Telomerase activity in human brain tumours

Mechanisms of telomere maintenance and associated therapeutic vulnerabilities in malignant gliomas

A majority of cancers (~85%) activate the enzyme telomerase to maintain telomere length over multiple rounds of cellular division. Telomerase-negative cancers activate a distinct, telomerase-independent mechanism of telomere maintenance termed alternative lengthening of telomeres (ALT). ALT uses homologous recombination to maintain telomere length and exhibits features of break-induced DNA replication. In malignant gliomas, the activation of either telomerase or ALT is nearly ubiquitous in pediatric and adult tumors, and the frequency with which these distinct telomere maintenance mechanisms (TMMs) is activated varies according to genetically defined glioma subtypes. In this review, we summarize the current state of the field of TMMs and their relevance to glioma biology and therapy. We review the genetic alterations and molecular mechanisms leading to telomerase activation or ALT induction in pediatric and adult gliomas. With this background, we review emerging evidence on strategies for targeting TMMs for glioma therapy. Finally, we comment on critical gaps and issues for moving the field forward to translate our improved understanding of glioma telomere maintenance into better therapeutic strategies for patients.

Alternative Lengthening of Telomeres in Pediatric High-Grade Glioma and Therapeutic Implications

Pediatric high-grade gliomas (pHGGs), including diffuse intrinsic pontine glioma (DIPG), are highly aggressive tumors with dismal prognoses despite multimodal therapy including surgery, radiation therapy, and chemotherapy. To achieve cellular immortality cancer cells must overcome replicative senescence and apoptosis by activating telomere maintenance mechanisms (TMMs) through the reactivation of telomerase activity or using alternative lengthening of telomere (ALT) pathways. Although the ALT phenotype is more prevalent in pHGGs compared to adult HGGs, the molecular pathway and the prognostic significance of ALT activation are not well understood in pHGGs. Here, we report the heterogeneity of TMM in pHGGs and their association with genetic alterations. Additionally, we show that sensitivity to the protein kinase ataxia telangiectasia- and RAD3-related protein (ATR) inhibitor and the ATR downstream target CHK1 is not specific to pHGG ALT-positive cells. Together, these findings underscore the need for novel therapeutic strategies to target ALT in pHGG tumors.

Telomerase: Key regulator of inflammation and cancer

The telomerase holoenzyme, which has a highly conserved role in maintaining telomere length, has long been regarded as a high-profile target in cancer therapy due to the high dependency of the majority of cancer cells on constitutive and elevated telomerase activity for sustained proliferation and immortality. In this review, we present the salient findings in the telomerase field with special focus on the association of telomerase with inflammation and cancer. The elucidation of extra-telomeric roles of telomerase in inflammation, reactive oxygen species (ROS) generation, and cancer development further complicated the design of anti-telomerase therapy. Of note, the discovery of the unique mechanism that underlies reactivation of the dormant telomerase reverse transcriptase TERT promoter in somatic cells not only enhanced our understanding of the critical role of TERT in carcinogenesis but also opens up new intervention ideas that enable the differential targeting of cancer cells only. Despite significant effort invested in developing telomerase-targeted therapeutics, devising efficacious cancer-specific telomerase/TERT inhibitors remains an uphill task. The latest discoveries of the telomere-independent functionalities of telomerase in inflammation and cancer can help illuminate the path of developing specific anti-telomerase/TERT therapeutics against cancer cells.

Alternative lengthening of telomeres is the major telomere maintenance mechanism in astrocytoma with isocitrate dehydrogenase 1 mutation

Purpose

Isocitrate dehydrogenase 1 (IDH1) mutations are associated with improved survival in gliomas. Depending on the IDH1 status, TERT promoter mutations affect prognosis. IDH1 mutations are associated with alpha-thalassemia/mental retardation syndrome X-linked (ATRX) mutations and alternative lengthening of telomeres (ALT), suggesting an interaction between IDH1 and telomeres. However, little is known how IDH1 mutations affect telomere maintenance.

Methods

We analyzed cell-specific telomere length (CS-TL) on a single cell level in 46 astrocytoma samples (WHO II-IV) by modified immune-quantitative fluorescence in situ hybridization, using endothelial cells as internal reference. In the same samples, we determined IDH1/TERT promoter mutation status and ATRX expression. The interaction of IDH1^R132H mutation and CS-TL was studied in vitro using an IDH1^R132H doxycycline-inducible glioma cell line system.

Results

Virtually all ALT^positive astrocytomas had normal TERT promoter and lacked ATRX expression. Further, all ALT^positive samples had IDH1^R132H mutations, resulting in a significantly longer CS-TL of IDH1^R132H gliomas, when compared to their wildtype counterparts. Conversely, TERT promotor mutations were associated with IDH^wildtype, ATRX expression, lack of ALT and short CS-TL. ALT, TERT promoter mutations, and CS-TL remained without prognostic significance, when correcting for IDH1 status. In vitro, overexpression of IDH^R132H in the glioma cell line LN319 resulted in downregulation of ATRX and rapid TERT-independent telomere lengthening consistent with ALT.

Conclusion

ALT is the major telomere maintenance mechanism in IDH^R132H mutated astrocytomas, while TERT promoter mutations were associated with IDH^wildtype glioma. IDH1^R132H downregulates ATRX expression in vitro resulting in ALT, which may contribute to the strong association of IDH1^R132H mutations, ATRX loss, and ALT.

Electronic supplementary material

The online version of this article (10.1007/s11060-020-03394-y) contains supplementary material, which is available to authorized users.

The inhibitory effects of ginsenoside Rd on the human glioma U251 cells and its underlying mechanisms

OBJECTIVE

The current study was designed to investigate the inhibitory effects of ginsenoside Rd (Gs-Rd) on human glioma U251 cells in vitro and its possible underlying mechanisms.

METHODS

The groups included blank control group, low concentration Gs-Rd treatment group (20 μM), mid concentration Gs-Rd treatment group (40 μM), and high concentration Gs-Rd treatment group (80 μM). The proliferative activity of human glioma U251 cells was detected by the MTT assay. Flow cytometry was performed to measure cell apoptosis of human glioma U251 cells. In addition, the ELISA assay was used to measure the telomerase activities in different groups on 24 hours, 48 hours, and 72 hours. Furthermore, real-time quantitative polymerase chain reaction (RT-PCR) and Western blot analysis were performed to measure the expression of Bcl-2, human telomerase catalytic subunit (hTERT), and caspase-3 in different groups on 48 hours at both messenger RNA (mRNA) and protein levels.

RESULTS

The proliferation of U251 cells was inhibited by Gs-Rd with different concentrations in the dose- and time-dependent manners. In addition, Gs-Rd promoted U251 cell apoptosis rate in a dose-dependent manner. Gs-Rd with different concentrations (20 μM, 40 μM, and 80 μM) significantly enhanced the expression of teleomerase on 24 hours and 48 hours. In addition, Gs-Rd with different concentrations significantly increased caspase-3 and decreased Bcl-2 and hTERT expressions at both mRNA and protein levels.

CONCLUSION

The Gs-Rd can remarkably inhibit the proliferation and promote cell apoptosis of human glioma U251 cells. The possible underlying mechanisms could be related to inhibiting telomerase activity, downregulating expression of Bcl-2 and hTERT, and upregulating expression of caspase-3 of human glioma U251 cells.

