**3.1.2 Single gene disorder**

Heart development is controlled by multiple genes regulating a complicated network of transcription regulation, translation regulation, and signal transduction pathways, ranging from a control of muscle growth, patterning to contractility, to name a few. However, mutations in only one or a few components of the cardiac gene network can result in the improper development of the heart. One type of heart defect could also be caused by different types of single gene disorders. Since the 1990s, researchers have identified more than 10 different single gene mutations that can lead to heart defects. To date, many genes responsible for several congenital heart defects have been identified (table 5).

**Transcription Factor Genes** transcribe and translate proteins that serve to interact cooperatively with each other to control gene expression.

 **NKX2-5, the NK family, on chromosome 5q35;** Homeobox-containing genes play critical roles in regulating tissue-specific gene expression essential for specification of heart muscle progenitors (Komuro & Izumo, 1993; Toko et al., 2002). Mutations in NKX2-5 result in loss of heart formation in the embryo and have been found in sporadic

Moreover, CHD that occur in multiple members of a family increases the incidence of CHD in familial lines, and support evidences of inherited genetic disorders towards the heart abnormities. Molecular genetics in conjunction with cytogenetics provide an opportunity to decipher the genetic basis and pathogenesis of CHD. With the rapid era of DNA sequencing and genetic discoveries, it is expected that genetic diagnosis and screening will become incorporated into standard practice in the near future. Consequently, it is imperative that cardiologists understand the basis for genetic disorders, and the medical and ethical implications relevant to the genetic information. Today, genetics are predisposed to malformation of the hearts and blood vessels, and account for the highest number of human birth defects. Thus, hereditary and congenital diseases are classified into three broad

Defects in chromosomes associated with CHD are diverse; some examples are aneuploidy or polyploidy, improper rearrangement during mitosis and meiosis, translocation, inversion or deletions. Importantly, certain chromosomes were reported to have a greater degree of significance and of percentages to heart development, and thus the same defects in different chromosomes may not result in similar defects (Table 4). About 0.30-2.0% of all live births have chromosomal defects, usually the chromosomal defects were aneuploidy and trisomy 21, 18, 13 (Dolk et al., 2010). Among all CHDs detected during infant period, the chromosomal defects account for approximately 6 - 10% (Ferencz et al., 1989; Tennstedt et al., 1999; Zhang et al., 2010). In Table 4, defects in chromosomes X, 3, 4, 5, 7, 8, 9, 10, 11, 13,

Nonetheless, the table summarizes the data reported by different studies, some conducted in different times and places. The incidence of CHD generally depends on multiple factors besides the type of genetic disorders and the chromosome where the disorders take place. The other factors include how many fetuses are conceived by the mothers, and how many of these fetuses reach term alive. Further, the affected number of fetuses also depends on the rate of the survival of the affected fetuses and the increased use of therapeutic

Heart development is controlled by multiple genes regulating a complicated network of transcription regulation, translation regulation, and signal transduction pathways, ranging from a control of muscle growth, patterning to contractility, to name a few. However, mutations in only one or a few components of the cardiac gene network can result in the improper development of the heart. One type of heart defect could also be caused by different types of single gene disorders. Since the 1990s, researchers have identified more than 10 different single gene mutations that can lead to heart defects. To date, many genes

**Transcription Factor Genes** transcribe and translate proteins that serve to interact

 **NKX2-5, the NK family, on chromosome 5q35;** Homeobox-containing genes play critical roles in regulating tissue-specific gene expression essential for specification of heart muscle progenitors (Komuro & Izumo, 1993; Toko et al., 2002). Mutations in NKX2-5 result in loss of heart formation in the embryo and have been found in sporadic

responsible for several congenital heart defects have been identified (table 5).

cooperatively with each other to control gene expression.

categories

abortion.

**3.1.2 Single gene disorder** 

**3.1.1 Chromosome defect** 

17, 18, 21 and 22 showed association with CHD.

CCVM. Although the contributions of these variants to the disease phenotype remains uncertain, the linkage between this gene disorder and the atrioventricular conduction defect, ASD, VSD or TOF, have been found (Elliott et al., 2003; McElhinney et al., 2003; Stallmeyer et al., 2010).


