Temporal requirement for high SMN expression in SMA mice

Motor pool selectivity of neuromuscular degeneration in type I spinal muscular atrophy is conserved between human and mouse

Spinal muscular atrophy (SMA) is caused by low levels of the survival motor neuron (SMN) protein. Even though SMN is ubiquitously expressed, the disease selectively affects motor neurons, leading to progressive muscle weakness. Even among motor neurons, certain motor units appear more clinically resistant to SMA. To quantitatively survey selective resistance, we studied extensive neuromuscular autopsies of Type I SMA patients and age-matched controls. We found highly divergent degrees of degeneration of neighboring motor units, even within individual cranial nerves or a single anatomical area such as the neck. Examination of a Type I SMA patient maintained on life support for 17 years found that most muscles were atrophied, but the diaphragm was strikingly preserved. Nevertheless, some resistant human muscles with preserved morphology displayed nearly complete conversion to slow Type I myofibers. Remarkably, a similar pattern of selective resistance was observed in the SMNΔ7 mouse model. Overall, differential motor unit vulnerability in human Type I SMA suggests the existence of potent, motor unit-specific disease modifiers. Mechanisms that confer selective resistance to SMA may represent therapeutic targets independent of the SMN protein, particularly in patients with neuromuscular weakness refractory to current treatments.

Isogenic patient-derived organoids reveal early neurodevelopmental defects in spinal muscular atrophy initiation

Whether neurodevelopmental defects underlie postnatal neuronal death in neurodegeneration is an intriguing hypothesis only recently explored. Here, we focus on spinal muscular atrophy (SMA), a neuromuscular disorder caused by reduced survival of motor neuron (SMN) protein levels leading to spinal motor neuron (MN) loss and muscle wasting. Using the first isogenic patient-derived induced pluripotent stem cell (iPSC) model and a spinal cord organoid (SCO) system, we show that SMA SCOs exhibit abnormal morphological development, reduced expression of early neural progenitor markers, and accelerated expression of MN progenitor and MN markers. Longitudinal single-cell RNA sequencing reveals marked defects in neural stem cell specification and fewer MNs, favoring mesodermal progenitors and muscle cells, a bias also seen in early SMA mouse embryos. Surprisingly, SMN2-to-SMN1 conversion does not fully reverse these developmental abnormalities. These suggest that early neurodevelopmental defects may underlie later MN degeneration, indicating that postnatal SMN-increasing interventions might not completely amend SMA pathology in all patients.

Our Journey from Individual Efforts to Nationwide Support: Implementing Newborn Screening for Spinal Muscular Atrophy in Serbia

Innovative treatments for spinal muscular atrophy (SMA) yield the utmost advantages only within the presymptomatic phase, underlining the significance of newborn screening (NBS). We aimed to establish statewide NBS for SMA in Serbia. Our stepwise implementation process involved technical validation of a screening assay, collaboration with patient organizations and medical professionals, a feasibility study, and negotiation with public health representatives. Over 12,000 newborns were tested during the 17-month feasibility study, revealing two unrelated SMA infants and one older sibling. All three children received therapeutic interventions during the presymptomatic phase and have shown no signs of SMA. No false-negative results were found among the negative test results. As frontrunners in this field in Serbia, we established screening and diagnostic algorithms and follow-up protocols and raised awareness among stakeholders about the importance of early disease detection, leading to the incorporation of NBS for SMA into the national program on 15 September 2023. Since then, 54,393 newborns have been tested, identifying six SMA cases and enabling timely treatment. Our study demonstrates that effective collaborations between academia, non-profit organizations, and industry are crucial in bringing innovative healthcare initiatives to fruition, and highlights the potential of NBS to revolutionize healthcare outcomes for presymptomatic SMA infants and their families.

Improved therapeutic approach for spinal muscular atrophy via ubiquitination-resistant survival motor neuron variant

BACKGROUND

Zolgensma is a gene-replacement therapy that has led to a promising treatment for spinal muscular atrophy (SMA). However, clinical trials of Zolgensma have raised two major concerns: insufficient therapeutic effects and adverse events. In a recent clinical trial, 30% of patients failed to achieve motor milestones despite pre-symptomatic treatment. In addition, more than 20% of patients showed hepatotoxicity due to excessive virus dosage, even after the administration of an immunosuppressant. Here, we aimed to test whether a ubiquitination-resistant variant of survival motor neuron (SMN), SMNK186R, has improved therapeutic effects for SMA compared with wild-type SMN (SMNWT).

METHODS

A severe SMA mouse model, SMA type 1.5 (Smn-/-; SMN2+/+; SMN∆7+/-) mice, was used to compare the differences in therapeutic efficacy between AAV9-SMNWT and AAV9-SMNK186R. All animals were injected within Postnatal Day (P) 1 through a facial vein or cerebral ventricle.

