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

Cellular Senescence in Diffuse Gliomas: From Physiopathology to Possible Treatments

Cellular senescence in gliomas is a complex process that is induced by aging and replication, ionizing radiation, oncogenic stress, and the use of temozolomide. However, the escape routes that gliomas must evade senescence and achieve cellular immortality are much more complex, in which the expression of telomerase and the alternative lengthening of telomeres, as well as the mutation of some proto-oncogenes or tumor suppressor genes, are involved. In gliomas, these molecular mechanisms related to cellular senescence can have a tumor-suppressing or promoting effect and are directly involved in tumor recurrence and progression. From these cellular mechanisms related to cellular senescence, it is possible to generate targeted senostatic and senolytic therapies that improve the response to currently available treatments and improve survival rates. This review aims to summarize the mechanisms of induction and evasion of cellular senescence in gliomas, as well as review possible treatments with therapies targeting pathways related to cellular senescence.

The Role of Glia Telomere Dysfunction in the Pathogenesis of Central Nervous System Diseases

Maintaining the telomere length is decisive for the viability and homeostasis process of all the cells of an organism, including human glial cells. Telomere shortening of microglial cells has been widely associated with the onset and progression of neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Additionally, traumatic brain injury appears to have a positive correlation with the telomere-shortening process of microglia, and telomere length can be used as a non-invasive biomarker for the clinical management of these patients. Moreover, telomere involvement through telomerase reactivation and homologous recombination also known as the alternative lengthening of telomeres (ALT) has been described in gliomagenesis pathways, and particular focus has been given in the translational significance of these mechanisms in gliomas diagnosis and prognostic classification. Finally, glia telomere shortening is implicated in some psychiatric diseases. Given that telomere dysfunction of glial cells is involved in the central nervous system (CNS) disease pathogenesis, it represents a promising drug target that could lead to the incorporation of new tools in the medicinal arsenal for the management of so far incurable conditions.

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.

Vorinostat, temozolomide or bevacizumab with irradiation and maintenance BEV/TMZ in pediatric high-grade glioma: A Children's Oncology Group Study

Background

Outcomes for children with high-grade gliomas (HGG) remain poor. This multicenter phase II trial evaluated whether concurrent use of vorinostat or bevacizumab with focal radiotherapy (RT) improved 1-year event-free survival (EFS) compared to temozolomide in children with newly diagnosed HGG who received maintenance temozolomide and bevacizumab.

Methods

Patients ≥ 3 and < 22 years with localized, non-brainstem HGG were randomized to receive RT (dose 54-59.4Gy) with vorinostat, temozolomide, or bevacizumab followed by 12 cycles of bevacizumab and temozolomide maintenance therapy.

Results

Among 90 patients randomized, the 1-year EFS for concurrent bevacizumab, vorinostat, or temozolomide with RT was 43.8% (±8.8%), 41.4% (±9.2%), and 59.3% (±9.5%), respectively, with no significant difference among treatment arms. Three- and five-year EFS for the entire cohort was 14.8% and 13.4%, respectively, with no significant EFS difference among the chemoradiotherapy arms. IDH mutations were associated with more favorable EFS (P = .03), whereas H3.3 K27M mutations (P = .0045) and alterations in PIK3CA or PTEN (P = .025) were associated with worse outcomes. Patients with telomerase- and alternative lengthening of telomeres (ALT)-negative tumors (n = 4) had an EFS of 100%, significantly greater than those with ALT or telomerase, or both (P = .002). While there was no difference in outcomes based on TERT expression, high TERC expression was associated with inferior survival independent of the telomere maintenance mechanism (P = .0012).

Conclusions

Chemoradiotherapy with vorinostat or bevacizumab is not superior to temozolomide in children with newly diagnosed HGG. Patients with telomerase- and ALT-negative tumors had higher EFS suggesting that, if reproduced, mechanism of telomere maintenance should be considered in molecular-risk stratification in future studies.

Cancer-associated SMARCAL1 loss-of-function mutations promote alternative lengthening of telomeres and tumorigenesis in telomerase-negative glioblastoma cells

BACKGROUND

Telomere maintenance mechanisms are required to enable the replicative immortality of malignant cells. While most cancers activate the enzyme telomerase, a subset of cancers uses telomerase-independent mechanisms termed alternative lengthening of telomeres (ALT). ALT occurs via homology-directed-repair mechanisms and is frequently associated with ATRX mutations. We previously showed that a subset of adult glioblastoma (GBM) patients with ATRX-expressing ALT-positive tumors harbored loss-of-function mutations in the SMARCAL1 gene, which encodes an annealing helicase involved in replication fork remodeling and the resolution of replication stress. However, the causative relationship between SMARCAL1 deficiency, tumorigenesis, and de novo telomere synthesis is not understood.

METHODS

We used a patient-derived ALT-positive GBM cell line with native SMARCAL1 deficiency to investigate the role of SMARCAL1 in ALT-mediated de novo telomere synthesis, replication stress, and gliomagenesis in vivo.

RESULTS

Inducible rescue of SMARCAL1 expression suppresses ALT indicators and inhibits de novo telomere synthesis in GBM and osteosarcoma cells, suggesting that SMARCAL1 deficiency plays a functional role in ALT induction in cancers that natively lack SMARCAL1 function. SMARCAL1-deficient ALT-positive cells can be serially propagated in vivo in the absence of detectable telomerase activity, demonstrating that the SMARCAL1-deficient ALT phenotype maintains telomeres in a manner that promotes tumorigenesis.

