Stem cells primed for action: polycomb repressive complexes restrain the expression of lineage-specific regulators in embryonic stem cells.

Stem cells are characterised by a capacity to self renew and generate progenycapable of differentiating along several defined lineage paths. Embryonic Stem (ES) cellsare derived from the inner cell mass (ICM) of early-stage embryos and can contribute toall tissues of the developing embryo. Discovering how ES cell pluripotency and lineageinduction is achieved is important for understanding normal development and forsuccessfully applying stem cell-based therapies. A series of recent studies have shownthat the chromatin profile of ES cells is unusual and have revealed a critical role for thePolycomb Repressive Complexes (PRCs) in maintaining pluripotency. In human andmouse ES cells many genes that encode transcription factors that are required for lineagespecification bind PRC2 and carry bivalent (or opposing) histone signatures, beingenriched for conventional indicators of active chromatin such as acetylated H3K9 andmethylated H3K4, while lying within domains of repressive trimethylated H3K27.Mutant ES cell...