sdRNA-D21, a fragment derived from small nucleolar RNA, is overexpressed in osteoarthritic cartilage and its abundance correlates with knee OA severity, suggesting a driver role in disease progression.

Mechanistically, sdRNA-D21 binds BAG2 mRNA and disrupts BAG2-mediated ER-phagy by preventing p62 recruitment, leading to accumulation of stressed ER and increased chondrocyte apoptosis.

A second mechanism shows sdRNA-D21 suppressing DEPTOR, which releases TRC8 to amplify ER stress signaling and further promote chondrocyte stress and death.

In a rat OA model, silencing sdRNA-D21 alleviates joint degeneration, supporting its active contribution to disease beyond a mere biomarker.

Functional studies indicate that increasing sdRNA-D21 promotes chondrocyte apoptosis, while knockdown improves cell survival and reduces OA-like damage in lab models.

The dual impairment of ER-phagy and heightened ER stress signaling forms a coherent pathway by which this single snoRNA-derived fragment drives cartilage degeneration.

This work shifts the view of non-coding RNA fragments from biomarkers to active disease drivers, laying a foundation for targeted interventions.

Therapeutic implications include antisense strategies to neutralize sdRNA-D21 or modulate BAG2-mediated ER-phagy and the DEPTOR-TRC8 axis, though delivery to cartilage and validation in larger patient cohorts remain challenges.