 **PROSIT240**, **also known as THRAP2**; An evolutionarily conserved THRAP genes encode a family of proteins that regulate embryonic development. Missense mutation PROSIT240 gene has been identified as a cause of transposition of the great arteries

 **CRELD1, cysteine-rich with EGF-like domains 1;** CRELD1 is the member of a family of matrix cellular proteins. Matrix cellular proteins contain epidermal growth factor-like repeats, and are grouped in a class of cysteine-rich domains that mediate interactions between proteins of diverse functions. Mutation in CRELD1 genes, locating on chromosome 3p25 locus, represents a vital gene position for AVSD (Guo et al., 2010;

 **EVC, EVC2;** This gene encodes a protein containing a leucine zipper and a transmembrane domain. The functions of EVC and EVC2, which share a promoter, are aligned in control limb, skeleton and teeth development. Mutation of this gene has been implicated in both Ellis-van Creveld syndrome and Weyers acrodental dysostosis, the disease locus mapped to chromosome 4p16 (Polymeropoulos et al., 1996). Ellis–van Creveld syndrome is an autosomal recessive disorder characterized by chondrodysplasia and CHD, typically a common atrium of the atrioventricular septal defect type or secundum type atrial septal defects (Ali et al., 2010; Hills et al., 2011; Tompson et al., 2007). Some heterozygous carriers of these mutations manifested Weyers acrodental dysostosis suggesting it is allelic with Ellis–van Creveld syndrome

 **TGFBR1 and TGFBR2, transforming growth factor receptor 1 and 2;** This gene encodes a member of the Ser/Thr protein kinase family and the TGFB receptor subfamily. Mutations in this gene have been associated with Marfan syndrome, Loeys–

**Extracellular Matrix Protein Genes** encode extracellular matrix proteins which cause

 **ELN, elastin;** This gene encodes a protein is one of the two components of elastic fibers. It resides in the Williams critical region on 7q11.23. Deletions and mutations in this gene are associated with Williams or Williams-Beuren syndrome in which the phenotype is comprised of characteristic endocrine, cognitive, and facial features in association with areas of arterial narrowing, most typically non-syndromic supravalvular AS (Micale et al., 2010; Rodriguez-Revenga et al., 2005; Arrington et al.,

 **FBN1, fibrillin 1;** This gene encodes a member of the fibrillin family. This fibrillin has long been assumed to be critical in the aortic wall and other connective tissues as a structural protein. Mutations in this gene are associated with Marfan syndrome (Brautbar et al., 2010; De Backer, 2009; Li et al., 2008). Marfan syndrome is an autosomal dominant disease of connective tissue principally involving the skeletal, ocular systems and cardiovascular malformation whose manifestations include mitral valve prolapse and regurgitation, presenting in infancy in the most severe cases, and progressive aneurismal dilation of the aortic root with the potential for catastrophic aortic dissection and rupture. Marfan syndrome was first mapped to chromosome 15 using traditional genetic linkage analysis (Dietz et al., 1991). Other studies have revealed that fibrillin has a regulatory role in TGF- signaling, and dysregulation of the pathway may instead

Deitz Aortic Aneurysm syndrome (Loeys et al., 2006; Singh et al., 2006).

congenital syndromes involving arteriopathies of different forms.

underlie Marfan pathogenesis (Neptune et al., 2003).

(Muncke et al., 2003).

(Riiz-Perez et al., 2000).

2006).

Zatyka et al., 2005; Robinson et al., 2003).

complex CHD including ASD, AVSD, TGA, PS, and TAPVR (Zhu et al., 2007; Grinberg & Millen, 2005).

**Cell signaling genes** produce proteins involved in cell signal transduction, which allow cells to respond to their environment and are therefore involved in regulation of many important biological functions.


