**2.3 LQT type 2 (LQT2)**

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*) encoding K+ 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

asymptomatic carrier [50]. These results are in parallel with clinical findings that LQT2 patients usually display symptoms when heart rate is slow. In addition, this report illustrated that electrophysiological abnormalities can be detected in hiPSC-CMs, although iPSCs are derived from asymptomatic carriers of *KCNH2* mutations. The application of IKr blockers (E4031 and sotalol) further prolonged the APD resulting in EADs, whereas Ca2+ channel blocker (nifedipine), IK,ATP channel opener (pinacidil and nicorandil), and IKr channel enhancer (PD-118057) reduced the APD and thus mitigated the formation of EAD in LQT2 hiPSC-CMs [48, 49]. Several novel pharmacological strategies including ICA-105574 (potent IKr activator) [51], chaperone modulator N-[N-(N-acetyl-L-leucyl)-L-leucyl]-L-norleucine (ALLN) [52], LUF7346 (hERG allosteric modulators) [53], as well as application of allele-specific RNA interference approach [54] have been attempts to rescue the LQT phenotype in LQT2 hiPSC-CMs. Correcting the mutation associated with LQT2 not only confirmed that mutation caused IKr reduction and APD prolongation but also suggested that trafficking defect as the pathological mechanism is responsible for the electrophysiological phenotype in LQT2 [51, 55].

## **2.4 LQT type 3 (LQT3)**

LQT type 3 (LQT3) is caused by gain-of-function mutations in SCN5A encoding α subunit of cardiac Na+ channels [56]. The gain-of-function SCN5A mutation results in augmented late or persistent Na+ current (INaL), which leads to prolongation of QT interval in ECG and proarrhythmia. LQT3 is the third most common LQTS accounting for 5–10% of all LQTS cases [56]. LQT3 patients exhibit longer QT duration at slower heart rate, thus LQT3 patients are at higher risk for cardiac events during rest or sleep [57]. LQT3 patients harboring V1763 M mutation in *SCN5A* [58] R1644H mutation in *SCN5A* [59] or F1473C mutation in *SCN5A* and a polymorphism (K897 T) in *KCNH2* [60] had prolonged QT interval, and in vitro models using hiPSC-CMs derived from all those LQT3 patients demonstrated prolonged APD resulting in the larger INa,L and altered biophysical properties of Na+ channels [58–60]. Mexiletine, a Na+ channel inhibitor commonly used in LQT3 therapy, lowered the INa,L and thereby rescued the APD prolongation phenotype [58, 59] and suppressed the occurrence of EAD [59] and also corrected the altered Na+ channel inactivation [60]. Incorporating the biophysics of Na+ channel and pharmacological analysis illustrated that the improper functioning of Na+ channel was responsible for LQT3 phenotypes rather than *KCNH2* polymorphism [60]. In addition to LQT3, mutation in *SCN5A* gene can cause BrS, and mixed phenotypes are often seen, which is also known as the "overlap syndrome." Loss in function of Na+ channel is often seen in BrS. Liang and co-workers had generated hiP-SCs from two BrS patients, one with double missense mutation (R620H and R811H) in *SCN5A* gene (BrS(p1)) and another with one-base pair deletion mutation in the *SCN5A* gene (BrS(p2)), and showed that BrS hiPSC-CMs derived from both patients had reduced Na+ current and increased triggered activity and abnormal Ca2+ handling [61]. These phenotypes were alleviated by correcting the mutation by CRISPR/Cas9 in hiPSCs derived from BrS (p2) [61]. Importantly, only BrS hiPSC-CMs harboring BrSassociated SCN5A-1795insD mutation displayed reduced Na+ current and upstroke velocity, but not with three sets of hiPSC-CMs derived from BrS patients who tested negative for mutations in the known BrS-associated genes suggesting the Na+ channel dysfunction may not be prerequisite for BrS [62]. In another study, Na+ current and upstroke velocity were reduced, but not the voltage-dependent inactivation in BrS hiPSC-CMs carrying the mutations R1638X and W156X [63].

