Diverse role of survival motor neuron protein

A link between agrin signalling and Cav3.2 at the neuromuscular junction in spinal muscular atrophy

SMN protein deficiency causes motoneuron disease spinal muscular atrophy (SMA). SMN-based therapies improve patient motor symptoms to variable degrees. An early hallmark of SMA is the perturbation of the neuromuscular junction (NMJ), a synapse between a motoneuron and muscle cell. NMJ formation depends on acetylcholine receptor (AChR) clustering triggered by agrin and its co-receptors lipoprotein receptor-related protein 4 (LRP4) and transmembrane muscle-specific kinase (MuSK) signalling pathway. We have previously shown that flunarizine improves NMJs in SMA model mice, but the mechanisms remain elusive. We show here that flunarizine promotes AChR clustering in cell-autonomous, dose- and agrin-dependent manners in C2C12 myotubes. This is associated with an increase in protein levels of LRP4, integrin-beta-1 and alpha-dystroglycan, three agrin co-receptors. Furthermore, flunarizine enhances MuSK interaction with integrin-beta-1 and phosphotyrosines. Moreover, the drug acts on the expression and splicing of Agrn and Cacna1h genes in a muscle-specific manner. We reveal that the Cacna1h encoded protein Cav3.2 closely associates in vitro with the agrin co-receptor LRP4. In vivo, it is enriched nearby NMJs during neonatal development and the drug increases this immunolabelling in SMA muscles. Thus, flunarizine modulates key players of the NMJ and identifies Ca_v3.2 as a new protein involved in the NMJ biology.

Pharmacotherapy for Spinal Muscular Atrophy in Babies and Children: A Review of Approved and Experimental Therapies

Spinal muscular atrophy (SMA) is an autosomal recessive degenerative neuromuscular disorder characterized by loss of spinal motor neurons leading to muscle weakness and atrophy that is caused by survival motor neuron (SMN) protein deficiency resulting from the biallelic loss of the SMN1 gene. The SMN2 gene modulates the SMA phenotype, as a small fraction of its transcripts are alternatively spliced to produce full-length SMN (fSMN) protein. SMN-targeted therapies increase SMN protein; mRNA therapies, nusinersen and risdiplam, increase the amount of fSMN transcripts alternatively spliced from the SMN2 gene, while gene transfer therapy, onasemnogene abeparvovec xioi, increases SMN protein by introducing the hSMN gene into various tissues, including spinal cord via an AAV9 vector. These SMN-targeted therapies have been found effective in improving outcomes and are approved for use in SMA in the US and elsewhere. This article discusses the clinical trial results for SMN-directed therapies with a focus on efficacy, side effects and treatment response predictors. It also discusses preliminary data from muscle-targeted trials, as single agents and in combination with SMN-targeted therapies, as well as other classes of SMA treatments.

Mitochondrial Dysfunction in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder caused by recessive mutations in the SMN1 gene, globally affecting ~8-14 newborns per 100,000. The severity of the disease depends on the residual levels of functional survival of motor neuron protein, SMN. SMN is a ubiquitously expressed RNA binding protein involved in a plethora of cellular processes. In this review, we discuss the effects of SMN loss on mitochondrial functions in the neuronal and muscular systems that are the most affected in patients with spinal muscular atrophy. Our aim is to highlight how mitochondrial defects may contribute to disease progression and how restoring mitochondrial functionality may be a promising approach to develop new therapies. We also collected from previous studies a list of transcripts encoding mitochondrial proteins affected in various SMA models. Moreover, we speculate that in adulthood, when motor neurons require only very low SMN levels, the natural deterioration of mitochondria associated with aging may be a crucial triggering factor for adult spinal muscular atrophy, and this requires particular attention for therapeutic strategies.

Mouse models of SMA show divergent patterns of neuronal vulnerability and resilience

Background

Spinal muscular atrophy (SMA) is a form of motor neuron disease affecting primarily children characterised by the loss of lower motor neurons (MNs). Breakdown of the neuromuscular junctions (NMJs) is an early pathological event in SMA. However, not all motor neurons are equally vulnerable, with some populations being lost early in the disease while others remain intact at the disease end-stage. A thorough understanding of the basis of this selective vulnerability will give critical insight into the factors which prohibit pathology in certain motor neuron populations and consequently help identify novel neuroprotective strategies.

Methods

To retrieve a comprehensive understanding of motor neuron susceptibility in SMA, we mapped NMJ pathology in 20 muscles from the Smn^2B/- SMA mouse model and cross-compared these data with published data from three other commonly used mouse models. To gain insight into the molecular mechanisms regulating selective resilience and vulnerability, we analysed published RNA sequencing data acquired from differentially vulnerable motor neurons from two different SMA mouse models.

Results

In the Smn^2B/- mouse model of SMA, we identified substantial NMJ loss in the muscles from the core, neck, proximal hind limbs and proximal forelimbs, with a marked reduction in denervation in the distal limbs and head. Motor neuron cell body loss was greater at T5 and T11 compared with L5. We subsequently show that although widespread denervation is observed in each SMA mouse model (with the notable exception of the Taiwanese model), all models have a distinct pattern of selective vulnerability. A comparison of previously published data sets reveals novel transcripts upregulated with a disease in selectively resistant motor neurons, including genes involved in axonal transport, RNA processing and mitochondrial bioenergetics.

Conclusions

Our work demonstrates that the Smn^2B/- mouse model shows a pattern of selective vulnerability which bears resemblance to the regional pathology observed in SMA patients. We found drastic differences in patterns of selective vulnerability across the four SMA mouse models, which is critical to consider during experimental design. We also identified transcript groups that potentially contribute to the protection of certain motor neurons in SMA mouse models.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00305-9.

The phospho-landscape of the survival of motoneuron protein (SMN) protein: relevance for spinal muscular atrophy (SMA)

Spinal muscular atrophy (SMA) is caused by low levels of the survival of motoneuron (SMN) Protein leading to preferential degeneration of lower motoneurons in the ventral horn of the spinal cord and brain stem. However, the SMN protein is ubiquitously expressed and there is growing evidence of a multisystem phenotype in SMA. Since a loss of SMN function is critical, it is important to decipher the regulatory mechanisms of SMN function starting on the level of the SMN protein itself. Posttranslational modifications (PTMs) of proteins regulate multiple functions and processes, including activity, cellular trafficking, and stability. Several PTM sites have been identified within the SMN sequence. Here, we map the identified SMN PTMs highlighting phosphorylation as a key regulator affecting localization, stability and functions of SMN. Furthermore, we propose SMN phosphorylation as a crucial factor for intracellular interaction and cellular distribution of SMN. We outline the relevance of phosphorylation of the spinal muscular atrophy (SMA) gene product SMN with regard to basic housekeeping functions of SMN impaired in this neurodegenerative disease. Finally, we compare SMA patient mutations with putative and verified phosphorylation sites. Thus, we emphasize the importance of phosphorylation as a cellular modulator in a clinical perspective as a potential additional target for combinatorial SMA treatment strategies.

Spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disorder caused by mutations in SMN1 (encoding survival motor neuron protein (SMN)). Reduced expression of SMN leads to loss of α-motor neurons, severe muscle weakness and often early death. Standard-of-care recommendations for multidisciplinary supportive care of SMA were established in the past few decades. However, improved understanding of the pathogenetic mechanisms of SMA has led to the development of different therapeutic approaches. Three treatments that increase SMN expression by distinct molecular mechanisms, administration routes and tissue biodistributions have received regulatory approval with others in clinical development. The advent of the new therapies is redefining standards of care as in many countries most patients are treated with one of the new therapies, leading to the identification of emerging new phenotypes of SMA and a renewed characterization of demographics owing to improved patient survival.

Natural history of Type 1 spinal muscular atrophy: a retrospective, global, multicenter study

BACKGROUND

ANCHOVY was a global, multicenter, chart-review study that aimed to describe the natural history of Type 1 spinal muscular atrophy (SMA) from a broad geographical area and provide further contextualization of results from the FIREFISH (NCT02913482) interventional study of risdiplam treatment in Type 1 SMA.

METHODS

Data were extracted from medical records of patients with first symptoms attributable to Type 1 SMA between 28 days and 3 months of age, genetic confirmation of SMA, and confirmed survival of motor neuron 2 copy number of two or unknown. The study period started on 1 January 2008 for all sites; study end dates were site-specific due to local treatment availabilities. Primary endpoints were time to death and/or permanent ventilation and proportion of patients achieving motor milestones. Secondary endpoints included time to initiation of respiratory and feeding support.

RESULTS

Data for 60 patients from nine countries across Asia, Europe and North and South America were analyzed. The median age (interquartile range [IQR]) for reaching death or permanent ventilation was ~ 7.3 (5.9-10.5) months. The median age (IQR) at permanent ventilation was ~ 12.7 (6.9-16.4) months and at death was ~ 41.2 (7.3-not applicable) months. No patients were able to sit without support or achieved any level of crawling, standing or walking.

INTERPRETATION

Findings from ANCHOVY were consistent with published natural history data on Type 1 SMA demonstrating the disease's devastating course, which markedly differed from risdiplam-treated infants (FIREFISH Part 2). The results provide meaningful additions to the literature, including a broader geographical representation.

Motor defects in a Drosophila model for spinal muscular atrophy result from SMN depletion during early neurogenesis

Spinal muscular atrophy (SMA) is the most common autosomal recessive neurodegenerative disease, and is characterised by spinal motor neuron loss, impaired motor function and, often, premature death. Mutations and deletions in the widely expressed survival motor neuron 1 (SMN1) gene cause SMA; however, the mechanisms underlying the selectivity of motor neuron degeneration are not well understood. Although SMA is degenerative in nature, SMN function during embryonic and early postnatal development appears to be essential for motor neuron survival in animal models and humans. Notwithstanding, how developmental defects contribute to the subversion of postnatal and adult motor function remains elusive. Here, in a Drosophila SMA model, we show that neurodevelopmental defects precede gross locomotor dysfunction in larvae. Furthermore, to specifically address the relevance of SMN during neurogenesis and in neurogenic cell types, we show that SMN knockdown using neuroblast-specific and pan-neuronal drivers, but not differentiated neuron or glial cell drivers, impairs adult motor function. Using targeted knockdown, we further restricted SMN manipulation in neuroblasts to a defined time window. Our aim was to express specifically in the neuronal progenitor cell types that have not formed synapses, and thus a time that precedes neuromuscular junction formation and maturation. By restoring SMN levels in these distinct neuronal population, we partially rescue the larval locomotor defects of Smn mutants. Finally, combinatorial SMN knockdown in immature and mature neurons synergistically enhances the locomotor and survival phenotypes. Our in-vivo study is the first to directly rescue the motor defects of an SMA model by expressing Smn in an identifiable population of Drosophila neuroblasts and developing neurons, highlighting that neuronal sensitivity to SMN loss may arise before synapse establishment and nerve cell maturation.

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality and leads to the degeneration of the nerves that control muscle function. Loss-of-function mutations in the widely expressed survival motor neuron 1 (SMN1) gene cause SMA, but how low levels of SMN protein cause the neuronal dysfunction is not known. Although SMA is a disease of nerve degeneration, SMN function during nerve cell development may be important, particularly in severe forms of SMA. Nevertheless, how the defects during development and throughout early life contribute to the disease is not well understood. We have previously demonstrated that SMN protein becomes enriched in neuroblasts, which are the cells that divide to produce neurons. In the present study, motor defects observed in our fly model for SMA could be rescued by restoring SMN in neuroblasts alone. In addition, we show that knocking down SMN in healthy flies within the same cell type causes impaired motor function. The present study shows that the manipulation of SMN in a developmentally important cell type can cause motor defects, indicating that a period of abnormal neurodevelopment may contribute to SMA.

SMN Enhances Pluripotent Genes Expression and Facilitates Cell Reprogramming

Survival motor neuron (SMN) plays important roles in snRNPs assembly and mRNA splicing. Deficiency of SMN causes spinal muscular atrophy (SMA), a leading genetic disease of childhood mortality. Previous studies have shown that SMN regulates stem cell self-renewal and pluripotency in Drosophila and in mouse, and is abundantly expressed in mouse embryonic stem cells (ESCs). However, whether SMN is required for the establishment of pluripotency is unclear. Herein, we show that SMN is gradually upregulated in pre-implantation mouse embryos and cultured cells undergoing cell reprogramming. Ectopic expression of SMN increased the cell reprogramming efficiency, whereas knockdown of SMN impeded iPSC colony formation. iPSCs could be derived from SMA model mice, but certain impairment in differentiation capacity may present. The ectopic overexpression of SMN in iPSCs can upregulate the expression levels of some pluripotent genes and restore the neuronal differentiation capacity of SMA-iPSCs. Taken together, our findings not only demonstrate the functional relevance of SMN and the establishment of cell pluripotency, but also propose its potential application in facilitating iPSC derivation.

Risdiplam in Types 2 and 3 spinal muscular atrophy: a randomised, placebo-controlled, dose-finding trial followed by 24 months of treatment

BACKGROUND

Spinal muscular atrophy (SMA) is caused by reduced levels of survival of motor neuron (SMN) protein due to deletions and/or mutations in the SMN1 gene. Risdiplam is an orally administered molecule that modifies SMN2 pre-mRNA splicing to increase functional SMN protein.

METHODS

SUNFISH Part 1 was a dose-finding study conducted in 51 individuals with Types 2 and 3 SMA aged 2-25 years. A dose-escalation method was used to identify the appropriate dose for the subsequent pivotal Part 2. Individuals were randomised (2:1) to risdiplam or placebo at escalating dose levels for a minimum 12-week, double-blind, placebo-controlled period, followed by treatment for 24 months. The dose selection for Part 2 was based on safety, tolerability, pharmacokinetic and pharmacodynamic data. Exploratory efficacy was also measured.

RESULTS

There was no difference in safety findings for all assessed dose levels. A dose-dependent increase in blood SMN protein was observed; a median two-fold increase was obtained within 4 weeks of treatment initiation at the highest dose level. The increase in SMN protein was sustained over 24 months of treatment. Exploratory efficacy showed improvement or stabilisation in motor function. The pivotal dose selected for Part 2 was 5 mg for patients with a body weight ≥20 kg or 0.25 mg/kg for patients <20 kg.

