Differential Impact of Distinct Long Qt Syndrome-1 (LQTS-1) C-Terminus Mutations on KCNQ1-KCNE1 Channel Trafficking and Gating

Abstract

KCNQ1/KCNE1 subunits together generate the slowly activating, delayed rectifier potassium current, IKs, important for human cardiac repolarization. Mutations throughout KCNQ1 lead to long QT syndrome-1 (LQTS-1), which predisposes patients to lethal ventricular arrhythmias. Many LQTS-1 mutations map to KCNQ1 C-terminus, but in many cases the mechanistic bases for their pathophysiological effects in heart are unknown. We compared the functional impact and mechanistic bases of six LQTS-1 mutations in distinct regions of KCNQ1 C-terminus that have been implicated in: PIP2 or calmodulin binding domains (R555H, L619M). Optical assays for channel surface density and electrophysiological recordings were carried out in Chinese Hamster Ovary (CHO)cells. All KCNQ1 mutants, except L619M, displayed a significant decrease in channel surface density, which was either partially (R366W, V524G, R539W,G589D) or fully (R555H) rescued by wild-type KCNQ1. G589D and L619M alone yielded no currents, while all other mutations moderately reduced current amplitude. When co-expressed with wild-type KCNQ1, G589D current amplitude was fully rescued whereas L619M exerted a dominant negative effect. When co-expressed with KCNE1: all mutants displayed reduced current amplitude compared to control, albeit to different extents; all except L619M displayed the slowly activating signature of IKs; three mutants displayed a large rightward shift in the activation curve which was either partially (R366W, R555H) or fully (R539W) recovered with wild-type KCNQ1. The data reveal a surprising heterogeneity of trafficking and gating mechanisms underlie KCNQ1 C-terminus mutations. Such information is essential for developing potential targeted therapies for LQTS-1 patients. We further introduce the first study of L619M, an unusual KCNQ1 mutant that trafficks normally to the cell surface, but is essentially non-functional and exerts a strong dominant-negative effect on wild-type KCNQ1 channels.