determinant of repeat addition processivity whereas template 5'-flanking single-stranded RNA and P2a.1 are critical for activity but not processivity. In comparison, requirements for the template sequence itself are confounding: different substitutions of the same position have strikingly different consequences, from improved processivity and activity to complete inactivation. We discovered that some altered-template sequences stabilize an alternative RNA conformation that precludes the pseudoknot by base-pairing of one pseudoknot strand to the template 3' end. Using mutations to reduce over-stability of the alternative conformation, we restore high activity and processivity to otherwise inactive altered-template telomerase ribonucleoproteins. In cells, over-stabilization or destabilization of the alternative state severely inhibited biogenesis of active telomerase. Our findings delineate roles for human telomerase RNA template-flanking regions, establish a biologically relevant pseudoknot-alternative RNA conformation, and expand the repertoire of human telomerase repeat synthesis.

Telomeres: Implications for Cancer Development

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

MYC drives overexpression of telomerase RNA (hTR/TERC) in prostate cancer

Telomerase consists of at least two essential elements, an RNA component hTR or TERC that contains the template for telomere DNA addition and a catalytic reverse transcriptase (TERT). While expression of TERT has been considered the key rate-limiting component for telomerase activity, increasing evidence suggests an important role for the regulation of TERC in telomere maintenance and perhaps other functions in human cancer. By using three orthogonal methods including RNAseq, RT-qPCR, and an analytically validated chromogenic RNA in situ hybridization assay, we report consistent overexpression of TERC in prostate cancer. This overexpression occurs at the precursor stage (e.g. high-grade prostatic intraepithelial neoplasia or PIN) and persists throughout all stages of disease progression. Levels of TERC correlate with levels of MYC (a known driver of prostate cancer) in clinical samples and we also show the following: forced reductions of MYC result in decreased TERC levels in eight cancer cell lines (prostate, lung, breast, and colorectal); forced overexpression of MYC in PCa cell lines, and in the mouse prostate, results in increased TERC levels; human TERC promoter activity is decreased after MYC silencing; and MYC occupies the TERC locus as assessed by chromatin immunoprecipitation (ChIP). Finally, we show that knockdown of TERC by siRNA results in reduced proliferation of prostate cancer cell lines. These studies indicate that TERC is consistently overexpressed in all stages of prostatic adenocarcinoma and that its expression is regulated by MYC. These findings nominate TERC as a novel prostate cancer biomarker and therapeutic target. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Telomerase and drug resistance in cancer

It is well known that a decreased expression or inhibited activity of telomerase in cancer cells is accompanied by an increased sensitivity to some drugs (e.g., doxorubicin, cisplatin, or 5-fluorouracil). However, the mechanism of the resistance resulting from telomerase alteration remains elusive. There are theories claiming that it might be associated with telomere shortening, genome instability, hTERT translocation, mitochondria functioning modulation, or even alterations in ABC family gene expression. However, association of those mechanisms, i.e., drug resistance and telomerase alterations, is not fully understood yet. We review the current theories on the aspect of the role of telomerase in cancer cells resistance to therapy. We believe that revealing/unravelling this correlation might significantly contribute to an increased efficiency of cancer cells elimination, especially the most difficult ones, i.e., drug resistant.

Current Insights to Regulation and Role of Telomerase in Human Diseases

The telomerase ribonucleoprotein complex has a pivotal role in regulating the proliferation and senescence of normal somatic cells as well as cancer cells. This complex is comprised mainly of telomerase reverse transcriptase (TERT), telomerase RNA component (TERC) and other associated proteins that function to elongate telomeres localized at the end of the chromosomes. While reactivation of telomerase is a major hallmark of most cancers, together with the synergistic activation of other oncogenic signals, deficiency in telomerase and telomeric proteins might lead to aging and senescence-associated disorders. Therefore, it is critically important to understand the canonical as well as non-canonical functions of telomerase through TERT to develop a therapeutic strategy against telomerase-related diseases. In this review, we shed light on the regulation and function of telomerase, and current therapeutic strategies against telomerase in cancer and age-related diseases.

