Functional analyses of three Csx/Nkx-2.5 mutations that cause human congenital heart disease

Embryonic cardio-respiratory impairments in rainbow trout (Oncorhynchus mykiss) following exposure to hydraulic fracturing flowback and produced water

During hydraulic fracturing, wastewaters - termed flowback and produced water (FPW) - are created as a by-product during hydrocarbon extraction. Given the large volumes of FPW that a single well can produce, and the history of FPW release to surface water bodies, it is imperative to understand the hazards that hydraulic fracturing and FPW pose to aquatic biota. Using rainbow trout embryos as model organisms, we investigated impacts to cardio-respiratory system development and function following acute (48 h) and sub-chronic (28-day) FPW exposure by examining occurrences of developmental deformities, rates of embryonic respiration (MO₂), and changes in expression of critical cardiac-specific genes. FPW-exposed embryos had significantly increased rates of pericardial edema, yolk-sac edema, and tail/trunk curvatures at hatch. Furthermore, when exposed at three days post-fertilization (dpf), acute 5% FPW exposures significantly increased embryonic MO₂ through development until 15 dpf, where a switch to significantly reduced MO₂ rates was subsequently recorded. A similar trend was observed during sub-chronic 1% FPW exposures. Interestingly, at certain specific developmental timepoints, previous salinity exposure seemed to affect embryonic MO₂; a result not previously observed. Following acute FPW exposures, embryonic genes for cardiac development and function were significantly altered, although at termination of sub-chronic exposures, significant changes to these same genes were not found. Together, our evidence of induced developmental deformities, modified embryonic MO₂, and altered cardiac transcript expression suggest that cardio-respiratory tissues are toxicologically targeted following FPW exposure in developing rainbow trout. These results may be helpful to regulatory bodies when developing hazard identification and risk management protocols concerning hydraulic fracturing activities.

A Novel Splicing Mutation c.335-1 G > A in the Cardiac Transcription Factor NKX2-5 Leads to Familial Atrial Septal Defect Through miR-19 and PYK2

Mutations of NKX2-5 largely contribute to congenital heart diseases (CHDs), especially atrial septal defect (ASD). We identified a novel heterozygous splicing mutation c.335-1G > A in NKX2-5 gene in an ASD family via whole exome sequencing (WES) and linkage analysis. Utilizing the human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (hiPSC-CMs) as a disease model, we showed that haploinsufficiency of NKX2-5 contributed to aberrant orchestration of apoptosis and proliferation in ASD patient-derived hiPSC-CMs. RNA-seq profiling and dual-luciferase reporter assay revealed that NKX2-5 acts upstream of PYK2 via miR-19a and miR-19b (miR-19a/b) to regulate cardiomyocyte apoptosis. Meanwhile, miR-19a/b are also downstream mediators of NKX2-5 during cardiomyocyte proliferation. The novel splicing mutation c.335-1G > A in NKX2-5 and its potential pathogenic roles in ASD were demonstrated. Our work provides clues not only for deep understanding of NKX2-5 in cardia development, but also for better knowledge in the molecular mechanisms of CHDs.

Precision medicine for heart failure based on molecular mechanisms: The 2019 ISHR Research Achievement Award Lecture

Heart failure is a leading cause of death, and the number of patients with heart failure continues to increase worldwide. To realize precision medicine for heart failure, its underlying molecular mechanisms must be elucidated. In this review summarizing the "The Research Achievement Award Lecture" of the 2019 XXIII ISHR World Congress held in Beijing, China, we would like to introduce our approaches for investigating the molecular mechanisms of cardiac hypertrophy, development, and failure, as well as discuss future perspectives.

Calcineurin Aβ-Specific Anchoring Confers Isoform-Specific Compartmentation and Function in Pathological Cardiac Myocyte Hypertrophy

BACKGROUND

The Ca²⁺/calmodulin-dependent phosphatase calcineurin is a key regulator of cardiac myocyte hypertrophy in disease. An unexplained paradox is how the β isoform of the calcineurin catalytic A-subunit (CaNAβ) is required for induction of pathological myocyte hypertrophy, despite calcineurin Aα expression in the same cells. It is unclear how the pleiotropic second messenger Ca²⁺ drives excitation-contraction coupling while not stimulating hypertrophy by calcineurin in the normal heart. Elucidation of the mechanisms conferring this selectivity in calcineurin signaling should reveal new strategies for targeting the phosphatase in disease.

METHODS

Primary adult rat ventricular myocytes were studied for morphology and intracellular signaling. New Förster resonance energy transfer reporters were used to assay Ca²⁺ and calcineurin activity in living cells. Conditional gene deletion and adeno-associated virus-mediated gene delivery in the mouse were used to study calcineurin signaling after transverse aortic constriction in vivo.

RESULTS

CIP4 (Cdc42-interacting protein 4)/TRIP10 (thyroid hormone receptor interactor 10) was identified as a new polyproline domain-dependent scaffold for CaNAβ2 by yeast 2-hybrid screen. Cardiac myocyte-specific CIP4 gene deletion in mice attenuated pressure overload-induced pathological cardiac remodeling and heart failure. Blockade of CaNAβ polyproline-dependent anchoring using a competing peptide inhibited concentric hypertrophy in cultured myocytes; disruption of anchoring in vivo using an adeno-associated virus gene therapy vector inhibited cardiac hypertrophy and improved systolic function after pressure overload. Live cell Förster resonance energy transfer biosensor imaging of cultured myocytes revealed that Ca²⁺ levels and calcineurin activity associated with the CIP4 compartment were increased by neurohormonal stimulation, but minimally by pacing. Conversely, Ca²⁺ levels and calcineurin activity detected by nonlocalized Förster resonance energy transfer sensors were induced by pacing and minimally by neurohormonal stimulation, providing functional evidence for differential intracellular compartmentation of Ca²⁺ and calcineurin signal transduction.

