Is Werdnig-Hoffmann disease a pure lower motor neuron disorder?

Disrupted individual-level morphological brain network in spinal muscular atrophy types 2 and 3

BACKGROUND AND OBJECTIVE

Spinal muscular atrophy (SMA) is one of the most common monogenic neuromuscular diseases, and the pathogenesis mechanisms, especially the brain network topological properties, remain unknown. This study aimed to use individual-level morphological brain network analysis to explore the brain neural network mechanisms in SMA.

METHODS

Individual-level gray matter (GM) networks were constructed by estimating the interregional similarity of GM volume distribution using both Kullback-Leibler divergence-based similarity (KLDs) and Jesen-Shannon divergence-based similarity (JSDs) measurements based on Automated Anatomical Labeling 116 and Hammersmith 83 atlases for 38 individuals with SMA types 2 and 3 and 38 age- and sex-matched healthy controls (HCs). The topological properties were analyzed by the graph theory approach and compared between groups by a nonparametric permutation test. Additionally, correlation analysis was used to assess the associations between altered topological metrics and clinical characteristics.

RESULTS

Compared with HCs, although global network topology remained preserved in individuals with SMA, brain regions with altered nodal properties mainly involved the right olfactory gyrus, right insula, bilateral parahippocampal gyrus, right amygdala, right thalamus, left superior temporal gyrus, left cerebellar lobule IV-V, bilateral cerebellar lobule VI, right cerebellar lobule VII, and vermis VII and IX. Further correlation analysis showed that the nodal degree of the right cerebellar lobule VII was positively correlated with the disease duration, and the right amygdala was negatively correlated with the Hammersmith Functional Motor Scale Expanded (HFMSE) scores.

CONCLUSIONS

Our findings demonstrated that topological reorganization may prioritize global properties over nodal properties, and disrupted topological properties in the cortical-limbic-cerebellum circuit in SMA may help to further understand the network pathogenesis underlying SMA.

Profile of cognitive abilities in spinal muscular atrophy type II and III: what is the role of motor impairment?

There has recently been some concern on possible cognitive impairment in patients with Spinal Muscular Atrophy (SMA). The aim of this study was to assess cognitive profiles in type II and III SMA with a focus on individual indexes and possible correlations with motor function. 57 type II and III individuals, aged 3.5-17 years, were consecutively enrolled in a prospective, multicentric study. Cognitive function was assessed using age-appropriate Weschler Scales. Motor function was concomitantly assessed using disease-specific functional scales. Only 2 individuals (3%) had a intellectual disability of mild degree while the others were within normal range, with no significant difference in relation to SMA type, gender or functional status. While the overall quotients were mostly within normal range, some indexes showed wider variability. A significant positive medium correlation was found between Processing Speed Index and motor functional scores. Working memory had lower scores in type III patients compared to type II. Intellectual disability is uncommon in type II and III SMA. Motor functional abilities may play a role in some of the items contributing to the overall cognitive profile.

Cerebellar structural, astrocytic, and neuronal abnormalities in the SMNΔ7 mouse model of spinal muscular atrophy

Spinalmuscular atrophy (SMA) is a neuromuscular disease that affects as many as 1 in 6000 individuals at birth, making it the leading genetic cause of infant mortality. A growing number of studies indicate that SMA is a multi-system disease. The cerebellum has received little attention even though it plays an important role in motor function and widespread pathology has been reported in the cerebella of SMA patients. In this study, we assessed SMA pathology in the cerebellum using structural and diffusion magnetic resonance imaging, immunohistochemistry, and electrophysiology with the SMNΔ7 mouse model. We found a significant disproportionate loss in cerebellar volume, decrease in afferent cerebellar tracts, selective lobule-specific degeneration of Purkinje cells, abnormal lobule foliation and astrocyte integrity, and a decrease in spontaneous firing of cerebellar output neurons in the SMA mice compared to controls. Our data suggest that defects in cerebellar structure and function due to decreased survival motor neuron (SMN) levels impair the functional cerebellar output affecting motor control, and that cerebellar pathology should be addressed to achieve comprehensive treatment and therapy for SMA patients.

Brain Magnetic Resonance Imaging (MRI) in Spinal Muscular Atrophy: A Scoping Review

BACKGROUND

5q Spinal Muscular Atrophy (SMA) is a prototypical lower motor neuron disorder. However, the characteristic early motor impairment raises the question on the scope of brain involvement with implications for further investigations on the brain as a potential therapeutic target.

OBJECTIVE

To review changes across the SMA clinical spectrum reported on brain magnetic resonance imaging (MRI).

METHODS

We conducted a scoping review of existing literature on PubMed and EMBASE. Two reviewers searched and retrieved relevant articles on magnetic resonance brain imaging in individuals with SMA censoring to April 2022. Full-text articles published in peer-reviewed journals or abstracts accepted to conferences in English and French were included.

RESULTS

Twelve articles were identified describing a total of 39 patients [age range: 11 days to 41 years old, type 0 (n = 5), type 1 (n = 4), type 2 (n = 2), type 3 (n = 22), type 4 (n = 6)]. All reported structural changes and did not explore other MRI modalities. In individuals with infantile onset SMA, cortical and subcortical brain abnormalities in white matter, basal ganglia, thalamus, hippocampus, and high intensity areas around lateral ventricles and thalami were reported over time. In individuals with later-onset SMA, reduced cerebellar and lobular volume were observed as well as increased grey matter density in motor areas.

