Identification of Greb1l as a genetic determinant of crisscross heart in mice showing torsion of the heart tube by shortage of progenitor cells

Despite their burden, most congenital defects remain poorly understood, due to lack of knowledge of embryological mechanisms. Here, we identify Greb1l mutants as a mouse model of crisscross heart. Based on 3D quantifications of shape changes, we demonstrate that torsion of the atrioventricular canal occurs together with supero-inferior ventricles at E10.5, after heart looping. Mutants phenocopy partial deficiency in retinoic acid signaling, which reflect overlapping pathways in cardiac precursors. Spatiotemporal gene mapping and cross-correlated transcriptomic analyses further reveal the role of Greb1l in maintaining a pool of dorsal pericardial wall precursor cells during heart tube elongation, likely by controlling ribosome biogenesis and cell differentiation. Consequently, we observe growth arrest and malposition of the outflow tract, which are predictive of abnormal tube remodeling in mutants. Our work on a rare cardiac malformation opens novel perspectives on the origin of a broader spectrum of congenital defects associated with GREB1L in humans.

Genetic profile and genotype-phenotype correlations in childhood cardiomyopathy

BACKGROUND

Genetic cardiomyopathy is a rare disease in childhood.

AIMS

To analyse clinical and genetic aspects of a paediatric cardiomyopathy population, and to establish genotype-phenotype correlations.

METHODS

We performed a retrospective study of all patients with idiopathic cardiomyopathy aged<18years in Southeast France. Secondary causes of cardiomyopathy were excluded. All data (clinical, echocardiography, genetic testing) were collected retrospectively. Patients were classified into six groups: hypertrophic cardiomyopathy; dilated cardiomyopathy; restrictive cardiomyopathy; left ventricular non-compaction; arrhythmogenic right ventricular dysplasia; and mixed cardiomyopathy. Patients who did not have a complete genetic test according to current scientific developments had another deoxyribonucleic acid blood sample during the study time. Genetic tests were considered positive if the variant found was classified as pathogenic, likely pathogenic or a variant of uncertain significance.

RESULTS

Eighty-three patients were included between 2005 and 2019. Most patients had hypertrophic cardiomyopathy (39.8%) or dilated cardiomyopathy (27.7%). The median age at diagnosis was 1.28years (interquartile range: 0.27-10.48years). Heart transplantation was performed in 30.1% of patients, and 10.8% died during follow-up. Among 64 patients with a complete genetic analysis, 64.1% had genetic anomalies, mostly in MYH7 (34.2%) and MYBPC3 (12.2%) genes. There were no differences in the whole cohort between genotype-positive and genotype-negative patients. In the hypertrophic cardiomyopathy group, 63.6% had a positive genetic test. Patients with a positive genetic test more often had extracardiac impact (38.1% vs. 8.3%; P=0.009), and more often required an implantable cardiac defibrillator (23.8% vs. 0%; P=0.025) or a heart transplant (19.1% vs. 0%; P=0.047).

CONCLUSIONS

In our population, children with cardiomyopathy had a high positive genetic test rate. Hypertrophic cardiomyopathy with a positive genetic test is associated with a worse outcome.

Prenatal screening and diagnosis of pulmonary artery anomalies

Congenital pulmonary artery anomalies are rare. Their antenatal diagnosis requires good knowledge of fetal cardiac anatomy because their clinical presentation varies depending on the type and severity of the underlying lesion. Screening of these vascular anomalies can be straightforward in some cases because of significant associated consequences that are detected easily on ultrasound, while other anomalies have considerably less obvious features. There may be an associated genetic syndrome. The aim of this review was to define anomalies of the main pulmonary artery and its branches and to propose, through the identification of suspicious findings during routine antenatal heart examination, an optimal screening method for the pulmonary artery pathway. We propose that pulmonary artery anomalies can be classified antenatally into four types of disorder. Herein we describe 14 cases subgrouped accordingly as: anomalies of the pulmonary valvular region, with stenosis or atresia of the valve (n = 4); conotruncal abnormalities (n = 4); anomalies associated with abnormal origin or course of the pulmonary artery (n = 4); and anomalies associated with abnormal growth of the pulmonary artery and its branches (n = 2). We highlight the need to differentiate the three-vessel view from the three-vessel-and-trachea view when assessing a fetus with a congenital pulmonary artery anomaly. © 2022 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Variations in the poly-histidine repeat motif of HOXA1 contribute to bicuspid aortic valve in mouse and zebrafish

Bicuspid aortic valve (BAV), the most common cardiovascular malformation occurs in 0.5-1.2% of the population. Although highly heritable, few causal mutations have been identified in BAV patients. Here, we report the targeted sequencing of HOXA1 in a cohort of BAV patients and the identification of rare indel variants in the homopolymeric histidine tract of HOXA1. In vitro analysis shows that disruption of this motif leads to a significant reduction in protein half-life and defective transcriptional activity of HOXA1. In zebrafish, targeting hoxa1a ortholog results in aortic valve defects. In vivo assays indicates that these variants behave as dominant negatives leading abnormal valve development. In mice, deletion of Hoxa1 leads to BAV with a very small, rudimentary non-coronary leaflet. We also show that 17% of homozygous Hoxa1^-1His knock-in mice present similar phenotype. Genetic lineage tracing in Hoxa1^-/- mutant mice reveals an abnormal reduction of neural crest-derived cells in the valve leaflet, which is caused by a failure of early migration of these cells.

Expanding the phenome and variome of the ROBO-SLIT pathway in congenital heart defects: toward improving the genetic testing yield of CHD

BACKGROUND

Recent studies have shown the implication of the ROBO-SLIT pathway in heart development. Within this study, we aimed to further assess the implication of the ROBO and SLIT genes mainly in bicuspid aortic valve (BAV) and other human congenital heart defects (CHD).

METHODS

We have analyzed a cohort of singleton exome sequencing data comprising 40 adult BAV patients, 20 pediatric BAV patients generated by the Pediatric Cardiac Genomics Consortium, 10 pediatric cases with tetralogy of Fallot (ToF), and one case with coarctation of the aorta. A gene-centered analysis of data was performed. To further advance the interpretation of the variants, we intended to combine more than 5 prediction tools comprising the assessment of protein structure and stability.

RESULTS

A total of 24 variants were identified. Only 4 adult BAV patients (10%) had missense variants in the ROBO and SLIT genes. In contrast, 19 pediatric cases carried variants in ROBO or SLIT genes (61%). Three BAV patients with a severe phenotype were digenic. Segregation analysis was possible for two BAV patients. For the homozygous ROBO4: p.(Arg776Cys) variant, family segregation was consistent with an autosomal recessive pattern of inheritance. The ROBO4: c.3001 + 3G > A variant segregates with the affected family members. Interestingly, these variants were also found in two unrelated patients with ToF highlighting that the same variant in the ROBO4 gene may underlie different cardiac phenotypes affecting the outflow tract development.

