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

Towards Computational Modeling of Ligand Binding to the ILPR G-Quadruplex

Using a combination of unconstrained and constrained molecular dynamics simulations, we have evaluated the binding affinities between two porphyrin derivatives (TMPyP4 and TEGPy) and the G-quadruplex (G4) of a DNA fragment modeling the insulin-linked polymorphic region (ILPR). Refining a well-established potential of mean force (PMF) approach to selections of constraints based on root-mean-square fluctuations results in an excellent agreement between the calculated and observed absolute free binding energy of TMPyP4. The binding affinity of IPLR-G4 toward TEGPy is predicted to be higher than that toward TMPyP4 by 2.5 kcal/mol, which can be traced back to stabilization provided by the polyether side chains of TMPyP4 that can nestle into the grooves of the quadruplex and form hydrogen bonds through the ether oxygen atoms. Because our refined methodology can be applied to large ligands with high flexibility, the present research opens an avenue for further ligand design in this important area.

Transcriptional regulation of telomeric repeat-containing RNA by acridine derivatives

Telomere is a specialized DNA-protein complex that plays an important role in maintaining chromosomal integrity. Shelterin is a protein complex formed by six different proteins, with telomeric repeat factors 1 (TRF1) and 2 (TRF2) binding to double-strand telomeric DNA. Telomeric DNA consists of complementary G-rich and C-rich repeats, which could form G-quadruplex and intercalated motif (i-motif), respectively, during cell cycle. Its G-rich transcription product, telomeric repeat-containing RNA (TERRA), is essential for telomere stability and heterochromatin formation. After extensive screening, we found that acridine derivative 2c and acridine dimer DI26 could selectively interact with TRF1 and telomeric i-motif, respectively. Compound 2c blocked the binding of TRF1 with telomeric duplex DNA, resulting in up-regulation of TERRA. Accumulated TERRA could bind with TRF1 at its allosteric site and further destabilize its binding with telomeric DNA. In contrast, DI26 could destabilize telomeric i-motif, resulting in down-regulation of TERRA. Both compounds exhibited anti-tumour activity for A549 cells, but induced different DNA damage pathways. Compound 2c significantly suppressed tumour growth in A549 xenograft mouse model. The function of telomeric i-motif structure was first studied with a selective binding ligand, which could play an important role in regulating TERRA transcription. Our results showed that appropriate level of TERRA transcript could be important for stability of telomere, and acridine derivatives could be further developed as anti-cancer agents targeting telomere. This research increased understanding for biological roles of telomeric i-motif, TRF1 and TERRA, as potential anti-cancer drug targets.

Analysis of hiPSCs differentiation toward hepatocyte-like cells upon extended exposition to oncostatin

The capability to produce and maintain functional human adult hepatocytes remains one of the major challenges for the use of in-vitro models toward liver cell therapy and industrial drug-screening applications. Among the suggested strategies to solve this issue, the use of human-induced pluripotent stem cells (hiPSCs), differentiated toward hepatocyte-like cells (HLCs) is promising. In this work, we propose a 31-day long protocol, that includes a final 14-day long phase of oncostatin treatment, as opposed to a 7-day treatment which led to the formation of a hepatic tissue functional for CYP1A2, CYP2B6, CYP2C8, CYP2D6, and CYP3A4. The production of albumin, as well as bile acid metabolism and transport, were also detected. Transcriptome profile comparisons and liver transcription factors (TFs) motif dynamics revealed increased expression of typical hepatic markers such as HNF1A and of important metabolic markers like PPARA. The performed analysis has allowed for the extraction of potential targets and pathways which would allow enhanced hepatic maturation in-vitro. From this investigation, NRF1 and SP3 appeared as transcription factors of importance. Complex epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) patterns were also observed during the differentiation process. Moreover, whole transcriptome analysis highlighted a response typical of the one observed in liver regeneration and hepatocyte proliferation. While a complete maturation of hepatocytes was yet to be obtained, the results presented in this work provide new insights into the process of liver development and highlight potential targets aimed to improve in-vitro liver regeneration.

