Tbx1 regulates fibroblast growth factors in the anterior heart field through a reinforcing autoregulatory loop involving forkhead transcription factors

Phase separation provides a mechanism to drive phenotype switching

Phenotypic switching plays a crucial role in cell fate determination across various organisms. Recent experimental findings highlight the significance of protein compartmentalization via liquid-liquid phase separation in influencing such decisions. However, the precise mechanism through which phase separation regulates phenotypic switching remains elusive. To investigate this, we established a mathematical model that couples a phase separation process and a gene expression process with feedback. We used the chemical master equation theory and mean-field approximation to study the effects of phase separation on the gene expression products. We found that phase separation can cause bistability and bimodality. Furthermore, phase separation can control the bistable properties of the system, such as bifurcation points and bistable ranges. On the other hand, in stochastic dynamics, the droplet phase exhibits double peaks within a more extensive phase separation threshold range than the dilute phase, indicating the pivotal role of the droplet phase in cell fate decisions. These findings propose an alternative mechanism that influences cell fate decisions through the phase separation process. As phase separation is increasingly discovered in gene regulatory networks, related modeling research can help build biomolecular systems with desired properties and offer insights into explaining cell fate decisions.

A regulatory variant impacting TBX1 expression contributes to basicranial morphology in Homo sapiens

Changes in gene regulatory elements play critical roles in human phenotypic divergence. However, identifying the base-pair changes responsible for the distinctive morphology of Homo sapiens remains challenging. Here, we report a noncoding single-nucleotide polymorphism (SNP), rs41298798, as a potential causal variant contributing to the morphology of the skull base and vertebral structures found in Homo sapiens. Screening for differentially regulated genes between Homo sapiens and extinct relatives revealed 13 candidate genes associated with basicranial development, with TBX1, implicated in DiGeorge syndrome, playing a pivotal role. Epigenetic markers and in silico analyses prioritized rs41298798 within a TBX1 intron for functional validation. CRISPR editing revealed that the 41-base-pair region surrounding rs41298798 modulates gene expression at 22q11.21. The derived allele of rs41298798 acts as an allele-specific enhancer mediated by E2F1, resulting in increased TBX1 expression levels compared to the ancestral allele. Tbx1-knockout mice exhibited skull base and vertebral abnormalities similar to those seen in DiGeorge syndrome. Phenotypic differences associated with TBX1 deficiency are observed between Homo sapiens and Neanderthals (Homo neanderthalensis). In conclusion, the regulatory divergence of TBX1 contributes to the formation of skull base and vertebral structures found in Homo sapiens.

Molecular Pathways and Animal Models of Semilunar Valve and Aortic Arch Anomalies

The great arteries of the vertebrate carry blood from the heart to the systemic circulation and are derived from the pharyngeal arch arteries. In higher vertebrates, the pharyngeal arch arteries are a symmetrical series of blood vessels that rapidly remodel during development to become the asymmetric aortic arch arteries carrying oxygenated blood from the left ventricle via the outflow tract. At the base of the aorta, as well as the pulmonary trunk, are the semilunar valves. These valves each have three leaflets and prevent the backflow of blood into the heart. During development, the process of aortic arch and valve formation may go wrong, resulting in cardiovascular defects, and these may, at least in part, be caused by genetic mutations. In this chapter, we will review models harboring genetic mutations that result in cardiovascular defects affecting the great arteries and the semilunar valves.

The miR-17-92 cluster in cardiac health and disease

MicroRNAs (miRs) are small noncoding RNAs that play important roles in both physiological and pathological processes through post-transcriptional regulation. The miR-17-92 cluster includes six individual members: miR-17, miR-18a, miR-19a, miR-19b-1, miR-20a, and miR-92a-1. The miR-17-92 cluster has been extensively studied and reported to broadly function in cancer biology, immunology, neurology, pulmonology, and cardiology. This review focuses on its roles in heart development and cardiac diseases. We briefly introduce the nature of the miR-17-92 cluster and its crucial roles in both normal development and the pathogenesis of various diseases. We summarize the recent progress in understanding the versatile roles of miR-17-92 during cardiac development, regeneration, and aging. Additionally, we highlight the indispensable roles of the miR-17-92 cluster in pathogenesis and therapeutic potential in cardiac birth defects and adult cardiac diseases.

Single-cell transcriptomics uncovers a non-autonomous Tbx1-dependent genetic program controlling cardiac neural crest cell development

Disruption of cardiac neural crest cells (CNCCs) results in congenital heart disease, yet we do not understand the cell fate dynamics as these cells differentiate to vascular smooth muscle cells. Here we performed single-cell RNA-sequencing of NCCs from the pharyngeal apparatus with the heart in control mouse embryos and when Tbx1, the gene for 22q11.2 deletion syndrome, is inactivated. We uncover three dynamic transitions of pharyngeal NCCs expressing Tbx2 and Tbx3 through differentiated CNCCs expressing cardiac transcription factors with smooth muscle genes. These transitions are altered non-autonomously by loss of Tbx1. Further, inactivation of Tbx2 and Tbx3 in early CNCCs results in aortic arch branching defects due to failed smooth muscle differentiation. Loss of Tbx1 interrupts mesoderm to CNCC cell-cell communication with upregulation and premature activation of BMP signaling and reduced MAPK signaling, as well as alteration of other signaling, and failed dynamic transitions of CNCCs leading to disruption of aortic arch artery formation and cardiac outflow tract septation.

TBX1 targets the miR-200-ZEB2 axis to induce epithelial differentiation and inhibit stem cell properties

TBX1, which encodes a T-box transcription factor, is considered a candidate gene for DiGeorge syndrome, velocardiofacial syndrome, and conotruncal anomaly face syndrome. Transduction of TBX1 decreases cell proliferation in epithelial cancer cells and Tbx1 ablation induces epithelial proliferation during palatal development. Here, we report that TBX1 regulates stem cell properties and epithelial differentiation through the transcriptional activation of microRNAs. Stable expression of TBX1 induces microRNA-200 (miR-200), whose members repress the epithelial-to-mesenchymal transition and induce epithelial differentiation. TBX1 rescues ZEB2-dependent transcriptional inhibition of the miR-200b/200a/429 cluster, whose promoter region contains conserved overlapping cis-regulatory motifs of the ZEB-binding E-box and TBX-binding element. Consequently, TBX1 activates the expression of both miR-200 and stemness-inhibitor miR-203 to inhibit their common targets, BMI1 and ZEB2. Moreover, Tbx1 ablation affects the differentiation of the palatal epithelium and perturbs the expression of miR-200, miR-203, and their target genes. We propose that TBX1 links stem cell properties and epithelial differentiation by inducing miR-200 and miR-203. Thus, targeting of the ZEB2-miR-200 axis by TBX1 may have potential therapeutic implications in miR-200-associated tumors and cleft palate.

