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

Autophagy in spinal muscular atrophy: from pathogenic mechanisms to therapeutic approaches

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder caused by the depletion of the ubiquitously expressed survival motor neuron (SMN) protein. While the genetic cause of SMA has been well documented, the exact mechanism(s) by which SMN depletion results in disease progression remain elusive. A wide body of evidence has highlighted the involvement and dysregulation of autophagy in SMA. Autophagy is a highly conserved lysosomal degradation process which is necessary for cellular homeostasis; defects in the autophagic machinery have been linked with a wide range of neurodegenerative disorders, including amyotrophic lateral sclerosis, Alzheimer's disease and Parkinson's disease. The pathway is particularly known to prevent neurodegeneration and has been suggested to act as a neuroprotective factor, thus presenting an attractive target for novel therapies for SMA patients. In this review, (a) we provide for the first time a comprehensive summary of the perturbations in the autophagic networks that characterize SMA development, (b) highlight the autophagic regulators which may play a key role in SMA pathogenesis and (c) propose decreased autophagic flux as the causative agent underlying the autophagic dysregulation observed in these patients.

Muscle: an independent contributor to the neuromuscular spinal muscular atrophy disease phenotype

Spinal muscular atrophy (SMA) is a pediatric-onset neuromuscular disorder caused by insufficient survival motor neuron (SMN) protein. SMN restorative therapies are now approved for the treatment of SMA; however, they are not curative, likely due to a combination of imperfect treatment timing, inadequate SMN augmentation, and failure to optimally target relevant organs. Here, we consider the implications of imperfect treatment administration, focusing specifically on outcomes for skeletal muscle. We examine the evidence that muscle plays a contributing role in driving neuromuscular dysfunction in SMA. Next, we discuss how SMN might regulate the health of myofibers and their progenitors. Finally, we speculate on therapeutic outcomes of failing to raise muscle SMN to healthful levels and present strategies to restore function to this tissue to ensure better treatment results.

A spinal muscular atrophy modifier implicates the SMN protein in SNARE complex assembly at neuromuscular synapses

Reduced survival motor neuron (SMN) protein triggers the motor neuron disease, spinal muscular atrophy (SMA). Restoring SMN prevents disease, but it is not known how neuromuscular function is preserved. We used model mice to map and identify an Hspa8G470R synaptic chaperone variant, which suppressed SMA. Expression of the variant in the severely affected mutant mice increased lifespan >10-fold, improved motor performance, and mitigated neuromuscular pathology. Mechanistically, Hspa8G470R altered SMN2 splicing and simultaneously stimulated formation of a tripartite chaperone complex, critical for synaptic homeostasis, by augmenting its interaction with other complex members. Concomitantly, synaptic vesicular SNARE complex formation, which relies on chaperone activity for sustained neuromuscular synaptic transmission, was found perturbed in SMA mice and patient-derived motor neurons and was restored in modified mutants. Identification of the Hspa8G470R SMA modifier implicates SMN in SNARE complex assembly and casts new light on how deficiency of the ubiquitous protein causes motor neuron disease.

Base editing rescue of spinal muscular atrophy in cells and in mice

Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality, arises from survival motor neuron (SMN) protein insufficiency resulting from SMN1 loss. Approved therapies circumvent endogenous SMN regulation and require repeated dosing or may wane. We describe genome editing of SMN2, an insufficient copy of SMN1 harboring a C6>T mutation, to permanently restore SMN protein levels and rescue SMA phenotypes. We used nucleases or base editors to modify five SMN2 regulatory regions. Base editing converted SMN2 T6>C, restoring SMN protein levels to wild type. Adeno-associated virus serotype 9-mediated base editor delivery in Δ7SMA mice yielded 87% average T6>C conversion, improved motor function, and extended average life span, which was enhanced by one-time base editor and nusinersen coadministration (111 versus 17 days untreated). These findings demonstrate the potential of a one-time base editing treatment for SMA.

Spinal muscular atrophy: state of the art and new therapeutic strategies

Spinal muscular atrophy (SMA) is a severe disorder of motor neurons and the most frequent cause of genetic mortality, due to respiratory complications. We are facing an exciting era with three available therapeutic options in a disease considered incurable for more than a century. However, the availability of effective approaches has raised up ethical, medical, and financial issues that are routinely faced by the SMA community. Each therapeutic strategy has its weaknesses and strengths and clinicians need to know them to optimize clinical care. In this review, the state of the art and the results and challenges of the new SMA therapeutic strategies are highlighted.

Advances and limitations for the treatment of spinal muscular atrophy

Spinal muscular atrophy (5q-SMA; SMA), a genetic neuromuscular condition affecting spinal motor neurons, is caused by defects in both copies of the SMN1 gene that produces survival motor neuron (SMN) protein. The highly homologous SMN2 gene primarily expresses a rapidly degraded isoform of SMN protein that causes anterior horn cell degeneration, progressive motor neuron loss, skeletal muscle atrophy and weakness. Severe cases result in limited mobility and ventilatory insufficiency. Untreated SMA is the leading genetic cause of death in young children. Recently, three therapeutics that increase SMN protein levels in patients with SMA have provided incremental improvements in motor function and developmental milestones and prevented the worsening of SMA symptoms. While the therapeutic approaches with Spinraza^®, Zolgensma^®, and Evrysdi^® have a clinically significant impact, they are not curative. For many patients, there remains a significant disease burden. A potential combination therapy under development for SMA targets myostatin, a negative regulator of muscle mass and strength. Myostatin inhibition in animal models increases muscle mass and function. Apitegromab is an investigational, fully human, monoclonal antibody that specifically binds to proforms of myostatin, promyostatin and latent myostatin, thereby inhibiting myostatin activation. A recently completed phase 2 trial demonstrated the potential clinical benefit of apitegromab by improving or stabilizing motor function in patients with Type 2 and Type 3 SMA and providing positive proof-of-concept for myostatin inhibition as a target for managing SMA. The primary goal of this manuscript is to orient physicians to the evolving landscape of SMA treatment.    

