Cellular and electrophysiological characterization of triadin knockout syndrome using induced pluripotent stem cell-derived cardiomyocytes

Triadin knockout syndrome (TKOS) is a malignant arrhythmia disorder caused by recessive null variants in TRDN-encoded cardiac triadin. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from two unrelated TKOS patients and an unrelated control. CRISPR-Cas9 gene editing was used to insert homozygous TRDN-p.D18fs∗13 into a control line to generate a TKOS model (TRDN-/-). Western blot confirmed total knockout of triadin in patient-specific and TRDN-/- iPSC-CMs. iPSC-CMs from both patients revealed a prolonged action potential duration (APD) at 90% repolarization, and this was normalized by protein replacement of triadin. APD prolongation was confirmed in TRDN-/- iPSC-CMs. TRDN-/- iPSC-CMs revealed that loss of triadin underlies decreased expression and co-localization of key calcium handling proteins, slow and decreased calcium release from the sarcoplasmic reticulum, and slow inactivation of the L-type calcium channel leading to frequent cellular arrhythmias, including early and delayed afterdepolarizations and APD alternans.

Suppression and Replacement Gene Therapy for KCNH2-Mediated Arrhythmias

BACKGROUND

KCNH2-mediated arrhythmia syndromes are caused by loss-of-function (type 2 long QT syndrome [LQT2]) or gain-of-function (type 1 short QT syndrome [SQT1]) pathogenic variants in the KCNH2-encoded K_v11.1 potassium channel, which is essential for the cardiac action potential.

METHODS

A dual-component "suppression-and-replacement" (SupRep) KCNH2 gene therapy was created by cloning into a single construct a custom-designed KCNH2 short hairpin RNA with ~80% knockdown (suppression) and a "short hairpin RNA-immune" KCNH2 cDNA (replacement). Induced pluripotent stem cell-derived cardiomyocytes and their CRISPR-Cas9 variant-corrected isogenic control (IC) induced pluripotent stem cell-derived cardiomyocytes were made for 2 LQT2- (G604S, N633S) and 1 SQT1- (N588K) causative variants. All variant lines were treated with KCNH2-SupRep or non-targeting control short hairpin RNA (shCT). The action potential duration (APD) at 90% repolarization (APD₉₀) was measured using FluoVolt voltage dye.

RESULTS

KCNH2-SupRep achieved variant-independent rescue of both pathologic phenotypes. For LQT2-causative variants, treatment with KCNH2-SupRep resulted in shortening of the pathologically prolonged APD₉₀ to near curative (IC-like) APD₉₀ levels (G604S IC, 471±25 ms; N633S IC, 405±55 ms) compared with treatment with shCT (G604S: SupRep-treated, 452±76 ms versus shCT-treated, 550±41 ms; P<0.0001; N633S: SupRep-treated, 399±105 ms versus shCT-treated, 577±39 ms, P<0.0001). Conversely, for the SQT1-causative variant, N588K, treatment with KCNH2-SupRep resulted in therapeutic prolongation of the pathologically shortened APD₉₀ (IC: 429±16 ms; SupRep-treated: 396±61 ms; shCT-treated: 274±12 ms).

CONCLUSIONS

We provide the first proof-of-principle gene therapy for correction of both LQT2 and SQT1. KCNH2-SupRep gene therapy successfully normalized the pathologic APD₉₀, thereby eliminating the pathognomonic feature of both LQT2 and SQT1.

Characterization of N-terminal RYR2 variants outside CPVT1 hotspot regions using patient iPSCs reveal pathogenesis and therapeutic potential

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a cardiac channelopathy causing ventricular tachycardia following adrenergic stimulation. Pathogenic variants in RYR2-encoded ryanodine receptor 2 (RYR2) cause CPVT1 and cluster into domains I–IV, with the most N-terminal domain involving residues 77–466. Patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated for RYR2-F13L, -L14P, -R15P, and -R176Q variants. Isogenic control iPSCs were generated using CRISPR-Cas9/PiggyBac. Fluo-4 Ca²⁺ imaging assessed Ca²⁺ handling with/without isoproterenol (ISO), nadolol (Nad), and flecainide (Flec) treatment. CPVT1 iPSC-CMs displayed increased Ca²⁺ sparking and Ca²⁺ transient amplitude following ISO compared with control. Combined Nad treatment/ISO stimulation reduced Ca²⁺ amplitude and sparking in variant iPSC-CMs. Molecular dynamic simulations visualized the structural role of these variants. We provide the first functional evidence that these most proximal N-terminal localizing variants alter calcium handling similar to CPVT1. These variants are located at the N-terminal domain and the central domain interface and could destabilize the RYR2 channel promoting Ca²⁺ leak-triggered arrhythmias.

