Lymphatic endothelial transcription factor Tbx1 promotes an immunosuppressive microenvironment to facilitate post-myocardial infarction repair.

The heart is an autoimmune-prone organ. It is crucial for the heart to keep injury-induced autoimmunity in check to avoid autoimmune-mediated inflammatory disease. However, little is known about how injury-induced autoimmunity is constrained in hearts. Here, we reveal an unknown intramyocardial immunosuppressive program driven by Tbx1, a DiGeorge syndrome disease gene that encodes a T-box transcription factor (TF). We found induced profound lymphangiogenic and immunomodulatory gene expression changes in lymphatic endothelial cells (LECs) after myocardial infarction (MI). The activated LECs penetrated the infarcted area and functioned as intramyocardial immune hubs to increase the numbers of tolerogenic dendritic cells (tDCs) and regulatory T (Treg) cells through the chemokine Ccl21 and integrin Icam1, thereby inhibiting the expansion of autoreactive CD8+ T cells and promoting reparative macrophage expansion to facilitate post-MI repair. Mimicking its timing and implementation may be an additional approach to treating autoimmunity-mediated cardiac diseases.

Significant improvement of cardiac outflow tract septation defects in a DiGeorge syndrome model after minoxidil treatment.

The T-BOX transcription factor TBX1 is essential for the development of the pharyngeal apparatus and it is haploinsufficient in DiGeorge syndrome (DGS), a developmental anomaly associated with congenital heart disease and other abnormalities. The murine model recapitulates the heart phenotype and showed collagen accumulation. We first used a cellular model to study gene expression during cardiogenic differentiation of WT and Tbx1-/- mouse embryonic stem cells. Then we used a mouse model of DGS to test whether interfering with collagen accumulation using an inhibitor of lysyl hydroxylase would modify the cardiac phenotype of the mutant. We found that loss of Tbx1 in a precardiac differentiation model was associated with up regulation of a subset of ECM-related genes, including several collagen genes. In the in vivo model, early prenatal treatment with Minoxidil, a lysyl hydroxylase inhibitor, ameliorated the cardiac outflow tract septation phenotype in Tbx1 mutant fetuses, but it had no effect on septation in WT fetuses. We conclude that TBX1 suppresses a defined subset of ECM-related genes. This function is critical for OFT septation because the inhibition of collagen cross-linking in the mutant reduces significantly the penetrance of septation defects.

Cardiopharyngeal mesoderm origins of musculoskeletal and connective tissues in the mammalian pharynx.

Cardiopharyngeal mesoderm (CPM) gives rise to muscles of the head and heart. Using genetic lineage analysis in mice, we show that CPM develops into a broad range of pharyngeal structures and cell types encompassing musculoskeletal and connective tissues. We demonstrate that CPM contributes to medial pharyngeal skeletal and connective tissues associated with both branchiomeric and somite-derived neck muscles. CPM and neural crest cells (NCC) make complementary mediolateral contributions to pharyngeal structures, in a distribution established in the early embryo. We further show that biallelic expression of the CPM regulatory gene Tbx1, haploinsufficient in 22q11.2 deletion syndrome patients, is required for the correct patterning of muscles with CPM-derived connective tissue. Our results suggest that CPM plays a patterning role during muscle development, similar to that of NCC during craniofacial myogenesis. The broad lineage contributions of CPM to pharyngeal structures provide new insights into congenital disorders and evolution of the mammalian pharynx.

Gene-environment interaction impacts on heart development and embryo survival.

Congenital heart disease (CHD) is the most common type of birth defect. In recent years, research has focussed on identifying the genetic causes of CHD. However, only a minority of CHD cases can be attributed to single gene mutations. In addition, studies have identified different environmental stressors that promote CHD, but the additive effect of genetic susceptibility and environmental factors is poorly understood. In this context, we have investigated the effects of short-term gestational hypoxia on mouse embryos genetically predisposed to heart defects. Exposure of mouse embryos heterozygous for Tbx1 or Fgfr1/Fgfr2 to hypoxia in utero increased the incidence and severity of heart defects while Nkx2-5+/- embryos died within 2 days of hypoxic exposure. We identified the molecular consequences of the interaction between Nkx2-5 and short-term gestational hypoxia, which suggest that reduced Nkx2-5 expression and a prolonged hypoxia-inducible factor 1α response together precipitate embryo death. Our study provides insight into the causes of embryo loss and variable penetrance of monogenic CHD, and raises the possibility that cases of foetal death and CHD in humans could be caused by similar gene-environment interactions.

