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Elucidation of ALG10B as a Novel Long-QT Syndrome-Susceptibility Gene. CIRCULATION. GENOMIC AND PRECISION MEDICINE 2023; 16:e003726. [PMID: 37071726 PMCID: PMC10844923 DOI: 10.1161/circgen.122.003726] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/21/2022] [Accepted: 11/04/2022] [Indexed: 02/24/2023]
Abstract
BACKGROUND Long-QT syndrome (LQTS) is characterized by QT prolongation and increased risk for syncope, seizures, and sudden cardiac death. The majority of LQTS stems from pathogenic mutations in KCNQ1, KCNH2, or SCN5A. However, ≈10% of patients with LQTS remain genetically elusive. We utilized genome sequencing to identify a novel LQTS genetic substrate in a multigenerational genotype-negative LQTS pedigree. METHODS Genome sequencing was performed on 5 affected family members. Only rare nonsynonymous variants present in all affected family members were considered. The candidate variant was characterized functionally in patient-derived induced pluripotent stem cell and gene-edited, variant corrected, isogenic control induced pluripotent stem cell-derived cardiomyocytes. RESULTS A missense variant (p.G6S) was identified in ALG10B-encoded α-1,2-glucosyltransferase B protein. ALG10B (alpha-1,2-glucosyltransferase B protein) is a known interacting protein of KCNH2-encoded Kv11.1 (HERG [human Ether-à-go-go-related gene]). Compared with isogenic control, ALG10B-p.G6S induced pluripotent stem cell-derived cardiomyocytes showed (1) decreased protein expression of ALG10B (p.G6S, 0.7±0.18, n=8 versus control, 1.25±0.16, n=9; P<0.05), (2) significant retention of HERG in the endoplasmic reticulum (P<0.0005), and (3) a significantly prolonged action potential duration confirmed by both patch clamp (p.G6S, 531.1±38.3 ms, n=15 versus control, 324.1±21.8 ms, n=13; P<0.001) and multielectrode assay (P<0.0001). Lumacaftor-a compound known to rescue HERG trafficking-shortened the pathologically prolonged action potential duration of ALG10B-p.G6S induced pluripotent stem cell-derived cardiomyocytes by 10.6% (n=31 electrodes; P<0.001). CONCLUSIONS Here, we demonstrate that ALG10B-p.G6S downregulates ALG10B, resulting in defective HERG trafficking and action potential duration prolongation. Therefore, ALG10B is a novel LQTS-susceptibility gene underlying the LQTS phenotype observed in a multigenerational pedigree. ALG10B mutation analysis may be warranted, especially in genotype-negative patients with an LQT2-like phenotype.
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How Functional Genomics Can Keep Pace With VUS Identification. Front Cardiovasc Med 2022; 9:900431. [PMID: 35859585 PMCID: PMC9291992 DOI: 10.3389/fcvm.2022.900431] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2022] [Accepted: 06/09/2022] [Indexed: 01/03/2023] Open
Abstract
Over the last two decades, an exponentially expanding number of genetic variants have been identified associated with inherited cardiac conditions. These tremendous gains also present challenges in deciphering the clinical relevance of unclassified variants or variants of uncertain significance (VUS). This review provides an overview of the advancements (and challenges) in functional and computational approaches to characterize variants and help keep pace with VUS identification related to inherited heart diseases.
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Mutation-Specific Differences in Kv7.1 ( KCNQ1) and Kv11.1 ( KCNH2) Channel Dysfunction and Long QT Syndrome Phenotypes. Int J Mol Sci 2022; 23:7389. [PMID: 35806392 PMCID: PMC9266926 DOI: 10.3390/ijms23137389] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2022] [Revised: 06/22/2022] [Accepted: 06/24/2022] [Indexed: 11/16/2022] Open
Abstract
The electrocardiogram (ECG) empowered clinician scientists to measure the electrical activity of the heart noninvasively to identify arrhythmias and heart disease. Shortly after the standardization of the 12-lead ECG for the diagnosis of heart disease, several families with autosomal recessive (Jervell and Lange-Nielsen Syndrome) and dominant (Romano-Ward Syndrome) forms of long QT syndrome (LQTS) were identified. An abnormally long heart rate-corrected QT-interval was established as a biomarker for the risk of sudden cardiac death. Since then, the International LQTS Registry was established; a phenotypic scoring system to identify LQTS patients was developed; the major genes that associate with typical forms of LQTS were identified; and guidelines for the successful management of patients advanced. In this review, we discuss the molecular and cellular mechanisms for LQTS associated with missense variants in KCNQ1 (LQT1) and KCNH2 (LQT2). We move beyond the "benign" to a "pathogenic" binary classification scheme for different KCNQ1 and KCNH2 missense variants and discuss gene- and mutation-specific differences in K+ channel dysfunction, which can predispose people to distinct clinical phenotypes (e.g., concealed, pleiotropic, severe, etc.). We conclude by discussing the emerging computational structural modeling strategies that will distinguish between dysfunctional subtypes of KCNQ1 and KCNH2 variants, with the goal of realizing a layered precision medicine approach focused on individuals.
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PO-630-04 ELUCIDATION OF ALG10B AS A NOVEL LONG QT SYNDROME SUSCEPTIBILITY GENE. Heart Rhythm 2022. [DOI: 10.1016/j.hrthm.2022.03.896] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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PO-646-07 KCNH2 VARIANTS IDENTIFIED AS PATHOGENIC, DAMAGING, AND DYSFUNCTIONAL DO NOT RELIABLY IDENTIFY LONG QT SYNDROME IN LARGE PATIENT BIOBANK. Heart Rhythm 2022. [DOI: 10.1016/j.hrthm.2022.03.196] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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Long QT Syndrome KCNH2 Variant Induces hERG1a/1b Subunit Imbalance in Patient-Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes. Circ Arrhythm Electrophysiol 2021; 14:e009343. [PMID: 33729832 PMCID: PMC8058932 DOI: 10.1161/circep.120.009343] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
[Figure: see text].
