Human telomerase contains evolutionarily conserved catalytic and structural subunits

Increased copy number of the TERT and TERC telomerase subunit genes in cancer cells

Telomerase is a ribonucleoprotein enzyme complex that adds telomeric repeats to the ends of chromosomes. The core telomerase components are the telomerase reverse transcriptase (TERT) catalytic subunit, and the telomerase RNA (TR) template subunit. In most cancers, telomerase is expressed at levels that are substantially higher than in normal cells. A known consequence of telomerase up-regulation which is considered to play a critical role in oncogenesis is maintenance of telomere length, and thus evasion by cancer cells of the normal limits on proliferation that are associated with the steady decrease in telomere length that accompanies proliferation of normal cells. It has also been suggested that telomerase up-regulation confers other advantages on cancer cells independent of its enzymatic activity. The mechanisms responsible for up-regulation of telomerase in cancer are incompletely understood. Here we review evidence suggesting that this frequently results from increased copy number of the genes encoding telomerase components. The TERT gene is located at human chromosome band 5p15.33, and the telomerase RNA component (TERC) gene that encodes TR is at 3q26.3. Chromosomal gains and gene amplifications involving chromosome arms 5p and 3q are among the most frequent in human tumors. Increased TERT and TERC gene dosage has been detected frequently in a variety of human cancers, and clonal evolution of cells with increased TERT or TERC copy number has been observed, suggesting a growth advantage in cells with increased TERT or TERC gene dosage.

Telomerase and cancer therapeutics

Telomerase is an attractive cancer target as it appears to be required in essentially all tumours for immortalization of a subset of cells, including cancer stem cells. Moreover, differences in telomerase expression, telomere length and cell kinetics between normal and tumour tissues suggest that targeting telomerase would be relatively safe. Clinical trials are ongoing with a potent and specific telomerase inhibitor, GRN163L, and with several versions of telomerase therapeutic vaccines. The prospect of adding telomerase-based therapies to the growing list of new anticancer products is promising, but what are the advantages and limitations of different approaches, and which patients are the most likely to respond?

SIRT1 acts as a nutrient-sensitive growth suppressor and its loss is associated with increased AMPK and telomerase activity

SIRT1, the mammalian homolog of SIR2 in Saccharomyces cerevisiae, is an NAD-dependent deacetylase implicated in regulation of lifespan. By designing effective short hairpin RNAs and a silent shRNA-resistant mutant SIRT1 in a genetically defined system, we show that efficient inhibition of SIRT1 in telomerase-immortalized human cells enhanced cell growth under normal and nutrient limiting conditions. Hematopoietic stem cells obtained from SIRT1-deficient mice also showed increased growth capacity and decreased dependency on growth factors. Consistent with this, SIRT1 inhibition was associated with increased telomerase activity in human cells. We also observed a significant increase in AMPK levels up on SIRT1 inhibition under glucose limiting conditions. Although SIRT1 suppression cooperated with hTERT to promote cell growth, either overexpression or suppression of SIRT1 alone had no effect on life span of human diploid fibroblasts. Our findings challenge certain models and connect nutrient sensing enzymes to the immortalization process. Furthermore, they show that in certain cell lineages, SIRT1 can act as a growth suppressor gene.

Telomerase redefined: Integrated regulation of hTR and hTERT for telomere maintenance and telomerase activity

Telomerase activity is dependent on the expression of 2 main core component genes, hTERT, which encodes the catalytic component and hTR (also called TERC), which encodes the RNA component. The correlation between telomerase activity and carcinogenesis has made this molecule of great interest in cancer research, however in order to fully understand the regulation of telomerase the mechanisms controlling both telomerase genes need to be studied. Some of these mechanisms of regulation have begun to emerge, however many more remain to be deciphered. For many years hTERT has been regarded as the limiting component of telomerase and much of the research in this field has focussed on its regulation, however it was clear from an early stage that hTR expression was also tightly regulated in normal cells and disease. More recently evidence from biochemistry, promoter studies and mouse models has been steadily increasing for a role for hTR as a limiting and essential component for telomerase activity and telomere maintenance. Perhaps the time has come to redefine our view of telomerase regulation. Knowledge of the mechanisms controlling both telomerase genes in normal systems and cancer may aid our understanding of the role of telomerase in carcinogenesis or highlight potential areas for therapeutic intervention. Here we review the essential requirement of hTR for telomere maintenance and telomerase activity in normal tissues and disease and focus on recent advances in our understanding of hTR regulation in relation to hTERT.

Identification of a new hTERT-derived HLA-A*0201 restricted, naturally processed CTL epitope

By the use of a neural network capable of performing quantitative predictions of peptides binding to HLA-A*0201 molecules, we identified a number of nonamer peptides derived from the catalytic subunit of telomerase, human telomerase reverse transcriptase (hTERT). Five nonimmunogenic peptides with measured binding affinities for HLA-A*0201 ranging from 155 to 1,298 nM were modified at the P1, P2 and P9 positions, respectively, to achieve stronger HLA-A*0201 binding. One peptide, mp30-38 (mp30), with an L to V substitution at position 9 was subsequently found to be immunogenic in mp30 immunized HLA-A*0201/H2K(b) or HHD transgenic mice. The T cell reactivity obtained was directed against both the mp30 and against the unmodified p30. Anti-mp30 specific T cells generated in HLA-A*0201 transgenic mice were dependent on TCR-CD8/MHC-I alpha3 binding and therefore not capable of recognizing mp30-pulsed human HLA-A*0201(+) cells or murine HLA-A*0201 transfectants. In order to show reactivity against naturally processed peptide in human tumor cells, an hTERT positive HLA-A*0201 negative colon carcinoma cell line (CCL220) was transfected with an HLA-A*0201/H2K(b) cDNA construct and used as target in ELISPOT and cytotoxicity assays. The data show that T cells from mp30 immunized HHD transgenic mice react specifically against the CCL220 transfectant indicating that p30 is naturally processed. In conclusion, we have identified a new CTL HLA-A*0201 restricted hTERT epitope, which is now, included in an ongoing phase 2 vaccine trial of patients with disseminated cancer.

Human bone marrow derived mesenchymal stem cells do not undergo transformation after long-term in vitro culture and do not exhibit telomere maintenance mechanisms

Significant improvement in the understanding of mesenchymal stem cell (MSC) biology has opened the way to their clinical use. However, concerns regarding the possibility that MSCs undergo malignant transformation have been raised. We investigated the susceptibility to transformation of human bone marrow (BM)-derived MSCs at different in vitro culture time points. MSCs were isolated from BM of 10 healthy donors and propagated in vitro until reaching either senescence or passage (P) 25. MSCs in the senescence phase were closely monitored for 8 to 12 weeks before interrupting the cultures. The genetic characterization of MSCs was investigated through array-comparative genomic hybridization (array-CGH), conventional karyotyping, and subtelomeric fluorescent in situ hybridization analysis both before and after prolonged culture. MSCs were tested for the expression of telomerase activity, human telomerase reverse transcriptase (hTERT) transcripts, and alternative lengthening of telomere (ALT) mechanism at different passages. A huge variability in terms of proliferative capacity and MSCs life span was noted between donors. In eight of 10 donors, MSCs displayed a progressive decrease in proliferative capacity until reaching senescence. In the remaining two MSC samples, the cultures were interrupted at P25 to pursue data analysis. Array-CGH and cytogenetic analyses showed that MSCs expanded in vitro did not show chromosomal abnormalities. Telomerase activity and hTERT transcripts were not expressed in any of the examined cultures and telomeres shortened during the culture period. ALT was not evidenced in the MSCs tested. BM-derived MSCs can be safely expanded in vitro and are not susceptible to malignant transformation, thus rendering these cells suitable for cell therapy approaches.

