A Comparative Study of SMN Protein and mRNA in Blood and Fibroblasts in Patients with Spinal Muscular Atrophy and Healthy Controls

Insights into spinal muscular atrophy from molecular biomarkers

Spinal muscular atrophy is a devastating motor neuron disease characterized by severe cases of fatal muscle weakness. It is one of the most common genetic causes of mortality among infants aged less than 2 years. Biomarker research is currently receiving more attention, and new candidate biomarkers are constantly being discovered. This review initially discusses the evaluation methods commonly used in clinical practice while briefly outlining their respective pros and cons. We also describe recent advancements in research and the clinical significance of molecular biomarkers for spinal muscular atrophy, which are classified as either specific or non-specific biomarkers. This review provides new insights into the pathogenesis of spinal muscular atrophy, the mechanism of biomarkers in response to drug-modified therapies, the selection of biomarker candidates, and would promote the development of future research. Furthermore, the successful utilization of biomarkers may facilitate the implementation of gene-targeting treatments for patients with spinal muscular atrophy.

Patient-specific responses to SMN2 splice-modifying treatments in spinal muscular atrophy fibroblasts

The availability of three therapies for the neuromuscular disease spinal muscular atrophy (SMA) highlights the need to match patients to the optimal treatment. Two of these treatments (nusinersen and risdiplam) target splicing of SMN2, but treatment outcomes vary from patient to patient. An incomplete understanding of the complex interactions among SMA genetics, SMN protein and mRNA levels, and gene-targeting treatments, limits our ability to explain this variability and identify optimal treatment strategies for individual patients. To address this, we analyzed responses to nusinersen and risdiplam in 45 primary fibroblast cell lines. Pre-treatment SMN2-FL, SMN2Δ7 mRNA, and SMN protein levels were influenced by SMN2 copy number, age, and sex. After treatment, SMN and mRNA levels were more heterogeneous. In 43% of patients, response to both therapies was similar, but in 57% one treatment led to a significantly higher SMN increase than the other treatment. Younger age, higher SMN2 copy number, and higher SMN levels before treatment predicted better in vitro efficacy. These findings showcase patient-derived fibroblasts as a tool for identifying molecular predictors for personalized treatment.

SMN2 gene copy number affects the incidence and prognosis of motor neuron diseases in Japan

BACKGROUND

The copy number status (CNS) of the survival motor neuron (SMN) gene may influence the risk and prognosis of amyotrophic lateral sclerosis (ALS) and lower motor neuron diseases (LMND) other than spinal muscular atrophy (SMA). However, previous studies of this association, mainly from Europe, have yielded controversial results, suggesting possible regional differences. Here, we investigated the effect of the SMN gene in Japanese patients with ALS and LMND.

METHODS

We examined the SMN copy numbers and clinical histories of 487 Japanese patients with sporadic ALS (281 men; mean age at onset 61.5 years), 50 with adult LMND (50 men; mean age at onset 58.4 years) and 399 Japanese controls (171 men; mean age 62.2 years). Patients with pathogenic mutations in ALS-causing genes were excluded. SMN1 and SMN2 copy numbers were determined using the droplet digital polymerase chain reaction.

RESULTS

The frequency of a copy number of one for the SMN2 gene was higher in patients with ALS (38.0%) than in healthy controls (30.8%) (odds ratio (OR) = 1.37, 95% confidence interval (CI) = 1.04-1.82, p < 0.05). The SMN2 copy number affected the survival time of patients with ALS (median time: 0 copies, 34 months; 1 copy, 39 months; 2 copies, 44 months; 3 copies, 54 months; log-rank test, p < 0.05). Cox regression analysis revealed that the SMN2 copy number was associated with increased mortality (hazard ratio = 0.84, 95% CI = 0.72-0.98, p < 0.05). Also, null SMN2 cases were significantly more frequent in the LMND group (12.0%) than in the control group (4.8%) (OR = 2.73, 95% CI = 1.06-6.98, p < 0.05).

CONCLUSIONS

Our findings suggest that SMN2 copy number reduction may adversely affect the onset and prognosis of MND, including ALS and LMND, in Japanese.

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.

