Human Genetics of Tricuspid Atresia and Univentricular Heart.

Tricuspid atresia (TA) is a rare congenital heart condition that presents with a complete absence of the right atrioventricular valve. Because of the rarity of familial and/or isolated cases of TA, little is known about the potential genetic abnormalities contributing to this condition. Potential responsible chromosomal abnormalities were identified in exploratory studies and include deletions in 22q11, 4q31, 8p23, and 3p as well as trisomies 13 and 18. In parallel, potential culprit genes include the ZFPM2, HEY2, NFATC1, NKX2-5, MYH6, and KLF13 genes. The aim of this chapter is to expose the genetic components that are potentially involved in the pathogenesis of TA in humans. The large variability in phenotypes and genotypes among cases of TA suggests a genetic network that involves many components yet to be unraveled.

Molecular Pathways and Animal Models of Tricuspid Atresia and Univentricular Heart.

The process of valve formation is a complex process that involves intricate interplay between various pathways at precise times. Although we have not completely elucidated the molecular pathways that lead to normal valve formation, we have identified a few major players in this process. We are now able to implicate TGF-ß, BMP, and NOTCH as suspects in tricuspid atresia (TA), as well as their downstream targets: NKX2-5, TBX5, NFATC1, GATA4, and SOX9. We know that the TGF-ß and the BMP pathways converge on the SMAD4 molecule, and we believe that this molecule plays a very important role to tie both pathways to TA. Similarly, we look at the NOTCH pathway and identify the HEY2 as a potential link between this pathway and TA. Another transcription factor that has been implicated in TA is NFATC1. While several mouse models exist that include part of the TA abnormality as their phenotype, no true mouse model can be said to represent TA. Bridging this gap will surely shed light on this complex molecular pathway and allow for better understanding of the disease process.

Monochorionic Twin Discordance for Horseshoe Lung and Tricuspid Atresia.

Background: The horseshoe lung is a congenital malformation in which the bases of the right and the left lung are fused. Case report: We describe a monochorionic twin gestation with malformation discordance. The abnormal twin had a horseshoe lung with hypoplasia of the right lung, tricuspid atresia, cleft lip, and a pelvic right kidney. Conclusion: The discordance of anomalies in this monochorionic twin suggests that a postzygotic mutation, epigenetic change, or environmental factors may be responsible for these malformations.

Identification and analysis of KLF13 variants in patients with congenital heart disease.

BACKGROUND: The protein Kruppel-like factor 13 (KLF13) is a member of the KLF family and has been identified as a cardiac transcription factor that is involved in heart development. However, the relationship between KLF13 variants and CHDs in humans remains largely unknown. The present study aimed to screen the KLF13 variants in CHD patients and genetically analyze the functions of these variants. METHODS: KLF13 variants were sequenced in a cohort of 309 CHD patients and population-matched healthy controls (n = 200) using targeted sequencing. To investigate the effect of variants on the functional properties of the KLF13 protein, the expression and subcellular localization of the protein, as well as the transcriptional activities of downstream genes and physical interactions with other transcription factors, were assessed. RESULTS: Two heterozygous variants, c.487C > T (P163S) and c.467G > A (S156N), were identified in two out of 309 CHD patients with tricuspid valve atresia and transposition of the great arteries, respectively. No variants were found among healthy controls. The variant c.467G > A (S156N) had increased protein expression and enhanced functionality compared with the wild type, without affecting the subcellular localization. The other variant, c.487C > T (P163S), did not show any abnormalities in protein expression or subcellular localization; however, it inhibited the transcriptional activities of downstream target genes and physically interacted with TBX5, another cardiac transcription factor. CONCLUSION: Our results show that the S156N and P163S variants may affect the transcriptional function of KLF13 and physical interaction with TBX5. These results identified KLF13 as a potential genetic risk factor for congenital heart disease.

Canonical wnt signaling regulates atrioventricular junction programming and electrophysiological properties.

