Substitution of a conserved alanine in the domain IIIS4–S5 linker of the cardiac sodium channel causes long QT syndrome

OBJECTIVE

Congenital long QT syndrome type 3 (LQT3) is an inherited cardiac arrhythmia disorder due to mutations in the cardiac sodium channel gene, SCN5A. Although most LQT3 mutations cause a persistent sodium current, increasing diversity in the disease mechanism is shown. Here we present the electrophysiological properties of the A1330T sodium channel mutation (DIIIS4-S5 linker). Like the A1330P, LQT3 mutation, A1330T, causes LQT3 in the absence of a persistent current.

METHODS

A1330T, A1330P and wild-type sodium channels were expressed in HEK-293 cells and characterized using the whole-cell configuration of the patch-clamp technique.

RESULTS

The A1330T mutation shifts positively the voltage-dependence of inactivation and speeds recovery from inactivation. Measurements of sodium window (I(Na, window)) currents revealed a positive shift of the I(Na, window) voltage range for both 1330 mutants, with in addition an increase in I(Na, window) magnitude for the A1330P mutant. Action potential (AP) clamp experiments revealed that these changes in I(Na, window) properties cause an increased inward current during the initial part of phase 4 repolarization of the AP.

CONCLUSIONS

Our findings indicate that the alanine at position 1330 in the DIIIS4-S5 linker of the cardiac sodium channel has a role in channel fast inactivation. Substitution by a threonine shifts the voltage range of I(Na, window) activity to more positive potentials. Here the counter-acting effect of outward K+ current is reduced and may delay AP repolarization, explaining the LQT3 phenotype.

Intragenic suppression of trafficking-defective KCNH2 channels associated with long QT syndrome

Mutations in the KCNH2 or human ether-a-go-go-related gene-encoded K(+) channel reduce functional KCNH2 current (I(KCNH2)) to cause long QT syndrome (LQT2) by multiple mechanisms, including defects in intracellular transport (trafficking). Trafficking-deficient, or class 2, LQT2 mutations reduce the Golgi processing and surface membrane expression of KCNH2 channel proteins. Drugs that associate with pore-S6 intracellular drug binding domain of KCNH2 channel proteins to cause high-affinity block of I(KCNH2) also can increase the processing of class 2 LQT2 channel proteins through the secretory pathway. We used a strategy of intragenic suppression to test the hypothesis that amino acid substitutions in the putative drug binding domain at residue Y652 could compensate for protein folding abnormalities caused by class 2 LQT2 mutations. We found that the Y652C substitution, and to lesser extent the Y652S substitution, resulted in intragenic suppression of the class 2 LQT2 G601S phenotype; these substitutions increased Golgi processing of G601S channel proteins. The Y652C substitution also caused intragenic suppression of the class 2 LQT2 V612L and F640V phenotypes but not the LQT2 N470D or F805C phenotypes. These are the first findings to demonstrate that a single amino acid substitution in the putative KCNH2 drug binding domain can cause intragenic suppression of several LQT2 mutations.

Defective assembly and trafficking of mutant HERG channels with C-terminal truncations in long QT syndrome

Mutations in the human ether-a-go-go-related gene (HERG) cause long QT syndrome type 2 (LQT2). HERG encodes a voltage-gated potassium channel consisting of four subunits. Tetrameric assembly is required for the formation of functional HERG channels. In the present work, we studied the role of assembly in HERG channel dysfunction of LQT2 mutations Q725X and R1014X, both of which cause truncations of the C-terminus of HERG channels. When expressed in HEK293 cells, Q725X did not generate HERG current, while R1014X generated HERG current with markedly reduced amplitude. Western blot analysis showed that both mutations caused defective trafficking of HERG channel proteins. Using sucrose gradient centrifugation we showed that wild type HERG and R1014X formed a tetrameric structure, whereas Q725X was expressed as a monomer. When coexpressed with wild type HERG, R1014X, but not Q725X, caused dominant negative suppression of wild type HERG current. Coimmunoprecipitation experiments showed that the lack of dominant negative effect by Q725X was due to failure of mutant subunits to coassemble with wild type subunits. These results suggest that the Q725X mutation causes HERG channel dysfunction by disruption of tetrameric assembly of HERG channels. In contrast, the R1014X mutation is capable of forming tetrameric structure, and it causes HERG channel dysfunction by defective trafficking of the mutant protein.

