Dystrophins DP71 and DP427 determine cell viability during proliferation and myofibre differentiation

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by mutations in the dystrophin gene. DMD manifests with progressive skeletal muscle wasting, cardiac dysfunction, and cognitive and neuropsychiatric symptoms. As the disease progresses, affected boys lose their ability to walk and die prematurely. All DMD patients lack the full-length DP427 dystrophin, whereas approximately 10% lose all dystrophins, including DP71, synthesized in various cell types, including myoblasts. There is evidence that the cognitive symptoms of DMD patients vary depending on the location of the mutation site and the number of missing dystrophins, and there is some indication that this may also be the case in skeletal muscle. We therefore investigated the roles of DP427 and DP71 during cell proliferation and fibre differentiation. We included paralogous utrophins in our analyses, particularly the UP395 isoform, owing to its presence in muscle and its partial ability to compensate for the lack of DP427. We demonstrated that DP71 and DP427 play important roles in cell viability during cell proliferation and fibre differentiation, respectively. Despite their different expression patterns, the absence of DP71 and DP427 resulted in a similar phenotype, including increased membrane permeability, mitochondrial aggregation, elevated ROS, and increased cyto- and genotoxicity, but induced substantially different transcriptome programs associated with impaired cell proliferation and structural reorganization of myotubes, respectively. The phenotype occurred independently of myofibre contraction, dysfunction of neuromuscular or myotendinous junctions, or other cell types. We further showed that the utrophin UP395 can partially compensate not only for DP427 but also for DP71. These results explain the observed differences in disease severity among DMD patients and suggest that individuals deficient in both DP71 and DP427 may require a different therapeutic approach than patients deficient in only DP427.