Skip to main content
PMC home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2013 Jul 1.
Published in final edited form as: Differentiation. 2012 Jun 17;84(1):117–130. doi: 10.1016/j.diff.2012.05.006

The Pathogenesis of Atrial and Atrioventricular Septal Defects with Special Emphasis on the Role of the Dorsal Mesenchymal Protrusion

Laura E Briggs 1, Jayant Kakarla 2, Andy Wessels 1,*
PMCID: PMC3389176  NIHMSID: NIHMS384176  PMID: 22709652

Abstract

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Keywords: atrioventricular septal defect, AVSD, atrial septal defect, ASD, dorsal mesenchymal protrusion, DMP, ostium primum defect, ostium secundum defect, second heart field, SHF

Introduction

Cardiac septation encompasses a complicated series of events in which structures derived from various precursor populations interact and fuse with one another to functionally and physically separate the different compartments of the four-chambered heart. Given the complexity of the process it may not be surprising that it is incomplete in a significant portion of the human population. In this review, we describe the anatomy of the atrioventricular (AV) junction, the origin and fate of tissues involved in septation, and will also discuss cardiac malformations that occur when the septation process goes awry.

Anatomy of the Four-Chambered Heart

The fully formed heart is a midline, four-chambered organ positioned within the middle mediastinum. Its two atria, each of which contains a venous component, a body, an appendage, and an AV valve vestibule, are physically and functionally separated by an atrial septum. The atrial septum is comprised of several components including the septum primum (SP) anchored by a muscular base and the septum secundum (SS) (Fig. 1A). The muscular base of the septum primum forms the floor, or antero-inferior rim, of the oval fossa while the septum secundum contributes to its superior margin.

Figure 1. Illustration of Atrial and Atrioventricular Septal Defects.

Figure 1

Open in a new tab

In Figure 1A, a mature, properly septated heart is depicted. The septum primum (green) has fused to the septum secundum (red), resulting in closure of the ostium secundum while interaction of the DMP (yellow), septum primum and major endocardial cushions (blue) results in closure of the ostium primum (OP). In Figure 1B, a patent foramen ovale is illustrated. Here, although the septum primum has not fused to the septum secundum, under normal hemodynamic conditions the two remain in contact to functionally partition the atria. If right atrial pressure were to exceed that of the left atrium, however, right-to-left shunting would result. In Figure 1C, an ostium secundum defect (OSD) is depicted; the flap valve (green) of the ostium secundum is not of sufficient length to prevent atrial communication. Figure 1D depicts an incomplete AVSD, a malformation in which atrial shunting is freely permitted due to persistence of the ostium primum. Complete AVSDs (1E) are characterized by both atrial and ventricular shunting is permitted. RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle; DMP, dorsal mesenchymal protrusion; SP, septum primum; SS, septum secundum; OSD, ostium secundum defect; OPD, ostium primum defect; PFO, patent foramen ovale; VSD, ventricular septal defect

Defects in the Atrioventricular Septal Complex

Patent Foramen Ovale (PFO)

In the adult heart, the superior and inferior caval veins return de-oxygenated venous blood from systemic circulation to the right atrium whereas the pulmonary vein delivers oxygenated, arterial blood from the pulmonic circulation to the left atrium. During fetal development, however, gas exchange occurs in the placenta and oxygenated blood is carried back to the fetus through the umbilical vein. After bypassing hepatic circulation via the ductus venosus, oxygenated blood enters the inferior caval vein where it joins, but streams separately from, deoxygenated blood returning from lower portions of the developing fetus (Barclay et al., 1939; Edelstone and Rudolph, 1979; Lind and Wegelius, 1949). At the junction of the right atrium and the inferior caval vein, a flap of tissue termed the Eustachian valve directs the more dorsally located, oxygenated blood across an interatrial communication termed the foramen ovale. The superior boundary of this communication is formed by the septum secundum while the septum primum acts as a flap valve of the foramen, allowing unidirectional shunting of oxygenated blood from the right to left atrium. The left ventricle and ascending aorta then deliver this blood to the two tissues with the highest oxygen demand, the developing brain and heart.

Shortly after birth and the establishment of pulmonic gas exchange, increased relative left to right atrial pressure pushes the septum primum against the septum secundum. Once in contact, these two structures fuse, resulting in permanent closure of the foramen ovale and two fully partitioned atria. Thus, the foramen ovale, whether patent or closed, is important for the delivery of oxygenated blood to tissues; the fetal patency of the foramen ovale ensures that vital structures receive blood with a higher oxygen content, while its closure postnatally helps maintain arterial oxygen saturation by preventing the mixture of venous and arterial blood.

In approximately 20–34% of the population, the septum primum and septum secundum fail to fuse, resulting in patency of the foramen ovale (Fig. 1B). The presence of a patent foramen ovale (PFO) does not necessarily imply active communication between the two atrial chambers. In fact, as long as left atrial pressure exceeds that of the right atrium and if the flap valve is of sufficient size to cover the boundaries of the oval fossa, the PFO remains functionally closed. Under certain hemodynamic conditions, however, the PFO may open and thus enable thrombi, air or any other substance to pass from venous to arterial circulation. It is by this mechanism that a PFO may become pathological and result in conditions such as cryptogenic stroke, migraine with aura, and decompression illness (Anzola et al., 1999; Choong et al., 2008; Dowson et al., 2008; Kim and Girardi, 2008; Sztajzel et al., 2002; Wilmshurst et al., 1989).

Ostium Secundum Defect

If the septum primum is of insufficient size to cover the entire oval fossa, and/or if the septum secundum fails to fully form, an ostium secundum defect will result (Fig. 1C). The clinical course of patients diagnosed with an isolated ostium secundum defect is often benign and may be associated with few, if any, functional limitations. Larger, more severe ostium secundum defects may lead to right atrial volume overload due to persistent left-to-right shunting and, as a result, increased risk of complications such as atrial fibrillation and pulmonary hypertension (Clark and Kugler, 1982; Engelfriet et al., 2007; Ruschhaupt et al., 1984). Eventually, increased pulmonic resistance can cause right-sided pressure to increase; subsequently, blood is shunted from right-to-left (Eisenmenger syndrome) and systemic oxygenation decreases (Engelfriet et al., 2007).

Ostium Primum Defect

The ostium primum defect is a less common, but more severe, possible consequence of aberrant septation. Although interatrial shunting is also observed in ostium primum defects, these malformations are now considered to be part of the family of abnormalities known as atrioventricular septal defects (AVSD) and therefore, will be discussed in the next section.

Atrioventricular Septal Defect (AVSD)

AVSDs represent a spectrum of cardiac malformations and include three subtypes: incomplete AVSD, transitional AVSD and complete AVSD (Jacobs et al., 2000). While all AVSDs are characterized by a common AV valve, the variants differ with respect to the level at which shunting between chambers is permitted as well as the morphology of the common AV valve (Anderson et al., 1998a).

Incomplete AVSDs (Fig. 1D) are characterized by the presence of distinct mitral and tricuspid annuli, or left and right valvar orifices (Jacobs et al., 2000). In transitional AVSDs, fusion of the anterior and posterior bridging leaflets results in a single valvar annulus; however, attachment of these leaflets to the ventricular septum creates, in effect, two distinct valvar orifices. In contrast, complete AVSDs (Fig. 1E) are characterized by the presence of a single, common AV valve orifice (Jacobs et al., 2000). All three variants generally demonstrate interatrial shunting due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve; however, shunting at the ventricular level will also be permitted if the bridging leaflets are attached to the leading edge of the atrial septum (Anderson et al., 1998a) (Fig. 1E). In transitional AVSDs, attachment of the common valve to the ventricular septum assists in separation of the right and left ventricle while in complete AVSDs, shunting at the ventricular level is freely permitted through a defect between the ventricular surface of the common valve and the interventricular septum (Jacobs et al., 2000).

Within these defects, the size of the ventricles is variable but important in determining whether the patient is a candidate for biventricular surgical repair (Bharati and Lev, 1973; Jegatheeswaran et al., 2010). In balanced AVSDs, both ventricles are appropriately sized while in unbalanced AVSDs, one ventricle is so small that the patient must be managed as having a functionally univentricular heart (Bharati and Lev, 1973; Jegatheeswaran et al., 2010). Complete AVSDs may also be classified according to the Rastelli classification system, which is based on the morphology, degree of bridging and chordal attachments of the superior leaflet (Rastelli et al., 1966).

The clinical presentation of AVSD is not uniform and depends on the size of the defect, the degree of shunting as well as the presence of other malformations. In general, untreated complete AVSD results in congestive heart failure within the first few months of life while the signs and symptoms of incomplete AVSD are subtler and may not manifest until later in life (Minich et al., 2010). Management of these defects requires surgical correction, which may be complicated by the presence of other cardiac abnormalities including dysplastic AV valve leaflets, isomeric atrial appendages, and tetralogy of Fallot or double outlet right ventricle (Shuhaiber et al., 2009).

Etiology of Abnormal Septation

Cardiac malformations, including septal defects, often occur in the context of a syndrome and in association with other abnormalities (Table 1). Holt-Oram syndrome, which results from mutations in TBX5, is characterized by atrial and ventricular septal defects as well as upper limb abnormalities (Basson et al., 1997; Li et al., 1997). Heterotaxy syndromes, or left-right laterality defects, often include septal defects as well as isomerism of the atrial appendages and may result from dysregulation of pathways involved in left-right patterning, such as PITX2, SHH and NODAL (Cunningham et al., 1998; Isaac et al., 1997; Levin et al., 1995; Piedra et al., 1998). 3p deletion syndrome, while rare, results in severe defects including AVSD as well as other congenital heart defects, microcephaly, low birth weight, and hypotonia; prior work has implicated CRELD1 and/or CRELD2 as the cause of atrioventricular septal defects in this deletion syndrome (Green et al., 2000; Robinson et al., 2003; Rupp et al., 2002).

Table 1.

Syndrome Gene OMIM # References
OSD Alagille 2 NOTCH2 OMIM: 610205 (McDaniell et al., 2006)
Axenfield-Rieger PITX2 OMIM: 602482 (Cunningham et al., 1998; Zetterqvist et al., 1971)
Cardio-facio-cutaneous KRAS, BRAF, MEK1, MEK2 OMIM: 115150 (Armour and Allanson, 2008; Niihori et al., 2006; Reynolds et al., 1986)
Fryns OMIM: 229850 (Bamforth et al., 1989; Lin et al., 2005)
Holt-Oram TBX5 OMIM: 142900 (Glauser et al., 1989; Holt and Oram, 1960; Newbury-Ecob et al., 1996; Sletten and Pierpont, 1996; Smith et al., 1979; Terrett et al., 1994)
Ivemark OMIM: 208530 (Calabro et al., 1988; Moller et al., 1967)
Noonan PTPN11 OMIM: 163950 (Burch et al., 1993; Croonen et al., 2008; Sznajer et al., 2007)
Ritscher-Schinzel OMIM: 220210 (Orstavik et al., 1998)
Rubinstein-Taybi CREBBP, EP300 OMIM: 180849 (Stevens and Bhakta, 1995)
Patau Ts13 (Musewe et al., 1990)
AVSD CHARGE CHD7 OMIM: 214800 (Issekutz et al., 2005)
Down Ts21 OMIM: 190685 (Freeman et al., 1998; Park et al., 1977; Stoll et al., 1998)
Ellis-van Creveld EVC, EVC2 OMIM: 225500 (Kamesui et al., 1997; McKusick et al., 1964; Ruiz-Perez and Goodship, 2009; Tompson et al., 2007)
Ivemark OMIM: 208530 (Moller et al., 1967)
Kaufman-McKusick MKKS OMIM: 236700 (Owens et al., 2009)
Ritscher-Schinzel OMIM: 220210 (Hoo et al., 1994; Ritscher et al., 1987)
Smith-Lemli-Opitz DHCR7 OMIM: 270400 (Ryan et al., 1998)
3p CRELD, CALL OMIM: 606217 (Drumheller et al., 1996; Green et al., 2000; Kozma et al., 2004; Robinson et al., 2003)
Open in a new tab

The most common genetic syndrome, Down syndrome (trisomy 21), is strongly associated with atrioventricular septal defect; approximately one-quarter of children with Down syndrome possess this malformation and roughly two-thirds of complete AVSD occur in association with trisomy 21 (Barlow et al., 2001). However, not all children with Down syndrome possess the defect, suggesting that its pathogenesis is multifactorial and may involve multiple genetic modifiers and/or an environmental component.

Genetic mutations may also result in non-syndromic septal defects, defects that occur in isolation or in absence of phenotypic features typically associated with a syndrome (Table 2). Non-syndromic AVSDs, which are estimated to occur in approximately one per 10,000 live births, are thought to occur as a result of multifactorial inheritance or sporadic mutations (Carmi et al., 1992; Sheffield et al., 1997). GATA4, CRELD2, BMP4, ALK2 and CFC1 are all genes suggested to play a role in non-syndromic AVSD (Garg et al., 2003; Maslen, 2004; Roessler et al., 2008; Smith et al., 2009). Ostium secundum defects have been linked to mutations in GATA4, its binding partners NKX2-5 and TBX5, and the downstream transcription target of GATA4 and TBX5, MYH6 (Biben et al., 2000; Ching et al., 2005; Garg et al., 2003; Okubo et al., 2004; Sarkozy et al., 2005; Schott et al., 1998).

Table 2.

