Non-core Subunits of the PRC2 Complex Are Collectively Required for Its Target-Site Specificity.

The Polycomb repressive complex 2 (PRC2) catalyzes H3K27 methylation across the genome, which impacts transcriptional regulation and is critical for establishment of cell identity. Because of its essential function during development and in cancer, understanding the delineation of genome-wide H3K27 methylation patterns has been the focus of intense investigation. PRC2 methylation activity is abundant and dispersed throughout the genome, but the highest activity is specifically directed to a subset of target sites that are stably occupied by the complex and highly enriched for H3K27me3. Here, we show, by systematically knocking out single and multiple non-core subunits of the PRC2 complex in mouse embryonic stem cells, that they each contribute to directing PRC2 activity to target sites. Furthermore, combined knockout of six non-core subunits reveals that, while dispensable for global H3K27 methylation levels, the non-core PRC2 subunits are collectively required for focusing H3K27me3 activity to specific sites in the genome.

Molecular Mechanisms Directing PRC2 Recruitment and H3K27 Methylation.

The polycomb repressive complex 2 (PRC2) is a chromatin-associated methyltransferase catalyzing mono-, di-, and trimethylation of lysine 27 on histone H3 (H3K27). This activity is required for normal organismal development and maintenance of gene expression patterns to uphold cell identity. PRC2 function is often deregulated in disease and is a promising candidate for therapeutic targeting in cancer. In this review, we discuss the molecular mechanisms proposed to take part in modulating PRC2 recruitment and shaping H3K27 methylation patterns across the genome. This includes consideration of factors influencing PRC2 residence time on chromatin and PRC2 catalytic activity with a focus on the mechanisms giving rise to regional preferences and differential deposition of H3K27 methylation. We further discuss existing evidence for functional diversity between distinct subsets of PRC2 complexes with the aim of extracting key concepts and highlighting major open questions toward a more complete understanding of PRC2 function.

Role of the Polycomb Repressive Complex 2 (PRC2) in Transcriptional Regulation and Cancer.

The chromatin environment is modulated by a machinery of chromatin modifiers, required for the specification and maintenance of cell fate. Many mutations in the machinery have been linked to the development and progression of cancer. In this review, we give a brief introduction to Polycomb group (PcG) proteins, their assembly into Polycomb repressive complexes (PRCs) and the normal physiological roles of these complexes with a focus on the PRC2. We review the many findings of mutations in the PRC2 coding genes, both loss-of-function and gain-of-function, associated with human cancers and discuss potential molecular mechanisms involved in the contribution of PRC2 mutations to cancer development and progression. Finally, we discuss some of the recent advances in developing and testing drugs targeting the PRC2 as well as emerging results from clinical trials using these drugs in the treatment of human cancers.

A yoctoliter-scale DNA reactor for small-molecule evolution.

The center of DNA three-way junctions, constituting a yoctoliter (10(-24) L) volume, is applied as an efficient reactor to create DNA-encoded libraries of chemical products. Amino acids and short peptides are linked to oligonucleotides via cleavable and noncleavable linkers. The oligonucleotide sequences contain two universal assembling domains at the center and a distal codon sequence specific for the attached building block. Stepwise self-assembly and chemical reactions of these conjugates in a combinatorial fashion create a library of pentapeptides in DNA three-way junctions in a single reaction vessel. We demonstrate the formation of an evenly distributed library of 100 peptides. Each library member contains a short synthetic peptide attached to a unique genetic code creating the necessary "genotype-phenotype" linkage essential to the process of in vitro molecular evolution. Selective enrichment of the [Leu]-enkephalin peptide from an original frequency of 1 in 10 million in a model library to a final frequency of 1.7% in only two rounds of affinity selection is described and demonstrates successful molecular evolution for a non-natural system.