Spinal Muscular Atrophy Therapeutics: Where do we Stand?

Innovative Genoceuticals in Human Gene Therapy Solutions: Challenges and Safe Clinical Trials of Orphan Gene Therapy Products

The success of gene therapy attempts is controversial and inconclusive. Currently, it is popular among the public, the scientific community, and manufacturers of Gene Therapy Medical Products. In the absence of any remedy or treatment options available for untreatable inborn metabolic orphan or genetic diseases, cancer, or brain diseases, gene therapy treatment by genoceuticals and T-cells for gene editing and recovery remains the preferred choice as the last hope. A new concept of "Genoceutical Gene Therapy" by using orphan 'nucleic acid-based therapy' aims to introduce scientific principles of treating acquired tissue damage and rare diseases. These Orphan Genoceuticals provide new scope for the 'genodrug' development and evaluation of genoceuticals and gene products for ideal 'gene therapy' use in humans with marketing authorization application (MAA). This perspective study focuses on the quality control, safety, and efficacy requirements of using 'nucleic acid-based and human cell-based new gene therapy' genoceutical products to set scientific advice on genoceutical-based 'orphan genodrug' design for clinical trials as per Western and European guidelines. The ethical Western FDA and European EMA guidelines suggest stringent legal and technical requirements on genoceutical medical products or orphan genodrug use for other countries to frame their own guidelines. The introduction section proposes lessknown 'orphan drug-like' properties of modified RNA/DNA, human cell origin gene therapy medical products, and their transgene products. The clinical trial section explores the genoceutical sources, FDA/EMA approvals for genoceutical efficacy criteria with challenges, and ethical guidelines relating to gene therapy of specific rare metabolic, cancer and neurological diseases. The safety evaluation of approved genoceuticals or orphan drugs is highlighted with basic principles and 'genovigilance' requirements (to observe any adverse effects, side effects, developed signs/symptoms) to establish their therapeutic use. Current European Union and Food and Drug Administration guidelines continuously administer fast-track regulatory legal framework from time to time, and they monitor the success of gene therapy medical product efficacy and safety. Moreover, new ethical guidelines on 'orphan drug-like genoceuticals' are updated for biodistribution of the vector, genokinetics studies of the transgene product, requirements for efficacy studies in industries for market authorization, and clinical safety endpoints with their specific concerns in clinical trials or public use.

From Junk to Function: LncRNAs in CNS Health and Disease

Recent advances in RNA sequencing technologies helped to uncover the existence of tens of thousands of long non-coding RNAs (lncRNAs) that arise from the dark matter of the genome. These lncRNAs were originally thought to be transcriptional noise but an increasing number of studies demonstrate that these transcripts can modulate protein-coding gene expression by a wide variety of transcriptional and post-transcriptional mechanisms. The spatiotemporal regulation of lncRNA expression is particularly evident in the central nervous system, suggesting that they may directly contribute to specific brain processes, including neurogenesis and cellular homeostasis. Not surprisingly, lncRNAs are therefore gaining attention as putative novel therapeutic targets for disorders of the brain. In this review, we summarize the recent insights into the functions of lncRNAs in the brain, their role in neuronal maintenance, and their potential contribution to disease. We conclude this review by postulating how these RNA molecules can be targeted for the treatment of yet incurable neurological disorders.

Small Activating RNAs: Towards the Development of New Therapeutic Agents and Clinical Treatments

Small double-strand RNA (dsRNA) molecules can activate endogenous genes via an RNA-based promoter targeting mechanism. RNA activation (RNAa) is an evolutionarily conserved mechanism present in diverse eukaryotic organisms ranging from nematodes to humans. Small activating RNAs (saRNAs) involved in RNAa have been successfully used to activate gene expression in cultured cells, and thereby this emergent technique might allow us to develop various biotechnological applications, without the need to synthesize hazardous construct systems harboring exogenous DNA sequences. Accordingly, this thematic issue aims to provide insights into how RNAa cellular machinery can be harnessed to activate gene expression leading to a more effective clinical treatment of various diseases.

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.

Spinal Muscular Atrophy (SMA) Subtype Concordance in Siblings: Findings From the Cure SMA Cohort

Background:

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by homozygous survival of motor neuron 1 (SMN1) gene disruption. Despite a genetic etiology, little is known about subtype concordance among siblings.

Objective:

To investigate subtype concordance among siblings with SMA.

Methods:

Cure SMA maintains a database of newly diagnosed patients with SMA, which was utilized for this research.

Results:

Among 303 sibships identified between 1996 and 2016, 84.8% were subtype concordant. Of concordant sibships, subtype distribution was as follows: Type I, 54.5%; Type II, 31.9%; Type III, 13.2%; Type IV, 0.4%. Subtype and concordance/discordance association was significant (Fisher’s exact test; p < 0.0001). Among discordant sibships (chi-square test, p < 0.0001), Types II/III (52.2%) and Types I/II (28.3%) were the most common pairs. No association was found between sibling sex and concordance. Our findings show that most siblings with SMA shared the same subtype concordance (most commonly Type I).

Conclusions:

These data are valuable for understanding familial occurrence of SMA subtypes, enabling better individual treatment and management planning in view of new treatment options and newborn screening initiatives.