TERT promoter-driven adenovirus vector for cancer gene therapy via systemic injection

Adenovirus vectors (Adv) are used widely in cancer gene therapy research. However, the clinical application of Adv currently is limited to local, intratumoral administration; systemic administration leads to redundant transgene expression in the liver and subsequent hepatotoxicity. Here we replaced the conventional cytomegalovirus (CMV) promoter of Adv with a tumor-specific telomere reverse transcriptase (TERT) promoter, to restrict expression of the Adv-transduced transgene to tumor tissue alone. We evaluated the therapeutic and side effects after systemic administration of Adv expressing herpes simplex virus thymidine kinase (Ad-HSVtk) in mice bearing Meth-A tumors. Although systemically injected CMV promoter-driven Ad-HSVtk lacked therapeutic effect, mice injected with 2x10(11) viral particles containing TERT promoter-driven Ad-HSVtk showed inhibited tumor growth and prolonged survival with minimal side effects. Our results suggest that Adv in which transgene expression is driven by the TERT promoter are a promising prototype of tumor-targeting vectors for effective and safe cancer gene therapy.

Epithelial molecular markers in the peripheral blood of patients with colorectal cancer

Despite the modest improvements in patient survival from colorectal cancer in the last few decades, the overall five-year survival rate remains at 40 to 45 percent. Surgical resection is the mainstay of treatment for colorectal cancer; however, nearly one-half of all patients who undergo a potentially curative resection will relapse because of undetected micrometastasis. The fact that the overall survival rate remains poor strongly suggests that the dissemination of these cells occurs early in the disease process and emphasizes the need for finding feasible diagnostic methods with sufficient sensitivity and specificity. The most commonly used technique for the detection of nucleic acid material of disseminated tumor cells is the polymerase chain reaction. We critically review the literature on DNA and messenger ribonucleic acid molecular markers that have been used for the detection of circulating tumor cells in the peripheral blood of patients with colorectal cancer and other solid tumors as appropriate for comparison. The cytokeratins, particularly cytokeratin 19 and cytokeratin 20, are the most investigated prognostic markers, but even for these questions remain about their clinical value, and hence most recent studies are utilizing a combination of factors. There is an urgent need for standardized isolation and analysis techniques to be adopted thus allowing large-scale, appropriately controlled, multicenter trials to be undertaken on the most promising candidate markers.

The expression of gene transcripts of telomere-associated genes in human breast cancer: correlation with clinico-pathological parameters and clinical outcome

BACKGROUND

Telomerase is a ribonucleoprotein enzyme that synthesises telomeres in human germ cells, embryogenesis and in cancer, maintaining chromosomal length, stability and cellular immortality. The hTERT gene is the rate-limiting determinant of telomerase reactivation during immortalization and malignant transformation. Telomeric DNA-binding proteins have been attracting increasing interest due to their essential role in the regulation of telomeric DNA length and in protecting against chromosomal end-to-end fusion. These proteins include hTR, TRF1, TRF2, TANK1, TANK2, POT1, TIN2, EST1, and TEP. This study represents the first comprehensive investigation of the mRNA expression of key telomere-related genes in human breast cancer.

METHODS

One hundred and twenty seven tumour tissues and 33 normal tissues were analyzed. Levels of transcription of hTERT, hTR, TRF1, TRF2, TANK1, TANK2, POT1, TIN2, EST1, and TEP1 were determined using real-time quantitative PCR. The mRNA expression of these genes was normalized against CK19 and was then analyzed against the pathological parameters and clinical outcome over a 10 year follow up period.

RESULTS

The mRNA expressions of hTERT, hTR, TANK1, EST1, and TEP1 were higher in tumour samples compared with normal breast tissue. This reached statistical significance for EST1 when comparing good prognosis tumours with normal breast tissue (means=11013 vs 1160, P=0.05). Both hTERT and TEP1 levels significantly predicted overall survival (P=0.012 and 0.005 respectively) and disease-free survival (P=0.0011 and 0.01 respectively). The mRNA levels of TANK2 and POT1 were lower in malignant tissues compared with non-malignant breast tissues and this difference reached statistical significance when comparing the levels in normal tissues with those in advanced tumours (P=0.0008 and P=0.038 respectively). Their levels fell further with increasing tumour's stage and were higher in tumours from patients who remained disease free compared with those who developed local recurrence or distant metastasis or died from breast cancer.TRF2 showed a trend similar to that of TANK2 and POT1. Furthermore, there was a highly significant correlation between TANK1 expression and that of hTERT, hTR, TRF1, TRF2 and EST1, (r=0.533, 0.586, 0.608, 0.644 and 0.551 respectively, P<0.001).

CONCLUSIONS

Genes encoding telomere-associated proteins display different patterns of mRNA expression in human breast cancer, and in normal breast tissue, suggesting different and sometimes opposing roles in mammary carcinogenesis. hTERT, hTR, TANK1, EST1 and TEP1 seem to be up-regulated, with hTERT and TEP1 correlating with clinical outcome. Conversely, TANK2 and POT1 transcription levels demonstrate a compelling trend to be lower in malignant tissues and lower still in those patients who develop recurrent disease suggesting that TANK2 and POT1 may act as tumour suppressor genes possibly by negatively regulating telomerase activity.

Telomerase activity in sputum and telomerase and its components in biopsies of advanced lung cancer

PURPOSE

In this study, we explored the diagnostic utility of sputum telomerase activity as a non-invasive biomarker of lung cancer. In biopsies of lung cancer, the relationship of telomerase activity to telomerase reverse transcriptase (hTERT) and telomerase RNA component (hTERC) and to c-Myc expression was also evaluated.

METHODS

Paired biopsy and sputum samples were evaluated for telomerase activity by the telomerase repeat amplification protocol (TRAP) assay in 34 cases of lung cancer and in 30 control subjects without any evidence of lung cancer. hTERT and hTERC transcript expression was evaluated in 42 cases of lung cancer and compared to telomerase activity and c-Myc transcript expression.

RESULTS

Telomerase activity was present in 85.2% of biopsies and in 67.6% of paired sputum with a good concordance. Three out of the 30 negative controls showed a weak telomerase activity, all of whom had sarcoidosis. Thus, sputum telomerase activity had sensitivity, specificity, Negative Predictive Value and Positive Predictive Value of 67.6%, 90%, 71% and 88.46%, respectively. The hTERT levels correlated to the telomerase activity but not to the c-Myc oncogene expression.

CONCLUSIONS

In lung cancer, sputum telomerase activity is a candidate non-invasive biomarker of malignancy.

Concise review: Telomere biology in normal and leukemic hematopoietic stem cells

The measurement of telomere length can give an insight into the replicative history of the cells in question. Much of the observed telomere loss occurs at the stem and progenitor cell level, even though these populations express the enzyme telomerase. Telomerase-transfected hematopoietic stem cells (HSC), although able to maintain telomere length, are still limited in terms of ability to undergo sequential transplantation, and other factors require to be addressed to achieve optimal levels of stem cell expansion. Unchecked telomere loss by HSC, meanwhile, would appear to play a significant role in the pathogenesis of bone marrow failure, as observed in the condition dyskeratosis congenita. This heterogeneous inherited condition appears to exhibit telomerase dysfunction as a common final pathogenic mechanism. Although less well-established for acquired marrow failure syndromes, mutations in key telomerase components have been described. The identification of the leukemic stem cell (LSC), along with the desire to target this population with anti-leukemia therapy, demands that telomerase biology be fully understood in this cell compartment. Future studies using primary selected LSC-rich samples are required. A better understanding of telomerase regulation in this population may allow effective targeting of the telomerase enzyme complex using small molecule inhibitors or additional novel approaches. Disclosure of potential conflicts of interest is found at the end of this article.

