Hamrick, Samantha K.

Introduction: Calmodulin (CaM)-mediated long QT syndrome (CaM-LQTS) is a genetic arrhythmia disorder characterized by a high prevalence of life-threatening ventricular arrhythmias occurring very early in life, frequent T-wave alternans, markedly prolonged QT intervals (QTc > 600 ms), and intermittent 2:1 AV block. Three distinct genes ( CALM1 , CALM2 , and CALM3 ) all encode for the identical CaM protein. Conventional LQTS pharmacotherapies fail to adequately protect against potentially lethal cardiac events in patients with CaM-LQTS. Here, we developed a CALM1 “ suppression-replacement (SupRep)” gene therapy with the goal of rescuing the prolonged cardiac action potential duration (APD) and increased Ca 2+ amplitude (ΔF/F0) in patient-derived induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from a patient with CALM1 -mediated LQTS. Methods: Custom designed CALM1 -targeting shRNAs were tested for knockdown efficiency using TSA201 cells and RT-qPCR. A dual-component SupRep CALM1 gene therapy was created by cloning into a single construct a custom-designed CALM1 shRNA that produces knockdown (suppression) and a “shRNA-immune” (shIMM) CALM1 cDNA (replacement). Patient-specific CALM1-F142L iPSC-CMs were generated from a patient with severe LQTS (QTc = 515 ms). FluoVolt voltage-sensing dye was used to measure the APD at 90% repolarization (APD 90 ). Calcium transient amplitude was measured using Fluo-4 indicator. Results: Overall, 6 unique candidate CALM1 -targeting shRNAs were tested for knockdown efficiency in a transient transfection CALM1 over-expression TSA201 cell system. One candidate shRNA had an 80% knockdown efficiency of CALM1. Following transfection with CALM1 -SupRep gene therapy, the APD 90 was significantly decreased in F142L iPSC-CMs compared to untreated cells (F142L: 665 ± 9 ms [n=40] vs F142L + SupRep: 353 ± 9 ms [n=20], p<0.0001). Ca 2+ amplitude was significantly decreased following SupRep transfection when compared to untreated cells (0.99 ± 0.04 vs 0.8 ± 0.06, p=0.02). Conclusions: Here, we provide the first proof-of-principle SupRep gene therapy for CALM1-mediated LQTS. CALM1 -SupRep gene therapy shortened significantly the pathologically prolonged APD 90 in CALM1 -F142L patient-derived iPSC-CMs.

Introduction: Multifocal ectopic Purkinje-related premature contractions (MEPPC), the most recent addition to the spectrum of SCN5A -mediated cardiac channelopathies, is characterized by frequent premature ventricular complexes (PVCs) originating from the Purkinje system, atrial arrhythmias, and a predilection for PVC-mediated dilated cardiomyopathy. Hypothesis: To investigate the electrophysiology phenotype of MEPPC in a novel, patient-specific, re-engineered cardiomyocyte model. Methods: Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from a 27-year-old male with a history of frequent, multifocal PVCs and atrial tachycardia, positive for a putative MEPPC-causative SCN5A variant (p.Q419P). A CRISPR-Cas9 Q419P variant-corrected isogenic control (IC) was created to establish the normalized electrophysiology recordings for that MEPPC model. The standard whole-cell patch clamp technique was used to measure Q419P-SCN5A- and IC-derived NaV1.5 sodium currents using voltage clamp configuration. In addition, action potential duration (APD) of the iPSC-CMs were measured using FluoVolt optical voltage-sensing dye. Results: Analysis of the current-voltage relationship revealed that neither peak sodium current densities nor late inward sodium current were impacted by Q419P-SCN5A. However, Q419P-SCN5A significantly right-shifted the V 1/2 in inactivation by 8.3 mV from -81.1±1.7 mV (IC, n=5) to -72.8±0.2 mV (Q419P-SCN5A, n=6, p<0.05 vs. IC) and left-shifted the V 1/2 in activation by -4.1 mV from -39.5±0.3 mV (IC, n=5) to -43.6±0.9 mV (Q419P-SCN5A, n=6, p<0.05 vs. IC) leading to a larger hyperpolarized window current. Optical APD90 measurement revealed no difference between the Q419P-containing (476 ± 23 ms) and the IC (474 ± 46 ms, p=0.867) cardiomyocytes. Conclusions: This is the first in-vitro disease model of MEPPC using patient-specific, re-engineered heart cells. Consistent with the findings in the non-cardiac HEK cell system, this MEPPC-causative variant contributes to increased window current in cardiomyocytes without affecting the cardiac APD. This novel cardiac cell model for MEPPC may facilitate new insights into the pathobiology of MEPPC and enable the testing of novel therapies for it.

