Compound heterozygous NOTCH1 mutations underlie impaired cardiogenesis in a patient with hypoplastic left heart syndrome

Deciphering Congenital Heart Disease Using Human Induced Pluripotent Stem Cells

Congenital heart disease (CHD) is a leading cause of birth defect-related death. Despite significant advances, the mechanisms underlying the development of CHD are complex and remain elusive due to a lack of efficient, reproducible, and translational model systems. Investigations relied on animal models have inherent limitations due to interspecies differences. Human induced pluripotent stem cells (iPSCs) have emerged as an effective platform for disease modeling. iPSCs allow for the production of a limitless supply of patient-specific somatic cells that enable advancement in cardiovascular precision medicine. Over the past decade, researchers have developed protocols to differentiate iPSCs to multiple cardiovascular lineages, as well as to enhance the maturity and functionality of these cells. With the development of physiologic three-dimensional cardiac organoids, iPSCs represent a powerful platform to mechanistically dissect CHD and serve as a foundation for future translational research.

Transcriptome studies of congenital heart diseases: identifying current gaps and therapeutic frontiers

Congenital heart disease (CHD) are genetically complex and comprise a wide range of structural defects that often predispose to - early heart failure, a common cause of neonatal morbidity and mortality. Transcriptome studies of CHD in human pediatric patients indicated a broad spectrum of diverse molecular signatures across various types of CHD. In order to advance research on congenital heart diseases (CHDs), we conducted a detailed review of transcriptome studies on this topic. Our analysis identified gaps in the literature, with a particular focus on the cardiac transcriptome signatures found in various biological specimens across different types of CHDs. In addition to translational studies involving human subjects, we also examined transcriptomic analyses of CHDs in a range of model systems, including iPSCs and animal models. We concluded that RNA-seq technology has revolutionized medical research and many of the discoveries from CHD transcriptome studies draw attention to biological pathways that concurrently open the door to a better understanding of cardiac development and related therapeutic avenue. While some crucial impediments to perfectly studying CHDs in this context remain obtaining pediatric cardiac tissue samples, phenotypic variation, and the lack of anatomical/spatial context with model systems. Combining model systems, RNA-seq technology, and integrating algorithms for analyzing transcriptomic data at both single-cell and high throughput spatial resolution is expected to continue uncovering unique biological pathways that are perturbed in CHDs, thus facilitating the development of novel therapy for congenital heart disease.

Rare variants in CAPN2 increase risk for isolated hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) is a severe congenital heart defect (CHD) characterized by hypoplasia of the left ventricle and aorta along with stenosis or atresia of the aortic and mitral valves. HLHS represents only ∼4%-8% of all CHDs but accounts for ∼25% of deaths. HLHS is an isolated defect (i.e., iHLHS) in 70% of families, the vast majority of which are simplex. Despite intense investigation, the genetic basis of iHLHS remains largely unknown. We performed exome sequencing on 331 families with iHLHS aggregated from four independent cohorts. A Mendelian-model-based analysis demonstrated that iHLHS was not due to single, large-effect alleles in genes previously reported to underlie iHLHS or CHD in >90% of families in this cohort. Gene-based association testing identified increased risk for iHLHS associated with variation in CAPN2 (p = 1.8 × 10-5), encoding a protein involved in functional adhesion. Functional validation studies in a vertebrate animal model (Xenopus laevis) confirmed CAPN2 is essential for cardiac ventricle morphogenesis and that in vivo loss of calpain function causes hypoplastic ventricle phenotypes and suggest that human CAPN2707C>T and CAPN21112C>T variants, each found in multiple individuals with iHLHS, are hypomorphic alleles. Collectively, our findings show that iHLHS is typically not a Mendelian condition, demonstrate that CAPN2 variants increase risk of iHLHS, and identify a novel pathway involved in HLHS pathogenesis.

Charting the Path: Navigating Embryonic Development to Potentially Safeguard against Congenital Heart Defects

Congenital heart diseases (CHDs) are structural or functional defects present at birth due to improper heart development. Current therapeutic approaches to treating severe CHDs are primarily palliative surgical interventions during the peri- or prenatal stages, when the heart has fully developed from faulty embryogenesis. However, earlier interventions during embryonic development have the potential for better outcomes, as demonstrated by fetal cardiac interventions performed in utero, which have shown improved neonatal and prenatal survival rates, as well as reduced lifelong morbidity. Extensive research on heart development has identified key steps, cellular players, and the intricate network of signaling pathways and transcription factors governing cardiogenesis. Additionally, some reports have indicated that certain adverse genetic and environmental conditions leading to heart malformations and embryonic death may be amendable through the activation of alternative mechanisms. This review first highlights key molecular and cellular processes involved in heart development. Subsequently, it explores the potential for future therapeutic strategies, targeting early embryonic stages, to prevent CHDs, through the delivery of biomolecules or exosomes to compensate for faulty cardiogenic mechanisms. Implementing such non-surgical interventions during early gestation may offer a prophylactic approach toward reducing the occurrence and severity of CHDs.

Mechanisms of Aortic Dilation in Patients With Bicuspid Aortic Valve: JACC State-of-the-Art Review

Bicuspid aortic valve is the most common congenital heart disease and exposes patients to an increased risk of aortic dilation and dissection. Aortic dilation is a slow, silent process, leading to a greater risk of aortic dissection. The prevention of adverse events together with optimization of the frequency of the required lifelong imaging surveillance are important for both clinicians and patients and motivated extensive research to shed light on the physiopathologic processes involved in bicuspid aortic valve aortopathy. Two main research hypotheses have been consolidated in the last decade: one supports a genetic basis for the increased prevalence of dilation, in particular for the aortic root, and the second supports the damaging impact on the aortic wall of altered flow dynamics associated with these structurally abnormal valves, particularly significant in the ascending aorta. Current opinion tends to rule out mutually excluding causative mechanisms, recognizing both as important and potentially clinically relevant.

Genetics of aortic valve disease

PURPOSE OF REVIEW

Aortic valve disease is a leading global cause of morbidity and mortality, posing an increasing burden on society. Advances in next-generation technologies and disease models over the last decade have further delineated the genetic and molecular factors that might be exploited in development of therapeutics for affected patients. This review describes several advances in the molecular and genetic understanding of AVD, focusing on bicuspid aortic valve (BAV) and calcific aortic valve disease (CAVD).

RECENT FINDINGS

Genomic studies have identified a myriad of genes implicated in the development of BAV, including NOTCH1 , SMAD6 and ADAMTS19 , along with members of the GATA and ROBO gene families. Similarly, several genes associated with the initiation and progression of CAVD, including NOTCH1 , LPA , PALMD , IL6 and FADS1/2 , serve as the launching point for emerging clinical trials.

SUMMARY

These new insights into the genetic contributors of AVD have offered new avenues for translational disease investigation, bridging molecular discoveries to emergent pharmacotherapeutic options. Future studies aimed at uncovering new genetic associations and further defining implicated molecular pathways are fuelling the new wave of drug discovery.

