A clinical study of chronic childhood spinal muscular atrophy. A review of 141 cases

Drug screening with human SMN2 reporter identifies SMN protein stabilizers to correct SMA pathology

Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality, is caused by reduced levels of functional survival motor neuron (SMN) protein. To identify therapeutic agents for SMA, we established a versatile SMN2-GFP reporter line by targeting the human SMN2 gene. We then screened a compound library and identified Z-FA-FMK as a potent candidate. Z-FA-FMK, a cysteine protease inhibitor, increased functional SMN through inhibiting the protease-mediated degradation of both full-length and exon 7-deleted forms of SMN. Further studies reveal that CAPN1, CAPN7, CTSB, and CTSL mediate the degradation of SMN proteins, providing novel targets for SMA. Notably, Z-FA-FMK mitigated mitochondriopathy and neuropathy in SMA patient-derived motor neurons and showed protective effects in SMA animal model after intracerebroventricular injection. E64d, another cysteine protease inhibitor which can pass through the blood-brain barrier, showed even more potent therapeutic effects after subcutaneous delivery to SMA mice. Taken together, we have successfully established a human SMN2 reporter for future drug discovery and identified the potential therapeutic value of cysteine protease inhibitors in treating SMA via stabilizing SMN proteins.

Relationships between long-term observations of motor milestones and genotype analysis results in childhood-onset Japanese spinal muscular atrophy patients

AIM

To clarify the long-term natural history of SMA in Japanese patients by investigating the peak motor milestones of cases 7months through 57years of age, in efforts to contribute to evaluating outcomes of new therapeutic interventions.

METHODS

We sub-classified 112 SMA type I-III cases into type Ia, type Ib, type IIa, type IIb, type IIIa and type IIIb, according to peak motor milestone achieved, and analyzed the SMN1, SMN2 and NAIP genes in relation to clinical subtypes.

RESULTS

In type I cases, there was a significant difference (p<0.0001), depending on whether or not head control was obtained, in the time of ventilation support being required. In type II cases as well, the time at which the ability to maintain the sitting position independently was lost also differed significantly (p<0.01) between those acquiring the ability to sit unaided within eight months after birth and those acquiring this ability after eight months of age. In type III cases, being able versus unable to climb stairs was associated with a significant difference (p=0.02) in the median time until loss of walking independently. Positive correlations were also seen between copy numbers and the clinical severity of SMA.

CONCLUSION

Our long-term results show peak motor milestone evaluations distinguishing between subtypes to be useful not only as outcome measures for assessing treatment efficacy in clinical trials but also for predicting the clinical courses of Japanese SMA patients.

Imaging Flow Cytometry Analysis to Identify Differences of Survival Motor Neuron Protein Expression in Patients With Spinal Muscular Atrophy

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Recapitulation of spinal motor neuron-specific disease phenotypes in a human cell model of spinal muscular atrophy

Establishing human cell models of spinal muscular atrophy (SMA) to mimic motor neuron-specific phenotypes holds the key to understanding the pathogenesis of this devastating disease. Here, we developed a closely representative cell model of SMA by knocking down the disease-determining gene, survival motor neuron (SMN), in human embryonic stem cells (hESCs). Our study with this cell model demonstrated that knocking down of SMN does not interfere with neural induction or the initial specification of spinal motor neurons. Notably, the axonal outgrowth of spinal motor neurons was significantly impaired and these disease-mimicking neurons subsequently degenerated. Furthermore, these disease phenotypes were caused by SMN-full length (SMN-FL) but not SMN-Δ7 (lacking exon 7) knockdown, and were specific to spinal motor neurons. Restoring the expression of SMN-FL completely ameliorated all of the disease phenotypes, including specific axonal defects and motor neuron loss. Finally, knockdown of SMN-FL led to excessive mitochondrial oxidative stress in human motor neuron progenitors. The involvement of oxidative stress in the degeneration of spinal motor neurons in the SMA cell model was further confirmed by the administration of N-acetylcysteine, a potent antioxidant, which prevented disease-related apoptosis and subsequent motor neuron death. Thus, we report here the successful establishment of an hESC-based SMA model, which exhibits disease gene isoform specificity, cell type specificity, and phenotype reversibility. Our model provides a unique paradigm for studying how motor neurons specifically degenerate and highlights the potential importance of antioxidants for the treatment of SMA.

