The role of BAP1 in early human placentation: Implications for trophoblast differentiation and preeclampsia

The human placenta is a specialized, temporary organ essential for fetal development and maternal adaptation during pregnancy. It serves as the interface between maternal and fetal tissues, enabling the exchange of nutrients, oxygen, hormones, and waste products while also performing endocrine and immune functions. Placentation begins with the blastocyst trophectoderm, where cytotrophoblasts differentiate into extravillous trophoblasts (EVTs), which invade the maternal endometrium and remodel spiral arteries, and syncytiotrophoblasts (STBs), which form the maternal–fetal interface. Disruptions in these processes are linked to pregnancy complications such as preeclampsia, a major cause of maternal and perinatal morbidity. Understanding early placentation is therefore critical for improving diagnosis and treatment. Advances in trophoblast stem cell lines and three-dimensional organoids that mimic placental structure and function now offer valuable tools to study these mechanisms. BRCA1-associated protein 1 (BAP1) is a nuclear deubiquitinase and tumor suppressor involved in cell cycle regulation, DNA repair, apoptosis, metabolism, and gene expression. As part of the Polycomb Repressive–Deubiquitinase (PR-DUB) complex, BAP1 removes ubiquitin from histone H2A, counteracting gene repression mediated by Polycomb complexes. Studies in mice show that BAP1 is essential for placental development, and its regulatory role appears conserved in humans. Interestingly, BAP1 must be downregulated during trophoblast invasion, similar to mechanisms seen in tumor metastasis. This thesis investigates BAP1’s role in early human placental development and how its dysregulation may contribute to pregnancy disorders. Using immortalized human trophoblast stem cells, primary villous cytotrophoblasts from first-trimester placentas, and placental tissues from clinical cohorts, we applied molecular, transcriptomic, proteomic, and epigenetic analyses to examine the effects of BAP1 knockdown and overexpression. Results show that BAP1 is a key regulator of trophoblast stemness and epithelial identity, with conserved functions across species. Sustained BAP1 expression maintains an undifferentiated state by strengthening cell–cell adhesion, while its downregulation promotes differentiation into EVT and STB lineages. Although loss of BAP1 increases proliferation without altering core stem-cell regulatory networks, overexpression reduces proliferation and enhances adhesion pathways. These findings suggest that BAP1 stabilizes epithelial features while ensuring proper differentiation when downregulated, offering insight into trophoblast dysfunction in early-onset preeclampsia (EO-PE). Integrated analyses identified pathways affected by BAP1 dysregulation, including glycoprotein metabolism and MAPK, Hippo, and JAK–STAT signaling. A gene signature derived from BAP1-overexpressing organoids distinguished EO-PE placentas from controls but not from intrauterine growth restriction or gestational diabetes, indicating disease specificity. Tissue studies also showed increased expression of BAP1 and PR-DUB partners ASXL2 and ASXL3 in EO-PE, suggesting abnormal PR-DUB activity is linked to severe early-onset disease. Overall, this work demonstrates that BAP1 orchestrates early placental development by maintaining epithelial identity and regulating trophoblast differentiation. Its controlled downregulation during invasion parallels processes in tumor biology, and BAP1-overexpressing trophoblast models reproduce key EO-PE features, providing a physiologically relevant platform to study placental disorders and identify potential therapeutic targets.