HERG 1a LQT2 C-terminus truncation mutants display hERG 1b-dependent dominant negative mechanisms

Background The human ether-à-go-go-related gene (hERG 1a) potassium channel is critical for cardiac repolarization. hERG 1b, another variant subunit, co-assembles with hERG 1a, modulates channel biophysical properties and plays an important role in repolarization. Mutations of hERG 1a lead to type 2 long QT syndrome (LQT2), and increased risk for fatal arrhythmias. The functional consequences of these mutations in the presence of hERG 1b are not known. Objective To investigate whether hERG 1a mutants exert dominant negative gating and trafficking defects when co-expressed with hERG 1b. Methods Electrophysiology, co-immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments in HEK293 cells and guinea pig cardiomyocytes were used to assess the mutants on gating and trafficking. Mutations of 1a-G965X and 1a-R1014X, relevant to gating and trafficking were introduced in the C-terminus region. Results The hERG 1a mutants when expressed alone did not result in decreased current amplitude. Compared to wild-type hERG 1a currents, 1a-G965X currents were significantly larger, whereas those produced by the 1a-R1014X mutant were similar in magnitude. Only when co-expressed with wild-type hERG 1a and 1b did a mutant phenotype emerge, with a marked reduction in surface expression, current amplitude, and a corresponding positive shift in the V_1/2 of the activation curve. Co-immunoprecipitation and FRET assays confirmed association of mutant and wild-type subunits. Conclusion Heterologously expressed hERG 1a C-terminus truncation mutants, exert a dominant negative gating and trafficking effect only when co-expressed with hERG 1b. These findings may have potentially profound implications for LQT2 therapy.