TBX1, a T-box transcription factor, is essential for pharyngeal apparatus development and marks cardiopharyngeal mesoderm (CPM) in various species. However, in mammals, we have an incomplete knowledge of the molecular pathways driving CPM diversification and of the role of TBX1 in this context. Using CPM-relevant in vitro differentiation of wild-type and Tbx1−/− mouse embryonic stem cells, we performed simultaneous single-nucleus RNA-seq and ATAC-seq at two stages, validated findings in embryos, and found that TBX1 loss affects gene expression and chromatin remodeling in a cell subpopulationspecific manner. TBX1 regulates chromatin accessibility and gene expression of distinct and evolutionarily conserved transcriptional modules for branchiomeric and cardiac development, and for tissue morphogenesis. Computational analyses predicted a feed-forward regulatory relationship between TBX1 and SIX factors. Notably, selected Tbx1 mutant CPM cell populations showed an altered differentiation trajectory, exhibiting activation of a mesothelial-like transcriptional program. We also observed cell death later in development. Thus, TBX1 is crucial for maintaining CPM transcriptional identity.

: 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.

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.