Molecular Pathways and Animal Models of Truncus Arteriosus.

In normal cardiovascular development in birds and mammals, the outflow tract of the heart is divided into two distinct channels to separate the oxygenated systemic blood flow from the deoxygenated pulmonary circulation. When the process of outflow tract septation fails, a single common outflow vessel persists resulting in a serious clinical condition known as persistent truncus arteriosus or common arterial trunk. In this chapter, we will review molecular pathways and the cells that are known to play a role in the formation and development of the outflow tract and how genetic manipulation of these pathways in animal models can result in common arterial trunk.

Human Genetics of Truncus Arteriosus.

Integrated human genetics and molecular/developmental biology studies have revealed that truncus arteriosus is highly associated with 22q11.2 deletion syndrome. Other congenital malformation syndromes and variants in genes encoding TBX, GATA, and NKX transcription factors and some signaling proteins have also been reported as its etiology.

Over-expression of Fgf8 in cardiac neural crest cells leads to persistent truncus arteriosus.

During cardiogenesis, the outflow tract undergoes a complicated morphogenesis, including the re-alignment of the great blood vessels, and the separation of aorta and pulmonary trunk. The deficiency of FGF8 in the morphogenesis of outflow tract has been well studied, however, the effect of over-dosed FGF8 on the development of outflow tract remains unknown. In this study, Rosa26R-Fgf8 knock-in allele was constitutively activated by Wnt1-cre transgene in the mouse neural crest cells presumptive for the endocardial cushion of outflow tract. Surprisingly, Wnt1-cre; Rosa26R-Fgf8 mouse embryos exhibited persistent truncus arteriosus and died prior to E15.5. The cardiac neural crest cells in Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus did not degenerate as in WT controls, but proliferated into a thickened endocardial cushion and then, blocked the blood outflow from cardiac chambers into the lungs, which resulted in the embryonic lethality. Although the spiral aorticopulmonary septum failed to form, the differentiaion of the endothelium and smooth muscle in the Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus were impacted little. However, lineage tracing assay showed that the neural crest derived cells aggregated in the cushion layer, but failed to differentiate into the endothelium of Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus. Further investigation displayed the reduced p-Akt and p-Erk immunostaining, and the decreased Bmp2 and Bmp4 transcription in the endothelium of Wnt1-cre; Rosa26R-Fgf8 truncus arteriosus. Our findings suggested that Fgf8 over-expression in cardiac neural crest impaired the formation of aorticopulmonary septum by suppressing the endothelial differentiation and stimulating the proliferation of endocardial cushion cells, which implicated a novel etiology of persistent truncus arteriosus.

Conditional inactivation of Foxc1 and Foxc2 in neural crest cells leads to cardiac abnormalities.

Cardiac neural crest cells (cNCCs) are required for normal heart development. cNCCs are a multipotent and migratory cell lineage that differentiates into multiple cell types. cNCCs migrate into the developing heart to contribute to the septation of the cardiac outflow tract (OFT). Foxc1 and Foxc2 are closely related members of the FOX (Forkhead box) transcription factor family and are expressed in cNCC during heart development. However, the precise role of Foxc1 and Foxc2 in cNCCs has yet to be fully described. We found that compound NCC-specific Foxc1;Foxc2 mutant embryos exhibited persistent truncus arteriosus (PTA), ventricular septal defects (VSDs), and thinning of the ventricular myocardium. Loss of Foxc1/c2 expression in cNCCs resulted in abnormal patterns of cNCC migration into the OFT without the formation of the aorticopulmonary septum. Further, loss of Foxc1 expression in cNCCs resulted in normal OFT development but abnormal ventricular septal formation. In contrast, loss of Foxc2 expression in NCCs led to no obvious cardiac abnormalities. Together, we provide evidence that Foxc1 and Foxc2 in cNCCs are cooperatively required for proper cNCC migration, the formation of the OFT septation, and the development of the ventricles. Our data also suggests that Foxc1 expression may play a larger role in ventricular development compared to Foxc2.

Functional characterization of a novel PBX1 de novo missense variant identified in a patient with syndromic congenital heart disease.