The SMA Clinical Trial Readiness Program: creation and evaluation of a program to enhance SMA trial readiness in the United States

Spinal muscular atrophy (SMA) is a rare neuromuscular disease with a rapidly evolving treatment landscape. To better meet the needs of trial sponsors and the patient community in the United States (US) in this evolving context, Cure SMA established a clinical trial readiness program for new and prospective SMA clinical trial sites. Program development was informed by a review of the SMA clinical trial landscape, successful NMD trial and care networks, and factors important to effective trial conduct in SMA. The program was piloted in 2018 with a virtual site readiness evaluation, a trial readiness toolkit, and a readiness program for physical therapists and clinical evaluators. Nine US research hospitals participated in the pilot. Cure SMA evaluated the pilot program and resources through feedback surveys, which supported the program's relevance and value. Since 2018, the program has been expanded with additional sites, new best practices toolkits, and workshops. In partnership with Cure SMA, SMA Europe is also extending programming to European countries. The program is significant as an example of a patient advocacy group working successfully with pharmaceutical companies, other patient advocacy organizations, and research hospitals to promote trial readiness, and may serve as a model for organizations in other regions and diseases.

Novel PGD strategy based on single sperm linkage analysis for carriers of single gene pathogenic variant and chromosome reciprocal translocation

PURPOSE

Preimplantation genetic diagnosis (PGD) analysis can be challenging for couples who carry more than one genetic condition. In this study, we describe a new PGD strategy to select which embryo(s) to transfer for two clinically challenging cases. Both cases lack essential family members for linkage analysis including de novo mutation combined with reciprocal translocation.

METHODS

Diverging from conventional method, we performed direct point mutation detection, quantitative analysis of gene copy number, combined with linkage analysis assisted by SNP information from single sperm (or polar bodies), thus establishing an all-in-one protocol for single embryonic cell preimplantation diagnosis for two co-existing genetic conditions (monogenic disease and chromosomal abnormality) on the NGS-based platform.

RESULTS

Using this newly developed method, 15 embryos from two cases were screened, and two embryos were determined as free of the monogenic disease and specific chromosomal abnormalities created by the prospective father's reciprocal translocations.

CONCLUSION

This novel PGD strategy could effectively select unaffected embryo(s) for couples affected with or carrying a monogenetic disease and a reciprocal chromosome translocation concurrently.

Genetic approaches to the treatment of inherited neuromuscular diseases

Inherited neuromuscular diseases are a heterogeneous group of developmental and degenerative disorders that affect motor unit function. Major challenges toward developing therapies for these diseases include heterogeneity with respect to clinical severity, age of onset and the primary cell type that is affected (e.g. motor neurons, skeletal muscle and Schwann cells). Here, we review recent progress toward the establishment of genetic therapies to treat inherited neuromuscular disorders that affect both children and adults with a focus on spinal muscular atrophy, Charcot-Marie-Tooth disease and spinal and bulbar muscular atrophy. We discuss clinical features, causative mutations and emerging approaches that are undergoing testing in preclinical models and in patients or that have received recent approval for clinical use. Many of these efforts employ antisense oligonucleotides to alter pre-mRNA splicing or diminish target gene expression and use viral vectors to replace expression of mutant genes. Finally, we discuss remaining challenges for optimizing the delivery and effectiveness of these approaches. In sum, therapeutic strategies for neuromuscular diseases have shown encouraging results, raising hope that recent strides will translate into significant clinical benefits for patients with these disorders.

Development and Clinical Translation of Approved Gene Therapy Products for Genetic Disorders

The field of gene therapy is striving more than ever to define a path to the clinic and the market. Twenty gene therapy products have already been approved and over two thousand human gene therapy clinical trials have been reported worldwide. These advances raise great hope to treat devastating rare and inherited diseases as well as incurable illnesses. Understanding of the precise pathomechanisms of diseases as well as the development of efficient and specific gene targeting and delivery tools are revolutionizing the global market. Currently, human cancers and monogenic disorders are indications number one. The elevated prevalence of genetic disorders and cancers, clear gene manipulation guidelines and increasing financial support for gene therapy in clinical trials are major trends. Gene therapy is presently starting to become commercially profitable as a number of gene and cell-based gene therapy products have entered the market and the clinic. This article reviews the history and development of twenty approved human gene and cell-based gene therapy products that have been approved up-to-now in clinic and markets of mainly North America, Europe and Asia.

Clinical translation of gene medicine

Gene therapy has recently witnessed accelerated progress as a new therapeutic strategy with the potential to treat a range of inherited and acquired diseases. Billions of dollars have been invested in basic and clinical research on gene medicine, with ongoing clinical trials focused on cancer, monogenic diseases, cardiovascular diseases and other refractory diseases. Advances addressing the inherent challenges of gene therapy, particularly those related to retaining the delivery efficacy and minimizing unwanted immune responses, provide the basis for the widespread clinical application of gene medicine. Several types of genes delivered by viral or non-viral delivery vectors have demonstrated encouraging results in both animals and humans. As augmented by clinical indications, gene medicine techniques have rapidly become a promising alternative to conventional therapeutic strategies because of their better clinical benefit and lower toxicities. Their application in the clinic has been extensive as a result of the approval of many gene therapy drugs in recent years. In this review, we provide a comprehensive overview of the clinical translation of gene medicine, focusing on the key events and latest progress made regarding clinical gene therapy products. We also discuss the gene types and non-viral materials with respect to developing gene therapeutics in clinical trials.