Craniofacial Phenotypes and Genetics of DiGeorge Syndrome

The 22q11.2 deletion is one of the most common genetic microdeletions, affecting approximately 1 in 4000 live births in humans. A 1.5 to 2.5 Mb hemizygous deletion of chromosome 22q11.2 causes DiGeorge syndrome (DGS) and velocardiofacial syndrome (VCFS). DGS/VCFS are associated with prevalent cardiac malformations, thymic and parathyroid hypoplasia, and craniofacial defects. Patients with DGS/VCFS manifest craniofacial anomalies involving the cranium, cranial base, jaws, pharyngeal muscles, ear-nose-throat, palate, teeth, and cervical spine. Most craniofacial phenotypes of DGS/VCFS are caused by proximal 1.5 Mb microdeletions, resulting in a hemizygosity of coding genes, microRNAs, and long noncoding RNAs. TBX1, located on chromosome 22q11.21, encodes a T-box transcription factor and is a candidate gene for DGS/VCFS. TBX1 regulates the fate of progenitor cells in the cranial and pharyngeal apparatus during embryogenesis. Tbx1-null mice exhibit the most clinical features of DGS/VCFS, including craniofacial phenotypes. Despite the frequency of DGS/VCFS, there has been a limited review of the craniofacial phenotypes of DGC/VCFS. This review focuses on these phenotypes and summarizes the current understanding of the genetic factors that impact DGS/VCFS-related phenotypes. We also review DGS/VCFS mouse models that have been designed to better understand the pathogenic processes of DGS/VCFS.

Developmental disorders caused by haploinsufficiency of transcriptional regulators: a perspective based on cell fate determination

Many human birth defects and neurodevelopmental disorders are caused by loss-of-function mutations in a single copy of transcription factor (TF) and chromatin regulator genes. Although this dosage sensitivity has long been known, how and why haploinsufficiency (HI) of transcriptional regulators leads to developmental disorders (DDs) is unclear. Here I propose the hypothesis that such DDs result from defects in cell fate determination that are based on disrupted bistability in the underlying gene regulatory network (GRN). Bistability, a crucial systems biology concept to model binary choices such as cell fate decisions, requires both positive feedback and ultrasensitivity, the latter often achieved through TF cooperativity. The hypothesis explains why dosage sensitivity of transcriptional regulators is an inherent property of fate decisions, and why disruption of either positive feedback or cooperativity in the underlying GRN is sufficient to cause disease. I present empirical and theoretical evidence in support of this hypothesis and discuss several issues for which it increases our understanding of disease, such as incomplete penetrance. The proposed framework provides a mechanistic, systems-level explanation of HI of transcriptional regulators, thus unifying existing theories, and offers new insights into outstanding issues of human disease. This article has an associated Future Leader to Watch interview with the author of the paper.

MicroRNA-451a attenuates angiotensin II-induced cardiac fibrosis and inflammation by directly targeting T-box1

Hypertension or angiotensin II (Ang II) induces cardiac inflammation and fibrosis, thus contributing to cardiac remodeling. MicroRNAs (miRNAs) are considered crucial regulators of cardiac homeostasis and remodeling in response to various types of stress. It has been reported that miR-451a is involved in regulating ischemic heart injury. However, its role in Ang II-induced cardiac fibrosis remains unknown. Cardiac remodeling was induced in mice by infusion of low-dose Ang II (490 ng/kg/min) with a minipump for 2 weeks. Echocardiography and histological examinations were performed to evaluate cardiac function and pathological changes. We observed that miR-451a expression was the most significantly downregulated in the hearts of Ang II-infused mice and in both primary cardiac myocytes and fibroblasts. Overexpression of miR-451a in mice significantly attenuated Ang II-induced cardiac fibrosis and inflammation. Conversely, knockdown of miR-451a in mice aggravated this effect. Bioinformatics analysis and a luciferase reporter assay revealed that TBX1 was a direct target of miR-451a. Mechanistically, miR-451a directly targeted TBX1 expression, which inhibited TGF-β1 production in both cardiac myocytes and fibroblasts, inactivating of TGF-β1/SMAD2/3 signaling, inhibiting myofibroblast differentiation and proinflammatory cytokine expression, and leading to attenuation of cardiac fibrosis and inflammation. In conclusion, these results indicate that miR-451a acts as a novel regulator of Ang II-induced cardiac fibrosis and inflammation by directly targeting TBX1, and may be a promising therapeutic target for treating hypertensive cardiac diseases.

Dysgenesis and Dysfunction of the Pancreas and Pituitary Due to FOXA2 Gene Defects

CONTEXT

Developmental disorders of the pituitary gland leading to congenital hypopituitarism can either be isolated or associated with extrapituitary abnormalities (syndromic hypopituitarism). A large number of syndromic hypopituitarism cases are linked to mutations in transcription factors. The forkhead box A2 (FOXA2) is a transcription factor that plays a key role in the central nervous system, foregut, and pancreatic development.

OBJECTIVE

This work aims to characterize 2 patients with syndromic hypopituitarism due to FOXA2 gene defects.

RESULTS

We report a novel heterozygous nonsense c.616C > T(p.Q206X) variant that leads to a truncated protein that lacks part of the DNA-binding domain of FOXA2, resulting in impaired transcriptional activation of the glucose transporter type 2 (GLUT2)-luciferase reporter. The patient is the sixth patient described in the literature with a FOXA2 mutation, and the first patient exhibiting pancreatic hypoplasia. We also report a second patient with a novel de novo 8.53 Mb deletion of 20p11.2 that encompasses FOXA2, who developed diabetes mellitus that responded to sulfonylurea treatment.

CONCLUSION

Our 2 cases broaden the molecular and clinical spectrum of FOXA2-related disease, reporting the first nonsense mutation and the first case of pancreatic dysgenesis.

Variants in a cis-regulatory element of TBX1 in conotruncal heart defect patients impair GATA6-mediated transactivation

Background

TBX1 (T-box transcription factor 1) is a major candidate gene that likely contributes to the etiology of velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS). Although the haploinsufficiency of TBX1 in both mice and humans results in congenital cardiac malformations, little has been elucidated about its upstream regulation. We aimed to explore the transcriptional regulation and dysregulation of TBX1.

Methods

Different TBX1 promoter reporters were constructed. Luciferase assays and electrophoretic mobility shift assays (EMSAs) were used to identify a cis-regulatory element within the TBX1 promoter region and its trans-acting factor. The expression of proteins was identified by immunohistochemistry and immunofluorescence. Variants in the cis-regulatory element were screened in conotruncal defect (CTD) patients. In vitro functional assays were performed to show the effects of the variants found in CTD patients on the transactivation of TBX1.

Results

We identified a cis-regulatory element within intron 1 of TBX1 that was found to be responsive to GATA6 (GATA binding protein 6), a transcription factor crucial for cardiogenesis. The expression patterns of GATA6 and TBX1 overlapped in the pharyngeal arches of human embryos. Transfection experiments and EMSA indicated that GATA6 could activate the transcription of TBX1 by directly binding with its GATA cis-regulatory element in vitro. Furthermore, sequencing analyses of 195 sporadic CTD patients without the 22q11.2 deletion or duplication identified 3 variants (NC_000022.11:g.19756832C > G, NC_000022.11:g.19756845C > T, and NC_000022.11:g. 19756902G > T) in the non-coding cis-regulatory element of TBX1. Luciferase assays showed that all 3 variants led to reduced transcription of TBX1 when incubated with GATA6.

Conclusions

Our findings showed that TBX1 might be a direct transcriptional target of GATA6, and variants in the non-coding cis-regulatory element of TBX1 disrupted GATA6-mediated transactivation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-01981-4.