**Cell signaling genes** produce proteins involved in cell signal transduction, which allow cells to respond to their environment and are therefore involved in regulation of many

 **JAG1, Jagged 1;** The jagged 1 protein encoded by JAG1 is the human homolog of the Drosophila jagged protein. Human jagged 1 is the ligand for the receptor NOTCH, which is essential in many organ developmental programs. Analysis of JAG1 expression during mammalian embryogenesis showed its high level of gene expression during the heart and vessel developing periods, and the finding was consistent with the crucial role of its patterning of the right heart and pulmonary vasculature (Loomes et al., 1999). Mutations in the jagged 1 protein cause Alagille syndrome, a complex disease characterized by liver problem, PS, and with or without TOF (Heritage et al., 2002;

 **NOTCH1, NOTCH2, The NOTCH family receptors;** The NOTCH gene encodes a single-pass transmembrane protein receptor that interacts with the ligands named Delta and Serrate/Jagged, and perform many cellular regulatory function. Mutations in NOTCH1 have been shown to cause autosomal-dominant aortic valve defects, and bicuspid (two-leaflet) aortic valve (Grag et al., 2005; McKellar et al., 2007; Mohamed et al., 2006). Because BAV is a risk factor for valve calcification, it has previously been hypothesized that calcification was due to increased blood flow turbulence across the valve leaflets (Robicsek et al., 2004), leading to progressive aortic stenosis and regurgitation in later life. Furthermore, mutation in NOTCH2 receptor was recently found to be able to cause Alagille syndrome even in the patients with no Jagged1

 **PTPN11;** The protein encoded by this gene is a member of the protein tyrosine phosphatase (PTP) family. PTPs are known to be signaling molecules that regulate a variety of cellular processes including cell growth, differentiation, mitotic cycle, and oncogenic transformation. Mutations in this gene are a cause of Noonan syndrome, located on chromosome 12q24 (Jamieson et al., 1994), it is an autosomal dominant disorder characterized by dysmorphic facial features, skeletal malformations, short stature, and cardiac abnormalities, most characteristic are PS, ASD, AVSD and

 **CFC1, cryptic family 1;** This gene encodes a member of the EGF-Cripto, Frl-1, and Cryptic (CFC) family. These proteins play key roles in intercellular signaling pathways during vertebrate embryogenesis. This protein is involved in left-right asymmetric morphogenesis during organ development. Mutations in this gene can cause autosomal visceral heterotaxy with complex CHD including TGA, septal defects and systemic vein anomalies (Goldmuntz et al., 2002; Ozcelik et al., 2006; Yan

 **SOS1, son of sevenless homolog 1;** This gene encodes a protein that is a guanine nucleotide exchange factor for RAS proteins, membrane proteins that bind guanine nucleotides and participate in signal transduction pathways. Mutations in this gene are associated with gingival fibromatosis 1 and Noonan syndrome (Serrano-Martin et al.,

hypertrophic cardiomyopathy (Jongmans et al., 2005; Sarkozy et al., 2003).

& Millen, 2005).

et al., 1999).

2008).

important biological functions.

McElhinney et al., 2002; Colliton et al., 2001).

mutations (El-Rassy et al., 2008; McDaniell et al., 2006).

complex CHD including ASD, AVSD, TGA, PS, and TAPVR (Zhu et al., 2007; Grinberg


**Extracellular Matrix Protein Genes** encode extracellular matrix proteins which cause congenital syndromes involving arteriopathies of different forms.


AD: autosomal dominant, AR: autosomal recessive, AVB: atrioventricular block; HCM: hypertrophic

Table 5. Gene abnormality and contiguous gene syndromes associated with congenital heart

Multifactor inheritance, also known as polygeny, relies on the concept of threshold limit, when the threshold limit of the combined genetic and environmental factors is reached, malformation results. Below the threshold level, the malformation is absent. One common

cardiomyopathy, HOS: Holt–Oram syndrome, SVAS: supravalvular aortic stenosis

**3.1.3 Polygenic / Multifactorial inheritance** 

anomalies.


