Telomerase is limiting the growth of acute myeloid leukemia cells

Mechanism of Human Telomerase Reverse Transcriptase (hTERT) Regulation and Clinical Impacts in Leukemia

The proliferative capacity and continuous survival of cells are highly dependent on telomerase expression and the maintenance of telomere length. For this reason, elevated expression of telomerase has been identified in virtually all cancers, including leukemias; however, it should be noted that expression of telomerase is sometimes observed later in malignant development. This time point of activation is highly dependent on the type of leukemia and its causative factors. Many recent studies in this field have contributed to the elucidation of the mechanisms by which the various forms of leukemias increase telomerase activity. These include the dysregulation of telomerase reverse transcriptase (TERT) at various levels which include transcriptional, post-transcriptional, and post-translational stages. The pathways and biological molecules involved in these processes are also being deciphered with the advent of enabling technologies such as next-generation sequencing (NGS), ribonucleic acid sequencing (RNA-Seq), liquid chromatography-mass spectrometry (LCMS/MS), and many others. It has also been established that TERT possess diagnostic value as most adult cells do not express high levels of telomerase. Indeed, studies have shown that prognosis is not favorable in patients who have leukemias expressing high levels of telomerase. Recent research has indicated that targeting of this gene is able to control the survival of malignant cells and therefore offers a potential treatment for TERT-dependent leukemias. Here we review the mechanisms of hTERT regulation and deliberate their association in malignant states of leukemic cells. Further, we also cover the clinical implications of this gene including its use in diagnostic, prognostic, and therapeutic discoveries.

Role of Telomeres and Telomeric Proteins in Human Malignancies and Their Therapeutic Potential

Telomeres are the ends of linear chromosomes comprised of repetitive nucleotide sequences in humans. Telomeres preserve chromosomal stability and genomic integrity. Telomere length shortens with every cell division in somatic cells, eventually resulting in replicative senescence once telomere length becomes critically short. Telomere shortening can be overcome by telomerase enzyme activity that is undetectable in somatic cells, while being active in germline cells, stem cells, and immune cells. Telomeres are bound by a shelterin complex that regulates telomere lengthening as well as protects them from being identified as DNA damage sites. Telomeres are transcribed by RNA polymerase II, and generate a long noncoding RNA called telomeric repeat-containing RNA (TERRA), which plays a key role in regulating subtelomeric gene expression. Replicative immortality and genome instability are hallmarks of cancer and to attain them cancer cells exploit telomere maintenance and telomere protection mechanisms. Thus, understanding the role of telomeres and their associated proteins in cancer initiation, progression and treatment is very important. The present review highlights the critical role of various telomeric components with recently established functions in cancer. Further, current strategies to target various telomeric components including human telomerase reverse transcriptase (hTERT) as a therapeutic approach in human malignancies are discussed.

Molecular Mechanism of Telomere Length Dynamics and Its Prognostic Value in Pediatric Cancers

BACKGROUND

We aimed to systematically evaluate telomere dynamics across a spectrum of pediatric cancers, search for underlying molecular mechanisms, and assess potential prognostic value.

METHODS

The fraction of telomeric reads was determined from whole-genome sequencing data for paired tumor and normal samples from 653 patients with 23 cancer types from the Pediatric Cancer Genome Project. Telomere dynamics were characterized as the ratio of telomere fractions between tumor and normal samples. Somatic mutations were gathered, RNA sequencing data for 330 patients were analyzed for gene expression, and Cox regression was used to assess the telomere dynamics on patient survival.

RESULTS

Telomere lengthening was observed in 28.7% of solid tumors, 10.5% of brain tumors, and 4.3% of hematological cancers. Among 81 samples with telomere lengthening, 26 had somatic mutations in alpha thalassemia/mental retardation syndrome X-linked gene, corroborated by a low level of the gene expression in the subset of tumors with RNA sequencing. Telomerase reverse transcriptase gene amplification and/or activation was observed in 10 tumors with telomere lengthening, including two leukemias of the E2A-PBX1 subtype. Among hematological cancers, pathway analysis for genes with expressions most negatively correlated with telomere fractions suggests the implication of a gene ontology process of antigen presentation by Major histocompatibility complex class II. A higher ratio of telomere fractions was statistically significantly associated with poorer survival for patients with brain tumors (hazard ratio = 2.18, 95% confidence interval = 1.37 to 3.46).