Unlocking the potential of induced pluripotent stem cells for neonatal disease modeling and drug development

Neonatal lung and heart diseases, albeit rare, can result in poor quality of life, often require long-term management and/or organ transplantation. For example, Congenital Heart Disease (CHD) is one of the most common type of congenital disabilities, affecting nearly 1% of the newborns, and has complex and multifactorial causes, including genetic predisposition and environmental influences. To develop new strategies for heart and lung regeneration in CHD and neonatal lung disease, human induced pluripotent stem cells (hiPSCs) provide a unique and personalized platform for future cell replacement therapy and high-throughput drug screening. Additionally, given the differentiation potential of iPSCs, cardiac cell types such as cardiomyocytes, endothelial cells, and fibroblasts and lung cell types such Type II alveolar epithelial cells can be derived in a dish to study the fundamental pathology during disease progression. In this review, we discuss the applications of hiPSCs in understanding the molecular mechanisms and cellular phenotypes of CHD (e.g., structural heart defect, congenital valve disease, and congenital channelopathies) and congenital lung diseases, such as surfactant deficiencies and Brain-Lung-Thyroid syndrome. We also provide future directions for generating mature cell types from iPSCs, and more complex hiPSC-based systems using three-dimensional (3D) organoids and tissue-engineering. With these potential advancements, the promise that hiPSCs will deliver new CHD and neonatal lung disease treatments may soon be fulfilled.

Use of Frogs as a Model to Study the Etiology of HLHS

A frog is a classical model organism used to uncover processes and regulations of early vertebrate development, including heart development. Recently, we showed that a frog also represents a useful model to study a rare human congenital heart disease, hypoplastic left heart syndrome. In this review, we first summarized the cellular events and molecular regulations of vertebrate heart development, and the benefit of using a frog model to study congenital heart diseases. Next, we described the challenges in elucidating the etiology of hypoplastic left heart syndrome and discussed how a frog model may contribute to our understanding of the molecular and cellular bases of the disease. We concluded that a frog model offers its unique advantage in uncovering the cellular mechanisms of hypoplastic left heart syndrome; however, combining multiple model organisms, including frogs, is needed to gain a comprehensive understanding of the disease.

Impaired Human Cardiac Cell Development due to NOTCH1 Deficiency

BACKGROUND

NOTCH1 pathogenic variants are implicated in multiple types of congenital heart defects including hypoplastic left heart syndrome, where the left ventricle is underdeveloped. It is unknown how NOTCH1 regulates human cardiac cell lineage determination and cardiomyocyte proliferation. In addition, mechanisms by which NOTCH1 pathogenic variants lead to ventricular hypoplasia in hypoplastic left heart syndrome remain elusive.

METHODS

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas9 genome editing was utilized to delete NOTCH1 in human induced pluripotent stem cells. Cardiac differentiation was carried out by sequential modulation of WNT signaling, and NOTCH1 knockout and wild-type differentiating cells were collected at day 0, 2, 5, 10, 14, and 30 for single-cell RNA-seq.

RESULTS

Human NOTCH1 knockout induced pluripotent stem cells are able to generate functional cardiomyocytes and endothelial cells, suggesting that NOTCH1 is not required for mesoderm differentiation and cardiovascular development in vitro. However, disruption of NOTCH1 blocks human ventricular-like cardiomyocyte differentiation but promotes atrial-like cardiomyocyte generation through shortening the action potential duration. NOTCH1 deficiency leads to defective proliferation of early human cardiomyocytes, and transcriptomic analysis indicates that pathways involved in cell cycle progression and mitosis are downregulated in NOTCH1 knockout cardiomyocytes. Single-cell transcriptomic analysis reveals abnormal cell lineage determination of cardiac mesoderm, which is manifested by the biased differentiation toward epicardial and second heart field progenitors at the expense of first heart field progenitors in NOTCH1 knockout cell populations.

CONCLUSIONS

NOTCH1 is essential for human ventricular-like cardiomyocyte differentiation and proliferation through balancing cell fate determination of cardiac mesoderm and modulating cell cycle progression. Because first heart field progenitors primarily contribute to the left ventricle, we speculate that pathogenic NOTCH1 variants lead to biased differentiation of first heart field progenitors, blocked ventricular-like cardiomyocyte differentiation, and defective cardiomyocyte proliferation, which collaboratively contribute to left ventricular hypoplasia in hypoplastic left heart syndrome.

Novel rare mutation in a conserved site of PTPRB causes human hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) is a rare but fatal birth defect in which the left side of the heart is underdeveloped. HLHS accounts for 2% to 4% of congenital heart anomalies. Whole genome sequencing (WGS) was conducted for a family trio consisting of a proband and his parents. A homozygous rare variant was detected in the PTPRB (Protein Tyrosine Phosphatase Receptor Type B) gene of the proband by functional annotation and co-segregation analysis. Sanger sequencing was used to confirm genotypes of the variant. The in silico prediction tools, including Mutation Taster, SpliceAI, and CADD, were used to predict the impact of the mutation. The allele frequencies across populations were compared based on multiple databases, including "1000 genomes" and "gnomAD". We used two vectors (pcMINI and pcDNA3.1) to generate a minigene construct to validate the mutational effect at the transcriptional level. Family-based WGS analyses showed that only a homozygous splice acceptor variant (NC_000012.12: g.70636068T>G, NM_001109754.4: c.56-2A>C, NG_029940.2: g.6373A>C) at the exon-intron border of PTPRB gene associates with HLHS. This variant is also within the region with the enhancer activity based on UCSC genome annotation. Genotyping and Sanger sequencing revealed that the proband's parents are heterozygous for this variant. Evolutionary conservation analysis revealed that the site (NC_000012.12: g.70636068) is extremely conserved across species, supporting the evolutionary functional constraints of the ancestral wild type (T). In silico tools universally predicted a deleterious or disease-causing impact of the mutation from T to G. The mutation was not found in the 1000 genomes and gnomAD databases, which indicates that this mutation is very rare in most human populations. A splicing assay indicated that the mutated minigene caused aberrant splicing of mRNA, in which a 3 bp missing in the second exon resulted in the deletion of one amino acid (NP_001103224.1:p.Glu19del) compared to the normal protein of PRPTB (also the VE-PTP). Structure prediction revealed that the deletion occurred within the C-region of the signal peptide of VE-PTP, suggesting signal peptide-related defects as a potential mechanism for the HLHS cellular pathogeny. We report a rare homozygous variant with splicing error in PTPRB associated with HLHS. Previous model species studies revealed conserved functions of PTPRB in cardiovascular and heart development in mice and zebrafish. Our study is the first report to show the association between PTPRB and HLHS in humans.

Congenital aortic valve stenosis: from pathophysiology to molecular genetics and the need for novel therapeutics

Congenital aortic valve stenosis (AVS) is one of the most common valve anomalies and accounts for 3%-6% of cardiac malformations. As congenital AVS is often progressive, many patients, both children and adults, require transcatheter or surgical intervention throughout their lives. While the mechanisms of degenerative aortic valve disease in the adult population are partially described, the pathophysiology of adult AVS is different from congenital AVS in children as epigenetic and environmental risk factors play a significant role in manifestations of aortic valve disease in adults. Despite increased understanding of genetic basis of congenital aortic valve disease such as bicuspid aortic valve, the etiology and underlying mechanisms of congenital AVS in infants and children remain unknown. Herein, we review the pathophysiology of congenitally stenotic aortic valves and their natural history and disease course along with current management strategies. With the rapid expansion of knowledge of genetic origins of congenital heart defects, we also summarize the literature on the genetic contributors to congenital AVS. Further, this increased molecular understanding has led to the expansion of animal models with congenital aortic valve anomalies. Finally, we discuss the potential to develop novel therapeutics for congenital AVS that expand on integration of these molecular and genetic advances.

