HCN4 and atrioventricular block: HCN4 mutations is associated with arrhythmias including Brugada syndrome (HCN4-V492F) (25), left ventricular noncompaction (HCN4-G482R, G811E) (16, 21), atrioventricular block (HCN4-G1097W) (11), and atrial fibrillation (HCN4-P257S, K530N) (8, 26).