CONCLUSIONS

Limited data on brain imaging in SMA highlights both cortical and subcortical involvement in SMA, supporting the hypothesis that changes are not restricted to lower motor neuron pathways. Further studies are needed to determine the extent and prevalence of structural and functional brain changes across SMA types.

Spinal muscular atrophy type 2 patient who survived 61 years: an autopsy case report

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease characterized by progressive muscle weakness due to degeneration of lower motor neurons in the anterior horn of the spinal cord. We analyzed autopsy findings of a male patient with SMA type 2 who survived until 61 years of age. Genetic analysis revealed a homozygous deletion of the survival motor neuron (SMN) gene 1 (SMN1) exon 7, confirming the diagnosis of SMA. Results of further analyses indicated that the patient had two copies of the genuine SMN gene 2 (SMN2) and one copy of a hybrid gene containing SMN2 exon 7 and SMN1 exon 8. Pathological examination revealed moderate neuronal loss of the anterior horn and appearance of heterotopic neurons in the lateral funiculus, whereas a few achromatic neurons were notably localized in the anterior horn of the lumbar segment. Microdysgenesis as a consequence of migration disturbance was found in the white matter of the frontal lobe, postulating the possibility of the maldevelopment of the nervous system.

Skin necrosis in spinal muscular atrophy: Case report and review of the literature

Spinal muscular atrophy (SMA) type 0 is the most severe phenotype of SMA and is characterized by hypotonia, muscle weakness, and respiratory distress. Cutaneous necrosis, first described in an SMA mouse model, can occur in patients with severe disease; the use of targeted treatment versus supportive measures in the setting of skin necrosis is debated. We present a male infant with SMA type 0 with cutaneous necrosis of proximal and distal limbs who improved with supportive care. The seven previously reported cases of SMA skin necrosis are reviewed.

Brain, cognition, and language development in spinal muscular atrophy type 1: a scoping review

AIM

To summarize the current knowledge on brain involvement in spinal muscular atrophy (SMA) type 1, focusing on brain pathology, cognition, and speech/language development.

METHOD

A scoping review was performed using the methodology of the Joanna Briggs Institute. Five databases and references from relevant articles were searched up to December 2019. Articles were screened on the basis of titles and abstracts. Full-text papers published in peer-reviewed journals in English were selected.

RESULTS

Nineteen articles met eligibility criteria. Eight case series/reports on brain pathology showed abnormalities in few SMA type 0/1 cases, supported by findings in three post-mortem examinations in mice. Four studies (three case-control, one cross-sectional) on cognition reported contradictory results, with impaired cognitive performances in recent, small groups with SMA type 1. Four studies (three cross-sectional, one observational) on speech/language showed that untreated SMA type 1 patients rarely achieve functional and intelligible speech, with data limited to parent reports/non-formal evaluations.

INTERPRETATION

Brain involvement is an under-investigated aspect of SMA type 1, requiring further exploration in longitudinal studies. A deeper knowledge of brain involvement would improve the interpretation of clinical phenotypes and the personalization of rehabilitation programmes supporting patients' autonomies and quality of life. Additionally, it may help to define further outcome measures testing the efficacy of current and new developing drugs on this domain.

WHAT THIS PAPER ADDS

Brain involvement is under-investigated in spinal muscular atrophy (SMA) type 1. Neuropathological data suggest progressive brain involvement in severe forms of SMA. Impaired cognitive performances are reported in small groups with SMA type 1. Data on language in those with SMA type 1 are limited to parent reports and non-formal assessments.

Functional Abnormalities of Cerebellum and Motor Cortex in Spinal Muscular Atrophy Mice

Spinal muscular atrophy (SMA) is a devastating genetic neuromuscular disease. Diffuse neuropathology has been reported in SMA patients and mouse models, however, functional changes in brain regions have not been studied. In the SMNΔ7 mouse model, we identified three types of differences in neuronal function in the cerebellum and motor cortex from two age groups: P7-9 (P7) and P11-14 (P11). Microelectrode array studies revealed significantly lower spontaneous firing and network activity in the cerebellum of SMA mice in both age groups, but it was more profound in the P11 group. In the motor cortex, however, neural activity was not different in either age group. Whole-cell patch-clamp was used to study the function of output neurons in both brain regions. In cerebellar Purkinje cells (PCs) of SMA mice, the input resistance was larger at P7, while capacitance was smaller at P11. In the motor cortex, no difference was observed in the passive membrane properties of layer V pyramidal neurons (PN5s). The action potential threshold of both types of output neurons was depolarized in the P11 group. We also observed lower spontaneous excitatory and inhibitory synaptic activity in PN5s and PCs respectively from P11 SMA mice. Overall, these differences suggest functional alterations in the neural network in these motor regions that change during development. Our results also suggest that neuronal dysfunction in these brain regions may contribute to the pathology of SMA. Comprehensive treatment strategies may consider motor regions outside of the spinal cord for better outcomes.

Cerebellar degeneration in adult spinal muscular atrophy patients

BACKGROUND

Spinal muscular atrophy (SMA) is a genetic motor neuron disease related to deletions in the SMN1 gene. There is mounting evidence that the disease is not restricted to motor neurons. In this neuroimaging study, we aimed to investigate the presence of in-vivo cerebellar damage in adult SMA patients not treated with disease-modifying treatment.

