Prolonged repolarization and triggered activity induced by adenoviral expression of HERG N629D in cardiomyocytes derived from stem cells

Objective: The long QT syndrome, N629D HERG mutation, alters the pore selectivity signature sequence, GFGN to GFGD. Heterologous co-expression of N629D and the wildtype HERG resulted in a relative loss of the selectivity of K+ over Na+, but its physiologic relevance has not been assessed in cardiac myocytes. Methods and results: Accordingly, N629D was overexpressed, via adenoviral gene transfer, in cardiomyocytes derived from mouse stem cells. Three I-Kr phenotypes were observed: (1) the wildtype-like I-Kr showed inward rectification and a positive tail current; (2) the N629D-like I-Kr showed outward rectification and an inward tail current; and (3) intermediate IKr showed a small outward tail current. Action potentials (AP) were paired with the IKr measurements in each cell. Resting membrane potential (RMP) was critically dependent on the I-Kr phenotype. The resting membrane potential of the cells was -61 +/- 5 mV (n = 40) in wildtype, -63 +/- 3 mV (n = 18) in wildtype-like IKr phenotype, -30 +/- 2 mV (n = 12) in N629D-like and - 47 +/- 2 mV (n 24) in intermediate phenotype (p < 0.00001). Triggered action potential durations (APD) were: 62 +/- 12 ms (n = 6) in wildtype, 65 +/- 11 ms (n = 6) in wildtype-like IKr phenotypes and 106 +/- 10 ms (n = 6) (p < 0.01) in intermediate I-Kr phenotypes. Lowering [K+](o) hyperpolarized wildtype cells and cells with a wildtype-like IKr phenotype, but depolarized those with intermediate phenotype (from - 45 +/- 1 to - 35 +/- 0.5 mV (n = 12), p < 0.01). In 6 of 12 cells, with intermediate phenotype, the hypokalemia-induced depolarization resulted in triggered activity. TTX suppressed this triggered activity. Conclusion: Overexpression of N629D in cardiomyocytes derived from stem cells results in phenotypic variability in IKr, which was the critical determinant of the resting membrane potential, action potential duration and arrhythmogenic response to low [K+](o). (C) 2003 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.