Point mutation in the HCN4 cardiac ion channel pore affecting synthesis, trafficking, and functional expression is associated with familial asymptomatic sinus bradycardia.
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.


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.
Nof
Eyal
E
Heart Institute, Chaim Sheba Medical Center, Tel Hashomer 52621 Israel. ernof@netvision.net.il
Luria
David
D
Brass
Dovrat
D
Marek
Dina
D
Lahat
Hadas
H
Reznik-Wolf
Haya
H
Pras
Elon
E
Dascal
Nathan
N
Eldar
Michael
M
Glikson
Michael
M
D000293:Adolescent; D000328:Adult; D000818:Animals; D001919:Bradycardia / Q000235:genetics* / Q000503:physiopathology; D002412:Cations / Q000378:metabolism*; D002460:Cell Line; D053447:Channelopathies / Q000235:genetics; D002648:Child; D054815:Cyclic Nucleotide-Gated Cation Channels; D005091:Exons / Q000235:genetics; D005260:Female; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015640:Ion Channel Gating; D007473:Ion Channels / Q000096:biosynthesis / Q000172:deficiency / Q000235:genetics* / Q000502:physiology; D017136:Ion Transport / Q000235:genetics*; D008297:Male; D008875:Middle Aged; D008958:Models, Molecular; D009124:Muscle Proteins / Q000096:biosynthesis / Q000172:deficiency / Q000235:genetics* / Q000502:physiology; D020125:Mutation, Missense; D010375:Pedigree; D017354:Point Mutation; D015221:Potassium Channels; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012849:Sinoatrial Node / Q000503:physiopathology*; D014981:Xenopus
D002412:Cations; D054815:Cyclic Nucleotide-Gated Cation Channels; C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D009124:Muscle Proteins; D015221:Potassium Channels; D011993:Recombinant Fusion Proteins