Natural history of denervation in SMA: relation to age, SMN2 copy number, and function

Spinal muscular atrophy--recent therapeutic advances for an old challenge

In the past decade, improved understanding of spinal muscular atrophy (SMA) aetiopathogenesis has brought us to a historical turning point: we are at the verge of development of disease-modifying treatments for this hitherto incurable disease. The increasingly precise delineation of molecular targets within the survival of motor neuron (SMN) gene locus has led to the development of promising therapeutic strategies. These novel avenues in treatment for SMA include gene therapy, molecular therapy with antisense oligonucleotides, and small molecules that aim to increase expression of SMN protein. Stem cell studies of SMA have provided an in vitro model for SMA, and stem cell transplantation could be used as a complementary strategy with a potential to treat the symptomatic phases of the disease. Here, we provide an overview of established data and novel insights into SMA pathogenesis, including discussion of the crucial function of the SMN protein. Preclinical evidence and recent advances from ongoing clinical trials are thoroughly reviewed. The final remarks are dedicated to future clinical perspectives in this rapidly evolving field, with a broad discussion on the comparison between the outlined therapeutic approaches and the remaining open questions.

Clinical characteristics of three subtypes of spinal muscular atrophy in children

BACKGROUND

The severity of spinal muscular atrophy (SMA) is highly variable and children with heterogeneous clinical features can be classified into three phenotypes (type I-III) on the basis of age of onset and maximum motor function achieved. The aim of this study was to compare the clinical characteristics of three phenotypes in children with SMA.

METHODS

One hundred and thirty-two SMA patients were classified as type I, II or III according to the SMA classification criteria. The clinical features, deletion of survival motor neuron 1 (SMN1) gene and electrophysiology were analyzed and compared. The survival and functional status were obtained through telephone follow up.

RESULTS

In our study, 90.6% of the patients lacked both copies of SMNl. No difference in the deletion frequency among the 3 groups was observed. Although most of the neurophysiological parameters showed no differences among the groups, the amplitudes of compound muscle action potential (CMAP) was lower in type III SMA. Absent sensory nerve action potential (SNAP) amplitude of the sural nerve was observed in 26 (25.4%) of the patients. The survival pattern and functional status of 66 cases were obtained. Two type II SMA patients could walk unaided during follow-up. The functional ability of lower extremities improved in 4 patients with type III SMA.

CONCLUSIONS

In this study, we confirm that EMG examination and homozygous deletion of SMN1 do not correlate with the subtypes. Motor function of patients with SMA type II and III can improve. A period of follow-up is necessary before rendering accurate classification and prognosis.

Upper limb evaluation and one-year follow up of non-ambulant patients with spinal muscular atrophy: an observational multicenter trial

Assessment of the upper limb strength in non-ambulant neuromuscular patients remains challenging. Although potential outcome measures have been reported, longitudinal data demonstrating sensitivity to clinical evolution in spinal muscular atrophy patients are critically lacking. Our study recruited 23 non-ambulant patients, 16 patients (males/females = 6/10; median age 15.4 years with a range from 10.7 to 31.1 years) with spinal muscular atrophy type II and 7 patients (males/females = 2/5; median age 19.9 years with a range from 8.3 to 29.9 years) with type III. The Brooke functional score was on median 3 with a range from 2 to 6. The average total vital capacity was 46%, and seven patients required non-invasive ventilation at night. Patients were assessed at baseline, 6 months, and 1 year using the Motor Function Measure and innovative devices MyoGrip, MyoPinch, and MoviPlate, which assess handgrip strength, key pinch strength, and hand/finger extension-flexion function, respectively. The study demonstrated the feasibility and reliability of these measures for all patients, and sensitivity to negative changes after the age of 14 years. The younger patients showed an increase of the distal force in the follow-up period. The distal force measurements and function were correlated to different functional scales. These data represent an important step in the process of validating these devices as potential outcome measures for future clinical trials.

Newborn screening for spinal muscular atrophy: Anticipating an imminent need

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality. Children with type I SMA typically die by the age of 2 years. Recent progress in gene modification and other innovative therapies suggest that improved outcomes may soon be forthcoming. In animal models, therapeutic intervention initiated before the loss of motor neurons alters SMA phenotype and increases lifespan. Presently, supportive care including respiratory, nutritional, physiatry, and orthopedic management can ameliorate clinical symptoms and improve survival rates if SMA is diagnosed early in life. Newborn screening could help optimize these potential benefits. A recent report demonstrated that SMA detection can be multiplexed at minimal additional cost with the assay for severe combined immunodeficiency, already implemented by many newborn screening programs. The public health community should remain alert to the rapidly changing developments in early detection and treatment of SMA.

The Genetics of Spinal Muscular Atrophy: Progress and Challenges

Spinal muscular atrophies (SMAs) are a group of inherited disorders characterized by motor neuron loss in the spinal cord and lower brainstem, muscle weakness, and atrophy. The clinical and genetic phenotypes incorporate a wide spectrum that is differentiated based on age of onset, pattern of muscle involvement, and inheritance pattern. Over the past several years, rapid advances in genetic technology have accelerated the identification of causative genes and provided important advances in understanding the molecular and biological basis of SMA and insights into the selective vulnerability of the motor neuron. Common pathophysiological themes include defects in RNA metabolism and splicing, axonal transport, and motor neuron development and connectivity. Together these have revealed potential novel treatment strategies, and extensive efforts are being undertaken towards expedited therapeutics. While a number of promising therapies for SMA are emerging, defining therapeutic windows and developing sensitive and relevant biomarkers are critical to facilitate potential success in clinical trials. This review incorporates an overview of the clinical manifestations and genetics of SMA, and describes recent advances in the understanding of mechanisms of disease pathogenesis and development of novel treatment strategies.

