GLP is critical for oogenesis exhibiting a G9A-independent role in transcriptional repression

GLP (EHMT1) functions as an H3K9me1 and H3K9me2 methyltransferase through its reportedly obligatory dimerization with G9A (EHMT2). Here, we investigated the role of GLP in oocyte and embryo development in comparison to G9A using oocyte-specific conditional knockout mouse models (G9a cKO, Glp cKO, G9a-Glp cDKO). Loss of GLP in oogenesis severely impairs oocyte maturation, fertilization and embryo development, resulting in lethality before embryonic day E12.5. In contrast, loss of G9A has a milder effect with a proportion of embryos producing viable offspring. The Glp cKO also showed loss of G9A protein and, hence, was phenotypically very similar to the G9a-Glp cDKO. H3K9me2 was equally depleted in all cKO genotypes, whereas H3K9me1 was decreased only in Glp cKO and G9a-Glp cDKO oocytes. Furthermore, the transcriptome, DNA methylome and proteome were markedly more affected in G9a-Glp cDKO than G9a cKO oocytes, demonstrating that in the absence of GLP there are widespread epigenetic and gene expression changes in the oocyte independent of H3K9me2. Gene dysregulation with coupled changes in DNA methylation suggest localised loss of chromatin repression, resulting in upregulated protein expression. Together, our findings demonstrate that GLP can function independently of G9A in the oocyte and is required for oocyte developmental competence. low input RNA-seq, BS-seq (PBAT) and H3K9me2 ChIP-seq (ULI-nChIP) of mouse oocytes (100-200 per sample) libraries were prepared. At least 3 samples per genotype. Genotypes used: Control, G9a cKO and G9a-Glp cDKO