#### **2.5 LQT type 7 (LQT7) or Andersen-Tawil syndrome (ATS)**

LQT type 7 (LQT7) or Andersen-Tawil syndrome (ATS) is a rare inherited cardiac disease associated with mutation in *KCNJ2* gene (ATS type 1) encoding inward

**69**

*Modelling of Genetic Cardiac Diseases*

rectifying K+

**2.7 Short QT (SQT)**

mutations in genes associated with K<sup>+</sup>

**3. Cardiomyopathy phenotypes in hiPSC-CMs**

**3.1 Hypertrophic cardiomyopathy (HCM)**

Na+

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

channel (Kir2.1) and accounts for ~70% of all ATS cases. However, the

genetic cause of the remaining 30% of ATS (ATS type 2) remains unknown. In ATS patients, QT interval prolongation is not common, but prominent U wave and QU interval in ECG could be hallmarks of ATS, and they experienced cardiac arrhythmias including non-sustained VT and torsade de pointes [64]. Kuroda and co-workers generated hiPSCs from ATS patients carrying R218W, R67W, and R218Q mutations in *KCNJ2* gene and showed strong arrhythmic events and higher incidence of irregular Ca2+ handling in ATS hiPSC-CMs, but flecainide and KB-R7943 (a reverse-mode

2.6 LQT type 8 (LQT8) or Timothy syndrome (TS) is a very rare genetic cardiac disease which results from mutation in *CACNA1C* gene encoding Ca2+ channel (CaV1.2). LQT8 is the most severe type of LQTS, which is characterized by markedly prolonged QT interval, severe ventricular arrhythmia, and multiorgan dysfunction [66]. hiPSC-CMs derived from TS patients recapitulated the disease phenotypes, but roscovitine rescued those abnormalities such as altered Ca2+ channel inactivation, prolonged APD, higher incidences of arrhythmias, and abnormal Ca2+ handling [67].

SQT is a rare inherited cardiac disease characterized by QT internal shortening, which is in contrast to QT prolongation observed in LQTS. SQT is associated with

lence of SQT is between 0.02–0.1% and 0.05% in adults and children, respectively [69]. Recently El-Battrawy and co-workers had generated hiPSCs from SQT type 1 patients carrying a mutation (N588K) in *KCNH2*, and hiPSC-CMs mimicked the clinical phenotype of SQT by showing a shortened APD as a result of increased IKr current densities [70]. In addition, SQT hiPSC-CMs exhibited abnormal Ca2+ transients and rhythmic activities, which are enhanced by carbachol, but quinidine alleviated those carbachol-induced arrhythmias and prolonged the APD [70].

Cardiomyopathies are diseases of cardiac muscle and associated with structural and/or functional abnormalities. The most common genetic cardiomyopathies are hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D). These genetic cardiomyopathies have been also extensively studied using hiPSC-CMs [71, 72].

HCM is one of the most common genetic cardiac diseases with an estimate prevalence of 1 in 500. HCM is characterized by unexplained symmetrical or asymmetrical left ventricular hypertrophy. Mutations in sarcomeric proteins account for ~60% of all HCM cases including mutation in β-myosin heavy chain (MYH7), cardiac myosinbinding protein C (MYBPC3), cardiac troponin I (cTnI), cardiac troponin T (cTnT), and tropomyosin (TPM1) [73]. Hypertrophy of myocytes and disarray of sarcomere are the histological hallmarks of HCM seen in cardiac biopsies from HCM patients [74], and these histological phenotypes are also observed in hiPSC-CM model of HCM [25, 75–77]. In addition, HCM hiPSC-CMs also demonstrated other hallmarks of HCM such as nuclear translocation of nuclear factor of activated T cells (NFAT) [75–77],

channel or Ca2+ channels [68]. The preva-

/Ca2+ exchanger inhibitor) were able to suppress those events [65].

**2.6 LQT type 8 (LQT8) or Timothy syndrome (TS)**

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

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

responsible for the electrophysiological phenotype in LQT2 [51, 55].