Inhibition of myostatin and related signaling pathways for the treatment of muscle atrophy in motor neuron diseases

Myostatin is a negative regulator of skeletal muscle growth secreted by skeletal myocytes. In the past years, myostatin inhibition sparked interest among the scientific community for its potential to enhance muscle growth and to reduce, or even prevent, muscle atrophy. These characteristics make it a promising target for the treatment of muscle atrophy in motor neuron diseases, namely, amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), which are rare neurological diseases, whereby the degeneration of motor neurons leads to progressive muscle loss and paralysis. These diseases carry a huge burden of morbidity and mortality but, despite this unfavorable scenario, several therapeutic advancements have been made in the past years. Indeed, a number of different curative therapies for SMA have been approved, leading to a revolution in the life expectancy and outcomes of SMA patients. Similarly, tofersen, an antisense oligonucleotide, is now undergoing clinical trial phase for use in ALS patients carrying the SOD1 mutation. However, these therapies are not able to completely halt or reverse progression of muscle damage. Recently, a trial evaluating apitegromab, a myostatin inhibitor, in SMA patients was started, following positive results from preclinical studies. In this context, myostatin inhibition could represent a useful strategy to tackle motor symptoms in these patients. The aim of this review is to describe the myostatin pathway and its role in motor neuron diseases, and to summarize and critically discuss preclinical and clinical studies of myostatin inhibitors in SMA and ALS. Then, we will highlight promises and pitfalls related to the use of myostatin inhibitors in the human setting, to aid the scientific community in the development of future clinical trials.

Molecular Pathogenesis and New Therapeutic Dimensions for Spinal Muscular Atrophy

The condition known as 5q spinal muscular atrophy (SMA) is a devastating autosomal recessive neuromuscular disease caused by a deficiency of the ubiquitous protein survival of motor neuron (SMN), which is encoded by the SMN1 and SMN2 genes. It is one of the most common pediatric recessive genetic diseases, and it represents the most common cause of hereditary infant mortality. After decades of intensive basic and clinical research efforts, and improvements in the standard of care, successful therapeutic milestones have been developed, delaying the progression of 5q SMA and increasing patient survival. At the same time, promising data from early-stage clinical trials have indicated that additional therapeutic options are likely to emerge in the near future. Here, we provide updated information on the molecular underpinnings of SMA; we also provide an overview of the rapidly evolving therapeutic landscape for SMA, including SMN-targeted therapies, SMN-independent therapies, and combinational therapies that are likely to be key for the development of treatments that are effective across a patient’s lifespan.

Effects of inhibitors of SLC9A-type sodium-protein exchangers on Survival Motor Neuron 2 (SMN2) mRNA splicing and expression

Spinal muscular atrophy (SMA) is an autosomal recessive, pediatric-onset disorder caused by the loss of spinal motor neurons thereby leading to muscle atrophy. SMA is caused by the loss of or mutations in the survival motor neuron 1 (SMN1) gene. SMN1 is duplicated in humans to give rise to the paralogous SMN2 gene. This paralog is nearly identical except for a cytosine to thymine (C-to-T) transition within an exonic splicing enhancer (ESE) element within exon 7. As a result, the majority of SMN2 transcripts lack exon 7 (SMNΔ7) which produces a truncated and unstable SMN protein. Since SMN2 copy number is inversely related to disease severity, it is a well-established target for SMA therapeutics development. 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an inhibitor of sodium/proton exchangers (NHEs), has previously been shown to increase exon 7 inclusion and SMN protein levels in SMA cells. In this study, NHE inhibitors were evaluated for their ability to modulate SMN2 expression. EIPA as well as 5-(N,N-hexamethylene)amiloride (HMA) increase exon 7 inclusion in SMN2 splicing reporter lines as well as in SMA fibroblasts. The EIPA-induced exon 7 inclusion occurs via a unique mechanism that does not involve previously identified splicing factors. Transcriptome analysis identified novel targets, including TIA1 and FABP3, for further characterization. EIPA and HMA are more selective at inhibiting the NHE5 isoform, which is expressed in fibroblasts as well as in neuronal cells. These results show that NHE5 inhibition increases SMN2 expression and may be a novel target for therapeutics development. Significance Statement This study demonstrates a molecular mechanism by which inhibitors of the sodium-protein exchanger increase the alternative splicing of SMN2 in spinal muscular atrophy cells. NHE5 selective inhibitors increase the inclusion of full-length SMN2 mRNAs by targeting TIA1 and FABP3 expression, which is distinct from other small molecule regulators of SMN2 alternative splicing. This study provides a novel means to increase full-length SMN2 expression and a novel target for therapeutics development.

Therapy development for spinal muscular atrophy: perspectives for muscular dystrophies and neurodegenerative disorders

BACKGROUND

Major efforts have been made in the last decade to develop and improve therapies for proximal spinal muscular atrophy (SMA). The introduction of Nusinersen/Spinraza™ as an antisense oligonucleotide therapy, Onasemnogene abeparvovec/Zolgensma™ as an AAV9-based gene therapy and Risdiplam/Evrysdi™ as a small molecule modifier of pre-mRNA splicing have set new standards for interference with neurodegeneration.

MAIN BODY

Therapies for SMA are designed to interfere with the cellular basis of the disease by modifying pre-mRNA splicing and enhancing expression of the Survival Motor Neuron (SMN) protein, which is only expressed at low levels in this disorder. The corresponding strategies also can be applied to other disease mechanisms caused by loss of function or toxic gain of function mutations. The development of therapies for SMA was based on the use of cell culture systems and mouse models, as well as innovative clinical trials that included readouts that had originally been introduced and optimized in preclinical studies. This is summarized in the first part of this review. The second part discusses current developments and perspectives for amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease, as well as the obstacles that need to be overcome to introduce RNA-based therapies and gene therapies for these disorders.

CONCLUSION

RNA-based therapies offer chances for therapy development of complex neurodegenerative disorders such as amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease. The experiences made with these new drugs for SMA, and also the experiences in AAV gene therapies could help to broaden the spectrum of current approaches to interfere with pathophysiological mechanisms in neurodegeneration.