METHODS

Twenty-five molecularly confirmed patients with SMA type III or IV and 25 healthy controls underwent MRI with cerebellar focused structural analysis by the CERES automated pipeline. Volumetry (total and gray matter-GM) as well as cortical thickness of the cerebellar lobules were compared in both groups. Full clinical and demographic data were then assessed for correlations with cerebellar imaging findings.

RESULTS

Volumes of cerebellar lobules VIIIB (right), IX and X were significantly smaller in patients with SMA. Lobule IX also had GM atrophy in comparison to controls. We found no significant correlation between clinical findings and cerebellar damage.

CONCLUSIONS

Neuroimaging detects cerebellar structural changes in adult SMA patients, suggesting that neurodegeneration is not confined to the lower motor neurons in the disease.

The spinal and cerebral profile of adult spinal-muscular atrophy: A multimodal imaging study

Spinal muscular atrophy (SMA) type III and IV are autosomal recessive, slowly progressive lower motor neuron syndromes. Nevertheless, wider cerebral involvement has been consistently reported in mouse models. The objective of this study is the characterisation of spinal and cerebral pathology in adult forms of SMA using multimodal quantitative imaging.

Methods

Twenty-five type III and IV adult SMA patients and 25 age-matched healthy controls were enrolled in a spinal cord and brain imaging study. Structural measures of grey and white matter involvement and diffusion parameters of white matter integrity were evaluated at each cervical spinal level. Whole-brain and region-of-interest analyses were also conducted in the brain to explore cortical thickness, grey matter density and tract-based white matter alterations.

Results

In the spinal cord, considerable grey matter atrophy was detected between C2-C6 vertebral levels. In the brain, increased grey matter density was detected in motor and extra-motor regions of SMA patients. No white matter pathology was identified neither at brain and spinal level.

Conclusions

Adult forms of SMA are associated with selective grey matter degeneration in the spinal cord with preserved white matter integrity. The observed increased grey matter density in the motor cortex may represent adaptive reorganisation.

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(SMA) type 3 and 4 is a lower motor neuron syndrome. Nevertheless, wider involvement of the nervous system might be possible.

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25 adults type 3 and 4 SMA patients were studied using brain and cervical spinal cord neuroimaging techniques.

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Grey matter atrophy was observed in the spinal cord. No white matter degeneration was present at brain and spinal level.

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Increased grey matter density was detected in cerebral motor regions and explained as compensatory mechanism.

Cardiac pathology in spinal muscular atrophy: a systematic review

Background

Hereditary proximal spinal muscular atrophy (SMA) is a severe neuromuscular disease of childhood caused by homozygous loss of function of the survival motor neuron (SMN) 1 gene. The presence of a second, nearly identical SMN gene (SMN2) in the human genome ensures production of residual levels of the ubiquitously expressed SMN protein. Alpha-motor neurons in the ventral horns of the spinal cord are most vulnerable to reduced SMN concentrations but the development or function of other tissues may also be affected, and cardiovascular abnormalities have frequently been reported both in patients and SMA mouse models.

Methods

We systematically reviewed reported cardiac pathology in relation to SMN deficiency. To investigate the relevance of the possible association in more detail, we used clinical classification systems to characterize structural cardiac defects and arrhythmias.

Conclusions

Seventy-two studies with a total of 264 SMA patients with reported cardiac pathology were identified, along with 14 publications on SMA mouse models with abnormalities of the heart. Structural cardiac pathology, mainly septal defects and abnormalities of the cardiac outflow tract, was reported predominantly in the most severely affected patients (i.e. SMA type 1). Cardiac rhythm disorders were most frequently reported in patients with milder SMA types (e.g. SMA type 3). All included studies lacked control groups and a standardized approach for cardiac evaluation.

The convergence to specific abnormalities of cardiac structure and function may indicate vulnerability of specific cell types or developmental processes relevant for cardiogenesis. Future studies would benefit from a controlled and standardized approach for cardiac evaluation in patients with SMA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13023-017-0613-5) contains supplementary material, which is available to authorized users.

Spectrum of neuropathophysiology in spinal muscular atrophy type I

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

Spinal muscular atrophy: journeying from bench to bedside

Spinal muscular atrophy (SMA) is a frequently fatal neuromuscular disorder and the most common inherited cause of infant mortality. SMA results from reduced levels of the survival of motor neuron (SMN) protein. Although the disease was first described more than a century ago, a precise understanding of its genetics was not obtained until the SMA genes were cloned in 1995. This was followed in rapid succession by experiments that assigned a role to the SMN protein in the proper splicing of genes, novel animal models of the disease, and the eventual use of the models in the pre clinical development of rational therapies for SMA. These successes have led the scientific and clinical communities to the cusp of what are expected to be the first truly promising treatments for the human disorder. Yet, important questions remain, not the least of which is how SMN paucity triggers a predominantly neuromuscular phenotype. Here we review how our understanding of the disease has evolved since the SMA genes were identified. We begin with a brief description of the genetics of SMA and the proposed roles of the SMN protein. We follow with an examination of how the genetics of the disease was exploited to develop genetically faithful animal models, and highlight the insights gained from their analysis. We end with a discussion of ongoing debates, future challenges, and the most promising treatments to have emerged from our current knowledge of the disease.