Impaired KCNQ1-KCNE1 and phosphatidylinositol-4,5-bisphosphate interaction underlies the long QT syndrome

Nearly a hundred different KCNQ1 mutations have been reported as leading to the cardiac long QT syndrome, characterized by prolonged QT interval, syncopes, and sudden death. We have previously shown that phosphatidylinositol-4,5-bisphosphate (PIP2) regulates the KCNQ1-KCNE1 complex. In the present study, we show that PIP2 affinity is reduced in three KCNQ1 mutant channels (R243H, R539W, and R555C) associated with the long QT syndrome. In giant excised patches, direct application of PIP2 on the cytoplasmic face of the three mutant channels counterbalances the loss of function. Reintroduction of a positive charge by application of methanethiosulfonate ethylammonium on the cytoplasmic face of R555C mutant channels also restores channel activity. The channel affinity for a soluble analog of PIP2 is decreased in the three mutant channels. By using a model that describes the KCNQ1-KCNE1 channel behavior and by fitting the relationship between the kinetics of deactivation and the current amplitude obtained in whole-cell experiments, we estimated the PIP2 binding and dissociation rates on wild-type and mutant channels. The dissociation rate of the three mutants was higher than for the wild-type channel, suggesting a decreased affinity for PIP2. PIP2 binding was magnesium-dependent, and the PIP2-dependent equilibrium constant in the absence of magnesium was higher with the wild-type than with the mutant channels. Altogether, our data suggest that a reduced PIP2 affinity of KCNQ1 mutants can lead to the long QT syndrome.

The therapeutic potential of neuronal KCNQ channel modulators

Neuronal KCNQ (Kv7) channels (KCNQ2-5 or Kv7.2-7.5, disclosed to date) were discovered by virtue of their homology with a known cardiac channel involved in long QT syndrome (KvLQT or KCNQ1, Kv7.1) and first disclosed in 1998. The involvement of KCNQ2 (Kv7.2) and KCNQ3 (Kv7.3) in a benign idiopathic neonatal epilepsy, KCNQ4 (Kv7.4) in a form of congenital deafness, and the discovery that neuronal KCNQ heteromultimers were among the molecular substrates of M-channels, resulted in a high level of interest for potential drug development strategies. A number of small-molecule modulators were quickly identified, including openers or activators such as the antiepileptic drug candidate retigabine and the structurally-related analgesic drug flupirtine (Katadolon trade mark Asta Medica), and a group of KCNQ channel inhibitors/blockers originally developed for cognition enhancement. All of these data have suggested a rich target profile for modulators of neuronal KCNQ channels, including a variety of neuronal hyperexcitability disorders and conditions for openers, such as the epilepsies, acute pain, neuropathic pain, migraine pain and some neurodegenerative and psychiatric disorders. KCNQ blockers could likewise have utility in disorders characterised by neuronal hypoactivity, including cognition enhancement and perhaps disorders of mood. Emerging patent literature suggests significant interest in neuronal KCNQ modulation in the pharmaceutical industry and significant chemical diversity concerning KCNQ modulation.

Intrinsic mechanism of the enhanced rate-dependent QT shortening in the R1623Q mutant of the LQT3 syndrome

OBJECTIVE

In the type 3 long QT syndrome (LQT3), arrhythmia events tend to occur at rest or during sleep. One of the mutations, R1623Q, is located in the voltage sensor of the cardiac sodium channel (hH1), and patients with R1623Q mutation have been also reported to show bradycardia-dependent cardiac events. Although the mutant channel has been characterized by inactivation gating defects, the intrinsic mechanism(s) that might explain why arrhythmia attack is most prevalent at slower heart rates has not been investigated.