Molecular mechanisms and animal models of spinal muscular atrophy

Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality, is characterized by the degeneration of spinal motor neurons and muscle atrophy. Although the genetic cause of SMA has been mapped to the Survival Motor Neuron1 (SMN1) gene, mechanisms underlying selective motor neuron degeneration in SMA remain largely unknown. Here we review the latest developments and our current understanding of the molecular mechanisms underlying SMA pathogenesis, focusing on the animal model systems that have been developed, as well as new diagnostic and treatment strategies that have been identified using these model systems. This article is part of a special issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

Spectrum of neuropathophysiology in spinal muscular atrophy type I

Neuropathologic findings within the central and peripheral nervous systems in patients with spinal muscular atrophy type I (SMA-I) were examined in relation to genetic, clinical, and electrophysiologic features. Five infants representing the full clinical spectrum of SMA-I were examined clinically for compound motor action potential amplitude and SMN2 gene copy number; morphologic analyses of postmortem central nervous system, neuromuscular junction, and muscle tissue samples were performed and SMN protein was assessed in muscle samples. The 2 clinically most severely affected patients had a single copy of the SMN2 gene; in addition to anterior horn cells, dorsal root ganglia, and thalamus, neuronal degeneration in them was widespread in the cerebral cortex, basal ganglia, pigmented nuclei, brainstem, and cerebellum. Two typical SMA-I patients and a milder case each had 2 copies of the SMN2 gene and more restricted neuropathologic abnormalities. Maturation of acetylcholine receptor subunits was delayed and the neuromuscular junctions were abnormally formed in the SMA-I patients. Thus, the neuropathologic findings in human SMA-I are similar to many findings in animal models; factors other than SMN2 copy number modify disease severity. We present a pathophysiologic model for SMA-I as a protein deficiency disease affecting a neuronal network with variable clinical thresholds. Because new treatment strategies improve survival of infants with SMA-I, a better understanding of these factors will guide future treatments.

A large animal model of spinal muscular atrophy and correction of phenotype

OBJECTIVES

Spinal muscular atrophy (SMA) is caused by reduced levels of survival motor neuron (SMN) protein, which results in motoneuron loss. Therapeutic strategies to increase SMN levels including drug compounds, antisense oligonucleotides, and scAAV9 gene therapy have proved effective in mice. We wished to determine whether reduction of SMN in postnatal motoneurons resulted in SMA in a large animal model, whether SMA could be corrected after development of muscle weakness, and the response of clinically relevant biomarkers.

METHODS

Using intrathecal delivery of scAAV9 expressing an shRNA targeting pig SMN1, SMN was knocked down in motoneurons postnatally to SMA levels. This resulted in an SMA phenotype representing the first large animal model of SMA. Restoration of SMN was performed at different time points with scAAV9 expressing human SMN (scAAV9-SMN), and electrophysiology measurements and pathology were performed.

RESULTS

Knockdown of SMN in postnatal motoneurons results in overt proximal weakness, fibrillations on electromyography indicating active denervation, and reduced compound muscle action potential (CMAP) and motor unit number estimation (MUNE), as in human SMA. Neuropathology showed loss of motoneurons and motor axons. Presymptomatic delivery of scAAV9-SMN prevented SMA symptoms, indicating that all changes are SMN dependent. Delivery of scAAV9-SMN after symptom onset had a marked impact on phenotype, electrophysiological measures, and pathology.

INTERPRETATION

High SMN levels are critical in postnatal motoneurons, and reduction of SMN results in an SMA phenotype that is SMN dependent. Importantly, clinically relevant biomarkers including CMAP and MUNE are responsive to SMN restoration, and abrogation of phenotype can be achieved even after symptom onset.

Spinal muscular atrophy: from tissue specificity to therapeutic strategies

Spinal muscular atrophy (SMA) is the most frequent genetic cause of death in infants and toddlers. All cases of spinal muscular atrophy result from reductions in levels of the survival motor neuron (SMN) protein, and so SMN upregulation is a focus of many preclinical and clinical studies. We examine four issues that may be important in planning for therapeutic success. First, neuromuscular phenotypes in the SMNΔ7 mouse model closely match those in human patients but peripheral disease manifestations differ, suggesting that endpoints other than mouse lifespan may be more useful in predicting clinical outcome. Second, SMN plays important roles in multiple central and peripheral cell types, not just motor neurons, and it remains unclear which of these cell types need to be targeted therapeutically. Third, should SMN-restoration therapy not be effective in all patients, blocking molecular changes downstream of SMN reduction may confer significant benefit, making it important to evaluate therapeutic targets other than SMN. Lastly, for patients whose disease progression is slowed, but who retain significant motor dysfunction, additional approaches used to enhance regeneration of the neuromuscular system may be of value.

Systems biology investigation of cAMP modulation to increase SMN levels for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA), a leading genetic cause of infant death worldwide, is an autosomal recessive disorder caused by the loss of SMN1 (survival motor neuron 1), which encodes the protein SMN. The loss of SMN1 causes a deficiency in SMN protein levels leading to motor neuron cell death in the anterior horn of the spinal cord. SMN2, however, can also produce some functional SMN to partially compensate for loss of SMN1 in SMA suggesting increasing transcription of SMN2 as a potential therapy to treat patients with SMA. A cAMP response element was identified on the SMN2 promoter, implicating cAMP activation as a step in the transcription of SMN2. Therefore, we investigated the effects of modulating the cAMP signaling cascade on SMN production in vitro and in silico. SMA patient fibroblasts were treated with the cAMP signaling modulators rolipram, salbutamol, dbcAMP, epinephrine and forskolin. All of the modulators tested were able to increase gem formation, a marker for SMN protein in the nucleus, in a dose-dependent manner. We then derived two possible mathematical models simulating the regulation of SMN2 expression by cAMP signaling. Both models fit well with our experimental data. In silico treatment of SMA fibroblasts simultaneously with two different cAMP modulators resulted in an additive increase in gem formation. This study shows how a systems biology approach can be used to develop potential therapeutic targets for treating SMA.

Spinal muscular atrophy: diagnosis and management in a new therapeutic era

Spinal muscular atrophy (SMA) describes a group of disorders associated with spinal motor neuron loss. In this review we provide an update regarding the most common form of SMA, proximal or 5q-SMA, and discuss the contemporary approach to diagnosis and treatment. Electromyography and muscle biopsy features of denervation were once the basis for diagnosis, but molecular testing for homozygous deletion or mutation of the SMN1 gene allows efficient and specific diagnosis. In combination with loss of SMN1, patients retain variable numbers of copies of a second similar gene, SMN2, which produces reduced levels of the survival motor neuron (SMN) protein that are insufficient for normal motor neuron function. Despite the fact that understanding of how ubiquitous reduction of SMN protein leads to motor neuron loss remains incomplete, several promising therapeutics are now being tested in early-phase clinical trials.