Spinal Muscular Atrophy and Common Therapeutic Advances

BACKGROUND

Spinal muscular atrophy (SMA) is an autosomal recessive destructive motor neuron disease which is characterized primarily by the degeneration of α-motor neurons in the ventral gray horn of the spinal cord. It mainly affects children and represents the most common reason of inherited infant mortality.

MATERIAL AND METHODS

We provide an overview of the recent therapeutic strategies for the treatment of SMA together with available and developing therapeutic strategies. For this purpose, Google Scholar and PubMed databases were searched for literature on SMA, therapy and treatment. Titles were reviewed and 96 were selected and assessed in this paper.

RESULT

Over the last two decades, different therapeutic strategies have been proposed for SMA. Some methods are in the pre-clinical, others the clinical phase.

CONCLUSION

By emergence of the new approaches, especially in gene therapy, effective treatment in the close future is probable.

Notch Signaling Mediates Astrocyte Abnormality in Spinal Muscular Atrophy Model Systems

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by the degeneration of spinal motor neurons and muscle atrophy. The disease is mainly caused by low level of the survival motor neuron (SMN) protein, which is coded by two genes, namely SMN1 and SMN2, but leads to selective spinal motor neuron degeneration when SMN1 gene is deleted or mutated. Previous reports have shown that SMN-protein-deficient astrocytes are abnormally abundant in the spinal cords of SMA model mice. However, the mechanism of the SMN- deficient astrocyte abnormality remains unclear. The purpose of this study is to identify the cellular signaling pathways associated with the SMN-deficient astrocyte abnormality and propose a candidate therapy tool that modulates signaling. In the present study, we found that the astrocyte density was increased around the central canal of the spinal cord in a mouse SMA model and we identified the dysregulation of Notch signaling which is a known mechanism that regulates astrocyte differentiation and proliferation, in the spinal cord in both early and late stages of SMA pathogenesis. Moreover, pharmacological inhibition of Notch signaling improved the motor functional deficits in SMA model mice. These findings indicate that dysregulated Notch signaling may be an underlying cause of SMA pathology.

The Alteration of Intrinsic Excitability and Synaptic Transmission in Lumbar Spinal Motor Neurons and Interneurons of Severe Spinal Muscular Atrophy Mice

Spinal muscular atrophy (SMA) is the leading genetic cause of death in infants. Studies with mouse models have demonstrated increased excitability and loss of afferent proprioceptive synapses on motor neurons (MNs). To further understand functional changes in the motor neural network occurring in SMA, we studied the intrinsic excitability and synaptic transmission of both MNs and interneurons (INs) from ventral horn in the lumbar spinal cord in the survival motor neuron (SMN)Δ7 mouse model. We found significant differences in the membrane properties of MNs in SMA mice compared to littermate controls, including hyperpolarized resting membrane potential, increased input resistance and decreased membrane capacitance. Action potential (AP) properties in MNs from SMA mice were also different from controls, including decreased rheobase current, increased amplitude and an increased afterdepolarization (ADP) potential. The relationship between AP firing frequency and injected current was reduced in MNs, as was the threshold current, while the percentage of MNs showing long-lasting potentiation (LLP) in the intrinsic excitability was higher in SMA mice. INs showed a high rate of spontaneous firing, and those from SMA mice fired at higher frequency. INs from SMA mice showed little difference in their input-output relationship, threshold current, and plasticity in intrinsic excitability. The changes observed in both passive membrane and AP properties suggest greater overall excitability in both MNs and INs in SMA mice, with MNs showing more differences. There were also changes of synaptic currents in SMA mice. The average charge transfer per post-synaptic current of spontaneous excitatory and inhibitory synaptic currents (sEPSCs/sIPSCs) were lower in SMA MNs, while in INs sIPSC frequency was higher. Strikingly in light of the known loss of excitatory synapses on MNs, there was no difference in sEPSC frequency in MNs from SMA mice compared to controls. For miniature synaptic currents, mEPSC frequency was higher in SMA MNs, while for SMA INs, both mEPSC and mIPSC frequencies were higher. In SMA-affected mice we observed alterations of intrinsic and synaptic properties in both MNs and INs in the spinal motor network that may contribute to the pathophysiology, or alternatively, may be a compensatory response to preserve network function.

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.

A Semi-Mechanistic Population Pharmacokinetic Model of Nusinersen: An Antisense Oligonucleotide for the Treatment of Spinal Muscular Atrophy

A pharmacokinetic (PK) model was developed for nusinersen, an antisense oligonucleotide (ASO) that is the first approved treatment for spinal muscular atrophy (SMA). The model was built with data from 92 nonhuman primates (NHPs) following intrathecal doses (0.3–7 mg) and characterized the PK in cerebrospinal fluid (CSF), plasma, total spinal cord, brain, and pons. The estimated volumes were 13.6, 937, 4.5, 53.8, and 2.11 mL, respectively. Global sensitivity analysis demonstrated that the CSF‐to‐plasma drug distribution rate (0.09 hour⁻¹) is a major determinant of the maximum nusinersen concentration in central nervous system (CNS) tissues. Physiological age‐based and body weight‐based allometric scaling was implemented with exponent values of −0.08 and 1 for the rate constants and the volume of distribution, respectively. Simulations of the scaled model were in agreement with clinical observations from 52 pediatric phase I PK profiles. The developed model can be used to guide the design of clinical trials with ASOs.

