Spinal muscular atrophy: going beyond the motor neuron

AAV9-Mediated Expression of SMN Restricted to Neurons Does Not Rescue the Spinal Muscular Atrophy Phenotype in Mice

Spinal muscular atrophy (SMA) is a neuromuscular disease mainly caused by mutations or deletions in the survival of motor neuron 1 (SMN1) gene and characterized by the degeneration of motor neurons and progressive muscle weakness. A viable therapeutic approach for SMA patients is a gene replacement strategy that restores functional SMN expression using adeno-associated virus serotype 9 (AAV9) vectors. Currently, systemic or intra-cerebrospinal fluid (CSF) delivery of AAV9-SMN is being explored in clinical trials. In this study, we show that the postnatal delivery of an AAV9 that expresses SMN under the control of the neuron-specific promoter synapsin selectively targets neurons without inducing re-expression in the peripheral organs of SMA mice. However, this approach is less efficient in restoring the survival and neuromuscular functions of SMA mice than the systemic or intra-CSF delivery of an AAV9 in which SMN is placed under the control of a ubiquitous promoter. This study suggests that further efforts are needed to understand the extent to which SMN is required in neurons and peripheral organs for a successful therapeutic effect.

New and Developing Therapies in Spinal Muscular Atrophy: From Genotype to Phenotype to Treatment and Where Do We Stand?

Spinal muscular atrophy (SMA) is a congenital neuromuscular disorder characterized by motor neuron loss, resulting in progressive weakness. SMA is notable in the health care community because it accounts for the most common cause of infant death resulting from a genetic defect. SMA is caused by low levels of the survival motor neuron protein (SMN) resulting from SMN1 gene mutations or deletions. However, patients always harbor various copies of SMN2, an almost identical but functionally deficient copy of the gene. A genotype–phenotype correlation suggests that SMN2 is a potent disease modifier for SMA, which also represents the primary target for potential therapies. Increasing comprehension of SMA pathophysiology, including the characterization of SMN1 and SMN2 genes and SMN protein functions, has led to the development of multiple therapeutic approaches. Until the end of 2016, no cure was available for SMA, and management consisted of supportive measures. Two breakthrough SMN-targeted treatments, either using antisense oligonucleotides (ASOs) or virus-mediated gene therapy, have recently been approved. These two novel therapeutics have a common objective: to increase the production of SMN protein in MNs and thereby improve motor function and survival. However, neither therapy currently provides a complete cure. Treating patients with SMA brings new responsibilities and unique dilemmas. As SMA is such a devastating disease, it is reasonable to assume that a unique therapeutic solution may not be sufficient. Current approaches under clinical investigation differ in administration routes, frequency of dosing, intrathecal versus systemic delivery, and mechanisms of action. Besides, emerging clinical trials evaluating the efficacy of either SMN-dependent or SMN-independent approaches are ongoing. This review aims to address the different knowledge gaps between genotype, phenotypes, and potential therapeutics.

Spinal Muscular Atrophy: Past, Present, and Future

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease caused by deletions or mutations in the survival motor neuron (SMN1) gene. SMA is characterized by loss of lower motor neurons (anterior horn cells) in the spinal cord and brainstem nuclei, leading to progressive symmetrical muscle weakness and atrophy. It affects approximately 1 in 6,000 to 1 in 10,000 individuals and is the most common inherited cause of childhood mortality, but this may soon change given recent developments. In December 2016, nusinersen, an antisense oligonucleotide drug, was approved by the United States Food and Drug Administration for the treatment of SMA, and in July 2018, SMA was added to the recommended uniform screening panel, a list of conditions that all states are encouraged to include in their newborn screening (NBS) panels. In this review, we begin with a brief clinical history of the diagnosis of SMA, discuss the current SMA clinical classification system, describe the current treatment, and discuss evolving treatment guidelines. We then discuss the path to include SMA in NBS programs as well as the controversies it engenders because the variability in age at symptom onset means early identification of asymptomatic patients who will not require therapy for years or decades. We also consider alternate population screening opportunities. Next, we consider experimental treatments. We conclude by supporting NBS for SMA with the caveat that a long-term follow-up registry is ethically essential to ensure that the benefits outweigh the harms for all screened infants, including those with milder and/or later-onset forms of SMA.

The Small-Molecule Flunarizine in Spinal Muscular Atrophy Patient Fibroblasts Impacts on the Gemin Components of the SMN Complex and TDP43, an RNA-Binding Protein Relevant to Motor Neuron Diseases

The motor neurodegenerative disease spinal muscular atrophy (SMA) is caused by alterations of the survival motor neuron 1 (SMN1) gene involved in RNA metabolism. Although the disease mechanisms are not completely elucidated, SMN protein deficiency leads to abnormal small nuclear ribonucleoproteins (snRNPs) assembly responsible for widespread splicing defects. SMN protein localizes in nuclear bodies that are lost in SMA and adult onset amyotrophic lateral sclerosis (ALS) patient cells harboring TDP-43 or FUS/TLS mutations. We previously reported that flunarizine recruits SMN into nuclear bodies and improves the phenotype of an SMA mouse model. However, the precise mode of action remains elusive. Here, a marked reduction of the integral components of the SMN complex is observed in severe SMA patient fibroblast cells. We show that flunarizine increases the protein levels of a subset of components of the SMN-Gemins complex, Gemins2-4, and markedly reduces the RNA and protein levels of the pro-oxydant thioredoxin-interacting protein (TXNIP) encoded by an mRNA target of Gemin5. We further show that SMN deficiency causes a dissociation of the localization of the SMN complex components from the same nuclear bodies. The accumulation of TDP-43 in SMN-positive nuclear bodies is also perturbed in SMA cells. Notably, TDP-43 is found to co-localize with SMN in nuclear bodies of flunarizine-treated SMA cells. Our findings indicate that flunarizine reverses cellular changes caused by SMN deficiency in SMA cells and further support the view of a common pathway in RNA metabolism underlying infantile and adult motor neuron diseases.

New treatments in spinal muscular atrophy: an overview of currently available data

Introduction: Spinal muscular atrophy (SMA) is one of the most common inherited neuromuscular disorders. It causes progressive muscle weakness and results in significant disability. Until recently, there were no drugs available for the treatment of SMA. Several phase 1-3 studies, including three double-blind randomized placebo-controlled studies have demonstrated the efficacy of disease-modifying approaches including gene replacement therapy, antisense oligonucleotides, and splicing modifiers.Areas covered: This article covers the publically available data on therapeutic strategies that address the underlying cause of SMA and clinical data available on approved treatments and drugs in the pipeline.Expert opinion: The newer therapeutic options in SMA have a good safety profile and deliver a therapeutic benefit in most patients. It is essential that the recommended standards of care are delivered along with the drugs for the best outcomes. No biomarkers to distinguish responders from non-responders are available; it is important that biomarkers be identified. Early treatment is essential for the maximum efficacy of the newly available treatments.

