MOLECULAR PATHOPHYSIOLOGY OF CONGENITAL LONG QT SYNDROME M. S. Bohnen, G. Peng, S. H. Robey, C. Terrenoire, V. Iyer, K. J. Sampson, and R. S. Kass Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York L Bohnen MS, Peng G, Robey SH, Terrenoire C, Iyer V, Sampson KJ, Kass RS. Molecular Pathophysiology of Congenital Long QT Syndrome. Physiol Rev 97: 89 –134, 2017. Published November 2, 2016; doi:10.1152/physrev.00008.2016.—Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syn- drome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) I Ks , the slow delayed rectifier current; 2) I Kr , the rapid delayed rectifier current; and 3) I Na , the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has ad- vanced understanding of ion channel structure-function relationships, physiology, and pharmaco- logical response to clinically employed and experimental pharmacological agents. Our understand- ing of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype- driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality. I. INTRODUCTION 89 II. I Ks DYSFUNCTION IN CONGENITAL... 91 III. I Kr DYSFUNCTION IN CONGENITAL... 99 IV. I Na DYSFUNCTION IN CONGENITAL... 106 V. OTHER SUBTYPES OF CONGENITAL LQTS 114 VI. GENOTYPE-DRIVEN CLINICAL... 116 VII. CONCLUSIONS 120 I. INTRODUCTION A genetic disorder disrupting electrical activity in the heart, congenital Long QT Syndrome (LQTS) can lead to life- threatening arrhythmias and sudden cardiac death. In the first cases of congenital LQTS, described in 1957, several children in one family presented with prolongation of the QT interval on the electrocardiogram (ECG) and congenital deafness (189). This came to be known as Jervell and Lange-Nielsen syndrome (JLNS), the autosomal recessive form of LQTS. The more common, autosomal dominant form of congenital LQTS that presents without deafness was first described in 1963 and 1964 in two separate cases and became known as Romano-Ward syndrome (346, 459). Since these initial patient descriptions, advances in our understanding of the mechanisms of cardiac electrical excitability at the tissue, cellular, and molecular level have yielded much insight into the pathophysiology of congenital LQTS. Clinically, congenital LQTS patients often first present after episodes of syncope and/or seizure, and the ECG reveals a prolonged QT interval. The ECG measures electrical activ- ity of the heart over time, at the patient’s body surface. The primary electrical signals observed include the P wave, which signifies atrial depolarization; the QRS complex, which arises from ventricular depolarization; and the T wave, due to ventricular repolarization (FIGURE 1A). The QT interval, therefore, reflects the time elapsed from the initiation of ventricular depolarization to the end of ventric- ular repolarization. The QT interval shortens with increas- ing heart rate thus requiring a normalization, or “correc- tion,” for heart rate. For a diagnosis of LQTS, this rate- corrected QT (QTc) interval prolongation on a 12-lead ECG generally is referenced as 470 ms for males and 480 ms for females. QTc also varies with age, and thus, an age-appropriate prolonged QTc interval in a patient aids the diagnosis of LQTS (209). However, diagnosis of LQTS based on absolute QT interval cutoffs can be challenging, since there is considerable overlap in the QTc distribution of affected patients and otherwise healthy individuals (193). Asymptomatic patients can have intervals beyond Physiol Rev 97: 89 –134, 2017 Published November 2, 2016; doi:10.1152/physrev.00008.2016 89 0031-9333/17 Copyright © 2017 the American Physiological Society Downloaded from journals.physiology.org/journal/physrev (171.243.000.161) on March 6, 2023.
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Molecular Pathophysiology of Congenital Long QT SyndromeMOLECULAR PATHOPHYSIOLOGY OF CONGENITAL LONG QT SYNDROME M. S. Bohnen, G. Peng, S. H. Robey, C. Terrenoire, V. Iyer, K. J. Sampson, and R. S. Kass
Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
L Bohnen MS, Peng G, Robey SH, Terrenoire C, Iyer V, Sampson KJ, Kass RS. Molecular Pathophysiology of Congenital Long QT Syndrome. Physiol Rev 97: 89–134, 2017. Published November 2, 2016; doi:10.1152/physrev.00008.2016.—Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these
channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syn- drome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has ad- vanced understanding of ion channel structure-function relationships, physiology, and pharmaco- logical response to clinically employed and experimental pharmacological agents. Our understand- ing of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype- driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.
I. INTRODUCTION 89 II. IKs DYSFUNCTION IN CONGENITAL... 91 III. IKr DYSFUNCTION IN CONGENITAL... 99 IV. INa DYSFUNCTION IN CONGENITAL... 106 V. OTHER SUBTYPES OF CONGENITAL LQTS 114 VI. GENOTYPE-DRIVEN CLINICAL... 116 VII. CONCLUSIONS 120
I. INTRODUCTION
A genetic disorder disrupting electrical activity in the heart, congenital Long QT Syndrome (LQTS) can lead to life- threatening arrhythmias and sudden cardiac death. In the first cases of congenital LQTS, described in 1957, several children in one family presented with prolongation of the QT interval on the electrocardiogram (ECG) and congenital deafness (189). This came to be known as Jervell and Lange-Nielsen syndrome (JLNS), the autosomal recessive form of LQTS. The more common, autosomal dominant form of congenital LQTS that presents without deafness was first described in 1963 and 1964 in two separate cases and became known as Romano-Ward syndrome (346, 459). Since these initial patient descriptions, advances in our understanding of the mechanisms of cardiac electrical excitability at the tissue, cellular, and molecular level have
yielded much insight into the pathophysiology of congenital LQTS.
