FRS2α-dependent cell fate transition during endocardial cushion morphogenesis.

Atrioventricular valve development requires endothelial-to-mesenchymal transition (EndMT) that induces cushion endocardial cells to give rise to mesenchymal cells crucial to valve formation. In the adult endothelium, deletion of the docking protein FRS2α induces EndMT by activating TGFß signaling in a miRNA let-7-dependent manner. To study the role of endothelial FRS2α during embryonic development, we generated mice with an inducible endothelial-specific deletion of Frs2α (FRS2αiECKO). Analysis of the FRS2αiECKO embryos uncovered a combination of impaired EndMT in AV cushions and defective maturation of AV valves leading to development of thickened, abnormal valves when Frs2α was deleted early (E7.5) in development. At the same time, no AV valve developmental abnormalities were observed after late (E10.5) deletion. These observations identify FRS2α as a pivotal controller of cell fate transition during both EndMT and post-EndMT valvulogenesis.

[Prenatal diagnosis and genetic analysis of a fetus with endocardial cushion defects].

OBJECTIVE: To perform prenatal diagnosis for a fetus with endocardial cushion defect and explore its mechanism. METHODS: The karotypes of the fetus and its parents were analyzed by routine G-banding. Their genomic DNA was also analyzed by array comparative genomic hybridization (aCGH). RESULTS: The fetus and its mother were found to have a karyotype of 46, XX, inv(8)(p21q24.1), while no karyotypic abnormality was detected for the father. aCGH has detected a 15.14 Mb deletion at 8p23.3-p22 and a 6.87 Mb duplication at 8q24.23-q24.3 in the fetus. CONCLUSION: The fetus was diagnosed with Rec8 syndrome. Its abnormal chromosomes have derived from the inv(8) carried by its mother. GATA4 and SOX7 may be the key genes for the endocardial cushion defect found in the fetus.

Cardiac neural crest ablation results in early endocardial cushion and hemodynamic flow abnormalities.

Cardiac neural crest cell (CNCC) ablation creates congenital heart defects (CHDs) that resemble those observed in many syndromes with craniofacial and cardiac consequences. The loss of CNCCs causes a variety of great vessel defects, including persistent truncus arteriosus and double-outlet right ventricle. However, because of the lack of quantitative volumetric measurements, less severe defects, such as great vessel size changes and valve defects, have not been assessed. Also poorly understood is the role of abnormal cardiac function in the progression of CNCC-related CHDs. CNCC ablation was previously reported to cause abnormal cardiac function in early cardiogenesis, before the CNCCs arrive in the outflow region of the heart. However, the affected functional parameters and how they correlate with the structural abnormalities were not fully characterized. In this study, using a CNCC-ablated quail model, we contribute quantitative phenotyping of CNCC ablation-related CHDs and investigate abnormal early cardiac function, which potentially contributes to late-stage CHDs. Optical coherence tomography was used to assay early- and late-stage embryos and hearts. In CNCC-ablated embryos at four-chambered heart stages, great vessel diameter and left atrioventricular valve leaflet volumes are reduced. Earlier, at cardiac looping stages, CNCC-ablated embryos exhibit abnormally twisted bodies, abnormal blood flow waveforms, increased retrograde flow percentage, and abnormal cardiac cushions. The phenotypes observed in this CNCC-ablation model were also strikingly similar to those found in an established avian fetal alcohol syndrome model, supporting the contribution of CNCC dysfunction to the development of alcohol-induced CHDs.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion.

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.

Atrioventricular valve regurgitation in the fetus with atrioventricular canal defect: transition from prenatal to postnatal life.

