GSE27827
Genomic prevalence of heterochromatic H3K9me2 and transcription do not discriminate pluripotent from terminally differentiated cells (ChIP-seq)
Cellular differentiation entails reprogramming of the transcriptome from a pluripotent to a unipotent fate. This process was suggested to coincide with a global increase of repressive heterochromatin, which results in a reduction of transcriptional plasticity and potential. Here we report the dynamics of the transcriptome and an abundant heterochromatic histone modification, dimethylation of histone H3 at lysine 9 (H3K9me2), during neuronal differentiation of embryonic stem cells. In contrast to the prevailing model, we find H3K9me2 to occupy over 50% of chromosomal regions already in stem cells. Marked are most genomic regions that are devoid of transcription and a subgroup of histone modifications. Importantly, no global increase occurs during differentiation, but discrete local changes of H3K9me2 particularly at genic regions can be detected. Mirroring the cell fate change many genes show altered expression upon differentiation. Quantitative sequencing of transcripts reveals however that the total number of active genes is equal between stem cells and several tested differentiated cell types. Together, these findings reveal high prevalence of a heterochromatic mark in stem cells and challenge the model of low abundance of epigenetic repression and resulting global transcription in stem cells.  Instead cellular differentiation entails local rather than global changes in epigenetic repression and transcriptional activity. Overall design: Our dataset comprises of 2 ChIP-seq samples using chromatin from embryonic stem (ES) cells and derived terminal neurons (TN)  which was immunoprecipitated, using antibodies against H3K27me3 and H3K4me2. For the same cells we generated H3K9me2 ChIP-chip samples in biological replicates. For ES cells, neurons and mouse embryonic fibroblasts (MEFs) we generated biological replicates of RNA-seq data.