CONCLUSION

Our results further reinforce the implication of the ROBO4 gene not only in BAV but also in ToF hence the importance of its inclusion in clinical genetic testing. The remaining ROBO and SLIT genes may be screened in patients with negative or inconclusive genetic tests.

Genetic and phenotypic continuum of HOXA genes: A case with double HOXA9/HOXA13 mutations

The HOXA genes cluster plays a key role in embryologic development. Mutations in HOXA genes have been linked to different human phenotypes, including developmental delay, limb anomalies, and urogenital malformations. The present study reported a clinical and genetic investigation of a female patient with polymalformative syndrome including left arm agenesis, bicornuate uterus and bicuspid aortic valve. Using whole exome sequencing, two heterozygous missense variants were identified. Of these, one was a novel variant in the HOXA13 gene [p.(Tyr290Ser)] and the second a heterozygous variant in the HOXA9 gene [p.(Ala102Pro)]. To the best of our knowledge, this is the first association of HOXA9/HOXA13 point mutations linked to a syndromic case. In conclusion, the present study suggested that the phenotypic spectrum of vertebral anomalies, anal atresia, cardiac defects, tracheo‑esophageal fistula, renal anomalies and limb abnormalities/hand‑foot‑genital syndrome may be attributable to the combination of different HOXA variants, particularly in patients with a severe clinical presentation. The current report contributed as well to the molecular understanding of HOXA genes‑related phenotypes via the identification of novel variant and genes associations.

Prognostic value of forward flow indices in primary mitral regurgitation due to mitral valve prolapse

Background

Degenerative mitral regurgitation (DMR) due to mitral valve prolapse (MVP) is a common valve disease associated with significant morbidity and mortality. Timing for surgery is debated for asymptomatic patients without Class I indication, prompting the search for novel parameters of early left ventricular (LV) systolic dysfunction.

Aims

To evaluate the prognostic impact of preoperative forward flow indices on the occurrence of post-operative LV systolic dysfunction.

Methods

We retrospectively included all consecutive patients with severe DMR due to MVP who underwent mitral valve repair between 2014 and 2019. LVOT_TVI, forward stroke volume index, and forward LVEF were assessed as potential risk factors for LVEF <50% at 6 months post-operatively.

Results

A total of 198 patients were included: 154 patients (78%) were asymptomatic, and 46 patients (23%) had hypertension. The mean preoperative LVEF was 69 ± 9%. 35 patients (18%) had LVEF ≤ 60%, and 61 patients (31%) had LVESD ≥40 mm. The mean post-operative LVEF was 59 ± 9%, and 21 patients (11%) had post-operative LVEF<50%. Based on multivariable analysis, LVOT_TVI was the strongest independent predictor of post-operative LV dysfunction after adjustment for age, sex, symptoms, LVEF, LV end systolic diameter, atrial fibrillation and left atrial volume index (0.75 [0.62-0.91], p < 0.01). The best sensitivity (81%) and specificity (63%) was obtained with LVOTTVI ≤15 cm based on ROC curve analysis.

Conclusion

LVOT_TVI represents an independent marker of myocardial performance impairment in the presence of severe DMR. LVOT_TVI could be an earlier marker than traditional echo parameters and aids in the optimization of the timing of surgery.

SCN5A Variants as Genetic Arrhythmias Triggers for Familial Bileaflet Mitral Valve Prolapse

Mitral valve prolapse (MVP) is a common valvular heart defect with variable outcomes. Several studies reported MVP as an underestimated cause of life-threatening arrhythmias and sudden cardiac death (SCD), mostly in young adult women. Herein, we report a clinical and genetic investigation of a family with bileaflet MVP and a history of syncopes and resuscitated sudden cardiac death. Using family based whole exome sequencing, we identified two missense variants in the SCN5A gene. A rare variant SCN5A:p.Ala572Asp and the well-known functional SCN5A:p.His558Arg polymorphism. Both variants are shared between the mother and her daughter with a history of resuscitated SCD and syncopes, respectively. The second daughter with prodromal MVP as well as her healthy father and sister carried only the SCN5A:p.His558Arg polymorphism. Our study is highly suggestive of the contribution of SCN5A mutations as the potential genetic cause of the electric instability leading to ventricular arrhythmias in familial MVP cases with syncope and/or SCD history.

Identification of non-synonymous variations in ROBO1 and GATA5 genes in a family with bicuspid aortic valve disease

Bicuspid aortic valve (BAV) is the most common congenital heart defect with a high index of heritability. Patients with BAV have different clinical courses and disease progression. Herein, we report three siblings with BAV and clinical differences. Their clinical presentations include moderate to severe aortic regurgitation, aortic stenosis, and ascending aortic aneurysm. Genetic investigation was carried out using Whole-Exome Sequencing for the three patients. We identified two non-synonymous variants in ROBO1 and GATA5 genes. The ROBO1: p.(Ser327Pro) variant is shared by the three BAV-affected siblings. The GATA5: p.(Gln3Arg) variant is shared only by the two brothers who presented BAV and ascending aortic aneurysm. Their sister, affected by BAV without aneurysm, does not harbor the GATA5: p.(Gln3Arg) variant. Both variants were absent in the patients' fourth brother who is clinically healthy with tricuspid aortic valve. To our knowledge, this is the first association of ROBO1 and GATA5 variants in familial BAV with a potential genotype-phenotype correlation. Our findings are suggestive of the implication of ROBO1 gene in BAV and the GATA5: p.(Gln3Arg) variant in ascending aortic aneurysm. Our family-based study further confirms the intrafamilial incomplete penetrance of BAV and the complex pattern of inheritance of the disease.

Molecular autopsy and clinical family screening in a case of sudden cardiac death reveals ACTN2 mutation related to hypertrophic/dilated cardiomyopathy and a novel LZTR1 variant associated with Noonan syndrome

Background

Genetic cardiac diseases are the main trigger of sudden cardiac death (SCD) in young adults. Hypertrophic cardiomyopathy (HCM) is the most prevalent cardiomyopathy and accounts for 0.5 to 1% of SCD cases per year.

Methods

Herein, we report a family with a marked history of SCD focusing on one SCD young adult case and one pediatric case with HCM.

Results

For the deceased young adult, postmortem whole‐exome sequencing (WES) revealed a missense variant in the ACTN2 gene: c.355G > A; p.(Ala119Thr) confirming the mixed hypertrophic/dilated cardiomyopathy phenotype detected in the autopsy. For the pediatric case, WES allowed us the identification of a novel frameshift variant in the LZTR1 gene: c.1745delT; p.(Val582Glyfs*10) which confirms a clinical suspicion of HCM related to Noonan syndrome.

Conclusion

The present study adds further evidence on the pathogenicity of ACTN2: p. Ala119Thr variant in SCD and expands the mutational spectrum of the LZTR1 gene related to Noonan syndrome.

Single Cell Approaches to Understand the Earliest Steps in Heart Development

PURPOSE OF REVIEW

Cardiac progenitors are the building blocks of the heart. Our knowledge, on how these progenitors build the heart, has considerably increased over the last two decades with the development of single cell approaches. We discuss the lessons learnt from clonal analyses and from single cell sequencing technologies on the understanding of the earliest steps of cardiac specification and lineage segregation.