Twenty-Five Years of Spinal Muscular Atrophy Research: From Phenotype to Genotype to Therapy, and What Comes Next

Twenty-five years ago, the underlying genetic cause for one of the most common and devastating inherited diseases in humans, spinal muscular atrophy (SMA), was identified. Homozygous deletions or, rarely, subtle mutations of SMN1 cause SMA, and the copy number of the nearly identical copy gene SMN2 inversely correlates with disease severity. SMA has become a paradigm and a prime example of a monogenic neurological disorder that can be efficiently ameliorated or nearly cured by novel therapeutic strategies, such as antisense oligonucleotide or gene replacement therapy. These therapies enable infants to survive who might otherwise have died before the age of two and allow individuals who have never been able to sit or walk to do both. The major milestones on the road to these therapies were to understand the genetic cause and splice regulation of SMN genes, the disease's phenotype-genotype variability, the function of the protein and the main affected cellular pathways and tissues, the disease's pathophysiology through research on animal models, the windows of opportunity for efficient treatment, and how and when to treat patients most effectively.This review aims to bridge our knowledge from phenotype to genotype to therapy, not only highlighting the significant advances so far but also speculating about the future of SMA screening and treatment.

Gene therapies for axonal neuropathies: Available strategies, successes to date, and what to target next

Nearly one-hundred loci in the human genome have been associated with different forms of Charcot-Marie-Tooth disease (CMT) and related inherited neuropathies. Despite this wealth of gene targets, treatment options are still extremely limited, and clear "druggable" pathways are not obvious for many of these mutations. However, recent advances in gene therapies are beginning to circumvent this challenge. Each type of CMT is a monogenic disorder, and the cellular targets are usually well-defined and typically include peripheral neurons or Schwann cells. In addition, the genetic mechanism is often also clear, with loss-of-function mutations requiring restoration of gene expression, and gain-of-function or dominant-negative mutations requiring silencing of the mutant allele. These factors combine to make CMT a good target for developing genetic therapies. Here we will review the state of relatively established gene therapy approaches, including viral vector-mediated gene replacement and antisense oligonucleotides for exon skipping, altering splicing, and gene knockdown. We will also describe earlier stage approaches for allele-specific knockdown and CRIPSR/Cas9 gene editing. We will next describe how these various approaches have been deployed in clinical and preclinical studies. Finally, we will evaluate various forms of CMT as candidates for gene therapy based on the current understanding of their genetics, cellular/tissue targets, validated animal models, and availability of patient populations and natural history data.

Drosophila SMN2 minigene reporter model identifies moxifloxacin as a candidate therapy for SMA

Spinal muscular atrophy is a rare and fatal neuromuscular disorder caused by the loss of alpha motor neurons. The affected individuals have mutated the ubiquitously expressed SMN1 gene resulting in the loss or reduction in the survival motor neuron (SMN) protein levels. However, an almost identical paralog exists in humans: SMN2. Pharmacological activation of SMN2 exon 7 inclusion by small molecules or modified antisense oligonucleotides is a valid approach to treat SMA. Here we describe an in vivo SMN2 minigene reporter system in Drosophila motor neurons that serves as a cost-effective, feasible, and stringent primary screening model for identifying chemicals capable of crossing the conserved Drosophila blood-brain barrier and modulating exon 7 inclusion. The model was used for the screening of 1100 drugs from the Prestwick Chemical Library, resulting in 2.45% hit rate. The most promising candidate drugs were validated in patient-derived fibroblasts where they proved to increase SMN protein levels. Among them, moxifloxacin modulated SMN2 splicing by promoting exon 7 inclusion. The recovery of SMN protein levels was confirmed by increased colocalization of nuclear gems with Cajal Bodies. Thus, a Drosophila-based drug screen allowed the discovery of an FDA-approved small molecule with the potential to become a novel therapy for SMA.

Recent advances of induced pluripotent stem cells application in neurodegenerative diseases

Stem cell is defined by its ability to self-renewal and generates differentiated functional cell types, which are derived from the embryo and various sources of postnatal animal. These cells can be divided according to their potential development into totipotent, unipotent, multipotent andpluripotent. Pluripotent is considered as the most important type due to its advantageous capability to create different cell types of the body in a similar behavior as embryonic stem cell. Induced pluripotent stem cells (iPSCs) are adult cells that maintain the characteristics of embryonic stem cells because it can be genetically reprogrammed to an embryonic stem cell-like state via express genes and transcription factors. Such cells provide an efficient pathway to explorehuman diseases and their corresponding therapy, particularly, neurodevelopmental disorders. Consequently, iPSCs can be investigated to check the specific mutations of neurodegenerative disease due to their unique ability to differentiate into neural cell types and/or neural organoids. The current review addresses the different neurodegenerative diseases model by using iPSCs approach such as Alzheimer's diseases (AD), Parkinson diseases (PD),multiplesclerosis(MS) and psychiatric disorders. We also highlight the importance of autophagy in neurodegenerative diseases.

SMN complex member Gemin3 self-interacts and has a functional relationship with ALS-linked proteins TDP-43, FUS and Sod1

The predominant motor neuron disease in infants and adults is spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), respectively. SMA is caused by insufficient levels of the Survival Motor Neuron (SMN) protein, which operates as part of the multiprotein SMN complex that includes the DEAD-box RNA helicase Gemin3/DDX20/DP103. C9orf72, SOD1, TDP-43 and FUS are ranked as the four major genes causing familial ALS. Accumulating evidence has revealed a surprising molecular overlap between SMA and ALS. Here, we ask the question of whether Drosophila can also be exploited to study shared pathogenic pathways. Focusing on motor behaviour, muscle mass and survival, we show that disruption of either TBPH/TDP-43 or Caz/FUS enhance defects associated with Gemin3 loss-of-function. Gemin3-associated neuromuscular junction overgrowth was however suppressed. Sod1 depletion had a modifying effect in late adulthood. We also show that Gemin3 self-interacts and Gem3^ΔN, a helicase domain deletion mutant, retains the ability to interact with its wild-type counterpart. Importantly, mutant:wild-type dimers are favoured more than wild-type:wild-type dimers. In addition to reinforcing the link between SMA and ALS, further exploration of mechanistic overlaps is now possible in a genetically tractable model organism. Notably, Gemin3 can be elevated to a candidate for modifying motor neuron degeneration.

Multifaceted roles of microRNAs: From motor neuron generation in embryos to degeneration in spinal muscular atrophy

Two crucial questions in neuroscience are how neurons establish individual identity in the developing nervous system and why only specific neuron subtypes are vulnerable to neurodegenerative diseases. In the central nervous system, spinal motor neurons serve as one of the best-characterized cell types for addressing these two questions. In this review, we dissect these questions by evaluating the emerging role of regulatory microRNAs in motor neuron generation in developing embryos and their potential contributions to neurodegenerative diseases such as spinal muscular atrophy (SMA). Given recent promising results from novel microRNA-based medicines, we discuss the potential applications of microRNAs for clinical assessments of SMA disease progression and treatment.