Reconstructing the molecular life history of gliomas

At the time of their clinical manifestation, the heterogeneous group of adult and pediatric gliomas carries a wide range of diverse somatic genomic alterations, ranging from somatic single-nucleotide variants to structural chromosomal rearrangements. Somatic abnormalities may have functional consequences, such as a decrease, increase or change in mRNA transcripts, and cells pay a penalty for maintaining them. These abnormalities, therefore, must provide cells with a competitive advantage to become engrained into the glioma genome. Here, we propose a model of gliomagenesis consisting of the following five consecutive phases that glioma cells have traversed prior to clinical manifestation: (I) initial growth; (II) oncogene-induced senescence; (III) stressed growth; (IV) replicative senescence/crisis; (V) immortal growth. We have integrated the findings from a large number of studies in biology and (neuro)oncology and relate somatic alterations and other results discussed in these papers to each of these five phases. Understanding the story that each glioma tells at presentation may ultimately facilitate the design of novel, more effective therapeutic approaches.

Alternative splicing of human telomerase reverse transcriptase in gliomas and its modulation mediated by CX-5461

Background

Glioma is a heterogeneous, invasive primary brain tumor with a wide range of patient survival and a lack of reliable prognostic biomarkers. Human telomerase reverse transcriptase (hTERT) has been reported in the presence of multiple transcripts in various tumor systems. The biological function and precise regulatory mechanisms of hTERT transcripts remain uncertain.

Methods

Alternative splicing of hTERT and telomerase activity were examined in 96 glioma specimens, including 38 glioblastomas (GBMs), 23 oligodendrogliomas (ODMs), and 35 oligoastrocytomas (OAMs). The correlation between telomerase activity or hTERT transcripts and patient clinical characteristics was investigated. We examined the regulation of alternative splicing of hTERT and telomerase activity by G-quadruplex stabilizer CX-5461 in GBM cells. The biological effects of CX-5461 on GBM cell lines, including inhibition of cell proliferation, effects on cell cycle/apoptosis, and telomere DNA damage were further explored.

Results

The β splicing was verified in human gliomas and hTERT+β was significantly correlated with higher telomerase activity, higher KPS, larger tumor size, and higher tumor grades. Meanwhile, glioma patients lacking hTERT+β expression or telomerase activity showed a significant survival benefit. Notably, CX-5461 altered hTERT splicing patterns, leading to an increase of hTERT-β transcript and a decrease of hTERT+β transcript expression, which inhibits telomerase activity. In addition, CX-5461 had cytotoxic effects on GBM cells and caused telomere DNA damage response, induced G2/M arrest and apoptosis.

Conclusions

The hTERT+β is verified to be correlated with clinical parameters in gliomas, and could serve as a prognostic marker or possibly therapeutic target for gliomas. CX-5461 can regulate the splicing pattern of hTERT, inhibit telomerase activity, and kill GBM cells.

Exploration of Involved Key Genes and Signaling Diversity in Brain Tumors

Brain tumors are becoming a major cause of death. The classification of brain tumors has gone through restructuring with regard to some criteria such as the presence or absence of a specific genetic alteration in the 2016 central nervous system World Health Organization update. Two categories of genes with a leading role in tumorigenesis and cancer induction include tumor suppressor genes and oncogenes; tumor suppressor genes are inactivated through a variety of mechanisms that result in their loss of function. As for the oncogenes, overexpression and amplification are the most common mechanisms of alteration. Important cell cycle genes such as p53, ATM, cyclin D2, and Rb have shown altered expression patterns in different brain tumors such as meningioma and astrocytoma. Some genes in signaling pathways have a role in brain tumorigenesis. These pathways include hedgehog, EGFR, Notch, hippo, MAPK, PI3K/Akt, and WNT signaling. It has been shown that telomere length in some brain tumor samples is shortened compared to that in normal cells. As the shortening of telomere length triggers chromosome instability early in brain tumors, it could lead to initiation of cancer. On the other hand, telomerase activity was positive in some brain tumors. It is suggestive that telomere length and telomerase activity are important diagnostic markers in brain tumors. This review focuses on brain tumors with regard to the status of oncogenes, tumor suppressors, cell cycle genes, and genes in signaling pathways as well as the role of telomere length and telomerase in brain tumors.

RNA interference mediated downregulation of human telomerase reverse transcriptase (hTERT) in LN18 cells

Human telomerase reverse transcriptase (hTERT) gene is a biomarker for the targeted therapy in various cancers. Presence of increased telomerase activity is a common feature of all cancers including glioblastoma. Both RNA and catalytic subunits of hTERT are the target sites for blocking its activity. The current study focuses on the expression of hTERT in glioblastoma and its regulation using two different novel siRNAs (small interfering RNA). Our patient data demonstrated increased expression of hTERT, which could be correlated with carcinogenesis in glioma. In vitro studies in siRNA transfected LN18 cells confirmed significant cell death (p < 0.05) as evidenced by MTT and trypan blue exclusion assay. These results were further supported by flow cytometry data, which showed significant increase in early and late apoptosis. The hTERT mRNA expression was effectively downregulated by 45 and 39 % with siRNA1 and siRNA2, respectively. These results were further confirmed by immunoblotting analysis (p < 0.05). Our results suggest that both the siRNAs effectively down regulated the expression of hTERT at mRNA and protein levels, thereby decreasing cell viability and proliferation rate. Hence siRNA mediated downregulation of hTERT could be a potential therapeutic avenue in glioblastoma.

Experimental Curative Fluorescence-guided Surgery of Highly Invasive Glioblastoma Multiforme Selectively Labeled With a Killer-reporter Adenovirus

Fluorescence-guided surgery (FGS) of cancer is an area of intense current interest. However, although benefits have been demonstrated with FGS, curative strategies need to be developed. Glioblastoma multiforme (GBM) is one of the most invasive of cancers and is not totally resectable using standard bright-light surgery (BLS) or current FGS strategies. We report here a curative strategy for FGS of GBM. In this study, telomerase-dependent adenovirus OBP-401 infection brightly and selectively labeled GBM with green fluorescent protein (GFP) for FGS in orthotopic nude mouse models. OBP-401-based FGS enabled curative resection of GBM without recurrence for at least 150 days, compared to less than 30 days with BLS.

Brain tumors: Special characters for research and banking

A brain tumor is an intracranial neoplasm within the brain or in the central spinal canal. Primary malignant brain tumors affect about 200,000 people worldwide every year. Brain cells have special characters. Due to the specific properties of brain tumors, including epidemiology, growth, and division, investigation of brain tumors and the interpretation of results is not simple. Research to identify the genetic alterations of human tumors improves our knowledge of tumor biology, genetic interactions, progression, and preclinical therapeutic assessment. Obtaining data for prevention, diagnosis, and therapy requires sufficient samples, and brain tumors have a wide range. As a result, establishing the bank of brain tumors is very important and essential.

TERT promoter mutations: a novel independent prognostic factor in primary glioblastomas

BACKGROUND

Activating somatic mutations in the promoter region of the telomerase reverse transcriptase gene (TERT) have been detected in several cancers. In this study we investigated the TERT promoter mutations and their impact on patient survival in World Health Organization grade IV glioblastoma multiforme (GBM).

METHODS

The TERT core promoter region containing the previously described mutations and a common functional polymorphism (rs2853669) was sequenced in tumors and blood samples from 192 GBM patients. O(6)-methylguanine-DNA methyltransferase (MGMT) promoter methylation status was assessed by pyrosequencing in 177 (92.2%) cases. Relevant clinical data were obtained from a prospectively maintained electronic database.

RESULTS

We detected specific (-124 C>T and -146 C>T) TERT promoter mutations in 143/178 (80.3%) primary GBM and 4/14 (28.6%) secondary GBM (P < .001). The presence of TERT mutations was associated with poor overall survival, and the effect was confined to the patients who did not carry the variant G-allele for the rs2853669 polymorphism. An exploratory analysis suggested that TERT mutations might be prognostic only in patients who had incomplete resections and no temozolomide chemotherapy.