Pre-B cell leukemia factor 1 (PBX1) is an essential developmental transcription factor, mutations in which have recently been associated with CAKUTHED syndrome, characterized by multiple congenital defects including congenital heart disease (CHD). During analysis of a whole-exome-sequenced cohort of heterogeneous CHD patients, we identified a de novo missense variant, PBX1:c.551G>C p.R184P, in a patient with tetralogy of Fallot with absent pulmonary valve and extra-cardiac phenotypes. Functional analysis of this variant by creating a CRISPR-Cas9 gene-edited mouse model revealed multiple congenital anomalies. Congenital heart defects (persistent truncus arteriosus and ventricular septal defect), hypoplastic lungs, hypoplastic/ectopic kidneys, aplastic adrenal glands and spleen, as well as atretic trachea and palate defects were observed in the homozygous mutant embryos at multiple stages of development. We also observed developmental anomalies in a proportion of heterozygous embryos, suggestive of a dominant mode of inheritance. Analysis of gene expression and protein levels revealed that although Pbx1 transcripts are higher in homozygotes, amounts of PBX1 protein are significantly decreased. Here, we have presented the first functional model of a missense PBX1 variant and provided strong evidence that p.R184P is disease-causal. Our findings also expand the phenotypic spectrum associated with pathogenic PBX1 variants in both humans and mice.

GATA6 mutations: Characterization of two novel patients and a comprehensive overview of the GATA6 genotypic and phenotypic spectrum.

The first human mutations in GATA6 were described in a cohort of patients with persistent truncus arteriosus, and the phenotypic spectrum has expanded since then. This study underscores the broad phenotypic spectrum by presenting two patients with de novo GATA6 mutations, both exhibiting complex cardiac defects, pancreatic, and other abnormalities. Furthermore, we provided a detailed overview of all published human genetic variation in/near GATA6 published to date and the associated phenotypes (n = 78). We conclude that the most common phenotypes associated with a mutation in GATA6 were structural cardiac and pancreatic abnormalities, with a penetrance of 87 and 60%, respectively. Other common malformations were gallbladder agenesis, congenital diaphragmatic hernia, and neurocognitive abnormalities, mostly developmental delay. Fifty-eight percent of the mutations were de novo, and these patients more often had an anomaly of intracardiac connections, an anomaly of the great arteries, and hypothyroidism, compared with those with inherited mutations. Functional studies mostly support loss-of-function as the pathophysiological mechanism. In conclusion, GATA6 mutations give a wide range of phenotypic defects, most frequently malformations of the heart and pancreas. This highlights the importance of detailed clinical evaluation of identified carriers to evaluate their full phenotypic spectrum.

Dysregulation of TBX1 dosage in the anterior heart field results in congenital heart disease resembling the 22q11.2 duplication syndrome.

Non-allelic homologous recombination events on chromosome 22q11.2 during meiosis can result in either the deletion (22q11.2DS) or duplication (22q11.2DupS) syndrome. Although the spectrum and frequency of congenital heart disease (CHD) are known for 22q11.2DS, there is less known for 22q11.2DupS. We now evaluated cardiac phenotypes in 235 subjects with 22q11.2DupS including 102 subjects we collected and 133 subjects that were previously reported as a confirmation and found 25% have CHD, mostly affecting the cardiac outflow tract (OFT). Previous studies have shown that global loss or gain of function (LOF; GOF) of mouse Tbx1, encoding a T-box transcription factor mapping to the region of synteny to 22q11.2, results in similar OFT defects. To further evaluate Tbx1 function in the progenitor cells forming the cardiac OFT, termed the anterior heart field, Tbx1 was overexpressed using the Mef2c-AHF-Cre driver (Tbx1 GOF). Here we found that all resulting conditional GOF embryos had a persistent truncus arteriosus (PTA), similar to what was previously reported for conditional Tbx1 LOF mutant embryos. To understand the basis for the PTA in the conditional GOF embryos, we found that proliferation in the Mef2c-AHF-Cre lineage cells before migrating to the heart, was reduced and critical genes were oppositely changed in this tissue in Tbx1 GOF embryos versus conditional LOF embryos. These results suggest that a major function of TBX1 in the AHF is to maintain the normal balance of expression of key cardiac developmental genes required to form the aorta and pulmonary trunk, which is disrupted in 22q11.2DS and 22q11.2DupS.

TBX20 loss-of-function mutation responsible for familial tetralogy of Fallot or sporadic persistent truncus arteriosus.