CONCLUSION

Because telomerase inhibitors are currently being explored as potential agents to treat pediatric cancer, these data are valuable because they identify a subpopulation of patients with reactivation of telomerase who are most likely to benefit from this novel therapeutic option.

Telomere dysfunction cooperates with epigenetic alterations to impair murine embryonic stem cell fate commitment

The precise relationship between epigenetic alterations and telomere dysfunction is still an extant question. Previously, we showed that eroded telomeres lead to differentiation instability in murine embryonic stem cells (mESCs) via DNA hypomethylation at pluripotency-factor promoters. Here, we uncovered that telomerase reverse transcriptase null (Tert^-/-) mESCs exhibit genome-wide alterations in chromatin accessibility and gene expression during differentiation. These changes were accompanied by an increase of H3K27me3 globally, an altered chromatin landscape at the Pou5f1/Oct4 promoter, and a refractory response to differentiation cues. Inhibition of the Polycomb Repressive Complex 2 (PRC2), an H3K27 tri-methyltransferase, exacerbated the impairment in differentiation and pluripotency gene repression in Tert^-/-mESCs but not wild-type mESCs, whereas inhibition of H3K27me3 demethylation led to a partial rescue of the Tert^-/- phenotype. These data reveal a new interdependent relationship between H3K27me3 and telomere integrity in stem cell lineage commitment that may have implications in aging and cancer.

Telomerase reverse transcriptase gene amplification in hematological malignancies

Background

Telomere is a complex DNA–protein structure located at the end of all eukaryotic chromosomes. The major role of human telomerase is to catalyze the addition of telomeric repeat sequences TTAGGG onto chromosome ends for stabilization of telomere length in attaining cellular immortality and may therefore be a critical step in carcinogenesis. Expression of significant levels of telomerase can dramatically increase proliferative life span and promote cellular immortality, thereby contributing to the malignant phenotype. The purpose of this study is to investigate telomerase reverse transcriptase (TERT) gene amplification in hematological neoplasms, e.g., multiple myeloma (MM), B-non-Hodgkin lymphoma (B-NHL), and acute myeloid leukemia (AML), using FISH technique and to evaluate its potential use as a prognostic marker.

Results

TERT amplification was detected in all groups of the participant patients (15 MM, 15 B-NHL, and 15 AML patients), with higher incidence in AML patients (53.3%). A significant association between the pattern of presentation and telomerase amplification was detected in 88.9% of the relapsed patients who demonstrated amplification of TERT. TERT amplification shows a significant association with p53 deletion and a highly significant association with poor prognosis.

Conclusions

TERT gene amplification is significantly associated with hematological malignancies and may play a critical role in carcinogenesis; thus, elucidation of their regulatory mechanism is highly demanding. Higher amplification was found in relapsed cases than de novo cases which highlight its potential implication in clinical analysis and disease monitoring. Moreover, our results suggest the future use of TERT gene as a potential prognostic marker that may aid in treatment decision and chemotherapy.

The Bromodomain Inhibitor JQ1 Enhances the Responses to All-trans Retinoic Acid in HL-60 and MV4-11 Leukemia Cells

All-trans retinoic acid (ATRA) is a highly effective treatment for acute promyelocytic leukemia (APL), a cytogenetically distinct subtype of acute myeloid leukemia (AML). However, ATRA-based treatment is not effective in other subtypes of AML. In non-APL AML, ATRA signaling pathway is impaired or downmodulated, and consequently fails to respond to pharmacological doses of ATRA. Therefore, complementary treatment strategies are needed to improve ATRA responsiveness in non-APL AML. In this study, we investigated the combined effect of ATRA and bromodomain inhibitor JQ1, proven to have potent anti-cancer activity mainly through inhibition of c-Myc. We showed that the combination of ATRA with JQ1 synergistically inhibited proliferation of AML cells. The synergistic growth inhibition was resulted from differentiation or apoptosis depending on the kind of AML cells. Concomitantly, the combined treatment of ATRA and JQ1 caused greater depletion of c-Myc and hTERT expression than each agent alone in AML cells. Taken together, these findings support the rationale for the use of the combination of ATRA and JQ1 as a therapeutic strategy for the treatment of AML.