METHODS

cDNA encoding either the wild-type or the R1623Q mutant of hH1 was stably transfected into HEK293 cells. I(Na) was recorded using a whole-cell patch-clamp technique at 23 degrees C.

RESULTS

A train of 50 depolarizing pulses from holding potentials (-120 and -80 mV) to -20 mV or a train of 50 action potential waveforms was applied at different frequencies. When using a rectangular waveform voltage clamp protocol, rate-dependent reduction of I(Na) was holding voltage-dependent but was not different between peak I(Na) and late I(Na). However, using the action potential clamp, preferential rate-dependent reduction of the phase 3 I(Na) was obvious as compared with peak I(Na). The discrepancy in the rate-dependent reduction between protocols was attributed to accelerated recovery from inactivation under non-equilibrium condition.

CONCLUSION

The rate dependency of phase 3 I(Na) under non-equilibrium gating is a novel mechanism to explain the enhanced rate-dependent QT-shortening in LQT3 patients. Our findings are important for genotype-phenotype correlations in LQT3 mutants as well as for understanding the function of S4 segment of domain IV region in the cardiac Na(+) channel.

Transmural dispersion of repolarization as a key factor of arrhythmogenicity in a novel intact heart model of LQT3

BACKGROUND

Congenital and acquired long QT syndrome (LQTS) are caused by abnormalities of ionic currents underlying ventricular repolarization. For a better understanding of the mechanisms by which functional electrical instability at the level of the whole heart leads to torsade de pointes (TdP), a novel model of LQT3 was developed and the role of transmural dispersion of repolarization for the development of proarrhythmia was evaluated.

METHODS AND RESULTS

In 11 Langendorff-perfused rabbit hearts, veratridine (0.1-0.5 microM), an inhibitor of sodium channel inactivation, led to a concentration-dependent increase in QT-interval and simultaneously recorded monophasic ventricular action potentials (MAPs) (p<0.05) and thereby mimicked LQT3. Veratridine reproducibly induced early afterdepolarizations (EADs) and TdP after lowering potassium concentration. In bradycardic (AV-blocked) hearts, the increase in MAP duration showed marked regional differences. It was significantly more pronounced on the left endocardium as compared to left or right epicardium. This resulted in a significant increase in dispersion of repolarization (24% at 0.1 microM, 92% at 0.25 microM, 208% at 0.5 microM; p<0.01). Left ventricular transmural dispersion of repolarization increased significantly more than interventricular dispersion (104 to 33 ms at 0.5 microM veratridine; p<0.05).

CONCLUSION

By inhibition of sodium channel inactivation, veratridine mimics LQT3 in this intact heart model. In bradycardic, hypokalemic hearts, it reproducibly induced EADs and TdP in the setting of significantly increased left ventricular transmural dispersion of repolarization. Based on these experimental data, reduction of transmural dispersion of repolarization may be considered an important target for the prevention of TdP in patients with LQT3.

Predicting drug–hERG channel interactions that cause acquired long QT syndrome

Avoiding drug-induced cardiac arrhythmia is recognized as a major hurdle in the successful development of new drugs. The most common problem is acquired long QT syndrome caused by drugs that block human ether-a-go-go-related-gene (hERG) K(+) channels, delay cardiac repolarization and increase the risk of torsades de pointes arrhythmia (TdP). Not all hERG channel blockers induce TdP because they can also modulate other channels that counteract the hERG channel-mediated effect. However, hERG channel blockade is an important indicator of potential pro-arrhythmic liability. The molecular determinants of hERG channel blockade have been defined using a site-directed mutagenesis approach. Combined with pharmacophore models, knowledge of the drug-binding site of hERG channels will facilitate in silico design efforts to discover drugs that are devoid of this rare, but potentially lethal, side-effect.

