A short antisense oligonucleotide masking a unique intronic motif prevents skipping of a critical exon in spinal muscular atrophy

RNA-targeted splice-correction therapy for neuromuscular disease

Splice-modulation therapy, whereby molecular manipulation of premessenger RNA splicing is engineered to yield genetic correction, is a promising novel therapy for genetic diseases of muscle and nerve-the prototypical example being Duchenne muscular dystrophy. Duchenne muscular dystrophy is the most common childhood genetic disease, affecting one in 3500 newborn boys, causing progressive muscle weakness, heart and respiratory failure and premature death. No cure exists for this disease and a number of promising new molecular therapies are being intensively studied. Duchenne muscular dystrophy arises due to mutations that disrupt the open-reading-frame in the DMD gene leading to the absence of the essential muscle protein dystrophin. Of all novel molecular interventions currently being investigated for Duchenne muscular dystrophy, perhaps the most promising method aiming to restore dystrophin expression to diseased cells is known as 'exon skipping' or splice-modulation, whereby antisense oligonucleotides eliminate the deleterious effects of DMD mutations by modulating dystrophin pre-messenger RNA splicing, such that functional dystrophin protein is produced. Recently this method was shown to be promising and safe in clinical trials both in The Netherlands and the UK. These trials studied direct antisense oligonucleotide injections into single peripheral lower limb muscles, whereas a viable therapy will need antisense oligonucleotides to be delivered systemically to all muscles, most critically to the heart, and ultimately to all other affected tissues including brain. There has also been considerable progress in understanding how such splice-correction methods could be applied to the treatment of related neuromuscular diseases, including spinal muscular atrophy and myotonic dystrophy, where defects of splicing or alternative splicing are closely related to the disease mechanism.

Trans-splicing-mediated improvement in a severe mouse model of spinal muscular atrophy

Spinal muscular atrophy is a leading genetic cause of infantile death and occurs in approximately 1/6000 live births. SMA is caused by the loss of Survival Motor Neuron-1 (SMN1), however, all patients retain at least one copy of a nearly identical gene called SMN2. While SMN2 and SMN1 are comprised of identical coding sequences, the majority of SMN2 transcripts are alternatively spliced, encoding a truncated protein that is unstable and nonfunctional. Considerable effort has focused upon modulating the SMN2 alternative splicing event since this would produce more wild-type protein. Recently we reported the development of an optimized trans-splicing system that involved the coexpression of a SMN2 trans-splicing RNA and an antisense RNA that blocks a downstream splice site in SMN2 pre-mRNA. Here, we demonstrate that in vivo delivery of the optimized trans-splicing vector increases an important SMN-dependent activity, snRNP assembly, in disease-relevant tissue in the SMA mouse model. A single injection of the vector into the intracerebral-ventricular space in SMA neonates also lessens the severity of the SMA phenotype in a severe SMA mouse model, extending survival approximately 70%. Collectively, these results provide the first in vivo demonstration that SMN2 trans-splicing leads to a lessening of the severity of the SMA phenotype and provide evidence for the power of this strategy for reprogramming genetic diseases at the pre-mRNA level.

RNA-targeting approaches for neuromuscular diseases

Although most molecular therapy strategies for genetic diseases are based on gene replacement, interesting alternative approaches target RNA. These strategies rely on the modification of the mutated gene's expression in vivo by modulating pre-mRNA splicing, mRNA stability or mRNA translation. Here, we review recent progress using these RNA-based approaches in the field of muscle and muscle-related genetic diseases. Different molecular tools, including modified antisense oligonucleotides, pre-mRNA trans-splicing molecules, ribozymes or chemical compounds have been used successfully on patient cells or animal models of disease. These diverse strategies show tremendous therapeutic potential and several clinical trials have been initiated with Duchenne muscular dystrophy patients with promising results.