Introduction: Pathogenic variants in PKP2 -encoded plakophilin 2 cause arrhythmogenic right ventricular cardiomyopathy (ARVC). Electrocardiographically, PKP2 -mediated ARVC may mimic RYR2 -mediated catecholaminergic polymorphic ventricular tachycardia (CPVT). We compare the underlying mechanism of arrhythmia in ARVC and CPVT using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Methods: iPSC-CMs were generated from 2 ARVC patients with different PKP2 variants [E149X (PKP2-1) and c.2146-1G>C (PKP2-2)], 1 patient with a CPVT variant [RyR2-R176Q (CPVT1)], and a wild-type (WT) control. Fluo-4 AM calcium dye was used to assess calcium kinetics in iPSC-CMs before and during treatment with 100 nM isoproterenol (ISO) at 8-10 min. Results: Compared to WT, PKP2 and RYR2 iPSC-CMs had a greater amplitude before (WT: 0.331±0.031, PKP2-1: 1.048±0.074, PKP2-2: 1.076±0.095, CPVT1: 0.966±0.061; P<0.0001) and during ISO (WT: 0.277±0.022, PKP2-1: 0.971±0.069, PKP2-2: 0.622±0.041, CPVT1: 0.756±0.051; P<0.0001). The Ca 2+ transient duration (CTD100) was longer in PKP2 and RYR2 iPSC-CMs compared to WT before (WT: 1.01±0.05 s, PKP2-1: 1.84±0.01 s, PKP2-2: 1.76±0.02 s, CPVT1: 1.81±0.02 s; P<0.0001) and during ISO (WT: 1.38±0.05 s, PKP2-1: 1.65±0.03 s, PKP2-2: 1.71±0.03 s, CPVT1: 1.61±0.05 s; P<0.001). Before (WT: 259±13 ms, PKP2-1: 538±23 ms, PKP2-2: 493±28 ms, CPVT1: 586±24 ms; P<0.0001) and during ISO (WT: 311±19 ms, PKP2-1: 406±21 ms, P=0.0172, PKP2-2: 487±28 ms, P<0.0001, CPVT1: 453±21 ms, P=0.0001), peak to 50% decay time was slower in PKP2 and RYR2 iPSC-CMs compared with WT. Upstroke time was longer in PKP2 and RYR2 iPSC-CMs compared to WT before (WT: 205±5 ms, PKP2-1: 539±13 ms, PKP2-2: 429±19 ms, CPVT1: 455±12 ms; P<0.0001) and during ISO (WT: 213±7 ms, PKP2-1: 375±16 ms, PKP2-2: 416±20 ms, CPVT1: 389±10 ms; P<0.0001). Conclusion: Here, iPSC-CMs, derived from patients with either PKP2 -mediated ARVC or RYR2 -mediated CPVT, display similar abnormal calcium handling properties such as greater amplitude, longer CTD100, and slower peak to 50% decay time and upstroke time compared to WT iPSC-CMs before and during ISO treatment. This may explain why patients with PKP2 -mediated ARVC often mimic the treadmill stress test seen in patients with CPVT1.

Background: Drug-induced QT prolongation (DI-QTP) is a clinical entity in which administration of a HERG/I Kr blocker such as dofetilide prolongs the cardiac action potential duration (APD) at the cellular level and the QT interval clinically, and increases the risk for a potentially lethal, QT-triggered ventricular arrhythmia. There may be a role for inhibition of I NaL to counter DI-QTP. Recently, we have shown that inhibition of serum and glucocorticoid regulated kinase-1 (SGK1) reduces the APD90 in induced pluripotent stem cell derived cardiomyocytes (iPSC-CM) from patients with LQT3 and ameliorates increases in I NaL . Objective: To test the efficacy of novel SGK1 inhibitors (SGK-I) in re-engineered cardiomyocyte models of dofetilide-induced APD prolongation. Methods: Normal iPSC-CMs were treated with dofetilide [5 nM] to produce a DI-QTP iPSC-CM model. The SGK1-I’s therapeutic efficacy for shortening the dofetilide-induced APD90 prolongation was compared to mexiletine. The APD90 values were recorded 4 hours after treatment using FluoVolt, a voltage-sensitive fluorescent dye. Results: The APD90 was significantly prolonged in normal iPSC-CMs treated with dofetilide (673 ± 8 ms with dofetilide vs 436 ± 4 ms with DMSO, p<0.0001). While 10 μM mexiletine shortened the average APD90 of dofetilide-treated normal iPSC-CMs from 673 ± 4 ms to 563 ± 8 ms (46% attenuation, p<0.0001), 30 nM of SGK-I shortened the APD90 from 673 ± 4 ms to 478 ± 10 ms (82% attenuation, p<0.0001). Conclusions: Therapeutically inhibiting SGK1 effectively shortens the cardiomyocyte APD in a human heart cell model of DI-QTP. The novel SGK1 inhibitor normalized the pathological APD prolongation almost fully (> 80%) in the iPSC-CM model treated with dofetilide. This pre-clinical data supports further development of this therapeutic strategy to counter and neutralize the threat of DI-QTP.