Gene OMIM # References
OSD ACTC1 OMIM: 102540 (Matsson et al., 2008; Monserrat et al., 2007)
CFC1 OMIM: 605194 (Ozcelik et al., 2006; Wang et al., 2011)
GATA4 OMIM: 600576 (Garg et al., 2003; Hirayama-Yamada et al., 2005; Reamon-Buettner and Borlak, 2005; Tomita- Mitchell et al., 2007)
MYH6 OMIM: 160710 (Ching et al., 2005)
NKX2-5 OMIM: 600584 (Benson et al., 1999; Gutierrez-Roelens et al., 2006; Reamon-Buettner and Borlak, 2004a; Schott et al., 1998)
TBX5 OMIM: 601620 (Garg et al., 2003; Reamon-Buettner and Borlak, 2004b)
TBX20 OMIM: 606061 (Kirk et al., 2007; Posch et al., 2010)
ZIC3 OMIM: 300265 (Ware et al., 2004)
AVSD ALK2 OMIM: 102576 (Smith et al., 2009)
BMP4 OMIM: 112262 (Posch et al., 2008)
CRELD1 OMIM: 607171 (Robinson et al., 2003; Zatyka et al., 2005)
GATA4 OMIM: 600576 (Reamon-Buettner and Borlak, 2005)
NKX2-5 OMIM: 600584 (Inga et al., 2005; Reamon-Buettner and Borlak, 2004a)
TBX5 OMIM: 601620 (Reamon-Buettner and Borlak, 2004b)
Open in a new tab

Between 1981 and 1989, a large case-control study, the Baltimore-Washington Infant Study, was conducted to assess risk factors for congenital cardiovascular defects. This study, which included 363 infants with AVSD (210 of whom were also diagnosed with Down syndrome), provided invaluable information regarding the contribution of non-inherited risk factors to the pathogenesis of septal defects. The results of this study, as well as others, are summarized here and in table 3. Although the mechanism is not fully understood, pre-gestational diabetes significantly increases the risk of both syndromic (Loffredo et al., 2001b) and non-syndromic complete AVSD (Loffredo et al., 2001a). Other potential risk factors for AVSD include urinary tract infection during the first trimester of pregnancy (Cleves et al., 2008), maternal use of ibuprofen (Ferencz et al., 1997), moderate to heavy maternal cigarette use (Ferencz et al., 1997; Malik et al., 2008), maternal use of cocaine (Ferencz et al., 1997) and maternal exposure to paint, varnishes, and ionizing radiation (Cleves et al., 2008; Ferencz et al., 1997; Malik et al., 2008). Ostium secundum defects have been linked to pre-pregnancy obesity (Gilboa et al., 2010), maternal use of β blockers (Caton et al., 2009), alcohol (Tikkanen and Heinonen, 1991), cigarette use (Kallen, 1999), and maternal age greater than 34 (Ferencz et al., 1997).

Table 3.

Non-Inherited Modifiers Type Odds Ratio (95% CI) References
OSD Advanced age 1.6 (1.0 – 2.5) (Ferencz et al., 1997)
Alcohol 1.9 (1.0 – 3.4) (Tikkanen and Heinonen, 1991)
β-blockers 2.6 (1.3 – 4.4) (Caton et al., 2009)
Cigarette 1.63 (1.04 – 2.57) (Kallen, 1999)
Obesity 1.29 (1.07 – 1.55) (Gilboa et al., 2010)
Thalidomide (Smithells and Newman, 1992)
AVSD Cigarette Non-syndromic 2.5 (1.21 – 5.19) (Ferencz et al., 1997; Malik et al., 2008)
Cocaine Non-syndromic 3.45 (1.05 – 11.40) (Ferencz et al., 1997)
Diabetes Non-syndromic 20.6 (5.6 – 76.4) (Loffredo et al., 2001a)
Syndromic 22.8 (7.4 – 70.5) (Loffredo et al., 2001b)
Ibuprofen Syndromic 2.49 (1.42 – 4.34) (Ferencz et al., 1997)
Ionizing radiation Non-syndromic 4.54 (1.36 – 15.18) (Ferencz et al., 1997)
Maternal illness Non-syndromic 2.29 (1.11 – 4.73) (Cleves et al., 2008)
Paint Syndromic 1.77 (1.19 – 2.63) (Ferencz et al., 1997)
Varnish Non-syndromic 4.54 (1.36 – 15.18) (Ferencz et al., 1997)
Open in a new tab

Much like their morphological features, classification, and clinical presentation, the etiology of septal defects is complex. These defects may arise from perturbation of any one component of the AV septal complex and development of each component is governed by multiple molecular pathways. In order to better understand the pathogenesis of these complicated defects, it’s important to review formation of the heart, the development of the AV canal and the components that comprise the AV septal complex.

Early Heart Development

In the vertebrate embryo, the heart is the first organ to become functional; its development commences at gastrulation with the formation of precardiac mesoderm. Cells composing this middle embryonic germ layer then migrate anterolaterally to establish bilateral primary heart fields. The two heart fields eventually combine to form the cardiac crescent and, as the mesodermal cells differentiate into myocardial and endocardial progenitors, the primary heart tube (Abu-Issa and Kirby, 2007). The linear tube is suspended from the foregut endoderm by the dorsal mesocardium and is oriented such that the arterial pole (outflow) is cranial to the venous pole (inflow), with bilateral venous tributaries draining into the latter (Drake et al., 2006; Manner, 2004). The two poles of the linear tube soon receive contributions from a second cardiogenic population; this population of cells is termed the Second Heart Field (SHF) and its addition leads to pronounced elongation of the linear tube (Kelly and Buckingham, 2002; Mjaatvedt et al., 2001; Moorman et al., 2007; Snarr et al., 2007a; Waldo et al., 2001; Zaffran and Kelly, 2012).

Soon thereafter, the central portion of dorsal mesocardium suspending the linear tube disintegrates, enabling the primary heart tube to loop to the right (Manner, 2004). Looping of the primary heart tube results in a more cranially and dorsally positioned inflow region. This inflow region, or venous pole of the primary heart tube, gives rise to the symmetrical common atrium which receives blood from the developing embryo through bilateral venous tributaries, the right and left superior caval veins.

At this point, the common atrium is separated from the left ventricle by the AV canal but the various components are still arranged in series. In order to progress from this “looped-tube” structure to a more mature, four-chambered configuration, changes must occur along the inner and outer curves of the looped heart. Along the outer curve, serial ballooning gives rise to the apical portions of the right and left ventricles (Davis, 1927; Lamers and Moorman, 2002; Moorman et al., 2003). It is through remodeling events along the inner curvature and at its junction with the AV canal and the outlet component that the atrial cavities come to be shared between ventricles. While the left atrium is able to communicate with the left ventricle from the outset, communication between the right atrium and the right ventricle requires expansion of the AV canal, the right side of which becomes incorporated into the right atrium as the vestibule of the tricuspid valve (Wessels et al., 1992).

Within just one embryonic day in murine development, the atrial entry point of the caval veins shifts from a caudal to dorsal position (Soufan et al., 2004). The junction between the systemic venous tributaries and the developing right atrium can be visualized by the appearance of the venous valves, bilaminar structures that merge cranially to form the septum spurium in the roof of the right atrium (Anderson et al., 2006). Simultaneously, the systemic venous tributaries are enveloped by primary myocardium as they are incorporated into the pericardial cavity (Soufan et al., 2004).

Eventually, the caval veins are further enclosed by primary myocardium and more caudally, canalization of the pulmonary pit leads to communication between the pulmonary vein and atrial cavity (Soufan et al., 2004). It should be noted that mouse and man differ with respect to the venous pole of the heart. In humans, the left caval vein regresses with the proximal portion giving rise to the coronary sinus while the rest of it forms the ligament of Marshall and the oblique vein (Wessels and Sedmera, 2003); if it fails to do so, persistent left superior caval vein results. In the mouse, however, persistence of the left superior caval vein is normal; the vessel is joined by the azygous vein as well as several cardiac veins before it drains into the right atrium (Wessels and Sedmera, 2003).

Components of Atrial and Atrioventricular Septation

Atrioventricular cushions

The primary heart tube consists of an outer layer of myocardium separated from an inner layer of endocardium by an acellular, extracellular matrix or, cardiac jelly. Around 8.5ED, when the linear tube begins to loop, the myocardium in the outflow tract and at the junction of the atria and left ventricle begins to secrete extracellular matrix components, including chondroitin sulfate and hyaluronan (DeVlaming et al., 2012; Kruithof et al., 2012; Manasek, 1970; Manasek et al., 1973). The production of lipophilic glycosaminoglycans results in localized formation of AV, or endocardial, cushions (DeVlaming et al., 2012; Kruithof et al., 2012; Manasek, 1975; Markwald et al., 1977; Markwald and Smith, 1972). The endocardial cells lining these cushions are unique in that they respond to growth factors released by underlying myocardium; after receiving these inductive cues, they undergo an epithelial-to-mesenchymal transition (EMT) and then migrate away from the surface and into the cushions (DeVlaming et al., 2012; Kruithof et al., 2012; Mjaatvedt et al., 1987; Runyan and Markwald, 1983).

This process of epithelial-to-mesenchymal transition results in the mesenchymalization of the two major endocardial cushions of the AV canal: the superior (ventral) endocardial cushion that is associated with the inner curvature of the looped heart and the inferior (dorsal) endocardial cushion that forms along its outer curvature. The two major cushions then expand and eventually fuse, separating the heart into left and right AV orifices. Following development of the major endocardial cushions, the lateral cushions appear later as lateral swellings of mesenchyme at the junction between the atria and ventricles (de Lange et al., 2004; Wessels et al., 1996). Each of the four cushions plays an important role in AV valvuloseptal development; the superior and inferior AV cushions contribute to the aortic leaflet of the mitral valve and the septal leaflet of the tricuspid valve (Snarr et al., 2008). The lateral cushions also make significant contributions to the AV valves; the right lateral cushion contributes to the anterosuperior and inferior leaflets of the tricuspid while the left lateral cushion is involved in formation of the mural leaflet of the mitral valve (Snarr et al., 2008).

Despite their significant contribution to valvuloseptal development, the lateral AV cushions have been largely ignored in most experimental studies and thus, relatively little is known regarding the molecular mechanisms that govern their delayed formation and maturation. Further studies comparing development of the major and lateral cushions are certainly needed as recent work indicates that the two are fundamentally different (Wessels et al., 2012).

Proper formation of the major cushions, which are centrally located between the interventricular septum and interatrial septum, is critical to cardiac septation as all components of the AV septal complex eventually fuse to these structures. Cushion abnormalities are observed in a number of genetically modified mouse models, including neurofibromin-1, retinoid X receptor alpha, and hyaluronan synthase-2 knockout mice (Camenisch et al., 2000; Gruber et al., 1996; Lakkis and Epstein, 1998). While defects of the endocardial cushions are variable with respect to etiology and morphological features, they are usually severe and result in embryonic lethality.

The Atrial Septa

The septum primum makes its first appearance in the looped heart as a muscular crescent in the roof of the common atrium. This true septal structure is capped on its leading edge by mesenchyme and protrudes into the atrial cavity toward the fusing endocardial cushions (Figs. 2A– D′). As it proceeds toward the superior and inferior endocardial cushions, the left and right atrial chambers communicate via the ostium primum. Eventually, the mesenchymal cap of the septum primum fuses with the endocardial cushions (Fig. 2C′), but before it does so, fenestrations form in the dorsal portion of the septum, at the site of its attachment to the atrial roof (Figs. 2C′, D′). This results in formation of the ostium secundum and the preservation of interatrial communication (Figs. 2E, E′). Fusion of the septum primum to the AV cushions and concomitant detachment from the roof of the atrial cavity enables the muscular septum primum to act as the flap valve of the oval foramen (Figs. 2E, E′).

Figure 2. Interaction of the mesenchymal cap of the septum primum and the endocardial cushions.

Figure 2

Open in a new tab

At 10.5ED, the AV canal is positioned to the left of midline (2A). The septum primum is capped by mesenchyme and protrudes toward the major endocardial cushions (2B). At this stage, the ostium primum is patent while the ostium secundum has yet to develop (2B). By 11.5ED, the mesenchymal cap of the septum primum has fused with the sAVC (*2C′). The ostium primum has yet to fully close while the ostium secundum is already forming, as demonstrated by the fenestrations in the septum primum (2D′). Figures 2E and 2E′ depict a specimen at 15ED; all components of the AV septal complex have fused. The septum secundum has formed, the ostium primum is fully closed and right to left shunting is permitted through the ostium secundum only. The septum primum acts as the flap valve of the foramen ovale and shortly after birth, will fuse with the septum secundum to fully partition the atria. MC, mesenchymal cap; SP, septum primum; SS, septum secundum; OP, ostium primum; OS, ostium secundum; DMP, dorsal mesenchymal protrusion

Following formation of the ostium secundum, an additional structure begins to develop at its cranial margin, the so-called septum secundum. In the developing human, the septum secundum is not a true septum; rather, it is formed by an infolding of dorsal atrial myocardium. This septum secundum does not become pronounced until the second trimester, by which time the right pulmonary veins have migrated to reach their definitive position in the atrial roof. By forming the superior boundary of the oval fossa, the septum secundum provides a structure against which the flap valve can close in postnatal life. In the mouse, the septum secundum, although small, is a true septum (Figs. 2E, E′). Thus, closure of the ostium secundum is largely accomplished by fusion of the cranial portion of the septum primum with the atrial wall.

Prior work indicates that the septum primum is a left-sided structure as the left-sided markers creatine kinase-B (CK-B) and Pitx2c are expressed within its myocardium (Meno et al., 1998; Wessels et al., 2000). Consistent with the importance of Pitx2c to left-sided development, the interatrial septum fails to properly develop in mice deficient for this transcription factor (Figs. 5B, D, D′). Absence of PITX2C results in isomerism of the right atrial appendages, AVSD, and abnormalities of the ventricular outflow tracts (Liu et al., 2002) (Figs. 5B, D, D′). The molecular mechanisms that drive the formation of the septum primum from a left-sided precursor population, however, are not fully understood.

