Human Genetics of Defects of Situs.

Defects of situs are associated with complex sets of congenital heart defects in which the normal concordance of asymmetric thoracic and abdominal organs is disturbed. The cellular and molecular mechanisms underlying the formation of the embryonic left-right axis have been investigated extensively in the past decade. This has led to the identification of mutations in at least 33 different genes in humans with heterotaxy and situs defects. Those mutations affect a broad range of molecular components, from transcription factors, signaling molecules, and chromatin modifiers to ciliary proteins. A substantial overlap of these genes is observed with genes associated with other congenital heart diseases such as tetralogy of Fallot and double-outlet right ventricle, d-transposition of the great arteries, and atrioventricular septal defects. In this chapter, we present the broad genetic heterogeneity of situs defects including recent human genomics efforts.

Human Genetics of Ventricular Septal Defect.

Ventricular septal defects (VSDs) are recognized as one of the commonest congenital heart diseases (CHD), accounting for up to 40% of all cardiac malformations, and occur as isolated CHDs as well as together with other cardiac and extracardiac congenital malformations in individual patients and families. The genetic etiology of VSD is complex and extraordinarily heterogeneous. Chromosomal abnormalities such as aneuploidy and structural variations as well as rare point mutations in various genes have been reported to be associated with this cardiac defect. This includes both well-defined syndromes with known genetic cause (e.g., DiGeorge syndrome and Holt-Oram syndrome) and so far undefined syndromic forms characterized by unspecific symptoms. Mutations in genes encoding cardiac transcription factors (e.g., NKX2-5 and GATA4) and signaling molecules (e.g., CFC1) have been most frequently found in VSD cases. Moreover, new high-resolution methods such as comparative genomic hybridization enabled the discovery of a high number of different copy number variations, leading to gain or loss of chromosomal regions often containing multiple genes, in patients with VSD. In this chapter, we will describe the broad genetic heterogeneity observed in VSD patients considering recent advances in this field.

Cardiac Transcription Factors and Regulatory Networks.

Cardiac development is a fine-tuned process governed by complex transcriptional networks, in which transcription factors (TFs) interact with other regulatory layers. In this chapter, we introduce the core cardiac TFs including Gata, Hand, Nkx2, Mef2, Srf, and Tbx. These factors regulate each other's expression and can also act in a combinatorial manner on their downstream targets. Their disruption leads to various cardiac phenotypes in mice, and mutations in humans have been associated with congenital heart defects. In the second part of the chapter, we discuss different levels of regulation including cis-regulatory elements, chromatin structure, and microRNAs, which can interact with transcription factors, modulate their function, or are downstream targets. Finally, examples of disturbances of the cardiac regulatory network leading to congenital heart diseases in human are provided.

Clinical Presentation and Therapy of Atrial Septal Defect.

Atrial septal defects (ASDs) occur in 1 of 1500 live births and constitute 6-10% of congenital heart defects. There is a female-to-male predominance of 2 to 1. According to their embryological origins, we can differentiate five different types of ASDs (see Fig. 23.1).

Clinical Presentation and Therapy of Ventricular Septal Defect.

Ventricular septal defects (VSDs) occur in 1.5-3.5 of 1000 live births and constitutes 20 % of congenital cardiac defects. There is no gender predominance.

Clinical Presentation and Therapy of Tetralogy of Fallot and Double-Outlet Right Ventricle.

Tetralogy of Fallot (TOF) is the most common cyanotic heart defect. TOF consists of the combination of four anomalies (Fig. 35.1): (1) a large malalignment ventricular septal defect, (2) an obstruction of the right ventricular outflow tract (usually infundibular and valvular pulmonary stenosis with a small pulmonary valve annulus and supravalvular stenosis, (3) an aorta that "overrides" the ventricular septal defect, and (4) right ventricular hypertrophy. TOF represents 4-8% of congenital heart defects. Specific variations of TOF include all forms of pulmonary atresia with VSD and absent pulmonary valve syndrome. In addition, the left and right main pulmonary arteries may be stenotic or hypoplastic. In these cases, there may be major aortopulmonary collateral arteries (MAPCAs) which are vessels arising from the aorta or the subclavian arteries that supply segments of the pulmonary arterial tree. Additional variations include an ASD (Pentalogy of Fallot), a right aortic arch, and coronary abnormalities.

Clinical Presentation and Therapy of Total Anomalous Pulmonary Venous Return.

