Telomerase activity reconstituted in vitro with purified human telomerase reverse transcriptase and human telomerase RNA component

A First-in-Class High-Throughput Screen to Discover Modulators of the Alternative Lengthening of Telomeres (ALT) Pathway

Telomeres are a protective cap that prevents chromosome ends from being recognized as double-stranded breaks. In somatic cells, telomeres shorten with each cell division due to the end replication problem, which eventually leads to senescence, a checkpoint proposed to prevent uncontrolled cell growth. Tumor cells avoid telomere shortening by activating one of two telomere maintenance mechanisms (TMMs): telomerase reactivation or alternative lengthening of telomeres (ALT). TMMs are a viable target for cancer treatment as they are not active in normal, differentiated cells. Whereas there is a telomerase inhibitor currently undergoing clinical trials, there are no known ALT inhibitors in development, partially because the complex ALT pathway is still poorly understood. For cancers such as neuroblastoma and osteosarcoma, the ALT-positive status is associated with an aggressive phenotype and few therapeutic options. Thus, methods that characterize the key biological pathways driving ALT will provide important mechanistic insight. We have developed a first-in-class phenotypic high-throughput screen to identify small-molecule inhibitors of ALT. Our screen measures relative C-circle level, an ALT-specific biomarker, to detect changes in ALT activity induced by compound treatment. To investigate epigenetic mechanisms that contribute to ALT, we screened osteosarcoma and neuroblastoma cells against an epigenetic-targeted compound library. Hits included compounds that target chromatin-regulating proteins and DNA damage repair pathways. Overall, the high-throughput C-circle assay will help expand the repertoire of potential ALT-specific therapeutic targets and increase our understanding of ALT biology.

Effects of hTERT transfection on the telomere and telomerase of Periplaneta americana cells in vitro

Telomere and telomerase are crucial factors in cell division and chromosome stability. Telomerase activity in most cells depends on the transcription control by the telomerase reverse transcriptase (TERT). The introduction of an exogenous human TERT (hTERT) in cultured cells could enhance telomerase activity and elongate the lifespan of various cells. Telomere elongation mechanisms vary between insects and are complex and unusual. Whether the use of exogenous hTERT can immortalize primary insect cells remains to be investigated. In this study, we used a recombinant virus expressing hTERT to infect primary cultured cells of Periplaneta americana and evaluated its effects on insect cell immortalization. We found that hTERT was successfully expressed and promoted the growth of P. americana cells, shortening their doubling time. This was due to the ability of hTERT to increase the activity of telomerase in P. americana cells, thus prolonging the telomeres. Our study lays the foundation for understanding the mechanisms of telomere elongation in P. americana, and suggests that the introduction of hTERT into insect cells could be an efficient way to establish certain insect cell lines.

SMG6 regulates DNA damage and cell survival in Hippo pathway kinase LATS2-inactivated malignant mesothelioma

Many genes responsible for Malignant mesothelioma (MM) have been identified as tumor suppressor genes and it is difficult to target these genes directly at a molecular level. We searched for the gene which showed synthetic lethal phenotype with LATS2, one of the MM causative genes and one of the kinases in the Hippo pathway. Here we showed that knockdown of SMG6 results in synthetic lethality in LATS2-inactivated cells. We found that this synthetic lethality required the nuclear translocation of YAP1 and TAZ. Both are downstream factors of the Hippo pathway. We also demonstrated that this synthetic lethality did not require SMG6 in nonsense-mediated mRNA decay (NMD) but in regulating telomerase reverse transcriptase (TERT) activity. In addition, the RNA-dependent DNA polymerase (RdDP) activity of TERT was required for this synthetic lethal phenotype. We confirmed the inhibitory effects of LATS2 and SMG6 on cell proliferation in vivo. The result suggests an interaction between the Hippo and TERT signaling pathways. We also propose that SMG6 and TERT are novel molecular target candidates for LATS2-inactivated cancers such as MM.

Telomere Maintenance and the cGAS-STING Pathway in Cancer

Cancer cells exhibit the unique characteristics of high proliferation and aberrant DNA damage response, which prevents cancer therapy from effectively eliminating them. The machinery required for telomere maintenance, such as telomerase and the alternative lengthening of telomeres (ALT), enables cancer cells to proliferate indefinitely. In addition, the molecules in this system are involved in noncanonical pro-tumorigenic functions. Of these, the function of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, which contains telomere-related molecules, is a well-known contributor to the tumor microenvironment (TME). This review summarizes the current knowledge of the role of telomerase and ALT in cancer regulation, with emphasis on their noncanonical roles beyond telomere maintenance. The components of the cGAS-STING pathway are summarized with respect to intercell communication in the TME. Elucidating the underlying functional connection between telomere-related molecules and TME regulation is important for the development of cancer therapeutics that target cancer-specific pathways in different contexts. Finally, strategies for designing new cancer therapies that target cancer cells and the TME are discussed.

Telomerase ribonucleoprotein and genome integrity-An emerging connection in protozoan parasites

Telomerase has an established role in telomere maintenance in eukaryotes. However, recent studies have begun to implicate telomerase in cellular roles beyond telomere maintenance. Specifically, evidence is emerging of cross-talks between telomerase mediated telomere homeostasis and DNA repair pathways. Telomere shortening due to the end replication problem is a constant threat to genome integrity in eukaryotic cells. This poses a particular problem in unicellular parasitic protists because their major virulence genes are located at the subtelomeric loci. Although telomerase is the major regulator of telomere lengthening in eukaryotes, it is less studied in the ancient eukaryotes, including clinically important human pathogens. Recent research is highlighting interplay between telomerase and the DNA damage response in human parasites. The importance of this interplay in pathogen virulence is only beginning to be illuminated, including the potential to highlight novel developmental regulation of telomerase in parasites who transition between multiple developmental stages throughout their life cycle. In this review, we will discuss the telomerase ribonucleoprotein enzyme and DNA repair pathways with emerging views in human parasites to give a broader perspective of the possible connection of telomere, telomerase, and DNA repair pathways across eukaryotic lineages and highlight their potential role in pathogen virulence. This article is categorized under: RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Single-Run Catalysis and Kinetic Control of Human Telomerase Holoenzyme

Genome stability in eukaryotic cells relies on proper maintenance of telomeres at the termini of linear chromosomes. Human telomerase holoenzyme is required for maintaining telomere stability in a majority of proliferative human cells, making it essential for control of cell division and aging, stem cell maintenance, and development and survival of tumor or cancer. A dividing human cell usually contains a limited number of active telomerase holoenzymes. Recently, we discovered that a human telomerase catalytic site undergoes catalysis-dependent shut-off and an inactive site can be reactivated by cellular fractions containing human intracellular telomerase-activating factors (hiTAFs). Such ON-OFF control of human telomerase activity suggests a dynamic switch between inactive and active pools of the holoenzymes. In this review, we will link the ON-OFF control to the thermodynamic and kinetic properties of human telomerase holoenzymes, and discuss its potential contributions to the maintenance of telomere length equilibrium. This treatment suggests probabilistic fluctuations in the number of active telomerase holoenzymes as well as the number of telomeres that are extended in a limited number of cell cycles, and may be an important component of a fully quantitative model for the dynamic control of telomerase activities and telomere lengths in different types of eukaryotic cells.