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.

Nusinersen: The First Option Beyond Supportive Care for Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by degeneration of spinal motor neurons and poses significant adverse outcome in affected population. Survival motor neuron 1 (SMN1) protein encoded by SMN1 gene located on 5q¹³ is critical for survival and functioning of motor neurons. Almost identical gene SMN2, present on the same chromosome, produces a small truncated protein (SMN2) because of skipping of exon 7 from translation due to translation silent C6U substitution in exon 7 of SMN2 pre-mRNA transcript. Only 10% of the SMN2 mRNAs produce full length SMN2 protein by including exon 7 in healthy individuals. A large deletion or sometimes a point mutation in SMN1 gene is responsible for SMA. In this case the number of copies of SMN2 genes in an individual determines the severity of disease (the more the number of copies the less severe the disease). Nusinersen (ISIS 396443) binds to intron splicing silencer-N1 (ISS-N1; a site present ten nucleotides down to the junction of exon 7 and intron 7), modulating the splicing of SMN2 pre-mRNA transcript to increase the inclusion of exon 7, thereby increasing the production of full length SMN2 protein. Major evidence of its efficacy came from a sham controlled phase 3 clinical study ENDEAR. The study was stopped early based on significantly favorable results in interim analysis and all the patients were transitioned to receive nusinersen in an ongoing open-label, phase 3 study, SHINE, which will evaluate the long-term efficacy, safety and tolerability of the drug. Nusinersen is globally the first drug approved (by the US FDA) for treatment of SMA in children and adults.

Targeting RNA structure in SMN2 reverses spinal muscular atrophy molecular phenotypes

Modification of SMN2 exon 7 (E7) splicing is a validated therapeutic strategy against spinal muscular atrophy (SMA). However, a target-based approach to identify small-molecule E7 splicing modifiers has not been attempted, which could reveal novel therapies with improved mechanistic insight. Here, we chose as a target the stem-loop RNA structure TSL2, which overlaps with the 5' splicing site of E7. A small-molecule TSL2-binding compound, homocarbonyltopsentin (PK4C9), was identified that increases E7 splicing to therapeutic levels and rescues downstream molecular alterations in SMA cells. High-resolution NMR combined with molecular modelling revealed that PK4C9 binds to pentaloop conformations of TSL2 and promotes a shift to triloop conformations that display enhanced E7 splicing. Collectively, our study validates TSL2 as a target for small-molecule drug discovery in SMA, identifies a novel mechanism of action for an E7 splicing modifier, and sets a precedent for other splicing-mediated diseases where RNA structure could be similarly targeted.

The Role of AMPK in Neuromuscular Biology and Disease

AMP-activated protein kinase (AMPK) is a primary regulator of cellular metabolism. Recent studies have revealed that AMPK also mediates the maintenance and plasticity of α-motoneurons, the neuromuscular junction, and skeletal muscle. Furthermore, AMPK stimulation by either genetic, pharmacological, or physiological approaches elicits beneficial phenotypic remodeling in neuromuscular disorders (NMDs). Here, we review the role of AMPK as a governor of neuromuscular biology, and present evidence for AMPK as an effective molecular target for therapeutic pursuit in the context of the most prevalent NMDs, including Duchenne muscular dystrophy, spinal muscular atrophy, and myotonic dystrophy type 1. This information may be useful for engineering AMPK-targeted pharmacological- or lifestyle-based strategies to treat disorders of the neuromuscular system.

Therapeutic strategies for spinal muscular atrophy: SMN and beyond

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder characterized by loss of motor neurons and muscle atrophy, generally presenting in childhood. SMA is caused by low levels of the survival motor neuron protein (SMN) due to inactivating mutations in the encoding gene SMN1. A second duplicated gene, SMN2, produces very little but sufficient functional protein for survival. Therapeutic strategies to increase SMN are in clinical trials, and the first SMN2-directed antisense oligonucleotide (ASO) therapy has recently been licensed. However, several factors suggest that complementary strategies may be needed for the long-term maintenance of neuromuscular and other functions in SMA patients. Pre-clinical SMA models demonstrate that the requirement for SMN protein is highest when the structural connections of the neuromuscular system are being established, from late fetal life throughout infancy. Augmenting SMN may not address the slow neurodegenerative process underlying progressive functional decline beyond childhood in less severe types of SMA. Furthermore, individuals receiving SMN-based treatments may be vulnerable to delayed symptoms if rescue of the neuromuscular system is incomplete. Finally, a large number of older patients living with SMA do not fulfill the present criteria for inclusion in gene therapy and ASO clinical trials, and may not benefit from SMN-inducing treatments. Therefore, a comprehensive whole-lifespan approach to SMA therapy is required that includes both SMN-dependent and SMN-independent strategies that treat the CNS and periphery. Here, we review the range of non-SMN pathways implicated in SMA pathophysiology and discuss how various model systems can serve as valuable tools for SMA drug discovery.

