Becker muscular dystrophy severity is linked to the structure of dystrophin

In-frame exon deletions of the Duchenne muscular dystrophy (DMD) gene produce internally truncated proteins that typically lead to Becker muscular dystrophy (BMD), a milder allelic disorder of DMD. We hypothesized that differences in the structure of mutant dystrophin may be responsible for the clinical heterogeneity observed in Becker patients and we studied four prevalent in-frame exon deletions, i.e. Delta 45-47, Delta 45-48, Delta 45-49 and Delta 45-51. Molecular homology modelling revealed that the proteins corresponding to deletions Delta 45-48 and Delta 45-51 displayed a similar structure (hybrid repeat) than the wild-type dystrophin, whereas deletions Delta 45-47 and Delta 45-49 lead to proteins with an unrelated structure (fractional repeat). All four proteins in vitro expressed in a fragment encoding repeats 16-21 were folded in a-helices and remained highly stable. Refolding dynamics were slowed and molecular surface hydrophobicity were higher in fractional repeat containing Delta 45-47 and Delta 45-49 deletions compared with hybrid repeat containing Delta 45-48 and Delta 45-51 deletions. By retrospectively collecting data for a series of French BMD patients, we showed that the age of dilated cardiomyopathy (DCM) onset was delayed by 11 and 14 years in Delta 45-48 and Delta 45-49 compared with Delta 45-47 patients, respectively. A clear trend toward earlier wheelchair dependency (minimum of 11 years) was also observed in Delta 45-47 and Delta 45-49 patients compared with Delta 45-48 patients. Muscle dystrophin levels were moderately reduced in most patients without clear correlation with the deletion type. Disease progression in BMD patients appears to be dependent on the deletion itself and associated with a specific structure of dystrophin at the deletion site.