Total anomalous pulmonary venous return (TAPVR) is rare (accounting for about 1% of all CHD) and can occur as a single lesion or in combination with other types of CHD (such as heterotaxy or HLHS). TAPVR is defined as an abnormal connection where all pulmonary veins do not drain into the left atrium but into the right atrium either directly or through a vein that is connected to the right atrium. TAPVR can be divided into four anatomic groups (Fig. 32.1): (1) supracardiac (about 55%), (2) cardiac (about 30%), (3) infracardiac (about 13%), and (4) mixed (very rare). In addition, it can be divided into two physiological types: nonobstructed and obstructed. Embryologically, all pulmonary veins usually connect to a pulmonary venous confluence that connects to the left atrium. If this connection does not occur, the pulmonary venous confluence connects to a systemic vein instead.

Human Genetics of Tetralogy of Fallot and Double-Outlet Right Ventricle.

Tetralogy of Fallot (TOF) and double-outlet right ventricle (DORV) are conotruncal defects resulting from disturbances of the second heart field and the neural crest, which can occur as isolated malformations or as part of multiorgan syndromes. Their etiology is multifactorial and characterized by overlapping genetic causes. In this chapter, we present the different genetic alterations underlying the two diseases, which range from chromosomal abnormalities like aneuploidies and structural mutations to rare single nucleotide variations affecting distinct genes. For example, mutations in the cardiac transcription factors NKX2-5, GATA4, and HAND2 have been identified in isolated TOF cases, while mutations of TBX5 and 22q11 deletion, leading to haploinsufficiency of TBX1, cause Holt-Oram and DiGeorge syndrome, respectively. Moreover, genes involved in signaling pathways, laterality determination, and epigenetic mechanisms have also been found mutated in TOF and/or DORV patients. Finally, genome-wide association studies identified common single nucleotide polymorphisms associated with the risk for TOF.

Clinical Presentation and Therapy of d-Transposition of the Great Arteries.

d-Transposition of the great arteries (d-TGA) is the most common form of congenital heart disease that presents with cyanosis in a newborn. The aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. It constitutes 3-5% of all congenital heart defects. In a simple d-TGA (about two-thirds of patients), there is no other cardiac abnormality other than a patent foramen ovale (PFO) and a patent ductus arteriosus (PDA). In a complex d-TGA additional cardiac abnormalities such as VSD, pulmonary stenosis or coronary abnormalities are present. About one-third to 40% of patients with d-TGA have an associated ventricular septal defect. Among patients with d-TGA, 6% of those with intact ventricular septum and 31% of those with ventricular septal defect have associated pulmonary stenosis. Coronary abnormalities are of importance with regard to the complexity of surgical repair.

Clinical Presentation and Therapy of Semilunar Valve and Aortic Arch Anomalies.

The following semilunar valve defects and aortic arch anomalies are called simple defects because there is a single problem that can be well described. Based on the degree of malformation and hemodynamic consequence, these simple lesions can however be life threatening immediately after birth. They all affect either the left or right outflow tract or the aortic arch.

Clinical Presentation and Therapy of Coronary Artery Anomalies.

There are two major coronary arteries that arise normally directly above the aortic valve in the sinus. The left main coronary artery (LCA or LMCA) arises from the left coronary sinus and divides shortly after its origin into the left anterior descending and the circumflex coronary arteries (LCX). Branches of the left anterior descending (LAD) coronary artery include the left conus, septal, and diagonal arteries. Branches of the circumflex coronary artery may include the sinus node artery, Kugel's artery, marginal arteries, and the left atrial circumflex artery (Fig. 47.1). The LAD follows the interventricular septum to the apex, the LCX turns posterior, follows the atrioventricular groove between the left atrium and ventricle to the coronary sinus. Branches of the right coronary artery (RCX) include the conal branch, the sinus node artery, an atrial branch, the right ventricular muscle branches (including the acute marginal branch), the posterior descending coronary artery, the atrioventricular node artery, and septal branches (Fig. 47.2). The RCX follows the atrioventricular groove between the right atrium and ventricle. The "dominant coronary artery" is the one giving rise to the posterior descending coronary artery. It originates from the right coronary artery in 80% of people.

Clinical Presentation and Therapy of Ebstein Anomaly.

Ebstein anomaly is a rare congenital heart defect, accounting for less than 1% of cardiac malformations and occurring in approximately 1 out of 210,000 live births. It is characterized by an abnormality of the tricuspid valve, where the valve is positioned lower than normal in the right ventricle. Although primarily a tricuspid valve defect, the right ventricle itself is often structurally abnormal and weakened (myopathic).

Clinical Presentation and Therapy of Arrhythmias.

The electrocardiogram (ECG) is one of the cornerstones of diagnostic investigations in pediatric or adult cardiology. The standard ECG includes 12 leads; there are 6 leads that are derived from electrodes from the arms and legs (Einthoven and Goldberger leads) and 6 precordial leads (Wilson leads).

Normal Cardiac Anatomy and Clinical Evaluation.

