Genetic mapping of chronic childhood-onset spinal muscular atrophy to chromosome 5q11.2-13.3

Diagnosis and clinical management of spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease characterized by degeneration of lower motor neurons, with resulting progressive muscle weakness. The clinical phenotype and disease severity can be varied and occupy a wide spectrum. Although many advances have been made regarding our understanding of SMA, no cure is yet available. The care of patients who have SMA can often be complex, with many medical issues to consider. When possible, a multidisciplinary team approach is effective. The current understanding of SMA, and the clinical management and rehabilitative care of patients who have SMA, are discussed in this article.

Fugu rubripes and human survival motor neuron genes: structural and functional similarities in comparative genome studies

The compactness of the Fugu rubripes (Fugu) genome has supported its use in comparative genome analysis. Nevertheless, as Fugu is distinct evolution-wise from humans, it is essential to determine the similarity between a Fugu gene and its human counterpart to confirm its potential for comparative genome analysis. We cloned and analyzed the Fugu survival motor neuron gene (fsmn) for similarities with human SMN gene (huSMN). The Fugu genome has a single fsmn that is 13.4 times smaller than huSMN. fsmn and huSMN are highly similar in their genome organization and tissue expression patterns. The functional domains of the Fugu smn and human SMN molecules are also highly conserved. In human MCF-7 cells, expression of fsmn protein resulted in the formation of "gems" in the cytoplasm and nucleus, similar to observations reported for huSMN protein. In these cells, fsmn RNA was also processed correctly and produced alternatively spliced transcripts like huSMN2. These findings indicate close structural and functional similarities between fsmn and huSMN, suggesting that regulation of the two genes may also be similar and supporting the use of fsmn in comparative genome studies for the identification of functional regulatory elements of huSMN.

Spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive neurodegenerative disease characterised by degeneration of spinal cord motor neurons, atrophy of skeletal muscles, and generalised weakness. It is caused by homozygous disruption of the survival motor neuron 1 (SMN1) gene by deletion, conversion, or mutation. Although no medical treatment is available, investigations have elucidated possible mechanisms underlying the molecular pathogenesis of the disease. Treatment strategies have been developed to use the unique genomic structure of the SMN1 gene region. Several candidate treatment agents have been identified and are in various stages of development. These and other advances in medical technology have changed the standard of care for patients with spinal muscular atrophy. In this Seminar, we provide a comprehensive review that integrates clinical manifestations, molecular pathogenesis, diagnostic strategy, therapeutic development, and evidence from clinical trials.

Survival motor neuron protein regulates apoptosis in an in vitro model of spinal muscular atrophy

Progressive spinal muscular atrophy (SMA), the most prevalent hereditary lower motor neuron disease, is caused by mutations in the telomeric copy of the survival of motor neuron (SMN1) gene. Unlike other cells, lower motor neurons cannot tolerate low levels of smn protein. However, it is unclear as to the nature of the cell death involved. There is evidence that lower motor neurons undergo apoptosis in SMA, leading to muscle weakness and wasting. This study investigated whether SMN1 regulation in a motor neuron model affected indices of apoptotic cell death. Decreased smn expression in neuroblastoma hybrid (NSC-34) cell lines by small interfering RNA (siRNA) was demonstrated at the mRNA and protein level. Smn-depleted cells showed elevated caspase-3 activity, decreased cell viability and increased percentage of TUNEL positive cells. Conversely, NSC-34 cell smn overexpression by adenoviral gene transfer decreased staurosporine-induced caspase-3 elevation and mitigated induced cell toxicity as assessed by 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. However, increased smn expression by itself did not increase cell viability. These data suggest not only that decreased smn levels increase apoptosis in an in vitro model of SMA, but also that increased smn can protect against neural injury.

SMN2 and NAIP gene dosages in Vietnamese patients with spinal muscular atrophy

BACKGROUND

The SMN1 gene is now recognized as a spinal muscular atrophy (SMA)-causing gene, while SMN2 and NAIP have been characterized as a modifying factor of the clinical severity of SMA. Gene dosage of SMN2 is associated with clinical severity of SMA. But the relationship between gene dosage of NAIP and clinical severity of SMA remains to be clarified, although complete deletion of NAIP is frequent in type I patients.

METHODS

To evaluate the contribution of the SMN2 and NAIP gene dosages to SMA, quantitative real-time polymerase chain reaction was used to measure copy numbers of SMN2 and NAIP in 34 Vietnamese SMA patients lacking SMN1 (13 type I, 11 type II and 10 type III patients).

RESULTS

The SMN2 copy number in type I patients was significantly lower than that in type II-III patients, which was compatible with the previous reports. In contrast, 25 out of 34 patients had only zero or one copy of NAIP, while 50 out of 52 controls had two or more copies. For NAIP (+) genotype, six out of 13 type I patients, eight out of 11 type II patients and six out of 10 type III patients carried one NAIP copy.

CONCLUSIONS

The SMN2 copy number was related to the clinical severity of SMA among Vietnamese patients. The presence of one NAIP copy, that is, heterozygous NAIP deletion, was common in Vietnamese SMA, regardless of clinical phenotype.

