Mutations in the gene encoding immunoglobulin mu-binding protein 2 cause spinal muscular atrophy with respiratory distress type 1

Spinal muscular atrophy: a deficiency in a ubiquitous protein; a motor neuron-specific disease

Spinal muscular atrophy (SMA) is a neurodegenerative disease in humans and the most common genetic cause of infant mortality. The disease results in motor neuron loss and skeletal muscle atrophy. Despite a range of disease phenotypes, SMA is caused by mutations in a single gene, the Survival of Motor Neuron 1 (SMN1) gene. Recent advances have shed light on functions of the protein product of this gene and the pathophysiology of the disease, yet, fundamental questions remain. This review attempts to highlight some of the recent advances made in the understanding of the disease and how loss of the ubiquitously expressed survival of motor neurons (SMN) protein results in the SMA phenotype. Answers to some of the questions raised may ultimately result in a viable treatment for SMA.

Genetics of familial and sporadic amyotrophic lateral sclerosis

Diseases affecting motor neurons, such as amyotrophic lateral sclerosis (Lou Gerhig's disease), hereditary spastic paraplegia and spinal bulbar muscular atrophy (Kennedy's disease) are a heterogeneous group of chronic progressive diseases and are among the most puzzling yet untreatable illnesses. Over the last decade, identification of mutations in genes predisposing to these disorders has provided the means to better understand their pathogenesis. The discovery 13 years ago of SOD1 mutations linked to ALS, which account for less than 2% of total cases, had a major impact in the field. However, despite intensive research effort, the pathways leading to the specific motor neurons degeneration in the presence of SOD1 mutations have not been fully identified. This review provides an overview of the genetics of both familial and sporadic forms of ALS.

Transplanted ALDHhiSSClo neural stem cells generate motor neurons and delay disease progression of nmd mice, an animal model of SMARD1

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an infantile autosomal-recessive motor neuron disease caused by mutations in the immunoglobulin micro-binding protein 2. We investigated the potential of a spinal cord neural stem cell population isolated on the basis of aldehyde dehydrogenase (ALDH) activity to modify disease progression of nmd mice, an animal model of SMARD1. ALDH(hi)SSC(lo) stem cells are self-renewing and multipotent and when intrathecally transplanted in nmd mice generate motor neurons properly localized in the spinal cord ventral horns. Transplanted nmd animals presented delayed disease progression, sparing of motor neurons and ventral root axons and increased lifespan. To further investigate the molecular events responsible for these differences, microarray and real-time reverse transcription-polymerase chain reaction analyses of wild-type, mutated and transplanted nmd spinal cord were undertaken. We demonstrated a down-regulation of genes involved in excitatory amino acid toxicity and oxidative stress handling, as well as an up-regulation of genes related to the chromatin organization in nmd compared with wild-type mice, suggesting that they may play a role in SMARD1 pathogenesis. Spinal cord of nmd-transplanted mice expressed high transcript levels for genes related to neurogenesis such as doublecortin (DCX), LIS1 and drebrin. The presence of DCX-expressing cells in adult nmd spinal cord suggests that both exogenous and endogenous neurogeneses may contribute to the observed nmd phenotype amelioration.

Dilated cardiomyopathy in the nmd mouse: transgenic rescue and QTLs that improve cardiac function and survival

Mutations in the immunoglobulin mu binding protein-2 (Ighmbp2) gene cause motor neuron disease and dilated cardiomyopathy (DCM) in the neuromuscular degeneration (nmd) mouse and spinal muscular atrophy with respiratory distress (SMARD1) in humans. To investigate the role of IGHMBP2 in the pathogenesis of DCM, we generated transgenic mice expressing the full-length Ighmbp2 cDNA specifically in myocytes under the control of the mouse titin promoter. This tissue-specific transgene increased the lifespan of nmd mice up to 8-fold by preventing primary DCM and showed complete functional correction as measured by ECG, echocardiography and plasma creatine kinase-MB. Double-transgenic nmd mice expressing Ighmbp2 both in myocytes and in neurons display correction of both DCM and motor neuron disease, resulting in an essentially wild-type appearance. Additionally, quantitative trait locus (QTL) analysis was undertaken to identify genetic modifier loci responsible for the preservation of cardiac function and a marked delay in the onset of cardiomyopathy in a CAST/EiJ backcross population. Three major CAST-derived cardiac modifiers of nmd were identified on chromosomes 9, 10 and 16, which account for over 26% of the genetic variance and that continue to suppress the exacerbation of cardiomyopathy, otherwise resulting in early death, as incipient B6.CAST congenics. Overall, our results verify the tissue-specific requirement for IGHMBP2 in cardiomyocyte maintenance and survival and describe genetic modifiers that can alter the course of DCM through cardiac functional adaptation and physical remodeling in response to changes in load and respiratory demand.

