G.P.75 Variable phenotype of del45–55 Becker patients correlated to nNOSμ mislocalization and RYR1 hypernitrosylation

Abstract Duchenne (DMD) and Becker (BMD) muscular dystrophies are muscle-wasting diseases caused by mutations in the DMD gene encoding dystrophin. Usually, out-of-frame deletions give rise to DMD whereas in-frame deletions result in BMD. BMD patients exhibit a less severe disease because an abnormal but functional dystrophin is produced. This is the rationale for attempts to correct the reading frame by using an exon-skipping strategy. In order to apply this approach to a larger number of patients, a strategy of exons 45–55 multi-skipping has been proposed, because it would correct the mRNA reading frame in almost 75% of DMD patients with a deletion. The resulting dystrophin lacks part of the binding site for the neuronal nitric oxide synthase (nNOSμ), which normally binds to spectrin-like repeats 16 and 17 of dystrophin. Knowing that these domains are encoded by exons 42–45, we investigated the nNOSμ status in muscle biopsies from twelve BMD patients with deletions of exons 45–55. We found a wide spectrum of nNOSμ expression and localization. The strictly cytosolic mislocalization of nNOSμμ was associated with the more severe phenotypes. The cytosolic NO production correlated with both hypernitrosylation of the sarcoplasmic reticulum calcium-release-channel RyR1 and release of calstabin-1, a central hub to Ca2+ signaling and contraction in muscle. This study shows that the terminal truncation of the nNOS binding domain in the “therapeutic” del45–55 dystrophin is not innocuous since, in some cases, it may perturb the nNOS dependent stability of the RyR1/calstabin-1 complex.