Molecular Analysis of the SMN1 and NAIP Genes in Iranian Patients with Spinal Muscular Atrophy

Introduction: Childhood-onset proximal spinal muscular atrophies (SMAs) are an autosomal recessive, clinically heterogeneous group of neuropathies characterised by the selective degeneration of anterior horn cells. SMA has an estimated incidence of 1 in 10,000 live births. The causative genes are survival motor neuron (SMN) gene and neuronal apoptosis inhibitory protein (NAIP) gene. Deletions of the telomeric copy of SMN gene (SMN1) have been reported in 88.5% to 95% of SMA cases, whereas the deletion rate for NAIP gene (NAIP) is between 20% and 50% depending on the disease severity. The main objective of this study was to genetically characterise the childhood onset of SMA in Iran. Materials and Methods: Molecular analysis was performed on a total of 75 patients with a clinical diagnosis of SMA. In addition to common PCR analysis for SMN1 exons 7 and 8, we analysed NAIP exons 4 and 5, along with exon 13, as a internal control, by bi-plex PCR. Results: The homozygous-deletion frequency rate for the telomeric copy of SMN exons 7 and 8 in all types of SMA was 97%. Moreover, exons 5 and 6 of NAIP gene were deleted in approximately 83% of all SMA types. Three deletion haplotypes were constructed by using SMN and NAIP genotypes. Haplotype A, in which both genes are deleted, was seen in approximately 83% of SMA types I and II but not type III. It was also found predominantly in phenotypically severe group with an early age of onset (i.e., less than 6-month-old). We also report 34 of our prenatal diagnosis. Conclusions: To our knowledge, the present study is the first one giving detailed information on SMN and NAIP deletion rates in Iranian SMA patients. Our results show that the frequency of SMN1 homozygous deletions in Iran is in agreement with previous studies in other countries. The molecular analysis of SMA-related gene deletion/s will be a useful tool for pre- and postnatal diagnostic. Key words: Deletion analysis, Iran, NAIP gene, Prenatal diagnosis, Spinal muscular atrophy, SMN gene

Molecular mechanisms of spinal muscular atrophy

Significant strides have been made during the past decade in the understanding of the molecular mechanisms that lead to the autosomal recessive motor neuron disease spinal muscular atrophy. Genetic studies revealed that spinal muscular atrophy is caused by mutation of the telomeric copy of the survival motor neuron gene ( SMN1), with all patients retaining at least one copy of the centromeric form of the gene, SMN2. SMN2 produces reduced amounts of full-length SMN messenger ribonucleic acid because of alterative splicing of SMN2 -derived transcripts, a process that is governed by specific cisand trans-acting factors. The resulting insufficient expression level of full-length SMN protein likely causes the disease manifestations of spinal muscular atrophy; however, the mechanism for the selective vulnerability of the motor unit to deficiency of this ubiquitously expressed protein remains unknown. It also remains unclear specifically when and where in the motor unit SMN is required. Despite the remaining questions, progress has been made in developing therapeutic strategies targeted to specific points along the pathogenetic pathway of spinal muscular atrophy. Histone deacetylase inhibitors will be discussed as an example.

Spinal muscular atrophy: clinical classification and disease heterogeneity

The clinical classification of spinal muscular atrophy, caused by deletion of the survival motor neuron 1 gene (SMN1), is based on age at onset and maximum function achieved. Evidence suggests that maximum function achieved is more closely related to life expectancy than age at onset. Therefore, it is important to wait for a period before assigning a patient to 1 of 5 classes of the disorder. Several diseases result from degeneration of the anterior horn cell but are not caused by SMN1. The classification for these conditions is evolving. This article offers an attempt at organizing one's thinking about this disease group.

The molecular basis of spinal muscular atrophy (SMA) in South African black patients

SMA is an autosomal recessive disorder that results in symmetrical muscle weakness and wasting due to degeneration of the anterior horns of the spinal cord. The gene for SMA, the survival motor neuron (SMN) gene is found on chromosome 5q13, in a region harbouring a 500kb duplication, resulting in two copies (a telomeric and a centromeric) of each of the genes found within the duplication. SMN1 is homozygously deleted in approximately 95% of patients worldwide. Results of the current study show that only 51% (42/92) of South African black SMA patients have homozygous deletions of the SMN1 gene. This frequency is significantly lower than observed in the South African white patient group and in other international populations. The pattern of deletions in the South African black patients is also significantly different. In order to elucidate the molecular basis of SMA in the black population, a dosage assay enabling the detection of SMN1 deletion heterozygotes was independently developed. This assay confirmed SMN1 heterozygosity in at least 70% of black non-deletion SMA patients. However, no second disease-causing mutation or a common chromosomal background for this mutation could be identified in these patients. The frequency of SMA in both the black and white population was also determined using the SMN1 gene dosage assay. Results showed that SMA is more common than previously thought with carrier rates of 1 in 50 and 1 in 23 and a predicted birth incidence of 1 in 3574 and 1 in 1945 in the black population and the white population, respectively. Development and incorporation of the SMN1 dosage assay into the molecular diagnostic service will increase the percentage of cases in which the diagnosis of SMA can be confirmed and allow preclinical and prenatal diagnosis. Further gene characterisation and functional studies would need to be performed in order to further define the molecular basis of SMA in the South African black population.

Coinheritance of mutated SMN1 and MECP2 genes in a child with phenotypic features of spinal muscular atrophy (SMA) type II and Rett syndrome

Spinal muscular atrophy (SMA) is a neuromuscular autosomal recessive disease characterized by progressive muscle weakness and atrophy combined with motor neuron degeneration caused by mutations in the SMN 1 gene locus (5q11.2-13.2). Rett syndrome (RS) is an X-linked dominant neurodevelopmental disorder caused by mutations in MECP2 (Xq28) and characterized by normal development until 6-12 months of age, followed by regression with loss of acquired skills, gradual onset of microcephaly, stereotypic hand movements and psychomotor delay. We report a 6-year-old girl who, at 2 years of age, presented with hypotonia, psychomotor delay, amyotrophy and areflexia of the lower extremities. Molecular DNA analysis (PCR-RFLP's) for SMA type II revealed that both exons 7 and 8 of SMN 1 gene were deleted. Over the past 4 years, onset of stereotypic hand-washing movements, epileptic seizures, microcephaly, hyperventilation/breath-holding attacks and severe psychomotor delay raised the suspicion of the coexistence of RS. DNA analysis (DGGE and sequencing) identified the hotspot missense mutation R306C (c.916C>T) in exon 4 of the MECP2 gene. The coinheritance of SMA and RS, two rare monogenic syndromes in the same patient, has not been previously reported. Thorough clinical evaluation in combination with DNA analysis, allowed accurate diagnosis, providing valuable information for the genetic counseling of the family.

