Functional Characterization of the A414G Loss-of-Function Mutation in <i>HCN4</i> Associated with Sinus Bradycardia

Abstract

Patients carrying the heterozygous A414G mutation in the HCN4 gene, which encodes the HCN4 protein, demonstrate moderate to severe bradycardia of the heart. Tetramers of HCN4 subunits compose the ion channels in the sinus node that carry the hyperpolarization-activated ‘funny’ current (If), also named the ‘pacemaker current’. If plays an essential modulating role in sinus node pacemaker activity. To assess the mechanism by which the A414G mutation results in sinus bradycardia, we first performed voltage clamp measurements on wild-type (WT) and heterozygous mutant HCN4 channels expressed in Chinese hamster ovary (CHO) cells. These experiments were performed at physiological temperature using the amphotericin-perforated patch-clamp technique. Next, we applied the experimentally observed mutation-induced changes in the HCN4 current of the CHO cells to If of the single human sinus node cell model developed by Fabbri and coworkers. The half-maximal activation voltage V1/2 of the heterozygous mutant HCN4 current was 19.9 mV more negative than that of the WT HCN4 current (p p f can explain the clinically observed sinus bradycardia in patients carrying the A414G HCN4 gene mutation.

Keywords

• sinoatrial node
• HCN4 channels
• hyperpolarization-activated current
• action potential
• cellular electrophysiology
• patch-clamp recordings