CONCLUSIONS

In this study, specific TERT promoter mutations were markers of primary GBM and predicted patient survival in conjunction with a common functional polymorphism. The prognostic impact of TERT mutations was absent in patients with complete resections and temozolomide chemotherapy. If confirmed in additional studies, these findings may have clinical implications, that is, TERT mutations appear to characterize tumors that require aggressive treatment.

On the Genesis of Neuroblastoma and Glioma

As the emergence of cancer is most frequent in proliferating tissues, replication errors are considered to be at the base of this disease. This review concentrates mainly on two neural cancers, neuroblastoma and glioma, with completely different backgrounds that are well documented with respect to their ontogeny. Although clinical data on other cancers of the nervous system are available, usually little can be said about their origins. Neuroblastoma is initiated in the embryo at a moment when the nervous system (NS) is in full expansion and occasionally genomic damage can lead to neoplasia. Glioma, to the contrary, occurs in the adult brain supposed to be mostly in a postmitotic state. According to current consensus, neural stem cells located in the subventricular zone (SVZ) in the adult are thought to accumulate enough genomic mutations to diverge on a carcinogenic course leading to diverse forms of glioma. After weighing the pros and cons of this current hypothesis in this review, it will be argued that this may be improbable, yielding to the original old concept of glial origin of glioma.

Prognostic significance of telomere maintenance mechanisms in pediatric high-grade gliomas

Children with high-grade glioma, including diffuse intrinsic pontine glioma (DIPG), have a poor prognosis despite multimodal therapy. Identifying novel therapeutic targets is critical to improve their outcome. We evaluated prognostic roles of telomere maintenance mechanisms in children with HGG, including DIPG. A multi-institutional retrospective study was conducted involving 50 flash-frozen HGG (35 non-brainstem; 15 DIPG) tumors from 45 children (30 non-brainstem; 15 DIPG). Telomerase activity, expression of hTERT mRNA (encoding telomerase catalytic component) and TERC (telomerase RNA template) and alternative lengthening of telomeres (ALT) mechanism were assayed. Cox Proportional Hazard regression analyses assessed association of clinical and pathological variables, TERC and hTERT levels, telomerase activity, and ALT use with progression-free or overall survival (OS). High TERC and hTERT expression was detected in 13/28 non-brainstem HGG samples as compared to non-neoplastic controls. High TERC and hTERT expression was identified in 13/15 and 11/15 DIPG samples, respectively, compared to controls. Evidence of ALT was noted in 3/11 DIPG and 10/19 non-brainstem HGG specimens. ALT and telomerase use were identified in 4/19 non-brainstem HGG and 2/11 DIPG specimens. In multivariable analyses, increased TERC and hTERT levels were associated with worse OS in patients with non-brainstem HGG, after controlling for tumor grade or resection extent. Children with HGG and DIPG, have increased hTERT and TERC expression. In children with non-brainstem HGG, increased TERC and hTERT expression levels are associated with a worse OS, making telomerase a promising potential therapeutic target in pediatric HGG.

TERT promoter mutations in primary and secondary glioblastomas

Telomerase reverse transcriptase (TERT) is up-regulated in a variety of human neoplasms. Mutations in the core promoter region of the TERT gene, which increases promoter activity, have been reported in melanomas and a variety of human neoplasms, including gliomas. In the present study, we screened for TERT promoter mutations by direct DNA sequencing in a population-based collection of 358 glioblastomas. TERT promoter mutations (C228T, C250T) were detected in 55 % glioblastomas analysed. Of these, 73 % had a C228T mutation, and 27 % had a C250T mutation; only one glioblastoma had both C228T and C250T mutations. TERT promoter mutations were significantly more frequent in primary (IDH1 wild-type) glioblastomas (187/322; 58 %) than in secondary (IDH1 mutated) glioblastomas (10/36, 28 %; P = 0.0056). They showed significant inverse correlations with IDH1 mutations (P = 0.0056) and TP53 mutations (P = 0.043), and a significant positive correlation with EGFR amplification (P = 0.048). Glioblastoma patients with TERT mutations showed a shorter survival than those without TERT mutations in univariate analysis (median, 9.3 vs. 10.5 months; P = 0.015) and multivariate analysis after adjusting for age and gender (HR 1.38, 95 % CI 1.01-1.88, P = 0.041). However, TERT mutations had no significant impact on patients' survival in multivariate analysis after further adjusting for other genetic alterations, or when primary and secondary glioblastomas were separately analysed. These results suggest that the prognostic value of TERT mutations for poor survival is largely due to their inverse correlation with IDH1 mutations, which are a significant prognostic marker of better survival in patients with secondary glioblastomas.

Upregulating mutations in the TERT promoter commonly occur in adult malignant gliomas and are strongly associated with total 1p19q loss

Telomere lengthening is one of the key events in most cancers, and depends largely on telomerase activation. Telomerase activation is a well-known phenomenon in gliomas; however, its mechanism remains obscure. In this study, we investigated the presence of mutations in the promoter of the telomerase reverse transcriptase (TERT) gene in a series of 546 gliomas. We found a high incidence of mutually exclusive mutations located at two hot spots, C228T and C250T, in all subtypes of gliomas (55 %). The frequency of mutation was particularly high among primary glioblastomas (70 %) and pure oligodendroglial tumors (74 %), while relatively low in diffuse astrocytomas and anaplastic astrocytomas (19 and 25 %, respectively). The expression level of TERT in tumors carrying those mutations was on average 6.1 times higher than that of wild-type tumors, indicating that the mutated promoter leads to upregulation of TERT. TERT promoter mutations were observed in almost all tumors harboring concurrent total 1p19q loss and IDH1/2 mutations (98 %). Otherwise TERT promoter mutations were mostly observed among IDH wild-type tumors. Most EGFR amplifications (92 %) were also associated with TERT promoter mutations. Our data indicate that mutation of the TERT promoter is one of the major mechanisms of telomerase activation in gliomas. The unique pattern of TERT promoter mutations in relation to other genetic alterations suggests that they play distinct roles in the pathogenesis of oligodendroglial and astrocytic tumors. Our results shed a new light on the role of telomerase activation in the development of adult gliomas.

Transcriptional differences between normal and glioma-derived glial progenitor cells identify a core set of dysregulated genes

Glial progenitor cells (GPCs) are a potential source of malignant gliomas. We used A2B5-based sorting to extract tumorigenic GPCs from human gliomas spanning World Health Organization grades II-IV. Messenger RNA profiling identified a cohort of genes that distinguished A2B5+ glioma tumor progenitor cells (TPCs) from A2B5+ GPCs isolated from normal white matter. A core set of genes and pathways was substantially dysregulated in A2B5+ TPCs, which included the transcription factor SIX1 and its principal cofactors, EYA1 and DACH2. Small hairpin RNAi silencing of SIX1 inhibited the expansion of glioma TPCs in vitro and in vivo, suggesting a critical and unrecognized role of the SIX1-EYA1-DACH2 system in glioma genesis or progression. By comparing the expression patterns of glioma TPCs with those of normal GPCs, we have identified a discrete set of pathways by which glial tumorigenesis may be better understood and more specifically targeted.

Telomerase activity in human brain tumors: astrocytoma and meningioma

Somatic cells do not have telomerase activity but immortalized cell lines and more than 85 % of the cancer cells show telomerase activation to prevent the telomere from progressive shortening. The activation of this enzyme has been found in a variety of human tumors and tumor-derived cell lines, but only few studies on telomerase activity in human brain tumors have been reported. Here, we evaluated telomerase activity in different grades of human astrocytoma and meningioma brain tumors. In this study, assay for telomerase activity performed on 50 eligible cases consisted of 26 meningioma, 24 astrocytoma according to the standard protocols. In the brain tissues, telomerase activity was positive in 39 (65 %) of 50 patients. One sample t test showed that the telomerase activity in meningioma and astrocytoma tumors was significantly positive entirely (P < 0.001). Also, grade I of meningioma and low grades of astrocytoma (grades I and II) significantly showed telomerase activity. According to our results, we suggest that activation of telomerase is an event that starts mostly at low grades of brain including meningioma and astrocytoma tumors.

