SBE6: a novel long-range enhancer involved in driving sonic hedgehog expression in neural progenitor cells.

The expression of genes with key roles in development is under very tight spatial and temporal control, mediated by enhancers. A classic example of this is the sonic hedgehog gene (Shh), which plays a pivotal role in the proliferation, differentiation and survival of neural progenitor cells both in vivo and in vitro. Shh expression in the brain is tightly controlled by several known enhancers that have been identified through genetic, genomic and functional assays. Using chromatin profiling during the differentiation of embryonic stem cells to neural progenitor cells, here we report the identification of a novel long-range enhancer for Shh-Shh-brain-enhancer-6 (SBE6)-that is located 100 kb upstream of Shh and that is required for the proper induction of Shh expression during this differentiation programme. This element is capable of driving expression in the vertebrate brain. Our study illustrates how a chromatin-focused approach, coupled to in vivo testing, can be used to identify new cell-type specific cis-regulatory elements, and points to yet further complexity in the control of Shh expression during embryonic brain development.

H4K16 acetylation marks active genes and enhancers of embryonic stem cells, but does not alter chromatin compaction.

Compared with histone H3, acetylation of H4 tails has not been well studied, especially in mammalian cells. Yet, H4K16 acetylation is of particular interest because of its ability to decompact nucleosomes in vitro and its involvement in dosage compensation in flies. Here we show that, surprisingly, loss of H4K16 acetylation does not alter higher-order chromatin compaction in vivo in mouse embryonic stem cells (ESCs). As well as peaks of acetylated H4K16 and KAT8 histone acetyltransferase at the transcription start sites of expressed genes, we report that acetylation of H4K16 is a new marker of active enhancers in ESCs and that some enhancers are marked by H3K4me1, KAT8, and H4K16ac, but not by acetylated H3K27 or EP300, suggesting that they are novel EP300 independent regulatory elements. Our data suggest a broad role for different histone acetylation marks and for different histone acetyltransferases in long-range gene regulation.

Intergenic Polycomb target sites are dynamically marked by non-coding transcription during lineage commitment.

Non-coding (nc) RNAs are involved both in recruitment of vertebrate Polycomb (PcG) proteins to chromatin, and in activation of PcG target genes. Here we investigate dynamic changes in the relationship between ncRNA transcription and recruitment of PcG proteins to chromatin during differentiation. Profiling of purified cell populations from different stages of a defined murine in vitro neural differentiation system shows that over 50% of regulated intergenic non-coding transcripts precisely correspond to PcG target sites. We designate these PcG recruiting elements as Transcribed Intergenic Polycomb (TIP) sites. The relationship between TIP transcription and PcG recruitment switches dynamically during differentiation between different states, in which transcription and PcG recruitment exclude each other, or in which both are present. Reporter assays show that transcribed TIP sites can repress a flanking gene. Knockdown experiments demonstrate that TIP ncRNAs are themselves required for repression of target genes both in cis and in trans. We propose that TIP transcription may ensure coordinated regulation of gene networks via dynamic switching and recruitment of PcG proteins both in cis and in trans during lineage commitment.