Neurofilament Phosphorylation during Development and Disease: Which Came First, the Phosphorylation or the Accumulation?

Posttranslational modification of proteins is a ubiquitous cellular mechanism for regulating protein function. Some of the most heavily modified neuronal proteins are cytoskeletal proteins of long myelinated axons referred to as neurofilaments (NFs). NFs are type IV intermediate filaments (IFs) that can be composed of four subunits, neurofilament heavy (NF-H), neurofilament medium (NF-M), neurofilament light (NF-L), and α-internexin. Within wild type axons, NFs are responsible for mediating radial growth, a process that determines axonal diameter. NFs are phosphorylated on highly conserved lysine-serine-proline (KSP) repeats located along the C-termini of both NF-M and NF-H within myelinated axonal regions. Phosphorylation is thought to regulate aspects of NF transport and function. However, a key pathological hallmark of several neurodegenerative diseases is ectopic accumulation and phosphorylation of NFs. The goal of this review is to provide an overview of the posttranslational modifications that occur in both normal and diseased axons. We review evidence that challenges the role of KSP phosphorylation as essential for radial growth and suggests an alternative role for NF phosphorylation in myelinated axons. Furthermore, we demonstrate that regulation of NF phosphorylation dynamics may be essential to avoiding NF accumulations.

Gemin3 is an essential gene required for larval motor function and pupation in Drosophila

The assembly of metazoan Sm-class small nuclear ribonucleoproteins (snRNPs) is an elaborate, step-wise process that takes place in multiple subcellular compartments. The initial steps, including formation of the core RNP, are mediated by the survival motor neuron (SMN) protein complex. Loss-of-function mutations in human SMN1 result in a neuromuscular disease called spinal muscular atrophy. The SMN complex is comprised of SMN and a number of tightly associated proteins, collectively called Gemins. In this report, we identify and characterize the fruitfly ortholog of the DEAD box protein, Gemin3. Drosophila Gemin3 (dGem3) colocalizes and interacts with dSMN in vitro and in vivo. RNA interference for dGem3 codepletes dSMN and inhibits efficient Sm core assembly in vitro. Transposon insertion mutations in Gemin3 are larval lethals and also codeplete dSMN. Transgenic overexpression of dGem3 rescues lethality, but overexpression of dSMN does not, indicating that loss of dSMN is not the primary cause of death. Gemin3 mutant larvae exhibit motor defects similar to previously characterized Smn alleles. Remarkably, appreciable numbers of Gemin3 mutants (along with one previously undescribed Smn allele) survive as larvae for several weeks without pupating. Our results demonstrate the conservation of Gemin3 protein function in metazoan snRNP assembly and reveal that loss of either Smn or Gemin3 can contribute to neuromuscular dysfunction.

Analysis of the mRNA transcripts of the survival motor neuron (SMN) gene in the tissue of an SMA fetus and the peripheral blood mononuclear cells of normals, carriers and SMA patients