CONCLUSIONS

SMARCAL1 deficiency is permissive to ALT and promotes gliomagenesis. Inducible rescue of SMARCAL1 in ALT-positive cell lines permits the dynamic modulation of ALT activity, which will be valuable for future studies aimed at understanding the mechanisms of ALT and identifying novel anticancer therapeutics that target the ALT phenotype.

Association of human telomerase reverse transcriptase promoter mutation with unfavorable prognosis in glioma: A systematic review and meta-analysis

Background

Glioma is one of the most malignant and aggressive tumors, with an extremely poor prognosis. Human telomerase reverse transcriptase (hTERT) promoter mutation is regarded as a risk factor in tumor growth. Although the prevalence of hTERT promoter (pTERT) mutation in gliomas has been investigated, the results are inconsistent. This meta-analysis aims to investigate the prognostic value of hTERT in glioma patients and its interaction with other biomarkers.

Materials and Methods

We searched 244 citations from four databases: PubMed (2000-2021), Web of Science (2000-2021), Embase (2010-2021), and Cochrane Library (2000-2021) with 28 articles included.

Results

We calculated hazard ratios (HRs) using the random effect model and the pooled result suggested that TERT promoter mutation predicted poorer overall survival (HR: 1.53, 95% confidence interval [CI]: 1.34-1.75, P < 0.001, I2: 49.9%, pheterogeneity:0.002) and progression-free survival (HR: 1.55, 95% CI: 1.27-1.88, P < 0.001, I2: 0.0%, pheterogeneity: 0.473). For subgroup analysis, we analyzed multiple factors including iso-citrate dehydrogenase (IDH) genotype, age, diagnosis, pTERT region, so as to locate the sources of heterogeneity. Interestingly, in IDH mutant subgroup, pTERT mutation became a beneficial prognostic factor (HR: 0.73, 95% CI: 0.57-0.93, I2: 22.3%, pheterogeneity: 0.277), which is contrary to the results in pooled analysis.

Conclusion

In general, pTERT mutation may result in shorter survival time in glioma patients, but longer survival time when glioma patients are combined with IDH mutation.

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.

Oncohistones and disrupted development in pediatric-type diffuse high-grade glioma

Recurrent, clonal somatic mutations in histone H3 are molecular hallmarks that distinguish the genetic mechanisms underlying pediatric and adult high-grade glioma (HGG), define biological subgroups of diffuse glioma, and highlight connections between cancer, development, and epigenetics. These oncogenic mutations in histones, now termed "oncohistones", were discovered through genome-wide sequencing of pediatric diffuse high-grade glioma. Up to 80% of diffuse midline glioma (DMG), including diffuse intrinsic pontine glioma (DIPG) and diffuse glioma arising in other midline structures including thalamus or spinal cord, contain histone H3 lysine 27 to methionine (K27M) mutations or, rarely, other alterations that result in a depletion of H3K27me3 similar to that induced by H3 K27M. This subgroup of glioma is now defined as diffuse midline glioma, H3K27-altered. In contrast, histone H3 Gly34Arg/Val (G34R/V) mutations are found in approximately 30% of diffuse glioma arising in the cerebral hemispheres of older adolescents and young adults, now classified as diffuse hemispheric glioma, H3G34-mutant. Here, we review how oncohistones modulate the epigenome and discuss the mutational landscape and invasive properties of histone mutant HGGs of childhood. The distinct mechanisms through which oncohistones and other mutations rewrite the epigenetic landscape provide novel insights into development and tumorigenesis and may present unique vulnerabilities for pHGGs. Lessons learned from these rare incurable brain tumors of childhood may have broader implications for cancer, as additional high- and low-frequency oncohistone mutations have been identified in other tumor types.

Commentary: Evaluating potential glioma serum biomarkers, with future applications

Systemic inflammation within malignant glioma is a topic of ongoing significance. In this commentary, we highlight recent findings from Gandhi et al and discuss alternative approaches. We present a counter argument with findings that IL-6 markers are controversial. We highlight the potential benefit of looking at microRNAs and other biomarkers. Finally, we present ideas for future application involving differentiation between radiation necrosis and recurrence. The commentary is intended to serve as a catalyst for further scientific discovery.

ALT Positivity in Human Cancers: Prevalence and Clinical Insights

Many exciting advances in cancer-related telomere biology have been made in the past decade. Of these recent advances, great progress has also been made with respect to the Alternative Lengthening of Telomeres (ALT) pathway. Along with a better understanding of the molecular mechanism of this unique telomere maintenance pathway, many studies have also evaluated ALT activity in various cancer subtypes. We first briefly review and assess a variety of commonly used ALT biomarkers. Then, we provide both an update on ALT-positive (ALT+) tumor prevalence as well as a systematic clinical assessment of the presently studied ALT+ malignancies. Additionally, we discuss the pathogenetic alterations in ALT+ cancers, for example, the mutation status of ATRX and DAXX, and their correlations with the activation of the ALT pathway. Finally, we highlight important ALT+ clinical associations within each cancer subtype and subdivisions within, as well as their prognoses. We hope this alternative perspective will allow scientists, clinicians, and drug developers to have greater insight into the ALT cancers so that together, we may develop more efficacious treatments and improved management strategies to meet the urgent needs of cancer patients.