Figure 5. Murine Models of Atrioventricular Septal Defect.

Figure 5

Open in a new tab

Figures 5A– D′ depict Pitx2c−/− specimens and wild-type controls. At 12ED in a normal mouse heart (5A), the septum primum with its mesenchymal cap can be seen protruding toward the endocardial cushions. At this stage, both the ostium primum and secundum are patent but the ostium primum is in the process of closing. In Pitx2c−/− specimens at 12ED (5B), however, no septum primum is observed and as such, the atria communicate freely. By 15ED in the normal mouse (5C, C′), the ostium primum has fully closed while the ostium secundum is patent. The right and left venous valves, which demarcate the orifice of systemic venous return, can be observed within the right atrium. In the Pitx2c−/− specimen at 15ED (5D, D′), the septum primum is absent and, consistent with right atrial isomerism, two sets of venous valves (one in each atrium) can be observed. In this model of AVSD, an ostium primum enables atrial shunting while a ventricular septal defect permits shunting at the ventricular level as well (5D, D′).

Figures 5E– H′ depict deletion of Smoothened from the anterior heart field, including the DMP, (Mef2c-AHF-cre; Smof/f) (5F, 5H, 5H′) and wild-type controls (Smof/f) (5E, 5G, 5G′). Here, at 13ED, the septum primum may be observed within the conditional knockout (5F); however, the ostium primum is still patent and the ostium secundum has not formed. Even two days later in developent, the primary septum is still attached to the roof of the atria and a large ostium primum defect can be observed in these mutants (5H, H′). MC, mesenchymal cap; SP, septum primum; SS, septum secundum; OP, ostium primum; OS, ostium secundum; iAVC, inferior atrioventricular cushion; sAVC, superior atrioventricular cushion; AVSD, atrioventricular septal defect

In humans, mutations in several genes, including NKX2-5, GATA4 and TBX20, have been linked to atrial septal defects and patency of the foramen ovale (Garg et al., 2003; Hirayama-Yamada et al., 2005; Kirk et al., 2007). Mice carrying heterozygous Nkx2-5 mutations demonstrate defective morphogenesis of the atrial septum, resulting in ostium secundum atrial septal defects and, secondary to increased mechanical stress, postnatal aneurysm of the septum primum (Biben et al., 2000). Adult mice heterozygous for Tbx20, which genetically and/or physically interacts with both Nkx2-5 and Gata4, also demonstrate abnormalities of the atrial septum, persistent foramen ovale, aneurysm of the septum primum, and dilated cardiomyopathy (Brown et al., 2005; Stennard et al., 2003; Stennard et al., 2005). Heterozygous Gata4 mice on an inbred C57 genetic background display severe cardiac anomalies including ostium secundum defect, AVSD and ventricular septal defect. Interestingly, these defects are not observed in mice on a mixed genetic background (Bisping et al., 2006; Pu et al., 2004; Rajagopal et al., 2007). Other mouse models with defective development of the septum primum include Pdgfrα−/− and podoplanin−/− mutants (Bax et al., 2010; Mahtab et al., 2009)(Douglas et al., 2009).

Although the septum primum is an important component of the AV septal complex, its development does not, at least upon superficial inspection, seem to be perturbed in all murine models of AVSD. Furthermore, abnormal development of the septum primum is responsible for defects located within the oval fossa (ostium secundum defects), and does not produce the ostium primum defect, indicating that maldevelopment of an additional structure, or structures, may be necessary for the pathogenesis of AVSD.

Dorsal Mesenchymal Protrusion

Nomenclature

The looping primary heart tube is attached to the developing embryo by mesocardial tissue located at its venous and arterial poles (Drake et al., 2006). Soon after looping is complete, the dorsal mesocardium persisting at the venous pole provides the portal of entry for an extracardiac population of mesenchyme to protrude into the common atrium (Figs. 3A, B). Like the endocardial cushions and cap of the septum primum, this structure, the dorsal mesenchymal protrusion (DMP), contributes to the AV mesenchymal complex (Snarr et al., 2007b; Wessels et al., 2000). The DMP resembles the structure first identified by Wilhelm His the elder as the “spina vestibuli” (His, 1880), or vestibular spine. Based on a series of immunohistochemical studies, we introduced the term DMP as it more accurately reflects the anatomical and histological characteristics of the structure (Wessels et al., 2000).

Figure 3. The DMP is derived from the SHF and eventually muscularizes to form the floor of the oval fossa.

Figure 3

Open in a new tab

At 9.5ED (3A, 3C), the SHF that gives rise to the DMP is located just dorsally to two symmetrical mesocardial reflections that flank the orifice of the primitive pulmonary vein. By 10.5ED (3B, 3D), the DMP protrudes past these reflections, to the right of the developing pulmonary vein and into the common atrium. The precursor population that gives rise to the DMP (3C) as well as the DMP itself (3D) can be visualized by staining with the known SHF marker Isl1. The DMP, which is contiguous with the mesenchymal cap of the septum primum has fused with the iAVC (3E) at 11.5ED. In order to form the muscular base of the oval fossa, the DMP must myocardialize; at 11.5ED, the DMP is just beginning to undergo mesenchymal-to-myocardial transition and as such, Isl1 expression (3F) is beginning to decrease while expression of NKX2-5 (3G) is upregulated. SHF, second heart field; DMP, dorsal mesenchymal protrusion; FG, foregut; iAVC, inferior atrioventricular cushion; PuV, pulmonary vein; r-MR, right mesocardial reflection; l-MR, left mesocardial reflection

The DMP is Derived From the Second Heart Field

Two bilateral primary heart fields coalesce to form the linear heart tube (Abu-Issa and Kirby, 2007). Over the last decade, an additional or “second” cardiogenic mesodermal precursor population, located anteromedial to the primary heart field in the cardiac crescent, has been identified as an important component of heart development (Abu-Issa and Kirby, 2007; Kelly and Buckingham, 2002; Mjaatvedt et al., 2001; Moorman et al., 2007; Snarr et al., 2007a; Waldo et al., 2001; Zaffran and Kelly, 2012). This population, termed the second heart field (SHF), significantly contributes to the arterial pole of the developing heart (Abu-Issa and Kirby, 2007; Cai et al., 2003; Kelly and Buckingham, 2002; Mjaatvedt et al., 2001; Verzi et al., 2005; Waldo et al., 2001; Zaffran and Kelly, 2012).

While the significance of the SHF to arterial pole development has been established for many years, more recent studies have revealed that this population of cells is also involved in development of the DMP (Snarr et al., 2007a). Earlier cell fate studies demonstrated that the DMP, unlike the mesenchyme of the AV cushions and cap of the septum primum, is not derived from the endocardium (Snarr et al., 2007b). Subsequent work revealed expression of the SHF-marker Isl1 within the developing DMP (Figs 3C, D).

Relationship to other AV Septal Components

After cardiac looping is complete, an endothelial invagination into the dorsal mesocardium demarcates the future orifice of the pulmonary vein (Lamers and Moorman, 2002). This site is flanked by the mesocardial reflections, two symmetrical structures that are also referred to as the pulmonary ridges (Soufan et al., 2004; Webb et al., 1998; Wessels et al., 2000) (Fig. 3A). The SHF population that gives rise to the dorsal mesenchymal protrusion resides just dorsal to these mesocardial reflections, in between the foregut and atrium, and can be visualized as a highly proliferative, ISL1 expressing population of mesenchyme (Snarr et al., 2007a) (Fig. 3A).

Soon thereafter, around 10.5ED, this prong of mesenchyme, which is continuous with the mesenchymal cap of the septum primum cranially, protrudes into the common atrium, to the right of the developing pulmonary vein (Snarr et al., 2007b; Webb et al., 1998) (Figs. 3B, D). By 11.5ED, all mesenchymal components of the AV septal complex have formed and the stage is set for their interaction (Webb et al., 1998). At this point, the major AV cushions are in close proximity but have not yet fused. Therefore, it is the DMP alone that bridges the gap between two endocardially-derived tissues; it is continuous with the inferior AV cushion ventrally as well as the mesenchymal cap of the septum primum cranially (Snarr et al., 2007b; Wessels et al., 2000) (Fig. 4A). The cap, in turn, is contiguous with the superior AV cushion (Snarr et al., 2007b) (Fig. 4A). While all three mesenchymal tissues that contribute to the AV septal complex are in continuity and are beginning to fuse, the ostium primum has yet to fully close (Fig. 2D, D′).

Figure 4. Relationship of the DMP to other Septal Components and Potential Mechanisms Leading to its Maldevelopment.

Figure 4

Open in a new tab

Figure 4A depicts the relationship of the DMP to the other mesenchymal components of the AV septal complex at 11.5ED. The DMP (blue) is continuous with the mesenchymal cap (MC, red) of the septum primum cranially as well as the inferior atrioventricular cushion (iAVC, green) ventrally. The cap of the septum primum is also in continuity with the superior atrioventricular cushion (sAVC, yellow). Thus, the sAVC and iAVC, which have yet to fuse, are continuous through the mesenchymal cap of the septum primum and the DMP. At 13ED (4B), all components of the septal complex have fused and the DMP is wedged between the sAVC (yellow) and iAVC (green) dorsally. At this point, the mesenchyme derived from the cap of the septum primum cannot be distinguished from that of the sAVC. Figure 4C depicts the proliferation of SHF cells that ultimately give rise to the DMP at 9.5ED. The DMP may fail to properly form due to decreased proliferation of these cells (4D), increased apoptosis (4E), or premature mesenchymal-to-myocardialization (4F). Figures 4G– H demonstrate that the Mef2c-AHF-cre transgene is expressed within the developing DMP. DMP, dorsal mesenchymal protrusion; iAVC, inferior atrioventricular cushion; sAVC, superior atrioventricular cushion; rlAVC, right lateral atrioventricular cushion; RV, right ventricle; LV, left ventricle; FG, foregut

By 13ED, however, fusion of the major cushions, the DMP and the cap of the septum primum results in closure of the primary atrial and the formation of separate right and left valvar orifices (Snarr et al., 2007b). Using the Tie2-Cre/R26R reporter system, the DMP can be visualized within this mesenchymal mass of mostly endocardially-derived tissues as a lacZ-negative, non-endocardial-derived population of mesenchymal cells wedged between the superior and inferior AV cushions dorsally (Snarr et al., 2007b) (Fig. 4B). Around this developmental stage, the cells comprising the DMP begin to undergo a mesenchymal-to-myocardial differentiation which is associated with a decrease in the level of ISL1 expression and an increase in expression of NKX2-5 (Snarr et al., 2007a) (Figs. 3E–G). This mesenchymal-to-myocardial differentiation of SHF-derived cells which, within the developing heart, only occurs in cells derived from the DMP, is important for maturation of the AV septal complex as it is the mechanism by which the muscular base of the septum primum, the rim of the oval fossa, is formed (Snarr et al., 2007a).

Potential Mechanisms Leading to Abnormal Development of the DMP

Prior studies have clearly indicated that the DMP is an essential component of the AV septal complex and thus, AV septation, but the molecular mechanisms underpinning its proper development and maturation are not fully understood. What is known, however, is that within less than half a day of murine embryonic development, a relatively large population of SHF cells protrudes from the area surrounding the foregut, past the mesocardial reflections and into the common atrium (Snarr et al., 2007b; Webb et al., 1998). This rapid expansion of cells is associated with intense proliferation of the SHF cells that ultimately give rise to the DMP (Fig. 4C). If this precursor population fails to proliferate, then the DMP may be hypoplastic (Fig. 4D). If the DMP fails to form, or if its proportions are abnormally small, it will be unable to partition the dorsal-inferior region of the AV junction.

Aberrant apoptosis is another mechanism that could contribute to maldevelopment of the DMP. At 13ED, after the DMP has fused with the inferior AV cushion, TUNEL staining demonstrates that programmed cell death occurs at the interface of these two mesenchymal tissues and within the DMP itself (Snarr et al., 2007b). If apoptosis were to occur prematurely, either within the DMP or within its SHF precursor population, the contribution of the DMP to the AV septal complex may be absent or abnormally small (Fig. 4E). Such apoptosis may also contribute to the formation of the so-called vestibular atrial septal defect (Sharratt et al., 2003).

The exact mechanism(s) by which the DMP protrudes into the common atrium have yet to be elucidated but may involve cell migration. Collective cell migration, the process by which a group of cells is transported to a new location, is important for organogenesis as well as numerous other developmental processes. While both the distance these cells travel as well as the number of cells involved is highly variable, an extracellular cue is generally required to initiate cell migration (Affolter and Weijer, 2005). Growth factors, cytokines and/or proteins from the extracellular matrix can act as this stimulus and may originate from a neighboring cell, and/or the extracellular matrix. These cues then initiate intracellular signaling pathways, including Rho, Rac and other members of the small GTPase family. Small GTPases facilitate migration by stimulating extension of lamellipodia, turnover of cell-substrate adhesion, and cell body contraction (Allen et al., 1997; Nobes and Hall, 1995; Zhao et al., 2000). If the cue that stimulates migration is not released and/or received, or if any other step involved in this complicated process is altered, collective cell migration will not occur and therefore, incomplete cardiac septation will result.

