Combination of SMN trans-splicing and a neurotrophic factor increases the life span and body mass in a severe model of spinal muscular atrophy

Alterations in insulin-like growth factor system in spinal muscular atrophy

INTRODUCTION/AIMS

Spinal muscular atrophy (SMA) is an inherited neuromuscular disease caused by survival motor neuron (SMN) protein deficiency. Insulin-like growth factor-I (IGF-I) is a myotrophic and neurotrophic factor that has been reported to be dysregulated in in vivo SMA model systems. However, detailed analyses of the IGF-I system in SMA patients are missing. In this study, we analyzed the components of the IGF-I system in serum and archived skeletal muscle biopsies of SMA patients.

METHODS

Serum IGF-I, IGF binding protein (IGFBP)-3, and IGFBP-5 levels were analyzed in 11 SMA patients and 13 healthy children by immunoradiometric and enzyme-linked immunosorbent assays. The expression of IGF-I, IGF-I receptor, and IGFBP-5 proteins was investigated by immunofluorescence analysis in the archived skeletal muscle biopsies of 9 SMA patients, 6 patients with non-SMA-related neuromuscular disease and atrophic fibers in muscle biopsy, and 4 controls.

RESULTS

A significant decrease in IGF-I levels (mean ± SD: -1.39 ± 1.46 vs. 0.017 ± 0.83, p = 0.02) and increase in IGFBP-5 levels (mean ± SD: 2358.5 ± 1617.4 ng/mL vs. 1003.4 ± 274.3 ng/mL, p=0.03) were detected in serum samples of SMA patients compared to healthy controls. Increased expression of IGF-I, IGF-I receptor, and IGFBP-5 was detected in skeletal muscle biopsies of SMA patients and non-SMA neuromuscular diseases, indicating atrophy-specific alterations in the pathway.

DISCUSSION

Our findings suggested that the components of the IGF-I system are altered in SMA patients at both the systemic and tissue-specific levels.

Mitochondrial Stress Responses in Duchenne muscular dystrophy: Metabolic Dysfunction or Adaptive Reprogramming?

Mitochondrial stress may be a secondary contributor to muscle weakness in inherited muscular dystrophies. Duchenne muscular dystrophy has received the majority of attention whereby most discoveries suggest mitochondrial ATP synthesis may be reduced. However, not all studies support this finding. Furthermore, some studies have reported increased mitochondrial reactive oxygen species and propensity for permeability transition pore formation as an inducer of apoptosis, although divergent findings have also been described. A closer examination of the literature suggests the degree and direction of mitochondrial stress responses may depend on the progression of the disease, the muscle type examined, the mouse model employed with regards to pre-clinical research, the precise metabolic pathways in consideration, and in some cases the in vitro technique used to assess a given mitochondrial bioenergetic function. One intent of this review is to provide careful considerations for future experimental designs to resolve the heterogeneous nature of mitochondrial stress during the progression of Duchenne muscular dystrophy. Such considerations have implications for other muscular dystrophies as well which are addressed briefly herein. A renewed perspective of the term 'mitochondrial dysfunction' is presented whereby stress responses might be re-explored in future investigations as direct contributors to myopathy vs an adaptive 'reprogramming' intended to maintain homeostasis in the face of disease stressors themselves. In so doing, the prospective development of mitochondrial enhancement therapies can be driven by advances in perspectives as much as experimental approaches when resolving the precise relationships between mitochondrial remodelling and muscle weakness in Duchenne and, indeed, other muscular dystrophies.

Systematic review and meta-analysis determining the benefits of in vivo genetic therapy in spinal muscular atrophy rodent models

Spinal muscular atrophy (SMA) is a severe childhood neuromuscular disease for which two genetic therapies, Nusinersen (Spinraza, an antisense oligonucleotide), and AVXS-101 (Zolgensma, an adeno-associated viral vector of serotype 9 AAV9), have recently been approved. We investigated the pre-clinical development of SMA genetic therapies in rodent models and whether this can predict clinical efficacy. We have performed a systematic review of relevant publications and extracted median survival and details of experimental design. A random effects meta-analysis was used to estimate and compare efficacy. We stratified by experimental design (type of genetic therapy, mouse model, route and time of administration) and sought any evidence of publication bias. 51 publications were identified containing 155 individual comparisons, comprising 2573 animals in total. Genetic therapies prolonged survival in SMA mouse models by 3.23-fold (95% CI 2.75-3.79) compared to controls. Study design characteristics accounted for significant heterogeneity between studies and greatly affected observed median survival ratios. Some evidence of publication bias was found. These data are consistent with the extended average lifespan of Spinraza- and Zolgensma-treated children in the clinic. Together, these results support that SMA has been particularly amenable to genetic therapy approaches and highlight SMA as a trailblazer for therapeutic development.

The development and improvement of ribonucleic acid therapy strategies

The biological understanding of RNA has evolved since the discovery of catalytic RNAs in the early 1980s and the establishment of RNA interference (RNAi) in the 1990s. RNA is no longer seen as the simple mid-product between transcription and translation but as potential molecules to be developed as RNA therapeutic drugs. RNA-based therapeutic drugs have gained recognition because of their ability to regulate gene expression and perform cellular functions. Various nucleobase, backbone, and sugar-modified oligonucleotides have been synthesized, as natural oligonucleotides have some limitations such as poor low nuclease resistance, binding affinity, poor cellular uptake, and toxicity, which affect their use as RNA therapeutic drugs. In this review, we briefly discuss different RNA therapeutic drugs and their internal connections, including antisense oligonucleotides, small interfering RNAs (siRNAs) and microRNAs (miRNAs), aptamers, small activating RNAs (saRNAs), and RNA vaccines. We also discuss the important roles of RNA vaccines and their use in the fight against COVID-19. In addition, various chemical modifications and delivery systems used to improve the performance of RNA therapeutic drugs and overcome their limitations are discussed.

In Search of a Cure: The Development of Therapeutics to Alter the Progression of Spinal Muscular Atrophy

Until the recent development of disease-modifying therapeutics, spinal muscular atrophy (SMA) was considered a devastating neuromuscular disease with a poor prognosis for most affected individuals. Symptoms generally present during early childhood and manifest as muscle weakness and progressive paralysis, severely compromising the affected individual’s quality of life, independence, and lifespan. SMA is most commonly caused by the inheritance of homozygously deleted SMN1 alleles with retention of one or more copies of a paralog gene, SMN2, which inversely correlates with disease severity. The recent advent and use of genetically targeted therapies have transformed SMA into a prototype for monogenic disease treatment in the era of genetic medicine. Many SMA-affected individuals receiving these therapies achieve traditionally unobtainable motor milestones and survival rates as medicines drastically alter the natural progression of this disease. This review discusses historical SMA progression and underlying disease mechanisms, highlights advances made in therapeutic research, clinical trials, and FDA-approved medicines, and discusses possible second-generation and complementary medicines as well as optimal temporal intervention windows in order to optimize motor function and improve quality of life for all SMA-affected individuals.

Therapeutic applications of trans-splicing

BACKGROUND

RNA trans-splicing joins exons from different pre-mRNA transcripts to generate a chimeric product. Trans-splicing can also occur at the protein level, with split inteins mediating the ligation of separate gene products to generate a mature protein.

