Sequence logic at enhancers governs a dual mechanism of endodermal organ fate induction by FOXA pioneer factors

FOXA pioneer transcription factors (TFs) associate with primed enhancers in endodermal organ precursors. Using a human stem cell model of pancreas differentiation, we here discover that only a subset of pancreatic enhancers is FOXA-primed, whereas the majority is unprimed and engages FOXA upon lineage induction. Primed enhancers are enriched for signal-dependent TF motifs and harbor abundant and strong FOXA motifs. Unprimed enhancers harbor fewer, more degenerate FOXA motifs, and FOXA recruitment to unprimed but not primed enhancers requires pancreatic TFs. Strengthening FOXA motifs at an unprimed enhancer near NKX6.1 renders FOXA recruitment pancreatic TF-independent, induces priming, and broadens the NKX6.1 expression domain. We make analogous observations about FOXA binding during hepatic and lung development. Our findings suggest a dual role for FOXA in endodermal organ development: First, FOXA facilitate signal-dependent lineage initiation via enhancer priming, and second, FOXA enforce organ cell type-specific gene expression via indirect recruitment by lineage-specific TFs. Overall design: Anaysis of gene expression patterns by RNA-Seq, chromatin accessability by ATAC-Seq, and TF binding sites and histone marker distribution through ChIP-Seq performed within hESC-based models of endoderm differentiation. Effects of genetic deletion of FOXA1 and FOXA2 TFs on each aspect are examined in the context of pancreas induction.