Point Mutation in the HCN4 Cardiac Ion Channel Pore Affecting Synthesis, Trafficking, and Functional Expression Is Associated With Familial Asymptomatic Sinus Bradycardia

Abstract

Background-The hyperpolarization-activated nucleotide-gated channel-HCN4 plays a major role in the diastolic depolarization of sinus atrial node cells. Mutant HCN4 channels have been found to be associated with inherited sinus bradycardia. Methods and Results-Sixteen members of a family with sinus bradycardia were evaluated. Evaluation included a clinical questionnaire, 12-lead ECGs, Holter monitoring, echocardiography, and treadmill exercise testing. Eight family members (5 males) were classified as affected. All affected family members were asymptomatic with normal exercise capacity during long-term follow-up. Electrophysiological testing performed on 2 affected family members confirmed significant isolated sinus node dysfunction. Segregation analysis suggested autosomal-dominant inheritance. Direct sequencing of the exons encoding HCN4 revealed a missense mutation, G480R, in the ion channel pore domain in all affected family members. Function analysis, including expression of HCN4 wild-type and G480R in Xenopus oocytes and human embryonic kidney 293 cells, revealed that mutant channels were activated at more negative voltages compared with wild-type channels. Synthesis and expression of the wild-type and mutant HCN4 channel on the plasma membrane tested in human embryonic kidney 293 cells using biotinylation and Western blot analysis demonstrated a reduction in synthesis and a trafficking defect in mutant compared with wild-type channels. Conclusions-We describe an inherited, autosomal-dominant form of sinus node dysfunction caused by a missense mutation in the HCN4 ion channel pore. Despite its critical location, this mutation carries a favorable prognosis without the need for pacemaker implantation during long-term follow-up.

Control of heart rate by cAMP sensitivity of HCN channels

“…In particular, the role of HCN channels as the dominant mechanism in heart rate regulation has repeatedly been called into question (4,23). Human genetic studies have suggested that HCN4 channels are important components of the SAN pacemaker machinery (9)(10)(11)(12). A contribution of I f to heart rate determination is further supported by the observation of a heart rate-lowering effect in both humans and rodents when I f is specifically blocked with IVA (13,14,24).…”

“…Although these biophysical properties seem to make f-channels ideal molecular targets for heart rate regulation, the contribution of the major I f -mediating cardiac isoforms HCN2 and HCN4 to SAN function remains highly controversial. Although human genetic (9)(10)(11)(12) and pharmacologic (5,13,14) studies suggest a significant role for HCN4 subunits in SAN pacemaking in humans and rodents, recent data from transgenic mouse models have challenged this view (15,16). Targeted deletion of HCN4 in adult mice was found to cause heart ratedependent sinus pauses but to have no effect on either basal or maximal heart rate or heart rate regulation (16).…”

Mechanism of Sinus Bradycardia in Carriers of the 1795insD Mutation in the SCN5A Gene

“…Together, the loss-offunction effects of the shifts in the voltage dependence of 1795insD 41 ± 1** 70 ± 1** 124 ± 3** 54 47 ± 1 77 ± 2 141 ± 3 40 Van den Berg et al [12] In HCN4 G480R 32 ± 8* 49 ± 12* 101 ± 21* 7 55 ± 9 73 ± 11 126 ± 17 8 Nof et al [13] A485V 37 ± 3* 58 ± 6* 117 ± 27 14 49 ± 11 77 ± 12 140 ± 33 5 Laish-Farkash et al [14] 695X 36 ± 6* 56 ± 5* 131 ± 17 7 47 ± 6 72 ± 10 157 ± 26 6 Schweizer et al [15] Minimum (Min), average (Avg), and maximum (Max) heart rate (HR) obtained with 24-hour Holter recording. Data are mean ± SEM.…”

“…Holter recording in 1795insD patients revealed sinus bradycardia with an ≈11% decrease in minimum, average, and maximum heart rate [12], as detailed in Table 1, which also shows the bradycardic effect of some typical HCN4 mutations [13][14][15] for comparison.…”

Expression of hyperpolarization-activated cyclic nucleotide-gated channel isoforms in a canine model of atrial fibrillation

“…Increased I f current activity may lead to an increase in myocardial tissue self-perpetuation, which commonly results in a tachyarrhythmia. The molecular propagators of the I f current are hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (11). Therefore, the inhibition of HCN channels has become a focus of study for the treatment of arrhythmia (6).…”