RECENT FINDINGS

While experiments were initially performed at the population level, the development of approaches to investigate heart development at the single cell resolution has clearly demonstrated that cardiac progenitors are highly heterogeneous, with different progenitors contributing to different cardiac regions and different cardiac cell types. Some critical transcriptional determinants have also been identified for cardiac progenitor specification. Single cell approaches have finally provided insights into the spatio-temporal specification of unipotent and multipotent cardiac progenitors and provided a framework for investigating congenital heart defects.

Identification of two variants in AGRN and RPL3L genes in a patient with catecholaminergic polymorphic ventricular tachycardia suggesting new candidate disease genes and digenic inheritance

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmogenic syndrome characterized by life‐threatening arrhythmias, a normal resting electrocardiogram and the absence of overt structural heart abnormalities. Mutations in RyR2 gene account for the large part of CPVT cases. Less frequently, mutations in CASQ2 gene have been linked to the recessive form of the disease. Overall, approximately 35% of CPVT patients remain without a genetic etiology implying that other genes might be found causative of the disease. Here, we present a 6‐year‐old boy born to first‐degree related parents, with a typical phenotype of CPVT and a family history of sudden cardiac death of his brother at 7 years. A trio‐based whole exome sequencing was performed, and we identified a homozygous variant in AGRN gene and a heterozygous variant in RPL3L gene. We hypothesized that the presence of the homozygous variant in AGRN accounts for the CPVT phenotype in this family and the heterozygous variant in RPL3L gene may act as a modifier gene. Further studies are needed to determine the role of these genes in CPVT.

Msx1 haploinsufficiency modifies the Pax9-deficient cardiovascular phenotype

BACKGROUND

Successful embryogenesis relies on the coordinated interaction between genes and tissues. The transcription factors Pax9 and Msx1 genetically interact during mouse craniofacial morphogenesis, and mice deficient for either gene display abnormal tooth and palate development. Pax9 is expressed specifically in the pharyngeal endoderm at mid-embryogenesis, and mice deficient for Pax9 on a C57Bl/6 genetic background also have cardiovascular defects affecting the outflow tract and aortic arch arteries giving double-outlet right ventricle, absent common carotid arteries and interruption of the aortic arch.

RESULTS

In this study we have investigated both the effect of a different genetic background and Msx1 haploinsufficiency on the presentation of the Pax9-deficient cardiovascular phenotype. Compared to mice on a C57Bl/6 background, congenic CD1-Pax9^-/- mice displayed a significantly reduced incidence of outflow tract defects but aortic arch defects were unchanged. Pax9^-/- mice with Msx1 haploinsufficiency, however, have a reduced incidence of interrupted aortic arch, but more cases with cervical origins of the right subclavian artery and aortic arch, than seen in Pax9^-/- mice. This alteration in arch artery defects was accompanied by a rescue in third pharyngeal arch neural crest cell migration and smooth muscle cell coverage of the third pharyngeal arch arteries. Although this change in phenotype could theoretically be compatible with post-natal survival, using tissue-specific inactivation of Pax9 to maintain correct palate development whilst inducing the cardiovascular defects was unable to prevent postnatal death in the mutant mice. Hyoid bone and thyroid cartilage formation were abnormal in Pax9^-/- mice.

CONCLUSIONS

Msx1 haploinsufficiency mitigates the arch artery defects in Pax9^-/- mice, potentially by maintaining the survival of the 3rd arch artery through unimpaired migration of neural crest cells to the third pharyngeal arches. With the neural crest cell derived hyoid bone and thyroid cartilage also being defective in Pax9^-/- mice, we speculate that the pharyngeal endoderm is a key signalling centre that impacts on neural crest cell behaviour highlighting the ability of cells in different tissues to act synergistically or antagonistically during embryo development.

Clinical insights into a tertiary care center cohort of patients with bicuspid aortic valve

Although bicuspid aortic valve (BAV) is one of the most common congenital heart diseases, clinical data associated with valve dysfunction are still limited. We evaluated clinical characteristics and echocardiography of French patients with BAV associated with leaking and stenosis degeneration. We initiated a prospective registry from 2014 to 2018 at a tertiary center. A total of 223 patients (168 males [75%], age 53 ± 17 years) were enrolled. Among these patients 83% had left-right coronary cusps fusion, 80% Sievers type 1 BAV and 49% showed aortic dilatation. Twenty-four patients (11%) had normal valve function, 66 patients (31%) had aortic stenosis (AS), 91 patients (41%) had aortic regurgitation (AR) and 40 patients (17%) had AR and AS. BAV phenotype did not predict neither AS nor AR (all p > 0.1). By multivariable analysis, age > 50 (41.6[10.3-248.2], p < 0.001) and presence of raphe/fusion (12.8[2.4-87.4], p < 0.001) were significantly associated with AS, whereas male gender was associated with AR (5[1.6-16.4], p = 0.005). In addition, leaking degeneration was observed at a much younger age than stenosis (44 ± 14 years vs. 66 ± 10 years, p < 0.01) and among patients with valve dysfunction younger age was independently associated with AR (1.9[1.85-1.94], p < 0.001). In this study we confirmed high prevalence of valve dysfunction at first diagnosis of BAV in a referred population. The degenerative process differs according to type of dysfunction and is mainly dependent on age and gender.

Outflow Tract Formation-Embryonic Origins of Conotruncal Congenital Heart Disease

Anomalies in the cardiac outflow tract (OFT) are among the most frequent congenital heart defects (CHDs). During embryogenesis, the cardiac OFT is a dynamic structure at the arterial pole of the heart. Heart tube elongation occurs by addition of cells from pharyngeal, splanchnic mesoderm to both ends. These progenitor cells, termed the second heart field (SHF), were first identified twenty years ago as essential to the growth of the forming heart tube and major contributors to the OFT. Perturbation of SHF development results in common forms of CHDs, including anomalies of the great arteries. OFT development also depends on paracrine interactions between multiple cell types, including myocardial, endocardial and neural crest lineages. In this publication, dedicated to Professor Andriana Gittenberger-De Groot and her contributions to the field of cardiac development and CHDs, we review some of her pioneering studies of OFT development with particular interest in the diverse origins of the many cell types that contribute to the OFT. We also discuss the clinical implications of selected key findings for our understanding of the etiology of CHDs and particularly OFT malformations.

Piezo1 is required for outflow tract and aortic valve development

AIMS

During embryogenesis, the onset of circulatory blood flow generates a variety of hemodynamic forces which reciprocally induce changes in cardiovascular development and performance. It has been known for some time that these forces can be detected by as yet unknown mechanosensory systems which in turn promote cardiogenic events such as outflow tract and aortic valve development. PIEZO1 is a mechanosensitive ion channel present in endothelial cells where it serves to detect hemodynamic forces making it an ideal candidate to play a role during cardiac development. We sought to determine whether PIEZO1 is required for outflow tract and aortic valve development.

