Translational and Regulatory Challenges for Exon Skipping Therapies

Several translational challenges are currently impeding the therapeutic development of antisense-mediated exon skipping approaches for rare diseases. Some of these are inherent to developing therapies for rare diseases, such as small patient numbers and limited information on natural history and interpretation of appropriate clinical outcome measures. Others are inherent to the antisense oligonucleotide (AON)-mediated exon skipping approach, which employs small modified DNA or RNA molecules to manipulate the splicing process. This is a new approach and only limited information is available on long-term safety and toxicity for most AON chemistries. Furthermore, AONs often act in a mutation-specific manner, in which case multiple AONs have to be developed for a single disease. A workshop focusing on preclinical development, trial design, outcome measures, and different forms of marketing authorization was organized by the regulatory models and biochemical outcome measures working groups of Cooperation of Science and Technology Action: "Networking towards clinical application of antisense-mediated exon skipping for rare diseases." The workshop included participants from patient organizations, academia, and members of staff from the European Medicine Agency and Medicine Evaluation Board (the Netherlands). This statement article contains the key outcomes of this meeting.status: publishe

ANTISENSE MEDIATED DYSTROPHIN READING FRAME RESTORATION

Exon skipping using antisense oligonucleotides (AONs) has successfully been used to reframe the mRNA in various DMD (Duchenne muscular dystrophy) patients carrying deletions and in the mdx mouse model. This study can be devided in two parts: in the first part we have tested the feasibility of the exon skipping approach for patients with small mutations in in-frame exons, while in the second part a quantitative comparison of exon skipping revealing techniques is addressed. We first identified 55 novel disease-causing point mutations. We selected 5 patients with nonsense or frameshifting mutations in exons 10, 16, 26, 33 and 34. Wild type and mutation specific 2‟OMePS AONs were tested in cell-free splicing assays and in cultured cells derived from the selected patients. The results obtained confirm cell-free splicing assay as an alternative system to test exon skipping propensity when patients‟ cells are unavailable. In myogenic cells, similar levels of exon skipping were observed for wild type and mutation specific AONs for exons 16, 26 and 33, while for exon 10 and exon 34 the efficiency of the AONs was significantly different. Interestingly, in some cases skipping efficiencies for mutated exons were quite dissimilar compared to what previously reported for the respective wild type exons. This behaviour may be related to effect of the mutations on exon skipping propensity and highlights the complexity of identifying optimal AONs for skipping exons with small mutations. In the second part we compared different techniques to reveal the exon skipping levels in the muscles of 7 different mdx mice. An absolute quantification of the dystrophin transcript amount was possible using a digital array. Results underline the low expression of the dytrophin gene and the amount needed to correctly quantify the exon skipping percentage

Progress in muscular dystrophy research with special emphasis on gene therapy

Duchenne muscular dystrophy (DMD) is an X-linked, progressive muscle-wasting disease caused by mutations in the DMD gene. Since the disease was described by physicians in the 19th century, information about the subject has been accumulated. One author (Sugita) was one of the coworkers who first reported that the serum creatine kinase (CK) level is elevated in progressive muscular dystrophy patients. Even 50 years after that first report, an elevated serum CK level is still the most useful marker in the diagnosis of DMD, a sensitive index of the state of skeletal muscle, and useful to evaluate therapeutic effects. In the latter half of this article, we describe recent progress in the therapy of DMD, with an emphasis on gene therapies, particularly exon skipping

Quantitative antisense screening and optimization for exon 51 skipping in Duchenne muscular dystrophy

International audienceDuchenne muscular dystrophy (DMD), the most common lethal genetic disorder, is caused by mutations in the dystrophin (DMD) gene. Exon skipping is a therapeutic approach that uses antisense oligonucleotides (AOs) to modulate splicing and restore the reading frame, leading to truncated, yet functional protein expression. In 2016, the US Food and Drug Administration (FDA) conditionally approved the first phosphorodiamidate morpholino oligomer (morpholino)-based AO drug, eteplirsen, developed for DMD exon 51 skipping. Eteplirsen remains controversial with insufficient evidence of its therapeutic effect in patients. We recently developed an in silico tool to design antisense morpholino sequences for exon skipping. Here, we designed morpholino AOs targeting DMD exon 51 using the in silico tool and quantitatively evaluated the effects in immortalized DMD muscle cells in vitro. To our surprise, most of the newly designed morpholinos induced exon 51 skipping more efficiently compared with the eteplirsen sequence. The efficacy of exon 51 skipping and rescue of dystrophin protein expression were increased by up to more than 12-fold and 7-fold, respectively, compared with the eteplirsen sequence. Significant in vivo efficacy of the most effective morpholino, determined in vitro, was confirmed in mice carrying the human DMD gene. These findings underscore the importance of AO sequence optimization for exon skipping

Advances in gene therapy for muscular dystrophies

Duchenne muscular dystrophy (DMD) is a recessive lethal inherited muscular dystrophy caused by mutations in the gene encoding dystrophin, a protein required for muscle fibre integrity. So far, many approaches have been tested from the traditional gene addition to newer advanced approaches based on manipulation of the cellular machinery either at the gene transcription, mRNA processing or translation levels. Unfortunately, despite all these efforts, no efficient treatments for DMD are currently available. In this review, we highlight the most advanced therapeutic strategies under investigation as potential DMD treatments