Patient-Reported Prevalence of Non-motor Symptoms Is Low in Adult Patients Suffering From 5q Spinal Muscular Atrophy

Background: 5q spinal muscular atrophy (SMA) is an autosomal recessive lower motoneuron disease caused by deletion or mutations in the survival motor neuron 1 gene (SMN1) which results in reduced expression of full-length SMN protein. The main symptoms are caused by spinal motor neuron demise leading to muscle atrophy, and medical care mostly refers to motor symptoms. However, new insights of recent studies in severe SMA type I revealed disease involvement of several non-motor regions, for example cardiac, vascular, sensory nerve involvement, and thalamic lesions. Non-motor symptoms (NMS) were previously described in many neurodegenerative diseases i.e., Parkinson's disease and, importantly, also amyotrophic lateral sclerosis. Methods: We screened for NMS in 70 adult patients with SMA type II (SMAII) and type III (SMAIII) and 59 age/sex-matched healthy controls (controls) in a multicenter cross-sectional study including 5 different centers with specialized expertise in medical health care of motoneuron diseases. We used a self-rating questionnaire including 30 different items of gastrointestinal, autonomic, neuropsychiatric, and sleep complaints [NMS questionnaire (NMSQuest)], which is a validated tool in Parkinson's disease. Results: Total NMS burden was low in adult SMA (median: 3 items) and not significantly different compared to controls (median: 2 items). Total NMS of SMA patients did not correlate with disease severity scores. However, the items "swallowing difficulties," "falling," and particularly "swelling legs" were significantly more frequently reported in SMA. Neuropsychiatric symptoms were reported in a frequency comparable to controls and were not significantly increased in SMA. Conclusion: Patient-reported prevalence of NMS in adult SMA was low, which does not argue for a clinically relevant multisystemic disorder in SMAII/III. Importantly, adult SMA patients do not seem to suffer more frequently from symptoms of depression or adaptive disorders compared to controls. Our results yield novel information on previously underreported symptoms and will help to improve the medical guidance of these patients.

Treating neonatal spinal muscular atrophy: A 21st century success story?

Severe spinal muscular atrophy is an autosomal recessive motor neuron disorder characterized by rapidly progressive hypotonia and weakness with respiratory complications and fatal outcome. It is caused by absence or pathogenic variants in the SMN1 gene. Knowledge and advances of the genetics of the disease allowed the development of tailored therapies that has changed clinical trajectories with evolving phenotypes. Several clinical investigations demonstrate that early diagnosis and intervention are essential for improved response to treatment and better prognosis. Therapeutic interventions that are effective at pre-symptomatic or early stages of the disease creates the need for awareness, expedite diagnosis and consideration of newborn screening programs.

Muscle regulates mTOR dependent axonal local translation in motor neurons via CTRP3 secretion: implications for a neuromuscular disorder, spinal muscular atrophy

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder, which causes dysfunction/loss of lower motor neurons and muscle weakness as well as atrophy. While SMA is primarily considered as a motor neuron disease, recent data suggests that survival motor neuron (SMN) deficiency in muscle causes intrinsic defects. We systematically profiled secreted proteins from control and SMN deficient muscle cells with two combined metabolic labeling methods and mass spectrometry. From the screening, we found lower levels of C1q/TNF-related protein 3 (CTRP3) in the SMA muscle secretome and confirmed that CTRP3 levels are indeed reduced in muscle tissues and serum of an SMA mouse model. We identified that CTRP3 regulates neuronal protein synthesis including SMN via mTOR pathway. Furthermore, CTRP3 enhances axonal outgrowth and protein synthesis rate, which are well-known impaired processes in SMA motor neurons. Our data revealed a new molecular mechanism by which muscles regulate the physiology of motor neurons via secreted molecules. Dysregulation of this mechanism contributes to the pathophysiology of SMA.

Low fat diets increase survival of a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disorder leading to paralysis and death. Recent evidence shows increased susceptibility to dyslipidemia and liver steatosis in patients. Here, we provide evidence that low fat diets nearly double survival in Smn^2B/− mice, a model for SMA, independent of changes in SMN levels, liver steatosis, or enhanced hepatic functions. Liver damage and ketone levels were reduced, implying a lower reliance on fatty acid oxidation. This preclinical proof of concept study provides grounds for controlled clinical investigation of dietary needs and offers evidence to inform nutritional guidelines specific to SMA.

Fetal Gene Therapy Using a Single Injection of Recombinant AAV9 Rescued SMA Phenotype in Mice

Symptoms of spinal muscular atrophy (SMA) disease typically begin in the late prenatal or the early postnatal period of life. The intrauterine (IU) correction of gene expression, fetal gene therapy, could offer effective gene therapy approach for early onset diseases. Hence, the overall goal of this study was to investigate the efficacy of human survival motor neuron (hSMN) gene expression after IU delivery in SMA mouse embryos. First, we found that IU-intracerebroventricular (i.c.v.) injection of adeno-associated virus serotype-9 (AAV9)-EGFP led to extensive expression of EGFP protein in different parts of the CNS with a great number of transduced neural stem cells. Then, to implement the fetal gene therapy, mouse fetuses received a single i.c.v. injection of a single-stranded (ss) or self-complementary (sc) AAV9-SMN vector that led to a lifespan of 93 (median of 63) or 171 (median 105) days for SMA mice. The muscle pathology and number of the motor neurons also improved in both study groups, with slightly better results coming from scAAV treatment. Consequently, fetal gene therapy may provide an alternative therapeutic approach for treating inherited diseases such as SMA that lead to prenatal death or lifelong irreversible damage.

Abnormal fatty acid metabolism is a core component of spinal muscular atrophy

OBJECTIVE

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder leading to paralysis and subsequent death in young children. Initially considered a motor neuron disease, extra-neuronal involvement is increasingly recognized. The primary goal of this study was to investigate alterations in lipid metabolism in SMA patients and mouse models of the disease.

METHODS

We analyzed clinical data collected from a large cohort of pediatric SMA type I-III patients as well as SMA type I liver necropsy data. In parallel, we performed histology, lipid analysis, and transcript profiling in mouse models of SMA.

RESULTS

We identify an increased susceptibility to developing dyslipidemia in a cohort of 72 SMA patients and liver steatosis in pathological samples. Similarly, fatty acid metabolic abnormalities were present in all SMA mouse models studied. Specifically, Smn2B/- mice displayed elevated hepatic triglycerides and dyslipidemia, resembling non-alcoholic fatty liver disease (NAFLD). Interestingly, this phenotype appeared prior to denervation.

INTERPRETATION

This work highlights metabolic abnormalities as an important feature of SMA, suggesting implementation of nutritional and screening guidelines in patients, as such defects are likely to increase metabolic distress and cardiovascular risk. This study emphasizes the need for a systemic therapeutic approach to ensure maximal benefits for all SMA patients throughout their life.

CHP1 reduction ameliorates spinal muscular atrophy pathology by restoring calcineurin activity and endocytosis