CONCLUSIONS

These results support a structural model for Ca²⁺ and CaNAβ compartmentation in cells based on an isoform-specific mechanism for calcineurin protein-protein interaction and localization. This mechanism provides an explanation for the specific role of CaNAβ in hypertrophy and its selective activation under conditions of pathologic stress. Disruption of CaNAβ polyproline-dependent anchoring constitutes a rational strategy for therapeutic targeting of CaNAβ-specific signaling responsible for pathological cardiac remodeling in cardiovascular disease deserving of further preclinical investigation.

An update on genetic variants of the NKX2-5

NKX2-5 is a homeodomain transcription factor that plays a crucial role in heart development. It is the first gene where a single genetic variant (GV) was found to be associated with congenital heart diseases in humans. In this study, we carried out a comprehensive survey of NKX2-5 GVs to build a unified, curated, and updated compilation of all available GVs. We retrieved a total of 1,380 unique GVs. From these, 970 had information on their frequency in the general population and 143 have been linked to pathogenic phenotypes in humans. In vitro effect was ascertained for 38 GVs. The homeodomain had the biggest cluster of pathogenic variants in the protein: 49 GVs in 60 residues, 23 in its third α-helix, where 11 missense variants may affect protein-DNA interaction or the hydrophobic core. We also pinpointed the likely location of pathogenic GVs in four linear motifs. These analyses allowed us to assign a putative explanation for the effect of 90 GVs. This study pointed to reliable pathogenicity for GVs in helix 3 of the homeodomain and may broaden the scope of functional and structural studies that can be done to better understand the effect of GVs in NKX2-5 function.

Congenital heart diseases: genetics, non-inherited risk factors, and signaling pathways

Background

Congenital heart diseases (CHDs) are the most common congenital anomalies with an estimated prevalence of 8 in 1000 live births. CHDs occur as a result of abnormal embryogenesis of the heart. Congenital heart diseases are associated with significant mortality and morbidity. The damage of the heart is irreversible due to a lack of regeneration potential, and usually, the patients may require surgical intervention. Studying the developmental biology of the heart is essential not only in understanding the mechanisms and pathogenesis of congenital heart diseases but also in providing us with insight towards developing new preventive and treatment methods.

Main body

The etiology of congenital heart diseases is still elusive. Both genetic and environmental factors have been implicated to play a role in the pathogenesis of the diseases. Recently, cardiac transcription factors, cardiac-specific genes, and signaling pathways, which are responsible for early cardiac morphogenesis have been extensively studied in both human and animal experiments but leave much to be desired. The discovery of novel genetic methods such as next generation sequencing and chromosomal microarrays have led to further study the genes, non-coding RNAs and subtle chromosomal changes, elucidating their implications to the etiology of congenital heart diseases. Studies have also implicated non-hereditary risk factors such as rubella infection, teratogens, maternal age, diabetes mellitus, and abnormal hemodynamics in causing CHDs.

These etiological factors raise questions on multifactorial etiology of CHDs. It is therefore important to endeavor in research based on finding the causes of CHDs. Finding causative factors will enable us to plan intervention strategies and mitigate the consequences associated with CHDs. This review, therefore, puts forward the genetic and non-genetic causes of congenital heart diseases. Besides, it discusses crucial signaling pathways which are involved in early cardiac morphogenesis. Consequently, we aim to consolidate our knowledge on multifactorial causes of CHDs so as to pave a way for further research regarding CHDs.

Conclusion

The multifactorial etiology of congenital heart diseases gives us a challenge to explicitly establishing specific causative factors and therefore plan intervention strategies. More well-designed studies and the use of novel genetic technologies could be the way through the discovery of etiological factors implicated in the pathogenesis of congenital heart diseases.

Cardio-respirometry disruption in zebrafish (Danio rerio) embryos exposed to hydraulic fracturing flowback and produced water

Hydraulic fracturing to extract oil and natural gas reserves is an increasing practice in many international energy sectors. Hydraulic fracturing flowback and produced water (FPW) is a hyper saline wastewater returned to the surface from a fractured well containing chemical species present in the initial fracturing fluid, geogenic contaminants, and potentially newly synthesized chemicals formed in the fracturing well environment. However, information on FPW toxicological mechanisms of action remain largely unknown. Both cardiotoxic and respirometric responses were explored in zebrafish (Danio rerio) embryos after either an acute sediment-free (FPW-SF) or raw/sediment containing (FPW-S) fraction exposure of 24 and 48 h at 2.5% and 5% dilutions. A 48 h exposure to either FPW fraction in 24-72 h post fertilization zebrafish embryos significantly increased occurrences of pericardial edema, yolk-sac edema, and tail/spine curvature. In contrast, larval heart rates significantly decreased after FPW fraction exposures. FPW-S, but not FPW-SF, at 2.5% doses significantly reduced embryonic respiration/metabolic rates (MO₂), while for 5% FPW, both fractions reduced MO₂. Expression of select cardiac genes were also significantly altered in each FPW exposure group, implicating a cardiovascular system compromise as the potential cause for reduced embryonic MO₂. Collectively, these results support our hypothesis that organics are major contributors to cardiac and respiratory responses to FPW exposure in zebrafish embryos. Our study is the first to investigate cardiac and respiratory sub-lethal effects of FPW exposure, demonstrating that FPW effects extend beyond initial osmotic stressors and verifies the use of respirometry as a potential marker for FPW exposure.

Genetics of Congenital Heart Defects: The NKX2-5 Gene, a Key Player

Congenital heart defects (CHDs) represent the biggest fraction of morbid congenital anomalies worldwide. Owing to their complex inheritance patterns and multifactorial etiologies, these defects are difficult to identify before complete manifestation. Research over the past two decades has established firmly the role of genetics in the development of these congenital defects. While syndromic CHDs are more straightforward, non-syndromic CHDs are usually characterized by multiple mutations that affect intricate inter-connected developmental pathways. Knock-out and gene expression studies in mice and other genetic models have been performed to elucidate the roles of these implicated genes. Functional analysis has not been able to resolve the complete picture, as increasingly more downstream effects are continuously being assigned to CHD mutant factors. NKX2-5, a cardiac transcription factor, has received much attention for its role in cardiac dysmorphogenesis. Approximately 50 different mutations in this gene have been identified to date, and only a few have been functionally characterized. The mutant NKX2-5 factor can regulate a number of off-targets downstream to facilitate CHD development. This review summarizes the genetic etiology of congenital heart defects and emphasizes the need for NKX2-5 mutation screening.