TERT Promoter Alterations in Glioblastoma: A Systematic Review

Simple Summary

Glioblastoma is the most common malignant primary brain tumor in adults. Glioblastoma accounts for 2 to 3 cases per 100,000 persons in North America and Europe. Glioblastoma classification is now based on histopathological and molecular features including isocitrate dehydrogenase (IDH) mutations. At the end of the 2000s, genome-wide sequencing of glioblastoma identified recurrent somatic genetic alterations involved in oncogenesis. Among them, the alterations in the promoter region of the telomerase reverse transcriptase (TERTp) gene are highly recurrent and occur in 70% to 80% of all glioblastomas, including glioblastoma IDH wild type and glioblastoma IDH mutated. This review focuses on recent advances related to physiopathological mechanisms, diagnosis, and clinical implications.

Abstract

Glioblastoma, the most frequent and aggressive primary malignant tumor, often presents with alterations in the telomerase reverse transcriptase promoter. Telomerase is responsible for the maintenance of telomere length to avoid cell death. Telomere lengthening is required for cancer cell survival and has led to the investigation of telomerase activity as a potential mechanism that enables cancer growth. The aim of this systematic review is to provide an overview of the available data concerning TERT alterations and glioblastoma in terms of incidence, physiopathological understanding, and potential therapeutic implications.

Alternative lengthening of telomeres in molecular subgroups of paediatric high-grade glioma

PURPOSE

The maintenance of telomere length prevents cancer cell senescence and occurs via two mutually exclusive mechanisms: (a) reactivation of telomerase expression and (b) activation of alternative lengthening of telomeres (ALT). ALT is frequently related to alterations on ATRX, a chromatin-remodelling protein. Recent data have identified different molecular subgroups of paediatric high-grade glioma (pHGG) with mutations of H3F3A, TERTp and ATRX; however, differences in telomere length among these molecular subgroups were not thoroughly examined.

METHODS

We investigated which genetic alterations trigger the ALT mechanism in 52 IDH-wildtype, 1p/19q-wildtype pHGG. Samples were analysed for telomere length using Tel-FISH. ATRX nuclear loss of expression was assessed by IHC, H3F3A and TERTp mutations by DNA sequencing, and TERTp methylation by MS-PCR.

RESULTS

Mutant H3.3 was found in 21 cases (40.3%): 19.2% with K27M mutation and 21.1% with G34R mutation. All H3.3G34R-mutated cases showed the ALT phenotype (100%); on the opposite, only 40% of the H3.3K27M-mutated showed ALT activation. ATRX nuclear loss was seen in 16 cases (30.7%), associated sometimes with the G34R mutation, and never with the K27M mutation. ATRX nuclear loss was always related to telomere elongation. TERTp C250T mutations were rare (5.4%) and were not associated with high intensity Tel-FISH signals, as TERTp hyper-methylation detected in 21% of the cases. H3.3/ATRX/TERTp-wildtype pHGG revealed all basal levels of telomere length.

CONCLUSION

Our results show a strong association between H3.3 mutations and ALT, and highlight the different telomeric profiles in histone-defined subgroups: H3.3-G34R mutants always trigger ALT to maintain telomere length, irrespective of ATRX status, whereas only some H3.3-K27M tumours activate ALT. These findings suggest that acquiring the gly34 mutation on H3.3 might suffice to trigger the ALT mechanism.

Clinical Relevance of Alternative Lengthening of Telomeres in Cancer

The alternative lengthening of telomere (ALT) is a pathway responsible for cell immortalization in some kinds of tumors. Since the first description of ALT is relatively recent in the oncology field, its mechanism remains elusive, but recent works address ALT-related proteins or cellular structures as potential druggable targets for more specific and efficient antitumor therapies. Moreover, some new generation compounds for antitelomerase therapy in cancer were able to provoke acquisition of ALT phenotype in treated tumors, enhancing the importance of studies on this alternative lengthening of the telomere. However, ALT has been implicated in different - sometimes opposite - outcomes, according to the tumor type studied. Then, in order to design and develop new drugs for ALT+ cancer in an effective way, it is crucial to understand its clinical implications. In this review, we gathered works published in the last two decades to highlight the clinical relevance of ALT on oncology.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Expression of tert Prevents ALT in Zebrafish Brain Tumors

The activation of a telomere maintenance mechanism (TMM) is an essential step in cancer progression to escape replicative senescence and apoptosis. Alternative lengthening of telomeres (ALT) is found in a subset of malignant brain tumors with poor outcomes. Here, we describe a model of juvenile zebrafish brain tumor that progressively develops ALT. We discovered that reduced expression of tert, linked to a widespread hypomethylation of the tert promoter and increase in Terra expression precedes ALT development. Surprisingly, expression of tert during juvenile brain tumor development led to reduced proliferation of tumor cells and prolonged survival. Most importantly, expression of tert reverted all ALT features and normalizes TERRA expression, promoted heterochromatin formation at telomeres, and attenuated telomeric DNA damage. These data suggest that the activity of telomerase goes beyond telomere maintenance and has profound consequences on genome stability.

