Human telomerase inhibition by 7-deaza-2'-deoxypurine nucleoside triphosphates

Highly Conductive Nucleotide Analogue Facilitates Base-Calling in Quantum-Tunneling-Based DNA Sequencing

Quantum-tunneling-based DNA sequencing is a single molecular technology that has great potential for achieving facile and high-throughput DNA sequencing. In principle, the sequence of DNA could be read out by the time trace of the tunnel current that can be changed according to molecular conductance of nucleobases passing through nanosized gap electrodes. However, efficient base-calling of four genetic alphabets has been seriously impeded due to the similarity of molecular conductance among canonical nucleotides. In this article, we demonstrate that replacement of canonical 2'-deoxyadenosine (dA) with a highly conductive dA analogue, 7-deaza dA, could expand the difference of molecular conductance between four genetic alphabets. Additionally, systematic evaluation of molecular conductance using a series of dA and dG analogues revealed that molecular conductance of the nucleotide is highly dependent on the HOMO level. Thus, the present study demonstrating that signal characteristics of the nucleotide can be modulated based on the HOMO level provides a widely applicable chemical approach and insight for facilitation of single molecular sensing as well as DNA sequencing based on quantum tunneling.

Telomerase as a Cancer Target. Development of New Molecules

Telomeres are the terminal part of the chromosome containing a long repetitive and non-codifying sequence that has as function protecting the chromosomes. In normal cells, telomeres lost part of such repetitive sequence in each mitosis, until telomeres reach a critical point, triggering at that time senescence and cell death. However, in most of tumor cells in each cell division a part of the telomere is lost, however the appearance of an enzyme called telomerase synthetize the segment that just has been lost, therefore conferring to tumor cells the immortality hallmark. Telomerase is significantly overexpressed in 80–95% of all malignant tumors, being present at low levels in few normal cells, mostly stem cells. Due to these characteristics, telomerase has become an attractive target for new and more effective anticancer agents. The capability of inhibiting telomerase in tumor cells should lead to telomere shortening, senescence and apoptosis. In this work, we analyze the different strategies for telomerase inhibition, either in development, preclinical or clinical stages taking into account their strong points and their caveats. We covered strategies such as nucleosides analogs, oligonucleotides, small molecule inhibitors, G-quadruplex stabilizers, immunotherapy, gene therapy, molecules that affect the telomere/telomerase associated proteins, agents from microbial sources, among others, providing a balanced evaluation of the status of the inhibitors of this powerful target together with an analysis of the challenges ahead.

Structural Basis of Telomerase Inhibition by the Highly Specific BIBR1532

BIBR1532 is a highly specific telomerase inhibitor, although the molecular basis for inhibition is unknown. Here we present the crystal structure of BIBR1532 bound to Tribolium castaneum catalytic subunit of telomerase (tcTERT). BIBR1532 binds to a conserved hydrophobic pocket (FVYL motif) on the outer surface of the thumb domain. The FVYL motif is near TRBD residues that bind the activation domain (CR4/5) of hTER. RNA binding assays show that the human TERT (hTERT) thumb domain binds the P6.1 stem loop of CR4/5 in vitro. hTERT mutations of the FVYL pocket alter wild-type CR4/5 binding and cause telomere attrition in cells. Furthermore, the hTERT FVYL mutations V1025F, N1028H, and V1090M are implicated in dyskeratosis congenita and aplastic anemia, further supporting the biological and clinical relevance of this novel motif. We propose that CR4/5 contacts with the telomerase thumb domain contribute to telomerase ribonucleoprotein assembly and promote enzymatic activity.

Enzymatic synthesis of a DNA triblock copolymer that is composed of natural and unnatural nucleotides

DNA molecules have come under the spotlight as potential templates for the fabrication of nanoscale products, such as molecular-scale electronic or photonic devices. Herein, we report an enhanced approach for the synthesis of oligoblock copolymer-type DNA by using the Klenow fragment exonuclease minus of E. coli DNA polymerase I (KF(-) ) in a multi-step reaction with natural and unnatural nucleotides. First, we confirmed the applicability of unnatural nucleotides with 7-deaza-nucleosides-which was expected because they were non-metalized nucleotides-on the unique polymerization process known as the "strand-slippage model". Because the length of the DNA sequence could be controlled by tuning the reaction time, analogous to a living polymerization reaction on this process, stepwise polymerization provided DNA block copolymers with natural and unnatural bases. AFM images showed that this DNA block copolymer could be metalized sequence-selectively. This approach could expand the utility of DNA as a template.

Oxidation in the nucleotide pool, the DNA damage response and cellular senescence: Defective bricks build a defective house

Activation of persistent DNA damage response (DDR) signaling is associated with the induction of a permanent proliferative arrest known as cellular senescence, a phenomenon intrinsically linked to both tissue aging as well as tumor suppression. The DNA damage observed in senescent cells has been attributed to elevated levels of reactive oxygen species (ROS), failing DNA damage repair processes, and/or oncogenic activation. It is not clear how labile molecules such as ROS are able to damage chromatin-bound DNA to a sufficient extent to invoke persistent DNA damage and DDR signaling. Recent evidence suggests that the nucleotide pool is a significant target for oxidants and that oxidized nucleotides, once incorporated into genomic DNA, can lead to the induction of a DNA strand break-associated DDR that triggers senescence in normal cells and in cells sustaining oncogene activation. Evasion of this DDR and resulting senescence is a key step in tumor progression. This review will explore the role of oxidation in the nucleotide pool as a major effector of oxidative stress-induced genotoxic damage and DDR in the context of cellular senescence and tumorigenic transformation.