Telomerase activity and telomere on stem progeny senescence

The end of linear chromosomes is formed of a special nucleoprotein heterochromatin structure with repetitive TTAGGG sequences called telomere. Telomere length is regulated by a special enzyme called telomerase, a specific DNA polymerase that adds new telomeric sequences to the chromosome ends. Telomerase consists of two parts; the central protein part and the accessory part which is a RNA component transported by the central part. Regulation of telomere length by this enzyme is a multi-stage process. Telomere length elongation is strongly influenced by the level of telomerase and has a strong correlation with the activity of telomerase enzyme. Human Telomerase Reverse Transcriptase (hTERT) gene expression plays an important role in maintaining telomere length and high proliferative property of cells. Except a low activity of telomerase enzyme in hematopoietic and few types of stem cells, most of somatic cells didn't showed telomerase activity. Moreover, cytokines are secretory proteins that control many aspects of hematopoiesis, especially immune responses and inflammation. Also, the induction of hTERT gene expression by cytokines is organized through the PI3K/AKT and NF/kB signaling pathways. In this review we have tried to talk about effects of immune cell cytokines on telomerase expression/telomere length and the induction of telomerase expression by cytokines.

Immortalization of human AE pre-leukemia cells by hTERT allows leukemic transformation

Human CD34+ hematopoietic stem and progenitor cells (HSPC) expressing fusion protein AML1-ETO (AE), generated by the t(8;21)(q22;q22) rearrangement, manifest enhanced self-renewal and dysregulated differentiation without leukemic transformation, representing a pre-leukemia stage. Enabling replicative immortalization via telomerase reactivation is a crucial step in cancer development. However, AE expression alone is not sufficient to maintain high telomerase activity to immortalize human HSPC cells, which may hamper transformation. Here, we investigated the cooperativity of telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, and AE in disease progression. Enforced expression of hTERT immortalized human AE pre-leukemia cells in a telomere-lengthening independent manner, and improved the pre-leukemia stem cell function by enhancing cell proliferation and survival. AE-hTERT cells retained cytokine dependency and multi-lineage differentiation potential similar to parental AE clones. Over the short-term, AE-hTERT cells did not show features of stepwise transformation, with no leukemogenecity evident upon initial injection into immunodeficient mice. Strikingly, after extended culture, we observed full transformation of one AE-hTERT clone, which recapitulated the disease evolution process in patients and emphasizes the importance of acquiring cooperating mutations in t(8;21) AML leukemogenesis. In summary, achieving unlimited proliferative potential via hTERT activation, and thereby allowing for acquisition of additional mutations, is a critical link for transition from pre-leukemia to overt disease in human cells. AE-hTERT cells represent a tractable model to study cooperating genetic lesions important for t(8;21) AML disease progression.

Maintenance of telomere length in AML

The importance of telomere length to human health, aging, and cancer continues to be underappreciated. This review examines some basics of telomere biology and relates how telomere function, telomerase activity, and mutations in TERC or TERT are involved in bone marrow failure, leukemias, and other cancers. Given the challenge to obtain accurate data on telomerase activity and telomere length in specific cell types, the situation in acute myeloid leukemia (AML) remains puzzling. In most cancers, telomerase levels are increased after cells have encountered a "telomere crisis," which is typically associated with poor prognosis. Cells emerging from "telomere crisis" have defective DNA damage responses, resulting, for example, from loss of p53. Such cells often express elevated telomerase levels as a result of point mutations in the TERT promoter or amplification of the TERT gene. While telomeres in AML blasts are typically shorter than expected for normal leukocytes, most AML cells do not show evidence of having gone through a "telomere crisis." In chronic myeloid leukemia (CML), the difference between the telomere length in nonmalignant T cells and malignant blasts from the same patient was found to correlate with the remaining duration of the chronic phase. This observation supports that a mitotic clock is ticking in CML stem cells and that disease progression in CML heralds the onset of a "telomere crisis." The presence of very short telomeres in tumor cells was found to predict disease progression in chronic lymphocytic leukemia, myeloma, and various solid tumors. In view of these findings longitudinal studies of telomere length in AML appear worthwhile.

