Targeted Suppression of a Dystrophin Pseudo-exon using Antisense Oligonucleotides

Abstract

Antisense oligonucleotide induced exon skipping is showing great promise as a potential therapy to restore functional dystrophin expression in Duchenne muscular dystrophy. Redirecting mRNA processing can excise one or more exons flanking frame-shifting deletions or duplications so that the reading frame is restored and a Becker muscular dystrophy-like transcript is induced. Here, we report the application of exon skipping to excise a diseasecausing 72 bp pseudo-exon from the mature dystrophin mRNA. This pseudo-exon arises from a single T>G base change in intron 47 that creates an acceptor site with a strong consensus splice site score. Although in-frame, this pseudo-exon contains several stop codons and its inclusion is consistent with a diagnosis of Duchenne muscular dystrophy. The 21 year old patient is no longer ambulant but can stand and sit unassisted, indicating leaky premRNA processing and the generation of some normal transcripts has delayed the muscle wasting to some extent. In cases where the DMD lesion activates one or more cryptic splice sites and results in the retention of intronic sequences in that mature mRNA, exon skipping has the potential to generate a normal gene transcript. Antisense oligonucleotides targeted to the mutant splice site were able to remove the pseudo-exon from the mature transcript and restore full-length dystrophin expression. This is in contrast to most exon-skipping strategies for Duchenne muscular dystrophy in which antisense oligonucleotides are used to restore expression but the induced Becker muscular dystrophy-like transcript is shorter and potentially less functional than the full-length mRNA. As such, pseudo-exons may be particularly amenable to exon skipping, especially when basal levels of expression have raised endogenous dystrophin levels, reducing disease severity and should mitigate immune responses to the induced normal isoform.