Dual SMN inducing therapies can rescue survival and motor unit function in symptomatic ∆7SMA mice

Spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by survival motor neuron (SMN) protein deficiency which results in motor neuron loss and muscle atrophy. SMA is caused by a mutation or deletion of the survival motor neuron 1 (SMN1) gene and retention of the nearly identical SMN2 gene. SMN2 contains a C to T change in exon 7 that results in exon 7 exclusion from 90% of transcripts. SMN protein lacking exon 7 is unstable and rapidly degraded. The remaining full-length transcripts from SMN2 are insufficient for normal motor neuron function leading to the development of SMA. Three different therapeutic approaches that increase full-length SMN (FL-SMN) protein production are approved for treatment of SMA patients. Studies in both animal models and humans have demonstrated increasing SMN levels prior to onset of symptoms provides the greatest therapeutic benefit. Treatment of SMA, after some motor neuron loss has occurred, is also effective but to a lesser degree. The SMN∆7 mouse model is a well characterized model of severe or type 1 SMA, dying at 14 days of age. Here we treated three groups of ∆7SMA mice starting before, roughly during, and after symptom onset to determine if combining two mechanistically distinct SMN inducing therapies could improve the therapeutic outcome both before and after motor neuron loss. We found, compared with individual therapies, that morpholino antisense oligonucleotide (ASO) directed against ISS-N1 combined with the small molecule compound RG7800 significantly increased FL-SMN transcript and protein production resulting in improved survival and weight of ∆7SMA mice. Moreover, when give late symptomatically, motor unit function was completely rescued with no loss in function at 100 days of age in the dual treatment group. We have therefore shown that this dual therapeutic approach successfully increases SMN protein and rescues motor function in symptomatic ∆7SMA mice.

ActRIIB:ALK4-Fc alleviates muscle dysfunction and comorbidities in murine models of neuromuscular disorders

Patients with neuromuscular disorders suffer from a lack of treatment options for skeletal muscle weakness and disease comorbidities. Here, we introduce as a potential therapeutic agent a heterodimeric ligand-trapping fusion protein, ActRIIB:ALK4-Fc, which comprises extracellular domains of activin-like kinase 4 (ALK4) and activin receptor type IIB (ActRIIB), a naturally occurring pair of type I and II receptors belonging to the TGF-β superfamily. By surface plasmon resonance (SPR), ActRIIB:ALK4-Fc exhibited a ligand binding profile distinctly different from that of its homodimeric variant ActRIIB-Fc, sequestering ActRIIB ligands known to inhibit muscle growth but not trapping the vascular regulatory ligand bone morphogenetic protein 9 (BMP9). ActRIIB:ALK4-Fc and ActRIIB-Fc administered to mice exerted differential effects - concordant with SPR results - on vessel outgrowth in a retinal explant assay. ActRIIB:ALK4-Fc induced a systemic increase in muscle mass and function in wild-type mice and in murine models of Duchenne muscular dystrophy (DMD), amyotrophic lateral sclerosis (ALS), and disuse atrophy. Importantly, ActRIIB:ALK4-Fc improved neuromuscular junction abnormalities in murine models of DMD and presymptomatic ALS and alleviated acute muscle fibrosis in a DMD model. Furthermore, in combination therapy ActRIIB:ALK4-Fc increased the efficacy of antisense oligonucleotide M12-PMO on dystrophin expression and skeletal muscle endurance in an aged DMD model. ActRIIB:ALK4-Fc shows promise as a therapeutic agent, alone or in combination with dystrophin rescue therapy, to alleviate muscle weakness and comorbidities of neuromuscular disorders.

SMN protein is required throughout life to prevent spinal muscular atrophy disease progression

Spinal muscular atrophy (SMA) is caused by the loss of the survival motor neuron 1 (SMN1) gene function. The related SMN2 gene partially compensates but produces insufficient levels of SMN protein due to alternative splicing of exon 7. Evrysdi™ (risdiplam), recently approved for the treatment of SMA, and related compounds promote exon 7 inclusion to generate full-length SMN2 mRNA and increase SMN protein levels. SMNΔ7 type I SMA mice survive without treatment for ~ 17 days. SMN2 mRNA splicing modulators increase survival of SMN∆7 mice with treatment initiated at postnatal day 3 (PND3). To define SMN requirements for adult mice, SMNΔ7 mice were dosed with a SMN2 mRNA splicing modifier from PND3 to PND40, then dosing was stopped. Mice not treated after PND40 showed progressive weight loss, necrosis, and muscle atrophy after ~ 20 days. Male mice presented a more severe phenotype than female mice. Mice dosed continuously did not show disease symptoms. The estimated half-life of SMN protein is 2 days indicating that the SMA phenotype reappeared after SMN protein levels returned to baseline. Although SMN protein levels decreased with age in mice and SMN protein levels were higher in brain than in muscle, our studies suggest that SMN protein is required throughout the life of the mouse and is especially essential in adult peripheral tissues including muscle. These studies indicate that drugs such as risdiplam will be optimally therapeutic when given as early as possible after diagnosis and potentially will be required for the life of an SMA patient.

Activation of Muscle-Specific Kinase (MuSK) Reduces Neuromuscular Defects in the Delta7 Mouse Model of Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is a motor neuron disease caused by insufficient levels of the survival motor neuron (SMN) protein. One of the most prominent pathological characteristics of SMA involves defects of the neuromuscular junction (NMJ), such as denervation and reduced clustering of acetylcholine receptors (AChRs). Recent studies suggest that upregulation of agrin, a crucial NMJ organizer promoting AChR clustering, can improve NMJ innervation and reduce muscle atrophy in the delta7 mouse model of SMA. To test whether the muscle-specific kinase (MuSK), part of the agrin receptor complex, also plays a beneficial role in SMA, we treated the delta7 SMA mice with an agonist antibody to MuSK. MuSK agonist antibody #13, which binds to the NMJ, significantly improved innervation and synaptic efficacy in denervation-vulnerable muscles. MuSK agonist antibody #13 also significantly increased the muscle cross-sectional area and myofiber numbers in these denervation-vulnerable muscles but not in denervation-resistant muscles. Although MuSK agonist antibody #13 did not affect the body weight, our study suggests that preservation of NMJ innervation by the activation of MuSK may serve as a complementary therapy to SMN-enhancing drugs to maximize the therapeutic effectiveness for all types of SMA patients.

