Association of BMI1 with Polycomb Bodies Is Dynamic and Requires PRC2/EZH2 and the Maintenance DNA Methyltransferase DNMT1

Abstract

Polycomb group (PcG) proteins are epigenetic chromatin modifiers involved in heritable gene repression. Two main PcG complexes have been characterized. Polycomb repressive complex 2 (PRC2) is thought to be involved in the initiation of gene silencing, whereas Polycomb repressive complex 1 (PRC1) is implicated in the stable maintenance of gene repression. Here, we investigate the kinetic properties of the binding of one of the PRC1 core components, BMI1, with PcG bodies. PcG bodies are unique nuclear structures located on regions of pericentric heterochromatin, found to be the site of accumulation of PcG complexes in different cell lines. We report the presence of at least two kinetically different pools of BMI1, a highly dynamic and a less dynamic fraction, which may reflect BMI1 pools with different binding capacities to these stable heterochromatin domains. Interestingly, PRC2 members EED and EZH2 appear to be essential for BMI1 recruitment to the PcG bodies. Furthermore, we demonstrate that the maintenance DNA methyltransferase DNMT1 is necessary for proper PcG body assembly independent of DNMT-associated histone deacetylase activity. Together, these results provide new insights in the mechanism for regulation of chromatin silencing by PcG proteins and suggest a highly regulated recruitment of PRC1 to chromatin.

Histone deacetylase inhibitors deplete enhancer of zeste 2 and associated polycomb repressive complex 2 proteins in human acute leukemia cells

“…Although not studied here, the binding to chromatin and gene silencing by PRC1 complex, which consists of >10 subunits, including BMI-1 and the HPC proteins, requires PRC2 complex -mediated trimethylation of H3K27 (6,42,43). Notably, PRC1, PRC2, and trimethylated H3K27 cooccupy >1,000 silenced genes, but 40 genes are derepressed in human embryonic fibroblasts depleted of PRC2 components and BMI-1 (44).…”

“…Of the two complexes, PRC2 is smaller and contains the PcG proteins enhancer of zeste 2 (EZH2), embryonic ectoderm development (EED) that serves to recruit to the chromatin the PRC1 complex, suppressor of zeste 12 (SUZ12), Yin Yang 1, and the histone-binding protein RbAp46 (2 -5). Recently, within the PRC2, EZH2 was shown to interact with and modulate the DNA methyltransferases DNMT1, DNMT3a, and DNMT3b, which affect their binding to the EZH2 target gene promoters (6,7). Thus, EZH2 is required for DNA methylation of EZH2-targeted promoters.…”

“…Thus, EZH2 is required for DNA methylation of EZH2-targeted promoters. EZH2 may serve as a recruitment platform for DNA methyltransferases, thereby mechanistically linking the two epigenetic silencing systems (6,7). EZH2 is crucial during embryonic development, as depletion of EZH2 from developing mouse embryos resulted in severe growth retardation and early embryonic lethality (8,9).…”

The polycomb group proteins, BMI-1 and EZH2, are tumour-associated antigens

“…This strong reactivity, together with the fact that the PcG proteins are known to be vitally involved in transcriptional control and carcinogenesis in humans, and may therefore be less susceptible to the development of 'immune escape' variants, made BMI-1 an attractive candidate for validation as a target for cancer immunotherapy. Recent studies showing that another polycomb protein, EZH2, which is essential for BMI-1 recruitment to the polycomb body in the nucleus (Hernandez-Munoz et al, 2005), is also highly expressed in many cancers and led us to extend our studies to include both BMI-1 and EZH2. As expression of these two proteins is often mutually exclusive, a sensible approach may be an attempt to target both.…”

“…They are important transcriptional repressors that regulate gene activity by the formation of multiprotein complexes, called PcG bodies, and the organisation of chromatin into an inaccessible structure that cannot bind transcription factors (Shao et al, 1999). Interestingly, EZH2 appears to be one of the PcG proteins essential for BMI-1 recruitment to the PcG bodies (Hernandez-Munoz et al, 2005). PcG gene expression is tightly regulated and differs between different tissues and cell types.…”