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An International Multicenter Evaluation of Inheritance Patterns, Arrhythmic Risks, and Underlying Mechanisms of CASQ2-Catecholaminergic Polymorphic Ventricular Tachycardia. Circulation 2020; 142:932-947. [PMID: 32693635 PMCID: PMC7484339 DOI: 10.1161/circulationaha.120.045723] [Citation(s) in RCA: 31] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
BACKGROUND Genetic variants in calsequestrin-2 (CASQ2) cause an autosomal recessive form of catecholaminergic polymorphic ventricular tachycardia (CPVT), although isolated reports have identified arrhythmic phenotypes among heterozygotes. Improved insight into the inheritance patterns, arrhythmic risks, and molecular mechanisms of CASQ2-CPVT was sought through an international multicenter collaboration. METHODS Genotype-phenotype segregation in CASQ2-CPVT families was assessed, and the impact of genotype on arrhythmic risk was evaluated using Cox regression models. Putative dominant CASQ2 missense variants and the established recessive CASQ2-p.R33Q variant were evaluated using oligomerization assays and their locations mapped to a recent CASQ2 filament structure. RESULTS A total of 112 individuals, including 36 CPVT probands (24 homozygotes/compound heterozygotes and 12 heterozygotes) and 76 family members possessing at least 1 presumed pathogenic CASQ2 variant, were identified. Among CASQ2 homozygotes and compound heterozygotes, clinical penetrance was 97.1% and 26 of 34 (76.5%) individuals had experienced a potentially fatal arrhythmic event with a median age of onset of 7 years (95% CI, 6-11). Fifty-one of 66 CASQ2 heterozygous family members had undergone clinical evaluation, and 17 of 51 (33.3%) met diagnostic criteria for CPVT. Relative to CASQ2 heterozygotes, CASQ2 homozygote/compound heterozygote genotype status in probands was associated with a 3.2-fold (95% CI, 1.3-8.0; P=0.013) increased hazard of a composite of cardiac syncope, aborted cardiac arrest, and sudden cardiac death, but a 38.8-fold (95% CI, 5.6-269.1; P<0.001) increased hazard in genotype-positive family members. In vitro turbidity assays revealed that p.R33Q and all 6 candidate dominant CASQ2 missense variants evaluated exhibited filamentation defects, but only p.R33Q convincingly failed to dimerize. Structural analysis revealed that 3 of these 6 putative dominant negative missense variants localized to an electronegative pocket considered critical for back-to-back binding of dimers. CONCLUSIONS This international multicenter study of CASQ2-CPVT redefines its heritability and confirms that pathogenic heterozygous CASQ2 variants may manifest with a CPVT phenotype, indicating a need to clinically screen these individuals. A dominant mode of inheritance appears intrinsic to certain missense variants because of their location and function within the CASQ2 filament structure.
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Long QT Syndrome Type 2: Emerging Strategies for Correcting Class 2 KCNH2 ( hERG) Mutations and Identifying New Patients. Biomolecules 2020; 10:E1144. [PMID: 32759882 PMCID: PMC7464307 DOI: 10.3390/biom10081144] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2020] [Revised: 07/25/2020] [Accepted: 07/27/2020] [Indexed: 12/15/2022] Open
Abstract
Significant advances in our understanding of the molecular mechanisms that cause congenital long QT syndrome (LQTS) have been made. A wide variety of experimental approaches, including heterologous expression of mutant ion channel proteins and the use of inducible pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from LQTS patients offer insights into etiology and new therapeutic strategies. This review briefly discusses the major molecular mechanisms underlying LQTS type 2 (LQT2), which is caused by loss-of-function (LOF) mutations in the KCNH2 gene (also known as the human ether-à-go-go-related gene or hERG). Almost half of suspected LQT2-causing mutations are missense mutations, and functional studies suggest that about 90% of these mutations disrupt the intracellular transport, or trafficking, of the KCNH2-encoded Kv11.1 channel protein to the cell surface membrane. In this review, we discuss emerging strategies that improve the trafficking and functional expression of trafficking-deficient LQT2 Kv11.1 channel proteins to the cell surface membrane and how new insights into the structure of the Kv11.1 channel protein will lead to computational approaches that identify which KCNH2 missense variants confer a high-risk for LQT2.
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LUF7244 plus Dofetilide Rescues Aberrant Kv11.1 Trafficking and Produces Functional IKv11.1. Mol Pharmacol 2020; 97:355-364. [DOI: 10.1124/mol.119.118190] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2019] [Accepted: 03/24/2020] [Indexed: 11/22/2022] Open
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2019 AHA/ACC/HRS focused update of the 2014 AHA/ACC/HRS guideline for the management of patients with atrial fibrillation. Heart Rhythm 2019; 16:e66-e93. [DOI: 10.1016/j.hrthm.2019.01.024] [Citation(s) in RCA: 132] [Impact Index Per Article: 26.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/18/2019] [Indexed: 02/08/2023]
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2019 AHA/ACC/HRS Focused Update of the 2014 AHA/ACC/HRS Guideline for the Management of Patients With Atrial Fibrillation: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society in Collaboration With the Society of Thoracic Surgeons. Circulation 2019; 140:e125-e151. [DOI: 10.1161/cir.0000000000000665] [Citation(s) in RCA: 1256] [Impact Index Per Article: 251.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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Visualizing Mutation-Specific Differences in the Trafficking-Deficient Phenotype of Kv11.1 Proteins Linked to Long QT Syndrome Type 2. Front Physiol 2018; 9:584. [PMID: 29875689 PMCID: PMC5974211 DOI: 10.3389/fphys.2018.00584] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2018] [Accepted: 05/02/2018] [Indexed: 02/05/2023] Open
Abstract
KCNH2 encodes the Kv11.1 α-subunit that underlies the rapidly activating delayed-rectifier K+ current in the heart. Loss-of-function KCNH2 mutations cause long QT syndrome type 2 (LQT2), and most LQT2-linked missense mutations inhibit the trafficking of Kv11.1 channel protein to the cell surface membrane. Several trafficking-deficient LQT2 mutations (e.g., G601S) generate Kv11.1 proteins that are sequestered in a microtubule-dependent quality control (QC) compartment in the transitional endoplasmic reticulum (ER). We tested the hypothesis that the QC mechanisms that regulate LQT2-linked Kv11.1 protein trafficking are mutation-specific. Confocal imaging analyses of HEK293 cells stably expressing the trafficking-deficient LQT2 mutation F805C showed that, unlike G601S-Kv11.1 protein, F805C-Kv11.1 protein was concentrated in several transitional ER subcompartments. The microtubule depolymerizing drug nocodazole differentially affected G601S- and F805C-Kv11.1 protein immunostaining. Nocodazole caused G601S-Kv11.1 protein to distribute into peripheral reticular structures, and it increased the diffuse immunostaining of F805C-Kv11.1 protein around the transitional ER subcompartments. Proteasome inhibition also affected the immunostaining of G601S- and F805C-Kv11.1 protein differently. Incubating cells in MG132 minimally impacted G601S-Kv11.1 immunostaining, but it dramatically increased the diffuse immunostaining of F805C-Kv11.1 protein in the transitional ER. Similar results were seen after incubating cells in the proteasome inhibitor lactacystin. Differences in the cellular distribution of G601S-Kv11.1 and F805C-Kv11.1 protein persisted in transfected human inducible pluripotent stem cell derived cardiomyocytes. These are the first data to visually demonstrate mutation-specific differences in the trafficking-deficient LQT2 phenotype, and this study has identified a novel way to categorize trafficking-deficient LQT2 mutations based on differences in intracellular retention.
