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.

Tbx1 represses Mef2c gene expression and is correlated with histone 3 deacetylation of the anterior heart field enhancer.

The TBX1 gene is haploinsufficient in 22q11.2 deletion syndrome (22q11.2DS), and genetic evidence from human patients and mouse models points to a major role of this gene in the pathogenesis of this syndrome. Tbx1 can activate and repress transcription, and previous work has shown that one of its functions is to negatively modulate cardiomyocyte differentiation. Tbx1 occupies the anterior heart field (AHF) enhancer of the Mef2c gene, which encodes a key cardiac differentiation transcription factor. Here, we show that increased dosage of Tbx1 correlates with downregulation of Mef2c expression and reduced acetylation of its AHF enhancer in cultured mouse myoblasts. Consistently, 22q11.2DS-derived and in vitro-differentiated human induced pluripotent stem cells (hiPSCs) expressed higher levels of MEF2C and showed increased AHF acetylation, compared with hiPSCs from a healthy donor. Most importantly, we show that in mouse embryos, loss of Tbx1 enhances the expression of the Mef2c-AHF-Cre transgene in a specific region of the splanchnic mesoderm, and in a dosage-dependent manner, providing an in vivo correlate of our cell culture data. These results indicate that Tbx1 regulates the Mef2c AHF enhancer by inducing histone deacetylation.