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Background: The ability to differentiate patient-specific human induced pluripotent stem cells in cardiac myocytes (hiPSC-CM) offers novel perspectives for cardiovascular research. A number of studies, that reported mainly on current-voltage curves used hiPSC-CM to model voltage-gated Na + channel (Na v ) dysfunction. However, the expression patterns and precise biophysical and pharmacological properties of Na v channels from hiPSC-CM remain unknown. Our objective was to study the characteristics of Na v channels from hiPSC-CM and assess the appropriateness of this novel cell model., Methods: We generated hiPSC-CM using the recently described monolayer-based differentiation protocol., Results: hiPSC-CM expressed cardiac-specific markers, exhibited spontaneous electrical and contractile activities, and expressed distinct Na v channels subtypes. Electrophysiological, pharmacological, and molecular characterizations revealed that, in addition to the main Na v 1.5 channel, the neuronal tetrodotoxin (TTX)-sensitive Na v 1.7 channel was also significantly expressed in hiPSC-CM. Most of the Na + currents were resistant to TTX block. Therapeutic concentrations of lidocaine, a class I antiarrhythmic drug, also inhibited Na + currents in a use-dependent manner. Na v 1.5 and Na v 1.7 expression and maturation patterns of hiPSC-CM and native human cardiac tissues appeared to be similar. The 4 Na v β regulatory subunits were expressed in hiPSC-CM, with β3 being the preponderant subtype., Conclusions: The findings indicated that hiPSC-CM robustly express Na v 1.5 channels, which exhibited molecular and pharmacological properties similar to those in native cardiac tissues. Interestingly, neuronal Na v 1.7 channels were also expressed in hiPSC-CM and are likely to be responsible for the TTX-sensitive Na v current., (Copyright © 2016 Canadian Cardiovascular Society. Published by Elsevier Inc. All rights reserved.)

Background: Mexiletine may protect patients with long QT syndrome (LQTS) type 3 from arrhythmias. However, we found an unusual in utero presentation of intermittent atrioventricular block and ventricular tachycardia (spontaneous or lidocaine-induced) in a fetus and his sibling with LQTS., Objective: The purpose of this study was to investigate the underlying channelopathy and functional alteration., Methods: Mutations were searched in KCNQ1, HERG, KCNE1, KCNE2, and SCN5A genes. In expressed mutants, whole-cell voltage clamp defined the electrophysiologic properties., Results: Novel missense mutations involving hERG (F627L) at the pore region and SCN5A (R43Q) at the N-terminus were found in the proband and in family members with prolonged QT interval. In oocytes injected with mRNA encoding hERG/ F627L, almost zero K(+) currents were elicited. In coinjected oocytes, the currents were decreased to half. In tsA201 cells transfected with SCN5A/R43Q, although the baseline kinetics of the Na current were similar to wild type, lidocaine caused a unique hyperpolarizing shift of the activation and increased the availability of Na currents at resting voltages. Window currents were enhanced due to a right shift of steady-state inactivation. These electrophysiologic alterations after lidocaine may lead to the development of ventricular tachycardia., Conclusion: We identified a novel hERG/F627L mutation that results in LQTS with fetal onset of atrioventricular block and ventricular tachycardia. A coexisting SCN5A/R43Q variant, although it per se does not prolong repolarization, contributes to the development of ventricular tachyarrhythmias after lidocaine. Patients with such latent lidocaine-induced phenotype who are given lidocaine or mexiletine may be at risk.

Background: 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., Objective: We explored whether the properties of this mutation could explain the pathology., Methods: 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., Results: 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., Conclusion: Decreased current levels caused by a trafficking defect may explain the long QT syndrome observed in our patient.