METHODS AND RESULTS

By analysing heart development in zebrafish we have determined that piezo1 is expressed in the developing outflow tract where it serves to detect hemodynamic forces. Consequently, disrupting Piezo1 signalling leads to defective outflow tract and aortic valve development and indicates this gene may be involved in the etiology of congenital heart diseases. Based on these findings, we analysed genomic data generated from patients who suffer from left ventricular outflow tract obstructions (LVOTO) and identified 3 probands who each harboured potentially pathogenic variants in PIEZO1. Subsequent in vitro and in vivo assays indicates that these variants behave as dominant negatives leading to an inhibition of normal PIEZO1 mechanosensory activity. Expressing these dominant negative PIEZO1 variants in zebrafish endothelium leads to defective aortic valve development.

CONCLUSION

These data indicate that the mechanosensitive ion channel piezo1 is required for outflow tract and aortic valve development.

Multiallelic rare variants support an oligogenic origin of sudden cardiac death in the young

Unexplained sudden death in the young is cardiovascular in most cases. Structural and conduction defects in cardiac-related genes can conspire to underlie sudden cardiac death. Here we report a clinical investigation and an extensive genetic assessment of a Tunisian family with sudden cardiac death in young members. In order to identify the family-genetic basis of sudden cardiac death, we performed Whole Exome Sequencing (WES), read depth copy-number-variation (CNV) screening and segregation analysis. We identify 6 ultra-rare pathogenic heterozygous variants in OBSCN, RYR2, DSC2, AKAP9, CACNA1C and RBM20 genes, and one homozygous splicing variant in TECRL gene consistent with an oligogenic model of inheritance. CNV analysis did not reveal any causative CNV consistent with the family phenotype. Overall, our results are highly suggestive for a cumulative effect of heterozygous missense variants as disease causation and to account for a greater disease severity among offspring. Our study further confirms the complexity of the inheritance of sudden cardiac death and highlights the utility of family-based WES and segregation analysis in the identification of family specific mutations within different cardiac genes pathways.

Krox20 Regulates Endothelial Nitric Oxide Signaling in Aortic Valve Development and Disease

Among the aortic valve diseases, the bicuspid aortic valve (BAV) occurs when the aortic valve has two leaflets (cusps), rather than three, and represents the most common form of congenital cardiac malformation, affecting 1–2% of the population. Despite recent advances, the etiology of BAV is poorly understood. We have recently shown that Krox20 is expressed in endothelial and cardiac neural crest derivatives that normally contribute to aortic valve development and that lack of Krox20 in these cells leads to aortic valve defects including partially penetrant BAV formation. Dysregulated expression of endothelial nitric oxide synthase (Nos3) is associated with BAV. To investigate the relationship between Krox20 and Nos3 during aortic valve development, we performed inter-genetic cross. While single heterozygous mice had normal valve formation, the compound Krox20+/−;Nos3+/− mice had BAV malformations displaying an in vivo genetic interaction between these genes for normal valve morphogenesis. Moreover, in vivo and in vitro experiments demonstrate that Krox20 directly binds to Nos3 proximal promoter to activate its expression. Our data suggests that Krox20 is a regulator of nitric oxide in endothelial-derived cells in the development of the aortic valve and concludes on the interaction of Krox20 and Nos3 in BAV formation.

Human pre-valvular endocardial cells derived from pluripotent stem cells recapitulate cardiac pathophysiological valvulogenesis

Genetically modified mice have advanced our understanding of valve development and disease. Yet, human pathophysiological valvulogenesis remains poorly understood. Here we report that, by combining single cell sequencing and in vivo approaches, a population of human pre-valvular endocardial cells (HPVCs) can be derived from pluripotent stem cells. HPVCs express gene patterns conforming to the E9.0 mouse atrio-ventricular canal (AVC) endocardium signature. HPVCs treated with BMP2, cultured on mouse AVC cushions, or transplanted into the AVC of embryonic mouse hearts, undergo endothelial-to-mesenchymal transition and express markers of valve interstitial cells of different valvular layers, demonstrating cell specificity. Extending this model to patient-specific induced pluripotent stem cells recapitulates features of mitral valve prolapse and identified dysregulation of the SHH pathway. Concurrently increased ECM secretion can be rescued by SHH inhibition, thus providing a putative therapeutic target. In summary, we report a human cell model of valvulogenesis that faithfully recapitulates valve disease in a dish.

There are few human models that can recapitulate valve development in vitro. Here, the authors derive human pre-valvular endocardial cells (HPVCs) from iPSCs and show they can recapitulate early valvulogenesis, and patient derived HPVCs have features of mitral valve prolapse and identified SHH dysregulation.

Transcriptome analysis of mouse and human sinoatrial node cells reveals a conserved genetic program

The rate of contraction of the heart relies on proper development and function of the sinoatrial node, which consists of a small heterogeneous cell population, including Tbx3⁺ pacemaker cells. Here, we have isolated and characterized the Tbx3⁺ cells from Tbx3 ^+/Venus knock-in mice. We studied electrophysiological parameters during development and found that Venus-labeled cells are genuine Tbx3⁺ pacemaker cells. We analyzed the transcriptomes of late fetal FACS-purified Tbx3⁺ sinoatrial nodal cells and Nppb-Katushka⁺ atrial and ventricular chamber cardiomyocytes, and identified a sinoatrial node-enriched gene program, including key nodal transcription factors, BMP signaling and Smoc2, the disruption of which in mice did not affect heart rhythm. We also obtained the transcriptomes of the sinoatrial node region, including pacemaker and other cell types, and right atrium of human fetuses, and found a gene program including TBX3, SHOX2, ISL1 and HOX family members, and BMP and NOTCH signaling components conserved between human and mouse. We conclude that a conserved gene program characterizes the sinoatrial node region and that the Tbx3 ^+/Venus allele provides a reliable tool for visualizing the sinoatrial node, and studying its development and function.

T-box genes and retinoic acid signaling regulate the segregation of arterial and venous pole progenitor cells in the murine second heart field

The arterial and venous poles of the mammalian heart are hotspots of congenital heart defects (CHD) such as those observed in 22q11.2 deletion (or DiGeorge) and Holt-Oram syndromes. These regions of the heart are derived from late differentiating cardiac progenitor cells of the Second Heart Field (SHF) located in pharyngeal mesoderm contiguous with the elongating heart tube. The T-box transcription factor Tbx1, encoded by the major 22q11.2 deletion syndrome gene, regulates SHF addition to both cardiac poles from a common progenitor population. Despite the significance of this cellular addition the mechanisms regulating the deployment of common progenitor cells to alternate cardiac poles remain poorly understood. Here we demonstrate that Tbx5, mutated in Holt-Oram syndrome and essential for venous pole development, is activated in Tbx1 expressing cells in the posterior region of the SHF at early stages of heart tube elongation. A subset of the SHF transcriptional program, including Tbx1 expression, is subsequently downregulated in Tbx5 expressing cells, generating a transcriptional boundary between Tbx1-positive arterial pole and Tbx5-positive venous pole progenitor cell populations. We show that normal downregulation of the definitive arterial pole progenitor cell program in the posterior SHF is dependent on both Tbx1 and Tbx5. Furthermore, retinoic acid (RA) signaling is required for Tbx5 activation in Tbx1-positive cells and blocking RA signaling at the time of Tbx5 activation results in atrioventricular septal defects at fetal stages. Our results reveal sequential steps of cardiac progenitor cell patterning and provide mechanistic insights into the origin of common forms of CHD.