Targeting G-quadruplex DNA with synthetic dendritic peptide: modulation of the proliferation of human cancer cells

Telomerase, a reverse transcriptase enzyme, is found to over express in most cancer cells. It elongates the telomere region by repeated adding of TTAGGG in the 3′-end and leads to excess cell proliferation which causes cancer. G-quadruplex (G4) formation can inhibit such telomere lengthening. So, stabilization of G4 structure as well as inhibition of telomerase activity is very promising approach in targeted cancer therapy. Herein, the aptitude of a synthetic dendritic peptide, C^δ2–(YEE)–E (peptide 1), to target specifically the human telomeric G4 DNA, dAGGG(TTAGGG)₃, has been evaluated. Both biochemical and biophysical techniques including gel mobility shift assay, isothermal titration calorimetry and fluorescence spectroscopy have been employed for the purpose. Circular dichroism study reveals that the targeting results an increase in thermal stability of G4 DNA. Interestingly, replacement of N-terminal tyrosine residue of peptide 1 by valine, C^δ2–(VEE)–E, (peptide 2) consequences in loss of its G4 DNA targeting ability, although both the peptides exhibit comparable affinity toward double-stranded DNA. Of note, peptide 1 causes cessation of growth of human cancer cells (HeLa and U2OS) and induces apoptosis in vitro. But it has no significant inhibitory effect on the growth of normal human embryonic kidney 293 cells. Mechanistically, Telomeric Repeat Amplification Protocol (TRAP) assay indicates that peptide 1 effectively inhibits the telomerase activity in human cell extracts. Overall, this study demonstrates the usefulness of a synthetic dendritic peptide as an inhibitor of tumor cell growth by inducing apoptosis upon targeting the telomeric G4 DNA.

A synthetic dendritic peptide, targeting human telomeric G4 DNA, inhibits the telomerase and lessens the proliferation of human cancer cells.

Sodium iodide symporter (NIS) in extrathyroidal malignancies: focus on breast and urological cancer

Background

Expression and function of sodium iodide symporter (NIS) is requisite for efficient iodide transport in thyrocytes, and its presence in cancer cells allows the use of radioiodine as a diagnostic and therapeutic tool in thyroid neoplasia. Discovery of NIS expression in extrathyroidal tissues, including transformed cells, has opened a novel field of research regarding NIS-expressing extrathyroidal neoplasia. Indeed, expression of NIS may be used as a biomarker for diagnostic, prognostic, and therapeutic purposes. Moreover, stimulation of endogenous NIS expression may permit the radioiodine treatment of extrathyroidal lesions by concentrating this radioisotope.

Results

This review describes recent findings in NIS research in extrathyroidal malignancies, focusing on breast and urological cancer, emphasizing the most relevant developments that may have clinical impact.

Conclusions

Given the recent progress in the study of NIS regulation as molecular basis for new therapeutic approaches in extrathyroidal cancers, particular attention is given to studies regarding the relationship between NIS and clinical-pathological aspects of the tumors and the regulation of NIS expression in the experimental models.

Telomerase-driven expression of the sodium iodide symporter (NIS) for in vivo radioiodide treatment of cancer: a new broad-spectrum NIS-mediated antitumor approach

CONTEXT

Telomerase promoters (hTERT and hTR) are useful for transcriptional targeting in gene therapy models of cancer. Telomerase-driven expression of the sodium iodide symporter (NIS) in tumor cells has been successfully used as a reporter gene in vivo using positron emission tomography (PET) imaging.

OBJECTIVE

The aim of this study was to investigate the NIS-mediated therapeutic effect of telomerase promoters in a wide variety of human cancer cell lines.

DESIGN AND METHODS

Promoter fragments from either hTERT or hTR were used to drive the expression of NIS in cell lines derived from melanoma (M14), breast (MDA-MB-231), colon (HT-29), lung (H460), ovarian (OVCAR-3), and thyroid (TPC-1) carcinomas. Iodide uptake assays, protein immunodetection, and clonigenic assays were used to confirm NIS functional expression and the (131)I-mediated cytopathic effect. Tumor xenografts in mice were infected with hTERT and hTR and then treated using radioiodide.

