**1. Introduction**

120 Congenital Heart Disease – Selected Aspects

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Congenital heart disease (CHD) is the most common type of birth defect, affecting 1% of all live births, and is the leading non-infectious cause of death in the first year of life [1,2]. CHD is a multifactorial complex disease, with environmental and genetic factors playing important roles. It has been recognized that environmental factors/insults during fetal development increase the risk of CHD, including viral infections with rubella [3], exposure to chemical teratogens such as retinoic acid , lithium, dilantin [4]and halogenated hydrocarbon [5]and maternal diseases including diabetes and systemic lupus erythematosus [1, 6]. Epidemiologic studies of CHD have demonstrated an increased recurrence risk for cardiac malformations in sequent pregnancies, supporting the existence of gene predispositions.

Great progress in molecular genetics and developmental biology has been made. Current genetic techniques for evaluation of congenital heart defects include cytogenetic techniques, fluorescence in situ hybridization (FISH) and DNA mutation analysis. Most methods employ polymerase chain reaction–based assays. Indirect screening methods, such as denaturing high-performance liquid chromatography or single-strand conformation polymorphism have been used extensively. More expensive exon-by-exon sequencing of genomic DNA has recently emerged [7, 8]. It has been accepted that the intricate process of cardiac morphogenesis is controlled by a network of highly conserved genetic and molecular pathways. The origins of CHD are diverse, such as abnormal chromosome structure (eg. duplication or deletion), gene mutations, single nucleotide polymorphisms, abnormal RNA, epigenetics and so on, and they are summarized in Figure 1.

In humans, heart development begins at 15 to 16 days of gestation with the migration of precardiac stem cells, in five steps:(1)migration of precardiac cells from the primitive streak and assembly of the paired cardiac crescents at the myocardial plate, (2) coalescence of the cardiac crescents to form the primitive heart tube, establishing the definitive heart, (3) cardiac looping, assurance of proper alignment of the future cardiac chambers, (4) septation and heart chambers formation, and (5) development of the cardiac conduction system and coronary vasculature [9-11]. The establishment of left-right asymmetry is very important to the normal development of heart [12, 13]. Secreted FGF, BMP, Nodal, and Wnt act as input signal of symmetric cardiac morphogenesis, BMP2, FGF8, Shh/Ihh, and Nodal function as positive regulators, whereas Wnt and Ser are negative regulators [14-16]. The cardiogenic

Molecular Mechanisms of Congenital Heart Disease 123

Disease genes of CHD identified to date are summarized in Table 1, and the functions of

disorder Causative genes Chromosome Location

D-TGA, DORV *CFC1* 2q21 D-TGA *PROSIT240* 12q24

ASD/VSD *GATA4* 8p23

Holt-Oram Syndromes *TBX5* 12q24 Alagille Syndromes *JAG1* 20p12 Char Syndromes (PDA) *TFAP2B* 6p12

CHARGE Syndromes *CHD7* 8q12 Ellis-van Creveld *EVC, EVC2* 4p16 Marfan Syndromes *FBN1* 15q21.1 Marfan-like Syndromes *TGFBR2* 3p22

*NKX2.5 JAG1* 

*CFC1 ACVR2B LEFTYA* 

*KRAS SOS1* 

*KRAS BRAF MEK1 MEK2* 

For example, NKX2-5, Homeobox-containing genes play critical roles in regulating tissuespecific gene expression essential for tissue differentiation, as well as determining the temporal and spatial patterns of development. It has been demonstrated that a Drosophila

Costello Syndromes *HRAS* 11p15.5

*NKX2.5* 5q34-q35

*CRELD1* 3p21

*ELN* 7q11

8q23 5q34-q35 20p12

Xq26 2q21

12q24 2p1.21 2p21

12p12.1 7q34 15q21 7q32

3p21.3-p221q42.1

these causative genes are summarized as following [25].

Tetralogy of Fallot *ZFPM2* 

Heterotaxy *ZIC3* 

Noonan Syndromes *PTPN11* 

Congenital heart defects Familial congenital heart

atrioventricular block)

Atrioventricular septal

Supravalvar aortic stenosis Syndromes

Cardiofaciocutaneous

Syndromes

Table 1. Causative genes of CHD

diseas(ASD,

defect

plate-specific expressed genes NKX2.5, SRF, GATA4, TBX5, and HAND2, compose the core regulatory network of cardiac morphogenesis, controlling heart looping, left-right symmetry and chambers formation. SRF regulates the differentiation of coronary vascular smooth muscle cells [17, 18]. Genes that involved in epicardial development include FOG-2, vascular cell adhesion molecule 1, integrins, erythropoietin, and erythropoietin receptor. Specific genes such as the NOTCH receptor, Jagged (JAG), WNT, transforming growth factor beta 2 (TGF ß2) and bone morphogenic proteins have been implicated in cardiac neural crest development in the mouse[12, 19-21]. Retinoic acid signal pathway is involved in the regulation of cardiac looping. Complex signal pathways are implicated in the crosstalk between endocardium and myocardium to form endocardial cushion and heart valves, including VEGF, NFATc1, Notch, Wnt/ß-catenin, BMP/TGF-ß, EGF, erbB, NF1 signal pathways [10, 22-24].

Fig. 1. Eteology of CHD