CONCLUSIONS

SUNFISH Part 1 demonstrated a two-fold increase in SMN protein after treatment with risdiplam. The observed safety profile supported the initiation of the pivotal Part 2 study. The long-term efficacy and safety of risdiplam is being assessed with ongoing treatment.

Newborn Screening for SMA – Can a Wait-and-See Strategy be Responsibly Justified in Patients With Four SMN2 Copies?

Background: Early treatment after genetic newborn screening for SMA significantly improves outcomes in infantile SMA. However, there is no consensus in the SMA treatment community about early treatment initiation in patients with four copies of SMN2. Objective: Approach to a responsible treatment strategy for SMA patients with four SMN2 copies detected in newborn screening. Methods: Inclusion criteria were a history of SMA diagnosed by NBS, age >  12 months at last examination, and diagnosis of four SMN2 copies at confirmatory diagnosis. Results: 21 patients with SMA and four SMN2 copies were identified in German screening projects over a three-year period. In three of them, the SMN2 copy number had to be corrected later, and three patients were lost to follow-up. Eight of the fifteen patients who were subject to long-term follow-up underwent presymptomatic therapy between 3 and 36 months of age and had no definite disease symptoms to date. Five of the other seven patients who underwent a strict follow-up strategy, showed clinical or electrophysiological disease onset between 1.5 and 4 years of age. In two of them, complete recovery was not achieved despite immediate initiation of treatment after the onset of the first symptoms. Conclusion: A remarkable proportion of patients with four copies of SMN2 develop irreversible symptoms within the first four years of life, if a wait-and-see strategy is followed. These data argue for a proactive approach, i.e., early initiation of treatment in this subgroup of SMA patients.

Structural Context of a Critical Exon of Spinal Muscular Atrophy Gene

Humans contain two nearly identical copies of Survival Motor Neuron genes, SMN1 and SMN2. Deletion or mutation of SMN1 causes spinal muscular atrophy (SMA), one of the leading genetic diseases associated with infant mortality. SMN2 is unable to compensate for the loss of SMN1 due to predominant exon 7 skipping, leading to the production of a truncated protein. Antisense oligonucleotide and small molecule-based strategies aimed at the restoration of SMN2 exon 7 inclusion are approved therapies of SMA. Many cis-elements and transacting factors have been implicated in regulation of SMN exon 7 splicing. Also, several structural elements, including those formed by a long-distance interaction, have been implicated in the modulation of SMN exon 7 splicing. Several of these structures have been confirmed by enzymatic and chemical structure-probing methods. Additional structures formed by inter-intronic interactions have been predicted by computational algorithms. SMN genes generate a vast repertoire of circular RNAs through inter-intronic secondary structures formed by inverted Alu repeats present in large number in SMN genes. Here, we review the structural context of the exonic and intronic cis-elements that promote or prevent exon 7 recognition. We discuss how structural rearrangements triggered by single nucleotide substitutions could bring drastic changes in SMN2 exon 7 splicing. We also propose potential mechanisms by which inter-intronic structures might impact the splicing outcomes.

Internal Introns Promote Backsplicing to Generate Circular RNAs from Spinal Muscular Atrophy Gene

Human survival motor neuron 1 (SMN1) codes for SMN, an essential housekeeping protein involved in most aspects of RNA metabolism. Deletions or mutations of SMN1 lead to spinal muscular atrophy (SMA), a devastating neurodegenerative disease linked to a high rate of infant mortality. SMN2, a near identical copy of SMN1 present in humans, cannot compensate for the loss of SMN1 due to predominant skipping of SMN2 exon 7. Restoration of SMN by splicing modulation of SMN2 exon 7 or gene replacement are currently approved therapies of SMA. Human SMN genes produce a vast repertoire of circular RNAs (circRNAs). However, the mechanism of SMN circRNA generation has not yet been examined in detail. For example, it remains unknown if forward splicing impacts backsplicing that generates circRNAs containing multiple exons. Here, we employed SMN as a model system to examine the impact of intronic sequences on the generation of circRNAs. We performed our experiments in HeLa cells transiently transfected with minigenes expressing three abundantly represented circRNAs containing two or more SMN exons. We observed an enhanced rate of circRNA generation when introns joining exons to be incorporated into circRNAs were present as compared to the intronless context. These results underscore the stimulatory effect of forward splicing in the generation of circRNAs containing multiple exons. These findings are consistent with the reported low abundance of SMN circRNAs comprised of single exons. We confirmed our findings using inducible HEK 293 cells stably expressing the SMN circRNAs. Our results support the role of the exon junction complex in the generation of the exon-only-containing circRNAs. We showed that SMN circRNAs were preferentially localized in the cytoplasm. These findings provide new insights regarding our understanding of circRNA generation and open avenues to uncover novel functions of the SMN genes.

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.

Combinatorial ASO-mediated therapy with low dose SMN and the protective modifier Chp1 is not sufficient to ameliorate SMA pathology hallmarks

Spinal muscular atrophy (SMA) is a devastating genetically inherited neuromuscular disorder characterized by the progressive loss of motor neurons in the spinal cord, leading to muscle atrophy and weakness. Although SMA is caused by homozygous mutations in SMN1, the disease severity is mainly determined by the copy number of SMN2, an almost identical gene that produces ~10% correctly spliced SMN transcripts. Recently, three FDA- and EMA-approved therapies that either increase correctly spliced SMN2 transcripts (nusinersen and risdiplam) or replace SMN1 (onasemnogen abeparvovec-xioi) have revolutionized the clinical outcome in SMA patients. However, for severely affected SMA individuals carrying only two SMN2 copies even a presymptomatic therapy might be insufficient to fully counteract disease development. Therefore, SMN-independent compounds supporting SMN-dependent therapies represent a promising therapeutic approach. Recently, we have shown a significant amelioration of SMA disease hallmarks in a severely affected SMA mouse carrying a mutant Chp1 allele when combined with low-dose of SMN antisense oligonucleotide (ASO) treatment. CHP1 is a direct interacting partner of PLS3, a strong protective modifier of SMA. Both proteins ameliorate impaired endocytosis in SMA and significantly restore pathological hallmarks in mice. Here, we aimed to pharmacologically reduce CHP1 levels in an ASO-based combinatorial therapy targeting SMN and Chp1. Chp1 modulation is a major challenge since its genetic reduction to ~50% has shown to ameliorate SMA pathology, while the downregulation below that level causes cerebellar ataxia. Efficacy and tolerability studies determined that a single injection of 30 μg Chp1-ASO4 in the CNS is a safe dosage that significantly reduced CHP1 levels to ~50% at postnatal day (PND)14. Unfortunately, neither electrophysiological predictors such as compound muscle action potential (CMAP) or motor unit number estimation (MUNE) nor histological hallmarks of SMA in neuromuscular junction (NMJ), spinal cord or muscle were ameliorated in SMA mice treated with Chp1-ASO4 compared to CTRL-ASO at PND21. Surprisingly, CHP1 levels were almost at control level 4-weeks post injection, indicating a rather short-term effect of the ASO. Therefore, we re-administrated Chp1-ASO4 by i.c.v. bolus injection at PND28. However, no significant improvement of SMA hallmarks were seen at 2 month-of-age either. In conclusion, in contrast to the protective effect of genetically-induced Chp1 reduction on SMA, combinatorial therapy with Chp1- and SMN-ASOs failed to significantly ameliorate the SMA pathology. Chp1-ASOs compared to SMN-ASO proved to have rather short-term effect and even reinjection had no significant impact on SMA progression, suggesting that further optimization of the ASO may be required to fully explore the combination.

