A Comparison of Three Electrophysiological Methods for the Assessment of Disease Status in a Mild Spinal Muscular Atrophy Mouse Model

Jia Li,Glenn D Rosen,Phillip G Zaworski,Seward B Rutkove,William D Arnold,Tom R Geisbush

doi:10.1371/journal.pone.0111428

Abstract

ObjectivesThere is a need for better, noninvasive quantitative biomarkers for assessing the rate of progression and possible response to therapy in spinal muscular atrophy (SMA). In this study, we compared three electrophysiological measures: compound muscle action potential (CMAP) amplitude, motor unit number estimate (MUNE), and electrical impedance myography (EIM) 50 kHz phase values in a mild mouse model of spinal muscular atrophy, the Smn1c/c mouse.MethodsSmn1c/c mice (N = 11) and wild type (WT) animals (−/−, N = 13) were measured on average triweekly until approximately 1 year of age. Measurements included CMAP, EIM, and MUNE of the gastrocnemius muscle as well as weight and front paw grip strength. At the time of sacrifice at one year, additional analyses were performed on the animals including serum survival motor neuron (SMN) protein levels and muscle fiber size.ResultsBoth EIM 50 kHz phase and CMAP showed strong differences between WT and SMA animals (repeated measures 2-way ANOVA, P<0.0001 for both) whereas MUNE did not. Both body weight and EIM showed differences in the trajectory over time (p<0.001 and p = 0.005, respectively). At the time of sacrifice at one year, EIM values correlated to motor neuron counts in the spinal cord and SMN levels across both groups of animals (r = 0.41, p = 0.047 and r = 0.57, p = 0.003, respectively), while CMAP did not. Motor neuron number in Smn1c/c mice was not significantly reduced compared to WT animals.ConclusionsEIM appears sensitive to muscle status in this mild animal model of SMA. The lack of a reduction in MUNE or motor neuron number but reduced EIM and CMAP values support that much of the pathology in these animals is distal to the cell body, likely at the neuromuscular junction or the muscle itself.

Highlights

A variety of mouse models of spinal muscular atrophy (SMA) have been developed over the past decade [1]
The impetus underlying the development of these various models is based on an effort to recapitulate the marked varying human disease severities, with the SMND7 model mimicking Type 1 SMA and Smn1c/c mimicking SMA Type 3
A variety of such biomarkers have been explored in both animals and humans, including serological biomarkers measures such as survival motor neuron (SMN) expression [8], functional measures [9], muscle imaging [10,11], as well as electrophysiologic markers, including motor unit number estimate (MUNE) and compound motor action potential amplitude (CMAP) [12,13,14]

Summary

Introduction

A variety of mouse models of spinal muscular atrophy (SMA) have been developed over the past decade [1] These include animals with relatively severe phenotypes, such as the survival motor neuron negative (Smn2/2), Smn2+/+, and SMND7 models [2,3,4], all of which die within the first 3 weeks of life, to relatively long lived models, including the Smn1c/c mouse, which develops only subtle motor deficits [5]. The impetus underlying the development of these various models is based on an effort to recapitulate the marked varying human disease severities, with the SMND7 model mimicking Type 1 SMA and Smn1c/c mimicking SMA Type 3. Since EIM is non-invasive, painless, and simple to apply, it may be especially useful in young children and is currently being investigated as a part of a multicenter study

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