The Jervell and Lange-Nielsen (JLN) syndrome (MIM 220400) is an inherited autosomal recessive disease characterized by a congenital bilateral deafness associated with a QT prolongation on the electrocardiogram, syncopal attacks due to ventricular arrhythmias and a high risk of sudden death. JLN syndrome is a rare disease, which seems to affect less than one percent of all deaf children. Linkage to chromosome 11p15.5 markers was found by analysing four consanguinous families. Recombinants allowed us to map the JLN gene between D11S922 and D11S4146, to a 6-cM interval where KVLQT1, a potassium channel gene causing Romano-Ward (RW) syndrome, the dominant form of long QT syndrome, has been previously localized. An homozygous deletion-insertion event (1244, -7 +8) in the C-terminal domain of this gene was detected in three affected children of two families. We found that KVLQT1 is expressed in the stria vascularis of mouse inner ear by in situ hybridization. Taken together, our data indicate that KVLQT1 is responsible for both JLN and RW syndromes and has a key role not only in the ventricular repolarization but also in normal hearing, probably via the control of endolymph homeostasis.

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.

Abstract-Calmodulin (CaM) is a calcium-sensing protein that binds to Na ϩ channels, with unknown functional consequences. Wild-type CaM produced a hyperpolarizing shift in the steady-state availability of expressed skeletal muscle (1) but not cardiac (hH1) Na ϩ channels. Mutant CaM 1234 did not alter the voltage dependence or the kinetics of gating of either 1 or hH1. Mutation of the highly conserved IQ motif in the carboxyl terminus of both isoforms (IQ/AA) slowed the kinetics of current decay and abolished the effect of wild-type CaM on 1, but did not alter hH1 currents. The IQ/AA mutation eliminated CaM binding to the carboxyl terminus of both 1 and hH1 channels. Inhibition of Ca 2ϩ /CaM kinase (CaM-K) slowed the current decay, the rate of entry into inactivation, and shifted the voltage dependence of hH1 in the depolarizing direction independent of CaM overexpression with no effect on 1 Na ϩ channels. CaM signaling modulates Na ϩ currents in an isoform-specific manner, via direct interaction with skeletal muscle Na ϩ channels and through CaM-K in the case of the cardiac isoform.

This study reports for the first time the critical role of the Na(v)1.5 N-terminal region in channel function and the dominant-negative effect of trafficking-defective channels occurring through α-subunit interaction.

Abstract-Membrane-associated guanylate kinase (MAGUK) proteins are major determinants of the organization of ion channels in the plasma membrane in various cell types. Here, we investigated the interaction between the MAGUK protein SAP97 and cardiac Kv4.2/3 channels, which account for a large part of the outward potassium current, I to , in heart. We found that the Kv4.2 and Kv4.3 channels C termini interacted with SAP97 via a SAL amino acid sequence. SAP97 and Kv4.3 channels were colocalized in the sarcolemma of cardiomyocytes. In CHO cells, SAP97 clustered Kv4.3 channels in the plasma membrane and increased the current independently of the presence of KChIP and dipeptidyl peptidase-like protein-6. Suppression of SAP97 by using short hairpin RNA inhibited I to in cardiac myocytes, whereas its overexpression by using an adenovirus increased I to . Kv4.3 channels without the SAL sequence were no longer regulated by Ca 2ϩ /calmodulin kinase (CaMK)II inhibitors. In cardiac myocytes, pull-down and coimmunoprecipitation assays showed that the Kv4 channel C terminus, SAP97, and CaMKII interact together, an interaction suppressed by SAP97 silencing and enhanced by SAP97 overexpression. In HEK293 cells, SAP97 silencing reproduced the effects of CaMKII inhibition on current kinetics and suppressed Kv4/CaMKII interactions. In conclusion, SAP97 is a major partner for surface expression and CaMKII-dependent regulation of cardiac Kv4 channels. Key Words: potassium channels Ⅲ cardiac myocytes Ⅲ MAGUK proteins Ⅲ calcium/calmodulin-dependent protein kinase Ⅲ dipeptidyl peptidase-like protein 6 I n the heart, the transient outward potassium current, I to , is among the main repolarizing currents that contribute to the early repolarization phase of the action potential and to its adaptation to changes in cardiac cycle length. 1,2 During cardiac diseases, I to is often altered, thus contributing to the risk of cardiac arrhythmias. 3,4 There is a general consensus that voltage-dependent Kv4.2 and Kv4.3 channels are the main molecular determinants of cardiac I to . [5][6][7][8] These ␣ subunits tether with several partners to form functional channels. The best-known partner of Kv4 channels is the Kv channel-interacting protein KChIP. 9 This protein assembles with the N terminus of the pore-forming Kv4 ␣ subunit and acts as a chaperone, regulating the channels surface expression and electrophysiological properties. 9,10 Dipeptidyl peptidase-like protein 6 (DPPX) is another subunit that regulates the activation and inactivation properties of Kv4 channels. 11,12 Membrane-associated guanylate kinase (MAGUK) proteins are important partners for the organization of several ion channels. 13,14 The MAGUK protein SAP97 is abundantly expressed in myocardium and interacts with voltagedependent Shaker channels Kv1.5 15,16 and K ir channels. 17 As in other tissues, SAP97 may regulate the targeting of cardiac ion channels in the sarcolemma. Indeed, in neonatal rat myocytes, SAP97 overexpression causes the clustering and immobilization of Kv1.5 ...

Abstract-The voltage-gated Kϩ channel KVLQT1 is essential for the repolarization phase of the cardiac action potential and for K ϩ homeostasis in the inner ear. Mutations in the human KCNQ1 gene encoding the ␣ subunit of the KVLQT1 channel cause the long-QT syndrome (LQTS). The autosomal dominant form of this cardiac disease, the Romano-Ward syndrome, is characterized by a prolongation of the QT interval, ventricular arrhythmias, and sudden death. The autosomal recessive form, the Jervell and Lange-Nielsen syndrome, also includes bilateral deafness. In the present study, we report the entire genomic structure of KCNQ1, which consists of 19 exons spanning 400 kb on chromosome 11p15.5. We describe the sequences of exon-intron boundaries and oligonucleotide primers that allow polymerase chain reaction (PCR) amplification of exons from genomic DNA. Two new (CA) n repeat microsatellites were found in introns 10 and 14. The present study provides helpful tools for the linkage analysis and mutation screening of the complete KCNQ1 gene. By use of these tools, five novel mutations were identified in LQTS patients by PCR-single-strand conformational polymorphism (SSCP) analysis in the C-terminal part of KCNQ1: two missense mutations, a 20-bp and 1-bp deletions, and a 1-bp insertion. Such mutations in the C-terminal domain of the gene may be more frequent than previously expected, because this region has not been analyzed so far. This could explain the low percentage of mutations found in large LQTS cohorts. (Circ Res. 1999;84:290-297.)

Based on genetic, histological, and functional evidence, we identified a new gene associated with DCM and observed mutations in 3-4% of cases in a population of European descent.

Background-Brugada syndrome (BrS) is caused mainly by mutations in the SCN5A gene, which encodes the ␣-subunit of the cardiac sodium channel Na v 1.5. However, Ϸ20% 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 (PϽ0.01). In contrast, overexpression of mutant E83D alone or E83DϩWT failed to increase I Na (PϽ0.05), 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. (Circ Cardiovasc Genet. 2011;4:261-268.)