Case report: birth after preimplantation genetic diagnosis of a subtle mutation in SMN1 gene

Spinal muscular atrophy (SMA) preimplantation genetic diagnosis (PGD) has been available since 1998. Protocols are based on the detection of the homozygous deletion of exon 7, which are present in 90-98% of SMA patients. A couple where the woman was a heterozygous carrier of the usual SMN1 Del7 mutation and the man was a heterozygous carrier of pMet263Arg substitution in exon 6 of SMN1 gene was referred for PGD. The usual PGD test being unsuitable for this couple, we developed a novel duplex polymerase chain reaction (PCR)-based PGD test for the detection of the mutation pMet263Arg by allele specific amplification, combined with the amplification of D5S641 extragenic polymorphic marker. PCR conditions were established using single control lymphoblasts and lymphocytes from the pMet263Arg substitution carrier. Amplification was obtained in 100% of the 86 single cells tested, amplification refractory mutation system (ARMS) PCR was specific in 100% of single cells tested and a complete genotype (mutation plus D5S641) was achieved in 88% of them. A PGD cycle was performed successfully and a pregnancy was obtained. An unaffected girl was born and postnatal diagnosis confirmed PGD results. This is the first PGD described for SMA because of another mutation than the major homozygous exon 7 deletion of SMN1. In the future, a similar strategy could be adopted for other subtle mutations of this gene.

Deletion analyses of SMN1 and NAIP genes in Malaysian spinal muscular atrophy patients

BACKGROUND

The survival motor neuron 1 (SMN1) gene has been recognized to be responsible for spinal muscular atrophy (SMA) because it is homozygously deleted in more than 90% of SMA patients, irrespective of their clinical severity, whereas the neuronal apoptosis inhibitory protein (NAIP) gene is now considered to be a modifying factor of the severity of SMA. In Malaysia, it remains to be elucidated whether deletion of the SMN1 gene is also a main cause of SMA or whether deletion of the NAIP gene is found in the SMA patients.

METHODS

To clarify the pathogenesis of SMA in Malaysia, a deletion analysis of the SMN1 and NAIP genes was performed in 24 Malaysian SMA patients. Deletion analysis of exons 7 and 8 of the SMN1 gene was performed according to the method described by van der Steege et al., while deletion analysis of exon 5 of the NAIP gene was performed according to a method described by Roy et al.

RESULTS

Homozygous deletion of SMN1 exon 7 and exon 8 were identified in 19 out of 24 patients (79%). As to the NAIP gene, deletion of exon 5 was detected in six out of 24 patients (25%). NAIP gene deletion was correlated with severity of the disease.

CONCLUSIONS

Deletion of the SMN1 exon 7 is a major cause of SMA in Malaysia, and NAIP gene deletions are not rare in type I SMA in Malaysia. The lower percentage of the SMN1 gene deletion may be due to the possibility that the present study included some patients without SMN1 gene abnormality and/or some patients with non-deletion type mutations in the SMN1 gene.

Spinal muscular atrophy and therapeutic prospects

The molecular genetic basis of spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disorder, is the loss of function of the survival motor neuron gene (SMN1). The SMN2 gene, a nearly identical copy of SMN1, has been detected as a promising target for SMA therapy. Both genes are ubiquitously expressed and encode identical proteins, but markedly differ in their splicing patterns: While SMN1 produces full-length (FL)-SMN transcripts only, the majority of SMN2 transcripts lacks exon 7. Transcriptional SMN2 activation or modulation of its splicing pattern to increase FL-SMN levels is believed to be clinically beneficial and therefore a crucial challenge in SMA research. Drugs such as valproic acid, phenylbutyrate, sodium butyrate, M344 and SAHA that mainly act as histone deacetylase inhibitors can mediate both: they stimulate the SMN2 gene transcription and/or restore the splicing pattern, thereby elevating the levels of FL-SMN2 protein. Preliminary phase II clinical trials and individual experimental curative approaches SMA patients show promising results. However, phase III double-blind placebo controlled clinical trials have to finally prove the efficacy of these drugs.

Molecular Analysis of Survival Motor Neuron and Neuronal Apoptosis Inhibitory Protein Genes in Macedonian Spinal Muscular Atrophy Patients

Spinal muscular atrophy (SMA) is classified according to the age of onset and severity of the clinical manifestations into: acute (Werding-Hoffman disease or type I), intermediate (type II) and juvenile (Kugelberg-Wilander disease or type III) forms. All three SMAs have been linked to markers at 5q11.2-q13.3. Two candidate genes deleted in SMA patients are the survival motor neuron (SMN) gene and the neuronal apoptosis inhibitory protein (NAIP) gene. We have performed molecular analyses of these genes in 30 unrelated Macedonian families (17 with type I, eight with type II and five with type III forms of the disease). Deletions of exons 7 and 8 of the SMN gene were found in 76.6% (23/30) of patients (94.1% in type I, 87.5% in type II). Among these 23 families, 19 had both exons deleted, while four had deletions only of exon 7. Deletions of exon 5 of the NAIP gene were found in 41.2% (7/17) patients with type I SMA and in 12.5% (1/8) of patients with type II SMA. No deletions of the SMN gene were found in 30 parents and 30 normal controls. We found 2/30 (6.7%) parents to be homozygous for the deletion of exon 5. Our data support the hypothesis that the telomeric SMN gene plays a major role in determining the clinical course of the disease, while the defects in the NAIP gene have only a modifying effect on the phenotype.

