Duchenne Muscular Dystrophy - Ongoing Research - Exon-skipping

Exon-skipping

Antisense oligonucleotides, structural analogs of DNA, are the basis of a potential therapy for patients afflicted with DMD. The compounds allow faulty parts of the dystrophin gene to be skipped when it is transcribed to RNA for protein production, permitting a still-truncated but more functional version of the protein to be produced.

Two kinds of antisense oligos, 2'-O-methyl phosphorothioate oligos (like drisapersen) and Morpholino oligos (like Eteplirsen), have been tested in early-phase clinical trials for DMD and have restored some dystrophin expression in muscles of DMD patients with a particular class of DMD-causing mutations. Further clinical trials are proceeding with these compounds.

Oligo-mediated exon skipping has resulted in clinical improvement in 12 patients in a Phase 1-2a study. On a standard test, the 6-minute walk test, patients whose performance had been declining instead improved, from 385 meters to 420 meters. DMD may result from mRNA that contains out-of-frame mutations (e.g. deletions, insertions or splice site mutations), resulting in frameshift or early termination so that in most muscle fibers no functional dystrophin is produced (though some revertant muscle fibers produce some dystrophin). In many cases an antisense oligonucleotide can be used to trigger skipping of an adjacent exon to restore the reading frame and production of partially functional dystrophin.

Patients with Becker's muscular dystrophy, which is milder than DMD, have a form of dystrophin which is functional even though it is shorter than normal dystrophin. In 1990 England et al. noticed that a patient with mild Becker muscular dystrophy was lacking 46% of his coding region for dystrophin. This functional, yet truncated, form of dystrophin gave rise to the notion that shorter dystrophin can still be therapeutically beneficial. Concurrently, Kole et al. had modified splicing by targeting pre-mRNA with antisense oligonucleotides (AONs). Kole demonstrated success using splice-targeted AONs to correct missplicing in cells removed from beta-thalassemia patients Wilton's group tested exon skipping for muscular dystrophy. Successful preclinical research led to the current efforts to use splice-modifying oligos to change DMD dystrophin to a more functional form of dystrophin, in effect converting Duchenne MD into Becker MD.

Though AONs hold promise, one of their major pitfalls is the need for periodic redelivery into muscles. Systemic delivery on a recurring basis is being tested in humans (http://clinicaltrials.gov/ct2/show/NCT00844597). To circumvent the requirement for periodic oligo delivery, a long-term exon-skip therapy is being explored. This therapy consists of modifying the U7 small nuclear RNA at the 5' end of the non-translated RNA to target regions within pre-mRNA. This has been shown to work in the DMD equivalent mouse, mdx.