Tbx1 regulates extracellular matrix-cell interactions in the second heart field.

Tbx1, the major candidate gene for DiGeorge or 22q11.2 deletion syndrome, is required for efficient incorporation of cardiac progenitors of the second heart field (SHF) into the heart. However, the mechanisms by which TBX1 regulates this process are still unclear. Here, we have used two independent models, mouse embryos and cultured cells, to define the role of TBX1 in establishing morphological and dynamic characteristics of SHF in the mouse. We found that loss of TBX1 impairs extracellular matrix (ECM)-integrin-focal adhesion (FA) signaling in both models. Mosaic analysis in embryos suggested that this function is non-cell autonomous, and, in cultured cells, loss of TBX1 impairs cell migration and FAs. Additionally, we found that ECM-mediated integrin signaling is disrupted upon loss of TBX1. Finally, we show that interfering with the ECM-integrin-FA axis between E8.5 and E9.5 in mouse embryos, corresponding to the time window within which TBX1 is required in the SHF, causes outflow tract dysmorphogenesis. Our results demonstrate that TBX1 is required to maintain the integrity of ECM-cell interactions in the SHF and that this interaction is critical for cardiac outflow tract development. More broadly, our data identifies a novel TBX1 downstream pathway as an important player in SHF tissue architecture and cardiac morphogenesis.

A dual role for Tbx1 in cardiac lymphangiogenesis through genetic interaction with Vegfr3.

The transcription factor TBX1 is the major gene implicated in 22q11.2 deletion syndrome (22q11.2DS). The complex clinical phenotype includes vascular anomalies and a recent report presented new cases of primary lymphedema in 22q11.2DS patients. We have previously shown that TBX1 is required for systemic lymphatic vessel development in prenatal mice and it is critical for their survival postnatally. Using loss-of-function genetics and transgenesis in the mouse, we show here a strong genetic interaction between Tbx1 and Vegfr3 in cardiac lymphangiogenesis. Intriguingly, we found that different aspects of the cardiac lymphatic phenotype in Tbx1-Vegfr3 compound heterozygotes were regulated independently by the two genes, with Tbx1 primarily regulating vessel numbers and Vegfr3 vessel morphology. Consistent with this observation, Tbx1Cre -activated expression of a Vegfr3 transgene rescued partially the cardiac lymphatic abnormalities in compound heterozygotes. Through time-controlled genetic experiments, we show that Tbx1 is activated and required in cardiac lymphatic endothelial cell (LEC) progenitors between E10.5 and E11.5. Furthermore, we found that it is also required later in development for the growth of the cardiac lymphatics. Finally, our study revealed a differential sensitivity between ventral and dorsal cardiac lymphatics to the effects of altered Tbx1 and Vegfr3 gene dosage, and we show that this likely results from an earlier requirement for Tbx1 in ventral cardiac LEC progenitors.

Pharyngeal epithelial deletion of Tbx1 causes caudal pharyngeal arch defect but not cardiac conotruncal anomaly.

TBX1 is a major disease gene of 22q11.2 deletion syndrome (22q11.2DS). It is expressed in all three germ layers of pharyngeal apparatus to control the complicated morphogenesis. The haploinsufficiency of pharyngeal endodermal or ectodermal, but not mesodermal Tbx1 causes aortic arch patterning defect. However, the mesodermal deletion of Tbx1 causes much severer pharyngeal and cardiovascular defect than either pharyngeal endodermal or ectodermal Tbx1 deletion does. It is inconsistent with the conventional thought that the invagination of pharyngeal epithelia drives pharyngeal segmentation. Therefore, we asked whether pharyngeal ectodermal and ectodermal Tbx1 can compensate the loss of each other. Here we carefully characterized pharyngeal epithelia-specific Fgf15Cre and Fgf15HspCre lines and used them to perform pharyngeal epithelia-specific deletion. Our data showed that the percentage of E18.5 Fgf15Cre;Tbx1flox/+ embryos with aortic arch patterning defects was similar to that of E10.5 Fgf15Cre;Tbx1flox/+ embryos with the 4th pharyngeal arch artery (PAA) defect, indicating that there is no significant recovery from the initial PAA defect, in contrast to germ line haploinsufficiency. Fgf15Cre;Tbx1flox/flox embryos had hypoplastic caudal pharyngeal arch and defective derivatives, but cardiac OFT development was not affected. The phenotypic spectrum of simultaneous Tbx1 deletion in both pharyngeal ectoderm and endoderm is strikingly similar to what presents with single pharyngeal endoderm or ectoderm-specific deletion of Tbx1. The absence of synergistic effect indicates intimate topographic interactions among pharyngeal endoderm and ectoderm, through which deletion of a gene in one tissue may disrupt the development of adjacent tissues and thereby lead to similar morphological phenotypes in either tissue-specific deletion.