Biomarkers in 5q-associated spinal muscular atrophy-a narrative review

5q-associated spinal muscular atrophy (SMA) is a rare genetic disease caused by mutations in the SMN1 gene, resulting in a loss of functional SMN protein and consecutive degeneration of motor neurons in the ventral horn. The disease is clinically characterized by proximal paralysis and secondary skeletal muscle atrophy. New disease-modifying drugs driving SMN gene expression have been developed in the past decade and have revolutionized SMA treatment. The rise of treatment options led to a concomitant need of biomarkers for therapeutic guidance and an improved disease monitoring. Intensive efforts have been undertaken to develop suitable markers, and numerous candidate biomarkers for diagnostic, prognostic, and predictive values have been identified. The most promising markers include appliance-based measures such as electrophysiological and imaging-based indices as well as molecular markers including SMN-related proteins and markers of neurodegeneration and skeletal muscle integrity. However, none of the proposed biomarkers have been validated for the clinical routine yet. In this narrative review, we discuss the most promising candidate biomarkers for SMA and expand the discussion by addressing the largely unfolded potential of muscle integrity markers, especially in the context of upcoming muscle-targeting therapies. While the discussed candidate biomarkers hold potential as either diagnostic (e.g., SMN-related biomarkers), prognostic (e.g., markers of neurodegeneration, imaging-based markers), predictive (e.g., electrophysiological markers) or response markers (e.g., muscle integrity markers), no single measure seems to be suitable to cover all biomarker categories. Hence, a combination of different biomarkers and clinical assessments appears to be the most expedient solution at the time.

Innovating spinal muscular atrophy models in the therapeutic era

Spinal muscular atrophy (SMA) is a severe, monogenetic, neuromuscular disease. A thorough understanding of its genetic cause and the availability of robust models has led to the development and approval of three gene-targeting therapies. This is a unique and exciting development for the field of neuromuscular diseases, many of which remain untreatable. The development of therapies for SMA not only opens the door to future therapeutic possibilities for other genetic neuromuscular diseases, but also informs us about the limitations of such treatments. For example, treatment response varies widely and, for many patients, significant disability remains. Currently available SMA models best recapitulate the severe types of SMA, and these models are genetically and phenotypically more homogeneous than patients. Furthermore, treating patients is leading to a shift in phenotypes with increased variability in SMA clinical presentation. Therefore, there is a need to generate model systems that better reflect these developments. Here, we will first discuss current animal models of SMA and their limitations. Next, we will discuss the characteristics required to future-proof models to assist the field in the development of additional, novel therapies for SMA.

Molecular Biomarkers for the Diagnosis, Prognosis, and Pharmacodynamics of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a progressive degenerative illness that affects 1 in every 6 to 11,000 live births. This autosomal recessive disorder is caused by homozygous deletion or mutation of the SMN1 gene (survival motor neuron). As a backup, the SMN1 gene has the SMN2 gene, which produces only 10% of the functional SMN protein. Nusinersen and risdiplam, the first FDA-approved medications, act as SMN2 pre-mRNA splicing modifiers and enhance the quantity of SMN protein produced by this gene. The emergence of new therapies for SMA has increased the demand for good prognostic and pharmacodynamic (response) biomarkers in SMA. This article discusses current molecular diagnostic, prognostic, and pharmacodynamic biomarkers that could be assessed in SMA patients' body fluids. Although various proteomic, genetic, and epigenetic biomarkers have been explored in SMA patients, more research is needed to uncover new prognostic and pharmacodynamic biomarkers (or a combination of biomarkers).

Homozygous COQ7 mutation: a new cause of potentially treatable distal hereditary motor neuropathy

Distal hereditary motor neuropathy represents a group of motor inherited neuropathies leading to distal weakness. We report a family of two brothers and a sister affected by distal hereditary motor neuropathy in whom a homozygous variant c.3G>T (p.1Met?) was identified in the COQ7 gene. This gene encodes a protein required for coenzyme Q10 biosynthesis, a component of the respiratory chain in mitochondria. Mutations of COQ7 were previously associated with severe multi-organ disorders characterized by early childhood onset and developmental delay. Using patient blood samples and fibroblasts derived from a skin biopsy, we investigated the pathogenicity of the variant of unknown significance c.3G>T (p.1Met?) in the COQ7 gene and the effect of coenzyme Q10 supplementation in vitro. We showed that this variation leads to a severe decrease in COQ7 protein levels in the patient's fibroblasts, resulting in a decrease in coenzyme Q10 production and in the accumulation of 6-demethoxycoenzyme Q10, the COQ7 substrate. Interestingly, such accumulation was also found in the patient's plasma. Normal coenzyme Q10 and 6-demethoxycoenzyme Q10 levels were restored in vitro by using the coenzyme Q10 precursor 2,4-dihydroxybenzoic acid, thus bypassing the COQ7 requirement. Coenzyme Q10 biosynthesis deficiency is known to impair the mitochondrial respiratory chain. Seahorse experiments showed that the patient's cells mainly rely on glycolysis to maintain sufficient ATP production. Consistently, the replacement of glucose by galactose in the culture medium of these cells reduced their proliferation rate. Interestingly, normal proliferation was restored by coenzyme Q10 supplementation of the culture medium, suggesting a therapeutic avenue for these patients. Altogether, we have identified the first example of recessive distal hereditary motor neuropathy caused by a homozygous variation in the COQ7 gene, which should thus be included in the gene panels used to diagnose peripheral inherited neuropathies. Furthermore, 6-demethoxycoenzyme Q10 accumulation in the blood can be used to confirm the pathogenic nature of the mutation. Finally, supplementation with coenzyme Q10 or derivatives should be considered to prevent the progression of COQ7-related peripheral inherited neuropathy in diagnosed patients.