The Multifunctional Contribution of FGF Signaling to Cardiac Development, Homeostasis, Disease and Repair

The current focus on cardiovascular research reflects society’s concerns regarding the alarming incidence of cardiac-related diseases and mortality in the industrialized world and, notably, an urgent need to combat them by more efficient therapies. To pursue these therapeutic approaches, a comprehensive understanding of the mechanism of action for multifunctional fibroblast growth factor (FGF) signaling in the biology of the heart is a matter of high importance. The roles of FGFs in heart development range from outflow tract formation to the proliferation of cardiomyocytes and the formation of heart chambers. In the context of cardiac regeneration, FGFs 1, 2, 9, 16, 19, and 21 mediate adaptive responses including restoration of cardiac contracting rate after myocardial infarction and reduction of myocardial infarct size. However, cardiac complications in human diseases are correlated with pathogenic effects of FGF ligands and/or FGF signaling impairment. FGFs 2 and 23 are involved in maladaptive responses such as cardiac hypertrophic, fibrotic responses and heart failure. Among FGFs with known causative (FGFs 2, 21, and 23) or protective (FGFs 2, 15/19, 16, and 21) roles in cardiac diseases, FGFs 15/19, 21, and 23 display diagnostic potential. The effective role of FGFs on the induction of progenitor stem cells to cardiac cells during development has been employed to boost the limited capacity of postnatal cardiac repair. To renew or replenish damaged cardiomyocytes, FGFs 1, 2, 10, and 16 were tested in (induced-) pluripotent stem cell-based approaches and for stimulation of cell cycle re-entry in adult cardiomyocytes. This review will shed light on the wide range of beneficiary and detrimental actions mediated by FGF ligands and their receptors in the heart, which may open new therapeutic avenues for ameliorating cardiac complications.

In Vivo and In Vitro Genetic Models of Congenital Heart Disease

Congenital cardiovascular malformations represent the most common type of birth defect and encompass a spectrum of anomalies that range from mild to severe. The etiology of congenital heart disease (CHD) is becoming increasingly defined based on prior epidemiologic studies that supported the importance of genetic contributors and technological advances in human genome analysis. These have led to the discovery of a growing number of disease-contributing genetic abnormalities in individuals affected by CHD. The ever-growing population of adult CHD survivors, which are the result of reductions in mortality from CHD during childhood, and this newfound genetic knowledge have led to important questions regarding recurrence risks, the mechanisms by which these defects occur, the potential for novel approaches for prevention, and the prediction of long-term cardiovascular morbidity in adult CHD survivors. Here, we will review the current status of genetic models that accurately model human CHD as they provide an important tool to answer these questions and test novel therapeutic strategies.

Genetic and Cellular Interaction During Cardiovascular Development Implicated in Congenital Heart Diseases

Congenital heart disease (CHD) is the most common life-threatening congenital anomaly. CHD occurs due to defects in cardiovascular development, and the majority of CHDs are caused by a multifactorial inheritance mechanism, which refers to the interaction between genetic and environmental factors. During embryogenesis, the cardiovascular system is derived from at least four distinct cell lineages: the first heart field, second heart field, cardiac neural crest, and proepicardial organ. Understanding the genes involved in each lineage is essential to uncover the genomic architecture of CHD. Therefore, we provide an overview of recent research progress using animal models and mutation analyses to better understand the molecular mechanisms and pathways linking cardiovascular development and CHD. For example, we highlight our recent work on genes encoding three isoforms of inositol 1,4,5-trisphosphate receptors (IP₃R1, 2, and 3) that regulate various vital and developmental processes, which have genetic redundancy during cardiovascular development. Specifically, IP₃R1 and 2 have redundant roles in the atrioventricular cushion derived from the first heart field lineage, whereas IP₃R1 and 3 exhibit redundancy in the right ventricle and the outflow tract derived from the second heart field lineage, respectively. Moreover, 22q11.2 deletion syndrome (22q11DS) is highly associated with CHD involving the outflow tract, characterized by defects of the cardiac neural crest lineage. However, our studies have shown that TBX1, a major genetic determinant of 22q11DS, was not expressed in the cardiac neural crest but rather in the second heart field, suggesting the importance of the cellular interaction between the cardiac neural crest and the second heart field. Comprehensive genetic analysis using the Japanese genome bank of CHD and mouse models revealed that a molecular regulatory network involving GATA6, FOXC1/2, TBX1, SEMA3C, and FGF8 was essential for reciprocal signaling between the cardiac neural crest and the second heart field during cardiovascular development. Elucidation of the genomic architecture of CHD using induced pluripotent stem cells and next-generation sequencing technology, in addition to genetically modified animal models and human mutation analyses, would facilitate the development of regenerative medicine and/or preventive medicine for CHD in the near future.

Heart Enhancers: Development and Disease Control at a Distance

Bound by lineage-determining transcription factors and signaling effectors, enhancers play essential roles in controlling spatiotemporal gene expression profiles during development, homeostasis and disease. Recent synergistic advances in functional genomic technologies, combined with the developmental biology toolbox, have resulted in unprecedented genome-wide annotation of heart enhancers and their target genes. Starting with early studies of vertebrate heart enhancers and ending with state-of-the-art genome-wide enhancer discovery and testing, we will review how studying heart enhancers in metazoan species has helped inform our understanding of cardiac development and disease.

The Drosophila Forkhead/Fox transcription factor Jumeau mediates specific cardiac progenitor cell divisions by regulating expression of the kinesin Nebbish

Forkhead (Fkh/Fox) domain transcription factors (TFs) mediate multiple cardiogenic processes in both mammals and Drosophila. We showed previously that the Drosophila Fox gene jumeau (jumu) controls three categories of cardiac progenitor cell division—asymmetric, symmetric, and cell division at an earlier stage—by regulating Polo kinase activity, and mediates the latter two categories in concert with the TF Myb. Those observations raised the question of whether other jumu-regulated genes also mediate all three categories of cardiac progenitor cell division or a subset thereof. By comparing microarray-based expression profiles of wild-type and jumu loss-of-function mesodermal cells, we identified nebbish (neb), a kinesin-encoding gene activated by jumu. Phenotypic analysis shows that neb is required for only two categories of jumu-regulated cardiac progenitor cell division: symmetric and cell division at an earlier stage. Synergistic genetic interactions between neb, jumu, Myb, and polo and the rescue of jumu mutations by ectopic cardiac mesoderm-specific expression of neb demonstrate that neb is an integral component of a jumu-regulated subnetwork mediating cardiac progenitor cell divisions. Our results emphasize the central role of Fox TFs in cardiogenesis and illustrate how a single TF can utilize different combinations of other regulators and downstream effectors to control distinct developmental processes.

Cardiopharyngeal Progenitor Specification: Multiple Roads to the Heart and Head Muscles

During embryonic development, the heart arises from various sources of undifferentiated mesodermal progenitors, with an additional contribution from ectodermal neural crest cells. Mesodermal cardiac progenitors are plastic and multipotent, but are nevertheless specified to a precise heart region and cell type very early during development. Recent findings have defined both this lineage plasticity and early commitment of cardiac progenitors, using a combination of single-cell and population analyses. In this review, we discuss several aspects of cardiac progenitor specification. We discuss their markers, fate potential in vitro and in vivo, early segregation and commitment, and also intrinsic and extrinsic cues regulating lineage restriction from multipotency to a specific cell type of the heart. Finally, we also discuss the subdivisions of the cardiopharyngeal field, and the shared origins of the heart with other mesodermal derivatives, including head and neck muscles.

Mutations in fibroblast growth factor (FGF8) and FGF10 identified in patients with conotruncal defects

Background

Conotruncal defects (CTDs) are a type of heterogeneous congenital heart diseases (CHDs), but little is known about their etiology. Increasing evidence has demonstrated that fibroblast growth factor (FGF) 8 and FGF10 may be involved in the pathogenesis of CTDs.

Methods

The variants of FGF8 and FGF10 in unrelated Chinese Han patients with CHDs (n = 585), and healthy controls (n = 319) were investigated. The expression and function of these patient-identified variants were detected to confirm the potential pathogenicity of the non-synonymous variants. The expression of FGF8 and FGF10 during the differentiation of human embryonic stem cells (hESCs) to cardiomyocytes and in Carnegie stage 13 human embryo was also identified.

