The molecular bases of spinal muscular atrophy

doi:10.1016/S0959-437X(02)00301-5

Current Opinion in Genetics & Development

Volume 12, Issue 3, 1 June 2002, Pages 294-298

https://doi.org/10.1016/S0959-437X(02)00301-5 Get rights and content

Abstract

Spinal muscular atrophy (SMA) is a common recessive autosomal disorder characterized by degeneration of motor neurons of the spinal cord. SMA is caused by mutations of the survival of motor neuron gene that encodes a multifunctional protein, and mouse models have been generated. These advances represent starting points towards an understanding of the pathophysiology of this disease and the design of therapeutic strategies in SMA.

Introduction

Spinal muscular atrophy (SMA) is characterized by the degeneration of motor neurons of the spinal cord associated with muscle paralysis and atrophy. Childhood SMA is a recessive autosomal disorder that represents one of the most common genetic causes of death in childhood (incidence 1:6000–10 000). Based on age of onset of symptoms, achievement of motor milestones and age at death, childhood SMA has been subdivided into three clinical types [1]. The acute form of Werdnig–Hoffmann disease (type I) is characterized by severe, generalized muscle weakness at birth or within the first 6 months. Death usually occurs within the first 2 years. Type II children are able to sit, although they cannot walk unaided, and they survive beyond 2 years. In type III SMA (or Kugelberg–Welander disease), patients have proximal muscle weakness, starting after the age of 18 months. The clinical features result from skeletal muscle denervation. The pathophysiology remains unknown and no curative treatment is available so far.

Section snippets

Genetic bases of SMA

The characterization of the SMA locus on chromosome 5q13 revealed a chromosomal region characterized by an inverted duplication, each element (∼500kb) containing several genes. The smallest deletions involving the telomeric copy of the Survival of motor neuron gene (SMN; renamed SMN1) and the presence of intragenic mutations of SMN1 in patients—including missense, non-sense or splice site mutations—have pinpointed SMN1 as the gene mutated in SMA 2., 3., 4., 5., 6., 7..

SMN1 is duplicated with a

SMN: a multifunctional protein

SMN is an ubiquitously expressed protein of 294 amino acids, a molecular weight of 38kDa, and no significant homology to any other protein. SMN associates with several proteins to form a large multiprotein complex. The SMN complex is found both in the cytoplasm and in the nucleus where it is concentrated in a structure known as ‘gems’ (for ‘gemini of coiled bodies’) most often associated with or identical to Cajal bodies (coiled bodies) depending on the cell type or tissue analyzed [15]. In

Mouse models of SMA

Early embryonic lethality of mice knocking out for the Smn gene has led to the adoption of sophisticated transgenic approaches to create mouse models (Fig. 2; 10•., 35., 36•., 40•., 41•.). One strategy was based on the generation of mice carrying a genomic organization similar to that of human SMA. It comprises the creation of two mouse lines: one carrying a deletion of Smn through homologous recombination and the other line carrying a transgene expressing the human SMN2 gene. Mice carrying

Conclusions

Potential therapeutic benefit in SMA will depend, in part, on the capacity of the remaining mutant motor neurons to re-innervate skeletal muscle fibers. A tetracycline-responsive gene system should allow the expression of wild-type SMN transgene at different stages of the disease developed by the SMA mouse models [44]. This approach will help in evaluating the repair capacity of motor neurons. Finding the answer to these questions may have profound implications for the development of therapy in

Acknowledgements

Work in our laboratory was supported by INSERM, the Association Française contre les Myopathies, the Fondation pour la Recherche Médicale, Families of SMA (USA), Andrew's Buddies (USA) and Genopole.