Spinal Muscular Atrophy: The Past, Present, and Future of Diagnosis and Treatment

Spinal muscular atrophy (SMA) is a lower motor neuron disease with autosomal recessive inheritance. The first cases of SMA were reported by Werdnig in 1891. Although the phenotypic variation of SMA led to controversy regarding the clinical entity of the disease, the genetic homogeneity of SMA was proved in 1990. Five years later, in 1995, the gene responsible for SMA, SMN1, was identified. Genetic testing of SMN1 has enabled precise epidemiological studies, revealing that SMA occurs in 1 of 10,000 to 20,000 live births and that more than 95% of affected patients are homozygous for SMN1 deletion. In 2016, nusinersen was the first drug approved for treatment of SMA in the United States. Two other drugs were subsequently approved: onasemnogene abeparvovec and risdiplam. Clinical trials with these drugs targeting patients with pre-symptomatic SMA (those who were diagnosed by genetic testing but showed no symptoms) revealed that such patients could achieve the milestones of independent sitting and/or walking. Following the great success of these trials, population-based newborn screening programs for SMA (more precisely, SMN1-deleted SMA) have been increasingly implemented worldwide. Early detection by newborn screening and early treatment with new drugs are expected to soon become the standards in the field of SMA.

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

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

Full-Length SMN Transcript in Extracellular Vesicles as Biomarker in Individuals with Spinal Muscular Atrophy Type 2 Treated with Nusinersen

BACKGROUND

Three therapeutic strategies have radically changed the therapeutic scenario for spinal muscular atrophy (SMA). However, therapeutic response differs between individuals. There is a need to identify biomarkers to further assess therapeutic response and to better understand which variables determine the extent of response.

METHODS

We conducted a study using an optimized digital droplet PCR-based method for the ultra-sensitive detection of SMN transcript in serum EVs from SMA 2 individuals treated with nusinersen over 14 months. In parallel, we investigated levels of serum and CSF neurofilament heavy chain (pNF-H) in the same cohort.

RESULTS

Expression of flSMN transcript in EVs of SMA 2 individuals prior to nusinersen was lower than in controls (0.40 vs 2.79 copies/ul; p < 0.05) and increased after 14 months of nusinersen (0.40 vs 1.11 copies/ul; p < 0.05). The increase in flSMN with nusinersen was significantly higher in younger individuals (p < 0.05). Serum pNF-h was higher in non-treated individuals with SMA 2 than in controls (230.72 vs 22.88 pg/ml; p < 0.05) and decreased with nusinersen (45.72 pg/ml at 6 months, 39.02 pg/ml at 14 months). CSF pNF-h in SMA 2 individuals also decreased with nusinersen (248.04 pg/ml prior to treatment, 197.10 pg/dl at 2 months, 104.43 pg/dl at 6 months, 131.03 pg/dl at 14 months).

CONCLUSIONS

We identified an increase of flSMN transcript in serum EVs of SMA 2 individuals treated with nusinersen that was more pronounced in the younger individuals. Our results indicate that flSMN transcript expression in serum EVs is a possible biomarker in SMA to predict or monitor the response to treatment.