Characterization of physical and functional anchor site interactions in human telomerase

Telomerase is a ribonucleoprotein reverse transcriptase (RT) that processively synthesizes telomeric repeats onto the ends of linear chromosomes to maintain genomic stability. It has been proposed that the N terminus of the telomerase protein subunit, telomerase RT (TERT), contains an anchor site that forms stable interactions with DNA to prevent enzyme-DNA dissociation during translocation and to promote realignment events that accompany each round of telomere synthesis. However, it is not known whether human TERT (hTERT) can directly interact with DNA in the absence of the telomerase RNA subunit. Here we use a novel primer binding assay to establish that hTERT forms stable and specific contacts with telomeric DNA in the absence of the human telomerase RNA component (hTR). We show that hTERT-mediated primer binding can be functionally uncoupled from telomerase-mediated primer extension. Our results demonstrate that the first 350 amino acids of hTERT have a critical role in regulating the strength and specificity of protein-DNA interactions, providing additional evidence that the TERT N terminus contains an anchor site. Furthermore, we establish that the RT domain of hTERT mediates important protein-DNA interactions. Collectively, these data suggest that hTERT contains distinct anchor regions that cooperate to help regulate telomerase-mediated DNA recognition and elongation.

The structure and function of telomerase reverse transcriptase

The structure and integrity of telomeres are essential for genome stability. Telomere dysregulation can lead to cell death, cell senescence, or abnormal cell proliferation. The maintenance of telomere repeats in most eukaryotic organisms requires telomerase, which consists of a reverse transcriptase (RT) and an RNA template that dictates the synthesis of the G-rich strand of telomere terminal repeats. Structurally, telomerase reverse transcriptase (TERT) contains unique and variable N- and C-terminal extensions that flank a central RT-like domain. The enzymology of telomerase includes features that are both similar to and distinct from those characteristic of other RTs. Two distinguishing features of TERT are its stable association with the telomerase RNA and its ability to repetitively reverse transcribe the template segment of RNA. Here we discuss TERT structure and function; its regulation by RNA-DNA, TERT-DNA, TERT-RNA, TERT-TERT interactions, and TERT-associated proteins; and the relationship between telomerase enzymology and telomere maintenance.

Telomerase (hTERT 611–626) serves as a tumor antigen in B-cell chronic lymphocytic leukemia and generates spontaneously antileukemic, cytotoxic T cells

OBJECTIVE

Human telomerase reverse transcriptase (hTERT) is the catalytic subunit of telomerase. In B-cell chronic lymphocytic leukemia (B-CLL), telomerase activity is increased in about 75% of patients. The aim of this study was to analyze whether B-CLL patients with telomerase-positive leukemic cells had naturally occurring, telomerase-specific T cells that might be utilized for immune-mediated lysis of autologous tumor cells.

METHODS

Spontaneous T-cell immunity and cytotoxicity against hTERT was explored in B-CLL. Nineteen of 25 B-CLL patients (76%) expressed hTERT (reverse transcriptase polymerase chain reaction) and 10 were selected for specific T-cell analysis against hTERT.

RESULTS

The stimulation index (SI) of T cells from seven telomerase-positive patients stimulated with a 16aa hTERT peptide (611-626) loaded onto dendritic cells (DC) was 33.9 +/- 15.4 (mean SI +/- standard error of mean) and 13.2 +/- 5.6 against a Ras control peptide (p = 0.05), whereas the corresponding SI values for three telomerase-negative patients were 5.3 +/- 5.3 against the hTERT 611-626 peptide and 10.3 +/- 6.5 against the Ras peptide, respectively; and for three healthy controls, 5.4 +/- 0.9 against the hTERT 611-626 peptide and 4.5 +/- 1.0 against the Ras peptide (both not significant). Blocking experiments revealed that the specific responses were major histocompatibility complex (MHC) class I and MHC class II restricted. DC pulsed with the hTERT-peptide generated MHC class I-restricted, hTERT-specific cytotoxic T lymphocytes in six of seven telomerase-positive patients; mean cytotoxicity of hTERT-stimulated T cells was 49.8% +/- 9.3% vs 13.1 +/- 2.9% for Ras-stimulated T cells (p < 0.05). In three of three telomerase-negative patients, no hTERT-specific cytotoxic T lymphocytes could be expanded.

CONCLUSION

Telomerase-positive B-CLL patients have spontaneously occurring cytotoxic hTERT-specific T cells. This antigen might be explored as a therapeutic vaccine in B-CLL.

The active site residue Valine 867 in human telomerase reverse transcriptase influences nucleotide incorporation and fidelity

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of human telomerase, contains conserved motifs common to retroviral reverse transcriptases and telomerases. Within the C motif of hTERT is the Leu866-Val867-Asp868-Asp869 tetrapeptide that includes a catalytically essential aspartate dyad. Site-directed mutagenesis of Tyr183 and Met184 residues in HIV-1 RT, residues analogous to Leu866 and Val867, revealed that they are key determinants of nucleotide binding, processivity and fidelity. In this study, we show that substitutions at Val867 lead to significant changes in overall enzyme activity and telomere repeat extension rate, but have little effect on polymerase processivity. All Val867 substitutions examined (Ala, Met, Thr) led to reduced repeat extension rates, ranging from approximately 20 to 50% of the wild-type rate. Reconstitution of V867M hTERT and telomerase RNAs (TRs) with mutated template sequences revealed the effect on extension rate was associated with a template copying defect specific to template A residues. Furthermore, the Val867 hTERT mutants also displayed increased nucleotide incorporation fidelity, implicating Val867 as a determinant of telomerase fidelity. These findings suggest that by evolving to have a valine at position 867, the wild-type hTERT protein may have partially compromised polymerase fidelity for optimal and rapid repeat synthesis.

Ischemic tissue repair by autologous bone marrow-derived stem cells: scientific basis and preclinical data

The success of therapies targeting acute myocardial ischemia and the aging of the population due to improved general medical care has resulted in an increasing population of patients with chronic myocardial ischemia and congestive heart failure who remain symptomatic despite having exhausted the currently available therapeutic options. In this chapter we review the scientific underpinnings of autologous bone marrow-derived cell therapy and the early clinical experience that has fuelled interest in this approach.

Specific telomere dysfunction induced by GRN163L increases radiation sensitivity in breast cancer cells

PURPOSE

Telomerase is expressed in 80-90% of tumor cells, but is absent in most somatic cells. The absence of telomerase activity results in progressive telomere shortening, leading to cellular senescence or death through deoxyribonucleic acid (DNA) damage signals. In addition, a role for telomerase in DNA damage repair has also been suggested. A specific telomerase inhibitor, GRN163L that is complementary to the template region of the telomerase ribonucleic acid component (hTR). We hypothesized that exposure to GRN163L, either through immediate inhibition of telomerase activity or through eventual telomere shortening and dysfunction, may enhance radiation sensitivity. Our goal was to test whether the treatment with GRN163L enhances sensitivity to irradiation (IR) in MDA-MB-231 breast cancer cells.

METHODS AND MATERIALS

The MDA-MB-231 breast cancer cells were treated with or without GRN163L for 2-42 days. Inhibition of telomerase activity and shortening of telomeres were confirmed. Cells were then irradiated and clonogenic assays were performed to show cell survival differences. In vivo studies using MDA-MB-231 xenografts were performed to corroborate the in vitro results.

