DNA hypermethylation within TERT promoter upregulates TERT expression in cancer

Replicative immortality is a hallmark of cancer governed by telomere maintenance. About 90% of human cancers maintain their telomeres by activating telomerase, driven by transcriptional upregulation of telomerase reverse transcriptase (TERT). Although TERT promoter mutations (TPMs) are a major cancer-associated genetic mechanism of TERT upregulation, many cancers exhibit TERT upregulation without TPMs. In this study, we described TERT Hypermethylated Oncological Region (THOR), a 433-bp genomic region encompassing 52 CpG sites located immediately upstream of the TERT core promoter, as a cancer-associated epigenetic mechanism of TERT upregulation. Unmethylated THOR repressed TERT promoter activity regardless of TPMs status, and hypermethylation of THOR counteracted this repressive function. THOR methylation analysis in 1,352 human tumors revealed frequent (>45%) cancer-associated DNA hypermethylation in 9 of 11 (82%) tumor types screened. Additionally, THOR hypermethylation - either independently or along with TPMs - accounted for how approximately 90% of human cancers can aberrantly activate telomerase. Thus, we propose THOR hypermethylation as a prevalent telomerase activating mechanism in cancer that can act independently or in conjunction with TPMs, further supporting the utility of THOR hypermethylation as a prognostic biomarker.