**2.4 LQT type 3 (LQT3)**

α subunit of cardiac Na+

in augmented late or persistent Na+

altered biophysical properties of Na+

also corrected the altered Na+

of Na+

of Na+

function of Na+

had reduced Na+

asymptomatic carrier [50]. These results are in parallel with clinical findings that LQT2 patients usually display symptoms when heart rate is slow. In addition, this report illustrated that electrophysiological abnormalities can be detected in hiPSC-CMs, although iPSCs are derived from asymptomatic carriers of *KCNH2* mutations. The application of IKr blockers (E4031 and sotalol) further prolonged the APD resulting in EADs, whereas Ca2+ channel blocker (nifedipine), IK,ATP channel opener (pinacidil and nicorandil), and IKr channel enhancer (PD-118057) reduced the APD and thus mitigated the formation of EAD in LQT2 hiPSC-CMs [48, 49]. Several novel pharmacological strategies including ICA-105574 (potent IKr activator) [51], chaperone modulator N-[N-(N-acetyl-L-leucyl)-L-leucyl]-L-norleucine (ALLN) [52], LUF7346 (hERG allosteric modulators) [53], as well as application of allele-specific RNA interference approach [54] have been attempts to rescue the LQT phenotype in LQT2 hiPSC-CMs. Correcting the mutation associated with LQT2 not only confirmed that mutation caused IKr reduction and APD prolongation but also suggested that trafficking defect as the pathological mechanism is

LQT type 3 (LQT3) is caused by gain-of-function mutations in SCN5A encoding

interval in ECG and proarrhythmia. LQT3 is the third most common LQTS accounting for 5–10% of all LQTS cases [56]. LQT3 patients exhibit longer QT duration at slower heart rate, thus LQT3 patients are at higher risk for cardiac events during rest or sleep [57]. LQT3 patients harboring V1763 M mutation in *SCN5A* [58] R1644H mutation in *SCN5A* [59] or F1473C mutation in *SCN5A* and a polymorphism (K897 T) in *KCNH2* [60] had prolonged QT interval, and in vitro models using hiPSC-CMs derived from all those LQT3 patients demonstrated prolonged APD resulting in the larger INa,L and

inhibitor commonly used in LQT3 therapy, lowered the INa,L and thereby rescued the APD prolongation phenotype [58, 59] and suppressed the occurrence of EAD [59] and

SCs from two BrS patients, one with double missense mutation (R620H and R811H) in *SCN5A* gene (BrS(p1)) and another with one-base pair deletion mutation in the *SCN5A* gene (BrS(p2)), and showed that BrS hiPSC-CMs derived from both patients

[61]. These phenotypes were alleviated by correcting the mutation by CRISPR/Cas9 in hiPSCs derived from BrS (p2) [61]. Importantly, only BrS hiPSC-CMs harboring BrS-

velocity, but not with three sets of hiPSC-CMs derived from BrS patients who tested negative for mutations in the known BrS-associated genes suggesting the Na+

upstroke velocity were reduced, but not the voltage-dependent inactivation in BrS

disease associated with mutation in *KCNJ2* gene (ATS type 1) encoding inward

LQT type 7 (LQT7) or Andersen-Tawil syndrome (ATS) is a rare inherited cardiac

associated SCN5A-1795insD mutation displayed reduced Na+

hiPSC-CMs carrying the mutations R1638X and W156X [63].

**2.5 LQT type 7 (LQT7) or Andersen-Tawil syndrome (ATS)**

dysfunction may not be prerequisite for BrS [62]. In another study, Na+

channel and pharmacological analysis illustrated that the improper functioning

channel is often seen in BrS. Liang and co-workers had generated hiP-

current and increased triggered activity and abnormal Ca2+ handling

 channel was responsible for LQT3 phenotypes rather than *KCNH2* polymorphism [60]. In addition to LQT3, mutation in *SCN5A* gene can cause BrS, and mixed phenotypes are often seen, which is also known as the "overlap syndrome." Loss in

channels [56]. The gain-of-function SCN5A mutation results

channels [58–60]. Mexiletine, a Na+

channel inactivation [60]. Incorporating the biophysics

current (INaL), which leads to prolongation of QT

channel

current and upstroke

channel

current and

**68**

rectifying K+ channel (Kir2.1) and accounts for ~70% of all ATS cases. However, the genetic cause of the remaining 30% of ATS (ATS type 2) remains unknown. In ATS patients, QT interval prolongation is not common, but prominent U wave and QU interval in ECG could be hallmarks of ATS, and they experienced cardiac arrhythmias including non-sustained VT and torsade de pointes [64]. Kuroda and co-workers generated hiPSCs from ATS patients carrying R218W, R67W, and R218Q mutations in *KCNJ2* gene and showed strong arrhythmic events and higher incidence of irregular Ca2+ handling in ATS hiPSC-CMs, but flecainide and KB-R7943 (a reverse-mode Na+ /Ca2+ exchanger inhibitor) were able to suppress those events [65].