RATIONALE: Proper patterning of the atrioventricular canal (AVC) is essential for delay of electrical impulses between atria and ventricles, and defects in AVC maturation can result in congenital heart disease. OBJECTIVE: To determine the role of canonical Wnt signaling in the myocardium during AVC development. METHODS AND RESULTS: We used a novel allele of ß-catenin that preserves ß-catenin's cell adhesive functions but disrupts canonical Wnt signaling, allowing us to probe the effects of Wnt loss of function independently. We show that the loss of canonical Wnt signaling in the myocardium results in tricuspid atresia with hypoplastic right ventricle associated with the loss of AVC myocardium. In contrast, ectopic activation of Wnt signaling was sufficient to induce formation of ectopic AV junction-like tissue as assessed by morphology, gene expression, and electrophysiological criteria. Aberrant AVC development can lead to ventricular pre-excitation, a characteristic feature of Wolff-Parkinson-White syndrome. We demonstrate that postnatal activation of Notch signaling downregulates canonical Wnt targets within the AV junction. Stabilization of ß-catenin protein levels can rescue Notch-mediated ventricular pre-excitation and dysregulated ion channel gene expression. CONCLUSIONS: Our data demonstrate that myocardial canonical Wnt signaling is an important regulator of AVC maturation and electric programming upstream of Tbx3. Our data further suggest that ventricular pre-excitation may require both morphological patterning defects, as well as myocardial lineage reprogramming, to allow robust conduction across accessory pathway tissue.

A case of Seckel syndrome with tricuspid atresia.

Seckel syndrome is an autosomal recessive disease presenting with marked growth retardation, microcephalic dwarfism, some facial and skeletal abnormalities. Tricuspid atresia is a rare and life threatening cyanotic congenital heart diseases, with an incidence of 1% to 3%. It is feature of the anatomically normally related great arteries with a large ventricular septum defect and stenosis of right ventricular outflow tract. Tricuspid atresia has never been reported in patients with Seckel syndrome. Here we report a 15-day-old girl baby diagnosed as having Seckel syndrome with tricuspid atresia.

A syndrome of tricuspid atresia in mice with a targeted mutation of the gene encoding Fog-2.

Tricuspid atresia (TA) is a common form of congenital heart disease, accounting for 1-3% of congenital cardiac disorders. TA is characterized by the congenital agenesis of the tricuspid valve connecting the right atrium to the right ventricle and both an atrial septal defect (ASD) and a ventricular septal defect (VSD). Some patients also have pulmonic stenosis, persistence of a left-sided superior vena cava or transposition of the great arteries. Most cases of TA are sporadic, but familial occurrences with disease in multiple siblings have been reported. Gata4 is a zinc-finger transcription factor with a role in early cardiac development. Gata4-deficient mice fail to form a ventral heart tube and die of circulatory failure at embryonic day (E) 8.5 (refs 6,7). Zfpm2 (also known as Fog-2) is a multi-zinc-finger protein that is co-expressed with Gata4 in the developing heart beginning at E8.5 (refs 8-10). Zfpm2 interacts specifically with the N-terminal zinc finger of Gata4 and represses Gata4-dependent transcription. Here we use targeted mutagenesis to explore the role of Zfpm2 in normal cardiac development. Zfpm2-deficient mice died of congestive heart failure at E13 with a syndrome of tricuspid atresia that includes an absent tricuspid valve, a large ASD, a VSD, an elongated left ventricular outflow tract, rightward displacement of the aortic valve and pulmonic stenosis. These mice also display hypoplasia of the compact zone of the left ventricle. Our findings indicate the importance of Zfpm2 in the normal looping and septation of the heart and suggest a genetic basis for the syndrome of tricuspid atresia.

Genetic and functional analyses detect one pathological NFATC1 mutation in a Chinese tricuspid atresia family.

BACKGROUND: Cardiac valvulogenesis is a highly conserved process among vertebrates and cause unidirectional flow of blood in the heart. It was precisely regulated by signal pathways such as VEGF, NOTCH, and WNT and transcriptional factors such as TWIST1, TBX20, NFATC1, and SOX9. Tricuspid atresia refers to morphological deficiency of the valve and confined right atrioventricular traffic due to tricuspid maldevelopment, and is one of the most common types of congenital valve defects. METHODS: We recruited a healthy couple with two fetuses aborted due to tricuspid atresia and identified related gene mutations using whole-exome sequencing. We then discussed the pathogenic significance of this mutation by bioinformatic and functional analyses. RESULTS: PROVEAN, PolyPhen, MutationTaster, and HOPE indicated the mutation could change the protein function and cause disease; Western blotting showed the expression of NFATC1 c.964G>A mutation was lower than the wild type. What's more, dual-luciferase reporter assay showed the transcriptional activity of NFATC1 was impact by mutation and the expression of downstream DEGS1 was influenced. CONCLUSION: Taken together, the c.964G>A mutation might be pathological and related to the occurrence of disease. Our research tended to deepen the understanding of etiology of tricuspid atresia and gene function of NFATC1, and provide some references or suggestions for genetic diagnosis of tricuspid atresia.

Prenatal diagnosis of tricuspid atresia: intrauterine course and outcome.

