Genetic homogeneity between acute and chronic forms of spinal muscular atrophy

Heterogeneity of subcellular localization and electrophoretic mobility of survival motor neuron (SMN) protein in mammalian neural cells and tissues

Spinal muscular atrophy is caused by defects in the survival motor neuron (SMN) gene. To better understand the patterns of expression of SMN in neuronal cells and tissues, we raised a polyclonal antibody (abSMN) against a synthetic oligopeptide from SMN exon 2. AbSMN immunostaining in neuroblastoma cells and mouse and human central nervous system (CNS) showed intense labeling of nuclear "gems," along with prominent nucleolar immunoreactivity in mouse and human CNS tissues. Strong cytoplasmic labeling was observed in the perikarya and proximal dendrites of human spinal motor neurons but not in their axons. Immunoblot analysis revealed a 34-kDa species in the insoluble protein fractions from human SY5Y neuroblastoma cells, embryonic mouse spinal cord cultures, and human CNS tissue. By contrast, a 38-kDa species was detected in the cytosolic fraction of SY5Y cells. We conclude that SMN protein is expressed prominently in both the cytoplasm and nucleus in multiple types of neurons in brain and spinal cord, a finding consistent with a role for SMN as a determinant of neuronal viability.

Coexistence of macular corneal dystrophy types I and II in a single sibship

BACKGROUND

Macular corneal dystrophy (MCD) is an inherited autosomal recessive disorder that has been subdivided into two primary immunophenotypes, MCD types I and II. The MCD type I gene has been localised previously to chromosome 16q22 and suggestive evidence provided that MCD type II gene is also linked to this region. Here an unusual family is reported where both MCD types I and II are found in a single sibship.

METHODS

Immunoreactivity to an anti-keratan sulphate monoclonal antibody (5-D-4) was evaluated in patients' serum and in corneal tissue obtained at keratoplasty. Chromosomal haplotypes were constructed using microsatellite repeat markers spanning the region of the MCD type I locus.

RESULTS

Immunological studies demonstrated that two of the affected siblings have MCD type II while one has MCD type I. Haplotype analysis suggests that all three affected sibs inherited one identical parental haplotype. However, the two MCD types differ in their alternative chromosome with both MCD type II children sharing an identical haplotype, different from their MCD type I sibling.

CONCLUSION

The findings in this study support the hypothesis that the genes for MCD types I and II co-localise to the same region of chromosome 16 and are likely to be due to allelic manifestations of the same abnormal gene.

Molecular basis of genetic heterogeneity: role of the clinical neurologist

Advances in molecular genetics have disclosed many different explanations for allelic heterogeneity, how different clinical syndromes arise from mutations in the same gene. The converse, how similar clinical syndromes arise from mutations of different genes on different chromosomes is called locus heterogeneity. Both, however, give rise to some disease-defining mutations, as in childhood spinal muscular atrophy or Duchenne muscular dystrophy. Nevertheless, new problems have been created, including what might be called "diagnosis by the number," diverse syndromes from mutations in the same gene without current explanation, or siblings with different clinical syndromes. These discoveries have transformed the clinical neurology of heritable diseases. They also provide clinicians with new responsibilities and opportunities in defining clinical syndromes and influencing the evolution of our clinical language.

Sequence of a 131-kb region of 5q13.1 containing the spinal muscular atrophy candidate genes SMN and NAIP

The spinal muscular atrophies (SMA), which are characterized by motor neuron loss and progressive paralysis, are among the most common autosomal recessive disorders. The SMA region of chromosome 5q13.1 is distinguished by variable amplification of genomic sequence incorporating a number of genes and pseudogenes. Recently, two SMA candidate genes mapping to this area were identified: survival motor neuron (SMN) and neuronal apoptosis inhibitory protein (NAIP). The telomeric copy of SMN (SMNtel) is deleted in over 95% of cases of SMA, with NAIP deletions primarily seen in type I SMA. We present here 131 kb of genomic sequence from 5q13.1 incorporating both NAIP and SMNtel in addition to revisions of the original NAIP cDNA sequence. The Alu-rich NAIP-SMNtel interval contains the microsatellite polymorphisms that are deleted in as many as 80% of type I SMA chromosomes, focusing attention on this region in the pathogenesis of type I SMA.

Identification and characterization of a mouse homologue of the spinal muscular atrophy-determining gene, survival motor neuron

Spinal muscular atrophy (SMA), the second most common fatal, autosomal recessive disease of infants, manifests as generalized muscle weakness. The most severe form (Type I, Werdnig-Hoffmann disease) is associated with quadriplegia, respiratory muscle paralysis and death in infancy. Less severe forms are classified as Type II and Type III, based on age of onset and ultimate motor disability. Some spinal motor neurons show chromatolysis and the number of these cells is decreased. Recently, SMA has been mapped to chromosome 5q11.2-13.3 (Gilliam et al., 1990), a region that contains three candidate genes: Survival Motor Neuron (SMN) (Lefebvre et al., 1995); Neuronal Apoptosis Inhibitory Protein (NAIP) (Roy et al., 1995); and p44, a subunit of transcription factor II H (TFIIH) (Carter et al., 1995; Bürglen et al., 1997). Homozygous deletions or deleterious mutations in SMN are present in all SMA patients, and in some affected individuals, deletions have been identified in one or both of the other genes. These extensive deletions may be associated with a more severe phenotype. We have identified and characterized the mouse homologue of SMN, MoSMN, which is 82% identical to SMN at the amino-acid level. Unlike the duplicated human SMN, MoSMN is present in single copy. Like its human counterpart, MoSMN is ubiquitously expressed, but unlike SMN, MoSMN does not appear to be alternatively spliced. In-situ hybridization analysis of the mouse nervous system revealed that MoSMN mRNA is expressed in spinal cord and throughout the brain, with relatively higher levels of expression in the hippocampus and cerebellum.