SOURCES OF DATA

Comprehensive literature search of published research papers and reviews using Pubmed.

AREAS OF AGREEMENT

Trans-splicing techniques have been used to target a wide range of diseases in both in vitro and in vivo models, resulting in RNA, protein and functional correction.

AREAS OF CONTROVERSY

Off-target effects can lead to therapeutically undesirable consequences. In vivo efficacy is typically low, and delivery issues remain a challenge.

GROWING POINTS

Trans-splicing provides a promising avenue for developing novel therapeutic approaches. However, much more research needs to be done before developing towards preclinical studies.

AREAS TIMELY FOR DEVELOPING RESEARCH

Increasing trans-splicing efficacy and specificity by rational design, screening and competitive inhibition of endogenous cis-splicing.

Drug Screening and Drug Repositioning as Promising Therapeutic Approaches for Spinal Muscular Atrophy Treatment

Spinal muscular atrophy (SMA) is the most common genetic disease affecting infants and young adults. Due to mutation/deletion of the survival motor neuron (SMN) gene, SMA is characterized by the SMN protein lack, resulting in motor neuron impairment, skeletal muscle atrophy and premature death. Even if the genetic causes of SMA are well known, many aspects of its pathogenesis remain unclear and only three drugs have been recently approved by the Food and Drug Administration (Nusinersen-Spinraza; Onasemnogene abeparvovec or AVXS-101-Zolgensma; Risdiplam-Evrysdi): although assuring remarkable results, the therapies show some important limits including high costs, still unknown long-term effects, side effects and disregarding of SMN-independent targets. Therefore, the research of new therapeutic strategies is still a hot topic in the SMA field and many efforts are spent in drug discovery. In this review, we describe two promising strategies to select effective molecules: drug screening (DS) and drug repositioning (DR). By using compounds libraries of chemical/natural compounds and/or Food and Drug Administration-approved substances, DS aims at identifying new potentially effective compounds, whereas DR at testing drugs originally designed for the treatment of other pathologies. The drastic reduction in risks, costs and time expenditure assured by these strategies make them particularly interesting, especially for those diseases for which the canonical drug discovery process would be long and expensive. Interestingly, among the identified molecules by DS/DR in the context of SMA, besides the modulators of SMN2 transcription, we highlighted a convergence of some targeted molecular cascades contributing to SMA pathology, including cell death related-pathways, mitochondria and cytoskeleton dynamics, neurotransmitter and hormone modulation.

Targeting Alternative Splicing as a Potential Therapy for Episodic Ataxia Type 2

Episodic ataxia type 2 (EA2) is an autosomal dominant neurological disorder characterized by paroxysmal attacks of ataxia, vertigo, and nausea that usually last hours to days. It is caused by loss-of-function mutations in CACNA1A, the gene encoding the pore-forming α1 subunit of P/Q-type voltage-gated Ca2+ channels. Although pharmacological treatments, such as acetazolamide and 4-aminopyridine, exist for EA2, they do not reduce or control the symptoms in all patients. CACNA1A is heavily spliced and some of the identified EA2 mutations are predicted to disrupt selective isoforms of this gene. Modulating splicing of CACNA1A may therefore represent a promising new strategy to develop improved EA2 therapies. Because RNA splicing is dysregulated in many other genetic diseases, several tools, such as antisense oligonucleotides, trans-splicing, and CRISPR-based strategies, have been developed for medical purposes. Here, we review splicing-based strategies used for genetic disorders, including those for Duchenne muscular dystrophy, spinal muscular dystrophy, and frontotemporal dementia with Parkinsonism linked to chromosome 17, and discuss their potential applicability to EA2.

AAV9-Stathmin1 gene delivery improves disease phenotype in an intermediate mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a devastating infantile genetic disorder caused by the loss of survival motor neuron (SMN) protein that leads to premature death due to loss of motor neurons and muscle atrophy. The approval of an antisense oligonucleotide therapy for SMA was an important milestone in SMA research; however, effective next-generation therapeutics will likely require combinatorial SMN-dependent therapeutics and SMN-independent disease modifiers. A recent cross-disease transcriptomic analysis identified Stathmin-1 (STMN1), a tubulin-depolymerizing protein, as a potential disease modifier across different motor neuron diseases, including SMA. Here, we investigated whether viral-based delivery of STMN1 decreased disease severity in a well-characterized SMA mouse model. Intracerebroventricular delivery of scAAV9-STMN1 in SMA mice at P2 significantly increased survival and weight gain compared to untreated SMA mice without elevating Smn levels. scAAV9-STMN1 improved important hallmarks of disease, including motor function, NMJ pathology and motor neuron cell preservation. Furthermore, scAAV9-STMN1 treatment restored microtubule networks and tubulin expression without affecting tubulin stability. Our results show that scAAV9-STMN1 treatment improves SMA pathology possibly by increasing microtubule turnover leading to restored levels of stable microtubules. Overall, these data demonstrate that STMN1 can significantly reduce the SMA phenotype independent of restoring SMN protein and highlight the importance of developing SMN-independent therapeutics for the treatment of SMA.

Insulin-like growth factor 1 signaling in motor neuron and polyglutamine diseases: From molecular pathogenesis to therapeutic perspectives

The pleiotropic peptide insulin-like growth factor 1 (IGF-I) regulates human body homeostasis and cell growth. IGF-I activates two major signaling pathways, namely phosphoinositide-3-kinase (PI3K)/protein kinase B (PKB/Akt) and Ras/extracellular signal-regulated kinase (ERK), which contribute to brain development, metabolism and function as well as to neuronal maintenance and survival. In this review, we discuss the general and tissue-specific effects of the IGF-I pathways. In addition, we present a comprehensive overview examining the role of IGF-I in neurodegenerative diseases, such as spinal and muscular atrophy, amyotrophic lateral sclerosis, and polyglutamine diseases. In each disease, we analyze the disturbances of the IGF-I pathway, the modification of the disease protein by IGF-I signaling, and the therapeutic strategies based on the use of IGF-I developed to date. Lastly, we highlight present and future considerations in the use of IGF-I for the treatment of these disorders.

Drug treatment for spinal muscular atrophy types II and III

BACKGROUND

Spinal muscular atrophy (SMA) is caused by a homozygous deletion of the survival motor neuron 1 (SMN1) gene on chromosome 5, or a heterozygous deletion in combination with a (point) mutation in the second SMN1 allele. This results in degeneration of anterior horn cells, which leads to progressive muscle weakness. Children with SMA type II do not develop the ability to walk without support and have a shortened life expectancy, whereas children with SMA type III develop the ability to walk and have a normal life expectancy. This is an update of a review first published in 2009 and previously updated in 2011.

OBJECTIVES

To evaluate if drug treatment is able to slow or arrest the disease progression of SMA types II and III, and to assess if such therapy can be given safely.

SEARCH METHODS

We searched the Cochrane Neuromuscular Specialised Register, CENTRAL, MEDLINE, Embase, and ISI Web of Science conference proceedings in October 2018. In October 2018, we also searched two trials registries to identify unpublished trials.