Nusinersen, an exon 7 inclusion drug for spinal muscular atrophy: A minireview

Spinal muscular atrophy is an autosomal recessive neuromuscular disease with incidence of 1 in 5000 to 10000 live births and is produced by homozygous deletion of exons 7 and 8 in the SMN1 gene. The SMN1 and SMN2 genes encode the survival motor neuron protein, a crucial protein for the preservation of motor neurons. Use of the newer drug, Nusinersen, from early infancy has shown improvement in clinical outcomes of spinal muscular atrophy patients.

Therapeutic Modulation of RNA Splicing in Malignant and Non-Malignant Disease

RNA splicing is the enzymatic process by which non-protein coding sequences are removed from RNA to produce mature protein-coding mRNA. Splicing is thereby a major mediator of proteome diversity as well as a dynamic regulator of gene expression. Genetic alterations disrupting splicing of individual genes or altering the function of splicing factors contribute to a wide range of human genetic diseases as well as cancer. These observations have resulted in the development of therapies based on oligonucleotides that bind to RNA sequences and modulate splicing for therapeutic benefit. In parallel, small molecules that bind to splicing factors to alter their function or modify RNA processing of individual transcripts are being pursued for monogenic disorders as well as for cancer.

Understanding European patient expectations towards current therapeutic development in spinal muscular atrophy

Following the 2017 approval of a first spinal muscular atrophy (SMA) treatment by the European Medicines Agency, SMA Europe launched a Europe-wide survey with the goal of understanding patients' treatment expectations, realities of daily living and access to clinical trials and therapy, and how this varied according to parameters such as age and disease severity. A response rate of 31% yielded 1474 completed surveys from 26 European countries. In line with findings from a 2015 SMA Europe-led survey, participants considered stabilization of their condition to be progress. Notably, responses indicated that the current classification of SMA at diagnosis by 'type' often does not reflect current mobility level. Large gaps in treatment access were identified that varied in particular between age and disease severity groups, yet there was high interest in clinical trial participation. In addition, alternative treatment options, including combination therapies, are now expectations. These perspectives should be central considerations through the research and development processes of new SMA therapies, through data generation and discussions on access to therapies. Results from this survey indicate that collaboration between stakeholders is essential to the foundation upon which innovative approaches for SMA treatments and access can be explored.

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.

Review of therapeutic options for spinal muscular atrophy

Spinal Muscular Atrophy (SMA) is uncommon genetic (autosomal recessive) disease that deteriorates neuromuscular function of the affected person's body by causing lower motor neuron damage, progress in muscle atrophy and in advanced cases leads to paralysis of muscles. Mainly skeletal and respiratory muscles are involved. SMA is present due to lack of SMA proteins, which are encoded by survival motor neuron-1 (SMN-1) genes. In mutation of SMN-1 genes, deficiency of SMN proteins occurs. SMA affects all age groups, but mainly and most severely children younger than 6 months of age. At present, risdiplam is a treatment option and the drug has been approved by the US Food Drug and Administration on 7 August 2020. The availability of the drug has led to increased financial, ethical and medical problems. SMA affected populations are regularly challenged to these issues.

Short-duration splice promoting compound enables a tunable mouse model of spinal muscular atrophy

We describe drug treatment paradigms that allow investigation of cellular and molecular pathogenesis at different stages of spinal muscular atrophy in a mouse model.

Spinal muscular atrophy (SMA) is a motor neuron disease and the leading genetic cause of infant mortality. SMA results from insufficient survival motor neuron (SMN) protein due to alternative splicing. Antisense oligonucleotides, gene therapy and splicing modifiers recently received FDA approval. Although severe SMA transgenic mouse models have been beneficial for testing therapeutic efficacy, models mimicking milder cases that manifest post-infancy have proven challenging to develop. We established a titratable model of mild and moderate SMA using the splicing compound NVS-SM2. Administration for 30 d prevented development of the SMA phenotype in severe SMA mice, which typically show rapid weakness and succumb by postnatal day 11. Furthermore, administration at day eight resulted in phenotypic recovery. Remarkably, acute dosing limited to the first 3 d of life significantly enhanced survival in two severe SMA mice models, easing the burden on neonates and demonstrating the compound as suitable for evaluation of follow-on therapies without potential drug–drug interactions. This pharmacologically tunable SMA model represents a useful tool to investigate cellular and molecular pathogenesis at different stages of disease.

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

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

AAV9-DOK7 gene therapy reduces disease severity in Smn2B/- SMA model mice

Spinal Muscular Atrophy (SMA) is an autosomal recessive neuromuscular disease caused by deletions or mutations in the survival motor neuron (SMN1) gene. An important hallmark of disease progression is the pathology of neuromuscular junctions (NMJs). Affected NMJs in the SMA context exhibit delayed maturation, impaired synaptic transmission, and loss of contact between motor neurons and skeletal muscle. Protection and maintenance of NMJs remains a focal point of therapeutic strategies to treat SMA, and the recent implication of the NMJ-organizer Agrin in SMA pathology suggests additional NMJ organizing molecules may contribute. DOK7 is an NMJ organizer that functions downstream of Agrin. The potential of DOK7 as a putative therapeutic target was demonstrated by adeno-associated virus (AAV)-mediated gene therapy delivery of DOK7 in Amyotrophic Lateral Sclerosis (ALS) and Emery Dreyefuss Muscular Dystrophy (EDMD). To assess the potential of DOK7 as a disease modifier of SMA, we administered AAV-DOK7 to an intermediate mouse model of SMA. AAV9-DOK7 treatment conferred improvements in NMJ architecture and reduced muscle fiber atrophy. Additionally, these improvements resulted in a subtle reduction in phenotypic severity, evidenced by improved grip strength and an extension in survival. These findings reveal DOK7 is a novel modifier of SMA.

