INA Loss-of-Function by Compound Variants in SCN5A from a Large Founder Population with Excess Sudden Cardiac Death

Abstract

Recent clinical investigations in a Dutch-German founder population with excess sudden cardiac death, revealed striking phenotypic heterogeneity: long QT-syndrome, cardiac conduction disease, (drug-induced) Brugada syndrome, isorhythmic atrioventricular dissociation and overlap. Ventricular tachyarrhythmia often occurred during mental or physical stress. DNA sequencing identified the pathogenic SCN5A deletion mutation c.4850_4852delTCT, encoding for Nav1.5-DelF1617, and a common polymorphism c.1673A>G, Nav1.5-H558R. In the present study, we performed whole-cell patch-clamping on Chinese hamster ovary cells, transiently transfected with wild-type (WT) Nav1.5, Nav1.5-DelF1617 or Nav1.5-DelF1617-H558R, combined with in-silico action potential (AP) simulations. Nav1.5-DelF1617 showed a significant peak INa density reduction compared to Nav1.5-WT, without changes in the steady-state activation and channels availability. The time constant of inactivation was slower in Nav1.5-DelF1617 and recovery from inactivation faster. The addition of H558R partially rescued the peak current density, without altering the decay rate of the fast inactivation, and slowing recovery from inactivation. No TTX-sensitive persistent current was detected. AP simulation confirmed a reduction of the phase 0 upstroke velocity for Nav1.5-DelF1617 and Nav1.5-DelF1617-H558R, due to a decreased peak INa. AP duration was slightly prolonged, especially at low pacing rate (1Hz). Sudden change in pacing rate or hypokalemia in mutant conditions did not evoke proarrhythmic responses. Taken together, our results demonstrated an INa loss-of-function by the compound variants Nav1.5-DelF1617 and Nav1.5-DelF1617-H558R, which fits partially with the clinical phenotype. Investigations on proarrhythmic triggers, such as cAMP stimulation of mutant INa, are ongoing.