[Modern principles of therapy for patients with spinal muscular atrophy]

Spinal muscular atrophy (SMA) is a common cause of childhood mortality among hereditary diseases of the central nervous system, which are caused by the processes of degeneration and death of motor neurons of the anterior horns of the spinal cord. An urgent issue of modern neurology is pathogenetic therapy for this group of patients, the purpose of which is to increase the level of motoneuron survival protein. We performed a search on current methods of treating SMA in Web of Science, Scopus, PubMed, Embase by the keywords: spinal muscular atrophy, neuromuscular diseases, pathogenetic therapy. Significant progress has been made in the treatment of SMA over the past 7 years. A major advance is the introduction of disease-modifying therapies using SMN2 splicing modulation or gene replacement therapy. At the moment, there are 3 FDA-approved drugs for pathogenetic therapy: Nusinersen, Risdiplam, Zolgensma. The article compares the drugs, evaluates their safety and effectiveness according to the available literature. Modern drugs for the pathogenetic therapy of SMA are highly effective and reduce the mortality rate. The results of clinical trials predict the emergence of new modern drugs. This suggests a favorable prognosis for the treatment of patients with SMA.

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

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

Spinal muscular atrophy

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

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

BACKGROUND

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

MAIN BODY

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

CONCLUSION

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

Molecular Biomarkers for Spinal Muscular Atrophy: A Systematic Review

Background

There is an unmet need for reliable biomarkers to predict disease severity, prognosis, and treatment effect in patients with spinal muscular atrophy (SMA). The purpose of this review is to evaluate the clinical utility of blood-based biomarkers in patients with SMA.

Methods

A systematic review of MEDLINE, DARE, PEDro, PsycINFO, Cochrane Database, LILACS, OTSeeker, SpeechBITE, CINAHL, Scopus, Science Direct, clinicaltrial.gov, OpenGrey, and Google Scholar was performed with the last search data of June 30, 2019.

Results

Survival motor neuron (SMN)-related biomarkers showed an important interpatient and cell variability with a wide overlap between SMA phenotypes and healthy controls. Several plasma protein analytes correlated with motor scores; however, validation studies are needed to rule out false positives. DNA methylation analysis distinguished between patients with mild/moderate SMA and healthy controls. Plasma phosphorylated neurofilament heavy chain (pNF-H) levels increased with disease severity and declined considerably after nusinersen treatment.

Conclusion

There is no sufficient evidence to support the clinical utility of SMN-related biomarkers to predict disease severity in SMA. pNF-H appears to be a promising biomarker of disease activity and treatment effect in SMA. Further studies should include longitudinal assessments of patients with SMA across functional groups and comparisons with age-matched healthy controls to evaluate the stability of putative biomarkers over time and in response to SMA therapeutics. PROSPERO registration: CRD42019139050.

Label-free quantitative proteomic analysis of extracellular vesicles released from fibroblasts derived from patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive disorder that represents a significant cause of infant mortality. SMA is characterized by reduced levels of the Survival Motor Neuron protein leading to the loss of alpha motor neurons in the spinal cord and brain stem as well as defects in peripheral tissues such as skeletal muscle and liver. With progress in promising therapies such as antisense oligonucleotide and gene replacement, there remains a need to better understand disease subtypes and develop biomarkers for improved diagnostics and therapeutic monitoring. In this study, we have examined the utility of extracellular vesicles as a source of biomarker discovery in patient-derived fibroblast cells. Proteome examination utilizing data-independent acquisition and ion mobility mass spectrometry identified 684 protein groups present in all biological replicates tested. Label-free quantitative analysis identified 116 statistically significant protein alterations compared to control cells, including several known SMA biomarkers. Protein level differences were also observed in regulators of Wnt signaling and Cajal bodies. Finally, levels of insulin growth factor binding protein-3 were validated as being significantly higher in extracellular vesicles isolated from SMA cells. We conclude that extracellular vesicles represent a promising source for SMA biomarker discovery as well as a relevant constituent for advancing our understanding of SMA pathophysiology.

Spinal Muscular Atrophy

Spinal muscular atrophy is one of the most common neuromuscular disorders of childhood and has high morbidity and mortality. Three different disease-modifying treatments were introduced in the last 4 years: nusinersen, onasemnogene abeparvovec, and risdiplam. These agents have demonstrated safety and efficacy, but their long-term benefits require further study. Newborn screening programs are enabling earlier diagnosis and treatment and better outcomes, but respiratory care and other supportive measures retain a key role in the management of spinal muscular atrophy. Ongoing efforts seek to optimize gene therapy vectors, explore new therapeutic targets beyond motor neurons, and evaluate the role of combination therapy.

