Overcoming the immortality of tumour cells by telomere and telomerase based cancer therapeutics – current status and future prospects

Formation of a unique end-to-end stacked pair of G-quadruplexes in the hTERT core promoter with implications for inhibition of telomerase by G-quadruplex-interactive ligands

The hTERT core promoter contains a G-rich region of 12 consecutive G-tracts, embracing 3 Sp1 binding sites, and has the potential to form multiple G-quadruplexes. From the 12 runs of guanines, 9 putative hTERT G-quadruplex-forming sequences were selected to assay for G-quadruplex formation and stability using circular dichroism and a Taq polymerase stop assay. Results from biophysical and chemical assays demonstrate an approximate inverse correlation between total loop size and structure stability. Investigation of the full-length hTERT G-rich sequence using a Taq polymerase stop assay and dimethyl sulfate footprinting revealed the formation of a unique end-to-end stacked G-quadruplex structure from this sequence. This structure consists of an all parallel G-quadruplex, formed by four consecutive G-tracts, linked to another, atypical G-quadruplex, formed by two pairs of consecutive G-tracts separated by a 26-base loop. This 26-base loop likely forms a stable hairpin structure, which would explain the unexpected stability of this G-quadruplex. Significantly, the formation of this tandem G-quadruplex structure in the full-length sequence masks all three Sp1 binding sites, which is predicted to produce significant inhibition of hTERT promoter activity. Furthermore, our study implies that inhibition of telomerase activity by some G-quadruplex ligands is not only produced by targeting telomeric G-quadruplexes but also by stabilization of the hTERT promoter G-quadruplexes.

The intrabody targeting of hTERT attenuates the immortality of cancer cells

hTERT (human telomerase reverse transcriptase) plays a key role in the process of cell immortalization. Overexpression of hTERT has been implicated in 85% of malignant tumors and offers a specific target for cancer therapy. In this paper, we describe an effective approach using a single-chain variable fragment (scFv) intrabody derived from monoclonal hybridoma directed against hTERT to attenuate the immortalization of human uterine cervix and hepatoma cells. The scFv we constructed had a high affinity to hTERT, and specifically neutralized over 70% of telomere synthesis activity, thereby inhibiting the viability and proliferation of the cancer cells. Our results indicate that this anti-hTERT intrabody is a promising tool to target hTERT and intervene in the immortalization process of cancer cells.

Effect of ellagic acid on the expression of human telomerase reverse transcriptase (hTERT) alpha+beta+ transcript in estrogen receptor-positive MCF-7 breast cancer cells

OBJECTIVES

To evaluate the potential of ellagic acid to inhibit the expression of human telomerase reverse transcriptase (hTERT) alpha+beta+ splice variant in MCF-7 breast cancer cells.

DESIGN AND METHODS

MCF-7 cells were incubated with ellagic acid (10(-)(9) M-10(-5) M) in the absence and in the presence of 17beta-estradiol (10(-8) M), a known inducer of hTERT transcription, and hTERT alpha+beta+ mRNA expression was quantified by real-time RT-PCR. 17beta-estradiol and ICI182780, a known estrogen antagonist, served as positive and negative controls respectively.

RESULTS

Ellagic acid, when alone, increased hTERT alpha+beta+ mRNA while its coexistence with 17beta-estradiol reduced significantly the 17beta-estradiol-induced increase in hTERT alpha+beta+ mRNA, implicating thus both its estrogenic and anti-estrogenic effects in breast cancer cells.

CONCLUSIONS

The potential of ellagic acid to down-regulate the 17beta-estradiol-induced hTERT alpha+beta+ mRNA expression may be a mechanism via which ellagic acid exerts, at least in part, its chemopreventive effects in breast cancer.