SELECTION CRITERIA

We sought all randomised or quasi-randomised trials that examined the efficacy of drug treatment for SMA types II and III. Participants had to fulfil the clinical criteria and have a homozygous deletion or hemizygous deletion in combination with a point mutation in the second allele of the SMN1 gene (5q11.2-13.2) confirmed by genetic analysis. The primary outcome measure was change in disability score within one year after the onset of treatment. Secondary outcome measures within one year after the onset of treatment were change in muscle strength, ability to stand or walk, change in quality of life, time from the start of treatment until death or full-time ventilation and adverse events attributable to treatment during the trial period. Treatment strategies involving SMN1-replacement with viral vectors are out of the scope of this review, but a summary is given in Appendix 1. Drug treatment for SMA type I is the topic of a separate Cochrane Review.

DATA COLLECTION AND ANALYSIS

We followed standard Cochrane methodology.

MAIN RESULTS

The review authors found 10 randomised, placebo-controlled trials of treatments for SMA types II and III for inclusion in this review, with 717 participants. We added four of the trials at this update. The trials investigated creatine (55 participants), gabapentin (84 participants), hydroxyurea (57 participants), nusinersen (126 participants), olesoxime (165 participants), phenylbutyrate (107 participants), somatotropin (20 participants), thyrotropin-releasing hormone (TRH) (nine participants), valproic acid (33 participants), and combination therapy with valproic acid and acetyl-L-carnitine (ALC) (61 participants). Treatment duration was from three to 24 months. None of the studies investigated the same treatment and none was completely free of bias. All studies had adequate blinding, sequence generation and reporting of primary outcomes. Based on moderate-certainty evidence, intrathecal nusinersen improved motor function (disability) in children with SMA type II, with a 3.7-point improvement in the nusinersen group on the Hammersmith Functional Motor Scale Expanded (HFMSE; range of possible scores 0 to 66), compared to a 1.9-point decline on the HFMSE in the sham procedure group (P < 0.01; n = 126). On all motor function scales used, higher scores indicate better function. Based on moderate-certainty evidence from two studies, the following interventions had no clinically important effect on motor function scores in SMA types II or III (or both) in comparison to placebo: creatine (median change 1 higher, 95% confidence interval (CI) -1 to 2; on the Gross Motor Function Measure (GMFM), scale 0 to 264; n = 40); and combination therapy with valproic acid and carnitine (mean difference (MD) 0.64, 95% CI -1.1 to 2.38; on the Modified Hammersmith Functional Motor Scale (MHFMS), scale 0 to 40; n = 61). Based on low-certainty evidence from other single studies, the following interventions had no clinically important effect on motor function scores in SMA types II or III (or both) in comparison to placebo: gabapentin (median change 0 in the gabapentin group and -2 in the placebo group on the SMA Functional Rating Scale (SMAFRS), scale 0 to 50; n = 66); hydroxyurea (MD -1.88, 95% CI -3.89 to 0.13 on the GMFM, scale 0 to 264; n = 57), phenylbutyrate (MD -0.13, 95% CI -0.84 to 0.58 on the Hammersmith Functional Motor Scale (HFMS) scale 0 to 40; n = 90) and monotherapy of valproic acid (MD 0.06, 95% CI -1.32 to 1.44 on SMAFRS, scale 0 to 50; n = 31). Very low-certainty evidence suggested that the following interventions had little or no effect on motor function: olesoxime (MD 2, 95% -0.25 to 4.25 on the Motor Function Measure (MFM) D1 + D2, scale 0 to 75; n = 160) and somatotropin (median change at 3 months 0.25 higher, 95% CI -1 to 2.5 on the HFMSE, scale 0 to 66; n = 19). One small TRH trial did not report effects on motor function and the certainty of evidence for other outcomes from this trial were low or very low. Results of nine completed trials investigating 4-aminopyridine, acetyl-L-carnitine, CK-2127107, hydroxyurea, pyridostigmine, riluzole, RO6885247/RG7800, salbutamol and valproic acid were awaited and not available for analysis at the time of writing. Various trials and studies investigating treatment strategies other than nusinersen (e.g. SMN2-augmentation by small molecules), are currently ongoing.

AUTHORS' CONCLUSIONS

Nusinersen improves motor function in SMA type II, based on moderate-certainty evidence. Creatine, gabapentin, hydroxyurea, phenylbutyrate, valproic acid and the combination of valproic acid and ALC probably have no clinically important effect on motor function in SMA types II or III (or both) based on low-certainty evidence, and olesoxime and somatropin may also have little to no clinically important effect but evidence was of very low-certainty. One trial of TRH did not measure motor function.

Selective vulnerability in neuronal populations in nmd/SMARD1 mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive motor neuron disease causing distal limb muscle atrophy that progresses proximally and is accompanied by diaphragmatic paralysis. Neuromuscular junction (NMJ) alterations have been reported in muscles of SMARD1 model mice, known as nmd mice, with varying degrees of severity, suggesting that different muscles are specifically and selectively resistant or susceptible to denervation. To evaluate the extent of NMJ pathology in a broad range of muscles, a panel of axial and appendicular muscles were isolated and immunostained from nmd mice. These analyses revealed that selective distal appendage muscles were highly vulnerable to denervation. Susceptibility to pathology was not limited to NMJ alterations, but included defects in myelination within those neurons innervating susceptible muscles. Interestingly, end plate fragmentation was present within all muscles independent of the extent of NMJ alterations, suggesting that end plate fragmentation is an early hallmark of SMARD1 pathogenesis. Expressing the full-length IGHMBP2 cDNA using an adeno-associated virus (AAV9) significantly decreased all aspects of muscle and nerve disease pathology. These results shed new light onto the pathogenesis of SMARD1 by identifying specific motor units that are resistant and susceptible to neurodegeneration in an important model of SMARD1.

Enhancing GABAergic Transmission Improves Locomotion in a Caenorhabditis elegans Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease characterized by degeneration of spinal motor neurons resulting in variable degrees of muscular wasting and weakness. It is caused by a loss-of-function mutation in the survival motor neuron (SMN1) gene. Caenorhabditis elegans mutants lacking SMN recapitulate several aspects of the disease including impaired movement and shorted life span. We examined whether genes previously implicated in life span extension conferred benefits to C. elegans lacking SMN. We find that reducing daf-2/insulin receptor signaling activity promotes survival and improves locomotor behavior in this C. elegans model of SMA. The locomotor dysfunction in C. elegans lacking SMN correlated with structural and functional abnormalities in GABAergic neuromuscular junctions (NMJs). Moreover, we demonstrated that reduction in daf-2 signaling reversed these abnormalities. Remarkably, enhancing GABAergic neurotransmission alone was able to correct the locomotor dysfunction. Our work indicated that an imbalance of excitatory/inhibitory activity within motor circuits and underlies motor system dysfunction in this SMA model. Interventions aimed at restoring the balance of excitatory/inhibitory activity in motor circuits could be of benefit to individuals with SMA.