Telomerase-based Cancer Therapeutics: A Review on their Clinical Trials

Telomeres are protective chromosomal ends that shield the chromosomes from DNA damage, exonucleolytic degradation, recombination, and end-to-end fusion. Telomerase is a ribonucleoprotein that adds TTAGGG tandem repeats to the telomeric ends. It has been observed that 85 to 90% of human tumors express high levels of telomerase, playing a crucial role in the development of cancers. Interestingly, the telomerase activity is generally absent in normal somatic cells. This selective telomerase expression has driven scientists to develop novel anti-cancer therapeutics with high specificity and potency. Several advancements have been made in this area, which is reflected by the enormous success of the anticancer agent Imetelstat. Since the discovery of Imetelstat, several research groups have contributed to enrich the therapeutic arsenal against cancer. Such contributions include the application of new classes of small molecules, peptides, and hTERT-based immunotherapeutic agents (p540, GV1001, GRNVAC1 or combinations of these such as Vx-001). Many of these therapeutic tools are under different stages of clinical trials and have shown promising outcomes. In this review, we highlight the current status of telomerase-based cancer therapeutics and the outcome of these investigations.

Accelerating the Production of Druggable Targets: Eukaryotic Cell-Free Systems Come into Focus

In the biopharmaceutical pipeline, protein expression systems are of high importance not only for the production of biotherapeutics but also for the discovery of novel drugs. The vast majority of drug targets are proteins, which need to be characterized and validated prior to the screening of potential hit components and molecules. A broad range of protein expression systems is currently available, mostly based on cellular organisms of prokaryotic and eukaryotic origin. Prokaryotic cell-free systems are often the system of choice for drug target protein production due to the simple generation of expression hosts and low cost of preparation. Limitations in the production of complex mammalian proteins appear due to inefficient protein folding and posttranslational modifications. Alternative protein production systems, so-called eukaryotic cell-free protein synthesis systems based on eukaryotic cell-lysates, close the gap between a fast protein generation system and a high quality of complex mammalian proteins. In this study, we show the production of druggable target proteins in eukaryotic cell-free systems. Functional characterization studies demonstrate the bioactivity of the proteins and underline the potential for eukaryotic cell-free systems to significantly improve drug development pipelines.

Biological Actions of the Hsp90-binding Immunophilins FKBP51 and FKBP52

Immunophilins are a family of proteins whose signature domain is the peptidylprolyl-isomerase domain. High molecular weight immunophilins are characterized by the additional presence of tetratricopeptide-repeats (TPR) through which they bind to the 90-kDa heat-shock protein (Hsp90), and via this chaperone, immunophilins contribute to the regulation of the biological functions of several client-proteins. Among these Hsp90-binding immunophilins, there are two highly homologous members named FKBP51 and FKBP52 (FK506-binding protein of 51-kDa and 52-kDa, respectively) that were first characterized as components of the Hsp90-based heterocomplex associated to steroid receptors. Afterwards, they emerged as likely contributors to a variety of other hormone-dependent diseases, stress-related pathologies, psychiatric disorders, cancer, and other syndromes characterized by misfolded proteins. The differential biological actions of these immunophilins have been assigned to the structurally similar, but functionally divergent enzymatic domain. Nonetheless, they also require the complementary input of the TPR domain, most likely due to their dependence with the association to Hsp90 as a functional unit. FKBP51 and FKBP52 regulate a variety of biological processes such as steroid receptor action, transcriptional activity, protein conformation, protein trafficking, cell differentiation, apoptosis, cancer progression, telomerase activity, cytoskeleton architecture, etc. In this article we discuss the biology of these events and some mechanistic aspects.

TERT promoter mutations in telomere biology

Telomere repeats at chromosomal ends, critical to genome integrity, are maintained through an elaborate network of proteins and pathways. Shelterin complex proteins shield telomeres from induction of DNA damage response to overcome end protection problem. A specialized ribonucleic protein, telomerase, maintains telomere homeostasis through repeat addition to counter intrinsic shortcomings of DNA replication that leads to gradual sequence shortening in successive mitoses. The biogenesis and recruitment of telomerase composed of telomerase reverse transcriptase (TERT) subunit and an RNA component, takes place through the intricate machinery that involves an elaborate number of molecules. The synthesis of telomeres remains a controlled and limited process. Inherited mutations in the molecules involved in the process directly or indirectly cause telomeropathies. Telomerase, while present in stem cells, is deactivated due to epigenetic silencing of the rate-limiting TERT upon differentiation in most of somatic cells with a few exceptions. However, in most of the cancer cells telomerase reactivation remains a ubiquitous process and constitutes one of the major hallmarks. Discovery of mutations within the core promoter of the TERT gene that create de novo binding sites for E-twenty-six (ETS) transcription factors provided a mechanism for cancer-specific telomerase reactivation. The TERT promoter mutations occur mainly in tumors from tissues with low rates of self-renewal. In melanoma, glioma, hepatocellular carcinoma, urothelial carcinoma and others, the promoter mutations have been shown to define subsets of patients with adverse disease outcomes, associate with increased transcription of TERT, telomerase reactivation and affect telomere length; in stem cells the mutations inhibit TERT silencing following differentiation into adult cells. The TERT promoter mutations cause an epigenetic switch on the mutant allele along with recruitment of pol II following the binding of GABPA/B1 complex that leads to mono-allelic expression. Thus, the TERT promoter mutations hold potential as biomarkers as well as future therapeutic targets.