Platinum(II) complexes with dipyridophenazine ligands as human telomerase inhibitors and luminescent probes for G-quadruplex DNA

A series of platinum(II) complexes containing dipyridophenazine (dppz) and C-deprotonated 2-phenylpyridine (N-CH) ligands were prepared and assayed for G-quadruplex DNA binding activities. [PtII(dppz-COOH)(N-C)]CF3SO3 (1; dppz-COOH = 11-carboxydipyrido[3,2-a:2',3'-c]phenazine) binds G-quadruplex DNA through an external end-stacking mode with a binding affinity of approximately 10(7) dm3 mol-1. G-quadruplex DNA binding is accompanied by up to a 293-fold increase in the intensity of photoluminescence at lambdamax = 512 nm. Using a biotinylated-primer extension telomerase assay, 1 was shown to be an effective inhibitor of human telomerase in vitro, with a telIC50 value of 760 nM.

Synthesis of a potent G-quadruplex-binding macrocyclic heptaoxazole

A novel G-quadruplex binder, L1H1-7OTD (shown in color by atom type), was developed. This macrocyclic heptaoxazole potently and selectively stabilizes telomeric DNA in an intramolecular antiparallel G-quadruplex conformation. L1H1-7OTD shows selective cytotoxicity toward HeLa cells, a telomerase-positive cell line.

Inhibition of dicing of guanosine-rich shRNAs by quadruplex-binding compounds

RNA interference is triggered by small hairpin precursors that are processed by the endonuclease dicer to yield active species such as siRNAs and miRNAs. To regulate the RNAi-mediated suppression of gene expression, we imagined a strategy that relies on the sequence-specific inhibition of shRNA precursor processing by immediate RNA-small molecule interactions. Here, we present a first step in this direction by augmenting shRNAs with guanosine-rich sequences that are prone to fold into four-stranded structures. The addition of small molecules that selectively bind to such quadruplex sequences should allow for the specific inhibition of dicing of shRNAs that contain suitable G-rich elements. In an attempt to find compounds that protect against dicer processing, we have examined the effects of quadruplex-binding compounds on the dicer processing of shRNAs containing G-quadruplexes. Although a variety of small molecules that are known to bind to quadruplexes inhibited in vitro dicing of shRNAs, only two substance classes, namely certain porphyrazines and bisquinolinium compounds, showed selective inhibition of G-rich shRNAs compared to control sequences lacking guanine-rich elements. The G-rich shRNAs displayed a potent knockdown of gene expression in mammalian cell culture, but the effect was not influenced by addition of the respective quadruplex-binding compounds.

In vitro and in vivo tumor suppressive activity induced by human telomerase transcriptase-targeting antisense oligonucleotides mediated by cationic liposomes

The objective of this study was to investigate the in vitro and in vivo influence of cationic liposomes on the tumor suppressive effect of antisense telomerase oligodeoxynucleotides to human cervical adenocarcinoma cells (HeLa). Antisense oligodeoxynucleotides (ASODN) against the human telomerase transcriptase (hTERT) served as telomerase inhibitors. The cholesterol derivative, 3beta [N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol), was synthesized, and cationic liposomes (CL) were prepared using DC-Chol and dioleoylphosphatidylethanolamine (DOPE). The in vitro IC50 of the CL-ASODN complex was 1.88 mumol/l, while the IC50 of the cells treated with free ASODNs or CL alone was 25.24 mumol/l and 55.18 mumol/l, respectively. The CL-ASODN complex inhibited HeLa cell growth for at least 120 h. In vivo, the CL-ASODN complex inhibited the tumor growth rate by 55.11%, which increased to 89.47% when CL-ASODN was combined with 5-fluorouracil treatment. ASODNs alone failed to induce tumor-suppressive activity, suggesting that CL prepared from DOPE and DC-Chol can significantly enhance the growth inhibitory effect of ASODN on tumor cells both in vitro and in vivo.

RNA interference-mediated validation of genes involved in telomere maintenance and evasion of apoptosis as cancer therapeutic targets

The discovery of new cancer-related therapeutic targets is mainly based on the identification of genes involved in pathways selectively exploited in cancer cells, including those leading to unlimited replicative potential, evasion of apoptosis, angiogenesis, tissue invasion and metastatic spread. Potentially, a gene--or a gene product--is recognized as a cancer target whether its modulation in experimental models can specifically modify or revert the cancer phenotype. As soon as RNA interference (RNAi)--a natural gene silencing mechanism--was demonstrated in mammalian cells, it rapidly became an essential means for gene knockdown in preclinical models, making it possible to define the role of several human genes and to identify those specifically involved in the onset and progression of cancer. Owing to its powerful gene-silencing properties, RNAi has been proposed as a useful tool to validate new therapeutic targets and to develop innovative anticancer therapies. This chapter summarizes the findings from recent studies relying on the use of RNAi-based approaches to functionally validate therapeutic targets related to two tumor hallmarks: the unlimited replicative potential (i.e., activation of telomere maintenance mechanisms) and evasion of apoptosis (i.e., up-regulation of anti-apoptotic factors).

