A Single Na
            +
            Channel Mutation Causing Both Long-QT and Brugada Syndromes

ΔKPQ, a three amino acid [lysine (K), proline (P), glutamine (Q)] deletion mutation of the human cardiac Na channel (hH1), which is one cause of long QT syndrome (LQT3), has impaired inactivation resulting in a late sodium current. To better understand inactivation in ΔKPQ, we applied a site-3 toxin anthopleurin A, which has been shown to inhibit inactivation from the open state with little or no effect on inactivation from the closed state(s) in wild-type hH1. In contrast to the effect of site-3 toxins on wild-type hH1, inactivation from closed state(s) in toxin-modified ΔKPQ demonstrated a large negative shift in the Na channel availability curve of nearly −14 mV. Recovery from inactivation showed that toxin-modified ΔKPQ channels recovered slightly faster than those in control, whereas development of inactivation at potentials negative to −80 mV showed that inactivation developed much more rapidly in toxin-modified ΔKPQ channels compared with control. An explanation for our results is that closed-state inactivation in toxin-modified ΔKPQ is enhanced by the mutated inactivation lid being positioned “closer” to its receptor resulting in an increased rate of association between the inactivation lid and its receptor.

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Induced pluripotent stem cells used to reveal drug actions in a long QT syndrome family with complex genetics

The Journal of General Physiology ◽

10.1085/jgp.201210899 ◽

2012 ◽

Vol 141 (1) ◽

pp. 61-72 ◽

Cited By ~ 130

Author(s):

Cecile Terrenoire ◽

Kai Wang ◽

Kelvin W. Chan Tung ◽

Wendy K. Chung ◽

Robert H. Pass ◽

...

Keyword(s):

Stem Cells ◽

Induced Pluripotent Stem Cells ◽

Long Qt Syndrome ◽

Pluripotent Stem Cells ◽

Na Channel ◽

Long Qt ◽

Complex Genetics ◽

Drug Actions ◽

Qt Syndrome ◽

Induced Pluripotent

Understanding the basis for differential responses to drug therapies remains a challenge despite advances in genetics and genomics. Induced pluripotent stem cells (iPSCs) offer an unprecedented opportunity to investigate the pharmacology of disease processes in therapeutically and genetically relevant primary cell types in vitro and to interweave clinical and basic molecular data. We report here the derivation of iPSCs from a long QT syndrome patient with complex genetics. The proband was found to have a de novo SCN5A LQT-3 mutation (F1473C) and a polymorphism (K897T) in KCNH2, the gene for LQT-2. Analysis of the biophysics and molecular pharmacology of ion channels expressed in cardiomyocytes (CMs) differentiated from these iPSCs (iPSC-CMs) demonstrates a primary LQT-3 (Na+ channel) defect responsible for the arrhythmias not influenced by the KCNH2 polymorphism. The F1473C mutation occurs in the channel inactivation gate and enhances late Na+ channel current (INaL) that is carried by channels that fail to inactivate completely and conduct increased inward current during prolonged depolarization, resulting in delayed repolarization, a prolonged QT interval, and increased risk of fatal arrhythmia. We find a very pronounced rate dependence of INaL such that increasing the pacing rate markedly reduces INaL and, in addition, increases its inhibition by the Na+ channel blocker mexiletine. These rate-dependent properties and drug interactions, unique to the proband’s iPSC-CMs, correlate with improved management of arrhythmias in the patient and provide support for this approach in developing patient-specific clinical regimens.

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