OBJECTIVE: To evaluate the intrauterine course and outcome of tricuspid atresia detected in the fetus. METHODS: This was a retrospective review of all confirmed cases of tricuspid atresia detected prenatally between 1998 and 2006 in three tertiary referral centers in Germany. RESULTS: Fifty-four cases of tricuspid atresia were detected prenatally during the study period and confirmed postnatally: 28 (51.9%) cases had a concordant ventriculoarterial connection of which 14 also had pulmonary outflow obstruction; 25 (46.3%) cases had a discordant ventriculoarterial connection of which 14 also had aortic outflow obstruction, six had pulmonary outflow tract obstruction and two had other associated intracardiac anomalies; and one (1.9%) had a common arterial trunk. The peak velocity index for veins in the ductus venosus was significantly elevated in 19 of the 37 (51.4%) cases assessed; however, this finding did not correlate with adverse intrauterine outcome. There were associated extracardiac anomalies in 12 cases: five with chromosomal anomalies, two with VACTERL association, one with unilateral renal agenesis, one with hypospadia, one with hydrothorax, one with megacystis and one with agenesis of the ductus venosus. Seventeen of the 54 (31.5%) cases underwent termination of pregnancy, two (3.7%) died in utero, two (3.7%) died in infancy and 33 (61.1%) children survived with a median follow-up of 26 (range, 12-120) months. Prenatal echocardiography correctly anticipated the postnatal course and the need for neonatal intervention in 29/35 (82.9%) continued pregnancies; in the remaining six (17.1%) cases the right outflow tract obstruction had been underestimated. CONCLUSIONS: Tricuspid atresia and the frequently associated intracardiac anomalies can be diagnosed in the fetus with considerable accuracy. A thorough search for extracardiac malformations should be performed in order to rule out chromosomal anomalies and multiple malformation syndromes. Elevated pulsatility in the ductus venosus does not indicate cardiac failure. The short-term overall survival in continued pregnancies in our study exceeded 89%, with the greatest rate of loss being in the first year of postnatal life.

Tetralogy of fallot and other congenital heart defects in Hey2 mutant mice.

Congenital malformations of the heart and circulatory system are the most common type of human birth defect. Recent studies have implicated the Notch signaling pathway in human cardiac development by demonstrating abnormalities of the JAG1 gene as the basis for Alagille syndrome and some cases of isolated tetralogy of Fallot or pulmonic stenosis. How the Notch pathway acts in cardiac development remains unknown, but the Hey family of basic helix-loop-helix (bHLH) transcription factors are candidates for mediating Notch signaling in the developing cardiovascular system. Here, we use gene targeting to determine the developmental functions of mouse Hey2, a Hey family member that is expressed during the embryonic development of the heart, arteries, and other organs. Homozygotes for the Hey2 mutant allele display a spectrum of cardiac malformations including ventricular septal defects, tetralogy of Fallot, and tricuspid atresia, defects that resemble those associated with mutations of human JAG1. These results establish Hey2 as an important regulator of cardiac morphogenesis and suggest a role for Hey2 in mediating or modulating Notch signaling in the developing heart.

Outcome after prenatal diagnosis of tricuspid atresia: a multicenter experience.

BACKGROUND: The outcome of prenatally diagnosed tricuspid atresia (TA) is undefined. We sought to characterize clinical and echocardiographic features of fetal TA and to determine factors associated with mortality. METHODS AND RESULTS: All fetuses with TA (n = 88) seen at 3 tertiary care institutions from 1990 to 2005 were reviewed. There were 58 liveborn infants (median gestational age 38 weeks, range 24-40 weeks), 4 in utero demises, 25 terminations of pregnancy, and 1 mother lost to follow-up. Obstruction was present at the pulmonary valve in 27 (45%), aortic valve in 6 (10%), and aortic arch in 15 (25%). Three neonates received compassionate care, 1 died with multiple extracardiac anomalies, 2 were lost to follow-up, and 52 liveborns were actively managed with Blalock-Taussig shunt (23), Norwood palliation (14), pulmonary artery band (10), bidirectional cavopulmonary connection (3), atrial septostomy (1), and right outflow stent (1). Of those actively managed, there were 7 (14%) of 52 who died. Kaplan-Meier estimates of survival were 91% at 1 month, 87% at 6 months, and 83% at 1 year with no subsequent deaths for 13 years. By multivariate analysis, 2 independent factors were associated with an increase in time-related mortality in the actively managed group: presence of chromosomal anomaly or syndrome (P = .005) and use of extracorporeal membrane oxygenation (P = .002). CONCLUSIONS: This is the largest study describing TA in fetus. Compared with published observations of TA diagnosed postnatally, antenatal diagnosis of TA appears to have similar short-term survival in pregnancies surviving to birth.

