Assessment of restoration of an internally deleted dystrophin as a surrogate biomarker for future clinical trials of a gene editing therapy in Duchenne muscular dystrophy

Project Summary Duchenne muscular dystrophy is a rare genetic muscle disease affecting ~16,000 patients in the US. It leads to premature death in the twenties to thirties and has no cure. Gene editing offers an innovative solution for Duchenne since it can be used to permanently remove patient mutations in the endogenous gene and restore an internally-deleted and functional dystrophin protein. The gene editing therapy described here, MyoDys45-55, uses adeno-associated virus (AAV) delivery of CRISPR/Cas9 to delete exons 45-55 in the DMD gene, correcting the reading frame for 50% of all Duchenne patients. The restored dystrophin is three times larger than the mini/micro- dystrophins introduced by gene replacement and is associated with a very mild or asymptomatic form of Becker muscular dystrophy. As MyoDys45-55 progresses towards clinical trials, preclinical data are critically needed to characterize the relationship between the amount of restored dystrophin and improvements in function. Here, pups from a humanized mouse model of Duchenne will be administered a range of AAV9-MyoDys45-55 doses and non-invasive functional testing, physiology, and quantification of molecular outcomes will be conducted. Analysis will be performed to assess how restoration of the gene edited dystrophin protein correlates to functional benefit. These data will be used to inform trial design (including dose selection) and to support the use of the exon 45-55 deleted dystrophin as a clinical biomarker and potential surrogate endpoint for accelerated approval. Importantly, there is clear regulatory precedent for this pathway since five other Duchenne drugs have received accelerated approval based on expression of different internally-deleted dystrophins. But FDA has also stressed the need for program-specific preclinical and clinical evidence. Here we will perform these preclinical studies to support clinical trials that we will initiate in the near future. This work is critical to advance MyoDys45-55 to patients as an innovative solution for a large proportion of the Duchenne population.