Impact of telomerase status on canine osteosarcoma patients

BACKGROUND

We demonstrated previously that canine osteosarcoma (OSA) cell lines and samples from clinical patients are predominantly telomerase positive. In contrast, the majority of OSA samples from human patients appear to be telomerase negative, maintaining telomere length by an alternative lengthening of telomeres (ALT) mechanism. The purpose of the current study was to examine the telomerase status of a large number of OSA samples from dogs and determine if telomerase status can serve as a prognostic factor.

HYPOTHESIS

The majority of clinical canine OSA appendicular lesions will be telomerase positive, and telomerase positivity will negatively impact disease outcome.

ANIMALS

Sixty-seven dogs with appendicular OSA presenting to the Colorado State University Animal Cancer Center for treatment.

METHODS

The Telomeric Repeat Amplification Protocol was performed on tissue samples from primary canine appendicular OSA to determine the presence of telomerase activity. Telomere restriction fragment (TRF) analysis was utilized to determine telomere length and detect ALT. Outcome data were obtained in a retrospective manner and correlated with telomerase status.

RESULTS

Seventy-three percent of canine OSA samples were telomerase positive. Telomerase status did not have an impact on disease-free interval or survival time. Nine of 10 telomerase-negative samples examined were consistent with an ALT phenotype, based on TRF analysis.

CONCLUSIONS AND CLINICAL IMPORTANCE

These results are consistent with the hypothesis that the majority of canine OSA are telomerase positive, suggesting that telomerase may be a valuable target for canine OSA therapy. Additionally, telomerase status does not appear to be a prognostic factor in canine OSA.

Over-expression of genes coding for proline oxidase, riboflavin kinase, cytochrome c oxidase and an MFS transporter induced by acriflavin in Trichophyton rubrum

Acriflavin (3,6-acridinediamine) and other acridine derivatives act in both prokaryotic and eukaryotic cells at the level of DNA-coiling enzymes (topoisomerases) causing the stabilization of the enzyme-DNA cleavable complex. In order to better understand the mode of action of acriflavin, Differential Display RT-PCR was used to isolate transcripts specifically over-expressed during exposure of Trichophyton rubrum mycelia to this drug. Five transcripts, whose differential expressions were confirmed by Northern blotting, revealed genes not previously described in this dermatophyte. Functional grouping identified putative enzymes possibly involved in the mitochondrial respiratory electron-transport chain and in iron transport. These results may be relevant to our understanding of the molecular events involved in the stress response of T. rubrum to acriflavin.

Antisense human telomerase reverse transcriptase could partially reverse malignant phenotypes of gastric carcinoma cell line in vitro