Autosomal recessive spinal muscular atrophy (SMA), the leading genetic cause of infant lethality, is caused by homozygous loss of the survival motor neuron 1 (SMN1) gene. SMA disease severity inversely correlates with the number of SMN2 copies, which in contrast to SMN1, mainly produce aberrantly spliced transcripts. Recently, the first SMA therapy based on antisense oligonucleotides correcting SMN2 splicing, namely SPINRAZA^TM, has been approved. Nevertheless, in type I SMA-affected individuals—representing 60% of SMA patients—the elevated SMN level may still be insufficient to restore motor neuron function lifelong. Plastin 3 (PLS3) and neurocalcin delta (NCALD) are two SMN-independent protective modifiers identified in humans and proved to be effective across various SMA animal models. Both PLS3 overexpression and NCALD downregulation protect against SMA by restoring impaired endocytosis; however, the exact mechanism of this protection is largely unknown. Here, we identified calcineurin-like EF-hand protein 1 (CHP1) as a novel PLS3 interacting protein using a yeast-two-hybrid screen. Co-immunoprecipitation and pull-down assays confirmed a direct interaction between CHP1 and PLS3. Although CHP1 is ubiquitously present, it is particularly abundant in the central nervous system and at SMA-relevant sites including motor neuron growth cones and neuromuscular junctions. Strikingly, we found elevated CHP1 levels in SMA mice. Congruently, CHP1 downregulation restored impaired axonal growth in Smn-depleted NSC34 motor neuron-like cells, SMA zebrafish and primary murine SMA motor neurons. Most importantly, subcutaneous injection of low-dose SMN antisense oligonucleotide in pre-symptomatic mice doubled the survival rate of severely-affected SMA mice, while additional CHP1 reduction by genetic modification prolonged survival further by 1.6-fold. Moreover, CHP1 reduction further ameliorated SMA disease hallmarks including electrophysiological defects, smaller neuromuscular junction size, impaired maturity of neuromuscular junctions and smaller muscle fibre size compared to low-dose SMN antisense oligonucleotide alone. In NSC34 cells, Chp1 knockdown tripled macropinocytosis whereas clathrin-mediated endocytosis remained unaffected. Importantly, Chp1 knockdown restored macropinocytosis in Smn-depleted cells by elevating calcineurin phosphatase activity. CHP1 is an inhibitor of calcineurin, which collectively dephosphorylates proteins involved in endocytosis, and is therefore crucial in synaptic vesicle endocytosis. Indeed, we found marked hyperphosphorylation of dynamin 1 in SMA motor neurons, which was restored to control level by the heterozygous Chp1 mutant allele. Taken together, we show that CHP1 is a novel SMA modifier that directly interacts with PLS3, and that CHP1 reduction ameliorates SMA pathology by counteracting impaired endocytosis. Most importantly, we demonstrate that CHP1 reduction is a promising SMN-independent therapeutic target for a combinatorial SMA therapy.

Applying Deep Neural Network Analysis to High-Content Image-Based Assays

The etiological underpinnings of many CNS disorders are not well understood. This is likely due to the fact that individual diseases aggregate numerous pathological subtypes, each associated with a complex landscape of genetic risk factors. To overcome these challenges, researchers are integrating novel data types from numerous patients, including imaging studies capturing broadly applicable features from patient-derived materials. These datasets, when combined with machine learning, potentially hold the power to elucidate the subtle patterns that stratify patients by shared pathology. In this study, we interrogated whether high-content imaging of primary skin fibroblasts, using the Cell Painting method, could reveal disease-relevant information among patients. First, we showed that technical features such as batch/plate type, plate, and location within a plate lead to detectable nuisance signals, as revealed by a pre-trained deep neural network and analysis with deep image embeddings. Using a plate design and image acquisition strategy that accounts for these variables, we performed a pilot study with 12 healthy controls and 12 subjects affected by the severe genetic neurological disorder spinal muscular atrophy (SMA), and evaluated whether a convolutional neural network (CNN) generated using a subset of the cells could distinguish disease states on cells from the remaining unseen control–SMA pair. Our results indicate that these two populations could effectively be differentiated from one another and that model selectivity is insensitive to batch/plate type. One caveat is that the samples were also largely separated by source. These findings lay a foundation for how to conduct future studies exploring diseases with more complex genetic contributions and unknown subtypes.

Peptide-conjugate antisense based splice-correction for Duchenne muscular dystrophy and other neuromuscular diseases

Duchenne muscular dystrophy (DMD) is an X-linked disorder characterized by progressive muscle degeneration, caused by the absence of dystrophin. Exon skipping by antisense oligonucleotides (ASOs) has recently gained recognition as therapeutic approach in DMD. Conjugation of a peptide to the phosphorodiamidate morpholino backbone (PMO) of ASOs generated the peptide-conjugated PMOs (PPMOs) that exhibit a dramatically improved pharmacokinetic profile. When tested in animal models, PPMOs demonstrate effective exon skipping in target muscles and prolonged duration of dystrophin restoration after a treatment regime. Herein we summarize the main pathophysiological features of DMD and the emergence of PPMOs as promising exon skipping agents aiming to rescue defective gene expression in DMD and other neuromuscular diseases. The listed PPMO laboratory findings correspond to latest trends in the field and highlight the obstacles that must be overcome prior to translating the animal-based research into clinical trials tailored to the needs of patients suffering from neuromuscular diseases.

AAV9 Vector: a Novel modality in gene therapy for spinal muscular atrophy

Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality, is characterized by the deterioration of alpha motor neurons in the brainstem and spinal cord. Currently, there is no cure for SMA, which calls for an urgent need to explore affordable and effective therapies and to maximize patients' independence and quality of life. Adeno-associated virus (AAV) vector, one of the most promising and well-investigated vehicles for delivering transgenes, is a compelling candidate for gene therapy. Some of the hallmarks of AAVs are their nonpathogenicity, inability to incur an immune response, potential to achieve robust transgene expression, and varied tropism for several tissues of the body. Recently, these features were harnessed in a clinical trial conducted by AveXis in SMA patients, where AAV9 was employed as a vehicle for one-time administration of the SMN gene, the causative gene in SMA. The trial demonstrated remarkable improvements in motor milestones and rates of survival in the patients. This review focuses on the advent of SMA gene therapy and summarizes different preclinical studies that were conducted leading up to the AAV9-SMA trial in SMA patients.

SMN deficiency causes pain hypersensitivity in a mild SMA mouse model through enhancing excitability of nociceptive dorsal root ganglion neurons

Spinal muscular atrophy (SMA) is a devastating motor neuron degeneration disease caused by a deficiency of the SMN protein. Majority of patients also suffer from chronic pain. However, the pathogenesis of pain in the context of SMA has never been explored. In this study, using various pain tests, we found that a mild SMA mouse model presents with multiple forms of pain hypersensitivity. Patch-clamp recording showed that nociceptive neurons in SMA mouse dorsal root ganglia (DRGs) are hyperexcitable and their sodium current densities are markedly increased. Using quantitative RT-PCR, western blotting and immunofluorescence, we observed enhanced expression of two main voltage-gated sodium channels Na_v1.7 and Na_v1.8 in SMA mouse DRGs, which is at least in part due to increase in both expression and phosphorylation of NF-κB p50/p65 heterodimer. Moreover, we revealed that plasma norepinephrine levels are elevated in SMA mice, which contributes to mechanical hypersensitivity via the β2-adrenergic receptor. Finally, we uncovered that β2-adrenergic signaling positively modulates expression as well as phosphorylation of p50 and p65 in SMA mouse DRGs. Therefore, our data demonstrate that SMA mice, similar to humans, also develop pain hypersensitivity, and highlight a peripheral signaling cascade that elicits the mechanical sensitization in the mouse model, suggesting potential targets for therapeutic intervention.