Genome-wide analysis reveals conserved transcriptional responses downstream of resting potential change in Xenopus embryos, axolotl regeneration, and human mesenchymal cell differentiation

Endogenous bioelectric signaling via changes in cellular resting potential (V_mem) is a key regulator of patterning during regeneration and embryogenesis in numerous model systems. Depolarization of V_mem has been functionally implicated in dedifferentiation, tumorigenesis, anatomical re‐specification, and appendage regeneration. However, no unbiased analyses have been performed to understand genome‐wide transcriptional responses to V_mem change in vivo. Moreover, it is unknown which genes or gene networks represent conserved targets of bioelectrical signaling across different patterning contexts and species. Here, we use microarray analysis to comparatively analyze transcriptional responses to V_mem depolarization. We compare the response of the transcriptome during embryogenesis (Xenopus development), regeneration (axolotl regeneration), and stem cell differentiation (human mesenchymal stem cells in culture) to identify common networks across model species that are associated with depolarization. Both subnetwork enrichment and PANTHER analyses identified a number of key genetic modules as targets of V_mem change, and also revealed important (well‐conserved) commonalities in bioelectric signal transduction, despite highly diverse experimental contexts and species. Depolarization regulates specific transcriptional networks across all three germ layers (ectoderm, mesoderm, and endoderm) such as cell differentiation and apoptosis, and this information will be used for developing mechanistic models of bioelectric regulation of patterning. Moreover, our analysis reveals that V_mem change regulates transcripts related to important disease pathways such as cancer and neurodegeneration, which may represent novel targets for emerging electroceutical therapies.

NKX2-5 mutations in an inbred consanguineous population: genetic and phenotypic diversity

NKX2-5 mutations are associated with different forms of congenital heart disease. Despite the knowledge gained from molecular and animal studies, genotype-phenotype correlations in humans are limited by the lack of large cohorts and the incomplete assessment of family members. We hypothesized that studying the role of NKX2-5 in inbred populations with homogeneous genetic backgrounds and high consanguinity rates such as Lebanon could help closing this gap. We sequenced NKX2-5 in 188 index CHD cases (25 with ASD). Five variants (three segregated in families) were detected in eleven families including the previously documented p.R25C variant, which was found in seven patients from different families, and in one healthy individual. In 3/5 familial dominant ASD cases, we identified an NKX2-5 mutation. In addition to the heterogeneity of NKX2-5 mutations, a diversity of phenotypes occurred within the families with predominant ASD and AV block. We did in fact identify a large prevalence of Sudden Cardiac Death (SCD) in families with truncating mutations, and two patients with coronary sinus disease. NKX2-5 is thus responsible for dominant familial ASD even in consanguineous populations, and a wide genetic and phenotypic diversity is characteristic of NKX2-5 mutations in the Lebanese population.

Prevalence and spectrum of GATA4 mutations associated with sporadic dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is the most frequent type of primary myocardial disorder responsible for substantial morbidity and mortality. DCM is the third most common cause of heart failure and the most common reason for heart transplantation. A recent study has implicated GATA4 mutation in the pathogenesis of familial DCM. However, the prevalence and spectrum of GATA4 mutations associated with sporadic DCM remain unclear. In this study, the coding exons and exon-intron boundaries of the GATA4 gene, which encodes a cardiac transcription factor crucial for normal cardiogenesis, were sequenced in 220 unrelated patients with sporadic DCM. A total of 200 unrelated ethnically-matched healthy individuals used as controls were genotyped. The functional characteristics of the mutant GATA4 were assayed in contrast to its wild-type counterpart using a luciferase reporter assay system. As a result, 3 novel heterozygous GATA4 mutations, p.V39L, p.P226Q and p.T279S, were identified in 3 unrelated patients with sporadic DCM, with a mutational prevalence of approximately 1.36%. The missense mutations were absent in 400 control chromosomes and the altered amino acids were completely conserved evolutionarily across species. Functional analysis showed that the GATA4 mutants were consistently associated with significantly decreased transcriptional activity and markedly reduced the synergistic activation between GATA4 and NKX2-5. This study firstly links GATA4 mutations to increased susceptibility to sporadic DCM and provides novel insight into the molecular etiology underlying DCM, suggesting the potential implications for the early prophylaxis and allele-specific treatment of this common form of cardiomyopathy.

Mutational spectrum of the NKX2-5 gene in patients with lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia in humans and is responsible for substantial morbidity and mortality worldwide. Emerging evidence indicates that abnormal cardiovascular development is involved in the pathogenesis of AF. In this study, the coding exons and splice sites of the NKX2-5 gene, which encodes a homeodomain-containing transcription factor essential for cardiovascular genesis, were sequenced in 146 unrelated patients with lone AF as well as the available relatives of the mutation carriers. A total of 700 unrelated ethnically matched healthy individuals used as controls were genotyped. The disease-causing potential of the identified NKX2-5 variations was predicted by MutationTaster and PolyPhen-2. The functional characteristics of the mutant NKX2-5 proteins were analyzed using a dual-luciferase reporter assay system. As a result, two heterozygous NKX2-5 mutations, including a previously reported p.E21Q and a novel p.T180A mutation, were identified in two families with AF transmitted in an autosomal dominant pattern. The mutations co-segregated with AF in the families with complete penetrance. The detected substitutions, which altered the amino acids highly conserved evolutionarily across species, were absent in 700 control individuals and were both predicted to be causative. Functional analyses demonstrated that the NKX2-5 mutants were associated with significantly decreased transcriptional activity compared with their wild-type counterpart. The findings expand the spectrum of NKX2-5 mutations linked to AF and provide additional evidence that dysfunctional NKX2-5 may confer vulnerability to AF, suggesting the potential benefit for the early prophylaxis and personalized treatment of AF.