Acute hypoxia differentially regulates K+ channels. Implications with respect to cardiac arrhythmia

The first ion channels demonstrated to be sensitive to changes in oxygen tension were K(+) channels in glomus cells of the carotid body. Since then a number of hypoxia-sensitive ion channels have been identified. However, not all K(+) channels respond to hypoxia alike. This has raised some debate about how cells detect changes in oxygen tension. Because ion channels respond rapidly to hypoxia it has been proposed that the channel is itself an oxygen sensor. However, channel function can also be modified by thiol reducing and oxidizing agents, implicating reactive oxygen species as signals in hypoxic events. Cardiac ion channels can also be modified by hypoxia and redox agents. The rapid and slow components of the delayed rectifier K(+) channel are differentially regulated by hypoxia and beta-adrenergic receptor stimulation. Mutations in the genes that encode the subunits for the channel are associated with Long QT syndrome and sudden cardiac death. The implications with respect to effects of hypoxia on the channel and triggering of cardiac arrhythmia will be discussed.

Divergent action potential morphologies reveal nonequilibrium properties of human cardiac Na channels

OBJECTIVE

Fast inward Na current (I(Na)) carried by the voltage-gated Na channel (Na(V)1.5) is critical for action potential (AP) propagation and the rapid upstroke of the cardiac AP. In addition, a small fraction of Na(V)1.5 channels remains open throughout the plateau of the AP, and this current is termed as late I(Na). In patients with mutant Na(V)1.5-based congenital long Q-T (LQT) syndrome, mutant channels pass more late I(Na) compared to wild-type channels in unaffected patients. Although LQT mutant Na(V)1.5 channels are well studied, there is no careful evaluation of the effects of cardiac APs on early and late current. This is important with the recent documentation of nonequilibrium I(Na).

METHODS

We measured AP-stimulated I(Na) through Na(V)1.5 wild-type and two LQT mutant channels (DeltaKPQ and N1325S). Three distinct AP morphologies were used: human embryonic stem cell-derived cardiac myocyte (hES-CM) APs with a relatively slow upstroke and canine endocardial and epicardial ventricular myocytes with rapid upstrokes.

RESULTS

All three APs elicited both early and late I(Na). For wild-type Na(V)1.5, the hES-CM AP elicits more early and late I(Na) than either the endocardial or epicardial AP. The mechanism for this difference is that the hES-CM has a relative slow dV/dt(max) that causes a maximal open channel probability. Slower upstroke stimulation also allows greater Na flux through wild-type and N1325S channels, but not the DeltaKPQ mutant.

CONCLUSIONS

The inherent gating properties of Na(V)1.5 provide natural tuning of optimal I(Na) density. Slower upstroke velocities can yield more I(Na) and Na flux in some Na(V)1.5 variants.

A novel SCN5A mutation manifests as a malignant form of long QT syndrome with perinatal onset of tachycardia/bradycardia

OBJECTIVE

Congenital long QT syndrome (LQTS) with in utero onset of the rhythm disturbances is associated with a poor prognosis. In this study we investigated a newborn patient with fetal bradycardia, 2:1 atrioventricular block and ventricular tachycardia soon after birth.

METHODS

Mutational analysis and DNA sequencing were conducted in a newborn. The 2:1 atrioventricular block improved to 1:1 conduction only after intravenous lidocaine infusion or a high dose of mexiletine, which also controlled the ventricular tachycardia.

RESULTS

A novel, spontaneous LQTS-3 mutation was identified in the transmembrane segment 6 of domain IV of the Na(v)1.5 cardiac sodium channel, with a G-->A substitution at codon 1763, which changed a valine (GTG) to a methionine (ATG). The proband was heterozygous but the mutation was absent in the parents and the sister. Expression of this mutant channel in tsA201 mammalian cells by site-directed mutagenesis revealed a persistent tetrodotoxin-sensitive but lidocaine-resistant current that was associated with a positive shift of the steady-state inactivation curve, steeper activation curve and faster recovery from inactivation. We also found a similar electrophysiological profile for the neighboring V1764M mutant. But, the other neighboring I1762A mutant had no persistent current and was still associated with a positive shift of inactivation.