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Functional Invalidation of Putative Sudden Infant Death Syndrome-Associated Variants in the KCNH2-Encoded Kv11.1 Channel. Circ Arrhythm Electrophysiol 2018; 11:e005859. [PMID: 29752375 PMCID: PMC11081002 DOI: 10.1161/circep.117.005859] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/14/2017] [Accepted: 03/12/2018] [Indexed: 12/14/2022]
Abstract
BACKGROUND Heterologous functional validation studies of putative long-QT syndrome subtype 2-associated variants clarify their pathological potential and identify disease mechanism(s) for most variants studied. The purpose of this study is to clarify the pathological potential for rare nonsynonymous KCNH2 variants seemingly associated with sudden infant death syndrome. METHODS Genetic testing of 292 sudden infant death syndrome cases identified 9 KCNH2 variants: E90K, R181Q, A190T, G294V, R791W, P967L, R1005W, R1047L, and Q1068R. Previous studies show R181Q-, P967L-, and R1047L-Kv11.1 channels function similar to wild-type Kv11.1 channels, whereas Q1068R-Kv11.1 channels accelerate inactivation gating. We studied the biochemical and biophysical properties for E90K-, G294V-, R791W-, and R1005W-Kv11.1 channels expressed in human embryonic kidney 293 cells; examined the electronic health records of patients who were genotype positive for the sudden infant death syndrome-linked KCNH2 variants; and simulated their functional impact using computational models of the human ventricular action potential. RESULTS Western blot and voltage-clamping analyses of cells expressing E90K-, G294V-, R791W-, and R1005W-Kv11.1 channels demonstrated these variants express and generate peak Kv11.1 current levels similar to cells expressing wild-type-Kv11.1 channels, but R791W- and R1005W-Kv11.1 channels accelerated deactivation and activation gating, respectively. Electronic health records of patients with the sudden infant death syndrome-linked KCNH2 variants showed that the patients had median heart rate-corrected QT intervals <480 ms and none had been diagnosed with long-QT syndrome or experienced cardiac arrest. Simulating the impact of dysfunctional gating variants predicted that they have little impact on ventricular action potential duration. CONCLUSIONS We conclude that these rare Kv11.1 missense variants are not long-QT syndrome subtype 2-causative variants and therefore do not represent the pathogenic substrate for sudden infant death syndrome in the variant-positive infants.
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Advancing precision medicine for the treatment of long-QT syndrome type 2: shedding light on lumacaftor. Eur Heart J 2017; 39:1456-1458. [DOI: 10.1093/eurheartj/ehx561] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Identification of Drug-Drug Interactions In Vitro: A Case Study Evaluating the Effects of Sofosbuvir and Amiodarone on hiPSC-Derived Cardiomyocytes. Toxicol Sci 2016; 154:174-182. [PMID: 27503387 DOI: 10.1093/toxsci/kfw153] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Drug-drug interactions pose a difficult drug safety problem, given the increasing number of individuals taking multiple medications and the relative complexity of assessing the potential for interactions. For example, sofosbuvir-based drug treatments have significantly advanced care for hepatitis C virus-infected patients, yet recent reports suggest interactions with amiodarone may cause severe symptomatic bradycardia and thus limit an otherwise extremely effective treatment. Here, we evaluated the ability of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) to recapitulate the interaction between sofosbuvir and amiodarone in vitro, and more generally assessed the feasibility of hiPSC-CMs as a model system for drug-drug interactions. Sofosbuvir alone had negligible effects on cardiomyocyte electrophysiology, whereas the sofosbuvir-amiodarone combination produced dose-dependent effects beyond that of amiodarone alone. By comparison, GS-331007, the primary circulating metabolite of sofosbuvir, had no effect alone or in combination with amiodarone. Further mechanistic studies revealed that the sofosbuvir-amiodarone combination disrupted intracellular calcium (Ca2+) handling and cellular electrophysiology at pharmacologically relevant concentrations, and mechanical activity at supra-pharmacological (30x Cmax) concentrations. These effects were independent of the common mechanisms of direct ion channel block and P-glycoprotein activity. These results support hiPSC-CMs as a comprehensive, yet scalable model system for the identification and evaluation of cardioactive pharmacodynamic drug-drug interactions.
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Abstract 394: Multiple Cellular Mechanisms Underlie the Trafficking-deficient Phenotype for Kv11.1 (hERG) Mutations Linked to Long QT Syndrome. Circ Res 2016. [DOI: 10.1161/res.119.suppl_1.394] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Type 2 Long QT syndrome (LQT2) is commonly caused by missense mutations that disrupt the trafficking of Kv11.1 channels. The goal of this study is to determine the cellular mechanisms that underlie the trafficking-deficient phenotype for LQT2-linked mutations located in different channel domains. We expressed the pore-domain mutation G601S and the cyclic nucleotide binding domain mutation F805C in human embryonic kidney 293 (HEK293) cells or human inducible pluripotent stem cell derived cardiomyocytes (hiPSC-CMs). HEK293 cells or hiPSC-CMs expressing G601S showed a diffuse intracellular anti-Kv11.1 staining pattern that colocalized with the transitional endoplasmic reticulum (ER) marker BAP31, but not markers for the rough ER (calnexin), ER-associated degradation compartment (derlin1), ER exit sites (Sec31), or the ER-Golgi intermediate compartment (ERGIC53). Culturing cells in the drug E-4031 (10 μM, 24 hrs), which corrects the G601s trafficking-deficient phenotype, decreased its colocalization with BAP31. Unlike G601S, the trafficking-deficient phenotype for F805C is not corrected by culturing cells in E-4031. Expressing F805C in HEK293 cells or hiPSC-CMs showed a very different anti-Kv11.1 staining pattern that consisted of several discrete aggregates. In hiPSC-CMs, the F805C aggregates partially overlapped with derlin1 staining and co-transfected green fluorescent protein (gfp) in the cytosol. Incubating cells in the proteasome inhibitor bortezomib (10 μM, 4 hrs) dispersed the F805C aggregates to generate a diffuse intracellular staining pattern. We conclude that G601S and F805C localize to distinct ER subcompartments: G601S is sequestered in the transitional ER and F805C localizes to an ER associated degradation subcompartment that is sensitive to proteasome inhibition.
