Abstract Body: Brugada Syndrome (BrS) is a genetic disease associated with ventricular arrhythmias and is one of the causes of sudden cardiac death. In particular, dysfunctional cardiac Na⁺ channels (SCN5A) represent the only mechanism supporting the autosomal dominant inheritance. A proband without previous medical history was referred to the intensive care unit after resuscitation showing ECG Type 1 BrS pattern and a significant hypokalemia (2.5 mEq/L). Genetic analysis identified a SCN5A de-novo mutation (S805L) in heterozygosis. This study investigates the causative association between the S805L mutation and the BrS event.
First, we expressed wild type (WT) and/or mutated (Hetero and Homo) channels in HEK293T. The application of I/V protocols (hp -120 mV) revealed reduced I_Na density in Homo and Hetero (-65% and -26% vs WT at -20 mV), indicating S805L as a loss-of-function (LOF) mutation. However, Hetero I_Na density was comparable to WT at more depolarized hp (-80 mV), suggesting alterations in the voltage dependent availability of the channel. Accordingly, steady-state inactivation curves of both Hetero and Homo I_Na were right shifted, indicating a gain-of-function (GOF) behaviour (Hetero availability: +13% vs WT at -80 mV). To investigate the possible role of hypokalemia in uncovering the LOF behaviour of the mutation, we reproduced in vitro the hypokalemic condition of the patient testing the effects of external K⁺ (K⁺_out) on diastolic membrane potential (E_diast) of paced guinea-pig cardiomyocytes. Cells were isolated from the Right Ventricular Outflow Tract, known as site of BrS onset. In comparison to a normokalemic condition, E_diast significantly hyperpolarized at 2.5 mM K⁺_out and the effect was greater at low pacing rates (1 Hz) (-16.5 mV vs E_diast at 5 mM K⁺ and 4 Hz). Thus, the combination of bradycardia and hypokalemia might represent a mechanism able to unmask the LOF effect of S805L mutation in Hetero conditions. Computational approach on the human ventricular action potential confirmed these findings. In conclusion, S805L-SCN5A is, at the same time, a LOF and GOF mutation causing reduced channel expression with increased channel availability. While the balance between GOF-LOF effects guaranties a “protected” phenotype, the hypokalemic-induced cell hyperpolarization might reduce the GOF, leading to the appearance of the BrS event.