Gain-of-Function Mutations in SCN5A Gene Lead to Type-3 Long QT Syndrome

Type-3 long QT syndrome, which is related to type 5 voltage-gated sodium channel alpha subunit (SCN5A) mutation, has been identified since 1995. LQTS mutation in SCN5A is a gain-of-function mutation producing late sodium current, INa,L. Brugada mutation in SCN5A is a loss-of-function causing INa decrease. Whereas, the mechanism for Dilated Cardiomyopathy mutations in SCN5A is still not fully understood. N1325S is one of the first series of mutations identified for type-3 LQTS. Our lab created a mouse model for LQTS by expressing SCN5A mutation N1325S in the mouse hearts (TG-NS) and a matched experimental control line with overexpression of wild- type SCN5A (TG-WT). There are some interesting findings in TG-NS mice: (i) Intracellular sodium (Na+) level is higher in TG-NS myocytes compared with TG-WT myocytes. (ii) Ca2+ handling is abnormal in TG-NS myocytes, but not in TG-WT myocytes. (iii) Apoptosis was also found in TG-NS mouse heart tissue, but not in TG-WT hearts. These results provoke the hypothesis that gain-of-function mutation N1325S in SCN5A leads to LQTS through abnormal cytosolic Ca2+ homeostasis. Another LQTS mutation in SCN5A R1193Q was identified in 2004 and the electrophysiological property is similar to other gain-of-function SCN5A mutations. The transgenic mouse model for this mutation was also established and the surface Electrocardiogram (ECG) results indicate longer corrected QT interval also present in transgenic mice carrying R1193Q mutation. Besides, quinidine, an anti-arrhythmic medication, can cause arrhythmic symptoms such as premature ventricular contraction (PVC), premature atrial contraction (PAC) and atrioventricular (AV) block in R1193Q transgenic mice.In order to further study the relationship between abnormal Ca2+ handling and the type of SCN5A mutation, either gain-of-function or loss-of-function, we have chosen HL-1 cells, a cell line with indefinite passages in culture with all the adult cardiac phenotypes. The similar abnormal Ca2+ handling was also identified in HL-1 cells e