A Direct Comparison of IV and ICV Delivery Methods for Gene Replacement Therapy in a Mouse Model of SMARD1

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an infantile autosomal recessive disease caused by the loss of the ubiquitously expressed IGHMBP2 gene. SMARD1 causes degeneration of alpha-motor neurons, resulting in distal muscle weakness, diaphragm paralysis, and respiratory malfunction. We have reported that delivery of a low dose of AAV9-IGHMBP2 to the CNS results in a significant rescue of the SMARD1 mouse model (nmd). To examine how a delivery route can impact efficacy, a direct comparison of intravenous (IV) and intracerebroventricular (ICV) delivery of AAV9-IGHMBP2 was performed. Using a low-dose, both IV and ICV delivery routes led to a significant extension in survival and increased body weight. Conversely, only ICV-treated animals demonstrated improvements in the hindlimb muscle, neuromuscular junction, and motor function. The hindlimb phenotype of IV-treated mice resembled the untreated nmd mice. We investigated whether the increased survival of IV-treated nmd mice was the result of a positive impact on the cardiac function. Our results revealed that cardiac function and pathology were similarly improved in IV- and ICV-treated mice. We concluded that while IV delivery of a low dose does not improve the hindlimb phenotype and motor function, partial restoration of cardiac performance is sufficient to significantly extend survival.

Optimization of trans-Splicing for Huntington's Disease RNA Therapy

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in exon 1 of the Huntingtin (HTT) gene. We have previously demonstrated that spliceosome-mediated trans-splicing is a viable molecular strategy to specifically reduce and repair mutant HTT (mtHTT). Here, the targeted tethering efficacy of the pre-mRNA trans-splicing modules (PTM) in HTT was optimized. Various PTMs that targeted the 3′ end of HTT intron 1 or the intron 1 branch point were shown trans-splice into an HTT mini-gene, as well as the endogenous HTT pre-mRNA. PTMs that specifically target the endogenous intron 1 branch point increased the trans-splicing efficacy from 1–5 to 10–15%. Furthermore, lentiviral expression of PTMs in a human HD patient iPSC-derived neural culture significantly reversed two previously established polyQ-length dependent phenotypes. These results suggest that pre-mRNA repair of mtHTT could hold therapeutic benefit and it demonstrates an alternative platform to correct the mRNA product produced by the mtHTT allele in the context of HD.

The SMA Trust: the role of a disease-focused research charity in developing treatments for SMA

SMA is a rare hereditary neuromuscular disease that causes weakness and muscle wasting as a result of the loss of spinal motor neurons. In its most severe form, SMA is the commonest genetic cause of death in infants, and children with less severe forms of SMA face the prospect of lifelong disability from progressive muscle wasting, loss of mobility and limb weakness. The initial discovery of the defective gene has been followed by major advances in our understanding of the genetic, cellular and molecular basis of SMA, providing the foundation for a range of approaches to treatment, including gene therapy, antisense oligonucleotide treatments and more traditional drug-based approaches to slow or halt disease progression. The approval by the US Food and Drug Administration (FDA) of Spinraza (nusinersen), the first targeted treatment for spinal muscular atrophy (SMA), is a historic moment. Disease-focused research charities, such as The SMA Trust (UK), continue to have a crucial role in promoting the development of additional treatments for SMA, both by funding translational research and by promoting links between researchers, people living with SMA and other stakeholders, including pharmaceutical companies and healthcare providers.

Comparison of independent screens on differentially vulnerable motor neurons reveals alpha-synuclein as a common modifier in motor neuron diseases

The term “motor neuron disease” encompasses a spectrum of disorders in which motor neurons are the primary pathological target. However, in both patients and animal models of these diseases, not all motor neurons are equally vulnerable, in that while some motor neurons are lost very early in disease, others remain comparatively intact, even at late stages. This creates a valuable system to investigate the factors that regulate motor neuron vulnerability. In this study, we aim to use this experimental paradigm to identify potential transcriptional modifiers. We have compared the transcriptome of motor neurons from healthy wild-type mice, which are differentially vulnerable in the childhood motor neuron disease Spinal Muscular Atrophy (SMA), and have identified 910 transcriptional changes. We have compared this data set with published microarray data sets on other differentially vulnerable motor neurons. These neurons were differentially vulnerable in the adult onset motor neuron disease Amyotrophic Lateral Sclerosis (ALS), but the screen was performed on the equivalent population of neurons from neurologically normal human, rat and mouse. This cross species comparison has generated a refined list of differentially expressed genes, including CELF5, Col5a2, PGEMN1, SNCA, Stmn1 and HOXa5, alongside a further enrichment for synaptic and axonal transcripts. As an in vivo validation, we demonstrate that the manipulation of a significant number of these transcripts can modify the neurodegenerative phenotype observed in a Drosophila line carrying an ALS causing mutation. Finally, we demonstrate that vector-mediated expression of alpha-synuclein (SNCA), a transcript decreased in selectively vulnerable motor neurons in all four screens, can extend life span, increase weight and decrease neuromuscular junction pathology in a mouse model of SMA. In summary, we have combined multiple data sets to identify transcripts, which are strong candidates for being phenotypic modifiers, and demonstrated SNCA is a modifier of pathology in motor neuron disease.

The term “motor neuron disease” refers to a group of disorders, causing progressive paralysis of affected patients due to the degeneration of motor neurons cells which control voluntary movements. Importantly, not all motor neurons appear to be affected in the same way, with those that control the face being affected less that those that control the abdomen. The reason why some motor neurons are more vulnerable is unknown; however, understanding this may provide new targets for therapeutics to slow motor neuron degeneration either as stand-alone therapeutics or in combination with SMN-inducing compounds. In this study, we analysed gene expression in different groups of motor neurons and compared this to previously published expression data to identify commonalities. One of the common transcripts was alpha-synuclein (SNCA), which was consistently expressed at lower levels in vulnerable motor neurons. Importantly, when SNCA levels were increased in a mouse model of motor neuron disease, the disease phenotype was significantly reduced, including an extension in survival and reduction in motor neuron pathology. Collectively, these results demonstrate that this approach can identify disease modifiers that can reduce disease severity in models of motor neuron disease and potentially identify new therapeutic targets.

Reprogramming the Dynamin 2 mRNA by Spliceosome-mediated RNA Trans-splicing

Dynamin 2 (DNM2) is a large GTPase, ubiquitously expressed, involved in membrane trafficking and regulation of actin and microtubule cytoskeletons. DNM2 mutations cause autosomal dominant centronuclear myopathy which is a rare congenital myopathy characterized by skeletal muscle weakness and histopathological features including nuclear centralization in absence of regeneration. No curative treatment is currently available for the DNM2-related autosomal dominant centronuclear myopathy. In order to develop therapeutic strategy, we evaluated here the potential of Spliceosome-Mediated RNA Trans-splicing technology to reprogram the Dnm2-mRNA in vitro and in vivo in mice. We show that classical 3′-trans-splicing strategy cannot be considered as accurate therapeutic strategy regarding toxicity of the pre-trans-splicing molecules leading to low rate of trans-splicing in vivo. Thus, we tested alternative strategies devoted to prevent this toxicity and enhance frequency of trans-splicing events. We succeeded to overcome the toxicity through a 5′-trans-splicing strategy which also allows detection of trans-splicing events at mRNA and protein levels in vitro and in vivo. These results suggest that the Spliceosome-Mediated RNA Trans-splicing strategy may be used to reprogram mutated Dnm2-mRNA but highlight the potential toxicity linked to the molecular tools which have to be carefully investigated during preclinical development.