Regulation of fetal gene expression in heart failure

During the processes leading to adverse cardiac remodeling and heart failure, cardiomyocytes react to neurohumoral stimuli and biomechanical stress by activating pathways that induce pathological hypertrophy. The gene expression patterns and molecular changes observed during cardiac hypertrophic remodeling bare resemblance to those observed during fetal cardiac development. The re-activation of fetal genes in the adult failing heart is a complex biological process that involves transcriptional, posttranscriptional and epigenetic regulation of the cardiac genome. In this review, the mechanistic actions of transcription factors, microRNAs and chromatin remodeling processes in regulating fetal gene expression in heart failure are discussed.

Mutation spectrum of the GATA4 gene in patients with idiopathic atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia associated with substantially increased morbidity and mortality. Growing evidence strongly implicates hereditary determinants in the pathogenesis of AF. However, AF is genetically heterogeneous and the genetic defects responsible for AF in the majority of cases remain to be identified. In this study, all the coding exons and splice junctions of GATA4, a gene encoding a zinc-finger transcription factor critical for normal cardiac morphogenesis, were sequenced in a cohort of 150 unrelated patients with idiopathic AF. The available relatives of the mutation carriers and a total of 200 unrelated ethnically matched healthy individuals used as controls were genotyped for the presence of mutations identified in index patients. The functional effect of the mutant GATA4 was characterized using a luciferase reporter assay system. As a result, two novel heterozygous GATA4 mutations (p.Y38D and p.P103A) were identified in 2 unrelated families with AF, respectively. In each family the mutation co-segregated with AF and was absent in the 400 control chromosomes. Functional analysis showed that the mutations of GATA4 were associated with a significantly decreased transcriptional activity. The findings expand the mutation spectrum of GATA4 linked to AF, and further support the notion that compromised GATA4 confers genetic susceptibility to AF.

Epigenetic mechanisms in cardiac development and disease

During mammalian development, cardiac specification and ultimately lineage commitment to a specific cardiac cell type is accomplished by the action of specific transcription factors (TFs) and their meticulous control on an epigenetic level. In this review, we detail how cardiac-specific TFs function in concert with nucleosome remodeling and histone-modifying enzymes to regulate a diverse network of genes required for processes such as cell growth and proliferation, or epithelial to mesenchymal transition (EMT), for instance. We provide examples of how several cardiac TFs, such as Nkx2.5, WHSC1, Tbx5, and Tbx1, which are associated with developmental and congenital heart defects, are required for the recruitment of histone modifiers, such as Jarid2, p300, and Ash2l, and components of ATP-dependent remodeling enzymes like Brg1, Baf60c, and Baf180. Binding of these TFs to their respective sites at cardiac genes coincides with a distinct pattern of histone marks, indicating that the precise regulation of cardiac gene networks is orchestrated by interactions between TFs and epigenetic modifiers. Furthermore, we speculate that an epigenetic signature, comprised of TF occupancy, histone modifications, and overall chromatin organization, is an underlying mechanism that governs cardiac morphogenesis and disease.

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.

Transactivation in Drosophila of human enhancers by human transcription factors involved in congenital heart diseases

BACKGROUND

The human transcription factors (TFs) GATA4, NKX2.5 and TBX5 form part of the core network necessary to build a human heart and are involved in Congenital Heart Diseases (CHDs). The human natriuretic peptide precursor A (NPPA) and α-myosin heavy chain 6 (MYH6) genes are downstream effectors involved in cardiogenesis that have been demonstrated to be in vitro targets of such TFs.

RESULTS

To study the interactions between these human TFs and their target enhancers in vivo, we overexpressed them in the whole Drosophila cardiac tube using the UAS/GAL4 system. We observed that all three TFs up-regulate their natural target enhancers in Drosophila and cause developmental defects when overexpressed in eyes and wings.

CONCLUSIONS

A strong potential of the present model might be the development of combinatorial and mutational assays to study the interactions between human TFs and their natural target promoters, which are not easily undertaken in tissue culture cells because of the variability in transfection efficiency, especially when multiple constructs are used. Thus, this novel system could be used to determine in vivo the genetic nature of the human mutant forms of these TFs, setting up a powerful tool to unravel the molecular genetic mechanisms that lead to CHDs.

GATA4 loss-of-function mutations in familial atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major source of the substantially increased morbidity and mortality. Growing studies demonstrate that genetic defects play pivotal roles in a subgroup of AF. However, AF is a genetically heterogeneous disorder and the molecular basis of AF in a majority of cases remains unknown.

METHODS

The whole coding region of the GATA4 gene, which encodes a zinc-finger transcription factor essential for cardiogenesis, was analyzed in 130 unrelated probands with AF in contrast to 200 unrelated ethnically matched healthy individuals used as controls. The available family members of the probands harboring the identified mutations were genotyped. The functional effect of the mutant GATA4 was characterized using a luciferase reporter assay system.

RESULTS

Two novel heterozygous GATA4 mutations, p.S70T and p.S160T, were identified in 2 unrelated families with AF inherited as an autosomal dominant trait, respectively, which co-segregated with AF in each family with complete penetrance. Functional analysis showed that the mutations of GATA4 were associated with a significantly decreased transcriptional activity.

CONCLUSION

The findings provide new insight into the molecular mechanism involved in the pathogenesis of AF, suggesting the potential implications in the genetic diagnosis and gene-specific therapy of this common arrhythmia.

The representation of heart development in the gene ontology

An understanding of heart development is critical in any systems biology approach to cardiovascular disease. The interpretation of data generated from high-throughput technologies (such as microarray and proteomics) is also essential to this approach. However, characterizing the role of genes in the processes underlying heart development and cardiovascular disease involves the non-trivial task of data analysis and integration of previous knowledge. The Gene Ontology (GO) Consortium provides structured controlled biological vocabularies that are used to summarize previous functional knowledge for gene products across all species. One aspect of GO describes biological processes, such as development and signaling. In order to support high-throughput cardiovascular research, we have initiated an effort to fully describe heart development in GO; expanding the number of GO terms describing heart development from 12 to over 280. This new ontology describes heart morphogenesis, the differentiation of specific cardiac cell types, and the involvement of signaling pathways in heart development. This work also aligns GO with the current views of the heart development research community and its representation in the literature. This extension of GO allows gene product annotators to comprehensively capture the genetic program leading to the developmental progression of the heart. This will enable users to integrate heart development data across species, resulting in the comprehensive retrieval of information about this subject. The revised GO structure, combined with gene product annotations, should improve the interpretation of data from high-throughput methods in a variety of cardiovascular research areas, including heart development, congenital cardiac disease, and cardiac stem cell research. Additionally, we invite the heart development community to contribute to the expansion of this important dataset for the benefit of future research in this area.