Telomere maintenance mechanisms in cancer: telomerase, ALT or lack thereof

Cancer cells acquire replicative immortality by activating a telomere maintenance mechanism (TMM), either the telomerase or the Alternative Lengthening of Telomeres (ALT) mechanism. ALT is frequently activated in tumors derived from mesenchymal cells, which are more frequent in childhood cancers. Recent studies showed that, occasionally, cancer cells can arise without any TMM activation. Here, we discuss the challenge in assessing which TMM is activated in tumors. We also evaluate the prevalence of ALT mechanism in pediatric cancers and review the associated survival prognosis in different tumor types. Finally, we discuss about possible anti-TMM therapies for new emerging cancer treatments.

Telomere Maintenance in Pediatric Cancer

Telomere length has been proposed as a biomarker of biological age and a risk factor for age-related diseases and cancer. Substantial progress has been made in recent decades in understanding the complex molecular relationships in this research field. However, the majority of telomere studies have been conducted in adults. The data on telomere dynamics in pediatric cancers is limited, and interpretation can be challenging, especially in cases where results are contrasting to those in adult entities. This review describes recent advances in the molecular characterization of structure and function of telomeres, regulation of telomerase activity in cancer pathogenesis in general, and highlights the key advances that have expanded our views on telomere biology in pediatric cancer, with special emphasis on the central role of telomere maintenance in neuroblastoma. Furthermore, open questions in the field of telomere maintenance research are discussed in the context of recently published literature.

Preclinical therapeutic targets in diffuse midline glioma

Diffuse midline gliomas (DMG) are rapidly fatal tumors of the midbrain in children, characterized by a diffuse growing pattern and high levels of intrinsic resistance to therapy. The location of these tumors, residing behind the blood-brain barrier (BBB), and the limited knowledge about the biology of these tumors, has hindered the development of effective treatment strategies. However, the introduction of diagnostic biopsies and the implementation of autopsy protocols in several large centers world-wide has allowed for a detailed characterization of these rare tumors. This has resulted in the identification of novel therapeutic targets, as well as major advances in understanding the biology of DMG in relation to therapy resistance. We here provide an overview of the cellular pathways and tumor-specific aberrations that have been targeted in preclinical DMG research, and discuss the advantages and limitations of these therapeutic strategies in relation to therapy resistance and BBB-penetration. Therewith, we aim to provide researchers with a framework for successful preclinical therapy development.

Telomere elongation via alternative lengthening of telomeres (ALT) and telomerase activation in primary metastatic medulloblastoma of childhood

PURPOSE

Elongation of telomeres is necessary for tumor cell immortalization and senescence escape; neoplastic cells use to alternative pathways to elongate telomeres: telomerase reactivation or a telomerase-independent mechanism termed alternative lengthening of telomeres (ALT). Telomerase and ALT pathway has been explored in adult and pediatric gliomas and medulloblastomas (MDBs); however, these mechanisms were not previously investigated in MDBs metastatic at the onset. Therefore, we analyzed the activation of telomerase and ALT pathway in a homogenous cohort of 43 pediatric metastatic medulloblastomas, to investigate whether telomere elongation could play a role in the biology of metastatic MDB.

METHODS

We evaluated telomeres length via telomere-specific fluorescence in situ hybridization (Telo-FISH); we assessed nuclear expression of ATRX by immunohistochemistry (IHC). H3F3A and TERT promoter mutations were analyzed by pyrosequencing, while UTSS methylation status was analyzed via methylation-specific-PCR (MS-PCR).

RESULTS

H3F3A mutations were absent in all MDBs, 30% of samples showed ATRX nuclear loss, 18.2% of cases were characterized by TERT promoter mutations, while 60.9% harboured TERT promoter hyper-methylation in the UTSS region. Elongation of telomeres was found in 42.8% of cases. Metastatic MDBs control telomere elongation via telomerase activation (10.7%), induced by TERT promoter mutations in association with UTSS hyper-methylation, and ALT mechanism (32.1%), triggered by ATRX inactivation. Among non-metastatic MDBs, only 5.9% (1/17) showed ATRX nuclear loss with activation of ALT.

CONCLUSIONS

Our metastatic cases frequently activate ALT pathway, suggesting that it is a common process for senescence escape in primary metastatic medulloblastomas. Furthermore, the activation of mechanisms for telomere elongation is not restricted to certain molecular subgroups in this high-risk group of MDBs.

Induced Telomere Damage to Treat Telomerase Expressing Therapy-Resistant Pediatric Brain Tumors

Brain tumors remain the leading cause of cancer-related deaths in children and often are associated with long-term sequelae among survivors of current therapies. Hence, there is an urgent need to identify actionable targets and to develop more effective therapies. Telomerase and telomeres play important roles in cancer, representing attractive therapeutic targets to treat children with poor-prognosis brain tumors such as diffuse intrinsic pontine glioma (DIPG), high-grade glioma (HGG), and high-risk medulloblastoma. We have previously shown that DIPG, HGG, and medulloblastoma frequently express telomerase activity. Here, we show that the telomerase-dependent incorporation of 6-thio-2'deoxyguanosine (6-thio-dG), a telomerase substrate precursor analogue, into telomeres leads to telomere dysfunction-induced foci (TIF) along with extensive genomic DNA damage, cell growth inhibition, and cell death of primary stem-like cells derived from patients with DIPG, HGG, and medulloblastoma. Importantly, the effect of 6-thio-dG is persistent even after drug withdrawal. Treatment with 6-thio-dG elicits a sequential activation of ATR and ATM pathways and induces G₂-M arrest. In vivo treatment of mice bearing medulloblastoma xenografts with 6-thio-dG delays tumor growth and increases in-tumor TIFs and apoptosis. Furthermore, 6-thio-dG crosses the blood-brain barrier and specifically targets tumor cells in an orthotopic mouse model of DIPG. Together, our findings suggest that 6-thio-dG is a promising novel approach to treat therapy-resistant telomerase-positive pediatric brain tumors. Mol Cancer Ther; 17(7); 1504-14. ©2018 AACR.