Telomerase activity is detected in more than 90% of examined tumors but not in most normal somatic cells. Among three subunits of human telomerase, human telomerase reverse transcriptase (hTERT) is the rate-limiting component for telomerase activity. Therefore, targeting hTERT represents a promising approach for diminishing telomerase function that will probably not cause substantial side effects on telomerase negative somatic cells. To explore the effects of antisense hTERT (ahTERT) on the malignant phenotypes of human SGC-7901 gastric cancer cell line in vitro, an antisense eukaryotic expression vector of hTERT was constructed by gene recombinant technology. Telomerase activity by telomeric repeat amplification protocol-ELISA, mRNA of telomerase subunits, c-myc and bcl-2 by reverse transcript-PCR, terminal restriction fragment (TRF) by Southern blot, cell cycle distribution by flow cytometry and protein expression of hTERT, c-myc and bcl-2 by Western blot were analyzed in SGC-7901 cells before and after transfection. Cloning efficiency assay in soft agar and tumorigenesis in nude mice were also examined and evaluated in the above cells. The results demonstrated that after ahTERT transfection, the proliferation of SGC-7901 cells was significantly inhibited. Further study showed that telomerase activity, telomere length, the mRNA and protein expression of hTERT, bcl-2 and c-myc were decreased in ahTERT-transfected cells. There were, however, no obvious effects on transcription of human telomerase RNA (hTR) and human telomerase associated protein1 (TP1) in both transfected and untransfected cells. Flow cytometric analysis displayed an accumulation of G0/G1 phase and a decreasing proliferation index (PI) in ahTERT-transfected cells. Moreover, no tumorigenicity was found after subcutaneous injection of ahTERT-transfected cells in nude mice, whereas palpable tumors were observed in mice injected with control cells. Our study indicates that exogenous ahTERT can inhibit proliferation and partially reverse malignant phenotypes of SGC-7901 cells via the suppression of telomerase activity, hTERT, c-myc and bcl-2 expression. Antisense technology targeted hTERT strategy might be a potential approach for gastric cancer therapy.

Gain at chromosomal region 5p15.33, containing TERT, is the most frequent genetic event in early stages of non-small cell lung cancer

Chromosomal imbalances resulting in altered gene dosage play a role in the molecular pathogenesis of non-small cell lung cancer (NSCLC), but the target genes remain to be identified. To identify early-stage genetic events that drive progression of NSCLC, we conducted a high-resolution array comparative genomic hybridization (CGH) study, using an array of 4,046 bacterial artificial chromosome clones to screen for DNA copy number changes associated with individual genes in 36 tumors obtained from patients in early stages of NSCLC. Multiple early genetic events occurring on chromosome 5p were identified, with a minimal detection region at 5p15.33 approximately 12. The most frequent finding involved gain of 5p15.33, observed in 15 of 19 stage I (A+B) cancers (79%) and in 28 of the total 36 NSCLC cases (78%). This locus harbors the genes TERT, SLC6A19, and SLC6A18 and is a telomeric boundary at bacterial artificial chromosome (BAC) clone 91_J20. Other potential candidate genes evidencing high numbers of genomic copy number changes (> or =40% of patients) included the following genes, encountered in >50% of 19 stage I (A+B) cancers: CEP72 and TPPP (14 of 19; 74%); AHRR, EXOC3 (previously SEC6L1), SLC9A3, LOC442126, ZDHHC11, BRD9, and TRIP13 (13/19; 68%); and CLPTM1L (alias CRR9), SLC6A3 (previously DAT1), and LOC401169 (10/19; 53%). Fluorescence in situ hybridization validated the array CGH findings. The gain of 5p15.33 is thus one of the most consistent alterations in the early stages of lung cancer, and a series of genes in the critical 5p15.33 region may be used as novel biomarkers for the early detection and classification of lung cancer.

Targeting telomerase and telomeres: a click chemistry approach towards highly selective G-quadruplex ligands

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, but is absent in most normal cells. Targeting telomere maintenance mechanisms could potentially halt tumour growth across a broad spectrum of cancer types, with little cytotoxic effect outside cancer cells. Here, we describe in detail a new class of G-quadruplex binding ligands synthesized using a click chemistry approach. These ligands comprise a 1,3-di(1,2,3-triazol-4-yl)benzene pharmacophore, and display high levels of selectivity for interaction with G-quadruplex DNA vs. duplex DNA. The ability of these ligands to inhibit the enzymatic activity of telomerase correlates with their ability to stabilize quadruplex DNA, and with estimates of affinity calculated by molecular modeling.

Thermodynamic analysis of quadruplex DNA-drug interaction

This work studies the binding properties of distamycin and its carbamoyl analog, containing four pyrrole units, with the [d(TGGGGT)](4) quadruplex by means of isothermal titration calorimetry (ITC). Analysis of the ITC data reveals that drug/quadruplex binding stoichiometry is 1:1 for both interactions and that distamycin analog gives approximately a 10-fold increase in the quadruplex affinity.