Spinal muscular atrophy (SMA) is a disorder characterized by degeneration of the anterior horn cells of the spinal cord. The gene most highly associated with SMA is the survival motor neuron (SMN) gene. In this study, we present an analysis of messenger RNA (mRNA) expression of the SMN gene in peripheral blood mononuclear cells in normal subjects, SMA carriers and patients from 20 SMA families. We found at least 6-8 different transcripts of SMN gene formed by alternative splicing involving exons 3, 5 and 7. We compared transcripts from the different types of SMA and found no definite differences in transcript patterns and amounts. Normal subjects with the telomeric SMN (SMN(T)) gene only had variable splicing resulting in several transcripts, the most dominant being a transcript containing all coding regions. However, SMA patients with the centromeric SMN (SMN(C)) gene only had a higher degree of splice variation and tended to show little or no exon 7. These results demonstrate that SMN(T) and SMN(C) genes participate in alternative splicing phenomena. The different splicing patterns support the view that the SMN(T) gene is responsible for SMA disease. We also analyzed the transcripts from several tissues of an SMA fetus who had a homozygous SMN(T) gene deletion. Different splicing patterns were also found in these tissues, and were similar to the splicing pattern of leukocytes. We compared the major transcripts from exons 4 to 8 of both the SMN(T) and SMN(C) genes and found that the relative proportion varied among normal subjects, SMA carriers and patients. This approach could be used as a novel diagnostic method. We suggest that analyzing the mRNA expression of the SMN gene in peripheral blood mononuclear cells offers an apparently reliable technique for separating SMA patients, carriers, and normal individuals.

Spinal muscular atrophy. Incidence in Iceland

Spinal muscular atrophy (SMA) is among the commonest degenerative disorders of the nervous system in childhood. This is an inherited autosomal recessive disease which results in premature death of anterior horn cells of the spinal cord and is manifested by progressive weakness and atrophy of skeletal muscles. Few studies have looked at the frequency of the disease in a defined population. We identified all patients diagnosed with SMA in Iceland during a 15-year period. The diagnosis is based on typical symptoms and supported by results of electromyography/nerve conduction studies and muscle biopsy. The average annual incidence was 13.7 per 100,000 live births for all types of SMA, which is similar to that reported in other population-based studies.

A clinical study of childhood spinal muscular atrophy in Sicily: a review of 75 cases

The aim of this study is to evaluate 75 patients affected by childhood-onset spinal muscular atrophy (SMA), using the diagnostic criteria and classification recently suggested by the International SMA Collaboration Consortium. Sex predominance, age of onset, clinical evaluation and other relevant clinical data of the disease are reported. These findings, as well as the role of the EMG and muscle biopsy as diagnostic tools, are discussed. The study suggests that the frequency of SMA in some areas of Sicily is high, possibly correlating with a relatively high gene frequency.

Association of progressive myoclonic epilepsy and spinal muscular atrophy

It is well known that certain hereditary diseases of the nervous system sometimes occur concurrently within particular families. This report presents a Yugoslav family of Hungarian origin in whom 2 brothers had progressive myoclonic epilepsy and proximal weakness and atrophy of muscles. Electromyography and muscle biopsy confirmed neurogenic atrophy. Electroencephalography disclosed paroxysmal spike-and-wave and polyspike-and-wave complexes with photic-induced myoclonic jerking. The combination of these clinical features is extremely rare and probably constitutes a clinical syndrome that has not been reported previously.

Chronic childhood spinal muscular atrophies in Algeria. A genetic study

This paper describes a genetic study of the chronic spinal muscular atrophies of late infancy and early childhood in Algeria. There were 50 index patients occurring in 44 kindreds and fourteen secondary cases. Genetic and nosological studies indicated that 52% of the patients constitute a genetically homogeneous subgroup with an age of onset between 3 and 24 months and an autosomal recessive mode of transmission. They also indicated that a large subgroup of index patients (48%) had a late age of onset, between 3 and 14 years. Such a large number of late presenting cases has not been reported in previous series. The majority of these cases are probably due to an autosomal recessive gene. A small proportion may represent new dominant mutations or nongenetic phenocopies. A possible sex influence on disease manifestations is discussed. A trend toward later male onset has been noted, and the degree of disability is more marked in males at or after the age of 10. Finally, some empirical risks for use in genetic counselling are presented.

Chronic proximal spinal muscular atrophy of childhood and adolescence: problems of classification and genetic counselling

Results obtained from a study of 354 cases of chronic proximal spinal muscular atrophy of childhood and adolescence suggest that the condition is not as homogeneous as it was previously thought. A tentative classification based on our results is proposed. Estimates of genetic risks are provided, taking into account the sex and age at clinical onset. In our opinion these factors are more reliable than the data hitherto available because they are based on a considerably larger series.