Clinically, congenital LQTS patients often first present after episodes of syncope and/or seizure, and the ECG reveals a prolonged QT interval. The ECG measures electrical activ- ity of the heart over time, at the patient’s body surface. The primary electrical signals observed include the P wave, which signifies atrial depolarization; the QRS complex, which arises from ventricular depolarization; and the T wave, due to ventricular repolarization (FIGURE 1A). The QT interval, therefore, reflects the time elapsed from the initiation of ventricular depolarization to the end of ventric- ular repolarization. The QT interval shortens with increas- ing heart rate thus requiring a normalization, or “correc- tion,” for heart rate. For a diagnosis of LQTS, this rate- corrected QT (QTc) interval prolongation on a 12-lead ECG generally is referenced as 470 ms for males and 480 ms for females. QTc also varies with age, and thus, an age-appropriate prolonged QTc interval in a patient aids the diagnosis of LQTS (209). However, diagnosis of LQTS based on absolute QT interval cutoffs can be challenging, since there is considerable overlap in the QTc distribution of affected patients and otherwise healthy individuals (193). Asymptomatic patients can have intervals beyond
Physiol Rev 97: 89–134, 2017 Published November 2, 2016; doi:10.1152/physrev.00008.2016
890031-9333/17 Copyright © 2017 the American Physiological Society Downloaded from journals.physiology.org/journal/physrev (171.243.000.161) on March 6, 2023.
these cutoff and develop no arrhythmias; similarly, QTc intervals below this cutoff can be seen in patients with es- tablished LQTS (with clinical arrhythmias and positive ge- netic testing) (16, 17, 338). Clinical scoring systems (368), as well as genetic testing, can be helpful to assist with the diagnosis of congenital LQTS (193), particularly when QT intervals are on the borderline (within 20 ms of these cut- offs) or when clinical history is equivocal. This review will focus primarily on the various forms of congenital LQTS.
Ion channels are the molecular entities underlying most ionic currents in the heart, allowing passive diffusion of ions across the cell membrane’s electrochemical gradients (FIGURE 2). A selectivity filter in the channel pore, deter- mined by distinct atomic components (152, 315), endows selective permeation of ions, such as Na, K, and Ca2
(246). Some ion channels exhibit voltage-dependent gating, where voltage-sensing domains respond to changes in mem- brane potential to cause channel opening or closing (247).
The ventricular cellular action potential results from the summation of a large number of ion channel currents and electrogenic pumps that control the cellular membrane po- tential (see Nerbonne and Kass review, Ref. 299), but in this review we will focus on three key ion channels that are well-established to be linked to LQTS and that are illus- trated in FIGURE 1B. The cellular resting membrane poten- tial is approximately 85 mV, determined largely by in- wardly rectifying K channels. Inward rectification is a property that hinders the outward flow of K as membrane potentials become positive, but passes K more efficiently at potentials negative to the K equilibrium potential where the flow of K would be inward. Hence, the term “inward rectification” is used to describe these channels (304). Ac- tivation of Nav1.5, the primary voltage-gated sodium chan- nel in the heart, leads to sodium influx (INa) and membrane depolarization. As the cell reaches approximately 40 mV, voltage-gated L-type calcium channels begin to open, lead- ing to calcium influx (ICa) (299). Concurrently during the upstroke of the action potential, potassium channels, in- cluding those carrying the IKs and IKr delayed rectifier cur- rents, begin to activate slowly. As the cell reaches 30 mV, INa inactivates almost completely. At this time, a brief and small repolarization of the membrane potential occurs via fast activation of the transient outward K current, Ito. In the plateau phase that follows, influx of Ca2 through volt- age-gated calcium channels is balanced mainly by IKs and IKr. The plateau phase ceases as calcium channels inactivate and outward potassium efflux persists, leading to a net out- ward membrane current, and cell repolarization back to the cellular resting potential. During this process, the Na-K- ATPase helps maintain intracellular concentrations of these key ions (304).
In LQTS patients, the QT is prolonged presumably due to prolongation of underlying action potential durations
IKs
IKr
A
B
FIGURE 1. ECG to cellular ionic currents. A: membrane depolar- ization and the rapid upstroke of the ventricular action potential give rise to the QRS complex. The duration of the QT-interval corre- sponds to the time to ventricular repolarization. The relatively stable membrane potential during the plateau phase of the action potential gives rise to a brief isoelectric period. Ventricular repolarization gives rise to the T-wave. B: time course of several ionic currents that underlie ventricular action potential morphology (currents not to scale). The rapidly activating and inactivating INa drives membrane depolarization. Two K currents, IKs and IKr, contribute most to the repolarizing current necessary to drive membrane potential back to rest.
