Poster abstracts

Poster number 90 submitted by Chitra Iyer

Increasing SMN in neurons is sufficient to rescue a severe mouse model of Spinal muscular atrophy

Chitra C. Iyer (Department of Molecular and Cellular Biochemistry, The Ohio State University, Columbus OH, USA), Vicki L. McGovern (Department of Molecular and Cellular Biochemistry, The Ohio State University, Columbus OH, USA), W. David Arnold (Department of Neurology, The Ohio State University, Columbus OH, USA), Jason D. Murray, Paul M. Janssen (Department of Physiology and Cell Biology, The Ohio State University, Columbus OH, USA), Brian K. Kaspar (Center for Gene Therapy, The Research Institute at Nationwide Childrens Hospital, Columbus OH, USA), Arthur H.M. Burghes (Department of Molecular and Cellular Biochemistry, Department of Neurology, Department of Molecular Genetics, The Ohio State University, Columbus OH, USA)

Abstract:
Spinal Muscular Atrophy (SMA) is an autosomal recessive disorder characterized by loss of lower motor neurons.It occurs due to deletion or mutation in the SMN1 gene and retention of the SMN2 gene.SMA is caused by a decrease in the levels of the SMN protein; complete absence of SMN is lethal.SMN is ubiquitous but a decrease in SMN causes degeneration of the motor neurons (MNs) and muscle atrophy.Hence understanding the spatial requirement of SMN is critical to localize the functional importance of SMN and for the development of therapies.Using tissue specific Cre-drivers and the Cre-loxP recombination system, we deleted mouse Smn in just the neurons or just the muscle; with SMN2 transgene in the background providing low SMN everywhere.As a reciprocal experiment, we restored normal levels of SMN in neurons or muscle with low SMN levels in other tissues.We observed that decreasing SMN in muscle has no phenotypic effect.This was corroborated by muscle function studies (twitch, tetanic and eccentric force).Replacement of Smn in muscle did not rescue SMA mice.Thus the muscle does not require high levels of SMN.SMN from two copies of SMN2 in the background is sufficient for normal muscle function.Deletion of Smn with Human Synapsin-iCre (all neurons plus vasculature) gave a phenotype similar to SMA while replacement gave rescue of the phenotype.Deletion of Smn with Nestin-Cre (all neurons, but weak in MNs) or with ChAT-Cre (all MNs) did not give an SMA phenotype and replacement did not rescue.Nestin-Cre together with ChAT-Cre (thus covering all neurons) gave an SMA phenotype upon deletion and rescue upon replacement.Electrophysiological measurements of the motor unit (MUNE: motor unit number estimate and CMAP: compound motor action potential) were performed on the deletion and rescue lines.These, along with weight and survival curves of deletion and replacement experiments show that SMN is a must in MNs.Hence increasing SMN in neurons is sufficient to rescue SMA mice.

References:
1. Burghes A.H.,and Beattie C.E. (2009) Spinal muscular atrophy : Why do low levels of survival motor neuron protein make motor neurons sick?, Nature Reviews. Neuroscience 10, 597-609.

2. Gavrilina, T. O., McGovern, V. L., Workman, E., Crawford, T. O., Gogliotti, R. G., DiDonato, C. J., Monani, U. R., Morris, G. E., and Burghes, A. H. (2008) Neuronal SMN expression corrects spinal muscular atrophy in severe SMA mice while muscle-specific SMN expression has no phenotypic effect, Human molecular genetics 17, 1063-1075.

Keywords: SMA, spinal muscular atrophy, MNs, motor neurons