Atrioventricular valve regurgitation (AVVR) is a clinically important element of the common atrioventricular canal defect. Cardiac preload and afterload increase from prenatal to postnatal life. These hemodynamic changes may increase the degree of regurgitation and affect management and prognosis. We sought to investigate the frequency of change in degree of AVVR from fetal to postnatal life in this patient population. Subjects who underwent both fetal and postnatal echocardiography within 4 weeks of life between January 2008 and September 2010 were included in the study. Degree of AVVR was assessed by color Doppler imaging and scored as 0 (no regurgitation), 1 (hemodynamically insignificant regurgitation), and 2 (hemodynamically important regurgitation). Forty-nine subjects were included. Mean gestational age at fetal echocardiogram was 34 ± 2.8 weeks; age at postnatal echocardiogram was a median of <24 h of age (range 0-24). After birth, 69 % subjects had no change, 8 % of subjects had a decrease, and 22 % subjects had an increase in AVVR grade. Five patients progressed from a fetal score 0 or 1 to postnatal score 2. Neither trisomy 21 nor heterotaxy syndrome were risk factors for progression of AVVR. In patients with AV canal defects, 90 % demonstrate no hemodynamically significant change in AVVR from fetal to postnatal life, whereas 10 % display a hemodynamically significant change. AVVR appreciated in utero is predictive of neonatal regurgitation in the majority of patients. These findings have implications for the counseling and management of the fetus with AV canal defect.

GATA5 interacts with GATA4 and GATA6 in outflow tract development.

Members of the GATA family of transcription factors are critical regulators of heart development and mutations in 2 of them, GATA4 and GATA6 are associated with outflow tract and septal defects in human. The heart expresses 3 GATA factors, GATA4, 5 and 6 in a partially overlapping pattern. Here, we report that compound Gata4/Gata5 and Gata5/Gata6 mutants die embryonically or perinatally due to severe congenital heart defects. Almost all Gata4(+/-)Gata5(+/-) mutant embryos have double outlet right ventricles (DORV), large ventricular septal defects (VSD) as well as hypertrophied mitral and tricuspid valves. Only 25% of double compound Gata4/Gata5 heterozygotes survive to adulthood and these mice have aortic stenosis. Compound loss of a Gata5 and a Gata6 allele also leads to DORVs associated with subaortic VSDs. Expression of several transcription factors important for endocardial and myocardial cell differentiation, such as Tbx20, Mef2c, Hey1 and Hand2, was reduced in compound heterozygote embryos. These findings suggest the existence of important genetic interactions between Gata5 and the 2 other cardiac GATA factors in endocardial cushion formation and outflow tract morphogenesis. The data identify GATA5 as a potential genetic modifier of congenital heart disease and provide insight for elucidating the genetic basis of an important class of human birth defects.

Increased regurgitant flow causes endocardial cushion defects in an avian embryonic model of congenital heart disease.

BACKGROUND: The relationship between changes in endocardial cushion and resultant congenital heart diseases (CHD) has yet to be established. It has been shown that increased regurgitant flow early in embryonic heart development leads to endocardial cushion defects, but it remains unclear how abnormal endocardial cushions during the looping stages might affect the fully septated heart. The goal of this study was to reproducibly alter blood flow in vivo and then quantify the resultant effects on morphology of endocardial cushions in the looping heart and on CHDs in the septated heart. METHODS: Optical pacing was applied to create regurgitant flow in embryonic hearts, and optical coherence tomography (OCT) was utilized to quantify regurgitation and morphology. Embryonic quail hearts were optically paced at 3 Hz (180 bpm, well above intrinsic rate 60-110 bpm) at stage 13 of development (3-4 weeks human) for 5 min. Pacing fatigued the heart and led to at least 1 h of increased regurgitant flow. Resultant morphological changes were quantified with OCT imaging at stage 19 (cardiac looping-4-5 weeks human) or stage 35 (4 chambered heart-8 weeks human). RESULTS: All paced embryos imaged at stage 19 displayed structural changes in cardiac cushions. The amount of regurgitant flow immediately after pacing was inversely correlated with cardiac cushion size 24-h post pacing (P value < .01). The embryos with the most regurgitant flow and smallest cushions after pacing had a decreased survival rate at 8 days (P < .05), indicating that those most severe endocardial cushion defects were lethal. Of the embryos that survived to stage 35, 17/18 exhibited CHDs including valve defects, ventricular septal defects, hypoplastic ventricles, and common AV canal. CONCLUSION: The data illustrate a strong inverse relationship in which regurgitant flow precedes abnormal and smaller cardiac cushions, resulting in the development of CHDs.

