MBNL Depletion Induces Muscle Stem Cell Activation and Myofibre Fusion while Impairing Myonuclear Maturation in Myotonic Dystrophy Type 1

Whether skeletal muscle in myotonic dystrophy type 1 (DM1) maintains its regenerative potential or shows inherent muscle stem cell (MuSC) defects remains unresolved. Here, we examine MuSC function in DM1 by combining an adult-onset Mbnl1/2 knockout mouse model with single-nucleus RNA sequencing (snRNA-seq) of human DM1 muscle biopsies. Loss of Mbnl in mouse myofibres triggers MuSC activation and fusion, resulting in central nuclear accumulation independent of degeneration. Lineage tracing confirms direct MuSC contribution to these nuclei, a process abolished upon MuSC ablation. In human DM1 specimens, we identify activated MuSCs and a myonuclear population with an immature transcriptional signature including expression of MuSC markers, and elevated DMPK levels. These findings imply that MuSCs in DM1 contribute to muscle repair by fusing into myofibres but give rise to immature myonuclei, likely due to toxic RNA effects on transcriptional reprogramming. Our study emphasises a previously overlooked role of MuSCs in DM1 pathology and identifies potential targets to support muscle regeneration.