Telomerase as a therapeutic target in glioblastoma

Glioblastoma is the most common primary malignant brain tumor in adults and it continues to have a dismal prognosis. The development of targeted therapeutics has been particularly challenging, in part due to a limited number of oncogenic mutations and significant intra-tumoral heterogeneity. TERT promoter mutations were first discovered in melanoma and later found to be present in up to 80% of glioblastoma samples. They are also frequent clonal alterations in this tumor. TERT promoter mutations are one of the mechanisms for telomerase reactivation, providing cancers with cellular immortality. Telomerase is a reverse transcriptase ribonucleoprotein complex that maintains telomere length in cells with high proliferative ability. In this article we present genomic and pre-clinical data that supports telomerase as a potential “Achilles’ heel” for glioblastoma. We also summarize prior experience with anti-telomerase agents and potential new approaches to tackle this target.

Cancer-specific loss of TERT activation sensitizes glioblastoma to DNA damage

Most glioblastomas (GBMs) achieve cellular immortality by acquiring a mutation in the telomerase reverse transcriptase (TERT) promoter. TERT promoter mutations create a binding site for a GA binding protein (GABP) transcription factor complex, whose assembly at the promoter is associated with TERT reactivation and telomere maintenance. Here, we demonstrate increased binding of a specific GABPB1L-isoform–containing complex to the mutant TERT promoter. Furthermore, we find that TERT promoter mutant GBM cells, unlike wild-type cells, exhibit a critical near-term dependence on GABPB1L for proliferation, notably also posttumor establishment in vivo. Up-regulation of the protein paralogue GABPB2, which is normally expressed at very low levels, can rescue this dependence. More importantly, when combined with frontline temozolomide (TMZ) chemotherapy, inducible GABPB1L knockdown and the associated TERT reduction led to an impaired DNA damage response that resulted in profoundly reduced growth of intracranial GBM tumors. Together, these findings provide insights into the mechanism of cancer-specific TERT regulation, uncover rapid effects of GABPB1L-mediated TERT suppression in GBM maintenance, and establish GABPB1L inhibition in combination with chemotherapy as a therapeutic strategy for TERT promoter mutant GBM.

Cancer-specific loss of TERT activation sensitizes glioblastoma to DNA damage

Most glioblastomas (GBMs) achieve cellular immortality by acquiring a mutation in the telomerase reverse transcriptase (TERT) promoter. TERT promoter mutations create a binding site for a GA binding protein (GABP) transcription factor complex, whose expression is associated with TERT reactivation and telomere maintenance. Here, using biochemical and cell biology approaches, we show direct evidence that a specific GABP complex containing the subunit protein GABPB1L forms predominantly at the mutant TERT promoter, leading to TERT re-expression. Furthermore, we find that TERT promoter mutant GBM cells, unlike wild-type cells, are immediately dependent on GABPB1L for proliferation in cell culture and post-tumor establishment in vivo. Notably, when combined with frontline temozolomide (TMZ) chemotherapy, GABPB1L knockdown and the associated TERT reduction lead to an impaired DNA damage response that results in profoundly reduced growth of intracranial GBM tumors. Together, these findings provide new insights into the mechanism of cancer-specific TERT regulation, uncover rapid effects of TERT suppression in GBM maintenance, and establish GABPB1L inhibition, alone or in combination with chemotherapy, as a therapeutic strategy for TERTp mutant GBM.

Telomerase reverse transcriptase gene amplification in hematological malignancies

Background Telomere is a complex DNA–protein structure located at the end of all eukaryotic chromosomes. The major role of human telomerase is to catalyze the addition of telomeric repeat sequences TTAGGG onto chromosome ends for stabilization of telomere length in attaining cellular immortality and may therefore be a critical step in carcinogenesis. Expression of significant levels of telomerase can dramatically increase proliferative life span and promote cellular immortality, thereby contributing to the malignant phenotype. The purpose of this study is to investigate telomerase reverse transcriptase (TERT) gene amplification in hematological neoplasms, e.g., multiple myeloma (MM), B-non-Hodgkin lymphoma (B-NHL), and acute myeloid leukemia (AML), using FISH technique and to evaluate its potential use as a prognostic marker. Results TERT amplification was detected in all groups of the participant patients (15 MM, 15 B-NHL, and 15 AML patients), with higher incidence in AML patients (53.3%). A significant association between the pattern of presentation and telomerase amplification was detected in 88.9% of the relapsed patients who demonstrated amplification of TERT. TERT amplification shows a significant association with p53 deletion and a highly significant association with poor prognosis. Conclusions TERT gene amplification is significantly associated with hematological malignancies and may play a critical role in carcinogenesis; thus, elucidation of their regulatory mechanism is highly demanding. Higher amplification was found in relapsed cases than de novo cases which highlight its potential implication in clinical analysis and disease monitoring. Moreover, our results suggest the future use of TERT gene as a potential prognostic marker that may aid in treatment decision and chemotherapy.

Cancer cells with alternative lengthening of telomeres do not display a general hypersensitivity to ATR inhibition

Telomere maintenance is a hallmark of cancer as it provides cancer cells with cellular immortality. A significant fraction of tumors uses the alternative lengthening of telomeres (ALT) pathway to elongate their telomeres and to gain an unlimited proliferation potential. Since the ALT pathway is unique to cancer cells, it represents a potentially valuable, currently unexploited target for anticancer therapies. Recently, it was proposed that ALT renders cells hypersensitive to ataxia telangiectasia-and RAD3-related (ATR) protein inhibitors (Flynn et al., Science 347, 273). Here, we measured the response of various ALT or telomerase positive cell lines to the ATR inhibitor VE-821. In addition, we compared the effect of the inhibitor on cell viability in an isogenic cell line, in which ALT was active or suppressed. In these experiments a general ATR inhibitor sensitivity of cells with ALT could not be confirmed. We rather propose that the observed variations in sensitivity reflect differences between cell lines that are unrelated to ALT.

Telomeric recombination induced by dysfunctional telomeres

Telomere maintenance is essential for cellular immortality, and most cancer cells maintain their telomeres through the enzyme telomerase. Telomeres and telomerase represent promising anticancer targets. However, 15% of cancer cells maintain their telomeres through alternative recombination-based mechanisms, and previous analyses showed that recombination-based telomere maintenance can be activated after telomerase inhibition. We determined whether telomeric recombination can also be promoted by telomere dysfunction. We report for the first time that telomeric recombination can be induced in human telomerase-positive cancer cells with dysfunctional telomeres.