Cell biology of cardiac cushion development.

The valves of the heart develop in the embryo from precursor structures called endocardial cushions. After cardiac looping, endocardial cushion swellings form and become populated by valve precursor cells formed by an epithelial-mesenchymal transition (EMT). Endocardial cushions subsequently undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. The developmental processes that mediate cushion formation include many prototypic cellular actions including adhesion, signaling, migration, secretion, replication, differentiation, and apoptosis. Cushion morphogenesis is unique in that these cellular possesses occur in a functioning organ where the cushions act as valves even while developing into definitive valvular structures. Cardiovascular defects are the most common congenital defects, and one of the most common causes of death during infancy. Thus, there is significant interest in understanding the mechanisms that underlie this complex developmental process. In this regard, substantial progress has been made by incorporating an understanding of cardiac morphology and cell biology with the rapidly expanding repertoire of molecular mechanisms gained through human genetics and research using animal models. This article reviews cardiac morphogenesis as it relates to heart valve formation and highlights selected growth factors, intracellular signaling mediators, and extracellular matrix components involved in the creation and remodeling of endocardial cushions into mature cardiac structures.

Is a cleft in the anterior leaflet of an otherwise normal mitral valve an atrioventricular canal malformation?

OBJECTIVES: This study sought to ascertain the surgical anatomy of a cleft in the left atrioventricular (AV) valve. BACKGROUND: Important morphologic differences exist between hearts with a cleft in the anterior leaflet of an otherwise normal mitral valve and those with a so-called cleft in the left AV valve when there is an AV septal defect, but it has been customary to link the lesions together on developmental grounds. METHODS: Eight autopsied specimens with a cleft in the aortic (or anterior) leaflet of the mitral valve were studied in detail, and echocardiograms from 21 patients with such a cleft were compared with the specimens and with findings typical of the so-called partial AV canal and other forms of AV septal defect. RESULTS: The structure and direction of the cleft, location of the papillary muscles within the left ventricle and AV junctional morphology of hearts with an otherwise normally structured mitral valve were significantly different from typical findings in hearts with AV septal defects. CONCLUSIONS: It is necessary to distinguish morphologically a cleft in an otherwise normally structured mitral valve in hearts with separate right and left AV junctions from the trifoliate left component of a common AV valve in hearts with an AV septal defect and a common AV junction because the disposition of the AV conduction tissues varies markedly between the lesions.

Left-right lineage analysis of AV cushion tissue in normal and laterality defective Xenopus hearts.

The majority of complex congenital heart defects occur in individuals who are afflicted by laterality disease. We hypothesize that the prevalence of valvuloseptal defects in this population is due to defective left-right patterning of the embryonic atrioventricular (AV) canal cushions, which are the progenitor tissue for valve and septal structures in the mature heart. Using embryos of the frog Xenopus laevis, this hypothesis was tested by performing left-right lineage analysis of myocytes and cushion mesenchyme cells of the superior and inferior cushion regions of the AV canal. Lineage analyses were conducted in both wild-type and laterality mutant embryos experimentally induced by misexpression of ALK4, a type I TGF-beta receptor previously shown to modulate left-right axis determination in Xenopus. We find that abnormalities in overall amount and left-right cell lineage composition are present in a majority of ALK4-induced laterality mutant embryos and that much variation in the nature of these abnormalities exists in embryos that exhibit the same overall body situs. We propose that these two parameters of cushion tissue formation-amount and left-right lineage origin-are important for normal processes of valvuloseptal morphogenesis and that defective allocation of cells in the AV canal might be causatively linked to the high incidence of valvuloseptal defects associated with laterality disease.

Increased adhesiveness of trisomy 21 cells and atrioventricular canal malformations in Down syndrome: a stochastic model.