CONCLUSIONS

These findings suggest that the Na(v)1.5/V1763M channel dysfunction and possible neighboring mutants contribute to a persistent inward current due to altered inactivation kinetics and clinically congenital LQTS with perinatal onset of arrhythmias that responded to lidocaine and mexiletine.

Novel C-terminus frameshift mutation, 1122fs/147, of HERG in LQT2: additional amino acids generated by frameshift cause accelerated inactivation

OBJECTIVE

The function of the C-terminus region of the human ether-a-go-go related gene (HERG) has not been well characterized except for its involvement in trafficking. To understand further the role of C-terminus region, we performed a functional analysis of a novel frameshift mutation (1122fs/147) identified in a Japanese long QT syndrome 2 (LQT2) patient who had recurrent episodes of syncope.

METHODS

Wild type (WT) and mutant HERG plasmids were transfected into human embryonic kidney (HEK-293) cells, and whole-cell current was recorded by the patch-clamp technique. Confocal microscopy was performed to examine the membrane distribution of channel protein using a green fluorescent protein tagged to the N-terminus of HERG.

RESULTS

The mutant 1122fs/147 alone could express current, but reduced density by 74% of control. No dominant negative effect was noted with co-expression of WT and 1122fs/147. Activation and deactivation time constants were not changed, while inactivation was accelerated in 1122fs/147 compared to WT, and V(1/2) of steady-state inactivation curve shifted by 11 mV in the negative direction. Current density of 1123stop mutant revealed 49% reduction compared to WT and showed no shift in steady-state inactivation. Confocal microscopy revealed reduced protein expression on the cell surface both in 1122fs/147 and 1123stop mutants compared to WT.

CONCLUSION

Frameshift mutation at the C-terminus region with additional 147 amino acids evoked a loss of function of the HERG channel. A negative shift in steady-state inactivation induced by the additional 147 amino acids and trafficking defect contribute to a reduced current amplitude of 1122fs/147.

HERG channel (dys)function revealed by dynamic action potential clamp technique

The human ether-a-go-go-related gene (HERG) encodes the rapid component of the cardiac delayed rectifier potassium current (I(Kr)). Per-Arnt-Sim domain mutations of the HERG channel are linked to type 2 long-QT syndrome. We studied wild-type and/or type 2 long-QT syndrome-associated mutant (R56Q) HERG current (I(HERG)) in HEK-293 cells, at both 23 and 36 degrees C. Conventional voltage-clamp analysis revealed mutation-induced changes in channel kinetics. To assess functional implication(s) of the mutation, we introduce the dynamic action potential clamp technique. In this study, we effectively replace the native I(Kr) of a ventricular cell (either a human model cell or an isolated rabbit myocyte) with I(HERG) generated in a HEK-293 cell that is voltage-clamped by the free-running action potential of the ventricular cell. Action potential characteristics of the ventricular cells were effectively reproduced with wild-type I(HERG), whereas the R56Q mutation caused a frequency-dependent increase of the action potential duration in accordance with the clinical phenotype. The dynamic action potential clamp approach also revealed a frequency-dependent transient wild-type I(HERG) component, which is absent with R56Q channels. This novel electrophysiological technique allows rapid and unambiguous determination of the effects of an ion channel mutation on the ventricular action potential and can serve as a new tool for investigating cardiac channelopathies.