Prognostic significance of telomerase-associated parameters in glioblastoma: effect of patient age

BACKGROUND

Glioblastoma multiforme (GBM) is a heterogeneous, highly aggressive primary brain tumor with strongly variable patient survival. Because reliable prognostic biomarkers are lacking, we investigated the relation between telomerase-associated parameters and the disease course.

METHODS

Telomerase-associated parameters were determined in 100 GBM tissues and associated with clinical characteristics and overall survival. Expressions of telomere length, telomerase activity (TA), and human telomerase reverse transcriptase (hTERT) were analyzed by quantitative PCR, telomeric repeat amplification protocol assay, and reverse transcriptase-PCR, respectively. Mutation status of isocitrate dehydrogenase (IDH)1 was determined by direct sequencing, and O(6)-methylguanine DNA methyltransferase (MGMT) promoter methylation by methylation-specific PCR.

RESULTS

Of 100 GBM tissues, 61 were positive for both hTERT mRNA and TA, with a highly significant correlation between both parameters (linear regression, P < .0001). Telomere length determination revealed a significant difference between the hTERT/TA-positive and -negative subgroups, with markedly longer telomeres in the hTERT/TA-negative cohort (unpaired Student's t-test, P = .0001). Accordingly, significantly shorter telomeres were detected in GBM tissues derived from older patients (>60 y at diagnosis, P < .0001). While no association of telomere parameters with MGMT promoter status was found, all tumors with IDH1 mutation (6/100) were negative for both hTERT expression and TA and harbored significantly longer telomeres. Patients with tumors lacking hTERT expression/TA showed a significant survival benefit (Kaplan-Meier test, both P < .01), which, however, was based exclusively on the younger patient subgroup (≤60 y, both P < .005; >60 y, both ns).

CONCLUSIONS

Telomerase activation is not an independent prognostic parameter in GBM but predicts aggressive tumor behavior solely in a younger patient cohort.

Association between glioma susceptibility loci and tumour pathology defines specific molecular etiologies

BACKGROUND

Genome-wide association studies have identified single-nucleotide polymorphisms (SNPs) at 7 loci influencing glioma risk: rs2736100 (TERT), rs11979158 and rs2252586 (EGFR), rs4295627 (CCDC26), rs4977756 (CDKN2A/CDKN2B), rs498872 (PHLDB1), and rs6010620 (RTEL1).

MATERIALS AND METHODS

We studied the relationship among these 7 glioma-risk SNPs and characteristics of tumors from 1374 patients, including grade, IDH (ie IDH1 or IDH2) mutation, EGFR amplification, CDKN2A-p16-INK4a homozygous deletion, 9p and 10q loss, and 1p-19q codeletion.

RESULTS

rs2736100 (TERT) and rs6010620 (RTEL1) risk alleles were associated with high-grade disease, EGFR amplification, CDKN2A-p16-INK4a homozygous deletion, and 9p and 10q deletion; rs4295627 (CCDC26) and rs498872 (PHLDB1) were associated with low-grade disease, IDH mutation, and 1p-19q codeletion. In contrast, rs4977756 (CDKN2A/B), rs11979158 (EGFR), and to a lesser extent, rs2252586 (EGFR) risk alleles were independent of tumor grade and genetic profile. Adjusting for tumor grade showed a significant association between rs2736100 and IDH status (P = .01), 10q loss (P = .02); rs4295627 and 1p-19q codeletion (P = .04), rs498872 and IDH (P = .02), 9p loss (P = .04), and 10q loss (P = .02). Case-control analyses stratified into 4 molecular classes (defined by 1p-19q status, IDH mutation, and EGFR amplification) showed an association of rs4295627 and rs498872 with IDH-mutated gliomas (P < 10(-3)) and rs2736100 and rs6010620 with IDH wild-type gliomas (P < 10(-3) and P = .03).

CONCLUSION

The frequency of EGFR and CDKN2A/B risk alleles were largely independent of tumor genetic profile, whereas TERT, RTEL1, CCDC26, and PHLDB1 variants were associated with different genetic profiles that annotate distinct molecular pathways. Our findings provide further insight into the biological basis of glioma etiology.

Telomere targeting with a new G4 ligand enhances radiation-induced killing of human glioblastoma cells

The aim of this study was to test in vitro the efficacy of TAC, an original G-quadruplex ligand, as a potential radiosensitizing agent for glioblastoma multiforme (GBM). Two human radioresistant telomerase-positive GBM cell lines (SF763 and SF767) were analyzed, with and without TAC treatment, for telomere length, cell proliferation, apoptosis, cell-cycle distribution, gene expression, cytogenetic aberrations, clonogenic survival assay, 53BP1 immunofluorescence staining, and γH2AX phosphorylation. We found that low concentrations of TAC (0.5 and 1 μmol/L) inhibited the proliferation of GBM cells in a concentration-dependent manner after only 1 week of treatment, with minimal effects on cell cycle and apoptosis. TAC treatment had no visible effect on average telomere length but modified expression levels of telomere-related genes (hTERT, TRF1, and TRF2) and induced concentration-dependent DNA damage response and dicentric chromosomes. Survival curves analysis showed that exposure to nontoxic, subapoptotic concentrations of TAC enhanced radiation-induced killing of GBM cells. Analysis of DNA repair after irradiation revealed delayed repair kinetics in GBM cells treated with TAC. Furthermore, the combined treatment (TAC and radiation) significantly increased the frequency of chromosomal aberrations as compared with radiation alone. These findings provide the first evidence that exposure to a G4 ligand radiosensitizes human glioblastoma cells and suggest the prospect of future therapeutic applications.

Histone deacetylase inhibitors such as sodium butyrate and trichostatin A induce apoptosis through an increase of the bcl-2-related protein Bad

The effects of sodium butyrate (SB) and trichostatin A (TSA) on cell proliferation andapoptosis against human glioma T98G, U251MG, and U877MG cells were investigated. Upon exposure to either SB or TSA, cell proliferation was reduced, and apoptosis detected by DNA fragmentation analysis and the cleavage of CPP32 was induced. Previously, we reported that SB increased the expression levels of p21 (WAF-1) and inhibited G1-S transition of the cell cycle. In this study, we showed that TSA also increased p21 expression, suggesting that histone deacetylase (HDAC) inhibitors may up-regulate p21 protein in common and thus arrest proliferation in the G1 phase of the cell cycle. To further determine the underlying molecular mechanisms of apoptosis with either SB or TSA treatment, we studied the expression levels of apoptosis-related proteins in human glioma cells. SB increased the expression of the Bad protein, although the expression of Bcl-2, Bcl-xL, Bax, and Fas was not changed by theaddition of SB. TSA treatment also up-regulated the expression of Bad protein. The results suggest that HDAC inhibitors such as SB and TSA induce apoptosis through an increase in Bad protein in human glioma cells in vitro.