Congenital heart disease (CHD), the most common form of developmental abnormality in humans, remains a leading cause of morbidity and mortality in neonates. Genetic defects have been recognized as the predominant causes of CHD. Nevertheless, CHD is of substantial genetic heterogeneity and the genetic defects underlying CHD in most cases remain unclear. In the current study, the coding regions and splicing junction sites of the TBX20 gene, which encodes a T-box transcription factor key to cardiovascular morphogenesis, were sequenced in 175 unrelated patients with CHD, and a novel heterozygous TBX20 mutation, p.K274X, was identified in an index patient with tetralogy of Fallot (TOF). Genetic analysis of the proband's available family members showed that his father, elder brother and son had also TOF. In addition, his father and elder brother had also atrial septal defect, and his niece had persistent truncus arteriosus and ventricular septal defect. Analysis of the pedigree revealed that the mutation co-segregated with CHD transmitted in an autosomal dominant fashion, with complete penetrance. The nonsense mutation, which was absent in the 800 control chromosomes, was predicted to produce a truncated protein with only the amino terminus and partial T-box domain left. Functional analyses by using a dual-luciferase reporter assay system showed that the mutant TBX20 lost the ability to transactivate the target gene ANF. Furthermore, the mutation reduced the synergistic activation between TBX20 and NKX2.5 as well as GATA4, two other transcriptional factors previously associated with various CHD, encompassing TOF. This study firstly links TBX20 loss-of-function mutation to familial TOF or sporadic persistent truncus arteriosus, providing novel insight into the molecular pathogenesis of CHD.

ASK1 mediates the teratogenicity of diabetes in the developing heart by inducing ER stress and inhibiting critical factors essential for cardiac development.

Maternal diabetes in mice induces heart defects similar to those observed in human diabetic pregnancies. Diabetes enhances apoptosis and suppresses cell proliferation in the developing heart, yet the underlying mechanism remains elusive. Apoptosis signal-regulating kinase 1 (ASK1) activates the proapoptotic c-Jun NH2-terminal kinase 1/2 (JNK1/2) leading to apoptosis, suggesting a possible role of ASK1 in diabetes-induced heart defects. We aimed to investigate whether ASK1 is activated in the heart and whether deleting the Ask1 gene blocks diabetes-induced adverse events and heart defect formation. The ASK1-JNK1/2 pathway was activated by diabetes. Deleting Ask1 gene significantly reduced the rate of heart defects, including ventricular septal defects (VSDs) and persistent truncus arteriosus (PTA). Additionally, Ask1 deletion diminished diabetes-induced JNK1/2 phosphorylation and its downstream transcription factors and endoplasmic reticulum (ER) stress markers. Consistent with this, caspase activation and apoptosis were blunted. Ask1 deletion blocked the increase in cell cycle inhibitors (p21 and p27) and the decrease in cyclin D1 and D3 and reversed diabetes-repressed cell proliferation. Ask1 deletion also restored the expression of BMP4, NKX2.5, and GATA5, Smad1/5/8 phosphorylation, whose mutations or deletion result in reduced cell proliferation, VSD, and PTA formation. We conclude that ASK1 may mediate the teratogenicity of diabetes through activating the JNK1/2-ER stress pathway and inhibiting cell cycle progression, thereby impeding the cardiogenesis pathways essential for ventricular septation and outflow tract development.

Positional mapping of PRKD1, NRP1 and PRDM1 as novel candidate disease genes in truncus arteriosus.

BACKGROUND: Truncus arteriosus (TA) is characterised by failure of septation of the outflow tract into aortic and pulmonary trunks and is associated with high morbidity and mortality. Although ranked among the least common congenital heart defects, TA provides an excellent model for the role of individual genes in cardiac morphogenesis as exemplified by TBX1 deficiency caused by point mutations or, more commonly, hemizygosity as part of the 22q11.2 deletion syndrome. The latter genetic lesion, however, is only observed in a proportion of patients with TA, which suggests the presence of additional disease genes. OBJECTIVE: To identify novel genes that cause Mendelian forms of TA. METHODS AND RESULTS: We exploited the occurrence of monogenic forms of TA in the Saudi population, which is characterised by high consanguinity, a feature conducive to the occurrence of Mendelian phenocopies of complex phenotypes as we and others have shown. Indeed, we demonstrate in two multiplex consanguineous families that we are able to map TA to regions of autozygosity in which whole-exome sequencing revealed homozygous truncating mutations in PRKD1 (encoding a kinase derepressor of MAF2) and NRP1 (encoding a coreceptor of vascular endothelial growth factor (VEGFA)). Previous work has demonstrated that Prkd1(-/-) is embryonic lethal and that its tissue-specific deletion results in abnormal heart remodelling, whereas Nrp1(-/-) develops TA. Surprisingly, molecular karyotyping to exclude 22q11.2 deletion syndrome in the replication cohort of 17 simplex TA cases revealed a de novo hemizygous deletion that encompasses PRDM1, deficiency of which also results in TA phenotype in mouse. CONCLUSIONS: Our results expand the repertoire of molecular lesions in chromatin remodelling and transcription factors that are implicated in the pathogenesis of congenital heart disease in humans and attest to the power of monogenic forms of congenital heart diseases as a complementary approach to dissect the genetics of these complex phenotypes.

