title: Correlation between SMN2 copy number and clinical phenotype of spinal muscular atrophy: three SMN2 copies fail to rescue some patients from the disease severity.
abstract: Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder that is characterized by degeneration of the anterior horn cells of the spinal cord, which leads to the axial and limb weakness associated with muscle atrophy. SMA is classified into three groups based on the clinical severity: type I (severe), type II (intermediate) and type III (mild). All three clinical subtypes of SMA are caused by mutations of the SMN1 gene. More than 95 % of SMA patients show homozygous deletion of SMN1. It is thought that SMN2, which is a highly homologous gene of SMN1, compensates for the SMN1 deletion to some degree. To clarify the relationship between SMN2 and the disease severity of SMA, we performed fluorescence-based quantitative polymerase chain reaction assay of the copy number of SMN2 in 27 patients (11 type I and 16 type II-III) homozygous for SMN1 deletion. The SMN2 copy number in type II-III patients was 3.1 +/- 0.3 (mean +/- SD), which is significantly higher than that observed in type I patients, 2.2 +/- 0.6 (P < 0.01). However, three of the 11 type I patients carried 3 SMN2 copies. A type I patient with 3 SMN2 copies was studied further. RT-PCR analysis of the patient showed a trace of full-length SMN2 mRNA species, but a large amount of the truncated SMN2 mRNA species lacking exon 7. In conclusion, SMN2 alleles are not functionally equivalent among SMA patients, although in general the SMN2 copy number is correlated with the severity of SMA. Genetic background influencing splicing mechanisms of the SMN2 gene may be more critical in some SMA patients.
authors.affiliation: Division of Public Health, Department of Environmental Health and Safety, Faculty of Medicine, Kobe University Graduate School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe, 650-0017, Japan.
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Harada
authors.forename: Retno
authors.initials: R
authors.lastname: Sutomo
authors.forename: Ahmad Hamim
authors.initials: AH
authors.lastname: Sadewa
authors.forename: Tomoko
authors.initials: T
authors.lastname: Akutsu
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Takeshima
authors.forename: Hiroko
authors.initials: H
authors.lastname: Wada
authors.forename: Masafumi
authors.initials: M
authors.lastname: Matsuo
authors.forename: Hisahide
authors.initials: H
authors.lastname: Nishio
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D017362:Cyclic AMP Response Element-Binding Protein; D005260:Female; D006239:Haplotypes / Q000235:genetics; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D009134:Muscular Atrophy, Spinal / Q000235:genetics*; D009419:Nerve Tissue Proteins / Q000235:genetics*; D010375:Pedigree; D010641:Phenotype; D012333:RNA, Messenger / Q000235:genetics; D016601:RNA-Binding Proteins; D055532:SMN Complex Proteins; D055533:Survival of Motor Neuron 1 Protein; D055540:Survival of Motor Neuron 2 Protein
chemical_list: D017362:Cyclic AMP Response Element-Binding Protein; D009419:Nerve Tissue Proteins; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D055532:SMN Complex Proteins; C526471:SMN1 protein, human; C526477:SMN2 protein, human; D055533:Survival of Motor Neuron 1 Protein; D055540:Survival of Motor Neuron 2 Protein