Telomerase structure paves the way for new cancer therapies

Inappropriate activation of a single enzyme, telomerase, is associated with the uncontrollable proliferation of cells observed in as many as 90% of all of human cancers. Since the mid-1990s, when telomerase activity was detected in human tumors, scientists have eyed the enzyme as an ideal target for developing broadly effective anticancer drugs. One of the missing links in the effort to identify such therapies has been the high-resolution structure of the enzyme, a powerful tool used for the identification and development of clinical drugs. A recent structure of the catalytic subunit of teleomerase from the Skordalakes laboratory, a major advancement in the field of telomeres, has opened the door to the development of new, broadly effective cancer drugs, as well as anti-aging therapies. Here we present a brief description of telomerase biology, current efforts to identify telomerase function modulators and the potential importance of the telomerase structure in future drug development.

Pharmacological intervention strategies for affecting telomerase activity: future prospects to treat cancer and degenerative disease

Telomerase enzyme is a ribonucleoprotein maintaining the length of the telomeres by adding G-rich repeats to the end of the eukaryotic chromosomes. Normal human somatic cells, cultured in vitro, have a strictly limited proliferative potential undergoing senescence after about 50-70 population doublings. In contrast, most of the tumor cells have unlimited replicative potential. Although the mechanisms of immortalization are not understood completely at a genetic level, the key role of the telomere/telomerase system in the process is clear. The DNA replication machinery is not able to replicate fully the DNA at the very end of the chromosomes; therefore, about 50-200 nucleotides are lost during each of the replication cycles resulting in a gradual decrease of telomere length. Critically short telomere induces senescence, subsequent crisis and cell death. In tumor cells, however, the telomerase enzyme prevents the formation of critically short telomeres, adding GGTTAG repeats to the 3' end of the chromosomes immortalizing the cells. Immortality is one of the hallmarks of cancer. Besides the catalytic activity dependent telomere maintenance, catalytic activity-independent effects of telomerase may also be involved in the regulation of cell cycle. The telomere/telomerase system offers two possibilities to intervene the proliferative activity of the cell: (1) inhibition the telomere maintenance by inhibiting the telomerase activity; (2) activating the residual telomerase enzyme or inducing telomerase expression. Whilst the former approach could abolish the limitless replicative potential of malignant cells, the activation of telomerase might be utilized for treating degenerative diseases. Here, we review the current status of telomerase therapeutics, summarizing the activities of those pharmacological agents which either inhibit or activate the enzyme. We also discuss the future opportunities and challenges of research on pharmacological intervention of telomerase activity.

Inhibition of human telomerase by diphosphates of acyclic nucleoside phosphonates

Diphosphates of the antiviral acyclic nucleoside phosphonates (ANPs) were evaluated in telomeric repeat amplification protocol (TRAP) for their ability to inhibit the extension of telomeres by human telomerase. Extracts from human leukaemia HL-60 cells were used as a source of the enzyme. Data show that the most effective compound studied was the guanine derivative PMEGpp (IC50 12.7+/-0.5 micromolL(-1) at 125 micromolL(-1) deoxynucleoside triphosphates (dNTPs)). The inhibitory effects of other PME, PMP and HPMP diphosphates on telomerase reverse transcriptase decreased in the order: (R)-PMPGpp>(R)-HPMPGpp>PMEDAPpp>(S)-PMPGpp>(S)-HPMPApp>PMEO-DAPypp>(R)-6-cyprPMPDAPpp>(R)-PMPApp>(R)-PMPDAPpp> or =PMEApp> or =PMECpp>PMETpp>(S)-PMPApp approximately 6-Me2PMEDAPpp. These results are consistent with the observed antineoplastic activities of the parental guanine (PMEG) and 2,6-diaminopurine (PMEDAP) PME-derivatives. Moreover, structure-activity relationship indicates enantioselectivity of some of these human telomerase inhibitors: (R)-isomers of the PMP-derivatives possess stronger inhibitory potency towards the enzyme than (S)-isomers. The data may contribute to the rational design of telomerase inhibitors based on the structure of acyclic nucleotide analogues.

Telomerase and telomere inhibitors in preclinical development

Telomerase is an enzyme required by actively dividing cells to maintain the ends of chromosomes (telomeres). It is present in germline tissue, stem cells and cancer cells, but is repressed in somatic cells. Efforts are underway to exploit this selective expression of telomerase in cancer therapeutics. This review describes the status of telomerase research, which although at present predominantly preclinical, has the potential to enter clinical research.

Molecular cloning of the canine telomerase reverse transcriptase gene and its expression in neoplastic and non-neoplastic cells

OBJECTIVE

To perform molecular cloning of the canine telomerase reverse transcriptase (TERT) gene and determine its expression in neoplastic and non-neoplastic cells.

SAMPLE POPULATION

9 canine tumor cell lines derived from various neoplasms, 16 primary canine tumors, and tissues from 15 normal canine organs.

PROCEDURE

Tumor cell lines were derived from canine tumors that included osteosarcoma, mammary gland adenocarcinoma, melanoma, acute lymphoblastic leukemia, lymphoma, and mastocytoma and a canine primary fibroblast culture. Canine TERT complementary DNA (cDNA) was amplified by use of polymerase chain reaction (PCR) and sequenced. Expression of TERT mRNA was examined by reverse transcription (RT)-PCR assay. Telomerase activity was measured by use of the telomeric repeat amplification protocol assay.

RESULTS

The canine TERT cDNA clone was 237 base pairs in length and contained a central region encoding the reverse transcriptase motif 2. Expression of TERT mRNA was detected in canine tumor cell lines that had telomerase activity but not in telomerase-negative canine primary fibroblasts. The TERT mRNA was detected in 13 of 16 canine tumor tissues and several normal tissues such as liver, ovary, lymph node, and thymus. A significant correlation between TERT expression level and telomerase activity was noted.

CONCLUSIONS AND CLINICAL RELEVANCE

Expression of TERT mRNA was closely associated with telomerase activity in neoplastic cells as well as some non-neoplastic cells from dogs. In addition to telomerase activity, expression of TERT mRNA can be used as a marker of tumor cells.