NKX2-5 is widely expressed in the developing heart but notably, is absent from the DMP as this group of ISL1-positive cells protrudes into the atrial cavity. After the DMP has fused with the other components of the AV septal complex, the DMP begins to muscularize (Snarr et al., 2007a). At this point, although many cells of this SHF-derived population are still ISL1 positive, a significant number also express NKX2-5 (Snarr et al., 2007a). Eventually, ISL1 expression is completely abolished as the DMP terminally differentiates into a myocardial phenotype, thereby forming the muscular base of the septum primum (Snarr et al., 2007a). In order for the DMP to migrate from the area surrounding the foregut to its final position within the heart, it must maintain a mesenchymal phenotype. If the cells that form the DMP prematurely differentiate into a myocardial phenotype, they may be unable to migrate into the atrial cavity and contribute to the AV septal complex (Goddeeris et al., 2008) (Fig. 4F).

Mouse Models of DMP Maldevelopment

For many years, maldevelopment of the endocardial cushions was thought to be the primary etiology of atrioventricular septal defects. More recently, however, analysis of AVSDs in mouse models, as well as observations in human fetuses with Down Syndrome (Blom et al., 2003), have revealed that defective development of the DMP also contributes to the pathogenesis of these defects.

AVSD is part of the spectrum of cardiac malformations observed in the Trisomy 16 mouse. (Epstein et al., 1985; Miyabara et al., 1982; Waller et al., 2000; Webb et al., 1999). Hypoplasia of the DMP has been implicated in the pathogenesis of this malformation in this mouse model (Snarr et al., 2007a; Webb et al., 1999). Although the mesenchymal cap of the septum primum is both substantial and continuous with the superior endocardial cushion in these mutants (Webb et al., 1999), septation is deficient caudally, where the DMP normally bridges the space between the cap and the inferior endocardial cushion. Consequently, these mice demonstrate AVSD as well underdevelopment of the septum primum. While the process of septation is obviously abnormal in trisomic specimens, the bridging leaflets of the common atrioventricular valve can be fused to one another, suggesting that fusion of the superior and inferior endocardial cushions is not necessarily aberrant in these mice (Webb et al., 1999). The cushions are, however, abnormal with respect to shape and volume and thus, may still play a role in the pathogenesis of AVSDs in the Ts16 mouse (Hiltgen et al., 1996; Webb et al., 1996; Webb et al., 1999).

Although polysomy in both the mouse (Ts16) and human (Ts21) is associated with AVSD, abnormalities involving the ventricular outflow tracts are frequently observed in only the murine version and include common arterial trunk, double outlet right ventricle, interrupted aortic arch and overriding aorta (Waller et al., 2000). Furthermore, the lesions seen in man and mouse differ with respect to valvar morphology as well as the connection between the atrioventricular junction and the ventricles (Anderson et al., 1998b).

The additional cardiac lesions observed in the murine model of Down syndrome may result from fundamental differences in which genes are triplicated. For example, the most widely used murine model of Down syndrome, Ts65Dn, is trisomic for only part of the Hsa21 syntenic region on Mmu16 (Li et al., 2007). Furthermore, the Ts65Dn mouse is trisomic for a region on Mmu17 that does not correspond to any region on Hsa21(Li et al., 2007). Regardless of whether the Ts16 mouse is a perfect model for studying cardiac malformations observed in the context of Down’s syndrome, morphological analysis of this transgenic mouse has provided invaluable insight into the role of the DMP in the pathogenesis of AVSD.

Recently, the Morrisey lab described defective development of the DMP and resultant AVSD in Wnt2−/− mutants (Tian et al., 2010). Prior work had demonstrated that Wnt signals through β-catenin to maintain ISL1 positive SHF progenitors at the anterior, or arterial, pole of the heart (Ai et al., 2007; Cohen et al., 2007; Kwon et al., 2007; Lin et al., 2007). Through subsequent experiments, it was determined that the ligand WNT2 regulates expansion of SHF progenitors at the venous pole of the heart, through a β-catenin dependent signaling mechanism (Tian et al., 2010). Importantly, this ligand is expressed in a manner that is spatiotemporally consistent with development of the DMP but is not expressed within the epicardium, endocardium, or myocardium of the AV canal or ventricles (Tian et al., 2010). Global deletion of Wnt2 leads to diminished numbers of ISL1+ SHF progenitors and reduced proliferation within the DMP precursor population; consequently Wnt2−/− mutants demonstrate hypoplasia of the DMP and AVSD (Tian et al., 2010). Interestingly, treatment of pregnant females with LiCl, an activator of Wnt/β-catenin signaling (Klein and Melton, 1996), at a stage consistent with DMP development rescues DMP proliferation in Wnt2−/− mutants, as well as the AVSD phenotype, and increases survival by approximately 5-fold (Tian et al., 2010). These results indicate that Wnt2 signaling is required for proliferation and expansion of the SHF population that gives rise to the DMP and confirms what had been described in the Ts16 mouse, that maldevelopment of the DMP is associated with AVSD.

In addition to defective development of the atrial septum, Pdgfrα−/− embryos also display abnormal development of the DMP. PDGFR-α is expressed within the SHF at the venous pole of the heart and is proposed to interact with NKX2-5 (Bax et al., 2010; Prall et al., 2007). Consistently, Pdgfrα−/− embryos demonstrate a hypoplastic DMP and increased expression of NKX2-5, suggesting that loss of PDGFR-α results in premature myocardial differentiation of the DMP (Stennard et al., 2003).

Using global knockout mice to study molecular mechanisms governing DMP development is often hampered by early embryonic lethality, which may be extracardiac in origin. Furthermore, due to lack of specificity, global knockouts are less informative for understanding molecular mechanisms that drive formation of a particular structure; if the DMP fails to develop in a traditional knockout, is it due to altered signaling within the DMP itself or does it stem from altered signaling in a nearby structure (i.e. mesocardial reflections) that contributes to DMP development through paracrine signaling? Many of these issues have been circumvented by the advent of site-specific recombinase technology, which has enabled the creation of transgenic mice whose genomes may be altered in a tissue and/or temporal-specific manner.

Gene expression within the SHF-derived DMP can be manipulated specifically through use of the Mef2c-AHF-Cre mouse, a transgenic mouse in which cre-recombinase is expressed within the SHF (Figures 4G and G′). This approach was used by Goddeeris et al. to determine the role of sonic hedgehog (Shh) signaling in the developing DMP (Goddeeris et al., 2008). Prior studies have implicated this signaling pathway in cardiac septation as Shh−/− mutants demonstrate AVSD (Goddeeris et al., 2008). Furthermore, loss of Shh from the pharyngeal endoderm, which is juxtaposed to the dorsal mesocardium during development of the DMP, also results in AVSD (Goddeeris et al., 2007).

First, to determine whether AV septation was dependent upon Shh signaling within the myocardium and/or endocardium, the Shh receptor smoothened (Rajagopal et al.) was conditionally deleted respectively, through generation of TnT-cre;Smoflox/− and/or Tie2-cre;Smoflox/− mutants. These mutants did not demonstrate defective septation, indicating that AV septation is not dependent on myocardial and/or endocardial Shh signaling (Goddeeris et al., 2008). However, AVSDs are observed upon deletion of Smo from the developing DMP (Figs. 5E– H′), further illustrating the important role this structure plays in AV septation (Goddeeris et al., 2008).

While the contribution of the DMP to the AV septal complex at 11.5ED was less extensive in Mef2c-AHF-cre; Smoflox/− mutants when compared to controls, neither proliferation nor apoptosis within the dorsal mesocardium differed significantly between groups (Goddeeris et al., 2008). Eventually the DMP muscularizes, but only after it has migrated into the atrial cavity and fused with the other components of the AV septal complex (Snarr et al., 2007a). Interestingly, Goodeeris et al. found ectopic myocardialization within the dorsal mesocardium of Mef2c-AHF-cre; Smoflox/− mutants. Furthermore, they describe decreased migration of explanted dorsal mesocardium upon treatment with the small molecular Smo inhibitor cyclopamine (Goddeeris et al., 2008). These results indicate that Shh signaling may be required to prevent premature mesenchymal-to-myocardial transition of the DMP and/or its proper migration.

Summary

Over the course of fetal development, the heart transforms from little more than a linear tube to a complex, four-chambered organ. Partitioning of these chambers requires the proper spatiotemporal formation, interaction, and fusion of several structures including the septum primum with its mesenchymal cap, the septum secundum, the atrioventricular cushions, and the dorsal mesenchymal protrusion.

The most severe consequence of abnormal septation is the AVSD. For decades, perturbation of endocardial cushion development was thought to be the primary etiology of these malformations, so much so that the term “endocardial cushion defect” has become synonymous with AVSD. A growing body of evidence, however, has demonstrated that perturbation of the DMP can also lead to AVSD. It is easy to understand how this structure may have been overlooked; the anatomical features of the venous pole of the developing heart are often difficult to identify and/or interpret, even in normal embryos. As our ability to manipulate the murine genome and analyze the 3-dimensional architecture of the developing heart improves, the role this structure plays in both future, and established, mouse models of AVSD requires further study as do the molecular mechanisms underpinning its proper formation and maturation.

Acknowledgments

The authors would like to thank Dr. Robert Anderson for helpful discussions during preparation of the manuscript. The authors also would like to acknowledge the financial support by grants 3P20RR016434-10S1 (A.W.), NIH-NHLBI grant R01HL084285 (L.E.B., A.W.), NIH-5T32-GM008716-12 (L.E.B.), American Heart Association grants 09GRNT2060075 (A.W.) and 11PRE7310036 (L.E.B.), and a Student Elective Prize from the Wellcome Trust (J.K.).