De novo rearrangements found in 2% of index patients with spinal muscular atrophy: mutational mechanisms, parental origin, mutation rate, and implications for genetic counseling

Spinal muscular atrophy (SMA) is a relatively common autosomal recessive neuromuscular disorder. We have identified de novo rearrangements in 7 (approximately 2%) index patients from 340 informative SMA families. In each, the rearrangements resulted in the absence of the telomeric copy of the survival motor neuron (SMN) gene (telSMN), in two cases accompanied by the loss of the neuronal apoptosis-inhibitory protein gene . Haplotype analysis revealed unequal recombination in four cases, with loss of markers Ag1-CA and C212, which are near the 5' ends of the SMN genes. In one case, an interchromosomal rearrangement involving both the SMN genes and a regrouping of Ag1-CA and C212 alleles must have occurred, suggesting either interchromosomal gene conversion or double recombination. In two cases, no such rearrangement was observed, but loss of telSMN plus Ag1-CA and C212 alleles in one case suggested intrachromosomal deletion or gene conversion. In six of the seven cases, the de novo rearrangement had occurred during paternal meiosis. Direct detection of de novo SMA mutations by molecular genetic means has allowed us to estimate for the first time the mutation rate for a recessive disorder in humans. The sex-averaged rate of 1.1 x 10(-4), arrived at in a proband-based approach, compares well with the rate of 0.9 x 10(-4) expected under a mutation-selection equilibrium for SMA. These findings have important implications for genetic counseling and prenatal diagnosis in that they emphasize the relevance of indirect genotype analysis in combination with direct SMN-gene deletion testing in SMA families.

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.

Correlation between deletion patterns of SMN and NAIP genes and the clinical features of spinal muscular atrophy in Japanese patients

We conducted molecular analysis of two candidate genes for spinal muscular atrophy (SMA), the survival motor neuron gene (SMN) and the neuronal apoptosis inhibitory protein gene (NAIP), in 16 Japanese patients with SMA and compared the phenotypic features of SMA in these patients with the corresponding genotypes. Exons 7 and/or 8 of SMN were homozygously deleted in 11 SMA type I (Werdnig-Hoffmann disease) patients, two SMA type II patients and one SMA type III patient. Exons 5 and 6 of NAIP were homozygously deleted in six SMA type I patients. No patient had a deletion in NAIP without a deletion in SMN. Mechanical ventilation was required during the first 7 months of life in the SMA type I patients who had a deletion in both SMN and NAIP. Ventilatory support was initiated within 2 years after birth in patients who had a deletion in SMN but not in NAIP. We detected homozygous deletion of exon 5 of NAIP in the unaffected mothers of two SMA type I patients. In these families, the patients exhibited a deletion in both SMN and NAIP. The parents and unaffected siblings of these patients did not have a deletion in SMN. The present findings support the hypothesis that SMN deletion plays an important role in the development of SMA and suggest that combined deletion of both SMN and NAIP may be relevant for determining the disease severity.

Inactivation of the survival motor neuron gene, a candidate gene for human spinal muscular atrophy, leads to massive cell death in early mouse embryos

Proximal spinal muscular atrophy is an autosomal recessive human disease of spinal motor neurons leading to muscular weakness with onset predominantly in infancy and childhood. With an estimated heterozygote frequency of 1/40 it is the most common monogenic disorder lethal to infants; milder forms represent the second most common pediatric neuromuscular disorder. Two candidate genes-survival motor neuron (SMN) and neuronal apoptosis inhibitory protein have been identified on chromosome 5q13 by positional cloning. However, the functional impact of these genes and the mechanism leading to a degeneration of motor neurons remain to be defined. To analyze the role of the SMN gene product in vivo we generated SMN-deficient mice. In contrast to the human genome, which contains two copies, the mouse genome contains only one SMN gene. Mice with homozygous SMN disruption display massive cell death during early embryonic development, indicating that the SMN gene product is necessary for cellular survival and function.

Gene deletions in Arab patients with spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive disorder characterized by degeneration of lower motor neurons. We have investigated the presence of survival motor neuron gene and neuronal apoptosis inhibitory protein gene deletions in 17 Arab and 1 Indian families with spinal muscular atrophy (15 type I and 3 type II). Homologous deletions were detected in exons 7 and 8 of the survival motor neuron gene and exon 5 of the neuronal apoptosis inhibitory protein gene in all patients with type I spinal muscular atrophy. Exon 13 of the neuronal apoptosis inhibitory protein gene was deleted in only one patient with type I spinal muscular atrophy. In two patients with type II spinal muscular atrophy, only exons 7 and 8 of the survival motor neuron gene were deleted whereas exons 5 and 13 of the neuronal apoptosis inhibitory protein gene were present. In another patient with spinal muscular atrophy type II, exons 7 and 8 of the survival motor neuron gene and exon 5 of the neuronal apoptosis inhibitory protein gene were deleted. This latter patient also had the Pierre Robin syndrome. No deletion was detected in healthy siblings or the parents. The deletions found in our patients are similar to those reported in other population groups.