RESULTS

Both promoters were selectively active in cancer cells that were effectively killed by exposure to (131)I. One single dose of 1 mCi (131)I markedly suppressed tumor growth of melanoma-derived tumor xenografts compared with controls. This effect was more modest in colon cancer-derived xenografts in part due to the reduced infectivity and the tumor cystic nature. The therapeutic effect of hTR promoter was found to be stronger than that of hTERT promoter.

CONCLUSIONS

These results demonstrate that telomerase-driven expression of NIS could potentially have applications for (131)I therapy of a wide variety of cancers. Additionally, this is the first study to report NIS-mediated (131)I therapy of melanoma tumors in vivo.

Ex vivo expanded telomerase-specific T cells are effective in an orthotopic mouse model for pancreatic adenocarcinoma

Telomerase activity is over-expressed in nearly all pancreatic carcinomas, but not in chronic pancreatitis. Here, we investigated various protocols for expansion of telomerase-specific T cells for adoptive cell transfer and their use in a syngeneic pancreatic carcinoma mouse model. Telomerase-specific T cells were generated by stimulation of splenocytes from peptide-immunized donor mice with either interleukin (IL)-2, IL-15, artificial antigen-presenting cells, anti-signalling lymphocyte activation molecule (SLAM) microbeads or allogeneic dendritic cells in combination with a limited dilution assay. T cells were tested for antigen specificity in vitro and for anti-tumour activity in syngeneic mice with orthotopically implanted tumours pretreated with cyclophosphamide. The immune cells from recipients were immunophenotyped. During a period of 2 weeks, the expansion approach using IL-2 was very successful in generating a high number of telomerase-specific CD8(+) T cells without losing their function after adoptive cell transfer. Significantly slower tumour growth rate and less metastasis were observed after adoptively transferring telomerase specific CD8(+) T cells, expanded using IL-2. Further investigations showed that anti-tumour efficacy was associated with a significant shift from naive CD8(+) T cells to CD8(+) central memory T cells, as well as recruitment of a high number of dendritic cells. Remarkable amounts of telomerase-specific T cells were detectable in the tumour. Generation of telomerase-specific T cells is feasible, whereat IL-2-based protocols seemed to be most effective and efficient. Antigen-specific T cells showed significant cytotoxic activity in a syngeneic, orthotopic mouse model, whereas central memory T cells but not effector memory T cells appear to be of high importance.

Strategies to explore new approaches in the investigation and treatment of osteosarcoma

Studies in osteosarcoma over the past 40 years have led to a steady improvement in the overall outcome of patients with osteosarcoma. In the year 2008, we can expect greater than 60% overall survival for newly diagnosed non-metastatic appendicular osteosarcoma. However, to achieve this current outcome, many patients are treated with aggressive cytotoxic chemotherapy and ultimately are not cured, and some patients who would be curable even without this aggressive approach are likely treated and cured. And finally, patients presenting with metastatic disease and those whose tumors recur after standard approaches continue to do very poorly. We believe that in order to continue to make progress in the treatment of this disease, we must achieve two main objectives. Firstly, we must find biomarkers that prospectively and accurately identify newly diagnosed non-metastatic patients who will not be cured with current modalities. We hope that the achievement of this goal will allow for innovative clinical studies in this high-risk population while not jeopardizing those patients who currently are cured using the available treatment approaches, and ultimately accelerate progress toward curing more patients. Secondly, we must develop entirely new approaches to the treatment of metastatic and recurrent osteosarcoma. Our approach has been to develop models of highly aggressive and less aggressive osteosarcoma, and to use these models to identify genetic alterations and signaling pathways that distinguish the two phenotypic behaviors. We have identified plasma membrane-cytoskeletal linker protein, ezrin, as one pathway that identifies aggressive biological behavior in mouse and dog osteosarcoma. Using ezrin as the initial discriminator, we have high ezrin expression to activation of mTOR signaling, suggesting a possible novel target for therapy of aggressive osteosarcoma. We have also linked beta4 integrin signaling to metastatic behavior that also appears to be linked to mTOR signaling. Most recently, we have identified a critical relationship between mTOR signaling and the IGF I signaling pathway that may help point the way to combination targeting therapy aimed at blocking both mTOR and IGF signaling in these tumors. Finally, we have proposed a novel clinical trial design to begin to test agents targeted at recurrent, metastatic disease, and this also will be discussed.

