A gain-of-function TBX20 mutation causes congenital atrial septal defects, patent foramen ovale and cardiac valve defects

MESP1 Mutations in Patients with Congenital Heart Defects

Identifying the genetic etiology of congenital heart disease (CHD) has been challenging despite being one of the most common congenital malformations in humans. We previously identified a microdeletion in a patient with a ventricular septal defect containing over 40 genes including MESP1 (mesoderm posterior basic helix-loop-helix transcription factor 1). Because of the importance of MESP1 as an early regulator of cardiac development in both in vivo and in vitro studies, we tested for MESP1 mutations in 647 patients with congenital conotruncal and related heart defects. We identified six rare, nonsynonymous variants not seen in ethnically matched controls and one likely race-specific nonsynonymous variant. Functional analyses revealed that three of these variants altered activation of transcription by MESP1. Two of the deleterious variants are located within the conserved HLH domain and thus impair the protein-protein interaction of MESP1 and E47. The third deleterious variant was a loss-of-function frameshift mutation. Our results suggest that pathologic variants in MESP1 may contribute to the development of CHD and that additional protein partners and downstream targets could likewise contribute to the wide range of causes for CHD.

TBX20 loss-of-function mutation associated with familial dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a major cause of congestive heart failure, sudden cardiac death and cardiac transplantation. Aggregating evidence highlights the genetic origin of DCM. However, DCM is a genetically heterogeneous disorder, and the genetic components underlying DCM in most cases remain unknown.

The coding regions and splicing junction sites of the

A novel heterozygous TBX20 mutation, p.F256I, was identified in a family with DCM transmitted in an autosomal dominant fashion, which co-segregated with DCM in the family with complete penetrance. The missense mutation was absent in 600 control chromosomes and the altered amino acid was completely conserved evolutionarily among various species. Functional assays revealed that the mutant TBX20 had significantly diminished transcriptional activity. Furthermore, the mutation markedly reduced the synergistic activation of TBX20 with NKX2-5 or GATA4.

This study links TBX20 loss-of-function mutation to idiopathic DCM in humans for the first time, providing novel insight into the molecular mechanism underpinning DCM.

A genome-wide association study of suicidal behavior

Genome wide array studies have reported limited success in identifying genetic markers conferring risk for suicidal behavior (SB). This may be attributable to study designs with primary outcome other than SB. We performed a GWAS on suicides and cases with a history of nonfatal suicide attempts compared with psychiatric controls and healthy volunteers. A consortium of USA, Canadian and German teams assembled two groups of cases (suicide attempters and suicides, N = 577) and non-attempter psychiatric and healthy controls (N = 1,233). Logistic regression was used to test genotype-suicidal behavior association. The test was repeated separating suicide attempt and completed suicide as outcomes. No SNP reached genome-wide significance, but several SNPs within STK3, ADAMTS14, PSME2, and TBX20 genes reached P < 1 × 10(-5) . The top SNPs for the suicide attempt analysis included two from DPP10, one from CTNNA3 and one from STK32B. In the suicide analysis we found seven SNPs from the TBX20 gene in the top hits. Pathway analysis identified the following pathways: "Cellular Assembly and Organization," "Nervous System Development and Function," "Cell Death and Survival," "Immunological Disease," "Infectious Disease," and "Inflammatory Response." The top genes in the SB analysis did not overlap with those in the ideation analysis. No genome wide significant results suggest that susceptibility to SB has genetic risk factors with smaller effect sizes. The strongest candidate genes, ADAMTS14, and PSME2 (both linked to inflammatory response), STK3 (neuronal cell death), and TBX20 (brainstem motor neuron development), have not been previously reported in association with suicide and warrant further study.

A novel variant in TBX20 (p.D176N) identified by whole-exome sequencing in combination with a congenital heart disease related gene filter is associated with familial atrial septal defect

Congenital heart disease (CHD) is the leading cause of birth defects, and its etiology is not completely understood. Atrial septal defect (ASD) is one of the most common defects of CHD. Previous studies have demonstrated that mutations in the transcription factor T-box 20 (TBX20) contribute to congenital ASD. Whole-exome sequencing in combination with a CHD-related gene filter was used to detect a family of three generations with ASD. A novel TBX20 mutation, c.526G>A (p.D176N), was identified and co-segregated in all affected members in this family. This mutation was predicted to be deleterious by bioinformatics programs (SIFT, Polyphen2, and MutationTaster). This mutation was also not presented in the current Single Nucleotide Polymorphism Database (dbSNP) or National Heart, Lung, and Blood Institute (NHLBI) Exome Sequencing Project (ESP). In conclusion, our finding expands the spectrum of TBX20 mutations and provides additional support that TBX20 plays important roles in cardiac development. Our study also provided a new and cost-effective analysis strategy for the genetic study in small CHD pedigree.