Background: Long QT syndrome (LQTS) often stems from pathogenic variants in KCNQ1 (LQT1), KCNH2 (LQT2), or SCN5A (LQT3) and is characterized cellularly by prolongation of the cardiac action potential duration (APD). We have shown that inhibition of serum and glucocorticoid regulated kinase-1 (SGK1) reduces I NaL and is proposed as a novel therapeutic strategy for LQT3. Objective: To test the efficacy of a potent and selective SGK1 inhibitor (SGK1-I) in human cardiomyocyte models of LQT1, LQT2, and LQT3. Methods: Induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs) were generated from patients with either LQT1, LQT2, or LQT3. The mexiletine (MEX)-sensitive SCN5A-P1332L iPSC-CMs were tested initially. A CRISPR/Cas9 P1332L variant-corrected isogenic control (IC) was used as a control. The SGK1-I’s therapeutic efficacy was compared to MEX and to a commercially available SGK1 inhibitor (EMD638683). The novel SGK-I was then tested for APD shortening in SCN5A-R1623Q, KCNH2-G604S and KCNQ1-V254M iPSC-CMs. The APD90 values were recorded 4 hours after treatment using FluoVolt. Results: The APD90 was significantly prolonged in SCN5A-P1332L iPSC-CMs compared to its IC (646 ± 7 ms vs 482 ± 23 ms, p<0.0001). MEX shortened the average APD90 to 560 ± 7 ms (52% attenuation). Interestingly, SGK1-I significantly shortened the APD to 518 ± 5 ms (78% attenuation) but did not further shorten the APD in the IC. SGK1-I also shortened the APD90 of the SCN5A-R1623Q iPSC-CMs from 753 ± 8 ms to 475 ± 19 ms compared to 558 ± 19 ms with MEX. The SGK1-I shortened the APD90 in KCNH2-G604S (666 ± 10ms to 574 ± 18ms for SGK1-I versus 538 ± 15 ms after MEX). EMD had limited APD shortening effects in SCN5A and KCNH2 iPSC-CMs. Interestingly, while neither MEX nor EMD reduced the APD90 in the KCNQ1-V254M iPSC-CMs, the novel SGK1-I reduced the APD90 from 544 ± 10 ms to 475 ± 11ms (p=0.0004). Conclusions: Therapeutically inhibiting SGK1 effectively shortens the cardiomyocyte APD in human heart cell models of the 3 major LQTS genotypes. The novel SGK1-I normalized the pathological APD prolongation almost fully (> 70%) in the patient-derived SCN5A-P1332L iPSC-CM model. These pre-clinical data support further development of SGK1-I as a novel, first-in-class therapy for patients with congenital LQTS.

Background: The transient outward current (Ito) that mediates early (phase 1) repolarization is conducted by the KCND3 -encoded Kv4.3 pore-forming α-subunit. KCND3 gain-of-function mutations have been reported previously as a pathogenic substrate for J wave syndromes (JWS), including the Brugada syndrome and early repolarization syndrome, as well as autopsy-negative sudden unexplained death (SUD). Acacetin, a natural flavone, is a potent Ito current blocker. Acacetin may be a novel therapeutic for KCND3 -mediated J wave syndrome. Methods: KCND3-V392I was identified in an 18-year-old male with J wave syndrome/early repolarization syndrome, and a history of cardiac arrest including ventricular tachycardia/ventricular fibrillation and atrial fibrillation/atrial flutter. Pathogenic KCND3 mutation was engineered by site-directed mutagenesis and co-expressed with wild-type KChIP2 in TSA201 cells. Gene-edited/variant-corrected isogenic control and patient-specific pluripotent stem cell–derived cardiomyocytes (iPSC-CMs) from the p. Val392Ile-KCND3-positive patient were generated. I to currents and action potentials were recorded before and after treatment with Acacetin using the whole cell patch-clamp and multielectrode array technique. Western blot and immunocytochemistry were performed to investigate KCND3 expression. Results: KCND3-V392I demonstrated a marked gain-of-function phenotype, increasing peak I to current density by 92.2% ( P <0.05 versus KCND3-WT). KCND3 expression was significantly increased in KCND3-V392I-derived iPSC-CMs ( P <0.05 versus isogenic control). While KCND3-WT revealed an IC50 of 7.2±1.0 µmol/L for acacetin effect, 30 µmol/L acacetin dramatically inhibited KCND3-V392I peak Ito current density by 96.2% ( P <0.05 versus before Acacetin). Ito was also increased by 60.9% in Kv4.3-V392I iPSC-CM ( P <0.05 versus isogenic control iPSC-CM). Ten micromoles per liter acacetin, a concentration approaching its IC50 value, inhibited Ito by ≈50% in patient-derived iPSC-CMs and reduced the accentuated action potential notch displayed in KCND3-V392I-derived iPSC-CMs. Conclusions: This preclinical study provides pharmacological and functional evidence to suggest that Acacetin may be a novel therapeutic for patients with KCND3 gain-of-function-associated J wave syndrome by inhibiting I to and abolishing the accentuated action potential notch in patient-derived iPSC-CMs.