Diagnosis and treatment of ALT tumors: is Trabectedin a new therapeutic option?

Telomeres are specialized nucleoprotein structures responsible for protecting chromosome ends in order to prevent the loss of genomic information. Telomere maintenance is required for achieving immortality by neoplastic cells. While most cancer cells rely on telomerase re-activation for linear chromosome maintenance and sustained proliferation, a significant population of cancers (10-15%) employs telomerase-independent strategies, collectively referred to as Alternative Lengthening of Telomeres (ALT). ALT mechanisms involve different types of homology-directed telomere recombination and synthesis. These processes are facilitated by loss of the ATRX or DAXX chromatin-remodeling factors and by abnormalities of the telomere nucleoprotein architecture. Although the functional consequences of telomerase and ALT up-regulation are similar in that they both prevent overall telomere shortening in tumors, these telomere maintenance mechanisms (TMMs) differ in several aspects which may account for their differential prognostic significance and response to therapy in various tumor types. Therefore, reliable methods for detecting telomerase activity and ALT are likely to become an important pre-requisite for the use of treatments targeting one or other of these mechanisms. However, the question whether ALT presence can confer sensitivity to rationally designed anti-cancer therapies is still open. Here we review the latest discoveries in terms of mechanisms of ALT activation and maintenance in human tumors, methods for ALT identification in cell lines and human tissues and biomarkers validation. Then, original results on sensitivity to rational based pre-clinical and clinical anti-tumor drugs in ALT vs hTERT positive cells will be presented.

Arsenic trioxide inhibits glioma cell growth through induction of telomerase displacement and telomere dysfunction

Glioblastomas are resistant to many kinds of treatment, including chemotherapy, radiation and other adjuvant therapies. As₂O₃ reportedly induces ROS generation in cells, suggesting it may be able to induce telomerase suppression and telomere dysfunction in glioblastoma cells. We show here that As₂O₃ induces ROS generation as well as telomerase phosphorylation in U87, U251, SHG4 and C6 glioma cells. It also induces translocation of telomerase from the nucleus to the cytoplasm, thereby decreasing total telomerase activity. These effects of As₂O₃ trigger an extensive DNA damage response at the telomere, which includes up-regulation of ATM, ATR, 53BP1, γ-H₂AX and Mer11, in parallel with telomere fusion and 3′-overhang degradation. This ultimately results in induction of p53- and p21-mediated cell apoptosis, G2/M cell cycle arrest and cellular senescence. These results provide new insight into the antitumor effects of As₂O₃ and can perhaps contribute to solving the problem of glioblastoma treatment resistance.

BMI-1 is a potential therapeutic target in diffuse intrinsic pontine glioma

Diffuse intrinsic pontine glioma (DIPG) is a poor-prognosis pediatric brain tumor. No effective curative therapy is currently available and no therapeutic advances have been made in several decades. BMI-1 is a member of the multimeric protein complex Polycomb repressor complex 1. It is highly expressed in a number of diseases and malignancies and has been implicated in self-renewal of normal and cancer cells, and in DNA damage signaling. The role of BMI-1 in DIPG is largely unknown. Here, we show that BMI-1 is highly expressed in tumor tissue samples of DIPG patients and in patient-derived cancer stem-like cells. BMI-1 downregulation leads to the inhibition of DIPG patient-derived neurosphere cell proliferation, cell cycle signaling, self-renewal, telomerase expression and activity, and suppresses DIPG cell migration. Moreover, targeted inhibition of BMI-1 sensitizes DIPG cells to radiomimetic drug-induced DNA damage. Together, our data validate BMI-1 as a potential therapeutic target to treat children with DIPG.

Epigenetic Regulation of Telomere Maintenance for Therapeutic Interventions in Gliomas

High-grade astrocytoma of WHO grade 4 termed glioblastoma multiforme (GBM) is a common human brain tumor with poor patient outcome. Astrocytoma demonstrates two known telomere maintenance mechanisms (TMMs) based on telomerase activity (TA) and on alternative lengthening of telomeres (ALT). ALT is associated with lower tumor grades and better outcome. In contrast to ALT, regulation of TA in tumors by direct mutation and epigenetic activation of the hTERT promoter is well established. Here, we summarize the genetic background of TMMs in non-malignant cells and in cancer, in addition to clinical and pathological features of gliomas. Furthermore, we present new evidence for epigenetic mechanisms (EMs) involved in regulation of ALT and TA with special emphasis on human diffuse gliomas as potential therapeutic drug targets. We discuss the role of TMM associated telomeric chromatin factors such as DNA and histone modifying enzymes and non-coding RNAs including microRNAs and long telomeric TERRA transcripts.