Ectopic expression of Hoxb1 induces cardiac and craniofacial malformations

Members of the large family of Hox transcription factors are encoded by genes whose tightly regulated expression in development and in space within different embryonic tissues confer positional identity from the neck to the tips of the limbs. Many structures of the face, head, and heart develop from cell populations expressing few or no Hox genes. Hoxb1 is the member of its chromosomal cluster expressed in the most rostral domain during vertebrate development, but never by the multipotent neural crest cell population anterior to the cerebellum. We have developed a novel floxed transgenic mouse line, CAG-Hoxb1,-EGFP (CAG-Hoxb1), which upon recombination by Cre recombinase conditionally induces robust Hoxb1 and eGFP overexpression. When induced within the neural crest lineage, pups die at birth. A variable phenotype develops from E11.5 on, associating frontonasal hypoplasia/aplasia, micrognathia/agnathia, major ocular and forebrain anomalies, and cardiovascular malformations. Neural crest derivatives in the body appear unaffected. Transcription of effectors of developmental signaling pathways (Bmp, Shh, Vegfa) and transcription factors (Pax3, Sox9) is altered in mutants. These outcomes emphasize that repression of Hoxb1, along with other paralog group 1 and 2 Hox genes, is strictly necessary in anterior cephalic NC for craniofacial, visual, auditory, and cardiovascular development.

Bmp2 and Notch cooperate to pattern the embryonic endocardium

Signaling interactions between the myocardium and endocardium pattern embryonic cardiac regions, instructing their development to fulfill specific functions in the mature heart. We show that ectopic Bmp2 expression in the mouse chamber myocardium changes the transcriptional signature of adjacent chamber endocardial cells into valve tissue, and enables them to undergo epithelial-mesenchyme transition. This induction is independent of valve myocardium specification and requires high levels of Notch1 activity. Biochemical experiments suggest that Bmp2-mediated Notch1 induction is achieved through transcriptional activation of the Notch ligand Jag1, and physical interaction of Smad1/5 with the intracellular domain of the Notch1 receptor. Thus, widespread myocardial Bmp2 and endocardial Notch signaling drive presumptive ventricular endocardium to differentiate into valve endocardium. Understanding the molecular basis of valve development is instrumental to designing therapeutic strategies for congenital heart valve defects.

Analysis of HOXB1 gene in a cohort of patients with sporadic ventricular septal defect

Ventricular septal defect (VSD) including outlet VSD of double outlet right ventricle (DORV) and perimembranous VSD are among the most common congenital heart diseases found at birth. HOXB1 encodes a homeodomain transcription factor essential for normal cardiac outflow tract development. The aim of the present study was to investigate the possible genetic effect of sequence variations in HOXB1 on VSD. The coding regions and splice junctions of the HOXB1 gene were sequenced in 57 unrelated VSD patients. As a result, a homozygous c.74_82dup (p.Pro28delinsHisSerAlaPro) variant was identified in one individual with DORV. We also identified five previously reported polymorphisms (rs35114525, rs12946855, rs14534040, rs12939811, and rs7207109) in 18 patients (12 DORV and 6 perimembranous VSD). Our study did not show any pathogenic alterations in the coding region of HOXB1 among patients with VSD. To our knowledge this is the first study investigating the role of HOXB1 in nonsyndromic VSD, which provide more insight on the etiology of this disease.

Asb2α-Filamin A Axis Is Essential for Actin Cytoskeleton Remodeling During Heart Development

RATIONALE

Heart development involves differentiation of cardiac progenitors and assembly of the contractile sarcomere apparatus of cardiomyocytes. However, little is known about the mechanisms that regulate actin cytoskeleton remodeling during cardiac cell differentiation.

OBJECTIVE

The Asb2α (Ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2) CRL5 (cullin 5 RING E3 ubiquitin ligase) triggers polyubiquitylation and subsequent degradation by the proteasome of FLNs (filamins). Here, we investigate the role of Asb2α in heart development and its mechanisms of action.

METHODS AND RESULTS

Using Asb2 knockout embryos, we show that Asb2 is an essential gene, critical to heart morphogenesis and function, although its loss does not interfere with the overall patterning of the embryonic heart tube. We show that the Asb2α E3 ubiquitin ligase controls Flna stability in immature cardiomyocytes. Importantly, Asb2α-mediated degradation of the actin-binding protein Flna marks a previously unrecognized intermediate step in cardiac cell differentiation characterized by cell shape changes and actin cytoskeleton remodeling. We further establish that in the absence of Asb2α, myofibrils are disorganized and that heartbeats are inefficient, leading to embryonic lethality in mice.

CONCLUSIONS

These findings identify Asb2α as an unsuspected key regulator of cardiac cell differentiation and shed light on the molecular and cellular mechanisms determining the onset of myocardial cell architecture and its link with early cardiac function. Although Flna is known to play roles in cytoskeleton organization and to be required for heart function, this study now reveals that its degradation mediated by Asb2α ensures essential functions in differentiating cardiac progenitors.

Krox20 defines a subpopulation of cardiac neural crest cells contributing to arterial valves and bicuspid aortic valve

Although cardiac neural crest cells are required at early stages of arterial valve development, their contribution during valvular leaflet maturation remains poorly understood. Here, we show in mouse that neural crest cells from pre-otic and post-otic regions make distinct contributions to the arterial valve leaflets. Genetic fate-mapping analysis of Krox20-expressing neural crest cells shows a large contribution to the borders and the interleaflet triangles of the arterial valves. Loss of Krox20 function results in hyperplastic aortic valve and partially penetrant bicuspid aortic valve formation. Similar defects are observed in neural crest Krox20-deficient embryos. Genetic lineage tracing in Krox20^-/- mutant mice shows that endothelial-derived cells are normal, whereas neural crest-derived cells are abnormally increased in number and misplaced in the valve leaflets. In contrast, genetic ablation of Krox20-expressing cells is not sufficient to cause an aortic valve defect, suggesting that adjacent cells can compensate this depletion. Our findings demonstrate a crucial role for Krox20 in arterial valve development and reveal that an excess of neural crest cells may be associated with bicuspid aortic valve.

