Trafficking-deficient long QT syndrome mutation KCNQ1-T587M confers severe clinical phenotype by impairment of KCNH2 membrane localization: Evidence for clinically significant IKr-IKs α-subunit interaction

BackgroundKCNQ1-T587M is a trafficking-deficient long QT syndrome (LQTS) missense mutation. Affected patients exhibit severe clinical phenotypes that are not explained by the mutant's effects on IKs. Previous work showed a KCNH2 and KCNQ1 α-subunit interaction that increases KCNH2 membrane localization and function.ObjectiveWe hypothesized that failure of trafficking-deficient KCNQ1-T587M to enhance KCNH2 membrane expression could reduce KCNH2 current versus wild-type KCNQ1 (KCNQ1-WT), contributing to the LQTS phenotype of KCNQ1-T587M carriers.MethodsPatch-clamp, protein biochemical studies, confocal imaging, and in vivo transfection of guinea pig cardiomyocytes were performed.ResultsKCNQ1-T587M failed to generate functional current when coexpressed with KCNE1 and caused haploinsufficiency when coexpressed with KCNQ1-WT/KCNE1. Coexpression of KCNQ1-WT with KCNH2 increased IKCNH2 versus KCNH2 alone (P <.05). Immunoblots and confocal microscopy indicated increased plasma membrane localization of KCNH2 α-subunits in cells cotransfected with KCNQ1-WT plasmid, while total KCNH2 protein synthesis and KCNH2 glycosylation remained unaffected, which suggests a chaperone effect of KCNQ1-WT to enhance the membrane localization of KCNH2. KCNH2 also coimmunoprecipitated with KCNQ1-WT. Although KCNQ1-T587M coprecipitated with KCNH2, the mutant was retained intracellularly and failed to increase KCNH2 membrane localization, abolishing the KCNQ1-WT chaperone function and reducing IKCNH2 upon coexpression substantially compared with coexpression with KCNQ1-WT (P <.05). In vivo transfection of KCNQ1-T587M in guinea pigs suppressed IKr in isolated cardiomyocytes.ConclusionThe trafficking-deficient LQTS mutation KCNQ1-T587M fails to show the chaperoning function that enhances KCNH2 membrane localization with KCNQ1-WT. This novel mechanism results in reduced IKCNH2, which would be expected to decrease repolarization reserve and synergize with reduced IKCNQ1 caused directly by the mutation, potentially explaining the malignant clinical phenotype in affected patients.