Telomerase in hematologic malignancies

PURPOSE OF REVIEW

The activation of telomere maintenance pathways has long been regarded as a key hallmark of cancer and this has propelled the development of novel inhibitors of telomerase. In this review, we detail the background biology on telomere maintenance in health and disease, then concentrate on the recent preclinical and clinical development behind targeting telomerase in blood cancers.

RECENT FINDINGS

Preclinical and clinical studies have shown that imetelstat, a competitive inhibitor of telomerase, has activity in certain hematologic malignancies, in particular the myeloproliferative neoplasms and acute myeloid leukemia.

SUMMARY

Telomerase inhibition has shown remarkable efficacy in myeloid malignancies, and current and future preclinical and clinical studies are necessary to comprehensively investigate its underlying mechanism of action. Future work should identify the potential genetic susceptibilities to telomerase inhibition therapy, and evaluate rational combinations of telomerase inhibitors with chemotherapy and other novel agents. Robust preclinical evaluation is essential to best translate these new agents successfully into our clinical treatment algorithm for myeloid and other blood cancers.

The Maintenance of Telomere Length in CD28+ T Cells During T Lymphocyte Stimulation

Telomerase activity is not readily detected in resting human T lymphocytes, however upon antigen presentation, telomerase is transiently upregulated. Presently, it is not known if telomerase activation is necessary for the proliferation of T cells or for the maintenance of telomere lengths. In this study, we found that telomerase activation is not required for the short- term proliferation of T cells and that telomeres progressively shorten in a heterogeneous population of T cells, even if telomerase is detected. By measuring telomerase activity at the single-cell level using quantitative ddPCR techniques (ddTRAP) and by monitoring changes in the shortest telomeres with more sensitive telomere length measurement assays, we show that only a subset of CD28+ T-cells have robust telomerase activity upon stimulation and are capable of maintaining their telomere lengths during induced proliferation. The study of this T-cell subset may lead to a better understanding on how telomerase is regulated and functions in immune cells.

Telomerase Activation in Hematological Malignancies

Telomerase expression and telomere maintenance are critical for cell proliferation and survival, and they play important roles in development and cancer, including hematological malignancies. Transcriptional regulation of the rate-limiting subunit of human telomerase reverse transcriptase gen (hTERT) is a complex process, and unveiling the mechanisms behind its reactivation is an important step for the development of diagnostic and therapeutic applications. Here, we review the main mechanisms of telomerase activation and the associated hematologic malignancies.

Therapeutic targeting of replicative immortality

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

Stem cell function and maintenance - ends that matter: role of telomeres and telomerase

Stem cell research holds a promise to treat and prevent age-related degenerative changes in humans. Literature is replete with studies showing that stem cell function declines with aging, especially in highly proliferative tissues/ organs. Among others, telomerase and telomere damage is one of the intrinsic physical instigators that drive agerelated degenerative changes. In this review we provide brief overview of telomerase-deficient aging affects in diverse stem cells populations. Furthermore, potential disease phenotypes associated with telomerase dysregulation in a specific stem cell population is also discussed in this review. Additionally, the role of telomerase in stem cell driven cancer is also briefly touched upon.

Epigenetic regulation of the human telomerase reverse transciptase gene: A potential therapeutic target for the treatment of leukemia (Review)

Telomerase activation is a critical step in human carcinogenesis through the maintenance of telomeres. Telomerase activity is primarily regulated by the human telomerase reverse transcriptase gene (hTERT), thus, an improved understanding of the transcriptional control of hTERT may provide potential therapeutic targets for the treatment of leukemia and other forms of cancer. Epigenetic modulation, a significant regulatory process in cell biology, has recently been shown to be involved in the regulation of the hTERT gene. Moreover, several epigenetic modifiers, including DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors, are now in pre- and early clinical trials of leukemia as monotherapies or in combination with other drugs, and have achieved significant clinical success. In the present review, the epigenetic mechanisms associated with telomerase activity in leukemia, and the therapeutic potential of an antitelomerase strategy that combines epigenetic modifiers with telomerase hTR subunit small molecule inhibitors are discussed.

