Paradoxical DNA methylation switches on telomerase across many cancers

Telomerase reactivation is a hallmark of cancer, letting malignant cells avoid the telomere shortening that would otherwise limit their division. The best-known cause is a pair of TERT promoter mutations (C228T and C250T), but many tumors express telomerase without them. This gap in understanding has drawn attention to epigenetic regulation, particularly a region called THOR (TERT Hypermethylated Oncological Region).

This is a narrative mini-review, not a new experiment. The authors synthesize studies from the late 1990s to 2024, summarized in a chronological table. The studies range from early cell-line work to the 2019 pan-cancer study that formally defined THOR. They use methods such as bisulfite sequencing, pyrosequencing, methylation-specific PCR and targeted sequencing. The central idea is that THOR, located upstream of the core promoter, behaves paradoxically. Its hypermethylation is associated with increased TERT expression, whereas the proximal promoter around the transcription start site stays hypomethylated and accessible. A proposed mechanism is that methylation in THOR blocks repressors such as CTCF, while an open proximal promoter with active histone marks lets RNA polymerase II drive transcription. This distal-versus-proximal distinction may explain older, seemingly contradictory findings that treated the promoter as a single unit.

The review reports THOR methylation across gliomas, hepatocellular, prostate, thyroid, gastric, breast and bladder cancers, and hematological malignancies including leukemias, B-cell lymphomas and cutaneous T-cell lymphoma. It often occurs in tumors lacking TERT promoter mutations. In bladder cancer, one cited study reported 60% TERT promoter mutations alongside 88% THOR hypermethylation. In hematological cancers, where promoter mutations are uncommon, THOR methylation may be a dominant activation route. Associations with aggressive behavior include prostate cancer relapse, gastric cancer lymph node metastasis and poor outcomes in papillary thyroid cancer. THOR also appears to distinguish malignant from benign cells in breast cancer and CTCL. Demethylation reduced TERT expression in thyroid cancer models.

The authors suggest THOR methylation could refine classification of 'TERT promoter wild-type' tumors and serve as a diagnostic or prognostic marker. Because it is epigenetic, it may be well suited to detection in circulating tumor DNA, though this is still an emerging concept. Therapeutically, DNA methyltransferase inhibitors act genome-wide and lack specificity, so locus-specific epigenome editing is proposed as a future strategy.

The review also notes that THOR is one part of a wider network that includes promoter mutations, copy-number gains, structural rearrangements and ALT (alternative lengthening of telomeres), not a universal mechanism. Key caveats are inconsistent assay design, CpG coverage and thresholds across studies, an unresolved relationship with TERT promoter mutations, and a lack of prospective clinical validation. The authors' framing of THOR as 'central and underrecognized' is a proposal, not an established fact.