Antitumor polycyclic acridines. 20. Search for DNA quadruplex binding selectivity in a series of 8,13-dimethylquino[4,3,2-kl]acridinium salts: telomere-targeted agents

The growth-inhibitory activities of an extensive series of quaternized quino[4,3,2- kl]acridinium salts against tumor cell lines in vitro have been measured and their biological properties interpreted in the light of differential binding to different DNA isoforms. Selectivity for quadruplex DNA binding and stabilization by compounds were explored through an array of methods: UV absorption and fluorescence emission spectroscopy, surface plasmon resonance, and competition dialysis. Quadruplex DNA interaction was further characterized through FRET and DNA polymerase arrest assays. Telomerase inhibition, inferred from the TRAP assay, is attributed to quadruplex stabilization, supported by the strong correlation (R(2) = 0.81) across the series between quadruplex DNA binding affinity and TRAP inhibition potency. Growth inhibition potency in the NCI60 human tumor cell line panel is more marked in compounds with greater DNA duplex binding affinity (R(2) = 0.82). Quantification of relative quadruplex and duplex binding affinity constants puts some of these ligands among the most selective quadruplex DNA interactive agents reported to date.

Transcriptional regulation of telomerase activity: roles of the the Ets transcription factor family

Telomerase maintains telomeres to preclude cell senescence. It remains elusive how telomerase activity is repressed in differentiated cells, but retained at high levels in stem cells and cancer. Recent studies suggest that the Ets transcription factor family, downstream of the mitogen signaling pathways of MAP kinase, regulates telomerase activity at the gene transcription level of human telomerase reverse transcriptase (hTERT). Several Ets transcription factors are involved in regulating hTERT gene expression, both directly and indirectly through the proto-oncogene c-myc. ER81 may mediate telomerase activation in telomerase-negative fibroblasts stimulated by oncogenes Her2/Neu, Ras, and Raf. Ets2 may also play an important role in regulating the hTERT gene; but further studies are required to decipher the mechanisms in the regulation of telomerase activity.

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.

GRSDB2 and GRS_UTRdb: databases of quadruplex forming G-rich sequences in pre-mRNAs and mRNAs

G-quadruplex motifs in the RNA play significant roles in key cellular processes and human disease. While sequences capable of forming G-quadruplexes in the pre-mRNA are involved in regulation of polyadenylation and splicing events in mammalian transcripts, the G-quadruplex motifs in the UTRs may help regulate mRNA expression. GRSDB2 is a second-generation database containing information on the composition and distribution of putative Quadruplex-forming G-Rich Sequences (QGRS) mapped in ∼29 000 eukaryotic pre-mRNA sequences, many of which are alternatively processed. The data stored in the GRSDB2 is based on computational analysis of NCBI Entrez Gene entries with the help of an improved version of the QGRS Mapper program. The database allows complex queries with a wide variety of parameters, including Gene Ontology terms. The data is displayed in a variety of formats with several additional computational capabilities. We have also developed a new database, GRS_UTRdb, containing information on the composition and distribution patterns of putative QGRS in the 5′- and 3′-UTRs of eukaryotic mRNA sequences. The goal of these experiments has been to build freely accessible resources for exploring the role of G-quadruplex structure in regulation of gene expression at post-transcriptional level. The databases can be accessed at the G-Quadruplex Resource Site at: http://bioinformatics.ramapo.edu/GQRS/.

Mechanisms of action of TGF-beta in cancer: evidence for Smad3 as a repressor of the hTERT gene

Transforming growth factor-beta (TGF-beta) induces cell differentiation and suppresses cell proliferation, but the mechanisms underlying the actions of TGF-beta remain to be fully elucidated. Recent studies suggest that TGF-beta suppresses neoplastic cell development by employing Smad3 protein to repress the gene of human telomerase reverse transcriptase (hTERT). In human breast cancer cells, TGF-beta induces rapid phosphorylation and subsequent entry of Smad3 into the nucleus. In the nucleus, Smad3 binds to the hTERT gene promoter directly and inhibits hTERT gene transcription activity. By interacting with c-myc, Smad3 also represses the c-myc gene. Thus, TGF-beta prevents continuous cell proliferation by switching off telomerase activity through Smad3 repression of the hTERT gene and the action of c-myc. Modulating the interface between Smad3 and the hTERT gene, and the potential feedback loop from telomeres to Smad3 via Smurf2, may represent a novel approach to regulate cell lifespan of proliferation.