Telomerase inhibitors and 'T-oligo' as cancer therapeutics: contrasting molecular mechanisms of cytotoxicity

Telomeres, the specialized structures that comprise the ends of chromosomes, form a closed structure, or t-loop, that is important in preventing genomic instability. Forced modulation of this structure, via overexpression of a dominant-negative form of telomere repeat binding factor 2, a protein critical for maintaining t-loop structure, for example, can result in the activation of DNA-damage responses, and ultimately cellular senescence or apoptosis. This response is also seen in normal somatic cells, where telomeres steadily decrease in length as cellular proliferation occurs owing to inefficient replication of terminal telomeric DNA. When telomere length becomes critically short, t-loop structure is compromised, and the cell undergoes senescence. Telomerase, the enzyme responsible for telomere length maintenance, is overexpressed in a majority of cancers. Its lack of expression in most normal somatic cells makes it an attractive target in designing cancer therapeutics. Compounds currently under development that seek to inhibit hTERT, the reverse transcriptase component of telomerase, include nucleoside analogs and the small molecule BIBR1532. Compounds inhibiting the RNA component of telomerase, hTERC, include peptide nucleic acids, 2-5A antisense oligonucleotides, and N3'-P5' thio-phosphoramidates. Recently, an oligonucleotide sharing sequence homology with terminal telomeric DNA, termed 'T-oligo', has shown cytotoxic effects in multiple cancers in culture and animal models. Independent of telomerase function, T-oligo is thought to mimic the DNA-damage response a cell normally experiences when the telomere t-loop structure becomes dysfunctional. In this review, the molecular mechanisms attributed to telomerase inhibitors and T-oligo, as well as their potential as cancer therapeutics, are discussed.

Telomerase and its potential for therapeutic intervention

Telomerase and telomeres are attractive targets for anticancer therapy. This is supported by the fact that the majority of human cancers express the enzyme telomerase which is essential to maintain their telomere length and thus, to ensure indefinite cell proliferation--a hallmark of cancer. Tumours have relatively shorter telomeres compared to normal cell types, opening the possibility that human cancers may be considerably more susceptible to killing by agents that inhibit telomere replication than normal cells. Advances in the understanding of the regulation of telomerase activity and the telomere structure, as well as the identification of telomerase and telomere associated binding proteins have opened new avenues for therapeutic intervention. Here, we review telomere and telomerase biology and the various approaches which have been developed to inhibit the telomere/telomerase complex over the past decade. They include inhibitors of the enzyme catalytic subunit and RNA component, agents that target telomeres, telomerase vaccines and drugs targeting binding proteins. The emerging role of telomerase in cancer stem cells and the implications for cancer therapy are also discussed.

Modulation of telomerase promoter tumor selectivity in the context of oncolytic adenoviruses

The telomerase RNA (hTR) and reverse transcriptase (hTERT) promoters are active in most cancer cells, but not in normal cells, and are useful for transcriptional targeting in gene therapy models. Telomerase-specific conditionally replicating adenoviruses (CRAd) are attractive vectors because they should selectively lyse tumor cells. Here, we compare CRAds, in which either the hTR or hTERT promoter controls expression of the adenovirus E1A gene. In replication-defective reporter adenoviruses, the hTR promoter was up to 57-fold stronger in cancer cells than normal cells and up to 49-fold stronger than hTERT. In normal cells, hTERT promoter activity was essentially absent. Doses of telomerase-specific CRAds between 1.8 and 28 infectious units per cell efficiently killed cancer cells, but normal cells required higher doses. However, CRAd DNA replication and E1A expression were detected in both cancer and normal cells. Overall, tumor specificity of the CRAds was limited compared with nonreplicating vectors. Surprisingly, both CRAds expressed similar E1A levels and functional behavior, despite known differentials between hTR and hTERT promoter activities, suggesting that the promoters are deregulated. Rapid amplification of cDNA ends analysis of hTR-/hTERT-E1A transcripts ruled out cryptic transcription from the vector backbone. Blocking E1A translation partially restored the hTR-/hTERT-E1A mRNA differential, evidencing feedback regulation by E1A.