Novel NKX2-5 mutations in patients with familial atrial septal defects

Atrial septal defect (ASD) is a common cardiovascular malformation and an important contributor to substantial morbidity and mortality. Increasing evidence demonstrates that mutated NKX2-5, a gene encoding a homeobox transcription factor crucial to cardiogenesis, is a significant genetic determinant for congenital ASD. Nevertheless, the genetic basis for ASD in a majority of ASD patients remains largely unknown. In the current study, the entire coding region of NKX2-5 was sequenced initially for 58 unrelated probands with familial ASD. The relatives of the probands harboring identified mutations and 200 unrelated control individuals were subsequently genotyped. Three novel heterozygous NKX2-5 mutations (p.P43GfsX59, p.C46 W, and p.S179F) were identified respectively in three families with autosomal dominantly inherited ASD. These mutations, absent in 200 control individuals, cosegregated with ASD in the families that had complete penetrance. The findings expand the spectrum of mutations in NKX2-5 linked to ASD and contribute to genetic counseling, clinical interventions, and prenatal prevention of ASD for individuals with genetic susceptibility.

NKX2-5: an update on this hypermutable homeodomain protein and its role in human congenital heart disease (CHD)

Congenital heart disease (CHD) is among the most prevalent and fatal of all birth defects. Deciphering its causes, however, is complicated, as many patients affected by CHD have no family history of the disease. There is also widespread heterogeneity of cardiac malformations within affected individuals. Nonetheless, there have been tremendous efforts toward a better understanding of the molecular and cellular events leading to CHD. Notably, certain cardiac-specific transcription factors have been implicated in mammalian heart development and disruption of their activity has been demonstrated in CHD. The homeodomain transcription factor NKX2-5 is an important member of this group. Indeed, more than 40 heterozygous NKX2-5 germline mutations have been observed in individuals with CHD, and these are spread along the coding region, with many shown to impact protein function. Thus, NKX2-5 appears to be hypermutable, yet the overall detection frequency in sporadic CHD is about 2% and NKX2-5 mutations are one-time detections with single-positives or private to families. Furthermore, there is lack of genotype-phenotype correlation, in which the same cardiac malformations have been exhibited in different NKX2-5 mutations or the same NKX2-5 mutation associated with diverse malformations. Here, we summarize published NKX2-5 germline mutations and explore different avenues in disease pathogenesis to support the notion of a multifactorial cause of CHD where possibly several genes and associated pathways are involved.

From molecular mechanisms of cardiac development to genetic substrate of congenital heart diseases

Congenital heart disease is one of the most important chapters in medicine because its incidence is increasing and nowadays it is close to 1.2%. Most congenital heart disorders are the result of defects during embryogenesis, which implies that they are due to alterations in genes involved in cardiac development. This review summarizes current knowledge regarding the molecular mechanisms involved in cardiac development in order to clarify the genetic basis of congenital heart disease.

Genetic screening of 104 patients with congenitally malformed hearts revealed a fresh mutation of GATA4 in those with atrial septal defects

We analysed the GATA binding protein 4 gene, or GATA4, along with the NK2 transcription factor related, locus 5 gene, or NKX2.5, to determine their genetic contribution to 104 sporadic patients in Indonesia with congenitally malformed hearts, 76 cases having atrial septal defect and 28 tetralogy of Fallot. We found only 1 novel mutation of GATA4 in those with atrial septal defects. Analysis of the genetic background of the parents of the patient showed for the first time that a new mutation of GATA4 can cause sporadic atrial septal defects. We failed to discover any other mutations of either the GATA4 or NKX2-5 genes, supporting the marked genetic heterogeneity of human congenital cardiac defects.

Carboxyl Terminus of NKX2.5 Impairs its Interaction with p300

The transcription factor Nkx2.5 plays critical roles in controlling cardiac-specific gene expression. Previous reports demonstrated that Nkx2.5 is only a modest transactivator due to the auto-inhibitory effect of its C-terminal domain. Deletion of the C-terminal domain, mimicking conformational change, evokes vigorous transactivation activity. Here, we show that a C-terminal defective mutant of Nkx2.5 improves the occupation of p300 at the ANF promoter compared with full-length Nkx2.5, leading to hyperacetylation of histone H4. We reveal that p300 is a cofactor of Nkx2.5, markedly potentiating Nkx2.5-dependent transactivation, whereas E1A antigen impairs Nkx2.5 activity. Furthermore, p300 can acetylate Nkx2.5 and display an acetyltransferase-independent mechanism to coactivate Nkx2.5. Physical interaction between the N-terminal activation domain of Nkx2.5 and the C/H3 domain of p300 are identified by GST pull-down assay. Point mutants of the N-terminal modify the transcriptional activity of Nkx2.5 and interaction with p300. Deletion of the C-terminal domain greatly facilitates p300 binding and improves the susceptibility of Nkx2.5 to histone deacetylase inhibitor. These results establish that p300 acts as an Nkx2.5 cofactor and facilitates increased Nkx2.5 activity by relieving the conformational impediment of its inhibitory C-terminal domain.