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.

Cellular senescence in cancer and aging

Cellular senescence, a state of irreversible growth arrest, can be triggered by multiple mechanisms including telomere shortening, the epigenetic derepression of the INK4a/ARF locus, and DNA damage. Together these mechanisms limit excessive or aberrant cellular proliferation, and so the state of senescence protects against the development of cancer. Recent evidence suggests that cellular senescence also may be involved in aging.

Targeting the Limitless Replicative Potential of Cancer: The Telomerase/Telomere Pathway: Fig. 1

The maintenance of telomeric DNA underlies the ability of tumors to possess unlimited replicative potential, one of the hallmarks of cancer. Telomere length and structure are maintained by the reverse transcriptase telomerase and a multiprotein telomere complex termed shelterin. Telomerase activity is elevated in the vast majority of tumors, and telomeres are critically shortened in tumors versus normal tissues, thus providing a compelling rationale to target the telomerase/telomere pathway for broad-spectrum cancer therapy. This strategy is supported by a variety of genetic-based target validation studies. Both telomerase inhibitors and telomere interactive molecules have shown stand-alone antitumor activity at nontoxic doses against a variety of human tumor xenografts in mice. These translational advances have resulted in the first antitelomerase agent, the oligonucleotide-based GRN163L targeting the telomerase RNA template, entering clinical evaluation. Additional translational approaches, such as targeting telomeres using G-quadruplex ligands, should result in antitelomere agents, such as RHPS4, entering the clinic in the near future. These prototype trials will be extremely informative in determining the role of the telomerase/telomere pathway in clinical oncology and, moreover, whether drugs targeting the unlimited replicative potential of cancer will find a place in cancer chemotherapy.

Mechanism of acridine-based telomerase inhibition and telomere shortening

The trisubstituted acridine compound BRACO-19 has been developed as a ligand for stabilising G-quadruplex structures. It is shown here that BRACO-19 produces short- and long-term growth arrest in cancer cell lines, and is significantly less potent in a normal cell line. BRACO-19 reduces telomerase activity and long-term telomere length attrition is observed. It is also shown that BRACO-19 binds to telomeric single-stranded overhang DNA, consistent with quadruplex formation, and the single-stranded protein hPOT1 has been shown to be displaced from the overhang in vitro and in cellular experiments. It is concluded that the cellular activity of BRACO-19 can be ascribed both to the uncapping of 3' telomere ends and to telomere shortening that may preferentially affect cells with short telomeres.

Targeting telomeres and telomerase

Telomeres and telomerase represent, at least in theory, an extremely attractive target for cancer therapy. The objective of this review is to present the latest view on the mechanism(s) of action of telomerase inhibitors, with an emphasis on a specific class of telomere ligands called G-quadruplex ligands, and to discuss their potential use in oncology.

Telomere biology of pediatric cancer

One of the hallmarks of cancer is limitless proliferative capacity, which is tightly associated with the ability to maintain telomeres. Over the last decade, the telomere biology of pediatric cancers has begun to be elucidated. Most pediatric leukemias and embryonal solid tumors activate the enzyme telomerase, a specialized reverse transcriptase that adds nucleotide repeats to telomeres. In general, high levels of tumor telomerase expression are associated with unfavorable outcome, although results vary according to tumor type. Some pediatric tumors, including osteosarcoma and glioblastoma multiforme, lack telomerase activity and maintain telomeres via a recombination-based mechanism called ALT (alternative lengthening of telomeres). Telomerase is a highly attractive therapeutic target for pediatric cancer because the enzyme plays a key role in conferring cellular immortality, is present in most tumors, and is relatively specific for cancer cells. Telomerase inhibitors have been evaluated in preclinical models of adult cancers, but few studies have been conducted on pediatric cancers. Further research is required to define how telomere biology can be used to clinical advantage in malignancies of childhood.