Heparan sulfate expression in the neural crest is essential for mouse cardiogenesis.

Impaired heparan sulfate (HS) synthesis in vertebrate development causes complex malformations due to the functional disruption of multiple HS-binding growth factors and morphogens. Here, we report developmental heart defects in mice bearing a targeted disruption of the HS-generating enzyme GlcNAc N-deacetylase/GlcN N-sulfotransferase 1 (NDST1), including ventricular septal defects (VSD), persistent truncus arteriosus (PTA), double outlet right ventricle (DORV), and retroesophageal right subclavian artery (RERSC). These defects closely resemble cardiac anomalies observed in mice made deficient in the cardiogenic regulator fibroblast growth factor 8 (FGF8). Consistent with this, we show that HS-dependent FGF8/FGF-receptor2C assembly and FGF8-dependent ERK-phosphorylation are strongly reduced in NDST1(-/-) embryonic cells and tissues. Moreover, WNT1-Cre/LoxP-mediated conditional targeting of NDST function in neural crest cells (NCCs) revealed that their impaired HS-dependent development contributes strongly to the observed cardiac defects. These findings raise the possibility that defects in HS biosynthesis may contribute to congenital heart defects in humans that represent the most common type of birth defect.

Functional interaction between Foxd3 and Pax3 in cardiac neural crest development.

The transcription factors Foxd3 and Pax3 are important early regulators of neural crest (NC) progenitor cell properties. Homozygous mutations of Pax3 or a homozygous NC-specific deletion of Foxd3 cause marked defects in most NC derivatives, but neither loss of both Foxd3 alleles nor loss of one Pax3 allele alone greatly affects overall development of cardiac NC derivatives. In contrast, compound mutant embryos homozygous for a NC-specific Foxd3 mutation and heterozygous for Pax3 have fully penetrant persistent truncus arteriosus, severe thymus hypoplasia, and midgestation lethality. Foxd3; Pax3 compound mutant embryos have increased cell death in the neural folds and a drastic early reduction of NC cells, with an almost complete absence of NC caudal to the first pharyngeal arch. The genetic interaction between these genes implicates gene dosage-sensitive roles for Foxd3 and Pax3 in cardiac NC progenitors. Foxd3 and Pax3 act together to affect survival and maintenance of cardiac NC progenitors, and loss of these progenitors catastrophically affects key aspects of later cardiovascular development.

The effect of p.Arg25Cys alteration in NKX2-5 on conotruncal heart anomalies: mutation or polymorphism?

Heterozygous mutations in the NKX2-5 gene of patients with various congenital heart defects have been reported. Most of the congenital heart defects associated with the mutations in the NKX2-5 gene are conotruncal heart anomalies, primarily the tetralogy of Fallot. In this study, the authors screened 72 Turkish children with conotruncal heart anomalies and 185 healthy control subjects to find the NKX2-5 alterations. They found one previously documented NKX2-5 missense alteration, heterozygous c.73C>T (p.Arg25Cys), in a 10-year-old boy with tetralogy of Fallot. The same heterozygous alteration was found also in the patient's healthy father and in two unrelated persons in the healthy control group. The current study shows for the first time the presence of p.Arg25Cys in healthy control subjects other than African Americans. These results show that no genetic support exists for the pathogenecity of this alteration, although a previous in vitro study and theoretical predictions suggest a structural/functional difference in the altered protein region.

Maternal diabetes induces congenital heart defects in mice by altering the expression of genes involved in cardiovascular development.

BACKGROUND: Congenital heart defects are frequently observed in infants of diabetic mothers, but the molecular basis of the defects remains obscure. Thus, the present study was performed to gain some insights into the molecular pathogenesis of maternal diabetes-induced congenital heart defects in mice. METHODS AND RESULTS: We analyzed the morphological changes, the expression pattern of some genes, the proliferation index and apoptosis in developing heart of embryos at E13.5 from streptozotocin-induced diabetic mice. Morphological analysis has shown the persistent truncus arteriosus combined with a ventricular septal defect in embryos of diabetic mice. Several other defects including defective endocardial cushion (EC) and aberrant myofibrillogenesis have also been found. Cardiac neural crest defects in experimental embryos were analyzed and validated by the protein expression of NCAM and PGP 9.5. In addition, the protein expression of Bmp4, Msx1 and Pax3 involved in the development of cardiac neural crest was found to be reduced in the defective hearts. The mRNA expression of Bmp4, Msx1 and Pax3 was significantly down-regulated (p < 0.001) in the hearts of experimental embryos. Further, the proliferation index was significantly decreased (p < 0.05), whereas the apoptotic cells were significantly increased (p < 0.001) in the EC and the ventricular myocardium of the experimental embryos. CONCLUSION: It is suggested that the down-regulation of genes involved in development of cardiac neural crest could contribute to the pathogenesis of maternal diabetes-induced congenital heart defects.