Motor Unit Number Estimation in Neuromuscular Disease

Motor unit number estimation (MUNE) is an electrophysiological method designed to quantify motor unit loss in target muscles of interest. Most of the techniques are noninvasive and are therefore well suited for longitudinal monitoring. In this brief review, we describe the more commonly used techniques and their applications in amyotrophic lateral sclerosis, poliomyelitis, spinal muscular atrophy and hereditary sensorimotor neuropathies. Findings in some of these studies offer important pathophysiological insights. Since conventional electrophysiologic methods are not sensible measures of motor neuronal loss, MUNE could play a potentially important role in the diagnosis, monitoring of disease progression and response to treatment in neuromuscular diseases in which motor unit loss is a major feature.

Reliability and validity of outcome measures of in-hospital and at-home visits in a randomized, double-blind, placebo-controlled trial for spinal muscular atrophy

We used at-home assessments in a clinical trial to relieve the visit burden for participants. A total of 57 patients with type II or III spinal muscular atrophy were enrolled and 10 of them (7 type II and 3 type III) received at-home assessments. The primary end points were Gross Motor Function Measure, Manual Muscle Test, and serum biomarker. The secondary endpoints were Modified Hammersmith Functional Motor Scale and forced vital capacity. The correlation coefficients and analysis of covariance showed good reliability and validity of all outcome measures. Except for Gross Motor Function Measure and Modified Hammersmith Functional Motor Scale, there were no significant differences in measures between in-hospital and at-home groups (intersubject) or among 3 patients who received both at-home and in-hospital visits (intrasubject). We concluded that at-home assessments could provide sufficient reliability in a controlled trial. This modification could help design a successful clinical trial for spinal muscular atrophy.

Subcellular transcriptome alterations in a cell culture model of spinal muscular atrophy point to widespread defects in axonal growth and presynaptic differentiation

Neuronal function critically depends on coordinated subcellular distribution of mRNAs. Disturbed mRNA processing and axonal transport has been found in spinal muscular atrophy and could be causative for dysfunction and degeneration of motoneurons. Despite the advances made in characterizing the transport mechanisms of several axonal mRNAs, an unbiased approach to identify the axonal repertoire of mRNAs in healthy and degenerating motoneurons has been lacking. Here we used compartmentalized microfluidic chambers to investigate the somatodendritic and axonal mRNA content of cultured motoneurons by microarray analysis. In axons, transcripts related to protein synthesis and energy production were enriched relative to the somatodendritic compartment. Knockdown of Smn, the protein deficient in spinal muscular atrophy, produced a large number of transcript alterations in both compartments. Transcripts related to immune functions, including MHC class I genes, and with roles in RNA splicing were up-regulated in the somatodendritic compartment. On the axonal side, transcripts associated with axon growth and synaptic activity were down-regulated. These alterations provide evidence that subcellular localization of transcripts with axonal functions as well as regulation of specific transcripts with nonautonomous functions is disturbed in Smn-deficient motoneurons, most likely contributing to the pathophysiology of spinal muscular atrophy.

Presynaptic localization of Smn and hnRNP R in axon terminals of embryonic and postnatal mouse motoneurons

Spinal muscular atrophy (SMA) is caused by deficiency of the ubiquitously expressed survival motoneuron (SMN) protein. SMN is crucial component of a complex for the assembly of spliceosomal small nuclear ribonucleoprotein (snRNP) particles. Other cellular functions of SMN are less characterized so far. SMA predominantly affects lower motoneurons, but the cellular basis for this relative specificity is still unknown. In contrast to nonneuronal cells where the protein is mainly localized in perinuclear regions and the nucleus, Smn is also present in dendrites, axons and axonal growth cones of isolated motoneurons invitro. However, this distribution has not been shown invivo and it is not clear whether Smn and hnRNP R are also present in presynaptic axon terminals of motoneurons in postnatal mice. Smn also associates with components not included in the classical SMN complex like RNA-binding proteins FUS, TDP43, HuD and hnRNP R which are involved in RNA processing, subcellular localization and translation. We show here that Smn and hnRNP R are present in presynaptic compartments at neuromuscular endplates of embryonic and postnatal mice. Smn and hnRNP R are localized in close proximity to each other in axons and axon terminals both invitro and invivo. We also provide new evidence for a direct interaction of Smn and hnRNP R invitro and invivo, particularly in the cytosol of motoneurons. These data point to functions of SMN beyond snRNP assembly which could be crucial for recruitment and transport of RNA particles into axons and axon terminals, a mechanism which may contribute to SMA pathogenesis.

Transcriptome profiling of spinal muscular atrophy motor neurons derived from mouse embryonic stem cells

Proximal spinal muscular atrophy (SMA) is an early onset, autosomal recessive motor neuron disease caused by loss of or mutation in SMN1 (survival motor neuron 1). Despite understanding the genetic basis underlying this disease, it is still not known why motor neurons (MNs) are selectively affected by the loss of the ubiquitously expressed SMN protein. Using a mouse embryonic stem cell (mESC) model for severe SMA, the RNA transcript profiles (transcriptomes) between control and severe SMA (SMN2^+/+;mSmn^−/−) mESC-derived MNs were compared in this study using massively parallel RNA sequencing (RNA-Seq). The MN differentiation efficiencies between control and severe SMA mESCs were similar. RNA-Seq analysis identified 3,094 upregulated and 6,964 downregulated transcripts in SMA mESC-derived MNs when compared against control cells. Pathway and network analysis of the differentially expressed RNA transcripts showed that pluripotency and cell proliferation transcripts were significantly increased in SMA MNs while transcripts related to neuronal development and activity were reduced. The differential expression of selected transcripts such as Crabp1, Crabp2 and Nkx2.2 was validated in a second mESC model for SMA as well as in the spinal cords of low copy SMN2 severe SMA mice. Furthermore, the levels of these selected transcripts were restored in high copy SMN2 rescue mouse spinal cords when compared against low copy SMN2 severe SMA mice. These findings suggest that SMN deficiency affects processes critical for normal development and maintenance of MNs.