The alternatively spliced LRRFIP1 Isoform-1 is a key regulator of the Wnt/β-catenin transcription pathway

The GC-rich Binding Factor 2/Leucine Rich Repeat in the Flightless 1 Interaction Protein 1 gene (GCF2/LRRFIP1) is predicted to be alternatively spliced in five different isoforms. Although important peptide sequence differences are expected to result from this alternative splicing, to date, only the gene transcription regulator properties of LRRFIP1-Iso5 were unveiled. Based on molecular, cellular and biochemical data, we show here that the five isoforms define two molecular entities with different expression profiles in human tissues, subcellular localizations, oligomerization properties and transcription enhancer properties of the canonical Wnt pathway. We demonstrated that LRRFIP1-Iso3, -4 and -5, which share over 80% sequence identity, are primarily located in the cell cytoplasm and form homo and hetero-multimers between each other. In contrast, LRRFIP1-Iso1 and -2 are primarily located in the cell nucleus in part thanks to their shared C-terminal domain. Furthermore, we showed that LRRFIP1-Iso1 is preferentially expressed in the myocardium and skeletal muscle. Using the in vitro Topflash reporter assay we revealed that among LRRFIP1 isoforms, LRRFIP1-Iso1 is the strongest enhancer of the β-catenin Wnt canonical transcription pathway thanks to a specific N-terminal domain harboring two critical tryptophan residues (W76, 82). In addition, we showed that the Wnt enhancer properties of LRRFIP1-Iso1 depend on its homo-dimerisation which is governed by its specific coiled coil domain. Together our study identified LRRFIP1-Iso1 as a critical regulator of the Wnt canonical pathway with a potential role in myocyte differentiation and myogenesis.

Mechanisms of retinoic acid signaling during cardiogenesis

Substantial experimental and epidemiological data have highlighted the interplay between nutritional and genetic factors in the development of congenital heart defects. Retinoic acid (RA), a derivative of vitamin A, plays a key role during vertebrate development including the formation of the heart. Retinoids bind to RA and retinoid X receptors (RARs and RXRs) which then regulate tissue-specific genes. Here, we will focus on the roles of RA signaling and receptors in gene regulation during cardiogenesis, and the consequence of deregulated retinoid signaling on heart formation and congenital heart defects.

Hoxa1 and Hoxb1 are required for pharyngeal arch artery development

Hox transcription factors play critical roles during early vertebrate development. Previous studies have revealed an overlapping function of Hoxa1 and Hoxb1 during specification of the rhombomeres from which neural crest cells emerge. A recent study on Hoxa1 mutant mice documented its function during cardiovascular development, however, the role of Hoxb1 is still unclear. Here we show using single and compound Hoxa1;Hoxb1 mutant embryos that reduction of Hoxa1 gene dosage in Hoxb1-null genetic background is sufficient to result in abnormal pharyngeal aortic arch (PAA) development and subsequently in great artery defects. Endothelial cells in the 4th PAAs of compound mutant differentiate normally whereas vascular smooth muscle cells of the vessels are absent in the defective PAAs. The importance of Hoxa1 and Hoxb1, and their interaction during specification of cardiac NCCs is demonstrated. Together, our data reveal a critical role for anterior Hox genes during PAA development, providing new mechanistic insights into the etiology of congenital heart defects.

WES/WGS Reporting of Mutations from Cardiovascular "Actionable" Genes in Clinical Practice: A Key Role for UMD Knowledgebases in the Era of Big Databases

High-throughput next-generation sequencing such as whole-exome and whole-genome sequencing are being rapidly integrated into clinical practice. The use of these techniques leads to the identification of secondary variants for which decisions about the reporting or not to the patient need to be made. The American College of Medical Genetics and Genomics recently published recommendations for the reporting of these variants in clinical practice for 56 "actionable" genes. Among these, seven are involved in Marfan Syndrome And Related Disorders (MSARD) resulting from mutations of the FBN1, TGFBR1 and 2, ACTA2, SMAD3, MYH11 and MYLK genes. Here, we show that mutations collected in UMD databases for MSARD genes (UMD-MSARD) are rarely reported, including the most frequent ones, in global scale initiatives for variant annotation such as the NHLBI GO Exome Sequencing Project (ESP), the Exome Aggregation Consortium (ExAC), and ClinVar. The predicted pathogenic mutations reported in global scale initiatives but absent in locus-specific databases (LSDBs) mainly correspond to rare events. UMD-MSARD databases are therefore the only resources providing access to the full spectrum of known pathogenic mutations. They are the most comprehensive resources for clinicians and geneticists to interpret MSARD-related variations not only primary variants but also secondary variants.

Actionable Genes, Core Databases, and Locus-Specific Databases

Adoption of next-generation sequencing (NGS) in a diagnostic context raises numerous questions with regard to identification and reports of secondary variants (SVs) in actionable genes. To better understand the whys and wherefores of these questioning, it is necessary to understand how they are selected during the filtering process and how their proportion can be estimated. It is likely that SVs are underestimated and that our capacity to label all true SVs can be improved. In this context, Locus-specific databases (LSDBs) can be key by providing a wealth of information and enabling classifying variants. We illustrate this issue by analyzing 318 SVs in 23 actionable genes involved in cancer susceptibility syndromes identified through sequencing of 572 participants selected for a range of atherosclerosis phenotypes. Among these 318 SVs, only 43.4% are reported in Human Gene Mutation Database (HGMD) Professional versus 71.4% in LSDB. In addition, 23.9% of HGMD Professional variants are reported as pathogenic versus 4.8% for LSDB. These data underline the benefits of LSDBs to annotate SVs and minimize overinterpretation of mutations thanks to their efficient curation process and collection of unpublished data.

Msx1CreERT2 knock-In allele: A useful tool to target embryonic and adult cardiac valves

Heart valve development begins with the endothelial-to-mesenchymal transition (EMT) of endocardial cells. Although lineage studies have demonstrated contributions from cardiac neural crest and epicardium to semilunar and atrioventricular (AV) valve formation, respectively, most valve mesenchyme derives from the endocardial EMT. Specific Cre mouse lines for fate-mapping analyses of valve endocardial cells are limited. Msx1 displayed expression in AV canal endocardium and cushion mesenchyme between E9.5 and E11.5, when EMT is underway. Additionally, previous studies have demonstrated that deletion of Msx1 and its paralog Msx2 results in hypoplastic AV cushions and impaired endocardial signaling. A knock-in tamoxifen-inducible Cre line was recently generated (Msx1CreERT2) and characterized during embryonic development and after birth, and was shown to recapitulate the endogenous Msx1 expression pattern. Here, we further analyze this knock-in allele and track the Msx1-expressing cells and their descendants during cardiac development with a particular focus on their contribution to the valves and their precursors. Thus, Msx1CreERT2 mice represent a useful model for lineage tracing and conditional gene manipulation of endocardial and mesenchymal cushion cells essential to understand mechanisms of valve development and remodeling.