Wnt5a is required for cardiac outflow tract septation in mice.

Lack of septation of the cardiac outflow tract (OFT) results in persistent truncus arteriosus (PTA), a form of congenital heart disease. The outflow myocardium expands through addition of cells originating from the pharyngeal mesoderm referred to as secondary/anterior heart field, whereas cardiac neural crest (CNC) cell-derived mesenchyme condenses to form an aortopulmonary septum. We show for the first time that a mutation in Wnt5a in mice leads to PTA. We provide evidence that Wnt5a is expressed in the pharyngeal mesoderm adjacent to CNC cells in both mouse and chicken embryos and in the myocardial cell layer of the conotruncus at the time when CNC cells begin to form the aortopulmonary septum in mice. Although expression domains of secondary heart field markers are not altered in Wnt5a mutant embryos, the expression of CNC cell marker PlexinA2 is significantly reduced. Stimulation of CNC cells with Wnt5a protein elicits Ca2+ transients, suggesting that CNC cells are capable of responding to Wnt5a. We propose a novel model in which Wnt5a produced in the OFT by cells originating from the pharyngeal mesoderm signals to adjacent CNC cells during formation of the aortopulmonary septum through a noncanonical pathway via localized intracellular increases in Ca2+.

Prenatal diagnosis of an unexpected interstitial 22q11.2 deletion causing truncus arteriosus and thymic hypoplasia in a ring 22 chromosome derived from a maternally inherited paracentric inversion.

OBJECTIVE: To present the prenatal diagnosis of an interstitial 22q11.2 deletion involving a ring 22 chromosome associated with truncus arteriosus and a hypoplastic thymus. CASE: Following the sonographic diagnosis of a cystic hygroma at 12 weeks of gestation, chromosome analysis revealed a ring 22 chromosome. RESULTS: Ring chromosomes typically result in the deletion of genetic material from the distal long and short arms of the affected chromosome. The presence of an interstitial deletion in a ring chromosome is therefore unusual. FISH analysis revealed an unexpected deletion involving the TUPLE1 gene in the DiGeorge/Velocardiofacial syndrome region in 22q11.2. Maternal chromosome analysis revealed the cause of the apparent interstitial deletion, a paracentric inversion in the long arm of chromosome 22, resulting in the distal long arm of 22q being located adjacent to the centromere and the proximal end being located near the telomere. The fetus was subsequently diagnosed with truncus arteriosus and a hypoplastic thymus, consistent with DiGeorge syndrome. CONCLUSION: The ring chromosome 22 found in the fetus appears to have been derived from a rearrangement of the mother's inverted 22, resulting in ring formation and loss of the end of the distal long arm of the inverted 22, including the TUPLE1 locus, causing DiGeorge syndrome in the fetus. The apparent interstitial deletion was actually a terminal deletion in a maternally inherited rearranged chromosome 22.

Chromosome 22q11 deletions in patients with conotruncal heart defects.

We performed this study to determine the frequency of 22q11 deletions and associated phenotypic features and abnormalities in conotruncal heart defects. Sixty-one patients with conotruncal heart defects, including tetralogy of Fallot (TOF; n = 32), pulmonary atresia/ventricular septal defect (PAVSD; n = 12), double-outlet right ventricle (DORV; n = 5), transposition of the great arteries (TGA; n = 4 ), truncus arteriosus (TA; n = 4), subpulmonary ventricular septal defect (SPVSD; n = 3), and interrupted aortic arch (IAA; n = 1), were enrolled in this study and screened for 22q11 deletions by the fluorescence in situ hybridization technique. Phenotypic features and associated abnormalities, including submucosal cleft palate, abnormal facies, square nose, nasal voice, abnormal ears, long and slender fingers, delayed development, mental retardation, delayed growth, short stature, and hypocalcemia, were examined in these patients. Nine of 61 patients (14.8%) had 22q11 deletions, including 100% of IAA, 50% of TA, 33.3% of SPVSD, 33.3% of PAVSD, and 3.1% of TOF. Deletions were not detected in DORV and TGA. In all patients with 22q11 deletions, > or =1 phenotypic features or associated abnormalities were observed. A subgroup of patients with IAA, TA, SPVSD, and PAVSD associated with phenotypic features or abnormalities warrants evaluation for the presence of 22q11 deletions.