SMN2 copy number predicts acute or chronic spinal muscular atrophy but does not account for intrafamilial variability in siblings

A new method for SMN1 and hybrid SMN gene analysis in spinal muscular atrophy using long-range PCR followed by sequencing

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by progressive loss of motor neurons in the spinal cord. Approximately 95% of SMA patients have a homozygous deletion of the survival motor neuron 1 (SMN1) gene, whereas 5% harbor compound heterozygous mutations such as an SMN1 deletion allele and an intragenic mutation in the other SMN1 allele. It is difficult to detect intragenic mutations in SMN1 because of the high degree of homology shared between SMN1 and SMN2. Current methods analyze a restricted region from exon 2a to exon 7 in SMN1. We propose a new, efficient long-range polymerase chain reaction (PCR) method for detecting intragenic mutations in SMN1 (exon 1-8) and hybrid SMN genes. We analyzed 20 unrelated SMA patients using SMN copy number analysis, and the new long-range PCR method followed by sequencing. We thus confirmed a novel mutation in SMN1 exon 1 (c.5C>T) in three patients with SMA type III who also had an SMN1 deletion allele. Moreover, we confirmed three hybrid SMN gene types in eight patients. We report a novel SMN1 mutation responsible for a relatively mild SMA phenotype and three hybrid SMN gene types in patients with SMA type III.

Analysis of the C9orf72 gene in spinal muscular atrophy patients

Spinal muscular atrophy and amyotrophic lateral sclerosis are both motor neuron disorders. Several studies have tried to establish a link between the two diseases but the subject is still under debate. In amyotrophic lateral sclerosis, large expansions of the hexanucleotide GGGGCC in intron 1 of the C9orf72 gene are responsible for a variable percentage of familial and sporadic cases. We investigated whether the number of the hexanucleotide repeat in C9orf72 was associated with the phenotype and the number of SMN2 copies in a group of 162 SMA patients. Conventional PCR, repeat primed-PCR and Southern blot were used to determine repeat number and characterize large expansions. Results showed that no pathological (> 30 repeats) or premutated alleles (20-30 repeats) were found. The allelic distribution of the C9orf72 gene in spinal muscular atrophy patients overlapped with the data obtained in our control population, discarding putative repeats that may be associated with the disease. No association was observed with either the SMA phenotype or the number of SMN2 copies. In conclusion, the involvement of C9orf72 as a genetic modifier in spinal muscular atrophy is unlikely. Current investigation of modifier genes in SMA and of the link between ALS and SMA should consider other possible candidates.

Synaptic defects in type I spinal muscular atrophy in human development

Childhood spinal muscular atrophy is an autosomal recessive neuromuscular disorder caused by alterations in the Survival Motor Neuron 1 gene that triggers degeneration of motor neurons within the spinal cord. Spinal muscular atrophy is the second most common severe hereditary disease of infancy and early childhood. In the most severe cases (type I), the disease appears in the first months of life, suggesting defects in fetal development. However, it is not yet known how motor neurons, neuromuscular junctions, and muscle interact in the neuropathology of the disease. We report the structure of presynaptic and postsynaptic apparatus of the neuromuscular junctions in control and spinal muscular atrophy prenatal and postnatal human samples. Qualitative and quantitative data from confocal and electron microscopy studies revealed changes in acetylcholine receptor clustering, abnormal preterminal accumulation of vesicles, and aberrant ultrastructure of nerve terminals in the motor endplates of prenatal type I spinal muscular atrophy samples. Fetuses predicted to develop milder type II disease had a similar appearance to controls. Postnatal muscle of type I spinal muscular atrophy patients showed persistence of the fetal subunit of acetylcholine receptors, suggesting a delay in maturation of neuromuscular junctions. We observed that pathology in the severe form of the disease starts in fetal development and that a defect in maintaining the initial innervation is an early finding of neuromuscular dysfunction. These results will improve our understanding of the spinal muscular atrophy pathogenesis and help to define targets for possible presymptomatic therapy for this disease.