Roles of telomeres and telomerase in cancer, and advances in telomerase-targeted therapies

Telomeres maintain genomic integrity in normal cells, and their progressive shortening during successive cell divisions induces chromosomal instability. In the large majority of cancer cells, telomere length is maintained by telomerase. Thus, telomere length and telomerase activity are crucial for cancer initiation and the survival of tumors. Several pathways that regulate telomere length have been identified, and genome-scale studies have helped in mapping genes that are involved in telomere length control. Additionally, genomic screening for recurrent human telomerase gene hTERT promoter mutations and mutations in genes involved in the alternative lengthening of telomeres pathway, such as ATRX and DAXX, has elucidated how these genomic changes contribute to the activation of telomere maintenance mechanisms in cancer cells. Attempts have also been made to develop telomere length- and telomerase-based diagnostic tools and anticancer therapeutics. Recent efforts have revealed key aspects of telomerase assembly, intracellular trafficking and recruitment to telomeres for completing DNA synthesis, which may provide novel targets for the development of anticancer agents. Here, we summarize telomere organization and function and its role in oncogenesis. We also highlight genomic mutations that lead to reactivation of telomerase, and mechanisms of telomerase reconstitution and trafficking that shed light on its function in cancer initiation and tumor development. Additionally, recent advances in the clinical development of telomerase inhibitors, as well as potential novel targets, will be summarized.

Telomerase repeat amplification protocol (TRAP) activity upon recombinant expression and purification of human telomerase in a bacterial system

Telomerase biogenesis is a highly regulated process that solves the DNA end-replication problem. Recombinant expression has so far been accomplished only within a eukaryotic background. Towards structural and functional analyses, we developed bacterial expression of human telomerase. Positive activity by the telomerase repeat amplification protocol (TRAP) was identified in cell extracts of Escherichia coli expressing a sequence-optimized hTERT gene, the full-length hTR RNA with a self-splicing hepatitis delta virus ribozyme, and the human heat shock complex of Hsp90, Hsp70, p60/Hop, Hsp40, and p23. The Hsp90 inhibitor geldanamycin did not affect post-assembly TRAP activity. By various purification methods, TRAP activity was also obtained upon expression of only hTERT and hTR. hTERT was confirmed by tandem mass spectrometry in a ∼120 kDa SDS-PAGE fragment from a TRAP-positive purification fraction. TRAP activity was also supported by hTR constructs lacking the box H/ACA small nucleolar RNA domain. End-point TRAP indicated expression levels within 3-fold of that from HeLa carcinoma cells, which is several orders of magnitude below detection by the direct assay. These results represent the first report of TRAP activity from a bacterium and provide a facile system for the investigation of assembly factors and anti-cancer therapeutics independently of a eukaryotic setting.

Identification of human telomerase assembly inhibitors enabled by a novel method to produce hTERT

Telomerase is the enzyme that maintains the length of telomeres. It is minimally constituted of two components: a core reverse transcriptase protein (hTERT) and an RNA (hTR). Despite its significance as an almost universal cancer target, the understanding of the structure of telomerase and the optimization of specific inhibitors have been hampered by the limited amount of enzyme available. Here, we present a breakthrough method to produce unprecedented amounts of recombinant hTERT and to reconstitute human telomerase with purified components. This system provides a decisive tool to identify regulators of the assembly of this ribonucleoprotein complex. It also enables the large-scale screening of small-molecules capable to interfere with telomerase assembly. Indeed, it has allowed us to identify a compound that inhibits telomerase activity when added prior to the assembly of the enzyme, while it has no effect on an already assembled telomerase. Therefore, the novel system presented here may accelerate the understanding of human telomerase assembly and facilitate the discovery of potent and mechanistically unique inhibitors.

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.

The telomerase reverse transcriptase subunit from the dimorphic fungus Ustilago maydis

In this study, we investigated the reverse transcriptase subunit of telomerase in the dimorphic fungus Ustilago maydis. This protein (Trt1) contains 1371 amino acids and all of the characteristic TERT motifs. Mutants created by disrupting trt1 had senescent traits, such as delayed growth, low replicative potential, and reduced survival, that were reminiscent of the traits observed in est2 budding yeast mutants. Telomerase activity was observed in wild-type fungus sporidia but not those of the disruption mutant. The introduction of a self-replicating plasmid expressing Trt1 into the mutant strain restored growth proficiency and replicative potential. Analyses of trt1 crosses in planta suggested that Trt1 is necessary for teliospore formation in homozygous disrupted diploids and that telomerase is haploinsufficient in heterozygous diploids. Additionally, terminal restriction fragment analysis in the progeny hinted at alternative survival mechanisms similar to those of budding yeast.

TERT promoter mutations in cancer development

Human telomerase reverse transcriptase (TERT) encodes a rate-limiting catalytic subunit of telomerase that maintains genomic integrity. TERT expression is mostly repressed in somatic cells with exception of proliferative cells in self-renewing tissues and cancer. Immortality associated with cancer cells has been attributed to telomerase over-expression. The precise mechanism behind the TERT activation in cancers has mostly remained unknown. The newly described germline and recurrent somatic mutations in melanoma and other cancers in the TERT promoter that create de novo E-twenty six/ternary complex factors (Ets/TCF) binding sites, provide an insight into the possible cause of tumor-specific increased TERT expression. In this review we discuss the discovery and possible implications of the TERT promoter mutations in melanoma and other cancers.

Telomerase trafficking and assembly in Xenopus oocytes

The core components of telomerase are telomerase RNA (TR) and telomerase reverse transcriptase (TERT). In vertebrate cells, TR and TERT have been reported to associate with intranuclear structures, including Cajal bodies and nucleoli as well as telomeres. Here, we examined the time course of both TR localization and assembly of TR with TERT in Xenopus oocytes. The major trafficking pathway for microinjected TR is through Cajal bodies into the nucleoplasm, with a fraction of TR found in nucleoli at later time points. Telomerase assembly precedes nucleolar localization of TR, and TR mutants that do not localize to nucleoli form active enzyme, indicating that localization of TR to nucleoli is not required for assembly with TERT. Assembly of telomerase coincides with Cajal-body localization; however, assembly is also unaffected by a CAB-box mutation (which significantly reduces association with Cajal bodies), suggesting that Cajal-body localization is not important for assembly. Our results suggest that assembly of TR with TERT occurs in the nucleoplasm. Unexpectedly, however, our experiments reveal that disruption of the CAB box does not eliminate early targeting to Cajal bodies, indicating that a role for Cajal bodies in telomerase assembly cannot be excluded on the basis of existing knowledge.

