Transcription factors and Tet1/2 enable Brd4-independent maintenance of stem cell identity

Histone acetylation and the acetyl-lysine reader Brd4 have recently been implicated in embryonic stem cell (ESC) proliferation and self-renewal. We found that na•ve pluripotent ESCs exhibit increased incorporation of glucose-derived carbons onto acetylated histones and elevations in H3K9ac and Brd4 recruitment at pluripotency gene promoters. Surprisingly, both H3K9 acetyltransferases, GCN5 and PCAF, and Brd4 recruitment were dispensable for proliferation, self-renewal and pluripotency of na•ve ESCs. Na•ve ESCs maintain gene expression by stabilizing Mediator at core pluripotency genes in a Brd4-independent manner. Brd4-independent proliferation could also be achieved in metastable ESCs by overexpression of pluripotency transcription factors. Under all conditions, self-renewal required the DNA methylcytosine oxidases Tet1 and Tet2. These data reveal that there is minimal dependence on Brd4 for self-renewal of na•ve ESCs. Instead, the relative levels of DNA methylation and transcription factor abundance determine the requirement for bromodomain recognition of histone acetylation to the maintenance of stem cell identity.