RESULTS

AAV9-SMNK186R-treated mice showed increased lifespan, body weight, motor neuron number, muscle weight and functional improvement in motor functions as compared with AAV9-SMNWT-treated mice. Lifespan increased by more than 10-fold in AAV9-SMNK186R-treated mice (144.8 ± 26.11 days) as compared with AAV9-SMNWT-treated mice (26.8 ± 1.41 days). AAV9-SMNK186R-treated mice showed an ascending weight pattern, unlike AAV9-SMNWT-treated mice, which only gained weight until P20 up to 5 g on average. Several motor function tests showed the improved therapeutic efficacy of SMNK186R. In the negative geotaxis test, AAV9-SMNK186R-treated mice turned their bodies in an upward direction successfully, unlike AAV9-SMNWT-treated mice, which failed to turn upwards from around P23. Hind limb clasping phenotype was rarely observed in AAV9-SMNK186R-treated mice, unlike AAV9-SMNWT-treated mice that showed clasping phenotype for more than 20 out of 30 s. At this point, the number of motor neurons (1.5-fold) and the size of myofibers (2.1-fold) were significantly increased in AAV9-SMNK186R-treated mice compared with AAV9-SMNWT-treated mice without prominent neurotoxicity. AAV9-SMNK186R had fewer liver defects compared with AAV9-SMNWT, as judged by increased proliferation of hepatocytes (P < 0.0001) and insulin-like growth factor-1 production (P < 0.0001). Especially, low-dose AAV9-SMNK186R (nine-fold) also reduced clasping time compared with SMNWT.

CONCLUSIONS

SMNK186R will provide improved therapeutic efficacy in patients with severe SMA with insufficient therapeutic efficacy. Low-dose treatment of SMA patients with AAV9-SMNK186R can reduce the adverse events of Zolgensma. Collectively, SMNK186R has value as a new treatment for SMA that improves treatment effectiveness and reduces adverse events simultaneously.

Therapeutic strategy for spinal muscular atrophy by combining gene supplementation and genome editing

Defect in the SMN1 gene causes spinal muscular atrophy (SMA), which shows loss of motor neurons, muscle weakness and atrophy. While current treatment strategies, including small molecules or viral vectors, have shown promise in improving motor function and survival, achieving a definitive and long-term correction of SMA's endogenous mutations and phenotypes remains highly challenging. We have previously developed a CRISPR-Cas9 based homology-independent targeted integration (HITI) strategy, enabling unidirectional DNA knock-in in both dividing and non-dividing cells in vivo. In this study, we demonstrated its utility by correcting an SMA mutation in mice. When combined with Smn1 cDNA supplementation, it exhibited long-term therapeutic benefits in SMA mice. Our observations may provide new avenues for the long-term and efficient treatment of inherited diseases.

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.

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.

The Proteome Signatures of Fibroblasts from Patients with Severe, Intermediate and Mild Spinal Muscular Atrophy Show Limited Overlap

Most research to characterise the molecular consequences of spinal muscular atrophy (SMA) has focused on SMA I. Here, proteomic profiling of skin fibroblasts from severe (SMA I), intermediate (SMA II), and mild (SMA III) patients, alongside age-matched controls, was conducted using SWATH mass spectrometry analysis. Differentially expressed proteomic profiles showed limited overlap across each SMA type, and variability was greatest within SMA II fibroblasts, which was not explained by SMN2 copy number. Despite limited proteomic overlap, enriched canonical pathways common to two of three SMA severities with at least one differentially expressed protein from the third included mTOR signalling, regulation of eIF2 and eIF4 signalling, and protein ubiquitination. Network expression clustering analysis identified protein profiles that may discriminate or correlate with SMA severity. From these clusters, the differential expression of PYGB (SMA I), RAB3B (SMA II), and IMP1 and STAT1 (SMA III) was verified by Western blot. All SMA fibroblasts were transfected with an SMN-enhanced construct, but only RAB3B expression in SMA II fibroblasts demonstrated an SMN-dependent response. The diverse proteomic profiles and pathways identified here pave the way for studies to determine their utility as biomarkers for patient stratification or monitoring treatment efficacy and for the identification of severity-specific treatments.

Suppression of the necroptotic cell death pathways improves survival in Smn2B/− mice

Spinal muscular atrophy (SMA) is a monogenic neuromuscular disease caused by low levels of the Survival Motor Neuron (SMN) protein. Motor neuron degeneration is the central hallmark of the disease. However, the SMN protein is ubiquitously expressed and depletion of the protein in peripheral tissues results in intrinsic disease manifestations, including muscle defects, independent of neurodegeneration. The approved SMN-restoring therapies have led to remarkable clinical improvements in SMA patients. Yet, the presence of a significant number of non-responders stresses the need for complementary therapeutic strategies targeting processes which do not rely solely on restoring SMN. Dysregulated cell death pathways are candidates for SMN-independent pathomechanisms in SMA. Receptor-interacting protein kinase 1 (RIPK1) and RIPK3 have been widely recognized as critical therapeutic targets of necroptosis, an important form of programmed cell death. In addition, Caspase-1 plays a fundamental role in inflammation and cell death. In this study, we evaluate the role of necroptosis, particularly RIPK3 and Caspase-1, in the Smn^2B/− mouse model of SMA. We have generated a triple mutant (TKO), the Smn^2B/−; Ripk3^−/−; Casp1^−/− mouse. TKO mice displayed a robust increase in survival and improved motor function compared to Smn^2B/− mice. While there was no protection against motor neuron loss or neuromuscular junction pathology, larger muscle fibers were observed in TKO mice compared to Smn^2B/− mice. Our study shows that necroptosis modulates survival, motor behavior and muscle fiber size independent of SMN levels and independent of neurodegeneration. Thus, small-molecule inhibitors of necroptosis as a combinatorial approach together with SMN-restoring drugs could be a future strategy for the treatment of SMA.

