633. A Reproducible and Reliable Non-Invasive Approach to Assess the Efficacy of Gene Therapy in mdx Mice with Relevance to Clinical Trials

The mdx mouse carries a naturally occurring nonsense mutation in the dystrophin gene and has become a critical animal model for research on Duchenne Muscular Dystrophy (DMD). Despite the extensive published studies on muscle structure and function in this model, we still lack appropriate protocols for evaluation of functional rescue in mdx mice using non-invasive tests relevant to the clinically relevant symptoms of DMD. Here we present for the first time results of a complex approach for the evaluation of locomotor and behavioral patterns of mdx mice during running wheel performance in a modified open field monitoring system. These results are examined in the same mice with force grip measurements and serum enzyme biomarker creatine kinase (CK) measurement. The comparison between juvenile congenic wild type (C57Bl10) and mdx mice indicates similarity in parameters such as rest time and the number of times entering and initiating use of the running wheel per day, but highly significant differences with regard to running wheel-associated locomotor activity characteristics including velocity, time, distance per run, and total distance per day. This suggests that exploratory behavior at low work rate is unaffected by dystrophin deficiency, but that the animals respond to as yet uncharacterized sensory inputs that limit their running speed and duration without preventing the desire to re-initiate after rest. This may be viewed as analogous to the early loss of function and metabolic reserve in the juvenile period of DMD disease progression, where boys are described by parents as frustrated at first being unable to keep pace with their friends. As presented in another poster by our group, blinded studies of the response to gene therapy indicates that it is unlikely that this phenotypic difference reflects the mislocalization of neuronal nitric oxide synthase (nNOS) in dystrophin deficiency. In further blinded studies, the open field data correlate with the differences in testing results between the two mouse strains observed in the force grip evaluation and serum levels of CK, offering possible leads for a further mechanistic dissection. These results demonstrate strong evidence in support of the running wheel open field system as a reliable and reproducible approach to assess the therapeutic efficacy in the mouse model of DMD. This approach also provides a platform for dissecting the physiological roles of dystrophin in supporting precisely measurable volitional activities in unrestricted animals, thereby offering potential improvements in the predictive power of preclinical studies of therapeutic efficacy to inform clinical trials.