Abstract 17271: An Alternatively Spliced Kcnh2 Variant Restores Cardiac Repolarization in a Model of Long QT Syndrome: Implications for Phenotypic Variation in Human Long QT Syndrome

Abstract

Introduction: Mutations in KCNH2 ( HERG1 , LQT-2) are a common cause of Long QT Syndrome (LQTS). Despite sharing the same mutation, affected family members often display marked differences in disease severity, including risk of sudden cardiac death. The genetic basis of this phenotypic variation is not known. We previously validated a zebrafish model of human LQT-2. Here, we use a repolarization-deficient zebrafish mutant ( breakdance , bre ) to probe the molecular basis of phenotypic variation. Methods: Bre is due to a recessive kchn2 mutation (I59S) within the cytoplasmic N-terminal domain. 5’-RACE and qPCR were used to clone and quantify novel N-terminal variants from zebrafish and human heart. Biophysical properties of zebrafish and human variants were characterized using voltage- and patch-clamp techniques. In vivo functional effects were determined by over-expressing the variants in knch2-null zebrafish embryos, and assaying for ability to restore ventricular repolarization by characterizing phenotypes of the embryonic heart at 48 hour-post-fertilization. Results: We identified a novel, alternatively spliced N-terminal variant, kcnh2 1c , with in-frame deletion of Exon 2-6. The 1c variant constitutes ~40% of the kcnh2 transcript, and is expressed 4X greater in atrium than ventricle. The 1c variant deletes the disease-causing mutation, I59S. Activation of kcnh2 1c channels is similar to WT, but deactivation is accelerated. In vivo , kcnh2 1c functions similar to WT, and is able to partially restore ventricular repolarization in knch2-null zebrafish. We identified 2 novel N-terminal variants in human heart that are also differentially expressed between the atrium and ventricle. Biophysical function of the human variants is similar to zebrafish 1c. Conclusion: Kcnh2 1c modulates cardiac repolarization in the bre mutant by excising the disease-causing mutation. Approximately 30% of LQT-2 mutations localize to the N-terminus, and are subject to alternative splicing. By excising N-terminal mutations human 1c variants may modulate repolarization and influence phenotypic expression in LQTS.