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.

OUP accepted manuscript

Spinal muscular atrophy (SMA) is a childhood motor neuron disease caused by anomalies in the SMN1 gene. Although therapeutics have been approved for the treatment of SMA, there is a therapeutic time window, after which efficacy is reduced. Hallmarks of motor unit pathology in SMA include loss of motor-neurons and neuromuscular junction (NMJs). Following an increase in Smn levels, it is unclear how much damage can be repaired and the degree to which normal connections are re-established. Here, we perform a detailed analysis of motor unit pathology before and after restoration of Smn levels. Using a Smn-inducible mouse model of SMA, we show that genetic restoration of Smn results in a dramatic reduction in NMJ pathology, with restoration of innervation patterns, preservation of axon and endplate number and normalized expression of P53-associated transcripts. Notably, presynaptic swelling and elevated Pmaip levels remained. We analysed the effect of either early or delayed treated of an antisense oligonucleotide (ASO) targeting SMN2 on a range of differentially vulnerable muscles. Following ASO administration, the majority of endplates appeared fully occupied. However, there was an underlying loss of axons and endplates, which was more prevalent following a delay in treatment. There was an increase in average motor unit size following both early and delayed treatment. Together this work demonstrates the remarkably regenerative capacity of the motor neuron following Smn restoration, but highlights that recovery is incomplete. This work suggests that there is an opportunity to enhance neuromuscular junction recovery following administration of Smn-enhancing therapeutics.

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.

Biodistribution of onasemnogene abeparvovec DNA, mRNA and SMN protein in human tissue

Spinal muscular atrophy type 1 (SMA1) is a debilitating neurodegenerative disease resulting from survival motor neuron 1 gene (SMN1) deletion/mutation. Onasemnogene abeparvovec (formerly AVXS-101) is a gene therapy that restores SMN production via one-time systemic administration. The present study demonstrates widespread biodistribution of vector genomes and transgenes throughout the central nervous system (CNS) and peripheral organs, after intravenous administration of an AAV9-mediated gene therapy. Two symptomatic infants with SMA1 enrolled in phase III studies received onasemnogene abeparvovec. Both patients died of respiratory complications unrelated to onasemnogene abeparvovec. One patient had improved motor function and the other died shortly after administration before appreciable clinical benefit could be observed. In both patients, onasemnogene abeparvovec DNA and messenger RNA distribution were widespread among peripheral organs and in the CNS. The greatest concentration of vector genomes was detected in the liver, with an increase over that detected in CNS tissues of 300-1,000-fold. SMN protein, which was low in an untreated SMA1 control, was clearly detectable in motor neurons, brain, skeletal muscle and multiple peripheral organs in treated patients. These data support the fact that onasemnogene abeparvovec has effective distribution, transduction and expression throughout the CNS after intravenous administration and restores SMN expression in humans.

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.

Spinal Muscular Atrophy

PURPOSE OF REVIEW

This article provides an overview of the pathophysiology and clinical presentations of spinal muscular atrophy (SMA) and reviews therapeutic developments, including US Food and Drug Administration (FDA)-approved gene-targeted therapies and mainstays of supportive SMA care.

RECENT FINDINGS

Over the past decades, an understanding of the role of SMN protein in the development and maintenance of the motor unit and the intricate genetics underlying SMA has led to striking developments in therapeutics with three FDA-approved treatments for SMA, one targeting SMN1 gene replacement (onasemnogene abeparvovec-xioi) and two others enhancing SMN protein production from the SMN2 gene (nusinersen and risdiplam). These therapies are most effective in infants treated at younger ages, and improvement is most striking in babies treated as neonates. Despite improvements in motor function, patients (especially those treated at older ages) continue to experience significant weakness and require continued close monitoring of respiratory and orthopedic symptoms.

SUMMARY

Striking therapeutic advancements have changed the clinical course of SMA dramatically, although supportive care continues to play an important role in patient care.

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.