Drug Discovery Approaches for Rare Neuromuscular Diseases

Rare neuromuscular diseases encompass many diverse and debilitating musculoskeletal disorders, ranging from ultra-orphan conditions that affect only a few families, to the so-called ‘common’ orphan diseases like Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA), which affect several thousand individuals worldwide. Increasingly, pharmaceutical and biotechnology companies, in an effort to improve productivity and rebuild dwindling pipelines, are shifting their business models away from the formerly popular ‘blockbuster’ strategy, with rare diseases being an area of increased focus in recent years. As a consequence of this paradigm shift, coupled with high-profile campaigns by not-for-profit organisations and patient advocacy groups, rare neuromuscular diseases are attracting considerable attention as new therapeutic areas for improved drug therapy. Much pioneering work has taken place to elucidate the underlying pathological mechanisms of many rare neuromuscular diseases. This, in conjunction with the availability of new screening technologies, has inspired the development of several truly innovative therapeutic strategies aimed at correcting the underlying pathology. A survey of medicinal chemistry approaches and the resulting clinical progress for new therapeutic agents targeting this devastating class of degenerative diseases is presented, using DMD and SMA as examples. Complementary strategies using small-molecule drugs and biological agents are included.

Observational study of spinal muscular atrophy type I and implications for clinical trials

OBJECTIVES

Prospective cohort study to characterize the clinical features and course of spinal muscular atrophy type I (SMA-I).

METHODS

Patients were enrolled at 3 study sites and followed for up to 36 months with serial clinical, motor function, laboratory, and electrophysiologic outcome assessments. Intervention was determined by published standard of care guidelines. Palliative care options were offered.

RESULTS

Thirty-four of 54 eligible subjects with SMA-I (63%) enrolled and 50% of these completed at least 12 months of follow-up. The median age at reaching the combined endpoint of death or requiring at least 16 hours/day of ventilation support was 13.5 months (interquartile range 8.1-22.0 months). Requirement for nutritional support preceded that for ventilation support. The distribution of age at reaching the combined endpoint was similar for subjects with SMA-I who had symptom onset before 3 months and after 3 months of age (p=0.58). Having 2 SMN2 copies was associated with greater morbidity and mortality than having 3 copies. Baseline electrophysiologic measures indicated substantial motor neuron loss. By comparison, subjects with SMA-II who lost sitting ability (n=10) had higher motor function, motor unit number estimate and compound motor action potential, longer survival, and later age when feeding or ventilation support was required. The mean rate of decline in The Children's Hospital of Philadelphia Infant Test for Neuromuscular Disorders motor function scale was 1.27 points/year (95% confidence interval 0.21-2.33, p=0.02).

CONCLUSIONS

Infants with SMA-I can be effectively enrolled and retained in a 12-month natural history study until a majority reach the combined endpoint. These outcome data can be used for clinical trial design.

SMA valiant trial: a prospective, double-blind, placebo-controlled trial of valproic acid in ambulatory adults with spinal muscular atrophy

INTRODUCTION

An open-label trial suggested that valproic acid (VPA) improved strength in adults with spinal muscular atrophy (SMA). We report a 12-month, double-blind, cross-over study of VPA in ambulatory SMA adults.

METHODS

There were 33 subjects, aged 20–55 years, included in this investigation. After baseline assessment, subjects were randomized to receive VPA (10–20 mg/kg/day) or placebo. At 6 months, patients were switched to the other group. Assessments were performed at 3, 6, and 12 months. The primary outcome was the 6-month change in maximum voluntary isometric contraction testing with pulmonary, electrophysiological, and functional secondary outcomes.

RESULTS

Thirty subjects completed the study. VPA was well tolerated, and compliance was good. There was no change in primary or secondary outcomes at 6 or 12 months.

CONCLUSIONS

VPA did not improve strength or function in SMA adults. The outcomes used are feasible and reliable and can be employed in future trials in SMA adults.

The motor neuron response to SMN1 deficiency in spinal muscular atrophy

INTRODUCTION

The purpose of this study was to measure and analyze motor unit number estimation (MUNE) values longitudinally in spinal muscular atrophy (SMA).

METHODS

Sixty-two children with SMA types 2 and 3 were observed prospectively for up to 42 months. Longitudinal electrophysiological data were collected, including compound motor action potential (CMAP), single motor unit action potential (SMUP), and MUNE.

RESULTS

Significant motor neuron loss and compensatory collateral reinnervation were noted at baseline. Over time, there was a significant mean increase in MUNE (4.92 units/year, P = 0.009), a mean decrease in SMUP amplitude (-6.32 μV/year, P = 0.10), and stable CMAP amplitude.

CONCLUSIONS

The unexpected longitudinal results differ from findings in amyotrophic lateral sclerosis studies, perhaps indicating that compensatory processes in SMA involve new motor unit development. A better understanding of the mechanisms of motor unit decline and compensation in SMA is important for assessing novel therapeutic strategies and for providing key insights into disease pathophysiology.

SMN-targeted therapeutics for spinal muscular atrophy: are we SMArt enough yet?

Spinal muscular atrophy (SMA) remains one of the most common and lethal autosomal recessive diseases. Homozygous deletion of survival of motor neuron 1 (SMN1) and resulting SMN protein deficiency manifests predominantly with motor neuron degeneration; however, a wealth of emerging data supports a broader influence of SMN deficiency in disease pathogenesis. In this issue of the JCI, Kariya and colleagues demonstrate the relatively selective impact of SMN depletion on the distal motor unit using a series of SMN2-expressing transgenic mice in which constitutive SMN knockdown follows variable periods of normal development. Their observations provide further insights regarding the temporal requirements for SMN in mice, renewing speculation about when and where repletion of SMN is necessary for optimal outcomes in SMA patients.

Requirement of enhanced Survival Motoneuron protein imposed during neuromuscular junction maturation

Spinal muscular atrophy is a common motor neuron disease caused by low survival motoneuron (SMN), a key protein in the proper splicing of genes. Restoring the protein is therefore a promising therapeutic strategy. Implementation of this strategy, however, depends on defining the temporal requirements for SMN. Here, we used controlled knockdown of SMN in transgenic mice to determine the precise postnatal stage requirements for this protein. Reducing SMN in neonatal mice resulted in a classic SMA-like phenotype. Unexpectedly, depletion of SMN in adults had relatively little effect. Insensitivity to low SMN emerged abruptly at postnatal day 17, which coincided with establishment of the fully mature neuromuscular junction (NMJ). Mature animals depleted of SMN eventually exhibited evidence of selective neuromuscular pathology that was made worse by traumatic injury. The ability to regenerate the mature NMJ in aged or injured SMN-depleted mice was grossly impaired, a likely consequence of the inability to meet the surge in demand for motoneuronal SMN that was seen in controls. Our results demonstrate that relative maturity of the NMJ determines the temporal requirement for the SMN protein. These observations suggest that the use of potent but potentially deleterious SMN-enhancing agents could be tapered in human patients once the neuromuscular system matures and reintroduced as needed to enhance SMN for remodeling aged or injured NMJs.

