CRISPR/Cas9 editing of directly reprogrammed myogenic progenitors restores dystrophin expression in a dystrophic mouse model

Genetic mutations in dystrophin manifest in Duchenne muscular dystrophy (DMD), the most prevalent form of a genetically inherited muscle disease. Dystrophin is expressed in skeletal muscle stem cells and fibers, playing a critical role in maintaining skeletal muscle structure, regeneration and function. Here, we report on direct reprogramming of fibroblasts from the Dmdmdx mouse model into induced myogenic progenitor cells (Dmdmdx iMPCs) utilizing transient MyoD overexpression in concert with small molecule treatment. Dmdmdx iMPCs proliferate extensively in vitro and express canonical skeletal muscle stem and progenitor cell markers including Pax7, Sox8 and Myf5. Furthermore, Dmdmdx iMPCs readily give rise to highly contractile and multinucleated myofibers that express a suite of mature skeletal muscle genes however lack dystrophin expression. Utilizing an exon-skipping based approach with CRISPR/Cas9 we report on a genetic correction of the dystrophin mutation in Dmdmdx-iMPCs and subsequent restoration of dystrophin protein expression in vitro. Furthermore, engraftment of genetically-corrected Dmdmdx iMPCs into dystrophic limb muscles of Dmdmdx mice restored dystrophin expression in vivo. Collectively, our findings report on a novel in vitro stem cell-based model for DMD and further establish a new approach to treat this disease via a combination of direct cellular reprogramming, genome engineering and stem cell transplantation methods. Overall design: Reprogramming of Dmdmdx fibroblasts into myogenic progenitor cells using ectopic MyoD overexpression in combination with small molecule (F/R/C) treatment