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

References

  1. Abu-Issa R, Kirby ML. Heart field: from mesoderm to heart tube. Annu Rev Cell Dev Biol. 2007;23:45–68. doi: 10.1146/annurev.cellbio.23.090506.123331. [DOI] [PubMed] [Google Scholar]
  2. Affolter M, Weijer CJ. Signaling to cytoskeletal dynamics during chemotaxis. Dev Cell. 2005;9:19–34. doi: 10.1016/j.devcel.2005.06.003. [DOI] [PubMed] [Google Scholar]
  3. Ai D, Fu X, Wang J, Lu MF, Chen L, Baldini A, Klein WH, Martin JF. Canonical Wnt signaling functions in second heart field to promote right ventricular growth. Proc Natl Acad Sci U S A. 2007;104:9319–9324. doi: 10.1073/pnas.0701212104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Allen WE, Jones GE, Pollard JW, Ridley AJ. Rho, Rac and Cdc42 regulate actin organization and cell adhesion in macrophages. J Cell Sci. 1997;110 (Pt 6):707–720. doi: 10.1242/jcs.110.6.707. [DOI] [PubMed] [Google Scholar]
  5. Anderson RH, Brown NA, Moorman AF. Development and structures of the venous pole of the heart. Dev Dyn. 2006;235:2–9. doi: 10.1002/dvdy.20578. [DOI] [PubMed] [Google Scholar]
  6. Anderson RH, Ho SY, Falcao S, Daliento L, Rigby ML. The diagnostic features of atrioventricular septal defect with common atrioventricular junction. Cardiol Young. 1998a;8:33–49. doi: 10.1017/s1047951100004613. [DOI] [PubMed] [Google Scholar]
  7. Anderson RH, Webb S, Brown NA. The mouse with trisomy 16 as a model of human hearts with common atrioventricular junction. Cardiovasc Res. 1998b;39:155–164. doi: 10.1016/s0008-6363(98)00037-6. [DOI] [PubMed] [Google Scholar]
  8. Anzola GP, Magoni M, Guindani M, Rozzini L, Dalla Volta G. Potential source of cerebral embolism in migraine with aura: a transcranial Doppler study. Neurology. 1999;52:1622–1625. doi: 10.1212/wnl.52.8.1622. [DOI] [PubMed] [Google Scholar]
  9. Armour CM, Allanson JE. Further delineation of cardio-facio-cutaneous syndrome: clinical features of 38 individuals with proven mutations. J Med Genet. 2008;45:249–254. doi: 10.1136/jmg.2007.054460. [DOI] [PubMed] [Google Scholar]
  10. Bamforth JS, Leonard CO, Chodirker BN, Chitayat D, Gritter HL, Evans JA, Keena B, Pantzar T, Friedman JM, Hall JG. Congenital diaphragmatic hernia, coarse facies, and acral hypoplasia: Fryns syndrome. Am J Med Genet. 1989;32:93–99. doi: 10.1002/ajmg.1320320120. [DOI] [PubMed] [Google Scholar]
  11. Barclay A, Barcroft J, Barron D, Franklin J. A radiographic demonstration of the circulation through the heart in the adult and in the foetus, and the identification of the ductus arteriosus. The British Journal of Radiology. 1939;12:505– 518. [Google Scholar]
  12. Barlow GM, Chen XN, Shi ZY, Lyons GE, Kurnit DM, Celle L, Spinner NB, Zackai E, Pettenati MJ, Van Riper AJ, Vekemans MJ, Mjaatvedt CH, Korenberg JR. Down syndrome congenital heart disease: a narrowed region and a candidate gene. Genet Med. 2001;3:91–101. doi: 10.1097/00125817-200103000-00002. [DOI] [PubMed] [Google Scholar]
  13. Basson CT, Bachinsky DR, Lin RC, Levi T, Elkins JA, Soults J, Grayzel D, Kroumpouzou E, Traill TA, Leblanc-Straceski J, Renault B, Kucherlapati R, Seidman JG, Seidman CE. Mutations in human TBX5 [corrected] cause limb and cardiac malformation in Holt-Oram syndrome. Nat Genet. 1997;15:30–35. doi: 10.1038/ng0197-30. [DOI] [PubMed] [Google Scholar]
  14. Bax NA, Bleyl SB, Gallini R, Wisse LJ, Hunter J, Van Oorschot AA, Mahtab EA, Lie-Venema H, Goumans MJ, Betsholtz C, Gittenberger-de Groot AC. Cardiac malformations in Pdgfralpha mutant embryos are associated with increased expression of WT1 and Nkx2.5 in the second heart field. Dev Dyn. 2010;239:2307–2317. doi: 10.1002/dvdy.22363. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Benson DW, Silberbach GM, Kavanaugh-McHugh A, Cottrill C, Zhang Y, Riggs S, Smalls O, Johnson MC, Watson MS, Seidman JG, Seidman CE, Plowden J, Kugler JD. Mutations in the cardiac transcription factor NKX2.5 affect diverse cardiac developmental pathways. J Clin Invest. 1999;104:1567–1573. doi: 10.1172/JCI8154. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Bharati S, Lev M. The spectrum of common atrioventricular orifice (canal) Am Heart J. 1973;86:553–561. doi: 10.1016/0002-8703(73)90148-8. [DOI] [PubMed] [Google Scholar]
  17. Biben C, Weber R, Kesteven S, Stanley E, McDonald L, Elliott DA, Barnett L, Koentgen F, Robb L, Feneley M, Harvey RP. Cardiac septal and valvular dysmorphogenesis in mice heterozygous for mutations in the homeobox gene Nkx2-5. Circ Res. 2000;87:888–895. doi: 10.1161/01.res.87.10.888. [DOI] [PubMed] [Google Scholar]
  18. Bisping E, Ikeda S, Kong SW, Tarnavski O, Bodyak N, McMullen JR, Rajagopal S, Son JK, Ma Q, Springer Z, Kang PM, Izumo S, Pu WT. Gata4 is required for maintenance of postnatal cardiac function and protection from pressure overload-induced heart failure. Proc Natl Acad Sci U S A. 2006;103:14471–14476. doi: 10.1073/pnas.0602543103. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Blom NA, Ottenkamp J, Wenink AG, Gittenberger-de Groot AC. Deficiency of the vestibular spine in atrioventricular septal defects in human fetuses with down syndrome. Am J Cardiol. 2003;91:180–184. doi: 10.1016/s0002-9149(02)03106-5. [DOI] [PubMed] [Google Scholar]
  20. Brown DD, Martz SN, Binder O, Goetz SC, Price BM, Smith JC, Conlon FL. Tbx5 and Tbx20 act synergistically to control vertebrate heart morphogenesis. Development. 2005;132:553–563. doi: 10.1242/dev.01596. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Burch M, Sharland M, Shinebourne E, Smith G, Patton M, McKenna W. Cardiologic abnormalities in Noonan syndrome: phenotypic diagnosis and echocardiographic assessment of 118 patients. J Am Coll Cardiol. 1993;22:1189–1192. doi: 10.1016/0735-1097(93)90436-5. [DOI] [PubMed] [Google Scholar]
  22. Cai CL, Liang X, Shi Y, Chu PH, Pfaff SL, Chen J, Evans S. Isl1 identifies a cardiac progenitor population that proliferates prior to differentiation and contributes a majority of cells to the heart. Dev Cell. 2003;5:877–889. doi: 10.1016/s1534-5807(03)00363-0. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Calabro A, Taraschi A, Lungarotti MS, Ferdinandi A. Familial situs inversus and congenital heart defects. Am J Med Genet. 1988;31:689–690. doi: 10.1002/ajmg.1320310325. [DOI] [PubMed] [Google Scholar]
  24. Camenisch TD, Spicer AP, Brehm-Gibson T, Biesterfeldt J, Augustine ML, Calabro A, Jr, Kubalak S, Klewer SE, McDonald JA. Disruption of hyaluronan synthase-2 abrogates normal cardiac morphogenesis and hyaluronan-mediated transformation of epithelium to mesenchyme. J Clin Invest. 2000;106:349–360. doi: 10.1172/JCI10272. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Carmi R, Boughman JA, Ferencz C. Endocardial cushion defect: further studies of “isolated” versus “syndromic” occurrence. Am J Med Genet. 1992;43:569–575. doi: 10.1002/ajmg.1320430313. [DOI] [PubMed] [Google Scholar]
  26. Caton AR, Bell EM, Druschel CM, Werler MM, Lin AE, Browne ML, McNutt LA, Romitti PA, Mitchell AA, Olney RS, Correa A. Antihypertensive medication use during pregnancy and the risk of cardiovascular malformations. Hypertension. 2009;54:63–70. doi: 10.1161/HYPERTENSIONAHA.109.129098. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Ching YH, Ghosh TK, Cross SJ, Packham EA, Honeyman L, Loughna S, Robinson TE, Dearlove AM, Ribas G, Bonser AJ, Thomas NR, Scotter AJ, Caves LS, Tyrrell GP, Newbury-Ecob RA, Munnich A, Bonnet D, Brook JD. Mutation in myosin heavy chain 6 causes atrial septal defect. Nat Genet. 2005;37:423–428. doi: 10.1038/ng1526. [DOI] [PubMed] [Google Scholar]
  28. Choong CK, Calvert PA, Falter F, Mathur R, Appleton D, Wells FC, Schofield PM, Crawford R. Life-threatening impending paradoxical embolus caught “red-handed”: successful management by multidisciplinary team approach. J Thorac Cardiovasc Surg. 2008;136:527–528. e528. doi: 10.1016/j.jtcvs.2007.10.090. [DOI] [PubMed] [Google Scholar]
  29. Clark EB, Kugler JD. Preoperative secundum atrial septal defect with coexisting sinus node and atrioventricular node dysfunction. Circulation. 1982;65:976–980. doi: 10.1161/01.cir.65.5.976. [DOI] [PubMed] [Google Scholar]
  30. Cleves MA, Malik S, Yang S, Carter TC, Hobbs CA. Maternal urinary tract infections and selected cardiovascular malformations. Birth Defects Res A Clin Mol Teratol. 2008;82:464–473. doi: 10.1002/bdra.20460. [DOI] [PubMed] [Google Scholar]
  31. Cohen ED, Wang Z, Lepore JJ, Lu MM, Taketo MM, Epstein DJ, Morrisey EE. Wnt/beta-catenin signaling promotes expansion of Isl-1-positive cardiac progenitor cells through regulation of FGF signaling. J Clin Invest. 2007;117:1794–1804. doi: 10.1172/JCI31731. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Croonen EA, van der Burgt I, Kapusta L, Draaisma JM. Electrocardiography in Noonan syndrome PTPN11 gene mutation--phenotype characterization. Am J Med Genet A. 2008;146:350–353. doi: 10.1002/ajmg.a.32140. [DOI] [PubMed] [Google Scholar]
  33. Cunningham ET, Jr, Eliott D, Miller NR, Maumenee IH, Green WR. Familial Axenfeld-Rieger anomaly, atrial septal defect, and sensorineural hearing loss: a possible new genetic syndrome. Arch Ophthalmol. 1998;116:78–82. doi: 10.1001/archopht.116.1.78. [DOI] [PubMed] [Google Scholar]
  34. Davis C. Development of the human heart from its first appearance to the stage found in embryos of twenty paired somites. contrib embryol. 1927;19:245– 284. [Google Scholar]
  35. de Lange FJ, Moorman AF, Anderson RH, Manner J, Soufan AT, de Gier-de Vries C, Schneider MD, Webb S, van den Hoff MJ, Christoffels VM. Lineage and morphogenetic analysis of the cardiac valves. Circ Res. 2004;95:645–654. doi: 10.1161/01.RES.0000141429.13560.cb. [DOI] [PubMed] [Google Scholar]
  36. DeVlaming A, Sauls K, Hajdu Z, Visconti R, Mehesh A, Levine R, Slaugenhaupt S, Hagege A, Chester A, Markward R, Norris R. Atrioventricular valve development: new perspectives on an old theme. Differentiation. 2012 doi: 10.1016/j.diff.2012.04.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Douglas YL, Mahtab EA, Jongbloed MR, Uhrin P, Zaujec J, Binder BR, Schalij MJ, Poelmann RE, Deruiter MC, Gittenberger-de Groot AC. Pulmonary vein, dorsal atrial wall and atrial septum abnormalities in podoplanin knockout mice with disturbed posterior heart field contribution. Pediatr Res. 2009;65:27–32. doi: 10.1203/PDR.0b013e31818bc11a. [DOI] [PubMed] [Google Scholar]
  38. Dowson A, Mullen MJ, Peatfield R, Muir K, Khan AA, Wells C, Lipscombe SL, Rees T, De Giovanni JV, Morrison WL, Hildick-Smith D, Elrington G, Hillis WS, Malik IS, Rickards A. Migraine Intervention With STARFlex Technology (MIST) trial: a prospective, multicenter, double-blind, sham-controlled trial to evaluate the effectiveness of patent foramen ovale closure with STARFlex septal repair implant to resolve refractory migraine headache. Circulation. 2008;117:1397–1404. doi: 10.1161/CIRCULATIONAHA.107.727271. [DOI] [PubMed] [Google Scholar]
  39. Drake CJ, Wessels A, Trusk T, Little CD. Elevated vascular endothelial cell growth factor affects mesocardial morphogenesis and inhibits normal heart bending. Dev Dyn. 2006;235:10–18. doi: 10.1002/dvdy.20539. [DOI] [PubMed] [Google Scholar]
  40. Drumheller T, McGillivray BC, Behrner D, MacLeod P, McFadden DE, Roberson J, Venditti C, Chorney K, Chorney M, Smith DI. Precise localisation of 3p25 breakpoints in four patients with the 3p-syndrome. J Med Genet. 1996;33:842–847. doi: 10.1136/jmg.33.10.842. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Edelstone DI, Rudolph AM. Preferential streaming of ductus venosus blood to the brain and heart in fetal lambs. Am J Physiol. 1979;237:H724–729. doi: 10.1152/ajpheart.1979.237.6.H724. [DOI] [PubMed] [Google Scholar]
