MBNL depletion drives stem cell fusion and immature myonuclear states in myotonic dystrophy type 1

Myotonic dystrophy type 1 is caused by the expression of expanded CTG repeats in the DMPK gene and the resulting loss of function of MBNL protein. Affected skeletal muscle displays abundant centrally located nuclei despite limited immune-cell–associated fibre necrosis, complicating interpretation of muscle damage and remodelling mechanisms. Here we show that muscle stem cells are activated and fuse with existing muscle fibres in myotonic dystrophy type 1. Single-nucleus transcriptomics in patient’s muscle identifies increased activated muscle stem cells and distinct myonuclear populations exhibiting transitional transcriptional states, including muscle stem cell associated markers and elevated DMPK expression. Myofibre-specific MBNL knockdown mouse models demonstrate that muscle stem cell fusion contributes to central nucleation, whereas their deletion does not improve myotonia or muscle strength. Together, these findings indicate that loss of MBNL function in muscle drives myonuclear accretion through stem cell-mediated fusion, giving rise to myonuclei with immature states in myotonic dystrophy type 1.