Extracellular
K+
K+
Cytosol
Na+
Na+
FIGURE 2. Schematic of a generic K and Na ion channel. Ion channels allow for selective permeation of ions through the plasma membrane down their electrochemical gradient. The classic K
channel consists of four identical pore-forming subunits, whereas each Na channel is formed by a single polypeptide with four homol- ogous domains.
BOHNEN ET AL.
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(APDs), most often caused by decreased repolarizing IKs or IKr activity, or persistent sodium influx that extends through the plateau phase. A loss of IKs or IKr function, or a gain of INa function, predisposes ventricular myocytes to early afterdepolarizations (EADs), and in some cases to de- layed afterdepolarizations (DADs) which may underlie de- generation into a characteristic sinusoidal wave pattern on the ECG, referred to as torsades de pointes, which may further regress into ventricular fibrillation and sudden car- diac death. EADs are driven in large part by calcium entry via L-type calcium channels during prolonged action poten- tial plateau phases, whereas DADs, which occur over the diastolic range of potentials after action potential repolar- ization, are caused by intracellular calcium overload, also a consequence of action potential prolongation (126). Addi- tionally, arrhythmic activity may result from altered refrac- toriness and impulse block, also putative consequences of prior APD prolongation. Thus treatment in LQTS aims to prevent malignant ventricular arrhythmia by shortening the QTc interval to minimize cardiac event rates.
Ion channels may interact with a variety of molecular enti- ties that contribute to their trafficking, stabilization, signal- ing, and function (299). Coordination among different ion channel types facilitates the ionic balance necessary for the generation of an action potential and normal electrical propagation through the heart. LQTS mutations may cause an increase or decrease in ion channel function, disrupting normal ionic balance leading to pathological electrical ac- tivity in the heart.
There are 15 subtypes of congenital LQTS, each associated with mutations on a different gene (15) (TABLE 1). The most common subtypes, LQT1, LQT2, and LQT3, account for the vast majority of congenital LQTS. LQT1 and LQT2 are associated with mutations in KCNQ1 and KCNH2 (which
encodes hERG), respectively, while LQT3 is associated with mutations in SCN5A, the gene coding for the Nav1.5 sodium channel alpha subunit. Disease association for vari- ants in these three proteins is supported by genome-wide association studies (300) and functional electrophysiologi- cal characterization of mutant channels. In addition, LQTS- associated mutations exist less frequently in other ion chan- nels, modulatory channel subunits, and signaling- or cyto- skeleton-associated proteins. Understanding the molecular mechanisms that cause LQTS allows for optimization of genotype-specific treatments. In this review, we discuss the molecular physiology, biology, and pathophysiology un- derlying congenital LQTS, and the cellular and molecular underpinnings of genotype-driven clinical management of LQTS.
II. IKs DYSFUNCTION IN CONGENITAL LQTS
A. Introduction
Of the different subtypes of inherited LQTS, subtypes 1, 5, and 11 are associated with mutations in proteins that par- ticipate in the macromolecular complex which generates and modulates the slow delayed rectifier potassium current (IKs), which plays a critical role in the repolarization of the cardiac action potential. Among all LQTS subtypes, LQT1 is the most common, representing 30-35% of all congenital LQTS (8). Upregulation of IKs during -adrenergic stimu- lation is critical to normal physiology by shortening ventric- ular APD and allowing for adequate diastolic filling in the context of an elevated heart rate (354). Cardiac events in patients with IKs-associated LQTS are often triggered by stress and exercise, consistent with the role of adrenergic stimulation in the regulation of IKs (350, 371). Insight into
Table 1. Subtypes of congenital LQTS and their associated genes, proteins, and effects on cardiac currents
LQT Subtype Gene Protein Current
LQT1 KCNQ1 KCNQ1 (Kv7.1) 2IKs
LQT2 KCNH2 hERG (Kv11.1) 2IKr
LQT3 SCN5A Nav1.5 1INa
LQT6 KCNE2 KCNE2 (MiRP1) 2IKr
LQT7 (Andersen-Tawil syndrome type 1) KCNJ2 Kir2.1 2IK1
LQT8 (Timothy syndrome) CACNA1C Cav1.2 1ICa
LQT9 CAV3 Caveolin 3 1INa
LQT10 SCN4B Nav1.5 4 1INa
LQT11 AKAP9 AKAP-9 (yotiao) 2IKs
LQT12 SNTA1 1-Syntrophin 1INa
LQT14 CALM1 Calmodulin Multichannel interactions LQT15 CALM2 Calmodulin Multichannel interactions
CONGENITAL LONG QT SYNDROME
91Physiol Rev • VOL 97 • JANUARY 2017 • www.prv.org Downloaded from journals.physiology.org/journal/physrev (171.243.000.161) on March 6, 2023.
the molecular mechanisms of disease mutations have greatly improved our understanding of LQTS pathophysi- ology and helped provide a first step to the future develop- ment of targeted therapies.
B. Physiology
IKs is an outward potassium current with unique kinetics and voltage dependence and plays a key role in the repolar- ization of the cardiac action potential (299). In 1969, the delayed rectifier potassium current in sheep Purkinje fibers was studied and shown to comprise two kinetically distinct components (305). These currents were later subjected to pharmacological dissection in guinea pig ventricular myo- cytes and identified as IKs and IKr (360).