RNA interference mediated downregulation of human telomerase reverse transcriptase (hTERT) in LN18 cells

Human telomerase reverse transcriptase (hTERT) gene is a biomarker for the targeted therapy in various cancers. Presence of increased telomerase activity is a common feature of all cancers including glioblastoma. Both RNA and catalytic subunits of hTERT are the target sites for blocking its activity. The current study focuses on the expression of hTERT in glioblastoma and its regulation using two different novel siRNAs (small interfering RNA). Our patient data demonstrated increased expression of hTERT, which could be correlated with carcinogenesis in glioma. In vitro studies in siRNA transfected LN18 cells confirmed significant cell death (p < 0.05) as evidenced by MTT and trypan blue exclusion assay. These results were further supported by flow cytometry data, which showed significant increase in early and late apoptosis. The hTERT mRNA expression was effectively downregulated by 45 and 39 % with siRNA1 and siRNA2, respectively. These results were further confirmed by immunoblotting analysis (p < 0.05). Our results suggest that both the siRNAs effectively down regulated the expression of hTERT at mRNA and protein levels, thereby decreasing cell viability and proliferation rate. Hence siRNA mediated downregulation of hTERT could be a potential therapeutic avenue in glioblastoma.

Piperlongumine induces gastric cancer cell apoptosis and G2/M cell cycle arrest both in vitro and in vivo

Recently, several studies have shown that piperlongumine (PL) can selectively kill cancer cells by targeting reactive oxygen species (ROS). However, the potential therapeutic effects and detailed mechanism of PL in gastric cancer are still not clear. In the current report, we found that PL significantly suppressed gastric cancer both in vitro and in vivo. PL obviously increased ROS generation in gastric cancer cells. Anti-oxidant glutathione (GSH) and N-acetyl-L-cysteine (NAC) can abrogate PL-induced gastric cancer cell death and proliferation inhibition. GADD45α was induced in PL-treated cancer cells and led to G2/M phase arrest, whereas genetic depletion of GADD45α by small interfering RNAs (siRNAs) could partly reverse PL-induced cell cycle arrest in gastric cancer cells. Interestingly, we also found that PL treatment decreased the expression of telomerase reverse transcriptase (TERT) gene, which plays an essential role in cancer initiation and progression. Our findings thus revealed a potential anti-tumor effect of PL on gastric cancer cells and may have therapeutic implications.

Telomere and Telomerase Therapeutics in Cancer

Telomerase is a reverse transcriptase capable of utilizing an integrated RNA component as a template to add protective tandem telomeric single strand DNA repeats, TTAGGG, to the ends of chromosomes. Telomere dysfunction and telomerase reactivation are observed in approximately 90% of human cancers; hence, telomerase activation plays a unique role as a nearly universal step on the path to malignancy. In the past two decades, multiple telomerase targeting therapeutic strategies have been pursued, including direct telomerase inhibition, telomerase interference, hTERT or hTERC promoter driven therapy, telomere-based approaches, and telomerase vaccines. Many of these strategies have entered clinical development, and some have now advanced to phase III clinical trials. In the coming years, one or more of these new telomerase-targeting drugs may be expected to enter the pharmacopeia of standard care. Here, we briefly review the molecular functions of telomerase in cancer and provide an update about the preclinical and clinical development of telomerase targeting therapeutics.

Lentivirus-Mediated Knockdown of CTHRC1 Inhibits Osteosarcoma Cell Proliferation and Migration

Collagen triple helix repeat containing-1 (CTHRC1), a secreted protein, is transiently expressed in the arterial wall in response to injury, indicating that it may contribute to vascular remodeling by limiting collagen matrix deposition and promoting cell migration. Recent studies showed that it is aberrantly upregulated in most human solid tumors, yet its role in osteosarcoma remains unclear. In this study, the authors investigated the role of CTHRC1 in human osteosarcoma tumorigenesis. The authors used lentivirus-mediated short hairpin RNA (shRNA) against CTHRC1 to limit its endogenous expression in U2OS and SW1353 cells. Interestingly, they found that depletion of CTHRC1 significantly inhibited cell proliferation and colony formation in U2OS and SW1353 cells. Flow cytometry assay showed that knockdown of CTHRC1 increased the cell percentage of G0/G1 phase, resulting in cell cycle arrest in U2OS cells. Moreover, CTHRC1 silencing induced the cell cycle arrest by a decrease in the cell percentage in G0/G1 phase and increased in G2/M phase in SW1353 cells. In addition, crystal violet staining suggested CTHRC1 silencing inhibited migration of U2OS and SW1353 cells. These results demonstrated that CTHRC1 might play an important role in osteosarcoma progression.