ASCA: aberrant subclavian artery, CPVD: congenital polyvalvular disease, CVM: cardiovascular malformations, DCM: dilated cardiomyopathy, RAA: right aortic arch, SVAS: supravalvular aortic stenosis (Ref: 1Douchin et al., 2000; 2Lichiardopol & Morta, 2004; 3Mazzanti & Cacciari, 1998; 4Poprawski et al., 2009; 5Tan & Yeo, 2009; 6Lin et al., 2007; 7Lizarraga et al., 1991; 8Musewe et al., 1990; 9Van Praagh et al., 1989; 10Matsuka et al., 1981; 11Musewe et al., 1990; 12Paladini et al., 2000; 13Weijerman et al., 2010; 14Berends et al., 2001; 15Rosias et al., 2001; 16Wilson et al., 1984; 17Akdeniz et al., 2009; 18Barisic et al., 2008; 19Selicorni et al., 2009; 20Digilio et al., 2003; 21Giltay et al., 1998; 22Roskers et al., 1990; 23Green et al., 2000; 24Shuib et al., 2009; 25Battaglia et al., 1999; 26Tsai et al., 1999; 27Strehle & Bantock, 2003; 28Huang et al., 2002; 29Hills et al., 2006; 30Devriendt et al., 1999; 31Wat et al., 2009; 32Lindstrand et al., 2010; 33Van Esch et al., 1999; 34Grossfeld et al., 2004; 35Mattina et al., 2009; 36Digilio et al., 2000; 37Movahhedian et al., 1991; 38Cody et al., 1999; 39Linnandivi et al., 2006; 40Eronen et al., 2002;41Ferrero et al., 2007; 42Edelman et al., 2007;43Potocki et al., 2003; 44Ballesta et al., 2008; 45Shprintzen, 2008)

Table 4. Chromosome abnormality associated with congenital heart anomalies and their percentages.

ASCA: aberrant subclavian artery, CPVD: congenital polyvalvular disease, CVM: cardiovascular

2007;43Potocki et al., 2003; 44Ballesta et al., 2008; 45Shprintzen, 2008)

percentages.

malformations, DCM: dilated cardiomyopathy, RAA: right aortic arch, SVAS: supravalvular aortic stenosis (Ref: 1Douchin et al., 2000; 2Lichiardopol & Morta, 2004; 3Mazzanti & Cacciari, 1998; 4Poprawski et al., 2009; 5Tan & Yeo, 2009; 6Lin et al., 2007; 7Lizarraga et al., 1991; 8Musewe et al., 1990; 9Van Praagh et al.,

1989; 10Matsuka et al., 1981; 11Musewe et al., 1990; 12Paladini et al., 2000; 13Weijerman et al., 2010; 14Berends et al., 2001; 15Rosias et al., 2001; 16Wilson et al., 1984; 17Akdeniz et al., 2009; 18Barisic et al., 2008; 19Selicorni et al., 2009; 20Digilio et al., 2003; 21Giltay et al., 1998; 22Roskers et al., 1990; 23Green et al., 2000; 24Shuib et al., 2009; 25Battaglia et al., 1999; 26Tsai et al., 1999; 27Strehle & Bantock, 2003; 28Huang et al.,

2002; 29Hills et al., 2006; 30Devriendt et al., 1999; 31Wat et al., 2009; 32Lindstrand et al., 2010; 33Van Esch et al., 1999; 34Grossfeld et al., 2004; 35Mattina et al., 2009; 36Digilio et al., 2000; 37Movahhedian et al., 1991; 38Cody et al., 1999; 39Linnandivi et al., 2006; 40Eronen et al., 2002;41Ferrero et al., 2007; 42Edelman et al.,

Table 4. Chromosome abnormality associated with congenital heart anomalies and their


AD: autosomal dominant, AR: autosomal recessive, AVB: atrioventricular block; HCM: hypertrophic cardiomyopathy, HOS: Holt–Oram syndrome, SVAS: supravalvular aortic stenosis

Table 5. Gene abnormality and contiguous gene syndromes associated with congenital heart anomalies.