Telomeres rather than telomerase a key target for anti-cancer therapy?

It was in the 1930s that telomeres (from the Greek telos = end and meros = part) were first recognized as essential structures at the ends of the chromosomes and were shown to be important for chromosomal stability (Muller HJ: The remaking of chromosomes. The Collecting Net-Woods Hole 1938: 13: 181-198, McClintock B, The stability of broken ends of chromosomes in Zea mays. Genetics 1041: 26: 234-282). However, it was only in 1978 that the first telomeric sequence was identified -- in the protocoa Tetrahymena, a single cell organism that at a certain stage of development has many identical minichromosomes with twice as many telomeres (Blackburn EH and Gall JG. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. J. Mol. Biol. 1978: 120: 33-53.). Today we know that telomeres form specialized, three-dimensional DNA-protein structures and fulfil important capping functions. Besides, telomeric DNA is essential as ''access DNA'' for those cells that are not able to counteract loss of DNA during replication because they do not express telomerase, the enzyme responsible for telomere length maintenance. Since telomerase is mostly found in tumor cells and inhibition correlates with telomere shortening and finally growth inhibition, telomerase and lately also the telomeres themselves have become attractive targets for anti-cancer therapy. This review aims to critically throw light on different therapeutical approaches and comes to the conclusion that telomeres may be the better targets for cancer therapeutics.

Senescence and telomere shortening induced by novel potent G-quadruplex interactive agents, quindoline derivatives, in human cancer cell lines

Agents stabilizing G-quadruplexes have the potential to interfere with telomere replication by blocking the elongation step catalysed by telomerase or telomerase-independent mechanism and could therefore act as antitumor agents. In this study, we found that quindoline derivatives interacted preferentially with intramolecular G-quadruplex structures and were novel potent telomerase inhibitors. Treatment with quindoline derivatives reproducibly inhibited telomerase activity in human leukemia K562 cells and colon cancer SW620 cells. N'-(10H-Indolo [3,2-b] quinolin-11-yl)-N, N-dimethyl-propane-1,3-diamine (SYUIQ-5), (one of quindoline derivatives), when added to K562 and SW620 cell culture at nonacute cytotoxic concentrations, increased time of population doublings of K562 and SW620 cells, induced a marked cessation in cell growth and cellular senescence phenotype after 35 and 18 days, respectively. Growth cessation was accompanied by a shortening of telomere length, and induction of p16, p21 and p27 protein expression. However, another compound SYUIQ-7 with greater IC(50) for telomerase had no obvious cellular effect in nonacute cytotoxic concentrations. These results indicate that quindoline derivatives as novel potent G-quadruplex interactive agents induce senescence and telomere shortening in cancer cells and therefore are promising agents for cancer treatment.

Transcriptional Repression of Telomerase RNA Gene Expression by c-Jun-NH2-Kinase and Sp1/Sp3

Telomerase is essential for immortalization of most human cancer cells. Expression of the core telomerase RNA (hTR) and reverse transcriptase (hTERT) subunits is mainly regulated by transcription. However, hTR transcriptional regulation remains poorly understood. We previously showed that the core hTR promoter is activated by Sp1 and is repressed by Sp3. Here, we show that the mitogen-activated protein kinase kinase kinase 1 (MEKK1)/c-Jun-NH(2)-kinase (JNK) pathway represses hTR expression by a mechanism that involves Sp1 and Sp3. Promoter activity was induced by the JNK inhibitor SP600125 and was repressed by activated MEKK1. Repression by MEKK1 was blocked by SP600125 or enhanced by coexpression of wild-type but not phosphoacceptor mutated JNK. SP600125 treatment also increased levels of endogenous hTR. Mutations in the hTR promoter Sp1/Sp3 binding sites attenuated SP600125-mediated promoter induction, whereas coexpression of MEKK1 with Sp3 enhanced hTR promoter repression. Chromatin immunoprecipitation showed that levels of immunoreactive Sp1 associated with the hTR promoter were low in comparison with Sp3 in control cells but increased after JNK inhibition with a reciprocal decrease in Sp3 levels. No corresponding changes in Sp1/Sp3 protein levels were detected. Thus, JNK represses hTR promoter activity and expression, apparently by enhancing repression through Sp3.