Results

Two probable deleterious variants (p.C10Y, p.R184H) of FGF8 and one deletion mutant (p.23_24del) of FGF10 were identified in three patients with CTD. Immunofluorescence suggested that variants did not affect the intracellular localization, whereas ELISA showed that the p.C10Y and p.23_24del variants reduced the amount of secreted FGF8 and FGF10, respectively. Quantitative RT-PCR and western blotting showed that the expression of FGF8 and FGF10 variants was increased compared with wild-type; however, their functions were reduced. And we found that FGF8 and FGF10 were expressed in the outflow tract (OFT) during human embryonic development, and were dynamically expressed during the differentiation of hESCs into cardiomyocytes.

Conclusion

Our results provided evidence that damaging variants of FGF8 and FGF10 were likely contribute to the etiology of CTD. This discovery expanded the spectrum of FGF mutations and underscored the pathogenic correlation between FGF mutations and CTD.

TBX1 Regulates Chondrocyte Maturation in the Spheno-occipital Synchondrosis

The synchondrosis in the cranial base is an important growth center for the craniofacial region. Abnormalities in the synchondroses affect the development of adjacent regions, including the craniofacial skeleton. Here, we report that the transcription factor TBX1, the candidate gene for DiGeorge syndrome, is expressed in mesoderm-derived chondrocytes and plays an essential and specific role in spheno-occipital synchondrosis development by inhibiting the expression of genes involved in chondrocyte hypertrophy and osteogenesis. In Tbx1-deficient mice, the spheno-occipital synchondrosis was completely mineralized at birth. TBX1 interacts with RUNX2, a master molecule of osteoblastogenesis and a regulator of chondrocyte maturation, and suppresses its transcriptional activity. Indeed, deleting Tbx1 triggers accelerated mineralization due to accelerated chondrocyte differentiation, which is associated with ectopic expression of downstream targets of RUNX2 in the spheno-occipital synchondrosis. These findings reveal that TBX1 acts as a regulator of chondrocyte maturation and osteogenesis during the spheno-occipital synchondrosis development. Thus, the tight regulation of endochondral ossification by TBX1 is crucial for the normal progression of chondrocyte differentiation in the spheno-occipital synchondrosis.

Chromosome 22q11.2 deletion syndrome and DiGeorge syndrome

Chromosome 22q11.2 deletion syndrome is the most common microdeletion syndrome in humans. The effects are protean and highly variable, making a unified approach difficult. Nevertheless, commonalities have been identified and white papers with recommended evaluations and anticipatory guidance have been published. This review will cover the immune system in detail and discuss both the primary features and the secondary features related to thymic hypoplasia. A brief discussion of the other organ system involvement will be provided for context. The immune system, percolating throughout the body can impact the function of other organs through allergy or autoimmune disease affecting organs in deleterious manners. Our work has shown that the primary effect of thymic hypoplasia is to restrict T cell production. Subsequent homeostatic proliferation and perhaps other factors drive a Th2 polarization, most obvious in adulthood. This contributes to atopic risk in this population. Thymic hypoplasia also contributes to low regulatory T cells and this may be part of the overall increased risk of autoimmunity. Collectively, the effects are complex and often age-dependent. Future goals of improving thymic function or augmenting thymic volume may offer a direct intervention to ameliorate infections, atopy, and autoimmunity.

Inositol 1,4,5-trisphosphate receptor 2 as a novel marker of vasculature to delineate processes of cardiopulmonary development

Congenital heart diseases (CHDs) involving the outflow tract (OFT), such as persistent truncus arteriosus (PTA), lead to mortality and morbidity with implications not only in the heart, but also in the pulmonary vasculature. The mechanisms of pulmonary artery (PA) development and the etiologies underlying PA disorders associated with CHD remain poorly understood partly because of a specific marker for PA development is nonexistent. The three subtypes of inositol 1,4,5-trisphosphate receptors (IP₃R1, 2, and 3) are intracellular Ca²⁺ channels that are essential for many tissues and organs. We discovered that IP₃R2 was expressed in the vasculature and heart during development using transgenic mice, in which a LacZ marker gene was knocked into the IP₃R2 locus. Whole-mount and section LacZ staining showed that IP₃R2-LacZ-positive cells were detectable exclusively in the smooth muscle cells, or tunica media, of PA, merging into αSMA-positive cells during development. Furthermore, our analyses suggested that IP₃R2-LacZ positive PA smooth muscle layers gradually elongate from the central PA to the peripheral PAs from E13.5 to E18.5, supporting the distal angiogenesis theory for the development of PA, whereas IP₃R2-LacZ was rarely expressed in smooth muscle cells in the pulmonary trunk. Crossing IP₃R-LacZ mice with mice hypomorphic for Tbx1 alleles revealed that PTA of Tbx1 mutants may result from agenesis or hypoplasia of the pulmonary trunk; thus, the left and right central to peripheral PAs connect directly to the dorsal side of the truncus arteriosus in these mutants. Additionally, we found hypercellular interstitial mesenchyme and delayed maturation of the lung endoderm in the Tbx1 mutant lungs. Our study identifies IP₃R2 as a novel marker for clear visualization of PA during development and can be utilized for studying cardiopulmonary development and disease.

Genome-wide association study identifies 14 previously unreported susceptibility loci for adolescent idiopathic scoliosis in Japanese

Adolescent idiopathic scoliosis (AIS) is the most common pediatric spinal deformity. Several AIS susceptibility loci have been identified; however, they could explain only a small proportion of AIS heritability. To identify additional AIS susceptibility loci, we conduct a meta-analysis of the three genome-wide association studies consisting of 79,211 Japanese individuals. We identify 20 loci significantly associated with AIS, including 14 previously not reported loci. These loci explain 4.6% of the phenotypic variance of AIS. We find 21 cis-expression quantitative trait loci-associated genes in seven of the fourteen loci. By a female meta-analysis, we identify additional three significant loci. We also find significant genetic correlations of AIS with body mass index and uric acid. The cell-type specificity analyses show the significant heritability enrichment for AIS in multiple cell-type groups, suggesting the heterogeneity of etiology and pathogenesis of AIS. Our findings provide insights into etiology and pathogenesis of AIS.

Adolescent idiopathic scoliosis (AIS) is a common pediatric disease leading to spinal deformities. Here, the authors report GWAS followed by genome-wide meta-analysis in up to 79,211 Japanese individuals, identifying 20 genetic loci for AIS, 14 of which were previously unreported, and perform in vitro validation for rs1978060.

In the line-up: deleted genes associated with DiGeorge/22q11.2 deletion syndrome: are they all suspects?

BACKGROUND

22q11.2 deletion syndrome (22q11DS), a copy number variation (CNV) disorder, occurs in approximately 1:4000 live births due to a heterozygous microdeletion at position 11.2 (proximal) on the q arm of human chromosome 22 (hChr22) (McDonald-McGinn and Sullivan, Medicine 90:1-18, 2011). This disorder was known as DiGeorge syndrome, Velo-cardio-facial syndrome (VCFS) or conotruncal anomaly face syndrome (CTAF) based upon diagnostic cardiovascular, pharyngeal, and craniofacial anomalies (McDonald-McGinn and Sullivan, Medicine 90:1-18, 2011; Burn et al., J Med Genet 30:822-4, 1993) before this phenotypic spectrum was associated with 22q11.2 CNVs. Subsequently, 22q11.2 deletion emerged as a major genomic lesion associated with vulnerability for several clinically defined behavioral deficits common to a number of neurodevelopmental disorders (Fernandez et al., Principles of Developmental Genetics, 2015; Robin and Shprintzen, J Pediatr 147:90-6, 2005; Schneider et al., Am J Psychiatry 171:627-39, 2014).

