A single administration of morpholino antisense oligomer rescues spinal muscular atrophy in mouse

Survival motor neuron protein in motor neurons determines synaptic integrity in spinal muscular atrophy

The inherited motor neuron disease spinal muscular atrophy (SMA) is caused by deficient expression of survival motor neuron (SMN) protein and results in severe muscle weakness. In SMA mice, synaptic dysfunction of both neuromuscular junctions (NMJs) and central sensorimotor synapses precedes motor neuron cell death. To address whether this synaptic dysfunction is due to SMN deficiency in motor neurons, muscle, or both, we generated three lines of conditional SMA mice with tissue-specific increases in SMN expression. All three lines of mice showed increased survival, weights, and improved motor behavior. While increased SMN expression in motor neurons prevented synaptic dysfunction at the NMJ and restored motor neuron somal synapses, increased SMN expression in muscle did not affect synaptic function although it did improve myofiber size. Together these data indicate that both peripheral and central synaptic integrity are dependent on motor neurons in SMA, but SMN may have variable roles in the maintenance of these different synapses. At the NMJ, it functions at the presynaptic terminal in a cell-autonomous fashion, but may be necessary for retrograde trophic signaling to presynaptic inputs onto motor neurons. Importantly, SMN also appears to function in muscle growth and/or maintenance independent of motor neurons. Our data suggest that SMN plays distinct roles in muscle, NMJs, and motor neuron somal synapses and that restored function of SMN at all three sites will be necessary for full recovery of muscle power.

Targeting mutant huntingtin for the development of disease-modifying therapy

Huntington's disease (HD) is a progressive and fatal neurodegenerative disease, and the most common inherited CAG repeat disorder. A polyglutamine expansion in the N-terminus of the huntingtin protein (HTT) leads to protein misfolding and downstream pathogenic processes culminating in widespread functional impairment and neurodegeneration in the striatum, cortex and other brain areas. To date, only symptomatic treatments are available that address motor, psychiatric and cognitive deficits. Here we review recent strategies for developing disease-modifying therapies designed to limit or abolish the pathogenic activities of the primary molecular target in HD, the mutant HTT protein itself.

Exon skipping for nonsense mutations in Duchenne muscular dystrophy: too many mutations, too few patients?

INTRODUCTION

Duchenne muscular dystrophy (DMD), one of the most common and lethal genetic disorders, is caused by mutations of the dystrophin gene. Removal of an exon or of multiple exons using antisense molecules has been demonstrated to allow synthesis of truncated 'Becker muscular dystrophy-like' dystrophin.

AREAS COVERED

Approximately 15% of DMD cases are caused by a nonsense mutation. Although patient databases have previously been surveyed for applicability to each deletion mutation pattern, this is not so for nonsense mutations. Here, we examine the world-wide database containing notations for more than 1200 patients with nonsense mutations. Approximately 47% of nonsense mutations can be potentially treated with single exon skipping, rising to 90% with double exon skipping, but to reach this proportion requires the development of exon skipping molecules targeting some 68 of dystrophin's 79 exons, with patient numbers spread thinly across those exons. In this review, we discuss progress and remaining hurdles in exon skipping and an alternative strategy, stop-codon readthrough.

EXPERT OPINION

Antisense-mediated exon skipping therapy is targeted highly at the individual patient and is a clear example of personalized medicine. An efficient regulatory path for drug approval will be a key to success.

Therapy development for spinal muscular atrophy in SMN independent targets

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder, leading to progressive muscle weakness, atrophy, and sometimes premature death. SMA is caused by mutation or deletion of the survival motor neuron-1 (SMN1) gene. An effective treatment does not presently exist. Since the severity of the SMA phenotype is inversely correlated with expression levels of SMN, the SMN-encoded protein, SMN is the most important therapeutic target for development of an effective treatment for SMA. In recent years, numerous SMN independent targets and therapeutic strategies have been demonstrated to have potential roles in SMA treatment. For example, some neurotrophic, antiapoptotic, and myotrophic factors are able to promote survival of motor neurons or improve muscle strength shown in SMA mouse models or clinical trials. Plastin-3, cpg15, and a Rho-kinase inhibitor regulate axonal dynamics and might reduce the influences of SMN depletion in disarrangement of neuromuscular junction. Stem cell transplantation in SMA model mice resulted in improvement of motor behaviors and extension of survival, likely from trophic support. Although most therapies are still under investigation, these nonclassical treatments might provide an adjunctive method for future SMA therapy.