Application of Targeted Radiotherapy/Gene Therapy to Bladder Cancer Cell Lines

OBJECTIVES

A targeted radiotherapy/gene therapy strategy for transitional cell carcinoma of bladder is described, using [131I]meta-iodobenzylguanidine ([131I]MIBG), a radionuclide combined with a tumour-seeking drug. The aim is to decrease side effects from radiation toxicity, while increasing radiation dose to tumour. This tumour cell kill approach is augmented by radiological bystander effects.

METHODS

The bladder cancer cell line EJ138 was transfected with a gene encoding the noradrenaline transporter (NAT) under the control of tumour-specific telomerase promoters. Resulting uptake of [131I]MIBG was assessed by gamma-counting of cell lysates, and NAT transgene expression by real-time RT-PCR. Cell kill of monolayers and disaggregated spheroids, dosed with [131I]MIBG, was assessed by clonogenic assay.

RESULTS

NAT gene transfected cells exhibited a significantly increased active uptake of [131I]MIBG, leading to dose-dependent cell kill. Clonogenic assay of disaggregated spheroids, a three-dimensional model, suggested cell kill via bystander effects.

CONCLUSIONS

Expression of a functional NAT after in vitro transfection of bladder cancer cells with the NAT gene under the control of telomerase promoters leads to active uptake of [131I]MIBG and dose-dependent cell kill. This strategy could produce a promising new treatment option for bladder cancer.

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

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

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

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

Combining a targeted radiotherapy and gene therapy approach for adenocarcinoma of prostate

A targeted radiotherapy/gene therapy approach for prostate cancer, using the radiopharmaceutical [(131)I]meta-iodobenzylguanidine ([(131)I]MIBG), would restrict the effects of radiotherapy to malignant cells, thereby increasing efficacy and decreasing morbidity of radiotherapy. Prostate cancer cells were transfected with a transgene encoding the noradrenaline transporter (NAT) under the control of tumour-specific telomerase promoters, enabling them to actively take up [(131)I]MIBG. This led to tumour-specific cell kill. This strategy has the advantage of generating a radiological bystander effect, leading to the destruction of neighbouring tumour cells that have escaped transfection. This targeted approach could be a promising tumour-specific treatment option for prostate cancer.

Chromosomal imbalances in primary and metastatic melanomas: over-representation of essential telomerase genes

Comparative genomic hybridization was used to map copy number abnormalities in 48 short-term cell cultures established from different stages and types of human melanoma. A variety of random and non-random chromosomal alterations were detected, with gains within chromosomes 20q, 7q, 7p, 20p, 6p and 17q and losses in 9p, 10q, 6q, 10p, 4q, and 11q being the most common observations. In addition, several other chromosomal loci were over- or under-represented in subgroups of melanomas. For example, sequences on 3q26 were over-represented in 33% and on 5p15.33 in 27% of cell cultures, reaching the level of amplification in 12% and 22%, respectively. These regions harbour the two essential genes for the enzyme telomerase: the telomerase reverse transcriptase gene (hTERT) on 5p15.33 and the telomerase RNA component gene (hTERC) on 3q26. Using fluorescence in situ hybridization and Southern blot analysis, both genes were shown to be over-represented or amplified in several melanomas. Interestingly, hTERT amplification was abundant in superficial spreading primary melanomas, subcutaneous metastases and malignant effusion-derived cells, but completely absent or very rare in primary nodular melanomas as well as brain, bone and lymph node metastases. Several chromosomes or chromosomal regions harbouring telomerase-suppressing activities (3p, 4, 6 and 10p) were frequently under-represented in melanomas. Our data suggest that genetic alterations at several chromosomal loci might facilitate activation of telomerase during the development of cutaneous malignant melanoma.