Circulating microRNAs as potential biomarkers and therapeutic targets in spinal muscular atrophy

Spinal muscular atrophy (SMA), a leading genetic cause of infant death, is a neurodegenerative disease characterized by the selective loss of particular groups of motor neurons (MNs) in the anterior horn of the spinal cord with progressive muscle wasting. SMA is caused by a deficiency of the survival motor neuron (SMN) protein due to a homozygous deletion or mutation of the SMN1 gene. However, the molecular mechanisms whereby the SMN complex regulates MN functions are not fully elucidated. Emerging studies on SMA pathogenesis have turned the attention of researchers to RNA metabolism, given that increasingly identified SMN-associated modifiers are involved in both coding and non-coding RNA (ncRNA) processing. Among various ncRNAs, microRNAs (miRNAs) are the most studied in terms of regulation of posttranscriptional gene expression. Recently, the discovery that miRNAs are critical to MN function and survival led to the study of dysregulated miRNAs in SMA pathogenesis. Circulating miRNAs have drawn attention as a readily available biomarker due to their property of being clinically detectable in numerous human biofluids through non-invasive approaches. As there are recent promising findings from novel miRNA-based medicines, this article presents an extensive review of the most up-to-date studies connecting specific miRNAs to SMA pathogenesis and the potential applications of miRNAs as biomarkers and therapeutic targets for SMA.

The Identification of Novel Biomarkers Is Required to Improve Adult SMA Patient Stratification, Diagnosis and Treatment

Spinal muscular atrophy (SMA) is currently classified into five different subtypes, from the most severe (type 0) to the mildest (type 4) depending on age at onset, best motor function achieved, and copy number of the SMN2 gene. The two recent approved treatments for SMA patients revolutionized their life quality and perspectives. However, upon treatment with Nusinersen, the most widely administered therapy up to date, a high degree of variability in therapeutic response was observed in adult SMA patients. These data, together with the lack of natural history information and the wide spectrum of disease phenotypes, suggest that further efforts are needed to develop precision medicine approaches for all SMA patients. Here, we compile the current methods for functional evaluation of adult SMA patients treated with Nusinersen. We also present an overview of the known molecular changes underpinning disease heterogeneity. We finally highlight the need for novel techniques, i.e., -omics approaches, to capture phenotypic differences and to understand the biological signature in order to revise the disease classification and device personalized treatments.

Intragenic and structural variation in the SMN locus and clinical variability in spinal muscular atrophy

Clinical severity and treatment response vary significantly between patients with spinal muscular atrophy. The approval of therapies and the emergence of neonatal screening programmes urgently require a more detailed understanding of the genetic variants that underlie this clinical heterogeneity. We systematically investigated genetic variation other than SMN2 copy number in the SMN locus. Data were collected through our single-centre, population-based study on spinal muscular atrophy in the Netherlands, including 286 children and adults with spinal muscular atrophy Types 1–4, including 56 patients from 25 families with multiple siblings with spinal muscular atrophy. We combined multiplex ligation-dependent probe amplification, Sanger sequencing, multiplexed targeted resequencing and digital droplet polymerase chain reaction to determine sequence and expression variation in the SMN locus. SMN1, SMN2 and NAIP gene copy number were determined by multiplex ligation-dependent probe amplification. SMN2 gene variant analysis was performed using Sanger sequencing and RNA expression analysis of SMN by droplet digital polymerase chain reaction. We identified SMN1–SMN2 hybrid genes in 10% of spinal muscular atrophy patients, including partial gene deletions, duplications or conversions within SMN1 and SMN2 genes. This indicates that SMN2 copies can vary structurally between patients, implicating an important novel level of genetic variability in spinal muscular atrophy. Sequence analysis revealed six exonic and four intronic SMN2 variants, which were associated with disease severity in individual cases. There are no indications that NAIP1 gene copy number or sequence variants add value in addition to SMN2 copies in predicting the clinical phenotype in individual patients with spinal muscular atrophy. Importantly, 95% of spinal muscular atrophy siblings in our study had equal SMN2 copy numbers and structural changes (e.g. hybrid genes), but 60% presented with a different spinal muscular atrophy type, indicating the likely presence of further inter- and intragenic variabilities inside as well as outside the SMN locus. SMN2 gene copies can be structurally different, resulting in inter- and intra-individual differences in the composition of SMN1 and SMN2 gene copies. This adds another layer of complexity to the genetics that underlie spinal muscular atrophy and should be considered in current genetic diagnosis and counselling practices.

Human Dermal Fibroblast: A Promising Cellular Model to Study Biological Mechanisms of Major Depression and Antidepressant Drug Response

BACKGROUND

Human dermal fibroblasts (HDF) can be used as a cellular model relatively easily and without genetic engineering. Therefore, HDF represent an interesting tool to study several human diseases including psychiatric disorders. Despite major depressive disorder (MDD) being the second cause of disability in the world, the efficacy of antidepressant drug (AD) treatment is not sufficient and the underlying mechanisms of MDD and the mechanisms of action of AD are poorly understood.

