Hidetada Yoshida

Evidence for a Single Nucleotide Polymorphism in the KCNQ1 Potassium Channel that Underlies Susceptibility to Life-Threatening Arrhythmias

Ion Channel Polymorphism and Cardiac Arrhythmia. Introduction: Congenital long QT syndrome (LQTS) is a genetically heterogeneous arrhythmogenic disorder caused by mutations in at least five different genes encoding cardiac ion channels. It was suggested recently that common polymorphisms of LQTS‐associated genes might modify arrhythmia susceptibility in potential gene carriers.

Novel KCNE3 mutation reduces repolarizing potassium current and associated with long QT syndrome.

Long QT syndrome (LQTS) is an inherited disease involving mutations in the genes encoding a number of cardiac ion channels and a membrane adaptor protein. Among the genes that are responsible for LQTS, KCNE1 and KCNE2 are members of the KCNE family of genes, and function as ancillary subunits of Kv channels. The third KCNE gene, KCNE3, is expressed in cardiac myocytes and interacts with KCNQ1 to change the channel properties. However, KCNE3 has never been linked to LQTS. To investigate the association between KCNE3 and LQTS, we conducted a genetic screening of KCNE3 mutations and single nucleotide polymorphisms (SNPs) in 485 Japanese LQTS probands using DHPLC‐WAVE system and direct sequencing. Consequently, we identified two KCNE3 missense mutations, located in the N‐ and C‐terminal domains. The functional effects of these mutations were examined by heterologous expression systems using CHO cells stably expressing KCNQ1. One mutation, p.R99λH was identified in a 76‐year‐old woman who suffered torsades de pointes (TdP) after administration of disopyramide. Another mutation, p.T4A was identified in a 16‐year‐old boy and 67‐year‐old woman. Although the boy carried another KCNH2 mutation, he was asymptomatic. On the other hand, the woman suffered from hypokalemia‐induced TdP. In a series of electrophysiological analyses, the KCNQ1(Q1)+KCNE3(E3)‐R99λH channel significantly reduced outward current compared to Q1+E3‐WT, though the current density of the Q1+E3‐T4A channel displayed no statistical significance. This is the first report of KCNE3 mutations associated with LQTS. Screening for variants in the KCNE3 gene is of clinical importance for LQTS patients. Hum Mutat 30, 557–563, 2009.

Adrenergic regulation of the rapid component of delayed rectifier K+ current: Implications for arrhythmogenesis in LQT2 patients

BACKGROUND KCNH2 gene mutations disrupting rapid component of I(K) (I(Kr)) underlie type 2 congenital long QT syndrome (LQT2). Startled auditory stimuli are specific symptomatic triggers in LQT2, thus suggesting fast arrhythmogenic mechanism. OBJECTIVE We investigated acute alpha(1A)- and cyclic adenosine monophosphate (cAMP)-related beta-adrenergic modulation of I(Kr) in HL-1 cardiomyocytes, wild type (WT)- and 2 LQT2-associated mutant Kv11.1 channels (Y43D- and K595E-Kv11.1) reconstituted in Chinese hamster ovary (CHO) cells. METHODS I(Kr) and Kv11.1 currents were recorded using the whole-cell patch-clamp technique and confocal microscopy of HL-1 cardiomyocytes transfected with green fluorescent protein (GFP)-tagged pleckstrin homology domain of phospholipase C-delta(1) visualized fluctuations of membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) content. RESULTS In HL-1 cardiomyocytes expressing human alpha(1A)-adrenoceptor, superfusion with phenylephrine significantly reduced I(Kr) amplitude, shifted current activation to more positive potentials, and accelerated kinetics of deactivation. Confocal images showed a decline of membrane PIP(2) content during phenylephrine exposure. Simultaneous application of adenylyl cyclase activator forskolin and phosphodiesterase inhibitor 3-isobutyl-1-methylxantine (IBMX) shifted I(Kr) activation to more negative potentials and decreased tail current amplitudes after depolarizations between +10 and +50 mV. In CHO cells, alpha(1A)-adrenoceptor activation downregulated WT-Kv11.1 channels and forskolin/IBMX produced a dual effect. Expressed alone, the Y43D-Kv11.1 or K595E-Kv11.1 channel had no measurable function. However, co-expression of WT-Kv11.1 and each mutant protein evoked currents with loss-of-function alterations but identical to WT-Kv11.1 alpha(1A)- and forskolin/IBMX-induced regulation. CONCLUSION Acute adrenergic regulation of at least 2 Kv11.1 mutant channels is preserved as in WT-Kv11.1 and native I(Kr). Suppression of alpha(1A)-adrenoceptor-related transduction might have therapeutic implications in some cases of LQT2.