New Treatments in Spinal Muscular Atrophy: Positive Results and New Challenges

Spinal muscular atrophy (SMA) is one of the most common autosomal recessive diseases with progressive weakness of skeletal and respiratory muscles, leading to significant disability. The disorder is caused by mutations in the survival motor neuron 1 (SMN1) gene and a consequent decrease in the SMN protein leading to lower motor neuron degeneration. Recently, Food and Drug Administration (FDA) and European Medical Agency (EMA) approved the antisense oligonucleotide nusinersen, the first SMA disease-modifying treatment and gene replacement therapy by onasemnogene abeparvovec. Encouraging results from phase II and III clinical trials have raised hope that other therapeutic options will enter soon in clinical practice. However, the availability of effective approaches has raised up ethical, medical and financial issues that are routinely faced by the SMA community. This review covers the available data and the new challenges of SMA therapeutic strategies.

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

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

Myostatin inhibition in combination with antisense oligonucleotide therapy improves outcomes in spinal muscular atrophy

BACKGROUND

Spinal muscular atrophy (SMA) is caused by genetic defects in the survival motor neuron 1 (SMN1) gene that lead to SMN deficiency. Different SMN-restoring therapies substantially prolong survival and function in transgenic mice of SMA. However, these therapies do not entirely prevent muscle atrophy and restore function completely. To further improve the outcome, we explored the potential of a combinatorial therapy by modulating SMN production and muscle-enhancing approach as a novel therapeutic strategy for SMA.

METHODS

The experiments were performed in a mouse model of severe SMA. A previously reported 25-mer morpholino antisense oligomer PMO25 was used to restore SMN expression. The adeno-associated virus-mediated expression of myostatin propeptide was used to block the myostatin pathway. Newborn SMA mice were treated with a single subcutaneous injection of 40 μg/g (therapeutic dose) or 10 μg/g (low-dose) PMO25 on its own or together with systemic delivery of a single dose of adeno-associated virus-mediated expression of myostatin propeptide. The multiple effects of myostatin inhibition on survival, skeletal muscle phenotype, motor function, neuromuscular junction maturation, and proprioceptive afferences were evaluated.

RESULTS

We show that myostatin inhibition acts synergistically with SMN-restoring antisense therapy in SMA mice treated with the higher therapeutic dose PMO25 (40 μg/g), by increasing not only body weight (21% increase in male mice at Day 40), muscle mass (38% increase), and fibre size (35% increase in tibialis anterior muscle in 3 month female SMA mice), but also motor function and physical performance as measured in hanging wire test (two-fold increase in time score) and treadmill exercise test (two-fold increase in running distance). In SMA mice treated with low-dose PMO25 (10 μg/g), the early application of myostatin inhibition prolongs survival (40% increase), improves neuromuscular junction maturation (50% increase) and innervation (30% increase), and increases both the size of sensory neurons in dorsal root ganglia (60% increase) and the preservation of proprioceptive synapses in the spinal cord (30% increase).

CONCLUSIONS

These data suggest that myostatin inhibition, in addition to the well-known effect on muscle mass, can also positively influence the sensory neural circuits that may enhance motor neurons function. While the availability of the antisense drug Spinraza for SMA and other SMN-enhancing therapies has provided unprecedented improvement in SMA patients, there are still unmet needs in these patients. Our study provides further rationale for considering myostatin inhibitors as a therapeutic intervention in SMA patients, in combination with SMN-restoring drugs.

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.

Motor transmission defects with sex differences in a new mouse model of mild spinal muscular atrophy

Background

Mouse models of mild spinal muscular atrophy (SMA) have been extremely challenging to generate. This paucity of model systems has limited our understanding of pathophysiological events in milder forms of the disease and of the effect of SMN depletion during aging.

Methods

A mild mouse model of SMA, termed Smn^2B/−;SMN2^+/−, was generated by crossing Smn^−/−;SMN2 and Smn^2B/2B mice. This new model was characterized using behavioral testing, histology, western blot, muscle-nerve electrophysiology as well as ultrasonography to study classical SMA features and extra-neuronal involvement.

Findings

Smn^2B/−;SMN2^+/− mice have normal survival, mild but sustained motor weakness, denervation and neuronal/neuromuscular junction (NMJ) transmission defects, and neurogenic muscle atrophy that are more prominent in male mice. Increased centrally located nuclei, intrinsic contractile and relaxation muscle defects were also identified in both female and male mice, with some male predominance. There was an absence of extra-neuronal pathology.

Interpretation

The Smn^2B/−;SMN2^+/− mouse provides a model of mild SMA, displaying some hallmark features including reduced weight, sustained motor weakness, electrophysiological transmission deficit, NMJ defects, and muscle atrophy. Early and prominent increase central nucleation and intrinsic electrophysiological deficits demonstrate the potential role played by muscle in SMA disease. The use of this model will allow for the understanding of the most susceptible pathogenic molecular changes in motor neurons and muscles, investigation of the effects of SMN depletion in aging, sex differences and most importantly will provide guidance for the currently aging SMA patients treated with the recently approved genetic therapies.

Funding

: This work was supported by Cure SMA/Families of SMA Canada (grant numbers KOT-1819 and KOT-2021); Muscular Dystrophy Association (USA) (grant number 575466); and Canadian Institutes of Health Research (CIHR) (grant number PJT-156379).

RNA-Targeted Therapies and High-Throughput Screening Methods

RNA-binding proteins (RBPs) are involved in regulating all aspects of RNA metabolism, including processing, transport, translation, and degradation. Dysregulation of RNA metabolism is linked to a plethora of diseases, such as cancer, neurodegenerative diseases, and neuromuscular disorders. Recent years have seen a dramatic shift in the knowledge base, with RNA increasingly being recognised as an attractive target for precision medicine therapies. In this article, we are going to review current RNA-targeted therapies. Furthermore, we will scrutinise a range of drug discoveries targeting protein-RNA interactions. In particular, we will focus on the interplay between Lin28 and let-7, splicing regulatory proteins and survival motor neuron (SMN) pre-mRNA, as well as HuR, Musashi, proteins and their RNA targets. We will highlight the mechanisms RBPs utilise to modulate RNA metabolism and discuss current high-throughput screening strategies. This review provides evidence that we are entering a new era of RNA-targeted medicine.