Identification of telomerase activity in gametocytes of Plasmodium falciparum

Telomerase, a specialized cellular reverse transcriptase, compensates the chromosome shortening during the replication of most eukaryotic cells and contributes to cellular immortalization in cell culture (in vitro) and cancerous cell (in vivo). In the present study, the telomerase activity in the gametocytes of Plasmodium falciparum was investigated. Here, we report for the first time, the presence of telomerase activity in the gametocytes of P. falciparum using P. falciparum telomere repeat amplification protocol (Pf-TRAP) assay and Southern blot hybridization. Telomerase inhibitors such as 7-deaza-dGTP and AZT-TP, when used with the cytoplasmic extract of gametocytes in the Pf-TRAP assay, efficiently inhibit the product, which confirms the presence of telomerase in the gametocytes. The presence of telomerase activity in the laboratory adapted local (eastern India) isolates of P. falciparum indicates that telomerase might be the major player in chromosomal end protection during replication. The finding suggests that telomerase can be a potent target for the transmission blocking vaccine and drugs for combating malaria caused by P. falciparum.

Molecular cloning of the feline telomerase reverse transcriptase (TERT) gene and its expression in cell lines and normal tissues

Telomerase is a kind of reverse transcriptase which synthesizes and elongates telomeres. Telomerase activity is detected in many naturally occurring tumors and its expression appears to play an important role in the immortalization of tumor cells. In this study, cDNA encoding the feline telomerase reverse transcriptase (TERT) gene was cloned partially from a feline lymphoma cell line. The clone obtained in this study was 237 bp long including a reverse transcriptase motif 2, and was shown to have amino acid sequence similarity of 81.0% and 58.2% with human and mouse TERT cDNAs, respectively. TERT mRNA expression was detected in telomerase-positive cells (FL74, FT-1, 3201, FKNp, FONp, and FYMp), and was not detected in telomerase-negative cells (normal fibroblasts and CRFK). TERT mRNA was detected in various normal tissues including the spleen, pancreas, stomach, cerebrum, testis, bone marrow, lymph node and thymus, and relatively high-level expression was observed in the small bowel and large bowel. No expression of TERT mRNA was detected in the liver, adrenal gland, urinary bladder and lung. The TERT cDNA clone and the results obtained in this study will be useful for further investigation of feline tumors.

Telomerase: biochemical considerations for enzyme and substrate

Telomerase extends chromosome ends by iterative reverse transcription of its RNA template. Following the addition of each telomeric repeat, the RNA template and the telomeric substrate reset their relative position in the active site provided by the telomerase reverse transcriptase (TERT). This step might require the formation of guanine-rich secondary structures in the nascent product. Results from numerous studies begin to delineate TERT sub-domains that orchestrate these events and support the model of cooperative action between distinct active sites within telomerase multimers. Natural telomere substrates are protein-DNA complexes that show an asymmetry between the two ends of a chromosome, possibly reflecting their differential mode of replication.

Mechanism of human telomerase inhibition by BIBR1532, a synthetic, non-nucleosidic drug candidate

Telomerase, a ribonucleoprotein acting as a reverse transcriptase, has been identified as a target for cancer drug discovery. The synthetic, non-nucleosidic compound, BIBR1532, is a potent and selective telomerase inhibitor capable of inducing senescence in human cancer cells (). In the present study, the mode of drug action was characterized. BIBR1532 inhibits the native and recombinant human telomerase, comprising the human telomerase reverse transcriptase and human telomerase RNA components, with similar potency primarily by interfering with the processivity of the enzyme. Enzyme-kinetic experiments show that BIBR1532 is a mixed-type non-competitive inhibitor and suggest a drug binding site distinct from the sites for deoxyribonucleotides and the DNA primer, respectively. Thus, BIBR1532 defines a novel class of telomerase inhibitor with mechanistic similarities to non-nucleosidic inhibitors of HIV1 reverse transcriptase.

Telomerase as a diagnostic and therapeutic target for cancer

Telomerase is a ribonucleoprotein enzyme responsible for the elongation of telomeres at the ends of chromosomes. It is widely expressed in most cancers, while absent from most normal somatic cells. Telomerase is partially responsible for the cellular immortalization that allows human cancers to progress indefinitely. Due to its widespread occurrence in cancer and its crucial role in the maintenance of the tumor, telomerase is an attractive target for cancer diagnosis and treatment.

Natural and pharmacological regulation of telomerase

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

Effects of nucleotide analogues on Euplotes aediculatus telomerase processivity: evidence for product-assisted translocation

Telomerase is a unique ribonucleoprotein that reverse transcribes a defined region of its RNA subunit onto the ends of eukaryotic chromosomes. The product of telomerase, telomeric DNA, is typically a G-rich repeated sequence, (TTTTGGGG)(n) in the ciliate Euplotes aediculatus and (TTAGGG)(n) in humans. Telomerase can extend oligonucleotide primers in vitro in a processive fashion. We used dNTP analogues to study the structure-activity relationship between substrate nucleotides and processivity of telomerase from E. aediculatus. Several analogues, including 2'-deoxyuridine triphosphate (dUTP), 2'-deoxyinosine triphosphate (dITP), and 7-deaza-2'-deoxyguanosine triphosphate (7-deaza-dGTP), were good substrates for telomerase with K(m) and V(max) values near those of the natural substrates, dTTP and dGTP. However, telomerase processivity was affected with these substrates, decreasing in the order dUTP > 7-deaza-dGTP > dITP. Telomerase did not completely reverse transcribe the template when dITP was the substrate, and it efficiently extended a primer by the addition of two repeats when 7-deaza-dGTP and dUTP were utilized. When the same nucleotide analogues were incorporated into the primers, no effects were observed except in the case of a 3'-terminal deoxyinosine. The data support a model that includes the formation of an intramolecular secondary structure within the product DNA to facilitate translocation. The most likely structure is a G-G hairpin.