Association of TBX20 gene polymorphism with congenital heart disease in Han Chinese neonates

As a transcription factor mainly expressed in cardiovascular system, T-box 20 (TBX20) plays an important role in embryonic cardiovascular system development and adult heart function. Previous studies have identified associations of two SNPs in the T-box DNA-binding domain of TBX20 with congenital heart disease (CHD) in two Caucasian families, but the associations of TBX20 mutations underlying the more common populations with CHD remain to be uncovered. In this study, 25 unrelated Chinese Han neonates with CHD and 25 healthy children as controls were investigated for TBX20 mutations. SNP genotyping was performed by PCR-DNA sequencing. The selected SNPs were well genotyped and SNP rs3999941 was found to be strongly associated with CHD (p = 0.007). The minor allele of rs3999941 showed a high-risk factor for CHD (OR 4.24; 95 % CI 1.41-12.71). Besides, we found a new SNP site located at the 657th nucleotide of the exon 5 of TBX20 gene which may also be associated with CHD, c.657A>C. The frequency was significantly different between two groups (p = 0.011), the minor allele of SNP c.657A>C also showed a risk factor for CHD (OR 2.56; 95 % CI 1.02-6.46). These findings suggested that the TC genotype of SNP rs3999941 and AC genotype of the new SNP c.657A>C in the TBX20 gene may be risk factors for CHD and thus screening of these SNPs may have some implications in the prevention and treatment of CHD in Han Chinese children.

Novel mutations in the transcriptional activator domain of the human TBX20 in patients with atrial septal defect

Background. The relevance of TBX20 gene in heart development has been demonstrated in many animal models, but there are few works that try to elucidate the effect of TBX20 mutations in human congenital heart diseases. In these studies, all missense mutations associated with atrial septal defect (ASD) were found in the DNA-binding T-box domain, none in the transcriptional activator domain. Methods. We search for TBX20 mutations in a group of patients with ASD or ventricular septal defect (VSD) using the High Resolution Melting (HRM) method and DNA sequencing. Results. We report three missense mutations (Y309D, T370O, and M395R) within the transcriptional activator domain of human TBX20 that were associated with ASD. Conclusions. This is the first association of TBX20 transcriptional activator domain missense mutations with ASD. These findings could have implications for diagnosis, genetic screening, and patient follow-up.

CCN1 mutation is associated with atrial septal defect

The genetic basis of congenital heart disease remains unknown in most of the cases. Recently, a novel mouse model shed new light on the role of CCN1/CYR61, a matricellular regulatory factor, in cardiac morphogenesis. In a candidate gene approach, we analyzed a cohort of 143 patients with atrial septal defects (ASD) by sequencing the coding exons of CCN1. In addition to three frequent polymorphisms, we identified an extremely rare novel heterozygous missense mutation (c.139C > T; p.R47W) in one patient with severe ASD. The mutation leads to an exchange of residues with quite different properties in a highly conserved position of the N-terminal insulin-like growth factor binding protein module. Further bioinformatic analysis, exclusion of known ASD disease genes as well as the exclusion of the mutation in a very high number of ethnically matched controls (more than 1,000 individuals) and in public genetic databases, indicates that the p.R47W variant is a probable disease-associated mutation. The report about ASD in mice in heterozygous Ccn 1 +/- animals strongly supports this notion. Our study is the first to suggest a relationship between a probable CCN1 mutation and ASD. Our purpose here was to draw attention to CCN1, a gene that we believe may be important for genetic analysis in patients with congenital heart disease.

A review of the diverse genetic disorders in the Lebanese population: highlighting the urgency for community genetic services

The review lists the genetic diseases reported in Lebanese individuals, surveys genetic programs and services, and highlights the absence of basic genetic health services at the individual and community level. The incidence of individual diseases is not determined, yet the variety of genetic diseases reported is tremendous, most of which follow autosomal recessive inheritance reflecting the social norms in the population, including high rates of consanguinity, which favor the increase in incidence of these diseases. Genetic services including all activities for the diagnosis, care, and prevention of genetic diseases at community level are extremely inadequate. Services are limited to some clinical and laboratory diagnostic services with no genetic counseling. These services are localized within the capital thus preventing their accessibility to high-risk communities. Screening programs, which are at the core of public health prevention services, are minimal and not nationally mandated. The absence of adequate genetic services is attributed to many factors undermining the importance of genetic diseases and their burden on society, the most important of which is genetic illiteracy at all levels of the population, including high-risk families, the general public, and most importantly health care providers and public health officials. Thus, a country like Lebanon, where genetic diseases are expected to be highly prevalent, is in utmost need for community genetics services. Strategies need to be developed to familiarize public health officials and medical professionals with medical genetics leading to a public health infrastructure that delivers community genetics services for the prevention and care of genetic disorders at community level.

Functionally significant, rare transcription factor variants in tetralogy of Fallot

Objective

Rare variants in certain transcription factors involved in cardiac development cause Mendelian forms of congenital heart disease. The purpose of this study was to systematically assess the frequency of rare transcription factor variants in sporadic patients with the cardiac outflow tract malformation tetralogy of Fallot (TOF).

