Clinical phenotypes as predictors of the outcome of skipping around DMD exon 45

Abstract

Objective: Exon-skipping therapies aim to convert Duchenne muscular dystrophy (DMD) into less severe Becker muscular dystrophy (BMD) by altering pre-mRNA splicing to restore an open reading frame, allowing translation of an internally deleted and partially functional dystrophin protein. The most common single exon deletion-exon 45 (Δ45)-may theoretically be treated by skipping of either flanking exon (44 or 46). We sought to predict the impact of these by assessing the clinical severity in dystrophinopathy patients.

Methods: Phenotypic data including clinical diagnosis, age at wheelchair use, age at loss of ambulation, and presence of cardiomyopathy were analyzed from 41 dystrophinopathy patients containing equivalent in-frame deletions.

Results: As expected, deletions of either exons 45 to 47 (Δ45-47) or exons 45 to 48 (Δ45-48) result in BMD in 97% (36 of 37) of subjects. Unexpectedly, deletion of exons 45 to 46 (Δ45-46) is associated with the more severe DMD phenotype in 4 of 4 subjects despite an in-frame transcript. Notably, no patients with a deletion of exons 44 to 45 (Δ44-45) were found within the United Dystrophinopathy Project database, and this mutation has only been reported twice before, which suggests an ascertainment bias attributable to a very mild phenotype.

Interpretation: The observation that Δ45-46 patients have typical DMD suggests that the conformation of the resultant protein may result in protein instability or altered binding of critical partners. We conclude that in DMD patients with Δ45, skipping of exon 44 and multiexon skipping of exons 46 and 47 (or exons 46-48) are better potential therapies than skipping of exon 46 alone.

Figure 1. DMD genotypes vs. age at last evaluation

The clinical diagnosis of patients is noted below each genotype class. Non-ambulant patients are marked by filled squares.

Figure 2. Molecular modeling of dystrophin proteins predicted to result from deletions of (A) exons 44-45 and (B) exons 45-46

Each is predicted to result in a hybrid repeat that contains three helices in a coiled-coil pattern, as opposed to a fractional repeat, which would be predicted to be less stable. (A) Δ44-45 is composed of R15 (pink), part of R16 (blue) and part of R17 (purple), R18 (yellow). (B) Δ45-46 is composed of R16 (blue), part of R17 (purple) and part of R18 (yellow), R19 (grey).