Current state of the art in hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) is a complex congenital heart condition in which a neonate is born with an underdeveloped left ventricle and associated structures. Without palliative interventions, HLHS is fatal. Treatment typically includes medical management at the time of birth to maintain patency of the ductus arteriosus, followed by three palliative procedures: most commonly the Norwood procedure, bidirectional cavopulmonary shunt, and Fontan procedures. With recent advances in surgical management of HLHS patients, high survival rates are now obtained at tertiary treatment centers, though adverse neurodevelopmental outcomes remain a clinical challenge. While surgical management remains the standard of care for HLHS patients, innovative treatment strategies continue to be developing. Important for the development of new strategies for HLHS patients is an understanding of the genetic basis of this condition. Another investigational strategy being developed for HLHS patients is the injection of stem cells within the myocardium of the right ventricle. Recent innovations in tissue engineering and regenerative medicine promise to provide important tools to both understand the underlying basis of HLHS as well as provide new therapeutic strategies. In this review article, we provide an overview of HLHS, starting with a historical description and progressing through a discussion of the genetics, surgical management, post-surgical outcomes, stem cell therapy, hemodynamics and tissue engineering approaches.

Translational potential of hiPSCs in predictive modeling of heart development and disease

Congenital heart disease (CHD) represents a major class of birth defects worldwide and is associated with cardiac malformations that often require surgical intervention immediately after birth. Despite the intense efforts from multicentric genome/exome sequencing studies that have identified several genetic variants, the etiology of CHD remains diverse and often unknown. Genetically modified animal models with candidate gene deficiencies continue to provide novel molecular insights that are responsible for fetal cardiac development. However, the past decade has seen remarkable advances in the field of human induced pluripotent stem cell (hiPSC)-based disease modeling approaches to better understand the development of CHD and discover novel preventative therapies. The iPSCs are derived from reprogramming of differentiated somatic cells to an embryonic-like pluripotent state via overexpression of key transcription factors. In this review, we describe how differentiation of hiPSCs to specialized cardiac cellular identities facilitates our understanding of the development and pathogenesis of CHD subtypes. We summarize the molecular and functional characterization of hiPSC-derived differentiated cells in support of normal cardiogenesis, those that go awry in CHD and other heart diseases. We illustrate how stem cell-based disease modeling enables scientists to dissect the molecular mechanisms of cell-cell interactions underlying CHD. We highlight the current state of hiPSC-based studies that are in the verge of translating into clinical trials. We also address limitations including hiPSC-model reproducibility and scalability and differentiation methods leading to cellular heterogeneity. Last, we provide future perspective on exploiting the potential of hiPSC technology as a predictive model for patient-specific CHD, screening pharmaceuticals, and provide a source for cell-based personalized medicine. In combination with existing clinical and animal model studies, data obtained from hiPSCs will yield further understanding of oligogenic, gene-environment interaction, pathophysiology, and management for CHD and other genetic cardiac disorders.

Probing single ventricle heart defects with patient-derived induced pluripotent stem cells and emerging technologies

Single ventricle heart defects (SVHDs) are a severe type of congenital heart disease with poorly understood pathogenic mechanisms. New research using patient-specific induced pluripotent stem cells (iPSCs) as a cellular model is beginning to uncover genetic and cellular etiologies of SVHDs. Hypoplastic left heart syndrome (HLHS) is a type of SVHD that is characterized by an underdeveloped left ventricle and other malformations in the left side of the heart. Hypoplastic right heart syndrome (HRHS), the second type of SVHD, is characterized by an underdeveloped right heart, including malformed tricuspid and pulmonary valves. Despite a noticeable lack of research on SVHD, emerging technologies offer a promising future to further probe the genetic and cellular mechanisms of these diseases. Pediatric cardiovascular research is at the dawn of a new era in terms of what can be discovered with patient-specific iPSCs in conjunction with other technologies (e.g., organoids, single-cell genomics, CRISPR/Cas9 genome editing). In this review, we present recent approaches and findings utilizing patient-specific iPSCs to identify cellular mechanisms responsible for improper cardiac organogenesis in HLHS and HRHS.

ETS1 and HLHS: Implications for the Role of the Endocardium

We have identified the ETS1 gene as the cause of congenital heart defects, including an unprecedented high frequency of HLHS, in the chromosomal disorder Jacobsen syndrome. Studies in Ciona intestinalis demonstrated a critical role for ETS1 in heart cell fate determination and cell migration, suggesting that the impairment of one or both processes can underlie the pathogenesis of HLHS. Our studies determined that ETS1 is expressed in the cardiac neural crest and endocardium in the developing murine heart, implicating one or both lineages in the development of HLHS. Studies in Drosophila and Xenopus demonstrated a critical role for ETS1 in regulating cardiac cell fate determination, and results in Xenopus provided further evidence for the role of the endocardium in the evolution of the “hypoplastic” HLHS LV. Paradoxically, these studies suggest that the loss of ETS1 may cause a cell fate switch resulting in the loss of endocardial cells and a relative abundance of cardiac myocytes. These studies implicate an “HLHS transcriptional network” of genes conserved across species that are essential for early heart development. Finally, the evidence suggests that in a subset of HLHS patients, the HLHS LV cardiac myocytes are, intrinsically, developmentally and functionally normal, which has important implications for potential future therapies.

Contribution of NOTCH1 genetic variants to bicuspid aortic valve and other congenital lesions

INTRODUCTION

Bicuspid aortic valve (BAV) affects 1% of the general population. NOTCH1 was the first gene associated with BAV. The proportion of familial and sporadic BAV disease attributed to NOTCH1 mutations has not been estimated.

AIM

The aim of our study was to provide an estimate of familial and sporadic BAV disease attributable to NOTCH1 mutations.

METHODS

The population of our study consisted of participants of the University of Leicester Bicuspid aoRtic vAlVe gEnetic research-8 pedigrees with multiple affected family members and 381 sporadic patients. All subjects underwent NOTCH1 sequencing. A systematic literature search was performed in the NCBI PubMed database to identify publications reporting NOTCH1 sequencing in context of congenital heart disease.

RESULTS

NOTCH1 sequencing in 36 subjects from 8 pedigrees identified one variant c.873C>G/p.Tyr291* meeting the American College of Medical Genetics and Genomics criteria for pathogenicity. No pathogenic or likely pathogenic NOTCH1 variants were identified in 381 sporadic patients. Literature review identified 64 relevant publication reporting NOTCH1 sequencing in 528 pedigrees and 9449 sporadic subjects. After excluding families with syndromic disease pathogenic and likely pathogenic NOTCH1 variants were detected in 9/435 (2.1%; 95% CI: 0.7% to 3.4%) of pedigrees and between 0.05% (95% CI: 0.005% to 0.10%) and 0.08% (95% CI: 0.02% to 0.13%) of sporadic patients. Incomplete penetrance of definitely pathogenic NOTCH1 mutations was observed in almost half of reported pedigrees.

CONCLUSIONS

Pathogenic and likely pathogenic NOTCH1 genetic variants explain 2% of familial and <0.1% of sporadic BAV disease and are more likely to associate with tetralogy of Fallot and hypoplastic left heart.

Endocardial Regulation of Cardiac Development

The endocardium is a specialized form of endothelium that lines the inner side of the heart chambers and plays a crucial role in cardiac development. While comparatively less studied than other cardiac cell types, much progress has been made in understanding the regulation of and by the endocardium over the past two decades. In this review, we will summarize what is currently known regarding endocardial origin and development, the relationship between endocardium and other cardiac cell types, and the various lineages that endocardial cells derive from and contribute to. These processes are driven by key molecular mechanisms such as Notch and BMP signaling. These pathways in particular have been well studied, but other signaling pathways and mechanical cues also play important roles. Finally, we will touch on the contribution of stem cell modeling in combination with single cell sequencing and its potential translational impact for congenital heart defects such as bicuspid aortic valves and hypoplastic left heart syndrome. The detailed understanding of cellular and molecular processes in the endocardium will be vital to further develop representative stem cell-derived models for disease modeling and regenerative medicine in the future.