The N-terminus of hTERT contains a DNA-binding domain and is required for telomerase activity and cellular immortalization

Telomerase defers the onset of telomere damage-induced signaling and cellular senescence by adding DNA onto chromosome ends. The ability of telomerase to elongate single-stranded telomeric DNA depends on the reverse transcriptase domain of TERT, and also relies on protein:DNA contacts outside the active site. We purified the N-terminus of human TERT (hTEN) from Escherichia coli, and found that it binds DNA with a preference for telomeric sequence of a certain length and register. hTEN interacted with the C-terminus of hTERT in trans to reconstitute enzymatic activity in vitro. Mutational analysis of hTEN revealed that amino acids Y18 and Q169 were required for telomerase activity in vitro, but not for the interaction with telomere DNA or the C-terminus. These mutants did not reconstitute telomerase activity in cells, maintain telomere length, or extend cellular lifespan. In addition, we found that T116/T117/S118, while dispensable in vitro, were required for cellular immortalization. Thus, the interactions of hTEN with telomere DNA and the C-terminus of hTERT are functionally separable from the role of hTEN in telomere elongation activity in vitro and in vivo, suggesting other roles for the protein and nucleic acid interactions of hTEN within, and possibly outside, the telomerase catalytic core.

A truncated form of p23 down-regulates telomerase activity via disruption of Hsp90 function

The Hsp90-associated protein p23 modulates Hsp90 activity during the final stages of the chaperone pathway to facilitate maturation of client proteins. Previous reports indicate that p23 cleavage induced by caspases during cell death triggers destabilization of client proteins. However, the specific role of truncated p23 (Delta p23) in this process and the underlying mechanisms remain to be determined. One such client protein, hTERT, is a telomerase catalytic subunit regulated by several chaperone proteins, including Hsp90 and p23. In the present study, we examined the effects of p23 cleavage on hTERT stability and telomerase activity. Our data showed that overexpression of Delta p23 resulted in a decrease in hTERT levels, and a down-regulation in telomerase activity. Serine phosphorylation of Hsp90 was significantly reduced in cells expressing high levels of Delta p23 compared with those expressing full-length p23. Mutation analyses revealed that two serine residues (Ser-231 and Ser-263) in Hsp90 are important for activation of telomerase, and down-regulation of telomerase activity by Delta p23 was associated with inhibition of cell growth and sensitization of cells to cisplatin. Our data aid in determining the mechanism underlying the regulation of telomerase activity by the chaperone complex during caspase-dependent cell death.

Targeting to the endoplasmic reticulum improves the folding of recombinant human telomerase reverse transcriptase

Telomerase is a specialized reverse transcriptase that catalyzes the addition of telomeric repeats, TTAGGG in all vertebrates, to the ends of chromosomes. The lack of recombinant purified human telomerase reverse transcriptase (hTERT) has hampered biochemical and structural studies. The primary problem in generating active recombinant hTERT appears to be protein folding, which may be due to the fact that telomerase is a multi-component ribonucleoprotein complex. When expressed in most heterologous systems, recombinant hTERT is largely insoluble. Here we describe a protein expression system using a baculovirus vector that can be used to prepare properly folded, enzymatically active, hTERT. In this system, the recombinant hTERT is directed to the endoplasmic reticulum (ER), which is rich in chaperones. This increases the expression of soluble recombinant hTERT, promoting proper folding using intrinsic ER chaperone proteins.

Induction of nitric oxide synthase-dependent telomere shortening after functional inhibition of Hsp90 in human tumor cells

In most cancer cells, the lengths of telomeres, the functional DNA-protein complexes located at chromosome ends, are maintained by the ribonucleoprotein telomerase. Hsp90 facilitates the assembly of telomerase and remains associated with the functional complex, implying a direct involvement of Hsp90 in telomere length regulation. In an effort to elucidate the effects of Hsp90 inhibition on function and viability of human prostate cancer cells, both pharmacological (radicicol) and genetic (small interfering RNA) approaches were utilized to target Hsp90. Depletion of functional Hsp90 caused dramatic telomere shortening followed by apoptosis. Of particular significance, these cells exhibit a high level of nitric oxide synthase (NOS)-dependent free radical production, and simultaneous treatment of cells with the NOS inhibitor L-NAME resulted in telomere elongation and prevention of apoptosis. In addition, we observe significant DNA damage assessed by telomere dysfunction, although in the absence of a classical DNA damage response. Overall, our data suggest a novel mechanism whereby inhibition of Hsp90 disrupts free radical homeostasis and contributes directly to telomere erosion, further implicating Hsp90 as a potential therapeutic target for cancer cells.

Telomeres and telomerase as targets for anticancer drug development

In most human cancers, the telomere erosion problem has been bypassed through the activation of a telomere maintenance system (usually activation of telomerase). Therefore, telomere and telomerase are attractive targets for anti-cancer therapeutic interventions. Here, we review a large panel of strategies that have been explored to date, from small inhibitors of the catalytic sub-unit of telomerase to anti-telomerase immunotherapy and gene therapy. The many positive results that are reported from anti-telomere/telomerase assays suggest a prudent optimism for a possible clinical application in a close future. However, we discuss some of the main limits for these approaches of antitumour drug development and why significant work remains before a clinically useful drug can be proposed to patients.

Soluble domains of telomerase reverse transcriptase identified by high-throughput screening

Telomerase is a ribonucleoprotein complex responsible for extending the ends of eukaryotic chromosomes. Structural and biophysical studies of this enzyme have been limited by the inability to produce large amounts of recombinant protein. Here we perform a high-throughput screen to map regions of the Tetrahymena thermophila TERT (Telomerase Reverse Transcriptase) protein that are overexpressed in a soluble form in Escherichia coli using a GFP-fusion system. Many of the soluble protein domains identified do not coincide with domains inferred from multiple sequence alignment, so screening for fluorescent colonies provided information not otherwise readily obtained. The method revealed an essential, independently folded N-terminal domain that was expressed and purified with high yield and found to be suitable for structural analysis. These results provide a tool for future structural and biophysical studies of TERT.

Differential expression of genes associated with telomere length homeostasis and oncogenesis in an avian model

Telomere-binding proteins, their interaction partners and transcription factors play a prominent role in telomere maintenance and telomerase activation. We examined mRNA expression levels of tankyrase 1 and 2, TRF1 and 2, c-myc, TERT and TR in Gallus domesticus, the domestic chicken, by quantitative real-time PCR, establishing expression profiles for three contrasting cell systems: the pluripotent gastrula, differentiated embryo fibroblasts and transformed DT40 cells. All seven genes were up-regulated in DT40 cells compared to telomerase-negative CEFs and a majority of the genes were also up-regulated in the gastrula relative to CEFs. Surprisingly, we found TERT and TR transcripts in CEFs, albeit at low levels. TRF1 was down-regulated in the six CEF cultures by the time of culture growth arrest. A marked increase in the TRF2:TRF1 ratio occurred at or near senescence in all of the CEF cultures studied, with the most elevated ratio found in a short-lived culture in which TRF1 mRNA levels decreased two-fold and TRF2 levels increased 21-fold. This culture also showed highly reduced, degraded telomeres by Southern blot analysis. These data suggest that genes involved in telomere maintenance and telomerase induction are expressed differentially in pluripotent, differentiated and transformed cell systems.