IKs is slowly activating and most prominent during the pla- teau and repolarizing phases of the cardiac action potential, where it contributes to counterbalancing calcium influx and repolarization (FIGURE 1B). Expression of IKs has been dem- onstrated in both human atrial and ventricular myocytes (198, 235, 457). In addition, it has also been measured in cardiomyocytes from a variety of non-human mammalian species including dogs (241, 406, 437, 442, 493) and rab- bits (352). On the other hand, IKs is expressed at very low levels or absent in mouse hearts (478), most likely because at very high heart rates in mouse heart (500 beats/min) this channel would have little or no time to activate and not affect cardiac electrophysiology in the mouse.
Importantly, IKs is subject to upregulation by -adrenergic stimulation to control APD in the face of sympathetic nerve activity (210, 444). During sympathetic activation, adren- ergic stimulation increases the outward IKs current, which counterbalances the concomitant increase in inward cal- cium current, prevents prolongation of the cardiac APD, and allows for adequate diastolic filling times between heart beats (386). However, insufficient IKs activation such as that seen in LQT1 results in failure to counterbalance the calcium influx, prolonging the action potential and increas- ing susceptibility to arrhythmia. This is consistent with ex- ercise being a key trigger of cardiac events seen in LQT1 patients (371).
C. Molecular Biology
The first known subtypes of inherited LQTS, LQT1-3, were distinguished by mapping to distinct chromosomes, with LQT1 mapping to chromosome 11 (191, 212, 213). Even- tually it was found that the KCNQ1 (KvLQT1) gene is responsible for LQT1 and encodes a potassium channel (451). The current conducted by this channel was rapidly activating and minimally inactivating, unlike any previ- ously known current in the heart, but soon it was shown that KCNQ1 together with the accessory protein KCNE1
(minK) generates IKs (36, 358). While KCNE1 had previ- ously been thought to be an independent potassium channel (167, 412), it was confirmed that KCNQ1 is actually the - or pore-forming subunit of IKs while KCNE1 is a critical - or modulatory subunit. Coexpression of KCNQ1 and KCNE1 generates the hallmark IKs current with slow acti- vation. KCNQ1 and KCNE1 have been shown to be ex- pressed in all four chambers in the heart (45), as well as the inner ear (301, 351), where IKs is thought to play a role in K secretion into the endolymph (68). This explains the observation that congenital deafness is a key feature of JLNS. In addition, KCNQ1 and KCNE1 are expressed else- where in the body, including the pancreas, the kidneys, and the brain (1).
1. KCNQ1, the pore-forming subunit
KCNQ1, like most voltage-gated potassium channels, consists of six transmembrane helices (451) (FIGURE 3A). Four subunits of KCNQ1 come together to form a chan- nel that is capable of voltage-dependent gating (FIGURE 3B). On each subunit, the helices S1–S4 comprise the voltage-sensing domain, where the S4 helix, with its pos- itively charged arginine residues, senses changes in mem- brane potential (310). Following the voltage-sensing do- main is the pore domain, which comprises the pore-loop, an extracellular segment containing the selectivity filter, and helices that line the pore, S5 and S6. Furthermore, the cytoplasmic loop between S4 and S5 plays important roles in the voltage sensor-to-pore coupling and in volt- age-dependent gating (82, 229, 498), which has been demonstrated in other voltage-gated channels as well (75, 122, 355). The cytoplasmic loop between S2 and S3 also plays a role in channel gating (498). The COOH- terminal domain (CTD) of KCNQ1 is large and contains four intracellular -helices referred to as A-D. A wide range of functions has been attributed to the CTD includ- ing calmodulin binding, interaction with -subunits and scaffolding proteins, as well as channel assembly and trafficking (159, 469).
2. KCNE1, the -subunit
Coexpression of KCNE1 with KCNQ1 leads to a drastic change in channel function to generate IKs. Most promi- nently, assembly with KCNE1 leads to a delay in the onset of activation, an increase in channel amplitude (FIGURE 3C), as well as a depolarizing shift in the current-voltage rela- tionship (not illustrated) (36, 358). This results in a channel that, compared with most other voltage-gated potassium channels, activates at more positive voltages and with slower kinetics. KCNE1 is a 129-amino acid protein that consists of a single transmembrane helix, with an extracel- lular NH2 terminus and intracellular COOH terminus (412). It is thought to have extensive contact with KCNQ1 including the voltage-sensing domain (37, 84, 203, 309,
BOHNEN ET AL.
92 Physiol Rev • VOL 97 • JANUARY 2017 • www.prv.org Downloaded from journals.physiology.org/journal/physrev (171.243.000.161) on March 6, 2023.
378, 479), the pore domain (84, 262, 311), as well as the CTD (161, 510). Previous crosslinking studies suggest that KCNE1 is located in a cleft between the voltage-sensing domain and pore domain of different KCNQ1 subunits (84, 479), underlying its ability to modulate KCNQ1 gating. With respect to the stoichiometry of KCNE1 to KCNQ1, some studies suggest a fixed 2:4 ratio (278, 326), while others suggest a flexibility in stoichiometry (293, 456) that allows for modulation of kinetics of assembled channels to provide another level of flexibility in channel function. Although three other members of the KCNE family, KCNE2-KCNE4, also are expressed in the heart (45) and are capable of modulating KCNQ1 activity (45, 154, 367, 422), whether they associate with KCNQ1 in vivo to contribute to potassium currents in the heart remains to be explored.