R-loop Mediated DNA Damage and Impaired DNA Repair in Spinal Muscular Atrophy

Defects in DNA repair pathways are a major cause of DNA damage accumulation leading to genomic instability and neurodegeneration. Efficient DNA damage repair is critical to maintain genomicstability and support cell function and viability. DNA damage results in the activation of cell death pathways, causing neuronal death in an expanding spectrum of neurological disorders, such as amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD), Alzheimer’s disease (AD), and spinal muscular atrophy (SMA). SMA is a neurodegenerative disorder caused by mutations in the Survival Motor Neuron 1 (SMN1) gene. SMA is characterized by the degeneration of spinal cord motor neurons due to low levels of the SMN protein. The molecular mechanism of selective motor neuron degeneration in SMA was unclear for about 20 years. However, several studies have identified biochemical and molecular mechanisms that may contribute to the predominant degeneration of motor neurons in SMA, including the RhoA/ROCK, the c-Jun NH₂-terminal kinase (JNK), and p53-mediated pathways, which are involved in mediating DNA damage-dependent cell death. Recent studies provided insight into selective degeneration of motor neurons, which might be caused by accumulation of R-loop-mediated DNA damage and impaired non-homologous end joining (NHEJ) DNA repair pathway leading to genomic instability. Here, we review the latest findings involving R-loop-mediated DNA damage and defects in neuron-specific DNA repair mechanisms in SMA and discuss these findings in the context of other neurodegenerative disorders linked to DNA damage.

Spinal Muscular Atrophy –Is Newborn Screening Too Late for Children with Two SMN2 Copies?

Background: Prompt treatment after genetic NBS for SMA substantially improves outcome in infantile SMA. However, deficiency of SMN-protein can cause damage of motor neurons even prior to birth. Objective: To describe the neurological status at the time of NBS and the reversibility of neurological deficits in a cohort of patients with only two copies of the SMN2 gene. Methods: We present motor, respiratory, and bulbar outcomes of 21 SMA patients identified in newborn screening projects in Germany. Inclusion criteria was initiation of SMN targeted medication at less than 6 weeks of age and a minimum age of 9 months at last examination. Results: Twelve patients (57%) developed completely normally, reaching motor milestones in time and having no bulbar or respiratory problems. Three children (14.5%) caught up after initial delay in motor development. Six patients (29%) developed proximal weakness despite early treatment: Three of them (14.5%) achieved the ability to walk with assistance and the other three (14.5%) showed an SMA type 2 phenotype at the age of 16–30 months. One patient (4.8%) had respiratory problems. Three children (14.5%) had mild chewing problems and two individuals (9.5%) needed feeding via gastrotube. Initial CHOP-INTEND values below 30 could be indicative of a less favourable outcome, whereas values above 50 could indicate a good outcome, however in-depth statistic due to the small case number is not predictive. Conclusion: More than 70% of SMA patients with two SMN2 copies can achieve independent ambulation with immediate initiation of therapy. However, caregivers and paediatricians must be informed about the possibility of less favourable outcomes when discussing therapeutic strategies.

Genome Integrity and Neurological Disease

Neurological complications directly impact the lives of hundreds of millions of people worldwide. While the precise molecular mechanisms that underlie neuronal cell loss remain under debate, evidence indicates that the accumulation of genomic DNA damage and consequent cellular responses can promote apoptosis and neurodegenerative disease. This idea is supported by the fact that individuals who harbor pathogenic mutations in DNA damage response genes experience profound neuropathological manifestations. The review article here provides a general overview of the nervous system, the threats to DNA stability, and the mechanisms that protect genomic integrity while highlighting the connections of DNA repair defects to neurological disease. The information presented should serve as a prelude to the Special Issue “Genome Stability and Neurological Disease”, where experts discuss the role of DNA repair in preserving central nervous system function in greater depth.

Protein Network Analysis Reveals a Functional Connectivity of Dysregulated Processes in ALS and SMA

Spinal Muscular Atrophy (SMA) and Amyotrophic Lateral Sclerosis (ALS) are neurodegenerative diseases which are characterized by the loss of motoneurons within the central nervous system. SMA is a monogenic disease caused by reduced levels of the Survival of motoneuron protein, whereas ALS is a multi-genic disease with over 50 identified disease-causing genes and involvement of environmental risk factors. Although these diseases have different causes, they partially share identical phenotypes and pathomechanisms. To analyze and identify functional connections and to get a global overview of altered pathways in both diseases, protein network analyses are commonly used. Here, we used an in silico tool to test for functional associations between proteins that are involved in actin cytoskeleton dynamics, fatty acid metabolism, skeletal muscle metabolism, stress granule dynamics as well as SMA or ALS risk factors, respectively. In network biology, interactions are represented by edges which connect proteins (nodes). Our approach showed that only a few edges are necessary to present a complex protein network of different biological processes. Moreover, Superoxide dismutase 1, which is mutated in ALS, and the actin-binding protein profilin1 play a central role in the connectivity of the aforementioned pathways. Our network indicates functional links between altered processes that are described in either ALS or SMA. These links may not have been considered in the past but represent putative targets to restore altered processes and reveal overlapping pathomechanisms in both diseases.

Identification of specific gene methylation patterns during motor neuron differentiation from spinal muscular atrophy patient-derived iPSC

Spinal muscular atrophy is a progressive motor neuron disorder caused by deletions or point mutations in the SMN1 gene. It is not known why motor neurons are particularly sensitive to a decrease in SMN protein levels and what factors besides SMN2 underlie the high clinical heterogeneity of the disease. Here we studied the methylation patterns of genes on sequential stages of motor neuron differentiation from induced pluripotent stem cells derived from the patients with SMA type I and II. The genes involved in the regulation of pluripotency, neural differentiation as well as those associated with spinal muscular atrophy development were included. The results show that the PAX6, HB9, CHAT, ARHGAP22, and SMN2 genes are differently methylated in cells derived from SMA patients compared to the cells of healthy individuals. This study clarifies the specificities of the disease pathogenesis and extends the knowledge of pathways involved in the SMA progression.