Methods and Results

We sequenced the coding, 5′UTR, and 3′UTR regions of twelve transcription factor genes implicated in cardiac outflow tract development (NKX2.5, GATA4, ISL1, TBX20, MEF2C, BOP/SMYD1, HAND2, FOXC1, FOXC2, FOXH, FOXA2 and TBX1) in 93 non-syndromic, non-Mendelian TOF cases. We also analysed Illumina Human 660W-Quad SNP Array data for copy number variants in these genes; none were detected. Four of the rare variants detected have previously been shown to affect transactivation in in vitro reporter assays: FOXC1 p.P297S, FOXC2 p.Q444R, FOXH1 p.S113T and TBX1 p.P43_G61del PPPPRYDPCAAAAPGAPGP. Two further rare variants, HAND2 p.A25_A26insAA and FOXC1 p.G378_G380delGGG, A488_491delAAAA, affected transactivation in in vitro reporter assays. Each of these six functionally significant variants was present in a single patient in the heterozygous state; each of the four for which parental samples were available were maternally inherited. Thus in the 93 TOF cases we identified six functionally significant mutations in the secondary heart field transcriptional network.

Significance

This study indicates that rare genetic variants in the secondary heart field transcriptional network with functional effects on protein function occur in 3–13% of patients with TOF. This is the first report of a functionally significant HAND2 mutation in a patient with congenital heart disease.

Cardiogenic genes expressed in cardiac fibroblasts contribute to heart development and repair

RATIONALE

Cardiac fibroblasts are critical to proper heart function through multiple interactions with the myocardial compartment, but appreciation of their contribution has suffered from incomplete characterization and lack of cell-specific markers.

OBJECTIVE

To generate an unbiased comparative gene expression profile of the cardiac fibroblast pool, identify and characterize the role of key genes in cardiac fibroblast function, and determine their contribution to myocardial development and regeneration.

METHODS AND RESULTS

High-throughput cell surface and intracellular profiling of cardiac and tail fibroblasts identified canonical mesenchymal stem cell and a surprising number of cardiogenic genes, some expressed at higher levels than in whole heart. While genetically marked fibroblasts contributed heterogeneously to interstitial but not cardiomyocyte compartments in infarcted hearts, fibroblast-restricted depletion of one highly expressed cardiogenic marker, T-box 20, caused marked myocardial dysmorphology and perturbations in scar formation on myocardial infarction.

CONCLUSIONS

The surprising transcriptional identity of cardiac fibroblasts, the adoption of cardiogenic gene programs, and direct contribution to cardiac development and repair provoke alternative interpretations for studies on more specialized cardiac progenitors, offering a novel perspective for reinterpreting cardiac regenerative therapies.

Exome sequencing identifies a novel variant in ACTC1 associated with familial atrial septal defect

BACKGROUND

The genetics of congenital heart disease (CHD) remain incompletely understood. Exome sequencing has been successfully used to identify disease-causing mutations in familial disorders in which candidate gene analyses and linkage mapping have failed.

METHODS

We studied a large family characterized by autosomal dominant isolated secundum atrial septal defect (ASD) (MIM No. 612794). Candidate gene resequencing and linkage analysis were uninformative.

RESULTS

Whole-exome sequencing of 2 affected family members identified 44 rare shared variants, including a nonsynonymous mutation (c.532A>T, p.M178L, NM_005159.4) in alpha-cardiac actin (ACTC1). This mutation was absent from 1834 internal controls as well as from the 1000 Genomes and the Exome Sequencing Project (ESP) databases, but predictions regarding its effect on protein function were divergent. However, p.M178L was the only rare mutation segregating with disease in our family.

CONCLUSIONS

Our results provide further evidence supporting a causative role for ACTC1 mutations in ASD. Massively parallel sequencing of the exome allows for the detection of novel rare variants causing CHD without the limitations of a candidate gene approach. When mutation prediction algorithms are not helpful, studies of familial disease can help distinguish rare pathologic mutations from benign variants. Consideration of the family history can lead to genetic insights into CHD.

Mutational analysis of the human MESP1 gene in patients with congenital heart disease reveals a highly variable sequence in exon 1

MESP1 represents an essential transcription factor to guarantee coordinated cardiac development. The expression of MESP1 is thought to be the first sign that a cell has been committed to the cardiac lineage. We analyzed the coding sequence of MESP1 in 215 patients with congenital heart disease. Our results show that the sequence of exon 1 is highly variable with up to seven alterations in individual samples. Five base pair positions (c.157_G>C A53P, rs6496598; c.174_A>C P58P, rs28377352; c.182_T>G L61R, rs28368490; c.669_C>G F223L, rs2305440; c.687_T>G P229P, rs2305441) are particularly variable. In almost half of the samples a 12 base pair insertion after position 55 (c.165_166insGTGCCGAGCCCC P55insVPSP, rs71934166) coding for VPSP was detected which was strongly correlated with the appearance of further amino acid changes (c.157_G>C A53P, c.182_T>G L61R, c.669_C>G F223L). Two missense mutations (c.33_G>C E11D, rs190259690; c.528_A>T T176S) were detected in two patients but were absent in the controls. The assessment of the biological activity of altered MESP1 proteins in a luciferase reporter assay showed an enhanced activity of the c.33_G>C E11D mutation and a reduction of the insertion without an accompanying change of c.182_T>G L61R. The modified biological properties of mutated MESP1 proteins might be associated with the appearance of certain pathological phenotypes of congenital heart disease.

