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Mechanism

ALT (alternative lengthening of telomeres)

Reference definition for a telomere-biology node.

Telomerase-independent lengthening
Two caveats govern every page on this site. (1) Telomere length is a contested aging biomarker — it is measured differently by different methods, varies between tissues, and "longer" is not straightforwardly "healthier"; changing a telomere-length or telomerase-activity number in a trial is not the same as slowing human aging. (2) Activating telomerase is not risk-free — telomerase expression is low in many differentiated normal tissues, while some normal proliferating cells can express it. Cancer can also use telomere maintenance; the included human records do not resolve long-term cancer risk from pharmacological activation. Tellingly, the FDA-approved telomerase inhibitor — imetelstat (RYTELO, 2024) — inhibits telomerase to treat cancer (lower-risk MDS), the opposite direction from the anti-aging "activation" narrative.

Definition

Category: Telomerase-independent lengthening

Also known as: ALT pathway, telomerase-independent lengthening

ALT is a telomerase-independent way that some cells maintain or lengthen telomeres, using recombination-based copying between telomeric sequences. It is used by a subset of human cancers, including certain sarcomas and gliomas. Telomeric copying has also been detected in normal mouse somatic tissues; these model findings do not establish ALT as a human anti-aging intervention.

Key points

  • ALT is a recombination-based mechanism, distinct from telomerase, and is a hallmark of a subset of tumours — a reminder that telomere lengthening is co-opted by cancer.
  • ALT-positive cancers show characteristic features (ALT-associated PML bodies, heterogeneous telomere lengths) used as biomarkers.
  • This node exists to keep the honest framing: telomere maintenance is something cancers actively acquire, by telomerase reactivation or by ALT.

Sourcing

Normal somatic-tissue copying: Neumann et al., Genes & Development 2013, DOI 10.1101/gad.205062.112, full original Results/Discussion and Methods read 2026-10-14. Human tumor features require the separate original tumor evidence; normal-mouse findings are not human treatment results.

Reference synthesis (mechanism context, not a human-trial evidence tier).