Cellular properties of C-terminal KCNH2 long QT syndrome mutations: Description and divergence from clinical phenotypes

BackgroundC-terminal KCNH2 mutations are commonly associated with a more benign clinical presentation, but mutations localized in close proximity may exhibit different clinical and biophysical phenotypes. The value of detailed cellular characterization of such mutant channels in vitro has not been studied with respect to clinical risk stratification of affected patients.ObjectiveThe purpose of this study was to study the cellular properties and clinical presentation of C-terminal KCNH2 missense mutations localized in close proximity.MethodsUnrelated female index patients with KCNH2 mutations and heterogeneous clinical presentation were identified. Mutations were studied in vitro with biophysical and molecular biology techniques.ResultsIonic currents from all three mutants were reduced compared with wild type. Coexpression experiments mimicking heterozygosity indicated haploinsufficiency as the mechanism of current suppression in all cases. One mutation (R954C) was associated with reversible QTc prolongation during macrolide treatment (QTc ∼600 ms). Biophysical properties included reduced current amplitude, accelerated deactivation, and altered activation voltage dependence. The patient affected by L955V suffered from recurrent syncope (QTc ∼460 ms), and this mutation led to greatly reduced current and reduced KCNH2 protein in plasma membrane preparations. Confocal microscopy supported these findings, suggesting aggregate formation and endoplasmic reticulum retention by L955V. The mutation carrier of G1036D (QTc ∼530 ms) was resuscitated from cardiac arrest, but biophysical characteristics were less strongly affected.ConclusionThe results of our study provide evidence that C-terminal mutations localized in proximity to each other may exhibit strongly different and poorly correlated clinical and cellular phenotypes. These findings provide evidence that even detailed characterization of long QT syndrome mutations may not provide additional definitive information for clinical risk stratification.