3. Molecular components of adrenergic stimulation
There have been considerable efforts to elucidate the mo- lecular pathway for the -adrenergic regulation of IKs. In
1988 it was shown that stimulation of PKA activity by a cAMP analog can upregulate the delayed rectifier current (444). Later it was shown that the scaffolding protein A-kinase anchoring protein 9 (AKAP-9), also known as yotiao, plays a central role in adrenergic regulation of IKs
by compartmentalizing key elements of the PKA signal- ing pathway, allowing for spatiotemporal control. AKAP-9 binds to the CTD of KCNQ1 and recruits sig- naling proteins including protein kinase A (PKA), protein phosphatase 1 (PP1) (255), adenylyl cyclase 9 (AC9) (238), and the phosphodiesterase PDE4D3 (419) (FIGURE
3A). Together these proteins form the IKs macromolecu- lar complex that can tightly control the phosphorylation state of the channel in response to adrenergic stimula- tion. PKA phosphorylates KCNQ1 at the S27 residue, adding a phosphate group and hence a change in charge to this residue, which leads to increased channel activa- tion and slower deactivation (226, 255) (FIGURE 3D). In addition, phosphorylation of AKAP-9 itself contributes to the PKA-mediated upregulation of IKs (78).
N
–40mV
–80mV
AKAP-9
AC9
cAMP
FIGURE 3. Molecular biology of IKs and regulation by PKA-mediated signaling. A: the IKs macromolecular complex, in- cluding KCNQ1, KCNE1, and associated scaffolding and signaling proteins. B and C: single pulse voltage-clamp recordings of KCNQ1 expressed alone or coex- pressed with KCNE1 in Xenopus oocytes. D: dialysis with 200 M cAMP and 0.2 M okadaic acid (OA) increases IKs am- plitude and slows…
Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
L Bohnen MS, Peng G, Robey SH, Terrenoire C, Iyer V, Sampson KJ, Kass RS. Molecular Pathophysiology of Congenital Long QT Syndrome. Physiol Rev 97: 89–134, 2017. Published November 2, 2016; doi:10.1152/physrev.00008.2016.—Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these
channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syn- drome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has ad- vanced understanding of ion channel structure-function relationships, physiology, and pharmaco- logical response to clinically employed and experimental pharmacological agents. Our understand- ing of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype- driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.
I. INTRODUCTION 89 II. IKs DYSFUNCTION IN CONGENITAL... 91 III. IKr DYSFUNCTION IN CONGENITAL... 99 IV. INa DYSFUNCTION IN CONGENITAL... 106 V. OTHER SUBTYPES OF CONGENITAL LQTS 114 VI. GENOTYPE-DRIVEN CLINICAL... 116 VII. CONCLUSIONS 120
I. INTRODUCTION
A genetic disorder disrupting electrical activity in the heart, congenital Long QT Syndrome (LQTS) can lead to life- threatening arrhythmias and sudden cardiac death. In the first cases of congenital LQTS, described in 1957, several children in one family presented with prolongation of the QT interval on the electrocardiogram (ECG) and congenital deafness (189). This came to be known as Jervell and Lange-Nielsen syndrome (JLNS), the autosomal recessive form of LQTS. The more common, autosomal dominant form of congenital LQTS that presents without deafness was first described in 1963 and 1964 in two separate cases and became known as Romano-Ward syndrome (346, 459). Since these initial patient descriptions, advances in our understanding of the mechanisms of cardiac electrical excitability at the tissue, cellular, and molecular level have
yielded much insight into the pathophysiology of congenital LQTS.
Clinically, congenital LQTS patients often first present after episodes of syncope and/or seizure, and the ECG reveals a prolonged QT interval. The ECG measures electrical activ- ity of the heart over time, at the patient’s body surface. The primary electrical signals observed include the P wave, which signifies atrial depolarization; the QRS complex, which arises from ventricular depolarization; and the T wave, due to ventricular repolarization (FIGURE 1A). The QT interval, therefore, reflects the time elapsed from the initiation of ventricular depolarization to the end of ventric- ular repolarization. The QT interval shortens with increas- ing heart rate thus requiring a normalization, or “correc- tion,” for heart rate. For a diagnosis of LQTS, this rate- corrected QT (QTc) interval prolongation on a 12-lead ECG generally is referenced as 470 ms for males and 480 ms for females. QTc also varies with age, and thus, an age-appropriate prolonged QTc interval in a patient aids the diagnosis of LQTS (209). However, diagnosis of LQTS based on absolute QT interval cutoffs can be challenging, since there is considerable overlap in the QTc distribution of affected patients and otherwise healthy individuals (193). Asymptomatic patients can have intervals beyond
Physiol Rev 97: 89–134, 2017 Published November 2, 2016; doi:10.1152/physrev.00008.2016
890031-9333/17 Copyright © 2017 the American Physiological Society Downloaded from journals.physiology.org/journal/physrev (171.243.000.161) on March 6, 2023.
these cutoff and develop no arrhythmias; similarly, QTc intervals below this cutoff can be seen in patients with es- tablished LQTS (with clinical arrhythmias and positive ge- netic testing) (16, 17, 338). Clinical scoring systems (368), as well as genetic testing, can be helpful to assist with the diagnosis of congenital LQTS (193), particularly when QT intervals are on the borderline (within 20 ms of these cut- offs) or when clinical history is equivocal. This review will focus primarily on the various forms of congenital LQTS.
