An essential role for Polycomb Repressive Complex 2 in the mouse ovary

Polycomb repressive complex 2 (PRC2) catalyses the repressive epigenetic modification of histone 3 lysine 27 tri-methylation (H3K27me3) and functions as a key epigenetic regulator during embryonic development. PRC2 is known to regulate the development of a range of tissues by transcriptional silencing of genes that control cell differentiation, but its roles in female germline and ovarian development remain unknown. Using a mouse model with hypomorphic embryonic ectoderm development (EED) function that reduced H3K27me3 in somatic and germ cells, we found that PRC2 was required for survival, with more than 95% of female animals dying before birth. Although surviving adult EED hypomorphic females appeared morphologically similar to controls and were fertile, Eedhypo/hypo adult ovaries were abnormal, with altered morphology characterised by abnormal follicles. Early Eedhypo/hypo and control fetal ovaries were morphologically similar, and germ cells entered meiosis normally. Immunofluorescent analyses of somatic and germline markers indicated that ovarian development in Eedhypo/hypo ovaries was similar to heterozygous and WT controls. However, TUNEL analyses revealed higher rates of apoptosis in the ovarian surface epithelium, and transcriptional analyses revealed changes in genes regulating epithelial and steroidogenic cell differentiation, possibly foreshadowing the defects observed in adult ovaries of hypomorphic females. While it was possible to analyse early-mid fetal ovarian development, postnatal stages were inaccessible due to the high level of lethality during late fetal stages. Despite this limitation, the data we were able to obtain reveal a novel role for EED in the ovary that is likely to alter ovarian development and ovarian function in adult animals.