The N-Terminal Juxtamembranous Domain of KCNQ1 Is Critical for Channel Surface Expression

Abstract

Abstract-N-terminal mutations in the KCNQ1 channel are frequently linked to fatal arrhythmias in newborn children and adolescents but the cellular mechanisms involved in this dramatic issue remain, however, to be discovered. Here, we analyzed the trafficking of a series of N-terminal truncation mutants and identified a critical trafficking motif of KCNQ1. This determinant is located in the juxtamembranous region preceding the first transmembrane domain of the protein. Three mutations (Y111C, L114P and P117L) implicated in inherited Romano-Ward LQT1 syndrome, are embedded within this domain. Reexpression studies in both COS-7 cells and cardiomyocytes showed that the mutant proteins fail to exit the endoplasmic reticulum. KCNQ1 subunits harboring Y111C or L114P exert a dominant negative effect on the wild-type KCNQ1 subunit by preventing plasma membrane trafficking of heteromultimeric channels. The P117L mutation had a less pronounced effect on the trafficking of heteromultimeric channels but altered the kinetics of the current. Furthermore, we showed that the trafficking determinant in KCNQ1 is structurally and functionally conserved in other KCNQ channels and constitutes a critical trafficking determinant of the KCNQ channel family.Computed structural predictions correlated the potential structural changes introduced by the mutations with impaired protein trafficking. In conclusion, our studies unveiled a new role of the N-terminus of KCNQ channels in their trafficking and its implication in severe forms of LQT1 syndrome. Key Words: channels Ⅲ KCNQ Ⅲ LQT Ⅲ membrane Ⅲ trafficking T he long QT syndrome (LQTS) is a cause of sudden cardiac death and is characterized by an increased QT interval on patients' ECG. The prolongation of the QT interval predisposes patients to cardiac arrhythmias known as torsades de pointes, eventually leading to ventricular fibrillation. Over the last decade, inherited mutations in genes encoding cardiac ionic channels or associated partners have been identified in 4 different congenital LQT syndromes (LQT1-4). For example, mutations in KCNQ1 have been associated with either autosomal dominant Romano-Ward (RW) or recessive Jervell and Lange-Nielsen (JLN) LQT1 syndromes. [1][2][3] KCNQ1 encodes the pore forming ␣-subunit of a voltage gated K ϩ channel, which associates with an accessory subunit, KCNE1, in the heart to form channels responsible for the slow component of the delayed repolarizing K ϩ current (I Ks ). 4,5 KCNQ1 mutations associated with LQT1 alter channel function through different mechanisms. Carboxy terminal mutations often disrupt channel assembly, alter regulatory subunit association and cause mistrafficking. 6 -9 Mutations in the pore region and transmembrane domains (TMDs), on the other hand, produce dominant negative effects on potassium permeation and KCNQ1 channel gating functions. 10 -12 Although more than a hundred different LQT1-causing mutations have been reported in KCNQ1, only a few are located in its cytoplasmic N-terminus. 2,3 In general, these mutations...

Low Incidence of Sudden Cardiac Death in a Swedish Y111C Type 1 Long-QT syndrome Population

“…17 None of the 80 individuals in the study population experienced SCD during the mean follow-up time of 28Ϯ20 years, with a mean of 25Ϯ20 years before the initiation of therapy. This finding is unexpected because the specific Y111C-KCNQ1 mutation has been shown to result in structural and functional effects on the K s channel leading to a severely reduced inward potassium current over the cell membrane in vitro, 9,10 characteristics that suggest a highly malignant mutation.…”

“…10 Indeed, the penetrance of the Y111C mutation in the Swedish population is unusually high for a LQT1 mutation. 1,18 The absolute majority (86%) of the Y111C mutation carriers exhibits phenotypic effects of the mutation in the form of a prolonged QTc, which usually corresponds to an increased risk of developing symptoms.…”

“…High-risk mutations have been identified within the LQT1 subgroup. [7][8][9][10][11] The Y111C-KCNQ1 (c.332AϾG) mutation has previously been described in 1 patient, a 36-year-old woman, who presented with syncope at the age of 3 years. 3 She experienced Ͼ30 syncopal episodes triggered by emotional or physical stress and had a QT interval, corrected for heart rate, of 520 ms. Electrophysiological studies subsequently demonstrated that the Y111C mutation causes intracellular sequestration of mutated ion channels in the endoplasmic reticulum that renders them inactive.…”

“…3 She experienced Ͼ30 syncopal episodes triggered by emotional or physical stress and had a QT interval, corrected for heart rate, of 520 ms. Electrophysiological studies subsequently demonstrated that the Y111C mutation causes intracellular sequestration of mutated ion channels in the endoplasmic reticulum that renders them inactive. 9 Moreover, the mutation exerts a strong dominantnegative effect on the coexpressed wild-type ion channels encoded by the unaffected allele, 10 leading to a severely reduced inward potassium current in vitro. 9,10 We have observed several Swedish families sharing the same Y111C-KCNQ1 mutation, expressing an apparent benign clinical phenotype.…”

“…9 Moreover, the mutation exerts a strong dominantnegative effect on the coexpressed wild-type ion channels encoded by the unaffected allele, 10 leading to a severely reduced inward potassium current in vitro. 9,10 We have observed several Swedish families sharing the same Y111C-KCNQ1 mutation, expressing an apparent benign clinical phenotype. In this study, we describe the phenotype of a Swedish Y111C population with a focus on life-threatening cardiac events.…”