RESULTS

We show that cells with shortened telomeres due to GRN163L enhance the effect on IR reducing survival by an additional 30% (p < 0.01). These results are confirmed in vivo, with a significant decrease in tumor growth in mice exposed to GRN163L.

CONCLUSIONS

We found that GRN163L is a promising adjuvant treatment in combination with radiation therapy that may improve the therapeutic index by enhancing the radiation sensitivity. These studies prompt further investigation as to whether this combination can be applied to other cancers and the clinic.

Small molecule, oligonucleotide-based telomerase template inhibition in combination with cytolytic therapy in an in vitro androgen-independent prostate cancer model

PURPOSE

Determine the efficacy and timing of small molecule oligonucleotide-based inhibitors to the enzyme telomerase in an in vitro model of androgen-independent, osseous prostate cancer.

MATERIALS AND METHODS

Telomerase was inhibited in prostate cancer cell lines C4-2/C4-2B and in controls by using small molecule antisense oligonucleotide-based inhibitors alone or in various combinations of small-dose Taxotere (sanofi-aventis, Bridgewater, NJ) and/or conditionally replication competent adenovirus (AD-BSP-E1a). After transfection and proliferation, telomerase telomeric repeat amplification protocol and telomere restriction fragment assays were performed, with specific times for evaluating telomere length. Specimens were stained for analysis with hematoxylin and eosin (H&E), terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL), and prostate-specific antigen (PSA).

RESULTS

C4-2/C4-2B cell lines had the shortest initial mean telomere length (approximately 2.5 kilobase [kb]) compared to PC-3 (approximately 5.5 kb). Dose-dependent inhibition of telomerase activity was seen using match oligonucleotide-based inhibitors to telomerase (50% inhibitory concentration 3-5 nm), whereas mismatch compound showed no telomerase inhibition. Significant growth delay and apoptosis in cell lines occurred after > 50 days of treatment. Cells treated with combination "triple therapy" (i.e., telomerase inhibitors, adenovirus, and Taxotere) had the highest amount of apoptosis. Compared to controls, all combination treatment groups had statistically significant reductions in prostate-specific antigen in the conditioned media.

CONCLUSIONS

Combining cytotoxic regimens with small molecule inhibitors to telomerase with oligonucleotide-based agents could be beneficial in controlling osseous hormone refractory prostate cancer, as evidenced by these in vitro, preclinical investigations. Telomerase inhibition needs to move into in vivo models and human studies.

Low-Dose Etoposide Enhances Telomerase-Dependent Adenovirus-Mediated Cytosine Deaminase Gene Therapy through Augmentation of Adenoviral Infection and Transgene Expression in a Syngeneic Bladder Tumor Model

The human telomerase reverse transcriptase (hTERT) promoter can selectively drive transgene expression in many telomerase-positive human cancer cells. Here we evaluated combination therapy of adenoviral vector Ad-hTERT-CD encoding E. coli cytosine deaminase (CD) driven by the hTERT promoter and low-dose etoposide (0.1 microg/mL) for treating bladder cancer. Ad-hTERT-CD conferred sensitivity to 5-fluorocytosine (5-FC) in bladder cancer cells, which could be enhanced by etoposide treatment, but not in normal cells. Such effect was correlated with up-regulation of hypoxia-inducible factor (HIF)-1alpha expression. By contrast, etoposide activated p53 and down-regulated hTERT promoter activity in normal cells. Etoposide also increased adenoviral infection via enhancement of coxsackie-adenovirus receptor expression on bladder cancer and normal cells. Combination index analysis revealed that combined therapy of Ad-hTERT-CD (10(9) plaque-forming units)/5-FC (200 mg/kg) with etoposide (2 mg/kg) synergistically suppressed tumor growth and prolonged survival in mice bearing syngeneic MBT-2 bladder tumors. This combination therapy regimen induced complete tumor regression and generated antitumor immunity in 75% of tumor-bearing mice. Furthermore, increased infiltrating CD4(+) and CD8(+) T cells and necrosis within tumors were found in mice receiving combination therapy of Ad-hTERT-CD and etoposide compared with those treated with either treatment alone. Thus, the potential high therapeutic index of the combination therapy may be an appealing therapeutic intervention for bladder cancer. Furthermore, because a majority of human tumors exhibit high telomerase activity, adenovirus-mediated CD gene therapy driven by the hTERT promoter in combination with low-dose etoposide may be applicable to a broad spectrum of cancers.

Functional characterization of natural telomerase mutations found in patients with hematologic disorders

Human telomerase hTERC RNA serves as a template for the catalytic hTERT protein to synthesize telomere repeats at chromosome ends. We have recently shown that some patients with bone marrow failure syndromes are heterozygous carriers for hTERC or hTERT mutations. These sequence variations usually lead to a compromised telomerase function by haploinsufficiency. Here, we provide functional characterization of an additional 8 distinct hTERT sequence variants and 5 hTERC variants that have recently been identified in patients with dyskeratosis congenita (DC) or aplastic anemia (AA). Among the mutations, 2 are novel telomerase variants that were identified in our cohort of patients. Whereas most of the sequence variants modulate telomerase function by haploinsufficiency, 2 hTERC variants with sequence changes located within the template region appear to act in a dominant-negative fashion. Inherited telomerase gene mutations, therefore, operate by various mechanisms to shorten telomere lengths, leading to limited marrow stem cell reserve and renewal capacity in patients with hematologic disorders.

Telomerase-based immunotherapy of cancer

The progression from the cloning of human telomerase reverse transcriptase (hTERT) in 1997 to the first clinical trials of hTERT as an antitumour immunotherapy target has been swift. hTERT is overexpressed in the vast majority of human cancers yet has limited expression in normal adult tissue. It plays a critical role in oncogenesis and may be expressed by cancer stem cells. However, despite being a self antigen, hTERT is immunogenic both in vitro and in vivo. Several Phase I studies of hTERT immunotherapy have been completed in patients with breast, prostate, lung and other cancers, and clinical and immunological results are encouraging. Immunotherapy induces functional, antitumour T cells in patients in the absence of clinical toxicity. The opportunity for vaccinating individuals as an immunoprevention strategy can also be envisioned for hTERT-based therapies.

The biogenesis and regulation of telomerase holoenzymes

Chromosome stability requires a dynamic balance of DNA loss and gain in each terminal tract of telomeric repeats. Repeat addition by a specialized reverse transcriptase, telomerase, has an important role in maintaining this equilibrium. Insights that have been gained into the cellular pathways for biogenesis and regulation of telomerase ribonucleoproteins raise new questions, particularly concerning the dynamic nature of this unique polymerase.

Characterization of novel alternative splicing sites in human telomerase reverse transcriptase (hTERT): analysis of expression and mutual correlation in mRNA isoforms from normal and tumour tissues

Background

Human telomerase reverse transcriptase (hTERT) is a key component for synthesis and maintenance of telomeres on chromosome ends and is required for the continued proliferation of cells. Estimation of hTERT expression therefore has broad relevance in oncology and stem cell research. Several splicing variants of hTERT have been described whose regulated expression contributes to the control of telomerase activity. Knowledge of the different hTERT mRNA isoforms and the ability to distinguish between them is an important issue when evaluating telomerase expression.

Results

By establishing cDNA-clone panels from lung and colon tissues, we could map hTERT clones individually for differences in DNA sequence. This made possible the identification of novel alternatively spliced sites as well as analysis of their frequency and mutual correlation in mRNA isoforms. Ten different alternatively spliced sites were detected, of which six were novel sites resulting from alternative splicing of intron 2 or 14. The majority of hTERT cDNA clones from normal and tumour lung and colon tissues encoded truncated proteins ending close after exon 2 or 6.

