Reprogramming into Cancer Stem Cell Figure 3. Global expression analysis of iESCs and iCSCs. (A) Gene expression microarray analysis among TIG1, iESCs, iCSCs, and iPSCs. Heat map (upper panel) shows differentially expressed genes of somatic and pluripotent genes. Scatter plots (lower panel) show comparison of global gene expression profiles. Yellow; genes in the Core module, Blue; genes in the PRC module, and Red; gene in the Myc module. (B) Expression analysis of pluripotent marker genes, somatic cell marker genes, and exogenous genes in TIG1, iPSCs, iESCs, 1st iCSCs, and 2nd iCSCs by RT-PCR analyses. (C) Expression of pluripotent marker proteins (Green) in iESCs, 1st iCSCs, and 2nd iCSCs by immunocytochemistry. Cell nuclei are counterstained blue with DAPI (Blue). Exo-, exogenous; Endo-, endogenous. doi:10.1371/journal.pone.0048699.g003 types through reprogramming more than we expected, and are necessary, but not sufficient, for defining iPSC identity. To reprogram somatic cells to iCSCs, up-regulation of the Myc module is a key event. Promoter regions bound by MYC are linked with histone H3 lysin4 trimethylation (H3K3me3), which is positively correlated with the formation of open chromatin, and gene activation as an epigenetic signature [9]. Furthermore, MYC interacts with histone acetyltransferases, which are associated with transcription activation complexes [14]. The Myc module facilitates cell metabolism by activating general genes. Genes in the Core module are also up-regulated through somatic repro- gramming to iCSCs. OCT4, SOX2, and NANOG, which are key Core module players, repress developmentally important home- odomain proteins through co-occupation of their target genes, while promote self-renewal and pluripotency by positive regulation of genes encoding components of key signaling pathways [15]. Taking these into consideration, the Core and Myc modules play a role in conferring the features of self-renewal and pluripotency on somatic nuclei, whereas continuously high activity of the PRC module in TIG1, iESCs, and iCSCs impedes the resetting of the somatic memory of some developmentally important home- odomain proteins (Fig. 4). The polycomb repressive complexes, PLOS ONE | www.plosone.org 5 November 2012 | Volume 7 | Issue 11 | e48699