Benzamide and 4-amino 1,8 naphthalimide treatment inhibit telomerase activity by down-regulating the expression of telomerase associated protein and inhibiting the poly(ADP-ribosyl)ation of telomerase reverse transcriptase in cultured cells

To test the role of poly(ADP-ribose) polymerase on the telomerase activity, we determined the telomerase activity in leukemic cells K562 treated with benzamide and 4-amino 1,8 naphthalimide (NAP), the inhibitors of PARP. We observed that both the agents inhibited telomerase activity in a dose-dependent manner. The doses of benzamide and NAP that inhibited telomerase activity to 50% of untreated control cells were 10.7 +/- 0.6 mm and 200 +/- 7 microm, respectively. Benzamide treatment (10 mm) inhibited telomerase activity in a time-dependent manner. We also tested the ability of benzamide to inhibit the telomerase activity in Chinese hamster V79 cells and observed similar inhibition of the telomerase activity. Expression of telomerase reverse transcriptase (TERT) and telomerase RNA component, detected by RT-PCR, remained unaltered by treatment with benzamide or NAP. On the contrary, the expression of telomerase associated protein (TEP1/TP1), as detected by RT-PCR and western blot analysis, was reduced by both the agents. Further, in K562 cells, immunoprecipitation with the anti-TERT IgG and probed anti-poly (ADP-ribose) IgG revealed that TERT was poly(ADP-ribosyl)ated in the physiological condition of cell growth and such poly(ADP-ribosyl)ation was inhibited by benzamide treatment. Decrease in TEP1/TP1 expression and poly(ADP-ribosyl)ation of TERT were correlated with the inhibition of PARP activity by benzamide, indicating that PARP had a role in telomerase activity through poly(ADP-ribosyl)ation of TERT and down-regulation of TEP1/TP1.

Telomerase: regulation, function and transformation

Work from several laboratories over the past decade indicates that the acquisition of constitutive telomerase expression is a critical step during the malignant transformation of human cells. Normal human cells transiently express low levels of telomerase, the ribonucleoprotein responsible for extending and maintaining telomeres, and exhibit telomere shortening after extended passage, whereas most cancers exhibit constitutive telomerase expression and maintain telomeres at stable lengths. These observations establish a direct connection between immortalization and stabilization of telomere structure. However, recent work suggests that telomerase also contributes to cancer development beyond its role in maintaining stable telomere lengths. In this review, we summarize recent observations that support the concept that telomerase plays multiple roles in facilitating human cell transformation.

The hTERT-protein and Ki-67 labelling index in recurrent and non-recurrent meningiomas

Meningiomas are considered as benign neoplasms affecting the coverings of the central nervous system and compromise approximately 20% of all intracranial tumours. However, a number of these tumours recur even after total resection. The aim of this study is to evaluate the prognostic significance for recurrence of the human telomerase catalytic subunit (hTERT) in the cells of meningiomas. The expression of hTERT-protein can be evaluated by immunohistochemical staining using a monoclonal antibody against hTERT (clone 44F42, NCL-L-hTERT). The interdependence between tumour recurrence and cell proliferation in this study is analysed by Ki-67 immunoreactivity (clone MIB-1). Archival material from 29 non-recurrent and 32 recurrent tumours has been evaluated, including specimens from World Health Organization (WHO) stages I (n = 73), II (n = 2) and III (n = 12). Although the tumours were categorized as benign meningiomas following the WHO classification, recurrence in 22 of 50 cases did not correlate with the tumour stage. For hTERT staining, the following results were found for nucleolar and total nuclear staining, respectively: non-recurrent meningiomas, 2.9% (+/- 7.7) and 3.0% (+/- 8.0); recurrent meningiomas at first resection, 16.8% (+/- 19.7) and 31.6% (+/- 30.2). Concerning the Ki-67 labelling index (LI): for the group of non-recurrent meningiomas, results were 2.1% (+/- 1.7) and for the recurrent group at first resection, 1.7% (+/- 2.0). A significant difference was seen for the hTERT staining (P < 0.001) between the non-recurrent and recurrent meningiomas, whereas no statistical significance was found for Ki-67. In conclusion hTERT-positive meningiomas had a high incidence for recurrence. Ki-67 was a good marker of cell proliferation status of the tumours, but did not correlate with recurrence; thus, hTERT alone seemed to be a potential predictor for recurrence.

Different role of functional domains of hTR in DNA binding to telomere and telomerase reconstruction

Even if template sequence of hTR played an essential role in telomere binding, a 326 nucleotide fragment of hTR containing template, pseudoknot, and CR4-5 domains is critical for both binding with telomeric DNA and reconstitution of telomerase activity. A functional study with antisense oligonucleotides suggested that targeted disruption of the template region efficiently abrogated both telomeric DNA binding and telomerase activity, whereas disruption of the CR4-5 region induced only loss of telomerase activity. hTR interacts with telomeric DNA via structural region composed of the template, pseudoknot, and CR4-5 domains, however, each structural domain plays a distinct role in telomere binding and telomerase activity reconstitution.

Nucleolin interacts with telomerase

Telomerase is a specialized reverse transcriptase composed of core RNA and protein subunits which plays essential roles in maintaining telomeres in actively dividing cells. Recent work indicates that telomerase shuttles between subcellular compartments during assembly and in response to specific stimuli. In particular, telomerase colocalizes with nucleoli in normal human fibroblasts. Here, we show that nucleolin, a major nucleolar phosphoprotein, interacts with telomerase and alters its subcellular localization. Nucleolin binds the human telomerase reverse transcriptase subunit (hTERT) through interactions with its RNA binding domain 4 and carboxyl-terminal RGG domain, and this binding also involves the telomerase RNA subunit hTERC. The protein-protein interaction between nucleolin and hTERT is critical for the nucleolar localization of hTERT. These findings indicate that interaction of hTERT and nucleolin participates in the dynamic intracellular localization of telomerase complex.

