Correlation of PLS3 expression with disease severity in children with spinal muscular atrophy

Analysis of SMN protein in umbilical cord blood and postnatal peripheral blood of neonates with SMA: a rationale for prompt treatment initiation to prevent SMA development

BACKGROUND

Spinal muscular atrophy (SMA) is a severe genetic neuromuscular disease caused by insufficient functional survival motor neuron protein (SMN). The SMN expression level in the spinal cord is highest during the 2nd trimester of the foetal period. We previously reported the SMN spot analysis in peripheral blood using imaging flow cytometry (IFC) as a biomarker of functional SMN protein expression. In this study, we analysed neonatal cord blood, postnatal peripheral blood, and maternal peripheral blood in presymptomatic five infants whose sibling has type 1 SMA to estimate prenatal and postnatal SMN dynamics before the onset of severe SMA.

RESULTS

Data from 37 untreated patients with SMA showed that SMN-spot+ cells were significantly correlated with SMA clinical classification and the copy numbers of the SMN2 gene. The range of values for cord blood, converted from each SMN2 copy number statistics, was - 0.7 to + 2.0 standard deviation (SD) (0.1-24.0%) for SMN-spot+ cells in patients with SMA. Subsequent analyses of the peripheral blood of neonates ranged from - 0.8 to + 0.8 SD (0.4-15.2%). The analysis of each maternal blood, converted from carrier statistics, ranged from - 0.2 to + 2.4 SD (1.4-25.2%). A correlation was observed between the cord blood and maternal peripheral blood.

CONCLUSIONS

This study suggests that the status of the motor neuron pool in the spinal cord can be presumed by cord blood SMN-spot+ cells and that SMN protein depletion determines the timing of disease onset. As the SMN spot analysis values tended to decrease with time after birth, they may eventually lead to the development of SMA. Furthermore, a correlation was found between the SMN spot analysis values of neonatal cord blood and maternal blood, which predicts disease severity after birth. In other words, the SMN protein supplied from the mother to the foetus may suppress the development of SMA in the infant at birth, and depletion of the SMN protein may occur after birth, causing the infant to develop SMA. Our findings demonstrated the effectiveness of newborn screening and the potential of maternally mediated treatment strategies by providing a rationale for prompt treatment initiation in SMA.

Cytoskeleton dysfunction of motor neuron in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by deletions or mutations of survival of motor neuron 1 (SMN1) gene. To date, the mechanism of selective cell death of motor neurons as a hallmark of SMA is still unclear. The severity of SMA is dependent on the amount of survival motor neuron (SMN) protein, which is an essential and ubiquitously expressed protein involved in various cellular processes including regulation of cytoskeletal dynamics. In this review, we discuss the effect of SMN ablation on cytoskeleton organization including actin dynamics, growth cone formation, axonal stability, neurite outgrowth, microtubule stability, synaptic vesicle dynamics and neurofilament protein release in SMA. We also summarized a list of critical proteins such as profilin-2 (PFN2), plastin-3 (PLS3), stathmin-1 (STMN1), microtubule-associated protein 1B (MAP1B) and neurofilament which play an important role in modulating cytoskeleton in SMA. Our aim is to highlight how cytoskeletal defects contribute to motor neuron degeneration in SMA disease progression and concentrating on cytoskeleton dynamics may be a promising approach to develop new therapy or biomarker.

Association among biomarkers, phenotypes, and motor milestones in Chinese patients with 5q spinal muscular atrophy types 1-3

Background

Biomarkers can be used to assess the severity of spinal muscular atrophy (5q SMA; SMA). Despite their potential, the relationship between biomarkers and clinical outcomes in SMA remains underexplored. This study aimed to assess the association among biomarkers, phenotypes, and motor milestones in Chinese patients diagnosed with SMA.

Methods

We collected retrospective clinical and follow-up data of disease-modifying therapy (DMT)-naïve patients with SMA at our center from 2019 to 2021. Four biomarkers were included: survival motor neuron 2 (SMN2) copies, neuronal apoptosis inhibitory protein (NAIP) copies, full-length SMN2 (fl-SMN2), and F-actin bundling protein plastin 3 (PLS3) transcript levels. Data were analyzed and stratified according to SMA subtype.

Results

Of the 123 patients, 30 were diagnosed with Type 1 (24.3%), 56 with Type 2 (45.5%), and 37 with Type 3 (30.1%). The mortality rate for Type 1 was 50%, with median survival times of 2 and 8 months for types 1a and 1b, respectively. All four biomarkers were correlated with disease severity. Notably, fl-SMN2 transcript levels increased with SMN2 copies and were higher in Type 2b than those in Type 2a (p = 0.028). Motor milestone deterioration was correlated with SMN2 copies, NAIP copies, and fl-SMN2 levels, while PLS3 levels were correlated with standing and walking function.

Discussion

Our findings suggest that SMN2 copies contribute to survival and that fl-SMN2 may serve as a valuable biomarker for phenotypic variability in SMA Type 2 subtypes. These insights can guide future research and clinical management of SMA.