Muscle fatigue in spinal muscular atrophy

We previously reported that patients with spinal muscular atrophy do not lose muscle strength over time as measured quantitatively. However, we noted that many patients with spinal muscular atrophy suffer from what they call fatigue. We wondered if we could measure fatigue during a single maximal voluntary contraction, whether fatigue might increase with time, independent of muscle strength, and whether increasing fatigue might correlate with loss of function in some patients. We measured fatigue during a single maximal voluntary contraction in a cohort of patients having spinal muscular atrophy using quantitative strength testing. We included only patients with spinal muscular atrophy aged 5 years or older, so they could follow instructions regarding muscle contraction, and who were followed for at least 2 years. Seventy-six children with spinal muscular atrophy and 24 untrained individuals, aged 5 to 57 years (mean, 16.8 years), were studied. There was no discernible abnormal fatigue in patients with spinal muscular atrophy compared to untrained controls using our methodology. Thus, spinal muscular atrophy may not be associated with fatiguability. Moreover, spinal muscular atrophy does not appear to cause progressive muscle fatigue with age or loss of function. It is possible that fatigue was undetectable by our methods. An alternative explanation is that what patients describe as fatigue may be caused by factors outside the neuromuscular system. Such factors may include chronic respiratory insufficiency with hypoventilation and carbon dioxide retention as well as chronic malnutrition and negative nitrogen balance.

Genomic variation and gene conversion in spinal muscular atrophy: implications for disease process and clinical phenotype

Autosomal recessive spinal muscular atrophy (SMA) is classified, on the basis of age at onset and severity, into three types: type I, severe; type II, intermediate; and type III, mild. The critical region in 5q13 contains an inverted repeat harboring several genes, including the survival motor neuron (SMN) gene, the neuronal apoptosis inhibitory protein (NAIP) gene, and the p44 gene, which encodes a transcription-factor subunit. Deletion of NAIP and p44 is observed more often in severe SMA, but there is no evidence that these genes play a role in the pathology of the disease. In > 90% of all SMA patients, exons 7 and 8 of the telomeric SMN gene (SMNtel) are not detectable, and this is also observed in some normal siblings and parents. Point mutations and gene conversions in SMNtel suggest that it plays a major role in the disease. To define a correlation between genotype and phenotype, we mapped deletions, using pulsed-field gel electrophoresis. Surprisingly, our data show that mutations in SMA types II and III, previously classed as deletions, are in fact due to gene-conversion events in which SMNtel is replaced by its centromeric counterpart, SMNcen. This results in a greater number of SMNcen copies in type II and type III patients compared with type I patients and enables a genotype/phenotype correlation to be made. We also demonstrate individual DNA-content variations of several hundred kilobases, even in a relatively isolated population from Finland. This explains why no consensus map of this region has been produced. This DNA variation may be due to a midisatellite repeat array, which would promote the observed high deletion and gene-conversion rate.

Extensive DNA deletion associated with severe disease alleles on spinal muscular atrophy homologues

Spinal muscular atrophy (SMA) is a motor neuron disease presenting with a wide spectrum of phenotypic variations. The primary cause of most, if not all, forms of childhood-onset spinal muscular atrophy appears to be the homozygous loss of the telomeric copy of the survival motor neuron (SMNT) gene. It is interesting that approximately half of all affected patients are likewise homozygous nulls for the neuronal apoptosis inhibitory protein (NAIP) gene and a somewhat lesser fraction for the basal transcription factor, p44 subunit (BTF2p44) gene. It has been proposed that homozygous loss of SMNT is the primary cause of spinal muscular atrophy while the loss of NAIP and perhaps other genes primarily affects the severity of disease manifestation. We explored this hypothesis by evaluating the extent of gene deletions in three multigenerational families with spinal muscular atrophy exhibiting dramatic intrafamilial phenotypic variation. Using somatic cell hybrid lines to sequester individual spinal muscular atrophy homologues, we show that homologues missing several contiguous genes correlate with "severe" disease alleles and homologues missing only SMNT correlate with "mild" disease alleles. These observations support the hypothesis that phenotypic severity among the childhood-onset spinal muscular atrophies is directly correlated with the extent of disease-specific deletions.

Molecular characterization of Br-cadherin, a developmentally regulated, brain-specific cadherin

Cadherins are a family of transmembrane proteins that play a crucial role in cell adhesion and in morphogenesis. Several of the cadherins are expressed in the nervous system, but none is neuron-specific. We characterize a new member of the cadherin family, Br-cadherin, which is present exclusively in the central nervous system. Although the Br-cadherin protein is confined to the central nervous system, its mRNA is present in several additional tissues, suggesting that there is posttranscriptional control of this gene's expression. Within the central nervous system, Br-cadherin appears to be expressed specifically by neurons. In the mouse, its expression becomes detectable during the first postnatal week, which corresponds temporally to the onset of synaptogenesis and dendrite outgrowth in the brain. This pattern of expression is consistent with a role for Br-cadherin in neuronal development, perhaps specifically with synaptogenesis.