Ion channels are the molecular entities underlying most ionic currents in the heart, allowing passive diffusion of ions across the cell membrane’s electrochemical gradients (FIGURE 2). A selectivity filter in the channel pore, deter- mined by distinct atomic components (152, 315), endows selective permeation of ions, such as Na, K, and Ca2
(246). Some ion channels exhibit voltage-dependent gating, where voltage-sensing domains respond to changes in mem- brane potential to cause channel opening or closing (247).
The ventricular cellular action potential results from the summation of a large number of ion channel currents and electrogenic pumps that control the cellular membrane po- tential (see Nerbonne and Kass review, Ref. 299), but in this review we will focus on three key ion channels that are well-established to be linked to LQTS and that are illus- trated in FIGURE 1B. The cellular resting membrane poten- tial is approximately 85 mV, determined largely by in- wardly rectifying K channels. Inward rectification is a property that hinders the outward flow of K as membrane potentials become positive, but passes K more efficiently at potentials negative to the K equilibrium potential where the flow of K would be inward. Hence, the term “inward rectification” is used to describe these channels (304). Ac- tivation of Nav1.5, the primary voltage-gated sodium chan- nel in the heart, leads to sodium influx (INa) and membrane depolarization. As the cell reaches approximately 40 mV, voltage-gated L-type calcium channels begin to open, lead- ing to calcium influx (ICa) (299). Concurrently during the upstroke of the action potential, potassium channels, in- cluding those carrying the IKs and IKr delayed rectifier cur- rents, begin to activate slowly. As the cell reaches 30 mV, INa inactivates almost completely. At this time, a brief and small repolarization of the membrane potential occurs via fast activation of the transient outward K current, Ito. In the plateau phase that follows, influx of Ca2 through volt- age-gated calcium channels is balanced mainly by IKs and IKr. The plateau phase ceases as calcium channels inactivate and outward potassium efflux persists, leading to a net out- ward membrane current, and cell repolarization back to the cellular resting potential. During this process, the Na-K- ATPase helps maintain intracellular concentrations of these key ions (304).
In LQTS patients, the QT is prolonged presumably due to prolongation of underlying action potential durations
IKs
IKr
A
B
FIGURE 1. ECG to cellular ionic currents. A: membrane depolar- ization and the rapid upstroke of the ventricular action potential give rise to the QRS complex. The duration of the QT-interval corre- sponds to the time to ventricular repolarization. The relatively stable membrane potential during the plateau phase of the action potential gives rise to a brief isoelectric period. Ventricular repolarization gives rise to the T-wave. B: time course of several ionic currents that underlie ventricular action potential morphology (currents not to scale). The rapidly activating and inactivating INa drives membrane depolarization. Two K currents, IKs and IKr, contribute most to the repolarizing current necessary to drive membrane potential back to rest.
Extracellular
K+
K+
Cytosol
Na+
Na+
FIGURE 2. Schematic of a generic K and Na ion channel. Ion channels allow for selective permeation of ions through the plasma membrane down their electrochemical gradient. The classic K
channel consists of four identical pore-forming subunits, whereas each Na channel is formed by a single polypeptide with four homol- ogous domains.
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(APDs), most often caused by decreased repolarizing IKs or IKr activity, or persistent sodium influx that extends through the plateau phase. A loss of IKs or IKr function, or a gain of INa function, predisposes ventricular myocytes to early afterdepolarizations (EADs), and in some cases to de- layed afterdepolarizations (DADs) which may underlie de- generation into a characteristic sinusoidal wave pattern on the ECG, referred to as torsades de pointes, which may further regress into ventricular fibrillation and sudden car- diac death. EADs are driven in large part by calcium entry via L-type calcium channels during prolonged action poten- tial plateau phases, whereas DADs, which occur over the diastolic range of potentials after action potential repolar- ization, are caused by intracellular calcium overload, also a consequence of action potential prolongation (126). Addi- tionally, arrhythmic activity may result from altered refrac- toriness and impulse block, also putative consequences of prior APD prolongation. Thus treatment in LQTS aims to prevent malignant ventricular arrhythmia by shortening the QTc interval to minimize cardiac event rates.
Ion channels may interact with a variety of molecular enti- ties that contribute to their trafficking, stabilization, signal- ing, and function (299). Coordination among different ion channel types facilitates the ionic balance necessary for the generation of an action potential and normal electrical propagation through the heart. LQTS mutations may cause an increase or decrease in ion channel function, disrupting normal ionic balance leading to pathological electrical ac- tivity in the heart.