Telomerase and cancer: time to move from a promising target to a clinical reality

The past 25 years have seen unparalleled advances in our understanding of the molecular basis of cancer. As a result, novel molecular targets have been identified that provide great potential for the development of new cancer diagnostics and therapies. Four key features of cancer cells distinguish them from their normal counterparts: loss of cell-cycle regulation, loss of control over invasion and metastasis, failure of apoptotic mechanisms, and bypass of senescence. This review examines our understanding of the bypass of senescence and the process of immortalization during carcinogenesis. In addition, the realistic opportunities for telomerase in cancer diagnostics and the challenges faced in clinical trial design for telomerase therapeutics are discussed.

Telomerase and cancer

The past few years have brought a flood of new information to the telomerase field. The identification of multiple components of both the telomere and telomerase, the understanding of the importance of telomere maintenance to the long term viability of cells, and the demonstration of the utility of telomerase inhibition in limiting tumor cell growth all convene to provide great enthusiasm for the prospects of targeting the telomerase enzyme in cancer. However, there is clearly much to be learned. Because tumor cells evolve under powerful selection, the emergence of non-telomerase based mechanisms for telomere maintenance should be examined closely. Additionally, the nature of telomerase regulation is currently only poorly understood. More work on the tumor specific regulation of telomerase activity might provide either more opportunities for telomerase inhibition, or more skepticism, as a tumor cell might possess mechanisms for upregulating telomerase activity in the presence of inhibitors. The potential for such regulation has already been observed in certain cell types (46). Currently, the field is intensively investigating the biology and applications of telomere and telomerase biology. In it are great hopes that these fundamental cellular processes might be manipulated to success in the treatment of cancer.

Telomere length and telomerase activity in canine mammary gland tumors

OBJECTIVE

To measure telomere length and telomerase activity in naturally occurring canine mammary gland tumors.

SAMPLE POPULATION

27 mammary gland tumor specimens obtained during resection or necropsy and 12 mammary gland tissue specimens obtained from healthy (control) dogs.

PROCEDURE

Telomere length in tissue specimens was measured by use of restriction endonuclease digestion and Southern blot analysis. Telomerase activity was measured by use of a telomeric repeat amplification protocol assay.

RESULTS

Telomere length in mammary gland tumors ranged from 11.0 to 21.6 kilobase pairs (kbp; mean +/- SEM, 14.5+/-0.5 kbp) but did not differ among tumor types. Telomeres in mammary gland tumors were slightly shorter than in normal tissue specimens, but telomere length could not be directly compared between groups, because mean age of dogs was significantly different between groups. Age was negatively correlated with telomere length in control dogs but was not significantly correlated with length in affected dogs. Telomerase activity was detected in 26 of 27 mammary gland tumors and in 4 of 12 normal tissue specimens. However, telomerase activity and telomere length were not correlated in tumor specimens.

CONCLUSIONS AND CLINICAL RELEVANCE

Telomere length is maintained in canine mammary gland tumors regardless of the age of the affected dog. Measurement of telomere length may be a useful tool for monitoring the in vivo effects of telomerase inhibitors in dogs with tumors.

Telomerase in endocrine and endocrine-dependent tumors

Telomerase, the ribonucleoprotein enzyme that elongates chromosomal ends, or telomeres, is repressed in most normal somatic cells but reactivated in transformed cells to compensate for the progressive erosion of the telomeres during cell divisions. In accordance with this hypothesis, the presence of telomerase activity has been reported in more than 90% of human cancers, whereas most normal tissues or benign tumors contain low or undetectable telomerase activity. Reactivation of telomerase has also been widely reported in endocrine neoplasms and in hormone-related cancers. In the present study, we review the most recent publications on telomerase in these types of tumors. The hormonal regulation of telomerase activity and the possible strategies for cancer therapy based on the inhibition of telomerase has also been discussed.

Telomerase, immortality and cancer

Replication of eukaryotic linear chromosomes is incomplete and leaves terminal gaps. The evolutionary widely distributed solution to this "end replication" is twofold: chromosome ends are capped with telomeres, bearing multiple copies of redundant telomeric sequences, and the telomerase enzyme can add (lost) telomeric repeats. Telomerase in humans, as in all mammals, is ubiquitous in all embryonic tissues. In adults, telomerase remains active in germs cells, and, although down-regulated in most somatic tissues, telomerase is active in regenerative tissues and notably, in tumor cells. Telomerase activity is linked to cellular proliferation, and its activation seems to be a mandatory step in carcinogenesis. In contrast to mammals, indeterminately growing multicellular organisms, like fish and crustaceae, maintain unlimited growth potential or 'immortality' in all somatic tissues throughout their entire life. Also this cell immortalization is brought about by maintaining telomerase expression. Disease prognosis for human tumors includes evaluation of cell proliferation, based on the detection of proliferation markers with monoclonal antibodies. The significance of the classical marker Ki-67, and of a novel marker repp-86 are compared with semiquantitative telomerase assays. For tumor therapy, telomerase inhibitors are attractive tools. Results with telomerase knock-out mice have revealed promise, but also risk of this approach. On the other side, telomerase stimulation is attractive for expanding the potential of cellular proliferation in vitro, with possible applications for transplantation of in vitro expanded human cells, for immortalizing primary human cells as improved tissue models, and for the isolation of otherwise intractable products, like genuine human monoclonal antibodies.