Background: The KCNH2 -encoded Kv11.1 hERG (human ether-a-go-go related gene) potassium channel is a critical regulator of cardiomyocyte action potential duration (APD). The majority of type 2 long-QT syndrome (LQT2) stems from trafficking defective KCNH2 mutations. Recently, Food and Drug Administration-approved cystic fibrosis protein trafficking chaperone, lumacaftor, has been proposed as novel therapy for LQT2. Here, we test the efficacy of lumacaftor treatment in patient-specific induced pluripotent stem cell-cardiomyocytes (iPSC-CMs) derived from 2 patients with known LQT2 trafficking defective mutations and a patient with novel KCNH2 variant, p.R685P. Methods: Patient-specific iPSC-CM models of KCNH2-G604S, KCNH2-N633S, and KCNH2-R685P were generated from 3 unrelated patients diagnosed with severe LQT2 (rate-corrected QT>500 ms). Lumacaftor efficacy was also tested by ANEPPS, FluoVolt, and ArcLight voltage dye-based APD90 measurements. Results: All 3 mutations were hERG trafficking defective in iPSC-CMs. While lumacaftor treatment failed to rescue the hERG trafficking defect in TSA201 cells, lumacaftor rescued channel trafficking for all mutations in the iPSC-CM model. All 3 mutations conferred a prolonged APD90 compared with control. While lumacaftor treatment rescued the phenotype of KCNH2-N633S and KCNH2-R685P, lumacaftor paradoxically prolonged the APD90 in KCNH2-G604S iPSC-CMs. Lumacaftor-mediated APD90 rescue was affected by rapidly activating delayed rectifier K+ current blocker consistent with the increase of rapidly activating delayed rectifier K+ current by lumacaftor is the underlying mechanism of the LQT2 rescue. Conclusions: While lumacaftor is an effective hERG channel trafficking chaperone and may be therapeutic for LQT2, we urge caution. Without understanding the functionality of the mutant channel to be rescued, lumacaftor therapy could be harmful.

We identified a potentially novel homozygous duplication involving the promoter region and exons 1–4 of the gene encoding type 2 cardiac ryanodine receptor ( RYR2 ) that is responsible for highly penetrant, exertion-related sudden deaths/cardiac arrests in the Amish community without an overt phenotype to suggest RYR2- mediated catecholaminergic polymorphic ventricular tachycardia (CPVT). Homozygous RYR2 duplication (RYR2-DUP) induced pluripotent stem cell cardiomyocytes (iPSC-CMs) were generated from 2 unrelated patients. There was no difference in baseline Ca 2+ handling measurements between WT-iPSC-CM and RYR2-DUP-iPSC-CM lines. However, compared with WT-iPSC-CMs, both patient lines demonstrated a dramatic reduction in caffeine-stimulated and isoproterenol-stimulated (ISO-stimulated) Ca 2+ transient amplitude, suggesting RyR2 loss of function. There was a greater than 50% reduction in RYR2 transcript/RyR2 protein expression in both patient iPSC-CMs compared with WT. Delayed afterdepolarization was observed in the RYR2-DUP-iPSC-CMs but not in the WT-iPSC-CMs. Compared with WT-iPSC-CMs, there was significantly elevated arrhythmic activity in the RYR2-DUP-iPSC-CMs in response to ISO. Nadolol, propranolol, and flecainide reduced erratic activity by 8.5-fold, 6.8-fold, and 2.4-fold, respectively, from ISO challenge. Unlike the gain-of-function mechanism observed in RYR2- mediated CPVT, the homozygous multiexon duplication precipitated a dramatic reduction in RYR2 transcription and RyR2 protein translation, a loss of function in calcium handling, and a calcium-induced calcium release apparatus that is insensitive to catecholamines and caffeine.