The effects of low levels of dystrophin on mouse muscle function and pathology.

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.

Comparison of skeletal muscle pathology and motor function of dystrophin and utrophin deficient mouse strains.

The genetic defect of mdx mice resembles that of Duchenne muscular dystrophy, although their functional performance and life expectancy is nearly normal. By contrast, mice lacking utrophin and dystrophin (mdx/utrn -/-) are severely affected and die prematurely. Mice with one utrophin allele (mdx/utrn +/-) are more severely affected than mdx mice, but outlive mdx/utrn -/- mice. We subjected mdx/utrn +/+, +/-, -/- and wild type males to a 12week functional test regime of four different functional tests. Mdx/utrn +/+ and +/- mice completed the regime, while mdx/utrn -/- mice died prematurely. Mdx/utrn +/- mice performed significantly worse compared to mdx/utrn +/+ mice in functional tests. Creatine kinase levels, percentage of fibrotic/necrotic tissue, morphology of neuromuscular synapses and expression of biomarker genes were comparable, whereas mdx/utrn +/- and -/- mice had increased levels of regenerating fibers. This makes mdx/utrn +/- mice valuable for testing the benefit of potential therapies on muscle function parameters.