ABSTRACT: Cell fate transitions involve integration of genomic information encoded by regulatory elements, such as enhancers, with the cellular environment. However, identification of the genomic sequences that control the earliest steps of human embryonic development represents a formidable challenge. Here we show that in human embryonic stem cells (hESCs) unique chromatin signatures identify two distinct classes of genomic elements, both of which are marked by the presence of chromatin regulators p300 and BRG1, and monomethylation of histone H3 at lysine 4 (H3K4me1). In addition, the elements of the first class are distinguished by the acetylation of histone H3 at lysine 27 (H3K27ac), overlap with previously characterized enhancers active in hESCs, and are generally located proximally to genes expressed in hESCs and in the epiblast. In contrast, the elements within the second class, which we termed “poised enhancers”, are distinguished by the absence of H3K27ac, enrichment of histone H3 lysine 27 trimethylation (H3K27me3) and are linked to genes inactive in hESCs and involved in orchestrating early steps in mammalian development, such as gastrulation, mesoderm formation and neurulation. Consistent with the poised identity, during differentiation of hESCs to neuroepithelium, a neuroectoderm-specific subset of these elements acquires a chromatin signature associated with active enhancers. Remarkably, when assayed in zebrafish embryos, human poised enhancer elements are able to direct cell type and stage specific expression patterns characteristic of their proximal developmental gene, even in the absence of sequence conservation in the fish genome. Our data demonstrate that enhancers are epigenetically pre-marked and suggest a heretofore unappreciated role of H3K27me3 at distal regulatory elements. Moreover, the unique chromatin signature associated with poised enhancers allowed us to uncover over 2,000 putative developmental regulatory sequences, thereby creating an invaluable resource for future studies and isolation of transient, rare cell populations representing early steps of human development.
