Abnormal calcium handling properties underlie familial hypertrophic cardiomyopathy pathology in patient-specific induced pluripotent stem cells

Abstract

Familial hypertrophic cardiomyopathy (HCM) is a prevalent hereditary cardiac disorder linked to arrhythmia and sudden cardiac death. While the causes of HCM have been identified as genetic mutations in the cardiac sarcomere, the pathways by which sarcomeric mutations engender myocyte hypertrophy and electrophysiological abnormalities are not understood. To elucidate the mechanisms underlying HCM development, we generated patient-specific induced pluripotent stem cell cardiomyocytes (iPSC-CMs) from a ten-member family cohort carrying a hereditary HCM missense mutation (Arg663His) in the MYH7 gene. Diseased iPSC-CMs recapitulated numerous aspects of the HCM phenotype including cellular enlargement and contractile arrhythmia at the single-cell level. Calcium (Ca(2+)) imaging indicated dysregulation of Ca(2+) cycling and elevation in intracellular Ca(2+) ([Ca(2+)](i)) are central mechanisms for disease pathogenesis. Pharmacological restoration of Ca(2+) homeostasis prevented development of hypertrophy and electrophysiological irregularities. We anticipate that these findings will help elucidate the mechanisms underlying HCM development and identify novel therapies for the disease.

Figure 1. Derivation and Characterization of Patient-Specific iPSCs

(A) Representative long-axis MRI images of the proband and a control matched family member at end systole and end diastole demonstrating asymmetric hypertrophy of the inferior wall. See also Figure S1A and Movie S1. (B) Confirmation of the Arg663His missense mutation on exon 18 of the MYH7 gene in HCM patients (II-1, III-1, III-2, III-3, and III-8) by PCR and sequence analysis. (C)Schematic pedigree of the proband carrying the Arg663His mutation in MYH7 recruited for this study (II-1) as well as her husband (II-2) and eight children (III-1 through III-8). Circles represent female family members and squares represent males. Solid symbols indicate clinical presentation of the HCM phenotype, whereas open symbols represent absence of presentation. “+” and “−” signs underneath family members indicate presence and absence of the Arg663His mutation, respectively. Two individuals (III-3 and III-8) were found to carry the Arg663His mutation but had yet to present the HCM phenotype due to young age. See also Table S1. (D) Brightfield and immunofluorescence microscopy photographs of patient-specific dermal fibroblasts, representative iPSC colony derived from patient-specific fibroblasts, alkaline phosphatase (AP) staining of pluripotent colonies, and markers of pluripotency including TRA-1-60, OCT4, TRA-1-81, SOX2, SSEA-4, and NANOG. See also Figures S1B–S1G. (E) Quantitative bisulfite pyrosequencing of the methylation status of Nanog and Oct4 promoter regions in ten patient-specific iPSC lines derived from five family members with HCM (II-1, III-1, III-2, III-3, and III-8) and five control matched family members (II-2, III-4, III-5, III-6, and III-7). Fibroblasts and H7 human ESCs were used as negative and positive controls, respectively. (F) Ratios of quantitative PCR values for total versus endogenous expression of the Yamanaka factors OCT4, SOX2, c-MYC, and KLF4 in patient-specific iPSC lines derived from five HCM (II-1, III-1, III-2, III-3, and III-8) and five control (II-2, III-4, III-5, III-6, and III-7) family members. Human ESC from the H7 line was used as controls. (G) Scatter plots comparing whole genome expression of patient-specific iPSC lines, H7 ESCs, and dermal fibroblasts. Genes with SD greater than 0.2 among all samples (n = 13,044) were selected for Pearson correlation analysis. Error bars represent SEM.

