Nucleosome organizations in induced pluripotent stem cells reprogrammed from somatic cells belonging to three different germ layers
Nucleosome organization determines chromatin state, which subsequently controls genes expression or silencing. Nucleosome remodeling occurs during somatic cell reprogramming, but it remains undetermined to what degree the re-established nucleosome organization resembles between induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs). We generated genome-wide nucleosome maps in mouse ESCs and in iPSCs reprogrammed from somatic cells belonging to three different germ layers using a secondary reprogramming system. Pairwise comparisons showed that the nucleosome organizations in the iPSCs, regardless of the iPSCs¿ tissue of origin, were nearly identical to the ESCs, but distinct to mouse embryonic fibroblasts (MEF). There is a canonical nucleosome arrangement of -1, nucleosome depletion region, +1, +2, +3, et al. nucleosomes around the transcription start sites of active genes whereas only a nucleosome occupies at silent transcriptional units. Transcription factor binding sites possessed characteristic nucleosomal architecture such that their access was governed by the rotational and translational settings of the nucleosome. Interestingly, the tissue-specific genes were highly expressed only in the parental somatic cells of the corresponding iPS cell line before reprogramming, but had a similar expression level in the all resultant iPSCs and ESCs. The re-established nucleosome landscape during nuclear reprogramming provides a conserved setting for accessibility of DNA sequences in mouse pluripotent stem cells. No persistent residual expression program or nucleosome positioning of the parental somatic cells that reflected their tissue of origin were passed onto the resulting mouse iPSCs. Overall design: Gene expression profiles of 5 cell lines with or without biological replicates