Discovery of a Novel Class of Survival Motor Neuron 2 Splicing Modifiers for the Treatment of Spinal Muscular Atrophy

Mechanistic Studies of Small Molecule Ligands Selective to RNA Single G Bulges

Small-molecule RNA binders have emerged as an important pharmacological modality. A profound understanding of the ligand selectivity, binding mode, and influential factors governing ligand engagement with RNA targets is the foundation for rational ligand design. Here, we report a novel class of coumarin derivatives exhibiting selective binding affinity towards single G RNA bulges. Harnessing the computational power of all-atom Gaussian accelerated Molecular Dynamics (GaMD) simulations, we unveiled a rare minor groove binding mode of the ligand with a key interaction between the coumarin moiety and the G bulge. This predicted binding mode is consistent with results obtained from structure-activity-relationship (SAR) studies and transverse relaxation measurements by NMR spectroscopy. We further generated 444 molecular descriptors from 69 coumarin derivatives and identified key contributors to the binding events, such as charge state and planarity, by lasso (least absolute shrinkage and selection operator) regression. Strikingly, small structure perturbations on these key contributors, such as the addition of a methyl group that disrupts the planarity of the ligand resulted in > 100-fold reduction in the binding affinity. Our work deepened the understanding of RNA-small molecule interactions and integrated a new generalizable platform for the rational design of selective small-molecule RNA binders.

The diversity of splicing modifiers acting on A-1 bulged 5'-splice sites reveals rules for rational drug design

Pharmacological modulation of RNA splicing by small molecules is an emerging facet of drug discovery. In this context, the SMN2 splicing modifier SMN-C5 was used as a prototype to understand the mode of action of small molecule splicing modifiers and propose the concept of 5'-splice site bulge repair. In this study, we combined in vitro binding assays and structure determination by NMR spectroscopy to identify the binding modes of four other small molecule splicing modifiers that switch the splicing of either the SMN2 or the HTT gene. Here, we determined the solution structures of risdiplam, branaplam, SMN-CX and SMN-CY bound to the intermolecular RNA helix epitope containing an unpaired adenine within the G-2A-1G+1U+2 motif of the 5'-splice site. Despite notable differences in their scaffolds, risdiplam, SMN-CX, SMN-CY and branaplam contact the RNA epitope similarly to SMN-C5, suggesting that the 5'-splice site bulge repair mechanism can be generalised. These findings not only deepen our understanding of the chemical diversity of splicing modifiers that target A-1 bulged 5'-splice sites, but also identify common pharmacophores required for modulating 5'-splice site selection with small molecules.

[Risdiplam for the treatment of spinal muscular atrophy]

Spinal muscular atrophy (SMA) is a devastating disease that is the leading genetic cause of death in infants and young children. It includes a broad spectrum of phenotypes that are classified into clinical groups based on the age of onset and maximum motor function achieved. The most common form of SMA is due to a defect in the survival motor neuron 1 gene (SMN1) localized to 5q11.2-q13.3. The development of clinical symptoms and disease progression is thought to be due to decreased levels of survival motor neuron (SMN) protein. SMA type 1 results in almost inevitable mortality within the first 2 years of life. The first two drugs approved globally for the treatment of SMA were the antisense oligonucleotide nusinersen (Spinraza), and the gene therapy onasemnogene abeparvovec-xioi (Zolgensma). Both interventions have approval and restrictions on use in different countries around the world. Despite these approved therapies, the medical unmet need in SMA (the majority of patients with SMA are not on a disease-modifying therapy) remains high with therapies in the pipeline to address some of the remaining limitations. The third and more recently approved drug for SMA is risdiplam (Evrysdi), an orally administered, centrally and peripherally distributed small molecule that modulates SMN2 pre-mRNA splicing toward the production of full-length SMN2 mRNA to increase functional SMN protein levels. In Russia the drug risdiplam was approved for use on November 26, 2020 with indications for the treatment of SMA in patients aged 2 months and older, and in 2023 the indications were expanded - use is allowed starting from the birth. Risdiplam is widely distributed into the CNS and peripheral tissues including muscles. Following risdiplam administration, SMN protein levels compared with baseline levels increase between 2- and 6-fold depending on the SMA phenotype treated. The risdiplam clinical development program currently has four ongoing clinical trials assessing its safety and efficacy. Clinical trials included more than 450 patients receiving risdiplam to date, has been well tolerated and no treatment-related safety findings leading to study withdrawal have been observed. Data from real clinical practice - more than 11.000 patients worldwide receive therapy with risdiplam, also confirm the safety and good tolerability of the drug.