RESULTS

The mechanistic relationships between heterozygously deleted 22q11.2 genes and 22q11DS phenotypes are still unknown. We assembled a comprehensive "line-up" of the 36 protein coding loci in the 1.5 Mb minimal critical deleted region on hChr22q11.2, plus 20 protein coding loci in the distal 1.5 Mb that defines the 3 Mb typical 22q11DS deletion. We categorized candidates based upon apparent primary cell biological functions. We analyzed 41 of these genes that encode known proteins to determine whether haploinsufficiency of any single 22q11.2 gene-a one gene to one phenotype correspondence due to heterozygous deletion restricted to that locus-versus complex multigenic interactions can account for single or multiple 22q11DS phenotypes.

CONCLUSIONS

Our 22q11.2 functional genomic assessment does not support current theories of single gene haploinsufficiency for one or all 22q11DS phenotypes. Shared molecular functions, convergence on fundamental cell biological processes, and related consequences of individual 22q11.2 genes point to a matrix of multigenic interactions due to diminished 22q11.2 gene dosage. These interactions target fundamental cellular mechanisms essential for development, maturation, or homeostasis at subsets of 22q11DS phenotypic sites.

Exome sequencing identifies a de novo FOXA2 variant in a patient with syndromic diabetes

OBJECTIVE

When diabetes is associated with congenital malformations, without autoimmune antibodies, a genetic cause is suspected. Here, we aimed to identify a defective gene that led to diabetes.

RESEARCH DESIGN AND METHODS

We performed an exome analysis of an index case and his healthy parents.

RESULTS

The child presented with childhood-onset diabetes, congenital hypopituitarism, cardiac malformation, and anal atresia. A DNA analysis revealed a heterozygous de novo pathogenic variant in the developmental transcription factor, forkhead box A2 (FOXA2). The mutation resided in the DNA-binding domain, which is highly conserved among species. Tridimensional molecular dynamics simulation modeling predicted an altered interaction between the mutated protein and DNA.

CONCLUSIONS

A defect in the FOXA2 DNA-binding domain was associated with childhood-onset diabetes and multiple congenital anomalies, which reflected the pleiotropic nature of the gene. This report extends the recently described phenotype of neonatal hypoglycemia to later-onset diabetes. We suggest to include FOXA2 analysis for neonatal hypoglycemia and to implement a long-term follow-up, particularly for the risk of diabetes.

Intronic Polymorphisms in Gene of Second Heart Field as Risk Factors for Human Congenital Heart Disease in a Chinese Population

Transcriptional factors and signaling factors in the second heart field (SHF) contribute to cardiac development. However, the associations of intronic gene variants in the SHF with congenital heart disease (CHD) remain ununderstood. Ten single nucleotide polymorphisms (SNPs) from our previous sequencing data were selected and then genotyped in 383 CHD patients and 384 healthy controls in a Chinese population. Genotype analyses revealed that minor alleles in TBX1: rs12165908 C > G [odds ratio (OR) = 2.64; 95% confidence interval (CI) = 1.87-3.73, p = 3.03 × 10^-8] and GATA6: rs143085291 C > T (OR = 2.49; 95% CI = 1.18-5.29, p = 0.01) increased CHD risk significantly. Meanwhile, FGF10: rs78454549 T > C and GATA4: rs13275657 A>G polymorphisms were significantly associated with increased risk of simple CHDs. The minor allele C in GATA4: rs17153694 T > C increased the risk of tetralogy of Fallot, whereas minor alleles in TBX1: rs41298006 G>A, FGF10: rs75629618 C>T, FGF10: rs10461755 G>A, FGF10: rs75632187 A>G, and FGF10: rs12518964 G > A were associated with increased risk of single ventricle. The minor allele T in rs143085291 in GATA6 enhancer decreased the transcription level in luciferase assay. Our findings suggest that intronic SNPs in transcriptional factors and signaling factors in the SHF are significantly associated with increased risk of different CHD types.

An interaction-based model for neuropsychiatric features of copy-number variants

Variably expressive copy-number variants (CNVs) are characterized by extensive phenotypic heterogeneity of neuropsychiatric phenotypes. Approaches to identify single causative genes for these phenotypes within each CNV have not been successful. Here, we posit using multiple lines of evidence, including pathogenicity metrics, functional assays of model organisms, and gene expression data, that multiple genes within each CNV region are likely responsible for the observed phenotypes. We propose that candidate genes within each region likely interact with each other through shared pathways to modulate the individual gene phenotypes, emphasizing the genetic complexity of CNV-associated neuropsychiatric features.

Deletion of the T-box transcription factor gene, Tbx1, in mice induces differential expression of genes associated with cleft palate in humans

OBJECTIVE

We examined the function of the T-box transcription factor 1 (TBX1) in palatogenesis.

DESIGN

Tbx1-knockout mice were histologically examined by hematoxylin and eosin staining. Next, secondary palatal shelves dissected from wild type or Tbx1-knockout mice embryos at embryonic day 13.5 were investigated with microarray analysis, gene ontology analysis, and real-time quantitative polymerase chain reaction. We performed gene profiling of developing palatal shelves from wild type and Tbx1-knockout embryos. We also analyzed the association of mouse genes linked to cleft palate with biological processes and compared the results with those of our ontology analysis of dysregulated genes in Tbx1-knockout palatal shelves.

RESULTS

Histological analysis of Tbx1-knockout palate with complete cleft palate at postnatal day 1 showed aplasia of secondary palates associated with a small mandible and a small tongue compared to wild type littermates. Gene ontology analysis indicated that genes associated with development of the nervous system, muscle, and biomineral tissue were dysregulated in Tbx1-knockout palatal shelves. Furthermore, in Tbx1-knockout palatal shelves, genes associated with human cleft palate, specifically, myosin heavy chain 3 (Myh3) and nebulin (Neb), were downregulated and gamma-aminobutyric acid type A receptor beta 3 subunit (Gabrb3) was upregulated.

CONCLUSIONS

Our findings demonstrate that TBX1 maintains normal growth and development of palatal shelves, mediated through the regulation of genes involved in muscle cell differentiation, nervous system development, and biomineral tissue development. Multiple factors in Tbx1-knockout mice may lead to various subtypes of cleft palate.

Gene expression profiling in the developing secondary palate in the absence of Tbx1 function

BACKGROUND

Microdeletion of chromosome 22q11 is associated with significant developmental anomalies, including disruption of the cardiac outflow tract, thymic/parathyroid aplasia and cleft palate. Amongst the genes within this region, TBX1 is a major candidate for many of these developmental defects. Targeted deletion of Tbx1 in the mouse has provided significant insight into the function of this transcription factor during early development of the cardiac and pharyngeal systems. However, less is known about its role during palatogenesis. To assess the influence of Tbx1 function on gene expression profile within the developing palate we performed a microarray screen using total RNA isolated from the secondary palate of E13.5 mouse embryos wild type, heterozygous and mutant for Tbx1.