TBX1 and Basal Cell Carcinoma: Expression and Interactions with Gli2 and Dvl2 Signaling.

Early events of basal cell carcinoma (BCC) tumorigenesis are triggered by inappropriate activation of SHH signaling, via the loss of Patched1 (Ptch1) or by activating mutations of Smoothened (Smo). TBX1 is a key regulator of pharyngeal development, mainly through expression in multipotent progenitor cells of the cardiopharyngeal lineage. This transcription factor is connected to several major signaling systems, such as FGF, WNT, and SHH, and it has been linked to cell proliferation and to the regulation of cell shape and cell dynamics. Here, we show that TBX1 was expressed in all of the 51 BCC samples that we have tested, while in healthy human skin it was only expressed in the hair follicle. Signal intensity and distribution was heterogeneous among tumor samples. Experiments performed on a cellular model of mouse BCC showed that Tbx1 is downstream to GLI2, a factor in the SHH signaling, and that, in turn, it regulates the expression of Dvl2, which encodes an adaptor protein that is necessary for the transduction of WNT signaling. Consistently, Tbx1 depletion in the cellular model significantly reduced cell migration. These results suggest that TBX1 is part of a core transcription network that promotes BCC tumorigenesis.

Vitamin B12 ameliorates the phenotype of a mouse model of DiGeorge syndrome.

Abstract: Pathological conditions caused by reduced dosage of a gene, such as gene haploinsufficiency, can potentially be reverted by enhancing the expression of the functional allele. In practice, low specificity of therapeutic agents, or their toxicity reduces their clinical applicability. Here, we have used a high throughput screening (HTS) approach to identify molecules capable of increasing the expression of the gene Tbx1, which is involved in one of the most common gene haploinsufficiency syndromes, the 22q11.2 deletion syndrome. Surprisingly, we found that one of the two compounds identified by the HTS is the vitamin B12. Validation in a mouse model demonstrated that vitamin B12 treatment enhances Tbx1 gene expression and partially rescues the haploinsufficiency phenotype. These results lay the basis for preclinical and clinical studies to establish the effectiveness of this drug in the human syndrome.

A defect in early myogenesis causes Otitis media in two mouse models of 22q11.2 Deletion Syndrome.

Otitis media (OM), the inflammation of the middle ear, is the most common disease and cause for surgery in infants worldwide. Chronic Otitis media with effusion (OME) often leads to conductive hearing loss and is a common feature of a number of craniofacial syndromes, such as 22q11.2 Deletion Syndrome (22q11.2DS). OM is more common in children because the more horizontal position of the Eustachian tube (ET) in infants limits or delays clearance of middle ear effusions. Some mouse models with OM have shown alterations in the morphology and angle of the ET. Here, we present a novel mechanism in which OM is caused not by a defect in the ET itself but in the muscles that control its function. Our results show that in two mouse models of 22q11.2DS (Df1/+ and Tbx1(+/-)) presenting with bi- or unilateral OME, the fourth pharyngeal arch-derived levator veli palatini muscles were hypoplastic, which was associated with an earlier altered pattern of MyoD expression. Importantly, in mice with unilateral OME, the side with the inflammation was associated with significantly smaller muscles than the contralateral unaffected ear. Functional tests examining ET patency confirmed a reduced clearing ability in the heterozygous mice. Our findings are also of clinical relevance as targeting hypoplastic muscles might present a novel preventative measure for reducing the high rates of OM in 22q11.2DS patients.

TBX1 protein interactions and microRNA-96-5p regulation controls cell proliferation during craniofacial and dental development: implications for 22q11.2 deletion syndrome.