Overview of Current Drugs and Molecules in Development for Spinal Muscular Atrophy Therapy

Spinal muscular atrophy (SMA) is a neurodegenerative disease primarily characterized by a loss of spinal motor neurons, leading to progressive paralysis and premature death in the most severe cases. SMA is caused by homozygous deletion of the survival motor neuron 1 (SMN1) gene, leading to low levels of SMN protein. However, a second SMN gene (SMN2) exists, which can be therapeutically targeted to increase SMN levels. This has recently led to the first disease-modifying therapy for SMA gaining formal approval from the US Food and Drug Administration (FDA) and European Medicines Agency (EMA). Spinraza (nusinersen) is a modified antisense oligonucleotide that targets the splicing of SMN2, leading to increased SMN protein levels, capable of improving clinical phenotypes in many patients. In addition to Spinraza, several other therapeutic approaches are currently in various stages of clinical development. These include SMN-dependent small molecule and gene therapy approaches along with SMN-independent strategies, such as general neuroprotective factors and muscle strength-enhancing compounds. For each therapy, we provide detailed information on clinical trial design and pharmacological/safety data where available. Previous clinical studies are also discussed to provide context on SMA clinical trial development and the insights these provided for the design of current studies.

Systemic nature of spinal muscular atrophy revealed by studying insurance claims

Objective

We investigated the presence of non-neuromuscular phenotypes in patients affected by Spinal Muscular Atrophy (SMA), a disorder caused by a mutation in the Survival of Motor Neuron (SMN) gene, and whether these phenotypes may be clinically detectable prior to clinical signs of neuromuscular degeneration and therefore independent of muscle weakness.

Methods

We utilized a de-identified database of insurance claims to explore the health of 1,038 SMA patients compared to controls. Two analyses were performed: (1) claims from the entire insurance coverage window; and (2) for SMA patients, claims prior to diagnosis of any neuromuscular disease or evidence of major neuromuscular degeneration to increase the chance that phenotypes could be attributed directly to reduced SMN levels. Logistic regression was used to determine whether phenotypes were diagnosed at significantly different rates between SMA patients and controls and to obtain covariate-adjusted odds ratios.

Results

Results from the entire coverage window revealed a broad spectrum of phenotypes that are differentially diagnosed in SMA subjects compared to controls. Moreover, data from SMA patients prior to their first clinical signs of neuromuscular degeneration revealed numerous non-neuromuscular phenotypes including defects within the cardiovascular, gastrointestinal, metabolic, reproductive, and skeletal systems. Furthermore, our data provide evidence of a potential ordering of disease progression beginning with these non-neuromuscular phenotypes.

Conclusions

Our data point to a direct relationship between early, detectable non-neuromuscular symptoms and SMN deficiency. Our findings are particularly important for evaluating the efficacy of SMN-increasing therapies for SMA, comparing the effectiveness of local versus systemically delivered therapeutics, and determining the optimal therapeutic treatment window prior to irreversible neuromuscular damage.

Temporal and tissue-specific variability of SMN protein levels in mouse models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a progressive motor neuron disease caused by deleterious variants in SMN1 that lead to a marked decrease in survival motor neuron (SMN) protein expression. Humans have a second SMN gene (SMN2) that is almost identical to SMN1. However, due to alternative splicing the majority of SMN2 messenger ribonucleic acid (mRNA) is translated into a truncated, unstable protein that is quickly degraded. Because the presence of SMN2 provides a unique opportunity for therapy development in SMA patients, the mechanisms that regulate SMN2 splicing and mRNA expression have been elucidated in great detail. In contrast, how much SMN protein is produced at different developmental time points and in different tissues remains under-characterized. In this study, we addressed this issue by determining SMN protein expression levels at three developmental time points across six different mouse tissues and in two distinct mouse models of SMA (‘severe’ Taiwanese and ‘intermediate’ Smn^2B/− mice). We found that, in healthy control mice, SMN protein expression was significantly influenced by both age and tissue type. When comparing mouse models of SMA, we found that, despite being transcribed from genetically different alleles, control SMN levels were relatively similar. In contrast, the degree of SMN depletion between tissues in SMA varied substantially over time and between the two models. These findings offer an explanation for the differential vulnerability of tissues and organs observed in SMA and further our understanding of the systemic and temporal requirements for SMN with direct relevance for developing effective therapies for SMA.

Molecular Mechanisms Underlying Sensory-Motor Circuit Dysfunction in SMA

Activation of skeletal muscle in response to acetylcholine release from the neuromuscular junction triggered by motor neuron firing forms the basis of all mammalian locomotion. Intricate feedback and control mechanisms, both from within the central nervous system and from sensory organs in the periphery, provide essential inputs that regulate and finetune motor neuron activity. Interestingly, in motor neuron diseases, such as spinal muscular atrophy (SMA), pathological studies in patients have identified alterations in multiple parts of the sensory-motor system. This has stimulated significant research efforts across a range of different animal models of SMA in order to understand these defects and their contribution to disease pathogenesis. Several recent studies have demonstrated that defects in sensory components of the sensory-motor system contribute to dysfunction of motor neurons early in the pathogenic process. In this review, we provide an overview of these findings, with a specific focus on studies that have provided mechanistic insights into the molecular processes that underlie dysfunction of the sensory-motor system in SMA. These findings highlight the role that cell types other than motor neurons play in SMA pathogenesis, and reinforce the need for therapeutic interventions that target and rescue the wide array of defects that occur in SMA.

Composition of the Survival Motor Neuron (SMN) Complex in Drosophila melanogaster

Spinal Muscular Atrophy (SMA) is caused by homozygous mutations in the human survival motor neuron 1 (SMN1) gene. SMN protein has a well-characterized role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), core components of the spliceosome. SMN is part of an oligomeric complex with core binding partners, collectively called Gemins. Biochemical and cell biological studies demonstrate that certain Gemins are required for proper snRNP assembly and transport. However, the precise functions of most Gemins are unknown. To gain a deeper understanding of the SMN complex in the context of metazoan evolution, we investigated its composition in Drosophila melanogaster Using transgenic flies that exclusively express Flag-tagged SMN from its native promoter, we previously found that Gemin2, Gemin3, Gemin5, and all nine classical Sm proteins, including Lsm10 and Lsm11, co-purify with SMN. Here, we show that CG2941 is also highly enriched in the pulldown. Reciprocal co-immunoprecipitation reveals that epitope-tagged CG2941 interacts with endogenous SMN in Schneider2 cells. Bioinformatic comparisons show that CG2941 shares sequence and structural similarity with metazoan Gemin4. Additional analysis shows that three other genes (CG14164, CG31950 and CG2371) are not orthologous to Gemins 6-7-8, respectively, as previously suggested. In D.melanogaster, CG2941 is located within an evolutionarily recent genomic triplication with two other nearly identical paralogous genes (CG32783 and CG32786). RNAi-mediated knockdown of CG2941 and its two close paralogs reveals that Gemin4 is essential for organismal viability.

Downregulation of Survivin contributes to cell-cycle arrest during postnatal cardiac development in a severe spinal muscular atrophy mouse model

Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality, characterized by progressive degeneration of spinal-cord motor neurons, leading to atrophy of skeletal muscles. However, accumulating evidence indicates that it is a multi-system disorder, particularly in its severe forms. Several studies delineated structural and functional cardiac abnormalities in SMA patients and mouse models, yet the abnormalities have been primarily attributed to autonomic dysfunction. Here, we show in a severe mouse model that its cardiomyocytes undergo G0/G1 cell-cycle arrest and enhanced apoptosis during postnatal development. Microarray and real-time RT-PCR analyses revealed that a set of genes associated with cell cycle and apoptosis were dysregulated in newborn pups. Of particular interest, the Birc5 gene, which encodes Survivin, an essential protein for heart development, was down-regulated even on pre-symptomatic postnatal day 0. Interestingly, cultured cardiomyocytes depleted of SMN recapitulated the gene expression changes including downregulation of Survivin and abnormal cell-cycle progression; and overexpression of Survivin rescued the cell-cycle defect. Finally, increasing SMN in SMA mice with a therapeutic antisense oligonucleotide improved heart pathology and recovered expression of deregulated genes. Collectively, our data demonstrate that the cardiac malfunction of the severe SMA mouse model is mainly a cell-autonomous defect, caused by widespread gene deregulation in heart tissue, particularly of Birc5, resulting in developmental abnormalities through cell-cycle arrest and apoptosis.

