The human centromeric survival motor neuron gene (SMN2) rescues embryonic lethality in Smn-/- mice and results in a mouse with spinal muscular atrophy

Abstract

Proximal spinal muscular atrophy (SMA) is a common motor neuron disease in humans and in its most severe form causes death by the age of 2 years. It is caused by defects in the telomeric survival motor neuron gene ( SMN1 ), but patients retain at least one copy of a highly homologous gene, centromeric SMN ( SMN2 ). Mice possess only one survival motor neuron gene ( Smn ) whose loss is embryonic lethal. Therefore, to obtain a mouse model of SMA we created transgenic mice that express human SMN2 and mated these onto the null Smn (-/-)background. We show that Smn (-/-); SMN2 mice carrying one or two copies of the transgene have normal numbers of motor neurons at birth, but vastly reduced numbers by postnatal day 5, and subsequently die. This closely resembles a severe type I SMA phenotype in humans and is the first report of an animal model of the disease. Eight copies of the transgene rescues this phenotype in the mice indicating that phenotypic severity can be modulated by SMN2 copy number. These results show that SMA is caused by insufficient SMN production by the SMN2 gene and that increased expression of the SMN2 gene may provide a strategy for treating SMA patients.

Baseline results of the NeuroNEXT spinal muscular atrophy infant biomarker study

“…Disease severity spans a wide range of phenotypes divided into five categories based upon maximal motor function: type 0, (neonates who present with severe hypotonia often with history of decreased fetal movements), type 1 (never sit independently), type 2 (sit but never stand independently), type 3 (ambulatory children), and type 4 (ambulatory adults) 9, 1011, 12, 13, 14. Thus, SMN2 copy number is a prognostic biomarker that predicts future clinical outcome.…”

An in vivo reporter system for measuring increased inclusion of exon 7 in SMN2 mRNA: potential therapy of SMA

“…Mice with eight copies of human SMN2 are phenotypically normal. [10][11][12][13] The promoters, introns, and flanking regions for SMN1 and SMN2 have been completely sequenced and are almost identical. Transcription assays comparing the SMN promoters implied that the two are transcriptionally equivalent.…”

Solving the puzzle of spinal muscular atrophy: What are the missing pieces?

“…However, other studies did not confirm these findings [Lutz et al, 2011]. Arguing against the existence of a therapeutic window is the observation that the loss of MNs is a late and end-stage event in SMA mouse models [Monani et al, 2000;Cifuentes Diaz et al, 2002]. If suitably stimulated, and if the dying back process has not reached the point of no return (but what this point would be is not known so far), surviving MNs could hopefully still be able to provide appropriate innervation to muscle fibers.…”

“…The 35.5 kb SMN2 transgene recapitulates normal SMN expression patterns in SMA mice suggesting that the $4.1 kb sequence upstream of the translation initiation site is sufficient for normal expression in vivo [Monani et al, 2000]. Both the human and mouse promoters have been systematically interrogated in a variety of cell types; however, most of this work has concentrated on the core promoter region [Echaniz-Laguna et al, 1999, Monani et al, 1999Germain-Desprez et al, 2001;Rouget et al, 2005].…”

“…While SMN expression diminishes after birth, it remains high in spinal motor neurons even into adult life [Tizzano et al, 1998; Bechade et al, 1999;Pagliardini et al, 2000;Giavazzi et al, 2006]. Except for humans, expression patterns relate to the SMN1 gene; in humans, it is difficult to associate SMN expression with one or the other copy gene so it is unclear whether regulation of the SMN1 and SMN2 genes is identical.The 35.5 kb SMN2 transgene recapitulates normal SMN expression patterns in SMA mice suggesting that the $4.1 kb sequence upstream of the translation initiation site is sufficient for normal expression in vivo [Monani et al, 2000]. Both the human and mouse promoters have been systematically interrogated in a variety of cell types; however, most of this work has concentrated on the core promoter region …”