Antisense oligonucleotides for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive disease affecting ∼1 in 10,000 live births. The most striking component is the loss of α-motor neurons in the ventral horn of the spinal cord, resulting in progressive paralysis and eventually premature death. There is no current treatment paradigm other than supportive care, though the past 15 years has seen a striking advancement in understanding of both SMA genetics and molecular mechanisms. A variety of disease-modifying interventions are rapidly bridging the translational gap from the laboratory to clinical trials, including the application of antisense oligonucleotide (ASO) therapy for the correction of aberrant RNA splicing characteristic of SMA. Survival motor neuron (SMN) is a ubiquitously expressed 38-kD protein. Humans have two genes that produce SMN, SMN1 and SMN2, the former of which is deleted or nonfunctional in the majority of patients with SMA. These two genes are nearly identical with one exception, a C to T transition (C6T) within exon 7 of SMN2. C6T disrupts a modulator of splicing, leading to the exclusion of exon 7 from ∼90% of the mRNA transcript. The resultant truncated Δ7SMN protein does not oligomerize efficiently and is rapidly degraded. SMA can therefore be considered a disease of too little SMN protein. A number of cis-acting splice modifiers have been identified in the region of exon 7, the steric block of which enhances the retention of the exon and a resultant full-length mRNA sequence. ASOs targeted to these splice motifs have shown impressive phenotype rescue in multiple SMA mouse models.

Molecular therapeutic strategies for spinal muscular atrophies: current and future clinical trials

BACKGROUND

Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease caused by mutations in the survival motor neuron gene (SMN1) and the leading genetic cause of infant mortality. Currently, there is no effective treatment other than supportive care.

OBJECTIVE

This article provides a general overview of the main aspects that need to be taken into account to design a more efficient clinical trial and to summarize the most promising molecular trials that are currently in development or are being planned for the treatment of SMA.

METHODS

A systematic review of the literature was performed, identifying key clinical trials involving novel molecular therapies in SMA. In addition, abstracts presented at the meetings of the Families of Spinal Muscular Atrophy were searched and the Families of Spinal Muscular Atrophy Web site was carefully analyzed. Finally, a selection of SMA clinical trials registered at clinical-trials.gov has been included in the article.

RESULTS

The past decade has seen a marked advancement in the understanding of both SMA genetics and molecular mechanisms. New molecules targeting SMN have shown promise in preclinical studies, and various clinical trials have started to test the drugs that were discovered through basic research.

CONCLUSIONS

Both preclinical and early clinical trial results involving novel molecular therapies suggest that the clinical care paradigm in SMA will soon change.

Spinal muscular atrophy: development and implementation of potential treatments

In neurodegenerative disorders, effective treatments are urgently needed, along with methods to determine whether treatment worked. In this review, we discuss the rapid progress in the understanding of recessive proximal spinal muscular atrophy and how this is leading to exciting potential treatments of the disease. Spinal muscular atrophy is caused by loss of the survival motor neuron 1 (SMN1) gene and reduced levels of SMN protein. The critical downstream targets of SMN deficiency that result in motor neuron loss are not known. However, increasing SMN levels has a marked impact in mouse models, and these therapeutics are rapidly moving toward clinical trials. Promising preclinical therapies, the varying degree of impact on the mouse models, and potential measures of treatment effect are reviewed. One key issue discussed is the variable outcome of increasing SMN at different stages of disease progression.

Electrophysiological Biomarkers in Spinal Muscular Atrophy: Preclinical Proof of Concept

Objective

Preclinical therapies that restore survival motor neuron (SMN) protein levels can dramatically extend survival in spinal muscular atrophy (SMA) mouse models. Biomarkers are needed to effectively translate these promising therapies to clinical trials. Our objective was to investigate electrophysiological biomarkers of compound muscle action potential (CMAP), motor unit number estimation (MUNE) and electromyography (EMG) using an SMA mouse model.

Methods

Sciatic CMAP, MUNE, and EMG were obtained in SMNΔ7 mice at ages 3–13 days and at 21 days in mice with SMN selectively reduced in motor neurons (ChAT^Cre). To investigate these measures as biomarkers of treatment response, measurements were obtained in SMNΔ7 mice treated with antisense oligonucleotide (ASO) or gene therapy.

Results

CMAP was significantly reduced in SMNΔ7 mice at days 6–13 (P < 0.01), and MUNE was reduced at days 7–13 (P < 0.01). Fibrillations were present on EMG in SMNΔ7 mice but not controls (P = 0.02). Similar findings were seen at 21 days in ChAT^Cre mice. MUNE in ASO-treated SMNΔ7 mice were similar to controls at day 12 and 30. CMAP reduction persisted in ASO-treated SMNΔ7 mice at day 12 but was corrected at day 30. Similarly, CMAP and MUNE responses were corrected with gene therapy to restore SMN.

Interpretation

These studies confirm features of preserved neuromuscular function in the early postnatal period and subsequent motor unit loss in SMNΔ7 mice. SMN restoring therapies result in preserved MUNE and gradual repair of CMAP responses. This provides preclinical evidence for the utilization of CMAP and MUNE as biomarkers in future SMA clinical trials.

Current advances in drug development in spinal muscular atrophy

PURPOSE OF REVIEW

Spinal muscular atrophy (SMA) is a pediatric neuromuscular condition characterized by progressive proximal muscle weakness. It is one of the most common genetic causes of infant mortality across different races and is caused by mutation of the survival of motor neuron 1 (SMN1) gene on chromosome 5q13.

RECENT FINDINGS

To date, there have been many therapeutics developments for SMA targeting various potential pathways such as increasing SMN gene expression, enhancing SMN2 exon 7 inclusion, neuroprotection, cell replacement, and gene therapy.

SUMMARY

Although SMA remains an incurable disease to date, recent advances in the field of basic and translational research have enhanced our understanding of the pathogenesis of the disease and opened new possibilities for therapeutic intervention. This article reviews and highlights past and current translational research on SMA therapeutics.