A cancer specific hypermethylation signature of the TERT promoter predicts biochemical relapse in prostate cancer: a retrospective cohort study

The identification of new biomarkers to differentiate between indolent and aggressive prostate tumors is an important unmet need. We examined the role of THOR (TERT Hypermethylated Oncological Region) as a diagnostic and prognostic biomarker in prostate cancer (PCa).We analyzed THOR in common cancers using genome-wide methylation arrays. Methylation status of the whole TERT gene in benign and malignant prostate samples was determined by MeDIP-Seq. The prognostic role of THOR in PCa was assessed by pyrosequencing on discovery and validation cohorts from patients who underwent radical prostatectomy with long-term follow-up data.Most cancers (n = 3056) including PCa (n = 300) exhibited hypermethylation of THOR. THOR was the only region within the TERT gene that is differentially methylated between normal and malignant prostate tissue (p < 0.0001). Also, THOR was significantly hypermethylated in PCa when compared to paired benign tissues (n = 164, p < 0.0001). THOR hypermethylation correlated with Gleason scores and was associated with tumor invasiveness (p = 0.0147). Five years biochemical progression free survival (BPFS) for PCa patients in the discovery cohort was 87% (95% CI 73-100) and 65% (95% CI 52-78) for THOR non-hypermethylated and hypermethylated cancers respectively (p = 0.01). Similar differences in BPFS were noted in the validation cohort (p = 0.03). Importantly, THOR was able to predict outcome in the challenging (Gleason 6 and 7 (3 + 4)) PCa (p = 0.007). For this group, THOR was an independent risk factor for BPFS with a hazard-ratio of 3.685 (p = 0.0247). Finally, THOR hypermethylation more than doubled the risk of recurrence across all PSA levels (OR 2.5, p = 0.02).

hTERT promoter methylation in meningiomas and central nervous hemangiopericytomas

In meningiomas, prognostic impact of mutations in the human telomerase reverse transcriptase (hTERT) promoter region was recently shown, while studies of promoter methylation and analyses of hemangiopericytomas are lacking. hTERT promoter methylation was analyzed in 78 meningioma and 38 meningeal hemangiopericytoma samples by methylation-specific polymerase chain reaction (MS-PCR) and compared with histopathological and clinical variables and with immunohistochemical hTERT expression. Promoter methylation was found in 62 samples (53 %) and tended to be higher in meningiomas (N = 19/41, 46 %) than in hemangiopericytomas (N = 8/33, 24 %, p = .057). In meningiomas, methylation was 16, 60 and 77 % in grade I, II and III tumors (p < .001) and higher in recurrent (N = 33/37, 89 %) than in primary diagnosed (N = 19/41, 46 %) tumors (OR 5.14, 95 % CI 1.34-19.71, p = .017). Univariate analyses showed shorter mean progression free and overall survival in methylated than in unmethylated individuals (26 vs. 100 months; p = .045 and 110 vs. 113 months; p = .025, respectively). Moreover, hTERT expression was found in 70 % (N = 53) and was more frequent in methylated than in unmethylated samples (78 vs. 52 %, OR 3.36, 95 % CI 1.20-9.40, p = .021). In hemangiopericytomas, methylation was similar in grade II (24 %) and III (25 %, p > .05) and in primary (24 %) and recurrent tumors (40 %, p > .05). hTERT expression was similar as compared to meningiomas (74 %, N = 28, p > .05) but was independent of promoter methylation (OR 4.26, 95 % CI 0.47-39.0, p = .199). In meningeal tumors, hTERT promoter methylation is more common than mutations and in meningiomas but not in hemangiopericytomas positively correlated with WHO grade and hTERT expression.

A molecular biology and phase II study of imetelstat (GRN163L) in children with recurrent or refractory central nervous system malignancies: a pediatric brain tumor consortium study

Telomerase activation is critical in many cancers including central nervous system (CNS) tumors. Imetelstat is an oligonucleotide that binds to the template region of the RNA component of telomerase, inhibiting its enzymatic activity. We conducted an investigator-sponsored molecular biology (MB) and phase II study to estimate inhibition of tumor telomerase activity and sustained responses by imetelstat in children with recurrent CNS malignancies. In the MB study, patients with recurrent medulloblastoma, high-grade glioma (HGG) or ependymoma undergoing resection received one dose of imetelstat as a 2-h intravenous infusion at 285 mg/m(2), 12-24 h before surgery. Telomerase activity was evaluated in fresh tumor from surgery. Post-surgery and in the phase II study, patients received imetelstat IV (days 1 and 8 q21-days) at 285 mg/m(2). Imetelstat pharmacokinetic and pharmacodynamic studies were performed. Of two evaluable patients on the MB trial, intratumoral telomerase activity was inhibited by 95 % compared to baseline archival tissue in one patient and was inevaluable in one patient. Forty-two patients (40 evaluable for toxicity) were enrolled: 9 medulloblastomas, 18 HGG, 4 ependymomas, 9 diffuse intrinsic pontine gliomas. Most common grade 3/4 toxicities included thrombocytopenia (32.5 %), lymphopenia (17.5 %), neutropenia (12.5 %), ALT (7.5 %) and AST (5 %) elevation. Two patients died of intratumoral hemorrhage secondary to thrombocytopenia leading to premature study closure. No objective responses were observed. Telomerase inhibition was observed in peripheral blood mononuclear cells (PBMCs) for at least 8 days. Imetelstat demonstrated intratumoral and PBMC target inhibition; the regimen proved too toxic in children with recurrent CNS tumors.

