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

Galina Yu Zheleznyakova,Anton V Kiselev,Helgi B Schiöth,Anna A Egorova,Rohit Chavan,Viktor G Vakharlovsky,Vladislav S Baranov,Mathias Rask-Andersen

doi:10.1186/1471-2350-12-96

Abstract

BackgroundSpinal 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.MethodsIn 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.ResultsComparison 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.ConclusionsWe 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.

Highlights

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)
DNA samples from the SMA patients carrying between one and four SMN2 copies were used as controls for the analysis
We found that the sample-to-sample variation after relative quantification analysis for the two SMN2 gene copies per genome was in the range of 0.73-1.32; for three copies per genome it was in range of 1.27-1.72; and for four copies per genome it was in range of 1.712.2

Summary

Introduction

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. 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. Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by degeneration of alpha motor neurons in the anterior horns of the spinal cord. SMA subdivides into types I, II, and III based on age of onset and clinical severity [2], and are all caused by mutations within the survival motor neuron gene (SMN1) located on chromosome 5q13. 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 [6,7,8]

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