Systematic Dissection of Sequence Features Affecting the Binding Specificity of a Pioneer Factor Reveals Binding Synergy Between FOXA1 and AP-1 [ChIP-seq]

Pioneer factors (PFs) are a subset of transcription factors (TFs) that can bind to nucleosomal DNA and invade closed chromatin. Despite that nucleosomes do not present a strong barrier to PF binding, PF can only bind to a small fraction of motifs in the genome. The underlying mechanism of such binding selectivity is not well understood. Here, we design a high-throughput assay named Chromatin Immunoprecipitation over Integrated Synthetic Oligonucleotides (ChIP-ISO) to systematically dissect the sequence features affecting the binding specificity of a classic PF, FOXA1, in A549 human lung carcinoma cells. This method involves integrating thousands of synthetic sequences containing FOXA1 motifs into a fixed genomic locus, followed by FOXA1 chromatin immunoprecipitation (ChIP) and amplicon sequencing. We find that 1) FOXA1 binding is affected by motif strength and co-binding TFs AP-1 and CEBPB, with AP-1 playing a major role in promoting FOXA1 binding, 2) FOXA1 binding in vivo and in vitro are poorly correlated, and FOXA1 and AP-1 show binding cooperativity in vitro, 3) FoxA1’s binding specificity is mostly determined by the local sequences, whereas chromatin context, including heterochromatin marks, only plays a minor role, and 4) AP-1 is at least partially responsible for differential binding of FOXA1 in different cell types. Finally, neural network analysis shows that AP-1 and CEBPB motifs are predictive of FOXA1 ChIP-seq peaks in A549, but not in some other cell types. In summary, combining ChIP-ISO with in vitro and in silico analyses, our study provides insights into the genetic rules underlying PF binding specificity and reveals a mechanism for its regulation during cell differentiation. Chromatin immunoprecipitation with sequencing (ChIP-seq) for TF FOXA1 and histone modifications H3K9me3 and H3K27me3 in WT A549 human lung carcinoma cells, as well as for TFs FOXA1 and FOSL2 in A549 cells induced with doxycycline to overexpress dominant-negative A-Fos.