**2.1 Catecholaminergic polymorphic ventricular tachycardia (CPVT)**

CPVT is an inherited cardiac disease with the prevalence of about 1:5000/10,000. This disease is characterized by premature ventricular contraction and/or polymorphic ventricular tachycardia (VT) induced by adrenergic stimulation in response to emotional stress or physical exercise in structurally normal heart. Over 150 mutations in ryanodine receptor type 2 (*RYR2* gene) are responsible for ~ 55% of CPVT type 1 cases (CPVT1), and mutation in calsequestrin 2 (*CASQ2* gene) CPVT accounts for 3–5% CPVT type 2 (CPVT2) cases [28, 29]. In addition, mutations in calmodulin (CALM1) genes and in triadin (TRDN) have been reported causing CPVT. *RYR2*, *CASQ2*, CALM1, and TRDN are involved in ECC, and mutation in any of these genes results in elevated intracellular Ca2+, which leads to abnormal Ca2+ handling and arrhythmias [28, 29]. In consistency with clinical phenotype, many hiPSC-CM model had demonstrated the exacerbation of electrophysiological and Ca2+ handling abnormalities upon adrenergic stimulation [26, 30–32]. Furthermore, Zhang and colleagues had modeled hiPSC-CMs harboring CPVT1-associated F2483I mutation in *RYR2* gene and demonstrated that CPVT1 hiPSC-CMs had longer and wandering Ca2+ sparks and smaller sarcoplasmic reticulum Ca2+ content [32]. Later on, the same group corrected this mutation using clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) gene editing technique and showed that this mutation is causative rather than associative to the disease [33]. hiPSC-CM model for CPVT has also been used in studying the efficacy of various drugs. Previously we had directly compared the clinical results from CPVT1 patients with dantrolene medication, and the clinical response of dantrolene was similar as in hiPSC-CMs from the same patients; dantrolene

**67**

lular K+

encoding K+

**2.3 LQT type 2 (LQT2)**

*Modelling of Genetic Cardiac Diseases*

**2.2 LQT type 1 (LQT1)**

ing α subunit of potassium (K+

*DOI: http://dx.doi.org/10.5772/intechopen.84965*

abolished or markedly reduced arrhythmias in patients and their hiPSC-CMs with certain mutation in *RYR2*, while it did not have any clinical effect with hiPSC-CMs or with other *RYR2* mutations [31]. Furthermore, an antiarrhythmic drug, flecainide, used to treat CPVT1 patients [34] was able to reduce the Ca2+ irregularities under adrenergic stimulation in CPVT1 hiPSC-CMs [30, 35]. CPVT2 patients harboring homozygous CASQ2-G112 + 5X mutation in *CASQ2* gene showed the rapid polymorphic VT under exercise stress test [36]. Adult rat ventricular myocytes were studied to understand the effect of *CASQ2* mutation in ECC, demonstrating that mutated CMs exhibited spontaneous extrasystolic Ca2+ elevations and delayed afterdepolarization (DADs) upon adrenergic stimulation [36]. Later, hiPSC-CM model harboring CASQ2-G112 + 5X mutation emulated these phenotypic features of disease, and AAV9-based gene delivery effectively prevents the development of adrenergic-induced DADs and triggered arrhythmias in CPVT2 hiPSC-CMs [37].

LQT type 1 (LQT1) is caused by loss-of-function mutation in *KCNQ1* gene encod-

(IKs). LQT1 is responsible for 30–35% of all LQTS cases [38]. LQT1 is characterized by prolongation of QT interval in ECG, which could lead to SCD due to VT, typically torsades de pointes [39]. hiPSC-CMs derived from LQT1 patients faithfully recapitulated the clinical hallmark by showing prolonged action potential duration (APD) which is analogous to QT duration in ECG, and reduced IKs current densities are held responsible for abnormal repolarization [40–42]. ML277, an IKs activator, increased the IKs amplitude by enhancing the activation of IKs, thus resulting in shortening of APD in LQT1 hiPSC-CMs [40]. In addition, adrenergic stimulation in LQT1 hiPSC-CMs induced the early afterdepolarization (EAD) [42], which is similar to arrhythmias triggered in LQT1 patients by exercise or emotional stress [39]. Clinically, β-blockers were effective in minimizing the risk of cardiac events in LQT1 patients [43]. Similar antiarrhythmic effect of β-blockers has been observed in LQT1 hiPSC-CMs [42]. Furthermore, hypokalemia is the electrolyte disturbance caused by lower K+

blood serum, which aggravates the QT prolongation and facilitates the development of hypokalemia-induced torsades de pointes in LQT1 patients [39, 44]. We successfully developed and mimicked these disease phenotypes in LQT1 hiPSC-CMs carrying G589D or IVS7-2A > G mutation in *KCNQ1* gene. Additionally, lowering the extracel-

relaxation [46] observed in our video image-based software analysis [47].