Tbx20 acts upstream of Wnt signaling to regulate endocardial cushion formation and valve remodeling during mouse cardiogenesis

Cardiac valves are essential to direct forward blood flow through the cardiac chambers efficiently. Congenital valvular defects are prevalent among newborns and can cause an immediate threat to survival as well as long-term morbidity. Valve leaflet formation is a rigorously programmed process consisting of endocardial epithelial-mesenchymal transformation (EMT), mesenchymal cell proliferation, valve elongation and remodeling. Currently, little is known about the coordination of the diverse signals that regulate endocardial cushion development and valve elongation. Here, we report that the T-box transcription factor Tbx20 is expressed in the developing endocardial cushions and valves throughout heart development. Ablation of Tbx20 in endocardial cells causes severe valve elongation defects and impaired cardiac function in mice. Our study reveals that endocardial Tbx20 is crucial for valve endocardial cell proliferation and extracellular matrix development, but is not required for initiation of EMT. Elimination of Tbx20 also causes aberrant Wnt/β-catenin signaling in the endocardial cushions. In addition, Tbx20 regulates Lef1, a key transcriptional mediator for Wnt/β-catenin signaling, in this developmental process. Our study suggests a model in which Tbx20 regulates the Wnt pathway to direct endocardial cushion maturation and valve elongation, and provides new insights into the etiology of valve defects in humans.

Tbx20 promotes cardiomyocyte proliferation and persistence of fetal characteristics in adult mouse hearts

While differentiated cardiomyocytes proliferate prior to birth, adult cardiomyocytes in mammals exhibit relatively little proliferative activity. The T-box transcription factor Tbx20 is necessary and sufficient to promote prenatal cardiomyocyte proliferation, and Tbx20 also is required for adult cardiac homeostasis. The ability of Tbx20 to promote post-natal and adult cardiomyocyte proliferation was examined in mice with cardiomyocyte-specific Tbx20 gain-of-function beginning in the fetal period. In adult hearts, increased Tbx20 expression promotes cardiomyocyte proliferation and results in increased numbers of small, cycling, mononucleated cardiomyocytes, marked by persistent expression of fetal contractile protein genes. In adult cardiomyocytes in vivo and in neonatal rat cardiomyocytes in culture, Tbx20 promotes the activation of BMP2/pSmad1/5/8 and PI3K/AKT/GSK3β/β-catenin signaling pathways concomitant with increased cell proliferation. Inhibition of PI3K/AKT/GSK3β/β-catenin signaling reduces, but does not eliminate, Tbx20-mediated increases in cell proliferation, providing evidence for parallel regulatory pathways downstream of BMP/Smad1/5/8 signaling in promoting cardiomyocyte proliferation after birth. Thus, Tbx20 overexpression beginning in the fetal period activates multiple cardiac proliferative pathways after birth and maintains adult cardiomyocytes in an immature state in vivo.

Congenital heart disease: emerging themes linking genetics and development

Although congenital heart disease (CHD) is the most common survivable birth defect, the etiology of most CHD remains unclear. Several lines of evidence from humans and vertebrate models have supported a genetic component for CHD, yet the extreme locus heterogeneity and lack of a distinct genotype-phenotype correlation have limited causative gene discovery. However, recent advances in genomic technologies are permitting detailed evaluation of the genetic abnormalities in large cohorts of CHD patients. This has led to the identification of copy-number variation and de novo mutations together accounting for up to 15% of CHD. Further, new strategies coupling human genetics with model organisms have provided mechanistic insights into the molecular and developmental pathways underlying CHD pathogenesis, notably chromatin remodeling and ciliary signaling.

A genome-wide association study identifies two risk loci for congenital heart malformations in Han Chinese populations

Congenital heart malformation (CHM) is the most common form of congenital human birth anomaly and is the leading cause of infant mortality. Although some causative genes have been identified, little progress has been made in identifying genes in which low-penetrance susceptibility variants occur in the majority of sporadic CHM cases. To identify common genetic variants associated with sporadic non-syndromic CHM in Han Chinese populations, we performed a multistage genome-wide association study (GWAS) in a total of 4,225 CHM cases and 5,112 non-CHM controls. The GWAS stage included 945 cases and 1,246 controls and was followed by 2-stage validation with 2,160 cases and 3,866 controls. The combined analyses identified significant associations (P < 5.0 × 10⁻⁸) at 1p12 (rs2474937 near TBX15; odds ratio (OR) = 1.40; P = 8.44 × 10⁻¹⁰) and 4q31.1 (rs1531070 in MAML3; OR = 1.40; P = 4.99 × 10⁻¹²). These results extend current knowledge of genetic contributions to CHM in Han Chinese populations.