Summary: Translational research for spinal muscular atrophy (SMA) should address the development of non-CNS and survival motor neuron (SMN)-independent therapeutic approaches to complement and enhance the benefits of CNS-directed and SMN-dependent therapies.

Correlation between SMA type and SMN2 copy number revisited: An analysis of 625 unrelated Spanish patients and a compilation of 2834 reported cases

Spinal muscular atrophy (SMA) is a neuromuscular disorder caused by loss or mutations in SMN1. According to age of onset, achieved motor abilities, and life span, SMA patients are classified into type I (never sit), II (never walk unaided) or III (achieve independent walking abilities). SMN2, the highly homologous copy of SMN1, is considered the most important phenotypic modifier of the disease. Determination of SMN2 copy number is essential to establish careful genotype-phenotype correlations, predict disease evolution, and to stratify patients for clinical trials. We have determined SMN2 copy numbers in 625 unrelated Spanish SMA patients with loss or mutation of both copies of SMN1 and a clear assignation of the SMA type by clinical criteria. Furthermore, we compiled data from relevant worldwide reports that link SMN2 copy number with SMA severity published from 1999 to date (2834 patients with different ethnic and geographic backgrounds). Altogether, we have assembled a database with a total of 3459 patients to delineate more universal prognostic rules regarding the influence of SMN2 copy number on SMA phenotype. This issue is crucial in the present scenario of therapeutic advances with the perspective of SMA neonatal screening and early diagnosis to initiate treatments.

Survival Motor Neuron Protein is Released from Cells in Exosomes: A Potential Biomarker for Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is caused by homozygous mutation of the survival motor neuron 1 (SMN1) gene. Disease severity inversely correlates to the amount of SMN protein produced from the homologous SMN2 gene. We show that SMN protein is naturally released in exosomes from all cell types examined. Fibroblasts from patients or a mouse model of SMA released exosomes containing reduced levels of SMN protein relative to normal controls. Cells overexpressing SMN protein released exosomes with dramatically elevated levels of SMN protein. We observed enhanced quantities of exosomes in the medium from SMN-depleted cells, and in serum from a mouse model of SMA and a patient with Type 3 SMA, suggesting that SMN-depletion causes a deregulation of exosome release or uptake. The quantity of SMN protein contained in the serum-derived exosomes correlated with the genotype of the animal, with progressively less protein in carrier and affected animals compared to wildtype mice. SMN protein was easily detectable in exosomes isolated from human serum, with a reduction in the amount of SMN protein in exosomes from a patient with Type 3 SMA compared to a normal control. Our results suggest that exosome-derived SMN protein may serve as an effective biomarker for SMA.

Cyclic tetrapeptide HDAC inhibitors as potential therapeutics for spinal muscular atrophy: Screening with iPSC-derived neuronal cells

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder that is caused by inactivating mutations in the Survival of motor neuron 1 (SMN1) gene, resulting in decreased SMN protein expression. Humans possess a paralog gene, SMN2, which contains a splicing defect in exon 7 leading to diminished expression of full-length, fully functional SMN protein. Increasing SMN2 expression has been a focus of therapeutic development for SMA. Multiple studies have reported the efficacy of histone deacetylase inhibitors (HDACi) in this regard. However, clinical trials involving HDACi have been unsatisfactory, possibly because previous efforts to identify HDACi to treat SMA have employed non-neuronal cells as the screening platform. To address this issue, we generated an SMA-patient specific, induced pluripotent stem cell (iPSC) derived neuronal cell line that contains homogenous Tuj1+neurons. We screened a small library of cyclic tetrapeptide HDACi using this SMA neuronal platform and discovered compounds that elevate SMN2 expression by an impressive twofold or higher. These candidates are also capable of forming gems intranuclearly in SMA neurons, demonstrating biological activity. Our study identifies new potential HDACi therapeutics for SMA screened using a disease-relevant SMA neuronal cellular model.

Discovery of a Small Molecule Probe That Post-Translationally Stabilizes the Survival Motor Neuron Protein for the Treatment of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is the leading genetic cause of infant death. We previously developed a high-throughput assay that employs an SMN2-luciferase reporter allowing identification of compounds that act transcriptionally, enhance exon recognition, or stabilize the SMN protein. We describe optimization and characterization of an analog suitable for in vivo testing. Initially, we identified analog 4m that had good in vitro properties but low plasma and brain exposure in a mouse PK experiment due to short plasma stability; this was overcome by reversing the amide bond and changing the heterocycle. Thiazole 27 showed excellent in vitro properties and a promising mouse PK profile, making it suitable for in vivo testing. This series post-translationally stabilizes the SMN protein, unrelated to global proteasome or autophagy inhibition, revealing a novel therapeutic mechanism that should complement other modalities for treatment of SMA.