Diaphragmatic paralysis due to spinal muscular atrophy. An unrecognised cause of respiratory failure in infancy?

An unusual form of spinal muscular atrophy presenting with respiratory failure was observed in four infants from two families. In one, whose death was attributed to pneumonia, the diagnosis was inferred retrospectively after two siblings died from an identical illness and were shown to have diaphragmatic paralysis and the typical electrophysiological and histological features of spinal muscular atrophy. Other signs of skeletal muscular weakness were absent or inconspicuous. The fourth, unrelated infant presented in an identical way but has survived for over a year on a ventilator. Two months after the onset of respiratory paralysis, more extensive skeletal muscular weakness was seen. Other infants, dying of unexplained respiratory illness, may have this disorder and some may be included in the miscellany of disorders that constitute the sudden infant death syndrome.

Chronic proximal spinal muscular atrophy of childhood and adolescence: sex influence

Segregation analysis was performed on 354 cases of chronic proximal spinal muscular atrophy of childhood and adolescence (CPSMA) in the total series and in a number of subgroups formed according to the age at onset and sex. The analysis provided evidence of sex influence in the series studied, particularly in a subgroup of the milder form of the disease with onset between the 37th month and 18th year of life. In the latter subgroup, females were affected much less frequently. This was particularly striking after age at onset of 8 years, and only exceptionally were females affected after the age of 13 years. These facts point to incomplete penetrance of the gene.

Succinic semialdehyde dehydrogenase deficiency

A coupled assay using [14C]4-aminobutyric acid and a direct assay using [14C]succinic semialdehyde have been designed to assay te activity of succinic semialdehyde dehydrogenase in a patient with 4-hydroxybutyric aciduria and family members. In the coupled assay less than 3% of control succinic semialdehyde dehydrogenase activity was found in lysates of lymphocytes isolated from whole blood of the patient. In the direct assay there was no detectable activity of the enzyme in lysates of isolated lymphocytes or cultured lymphoblasts. Results indicated the parents to be heterozygous carriers carriers of the abnormal gene, consistent with an autosomal recessive inheritance.

Classification of spinal muscular atrophies

Clinical heterogeneity within the spinal muscular atrophies (SMA) has long been a source of confusion for questions of prognosis and genetic counselling. Comprehensive clinical and genetic analyses of 240 consecutive index cases from two English centres (The English SMA Study) have enabled some nosological questions to be resolved. The different SMA syndromes can be discriminated by (a) age at the first clinical signs of the disease, (b) pattern of muscle involvement, (c) age at death of other patients within an affected kindred, and (d) genetic evidence. Seven different SMA syndromes can be defined clinically and genetically; thirteen mutant genes are incriminated. Prevalence and incidence figures are presented. SMA type I (Werdnig-Hoffman disease) and chronic childhood SMA together comprise 74% of all SMA cases. The classification of the spinal muscular atrophies presented also provides the differential diagnosis for newly presenting cases.

Chronic form of childhood spinal muscular atrophy. Are the problems of its genetics really solved?

The authors discuss the differences between the two large series of chronic childhood spinal muscular atrophies (SMA)--their own comprising 273 cases, and that of Pearn et al. comprising 141 cases. The main difference concerns the predominance of males in the clinically milder later-onset group in the present series. The data of Pearn et al. (1978a, b) are quite different. The reason for the discrepancies is apparently a different selection of material. The present material is highly selective in favour of chronic cases, and Kugelberg-Welander cases are well-represented, whereas the percentage of Kugelberg-Welander cases in the material of Pearn et al. was very small. Differences in selection also appear to be responsible for discordance in observations regarding influence of sex on the course of the disease. The present data seem to support the view that most of the cases revealing chronic forms of SMA (both mild and severe) are not distinct genetically. However, the possible existence of a distinct subgroup in which sex influence is strongly expressed is not excluded.