Muscle-specific SMN reduction reveals motor neuron-independent disease in spinal muscular atrophy models

Paucity of the survival motor neuron (SMN) protein triggers the oft-fatal infantile-onset motor neuron disorder, spinal muscular atrophy (SMA). Augmenting the protein is one means of treating SMA and recently led to FDA approval of an intrathecally delivered SMN-enhancing oligonucleotide currently in use. Notwithstanding the advent of this and other therapies for SMA, it is unclear whether the paralysis associated with the disease derives solely from dysfunctional motor neurons that may be efficiently targeted by restricted delivery of SMN-enhancing agents to the nervous system, or stems from broader defects of the motor unit, arguing for systemic SMN repletion. We investigated the disease-contributing effects of low SMN in one relevant peripheral organ - skeletal muscle - by selectively depleting the protein in only this tissue. We found that muscle deprived of SMN was profoundly damaged. Although a disease phenotype was not immediately obvious, persistent low levels of the protein eventually resulted in muscle fiber defects, neuromuscular junction abnormalities, compromised motor performance, and premature death. Importantly, restoring SMN after the onset of muscle pathology reversed disease. Our results provide the most compelling evidence yet for a direct contributing role of muscle in SMA and argue that an optimal therapy for the disease must be designed to treat this aspect of the dysfunctional motor unit.

Survival motor neuron protein regulates oxidative stress and inflammatory response in microglia of the spinal cord in spinal muscular atrophy

The deficiency of survival motor neuron (SMN) protein can result in the onset of spinal muscular atrophy (SMA), an autosomal recessive disorder characterized by a progressive loss of motor neurons and skeletal muscle atrophy. The mechanism underlying SMA pathology remains unclear. Here, we demonstrate that SMN protein regulates oxidative stress and inflammatory response in microglia. Antisense oligonucleotide, which increases SMN protein expression (SMN-ASO), attenuated SMA model mice phenotypes and suppressed the activation of microglia in the spinal cord. The expression of oxidative stress marker in microglia was decreased by SMN-ASO injection in SMA model mice. Increased reactive oxygen species production and subsequent antioxidative stress reaction was observed in SMN protein-depleted RAW264.7. Furthermore, nuclear factor kappa B (NFκB) and c-Jun amino terminal kinase (JNK) signaling, which mainly mediate the inflammatory response, are activated in SMN protein-depleted RAW264.7. Tumor necrosis factor-α (TNF-α) production is also increased in SMN protein-depleted RAW264.7. These findings suggest that SMN protein regulates oxidative stress and inflammatory response in microglia, supporting current claims that microglia can be an effective target for SMA therapy.

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.

Novel challenges in spinal muscular atrophy - How to screen and whom to treat?

In recent years, disease‐modifying and life‐prolonging therapies for spinal muscular atrophy (SMA) have been developed. However, patients are currently diagnosed with significant delay and therapies are often administered in advanced stages of motor neuron degeneration, showing limited effects. Methods to identify children in presymptomatic stages are currently evaluated in newborn screening programs. Yet, not all children develop symptoms shortly after birth raising the question whom to treat and when to initiate therapy. Finally, monitoring disease progression becomes essential to individualize management. Here, we review the literature on screening approaches, strategies to predict disease severity, and biomarkers to monitor therapy.

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

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

Mild SMN missense alleles are only functional in the presence of SMN2 in mammals

Spinal muscular atrophy (SMA) is caused by reduced levels of full-length SMN (FL-SMN). In SMA patients with one or two copies of the Survival Motor Neuron 2 (SMN2) gene there are a number of SMN missense mutations that result in milder-than-predicted SMA phenotypes. These mild SMN missense mutation alleles are often assumed to have partial function. However, it is important to consider the contribution of FL-SMN as these missense alleles never occur in the absence of SMN2. We propose that these patients contain a partially functional oligomeric SMN complex consisting of FL-SMN from SMN2 and mutant SMN protein produced from the missense allele. Here we show that mild SMN missense mutations SMND44V, SMNT74I or SMNQ282A alone do not rescue mice lacking wild-type FL-SMN. Thus, missense mutations are not functional in the absence of FL-SMN. In contrast, when the same mild SMN missense mutations are expressed in a mouse containing two SMN2 copies, functional SMN complexes are formed with the small amount of wild-type FL-SMN produced by SMN2 and the SMA phenotype is completely rescued. This contrasts with SMN missense alleles when studied in C. elegans, Drosophila and zebrafish. Here we demonstrate that the heteromeric SMN complex formed with FL-SMN is functional and sufficient to rescue small nuclear ribonucleoprotein assembly, motor neuron function and rescue the SMA mice. We conclude that mild SMN missense alleles are not partially functional but rather they are completely non-functional in the absence of wild-type SMN in mammals.

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.

Spinal muscular atrophy

Autosomal-recessive proximal spinal muscular atrophy (Werdnig-Hoffmann, Kugelberg-Welander) is caused by mutation of the SMN1 gene, and the clinical severity correlates with the number of copies of a nearly identical gene, SMN2. The SMN protein plays a critical role in spliceosome assembly and may have other cellular functions, such as mRNA transport. Cell culture and animal models have helped to define the disease mechanism and to identify targets for therapeutic intervention. The main focus for developing treatment has been to increase SMN levels, and accomplishing this with small molecules, oligonucleotides, and gene replacement has been quite. An oligonucleotide, nusinersen, was recently approved for treatment in patients, and confirmatory studies of other agents are now under way.