Missense mutation clustering in the survival motor neuron gene: a role for a conserved tyrosine and glycine rich region of the protein in RNA metabolism?

The Survival Motor Neuron (SMN) gene shows deletions in the majority of patients with Spinal Muscular Atrophy (SMA), a disease of motor neuron degeneration. To date only two missense mutations have been reported in SMN in patients with SMA. The fact that no SMN-homologues have been forthcoming from data-base searching has resulted in a lack of hypotheses concerning the structural and functional consequences of these mutations. Recently SMN has been shown to interact with heterogeneous nuclear ribonucleoproteins (hnRNPs) suggesting a role in mRNA metabolism. We describe a novel missense mutation and the subsequent identification of a triplicated tyrosine-glycine (Y-G) peptide sequence at the C-terminal of SMN which encompasses each of the three predicted amino acid sequence substitutions. We have identified apparent orthologues of SMN in Caenorhabditis elegans and Schizosaccharomyces pombe. These sequences retain the highly conserved Y-G motif and provide additional support for a role of SMN in mRNA metabolism.

cDNA isolation, expression, and chromosomal localization of the mouse survival motor neuron gene (Smn)

Spinal muscular atrophy (SMA) is a frequent autosomal recessive disease in human characterized by degeneration of motor neurons of the spinal cord. The genomic region containing the defective gene (5q13) is particularly unstable and prone to large-scale deletions whose characterization led to the identification of the survival motor neuron (SMN) gene, the SMA determining gene encoding a hitherto unknown protein. As an initial step toward the generation of a murine model for SMA, we identified and characterized a full-length murine Smn cDNA. The coding sequence of the mouse Smn gene was found to be 82% identical, at the amino acid level, with the human SMN coding sequence. The Smn locus was mapped to the segment of mouse chromosome 13 exhibiting conservation of synteny with human chromosome 5q11-q23, which contains the SMN gene. However, no evidence for a duplication of the Smn gene was found in the mouse, suggesting that the duplication reported in human is a recent evolutionary event.

A multicopy transcription-repair gene, BTF2p44, maps to the SMA region and demonstrates SMA associated deletions

The childhood-onset spinal muscular atrophies are a clinically heterogeneous group of autosomal recessive disorders characterized by selective degeneration of the anterior horn cells with subsequent weakness and atrophy of limb muscles. The disease locus has been mapped to a region of chromosome 5q13 characterized by genetic instability and DNA duplication. Among the duplicated genes in this region, SMNT (telomeric copy; survival motor neuron) is thought to be the major disease determining gene since it is missing in the majority of SMA patients and since small, intragenic mutations in the gene have been associated with the disorder. Approximately half of the severely affected SMA I patients are also missing both homologues of a neighboring gene, the neuronal apoptosis inhibitory protein (NAIP). These data indicate that loss of NAIP may affect disease severity and further, that the molecular events underlying the childhood-onset SMAs are complex, possibly involving multiple genes. We report a third multicopy gene in the SMA region, encoding the p44 subunit of basal transcription factor II (BTF2p44). One copy of this transcription-repair gene is deleted in at least 15% of all SMA cases.

Exclusion of the beta-subunit of type II calmodulin kinase for the wobbler spinal muscular atrophy gene

The wobbler mouse (wr) is an attractive model for studying motor neuron disease but the genetic defect is unknown. The beta-subunit of calmodulin kinase II (beta-CaMK II) is a good candidate for the wr mutation because of its chromosomal localization and tissue distribution. In this report, we found normal levels of CaM KII mRNA and enzyme activity making it highly unlikely that a mutation in the beta-CaM KII gene is the cause of the wr phenotype.

Motor nerve conduction studies on children with spinal muscular atrophy

Median and posterior tibial motor nerve conduction studies were performed on 10 children with spinal muscular atrophy (SMA). Three patients with SMA type I, in whom rapid deterioration occurred, showed reduced motor nerve conduction velocity and a remarkably low M-wave amplitude in both nerves. In type II and III patients, the motor nerve conduction velocity was normal in the median nerve, although the M-wave amplitude was small in the tibial nerve. In four patients, a reduction of the M-wave amplitude was observed as clinical symptoms advanced. These findings may suggest that motor conduction studies in spinal muscular atrophy provide complementary information for understanding the pathogenesis and are also useful to clarify the heterogeneity of this disease.

Mitochondrial myopathy simulating spinal muscular atrophy

A patient with a severe progressive neuromuscular disorder resembling spinal muscular atrophy is reported. The initial muscle biopsy was consistent with a denervating process. DNA analysis did not reveal deletions in exons 7 and 8 of the survival motor neuron gene. Histology, histochemistry, and biochemistry of a second muscle biopsy suggested mitochondrial myopathy accompanying the denervating features. Immunohistochemistry using anti-DNA antibodies revealed only nuclear staining in skeletal muscle, suggesting mitochondrial DNA depletion. In patients with clinical features of spinal muscular atrophy and no deletions in the survival motor neuron gene, mitochondrial DNA depletion should be considered.