Safety and Treatment Effects of Nusinersen in Longstanding Adult 5q-SMA Type 3 - A Prospective Observational Study

Objective:

Spinal muscular atrophy (SMA) is a progressive autosomal recessive motor neuron disease caused by loss of the SMN1 gene. Based on randomized clinical trials in children with SMA type 1 and 2, Nusinersen has been approved as the first treatment for all types of SMA, including adults with SMA type 3.

Methods:

We evaluated the safety and treatment effects of Nusinersen in longstanding adult 5q-SMA type 3. Patients were treated with intrathecal loading doses at day 1, 14, 28 and 63, followed by maintenance dose every four months up to 300 days. We monitored the patients within SMArtCARE, a prospective open-label outcome study for disease progression, side effects and treatment efficacy, encompassing clinical examination including MRC sum score, vital capacity in sitting position (VC, VC % pred.), ALS Functional Rating Scale (ALS-FRS), 6-Minute-Walk-Test (6MWT), Revised Upper Limb Module (RULM), and Hammersmith Functional Rating Scale (HFMSE). We also measured biomarkers in the spinal fluid (phosphorylated neurofilament heavy chain pNFH, neuron-specific enolase NSE, proteins, ß-Amyloid 1–40, ß-Amyloid 1–42, tau and phospho-tau) and creatine kinase (CK). Assessments were performed at baseline, day 63 (V4), day 180 (V5) and day 300 (V6). For statistical analysis, we compared baseline to V4, V5 and V6, using the paired sample t-test. When there were significant differences, we added cohen's d and effect size r for evaluation of clinical meaningfulness.

Results:

19 patients were included, 17 of them have completed the observation period of 10 months (day 300, V6). Patients were aged 18 to 59 years with disease duration ranging from 6 to 53 years. Except for the 6MWT, the RULM and the peak cough flow, there were no relevant significant changes in all functional outcome assessments at V4, V5 or V6, compared to baseline. For the 6MWT, there was a statistically significant improvement at visit 5 and at visit 6. RULM-score increased significantly at V6, and peak cough flow at visit 5. In biomarker studies, there was a significant decline in NSE and pTAU as well as a slight increase in proteins. In safety analysis, overall, Nusinersen applications were well tolerated. Eleven patients reported adverse events that were related to the study procedures, comprising back pain in seven patients and post-lumbar-puncture headache following intrathecal administration in four patients. Post-lumbar-puncture headache was reported in three females and one male, in total eleven times of 108 punctures (10%). No serious adverse events occurred.

Conclusions:

This prospective observational study indicates a mild treatment effect in adults with long-standing SMA3 after 10 months of treatment with Nusinersen, which had never occurred in the natural history of the disease. In our cohort, the most significant outcome measures were the 6MWT with statistically significant changes after day 180 and day 300, RULM after day 300 and peak cough flow after day 180.

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.

AAV9-mediated delivery of miR-23a reduces disease severity in Smn2B/-SMA model mice

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by deletions or mutations in survival motor neuron 1 (SMN1). The molecular mechanisms underlying motor neuron degeneration in SMA remain elusive, as global cellular dysfunction obscures the identification and characterization of disease-relevant pathways and potential therapeutic targets. Recent reports have implicated microRNA (miRNA) dysregulation as a potential contributor to the pathological mechanism in SMA. To characterize miRNAs that are differentially regulated in SMA, we profiled miRNA levels in SMA induced pluripotent stem cell (iPSC)-derived motor neurons. From this array, miR-23a downregulation was identified selectively in SMA motor neurons, consistent with previous reports where miR-23a functioned in neuroprotective and muscle atrophy-antagonizing roles. Reintroduction of miR-23a expression in SMA patient iPSC-derived motor neurons protected against degeneration, suggesting a potential miR-23a-specific disease-modifying effect. To assess this activity in vivo, miR-23a was expressed using a self-complementary adeno-associated virus serotype 9 (scAAV9) viral vector in the Smn2B/- SMA mouse model. scAAV9-miR-23a significantly reduced the pathology in SMA mice, including increased motor neuron size, reduced neuromuscular junction pathology, increased muscle fiber area, and extended survival. These experiments demonstrate that miR-23a is a novel protective modifier of SMA, warranting further characterization of miRNA dysfunction in SMA.

Nerve sprouting capacity in a pharmacologically induced mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by loss-of-function mutations in the survival of motoneuron gene 1 (SMN1). SMA is characterized by motoneuron death, skeletal muscle denervation and atrophy. Disease severity inversely correlates with copy number of a second gene (SMN2), which harbors a splicing defect that causes the production of inadequate levels of functional SMN protein. Small molecules that modify SMN2 splicing towards increased production of functional SMN significantly ameliorate SMA phenotypes in mouse models of severe SMA. At suboptimal doses, splicing modifiers, such as SMN-C1, have served to generate mice that model milder SMA, referred to as pharmacological SMA mice, which survive into early adulthood. Nerve sprouting at endplates, known as terminal sprouting, is key to normal muscle fiber reinnervation following nerve injury and its promotion might mitigate neuromuscular symptoms in mild SMA. Sprouting has been difficult to study in severe SMA mice due to their short lifespan. Here, we show that pharmacological SMA mice are capable of terminal sprouting following reinnervation that is largely SMN-C1 dose-independent, but that they display a reinnervation delay that is critically SMN-C1 dose-dependent. Data also suggest that SMN-C1 can induce by itself a limited terminal sprouting response in SMA and wild-type normally-innervated endplates.