Conclusion

The increased complexity in telomerase expression revealed here has implications for our understanding of telomerase regulation and for the choice of suitable methods for addressing hTERT expression.

The finger subdomain of yeast telomerase cooperates with Pif1p to limit telomere elongation

Telomere synthesis depends on telomerase, which contains an RNA subunit linked to a specialized reverse transcriptase subunit and several associated proteins. Here we report the characterization of four mutations in the yeast reverse transcriptase subunit Est2p that cause an overelongation of telomeres and an increase in the association of Est1p with telomeres during S phase. These 'up-mutations' are clustered in the finger subdomain of the reverse transcriptase. We show that the catalytic properties of the up-mutant telomerases are not improved in vitro. In vivo, the up-mutations neither bypass the activation step governed by Cdc13p nor do they uncouple telomerase from the Rap1p inhibition pathway. In the presence of the up-mutations, however, the ability of the Pif1p helicase to decrease telomere length and to inhibit the association of Est1p with telomeres is impaired. In addition, Pif1p associates in vivo with the telomerase RNA (TLC1) in a way that depends on the finger subdomain. We propose that, in addition to its catalytic role, the finger subdomain of Est2p facilitates the action of Pif1p at telomeres.

Regulation of cellular immortalization and steady-state levels of the telomerase reverse transcriptase through its carboxy-terminal domain

Telomerase maintains cell viability and chromosomal stability through the addition of telomere repeats to chromosome ends. The reactivation of telomerase through the upregulation of TERT, the telomerase protein subunit, is an important step during cancer development, yet TERT protein function and regulation remain incompletely understood. Despite its close sequence similarity to human TERT (hTERT), we find that mouse TERT (mTERT) does not immortalize primary human fibroblasts. Here we exploit these differences in activity to understand TERT protein function by creating chimeric mouse-human TERT proteins. Through the analysis of these chimeric TERT proteins, we find that sequences in the human carboxy-terminal domain are critical for telomere maintenance in human fibroblasts. The substitution of the human carboxy-terminal sequences into the mouse TERT protein is sufficient to confer immortalization and maintenance of telomere length and function. Strikingly, we find that hTERT protein accumulates to markedly higher levels than does mTERT protein and that the sequences governing this difference in protein regulation also reside in the carboxy-terminal domain. These elevated protein levels, which are characteristic of hTERT, are necessary but not sufficient for telomere maintenance because stabilized mTERT mutants cannot immortalize human cells. Thus, the TERT carboxy terminus contains sequences that regulate TERT protein levels and determinants that are required for productive action on telomere ends.

The role of telomeres and telomerase in the pathology of human cancer and aging

Cellular senescence, the state of permanent growth arrest, is the inevitable fate of replicating normal somatic cells. Postulated to underlie this finite replicative span is the physiology of telomeres, which constitute the ends of chromosomes. The repetitive sequences of these DNA-protein complexes progressively shorten with each mitosis. When the critical length is bridged, telomeres trigger DNA repair and cell cycle checkpoint mechanisms that result in chromosomal fusions, cell cycle arrest, senescence and/or apoptosis. Should senescence be bypassed at such time, continued cell divisions in the face of dysfunctional telomeres and activated DNA repair machinery can result in the genomic instability favourable for oncogenesis. The longevity and malignant progression of the thus transformed cell requires coincident telomerase expression or other means to negate the constitutional telomeric loss. Practically then, telomeres and telomerase may represent plausible prognostic and screening cancer markers. Furthermore, if the argument is extended, with assumptions that telomeric attrition is indeed the basis of cellular senescence and that accumulation of the latter equates to aging at the organismal level, then telomeres may well explain the increased incidence of cancer with human aging.

Cytotoxic T cell responses to human telomerase reverse transcriptase in patients with hepatocellular carcinoma

Human telomerase reverse transcriptase, hTERT, has been identified as the catalytic enzyme required for telomere elongation. hTERT is expressed in most tumor cells but seldom expressed in most human adult cells. It has been reported that 80% to 90% of hepatocellular carcinomas (HCCs) express hTERT, making the enzyme a potential target in immunotherapy for HCC. In the current study, we identified hTERT-derived, HLA-A*2402-restricted cytotoxic T cell (CTL) epitopes and analyzed hTERT-specific CTL responses in patients with HCC. Peptides containing the epitopes showed high affinity to bind HLA-A*2402 in a major histocompatibility complex binding assay and were able to induce hTERT-specific CTLs in both hTERT cDNA-immunized HLA-A*2402/Kb transgenic mice and patients with HCC. The CTLs were able to kill hepatoma cell lines depending on hTERT expression levels in an HLA-A*2402-restricted manner and induced irrespective of hepatitis viral infection. The number of single hTERT epitope-specific T cells detected by ELISPOT assay was 10 to 100 specific cells per 3 x 10(5) PBMCs, and positive T cell responses were observed in 6.9% to 12.5% of HCC patients. hTERT-specific T cell responses were observed even in the patients with early stages of HCC. The frequency of hTERT/tetramer+ CD8+ T cells in the tumor tissue of patients with HCC was quite high, and they were functional. In conclusion, these results suggest that hTERT is an attractive target for T-cell-based immunotherapy for HCC, and the identified hTERT epitopes may be valuable both for immunotherapy and for analyzing host immune responses to HCC.

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.

Strict control of telomerase activation using Cre-mediated inversion

Background

Human cells appear exquisitely sensitive to the levels of hTERT expression, the telomerase reverse transcriptase. In primary cells that do not express hTERT, telomeres erode with each successive cell division, leading to the eventual loss of telomere DNA, an induction of a telomere DNA damage response, and the onset of cellular senescence or crisis. In some instances, an average of less than one appropriately spliced hTERT transcript per cell appears sufficient to restore telomerase activity and telomere maintenance, and overcome finite replicative capacity.

Results

To underscore this sensitivity, we showed that a widely used system of transcriptional induction involving ecdysone (muristerone) led to sufficient expression of hTERT to immortalize human fibroblasts, even in the absence of induction. To permit tightly regulated expression of hTERT, or any other gene of interest, we developed a method of transcriptional control using an invertible expression cassette flanked by antiparallel loxP recombination sites. When introduced into human fibroblasts with the hTERT cDNA positioned in the opposite orientation relative to a constitutively active promoter, no telomerase activity was detected, and the cell population retained a mortal phenotype. Upon inversion of the hTERT cDNA to a transcriptionally competent orientation via the action of Cre recombinase, cells acquired telomerase activity, telomere DNA was replenished, and the population was immortalized. Further, using expression of a fluorescent protein marker, we demonstrated the ability to repeatedly invert specific transcripts between an active and inactive state in an otherwise isogenic cell background.

Conclusion

This binary expression system thus provides a useful genetic means to strictly regulate the expression of a given gene, or to control the expression of at least two different genes in a mutually exclusive manner.

Prognostic implication of telomerase activity in patients with brain tumors

Telomerase adds telomeric repeats to the ends of telomeres to compensate for their progressive loss. A favorable prognosis is associated with low or no telomerase in some tumors. The authors investigated whether telomerase activity is associated with survival of patients with brain tumors. Sixty-two consecutive patients with brain tumors underwent surgery, and their surgical specimens were investigated. The patients were pathologically categorized as group I (aggressive group) and group II (non-aggressive group). Telomerase activity was examined by the telomeric repeat amplification protocol (TRAP) assay. The median time was calculated in association with overall survival and progression-free survival in each group. The significant difference was noted in telomerase activity between high-grade gliomas and lowgrade gliomas (p=0.022). Telomerase activity was significantly associated with the median overall survival and progression-free survival in all tumors of the aggressive group. On the other hand, the median overall survival in the non-aggressive group was not dependent on telomerase activity, while the median progression-free survival was. Our data suggests that telomerase is an important prognostic indicator of survival in patients with brain tumors.