Molecular basis of phenotypic heterogeneity in siblings with spinal muscular atrophy

We report on a family with childhood-onset spinal muscular atrophy with intrafamilial phenotypic variation. Typical of a large majority of such patients, both the child with spinal muscular atrophy type I and the child with type II were missing both copies of the survival motor neuron telomeric gene (SMN(T)). The more severely affected child, however, showed genotypic evidence consistent with the de novo loss of DNA sequence in addition to that inherited by both affected children. These data suggest that the intrafamilial phenotypic variation in this family results from a new mutation event in the more severely affected child. Examples of intrafamilial phenotypic variability are quite rare, but some reports exist in the spinal muscular atrophy literature. We present evidence that one explanation for this phenomenon is the occurrence of de novo deletion events at the highly unstable disease locus.

The genetic basis of neuromuscular disorders

In the last decade, our knowledge of human diseases genes has been growing rapidly as a result of the availability of resources and techniques for mapping and sequencing the human genome. New disease genes are now reported almost weekly. This review illustrates how the identification of genes involved in neuromuscular disorders has led to the characterization of not only novel genes, but also of a variety of different types of genetic mutation. These observations, which include high deletion frequencies, unstable tandem repeat sequences, genomic duplications and triplet repeat expansions, have facilitated the identification of similar types of mutation in other genetic disorders.

Spinal muscular atrophy in childhood

Diagnosis and classification of spinal muscular atrophy (SMA) in childhood are based on clinical, electrophysiological, and histological studies. The concept of maturational arrest of motoneurons and their targets (muscle cells in SMA type I) is documented by ultrastructural and immunohistochemical data. The prolongated or markedly delayed process of muscle cell and motoneuron elimination by apoptosis seen in SMA type I is discussed according to the new finding of a gene for a neuronal apoptosis inhibitory protein that is partially deleted in children with spinal muscular atrophy.

Analysis of chromosome 5q13 genes in amyotrophic lateral sclerosis: homozygous NAIP deletion in a sporadic case

Although defects in the gene encoding the enzyme cytosolic copper/zinc superoxide dismutase (SOD1) have been reported in 20% of familial amyotrophic lateral sclerosis (ALS) patients, the etiology of the remaining familial cases and the more common sporadic form of the disease remains unknown. Recently, deletions of the neuronal apoptosis inhibitory protein gene NAIP, of the survival motor neuron gene SMN, and of a further cDNA fragment, XS2G3, have been reported in childhood-onset proximal spinal muscular atrophy (SMA), another disorder with pathology restricted to the motor system. We have therefore investigated the possibility of alterations in SMN and NAIP in 154 patients with ALS (135 sporadic cases, 17 familial cases). None of these patients revealed mutations in SMN by single-strand conformation polymorphism analysis. A single patient revealed a partial deletion of NAIP, with a homozygous absence of NAIP exon 5. While it is possible that this individual is one of the rare carriers of SMA who show NAIP deletions, a further explanation is that the NAIP deletion is in some way contributing to the ALS phenotype in this individual.

Large scale deletions of the 5q13 region are specific to Werdnig-Hoffmann disease

Spinal muscular atrophy (SMA) is characterised by degeneration of anterior horn cells of the spinal cord and represents the second most common, lethal, autosomal recessive disorder after cystic fibrosis. Based on the criteria of the Internatinal SMA Consortium, childhood SMAs are classified into type I (Werdnig-Hoffmann disease), type II (intermediate form), and type III (Kugelberg-Welander disease). Recently, two genes have been found to be associated with SMA. The survival motor neurone gene (SMN) is an SMA determining gene as it is absent in 98.6% of patients. A second gene, XS2G3, or the highly homologous neuronal apoptosis inhibitory protein gene (NAIP) have been found to be more frequently deleted in type I than in the milder forms (types II and III). We investigated the correlation between the clinical phenotype and the genotype at this loci. A total of 106 patients were classified into type I (44), type II (31), and type III (31) and analysed using SMN, markers C212 and C272, and NAIP mapping upstream and downstream from SMN respectively. The combined analysis of all markers showed a large proportion of type I patients (43%) carried deletions of both SMN and its flanking markers (C212/272) and NAIP exon 5), as compared with none of the patients with type II or III SMA. The presence of large scale deletions involving these loci is specific to Werdnig-Hoffman disease (type I) and allows one to predict the severity of the disease in our series.

Deletions in the SMN gene in infantile and adult spinal muscular atrophy patients from the same family

Recently, a gene determining spinal muscular atrophy (SMA), termed survival motor neuron (SMN) gene, has been isolated from the 5q13 region. This gene has been found to be deleted in most patients with childhood-onset SMA. We have studied the SMN gene in a clinically heterogeneous family, including one patient affected by infantile chronic SMA and three subjects with mild adult-onset muscle weakness. Deletions in the SMN gene were detected in all of these patients, indicating that the childhood and adult SMAs are genetically homogeneous in this family. Genotyping of the family members established that the three mildly affected individuals were homozygous for the same haplotype from the SMA region, whereas the more severely affected patient was heterozygous with one different haplotype.

Diagnosis in neuromuscular diseases

The diagnosis of neuromuscular diseases can be challenging and successful in the majority of patients, due to advancements in electrophysiology, muscle and nerve biopsy immunohistochemistry, and cytogenetics. This article reviews diverse topics, highlighting these recent achievements, with an emphasis on how they affect the clinical and laboratory diagnosis of specific neuromuscular disorders.