Restoring SMN Expression: An Overview of the Therapeutic Developments for the Treatment of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder and one of the most common genetic causes of infant death. It is characterized by progressive weakness of the muscles, loss of ambulation, and death from respiratory complications. SMA is caused by the homozygous deletion or mutations in the survival of the motor neuron 1 (SMN1) gene. Humans, however, have a nearly identical copy of SMN1 known as the SMN2 gene. The severity of the disease correlates inversely with the number of SMN2 copies present. SMN2 cannot completely compensate for the loss of SMN1 in SMA patients because it can produce only a fraction of functional SMN protein. SMN protein is ubiquitously expressed in the body and has a variety of roles ranging from assembling the spliceosomal machinery, autophagy, RNA metabolism, signal transduction, cellular homeostasis, DNA repair, and recombination. Motor neurons in the anterior horn of the spinal cord are extremely susceptible to the loss of SMN protein, with the reason still being unclear. Due to the ability of the SMN2 gene to produce small amounts of functional SMN, two FDA-approved treatment strategies, including an antisense oligonucleotide (AON) nusinersen and small-molecule risdiplam, target SMN2 to produce more functional SMN. On the other hand, Onasemnogene abeparvovec (brand name Zolgensma) is an FDA-approved adeno-associated vector 9-mediated gene replacement therapy that can deliver a copy of the human SMN1. In this review, we summarize the SMA etiology, the role of SMN, and discuss the challenges of the therapies that are approved for SMA treatment.

How does risdiplam compare with other treatments for Types 1-3 spinal muscular atrophy: a systematic literature review and indirect treatment comparison

Aim: To conduct indirect treatment comparisons between risdiplam and other approved treatments for spinal muscular atrophy (SMA). Patients & methods: Individual patient data from risdiplam trials were compared with aggregated data from published studies of nusinersen and onasemnogene abeparvovec, accounting for heterogeneity across studies. Results: In Type 1 SMA, studies of risdiplam and nusinersen included similar populations. Indirect comparison results found improved survival and motor function with risdiplam versus nusinersen. Comparison with onasemnogene abeparvovec in Type 1 SMA and with nusinersen in Types 2/3 SMA was challenging due to substantial differences in study populations; no concrete conclusions could be drawn from the indirect comparison analyses. Conclusion: Indirect comparisons support risdiplam as a superior alternative to nusinersen in Type 1 SMA.

Identification and functional characterization of transcriptional activators in human cells

Transcription is orchestrated by thousands of transcription factors (TFs) and chromatin-associated proteins, but how these are causally connected to transcriptional activation is poorly understood. Here, we conduct an unbiased proteome-scale screen to systematically uncover human proteins that activate transcription in a natural chromatin context. By combining interaction proteomics and chemical inhibitors, we delineate the preference of these transcriptional activators for specific co-activators, highlighting how even closely related TFs can function via distinct cofactors. We also identify potent transactivation domains among the hits and use AlphaFold2 to predict and experimentally validate interaction interfaces of two activation domains with BRD4. Finally, we show that many novel activators are partners in fusion events in tumors and functionally characterize a myofibroma-associated fusion between SRF and C3orf62, a potent p300-dependent activator. Our work provides a functional catalog of potent transactivators in the human proteome and a platform for discovering transcriptional regulators at genome scale.

Safety and efficacy of once-daily risdiplam in type 2 and non-ambulant type 3 spinal muscular atrophy (SUNFISH part 2): a phase 3, double-blind, randomised, placebo-controlled trial

BACKGROUND

Risdiplam is an oral small molecule approved for the treatment of patients with spinal muscular atrophy, with approval for use in patients with type 2 and type 3 spinal muscular atrophy granted on the basis of unpublished data. The drug modifies pre-mRNA splicing of the SMN2 gene to increase production of functional SMN. We aimed to investigate the safety and efficacy of risdiplam in patients with type 2 or non-ambulant type 3 spinal muscular atrophy.

METHODS

In this phase 3, randomised, double-blind, placebo-controlled study, patients aged 2-25 years with confirmed 5q autosomal recessive type 2 or type 3 spinal muscular atrophy were recruited from 42 hospitals in 14 countries across Europe, North America, South America, and Asia. Participants were eligible if they were non-ambulant, could sit independently, and had a score of at least 2 in entry item A of the Revised Upper Limb Module. Patients were stratified by age and randomly assigned (2:1) to receive either daily oral risdiplam, at a dose of 5·00 mg (for individuals weighing ≥20 kg) or 0·25 mg/kg (for individuals weighing <20 kg), or daily oral placebo (matched to risdiplam in colour and taste). Randomisation was conducted by permutated block randomisation with a computerised system run by an external party. Patients, investigators, and all individuals in direct contact with patients were masked to treatment assignment. The primary endpoint was the change from baseline in the 32-item Motor Function Measure total score at month 12. All individuals who were randomly assigned to risdiplam or placebo, and who did not meet the prespecified missing item criteria for exclusion, were included in the primary efficacy analysis. Individuals who received at least one dose of risdiplam or placebo were included in the safety analysis. SUNFISH is registered with ClinicalTrials.gov, NCT02908685. Recruitment is closed; the study is ongoing.

FINDINGS

Between Oct 9, 2017, and Sept 4, 2018, 180 patients were randomly assigned to receive risdiplam (n=120) or placebo (n=60). For analysis of the primary endpoint, 115 patients from the risdiplam group and 59 patients from the placebo group were included. At month 12, the least squares mean change from baseline in 32-item Motor Function Measure was 1·36 (95% CI 0·61 to 2·11) in the risdiplam group and -0·19 (-1·22 to 0·84) in the placebo group, with a treatment difference of 1·55 (0·30 to 2·81, p=0·016) in favour of risdiplam. 120 patients who received risdiplam and 60 who received placebo were included in safety analyses. Adverse events that were reported in at least 5% more patients who received risdiplam than those who received placebo were pyrexia (25 [21%] of 120 patients who received risdiplam vs ten [17%] of 60 patients who received placebo), diarrhoea (20 [17%] vs five [8%]), rash (20 [17%] vs one [2%]), mouth and aphthous ulcers (eight [7%] vs 0), urinary tract infection (eight [7%] vs 0), and arthralgias (six [5%] vs 0). The incidence of serious adverse events was similar between treatment groups (24 [20%] of 120 patients in the risdiplam group; 11 [18%] of 60 patients in the placebo group), with the exception of pneumonia (nine [8%] in the risdiplam group; one [2%] in the placebo group).

INTERPRETATION

Risdiplam resulted in a significant improvement in motor function compared with placebo in patients aged 2-25 years with type 2 or non-ambulant type 3 spinal muscular atrophy. Our exploratory subgroup analyses showed that motor function was generally improved in younger individuals and stabilised in older individuals, which requires confirmation in further studies. SUNFISH part 2 is ongoing and will provide additional evidence regarding the long-term safety and efficacy of risdiplam.