Histone deacetylase inhibitors as therapeutics for polyglutamine disorders

During the past 5 years, gene expression studies in cell culture, animal models and in the brains of patients have shown that the perturbation of transcription frequently results in neuronal dysfunction in polyglutamine repeat diseases such as Huntington's disease. Histone deacetylases act as repressors of transcription through interactions with co-repressor complexes, which leads to chromatin remodelling. Aberrant interactions between polyglutamine proteins and regulators of transcription could be one mechanism by which transcriptional dysregulation occurs. Here, we discuss the potential therapeutic pathways through which histone deacetylase inhibitors might act to correct the aberrant transcription observed in Huntington's disease and other polyglutamine repeat diseases.

Spinal muscular atrophy: from gene to therapy

The molecular basis of spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disorder, is the homozygous loss of the survival motor neuron gene 1 (SMN1). A nearly identical copy of the SMN1 gene, called SMN2, modulates the disease severity. The functional difference between both genes is a translationally silent mutation that, however, disrupts an exonic splicing enhancer causing exon 7 skipping in most SMN2 transcripts. Only 10% of SMN2 transcripts encode functional full-length protein identical to SMN1. Transcriptional activation, facilitation of correct SMN2 splicing, or stabilization of the protein are considered as strategies for SMA therapy. Among various drugs, histone deacetylase inhibitors such as valproic acid (VPA) or 4-phenylbutyrate (PBA) have been shown to increase SMN2-derived RNA and protein levels. Recently, in vivo activation of the SMN gene was shown in VPA-treated SMA patients and carriers. Clinical trials are underway to investigate the effect of VPA and PBA on motor function in SMA patients.

Therapeutics development for spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive motor neuron disease that is the leading inherited cause of infant and early childhood mortality. Spinal muscular atrophy is caused by mutation of the telomeric copy of the survival motor neuron gene (SMN1), but all patients retain a centromeric copy of the gene, SMN2. SMN2 produces reduced amounts of full-length SMN mRNA, and spinal muscular atrophy likely results from insufficient levels of SMN protein in motor neurons. The SMN protein plays a well-established role in assembly of the spliceosome and may also mediate mRNA trafficking in the axon and nerve terminus of neurons. In patients, spinal muscular atrophy disease severity correlates inversely with increased SMN2 gene copy number and, in transgenic mice lacking endogenous SMN, increasing SMN2 gene copy number from two to eight prevents the SMA disease phenotype. These observations suggest that increasing SMN expression levels may be beneficial to SMA patients. Currently pursued therapeutic strategies for SMA include induction of SMN2 gene expression, modulation of splicing of SMN2-derived transcripts, stabilization of SMN protein, neuroprotection of SMN deficit neurons, and SMN1 gene replacement. Early clinical trials of candidate therapeutics are now ongoing in SMA patients. Clinical trials in this disease present a unique set of challenges, including the development of meaningful outcome measures and disease biomarkers.

Is spinal muscular atrophy the result of defects in motor neuron processes?

The hereditary neurodegenerative disease spinal muscular atrophy (SMA) with childhood onset is one of the most common genetic causes of infant mortality. The disease is characterized by selective loss of spinal cord motor neurons leading to muscle atrophy and is the result of mutations in the survival motor neuron (SMN) gene. The SMN protein has been implicated in diverse nuclear processes including splicing, ribosome formation and gene transcription. Even though the genetic basis of SMA is well understood, it is not clear how defects in these ubiquitous processes result in motor neuron degeneration leaving other tissues unaffected. Recent evidence from animal and cell culture models of SMA points to roles for SMN in neurite outgrowth and axonal transport. Disruption of these functions might be particularly detrimental to motor neurons given their high metabolic demands and precise connectivity requirements, thus providing a possible explanation for the specificity of motor neuron susceptibility in SMA. Understanding the molecular mechanisms of SMN activity in neuronal processes may generate new targets for future therapeutic strategies.

Gemins modulate the expression and activity of the SMN complex

Reduction in the expression of the survival of motor neurons (SMN) protein results in spinal muscular atrophy (SMA), a common motor neuron degenerative disease. SMN is part of a large macromolecular complex (the SMN complex) that includes at least six additional proteins called Gemins (Gemin2-7). The SMN complex is expressed in all cells and is present throughout the cytoplasm and in the nucleus where it is concentrated in Gems. The SMN complex plays an essential role in the production of spliceosomal small nuclear ribonucleoproteins (snRNPs) and likely other RNPs. To study the roles of the individual proteins, we systematically reduced the expression of SMN and each of the Gemins (2-6) by RNA interference. We show that the reduction of SMN leads to a decrease in snRNP assembly, the disappearance of Gems, and to a drastic reduction in the amounts of several Gemins. Moreover, reduction of Gemin2 or Gemin6 strongly decreases the activity of the SMN complex. These findings demonstrate that other components of the SMN complex, in addition to SMN, are critical for the activity of the complex and suggest that Gemin2 and Gemin6 are potentially important modifiers of SMA as well as potential disease genes for non-SMN motor neuron diseases.

