High Histone Acetylation and Decreased Polycomb Repressive Complex 2 Member Levels Regulate Gene Specific Transcriptional Changes During Early Embryonic Stem Cell Differentiation Induced by Retinoic Acid

¹ Cancer Biology Program, University of Wisconsin - Madison, Madison, Wisconsin, USA
² Department of Pathology and Laboratory Medicine, University of Wisconsin - Madison, Madison, Wisconsin, USA

^* To whom correspondence should be addressed. E-mail: mkfritsch{at}wisc.edu.

	Abstract

Histone modifications play a crucial role during embryonic stem (ES) cell differentiation. During differentiation, binding of Polycomb repressive complex 2 (PRC2), which mediates lysine 27 histone H3 trimethylation (K27me3), is lost on developmental genes that are transcriptionally induced. We observed a global decrease in K27me3 in as little as 3 days after differentiation of mouse ES cells induced by retinoic acid (RA) treatment. The global levels of the histone K27 methyltransferase, EZH2, also decreased with RA treatment. A loss of EZH2 binding and K27me3 was observed locally on PRC2 target genes induced after 3 days of RA, including Nestin. In contrast, direct RA responsive genes that are rapidly induced, such as Hoxa1, showed a loss of EZH2 binding and K27me3 after only a few hours of RA treatment. Following differentiation induced by LIF withdrawal without RA, Hoxa1 is not transcriptionally activated. SiRNA mediated knockdown of EZH2 resulted in loss of K27me3 during LIF withdrawal but the Hoxa1 gene remained transcriptionally silent after loss of this repressive mark. Induction of histone hyperacetylation overrode the repressive K27me3 modification and resulted in Hoxa1 gene expression. Together, these data show that there are multiple temporal phases of de-repression of PRC2 target genes during ES cell differentiation and that other epigenetic marks (specifically increased acetylation of histones H3 and H4), in addition to derepression, are important for gene specific transcriptional activation. This report demonstrates the temporal interplay of various epigenetic changes in regulating gene expression during early ES cell differentiation.