FUNDING

F Hoffmann-La Roche.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Sensory neuron transcriptomes reveal complex neuron-specific function and regulation of mec-2/Stomatin splicing

The function and identity of a cell is shaped by transcription factors controlling transcriptional networks, and further shaped by RNA binding proteins controlling post-transcriptional networks. To overcome limitations inherent to analysis of sparse single-cell post-transcriptional data, we leverage the invariant Caenorhabditis elegans cell lineage, isolating thousands of identical neuron types from thousands of isogenic individuals. The resulting deep transcriptomes facilitate splicing network analysis due to increased sequencing depth and uniformity. We focus on mechanosensory touch-neuron splicing regulated by MEC-8/RBPMS. We identify a small MEC-8-regulated network, where MEC-8 establishes touch-neuron isoforms differing from default isoforms found in other cells. MEC-8 establishes the canonical long mec-2/Stomatin isoform in touch neurons, but surprisingly the non-canonical short isoform predominates in other neurons, including olfactory neurons, and mec-2 is required for olfaction. Forced endogenous isoform-specific expression reveals that the short isoform functions in olfaction but not mechanosensation. The long isoform is functional in both processes. Remarkably, restoring the long isoform completely rescues mec-8 mutant mechanosensation, indicating a single MEC-8 touch-neuron target is phenotypically relevant. Within the long isoform we identify a cassette exon further diversifying mec-2 into long/extra-long isoforms. Neither is sufficient for mechanosensation. Both are simultaneously required, likely functioning as heteromers to mediate mechanosensation.

A novel CARM1-HuR axis involved in muscle differentiation and plasticity misregulated in spinal muscular atrophy

Spinal muscular atrophy (SMA) is characterized by the loss of alpha motor neurons in the spinal cord and a progressive muscle weakness and atrophy. SMA is caused by loss-of-function mutations and/or deletions in the survival of motor neuron (SMN) gene. The role of SMN in motor neurons has been extensively studied, but its function and the consequences of its loss in muscle has also emerged as a key aspect of SMA pathology. In this study, we explore the molecular mechanisms involved in muscle defects in SMA. First, we show in C2C12 myoblasts, that arginine methylation by CARM1 controls myogenic differentiation. More specifically, the methylation of HuR on K217 regulates HuR levels and subcellular localization during myogenic differentiation, and the formation of myotubes. Furthermore, we demonstrate that SMN and HuR interact in C2C12 myoblasts. Interestingly, the SMA-causing E134K point mutation within the SMN Tudor domain, and CARM1 depletion, modulate the SMN-HuR interaction. In addition, using the Smn2B/- mouse model, we report that CARM1 levels are markedly increased in SMA muscles and that HuR fails to properly respond to muscle denervation, thereby affecting the regulation of its mRNA targets. Altogether, our results show a novel CARM1-HuR axis in the regulation of muscle differentiation and plasticity as well as in the aberrant regulation of this axis caused by the absence of SMN in SMA muscle. With the recent developments of therapeutics targeting motor neurons, this study further indicates the need for more global therapeutic approaches for SMA.

Fine-Tuning of mTOR mRNA and Nucleolin Complexes by SMN

Increasing evidence points to the Survival Motor Neuron (SMN) protein as a key determinant of translation pathway. Besides its role in RNA processing and sorting, several works support a critical implication of SMN in ribosome biogenesis. We previously showed that SMN binds ribosomal proteins (RPs) as well as their encoding transcripts, ensuring an appropriate level of locally synthesized RPs. SMN impacts the translation machinery in both neural and non-neural cells, in agreement with the concept that SMN is an essential protein in all cell types. Here, we further assessed the relationship between SMN and translation-related factors in immortalized human fibroblasts. We focused on SMN-nucleolin interaction, keeping in mind that nucleolin is an RNA-binding protein, highly abundant within the nucleolus, that exhibits a central role in ribosomes production. Nucleolin may also affects translation network by binding the mammalian target of rapamycin (mTOR) mRNA and promoting its local synthesis. In this regard, for the first time we provided evidence that SMN protein itself associates with mTOR transcript. Collectively, we found that: (1) SMN coexists with nucleolin-mTOR mRNA complexes at subcellular level; (2) SMN deficiency impairs nucleolar compartmentalization of nucleolin, and (3) this event correlates with the nuclear retention of mTOR mRNA. These findings suggest that SMN may regulate not only structural components of translation machinery, but also their upstream regulating factors.

[Newborn screening program for spinal muscular atrophy]

BACKGROUND

The introduction of a comprehensive newborn screening program for spinal muscular atrophy (SMA), specifically for 5q-SMA, is planned for the end of 2021 in Germany. Several targeted treatment options have become available for all patients with SMA.

MATERIAL AND METHODS

Newborn screening for 5q-SMA is based on the detection of a homozygous deletion of exon 7 in the SMN1 gene by molecular genetic analysis from the dried blood card. In all cases a second blood sample must be drawn as a part of confirmation diagnostics including the determination of the SMN2 copy numbers.

RESULTS

Insights from pilot projects performed in parts of Germany are presented. Advantages and disadvantages of the screening project are discussed.

CONCLUSION

Consultation and treatment should be carried out in a department of neuropediatrics with experience in the treatment of children with 5q-SMA, which is able to provide all current treatment options for the child, so that, when necessary, the treatment can be started within the first month of life.

Protein expression reveals a molecular sexual identity of avian primordial germ cells at pre-gonadal stages

In poultry, in vitro propagated primordial germ cells (PGCs) represent an important tool for the cryopreservation of avian genetic resources. However, several studies have highlighted sexual differences exhibited by PGCs during in vitro propagation, which may compromise their reproductive capacities. To understand this phenomenon, we compared the proteome of pregonadal migratory male (ZZ) and female (ZW) chicken PGCs propagated in vitro by quantitative proteomic analysis using a GeLC-MS/MS strategy. Many proteins were found to be differentially abundant in chicken male and female PGCs indicating their early sexual identity. Many of the proteins more highly expressed in male PGCs were encoded by genes localised to the Z sex chromosome. This suggests that the known lack of dosage compensation of the transcription of Z-linked genes between sexes persists at the protein level in PGCs, and that this may be a key factor of their autonomous sex differentiation. We also found that globally, protein differences do not closely correlate with transcript differences indicating a selective translational mechanism in PGCs. Male and female PGC expressed protein sets were associated with differential biological processes and contained proteins known to be biologically relevant for male and female germ cell development, respectively. We also discovered that female PGCs have a higher capacity to uptake proteins from the cell culture medium than male PGCs. This study presents the first evidence of an early predetermined sex specific cell fate of chicken PGCs and their sexual molecular specificities which will enable the development of more precise sex-specific in vitro culture conditions for the preservation of avian genetic resources.

Pathogenesis and therapeutic targets in spinal muscular atrophy (SMA)

Autosomal-recessive spinal muscular atrophy (SMA) is characterized by the loss of specific motor neurons of the spinal cord and skeletal muscle atrophy. SMA is caused by mutations or deletions of the survival motor neuron 1 (SMN1) gene, and disease severity correlates with the expression levels of the nearly identical copy gene, SMN2. Both genes ubiquitously express SMN protein, but SMN2 generates only low levels of protein that do not fully compensate for the loss-of-function of SMN1. SMN protein forms a multiprotein complex essential for the cellular assembly of ribonucleoprotein particles involved in diverse aspects of RNA metabolism. Other studies using animal models revealed a spatio-temporal requirement of SMN that is high during the development of the neuromuscular system and later, in the general maintenance of cellular and tissues homeostasis. These observations define a period for maximum therapeutic efficiency of SMN restoration, and suggest that cells outside the central nervous system may also participate in the pathogenesis of SMA. Finally, recent innovative therapies have been shown to mitigate SMN deficiency and have been approved to treat SMA patients. We briefly review major findings from the past twenty-five years of SMA research. © 2020 French Society of Pediatrics. Published by Elsevier Masson SAS. All rights reserved.