Whole Genome Sequencing in Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a genetically complex disorder. Whole genome sequencing enables comprehensive scrutiny of single nucleotide variants and small insertions/deletions, within both coding and regulatory regions of the genome, revolutionizing susceptibility-gene discovery research. Because millions of rare variants comprise an individual genome, identification of alleles linked to HLHS necessitates filtering algorithms based on various parameters, such as inheritance, enrichment, omics data, known genotype–phenotype associations, and predictive or experimental modeling. In this brief review, we highlight family and cohort-based strategies used to analyze whole genome sequencing datasets and identify HLHS candidate genes. Key findings include compound and digenic heterozygosity among several prioritized genes and genetic associations between HLHS and bicuspid aortic valve or cardiomyopathy. Together with findings of independent genomic investigations, MYH6 has emerged as a compelling disease gene for HLHS and other left-sided congenital heart diseases.

Delving into the Molecular World of Single Ventricle Congenital Heart Disease

PURPOSE OF REVIEW

Given a general lack of emphasis on the molecular underpinnings of single ventricle (SV) congenital heart diseases (CHD), our review highlights and summarizes recent advances in uncovering the genetic and molecular mechanisms in SV CHD etiology.

RECENT FINDINGS

While common SV-associated genetic mutations were found in key cardiac transcription factors, other mutations were sporadic. With advances in genetic sequencing technologies and animal models, more disease-associated factors have been identified to act in critical cardiac signaling pathways such as NOTCH, Wnt, and TGF signaling. Recent studies have also revealed that different cardiac lineages play different roles in disease pathogenesis. SV defects are attributed to complex combinations of genetic mutations, indicating that sophisticated spatiotemporal regulation of gene transcription networks and functional cellular pathways govern disease progression. Future studies will warrant in-depth investigations into better understanding how different genetic factors converge to influence common downstream cellular pathways, resulting in SV abnormalities.

CELSR1 Risk Alleles in Familial Bicuspid Aortic Valve and Hypoplastic Left Heart Syndrome

BACKGROUND

Whole-genome sequencing in families enables deciphering of congenital heart disease causes. A shared genetic basis for familial bicuspid aortic valve (BAV) and hypoplastic left heart syndrome (HLHS) was postulated.

METHODS

Whole-genome sequencing was performed in affected members of 6 multiplex BAV families, an HLHS cohort of 197 probands and 546 relatives, and 813 controls. Data were filtered for rare, predicted-damaging variants that cosegregated with familial BAV and disrupted genes associated with congenital heart disease in humans and mice. Candidate genes were further prioritized by rare variant burden testing in HLHS cases versus controls. Modifier variants in HLHS proband-parent trios were sought to account for the severe developmental phenotype.

RESULTS

In 5 BAV families, missense variants in 6 ontologically diverse genes for structural (SPTBN1, PAXIP1, and FBLN1) and signaling (CELSR1, PLXND1, and NOS3) proteins fulfilled filtering metrics. CELSR1, encoding cadherin epidermal growth factor laminin G seven-pass G-type receptor, was identified as a candidate gene in 2 families and was the only gene demonstrating rare variant enrichment in HLHS probands (P=0.003575). HLHS-associated CELSR1 variants included 16 missense, one splice site, and 3 noncoding variants predicted to disrupt canonical transcription factor binding sites, most of which were inherited from a parent without congenital heart disease. Filtering whole-genome sequencing data for rare, predicted-damaging variants inherited from the other parent revealed 2 cases of CELSR1 compound heterozygosity, one case of CELSR1-CELSR3 synergistic heterozygosity, and 4 cases of CELSR1-MYO15A digenic heterozygosity.

CONCLUSIONS

CELSR1 is a susceptibility gene for familial BAV and HLHS, further implicating planar cell polarity pathway perturbation in congenital heart disease.

Sequential Defects in Cardiac Lineage Commitment and Maturation Cause Hypoplastic Left Heart Syndrome

BACKGROUND

Complex molecular programs in specific cell lineages govern human heart development. Hypoplastic left heart syndrome (HLHS) is the most common and severe manifestation within the spectrum of left ventricular outflow tract obstruction defects occurring in association with ventricular hypoplasia. The pathogenesis of HLHS is unknown, but hemodynamic disturbances are assumed to play a prominent role.

METHODS

To identify perturbations in gene programs controlling ventricular muscle lineage development in HLHS, we performed whole-exome sequencing of 87 HLHS parent-offspring trios, nuclear transcriptomics of cardiomyocytes from ventricles of 4 patients with HLHS and 15 controls at different stages of heart development, single cell RNA sequencing, and 3D modeling in induced pluripotent stem cells from 3 patients with HLHS and 3 controls.

RESULTS

Gene set enrichment and protein network analyses of damaging de novo mutations and dysregulated genes from ventricles of patients with HLHS suggested alterations in specific gene programs and cellular processes critical during fetal ventricular cardiogenesis, including cell cycle and cardiomyocyte maturation. Single-cell and 3D modeling with induced pluripotent stem cells demonstrated intrinsic defects in the cell cycle/unfolded protein response/autophagy hub resulting in disrupted differentiation of early cardiac progenitor lineages leading to defective cardiomyocyte subtype differentiation/maturation in HLHS. Premature cell cycle exit of ventricular cardiomyocytes from patients with HLHS prevented normal tissue responses to developmental signals for growth, leading to multinucleation/polyploidy, accumulation of DNA damage, and exacerbated apoptosis, all potential drivers of left ventricular hypoplasia in absence of hemodynamic cues.

CONCLUSIONS

Our results highlight that despite genetic heterogeneity in HLHS, many mutations converge on sequential cellular processes primarily driving cardiac myogenesis, suggesting novel therapeutic approaches.

Hypoplastic left heart syndrome (HLHS): molecular pathogenesis and emerging drug targets for cardiac repair and regeneration

INTRODUCTION

Hypoplastic left heart syndrome (HLHS) is a severe developmental defect characterized by the underdevelopment of the left ventricle along with aortic and valvular defects. Multiple palliative surgeries are required for survival. Emerging studies have identified potential mechanisms for the disease onset, including genetic and hemodynamic causes. Genetic variants associated with HLHS include transcription factors, chromatin remodelers, structural proteins, and signaling proteins necessary for normal heart development. Nonetheless, current therapies are being tested clinically and have shown promising results at improving cardiac function in patients who have undergone palliative surgeries.

AREAS COVERED

We searched PubMed and clinicaltrials.gov to review most of the mechanistic research and clinical trials involving HLHS. This review discusses the anatomy and pathology of HLHS hearts. We highlight some of the identified genetic variants that underly the molecular pathogenesis of HLHS. Additionally, we discuss some of the emerging therapies and their limitations for HLHS.

EXPERT OPINION

While HLHS etiology is largely obscure, palliative therapies remain the most viable option for the patients. It is necessary to generate animal and stem cell models to understand the underlying genetic causes directly leading to HLHS and facilitate the use of gene-based therapies to improve cardiac development and regeneration.