Homoharringtonine-Induced Apoptosis of Human Leukemia HL-60 Cells is Associated with Down-Regulation of Telomerase

Homoharringtonine (HHT), first isolated from the Chinese evergreen Cephalotaxus Harringtonia, has been shown inhibiting activity in leukemia in initial studies in China and in later studies in the US, but the detailed mechanism of action is still unclear. The goal of the experiments shown here is to explore the effect of HHT on the telomerase activity and apoptosis of human leukemia HL-60 cells. The telomerase activity of HL-60 cells was examined by the telomeric repeat amplification protocol (TRAP) — an enzyme-linked immunosorbent assay (ELISA). Apoptosis was analyzed by morphological observation, DNA agarose gel electrophoresis, flow cytometry (FCM), and TdT-mediated dUTP-biotin nick end labeling (TUNEL). After treatment with HHT at 5–500 μg/l for 48 hours, the level of telomerase activity in HL-60 cells decreased in a dose-and time-dependent manner. Simultaneously, HL-60 cells underwent apoptosis. In conclusion, our data suggest that HHT can inhibit the telomerase content of HL-60 cells effectively and induce apoptosis.

Knockdown of hTERT alters biophysical properties of K562 cells resulting in decreased migration rate in vitro

It has been shown that 90% of tumors, including hematological malignant tumors and leukemia, have much higher levels of telomerase expression than normal cells. To investigate the effect of telomerase on leukemia cells, we transfected K562, a human erythroleukemia cell line with an antisense-hTERT (human telomerase reverse transcriptase) cDNA vector, and examined the biological and biophysical properties of the stably transfected cells (referred to as KAT). Un-transfected cells (K562) and cells transfected with the empty vector (referred to as KC) were used as controls. Cell growth curve and (3)H-TdR test showed that the growth rate and DNA synthesis of KAT decreased compared with those of K562 and KC cells. Apoptosis and cell cycle distribution in KAT cells under normal culture condition were similar to those of K562 and KC cells, but changed after serum deprivation. KAT cells had significantly different biophysical characteristics from K562 and KC in terms of cell electrophoresis, membrane fluidity, membrane fluidity, and viscoelasticity. Furthermore, the transendothelial migration rate of KAT was much lower than those of K562 and KC cells. Confocal microscopy showed that KAT cells had higher F-actin content, suggesting the reorganization of cytoskeleton. Flow cytometry analysis revealed a lowered intracellular calcium concentration and CD71 expression, explaining the high F-actin content in KAT cells. In conclusion, we found that the knockdown of hTERT in K562 cells changed their cytoskeleton and biophysical features, and reduced the cell migration.

hTERT promotes imatinib resistance in chronic myeloid leukemia cells: therapeutic implications

Imatinib mesylate has shown remarkable efficacy in the treatment of patients in the chronic phase of chronic myeloid leukemia. However, despite an overall significant hematological and cytogenetic response, imatinib therapy may favor the emergence of drug-resistant clones, ultimately leading to relapse. Some imatinib resistance mechanisms had not been fully elucidated yet. In this study we used sensitive and resistant sublines from a Bcr-Abl positive cell line to investigate the putative involvement of telomerase in the promotion of imatinib resistance. We showed that sensitivity to imatinib can be partly restored in imatinib-resistant cells by targeting telomerase expression, either by the introduction of a dominant-negative form of the catalytic protein subunit of the telomerase (hTERT) or by the treatment with all-trans-retinoic acid, a clinically used drug. Furthermore, we showed that hTERT overexpression favors the development of imatinib resistance through both its antiapoptotic and telomere maintenance functions. Therefore, combining antitelomerase strategies to imatinib treatment at the beginning of the treatment should be promoted to reduce the risk of imatinib resistance development and increase the probability of eradicating the disease.

Functional p53 is required for effective execution of telomerase inhibition in BCR-ABL-positive CML cells

OBJECTIVE

In chronic myeloid leukemia (CML), increased cellular turnover of hematopoietic cells driven by the oncogene BCR-ABL leads to accelerated telomere shortening despite increased telomerase activity. It has been postulated that shortened telomeres, particularly in the context of increased telomerase activity, might facilitate accumulation of genetic aberrations and, consequently, disease progression from chronic phase to accelerated phase and blast crisis. Therefore, inhibition of telomerase might be a promising approach in CML therapy.