Spinal muscular atrophy: A changing phenotype beyond the clinical trials

Spinal muscular atrophy is a monogenic, progressive motor neuron disorder caused by deletion or mutation in the SMN1 gene. A broad range of phenotypic severity, from very weak infants (Type 1) to ambulant children (type 3), is modified mainly by the number of copies of the "backup" SMN2 gene. Since the discovery of the role of both genes, basic research into the pathobiology of SMA, with in vitro and animal model studies, has identified therapeutic targets. Development of clinical outcome measures, natural history studies and standard of care guidelines have contributed to the development of protocols for therapeutic drugs now under clinical investigation. Following regulatory approval of the first drug treatment for SMA in the US (December, 2016) and marketing authorization in Europe (June, 2017), the prospects for care of these patients have changed. The evolution of the phenotype of SMA now needs to be considered beyond the clinical trials. This perspective review discusses potential new trajectories in the phenotype of SMA and the need for multidisciplinary teams to prepare for this changing landscape.

Combining Engineered Nucleases with Adeno-associated Viral Vectors for Therapeutic Gene Editing

With the recent advent of several generations of targeted DNA nucleases, most recently CRISPR/Cas9, genome editing has become broadly accessible across the biomedical community. Importantly, the capacity of these nucleases to modify specific genomic loci associated with human disease could render new classes of genetic disease, including autosomal dominant or even idiopathic disease, accessible to gene therapy. In parallel, the emergence of adeno-associated virus (AAV) as a clinically important vector raises the possibility of integrating these two technologies towards the development of gene editing therapies. Though clear challenges exist, numerous proof-of-concept studies in preclinical models offer exciting promise for the future of gene therapy.

The Antisense Transcript SMN-AS1 Regulates SMN Expression and Is a Novel Therapeutic Target for Spinal Muscular Atrophy

The neuromuscular disorder spinal muscular atrophy (SMA), the most common inherited killer of infants, is caused by insufficient expression of survival motor neuron (SMN) protein. SMA therapeutics development efforts have focused on identifying strategies to increase SMN expression. We identified a long non-coding RNA (lncRNA) that arises from the antisense strand of SMN, SMN-AS1, which is enriched in neurons and transcriptionally represses SMN expression by recruiting the epigenetic Polycomb repressive complex-2. Targeted degradation of SMN-AS1 with antisense oligonucleotides (ASOs) increases SMN expression in patient-derived cells, cultured neurons, and the mouse central nervous system. SMN-AS1 ASOs delivered together with SMN2 splice-switching oligonucleotides additively increase SMN expression and improve survival of severe SMA mice. This study is the first proof of concept that targeting a lncRNA to transcriptionally activate SMN2 can be combined with SMN2 splicing modification to ameliorate SMA and demonstrates the promise of combinatorial ASOs for the treatment of neurogenetic disorders.

Small Molecules in Development for the Treatment of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease resulting from pathologically low levels of survival motor neuron (SMN) protein. The majority of mRNA from the SMN2 allele undergoes alternative splicing and excludes critical codons, causing an SMN protein deficiency. While there is currently no FDA-approved treatment for SMA, early therapeutic efforts have focused on testing repurposed drugs such as phenylbutyrate (2), valproic acid (3), riluzole (6), hydroxyurea (7), and albuterol (9), none of which has demonstrated clinical effectiveness. More recently, clinical trials have focused on novel small-molecule compounds identified from high-throughput screening and medicinal chemistry optimization such as olesoxime (11), CK-2127107, RG7800, LMI070, and RG3039 (17). In this paper, we review both repurposed drugs and small-molecule compounds discovered following medicinal chemistry optimization for the potential treatment of SMA.

Activin Receptor Type IIB Inhibition Improves Muscle Phenotype and Function in a Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a devastating neurodegenerative disorder that causes progressive muscle atrophy and weakness. Using adeno-associated virus-mediated gene transfer, we evaluated the potential to improve skeletal muscle weakness via systemic, postnatal inhibition of either myostatin or all signaling via the activin receptor type IIB (ActRIIB). After demonstrating elevated p-SMAD3 content and differential content of ActRIIB ligands, 4-week-old male C/C SMA model mice were treated intraperitoneally with 1x10¹² genome copies of pseudotype 2/8 virus encoding a soluble form of the ActRIIB extracellular domain (sActRIIB) or protease-resistant myostatin propeptide (dnMstn) driven by a liver specific promoter. At 12 weeks of age, muscle mass and function were improved in treated C/C mice by both treatments, compared to controls. The fast fiber type muscles had a greater response to treatment than did slow muscles, and the greatest therapeutic effects were found with sActRIIB treatment. Myostatin/activin inhibition, however, did not rescue C/C mice from the reduction in motor unit numbers of the tibialis anterior muscle. Collectively, this study indicates that myostatin/activin inhibition represents a potential therapeutic strategy to increase muscle mass and strength, but not neuromuscular junction defects, in less severe forms of SMA.

Commonality amid diversity: Multi-study proteomic identification of conserved disease mechanisms in spinal muscular atrophy

The neuromuscular disease spinal muscular atrophy (SMA) is a leading genetic cause of infant mortality, resulting from low levels of full-length survival motor neuron (SMN) protein. Despite having a good understanding of the underlying genetics of SMA, the molecular pathways downstream of SMN that regulate disease pathogenesis remain unclear. The identification of molecular perturbations downstream of SMN is required in order to fully understand the fundamental biological role(s) for SMN in cells and tissues of the body, as well as to develop a range of therapeutic targets for developing novel treatments for SMA. Recent developments in proteomic screening technologies have facilitated proteome-wide investigations of a range of SMA models and tissues, generating novel insights into disease mechanisms by highlighting conserved changes in a range of molecular pathways. Comparative analysis of distinct proteomic datasets reveals conserved changes in pathways converging on GAP43, GAPDH, NCAM, UBA1, LMNA, ANXA2 and COL6A3. Proteomic studies therefore represent a leading tool with which to dissect the molecular mechanisms of disease pathogenesis in SMA, serving to identify potentially attractive targets for the development of novel therapies.