Cyclic induction of senescence with intermittent AZT treatment accelerates both apoptosis and telomere loss

BACKGROUND

3'-azido-2',3'-dideoxythymidine (AZT) is phosphorylated intracellularly to 3'-azido-3'-deoxythymidine-5'-triphosphate (AZT-TP), which is incorporated into telomeric DNA, thereby blocking chain elongation. AZT is also known to inhibit reverse transcriptase, as well as other cellular enzymes including DNA polymerase gamma, thymidine kinase, and telomerase.

METHODS

We induced cancer cell senescence by treating MCF-7 cells with AZT in dosages of IC10 and IC20 for an extended period (about 120 population doublings (PD)). We then investigated the sequential changes in cellular growth, expression of telomerase subunits and transcription factors (c-Myc, Mad1), telomerase activity and telomere length.

RESULTS

Senescence, apoptosis, growth delay, inhibition of telomerase activity and shortening of telomere length were all observed in a dose- and time-dependent manner. After the onset of senescence, the apoptosis rate increased slowly during early PDs. In contrast to senescence, the apoptotic rate showed little change after AZT removal, while it increased suddenly and significantly in a dose-dependent manner upon the second introduction of AZT. Continuous shortening of the telomeric length was observed with AZT, and, upon re-exposure to AZT, shortening of the telomere occurred more rapidly than with first exposure. Of the telomerase subunits, telomerase reverse transcriptase (hTERT) and c-Myc were the first to show a reduction in activity after AZT treatment, followed by changes in hTER , Mad1 and hTEP-1.

CONCLUSION

Cyclic treatment with AZT initially suppressed hTERT and c-Myc, followed by suppression of hTER, Mad1 and hTEP-1. Furthermore, the treatment accelerated both telomere loss and apoptosis, even when administered at a senescence-inducing dosage level.

Increased susceptibility of vault poly(ADP-ribose) polymerase-deficient mice to carcinogen-induced tumorigenesis

Vault poly(ADP-ribose) polymerase (VPARP) and telomerase-associated protein 1 (TEP1) are components of the vault ribonucleoprotein complex. Vaults have been implicated in multidrug resistance of human tumors and are thought to be involved in macromolecular assembly and/or transport. Previous studies showed that VPARP-deficient mice were viable, fertile, and did not display any vault-related or telomerase-related phenotype, whereas disruption of telomerase-associated protein 1 in mice led to reduced stability of the vault RNA and affected its stable association with vaults, although there were no telomerase-related changes. In this study, we evaluated the susceptibility of Vparp-/- and Tep1-/- mice to dimethylhydrazine-induced colon tumorigenesis and urethane-induced lung tumorigenesis. Mice received i.p. injections of either 1 g/kg body weight of urethane twice a week for 2 weeks or 20 mg/kg body weight of dimethylhydrazine once a week for 10 weeks and were analyzed after 10 and 60 weeks, respectively. The colon tumor incidence and multiplicity were significantly higher and colon tumor latency was significantly shorter in Vparp-/- mice compared with wild-type mice. Increased colon tumor incidence, multiplicity, and reduced tumor latency were also seen in Tep1-/- mice, however, these results were statistically not significant. Lung tumor multiplicities were increased in both Vparp-/- and Tep1-/- mice but were not significant. The increase in carcinogen-induced tumors in VPARP-deficient mice is the only phenotype observed to date, and suggests a possible role for VPARP, directly or indirectly, in chemically induced neoplasia.

Preliminary analysis of two and three dimensional crystals of vault ribonucleoprotein particles

Vaults are large ribonucleoprotein particles found in a wide variety of eukaryotes. When imaged by electron-microscopy vaults present a strikingly conserved barrel-shaped structure with an invaginated waist and two protruding caps. In this work, we present two dimensional (2D) and three dimensional (3D) crystals of naturally produced vaults in murine and monkey cells, respectively. The 2D-crystals presented a hexagonal packing with the lattice parameter defined by the diameter of the vault barrel. Fourier transforms from images of the negatively stained 2D-crystals showed spots till about 45 A resolution. The 3D-crystals reached about 0.15 x 0.15 x 0.02 mm3 in size and presented a flat triangular morphology with well-developed faces. The preliminary characterization of these 3D-crystals, which diffract very weakly to approximately 10 A resolution, suggests a trigonal packing with the R32 space group symmetry. The 3D-crystals appear to be formed by adding layers of vaults, which retain the hexagonal organization seen in the 2D-crystals, with relative shifts that maximize the interdigitation of particles in adjacent layers. Accurate crystal symmetry in the 2D- and 3D-crystals requires neighbor particles interacting according to a 6-fold and a 3-fold dihedral symmetry, respectively. Compatibility with the reported 8-fold symmetry would imply multiples of 24-fold rotational symmetry, in agreement with the recently proposed 48-fold dihedral symmetry for reconstituted recombinant vaults.

Dissociation of telomerase activity and telomere length maintenance in primitive human hematopoietic cells

Primitive human hematopoietic cells have low endogenous telomerase activity, yet telomeres are not maintained. In contrast, ectopic telomerase expression in fibroblasts and other cells leads to telomere length maintenance or elongation. It is unclear whether this disparity can be attributed to telomerase level or stems from fundamentally different telomere biology. Here, we show that telomerase overexpression does not prevent proliferation-associated telomere shortening in human hematopoietic cells, pointing to the existence of cell type-specific differences in telomere dynamics. Furthermore, we observed eventual stabilization of telomere length without detectable changes in telomerase activity during establishment of two leukemic cell lines from normal cord blood cells, indicating that additional cooperating events are required for telomere maintenance in immortalized human hematopoietic cells.

Modeling aging and cancer in the telomerase knockout mouse

The telomerase deficient mouse has been invaluable in providing insights into basic questions pertaining to consequences of telomere dysfunction during aging and cancer in the context of the mammalian organism. Studies using this mouse model have demonstrated that cellular responses to telomere dysfunction are fundamentally conserved in both humans and mice, and that the tight regulation of telomere length and telomerase activity in somatic cells may be important in mediating the balance between aging and cancer. Here, I discuss the use of the telomerase null mouse for understanding the contrasting roles of telomeres and telomerase in organismal aging and cancer.

Telomerase reverse transcriptase mRNA expression in peripheral lymphocytes of patients with chronic HBV and HCV infections

Telomerase activity is present at low levels in peripheral lymphocytes (PL) and is upregulated upon activation, possibly protecting PL from telomere shortening. As decreased telomere length is considered a sign of cellular senescence, telomerase may, therefore, play an important role on immune function, organ regeneration and carcinogenesis. So far, quantification of human telomerase reverse transcriptase levels (hTERT) in PL, has not been reported. We determined hTERT mRNA levels in PL of hepatitis B virus (HBV) and hepatitis C virus (HCV) patients, in an attempt to address whether hTERT transcripts in PL are altered in these viral diseases, which are characterized by immune dysfunction and increased incidence of hepatocarcinogenesis. hTERT mRNA levels in PL of HBV (n = 17), HCV (n = 24) patients and healthy controls (n = 22) were quantified by real-time polymerase chain reaction. We observed significantly lower hTERT mRNA levels in HBV and HCV patients compared with healthy individuals (P < 0.05). hTERT mRNA levels were not associated with the patients' clinical status (inactive, hepatitis and cirrhosis). Also no correlation was observed between hTERT mRNA expression, and HBV and HCV replicative activity. In the inactive group (n = 18) we observed a negative correlation between hTERT mRNA expression and disease duration (rs = -0.52, P < 0.03). We performed for the first time an accurate quantification of hTERT mRNA expression in PL of HBV and HCV patients. The observed low levels of hTERT mRNA expression in the above patients may suggest its involvement in the immunopathogenesis of chronic viral hepatitis.

