Rewiring Fibroblast–Muscle Axis Drives Progressive Pathology in Bethlem Myopathy

Collagen VI–related myopathies, including Bethlem myopathy (BM), are progressive muscle disorders, but the mechanisms driving age-dependent disease progression remain poorly understood. Here, we used a zebrafish BM model carrying an exon-skipping mutation that generates a shorter collagen VI α1 chain and disrupts supramolecular assembly, recapitulating key features of the human disease. We further demonstrated that this model reproduces disease progression, with worsening muscle wasting, increased myofiber size variability, and age-associated skeletal deformities consistent with secondary consequences of muscle dysfunction rather than intrinsic bone defects. Single-nucleus RNA sequencing of trunk skeletal muscle revealed an early shift in cellular composition, with reduced myonuclei and increased fibroblast abundance, indicative of disease-associated aging. Myonuclei activated stress and quality control pathways, including autophagy and mitophagy, along with metabolic rewiring. In contrast, fibroblasts displayed early translational activation followed by progressive proteostatic and endoplasmic reticulum stress. At later stages, fibroblasts adopted a pro-fibrotic state, driving extracellular matrix remodeling and enhanced muscle–fibroblast communication. Consistently, analyses at the protein level confirmed early intracellular retention of the mutant protein, along with increased extracellular matrix deposition and fibrotic tissue formation in BM muscle. Among the three tested drugs targeting ER-stress and protein degradation, only TUDCA significantly ameliorated collagen VI deposition in the extracellular space in larvae. These findings identify fibroblasts as key drivers of disease progression and potential therapeutic targets.