Genetics of congenital heart disease: the glass half empty

Congenital heart disease (CHD) is the most common congenital anomaly in newborn babies. Cardiac malformations have been produced in multiple experimental animal models, by perturbing selected molecules that function in the developmental pathways involved in myocyte specification, differentiation, or cardiac morphogenesis. In contrast, the precise genetic, epigenetic, or environmental basis for these perturbations in humans remains poorly understood. Over the past few decades, researchers have tried to bridge this knowledge gap through conventional genome-wide analyses of rare Mendelian CHD families, and by sequencing candidate genes in CHD cohorts. Although yielding few, usually highly penetrant, disease gene mutations, these discoveries provided 3 notable insights. First, human CHD mutations impact a heterogeneous set of molecules that orchestrate cardiac development. Second, CHD mutations often alter gene/protein dosage. Third, identical pathogenic CHD mutations cause a variety of distinct malformations, implying that higher order interactions account for particular CHD phenotypes. The advent of contemporary genomic technologies including single nucleotide polymorphism arrays, next-generation sequencing, and copy number variant platforms are accelerating the discovery of genetic causes of CHD. Importantly, these approaches enable study of sporadic cases, the most common presentation of CHD. Emerging results from ongoing genomic efforts have validated earlier observations learned from the monogenic CHD families. In this review, we explore how continued use of these technologies and integration of systems biology is expected to expand our understanding of the genetic architecture of CHD.

The molecular genetics of congenital heart disease: a review of recent developments

PURPOSE OF REVIEW

Our understanding of the interactions of genes and pathways during heart development continues to expand with our knowledge of the genetic basis of congenital heart disease. Along with the discovery of specific genes that cause lesions, recent research has focused on the interactions of some previously identified genes. This review focuses on the progress made during the last year.

RECENT FINDINGS

T-box, NK, and GATA transcription factors have known associations with a variety of syndromic and isolated congenital heart defects. Discovery of novel interactions of GATA and T-box transcription factors highlights the direction of recent research. In addition, the critical yet somewhat redundant roles of nkx2.5 and nkx2.7, along with the interaction of nkx2.7 with tbx20, have been elucidated. The contributions of still other transcription factor classes are being elucidated. Further understanding of 22q11.2 deletion and microduplication syndromes and their genetic interactions has also been studied. Recent work also highlights PTPN11 and NOTCH1 in Noonan syndrome.

SUMMARY

The recent developments in the genetics of congenital heart disease are reviewed. In many cases, it is the novel interactions of previously known genes that highlight this year's developments. These interactions will ultimately lead to better understanding of downstream transcriptional or signaling pathways.

T-box factors: insights into the evolutionary emergence of the complex heart

The heart as a functional organ first appeared in bilaterians as a single peristaltic pump and evolved through arthropods, fish, amphibians, and finally mammals into a four-chambered engine controlling blood-flow within the body. The acquisition of cardiac complexity in the evolving heart was a product of gene duplication events and the co-option of novel signaling pathways to an ancestral cardiac-specific gene network. T-box factors belong to an evolutionary conserved family of transcriptional regulators with diverse roles in development. Their regulatory functions are integral in the initiation and potentiation of heart development, and mutations in these genes are associated with congenital heart defects. In this review we will discuss the evolutionary conserved cardiac regulatory functions of this family as well as their implication in disease in an aim to facilitate future gene-targeted and regenerative therapeutic remedies.

Cooperative and antagonistic roles for Irx3 and Irx5 in cardiac morphogenesis and postnatal physiology

The Iroquois homeobox (Irx) homeodomain transcription factors are important for several aspects of embryonic development. In the developing heart, individual Irx genes are important for certain postnatal cardiac functions, including cardiac repolarization (Irx5) and rapid ventricular conduction (Irx3). Irx genes are expressed in dynamic and partially overlapping patterns in the developing heart. Here we show in mice that Irx3 and Irx5 have redundant function in the endocardium to regulate atrioventricular canal morphogenesis and outflow tract formation. Our data suggest that direct transcriptional repression of Bmp10 by Irx3 and Irx5 in the endocardium is required for ventricular septation. A postnatal deletion of Irx3 and Irx5 in the myocardium leads to prolongation of atrioventricular conduction, due in part to activation of expression of the Na(+) channel protein Nav1.5. Surprisingly, combined postnatal loss of Irx3 and Irx5 results in a restoration of the repolarization gradient that is altered in Irx5 mutant hearts, suggesting that postnatal Irx3 activity can be repressed by Irx5. Our results have uncovered complex genetic interactions between Irx3 and Irx5 in embryonic cardiac development and postnatal physiology.