VarSAn: associating pathways with a set of genomic variants using network analysis

There is a pressing need today to mechanistically interpret sets of genomic variants associated with diseases. Here we present a tool called ‘VarSAn’ that uses a network analysis algorithm to identify pathways relevant to a given set of variants. VarSAn analyzes a configurable network whose nodes represent variants, genes and pathways, using a Random Walk with Restarts algorithm to rank pathways for relevance to the given variants, and reports P-values for pathway relevance. It treats non-coding and coding variants differently, properly accounts for the number of pathways impacted by each variant and identifies relevant pathways even if many variants do not directly impact genes of the pathway. We use VarSAn to identify pathways relevant to variants related to cancer and several other diseases, as well as drug response variation. We find VarSAn's pathway ranking to be complementary to the standard approach of enrichment tests on genes related to the query set. We adopt a novel benchmarking strategy to quantify its advantage over this baseline approach. Finally, we use VarSAn to discover key pathways, including the VEGFA-VEGFR2 pathway, related to de novo variants in patients of Hypoplastic Left Heart Syndrome, a rare and severe congenital heart defect.

The role of glucose in physiological and pathological heart formation

Cells display distinct metabolic characteristics depending on its differentiation stage. The fuel type of the cells serves not only as a source of energy but also as a driver of differentiation. Glucose, the primary nutrient to the cells, is a critical regulator of rapidly growing embryos. This metabolic change is a consequence as well as a cause of changes in genetic program. Disturbance of fetal glucose metabolism such as in diabetic pregnancy is associated with congenital heart disease. In utero hyperglycemia impacts the left-right axis establishment, migration of cardiac neural crest cells, conotruncal formation and mesenchymal formation of the cardiac cushion during early embryogenesis and causes cardiac hypertrophy in late fetal stages. In this review, we focus on the role of glucose in cardiogenesis and the molecular mechanisms underlying heart diseases associated with hyperglycemia.

From Stem Cells to Populations-Using hiPSC, Next-Generation Sequencing, and GWAS to Explore the Genetic and Molecular Mechanisms of Congenital Heart Defects

Congenital heart defects (CHD) are developmental malformations affecting the heart and the great vessels. Early heart development requires temporally regulated crosstalk between multiple cell types, signaling pathways, and mechanical forces of early blood flow. While both genetic and environmental factors have been recognized to be involved, identifying causal genes in non-syndromic CHD has been difficult. While variants following Mendelian inheritance have been identified by linkage analysis in a few families with multiple affected members, the inheritance pattern in most familial cases is complex, with reduced penetrance and variable expressivity. Furthermore, most non-syndromic CHD are sporadic. Improved sequencing technologies and large biobank collections have enabled genome-wide association studies (GWAS) in non-syndromic CHD. The ability to generate human to create human induced pluripotent stem cells (hiPSC) and further differentiate them to organotypic cells enables further exploration of genotype–phenotype correlations in patient-derived cells. Here we review how these technologies can be used in unraveling the genetics and molecular mechanisms of heart development.

Exploration and validation of hub genes and pathways in the progression of hypoplastic left heart syndrome via weighted gene co-expression network analysis

Background

Despite significant progress in surgical treatment of hypoplastic left heart syndrome (HLHS), its mortality and morbidity are still high. Little is known about the molecular abnormalities of the syndrome. In this study, we aimed to probe into hub genes and key pathways in the progression of the syndrome.

Methods

Differentially expressed genes (DEGs) were identified in left ventricle (LV) or right ventricle (RV) tissues between HLHS and controls using the GSE77798 dataset. Then, weighted gene co-expression network analysis (WGCNA) was performed and key modules were constructed for HLHS. Based on the genes in the key modules, protein–protein interaction networks were conducted, and hub genes and key pathways were screened. Finally, the GSE23959 dataset was used to validate hub genes between HLHS and controls.

Results

We identified 88 and 41 DEGs in LV and RV tissues between HLHS and controls, respectively. DEGs in LV tissues of HLHS were distinctly involved in heart development, apoptotic signaling pathway and ECM receptor interaction. DEGs in RV tissues of HLHS were mainly enriched in BMP signaling pathway, regulation of cell development and regulation of blood pressure. A total of 16 co-expression network were constructed. Among them, black module (r = 0.79 and p value = 2e−04) and pink module (r = 0.84 and p value = 4e−05) had the most significant correlation with HLHS, indicating that the two modules could be the most relevant for HLHS progression. We identified five hub genes in the black module (including Fbn1, Itga8, Itga11, Itgb5 and Thbs2), and five hub genes (including Cblb, Ccl2, Edn1, Itgb3 and Map2k1) in the pink module for HLHS. Their abnormal expression was verified in the GSE23959 dataset.

Conclusions

Our findings revealed hub genes and key pathways for HLHS through WGCNA, which could play key roles in the molecular mechanism of HLHS.

Mending a broken heart: In vitro, in vivo and in silico models of congenital heart disease

Birth defects contribute to ∼0.3% of global infant mortality in the first month of life, and congenital heart disease (CHD) is the most common birth defect among newborns worldwide. Despite the significant impact on human health, most treatments available for this heterogenous group of disorders are palliative at best. For this reason, the complex process of cardiogenesis, governed by multiple interlinked and dose-dependent pathways, is well investigated. Tissue, animal and, more recently, computerized models of the developing heart have facilitated important discoveries that are helping us to understand the genetic, epigenetic and mechanobiological contributors to CHD aetiology. In this Review, we discuss the strengths and limitations of different models of normal and abnormal cardiogenesis, ranging from single-cell systems and 3D cardiac organoids, to small and large animals and organ-level computational models. These investigative tools have revealed a diversity of pathogenic mechanisms that contribute to CHD, including genetic pathways, epigenetic regulators and shear wall stresses, paving the way for new strategies for screening and non-surgical treatment of CHD. As we discuss in this Review, one of the most-valuable advances in recent years has been the creation of highly personalized platforms with which to study individual diseases in clinically relevant settings.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Decoding Genetics of Congenital Heart Disease Using Patient-Derived Induced Pluripotent Stem Cells (iPSCs)

Congenital heart disease (CHD) is the most common cause of infant death associated with birth defects. Recent next-generation genome sequencing has uncovered novel genetic etiologies of CHD, from inherited and de novo variants to non-coding genetic variants. The next phase of understanding the genetic contributors of CHD will be the functional illustration and validation of this genome sequencing data in cellular and animal model systems. Human induced pluripotent stem cells (iPSCs) have opened up new horizons to investigate genetic mechanisms of CHD using clinically relevant and patient-specific cardiac cells such as cardiomyocytes, endothelial/endocardial cells, cardiac fibroblasts and vascular smooth muscle cells. Using cutting-edge CRISPR/Cas9 genome editing tools, a given genetic variant can be corrected in diseased iPSCs and introduced to healthy iPSCs to define the pathogenicity of the variant and molecular basis of CHD. In this review, we discuss the recent progress in genetics of CHD deciphered by large-scale genome sequencing and explore how genome-edited patient iPSCs are poised to decode the genetic etiologies of CHD by coupling with single-cell genomics and organoid technologies.

Genetic Etiology of Left-Sided Obstructive Heart Lesions: A Story in Development

Congenital heart disease is the most common congenital defect observed in newborns. Within the spectrum of congenital heart disease are left‐sided obstructive lesions (LSOLs), which include hypoplastic left heart syndrome, aortic stenosis, bicuspid aortic valve, coarctation of the aorta, and interrupted aortic arch. These defects can arise in isolation or as a component of a defined syndrome; however, nonsyndromic defects are often observed in multiple family members and associated with high sibling recurrence risk. This clear evidence for a heritable basis has driven a lengthy search for disease‐causing variants that has uncovered both rare and common variants in genes that, when perturbed in cardiac development, can result in LSOLs. Despite advancements in genetic sequencing platforms and broadening use of exome sequencing, the currently accepted LSOL‐associated genes explain only 10% to 20% of patients. Further, the combinatorial effects of common and rare variants as a cause of LSOLs are emerging. In this review, we highlight the genes and variants associated with the different LSOLs and discuss the strengths and weaknesses of the present genetic associations. Furthermore, we discuss the research avenues needed to bridge the gaps in our current understanding of the genetic basis of nonsyndromic congenital heart disease.