OBJECTIVE

The aim of this review is to highlight the potential of HDF in the study of cellular mechanisms involved in MDD pathophysiology and in the action of AD response.

METHODS

The first part is a systematic review following PRISMA guidelines on the use of HDF in MDD research. The second part reports the mechanisms and molecules both present in HDF and relevant regarding MDD pathophysiology and AD mechanisms of action.

RESULTS

HDFs from MDD patients have been investigated in a relatively small number of works and most of them focused on the adrenergic pathway and metabolism-related gene expression as compared to HDF from healthy controls. The second part listed an important number of papers demonstrating the presence of many molecular processes in HDF, involved in MDD and AD mechanisms of action.

CONCLUSION

The imbalance in the number of papers between the two parts highlights the great and still underused potential of HDF, which stands out as a very promising tool in our understanding of MDD and AD mechanisms of action.

Multifaceted roles of microRNAs: From motor neuron generation in embryos to degeneration in spinal muscular atrophy

Two crucial questions in neuroscience are how neurons establish individual identity in the developing nervous system and why only specific neuron subtypes are vulnerable to neurodegenerative diseases. In the central nervous system, spinal motor neurons serve as one of the best-characterized cell types for addressing these two questions. In this review, we dissect these questions by evaluating the emerging role of regulatory microRNAs in motor neuron generation in developing embryos and their potential contributions to neurodegenerative diseases such as spinal muscular atrophy (SMA). Given recent promising results from novel microRNA-based medicines, we discuss the potential applications of microRNAs for clinical assessments of SMA disease progression and treatment.

Age-dependent SMN expression in disease-relevant tissue and implications for SMA treatment

BACKGROUNDSpinal muscular atrophy (SMA) is caused by deficient expression of survival motor neuron (SMN) protein. New SMN-enhancing therapeutics are associated with variable clinical benefits. Limited knowledge of baseline and drug-induced SMN levels in disease-relevant tissues hinders efforts to optimize these treatments.METHODSSMN mRNA and protein levels were quantified in human tissues isolated during expedited autopsies.RESULTSSMN protein expression varied broadly among prenatal control spinal cord samples, but was restricted at relatively low levels in controls and SMA patients after 3 months of life. A 2.3-fold perinatal decrease in median SMN protein levels was not paralleled by comparable changes in SMN mRNA. In tissues isolated from nusinersen-treated SMA patients, antisense oligonucleotide (ASO) concentration and full-length (exon 7 including) SMN2 (SMN2-FL) mRNA level increases were highest in lumbar and thoracic spinal cord. An increased number of cells showed SMN immunolabeling in spinal cord of treated patients, but was not associated with an increase in whole-tissue SMN protein levels.CONCLUSIONSA normally occurring perinatal decrease in whole-tissue SMN protein levels supports efforts to initiate SMN-inducing therapies as soon after birth as possible. Limited ASO distribution to rostral spinal and brain regions in some patients likely limits clinical response of motor units in these regions for those patients. These results have important implications for optimizing treatment of SMA patients and warrant further investigations to enhance bioavailability of intrathecally administered ASOs.FUNDINGSMA Foundation, SMART, NIH (R01-NS096770, R01-NS062869), Ionis Pharmaceuticals, and PTC Therapeutics. Biogen provided support for absolute real-time RT-PCR.

Validation of a Fast, Robust, Inexpensive, Two-Tiered Neonatal Screening Test algorithm on Dried Blood Spots for Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is one of the leading genetic causes of infant mortality with an incidence of 1:10,000. The recently-introduced antisense oligonucleotide treatment improves the outcome of this disease, in particular when applied at an early stage of progression. The genetic cause of SMA is, in >95% of cases, a homozygous deletion of the survival motor neuron 1 (SMN1) gene, which makes the low-cost detection of SMA cases as part of newborn screening programs feasible. We developed and validated a new SALSA MC002 melting curve assay that detects the absence of the SMN1 exon 7 DNA sequence without detecting asymptomatic carriers and reliably discriminates SMN1 from its genetic homolog SMN2 using crude extracts from newborn screening cards. Melting curve analysis shows peaks specific for both the SMN1 gene and the disease modifying SMN2 homolog. The detection of the SMN2 homolog, of which the only clinically relevant difference from the SMN1 gene is a single nucleotide in exon 7, was only used to confirm a correct reaction in samples that lacked the SMN1 gene, and not for SMN2 quantification. We retrieved 47 DBS samples from children with genetically-confirmed SMA, after informed consent from parents, and 375 controls from the national archive of the Dutch National Institute for Public Health and the Environment (RIVM). The assay correctly identified all anonymized and randomized SMA and control samples (i.e., sensitivity and specificity of 100%), without the detection of carriers, on the three most commonly-used PCR platforms with melting curve analysis. This test's concordance with the second-tier 'golden standard' P021 SMA MLPA test was 100%. Using the new P021-B1 version, crude extracts from DBS cards could also be used to determine the SMN2 copy number of SMA patients with a high level of accuracy. The MC002 test showed the feasibility and accuracy of SMA screening in a neonatal screening program.