MATERIAL AND METHODS

To investigate the therapeutic potential of telomerase inhibition in this model disorder, we used a small molecule telomerase inhibitor, BIBR1532 as well as expression of a dominant-negative mutant of hTERT (DNhTERT-IRES-GFP) in the p53-negative CML blast crisis cell line K562 and characterized the effects in long-term culture. Furthermore, we expressed an inducible p53 construct (vector pBabe-p53ER(tam)) via retroviral transduction in cells with critically short telomeres and in cells with a normal telomere length to explain the role of the tumor suppressor in response to critical telomere shortening in BCR-ABL-positive cells.

RESULTS

BIBR1532-treated bulk cultures did not show altered growth kinetics despite significant telomere shortening to a critical length of approximately 5 kb. In comparison, DNhTERT-expressing clones either lost telomere length, leading to a significant but transient slow down in proliferation but eventually all escaped senescence/crisis (group I) or, alternatively, remained virtually unaffected despite measurable telomerase inhibition (group II). Further analyses of group I clones revealed impaired DNA damage response and an accumulation of dicentric chromosomes. However, upon restoration of p53 in telomerase-negative K562 clones with critically short telomeres, immediate reinduction of apoptosis and complete eradication of cells was observed, whereas vector control cells continued to escape from crisis.

CONCLUSIONS

These results suggest that the success of strategies aimed at telomerase inhibition in CML is highly dependent on the presence of functional p53 and should be explored preferentially in chronic phase CML.

Targeting telomerase in hematologic malignancy

Over the past two decades, it has become increasingly apparent that telomerase-mediated telomere maintenance plays a crucial role in hematopoiesis. Supporting evidence is underscored by recent findings of mutations in genes involved in telomerase-mediated telomere maintenance that contribute to the pathogenesis of bone marrow failure syndromes. More recently described telomere-independent functions of telomerase are also likely to contribute to both normal hematopoiesis and hematologic diseases. The high levels of telomerase detected in aggressive leukemias have fueled fervent investigation into diverse approaches to targeting telomerase in hematologic malignancies. Successful preclinical investigations that employed genetic strategies, oligonucleotides, small-molecule inhibitors and immunotherapy have resulted in a rapid translation to clinical trials. Further investigation of telomere-independent functions of telomerase and detailed preclinical studies of telomerase inhibition in both normal and malignant hematopoiesis will be invaluable for refining treatments to effectively and safely exploit telomerase as a therapeutic target in hematologic malignancies.

Imetelstat (GRN163L)--telomerase-based cancer therapy

Telomeres and telomerase play essential roles in the regulation of the lifespan of human cells. While normal human somatic cells do not or only transiently express telomerase and therefore shorten their telomeres with each cell division, most human cancer cells typically express high levels of telomerase and show unlimited cell proliferation. High telomerase expression allows cells to proliferate and expand long-term and therefore supports tumor growth. Owing to the high expression and its role, telomerase has become an attractive diagnostic and therapeutic cancer target. Imetelstat (GRN163L) is a potent and specific telomerase inhibitor and so far the only drug of its class in clinical trials. Here, we report on the structure and the mechanism of action of imetelstat as well as about the preclinical and clinical data and future prospects using imetelstat in cancer therapy.

Telomeres and telomerase in chronic myeloid leukaemia: impact for pathogenesis, disease progression and targeted therapy

Telomeres are specialized structures localized at the end of human chromosomes. Due to the end replication problem, each cell division results in a loss of telomeric repeats in normal somatic cells. In germ line and stem cells, the multicomponent enzyme telomerase maintains the length of telomere repeats. However, elevated telomerase activity has also been reported in the majority of solid tumours as well as in acute and chronic leukaemia. Chronic myeloid leukaemia (CML) serves as a model disease to study telomere biology in clonal myeloproliferative disorders. In CML, telomere shortening correlates with disease stage, duration of chronic phase (CP), prognosis measured by the Hasford risk score and the response to disease-modifying therapeutics such as the tyrosine kinase inhibitor Imatinib. In addition, telomerase activity (TA) is already increased in CP CML and further upregulated with disease progression to accelerated phase and blast crisis (BC). Furthermore, a correlation of TA with increased genetic instability as well as a shorter survival of the patients has been reported. Here, we review the current state of knowledge of the role of telomere and telomerase biology in CML and discuss the possible impact of novel treatment approaches.

