A Cbx8-containing Polycomb complex facilitates gene activation during ES cell differentiation [Agilent Array]

Polycomb proteins play an essential role in maintaining the repression of developmental genes in self-renewing embryonic stem cells. The exact mechanism allowing the derepression of polycomb target genes during cell differentiation remains unclear. Here, we show that several differentiation genes transiently recruit a Cbx8-containing Polycomb repressive complex (PRC) 1 during their early activation. Depletion of Cbx8 partially impairs the transcriptional activation of these genes. This correlates with a reduction in low but detectable levels of histone H3 lysine 27 acetylation. Prolonged gene activation results in eviction of PRC1 despite persisting H3K27me3. The composition of PRC1 is highly modular and changes when ES cells commit to differentiation. We further demonstrate that the exchange of Cbx7 for Cbx8 is required for the effective activation of differentiation genes. Taken together our results establish a function for a Cbx8-containing complex in facilitating the transition from a Polycomb-repressed chromatin state to an active state. As this affects several key regulatory differentiation genes this mechanism is likely to contribute to the robust execution of differentiation programs. Four biological replicates were used for each condition (untreated and after 72h of retinoic acid stimulation) and samples were prepared and hybridized to SurePrint G3 Gene Expression Microarrays (Agilent technologies) following the supplier's instructions. Analyses were essentially performed as described (Uribesalgo et al. 2011) selecting differentially expressed probes with a FDR of 0.05 and fold change of > 1.5.