#### **3.1.3 Polygenic / Multifactorial inheritance**

Multifactor inheritance, also known as polygeny, relies on the concept of threshold limit, when the threshold limit of the combined genetic and environmental factors is reached, malformation results. Below the threshold level, the malformation is absent. One common

 **Maternal connective tissue diseases;** Connective tissue disease is a group of multisystem disorder, such as systemic lupus erythematosus (SLE), which have been associated with congenital complete atrioventricular heart block in offspring. With regard to maternal anti-Ro and anti-La autoantibodies can transmit from a mother to the fetus, which causes a fetal inflammatory response that damages the AV nodal and myocardial tissue in susceptible fetus' which may result in myocarditis, endocardial fibroelastosis and cardiac arrhythmias (Buyon et al., 2009; Clancy & Buyon, 2004). **Maternal rubella;** Women who contract rubella during pregnancy have a high risk of having a baby with congenital rubella syndrome (CRS) which will cause effects such as miscarriage, stillbirth, and a series of birth defects. The risk of fetal infection varies according to the time of onset of maternal infection. Infection rates are highest during the first trimester. The most common manifestations of CRS are congenital cataracts, sensorineural deafness, and congenital heart defects (especially PDA). When the heart is targeted, there is direct viral damage to the myocardium, affecting primarily the left atrium and the heart septa, leading to thrombosis, necrosis, and hemorrhage that cause

 **Maternal febrile illness**; Influenza during the first trimester of pregnancy is associated with febrile illness, which appears to cause more right-sided obstructive heart defects, especially TA and PA, some left obstructive defects and VSD (Oster et al., 2011; Tikkanen & Heinonen, 1991; Yu et al., 2008; Botto et al., 2001). In both hyperthermia and infection there have been documented biological effects on developmental apoptosis pathways. It has been suggested that altered apoptosis may cause birth defects, and apoptosis is known to be involved in cardiac morphogenesis, such as in the

 **Maternal Stress;** Intense maternal stress during the periconceptional period was associated with increased risk of delivering infants with certain congenital anomalies particularly with conotruncal heart defects and neural tube defects (Carmichael &

 **Maternal obesity;** Many studies have examined the association between maternal prepregnancy and during pregnant obesity (elevated BMI >25.0 Kg/m2) with CHD such as ASD, VSD, conotruncal defects and right ventricular outflow tract defects (Cedergren & Kallen, 2003; Mills et al., 2010; Oddy et al., 2009; Gilboa et al., 2010). Several aspects of such potential associations between obesity and heart defects remain unclear due to studies of obesity and heart defects which are difficult to assess and compare because of the possibility of bias in obesity that may associated with unrecognized diabetes. While some literature found no association between maternal

Consumption of many drugs, such as thalidomide and isotretinoin, during early gestationcan interfere with the normal cardiogenesis of the fetus. This list of definite and

Some studies suggest that drinking alcohol or using cocaine, especially during the

weight and isolated CHDs (Khalil et al., 2008; Watkins & Botto, 2001).

of PDA, PS, and ASD (De Santis et al., 2006; Webster, 1998).

development of the cardiac outflow tract.

Shaw, 2000; Adams et al., 1989).

**3.2.2 Maternal drug and medical use** 

**3.2.3 Maternal drugs abuse** 

potential human cardiac teratogens was showed in table 6.

pregnancy, can increase the risk of congenital heart defects (table 6).

key risk is that the babies are genetically oriented towards some level of atypical cardiovascular formation and/or development, together with the exposure to other causative factors. Different stages of cardiac development possess various degrees of vulnerability to environmental factors. Some clues to multifactorial inheritances are a reason for CHD, including a lack of consistent CHD people in the pedigree of the family, and an occasional abnormality with no recognizable pattern in the pedigree of the family.

### **3.2 Maternal factors**

Various teratogenic agents have been implicated as the etiologic agents of CHD. For example, women who have insulin-dependent diabetes mellitus, and those who take certain medications, such as acne and epilepsy medication, have a higher risk for having babies with CHD. Women with drug or alcohol abuse also have predisposing risks. The basic biological principle mechanism of teratogens action that cause CHD include susceptible stage of organogenesis development, genetic differences in susceptibility, dose response relationships, and specific actions of the teratogenic agent. The highest degree of embryonic and fetal sensitivity or susceptibility to adverse effects of exposure to teratogens occurs during the first trimester, especially during the 2nd to 8th week of embryonic life. Dose response relationship implies that for each teratogen there is a dose threshold, theoretic dose below which no adverse effects can be observed.