Nusinersen treatment of spinal muscular atrophy: current knowledge and existing gaps

Spinal muscular atrophy (SMA) is a recessive disorder caused by a mutation in the survival motor neuron 1 gene (SMN1); it affects 1 in 11 000 newborn infants. The most severe and most common form, type 1 SMA, is associated with early mortality in most cases and severe disability in survivors. Nusinersen, an antisense oligonucleotide, promotes production of full-length protein from the pseudogene SMN2. Nusinersen treatment prolongs survival of patients with type 1 SMA and allows motor milestone acquisition. Patients with type 2 SMA also show progress on different motor scales after nusinersen treatment. Nusinersen was recently approved by the European Medicines Agency and the US Food and Drug Administration; it is now reimbursed in several European countries and in the USA. In Australia, the transition from expanded access programme to commercial availability is coming soon. In New Zealand, an expanded access programme is opened, and in Canada price negotiation for the treatment is in progress. In this review we exemplify the clinical benefit of nusinersen in subgroups of patients with SMA. Nusinersen represents the first efficacious marked approved drug in type 1 and type 2 SMA. Different knowledge gaps, such as results in older patients, in patients with permanent ventilation, in patients with neonatal forms, or in patients after spinal fusion, still need to be addressed. WHAT THIS PAPER ADDS: Identifies gaps in knowledge about the efficacy of nusinersen in broader populations of patients with spinal muscular atrophy. Identifies open questions in populations of patients where proof of efficacy is available.

An Integrated Safety Analysis of Infants and Children with Symptomatic Spinal Muscular Atrophy (SMA) Treated with Nusinersen in Seven Clinical Trials

BACKGROUND

Treatment with nusinersen has demonstrated significant and clinically meaningful benefits in clinical trials in infants and children with spinal muscular atrophy (SMA).

OBJECTIVE

The objective of this analysis was to characterize the safety of nusinersen across the clinical trial program in infants and children with symptomatic SMA.

METHODS

An integrated safety analysis evaluated end of study data from seven completed clinical trials that enrolled infants and children with symptomatic SMA who were treated with intrathecal nusinersen or underwent sham procedures. Two of the studies were conducted in symptomatic infants with infantile-onset SMA (most likely to develop SMA type I or II) and the remaining five in symptomatic children and adolescents with later-onset SMA (have or are most likely to develop SMA type II or III). Safety assessments included incidence of adverse events (AEs), physical and neurological examinations, vital signs, clinical laboratory tests (serum chemistry, hematology, and urinalysis), and electrocardiograms.

RESULTS

Data were analyzed from 323 infants and children, including 240 treated with nusinersen (100 with infantile-onset SMA and 140 with later-onset SMA) and 83 who underwent sham procedures (41 infantile-onset, 42 later-onset). Median (range) exposure to nusinersen was 449.0 (6-1538) days (375.9 participant-years). The most common AEs with nusinersen were pyrexia, upper respiratory tract infection, nasopharyngitis, vomiting, headache, and constipation. The incidence of serious AEs was lower with nusinersen than with the sham procedure (41% vs. 61%). The overall incidence of respiratory, thoracic, and mediastinal AEs was higher in participants with symptomatic infantile-onset SMA than those with symptomatic later-onset SMA and similar in nusinersen- versus sham procedure-treated participants. Rates of post-lumbar puncture syndrome and related events were higher with nusinersen versus sham procedure in later-onset SMA participants. No abnormal patterns or trends in laboratory test results were observed.

CONCLUSIONS

Nusinersen demonstrated a favorable safety profile in children with symptomatic infantile- and later-onset SMA. Most reported AEs and serious AEs were consistent with the nature and frequency of events typically seen with SMA or in the context of lumbar puncture procedures.

REGISTRATION

NCT01494701, NCT01703988, NCT01839656, NCT02193074, NCT02292537, NCT01780246, NCT02052791.

Risdiplam distributes and increases SMN protein in both the central nervous system and peripheral organs

Spinal muscular atrophy (SMA) is a rare, inherited neuromuscular disease caused by deletion and/or mutation of the Survival of Motor Neuron 1 (SMN1) gene. A second gene, SMN2, produces low levels of functional SMN protein that are insufficient to fully compensate for the lack of SMN1. Risdiplam (RG7916; RO7034067) is an orally administered, small-molecule SMN2 pre-mRNA splicing modifier that distributes into the central nervous system (CNS) and peripheral tissues. To further explore risdiplam distribution, we assessed in vitro characteristics and in vivo drug levels and effect of risdiplam on SMN protein expression in different tissues in animal models. Total drug levels were similar in plasma, muscle, and brain of mice (n = 90), rats (n = 148), and monkeys (n = 24). As expected mechanistically based on its high passive permeability and not being a human multidrug resistance protein 1 substrate, risdiplam CSF levels reflected free compound concentration in plasma in monkeys. Tissue distribution remained unchanged when monkeys received risdiplam once daily for 39 weeks. A parallel dose-dependent increase in SMN protein levels was seen in CNS and peripheral tissues in two SMA mouse models dosed with risdiplam. These in vitro and in vivo preclinical data strongly suggest that functional SMN protein increases seen in patients' blood following risdiplam treatment should reflect similar increases in functional SMN protein in the CNS, muscle, and other peripheral tissues.

Impact of missense mutations in survival motor neuron protein (SMN1) leading to Spinal Muscular Atrophy (SMA): A computational approach

Spinal muscular atrophy (SMA) is a neuromuscular disorder caused by the mutations in survival motor neuron 1 gene (SMN1). The molecular pathology of missense mutations in SMN1 is not thoroughly investigated so far. Therefore, we collected all missense mutations in the SMN1 protein, using all possible search terms, from three databases (PubMed, PMC and Google Scholar). All missense mutations were subjected to in silico pathogenicity, conservation, and stability analysis tools. We used statistical analysis as a QC measure for validating the specificity and accuracy of these tools. PolyPhen-2 demonstrated the highest specificity and accuracy. While PolyPhen-1 showed the highest sensitivity; overall, PolyPhen2 showed better measures in comparison to other in silico tools. Three mutations (D44V, Y272C, and Y277C) were identified as the most pathogenic and destabilizing. Further, we compared the physiochemical properties of the native and the mutant amino acids and observed loss of H-bonds and aromatic stacking upon the cysteine to tyrosine substitution, which led to the loss of aromatic rings and may reduce protein stability. The three mutations were further subjected to Molecular Dynamics Simulation (MDS) analysis using GROMACS to understand the structural changes. The Y272C and Y277C mutants exhibited maximum deviation pattern from the native protein as compared to D44V mutant. Further MDS analysis predicted changes in the stability that may have been contributed due to the loss of hydrogen bonds as observed in intramolecular hydrogen bond analysis and physiochemical analysis. A loss of function/structural impact was found to be severe in the case of Y272C and Y277C mutants in comparison to D44V mutation. Correlating the results from in silico predictions, physiochemical analysis, and MDS, we were able to observe a loss of stability in all the three mutants. This combinatorial approach could serve as a platform for variant interpretation and drug design for spinal muscular dystrophy resulting from missense mutations.