Molecular analysis of the SMN and NAIP genes in Spanish spinal muscular atrophy (SMA) families and correlation between number of copies of cBCD541 and SMA phenotype

Spinal muscular atrophy is an autosomal recessive disorder which affects about 1 in 10,000 individuals. The three clinical forms of SMA were mapped to the 5q13 region. Three candidate genes have been isolated and shown to be deleted in SMA patients: the Survival Motor Neuron gene (SMN), the Neuronal Apoptosis Inhibitory Protein gene (NAIP) and the XS2G3 cDNA. In this report we present the molecular analysis of the SMN exons 7 and 8 and NAIP exon 5 in 65 Spanish SMA families. NAIP was mostly deleted in type I patients (67.9%) and SMN was deleted in 92.3% of patients with severe and milder forms. Most patients who lacked the NAIP gene also lacked the SMN gene, but we identified one type II patient deleted for NAIP exon 5 but not for SMN exons 7 and 8. Two other patients carried deletions of NAIP exon 5 and SMN exon 7 but retained the SMN exon 8. Three polymorphic variants from the SMN gene, showing changes on the sequence of the centromeric (cBCD541) and telomeric copies of the SMN gene, were found. In addition, we show several genetic rearrangements of the telomeric SMN gene, which include duplication of this gene in one normal chromosome, and putative gene conversion events in affected and normal chromosomes. Altogether these results corroborate the high genetic variability of the SMA region. Finally, we have determined the ratio between the number of centromeric and telomeric copies of the SMN gene in parents of SMA patients, showing that the majority of parents of types II and III patients carried three or more copies of the cBCD541 gene; we suggest a relationship between the number of copies of cBCD541 and the disease phenotype.

Gene deletions in spinal muscular atrophy

Two candidate genes (NAIP and SMN) have recently been reported for childhood onset spinal muscular atrophy (SMA). Although affected subjects show deletions of these genes, these deletions can lead to either a very mild or a severe phenotype. We have analysed a large number of clinically well defined patients, carriers, and normal controls to assess the frequency and extent of deletions encompassing both of these genes. A genotype analysis indicates that more extensive deletions are seen in the severe form of SMA than in the milder forms. In addition, 1 center dot 9% of phenotypically normal carriers are deleted for the NAIP gene; no carriers were deleted for the SMN gene. Our data suggest that deletions in both of these genes, using the currently available assays, are associated with both a severe and very mild phenotype.

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

Spinal muscular atrophy (SMA) is a frequent autosomal recessive disease characterized by degeneration of the motor neurons of the spinal cord causing proximal paralysis with muscle atrophy. The region on chromosome 5q13 encompassing the disease gene is particularly unstable and prone to large-scale deletions whose characterization recently led to the identification of the survival motor neuron (SMN) gene. We now present a genetic analysis of 54 unrelated Spanish SMA families that has revealed a 4-basepair (bp) deletion (AGAG) in exon 3 of SMN in four unrelated patients. This deletion, which results in a frameshift and a premature stop codon, occurs on the same haplotype background, suggesting that a single mutational event is involved in the four families. The other patients showed either deletions of the SMN gene (49/54) or a gene conversion event changing SMN exon 7 into its highly homologous copy (cBCD541, 1/54). This observation gives strong support to the view that mutations of the SMN gene are responsible for the SMA phenotype as it is the first frameshift mutation reported in SMA.

Scoliosis and lung function in spinal muscular atrophy

The notes and radiographs of 43 patients with a confirmed diagnosis of spinal muscular atrophy were reviewed. A significant inverse linear relationship between the severity of scoliosis and the percentage of predicted vital capacity and peak flow was found. The patients who stood had a significantly better lung function than patients who were confined to a wheelchair, and their scoliosis deteriorated significantly more slowly. Sixteen patients underwent surgical spinal stabilisation, 4 with Harrington instrumentation and 12 with segmental spinal instrumentation, at an average age of 12 years and 11 months. The average curve correction achieved was 40%. The decline in lung function seen pre-operatively was not only reversed, but a significant improvement was found at final follow-up.

Prenatal prediction in families with autosomal recessive proximal spinal muscular atrophy (5q11.2-q13.3): molecular genetics and clinical experience in 109 cases

Prenatal prediction in families at risk for autosomal recessive proximal spinal muscular atrophy (SMA) mainly of type I is often requested due to the high incidence and the fetal outcome of the disease. So far, only indirect genotype analysis can be performed in SMA families, since the gene has not yet been identified. We present our experience of 109 prenatal diagnoses obtained in 91 families by use of single- and multi-locus polymorphic microsatellites of the region 5q11.2-q13.3. The marker combinations and specific features of the closest microsatellites are described in detail. From 137 requests for prenatal prediction of SMA between October 1991 and August 1994, 28 families were excluded, mostly because the clinical diagnosis was uncertain or doubtful. Others had to be classified as 'SMA-variants' or showed autosomal dominant transmission of SMA. Of the 109 prenatal diagnoses performed, 29 fetuses were diagnosed to be at high risk (> 99 per cent) of developing the disease, while in seven additional pregnancies no exact prediction could be made due to a recombination event in one parental haplotype. Altogether, recombinations between closely flanking markers were observed in 14 cases. In 35 cases, the parents decided to terminate the pregnancy. Of the remaining pregnancies, 32 could be followed beyond term. All infants were reported to develop normally without signs of SMA. Two children were born with transverse reduction defects of one hand, which was most likely related to early chorionic villus sampling at 9 and 10 weeks' gestation. No further abnormalities could be detected. The limits of indirect genotype analysis and the problems of diagnostic accuracy and heterogeneity of proximal SMA are discussed.