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IK1-enhanced human-induced pluripotent stem cell-derived cardiomyocytes: an improved cardiomyocyte model to investigate inherited arrhythmia syndromes. Am J Physiol Heart Circ Physiol 2016; 310:H1611-21. [PMID: 27059077 DOI: 10.1152/ajpheart.00481.2015] [Citation(s) in RCA: 79] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/18/2015] [Accepted: 04/01/2016] [Indexed: 01/05/2023]
Abstract
Currently available induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) do not ideally model cellular mechanisms of human arrhythmic disease due to lack of a mature action potential (AP) phenotype. In this study, we create and characterize iPS-CMs with an electrically mature AP induced by potassium inward rectifier (IK1) enhancement. The advantages of IK1-enhanced iPS-CMs include the absence of spontaneous beating, stable resting membrane potentials at approximately -80 mV and capability for electrical pacing. Compared with unenhanced, IK1-enhanced iPS-CMs calcium transient amplitudes were larger (P < 0.05) with a typical staircase pattern. IK1-enhanced iPS-CMs demonstrated a twofold increase in cell size and membrane capacitance and increased DNA synthesis compared with control iPS-CMs (P < 0.05). Furthermore, IK1-enhanced iPS-CMs expressing the F97C-CAV3 long QT9 mutation compared with wild-type CAV3 demonstrated an increase in AP duration and late sodium current. IK1-enhanced iPS-CMs represent a more mature cardiomyocyte model to study arrhythmia mechanisms.
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Molecular pathogenesis of long QT syndrome type 2. J Arrhythm 2016; 32:373-380. [PMID: 27761161 PMCID: PMC5063260 DOI: 10.1016/j.joa.2015.11.009] [Citation(s) in RCA: 69] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2015] [Revised: 10/21/2015] [Accepted: 11/17/2015] [Indexed: 12/26/2022] Open
Abstract
The molecular mechanisms underlying congenital long QT syndrome (LQTS) are now beginning to be understood. New insights into the etiology and therapeutic strategies are emerging from heterologous expression studies of LQTS-linked mutant proteins, as well as inducible pluripotent stem cell derived cardiomyocytes (iPSC-CMs) from LQTS patients. This review focuses on the major molecular mechanism that underlies LQTS type 2 (LQT2). LQT2 is caused by loss of function (LOF) mutations in KCNH2 (also known as the human Ether-à-go-go-Related Gene or hERG). Most LQT2-linked mutations are missense mutations and functional studies suggest that ~90% of them disrupt the intracellular transport (trafficking) of KCNH2-encoded Kv11.1 proteins to the cell membrane. Trafficking deficient LQT2 mutations disrupt Kv11.1 protein folding and misfolded Kv11.1 proteins are retained in the endoplasmic reticulum (ER) until they are degraded in the ER associated degradation pathway (ERAD). This review focuses on the quality control mechanisms in the ER that contribute to the folding and ERAD of Kv11.1 proteins; the mechanism for ER export of Kv11.1 proteins in the secretory pathway; different subclasses of trafficking deficient LQT2 mutations; and strategies being developed to mitigate or correct trafficking deficient LQT2-related phenotypes.
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In Memoriam: Harry A. Fozzard, MD, April 22, 1931–December 9, 2014. Heart Rhythm 2015; 12:855. [DOI: 10.1016/j.hrthm.2015.01.010] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/07/2015] [Indexed: 10/23/2022]
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Harry A. Fozzard, MD: 1931–2014. Circ Res 2015; 116:552-3. [PMID: 25821880 DOI: 10.1161/circresaha.115.306038] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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Abstract
Background The mouse ether‐a‐go‐go‐related gene 1a (mERG1a, mKCNH2) encodes mERG K+ channels in mouse cardiomyocytes. The mERG channels and their human analogue, hERG channels, conduct IKr. Mutations in hERG channels reduce IKr to cause congenital long‐QT syndrome type 2, mostly by decreasing surface membrane expression of trafficking‐deficient channels. Three cDNA sequences were originally reported for mERG channels that differ by 1 to 4 amino acid residues (mERG‐London, mERG‐Waterston, and mERG‐Nie). We characterized these mERG channels to test the postulation that they would differ in their protein trafficking and biophysical function, based on previous findings in long‐QT syndrome type 2. Methods and Results The 3 mERG and hERG channels were expressed in HEK293 cells and neonatal mouse cardiomyocytes and were studied using Western blot and whole‐cell patch clamp. We then compared our findings with the recent sequencing results in the Welcome Trust Sanger Institute Mouse Genomes Project (WTSIMGP). Conclusions First, the mERG‐London channel with amino acid substitutions in regions of highly ordered structure is trafficking deficient and undergoes temperature‐dependent and pharmacological correction of its trafficking deficiency. Second, the voltage dependence of channel gating would be different for the 3 mERG channels. Third, compared with the WTSIMGP data set, the mERG‐Nie clone is likely to represent the wild‐type mouse sequence and physiology. Fourth, the WTSIMGP analysis suggests that substrain‐specific sequence differences in mERG are a common finding in mice. These findings with mERG channels support previous findings with hERG channel structure–function analyses in long‐QT syndrome type 2, in which sequence changes in regions of highly ordered structure are likely to result in abnormal protein trafficking.
