Non-invasive MRI and spectroscopy of mdx mice reveal temporal changes in dystrophic muscle imaging and in energy deficits

Abstract

In Duchenne muscular dystrophy (DMD), a genetic disruption of dystrophin protein expression results in repeated muscle injury and chronic inflammation. Magnetic resonance imaging shows promise as a surrogate outcome measure in both DMD and rehabilitation medicine that is capable of predicting clinical benefit years in advance of functional outcome measures. The mdx mouse reproduces the dystrophin deficiency that causes DMD and is routinely used for preclinical drug testing. There is a need to develop sensitive, non-invasive outcome measures in the mdx model that can be readily translatable to human clinical trials. Here we report the use of magnetic resonance imaging and spectroscopy techniques for the non-invasive monitoring of muscle damage in mdx mice. Using these techniques, we studied dystrophic mdx muscle in mice from 6 to 12 weeks of age, examining both the peak disease phase and natural recovery phase of the mdx disease course. T2 and fat-suppressed imaging revealed significant levels of tissue with elevated signal intensity in mdx hindlimb muscles at all ages; spectroscopy revealed a significant deficiency of energy metabolites in 6-week-old mdx mice. As the mdx mice progressed from the peak disease stage to the recovery stage of disease, each of these phenotypes was either eliminated or reduced, and the cross-sectional area of the mdx muscle was significantly increased when compared to that of wild-type mice. Histology indicates that hyper-intense MRI foci correspond to areas of dystrophic lesions containing inflammation as well as regenerating, degenerating and hypertrophied myofibers. Statistical sample size calculations provide several robust measures with the ability to detect intervention effects using small numbers of animals. These data establish a framework for further imaging or preclinical studies, and they support the development of MRI as a sensitive, non-invasive outcome measure for muscular dystrophy. CRH is supported by the National Institutes of Health's (http://www.nih.gov/) 5T32AR056993 and 5R24HD050846-02 grants. PW and this work were supported in part by the National Institutes of Health's G12MD007597 and United States Army Medical Research and Materiel Command (http://mrmc.amedd. army.mil/) W81XWH 10-1-0767 grants. KN is supported by National Institutes of Health's K26OD011171 and P50AR060836 grants, a Muscular Dystrophy Association (http://mda.org/) translational grant 30000783/4736, and the United States Department of Defense (http://www.defense.gov/) grants W81XWH-05-1- 0659, W81XWH-11-1-0782, W81XWH-11-1-0330, and W81XWH-11-1-0782. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.