Is spinal muscular atrophy a disease of the motor neurons only: pathogenesis and therapeutic implications?

Spinal muscular atrophy (SMA) is a genetic neurological disease that causes infant mortality; no effective therapies are currently available. SMA is due to homozygous mutations and/or deletions in the survival motor neuron 1 gene and subsequent reduction of the SMN protein, leading to the death of motor neurons. However, there is increasing evidence that in addition to motor neurons, other cell types are contributing to SMA pathology. In this review, we will discuss the involvement of non-motor neuronal cells, located both inside and outside the central nervous system, in disease onset and progression. Even if SMN restoration in motor neurons is needed, it has been shown that optimal phenotypic amelioration in animal models of SMA requires a more widespread SMN correction. It has been demonstrated that non-motor neuronal cells are also involved in disease pathogenesis and could have important therapeutic implications. For these reasons it will be crucial to take this evidence into account for the clinical translation of the novel therapeutic approaches.

Rescue of a Mouse Model of Spinal Muscular Atrophy With Respiratory Distress Type 1 by AAV9-IGHMBP2 Is Dose Dependent

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive disease occurring during childhood. The gene responsible for disease development is a ubiquitously expressed protein, IGHMBP2. Mutations in IGHMBP2 result in the loss of α-motor neurons leading to muscle atrophy in the distal limbs accompanied by respiratory complications. Although genetically and clinically distinct, proximal SMA is also caused by the loss of a ubiquitously expressed gene (SMN). Significant preclinical success has been achieved in proximal SMA using viral-based gene replacement strategies. We leveraged the technologies employed in SMA to demonstrate gene replacement efficacy in an SMARD1 animal model. Intracerebroventricular (ICV) injection of single-stranded AAV9 expressing the full-length cDNA of IGHMBP2 in a low dose led to a significant level of rescue in treated SMARD1 animals. Consistent with drastically increased survival, weight gain, and strength, the rescued animals demonstrated a significant improvement in muscle, NMJ, motor neurons, and axonal pathology. In addition, increased levels of IGHMBP2 in lumbar motor neurons verified the efficacy of the virus to transduce the target tissues. Our results indicate that AAV9-based gene replacement is a viable strategy for SMARD1, although dosing effects and potential negative impacts of high dose and ICV injection should be thoroughly investigated.

Developing therapies for spinal muscular atrophy

Spinal muscular atrophy is an autosomal-recessive pediatric neurodegenerative disease characterized by loss of spinal motor neurons. It is caused by mutation in the gene survival of motor neuron 1 (SMN1), leading to loss of function of the full-length SMN protein. SMN has a number of functions in neurons, including RNA splicing and snRNP biogenesis in the nucleus, and RNA trafficking in neurites. The expression level of full-length SMN protein from the SMN2 locus modifies disease severity. Increasing full-length SMN protein by a small amount can lead to significant improvements in the neurological phenotype. Currently available interventions for spinal muscular atrophy patients are physical therapy and orthopedic, nutritional, and pulmonary interventions; these are palliative or supportive measures and do not address the etiology of the disease. In the past decade, there has been a push for developing therapeutics to improve motor phenotypes and increase life span of spinal muscular atrophy patients. These therapies are aimed primarily at restoration of full-length SMN protein levels, but other neuroprotective treatments have been investigated as well. Here, we discuss recent advances in basic and clinical studies toward finding safe and effective treatments of spinal muscular atrophy using gene therapy, antisense oligonucleotides, and other small molecule modulators of SMN expression.

Chimeric RNAs as potential biomarkers for tumor diagnosis

Cancers claim millions of lives each year. Early detection that can enable a higher chance of cure is of paramount importance to cancer patients. However, diagnostic tools for many forms of tumors have been lacking. Over the last few years, studies of chimeric RNAs as biomarkers have emerged. Numerous reports using bioinformatics and screening methodologies have described more than 30,000 expressed sequence tags (EST) or cDNA sequences as putative chimeric RNAs. While cancer cells have been well known to contain fusion genes derived from chromosomal translocations, rearrangements or deletions, recent studies suggest that trans-splicing in cells may be another source of chimeric RNA production. Unlike cis-splicing, trans-splicing takes place between two pre-mRNA molecules, which are in most cases derived from two different genes, generating a chimeric non-co-linear RNA. It is possible that trans-splicing occurs in normal cells at high frequencies but the resulting chimeric RNAs exist only at low levels. However the levels of certain RNA chimeras may be elevated in cancers, leading to the formation of fusion genes. In light of the fact that chimeric RNAs have been shown to be overrepresented in various tumors, studies of the mechanisms that produce chimeric RNAs and identification of signature RNA chimeras as biomarkers present an opportunity for the development of diagnoses for early tumor detection.

SMN regulates axonal local translation via miR-183/mTOR pathway

Reduced expression of SMN protein causes spinal muscular atrophy (SMA), a neurodegenerative disorder leading to motor neuron dysfunction and loss. However, the molecular mechanisms by which SMN regulates neuronal dysfunction are not fully understood. Here, we report that reduced SMN protein level alters miRNA expression and distribution in neurons. In particular, miR-183 levels are increased in neurites of SMN-deficient neurons. We demonstrate that miR-183 regulates translation of mTor via direct binding to its 3' UTR. Interestingly, local axonal translation of mTor is reduced in SMN-deficient neurons, and this can be recovered by miR-183 inhibition. Finally, inhibition of miR-183 expression in the spinal cord of an SMA mouse model prolongs survival and improves motor function of Smn-mutant mice. Together, these observations suggest that axonal miRNAs and the mTOR pathway are previously unidentified molecular mechanisms contributing to SMA pathology.

Gene therapy: a promising approach to treating spinal muscular atrophy

Spinal muscular atrophy (SMA) is a severe autosomal recessive disease caused by a genetic defect in the survival motor neuron 1 (SMN1) gene, which encodes SMN, a protein widely expressed in all eukaryotic cells. Depletion of the SMN protein causes muscle weakness and progressive loss of movement in SMA patients. The field of gene therapy has made major advances over the past decade, and gene delivery to the central nervous system (CNS) by in vivo or ex vivo techniques is a rapidly emerging field in neuroscience. Despite Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis being among the most common neurodegenerative diseases in humans and attractive targets for treatment development, their multifactorial origin and complicated genetics make them less amenable to gene therapy. Monogenic disorders resulting from modifications in a single gene, such as SMA, prove more favorable and have been at the fore of this evolution of potential gene therapies, and results to date have been promising at least. With the estimated number of monogenic diseases standing in the thousands, elucidating a therapeutic target for one could have major implications for many more. Recent progress has brought about the commercialization of the first gene therapies for diseases, such as pancreatitis in the form of Glybera, with the potential for other monogenic disease therapies to follow suit. While much research has been carried out, there are many limiting factors that can halt or impede translation of therapies from the bench to the clinic. This review will look at both recent advances and encountered impediments in terms of SMA and endeavor to highlight the promising results that may be applicable to various associated diseases and also discuss the potential to overcome present limitations.

