Renadirsen, a Novel 2'OMeRNA/ENA® Chimera Antisense Oligonucleotide, Induces Robust Exon 45 Skipping for Dystrophin In Vivo

Successful skipping of abnormal pseudoexon by antisense oligonucleotides in vitro for a patient with beta-propeller protein-associated neurodegeneration

Pathogenic variants in WDR45 on chromosome Xp11 cause neurodegenerative disorder beta-propeller protein-associated neurodegeneration (BPAN). Currently, there is no effective therapy for BPAN. Here we report a 17-year-old female patient with BPAN and show that antisense oligonucleotide (ASO) was effective in vitro. The patient had developmental delay and later showed extrapyramidal signs since the age of 15 years. MRI findings showed iron deposition in the globus pallidus and substantia nigra on T2 MRI. Whole genome sequencing and RNA sequencing revealed generation of pseudoexon due to inclusion of intronic sequences triggered by an intronic variant that is remote from the exon-intron junction: WDR45 (OMIM #300526) chrX(GRCh37):g.48935143G > C, (NM_007075.4:c.235 + 159C > G). We recapitulated the exonization of intron sequences by a mini-gene assay and further sought antisense oligonucleotide that induce pseudoexon skipping using our recently developed, a dual fluorescent splicing reporter system that encodes two fluorescent proteins, mCherry, a transfection marker designed to facilitate evaluation of exon skipping and split eGFP, a splicing reaction marker. The results showed that the 24-base ASO was the strongest inducer of pseudoexon skipping. Our data presented here have provided supportive evidence for in vivo preclinical studies.

Therapeutic approaches for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a monogenic muscle-wasting disorder and a priority candidate for molecular and cellular therapeutics. Although rare, it is the most common inherited myopathy affecting children and so has been the focus of intense research activity. It is caused by mutations that disrupt production of the dystrophin protein, and a plethora of drug development approaches are under way that aim to restore dystrophin function, including exon skipping, stop codon readthrough, gene replacement, cell therapy and gene editing. These efforts have led to the clinical approval of four exon skipping antisense oligonucleotides, one stop codon readthrough drug and one gene therapy product, with other approvals likely soon. Here, we discuss the latest therapeutic strategies that are under development and being deployed to treat DMD. Lessons from these drug development programmes are likely to have a major impact on the DMD field, but also on molecular and cellular medicine more generally. Thus, DMD is a pioneer disease at the forefront of future drug discovery efforts, with these experimental treatments paving the way for therapies using similar mechanisms of action being developed for other genetic diseases.

Next steps for the optimization of exon therapy for Duchenne muscular dystrophy

INTRODUCTION

It is established that the exon-skipping approach can restore dystrophin in Duchenne muscular dystrophy (DMD) patients. However, dystrophin restoration levels are low, and the field is evolving to provide solutions for improved exon skipping. DMD is a neuromuscular disorder associated with chronic muscle tissue loss attributed to the lack of dystrophin, which causes muscle inflammation, fibrosis formation, and impaired regeneration. Currently, four antisense oligonucleotides (AONs) based on phosphorodiamidate morpholino oligomer (PMO) chemistry are approved by US Food and Drug Administration for exon skipping therapy of eligible DMD patients.

AREAS COVERED

This review describes a preclinical and clinical experience with approved and newly developed AONs for DMD, outlines efforts that have been done to enhance AON efficiency, reviews challenges of clinical trials, and summarizes the current state of the exon skipping approach in the DMD field.

EXPERT OPINION

The exon skipping approach for DMD is under development, and several chemical modifications with improved properties are under (pre)-clinical investigation. Despite existing advantages of these modifications, their safety and effectiveness have to be examined in clinical trials, which are planned or ongoing. Furthermore, we propose clinical settings using natural history controls to facilitate studying the functional effect of the therapy.

Synthesis and Properties of Oligonucleotides Containing 2'-O,4'-C-Methyleneoxy-Bridged Pyrimidine Derivatives

In this study, 2'-O,4'-C-methyleneoxy-bridged nucleic acid, a unique consecutive three-acetal-containing nucleic acid (TaNA), was designed. Pyrimidine derivatives of methylated TaNA (Me-TaNA) were also synthesized and introduced into oligonucleotides via solid-phase synthesis. The Me-TaNA-modified oligonucleotides exhibited higher stabilities when forming duplexes with single-stranded RNA or triplexes with double-stranded DNA, relative to the natural oligonucleotides and modified oligonucleotides containing another 2',4'-bridged 5-methyluridine, such as 2',4'-BNA/LNA and 2',4'-ENA. Furthermore, Me-TaNA within oligonucleotides significantly enhanced nuclease resistance.