There are 15 subtypes of congenital LQTS, each associated with mutations on a different gene (15) (TABLE 1). The most common subtypes, LQT1, LQT2, and LQT3, account for the vast majority of congenital LQTS. LQT1 and LQT2 are associated with mutations in KCNQ1 and KCNH2 (which
encodes hERG), respectively, while LQT3 is associated with mutations in SCN5A, the gene coding for the Nav1.5 sodium channel alpha subunit. Disease association for vari- ants in these three proteins is supported by genome-wide association studies (300) and functional electrophysiologi- cal characterization of mutant channels. In addition, LQTS- associated mutations exist less frequently in other ion chan- nels, modulatory channel subunits, and signaling- or cyto- skeleton-associated proteins. Understanding the molecular mechanisms that cause LQTS allows for optimization of genotype-specific treatments. In this review, we discuss the molecular physiology, biology, and pathophysiology un- derlying congenital LQTS, and the cellular and molecular underpinnings of genotype-driven clinical management of LQTS.
II. IKs DYSFUNCTION IN CONGENITAL LQTS
A. Introduction
Of the different subtypes of inherited LQTS, subtypes 1, 5, and 11 are associated with mutations in proteins that par- ticipate in the macromolecular complex which generates and modulates the slow delayed rectifier potassium current (IKs), which plays a critical role in the repolarization of the cardiac action potential. Among all LQTS subtypes, LQT1 is the most common, representing 30-35% of all congenital LQTS (8). Upregulation of IKs during -adrenergic stimu- lation is critical to normal physiology by shortening ventric- ular APD and allowing for adequate diastolic filling in the context of an elevated heart rate (354). Cardiac events in patients with IKs-associated LQTS are often triggered by stress and exercise, consistent with the role of adrenergic stimulation in the regulation of IKs (350, 371). Insight into
Table 1. Subtypes of congenital LQTS and their associated genes, proteins, and effects on cardiac currents
LQT Subtype Gene Protein Current
LQT1 KCNQ1 KCNQ1 (Kv7.1) 2IKs
LQT2 KCNH2 hERG (Kv11.1) 2IKr
LQT3 SCN5A Nav1.5 1INa
LQT6 KCNE2 KCNE2 (MiRP1) 2IKr
LQT7 (Andersen-Tawil syndrome type 1) KCNJ2 Kir2.1 2IK1
LQT8 (Timothy syndrome) CACNA1C Cav1.2 1ICa
LQT9 CAV3 Caveolin 3 1INa
LQT10 SCN4B Nav1.5 4 1INa
LQT11 AKAP9 AKAP-9 (yotiao) 2IKs
LQT12 SNTA1 1-Syntrophin 1INa
LQT14 CALM1 Calmodulin Multichannel interactions LQT15 CALM2 Calmodulin Multichannel interactions
CONGENITAL LONG QT SYNDROME
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the molecular mechanisms of disease mutations have greatly improved our understanding of LQTS pathophysi- ology and helped provide a first step to the future develop- ment of targeted therapies.
B. Physiology
IKs is an outward potassium current with unique kinetics and voltage dependence and plays a key role in the repolar- ization of the cardiac action potential (299). In 1969, the delayed rectifier potassium current in sheep Purkinje fibers was studied and shown to comprise two kinetically distinct components (305). These currents were later subjected to pharmacological dissection in guinea pig ventricular myo- cytes and identified as IKs and IKr (360).
IKs is slowly activating and most prominent during the pla- teau and repolarizing phases of the cardiac action potential, where it contributes to counterbalancing calcium influx and repolarization (FIGURE 1B). Expression of IKs has been dem- onstrated in both human atrial and ventricular myocytes (198, 235, 457). In addition, it has also been measured in cardiomyocytes from a variety of non-human mammalian species including dogs (241, 406, 437, 442, 493) and rab- bits (352). On the other hand, IKs is expressed at very low levels or absent in mouse hearts (478), most likely because at very high heart rates in mouse heart (500 beats/min) this channel would have little or no time to activate and not affect cardiac electrophysiology in the mouse.
Importantly, IKs is subject to upregulation by -adrenergic stimulation to control APD in the face of sympathetic nerve activity (210, 444). During sympathetic activation, adren- ergic stimulation increases the outward IKs current, which counterbalances the concomitant increase in inward cal- cium current, prevents prolongation of the cardiac APD, and allows for adequate diastolic filling times between heart beats (386). However, insufficient IKs activation such as that seen in LQT1 results in failure to counterbalance the calcium influx, prolonging the action potential and increas- ing susceptibility to arrhythmia. This is consistent with ex- ercise being a key trigger of cardiac events seen in LQT1 patients (371).
