In-frame exon deletions of the DMD gene produce internally truncated proteins that typically lead to Becker muscular dystrophy (BMD), a milder allelic disorder of Duchenne muscular dystrophy (DMD). In this study, the suthors hypothesized that differences in the structure of mutant dystrophin may be responsible for the clinical heterogeneity observed in Becker patients and they studied four prevalent in-frame exon deletions, i.e. Δ45-47, Δ45-48, Δ45-49 and Δ45-51. Molecular homology modelling revealed that the proteins corresponding to deletions Δ45-48 and Δ45-51 displayed a similar structure (hybrid repeat) than the wild type dystrophin, whereas deletions Δ45-47 and Δ45-49 lead to proteins with an unrelated structure (fractional repeat). All four proteins in vitro expressed in a fragment encoding repeats 16 to 21 were folded in alpha-helices and remained highly stable. Refolding dynamics were slowed and molecular surface hydrophobicity were higher in fractional repeat containing Δ45-47 and Δ45-49 deletions compared to hybrid repeat containing Δ45-48 and Δ45-51 deletions. By retrospectively collecting data for a series of French BMD patients, the authors showed that the age of dilated cardiomyopathy onset was delayed by 11 and 14 years in Δ45-48 and Δ45-49 compared to Δ45-47 patients, respectively. A clear trend toward earlier wheelchair dependency (minimum of 11 years) was also observed in Δ45-47 and Δ45-49 patients compared to Δ45-48 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.
