Pluto Bioinformatics

GSE93941: MLL2 conveys transcription-independent H3K4me3 in the oocyte

Bulk RNA sequencing

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. SOURCE: Felix Krueger (felix.krueger@babraham.ac.uk) - The Babraham Institute