Specific inhibition of myostatin activation is beneficial in mouse models of SMA therapy

Spinal muscular atrophy (SMA) is a neuromuscular disease characterized by loss of α-motor neurons, leading to profound skeletal muscle atrophy. Patients also suffer from decreased bone mineral density and increased fracture risk. The majority of treatments for SMA, approved or in clinic trials, focus on addressing the underlying cause of disease, insufficient production of full-length SMN protein. While restoration of SMN has resulted in improvements in functional measures, significant deficits remain in both mice and SMA patients following treatment. Motor function in SMA patients may be additionally improved by targeting skeletal muscle to reduce atrophy and improve muscle strength. Inhibition of myostatin, a negative regulator of muscle mass, offers a promising approach to increase muscle function in SMA patients. Here we demonstrate that muSRK-015P, a monoclonal antibody which specifically inhibits myostatin activation, effectively increases muscle mass and function in two variants of the pharmacological mouse model of SMA in which pharmacologic restoration of SMN has taken place either 1 or 24 days after birth to reflect early or later therapeutic intervention. Additionally, muSRK-015P treatment improves the cortical and trabecular bone phenotypes in these mice. These data indicate that preventing myostatin activation has therapeutic potential in addressing muscle and bone deficiencies in SMA patients. An optimized variant of SRK-015P, SRK-015, is currently in clinical development for treatment of SMA.

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.

New and developing therapies in spinal muscular atrophy

Great progress has been made in the clinical translation of several therapeutic strategies for spinal muscular atrophy (SMA), including measures to selectively address Survival Motor Neuron (SMN) protein deficiency with SMN1 gene replacement or modulation of SMN2 encoded protein levels, as well as neuroprotective approaches and supporting muscle strength and function. This review highlights these novel therapies. This is particularly vital with the advent of the first disease modifying therapy, which has brought to the fore an array of questions surrounding who, how and when to treat, and stimulated challenges in resource limited healthcare systems to streamline access for those eligible for drug therapy. The overhaul of the landscape for all those involved in SMA extends to the design of further drug trials and the necessity of multidisciplinary supportive care to potentiate the effects of disease modifying medications. The impact of respiratory complications in SMA is central to management in the current era of emerging novel therapies. These fundamental changes in our knowledge and management approach to those with SMA are explored further in this review.

New Directions for SMA Therapy

Spinal muscular atrophy (SMA) is a severe disorder of motor neurons and the most frequent genetic cause of mortality in childhood, due to respiratory complications. The disease occurs due to mutations in the survival motor neuron 1 (SMN1) gene that leads to a reduction in the SMN protein, causing degeneration of lower motor neurons, muscle weakness and atrophy. Recently, the Food and Drug Administration (FDA) and the European Medical Agency (EMA) approved the antisense oligonucleotide nusinersen, the first disease-modifying treatment for SMA. Encouraging results from SMN1 gene therapy studies have raised hope for other therapeutic approaches that might arise in the coming years. However, nusinersen licensing has created ethical, medical, and financial implications that will need to be addressed. In this review, the history and challenges of the new SMA therapeutic strategies are highlighted.

Analysis of Azithromycin Monohydrate as a Single or a Combinatorial Therapy in a Mouse Model of Severe Spinal Muscular Atrophy

BACKGROUND

Spinal muscular atrophy (SMA) is a neurodegenerative autosomal recessive disorder characterized by the loss of α-motor neurons. A variety of molecular pathways are being investigated to elevate SMN protein expression in SMA models and in the clinic. One of these approaches involves stabilizing the SMNΔ7 protein by inducing translational read-through. Previous studies have demonstrated that functionality and stability are partially restored to the otherwise unstable SMNΔ7 by the addition of non-specific C-terminal peptide sequences, or by inducing a similar molecular event through the use of read-through inducing compounds such as aminoglycosides.

OBJECTIVE

The objective was to determine the efficacy of the macrolide Azithromycin (AZM), an FDA approved read-through-inducing compound, in the well-established severe mouse model of SMA.

METHODS

Initially, dosing regimen following ICV administrations of AZM at different post-natal days and concentrations was determined by their impact on SMN levels in disease-relevant tissues. Selected dose was then tested for phenotypic parameters changes as compared to the appropriate controls and in conjugation to another therapy.

RESULTS

AZM increases SMN protein in disease relevant tissues, however, this did not translate into similar improvements in the SMA phenotype in a severe mouse model of SMA. Co-administration of AZM and a previously developed antisense oligonucleotide that increases SMN2 splicing, resulted in an improvement in the SMA phenotype beyond either AZM or ASO alone, including a highly significant extension in survival with improvement in body weight and movement.

CONCLUSIONS

It is important to explore various approaches for SMA therapeutics, hence compounds that specifically induce SMNΔ7 read-through, without having prohibitive toxicity, may provide an alternative platform for a combinatorial treatment. Here we established that AZM activity at a low dose can increase SMN protein in disease-relevant animal model and can impact disease severity.

Advances in modeling and treating spinal muscular atrophy

PURPOSE OF REVIEW

Spinal muscular atrophy (SMA) is an inherited childhood neurodegenerative disorder caused by ubiquitous deficiency of the survival motor neuron (SMN) protein - the hallmarks of which are the selective loss of motor neurons and skeletal muscle atrophy. Here, we highlight recent progress in the understanding of SMA pathology and in the development of therapeutic approaches for its treatment.

RECENT FINDINGS

Phenotypic characterization of mouse models of the disease, combined with analysis of SMN restoration or depletion in a spatially and temporally controlled manner, has yielded key insights into the normal requirement of SMN and SMA pathophysiology. Increasing evidence indicates a higher demand for SMN during neuromuscular development and extends the pathogenic effects of SMN deficiency beyond motor neurons to include additional cells both within and outside the nervous system. These findings have been paralleled by preclinical development of powerful approaches for increasing SMN expression through gene therapy or splicing modulation that are now in human trials.

SUMMARY

Along with the availability of SMN-upregulating drugs, identification of the specific cell types in which SMN deficiency induces the disease and delineation of the window of opportunity for effective treatment are key advances in the ongoing path to SMA therapy.