Cardiac IKr Channels Minimally Comprise hERG 1a and 1b Subunits

Previous studies suggest native cardiac IKr channels are composed of alpha subunits encoded solely by the 1a transcript of the ERG1 gene. Using isoform-specific ERG1 antibodies, we have new evidence that subunits encoded by an alternate transcript, ERG1b, are also expressed in rat, canine, and human heart. The ERG1a and -1b subunits associate in vivo where they localize to the T tubules of ventricular myocytes. These data indicate native ventricular IKr channels are heteromers containing two alpha subunit types, ERG1a and -1b. The hERG1b-specific exon thus represents a novel target to screen for mutations causing type 2 long QT syndrome. These findings also suggest phenotypic analyses of existing type 2 long QT syndrome mutations, especially those exclusive to the hERG1a amino terminus, should be carried out in systems expressing both subunits.

Acquired QT interval prolongation and HERG: implications for drug discovery and development

Putative interactions between the Human Ether-a-go-go Related Gene (HERG), QT interval prolongation and Torsades de Pointes (TdP) are now integral components of any discussion on drug safety. HERG encodes for the inwardly rectifying potassium channel (I(Kr)), which is essential to the maintenance of normal cardiac function. HERG channel mutations are responsible for one form of familial long QT syndrome, a potentially deadly inherited cardiac disorder associated with TdP. Moreover, drug-induced (acquired) QT interval prolongation has been associated with an increase in the incidence of sudden unexplained deaths, with HERG inhibition implicated as the underlying cause. Subsequently, a number of non-cardiovascular drugs which induce QT interval prolongation and/or TdP have been withdrawn. However, a definitive link between HERG, QT interval prolongation and arrhythmogenesis has not been established. Nevertheless, this area is subject to ever increasing regulatory scrutiny. Here we review the relationship between HERG, long QT syndrome and TdP, together with a summary of the associated regulatory issues, and developments in pre-clinical screening.

Human cardiac inwardly rectifying current IKir2.2 is upregulated by activation of protein kinase A

OBJECTIVE

The cardiac inwardly rectifying potassium current IK1 and its molecular correlates Kir2.1 and Kir2.2 play an important role in cardiac repolarisation and in the pathogenesis of hereditary long-QT syndrome (LQTS-7). Protein kinases A (PKA) and C (PKC) are key enzymes in adrenergic signal transduction, inducing arrhythmias in heart disease. This study investigated the regulation of Kir2.2 (KCNJ12) by PKA.

METHODS

Cloned Kir2.2 channels were expressed heterologously in Xenopus oocytes and currents were measured with the double-electrode voltage-clamp technique.

RESULTS

After activation of PKA by forskolin (100 micromol/l) or Ro-20-1724 (100 micromol/l), wild type currents at -120 mV were increased by 93.7% and 79.0%, respectively. Coapplication of the PKA inhibitor KT-5720 (2.5 micromol/l) attenuated this effect. No significant changes were apparent after mutation of the single PKA consensus site S430. In addition, removal of all four PKC consensus sites in Kir2.2 induced a phorbolester-mediated current increase which could be suppressed by PKA inhibitors H-89 (50 micromol/l) and KT-5720 (2.5 micromol/l).

CONCLUSIONS

This study demonstrates antagonistic effects of PKA and PKC in the regulation of Kir2.2. Phosphorylation by PKC has been shown to cause an inhibition of Kir2.2 currents, whereas activation of PKA leads to current upregulation.

Acquired long QT syndrome and monomorphic ventricular tachycardia after alternative treatment with cesium chloride for brain cancer

Individuals searching for symptomatic relief or a potential cure are increasingly seeking and using nontraditional therapies for their various diseases. Little is known about the potential adverse effects that patients may encounter while undergoing these alternative treatments. Cesium chloride is an unregulated agent that has been reported to have antineoplastic properties. Cesium chloride is advertised as an alternative agent for many different types of cancers and can be purchased easily on the Internet. Recently, QT prolongation and polymorphic ventricular tachycardia were reported in several patients taking cesium chloride as alternative treatment for cancer. We report acquired QT prolongation and sustained monomorphic ventricular tachycardia in a patient who self-initiated and completed a course of cesium chloride as adjunctive treatment for brain cancer.

