BACKGROUND: Gliomas are a deadly group of childhood and adult cancers associated with high relapse rate following current therapies. Limitless self-renewal is a hallmark of cancer recurrence and is controlled by telomerase activation and telomere maintenance. We have recently uncovered THOR (TERT Hypermethylated Oncological Region) which is paradoxically hypermethylated in gliomas with telomerase activation. In order to further explore the biological impact of THOR hypermethylation on self renewal and telomere maintenance of gliomas we undertook a stepwise approach. METHODS: RESULTS: Mapping of the human TERT promoter reveals that THOR spans 432 BP and comprises 52 CG sites. In contrast, the area where mutations in TERT promoter were uncovered is permanently hypomethylated. Promoter driven expression was analysed through luciferase assays and unveiled a repressive effect of THOR on the TERT promoter. Moreover, TERT mutations promoted hyperactivation of the reporter gene providing explanations for the methylation pattern observed in malignant gliomas. THOR methylation increases in gliomas as they transform from low to high grade and from primary tumor to established cell lines (p < 0.001). Analysis of allelic Tert expression reveals that THOR is initally methylated in the mutant allele and throughout tumor progression, the other allele eventually becomes methylated. This correlates with higher TERT expression. In contrast, tumors utilizing alternative lengthening of telomeres (ALT) lack THOR methylation and TERT mutations. Glioma stem cells (n = 32) are addicted to telomerase and have higher THOR methylation than the bulk tumor. Strikingly, glioblastomas which activate ALT lack this phenotype in their stem cells compartment. Whole exome sequencing reveals multiple ALT related mutations (TP53 and ATRX) which are present in the mature tumor cells subpopulation and absent in the TERT expressing stem cell subpopulation. THOR demethylation with Decitabine results in loss of telomerase activation only in tumor cells and not in normal and embryonic stem cells which lack THOR methylation. Combining telomerase inhibition with decitabine result in permanent loss of self renewal capacity of patient derived cell lines in vitro and lack of tumor formation in vivo. CONCLUSIONS: We offer a model and a novel classification of gliomas based on THOR hypermethylation and alterations in the telomere maintenance pathway. We also suggest that intratumoral heterogeneity in telomere maintenance might be the result of secondary hits in the non-stem cell compartment Finally, we propose that THOR demethylation can be a safe and viable option for exhausting the self renewal capacity of gliomas. SECONDARY CATEGORY: Pediatrics.