The intriguing case of motor neuron disease: ALS and SMA come closer

MNDs (motor neuron diseases) form a heterogeneous group of pathologies characterized by the progressive degeneration of motor neurons. More and more genetic factors associated with MND encode proteins that have a function in RNA metabolism, suggesting that disturbed RNA metabolism could be a common underlying problem in several, perhaps all, forms of MND. In the present paper we review recent developments showing a functional link between SMN (survival of motor neuron), the causative factor of SMA (spinal muscular atrophy), and FUS (fused in sarcoma), a genetic factor in ALS (amyotrophic lateral sclerosis). SMN is long known to have a crucial role in the biogenesis and localization of the spliceosomal snRNPs (small nuclear ribonucleoproteins), which are essential assembly modules of the splicing machinery. Now we know that FUS interacts with SMN and pathogenic FUS mutations have a significant effect on snRNP localization. Together with other recently published evidence, this finding potentially links ALS pathogenesis to disturbances in the splicing machinery, and implies that pre-mRNA splicing may be the common weak point in MND, although other steps in mRNA metabolism could also play a role. Certainly, further comparison of the RNA metabolism in different MND will greatly help our understanding of the molecular causes of these devastating diseases.

Spinal muscular atrophies

Spinal muscular atrophies (SMA) are genetic disorders characterized by degeneration of lower motor neurons. The most frequent form is caused by mutations of the survival motor neuron 1 gene (SMN1). The identification of this gene greatly improved diagnostic testing and family-planning options of SMA families. SMN plays a key role in metabolism of RNA. However, the link between RNA metabolism and motor neuron degeneration remains unknown. A defect in mRNA processing likely generates either a loss of function of some critical RNA or abnormal transcripts with toxic property for motor neurons. Mutations of SMN in various organisms highlighted an essential role of SMN in motor axon and neuromuscular junction development or maintenance. The quality of life of patients has greatly improved over recent decades through the improvement of care and management of patients. In addition, major advances in translational research have been made in the field of SMA. Various therapeutic strategies have been successfully developed aiming at acting on SMN2, a partially functional copy of the SMN1 gene which remains present in patients. Drugs have been identified and some are already at preclinical stages. Identifying molecules involved in the SMA degenerative process should represent additional attractive targets for therapeutics in SMA.

SMN deficiency disrupts brain development in a mouse model of severe spinal muscular atrophy

Reduced expression of the survival motor neuron (SMN) gene causes the childhood motor neuron disease spinal muscular atrophy (SMA). Low levels of ubiquitously expressed SMN protein result in the degeneration of lower motor neurons, but it remains unclear whether other regions of the nervous system are also affected. Here we show that reduced levels of SMN lead to impaired perinatal brain development in a mouse model of severe SMA. Regionally selective changes in brain morphology were apparent in areas normally associated with higher SMN levels in the healthy postnatal brain, including the hippocampus, and were associated with decreased cell density, reduced cell proliferation and impaired hippocampal neurogenesis. A comparative proteomics analysis of the hippocampus from SMA and wild-type littermate mice revealed widespread modifications in expression levels of proteins regulating cellular proliferation, migration and development when SMN levels were reduced. This study reveals novel roles for SMN protein in brain development and maintenance and provides the first insights into cellular and molecular pathways disrupted in the brain in a severe form of SMA.

Alternative splicing events are a late feature of pathology in a mouse model of spinal muscular atrophy

Spinal muscular atrophy is a severe motor neuron disease caused by inactivating mutations in the SMN1 gene leading to reduced levels of full-length functional SMN protein. SMN is a critical mediator of spliceosomal protein assembly, and complete loss or drastic reduction in protein leads to loss of cell viability. However, the reason for selective motor neuron degeneration when SMN is reduced to levels which are tolerated by all other cell types is not currently understood. Widespread splicing abnormalities have recently been reported at end-stage in a mouse model of SMA, leading to the proposition that disruption of efficient splicing is the primary mechanism of motor neuron death. However, it remains unclear whether splicing abnormalities are present during early stages of the disease, which would be a requirement for a direct role in disease pathogenesis. We performed exon-array analysis of RNA from SMN deficient mouse spinal cord at 3 time points, pre-symptomatic (P1), early symptomatic (P7), and late-symptomatic (P13). Compared to littermate control mice, SMA mice showed a time-dependent increase in the number of exons showing differential expression, with minimal differences between genotypes at P1 and P7, but substantial variation in late-symptomatic (P13) mice. Gene ontology analysis revealed differences in pathways associated with neuronal development as well as cellular injury. Validation of selected targets by RT-PCR confirmed the array findings and was in keeping with a shift between physiologically occurring mRNA isoforms. We conclude that the majority of splicing changes occur late in SMA and may represent a secondary effect of cell injury, though we cannot rule out significant early changes in a small number of transcripts crucial to motor neuron survival.