Disruption of CXCR4 signaling in pharyngeal neural crest cells causes DiGeorge syndrome-like malformations

DiGeorge syndrome (DGS) is a congenital disease causing cardiac outflow tract anomalies, craniofacial dysmorphogenesis, thymus hypoplasia, and mental disorders. It results from defective development of neural crest cells (NCs) that colonize the pharyngeal arches and contribute to lower jaw, neck and heart tissues. Although TBX1 has been identified as the main gene accounting for the defects observed in human patients and mouse models, the molecular mechanisms underlying DGS etiology are poorly identified. The recent demonstrations that the SDF1/CXCR4 axis is implicated in NC chemotactic guidance and impaired in cortical interneurons of mouse DGS models prompted us to search for genetic interactions between Tbx1, Sdf1 (Cxcl12) and Cxcr4 in pharyngeal NCs and to investigate the effect of altering CXCR4 signaling on the ontogeny of their derivatives, which are affected in DGS. Here, we provide evidence that Cxcr4 and Sdf1 are genetically downstream of Tbx1 during pharyngeal NC development and that reduction of CXCR4 signaling causes misrouting of pharyngeal NCs in chick and dramatic morphological alterations in the mandibular skeleton, thymus and cranial sensory ganglia. Our results therefore support the possibility of a pivotal role for the SDF1/CXCR4 axis in DGS etiology.

Krox20 heterozygous mice: A model of aortic regurgitation associated with decreased expression of fibrillar collagen genes

BACKGROUND

The mechanism involved in the onset of aortic valve (AoV) disease remains unclear despite its poor prognosis and frequency. Recently, we reported that Krox20 (EGR2 in humans) is involved in AoV development and dysfunction.

AIM

Analyze Krox20 heterozygous mice (Krox20(+/-)) to discover whether incomplete expression of Krox20 can cause valvular diseases.

METHODS

Transcriptional levels of Col1a2/COL1A2 and Krox20/EGR2 in AoVs from Krox20(+/-) mice and human patients operated on for severe aortic regurgitation were evaluated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Human control valves were obtained from three transplanted patients without AoV disease. Twenty-one heterozygous Krox20(+/-) mice were compared with 35 controls at different ages. Three independent measurements of valve thickness were performed on magnified tissue sections using Image J software. In vivo valve structure and function were evaluated using the high-frequency Vevo(®) 2100 echocardiogram.

RESULTS

qRT-PCR analysis using AoVs from patients with severe aortic regurgitation showed a decrease in EGR2 expression associated with significant downregulation of COL1A2 expression (P<0.05). Similar results were observed in the AoVs of Krox20(+/-) mice. Anatomical examination revealed that incomplete invalidation of Krox20 caused significant thickening of the aortic leaflet compared with controls (145±22 vs. 75±24μm; P=0.01). Within the mutant group, this thickening worsened significantly over time (Krox20(+/-) mice aged>7 vs.<7months: 136±48 vs. 102±41μm; P<0.001). Moreover, the aortic leaflets of embryonic day 18.5 Krox20(+/-) embryos were significantly more thickened than those from controls, suggesting that this disease begins during embryonic development. Echo-Doppler analysis showed a significant increase in AoV dysfunction in heterozygous versus control mice (53% vs. 17%; P<0.001), suggesting a tight relationship between valve architecture and function. Morphometric analysis revealed that the most severe AoV dysfunction was always associated with the most thickened valves. Classic histological analysis revealed that mutant AoVs had extracellular matrix disorganization, with features of human myxomatous degeneration, including excess of proteoglycan deposition in spongiosa and reduction of collagen fibre in fibrosa, but no calcification.

CONCLUSION

Decreased expression of Krox20 in mice causes degeneration of the aortic leaflets and disorganization of the extracellular matrix, causing valvular dysfunction.

An uncommon cause of tricuspid regurgitation: three-dimensional echocardiographic incremental value, surgical and genetic insights

Congenital tricuspid valve disease is a rare defect that includes regurgitation, stenosis and Ebstein's anomaly. We report a case of severe tricuspid regurgitation associated with functional mitral regurgitation in a 47-year-old man with congestive heart failure. Transthoracic echocardiography (TTE) ruled out any Ebstein's anomaly. Three-dimensional TTE revealed a 'tricuspid hole' into the anterior leaflet that was only attached to the tricuspid annulus next to both anteroseptal and anteroposterior commissures. There was no sign of leaflet tear or perforation. The surgical repair of the tricuspid and mitral valves was performed with an optimal result. No sign of endocarditis or rheumatic disease was observed during the intervention. Sequence analysis of GATA4, HEY2 and ZFPM2 genes was performed, but no causative mutation was identified.

Cardiac outflow morphogenesis depends on effects of retinoic acid signaling on multiple cell lineages

BACKGROUND

Retinoic acid (RA), the bioactive derivative of vitamin A, is essential for vertebrate heart development. Both excess and reduced RA signaling lead to cardiovascular malformations affecting the outflow tract (OFT). To address the cellular mechanisms underlying the effects of RA signaling during OFT morphogenesis, we used transient maternal RA supplementation to rescue the early lethality resulting from inactivation of the murine retinaldehyde dehydrogenase 2 (Raldh2) gene.

RESULTS

By embryonic day 13.5, all rescued Raldh2(-/-) hearts exhibit severe, reproducible OFT septation defects, although wild-type and Raldh2(+/-) littermates have normal hearts. Cardiac neural crest cells (cNCC) were present in OFT cushions of Raldh2(-/-) mutant embryos but ectopically located in the periphery of the endocardial cushions, rather than immediately underlying the endocardium. Excess mesenchyme was generated by Raldh2(-/-) mutant endocardium, which displaced cNCC derivatives from their subendocardial, medial position.

CONCLUSIONS

RA signaling affects not only cNCC numbers but also their position relative to endocardial mesenchyme during the septation process. Our study shows that inappropriate coordination between the different cell types of the OFT perturbs its morphogenesis and leads to a severe congenital heart defect, persistent truncus arteriosus.

Hoxb1 regulates proliferation and differentiation of second heart field progenitors in pharyngeal mesoderm and genetically interacts with Hoxa1 during cardiac outflow tract development

Outflow tract (OFT) anomalies are among the most common congenital heart defects found at birth. The embryonic OFT grows by the progressive addition of cardiac progenitors, termed the second heart field (SHF), which originate from splanchnic pharyngeal mesoderm. Development of the SHF is controlled by multiple intercellular signals and transcription factors; however the relationship between different SHF regulators remains unclear. We have recently shown that Hoxa1 and Hoxb1 are expressed in a sub-population of the SHF contributing to the OFT. Here, we report that Hoxb1 deficiency results in a shorter OFT and ventricular septal defects (VSD). Mechanistically, we show that both FGF/ERK and BMP/SMAD signaling, which regulate proliferation and differentiation of cardiac progenitor cells and OFT morphogenesis, are enhanced in the pharyngeal region in Hoxb1 mutants. Absence of Hoxb1 also perturbed SHF development through premature myocardial differentiation. Hence, the positioning and remodeling of the mutant OFT is disrupted. Hoxa1(-/-) embryos, in contrast, have low percentage of VSD and normal SHF development. However, compound Hoxa1(-/-); Hoxb1(+/-) embryos display OFT defects associated with premature SHF differentiation, demonstrating redundant roles of these factors during OFT development. Our findings provide new insights into the gene regulatory network controlling SHF and OFT formation.