Gonorazky H, Cohn RD, Campbell C. Treating pediatric neuromuscular disorders: The future is now

Pediatric neuromuscular diseases encompass all disorders with onset in childhood and where the primary area of pathology is in the peripheral nervous system. These conditions are largely genetic in etiology, and only those with a genetic underpinning will be presented in this review. This includes disorders of the anterior horn cell (e.g., spinal muscular atrophy), peripheral nerve (e.g., Charcot-Marie-Tooth disease), the neuromuscular junction (e.g., congenital myasthenic syndrome), and the muscle (myopathies and muscular dystrophies). Historically, pediatric neuromuscular disorders have uniformly been considered to be without treatment possibilities and to have dire prognoses. This perception has gradually changed, starting in part with the discovery and widespread application of corticosteroids for Duchenne muscular dystrophy. At present, several exciting therapeutic avenues are under investigation for a range of conditions, offering the potential for significant improvements in patient morbidities and mortality and, in some cases, curative intervention. In this review, we will present the current state of treatment for the most common pediatric neuromuscular conditions, and detail the treatment strategies with the greatest potential for helping with these devastating diseases.

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.

Genetic screening of spinal muscular atrophy using a real-time modified COP-PCR technique with dried blood-spot DNA

BACKGROUND

Spinal muscular atrophy (SMA) is a common neuromuscular disorder caused by mutations in SMN1. More than 95% of SMA patients carry homozygous SMN1 deletion. SMA is the leading genetic cause of infant death, and has been considered an incurable disease. However, a recent clinical trial with an antisense oligonucleotide drug has shown encouraging clinical efficacy. Thus, early and accurate detection of SMN1 deletion may improve prognosis of many infantile SMA patients.

METHODS

A total of 88 DNA samples (37 SMA patients, 12 carriers and 39 controls) from dried blood spots (DBS) on filter paper were analyzed. All participants had previously been screened for SMN genes by PCR restriction fragment length polymorphism (PCR-RFLP) using DNA extracted from freshly collected blood. DNA was extracted from DBS that had been stored at room temperature (20-25°C) for 1week to 5years. To ensure sufficient quality and quantity of DNA samples, target sequences were pre-amplified by conventional PCR. Real-time modified competitive oligonucleotide priming-PCR (mCOP-PCR) with the pre-amplified PCR products was performed for the gene-specific amplification of SMN1 and SMN2 exon 7.

RESULTS

Compared with PCR-RFLP using DNA from freshly collected blood, results from real-time mCOP-PCR using DBS-DNA for detection of SMN1 exon 7 deletion showed a sensitivity of 1.00 (CI [0.87, 1.00])] and specificity of 1.00 (CI [0.90, 1.00]), respectively.

CONCLUSION

We combined DNA extraction from DBS on filter paper, pre-amplification of target DNA, and real-time mCOP-PCR to specifically detect SMN1 and SMN2 genes, thereby establishing a rapid, accurate, and high-throughput system for detecting SMN1-deletion with practical applications for newborn screening.

Gestational Age-Dependent Increase of Survival Motor Neuron Protein in Umbilical Cord-Derived Mesenchymal Stem Cells

BACKGROUND

Spinal muscular atrophy (SMA) is the most common genetic neurological disease leading to infant death. It is caused by loss of survival motor neuron (SMN) 1 gene and subsequent reduction of SMN protein in motor neurons. Because SMN is ubiquitously expressed and functionally linked to general RNA metabolism pathway, fibroblasts (FBs) are most widely used for the assessment of SMN expression in SMA patients but usually isolated from skin biopsy samples after the onset of overt symptoms. Although recent translational studies of SMN-targeted therapies have revealed the very limited time window for effective SMA therapies during perinatal period, the exact time point when SMN shortage became evident is unknown in human samples. In this study, we analyzed SMN mRNA and protein expression during perinatal period by using umbilical cord-derived mesenchymal stem cells (UC-MSCs) obtained from preterm and term infants.

METHODS

UC-MSCs were isolated from 16 control infants delivered at 22-40 weeks of gestation and SMA fetus aborted at 19 weeks of gestation (UC-MSC-Control and UC-MSC-SMA). FBs were isolated from control volunteer and SMA patient (FB-Control and FB-SMA). SMN mRNA and protein expression in UC-MSCs and FBs was determined by RT-qPCR and Western blot.

RESULTS

UC-MSC-Control and UC-MSC-SMA expressed the comparable level of MSC markers on their cell surface and were able to differentiate into adipocytes, osteocytes, and chondrocytes. At steady state, SMN mRNA and protein expression was decreased in UC-MSC-SMA compared to UC-MSC-Control, as observed in FB-SMA and FB-Control. In response to histone deacetylase inhibitor valproic acid, SMN mRNA and protein expression in UC-MSC-SMA and FB-SMA was increased. During perinatal development from 22 to 40 weeks of gestation, SMN mRNA and protein expression in UC-MSC-Control was positively correlated with gestational age.

CONCLUSION

UC-MSCs isolated from 17 fetus/infant of 19-40 weeks of gestation are expressed functional SMN mRNA and protein. SMN mRNA and protein expression in UC-MSCs is increased with gestational age during perinatal development.