RESULTS

Expression-level filtering and statistical analysis revealed a total of 577 genes differentially expressed across genotypes. Data were clustered into 3 groups based on comparison between genotypes. Group A was composed of differentially expressed genes in mutant compared to wild type (n = 89); Group B included differentially expressed genes in heterozygous compared to wild type (n = 400) and Group C included differentially expressed genes in mutant compared to heterozygous (n = 88). High-throughput quantitative real-time PCR (RT-PCR) confirmed a total of 27 genes significantly changed between wild type and mutant; and 27 genes between heterozygote and mutant. Amongst these, the majority were present in both groups A and C (26 genes). Associations existed with hypertrophic cardiomyopathy, cardiac muscle contraction, dilated cardiomyopathy, focal adhesion, tight junction and calcium signalling pathways. No significant differences in gene expression were found between wild type and heterozygous palatal shelves.

CONCLUSIONS

Significant differences in gene expression profile within the secondary palate of wild type and mutant embryos is consistent with a primary role for Tbx1 during palatogenesis.

Continuous addition of progenitors forms the cardiac ventricle in zebrafish

The vertebrate heart develops from several progenitor lineages. After early-differentiating first heart field (FHF) progenitors form the linear heart tube, late-differentiating second heart field (SHF) progenitors extend the atrium and ventricle, and form inflow and outflow tracts (IFT/OFT). However, the position and migration of late-differentiating progenitors during heart formation remains unclear. Here, we track zebrafish heart development using transgenics based on the cardiopharyngeal gene tbx1. Live imaging uncovers a tbx1 reporter-expressing cell sheath that continuously disseminates from the lateral plate mesoderm towards the forming heart tube. High-speed imaging and optogenetic lineage tracing corroborates that the zebrafish ventricle forms through continuous addition from the undifferentiated progenitor sheath followed by late-phase accrual of the bulbus arteriosus (BA). FGF inhibition during sheath migration reduces ventricle size and abolishes BA formation, refining the window of FGF action during OFT formation. Our findings consolidate previous end-point analyses and establish zebrafish ventricle formation as a continuous process.

Late-differentiating second heart field progenitors contribute to atrium, ventricle, and outflow tract in the zebrafish heart but how remains unclear. Here, the authors image heart formation in transgenics based on the cardiopharyngeal gene tbx1 and show that progenitors are continuously added.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

An FGF-driven feed-forward circuit patterns the cardiopharyngeal mesoderm in space and time

In embryos, multipotent progenitors divide to produce distinct progeny and express their full potential. In vertebrates, multipotent cardiopharyngeal progenitors produce second-heart-field-derived cardiomyocytes, and branchiomeric skeletal head muscles. However, the mechanisms underlying these early fate choices remain largely elusive. The tunicate Ciona emerged as an attractive model to study early cardiopharyngeal development at high resolution: through two asymmetric and oriented divisions, defined cardiopharyngeal progenitors produce distinct first and second heart precursors, and pharyngeal muscle (aka atrial siphon muscle, ASM) precursors. Here, we demonstrate that differential FGF-MAPK signaling distinguishes between heart and ASM precursors. We characterize a feed-forward circuit that promotes the successive activations of essential ASM determinants, Hand-related, Tbx1/10 and Ebf. Finally, we show that coupling FGF-MAPK restriction and cardiopharyngeal network deployment with cell divisions defines the timing of gene expression and permits the emergence of diverse cell types from multipotent progenitors.

To roll the eyes and snap a bite - function, development and evolution of craniofacial muscles

Craniofacial muscles, muscles that move the eyes, control facial expression and allow food uptake and speech, have long been regarded as a variation on the general body muscle scheme. However, evidence has accumulated that the function of head muscles, their developmental anatomy and the underlying regulatory cascades are distinct. This article reviews the key aspects of craniofacial muscle and muscle stem cell formation and discusses how this differs from the trunk programme of myogenesis; we show novel RNAseq data to support this notion. We also trace the origin of head muscle in the chordate ancestors of vertebrates and discuss links with smooth-type muscle in the primitive chordate pharynx. We look out as to how the special properties of head muscle precursor and stem cells, in particular their competence to contribute to the heart, could be exploited in regenerative medicine.

Regulation of Sema3c and the Interaction between Cardiac Neural Crest and Second Heart Field during Outflow Tract Development

The cardiac neural crest cells (cNCCs) and the second heart field (SHF) play key roles in development of the cardiac outflow tract (OFT) for establishment of completely separated pulmonary and systemic circulations in vertebrates. A neurovascular guiding factor, Semaphorin 3c (Sema3c), is required for the development of the OFT, however, its regulation of the interaction between cNCCs and SHF remains to be determined. Here, we show that a Sema3c is a candidate that mediates interaction between cNCCs and the SHF during development of the OFT. Foxc1/c2 directly activates the transcription of Sema3c in the OFT, whereas, a hypomorph of Tbx1, a key SHF transcription factor, resulted in the ectopic expression of Sema3c in the pharyngeal arch region. Fgf8, a downstream secreted factor of Tbx1, inhibited the expression of Sema3c in cNCCs via activation of ERK1/2 signaling. Blocking of FGF8 caused ectopic expression of SEMA3C and a migration defect of cNCCs, resulting in abnormal chick pharyngeal arch development. These results suggest that proper spatio-temporal expression of Sema3c, regulated positively by Foxc1/c2 and negatively by the Tbx1-Fgf8 cascade, respectively, is essential for the interaction between cNCCs and the SHF that correctly navigates cNCCs towards the OFT, composed of SHF-derived cells.

22q11.2 Deletion Syndrome: Laboratory Diagnosis and TBX1 and FGF8 Mutation Screening

Velocardiofacial syndrome is one of the recognized forms of chromosome 22q11.2 deletion syndrome (22q11.2 DS) and has an incidence of 1 of 4,000 to 1 of 6,000 births. Nevertheless, the 22q11 deletion is not found in several patients with a 22q11.2 DS phenotype. In this situation, other chromosomal aberrations and/or mutations in the T-box 1 transcription factor C (TBX1) gene have been detected in some patients. A similar phenotype to that of the 22q11.2 DS has been reported in animal models with mutations in fibroblast growth factor 8 (Fgf8) gene. To date, FGF8 mutations have not been investigated in humans. We tested a strategy to perform laboratory testing to reduce costs in the investigation of patients presenting with the 22q11.2 DS phenotype. A total of 109 individuals with clinical suspicion were investigated using GTG-banding karyotype, fluorescence in situ hybridization, and/or multiplex ligation-dependent probe amplification. A conclusive diagnosis was achieved in 33 of 109 (30.2%) cases. In addition, mutations in the coding regions of TBX1 and FGF8 genes were investigated in selected cases where 22q11.2 deletion had been excluded, and no pathogenic mutations were detected in both genes. This study presents a strategy for molecular genetic characterization of patients presenting with the 22q11.2 DS using different laboratory techniques. This strategy could be useful in different countries, according to local resources. Also, to our knowledge, this is the first investigation of FGF8 gene in humans with this clinical suspicion.

T-Box Genes in the Kidney and Urinary Tract

T-box (Tbx) genes encode an ancient group of transcription factors that play important roles in patterning, specification, proliferation, and differentiation programs in vertebrate organogenesis. This is testified by severe organ malformation syndromes in mice homozygous for engineered null alleles of specific T-box genes and by the large number of human inherited organ-specific diseases that have been linked to mutations in these genes. One of the organ systems that has not been associated with loss of specific T-box gene function in human disease for long is the excretory system. However, this has changed with the finding that mutations in TBX18, a member of a vertebrate-specific subgroup within the Tbx1-subfamily of T-box transcription factor genes, cause congenital anomalies of the kidney and urinary tract, predominantly hydroureter and ureteropelvic junction obstruction. Gene expression analyses, loss-of-function studies, and lineage tracing in the mouse suggest a primary role for this transcription factor in specifying the ureteric mesenchyme in the common anlage of the kidney, the ureter, and the bladder. We review the function of Tbx18 in ureterogenesis and discuss the body of evidence that Tbx18 and other members of the T-box gene family, namely, Tbx1, Tbx2, Tbx3, and Tbx20, play additional roles in development and homeostasis of other components of the excretory system in vertebrates.