Telomerase regulation in hematological cancers: a matter of stemness?

Human telomerase is a nuclear ribonucleoprotein enzyme complex that catalyzes the synthesis and extension of telomeric DNA. This enzyme is highly expressed and active in most malignant tumors while it is usually not or transiently detectable in normal somatic cells, suggesting that it plays an important role in cellular immortalization and tumorigenesis. As most leukemic cells are generally telomerase-positive and have often shortened telomeres, our understanding of how telomerase is deregulated in these diseases could help to define novel therapies targeting the telomere/telomerase complex. Nonetheless, considering that normal hematopoietic stem cells and some of their progeny do express a functional telomerase, it is tempting to consider such an activity in leukemias as a sustained stemness feature and important to understand how telomere length and telomerase activity are regulated in the various forms of leukemias.

Short telomeres and high telomerase activity in T-cell prolymphocytic leukemia

To test the role of telomere biology in T-cell prolymphocytic leukemia (T-PLL), a rare aggressive disease characterized by the expansion of a T-cell clone derived from immuno-competent post-thymic T-lymphocytes, we analyzed telomere length and telomerase activity in subsets of peripheral blood leukocytes from 11 newly diagnosed or relapsed patients with sporadic T-PLL. Telomere length values of the leukemic T cells (mean+/-s.d.: 1.53+/-0.65 kb) were all below the 1st percentile of telomere length values observed in T cells from healthy age-matched controls whereas telomere length of normal T- and B cells fell between the 1st and 99th percentile of the normal distribution. Leukemic T cells exhibited high levels of telomerase and were sensitive to the telomerase inhibitor BIBR1532 at doses that showed no effect on normal, unstimulated T cells. Targeting the short telomeres and telomerase activity in T-PLL seems an attractive strategy for the future treatment of this devastating disease.

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.

Telomerase expression in B-cell chronic lymphocytic leukemia predicts survival and delineates subgroups of patients with the same igVH mutation status and different outcome

Activation of telomerase reverse transcriptase (hTERT) is essential for unlimited cell growth and plays a critical role in tumorigenesis. We investigated hTERT gene expression in 134 B-cell chronic lymphocytic leukemia (B-CLL) cases and evaluated its prognostic value with other prognostic markers (IgVH mutation status, CD38 and ZAP-70 expression). Real-time PCR assays to quantify either all hTERT transcripts (AT) or only the full length (FL) transcript encoding the functional protein were developed. hTERT-AT levels strongly correlated with hTERT-FT levels (r=0.743, P<0.0001); both inversely correlated with the percentage of IgVH mutation (P<0.005) and were significantly higher in unmutated than in mutated cases (P=0.004 and P=0.001, respectively). The hTERT values which best discriminated between the unmutated and mutated IgVH cases were 150 and 40 copies for hTERT-AT and hTERT-FL, respectively. Using these cut-off values, there was a significant difference in the survival of patients with high or low hTERT levels (P<0.0001). Unmutated cases with low hTERT levels had an overall survival close to mutated cases with high hTERT levels. Thus, this work identifies hTERT-RNA level as a new prognostic marker in B-CLL, and may be used to identify previously unrecognized patient groups with the same IgVH mutation status and different disease outcomes.

Telomerase targeting by retinoids in cells from patients with myeloid leukemias of various subtypes, not only APL

Numerous strategies have been proposed to specifically inhibit telomerase (human telomerase reverse transcriptase (hTERT)) but to date only a few are clinically relevant in anticancer therapy. Recently, we have shown that long-term treatment with all-trans retinoic acid (ATRA), a compound clinically approved for differentiation therapy of acute promyelocytic leukemia (APL), represses hTERT in differentiation-resistant APL cell lines leading to telomere shortening and death. This signaling requires the co-activation of the retinoic acid receptor alpha (RARalpha) and the retinoic X receptor (RXR). In contrast to differentiation-therapy, which is only successful in this subtype of leukemia, the telomerase-targeted pathway could also be of use in non-APL. Here, we demonstrate that repression of hTERT occurs in fresh blasts cells from patients with myeloid leukemias of various subtypes exposed ex vivo to ATRA or synthetic retinoids. These results support the idea that, by hTERT targeting, retinoids can induce telomere shortening and cell death and their integration in therapy protocols for myeloid leukemias refractory to maturation should be considered.