Drug Insight: cancer cell immortality—telomerase as a target for novel cancer gene therapies

Rapid advances in our understanding of the molecular basis of cancer development and progression over the past three decades have led to the design of new potential cancer therapies. High throughput target validation and expression studies are expected to yield a powerful arsenal of new cancer treatments, but untangling the complex pathways underlying the major cancer phenotypes remains a significant challenge. A considerable body of evidence in recent years implicates deregulated expression of a single multi-component enzyme, telomerase, as a causative factor at the heart of immortalization in the vast majority of human tumors. This review highlights the potential of telomerase as a target for novel cancer therapies. The potential of exploiting the selectivity of the telomerase family of genes within cancer cells to develop gene therapy strategies is discussed, and the progress towards translating these novel therapeutics from the laboratory to the clinic is reviewed.

Function of HSP90 and p23 in the telomerase complex of thyroid tumors

Recently, studies on endocrine tumors revealed a potential role of telomerase in the dedifferentiation and/or malignant transition of these tumors. Telomerase is a ribonucleoprotein complex that includes the telomerase RNA component (hTR), the telomerase-associated protein (TP1), and the telomerase catalytic subunit (hTERT). Previously, the chaperones p23 and HSP90 have been described as additional telomerase regulators. To test whether the interactions of these genes are reflected in the dedifferentiation of thyroid tumors, we determined their mRNA and/or protein expression in 30 normal (tumor-free) thyroid tissues (NT), 35 follicular adenomas (FAD), 42 papillary carcinomas (PTC), 38 follicular carcinomas (FTC), 25 poorly differentiated carcinomas (PDTC), and 34 undifferentiated carcinomas (UTC). We then compared the results with telomerase activity. RT-PCR analysis revealed that TP1 was ubiquitously expressed. hTR was found in 50-94% of malignant tumors, in contrast to 7% of NT and 26% of FAD. hTERT was clearly associated with aggressive biological behavior. Ninety-two to 100% of the malignant tumors were positive for hTERT protein, whereas NT and FAD were negative in 100% and 94%, respectively. HSP90 mRNA and protein showed a close relationship to hTERT. p23 protein was negative in NT and positive in 3% of FAD, 39% of FTC, 40% of PTC, 44% of PDTC and 47% of UTC. High telomerase activity was measurable in hTERT and HSP90-positive tissues only. Our data show that the common expression of hTERT and HSP90 regulates telomerase activity in thyroid carcinomas. Chaperone p23 is involved in the telomeric complex to a lesser extent, but its expression is stronger in carcinomas than in non-malignant thyroid tissues. The expression profile of telomerase components represents an additional prognostic marker that may identify more aggressive thyroid tumors.

A human telomerase-associated nuclease

Ciliate and yeast telomerase possess a nucleolytic activity capable of removing DNA from the 3' end of a single-stranded oligonucleotide substrate. The nuclease activity is thought to assist in enzyme proofreading and/or processivity. Herein, we report a previously uncharacterized human telomerase-associated nuclease activity that shares several properties with ciliate and yeast telomerases. Partially purified human telomerase, either from cell extracts or recombinantly produced, demonstrated an ability to remove 3' nontelomeric nucleotides from a substrate containing 5' telomeric DNA, followed by extension of the newly exposed telomeric sequence. This cleavage/extension activity was apparent at more than one position within the telomeric DNA and was influenced by sequences 5' to the telomeric/nontelomeric boundary and by substitution with a methylphosphonate moiety at the telomeric/nontelomeric DNA boundary. Our data suggest that human telomerase is associated with an evolutionarily conserved nucleolytic activity and support a model in which telomerase-substrate interactions can occur distal from the 3' primer end.

Tetrahymena telomerase is active as a monomer

Telomerase is an enzyme that utilizes an internal RNA molecule as a template for the extension of chromosomal DNA ends. The catalytic core of telomerase consists of the RNA subunit and a protein reverse transcriptase subunit, known as telomerase reverse transcriptase (TERT). It has previously been shown that both yeast and human telomerase can form dimers or multimers in which one RNA in the complex can influence the activity of another. To test the proposal that dimerization might be essential for telomerase activity, we sought to determine whether Tetrahymena thermophila telomerase is active as a dimer or a monomer. Recombinant Tetrahymena telomerase eluted from a gel filtration column at the size of a monomeric complex (one RNA plus one TERT), and those fractions showed processive telomerase activity. We were unable to detect dimerization of Tetrahymena telomerase by coprecipitation experiments, by using tags on either the TERT protein or telomerase RNA. Therefore, a majority, if not all, of the recombinant Tetrahymena telomerase in our reconstitution system is present as a monomeric complex. We were also unable to detect dimerization of native telomerase from mating and vegetative Tetrahymena cell extracts. These results demonstrate that Tetrahymena telomerase does not need to dimerize to be active and processive.

Significance of immunological detection of human telomerase reverse transcriptase: re-evaluation of expression and localization of human telomerase reverse transcriptase

Human telomerase reverse transcriptase (hTERT) is a catalytic subunit of telomerase and is a potentially useful diagnostic marker for cancers. There have been few studies in which immunological detection of hTERT has been attempted and its subcellular localization has not been precisely defined. In the present study, we re-evaluated expression and localization of hTERT in cancer and normal cells using a newly developed antibody. Immunohistochemistry revealed that hTERT is expressed in approximately 80% of gynecological cancers, but some premalignant lesions exhibited weak expression of hTERT. Interestingly, not only nuclei but also cytoplasm of cancer cells were positive for hTERT staining. This finding was supported by the results of Western blot analysis of cell lines, in which both nuclear and cytoplasmic extracts exhibited significant hTERT bands. Cytoplasmic hTERT in cancer cells may be functional because the telomeric repeat amplification protocol assay of cytoplasmic extracts showed high levels of telomerase activity. Unexpectedly, not all normal primary cells and telomerase-negative cancer cell lines lacked hTERT expression; some exhibited weak TERT signals. In Western analysis, hTERT signals did not always correlate with telomerase activity of the various cell types. These findings suggest that functional hTERT is expressed in both the nucleus and cytoplasm of cancer cells and that hTERT expression does not strictly reflect telomerase activity. Further analysis is needed to clarify the biological significance of cytoplasmic hTERT.

Telomerase maintains telomere structure in normal human cells

In normal human cells, telomeres shorten with successive rounds of cell division, and immortalization correlates with stabilization of telomere length. These observations suggest that human cancer cells achieve immortalization in large part through the illegitimate activation of telomerase expression. Here, we demonstrate that the rate-limiting telomerase catalytic subunit hTERT is expressed in cycling primary presenescent human fibroblasts, previously believed to lack hTERT expression and telomerase activity. Disruption of telomerase activity in normal human cells slows cell proliferation, restricts cell lifespan, and alters the maintenance of the 3' single-stranded telomeric overhang without changing the rate of overall telomere shortening. Together, these observations support the view that telomerase and telomere structure are dynamically regulated in normal human cells and that telomere length alone is unlikely to trigger entry into replicative senescence.

