Asf1a resolves bivalent chromatin domains for the induction of lineage-specific genes during mouse embryonic stem cell differentiation

Bivalent chromatin domains containing both repressive H3K27me3 and active H3K4me3 modifications are barriers for the expression of lineage-specific genes in embryonic stem (ES) cells and must be resolved for the transcription induction of these genes during differentiation, a process that remains largely unknown. Here, we show that Asf1a, a histone chaperone involved in both nucleosome assembly and disassembly, regulates the resolution of bivalent domains and activation of lineage-specific genes during mouse ES cell differentiation. Deletion of Asf1a does not affect the silencing of pluripotent genes, but compromises the expression of lineage-specific genes during ES cell differentiation. Mechanistically, the Asf1a-histone interaction, but not the role of Asf1a in nucleosome assembly is required for gene transcription. Asf1a is recruited to several bivalent promoters, partially through association with transcription factors, and mediates nucleosome disassembly during differentiation. We suggest that Asf1a-mediated nucleosome disassembly provides a means for resolution of bivalent domain barriers for induction of lineage-specific genes during differentiation. To analyze how Asf1a regulates bivalent gene expression during ES differentiation, we performed RNA-seq, H3K4me3 and H3K27me3 ChIP-seq in WT and Asf1a KO mouse ES cells and differentiated neural precursors