Neuroprotective Effect of Non-viral Gene Therapy Treatment Based on Tetanus Toxin C-fragment in a Severe Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a hereditary childhood disease that causes paralysis and progressive degeneration of skeletal muscles and spinal motor neurons. SMA is associated with reduced levels of full-length Survival of Motor Neuron (SMN) protein, due to mutations in the Survival of Motor Neuron 1 gene. Nowadays there are no effective therapies available to treat patients with SMA, so our aim was to test whether the non-toxic carboxy-terminal fragment of tetanus toxin heavy chain (TTC), which exhibits neurotrophic properties, might have a therapeutic role or benefit in SMA. In this manuscript, we have demonstrated that TTC enhance the SMN expression in motor neurons “in vitro” and evaluated the effect of intramuscular injection of TTC-encoding plasmid in the spinal cord and the skeletal muscle of SMNdelta7 mice. For this purpose, we studied the weight and the survival time, as well as, the survival and cell death pathways and muscular atrophy. Our results showed that TTC treatment reduced the expression of autophagy markers (Becn1, Atg5, Lc3, and p62) and pro-apoptotic genes such as Bax and Casp3 in spinal cord. In skeletal muscle, TTC was able to downregulate the expression of the main marker of autophagy, Lc3, to wild-type levels and the expression of the apoptosis effector protein, Casp3. Regarding the genes related to muscular atrophy (Ankrd1, Calm1, Col19a1, Fbox32, Mt2, Myod1, NogoA, Pax7, Rrad, and Sln), TTC suggest a compensatory effect for muscle damage response, diminished oxidative stress and modulated calcium homeostasis. These preliminary findings suggest the need for further experiments to depth study the effect of TTC in SMA disease.

Opening the window: The case for carrier and perinatal screening for spinal muscular atrophy

Spinal muscular atrophy (SMA) is the most common genetically inherited neurodegenerative disease that leads to infant mortality worldwide. SMA is caused by genetic deletion or mutation in the survival of motor neuron 1 (SMN1) gene, which results in a deficiency in SMN protein. For reasons that are still unclear, SMN protein deficiency predominantly affects α-motor neurons, resulting in their degeneration and subsequent paralysis of limb and trunk muscles, progressing to death in severe cases. Emerging evidence suggests that SMN protein deficiency also affects the heart, autonomic nervous system, skeletal muscle, liver, pancreas and perhaps many other organs. Currently, there is no cure for SMA. Patient treatment includes respiratory care, physiotherapy, and nutritional management, which can somewhat ameliorate disease symptoms and increase life span. Fortunately, several novel therapies have advanced to human clinical trials. However, data from studies in animal models of SMA indicate that the greatest therapeutic benefit is achieved through initiating treatment as early as possible, before widespread loss of motor neurons has occurred. In this review, we discuss the merit of carrier and perinatal patient screening for SMA considering the efficacy of emerging therapeutics and the physical, emotional and financial burden of the disease on affected families and society.

Selective Neuromuscular Denervation in Taiwanese Severe SMA Mouse Can Be Reversed by Morpholino Antisense Oligonucleotides

Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease caused by deficiency of the survival of motor neuron (SMN) protein, which leads to synaptic defects and spinal motor neuron death. Neuromuscular junction (NMJ) abnormalities have been found to be involved in SMA pathogenesis in the SMNΔ7 SMA mouse model. However, whether similar NMJ pathological findings present in another commonly used mouse model, the Taiwanese SMA mouse, has not been fully investigated. To examine the NMJs of the Taiwanese severe SMA mouse model (Smn-/-; SMN2tg/0), which is characterized by severe phenotype and death before postnatal day (P) 9, we investigated 25 axial and appendicular muscles from P1 to P9. We labelled the muscles with anti-neurofilament and anti-synaptophysin antibodies for nerve terminals and α-bungarotoxin for acetylcholine receptors (AChRs). We found that severe NMJ denervation (<50% fully innervated endplates) selectively occurred in the flexor digitorum brevis 2 and 3 (FDB-2/3) muscles from P5, and an increased percentage of fully denervated endplates correlated with SMA progression. Furthermore, synaptophysin signals were absent at the endplate compared to control littermate mice, suggesting that vesicle transport might only be affected at the end stage. Subsequently, we treated the Taiwanese severe SMA mice with morpholino (MO) antisense oligonucleotides (80 μg/g) via subcutaneous injection at P0. We found that MO significantly reversed the NMJ denervation in FDB-2/3 muscles and extended the survival of Taiwanese severe SMA mice. We conclude that early NMJ denervation in the FDB-2/3 muscles of Taiwanese severe SMA mice can be reversed by MO treatment. The FDB-2/3 muscles of Taiwanese severe SMA mice provide a very sensitive platform for assessing the effectiveness of drug treatments in SMA preclinical studies.

Spinal Muscular Atrophy Patient iPSC-Derived Motor Neurons Have Reduced Expression of Proteins Important in Neuronal Development

Spinal muscular atrophy (SMA) is an inherited neuromuscular disease primarily characterized by degeneration of spinal motor neurons, and caused by reduced levels of the SMN protein. Previous studies to understand the proteomic consequences of reduced SMN have mostly utilized patient fibroblasts and animal models. We have derived human motor neurons from type I SMA and healthy controls by creating their induced pluripotent stem cells (iPSCs). Quantitative mass spectrometry of these cells revealed increased expression of 63 proteins in control motor neurons compared to respective fibroblasts, whereas 30 proteins were increased in SMA motor neurons vs. their fibroblasts. Notably, UBA1 was significantly decreased in SMA motor neurons, supporting evidence for ubiquitin pathway defects. Subcellular distribution of UBA1 was predominantly cytoplasmic in SMA motor neurons in contrast to nuclear in control motor neurons; suggestive of neurodevelopmental abnormalities. Many of the proteins that were decreased in SMA motor neurons, including beta III-tubulin and UCHL1, were associated with neurodevelopment and differentiation. These neuron-specific consequences of SMN depletion were not evident in fibroblasts, highlighting the importance of iPSC technology. The proteomic profiles identified here provide a useful resource to explore the molecular consequences of reduced SMN in motor neurons, and for the identification of novel biomarker and therapeutic targets for SMA.

Pharmacologically induced mouse model of adult spinal muscular atrophy to evaluate effectiveness of therapeutics after disease onset

Spinal muscular atrophy (SMA) is a genetic disease characterized by atrophy of muscle and loss of spinal motor neurons. SMA is caused by deletion or mutation of the survival motor neuron 1 (SMN1) gene, and the nearly identical SMN2 gene fails to generate adequate levels of functional SMN protein due to a splicing defect. Currently, several therapeutics targeted to increase SMN protein are in clinical trials. An outstanding issue in the field is whether initiating treatment in symptomatic older patients would confer a therapeutic benefit, an important consideration as the majority of patients with milder forms of SMA are diagnosed at an older age. An SMA mouse model that recapitulates the disease phenotype observed in adolescent and adult SMA patients is needed to address this important question. We demonstrate here that Δ7 mice, a model of severe SMA, treated with a suboptimal dose of an SMN2 splicing modifier show increased SMN protein, survive into adulthood and display SMA disease-relevant pathologies. Increasing the dose of the splicing modifier after the disease symptoms are apparent further mitigates SMA histopathological features in suboptimally dosed adult Δ7 mice. In addition, inhibiting myostatin using intramuscular injection of AAV1-follistatin ameliorates muscle atrophy in suboptimally dosed Δ7 mice. Taken together, we have developed a new murine model of symptomatic SMA in adolescents and adult mice that is induced pharmacologically from a more severe model and demonstrated efficacy of both SMN2 splicing modifiers and a myostatin inhibitor in mice at later disease stages.

Chronic Treatment with the AMPK Agonist AICAR Prevents Skeletal Muscle Pathology but Fails to Improve Clinical Outcome in a Mouse Model of Severe Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder characterized by spinal and brainstem motor neuron (MN) loss and skeletal muscle paralysis. Currently, there is no effective treatment other than supportive care to ameliorate the quality of life of patients with SMA. Some studies have reported that physical exercise, by improving muscle strength and motor function, is potentially beneficial in SMA. The adenosine monophosphate-activated protein kinase agonist 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) has been reported to be an exercise mimetic agent that is able to regulate muscle metabolism and increase endurance both at rest and during exercise. Chronic AICAR administration has been shown to ameliorate the dystrophic muscle phenotype and motor behavior in the mdx mouse, a model of Duchenne muscular dystrophy. Here, we investigated whether chronic AICAR treatment was able to elicit beneficial effects on motor abilities and neuromuscular histopathology in a mouse model of severe SMA (the SMNΔ7 mouse). We report that AICAR improved skeletal muscle atrophy and structural changes found in neuromuscular junctions of SMNΔ7 animals. However, although AICAR prevented the loss of glutamatergic excitatory synapses on MNs, this compound was not able to mitigate MN loss or the microglial and astroglial reaction occurring in the spinal cord of diseased mice. Moreover, no improvement in survival or motor performance was seen in SMNΔ7 animals treated with AICAR. The beneficial effects of AICAR in SMA found in our study are SMN-independent, as no changes in the expression of this protein were seen in the spinal cord and skeletal muscle of diseased animals treated with this compound.

Mechanistic principles of antisense targets for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a major neurodegenerative disorder of children and infants. SMA is primarily caused by low levels of SMN protein owing to deletions or mutations of the SMN1 gene. SMN2, a nearly identical copy of SMN1, fails to compensate for the loss of the production of the functional SMN protein due to predominant skipping of exon 7. Several compounds, including antisense oligonucleotides (ASOs) that elevate SMN protein from SMN2 hold the promise for treatment. An ASO-based drug currently under Phase III clinical trial employs intronic splicing silencer N1 (ISS-N1) as its target. Cumulative studies on ISS-N1 reveal a wealth of information with significance to the overall therapeutic development for SMA. Here, the authors summarize the mechanistic principles behind various antisense targets currently available for SMA therapy.