Neural Tumor-Initiating Cells Have Distinct Telomere Maintenance and Can be Safely Targeted for Telomerase Inhibition

Abstract

Purpose: Cancer recurrence is one of the major setbacks in oncology. Maintaining telomeres is essential for sustaining the limitless replicative potential of such cancers. Because telomerase is thought to be active in all tumor cells and normal stem cells, telomerase inhibition may be nonspecific and have detrimental effects on tissue maintenance and development by affecting normal stem cell self-renewal.Methods: We examined telomerase activity, telomere maintenance, and stem cell maturation in tumor subpopulations from freshly resected gliomas, long-term, primary, neural tumor-initiating cells (TIC) and corresponding normal stem cell lines. We then tested the efficacy of the telomerase inhibitor Imetelstat on propagation and self-renewal capacity of TIC and normal stem cells in vitro and in vivo.Results: Telomerase was undetectable in the majority of tumor cells and specific to the TIC subpopulation that possessed critically short telomeres. In contrast, normal tissue stem cells had longer telomeres and undetectable telomerase activity and were insensitive to telomerase inhibition, which results in proliferation arrest, cell maturation, and DNA damage in neural TIC. Significant survival benefit and late tumor growth arrest of neuroblastoma TIC were observed in a xenograft model (P ¼ 0.02). Furthermore, neural TIC exhibited irreversible loss of self-renewal and stem cell capabilities even after cessation of treatment in vitro and in vivo.Conclusions: TIC exhaustion with telomerase inhibition and lack of telomerase dependency in normal stem cells add new dimensions to the telomere hypothesis and suggest that targeting TIC with telomerase inhibitors may represent a specific and safe therapeutic approach for tumors of neural origin.

BRAF-KIAA1549 Fusion Predicts Better Clinical Outcome in Pediatric Low-Grade Astrocytoma

“…To test this hypothesis, we overexpressed BRAF in hTERT-immortalized human astrocytes, as previously published by our group (14), and reversed the hTERT immortalization by telomerase inhibition (ref. 24, Fig. 2).…”

“…Telomerase inhibition was achieved by treatment with Imetelstat (5 mmol/L, Geron Corp). Mismatch (MIS) scrambled RNA served as treatment control as previously reported (24). Further information of experimental procedures, b-galactosidase activity, and immunofluorescence are available in Supplementary Methods.…”

Neuroblastoma stem cells – mechanisms of chemoresistance and histonedeacetylase inhibitors

“…Furthermore, normal tissue stem cells lack telomerase activity and therefore are insensitive to telomerase inhibition, which is a highly specific and effective anti-CSC therapy. Furthermore, as opposed to conventional chemotherapies, telomerase inhibition resulted in CSC exhaustion by irreversibly altering their self-renewal kapacity [59].…”

Promises and challenges of exhausting pediatric neural cancer stem cells

“…In addition, telomerase inhibition by the specific inhibitor imetelstat resulted in proliferation arrest, cell maturation, and DNA damage in pediatric neural CSCs. Strikingly, CSCs exhibited irreversible loss of self-renewal and stem cell capabilities even after cessation of treatment in vitro and in vivo in a neuroblastoma model (49). This unique observation may indicate that these CSCs have lost the ability to cause tumor recurrence.…”

“…Another illustration that highlights the differences between normal and malignant stem cells is the observation that telomerase activation is specific and critical for neuroblastoma and glioblastoma CSCs but is not required by normal neural stem cells (49). These differences could be used as an Achilles' heel to treat CSCs while preserving the normal brain.…”