T-box transcription factor TBX1 is the major candidate gene for 22q11.2 deletion syndrome (22q11.2DS, DiGeorge syndrome/Velo-cardio-facial syndrome), whose phenotypes include craniofacial malformations such as dental defects and cleft palate. In this study, Tbx1 was conditionally deleted or over-expressed in the oral and dental epithelium to establish its role in odontogenesis and craniofacial developmental. Tbx1 lineage tracing experiments demonstrated a specific region of Tbx1-positive cells in the labial cervical loop (LaCL, stem cell niche). We found that Tbx1 conditional knockout (Tbx1(cKO)) mice featured microdontia, which coincides with decreased stem cell proliferation in the LaCL of Tbx1(cKO) mice. In contrast, Tbx1 over-expression increased dental epithelial progenitor cells in the LaCL. Furthermore, microRNA-96 (miR-96) repressed Tbx1 expression and Tbx1 repressed miR-96 expression, suggesting that miR-96 and Tbx1 work in a regulatory loop to maintain the correct levels of Tbx1. Cleft palate was observed in both conditional knockout and over-expression mice, consistent with the craniofacial/tooth defects associated with TBX1 deletion and the gene duplication that leads to 22q11.2DS. The biochemical analyses of TBX1 human mutations demonstrate functional differences in their transcriptional regulation of miR-96 and co-regulation of PITX2 activity. TBX1 interacts with PITX2 to negatively regulate PITX2 transcriptional activity and the TBX1 N-terminus is required for its repressive activity. Overall, our results indicate that Tbx1 regulates the proliferation of dental progenitor cells and craniofacial development through miR-96-5p and PITX2. Together, these data suggest a new molecular mechanism controlling pathogenesis of dental anomalies in human 22q11.2DS.

p53 Suppression partially rescues the mutant phenotype in mouse models of DiGeorge syndrome.

T-box 1 (Tbx1), a gene encoding a T-box transcription factor, is required for embryonic development in humans and mice. Half dosage of this gene in humans causes most of the features of the DiGeorge or Velocardiofacial syndrome phenotypes, including aortic arch and cardiac outflow tract abnormalities. Here we found a strong genetic interaction between Tbx1 and transformation related protein 53 (Trp53). Indeed, genetic ablation of Trp53, or pharmacological inhibition of its protein product p53, rescues significantly the cardiovascular defects of Tbx1 heterozygous and hypomorphic mutants. We found that the Tbx1 and p53 proteins do not interact directly but both occupy a genetic element of Gbx2, which is required for aortic arch and cardiac outflow tract development, and is a known genetic interactor of Tbx1. We found that Gbx2 expression is down-regulated in Tbx1(+/-) embryos and is restored to normal levels in Tbx1(+/-);Trp53(+/-) embryos. In addition, we found that the genetic element that binds both Tbx1 and p53 is highly enriched in H3K27 trimethylation, and upon p53 suppression H3K27me3 levels are reduced, along with Ezh2 enrichment. This finding suggests that the rescue of Gbx2 expression in Tbx1(+/-);Trp53(+/-) embryos is due to reduction of repressive chromatin marks. Overall our data identify unexpected genetic interactions between Tbx1 and Trp53 and provide a proof of principle that developmental defects associated with reduced dosage of Tbx1 can be rescued pharmacologically.

Coronary stem development in wild-type and Tbx1 null mouse hearts.

BACKGROUND: Coronary artery (CA) stems connect the ventricular coronary tree with the aorta. Defects in proximal CA patterning are a cause of sudden cardiac death. In mice lacking Tbx1, common arterial trunk is associated with an abnormal trajectory of the proximal left CA. Here we investigate CA stem development in wild-type and Tbx1 null embryos. RESULTS: Genetic lineage tracing reveals that limited outgrowth of aortic endothelium contributes to proximal CA stems. Immunohistochemistry and fluorescent tracer injections identify a periarterial vascular plexus present at the onset of CA stem development. Transplantation experiments in avian embryos indicate that the periarterial plexus originates in mesenchyme distal to the outflow tract. Tbx1 is required for the patterning but not timing of CA stem development and a Tbx1 reporter allele is expressed in myocardium adjacent to the left but not right CA stem. This expression domain is maintained in Sema3c(-/-) hearts with a common arterial trunk and leftward positioned CA. Ectopic myocardial differentiation is observed on the left side of the Tbx1(-/-) common arterial trunk. CONCLUSIONS: A periarterial plexus bridges limited outgrowth of the aortic endothelium with the ventricular plexus during CA stem development. Molecular differences associated with left and right CA stems provide new insights into the etiology of CA patterning defects.