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2014 AHA/ACC/HRS guideline for the management of patients with atrial fibrillation: a report of the American College of Cardiology/American Heart Association Task Force on practice guidelines and the Heart Rhythm Society. Circulation 2014; 130:e199-267. [PMID: 24682347 PMCID: PMC4676081 DOI: 10.1161/cir.0000000000000041] [Citation(s) in RCA: 900] [Impact Index Per Article: 90.0] [Reference Citation Analysis] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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Calcium transients closely reflect prolonged action potentials in iPSC models of inherited cardiac arrhythmia. Stem Cell Reports 2014; 3:269-81. [PMID: 25254341 PMCID: PMC4175159 DOI: 10.1016/j.stemcr.2014.06.003] [Citation(s) in RCA: 77] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2013] [Revised: 06/02/2014] [Accepted: 06/03/2014] [Indexed: 12/12/2022] Open
Abstract
Long-QT syndrome mutations can cause syncope and sudden death by prolonging the cardiac action potential (AP). Ion channels affected by mutations are various, and the influences of cellular calcium cycling on LQTS cardiac events are unknown. To better understand LQTS arrhythmias, we performed current-clamp and intracellular calcium ([Ca(2+)]i) measurements on cardiomyocytes differentiated from patient-derived induced pluripotent stem cells (iPS-CM). In myocytes carrying an LQT2 mutation (HERG-A422T), APs and [Ca(2+)]i transients were prolonged in parallel. APs were abbreviated by nifedipine exposure and further lengthened upon releasing intracellularly stored Ca(2+). Validating this model, control iPS-CM treated with HERG-blocking drugs recapitulated the LQT2 phenotype. In LQT3 iPS-CM, expressing NaV1.5-N406K, APs and [Ca(2+)]i transients were markedly prolonged. AP prolongation was sensitive to tetrodotoxin and to inhibiting Na(+)-Ca(2+) exchange. These results suggest that LQTS mutations act partly on cytosolic Ca(2+) cycling, potentially providing a basis for functionally targeted interventions regardless of the specific mutation site.
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2014 AHA/ACC/HRS guideline for the management of patients with atrial fibrillation: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on practice guidelines and the Heart Rhythm Society. Circulation 2014; 130:2071-104. [PMID: 24682348 DOI: 10.1161/cir.0000000000000040] [Citation(s) in RCA: 1523] [Impact Index Per Article: 152.3] [Reference Citation Analysis] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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Human Induced Pluripotent Stem Cell Derived Cardiomyocytes (HIPS-CM's): An Expression Model System for Investigating Cardiac Channelopathies. Biophys J 2014. [DOI: 10.1016/j.bpj.2013.11.3078] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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Pharmacological correction of long QT-linked mutations in KCNH2 (hERG) increases the trafficking of Kv11.1 channels stored in the transitional endoplasmic reticulum. Am J Physiol Cell Physiol 2013; 305:C919-30. [PMID: 23864605 DOI: 10.1152/ajpcell.00406.2012] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
KCNH2 encodes Kv11.1 and underlies the rapidly activating delayed rectifier K(+) current (IKr) in the heart. Loss-of-function KCNH2 mutations cause the type 2 long QT syndrome (LQT2), and most LQT2-linked missense mutations inhibit the trafficking of Kv11.1 channels. Drugs that bind to Kv11.1 and block IKr (e.g., E-4031) can act as pharmacological chaperones to increase the trafficking and functional expression for most LQT2 channels (pharmacological correction). We previously showed that LQT2 channels are selectively stored in a microtubule-dependent compartment within the endoplasmic reticulum (ER). We tested the hypothesis that pharmacological correction promotes the trafficking of LQT2 channels stored in this compartment. Confocal analyses of cells expressing the trafficking-deficient LQT2 channel G601S showed that the microtubule-dependent ER compartment is the transitional ER. Experiments with E-4031 and the protein synthesis inhibitor cycloheximide suggested that pharmacological correction promotes the trafficking of G601S stored in this compartment. Treating cells in E-4031 or ranolazine (a drug that blocks IKr and has a short half-life) for 30 min was sufficient to cause pharmacological correction. Moreover, the increased functional expression of G601S persisted 4-5 h after drug washout. Coexpression studies with a dominant-negative form of Rab11B, a small GTPase that regulates Kv11.1 trafficking, prevented the pharmacological correction of G601S trafficking from the transitional ER. These data suggest that pharmacological correction quickly increases the trafficking of LQT2 channels stored in the transitional ER via a Rab11B-dependent pathway, and we conclude that the pharmacological chaperone activity of drugs like ranolazine might have therapeutic potential.
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Management of patients with atrial fibrillation (compilation of 2006 ACCF/AHA/ESC and 2011 ACCF/AHA/HRS recommendations): a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol 2013; 127:1916-26. [PMID: 23558044 DOI: 10.1161/cir.0b013e318290826d] [Citation(s) in RCA: 150] [Impact Index Per Article: 13.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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Human embryonic stem cell-derived cardiomyocytes: drug discovery and safety pharmacology. Expert Opin Drug Discov 2013; 2:739-53. [PMID: 23488962 DOI: 10.1517/17460441.2.5.739] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Human embryonic stem cells (hESCs) can provide potentially unlimited quantities of a wide range of human cell types that can be used in drug discovery and development, basic research and regenerative medicine. In this review, the authors describe the differentiation of hESCs into cardiomyocytes and outline the properties of hESC-derived cardiomyocytes (hESC-CMs), including their cardiac-type action potentials and contractile characteristics. In vitro cellular assays using hESC-CMs, which can be genetically engineered to create target-specific reporters as well as human disease models, will have applications at multiple stages of the drug discovery process. Furthermore, cardiac safety pharmacology assays evaluating the risk of proarrhythmic side effects associated with QT prolongation may be enhanced in their predictive value with the use of hESC-CMs.