Inhibition of human telomerase by 7-deaza-2'-deoxyguanosine nucleoside triphosphate analogs: potent inhibition by 6-thio-7-deaza-2'-deoxyguanosine 5'-triphosphate

We have examined analogs of the previously reported 7-deaza-2'-deoxypurine nucleoside triphosphate series of human telomerase inhibitors. Two new telomerase-inhibiting nucleotides are reported: 6-methoxy-7-deaza-2'-deoxyguanosine 5'-triphosphate (OMDG-TP) and 6-thio-7-deaza-2'-deoxyguanosine 5'-triphosphate (TDG-TP). In particular, TDG-TP is a very potent inhibitor of human telomerase with an IC(50) of 60 nM. TDG-TP can substitute for dGTP as a substrate for telomerase, but only at relatively high concentrations. Under conditions in which TDG-TP is the only available guanosine substrate, telomerase becomes nonprocessive, synthesizing short products that appear to contain only one to three TDG residues. Similarly, the less potent telomerase inhibitor OMDG-TP gives rise to short telomerase products, but less efficiently than TDG-TP. We show here that TDG-TP, and to a lesser extent OMDG-TP, can serve as substrates for both templated (Klenow exo) and nontemplated (terminal transferase) DNA polymerases. For either polymerase, the products arising from TDG-TP are relatively short, and give rise to bands of unusual mobility under PAGE conditions. These anomalous bands revert, under treatment with DTT, to normal mobility bands, indicating that these products may contain thiol-labile disulfide linkages involving the incorporated TDG residues. This observation of potential TDG-crosslinks may have bearing on the mechanism of telomerase inhibition by this nucleotide analog.

Telomerase inhibitors: targeting the vulnerable end of cancer?

In the past decade, a great deal has been learnt about the maintenance of telomeres in mammalian cells by the specialized reverse transcriptase, telomerase, and its associated proteins. The catalytic component of telomerase, hTERT, appears to be selectively activated in the vast majority of tumors relative to most somatic cells suggesting that its inhibition may result in antitumor effects. Although beset with some unusual issues as a drug target, recent 'target validation' studies using hTERT dominant-negative and antisense approaches strongly support the view that potent and selective telomerase inhibitors will induce inhibitory effects on tumors, especially in those possessing relatively short telomeres. Inhibitory strategies have focused on three main areas: antisense molecules (oligonucleotides, RNA molecules, ribozymes and peptide nucleic acids) directed against the hTR RNA component of telomerase, small molecule reverse transcriptase inhibitors (e.g. azidothymidine), and, probably most advanced, small molecules capable of interacting with and stabilizing four-stranded (G-quadruplex) structures formed by telomeres. G-quadruplex interactive agents that inhibit telomerase at sub-micromolar concentrations in cell-free assays have been described. Lead optimization and preclinical whole-cell and animal antitumor and pharmacology studies are now progressing which should result in the first generation of telomerase inhibitors being evaluated in the clinic within the next few years.

Modified telomeric repeat amplification protocol: a quantitative radioactive assay for telomerase without using electrophoresis

A polymerase chain reaction (PCR)-based radioactive telomerase assay was developed in our laboratory which is quantitative and does not require electrophoretic evaluation (designated as TP-TRAP; it utilizes two reverse primers). The main steps of the assay include (1) extension of a 20-mer oligonucleotide substrate (MTS) by telomerase, (2) amplification of the telomerase products in the presence of [(3)H]dTTP using the substrate oligonucleotide and two reverse primers (RPC3, 38 mer; RP, 20 mer), (3) isolation of the amplified radioactive dsDNA by precipitation and filtration, (4) determination of the radioactivity of the acid-insoluble DNA. The length of the telomerase products does not increase on amplification. This valuable feature of the assay is achieved by utilization of the two reverse primers and a highly specific PCR protocol. The assay is linear, accurate, and suitable for cell-biological studies where slight quantitative differences in telomerase activity must be detected. The assay is also suitable for screening and characterization of telomerase inhibitors, as shown with a chemically modified oligonucleotide reverse transcriptase inhibitor [(s(4)dU)(35)].

DNA G-quadruplexes, telomere-specific proteins and telomere-associated enzymes as potential targets for new anticancer drugs

Telomeres and telomerase have been subjects to a tremendous attention from scientists and oncologists during the past 5 years. This interest has been motivated by the potential of telomerase as a tumor marker for the diagnosis and the prognosis of cancer. The possible use of telomerase or telomeres as new targets for anticancer drugs also triggered investigations. The expression of telomerase was found in overall 85% of cancers. Telomerase is early expressed during oncogenesis with a gradient indicating that a high level of telomerase expression could be associated with a bad prognosis. Therefore, drugs targeting telomerase and telomeres might be useful in many human tumors with little restrictions regarding the tumor type or on the stage of the disease. Moreover, since telomerase is not or slightly expressed in normal cells, it has been postulated that drugs targeting telomerase would induce low toxicity. The race for the discovery of telomerase inhibitors has started while the identification of the components controlling telomerase, telomeres, cell survival, senescence, and apoptosis was still in progress. The recent identification of components regulating telomere length and telomerase expression (TRF1, TRF2, and tankyrase) opened a variety of new opportunities to control telomerase/telomere interactions. Meanwhile, a proof of principle was provided that changing telomere interactions with telomere binding proteins by chemical or biological means can induce cancer cell death. Interestingly, recent data challenge the old paradigm which suggested that a long exposure to telomerase and telomere inhibitors is necessary to induce anticancer effects. In this paper, we review the most recent information concerning the regulation of telomere length and telomerase expression, with emphasis on mechanisms that might translate into new drug discovery.

