LETTER
LQT1-phenotypes in hiPSC: Are we measuring the right thing? Jervell and Lange-Nielsen syndrome presents with deafness and syncopal attacks. Prolongation of QT-duration was identified as a hallmark of the disease, which became the first “long-QT syndrome” (LQT-syndrome). Genetic studies revealed a mutation in a gene encoding a voltage potassium channel, which was called KVLQT1. When coexpressed with the correct accessory subunit, KVLQT1 was shown to generate a slowly activating component of the delayed rectifier potassium current (IKs) (1, 2). The role of IKs for cardiac action potential (AP) configuration has remained enigmatic for some time. IKs-Blocker did not affect AP configuration in multicellular preparations of dog hearts, despite the fact that both compounds effectively block IKs in ventricular myocytes (3). Thus, failure of IKs-block to prolong AP duration (APD) must be the result of some peculiar properties of IKs. The activation time constant of IKs in dog ventricular myocytes is so slow that no IKs was generated during the first 300 ms of voltage steps in the range of a cardiac APplateau (up to +30 mV). β-Adrenoceptorstimulation increases IKs conductance and accelerates activation kinetics (3). Only under this condition, the channel generates sufficient current so that IKs-blockade prolongs APD. Accordingly, pharmacological IKs-block prolonged QTc in awake dogs and mimicked the phenotype of LQT-syndrome (3). Collectively, these results suggest that a certain amount of sympathetic tone is needed to activate enough IKs to contribute to repolarization. The main findings of this seminal work in dogs was later confirmed
E1968 | PNAS | April 21, 2015 | vol. 112 | no. 16
in left ventricles from undiseased humans (4). These findings are in stark contrast to results recently published in PNAS by Zhang et al. (5). Here, baseline APD, i.e., in the absence of β-adrenoceptor stimulation, was markedly longer in human induced pluripotent stem cell-derived cardiomymocytes (hiPSC-CM) from patients with a LQT1 mutation than in cells from controls. At first glance, this finding could simply indicate that repolarization reserve in hiPSC-CM is profoundly different from native human ventricular cardiomyocytes, with a much larger impact of unstimulated IKs compared with other potassium currents. However, this interpretation is at odds with the wellknown slow-activation kinetics of (even stimulated) IKs (4) and solid data showing that effects of IKs-block, even in the presence of β-adrenoceptorstimulation, are restricted to the late repolarization phase, whereas the initial AP phase remained unaffected. In the study by Zhang et al. (5), the AP prolongation, both in a patient-derived hiPSCCM line and in hiPSC-CM in which IKs was genetically suppressed, was associated with more positive AP amplitude and an elevated plateau phase. This result can only be explained by suppression of a very rapidly activated repolarizing current. Indeed, figure 5A of ref. 5 shows IKs-blocker–sensitive currents in control hiPSC-CM that activate almost instantaneously, very different from classic IKs. Thus, we would like to raise a word of caution with regard to the interpretation that the difference between the patient-derived
hiPSC-CM and controls in the study by Zhang et al. (5) were indeed a result of differences in IKs and, conversely, indicate a role of IKs in these cells. Such a conclusion would require a thorough explanation why IKs expressed in hiPSC-CM should fundamentally differ from the well-characterized IKs in native cardiomyocytes or heterologous expression systems. Torsten Christa,b,1, András Horvatha,b, and Thomas Eschenhagena,b a
Department of Experimental Pharmacology and Toxicology, University Medical Center Hamburg-Eppendorf, 20246 Hamburg, Germany; and bDZHK (German Centre for Cardiovascular Research), German Centre for Cardiovascular Research, partner site Hamburg/Kiel/Lübeck, 20246 Hamburg, Germany 1 Barhanin J, et al. (1996) K(V)LQT1 and lsK (minK) proteins associate to form the I(Ks) cardiac potassium current. Nature 384(6604):78–80. 2 Sanguinetti MC, et al. (1996) Coassembly of K(V)LQT1 and minK (IsK) proteins to form cardiac I(Ks) potassium channel. Nature 384(6604):80–83. 3 Volders PG, et al. (2003) Probing the contribution of IKs to canine ventricular repolarization: Key role for beta-adrenergic receptor stimulation. Circulation 107(21):2753–2760. 4 Jost N, et al. (2005) Restricting excessive cardiac action potential and QT prolongation: A vital role for IKs in human ventricular muscle. Circulation 112(10):1392–1399. 5 Zhang M, et al. (2014) Recessive cardiac phenotypes in induced pluripotent stem cell models of Jervell and Lange-Nielsen syndrome: Disease mechanisms and pharmacological rescue. Proc Natl Acad Sci USA 111(50):E5383–E5392.