Expressed cadherin pseudogenes are localized to the critical region of the spinal muscular atrophy gene

Low-copy repeats have been associated with genomic rearrangements and have been implicated in the generation of mutations in several diseases. Here we characterize a subset of low-copy repeats in the spinal muscular atrophy (SMA) region in human chromosome 5q13. We show that this repeated sequence, named c41-cad, is a highly expressed pseudogene derived from an intact neuronal cadherin gene, Br-cadherin, situated on 5p13-14. Br-cadherin is expressed specifically in the brain, whereas the c41-cad transcripts are 10-15 times more abundant and are present in all tissues examined. We speculate that the c41-cad repeats, separately or in concert with other repeats in the SMA region, are involved in the pathogenesis of SMA by promoting rearrangements and deletions.

Refined physical map of the spinal muscular atrophy gene (SMA) region at 5q13 based on YAC and cosmid contiguous arrays

The gene for the autosomal recessive neurodegenerative disorder spinal muscular atrophy has been mapped to a region of 5q13 flanked proximally by CMS-1 and distally by D5S557. We present a 2-Mb yeast artificial chromosome (YAC) contig constructed from three libraries encompassing the D5S435/D5S629/CMS-1-SMA-D5S557/D5S112 interval. The D5S629/CMS-1-SMA-D5S557 interval is unusual insofar as chromosome 5-specific repetitive sequences are present and many of the simple tandem repeats (STR) are located at multiple loci that are unstable in our YAC clones. A long-range restriction map that demonstrates the SMA-containing interval to be 550 kb is presented. Moreover, a 210-kb cosmid array from both a YAC-specific and a chromosome 5-specific cosmid library encompassing the multilocus STRs CATT-1, CMS-1, D5F149, D5F150, and D5F153 has been assembled. We have recently reported strong linkage disequilibrium with Type I SMA for two of these STRs, indicating that the gene is located in close proximity to or within our cosmid clone array.

Neonatal spinal muscular atrophy with diaphragmatic paralysis is unlinked to 5q11.2-q13

Two sibs affected by the severe neonatal form of spinal muscular atrophy (SMA) with diaphragmatic paralysis are described. The two sibs were discordant for the haplotypes determined by DNA markers flanking the SMA locus. This supports non-linkage of SMA to chromosome 5 in this family and indicates that the uncommon SMA type I variant associated with early onset respiratory failure maps outside the 5q11.2-q13.3 region.

Genomic rearrangements in childhood spinal muscular atrophy: linkage disequilibrium with a null allele

Autosomal recessive childhood onset spinal muscular atrophy has been mapped to chromosome 5q13. We report the analysis of a polymorphic microsatellite which shows linkage disequilibrium with the disease. The linkage disequilibrium is observed with a null allele which is seen as the non-inheritance of alleles from one or both parents. The inheritance of a null allele was observed in 26 out of 36 (72%) informative childhood onset spinal muscular atrophy (SMA) families tested, of all types of severity and from a variety of ethnic backgrounds. In seven families segregating for the severe Werdnig-Hoffmann or SMA type I, no alleles were inherited from either parent using this microsatellite. This null allele may represent a deletion which is either closely associated with, or causes, the disease.

Identification and characterization of a spinal muscular atrophy-determining gene

Spinal muscular atrophy (SMA) is a common fatal autosomal recessive disorder characterized by degeneration of lower motor neurons, leading to progressive paralysis with muscular atrophy. The gene for SMA has been mapped to chromosome 5q13, where large-scale deletions have been reported. We describe here the inverted duplication of a 500 kb element in normal chromosomes and narrow the critical region to 140 kb within the telomeric region. This interval contains a 20 kb gene encoding a novel protein of 294 amino acids. An highly homologous gene is present in the centromeric element of 95% of controls. The telomeric gene is either lacking or interrupted in 226 of 229 patients, and patients retaining this gene (3 of 229) carry either a point mutation (Y272C) or short deletions in the consensus splice sites of introns 6 and 7. These data suggest that this gene, termed the survival motor neuron (SMN) gene, is an SMA-determining gene.

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.

The profile of major congenital abnormalities in the United Arab Emirates (UAE) population

The aim of this study was to establish the profile of major congenital malformations in the United Arab Emirates (UAE) population which has a high rate of consanguinity. All births with birth weight above 500 g in the three hospitals in the Al Ain Medical District of UAE were prospectively studied from January 1992 to January 1994. About 98% of the births in the district occur in these three hospitals. Detailed family history and clinical and relevant laboratory investigations were recorded in each case. Necropsy was not permitted. The major malformations were classified as multiple or isolated single system abnormalities as well as genetic or non-genetic disorders. Of the 16,419 births which occurred during the two year period, 173 (10.5/1000 births) had major malformations, 90 (52%) had multiple malformations, and 83 (47.97%) had involvement of a single system. Of the infants with multiple malformations, 43 had recognised syndromes, most of which are autosomal recessive disorders with a high frequency of rare syndromes. Twenty eight (31%) had chromosomal abnormalities. The most common systems involved in infants with isolated single system malformations include gastrointestinal (33), central nervous system (17), and cardiovascular (10). While the consanguinity rate was similar (57% v 54%), the frequency of first cousin marriages was much higher (51% v 30%) in the study group compared with the figures for the general population. The consanguinity rate was highest among the syndrome cases, and related parents were more likely to have infants with multiple malformations than an isolated single system abnormality with a relative risk of 1.69 (95% CL 1.27-2.24). Genetic factors could be implicated in 116 (67%) of the 173 cases of major malformations and 49 (28%) were potentially preventable. The study suggests that genetic disorders account for a significant proportion of congenital malformation in the UAE and, thus, a genetic service should be provided as part of the preventive cae programme.

