Cell Reprogramming Requires Silencing of a Core Subset of Polycomb Targets

Abstract

Transcription factor (TF)–induced reprogramming of somatic cells into induced pluripotent stem cells (iPSC) is associated with genome-wide changes in chromatin modifications. Polycomb-mediated histone H3 lysine-27 trimethylation (H3K27me3) has been proposed as a defining mark that distinguishes the somatic from the iPSC epigenome. Here, we dissected the functional role of H3K27me3 in TF–induced reprogramming through the inactivation of the H3K27 methylase EZH2 at the onset of reprogramming. Our results demonstrate that surprisingly the establishment of functional iPSC proceeds despite global loss of H3K27me3. iPSC lacking EZH2 efficiently silenced the somatic transcriptome and differentiated into tissues derived from the three germ layers. Remarkably, the genome-wide analysis of H3K27me3 in Ezh2 mutant iPSC cells revealed the retention of this mark on a highly selected group of Polycomb targets enriched for developmental regulators controlling the expression of lineage specific genes. Erasure of H3K27me3 from these targets led to a striking impairment in TF–induced reprogramming. These results indicate that PRC2-mediated H3K27 trimethylation is required on a highly selective core of Polycomb targets whose repression enables TF–dependent cell reprogramming.

The Polycomb Protein Ezh2 Impacts on Induced Pluripotent Stem Cell Generation

“…The results by Fragola et al are also in contrast to the shRNA knockdown studies reported here on primary iPS cells and on a doxycylin-inducible secondary iPS cell system [17]. Additionally, it also contrasts findings by the same authors on shRNA-induced knockdown of the PRC2 component EED, which reduced iPS cell generation by over 80% [24]. Thus, the result on Ezh2-deficient iPS cells might be rather methodically, in that Ezh2 inactivation could have occurred after reprogramming.…”

“…In a recent paper, Fragola et al [24] generated iPS cells from MEF with a conditional Ezh2 knockout allele for the deletion of the catalytic Ezh2 SET domain. A cellpermeable TAT-Cre recombinase was used to inactivate Ezh2.…”

“…iPS cell generation is regulated by a series of complex processes that are increasingly being better understood [14][15][16][17][18][19]. Extensive epigenetic reorganization occurs during reprogramming, and recent studies indicate that activities of epigenetic modifiers play an important function in reprogramming, and thus, the role of PcG proteins in iPS cell generation is now beginning to be studied in detail [15,[19][20][21][22][23][24].…”

H3K27me3 may be associated with Oct4 and Sox2 in mouse preimplantation embryos

“…In murine Oct4(+)Sca-1(+)Lin(-)CD45(-) very small embryonic-like stem cells, similarly to other pluripotent stem cells, the pluripotent states are maintained through an Ezh2-dependent bivalent domain-mediated epigenetic mechanism (Shin et al, 2012). At the onset of transcription factor-induced reprogramming, iPS cells lacking Ezh2 efficiently silenced the somatic transcriptome and differentiated into tissues derived from the 3 germ layers (Fragola et al, 2013).…”

823: 3-deazaneplanocin A (DZNep), an inhibitor of histone methyltransferase EZH2, induces apoptosis and reduces cell migration in chondrosarcoma cells

“…In particular, EZH2, the catalytic subunit of Polycomb Repressive Complex 2 (PRC2), induces histone methyltransferase activity primarily by trimethylating histone H3 at lysine 27 (H3K27me3), hence mediating gene silencing. PcGs are crucial in the chromatin control of stem cell self-renewal and differentiation [1][2][3][4][5][6][7]. They also play a crucial role in malignant progression and are implicated in cancer metastasis [8].…”