Biochemical aspects of telomerase function

Arthur Kornberg "never met a dull enzyme" (For the Love of Enzymes: The Odyssey of a Biochemist, Harvard University Press, 1989) and telomerase is no exception. Telomerase is a remarkable polymerase that uses an internal RNA template to reverse-transcribe telomere DNA, one nucleotide at a time, onto telomeric, G-rich single-stranded DNA. In the 17 years since its discovery, the characterization of telomerase enzyme components has uncovered a highly conserved family of telomerase reverse transcriptases that, together with the telomerase RNA, appear to comprise the enzymatic core of telomerase. While not as comprehensively understood as yet, some telomerase-associated proteins also serve crucial roles in telomerase function in vivo, such as telomerase ribonudeoprotein (RNP) assembly, recruitment to the telomere, and the coordination of DNA replication at the telomere. A selected overview of the biochemical properties of this unique enzyme, in vitro and in vivo, will be presented.

Telomerase and tumorigenesis

The unique biology of telomeres and telomerase plays important roles in many aspects of mammalian cell physiology. Over the past decade, several lines of evidence have confirmed that the maintenance of telomeres and telomerase participate actively in the pathogenesis of human cancer. Specifically, activation of telomerase is strongly associated with cancer, and recent observations confirm that telomeres and telomerase perform important roles in both suppressing and facilitating malignant transformation by regulating genomic stability and cell lifespan. In addition, recent evidence suggests that telomerase activation contributes to tumorigenesis independently of its role in maintaining telomere length. Here we review recent developments in our understanding of the relationships among telomeres, telomerase, and cancer.

Design of a ribozyme targeting human telomerase reverse transcriptase and cloning of it’s gene

AIM: To design a hammerhead ribozyme targeting human telomerase reverse transcriptase (hTERT) and clone it’s gene for future use in the study of tumor gene therapy.

METHODS: Using the software RNAstructure, the secondary structure of hTERT mRNA was predicted and the cleavage site of ribozyme was selected. A hammerhead ribozyme targeting this site was designed and bimolecular fold between the ribozyme and hTERT was predicted. The DNA encoding the ribozyme was synthesized and cloned into pGEMEX-1 and the sequence of the ribozyme gene was confirmed by DNA sequencing.

RESULTS: Triplet GUC at 1742 of hTERT mRNA was chosen as the cleavage site of the ribozyme. The designed ribozyme was comprised of 22 nt catalytic core and 17 nt flanking sequence. Computer-aided prediction suggested that the ribozyme and hTERT mRNA could cofold into a proper conformation. Endonuclease restriction and DNA sequencing confirmed the correct insertion of the ribozyme gene into the vector pGEMEX-1.

CONCLUSION: This fundamental work of successful designing and cloning of an anti-hTERT hammerhead ribozyme has paved the way for further study of inhibiting tumor cell growth by cleaving hTERT mRNA with ribozyme.

Reconstitution of telomerase activity utilizing human catalytic subunit expressed in insect cells

Telomerase is a specialized reverse transcriptase responsible for maintaining the termini of linear chromosomes. The human enzyme is a ribonucleoprotein complex minimally comprising a catalytic protein moiety (hTERT) and an RNA subunit (hTR) which acts as the template for the reverse transcriptase reaction. Here we report expression of recombinant hTERT protein in insect cells utilizing a baculovirus expression system. The recombinant hTERT protein reconstitutes telomerase activity in the presence of hTR, either when co-expressed in insect cells or when added in vitro. Reconstitution of telomerase activity using this system will facilitate further analysis of the biochemical and biophysical properties of this enzyme.

Identification of serum anti-human telomerase reverse transcriptase (hTERT) auto-antibodies during progression to hepatocellular carcinoma

Human telomerase reverse transcriptase, hTERT, is the catalytic component of human telomerase. Expression of hTERT confers telomerase activity, indicating that hTERT is the rate-limiting component of human telomerase. Here we report the detection of anti-hTERT auto-antibodies in the sera derived patients with hepatocellular carcinoma using recombinant, purified hTERT as an antigen in an enzyme linked immunosorbent assay (ELISA). The levels of anti-hTERT antibodies in serum correlated with progression to hepatocellular carcinoma. In contrast, we detected only low levels of anti-hTERT auto-antibodies in the sera derived from 18 normal volunteers. The observation of hTERT auto-antibodies in the sera derived from cancer patients suggests that such auto-antibodies constitute novel and specific tumor marker.

Nucleolar localization of hTERT protein is associated with telomerase function

Telomerase is a ribonucleoprotein (RNP) complex that prevents telomeric erosion in eukaryotic cells. Although there are also other associated proteins in this complex, the catalytic activity of this complex is composed of two components. One is a reverse transcriptase subunit, TERT (telomerase reverse transcriptase); another is an RNA template subunit, TR (telomerase RNA). However, where these two parts are assembled in mammalian cells is unclear. In the present study, we investigated the intracellular distribution of human TERT (hTERT) protein and observed that hTERT protein in individual cells could concentrate in or be excluded from the nucleolus. Further we have identified a nucleolar targeting signal in the hTERT protein. Point mutations that disrupted this signal region interrupted telomerase RNP complex formation, decreased telomerase activity, and caused telomere shortening in cells transfected with mutated hTERT. Our results indicate that the amino acid sequence of the extreme N-terminus (1-15) of hTERT, which targets nucleolar localization of the protein, is required for full telomerase function.

Differential regulation of telomerase activity by six telomerase subunits

Telomerase is a specialized reverse transcriptase responsible for synthesizing telomeric DNA at the ends of chromosomes. Six subunits composing the telomerase complex have been cloned: hTR (human telomerase RNA), TEP1 (telomerase-associated protein 1), hTERT (human telomerase reverse transcriptase), hsp90 (heat shock protein 90), p23, and dyskerin. In this study, we investigated the role of each the telomerase subunit on the activity of telomerase. Through down- or upregulation of telomerase, we found that only hTERT expression changed proportionally with the level of telomerase activity. The other components, TEP1, hTR, hsp90, p23, and dyskerin remained at high and unchanged levels throughout modulation. In vivo and in vitro experiments with antisense oligonucleotides against each telomerase component were also performed. Telomerase activity was decreased or abolished by antisense treatment. To correlate clinical sample status, four pairs of normal and malignant tissues from patients with oral cancer were examined. Except for the hTERT subunit, which showed differential expression in normal and cancer tissues, all other components were expressed in both normal and malignant tissues. We conclude that hTERT is a regulatable subunit, whereas the other components are expressed more constantly in cells. Although hTERT has a rate-limiting effect on enzyme activity, the other telomerase subunits (hTR, TEP1, hsp90, p23, dyskerin) participated in full enzyme activity. We hypothesize that once hTERT is expressed, all other telomerase subunits can be assembled to form a highly active holoenzyme.