SMN regulation in SMA and in response to stress: new paradigms and therapeutic possibilities

Low levels of the survival of motor neuron (SMN) protein cause the neurodegenerative disease spinal muscular atrophy (SMA). SMA is a pediatric disease characterized by spinal motor neuron degeneration. SMA exhibits several levels of severity ranging from early antenatal fatality to only mild muscular weakness, and disease prognosis is related directly to the amount of functional SMN protein that a patient is able to express. Current therapies are being developed to increase the production of functional SMN protein; however, understanding the effect that natural stresses have on the production and function of SMN is of critical importance to ensuring that these therapies will have the greatest possible effect for patients. Research has shown that SMN, both on the mRNA and protein level, is highly affected by cellular stress. In this review we will summarize the research that highlights the roles of SMN in the disease process and the response of SMN to various environmental stresses.

Splice-Switching Therapy for Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a genetic disorder with severity ranging from premature death in infants to restricted motor function in adult life. Despite the genetic cause of this disease being known for over twenty years, only recently has a therapy been approved to treat the most severe form of this disease. Here we discuss the genetic basis of SMA and the subsequent studies that led to the utilization of splice switching oligonucleotides to enhance production of SMN protein, which is absent in patients, through a mechanism of exon inclusion into the mature mRNA. Whilst approval of oligonucleotide-based therapies for SMA should be celebrated, we also discuss some of the limitations of this approach and alternate genetic strategies that are currently underway in clinical trials.

Spinal Muscular Atrophy: From Defective Chaperoning of snRNP Assembly to Neuromuscular Dysfunction

Spinal Muscular Atrophy (SMA) is a neuromuscular disorder that results from decreased levels of the survival motor neuron (SMN) protein. SMN is part of a multiprotein complex that also includes Gemins 2–8 and Unrip. The SMN-Gemins complex cooperates with the protein arginine methyltransferase 5 (PRMT5) complex, whose constituents include WD45, PRMT5 and pICln. Both complexes function as molecular chaperones, interacting with and assisting in the assembly of an Sm protein core onto small nuclear RNAs (snRNAs) to generate small nuclear ribonucleoproteins (snRNPs), which are the operating components of the spliceosome. Molecular and structural studies have refined our knowledge of the key events taking place within the crowded environment of cells and the numerous precautions undertaken to ensure the faithful assembly of snRNPs. Nonetheless, it remains unclear whether a loss of chaperoning in snRNP assembly, considered as a “housekeeping” activity, is responsible for the selective neuromuscular phenotype in SMA. This review thus shines light on in vivo studies that point toward disturbances in snRNP assembly and the consequential transcriptome abnormalities as the primary drivers of the progressive neuromuscular degeneration underpinning the disease. Disruption of U1 snRNP or snRNP assembly factors other than SMN induces phenotypes that mirror aspects of SMN deficiency, and splicing defects, described in numerous SMA models, can lead to a DNA damage and stress response that compromises the survival of the motor system. Restoring the correct chaperoning of snRNP assembly is therefore predicted to enhance the benefit of SMA therapeutic modalities based on augmenting SMN expression.

Cure SMA and our patient community celebrate the first approved drug for SMA

Cure SMA is dedicated to the treatment and cure of spinal muscular atrophy (SMA)-a disease affecting motor neurons, that robs patients of their ability to walk, eat and even breathe. Since 1984, we have directed and invested in comprehensive research that has shaped the scientific community's understanding of SMA. On 23 December, 2016, the Food and Drug Administration (FDA) announced approval of Spinraza, a treatment developed by Biogen and Ionis, making it the first-ever approved therapy for SMA. Cure SMA provided early research funding in 2003 leading to the discovery of ISS-N1 sequence, now targeted by Spinraza. We are pleased that our strategy of providing seed funding for research to either identify new therapeutic strategies or de-risk early stage ones, has proven successful with Spinraza's approval. The approval of Spinraza provides great hope to the SMA community and represents decades of hard work and perseverance by families, researchers, pharmaceutical companies and the FDA. Our hope is that Spinraza is the leading edge of a robust drug pipeline, and with our deep expertise in every aspect of SMA, we remain committed to do everything we can to support research and drug development to achieve the greatest possible effect for each and every SMA patient.

Compensatory axon sprouting for very slow axonal die-back in a transgenic model of spinal muscular atrophy type III

KEY POINTS

Smn+/- transgenic mouse is a model of the mildest form of spinal muscular atrophy. Although there is a loss of spinal motoneurons in 11-month-old animals, muscular force is maintained. This maintained muscular force is mediated by reinnervation of the denervated fibres by surviving motoneurons. The spinal motoneurons in these animals do not show an increased susceptibility to death after nerve injury and they retain their regenerative capacity. We conclude that the hypothesized immaturity of the neuromuscular system in this model cannot explain the loss of motoneurons by systematic die-back.