Developmental changes in ventricular diastolic function correlate with changes in ventricular myoarchitecture in normal mouse embryos.

Both genetic and epigenetic factors, such as abnormal hemodynamics, affect cardiac morphogenesis and the pathogenesis of congenital heart disease. Diastolic function is an important determinant of cardiac function, and tools for evaluating diastolic function in the embryo would be very valuable for assessment of cardiac performance. Using histological measurements of ventricular myoarchitecture, Doppler assessment of ventricular inflow velocities, and direct measurement of ventricular pressure, we investigated developmental changes of ventricular diastolic function in the mouse embryos from embryonic days 9.5 to 19.5. Regression analysis showed that peak velocity of A wave (an index of passive compliance) correlated with the area of trabecular myocardium in right ventricle (RV) (r2=0.92, P<0.0001) and left ventricle (LV) (r2=0.93, P<0.0001). Peak velocity of E wave (an index of active relaxation) exponentially correlated with the area of compact myocardium in RV (r2=0.98, P<0.0001) and LV (r2=0.97, P<0.0001). We used these techniques to analyze FOG-2 null embryos. FOG-2 null embryos had thin compact myocardium, higher EDP and E/A ratio, smaller -dP/dt, and diminished sucking pressure than wild-type littermates, indicating that decreased ventricular diastolic function might be the primary cause of embryonic lethality. In conclusion, during embryogenesis the development of compact myocardium tightly regulates the development of ventricular distensibility. Our study in normal mice forms the basis for future studies of embryonic cardiac function in genetically manipulated mice with abnormalities of the cardiovascular system.

Tricuspid atresia and 22q11 deletion.

Tricuspid atresia has not been reported in 22q11 microdeletions causing DiGeorge and velo-cardio-facial syndromes. We investigated the prevalence of 22q11 hemizygosity in 26 children with tricuspid atresia. Fluorescent hybridization with the Sc11.1 probe demonstrated a 22q11 microdeletion in 2 patients, one with and another without transposition of the great arteries. Both deletion patients had minor facial anomalies characteristic of DiGeorge syndrome. The present observations suggest that tricuspid atresia should be included in the list of cardiac malformations seen in del22q11 syndromes.

Surgical management of atrio ventricular septal defects with normal caryotype.

OBJECTIVE: Atrio ventricular septal defects (AVSD) with normal caryotype represent in average 25% of AVSD. They constitute a more complex group of patients characterized by frequent left sided heart obstructive lesions, raising the problem of the appropriate indications between biventricular and univentricular procedures. METHODS: Sixty-nine consecutive patients, who had AVSD with normal caryotype underwent surgery. According to the anatomical complexity there were 22 intermediate AVSD, 36 complete AVSD and 11 complex AVSD. Associated lesions were present in 68% of the patients including left sided heart anomalies in 57%. According to the size of the left ventricle (LV) evaluated on the LV/RV end diastolic diameter ratio calculated at 2D echocardiogram: right ventricular (RV) dominance was found in 29%; with border line LV in 13 patients and truly hypoplastic LV in 7 patients. Biventricular repair was always favored in case of border line LV and precluded when the LV/RV ratio was less than 0.33. RESULTS: There were 57 biventricular repairs with 10 years actuarial survival of 70 +/- 8% and respectively, 100% in the complex AVSD, 86% in the intermediate AVSD and 51% in the complete AVSD (P = 0.006). The risk factors for biventricular repair were the association to a subaortic stenosis (P = 0.01) and the severity of the mitral valve lesions (P = 0.03) that led to 38% reoperation. There were 12 univentricular palliation with 10 years survival of 66 +/- 14%. The risk factor for univentricular palliation was the association to a severe pre-operative mitral regurgitation (P = 0.005). CONCLUSIONS: Biventricular repair should be precluded in patients presenting with subaortic stenosis. Severe mitral valve anomalies lead to elevated mortality and morbidity with frequent reoperations. Univentricular repair might have larger indications and cardiac transplantation might be considered in patients with truly hypoplastic LV presenting with severe pre-operative AV valve regurgitation.

Tricuspid atresia associated with common arterial trunk and 22q11 chromosome deletion.

The case of a four-months old male with coexistent tricuspid atresia and common arterial trunk is presented. The diagnosis was made by cardiac catheterization and selective angiocardiography. Clinical considerations are discussed and the review of the available literature reveals this patient to be the tenth case reported of this very unusual association of cardiovascular defects, and the first with positive deletion of the 22q11 chromosome.