Profilin Isoforms in Health and Disease - All the Same but Different

Profilins are small actin binding proteins, which are structurally conserved throughout evolution. They are probably best known to promote and direct actin polymerization. However, they also participate in numerous cell biological processes beyond the roles typically ascribed to the actin cytoskeleton. Moreover, most complex organisms express several profilin isoforms. Their cellular functions are far from being understood, whereas a growing number of publications indicate that profilin isoforms are involved in the pathogenesis of various diseases. In this review, we will provide an overview of the profilin family and "typical" profilin properties including the control of actin dynamics. We will then discuss the profilin isoforms of higher animals in detail. In terms of cellular functions, we will focus on the role of Profilin 1 (PFN1) and Profilin 2a (PFN2a), which are co-expressed in the central nervous system. Finally, we will discuss recent findings that link PFN1 and PFN2a to neurological diseases, such as amyotrophic lateral sclerosis (ALS), Fragile X syndrome (FXS), Huntington's disease and spinal muscular atrophy (SMA).

Drug Discovery of Spinal Muscular Atrophy (SMA) from the Computational Perspective: A Comprehensive Review

Spinal muscular atrophy (SMA), one of the leading inherited causes of child mortality, is a rare neuromuscular disease arising from loss-of-function mutations of the survival motor neuron 1 (SMN1) gene, which encodes the SMN protein. When lacking the SMN protein in neurons, patients suffer from muscle weakness and atrophy, and in the severe cases, respiratory failure and death. Several therapeutic approaches show promise with human testing and three medications have been approved by the U.S. Food and Drug Administration (FDA) to date. Despite the shown promise of these approved therapies, there are some crucial limitations, one of the most important being the cost. The FDA-approved drugs are high-priced and are shortlisted among the most expensive treatments in the world. The price is still far beyond affordable and may serve as a burden for patients. The blooming of the biomedical data and advancement of computational approaches have opened new possibilities for SMA therapeutic development. This article highlights the present status of computationally aided approaches, including in silico drug repurposing, network driven drug discovery as well as artificial intelligence (AI)-assisted drug discovery, and discusses the future prospects.

Harmony Lost: Cell-Cell Communication at the Neuromuscular Junction in Motor Neuron Disease

The neuromuscular junction (NMJ) is a specialized synapse that is the point of connection between motor neurons and skeletal muscle. Although developmental studies have established the importance of cell-cell communication at the NMJ for the integrity and full functionality of this synapse, the contribution of this structure as a primary driver in motor neuron disease pathogenesis remains uncertain. Here, we consider the biology of the NMJ and review emerging lines of investigation that are highlighting the importance of cell-cell interaction at the NMJ in spinal muscular atrophy (SMA), X-linked spinal and bulbar muscular atrophy (SBMA), and amyotrophic lateral sclerosis (ALS). Ongoing research may reveal NMJ targets and pathways whose therapeutic modulation will help slow the progression of motor neuron disease, offering a novel treatment paradigm for ALS, SBMA, SMA, and related disorders.

Local Translation Across Neural Development: A Focus on Radial Glial Cells, Axons, and Synaptogenesis

In the past two decades, significant progress has been made in our understanding of mRNA localization and translation at distal sites in axons and dendrites. The existing literature shows that local translation is regulated in a temporally and spatially restricted manner and is critical throughout embryonic and post-embryonic life. Here, recent key findings about mRNA localization and local translation across the various stages of neural development, including neurogenesis, axon development, and synaptogenesis, are reviewed. In the early stages of development, mRNAs are localized and locally translated in the endfeet of radial glial cells, but much is still unexplored about their functional significance. Recent in vitro and in vivo studies have provided new information about the specific mechanisms regulating local translation during axon development, including growth cone guidance and axon branching. Later in development, localization and translation of mRNAs help mediate the major structural and functional changes that occur in the axon during synaptogenesis. Clinically, changes in local translation across all stages of neural development have important implications for understanding the etiology of several neurological disorders. Herein, local translation and mechanisms regulating this process across developmental stages are compared and discussed in the context of function and dysfunction.

What Genetics Has Told Us and How It Can Inform Future Experiments for Spinal Muscular Atrophy, a Perspective

Proximal spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder characterized by motor neuron loss and subsequent atrophy of skeletal muscle. SMA is caused by deficiency of the essential survival motor neuron (SMN) protein, canonically responsible for the assembly of the spliceosomal small nuclear ribonucleoproteins (snRNPs). Therapeutics aimed at increasing SMN protein levels are efficacious in treating SMA. However, it remains unknown how deficiency of SMN results in motor neuron loss, resulting in many reported cellular functions of SMN and pathways affected in SMA. Herein is a perspective detailing what genetics and biochemistry have told us about SMA and SMN, from identifying the SMA determinant region of the genome, to the development of therapeutics. Furthermore, we will discuss how genetics and biochemistry have been used to understand SMN function and how we can determine which of these are critical to SMA moving forward.

High Concentration of an ISS-N1-Targeting Antisense Oligonucleotide Causes Massive Perturbation of the Transcriptome

Intronic splicing silencer N1 (ISS-N1) located within Survival Motor Neuron 2 (SMN2) intron 7 is the target of a therapeutic antisense oligonucleotide (ASO), nusinersen (Spinraza), which is currently being used for the treatment of spinal muscular atrophy (SMA), a leading genetic disease associated with infant mortality. The discovery of ISS-N1 as a promising therapeutic target was enabled in part by Anti-N1, a 20-mer ASO that restored SMN2 exon 7 inclusion by annealing to ISS-N1. Here, we analyzed the transcriptome of SMA patient cells treated with 100 nM of Anti-N1 for 30 h. Such concentrations are routinely used to demonstrate the efficacy of an ASO. While 100 nM of Anti-N1 substantially stimulated SMN2 exon 7 inclusion, it also caused massive perturbations in the transcriptome and triggered widespread aberrant splicing, affecting expression of essential genes associated with multiple cellular processes such as transcription, splicing, translation, cell signaling, cell cycle, macromolecular trafficking, cytoskeletal dynamics, and innate immunity. We validated our findings with quantitative and semiquantitative PCR of 39 candidate genes associated with diverse pathways. We also showed a substantial reduction in off-target effects with shorter ISS-N1-targeting ASOs. Our findings are significant for implementing better ASO design and dosing regimens of ASO-based drugs.

Genomic Variability in the Survival Motor Neuron Genes (SMN1 and SMN2): Implications for Spinal Muscular Atrophy Phenotype and Therapeutics Development

Spinal muscular atrophy (SMA) is a leading genetic cause of infant death worldwide that is characterized by loss of spinal motor neurons leading to muscle weakness and atrophy. SMA results from the loss of survival motor neuron 1 (SMN1) gene but retention of its paralog SMN2. The copy numbers of SMN1 and SMN2 are variable within the human population with SMN2 copy number inversely correlating with SMA severity. Current therapeutic options for SMA focus on increasing SMN2 expression and alternative splicing so as to increase the amount of SMN protein. Recent work has demonstrated that not all SMN2, or SMN1, genes are equivalent and there is a high degree of genomic heterogeneity with respect to the SMN genes. Because SMA is now an actionable disease with SMN2 being the primary target, it is imperative to have a comprehensive understanding of this genomic heterogeneity with respect to hybrid SMN1–SMN2 genes generated by gene conversion events as well as partial deletions of the SMN genes. This review will describe this genetic heterogeneity in SMA and its impact on disease phenotype as well as therapeutic efficacy.

