(90) BAP1 Regulates Maternal-to-zygotic Transition by Preventing Pervasive H2AK119ub1 and Ectopic H3K27me3 in Oocytes.

Jin-Wen Kang^1,2, Peiyao Liu^1,2, Lauryn Cook^1,2, Shoko Ichimura^1,2, Zhiyuan Chen^1,2

1. Reproductive Sciences Center, Division of Developmental Biology, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH 45229, USA;

2. Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, OH 45267, USA;

Abstract Text:

Accumulating evidence indicates that oocyte-derived post-translational histone modifications modulate early embryonic development. Two key modifications, histone H2A lysine 119 (H2AK119) monoubiquitylation and histone H3 lysine 27 (H3K27) trimethylation are hallmarks of heterochromatin and essential for Polycomb-mediated gene silencing. Unlike somatic cells, mouse oocytes exhibit remarkably broad H2AK119ub1 and H3K27me3 domains. Depletion either modification alters oocyte transcriptome, compromises chromatin architecture, and impairs embryonic development. Although it is well established that the Polycomb group proteins establish these histone marks, it remains unknown which factor(s) counteract Polycomb activity to prevent excessive H2AK119ub1 and H3K27me3 during oogenesis. Here, we report that BAP1-mediated H2A deubiquitylation is crucial to constrain pervasive H2AK119ub1 and prevent ectopic H3K27me3 domains in oocytes. Loss of maternal BAP1 disrupts the maternal-to-zygotic transition and leads to severe preimplantation defects.

BAP1 is a ubiquitin carboxyl-terminal hydrolase that specifically removes monoubiquitin from H2AK119ub1. Bap1 and its cofactors Asxl1, Foxk2, Ogt, and Hcfc1 are highly expressed in both growing and fully grown oocytes. To study its function in oogenesis, we conditionally depleted Bap1 starting from primordial follicles using a Gdf9-iCre, Bap1^flox/flox mouse model. The Bap1^flox/flox mice without Gdf9-iCre served as the controls. We found that BAP1 depletion does not affect folliculogenesis, as comparable numbers of fully grown oocytes (FGOs) and MII eggs were retrieved from knockout (KO) and control mice. However, immunofluorescence analyses indicated that H2AK119ub1 level in KO FGOs was remarkably higher than that in control oocytes. Quantitative CUT&RUN analyses further revealed genome-wide pervasive increase of H2AK119ub1 in KO FGOs. In contrast to H2AK119ub1, increase of H3K27me3 is modest in KO oocytes. Notably, a subset of genes gained ectopic H3K27me3 at gene bodies, correlating with their downregulation. RNA-seq revealed 618 genes significantly downregulated in KO FG oocytes, while only 83 genes were upregulated, consistent with BAP1’s role as a transcriptional activator.

To assess developmental consequences, we fertilized BAP1 KO oocytes with wild-type sperm. The maternal KO embryos exhibited developmental delay around 4- and 8-cell stages, with only ~10-20% of them reaching the blastocyst stage by 96 hours post fertilization. RNA-seq analyses at late 1-cell, early and late 2-cell revealed defects in both maternal decay and zygotic genome activation (ZGA). Specifically, maternal genes such as Btg4, Pabpn1l, Zfp36l2, and Zar1 were up-regulated, whereas ZGA genes including Nr5a2, Rarg, Obox3/6, and Dppa2/4 were down-regulated in maternal KO embryos. Notably, 455 (73%) out of the 618 down-regulated genes in KO FGOs were also down-regulated in late 1-cell embryos, suggesting that aberrant transcriptomes in KO FGOs are inherited in zygotes.

Together, these results demonstrate that BAP1 is required to constrain pervasive H2AK119ub1 and prevent aberrant H3K27me3 deposition in oocytes. Loss of BAP1 alters oocyte transcriptome and causes maternal-to-zygotic transition failure, ultimately compromising preimplantation development.