Figure 2. Generation and Characterization of Patient-Specific HCM iPSC-CMs

(A) Representative immunostaining for cardiac troponin T and F-actin demonstrating increased cellular size and multinucleation in HCM iPSC-CMs as compared to control iPSC-CMs. See also Figures S2A–S2D. (B) Quantification of cell size for five control iPSC-CM lines (II-2, III-4, III-5, III-6, and III-7) (n = 55 per patient line) and five HCM iPSC-CM lines (II-1, III-1, III-2, III-3, and III-8) (n = 59 per patient line) 40 days after induction of cardiac differentiation. (C) Quantification of multinucleation in control (n = 55, 5 control subject lines) and HCM (n = 59, 5 patient lines) iPSC-CMs. (D) Representative immunofluorescence staining reveals elevated ANF expression in the MLC2v-positive ventricular HCM iPSC-CMs as compared to controls. (E) Changes in ANF gene expression as measured by single-cell quantitative PCR in control and HCM iPSC-CMs at days 20, 30, and 40 after induction of cardiac differentiation (n = 32 per time point, 5 control subject and 5 patient lines). (F) Quantification of MYH7/MYH6 expression ratio in HCM iPSC-CMs and controls (n = 32 per time point, 5 control subject and 5 patient lines). (G) Representative immunofluorescence staining images revealing nuclear translocation of NFATC4 in HCM iPSC-CMs. (H) Percentage of cardiomyocytes exhibiting positive NFATC4 staining in control (n = 187, 5 control subject lines) and HCM (n = 169, 5 patient lines) iPSC-CMs. (I) Quantification of cell size in control and HCM iPSC-CMs after treatment with calcineurin inhibitors CsA and FK506 for 5 continuous days (n = 50, 5 control subject and 5 patient lines per group). See also Figures S2E–S2I. (J) Heat map representations of gene expression in single control and HCM iPSC-CMs for genes associated with cardiac hypertrophy at days 20, 30, and 40 after induction of cardiac differentiation. See also Figure S3A. ^*p < 0.05 HCM versus control, ^**p < 0.0001 HCM versus control. Error bars represent SEM.

Figure 3. Assessment of Arrhythmia and Irregular Ca²⁺ Regulation in HCM iPSC-CMs

(A) Electrophysiological measurements of spontaneous action potentials in control and HCM iPSC-CMs measured by patch clamp in current-clamp mode. Boxes indicate underlined portions of HCM iPSC-CM waveforms at expanded timescales demonstrating DAD-like arrhythmias. See also Figures S3B–S3H. (B) Quantification of DAD occurrence in control (n = 144, 5 control subject lines) and HCM (n = 131, 5 patient lines) iPSC-CMs. DAD rate is defined as total DADs/total beats. (C) Quantification of percentage of control (n = 144, 5 control subject lines) and HCM (n = 131, 5 patient lines) iPSC-CMs exhibiting putative DADs. (D) Representative line-scan images and spontaneous Ca²⁺ transients in control and HCM iPSC-CMs. Red arrows indicate tachyarrhythmia-like waveforms observed in HCM cells but not control. See also Figures S4A–S4E and Movie S2. (E) Quantification of percentages for control and HCM iPSC-CMs exhibiting irregular Ca²⁺ transients at days 20, 30, and 40 after induction of cardiac differentiation (n = 50, 5 control subject and 5 patient lines per time point). (F) Representative line-scan images and spontaneous Ca²⁺ transients for H9 hESC-CMs and hESC-CMs stably transduced with lentivirus driving expression of wild-type MYH7 or mutant MYH7 carrying the Arg663His mutation. Red arrowheads indicate irregular Ca²⁺ waveforms. (G) Quantification of cells exhibiting irregular Ca²⁺ transients in WA09 hESC-CMs, hESC-CMs overexpressing wild-type MYH7, and hESC-CMs overexpressing MYH7 carrying the Arg663His mutation (n = 40, 5 lines per group). (H) Spontaneous action potentials recorded in current-clamp mode for hESC-CMs, hESC-CMs overexpressing wild-type MYH7, and hESC-CMs overexpressing MYH7 carrying the Arg663His mutation. Red arrowheads indicate DAD-like waveforms. (I) Quantification of cells exhibiting DAD-like waveforms in hESC-CMs, hESC-CMs stably transduced with lentivirus driving expression of wild-type MYH7, or mutant MYH7 carrying the Arg663His mutation (n = 20, 5 lines per group). Error bars represent SEM.

Figure 4. Assessment of Intracellular Ca²⁺ Content in HCM iPSC-CMs

(A) Quantification of baseline Fluo-4 Ca²⁺ dye intensities for control (n = 122,4 control subject lines) and HCM (n = 105,4patient lines) iPSC-CMs. See also Figures S4F–S4N. (B) Representative Ca²⁺ transients of control and HCM iPSC-CMs using the Indo-1 ratiometric Ca²⁺ dye. (C) Quantification of resting Ca²⁺ levels by measurement of Indo-1 ratio in control (n = 21, 5 control subject lines) and HCM (n = 30, 5 patient lines) iPSC-CMs. (D) Representative Ca²⁺ transient traces from control and HCM iPSC-CMs followed by caffeine exposure. (E) Mean peak amplitudes of ΔF/F0 ratios after caffeine administration representing release of SR Ca²⁺ load for control (n = 36, 5 control subject lines) and HCM (n = 44, 5 patient lines) iPSC-CMs. (F) Quantification of irregular Ca²⁺ transients for control iPSC-CMs cultured in normal Ca²⁺ media (1.8 mM; n = 280, 5 control subject lines) and high Ca²⁺ media (3.6 mM; n = 67, 5 control subject lines) for 2 hr. (G) Quantification of DADs for control iPSC-CMs cultured in normal Ca²⁺ media (1.8 mM; n = 144, 5 control subject lines) and high Ca²⁺ media (3.6 mM; n = 32, 5control subject lines) for 2 hr. ^*p < 0.05 HCM versus control, ^**p < 0.01 High Ca²⁺ versus normal Ca²⁺. Error bars represent SEM.