The ultrastructure of peripheral nerve, motor end-plate and skeletal muscle in patients suffering from spinal muscular atrophy with respiratory distress type 1 (SMARD1)

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is genetically and clinically distinct from classic spinal muscular atrophy (SMA1). It results from mutations in the gene encoding immunoglobulin mu-binding protein 2 (IGHMBP2) on chromosome 11q13. Patients develop distally pronounced muscular weakness and early involvement of the diaphragm, resulting in respiratory failure. Sensory and autonomic nerves are also affected at later stages of the disease. We investigated peripheral nerves, skeletal muscles and neuromuscular junctions (NMJ) ultrastructurally in five unrelated patients and three siblings with genetically confirmed SMARD1. In mixed motor and sensory nerves we detected Wallerian degeneration and axonal atrophy similar to the ultrastructural findings described in SMA1. Isolated axonal atrophy was evident in purely sensory nerves. All investigated NMJ of patients with SMARD1 were dysmorphic and lacked a terminal axon. Moreover, we also observed characteristics of neuropathies, such as abnormalities in myelination, that have not been described in spinal muscular atrophies so far. Based on these findings we conclude that impairment of IGHMBP2 function leads to axonal degeneration, abnormal myelin formation, and motor end-plate degeneration.

Emerging pathways for hereditary axonopathies

Motor neurons are affected in a number of neurological diseases. Their unifying pathological signature is degeneration of extended projecting axons and loss of motor neurons in the prefrontal cortex and/or the spinal cord. Based on clinical criteria, hereditary forms have been traditionally divided into distinct entities, such as familial amyotrophic lateral sclerosis, hereditary motor neuropathy, spinal muscular atrophy, familial spinal paraplegia, and Charcot-Marie-Tooth disease type 2, also known as hereditary motor and sensory neuropathy II. Genetic research of the last decade has revealed remarkable heterogeneity within these disorders. Most of the identified genes to date cause disease in a classic Mendelian inheritance pattern with a high phenotypic penetrance. This rich source of molecular genetic data has already provided insight into the underlying major pathways of these diseases and should continue to do so in the future. This review attempts to cross the traditional clinical classifications in order to draw an emerging picture of common pathways between causative genes, providing a different perspective of this rapidly growing scientific field.

Treatment with trkC agonist antibodies delays disease progression in neuromuscular degeneration (nmd) mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal autosomal recessive disorder seen in infants. It is characterized by lower motor neuron degeneration, progressive muscle paralysis and respiratory failure, for which no effective treatment exists. The phenotype of neuromuscular degeneration (nmd) mice closely resembles the human SMARD1. The identification of the mutated mouse gene in nmd mice, Ighmbp2, led to the discovery of mutations of the homologous gene in humans with SMARD1. We have studied the nmd mouse model with in vivo electrophysiological techniques and evaluated the efficacy of Mab2256, a monoclonal antibody with agonist effect on the tyrosine kinase receptor C, trkC, on disease progression in nmd mice. Treatment with Mab2256 resulted in a significant but transient improvement of muscle strength in nmd mice, as well as normalization of the neuromuscular depression during high-frequency nerve stimulation. These results suggest the potential of using monoclonal agonist antibodies for neurotrophin receptors in lower motor neuron diseases such as SMARD1.