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

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Dual transcriptional activator and repressor roles of TBX20 regulate adult cardiac structure and function

The ongoing requirement in adult heart for transcription factors with key roles in cardiac development is not well understood. We recently demonstrated that TBX20, a transcriptional regulator required for cardiac development, has key roles in the maintenance of functional and structural phenotypes in adult mouse heart. Conditional ablation of Tbx20 in adult cardiomyocytes leads to a rapid onset and progression of heart failure, with prominent conduction and contractility phenotypes that lead to death. Here we describe a more comprehensive molecular characterization of the functions of TBX20 in adult mouse heart. Coupling genome-wide chromatin immunoprecipitation and transcriptome analyses (RNA-Seq), we identified a subset of genes that change expression in Tbx20 adult cardiomyocyte-specific knockout hearts which are direct downstream targets of TBX20. This analysis revealed a dual role for TBX20 as both a transcriptional activator and a repressor, and that each of these functions regulates genes with very specialized and distinct molecular roles. We also show how TBX20 binds to its targets genome-wide in a context-dependent manner, using various cohorts of co-factors to either promote or repress distinct genetic programs within adult heart. Our integrative approach has uncovered several novel aspects of TBX20 and T-box protein function within adult heart. Sequencing data accession number (http://www.ncbi.nlm.nih.gov/geo): GSE30943.

Human gene copy number spectra analysis in congenital heart malformations

The clinical significance of copy number variants (CNVs) in congenital heart disease (CHD) continues to be a challenge. Although CNVs including genes can confer disease risk, relationships between gene dosage and phenotype are still being defined. Our goal was to perform a quantitative analysis of CNVs involving 100 well-defined CHD risk genes identified through previously published human association studies in subjects with anatomically defined cardiac malformations. A novel analytical approach permitting CNV gene frequency "spectra" to be computed over prespecified regions to determine phenotype-gene dosage relationships was employed. CNVs in subjects with CHD (n = 945), subphenotyped into 40 groups and verified in accordance with the European Paediatric Cardiac Code, were compared with two control groups, a disease-free cohort (n = 2,026) and a population with coronary artery disease (n = 880). Gains (≥200 kb) and losses (≥100 kb) were determined over 100 CHD risk genes and compared using a Barnard exact test. Six subphenotypes showed significant enrichment (P ≤ 0.05), including aortic stenosis (valvar), atrioventricular canal (partial), atrioventricular septal defect with tetralogy of Fallot, subaortic stenosis, tetralogy of Fallot, and truncus arteriosus. Furthermore, CNV gene frequency spectra were enriched (P ≤ 0.05) for losses at: FKBP6, ELN, GTF2IRD1, GATA4, CRKL, TBX1, ATRX, GPC3, BCOR, ZIC3, FLNA and MID1; and gains at: PRKAB2, FMO5, CHD1L, BCL9, ACP6, GJA5, HRAS, GATA6 and RUNX1. Of CHD subjects, 14% had causal chromosomal abnormalities, and 4.3% had likely causal (significantly enriched), large, rare CNVs. CNV frequency spectra combined with precision phenotyping may lead to increased molecular understanding of etiologic pathways.

Transcription factor pathways and congenital heart disease

Congenital heart disease is a major cause of morbidity and mortality throughout life. Mutations in numerous transcription factors have been identified in patients and families with some of the most common forms of cardiac malformations and arrhythmias. This review discusses transcription factor pathways known to be important for normal heart development and how abnormalities in these pathways have been linked to morphological and functional forms of congenital heart defects. A comprehensive, current list of known transcription factor mutations associated with congenital heart disease is provided, but the review focuses primarily on three key transcription factors, Nkx2-5, GATA4, and Tbx5, and their known biochemical and genetic partners. By understanding the interaction partners, transcriptional targets, and upstream activators of these core cardiac transcription factors, additional information about normal heart formation and further insight into genes and pathways affected in congenital heart disease should result.

Tbx20 regulation of cardiac cell proliferation and lineage specialization during embryonic and fetal development in vivo

TBX20 gain-of-function mutations in humans are associated with congenital heart malformations and myocardial defects. However the effects of increased Tbx20 function during cardiac chamber development and maturation have not been reported previously. CAG-CAT-Tbx20 transgenic mice were generated for Cre-dependent induction of Tbx20 in myocardial lineages in the developing heart. βMHCCre-mediated overexpression of Tbx20 in fetal ventricular cardiomyocytes results in increased thickness of compact myocardium, induction of cardiomyocyte proliferation, and increased expression of Bmp10 and pSmad1/5/8 at embryonic day (E) 14.5. βMHCCre-mediated Tbx20 overexpression also leads to increased expression of cardiac conduction system (CCS) genes Tbx5, Cx40, and Cx43 throughout the ventricular myocardium. In contrast, Nkx2.5Cre mediated overexpression of Tbx20 in the embryonic heart results in reduced cardiomyocyte proliferation, increased expression of a cell cycle inhibitor, p21(CIP1), and decreased expression of Tbx2, Tbx5, and N-myc1 at E9.5, concomitant with decreased phospho-ERK1/2 expression. Together, these analyses demonstrate that Tbx20 differentially regulates cell proliferation and cardiac lineage specification in embryonic versus fetal cardiomyocytes. Induction of pSmad1/5/8 at E14.5 and inhibition of dpERK expression at E9.5 are consistent with selective Tbx20 regulation of these pathways in association with stage-specific effects on cardiomyocyte proliferation. Together, these in vivo data support distinct functions for Tbx20 in regulation of cardiomyocyte lineage maturation and cell proliferation at embryonic and fetal stages of heart development.

