Direct Reprogramming of Human DMD Fibroblasts into Myotubes for In Vitro Evaluation of Antisense-Mediated Exon Skipping and Exons 45-55 Skipping Accompanied by Rescue of Dystrophin Expression

Antisense oligonucleotide-mediated exon skipping is a promising therapeutic approach for the treatment of various genetic diseases and a therapy which has gained significant traction in recent years following FDA approval of new antisense-based drugs. Exon skipping for Duchenne muscular dystrophy (DMD) works by modulating dystrophin pre-mRNA splicing, preventing incorporation of frame-disrupting exons into the final mRNA product while maintaining the open reading frame, to produce a shortened-yet-functional protein as seen in milder Becker muscular dystrophy (BMD) patients. Exons 45-55 skipping in dystrophin is potentially applicable to approximately 47% of DMD patients because many mutations occur within this "mutation hotspot." In addition, patients naturally harboring a dystrophin exons 45-55 in-frame deletion mutation have an asymptomatic or exceptionally mild phenotype compared to shorter in-frame deletion mutations in this region. As such, exons 45-55 skipping could transform the DMD phenotype into an asymptomatic or very mild BMD phenotype and rescue nearly a half of DMD patients. In addition, this strategy is potentially applicable to some BMD patients as well, who have in-frame deletion mutations in this region. As the degree of exon skipping correlates with therapeutic outcomes, reliable measurements of exon skipping efficiencies are essential to the development of novel antisense-mediated exon skipping therapeutics. In the case of DMD, researchers have often relied upon human muscle fibers obtained from muscle biopsies for testing; however, this method is highly invasive and patient myofibers can display limited proliferative ability. To overcome these challenges, researchers can generate myofibers from patient fibroblast cells by transducing the cells with a viral vector containing MyoD, a myogenic regulatory factor. Here, we describe a methodology for assessing dystrophin exons 45-55 multiple skipping efficiency using antisense oligonucleotides in human muscle cells derived from DMD patient fibroblast cells.