Genome-wide analysis shows association of epigenetic changes in regulators of Rab and Rho GTPases with spinal muscular atrophy severity

Spinal muscular atrophy (SMA) is a monogenic disorder that is subdivided into four different types and caused by survival motor neuron gene 1 (SMN1) deletion. Discordant cases of SMA suggest that there exist additional severity modifying factors, apart from the SMN2 gene copy number. Here we performed the first genome-wide methylation profiling of SMA patients and healthy individuals to study the association of DNA methylation status with the severity of the SMA phenotype. We identified strong significant differences in methylation level between SMA patients and healthy controls in CpG sites close to the genes CHML, ARHGAP22, CYTSB, CDK2AP1 and SLC23A2. Interestingly, the CHML and ARHGAP22 genes are associated with the activity of Rab and Rho GTPases, which are important regulators of vesicle formation, actin dynamics, axonogenesis, processes that could be critical for SMA development. We suggest that epigenetic modifications may influence the severity of SMA and that these novel genetic positions could prove to be valuable biomarkers for the understanding of SMA pathogenesis.

Raising obstetricians' awareness of spinal muscular atrophy: towards early detection and reproductive planning

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder that is caused by degeneration of α motor neurons in the spinal cord anterior horns. This degeneration can lead to progressive atrophy of proximal muscles, weakness, respiratory failure and death in severe cases. SMA is the most common neuromuscular disease of childhood and one of the main causes of infant death, with no cure in sight. This review highlights the impact of the disease in families, summarizes genetics and ultrasound advances, discusses how obstetricians can work towards its early detection and explores the options for reproductive planning.

Evaluation of fetal nuchal translucency in 98 pregnancies at risk for severe spinal muscular atrophy: possible relevance of the SMN2 copy number

OBJECTIVE

To study fetal nuchal translucency (NT) thickness as a possible early marker in fetuses at risk for severe spinal muscular atrophy (SMA). To investigate the significance of the survival motor neuron (SMN) 2 gene copy number in affected fetuses.

METHODS

We performed 2D-ultrasound in 98 pregnancies at risk for SMA, all of which underwent prenatal molecular testing of the SMN1 gene. Crown-rump length (CRL) and NT measurements were obtained in all cases before chorionic villus sampling. Fetuses were diagnosed as healthy, carriers or affected according to the SMN1 molecular testing results. SMN2 copies were also tested in all affected fetuses.

RESULTS

Nineteen fetuses were predicted to be affected due to the absence of the SMN1 gene, 18 of which had two SMN2 copies. Mean CRL and NT values did not differ between healthy, carrier and affected fetuses. In the remaining affected case who had only one SMN2 copy, the ultrasound examination showed a NT value of 4.98 mm and findings compatible with hypoplastic left heart.

CONCLUSIONS

Most affected SMA fetuses have normal NT values. Our findings support the idea that SMN2 copy number in SMA fetuses is relevant for the development of congenital heart defects and increased NT values.

Identification of bidirectional gene conversion between SMN1 and SMN2 by simultaneous analysis of SMN dosage and hybrid genes in a Chinese population

Spinal muscular atrophy (SMA) is a neurodegenerative disease characterized by programmed motoneuron death. The survival motor neuron 1 (SMN1) gene is an SMA-determining gene and SMN2 represents an SMA-modifying gene. Here, we applied capillary electrophoresis to quantify the SMN gene dosage in 163 normal individuals, 94 SMA patients and 138 of their parents. We further quantified exons 7 and 8 in SMN1 and SMN2. We found that the SMA patients carried the highest SMN2 copies, which was inversely correlated with disease severity among its three subtypes. Increased SMN1 was significantly associated with decreased SMN2 in the normal group. We also observed that parents of type I SMA patients had significantly fewer SMN2 copies than those of types II and III patients. The hybrid SMN genes were detected in two normal individuals and one patient and her mother. These results imply that increased SMN2 copies in SMA patient group might be derived from SMN1-to-SMN2 conversion, whereas the trend that normal individuals with higher SMN1 copies simultaneously carry fewer SMN2 copies suggested a reverse conversion, SMN2-to-SMN1. Together with the identification of hybrid SMN genes, our data provided additional evidence to support that SMN1 and SMN2 gene loci are interchangeable between population groups.

