**1. The importance of hiPSC-CMs**

Cardiovascular diseases (CVDs) are the major causes of premature death and chronic disability worldwide [1]. Among CVD-related deaths, the occurrence of inherited lethal arrhythmias is the main reason for sudden cardiac death (SCD) in cardiac patients especially at young age [2]. Although many risk factors associated with SCD have been identified and understanding of pathogenesis of many cardiac diseases is progressing, the considerable number of cardiac patients still suffers SCD without warning, and we are still far from disease-specific treatment. Heterogeneous and multifactorial natures of genetic cardiac diseases are reasons for these complications. Furthermore, founder mutations causing cardiac disease have been reported in Finland [3], the Netherlands [4], and South Africa [5]. Not only disease phenotypes vary among different mutations, but also these vary among individuals carrying the same mutation. For example, long QT syndrome (LQTS) patients demonstrate a wide range of clinical phenotypes even among family members with the identical mutation [6]. Despite carrying the same gene variant resulting in cardiac disease, patients

often demonstrate the wide spectrum of clinical outcomes ranging from the absence of distinct electrocardiogram (ECG) abnormalities and being lifelong asymptomatic to clear abnormalities in ECG (e.g., prolonged QT interval and arrhythmias) and premature SCD. In addition, SCD could also be the first manifestation of cardiac disease. These suggest that the type of genetic mutation cannot always be the sole factor that dictates the prognosis of disease and clinical phenotype in all individuals who carry it [7]. Thus, genetic cardiac diseases exhibit the incomplete penetrance and differ among genetic cardiac diseases. For example, Brugada syndrome (BrS) has a penetration range from 12.5 to 50%; mean penetrance of LQTS is ~40%, while overall penetrance of catecholaminergic polymorphic ventricular tachycardia (CPVT) is 78% [7]. Another convoluting factor that hinders the genotype-phenotype correlation is variable expressivity within one phenotype because some mutation carriers display all the phenotypic symptoms, whereas some only display part of mutation-specific phenotypes [8]. The clinical heterogeneity of genetic cardiac diseases suggests that ultimate disease severity (i.e., penetrance and expressivity) does not solely depend on one particular gene causing cardiac disease, but instead results from the combination of many modifying factors such as age, gender, and environmental and lifestyle factors, which either exacerbate or protect against disease [9]. In addition, patients carrying more than one disease-causing mutations (i.e., not polymorphisms) either in the same gene or different genes yield to more severe clinical disease including earlier onset of disease, early heart failure, and premature SCD [10]. Besides these, some of the cardiac diseases overlap their phenotypes with other cardiac diseases (**Figure 1**). For example, mutations in cardiac sodium (Na+ ) channel gene, SCN5A, are associated with type 3 long QT (LQT3), BrS, cardiac conduction diseases, and sinus node dysfunction [11]. These incomplete penetrance, variable expressivity, and phenotypic overlap impede the complete understanding of diseases' mechanism as well as disease-specific treatment. Furthermore, the treatment therapies are mainly targeted for symptomatic patients to prevent and counteract the symptoms, but treatments in asymptomatic individuals are still of concern with variable opinions. Nevertheless, pharmacological therapies have been resulted in poor outcomes in the

#### **Figure 1.**

*Heterogeneity of genetic cardiac diseases. (A) Overlapping genes causing channelopathies [27]. Brugada syndrome (BrS), long QT syndrome (LQTS), short QT syndrome (SQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT) (ref). (B) Overlapping genes causing cardiomyopathies [72]. Arrhythmogenic right ventricular cardiomyopathy (ARVC), dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM), left ventricular non-compaction cardiomyopathy (LVNC).*

**65**

**Figure 2.**

*were able to mitigate these abnormalities in hiPSC-CMs.*

*Modelling of Genetic Cardiac Diseases*

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

cardiac diseases [12]. So far, implantable cardioverter-defibrillator (ICD) is the only proven therapy for preventing detrimental consequences in cardiac patients with high risk of SCD [13]. However, ICD implantation is associated with its own complications and lower quality of life [14]. There are large groups of asymptomatic cardiac patients who do not have risk factors, which shift them into high-risk category as candidate for ICD implantation, but suffer SCD. Thus, the management for asymptomatic patients carrying pathogenic variant is the most challenging since SCD could be the first manifestation of disease [15, 16]. The clinical management of most cardiac diseases is suboptimal due to lack of comprehensive knowledge of mutations and possible mechanism involved. Thus, the mechanism of how mutation leads to modify the normal cardiac physiology and engender lethal arrhythmias should be deciphered

The prior cardiovascular research and drug screening have mostly been performed in animal models through knock-in/knock-out approaches. Although animal models have provided some fundamental information and led to many discoveries in genetic cardiac disease, physiological and pharmacological results cannot directly extrapolate from animals to humans because of some fundamental differences that exist between animal and human cardiac physiology [17]. For example, the resting heart rate of human is 75 bpm, while that of rat is 300 bpm, and the animal (mice and rats) can tolerate 6–400-fold higher concentration of some drugs compared to human [18]. The animal models become even worse when studying human cardiomyopathies due to mutations in contractile proteins, which are not highly expressed in mouse or rat. Therefore, it is more complicated to extrapolate physiological and pharmacological results from animal to human [17, 18]. Furthermore, most of cardiovascular drug screening and toxicology studies were performed in non-cardiac cell lines or animals, which do not accurately represent human CMs. Thus, considerable amount of cardiovascular drugs were withdrawn from market due to off-target effects [19]. Therefore, human tissues are required to study the human cardiac diseases and drug

*hiPSC-CM-based modeling of human genetic cardiac diseases. Human-induced pluripotent stem cells (hiPSCs) can be differentiated into hiPSC-derived cardiomyocytes (hiPSC-CMs). There are at least three subtypes of hiPSC-CMs, namely, ventricular-like, atrial-like, and nodal-like hiPSC-CMs. hiPSC-CMs derived from cardiac patients carrying genetic mutation recapitulate calcium and electrical abnormalities (early afterdepolarization (EAD) and delayed afterdepolarization (DAD)). Newly emerging gene editing techniques* 

so that the promising prevention and treatment could be established.