References and recommended reading

Papers of particular interest, published within the annual period of review,have been highlighted as:

• of special interest
•• of outstanding interest

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Cited by (96)

Glial cells involvement in spinal muscular atrophy: Could SMA be a neuroinflammatory disease?
2020, Neurobiology of Disease
Citation Excerpt :
In the majority of cases, the genetic culprits are biallelic deletions or mutations in the Survival Motor Neuron 1 gene (SMN1) on chromosome 5 (5q13.2), leading to deficiency of the SMN protein (Frugier et al., 2002). Among them, the most common mutation (present in almost 95% of the patient with SMA) is the deletion of exon 7 in SMN1 (Frugier et al., 2002). Although its biological roles have not been completely elucidated yet, it is known that SMN, in collaboration with partner proteins, catalyzes the assembly of snRNPs (which are building blocks for pre-mRNA splicing) (Juntas Morales et al., 2017).
Spinal muscular atrophy (SMA) is a severe, inherited disease characterized by the progressive degeneration and death of motor neurons of the anterior horns of the spinal cord, which results in muscular atrophy and weakness of variable severity. Its early-onset form is invariably fatal in early childhood, while milder forms lead to permanent disability, physical deformities and respiratory complications. Recently, two novel revolutionary therapies, antisense oligonucleotides and gene therapy, have been approved, and might prove successful in making long-term survival of these patients likely. In this perspective, a deep understanding of the pathogenic mechanisms and of their impact on the interactions between motor neurons and other cell types within the central nervous system (CNS) is crucial. Studies using SMA animal and cellular models have taught us that the survival and functionality of motor neurons is highly dependent on a whole range of other cell types, namely glial cells, which are responsible for a variety of different functions, such as neuronal trophic support, synaptic remodeling, and immune surveillance. Thus, it emerges that SMA is likely a non-cell autonomous, multifactorial disease in which the interaction of different cell types and disease mechanisms leads to motor neurons failure and loss. This review will introduce the different glial cell types in the CNS and provide an overview of the role of glial cells in motor neuron degeneration in SMA. Furthermore, we will discuss the relevance of these findings so far and the potential impact on the success of available therapies and on the development of novel ones.
Cellular bases of the RNA metabolism dysfunction in motor neurons of a murine model of spinal muscular atrophy: Role of Cajal bodies and the nucleolus
2017, Neurobiology of Disease
Citation Excerpt :
Thus, the majority of the mRNA transcripts from SMN2 generate an alternatively spliced isoform that lacks exon 7 and encodes a truncated form of the SMN protein (SMN∆7) that is rapidly degraded (Cho and Dreyfuss, 2010). Only ~ 10% of the SMN2 transcripts undergo a normal splicing and encode the full-length SMN protein (Coady and Lorson, 2011; Frugier et al., 2002; Lefebvre et al., 1995; Monani, 2005). The best-known functions of SMN are the biogenesis of spliceosomal snRNPs, the axonal transport of certain mRNAs and the formation and maintenance of the neuromuscular junctions (Goulet et al., 2013).
Spinal muscular atrophy (SMA) is caused by a homozygous deletion or mutation in the survival motor neuron 1 (SMN1) gene that leads to reduced levels of SMN protein resulting in degeneration of motor neurons (MNs). The best known functions of SMN is the biogenesis of spliceosomal snRNPs. Linked to this function, Cajal bodies (CBs) are involved in the assembly of spliceosomal (snRNPs) and nucleolar (snoRNPs) ribonucleoproteins required for pre-mRNA and pre-rRNA processing. Recent studies support that the interaction between CBs and nucleoli, which are especially prominent in neurons, is essential for the nucleolar rRNA homeostasis.
We use the SMN∆7 murine model of type I SMA to investigate the cellular basis of the dysfunction of RNA metabolism in MNs. SMN deficiency in postnatal MNs produces a depletion of functional CBs and relocalization of coilin, which is a scaffold protein of CBs, in snRNP-free perinucleolar caps or within the nucleolus. Disruption of CBs is the earliest nuclear sign of MN degeneration. We demonstrate that depletion of CBs, with loss of CB-nucleolus interactions, induces a progressive nucleolar dysfunction in ribosome biogenesis. It includes reorganization and loss of nucleolar transcription units, segregation of dense fibrillar and granular components, retention of SUMO-conjugated proteins in intranucleolar bodies and a reactive, compensatory, up-regulation of mature 18S rRNA and genes encoding key nucleolar proteins, such as upstream binding factor, fibrillarin, nucleolin and nucleophosmin.
We propose that CB depletion and nucleolar alterations are essential components of the dysfunction of RNA metabolism in SMA.
Combination of valproic acid and morpholino splice-switching oligonucleotide produces improved outcomes in spinal muscular atrophy patient-derived fibroblasts
2017, Neurochemistry International
Citation Excerpt :
Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease that affects between 1/6000–1/10,000 children (Frugier et al., 2002; Smith et al., 2007; Tisdale and Pellizzoni, 2015).
Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality worldwide, is characterised by the homozygous loss of the survival motor neuron 1 (SMN1) gene. The consequent degeneration of spinal motor neurons and progressive atrophy of voluntary muscle groups results in paralysis and eventually premature infantile death. Humans possess a second nearly identical copy of SMN1, known as SMN2. However, SMN2 produces only 10–20% functional SMN protein due to aberrant splicing of its pre-mRNA that leads to the exclusion of exon 7. This level of SMN is insufficient to rescue the phenotype. Recently developed splice-switching antisense oligonuclotides (SSO) have shown great promise in correcting the aberrant splicing of SMN2 towards producing functional SMN protein. Several FDA approved drugs are being repurposed for SMA treatment including valproic acid (VPA), a histone deacetylase inhibitor, which has been shown to increase overall SMN2 expression. In this study, we have characterised the effects of single and combined treatment of VPA and a SSO based on phosphorodiamidate morpholino oligomer (PMO) chemistry. We conjugated both VPA and PMO to a single cell-penetrating peptide (Apolipoprotein E (ApoE)) for their simultaneous intracellular delivery. Treatment of SMA Type I patient-derived fibroblasts with the conjugates showed no additive increase in the level of full-length SMN2 mRNA expression over both 4 and 16 h treatments indicating that conjugation of VPA to ApoE-PMO has limited benefit. However, treatment with a combination of VPA and ApoE-PMO induced more favourable splice switching activity than either agent alone, promoting exon 7 inclusion in SMN2 transcripts. Our results suggest that combination therapy of VPA and ApoE-PMO is superior in upregulating SMN2 production in vitro, as compared to singular treatment of each compound at both transcriptional and protein levels. This study provides the first indication of a novel dual therapy approach for the potential treatment of SMA.
Imaging Flow Cytometry Analysis to Identify Differences of Survival Motor Neuron Protein Expression in Patients With Spinal Muscular Atrophy
2016, Pediatric Neurology
Citation Excerpt :
SMN2 is a highly homologous copy of SMN1 that, although duplicated and present in all SMA patients, is unable to compensate for defects in SMN1. The result is that SMA patients have low levels of full-length SMN.4 Accordingly, previous studies using Western blotting and immunocytochemistry have demonstrated that the expression of SMN is inversely correlated with the phenotypic severity of SMA.5,6
Spinal muscular atrophy is a neurodegenerative disorder caused by the deficient expression of survival motor neuron protein in motor neurons. A major goal of disease-modifying therapy is to increase survival motor neuron expression. Changes in survival motor neuron protein expression can be monitored via peripheral blood cells in patients; therefore we tested the sensitivity and utility of imaging flow cytometry for this purpose.
After the immortalization of peripheral blood lymphocytes from a human healthy control subject and two patients with spinal muscular atrophy type 1 with two and three copies of SMN2 gene, respectively, we used imaging flow cytometry analysis to identify significant differences in survival motor neuron expression. A bright detail intensity analysis was used to investigate differences in the cellular localization of survival motor neuron protein.
Survival motor neuron expression was significantly decreased in cells derived from patients with spinal muscular atrophy relative to those derived from a healthy control subject. Moreover, survival motor neuron expression correlated with the clinical severity of spinal muscular atrophy according to SMN2 copy number. The cellular accumulation of survival motor neuron protein was also significantly decreased in cells derived from patients with spinal muscular atrophy relative to those derived from a healthy control subject.
The benefits of imaging flow cytometry for peripheral blood analysis include its capacities for analyzing heterogeneous cell populations; visualizing cell morphology; and evaluating the accumulation, localization, and expression of a target protein. Imaging flow cytometry analysis should be implemented in future studies to optimize its application as a tool for spinal muscular atrophy clinical trials.
Whole Genome Sequencing in the Molecular Pathology Laboratory
2016, Diagnostic Molecular Pathology: A Guide to Applied Molecular Testing
With the introduction of massively parallel sequencing (including whole exome sequencing and whole genome sequencing), the field of molecular diagnostics is moving incredibly rapidly toward platforms that will interrogate many or all genes simultaneously. These approaches have many benefits and limitations that are being identified and overcome simultaneously with their clinical deployment. Variant interpretation is and will likely continue to be a challenge for the foreseeable future, particularly as we begin to consider the clinical relevance of noncoding variants as part of the scope of clinical diagnostic evaluations. This chapter takes a broad look at the scope of whole genome sequencing and how it may be incorporated into molecular diagnostic testing.
A novel evaluation method of survival motor neuron protein as a biomarker of spinal muscular atrophy by imaging flow cytometry
2014, Biochemical and Biophysical Research Communications
Spinal muscular atrophy (SMA) is caused by mutations within the survival motor neuron 1 (SMN1) gene. These mutations result in the reduction of survival motor neuron (SMN) protein expression and SMN complex in spinal motor neurons and other tissues. SMN protein has been used as a therapeutic biomarker in recent SMA clinical studies using enzyme-linked immunosorbent assay (ELISA). Here, we investigated whether imaging flow cytometry can be a viable source of quantitative information on the SMN protein. Using a FlowSight imaging flow cytometer (Merck-Millipore, Germany), we demonstrated that imaging flow cytometry could successfully identify different expression patterns and subcellular localization of SMN protein in healthy human fibroblasts and SMA patient-derived fibroblasts. In addition, we could also evaluate the therapeutic effects of SMN protein expression by valproic acid treatment of SMA patient-derived cells in vitro. Therefore, we suggest that imaging flow cytometry technology has the potential for identifying SMN protein expression level and pattern as an evaluation tool of clinical studies.