Lack of telomerase activity in lung carcinoids is dependent on human telomerase reverse transcriptase transcription and alternative splicing and is associated with long telomeres

PURPOSE

Preliminary evidence indicates that telomerase activity is significantly less expressed in typical carcinoids than in large cell neuroendocrine carcinomas or in small cell lung cancers. Knowledge of the mechanisms by which telomerase is differentially regulated in neuroendocrine lung tumors is important for a better understanding of the pathogenesis of these malignancies.

EXPERIMENTAL DESIGN

We investigated telomerase activity in 86 neuroendocrine lung tumors and correlated the enzyme activity with the expression of the enzyme subunits [human RNA component (hTR), human telomerase reverse transcriptase (hTERT), and alternatively spliced hTERT variants], with the telomere-associated protein human protection of telomere-1, and with the telomere length pattern.

RESULTS

A significantly (P = 0.0001) lower frequency of telomerase-positive cases was found in typical carcinoids (14%) than in large cell neuroendocrine carcinomas (87%) and small cell lung cancers (92%). hTR was constitutively expressed in all carcinoids. Telomerase-negative carcinoids were characterized by the absence of any hTERT transcript, only displayed the beta(-) alternatively spliced variant, or concomitantly expressed the alpha(+)beta(+) full-length message with different combinations of alternatively spliced variants. However, in these tumors, a more abundant level of alternatively spliced transcripts than that of the alpha(+)beta(+) full-length transcript was generally found. No significant difference was observed in human protection of telomere-1 expression between telomerase-negative and telomerase-positive carcinoids. Telomeres were significantly (P < 0.05) longer in telomerase-negative carcinoids than in telomerase-positive carcinoids (median value, 9.15 versus 4.47 kb). However, alternative lengthening of telomeres, as shown by associated promyelocytic leukemia bodies, was not observed in these tumors.

CONCLUSIONS

Our results indicate that telomerase is repressed in most lung carcinoids and that hTERT transcription and alternative splicing play a role in such a negative regulation. Moreover, the absence of any telomerase maintenance mechanism may contribute to the favorable prognosis of this malignancy.

Direct evidence for cooperating genetic events in the leukemic transformation of normal human hematopoietic cells

Although genetic abnormalities associated with hematological malignancies are readily identified, the natural history of human leukemia cannot be observed because initiating and subsequent transforming events occur before clinical presentation. Furthermore, it has not been possible to study leukemogenesis in vitro as normal human cells do not spontaneously transform. Thus, the nature and sequence of genetic changes required to convert human hematopoietic cells into leukemia cells have never been directly examined. We have developed a system where the first step in the leukemogenic process is an engineered disruption of differentiation and self-renewal due to expression of the TLS-ERG oncogene, followed in some cases by overexpression of hTERT. In two of 13 experiments, transduced cells underwent step-wise transformation and immortalization through spontaneous acquisition of additional changes. The acquired karyotypic abnormalities and alterations including upregulation of Bmi-1 and telomerase all occur in acute myeloid leukemia (AML), establishing the relevance of this system. One resultant cell line studied in depth exhibits cellular properties characteristic of AML, notably a hierarchical organization initiated by leukemic stem cells that differentiate abnormally. These findings provide direct evidence for multiple cooperating events in human leukemogenesis, and provide a foundation for studying the genetic changes that occur during leukemic initiation and progression.

An anchor site-type defect in human telomerase that disrupts telomere length maintenance and cellular immortalization

Telomerase-mediated telomeric DNA synthesis is important for eukaryotic cell immortality. Telomerase adds tracts of short telomeric repeats to DNA substrates using a unique repeat addition form of processivity. It has been proposed that repeat addition processivity is partly regulated by a telomerase reverse transcriptase (TERT)-dependent anchor site; however, anchor site-mediating residues have not been identified in any TERT. We report the characterization of an N-terminal human TERT (hTERT) RNA interaction domain 1 (RID1) mutation that caused telomerase activity defects consistent with disruption of a template-proximal anchor site, including reduced processivity on short telomeric primers and reduced activity on substrates with nontelomeric 5' sequences, but not on primers with nontelomeric G-rich 5' sequences. This mutation was located within a subregion of RID1 previously implicated in biological telomerase functions unrelated to catalytic activity (N-DAT domain). Other N-DAT and C-terminal DAT (C-DAT) mutants and a C-terminally tagged hTERT-HA variant were defective in elongating short telomeric primers, and catalytic phenotypes of DAT variants were partially or completely rescued by increasing concentrations of DNA primers. These observations imply that RID1 and the hTERT C terminus contribute to telomerase's affinity for its substrate, and that RID1 may form part of the human telomerase anchor site.

Human telomerase reverse transcriptase immortalizes bovine lens epithelial cells and suppresses differentiation through regulation of the ERK signaling pathway

Telomerase is a specialized reverse transcriptase that extends telomeres of eukaryotic chromosomes. The functional telomerase complex contains a telomerase reverse transcriptase catalytic subunit and a telomerase template RNA. We have previously demonstrated that human telomerase reverse transcriptase (hTERT) catalytic subunit is functionally compatible with a telomerase template RNA from rabbit. In this study, we show that hTERT is also functionally compatible with a telomerase template RNA from bovine. Introduction of hTERT into bovine lens epithelial cells (BLECs) provides the transfected cells telomerase activity. The expressed hTERT in BLECs supports normal growth of the transfected cells for 108 population doublings so far, and these cells are still extremely healthy in both morphology and growth. In contrast, the vector-transfected cells display growth crisis after 20 population doublings. These cells run into cellular senescence due to shortening of the telomeres and also commit differentiation as indicated by the accumulation of the differentiation markers, beta-crystallin and filensin. hTERT prevents the occurrence of both events. By synthesizing new telomere, hTERT prevents replicative senescence, and through regulation of MEK/ERK, protein kinase C, and protein kinase A and eventual suppression of the MEK/ERK signaling pathway, hTERT inhibits differentiation of BLECs. Our finding that hTERT can suppress RAS/RAF/MEK/ERK signaling pathway to prevent differentiation provides a novel mechanism to explain how hTERT regulates cell differentiation.

Protein coding potential of retroviruses and other transposable elements in vertebrate genomes

We suggest an annotation strategy for genes encoded by retroviruses and transposable elements (RETRA genes) based on a set of marker protein domains. Usually RETRA genes are masked in vertebrate genomes prior to the application of automated gene prediction pipelines under the assumption that they provide no selective advantage to the host. Yet, we show that about 1000 genes in four vertebrate gene sets analyzed contain at least one RETRA gene marker domain. Using the conservation of genomic neighborhood (synteny), we were able to discriminate between RETRA genes with putative functionality in the vertebrates and those that probably function only in the context of mobile elements. We identified 35 such genes in human, along with their corresponding mouse and rat orthologs; which included almost all known human genes with similarity to mobile elements. The results also imply that the vast majority of the remaining RETRA genes in current gene sets are unlikely to encode vertebrate functions. To automatically annotate RETRA genes in other vertebrate genomes, we provide as a tool a set of marker protein domains and a manually refined list of domesticated or ancestral RETRA genes for rescuing genes with vertebrate functions.