In vivo inhibition of lung cancer by GRN163L: a novel human telomerase inhibitor

Differential regulation of telomerase activity in normal and tumor cells provides a rationale for the design of new classes of telomerase inhibitors. The telomerase enzyme complex presents multiple potential sites for the development of inhibitors. GRN163L, a telomerase enzyme antagonist, is a lipid-modified 13-mer oligonucleotide N3' --> P5'-thio-phosphoramidate, complementary to the template region of telomerase RNA (hTR). We evaluated both the in vitro and in vivo effects of GRN163L using A549-luciferase (A549-Luc) human lung cancer cells expressing a luciferase reporter. GRN163L (1 micromol/L) effectively inhibits telomerase activity of A549-Luc cells, resulting in progressive telomere shortening. GRN163L treatment also reduces colony formation in soft agar assays. Surprisingly, after only 1 week of treatment with GRN163L, A549-Luc cells were unable to form robust colonies in the clonal efficiency assay, whereas the mismatch control compound had no effect. Finally, we show that in vivo treatment with GRN163L is effective in preventing lung metastases in xenograft animal models. These in vitro and in vivo data support the development of GRN163L as a therapeutic for the treatment of cancer.

Identification and functional characterization of 2 variant alleles of the telomerase RNA template gene (TERC) in a patient with dyskeratosis congenita

Heterozygous mutations of the human telomerase RNA template gene (TERC) have been described in patients with acquired aplastic anemia and the autosomal dominant form of dyskeratosis congenita (DKC). Patients with mutations in both TERC alleles have not yet been reported. Here, we report a patient with DKC who inherited 2 distinct TERC sequence variants from her parents; a deletion (216_229del) in one and a point mutation (37A>G) in the other allele of the TERC gene. Her marrow was hypocellular and showed an abnormal clone [46, XX t(7;21)(q34;q22)]. The telomere lengths in leukocytes of the patient and her relatives were shorter than those of the age-matched controls and were progressively shorter in subsequent generations of family members with the 216_229del allele. Telomerase enzymatic levels in lymphocytes from the patient were approximately half of those measured in healthy controls. The 216_229del mutation failed to reconstitute telomerase activity in transfected cells, but, when coexpressed with the 37A>G variant, telomerase activity was only modestly suppressed. These clinical and laboratory findings support the concept that telomerase levels in human hematopoietic stem cells are tightly controlled as even moderately reduced levels result in accelerated telomere shortening and eventual marrow failure.

Telomere loss, senescence, and genetic instability in CD4+ T lymphocytes overexpressing hTERT

Little is known about the long-term consequences of overexpression of the human telomerase reverse transcriptase (hTERT) gene in T lymphocytes. To address this issue, we transduced polyclonal as well as clonally derived populations of naive and memory CD44 T cells from 2 healthy donors (aged 24 and 34 years) with retroviral vectors encoding green fluorescence protein (GFP) and hTERT (GFP-hTERT) or GFP alone. After transduction, cells were sorted on the basis of GFP expression and cultured in vitro until senescence. T cells transduced with hTERT exhibited high stable telomerase activity throughout the culture period. Relative to GFP controls, minor changes in overall gene expression were observed yet the proliferative lifespan of the hTERT-transduced populations was significantly increased and the rate of telomere loss was lower. Nevertheless, hTERT-transduced cells showed progressive telomere loss and had shorter telomeres at senescence than controls (2.3 +/- 0.3 kilobase [kb] versus 3.4 +/- 0.1 kb). Furthermore, a population of cells with 4N DNA consisting of binucleated cells with connected nuclei emerged in the hTERT-transduced cells prior to senescence. We conclude that overexpression of hTERT in CD4+ T cells provides a proliferative advantage independent of the average telomere length but does not prevent eventual genetic instability and replicative senescence.

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.