Cardiac electrophysiological characteristics of the mdx 5cv mouse model of Duchenne muscular dystrophy

Electrocardiographic features of children with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a clinically common X-linked recessive myopathy, which is caused by mutation of the gene encoding dystrophin on chromosome Xp21. The onset of heart injury in children with DMD is inconspicuous, and the prognosis is poor once it develops to the stage of heart failure. Cardiovascular complications remain an important cause of death in this patient population. At present, population and animal studies have suggested that Electrocardiogram (ECG) changes may be the initial manifestation of cardiac involvement in children with DMD. Relevant clinical studies have also confirmed that significant abnormal ECG changes already exist in DMD patients before cardiomegaly and/or LVEF decrease. With increases in age and decreases in cardiac function, the proportion of ECG abnormalities in DMD patients increase significantly. Some characteristic ECG changes, such as ST-segment changes, T wave inversion, Q wave at the inferolateral leads, LBBB and SDANN, have a certain correlation with the indexes of cardiac remodeling or impaired cardiac function in DMD patients, while VT and LBBB have demonstrated relatively good predictive value for the occurrence of long-term DCM and/or adverse cardiovascular events or even death in DMD patients. The present review discusses the electrocardiographic features in children with DMD.

Animal Models to Study Cardiac Arrhythmias

Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide, accounting for 10% to 15% of all deaths. Although most arrhythmias are due to acquired heart disease, inherited channelopathies and cardiomyopathies disproportionately affect children and young adults. Arrhythmogenesis is complex, involving anatomic structure, ion channels and regulatory proteins, and the interplay between cells in the conduction system, cardiomyocytes, fibroblasts, and the immune system. Animal models of arrhythmia are powerful tools for studying not only molecular and cellular mechanism of arrhythmogenesis but also more complex mechanisms at the whole heart level, and for testing therapeutic interventions. This review summarizes basic and clinical arrhythmia mechanisms followed by an in-depth review of published animal models of genetic and acquired arrhythmia disorders.

Low human dystrophin levels prevent cardiac electrophysiological and structural remodelling in a Duchenne mouse model

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder caused by loss of dystrophin. This lack also affects cardiac structure and function, and cardiovascular complications are a major cause of death in DMD. Newly developed therapies partially restore dystrophin expression. It is unclear whether this will be sufficient to prevent or ameliorate cardiac involvement in DMD. We here establish the cardiac electrophysiological and structural phenotype in young (2-3 months) and aged (6-13 months) dystrophin-deficient mdx mice expressing 100% human dystrophin (hDMD), 0% human dystrophin (hDMDdel52-null) or low levels (~ 5%) of human dystrophin (hDMDdel52-low). Compared to hDMD, young and aged hDMDdel52-null mice displayed conduction slowing and repolarisation abnormalities, while only aged hDMDdel52-null mice displayed increased myocardial fibrosis. Moreover, ventricular cardiomyocytes from young hDMDdel52-null animals displayed decreased sodium current and action potential (AP) upstroke velocity, and prolonged AP duration at 20% and 50% of repolarisation. Hence, cardiac electrical remodelling in hDMDdel52-null mice preceded development of structural alterations. In contrast to hDMDdel52-null, hDMDdel52-low mice showed similar electrophysiological and structural characteristics as hDMD, indicating prevention of the cardiac DMD phenotype by low levels of human dystrophin. Our findings are potentially relevant for the development of therapeutic strategies aimed at restoring dystrophin expression in DMD.

Cardiac lesions in Duchenne muscular dystrophy model rats with out-of-frame Dmd gene mutation mediated by CRISPR/Cas9 system

Duchenne muscular dystrophy (DMD) is a progressive muscular disorder caused by X-chromosomal DMD gene mutations. Recently, a new CRISPR/Cas9-mediated DMD rat model (cDMDR) was established and is expected to show cardiac lesions similar to those in humans. We therefore investigated the pathological and pathophysiological features of the cardiac lesions and their progression in cDMDR. For our cDMDR, Dmd-mutated rats (W-Dmd^em1Kykn) were obtained. Dmd heterozygous-deficient females and wild-type (WT) males were mated, and male offspring including WT as controls were used. (1) Hearts were collected at 3, 5, and 10 months of age, and HE- and Masson’s trichrome-stained specimens were observed. (2) Electrocardiogram (ECG) recordings were made and analyzed at 3, 5, and 8 months of age. (3) Echocardiography was performed at 9 months of age. In cDMDR rats, (1) degeneration/necrosis of cardiomyocytes and myocardial fibrosis prominent in the right ventricular wall and the outer layer of the left ventricular wall were observed. Fibrosis became more prominent with aging. (2) Lower P wave amplitudes and greater R wave amplitudes were detected. PR intervals tended to be shorter. QT intervals were longer at 3 months but tended to be shorter at 8 months. Sinus irregularity and premature ventricular contraction were observed at 8 months. (3) Echocardiography indicated myocardial sclerosis and a tendency of systolic dysfunction. Pathological and pathophysiological changes occurred in cDMDR rat hearts and progressed with aging, which is, to some extent, similar to what occurs in humans. Thus, cDMDR could be a valuable model for studying cardiology of human DMD.