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View full text

ReviewThe molecular bases of spinal muscular atrophy

Abstract

Introduction

Section snippets

Genetic bases of SMA

SMN: a multifunctional protein

Mouse models of SMA

Conclusions

Acknowledgements

References and recommended reading

Neuromuscular Disord

Cell

Am J Hum Genet

Am J Hum Genet

Cell

J Biol Chem

Cell

J Biol Chem

Curr Biol

Mol Cell

J Biol Chem

Cell

FEBS Lett

J Biol Chem

FEBS Lett

Lancet

Cell

De novo and inherited deletions of the 5q13 region in spinal muscular atrophies

Science

Frameshift mutation in the survival motor neuron gene in a severe case of SMA type I

Hum Mol Genet

A frame-shift deletion in the survival motor neuron gene in Spanish spinal muscular atrophy patients

Nat Genet

Missense mutations in exon 6 of the survival motor neuron gene in patients with spinal muscular atrophy (SMA)

Hum Mol Genet

An 11 base pair duplication in exon 6 of the SMN gene produces a type I spinal muscular atrophy (SMA) phenotype: further evidence for SMN as the primary SMA-determining gene

Hum Mol Genet

A single nucleotide in the SMN gene regulates splicing and is responsible for spinal muscular atrophy

Proc Natl Acad Sci USA

An exonic enhancer is required for inclusion of an essential exon in the SMA-determining gene SMN

Hum Mol Genet

Deletion of murine smn exon 7 directed to skeletal muscle leads to severe muscular dystrophy

J Cell Biol

Review
The molecular bases of spinal muscular atrophy