G.O.5 Partial functionality of a Mini-dysferlin molecule identified in a patient affected with moderately severe primary dysferlinopathy

Identification of Novel Antisense-Mediated Exon Skipping Targets in DYSF for Therapeutic Treatment of Dysferlinopathy

Dysferlinopathy is a progressive myopathy caused by mutations in the dysferlin (DYSF) gene. Dysferlin protein plays a major role in plasma-membrane resealing. Some patients with DYSF deletion mutations exhibit mild symptoms, suggesting some regions of DYSF can be removed without significantly impacting protein function. Antisense-mediated exon-skipping therapy uses synthetic molecules called antisense oligonucleotides to modulate splicing, allowing exons harboring or near genetic mutations to be removed and the open reading frame corrected. Previous studies have focused on DYSF exon 32 skipping as a potential therapeutic approach, based on the association of a mild phenotype with the in-frame deletion of exon 32. To date, no other DYSF exon-skipping targets have been identified, and the relationship between DYSF exon deletion pattern and protein function remains largely uncharacterized. In this study, we utilized a membrane-wounding assay to evaluate the ability of plasmid constructs carrying mutant DYSF, as well as antisense oligonucleotides, to rescue membrane resealing in patient cells. We report that multi-exon skipping of DYSF exons 26–27 and 28–29 rescues plasma-membrane resealing. Successful translation of these findings into the development of clinical antisense drugs would establish new therapeutic approaches that would be applicable to ∼5%–7% (exons 26–27 skipping) and ∼8% (exons 28–29 skipping) of dysferlinopathy patients worldwide.

New developments in exon skipping and splice modulation therapies for neuromuscular diseases

INTRODUCTION

Antisense oligonucleotide (AON) therapy is a form of treatment for genetic or infectious diseases using small, synthetic DNA-like molecules called AONs. Recent advances in the development of AONs that show improved stability and increased sequence specificity have led to clinical trials for several neuromuscular diseases. Impressive preclinical and clinical data are published regarding the usage of AONs in exon-skipping and splice modulation strategies to increase dystrophin production in Duchenne muscular dystrophy (DMD) and survival of motor neuron (SMN) production in spinal muscular atrophy (SMA).

AREAS COVERED

In this review, we focus on the current progress and challenges of exon-skipping and splice modulation therapies. In addition, we discuss the recent failure of the Phase III clinical trials of exon 51 skipping (drisapersen) for DMD.

EXPERT OPINION

The main approach of AON therapy in DMD and SMA is to rescue ('knock up' or increase) target proteins through exon skipping or exon inclusion; conversely, most conventional antisense drugs are designed to knock down (inhibit) the target. Encouraging preclinical data using this 'knock up' approach are also reported to rescue dysferlinopathies, including limb-girdle muscular dystrophy type 2B, Miyoshi myopathy, distal myopathy with anterior tibial onset and Fukuyama congenital muscular dystrophy.

Crystal structures of the human Dysferlin inner DysF domain

BACKGROUND

Mutations in dysferlin, the first protein linked with the cell membrane repair mechanism, causes a group of muscular dystrophies called dysferlinopathies. Dysferlin is a type two-anchored membrane protein, with a single C terminal trans-membrane helix, and most of the protein lying in cytoplasm. Dysferlin contains several C2 domains and two DysF domains which are nested one inside the other. Many pathogenic point mutations fall in the DysF domain region.

RESULTS

We describe the crystal structure of the human dysferlin inner DysF domain with a resolution of 1.9 Ångstroms. Most of the pathogenic mutations are part of aromatic/arginine stacks that hold the domain in a folded conformation. The high resolution of the structure show that these interactions are a mixture of parallel ring/guanadinium stacking, perpendicular H bond stacking and aliphatic chain packing.

CONCLUSIONS

The high resolution structure of the Dysferlin DysF domain gives a template on which to interpret in detail the pathogenic mutations that lead to disease.

Opportunities and challenges for the development of antisense treatment in neuromuscular disorders

INTRODUCTION

Neuromuscular disorders are diseases of the musculature and/or the nervous system, generally leading to loss of muscle function. They are a frequent cause of disability and treatment options are often only symptomatic. Interestingly, for a number of neuromuscular disorders the application of antisense oligonucleotides has therapeutic potential.

AREAS COVERED

The authors describe how this approach is exploited for different neuromuscular diseases, focusing on literature published in the past 10 years. For each disease the opportunities of this approach, the state of the art, and current challenges are described.

EXPERT OPINION

A lot of progress has been made in the development of antisense-mediated approaches during recent years and they may become clinically applicable in the near future.

Efficient bypass of mutations in dysferlin deficient patient cells by antisense-induced exon skipping

Mutations in DYSF encoding dysferlin cause primary dysferlinopathies, autosomal recessive diseases that mainly present clinically as Limb Girdle Muscular Dystrophy type 2B and Miyoshi myopathy. More than 350 different sequence variants have been reported in DYSF. Like dystrophin, the size of the dysferlin mRNA is above the limited packaging size of AAV vectors. Alternative strategies to AAV gene transfer in muscle cells must then be addressed for patients. A gene therapy approach for Duchenne muscular dystrophy was recently developed, based on exon-skipping strategy. Numerous sequences are recognized by splicing protein complexes and, when specifically blocked by antisense oligoucleotides (AON), the corresponding exon is skipped. We hypothesized that this approach could be useful for patients affected with dysferlinopathies. To confirm this assumption, exon 32 was selected as a prioritary target for exon skipping strategy. This option was initially driven by the report from Sinnreich and colleagues of a patient with a very mild and late-onset phenotype associated to a natural skipping of exon 32. Three different antisense oligonucleotides were tested in myoblasts generated from control and patient MyoD transduced fibroblasts, either as oligonucleotides or after incorporation into lentiviral vectors. These approaches led to a high efficiency of exon 32 skipping. Therefore, these results seem promising, and could be applied to several other exons in the DYSF gene. Patients carrying mutations in exons whose the in-frame suppression has been proven to have no major consequences on the protein function, might benefit of exon-skipping based gene correction.

Therapeutic exon skipping for dysferlinopathies?

Antisense-mediated exon skipping is a promising therapeutic approach for Duchenne muscular dystrophy (DMD) currently tested in clinical trials. The aim is to reframe dystrophin transcripts using antisense oligonucleotides (AONs). These hide an exon from the splicing machinery to induce exon skipping, restoration of the reading frame and generation of internally deleted, but partially functional proteins. It thus relies on the characteristic of the dystrophin protein, which has essential N- and C-terminal domains, whereas the central rod domain is largely redundant. This approach may also be applicable to limb-girdle muscular dystrophy type 2B (LGMD2B), Myoshi myopathy (MM) and distal myopathy with anterior tibial onset (DMAT), which are caused by mutations in the dysferlin-encoding DYSF gene. Dysferlin has a function in repairing muscle membrane damage. Dysferlin contains calcium-dependent C2 lipid binding (C2) domains and an essential transmembrane domain. However, mildly affected patients in whom one or a large number of DYSF exons were missing have been described, suggesting that internally deleted dysferlin proteins can be functional. Thus, exon skipping might also be applicable as a LGMD2B, MM and DMAT therapy. In this study we have analyzed the dysferlin protein domains and DYSF mutations and have described what exons are promising targets with regard to applicability and feasibility. We also show that DYSF exon skipping seems to be as straightforward as DMD exon skipping, as AONs to induce efficient skipping of four DYSF exons were readily identified.