Project description:Trimethylation of histone 3 lysine 4 (H3K4me3) is classically thought of as a mark of active promoters and yet it occurs at untranscribed domains. Partial redundancy of H3K4 methyltransferases has made it difficult to delineate the mechanisms underlying genomic targeting of H3K4me3. The oocyte provides an attractive system to investigate this, because extensive acquisition of H3K4me3 occurs in a non-dividing cell and ablation of a single H3K4 methyltransferase, Mll2, prevents most H3K4me3. We developed low-input chromatin immunoprecipitation to interrogate promoter associated histone modifications H3K4me3, H3K27ac and H3K27me3 throughout oogenesis. In non-growing oocytes, H3K4me3 was restricted to transcriptionally active promoters, but as oogenesis progresses, H3K4me3 accumulates in a transcription-independent manner: targeted to broad inter-genic regions, putative enhancers, and transcriptionally silent H3K27me3-marked promoters. Consequently, thousands of bivalent domains are established during oogenesis. Ablation of Mll2 resulted in loss of transcription-independent H3K4me3, with limited effects on transcription-coupled H3K4me3 or gene expression. Deletion of Dnmt3a/b showed that DNA methylation protects regions from acquiring H3K4me3. Our findings show that there are two independent mechanisms of targeting H3K4me3 to genomic elements, with MLL2 recruited to unmethylated CpG-rich regions independently of transcription.
Project description:The spatiotemporal regulation of gene expression is central for cell-lineage specification during embryonic development and is achieved through the combinatorial action of transcription factors/co-factors and the epigenetic states at cis-regulatory elements. Previously, we reported that Mll2 (KMT2B)/COMPASS is responsible for the implementation of H3K4me3 at promoters of bivalent genes. Here, we show that Mll2/COMPASS can also implements H3K4me3 at some of the non-TSS regulatory elements, a subset of which share epigenetic signatures of active enhancers. Our mechanistic studies reveal that the association of Mll2’s CXXC domain with CpG-rich regions plays an instrumental role for chromatin targeting and subsequent implementation of H3K4me3. Although Mll2/COMPASS is required for H3K4me3 implementation on thousands of sites, it appears to be essential for the expression of a subset of genes, including those functioning in the control of transcriptional programs during embryonic development, indicating that not all H3K4 trimethylations implemented by MLL2/COMPASS are functionally equivalent.