Two independent regions of human telomerase reverse transcriptase are important for its oligomerization and telomerase activity

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, contains motifs conserved among reverse transcriptases. Several nucleic acid-dependent polymerases that share a "fingers, palm, and thumb substructure" were shown to oligomerize. Here we demonstrate that hTERT also has this ability using partially purified recombinant hTERTs and mammalian cells co-expressing differently tagged hTERTs. Human template RNA (hTR), by contrast, has no effect on the structural oligomerization of hTERTs. Therefore, hTERT has an intrinsic ability of oligomerization in the absence of hTR. We identified two separate regions as essential for the oligomerization. The regions, amino acids 301-538 (amino-terminal region) and amino acids 914-928 (carboxyl-terminal region), are outside the fingers and palm substructure covering motif T to D and interact with each other in vivo. A substituted mutant of hTERT, hTERT-D712A-V713I, which was reported as a dominant negative form of hTERT, bound to the wild-type hTERT and inhibited its telomerase activity transiently expressed in telomerase-negative finite normal human fibroblast. The truncated forms of hTERT containing the binding region to the wild-type hTERT partially inhibited the telomerase activity, probably by preventing the wild-type hTERT from forming an oligomer. Taken together, the oligomerization of hTERT is an important step for telomerase activity.

Natural and pharmacological regulation of telomerase

The extremities of eukaryotic chromosomes are called telomeres. They have a structure unlike the bulk of the chromosome, which allows the cell DNA repair machinery to distinguish them from 'broken' DNA ends. But these specialised structures present a problem when it comes to replicating the DNA. Indeed, telomeric DNA progressively erodes with each round of cell division in cells that do not express telomerase, a specialised reverse transcriptase necessary to fully duplicate the telomeric DNA. Telomerase is expressed in tumour cells but not in most somatic cells and thus telomeres and telomerase may be proposed as attractive targets for the discovery of new anticancer agents.

Telomerase in the human organism

The intent of this review is to describe what is known and unknown about telomerase in somatic cells of the human organism. First, we consider the telomerase enzyme. Human telomerase ribonucleoproteins undergo at least three stages of cellular biogenesis: accumulation, catalytic activation and recruitment to the telomere. Next, we describe the patterns of telomerase regulation in the human soma. Telomerase activation in some cell types appears to offset proliferation-dependent telomere shortening, delaying but not defeating the inherent mitotic clock. Finally, we elaborate the connection between telomerase misregulation and human disease, in the contexts of inappropriate telomerase activation and telomerase deficiency. We discuss how our current perspectives on telomerase function could be applied to improving human health.

Complex regulatory mechanisms of telomerase activity in normal and cancer cells: how can we apply them for cancer therapy?

Telomerase activation is observed in almost 90% of human cancers but not in normal tissues of somatic origin and thus is a critical step for multistep carcinogenesis. A more thorough understanding of telomerase regulation may provide not only a molecular basis of cancer progression but also as a way to manipulate telomerase activity as a potential therapeutic modality. Recent progress in studies on telomerase regulation has shown that telomerase activation is achieved at various steps, including transcriptional and post-transcriptional levels of the telomerase reverse transcriptase (hTERT) gene. Although a number of potentially important mechanisms of telomerase activation have been proposed, none of the current models can fully explain tumor-specific activation of telomerase, suggesting a need for further extensive analysis. This review includes a summary of recent works on telomerase regulation and a discussion of how we can overcome this situation.

N-terminal domains of the human telomerase catalytic subunit required for enzyme activity in vivo

Most tumor cells depend upon activation of the ribonucleoprotein enzyme telomerase for telomere maintenance and continual proliferation. The catalytic activity of this enzyme can be reconstituted in vitro with the RNA (hTR) and catalytic (hTERT) subunits. However, catalytic activity alone is insufficient for the full in vivo function of the enzyme. In addition, the enzyme must localize to the nucleus, recognize chromosome ends, and orchestrate telomere elongation in a highly regulated fashion. To identify domains of hTERT involved in these biological functions, we introduced a panel of 90 N-terminal hTERT substitution mutants into telomerase-negative cells and assayed the resulting cells for catalytic activity and, as a marker of in vivo function, for cellular proliferation. We found four domains to be essential for in vitro and in vivo enzyme activity, two of which were required for hTR binding. These domains map to regions defined by sequence alignments and mutational analysis in yeast, indicating that the N terminus has also been functionally conserved throughout evolution. Additionally, we discovered a novel domain, DAT, that "dissociates activities of telomerase," where mutations left the enzyme catalytically active, but was unable to function in vivo. Since mutations in this domain had no measurable effect on hTERT homomultimerization, hTR binding, or nuclear targeting, we propose that this domain is involved in other aspects of in vivo telomere elongation. The discovery of these domains provides the first step in dissecting the biological functions of human telomerase, with the ultimate goal of targeting this enzyme for the treatment of human cancers.

Tetrahymena proteins p80 and p95 are not core telomerase components

Telomeres provide stability to eukaryotic chromosomes and consist of tandem DNA repeat sequences. Telomeric repeats are synthesized and maintained by a specialized reverse transcriptase, termed telomerase. Tetrahymena thermophila telomerase contains two essential components: Tetrahymena telomerase reverse transcriptase (tTERT), the catalytic protein component, and telomerase RNA that provides the template for telomere repeat synthesis. In addition to these two components, two proteins, p80 and p95, were previously found to copurify with telomerase activity and to interact with tTERT and telomerase RNA. To investigate the role of p80 and p95 in the telomerase enzyme, we tested the interaction of p80, p95, and tTERT in several different recombinant expression systems and in Tetrahymena extracts. Immunoprecipitation of recombinant proteins showed that although p80 and p95 associated with each other, they did not associate with tTERT. In in vitro transcription and translation lysates, tTERT was associated with telomerase activity, but p80 and p95 were not. p80 bound telomerase RNA as well as several other unrelated RNAs, suggesting p80 has a general affinity for RNA. Immunoprecipitations from Tetrahymena extracts also showed no evidence for an interaction between the core tTERT/telomerase RNA complex and the p80 and p95 proteins. These data suggest that p80 and p95 are not associated with the bulk of active telomerase in Tetrahymena.