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Background  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.    Methods  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.    Results  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.    Conclusions  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.

Genetic and expression studies of SMN2 gene in Russian patients with spinal muscular atrophy type II and III