SMN, the product of the spinal muscular atrophy-determining gene, is expressed widely but selectively in the developing human forebrain

The expression pattern of the survival motor neuron (SMN) protein has been investigated immunohistochemically in the human fetal forebrain from 14 to 38 weeks of gestation. Mutations in the SMN gene cause spinal muscular atrophy (SMA), an autosomal recessive disease characterized by degeneration of lower motor neurons in the spinal cord leading to progressive muscle wasting. SMN is a multifunctional protein and has been implicated in diverse cytoplasmic and nuclear processes. The monoclonal murine SMN antibody used in this study recognized a major band at approximately 34 kDa. In spinal cord anterior horn motor neurons at 13 weeks of gestation, the soma, proximal neurites, and nucleus were immunostained. In the nucleus, SMN immunolabeling was observed at the nuclear membrane, at the nucleolus, and at dot-like structures in the nucleoplasm likely to be coiled bodies and gems. In the fetal forebrain, SMN was immunodetected as early as 14 weeks of gestation. From 14 to 24 weeks of gestation, intense immunostaining was observed in the basal nucleus of Meynert, a major source of cholinergic afferents to the cortex. Less intensely labeled cells at lower packing density were also observed in the thalamus, reticular and perireticular nucleus, globus pallidus, hippocampus, amygdala, and enthorinal cortex. Immunolabeled cells were still detectable at 38 gestational weeks, the latest time point investigated. These findings provide an anatomical basis for future investigations of SMN functions during brain development and for the neuropathological characterization of severe SMA cases.

Finger cold-induced vasodilatation, sympathetic skin response, and R-R interval variation in patients with progressive spinal muscular atrophy

To elucidate autonomic function in spinal muscular atrophy, we evaluated finger cold-induced vasodilatation, sympathetic skin response, and R-R interval variation in 10 patients with spinal muscular atrophy: 7 of type 1, 2 of type 2, and 1 of type 3. Results of finger cold-induced vasodilatation, sympathetic skin response, and R-R interval variation were compared with those of healthy children. Finger cold-induced vasodilatation was abnormal in 6 of 10 patients with spinal muscular atrophy; it was normal in the healthy children. The mean sympathetic skin response latency and amplitude did not differ significantly from those of the healthy children. Amplitudes of sympathetic skin response to sound stimulation were absent or low in all six patients with spinal muscular atrophy. No significant difference was found in the mean R-R interval variation of patients with spinal muscular atrophy and healthy children. Results show that some patients with spinal muscular atrophy have autonomic dysfunction, especially sympathetic nerve hyperactivity, that resembles dysfunction observed in amyotrophic lateral sclerosis.

Extended phenotype of pontocerebellar hypoplasia with infantile spinal muscular atrophy

Pontocerebellar hypoplasia (PCH) is rarely associated with anterior horn cell disease and designated as PCH-1. This phenotype is characterized by severe muscle weakness and hypotonia starting prenatally or at birth with a life span not exceeding a few months in most cases. Milder disease courses with later onset and longer survival are normally not diagnosed as PCH-1. We describe the clinical and neuroradiological findings in nine patients out of six siblingships with evidence of cerebellar defects and early onset spinal muscular atrophy (SMA), representing a broad spectrum of clinical variability. In all patients, the diagnosis of SMA (Werdnig-Hoffmann disease) was made on the basis of electrophysiological data and muscle biopsy; however, genetic testing failed to confirm the diagnosis of infantile SMA with a gene defect on chromosome 5q and resulted in clinical reevaluation. Age at onset was after a normal period in the first months of life in three siblingships and pre- and postnatally in the other three families. Life span was 2-4 years in patients with later onset, and age at death occurred after birth or within months in the more severe group. Two siblingships showed discordant ages at death despite similar treatment. In contrast to the previous definition of PCH-1, our observations suggest the existence of milder phenotypes with pontocerebellar hypoplasia or olivopontocerebellar atrophy in combination with anterior horn cell loss. A pontine involvement is not necessarily seen by neuroimaging methods. The genetic basis of PCH-1 remains to be determined. The gene locus for infantile SMA on chromosome 5q could be excluded by linkage studies. Parental consanguinity and affected siblings make autosomal recessive inheritance most likely.

Spinal muscular atrophy: present state

Spinal muscular atrophy (SMA) is a hereditary neurodegenerative disease caused by homozygous deletions or mutations in the SMN1 gene on Chr.5q13. SMA spans from severe Werdnig-Hoffmann disease (SMA 1) to relatively benign Kugelberg-Welander disease (SMA 3). Onset before birth possibly aggravates the clinical course, because immature motoneurons do not show compensatory sprouting and collateral reinnervation, and motor units in SMA 1, in contrast to those in SMA 3, are not enlarged. Genetic evidence indicates that SMN2, a gene 99% identical to SMN1, can attenuate SMA severity: in patients, more SMN2 copies and higher SMN protein levels are correlated with milder SMA. There is evidence that SMN plays a role in motoneuron RNA metabolism, but it has also been linked to apoptosis. Several mouse models with motoneuron disease have been successfully treated with neurotrophic factors. None of these models is, however, homologous to SMA. Recently, genetic mouse models of SMA have been created by introducing human SMN2 transgenes into Smn knockout mice or by targeting the Smn gene knockout to neurons. These mice not only provide important insights into the pathogenesis of SMA but are also crucial for testing new therapeutic strategies. These include SMN gene transfer, molecules capable to up-regulate SMN expression and trophic or antiapoptotic factors.

