Intrinsic dysfunction in muscle stem cells lacking dystrophin begins during secondary myogenesis.

Loss of dystrophin causes Duchenne Muscular Dystrophy (DMD), a neuromuscular disease characterized by muscle fragility and muscle stem cell (MuSC) impairment. Conventional understanding is that DMD manifests after birth from cumulative muscle damage. Here, examination of mdx mouse embryos lacking dystrophin reveals no impairment of the primary myogenic program. By contrast, histological and single cell RNA-sequencing analysis during secondary myogenesis uncovers an increase in the proportion of fetal (f) MuSCs and a marked reduction in myogenic progenitors and myocytes, leading to fewer smaller-caliber myofibers. Wild type fMuSCs express full-length dystrophin that interacts with MARK2, whereas mdx fMuSCs downregulate MARK2 and NUMB, exhibiting reduced PARD3 polarization. Strikingly, deletion of the Numb Associated Kinase, AAK1, rescues polarization of NUMB and myogenic progenitor generation in mdx fetal muscle. Together, our results elucidate an acute disease pathology during DMD fetal development and the potential for therapeutic intervention by targeting AAK1.