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Effect of microculture on cell metabolism and biochemistry: do cells get stressed in microchannels? Anal Chem 2013; 85:1562-70. [PMID: 23327437 DOI: 10.1021/ac3027228] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Microfluidics is emerging as a promising platform for cell culture, enabling increased microenvironment control and potential for integrated analysis compared to conventional macroculture systems such as well plates and Petri dishes. To advance the use of microfluidic devices for cell culture, it is necessary to better understand how miniaturization affects cell behavior. In particular, microfluidic devices have significantly higher surface-area-to-volume ratios than conventional platforms, resulting in lower volumes of media per cell, which can lead to cell stress. We investigated cell stress under a variety of culture conditions using three cell lines: parental HEK (human embryonic kidney) cells and transfected HEK cells that stably express wild-type (WT) and mutant (G601S) human ether-a-go-go related gene (hERG) potassium channel protein. These three cell lines provide a unique model system through which to study cell-type-specific responses in microculture because mutant hERG is known to be sensitive to environmental conditions, making its expression a particularly sensitive readout through which to compare macro- and microculture. While expression of WT-hERG was similar in microchannel and well culture, the expression of mutant G601S-hERG was reduced in microchannels. Expression of the endoplasmic reticulum (ER) stress marker immunoglobulin binding protein (BiP) was upregulated in all three cell lines in microculture. Using BiP expression, glucose consumption, and lactate accumulation as readouts we developed methods for reducing ER stress including properly increasing the frequency of media replacement, reducing cell seeding density, and adjusting the serum concentration and buffering capacity of culture medium. Indeed, increasing the buffering capacity of culture medium or frequency of media replacement partially restored the expression of the G601S-hERG in microculture. This work illuminates how biochemical properties of cells differ in macro- and microculture and suggests strategies that can be used to modify cell culture protocols for future studies involving miniaturized culture platforms.
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Cellular Mechanism for the Pharmacological Correction of hERG Mutations Linked to the Long QT Syndrome. Biophys J 2013. [DOI: 10.1016/j.bpj.2012.11.1496] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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Mechanism of loss of Kv11.1 K+ current in mutant T421M-Kv11.1-expressing rat ventricular myocytes: interaction of trafficking and gating. Circulation 2012; 126:2809-18. [PMID: 23136156 DOI: 10.1161/circulationaha.112.118018] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
BACKGROUND Type 2 long QT syndrome involves mutations in the human ether a-go-go-related gene (hERG or KCNH2). T421M, an S1 domain mutation in the Kv11.1 channel protein, was identified in a resuscitated patient. We assessed its biophysical, protein trafficking, and pharmacological mechanisms in adult rat ventricular myocytes. METHODS AND RESULTS Isolated adult rat ventricular myocytes were infected with wild-type (WT)-Kv11.1- and T421M-Kv11.1-expressing adenovirus and analyzed with the use of patch clamp, Western blot, and confocal imaging techniques. Expression of WT-Kv11.1 or T421M-Kv11.1 produced peak tail current (I(Kv11.1)) of 8.78±1.18 and 1.91±0.22 pA/pF, respectively. Loss of mutant I(Kv11.1) resulted from (1) a partially trafficking-deficient channel protein with reduced cell surface expression and (2) altered channel gating with a positive shift in the voltage dependence of activation and altered kinetics of activation and deactivation. Coexpression of WT+T421M-Kv11.1 resulted in heterotetrameric channels that remained partially trafficking deficient with only a minimal increase in peak I(Kv11.1) density, whereas the voltage dependence of channel gating became WT-like. In the adult rat ventricular myocyte model, both WT-Kv11.1 and T421M-Kv11.1 channels responded to β-adrenergic stimulation by increasing I(Kv11.1). CONCLUSIONS The T421M-Kv11.1 mutation caused a loss of I(Kv11.1) through interactions of abnormal protein trafficking and channel gating. Furthermore, for coexpressed WT+T421M-Kv11.1 channels, different dominant-negative interactions govern protein trafficking and ion channel gating, and these are likely to be reflected in the clinical phenotype. Our results also show that WT and mutant Kv11.1 channels responded to β-adrenergic stimulation.
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High purity human-induced pluripotent stem cell-derived cardiomyocytes: electrophysiological properties of action potentials and ionic currents. Am J Physiol Heart Circ Physiol 2011; 301:H2006-17. [PMID: 21890694 DOI: 10.1152/ajpheart.00694.2011] [Citation(s) in RCA: 545] [Impact Index Per Article: 41.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Human-induced pluripotent stem cells (hiPSCs) can differentiate into functional cardiomyocytes; however, the electrophysiological properties of hiPSC-derived cardiomyocytes have yet to be fully characterized. We performed detailed electrophysiological characterization of highly pure hiPSC-derived cardiomyocytes. Action potentials (APs) were recorded from spontaneously beating cardiomyocytes using a perforated patch method and had atrial-, nodal-, and ventricular-like properties. Ventricular-like APs were more common and had maximum diastolic potentials close to those of human cardiac myocytes, AP durations were within the range of the normal human electrocardiographic QT interval, and APs showed expected sensitivity to multiple drugs (tetrodotoxin, nifedipine, and E4031). Early afterdepolarizations (EADs) were induced with E4031 and were bradycardia dependent, and EAD peak voltage varied inversely with the EAD take-off potential. Gating properties of seven ionic currents were studied including sodium (I(Na)), L-type calcium (I(Ca)), hyperpolarization-activated pacemaker (I(f)), transient outward potassium (I(to)), inward rectifier potassium (I(K1)), and the rapidly and slowly activating components of delayed rectifier potassium (I(Kr) and I(Ks), respectively) current. The high purity and large cell numbers also enabled automated patch-clamp analysis. We conclude that these hiPSC-derived cardiomyocytes have ionic currents and channel gating properties underlying their APs and EADs that are quantitatively similar to those reported for human cardiac myocytes. These hiPSC-derived cardiomyocytes have the added advantage that they can be used in high-throughput assays, and they have the potential to impact multiple areas of cardiovascular research and therapeutic applications.
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Trafficking-deficient hERG K⁺ channels linked to long QT syndrome are regulated by a microtubule-dependent quality control compartment in the ER. Am J Physiol Cell Physiol 2011; 301:C75-85. [PMID: 21490315 DOI: 10.1152/ajpcell.00494.2010] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The human ether-a-go-go related gene (hERG) encodes the voltage-gated K(+) channel that underlies the rapidly activating delayed-rectifier current in cardiac myocytes. hERG is synthesized in the endoplasmic reticulum (ER) as an "immature" N-linked glycoprotein and is terminally glycosylated in the Golgi apparatus. Most hERG missense mutations linked to long QT syndrome type 2 (LQT2) reduce the terminal glycosylation and functional expression. We tested the hypothesis that a distinct pre-Golgi compartment negatively regulates the trafficking of some LQT2 mutations to the Golgi apparatus. We found that treating cells in nocodazole, a microtubule depolymerizing agent, altered the subcellular localization, functional expression, and glycosylation of the LQT2 mutation G601S-hERG differently from wild-type hERG (WT-hERG). G601S-hERG quickly redistributed to peripheral compartments that partially colocalized with KDEL (Lys-Asp-Glu-Leu) chaperones but not calnexin, Sec31, or the ER golgi intermediate compartment (ERGIC). Treating cells in E-4031, a drug that increases the functional expression of G601S-hERG, prevented the accumulation of G601S-hERG to the peripheral compartments and increased G601S-hERG colocalization with the ERGIC. Coexpressing the temperature-sensitive mutant G protein from vesicular stomatitis virus, a mutant N-linked glycoprotein that is retained in the ER, showed it was not restricted to the same peripheral compartments as G601S-hERG at nonpermissive temperatures. We conclude that the trafficking of G601S-hERG is negatively regulated by a microtubule-dependent compartment within the ER. Identifying mechanisms that prevent the sorting or promote the release of LQT2 channels from this compartment may represent a novel therapeutic strategy for LQT2.
