Pacemaker Channel Dysfunction in a Patient with Sinus Node Disease

Pacemaker channel dysfunction in a patient with sinus node disease Eric Schulze-Bahr, … , Olaf Pongs, Dirk Isbrandt J Clin Invest. 2003;111(10):1537-1545. https://doi.org/10.1172/JCI16387. Article Cardiology The cardiac pacemaker current If is a major determinant of diastolic depolarization in sinus nodal cells and has a key role in heartbeat generation. Therefore, we hypothesized that some forms of “idiopathic” sinus node dysfunction (SND) are related to inherited dysfunctions of cardiac pacemaker ion channels. In a candidate gene approach, a heterozygous 1-bp deletion (1631delC) in exon 5 of the human HCN4 gene was detected in a patient with idiopathic SND. The mutant HCN4 protein (HCN4-573X) had a truncated C-terminus and lacked the cyclic nucleotide–binding domain. COS-7 cells transiently transfected with HCN4-573X cDNA indicated normal intracellular trafficking and membrane integration of HCN4-573X subunits. Patch-clamp experiments showed that HCN4-573X channels mediated If-like currents that were insensitive to increased cellular cAMP levels. Coexpression experiments showed a dominant-negative effect of HCN4- 573X subunits on wild-type subunits. These data indicate that the cardiac If channels are functionally expressed but with altered biophysical properties. Taken together, the clinical, genetic, and in vitro data provide a likely explanation for the patient’s sinus bradycardia and the chronotropic incompetence. Find the latest version: https://jci.me/16387/pdf Pacemaker channel dysfunction in a patient with sinus node disease Eric Schulze-Bahr,1,2 Axel Neu,3 Patrick Friederich,4 U. Benjamin Kaupp,5 Günter Breithardt,1,2 Olaf Pongs,3 and Dirk Isbrandt3 1Genetics of Arrhythmias, Molecular Cardiology Section, Institute for Arteriosclerosis Research, University of Münster, Münster, Germany 2Department of Cardiology and Angiology, Hospital of the University of Münster, Münster, Germany 3Institute for Neural Signal Transduction, Center for Neurobiology Hamburg (ZMNH), University of Hamburg, Hamburg, Germany 4Department of Anesthesiology, University Hospital Hamburg, Hamburg, Germany 5Institute for Biological Information Processing, Research Center Jülich, Jülich, Germany The cardiac pacemaker current If is a major determinant of diastolic depolarization in sinus nodal cells and has a key role in heartbeat generation. Therefore, we hypothesized that some forms of “idiopathic” sinus node dysfunction (SND) are related to inherited dysfunctions of cardiac pacemaker ion channels. In a candidate gene approach, a heterozygous 1-bp deletion (1631delC) in exon 5 of the human HCN4 gene was detected in a patient with idiopathic SND. The mutant HCN4 protein (HCN4-573X) had a truncated C-terminus and lacked the cyclic nucleotide–binding domain. COS-7 cells transiently transfected with HCN4-573X cDNA indicated normal intracellular trafficking and membrane integration of HCN4-573X subunits. Patch-clamp experiments showed that HCN4-573X channels mediated If-like currents that were insensitive to increased cellular cAMP levels. Coexpression experiments showed a dominant-negative effect of HCN4-573X subunits on wild-type subunits. These data indicate that the cardiac If channels are functionally expressed but with altered biophysical properties. Taken together, the clinical, genetic, and in vitro data provide a likely explanation for the patient’s sinus bradycardia and the chronotropic incompetence. This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org. J. Clin. Invest. 111:1537–1545 (2003). doi:10.1172/JCI200316387. Introduction some instances (2–7). These findings and observa- Sinus node dysfunction (SND) is the major cause tions from twin studies indicate a role for genetic fac- necessitating pacemaker implantation and accounts tors in SND and heart rate control (8, 9). for approximately half of all patients requiring a The sinoatrial node (SAN) is the physiological pace- pacemaker (1). The disease commonly occurs in maker in the human heart and is predominantly adults with acquired heart disease, during antiar- responsible for autonomous heart beat generation. rhythmic therapy, or after surgically corrected, con- Heart rate control is achieved through control of SAN genital heart disease. In a significant portion of pacemaking by the autonomic nervous system; cholin- patients, however, SND appears in the absence of ergic or β-adrenergic stimulation either slows or accel- identifiable cardiac abnormalities or other associat- erates the spontaneous SAN activity (10). Pacemaking ed conditions (“idiopathic” SND) (2). A familial activity depends on a phase of spontaneous, slow mem- occurrence of idiopathic SND has been reported in brane depolarization