Transcriptional control in cardiac progenitors: Tbx1 interacts with the BAF chromatin remodeling complex and regulates Wnt5a.

Mutations of the Wnt5a gene, encoding a ligand of the non-canonical Wnt pathway, and the Ror2 gene, encoding its receptor, have been found in patients with cardiac outflow tract defects. We found that Wnt5a is expressed in the second heart field (SHF), a population of cardiac progenitor cells destined to populate the cardiac outflow tract and the right ventricle. Because of cardiac phenotype similarities between Wnt5a and Tbx1 mutant mice, we tested potential interactions between the two genes. We found a strong genetic interaction in vivo and determined that the loss of both genes caused severe hypoplasia of SHF-dependent segments of the heart. We demonstrated that Wnt5a is a transcriptional target of Tbx1 and explored the mechanisms of gene regulation. Tbx1 occupies T-box binding elements within the Wnt5a gene and interacts with the Baf60a/Smarcd1 subunit of a chromatin remodeling complex. It also interacts with the Setd7 histone H3K4 monomethyltransferase. Tbx1 enhances Baf60a occupation at the Wnt5a gene and enhances its H3K4 monomethylation status. Finally, we show that Baf60a is required for Tbx1-driven regulation of target genes. These data suggest a model in which Tbx1 interacts with, and probably recruits a specific subunit of, the BAF complex as well as histone methylases to activate or enhance transcription. We speculate that this may be a general mechanism of T-box function and that Baf60a is a key component of the transcriptional control in cardiac progenitors.

Tbx1 is a negative modulator of Mef2c.

The developmental role of the T-box transcription factor Tbx1 is exquisitely dosage-sensitive. In this study, we performed a microarray-based transcriptome analysis of E9.5 embryo tissues across a previously generated Tbx1 mouse allelic series. This analysis identified several genes whose expression was affected by Tbx1 dosage. Interestingly, we found that the expression of the gene encoding the cardiogenic transcription factor Mef2c was negatively correlated to Tbx1 dosage. In vivo data revealed Mef2c up-regulation in the second heart field (SHF) of Tbx1 null mutant embryos compared with wild-type littermates at E9.5. Conversely, Mef2c expression was decreased in the SHF and in somites of Tbx1 gain-of-function mutants. These results are consistent with the described role of Tbx1 in suppressing cardiac progenitor cell differentiation and indicate also a negative effect of Tbx1 on Mef2c during skeletal muscle differentiation. We show that Tbx1 occupies conserved regulatory regions of the Mef2c locus, suggesting a direct effect on Mef2c transcription. However, we also show that Tbx1 interferes with the Gata4→ Mef2c regulatory pathway. Overall, our study uncovered a target of Tbx1 with critical developmental roles, so highlighting the power of the dosage gradient approach that we used.

Endothelial gene regulatory elements associated with cardiopharyngeal lineage differentiation.

Endothelial cells (EC) differentiate from multiple sources, including the cardiopharyngeal mesoderm, which gives rise also to cardiac and branchiomeric muscles. The enhancers activated during endothelial differentiation within the cardiopharyngeal mesoderm are not completely known. Here, we use a cardiogenic mesoderm differentiation model that activates an endothelial transcription program to identify endothelial regulatory elements activated in early cardiogenic mesoderm. Integrating chromatin remodeling and gene expression data with available single-cell RNA-seq data from mouse embryos, we identify 101 putative regulatory elements of EC genes. We then apply a machine-learning strategy, trained on validated enhancers, to predict enhancers. Using this computational assay, we determine that 50% of these sequences are likely enhancers, some of which are already reported. We also identify a smaller set of regulatory elements of well-known EC genes and validate them using genetic and epigenetic perturbation. Finally, we integrate multiple data sources and computational tools to search for transcriptional factor binding motifs. In conclusion, we show EC regulatory sequences with a high likelihood to be enhancers, and we validate a subset of them using computational and cell culture models. Motif analyses show that the core EC transcription factors GATA/ETS/FOS is a likely driver of EC regulation in cardiopharyngeal mesoderm.