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2011 ACCF/AHA/HRS Focused Update on the Management of Patients With Atrial Fibrillation (Update on Dabigatran). Circulation 2011; 123:1144-50. [DOI: 10.1161/cir.0b013e31820f14c0] [Citation(s) in RCA: 351] [Impact Index Per Article: 27.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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The common African American polymorphism SCN5A-S1103Y interacts with mutation SCN5A-R680H to increase late Na current. Physiol Genomics 2011; 43:461-6. [PMID: 21385947 DOI: 10.1152/physiolgenomics.00198.2010] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
The common polymorphism SCN5A-S1103Y (∼13% allelic frequency in African Americans) is a risk factor for arrhythmia, sudden unexplained death (SUD), and sudden infant death syndrome. Prompted by a case of autopsy-negative SUD in a 23-year-old African American man who collapsed while playing football, we hypothesized that S1103Y interacted with other SCN5A variants to pathologically modify sodium current (I(Na)). Mutational analysis of arrhythmia-associated genes in the victim revealed the variants SCN5A-R680H and SCN5A-S1103Y. These variants were made both separately and in the same cDNA construct of the alternative splice variant backgrounds (SCN5A-Q1077del and Q1077) and expressed in HEK293 cells. In the most abundant SCN5A-Q1077del, late I(Na) for S1103Y alone was not significantly different from wild type (WT). However, late I(Na) for R680H, R680H+S1103Y (coexpressed), and R680H/S1103Y (on the same cDNA) was increased 2.1-, 3.4-, and 3.6-fold, respectively, compared with WT. Intracellular acidosis (pH 6.7) increased late I(Na) for S1103Y, R680H, R680H+S1103Y, and R680H/S1103Y by 2.2-, 2.4-, 5.0-, and 5.5-fold, respectively, compared with WT at pH 6.7. Expression in the less abundant SCN5A-Q1077 showed no increased late I(Na). This is the initial report of a functional interaction for the common polymorphism S1103Y with another mutation in the major transcript Q1077del of SCN5A. The "double hit" and environmental factor of acidosis may have converged to cause arrhythmic sudden death in this case.
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2011 ACCF/AHA/HRS Focused Update on the Management of Patients With Atrial Fibrillation (Update on Dabigatran). Heart Rhythm 2011; 8:e1-8. [DOI: 10.1016/j.hrthm.2011.01.032] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/18/2011] [Indexed: 11/24/2022]
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2011 ACCF/AHA/HRS focused update on the management of patients with atrial fibrillation (update on dabigatran). A report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. Heart Rhythm 2011; 57:1330-7. [PMID: 21324629 DOI: 10.1016/j.jacc.2011.01.010] [Citation(s) in RCA: 169] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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The Trafficking of Mutant HERG K+ Channels Linked to Long QT Syndrome are Regulated by a Subdomain in the Endoplasmic Reticulum. Biophys J 2011. [DOI: 10.1016/j.bpj.2010.12.2526] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
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Small GTPase Rab11b regulates degradation of surface membrane L-type Cav1.2 channels. Am J Physiol Cell Physiol 2011; 300:C1023-33. [PMID: 21248079 DOI: 10.1152/ajpcell.00288.2010] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
L-type Ca(2+) channels (LTCCs) play a critical role in Ca(2+)-dependent signaling processes in a variety of cell types. The number of functional LTCCs at the plasma membrane strongly influences the strength and duration of Ca(2+) signals. Recent studies demonstrated that endosomal trafficking provides a mechanism for dynamic changes in LTCC surface membrane density. The purpose of the current study was to determine whether the small GTPase Rab11b, a known regulator of endosomal recycling, impacts plasmalemmal expression of Ca(v)1.2 LTCCs. Disruption of endogenous Rab11b function with a dominant negative Rab11b S25N mutant led to a significant 64% increase in peak L-type Ba(2+) current (I(Ba,L)) in human embryonic kidney (HEK)293 cells. Short-hairpin RNA (shRNA)-mediated knockdown of Rab11b also significantly increased peak I(Ba,L) by 66% compared when with cells transfected with control shRNA, whereas knockdown of Rab11a did not impact I(Ba,L). Rab11b S25N led to a 1.7-fold increase in plasma membrane density of hemagglutinin epitope-tagged Ca(v)1.2 expressed in HEK293 cells. Cell surface biotinylation experiments demonstrated that Rab11b S25N does not significantly impact anterograde trafficking of LTCCs to the surface membrane but rather slows degradation of plasmalemmal Ca(v)1.2 channels. We further demonstrated Rab11b expression in ventricular myocardium and showed that Rab11b S25N significantly increases peak I(Ba,L) by 98% in neonatal mouse cardiac myocytes. These findings reveal a novel role for Rab11b in limiting, rather than promoting, the plasma membrane expression of Ca(v)1.2 LTCCs in contrast to its effects on other ion channels including human ether-a-go-go-related gene (hERG) K(+) channels and cystic fibrosis transmembrane conductance regulator. This suggests Rab11b differentially regulates the trafficking of distinct cargo and extends our understanding of how endosomal transport impacts the functional expression of LTCCs.