Motor Neuron Gene Therapy: Lessons from Spinal Muscular Atrophy for Amyotrophic Lateral Sclerosis

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are severe nervous system diseases characterized by the degeneration of lower motor neurons. They share a number of additional pathological, cellular, and genetic parallels suggesting that mechanistic and clinical insights into one disorder may have value for the other. While there are currently no clinical ALS gene therapies, the splice-switching antisense oligonucleotide, nusinersen, was recently approved for SMA. This milestone was achieved through extensive pre-clinical research and patient trials, which together have spawned fundamental insights into motor neuron gene therapy. We have thus tried to distil key information garnered from SMA research, in the hope that it may stimulate a more directed approach to ALS gene therapy. Not only must the type of therapeutic (e.g., antisense oligonucleotide vs. viral vector) be sensibly selected, but considerable thought must be applied to the where, which, what, and when in order to enhance treatment benefit: to where (cell types and tissues) must the drug be delivered and how can this be best achieved? Which perturbed pathways must be corrected and can they be concurrently targeted? What dosing regime and concentration should be used? When should medication be administered? These questions are intuitive, but central to identifying and optimizing a successful gene therapy. Providing definitive solutions to these quandaries will be difficult, but clear thinking about therapeutic testing is necessary if we are to have the best chance of developing viable ALS gene therapies and improving upon early generation SMA treatments.

Emerging therapies and challenges in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a hereditary neurodegenerative disease with severity ranging from progressive infantile paralysis and premature death (type I) to limited motor neuron loss and normal life expectancy (type IV). Without disease‐modifying therapies, the impact is profound for patients and their families. Improved understanding of the molecular basis of SMA, disease pathogenesis, natural history, and recognition of the impact of standardized care on outcomes has yielded progress toward the development of novel therapeutic strategies and are summarized. Therapeutic strategies in the pipeline are appraised, ranging from SMN1 gene replacement to modulation of SMN2 encoded transcripts, to neuroprotection, to an expanding repertoire of peripheral targets, including muscle. With the advent of preliminary trial data, it can be reasonably anticipated that the SMA treatment landscape will transform significantly. Advancement in presymptomatic diagnosis and screening programs will be critical, with pilot newborn screening studies underway to facilitate preclinical diagnosis. The development of disease‐modifying therapies will necessitate monitoring programs to determine the long‐term impact, careful evaluation of combined treatments, and further acceleration of improvements in supportive care. In advance of upcoming clinical trial results, we consider the challenges and controversies related to the implementation of novel therapies for all patients and set the scene as the field prepares to enter an era of novel therapies. Ann Neurol 2017;81:355–368

The Power of Human Protective Modifiers: PLS3 and CORO1C Unravel Impaired Endocytosis in Spinal Muscular Atrophy and Rescue SMA Phenotype

Homozygous loss of SMN1 causes spinal muscular atrophy (SMA), the most common and devastating childhood genetic motor-neuron disease. The copy gene SMN2 produces only ∼10% functional SMN protein, insufficient to counteract development of SMA. In contrast, the human genetic modifier plastin 3 (PLS3), an actin-binding and -bundling protein, fully protects against SMA in SMN1-deleted individuals carrying 3-4 SMN2 copies. Here, we demonstrate that the combinatorial effect of suboptimal SMN antisense oligonucleotide treatment and PLS3 overexpression-a situation resembling the human condition in asymptomatic SMN1-deleted individuals-rescues survival (from 14 to >250 days) and motoric abilities in a severe SMA mouse model. Because PLS3 knockout in yeast impairs endocytosis, we hypothesized that disturbed endocytosis might be a key cellular mechanism underlying impaired neurotransmission and neuromuscular junction maintenance in SMA. Indeed, SMN deficit dramatically reduced endocytosis, which was restored to normal levels by PLS3 overexpression. Upon low-frequency electro-stimulation, endocytotic FM1-43 (SynaptoGreen) uptake in the presynaptic terminal of neuromuscular junctions was restored to control levels in SMA-PLS3 mice. Moreover, proteomics and biochemical analysis revealed CORO1C, another F-actin binding protein, whose direct binding to PLS3 is dependent on calcium. Similar to PLS3 overexpression, CORO1C overexpression restored fluid-phase endocytosis in SMN-knockdown cells by elevating F-actin amounts and rescued the axonal truncation and branching phenotype in Smn-depleted zebrafish. Our findings emphasize the power of genetic modifiers to unravel the cellular pathomechanisms underlying SMA and the power of combinatorial therapy based on splice correction of SMN2 and endocytosis improvement to efficiently treat SMA.

Pharmacokinetics, pharmacodynamics, and efficacy of a small-molecule SMN2 splicing modifier in mouse models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by the loss or mutation of both copies of the survival motor neuron 1 (SMN1) gene. The related SMN2 gene is retained, but due to alternative splicing of exon 7, produces insufficient levels of the SMN protein. Here, we systematically characterize the pharmacokinetic and pharmacodynamics properties of the SMN splicing modifier SMN-C1. SMN-C1 is a low-molecular weight compound that promotes the inclusion of exon 7 and increases production of SMN protein in human cells and in two transgenic mouse models of SMA. Furthermore, increases in SMN protein levels in peripheral blood mononuclear cells and skin correlate with those in the central nervous system (CNS), indicating that a change of these levels in blood or skin can be used as a non-invasive surrogate to monitor increases of SMN protein levels in the CNS. Consistent with restored SMN function, SMN-C1 treatment increases the levels of spliceosomal and U7 small-nuclear RNAs and corrects RNA processing defects induced by SMN deficiency in the spinal cord of SMNΔ7 SMA mice. A 100% or greater increase in SMN protein in the CNS of SMNΔ7 SMA mice robustly improves the phenotype. Importantly, a ∼50% increase in SMN leads to long-term survival, but the SMA phenotype is only partially corrected, indicating that certain SMA disease manifestations may respond to treatment at lower doses. Overall, we provide important insights for the translation of pre-clinical data to the clinic and further therapeutic development of this series of molecules for SMA treatment.