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Extreme N-terminal RyR2 variants alter calcium handling similar to classical CPVT1

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Abnormal Ca²⁺ kinetics as well as uncontrolled Ca²⁺ release underlies CPVT1

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In vitro arrhythmia studies with iPSCs show nadolol is an effective treatment

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In silico 3D modeling of RYR2 revealed pathogenicity of N-terminal variants

A phenotype-enhanced variant classification framework to decrease the burden of missense variants of uncertain significance in type 1 long QT syndrome

BACKGROUND

Pathogenic/likely pathogenic (P/LP) variants in the KCNQ1-encoded Kv7.1 potassium channel cause type 1 long QT syndrome (LQT1). Despite the revamped 2015 American College of Medical Genetics (ACMG) variant interpretation guidelines, the burden of KCNQ1 variants of uncertain significance (VUS) in patients with LQTS remains ∼30%.

OBJECTIVE

The purpose of this study was to determine whether a phenotype-enhanced (PE) variant classification approach could reduce the VUS burden in LQTS genetic testing.

METHODS

Retrospective analysis was performed on 79 KCNQ1 missense variants in 356 patients from Mayo Clinic and an independent cohort of 42 variants in 225 patients from Amsterdam University Medical Center (UMC). Each variant was classified initially using the ACMG guidelines and then readjudicated using a PE-ACMG framework that incorporated the LQTS clinical diagnostic Schwartz score plus 4 "LQT1-defining features": broad-based/slow upstroke T waves, syncope/seizure during exertion, swimming-associated events, and a maladaptive LQT1 treadmill stress test.

RESULTS

According to the ACMG guidelines, Mayo Clinic variants were classified as follows: 17 of 79 P variants (22%), 34 of 79 LP variants (43%), and 28 of 79 VUS (35%). Similarly, for Amsterdam UMC, the variant distribution was 9 of 42 P variants (22%), 14 of 42 LP variants (33%), and 19 of 42 variants VUS (45%). After PE-ACMG readjudication, the total VUS burden decreased significantly from 28 (35%) to 13 (16%) (P = .0007) for Mayo Clinic and from 19 (45%) to 12 (29%) (P = .02) for Amsterdam UMC.

CONCLUSION

Phenotype-guided variant adjudication decreased significantly the VUS burden of LQT1 case-derived KCNQ1 missense variants in 2 independent cohorts. This study demonstrates the value of incorporating LQT1-specific phenotype/clinical data to aid in the interpretation of KCNQ1 missense variants identified during genetic testing for LQTS.

Perceptions of Barriers and Facilitators to Becoming a Medical Professional Among Underrepresented Undergraduate and Postbaccalaureate Learners

Objective

To assess underrepresented undergraduate and postbaccalaureate learners’ perceptions of (1) the medical field, (2) barriers that might prevent individuals from pursuing professional medical careers, and (3) resources that assist in overcoming these barriers.

Participants and Methods

A qualitative study with focus groups was designed to achieve the objective. Participants were recruited from a community initiative to provide early exploration of the medical field to disadvantaged and minority individuals. Thirty-five individuals voluntarily participated in semistructured interviews. Audio from the interviews was analyzed using a qualitative descriptive approach and thematic analysis. This study was conducted from October 20, 2018, to April 6, 2019.

Results

Participants identified multiple characteristics related to the health care work environment and desirable attributes of health care personnel. The following barriers were identified: financial burden, lacking knowledge of the path to becoming a medical professional, inadequate social support, and lacking the metrics of a competitive candidate. Resources identified by participants to overcome barriers included professional networks and programmatic considerations.

Conclusion

The study participants discussed negative and positive aspects of the health care environment, such as implicit and explicit biases and attributes that promote or sustain success. Participants expounded on financial, academic, social, and personal factors as barriers to success. In regard to resources that were believed to be helpful to mitigate barriers and promote success, participants commented on activities that simulate a professional medical environment, include networking with medical personnel, support well-being, and provide exposure to structured information on the process of obtaining professional medical training.

Suppression-Replacement KCNQ1 Gene Therapy for Type 1 Long QT Syndrome

BACKGROUND

Type 1 long QT syndrome (LQT1) is caused by loss-of-function variants in the KCNQ1-encoded K_v7.1 potassium channel α-subunit that is essential for cardiac repolarization, providing the slow delayed rectifier current. No current therapies target the molecular cause of LQT1.