LQT type 2 (LQT2) is an LQTS subtype, which is caused by loss-of-function mutations in *KCNH2* gene also known as human ether-a-go-go-related gene (*hERG*)

 channel mediating rapid delayed rectifier K current (IKr). LQT2 is responsible for approximately 25–30% of all LQTS cases [38]. Similar to LQT1, LQT2 patients also exhibit the prolongation of QT interval and torsades de pointes. As in LQT1 hiPSC-CM model, LQT2 hiPSC-CMs also recapitulated clinical phenotypes by displaying longer APD resulted from reduced IKr current densities and enhanced EAD following the adrenergic stimulation [48–50]. Our early study of LQT2 hiPSC-CMs carrying R176W mutation in *KCNH2* gene demonstrated the reduced IKr current densities, prolonged repolarization, and increased arrhythmogenicity although the donor is an

 concentration prolonged APDs and induced the formation of EADs in LQT1 hiPSC-CMs [45]. Both G589D- and IVS7-2A > G-specific LQT1 hiPSC-CMs displayed longer APD and higher Ca2+ abnormalities in baseline; G589D hiPSC-CMs demonstrated prolonged contraction, while IVS7-2A > G hiPSC-CMs showed impaired

) channel mediating slow delayed rectifier K+

current

level in

#### *Modelling of Genetic Cardiac Diseases DOI: http://dx.doi.org/10.5772/intechopen.84965*

*Visions of Cardiomyocyte - Fundamental Concepts of Heart Life and Disease*

**2. Channelopathy phenotypes in hiPSC-CMs**

testing. However, the human sample exhibits some of the major challenges: there is limited supply of human cardiac biopsies, and it involves complex procedures and ethical issues. In addition, these cardiac biopsies are typically obtained from the end stage of cardiac diseases; hence it is not possible to understand the mechanism of cardiac diseases [20, 21]. These obstacles are mostly overcome by the groundbreaking discovery of reprogramming adult somatic cells into induced pluripotent stem cells (iPSCs) [22, 23] which can be differentiated into cardiomyocytes (CMs) (hiPSC-CMs) [24–26]. The main advantages of hiPSC-CMs are iPSCs can be generated at any period of a patient's life, they have unlimited supply, and these retain the same genetic information as the donor, i.e., hiPSC-CMs are patient specific (**Figure 2**). These are superior features of hiPSC-CMs to the conventional in vitro modeling of cardiac diseases. In addition, hiPSC-CMs can be cultured for several months, which enable us to study acute and chronic effect of mutation and drugs on CMs. Thus, hiPSC-CMs not only provide the platform to investigate the mutation-specific mechanism but also assist to anticipate drug response on an individual basis and guide us to personalized

Channelopathy cardiac diseases are caused by mutations in cardiac ion channels located in the cellular membrane or organelles. Mutations in ion channels result in misbalance of fine-tuning ion exchange during excitation-contraction coupling (ECC), which could lead to cardiac arrhythmias and SCD in the worst case. The main cardiac channelopathies are CPVT, LQTS, BrS, and short QT syndromes (SQTS) [27]. These cardiac channelopathies have been extensively studied using

**2.1 Catecholaminergic polymorphic ventricular tachycardia (CPVT)**

CPVT is an inherited cardiac disease with the prevalence of about

1:5000/10,000. This disease is characterized by premature ventricular contraction and/or polymorphic ventricular tachycardia (VT) induced by adrenergic stimulation in response to emotional stress or physical exercise in structurally normal heart. Over 150 mutations in ryanodine receptor type 2 (*RYR2* gene) are responsible for ~ 55% of CPVT type 1 cases (CPVT1), and mutation in calsequestrin 2 (*CASQ2* gene) CPVT accounts for 3–5% CPVT type 2 (CPVT2) cases [28, 29]. In addition, mutations in calmodulin (CALM1) genes and in triadin (TRDN) have been reported causing CPVT. *RYR2*, *CASQ2*, CALM1, and TRDN are involved in ECC, and mutation in any of these genes results in elevated intracellular Ca2+, which leads to abnormal Ca2+ handling and arrhythmias [28, 29]. In consistency with clinical phenotype, many hiPSC-CM model had demonstrated the exacerbation of electrophysiological and Ca2+ handling abnormalities upon adrenergic stimulation [26, 30–32]. Furthermore, Zhang and colleagues had modeled hiPSC-CMs harboring CPVT1-associated F2483I mutation in *RYR2* gene and demonstrated that CPVT1 hiPSC-CMs had longer and wandering Ca2+ sparks and smaller sarcoplasmic reticulum Ca2+ content [32]. Later on, the same group corrected this mutation using clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) gene editing technique and showed that this mutation is causative rather than associative to the disease [33]. hiPSC-CM model for CPVT has also been used in studying the efficacy of various drugs. Previously we had directly compared the clinical results from CPVT1 patients with dantrolene medication, and the clinical response of dantrolene was similar as in hiPSC-CMs from the same patients; dantrolene

**66**

medicine in future.

hiPSC-CMs and described below.

abolished or markedly reduced arrhythmias in patients and their hiPSC-CMs with certain mutation in *RYR2*, while it did not have any clinical effect with hiPSC-CMs or with other *RYR2* mutations [31]. Furthermore, an antiarrhythmic drug, flecainide, used to treat CPVT1 patients [34] was able to reduce the Ca2+ irregularities under adrenergic stimulation in CPVT1 hiPSC-CMs [30, 35]. CPVT2 patients harboring homozygous CASQ2-G112 + 5X mutation in *CASQ2* gene showed the rapid polymorphic VT under exercise stress test [36]. Adult rat ventricular myocytes were studied to understand the effect of *CASQ2* mutation in ECC, demonstrating that mutated CMs exhibited spontaneous extrasystolic Ca2+ elevations and delayed afterdepolarization (DADs) upon adrenergic stimulation [36]. Later, hiPSC-CM model harboring CASQ2-G112 + 5X mutation emulated these phenotypic features of disease, and AAV9-based gene delivery effectively prevents the development of adrenergic-induced DADs and triggered arrhythmias in CPVT2 hiPSC-CMs [37].