Expression of telomeres in astrocytoma WHO grade 2 to 4: TERRA level correlates with telomere length, telomerase activity, and advanced clinical grade

Cancer cells bypass replicative senescence, the major barrier to tumor progression, by using telomerase or alternative lengthening of telomeres (ALT) as telomere maintenance mechanisms (TMMs). Correlation between ALT and patient survival was demonstrated for high-grade astrocytomas. Transcription from subtelomeres produces telomeric repeat-containing RNA (TERRA), a natural inhibitor of telomerase activity (TA). This led us to evaluate correlations of TERRA and TMM with tumor grade and outcome in astrocytoma patients. SYBR Green real-time reverse transcription-polymerase chain reaction assays for quantitation of total and chromosome 2p and 18p specific TERRA levels were developed. Tumor samples from 46 patients with astrocytoma grade 2 to 4, tissue controls, and cell lines were assessed. TMMs were evaluated by measuring TA and by detecting long telomeres due to ALT. In glioblastoma multiforme (GBM) grade 4, total TERRA levels were similar to cell lines but 14-, 31-, and 313-fold lower compared with grade 3, grade 2, and nonmalignant tissue, respectively. Total TERRA levels differed from chromosomal levels. Low 2p TERRA levels correlated with dense promoter methylation of subtelomeric CpG islands, indicating that TERRA expression in gliomas may be chromosome specific and epigenetically regulated. Total TERRA levels correlated with diagnosis, with low or absent TA and the presence of ALT, and were tentatively associated with favorable patient prognosis in our cohort (P = .06). TA and short telomeres identified a subset of GBM with a median survival of only 14.8 months. TERRA and TA may be prognostic in astrocytic tumors.

Molecular targeting of neural cancer stem cells: TTAGGG, you're it!

Telomerase is an important mechanism by which cancers escape replicative senescence. In neural tumors, cancer stem cells express telomerase, suggesting that this may explain their preferential tumorigenesis. Oligonucleotide telomerase targeting selectively disrupts cancer stem cell growth through the induction of differentiation, adding to the armamentarium of anticancer stem cell therapies.

Genetic risk profiles identify different molecular etiologies for glioma

PURPOSE

Genome-wide association studies have recently identified single-nucleotide polymorphisms (SNP) in five loci at 5p15.33 (rs2736100, TERT), 8q24.21 (rs4295627, CCDC26), 9p21.3 (rs4977756, CDKN2A/CDKN2B), 20q13.33 (rs6010620, RTEL1), and 11q23.3 (rs498872, PHLDB1) to be associated with glioma risk. Because gliomas are heterogeneous in histology, molecular alterations, and clinical behavior, we have investigated these polymorphisms for potential correlations with tumor histology and patient survival.

EXPERIMENTAL DESIGN

We studied the relationship between SNPs and glioma subtype in two large patient cohorts from France and Germany, totaling 1,577 patients, as well as the relationship between SNP genotype and overall survival.

RESULTS

In both cohorts, the frequencies of rs2736100 and rs6010620 risk genotypes were highly correlated with high-grade disease (P < 0.001), whereas rs4295627 and rs498872 risk genotypes were inversely related to tumor grade (P < 0.001). These data show that genetic variations at these loci have subtype-specific effects on the risk of developing glioma. In contrast, the rs4977756 genotype was not correlated with tumor grade, consistent with the causal variant having a generic influence on glioma development. None of the five SNPs was associated with prognosis independent of tumor grade.

CONCLUSIONS

Our findings provide novel insight into etiologic pathways in the different glioma subtypes.

The telomerase antagonist, imetelstat, efficiently targets glioblastoma tumor-initiating cells leading to decreased proliferation and tumor growth

PURPOSE

Telomerase activity is one of the hallmarks of cancer and is a highly relevant therapeutic target. The effects of a novel human telomerase antagonist, imetelstat, on primary human glioblastoma (GBM) tumor-initiating cells were investigated in vitro and in vivo.

EXPERIMENTAL DESIGN

Tumor-initiating cells were isolated from primary GBM tumors and expanded as neurospheres in vitro. The GBM tumor-initiating cells were treated with imetelstat and examined for the effects on telomerase activity levels, telomere length, proliferation, clonogenicity, and differentiation. Subsequently, mouse orthotopic and subcutaneous xenografts were used to assess the in vivo efficacy of imetelstat.

RESULTS

Imetelstat treatment produced a dose-dependent inhibition of telomerase (IC(50) 0.45 micromol/L). Long-term imetelstat treatment led to progressive telomere shortening, reduced rates of proliferation, and eventually cell death in GBM tumor-initiating cells. Imetelstat in combination with radiation and temozolomide had a dramatic effect on cell survival and activated the DNA damage response pathway. Imetelstat is able to cross the blood-brain barrier in orthotopic GBM xenograft tumors. Fluorescently labeled GBM tumor cells isolated from orthotopic tumors, following systemic administration of imetelstat (30 mg/kg every day for three days), showed approximately 70% inhibition of telomerase activity. Chronic systemic treatment produced a marked decrease in the rate of xenograft subcutaneous tumor growth.

CONCLUSION

This preclinical study supports the feasibility of testing imetelstat in the treatment of GBM patients, alone or in combination with standard therapies.

Cellular immortality in brain tumours: an integration of the cancer stem cell paradigm

Brain tumours are a diverse group of neoplasms that continue to present a formidable challenge in our attempt to achieve curable intervention. Our conceptual framework of human brain cancer has been redrawn in the current decade. There is a gathering acceptance that brain tumour formation is a phenotypic outcome of dysregulated neurogenesis, with tumours viewed as abnormally differentiated neural tissue. In relation, there is accumulating evidence that brain tumours, similar to leukaemia and many solid tumours, are organized as a developmental hierarchy which is maintained by a small fraction of cells endowed with many shared properties of tissue stem cells. Proof that neurogenesis persists throughout adult life, compliments this concept. Although the cancer cell of origin is unclear, the proliferative zones that harbour stem cells in the embryonic, post-natal and adult brain are attractive candidates within which tumour-initiation may ensue. Dysregulated, unlimited proliferation and an ability to bypass senescence are acquired capabilities of cancerous cells. These abilities in part require the establishment of a telomere maintenance mechanism for counteracting the shortening of chromosomal termini. A strategy based upon the synthesis of telomeric repeat sequences by the ribonucleoprotein telomerase, is prevalent in approximately 90% of human tumours studied, including the majority of brain tumours. This review will provide a developmental perspective with respect to normal (neurogenesis) and aberrant (tumourigenesis) cellular turnover, differentiation and function. Within this context our current knowledge of brain tumour telomere/telomerase biology will be discussed with respect to both its developmental and therapeutic relevance to the hierarchical model of brain tumourigenesis presented by the cancer stem cell paradigm.

Synthesis, characterization and deepening in the comprehension of the biological action mechanisms of a new nickel complex with antiproliferative activity

Thiosemicarbazones are versatile organic compounds that present considerable pharmaceutical interest because of a wide range of properties. In our laboratory we synthesised some new metal-complexes with thiosemicarbazones derived from natural aldehydes which showed peculiar biological activities. In particular, a nickel complex [Ni(S-tcitr)(2)] (S-tcitr=S-citronellalthiosemicarbazonate) was observed to induce an antiproliferative effect on U937, a human histiocytic lymphoma cell line, at low concentrations (IC(50)=14.4microM). Therefore, we decided to study the interactions of this molecule with various cellular components and to characterise the induced apoptotic pathway. Results showed that [Ni(S-tcitr)(2)] causes programmed cell death via down-regulation of Bcl-2, alteration of mitochondrial membrane potential and caspase-3 activity, regardless of p53 function. The metal complex is not active on G(0) cells (i.e. fresh leukocytes) but is able to induce perturbation of the cell cycle on stimulated lymphocytes and U937 cells, in which a G(2)/M block was detected. It reaches the nucleus where it induces, at low concentrations (2.5-5.0microM), DNA damage, which could be partially ascribed to oxidative stress. [Ni(S-tcitr)(2)] is moreover able to strongly reduce the telomerase activity. Although the biological target of this metal complex is still unknown, the reported data suggest that [Ni(S-tcitr)(2)] could be a good model for the synthesis of new metal thiosemicarbazones with specific biological activity.

