Written in development and lost in ageing: the grammar of cellular identity.

Ageing reflects progressive decanalization of mammalian cell identity driven by erosion of a regulatory grammar established during development, with most age-related DNA methylation gain occurring at PRC2-bound regions.

• Why it matters: Understanding the mechanistic basis of ageing remains unresolved despite numerous theories, and clarifying how cellular identity deteriorates could inform interventions to preserve tissue function and lifespan.
• What they did: The study extends Waddington's epigenetic landscape to show development establishes chromatin-based identity constraints, which are gradually lost with age due to limited fidelity in maintenance mechanisms, especially involving PRC2 and H3K4/36 methylation.
• The result: This model explains cross-tissue ageing signatures and the robustness of epigenetic clocks, and it suggests that stabilizing chromatin architecture may help preserve cellular identity and function during ageing.