Genetic lineage tracing analysis of anterior Hox expressing cells

Cell lineage studies have been widely used in developmental biology to establish which cells, and how many cells, in the early embryo will give rise to a specific structure and its derivatives. Several methods have been developed to label progenitor cells in the early embryo. Here, we describe the genetic tracing approach that relies on the use of the recombinase to genetically and permanently label progenitor cells as well as their progeny through specific activation of a conditional reporter gene, the ROSA26 reporter mouse. Labeling of progenitor cells is spatially controlled by the use of a tissue-specific promoter driving Cre, the Hoxb1 (IRES-Cre/+)  and the Hoxa1-enhIII-cre. ROSA26R mice and Hoxb1 (IRES-Cre/+) or Hoxa1-enhIII-cremice are crossed together to generate embryos at different stages of development. Embryos are collected and dissected at a specific stage of development and fixed in paraformaldehyde. To follow Hoxb1 (+) and Hoxa1 (+) progeny, X-gal staining is performed to detect β-galactosidase activity in embryos or developing organ such as the heart. Finally, X-gal-positive cells are observed on whole-mount embryos or dissected organ to determine the lineage contribution of Hoxa1 and Hoxb1 during development.

Loss of Krox20 results in aortic valve regurgitation and impaired transcriptional activation of fibrillar collagen genes

AIMS

Heart valve maturation is achieved by the organization of extracellular matrix (ECM) and the distribution of valvular interstitial cells. However, the factors that regulate matrix components required for valvular structure and function are unknown. Based on the discovery of its specific expression in cardiac valves, we aimed to uncover the role of Krox20 (Egr-2) during valve development and disease.

METHODS AND RESULTS

Using series of mouse genetic tools, we demonstrated that loss of function of Krox20 caused significant hyperplasia of the semilunar valves, while atrioventricular valves appeared normal. This defect was associated with an increase in valvular interstitial cell number and ECM volume. Echo Doppler analysis revealed that adult mutant mice had aortic insufficiency. Defective aortic valves (AoVs) in Krox20(-/-) mice had features of human AoV disease, including excess of proteoglycan deposition and reduction of collagen fibres. Furthermore, examination of diseased human AoVs revealed decreased expression of KROX20. To identify downstream targets of Krox20, we examined expression of fibrillar collagens in the AoV leaflets at different stages in the mouse. We found significant down-regulation of Col1a1, Col1a2, and Col3a1 in the semilunar valves of Krox20 mutant mice. Utilizing in vitro and in vivo experiments, we demonstrated that Col1a1 and Col3a1 are direct targets of Krox20 activation in interstitial cells of the AoV.

CONCLUSION

This study identifies a previously unknown function of Krox20 during heart valve development. These results indicate that Krox20-mediated activation of fibrillar Col1a1 and Col3a1 genes is crucial to avoid postnatal degeneration of the AoV leaflets.

Early cardiac development: a view from stem cells to embryos

From the 1920s, early cardiac development has been studied in chick and, later, in mouse embryos in order to understand the first cell fate decisions that drive specification and determination of the endocardium, myocardium, and epicardium. More recently, mouse and human embryonic stem cells (ESCs) have demonstrated faithful recapitulation of early cardiogenesis and have contributed significantly to this research over the past few decades. Derived almost 15 years ago, human ESCs have provided a unique developmental model for understanding the genetic and epigenetic regulation of early human cardiogenesis. Here, we review the biological concepts underlying cell fate decisions during early cardiogenesis in model organisms and ESCs. We draw upon both pioneering and recent studies and highlight the continued role for in vitro stem cells in cardiac developmental biology.

ISL1 directly regulates FGF10 transcription during human cardiac outflow formation

The LIM homeodomain gene Islet-1 (ISL1) encodes a transcription factor that has been associated with the multipotency of human cardiac progenitors, and in mice enables the correct deployment of second heart field (SHF) cells to become the myocardium of atria, right ventricle and outflow tract. Other markers have been identified that characterize subdomains of the SHF, such as the fibroblast growth factor Fgf10 in its anterior region. While functional evidence of its essential contribution has been demonstrated in many vertebrate species, SHF expression of Isl1 has been shown in only some models. We examined the relationship between human ISL1 and FGF10 within the embryonic time window during which the linear heart tube remodels into four chambers. ISL1 transcription demarcated an anatomical region supporting the conserved existence of a SHF in humans, and transcription factors of the GATA family were co-expressed therein. In conjunction, we identified a novel enhancer containing a highly conserved ISL1 consensus binding site within the FGF10 first intron. ChIP and EMSA demonstrated its direct occupation by ISL1. Transcription mediated by ISL1 from this FGF10 intronic element was enhanced by the presence of GATA4 and TBX20 cardiac transcription factors. Finally, transgenic mice confirmed that endogenous factors bound the human FGF10 intronic enhancer to drive reporter expression in the developing cardiac outflow tract. These findings highlight the interest of examining developmental regulatory networks directly in human tissues, when possible, to assess candidate non-coding regions that may be responsible for congenital malformations.

A retinoic acid responsive Hoxa3 transgene expressed in embryonic pharyngeal endoderm, cardiac neural crest and a subdomain of the second heart field

A transgenic mouse line harbouring a β-galacdosidase reporter gene controlled by the proximal 2 kb promoter of Hoxa3 was previously generated to investigate the regulatory cues governing Hoxa3 expression in the mouse. Examination of transgenic embryos from embryonic day (E) 8.0 to E15.5 revealed regionally restricted reporter activity in the developing heart. Indeed, transgene expression specifically delineated cells from three distinct lineages: a subpopulation of the second heart field contributing to outflow tract myocardium, the cardiac neural crest cells and the pharyngeal endoderm. Manipulation of the Retinoic Acid (RA) signaling pathway showed that RA is required for correct expression of the transgene. Therefore, this transgenic line may serve as a cardiosensor line of particular interest for further analysis of outflow tract development.

Expression of Slit and Robo genes in the developing mouse heart

Development of the mammalian heart is mediated by complex interactions between myocardial, endocardial, and neural crest-derived cells. Studies in Drosophila have shown that the Slit-Robo signaling pathway controls cardiac cell shape changes and lumen formation of the heart tube. Here, we demonstrate by in situ hybridization that multiple Slit ligands and Robo receptors are expressed in the developing mouse heart. Slit3 is the predominant ligand transcribed in the early mouse heart and is expressed in the ventral wall of the linear heart tube and subsequently in chamber but not in atrioventricular canal myocardium. Furthermore, we identify that the homeobox gene Nkx2-5 is required for early ventral restriction of Slit3 and that the T-box transcription factor Tbx2 mediates repression of Slit3 in nonchamber myocardium. Our results suggest that patterned Slit-Robo signaling may contribute to the control of oriented cell growth during chamber morphogenesis of the mammalian heart.