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

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.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 is one of the leading genetic causes of infant mortality, with an incidence of about 1 in 8000 live births and with a carrier frequency of about 1 in 40 [1]. 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. Approximately 95 percent of these patients have a homozygous deletion of exon 7 in the SMN1 gene. Small intragenic mutations have been found in the remaining affected individuals who do not lack both copies of SMN1 [3]. SMN2 is an almost identical centromeric SMN1 copy, with 8 nucleotide substitutions: five in the introns and three in exons 6, 7 and 8 [4]. Single-nucleotide tra