Floppy infant syndrome

Floppiness/hypotonia is a common neurologic symptom in infancy. A variety of neuromuscular disorders and central nervous system (CNS) disorders cause floppy infant syndrome (FIS). CNS disorders are the much more common causes of the syndrome than neuromuscular disorders. On long-term follow up, cerebral palsy and mental retardation turn out to be the 2 most common causes of FIS. This review focuses on neuromuscular causes of FIS. With the advent of molecular diagnosis, a few conditions can be diagnosed by DNA analysis of the peripheral lymphocytes (myotonic dystrophy, spinal muscular atrophy); however, for the most part, electrodiagnostic studies and muscle biopsy remain as essential diagnostic tools for FIS. Immunohistochemical study of the biopsied muscle also improves diagnostic capability. Management for most conditions remains supportive.

Spinal muscular atrophy and progressive myoclonic epilepsy: one case report and characteristics of the epileptic syndrome

INTRODUCTION

Spinal muscular atrophies (SMAs) are a group of degenerative diseases primarily affecting the anterior horn cells of the spinal cord and motor cells of cranial nerve nuclei. Even if the clinical picture is mainly dominated by the diffuse muscular atrophy, in some cases, patients may show associated, atypical clinical features ("SMA plus"). In particular, the association of SMA and progressive myoclonic epilepsy (PME) has been rarely described.

CASE REPORT

We present the clinical and electrophysiological data of a boy with childhood-onset SMA associated with PME and reviewed cases of the literature.

CONCLUSION

The association of SMA with PME may constitute a separate and, probably, genetically independent syndrome with unique clinical and electroencephalographic findings or, at least, a variant of a neurodegenerative or metabolic disease, due to yet unknown causes.

Reduced expression of nicotinic AChRs in myotubes from spinal muscular atrophy I patients

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by degeneration of motoneurons and skeletal muscle atrophy. In its most severe form, it leads to death before the age of 2 years. While primary degeneration of motor neurons is well established in this disease, and this results in neurogenic atrophy of skeletal muscle, we have previously reported evidence for a primary muscle defect. In this study, we used primary cultures of embryonic human skeletal muscle cells from patients with SMA and from controls to examine the effects of muscle fiber differentiation in the absence of a nerve component. Cultured SMA skeletal muscle cells are unable to fuse correctly to form multinuclear myotubes, the precursors of the myofibers. We also show that agrin-induced aggregates of nicotinic acetylcholine receptors, one of the earliest steps of neuromuscular junction formation, cannot be visualized by confocal microscopy on cells from SMA patients. In binding experiments, we demonstrate that this lack of clustering is due to defective expression of the nicotinic acetylcholine receptors in the myotubes of SMA patients whereas the affinity of alpha-bungarotoxin for its receptor remains unchanged regardless of muscle cell type (SMA or control). These observations suggest that muscle cells from SMA patients have intrinsic abnormalities that may affect proper formation of the neuromuscular junction.

Multiplex nested PCR for preimplantation genetic diagnosis of spinal muscular atrophy

OBJECTIVE

Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder caused in most patients by homozygous deletion of the SMN1 gene. For a carrier couple at a 25% risk of affected offspring, preimplantation genetic diagnosis (PGD) offers an alternative to prenatal diagnosis and termination of affected pregnancies. Our objective was to develop an accurate and reliable single-cell multiplex nested PCR analysis for PGD of SMA.

METHODS

The method was developed on single blood leukocytes, obtained from healthy controls and an adult SMA type III patient with a known homozygous deletion of SMN1 exon 7 and 8. Multiplex nested PCR on single cells was used to co-amplify exons 7 and 8 of SMN. Additional multiplexing was performed with the ZFX/ZFY gene for sexing. Following successful establishment of the multiplex nested PCR protocol in single leukocytes, the technique was employed for PGD in 4 patients for a total of 7 cycles. In 2 patients, sexing was simultaneously performed using ZFX/ZFY.

RESULTS

220 single leukocytes from a normal individual and 220 from an SMA patient were analyzed. Exon 7 of SMN1 was amplified in 99% of normal single leukocytes and in none of the SMA-affected leukocytes. Exon 7 of SMN2 was amplified in 100% of both normal and SMA-affected leukocytes. Exon 8 of SMN1 was amplified in 98% of normal cells and in none of the SMA-affected leukocytes. Exon 8 of SMN2 was amplified in 96% of both normal and SMA-affected leukocytes. Amplification efficiency was 99% for ZFX/ZFY. There were no false-negative results and no contamination was detected in all wash-drop blanks tested. Seven PGD cycles were performed in 4 SMA-carrier couples with successful molecular analysis of 34 embryos and a total of 15 normal embryos transferred in 7 cycles. One clinical pregnancy has resulted in the delivery of a healthy male. Amniocentesis performed at 17 weeks confirmed the correct diagnosis for both SMA and sexing.

CONCLUSIONS

These results suggest that our multiplex nested PCR protocol offers an efficient and accurate method for PGD of SMA while enabling the simultaneous analysis of an additional loci.

Significant increase in the number of the SMN2 gene copies in an adult-onset Type III spinal muscular atrophy patient with homozygous deletion of the NAIP gene

The patient was a 57-year-old Japanese man who gradually developed muscle atrophy and weakness in the trunk and limbs since the age of 20 years and was wheelchair bound at the age of 56 years. The gene copy number assay confirmed the combined homozygous deletion of the survival motor neuron (SMN) 1 and neuronal apoptosis inhibitory protein (NAIP) genes and showed the presence of 4 copies of the SMN2 gene. In this patient, the significant increase in the number of the SMN2 gene copies should compensate for the homozygous deletion of the SMN1 gene and make his disease milder despite the absence of the NAIP gene. Taken together with our previous data, we may reasonably hypothesize that the SMN2 gene copy number is more critical in determining the severity of the disease compared to the NAIP genotype.