The impact of three SMN2 gene copies on clinical characteristics and effect of disease-modifying treatment in patients with spinal muscular atrophy: a systematic literature review

Objective

To review the clinical characteristics and effect of treatment in patients with spinal muscular atrophy (SMA) and three copies of the SMN2 gene.

Methods

We conducted a literature search in October 2022 to identify English-language clinical research on SMA that included SMN2 copy number according to PRISMA guidelines.

Results

Our search identified 44 studies examining the impact of three SMN2 copies on clinical characteristics (21 on phenotype, 13 on natural history, and 15 on functional status and other signs/symptoms). In children with type I SMA or presymptomatic infants with an SMN1 deletion, three SMN2 copies was associated with later symptom onset, slower decline in motor function and longer survival compared with two SMN2 copies. In patients with SMA type II or III, three SMN2 copies is associated with earlier symptom onset, loss of ambulation, and ventilator dependence compared with four SMN2 copies. Eleven studies examined treatment effects with nusinersen (nine studies), onasemnogene abeparvovec (one study), and a range of treatments (one study) in patients with three SMN2 copies. In presymptomatic infants, early treatment delayed the onset of symptoms and maintained motor function in those with three SMN2 copies. The impact of copy number on treatment response in symptomatic patients is still unclear.

Conclusion

SMN2 copy number is strongly correlated with SMA phenotype in patients with SMN1 deletion, while no correlation was found in patients with an SMN1 mutation. Patients with three SMN2 copies show a highly variable clinical phenotype. Early initiation of treatment is highly effective in presymptomatic patients with three SMN2 copies.

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

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

Role of circulating biomarkers in spinal muscular atrophy: insights from a new treatment era

Spinal muscular atrophy (SMA) is a lower motor neuron disease due to biallelic mutations in the SMN1 gene on chromosome 5. It is characterized by progressive muscle weakness of limbs, bulbar and respiratory muscles. The disease is usually classified in four different phenotypes (1-4) according to age at symptoms onset and maximal motor milestones achieved. Recently, three disease modifying treatments have received approval from the Food and Drug Administration (FDA) and the European Medicines Agency (EMA), while several other innovative drugs are under study. New therapies have been game changing, improving survival and life quality for SMA patients. However, they have also intensified the need for accurate biomarkers to monitor disease progression and treatment efficacy. While clinical and neurophysiological biomarkers are well established and helpful in describing disease progression, there is a great need to develop more robust and sensitive circulating biomarkers, such as proteins, nucleic acids, and other small molecules. Used alone or in combination with clinical biomarkers, they will play a critical role in enhancing patients' stratification for clinical trials and access to approved treatments, as well as in tracking response to therapy, paving the way to the development of individualized therapeutic approaches. In this comprehensive review, we describe the foremost circulating biomarkers of current significance, analyzing existing literature on non-treated and treated patients with a special focus on neurofilaments and circulating miRNA, aiming to identify and examine their role in the follow-up of patients treated with innovative treatments, including gene therapy.

Identifying Biomarkers of Spinal Muscular Atrophy for Further Development

BACKGROUND

Spinal muscular atrophy (SMA) is caused by bi-allelic, recessive mutations of the survival motor neuron 1 (SMN1) gene and reduced expression levels of the survival motor neuron (SMN) protein. Degeneration of alpha motor neurons in the spinal cord causes progressive skeletal muscle weakness. The wide range of disease severities, variable rates of decline, and heterogenous clinical responses to approved disease-modifying treatment remain poorly understood and limit the ability to optimize treatment for patients. Validation of a reliable biomarker(s) with the potential to support early diagnosis, inform disease prognosis and therapeutic suitability, and/or confirm response to treatment(s) represents a significant unmet need in SMA.

OBJECTIVES

The SMA Multidisciplinary Biomarkers Working Group, comprising 11 experts in a variety of relevant fields, sought to determine the most promising candidate biomarker currently available, determine key knowledge gaps, and recommend next steps toward validating that biomarker for SMA.

METHODS

The Working Group engaged in a modified Delphi process to answer questions about candidate SMA biomarkers. Members participated in six rounds of reiterative surveys that were designed to build upon previous discussions.

RESULTS

The Working Group reached a consensus that neurofilament (NF) is the candidate biomarker best poised for further development. Several important knowledge gaps were identified, and the next steps toward filling these gaps were proposed.

CONCLUSIONS

NF is a promising SMA biomarker with the potential for prognostic, predictive, and pharmacodynamic capabilities. The Working Group has identified needed information to continue efforts toward the validation of NF as a biomarker for SMA.

Sex Difference in Spinal Muscular Atrophy Patients - are Males More Vulnerable?

BACKGROUND

Sex is a significant risk factor in many neurodegenerative disorders. A better understanding of the molecular mechanisms behind sex differences could help develop more targeted therapies that would lead to better outcomes. Untreated spinal muscular atrophy (SMA) is the leading genetic motor disorder causing infant mortality. SMA has a broad spectrum of severity ranging from prenatal death to infant mortality to normal lifespan with some disability. Scattered evidence points to a sex-specific vulnerability in SMA. However, the role of sex as a risk factor in SMA pathology and treatment has received limited attention.