Telomerase: a potential diagnostic and therapeutic tool in canine oncology

In recent years there has been considerable interest in telomerase as a target for therapeutic intervention in oncology. This largely stems from the vast number of studies that have demonstrated expression and activity of the enzyme telomerase in the majority of human cancer tissues with little or no activity detectable in normal somatic tissues. These studies have led to an interest in the role of telomerase in cancers associated with domesticated species, in particular tumors that affect dogs. This article reviews the biology of telomerase and the biological significance of telomerase activity in canine tumors and discusses the clinical implications of telomerase expression in canine cancers with regard to therapeutics and diagnostics.

Evaluation of malignancy and prognosis of gastrointestinal stromal tumors: a review

This paper reviews data on the prognosis of gastrointestinal stromal tumors (GISTs). These tumors are specific KIT-expressing and KIT-signaling-driven mesenchymal tumors, many of which have KIT-activating mutations. GISTs occur in the entire gastrointestinal (GI) tract and may also arise from the omentum, mesenteries, and retroperitoneum. They range from small benign tumors to sarcomas at all sites of occurrence. A KIT tyrosine kinase inhibitor, STI-571 (imatinib [Gleevec]; Novartis, Basel, Switzerland), has recently shown promise in the treatment of metastatic GISTs. Understanding the natural history of GIST before introduction of STI-571 will help assess the impact and position of this new treatment. The frequency of benign versus malignant GISTs varies between sites. Benign GISTs outnumber malignant GISTs in the stomach, whereas malignant GISTs are more common in the intestines. Tumors that have metastasized at presentation have a very poor prognosis. Traditionally, the 3 key prognostic factors have been mitotic rate, tumor size, and site. Tumors that are small (< or =2 cm) and show mitotic activity not exceeding 5 mitoses per 50 high-power fields (HPFs) have an excellent prognosis, probably independent of site, although this has not been shown specifically for all sites. In the stomach, most epithelioid GISTs are benign, provided that mitotic counts do not exceed 5/50 HPFs. However, a small proportion of tumors apparently lacking mitotic activity do metastasize. Tumors with a mitotic rate >5/50 HPFs usually have a malignant behavior. The Ki67 index may help identify tumors with malignant potential, but large site-specific series are not yet available. Genetic markers, including DNA-copy number changes, telomerase activity, and KIT mutation status, may be useful in more accurately identifying tumors with malignant potential.

Telomere maintenance as a target for anticancer drug discovery

Maintenance of telomeres--specialized complexes that protect the ends of chromosomes--is undertaken by the enzyme complex telomerase, which is a key factor that is activated in more than 80% of cancer cells that have been examined so far, but is absent in most normal cells. So, targeting telomere-maintenance mechanisms could potentially half tumour growth across a broad spectrum of tumour types, with little cytotoxic effect outside tumours. Here, we describe the current understanding of telomere biology, and the application of this knowledge to the development of anticancer drugs.

Markers of senescence?

The specific identification of cellular senescence in clinical material has important implications for determining the role of senescence in age-related pathologies and in neoplasia in certain tumours. One suggested marker of senescence is the histochemical identification of a specific beta-galactosidase enzyme operative at pH 6.0. However, recent data indicate that this enzyme may not be specific for senescence in all tissues and probably represents the expression of endogenous lysosomal acid beta-galactosidase, which is expressed by a variety of differentiated cell types.

Cell senescence and telomere shortening induced by a new series of specific G-quadruplex DNA ligands

Telomeres of human chromosomes contain a G-rich 3'-overhang that adopts an intramolecular G-quadruplex structure in vitro which blocks the catalytic reaction of telomerase. Agents that stabilize G-quadruplexes have the potential to interfere with telomere replication by blocking the elongation step catalyzed by telomerase and can therefore act as antitumor agents. We have identified by Fluorescence Resonance Energy Transfer a new series of quinoline-based G-quadruplex ligands that also exhibit potent and specific anti-telomerase activity with IC50 in the nanomolar concentration range. Long term treatment of tumor cells at subapoptotic dosage induces a delayed growth arrest that depends on the initial telomere length. This growth arrest is associated with telomere erosion and the appearance of the senescent cell phenotype (large size and expression of beta-galactosidase activity). Our data show that a G-quadruplex interacting agent is able to impair telomerase function in a tumor cell thus providing a basis for the development of new anticancer agents.