Antisense Oligonucleotide (AON)-based Therapy for Leber Congenital Amaurosis Caused by a Frequent Mutation in CEP290

Leber congenital amaurosis (LCA) is the most severe form of inherited retinal degeneration, with an onset in the first year of life. The most frequent mutation that causes LCA, present in at least 10% of individuals with LCA from North-American and Northern-European descent, is an intronic mutation in CEP290 that results in the inclusion of an aberrant exon in the CEP290 mRNA. Here, we describe a genetic therapy approach that is based on antisense oligonucleotides (AONs), small RNA molecules that are able to redirect normal splicing of aberrantly processed pre-mRNA. Immortalized lymphoblastoid cells of individuals with LCA homozygously carrying the intronic CEP290 mutation were transfected with several AONs that target the aberrant exon that is incorporated in the mutant CEP290 mRNA. Subsequent RNA isolation and reverse transcription-PCR analysis revealed that a number of AONs were capable of almost fully redirecting normal CEP290 splicing, in a dose-dependent manner. Other AONs however, displayed no effect on CEP290 splicing at all, indicating that the rescue of aberrant CEP290 splicing shows a high degree of sequence specificity. Together, our data show that AON-based therapy is a promising therapeutic approach for CEP290-associated LCA that warrants future research in animal models to develop a cure for this blinding disease.

Targeting RNA-splicing for SMA treatment

The central dogma of DNA-RNA-protein was established more than 40 years ago. However, important biological processes have been identified since the central dogma was developed. For example, methylation is important in the regulation of transcription. In contrast, proteins, are more complex due to modifications such as phosphorylation, glycosylation, ubiquitination, or cleavage. RNA is the mediator between DNA and protein, but it can also be modulated at several levels. Among the most profound discoveries of RNA regulation is RNA splicing. It has been estimated that 80% of pre-mRNA undergo alternative splicing, which exponentially increases biological information flow in cellular processes. However, an increased number of regulated steps inevitably accompanies an increased number of errors. Abnormal splicing is often found in cells, resulting in protein dysfunction that causes disease. Splicing of the survival motor neuron (SMN) gene has been extensively studied during the last two decades. Accumulating knowledge on SMN splicing has led to speculation and search for spinal muscular atrophy (SMA) treatment by stimulating the inclusion of exon 7 into SMN mRNA. This mini-review summaries the latest progress on SMN splicing research as a potential treatment for SMA disease.

Spliceosomal small nuclear ribonucleoprotein biogenesis defects and motor neuron selectivity in spinal muscular atrophy

The SMN protein is essential and participates in the assembly of macromolecular complexes of RNA and protein in all cells. The best-characterized function of SMN is as an assembler of spliceosomal small nuclear ribonucleoproteins (snRNPs). SMN performs this function as part of a complex with several other proteins called Gemins. snRNPs are assembled in the cytoplasm in a stepwise manner and then are imported to the nucleus where they participate globally in the splicing of pre-mRNA. Mutations in the SMN1 gene result in the motor neuron disease, spinal muscular atrophy (SMA). Most of these mutations result in a reduction in the expression levels of the SMN protein, which, in turn, results in a reduction in snRNP assembly capacity. This review highlights current studies that have investigated the mechanism of SMN-dependent snRNP assembly, as well as the downstream effects on pre-mRNA splicing that result from a decrease in SMN. This article is part of a Special Issue entitled "RNA-Binding Proteins".