Polycomb-mediated genome architecture enables long-range spreading of H3K27 methylation [HiChIP]

Polycomb-group proteins play critical roles in gene silencing through the deposition of histone H3 lysine 27 trimethylation (H3K27me3) and chromatin compaction. This process is essential for embryonic stem cells (ESCs) pluripotency, differentiation, and development. Polycomb repressive complex 2 (PRC2) can both read and write H3K27me3, enabling processive spread of H3K27me3 on linear genome and epigenetic memory. Long-range Polycomb-associated DNA contacts have also been described, but their regulation and role in gene silencing remains unclear. Here, we develop H3K27me3 HiChIP and apply optical reconstruction of chromatin architecture to reveal long-range Polycomb-associated DNA loops that span tens to hundreds of megabases and across multiple topological associated domains in mouse ESCs and human induced pluripotent stem cells. H3K27me3 loop anchors are enriched for Polycomb nucleation points and coincide with key developmental genes, such as Hmx1, Wnt6 and Hoxa. Genetic deletion of H3K27me3 loop anchors causes spatially proximal partner chromosomal loci to break apart, and alters H3K27me3 deposition, both locally and megabases away on the same chromosome. A selective EZH2 mutant deficient in RNA binding but intact H3K27me3 enzymatic activity leads to global alteration in H3K27me3 loops, decreased spatial proximity, and failure to spread Polycomb from PRC2 nucleation points to partner loci at developmental genes. Together, these results suggest PRC2 acts as a “genomic wormhole”, using RNA binding to enhance long range chromosome folding and H3K27me3 spreading. Developmental gene loci have novel roles in Polycomb spreading, emerging as key architectural elements of the epigenome. H3K27ac and H3K27me3 HiChP of mouse embryonic stem cells (mESCs), wild-type and RNA-binding deficient EZH2 mutant human induced pluripotent stem cells (iPSCs).