Spinal muscular atrophy (SMA) is caused by the loss of functional survival motor neuron 1 (SMN1) protein. This ubiquitously expressed protein is a component of a novel complex immunodetected in both the cytoplasm and the nucleus, which is associated with complexes involved in mRNA splicing, ribosome biogenesis and transcription. Here, we study a mutant protein corresponding to the N-terminal half of the protein that is encoded by the SMA frameshift mutation SMN 472del5. We show by confocal microscopy that the resulting mutant protein exhibits various distribution patterns in different transiently transfected COS cells. The mutant distributes into the nucleoplasm and/or the nucleolus, whereas the normal SMN protein accumulates at discrete nucleocytoplasmic dot-like structures previously named gems/Cajal bodies. The cell population with the nucleolar distribution is enriched upon treatment with mimosine, a synchronizing drug in late G(1) phase. Co-immunoprecipitation studies carried out on nuclear extracts reveal that both the endogenous SMN and mutant proteins are associated with complexes containing two major non-ribosomal nucleolar proteins, namely nucleolin and protein B23, and that the association is mediated, by among other things, RNA moieties. Both the association of the SMN protein with nucleolin-containing complexes and the nucleolin/B23 complex are disrupted in fibroblasts derived from a type I SMA patient harboring a homozygous SMN1 gene deletion. These findings suggest that altered assembly and/or stability of ribonucleoprotein complexes may contribute to the pathophysiological processes in SMA.

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder that results in the degeneration of spinal motor neurons. SMA is caused by alterations of the survival motor neuron (SMN) gene which encodes a novel protein of hitherto unclear function. The SMN protein associates with ribonucleoprotein particles involved in RNA processing and exhibits an RNA-binding capacity. We have isolated the zebrafish Danio rerio and nematode Caenorhabditis elegans orthologues and have found that the RNA-binding capacity is conserved across species. Purified recombinant SMN proteins from both species showed selectivity to poly(G) homopolymer RNA in vitro, similar to that of the human protein. Studying deletions of the zebrafish SMN protein, we defined an RNA-binding element in exon 2a, which is highly conserved across species, and revealed that its binding activity is modulated by protein domains encoded by exon 2b and exon 3. Finally, the deleted recombinant zebrafish protein mimicking an SMA frameshift mutation showed a dramatic change in vitro in the formation of the RNA-protein complexes. These observations indicate that the RNA-binding capacity of SMN is an evolutionarily conserved function and further support the view that defects in RNA metabolism most likely account for the pathogenesis of SMA.

Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder characterized by degeneration of motor neurons of the spinal cord and muscular atrophy. SMA is caused by alterations to the survival of motor neuron (SMN) gene, the function of which has hitherto been unclear. Here, we present immunoblot analyses showing that normal SMN protein expression undergoes a marked decay in the postnatal period compared with fetal development. Morphological and immunohistochemical analyses of the SMN protein in human fetal tissues showed a general distribution in the cytoplasm, except in muscle cells, where SMN protein was immunolocalized to large cyto-plasmic dot-like structures and was tightly associated with membrane-free heavy sedimenting complexes. These cytoplasmic structures were similar in size to gem. The SMN protein was markedly deficient in tissues derived from type I SMA fetuses, including skeletal muscles and, as previously shown, spinal cord. While our data do not help decide whether SMA results from impaired SMN expression in spinal cord, skeletal muscle or both, they suggest a requirement for SMN protein during embryo-fetal development.

Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder characterized by degeneration of motor neurons of the spinal cord. Three different forms of childhood SMA have been recognized on the basts of age at onset and clinical course: Werdnig-Hoffmann disease (type l), the intermediate form (type II) and Kugelberg-Welander disease (type III) 1 . A gene termed ‘survival of motor neuron’ ( SMN ) has been recognized as the disease-causing gene in SMA 2–6 . SMN encodes a protein located within a novel nuclear structure and interacts with RNA-binding proteins 7 . To elucidate the molecular mechanism underlying the pathogenesis of the disease, we examined the expression of the SMN gene in both controls and SMA patients by western blot and immunohistochemical analyses using antibodies raised against the SMN protein. The present study shows a marked deficiency of the SMN protein in SMA.

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.

Les amyotrophies spinales proximales (SMA) sont des maladies neuromusculaires caractérisées par une dégénérescence des neurones moteurs de la come antérieure de la moelle épinière, responsable d'une paralysie musculaire progressive, avec amyotrophie. Leur fréquence les place au 2e rang des affections récessives autosomiques fatales, après la mucoviscidose (incidence 1/6000). Une stratégie dite de génétique inverse a permis l'identification du gène responsable des SMA. Ce gène, appelé SMN (survival motor neurone), code pour une protéine sans homologie avec des protéines de fonction connue. Il est désormais possible d'appréhender les bases moléculaires de la dégénérescence motoneuronale qui caractérise ces maladies.