Cardiomyocyte apoptosis in the right auricle of patients with ostium secundum atrial septal defect diseases

Ostium secundum atrial septal defect (osASD) is one of the most commonly occurring cardiac malformations. Although some embryological pathways have been elucidated, the molecular etiologies of ASD are not fully understood. Previous microarray analysis in our laboratory identified differentially expressed genes between osASD and normal right auricular myocardium. Of the 1056 differentially expressed genes, 14 genes were related to apoptosis: eight pro-apoptotic genes were up-regulated and six anti-apoptotic genes were down-regulated in ASD patients. In the current study, we utilized semi-quantitative RT-PCR, electron microscopy, TUNEL and flow cytometry to further understand the role of apoptosis in the atrium of osASD patients. RT-PCR results confirmed differential expression data from previous microarray studies. Additionally, while apoptosis was detected in the right auricular myocardium of all osASD patients, it was absent in controls. These data suggested apoptosis may play an important role in the pathogenesis of osASD or possibly occurs as a consequence of volume overload and hemodynamic changes in right atrium of osASD patients.

Transcription factors and congenital heart defects

Although there have been important advances in diagnostic modalities and therapeutic strategies for congenital heart defects (CHD), these malformations still lead to significant morbidity and mortality in the human population. Over the past 10 years, characterization of the genetic causes of CHD has begun to elucidate some of the molecular causes of these defects. Linkage analysis and candidate-gene approaches have been used to identify gene mutations that are associated with both familial and sporadic cases of CHD. Complementation of the human studies with developmental studies in mouse models provides information for the roles of these genes in normal development as well as indications for disease pathogenesis. Biochemical analysis of these gene mutations has provided further insight into the molecular effects of these genetic mutations. Here we review genetic, developmental, and biochemical studies of six cardiac transcription factors that have been identified as genetic causes for CHD in humans.

hhLIM is involved in cardiomyogenesis of embryonic stem cells

Proteins of the LIM family play important roles in a variety of fundamental biological processes including cell lineage specification and organ development. Here we examined the function in cardiogenesis of a new member of the LIM family, hhLIM, by molecular analysis of early stages of cardiomyocyte differentiation in hhLIM-deficient P19 cell line and P19 cells stably overexpressing hhLIM. The results indicate that hhLIM is a potent transcriptional activator of several cardiac muscle-specific genes. Inhibition of hhLIM expression by antisense transcripts can interfere with expression of cardiac muscle genes and block development of beating cardiomyocytes in P19 embryonic stem cells. Overexpression of hhLIM in P19 cells can enhance expression of cardiac marker genes Nkx2.5 and GATA-4 and potentiate development of cardiomyocyte-like morphology. These findings suggest that, in addition to its role in activation of the cardiac genetic program, hhLIM may be the nuclear target of inductive factor for precardiac cells.

Delineation of a 2.2 Mb microdeletion at 5q35 associated with microcephaly and congenital heart disease

Fine mapping of chromosomal deletions and genotype-phenotype comparisons of clinically well-defined patients can be used to confirm or reveal loci and genes associated with human disorders. Eleven patients with cytogenetically visible deletions involving the terminal region of chromosome 5q have been described, but the extent of the deletion was determined only in one case. In this study we describe a 15-year-old boy with Ebstein anomaly, atrial septal defect (ASD), atrioventricular (AV) conduction defect, and microcephaly. He had an apparently balanced paracentric inversion of chromosome 5, with the karyotype 46, XY,inv(5)(q13q35) de novo. Further mapping of the chromosome breakpoints using fluorescence in situ hybridization (FISH) revealed a 2.2 Mb microdeletion at the 5q35 breakpoint, which spans 16 genes, including the cardiac homeobox transcription factor gene NKX2-5. The current data suggest that haploinsufficiency of NKX2-5 cause Ebstein anomaly and support previous results showing that NKX2-5 mutations cause ASD and AV conduction defect. Furthermore, we suggest presence of a new microcephaly locus within a 2.2 Mb region at 5q35.1-q35.2.

Pendrin is a novel in vivo downstream target gene of the TTF-1/Nkx-2.1 homeodomain transcription factor in differentiated thyroid cells

Thyroid transcription factor gene 1 (TTF-1) is a homeobox-containing gene involved in thyroid organogenesis. During early thyroid development, the homeobox gene Nkx-2.5 is expressed in thyroid precursor cells coincident with the appearance of TTF-1. The aim of this study was to investigate the molecular mechanisms underlying thyroid-specific gene expression. We show that the Nkx-2.5 C terminus interacts with the TTF-1 homeodomain and, moreover, that the expression of a dominant-negative Nkx-2.5 isoform (N188K) in thyroid cells reduces TTF-1-driven transcription by titrating TTF-1 away from its target DNA. This process reduced the expression of several thyroid-specific genes, including pendrin and thyroglobulin. Similarly, down-regulation of TTF-1 by RNA interference reduced the expression of both genes, whose promoters are sensitive to and directly associate with TTF-1 in the chromatin context. In conclusion, we demonstrate that pendrin and thyroglobulin are downstream targets in vivo of TTF-1, whose action is a prime factor in controlling thyroid differentiation in vivo.

Cardiac transcription factor Csx/Nkx2-5: Its role in cardiac development and diseases

During the past decade, an emerging body of evidence has accumulated that cardiac transcription factors control a cardiac gene program and play a critical role in transcriptional regulation during cardiogenesis and during the adaptive process in adult hearts. Especially, an evolutionally conserved homeobox transcription factor Csx/Nkx2-5 has been in the forefront in the field of cardiac biology, providing molecular insights into the mechanisms of cardiac development and diseases. Csx/Nkx2-5 is indispensable for normal cardiac development, and mutations of the gene are associated with human congenital heart diseases (CHD). In the present review, the regulation of a cardiac gene program by Csx/Nkx2-5 is summarized, with an emphasis on its role in the cardiac development and diseases.