Spatial genomic heterogeneity in diffuse intrinsic pontine and midline high-grade glioma: implications for diagnostic biopsy and targeted therapeutics

Introduction

Diffuse intrinsic pontine glioma (DIPG) and midline high-grade glioma (mHGG) are lethal childhood brain tumors. Spatial genomic heterogeneity has been well-described in adult HGG but has not been comprehensively characterized in pediatric HGG. We performed whole exome sequencing on 38-matched primary, contiguous, and metastatic tumor sites from eight children with DIPG (n = 7) or mHGG (n = 1) collected using a unique MRI-guided autopsy protocol. Validation was performed using Sanger sequencing, Droplet Digital polymerase-chain reaction, immunohistochemistry, and fluorescent in-situ hybridization.

Results

Median age at diagnosis was 6.1 years (range: 2.9–23.3 years). Median overall survival was 13.2 months (range: 11.2–32.2 months). Contiguous tumor infiltration and distant metastases were observed in seven and six patients, respectively, including leptomeningeal dissemination in three DIPGs. Histopathological heterogeneity was evident in seven patients, including intra-pontine heterogeneity in two DIPGs, ranging from World Health Organization grade II to IV astrocytoma. We found conservation of heterozygous K27M mutations in H3F3A (n = 4) or HIST1H3B (n = 3) across all primary, contiguous, and metastatic tumor sites in all DIPGs. ACVR1 (n = 2), PIK3CA (n = 2), FGFR1 (n = 2), and MET (n = 1) were also intra-tumorally conserved. ACVR1 was co-mutated with HIST1H3B (n = 2). In contrast, PDGFRA amplification and mutation were spatially heterogeneous, as were mutations in BCOR (n = 1), ATRX (n = 2), and MYC (n = 1). TP53 aberrations (n = 3 patients) varied by type and location between primary and metastatic tumors sites but were intra-tumorally conserved.

Conclusion

Spatial conservation of prognostically-relevant and therapeutically-targetable somatic mutations in DIPG and mHGG contrasts the significant heterogeneity of driver mutations seen in adult HGG and supports uniform implementation of diagnostic biopsy in DIPG and mHGG to classify molecular risk groups and guide therapeutic strategy.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0269-0) contains supplementary material, which is available to authorized users.

A pilot study of bevacizumab-based therapy in patients with newly diagnosed high-grade gliomas and diffuse intrinsic pontine gliomas

Although bevacizumab has not proven effective in adults with newly diagnosed high-grade gliomas (HGG), feasibility in newly diagnosed children with diffuse intrinsic pontine gliomas (DIPG) or HGG has not been reported in a prospective study. In a safety and feasibility study, children and young adults with newly diagnosed HGG received radiotherapy (RT) with bevacizumab (10 mg/kg: days 22, 36) and temozolomide (75-90 mg/m(2)/day for 42 days) followed by bevacizumab (10 mg/kg, days 1, 15), irinotecan (125 mg/m(2), days 1, 15) and temozolomide (150 mg/m(2)/day days 1-5). DIPG patients did not receive temozolomide. Telomerase activity, quality of life (QOL), and functional outcomes were assessed. Among 27 eligible patients (15 DIPG, 12 HGG), median age 10 years (range 3-29 years), 6 discontinued therapy for toxicity: 2 during RT (grade 4 thrombocytopenia, grade 3 hepatotoxicity) and 4 during maintenance therapy (grade 3: thrombosis, hypertension, skin ulceration, and wound dehiscence). Commonest ≥grade 3 toxicities included lymphopenia, neutropenia and leukopenia. Grade 3 hypertension occurred in 2 patients. No intracranial hemorrhages occurred. For DIPG patients, median overall survival (OS) was 10.4 months. For HGG patients, 3-year progression free survival and OS were 33 % (SE ± 14 %) and 50 % (SE ± 14 %), respectively. All 3 tested tumor samples, demonstrated histone H3.3K27M (n = 2 DIPG) or G34R (n = 1 HGG) mutations. QOL scores improved over the course of therapy. A bevacizumab-based regimen is feasible and tolerable in newly diagnosed children and young adults with HGG and DIPG.

ALTernative Telomere Maintenance and Cancer

Activation of a telomere maintenance mechanism (TMM) is permissive for replicative immortality and a hallmark of human cancer. While most cancers rely on reactivation of telomerase, a significant fraction utilizes the recombination dependent alternative lengthening of telomeres (ALT) pathway. ALT is enriched in tumors of mesenchymal origin, including those arising from bone, soft tissue, and the nervous system, and usually portends a poor prognosis. Recent insights into the mechanisms of ALT are uncovering novel avenues to exploit vulnerabilities and may facilitate clinical development of ALT detection assays and personalized treatment decisions based on TMM status. Treatments targeting ALT may hold promise for a broadly applicable therapeutic modality specific to mesenchymal lineage tumors, something that has thus far remained elusive.