OBJECTIVE

Systematically investigate sex differences in the incidence, symptom severity, motor function of patients with different types of SMA, and in the development of SMA1 patients.

METHODS

Aggregated data of SMA patients were obtained from the TREAT-NMD Global SMA Registry and the Cure SMA membership database by data enquiries. Data were analyzed and compared with publicly available standard data and data from published literature.

RESULTS

The analysis of the aggregated results from the TREAT-NMD dataset revealed that the male/female ratio was correlated to the incidence and prevalence of SMA from different countries; and for SMA patients, more of their male family members were affected by SMA. However, there was no significant difference of sex ratio in the Cure SMA membership dataset. As quantified by the clinician severity scores, symptoms were more severe in males than females in SMA types 2 and 3b. Motor function scores measured higher in females than males in SMA types 1, 3a and 3b. The head circumference was more strongly affected in male SMA type 1 patients.

CONCLUSIONS

The data in certain registry datasets suggest that males may be more vulnerable to SMA than females. The variability observed indicates that more investigation is necessary to fully understand the role of sex differences in SMA epidemiology, and to guide development of more targeted treatments.

Investigation on the Effects of Modifying Genes on the Spinal Muscular Atrophy Phenotype

Introduction  Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by the degeneration of motor neurons, muscle weakness, and atrophy that leads to infant's death. The duplication of exon 7/8 in the SMN2 gene reduces the clinical severity of disease, and it is defined as modifying effect. In this study, we aim to investigate the expression of modifying genes related to the prognosis of SMA like PLS3 , PFN2 , ZPR1 , CORO1C , GTF2H2 , NRN1 , SERF1A , NCALD , NAIP , and TIA1.

Methods  Seventeen patients, who came to Trakya University, Faculty of Medicine, Medical Genetics Department, with a preliminary diagnosis of SMA disease, and eight healthy controls were included in this study after multiplex ligation-dependent probe amplification analysis. Gene expression levels were determined by real-time reverse transcription polymerase chain reaction and delta–delta CT method by the isolation of RNA from peripheral blood of patients and controls.

Results   SERF1A and NAIP genes compared between A group and B + C + D groups, and A group of healthy controls, showed statistically significant differences ( p  = 0.037, p  = 0.001).

Discussion   PLS3, NAIP , and NRN1 gene expressions related to SMA disease have been reported before in the literature. In our study, the expression levels of SERF1A , GTF2H2 , NCALD , ZPR1 , TIA1 , PFN2 , and CORO1C genes have been studied for the first time in SMA patients.

Evaluation of exonic copy numbers of SMN1 and SMN2 genes in SMA

SMA is a neuromuscular disease and occurs primarily through autosomal recessive inheritance. Identification of deletions in the SMN1 gene especially in the exon 7 and exon 8 regions (hot spot), are used in carrier testing. The exact copy numbers of those exons in the SMN1 and SMN2 genes in 113 patients who presented with a pre-diagnosis of SMA were determined using MLPA method. We aimed to reveal both the most common copy number profiles of different SMA types. It was found that the frequency of homozygous deletions in SMN1 was 15.9%, while heterozygous deletions was 16.9%. The most common SMN-MLPA profile was 0-0-3-3. In the cases with homozygous deletion, SMA type III diagnosis was observed most frequently (44%), and the rate of consanguineous marriage was found 33%. Two cases with the same exonic copy number profile but with different clinical subtypes were identified in a family. We also detected distinct exonic deletion and duplication MLPA profiles for the first time. We created "the SMA signature" that can be added to patient reports. Furthermore, our data are important for revealing potential local profiles of SMA and describing the disease in genetic reports in a way that is clear and comprehensive.

Update on Biomarkers in Spinal Muscular Atrophy

The availability of disease modifying therapies for spinal muscular atrophy (SMA) has created an urgent need to identify clinically meaningful biomarkers. Biomarkers present a means to measure and evaluate neurological disease across time. Changes in biomarkers provide insight into disease progression and may reveal biologic, physiologic, or pharmacologic phenomena occurring prior to clinical detection. Efforts to identify biomarkers for SMA, a genetic motor neuron disease characterized by motor neuron degeneration and weakness, have culminated in a number of putative molecular and physiologic markers that evaluate biological media (eg, blood and cerebrospinal fluid [CSF]) or nervous system function. Such biomarkers include SMN2 copy number, SMN mRNA and protein levels, neurofilament proteins (NFs), plasma protein analytes, creatine kinase (CK) and creatinine (Crn), and various electrophysiology and imaging measures. SMN2 copy number inversely correlates with disease severity and is the best predictor of clinical outcome in untreated individuals. SMN mRNA and protein are commonly measured in the blood or CSF of patients receiving SMA therapies, particularly those aimed at increasing SMN protein expression, and provide insight into current disease state. NFs have proven to be robust prognostic, disease progression, and pharmacodynamic markers for SMA infants undergoing treatment, but less so for adolescents and adults. Select plasma proteins are altered in SMA individuals and may track response to therapy. CK and Crn from blood correlate with motor function and disease severity status and are useful for predicting which individuals will respond to therapy. Electrophysiology measures comprise the most reliable means for monitoring motor function throughout disease course and are sensitive enough to detect neuromuscular changes before overt clinical manifestation, making them robust predictive and pharmacodynamic biomarkers. Finally, magnetic resonance imaging and muscle ultrasonography are non-invasive techniques for studying muscle structure and physiology and are useful diagnostic tools, but cannot reliably track disease progression. Importantly, biomarkers can provide information about the underlying mechanisms of disease as well as reveal subclinical disease progression, allowing for more appropriate timing and dosing of therapy for individuals with SMA. Recent therapeutic advancements in SMA have shown promising results, though there is still a great need to identify and understand the impact of biomarkers in modulating disease onset and progression.

