Pathogenesis of lethal cardiac arrhythmias in Mecp2 mutant mice: implication for therapy in Rett syndrome

Abstract

Rett syndrome is a neurodevelopmental disorder typically caused by mutations in methyl-CpG-binding protein 2 (MECP2) in which 26% of deaths are sudden and of unknown cause. To explore the hypothesis that these deaths may be due to cardiac dysfunction, we characterized the electrocardiograms in 379 people with Rett syndrome and found that 18.5% show prolongation of the corrected QT interval (QTc), an indication of a repolarization abnormality that can predispose to the development of an unstable fatal cardiac rhythm. Male mice lacking MeCP2 function, Mecp2(Null/Y), also have prolonged QTc and show increased susceptibility to induced ventricular tachycardia. Female heterozygous null mice, Mecp2(Null/+), show an age-dependent prolongation of QTc associated with ventricular tachycardia and cardiac-related death. Genetic deletion of MeCP2 function in only the nervous system was sufficient to cause long QTc and ventricular tachycardia, implicating neuronally mediated changes to cardiac electrical conduction as a potential cause of ventricular tachycardia in Rett syndrome. The standard therapy for prolonged QTc in Rett syndrome, β-adrenergic receptor blockers, did not prevent ventricular tachycardia in Mecp2(Null/Y) mice. To determine whether an alternative therapy would be more appropriate, we characterized cardiomyocytes from Mecp2(Null/Y) mice and found increased persistent sodium current, which was normalized when cells were treated with the sodium channel-blocking anti-seizure drug phenytoin. Treatment with phenytoin reduced both QTc and sustained ventricular tachycardia in Mecp2(Null/Y) mice. These results demonstrate that cardiac abnormalities in Rett syndrome are secondary to abnormal nervous system control, which leads to increased persistent sodium current. Our findings suggest that treatment in people with Rett syndrome would be more effective if it targeted the increased persistent sodium current to prevent lethal cardiac arrhythmias.

Figures

Fig. 1. Mecp2 deficiency is associated with LQT

(A) A representative ECG from a human patient with RTT with the R-R and QT interval identified. (B) A scatter plot of QTc interval as a function of age in years for 379 RTT patients. The red dashed line indicates the cutoff (450ms) for prolonged QTc. (C) A representative ECG from a wild type (WT) mouse at baseline. (D) A representative ECG from a Mecp2Null/Y male mouse at 2 months old. (E–H) Quantification of the ECGs show increased QTc and QRS (F) intervals in Mecp2Null/Y mice. Young Mecp2+/− female mice show normal QTc (G) and QRS (H). However, older female Mecp2+/− mice show LQT (G) and increased QRS duration (H). * P<0.05, ** P<0.01, ***P<0.001. N, number of mice as indicated within bars.

Fig. 2. Mecp2 deficiency is associated with predisposition to induced VT and arrhythmia-induced death in mice

(A) Representative surface ECG tracings of wild type (top) and male Mecp2Null/Y mice (middle) after pacing (bottom). (B) Incidence of sustained VT after pacing in Mecp2Null/Y and WT mice. (C) Arrhythmia duration of any type in Mecp2Null/Y and WT mice. (D) Representative surface ECG tracings from wild type (top) and Mecp2Null/+ mice (middle) after pacing (bottom). (E) Age-dependent susceptibility of sustained VT. (F) Incidence of sudden cardiac death (SCD) in WT and Mecp2Null/+ mice at 10 months. 2 of 7 Mecp2Null/+− mice developed ventricular tachycardia and ventricular fibrillation after intracardiac pacing, causing death. (G) ECG of a 10 months old Mecp2+/− mouse showing development of sustained VT leading to asystole at 8 minutes after pacing. ** P<0.01, ***P<0.001. N is indicated within the bars of the graphs.

Fig. 3. Male mice with nervous system-specific conditional knockout of MeCP2 (NKO) have LQT and inducible arrhythmias

(A) Mecp2 mRNA expression in brain and heart normalized to WT. * P<0.05 FLOX compared to WT, *** P<0.01 NKO compared with WT. (B) QTc duration and (C) VT incidence of WT, FLOX, and NKO male mice. *P<0.05, **P<0.01. N is shown within the bars of the graphs. NS, non-significant. (D) Example of an ECG from one of two NKO mice that died of ventricular arrhythmia, showing the ECG 1 mintute and 10 minute after PES-induced VT.

Figure 4. Block of INa prevents pacing-induced arrhythmias

(A) Effect of treatment with β-adrenergic receptor blocker (BB) propranolol on heart rate in WT and Mecp2Null/Y mice. (B) Effect of propranolol on QTc intervals in WT and Mecp2Null/Y mice. (C) Effect of propranolol on PES-induced arrhythmias in WT and Mecp2Null/Y mice. Two Mecp2Null/Y mice developed severe atrioventricular block (AVB) and the other three developed sustained VT or ventricular fibrillation (VT/VF). (D and E) Effect of PHT on late phase INa in Mecp2Null/Y cardiomyocytes, WT control cardiomyocytes, and NKO cardiomycytes. (F) Effect of acute PHT injection on heart rate in WT and Mecp2Null/Y mice. (G) Effect of PHT on QTc interval in WT and Mecp2Null/Y mice. (H) Effect of PHT onarrhythmias in Mecp2Null/Y and WT mice. * P<0.05, ** P<0.01. N is indicated in bar graphs. NS, non-significant. pA=pico-Amperes, pA/s = pA per second, the integrated magnitude of the current from 350–800ms after depolarizing pulse.