Genetic intratumour heterogeneity in high-grade brain tumours is associated with telomere-dependent mitotic instability

Glioblastoma multiforme (GBM) and other high-grade brain tumours are typically characterized by complex chromosome abnormalities and extensive intratumour cytogenetic heterogeneity. The mechanisms behind this diversity have been little explored. In this study, we analysed the pattern of chromosome segregation at mitosis in 20 brain tumours. We found an abnormal segregation of chromatids at mitosis through anaphase bridging (10-25% of anaphase cells) in all 10 GBMs. Anaphase bridging was also found in two medulloblastomas (7-15%), one anaplastic astrocytoma (17%) and one oligodendroglioma (6%). These tumours showed a relatively high degree of cytogenetic complexity and heterogeneity. In contrast, cell division abnormalities were not found in low-grade brain tumours with less complex karyotypes, including two pilocytic astrocytomas and two ependymomas. Further analysis of two GBMs by fluorescence in situ hybridization with telomeric repeat probes revealed excessive shortening of TTAGGG repeats, indicating dysfunctional protection of chromosome ends. In xenografts established from these GBMs, there was a gradual reduction in cytogenetic heterogeneity through successive passages as the proportion of abnormally short telomeres was reduced and the frequency of anaphase bridges decreased from >25% to 0. However, bridging could be reintroduced in late-passage xenograft cells by pharmacological induction of telomere shortening, using a small-molecule telomerase inhibitor. Telomere-dependent abnormal segregation of chromosomes at mitosis is thus a common phenomenon in high-grade brain tumours and may be one important factor behind cytogenetic intratumour diversity in GBM.

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.

A novel glioblastoma cancer gene therapy using AAV-mediated long-term expression of human TERT C-terminal polypeptide

Glioblastoma multiforme is the most aggressive form of human brain tumor, which has no effective cure. Previously, we have demonstrated that overexpression of the C-terminal fragment of the human telomerase reverse transcriptase (hTERTC27) inhibits the growth and tumorigenicity of human cervical cancer HeLa cells. In this study, the therapeutic effect and molecular mechanisms of hTERTC27-mediated cancer gene therapy were further explored in vivo in established human glioblastoma xenografts in nude mice. We showed that intratumoral injection of adeno-associated virus carrying hTERTC27 (rAAV-hTERTC27) is highly effective in reducing the growth of the subcutaneously transplanted glioblastoma tumors. Histological analyses showed that rAAV-hTERTC27 treatment leads to profound necrosis, apoptosis, infiltration of polymorphonuclear neutrophils and reduced microvessel density in the tumor samples. To study the molecular mechanism of rAAV-hTERTC27-mediated antitumor effects, we analyzed the global gene expression profiles of the rAAV-hTERTC27-treated tumor tissues and cell line as compared with that of the control rAAV-green fluorescent protein-treated samples by DNA microarray. Our results suggest that hTERTC27 exerts its effect through complex mechanisms, which involve genes regulating apoptosis, cell adhesion, cell cycle, immune responses, metabolism, signal transduction, transport, transcription and telomere maintenance.

Study of the telomerase hTERT fraction, PCNA and CD34 expression on pituitary adenomas. Association with clinical and demographic characteristics

To determine the frequency of human telomerase reverse transcriptase (hTERT) catalytic fraction expression and its association with clinical and demographic characteristics of the patient, as well as with the expression of CD34 and proliferating cell nuclear antigen (PCNA) indexes on adenohypophyseal hormone tissues. A transverse study was realized with 49 cases of hypophyseal adenoma with analysis type cases and controls. The different adenohypophyseal hormones [prolactin (PRL), growth hormone (GH), follicle stimulating hormone, luteinizing hormone, thyroid gland stimulant hormone, adrenocorticotropic hormone (ACTH)], the catalytic fraction of the telomerase hTERT, the PCNA index and the CD34 density were determined by means of immunohistochemical techniques. The clinical, demographic and histopathological characteristics of the patients with and without hTERT expression were compared by means of Pearson's Chi-squared, Fisher's exact test and Mann-Whitney's U. Twenty-eight point six percent of the adenomas had positive expression for hTERT. The variables significantly correlated with hTERT's expression were younger age of presentation, diagnostic of adenoma producer, higher PCNA index, higher CD34 density, increased GH on serum and the expression on PRL tissue, GH and ACTH. Tobacco history had a negative association with hTERT's expression. The telomerase could be a marker of cellular proliferation associated with angiogenesis and hormonal activity. Evaluation of these variables could provide information about their biological behavior.

Telomerase subunits expression variation between biopsy samples and cell lines derived from malignant glioma

Although scientific advances have recognised the prognostic power of telomerase activity in different cancers, as yet there has been no investigation regarding the expression variation of telomerase subunits in glioma tissues and cell lines. In this study, a recurrent anaplastic ependymoma and seven glioblastoma biopsy samples, four cell lines and four controls including two normal brain tissues were analysed for telomerase subunit expression profiles together with telomerase activity. Since telomerase activity is linked to tumourgenesis, the genes were analysed with respect to their expression variation. TEP1 was expressed in all glioma cell lines and 70% of glioblastoma tissues, in addition to the control brain tissues. Tankyrase was expressed in 85% of the glioblastoma tissues and was down-regulated in the recurrent anaplastic ependymoma tissue control cell lines. However, it was expressed in the control tissues. Dyskerin was expressed in all cell lines and tissues apart from U87-MG and NHA cells and the recurrent anaplastic ependymoma tissue. As expected, PARP1 and GAPDH showed constitutive expression throughout all cell lines and tissues since both are known to be housekeeping genes. hTERT was expressed in all glioma cell lines and tissues but was absent in the control cells and tissues. Telomerase activity was absent in IPDDC-A2 cells and 57% of the glioblastoma tissues. These results suggest that hTERT expression and not telomerase activity possibly represents a simple and reliable biological diagnostic tool.

Human telomerase RNA degradation by 2'-5'-linked oligoadenylate antisense chimeras in a cell-free system, cultured tumor cells, and murine xenograft models

Ribonuclease L (RNase L) is a latent single-stranded RNA-directed endoribonuclease that is activated on binding to short 2'-5'-linked oligoadenylates (2-5A), a feature that has led to its use in antisense therapeutic strategies. By attaching a 2-5A moiety to the 5' terminus of standard antisense oligonucleotides, it is possible to activate RNase L and guide it to specific RNAs for degradation. These 2-5A antisense chimeras have been used successfully to target a variety of cellular and viral RNAs. Telomerase is a nuclear ribonucleoprotein complex that elongates telomeric DNA and contributes to cellular immortalization. Telomerase is composed of a protein catalytic subunit and an RNA (hTR or TERC) component, both of which are critical for holoenzyme activity. We describe the characterization of 2-5A antisense chimeras targeting the hTR component of telomerase (2-5A antihTR). Newly designed 2-5A anti-hTR molecules were assayed for their abilities to selectively degrade hTR in a cell-free system. Of the five chimeras tested, one (RBI011) degraded hTR by 97%, and two others (RBI013 and RBI009) were also found to be highly active (73-76% degradation). The ability of transfected RBI011, and its homolog RBI254, to degrade hTR in cultured tumor cells was assessed by real-time RT-PCR. In these studies, RBI011 and RBI254 effectively degraded hTR in a variety of hTR-positive tumor cell lines. The hTR degradation studies were extended to growth assays to determine whether hTR ablation affected tumor cell viability or proliferation. RBI254 treatment resulted in reduced tumor cell viability over the course of 4-day growth assays, effects that were augmented by cotreatment with interferon-beta. To extend these results to an in vivo system, nude mice were implanted subcutaneously or orthotopically with hTR-positive prostate tumors and treated with RBI254. RBI254-treated mice exhibited enhanced tumor cell apoptosis and reduced tumor volume as compared with controls. These findings demonstrated the effectiveness of highly active forms of 2-5A antisense against hTR, and also highlight the usefulness of the cell-free system in predicting chimera efficacy before to inception of cell-based and in vivo studies.