Anaesthesia for caesarean section in spinal muscular atrophy type III

We describe the conduct of general anaesthesia for a patient with spinal muscular atrophy Type III (Kugelberg-Welander disease) undergoing elective caesarean section. Apart from a delayed return of skeletal muscle power following non-depolarising neuromuscular blockade the procedure was uneventful. We found no previously published reports of general anaesthesia for caesarean section in this condition in the English language literature. We review the available literature and discuss the potential anaesthetic problems in the management of obstetric patients with this degenerative neuromuscular disorder.

Spinal muscular atrophy

Spinal muscular atrophy is a common genetic disease of the motor neuron (frequency of eight cases per 100,000 live births) with a high mortality during infancy and no known treatment. Death is caused by severe and progressive restrictive lung disease. New information regarding the nature and function of the SMN protein and the availability of new pharmacologic agents now make it possible to consider clinical trials in this disease. Rehabilitation and proper management of medical complications have improved both the quality and duration of life for children with spinal muscular atrophy.

Survival Motor Neuron (SMN) Genes for Molecular Allele-Specific Amplification of Exon 7 in the Diagnosis of Spinal Muscular Atrophy

There are two highly homologous survival motor neuron (SMN) genes in humans but molecular defects in the SMN1 gene cause spinal muscular atrophy (SMA). More than 90% of SMA patients are shown to have a homozygous deletion of exon 7 in the SMN1 gene. Therefore, a simple test for exon 7 deletion would be very useful in the molecular diagnosis of SMA. However, limited methods are available, and most of these methods utilize expensive instruments and consumables. Here, we describe a simple allele-specific PCR test, which can be performed using standard equipment in DNA laboratories. The principle of the test is based on a single nucleotide difference (C versus T) between the exon 7 of SMN1 and SMN2 genes. Using allele-specific primers, two PCR amplifications are performed for each sample to amplify a 404-bp diagnostic fragment, and consequent electrophoresis of PCR products on agarose gel provides definitive information concerning the exon 7 deletion To rule out false negatives, a 500-bp fragment from the N-acetyltransferase gene was coamplified as an internal control in each test. We have, so far, analyzed 41 SMA samples with our method, and tested the validity of results using an independent restriction fragment length polymorphism (RFLP) method. Genotyping results obtained by both methods were in complete agreement for all of the samples analyzed. Our method can also be used to detect heterozygous deletion of exon 7 in SMN genes, if the relative intensities of the diagnostic and internal control bands are determined.

Duplex PCR for preimplantation genetic diagnosis (PGD) of spinal muscular atrophy

The main difficulty in developing a molecular diagnosis of spinal muscular atrophy (SMA) resides in the specific genomic structure of the locus. Indeed, two highly homologous survival motor neurone genes, SMN1 and SMN2, are present at the locus. The detection of the homozygous deletion of exons 7 and 8 of the SMN1 gene, which is present in 90 to 98% of the patients, is based on methods highlighting 1 of the 8 nucleotidic mismatches existing between these 2 genes. In order to offer preimplantation genetic diagnosis (PGD) for SMA, we developed a new allele-specific amplification method. The main disadvantage of our previously described strategy resided in the possibility of diagnosing, in case of amplification failure, an unaffected embryo as affected. We present here a new PGD-SMA method. We established the conditions for three different duplex PCRs, allowing the specific detection of the SMN1 gene and one polymorphic marker, either D5S629, D5S1977, or D5S641. Of the 60 to 90 single cells tested, the PCR efficiency varied from 98 to 100% with a complete genotype obtained in a range between 81 and 87% with a global allele drop-out rate of 9%. Such a test was used to perform 1 PGD cycle for which 7 embryos could be analysed. All the embryos were fully diagnosed, six as unaffected and one as affected. Four embryos were transferred, but no pregnancy ensued.

A simple method for diagnosis of autosomal recessive spinal muscular atrophy by denaturing high-performance liquid chromatography

Autosomal recessive spinal muscular atrophy is caused by mutations in the survival motoneuron (SMN) gene. There are two nearly identical copies of this gene present on chromosome 5q13; however, only the telomeric copy of this gene is affected in spinal muscular atrophy. In this study, we describe a new method to detect SMN gene deletion by denaturing high-performance liquid chromatography, which is also simple to perform but is faster and more specific.

Genetic diagnosis of Werdnig-Hoffmann disease: a problem for application to prenatal diagnosis

We report a floppy infant with Werdnig-Hoffmann disease (spinal muscular atrophy: SMA type 1) and Klinefelter syndrome. After genetic counseling with parents, a genetic diagnosis using DNA from the infant's peripheral blood mononuclear cells was performed. The parents' deletion of exons 7 and 8 of the survival motor neuron (smn) gene and exons 4 and 5 of the neuronal apoptosis inhibitory protein (naip) gene were noted in the infant, so he was confirmed to have SMA type 1. The parents wanted to receive a prenatal diagnosis on the next pregnancy. However this genetic test is achieved by confirming that a specific band can not be detected by PCR. Therefore, this method should be applied with great care to prenatal diagnosis using chorionic villi, which may be contaminated with maternal tissue.