Identification and Optimization of RNA-Splicing Modulators as Huntingtin Protein-Lowering Agents for the Treatment of Huntington's Disease

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat in exon 1 of the huntingtin (HTT) gene. We report the design of a series of HTT pre-mRNA splicing modulators that lower huntingtin (HTT) protein, including the toxic mutant huntingtin (mHTT), by promoting insertion of a pseudoexon containing a premature termination codon at the exon 49-50 junction. The resulting transcript undergoes nonsense-mediated decay, leading to a reduction of HTT mRNA transcripts and protein levels. The starting benzamide core was modified to pyrazine amide and further optimized to give a potent, CNS-penetrant, and orally bioavailable HTT-splicing modulator 27. This compound reduced canonical splicing of the HTT RNA exon 49-50 and demonstrated significant HTT-lowering in both human HD stem cells and mouse BACHD models. Compound 27 is a structurally diverse HTT-splicing modulator that may help understand the mechanism of adverse effects such as peripheral neuropathy associated with branaplam.

Phototoxicity─Medicinal Chemistry Strategies for Risk Mitigation in Drug Discovery

Phototoxicity is a common safety concern encountered by project teams in pharmaceutical research and has the potential to stop progression of an otherwise promising candidate molecule. This perspective aims to provide an overview of the approaches toward mitigation of phototoxicity that medicinal chemists have taken during the lead optimization phase in the context of regulatory standards for photosafety evaluation. Various strategies are laid out based on available literature examples in order to highlight how structural modification can be utilized toward successful mitigation of a phototoxicity liability. A proposed flowchart is presented as a guidance tool to be used by the practicing medicinal chemist when facing a phototoxicity risk. The description of available tools to consider in the drug design process will include an overview of the evolution of in silico methods and their application as well as structure alerts for consideration as potential phototoxicophores.

Reversible Control of RNA Splicing by Photoswitchable Small Molecules

Dynamics are intrinsic to both RNA function and structure. Yet, the available means to precisely provide RNA-based processes with spatiotemporal resolution are scarce. Here, our work pioneers a reversible approach to regulate RNA splicing within primary patient-derived cells by synthetic photoswitches. Our small molecule enables conditional real-time control at mRNA and protein levels. NMR experiments, together with theoretical calculations, photochemical characterization, fluorescence polarization measurements, and living cell-based assays, confirmed light-dependent exon inclusion as well as an increase in the target functional protein. Therefore, we first demonstrated the potential of photopharmacology modulation in splicing, tweaking the current optochemical toolkit. The timeliness on the consolidation of RNA research as the driving force toward therapeutical innovation holds the promise that our approach will contribute to redrawing the vision of RNA.

Genetically modified animal models of hereditary diseases for testing of gene-directed therapy

Disease-causing genes have been identified for many severe muscular and neurological genetic disorders. Advances in the gene therapy field offer promising solutions for drug development to treat these life-threatening conditions. Depending on how the mutation affects the function of the gene product, different gene therapy approaches may be beneficial. Gene replacement therapy is appropriate for diseases caused by mutations that result in the deficiency of the functional protein. Gene suppression strategy is suggested for disorders caused by the toxic product of the mutant gene. Splicing modulators, genome editing, and base editing techniques can be applied to disorders with different types of underlying mutations. Testing potential drugs in animal models of human diseases is an indispensable step of development. Given the specific gene therapy approach, appropriate animal models can be generated using a variety of technologies ranging from transgenesis to precise genome editing. In this review, we discuss technologies used to generate small and large animal models of the most common muscular and neurological genetic disorders. We specifically focus on animal models that were used to test gene therapies based on adeno-associated vectors and antisense nucleotides.

