How Cancer Hijacks Telomere Repair to Achieve Immortality

Telomeres—repetitive TTAGGG sequences capping human chromosomes—normally shorten with each cell division, eventually triggering senescence or apoptosis as a built-in tumor-suppression mechanism. The shelterin protein complex (TRF1, TRF2, RAP1, TIN2, TPP1, POT1) stabilizes telomere architecture, forming T-loops and D-loops that prevent DNA damage response (DDR) machinery from mistaking chromosome ends for double-strand breaks. When telomeres erode critically, shelterin proteins dissociate, DDR activates, and the cell halts division—a process cancer must override to proliferate indefinitely.

This 2025 review from Wroclaw Medical University examines precisely how cancer accomplishes that override. Approximately 85–90% of tumors reactivate telomerase, a reverse transcriptase composed of the catalytic TERT subunit and the RNA template TERC. TERT overexpression arises via promoter mutations, epigenetic methylation changes, chromosomal translocations (notably in B-cell neoplasms), or transcriptional activation by oncogenes including MYC, NF-κB, and β-catenin. The remaining 10–15% of cancers—particularly sarcomas and glioblastomas—use the ALT pathway, an HR-based mechanism that copies telomeric sequence from existing telomeres without telomerase. ALT is strongly associated with loss-of-function mutations in the ATRX/DAXX chromatin-remodeling complex and SMARCAL1, which together promote G-quadruplex and R-loop formation, driving recombination-based telomere elongation and contributing to therapy resistance.

The review devotes considerable attention to the bidirectional interplay between telomere maintenance and DNA repair. Shelterin components actively suppress homologous recombination (HR) and non-homologous end joining (NHEJ) at telomeres, while paradoxically, cancer cells co-opt HR machinery to fuel ALT. Telomere dysfunction caused by critically short telomeres or shelterin disruption (as seen in EBV-driven Hodgkin lymphoma via LMP1 oncoprotein) can trigger breakage-fusion-bridge cycles, chromosomal instability, and ultimately malignant transformation. Cancer cells bearing 'T-stump' telomeres—nearly devoid of repeats yet still bound by TRF1/TRF2—illustrate how high telomerase activity and defective checkpoints allow survival despite extreme telomere erosion.

Therapeutic strategies reviewed span multiple modalities. Telomerase inhibitors (e.g., imetelstat, a competitive oligonucleotide inhibitor of TERC) have shown clinical activity in myelofibrosis and myelodysplastic syndromes. G-quadruplex-stabilizing ligands selectively disrupt telomere structure and impair telomerase access. TERT promoter-targeted approaches, including CRISPR-based editing and transcriptional repression, are under preclinical investigation. For ALT-positive tumors, targeting HR factors (RAD51, BRCA1/2) or exploiting synthetic lethality with PARP inhibitors offers emerging avenues. Immunotherapy leveraging TERT-derived peptides as tumor-specific antigens is in early-phase trials. Natural compounds—including EGCG, curcumin, and quercetin—show telomerase-inhibitory activity in vitro, though clinical translation remains limited.

The authors candidly address key limitations: telomerase inhibitors require a lag period before telomere shortening causes tumor regression; normal stem cells expressing telomerase (gut epithelium, hematopoietic cells) may suffer off-target toxicity; and ALT-positive tumors can be especially refractory because they bypass telomerase entirely. Biomarkers such as telomere length, TERT promoter mutation status, ALT-associated PML bodies, and C-circles are highlighted as tools to guide patient selection and monitor therapeutic response. The review concludes that a precision medicine framework—matching patients to telomere-targeting strategies based on their tumor's specific maintenance mechanism—represents the most promising path forward.