  42. Engelfriet PM, Duffels MG, Moller T, Boersma E, Tijssen JG, Thaulow E, Gatzoulis MA, Mulder BJ. Pulmonary arterial hypertension in adults born with a heart septal defect: the Euro Heart Survey on adult congenital heart disease. Heart. 2007;93:682–687. doi: 10.1136/hrt.2006.098848. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Epstein CJ, Cox DR, Epstein LB. Mouse trisomy 16: an animal model of human trisomy 21 (Down syndrome) Ann N Y Acad Sci. 1985;450:157–168. doi: 10.1111/j.1749-6632.1985.tb21490.x. [DOI] [PubMed] [Google Scholar]
  44. Ferencz C, Correa-Villasenor A, Loffredo C. Genetic and Environmental Risk Factors of Major Cardiovascular Malformations: The Baltimore-Washington Infant Study - 1981–1989. Futura Publishing Co. Inc; Armonk, NY: 1997. [Google Scholar]
  45. Freeman SB, Taft LF, Dooley KJ, Allran K, Sherman SL, Hassold TJ, Khoury MJ, Saker DM. Population-based study of congenital heart defects in Down syndrome. Am J Med Genet. 1998;80:213–217. [PubMed] [Google Scholar]
  46. Garg V, Kathiriya IS, Barnes R, Schluterman MK, King IN, Butler CA, Rothrock CR, Eapen RS, Hirayama-Yamada K, Joo K, Matsuoka R, Cohen JC, Srivastava D. GATA4 mutations cause human congenital heart defects and reveal an interaction with TBX5. Nature. 2003;424:443–447. doi: 10.1038/nature01827. [DOI] [PubMed] [Google Scholar]
  47. Gilboa SM, Correa A, Botto LD, Rasmussen SA, Waller DK, Hobbs CA, Cleves MA, Riehle-Colarusso TJ. Association between prepregnancy body mass index and congenital heart defects. Am J Obstet Gynecol. 2010;202:51 e51–51 e10. doi: 10.1016/j.ajog.2009.08.005. [DOI] [PubMed] [Google Scholar]
  48. Glauser TA, Zackai E, Weinberg P, Clancy R. Holt-Oram syndrome associated with the hypoplastic left heart syndrome. Clin Genet. 1989;36:69–72. doi: 10.1111/j.1399-0004.1989.tb03369.x. [DOI] [PubMed] [Google Scholar]
  49. Goddeeris MM, Rho S, Petiet A, Davenport CL, Johnson GA, Meyers EN, Klingensmith J. Intracardiac septation requires hedgehog-dependent cellular contributions from outside the heart. Development. 2008;135:1887–1895. doi: 10.1242/dev.016147. [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Goddeeris MM, Schwartz R, Klingensmith J, Meyers EN. Independent requirements for Hedgehog signaling by both the anterior heart field and neural crest cells for outflow tract development. Development. 2007;134:1593–1604. doi: 10.1242/dev.02824. [DOI] [PubMed] [Google Scholar]
  51. Green EK, Priestley MD, Waters J, Maliszewska C, Latif F, Maher ER. Detailed mapping of a congenital heart disease gene in chromosome 3p25. J Med Genet. 2000;37:581–587. doi: 10.1136/jmg.37.8.581. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Gruber PJ, Kubalak SW, Pexieder T, Sucov HM, Evans RM, Chien KR. RXR alpha deficiency confers genetic susceptibility for aortic sac, conotruncal, atrioventricular cushion, and ventricular muscle defects in mice. J Clin Invest. 1996;98:1332–1343. doi: 10.1172/JCI118920. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Gutierrez-Roelens I, De Roy L, Ovaert C, Sluysmans T, Devriendt K, Brunner HG, Vikkula M. A novel CSX/NKX2-5 mutation causes autosomal-dominant AV block: are atrial fibrillation and syncopes part of the phenotype? Eur J Hum Genet. 2006;14:1313–1316. doi: 10.1038/sj.ejhg.5201702. [DOI] [PubMed] [Google Scholar]
  54. Hiltgen GG, Markwald RR, Litke LL. Morphogenetic alterations during endocardial cushion development in the trisomy 16 (Down syndrome) mouse. Pediatr Cardiol. 1996;17:21–30. doi: 10.1007/BF02505807. [DOI] [PubMed] [Google Scholar]
  55. Hirayama-Yamada K, Kamisago M, Akimoto K, Aotsuka H, Nakamura Y, Tomita H, Furutani M, Imamura S, Takao A, Nakazawa M, Matsuoka R. Phenotypes with GATA4 or NKX2.5 mutations in familial atrial septal defect. Am J Med Genet A. 2005;135:47–52. doi: 10.1002/ajmg.a.30684. [DOI] [PubMed] [Google Scholar]
  56. His W. Die Area interposita, die Eustachi’sche Klappe und die Spina vestibuli. Leipzig, Germany: 1880. [Google Scholar]
  57. Holt M, Oram S. Familial heart disease with skeletal malformations. Br Heart J. 1960;22:236–242. doi: 10.1136/hrt.22.2.236. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Hoo JJ, Kreiter M, Halverson N, Perszyk A. 3C (cranio-cerebello-cardiac) syndrome: a recently delineated and easily recognizable congenital malformation syndrome. Am J Med Genet. 1994;52:66–69. doi: 10.1002/ajmg.1320520113. [DOI] [PubMed] [Google Scholar]
  59. Inga A, Reamon-Buettner SM, Borlak J, Resnick MA. Functional dissection of sequence-specific NKX2-5 DNA binding domain mutations associated with human heart septation defects using a yeast-based system. Hum Mol Genet. 2005;14:1965–1975. doi: 10.1093/hmg/ddi202. [DOI] [PubMed] [Google Scholar]
  60. Isaac A, Sargent MG, Cooke J. Control of vertebrate left-right asymmetry by a snail-related zinc finger gene. Science. 1997;275:1301–1304. doi: 10.1126/science.275.5304.1301. [DOI] [PubMed] [Google Scholar]
  61. Issekutz KA, Graham JM, Jr, Prasad C, Smith IM, Blake KD. An epidemiological analysis of CHARGE syndrome: preliminary results from a Canadian study. Am J Med Genet A. 2005;133A:309–317. doi: 10.1002/ajmg.a.30560. [DOI] [PubMed] [Google Scholar]
  62. Jacobs JP, Burke RP, Quintessenza JA, Mavroudis C. Congenital Heart Surgery Nomenclature and Database Project: atrioventricular canal defect. Ann Thorac Surg. 2000;69:S36–43. doi: 10.1016/s0003-4975(99)01235-7. [DOI] [PubMed] [Google Scholar]
  63. Jegatheeswaran A, Pizarro C, Caldarone CA, Cohen MS, Baffa JM, Gremmels DB, Mertens L, Morell VO, Williams WG, Blackstone EH, McCrindle BW, Overman DM. Echocardiographic definition and surgical decision-making in unbalanced atrioventricular septal defect: a Congenital Heart Surgeons’ Society multiinstitutional study. Circulation. 2010;122:S209–215. doi: 10.1161/CIRCULATIONAHA.109.925636. [DOI] [PubMed] [Google Scholar]
  64. Kallen K. Maternal smoking and congenital heart defects. Eur J Epidemiol. 1999;15:731–737. doi: 10.1023/a:1007671631188. [DOI] [PubMed] [Google Scholar]
  65. Kamesui T, Seki M, Tsubota M, Endo M, Watanabe S, Sato H. A case of Ellis-van Creveld syndrome with partial atrioventricular septal defect and double orifice mitral valve. Nihon Kyobu Geka Gakkai Zasshi. 1997;45:589–593. [PubMed] [Google Scholar]
  66. Kelly RG, Buckingham ME. The anterior heart-forming field: voyage to the arterial pole of the heart. Trends Genet. 2002;18:210–216. doi: 10.1016/s0168-9525(02)02642-2. [DOI] [PubMed] [Google Scholar]
  67. Kim RJ, Girardi LN. “Lots of clots”: multiple thromboemboli including a huge paradoxical embolus in a 29-year old man. Int J Cardiol. 2008;129:e50–52. doi: 10.1016/j.ijcard.2007.06.116. [DOI] [PubMed] [Google Scholar]
  68. Kirk EP, Sunde M, Costa MW, Rankin SA, Wolstein O, Castro ML, Butler TL, Hyun C, Guo G, Otway R, Mackay JP, Waddell LB, Cole AD, Hayward C, Keogh A, Macdonald P, Griffiths L, Fatkin D, Sholler GF, Zorn AM, Feneley MP, Winlaw DS, Harvey RP. Mutations in cardiac T-box factor gene TBX20 are associated with diverse cardiac pathologies, including defects of septation and valvulogenesis and cardiomyopathy. Am J Hum Genet. 2007;81:280–291. doi: 10.1086/519530. [DOI] [PMC free article] [PubMed] [Google Scholar]
  69. Klein PS, Melton DA. A molecular mechanism for the effect of lithium on development. Proc Natl Acad Sci U S A. 1996;93:8455–8459. doi: 10.1073/pnas.93.16.8455. [DOI] [PMC free article] [PubMed] [Google Scholar]
  70. Kozma C, Slavotinek AM, Meck JM. Segregation of a t(1;3) translocation in multiple affected family members with both types of adjacent-1 segregants. Am J Med Genet A. 2004;124A:118–128. doi: 10.1002/ajmg.a.20332. [DOI] [PubMed] [Google Scholar]
  71. Kruithof B, Duim S, Moerkamp A, Goumans M. TGFbeta and BMP signaling in cardiac cushion formation; lessons from mice and chicken. Differentiation. 2012 doi: 10.1016/j.diff.2012.04.003. [DOI] [PubMed] [Google Scholar]
  72. Kwon C, Arnold J, Hsiao EC, Taketo MM, Conklin BR, Srivastava D. Canonical Wnt signaling is a positive regulator of mammalian cardiac progenitors. Proc Natl Acad Sci U S A. 2007;104:10894–10899. doi: 10.1073/pnas.0704044104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  73. Lakkis MM, Epstein JA. Neurofibromin modulation of ras activity is required for normal endocardial-mesenchymal transformation in the developing heart. Development. 1998;125:4359–4367. doi: 10.1242/dev.125.22.4359. [DOI] [PubMed] [Google Scholar]
  74. Lamers WH, Moorman AF. Cardiac septation: a late contribution of the embryonic primary myocardium to heart morphogenesis. Circ Res. 2002;91:93–103. doi: 10.1161/01.res.0000027135.63141.89. [DOI] [PubMed] [Google Scholar]
  75. Levin M, Johnson RL, Stern CD, Kuehn M, Tabin C. A molecular pathway determining left-right asymmetry in chick embryogenesis. Cell. 1995;82:803–814. doi: 10.1016/0092-8674(95)90477-8. [DOI] [PubMed] [Google Scholar]
  76. Li QY, Newbury-Ecob RA, Terrett JA, Wilson DI, Curtis AR, Yi CH, Gebuhr T, Bullen PJ, Robson SC, Strachan T, Bonnet D, Lyonnet S, Young ID, Raeburn JA, Buckler AJ, Law DJ, Brook JD. Holt-Oram syndrome is caused by mutations in TBX5, a member of the Brachyury (T) gene family. Nat Genet. 1997;15:21–29. doi: 10.1038/ng0197-21. [DOI] [PubMed] [Google Scholar]
  77. Li Z, Yu T, Morishima M, Pao A, LaDuca J, Conroy J, Nowak N, Matsui S, Shiraishi I, Yu YE. Duplication of the entire 22.9 Mb human chromosome 21 syntenic region on mouse chromosome 16 causes cardiovascular and gastrointestinal abnormalities. Hum Mol Genet. 2007;16:1359–1366. doi: 10.1093/hmg/ddm086. [DOI] [PubMed] [Google Scholar]
  78. Lin AE, Pober BR, Mullen MP, Slavotinek AM. Cardiovascular malformations in Fryns syndrome: is there a pathogenic role for neural crest cells? Am J Med Genet A. 2005;139:186–193. doi: 10.1002/ajmg.a.31023. [DOI] [PubMed] [Google Scholar]
  79. Lin L, Cui L, Zhou W, Dufort D, Zhang X, Cai CL, Bu L, Yang L, Martin J, Kemler R, Rosenfeld MG, Chen J, Evans SM. Beta-catenin directly regulates Islet1 expression in cardiovascular progenitors and is required for multiple aspects of cardiogenesis. Proc Natl Acad Sci U S A. 2007;104:9313–9318. doi: 10.1073/pnas.0700923104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  80. Lind J, Wegelius C. Angiocardiographic studies on the human foetal circulation; a preliminary report. Pediatrics. 1949;4:391–400. [PubMed] [Google Scholar]
  81. Liu C, Liu W, Palie J, Lu MF, Brown NA, Martin JF. Pitx2c patterns anterior myocardium and aortic arch vessels and is required for local cell movement into atrioventricular cushions. Development. 2002;129:5081–5091. doi: 10.1242/dev.129.21.5081. [DOI] [PubMed] [Google Scholar]
  82. Loffredo CA, Hirata J, Wilson PD, Ferencz C, Lurie IW. Atrioventricular septal defects: possible etiologic differences between complete and partial defects. Teratology. 2001a;63:87–93. doi: 10.1002/1096-9926(200102)63:2<87::AID-TERA1014>3.0.CO;2-5. [DOI] [PubMed] [Google Scholar]
  83. Loffredo CA, Wilson PD, Ferencz C. Maternal diabetes: an independent risk factor for major cardiovascular malformations with increased mortality of affected infants. Teratology. 2001b;64:98–106. doi: 10.1002/tera.1051. [DOI] [PubMed] [Google Scholar]
  84. Mahtab EA, Vicente-Steijn R, Hahurij ND, Jongbloed MR, Wisse LJ, DeRuiter MC, Uhrin P, Zaujec J, Binder BR, Schalij MJ, Poelmann RE, Gittenberger-de Groot AC. Podoplanin deficient mice show a RhoA-related hypoplasia of the sinus venosus myocardium including the sinoatrial node. Dev Dyn. 2009;238:183–193. doi: 10.1002/dvdy.21819. [DOI] [PubMed] [Google Scholar]
  85. Malik S, Cleves MA, Honein MA, Romitti PA, Botto LD, Yang S, Hobbs CA. Maternal smoking and congenital heart defects. Pediatrics. 2008;121:e810–816. doi: 10.1542/peds.2007-1519. [DOI] [PubMed] [Google Scholar]
  86. Manasek FJ. Sulfated extracellular matrix production in the embryonic heart and adjacent tissues. J Exp Zool. 1970;174:415–439. doi: 10.1002/jez.1401740406. [DOI] [PubMed] [Google Scholar]
