The epigenetic basis for embryonic stem cell pluripotency.

As well as having the remarkable ability to differentiate into all of the cell types in the embryo, embryonic stem (ES) cells also have the capacity to divide and self-renew. Maintenance of pluripotency through repeated cell divisions indicates that the developmental plasticity of ES cells has a specific epigenetic basis. We propose that tightly localised regions of histone modification are formed in ES cells by binding of sequence-specific transcription factors at genes that are destined for expression at later stages of differentiation. These 'early transcription competence marks' would help to maintain pluripotency by preventing the spread of repressive chromatin modifications. We further propose that the presence of discrete histone modification marks in pluripotent cells facilitates the binding of lineage-specific and general transcription factors to the marked regions as ES cells commit to different fates. By helping to organise the precisely timed responses of genes to the signals that determine lineage choice, the gene-specific localised epigenetic marks would play a key role in the establishment of complex gene expression programmes in differentiating cells.