Biochemical and Functional Comparisons of mdx and Sgcg(-/-) Muscular Dystrophy Mouse Models

Mouse models have provided an essential platform to investigate facets of human diseases, from etiology, diagnosis, and prognosis, to potential treatments. Muscular dystrophy (MD) is the most common human genetic disease occurring in approximately 1 in 2500 births. The mdx mouse, which is dystrophin-deficient, has long been used to model this disease. However, this mouse strain displays a rather mild disease course compared to human patients. The mdx mice have been bred to additional genetically engineered mice to worsen the disease. Alternatively, other genes which cause human MD have been genetically disrupted in mice. We are now comparing disease progression from one of these alternative gene disruptions, the γ-sarcoglycan null mouse Sgcg^−/− on the DBA2/J background, to the mdx mouse line. This paper aims to assess the time-course severity of the disease in the mouse models and determine which is best for MD research. The Sgcg^−/− mice have a more severe phenotype than the mdx mice. Muscle function was assessed by plethysmography and echocardiography. Histologically the Sgcg^−/− mice displayed increased fibrosis and variable fiber size. By quantitative Evan's blue dye uptake and hydroxyproline content two key disease determinants, membrane permeability and fibrosis respectively, were also proven worse in the Sgcg^−/− mice.

Inhibition of CaMKII phosphorylation of RyR2 prevents inducible ventricular arrhythmias in mice with Duchenne muscular dystrophy

BACKGROUND

Ventricular tachycardia (VT) is the second most common cause of death in patients with Duchenne muscular dystrophy (DMD). Recent studies have implicated enhanced sarcoplasmic reticulum (SR) Ca(2+) leak via type 2 ryanodine receptor (RyR2) as a cause of VT in the mdx mouse model of DMD. However, the signaling mechanisms underlying induction of SR Ca(2+) leak and VT are poorly understood.

OBJECTIVE

To test whether enhanced Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2 underlies SR Ca(2+) leak and induction of VT in mdx mice.

METHODS

Programmed electrical stimulation was performed on anesthetized mice and confocal imaging of Ca(2+) release events in isolated ventricular myocytes.

RESULTS

Programmed electrical stimulation revealed inducible VT in mdx mice, which was inhibited by CaMKII inhibition or mutation S2814A in RyR2. Myocytes from mdx mice exhibited more Ca(2+) sparks and Ca(2+) waves compared with wild-type mice, in particular at faster pacing rates. Arrhythmogenic Ca(2+) waves were inhibited by CaMKII but not by protein kinase A inhibition. Moreover, mutation S2814A but not S2808A in RyR2 suppressed spontaneous Ca(2+) waves in myocytes from mdx mice.

CONCLUSIONS

CaMKII blockade and genetic inhibition of RyR2-S2814 phosphorylation prevent VT induction in a mouse model of DMD. In ventricular myocytes from mdx mice, spontaneous Ca(2+) sparks and Ca(2+) waves can be suppressed by CaMKII inhibition or mutation S2814A in RyR2. Thus, the inhibition of CaMKII-induced SR Ca(2+) leak might be a new strategy to prevent arrhythmias in patients with DMD without heart failure.

Dystrophin is required for the normal function of the cardio-protective K(ATP) channel in cardiomyocytes

Duchenne and Becker muscular dystrophy patients often develop a cardiomyopathy for which the pathogenesis is still unknown. We have employed the murine animal model of Duchenne muscular dystrophy (mdx), which develops a cardiomyopathy that includes some characteristics of the human disease, to study the molecular basis of this pathology. Here we show that the mdx mouse heart has defects consistent with alteration in compounds that regulate energy homeostasis including a marked decrease in creatine-phosphate (PC). In addition, the mdx heart is more susceptible to anoxia than controls. Since the cardio-protective ATP sensitive potassium channel (K_ATP) complex and PC have been shown to interact we investigated whether deficits in PC levels correlate with other molecular events including K_ATP ion channel complex presence, its functionality and interaction with dystrophin. We found that this channel complex is present in the dystrophic cardiac cell membrane but its ability to sense a drop in the intracellular ATP concentration and consequently open is compromised by the absence of dystrophin. We further demonstrate that the creatine kinase muscle isoform (CKm) is displaced from the plasma membrane of the mdx cardiac cells. Considering that CKm is a determinant of K_ATP channel complex function we hypothesize that dystrophin acts as a scaffolding protein organizing the K_ATP channel complex and the enzymes necessary for its correct functioning. Therefore, the lack of proper functioning of the cardio-protective K_ATP system in the mdx cardiomyocytes may be part of the mechanism contributing to development of cardiac disease in dystrophic patients.