  87. Manasek FJ. The extracellular matrix: a dynamic component of the developing embryo. Curr Top Dev Biol. 1975;10:35–102. doi: 10.1016/s0070-2153(08)60038-1. [DOI] [PubMed] [Google Scholar]
  88. Manasek FJ, Reid M, Vinson W, Seyer J, Johnson R. Glycosaminoglycan synthesis by the early embryonic chick heart. Dev Biol. 1973;35:332–348. doi: 10.1016/0012-1606(73)90028-6. [DOI] [PubMed] [Google Scholar]
  89. Manner J. On rotation, torsion, lateralization, and handedness of the embryonic heart loop: new insights from a simulation model for the heart loop of chick embryos. Anat Rec A Discov Mol Cell Evol Biol. 2004;278:481–492. doi: 10.1002/ar.a.20036. [DOI] [PubMed] [Google Scholar]
  90. Markwald RR, Fitzharris TP, Manasek FJ. Structural development of endocardial cushions. Am J Anat. 1977;148:85–119. doi: 10.1002/aja.1001480108. [DOI] [PubMed] [Google Scholar]
  91. Markwald RR, Smith WN. Distribution of mucosubstances in the developing rat heart. J Histochem Cytochem. 1972;20:896–907. doi: 10.1177/20.11.896. [DOI] [PubMed] [Google Scholar]
  92. Maslen CL. Molecular genetics of atrioventricular septal defects. Curr Opin Cardiol. 2004;19:205–210. doi: 10.1097/00001573-200405000-00003. [DOI] [PubMed] [Google Scholar]
  93. Matsson H, Eason J, Bookwalter CS, Klar J, Gustavsson P, Sunnegardh J, Enell H, Jonzon A, Vikkula M, Gutierrez I, Granados-Riveron J, Pope M, Bu’Lock F, Cox J, Robinson TE, Song F, Brook DJ, Marston S, Trybus KM, Dahl N. Alpha-cardiac actin mutations produce atrial septal defects. Hum Mol Genet. 2008;17:256–265. doi: 10.1093/hmg/ddm302. [DOI] [PubMed] [Google Scholar]
  94. McDaniell R, Warthen DM, Sanchez-Lara PA, Pai A, Krantz ID, Piccoli DA, Spinner NB. NOTCH2 mutations cause Alagille syndrome, a heterogeneous disorder of the notch signaling pathway. Am J Hum Genet. 2006;79:169–173. doi: 10.1086/505332. [DOI] [PMC free article] [PubMed] [Google Scholar]
  95. McKusick VA, Egeland JA, Eldridge R, Krusen DE. DWARFISM IN THE AMISH I. THE ELLIS-VAN CREVELD SYNDROME. Bull Johns Hopkins Hosp. 1964;115:306–336. [PubMed] [Google Scholar]
  96. Meno C, Shimono A, Saijoh Y, Yashiro K, Mochida K, Ohishi S, Noji S, Kondoh H, Hamada H. lefty-1 is required for left-right determination as a regulator of lefty-2 and nodal. Cell. 1998;94:287–297. doi: 10.1016/s0092-8674(00)81472-5. [DOI] [PubMed] [Google Scholar]
  97. Minich LL, Atz AM, Colan SD, Sleeper LA, Mital S, Jaggers J, Margossian R, Prakash A, Li JS, Cohen MS, Lacro RV, Klein GL, Hawkins JA. Partial and transitional atrioventricular septal defect outcomes. Ann Thorac Surg. 2010;89:530–536. doi: 10.1016/j.athoracsur.2009.10.047. [DOI] [PMC free article] [PubMed] [Google Scholar]
  98. Miyabara S, Gropp A, Winking H. Trisomy 16 in the mouse fetus associated with generalized edema and cardiovascular and urinary tract anomalies. Teratology. 1982;25:369–380. doi: 10.1002/tera.1420250314. [DOI] [PubMed] [Google Scholar]
  99. Mjaatvedt CH, Lepera RC, Markwald RR. Myocardial specificity for initiating endothelial-mesenchymal cell transition in embryonic chick heart correlates with a particulate distribution of fibronectin. Dev Biol. 1987;119:59–67. doi: 10.1016/0012-1606(87)90206-5. [DOI] [PubMed] [Google Scholar]
  100. Mjaatvedt CH, Nakaoka T, Moreno-Rodriguez R, Norris RA, Kern MJ, Eisenberg CA, Turner D, Markwald RR. The outflow tract of the heart is recruited from a novel heart-forming field. Dev Biol. 2001;238:97–109. doi: 10.1006/dbio.2001.0409. [DOI] [PubMed] [Google Scholar]
  101. Moller JH, Nakib A, Anderson RC, Edwards JE. Congenital cardiac disease associated with polysplenia. A developmental complex of bilateral “left-sidedness”. Circulation. 1967;36:789–799. doi: 10.1161/01.cir.36.5.789. [DOI] [PubMed] [Google Scholar]
  102. Monserrat L, Hermida-Prieto M, Fernandez X, Rodriguez I, Dumont C, Cazon L, Cuesta MG, Gonzalez-Juanatey C, Peteiro J, Alvarez N, Penas-Lado M, Castro-Beiras A. Mutation in the alpha-cardiac actin gene associated with apical hypertrophic cardiomyopathy, left ventricular non-compaction, and septal defects. Eur Heart J. 2007;28:1953–1961. doi: 10.1093/eurheartj/ehm239. [DOI] [PubMed] [Google Scholar]
  103. Moorman A, Webb S, Brown NA, Lamers W, Anderson RH. Development of the heart: (1) formation of the cardiac chambers and arterial trunks. Heart. 2003;89:806–814. doi: 10.1136/heart.89.7.806. [DOI] [PMC free article] [PubMed] [Google Scholar]
  104. Moorman AF, Christoffels VM, Anderson RH, van den Hoff MJ. The heart-forming fields: one or multiple? Philos Trans R Soc Lond B Biol Sci. 2007;362:1257–1265. doi: 10.1098/rstb.2007.2113. [DOI] [PMC free article] [PubMed] [Google Scholar]
  105. Musewe NN, Alexander DJ, Teshima I, Smallhorn JF, Freedom RM. Echocardiographic evaluation of the spectrum of cardiac anomalies associated with trisomy 13 and trisomy 18. J Am Coll Cardiol. 1990;15:673–677. doi: 10.1016/0735-1097(90)90644-5. [DOI] [PubMed] [Google Scholar]
  106. Newbury-Ecob RA, Leanage R, Raeburn JA, Young ID. Holt-Oram syndrome: a clinical genetic study. J Med Genet. 1996;33:300–307. doi: 10.1136/jmg.33.4.300. [DOI] [PMC free article] [PubMed] [Google Scholar]
  107. Niihori T, Aoki Y, Narumi Y, Neri G, Cave H, Verloes A, Okamoto N, Hennekam RC, Gillessen-Kaesbach G, Wieczorek D, Kavamura MI, Kurosawa K, Ohashi H, Wilson L, Heron D, Bonneau D, Corona G, Kaname T, Naritomi K, Baumann C, Matsumoto N, Kato K, Kure S, Matsubara Y. Germline KRAS and BRAF mutations in cardio-facio-cutaneous syndrome. Nat Genet. 2006;38:294–296. doi: 10.1038/ng1749. [DOI] [PubMed] [Google Scholar]
  108. Nobes CD, Hall A. Rho, rac, and cdc42 GTPases regulate the assembly of multimolecular focal complexes associated with actin stress fibers, lamellipodia, and filopodia. Cell. 1995;81:53–62. doi: 10.1016/0092-8674(95)90370-4. [DOI] [PubMed] [Google Scholar]
  109. Okubo A, Miyoshi O, Baba K, Takagi M, Tsukamoto K, Kinoshita A, Yoshiura K, Kishino T, Ohta T, Niikawa N, Matsumoto N. A novel GATA4 mutation completely segregated with atrial septal defect in a large Japanese family. J Med Genet. 2004;41:e97. doi: 10.1136/jmg.2004.018895. [DOI] [PMC free article] [PubMed] [Google Scholar]
  110. Orstavik KH, Bechensteen AG, Fugelseth D, Orderud W. Sibs with Ritscher-Schinzel (3C) syndrome and anal malformations. Am J Med Genet. 1998;75:300–303. [PubMed] [Google Scholar]
  111. Owens GE, Gomez-Fifer C, Gelehrter S, Owens ST. Outcomes for patients with unbalanced atrioventricular septal defects. Pediatr Cardiol. 2009;30:431–435. doi: 10.1007/s00246-008-9376-z. [DOI] [PubMed] [Google Scholar]
  112. Ozcelik C, Bit-Avragim N, Panek A, Gaio U, Geier C, Lange PE, Dietz R, Posch MG, Perrot A, Stiller B. Mutations in the EGF-CFC gene cryptic are an infrequent cause of congenital heart disease. Pediatr Cardiol. 2006;27:695–698. doi: 10.1007/s00246-006-1082-0. [DOI] [PubMed] [Google Scholar]
  113. Park SC, Mathews RA, Zuberbuhler JR, Rowe RD, Neches WH, Lenox CC. Down syndrome with congenital heart malformation. Am J Dis Child. 1977;131:29–33. doi: 10.1001/archpedi.1977.02120140031003. [DOI] [PubMed] [Google Scholar]
  114. Piedra ME, Icardo JM, Albajar M, Rodriguez-Rey JC, Ros MA. Pitx2 participates in the late phase of the pathway controlling left-right asymmetry. Cell. 1998;94:319–324. doi: 10.1016/s0092-8674(00)81475-0. [DOI] [PubMed] [Google Scholar]
  115. Posch MG, Gramlich M, Sunde M, Schmitt KR, Lee SH, Richter S, Kersten A, Perrot A, Panek AN, Al Khatib IH, Nemer G, Megarbane A, Dietz R, Stiller B, Berger F, Harvey RP, Ozcelik C. A gain-of-function TBX20 mutation causes congenital atrial septal defects, patent foramen ovale and cardiac valve defects. J Med Genet. 2010;47:230–235. doi: 10.1136/jmg.2009.069997. [DOI] [PMC free article] [PubMed] [Google Scholar]
  116. Prall OW, Menon MK, Solloway MJ, Watanabe Y, Zaffran S, Bajolle F, Biben C, McBride JJ, Robertson BR, Chaulet H, Stennard FA, Wise N, Schaft D, Wolstein O, Furtado MB, Shiratori H, Chien KR, Hamada H, Black BL, Saga Y, Robertson EJ, Buckingham ME, Harvey RP. An Nkx2-5/Bmp2/Smad1 negative feedback loop controls heart progenitor specification and proliferation. Cell. 2007;128:947–959. doi: 10.1016/j.cell.2007.01.042. [DOI] [PMC free article] [PubMed] [Google Scholar]
  117. Pu WT, Ishiwata T, Juraszek AL, Ma Q, Izumo S. GATA4 is a dosage-sensitive regulator of cardiac morphogenesis. Dev Biol. 2004;275:235–244. doi: 10.1016/j.ydbio.2004.08.008. [DOI] [PubMed] [Google Scholar]
  118. Rajagopal SK, Ma Q, Obler D, Shen J, Manichaikul A, Tomita-Mitchell A, Boardman K, Briggs C, Garg V, Srivastava D, Goldmuntz E, Broman KW, Benson DW, Smoot LB, Pu WT. Spectrum of heart disease associated with murine and human GATA4 mutation. J Mol Cell Cardiol. 2007;43:677–685. doi: 10.1016/j.yjmcc.2007.06.004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  119. Rastelli G, Kirklin JW, Titus JL. Anatomic observations on complete form of persistent common atrioventricular canal with special reference to atrioventricular valves. Mayo Clin Proc. 1966;41:296–308. [PubMed] [Google Scholar]
  120. Reamon-Buettner SM, Borlak J. Somatic NKX2-5 mutations as a novel mechanism of disease in complex congenital heart disease. J Med Genet. 2004a;41:684–690. doi: 10.1136/jmg.2003.017483. [DOI] [PMC free article] [PubMed] [Google Scholar]
  121. Reamon-Buettner SM, Borlak J. TBX5 mutations in non-Holt-Oram syndrome (HOS) malformed hearts. Hum Mutat. 2004b;24:104. doi: 10.1002/humu.9255. [DOI] [PubMed] [Google Scholar]
  122. Reamon-Buettner SM, Borlak J. GATA4 zinc finger mutations as a molecular rationale for septation defects of the human heart. J Med Genet. 2005;42:e32. doi: 10.1136/jmg.2004.025395. [DOI] [PMC free article] [PubMed] [Google Scholar]
  123. Reynolds JF, Neri G, Herrmann JP, Blumberg B, Coldwell JG, Miles PV, Opitz JM. New multiple congenital anomalies/mental retardation syndrome with cardio-facio-cutaneous involvement--the CFC syndrome. Am J Med Genet. 1986;25:413–427. doi: 10.1002/ajmg.1320250303. [DOI] [PubMed] [Google Scholar]
  124. Ritscher D, Schinzel A, Boltshauser E, Briner J, Arbenz U, Sigg P. Dandy-Walker(like) malformation, atrio-ventricular septal defect and a similar pattern of minor anomalies in 2 sisters: a new syndrome? Am J Med Genet. 1987;26:481–491. doi: 10.1002/ajmg.1320260227. [DOI] [PubMed] [Google Scholar]
  125. Robinson SW, Morris CD, Goldmuntz E, Reller MD, Jones MA, Steiner RD, Maslen CL. Missense mutations in CRELD1 are associated with cardiac atrioventricular septal defects. Am J Hum Genet. 2003;72:1047–1052. doi: 10.1086/374319. [DOI] [PMC free article] [PubMed] [Google Scholar]
  126. Roessler E, Ouspenskaia MV, Karkera JD, Velez JI, Kantipong A, Lacbawan F, Bowers P, Belmont JW, Towbin JA, Goldmuntz E, Feldman B, Muenke M. Reduced NODAL signaling strength via mutation of several pathway members including FOXH1 is linked to human heart defects and holoprosencephaly. Am J Hum Genet. 2008;83:18–29. doi: 10.1016/j.ajhg.2008.05.012. [DOI] [PMC free article] [PubMed] [Google Scholar]
  127. Ruiz-Perez VL, Goodship JA. Ellis-van Creveld syndrome and Weyers acrodental dysostosis are caused by cilia-mediated diminished response to hedgehog ligands. Am J Med Genet C Semin Med Genet. 2009;151C:341–351. doi: 10.1002/ajmg.c.30226. [DOI] [PubMed] [Google Scholar]
  128. Runyan RB, Markwald RR. Invasion of mesenchyme into three-dimensional collagen gels: a regional and temporal analysis of interaction in embryonic heart tissue. Dev Biol. 1983;95:108–114. doi: 10.1016/0012-1606(83)90010-6. [DOI] [PubMed] [Google Scholar]
  129. Rupp PA, Fouad GT, Egelston CA, Reifsteck CA, Olson SB, Knosp WM, Glanville RW, Thornburg KL, Robinson SW, Maslen CL. Identification, genomic organization and mRNA expression of CRELD1, the founding member of a unique family of matricellular proteins. Gene. 2002;293:47–57. doi: 10.1016/s0378-1119(02)00696-0. [DOI] [PubMed] [Google Scholar]