Antisense oligonucleotide-mediated inhibition of hTERT, but not hTERC, induces rapid cell growth decline and apoptosis in the absence of telomere shortening in human prostate cancer cells

Recent evidence points to a novel function of human telomerase reverse transcriptase (hTERT) in promoting tumour cell survival, which might be independent of the telomere-elongating activity of the enzyme. To test this hypothesis, we evaluated comparatively the effects of telomerase inhibition, accomplished through antisense oligonucleotide-mediated interference with hTERT or human telomerase RNA component (hTERC), on the proliferative potential of DU145 human prostate cancer cells. Exposure of cells to a 2'-O-methyl-RNA phosphorothioate oligonucleotide targeting a splicing site within hTERT pre-mRNA induced almost complete inhibition of telomerase activity as a consequence of a marked reduction of the hTERT mRNA expression level, an early decline of DU145 cell growth and apoptotic cell death without any appreciable telomere shortening. Conversely, exposure of DU145 cells to a 2'-O-methyl-RNA phosphorothioate oligonucleotide targeting the template region of hTERC failed to interfere with cell proliferation in spite of the almost complete abrogation of telomerase activity. These results extend and corroborate earlier evidence in favour of an enzymatic activity-independent mechanism by which hTERT maintains tumour cell survival and proliferation.

The evidences of human orphan receptor COUP-TFII inhibiting telomerase activity through decreasing hTERT transcription

Activation of hTERT, the human telomerase catalytic subunit, has been implicated as the critical event in triggering telomerase activity of cancer cells, but the details of regulation of hTERT need to be elucidated. By screening HeLa cDNA library with hTERT promoter-based yeast one-hybrid assay, one of positive clones, which potentially interacted with hTERT promoter, contained the full sequences of COUP-TFII cDNA. EMSA showed that the prepared His-tagged COUP-TFII could firmly bind to -201 to +35 fragment of hTERT promoter. The precise binding sites were confirmed by DNase I footprint analysis and proved to involve the E-box motif of hTERT promoter. Luciferase reporter assays indicated that COUP-TFII could suppress the transcription of hTERT promoter, and the inhibition to some extent could be reversed by co-transfection of c-myc. Stable introduction of COUP-TFII into HeLa cells decreased both endogenous hTERT expression and telomerase activity. The results suggested that the human COUP-TFII could specifically interact with hTERT promoter, in part via the E-box, and suppress the expression of hTERT.

Does the reservoir for self-renewal stem from the ends?

Stem cell research is a burgeoning field with an alluring potential for therapeutic intervention, and thus begs a critical understanding of the long-term consequences of stem cell replacement. Operationally, a stem cell may be defined as a rarely dividing cell with the capacity for self-renewal throughout the lifetime of the organism, and an ability to reconstitute its appropriate lineages via proliferation and differentiation. In many differentiated normal and cancer cell types, the maintenance of telomeres plays a pivotal role in their continued division potential. Taken together with the presence of the enzymatic activity responsible for telomere addition, telomerase, in several progenitor cell lineages, it is presumed that telomere maintenance will be critical for the replenishment of stem cells or their successors. The purpose of this review is to discuss the role of telomere length maintenance in self-renewal, and the consequent challenges and potential pitfalls to the manipulation of normal and cancer-derived stem cells.

Telomerized presenescent osteoblasts prevent bone mass loss in vivo

Previously, we showed that human osteoblasts expressing the human telomerase reverse transcriptase (hTERT) gene exhibited specific survival advantages--the result of breaching the replicative senescence barrier and maintaining the phenotypic and functional properties of primary osteoblasts in vitro over the total replicative capacity of primary osteoblasts. We postulated that rejuvenated osteoblasts may have a potential to correct bone loss or osteopenia in age-related osteoporotic diseases. In the present study, we studied whether telomerized presenescent osteoblasts prevent bone mass loss in vivo. After obtaining the informed consent from a patient with osteoarthritis who underwent the arthroplastic knee surgery, osteoblastic cells were isolated from donor bone sample. We transfected the gene encoding hTERT into human osteoblastic cells. Human bone fragments from a donor were incubated with human hTERT-transfected presenescent (in vitro aged) osteoblasts or mock-transfected presenescent osteoblasts in culture medium containing Matrigel. We subcutaneously implanted human bone fragments with telomerized presenescent osteoblasts or primary presenescent osteoblasts as three-dimensional Matrigel xenografts in severe combined immunodeficiency (SCID) mice (each group: six mice) and analyzed the grafts at 6 weeks after implantation. We also determined whether telomerized osteoblasts affect the bone-forming capacity in vivo, using a well-established mouse transplantation model in which ceramic hydroxyapatite/tricalcium phosphate particles are used as carrier vehicle. Telomerized presenescent osteoblasts were rejuvenated, and maintained the functional properties of young osteoblasts in vitro. Bone mineral content (BMC) and bone mineral density (BMD) were measured by ash weight and dual-energy X-ray absorptiometry, respectively. Whereas BMC and BMD of human bone fragments, which were inoculated with aged osteoblasts in SCID mice, decreased with time, telomerized presenescent osteoblasts maintained the BMC and BMD of human bone fragments, indicating that telomerized and rejuvenated osteoblasts may be functional to prevent bone mass loss in vivo. In xenogenic transplants, telomerized osteoblasts generated more bone tissue with lamellar bone structure and cellular components, than did control osteoblasts. These findings suggest that telomerized/rejuvenated presenescent osteoblasts may be used in the development of tissue engineering or cell-based therapy for bone regeneration and repair.

Telomere length and telomerase activity: variations with advancing age and potential role in childhood malignancies

Telomeres, representing the chromosome nucleoprotein tails, shorten during each cell division due to the inability of conventional DNA polymerases to completely replicate the chromosome termini. When telomeres become critically short, cells are directed to exit from the cell division cycle (replicative senescence). Telomerase is a reverse transcriptase that synthesizes telomeric sequences, thereby prolonging the lifespan of cells. Telomere length and telomerase activity expression vary significantly in different normal somatic tissues and age groups. In many childhood malignancies (ie, acute leukemias and solid tumors), telomere length and telomerase activity of the malignant cell population may be correlated with the disease outcome and thus may be promising tools in evaluating prognosis and monitoring treatment progress. Finally, telomerase inhibition by using several strategies (ie, antisense oligonucleotides) represents a potentially valuable target for antitumor therapy in the near future.

Vault poly(ADP-ribose) polymerase is associated with mammalian telomerase and is dispensable for telomerase function and vault structure in vivo

Vault poly(ADP-ribose) polymerase (VPARP) was originally identified as a minor protein component of the vault ribonucleoprotein particle, which may be involved in molecular assembly or subcellular transport. In addition to the association of VPARP with the cytoplasmic vault particle, subpopulations of VPARP localize to the nucleus and the mitotic spindle, indicating that VPARP may have other cellular functions. We found that VPARP was associated with telomerase activity and interacted with exogenously expressed telomerase-associated protein 1 (TEP1) in human cells. To study the possible role of VPARP in telomerase and vault complexes in vivo, mVparp-deficient mice were generated. Mice deficient in mVparp were viable and fertile for up to five generations, with no apparent changes in telomerase activity or telomere length. Vaults purified from mVparp-deficient mouse liver appeared intact, and no defect in association with other vault components was observed. Mice deficient in mTep1, whose disruption alone does not affect telomere function but does affect the stability of vault RNA, showed no additional telomerase or telomere-related phenotypes when the mTep1 deficiency was combined with an mVparp deficiency. These data suggest that murine mTep1 and mVparp, alone or in combination, are dispensable for normal development, telomerase catalysis, telomere length maintenance, and vault structure in vivo.