Telomerase. A target for anticancer therapy

Telomerase is absent in most normal tissues, but is abnormally reactivated in all major cancer types. Telomerase enables tumor cells to maintain telomere length, allowing indefinite replicative capacity. Albeit not sufficient in itself to induce neoplasia, telomerase is believed to be necessary for cancer cells to grow without limit. The presence of telomerase has been detected in virtually all cancer types including the most prevalent cancers of the prostate, breast, lung, colon, bladder, uterus, ovary, and pancreas as well as in lymphomas, leukemias, and melanomas. In addition, data from cancer patients indicate that telomerase levels correlate with clinical outcome in neuroblastomas, leukemias, and prostate, gastric, and breast cancers. Studies using an antisense to the human telomerase RNA component demonstrate that telomerase in human tumor lines can be blocked ex vivo. In these experiments, telomerase inhibition led to telomere shortening and cancer cell death, validating telomerase as a target for anticancer therapy. Telomerase is a uniquely appealing target for drug discovery because its dichotomic expression in normal versus cancer cells suggests that no serious side effects would result from a treatment abrogating telomerase activity. A variety of approaches to telomerase inhibition are being investigated and are discussed.

Telomeres, telomerase, and cancer: life on the edge of genomic stability

The presence of telomerase activity in most human tumors, but not in many normal somatic tissues, has raised considerable interest in telomerase as a possible anticancer therapy. Recent advances in the cloning and characterization of mammalian telomerase components have paved the way for a more detailed understanding of the role of telomerase and telomere length maintenance in cell proliferation. Here, we summarize the most recent biochemical and genetic evidence suggesting that telomere length maintenance by telomerase is critical to the proliferative ability of some immortalized mammalian cells in culture and in vivo.

Recent advances in the development of telomerase inhibitors for the treatment of cancer

Telomerase is an holoenzyme responsible for the maintenance of telomeres, the protein-nucleic acid structures which exist at the ends of eukaryotic chromosomes that serve to protect chromosomal stability and integrity. Telomerase activity is essential for the sustained proliferation of most immortal cells, including cancer cells. Since the discovery that telomerase activity is expressed in 85 - 90% of all human tumours and tumour-derived cell lines but not in most normal somatic cells, telomerase has become the focus of much attention as a novel and potentially highly-specific target for the development of new anticancer chemotherapeutics. Herein we review recent advances in the development of telomerase inhibitors for the treatment of cancer. To date, these have included antisense strategies, reverse transcriptase inhibitors and compounds capable of interacting with high-order telomeric DNA tetraplex ('G-quadruplex') structures to prevent enzyme access to the necessary linear telomere substrate. In addition, a number of telomerase-inhibitory therapies have been shown to synergistically enhance the effects of clinically-established anticancer drugs. Critical appraisal of each individual approach is provided, together with highlighted areas of likely future development. We also review recent developments in telomere and telomerase biology, of which a more detailed understanding would be essential in order to further develop the present classes of telomerase inhibitors into viable, clinically applicable therapies.

Telomerase inhibition by peptide nucleic acids reverses 'immortality' of transformed human cells

Telomerase activity, the ability to add telomeric repeats to the ends of chromosomes, has been detected in most immortal cell lines including tumor cells, but is low or absent in most diploid, mortal cells such as those of somatic tissues. Peptide nucleic acids (PNAs), analogs of DNA or RNA which bind to complementary nucleic acids with very high affinity, were co-electroporated into immortal human cells along with a selectable plasmid. Introduction of PNAs inverse-complementary to telomerase RNA effectively inhibited telomerase activity in intact cells, shortened telomeres, reduced colony size, and arrested cell proliferation after a lag period of 5-30 cell generations, consistent with suppression of their 'immortality'. Electroporation of selection plasmid alone had no effect, while PNAs of altered sequence were markedly less effective in each assay. This constitutes the first demonstration of cell growth arrest through telomerase inhibition, upon treatment of intact cells with an exogenous compound which can be efficiently delivered in vivo. The phenotype of telomerase-inhibited transformed cells differs from senescence of normal diploid fibroblasts, but rather resembles the crisis state of incompletely transformed cells.

Enzymatic activity of endogenous telomerase associated with intact nuclei from human leukemia CEM cells

Telomerase, a telomere-specific DNA polymerase and novel target for chemotherapeutic intervention, is found in many types of cancers. Telomerase activity is typically assayed using an exogenous primer and cellular extracts as the source of enzyme. Since the nuclear organization might affect telomerase function, we developed a system in which telomerase in intact nuclei catalyzes primer extension. Telomerase activity in isotonically isolated nuclei from human CEM cells shows low processivity (addition of up to four TTAGGG repeats). In contrast, telomerase activity which leaks into a 500 g postnuclear supernatant and the activity in a CHAPS extract are highly processive. The nucleotide inhibitor, 7-deaza-dGTP, seems to be more inhibitory against the nuclei-associated enzyme compared to telomerase from cytoplasmic extracts. However, 7-deaza-dATP and ddGTP are less inhibitory against nuclei-associated telomerase. The results suggest that the association of telomerase with the nuclear chromatin affects telomerase activity. Examination of telomerase activity in a more natural nuclear environment may shed new light on the telomerase function and provide a useful system for the evaluation of new telomerase inhibitors.

The cytotoxic effects of single-stranded telomere mimics on OMA-BL1 cells

Telomerase is a ribonucleoprotein that adds 5'-d(TTAGGG)-3' hexameric repeats onto the 3' ends of chromosomes. High telomerase activity has been associated with immortal cells, transformed cells, mitogenic stimulation, and proliferative diseases. It is not clear what phenotype would be observed by transient inhibition of telomerase. Studies were designed to inhibit telomerase activity using a series of S-ODN telomere sequence motifs. The studies evaluated the length, hydrogen bonding, and sequence requirements of telomerase inhibition using the TRAP assay and a bioassay measuring cell viability following exposure to the compounds. In addition, we have also studied the role of the 3' end and secondary structure of telomere mimics on telomerase inhibition. Observations reveal that sensitivity to the S-ODNs may not require hybridization to an antisense target but required guanine nucleotides on the 3' end for cells in culture and telomerase inhibition in vitro. The importance of H bonding and the requirement for a free 3' end for the activity of these compounds has also been demonstrated. However, transient inhibition of telomerase is not cytotoxic to all immortal cells and is not sufficient to explain the mechanism of cytotoxicity of these short oligonucleotides.