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2011 ACCF/AHA/HRS Focused Update on the Management of Patients With Atrial Fibrillation (Updating the 2006 Guideline). Circulation 2011; 123:104-23. [PMID: 21173346 DOI: 10.1161/cir.0b013e3181fa3cf4] [Citation(s) in RCA: 145] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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2011 ACCF/AHA/HRS Focused Update on the Management of Patients With Atrial Fibrillation (Updating the 2006 Guideline). Heart Rhythm 2011; 8:157-76. [PMID: 21182985 DOI: 10.1016/j.hrthm.2010.11.047] [Citation(s) in RCA: 84] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/22/2010] [Indexed: 10/18/2022]
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Microfluidic cell culture and its application in high-throughput drug screening: cardiotoxicity assay for hERG channels. ACTA ACUST UNITED AC 2010; 16:101-11. [PMID: 21131594 DOI: 10.1177/1087057110386218] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Evaluation of drug cardiotoxicity is essential to the safe development of novel pharmaceuticals. Assessing a compound's risk for prolongation of the surface electrocardiographic QT interval and hence risk for life-threatening arrhythmias is mandated before approval of nearly all new pharmaceuticals. QT prolongation has most commonly been associated with loss of current through hERG (human ether-a-go-go related gene) potassium ion channels due to direct block of the ion channel by drugs or occasionally by inhibition of the plasma membrane expression of the channel protein. To develop an efficient, reliable, and cost-effective hERG screening assay for detecting drug-mediated disruption of hERG membrane trafficking, the authors demonstrate the use of microfluidic-based systems to improve throughput and lower cost of current methods. They validate their microfluidics array platform in polystyrene (PS), cyclo-olefin polymer (COP), and polydimethylsiloxane (PDMS) microchannels for drug-induced disruption of hERG trafficking by culturing stably transfected HEK cells that overexpressed hERG (WT-hERG) and studying their morphology, proliferation rates, hERG protein expression, and response to drug treatment. Results show that WT-hERG cells readily proliferate in PS, COP, and PDMS microfluidic channels. The authors demonstrated that conventional Western blot analysis was possible using cell lysate extracted from a single microchannel. The Western blot analysis also provided important evidence that WT-hERG cells cultured in microchannels maintained regular (well plate-based) expression of hERG. The authors further show that experimental procedures can be streamlined by using direct in-channel immunofluorescence staining in conjunction with detection using an infrared scanner. Finally, treatment of WT-hERG cells with 5 different drugs suggests that PS (and COP) microchannels were more suitable than PDMS microchannels for drug screening applications, particularly for tests involving hydrophobic drug molecules.
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Abstract
BACKGROUND Genetic long QT (LQT) syndrome is a life-threatening disorder caused by mutations that result in prolongation of cardiac repolarization. Recent work has demonstrated that a zebrafish model of LQT syndrome faithfully recapitulates several features of human disease, including prolongation of ventricular action potential duration, spontaneous early afterdepolarizations, and 2:1 atrioventricular block in early stages of development. Because of their transparency, small size, and absorption of small molecules from their environment, zebrafish are amenable to high-throughput chemical screens. We describe a small-molecule screen using the zebrafish KCNH2 mutant breakdance to identify compounds that can rescue the LQT type 2 phenotype. METHODS AND RESULTS Zebrafish breakdance embryos were exposed to test compounds at 48 hours of development and scored for rescue of 2:1 atrioventricular block at 72 hours in a 96-well format. Only compounds that suppressed the LQT phenotype in 3 of 3 fish were considered hits. Screen compounds were obtained from commercially available small-molecule libraries (Prestwick and Chembridge). Initial hits were confirmed with dose-response testing and time-course studies. Optical mapping with the voltage-sensitive dye di-4 ANEPPS was performed to measure compound effects on cardiac action potential durations. Screening of 1200 small molecules resulted in the identification of flurandrenolide and 2-methoxy-N-(4-methylphenyl) benzamide (2-MMB) as compounds that reproducibly suppressed the LQT phenotype. Optical mapping confirmed that treatment with each compound caused shortening of ventricular action potential durations. Structure activity studies and steroid receptor knockdown suggest that flurandrenolide functions via the glucocorticoid signaling pathway. CONCLUSIONS Using a zebrafish model of LQT type 2 syndrome in a high-throughput chemical screen, we have identified 2 compounds, flurandrenolide and the novel compound 2-MMB, as small molecules that rescue the zebrafish LQT type 2 syndrome by shortening the ventricular action potential duration. We provide evidence that flurandrenolide functions via the glucocorticoid receptor-mediated pathway. These 2 molecules and future discoveries from this screen should yield novel tools for the study of cardiac electrophysiology and may lead to novel therapeutics for human LQT patients.
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Comparison of mERG and hERG Clones Reveals Differences in Protein Trafficking and Functionality: Structural Implications. Heart Rhythm 2010. [DOI: 10.1016/j.hrthm.2010.09.025] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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Properties of WT and mutant hERG K(+) channels expressed in neonatal mouse cardiomyocytes. Am J Physiol Heart Circ Physiol 2010; 298:H1842-9. [PMID: 20363883 DOI: 10.1152/ajpheart.01236.2009] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mutations in human ether-a-go-go-related gene 1 (hERG) are linked to long QT syndrome type 2 (LQT2). hERG encodes the pore-forming alpha-subunits that coassemble to form rapidly activating delayed rectifier K(+) current in the heart. LQT2-linked missense mutations have been extensively studied in noncardiac heterologous expression systems, where biogenic (protein trafficking) and biophysical (gating and permeation) abnormalities have been postulated to underlie the loss-of-function phenotype associated with LQT2 channels. Little is known about the properties of LQT2-linked hERG channel proteins in native cardiomyocyte systems. In this study, we expressed wild-type (WT) hERG and three LQT2-linked mutations in neonatal mouse cardiomyocytes and studied their electrophysiological and biochemical properties. Compared with WT hERG channels, the LQT2 missense mutations G601S and N470D hERG exhibited altered protein trafficking and underwent pharmacological correction, and N470D hERG channels gated at more negative voltages. The DeltaY475 hERG deletion mutation trafficked similar to WT hERG channels, gated at more negative voltages, and had rapid deactivation kinetics, and these properties were confirmed in both neonatal mouse cardiomyocyte and human embryonic kidney (HEK)-293 cell expression systems. Differences between the cardiomyocytes and HEK-293 cell expression systems were that hERG current densities were reduced 10-fold and deactivation kinetics were accelerated 1.5- to 2-fold in neonatal mouse cardiomyocytes. An important finding of this work is that pharmacological correction of trafficking-deficient LQT2 mutations, as a potential innovative approach to therapy, is possible in native cardiac tissue.
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