Severe spinal muscular atrophy variant associated with congenital bone fractures

Infantile autosomal recessive spinal muscular atrophy (type I) represents a lethal disorder leading to progressive symmetric muscular atrophy of limb and trunk muscles. Ninety-six percent cases of spinal muscular atrophy type I are caused by deletions or mutations in the survival motoneuron gene (SMNI) on chromosome 5q11.2-13.3. However, a number of chromosome 5q-negative patients with additional clinical features (respiratory distress, cerebellar hypoplasia) have been designated in the literature as infantile spinal muscular atrophy plus forms. In addition, the combination of severe spinal muscular atrophy and neurogenic arthrogryposis has been described. We present clinical, molecular, and autopsy findings of a newborn boy presenting with generalized muscular atrophy in combination with congenital bone fractures and extremely thin ribs but without contractures.

Prenatal prediction of childhood-onset spinal muscular atrophy (SMA) in Turkish families

Childhood-onset spinal muscular atrophy (SMA) is one of the most common neurodegenerative genetic disorders. SMN1 is the SMA-determining gene deleted or mutated in the majority of SMA cases. There is no effective cure or treatment for this disease yet. Thus, the availability of prenatal testing is important. Here we report prenatal prediction for 68 fetuses in 63 Turkish SMA families using direct deletion analysis of the SMN1 gene by restriction digestion. The genotype of the index case was known in 40 families (Group A) but unknown in the remaining 23 families (Group B). A total of ten fetuses were predicted to be affected. Eight of these fetuses were derived from Group A and two of these fetuses were from Group B families. Two fetuses from the same family in Group A had the SMNhyb1 gene in addition to homozygous deletion of the NAIP gene. One fetus from Group A was homozygously deleted for only exon 8 of the SMN2 gene, and further analysis showed the presence of both the SMN1 and SMNhyb1 genes but not the SMN2 gene. In addition, one carrier with a homozygous deletion of only exon 8 of the SMN1 gene was detected to have a SMNhyb2 gene, which was also found in the fetus. To our knowledge, these are the first prenatal cases with SMNhyb genes. Follow-up studies demonstrated that the prenatal predictions and the phenotype of the fetuses correlated well in 33 type I pregnancies demonstrating that a careful molecular analysis of the SMN genes is very useful in predicting the phenotype of the fetus in families at risk for SMA.

Spinal muscular atrophy: recent advances and future prospects

Spinal muscular atrophies (SMA) are characterized by degeneration of lower motor neurons associated with muscle paralysis and atrophy. Childhood SMA is a frequent recessive autosomal disorder and represents one of the most common genetic causes of death in childhood. Mutations of the SMN1 gene are responsible for SMA. The knowledge of the genetic basis of SMA, a better understanding of SMN function, and the recent generation of SMA mouse models represent major advances in the field of SMA. These are starting points towards understanding the pathophysiology of SMA and developing therapeutic strategies for this devastating neurodegenerative disease, for which no curative treatment is known so far.

Spinal muscular atrophy among the Roma (Gypsies) in Bulgaria and Hungary

Spinal muscular atrophy is one of the most common autosomal recessive disorders, classified into three major clinical forms. It is caused mainly by deletions or gene conversions of the telomeric survival motor neuron gene (SMN1) on human chromosome 5. We have conducted molecular studies of the disorder in genetically isolated Romani (Gypsy) communities in Bulgaria and Hungary, where spinal muscular atrophy appears to have different prevalence and both mild and severe spinal muscular atrophy phenotypes have been diagnosed. We have observed three distinct genetic defects which, in different combinations, lead to different forms of the disease. The similar chromosomal background on which the different mutations occur suggests a common origin and founder effect, with rearrangements of a single ancestral chromosome resulting in a diversity of molecular defects.

Gemin5, a novel WD repeat protein component of the SMN complex that binds Sm proteins

The survival of motor neurons (SMN) protein is the product of the disease gene of spinal muscular atrophy and is found both in the cytoplasm and the nucleus, where it is concentrated in gems. SMN is part of a multi-protein complex that includes Gemin2, Gemin3, and Gemin4. The SMN complex plays an important role in the cytoplasmic assembly of small nuclear ribonucleoproteins (snRNPs) and likely other RNPs in pre-mRNA splicing and in the assembly of transcriptosomes. Here, we report the identification of an additional component of the SMN complex, a novel WD repeat protein termed Gemin5. Gemin5 binds SMN directly and is a component of the SMN complex. Furthermore, Gemin5 interacts with several of the snRNP core proteins including SmB, SmD1, SmD2, SmD3, and SmE, suggesting that it participates in the activities of the SMN complex in snRNP assembly. Immunolocalization studies demonstrate that Gemin5 is found in the cytoplasm and in the nucleus, where it colocalizes with SMN in gems. The presence of 13 WD repeat domains in the amino-terminal half of Gemin5 and a coiled-coil motif near its carboxyl terminus indicate that it may form a large heteromeric complex and engage in multiple interactions.

Molecular analysis of spinal muscular atrophy and modification of the phenotype by SMN2

PURPOSE

This study describes SMN1 deletion frequency, carrier studies, and the effect of the modifying SMN2 gene on the spinal muscular atrophy (SMA) phenotype. A novel allele-specific intragenic mutation panel increases the sensitivity of SMN1 testing.

METHODS

From 1995 to 2001, 610 patients were tested for SMN1 deletions and 399 relatives of probands have been tested for carrier status. SMN2 copy number was compared between 52 type I and 90 type III patients, and between type I and type III patients with chimeric SMN genes. A fluorescent allele-specific polymerase chain reaction (PCR) -based strategy detected intragenic mutations in potential compound heterozygotes and was used on 366 patients.

RESULTS

Less than half of the patients tested were homozygously deleted for SMN1. A PCR-based panel detected the seven most common intragenic mutations. SMN2 copy number was significantly different between mild and severely affected patients.

CONCLUSIONS

SMN1 molecular testing is essential for the diagnosis of SMA and allows for accurate carrier testing. Screening for intragenic mutations in SMN1 increases the sensitivity of diagnostic testing. Finally, SMN2 copy number is conclusively shown to ameliorate the phenotype and provide valuable prognostic information.

A single strand conformation polymorphism-based carrier test for spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive disorder with a newborn prevalence of 1 in 10,000, and a carrier frequency of 1 in 40-60 individuals. The SMA locus has been mapped to chromosome 5q11.2-13. The disease is caused by a deletion of the SMN gene, often encompassing other genes and microsatellite markers. The SMN gene is present in two highly homologous copies, SMN1 and SMN2, differing at five nucleotide positions. Only homozygous SMN1 mutations cause the disease. The sequence similarity between the SMN1 and SMN2 genes can make molecular diagnosis and carrier identification difficult. We developed a sensitive and reliable molecular test for SMN1 carrier identification, by setting up a nonradioactive single strand conformation polymorphism (SSCP)-based method, which allows for the quantification of the amount of the SMN1 gene product with respect to a control gene. The assay was validated in 56 obligate (ascertained) carriers and 20 (ascertained) noncarriers. The sensitivity of the test is 96.4%, and its specificity, 98%. In addition, 6 of 7 SMA patients without homozygous deletions presented with a heterozygous deletion, suggesting a concomitant undetected point mutation on the nondeleted SMN1 allele. Therefore, the present test is effective for detecting compound hemizygote patients, for testing carriers in SMA families, and for screening for SMA heterozygotes in the general population.

The gene copy ratios of SMN1/SMN2 in Japanese carriers with type I spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive neurodegenerative disorder with progressive weakness and atrophy of voluntary muscles. The survival motor neuron gene (SMN) is present in two highly homologous copies (SMN1 and SMN2) on chromosome 5q13. Homozygous deletion of exons 7 and 8 of SMN1 is responsible for spinal muscular atrophy. In spinal muscular atrophy patients, SMN2 partially compensates for the lack of SMN1. Previously, we reported the relatively high incidence of a large deletion including the SMN1 region in Japanese spinal muscular atrophy type I patients. In order to further establish the genetic background of Japanese spinal muscular atrophy type I patients, we investigated the SMN1/SMN2 ratio in the carriers. In normal individuals, there is one copy of each gene on the chromosome (the SMN1/SMN2 ratio was 1). Among 15 carriers (14 parents and one carrier sibling of Japanese type I spinal muscular atrophy patients with homozygous deletion of exons 7 and 8 of SMN1), we found that the SMN1/SMN2 ratio was 0.5 or 1 in 11 (73.3%) carriers. The remaining four carriers had an SMN1/SMN2 ratio of 1/3. This finding supports the idea that deletion rather than conversion is the main genetic event in type I spinal muscular atrophy. In addition, the ratio of SMN1/SMN2 among Japanese carriers, which was thought to be higher than that of the Western population, was compatible with the results obtained in Western populations. For further insight into the characteristic genetic background of spinal muscular atrophy in Japanese, determination of the gene copy number is essential.

Genetic and physical delineation of the region overlapping the progressive motor neuropathy (pmn) locus on mouse chromosome 13

The mouse autosomal recessive mutation progressive motor neuropathy (pmn) results in early onset motor neuron disease with rapidly progressing hindlimb paralysis, severe muscular wasting, and death at 4--6 weeks of age. pmn is thus considered a good animal model for motor neuron diseases and the characterization of the causative gene should help in understanding the biological causes of human spinal muscular atrophies. Here we report the generation of a physical map based on a high-resolution and high-density genetic map encompassing the pmn locus on mouse chromosome 13. We have positioned the pmn locus and a cluster of markers cosegregating with it within a genetic interval of 0.30 cM, delineated by two clusters of markers. We have constructed an approximately 850-kb contig of BACs spanning the pmn critical region. This BAC contig contains the breakpoint of synteny between mouse chromosome 13 and human 1q and 7p regions and lays the foundation for identifying at the molecular level such a breakpoint region. The physical and genetic maps provided a support for the identification of five transcription units positioned in the nonrecombinant interval, and constitute invaluable tools for the identification of other candidate genes for the pmn mutation.

Allele-specific amplification for preimplantation genetic diagnosis (PGD) of spinal muscular atrophy

We have developed a new allele-specific amplification method for the preimplantation genetic diagnosis (PGD) of spinal muscular atrophy (SMA; Werdnig-Hoffmann disease) from a single cell. This method is based on the detection of the deletion of exon 7 of the telomeric copy of the survival motor neurone (SMN(t)) gene. An oligonucleotide was designed to be specific to the SMN(t) nucleotidic sequence with exonic mismatch G (for SMN(t))-->A (for SMN(c)) at its 3' end. This test produces reliable PCR products in 95% of single lymphoblasts (85/88) tested as well as in 16/16 blastomeres from normal controls. Specificity analysis showed that we were able to detect homozygous deletion of the SMN(t) gene in 99% of single lymphoblasts (103/104) from a SMA patient. No contamination was detected in 68 blanks tested. Multiple cell and DNA dilution analysis revealed that the test is accurate and specific up to 100 pg DNA and should thus also be suitable for PGD at the blastocyst stage. This rapid procedure requires a single round of fluorescent PCR and no restriction digestion, while previously described single cell methods include nested PCR followed by restriction enzyme digestion. Two PGD cycles for SMA using this procedure were performed in our centre.