Temporal and tissue-specific variability of SMN protein levels in mouse models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a progressive motor neuron disease caused by deleterious variants in SMN1 that lead to a marked decrease in survival motor neuron (SMN) protein expression. Humans have a second SMN gene (SMN2) that is almost identical to SMN1. However, due to alternative splicing the majority of SMN2 messenger ribonucleic acid (mRNA) is translated into a truncated, unstable protein that is quickly degraded. Because the presence of SMN2 provides a unique opportunity for therapy development in SMA patients, the mechanisms that regulate SMN2 splicing and mRNA expression have been elucidated in great detail. In contrast, how much SMN protein is produced at different developmental time points and in different tissues remains under-characterized. In this study, we addressed this issue by determining SMN protein expression levels at three developmental time points across six different mouse tissues and in two distinct mouse models of SMA (‘severe’ Taiwanese and ‘intermediate’ Smn^2B/− mice). We found that, in healthy control mice, SMN protein expression was significantly influenced by both age and tissue type. When comparing mouse models of SMA, we found that, despite being transcribed from genetically different alleles, control SMN levels were relatively similar. In contrast, the degree of SMN depletion between tissues in SMA varied substantially over time and between the two models. These findings offer an explanation for the differential vulnerability of tissues and organs observed in SMA and further our understanding of the systemic and temporal requirements for SMN with direct relevance for developing effective therapies for SMA.

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

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

Lumbosacral ventral spinal nerve root atrophy identified on MRI in a case of spinal muscular atrophy type II

Spinal muscular atrophies are rare genetic disorders most often caused by homozygous deletion mutations in SMN1 that lead to progressive neurodegeneration of anterior horn cells. Ventral spinal root atrophy is a consistent pathological finding in post-mortem examinations of patients who suffered from various subtypes of spinal muscular atrophy; however, corresponding radiographic findings have not been previously reported. We present a patient with hypotonia and weakness who was found to have ventral spinal root atrophy in the lumbosacral region on MRI and was subsequently diagnosed with spinal muscular atrophy. More systematic analyses of imaging studies in spinal muscular atrophy will help determine whether such findings have the potential to serve as reliable diagnostic markers for clinical evaluations or as outcome measure for clinical trials.

Evaluation of potential effects of Plastin 3 overexpression and low-dose SMN-antisense oligonucleotides on putative biomarkers in spinal muscular atrophy mice

OBJECTIVES

Spinal muscular atrophy (SMA) is a devastating motor neuron disorder caused by homozygous loss of the survival motor neuron 1 (SMN1) gene and insufficient functional SMN protein produced by the SMN2 copy gene. Additional genetic protective modifiers such as Plastin 3 (PLS3) can counteract SMA pathology despite insufficient SMN protein. Recently, Spinraza, an SMN antisense oligonucleotide (ASO) that restores full-length SMN2 transcripts, has been FDA- and EMA-approved for SMA therapy. Hence, the availability of biomarkers allowing a reliable monitoring of disease and therapy progression would be of great importance. Our objectives were (i) to analyse the feasibility of SMN and of six SMA biomarkers identified by the BforSMA study in the Taiwanese SMA mouse model, (ii) to analyse the effect of PLS3 overexpression on these biomarkers, and (iii) to assess the impact of low-dose SMN-ASO therapy on the level of SMN and the six biomarkers.

METHODS

At P10 and P21, the level of SMN and six putative biomarkers were compared among SMA, heterozygous and wild type mice, with or without PLS3 overexpression, and with or without presymptomatic low-dose SMN-ASO subcutaneous injection. SMN levels were measured in whole blood by ECL immunoassay and of six SMA putative biomarkers, namely Cartilage Oligomeric Matrix Protein (COMP), Dipeptidyl Peptidase 4 (DPP4), Tetranectin (C-type Lectin Family 3 Member B, CLEC3B), Osteopontin (Secreted Phosphoprotein 1, SPP1), Vitronectin (VTN) and Fetuin A (Alpha 2-HS Glycoprotein, AHSG) in plasma.