Phosphatidylinositol 3-kinase-Akt pathway plays a critical role in early cardiomyogenesis by regulating canonical Wnt signaling

We have recently reported that activation of phosphatidylinositol 3-kinase (PI3K) plays a critical role in the early stage of cardiomyocyte differentiation of P19CL6 cells. We here examined molecular mechanisms of how PI3K is involved in cardiomyocyte differentiation. DNA chip analysis revealed that expression levels of Wnt-3a were markedly increased and that the Wnt/beta-catenin pathway was activated temporally during the early stage of cardiomyocyte differentiation of P19CL6 cells. Activation of the Wnt/beta-catenin pathway during this period was required and sufficient for cardiomyocyte differentiation of P19CL6 cells. Inhibition of the PI3K/Akt pathway suppressed the Wnt/beta-catenin pathway by activation of glycogen synthase kinase-3beta (GSK-3beta) and degradation of beta-catenin. Suppression of cardiomyocyte differentiation by inhibiting the PI3K/Akt pathway was rescued by forced expression of a nonphosphorylated, constitutively active form of beta-catenin. These results suggest that the PI3K pathway regulates cardiomyocyte differentiation through suppressing the GSK-3beta activity and maintaining the Wnt/beta-catenin activity.

Gene expression in pediatric heart disease with emphasis on conotruncal defects

Developmental abnormalities of the heart are the underlying cause of many congenital heart malformations. The embryological development of the integrated cardiovascular tissue is the result of multiple tissue and cell-to-cell interactions involving temporal and spatial events under genetic control. Recent technological advances, like microarray analysis of gene expression, are providing new tools to aid in deciphering the complex networks of gene expression that regulate cardiac development. Here, we review our current understanding of the genetics of congenital heart disorders with emphasis on gene expression studies and report preliminary data from infants with conotruncal defects. We report our microarray analysis showing over- and underexpression of individual genes and gene network interactions from dysplastic pulmonic tissue from two infants with tetralogy of Fallot compared with normal pulmonic tissue from an unaffected control infant.

Functional dissection of sequence-specific NKX2-5 DNA binding domain mutations associated with human heart septation defects using a yeast-based system

Human heart development requires an orderly coordination of transcriptional programs, with the homeodomain protein NKX2-5 being one of the key transcription factors required for the differentiation of mesodermal progenitor cells. Indeed, lack of Nkx2-5 in mice arrests heart development prior to looping, resulting in embryonic lethality. There are 28 germline NKX2-5 mutations identified in humans that are associated with congenital heart disease, and we recently reported multiple somatic mutations in patients with complex cardiac malformations. To address the functional consequences of single and multiple mutations of NKX2-5, we developed a functional assay in the budding yeast Saccharomyces cerevisiae, which could determine transactivation capacity and specificity of expressed NKX2-5 alleles towards targeted response element (RE) sequences. We focused on mutants of the third helix, which provides DNA binding specificity, and characterized mutations that were highly associated with either ventricular (VSD) or atrioventricular (AVSD) septal defects. Individual mutants exhibited partial to complete loss of function and differences in transactivation capacity between the various REs. The mutants also exhibited gene dosage rather than dominant effects on transcription. Surprisingly, all AVSD patients (22/23) had a single K183E mutation in the DNA binding domain, which resulted in transcriptional inactivation. None of the VSD patients had this mutation; yet 14/29 had at least one mutation in the third helix leading to either inactivation or reduction of NKX2-5 transactivation. Therefore, mutations of somatic origin in the binding domains of NKX2-5 were associated specifically with AVSD or VSD and resulted in loss of protein function.

Novel NKX2-5 mutations in diseased heart tissues of patients with cardiac malformations

NKX2-5 is a homeodomain-containing transcription factor important in cardiac development. Familial mutations in the NKX2-5 gene are associated with cardiac abnormalities, but mutations are rare in sporadic cases. We studied the pathology and molecular genetics of NKX2-5 in diseased heart tissues of 68 patients with complex congenital heart disease (CHD), particularly atrial (ASD), ventricular (VSD), and atrioventricular septal defects (AVSD). We also studied DNA extracted from 16 normal hearts, as well as lymphocytic DNA from 50 healthy volunteers, 7 families, and 4 unrelated individuals with CHD. Direct sequencing revealed 53 NKX2-5 mutations in the diseased heart tissues, including nonsynonymous substitutions in the homeodomain of NKX2-5. We found common mutations among unrelated patients, but certain mutations were specific to VSDs and AVSDs. Many patients had multiple NKX2-5 mutations, up to 14 nonsynonymous mutations per patient in VSDs. Importantly, these nonsynonymous mutations were mainly absent in normal heart tissues of the same CHD patients, thus indicating somatic origin and mosaicism of mutations. Further, observed mutations were completely absent in normal hearts and lymphocytic DNA of healthy individuals. Our findings provide new insights for somatic NKX2-5 mutations to be of importance in congenital heart disease.

Heat Shock Transcription Factor 1 Protects Cardiomyocytes From Ischemia/Reperfusion Injury

Background— Because cardiomyocyte death causes heart failure, it is important to find the molecules that protect cardiomyocytes from death. The death trap is a useful method to identify cell-protective genes.

Methods and Results— In this study, we isolated the heat shock transcription factor 1 (HSF1) as a protective molecule by the death trap method. Cell death induced by hydrogen peroxide was prevented by overexpression of HSF1 in COS7 cells. Thermal preconditioning at 42°C for 60 minutes activated HSF1, which played a critical role in survival of cardiomyocytes from oxidative stress. In the heart of transgenic mice overexpressing a constitutively active form of HSF1, ischemia followed by reperfusion-induced ST-segment elevation in ECG was recovered faster, infarct size was smaller, and cardiomyocyte death was less than wild-type mice. Protein kinase B/Akt was more strongly activated, whereas Jun N-terminal kinase and caspase 3 were less activated in transgenic hearts than wild-type ones.

Conclusions— These results suggest that HSF1 protects cardiomyocytes from death at least in part through activation of Akt and inactivation of Jun N-terminal kinase and caspase 3.

