ALT (alternative lengthening of telomeres): telomere mechanism

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 is silenced in most somatic cells and reactivated in most cancers, so its activation raises an unresolved cancer-risk question. Tellingly, the only FDA-approved telomerase drug — 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 homologous-recombination-based copying between telomeric sequences. It is found in a subset of human cancers (particularly certain sarcomas and gliomas) rather than in normal aging tissue, and is important context for why 'lengthening telomeres' is not a uniformly benign goal.

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

Standard ALT reviews (Cesare & Reddel, 'Alternative lengthening of telomeres'). Review-level description.

Reference synthesis (tier 4); verification: review_level_2026-07-12.