Location score and haplotype analyses of the locus for autosomal recessive spastic ataxia of Charlevoix-Saguenay, in chromosome region 13q11.
Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a clinically homogeneous form of early-onset familial spastic ataxia with prominent myelinated retinal nerve fibers. More than 300 patients have been identified, and most of their families originated in the Charlevoix-Saguenay region of northeastern Quebec, where the carrier prevalence has been estimated to be 1/22. Consistent with the hypothesis of a founder effect, we observed excess shared homozygosity at 13q11, among patients in a genomewide scan of 12 families. Analysis of 19 pedigrees demonstrated very tight linkage between the ARSACS locus and an intragenic polymorphism of the gamma-sarcoglycan (SGCG) gene, but genomic DNA sequence analysis of all eight exons of SGCG revealed no disease-causing mutation. On the basis of haplotypes composed of seven marker loci that spanned 11.1 cM, the most likely position of the ARSACS locus was 0.42 cM distal to the SGCG polymorphism. Two groups of ARSACS-associated haplotypes were identified: a large group that carries a common SGCG allele and a small group that carries a rare SGCG allele. The haplotype groups do not appear to be closely related. Therefore, although chromosomes within each haplotype group may harbor a single ARSACS mutation identical by descent, the two mutations could have independent origins. (+info)
Ecto-ATPase activity of alpha-sarcoglycan (adhalin).
alpha-Sarcoglycan is a component of the sarcoglycan complex of dystrophin-associated proteins. Mutations of any of the sarcoglycan genes cause specific forms of muscular dystrophies, collectively termed sarcoglycanopathies. Importantly, a deficiency of any specific sarcoglycan affects the expression of the others. Thus, it appears that the lack of sarcoglycans deprives the muscle cell of an essential, yet unknown function. In the present study, we provide evidence for an ecto-ATPase activity of alpha-sarcoglycan. alpha-Sarcoglycan binds ATP in a Mg2+-dependent and Ca2+-independent manner. The binding is inhibited by 3'-O-(4-benzoyl)benzoyl ATP and ADP. Sequence analysis reveals the existence of a consensus site for nucleotide binding in the extracellular domain of the protein. An antibody against this sequence inhibits the binding of ATP. A dystrophin.dystrophin-associated protein preparation demonstrates a Mg-ATPase activity that is inhibited by the antibody but not by inhibitors of endo-ATPases. In addition, we demonstrate the presence in the sarcolemmal membrane of a P2X-type purinergic receptor. These data suggest that alpha-sarcoglycan may modulate the activity of P2X receptors by buffering the extracellular ATP concentration. The absence of alpha-sarcoglycan in sarcoglycanopathies leaves elevated the concentration of extracellular ATP and the persistent activation of P2X receptors, leading to intracellular Ca2+ overload and muscle fiber death. (+info)
Membrane targeting and stabilization of sarcospan is mediated by the sarcoglycan subcomplex.
The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that spans the muscle plasma membrane and forms a link between the F-actin cytoskeleton and the extracellular matrix. The proteins of the DGC are structurally organized into distinct subcomplexes, and genetic mutations in many individual components are manifested as muscular dystrophy. We recently identified a unique tetraspan-like dystrophin-associated protein, which we have named sarcospan (SPN) for its multiple sarcolemma spanning domains (Crosbie, R.H., J. Heighway, D.P. Venzke, J.C. Lee, and K.P. Campbell. 1997. J. Biol. Chem. 272:31221-31224). To probe molecular associations of SPN within the DGC, we investigated SPN expression in normal muscle as a baseline for comparison to SPN's expression in animal models of muscular dystrophy. We show that, in addition to its sarcolemma localization, SPN is enriched at the myotendinous junction (MTJ) and neuromuscular junction (NMJ), where it is a component of both the dystrophin- and utrophin-glycoprotein complexes. We demonstrate that SPN is preferentially associated with the sarcoglycan (SG) subcomplex, and this interaction is critical for stable localization of SPN to the sarcolemma, NMJ, and MTJ. Our experiments indicate that assembly of the SG subcomplex is a prerequisite for targeting SPN to the sarcolemma. In addition, the SG- SPN subcomplex functions to stabilize alpha-dystroglycan to the muscle plasma membrane. Taken together, our data provide important information about assembly and function of the SG-SPN subcomplex. (+info)
Further evidence for the organisation of the four sarcoglycans proteins within the dystrophin-glycoprotein complex.
Based on the pattern of distribution of the SG proteins in patients with LGMD2C and 2D, and on the observed decreased abundance of dystrophin through WB in some sarcoglycans (SG) patients, we have recently suggested that alpha, beta and delta subunits of sarcoglycan complex might be more closely associated and that gamma-SG might interact more directly with dystrophin. Two additional SG patients here reported give further support to these suggestions: an LGMD2F patient showed patchy labelling for gamma-SG, despite the lack of staining of the other three SG proteins; an LGMD2C boy showed deficiency in dystrophin by means of WB and IF, comparable with an DMD manifesting carrier. These two patients represent further evidence of a closer relation of alpha, beta and delta-SG than of gamma-SG and of the possible association of gamma-SG with dystrophin. In addition the LGMD2C patient illustrates the potential risk of misdiagnosis using only dystrophin analysis, in cases with no positive family history, or when DNA analysis is not informative. (+info)
Delineation of genomic deletion in cardiomyopathic hamster.