ABSTRACT

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder in humans and is the leading genetic cause of infantile death. Patients lack the SMN1 gene with the severity of the disease depending on the number of copies of the highly homologous SMN2 gene. Although motoneuron death in the Smn+/- transgenic mouse model of the mildest form of SMA, SMA type III, has been reported, we have used retrograde tracing of sciatic and femoral motoneurons in the hindlimb with recording of muscle and motor unit isometric forces to count the number of motoneurons with intact neuromuscular connections. Thereby, we investigated whether incomplete maturation of the neuromuscular system induced by survival motoneuron protein (SMN) defects is responsible for die-back of axons relative to survival of motoneurons. First, a reduction of ∼30% of backlabelled motoneurons began relatively late, at 11 months of age, with a significant loss of 19% at 7 months. Motor axon die-back was affirmed by motor unit number estimation. Loss of functional motor units was fully compensated by axonal sprouting to retain normal contractile force in four hindlimb muscles (three fast-twitch and one slow-twitch) innervated by branches of the sciatic nerve. Second, our evaluation of whether axotomy of motoneurons in the adult Smn+/- transgenic mouse increases their susceptibility to cell death demonstrated that all the motoneurons survived and they sustained their capacity to regenerate their nerve fibres. It is concluded the systematic die-back of motoneurons that innervate both fast- and slow-twitch muscle fibres is not related to immaturity of the neuromuscular system in SMA.

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

ADAMTS13 deficiency promotes microthrombosis in a murine model of diet-induced liver steatosis

ADAMTS13 cleaves ultralarge multimeric von Willebrand Factor (VWF), thereby preventing formation of platelet-rich microthrombi. ADAMTS13 is mainly produced by hepatic stellate cells, and numerous studies have suggested a functional role of ADAMTS13 in the pathogenesis of liver diseases. The aim of our study was to investigate a potential role of ADAMTS13 in formation of hepatic microthrombi and development of non-alcoholic steatohepatitis (NASH), and furthermore to evaluate whether plasmin can compensate for the absence of ADAMTS13 in removal of thrombi. Therefore, we used a model of high-fat diet-induced steatosis in Adamts13 deficient (Adamts13^-/-) and wild-type (WT) control mice. Microthrombi were more abundant in the liver of obese Adamts13^-/- as compared to obese WT or to lean Adamts13^-/- mice. Obese Adamts13^-/- mice displayed lower platelet counts and higher prevalence of ultra-large VWF multimers. Hepatic plasmin-α2-antiplasmin complex levels were comparable for obese WT and Adamts13^-/- mice and were lower for lean Adamts13^-/- than WT mice, not supporting marked activation of the fibrinolytic system. High fat diet feeding, as compared to normal chow, resulted in enhanced liver triglyceride levels for both genotypes (p < 0.0001) and steatosis (p < 0.0001 for WT mice, p = 0.002 for Adamts13^-/- mice) without differences between the genotypes. Expression of markers of inflammation, oxidative stress, steatosis and fibrosis was affected by diet, but not by genotype. Thus, our data confirm that obesity promotes NASH, but do not support a detrimental role of ADAMTS13 in its development. However, Adamts13 deficiency in obese mice promotes hepatic microthrombosis, whereas a compensatory role of plasmin in removal of microthrombi in the absence of ADAMTS13 could not be demonstrated.

Systemic restoration of UBA1 ameliorates disease in spinal muscular atrophy

The autosomal recessive neuromuscular disease spinal muscular atrophy (SMA) is caused by loss of survival motor neuron (SMN) protein. Molecular pathways that are disrupted downstream of SMN therefore represent potentially attractive therapeutic targets for SMA. Here, we demonstrate that therapeutic targeting of ubiquitin pathways disrupted as a consequence of SMN depletion, by increasing levels of one key ubiquitination enzyme (ubiquitin-like modifier activating enzyme 1 [UBA1]), represents a viable approach for treating SMA. Loss of UBA1 was a conserved response across mouse and zebrafish models of SMA as well as in patient induced pluripotent stem cell-derive motor neurons. Restoration of UBA1 was sufficient to rescue motor axon pathology and restore motor performance in SMA zebrafish. Adeno-associated virus serotype 9-UBA1 (AAV9-UBA1) gene therapy delivered systemic increases in UBA1 protein levels that were well tolerated over a prolonged period in healthy control mice. Systemic restoration of UBA1 in SMA mice ameliorated weight loss, increased survival and motor performance, and improved neuromuscular and organ pathology. AAV9-UBA1 therapy was also sufficient to reverse the widespread molecular perturbations in ubiquitin homeostasis that occur during SMA. We conclude that UBA1 represents a safe and effective therapeutic target for the treatment of both neuromuscular and systemic aspects of SMA.