Transcriptional and epigenetic characterization of a new in vitro platform to model the formation of human pharyngeal endoderm.

BACKGROUND: The Pharyngeal Endoderm (PE) is an extremely relevant developmental tissue, serving as the progenitor for the esophagus, parathyroids, thyroids, lungs, and thymus. While several studies have highlighted the importance of PE cells, a detailed transcriptional and epigenetic characterization of this important developmental stage is still missing, especially in humans, due to technical and ethical constraints pertaining to its early formation. RESULTS: Here we fill this knowledge gap by developing an in vitro protocol for the derivation of PE-like cells from human Embryonic Stem Cells (hESCs) and by providing an integrated multi-omics characterization. Our PE-like cells robustly express PE markers and are transcriptionally homogenous and similar to in vivo mouse PE cells. In addition, we define their epigenetic landscape and dynamic changes in response to Retinoic Acid by combining ATAC-Seq and ChIP-Seq of histone modifications. The integration of multiple high-throughput datasets leads to the identification of new putative regulatory regions and to the inference of a Retinoic Acid-centered transcription factor network orchestrating the development of PE-like cells. CONCLUSIONS: By combining hESCs differentiation with computational genomics, our work reveals the epigenetic dynamics that occur during human PE differentiation, providing a solid resource and foundation for research focused on the development of PE derivatives and the modeling of their developmental defects in genetic syndromes.

Conditional and constitutive expression of a Tbx1-GFP fusion protein in mice.

BACKGROUND: Velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS) is caused by a 1.5-3 Mb microdeletion of chromosome 22q11.2, frequently referred to as 22q11.2 deletion syndrome (22q11DS). This region includes TBX1, a T-box transcription factor gene that contributes to the etiology of 22q11DS. The requirement for TBX1 in mammalian development is dosage-sensitive, such that loss-of-function (LOF) and gain-of-function (GOF) of TBX1 in both mice and humans results in disease relevant congenital malformations. RESULTS: To further gain insight into the role of Tbx1 in development, we have targeted the Rosa26 locus to generate a new GOF mouse model in which a Tbx1-GFP fusion protein is expressed conditionally using the Cre/LoxP system. Tbx1-GFP expression is driven by the endogenous Rosa26 promoter resulting in ectopic and persistent expression. Tbx1 is pivotal for proper ear and heart development; ectopic activation of Tbx1-GFP in the otic vesicle by Pax2-Cre and Foxg1-Cre represses neurogenesis and produces morphological defects of the inner ear. Overexpression of a single copy of Tbx1-GFP using Tbx1Cre/+ was viable, while overexpression of both copies resulted in neonatal lethality with cardiac outflow tract defects. We have partially rescued inner ear and heart anomalies in Tbx1Cre/- null embryos by expression of Tbx1-GFP. CONCLUSIONS: We have generated a new mouse model to conditionally overexpress a GFP-tagged Tbx1 protein in vivo. This provides a useful tool to investigate in vivo direct downstream targets and protein binding partners of Tbx1.

A phenotypic rescue approach identifies lineage regionalization defects in a mouse model of DiGeorge syndrome.

TBX1 is a key regulator of pharyngeal apparatus (PhAp) development. Vitamin B12 (vB12) treatment partially rescues aortic arch patterning defects of Tbx1+/- embryos. Here, we show that it also improves cardiac outflow tract septation and branchiomeric muscle anomalies of Tbx1 hypomorphic mutants. At the molecular level, in vivo vB12 treatment enabled us to identify genes that were dysregulated by Tbx1 haploinsufficiency and rescued by treatment. We found that SNAI2, also known as SLUG, encoded by the rescued gene Snai2, identified a population of mesodermal cells that was partially overlapping with, but distinct from, ISL1+ and TBX1+ populations. In addition, SNAI2+ cells were mislocalized and had a greater tendency to aggregate in Tbx1+/- and Tbx1-/- embryos, and vB12 treatment restored cellular distribution. Adjacent neural crest-derived mesenchymal cells, which do not express TBX1, were also affected, showing enhanced segregation from cardiopharyngeal mesodermal cells. We propose that TBX1 regulates cell distribution in the core mesoderm and the arrangement of multiple lineages within the PhAp.