Spinal muscular atrophy (SMA) is caused by mutations of the survival motor neuron 1 (SMN1) gene, retention of the survival motor neuron 2 (SMN2) gene, and insufficient expression of full-length survival motor neuron (SMN) protein. Since SMA patients have at least one copy of SMN2, drug discovery campaigns have sought to identify SMN2 inducers. C5-substituted quinazolines increase SMN2 promoter activity in cell based assays and a derivative, RG3039, has progressed to clinical testing. It is orally bioavailable, brain-penetrant and has been shown to be an inhibitor of the mRNA decapping enzyme, DcpS. The pharmacological characterisation of RG3039, reported in two new studies in Human Molecule Genetics, demonstrates that RG3039 can extend survival, improve function, and impact neuromuscular pathology in three SMA mouse models of varying disease severity.
In the first paper, by Dr. DiDonato et al., report the results obtained in two SMA mouse models of varying disease severity (Taiwanese 5058 Hemi and 2B/- SMA mice). In 2B/- SMA mice, RG3039 provides >600% survival benefit (median 18 days to >112 days) when dosing began at P4, highlighting the importance of early intervention. They determined the minimum effective dose and the associated pharmacokinetic (PK) and exposure relationship of RG3039 and DcpS inhibition ex-vivo. These data support the long PK half-life with extended pharmacodynamic outcome of RG3039 in 2B/- SMA mice. In motor neurons, RG3039 significantly increased the number of gems and cells with gems, which is used as an indirect measure of SMN levels. These studies contribute to dose selection and exposure estimates for the first studies with RG3039 in human subjects.
The second paper by Dr. Sumner et al., shows that RG3039 distributed to central nervous system tissues where it robustly inhibited DcpS enzyme activity, but minimally activated SMN expression or the assembly of small nuclear ribonucleoproteins. Nonetheless, treated SMA mice showed a dose-dependent increase in survival, weight, and motor function. This was associated with improved motor neuron somal and neuromuscular junction synaptic innervation and function and increased muscle size. RG3039 also enhanced survival of conditional SMA mice in which SMN had been genetically-restored to motor neurons. As this systemically delivered drug may have therapeutic benefits that extend beyond motor neurons, it could act additively with SMN-restoring therapies delivered directly to the central nervous system such as antisense oligonucleotides or gene therapy.

Pompe disease is a neuromuscular disease resulting from deficiency in acid α-glucosidase (GAA), results in cardiac, skeletal muscle, and central nervous system (CNS) pathology. Enzyme replacement therapy (ERT) has been shown to partially correct cardiac and skeletal muscle dysfunction. However, ERT does not cross the blood-brain barrier and progressive CNS pathology ensues. Here, the authors tested the hypothesis that intrapleural administration of recombinant adeno-associated virus (rAAV9)-GAA driven by a cytomegalovirus (CMV) or desmin (DES) promoter would improve cardiac and respiratory function in Gaa-/- mice through a direct effect and retrograde transport to motoneurons. Cardiac magnetic resonance imaging revealed significant improvement in ejection fraction in rAAV9-GAA-treated animals. Inspiratory phrenic and diaphragm activity was examined at baseline and during hypercapnic respiratory challenge. Mice treated with AAV9 had greater relative inspiratory burst amplitude during baseline conditions when compared with Gaa-/-. In addition, efferent phrenic burst amplitude was significantly correlated with diaphragm activity in both AAV9-DES and AAV9-CMV groups but not in Gaa-/-. This is the first study to indicate improvements in cardiac, skeletal muscle, and respiratory neural output following rAAV administration in Pompe disease. These results further implicate a role for the CNS in Pompe disease pathology and the critical need to target the neurologic aspects in developing therapeutic strategies.

Myotonic dystrophy type 1 is an autosomal dominant condition caused by a trinucleotide CTG repeat expansion in the 3' untranslated region of the dystrophia myotonica protein kinase gene. The phenotypic features of myopathic facies, generalized weakness, and myotonia are thought to be dependent on repeat number, with larger expansions generally leading to earlier and/or more severe disease. The vast majority of individuals are heterozygous for an expanded allele and an allele in the normal range. In this clinical report, two brothers with congenital myotonic dystrophy type 1are described. The younger of the two siblings is one of only 13 homozygous patients ever reported in the literature. He carries two expanded alleles: one with 1,170 repeats and the other with >100 repeats. His clinical picture is represented in relation to his more severely affected heterozygous brother as well as other published homozygous cases. Finally, the inconsistency between repeat size and symptomatic expression as it applies to the current proposed mechanisms of myotonic dystrophy type 1 pathogenicity is discussed.

Duchenne muscular dystrophy is due to a mutation on the X-chromosome, therefore it rarely affects women, unless there is an unequal lyonisation of the X-chromosome containing the normal dystrophin gene. This study reports the unique situation of a symptomatic Duchenne muscular dystrophy woman who was transplanted with myoblasts received from her asymptomatic monozygotic twin sister 20 years ago. Specific dynamometry was performed to possibly detect a long-term effect of this cell therapy. Long-term safety of myoblast transplantation was established by this exceptional case. However, long-term efficacy could not be definitively asserted for this patient, in spite of several clues suggesting beneficial effects.

Genome editing with engineered nucleases has recently emerged as an approach to correct genetic mutations by enhancing homologous recombination with a DNA repair template. However, many genetic diseases, such as Duchenne muscular dystrophy (DMD), can be treated simply by correcting a disrupted reading frame. In this study, the authors show that genome editing with transcription activator-like effector nucleases (TALENs), without a repair template, can efficiently correct the reading frame and restore the expression of a functional dystrophin protein that is mutated in DMD. TALENs were engineered to mediate highly efficient gene editing at exon 51 of the dystrophin gene. This led to restoration of dystrophin protein expression in cells from Duchenne patients, including skeletal myoblasts and dermal fibroblasts that were reprogrammed to the myogenic lineage by MyoD. Finally, exome sequencing of cells with targeted modifications of the dystrophin locus showed no TALEN-mediated off-target changes to the protein-coding regions of the genome, as predicted by in silico target site analysis. This strategy integrates the rapid and robust assembly of active TALENs with an efficient gene-editing method for the correction of genetic diseases caused by mutations in non-essential coding regions that cause frameshifts or premature stop codons.

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Reading Frame Correction by Targeted Genome Editing Restores Dystrophin Expression in Cells From Duchenne Muscular Dystrophy Patients. Ousterout DG, Perez-Pinera P, Thakore PI, et al. Mol Ther. 2013 Jun 4. doi: 10.1038/mt.2013.111. [Epub ahead of print]

Charcot-Marie-Tooth (CMT) disease is a genetically heterogeneous condition with >50 genes now being identified. Thanks to new technological developments, namely, exome sequencing, the ability to identify additional rare genes in CMT has been drastically improved. In this short report, the authors present data suggesting that MARS is a very rare novel cause of late-onset CMT2. This is supported by strong functional and evolutionary evidence, yet the absence of additional unrelated cases warrant future studies to substantiate this conclusion.