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

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

Newborn Screening for Spinal Muscular Atrophy: Ontario Testing and Follow-up Recommendations

BACKGROUND

Spinal muscular atrophy (SMA) is characterized by the progressive loss of motor neurons causing muscle atrophy and weakness. Nusinersen, the first effective SMA therapy was approved by Health Canada in June 2017 and has been added to the provincial formulary of all but one Canadian province. Access to this effective therapy has triggered the inclusion of SMA in an increasing number of Newborn Screening (NBS) programs. However, the range of disease-modifying SMN2 gene copy numbers encountered in survival motor neuron 1 (SMN1)-null individuals means that neither screen-positive definition nor resulting treatment decisions can be determined by SMN1 genotype alone. We outline an approach to this challenge, one that specifically addresses the case of SMA newborns with four copies of SMN2.

OBJECTIVES

To develop a standardized post-referral evaluation pathway for babies with a positive SMA NBS screen result.

METHODS

An SMA NBS pilot trial in Ontario using first-tier MassARRAY and second-tier multi-ligand probe amplification (MLPA) was launched in January 2020. Prior to this, Ontario pediatric neuromuscular disease and NBS experts met to review the evidence regarding the diagnosis and treatment of children with SMA as it pertained to NBS. A post-referral evaluation algorithm was developed, outlining timelines for patient retrieval and management.

CONCLUSIONS

Ontario's pilot NBS program has created a standardized path to facilitate early diagnosis of SMA and initiation of treatment. The goal is to provide timely access to those SMA infants in need of therapy to optimize motor function and prolong survival.

Unique, Gender-Dependent Serum microRNA Profile in PLS3 Gene-Related Osteoporosis

Plastin 3 (PLS3), encoded by PLS3, is a newly recognized regulator of bone metabolism, and mutations in the encoding gene result in severe childhood-onset osteoporosis. Because it is an X chromosomal gene, PLS3 mutation-positive males are typically more severely affected whereas females portray normal to increased skeletal fragility. Despite the severe skeletal pathology, conventional metabolic bone markers tend to be normal and are thus insufficient for diagnosing or monitoring patients. Our study aimed to explore serum microRNA (miRNA) concentrations in subjects with defective PLS3 function to identify novel markers that could differentiate subjects according to mutation status and give insight into the molecular mechanisms by which PLS3 regulates skeletal health. We analyzed fasting serum samples for a custom-designed panel comprising 192 miRNAs in 15 mutation-positive (five males, age range 8-76 years, median 41 years) and 14 mutation-negative (six males, age range 8-69 years, median 40 years) subjects from four Finnish families with different PLS3 mutations. We identified a unique miRNA expression profile in the mutation-positive subjects with seven significantly upregulated or downregulated miRNAs (miR-93-3p, miR-532-3p, miR-133a-3p, miR-301b-3p, miR-181c-5p, miR-203a-3p, and miR-590-3p; p values, range .004-.044). Surprisingly, gender subgroup analysis revealed the difference to be even more distinct in female mutation-positive subjects (congruent p values, range .007-.086) than in males (p values, range .127-.843) in comparison to corresponding mutation-negative subjects. Although the seven identified miRNAs have all been linked to bone metabolism and two of them (miR-181c-5p and miR-203a-3p) have bioinformatically predicted targets in the PLS3 3' untranslated region (3'-UTR), none have previously been reported to associate with PLS3. Our results indicate that PLS3 mutations are reflected in altered serum miRNA levels and suggest there is crosstalk between PLS3 and these miRNAs in bone metabolism. These provide new understanding of the pathomechanisms by which mutations in PLS3 lead to skeletal disease and may provide novel avenues for exploring miRNAs as biomarkers in PLS3 osteoporosis or as target molecules in future therapeutic applications. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Management and current status of spinal muscular atrophy: a retrospective multicentre claims database analysis

Introduction

The interest in patient demographics and disease management has increased in the past years due to their utility in developing measures that allow healthcare providers to reflect disease complexity.

Objective

To revise the current status of spinal muscular atrophy in the region of Catalonia, and to validate the utility of the database for this aim.

Methods

Five hundred twenty-four patients diagnosed with a spinal muscular atrophy were identified in the region of Catalonia via the novel program of data analysis for research and innovation in health (PADRIS). Patient records included in the analysis corresponded to primary care, hospital, emergency room, extended care and mental health admissions between 2007 and 2017.

