Background KVLQT1 , the gene encoding the α-subunit of a cardiac potassium channel, is the most common cause of the dominant form of long-QT syndrome (LQT1-type), the Romano-Ward syndrome (RWS). The overall phenotype of RWS is characterized by a prolonged QT interval on the ECG and cardiac ventricular arrhythmias leading to recurrent syncopes and sudden death. However, there is considerable variability in the clinical presentation, and potential severity is often difficult to evaluate. To analyze the relationship between phenotypes and underlying defects in KVLQT1 , we investigated mutations in this gene in 20 RWS families originating from France. Methods and Results By PCR-SSCP analysis, 16 missense mutations were identified in KVLQT1 , 11 of them being novel. Fifteen mutations, localized in the transmembrane domains S2-S3, S4-S5, P, and S6, were associated with a high percentage of symptomatic carriers (55 of 95, or 58%) and sudden deaths (23 of 95, or 24%). In contrast, a missense mutation, Arg ⁵⁵⁵ Cys, identified in the C-terminal domain in 3 families, was associated with a significantly less pronounced QT prolongation (459±33 ms, n=41, versus 480±32 ms, n=70, P =.0012), and significantly lower percentages of symptomatic carriers (7 of 44, or 16%, P <.001) and sudden deaths (2 of 44, or 5%, P <.01). Most of the cardiac events occurring in these 3 families were triggered by drugs known to affect ventricular repolarization. Conclusions Our data show a wide KVLQT1 allelic heterogeneity among 20 families in which KVLQT1 causes RWS. We describe the first missense mutation in the C-terminal domain of KVLQT1, which is clearly associated with a fruste phenotype, which could be a favoring factor of acquired LQT syndrome.

Mutations in the delayed rectifier K+ channel subunit KvLQT1 have been identified as responsible for both Romano–Ward (RW) and Jervell and Lange‐Nielsen (JLN) inherited long QT syndromes. We report the molecular cloning of a human KvLQT1 isoform that is expressed in several human tissues including heart. Expression studies revealed that the association of KvLQT1 with another subunit, IsK, reconstitutes a channel responsible for the I Ks current involved in ventricular myocyte repolarization. Six RW and two JLN mutated KvLQT1 subunits were produced and co‐expressed with IsK in COS cells. All the mutants, except R555C, fail to produce functional homomeric channels and reduce the K+ current when co‐expressed with the wild‐type subunit. Thus, in both syndromes, the main effect of the mutations is a dominant‐negative suppression of KvLQT1 function. The JLN mutations have a smaller dominant‐negative effect, in agreement with the fact that the disease is recessive. The R555C subunit forms a functional channel when expressed with IsK, but with altered gating properties. The voltage dependence of the activation is strongly shifted to more positive values, and deactivation kinetics are accelerated. This finding indicates the functional importance of a small positively charged cytoplasmic region of the KvLQT structure where two RW and one JLN mutations have been found to take place.

Background—: Brugada syndrome (BrS) is caused mainly by mutations in the SCN5A gene, which encodes the α-subunit of the cardiac sodium channel Na _v 1.5. However, ≈20% of probands have SCN5A mutations, suggesting the implication of other genes. MOG1 recently was described as a new partner of Na _v 1.5, playing a potential role in the regulation of its expression and trafficking. We investigated whether mutations in MOG1 could cause BrS. Methods and Results—: MOG1 was screened by direct sequencing in patients with BrS and idiopathic ventricular fibrillation. A missense mutation p.Glu83Asp (E83D) was detected in a symptomatic female patient with a type-1 BrS ECG but not in 281 controls. Wild type (WT)- and mutant E83D-MOG1 were expressed in HEK Na _v 1.5 stable cells and studied using patch-clamp assays. Overexpression of WT-MOG1 alone doubled sodium current (I _Na ) density compared to control conditions ( P <0.01). In contrast, overexpression of mutant E83D alone or E83D+WT failed to increase I _Na ( P <0.05), demonstrating the dominant-negative effect of the mutant. Microscopy revealed that Na _v 1.5 channels failed to properly traffic to the cell membrane in the presence of the mutant. Silencing endogenous MOG1 demonstrated a 54% decrease in I _Na density. Conclusions—: Our results support the hypothesis that dominant-negative mutations in MOG1 can impair the trafficking of Na _v 1.5 to the membrane, leading to I _Na reduction and clinical manifestation of BrS. Moreover, silencing MOG1 reduced I _Na , demonstrating that MOG1 is likely to be important in the surface expression of Na _v 1.5 channels. All together, our data support MOG1 as a new susceptibility gene for BrS.

Calmodulin (CaM) is a calcium-sensing protein that binds to Na+ channels, with unknown functional consequences. Wild-type CaM produced a hyperpolarizing shift in the steady-state availability of ex...

Brugada syndrome (BrS) is an autosomal-inherited cardiac arrhythmia characterized by an ST-segment elevation in the right precordial leads of the electrocardiogram and an increased risk of syncope and sudden death. SCN5A, encoding the cardiac sodium channel Na_v1.5, is the main gene involved in BrS. Despite the fact that several mutations have been reported in the N-terminus of Na_v1.5, the functional role of this region remains unknown. We aimed to characterize two BrS N-terminal mutations, R104W and R121W, a construct where this region was deleted, ΔNter, and a construct where only this region was present, Nter. Patch-clamp recordings in HEK293 cells demonstrated that R104W, R121W, and ΔNter abolished the sodium current I_Na. Moreover, R104W and R121W mutations exerted a strong dominant-negative effect on wild-type (WT) channels. Immunocytochemistry of rat neonatal cardiomyocytes revealed that both mutants were mostly retained in the endoplasmic reticulum and that their co-expression with WT channels led to WT channel retention. Furthermore, co-immunoprecipitation experiments showed that Na_v1.5-subunits were interacting with each other, even when mutated, deciphering the mutation dominant-negative effect. Both mutants were mostly degraded by the ubiquitin–proteasome system, while ΔNter was addressed to the membrane, and Nter expression induced a two-fold increase in I_Na. In addition, the co-expression of N-terminal mutants with the gating-defective but trafficking-competent R878C-Na_v1.5 mutant gave rise to a small I_Na. This study reports for the first time the critical role of the Na_v1.5 N-terminal region in channel function and the dominant-negative effect of trafficking-defective channels occurring through α-subunit interaction.