HCN4 gain-of-function mutation increases intrinsic heart rate and limits maladaptive remodeling under pressure overload

Introduction: Gain-of-function (GOF) mutations in the cardiac pacemaker channel HCN4 have been associated with inappropriate sinus tachycardia in human patients. Chronic tachycardia is generally associated with adverse cardiac remodeling and cardiomyopathy, but whether enhanced HCN4 activity can induce or modulate such remodeling remains unknown. We aimed to investigate the influence of an HCN4 gain-of-function mutation on cardiac function and structure under baseline and pressure overload conditions. Methods and Results: We generated HCN4(Y527F) knock-in mice (HCN4F) carrying a GOF mutation in the C-Linker of HCN4 channels, which shifts their activation curves to more positive potentials. Electrophysiological recordings confirmed increased channel availability at physiological membrane potentials. Telemetric ECGs and in vivo electrophysiological studies revealed an elevated mean and intrinsic heart rate, faster sinus node and atrioventricular conduction in HCN4F mice, but no spontaneous arrhythmias. In HCN4F mice, the heart rate histogram was truncated at lower heart rates, indicating fewer low-rate intervals and more frequent periods of elevated heart rate, while maximal heart rates remained comparable between the two phenotypes. Histological analysis did not reveal structural changes consistent with tachycardia-induced cardiomyopathy. Cardiac morphology, fibrosis, and contractility were indistinguishable between genotypes up to 12 months of age. Following transverse aortic constriction, both WT and HCN4F mice developed left ventricular hypertrophy, but HCN4F hearts exhibited less chamber dilatation, smaller left ventricular lumen, and preserved systolic function compared to WT. Gene expression and RNA-sequencing analyses revealed activation of a typical hypertrophic gene program in both genotypes, but distinct remodeling signatures. Discussion: The HCN4(Y527F) gain-of-function mutation increases intrinsic heart rate without inducing structural or functional deterioration. Under pressure overload, it even confers protection against maladaptive dilatation and contractile dysfunction. These findings challenge the concept that persistent inappropriately elevated heart rate is necessarily detrimental and suggest that enhanced HCN4 activity may facilitate adaptive cardiac responses to stress.