The Novel cAMP-Insensitive HCN4-695X Mutation is Associated with Marked Sinus Bradycardia but Regular Autonomic Rate Control

Abstract

The mammalian genome encodes four HCN channels (HCN1-4), which in response to hyperpolarization activate the pacemaker current If. HCN4, undoubtedly the dominant HCN isotype in the sinoatrial node was suggested to be critical for generation and regulation of heart rhythm, but the precise implication is still unresolved. Here we report a novel HCN4 mutation that may allow for better understanding how HCN4 contributes to heart rate regulation in humans.In a candidate gene approach, we identified a HCN4 mutation in a 45 year-old female referred to our clinic with marked sinus bradycardia (heart rate 41 bpm). Clinical evaluation including echocardiography, dobutamine-stress MRI, treadmill-test and 24h-holter recording showed regular chronotropic competence, normal heart rate variability and no signs of ischemic or structural heart disease. DNA sequence analysis revealed a heterozygous insertion of 13 nucleotides in exon 6 of the HCN4 gene (HCN4-695X) leading to a truncated cyclic nucleotide binging domain. Biophysical properties measured by whole-cell patch-clamp technique of human embryonic kidney cells demonstrated that mutant channels were insensitive towards cAMP. Co-expression of mutant and wildtype subunits induced currents with properties nearly identical to currents produced by homomeric mutant channels, indicating that HCN4-695X subunits affect HCN4 currents in a dominant-negative mode. Pedigree analysis confirmed seven additional mutant-carriers characterized by a similar clinical phenotype, notably sinus node dysfunction with maintained chronotropic competence.In conclusion we report a family with inherited sinus bradycardia linked to a novel cAMP-insensitive HCN4 mutation that induces a dominant-negative effect in the heterozygous conformation. Importantly regular chronotropic competence and normal heart rate variability of mutant-carriers indicate that mutant HCN4 channels can be substituted in-vivo, questioning their critical role for autonomic heart rate control in humans.