A phase 1 healthy male volunteer single escalating dose study of the pharmacokinetics and pharmacodynamics of risdiplam (RG7916, RO7034067), a SMN2 splicing modifier

AIMS

Risdiplam (RG7916, RO7034067) is an orally administered, centrally and peripherally distributed, survival of motor neuron 2 (SMN2) mRNA splicing modifier for the treatment of spinal muscular atrophy (SMA). The objectives of this entry-into-human study were to assess the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics of risdiplam, and the effect of the strong CYP3A inhibitor itraconazole on the PK of risdiplam in healthy male volunteers.

METHODS

Part 1 had a randomized, double-blind, adaptive design with 25 subjects receiving single ascending oral doses of risdiplam (ranging from 0.6-18.0 mg, n = 18) or placebo (n = 7). A Bayesian framework was applied to estimate risdiplam's effect on SMN2 mRNA. The effect of multiple doses of itraconazole on the PK of risdiplam was also assessed using a two-period cross-over design (n = 8).

RESULTS

Risdiplam in the fasted or fed state was well tolerated. Risdiplam exhibited linear PK over the dose range with a multi-phasic decline with a mean terminal half-life of 40-69 h. Food had no relevant effect, and itraconazole had only a minor effect on plasma PK indicating a low fraction of risdiplam metabolized by CYP3A. The highest tested dose of 18.0 mg risdiplam led to approximately 41% (95% confidence interval 27-55%) of the estimated maximum increase in SMN2 mRNA.

CONCLUSIONS

Risdiplam was well tolerated and proof of mechanism was demonstrated by the intended shift in SMN2 splicing towards full-length SMN2 mRNA. Based on these data, Phase 2/3 studies of risdiplam in patients with SMA are now ongoing.

Prenatal aspects in spinal muscular atrophy: From early detection to early presymptomatic intervention

With the recent advances in spinal muscular atrophy therapies, the complete scenario of standard of care and following up is changing not only in the clinical field with new phenotypes emerging but also with new expectations for patients, caregivers and health providers. The actual evidence indicates that early intervention and treatment is crucial for better response and prognosis. Knowledge of the prenatal and pre-symptomatic postnatal stages of the disease are becoming essential to consider the opportunities of timely diagnosis and to decide the earliest therapeutic intervention.

Exercise biology of neuromuscular disorders

Neuromuscular disorders (NMDs) are chronic conditions that affect the neuromuscular system. Many NMDs currently have no cure; however, as more effective therapies become available for NMD patients, these individuals will exhibit improved health and/or prolonged lifespans. As a result, persons with NMDs will likely desire to engage in a more diverse variety of activities of daily living, including increased physical activity or exercise. Therefore, there is a need to increase our knowledge of the effects of acute exercise and chronic training on the neuromuscular system in NMD contexts. Here, we discuss the disease mechanisms and exercise biology of Duchenne muscular dystrophy (DMD), spinal muscular atrophy (SMA), and myotonic dystrophy type 1 (DM1), which are among the most prevalent NMDs in children and adults. Evidence from clinical and preclinical studies are reviewed, with emphasis on the functional outcomes of exercise, as well as on the putative cellular mechanisms that drive exercise-induced remodelling of the neuromuscular system. Continued investigation of the molecular mechanisms of exercise adaptation in DMD, SMA, and DM1 will assist in enhancing our understanding of the biology of these most prevalent NMDs. This information may also be useful for guiding the development of novel therapeutic targets for future pursuit.

Therapeutic advances in 5q-linked spinal muscular atrophy

Spinal muscular atrophy (SMA) is a severe and clinically-heterogeneous motor neuron disease caused, in most cases, by a homozygous mutation in the SMN1 gene. Regarding the age of onset and motor involvement, at least four distinct clinical phenotypes have been recognized. This clinical variability is, in part, related to the SMN2 copy number. By now, only supportive therapies have been available. However, promising specific therapies are currently being developed based on different mechanisms to increase the level of SMN protein; in particular, intrathecal antisense oligonucleotides that prevent the skipping of exon 7 during SMN2 transcription, and intravenous SMN1 insertion using viral vector. These therapeutic perspectives open a new era in the natural history of the disease. In this review, we intend to discuss the most recent and promising therapeutic strategies, with special consideration to the pathogenesis of the disease and the mechanisms of action of such therapies.

Light modulation ameliorates expression of circadian genes and disease progression in spinal muscular atrophy mice

Physiology and behaviour are critically dependent on circadian regulation via a core set of clock genes, dysregulation of which leads to metabolic and sleep disturbances. Metabolic and sleep perturbations occur in spinal muscular atrophy (SMA), a neuromuscular disorder caused by loss of the survival motor neuron (SMN) protein and characterized by motor neuron loss and muscle atrophy. We therefore investigated the expression of circadian rhythm genes in various metabolic tissues and spinal cord of the Taiwanese Smn-/-;SMN2 SMA animal model. We demonstrate a dysregulated expression of the core clock genes (clock, ARNTL/Bmal1, Cry1/2, Per1/2) and clock output genes (Nr1d1 and Dbp) in SMA tissues during disease progression. We also uncover an age- and tissue-dependent diurnal expression of the Smn gene. Importantly, we observe molecular and phenotypic corrections in SMA mice following direct light modulation. Our study identifies a key relationship between an SMA pathology and peripheral core clock gene dysregulation, highlights the influence of SMN on peripheral circadian regulation and metabolism and has significant implications for the development of peripheral therapeutic approaches and clinical care management of SMA patients.

Evidence in focus: Nusinersen use in spinal muscular atrophy: Report of the Guideline Development, Dissemination, and Implementation Subcommittee of the American Academy of Neurology

OBJECTIVE

To identify the level of evidence for use of nusinersen to treat spinal muscular atrophy (SMA) and review clinical considerations regarding use.

METHODS

The author panel systematically reviewed nusinersen clinical trials for patients with SMA and assigned level of evidence statements based on the American Academy of Neurology's 2017 therapeutic classification of evidence scheme. Safety information, regulatory decisions, and clinical context were also reviewed.

RESULTS

Four published clinical trials were identified, 3 of which were rated above Class IV. There is Class III evidence that in infants with homozygous deletions or mutations of SMN1, nusinersen improves the probability of permanent ventilation-free survival at 24 months vs a well-defined historical cohort. There is Class I evidence that in term infants with SMA and 2 copies of SMN2, treatment with nusinersen started in individuals younger than 7 months results in a better motor milestone response and higher rates of event-free survival than sham control. There is Class I evidence that in children aged 2-12 years with SMA symptom onset after 6 months of age, nusinersen results in greater improvement in motor function at 15 months than sham control. Nusinersen was safe and well-tolerated.

