The Effects of Low Levels of Dystrophin on Mouse Muscle Function and Pathology

Abstract

Duchenne muscular dystrophy (DMD) is a severe progressive muscular disorder caused by reading frame disrupting mutations in the DMD gene, preventing the synthesis of functional dystrophin. As dystrophin provides muscle fiber stability during contractions, dystrophin negative fibers are prone to exercise-induced damage. Upon exhaustion of the regenerative capacity, fibers will be replaced by fibrotic and fat tissue resulting in a progressive loss of function eventually leading to death in the early thirties. With several promising approaches for the treatment of DMD aiming at dystrophin restoration in clinical trials, there is an increasing need to determine more precisely which dystrophin levels are sufficient to restore muscle fiber integrity, protect against muscle damage and improve muscle function.To address this we generated a new mouse model (mdx-Xist Δhs) with varying, low dystrophin levels (3–47%, mean 22.7%, stdev 12.1, n = 24) due to skewed X-inactivation. Longitudinal sections revealed that within individual fibers, some nuclei did and some did not express dystrophin, resulting in a random, mosaic pattern of dystrophin expression within fibers. Mdx-Xist Δhs, mdx and wild type females underwent a 12 week functional test regime consisting of different tests to assess muscle function at base line, or after chronic treadmill running exercise. Overall, mdx-Xist Δhs mice with 3–14% dystrophin outperformed mdx mice in the functional tests. Improved histopathology was observed in mice with 15–29% dystrophin and these levels also resulted in normalized expression of pro-inflammatory biomarker genes, while for other parameters >30% of dystrophin was needed. Chronic exercise clearly worsened pathology, which needed dystrophin levels >20% for protection. Based on these findings, we conclude that while even dystrophin levels below 15% can improve pathology and performance, levels of >20% are needed to fully protect muscle fibers from exercise-induced damage.

Retracted: ‘The effects of Duchenne muscular dystrophy on the performance of the stomatognathic system: case–control study’, by Ferreira B., Da Silva G.P., Gonçalves C.R., et al.

“…1,2 The symptoms of this disease occur in infancy, with the necrosis of skeletal muscle fibers, muscle fibrosis, and the infiltration of adipose tissue. 3,4 In dystrophic muscle, the degenerative process occurs by oxidative stress and the loss of homeostasis combined with the synthesis of free radicals that contribute to skeletal muscle necrosis.…”

Adaptive Immune Response Impairs the Efficacy of Autologous Transplantation of Engineered Stem Cells in Dystrophic Dogs

“…[21][22][23][24][25] Moreover, low dystrophin levels in transgenic mice models and patient cohorts have been demonstrated to bring pathological and functional benefits, at least slowing down or preventing disease development. 25,26 In addition, the contribution from soluble paracrine molecules secreted from the injected cells rather than from the presence on dystrophin has to be mentioned. It is likely that in the first year of treatment the amelioration seen in the GRMD dogs injected with autologous LV ex6-8 133+musSCs was due to two uneven components: (i) a burst release of stimulating and restoring muscle factors from injected cells and the recipient tissue, and (ii) an initial low dystrophin expression.…”

Genetic Therapeutic Approaches for Duchenne Muscular Dystrophy

“…All these approaches have many hurdles to overcome if a comprehensive and effective treatment for DMD is to be found. All muscles including cardiac, skeletal, and smooth muscle need to be targeted; reports have suggested that dystrophin levels of more than 20% in mice are needed to protect muscles from exercise-induced damage (van Putten et al, 2012) and a minimum of 30% of normal dystrophin levels needs to be present uniformly in all myofibers to prevent muscular dystrophy in humans (Neri et al, 2007). Therefore any potential treatment would need to be administered systemically.…”