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

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

(**Figure 1**). For example, mutations in cardiac sodium (Na+

are associated with type 3 long QT (LQT3), BrS, cardiac conduction diseases, and sinus node dysfunction [11]. These incomplete penetrance, variable expressivity, and phenotypic overlap impede the complete understanding of diseases' mechanism as well as disease-specific treatment. Furthermore, the treatment therapies are mainly targeted for symptomatic patients to prevent and counteract the symptoms, but treatments in asymptomatic individuals are still of concern with variable opinions. Nevertheless, pharmacological therapies have been resulted in poor outcomes in the

*Heterogeneity of genetic cardiac diseases. (A) Overlapping genes causing channelopathies [27]. Brugada syndrome (BrS), long QT syndrome (LQTS), short QT syndrome (SQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT) (ref). (B) Overlapping genes causing cardiomyopathies [72]. Arrhythmogenic right ventricular cardiomyopathy (ARVC), dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM), left ventricular non-compaction cardiomyopathy (LVNC).*

often demonstrate the wide spectrum of clinical outcomes ranging from the absence of distinct electrocardiogram (ECG) abnormalities and being lifelong asymptomatic to clear abnormalities in ECG (e.g., prolonged QT interval and arrhythmias) and premature SCD. In addition, SCD could also be the first manifestation of cardiac disease. These suggest that the type of genetic mutation cannot always be the sole factor that dictates the prognosis of disease and clinical phenotype in all individuals who carry it [7]. Thus, genetic cardiac diseases exhibit the incomplete penetrance and differ among genetic cardiac diseases. For example, Brugada syndrome (BrS) has a penetration range from 12.5 to 50%; mean penetrance of LQTS is ~40%, while overall penetrance of catecholaminergic polymorphic ventricular tachycardia (CPVT) is 78% [7]. Another convoluting factor that hinders the genotype-phenotype correlation is variable expressivity within one phenotype because some mutation carriers display all the phenotypic symptoms, whereas some only display part of mutation-specific phenotypes [8]. The clinical heterogeneity of genetic cardiac diseases suggests that ultimate disease severity (i.e., penetrance and expressivity) does not solely depend on one particular gene causing cardiac disease, but instead results from the combination of many modifying factors such as age, gender, and environmental and lifestyle factors, which either exacerbate or protect against disease [9]. In addition, patients carrying more than one disease-causing mutations (i.e., not polymorphisms) either in the same gene or different genes yield to more severe clinical disease including earlier onset of disease, early heart failure, and premature SCD [10]. Besides these, some of the cardiac diseases overlap their phenotypes with other cardiac diseases

) channel gene, SCN5A,

**64**

**Figure 1.**

cardiac diseases [12]. So far, implantable cardioverter-defibrillator (ICD) is the only proven therapy for preventing detrimental consequences in cardiac patients with high risk of SCD [13]. However, ICD implantation is associated with its own complications and lower quality of life [14]. There are large groups of asymptomatic cardiac patients who do not have risk factors, which shift them into high-risk category as candidate for ICD implantation, but suffer SCD. Thus, the management for asymptomatic patients carrying pathogenic variant is the most challenging since SCD could be the first manifestation of disease [15, 16]. The clinical management of most cardiac diseases is suboptimal due to lack of comprehensive knowledge of mutations and possible mechanism involved. Thus, the mechanism of how mutation leads to modify the normal cardiac physiology and engender lethal arrhythmias should be deciphered so that the promising prevention and treatment could be established.

The prior cardiovascular research and drug screening have mostly been performed in animal models through knock-in/knock-out approaches. Although animal models have provided some fundamental information and led to many discoveries in genetic cardiac disease, physiological and pharmacological results cannot directly extrapolate from animals to humans because of some fundamental differences that exist between animal and human cardiac physiology [17]. For example, the resting heart rate of human is 75 bpm, while that of rat is 300 bpm, and the animal (mice and rats) can tolerate 6–400-fold higher concentration of some drugs compared to human [18]. The animal models become even worse when studying human cardiomyopathies due to mutations in contractile proteins, which are not highly expressed in mouse or rat. Therefore, it is more complicated to extrapolate physiological and pharmacological results from animal to human [17, 18]. Furthermore, most of cardiovascular drug screening and toxicology studies were performed in non-cardiac cell lines or animals, which do not accurately represent human CMs. Thus, considerable amount of cardiovascular drugs were withdrawn from market due to off-target effects [19]. Therefore, human tissues are required to study the human cardiac diseases and drug

#### **Figure 2.**

*hiPSC-CM-based modeling of human genetic cardiac diseases. Human-induced pluripotent stem cells (hiPSCs) can be differentiated into hiPSC-derived cardiomyocytes (hiPSC-CMs). There are at least three subtypes of hiPSC-CMs, namely, ventricular-like, atrial-like, and nodal-like hiPSC-CMs. hiPSC-CMs derived from cardiac patients carrying genetic mutation recapitulate calcium and electrical abnormalities (early afterdepolarization (EAD) and delayed afterdepolarization (DAD)). Newly emerging gene editing techniques were able to mitigate these abnormalities 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 medicine in future.