Autosomal-Recessive Charcot-Marie-Tooth Diseases

In certain countries around the Mediterranean basin such as Algeria, which have a high prevalence of consanguineous marriages, autosomal-recessive (AR) inheritance may account for more than 50% of all forms of Charcot-Marie-Tooth (CMT) disease. Like with the dominant forms, it is usual to differentiate the demyelinating forms (CMT 4 corresponding to autosomal-recessive CMT 1 [AR-CMT 1] from the axonal forms [AR-CMT 2]). Genetic analysis of large families with recessive transmission has uncovered novel CMT genotypes (genes: GDAP 1, MTMR 2, MTMR 13, KIAA1985, NDGR1, periaxi, lamin). The clinical and especially the histologic phenotypes often indicate that a specific gene is implicated. We present and discuss microscopic lesions seen on nerve biopsies from patients in a number of consanguineous Algerian families, and we outline the characteristic lesions that would prompt a search for mutations in genes such as MTMR 2, MTMR 13, KIAA1985, periaxin for CMT 4, and lamin for AR-CMT 2. Like with the dominant forms, there are undoubtedly many more mutations of other genes to be discovered.

A new mutation of IGHMBP2 gene in spinal muscular atrophy with respiratory distress type 1

This report presents a new mutation in the first Japanese female infant with spinal muscular atrophy with respiratory distress type 1. She manifested the characteristic clinical features, including early-onset respiratory failure due to diaphragmatic paralysis and severe distal muscle weakness. Muscle biopsy in the femoral muscle indicated massive neurogenic changes. Sural nerve biopsy disclosed a moderate reduction of myelinated fibers, predominantly reduced large fibers. She had a novel homozygous missense mutation 2685 C -->A, leading to a T879K substitution in the immunoglobulin mu-binding protein 2 gene. Both parents were heterozygous for this mutation.

Inherited motor neuron disease in domestic cats: a model of spinal muscular atrophy

Juvenile-onset spinal muscular atrophy was observed in an extended family of purebred domestic cats as a fully penetrant, simple autosomal recessive trait. Affected kittens exhibited tremor, proximal muscle weakness, and muscle atrophy beginning at ~4 mo of age. Apparent loss of function was rapid initially but progressed slowly after 7-8 mo of age, and variably disabled cats lived for at least 8 y. Electromyography and microscopic examination of muscle and nerve biopsies were consistent with denervation atrophy as a result of a central lesion. There was astrogliosis and dramatic loss of motor neurons in ventral but not dorsal horn gray matter of spinal cord and loss of axons in ventral horn nerve roots. These phenotypic findings were similar to mild forms (type III) of spinal muscular atrophy in humans caused by survival of motor neuron mutations, but molecular analysis excluded feline survival of motor neuron as the disease gene in this family. A breeding colony has been established for further investigation of this naturally occurring large-animal model of inherited motor neuron disease.

Type I spinal muscular atrophy can mimic sensory-motor axonal neuropathy

Spinal muscular atrophy is a group of allelic autosomal recessive disorders characterized by progressive motoneuron loss, symmetric weakness, and skeletal muscle atrophy. It is traditionally considered a pure lower motoneuron disorder, for which a current definitive diagnosis is now possible by molecular genetic testing. We report two newborns with a clinical phenotype consistent with that of spinal muscular atrophy type I and nerve conduction studies and electromyography suggesting more extensive sensory involvement than classically described with spinal muscular atrophy. Molecular testing confirmed spinal muscular atrophy in patient 1 but not in patient 2. Thus, in the setting of a suspected congenital axonal neuropathy, molecular testing might be necessary to distinguish spinal muscular atrophy type I from infantile polyneuropathy.

Genomic rearrangements at the IGHMBP2 gene locus in two patients with SMARD1

Autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) is caused by mutations in the immunoglobulin mu-binding protein 2 (IGHMBP2) gene. Patients affected by the infantile form of SMARD1 present with early onset respiratory distress. So far, patients with neither juvenile onset nor with larger deletions/rearrangements in IGHMBP2 have been reported. In this study, we investigated one patient with infantile (4 months) and another with juvenile (4.3 years) onset of respiratory distress. Direct sequencing of all exons and flanking intron sequences in both patients revealed a mutation on only one allele. In both patients, we identified genomic rearrangements of the other allele of IGHMBP2 by means of Southern blotting. Putative breakpoints were confirmed by polymerase chain reaction on genomic and cDNA. The patient with juvenile onset had an Alu/Alu mediated rearrangement, which resulted in the loss of aproximately 18.5 kb genomic DNA. At the mRNA level, this caused an in-frame deletion of exons 3-7. The patient with infantile onset had a complex rearrangement with two deletions and an inversion between intron 10 and 14. This rearrangement led to a frameshift at the mRNA level. Our results show that SMARD1 can be caused by genomic rearrangements at the IGHMBP2 gene locus. This may be missed by mere sequence analysis. Additionally, we demonstrate that juvenile onset SMARD1 may also be caused by mutations of IGHMBP2. The complex nature of the genomic rearrangement in the patient with infantile SMARD1 is discussed and a deletion mechanism is proposed.