Systemic administration of a recombinant AAV1 vector encoding IGF-1 improves disease manifestations in SMA mice

Spinal muscular atrophy is a progressive motor neuron disease caused by a deficiency of survival motor neuron. In this study, we evaluated the efficacy of intravenous administration of a recombinant adeno-associated virus (AAV1) vector encoding human insulin-like growth factor-1 (IGF-1) in a severe mouse model of spinal muscular atrophy. Measurable quantities of human IGF-1 transcripts and protein were detected in the liver (up to 3 months postinjection) and in the serum indicating that IGF-1 was secreted from the liver into systemic circulation. Spinal muscular atrophy mice administered AAV1-IGF-1 on postnatal day 1 exhibited a lower extent of motor neuron degeneration, cardiac and muscle atrophy as well as a greater extent of innervation at the neuromuscular junctions compared to untreated controls at day 8 posttreatment. Importantly, treatment with AAV1-IGF-1 prolonged the animals' lifespan, increased their body weights and improved their motor coordination. Quantitative polymerase chain reaction and western blot analyses showed that AAV1-mediated expression of IGF-1 led to an increase in survival motor neuron transcript and protein levels in the spinal cord, brain, muscles, and heart. These data indicate that systemically delivered AAV1-IGF-1 can correct several of the biochemical and behavioral deficits in spinal muscular atrophy mice through increasing tissue levels of survival motor neuron.

Cis-splicing and translation of the pre-trans-splicing molecule combine with efficiency in spliceosome-mediated RNA trans-splicing

Muscular dystrophies are a group of genetically distinct diseases for which no treatment exists. While gene transfer approach is being tested for several of these diseases, such strategies can be hampered when the size of the corresponding complementary DNA (cDNA) exceeds the packaging capacity of adeno-associated virus vectors. This issue concerns, in particular, dysferlinopathies and titinopathies that are due to mutations in the dysferlin (DYSF) and titin (TTN) genes. We investigated the efficacy of RNA trans-splicing as a mode of RNA therapy for these two types of diseases. Results obtained with RNA trans-splicing molecules designed to target the 3' end of mouse titin and human dysferlin pre-mRNA transcripts indicated that trans-splicing of pre-mRNA generated from minigene constructs or from the endogenous genes was achieved. Collectively, these results provide the first demonstration of DYSF and TTN trans-splicing reprogramming in vitro and in vivo. However, in addition to these positive results, we uncovered a possible issue of the technique in the form of undesirable translation of RNA pre-trans-splicing molecules, directly from open reading frames present on the molecule or associated with internal alternative cis-splicing. These events may hamper the efficiency of the trans-splicing process and/or lead to toxicity.

Investigation of a recombinant SMN protein delivery system to treat spinal muscular atrophy

Spinal muscular atrophy (SMA), the most common genetic cause of infant death, is a neurodegenerative disorder affecting motor neurons. SMA results from a loss in full-length survival of motor neuron (SMN) protein due to deletions/mutations in the SMN1 gene. In this study, we assessed the ability of cell-penetrating peptides (CPP) to deliver recombinant SMN protein to cultured neurons as a prelude for a potential therapeutic to treat SMA. Firstly, we confirmed that E. coli produced recombinant GFP protein fused to TAT (YGRKKRRQRRR; TAT-GFP) transduced rat cortical neurons in a concentration dependent manner. However, due to low yields of recombinant TATSMN protein obtainable from E. coli, we investigated the potential of a modified TAT (TATκ: YARKAARQARA) or R9 (RRRRRRRRR) peptide downstream of the fibronectin (FIB) secretory signal peptide to generate recombinant CPP-fused SMN protein. While U251 cells transduced with an adenoviral vector expressing CMV-FIB-TATκ-SMN secreted recombinant TATκ-SMN protein, we did not detect TATκ-SMN protein transduction of cortical neurons. Further, purified TATκ-SMN was unable to transduce SH-SY5Y cells, nor block apoptosis following LY294002 treatment of these cells. Our findings indicate that TATκ is not a suitable CPP to deliver SMN protein to neurons. Nonetheless, we have developed a novel method to generate full-length recombinant SMN protein using a mammalian expression system, which can be used to explore the application of other CPPs to deliver SMN protein as a treatment for SMA.

More than a bystander: the contributions of intrinsic skeletal muscle defects in motor neuron diseases

Spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), and spinal-bulbar muscular atrophy (SBMA) are devastating diseases characterized by the degeneration of motor neurons. Although the molecular causes underlying these diseases differ, recent findings have highlighted the contribution of intrinsic skeletal muscle defects in motor neuron diseases. The use of cell culture and animal models has led to the important finding that muscle defects occur prior to and independently of motor neuron degeneration in motor neuron diseases. In SMA for instance, the muscle specific requirements of the SMA disease-causing gene have been demonstrated by a series of genetic rescue experiments in SMA models. Conditional ALS mouse models expressing a muscle specific mutant SOD1 gene develop atrophy and muscle degeneration in the absence of motor neuron pathology. Treating SBMA mice by over-expressing IGF-1 in a skeletal muscle-specific manner attenuates disease severity and improves motor neuron pathology. In the present review, we provide an in depth description of muscle intrinsic defects, and discuss how they impact muscle function in these diseases. Furthermore, we discuss muscle-specific therapeutic strategies used to treat animal models of SMA, ALS, and SBMA. The study of intrinsic skeletal muscle defects is crucial for the understanding of the pathophysiology of these diseases and will open new therapeutic options for the treatment of motor neuron diseases.

Spinal muscular atrophy: development and implementation of potential treatments

In neurodegenerative disorders, effective treatments are urgently needed, along with methods to determine whether treatment worked. In this review, we discuss the rapid progress in the understanding of recessive proximal spinal muscular atrophy and how this is leading to exciting potential treatments of the disease. Spinal muscular atrophy is caused by loss of the survival motor neuron 1 (SMN1) gene and reduced levels of SMN protein. The critical downstream targets of SMN deficiency that result in motor neuron loss are not known. However, increasing SMN levels has a marked impact in mouse models, and these therapeutics are rapidly moving toward clinical trials. Promising preclinical therapies, the varying degree of impact on the mouse models, and potential measures of treatment effect are reviewed. One key issue discussed is the variable outcome of increasing SMN at different stages of disease progression.

Pathological impact of SMN2 mis-splicing in adult SMA mice

Loss-of-function mutations in SMN1 cause spinal muscular atrophy (SMA), a leading genetic cause of infant mortality. The related SMN2 gene expresses suboptimal levels of functional SMN protein, due to a splicing defect. Many SMA patients reach adulthood, and there is also adult-onset (type IV) SMA. There is currently no animal model for adult-onset SMA, and the tissue-specific pathogenesis of post-developmental SMN deficiency remains elusive. Here, we use an antisense oligonucleotide (ASO) to exacerbate SMN2 mis-splicing. Intracerebroventricular ASO injection in adult SMN2-transgenic mice phenocopies key aspects of adult-onset SMA, including delayed-onset motor dysfunction and relevant histopathological features. SMN2 mis-splicing increases during late-stage disease, likely accelerating disease progression. Systemic ASO injection in adult mice causes peripheral SMN2 mis-splicing and affects prognosis, eliciting marked liver and heart pathologies, with decreased IGF1 levels. ASO dose–response and time-course studies suggest that only moderate SMN levels are required in the adult central nervous system, and treatment with a splicing-correcting ASO shows a broad therapeutic time window. We describe distinctive pathological features of adult-onset and early-onset SMA.