Based on the finding that fetal trisomy 21 fibroblasts explanted from lungs and endocardial-cushion-derived structures appear more adhesive in vitro than those from normal control individuals, we present a stochastic model for atrioventricular (AV) canal malformations in Down syndrome (DS). Computer simulations were performed to model the normal anatomic sequences of cushion-to-cushion and cushion-to-septum fusion in AV canal development. In these simulations, random-walking endocardial cells were allowed to migrate, divide, and adhere with programmable probabilities. Low values of intercellular adhesiveness engendered simulations resembling normal AV canal development; higher values of adhesiveness yielded deficiencies of AV canal development as seen in DS. Moderately high levels of adhesiveness resulted in abnormalities in only a proportion of multiple, independently performed simulations. The model successfully predicts the temporospatial sequence of anatomic events in cushion-to-septum fusion, clinical variability among individuals with the same genotype based on chance alone, and amplified developmental instability as observed in individuals with DS.

Genetic aspects of atrioventricular septal defects.

Formation of the atrioventricular canal (AVC) results from complex interactions of components of the extracellular matrix. In response to signaling molecules, endothelial/mesenchymal transformations are crucial to normal development of the AVC. Atrioventricular septal defects (AVSDs) can result from arrest or interruption of normal endocardial cushion development. The presence of AVSDs has been associated with chromosome abnormalities, laterality or left-right axis abnormalities, and a variety of syndromes. An AVSD susceptibility gene has been identified in a large kindred with many affected members. Studies of transcription factors and signaling molecules in heart development over the past decade are paving the way for our understanding of the heterogeneous mechanisms of causation of AVSDs.

Molecular determinants of atrial and ventricular septal defects and patent ductus arteriosus.

Septation defects and patent ductus arteriosus are the most common human cardiovascular malformations (CVMs). Genetic factors play a major part in the origin of these malformations. Recent molecular analyses have shed light on several mendelian forms. In the autosomal dominant Holt-Oram syndrome, both atrial and ventricular septal defects are inherited in association with limb deformity as a result of mutations in the gene encoding the TBX5 transcription factor. Mutations in the NKX2.5 transcription factor gene cause autosomal dominant familial atrial septal defects in association with progressive atrioventricular block as well as complex congenital heart disease. Common atrial syndromes in autosomal dominant Ellis-van Creveld syndrome arise in the context of axial skeletal and limb malformation as a result of mutations in the EVC gene, whose function is unknown. Patent ductus arteriosus occurs in several syndromic forms of congenital heart disease, including Holt-Oram syndrome. Recent analyses of autosomal dominant Char syndrome, which includes, with variable penetrance, patent ductus arteriosus as well as craniofacial and hand malformations, have shown that the syndrome is caused by mutations in the TFAP2B transcription factor gene. Ongoing analyses are poised to determine the contribution of these genes as well as others yet to be identified to common, sporadic forms of congenital heart disease.

Developmental aspects of atrioventricular septal defects.

Three human embryos with an atrioventricular septal defect were studied. Their morphology was compared with that of 67 autopsy specimens, in which particular attention was paid to the septal attachments of the bridging leaflets. The malformed embryos showed deficiency of the inlet component of the ventricular septum. They had distinct superior and inferior bridging leaflets, which were nearly completely muscular. Myocardial undermining had taken place at two independent sites but had not been able to lead to the formation of a valve of mitral morphology. Normal delamination of myocardium to form the leaflets could not continue directly below the aortic root because the rim of the inlet septum had a more apical position. From this, we conclude that the deficiency of the inlet septum is the cause of the typical morphology of the left valve in these hearts. The role of endocardial cushion tissue is probably restricted to glueing together myocardial structures, thus determining the variable septal attachment of the bridging leaflets in atrioventricular septal defect.

[Embryologic and pathogenetic aspects of the common atrioventricular canal development]. / Embriologicheskie i patogeneticheskie aspekty razvitiia obshchego atrioventrikuliarnogo kanala.

Atrioventricular canal (AVC) is an inherited defect the embryological basis of which is deficiency of the affluent part of the interventricular septum (IVS). Folds of the atrioventricular (AV) valves are formed from the myocardium and not from the endocardial thickening but much later than the IVS formation. Under the conditions of the affluent part of IVS the mode of connection of the anterior fold of the left AV valve creates the narrowing of the left ventricular effluent part. Endocardial thickenings play a role of a glue fixing corresponding structural components of AV valves and primary heart partitions. The degree of sticking together determines great variants of the defect anatomy. Important deficiency of the affluent part of IVS is possible this making the function difficult due to space change of the endocardial thickenings. The common valve ring with freely floating bridge-like folds is frequently revealed in such case. The notion "deficiency of the endocardial thickenings" has the only manifestation as an isolated splitting of the anterior fold of the mitral valve and exhibits main features of AVC.