Results

58.02% of patients with a SMA diagnosis were males while 40.84% were females. Average age of diagnosis was 38.31 ± 24.49 years ±SD. Significantly lower was the age of diagnosis of spinal muscular atrophy type I, 1.81 ± 3.01 years. An average of 22 patients died per year during the study period, with a mean decease age of 62.96 ± 25.41 years. Patients were generally attended in hospitals, and the use of healthcare resources was focused on resolving respiratory issues and scoliosis. The highest ratio of admissions per patient was registered in those aged 0 to 4 years. Patients presented a higher risk than the general population and a higher frequency of multimorbidites.

Conclusions

Patients exhibited similar characteristics to prior European studies. Multiple admissions in younger patients, mostly due to respiratory issues, have a central role in increasing medical costs of SMA. Equally, the higher risk of patients and increased number of multimorbidity groups translate in an elevated number of admissions in health centres and ER, deriving in higher expenses.

A novel nonsense variant in PLS3 causes X-linked osteoporosis in a Chinese family

Osteoporosis is a complex bone metabolic disorder. Genetic factors play an important role in the development of osteoporosis. Mutations in more than 15 genes have been identified to be responsible for osteoporosis to date. Most recently, the gene PLS3 encoding plastin 3 was recognized to be involved in X-linked osteoporosis. Here, we recruited a four-generation Chinese family with X-linked osteoporosis, which had its onset in childhood and was characterized by peripheral fractures and low bone mineral density. All affected individuals shared a nonsense variant (c.244C > T) in exon 4 of PLS3 on Xq23. The variant in affected individuals segregated with the osteoporosis phenotype. By restriction analysis using Dra I, this variant was confirmed in all affected individuals but was not detected in unaffected family members or in 100 unrelated Chinese male controls. The variant was predicted to cause a premature termination of messenger RNA (mRNA) translation (p.Gln82*). The mutant mRNA degraded via the mechanism of "nonsense-mediated mRNA decay." In the present study, we identified a novel nonsense variant of PLS3 in early-onset X-linked osteoporosis and provided a novel insight into the molecular mechanism underlying the pathogenesis of osteoporosis.

Biomarkers and the Development of a Personalized Medicine Approach in Spinal Muscular Atrophy

Recent unprecedented advances in treatment for spinal muscular atrophy (SMA) enabled patients to access the first approved disease modifying therapy for the condition. There are however many uncertainties, regarding timing of treatment initiation, response to intervention, treatment effects and long-term outcomes, which are complicated by the evolving phenotypes seen in the post-treatment era for patients with SMA. Biomarkers of disease, with diagnostic, prognostic, predictive, and pharmacodynamic value are thus urgently required, to facilitate a wider understanding in this dynamic landscape. A spectrum of these candidate biomarkers, will be evaluated in this review, including genetic, epigenetic, proteomic, electrophysiological, and imaging measures. Of these, SMN2 appears to be the most significant modifier of phenotype to date, and its use in prognostication shows considerable clinical utility. Longitudinal studies in patients with SMA highlight an emerging role of circulatory markers such as neurofilament, in tracking disease progression and response to treatment. Furthermore, neurophysiological biomarkers such as CMAP and MUNE values show considerable promise in the real word setting, in following the dynamic response and output of the motor unit to therapeutic intervention. The specific value for these possible biomarkers across diagnosis, prognosis, prediction of treatment response, efficacy, and safety will be central to guide future patient-targeted treatments, the design of clinical trials, and understanding of the pathophysiological mechanisms of disease and intervention.

Analysis of FUS, PFN2, TDP-43, and PLS3 as potential disease severity modifiers in spinal muscular atrophy

Objective

To investigate mutations in genes that are potential modifiers of spinal muscular atrophy (SMA) severity.

Methods

We performed a hypothesis-based search into the presence of variants in fused in sarcoma (FUS), transactive response DNA-binding protein 43 (TDP-43), plastin 3 (PLS3), and profilin 2 (PFN2) in a cohort of 153 patients with SMA types 1–4, including 19 families. Variants were detected with targeted next-generation sequencing and confirmed with Sanger sequencing. Functional effects of the identified variants were analyzed in silico and for PLS3, by analyzing expression levels in peripheral blood.

Results

We identified 2 exonic variants in FUS exons 5 and 6 (p.R216C and p.S135N) in 2 unrelated patients, but clinical effects were not evident. We identified 8 intronic variants in PLS3 in 33 patients. Five PLS3 variants (c.1511+82T>C; c.748+130 G>A; c.367+182C>T; c.891-25T>C (rs145269469); c.1355+17A>G (rs150802596)) potentially alter exonic splice silencer or exonic splice enhancer sites. The variant c.367+182C>T, but not RNA expression levels, corresponded with a more severe phenotype in 1 family. However, this variant or level of PLS3 expression did not consistently correspond with a milder or more severe phenotype in other families or the overall cohort. We found 3 heterozygous, intronic variants in PFN2 and TDP-43 with no correlation with clinical phenotype or effects on splicing.

Conclusions

PLS3 and FUS sequence variants do not modify SMA severity at the population level. Specific variants in individual patients or families do not consistently correlate with disease severity.

