Hypokalemia and bradycardia unmask the loss-of-function phenotype of a Brugada Syndrome SCN5A mutation

Background and aims: Loss-of-function (LOF) mutations of the cardiac Na+ channel (SCN5A) are causatively associated with the Brugada Syndrome (BrS). However, the onset of Ventricular Fibrillation (VF) is a rare event, and critical factors favoring the pathological phenotype remain often elusive. This study explores how concomitant triggering conditions may impact on VF onset in a symptomatic proband carrying the S805L/SCN5A BrS mutation. Methods: Clinical, in-vitro, numerical, and structural analyses were performed. Results: A 67-year-old male was resuscitated after cardiac arrest, and clinical analysis upon hospitalization revealed severe hypokalemia (2.5 mEq/L). The ECG showed a coved type-I BrS pattern and the SCN5A mutation (S805L) was identified. Patch-clamp studies carried out in a heterologous expression system (HEK293 cells) revealed that WT/S805L channels exhibit two different phenotypes (normal and LOF); the main parameter controlling this distribution is the cell membrane potential. A protected/normal behavior was observed at -80 mV; conversely, LOF occurred at more negative potentials (-100/-120 mV). Further analyses in isolated outflow tract ventricular cardiomyocytes showed that hypokalemia (and bradycardia) induced diastolic potential hyperpolarization, thus favoring the Na+ current LOF. Computational and molecular modeling confirmed our findings and revealed the structural determinant of this alteration. Conclusion: WT/S805L Na+ channels exhibit either a LOF or a wild-type-like behavior depending on the membrane potential. Since hypokalemia and slow pacing rate induce cell hyperpolarization and the associated LOF, they represent concurrent elements creating the scenario responsible for the VF and cardiac arrest. These results may represent an interpretative paradigm applicable to other BrS mutations.