Label-free proteomics identifies Calreticulin and GRP75/Mortalin as peripherally accessible protein biomarkers for spinal muscular atrophy

BACKGROUND

Spinal muscular atrophy (SMA) is a neuromuscular disease resulting from mutations in the survival motor neuron 1 (SMN1) gene. Recent breakthroughs in preclinical research have highlighted several potential novel therapies for SMA, increasing the need for robust and sensitive clinical trial platforms for evaluating their effectiveness in human patient cohorts. Given that most clinical trials for SMA are likely to involve young children, there is a need for validated molecular biomarkers to assist with monitoring disease progression and establishing the effectiveness of therapies being tested. Proteomics technologies have recently been highlighted as a potentially powerful tool for such biomarker discovery.

METHODS

We utilized label-free proteomics to identify individual proteins in pathologically-affected skeletal muscle from SMA mice that report directly on disease status. Quantitative fluorescent western blotting was then used to assess whether protein biomarkers were robustly changed in muscle, skin and blood from another mouse model of SMA, as well as in a small cohort of human SMA patient muscle biopsies.

RESULTS

By comparing the protein composition of skeletal muscle in SMA mice at a pre-symptomatic time-point with the muscle proteome at a late-symptomatic time-point we identified increased expression of both Calreticulin and GRP75/Mortalin as robust indicators of disease progression in SMA mice. We report that these protein biomarkers were consistently modified in different mouse models of SMA, as well as across multiple skeletal muscles, and were also measurable in skin biopsies. Furthermore, Calreticulin and GRP75/Mortalin were measurable in muscle biopsy samples from human SMA patients.

CONCLUSIONS

We conclude that label-free proteomics technology provides a powerful platform for biomarker identification in SMA, revealing Calreticulin and GRP75/Mortalin as peripherally accessible protein biomarkers capable of reporting on disease progression in samples of muscle and skin.

Solving the puzzle of spinal muscular atrophy: what are the missing pieces?

Spinal muscular atrophy (SMA) is an autosomal recessive, lower motor neuron disease. Clinical heterogeneity is pervasive: three infantile (type I-III) and one adult-onset (type IV) forms are recognized. Type I SMA is the most common genetic cause of death in infancy and accounts for about 50% of all patients with SMA. Most forms of SMA are caused by mutations of the survival motor neuron (SMN1) gene. A second gene that is 99% identical to SMN1 (SMN2) is located in the same region. The only functionally relevant difference between the two genes identified to date is a C → T transition in exon 7 of SMN2, which determines an alternative spliced isoform that predominantly excludes exon 7. Thus, SMN2 genes do not produce sufficient full length SMN protein to prevent the onset of the disease. Since the identification of the causative mutation, biomedical research of SMA has progressed by leaps and bounds: from clues on the function of SMN protein, to the development of different models of the disease, to the identification of potential treatments, some of which are currently in human trials. The aim of this review is to elucidate the current state of knowledge, emphasizing how close we are to the solution of the puzzle that is SMA, and, more importantly, to highlight the missing pieces of this puzzle. Filling in these gaps in our knowledge will likely accelerate the development and delivery of efficient treatments for SMA patients and be a prerequisite towards achieving our final goal, the cure of SMA.

Small molecule suppressors of Drosophila kinesin deficiency rescue motor axon development in a zebrafish model of spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is the most common inherited motor neuropathy and the leading hereditary cause of infant mortality. Currently there is no effective treatment for the disease, reflecting a need for pharmacologic interventions that restore performance of dysfunctional motor neurons or suppress the consequences of their dysfunction. In a series of assays relevant to motor neuron biology, we explored the activities of a collection of tetrahydroindoles that were reported to alter the metabolism of amyloid precursor protein (APP). In Drosophila larvae the compounds suppressed aberrant larval locomotion due to mutations in the Khc and Klc genes, which respectively encode the heavy and light chains of kinesin-1. A representative compound of this class also suppressed the appearance of axonal swellings (alternatively termed axonal spheroids or neuritic beads) in the segmental nerves of the kinesin-deficient Drosophila larvae. Given the importance of kinesin-dependent transport for extension and maintenance of axons and their growth cones, three members of the class were tested for neurotrophic effects on isolated rat spinal motor neurons. Each compound stimulated neurite outgrowth. In addition, consistent with SMA being an axonopathy of motor neurons, the three axonotrophic compounds rescued motor axon development in a zebrafish model of SMA. The results introduce a collection of small molecules as pharmacologic suppressors of SMA-associated phenotypes and nominate specific members of the collection for development as candidate SMA therapeutics. More generally, the results reinforce the perception of SMA as an axonopathy and suggest novel approaches to treating the disease.

Spinal muscular atrophy: from gene discovery to clinical trials

Spinal muscular atrophy (SMA) is a common neuromuscular disorder with autosomal recessive inheritance, resulting in the degeneration of motor neurons. The incidence of the disease has been estimated at 1 in 6000-10,000 newborns with a carrier frequency of 1 in 40-60. SMA is caused by mutations of the SMN1 gene, located on chromosome 5q13. The gene product, survival motor neuron (SMN) plays critical roles in a variety of cellular activities. SMN2, a homologue of SMN1, is retained in all SMA patients and generates low levels of SMN, but does not compensate for the mutated SMN1. Genetic analysis demonstrates the presence of homozygous deletion of SMN1 in most patients, and allows screening of heterozygous carriers in affected families. Considering high incidence of carrier frequency in SMA, population-wide newborn and carrier screening has been proposed. Although no effective treatment is currently available, some treatment strategies have already been developed based on the molecular pathophysiology of this disease. Current treatment strategies can be classified into three major groups: SMN2-targeting, SMN1-introduction, and non-SMN targeting. Here, we provide a comprehensive and up-to-date review integrating advances in molecular pathophysiology and diagnostic testing with therapeutic developments for this disease including promising candidates from recent clinical trials.

