Unexpected Impairment of INa current Underpins Reentrant Arrhythmias in a Knock-in Swine Model of Timothy Syndrome

Timothy Syndrome 1 (TS1) is a multiorgan form of Long QT Syndrome associated with life-threatening cardiac arrhythmias, the organ-level dynamics of which remain unclear. Here, we developed and characterized a novel porcine model of TS1 carrying the causative p.Gly406Arg mutation in CACNA1C , known to impair CaV1.2 channel inactivation. Our model fully recapitulated the human disease with prolonged QT interval and arrhythmic mortality. Electroanatomical mapping revealed the presence of a functional substrate vulnerable to reentry, stemming from an unforeseen constitutional slowing of cardiac activation. This signature substrate of TS1 was reliably identified using reentry vulnerability index, which we further demonstrate can be used as a benchmark for predicting treatment efficacy, as shown by pharmacological shortening of action potential using HERG channel agonist ICA-105574. Notably, in vitro experiments showed that TS1 cardiomyocytes display Ca 2+ overload and decreased peak INa current, providing a rationale for the arrhythmogenic slowing of impulse propagation in vivo.