METHODS

A dual-component suppression-and-replacement (SupRep) KCNQ1 gene therapy was created by cloning a KCNQ1 short hairpin RNA and a short hairpin RNA-immune KCNQ1 cDNA modified with synonymous variants in the short hairpin RNA target site, into a single construct. The ability of KCNQ1-SupRep gene therapy to suppress and replace LQT1-causative variants in KCNQ1 was evaluated by means of heterologous expression in TSA201 cells. For a human in vitro cardiac model, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from 4 patients with LQT1 (KCNQ1-Y171X, -V254M, -I567S, and -A344A/spl) and an unrelated healthy control. CRISPR-Cas9 corrected isogenic control iPSC-CMs were made for 2 LQT1 lines (correction of KCNQ1-V254M and KCNQ1-A344A/spl). FluoVolt voltage dye was used to measure the cardiac action potential duration (APD) in iPSC-CMs treated with KCNQ1-SupRep.

RESULTS

In TSA201 cells, KCNQ1-SupRep achieved mutation-independent suppression of wild-type KCNQ1 and 3 LQT1-causative variants (KCNQ1-Y171X, -V254M, and -I567S) with simultaneous replacement of short hairpin RNA-immune KCNQ1 as measured by allele-specific quantitative reverse transcription polymerase chain reaction and Western blot. Using FluoVolt voltage dye to measure the cardiac APD in the 4 LQT1 patient-derived iPSC-CMs, treatment with KCNQ1-SupRep resulted in shortening of the pathologically prolonged APD at both 90% and 50% repolarization, resulting in APD values similar to those of the 2 isogenic controls.

CONCLUSIONS

This study provides the first proof-of-principle gene therapy for complete correction of long QT syndrome. As a dual-component gene therapy vector, KCNQ1-SupRep successfully suppressed and replaced KCNQ1 to normal wild-type levels. In TSA201 cells, cotransfection of LQT1-causative variants and KCNQ1-SupRep caused mutation-independent suppression and replacement of KCNQ1. In LQT1 iPSC-CMs, KCNQ1-SupRep gene therapy shortened the APD, thereby eliminating the pathognomonic feature of LQT1.

Corrected QT Interval–Polygenic Risk Score and Its Contribution to Type 1, Type 2, and Type 3 Long-QT Syndrome in Probands and Genotype-Positive Family Members

Background:

Long-QT syndrome (LQTS) is characterized by a prolonged heart rate–corrected QT interval (QTc). Genome-wide association studies identified common genetic variants that collectively explain ≈8% to 10% of QTc variation in the general population.

Methods:

Overall, 423 patients with LQT1, LQT2, or LQT3 were genotyped for 61 QTc-associated genetic variants used in a prototype QTc–polygenic risk score (QTc-PRS). A weighted QTc-PRS (range, 0–154.8 ms) was calculated for each patient, and the FHS (Framingham Heart Study) population-based reference cohort (n=853).

Results:

The average QTc-PRS in LQTS was 88.0±7.2 and explained only ≈2.0% of the QTc variability. The QTc-PRS in LQTS probands (n=137; 89.3±6.8) was significantly greater than both FHS controls (87.2±7.4, difference-in-means±SE: 2.1±0.7, P <0.002) and LQTS genotype-positive family members (87.5±7.4, difference-in-mean, 1.8±.7, P <0.009). There was no difference in QTc-PRS between symptomatic (n=156, 88.6±7.3) and asymptomatic patients (n=267; 87.7±7.2, difference-in-mean, 0.9±0.7, P=0.15). LQTS patients with a QTc≥480 ms (n=120) had a significantly higher QTc-PRS (89.3±6.7) than patients with a QTc<480 ms (n=303, 87.6±7.4, difference-in-mean, 1.7±0.8, P <0.05). There was no difference in QTc-PRS or QTc between genotypes.

Conclusions:

The QTc-PRS explained <2% of the QTc variability in our LQT1, LQT2, and LQT3 cohort, contributing 5× less to their QTc value than in the general population. This prototype QTc-PRS does not distinguish/predict the clinical outcomes of individuals with LQTS.

Triadin Knockout Syndrome Is Absent in a Multi-Center Molecular Autopsy Cohort of Sudden Infant Death Syndrome and Sudden Unexplained Death in the Young and Is Extremely Rare in the General Population

BACKGROUND

Triadin knockout syndrome (TKOS) is a potentially lethal arrhythmia disorder caused by recessively inherited null variants in TRDN-encoded cardiac triadin. Despite its malignant phenotype, the prevalence of TKOS in sudden infant death syndrome and sudden unexplained death in the young is unknown.

METHODS

Exome sequencing was performed on 599 sudden infant death syndrome and 258 sudden unexplained death in the young cases. Allele frequencies of all TRDN null variants identified in the cardiac-specific isoform of TRDN in the Genome Aggregation Database were used to determine the estimated prevalence and ethnic distribution of TKOS.

RESULTS

No triadin null individuals were identified in 599 sudden infant death syndrome and 258 sudden unexplained death in the young exomes. Using the Genome Aggregation Database, we estimate the overall prevalence of TKOS to be ≈1:22.7 million individuals. However, TKOS prevalence is 5.5-fold higher in those of African descent (≈1:4.1 million).