  130. Ruschhaupt DG, Khoury L, Thilenius OG, Replogle RL, Arcilla RA. Electrophysiologic abnormalities of children with ostium secundum atrial septal defect. Am J Cardiol. 1984;53:1643–1647. doi: 10.1016/0002-9149(84)90594-0. [DOI] [PubMed] [Google Scholar]
  131. Ryan AK, Bartlett K, Clayton P, Eaton S, Mills L, Donnai D, Winter RM, Burn J. Smith-Lemli-Opitz syndrome: a variable clinical and biochemical phenotype. J Med Genet. 1998;35:558–565. doi: 10.1136/jmg.35.7.558. [DOI] [PMC free article] [PubMed] [Google Scholar]
  132. Sarkozy A, Conti E, Neri C, D’Agostino R, Digilio MC, Esposito G, Toscano A, Marino B, Pizzuti A, Dallapiccola B. Spectrum of atrial septal defects associated with mutations of NKX2.5 and GATA4 transcription factors. J Med Genet. 2005;42:e16. doi: 10.1136/jmg.2004.026740. [DOI] [PMC free article] [PubMed] [Google Scholar]
  133. Schott JJ, Benson DW, Basson CT, Pease W, Silberbach GM, Moak JP, Maron BJ, Seidman CE, Seidman JG. Congenital heart disease caused by mutations in the transcription factor NKX2-5. Science. 1998;281:108–111. doi: 10.1126/science.281.5373.108. [DOI] [PubMed] [Google Scholar]
  134. Sharratt GP, Webb S, Anderson RH. The vestibular defect: an interatrial communication due to a deficiency in the atrial septal component derived from the vestibular spine. Cardiol Young. 2003;13:184–190. doi: 10.1017/s1047951103000349. [DOI] [PubMed] [Google Scholar]
  135. Sheffield VC, Pierpont ME, Nishimura D, Beck JS, Burns TL, Berg MA, Stone EM, Patil SR, Lauer RM. Identification of a complex congenital heart defect susceptibility locus by using DNA pooling and shared segment analysis. Hum Mol Genet. 1997;6:117–121. doi: 10.1093/hmg/6.1.117. [DOI] [PubMed] [Google Scholar]
  136. Shuhaiber JH, Ho SY, Rigby M, Sethia B. Current options and outcomes for the management of atrioventricular septal defect. Eur J Cardiothorac Surg. 2009;35:891–900. doi: 10.1016/j.ejcts.2009.01.009. [DOI] [PubMed] [Google Scholar]
  137. Sletten LJ, Pierpont ME. Variation in severity of cardiac disease in Holt-Oram syndrome. Am J Med Genet. 1996;65:128–132. doi: 10.1002/(SICI)1096-8628(19961016)65:2<128::AID-AJMG9>3.0.CO;2-O. [DOI] [PubMed] [Google Scholar]
  138. Smith AT, Sack GH, Jr, Taylor GJ. Holt-Oram syndrome. J Pediatr. 1979;95:538–543. doi: 10.1016/s0022-3476(79)80758-1. [DOI] [PubMed] [Google Scholar]
  139. Smith KA, Joziasse IC, Chocron S, van Dinther M, Guryev V, Verhoeven MC, Rehmann H, van der Smagt JJ, Doevendans PA, Cuppen E, Mulder BJ, Ten Dijke P, Bakkers J. Dominant-negative ALK2 allele associates with congenital heart defects. Circulation. 2009;119:3062–3069. doi: 10.1161/CIRCULATIONAHA.108.843714. [DOI] [PubMed] [Google Scholar]
  140. Smithells RW, Newman CG. Recognition of thalidomide defects. J Med Genet. 1992;29:716–723. doi: 10.1136/jmg.29.10.716. [DOI] [PMC free article] [PubMed] [Google Scholar]
  141. Snarr BS, Kern CB, Wessels A. Origin and fate of cardiac mesenchyme. Dev Dyn. 2008;237:2804–2819. doi: 10.1002/dvdy.21725. [DOI] [PubMed] [Google Scholar]
  142. Snarr BS, O’Neal JL, Chintalapudi MR, Wirrig EE, Phelps AL, Kubalak SW, Wessels A. Isl1 expression at the venous pole identifies a novel role for the second heart field in cardiac development. Circ Res. 2007a;101:971–974. doi: 10.1161/CIRCRESAHA.107.162206. [DOI] [PubMed] [Google Scholar]
  143. Snarr BS, Wirrig EE, Phelps AL, Trusk TC, Wessels A. A spatiotemporal evaluation of the contribution of the dorsal mesenchymal protrusion to cardiac development. Dev Dyn. 2007b;236:1287–1294. doi: 10.1002/dvdy.21074. [DOI] [PMC free article] [PubMed] [Google Scholar]
  144. Soufan AT, van den Hoff MJ, Ruijter JM, de Boer PA, Hagoort J, Webb S, Anderson RH, Moorman AF. Reconstruction of the patterns of gene expression in the developing mouse heart reveals an architectural arrangement that facilitates the understanding of atrial malformations and arrhythmias. Circ Res. 2004;95:1207–1215. doi: 10.1161/01.RES.0000150852.04747.e1. [DOI] [PubMed] [Google Scholar]
  145. Stennard FA, Costa MW, Elliott DA, Rankin S, Haast SJ, Lai D, McDonald LP, Niederreither K, Dolle P, Bruneau BG, Zorn AM, Harvey RP. Cardiac T-box factor Tbx20 directly interacts with Nkx2-5, GATA4, and GATA5 in regulation of gene expression in the developing heart. Dev Biol. 2003;262:206–224. doi: 10.1016/s0012-1606(03)00385-3. [DOI] [PubMed] [Google Scholar]
  146. Stennard FA, Costa MW, Lai D, Biben C, Furtado MB, Solloway MJ, McCulley DJ, Leimena C, Preis JI, Dunwoodie SL, Elliott DE, Prall OW, Black BL, Fatkin D, Harvey RP. Murine T-box transcription factor Tbx20 acts as a repressor during heart development, and is essential for adult heart integrity, function and adaptation. Development. 2005;132:2451–2462. doi: 10.1242/dev.01799. [DOI] [PubMed] [Google Scholar]
  147. Stevens CA, Bhakta MG. Cardiac abnormalities in the Rubinstein-Taybi syndrome. Am J Med Genet. 1995;59:346–348. doi: 10.1002/ajmg.1320590313. [DOI] [PubMed] [Google Scholar]
  148. Stoll C, Alembik Y, Dott B, Roth MP. Study of Down syndrome in 238,942 consecutive births. Ann Genet. 1998;41:44–51. [PubMed] [Google Scholar]
  149. Sznajer Y, Keren B, Baumann C, Pereira S, Alberti C, Elion J, Cave H, Verloes A. The spectrum of cardiac anomalies in Noonan syndrome as a result of mutations in the PTPN11 gene. Pediatrics. 2007;119:e1325–1331. doi: 10.1542/peds.2006-0211. [DOI] [PubMed] [Google Scholar]
  150. Sztajzel R, Genoud D, Roth S, Mermillod B, Le Floch-Rohr J. Patent foramen ovale, a possible cause of symptomatic migraine: a study of 74 patients with acute ischemic stroke. Cerebrovasc Dis. 2002;13:102–106. doi: 10.1159/000047758. [DOI] [PubMed] [Google Scholar]
  151. Terrett JA, Newbury-Ecob R, Cross GS, Fenton I, Raeburn JA, Young ID, Brook JD. Holt-Oram syndrome is a genetically heterogeneous disease with one locus mapping to human chromosome 12q. Nat Genet. 1994;6:401–404. doi: 10.1038/ng0494-401. [DOI] [PubMed] [Google Scholar]
  152. Tian Y, Yuan L, Goss AM, Wang T, Yang J, Lepore JJ, Zhou D, Schwartz RJ, Patel V, Cohen ED, Morrisey EE. Characterization and in vivo pharmacological rescue of a Wnt2-Gata6 pathway required for cardiac inflow tract development. Dev Cell. 2010;18:275–287. doi: 10.1016/j.devcel.2010.01.008. [DOI] [PMC free article] [PubMed] [Google Scholar]
  153. Tikkanen J, Heinonen OP. Risk factors for ventricular septal defect in Finland. Public Health. 1991;105:99–112. doi: 10.1016/s0033-3506(05)80283-5. [DOI] [PubMed] [Google Scholar]
  154. Tomita-Mitchell A, Maslen CL, Morris CD, Garg V, Goldmuntz E. GATA4 sequence variants in patients with congenital heart disease. J Med Genet. 2007;44:779–783. doi: 10.1136/jmg.2007.052183. [DOI] [PMC free article] [PubMed] [Google Scholar]
  155. Tompson SW, Ruiz-Perez VL, Blair HJ, Barton S, Navarro V, Robson JL, Wright MJ, Goodship JA. Sequencing EVC and EVC2 identifies mutations in two-thirds of Ellis-van Creveld syndrome patients. Hum Genet. 2007;120:663–670. doi: 10.1007/s00439-006-0237-7. [DOI] [PubMed] [Google Scholar]
  156. Verzi MP, McCulley DJ, De Val S, Dodou E, Black BL. The right ventricle, outflow tract, and ventricular septum comprise a restricted expression domain within the secondary/anterior heart field. Dev Biol. 2005;287:134–145. doi: 10.1016/j.ydbio.2005.08.041. [DOI] [PubMed] [Google Scholar]
  157. Waldo KL, Kumiski DH, Wallis KT, Stadt HA, Hutson MR, Platt DH, Kirby ML. Conotruncal myocardium arises from a secondary heart field. Development. 2001;128:3179–3188. doi: 10.1242/dev.128.16.3179. [DOI] [PubMed] [Google Scholar]
  158. Waller BR, 3rd, McQuinn T, Phelps AL, Markwald RR, Lo CW, Thompson RP, Wessels A. Conotruncal anomalies in the trisomy 16 mouse: an immunohistochemical analysis with emphasis on the involvement of the neural crest. Anat Rec. 2000;260:279–293. doi: 10.1002/1097-0185(20001101)260:3<279::AID-AR65>3.0.CO;2-2. [DOI] [PubMed] [Google Scholar]
  159. Wang B, Wang J, Liu S, Han X, Xie X, Tao Y, Yan J, Ma X. CFC1 mutations in Chinese children with congenital heart disease. Int J Cardiol. 2011;146:86–88. doi: 10.1016/j.ijcard.2009.07.034. [DOI] [PubMed] [Google Scholar]
  160. Ware SM, Peng J, Zhu L, Fernbach S, Colicos S, Casey B, Towbin J, Belmont JW. Identification and functional analysis of ZIC3 mutations in heterotaxy and related congenital heart defects. Am J Hum Genet. 2004;74:93–105. doi: 10.1086/380998. [DOI] [PMC free article] [PubMed] [Google Scholar]
  161. Webb S, Anderson RH, Brown NA. Endocardial cushion development and heart loop architecture in the trisomy 16 mouse. Dev Dyn. 1996;206:301–309. doi: 10.1002/(SICI)1097-0177(199607)206:3<301::AID-AJA7>3.0.CO;2-M. [DOI] [PubMed] [Google Scholar]
  162. Webb S, Anderson RH, Lamers WH, Brown NA. Mechanisms of deficient cardiac septation in the mouse with trisomy 16. Circ Res. 1999;84:897–905. doi: 10.1161/01.res.84.8.897. [DOI] [PubMed] [Google Scholar]
  163. Webb S, Brown NA, Anderson RH. Formation of the atrioventricular septal structures in the normal mouse. Circ Res. 1998;82:645–656. doi: 10.1161/01.res.82.6.645. [DOI] [PubMed] [Google Scholar]
  164. Wessels A, Anderson RH, Markwald RR, Webb S, Brown NA, Viragh S, Moorman AF, Lamers WH. Atrial development in the human heart: an immunohistochemical study with emphasis on the role of mesenchymal tissues. Anat Rec. 2000;259:288–300. doi: 10.1002/1097-0185(20000701)259:3<288::AID-AR60>3.0.CO;2-D. [DOI] [PubMed] [Google Scholar]
  165. Wessels A, Markman MW, Vermeulen JL, Anderson RH, Moorman AF, Lamers WH. The development of the atrioventricular junction in the human heart. Circ Res. 1996;78:110–117. doi: 10.1161/01.res.78.1.110. [DOI] [PubMed] [Google Scholar]
  166. Wessels A, Sedmera D. Developmental anatomy of the heart: a tale of mice and man. Physiol Genomics. 2003;15:165–176. doi: 10.1152/physiolgenomics.00033.2003. [DOI] [PubMed] [Google Scholar]
  167. Wessels A, van den Hoff M, Adamo R, Phelps A, Lockhart M, Sauls K, Briggs L, Norris RA, van Wijk B, Perez-Pomares J, Dettman R, Burch J. Epicardially-derived Fibroblasts Preferentially Contribute to the Parietal Leaflets of the Atrioventricular Valves in the Murine Heart. Developmental Biology. 2012 doi: 10.1016/j.ydbio.2012.04.020. In press. [DOI] [PMC free article] [PubMed] [Google Scholar]
  168. Wessels A, Vermeulen JL, Verbeek FJ, Viragh S, Kalman F, Lamers WH, Moorman AF. Spatial distribution of “tissue-specific” antigens in the developing human heart and skeletal muscle. III. An immunohistochemical analysis of the distribution of the neural tissue antigen G1N2 in the embryonic heart; implications for the development of the atrioventricular conduction system. Anat Rec. 1992;232:97–111. doi: 10.1002/ar.1092320111. [DOI] [PubMed] [Google Scholar]
  169. Wilmshurst PT, Byrne JC, Webb-Peploe MM. Relation between interatrial shunts and decompression sickness in divers. Lancet. 1989;2:1302–1306. doi: 10.1016/s0140-6736(89)91911-9. [DOI] [PubMed] [Google Scholar]
  170. Zaffran S, Kelly R. New developments in the second heart field. Differentiation. 2012 doi: 10.1016/j.diff.2012.03.003. [DOI] [PubMed] [Google Scholar]
  171. Zatyka M, Priestley M, Ladusans EJ, Fryer AE, Mason J, Latif F, Maher ER. Analysis of CRELD1 as a candidate 3p25 atrioventicular septal defect locus (AVSD2) Clin Genet. 2005;67:526–528. doi: 10.1111/j.1399-0004.2005.00435.x. [DOI] [PubMed] [Google Scholar]
  172. Zetterqvist P, Turesson I, Johansson BW, Laurell S, Ohlsson NM. Dominant mode of inheritance in atrial septal defect. Clin Genet. 1971;2:78–86. doi: 10.1111/j.1399-0004.1971.tb00259.x. [DOI] [PubMed] [Google Scholar]
  173. Zhao ZS, Manser E, Loo TH, Lim L. Coupling of PAK-interacting exchange factor PIX to GIT1 promotes focal complex disassembly. Mol Cell Biol. 2000;20:6354–6363. doi: 10.1128/mcb.20.17.6354-6363.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES