The hyperpolarization-activated cation current I_f is a key determinant for cardiac pacemaker activity. It is conducted by subunits of the hyperpolarization-activated cyclic nucleotide–gated (HCN) channel family, of which HCN4 is predominant in mammalian heart. Both loss-of-function and gain-of-function mutations of the HCN4 gene are associated with sinus node dysfunction in humans; however, their functional impact is not fully understood yet. Here, we sought to characterize a HCN4 V759I variant detected in a patient with a family history of sick sinus syndrome. The genomic analysis yielded a mono-allelic HCN4 V759I variant in a 49-year-old woman presenting with a family history of sick sinus syndrome. This HCN4 variant was previously classified as putatively pathogenic because genetically linked to sudden infant death syndrome and malignant epilepsy. However, detailed electrophysiological and cell biological characterization of HCN4 V759I in Xenopus laevis oocytes and embryonic rat cardiomyocytes, respectively, did not reveal any obvious abnormality. Voltage dependence and kinetics of mutant channel activation, modulation of cAMP-gating by the neuronal HCN channel auxiliary subunit PEX5R, and cell surface expression were indistinguishable from wild-type HCN4. In good agreement, the clinically likewise affected mother of the patient does not exhibit the reported HCN4 variance. HCN4 V759I resembles an innocuous genetic HCN channel variant, which is not sufficient to disturb cardiac pacemaking. Once more, our work emphasizes the importance of careful functional interpretation of genetic findings not only in the context of hereditary cardiac arrhythmias.

超极化激活的阳离子电流I _f是心脏起搏器活动的关键决定因素。它由超极化激活的环核苷酸门控 (HCN) 通道家族的亚基进行，其中 HCN4 在哺乳动物心脏中占主导地位。HCN4 基因的功能丧失和功能获得突变都与人类的窦房结功能障碍有关；然而，它们的功能影响尚未完全了解。在这里，我们试图表征在具有病窦综合征家族史的患者中检测到的 HCN4 V759I 变异。基因组分析在一名有病窦综合征家族史的 49 岁女性中产生了单等位基因 HCN4 V759I 变异。这种 HCN4 变异以前被归类为假定的致病性，因为在遗传上与婴儿猝死综合征和恶性癫痫有关。然而，爪蟾卵母细胞和胚胎大鼠心肌细胞分别未显示任何明显异常。突变通道激活的电压依赖性和动力学、神经元 HCN 通道辅助亚基 PEX5R 对 cAMP 门控的调节以及细胞表面表达与野生型 HCN4 无法区分。非常一致的是，临床上同样受影响的患者母亲没有表现出报告的 HCN4 变异。HCN4 V759I 类似于一种无害的遗传 HCN 通道变体，不足以干扰心脏起搏。我们的工作再次强调了对遗传发现进行仔细功能解释的重要性，而不仅仅是在遗传性心律失常的背景下。