DNA/RNA helicase gene mutations in a form of juvenile amyotrophic lateral sclerosis (ALS4)

Juvenile amyotrophic lateral sclerosis (ALS4) is a rare autosomal dominant form of juvenile amyotrophic lateral sclerosis (ALS) characterized by distal muscle weakness and atrophy, normal sensation, and pyramidal signs. Individuals affected with ALS4 usually have an onset of symptoms at age <25 years, a slow rate of progression, and a normal life span. The ALS4 locus maps to a 1.7-Mb interval on chromosome 9q34 flanked by D9S64 and D9S1198. To identify the molecular basis of ALS4, we tested 19 genes within the ALS4 interval and detected missense mutations (T3I, L389S, and R2136H) in the Senataxin gene (SETX). The SETX gene encodes a novel 302.8-kD protein. Although its function remains unknown, SETX contains a DNA/RNA helicase domain with strong homology to human RENT1 and IGHMBP2, two genes encoding proteins known to have roles in RNA processing. These observations of ALS4 suggest that mutations in SETX may cause neuronal degeneration through dysfunction of the helicase activity or other steps in RNA processing.

Hot-spot residue in small heat-shock protein 22 causes distal motor neuropathy

Distal hereditary motor neuropathies are pure motor disorders of the peripheral nervous system resulting in severe atrophy and wasting of distal limb muscles. In two pedigrees with distal hereditary motor neuropathy type II linked to chromosome 12q24.3, we identified the same mutation (K141N) in small heat-shock 22-kDa protein 8 (encoded by HSPB8; also called HSP22). We found a second mutation (K141E) in two smaller families. Both mutations target the same amino acid, which is essential to the structural and functional integrity of the small heat-shock protein alphaA-crystallin. This positively charged residue, when mutated in other small heat-shock proteins, results in various human disorders. Coimmunoprecipitation experiments showed greater binding of both HSPB8 mutants to the interacting partner HSPB1. Expression of mutant HSPB8 in cultured cells promoted formation of intracellular aggregates. Our findings provide further evidence that mutations in heat-shock proteins have an important role in neurodegenerative disorders.

Identifying Functional Genetic Variants in DNA Repair Pathway Using Protein Conservation Analysis

The role of DNA repair in initiation, promotion, and progression of malignancy suggests that variations in DNA repair genes confer altered cancer risk. Accordingly, DNA repair gene variants have been studied extensively in the context of cancer predisposition. Single nucleotide polymorphisms (SNPs) are the most common genetic variations in the human genome. A fraction of SNPs are located within the genes, which are likely to alter the gene expression and function. SNPs that change the encoded amino acid sequence of the proteins (non-synonymous; nsSNPs) are potentially genetic disease determinant variations. However, as not all amino acid substitutions are supposed to lead to a change in protein function, it will be necessary to have a priori prediction and determination of the functional consequences of amino acid substitutions per se, and then together with other genetic and environmental factors to study their possible association with a trait. Here we report the analysis of nsSNPs in 88 DNA repair genes and their functional evaluation based on the conservation of amino acids among the protein family members. Our analysis demonstrated that &gt;30% of variants of DNA repair proteins are highly likely to affect the function of the proteins drastically. In this study, we have shown that three nsSNPs, which were predicted to have functional consequences (XRCC1-R399Q, XRCC3-T241M, XRCC1-R280H), were already found to be associated with cancer risk. The strategy developed and applied in this study has the potential to identify functional protein variants of DNA repair pathway that may be associated with cancer predisposition.