Two heterozygous mutations in NFATC1 in a patient with Tricuspid Atresia.

Tricuspid Atresia (TA) is a rare form of congenital heart disease (CHD) with usually poor prognosis in humans. It presents as a complete absence of the right atrio-ventricular connection secured normally by the tricuspid valve. Defects in the tricuspid valve are so far not associated with any genetic locus, although mutations in numerous genes were linked to multiple forms of congenital heart disease. In the last decade, Knock-out mice have offered models for cardiologists and geneticists to study the causes of congenital disease. One such model was the Nfatc1(-/-) mice embryos which die at mid-gestation stage due to a complete absence of the valves. NFATC1 belongs to the Rel family of transcription factors members of which were shown to be implicated in gene activation, cell differentiation, and organogenesis. We have previously shown that a tandem repeat in the intronic region of NFATC1 is associated with ventricular septal defects. In this report, we unravel for the first time a potential link between a mutation in NFATC1 and TA. Two heterozygous missense mutations were found in the NFATC1 gene in one indexed-case out of 19 patients with TA. The two amino-acids changes were not found neither in other patients with CHDs, nor in the control healthy population. Moreover, we showed that these mutations alter dramatically the normal function of the protein at the cellular localization, DNA binding and transcriptional levels suggesting they are disease-causing.

Differential duplication of an intronic region in the NFATC1 gene in patients with congenital heart disease.

Most forms of congenital heart disease (CHD) result from aberrations in cardiac morphogenesis including errors in septation, valve formation, and proper patterning of the great vessels. Transcription factors are key proteins that dictate mRNA synthesis rate and subsequent protein production in most eukaryotes. NFATC1 belongs to the Rel family of transcription factors. In mice, it is expressed in the embryonic heart and is restricted to the endocardium where it plays a major role in valve formation. To establish a role for NFATC1 in CHD, we started screening for mutations in the exons encoding the DNA-binding domain of NFATC1 in patients enrolled in our study on CHD in Lebanon. DNA was extracted from patients with pulmonary stenosis (PS), tricuspid atresia (TA) and ventricular septal defect (VSD). PCR amplification and DNA sequencing were done on the patients and their parents and (or) siblings. PCR amplification of the exon 7 region showed that 2 bands are obtained in 57% of patients with CHD (32/56) and in 45% of their healthy parents and (or) siblings. Sequencing of the 2 bands revealed that both are amplicons of the exon 7 region, and that the additional band harbors an additional 44 nucleotides segment in the intronic region. The homozygous form of this allele was only present in patients with VSD (2/21). A screen of a pool of 81 healthy, unrelated individuals showed no presence for the homozygous form of this allele, suggesting that NFATC1 is a potential VSD-susceptibility gene.

ZFPM2/FOG2 and HEY2 genes analysis in nonsyndromic tricuspid atresia.

Tricuspid atresia (TriAt), the third most common cyanotic congenital heart defect (CHD), consists of complete lack of tricuspid valve formation, with no connection between the right atrium and the right ventricle. To date, the genetic mechanism responsible of TriAt is still obscure. However, animal models have suggested a role of cardiogenic Zfpm2/Fog2 and Hey2 genes in the pathogenesis of TriAt. Therefore, we screened 40 individuals affected by nonsyndromic TriAt for ZFPM2/FOG2 and HEY2 gene mutations. No pathogenetic mutation has been identified, thus failing to demonstrate a major role of ZFPM2/FOG2 and HEY2 genes in the pathogenesis of human TriAt.

Detailed four-way comparative mapping and gene order analysis of the canine ctvm locus reveals evolutionary chromosome rearrangements.

Canine tricuspid valve malformation (CTVM) maps to canine chromosome 9 (CFA9), in a region syntenic with gene-dense human chromosome 17q. To define synteny blocks, we analyzed 148 markers on CFA9 using radiation hybrid mapping and established a four-way comparative map for human, mouse, rat, and dog. We identified a large number of rearrangements, allowing us to reconstruct the evolutionary history of individual synteny blocks and large chromosomal segments. A most parsimonious rearrangement scenario for all four species reveals that human chromosome 17q differs from CFA9 and the syntenic rodent chromosomes through two macroreversals of 9.2 and 23 Mb. Compared to a recovered ancestral gene order, CFA9 has undergone 11 reversals of <3 Mb and 2 reversals of >3 Mb. Interspecies reuse of breakpoints for micro- and macrorearrangements was observed. Gene order and content of the ctvm interval are best extrapolated from murine data, showing that multispecies genome rearrangement scenarios contribute to identifying gene content in canine mapping studies.