A novel cDNA detects homozygous microdeletions in greater than 50% of type I spinal muscular atrophy patients

Spinal muscular atrophy (SMA) is the second most common lethal, autosomal recessive disease in Caucasians (after cystic fibrosis). Childhood SMAs are divided into three groups (type I, II and III), which are allelic variants of the same locus in a region of approximately 850 kb in chromosome 5q12-q13, containing multiple copies of a novel, chromosome 5-specific repeat as well as many atypical pseudogenes. This has hampered the identification of candidate genes. We have identified several coding sequences unique to the SMA region. A genomic fragment detected by one cDNA is homozygously deleted in 17/29 (58%) of type I SMA patients. Of 235 unaffected individuals examined, only two showed the deletion and both are carriers of SMA. Our results suggest that deletion of at least part of this novel gene is directly related to the phenotype of SMA.

Lethal congenital contracture syndrome (LCCS), a fetal anterior horn cell disease, is not linked to the SMA 5q locus

The lethal congenital contracture syndrome (LCCS) is an autosomal recessive syndrome (McKusick 253310) leading to perinatal death owing to early onset degeneration of the anterior horn motor neurones of the spinal cord. The neuropathological findings in the LCCS closely resemble those of spinal muscular atrophy (SMA). Since all the three types of SMA have been localised to the same gene locus on the long arm of chromosome 5, we analysed samples from seven families with 10 LCCS fetuses with the microsatellite markers assigned to the SMA 5q region. Linkage analyses between the SMA linked DNA markers and the disease allele in the LCCS families excluded the critical chromosomal region around the SMA locus as the critical chromosomal region for the LCCS locus.

Fatty acid oxidation abnormalities in childhood-onset spinal muscular atrophy: primary or secondary defect(s)?

The purpose of this study was to further identify and quantify the fatty acid oxidation abnormalities in spinal muscular atrophy, correlate these with disease severity, and identify specific underlying defect(s). Fifteen children with spinal muscular atrophy (3 type I, 8 type II, 4 type III) were studied. Serum carnitine total/free ratios demonstrated a tendency toward an increased esterified fraction ranging 35-58% of total carnitine (normal: 25-30% of total) in younger children with types I and II. The remaining type II and III patients, older than 23 months of age at sampling, had normal esterified carnitine levels. Urinary organic acid analysis demonstrated mild to moderate medium-chain dicarboxylic aciduria in type I patients and normal, mild, or moderate increases in short-chain and medium-chain organic acids in type II patients. In the type III group, the organic acids were normal except for one patient with mild medium-chain dicarboxylic aciduria. Muscle intramitochondrial beta-oxidation was measured in 5 children (2 type I, 2 type II, and 1 type III) and a significant reduction in the activities of short-chain L-3-hydroxyacyl-CoA dehydrogenase, long-chain L-3-hydroxyacyl-CoA dehydrogenase, acetoacetyl-CoA thiolase, and 3-ketoacyl-CoA thiolase were found; however, normal crotonase activity was documented. Most strikingly, there was a marked increase (3- to 5-fold) in the activity ratios of crotonase to L-3-hydroxyacyl-CoA dehydrogenase and thiolase activities with both short- and long-chain substrates. The combined abnormalities suggest a defect in a mitochondrial multifunctional enzyme complex, distinct from the trifunctional enzyme. These abnormalities may be either primary or secondary and may respond to dietary measures to reduce the dependence on fatty acid oxidation.

Prenatal diagnosis of spinal muscular atrophy in Russia

Ninety-two families with spinal muscular atrophy (SMA) applied for genetic counselling and further prenatal diagnosis. To minimize expenses, only one tightly linked informative marker was determined in the course of preliminary examination, and non-radioactive allele detection was preferably used. Four prenatal diagnoses of SMA type I, four of SMA type II, and one of SMA type III were made. This trial programme shows the considerable requirements, importance, and potential effectiveness of prenatal prediction of SMA in Russia.

Spinal muscular atrophy in trizygotic triplets

The clinical, electrophysiological, pathological and genetic findings in trizygotic triplets with spinal muscular atrophy (SMA) are reported. The first child was clinically affected shortly after birth and the third one first showed symptoms at 1 month of age. Electromyography and a muscle biopsy provided evidence of lower motor neuron disease. The second child remains clinically normal, but electromyography showed fibrillation potentials and regular spontaneous motor unit activity at rest. Genetic linkage analysis revealed that the two siblings with typical type 1 SMA had the same chromosome 5q haplotype, and that the second child had a different haplotype. It is considered that in this family there is a link to SMA 5q and there is little possibility that the second child is affected. These data emphasize the need to adhere to strict clinical criteria for the diagnosis of chromosome 5q SMA.