Disruption of snRNP biogenesis factors Tgs1 and pICln induces phenotypes that mirror aspects of SMN-Gemins complex perturbation in Drosophila, providing new insights into spinal muscular atrophy

The neuromuscular disorder, spinal muscular atrophy (SMA), results from insufficient levels of the survival motor neuron (SMN) protein. Together with Gemins 2-8 and Unrip, SMN forms the large macromolecular SMN-Gemins complex, which is known to be indispensable for chaperoning the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs). It remains unclear whether disruption of this function is responsible for the selective neuromuscular degeneration in SMA. In the present study, we first show that loss of wmd, the Drosophila Unrip orthologue, has a negative impact on the motor system. However, due to lack of a functional relationship between wmd/Unrip and Gemin3, it is likely that Unrip joined the SMN-Gemins complex only recently in evolution. Second, we uncover that disruption of either Tgs1 or pICln, two cardinal players in snRNP biogenesis, results in viability and motor phenotypes that closely resemble those previously uncovered on loss of the constituent members of the SMN-Gemins complex. Interestingly, overexpression of both factors leads to motor dysfunction in Drosophila, a situation analogous to that of Gemin2. Toxicity is conserved in the yeast S. pombe where pICln overexpression induces a surplus of Sm proteins in the cytoplasm, indicating that a block in snRNP biogenesis is partly responsible for this phenotype. Importantly, we show a strong functional relationship and a physical interaction between Gemin3 and either Tgs1 or pICln. We propose that snRNP biogenesis is the pathway connecting the SMN-Gemins complex to a functional neuromuscular system, and its disturbance most likely leads to the motor dysfunction that is typical in SMA.

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

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

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

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

Systemic, postsymptomatic antisense oligonucleotide rescues motor unit maturation delay in a new mouse model for type II/III spinal muscular atrophy

Clinical presentation of spinal muscular atrophy (SMA) ranges from a neonatal-onset, very severe disease to an adult-onset, milder form. SMA is caused by the mutation of the Survival Motor Neuron 1 (SMN1) gene, and prognosis inversely correlates with the number of copies of the SMN2 gene, a human-specific homolog of SMN1. Despite progress in identifying potential therapies for the treatment of SMA, many questions remain including how late after onset treatments can still be effective and what the target tissues should be. These questions can be addressed in part with preclinical animal models; however, modeling the array of SMA severities in the mouse, which lacks SMN2, has proven challenging. We created a new mouse model for the intermediate forms of SMA presenting with a delay in neuromuscular junction maturation and a decrease in the number of functional motor units, all relevant to the clinical presentation of the disease. Using this new model, in combination with clinical electrophysiology methods, we found that administering systemically SMN-restoring antisense oligonucleotides (ASOs) at the age of onset can extend survival and rescue the neurological phenotypes. Furthermore, these effects were also achieved by administration of the ASOs late after onset, independent of the restoration of SMN in the spinal cord. Thus, by adding to the limited repertoire of existing mouse models for type II/III SMA, we demonstrate that ASO therapy can be effective even when administered after onset of the neurological symptoms, in young adult mice, and without being delivered into the central nervous system.

SMN expression is required in motor neurons to rescue electrophysiological deficits in the SMNΔ7 mouse model of SMA

Proximal spinal muscular atrophy (SMA) is the most frequent cause of hereditary infant mortality. SMA is an autosomal recessive neuromuscular disorder that results from the loss of the Survival Motor Neuron 1 (SMN1) gene and retention of the SMN2 gene. The SMN2 gene produces an insufficient amount of full-length SMN protein that results in loss of motor neurons in the spinal cord and subsequent muscle paralysis. Previously we have shown that overexpression of human SMN in neurons in the SMA mouse ameliorates the SMA phenotype while overexpression of human SMN in skeletal muscle had no effect. Using Cre recombinase, here we show that either deletion or replacement of Smn in motor neurons (ChAT-Cre) significantly alters the functional output of the motor unit as measured with compound muscle action potential and motor unit number estimation. However ChAT-Cre alone did not alter the survival of SMA mice by replacement and did not appreciably affect survival when used to deplete SMN. However replacement of Smn in both neurons and glia in addition to the motor neuron (Nestin-Cre and ChAT-Cre) resulted in the greatest improvement in survival of the mouse and in some instances complete rescue was achieved. These findings demonstrate that high expression of SMN in the motor neuron is both necessary and sufficient for proper function of the motor unit. Furthermore, in the mouse high expression of SMN in neurons and glia, in addition to motor neurons, has a major impact on survival.

Disease mechanisms and therapeutic approaches in spinal muscular atrophy

Motor neuron diseases are neurological disorders characterized primarily by the degeneration of spinal motor neurons, skeletal muscle atrophy, and debilitating and often fatal motor dysfunction. Spinal muscular atrophy (SMA) is an autosomal-recessive motor neuron disease of high incidence and severity and the most common genetic cause of infant mortality. SMA is caused by homozygous mutations in the survival motor neuron 1 (SMN1) gene and retention of at least one copy of the hypomorphic gene paralog SMN2. Early studies established a loss-of-function disease mechanism involving ubiquitous SMN deficiency and suggested SMN upregulation as a possible therapeutic approach. In recent years, greater knowledge of the central role of SMN in RNA processing combined with deep characterization of animal models of SMA has significantly advanced our understanding of the cellular and molecular basis of the disease. SMA is emerging as an RNA disease not limited to motor neurons, but one that involves dysfunction of motor circuits that comprise multiple neuronal subpopulations and possibly other cell types. Advances in SMA research have also led to the development of several potential therapeutics shown to be effective in animal models of SMA that are now in clinical trials. These agents offer unprecedented promise for the treatment of this still incurable neurodegenerative disease.