Tbx20 regulates a genetic program essential to adult mouse cardiomyocyte function

Human mutations in or variants of TBX20 are associated with congenital heart disease, cardiomyopathy, and arrhythmias. To investigate whether cardiac disease in patients with these conditions results from an embryonic or ongoing requirement for Tbx20 in myocardium, we ablated Tbx20 specifically in adult cardiomyocytes in mice. This ablation resulted in the onset of severe cardiomyopathy accompanied by arrhythmias, with death ensuing within 1 to 2 weeks of Tbx20 ablation. Accounting for this dramatic phenotype, we identified molecular signatures that posit Tbx20 as a central integrator of a genetic program that maintains cardiomyocyte function in the adult heart. Expression of a number of genes encoding critical transcription factors, ion channels, and cytoskeletal/myofibrillar proteins was downregulated consequent to loss of Tbx20. Genome-wide ChIP analysis of Tbx20-binding regions in the adult heart revealed that many of these genes were direct downstream targets of Tbx20 and uncovered a previously undescribed DNA-binding site for Tbx20. Bioinformatics and in vivo functional analyses revealed a cohort of transcription factors that, working with Tbx20, integrated multiple environmental signals to maintain ion channel gene expression in the adult heart. Our data provide insight into the mechanisms by which mutations in TBX20 cause adult heart disease in humans.

Two novel HAND1 mutations in Chinese patients with ventricular septal defect

BACKGROUND

The HAND1 gene encodes a basic helix-loop-helix (bHLH) transcription factor which plays an essential role in the development of heart. Mutations in HAND1 have been identified in congenital heart disease (CHD) patients with hypoplastic hearts and septal defects. The spectrum of CHD relating to HAND1 mutations needs further study.

METHODS AND RESULTS

We screened HAND1 coding regions for mutations in 498 Chinese patients with CHD and 250 control subjects. We identified two novel non-synonymous mutations, c.217G>A (p.Gly73Ser) and c.456G>T (p.Lys152Asn), in the patients with ventricular septal defect (VSD). The two mutations were located in HAND1 evolutionarily conserved residues and enhanced the capability of HAND1 to form homodimers.

CONCLUSIONS

This is the first report of mutations in the HAND1 gene in Chinese patients with VSD and provides new insight into the etiology of VSD.

Loss of Gata5 in mice leads to bicuspid aortic valve

Bicuspid aortic valve (BAV), the leading congenital heart disease, occurs in 1%-2% of the population. Genetic studies suggest that BAV is an autosomal-dominant disease with reduced penetrance. However, only 1 gene, NOTCH1, has been linked to cases of BAV. Here, we show that targeted deletion of Gata5 in mice leads to hypoplastic hearts and partially penetrant BAV formation. Endocardial cell-specific inactivation of Gata5 led to BAV, similar to that observed in Gata5-/- mice. In all cases, the observed BAVs resulted from fusion of the right-coronary and noncoronary leaflets, the subtype associated with the more severe valve dysfunction in humans. Neither endocardial cell proliferation nor cushion formation was altered in the absence of Gata5. Rather, defective endocardial cell differentiation, resulting from the deregulation of several components of the Notch pathway and other important endocardial cell regulators, may be the underlying mechanism of disease. The results unravel a critical cell-autonomous role for endocardial Gata5 in aortic valve formation and identify GATA5 as a potential gene responsible for congenital heart disease in humans. Mice with mutated Gata5 alleles represent unique models to dissect the mechanisms underlying degenerative aortic valve disease and to develop much-needed preventive and therapeutic interventions.

Alternative splicing of T-box transcription factor genes

T-box (TBX) transcription factors are an ancient gene family with critical roles in embryogenesis. Currently, TBX3, TBX5, and TBX20 are TBX genes defined to have multiple protein isoforms created by alternative splicing and characterized by expression and functional studies. These proteins are important for development as mutations lead to severe developmental disorders in humans and mice. Cumulative studies suggest that alternative splicing of these genes can regulate TBX activities during multiple biological processes including cardiogenesis, limb development, and cancer mechanisms. This mini-review focuses on how alternative splicing adds complexity to transcriptional regulation of target genes controlled by TBX transcription factors.