A trafficking defective, Brugada syndrome-causing SCN5A mutation rescued by drugs

BACKGROUND

The human cardiac SCN5A gene encodes for the alpha subunit of the human cardiac voltage-dependent sodium channel hNav1.5 [Neuron 28 (2) (2000) 365] and carries inward Na current (INa). Mutations in SCN5A cause arrhythmia syndromes including Brugada syndrome (BrS) and congenital long QT syndrome subtype 3 (LQT3). Here, we report a trafficking defective BrS-causing SCN5A mutation that was drug-rescued.

METHODS AND RESULTS

A 14-year-old Caucasian male was diagnosed with BrS with typical ECG pattern for BrS and ventricular fibrillation was easily induced. He also had significant HV interval delay ( approximately 65 ms) and high (31 J) defibrillation thresholds (DFTs). Genomic analysis revealed the SCN5A mutation (G1743R). We engineered G1743R into the cardiac Na channel and transfected HEK-293 cells for functional studies. The mutant channel yielded nearly undetectable sodium channel currents. Coexpression with the beta1 subunit, or incubation at low temperature did not increase current density. However, mexiletine, a sodium channel blocker, increased current density 93-fold in G1743R, but only twofold in WT.

CONCLUSIONS

This study identifies an expression-defective BrS mutation in SCN5A with pharmacological rescue. The profoundly decreased sodium current associated with the G1743R suggests a molecular basis for the delayed His-Purkinje conduction and elevated DFTs observed in the proband. Whether the mutant channel may be rescued in vivo by mexiletine and normalize the patient's electrophysiologic parameters remains to be tested.

A cardiac arrhythmia syndrome caused by loss of ankyrin-B function

220-kDa ankyrin-B is required for coordinated assembly of Na/Ca exchanger, Na/K ATPase, and inositol trisphosphate (InsP(3)) receptor at transverse-tubule/sarcoplasmic reticulum sites in cardiomyocytes. A loss-of-function mutation of ankyrin-B identified in an extended kindred causes a dominantly inherited cardiac arrhythmia, initially described as type 4 long QT syndrome. Here we report the identification of eight unrelated probands harboring ankyrin-B loss-of-function mutations, including four previously undescribed mutations, whose clinical features distinguish the cardiac phenotype associated with loss of ankyrin-B activity from classic long QT syndromes. Humans with ankyrin-B mutations display varying degrees of cardiac dysfunction including bradycardia, sinus arrhythmia, idiopathic ventricular fibrillation, catecholaminergic polymorphic ventricular tachycardia, and risk of sudden death. However, a prolonged rate-corrected QT interval was not a consistent feature, indicating that ankyrin-B dysfunction represents a clinical entity distinct from classic long QT syndromes. The mutations are localized in the ankyrin-B regulatory domain, which distinguishes function of ankyrin-B from ankyrin-G in cardiomyocytes. All mutations abolish ability of ankyrin-B to restore abnormal Ca(2+) dynamics and abnormal localization and expression of Na/Ca exchanger, Na/K ATPase, and InsP(3)R in ankyrin-B(+/-) cardiomyocytes. This study, considered together with the first description of ankyrin-B mutation associated with cardiac dysfunction, supports a previously undescribed paradigm for human disease due to abnormal coordination of multiple functionally related ion channels and transporters, in this case the Na/K ATPase, Na/Ca exchanger, and InsP(3) receptor.

