Local dystrophin restoration with antisense oligonucleotide PRO051.

BACKGROUND: Duchenne's muscular dystrophy is associated with severe, progressive muscle weakness and typically leads to death between the ages of 20 and 35 years. By inducing specific exon skipping during messenger RNA (mRNA) splicing, antisense compounds were recently shown to correct the open reading frame of the DMD gene and thus to restore dystrophin expression in vitro and in animal models in vivo. We explored the safety, adverse-event profile, and local dystrophin-restoring effect of a single, intramuscular dose of an antisense oligonucleotide, PRO051, in patients with this disease. METHODS: Four patients, who were selected on the basis of their mutational status, muscle condition, and positive exon-skipping response to PRO051 in vitro, received a dose of 0.8 mg of PRO051 injected into the tibialis anterior muscle. A biopsy was performed 28 days later. Safety measures, composition of mRNA, and dystrophin expression were assessed. RESULTS: PRO051 injection was not associated with clinically apparent adverse events. Each patient showed specific skipping of exon 51 and sarcolemmal dystrophin in 64 to 97% of myofibers. The amount of dystrophin in total protein extracts ranged from 3 to 12% of that found in the control specimen and from 17 to 35% of that of the control specimen in the quantitative ratio of dystrophin to laminin alpha2. CONCLUSIONS: Intramuscular injection of antisense oligonucleotide PRO051 induced dystrophin synthesis in four patients with Duchenne's muscular dystrophy who had suitable mutations, suggesting that further studies might be feasible.

Targeted exon skipping as a potential gene correction therapy for Duchenne muscular dystrophy.

Duchenne muscular dystrophy is primarily caused by frame-disrupting mutations in the Duchenne muscular dystrophy gene which abort dystrophin synthesis. We have explored a gene correction therapy aimed at restoration of the reading frame in Duchenne muscular dystrophy patients. Through the binding of antisense oligoribonucleotides to exon-internal sequences in the pre-mRNA, the splicing can be manipulated in such a manner that the targeted exon is skipped and a slightly shorter, but in-frame, transcript is generated. We recently showed that antisense oligoribonucleotide-mediated skipping of exon 46 efficiently induced dystrophin synthesis in cultured muscle cells from Duchenne muscular dystrophy patients carrying an exon 45 deletion. In this study we have identified antisense oligoribonucleotides with which the skipping of 11 other Duchenne muscular dystrophy exons could be induced in cultured human muscle cells. The targeted skipping of only one particular exon may restore the reading frame in a series of patients with different mutations. Accordingly, these antisense oligoribonucleotides would allow correction of over 50% of deletions and 22% of duplications reported in the Leiden DMD-mutation Database.

Antisense-induced multiexon skipping for Duchenne muscular dystrophy makes more sense.

Dystrophin deficiency, which leads to severe and progressive muscle degeneration in patients with Duchenne muscular dystrophy (DMD), is caused by frameshifting mutations in the dystrophin gene. A relatively new therapeutic strategy is based on antisense oligonucleotides (AONs) that induce the specific skipping of a single exon, such that the reading frame is restored. This allows the synthesis of a largely functional dystrophin, associated with a milder Becker muscular dystrophy phenotype. We have previously successfully targeted 20 different DMD exons that would, theoretically, be beneficial for >75% of all patients. To further enlarge this proportion, we here studied the feasibility of double and multiexon skipping. Using a combination of AONs, double skipping of exon 43 and 44 was induced, and dystrophin synthesis was restored in myotubes from one patient affected by a nonsense mutation in exon 43. For another patient, with an exon 46-50 deletion, the therapeutic double skipping of exon 45 and 51 was achieved. Remarkably, in control myotubes, the latter combination of AONs caused the skipping of the entire stretch of exons from 45 through 51. This in-frame multiexon skipping would be therapeutic for a series of patients carrying different DMD-causing mutations. In fact, we here demonstrate its feasibility in myotubes from a patient with an exon 48-50 deletion. The application of multiexon skipping may provide a more uniform methodology for a larger group of patients with DMD.

Targeted exon skipping in transgenic hDMD mice: A model for direct preclinical screening of human-specific antisense oligonucleotides.

The therapeutic potential of frame-restoring exon skipping by antisense oligonucleotides (AONs) has recently been demonstrated in cultured muscle cells from a series of Duchenne muscular dystrophy (DMD) patients. To facilitate clinical application, in vivo studies in animal models are required to develop safe and efficient AON-delivery methods. However, since exon skipping is a sequence-specific therapy, it is desirable to target the human DMD gene directly. We therefore set up human sequence-specific exon skipping in transgenic mice carrying the full-size human gene (hDMD). We initially compared the efficiency and toxicity of intramuscular AON injections using different delivery reagents in wild-type mice. At a dose of 3.6 nmol AON and using polyethylenimine, the skipping levels accumulated up to 3% in the second week postinjection and lasted for 4 weeks. We observed a correlation of this long-term effect with the intramuscular persistence of the AON. In regenerating myofibers higher efficiencies (up to 9%) could be obtained. Finally, using the optimized protocols in hDMD mice, we were able to induce the specific skipping of human DMD exons without affecting the endogenous mouse gene. These data highlight the high sequence specificity of this therapy and present the hDMD mouse as a unique model to optimize human-specific exon skipping in vivo.

Therapeutic antisense-induced exon skipping in cultured muscle cells from six different DMD patients.

The dystrophin deficiency leading to the severely progressing muscle degeneration in Duchenne muscular dystrophy (DMD) patients is caused by frame-shifting mutations in the DMD gene. We are developing a reading frame correction therapy aimed at the antisense-induced skipping of targeted exons from the pre-mRNA. Despite introducing a (larger) deletion, an in-frame transcript is generated that allows the synthesis of a slightly shorter, but largely functional dystrophin as found in the mostly milder Becker muscular dystrophy (BMD). We have recently demonstrated both the efficacy and high efficiency of the antisense-induced skipping of numerous exons from the DMD transcript in control muscle cells. In principle, this would restore the reading frame in over 75% of the patients reported in the Leiden DMD mutation database. In this study, we in fact demonstrate the broad therapeutic applicability of this strategy in cultured muscle cells from six DMD patients carrying different deletions and a nonsense mutation. In each case, the specific skipping of the targeted exon was induced, restoring dystrophin synthesis in over 75% of cells. The protein was detectable as soon as 16 h post-transfection, then increased to significant levels at the membrane within 2 days, and was maintained for at least a week. Finally, its proper function was further suggested by the restored membranal expression of four associated proteins from the dystrophin-glycoprotein complex. These results document important progress towards a clinically applicable, small-molecule based therapy.