Nε-lysine acetylation determines dissociation from GAP junctions and lateralization of connexin 43 in normal and dystrophic heart

Wanting to explore the epigenetic basis of Duchenne cardiomyopathy, we found that global histone acetylase activity was abnormally elevated and the acetylase P300/CBP-associated factor (PCAF) coimmunoprecipitated with connexin 43 (Cx43), which was N(ε)-lysine acetylated and lateralized in mdx heart. This observation was paralleled by Cx43 dissociation from N-cadherin and zonula occludens 1, whereas pp60-c-Src association was unaltered. In vivo treatment of mdx with the pan-histone acetylase inhibitor anacardic acid significantly reduced Cx43 N(ε)-lysine acetylation and restored its association to GAP junctions (GJs) at intercalated discs. Noteworthy, in normal as well as mdx mice, the class IIa histone deacetylases 4 and 5 constitutively colocalized with Cx43 either at GJs or in the lateralized compartments. The class I histone deacetylase 3 was also part of the complex. Treatment of normal controls with the histone deacetylase pan-inhibitor suberoylanilide hydroxamic acid (MC1568) or the class IIa-selective inhibitor 3-{4-[3-(3-fluorophenyl)-3-oxo-1-propen-1-yl]-1-methyl-1H-pyrrol-2-yl}-N-hydroxy-2-propenamide (MC1568) determined Cx43 hyperacetylation, dissociation from GJs, and distribution along the long axis of ventricular cardiomyocytes. Consistently, the histone acetylase activator pentadecylidenemalonate 1b (SPV106) hyperacetylated cardiac proteins, including Cx43, which assumed a lateralized position that partly reproduced the dystrophic phenotype. In the presence of suberoylanilide hydroxamic acid, cell to cell permeability was significantly diminished, which is in agreement with a Cx43 close conformation in the consequence of hyperacetylation. Additional experiments, performed with Cx43 acetylation mutants, revealed, for the acetylated form of the molecule, a significant reduction in plasma membrane localization and a tendency to nuclear accumulation. These results suggest that Cx43 N(ε)-lysine acetylation may have physiopathological consequences for cell to cell coupling and cardiac function.

Voluntary exercise induces structural remodeling in the hearts of dystrophin-deficient mice

In this exploratory study, we test the hypothesis that voluntary exercise affects the progression of dystrophic changes in the left ventricle of the heart. Wild-type (C57BL/10ScSn) and dystrophin-deficient (mdx) mice, aged 7 weeks, were divided into sedentary and exercise-treated groups and tested for differences in cardiac histomorphometry. Exercised mdx mice were found to exhibit significantly enlarged ventricles and thinner lateral ventricular walls than sedentary mdx mice (P < 0.05). Trichrome staining indicated the presence of fibrotic lesions in the left ventricular myocardium in 20% of the exercised mdx group. Fibrotic lesions were not found in control or sedentary mdx mice. No histomorphometric differences were found between treatment groups in wild-type mice. Our findings suggest voluntary exercise may accelerate the progression of ventricular dilation and fibrosis in young mdx mice. The effects of exercise on cardiac remodeling should be considered during the treatment of cardiac disease in dystrophin-deficient patients.

Phenotyping cardiac gene therapy in mice

Heart disease is the leading health problem of industrialized countries. The development of gene therapies tailored towards the heart has grown exponentially over the past decade. Murine models of heart diseases have played a pivotal role in testing novel cardiac gene therapy approaches. Unfortunately, the small body size and rapid heart rate of mice present a great challenge to heart function evaluation. Here we outline the commonly used cardiac phenotyping methods of treadmill exercise regimen, full 12-lead electrocardiographic assay and left ventricular catheterization hemodynamic assay. Application of these protocols will allow critical testing of gene therapy efficacy in mouse models of heart diseases.

High incidence of electrocardiogram abnormalities in young patients with duchenne muscular dystrophy

Electrocardiogram abnormalities are reported to be complicated in Duchenne muscular dystrophy. Although Duchenne muscular dystrophy can be genetically diagnosed in young patients, extensive electrocardiogram studies have not been reported. Here, electrocardiogram abnormalities were examined in Duchenne muscular dystrophy cases with dystrophin gene mutations. Sixty-nine patients, aged </=18 years, received 136 electrocardiogram examinations. Sixty-four patients (91.3%) displayed one or more abnormalities. Furthermore, patients adolescent <10 years (84.8% of patients) displayed electrocardiogram abnormalities, and the most common abnormality was deep Q-waves. Remarkably, the abnormality incidence of both deep Q-waves and low RV5 + SV1 (R-wave V5 + S-wave V1) were significantly high in adolescent patients. Although the patterns or positions of dystrophin gene mutations were compared with electrocardiogram abnormalities, no predisposing mutation was disclosed. These results indicate that electrocardiogram abnormalities in Duchenne muscular dystrophy are a result of dystrophin deficiency, regardless of types of gene mutations. The disease can be divided into two types: age-dependent and age-independent. Deep Q-waves and low RV5 + SV1 are proposed as markers of age-dependent cardiac complications.