Inherited sodium channelopathies: a continuum of channel dysfunction

Voltage-gated sodium channels are transmembrane proteins that produce the ionic current responsible for the rising phase of the cardiac action potential and play a fundamental role in the initiation, propagation, and maintenance of normal cardiac rhythm. Inherited mutations in SCN5A, the gene encoding the pore-forming subunit of the cardiac Na+ channel, have been associated with distinct cardiac rhythm syndromes: the congenital long QT syndrome, Brugada syndrome, and isolated conduction disease. Electrophysiologic characterization of heterologously expressed mutant Na+ channels have revealed gating defects that, in many cases, can explain the distinct phenotype associated with the rhythm disorder. However, recent studies have revealed significant overlap between aberrant rhythm phenotypes, and single mutations have been identified that evoke multiple rhythm disorders with common gating lesions. These new insights enhance understanding of the structure-function relationships of voltage-gated Na+ channels, and also highlight the complexities involved in linking single mutations, ion-channel behavior, and cardiac rhythm.

KCNE2 protein is expressed in ventricles of different species, and changes in its expression contribute to electrical remodeling in diseased hearts

BACKGROUND

Mutations in KCNE2 have been linked to long-QT syndrome (LQT6), yet KCNE2 protein expression in the ventricle and its functional role in native channels are not clear.

METHODS AND RESULTS

We detected KCNE2 protein in human, dog, and rat ventricles in Western blot experiments. Immunocytochemistry confirmed KCNE2 protein expression in ventricular myocytes. To explore the functional role of KCNE2, we studied how its expression was altered in 2 models of cardiac pathology and whether these alterations could help explain observed changes in the function of native channels, for which KCNE2 is a putative auxiliary (beta) subunit. In canine ventricle injured by coronary microembolizations, the rapid delayed rectifier current (I(Kr)) density was increased. Although the protein level of ERG (I(Kr) pore-forming, alpha, subunit) was not altered, the KCNE2 protein level was markedly reduced. These data are consistent with the effect of heterologously expressed KCNE2 on ERG and suggest that in canine ventricle, KCNE2 may associate with ERG and suppress its current amplitude. In aging rat ventricle, the pacemaker current (I(f)) density was increased. There was a significant increase in the KCNE2 protein level, whereas changes in the alpha-subunit (HCN2) were not significant. These data are consistent with the effect of heterologously expressed KCNE2 on HCN2 and suggest that in aging rat ventricle, KCNE2 may associate with HCN2 and enhance its current amplitude.

CONCLUSIONS

KCNE2 protein is expressed in ventricles, and it can play diverse roles in ventricular electrical activity under (patho)physiological conditions.

An LQT mutant minK alters KvLQT1 trafficking

Cardiac I(Ks), the slowly activated delayed-rectifier K(+) current, is produced by the protein complex composed of alpha- and beta-subunits: KvLQT1 and minK. Mutations of genes encoding KvLQT1 and minK are responsible for the hereditary long QT syndrome (loci LQT1 and LQT5, respectively). MinK-L51H fails to traffic to the cell surface, thereby failing to produce effective I(Ks). We examined the effects that minK-L51H and an endoplasmic reticulum (ER)-targeted minK (minK-ER) exerted over the electrophysiology and biosynthesis of coexpressed KvLQT1. Both minK-L51H and minK-ER were sequestered primarily in the ER as confirmed by lack of plasma membrane expression. Glycosylation and immunofluorescence patterns of minK-L51H were qualitatively different for minK-ER, suggesting differences in trafficking. Cotransfection with the minK mutants resulted in reduced surface expression of KvLQT1 as assayed by whole cell voltage clamp and immunofluorescence. MinK-L51H reduced current amplitude by 91% compared with wild-type (WT) minK/KvLQT1, and the residual current was identical to KvLQT1 without minK. The phenotype of minK-L51H on I(Ks) was not dominant because coexpressed WT minK rescued the current and surface expression. Collectively, our data suggest that ER quality control prevents minK-L51H/KvLQT1 complexes from trafficking to the plasma membrane, resulting in decreased I(Ks). This is the first demonstration that a minK LQT mutation is capable of conferring trafficking defects onto its associated alpha-subunit.

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 IKr phenotypes were observed: (1) the wildtype-like IKr showed inward rectification and a positive tail current; (2) the N629D-like IKr 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 IKr 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 IKr 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.