Is There Any Clinical Utility to Genetic Testing for Patients With Congenital Heart Disease?

Genetic diagnosis is becoming increasingly sophisticated, with the ability to identify even fine differences in patients with a wide variety of congenital heart lesions. Although we have an incomplete understanding of the clinical consequences of most genetic findings, some categories of mutations can have important implications for disease recurrence and prognosis. Consideration of the biology underlying a genetic deficiency, when known, can be useful in the clinical management of some patients.

Genetic Association Between Hypoplastic Left Heart Syndrome and Cardiomyopathies

BACKGROUND

Hypoplastic left heart syndrome (HLHS) with risk of poor outcome has been linked to MYH6 variants, implicating overlap in genetic etiologies of structural and myopathic heart disease.

METHODS

Whole genome sequencing was performed in 197 probands with HLHS, 43 family members, and 813 controls. Data were filtered for rare, segregating variants in 3 index families comprised of an HLHS proband and relative(s) with cardiomyopathy. Whole genome sequencing data from cases and controls were compared for rare variant burden across 56 cardiomyopathy genes utilizing a weighted burden test approach, accounting for multiple testing using a Bonferroni correction.

RESULTS

A pathogenic MYBPC3 nonsense variant was identified in the first proband who underwent cardiac transplantation for diastolic heart failure, her father with left ventricular noncompaction, and 2 fourth-degree relatives with hypertrophic cardiomyopathy. A likely pathogenic RYR2 missense variant was identified in the second proband, a second-degree relative with aortic dilation, and a fourth-degree relative with dilated cardiomyopathy. A pathogenic RYR2 exon 3 in-frame deletion was identified in the third proband diagnosed with catecholaminergic polymorphic ventricular tachycardia and his father with left ventricular noncompaction and catecholaminergic polymorphic ventricular tachycardia. To further investigate HLHS-cardiomyopathy gene associations in cases versus controls, rare variant burden testing of 56 genes revealed enrichment in MYH6 (P=0.000068). Rare, predicted-damaging MYH6 variants were identified in 10% of probands in our cohort-4 with familial congenital heart disease, 4 with compound heterozygosity (3 with systolic ventricular dysfunction), and 4 with MYH6-FLNC synergistic heterozygosity.

CONCLUSIONS

Whole genome sequencing in multiplex families, proband-parent trios, and case-control cohorts revealed defects in cardiomyopathy-associated genes in patients with HLHS, which may portend impaired functional reserve of the single-ventricle circulation.

Patient-specific genomics and cross-species functional analysis implicate LRP2 in hypoplastic left heart syndrome

Congenital heart diseases (CHDs), including hypoplastic left heart syndrome (HLHS), are genetically complex and poorly understood. Here, a multidisciplinary platform was established to functionally evaluate novel CHD gene candidates, based on whole-genome and iPSC RNA sequencing of a HLHS family-trio. Filtering for rare variants and altered expression in proband iPSCs prioritized 10 candidates. siRNA/RNAi-mediated knockdown in healthy human iPSC-derived cardiomyocytes (hiPSC-CM) and in developing Drosophila and zebrafish hearts revealed that LDL receptor-related protein LRP2 is required for cardiomyocyte proliferation and differentiation. Consistent with hypoplastic heart defects, compared to patents the proband's iPSC-CMs exhibited reduced proliferation. Interestingly, rare, predicted-damaging LRP2 variants were enriched in a HLHS cohort; however, understanding their contribution to HLHS requires further investigation. Collectively, we have established a multi-species high-throughput platform to rapidly evaluate candidate genes and their interactions during heart development, which are crucial first steps toward deciphering oligogenic underpinnings of CHDs, including hypoplastic left hearts.

Modeling Congenital Heart Disease Using Pluripotent Stem Cells

Congenital heart disease (CHD) represents a major class of birth defects worldwide and is associated with cardiac malformations that often require immediate surgery upon birth. Significant efforts are underway to better understand how CHD manifests through basic science approaches. Recently, human-induced pluripotent stem cells (hiPSCs) have emerged as a means by which to interrogate CHD phenotypes mechanistically. PURPOSE OF REVIEW: To review recent studies and results utilizing hiPSCs and their cardiovascular derivative cell types to better understand the mechanisms for various forms of CHD. RECENT FINDINGS: Recent studies demonstrate that hiPSC-derived cardiomyocytes can replicate the genetic and epigenetic abnormalities that ultimately lay the cellular foundation for CHD phenotypes. Such irregularities manifest in vitro through defects in hiPSC differentiation, signaling, and transcriptional activity. Use of hiPSC-derived cells to understand CHD may ultimately lead to the development of preemptive screening approaches to identify CHD early in utero and innovative therapies to alleviate symptoms after birth.

6q25.1 (TAB2) microdeletion is a risk factor for hypoplastic left heart: a case report that expands the phenotype

Introduction

Hypoplastic left heart syndrome (HLHS) is a rare but devastating congenital heart defect (CHD) accounting for 25% of all infant deaths due to a CHD. The etiology of HLHS remains elusive, but there is increasing evidence to support a genetic cause for HLHS; in particular, this syndrome is associated with abnormalities in genes involved in cardiac development. Consistent with the involvement of heritable genes in structural heart abnormalities, family members of HLHS patients have a higher incidence of both left- and right-sided valve abnormalities, including bicuspid aortic valve (BAV).

Case presentation

We previously described (Am J Med Genet A 173:1848–1857, 2017) a 4-generation family with a 6q25.1 microdeletion encompassing TAB2, a gene known to play an important role in outflow tract and cardiac valve formation during embryonic development. Affected adult family members have short stature, dysmorphic facial features, and multiple valve dysplasia, including BAV. This follow-up report includes previously unpublished details of the cardiac phenotype of affected family members. It also describes a baby recently born into this family who was diagnosed prenatally with short long bones, intrauterine growth restriction (IUGR), and HLHS. He was the second family member to have HLHS; the first died several decades ago. Postnatal genetic testing confirmed the baby had inherited the familial TAB2 deletion.

Conclusions

Our findings suggest TAB2 haploinsufficiency is a risk factor for HLHS and expands the phenotypic spectrum of this microdeletion syndrome. Chromosomal single nucleotide polymorphism (SNP) microarray analysis and molecular testing for a TAB2 loss of function variant should be considered for individuals with HLHS, particularly in those with additional non-cardiac findings such as IUGR, short stature, and/or dysmorphic facial features.

Rare Variants in Genes Associated With Cardiomyopathy Are Not Common in Hypoplastic Left Heart Syndrome Patients With Myocardial Dysfunction

Myocardial dysfunction is a known risk factor for morbidity and mortality in hypoplastic left heart syndrome (HLHS). Variants in some transcription factor and contractility genes, which are known to cause cardiomyopathy, have previously been associated with impaired right ventricular function in some HLHS patients. The care of HLHS patients is resource demanding. Identifying genetic variants associated with myocardial dysfunction would be helpful in tailoring the follow-up and therapeutic strategies. We tested whether a commercial cardiomyopathy gene panel could serve as a diagnostic tool in a Finnish cohort of HLHS patients with impaired right ventricular function to identify potentially pathogenic variants associated with poor prognosis. None of the patients had pathogenic or likely pathogenic variants in the studied cardiomyopathy-associated genes. Thus, our approach of performing a cardiomyopathy gene panel to identify pathogenic variants as directly causal or as modifiers for worse outcomes in hypoplastic left heart syndrome is not useful in clinical practice at the moment.