Measurement of telomerase activity in dog tumors

Telomeres are specific structures present at the end of liner chromosomes. DNA polymerase can not synthesize the end of liner DNA and, as a result, the telomeres become progressively shortened by successive cell divisions. To overcome the end replication problem, telomerase adds new telomeric sequences to the end of chromosomal DNA. The enzyme activity is undetectable in most normal human adult somatic cells, in which shortening of the telomere is thought to limit the somatic-cell life span. In contrast to normal somatic cells, many human tumors possess telomerase activity. The present study looked at whether telomerase activity might serve as a marker for canine tumors. Telomerase activity was measured using the telomeric repeat amplification protocol assay. Normal dog somatic tissues showed little or no telomerase activity, while normal testis exhibited a high level of telomerase activity. We measured telomerase activity in tumor samples from 45 dogs; 21 mammary gland tumors, 16 tumors developed in the skin and oral cavity, 7 vascular tumors and 1 Sertoli cell tumor. Greater than 95% of the tumor samples contained telomerase activity (3-924 U/2 micrograms protein). The results obtained in this study indicated that telomerase should be a useful diagnostic marker for a variety of dog tumors, and it may serve as a target for antitumor chemotherapy.

The telomerase challenge - an unusual problem in drug discovery

Telomerase is a ribonucleoprotein that is responsible for maintaining telomere length. The observation that telomerase activity is found in many types of tumors, but not in adjacent normal tissue, has led to the hypothesis that telomerase is a novel target for chemotherapy. Inhibitors of telomerase activity are essential to validate this hypothesis, and their design presents special opportunities and challenges.

Telomerase targeting in cancer treatment: new developments

Telomerase, a ribonucleoprotein expressed in 85% of advanced cancers but not in most somatic cells, compensates for telomeric DNA erosion and as such stabilizes cell immortality. Telomerase inhibition might restore mortality in tumor cells. Recent progress is illustrated in studies on telomerase and telomere targeting with differentiation induction, reverse transcriptase inhibitors, promoter down regulation, antisense inhibition, and blockage of telomere/telomerase interactions. Also, new developments are described indicating that anti-telomerase treatment can induce apoptosis in tumor cells and can chemosensitize drug-resistant cell lines. Implications of these findings for anti-telomerase-based therapeutic applications, in particular in combination therapies, are discussed. Copyright 1999 Harcourt Publishers Ltd.

Telomerase activity in gastrointestinal stromal tumors

BACKGROUND

Telomerase activity has been observed in 80-90% of carcinomas derived from various organs. However, to the authors' knowledge this report is the first assessment of telomerase activity in gastrointestinal stromal tumors (GISTs).

METHODS

Telomerase activity was analyzed by the telomerase repeat amplification protocol assay in 29 tumors from 26 patients (23 primary tumors from 22 patients, 1 pair of primary and metastatic tumors from 1 patient, and 4 metastatic tumors from 3 patients). Phenotypes, tumor cell proliferation, and overexpression of p53 protein were evaluated immunohistochemically.

RESULTS

Seven of 24 primary tumors (29%) and 5 of 5 metastatic tumors (100%) showed telomerase activity. Telomerase activity positive (+) GISTs were significantly larger (P < 0.05) and showed a significantly higher rate of proliferation than telomerase activity negative (-) tumors (P < 0.0001). All telomerase activity (+) GISTs were classified histologically as high risk tumors. Conversely, 15 of the 17 telomerase (-) GISTs were classified histologically as low risk tumors (P < 0.0001). With regard to p53 immunoreactivity, two and seven telomerase activity (+) tumors showed diffuse and sporadic positivity, respectively, whereas only five telomerase activity (-) tumors showed only focal or sporadic positivity. Telomerase activity was correlated significantly with poor prognosis (P < 0.05) in the patients in whom the primary GISTs were evaluated (n = 23).

CONCLUSIONS

Telomerase activity may be a useful marker for evaluating the malignant potential of GIST. A distinct subgroup of GISTs is a target for therapy with a telomerase inhibitor.

Detection of telomerase activity in Chinese hamster V79 cells and its inhibition by 7-deaza-deoxy guanosine triphosphate and (TTAGGG)4 in vitro

To investigate the nature of telomerase activity and its inhibition in Chinese hamster V79 cells, we have detected telomerase activity in Chinese hamster cells using Telomeric Repeat Amplification Protocol (TRAP) assay. We have further studied inhibition characteristics of this enzyme in vitro by nucleotide analogue 7-deaza-2'-deoxy guanosine triphosphate (7-deaza-dGTP) and oligonucleotide (TTAGGG)4. Both the inhibitors inhibited the telomerase activity in a dose dependent manner. To attain 50% inhibition of the telomerase activity, we needed about 4.5 microM of 7-deaza-dGTP. Similarly, preincubation at 37 degreesC of the cell extract with 1.25 x 10(-3) microgram oligonucleotide (TTAGGG)4 showed 50% inhibition of the control value. Inhibition of telomerase activity by 7-deaza-dGTP could be due to the incorporation of the modified nucleotide in the telomeric repeat and thus altering the further binding/extension by the enzyme. (TTAGGG)4 could have possibly interacted with RNA component of telomerase and inhibited its activity.

Clinical evaluation of biologically targeted drugs: obstacles and opportunities

Recent insights into the molecular mechanisms of cancer have indicated that a variety of fundamental cellular processes are dysregulated in malignant cells. These processes include cell cycle control, signal transduction pathways, apoptosis, telomere stability, angiogenesis, and interactions with the extracellular matrix. Remarkable advances in molecular genetics, enzymology, and medicinal chemistry have permitted the design of compounds that modulate some of these processes with specificity that was unimaginable a decade ago. As these novel, biologically targeted compounds enter the clinic, they will require a strategy for clinical evaluation and development different from that used commonly for cytotoxic antineoplastic agents. This review examines the development of cancer drugs directed against angiogenesis, metastasis, signal transduction, telomerase, and molecular message (antisense), outlines strategies for the clinical testing of agents directed at these processes, and contrasts these efforts with traditional approaches to cancer drug testing.