Histologic and transcriptional assessment of a mild SMA model

Spinal muscular atrophy (SMA) is caused by survival of motor neuron (SMN) deficiency, leading to specific motor neuron attrition. The time course and molecular pathophysiologic etiology of motor neuron loss observed in SMA remains obscure. Mice heterozygous for Smn show up to 50% motor neuron attrition by 6 months of age and are used as a model for mild SMA in humans. To determine both the rate of cellular loss and the molecular events underlying motor neuron degeneration in SMA, motor neuron counts and mRNA quantification were performed in spinal cords of Smn(+/-) mice and wild-type littermates. Surprisingly, despite the chronic, subclinical nature of motor neuron loss, we find that the bulk of the loss occurs by 5 weeks of age. RNA isolated from the spinal cords of 5 week-old Smn(+/-) mice subjected to microarray analysis reveal alterations in genes involved in RNA metabolism, apoptosis and transcriptional regulation including a general perturbation of transcripts coding for calcium binding proteins. A subset of these changes in expression was further characterized by semi-quantitative RT-PCR and Western blot analysis at various time points. Taken together, these results indicate that spinal cord cells present the first signs of the apoptotic process consistent with a response to the stress of Smn depletion. A picture of comparatively rapid neuronal attrition in spite of the very mild nature of SMA is obtained. Furthermore, changes occur, which may be reactive to and not causative of the cellular loss, involving central cellular functions as well as calcium modulating proteins.

Aberrant splicing in neurological diseases

Splicing of precursor messenger RNA (pre-mRNA) removes the intervening sequences (introns) and joins the expressed regions (exons) in the nucleus, before an intron-containing eukaryotic mRNA transcript can be exported and translated into proteins in the cytoplasm. While some sequences are always included or excluded (constitutive splicing), others can be selectively used (alternative splicing) in this process. Particularly by alternative splicing, up to tens of thousands of variant transcripts can be produced from a single gene, which contributes greatly to the proteomic diversity for such complex cellular functions as 'wiring' neurons in the nervous system. Disruption of this process leads to aberrant splicing, which accounts for the defects of up to 50% of mutations that cause certain human genetic diseases. In this review, we describe the different mechanisms of aberrant splicing that cause or have been associated with neurological diseases.

Recent developments in the treatment of Duchenne muscular dystrophy and spinal muscular atrophy

Pediatric neuromuscular disorders comprise a large variety of disorders that can be classified based on their neuroanatomical localization, patterns of weakness, and laboratory test results. Over the last decade, the field of translational research has been active with many ongoing clinical trials. This is particularly so in two common pediatric neuromuscular disorders: Duchenne muscular dystrophy and spinal muscular atrophy. Although no definitive therapy has yet been found, numerous active areas of research raise the potential for novel therapies in these two disorders, offering hope for improved quality of life and life expectancy for affected individuals.

Survival of patients with spinal muscular atrophy type 1

BACKGROUND

Spinal muscular atrophy type 1 (SMA1) is a progressive disease and is usually fatal in the first year of life.

METHODS

A retrospective chart review was performed of SMA1 patients and their outcomes according to the following choices: letting nature take its course (NT); tracheostomy and invasive mechanical ventilation (TV); continuous noninvasive respiratory muscle aid (NRA), including noninvasive ventilation; and mechanically assisted cough.

RESULTS

Of 194 consecutively referred patients enrolled in this study (103 males, 91 females), NT, TV, and NRA were chosen for 121 (62.3%), 42 (21.7%), and 31 (16%) patients, respectively. Survival at ages 24 and 48 months was higher in TV than NRA users: 95% (95% confidence interval: 81.8%-98.8%) and 67.7% (95% confidence interval: 46.7%-82%) at age 24 months (P < .001) and 89.43% and 45% at age 48 months in the TV and NRA groups, respectively (P < .001). The choice of TV decreased from 50% (1992-1998) to 12.7% (2005-2010) (P < .005) with a nonstatistically significant increase for NT from 50% to 65%. The choice of NRA increased from 8.1% (1999-2004) to 22.7% (2005-2010) (P < .001).

CONCLUSIONS

Long-term survival outcome is determined by the choice of the treatment. NRA and TV can prolong survival, with NRA showing a lower survival probability at ages 24 and 48 months.

Type III spinal muscular atrophy mimicking muscular dystrophies

Types III and IV spinal muscular atrophy represent a diagnostic challenge due to the great variability in their presentation. We report a series of eight patients with type III spinal muscular atrophy who were followed for a long time for possible muscular dystrophy or myopathy, confirming its clinical heterogeneity and propensity to delayed diagnosis. Clinical examination revealed heterogeneous findings, where the diagnosis of type III spinal muscular atrophy was not immediately apparent in many patients as their clinical and laboratory abnormalities were consistent with muscular dystrophy or myopathy. The presence of dystrophic features such as hypertrophy of the calves, weakness of the limb girdle, high serum creatine kinase levels, and myopathic histopathology should not divert attention from the possibility of spinal muscular atrophy. It is strongly recommended to give variable presentations enough thought and to consider the autosomal recessive type III spinal muscular atrophy in the diagnostic evaluation.

SMA-EUROPE workshop report: Opportunities and challenges in developing clinical trials for spinal muscular atrophy in Europe

Spinal muscular atrophy (SMA) is the most common lethal recessive disease in childhood, and there is currently no effective treatment to halt disease progression. The translation of scientific advances into effective therapies is hampered by major roadblocks in clinical trials, including the complex regulatory environment in Europe, variations in standards of care, patient ascertainment and enrolment, a narrow therapeutic window and a lack of biomarkers of efficacy. In this context, SMA-Europe organized its first international workshop in July 2012 in Rome, gathering 34 scientists, clinicians and representatives of patient organizations to establish recommendations for improving clinical trials for SMAa.

Neurotransmitter release in motor nerve terminals of a mouse model of mild spinal muscular atrophy

Spinal muscular atrophy is a genetic disease which severity depends on the amount of SMN protein, the product of the genes SMN1 and SMN2. Symptomatology goes from severe neuromuscular impairment leading to early death in infants to slow progressing motor deficits during adulthood. Much of the knowledge about the pathophysiology of SMA comes from studies using genetically engineered animal models of the disease. Here we investigated one of the milder models, the homozygous A2G SMA mice, in which the level of the protein is restored to almost normal levels by the addition of a mutated transgene to the severe SMN-deficient background. We examined neuromuscular function and found that calcium-dependent neurotransmitter release was significantly decreased. In addition, the amplitude of spontaneous endplate potentials was decreased, the morphology of NMJ altered, and slight changes in short-term synaptic plasticity were found. In spite of these defects, excitation contraction coupling was well preserved, possibly due to the safety factor of this synapse. These data further support that the quasi-normal restoration of SMN levels in severe cases preserves neuromuscular function, even when neurotransmitter release is significantly decreased at motor nerve terminals. Nevertheless, this deficit could represent a greater risk of motor impairment during aging or after injuries.