Survival motor neuron protein modulates neuron-specific apoptosis

Spinal muscular atrophy (SMA) is attributed to mutations in the SMN1 gene, leading to loss of spinal cord motor neurons. The neurotropic Sindbis virus vector system was used to investigate a role for the survival motor neuron (SMN) protein in regulating neuronal apoptosis. Here we show that SMN protects primary neurons and differentiated neuron-like stem cells, but not cultured cell lines from virus-induced apoptotic death. SMN also protects neurons in vivo and increases survival of virus-infected mice. SMN mutants (SMNDelta7 and SMN-Y272C) found in patients with SMA not only lack antiapoptotic activity but also are potently proapoptotic, causing increased neuronal apoptosis and animal mortality. Full-length SMN is proteolytically processed in brains undergoing apoptosis or after ischemic injury. Mutation of an Asp-252 of SMN abolished cleavage of SMN and increased the antiapoptotic function of full-length SMN in neurons. Taken together, deletions or mutations of the C terminus of SMN that result from proteolysis, splicing (SMNDelta7), or germ-line mutations (e.g., Y272C), produce a proapoptotic form of SMN that may contribute to neuronal death in SMA and perhaps other neurodegenerative disorders.

Subcellular distribution of survival motor neuron (SMN) protein: possible involvement in nucleocytoplasmic and dendritic transport

Spinal muscular atrophy (SMA) is among the most common recessive autosomal diseases and is characterized by the loss of spinal motor neurons. A gene termed 'Survival of Motor Neurons' (SMN) has been identified as the SMA-determining gene. Recent work indicates the involvement of the SMN protein and its associated protein SIP1 in spliceosomal snRNP biogenesis. However, the function of SMN remains unknown. Here, we have studied the subcellular localization of SMN in the rat spinal cord and more generally in the central nervous system (CNS), by light fluorescence and electron microscopy. SMN immunoreactivity (IR) was found in the different regions of the spinal cord but also in various regions of the CNS such as the brainstem, cerebellum, thalamus, cortex and hippocampus. In most neurons, we observed a speckled labelling of the cytoplasm and a discontinuous staining of the nuclear envelope. For some neurons (e.g. brainstem nuclei, dentate gyrus, cortex: layer V) and, in particular in motoneurons, SMN-IR was also present as prominent nuclear dot-like-structures. In these nuclear dots, SMN colocalized with SIP1 and with fibrillarin, a marker of coiled bodies. Ultrastructural studies in the anterior horn of the spinal cord confirmed the presence of SMN in the coiled bodies and also revealed the protein at the external side of nuclear pores complexes, in association with polyribosomes, and in dendrites, associated with microtubules. These localizations suggest that, in addition to its involvement in the spliceosome biogenesis, the SMN protein could also play a part in nucleocytoplasmic and dendritic transport.

Expression of the SMN gene, the spinal muscular atrophy determining gene, in the mammalian central nervous system

The survival motor neuron (SMN) gene is the putative disease gene for human spinal muscular atrophy (SMA), an autosomal recessive disorder characterized by progressive degeneration of lower motor neurons. Two copies of the gene, centromeric and telomeric, are present in the same 5q13 chromosomal region in humans. However, only the telomeric gene is affected in SMA. The SMN gene(s) encode(s) a novel protein of unknown function. To gain insights into the role of SMN in neurons, we have identified the SMN gene ortholog in the rat, and investigated SMN expression in the CNS of rat, monkey and humans by immunocytochemistry and in situ hybridization experiments. Antibodies against the SMN amino-terminus specifically recognized a single protein identical to the in vitro translation products of human and rat SMN cDNAs. The SMN gene transcript and product were widely but unevenly expressed throughout cerebral and spinal cord areas. The SMN protein was localized mainly in the cytoplasm of specific neuronal systems, and it was particularly expressed in lower motor neurons of newborn and adult animals. Likewise, a strong hybridization signal was detected in lamina IX of the spinal ventral horn. These results support the relevance of SMN for the motor neuron function and the pathogenetic role of the SMN gene in the neuronal degeneration associated with SMA.

Clinical and molecular genetic features of congenital spinal muscular atrophy

A neonate presented with the fetal hypokinesia sequence and signs of spinal muscular atrophy (SMA). Severe pathological changes including ballooned neurons and neuronophagia were found not only in the motor nerve nuclei but also in the thalamic, cerebellar, and brainstem nuclei as well as in the dorsal root ganglia. Direct DNA analysis showed the presence of a chimeric SMN gene, with a rearrangement occurring between exon 7 of the centromeric SMN gene and exon 8 of the telomeric SMN gene. Circumstantial evidence suggests that only a single copy of this gene is present, with transcriptional characteristics of a centromeric SMN gene. In addition, a homozygous deletion in the NAIP genes was demonstrated. This observation demonstrates that at least some cases with fetal hypokinesia and SMA may represent the severe end of a spectrum of disorders caused by deletions in the SMA locus on chromosome 5q13. In addition, these findings are compatible with a modifying role for the centromeric SMN genes and the NAIP genes in the severity of the SMA phenotype.

Synaptic alterations of anterior horn cells in Werdnig-Hoffmann disease

The most characteristic neuropathologic feature of Werdnig-Hoffmann disease is degenerative change in the anterior horn cells of the spinal cord, the mechanisms of which have not yet been clearly determined. To assess the synaptic changes in the motor neurons, we examined immunoreactivities for synaptophysin in the spinal cord of 11 patients with Werdnig-Hoffmann disease. Decreased synaptophysin immunoreactivity was observed in the anterior horn cell column in all patients with Werdnig-Hoffmann disease and correlated with the degree of degenerative change in the motor neurons. Synaptophysin immunoreactivity was relatively preserved on the surface of the residual anterior horn cells. Both atrophic neurons and chromatolytic neurons had dense accumulations of synaptophysin-immunoreactive products on the surface of the cell body and their proximal processes. These observations suggest that synaptic changes in the anterior horn cell column are secondary to the degenerative processes of the anterior horn cells.