Right ventricular failure in congenital heart disease

PURPOSE OF REVIEW

We aim to review select literature pertaining to congenital heart disease (CHD)-induced right ventricular (RV) function and failure.

RECENT FINDINGS

We review recent findings pertaining to children and adults with repaired tetralogy of Fallot (rTOF), systemic RV and hypoplastic left heart syndrome (HLHS). We emphasize pathophysiological mechanisms contributing to RV dysfunction in these conditions, the risk factors for adverse outcomes and the continuing challenges in treating these patients. We discuss how recent pathology findings, as well as developments in imaging and computer modeling have broadened our understanding of the pathophysiology of these conditions. We further review developments in the molecular and cellular basis of RV failure; and in particular, the RV molecular response to stress in repaired tetralogy of Fallot (rTOF). We highlight some of the genetic complexities in HLHS and how these may influence the long-term outcomes in these patients.

SUMMARY

Recent literature has led to new understandings in the pathology, pathophysiology, risk factors for adverse outcomes, molecular and genetic basis for RV dysfunction and failure in CHD. Although these findings provide new therapeutic targets, the treatment of RV failure at this time remains limited.

Molecular characterization of hepatic epithelioid hemangioendothelioma reveals alterations in various genes involved in DNA repair, epigenetic regulation, signaling pathways, and cell cycle control

Epithelioid hemangioendotheliomas (EHE) of the liver are rare, low-malignant vascular tumors whose molecular pathogenesis is incompletely understood. The diagnosis of EHE is challenging, and the course of the disease can be highly variable. Therapeutic options for EHE are limited, including resection of primary and metastatic tumors, organ transplantation and rather ineffective systemic approaches. Driver mutations have been reported (fusion transcripts of either YAP-TFE3 or WWTR1-CAMTA1) but comprehensive molecular profiling has not been performed. Our aim was to molecularly characterize hepatic EHE to identify new molecular targets. Eight primary hepatic EHE were analyzed by next-generation sequencing using a 409-gene panel. The majority of primary hepatic EHE revealed a low number of mutations. Genes that were mutated primarily are involved in DNA repair, epigenetic regulation, signaling pathways and cell cycle control, indicating that EHE present with mutations in various functions. Although only detecting a low mutation rate, a comparison with comprehensive databases (target db V3) revealed mutations in five genes with putative therapeutical options. Therefore, our findings help to shed light on the molecular background of EHE and might pave the way to new therapeutic approaches.

Loss of function, missense, and intronic variants in NOTCH1 confer different risks for left ventricular outflow tract obstructive heart defects in two European cohorts

Loss of function variants in NOTCH1 cause left ventricular outflow tract obstructive defects (LVOTO). However, the risk conferred by rare and noncoding variants in NOTCH1 for LVOTO remains largely uncharacterized. In a cohort of 49 families affected by hypoplastic left heart syndrome, a severe form of LVOTO, we discovered predicted loss of function NOTCH1 variants in 6% of individuals. Rare or low-frequency missense variants were found in 16% of families. To make a quantitative estimate of the genetic risk posed by variants in NOTCH1 for LVOTO, we studied associations of 400 coding and noncoding variants in NOTCH1 in 1,085 cases and 332,788 controls from the UK Biobank. Two rare intronic variants in strong linkage disequilibrium displayed significant association with risk for LVOTO amongst European-ancestry individuals. This result was replicated in an independent analysis of 210 cases and 68,762 controls of non-European and mixed ancestry. In conclusion, carrying rare predicted loss of function variants in NOTCH1 confer significant risk for LVOTO. In addition, the two intronic variants seem to be associated with an increased risk for these defects. Our approach demonstrates the utility of population-based data sets in quantifying the specific risk of individual variants for disease-related phenotypes.

The Genetic Landscape of Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is one of the most lethal congenital heart defects, and remains clinically challenging. While surgical palliation allows most HLHS patients to survive their critical heart disease with a single-ventricle physiology, many will suffer heart failure, requiring heart transplantation as the only therapeutic course. Current paradigm suggests HLHS is largely of hemodynamic origin, but recent findings from analysis of the first mouse model of HLHS showed intrinsic cardiomyocyte proliferation and differentiation defects underlying the left ventricular (LV) hypoplasia. The findings of similar defects of lesser severity in the right ventricle suggest this could contribute to the heart failure risks in surgically palliated HLHS patients. Analysis of 8 independent HLHS mouse lines showed HLHS is genetically heterogeneous and multigenic in etiology. Detailed analysis of the Ohia mouse line accompanied by validation studies in CRISPR gene-targeted mice revealed a digenic etiology for HLHS. Mutation in Sap130, a component of the HDAC repressor complex, was demonstrated to drive the LV hypoplasia, while mutation in Pcdha9, a protocadherin cell adhesion molecule played a pivotal role in the valvular defects associated with HLHS. Based on these findings, we propose a new paradigm in which complex CHD such as HLHS may arise in a modular fashion, mediated by multiple mutations. The finding of intrinsic cardiomyocyte defects would suggest hemodynamic intervention may not rescue LV growth. The profound genetic heterogeneity and oligogenic etiology indicated for HLHS would suggest that the genetic landscape of HLHS may be complex and more accessible in clinical studies built on a familial study design.

Genetics of bicuspid aortic valve aortopathy

PURPOSE OF REVIEW

The incidence of aortic dilation and acute complications (rupture and dissection) is higher in patients with a bicuspid aortic valve (BAV), the most frequent congenital heart defect.The present review focuses on the current knowledge in the genetics of BAV, emphasizing the clinical implications for early detection and personalized care.

RECENT FINDINGS

BAV is a highly heritable trait, but the genetic causes remain largely elusive. NOTCH1 is the only proven candidate gene to be associated with both familial and sporadic BAV. Other genes have been reported to be associated with BAV, but some of these associations may result from coexisting disease.The application of modern high-throughput technologies (next generation sequencing, genome-wide copy number and genome-wide methylation arrays) have begun to dissect the genetic heterogeneity underlying BAV as well as the diverse molecular pathways involved in the progression of BAV aortopathy.

SUMMARY

The clinical variability seen in BAV aortopathy, in terms of phenotype and natural/clinical history, suggests complex interactions between primary genetic defects, other modifier genes, epigenetic factors (DNA methylation or histone modifications, microRNA) and environmental factors (disturbed flow). Integrated, more comprehensive studies are needed for elucidating these connections to develop more individualized and accurate risk assessment methods.

MEF2C loss-of-function mutation contributes to congenital heart defects

Congenital heart disease (CHD) is the most common type of developmental abnormality in humans, and is a leading cause for substantially increased morbidity and mortality in affected individuals. Increasing studies demonstrates a pivotal role of genetic defects in the pathogenesis of CHD, and presently mutations in more than 60 genes have been associated with CHD. Nevertheless, CHD is of pronounced genetic heterogeneity, and the genetic basis underpinning CHD in a large proportion of patients remains unclear. In the present study, the whole coding exons and splicing donors/acceptors of the MEF2C gene, which codes for a transcription factor essential for normal cardiovascular development, were sequenced in 200 unrelated patients affected with CHD, and a novel heterozygous missense mutation, p.L38P, was identified in an index patient with patent ductus arteriosus (PDA) and ventricular septal defect (VSD). Genetic scan of the mutation carrier's family members available showed that the mutation was present in all affected family members but absent in unaffected family members. Analysis of the proband's pedigree revealed that the mutation co-segregated with PDA, which was transmitted as an autosomal dominant trait with complete penetrance. The mutation changed the amino acid that was completely conserved evolutionarily, and did not exist in 300 unrelated, ethnically-matched healthy individuals used as controls. Functional deciphers by using a dual-luciferase reporter assay system unveiled that the mutant MEF2C protein had a significantly reduced transcriptional activity. Furthermore, the mutation significantly diminished the synergistic activation between MEF2C and GATA4, another cardiac core transcription factor that has been causally linked to CHD. In conclusion, this is the first report on the association of a MEF2C loss-of-function mutation with an increased vulnerability to CHD in humans, which provides novel insight into the molecular mechanisms underlying CHD, implying potential implications for early diagnosis and timely prophylaxis of CHD.