Spinal muscular atrophy due to a "de novo" 1.3 Mb deletion: implication for genetic counseling

We report a 3-year-old female with type I spinal muscular atrophy (SMA) born to a young and non-consanguineous couple. The child presented at two months of life with intense muscle weakness affecting predominantly proximal portions of the limbs, especially the legs, muscle hypotonia, fasciculation of the tongue, and severe respiratory muscle involvement. She remained in an intensive care unit with an assisted ventilation system from the fourth month of life. She died at 3 years of age from pulmonary infection. Molecular analysis confirmed the diagnosis of SMA but revealed that only the father was an asymptomatic carrier. Because SMN1 is mapped in a complex region containing repetitive elements due to an inverted duplication of approximately 500 kb, we carry out an SNP array and detected a 1.3 Mb deletion including the SMN1 and SMN2 genes that explain the disease.

Public attitudes regarding a pilot study of newborn screening for spinal muscular atrophy

A population-based pilot study of newborns screening for a rare genetic condition, spinal muscular atrophy (SMA), is being conducted with funding from the National Institutes of Health. The first component of the study is to assess the ethical, legal, and social implications of population-based pilot studies with a focus on public engagement and parental decision-making for the proposed opt-out approach in this research. We conducted focus groups with members of the general public to ascertain attitudes about the pilot study and acceptability of an opt-out approach in two states, Colorado and Utah, where the pilot screening is being proposed (N = 70). We developed an informational video for the project and showed it to the groups prior to the discussion in order to inform participants about population-based research, newborn screening (NBS), permission/consent models, and SMA. Results indicated support for the conduct of pilot studies that is consistent with the current standard of practice for similar population-based programs. There was support for an opt-out approach for parental decision-making; however there was limited parental knowledge about population-based research, NBS and SMA. In general, our participants considered this pilot study to be low risk and of potential benefit to infants and families. The majority of participants were supportive of an opt-out approach with information delivered through various avenues

Synaptic defects in type I spinal muscular atrophy in human development

Childhood spinal muscular atrophy is an autosomal recessive neuromuscular disorder caused by alterations in the Survival Motor Neuron 1 gene that triggers degeneration of motor neurons within the spinal cord. Spinal muscular atrophy is the second most common severe hereditary disease of infancy and early childhood. In the most severe cases (type I), the disease appears in the first months of life, suggesting defects in fetal development. However, it is not yet known how motor neurons, neuromuscular junctions, and muscle interact in the neuropathology of the disease. We report the structure of presynaptic and postsynaptic apparatus of the neuromuscular junctions in control and spinal muscular atrophy prenatal and postnatal human samples. Qualitative and quantitative data from confocal and electron microscopy studies revealed changes in acetylcholine receptor clustering, abnormal preterminal accumulation of vesicles, and aberrant ultrastructure of nerve terminals in the motor endplates of prenatal type I spinal muscular atrophy samples. Fetuses predicted to develop milder type II disease had a similar appearance to controls. Postnatal muscle of type I spinal muscular atrophy patients showed persistence of the fetal subunit of acetylcholine receptors, suggesting a delay in maturation of neuromuscular junctions. We observed that pathology in the severe form of the disease starts in fetal development and that a defect in maintaining the initial innervation is an early finding of neuromuscular dysfunction. These results will improve our understanding of the spinal muscular atrophy pathogenesis and help to define targets for possible presymptomatic therapy for this disease.

Pathophysiological insights derived by natural history and motor function of spinal muscular atrophy

OBJECTIVE

To examine the natural history of spinal muscular atrophy (SMA) to gain further insight into the clinical course and pathogenesis.

STUDY DESIGN

Survival pattern, age of onset, and ambulatory status were retrospectively analyzed in 70 patients with SMA with deletions of the survival motor neuron 1 genes that presented to a specialized neuromuscular clinic. The Kaplan-Meier method was used to obtain survival curves. Hammersmith Functional Motor Scale-Expanded and abductor pollicis brevis compound muscle action potential amplitudes were assessed in 25 of the surviving cohort and correlated with survival motor neuron 2 copy number.

RESULTS

Survival probabilities at ages 1, 2, 4, 10, 20, and 40 years were 40%, 25%, 6%, and 0%, respectively, for patients with SMA type 1; 100%, 100%, 97%, 93%, 93%, and 52% for patients with SMA type 2 and all patients with SMA type 3 were alive (age range 7-33 years). There were significant associations between age of onset and long-term outcome, specifically survival in SMA type 1 (P < .01) and Hammersmith Functional Motor Scale-Expanded (P < .0001), and compound muscle action potential (P = .001) in SMA types 2 and 3. Motor function in patients with long-standing SMA reduced over prolonged periods or remained stable. Survival motor neuron 2 copy number related to continuing changes in motor function with age.

CONCLUSION

The natural history of SMA suggests considerable early loss of motor neurons, with severity related to differences in the number of remaining motor neurons. As the ensuing chronic course in milder phenotypes suggests relative stability of remaining motor neurons, the maximal therapeutic window presents early.

SMN is required for sensory-motor circuit function in Drosophila

Spinal muscular atrophy (SMA) is a lethal human disease characterized by motor neuron dysfunction and muscle deterioration due to depletion of the ubiquitous survival motor neuron (SMN) protein. Drosophila SMN mutants have reduced muscle size and defective locomotion, motor rhythm, and motor neuron neurotransmission. Unexpectedly, restoration of SMN in either muscles or motor neurons did not alter these phenotypes. Instead, SMN must be expressed in proprioceptive neurons and interneurons in the motor circuit to nonautonomously correct defects in motor neurons and muscles. SMN depletion disrupts the motor system subsequent to circuit development and can be mimicked by the inhibition of motor network function. Furthermore, increasing motor circuit excitability by genetic or pharmacological inhibition of K(+) channels can correct SMN-dependent phenotypes. These results establish sensory-motor circuit dysfunction as the origin of motor system deficits in this SMA model and suggest that enhancement of motor neural network activity could ameliorate the disease.