Molecular Factors Involved in Spinal Muscular Atrophy Pathways as Possible Disease-modifying Candidates

Spinal Muscular Atrophy (SMA) is a neuromuscular disorder caused by mutations in the SMN1 gene. Being a monogenic disease, it is characterized by high clinical heterogeneity. Variations in penetrance and severity of symptoms, as well as clinical discrepancies between affected family members can result from modifier genes influence on disease manifestation. SMN2 gene copy number is known to be the main phenotype modifier and there is growing evidence of additional factors contributing to SMA severity. Potential modifiers of spinal muscular atrophy can be found among the wide variety of different factors, such as multiple proteins interacting with SMN or promoting motor neuron survival, epigenetic modifications, transcriptional or splicing factors influencing SMN2 expression. Study of these factors enables to reveal mechanisms underlying SMA pathology and can have pronounced clinical application.

Interventions Targeting Glucocorticoid-Krüppel-like Factor 15-Branched-Chain Amino Acid Signaling Improve Disease Phenotypes in Spinal Muscular Atrophy Mice

The circadian glucocorticoid-Krüppel-like factor 15-branched-chain amino acid (GC-KLF15-BCAA) signaling pathway is a key regulatory axis in muscle, whose imbalance has wide-reaching effects on metabolic homeostasis. Spinal muscular atrophy (SMA) is a neuromuscular disorder also characterized by intrinsic muscle pathologies, metabolic abnormalities and disrupted sleep patterns, which can influence or be influenced by circadian regulatory networks that control behavioral and metabolic rhythms. We therefore set out to investigate the contribution of the GC-KLF15-BCAA pathway in SMA pathophysiology of Taiwanese Smn^−/−;SMN2 and Smn^2B/− mouse models. We thus uncover substantial dysregulation of GC-KLF15-BCAA diurnal rhythmicity in serum, skeletal muscle and metabolic tissues of SMA mice. Importantly, modulating the components of the GC-KLF15-BCAA pathway via pharmacological (prednisolone), genetic (muscle-specific Klf15 overexpression) and dietary (BCAA supplementation) interventions significantly improves disease phenotypes in SMA mice. Our study highlights the GC-KLF15-BCAA pathway as a contributor to SMA pathogenesis and provides several treatment avenues to alleviate peripheral manifestations of the disease. The therapeutic potential of targeting metabolic perturbations by diet and commercially available drugs could have a broader implementation across other neuromuscular and metabolic disorders characterized by altered GC-KLF15-BCAA signaling.

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SMA is a neuromuscular disease characterized by motoneuron loss, muscle abnormalities and metabolic perturbations.

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The regulatory GC-KLF15-BCAA pathway is dysregulated in serum and skeletal muscle of SMA mice during disease progression.

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Modulating GC-KLF15-BCAA signaling by pharmacological, dietary and genetic interventions improves phenotype of SMA mice.

Spinal muscular atrophy (SMA) is a devastating and debilitating childhood genetic disease. Although nerve cells are mainly affected, muscle is also severely impacted. The normal communication between the glucocorticoid (GC) hormone, the protein KLF15 and the dietary branched-chain amino acids (BCAAs) maintains muscle and whole-body health. In this study, we identified an abnormal activity of GC-KLF15- BCAA in blood and muscle of SMA mice. Importantly, targeting GC-KLF15-BCAA activity with an existing drug or a specific diet improved disease progression in SMA mice. Our research uncovers GCs, KLF15 and BCAAs as therapeutic targets to ameliorate SMA muscle and whole-body health.

Therapeutic strategies for spinal muscular atrophy: SMN and beyond

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder characterized by loss of motor neurons and muscle atrophy, generally presenting in childhood. SMA is caused by low levels of the survival motor neuron protein (SMN) due to inactivating mutations in the encoding gene SMN1. A second duplicated gene, SMN2, produces very little but sufficient functional protein for survival. Therapeutic strategies to increase SMN are in clinical trials, and the first SMN2-directed antisense oligonucleotide (ASO) therapy has recently been licensed. However, several factors suggest that complementary strategies may be needed for the long-term maintenance of neuromuscular and other functions in SMA patients. Pre-clinical SMA models demonstrate that the requirement for SMN protein is highest when the structural connections of the neuromuscular system are being established, from late fetal life throughout infancy. Augmenting SMN may not address the slow neurodegenerative process underlying progressive functional decline beyond childhood in less severe types of SMA. Furthermore, individuals receiving SMN-based treatments may be vulnerable to delayed symptoms if rescue of the neuromuscular system is incomplete. Finally, a large number of older patients living with SMA do not fulfill the present criteria for inclusion in gene therapy and ASO clinical trials, and may not benefit from SMN-inducing treatments. Therefore, a comprehensive whole-lifespan approach to SMA therapy is required that includes both SMN-dependent and SMN-independent strategies that treat the CNS and periphery. Here, we review the range of non-SMN pathways implicated in SMA pathophysiology and discuss how various model systems can serve as valuable tools for SMA drug discovery.