Synaptophysin expression in the anterior horn of Werdnig-Hoffmann disease

This report concerns the study of synaptophysin (SP) expression in the anterior horn in four cases of Werdnig-Hoffmann disease (WHD). All patients had distinct anterior horn cell degeneration, and died before the age of one year. Normal spinal cords from five age-matched children served as controls. Five cases of sporadic amyotrophic lateral sclerosis (S-ALS), three cases of lower motor neuron disease (L-MND), three cases of peripheral neuropathy with axonal reaction, and six adult cases with normal spinal cords were included for comparison. Immunohistochemical techniques were used throughout. The results show that normal spinal cords of children have similar SP immunoreactivity patterns as those of normal adults. We also found that despite relatively preserved or slightly increased SP immunoreactivity on the surface of the cell body and proximal processes of the remaining neurons, there was a diffuse decrease of immunoreaction product deposits in the anterior horn neuropil of the WHD cases. The ballooned neurons in the anterior horns of patients with WHD, S-ALS, L-MND, and axonal reaction had few SP immunoreactive dots or granules around the cell bodies and proximal processes. The perikarya of some ballooned neurons of the children with WHD was diffusely stained for SP. There was no SP immunoreactive structures within the empty cell beds of these patients. The observed decrease in SP expression around ballooned neurons in these disorders is indicative of a disconnection of presynaptic terminals of afferent fibers from the proximal portion of the swollen degenerated anterior horn cells.

The gene for neuronal apoptosis inhibitory protein is partially deleted in individuals with spinal muscular atrophy

The spinal muscular atrophies (SMAs), characterized by spinal cord motor neuron depletion, are among the most common autosomal recessive disorders. One model of SMA pathogenesis invokes an inappropriate persistence of normally occurring motor neuron apoptosis. Consistent with this hypothesis, the novel gene for neuronal apoptosis inhibitory protein (NAIP) has been mapped to the SMA region of chromosome 5q13.1 and is homologous with baculoviral apoptosis inhibitor proteins. The two first coding exons of this gene are deleted in approximately 67% of type I SMA chromosomes compared with 2% of non-SMA chromosomes. Furthermore, RT-PCR analysis reveals internally deleted and mutated forms of the NAIP transcript in type I SMA individuals and not in unaffected individuals. These findings suggest that mutations in the NAIP locus may lead to a failure of a normally occurring inhibition of motor neuron apoptosis resulting in or contributing to the SMA phenotype.

Exclusion of the gene locus for spinal muscular atrophy on chromosome 5q in a family with infantile olivopontocerebellar atrophy (OPCA) and anterior horn cell degeneration

Two sisters with infantile OPCA plus spinal muscular atrophy (SMA) are reported. Both showed severe hypotonia and psychomotor delay from birth, and in addition, nystagmoid eye movements and vision impairment were evident. Cerebellar hypoplasia with cystic dilatation was seen by neuro-imaging methods. Pathoanatomically, a marked cerebellar hypoplasia and neuronal loss in the basal ganglia, brainstem and anterior horns were found in the deceased girl. Linkage studies with polymorphic markers of the region 5q11.2-q13.3 flanking the gene locus for infantile SMA showed identical parental haplotypes in the patients and their older healthy sister. It can be concluded that the gene locus for infantile SMA on chromosome 5q is not responsible for infantile OPCA plus anterior horn cell degeneration in the described family which might apply to this disorder in general.

Childhood progressive spinal muscular atrophy with facioscapulo-humeral predominance, sensory and autonomic involvement and optic atrophy

A female child of healthy parents developed rotary nystagmus at the age of 15 months. Ophthalmoscopy disclosed incomplete optic atrophy. Blood tests, EEG and CT scans were normal. At 20 months progressive muscular weakness and wasting with limb-girdle distribution commenced, followed later by disturbance of gait. From muscle and nerve biopsy the diagnosis of a peripheral neuropathy with neurogenic muscular atrophy was made. No mental change occurred. At 23 months she sustained cardiac arrest and was resuscitated; thereafter, she remained in a vegetative state and expired 9 months later. Her brain was markedly atrophic and firm. Diffuse old ischemic necroses and neuronal loss with gliosis were found in the cortex, the neostriatum, the thalamus, parts of the lower brainstem, and the cerebellum. Her optic nerves and tracts showed complete atrophy. The spinal cord exhibited degeneration and loss of motor neurons with cervical accentuation. The intermediolateral nuclei, the dorsal nuclei and the spinal ganglia were also involved. There was demyelination of the posterior funiculi, the pyramidal tracts, and the sciatic, peroneal, sural, and superior frontal nerve. The voluntary muscles exhibited large group atrophy with liposclerotic change and limb-girdle predominance. The neck, tongue and ocular muscles were also involved, as were, to a less extent, the lower limbs. Although the loss of motor neurons in the spinal cord and at the bulbar level with the typical pattern of neurogenic muscular atrophy, as well as its distribution, resemble the facioscapulo-humoral type of heredity motor neuropathy (HMN), early onset, rapid course, sensory and autonomic involvement, and atrophy of the optic nerve do not fit this or any one type of HMN.

