Coordinated regulation of chromatin modifiers reflects organised epigenetic programming in mouse oocytes

Epigenetic modifications provide mechanisms for influencing gene expression, regulating cell differentiation and maintaining long-term memory of cellular identity and function. As oocytes transmit epigenetic information to offspring, correct establishment of the oocyte epigenome is important for normal offspring development. Oocyte epigenetic programming is highly complex, involving a range of epigenetic modifiers which interact to establish a specific distribution of DNA methylation and histone modifications. Disruptions to oocyte epigenetic programming can alter epigenetic memory and prevent normal developmental outcomes in the next generation. Therefore, it is critical that we further our understanding of the interdependent relationships between various epigenetic modifiers and modifications during oogenesis.

In this study we investigated the spatial and temporal distribution of a range of epigenetic modifiers and modifications in growing oocytes of primordial to antral follicles. We provide comprehensive immunofluorescent profiles of SETD2, H3K36me3, KDM6A, RBBP7, H3K27me3, DNMT3A and DNMT3L and compare these profiles to our previously published profiles of the Polycomb Repressive Complex 2 components EED, EZH2 and SUZ12 in growing oocytes of wildtype mice. In addition, we examined the nuclear levels and spatial distribution of these epigenetic modifiers and modifications in oocytes that lacked the essential Polycomb Repressive Complex 2 subunit, EED. Notably, histone remodelling in primary-secondary follicle oocytes preceded upregulation of DNMT3A and DNMT3L in secondary-antral follicle oocytes. Moreover, loss of EED and H3K27me3 led to significantly increased levels of the H3K36me3 methyltransferase SETD2 during early-mid oocyte growth, although the average levels of H3K36me3 were unchanged.

Overall, these data demonstrate that oocyte epigenetic programming is a highly ordered process, with histone remodelling in early growing oocytes preceding de novo DNA methylation in secondary-antral follicle oocytes. These results indicate that tight temporal and spatial regulation of histone modifiers and modifications is essential to ensure correct establishment of the unique epigenome present in fully grown oocytes. Further understanding of the temporal and spatial relationships between different epigenetic modifications and how they interact is essential for understanding how germline epigenetic programming affects inheritance and offspring development in mammals, including humans.