RESULTS

SMN levels were significantly discernible between SMA, heterozygous and wild type mice. However, no significant differences were measured upon low-dose SMN-ASO treatment compared to untreated animals. Of the six biomarkers, only COMP and DPP4 showed high and SPP1 moderate correlation with the SMA phenotype. PLS3 overexpression neither influenced the SMN level nor the six biomarkers, supporting the hypothesis that PLS3 acts as an independent protective modifier.

Advances in therapy for spinal muscular atrophy: promises and challenges

Spinal muscular atrophy (SMA) is a devastating motor neuron disease that predominantly affects children and represents the most common cause of hereditary infant mortality. The condition results from deleterious variants in SMN1, which lead to depletion of the survival motor neuron protein (SMN). Now, 20 years after the discovery of this genetic defect, a major milestone in SMA and motor neuron disease research has been reached with the approval of the first disease-modifying therapy for SMA by US and European authorities - the antisense oligonucleotide nusinersen. At the same time, promising data from early-stage clinical trials of SMN1 gene therapy have indicated that additional therapeutic options are likely to emerge for patients with SMA in the near future. However, the approval of nusinersen has generated a number of immediate and substantial medical, ethical and financial implications that have the potential to resonate beyond the specific treatment of SMA. Here, we provide an overview of the rapidly evolving therapeutic landscape for SMA, highlighting current achievements and future opportunities. We also discuss how these developments are providing important lessons for the emerging second generation of combinatorial ('SMN-plus') therapies that are likely to be required to generate robust treatments that are effective across a patient's lifespan.

Muscle strength and motor function throughout life in a cross-sectional cohort of 180 patients with spinal muscular atrophy types 1c-4

BACKGROUND AND PURPOSE

Natural history studies in spinal muscular atrophy (SMA) have primarily focused on infants and children. Natural history studies encompassing all age groups and SMA types are important for the interpretation of treatment effects of recently introduced survival motor neuron gene-augmenting therapies.

METHODS

We conducted a cross-sectional study to investigate muscle strength, Hammersmith Functional Motor Scale (Expanded) score and the patterns of muscle weakness in relation to age and SMA type.

RESULTS

We included 180 patients with SMA types 1-4 in the age range 1-77.5 years with median disease duration of 18 (range 0-65.8) years. With the exception of the early phases of disease in which children with SMA types 2 and 3 may achieve new motor skills and show a temporary increase in muscle strength, cross-sectional data suggested that declining muscle strength and loss of motor skills over time are characteristic of all SMA types. Mean loss of strength was at least 1 point on the Medical Research Council score and 0.5 point on the Hammersmith Functional Motor Scale (Expanded) score per year. Trend lines compatible with deterioration of motor function and muscle strength started in childhood and continued into adulthood. The age at loss of specific motor skills was associated with disease severity. Triceps, deltoid, iliopsoas and quadriceps were the weakest muscles in all patients. Hierarchical cluster analysis did not show a segmental distribution of muscle weakness as suggested previously.

CONCLUSIONS

Progressive muscle weakness and loss of motor function are characteristic of all SMA types and all ages.

Survival Motor Neuron Protein is Released from Cells in Exosomes: A Potential Biomarker for Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is caused by homozygous mutation of the survival motor neuron 1 (SMN1) gene. Disease severity inversely correlates to the amount of SMN protein produced from the homologous SMN2 gene. We show that SMN protein is naturally released in exosomes from all cell types examined. Fibroblasts from patients or a mouse model of SMA released exosomes containing reduced levels of SMN protein relative to normal controls. Cells overexpressing SMN protein released exosomes with dramatically elevated levels of SMN protein. We observed enhanced quantities of exosomes in the medium from SMN-depleted cells, and in serum from a mouse model of SMA and a patient with Type 3 SMA, suggesting that SMN-depletion causes a deregulation of exosome release or uptake. The quantity of SMN protein contained in the serum-derived exosomes correlated with the genotype of the animal, with progressively less protein in carrier and affected animals compared to wildtype mice. SMN protein was easily detectable in exosomes isolated from human serum, with a reduction in the amount of SMN protein in exosomes from a patient with Type 3 SMA compared to a normal control. Our results suggest that exosome-derived SMN protein may serve as an effective biomarker for SMA.