An Antisense Oligonucleotide against a Splicing Enhancer Sequence within Exon 1 of the MSTN Gene Inhibits Pre-mRNA Maturation to Act as a Novel Myostatin Inhibitor

Antisense oligonucleotides (ASOs) are agents that modulate gene function. ASO-mediated out-of-frame exon skipping has been employed to suppress gene function. Myostatin, encoded by the MSTN gene, is a potent negative regulator of skeletal muscle growth. ASOs that induce skipping of out-of-frame exon 2 of the MSTN gene have been studied for their use in increasing muscle mass. However, no ASOs are currently available for clinical use. We hypothesized that ASOs against the splicing enhancer sequence within exon 1 of the MSTN gene would inhibit maturation of pre-mRNA, thereby suppressing gene function. To explore this hypothesis, ASOs against sequences of exon 1 of the MSTN gene were screened for their ability to reduce mature MSTN mRNA levels. One screened ASO, named KMM001, decreased MSTN mRNA levels in a dose-dependent manner and reciprocally increased MSTN pre-mRNA levels. Accordingly, KMM001 decreased myostatin protein levels. KMM001 inhibited SMAD-mediated myostatin signaling in rhabdomyosarcoma cells. Remarkably, it did not decrease GDF11 mRNA levels, indicating myostatin-specific inhibition. As expected, KMM001 enhanced the proliferation of human myoblasts. We conclude that KMM001 is a novel myostatin inhibitor that inhibits pre-mRNA maturation. KMM001 has great promise for clinical applications and should be examined for its ability to treat various muscle-wasting conditions.

Advanced Gene-Targeting Therapies for Motor Neuron Diseases and Muscular Dystrophies

Gene therapy is a revolutionary, cutting-edge approach to permanently ameliorate or amend many neuromuscular diseases by targeting their genetic origins. Motor neuron diseases and muscular dystrophies, whose genetic causes are well known, are the frontiers of this research revolution. Several genetic treatments, with diverse mechanisms of action and delivery methods, have been approved during the past decade and have demonstrated remarkable results. However, despite the high number of genetic treatments studied preclinically, those that have been advanced to clinical trials are significantly fewer. The most clinically advanced treatments include adeno-associated virus gene replacement therapy, antisense oligonucleotides, and RNA interference. This review provides a comprehensive overview of the advanced gene therapies for motor neuron diseases (i.e., amyotrophic lateral sclerosis and spinal muscular atrophy) and muscular dystrophies (i.e., Duchenne muscular dystrophy, limb-girdle muscular dystrophy, and myotonic dystrophy) tested in clinical trials. Emphasis has been placed on those methods that are a few steps away from their authoritative approval.

Antisense Oligonucleotides Conjugated with Lipophilic Compounds: Synthesis and In Vitro Evaluation of Exon Skipping in Duchenne Muscular Dystrophy

Our groups previously reported that conjugation at 3′-end with ursodeoxycholic acid (UDCA) significantly enhanced in vitro exon skipping properties of ASO 51 oligonucleotide targeting the human DMD exon 51. In this study, we designed a series of lipophilic conjugates of ASO 51, to explore the influence of the lipophilic moiety on exon skipping efficiency. To this end, three bile acids and two fatty acids have been derivatized and/or modified and conjugated to ASO 51 by automatized solid phase synthesis. We measured the melting temperature (T_m) of lipophilic conjugates to evaluate their ability to form a stable duplex with the target RNA. The exon skipping efficiency has been evaluated in myogenic cell lines first in presence of a transfection agent, then in gymnotic conditions on a selection of conjugated ASO 51. In the case of 5′-UDC-ASO 51, we also evaluated the influence of PS content on exon skipping efficiency; we found that it performed better exon skipping with full PS linkages. The more efficient compounds in terms of exon skipping were found to be 5′-UDC- and 5′,3′-bis-UDC-ASO 51.