Spinal muscular atrophy: an update on therapeutic progress

Humans have two nearly identical copies of survival motor neuron gene: SMN1 and SMN2. Deletion or mutation of SMN1 combined with the inability of SMN2 to compensate for the loss of SMN1 results in spinal muscular atrophy (SMA), a leading genetic cause of infant mortality. SMA affects 1 in ~6000 live births, a frequency much higher than in several genetic diseases. The major known defect of SMN2 is the predominant exon 7 skipping that leads to production of a truncated protein (SMNΔ7), which is unstable. Therefore, SMA has emerged as a model genetic disorder in which almost the entire disease population could be linked to the aberrant splicing of a single exon (i.e. SMN2 exon 7). Diverse treatment strategies aimed at improving the function of SMN2 have been envisioned. These strategies include, but are not limited to, manipulation of transcription, correction of aberrant splicing and stabilization of mRNA, SMN and SMNΔ7. This review summarizes up to date progress and promise of various in vivo studies reported for the treatment of SMA.

Spinal muscular atrophy: from gene discovery to clinical trials

Spinal muscular atrophy (SMA) is a common neuromuscular disorder with autosomal recessive inheritance, resulting in the degeneration of motor neurons. The incidence of the disease has been estimated at 1 in 6000-10,000 newborns with a carrier frequency of 1 in 40-60. SMA is caused by mutations of the SMN1 gene, located on chromosome 5q13. The gene product, survival motor neuron (SMN) plays critical roles in a variety of cellular activities. SMN2, a homologue of SMN1, is retained in all SMA patients and generates low levels of SMN, but does not compensate for the mutated SMN1. Genetic analysis demonstrates the presence of homozygous deletion of SMN1 in most patients, and allows screening of heterozygous carriers in affected families. Considering high incidence of carrier frequency in SMA, population-wide newborn and carrier screening has been proposed. Although no effective treatment is currently available, some treatment strategies have already been developed based on the molecular pathophysiology of this disease. Current treatment strategies can be classified into three major groups: SMN2-targeting, SMN1-introduction, and non-SMN targeting. Here, we provide a comprehensive and up-to-date review integrating advances in molecular pathophysiology and diagnostic testing with therapeutic developments for this disease including promising candidates from recent clinical trials.

Targeting RNA splicing for disease therapy

Splicing of pre-messenger RNA into mature messenger RNA is an essential step for the expression of most genes in higher eukaryotes. Defects in this process typically affect cellular function and can have pathological consequences. Many human genetic diseases are caused by mutations that cause splicing defects. Furthermore, a number of diseases are associated with splicing defects that are not attributed to overt mutations. Targeting splicing directly to correct disease-associated aberrant splicing is a logical approach to therapy. Splicing is a favorable intervention point for disease therapeutics, because it is an early step in gene expression and does not alter the genome. Significant advances have been made in the development of approaches to manipulate splicing for therapy. Splicing can be manipulated with a number of tools including antisense oligonucleotides, modified small nuclear RNAs (snRNAs), trans-splicing, and small molecule compounds, all of which have been used to increase specific alternatively spliced isoforms or to correct aberrant gene expression resulting from gene mutations that alter splicing. Here we describe clinically relevant splicing defects in disease states, the current tools used to target and alter splicing, specific mutations and diseases that are being targeted using splice-modulating approaches, and emerging therapeutics.

SMA-MAP: a plasma protein panel for spinal muscular atrophy

Objectives

Spinal Muscular Atrophy (SMA) presents challenges in (i) monitoring disease activity and predicting progression, (ii) designing trials that allow rapid assessment of candidate therapies, and (iii) understanding molecular causes and consequences of the disease. Validated biomarkers of SMA motor and non-motor function would offer utility in addressing these challenges. Our objectives were (i) to discover additional markers from the Biomarkers for SMA (BforSMA) study using an immunoassay platform, and (ii) to validate the putative biomarkers in an independent cohort of SMA patients collected from a multi-site natural history study (NHS).

Methods

BforSMA study plasma samples (N = 129) were analyzed by immunoassay to identify new analytes correlating to SMA motor function. These immunoassays included the strongest candidate biomarkers identified previously by chromatography. We selected 35 biomarkers to validate in an independent cohort SMA type 1, 2, and 3 samples (N = 158) from an SMA NHS. The putative biomarkers were tested for association to multiple motor scales and to pulmonary function, neurophysiology, strength, and quality of life measures. We implemented a Tobit model to predict SMA motor function scores.

Results

12 of the 35 putative SMA biomarkers were significantly associated (p<0.05) with motor function, with a 13^th analyte being nearly significant. Several other analytes associated with non-motor SMA outcome measures. From these 35 biomarkers, 27 analytes were selected for inclusion in a commercial panel (SMA-MAP) for association with motor and other functional measures.

Conclusions

Discovery and validation using independent cohorts yielded a set of SMA biomarkers significantly associated with motor function and other measures of SMA disease activity. A commercial SMA-MAP biomarker panel was generated for further testing in other SMA collections and interventional trials. Future work includes evaluating the panel in other neuromuscular diseases, for pharmacodynamic responsiveness to experimental SMA therapies, and for predicting functional changes over time in SMA patients.

Restoration of SMN to Emx-1 expressing cortical neurons is not sufficient to provide benefit to a severe mouse model of Spinal Muscular Atrophy

Spinal Muscular Atrophy (SMA), an autosomal recessive neuromuscular disorder, is a leading genetic cause of infant mortality. SMA is caused by the homozygous loss of Survival Motor Neuron-1 (SMN1). However, low, but essential, levels of SMN protein are produced by a nearly identical copy gene called SMN2. Detailed analysis of neuromuscular junctions in SMA mice has revealed a selective vulnerability in a subset of muscle targets, suggesting that while SMN is reduced uniformly, the functional deficits manifest sporadically. Additionally, in severe SMA models, it is becoming increasing apparent that SMA is not restricted solely to motor neurons. Rather, additional tissues including the heart, vasculature, and the pancreas contribute to the complete SMA-associated pathology. Recently, transgenic models have been utilized to examine the tissue-specific requirements of SMN, including selective depletion and restoration of SMN in motor neurons. To determine whether the cortical neuronal populations expressing the Emx-1 promoter are involved in SMA pathology, we generated a novel SMA mouse model in which SMN expression was specifically induced in Emx-1 expressing cortical neurons utilizing an Emx-1-Cre transgene. While SMN expression was robust in the central nervous system as expected, SMA mice did not live longer. Weight and time-to-right motor function were not significantly improved.

