Characterization of survival motor neuron (SMN(T)) gene deletions in asymptomatic carriers of spinal muscular atrophy

Department of Genetics and Development, College of Physicians and Surgeons at Columbia University, New York, NY 10032, USA.
Human Molecular Genetics (Impact Factor: 6.39). 04/1996; 5(3):359-65. DOI: 10.1093/hmg/5.3.359
Source: PubMed


Previous reports have established that the telomeric copy of the survival motor neuron (SMNT) gene and the intact copy of the neuronal apoptosis inhibitory protein (NAIP) gene are preferentially deleted in patients with spinal muscular atrophy (SMA). Although deletions or mutations in the SMNT gene are most highly correlated with SMA, it is not clear to what extent NAIP or other genes influence the SMA phenotype, or whether a small fraction of SMA patients actually have functional copies of both SMNT and NAIP. To evaluate further the part of SMNT in the development of SMA, we analyzed 280 asymptomatic SMA family members for the presence or absence of SMNT exons 7 and 8. We report the following observations: (i) 4% of the sample harbored a polymorphic variant of SMNT exon 7 that looks like a homozygous deletion; (ii) approximately 1% of the parents are homozygously deleted for both exons 7 and 8; (iii) one asymptomatic parent lacking both copies of SMNT exons 7 and 8 displays a 'subclinical phenotype' characterized by mild neurogenic pathology; (iv) another asymptomatic parent lacking both SMNT exons showed no signs of motor neuron disorder by clinical and neurodiagnostic analyses. The demonstration of polymorphic variants of exon 7 that masquerade as homozygous nulls, and the identification of SMA parents who harbor two disease alleles, serve as a caution to those conducting prenatal tests with these markers.

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    • "Although SMA-affected siblings usually develop similar disease severity in terms of their age at onset and the progression of disease [131], a small proportion of individuals with homozygous SMN1 mutation are fully asymptomatic despite carrying an identical number of SMN2 copies as their affected siblings, suggesting the influence of modifier genes [132, 133]. The first potential SMN-independent disease modifier, plastin-3, was recently identified from six SMA-discordant families with eight fully asymptomatic females who had inherited the same SMN1 and SMN2 alleles as their affected siblings [18]. "
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    ABSTRACT: Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder, leading to progressive muscle weakness, atrophy, and sometimes premature death. SMA is caused by mutation or deletion of the survival motor neuron-1 (SMN1) gene. An effective treatment does not presently exist. Since the severity of the SMA phenotype is inversely correlated with expression levels of SMN, the SMN-encoded protein, SMN is the most important therapeutic target for development of an effective treatment for SMA. In recent years, numerous SMN independent targets and therapeutic strategies have been demonstrated to have potential roles in SMA treatment. For example, some neurotrophic, antiapoptotic, and myotrophic factors are able to promote survival of motor neurons or improve muscle strength shown in SMA mouse models or clinical trials. Plastin-3, cpg15, and a Rho-kinase inhibitor regulate axonal dynamics and might reduce the influences of SMN depletion in disarrangement of neuromuscular junction. Stem cell transplantation in SMA model mice resulted in improvement of motor behaviors and extension of survival, likely from trophic support. Although most therapies are still under investigation, these nonclassical treatments might provide an adjunctive method for future SMA therapy.
    Full-text · Article · May 2012 · Neural Plasticity
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    • "The importance of the SMN2 gene copy number determination can be further illustrated by our findings in families that have unaffected relatives with a homozygous loss of the SMN1 gene. Being an exceptionally rare event, homozygous deletion of SMN1 exon 7 and 8 in unaffected relatives of SMA patients has been reported several times in American, German and Polish SMA families [21-24]. In this study we were able, for the first time, to report two cases of asymptomatic individuals from Russian SMA families. "
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    ABSTRACT: 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. 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. 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. 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.
    Full-text · Article · Jul 2011 · BMC Medical Genetics
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    • "Interestingly, there are some patients diagnosed as having Kugelberg-Welander disease (KW; also known as "spinal muscular atrophy type III") who have slightly elevated serum CK levels and rimmed vacuoles in their muscle biopsies, thus presenting with a phenotype resembling that of the LGMD family (LG61) described here (Fukahara et al. 1980; Aubry et al. 1995; Lefebvre et al. 1995; Rodrigues et al. 1996). In most of the KW patients, the disease is due to mutations in the survival motor-neuron (SMN) gene; however, in 5%-10% of these patients, KW seems to be caused by mutations at another locus (Aubry et al. 1995; Lefebvre et al. 1995; Rodrigues et al. 1996; Wang et al. 1996). The possibility that the pathology in these cases also is caused by mutations in the LGMD2G gene should be considered. "
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    ABSTRACT: The group of autosomal recessive (AR) muscular dystrophies includes, among others, two main clinical entities, the limb-girdle muscular dystrophies (LGMDs) and the distal muscular dystrophies. The former are characterized mainly by muscle wasting of the upper and lower limbs, with a wide range of clinical severity. This clinical heterogeneity has been demonstrated at the molecular level, since the genes for six AR forms have been cloned and/or have been mapped to 15q15.1 (LGMD2A), 2p12-16 (LGMD2B), 13q12 (LGMD2C), 17q12-q21.33 (LGMD2D),4q12 (LGMD2E), and 5q33-34 (LGMD2F). The AR distal muscular dystrophies originally included two subgroups, Miyoshi myopathy, characterized mainly by extremely elevated serum creatine kinase (CK) activity and by a dystrophic muscle pattern, and Nonaka myopathy, which is distinct from the others because of the normal to slightly elevated serum CK levels and a myopathic muscle pattern with rimmed vacuoles. With regard to our unclassified AR LGMD families, analysis of the affected sibs from one of them (family LG61) revealed some clinical and laboratory findings (early involvement of the distal muscles, mildly elevated serum CK levels, and rimmed vacuoles in muscle biopsies) that usually are not observed in the analysis of patients with LGMD2A-LGMD2F. In the present investigation, through a genomewide search in family LG61, we demonstrated linkage of the allele causing this form of muscular dystrophy to a 3-cM region on 17q11-12. We suggest that this form, which, interestingly, clinically resembles AR Kugelberg-Welander disease, should be classified as LGMD2G. In addition, our results indicate the existence of still another locus causing severe LGMD.
    Full-text · Article · Aug 1997 · The American Journal of Human Genetics
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