Cardiac calcium channel abnormalities in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe inherited disease characterised by progressive skeletal muscle weakness that leads to loss of ambulation, respiratory failure, and premature death. Besides skeletal muscle degeneration, DMD is also associated with cardiovascular complications (cardiomyopathy development and cardiac arrhythmias), which significantly contribute to the morbidity and mortality observed. Since the specific mechanisms responsible for the cardiac complications in DMD are poorly understood, current therapy approaches are not targeted, and only induce very limited benefit. Thus, in DMD patients, cardiomyopathy development can neither be prevented, nor can its progression effectively be slowed, and current cardiac arrhythmia management is unsatisfactory. The future challenge will be to identify and validate new and better therapeutic targets. The successful exploration of such targets requires a more detailed understanding of the pathophysiology in the DMD heart. Recent research has pointed at abnormal expression and/or function of voltage-dependent ion channels in cardiomyocytes from DMD hearts as possible contributing factors to disease pathophysiology. In example, we could recently show that currents through L-type Ca2+ channels were significantly increased, and current inactivation was impaired in dystrophic cardiomyocytes. There is also increasing evidence for the intriguing view that ion channel abnormalities do already exist prior to cardiomyopathy development. These may even represent causes or underlying mechanisms of cardiomyopathy formation in DMD. Thus, ion channel modulation emerges as promising strategy for disease prevention. In the planned project, we will test the hypothesis that L-type Ca2+ channels represent a promising new therapeutic target in DMD patients to inhibit or even prevent cardiomyopathy development and improve arrhythmia management. Therefore, we will first accurately characterise the Ca2+ channel abnormalities and their potential functional consequences in adult dystrophic cardiomyocytes. This will include studies to elucidate the underlying molecular mechanism(s). Thereafter, the effects of pharmacological interventions with Ca2+ channel modulators will be tested by drug-treatment experiments with dystrophic mice. For these studies, cardiomyocytes and hearts derived from mouse models for DMD will be investigated mainly by electrophysiological techniques, but also by intracellular Ca2+ measurements, biochemistry and molecular biology techniques, as well as pharmacological experiments. The spectrum of methods to be used will additionally comprise cardiomyopathy assessment in dystrophic mice by testing heart function in vivo and by analyses of extracted hearts. Together, our studies should provide basic knowledge for the development of new evidence-based treatment strategies to combat the cardiac complications observed in DMD patients.

The project studies have exposed significant ion channel and calcium (Ca) handling abnormalities in dystrophic cardiomyocytes. These represent potential new therapeutic targets for the treatment of the cardiovascular complications associated with Duchenne muscular dystrophy (DMD). DMD patients are faced with loss of ambulation, respiratory failure, and premature death. Besides skeletal muscle degeneration, DMD patients also show severe cardiovascular complications. Among those, cardiac arrhythmias and dilated cardiomyopathy development significantly contribute to the morbidity and mortality. Since the specific mechanisms causing these complications in DMD are unknown, current therapy approaches are not targeted, and their effectiveness is limited. In the present project, we could show that abnormal function of ion channels and impaired intracellular Ca handling are important features of dystrophic cardiomyocytes likely causative for cardiovascular complications associated with DMD. In particular, we found reduced sodium and T-type Ca channel activity in the dystrophic heart, which may explain conduction defects and associated arrhythmias in DMD patients. Further, we showed that intracellular Ca transients of dystrophic cardiomyocytes were abnormally small and had a slowed decay phase. This suggested impaired Ca release from intracellular stores and perturbed Ca removal from the cytosol after release. Importantly, these Ca handling abnormalities explain impaired function in the dystrophic heart. The named findings represented a starting point for testing new targeted and evidence-based therapeutic strategies. Here, we could show that treatment of animal models for DMD with the clinically approved drug ivabradine resulted in improved cardiac function. This was due to a rescue of Ca handling impairments in dystrophic cardiomyocytes by the drug. As second therapeutic approach, we applied a gene therapy strategy. Micro-dystrophin transfer to dystrophic mice rescued abnormally reduced Na channel activity in dystrophic cardiomyocytes. This should diminish arrhythmia risk in the dystrophic heart. The findings obtained in the course of the present project are directly relevant for the clinic, because successful therapeutic target validation, followed by promising animal treatment experiments, will trigger the initiation of clinical studies. This may finally lead to successful treatment of the cardiovascular complications in DMD patients, and/or improved cardiac arrhythmia management. Besides clinical relevance, successful new therapy development would also have considerable economic impact.