Association of Telomerase Reverse Transcriptase Promoter Mutations with the Prognosis of Glioma Patients: a Meta-Analysis

Previous studies have found that telomerase reverse transcriptase (TERT) has vital roles in the development of malignant diseases including glioma. The occurrence of TERT promoter mutations in gliomas is frequent. So far, several studies on the association between TERT promoter mutations and prognosis of gliomas had been published, but the conclusion was still not uncertain. The aim of the present meta-analysis was to assess the association between TERT promoter mutations and survival of glioma patients by pooling data from published studies. PubMed, Embase, and Web of Science were searched for articles on the association between TERT promoter mutations and survival of glioma patients until June 30, 2015. Hazard ratios (HR) and the 95% confidence intervals (CIs) were utilized to analyze the prognosis of glioma patients with TERT promoter mutations. Heterogeneity of included studies was assessed using Cochrane's Q test and I (2) method. Eleven studies with a total of 3,444 glioma patients were finally included into the meta-analysis. Nine studies reported the HRs adjusting for other confounding factors. Meta-analysis of total 11 studies suggested that TERT promoter mutations were significantly associated with worse prognosis of patients with gliomas (HR = 2.07, 95% CI = 1.58-2.71, P < 0.00001). Meta-analysis of nine studies with adjusted outcomes suggested that TERT promoter mutations were independently associated with worse prognosis of patients with gliomas (HR = 2.28, 95% CI = 1.72-3.01, P < 0.00001). In conclusion, TERT promoter mutation is a promising biomarker for predicting worse prognosis for patients with gliomas. More prospective well-designed cohort studies are needed to further validate its prognostic role in gliomas.

Understanding genetics in neuroimaging

Gene expression is a process of DNA sequence reading into protein synthesis. In cases of problems in DNA repair/apoptosis mechanisms, cells accumulate genomic abnormalities and pass them through generations of cells. The accumulation of mutations causes diseases and even tumors. In addition to cancer, many other neurologic conditions have been associated with genetic mutations. Some trials are testing patients with epigenetic treatments. Epigenetic therapy must be used with caution because epigenetic processes and changes happen constantly in normal cells, giving rise to drug off-target effects. Scientists are making progress in specifically targeting abnormal cells with minimal damage to normal ones.

Telomere maintenance mechanisms in cancer: clinical implications

The presence of immortal cell populations with an up-regulated telomere maintenance mechanism (TMM) is an almost universal characteristic of cancers, whereas normal somatic cells are unable to prevent proliferation-associated telomere shortening and have a limited proliferative potential. TMMs and related aspects of telomere structure and function therefore appear to be ideal targets for the development of anticancer therapeutics. Such treatments would be targeted to a specific cancer-related molecular abnormality, and also be broad-spectrum in that they would be expected to be potentially applicable to most cancers. However, the telomere biology of normal and malignant human cells is a relatively young research field with large numbers of unanswered questions, so the optimal design of TMM-targeted therapeutic approaches remains unclear. This review outlines the opportunities and challenges presented by telomeres and TMMs for clinical management of cancer.

Practical molecular pathologic diagnosis of infiltrating gliomas

Recent advances in molecular diagnostics have led to better understanding of glioma tumorigenesis and biology. Numerous glioma biomarkers with diagnostic, prognostic, and predictive value have been identified. Although some of these markers are already part of the routine clinical management of glioma patients, data regarding others are limited and difficult to apply routinely. In addition, multiple methods for molecular subclassification have been proposed either together with or as an alternative to the current morphologic classification and grading scheme. This article reviews the literature regarding glioma biomarkers and offers a few practical suggestions.

Alternative lengthening of telomeres is enriched in, and impacts survival of TP53 mutant pediatric malignant brain tumors

Although telomeres are maintained in most cancers by telomerase activation, a subset of tumors utilize alternative lengthening of telomeres (ALT) to sustain self-renewal capacity. In order to study the prevalence and significance of ALT in childhood brain tumors we screened 517 pediatric brain tumors using the novel C-circle assay. We examined the association of ALT with alterations in genes found to segregate with specific histological phenotypes and with clinical outcome. ALT was detected almost exclusively in malignant tumors (p = 0.001). ALT was highly enriched in primitive neuroectodermal tumors (12 %), choroid plexus carcinomas (23 %) and high-grade gliomas (22 %). Furthermore, in contrast to adult gliomas, pediatric low grade gliomas which progressed to high-grade tumors did not exhibit the ALT phenotype. Somatic but not germline TP53 mutations were highly associated with ALT (p = 1.01 × 10(-8)). Of the other alterations examined, only ATRX point mutations and reduced expression were associated with the ALT phenotype (p = 0.0005). Interestingly, ALT attenuated the poor outcome conferred by TP53 mutations in specific pediatric brain tumors. Due to very poor prognosis, one year overall survival was quantified in malignant gliomas, while in children with choroid plexus carcinoma, five year overall survival was investigated. For children with TP53 mutant malignant gliomas, one year overall survival was 63 ± 12 and 23 ± 10 % for ALT positive and negative tumors, respectively (p = 0.03), while for children with TP53 mutant choroid plexus carcinomas, 5 years overall survival was 67 ± 19 and 27 ± 13 % for ALT positive and negative tumors, respectively (p = 0.07). These observations suggest that the presence of ALT is limited to a specific group of childhood brain cancers which harbor somatic TP53 mutations and may influence the outcome of these patients. Analysis of ALT may contribute to risk stratification and targeted therapies to improve outcome for these children.