Reactivation of telomerase in cancer

Activation of telomerase is a critical step in the development of about 85 % of human cancers. Levels of Tert, which encodes the reverse transcriptase subunit of telomerase, are limiting in normal somatic cells. Tert is subjected to transcriptional, post-transcriptional and epigenetic regulation, but the precise mechanism of how telomerase is re-activated in cancer cells is poorly understood. Reactivation of the Tert promoter involves multiple changes which evolve during cancer progression including mutations and chromosomal re-arrangements. Newly described non-coding mutations in the Tert promoter region of many cancer cells (19 %) in two key positions, C250T and C228T, have added another layer of complexity to telomerase reactivation. These mutations create novel consensus sequences for transcription factors which can enhance Tert expression. In this review, we will discuss gene structure and function of Tert and provide insights into the mechanisms of Tert reactivation in cancers, highlighting the contribution of recently identified Tert promoter mutations.

Shortened leukocyte telomere length in type 2 diabetes mellitus: genetic polymorphisms in mitochondrial uncoupling proteins and telomeric pathways

Current debate in type 2 diabetes (T2DM) has focused on shortened leukocyte telomere length (LTL) as the result of a number of possible causes, including polymorphisms in mitochondrial uncoupling proteins (UCPs) leading to oxidative stress, telomere regulatory pathway gene polymorphisms, or as a direct result of associated cardiovascular complications inducing tissue organ inflammation and oxidative stress. There is evidence that a heritable shorter telomere trait is a risk factor for development of T2DM. This review discusses the contribution and balance of genetic regulation of UCPs and telomere pathways in the context of T2DM. We discuss genotypes that are well known to influence the shortening of LTL, in particular OBFC1 and telomerase genotypes such as TERC. Interestingly, the interaction between short telomeres and T2DM risk appears to involve mitochondrial dysfunction as an intermediate process. A hypothesis is presented that genetic heterogeneity within UCPs may directly affect oxidative stress that feeds back to influence the fine balance of telomere regulation, cell cycle regulation and diabetes risk and/or metabolic disease progression.

Elevated levels of TRF2 induce telomeric ultrafine anaphase bridges and rapid telomere deletions

The shelterin protein TRF2 is essential for chromosome-end protection. Depletion of TRF2 causes chromosome end-to-end fusions, initiating genomic instability that can be cancer promoting. Paradoxically, significant increased levels of TRF2 are observed in a subset of human cancers. Experimental overexpression of TRF2 has also been shown to induce telomere shortening, through an unknown mechanism. Here we report that TRF2 overexpression results in replication stalling in duplex telomeric repeat tracts and the subsequent formation of telomeric ultrafine anaphase bridges (UFBs), ultimately leading to stochastic loss of telomeric sequences. These TRF2 overexpression-induced telomere deletions generate chromosome fusions resembling those detected in human cancers and in mammalian cells containing critically shortened telomeres. Therefore, our findings have uncovered a second pathway by which altered TRF2 protein levels can induce end-to-end fusions. The observations also provide mechanistic insight into the molecular basis of genomic instability in tumour cells containing significantly increased TRF2 levels.

G-protein-coupled receptor 137 accelerates proliferation of urinary bladder cancer cells in vitro

Urinary bladder cancer is a worldwide concern because of its level of incidence and recurrence. To search an effective therapeutic strategy for urinary bladder cancer, it is important to identify proteins involved in tumorigenesis that could serve as potential targets for diagnosis and treatment. G-protein-coupled receptors (GPRs) constitute a large protein family of receptors that sense molecules outside the cell and activate signal transduction pathways and cellular responses inside the cell. GPR137 is a newly discovered human gene encoding orphan GPRs. In this study, we aimed to investigate the physiological role of GPR137 in urinary bladder cancer. The effect of GPR137 on cell growth was examined via an RNA interference (RNAi) lentivirus system in two human urinary bladder cancer cell lines BT5637 and T24. Lentivirus-mediated RNAi could specifically suppressed GPR137 expression in vitro, resulting in alleviated cell viability and impaired colony formation, as well as blocks G0/G1 and S phases of the cell cycle. These results suggested GPR137 as an essential player in urinary bladder cancer cell growth, and it may serve as a potential target for gene therapy in the treatment of urinary bladder cancer.

