Altered HCN4 Channel C-Linker Interaction is Associated with Familial Tachycardia-Bradycardia Syndrome and Atrial Fibrillation

HCN channels underlie the pacemaker current If, involved in generation, regulation, and stabilization of sinus rhythm. HCN4 represents the dominant isotype in the sinoatrial node and channel dysfunction is associated with inherited sinus node bradycardia. Here, we report a previously undescribed HCN4 gene mutation that replaced the positively charged lysine 530 with an asparagine (HCN4-K530N) in a highly conserved region of the C-linker. Six members of a German family carrying the HCN4-K530N mutation developed tachycardia-bradycardia syndrome and persistent atrial fibrillation in an age-dependent fashion. HEK293 cell recordings using whole-cell patch clamp electrophysiology revealed that homomeric HCN4-K530N mutant and wild type channels activate at similar potentials and respond equally upon binding of cAMP. Heteromeric channels, in contrast, showed a significant hyperpolarizing shift in the half-maximal activation voltage. Moreover, the effect of cAMP on channel activation and deactivation properties was significantly increased in heteromeric channels. A comparison of mutant and wild type C-linker domain models suggests that altered subunit interactions between the A’ and B’ helices and the C’ and D’ helices of the neighboring subunit may change the intersubunit structural dynamics in heterotetramers of HCN4 wild type and mutant subunits, enforcing inhibition of channel activity by the nucleotide free cyclic nucleotide binding domain in the heterozygous situation. Thus, altered interaction of side chains is tolerated in homomeric mutant channels but interferes with wild type subunits in the heteromeric complex leading to f-channel dysfunction that promotes the development of tachycardia-bradycardia syndrome and persistent atrial fibrillation.