Characterization of the novel interaction between muskelin and TBX20, a critical cardiogenic transcription factor

The genetic regulation necessary for the formation of a four-chambered heart is tightly regulated by transcription factors such as TBX20, a member of the T-box (TBX) transcription factor family. TBX20 is critical for proper cardiogenesis and is expressed in the heart throughout development. Missense mutations in TBX20 have been found in patients with congenital heart defects (CHD). Characterization of modifiers of TBX20 activity will help elucidate the genetic mechanisms of heart development and CHD. A yeast two-hybrid assay screening an embryonic mouse heart cDNA library with TBX20b as bait was used to identify potential modifiers of TBX20 activity and identified an interaction with muskelin (MKLN1), a primarily cytoplasmic protein with potential roles in signal transduction machinery scaffolding and nucleocytoplasmic protein shuttling. In cellular studies, MKLN1 directly binds to the T-box DNA-binding domain of only the TBX20b isoform by its kelch repeats domain. Immunostaining of mammalian cells transfected with tagged TBX20b and MKLN1 revealed colocalization primarily in the cytoplasm. Immunohistochemistry analysis of embryonic mouse hearts reveals coexpression in the developing endocardial valvular and myocardial interventricular cells. This novel interaction between TBX20b and MKLN1 may help elucidate new regulatory mechanisms within heart development.

What chick and mouse models have taught us about the role of the endocardium in congenital heart disease

Specific cell and tissue interactions drive the formation and function of the vertebrate cardiovascular system. Although much attention has been focused on the muscular components of the developing heart, the endocardium plays a key role in the formation of a functioning heart. Endocardial cells exhibit heterogeneity that allows them to participate in events such as the formation of the valves, septation of the outflow tract, and trabeculation. Here we review, the contributions of the endocardium to cardiovascular development and outline useful approaches developed in the chick and mouse that have revealed endocardial cell heterogeneity, the signaling molecules that direct endocardial cell behavior, and how these insights have contributed to our understanding of cardiovascular development and disease.

Xenopus: An emerging model for studying congenital heart disease

Congenital heart defects affect nearly 1% of all newborns and are a significant cause of infant death. Clinical studies have identified a number of congenital heart syndromes associated with mutations in genes that are involved in the complex process of cardiogenesis. The African clawed frog, Xenopus, has been instrumental in studies of vertebrate heart development and provides a valuable tool to investigate the molecular mechanisms underlying human congenital heart diseases. In this review, we discuss the methodologies that make Xenopus an ideal model system to investigate heart development and disease. We also outline congenital heart conditions linked to cardiac genes that have been well studied in Xenopus and describe some emerging technologies that will further aid in the study of these complex syndromes.

Common variants in 22 loci are associated with QRS duration and cardiac ventricular conduction

QRS interval on the electrocardiogram reflects ventricular depolarization and conduction time, and is a risk factor for mortality, sudden death, and heart failure. We performed a genome-wide association meta-analysis in 40,407 European-descent individuals from 14 studies, with further genotyping in 7170 additional Europeans, and identified 22 loci associated with QRS duration (P < 5 × 10⁻⁸). These loci map in or near genes in pathways with established roles in ventricular conduction such as sodium channels, transcription factors, and calcium-handling proteins, but also point to novel biologic processes, such as kinase inhibitors and genes related to tumorigenesis. We demonstrate that SCN10A, a gene at our most significant locus, is expressed in the mouse ventricular conduction system, and treatment with a selective SCN10A blocker prolongs QRS duration. These findings extend our current knowledge of ventricular depolarization and conduction.

Identification of GATA6 sequence variants in patients with congenital heart defects

Although the etiology for the majority of congenital heart disease (CHD) remains poorly understood, the known genetic causes are often the result of mutations in cardiac developmental genes. GATA6 encodes for a cardiac transcription factor, which is broadly expressed in the developing heart and is critical for normal cardiac morphogenesis, making it a candidate gene for congenital heart defects in humans. The objective of this study was to determine the frequency of GATA6 sequence variants in a population of individuals with a spectrum of cardiac malformations. The coding regions of GATA6 were sequenced in 310 individuals with CHD. We identified two novel sequence variations in GATA6 that altered highly conserved amino acid residues (A178V and L198V) and were not found in a control population. These variants were identified in two individuals (one with tetralogy of Fallot and the other with an atrioventricular septal defect in the setting of complex CHD). Biochemical studies demonstrate that the GATA6 A178V mutant protein results in increased transactivation ability when compared with wild-type GATA6. These data suggest that nonsynonymous GATA6 sequence variants are infrequently found in individuals with CHD.

Molecular genetics of congenital atrial septal defects

Congenital heart defects (CHD) are the most common developmental errors in humans, affecting 8 out of 1,000 newborns. Clinical diagnosis and treatment of CHD has dramatically improved in the last decades. Hence, the majority of CHD patients are now reaching reproductive age. While the risk of familial recurrence has been evaluated in various population studies, little is known about the genetic pathogenesis of CHD. In recent years significant progress has been made in uncovering genetic processes during cardiac development. Data from human genetic studies in CHD patients indicate that the genetic aetiology was presumably underestimated in the past. Inherited mutations in genes encoding cardiac transcription factors and sarcomeric proteins were found as an underlying cause for familial recurrence of non-syndromic CHD in humans, in particular cardiac septal defects. Notably, the cardiac phenotypes most frequently seen in mutation carriers are ostium secundum atrial septal defects (ASDII). This review outlines experimental approaches employed for the detection of CHD-related genes in humans and summarizes recent findings in molecular genetics of congenital cardiac septal defects with an emphasis on ASDII.