Muscular degeneration in Duchenne's dystrophy may be caused by a mitochondrial defect

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Evaluation of muscle oxygenation by near-infrared spectroscopy in patients with Becker muscular dystrophy

Several authors have reported alterations in vasodilation during effort in patients with dystrophinopathies, in which a lack of neuronal NO synthase is thought to lead to functional muscle ischemia. In order to determine changes in muscle oxygenation during effort in patients with Becker muscular dystrophy (BMD) and assess the parameters' links with disease severity and functional status, 10 BMD patients and 10 age-matched controls performed isokinetic, constant-load knee extension exercises at (i) 20% of their extensors' peak torque (i.e. the same relative load) and (ii) the same absolute load (20 Nm). Muscle oxygenation was evaluated noninvasively using near-infrared spectroscopy (NIRS), with the time course of deoxygenation as the main criterion. As expected, BMD patients displayed a lower peak torque than controls (-62%). During both types of exercise, initial muscle deoxygenation was faster (by 27-41%) in BMD patients than in controls. Greater disease severity (according to the Motor Function Measure) and functional impairment (walking endurance) were associated with a faster second deoxygenation phase (τ). The validity and relevance of muscle deoxygenation parameters and the alteration of vasodilatation by nNOS deficiency in dystrophinopathies should be assessed by further studies.

Transcriptomic analysis of dystrophin RNAi knockdown reveals a central role for dystrophin in muscle differentiation and contractile apparatus organization

BACKGROUND

Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disorder caused by mutations in the dystrophin gene. DMD has a complex and as yet incompletely defined molecular pathophysiology hindering development of effective ameliorative approaches. Transcriptomic studies so far conducted on dystrophic cells and tissues suffer from non-specific changes and background noise due to heterogeneous comparisons and secondary pathologies. A study design in which a perfectly matched control cell population is used as reference for transcriptomic studies will give a much more specific insight into the effects of dystrophin deficiency and DMD pathophysiology.

RESULTS

Using RNA interference (RNAi) to knock down dystrophin in myotubes from C57BL10 mice, we created a homogenous model to study the transcriptome of dystrophin-deficient myotubes. We noted significant differences in the global gene expression pattern between these myotubes and their matched control cultures. In particular, categorical analyses of the dysregulated genes demonstrated significant enrichment of molecules associated with the components of muscle cell contractile unit, ion channels, metabolic pathways and kinases. Additionally, some of the dysregulated genes could potentially explain conditions and endophenotypes associated with dystrophin deficiency, such as dysregulation of calcium homeostasis (Pvalb and Casq1), or cardiomyopathy (Obscurin, Tcap). In addition to be validated by qPCR, our data gains another level of validity by affirmatively reproducing several independent studies conducted previously at genes and/or protein levels in vivo and in vitro.

CONCLUSION

Our results suggest that in striated muscles, dystrophin is involved in orchestrating proper development and organization of myofibers as contractile units, depicting a novel pathophysiology for DMD where the absence of dystrophin results in maldeveloped myofibers prone to physical stress and damage. Therefore, it becomes apparent that any gene therapy approaches for DMD should target early stages in muscle development to attain a maximum clinical benefit. With a clear and specific definition of the transcriptome of dystrophin deficiency, manipulation of identified dysregulated molecules downstream of dystrophin may lead to novel ameliorative approaches for DMD.

A drug inhibits the mitochondrial protease inducing calmitine deficiency in skeletal muscle of patients with Duchenne's muscular dystrophy and dy/dy dystrophic mice

This study demonstrates that the cause of calmitine deficiency in dy/dy dystrophic mice and patients with Duchenne's muscular dystrophy (DMD) is the same; i.e., the absence of an inhibitor of calmitine-specific mitochondrial protease. This inhibitor, which is present in control mice and control subjects, prevented degradation of the protein. It is also shown that a drug (IP96) was capable in vitro of inhibiting calmitine-specific mitochondrial protease from muscle of DMD patients and dy/dy mice. This drug was also active in vivo in an experimental model of myopathy created in the normal mouse by a single injection of chlorpromazine, a myotoxic drug, which induced temporary calmitine degradation. Thus, it seems quite likely that IP96 prevents calmitine degradation by inhibiting the specific protease.

Muscle regeneration and mitochondrial calmitine increase in the dystrophic dy/dy mouse after intramuscular chlorpromazine injection

We studied the effect of chlorpromazine injection on the gastrocnemius muscles of C57BL/6J dy/dy dystrophic mice. Changes in mitochondrial calmitine concentrations and differences in microscopy studies, fibre typing and morphometry were compared in gastrocnemius muscles of dystrophic and control mice before and 2 and 21 days after injection. In both cases, calmitine reduction associated with muscle degeneration was observed 2 days after drug injection. Calmitine then increased, reaching a level at day 21 nearly identical to that of controls before injection. This increase was associated with muscle regeneration. These results clearly indicate that dystrophic mouse muscle can regenerate calmitine after drug-induced damage.