Original article A mutation in the human cardiac sodium channel (E161K) contributes to sick sinus syndrome, conduction disease and Brugada syndrome in two families Jeroen P.P. Smits a,b,1 , Tamara T. Koopmann a,1 , Ronald Wilders c , Marieke W. Veldkamp a , Tobias Opthof a,d , Zahir A. Bhuiyan a,e , Marcel M.A.M. Mannens e , Jeffrey R. Balser b , Hanno L. Tan a , Connie R. Bezzina a,e, *, Arthur A.M. Wilde a a Experimental and Molecular Cardiology Group, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands b Departments of Anesthesiology and Pharmacology, Vanderbilt University School of Medicine, Nashville, TN, USA c Department of Physiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands d Department of Medical Physiology, University Medical Center, Utrecht, The Netherlands e Department of Clinical Genetics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands Received 5 August 2004; received in revised form 4 February 2005; accepted 8 February 2005 Available online 01 April 2005 Abstract Background. – Mutations in the gene encoding the human cardiac sodium channel (SCN5A) have been associated with three distinct cardiac arrhythmia disorders: the long QT syndrome, the Brugada syndrome and cardiac conduction disease. Here we report the biophysical features of a novel sodium channel mutation, E161K, which we identified in individuals of two non-related families with symptoms of bradycardia, sinus node dysfunction, generalized conduction disease and Brugada syndrome, or combinations thereof. Methods and results. – Wild-type (WT) or E161K sodium channel a-subunit and b-subunit were cotransfected into tsA201 cells to study the functional consequences of mutant sodium channels. Characterization of whole-cell sodium current (I Na ) using the whole cell patch-clamp technique revealed that the E161K mutation caused an almost threefold reduction in current density (P < 0.001), and an 11.9 mV positive shift of the voltage-dependence of activation (P < 0.0001). The inactivation properties of mutant and WT sodium channels were similar. These results suggest an overall reduction of E161K I Na . Incorporation of the experimental findings into computational models demonstrate atrial and ventricular conduction slowing as well as a reduction in sinus rate by slowing of the diastolic depolarization rate and upstroke velocity of the sinus node action potential. This reduction in sinus rate was aggravated by application of acetylcholine, simulating the dominant vagal tone during night. Conclusion. – Our experimental and computational analysis of the E161K mutation suggests that a loss of sodium channel function is not only associated with Brugada syndrome and conduction disease, but may also cause sinus node dysfunction in carriers of this mutation. © 2005 Elsevier Ltd. All rights reserved. Keywords: Arrhythmia; Sinus node dysfunction; Brugada syndrome; Conduction disease; Electrophysiology; Ion channel; Sodium channel; Genetics; Mutation 1. Introduction Cardiac arrhythmias in the absence of structural abnor- malities form an extending group of cardiac diseases. These so-called ’primary electrical diseases’ of the heart are of a hereditary nature and can be associated with specific muta- tions in genes mostly encoding ion channel proteins [1,2]. The consequences of such mutations are alterations in the biophysical properties of ion channel proteins, which may affect normal cardiac electrophysiology and render the heart susceptible to the development of life-threatening arrhyth- mias [1,2]. Mutations in the gene encoding the pore forming a-subunit of the human cardiac sodium channel (SCN5A) have been * Corresponding author. Present address: Department of Experimental Cardiology, Room M0-107, AMC, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. Tel.: +31 20 566 3265; fax: +31 20 697 5458. E-mail address: C.R.Bezzina@amc.uva.nl (C.R. Bezzina). 1 These authors contributed equally to this study. Journal of Molecular and Cellular Cardiology 38 (2005) 969–981 www.elsevier.com/locate/yjmcc 0022-2828/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.yjmcc.2005.02.024