Two Forkhead transcription factors regulate cardiac progenitor specification by controlling the expression of receptors of the fibroblast growth factor and Wnt signaling pathways

Cardiogenesis involves the coordinated regulation of multiple biological processes by a finite set of transcription factors (TFs). Here, we show that the Forkhead TFs Checkpoint suppressor homologue (CHES-1-like) and Jumeau (Jumu), which govern cardiac progenitor cell divisions by regulating Polo kinase activity, play an additional, mutually redundant role in specifying the cardiac mesoderm (CM) as eliminating the functions of both Forkhead genes in the same Drosophila embryo results in defective hearts with missing hemisegments. This process is mediated by the Forkhead TFs regulating the fibroblast growth factor receptor Heartless (Htl) and the Wnt receptor Frizzled (Fz): CHES-1-like and jumu exhibit synergistic genetic interactions with htl and fz in CM specification, thereby implying that they function through the same genetic pathways, and transcriptionally activate the expression of both receptor-encoding genes. Furthermore, ectopic overexpression of either htl or fz in the mesoderm partially rescues the defective CM specification phenotype in embryos lacking both Forkhead genes. Together, these data emphasize the functional redundancy that leads to robustness in the cardiac progenitor specification process, and illustrate the pleiotropic functions of Forkhead TFs in different aspects of cardiogenesis.

Summary: Checkpoint suppressor homologue and Jumeau, which are known to govern cardiac progenitor cell divisions, play additional, mutually redundant roles in specifying cardiac mesoderm in Drosophila.

Forkhead box transcription factors in embryonic heart development and congenital heart disease

Embryonic heart development is a very complicated process regulated precisely by a network composed of many genes and signaling pathways in time and space. Forkhead box (Fox, FOX) proteins are a family of transcription factors characterized by the presence of an evolutionary conserved "forkhead"or "winged-helix" DNA-binding domain and able to organize temporal and spatial gene expression during development. They are involved in a wide variety of cellular processes, such as cell cycle progression, proliferation, differentiation, migration, metabolism and DNA damage response. An abundance of studies in model organisms and systems has established that Foxa2, Foxc1/c2, Foxh1 and Foxm1, Foxos and Foxps are important components of the signaling pathways that instruct cardiogenesis and embryonic heart development, playing paramount roles in heart development. The previous studies also have demonstrated that mutations in some of the forkhead box genes and the aberrant expression of forkhead box gene are heavily implicated in the congenital heart disease (CHD) of humans. This review primarily focuses on the current understanding of heart development regulated by forkhead box transcription factors and molecular genetic mechanisms by which forkhead box factors modulate heart development during embryogenesis and organogenesis. This review also summarizes human CHD related mutations in forkhead box genes as well as the abnormal expression of forkhead box gene, and discusses additional possible regulatory mechanisms of the forkhead box genes during embryonic heart development that warrant further investigation.

Head muscle development

The developmental paths that lead to the formation of skeletal muscles in the head are distinct from those operating in the trunk. Craniofacial muscles are associated with head and neck structures. In the embryo, these structures derive from distinct mesoderm populations. Distinct genetic programs regulate different groups of muscles within the head to generate diverse muscle specifications. Developmental and lineage studies in vertebrates and invertebrates demonstrated an overlap in progenitor populations derived from the pharyngeal mesoderm that contribute to certain head muscles and the heart. These studies reveal that the genetic program controlling pharyngeal muscles overlaps with that of the heart. Indeed cardiac and craniofacial birth defects are often linked. Recent studies suggest that early chordates, the last common ancestor of tunicates and vertebrates, had an ancestral pharyngeal mesoderm lineage that later during evolution gave rise to both heart and craniofacial structures. This chapter summarizes studies related to the origins, signaling, genetics, and evolution of the head musculature, highlighting its heterogeneous characteristics in all these aspects.

Akt1 signaling coordinates BMP signaling and β-catenin activity to regulate second heart field progenitor development

Second heart field (SHF) progenitors exhibit continued proliferation and delayed differentiation, which are modulated by FGF4/8/10, BMP and canonical Wnt/β-catenin signaling. PTEN-Akt signaling regulates the stem cell/progenitor cell homeostasis in several systems, such as hematopoietic stem cells, intestinal stem cells and neural progenitor cells. To address whether PTEN-Akt signaling is involved in regulating cardiac progenitors, we deleted Pten in SHF progenitors. Deletion of Pten caused SHF expansion and increased the size of the SHF derivatives, the right ventricle and the outflow tract. Cell proliferation of cardiac progenitors was enhanced, whereas cardiac differentiation was unaffected by Pten deletion. Removal of Akt1 rescued the phenotype and early lethality of Pten deletion mice, suggesting that Akt1 was the key downstream target that was negatively regulated by PTEN in cardiac progenitors. Furthermore, we found that inhibition of FOXO by Akt1 suppressed the expression of the gene encoding the BMP ligand (BMP7), leading to dampened BMP signaling in the hearts of Pten deletion mice. Cardiac activation of Akt also increased the Ser552 phosphorylation of β-catenin, thus enhancing its activity. Reducing β-catenin levels could partially rescue heart defects of Pten deletion mice. We conclude that Akt signaling regulates the cell proliferation of SHF progenitors through coordination of BMP signaling and β-catenin activity.

Evolutionary and developmental origins of the cardiac neural crest: building a divided outflow tract

The cardiac neural crest cells (CNCCs) have played an important role in the evolution and development of the vertebrate cardiovascular system: from reinforcement of the developing aortic arch arteries early in vertebrate evolution, to later orchestration of aortic arch artery remodeling into the great arteries of the heart, and finally outflow tract septation in amniotes. A critical element necessary for the evolutionary advent of outflow tract septation was the co-evolution of the cardiac neural crest cells with the second heart field. This review highlights the major transitions in vertebrate circulatory evolution, explores the evolutionary developmental origins of the CNCCs from the third stream cranial neural crest, and explores candidate signaling pathways in CNCC and outflow tract evolution drawn from our knowledge of DiGeorge Syndrome.

Loss of Tbx1 induces bone phenotypes similar to cleidocranial dysplasia

T-box transcription factor, TBX1, is the major candidate gene for 22q11.2 deletion syndrome (DiGeorge/ Velo-cardio-facial syndrome) characterized by facial defects, thymus hypoplasia, cardiovascular anomalies and cleft palates. Here, we report that the loss of Tbx1 in mouse (Tbx1(-/-)) results in skeletal abnormalities similar to those of cleidocranial dysplasia (CCD) in humans, which is an autosomal-dominant skeletal disease caused by mutations in RUNX2. Tbx1(-/-) mice display short stature, absence of hyoid bone, failed closure of fontanelle, bifid xiphoid process and hypoplasia of clavicle and zygomatic arch. A cell-type-specific deletion of Tbx1 in osteochondro-progenitor (Tbx1(OPKO)) or mesodermal (Tbx1(MKO)) lineage partially recapitulates the Tbx1(-/-) bone phenotypes. Although Tbx1 expression has not been previously reported in neural crest, inactivation of Tbx1 in the neural crest lineage (Tbx1(NCKO)) leads to an absence of the body of hyoid bone and postnatal lethality, indicating an unanticipated role of Tbx1 in neural crest development. Indeed, Tbx1 is expressed in the neural crest-derived hyoid bone primordium, in addition to mesoderm-derived osteochondral progenitors. Ablation of Tbx1 affected Runx2 expression in calvarial bones and overexpression of Tbx1 induced Runx2 expression in vitro. Taken together, our current studies reveal that Tbx1 is required for mesoderm- and neural crest-derived osteoblast differentiation and normal skeletal development. TBX1 mutation could lead to CCD-like bone phenotypes in human.

