**2.2 LQT type 1 (LQT1)**

LQT type 1 (LQT1) is caused by loss-of-function mutation in *KCNQ1* gene encoding α subunit of potassium (K+ ) channel mediating slow delayed rectifier K+ current (IKs). LQT1 is responsible for 30–35% of all LQTS cases [38]. LQT1 is characterized by prolongation of QT interval in ECG, which could lead to SCD due to VT, typically torsades de pointes [39]. hiPSC-CMs derived from LQT1 patients faithfully recapitulated the clinical hallmark by showing prolonged action potential duration (APD) which is analogous to QT duration in ECG, and reduced IKs current densities are held responsible for abnormal repolarization [40–42]. ML277, an IKs activator, increased the IKs amplitude by enhancing the activation of IKs, thus resulting in shortening of APD in LQT1 hiPSC-CMs [40]. In addition, adrenergic stimulation in LQT1 hiPSC-CMs induced the early afterdepolarization (EAD) [42], which is similar to arrhythmias triggered in LQT1 patients by exercise or emotional stress [39]. Clinically, β-blockers were effective in minimizing the risk of cardiac events in LQT1 patients [43]. Similar antiarrhythmic effect of β-blockers has been observed in LQT1 hiPSC-CMs [42]. Furthermore, hypokalemia is the electrolyte disturbance caused by lower K+ level in blood serum, which aggravates the QT prolongation and facilitates the development of hypokalemia-induced torsades de pointes in LQT1 patients [39, 44]. We successfully developed and mimicked these disease phenotypes in LQT1 hiPSC-CMs carrying G589D or IVS7-2A > G mutation in *KCNQ1* gene. Additionally, lowering the extracellular K+ concentration prolonged APDs and induced the formation of EADs in LQT1 hiPSC-CMs [45]. Both G589D- and IVS7-2A > G-specific LQT1 hiPSC-CMs displayed longer APD and higher Ca2+ abnormalities in baseline; G589D hiPSC-CMs demonstrated prolonged contraction, while IVS7-2A > G hiPSC-CMs showed impaired relaxation [46] observed in our video image-based software analysis [47].
