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We show that PRC1 functions as a master regulator of mouse ESC genome architecture by organizing genes in three-dimensional interaction networks. The strongest spatial network is composed of the four Hox gene clusters and early developmental transcription factor genes, the majority of which contact poised enhancers. Removal of Polycomb repression leads to disruption of promoter-promoter contacts in the Hox gene network. In contrast, promoter-enhancer contacts are maintained in the absence of Polycomb repression, with accompanying widespread acquisition of active chromatin signatures at network enhancers and pronounced transcriptional upregulation of network genes. Thus, PRC1 physically constrains developmental transcription factor genes and their enhancers in a silenced but poised spatial network. We propose that the selective release of genes from this spatial network underlies cell fate specification during early embryonic development.

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pressed in 3T3 mouse fibroblast cells, PRC17 induced growth in low serum, loss of contact inhibition, and tumor formation in nude mice. The PRC17 gene product contains a GTPase-activating protein (GAP) catalytic core motif found in various Rab/Ypt GAPs, including RN-Tre. Similar to RN-Tre, we found that PRC17 protein interacts directly with Rab5 and stimulates its GTP hydrolysis. Point mutations that alter conserved amino acid residues within the PRC17 GAP domain abolished its transforming abilities, suggesting that GAP activity is essential for its oncogenic function. Whereas PRC17 is amplified in 15% of prostate cancers, it is highly overexpressed in approximately one-half of metastatic prostate tumors. The potent oncogenic activity of PRC17 is likely to influence the tumorigenic phenotype of these prostate cancers.