Evaluating Long-QT Type 8 Mutations in Induced Pluripotent Stem Cell Derived Cardiomyocytes

Abstract

Long QT syndrome type 8 (LQT8) is a severe disorder caused by a point mutation in CaV1.2 L-type Ca2+ channels, resulting in a prolonged QT interval, arrhythmia, and sudden cardiac death. The canonical and most common mutation, G406R, occurs most often within the mutually exclusive exon 8a, found in 20% of cardiac CaV1.2 variants. Yet, even heterozygous expression (10% of cardiac channels) of G406R is hugely deleterious. Despite this limited expression, the patients exhibit severe symptoms, which are often fatal in early childhood. Conversely, some recently discovered CaV1.2 mutations, occur in constitutive exons, resulting in an overall higher expression (50%). We therefore compared the biophysical effects of LQT8 mutations and evaluated the impact of each mutation on the action potential profile of induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs). Patch clamp recordings showed the mutations differentially altered Ca2+ dependent inactivation (CDI), and voltage dependent inactivation (VDI). In addition, action potential recordings demonstrated significant prolongation in iPSC-CMs harboring each mutation, consistent with LQT8. Moreover, optical mapping demonstrated readily inducible arrhythmic behavior in each mutant iPSC-CM line, displayed as a spiral pattern in the propagation of the action potential across the coverslip. Thus, LQT8 iPSC-CMs recapitulate not only the expected AP prolongation of LQT8, but the arrhythmogenic episodes seen in patients, making them an ideal model system to study this disease.