ATAC-seq of wild-type, Pbx1/Pbx2 double-knockout and pMsgn1-mut EpiSCs differentiated in vitro to pre-somitic mesoderm in the first 48 hours of differentiation.

In vertebrates, body axis elongation is fuelled by bipotent neuromesodermal progenitors (NMPs), which support the development of both spinal cord and paraxial mesoderm (PM). WNT signalling sustains both NMP expansion and PM differentiation, but the mechanism by which it distinguishes between these alternative fates is unknown. HOX transcription factors have been historically implicated in axial elongation, with their sequential activation playing a fundamental role in timing PM development. PBX1 and PBX2 are obligate anterior HOX cofactors, and therefore they represent prominent candidates for controlling the distinct response to individual HOX factors. In our work, we have demonstrated that PBX/HOX complexes establish a permissive chromatin landscape for de novo recruitment of the WNT-effector LEF1 on PM genes, including the master regulator Mesogenin1 (Msgn1). To assess the PBX-dependent changes in chromatin accessibility during PM differentiation, we performed ATAC-seq of wild-type (WT) and Pbx1/Pbx2 double-knockout (Pbx1/2-DKO) EpiSCs differentiated in vitro to pre-somitic mesoderm (PSM) at different time-points (EpiSCs, 6 hours, 12 hours, 24 hours, 36 hours, 48 hours). To assess the direct consequence of PBX/HOX binding on chromatin accessibility, we employed the CRISPR/Cas9 technology to generate lines carrying base-pair substitutions on the Msgn1 promoter (pMsgn1-mut) that abrogate the recruitment of PBX/HOX complexes, and we performed ATAC-seq of pMsgn1-mut EpiSCs differentiated in vitro to PSM at different time-points (12 hours, 24 hours, 36 hours, 48 hours).