DNA damage induces nuclear actin filament assembly by Formin -2 and Spire-½ that promotes efficient DNA repair. [corrected]

Actin filaments assemble inside the nucleus in response to multiple cellular perturbations, including heat shock, protein misfolding, integrin engagement, and serum stimulation. We find that DNA damage also generates nuclear actin filaments-detectable by phalloidin and live-cell actin probes-with three characteristic morphologies: (i) long, nucleoplasmic filaments; (ii) short, nucleolus-associated filaments; and (iii) dense, nucleoplasmic clusters. This DNA damage-induced nuclear actin assembly requires two biologically and physically linked nucleation factors: Formin-2 and Spire-1/Spire-2. Formin-2 accumulates in the nucleus after DNA damage, and depletion of either Formin-2 or actin's nuclear import factor, importin-9, increases the number of DNA double-strand breaks (DSBs), linking nuclear actin filaments to efficient DSB clearance. Nuclear actin filaments are also required for nuclear oxidation induced by acute genotoxic stress. Our results reveal a previously unknown role for nuclear actin filaments in DNA repair and identify the molecular mechanisms creating these nuclear filaments.

The Shelterin TIN2 Subunit Mediates Recruitment of Telomerase to Telomeres

Dyskeratosis Congenita (DC) is a heritable multi-system disorder caused by abnormally short telomeres. Clinically diagnosed by the mucocutaneous symptoms, DC patients are at high risk for bone marrow failure, pulmonary fibrosis, and multiple types of cancers. We have recapitulated the most common DC-causing mutation in the shelterin component TIN2 by introducing a TIN2-R282H mutation into cultured telomerase-positive human cells via a knock-in approach. The resulting heterozygous TIN2-R282H mutation does not perturb occupancy of other shelterin components on telomeres, result in activation of telomeric DNA damage signaling or exhibit other characteristics indicative of a telomere deprotection defect. Using a novel assay that monitors the frequency and extension rate of telomerase activity at individual telomeres, we show instead that telomerase elongates telomeres at a reduced frequency in TIN2-R282H heterozygous cells; this recruitment defect is further corroborated by examining the effect of this mutation on telomerase-telomere co-localization. These observations suggest a direct role for TIN2 in mediating telomere length through telomerase, separable from its role in telomere protection.

The shelterin complex protects telomeres from being processed by the DNA damage repair machinery, and also regulates telomerase access and activity at telomeres. The only shelterin subunit known to promote telomerase function is TPP1, which mediates telomerase recruitment to telomeres and stimulates telomerase processivity. Mutations in shelterin components cause Dyskeratosis Congenita (DC) and related disease syndromes due to the inability to maintain telomere homeostasis. In this study, we have identified TIN2-R282H, the most common DC-causing mutation in shelterin subunit TIN2, as a separation-of-function mutant which impairs telomerase recruitment to telomeres, but not chromosome end protection. The telomerase recruitment defect conferred by TIN2-R282H is likely through a mechanism independent of TIN2’s role in anchoring TPP1 at telomeres, since TPP1 localization to telomeres is unaffected by the mutation.

Therapeutic targeting of replicative immortality

One of the hallmarks of malignant cell populations is the ability to undergo continuous proliferation. This property allows clonal lineages to acquire sequential aberrations that can fuel increasingly autonomous growth, invasiveness, and therapeutic resistance. Innate cellular mechanisms have evolved to regulate replicative potential as a hedge against malignant progression. When activated in the absence of normal terminal differentiation cues, these mechanisms can result in a state of persistent cytostasis. This state, termed “senescence,” can be triggered by intrinsic cellular processes such as telomere dysfunction and oncogene expression, and by exogenous factors such as DNA damaging agents or oxidative environments. Despite differences in upstream signaling, senescence often involves convergent interdependent activation of tumor suppressors p53 and p16/pRB, but can be induced, albeit with reduced sensitivity, when these suppressors are compromised. Doses of conventional genotoxic drugs required to achieve cancer cell senescence are often much lower than doses required to achieve outright cell death. Additional therapies, such as those targeting cyclin dependent kinases or components of the PI3K signaling pathway, may induce senescence specifically in cancer cells by circumventing defects in tumor suppressor pathways or exploiting cancer cells’ heightened requirements for telomerase. Such treatments sufficient to induce cancer cell senescence could provide increased patient survival with fewer and less severe side effects than conventional cytotoxic regimens. This positive aspect is countered by important caveats regarding senescence reversibility, genomic instability, and paracrine effects that may increase heterogeneity and adaptive resistance of surviving cancer cells. Nevertheless, agents that effectively disrupt replicative immortality will likely be valuable components of new combinatorial approaches to cancer therapy.