Connecting teratogen-induced congenital heart defects to neural crest cells and their effect on cardiac function

Neural crest cells play many key roles in embryonic development, as demonstrated by the abnormalities that result from their specific absence or dysfunction. Unfortunately, these key cells are particularly sensitive to abnormalities in various intrinsic and extrinsic factors, such as genetic deletions or ethanol-exposure that lead to morbidity and mortality for organisms. This review discusses the role identified for a segment of neural crest in regulating the morphogenesis of the heart and associated great vessels. The paradox is that their derivatives constitute a small proportion of cells to the cardiovascular system. Findings supporting that these cells impact early cardiac function raises the interesting possibility that they indirectly control cardiovascular development at least partially through regulating function. Making connections between insults to the neural crest, cardiac function, and morphogenesis is more approachable with technological advances. Expanding our understanding of early functional consequences could be useful in improving diagnosis and testing therapies.

Prdm1 functions in the mesoderm of the second heart field, where it interacts genetically with Tbx1, during outflow tract morphogenesis in the mouse embryo

Congenital heart defects affect at least 0.8% of newborn children and are a major cause of lethality prior to birth. Malformations of the arterial pole are particularly frequent. The myocardium at the base of the pulmonary trunk and aorta and the arterial tree associated with these great arteries are derived from splanchnic mesoderm of the second heart field (SHF), an important source of cardiac progenitor cells. These cells are controlled by a gene regulatory network that includes Fgf8, Fgf10 and Tbx1. Prdm1 encodes a transcriptional repressor that we show is also expressed in the SHF. In mouse embryos, mutation of Prdm1 affects branchial arch development and leads to persistent truncus arteriosus (PTA), indicative of neural crest dysfunction. Using conditional mutants, we show that this is not due to a direct function of Prdm1 in neural crest cells. Mutation of Prdm1 in the SHF does not result in PTA, but leads to arterial pole defects, characterized by mis-alignment or reduction of the aorta and pulmonary trunk, and abnormalities in the arterial tree, defects that are preceded by a reduction in outflow tract size and loss of caudal pharyngeal arch arteries. These defects are associated with a reduction in proliferation of progenitor cells in the SHF. We have investigated genetic interactions with Fgf8 and Tbx1, and show that on a Tbx1 heterozygote background, conditional Prdm1 mutants have more pronounced arterial pole defects, now including PTA. Our results identify PRDM1 as a potential modifier of phenotypic severity in TBX1 haploinsufficient DiGeorge syndrome patients.

Mammalian TBX1 preferentially binds and regulates downstream targets via a tandem T-site repeat

Haploinsufficiency or mutation of TBX1 is largely responsible for the etiology of physical malformations in individuals with velo-cardio-facial/DiGeorge syndrome (VCFS/DGS/22q11.2 deletion syndrome). TBX1 encodes a transcription factor protein that contains an evolutionarily conserved DNA binding domain termed the T-box that is shared with other family members. All T-box proteins, examined so far, bind to similar but not identical consensus DNA sequences, indicating that they have specific binding preferences. To identify the TBX1 specific consensus sequence, Systematic Evolution of Ligands by Exponential Enrichment (SELEX) was performed. In contrast to other TBX family members recognizing palindrome sequences, we found that TBX1 preferentially binds to a tandem repeat of 5′-AGGTGTGAAGGTGTGA-3′. We also identified a second consensus sequence comprised of a tandem repeat with a degenerated downstream site. We show that three known human disease-causing TBX1 missense mutations (F148Y, H194Q and G310S) do not alter nuclear localization, or disrupt binding to the tandem repeat consensus sequences, but they reduce transcriptional activity in cell culture reporter assays. To identify Tbx1-downstream genes, we performed an in silico genome wide analysis of potential cis-acting elements in DNA and found strong enrichment of genes required for developmental processes and transcriptional regulation. We found that TBX1 binds to 19 different loci in vitro, which may correspond to putative cis-acting binding sites. In situ hybridization coupled with luciferase gene reporter assays on three gene loci, Fgf8, Bmper, Otog-MyoD, show that these motifs are directly regulated by TBX1 in vitro. Collectively, the present studies establish new insights into molecular aspects of TBX1 binding to DNA. This work lays the groundwork for future in vivo studies, including chromatin immunoprecipitation followed by next generation sequencing (ChIP-Seq) to further elucidate the molecular pathogenesis of VCFS/DGS.

Tbx1 modulates endodermal and mesodermal differentiation from mouse induced pluripotent stem cells

The T-box transcriptional factor (Tbx) family of transcriptional factors has distinct roles in a wide range of embryonic differentiation or response pathways. Tbx1, a T-box transcription factor, is an important gene for the human congenital disorder 22q11.2 deletion syndrome. Induced pluripotent stem cell (iPSC) technology offers new opportunities for both elucidation of the pathogenesis of diseases and the development of stem-cell-based therapies. In this study, we generated iPSCs from Tbx1(-/-) and Tbx1(+/+) fibroblasts and investigated the spontaneous differentiation potential of iPSCs by detailed lineage analysis of the iPSC-derived embryoid bodies. Undifferentiated Tbx1(-/-) and Tbx1(+/+) iPSCs showed similar expression levels of pluripotent markers. The ability of the Tbx1(-/-) iPSCs to generate endodermal and mesodermal lineages was compromised upon spontaneous differentiation into embryonic bodies. Restoration of Tbx1 expression in the Tbx1(-/-) iPSCs to normal levels using an inducible lentiviral system rescued these cells from the potential of defective differentiation. Interestingly, overexpression of Tbx1 in the Tbx1(-/-) iPSCs to higher levels than in the Tbx1(+/+) iPSCs again led to a defective differentiation potential. Additionally, we observed that expression of fibroblast growth factor (FGF) 10 and FGF8 was downregulated in the Tbx1(-/-) iPSC-derived cells, which suggests that Tbx1 regulates the expression of FGFs. Taken together, our results implicated the Tbx1 level as an important determinant of endodermal and mesodermal lineage differentiation during embryonic development.

Understanding the role of Tbx1 as a candidate gene for 22q11.2 deletion syndrome

22q11.2 deletion syndrome (22q11.2DS) is caused by a commonly occurring microdeletion on chromosome 22. Clinical findings include cardiac malformations, thymic and parathyroid hypoplasia, craniofacial dysmorphisms, and dental defects. These phenotypes are due mainly to abnormal development of the pharyngeal apparatus. Targeted deletion studies in mice and analysis of naturally occurring mutations in humans have implicated Tbx1 as a candidate gene for 22q11.2DS. Tbx1 belongs to an evolutionarily conserved T-box family of transcription factors, whose expression is precisely regulated during embryogenesis, and it appears to regulate the proliferation and differentiation of various progenitor cells during organogenesis. In this review, we discuss the mechanisms of Tbx1 during development of the heart, thymus and parathyroid glands, as well as during formation of the palate, teeth, and other craniofacial features.