SCN5A-C683R exhibits combined gain-of-function and loss-of-function properties related to adrenaline-triggered ventricular arrhythmia.

New Findings: What is the role of SCN5A-C683R? SCN5A-C683R is a novel variant associated with an uncommon phenotype of adrenaline-triggered ventricular arrhythmia in the absence of a distinct ECG phenotype. What is the main finding and its importance? Functional studies demonstrated that Na 1.5/C683R results in a mixed electrophysiological phenotype with gain-of-function (GOF) and loss-of-function (LOF) properties compared with Na 1.5/wild type. Gain-of-function properties are characterized by a significant increase of the maximal current density and a hyperpolarizing shift of the steady-state activation. The LOF effect of Na 1.5/C683R is characterized by increased closed-state inactivation. Electrophysiological properties and clinical manifestation of SCN5A-C683R are different from long-QT-3 or Brugada syndrome and might represent a distinct inherited arrhythmia syndrome.

Abstract: Mutations of SCN5Ahave been identified as the genetic substrate of various inherited arrhythmia syndromes, including long-QT-3 and Brugada syndrome. We recently identified a novel SCN5A variant (C683R) in two genetically unrelated families. The index patients of both families experienced adrenaline-triggered ventricular arrhythmia with cardiac arrest but did not show a specific ECG phenotype, raising the hypothesis that SCN5A-C683R might be a susceptibility variant and the genetic substrate of distinct inherited arrhythmia. We conducted functional cellular studies to characterize the electrophysiological properties of Na 1.5/C683R in order to explore the potential pathogenicity of this novel variant. The C683R variant was engineered by site-directed mutagenesis. Na 1.5/wild type (WT) and Na 1.5/C683R were expressed in tsA201 cells. Electrophysiological characterization of C683R was performed using the whole-cell patch-clamp technique. Adrenergic stimulation was mimicked by exposure to the protein kinase A activator 8-CPT-cAMP. The impact of β-blockers was tested by exposing Na 1.5/WT and Na 1.5/C683R currents to propranolol and nadolol. C683R resulted in a co-association of gain-of-function and loss-of-function properties of Na 1.5. Gain-of-function properties were characterized by a significant increase of the maximal Na 1.5 current density compared with Na 1.5/WT (861 ± 309 vs. 627 ± 489 pA/pF; P < 0.05, n ≥ 9) that was potentiated in Na 1.5/C683R with 8-CPT-cAMP stimulation (869 ± 287 vs. 607 ± 320 pA/pF; P < 0.05, n ≥ 12). C683R also resulted in a significant hyperpolarizing shift in the voltage of steady-state activation (-65.4 ± 3.0 vs. -57.2 ± 4.8 mV; P < 0.001), resulting in an increased window current compared with WT. The loss-of-function effect of Na 1.5/C683R was characterized by significantly increased closed-state inactivation compared with Na 1.5/WT (P < 0.05). C683R is a novel SCN5A variant resulting in a co-association of gain-of-function and loss-of-function properties of the cardiac sodium channel Na 1.5. The phenotype is characterized by adrenaline-triggered ventricular arrhythmias. Electrophysiological properties and clinical manifestations are different from long-QT-3 or Brugada syndrome and might represent a distinct inherited arrhythmia syndrome.