Cardiomyopathic hamster is a representative animal model for autosomal recessive cardiomyopathy. We have previously shown that the transcript of delta-sarcoglycan is missing in the heart of cardiomyopathic hamster due to genomic deletion. Here we define the normal genomic region deleted in cardiomyopathic hamster, which spans about 30 kb interval and includes the two first exons of the delta-sarcoglycan gene. RNA blot analysis using genomic DNA fragments covering the entire deletion as probes failed to detect any transcript other than delta-sarcoglycan in normal hamster heart, suggesting that delta-sarcoglycan is the only transcript defective in the heart of cardiomyopathic hamster. (+info)
rAAV vector-mediated sarcogylcan gene transfer in a hamster model for limb girdle muscular dystrophy.
The limb girdle muscular dystrophies (LGMD) are a genetically and phenotypically heterogeneous group of degenerative neuromuscular diseases. A subset of the genetically recessive forms of LGMD are caused by mutations in the four muscle sarcoglycan genes (alpha, beta, gamma and delta). The coding sequences of all known sarcoglycan genes are smaller than 2 kb, and thus can be readily packaged in recombinant adeno-associated virus (rAAV) vectors. Previously, we have demonstrated highly efficient and sustained transduction in mature muscle tissue of immunocompetent animals with rAAV vectors. In this report, we utilize recombinant AAV containing the delta-sarcoglycan gene for genetic complementation of muscle diseases using a delta-sarcoglycan-deficient hamster model (Bio 14.6). We show efficient delivery and widespread expression of delta-sarcoglycan after a single intramuscular injection. Importantly, rAAV vector containing the human delta-sarcoglycan cDNA restored secondary biochemical deficiencies, with correct localization of other sarcoglycan proteins to the muscle fiber membrane. Interestingly, restoration of alpha-, as well as beta-sarcoglycan was homogeneous and properly localized throughout transduced muscle, and appeared unaffected by dramatic overexpression of delta-sarcoglycan in the cytoplasm of some myofibers. These results support the feasibility of rAAV vector's application to treat LGMD by means of direct in vivo gene transfer. (+info)
alpha-Dystroglycan is a laminin receptor involved in extracellular matrix assembly on myotubes and muscle cell viability.
alpha-Dystroglycan (alpha-DG) is a laminin-binding protein and member of a glycoprotein complex associated with dystrophin that has been implicated in the etiology of several muscular dystrophies. To study the function of DG, C2 myoblasts were transfected stably with an antisense DG expression construct. Myotubes from two resulting clones (11F and 11E) had at least a 40-50% and 80-90% reduction, respectively, in alpha-DG but normal or near normal levels of alpha-sarcoglycan, integrin beta1 subunit, acetylcholine receptors (AChRs), and muscle-specific kinase (MuSK) when compared with parental C2 cells or three clones (11A, 9B, and 10C) which went through the same transfection and selection procedures but expressed normal levels of alpha-DG. Antisense DG-expressing myoblasts proliferate at the same rate as parental C2 cells and differentiate into myotubes, however, a gradual loss of cells was observed in these cultures. This loss correlates with increased apoptosis as indicated by greater numbers of nuclei with condensed chromatin and more nuclei labeled by the TUNEL method. Moreover, there was no sign of increased membrane permeability to Trypan blue as would be expected with necrosis. Unlike parental C2 myotubes, 11F and 11E myotubes had very little laminin (LN) on their surfaces; LN instead tended to accumulate on the substratum between myotubes. Exogenous LN bound to C2 myotubes and was redistributed into plaques along with alpha-DG on their surfaces but far fewer LN/alpha-DG plaques were seen after LN addition to 11F or 11E myotubes. These results suggest that alpha-DG is a functional LN receptor in situ which is required for deposition of LN on the cell and, further, implicate alpha-DG in the maintenance of myotube viability. (+info)
Statistically significant differences in the number of CD24 positive muscle fibers and satellite cells between sarcoglycanopathy and age-matched Becker muscular dystrophy patients.
OBJECT: The aim of this study was to reveal variations in the patterns of expression of the cell surface proteins in regenerating fibers and those in the number of satellite cells to gain an understanding of the pathological processes involved in sarcoglycanopathy. METHODS: We have reported that there is a reduction of the beta-1 subunit of laminin, heparan sulfate proteoglycan (HSPG), and HCAM (CD44) in Japanese patients with sarcoglycanopathy. Here, we investigated immunohistochemically the expression of the neural cell adhesion molecule (NCAM), which is a marker for human regenerating muscle and satellite cell, and CD24, which appears to be expressed in the early stages of the regeneration process. PATIENTS: We investigated six Japanese patients with sarcoglycanopathy, and compared to age-matched Becker muscular dystrophy. RESULTS: We found that the incidences of muscle fibers with increased NCAM were not statistically different between the two groups. However, the incidences of muscle fibers with increased CD24 and those of NCAM positive satellite cells were very low in sarcoglycanopathy and were statistically different between sarcoglycanopathy and age-matched Becker muscular dystrophies. CONCLUSION: The poor expression of CD24 and the fewer satellite cells in sarcoglycanopathy without significant difference in the number of total regenerating fibers suggest that a different regeneration process is involved in sarcoglycanopathy compared to that in other types of muscular dystrophy. (+info)