Loss of SETD1B results in the redistribution of genomic H3K4me3 in the oocyte

Histone 3 lysine 4 trimethylation (H3K4me3) is an epigenetic mark found at gene promoters and CpG islands. H3K4me3 is essential for mammalian development, yet mechanisms underlying its genomic targeting are poorly understood. H3K4me3 methyltransferases SETD1B and MLL2 are essential for oogenesis. We investigated changes in H3K4me3 in Setd1b conditional knockout (cKO) GV oocytes using ultra-low input ChIP-seq, in conjunction with DNA methylation and gene expression analysis. Setd1b cKO oocytes showed a redistribution of H3K4me3, with a marked loss at active gene promoters associated with downregulated gene expression. Remarkably, many regions gained H3K4me3 in Setd1b cKOs, in particular those that were DNA hypomethylated, transcriptionally inactive and CpG-rich - hallmarks of MLL2 targets. Thus, loss of SETD1B appears to enable enhanced MLL2 activity. Our work reveals two distinct, complementary mechanisms of genomic targeting of H3K4me3 in oogenesis, with SETD1B linked to gene expression in the oogenic program and MLL2 to CpG content.