Intracardiac flow dynamics regulate atrioventricular valve morphogenesis.

AIMS: Valvular heart disease is responsible for considerable morbidity and mortality. Cardiac valves develop as the heart contracts, and they function throughout the lifetime of the organism to prevent retrograde blood flow. Their precise morphogenesis is crucial for cardiac function. Zebrafish is an ideal model to investigate cardiac valve development as it allows these studies to be carried out in vivo through non-invasive imaging. Accumulating evidence suggests a role for contractility and intracardiac flow dynamics in cardiac valve development. However, these two factors have proved difficult to uncouple, especially since altering myocardial function affects the intracardiac flow pattern. METHODS AND RESULTS: Here, we describe novel zebrafish models of developmental valve defects. We identified two mutant alleles of myosin heavy chain 6 that can be raised to adulthood despite having only one functional chamber-the ventricle. The adult mutant ventricle undergoes remodelling, and the atrioventricular (AV) valves fail to form four cuspids. In parallel, we characterized a novel mutant allele of southpaw, a nodal-related gene involved in the establishment of left-right asymmetry, which exhibits randomized heart and endoderm positioning. We first observed that in southpaw mutants the relative position of the two cardiac chambers is altered, affecting the geometry of the heart, while myocardial function appears unaffected. Mutant hearts that loop properly or exhibit situs inversus develop normally, whereas midline, unlooped hearts exhibit defects in their transvalvular flow pattern during AV valve development as well as defects in valve morphogenesis. CONCLUSION: Our data indicate that intracardiac flow dynamics regulate valve morphogenesis independently of myocardial contractility.

Neural crest cells are required for correct positioning of the developing outflow cushions and pattern the arterial valve leaflets.

AIMS: Anomalies of the arterial valves, principally bicuspid aortic valve (BAV), are the most common congenital anomalies. The cellular mechanisms that underlie arterial valve development are poorly understood. While it is known that the valve leaflets derive from the outflow cushions, which are populated by cells derived from the endothelium and neural crest cells (NCCs), the mechanism by which these cushions are sculpted to form the leaflets of the arterial valves remains unresolved. We set out to investigate how NCCs participate in arterial valve formation, reasoning that disrupting NCC within the developing outflow cushions would result in arterial valve anomalies, in the process elucidating the normal mechanism of arterial valve leaflet formation. METHODS AND RESULTS: By disrupting Rho kinase signalling specifically in NCC using transgenic mice and primary cultures, we show that NCC condensation within the cardiac jelly is required for correct positioning of the outflow cushions. Moreover, we show that this process is essential for normal patterning of the arterial valve leaflets with disruption leading to a spectrum of valve leaflet patterning anomalies, abnormal positioning of the orifices of the coronary arteries, and abnormalities of the arterial wall. CONCLUSION: NCCs are required at earlier stages of arterial valve development than previously recognized, playing essential roles in positioning the cushions, and patterning the valve leaflets. Abnormalities in the process of NCC condensation at early stages of outflow cushion formation may provide a common mechanism underlying BAV, and also explain the link with arterial wall anomalies and outflow malalignment defects.

Cardiac neural crest orchestrates remodeling and functional maturation of mouse semilunar valves.

Congenital anomalies of the aortic valve are common and are associated with progressive valvular insufficiency and/or stenosis. In addition, aneurysm, coarctation, and dissection of the ascending aorta and aortic arch are often associated conditions that complicate patient management and increase morbidity and mortality. These associated aortopathies are commonly attributed to turbulent hemodynamic flow through the malformed valve leading to focal defects in the vessel wall. However, numerous surgical and pathological studies have identified widespread cystic medial necrosis and smooth muscle apoptosis throughout the aortic arch in affected patients. Here, we provide experimental evidence for an alternative model to explain the association of aortic vessel and valvular disease. Using mice with primary and secondary cardiac neural crest deficiencies, we have shown that neural crest contribution to the outflow endocardial cushions (the precursors of the semilunar valves) is required for late gestation valvular remodeling, mesenchymal apoptosis, and proper valve architecture. Neural crest was also shown to contribute to the smooth muscle layer of the wall of the ascending aorta and aortic arch. Hence, defects of cardiac neural crest can result in functionally abnormal semilunar valves and concomitant aortic arch artery abnormalities.