A Chemical Biology Perspective to Therapeutic Regulation of RNA Splicing in Spinal Muscular Atrophy (SMA)

Manipulation of RNA splicing machinery has emerged as a drug modality. Here, we illustrate the potential of this novel paradigm to correct aberrant splicing events focused on the recent therapeutic advances in spinal muscular atrophy (SMA). SMA is an incurable neuromuscular disorder and at present the primary genetic cause of early infant death. This Review summarizes the exciting journey from the first reported SMA cases to the currently approved splicing-switching treatments, i.e., antisense oligonucleotides and small-molecule modifiers. We emphasize both chemical structures and molecular bases for recognition. We briefly discuss the advantages and disadvantages of these treatments and include the remaining challenges and future directions. Finally, we also predict that these success stories will contribute to further therapies for human diseases by RNA-splicing control.

Risdiplam: an investigational motor neuron-2 (SMN-2) splicing modifier for spinal muscular atrophy (SMA)

INTRODUCTION

Spinal muscular atrophy (SMA) is a rare autosomal recessive neuromuscular disease which is characterized by muscle atrophy and early death in most patients. Risdiplam is the third overall and first oral drug approved for SMA with disease-modifying potential. Risdiplam acts as a survival motor neuron 2 (SMN2) pre-mRNA splicing modifier with satisfactory safety and efficacy profile. This review aims to critically appraise the place of risdiplam in the map of SMA therapeutics.

AREAS COVERED

This review gives an overview of the current market for SMA and presents the mechanism of action and the pharmacological properties of risdiplam. It also outlines the development of risdiplam from early preclinical stages through to the most recently published results from phase 2/3 clinical trials. Risdiplam has proved its efficacy in pivotal trials for SMA Types 1, 2, and 3 with a satisfactory safety profile.

EXPERT OPINION

In the absence of comparative data with the other two approved drugs, the role of risdiplam in the treatment algorithm of affected individuals is examined in three different patient populations based on the age and diagnosis method (newborn screening or clinical, symptom-driven diagnosis). Long-term data and real-world data will play a fundamental role in its future.

CRISPR-mediated Enzyme Fragment Complementation Assay for Quantification of the Stability of Splice Isoforms

Small-molecule splicing modulators exemplified by an FDA-approved drug, risdiplam, are a new pharmacological modality for regulating the expression and stability of splice isoforms. We report a CRISPR-mediated enzyme fragment complementation (EFC) assay to quantify the splice isoform stability. The EFC assay harnessed a 42 amino acid split of a β-galactosidase (designate α-tag), which could be fused at the termini of the target genes using CRISPR/cas9. The α-tagged splice isoform would be quantified by measuring the enzymatic activity upon complementation with the rest of β-galactosidase. This EFC assay retained all the sequences of introns and exons of the target gene in the native genomic environment that recapitulates the cell biology of the diseases of interest. For a proof-of-concept, we developed a CRISPR-mediated EFC assay targeting the exon 7 of the survival of motor neuron 2 (SMN2) gene. The EFC assay compatible with 384-well plates robustly quantified the splicing modulation activity of small molecules. In this study, we also discovered that a coumarin derivative, compound 4, potently modulate SMN2 splicing at as low as 1.1 nM.

Principles and correction of 5'-splice site selection

In Eukarya, immature mRNA transcripts (pre-mRNA) often contain coding sequences, or exons, interleaved by non-coding sequences, or introns. Introns are removed upon splicing, and further regulation of the retained exons leads to alternatively spliced mRNA. The splicing reaction requires the stepwise assembly of the spliceosome, a macromolecular machine composed of small nuclear ribonucleoproteins (snRNPs). This review focuses on the early stage of spliceosome assembly, when U1 snRNP defines each intron 5'-splice site (5'ss) in the pre-mRNA. We first introduce the splicing reaction and the impact of alternative splicing on gene expression regulation. Thereafter, we extensively discuss splicing descriptors that influence the 5'ss selection by U1 snRNP, such as sequence determinants, and interactions mediated by U1-specific proteins or U1 small nuclear RNA (U1 snRNA). We also include examples of diseases that affect the 5'ss selection by U1 snRNP, and discuss recent therapeutic advances that manipulate U1 snRNP 5'ss selectivity with antisense oligonucleotides and small-molecule splicing switches.

Alternative Splicing Role in New Therapies of Spinal Muscular Atrophy

It has been estimated that 80% of the pre-mRNA undergoes alternative splicing, which exponentially increases the flow of biological information in cellular processes and can be an attractive therapeutic target. It is a crucial mechanism to increase genetic diversity. Disturbed alternative splicing is observed in many disorders, including neuromuscular diseases and carcinomas. Spinal Muscular Atrophy (SMA) is an autosomal recessive neurodegenerative disease. Homozygous deletion in 5q13 (the region coding for the motor neuron survival gene (SMN1)) is responsible for 95% of SMA cases. The nearly identical SMN2 gene does not compensate for SMN loss caused by SMN1 gene mutation due to different splicing of exon 7. A pathologically low level of survival motor neuron protein (SMN) causes degeneration of the anterior horn cells in the spinal cord with associated destruction of α-motor cells and manifested by muscle weakness and loss. Understanding the regulation of the SMN2 pre-mRNA splicing process has allowed for innovative treatment and the introduction of new medicines for SMA. After describing the concept of splicing modulation, this review will cover the progress achieved in this field, by highlighting the breakthrough accomplished recently for the treatment of SMA using the mechanism of alternative splicing.

Systematically Studying the Effect of Small Molecules Interacting with RNA in Cellular and Preclinical Models

The interrogation and manipulation of biological systems by small molecules is a powerful approach in chemical biology. Ideal compounds selectively engage a target and mediate a downstream phenotypic response. Although historically small molecule drug discovery has focused on proteins and enzymes, targeting RNA is an attractive therapeutic alternative, as many disease-causing or -associated RNAs have been identified through genome-wide association studies. As the field of RNA chemical biology emerges, the systematic evaluation of target validation and modulation of target-associated pathways is of paramount importance. In this Review, through an examination of case studies, we outline the experimental characterization, including methods and tools, to evaluate comprehensively the impact of small molecules that target RNA on cellular phenotype.

Use of Intramolecular 1,5-Sulfur-Oxygen and 1,5-Sulfur-Halogen Interactions in the Design of N-Methyl-5-aryl-N-(2,2,6,6-tetramethylpiperidin-4-yl)-1,3,4-thiadiazol-2-amine SMN2 Splicing Modulators

Spinal muscular atrophy (SMA) is a debilitating neuromuscular disease caused by low levels of functional survival motor neuron protein (SMN) resulting from a deletion or loss of function mutation of the survival motor neuron 1 (SMN1) gene. Branaplam (1) elevates levels of full-length SMN protein in vivo by modulating the splicing of the related gene SMN2 to enhance the exon-7 inclusion and increase levels of the SMN. The intramolecular hydrogen bond present in the 2-hydroxyphenyl pyridazine core of 1 enforces a planar conformation of the biaryl system and is critical for the compound activity. Scaffold morphing revealed that the pyridazine could be replaced by a 1,3,4-thiadiazole, which provided additional opportunities for a conformational constraint of the biaryl through intramolecular 1,5-sulfur-oxygen (S···O) or 1,5-sulfur-halogen (S···X) noncovalent interactions. Compound 26, which incorporates a 2-fluorophenyl thiadiazole motif, demonstrated a greater than 50% increase in production of full-length SMN protein in a mouse model of SMA.

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.

Structural basis of a small molecule targeting RNA for a specific splicing correction

Splicing modifiers promoting SMN2 exon 7 inclusion have the potential to treat spinal muscular atrophy, the leading genetic cause of infantile death. These small molecules are SMN2 exon 7 selective and act during the early stages of spliceosome assembly. Here, we show at atomic resolution how the drug selectively promotes the recognition of the weak 5' splice site of SMN2 exon 7 by U1 snRNP. The solution structure of the RNA duplex formed following 5' splice site recognition in the presence of the splicing modifier revealed that the drug specifically stabilizes a bulged adenine at this exon-intron junction and converts the weak 5' splice site of SMN2 exon 7 into a stronger one. The small molecule acts as a specific splicing enhancer cooperatively with the splicing regulatory network. Our investigations uncovered a novel concept for gene-specific alternative splicing correction that we coined 5' splice site bulge repair.

Disruption of RNA Metabolism in Neurological Diseases and Emerging Therapeutic Interventions

RNA binding proteins are critical to the maintenance of the transcriptome via controlled regulation of RNA processing and transport. Alterations of these proteins impact multiple steps of the RNA life cycle resulting in various molecular phenotypes such as aberrant RNA splicing, transport, and stability. Disruption of RNA binding proteins and widespread RNA processing defects are increasingly recognized as critical determinants of neurological diseases. Here, we describe distinct mechanisms by which the homeostasis of RNA binding proteins is compromised in neurological disorders through their reduced expression level, increased propensity to aggregate or sequestration by abnormal RNAs. These mechanisms all converge toward altered neuronal function highlighting the susceptibility of neurons to deleterious changes in RNA expression and the central role of RNA binding proteins in preserving neuronal integrity. Emerging therapeutic approaches to mitigate or reverse alterations of RNA binding proteins in neurological diseases are discussed.

More than a messenger: Alternative splicing as a therapeutic target

Alternative splicing of pre-mRNA is an essential post- and co-transcriptional mechanism of gene expression regulation that produces multiple mature mRNA transcripts from a single gene. Genetic mutations that affect splicing underlie numerous devastating diseases. The complexity of splicing regulation allows for multiple therapeutic approaches to correct disease-associated mis-splicing events. In this review, we first highlight recent findings from therapeutic strategies that have used splice switching antisense oligonucleotides and small molecules that bind directly to RNA. Second, we summarize different genetic and chemical approaches to target components of the spliceosome to correct splicing defects in pathological conditions. Finally, we present an overview of compounds that target kinases and accessory pathways that intersect with the splicing machinery. Advancements in the understanding of disease-specific defects caused by mis-regulation of alternative splicing will certainly increase the development of therapeutic options for the clinic. This article is part of a Special Issue entitled: RNA structure and splicing regulation edited by Francisco Baralle, Ravindra Singh and Stefan Stamm.

Splice-switching small molecules: A new therapeutic approach to modulate gene expression

Manipulating alternative RNA splicing events with small molecules is emerging as a viable mechanism for the development of therapeutics. A salient challenge in the field is understanding the molecular determinants defining the selectivity of splice-switching events and their mechanisms of action. In this review, the current state-of-the-art in splice-switching small molecules is described. Three examples of splice-switching small molecules are presented, and the differences in their modes of action compared.

Pharmacologic normalization of pathogenic dosage underlying genetic diseases: an overview of the literature and path forward

Most monogenic disorders are caused by a pathologic deficit or excess of a single transcript and/or protein. Given that small molecules, including drugs, can affect levels of mRNA and protein, the pharmacologic normalization of such pathogenic dosage represents a possible therapeutic approach for such conditions. Here, we review the literature exploring pharmacologic modulation of mRNA and/or protein levels for disorders with paralogous modifier genes, for haploinsufficient disorders (insufficient gene-product), as well as toxic gain-of-function disorders (surplus or pathologic gene-product). We also discuss challenges facing the development of rare disease therapy by pharmacologic modulation of mRNA and protein. Finally, we lay out guiding principles for selection of disorders which may be amenable to this approach.

Rewriting the (tran)script: Application to spinal muscular atrophy

Targeting RNA drastically expands our target space to therapeutically modulate numerous cellular processes implicated in human diseases. Of particular interest, drugging pre-mRNA splicing appears a very viable strategy; to control levels of splicing product by promoting the inclusion or exclusion of exons. After describing the concept of "splicing modulation", this chapter will cover the outstanding progress achieved in this field, by highlighting the breakthrough accomplished recently for the treatment of spinal muscular atrophy using two therapeutic modalities: splice switching oligonucleotides and small molecules. This review discusses the vital but feasible requirement for such drugs to deliver selectivity, and critical safety aspects are highlighted. Transformational medicines such as those developed to treat SMA are likely just the beginning of this story.

Discovery of Small Molecule Splicing Modulators of Survival Motor Neuron-2 (SMN2) for the Treatment of Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA), a rare neuromuscular disorder, is the leading genetic cause of death in infants and toddlers. SMA is caused by the deletion or a loss of function mutation of the survival motor neuron 1 (SMN1) gene. In humans, a second closely related gene SMN2 exists; however it codes for a less stable SMN protein. In recent years, significant progress has been made toward disease modifying treatments for SMA by modulating SMN2 pre-mRNA splicing. Herein, we describe the discovery of LMI070/branaplam, a small molecule that stabilizes the interaction between the spliceosome and SMN2 pre-mRNA. Branaplam (1) originated from a high-throughput phenotypic screening hit, pyridazine 2, and evolved via multiparameter lead optimization. In a severe mouse SMA model, branaplam treatment increased full-length SMN RNA and protein levels, and extended survival. Currently, branaplam is in clinical studies for SMA.

Discovery of Risdiplam, a Selective Survival of Motor Neuron-2 ( SMN2) Gene Splicing Modifier for the Treatment of Spinal Muscular Atrophy (SMA)

SMA is an inherited disease that leads to loss of motor function and ambulation and a reduced life expectancy. We have been working to develop orally administrated, systemically distributed small molecules to increase levels of functional SMN protein. Compound 2 was the first SMN2 splicing modifier tested in clinical trials in healthy volunteers and SMA patients. It was safe and well tolerated and increased SMN protein levels up to 2-fold in patients. Nevertheless, its development was stopped as a precautionary measure because retinal toxicity was observed in cynomolgus monkeys after chronic daily oral dosing (39 weeks) at exposures in excess of those investigated in patients. Herein, we describe the discovery of 1 (risdiplam, RG7916, RO7034067) that focused on thorough pharmacology, DMPK and safety characterization and optimization. This compound is undergoing pivotal clinical trials and is a promising medicine for the treatment of patients in all ages and stages with SMA.

Genetic therapies for inherited neuromuscular disorders

Inherited neuromuscular disorders encompass a broad group of genetic conditions, and the discovery of these underlying genes has expanded greatly in the past three decades. The discovery of such genes has enabled more precise diagnosis of these disorders and the development of specific therapeutic approaches that target the genetic basis and pathophysiological pathways. Such translational research has led to the approval of two genetic therapies by the US Food and Drug Administration: eteplirsen for Duchenne muscular dystrophy and nusinersen for spinal muscular atrophy, which are both antisense oligonucleotides that modify pre-mRNA splicing. In this Review we aim to discuss new genetic therapies and ongoing clinical trials for Duchenne muscular dystrophy, spinal muscular atrophy, and other less common childhood neuromuscular disorders.

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.