CLINICAL CONTEXT

Evidence of efficacy is currently highest for treatment of infantile- and childhood-onset SMA in the early and middle symptomatic phases. While approved indications for nusinersen use in North America and Europe are broad, payer coverage for populations outside those in clinical trials remain variable. Evidence, availability, cost, and patient preferences all influence decision-making regarding nusinersen use.

Impaired myogenic development, differentiation and function in hESC-derived SMA myoblasts and myotubes

Spinal muscular atrophy (SMA) is a severe genetic disorder that manifests in progressive neuromuscular degeneration. SMA originates from loss-of-function mutations of the SMN1 (Survival of Motor Neuron 1) gene. Recent evidence has implicated peripheral deficits, especially in skeletal muscle, as key contributors to disease progression in SMA. In this study we generated myogenic cells from two SMA-affected human embryonic stem cell (hESC) lines with deletion of SMN1 bearing two copies of the SMN2 gene and recapitulating the molecular phenotype of Type 1 SMA. We characterized myoblasts and myotubes by comparing them to two unaffected, control hESC lines and demonstrate that SMA myoblasts and myotubes showed altered expression of various myogenic markers, which translated into an impaired in vitro myogenic maturation and development process. Additionally, we provide evidence that these SMN1 deficient cells display functional deficits in cholinergic calcium signaling response, glycolysis and oxidative phosphorylation. Our data describe a novel human myogenic SMA model that might be used for interrogating the effect of SMN depletion during skeletal muscle development, and as model to investigate biological mechanisms targeting myogenic differentiation, mitochondrial respiration and calcium signaling processes in SMA muscle cells.

A continuous repetitive task to detect fatigability in spinal muscular atrophy

Background

To determine the value of a continuous repetitive task to detect and quantify fatigability as additional dimension of impaired motor function in patients with hereditary proximal spinal muscular atrophy (SMA).

Results

In this repeated measure case-control study 52 patients with SMA types 2–4, 17 healthy and 29 disease controls performed five consecutive rounds of the Nine-Hole Peg test to determine the presence of fatigability. We analysed differences in test performance and associations with disease characteristics. Five patients with SMA type 2 (22%) and 1 disease control (3%) could not finish five rounds due to fatigue (p = 0.01). Patients with SMA type 2 performed the test significantly more slowly than all other groups (p < 0.005) and disease controls were slower than healthy controls (p < 0.05). Patients with SMA type 2 performed round five 27% slower than round one, while healthy controls performed round five 14% faster than round one (p = 0.005). There was no difference between SMA type 3a, type 3b/4 or disease controls and healthy controls (p > 0.4). Time needed to complete each round during the five-round task increased in 15 patients with SMA type 2 (65%), 4 with type 3a (36%), 4 with type 3b/4 (22%), 9 disease controls (31%) and 1 healthy control (6%). There was no effect of age at disease onset or disease duration in SMA type 2 (p = 0.39). Test-retest reliability was high.

Conclusion

Fatigability of remaining arm function is a feature of SMA type 2 and can be determined with continuous repetitive tasks.

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.

Dysregulation of Mdm2 and Mdm4 alternative splicing underlies motor neuron death in spinal muscular atrophy

Van Alstyne et al. show that loss of SMN-dependent regulation of Mdm2 and Mdm4 alternative splicing underlies p53-mediated death of motor neurons in SMA, establishing a causal link between snRNP dysfunction and neurodegeneration.

Ubiquitous deficiency in the survival motor neuron (SMN) protein causes death of motor neurons—a hallmark of the neurodegenerative disease spinal muscular atrophy (SMA)—through poorly understood mechanisms. Here, we show that the function of SMN in the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs) regulates alternative splicing of Mdm2 and Mdm4, two nonredundant repressors of p53. Decreased inclusion of critical Mdm2 and Mdm4 exons is most prominent in SMA motor neurons and correlates with both snRNP reduction and p53 activation in vivo. Importantly, increased skipping of Mdm2 and Mdm4 exons regulated by SMN is necessary and sufficient to synergistically elicit robust p53 activation in wild-type mice. Conversely, restoration of full-length Mdm2 and Mdm4 suppresses p53 induction and motor neuron degeneration in SMA mice. These findings reveal that loss of SMN-dependent regulation of Mdm2 and Mdm4 alternative splicing underlies p53-mediated death of motor neurons in SMA, establishing a causal link between snRNP dysfunction and neurodegeneration.

Two breakthrough gene-targeted treatments for spinal muscular atrophy: challenges remain

The motor neuron disease spinal muscular atrophy (SMA) is caused by recessive, loss-of-function mutations of the survival motor neuron 1 gene (SMN1). Alone, such mutations are embryonically lethal, but SMA patients retain a paralog gene, SMN2, that undergoes alternative pre-mRNA splicing, producing low levels of SMN protein. By mechanisms that are not well understood, reduced expression of the ubiquitously expressed SMN protein causes an early-onset motor neuron disease that often results in infantile or childhood mortality. Recently, striking clinical improvements have resulted from two novel treatment strategies to increase SMN protein by (a) modulating the splicing of existing SMN2 pre-mRNAs using antisense oligonucleotides, and (b) transducing motor neurons with self-complementary adeno-associated virus 9 (scAAV9) expressing exogenous SMN1 cDNA. We review the recently published clinical trial results and discuss the differing administration, tissue targeting, and potential toxicities of these two therapies. We also focus on the challenges that remain, emphasizing the many clinical and biologic questions that remain open. Answers to these questions will enable further optimization of these remarkable SMA treatments as well as provide insights that may well be useful in application of these therapeutic platforms to other diseases.

Motor neuron biology and disease: A current perspective on infantile-onset spinal muscular atrophy

Infantile-onset spinal muscular atrophy (SMA) is a prototypical disease in which to investigate selective neurodegenerative phenotypes. Caused by low levels of the ubiquitously expressed Survival Motor Neuron (SMN) protein, the disease mainly targets the spinal motor neurons. This selective phenotype remains largely unexplained, but has not hindered the development of SMN repletion as a means to a treatment. Here we chronicle recent advances in the area of SMA biology. We provide a brief background to the disease, highlight major advances that have shaped our current understanding of SMA, trace efforts to treat the condition, discuss the outcome of two promising new therapies and conclude by considering contemporary as well as new challenges stemming from recent successes within the field.

Insights into the pharmaceuticals and mechanisms of neurological orphan diseases: Current Status and future expectations

Several rare or orphan diseases have been characterized that singly affect low numbers of people, but cumulatively reach ∼6%-10% of the population in Europe and in the United States. Human genetics has shown to be broadly effective when evaluating subjacent genetic defects such as orphan genetic diseases, but on the other hand, a modest progress has been achieved toward comprehending the molecular pathologies and designing new therapies. Chemical genetics, placed at the interface of chemistry and genetics, could be employed to understand the molecular mechanisms of subjacent illnesses and for the discovery of new remediation processes. This review debates current progress in chemical genetics, and how a variety of compounds and reaction mechanisms can be used to study and ultimately treat rare genetic diseases. We focus here on a study involving Amyotrophic lateral sclerosis (ALS), Duchenne Muscular Dystrophy (DMD), Spinal muscular atrophy (SMA) and Familial Amyloid Polyneuropathy (FAP), approaching different treatment methods and the reaction mechanisms of several compounds, trying to elucidate new routes capable of assisting in the treatment profile.