Summary: Translational research for spinal muscular atrophy (SMA) should address the development of non-CNS and survival motor neuron (SMN)-independent therapeutic approaches to complement and enhance the benefits of CNS-directed and SMN-dependent therapies.

Longitudinal assessments in discordant twins with SMA

We report longitudinal clinical and neurophysiological assessments in twins affected by spinal muscular atrophy (SMA) with discordant phenotypes. The boy had the homozygous deletion of SMN1, a typical type 1 SMA course, and died at the age of eight months. His twin sister, asymptomatic at the time of the diagnosis in her brother, had the same genetic defect but she developed clinical and electrophysiological signs of type 2 SMA. The reduction of tendon reflexes was the first clinical sign at the age of 4 months, followed within few weeks, by a mild decrement in the amplitude of the compound motor action potentials. After the age of 9 months, she showed a sudden clinical and electrophysiological deterioration. Among molecular tests, we determined SMN2 copy number, SMN2 and Plastin 3 transcript levels in peripheral blood, and observed no relevant differences between twins.

Transcript levels of plastin 3 and neuritin 1 modifier genes in spinal muscular atrophy siblings

BACKGROUND

In single gene disorders, patients with the same genotype may have variations in severity. One of the main factors affecting disease severity is modifier genes. Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by degeneration of alpha motor neurons. Plastin 3 (PLS3) is a phenotypic modifier of SMA, and neuritin 1 (NRN1) has also been suggested as a possible modifier gene. The aim of the present study was therefore to analyze PLS3 and NRN1 expression in SMA siblings in four families.

METHODS

The study group consisted of four SMA families with seven with discordant phenotype and two affected siblings. Total RNA was isolated from whole blood. PLS3 and NRN1 expression was analyzed on quantitative real-time polymerase chain reaction.

RESULTS

In family 1 only NRN1 expression was increased in the mildly affected sister. In family 2 only PLS3 had a modifier effect. Family 3, which had type III siblings with identical clinical phenotypes, had similar PLS3 expression between the siblings but no NRN1 expression. In family 4, neither PLS3 nor NRN1 had any correlation with severity.

CONCLUSION

On analysis of the expression of NRN1 in SMA patients for the first time, NRN1 could be a potential modifier gene. PLS3 expression does not always modify SMA phenotype. In patients with no modifier effect of known genes, genome sequencing and transcriptome analysis are promising for the identification of novel modifiers and understanding of SMA pathophysiology.

A rare variant (c.863G>T) in exon 7 of SMN1 disrupts mRNA splicing and is responsible for spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by deletion or mutation of SMN1 (survival motor neuron 1). SMN exon 7 splicing is regulated by a number of exonic and intronic regulatory sequences and the trans-factors that bind them. Variants located in or near these regulated regions should be evaluated to determine their effect on splicing. We identified the rare variant c.863G>T (r.835_*3del, p.Gly279Glufs*5) in exon 7 of SMN1 in three patients affected with type I or type II SMA. Most of the SMN1 transcripts exhibited complete loss of exon 7 in vivo. The ex vivo splicing assay demonstrated that the variant disrupts inclusion of exon 7 (~85%) in the SMN1 mRNA; replacement with various bases yielded a variety of splicing effects in SMN1 and SMN2 pre-mRNA. The c.863G>T (r.835_*3del, p.Gly279Glufs*5) variant is located in a region that includes binding sites for multiple splicing factors including Tra2β1. Thus, the variant disrupts Tra2β1 binding, but does not affect binding of hnRNP A1. These findings demonstrate how rare variants influence pre-mRNA splicing of SMN and reveal the functional influence of c.863G>T (r.835_*3del, p.Gly279Glufs*5) variant in patients with SMA.

Copy Number Variations in the Survival Motor Neuron Genes: Implications for Spinal Muscular Atrophy and Other Neurodegenerative Diseases

Proximal spinal muscular atrophy (SMA), a leading genetic cause of infant death worldwide, is an early-onset, autosomal recessive neurodegenerative disease characterized by the loss of spinal α-motor neurons. This loss of α-motor neurons is associated with muscle weakness and atrophy. SMA can be classified into five clinical grades based on age of onset and severity of the disease. Regardless of clinical grade, proximal SMA results from the loss or mutation of SMN1 (survival motor neuron 1) on chromosome 5q13. In humans a large tandem chromosomal duplication has lead to a second copy of the SMN gene locus known as SMN2. SMN2 is distinguishable from SMN1 by a single nucleotide difference that disrupts an exonic splice enhancer in exon 7. As a result, most of SMN2 mRNAs lack exon 7 (SMNΔ7) and produce a protein that is both unstable and less than fully functional. Although only 10–20% of the SMN2 gene product is fully functional, increased genomic copies of SMN2 inversely correlates with disease severity among individuals with SMA. Because SMN2 copy number influences disease severity in SMA, there is prognostic value in accurate measurement of SMN2 copy number from patients being evaluated for SMA. This prognostic value is especially important given that SMN2 copy number is now being used as an inclusion criterion for SMA clinical trials. In addition to SMA, copy number variations (CNVs) in the SMN genes can affect the clinical severity of other neurological disorders including amyotrophic lateral sclerosis (ALS) and progressive muscular atrophy (PMA). This review will discuss how SMN1 and SMN2 CNVs are detected and why accurate measurement of SMN1 and SMN2 copy numbers is relevant for SMA and other neurodegenerative diseases.

