Abstract 15907: Disruption of HCN2 Leads to Laterality Defects and Cardiac Arrhythmia

Abstract

Background: Congenital heart defects (CHDs) encompass a large numbers of cardiovascular malformations, and remain the major cause of infant mortality among all types of birth defects. However, molecular mechanisms underlying CHDs remain elusive, largely owning to the complexity of the diseases and lack of animal models that can reproduce the pathophysiological conditions in a laboratory setting. The hyperpolarization-activated, cyclic nucleotide-gated cation channels (HCN) are responsible for generating spontaneous pacemaker activities in cardiac and central nervous systems. These channels are also detected in other cell types such as ventricular myocytes. HCN currents recorded from neonatal cells have an activation threshold of -70 mV while those recorded from adult cells are activated at -110 mV. This difference indicates that HCN activity might be important in early development. However, roles of HCN channels in cardiogenesis and development are not fully understood. Methods and Results: We created a HCN2 conditional knockout (HCN2KO) model in which the full-length HCN2 was disrupted. Two KO lines were subsequently derived from this model. The first line was weaned by 21 days and they all died by 4-5 wks of age. Maternal ultrasound study revealed that these KO mice developed fetal arrhythmia and had an underdeveloped left side in their hearts. The second line was able to live on under our special diet/care. These mice displayed a slower growth rate (1.1±0.2 g/wk) and lower body weight (15.1±2.0 g) relative to their age-matched WT controls (2.2±0.1 g/wk and 27.5±1.5 g; n=9-10, p<0.05). Echocardiography and tissue staining data suggested that KO hearts had laterality defects compared to their size-matched WT controls. The survived KO mice were able to maintain cardiac function by developing much thicker anterior and posterior walls to sustain blood-pumping (n=6, p<0.05). Electrocardiographic results indicated that the average heart rate recorded from KO mice was ~100 bpm slower than their age-matched WT controls (n=5-6, p<0.05). Conclusions: These novel findings indicate that HCN2 is indispensable in mouse cardiogenesis and development. Our KO models are therefore innovative platforms for future CHD research.