Infantile progressive spinal muscular atrophy with ophthalmoplegia and pyramidal symptoms

Two siblings presented with identical features of progressive peripheral paralysis of the lower motor neuron type, pyramidal signs, cranial nerve palsy which included external ocular palsy and deafness, and internal ocular palsy; both died before 1 year of age. Pathologic examination of the central nervous system in both patients revealed degeneration and loss of spinal and cranial nerve motor nuclei, including the oculomotor nucleus. In addition, there was degeneration of the Edinger-Westphal nuclei and demyelination of the corticospinal tract under the midbrain. Although spinal cord lesions were indistinguishable from those of Werdnig-Hoffmann disease, the 2 patients are not considered to have Werdnig-Hoffmann disease from the clinicopathologic findings.

Immunocytochemical and ultrastructural studies of Werdnig-Hoffmann disease

Neuronal alterations in two cases of Werdnig-Hoffmann disease (WH) were investigated immunocytochemically and ultrastructurally. Ballooned neurons (BNs) were found in anterior horn, Clarke's column, dorsal root ganglion and thalamus. Anti-phosphorylated neurofilament antibodies preferentially stained the peripheral perikarya and proximal neuronal processes of BNs, whereas anti-ubiquitin antibodies preferentially stained the central perikarya of BNs. Ultrastructurally, BNs showed degenerative changes ranging from a diffuse increase of neurofilaments to a centrally accentuated accumulation of mitochondria and vesicular or membranous profiles. Our studies suggest that ubiquitinated degradation products accumulate in the center of the BN's perikaryon and displace aberrantly phosphorylated neurofilaments to the periphery. BNs in WH probably reflect an intrinsic alteration in the metabolism of neurofilaments that is associated with regressive changes in the neuron and eventually neuronal death.

Ubiquitin and phosphorylated neurofilament epitopes in ballooned neurons of the extraocular muscle nuclei in a case of Werdnig-Hoffmann disease

The extraocular muscle nuclei in one case of Werdnig-Hoffmann disease were examined immunocytochemically using antibodies against phosphorylated neurofilament (pNF) and ubiquitin (UBQ). The oculomotor and trochlear nuclei showed several chromatolytic ballooned neurons. All ballooned neurons contained epitopes of pNF and UBQ. pNF were present mainly in the periphery of the cell in a ring-like shape and were occasionally seen in the center of some cells. On the other hand, the structures stained by the antibody to UBQ were small vesicles or granules and most of them were aggregated in the center of the cell. These distribution patterns of pNF and UBQ may be unique in Werdnig-Hoffmann disease, since similar patterns were reported in other types of neurons of Werdnig-Hoffmann disease but were not seen in two other motor neuron diseases: classical amyotrophic lateral sclerosis, and familial amyotrophic lateral sclerosis with posterior column and spinocerebellar tract involvement.

Sensory neuron degeneration in familial Kugelberg-Welander disease

A 53 year old man developed symptoms of motor neuron disease in childhood. There was a family history of a similar disorder and it was felt to represent a form of Kugelberg-Welander disease. In addition to the motor deficits, sensory abnormalities in his legs were documented during life. Autopsy revealed anterior horn cell loss throughout the length of the spinal cord, with preservation of the phrenic nucleus. The lumbar dorsal root ganglia showed active degeneration of sensory neurons, with nuclear changes exceeding cytoplasmic ones. The fasciculus gracilis showed Wallerian degeneration. The findings provide direct evidence that sensory neurons can degenerate in some forms of motor neuron disease, and that the "demyelination" or "degeneration" of posterior columns sometimes seen in the various forms of motor neuron disease may actually be secondary to cell body disease in the dorsal root ganglia.

Peripheral nerve involvement in Werdnig-Hoffmann disease

The number of large myelinated axons was markedly decreased in almost all the intramuscular nerve bundles included in 32 muscle biopsies from patients with Werdnig-Hoffmann disease compared to that in normals. The morphometric analysis of peripheral nerves in 5 epon-embedded sections also showed a selective loss of larger myelinated fibers. The ultrastructural findings of the nerves were similar to those seen in Wallerian degeneration including axonal degeneration, myelin breakdown with phagocytosis, Schwann cell proliferation forming Schwann cell columns, axonal sprouting and probable remyelination. The earlier and more striking peripheral nerve involvement than that previously believed was not different from that seen in amyotrophic lateral sclerosis (ALS). The earlier damage to the peripheral nerves probably resulted from a degeneration of the anterior horn cells or anterior spinal roots as in ALS rather than from a dying-back process.

Chromatolytic neurons in Werdnig-Hoffmann disease contain phosphorylated neurofilaments

Two cases of Werdnig-Hoffman disease are described. Chromatolytic neurons were found within the ventral horns of the spinal cord as well as in dorsal root ganglia, Clarke's column, cranial nerve nuclei and thalamus. Immunohistochemical staining demonstrated phosphorylated neurofilament antigen within the cytoplasm of the chromatolytic neurons. This finding suggests that a defect in slow axonal transport or in the regulation of neurofilament phosphorylation may play a role in the pathogenesis of this disorder.