Quantitative assessment of telomerase components in cancer cell lines

Besides its canonical function of catalyzing the formation of telomeric repeats, many groups have recently reported non-canonical functions of hTERT in particular, and telomerase in general. Regulating transcription is the central basis of non-canonical functions of telomerase. However, unlike reverse transcriptase activity of telomerase that requires only a few molecules of enzymatically active hTERT, non-canonical functions of hTERT or other telomerase components theoretically require several hundred copies. Here, we provide the first direct quantification of all the telomerase components in human cancer cell lines. We demonstrate that telomerase components do not exist in a 1:1 stoichiometric ratio, and there are several hundred copies of hTERT in cells. This provides the molecular basis of hTERT to function in other signaling cascades, including transcription.

Autophagy-independent senescence and genome instability driven by targeted telomere dysfunction

Telomere dysfunction plays a complex role in tumorigenesis. While dysfunctional telomeres can block the proliferation of incipient cancer clones by inducing replicative senescence, fusion of dysfunctional telomeres can drive genome instability and oncogenic genomic rearrangements. Therefore, it is important to define the regulatory pathways that guide these opposing effects. Recent work has shown that the autophagy pathway regulates both senescence and genome instability in various contexts. Here, we apply models of acute telomere dysfunction to determine whether autophagy modulates the resulting genome instability and senescence responses. While telomere dysfunction rapidly induces autophagic flux in human fibroblast cell lines, inhibition of the autophagy pathway does not have a significant impact upon the transition to senescence, in contrast to what has previously been reported for oncogene-induced senescence. Our results suggest that this difference may be explained by disparities in the development of the senescence-associated secretory phenotype. We also show that chromosome fusions induced by telomere dysfunction are comparable in autophagy-proficient and autophagy-deficient cells. Altogether, our results highlight the complexity of the senescence-autophagy interface and indicate that autophagy induction is unlikely to play a significant role in telomere dysfunction-driven senescence and chromosome fusions.

An antiapoptotic role for telomerase RNA in human immune cells independent of telomere integrity or telomerase enzymatic activity

Telomerase is a ribonucleoprotein complex that adds telomeric DNA to the ends of linear chromosomes. It contains two core canonical components: the essential RNA component, hTR, which provides the template for DNA synthesis, and the reverse transcriptase protein component, hTERT. Low telomerase activity in circulating peripheral blood mononuclear cells has been associated with a variety of diseases. It is unknown, however, whether telomerase, in addition to its long-term requirement for telomere maintenance, is also necessary for short-term immune cell proliferation and survival. We report that overexpression of enzymatically inactive hTR mutants protected against dexamethasone-induced apoptosis in stimulated CD4 T cells. Furthermore, hTR knockdown reproducibly induced apoptosis in the absence of any detectable telomere shortening or DNA damage response. In contrast, hTERT knockdown did not induce apoptosis. Strikingly, overexpression of hTERT protein caused apoptosis that was rescued by overexpression of enzymatically inactive hTR mutants. Hence, we propose that hTR can function as a noncoding RNA that protects from apoptosis independent of its function in telomerase enzymatic activity and long-term telomere maintenance in normal human immune cells. These results imply that genetic or environmental factors that alter hTR levels can directly affect immune cell function to influence health and disease.

Silencing of the hTERT gene by shRNA inhibits colon cancer SW480 cell growth in vitro and in vivo

Human telomerase reverse transcriptase (hTERT) is the key enzyme responsible for synthesizing and maintaining the telomeres on the ends of chromosomes, and it is essential for cell proliferation. This has made hTERT a focus of oncology research and an attractive target for anticancer drug development. In this study, we designed a small interfering RNA (siRNA) targeting the catalytic subunit of hTERT and tested its effects on the growth of telomerase-positive human colon carcinoma SW480 cells in vitro, as well as on the tumorigenicity of these cells in nude mice. Transient and stable transfection of hTERT siRNA into colon cancer SW480 cells suppressed hTERT expression, reduced telomerase activity and inhibited cell growth and proliferation. Knocking down hTERT expression in SW480 tumors xenografted into nude mice significantly slowed tumor growth and promoted tumor cell apoptosis. Our results suggest that hTERT is involved in carcinogenesis of human colon carcinoma, and they highlight the therapeutic potential of a hTERT knock-down approach.

The use of transformed IMR90 cell model to identify the potential extra-telomeric effects of hTERT in cell migration and DNA damage response

BACKGROUND

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomesase, is responsible for telomere maintenance and its reactivation is implicated in almost 90% human cancers. Recent evidences show that hTERT is essential for neoplastic transformation independent of its canonical function. However, the roles of hTERT in the process remain elusive. In the current work, we explore the extra-telomeric role of hTERT in the neoplastic transformation of fibroblast IMR90.