CONCLUSIONS

TKOS is an exceedingly rare clinical entity that does not contribute meaningfully to either sudden infant death syndrome or sudden unexplained death in the young. However, despite its rarity and absence in large sudden death cohorts, TKOS remains a malignant and potentially lethal disorder which requires further research to better care for these patients.

MRAS Variants Cause Cardiomyocyte Hypertrophy in Patient-Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes: Additional Evidence for MRAS as a Definitive Noonan Syndrome-Susceptibility Gene

BACKGROUND

MRAS was identified recently as a novel Noonan syndrome (NS)-susceptibility gene. Phenotypically, both patients with NS, harboring pathogenic MRAS variants, displayed severe cardiac hypertrophy. This study aimed to demonstrate both the necessity and sufficiency of a patient-specific variant (p.Gly23Val-MRAS) to cause NS through the generation and characterization of patient-specific, isogenic control, and disease modeled induced pluripotent stem cell (iPSC) lines.

METHODS

iPSCs were derived from a patient with a p.Gly23Val-MRAS variant to assess the effect of MRAS variants on pathogenesis of NS-associated cardiac hypertrophy. CRISPR/Cas9 gene editing was used to correct the pathogenic p.Gly23Val-MRAS variant in patient cells (isogenic control) and to introduce the pathogenic variant into unrelated control cells (disease modeled) to determine the necessity and sufficiency of the p.Gly23Val-MRAS variant to elicit the disease phenotype in iPSC-derived cardiomyocytes (iPSC-CMs). iPSC-CMs were analyzed by microscopy and immunofluroesence, single-cell RNAseq, Western blot, room temperature-quantitative polymerase chain reaction, and live-cell calcium imaging to define an in vitro phenotype of MRAS-mediated cardiac hypertrophy.

RESULTS

Compared with controls, both patient and disease modeled iPSC-CMs were significantly larger and demonstrated changes in gene expression and intracellular pathway signaling characteristic of cardiac hypertrophy. Additionally, patient and disease modeled iPSC-CMs displayed impaired Ca²⁺ handling, including increased frequency of irregular Ca²⁺ transients and changes in Ca²⁺ handling kinetics.

CONCLUSIONS

p.Gly23Val-MRAS is both necessary and sufficient to elicit a cardiac hypertrophy phenotype in iPSC-CMs that includes increased cell size, changes in cardiac gene expression, and abnormal calcium handling-providing further evidence to establish the monogenetic pathogenicity of p.Gly23Val-MRAS in NS with cardiac hypertrophy.

Characterization of the CACNA1C-R518C Missense Mutation in the Pathobiology of Long-QT Syndrome Using Human Induced Pluripotent Stem Cell Cardiomyocytes Shows Action Potential Prolongation and L-Type Calcium Channel Perturbation

BACKGROUND

The CACNA1C-encoded cardiac L-type calcium channel (Cav1.2) is essential for cardiocyte action potential duration (APD). We previously reported the CACNA1C-p.R518C variant associated with prolonged QT intervals, cardiomyopathy, and sudden cardiac death in several pedigrees.

METHODS

To characterize a patient-derived human induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) CACNA1C-p.R518C model, CACNA1C-p.R518C hiPSC-CMs were generated from a 13-year-old man (QTc, >480 ms) with a family history of sudden cardiac death. An isogenic hiPSC-CM gene-corrected control was created using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9). APD and calcium handling were assessed by live cell imaging with Arclight voltage and Fluo-4 calcium indicators, respectively. The APD and L-type calcium channel biophysical properties were further assessed by whole-cell patch clamp technique.

RESULTS

The Bazett formula-corrected, Arclight-measured APD₉₀ of CACNA1C-p.R518C hiPSC-CMs was significantly longer (622±11 ms; n=92) than the isogenic control hiPSC-CMs (453±5 ms; n=62; P<0.0001). Patch clamp assessment of CACNA1C-p.R518C hiPSC-CMs paced at 1 Hz confirmed a prolonged APD₉₀ (689±29 ms; n=10) compared with the patient's isogenic control hiPSC-CMs (434±30 ms; n=8; P<0.05). Fluo-4-measured calcium transient decay time suggested calcium mishandling in CACNA1C-p.R518C. Patch clamp analysis revealed increased L-type calcium channel window current, slow decay time at various voltages, and increased late calcium current for CACNA1C-p.R518C hiPSC-CMs when compared with isogenic control hiPSC-CMs.

CONCLUSIONS

Using patient-specific hiPSC-CM mutant and isogenic control lines, we demonstrate that the CACNA1C-p.R518C variant is the self-sufficient, monogenetic substrate for the patient's long-QT syndrome phenotype. These data further bolster the conclusion that CACNA1C is a bona fide, definite evidence long-QT syndrome susceptibility gene.