Pharmacological Targeting of Gain-of-function KCNQ1 Mutations Predisposing to Atrial Fibrillation

Abstract

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in adults. The discovery of mutations in familial AF illustrated the contribution of specific genetic factors to AF susceptibility and suggested molecular mechanisms for some heritable forms of this common arrhythmia. This dissertation focused on the first identified causative familial AF mutation (S140G) in the voltage gated potassium channel gene KCNQ1. Because transgenic S140G mouse models were inadequate for reproducing an AF-prone substrate, we first developed methods for high yield isolation, extended culture, and transfection of adult atrial myocytes suitable for electrophysiological experiments. Using a novel serial sampling technique, we were able to achieve reproducible high yields of both ventricular and atrial myocytes. Adenovirus-mediated transduction proved most efficient in rabbit myocyte preparations with high titers and robust expression of transgenes. Using this rabbit adult atrial myocyte model system, we demonstrated that S140G-IKs expressing myocytes had triggered activity at low frequency stimulation and a shorter action potential duration at higher frequency with hyperpolarized resting membrane potential compared to WT-IKs expressing myocytes. We hypothesized that KCNQ1 mutations predisposing to AF encode potassium channels with distinct pharmacological properties that could render them susceptible to selective inhibition. Consistent with our hypothesis, we observed that S140G-IKs exhibits enhanced sensitivity to the IKs-selective blocker, HMR-1556. This enhancement was correlated with the emergence of an additional high affinity state, which was also observed with V141M-IKs, a neighboring gain-of-function AF-associated mutation. Using a concentration that predominantly inhibits the high affinity state, we demonstrated that HMR-1556 effectively suppressed HET-IKs amplitude to a level that was not significantly different from WT-IKs. Further, this drug concentration attenuated the use-dependent accumulation of HET-IKs that occurs during repetitive pulsing. Significantly, we demonstrated that HMR-1556 can mitigate the S140G-IKs induced APD shortening in cultured adult rabbit atrial myocytes without affecting action potentials in myocytes expressing WT-IKs. These findings offer evidence supporting the potential for genotype-specific therapy of familial AF.