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Telomerase inhibition impairs tumor growth in glioblastoma xenografts

Telomerase is a specialized DNA polymerase that is required to replicate the ends of linear chromosomes, the telomeres. The majority of human cancers express high levels of telomerase activity that is permissive for tumor growth because it provides cells with an extended proliferative potential. Additionally, telomerase exerts cell growth promoting functions and favors cell survival. Human glioblastoma multiforme (GBM) cells express high level of telomerase activity owing to the overexpression of human telomerase reverse transcriptase (hTERT), the limiting subunit of the enzyme. Here we used retroviral mediated RNA interference to dampen down telomerase activity in two distinct human GBM cell lines, U87MG and TB10. Substantial decrease of hTERT mRNA and telomerase activity had only minimal effects on telomere length maintenance, cell growth and survival in vitro. On the contrary, development of tumors upon subcutaneously grafting of U87MG and TB10 cells and intracranial implantation of U87MG cells in nude athymic mice was strongly reduced by telomerase inhibition.

Establishing tumor cell lines from aggressive telomerase-positive chordomas of the skull base

✓ Permanent cell cultures are invaluable tools for understanding the biological characteristics of tumors. In the present study the authors report on the establishment of permanent human cell lines from three cases of aggressive chordomas of the clival region. All of the parental tumors showed telomerase activity. Cultured chordoma cells had a doubling time of 5 to 7 days and grew as a monolayer of cells that retained both the immunophenotype and the p53 status of the parental tumor. In vitro, chordoma cells overexpressed telomerase, supporting the hypothesis that this enzyme is required for the immortalization process.

Autophagic cell death of malignant glioma cells induced by a conditionally replicating adenovirus

BACKGROUND

Conditionally replicating adenoviruses (CRAds) can be engineered to replicate selectively in cancer cells and cause cancer-specific cell lysis; thus they are considered a promising cancer therapy.

METHODS

To elucidate the mechanisms by which CRAds induce cancer-specific cell death, we infected normal human fibroblasts (MRC5, telomerase negative), human malignant glioma (U373-MG and U87-MG), human cervical cancer (HeLa), and human prostate cancer (PC3) cells (all telomerase positive) with CRAds regulated by the human telomerase reverse transcriptase promoter (hTERT-Ad) or control nonreplicating adenoviruses (Ad-GFP). Nonapoptotic autophagy was assessed in Ad-GFP- and hTERT-Ad-infected cells by examining cell morphology, the development of acidic vesicular organelles, and the conversion of microtubule-associated protein 1 light chain 3 from the cytoplasmic form to the autophagosome membrane form; signaling via mammalian target of rapamycin (mTOR), an autophagy-associated molecule, was monitored by western blot analysis. We also compared the growth of subcutaneous gliomas in nude mice that were treated by intratumoral injection with Ad-GFP or hTERT-Ad. Survival of athymic mice carrying intracranial gliomas treated by intratumoral injection with Ad-GFP or hTERT-Ad was compared by using the Kaplan-Meier method and the Cox-Mantel log-rank analysis. All statistical tests were two-sided.

RESULTS

hTERT-Ad induced tumor-specific autophagic cell death in tumor cells and in subcutaneous gliomas. hTERT-Ad-induced autophagy was associated with hTERT-Ad infection kinetics. The mTOR signaling pathway was suppressed in tumor cells and in subcutaneous gliomas treated with hTERT-Ad compared with GFP-Ad or no treatment as shown by reduced phosphorylation of mTOR's downstream target p70S6 kinase (p70S6K). hTERT-Ad treatment of mice (n = 7) slowed growth of subcutaneous gliomas (mean tumor volume = 39 mm3, 95% confidence interval [CI] = 23 to 54 mm3) compared with GFP-Ad treatment (n = 7) (mean tumor volume = 200 mm3, 95% CI = 149 to 251 mm3) at day 7 (volume difference = 161 mm3, 95% CI = 126 to 197 mm3; P < .001). Mice carrying intracranial tumors that were treated with three intratumoral injections of hTERT-Ad survived longer (53 days) than after treatment with GFP-Ad (29 days) (seven mice per group, difference = 24 days, 95% CI = 20 to 28 days; P < .001).

CONCLUSIONS

hTERT-Ad may kill telomerase-positive cancer cells by inducing autophagic cell death.

Use of replication-competent retroviral vectors in an immunocompetent intracranial glioma model

OBJECT

The authors had previously reported on a replication-competent retrovirus (RCR) that has been demonstrated to be stable, capable of effective transduction, and able to prolong survival in an intracranial tumor model in nude mice. The purpose of this study was further investigation of this gene therapy option.

METHODS

The transduction efficiency of RCR in RG2, an immunocompetent intracranial tumor model, was tested in Fischer 344 rats. The immune response to the RCR vector was expressed by the quantification of CD4, CD8, and CD11/b in tumors. The pharmaceutical efficacy of the suicide gene CD in converting prodrug 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU) was measured using fluorine-19 nuclear magnetic resonance (19F-NMR) spectroscopy. Animal survival data were plotted on Kaplan-Meier survival curves. Finally, the biodistribution of RCR was determined using quantitative real-time polymerase chain reaction (RT-PCR) for the detection of retroviral env gene. There was no evidence of viral transduction in normal brain cells. Neither severe inflammation nor immunoreaction occurred after intracranial injection of RCR-green fluorescent protein compared with phosphate-buffered saline (PBS). The 19F-NMR spectroscopy studies demonstrated that RCR-CD was able to convert 5-FC to 5-FU effectively in vitro. The infection of RG2 brain tumors with RCR-CD and their subsequent treatment with 5-FC significantly prolonged survival compared with that in animals with RG2 transduced tumors treated with PBS. In contrast to the nude mouse model, evidence of virus dissemination to the systemic organs after intracranial injection was not detected using RT-PCR.

CONCLUSIONS

The RCR-mediated suicide gene therapy described in this paper effectively transduced malignant gliomas in an immunocompetent in vivo rodent model, prolonging survival, without evidence of severe intracranial inflammation, and without local transduction of normal brain cells or systemic organs.

Telomerase inhibition by stable RNA interference impairs tumor growth and angiogenesis in glioblastoma xenografts

Telomerase is highly expressed in advanced stages of most cancers where it allows the clonal expansion of transformed cells by counteracting telomere erosion. Telomerase may also contribute to tumor progression through still undefined cell growth-promoting functions. Here, we inhibited telomerase activity in 2 human glioblastoma (GBM) cell lines, TB10 and U87MG, by targeting the catalytic subunit, hTERT, via stable RNA interference (RNAi). Although the reduction in telomerase activity had no effect on GBM cell growth in vitro, the development of tumors in subcutaneously and intracranially grafted nude mice was significantly inhibited by antitelomerase RNAi. The in vivo effect was observed within a relatively small number of population doublings, suggesting that telomerase inhibition may hinder cancer cell growth in vivo prior to a substantial shortening of telomere length. Tumor xenografts that arose from telomerase-inhibited GBM cells also showed a less-malignant phenotype due both to the absence of massive necrosis and to reduced angiogenesis.