Genetic and expression studies of SMN2 gene in Russian patients with spinal muscular atrophy type II and III

Background

Spinal muscular atrophy (SMA type I, II and III) is an autosomal recessive neuromuscular disorder caused by mutations in the survival motor neuron gene (SMN1). SMN2 is a centromeric copy gene that has been characterized as a major modifier of SMA severity. SMA type I patients have one or two SMN2 copies while most SMA type II patients carry three SMN2 copies and SMA III patients have three or four SMN2 copies. The SMN1 gene produces a full-length transcript (FL-SMN) while SMN2 is only able to produce a small portion of the FL-SMN because of a splice mutation which results in the production of abnormal SMNΔ7 mRNA.

Methods

In this study we performed quantification of the SMN2 gene copy number in Russian patients affected by SMA type II and III (42 and 19 patients, respectively) by means of real-time PCR. Moreover, we present two families consisting of asymptomatic carriers of a homozygous absence of the SMN1 gene. We also developed a novel RT-qPCR-based assay to determine the FL-SMN/SMNΔ7 mRNA ratio as SMA biomarker.

Results

Comparison of the SMN2 copy number and clinical features revealed a significant correlation between mild clinical phenotype (SMA type III) and presence of four copies of the SMN2 gene. In both asymptomatic cases we found an increased number of SMN2 copies in the healthy carriers and a biallelic SMN1 absence. Furthermore, the novel assay revealed a difference between SMA patients and healthy controls.

Conclusions

We suggest that the SMN2 gene copy quantification in SMA patients could be used as a prognostic tool for discrimination between the SMA type II and SMA type III diagnoses, whereas the FL-SMN/SMNΔ7 mRNA ratio could be a useful biomarker for detecting changes during SMA pharmacotherapy.

Treatment of spinal muscular atrophy cells with drugs that upregulate SMN expression reveals inter- and intra-patient variability

Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder caused by mutations in the SMN1 gene. The homologous copy (SMN2) is always present in SMA patients. SMN1 gene transcripts are usually full-length (FL), but exon 7 is spliced out in a high proportion of SMN2 transcripts (delta7) (Δ7). Advances in drug therapy for SMA have shown that an increase in SMN mRNA and protein levels can be achieved in vitro. We performed a systematic analysis of SMN expression in primary fibroblasts and EBV-transformed lymphoblasts from seven SMA patients with varying clinical severity and different SMN1 genotypes to determine expression differences in two accessible tissues (skin and blood). The basal expression of SMN mRNA FL and Δ7 in fibroblasts and lymphoblasts was analyzed by quantitative real-time PCR. The FL-SMN and FL/Δ7 SMN ratios were higher in control cells than in patients. Furthermore, we investigated the response of these cell lines to hydroxyurea, valproate and phenylbutyrate, drugs previously reported to upregulate SMN2. The response to treatments with these compounds was heterogeneous. We found both intra-patient and inter-patient variability even within haploidentical siblings, suggesting that tissue and individual factors may affect the response to these compounds. To optimize the stratification of patients in clinical trials, in vitro studies should be performed before enrolment so as to define each patient as a responder or non-responder to the compound under investigation.

Accuracy of marker analysis, quantitative real-time polymerase chain reaction, and multiple ligation-dependent probe amplification to determine SMN2 copy number in patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by absence of or mutations in the survival motor neuron1 gene (SMN1). All SMA patients have a highly homologous copy of SMN1, the SMN2 gene. Severe (type I) SMA patients present one or two SMN2 copies, whereas milder chronic forms (type II-III) usually have three or four SMN2 copies. SMN2 dosage is important to stratify patients for motor function tests and clinical trials. Our aim was to compare three methods, marker analysis, real-time quantitative polymerase chain reaction using the LightCycler instrument, and multiple ligation-dependent probe amplification (MLPA), to characterize their accuracy in quantifying SMN2 genes. We studied a group of 62 genetically confirmed SMA patients, 54 with homozygous absence of exons 7 and 8 of SMN1 and 8 with SMN2-SMN1 hybrid genes. A complete correlation using the three methods was observed in 32 patients (51.6%). In the remaining 30 patients, discordances between the three methods were found, including under or overestimation of SMN2 copies by marker analysis with respect to the quantitative methods (LightCycler and MLPA) because of lack of informativeness of markers, 3' deletions of SMN genes, and breakpoints in SMN2-SMN1 hybrid genes. The technical limitations and advantages and disadvantages of these methods are discussed. We conclude that the three methods complement each other in estimating the SMN2 copy number in most cases. However, MLPA offers additional information to characterize SMA cases with particular rearrangements such as partial deletions and hybrid genes.

Plastin 3 expression in discordant spinal muscular atrophy (SMA) siblings

Spinal muscular atrophy (SMA) is caused by loss or mutations of the survival motor neuron 1 gene (SMN1). Its highly homologous copy, SMN2, is present in all SMA cases and is a phenotypic modifier. There are cases where asymptomatic siblings of typical SMA patients possess a homozygous deletion of SMN1 just like their symptomatic brothers or sisters. Plastin 3 (PLS3) when over expressed in lymphoblasts from females has been suggested to act as a genetic modifier of SMA. We studied PLS3 expression in four Spanish SMA families with discordant siblings haploidentical for the SMA locus. We excluded PLS3 as a possible modifier in two of our families with female discordant siblings. In the remaining two, we observed small differences in PLS3 expression between male and female discordant siblings. Indeed, we found that values of PLS3 expression in lymphoblasts and peripheral blood ranged from 12 to 200-fold less than those in fibroblasts. These findings warrant further investigation in motor neurons derived from induced pluripotential stem cells of these patients.

Insights into genotype-phenotype correlations in spinal muscular atrophy: a retrospective study of 103 patients

Spinal muscular atrophy (SMA) is an autosomal recessive disorder associated with homozygous deletion of the survival motor neuron 1 gene (SMN1). Its centromeric copy gene, SMN2, is the major modifying factor. However, the genotype-phenotype correlation is incomplete and is therefore not useful in clinical practice. We studied a cohort of 103 patients in order to refine this correlation. In addition to standard disease severity data, we collected three additional criteria: age at death; brainstem involvement; and loss of ambulation. Gene dosage analysis was conducted by multiplex ligation-dependent probe amplification (MLPA). SMN2 copynumber was highly correlated with survival duration in SMA type I and ambulation conservation or loss in type III. Among SMA severity groups, it was not significantly different in cases with brainstem involvement. Although the SMN2 copynumber could provide prognostic indications, clinical discrepancies still exist among patients, suggesting the existence of unidentified modifying factors.

Combination of SMN trans-splicing and a neurotrophic factor increases the life span and body mass in a severe model of spinal muscular atrophy

Spinal muscular atrophy (SMA), a neurodegenerative disease, is the second most common genetic disorder and the leading genetic cause of infantile death. SMA arises from the loss of Survival Motor Neuron-1 (SMN1), leading to degeneration of lower motor neurons and, consequently, the atrophy of voluntary muscles. A duplicated copy gene called SMN2 exists in humans. SMN2 is unable to fully compensate for the loss of SMN1 because it produces very low levels of functional SMN protein due to an alternative splicing event. A C/T transition in SMN2 exon 7 results in a transcript lacking exon 7 and, therefore, creates a truncated SMN protein that cannot fully compensate for the loss of SMN1. However, SMN2 is an ideal target for therapeutic strategies that redirect this critical splicing event. Previously, we developed the first trans-splicing strategy to increase the full-length mRNA and functional SMN protein from the SMN2 gene. To improve the trans-splicing efficacy, we then developed a single-vector system that expressed a trans-splicing RNA (tsRNA) and an antisense blocking the downstream splice site. This single vector greatly enhanced trans-splicing of SMN2 transcripts in vitro and in vivo. In this report, we have added a neurotrophic factor [insulin-like growth factor (IGF)-1] to this single vector to determine whether neuroprotection and SMN induction provide greater protection in an SMA animal model. Intracerebroventricular injection of the trans-splicing/IGF vector significantly increased SMN protein in brain and spinal cord of SMAΔ7 mice and lessened the severity of disease in a more severe mouse model as evidenced by an extension of life span and increased body mass.

A degron created by SMN2 exon 7 skipping is a principal contributor to spinal muscular atrophy severity

Spinal muscular atrophy (SMA) is caused by homozygous survival of motor neurons 1 (SMN1) gene deletions, leaving a duplicate gene, SMN2, as the sole source of SMN protein. However, most of the mRNA produced from SMN2 pre-mRNA is exon 7-skipped ( approximately 80%), resulting in a highly unstable and almost undetectable protein (SMNDelta7). We show that this splicing defect creates a potent degradation signal (degron; SMNDelta7-DEG) at SMNDelta7's C-terminal 15 amino acids. The S270A mutation inactivates SMNDelta7-DEG, generating a stable SMNDelta7 that rescues viability of SMN-deleted cells. These findings explain a key aspect of the SMA disease mechanism, and suggest new treatment approaches based on interference with SMNDelta7-DEG activity.

Treatment strategies for spinal muscular atrophy

Progress in understanding the genetic basis and pathophysiology of spinal muscular atrophy (SMA), along with continuous efforts in finding a way to increase survival motor neuron (SMN) protein levels have resulted in several strategies that have been proposed as potential directions for efficient drug development. Here we provide an overview on the current status of the following approaches: 1) activation of SMN2 gene and increasing full length SMN2 transcript level, 2) modulating SMN2 splicing, 3) stabilizing SMN mRNA and SMN protein, 4) development of neurotrophic, neuroprotective and anabolic compounds and 5) stem cell and gene therapy. The new preclinical advances warrant a cautious optimism for emergence of an effective treatment in the very near future.

A population-based study of genotypic and phenotypic variability in children with spinal muscular atrophy

AIMS

To describe the occurrence of spinal muscular atrophy (SMA) in childhood; to evaluate if any of the genes in the SMA region on chromosome 5q13 correlates with disease severity; to make genotype-phenotype correlations; to evaluate the variability of different disease alleles in carriers and the sensitivity of multiplex ligation-dependent probe amplification (MLPA) for detecting carriers.

METHODS

In a population-based study from Western Sweden MLPA was used to determine the copy-numbers of several genes in the SMA region (SMN1, SMN2, BIRC1, GTF2H2 and SERF1A) in SMA-patients and their parents.

RESULTS

We estimated the incidence of SMN1-related SMA in childhood at 1 in 11 800 live births and confirmed the relationship between the number of SMN2 copies and the severity of disease. No other direct relationships were found. All but one of the analysed parents were confirmed as carriers by MLPA analysis. A total of at least 30 different disease alleles were identified and no specific disease allele represented more than 15% of the total.

CONCLUSION

The childhood incidence of SMA in the Swedish population is around 1 in 12,000 live births and it is unlikely that there is any founder effect involved in SMA in western Sweden.

Combination of SMN2 copy number and NAIP deletion predicts disease severity in spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by mutations in the SMN1 gene. The SMN2 gene is highly homologous to SMN1 and has been reported to be correlated with severity of the disease. The clinical presentation of SMA varies from severe to mild, with three clinical subtypes (type I, type II, and type III) that are assigned according to age of onset and severity of the disease. Here, we aim to investigate the potential association between the number of copies of SMN2 and the deletion in the NAIP gene with the clinical severity of SMA in patients of Malaysian origin. Forty-two SMA patients (14 of type I, 20 type II, and 8 type III) carrying deletions of the SMN1 gene were enrolled in this study. SMN2 copy number was determined by fluorescence-based quantitative polymerase chain reaction assay. Twenty-nine percent of type I patients carried one copy of SMN2, while the remaining 71% carried two copies. Among the type II and type III SMA patients, 29% of cases carried two copies of the gene, while 71% carried three or four copies of SMN2. Deletion analysis of NAIP showed that 50% of type I SMA patients had a homozygous deletion of exon 5 of this gene and that only 10% of type II SMA cases carried a homozygous deletion, while all type III patients carried intact copies of the NAIP gene. We conclude that there exists a close relationship between SMN2 copy number and SMA disease severity, suggesting that the determination of SMN2 copy number may be a good predictor of SMA disease type. Furthermore, NAIP gene deletion was found to be associated with SMA severity. In conclusion, combining the analysis of deletion of NAIP with the assessment of SMN2 copy number increases the value of this tool in predicting the severity of SMA.

Spinal muscular atrophy during human development: where are the early pathogenic findings?

Spinal muscular atrophy (SMA) is an autosomal recessive disorder that affects motor neurons. It is caused by mutations in the survival motor neuron gene 1 (SMN1). The SMN2 gene, which is the highly homologous SMN1 copy that is present in all patients, is unable to prevent the disease. SMA patients can be classified into four groups based on age at onset and acquired milestones (type I or severe acute disease, with onset before 6 months; type II, before 18 months; type III, after 18 months and type IV, in adult life). The human developmental period is believed to play an essential role in SMA pathogenesis. However, the neuropathologic study of SMA comes largely from postnatal necropsy samples, which describe the end-stage of the disease. With the exception of severe congenital SMA (or Type 0 SMA), type I patients tend to present a short but variable presymptomatic period after birth. Our main interest lies in studying SMA during human development so as to gain insight into the mechanism of the disease in the prenatal-presymptomatic stage. In fetuses of 12-15 weeks' gestational age we systematically studied histology, cell death and gene expression in spinal cord and muscle, the key tissues involved in the disease. Furthermore, ultrasound parameters were investigated at these stages. These studies may help to delineate an early intervention in SMA, in particular during the potential therapeutic window.

Survival motor neuron gene 2 silencing by DNA methylation correlates with spinal muscular atrophy disease severity and can be bypassed by histone deacetylase inhibition

Spinal muscular atrophy (SMA), a common neuromuscular disorder, is caused by homozygous absence of the survival motor neuron gene 1 (SMN1), while the disease severity is mainly influenced by the number of SMN2 gene copies. This correlation is not absolute, suggesting the existence of yet unknown factors modulating disease progression. We demonstrate that the SMN2 gene is subject to gene silencing by DNA methylation. SMN2 contains four CpG islands which present highly conserved methylation patterns and little interindividual variations in SMN1-deleted SMA patients. The comprehensive analysis of SMN2 methylation in patients suffering from severe versus mild SMA carrying identical SMN2 copy numbers revealed a correlation of CpG methylation at the positions -290 and -296 with the disease severity and the activity of the first transcriptional start site of SMN2 at position -296. These results provide first evidence that SMN2 alleles are functionally not equivalent due to differences in DNA methylation. We demonstrate that the methyl-CpG-binding protein 2, a transcriptional repressor, binds to the critical SMN2 promoter region in a methylation-dependent manner. However, inhibition of SMN2 gene silencing conferred by DNA methylation might represent a promising strategy for pharmacologic SMA therapy. We identified histone deacetylase (HDAC) inhibitors including vorinostat and romidepsin which are able to bypass SMN2 gene silencing by DNA methylation, while others such as valproic acid and phenylbutyrate do not, due to HDAC isoenzyme specificities. These findings indicate that DNA methylation is functionally important regarding SMA disease progression and pharmacological SMN2 gene activation which might have implications for future SMA therapy regimens.

Pathogenesis of proximal autosomal recessive spinal muscular atrophy

Although it is known that deletions or mutations of the SMN1 gene on chromosome 5 cause decreased levels of the SMN protein in subjects with proximal autosomal recessive spinal muscular atrophy (SMA), the exact sequence of pathological events leading to selective motoneuron cell death is not fully understood yet. In this review, new findings regarding the dual cellular role of the SMN protein (translocation of beta-actin to axonal growth cones and snRNP biogenesis/pre-mRNA splicing) were integrated with recent data obtained by detailed neuropathological examination of SMA and control subjects. A presumptive series of 10 pathogenetic events for SMA is proposed as follows: (1) deletions or mutations of the SMN1 gene, (2) increased SMN mRNA decay and reduction in full-length functional SMN protein, (3) impaired motoneuron axono- and dendrogenesis, (4) failure of motoneurons to form synapses with corticospinal fibers from upper motoneurons, (5) abnormal motoneuron migration towards ventral spinal roots, (6) inappropriate persistence of motoneuron apoptosis due to impaired differentiation and motoneuron displacement, (7) substantial numbers of motoneurons continuing to migrate abnormally ("heterotopic motoneurons") and entering into the ventral roots, (8) attracted glial cells following these heterotopic motoneurons, which form the glial bundles of ventral roots, (9) impaired axonal transport of actin, causing remaining motoneurons to become chromatolytic, and (10) eventual death of all apoptotic, heterotopic and chromatolytic neurons, with apoptosis being more rapid and predominating in the earlier stages, with death of heterotopic and chromatolytic neurons occurring more slowly by necrosis during the later stages of SMA. According to this model, the motoneuron axonopathy is more important for pathogenesis than the ubiquitous nuclear splicing deficit. It is also supposed that individually variable levels of SMN protein, together with influences of other phenotype modifier genes and their products, cause the clinical SMA spectrum through differential degree of motoneuron functional loss.

Unaffected patients with a homozygous absence of the SMN1 gene

In this report, we present three families in which we identified asymptomatic carriers of a homozygous absence of the SMN1 gene. In the first family, the bialleleic deletion was found in three of four siblings: two affected brothers (SMA type 3a and 3b) and a 25-years-old asymptomatic sister. All of them have four SMN2 copies. In the second family, four of six siblings are affected (three suffer from SMA2 and one from SMA3a), each with three SMN2 copies. The clinically asymptomatic 47-year-old father has the biallelic deletion and four SMN2 copies. In the third family, the biallelic SMN1 absence was found in a girl affected with SMA1 and in her healthy 53-years-old father who had five SMN2 copies. Our findings as well as those of other authors show that an increased number of SMN2 copies in healthy carriers of the biallelic SMN1 deletion is an important SMA phenotype modifier, but probably not the only one.

Abnormal motoneuron migration, differentiation, and axon outgrowth in spinal muscular atrophy

The role of heterotopic (migratory) motoneurons (HMN) in the pathogenesis of spinal muscular atrophy (SMA) is still controversial. We examined the occurrence and amount of HMN in spinal cord tissue from eight children with SMA (six with SMA-I and two with SMA-II). All affected subjects were carrying a homozygous deletion of exon 7 in the SMN1 gene. Unlike controls, virtually free from HMN, all SMA subjects showed a significant number of HMN at all levels of the spinal cord. Heterotopic neurons were hyperchromatic, located mostly in the ventral white matter and had no axon or dendrites. More than half of the HMN were very undifferentiated, as judged from their lack of immunoreactivity for NeuN and MAP2 proteins. Small numbers of more differentiated heterotopic neurons were also found in the dorsal and lateral white matter region. As confirmed by ultrastructural analysis, in situ end labeling (ISEL) and CD68 immunoreactivity, HMN in the ventral outflow were found to have no synapses, to activate microglial cells, and to eventually die by necrosis. An unbiased quantitative analysis showed a significant negative correlation between age of SMA subjects (a reflection of the clinical severity) and the number of HMN. Subjects who died at older ages had increased number of GFAP-positive astrocytes. Complementing our previous report on motoneuron apoptosis within the ventral horns in SMA, we now propose that abnormal migration, differentiation, and lack of axonal outgrowth may induce motoneuron apoptosis predominantly during early stages, whereas a slower necrosis-like cell death of displaced motoneurons which "escaped" apoptosis characterizes later stages of SMA.

Investigation of the role of SMN1 and SMN2 haploinsufficiency as a risk factor for Hirayama's disease: Clinical, neurophysiological and genetic characteristics in a Spanish series of 13 patients

OBJECTIVE

The effect of the number of copies in the SMN1 and SMN2 genes - the most extensively studied susceptibility and modifying genetic factors in adult onset motor neuron diseases - as a genetic risk factor for Hirayama's disease (HirD) has never been studied. The purpose of this study was to investigate the influence of the number of copies of the SMN1/SMN2 genes on the resulting phenotype in 13 HirD Spanish patients.

PATIENTS AND METHODS

We performed a qualitative and quantitative SMN1/SMN2 gene analysis in 13 unrelated HirD patients. The phenotype-genotype correlation was investigated, paying particular attention to the effect of the SMN1/SMN2 copy number on the disease's phenotype.

RESULTS

No patient had a homozygous deletion of the SMN1 or SMN2. No differences were found when comparing the SMN1 and SMN2 copy number distributions of the healthy population and HirD patients, and they do not therefore appear to be a susceptibility factor. There was also no correlation found between the number of copies of the SMN1 and SMN2 and the severity of the resulting phenotype.

CONCLUSION

Our results suggest that SMN1 and SMN2 are not predisposing factors for HirD and therefore support a lack of association between these genes and the resulting phenotype.

A novel mutation at the N-terminal of SMN Tudor domain inhibits its interaction with target proteins

Although most patients with spinal muscular atrophy (SMA) are homozygous for deletion of the SMN1 gene, some patients bear one SMN1 copy with a subtle mutation. Detection of such an intragenic mutation may be helpful not only in confirming diagnosis but also in elucidating functional domains of the SMN protein. In this study, we identified a novel mutation in SMN1 of two Japanese patients with type I SMA. DHPLC and sequencing analysis revealed that they harbored a point mutation in SMN1 exon 3, 275G > C, leading to tryptophan-to-serine substitution at amino acid 92 (W92S) at the Nterminal of SMN Tudor domain. In-vitro protein binding assays showed that the mutation severely reduced interaction of the domain with SmB protein and fibrillarin, suggesting that it impairs the critical function of SMN. In conclusion, we reported here that a novel mutation, W92S, in the Tudor domain affects the interaction of SMN with the target proteins.

Deficiency of the zinc finger protein ZPR1 causes defects in transcription and cell cycle progression

The zinc finger protein ZPR1 is present in both the cytoplasm and nucleoplasm. Cell cycle analysis demonstrates that ZPR1 undergoes major changes in subcellular distribution during proliferation. ZPR1 is diffusely localized throughout the cell during the G(1) and G(2)/M phases of the cell cycle. In contrast, ZPR1 redistributes to the nucleus during S phase and ZPR1 exhibits prominent co-localization with the survival motor neurons protein and the histone gene-specific transcription factor NPAT in subnuclear foci, including Cajal bodies that associate with histone gene clusters. ZPR1 deficiency causes disruption of survival motor neurons and NPAT localization within the nucleus, blocks S phase progression, and arrests cells in both the G(1) and G(2) phases of the cell cycle. These changes in subnuclear architecture and cell cycle progression may be caused by transcriptional defects in ZPR1-deficient cells, including decreased histone gene expression.

Spinal muscular atrophy genotyping by gene dosage using multiple ligation-dependent probe amplification

Spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by degeneration of the anterior horn cells of the spinal cord, causing symmetric proximal muscle weakness. SMA is classified in three clinical types, SMA I, SMA II, and SMA III, based on the severity of the symptoms and the age of onset. About 95% of SMA cases are caused by homozygous deletion of the survival motor neuron 1 (SMN1) gene (5q13), or its conversion to SMN2. The molecular diagnosis of this disease is usually carried out by a polymerase chain reaction-restriction fragment length polymorphism approach able to evidence the absence of both SMN1 copies. However, this approach is not able to identify heterozygous healthy carriers, which show a very high frequency in general population (1:50). We used the multiple ligation-dependent probe amplification (MLPA) approach for the molecular diagnosis of SMA in 19 affected patient and in 57 individuals at risk to become healthy carriers. This analysis detected the absence of the homozygous SMN1 in all the investigated cases, and allowed to discriminate between SMN1 deletion and conversion to SMN2 on the basis of the size showed by the peaks specific for the different genes mapped within the SMA critical region. Moreover, MLPA analysis evidenced a condition of the absence of the heterozygous SMN1 in 33 out of the 57 relatives of the affected patients, demonstrating the usefulness of this approach in the identification of healthy carriers. Thus, the MLPA technique represents an easy, low cost, and high throughput system in the molecular diagnosis of SMA, both in affected patients and in healthy carriers.