Author contributions: T.C., A.H., and T.E. wrote the paper. The authors declare no conflict of interest. 1
To whom correspondence should be addressed. Email: t.christ@ uke.de.
www.pnas.org/cgi/doi/10.1073/pnas.1503347112
LQT1-phenotypes in hiPSC: Are we measuring the right thing? Jervell and Lange-Nielsen syndrome presents with deafness and syncopal attacks. Prolongation of QT-duration was identified as a hallmark of the disease, which became the first “long-QT syndrome” (LQT-syndrome). Genetic studies revealed a mutation in a gene encoding a voltage potassium channel, which was called KVLQT1. When coexpressed with the correct accessory subunit, KVLQT1 was shown to generate a slowly activating component of the delayed rectifier potassium current (IKs) (1, 2). The role of IKs for cardiac action potential (AP) configuration has remained enigmatic for some time. IKs-Blocker did not affect AP configuration in multicellular preparations of dog hearts, despite the fact that both compounds effectively block IKs in ventricular myocytes (3). Thus, failure of IKs-block to prolong AP duration (APD) must be the result of some peculiar properties of IKs. The activation time constant of IKs in dog ventricular myocytes is so slow that no IKs was generated during the first 300 ms of voltage steps in the range of a cardiac APplateau (up to +30 mV). β-Adrenoceptorstimulation increases IKs conductance and accelerates activation kinetics (3). Only under this condition, the channel generates sufficient current so that IKs-blockade prolongs APD. Accordingly, pharmacological IKs-block prolonged QTc in awake dogs and mimicked the phenotype of LQT-syndrome (3). Collectively, these results suggest that a certain amount of sympathetic tone is needed to activate enough IKs to contribute to repolarization. The main findings of this seminal work in dogs was later confirmed
E1968 | PNAS | April 21, 2015 | vol. 112 | no. 16
in left ventricles from undiseased humans (4). These findings are in stark contrast to results recently published in PNAS by Zhang et al. (5). Here, baseline APD, i.e., in the absence of β-adrenoceptor stimulation, was markedly longer in human induced pluripotent stem cell-derived cardiomymocytes (hiPSC-CM) from patients with a LQT1 mutation than in cells from controls. At first glance, this finding could simply indicate that repolarization reserve in hiPSC-CM is profoundly different from native human ventricular cardiomyocytes, with a much larger impact of unstimulated IKs compared with other potassium currents. However, this interpretation is at odds with the wellknown slow-activation kinetics of (even stimulated) IKs (4) and solid data showing that effects of IKs-block, even in the presence of β-adrenoceptorstimulation, are restricted to the late repolarization phase, whereas the initial AP phase remained unaffected. In the study by Zhang et al. (5), the AP prolongation, both in a patient-derived hiPSCCM line and in hiPSC-CM in which IKs was genetically suppressed, was associated with more positive AP amplitude and an elevated plateau phase. This result can only be explained by suppression of a very rapidly activated repolarizing current. Indeed, figure 5A of ref. 5 shows IKs-blocker–sensitive currents in control hiPSC-CM that activate almost instantaneously, very different from classic IKs. Thus, we would like to raise a word of caution with regard to the interpretation that the difference between the patient-derived
hiPSC-CM and controls in the study by Zhang et al. (5) were indeed a result of differences in IKs and, conversely, indicate a role of IKs in these cells. Such a conclusion would require a thorough explanation why IKs expressed in hiPSC-CM should fundamentally differ from the well-characterized IKs in native cardiomyocytes or heterologous expression systems. Torsten Christa,b,1, András Horvatha,b, and Thomas Eschenhagena,b a
Department of Experimental Pharmacology and Toxicology, University Medical Center Hamburg-Eppendorf, 20246 Hamburg, Germany; and bDZHK (German Centre for Cardiovascular Research), German Centre for Cardiovascular Research, partner site Hamburg/Kiel/Lübeck, 20246 Hamburg, Germany 1 Barhanin J, et al. (1996) K(V)LQT1 and lsK (minK) proteins associate to form the I(Ks) cardiac potassium current. Nature 384(6604):78–80. 2 Sanguinetti MC, et al. (1996) Coassembly of K(V)LQT1 and minK (IsK) proteins to form cardiac I(Ks) potassium channel. Nature 384(6604):80–83. 3 Volders PG, et al. (2003) Probing the contribution of IKs to canine ventricular repolarization: Key role for beta-adrenergic receptor stimulation. Circulation 107(21):2753–2760. 4 Jost N, et al. (2005) Restricting excessive cardiac action potential and QT prolongation: A vital role for IKs in human ventricular muscle. Circulation 112(10):1392–1399. 5 Zhang M, et al. (2014) Recessive cardiac phenotypes in induced pluripotent stem cell models of Jervell and Lange-Nielsen syndrome: Disease mechanisms and pharmacological rescue. Proc Natl Acad Sci USA 111(50):E5383–E5392.
Author contributions: T.C., A.H., and T.E. wrote the paper. The authors declare no conflict of interest. 1
To whom correspondence should be addressed. Email: t.christ@ uke.de.
www.pnas.org/cgi/doi/10.1073/pnas.1503347112
