The transcription factor TBXT is crucial for mesoderm differentiation across vertebrates and promotes development of the primitive streak. While TBXT promotes epithelial-mesenchymal transition (EMT) both during gastrulation and in cancer contexts, its precise role – particularly in humans – remains unclear. Here, Emily Bulger, Benoit Bruneau and colleagues use a highly tractable human induced pluripotent stem cell (hiPSC) 2D gastruloid model to show that TBXT dosage temporally regulates EMT without compromising initial mesodermal specification. First, they generated a hiPSC allelic series: TBXT-Wild Type, TBXT-heterozygous and TBXT knockout. In gastruloids, the authors find that primary germ layer specification can still take place under conditions of reduced (heterozygous) or absent (knockout) TBXT – consistent with previous studies in mice. Using single-nucleus sequencing and chromatin accessibility assays, they also show that TBXT dosage influences the expression of genes associated with EMT without affecting initial cell identity. Reduction or absence of TBXT hampers cell motility and delays the onset of EMT, partly by sustaining cell-cell adhesion proteins. Overall, this work reveals that activation of a mesodermal transcriptional profile can be uncoupled from EMT progression, providing new insights into how TBXT regulates early human gastrulation.