The heart is positioned in the middle, superior, and posterior regions of the mediastinum. Although it is a midline structure, the apex of the heart is typically situated to the left of the midline (Fig. 4.1).

Clinical Presentation and Therapy of Atrioventricular Septal Defect.

Atrioventricular septal defects (AVSDs) consist of a number of cardiac malformations that result from abnormal development of the endocardial cushions. AVSDs occur in 0.19 of 1000 live births and constitute 4-5 % of congenital heart defects. AVSDs can be categorized as incomplete (or partial) or complete, and intermediate or transitional.

Clinical Presentation and Therapy of Anomalies of the Situs.

Situs abnormalities may occur in many and most often more complex congenital cardiac malformations. These conditions are collectively referred to as heterotaxy syndromes, derived from the Greek words "heteros" meaning different and "taxos" meaning orientation or arrangement. Clinically, heterotaxy spectrum encompasses defects in the left-right laterality and arrangement of visceral organs. "Situs" is derived from Latin and is the place where something exists or originates. In human anatomy, situs can be solitus (derived from Latin, meaning "normal"), inversus, or ambiguus. Heterotaxy syndrome represents an intermediate arrangement of internal organs between situs solitus and situs inversus, also known as "situs ambiguous." Situs ambiguus describes an abnormal distribution of major visceral organs within the chest and abdomen. The determination of situs as normal, inversus, or ambiguus is primarily based on the location of unpaired organs such as the spleen, liver, stomach, and intestines. Diagnosis is made by clinical examination, echocardiography, a chest X-ray (position of the heart, stomach, and liver), and ultrasound of the abdominal organs. Situs is considered solitus if the left atrium, spleen, stomach, and the trilobed lung are on the left side and the liver and bilobed lung are on the right side. Situs ambiguus is present if the location of unpaired structures is random or indeterminate even after detailed and appropriate imaging. Situs inversus results when the arrangement of the thoracic and abdominal organs is mirrored. Individuals with situs inversus or situs solitus do not experience fatal dysfunction of their organ systems, as general anatomy and morphology of the abdominothoracic organ-vessel systems are conserved.

Clinical Presentation and Therapy of Tricuspid Atresia and Univentricular Heart.

Although the terms "single ventricle" and "univentricular heart" are frequently used to describe a variety of complex congenital heart defects, in fact, nearly all hearts have two ventricles, although one of the two may be too small to be functional. A better term for these hearts would therefore be "functional single ventricle."

Clinical Presentation and Therapy of Truncus Arteriosus.

Truncus arteriosus (TA, also known as common arterial trunk) consists of only one great artery ("the truncus") with a semilunar valve (truncus valve) arising from the heart and an additional ventricular septal defect and (Fig. 50.1). This great artery is positioned above the ventricular septal defect and gives rise to the coronary arteries, the pulmonary arteries, and the aortic arch. Historically, TA has been classified by Collet and Edwards in three types, where in type I there was a common pulmonary artery truncus, in type II the left and right PA arise separately but close to each other, in type III both PA arise independently; in addition, there was a type IV that was later characterized as pulmonary atresia with VSD and major aortopulmonary collateral arteries arising from the descending aorta.

Clinical Presentation and Therapy of Hypoplastic Left Heart Syndrome.

Hypoplastic left heart syndrome (HLHS) is a complex congenital heart defect characterized by several abnormalities that result in a significantly underdeveloped left ventricle and severe hypoplasia of the ascending aorta, often leading to retrograde perfusion. These abnormalities include aortic valve atresia or severe stenosis, accompanied by a severely hypoplastic aortic valve annulus (Fig. 59.1). Mitral valve atresia, hypoplasia, and/or stenosis with a hypoplastic valve annulus with or without a ventricular septal defect can also contribute to the development of HLHS. Endocardial fibroelastosis and sinusoids may be present as well. The interatrial septum can either be closed or the foramen ovale severely stenotic. Other malformations, such as anomalous pulmonary venous drainage or variations of the systemic veins, may coexist. It is also common to observe a coarctation of the aorta in these cases.

Clinical Presentation and Therapy of Cardiomyopathies.

Cardiomyopathies are a group of diseases that primarily affect the heart muscle, leading to mechanical or electrical dysfunction of the heart. They can be categorized into primary and secondary forms. Primary cardiomyopathies can be further classified as congenital, acquired, or mixed. In terms of the heart muscle itself, there are five distinct types of cardiomyopathies: hypertrophic cardiomyopathy, dilated or congestive cardiomyopathy, restrictive cardiomyopathy, arrhythmogenic (right ventricular) dysplasia, and noncompaction cardiomyopathy. While cardiomyopathies primarily affect the heart, they can also have systemic manifestations, impacting other organs and potentially causing progressive debilitation, heart failure, or even death.