Down's syndrome-like cardiac developmental defects in embryos of the transchromosomic Tc1 mouse.

AIMS: Cardiac malformations are prevalent in trisomies of human chromosome 21 [Down's syndrome (DS)], affecting normal chamber separation in the developing heart. Efforts to understand the aetiology of these defects have been severely hampered by the absence of an accurate mouse model. Such models have proved challenging to establish because synteny with human chromosome Hsa21 is distributed across three mouse chromosomes. None of those engineered so far accurately models the full range of DS cardiac phenotypes, in particular the profound disruptions resulting from atrioventricular septal defects (AVSDs). Here, we present analysis of the cardiac malformations exhibited by embryos of the transchromosomic mouse line Tc(Hsa21)1TybEmcf (Tc1) which contains more than 90% of chromosome Hsa21 in addition to the normal diploid mouse genome. METHODS AND RESULTS: Using high-resolution episcopic microscopy and three-dimensional (3D) modelling, we show that Tc1 embryos exhibit many of the cardiac defects found in DS, including balanced AVSD with single and separate valvar orifices, membranous and muscular ventricular septal defects along with outflow tract and valve leaflet abnormalities. Frequencies of cardiac malformations (ranging from 38 to 55%) are dependent on strain background. In contrast, no comparable cardiac defects were detected in embryos of the more limited mouse trisomy model, Dp(16Cbr1-ORF9)1Rhr (Ts1Rhr), indicating that trisomy of the region syntenic to the Down's syndrome critical region, including the candidate genes DSCAM and DYRK1A, is insufficient to yield DS cardiac abnormalities. CONCLUSION: The Tc1 mouse line provides a suitable model for studying the underlying genetic causes of the DS AVSD cardiac phenotype.

Endothelial expression of bone morphogenetic protein receptor type 1a is required for atrioventricular valve formation.

BACKGROUND: Atrioventricular canal defects account for 4% of all congenital heart anomalies. They arise from failure of endocardial cushion formation, a process dependent on transition of endothelial cells into clustered mesenchymal cells in the mid-atrioventricular septum. To date, the genetic signals necessary for atrioventricular canal defects are poorly understood. We hypothesized that bone morphogenetic protein signaling in cardiac endothelial cells may be crucial to this process. METHODS: To study the role of bone morphogenetic protein receptors (Bmpr) in the developing heart, we created knockout mice with inactivation of Bmpr1a selectively in endocardium. Two strains of null mice were created: one with constitutive endothelial-specific knockout of Bmpr1a and one with time-inducible, endothelial-specific knockout of Bmpr1a. Embryos and animals were analyzed by microscopy, RNA in situ hybridization, and microangiography. RESULTS: Animals with null mutation of Bmpr1a in endothelium were embryonic lethal at E11.5 to 12.0 and demonstrated absence of endocardial cushion formation. Embryos failed to form atrioventricular valves and adjacent septa. Endocardial knockout of Bmpr1a did not affect development of the outflow tract or aortic arches. Using time-inducible, cell-specific knockout mice, we show that Bmpr1a has two functions in the developing atrioventricular canal: to induce endocardial endothelial-mesenchymal transition, and to pattern the septal mesenchyme into endocardial cushions. We demonstrate that these processes are temporally linked to expression of the transcription factors Id1 and Id3. CONCLUSIONS: Endocardial cushion formation is dependent on cell-specific expression of Bmpr1a. Our results suggest that Bmpr1a-mediated signaling is a crucial pathway involved in pathogenesis of atrioventricular septal and valve malformations, which are among the most common congenital heart defects in humans.