Congenital form of spinal muscular atrophy predominantly affecting the lower limbs: a clinical and muscle MRI study

We describe clinical and muscle magnetic resonance imaging (MRI) findings in 11 cases (three familial and eight sporadic) with the form of spinal muscular atrophy characterised by predominant involvement of the lower limbs with weakness of the proximal and distal muscles and marked atrophy of the distal leg and foot muscles. All patients presented at birth with talipes, which were in extension in seven of the 11. Arm muscle and function were preserved and lower limbs appeared to be disproportionately shorter compared to trunk and upper limbs. Functional abilities were markedly affected and only one of the 11 is able to walk independently for long distances, while six require support of crutches and two use callipers for walking. One child lost ambulation following a fall. The course of the disease is relatively stable and the progression of disability appeared to be related mostly to increased contractures rather than to loss of muscle strength. Respiratory and cardiac function were well preserved. A neurogenic disorder was suggested by electromyography and/or muscle biopsy in all patients, while motor nerve conduction was consistently normal. Muscle MRI of the thighs revealed diffuse atrophic appearance with relative hypertrophy of the adductor longus and of the semitendinosus. Genetic studies excluded the involvement of the survival motor neuron gene but none of these families was sufficiently informative to study linkage to the locus on chromosome 12q23-q24 previously found to be involved in patients with similar phenotype. In our experience this form of spinal muscular atrophy affecting predominantly the lower limbs is a relatively common form and should be considered in the differential diagnosis of infants with talipes and weakness in the lower limbs. The identical clinical and imaging features of the sporadic and familial cases suggest that these cases are likely to be affected by the same condition.

Transgenic rescue of neurogenic atrophy in the nmd mouse reveals a role for Ighmbp2 in dilated cardiomyopathy

Immunoglobulin mu binding protein 2 (IGHMBP2) is a DNA/RNA helicase with a putative role in transcriptional regulation and splicing. A recessive mutation of the Ighmbp2 gene in neuromuscular degeneration (nmd) mice causes progressive neurogenic atrophy of limb muscles. Affected mice show significant loss of motor neurons with large caliber axons and a moderate reduction of neurons with small caliber axons in the ventral nerve roots of the spinal cord. To investigate the role of Ighmbp2 in the pathogenesis of neuromuscular degeneration, we generated two independent lines of transgenic mice expressing the full-length Ighmbp2 cDNA specifically in neurons. Histopathological evaluation of L4 ventral nerve roots revealed that transgenic expression of the Ighmbp2 cDNA prevented primary motor neuron degeneration, while restoring the normal axonal morphology and density in nmd mice. A similar neuronal improvement is found in mutant mice carrying the CAST/EiJ-derived modifier of nmd (Mnm(C)). Intriguingly, both the transgenic and modified nmd mice went on to develop a previously unobserved cardiac and skeletal myopathy. Necropsy of nmd mice in end-stage heart failure revealed a primary dilated cardiomyopathy with secondary respiratory failure that was confirmed by in vivo ECG and echocardiographic measures. Our results suggest that reduced levels of IGHMBP2 in nmd mice compromise the integrity and function not only of motor neurons but also of skeletal and cardiac myocytes. These findings highlight the important role of IGHMBP2 in the maintenance and survival of these terminally differentiated cell types.

Refined genetic mapping of autosomal recessive chronic distal spinal muscular atrophy to chromosome 11q13.3 and evidence of linkage disequilibrium in European families

Chronic distal spinal muscular atrophy (Chronic DSMA, MIM (*)607088) is a rare autosomal recessive disorder characterized by a progressive motor weakness and muscular atrophy, predominating in the distal parts of the limbs. A form of Chronic DSMA gene has been previously mapped to chromosome 11q13 in the 10.3 cM interval defined by loci D11S1889 and D11S1321. By linkage analysis in 12 European Chronic DSMA families, we showed that a disease gene maps to chromosome 11q13.3 (Z(max)=6.66 at theta=0.00 at the DSM4 locus) and suggested that this condition is genetically homogeneous. Recombination events allowed us to reduce the genetic interval to a 2.6 cM region, telomeric to the IGHMBP2 gene, excluding this gene as the disease causing gene in Chronic DSMA. Moreover, partial linkage disequilibrium was found between three rare alleles at loci D11S1369, DSM4 and D11S4184 and the mutant chromosome in European patients. Analysis of the markers at these loci strongly suggests that most Chronic DSMA chromosomes are derived from a single ancestor. Refinement of the Chronic DSMA locus will hopefully allow to test candidate genes and lead to identification of the disease-causing mutations.

Axonal defects in mouse models of motoneuron disease

Human motoneuron disease is characterized by loss of motor endplates, axonal degeneration, and cell death of motoneurons. The identification of the underlying gene defects for familial ALS, spinal muscular atrophy (SMA), and spinal muscular atrophy with respiratory distress (SMARD) has pointed to distinct pathophysiological mechanisms that are responsible for the various forms of the disease. Accumulating evidence from mouse models suggests that enhanced vulnerability and sensitivity to proapoptotic stimuli is only responsible for some but not all forms of motoneuron disease. Mechanisms that modulate microtubule assembly and the axonal transport machinery are defective in several spontaneous and ENU (ethylnitrososurea) mutagenized mouse models but also in patients with mutations in the p150 subunit of dynactin. Recent evidence suggests that axonal growth defects contribute significantly to the pathophysiology of spinal muscular atrophy. Reduced levels of the survival motoneuron protein that are responsible for SMA lead to disturbed RNA processing in motoneurons. This could also affect axonal transport of mRNAs for beta-actin and other proteins that play an essential role in axon growth and synaptic function. The local translation of specific proteins might be affected, because developing motoneurons contain ribosome-like structures in distal axons and growth cones. Altogether, the evidence from these mouse models and the new genetic data from patients suggest that axon growth and maintenance involves a variety of mechanisms, including microtubule assembly and axonal transport of proteins and ribonucleoproteins (RNPs). Thus, defects in axon maintenance could play a leading role in the development of several forms of human motoneuron disease.

Senataxin, the ortholog of a yeast RNA helicase, is mutant in ataxia-ocular apraxia 2

Ataxia-ocular apraxia 2 (AOA2) was recently identified as a new autosomal recessive ataxia. We have now identified causative mutations in 15 families, which allows us to clinically define this entity by onset between 10 and 22 years, cerebellar atrophy, axonal sensorimotor neuropathy, oculomotor apraxia and elevated alpha-fetoprotein (AFP). Ten of the fifteen mutations cause premature termination of a large DEAxQ-box helicase, the human ortholog of yeast Sen1p, involved in RNA maturation and termination.

Infantile spinal muscular atrophy with respiratory distress type 1 (SMARD1)

Autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) is the second anterior horn cell disease in infants in which the genetic defect has been defined. SMARD1 results from mutations in the gene encoding the immunoglobulin micro-binding protein 2 (IGHMBP2) on chromosome 11q13. Our aim was to review the clinical features of 29 infants affected with SMARD1 and report on 26 novel IGHMBP2 mutations. Intrauterine growth retardation, weak cry, and foot deformities were the earliest symptoms of SMARD1. Most patients presented at the age of 1 to 6 months with respiratory distress due to diaphragmatic paralysis and progressive muscle weakness with predominantly distal lower limb muscle involvement. Sensory and autonomic nerves are also affected. Because of the poor prognosis, there is a demand for prenatal diagnosis, and clear diagnostic criteria for infantile SMARD1 are needed. The diagnosis of SMARD1 should be considered in infants with non-5q spinal muscular atrophy, neuropathy, and muscle weakness and/or respiratory distress of unclear cause. Furthermore, consanguineous parents of a child with sudden infant death syndrome should be examined for IGHMBP2 mutations.

Mapping of the bovine spinal muscular atrophy locus to Chromosome 24

A hereditary form of spinal muscular atrophy (SMA) caused by an autosomal recessive gene has been reported for American Brown-Swiss cattle and in advanced backcrosses between American Brown-Swiss and many European brown cattle breeds. Bovine SMA (bovSMA) bears remarkable resemblance to the human SMA (SMA1). Affected homozygous calves also show progressive symmetric weakness and neurogenic atrophy of proximal muscles. The condition is characterized by severe muscle atrophy, quadriparesis, and sternal recumbency as result of neurogenic atrophy. We report on the localization of the gene causing bovSMA within a genomic interval between the microsatellite marker URB031 and the telomeric end of bovine Chromosome (Chr) 24 (BTA24). Linkage analysis of a complex pedigree of German Braunvieh cattle revealed a recombination fraction of 0.06 and a three-point lod score of 11.82. The results of linkage and haplotyping analysis enable a marker-assisted selection against bovSMA based on four microsatellite markers most telomeric on BTA24 to a moderate accuracy of 89-94%. So far, this region is not orthologous to any human chromosome segments responsible for twelve distinct disease phenotypes of autosomal neuropathies. Our results indicate the apoptosis-inhibiting protein BCL2 as the most promising positional candidate gene causing bovSMA. Our findings offer an attractive animal model for a better understanding of human forms of SMA and for a probable anti-apoptotic synergy of SMN-BCL2 aggregates in mammals.

Solution structure of the R3H domain from human Smubp-2

The R3H domain is a conserved sequence motif, identified in over 100 proteins, that is thought to be involved in polynucleotide-binding, including DNA, RNA and single-stranded DNA. In this work the 3D structure of the R3H domain from human Smubp-2 was determined by NMR spectroscopy. It is the first 3D structure determination of an R3H domain. The fold presents a small motif, consisting of a three-stranded antiparallel beta-sheet and two alpha-helices, which is related to the structures of the YhhP protein and the C-terminal domain of the translational initiation factor IF3. The similarities are non-trivial, as the amino acid identities are below 10%. Three conserved basic residues cluster on the same face of the R3H domain and could play a role in nucleic acid recognition. An extended hydrophobic area at a different site of the molecular surface could act as a protein-binding site. A strong correlation between conservation of hydrophobic amino acids and side-chain solvent protection indicates that the structure of the Smubp-2 R3H domain is representative of R3H domains in general.

Hereditary motor neuropathies and motor neuron diseases: which is which

When Charcot first defined amyotrophic lateral sclerosis (ALS) he used the clinical and neuropathological pattern of vulnerability as a guideline. Similarly other motor neuron diseases such as the spinal muscular atrophies (SMA) and the motor neuropathies (MN) were grouped following clinical criteria. However, ever since the etiology of these diseases has started to be disclosed by genetics, we have learnt that the limits of the syndromes are not as well defined as our forefathers thought. A mutation leading to ALS can also be associated with the clinical picture of spinal muscular atrophy; even more unexpected is the overlap of the so-called motor neuropathies with the clinical syndrome of slowly progressive ALS or that primary lateral sclerosis (PLS) can be caused by the same gene as that responsible for some cases of ALS. In this review we summarise recent work showing that there is a considerable overlap between CMT, MN, SMA, ALS and PLS. Insights into these phenotypes should lead to study of the variants of motor neuron disease and possibly to a reclassification. This comprehensive review should help to improve understanding of the pathogenesis of motor neuron degeneration and finally may aid the research for urgently needed new treatment strategies, perhaps with validity for the entire group of motor neuron diseases.

Missense mutation in the tubulin-specific chaperone E (Tbce) gene in the mouse mutant progressive motor neuronopathy, a model of human motoneuron disease

Progressive motor neuronopathy (pmn) mutant mice have been widely used as a model for human motoneuron disease. Mice that are homozygous for the pmn gene defect appear healthy at birth but develop progressive motoneuron disease, resulting in severe skeletal muscle weakness and respiratory failure by postnatal week 3. The disease starts at the motor endplates, and then leads to axonal loss and finally to apoptosis of the corresponding cell bodies. We localized the genetic defect in pmn mice to a missense mutation in the tubulin-specific chaperone E (Tbce) gene on mouse chromosome 13. The human orthologue maps to chromosome 1q42.3. The Tbce gene encodes a protein (cofactor E) that is essential for the formation of primary alpha-tubulin and beta-tubulin heterodimeric complexes. Isolated motoneurons from pmn mutant mice exhibit shorter axons and axonal swelling with irregularly structured beta-tubulin and tau immunoreactivity. Thus, the pmn gene mutation provides the first genetic evidence that alterations in tubulin assembly lead to retrograde degeneration of motor axons, ultimately resulting in motoneuron cell death.

Untranslated element in neurofilament mRNA has neuropathic effect on motor neurons of transgenic mice

Studies of experimental motor neuron degeneration attributable to expression of neurofilament light chain (NF-L) transgenes have raised the possibility that the neuropathic effects result from overexpression of NF-L mRNA, independent of NF-L protein effects (Cañete-Soler et al., 1999). The present study was undertaken to test for an RNA-mediated pathogenesis. Transgenic mice were derived using either an enhanced green fluorescent protein reporter construct or modified chimeric constructs that differ only in their 3' untranslated regions (UTRs). Motor function and spinal cord histology were normal in mice expressing the unmodified reporter transgene. In mice expressing a chimeric transgene in which sequence of NF-L 3' UTR was inserted into the 3' UTR of the reporter transgene, we observed growth retardation and reduced kinetic activity during postnatal development. Older mice developed impairment of motor function and atrophy of nerve fibers in the ventral roots. A similar but more severe phenotype was observed when the chimeric transgene contained a 36 bp c-myc insert in an mRNA destabilizing element of the NF-L sequence. Our results suggest that neuropathic effects of overexpressing NF-L can occur at the level of transgene RNA and are mediated by sequences in the NF-L 3' UTR.

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.

Motor neurone disease

Motor neurone disease (MND), or amyotrophic lateral sclerosis (ALS), is a neurodegenerative disorder of unknown aetiology. Progressive motor weakness and bulbar dysfunction lead to premature death, usually from respiratory failure. Confirming the diagnosis may initially be difficult until the full clinical features are manifest. For all forms of the disease there is a significant differential diagnosis to consider, including treatable conditions, and therefore specialist neurological opinion should always be sought. Clear genetic inheritance has been demonstrated in a minority of patients with familial ALS but elucidation of the biological basis of genetic subtypes is also providing important information which may lead to treatments for sporadic forms of the disease. In the absence of curative or disease modifying therapy, management is supportive and requires a multidisciplinary approach. If, as seems likely, complex inherited and environmental factors contribute to the pathogenesis of MND, future treatment may involve a combination of molecular based treatments or restoration of cellular integrity using stem cell grafts.

Mutation analysis of 12 candidate genes for distal hereditary motor neuropathy type II (distal HMN II) linked to 12q24.3

Distal hereditary motor neuropathies (distal HMNs) are characterized by degeneration of anterior horn cells of the spinal cord resulting in muscle weakness and atrophy. Distal HMN type II is genetically linked to chromosome 12q24.3 and located within a 13 cM region flanked by D12S86 and D12S340. We previously excluded 5 positional and functional candidate genes for distal HMN II. Here, we report the exclusion of 12 additional candidate genes localized within the distal HMN II region; the genes include musashi (Drosophila) homolog 1 (MSI1), protein inhibitor of neuronal nitric oxide synthase (PIN), peripherin (PRPH), tubulin alpha ubiquitous (K-ALPHA-1), tubulin alpha 3 (TUBA3), tubulin alpha 6 (TUBA6), splicing factor arginine/serine-rich 9 (SFRS9), U5 snRNP 100 kd (U5- 100K), putative chemokine receptor, GTP-binding protein (HM74), MondoA, cut (Drosophila)-like homeobox 2 (CUX2) and ADP-ribosylation factor 3 (ARF3).

Mapping of autosomal recessive chronic distal spinal muscular atrophy to chromosome 11q13

Distal spinal muscular atrophy is a heterogeneous group of neuromuscular disorders caused by progressive anterior horn cell degeneration and characterized by progressive motor weakness and muscular atrophy, predominantly in the distal parts of the limbs. Here we report on chronic autosomal recessive distal spinal muscular atrophy in a large, inbred family with onset at various ages. Because this condition had some of the same clinical features as spinal muscular atrophy with respiratory distress, we tested the disease gene for linkage to chromosome 11q and mapped the disease locus to chromosome 11q13 in the genetic interval that included the spinal muscular atrophy with respiratory distress gene (D11S1889-D11S1321, Z(max) = 4.59 at theta = 0 at locus D11S4136). The sequencing of IGHMBP2, the human homologue of the mouse neuromuscular degeneration gene (nmd) that accounts for spinal muscular atrophy with respiratory distress, failed to detect any mutation in our chronic distal spinal muscular atrophy patients, suggesting that spinal muscular atrophy with respiratory distress and chronic distal spinal muscular atrophy are caused by distinct genes located in the same chromosomal region. In addition, the high intrafamilial variability in age at onset raises the question of whether nonallelic modifying genes could be involved in chronic distal spinal muscular atrophy.