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Haematopoietic cells arise from endothelial cells within the dorsal aorta of the embryo via a process called the endothelial-haematopoietic transition (EHT). This process crucially depends on the transcription factor RUNX1 which rapidly activates the expression of genes essential for haematopoietic development. Using an inducible version of RUNX1 in a mouse embryonic stem cell differentiation model we showed that prior to the EHT, haematopoietic genes are primed by the binding of the transcription factor FLI1. Once expressed, RUNX1 relocates FLI1 towards its binding sites. However, the nature of the transcription factor assemblies recruited by RUNX1 to reshape the chromatin landscape and initiate mRNA synthesis are unclear. Here, we performed genome-wide analyses of RUNX1-dependent binding of factors associated with transcription elongation to address this question. We demonstrate that RUNX1 induction moves FLI1 from distal ETS/GATA sites to RUNX1/ETS sites and recruits the basal transcription factors CDK9, BRD4, the Mediator complex and the looping factor LDB1. Our study explains how the expression of a single transcription factor can drive rapid and replication independent transitions in cellular shape which are widely observed in development and disease.

The differentiation from multipotent hematopoietic stem cells (HSC) to mature and functional blood cells requires the finely tuned regulation of gene expression at each stage of development. Specific transcription factors play a key role in this process as they modulate the expression of their target genes in an exquisitely lineage-specific manner. A large number of important transcriptional regulators have been identified which establish and maintain specific gene expression patterns during hematopoietic development. Hematopoiesis is therefore a paradigm for investigating how transcription factors function in mammalian cells, thanks also to the evolution of genome-wide and the next-generation sequencing technologies. In this review, we focus on the current knowledge of the biological and functional properties of the hematopoietic master regulator RUNX1 (also known as AML1, CBFA2, PEBP2aB) transcription factor and its main downstream target PU.1. We will outline their relationship in determining the fate of the myeloid lineage during normal stem cell development and under conditions when hematopoietic development is subverted by leukemic transformation.