Wnt11 regulates cardiac chamber development and disease during perinatal maturation

Ventricular chamber growth and development during perinatal circulatory transition is critical for functional adaptation of the heart. However, the chamber-specific programs of neonatal heart growth are poorly understood. We used integrated systems genomic and functional biology analyses of the perinatal chamber specific transcriptome and we identified Wnt11 as a prominent regulator of chamber-specific proliferation. Importantly, downregulation of Wnt11 expression was associated with cyanotic congenital heart defect (CHD) phenotypes and correlated with O2 saturation levels in hypoxemic infants with Tetralogy of Fallot (TOF). Perinatal hypoxia treatment in mice suppressed Wnt11 expression and induced myocyte proliferation more robustly in the right ventricle, modulating Rb1 protein activity. Wnt11 inactivation was sufficient to induce myocyte proliferation in perinatal mouse hearts and reduced Rb1 protein and phosphorylation in neonatal cardiomyocytes. Finally, downregulated Wnt11 in hypoxemic TOF infantile hearts was associated with Rb1 suppression and induction of proliferation markers. This study revealed a previously uncharacterized function of Wnt11-mediated signaling as an important player in programming the chamber-specific growth of the neonatal heart. This function influences the chamber-specific development and pathogenesis in response to hypoxia and cyanotic CHDs. Defining the underlying regulatory mechanism may yield chamber-specific therapies for infants born with CHDs.

Induced pluripotent stem cell modelling of HLHS underlines the contribution of dysfunctional NOTCH signalling to impaired cardiogenesis

Hypoplastic left heart syndrome (HLHS) is among the most severe forms of congenital heart disease. Although the consensus view is that reduced flow through the left heart during development is a key factor in the development of the condition, the molecular mechanisms leading to hypoplasia of left heart structures are unknown. We have generated induced pluripotent stem cells (iPSC) from five HLHS patients and two unaffected controls, differentiated these to cardiomyocytes and identified reproducible in vitro cellular and functional correlates of the HLHS phenotype. Our data indicate that HLHS-iPSC have a reduced ability to give rise to mesodermal, cardiac progenitors and mature cardiomyocytes and an enhanced ability to differentiate to smooth muscle cells. HLHS-iPSC-derived cardiomyocytes are characterised by a lower beating rate, disorganised sarcomeres and sarcoplasmic reticulum and a blunted response to isoprenaline. Whole exome sequencing of HLHS fibroblasts identified deleterious variants in NOTCH receptors and other genes involved in the NOTCH signalling pathway. Our data indicate that the expression of NOTCH receptors was significantly downregulated in HLHS-iPSC-derived cardiomyocytes alongside NOTCH target genes confirming downregulation of NOTCH signalling activity. Activation of NOTCH signalling via addition of Jagged peptide ligand during the differentiation of HLHS-iPSC restored their cardiomyocyte differentiation capacity and beating rate and suppressed the smooth muscle cell formation. Together, our data provide firm evidence for involvement of NOTCH signalling in HLHS pathogenesis, reveal novel genetic insights important for HLHS pathology and shed new insights into the role of this pathway during human cardiac development.

Hypoplastic Left Heart Syndrome Sequencing Reveals a Novel NOTCH1 Mutation in a Family with Single Ventricle Defects

Hypoplastic left heart syndrome (HLHS) has been associated with germline mutations in 12 candidate genes and a recurrent somatic mutation in HAND1 gene. Using targeted and whole exome sequencing (WES) of heart tissue samples from HLHS patients, we sought to estimate the prevalence of somatic and germline mutations associated with HLHS. We performed Sanger sequencing of the HAND1 gene on 14 ventricular (9 LV and 5 RV) samples obtained from HLHS patients, and WES of 4 LV, 2 aortic, and 4 matched PBMC samples, analyzing for sequence discrepancy. We also screened for mutations in the 12 candidate genes implicated in HLHS. We found no somatic mutations in our HLHS cohort. However, we detected a novel germline frameshift/stop-gain mutation in NOTCH1 in a HLHS patient with a family history of both HLHS and hypoplastic right heart syndrome (HRHS). Our study, involving one of the first familial cases of single ventricle defects linked to a specific mutation, strengthens the association of NOTCH1 mutations with HLHS and suggests that the two morphologically distinct single ventricle conditions, HLHS and HRHS, may share a common molecular and cellular etiology. Finally, somatic mutations in the LV are an unlikely contributor to HLHS.

The complex genetics of hypoplastic left heart syndrome

Congenital heart disease (CHD) affects up to 1% of live births. Although a genetic etiology is indicated by an increased recurrence risk, sporadic occurrence suggests that CHD genetics is complex. Here, we show that hypoplastic left heart syndrome (HLHS), a severe CHD, is multigenic and genetically heterogeneous. Using mouse forward genetics, we report what is, to our knowledge, the first isolation of HLHS mutant mice and identification of genes causing HLHS. Mutations from seven HLHS mouse lines showed multigenic enrichment in ten human chromosome regions linked to HLHS. Mutations in Sap130 and Pcdha9, genes not previously associated with CHD, were validated by CRISPR-Cas9 genome editing in mice as being digenic causes of HLHS. We also identified one subject with HLHS with SAP130 and PCDHA13 mutations. Mouse and zebrafish modeling showed that Sap130 mediates left ventricular hypoplasia, whereas Pcdha9 increases penetrance of aortic valve abnormalities, both signature HLHS defects. These findings show that HLHS can arise genetically in a combinatorial fashion, thus providing a new paradigm for the complex genetics of CHD.

Genetic basis of aortic valvular disease

PURPOSE OF REVIEW

Aortic valve disease is relatively common and encompasses both congenital and acquired forms. Bicuspid aortic valve (BAV) is the most common type of cardiac malformation and predisposes to the development of calcific aortic valve disease (CAVD). Since the description of the link between NOTCH1, BAV and CAVD approximately a decade ago, there have been significant advances in the genetic and molecular understanding of these diseases.

RECENT FINDINGS

Recent work has defined the congenital cardiac phenotypes linked to mutations in NOTCH1, and in addition, novel etiologic genes for BAV have been discovered using new genetic technologies in humans. Furthermore, several mouse models of BAV have been described defining the role of endothelial Notch1 in aortic valve morphogenesis, whereas others have implicated new genes. These murine models along with other cell-based studies have led to molecular insights in the pathogenesis of CAVD.

SUMMARY

These findings provide important insights into the molecular and genetic basis of aortic valve malformations, including BAV, specifically highlighting the etiologic role of endothelial cells. In addition, numerous investigations in to the mechanisms of CAVD demonstrate the importance of developmental origins and signaling pathways as well as communication between valve endothelial cells and the underlying interstitial cells in valve disease onset and progression.