Background— The electrocardiographic short QT-interval syndrome forms a distinct clinical entity presenting with a high rate of sudden death and exceptionally short QT intervals. The disorder has recently been linked to gain-of-function mutation in KCNH2 . The present study demonstrates that this disorder is genetically heterogeneous and can also be caused by mutation in the KCNQ1 gene. Methods and Results— A 70-year man presented with idiopathic ventricular fibrillation. Both immediately after the episode and much later, his QT interval was abnormally short without any other physical or electrophysiological anomalies. Analysis of candidate genes identified a g919c substitution in KCNQ1 encoding the K ⁺ channel KvLQT1. Functional studies of the KvLQT1 V307L mutant (alone or coexpressed with the wild-type channel, in the presence of IsK) revealed a pronounced shift of the half-activation potential and an acceleration of the activation kinetics leading to a gain of function in I _Ks . When introduced in a human action potential computer model, the modified biophysical parameters predicted repolarization shortening. Conclusions— We present an alternative molecular mechanism for the short QT-interval syndrome. Functional and computational studies of the KCNQ1 V307L mutation identified in a patient with this disorder favor the association of short QT with mutation in KCNQ1 .

Background— TheSCN5Agene encoding the human cardiac sodium channel α subunit plays a key role in cardiac electrophysiology. Mutations inSCN5Alead to a large spectrum of phenotypes, including long-QT syndrome, Brugada syndrome, and isolated progressive cardiac conduction defect (Lenègre disease). Methods and Results— In the present study, we report the identification of a novel singleSCN5Amissense mutation causing either Brugada syndrome or an isolated cardiac conduction defect in the same family. A G-to-T mutation at position 4372 was identified by direct sequencing and was predicted to change a glycine for an arginine (G1406R) between the DIII-S5 and DIII-S6 domain of the sodium channel protein. Among 45 family members, 13 were carrying the G1406RSCN5Amutation. Four individuals from 2 family collateral branches showed typical Brugada phenotypes, including ST-segment elevation in the right precordial leads and right bundle branch block. One symptomatic patient with the Brugada phenotype required implantation of a cardioverter-defibrillator. Seven individuals from 3 other family collateral branches had isolated cardiac conduction defects but no Brugada phenotype. Three flecainide test were negative. One patient with an isolated cardiac conduction defect had an episode of syncope and required pacemaker implantation. An expression study of the G1406R-mutatedSCN5Ashowed no detectable Na⁺current but normal protein trafficking. Conclusions— We conclude that the same mutation in theSCN5Agene can lead either to Brugada syndrome or to an isolated cardiac conduction defect. Our findings suggest that modifier gene(s) may influence the phenotypic consequences of aSCN5Amutation.

Phosphatidylinositol‐4,5‐bisphosphate (PIP2) is a major signaling molecule implicated in the regulation of various ion transporters and channels. Here we show that PIP2 and intracellular MgATP control the activity of the KCNQ1/KCNE1 potassium channel complex. In excised patch–clamp recordings, the KCNQ1/KCNE1 current decreased spontaneously with time. This rundown was markedly slowed by cytosolic application of PIP2 and fully prevented by application of PIP2 plus MgATP. PIP2‐dependent rundown was accompanied by acceleration in the current deactivation kinetics, whereas the MgATP‐dependent rundown was not. Cytosolic application of PIP2 slowed deactivation kinetics and also shifted the voltage dependency of the channel activation toward negative potentials. Complex changes in the current characteristics induced by membrane PIP2 was fully restituted by a model originally elaborated for ATP‐regulated two transmembrane‐domain potassium channels. The model is consistent with stabilization by PIP2 of KCNQ1/KCNE1 channels in the open state. Our data suggest a striking functional homology between a six transmembrane‐domain voltage‐gated channel and a two transmembrane‐domain ATP‐gated channel.

The voltage-gated KCNQ1 potassium channel regulates key physiological functions in a number of tissues. In the heart, KCNQ1 alpha-subunits assemble with KCNE1 beta-subunits forming a channel complex constituting the delayed rectifier current I(Ks). In epithelia, KCNQ1 channels participate in controlling body electrolyte homeostasis. Several regulatory mechanisms of the KCNQ1 channel complexes have been reported, including protein kinase A (PKA)-phosphorylation and beta-subunit interactions. However, the mechanisms controlling the membrane density of KCNQ1 channels have attracted less attention. Here we demonstrate that KCNQ1 proteins expressed in HEK293 cells are down-regulated by Nedd4/Nedd4-like ubiquitin-protein ligases. KCNQ1 and KCNQ1/KCNE1 currents were reduced upon co-expression of Nedd4-2, the isoform among the nine members of the Nedd4/Nedd4-like family displaying the highest expression level in human heart. In vivo expression of a catalytically inactive form of Nedd4-2, able to antagonize endogenous Nedd4-2 in guinea-pig cardiomyocytes, increased I(Ks) significantly, but did not modify I(K1). Concomitant with the reduction in current induced by Nedd4-2, an increased ubiquitylation as well as a decreased total level of KCNQ1 proteins were observed in HEK293 cells. Pull-down and co-immunoprecipitation experiments showed that Nedd4-2 interacts with the C-terminal part of KCNQ1. The Nedd4/Nedd4-like-mediated regulation of the KCNQ1 channel complexes is strictly dependent on a PY motif located in the distal part of the C-terminal domain. When this motif was mutated, the current and ubiquitylation levels were unaffected by Nedd4-2, and Nedd4-2 proteins were neither pulled-down nor co-immunoprecipitated. These results suggest that KCNQ1 internalization and stability is physiologically regulated by its Nedd4/Nedd4-like-dependent ubiquitylation. This mechanism may thereby be important in regulating the surface density of the KCNQ1 channels in cardiomyocytes and other cell types.