C. Molecular Biology
The first known subtypes of inherited LQTS, LQT1-3, were distinguished by mapping to distinct chromosomes, with LQT1 mapping to chromosome 11 (191, 212, 213). Even- tually it was found that the KCNQ1 (KvLQT1) gene is responsible for LQT1 and encodes a potassium channel (451). The current conducted by this channel was rapidly activating and minimally inactivating, unlike any previ- ously known current in the heart, but soon it was shown that KCNQ1 together with the accessory protein KCNE1
(minK) generates IKs (36, 358). While KCNE1 had previ- ously been thought to be an independent potassium channel (167, 412), it was confirmed that KCNQ1 is actually the - or pore-forming subunit of IKs while KCNE1 is a critical - or modulatory subunit. Coexpression of KCNQ1 and KCNE1 generates the hallmark IKs current with slow acti- vation. KCNQ1 and KCNE1 have been shown to be ex- pressed in all four chambers in the heart (45), as well as the inner ear (301, 351), where IKs is thought to play a role in K secretion into the endolymph (68). This explains the observation that congenital deafness is a key feature of JLNS. In addition, KCNQ1 and KCNE1 are expressed else- where in the body, including the pancreas, the kidneys, and the brain (1).
1. KCNQ1, the pore-forming subunit
KCNQ1, like most voltage-gated potassium channels, consists of six transmembrane helices (451) (FIGURE 3A). Four subunits of KCNQ1 come together to form a chan- nel that is capable of voltage-dependent gating (FIGURE 3B). On each subunit, the helices S1–S4 comprise the voltage-sensing domain, where the S4 helix, with its pos- itively charged arginine residues, senses changes in mem- brane potential (310). Following the voltage-sensing do- main is the pore domain, which comprises the pore-loop, an extracellular segment containing the selectivity filter, and helices that line the pore, S5 and S6. Furthermore, the cytoplasmic loop between S4 and S5 plays important roles in the voltage sensor-to-pore coupling and in volt- age-dependent gating (82, 229, 498), which has been demonstrated in other voltage-gated channels as well (75, 122, 355). The cytoplasmic loop between S2 and S3 also plays a role in channel gating (498). The COOH- terminal domain (CTD) of KCNQ1 is large and contains four intracellular -helices referred to as A-D. A wide range of functions has been attributed to the CTD includ- ing calmodulin binding, interaction with -subunits and scaffolding proteins, as well as channel assembly and trafficking (159, 469).
2. KCNE1, the -subunit
Coexpression of KCNE1 with KCNQ1 leads to a drastic change in channel function to generate IKs. Most promi- nently, assembly with KCNE1 leads to a delay in the onset of activation, an increase in channel amplitude (FIGURE 3C), as well as a depolarizing shift in the current-voltage rela- tionship (not illustrated) (36, 358). This results in a channel that, compared with most other voltage-gated potassium channels, activates at more positive voltages and with slower kinetics. KCNE1 is a 129-amino acid protein that consists of a single transmembrane helix, with an extracel- lular NH2 terminus and intracellular COOH terminus (412). It is thought to have extensive contact with KCNQ1 including the voltage-sensing domain (37, 84, 203, 309,
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378, 479), the pore domain (84, 262, 311), as well as the CTD (161, 510). Previous crosslinking studies suggest that KCNE1 is located in a cleft between the voltage-sensing domain and pore domain of different KCNQ1 subunits (84, 479), underlying its ability to modulate KCNQ1 gating. With respect to the stoichiometry of KCNE1 to KCNQ1, some studies suggest a fixed 2:4 ratio (278, 326), while others suggest a flexibility in stoichiometry (293, 456) that allows for modulation of kinetics of assembled channels to provide another level of flexibility in channel function. Although three other members of the KCNE family, KCNE2-KCNE4, also are expressed in the heart (45) and are capable of modulating KCNQ1 activity (45, 154, 367, 422), whether they associate with KCNQ1 in vivo to contribute to potassium currents in the heart remains to be explored.
3. Molecular components of adrenergic stimulation
There have been considerable efforts to elucidate the mo- lecular pathway for the -adrenergic regulation of IKs. In
1988 it was shown that stimulation of PKA activity by a cAMP analog can upregulate the delayed rectifier current (444). Later it was shown that the scaffolding protein A-kinase anchoring protein 9 (AKAP-9), also known as yotiao, plays a central role in adrenergic regulation of IKs
by compartmentalizing key elements of the PKA signal- ing pathway, allowing for spatiotemporal control. AKAP-9 binds to the CTD of KCNQ1 and recruits sig- naling proteins including protein kinase A (PKA), protein phosphatase 1 (PP1) (255), adenylyl cyclase 9 (AC9) (238), and the phosphodiesterase PDE4D3 (419) (FIGURE
3A). Together these proteins form the IKs macromolecu- lar complex that can tightly control the phosphorylation state of the channel in response to adrenergic stimula- tion. PKA phosphorylates KCNQ1 at the S27 residue, adding a phosphate group and hence a change in charge to this residue, which leads to increased channel activa- tion and slower deactivation (226, 255) (FIGURE 3D). In addition, phosphorylation of AKAP-9 itself contributes to the PKA-mediated upregulation of IKs (78).
N
–40mV
–80mV
AKAP-9
AC9
cAMP
FIGURE 3. Molecular biology of IKs and regulation by PKA-mediated signaling. A: the IKs macromolecular complex, in- cluding KCNQ1, KCNE1, and associated scaffolding and signaling proteins. B and C: single pulse voltage-clamp recordings of KCNQ1 expressed alone or coex- pressed with KCNE1 in Xenopus oocytes. D: dialysis with 200 M cAMP and 0.2 M okadaic acid (OA) increases IKs am- plitude and slows…
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