Missense mutations in the homeodomain of HOXD13 are associated with brachydactyly types D and E

HOXD13, the most 5' gene of the HOXD cluster, encodes a homeodomain transcription factor with important functions in limb patterning and growth. Heterozygous mutations of human HOXD13, encoding polyalanine expansions or frameshifts, are believed to act by dominant negative or haploinsufficiency mechanisms and are predominantly associated with synpolydactyly phenotypes. Here, we describe two mutations of HOXD13 (923C-->G encoding Ser308Cys and 940A-->C encoding Ile314Leu) that cause missense substitutions within the homeodomain. Both are associated with distinctive limb phenotypes in which brachydactyly of specific metacarpals, metatarsals, and phalangeal bones is the most constant feature, exhibiting overlap with brachydactyly types D and E. We investigated the binding of synthetic mutant proteins to double-stranded DNA targets in vitro. No consistent differences were found for the Ser308Cys mutation compared with the wild type, but the Ile314Leu mutation (which resides at the 47th position of the homeodomain) exhibited increased affinity for a target containing the core recognition sequence 5'-TTAC-3' but decreased affinity for a 5'-TTAT-3' target. Molecular modeling of the Ile314Leu mutation indicates that this mixed gain and loss of affinity may be accounted for by the relative positions of methyl groups in the amino acid side chain and target base.

Dual effects of the homeobox transcription factor Csx/Nkx2-5 on cardiomyocytes

A homeobox-containing transcription factor Csx/Nkx2-5 is an important regulator of cardiac development. Many different human CSX/NKX2-5 mutations have been reported to cause congenital heart disease. We here examined the effects of three representative CSX/NKX2-5 mutations on cardiomyocyte differentiation and death with the use of the P19CL6 cardiomyogenic cell lines. Stable overexpression of wild-type CSX/NKX2-5 enhanced expression of cardiac-specific genes such as MEF2C and MLC2v, the promoter activity of the atrial natriuretic peptide gene, and the terminal differentiation of P19CL6 into cardiomyocytes, while all CSX/NKX2-5 mutants attenuated them by different degrees. When exposed to H(2)O(2) or cultured without change of the medium, many differentiated P19CL6 cells overexpressing the mutants, especially the mutant which lacks the carboxyl terminal region just after the homeodomain, were dead, while most of the cells overexpressing wild-type CSX/NKX2-5 survived. Overexpression of the carboxyl terminus-deleted mutant down-regulated expression of an anti-apoptotic protein Bcl-x(L) and up-regulated that of a pro-apoptotic protein CAS, while in the cells overexpressing wild-type CSX/NKX2-5, expression of a pro-apoptotic protein RIP was reduced. Furthermore, overexpression of wild-type CSX/NKX2-5 decreased the number of H(2)O(2)-induced TUNEL-positive cultured cardiomyocytes of neonatal rats, whereas overexpression of the mutants enhanced it. These results suggest that Csx/Nkx2-5 not only regulates expression of cardiac-specific genes but protects cardiomyocytes from stresses and that cell death may be another cause for the cardiac defects induced by human CSX/NKX2-5 mutations.

Csx/Nkx2-5 is required for homeostasis and survival of cardiac myocytes in the adult heart

Csx/Nkx2-5, which is essential for cardiac development of the embryo, is abundantly expressed in the adult heart. We here examined the role of Csx/Nkx2-5 in the adult heart using two kinds of transgenic mice. Transgenic mice that overexpress a dominant negative mutant of Csx/Nkx2-5 (DN-TG mice) showed degeneration of cardiac myocytes and impairment of cardiac function. Doxorubicin induced more marked cardiac dysfunction in DN-TG mice and less in transgenic mice that overexpress wild type Csx/Nkx2-5 (WT-TG mice) compared with non-transgenic mice. Doxorubicin induced cardiomyocyte apoptosis, and the number of apoptotic cardiomyocytes was high in the order of DN-TG mice, non-transgenic mice, and WT-TG mice. Overexpression of the dominant negative mutant of Csx/Nkx2-5 induced apoptosis in cultured cardiomyocytes, while expression of wild type Csx/Nkx2-5 protected cardiomyocytes from doxorubicin-induced apoptotic death. These results suggest that Csx/Nkx2-5 plays a critical role in maintaining highly differentiated cardiac phenotype and in protecting the heart from stresses including doxorubicin.

Novel point mutation in the cardiac transcription factor CSX/NKX2.5 associated with congenital heart disease

The homeobox transcription factor CSX/NKX2.5, which is a vertebrate homologue of the Drosophila gene tinman, is essential for cardiac development. It is expressed in the early cardiac mesoderm and in heart muscle lineage throughout life. Homozygous deletion of CSX/NKX2.5 causes early embryonic lethality in mice because cardiac development is arrested at the linear heart tube stage. Heterozygous mutation of human CSX/NKX2.5 has been associated with various congenital heart diseases such as atrial septal defect (ASD), ventricular septal defect, tetralogy of Fallot, and tricuspid valve abnormalities, including Ebstein's anomaly. Additionally, CSX/NKX2.5 mutation causes atrioventricular (AV) conduction block with or without associated congenital heart diseases. Ten different heterozygous mutations have been already reported and a new point mutation, which is a C-to-A transition (Cys264ter) at nucleotide 901 of CSX/NKX2.5, results in the production of a truncated protein occurring COOH-terminal to the homeodomain of CSX/NKX2.5. The mutation was found in a patient with familial ASD and first-degree AV block; 4 members from 3 generations had secundum-type ASD and first-degree AV block.

Molecular evolution of the homeodomain family of transcription factors

The homeodomain family of transcription factors plays a fundamental role in a diverse set of functions that include body plan specification, pattern formation and cell fate determination during metazoan development. Members of this family are characterized by a helix-turn-helix DNA-binding motif known as the homeodomain. Homeodomain proteins regulate various cellular processes by specifically binding to the transcriptional control region of a target gene. These proteins have been conserved across a diverse range of species, from yeast to human. A number of inherited human disorders are caused by mutations in homeodomain-containing proteins. In this study, we present an evolutionary classification of 129 human homeodomain proteins. Phylogenetic analysis of these proteins, whose sequences were aligned based on the three-dimensional structure of the homeodomain, was performed using a distance matrix approach. The homeodomain proteins segregate into six distinct classes, and this classification is consistent with the known functional and structural characteristics of these proteins. An ancestral sequence signature that accurately describes the unique sequence characteristics of each of these classes has been derived. The phylogenetic analysis, coupled with the chromosomal localization of these genes, provides powerful clues as to how each of these classes arose from the ancestral homeodomain.