Conditionally replicative adenoviral vectors for malignant glioma

High-grade gliomas constitute an important challenge to modern medicine, and although great effort has been made to prolong patient survival, the prognosis for this disease remains poor. Due to recent discoveries in the molecular basis of gliomas, gene therapy is becoming a promising alternative. In this review, we discuss the use of conditionally replicative adenoviral vectors (CRAd) and their applications in neuro-oncology. Such vectors, when rendered conditionally replicative via transductional and transcriptional modifications, offer great promise for patients with malignant brain tumours. We review data from preclinical and clinical studies utilising such vectors and discuss the limitations and future perspectives of CRAd oncolytic therapy for malignant glioma.

Expression of transcription factor E2F1 and telomerase in glioblastomas: mechanistic linkage and prognostic significance

BACKGROUND

Several tumor suppressor pathways have been identified as modulators of telomerase function. We examined the functional role of the retinoblastoma-E2F1 pathway in regulating telomerase activity in malignant gliomas.

METHODS

Adenovirus vectors were used to transfer cDNAs into human glioblastoma and sarcoma cells. Telomerase activity was assessed with a telomere repeat amplification protocol. Promoter activity in cancer cells was assessed with promoter-luciferase reporter constructs. Promoter binding was assessed with the chromatin immunoprecipitation (ChIP) assay. We isolated astrocytes from E2F1 transgenic mice and normal mice for in vivo studies. We evaluated the expression of E2F1 and hTERT (the catalytic subunit of human telomerase) mRNAs by reverse transcriptase-polymerase chain reaction and proteins in human glioblastoma samples by immunoblot analysis. Associations between survival among 61 glioblastoma multiforme patients and expression of E2F1 and hTERT mRNA and protein were examined with Kaplan-Meier analysis, the log-rank test, and Cox proportional hazards regression models. All statistical tests were two-sided.

RESULTS

Ectopic E2F1 expression increased hTERT promoter activity in cancer cells. We detected an interaction between E2F1 protein and the hTERT promoter. Transgenic E2F1 astrocytes contained functional telomerase protein. E2F1 mRNA expression and hTERT mRNA expression were statistically significantly correlated in human glioblastoma specimens (R = .8; P < .001). Longer median survival was statistically significantly associated with lower E2F1 mRNA expression in tumors (103.6 weeks) rather than with higher expression (46.1 weeks) (difference = 57.5 weeks; 95% confidence interval [CI] = 14.7 to 159.7; log-rank P = .002). E2F1 mRNA was the only factor that was statistically significantly associated with overall survival in a multivariable model (P = .04). Among 27 patients with glioblastoma multiforme samples, the expression of E2F1 protein was statistically significantly associated with survival (log-rank P < .001).

CONCLUSIONS

E2F1 may participate in telomerase activity regulation in malignant glioma cells. Its expression appears to be strongly associated with the survival of patients with malignant brain tumors.

Therapeutic efficacy of PUMA for malignant glioma cells regardless of p53 status

Replacement of the p53 tumor suppressor gene is a rational approach to the management of malignant gliomas because p53 is frequently mutated or inactivated in these cancers. Major weaknesses of this approach are that malignant gliomas are mixtures of cells with wild-type and mutant p53, and that tumor cells exhibiting wildtype p53 are resistant to p53 gene transfer. An effective alternative is needed to overcome these difficulties. p53-upregulated modulator of apoptosis (PUMA) was identified as a p53-inducible proapoptotic molecule. Our purpose was to elucidate a role for PUMA in p53 gene therapy and to investigate whether PUMA is an efficient substitute for p53 in cancer therapy. We demonstrated that PUMA was upregulated in mutant p53 malignant glioma cells (U373-MG and T98G) undergoing apoptosis but was not upregulated in apoptosis-resistant wild-type p53 malignant glioma cells (U87-MG and D54) after adenoviral transfer of p53. Overexpression of PUMA resulted in massive apoptosis associated with mitochondrial damage and caspase-3 activation in all tumor cells tested. Use of the human telomerase reverse transcriptase (hTERT) promoter system induced apoptosis only in malignant glioma cells with telomerase activity, while sparing normal cells lacking telomerase. The ability of PUMA to induce apoptosis was greater than that of caspase-6 or caspase-8 transfer, using the same system. Moreover, exogenous expression of PUMA under the hTERT promoter system significantly suppressed the growth of subcutaneous U87-MG tumors in nude mice and did not induce apoptosis in surrounding nontumor tissues. These results indicate that PUMA, which is regulated under a tumor-specific expression system such as the hTERT promoter, may be better than p53 as a therapeutic tool for malignant gliomas.

mRNA quantification and clinical evaluation of telomerase reverse transcriptase subunit (hTERT) in intracranial tumours of patients in the island of Crete

Telomerase is a reverse transcriptase that maintains telomeres by adding telomeric TTAGGG repeats to the ends of human chromosomes. The aim of this study was to evaluate quantitatively the mRNA expression of telomerase catalytic subunit (hTERT) in different types of intracranial tumours in relation to their histologic pattern and grade and correlate it with progression-free (PFS) and overall survival (OS) of patients. Human telomerase reverse transcriptase mRNA levels were estimated by the use of real time RT–PCR in 68 samples of intracranial tumours. It revealed statistical correlation between hTERT mRNA expression levels and the grade of the tumours (P<0.001). Patients having negative expression of hTERT mRNA had statistically longer PFS (P=0.031) and OS (P=0.047). Cox univariate regression analysis revealed that hTERT mRNA-positive patients had a high and statistically significant risk of relapse (hazard ratio (HR) of 2.24 and P=0.038). In the Cox multivariate regression model, the levels of hTERT mRNA were adjusted for tumour grade and patients age, and since there was statistically significant relationship between the levels of hTERT mRNA and the grade of the tumours (P=0.003 or P=0.006, respectively), hTERT mRNA levels could not be considered as an independent prognostic factor for PFS or OS.

Telomerase activity predicts malignancy in percutaneous image-guided needle biopsy specimens of the abdomen and pelvis

PURPOSE

To determine prospectively if assessment of telomerase activity in percutaneous needle biopsy specimens improves sensitivity and specificity in the diagnosis of abdominal and pelvic malignancy.

MATERIALS AND METHODS

The study was approved by the institutional review board, and written informed consent was obtained from all patients. A prospective double-blinded design was used to assess telomerase activity in abdominal and pelvic biopsy specimens from 99 patients (64 men, 35 women; age range, 22-87 years). After the clinical sample was retrieved, a study specimen from an extra needle pass was divided and independently analyzed for cytologic characteristics and telomerase activity. The final diagnosis was based on chart review at a minimum 1-year follow-up. Statistical analyses included sensitivity, specificity, and accuracy of cytologic examination and/or telomerase activity in predicting malignancy.

RESULTS

Data from study specimens indicated that the sensitivity, specificity, and accuracy of telomerase activity (n=99) in predicting malignancy were 55%, 79%, and 60%, respectively. For cytologic examination (n=86), the sensitivity, specificity, and accuracy in predicting malignancy were 74%, 94%, and 78%, respectively. Combining the two tests (n=86) and classifying a positive reading with either test as malignant improved sensitivity (83%) (P <.05) without altering specificity (76%). In 20 patients who had clinical sample reports that were classified as indeterminate, telomerase activity (n=20) yielded a higher sensitivity (62%) (P <.05) and similar specificity (86%) compared with cytologic examination (n=15), which yielded a sensitivity of 11% and a specificity of 83%.

CONCLUSION

In percutaneous biopsy specimens of the abdomen and pelvis, the combination of cytologic examination and telomerase activity yielded an increased sensitivity in predicting malignancy. In addition, assessing telomerase activity can help identify cancer even when cytologic results are indeterminate.