Study to Evaluate the Effect of Eleclazine on QT, Safety, and Tolerability in Participants With Long QT2 Syndrome

Key Inclusion Criteria: * Participants with an established diagnosis of LQT2 (by genotype testing) * Mean (of triplicate) QTc interval ≥ 480 msec for at least four out of seven time points, determined by standard 12-lead electrocardiogram (ECG), at screening Key Exclusion Criteria: * Known mutations associated with long QT syndrome type 1 or long QT syndrome type 3 * Known or suspected history of seizures or epilepsy * History of heart failure defined as New York Heart Association (NYHA) Class IV and/or known left ventricular ejection fraction (EF) ≤ 45% * Body mass index (BMI) ≥ 36 kg/m\^2 at screening * Severe renal impairment at screening (defined as an estimated glomerular filtration rate (eGFR) \< 30 mL/min/1.73m\^2, using the 4 variable modification of diet in renal disease (MDRD) equation), as determined by the study center * Abnormal liver function tests at screening, defined as alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \> 2 x upper limit of normal (ULN), or total bilirubin \> 1.5 x ULN * An aborted cardiac arrest (ACA), implantable cardioverter-defibrillator (ICD) implantation, syncopal episode, or appropriate ICD therapy within 3 months prior to screening * Any other condition or circumstance that in the opinion of the investigator would preclude compliance with the study protocol. Note: Other protocol defined Inclusion/Exclusion criteria may apply.