Inhibition of telomerase activity of melanoma cells in vitro by antisense oligonucleotides

The effect of antisense oligonucleotides complementary to the RNA component of human telomerase on telomerase activity in cell extracts of the melanoma cell line SK-Mel-28 has been studied. It has been shown that the antisense oligonucleotide complementary to the hTR component in the region of the template synthesis of telomeric repeats is the most efficient inhibitor of telomerase activity in comparison with other antisense oligonucleotides. Pronounced inhibition of telomerase activity was observed at the oligonucleotide (Tel P5) concentration in the reaction mixture of about 5 nM. Complete inhibition of the enzyme occurs at the oligonucleotide concentration in the sample of about 20 nM.

Effect of DNA secondary structure on human telomerase activity

Telomeres are specialized DNA-protein complexes located at the chromosome ends. The guanine-rich telomeric sequences have the ability to form G-quadruplex structures under physiological ionic conditions in vitro. Human telomeres are maintained through addition of TTAGGG repeats by the enzyme telomerase. To determine a correlation between DNA secondary structure and human telomerase, telomerase activity in the presence of various metal cations was monitored. Telomerase synthesized a larger proportion of products corresponding to four, five, eight, and nine full repeats of TTAGGG in 100 mM K+ and to a lesser extent in 100 mM Na+ when a d(TTAGGG)3 input primer was used. A more even product distribution was observed when the reaction mixture contained no added Na+ or K+. Increasing concentrations of Cs+ resulted in a loss of processivity but not in the distinct manner observed in K+. When the input primer contained 7-deaza-dG, the product distribution resembled that of reactions without K+ even in the presence of 100 mM K+. Native polyacrylamide gel electrophoresis indicated that d(TTAGGG)4, d(TTAGGG)5, d(TTAGGG)8, and d(TTAGGG)9 formed compact structures in the presence of K+. The oligonucleotide d(TTAGGG)4 had a UV spectrum characteristic of that of the G-quadruplex only in the presence of K+ and Na+. A reasonable explanation for these results is that four, five, eight, and nine repeats of TTAGGG form DNA secondary structures which promote dissociation of the primer from telomerase. This suggests that telomerase activity in cells can be modulated by the secondary structure of the DNA template. These findings are of probable relevance to the concept of telomerase as a therapeutic target for drug design.

Euplotes telomerase: evidence for limited base-pairing during primer elongation and dGTP as an effector of translocation

The telomeric sequence repeats at the ends of eukaryotic chromosomes are maintained by the ribonucleoprotein enzyme telomerase. Telomeric DNA primers are bound by telomerase both at the active site, which includes base-pairing with the RNA template, and at a second anchor site. The stabilities of Euplotes aediculatus primer-telomerase complexes were determined by measuring their dissociation rates (koff), using an assay involving photo-cross-linking at the anchor site. The primer length was varied, and mismatched substitutions were introduced in a systematic manner. We observed that koff does not scale with primer length as expected for accumulated primer-template base-pairing. This suggests that telomerase maintains a more-or-less constant number of base pairs, similar to the transcription bubble maintained by RNA polymerase. An upper limit was estimated by comparing the experimental koff for the primer-telomerase complex to that of a model DNA-RNA duplex. All the binding energy could be attributed to 10 or 11 base pairs; alternatively, there could be <10 base pairs, with the remaining energy contributed by other parts of telomerase. Most primers exhibited biphasic dissociation kinetics, with variations in both the amount in each phase and the rate for each phase. Since the cross-links monitored in the dissociation assay were all formed with the 5' region of the primer, the two phases may arise from different base-pairing registers with the RNA template, possibly representing pre- and post-translocation complexes. A shift from slow phase to fast phase dissociation was observed in the presence of dGTP, which may implicate dGTP as a positive effector of translocation.

Structural polymorphism in a tubercidin analogue of the DNA double helix

A high-angle X-ray fibre diffraction study of a tubercidin analogue of the poly[d(A-T)].poly[d(A-T)] DNA double helix has been carried out using station 7.2 at the Daresbury Laboratory synchrotron radiation source. The polymer has been studied for a wide range of salt strengths and hydration conditions and exhibits conformational polymorphism that is quite distinct from that observed for the unmodified polymer. The replacement of deoxyadenosine by deoxytubercidin in the polynucleotide causes only slight alterations to the structure of A-DNA, but significantly alters the structure of the B conformation. Additionally, the modified polymer does not, in any conditions yet identified, adopt the D conformation. In conditions which would normally favour the D conformation of poly[d(A-T)].poly[d(A-T)], the modified polymer adopts an unusual conformation which is designated here as the K conformation. These observations are important for an understanding of major groove interactions involved in the stabilisation of particular DNA conformations and also more generally for an insight into the pharmacological activity of tubercidin which following its incorporation into nucleic acids may cause stereochemical distortions of the DNA double helix.

Preclinical and clinical strategies for development of telomerase and telomere inhibitors

BACKGROUND

Telomerase is an important enzyme whose activity has been convincingly demonstrated in humans recently. It is required for maintenance of ends of chromosomes (telomeres) during cell division. Since its presence has been selectively demonstrated in dividing cells including tumor cells, it has generated considerable excitement as a potential anti-cancer strategy.

DESIGN

In this article, we review the current relevant biology of the enzyme, the challenges encountered in the preclinical phase of target development and the current efforts that focus on telomeres and telomerase as therapeutic targets. We also speculate on the potential toxicities and mechanisms of resistance that may be encountered during use of such therapies.