RESULTS

Here we established transformed IMR90 cells by co-expression of three oncogenic factors, namely, H-Ras, SV40 Large-T antigen and hTERT (RSH). The RSH-transformed cells acquired hallmarks of cancer, such as they can grow under anchorage independent conditions; self-sufficient in growth signals; attenuated response to apoptosis; and possessed recurrent chromosomal abnormalities. Furthermore, the RSH-transformed cells showed enhanced migration capability which was also observed in IMR90 cells expressing hTERT alone, indicating that hTERT plays a role in cell migration, and thus possibly contribute to their metastatic potential during tumor transformation. This notion was further supported by our microarray analysis. In addition, we found that Ku70 were exclusively upregulated in both RSH-transformed IMR90 cells and hTERT-overexpressing IMR90 cells, suggesting the potential role of hTERT in DNA damage response (DDR).

CONCLUSIONS

Collectively, our study revealed the extra-telomeric effects of hTERT in cell migration and DDR during neoplastic transformation.

A self-regulating template in human telomerase

Telomerase is a specialized reverse transcriptase (RT) containing an intrinsic telomerase RNA (TR) component. It synthesizes telomeric DNA repeats, (GGTTAG)n in humans, by reiteratively copying a precisely defined, short template sequence from the integral TR. The specific mechanism of how the telomerase active site uses this short template region accurately and efficiently during processive DNA repeat synthesis has remained elusive. Here we report that the human TR template, in addition to specifying the DNA sequence, is embedded with a single-nucleotide signal to pause DNA synthesis. After the addition of a dT residue to the DNA primer, which is specified by the 49 rA residue in the template, telomerase extends the DNA primer with three additional nucleotides and then pauses DNA synthesis. This sequence-defined pause site coincides precisely with the helix paired region 1 (P1)-defined physical template boundary and precludes the incorporation of nontelomeric nucleotides from residues outside the template region. Furthermore, this sequence-defined pausing mechanism is a key determinant, in addition to the P1-defined template boundary, for generating the characteristic 6-nt ladder banding pattern of telomeric DNA products in vitro. In the absence of the pausing signal, telomerase stalls nucleotide addition at multiple sites along the template, generating DNA products with heterogeneous terminal repeat registers. Our findings demonstrate that this unique self-regulating mechanism of the human TR template is essential for high-fidelity synthesis of DNA repeats.

Telomere extension by telomerase and ALT generates variant repeats by mechanistically distinct processes

Telomeres are terminal repetitive DNA sequences on chromosomes, and are considered to comprise almost exclusively hexameric TTAGGG repeats. We have evaluated telomere sequence content in human cells using whole-genome sequencing followed by telomere read extraction in a panel of mortal cell strains and immortal cell lines. We identified a wide range of telomere variant repeats in human cells, and found evidence that variant repeats are generated by mechanistically distinct processes during telomerase- and ALT-mediated telomere lengthening. Telomerase-mediated telomere extension resulted in biased repeat synthesis of variant repeats that differed from the canonical sequence at positions 1 and 3, but not at positions 2, 4, 5 or 6. This indicates that telomerase is most likely an error-prone reverse transcriptase that misincorporates nucleotides at specific positions on the telomerase RNA template. In contrast, cell lines that use the ALT pathway contained a large range of variant repeats that varied greatly between lines. This is consistent with variant repeats spreading from proximal telomeric regions throughout telomeres in a stochastic manner by recombination-mediated templating of DNA synthesis. The presence of unexpectedly large numbers of variant repeats in cells utilizing either telomere maintenance mechanism suggests a conserved role for variant sequences at human telomeres.

Dynamics of human telomerase RNA structure revealed by antisense oligonucleotide technique

Telomeres are the nucleoprotein complexes that cap the linear chromosome ends. Telomerase is a ribonucleoprotein that maintains telomere length in stem, embryonic and cancer cells. Somatic cells don't contain active telomerase and telomere function as mitotic clock and telomere length determines the number of cell divisions. Telomerase RNA (TER) contains the template for telomere synthesis and serves as a structural scaffold for holoenzyme assembly. We compared different oligonucleotide based methods for telomerase RNA inhibition, such as antisense oligonucleotides, knockdown by transient siRNA transfection and silencing by miRNA derived from short expressed RNA hairpin in HEK293 cells. All of these methods were applied to different TER regions. Our results revealed that CR2/CR3 domain of TER is accessible in vitro and in vivo and could serve as an optimal site for oligonucleotide-based telomerase silencing.