A new C-terminal hERG mutation A915fs+47X associated with symptomatic LQT2 and auditory-trigger syncope.
ABSTRACT A novel mutation of hERG (A915fs+47X) was discovered in a 32-year-old woman with torsades de pointes, long QTc interval (515 ms), and syncope upon auditory trigger.
We explored whether the properties of this mutation could explain the pathology.
Whole-cell A915fs+47X (del) and wild-type (WT) currents were recorded in transiently transfected COS7 cells or Xenopus oocytes. Western blots and sedimentation analysis of del/WT hERG were used to analyze protein expression, assembly, and trafficking.
The tail current density at -40 mV after a 2-s depolarization to +40 mV in COS7 cells expressing del was 36% of that for WT. Inactivation was 1.9-fold to 2.8-fold faster in del versus WT between -60 and +60 mV. In the range -60 to -10 mV, we found that a nondeactivating fraction of current was increased in del at the expense of a rapidly deactivating fraction, with a slowly deactivating fraction being unchanged. In Xenopus oocytes, expression of del alone produced 38% of WT currents, whereas coexpression of 1/2 WT + 1/2 del produced 49.8%. Furthermore, the expression of del protein at the cell surface was reduced by about 50%. This suggests that a partial trafficking defect of del contributes to the reduction in del current densities and to the dominant negative effect when coexpressed with WT. In model simulations, the mutation causes a 10% prolongation of action potential duration.
Decreased current levels caused by a trafficking defect may explain the long QT syndrome observed in our patient.
Article: Two components of the delayed rectifier K+ current in ventricular myocytes of the guinea pig type. Theoretical formulation and their role in repolarization.[show abstract] [hide abstract]
ABSTRACT: Two distinct delayed rectifier K+ currents, IKr and IKs, were found recently in ventricular cells. We formulated these currents theoretically and investigated their roles in action potential repolarization and the restitution of action potential duration (APD). The Luo-Rudy (L-R) model of the ventricular action potential was used in the simulations. The single delayed rectifier K+ current in the model was replaced by IKr and IKs. Our results show that IKs is the major outward current during the plateau repolarization. A specific block of either IKr or IKs can effectively prolong APD to the same degree. Therefore, either channel provides a target for class III antiarrhythmic drugs. In the simulated guinea pig ventricular cell, complete block of IKr does not result in early afterdepolarizations (EADs). In contrast, > 80% block of IKs results in abnormal repolarization and EADs. This behavior reflects the high IKs-to-IKr density ratio (approximately 8:1) in this cell and can be reversed (ie, IKr block can cause EADs) by reducing the ratio of IKs to IKr. The computed APD restitution curve is consistent with the experimental behavior, displaying fast APD variation at short diastolic intervals (DIs) and downward shift at longer DIs with the decrease of basic drive cycle length (BCL). Examining the ionic currents and their underlying kinetic processes, we found that activation of both IKr and IKs is the primary determinant of the APD restitution at shorter DIs, with Ca2+ current through L-type channels (ICa) playing a minor role. The rate of APD change depends on the relative densities of IKr and IKs; it increases when the IKr-to-IKs density ratio is large. The BCL-dependent shift of restitution at longer DIs is primarily attributed to long-lasting changes in [Ca2+]i. This in turn causes different degrees of Ca(2+)-dependent inactivation of ICa and different degrees of Ca(2+)-dependent conductance of IKs at very long DIs (> 5 s) for different BCLs. This BCL dependence of ICa and IKs that is secondary to long-lasting changes in [Ca2+]i is responsible for APD changes at long DIs and can be viewed as a "memory property" of cardiac cells.Circulation Research 08/1995; 77(1):140-52. · 9.49 Impact Factor
Article: HERG channel dysfunction in human long QT syndrome. Intracellular transport and functional defects.[show abstract] [hide abstract]
ABSTRACT: Mutations in HERG are associated with human chromosome 7-linked congenital long QT (LQT-2) syndrome. We used electrophysiological, biochemical, and immunohistochemical methods to study the molecular mechanisms of HERG channel dysfunction caused by LQT-2 mutations. Wild type HERG and LQT-2 mutations were studied by stable and transient expression in HEK 293 cells. We found that some mutations (Y611H and V822M) caused defects in biosynthetic processing of HERG channels with the protein retained in the endoplasmic reticulum. Other mutations (I593R and G628S) were processed similarly to wild type HERG protein, but these mutations did not produce functional channels. In contrast, the T474I mutation expressed HERG current but with altered gating properties. These findings suggest that the loss of HERG channel function in LQT-2 mutations is caused by multiple mechanisms including abnormal channel processing, the generation of nonfunctional channels, and altered channel gating.Journal of Biological Chemistry 09/1998; 273(33):21061-6. · 4.77 Impact Factor
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ABSTRACT: The tertiary amine lidocaine is used clinically for preventing cardiac arrhythmias, and has been widely studied on mammalian tissue. Xenopus oocytes were used as an expression system to study the effect of lidocaine on a sodium (Na) channel, derived from a full-length human heart (hH1) cDNA clone. The concentration dependence of the lidocaine block of hH1 Na current was consistent with a binding stoichiometry of 1:1. At low frequency stimulation, and at holding potentials < or = 100 mV, the IC50 was 226 microM, comparable to values found in mammalian cardiac cells. Lidocaine also shifted the steady-state inactivation of hH1 Na current to hyperpolarized potentials in a dose-dependent manner. Our experiments suggest that lidocaine block is state dependent, with high affinity for an inactivated state (KI = 11 microM) and low affinity for the resting state (KR = 3.9 mM). The quaternary amine derivative of lidocaine, QX-314, had no effect on Na current at an extracellular concentration of 1 mM.Journal of Molecular and Cellular Cardiology 12/1992; 24(11):1231-6. · 5.17 Impact Factor