Figure 5. Exacerbation of the HCM Phenotype by Positive Inotropic Stress

(A) Inotropic stimulation of control (n = 50, 5 control subject lines) and HCM (n = 50, 5 patient lines) iPSC-CMs by the β-agonist isoproterenol accelerated presentation of cellular hypertrophy in HCM iPSC-CMs as compared to control counterparts. Coadministration of the β-blocker propranolol prevented catecholamine-induced hypertrophy in HCM iPSC-CMs. (B) Representative traces of MEA recordings for HCM and control multicellular iPSC-CM preparations. Treatment with 200 nM ISO increased both the beating rate and the prevalence of putative DADs. Red arrowheads indicate putative DAD waveforms. See also Table S2 for baseline MEA measurements. (C) Quantification of DAD waveforms in multicellular preparations of HCM iPSC-CMs before and after treatment with isoproterenol. (D) Quantification of putative DAD rate in multicellular preparations of HCM iPSC-CMs before and after treatment with isoproterenol. (E) Representative Ca²⁺ line scans and waveforms in control and HCM iPSC-CMs after positive inotropic stimulation by isoproterenol. Red arrowheads indicate abnormal Ca²⁺ waveforms. (F) Quantification of control (n=50, 5control subject lines) and HCM(n=50, 5patient lines) iPSC-CMs exhibiting irregular Ca²⁺ transients in response to treatment by isoproterenol and coadministration of propranolol. (G) Electrophysiological measurement of spontaneous action potentials and arrhythmia in control and HCM iPSC-CMs at baseline, followed by positive inotropic stimulation by isoproterenol. Red arrows indicate DAD-like waveforms. (H) Quantification of DAD rate in control and HCM iPSC-CMs after isoproterenol administration (total DADs/total beats). ^*p < 0.05 HCM versus control, ^**p < 0.001 HCM versus control, ^##p < 0.01 iso + pro versus iso. Error bars represent SEM.

Figure 6. Treatment of HCM iPSC-CMs by Ca²⁺ Channel Inhibition Significantly Mitigates Development of the HCM Phenotype

(A) Representative immunostaining images of HCM iPSC-CMs treated with 0 nM, 50 nM, and 100 nM of the L-type Ca²⁺ channel blocker verapamil for 5 continuous days beginning 25days after induction of cardiac differentiation. Quantification of relative cell sizes for HCM iPSC-CMs treated with verapamil (n =50, 5 patient lines per treatment group). (B) Representative Ca²⁺ line scan images and waveforms of HCM iPSC-CMs treated with 0 nM, 50 nM, and 100 nM of verapamil for 5 continuous days. Quantification of percentages of HCM iPSC-CMs found to exhibit irregular Ca²⁺ transients after treatment with verapamil (n = 40, 5 patient lines per treatment group). (C) Representative electrophysiological recordings of spontaneous action potentials in HCM iPSC-CMs treated with 0 nM, 50 nM, and 100 nM of verapamil for 5 continuous days. Quantification of DAD frequencies in HCM iPSC-CMs after treatment with verapamil (n = 25, 5 patient lines per treatment group). See also Figures S5A–S5F. (D) Plots of average beat frequencies for control and HCM iPSC-CM EBs against nifedipine concentration fit to the Hill equation. Quantification of control and HCM IC₅₀ values for spontaneous beat frequency against nifedipine (HCM, n = 8, 5 patient lines, control n = 13, 5 control subject lines). (E) Schematic for development of the HCM phenotype as caused by HCM mutations in MYH7. Red boxes indicate potential methods to mitigate development of the disease. See also Figures S6A–S6E, Table S3, and Movie S3. ^*p < 0.01 untreated versus 50 nM verapamil versus 100 nM verapamil. ^**p < 0.001 HCM versus control. Error bars represent SEM.