that occurs between cardiac action potentials (i.e., during diastole). Several ionic currents Received for publication July 10, 2002, and accepted in revised form apparently contribute to the time course and slope of February 18, 2003. diastolic membrane depolarizations (e.g., the T-type and Address correspondence to: Eric Schulze-Bahr, AG Genetics L-type Ca2+ currents), some of which are directly regu- of Arrhythmias, Molekular-Kardiologie, Institut für lated by the autonomic nervous system (10). In particu- Arterioskleroseforschung an der Westfälischen Wilhelms- lar, the activity of the hyperpolarization-activated non- Universität Münster, Domagkstrasse 3, D-48149 Münster, Germany. Phone: 49-251-83-52982; Fax: 49-251-83-52980; selective cationic current If is cAMP regulated (11). This E-mail: [email protected]. involves a direct interaction of cAMP with the cyclic Conflict of interest: The authors have declared that no conflict of nucleotide–binding domain (cNBD) of If channels (11, interest exists. 12) and elicits a positive shift in the voltage dependence Nonstandard abbreviations used: sinus node dysfunction (SND); sinoatrial node (SAN); cyclic nucleotide–binding domain of If channel activation accompanied by an increase in (cNBD); hyperpolarization-activated cyclic nucleotide-gated current amplitude. Eventually, this leads to accelerated channels (HCN); beats per minute (bpm); dual-chamber (DDD); diastolic depolarization and electrical impulse genera- electrocardiography (ECG); CMV promoter (PCMV); human tion in SAN. Conversely, a decrease in cellular cAMP lev- elongation factor 1α promoter (PEF-1α); enhanced green fluorescent protein (EGFP); internal ribosomal entry site (IRES); midpoint els (e.g., during muscarinic stimulation) leads to a potential (V1/2). decrease in If channel activity and thereby slows the The Journal of Clinical Investigation | May 2003 | Volume 111 | Number 10 1537 heart rate. Apparently, cAMP sensitivity of pacemaker high-grade atrioventricular conduction block), were not channels represents a key element in the response of observed during pacemaker interrogation. During exer- heart rate to sympathetic stimulation (13, 14). cise electrocardiography (ECG), the patient had sinus Hyperpolarization-activated cyclic nucleotide-gated rhythm, and the maximal heart rate was 101 bpm at a (HCN) subunits are the molecular components of If maximal work load of 150 W. This was markedly below channels. Two of the four known genes encoding HCN the predicted maximal heart rate when adapted for age channel subunits (15–19), HCN2 and HCN4, are pre- and gender (150–184 bpm) (Figure 1d) (25). dominantly expressed in the heart (20). Their in vitro The family history of the female index patient was properties closely resemble those of native If channels unremarkable; three of her four children (between 32 (15–18, 21–23). Therefore, HCN genes have been sug- and 39 years of age) that were available for clinical and gested as candidates underlying disorders of heartbeat genetic investigations had normal 12-lead ECGs with generation (13, 17). sinus rhythms (65, 75, and 65 bpm, respectively) and In a previous study, we excluded the presence of exercise ECGs with regular chronotropic responses. HCN2 mutations in a series of ten patients with SND In addition to this family, nine other unrelated (24). Here, we report on the investigation of the HCN4 index families with idiopathic SND were available for gene in the same population with idiopathic SND. In genetic investigations; three had familial sinus brady- one index patient, a heterozygous 1-bp deletion muta- cardia (without atrial fibrillation), two had “lone atri- tion was detected that gave rise to a C-terminally trun- al fibrillation,” three had “sporadic” SND, and one cated HCN4 protein (HCN4-573X) lacking the C-ter- previously reported large family had autosomal dom- minal cNBD domain. In vitro–expressed HCN4-573X inant sinoatrial disease (24). channels mediated If -like currents comparable to Genomic structure, chromosomal localization, and those of wild-type HCN4 but were unable to respond mutation detection of the HCN4 gene to increases in intracellular cAMP levels. In coexpression experiments with wild-type subunits, current A BLAST search of the National Center for Biotechnolo- properties of HCN4-573X subunits were predomi- gy Information GenBank high-throughput genomic nant. It is likely that altered If channel properties in sequences database using the HCN4 cDNA (GenBank the index patient are associated with the observed accession number AJ238850) as a probe identified a clinical SND phenotype. Our investigation supports the hypothesis that idiopathic SND is in part geneti- cally determined and may be linked to mutations

Load more