Trans-splicing correction of tau isoform imbalance in a mouse model of tau mis-splicing

Abnormal metabolism of the tau protein is central to the pathogenesis of a number of dementias, including Alzheimer's disease. Aberrant alternative splicing of exon 10 in the tau pre-mRNA resulting in an imbalance of tau isoforms is one of the molecular causes of the inherited tauopathy, FTDP-17. We showed previously in heterologous systems that exon 10 inclusion in tau mRNA could be modulated by spliceosome-mediated RNA trans-splicing (SMaRT). Here, we evaluated the potential of trans-splicing RNA reprogramming to correct tau mis-splicing in differentiated neurons in a mouse model of tau mis-splicing, the htau transgenic mouse line, expressing the human MAPT gene in a null mouse Mapt background. Trans-splicing molecules designed to increase exon 10 inclusion were delivered to neurons using lentiviral vectors. We demonstrate reprogramming of tau transcripts at the RNA level after transduction of cultured neurons or after direct delivery and long-term expression of viral vectors into the brain of htau mice in vivo. Tau RNA trans-splicing resulted in an increase in exon 10 inclusion in the mature tau mRNA. Importantly, we also show that the trans-spliced product is translated into a full-length chimeric tau protein. These results validate the potential of SMaRT to correct tau mis-splicing and provide a framework for its therapeutic application to neurodegenerative conditions linked to aberrant RNA processing.

Spinal muscular atrophy and the antiapoptotic role of survival of motor neuron (SMN) protein

Spinal muscular atrophy (SMA) is a devastating and often fatal neurodegenerative disease that affects spinal motor neurons and leads to progressive muscle wasting and paralysis. The survival of motor neuron (SMN) gene is mutated or deleted in most forms of SMA, which results in a critical reduction in SMN protein. Motor neurons appear particularly vulnerable to reduced SMN protein levels. Therefore, understanding the functional role of SMN in protecting motor neurons from degeneration is an essential prerequisite for the design of effective therapies for SMA. To this end, there is increasing evidence indicating a key regulatory antiapoptotic role for the SMN protein that is important in motor neuron survival. The aim of this review is to highlight key findings that support an antiapoptotic role for SMN in modulating cell survival and raise possibilities for new therapeutic approaches.

RNA trans-splicing for genodermatoses

Spliceosome-mediated RNA trans-splicing (SMaRT) is a tool that facilitates the recombination of two distinct pre-mRNA molecules. Its application for gene therapeutic purposes has been hindered by laborious procedures to identify gene-specific molecules. We have established a screening method for the identification of highly functional RNA trans-splicing molecules based on fluorescence reporters, facilitating the generation of most potent therapeutic molecules for the correction of any gene of interest.

SMN-inducing compounds for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infant mortality. A neurodegenerative disease, it is caused by loss of SMN1, although low, but essential, levels of SMN protein are produced by the nearly identical gene SMN2. While no effective treatment or therapy currently exists, a new wave of therapeutics has rapidly progressed from cell-based and preclinical animal models to the point where clinical trials have initiated for SMA-specific compounds. There are several reasons why SMA has moved relatively rapidly towards novel therapeutics, including: SMA is monogenic; the molecular understanding of SMN gene regulation has been building for nearly 20 years; and all SMA patients retain one or more copies of SMN2 that produces low levels of full-length, fully functional SMN protein. This review primarily focuses upon the biology behind the disease and examines SMN1- and SMN2-targeted therapeutics.

IPLEX administration improves motor neuron survival and ameliorates motor functions in a severe mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is an inherited neurodegenerative disorder and the first genetic cause of death in childhood. SMA is caused by low levels of survival motor neuron (SMN) protein that induce selective loss of α-motor neurons (MNs) in the spinal cord, resulting in progressive muscle atrophy and consequent respiratory failure. To date, no effective treatment is available to counteract the course of the disease. Among the different therapeutic strategies with potential clinical applications, the evaluation of trophic and/or protective agents able to antagonize MNs degeneration represents an attractive opportunity to develop valid therapies. Here we investigated the effects of IPLEX (recombinant human insulinlike growth factor 1 [rhIGF-1] complexed with recombinant human IGF-1 binding protein 3 [rhIGFBP-3]) on a severe mouse model of SMA. Interestingly, molecular and biochemical analyses of IGF-1 carried out in SMA mice before drug administration revealed marked reductions of IGF-1 circulating levels and hepatic mRNA expression. In this study, we found that perinatal administration of IPLEX, even if does not influence survival and body weight of mice, results in reduced degeneration of MNs, increased muscle fiber size and in amelioration of motor functions in SMA mice. Additionally, we show that phenotypic changes observed are not SMN-dependent, since no significant SMN modification was addressed in treated mice. Collectively, our data indicate IPLEX as a good therapeutic candidate to hinder the progression of the neurodegenerative process in SMA.

Insulin-like growth factors in the peripheral nervous system

Insulin-like growth factors (IGFs) play an integral role in development, growth, and survival. This article details the current understanding of the effects of IGFs in the peripheral nervous system (PNS) during health and disease, and introduces how the IGF system regulates PNS development and impacts growth and survival of PNS cells. Also discussed are implications of IGF signaling in neurodegeneration and the status and prospects of IGF therapies for PNS conditions. There is substantial support for the application of IGF therapies in the treatment of PNS injury and disease.

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.

Spliceosome-mediated trans-splicing: the therapeutic cut and paste

Spliceosome-mediated RNA trans-splicing (SMaRT) is an RNA-based technology to reprogram genes for diagnostic and therapeutic purposes. For the correction of genetic diseases, SMaRT offers several advantages over traditional gene-replacement strategies. SMaRT protocols have recently been used for in vitro phenotypic correction of a variety of genetic disorders, ranging from epidermolysis bullosa to neurodegenerative diseases. In vivo studies are currently bringing trans-splicing RNA therapy toward clinical application. In this review, we summarize the progress made toward the medical use of SMaRT and provide an outlook on its upcoming applications.

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.

Decreasing disease severity in symptomatic, Smn(-/-);SMN2(+/+), spinal muscular atrophy mice following scAAV9-SMN delivery

Spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disorder, is the leading genetic cause of infant mortality. SMA is caused by the homozygous loss of Survival Motor Neuron-1 (SMN1). In humans, a nearly identical copy gene is present, SMN2. SMN2 is retained in all SMA patients and encodes the same protein as SMN1. However, SMN1 and SMN2 differ by a silent C-to-T transition at the 5' end of exon 7, causing alternative splicing of SMN2 transcripts and low levels of full-length SMN. SMA is monogenic and therefore well suited for gene-replacement strategies. Recently, self-complementary adeno-associated virus (scAAV) vectors have been used to deliver the SMN cDNA to an animal model of disease, the SMNΔ7 mouse. In this study, we examine a severe model of SMA, Smn(-/-);SMN2(+/+), to determine whether gene replacement is viable in a model in which disease development begins in utero. Using two delivery paradigms, intracerebroventricular injections and intravenous injections, we delivered scAAV9-SMN and demonstrated a two to four fold increase in survival, in addition to improving many of the phenotypic parameters of the model. This represents the longest extension in survival for this severe model for any therapeutic intervention and suggests that postsymptomatic treatment of SMA may lead to significant improvement of disease severity.