Assembly of higher-order SMN oligomers is essential for metazoan viability and requires an exposed structural motif present in the YG zipper dimer

Protein oligomerization is one mechanism by which homogenous solutions can separate into distinct liquid phases, enabling assembly of membraneless organelles. Survival Motor Neuron (SMN) is the eponymous component of a large macromolecular complex that chaperones biogenesis of eukaryotic ribonucleoproteins and localizes to distinct membraneless organelles in both the nucleus and cytoplasm. SMN forms the oligomeric core of this complex, and missense mutations within its YG box domain are known to cause Spinal Muscular Atrophy (SMA). The SMN YG box utilizes a unique variant of the glycine zipper motif to form dimers, but the mechanism of higher-order oligomerization remains unknown. Here, we use a combination of molecular genetic, phylogenetic, biophysical, biochemical and computational approaches to show that formation of higher-order SMN oligomers depends on a set of YG box residues that are not involved in dimerization. Mutation of key residues within this new structural motif restricts assembly of SMN to dimers and causes locomotor dysfunction and viability defects in animal models.

Combining multiomics and drug perturbation profiles to identify muscle-specific treatments for spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disorder caused by loss of survival motor neuron (SMN) protein. While SMN restoration therapies are beneficial, they are not a cure. We aimed to identify potentially novel treatments to alleviate muscle pathology combining transcriptomics, proteomics, and perturbational data sets. This revealed potential drug candidates for repurposing in SMA. One of the candidates, harmine, was further investigated in cell and animal models, improving multiple disease phenotypes, including lifespan, weight, and key molecular networks in skeletal muscle. Our work highlights the potential of multiple and parallel data-driven approaches for the development of potentially novel treatments for use in combination with SMN restoration therapies.

The potential role of miRNA therapies in spinal muscle atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by low levels of full-length survival motor neuron (SMN) protein due to the loss of the survival motor neuron 1 (SMN1) gene and inefficient splicing of the survival motor neuron 2 (SMN2) gene, which mostly affects alpha motor neurons of the lower spinal cord. Despite the U.S. Food and Drug Administration (FDA) approved SMN-dependent therapies including Nusinersen, Zolgensma® and Evrysdi™, SMA is still a devastating disease as these existing expensive drugs may not be sufficient and thus, remains a need for additional therapies. The involvement of microRNAs (miRNAs) in SMA is expanding because miRNAs are important mediators of gene expression as each miRNA could target a number of genes. Hence, miRNA-based therapy could be utilized in treating this genetic disorder. However, the delivery of miRNAs into the target cells remains an obstacle in SMA, as there is no effective delivery system to date. This review highlights the potential strategies for intracellular miRNA delivery into target cells and current challenges in miRNA delivery. Furthermore, we provide the future prospects of miRNA-based therapeutic strategies in SMA.

Skin necrosis in spinal muscular atrophy: Case report and review of the literature

Spinal muscular atrophy (SMA) type 0 is the most severe phenotype of SMA and is characterized by hypotonia, muscle weakness, and respiratory distress. Cutaneous necrosis, first described in an SMA mouse model, can occur in patients with severe disease; the use of targeted treatment versus supportive measures in the setting of skin necrosis is debated. We present a male infant with SMA type 0 with cutaneous necrosis of proximal and distal limbs who improved with supportive care. The seven previously reported cases of SMA skin necrosis are reviewed.

Risdiplam in Type 1 Spinal Muscular Atrophy

BACKGROUND

Type 1 spinal muscular atrophy is a rare, progressive neuromuscular disease that is caused by low levels of functional survival of motor neuron (SMN) protein. Risdiplam is an orally administered, small molecule that modifies SMN2 pre-messenger RNA splicing and increases levels of functional SMN protein.

METHODS

We report the results of part 1 of a two-part, phase 2-3, open-label study of risdiplam in infants 1 to 7 months of age who had type 1 spinal muscular atrophy, which is characterized by the infant not attaining the ability to sit without support. Primary outcomes were safety, pharmacokinetics, pharmacodynamics (including the blood SMN protein concentration), and the selection of the risdiplam dose for part 2 of the study. Exploratory outcomes included the ability to sit without support for at least 5 seconds.

RESULTS

A total of 21 infants were enrolled. Four infants were in a low-dose cohort and were treated with a final dose at month 12 of 0.08 mg of risdiplam per kilogram of body weight per day, and 17 were in a high-dose cohort and were treated with a final dose at month 12 of 0.2 mg per kilogram per day. The baseline median SMN protein concentrations in blood were 1.31 ng per milliliter in the low-dose cohort and 2.54 ng per milliliter in the high-dose cohort; at 12 months, the median values increased to 3.05 ng per milliliter and 5.66 ng per milliliter, respectively, which represented a median of 3.0 times and 1.9 times the baseline values in the low-dose and high-dose cohorts, respectively. Serious adverse events included pneumonia, respiratory tract infection, and acute respiratory failure. At the time of this publication, 4 infants had died of respiratory complications. Seven infants in the high-dose cohort and no infants in the low-dose cohort were able to sit without support for at least 5 seconds. The higher dose of risdiplam (0.2 mg per kilogram per day) was selected for part 2 of the study.

CONCLUSIONS

In infants with type 1 spinal muscular atrophy, treatment with oral risdiplam led to an increased expression of functional SMN protein in the blood. (Funded by F. Hoffmann-La Roche; ClinicalTrials.gov number, NCT02913482.).

Population pharmacokinetics-based recommendations for a single delayed or missed dose of nusinersen

Nusinersen is an antisense oligonucleotide approved for the treatment of spinal muscular atrophy. The drug is given intrathecally at 12 mg, beginning with 3 loading doses at 2-week intervals, a fourth loading dose 30 days thereafter, and maintenance doses at 4-month intervals. This population pharmacokinetic model was developed to clarify how to maintain targeted nusinersen exposure after an unforeseen one-time delay or missed dose. Simulations demonstrated that the impact of a one-time delay in dosing or a missed dose on median cerebrospinal fluid exposures depended on duration of interruption and the regimen phase in which it occurred. Delays in loading doses delayed reaching the peak trough concentration by approximately the duration of the interruption. Resumption of the regimen as soon as possible resulted in achieving steady state trough concentration upon completion of the loading phase. A short delay (30-90 days) during the maintenance phase led to prolonged lower median cerebrospinal fluid concentration if all subsequent doses were shifted by the same 4-month interval. However, administration of the delayed dose, followed by the subsequent dose as originally scheduled, rapidly restored trough concentration. If a dose must be delayed, patients should return to the original dosing schedule as soon as possible.

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.