Plastin 3 Expression Does Not Modify Spinal Muscular Atrophy Severity in the ∆7 SMA Mouse

Spinal muscular atrophy is caused by loss of the SMN1 gene and retention of SMN2. The SMN2 copy number inversely correlates with phenotypic severity and is a modifier of disease outcome. The SMN2 gene essentially differs from SMN1 by a single nucleotide in exon 7 that modulates the incorporation of exon 7 into the final SMN transcript. The majority of the SMN2 transcripts lack exon 7 and this leads to a SMN protein that does not effectively oligomerize and is rapidly degraded. However the SMN2 gene does produce some full-length SMN and the SMN2 copy number along with how much full-length SMN the SMN2 gene makes correlates with severity of the SMA phenotype. However there are a number of discordant SMA siblings that have identical haplotypes and SMN2 copy number yet one has a milder form of SMA. It has been suggested that Plastin3 (PLS3) acts as a sex specific phenotypic modifier where increased expression of PLS3 modifies the SMA phenotype in females. To test the effect of PLS3 overexpression we have over expressed full-length PLS3 in SMA mice. To ensure no disruption of functionality or post-translational processing of PLS3 we did not place a tag on the protein. PLS3 protein was expressed under the Prion promoter as we have shown previously that SMN expression under this promoter can rescue SMA mice. High levels of PLS3 mRNA were expressed in motor neurons along with an increased level of PLS3 protein in total spinal cord, yet there was no significant beneficial effect on the phenotype of SMA mice. Specifically, neither survival nor the fundamental electrophysiological aspects of the neuromuscular junction were improved upon overexpression of PLS3 in neurons.

Decay in survival motor neuron and plastin 3 levels during differentiation of iPSC-derived human motor neurons

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by mutations in Survival Motor Neuron 1 (SMN1), leading to degeneration of alpha motor neurons (MNs) but also affecting other cell types. Induced pluripotent stem cell (iPSC)-derived human MN models from severe SMA patients have shown relevant phenotypes. We have produced and fully characterized iPSCs from members of a discordant consanguineous family with chronic SMA. We differentiated the iPSC clones into ISL-1+/ChAT+ MNs and performed a comparative study during the differentiation process, observing significant differences in neurite length and number between family members. Analyses of samples from wild-type, severe SMA type I and the type IIIa/IV family showed a progressive decay in SMN protein levels during iPSC-MN differentiation, recapitulating previous observations in developmental studies. PLS3 underwent parallel reductions at both the transcriptional and translational levels. The underlying, progressive developmental decay in SMN and PLS3 levels may lead to the increased vulnerability of MNs in SMA disease. Measurements of SMN and PLS3 transcript and protein levels in iPSC-derived MNs show limited value as SMA biomarkers.

Actin-bundling protein plastin 3 is a regulator of ectoplasmic specialization dynamics during spermatogenesis in the rat testis

Ectoplasmic specialization (ES) is an actin-rich adherens junction in the seminiferous epithelium of adult mammalian testes. ES is restricted to the Sertoli-spermatid (apical ES) interface, as well as the Sertoli cell-cell (basal ES) interface at the blood-testis barrier (BTB). ES is typified by the presence of an array of bundles of actin microfilaments near the Sertoli cell plasma membrane. These actin microfilament bundles require rapid debundling to convert them from a bundled to branched/unbundled configuration and vice versa to confer plasticity to support the transport of 1) spermatids in the adluminal compartment and 2) preleptotene spermatocytes at the BTB while maintaining cell adhesion. Plastin 3 is one of the plastin family members abundantly found in yeast, plant and animal cells that confers actin microfilaments their bundled configuration. Herein, plastin 3 was shown to be a component of the apical and basal ES in the rat testis, displaying spatiotemporal expression during the epithelial cycle. A knockdown (KD) of plastin 3 in Sertoli cells by RNA interference using an in vitro model to study BTB function showed that a transient loss of plastin 3 perturbed the Sertoli cell tight junction-permeability barrier, mediated by changes in the localization of basal ES proteins N-cadherin and β-catenin. More importantly, these changes were the result of an alteration of the actin microfilaments, converting from their bundled to branched configuration when examined microscopically, and validated by biochemical assays that quantified actin-bundling and polymerization activity. Moreover, these changes were confirmed by studies in vivo by plastin 3 KD in the testis in which mis-localization of N-cadherin and β-catenin was also detected at the BTB, concomitant with defects in the transport of spermatids and phagosomes and a disruption of cell adhesion most notably in elongated spermatids due to a loss of actin-bundling capability at the apical ES, which in turn affected localization of adhesion protein complexes at the site. In summary, plastin 3 is a regulator of actin microfilament bundles at the ES in which it dictates the configuration of the filamentous actin network by assuming either a bundled or unbundled/branched configuration via changes in its spatiotemporal expression during the epithelial cycle.