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.

Single cell transcriptome atlas of mouse mammary epithelial cells across development.

BACKGROUND: Heterogeneity within the mouse mammary epithelium and potential lineage relationships have been recently explored by single-cell RNA profiling. To further understand how cellular diversity changes during mammary ontogeny, we profiled single cells from nine different developmental stages spanning late embryogenesis, early postnatal, prepuberty, adult, mid-pregnancy, late-pregnancy, and post-involution, as well as the transcriptomes of micro-dissected terminal end buds (TEBs) and subtending ducts during puberty. METHODS: The single cell transcriptomes of 132,599 mammary epithelial cells from 9 different developmental stages were determined on the 10x Genomics Chromium platform, and integrative analyses were performed to compare specific time points. RESULTS: The mammary rudiment at E18.5 closely aligned with the basal lineage, while prepubertal epithelial cells exhibited lineage segregation but to a less differentiated state than their adult counterparts. Comparison of micro-dissected TEBs versus ducts showed that luminal cells within TEBs harbored intermediate expression profiles. Ductal basal cells exhibited increased chromatin accessibility of luminal genes compared to their TEB counterparts suggesting that lineage-specific chromatin is established within the subtending ducts during puberty. An integrative analysis of five stages spanning the pregnancy cycle revealed distinct stage-specific profiles and the presence of cycling basal, mixed-lineage, and 'late' alveolar intermediates in pregnancy. Moreover, a number of intermediates were uncovered along the basal-luminal progenitor cell axis, suggesting a continuum of alveolar-restricted progenitor states. CONCLUSIONS: This extended single cell transcriptome atlas of mouse mammary epithelial cells provides the most complete coverage for mammary epithelial cells during morphogenesis to date. Together with chromatin accessibility analysis of TEB structures, it represents a valuable framework for understanding developmental decisions within the mouse mammary gland.

Reduced PRC2 function alters male germline epigenetic programming and paternal inheritance.

BACKGROUND: Defining the mechanisms that establish and regulate the transmission of epigenetic information from parent to offspring is critical for understanding disease heredity. Currently, the molecular pathways that regulate epigenetic information in the germline and its transmission to offspring are poorly understood. RESULTS: Here we provide evidence that Polycomb Repressive Complex 2 (PRC2) regulates paternal inheritance. Reduced PRC2 function in mice resulted in male sub-fertility and altered epigenetic and transcriptional control of retrotransposed elements in foetal male germ cells. Males with reduced PRC2 function produced offspring that over-expressed retrotransposed pseudogenes and had altered preimplantation embryo cleavage rates and cell cycle control. CONCLUSION: This study reveals a novel role for the histone-modifying complex, PRC2, in paternal intergenerational transmission of epigenetic effects on offspring, with important implications for understanding disease inheritance.

Pharmacological inhibition of EZH2 disrupts the female germline epigenome.

Background: Recently discovered drugs that target epigenetic modifying complexes are providing new treatment options for a range of cancers that affect patients of reproductive age. Although these drugs provide new therapies, it is likely that they will also affect epigenetic programming in sperm and oocytes. A promising target is Enhancer of Zeste 2 (EZH2), which establishes the essential epigenetic modification, H3K27me3, during development. Results: In this study, we demonstrate that inhibition of EZH1/2 with the clinically relevant drug, tazemetostat, severely depletes H3K27me3 in growing oocytes of adult female mice. Moreover, EZH2 inhibition depleted H3K27me3 in primary oocytes and in fetal oocytes undergoing epigenetic reprogramming. Surprisingly, once depleted, H3K27me3 failed to recover in growing oocytes or in fetal oocytes. Conclusion: Together, these data demonstrate that drugs targeting EZH2 significantly affect the germline epigenome and, based on genetic models with oocyte-specific loss of EZH2 function, are likely to affect outcomes in offspring.

Loss of maternal EED results in postnatal overgrowth.

BACKGROUND: Investigating how epigenetic information is transmitted through the mammalian germline is the key to understanding how this information impacts on health and disease susceptibility in offspring. EED is essential for regulating the repressive histone modification, histone 3 lysine 27 tri-methylation (H3K27me3) at many developmental genes. RESULTS: In this study, we used oocyte-specific Zp3-Cre recombinase (Zp3Cre) to delete Eed specifically in mouse growing oocytes, permitting the study of EED function in oocytes and the impact of depleting EED in oocytes on outcomes in offspring. As EED deletion occurred only in growing oocytes and females were mated to normal wild type males, this model allowed the study of oocyte programming without confounding factors such as altered in utero environment. Loss of EED from growing oocytes resulted in a significant overgrowth phenotype that persisted into adult life. Significantly, this involved increased adiposity (total fat) and bone mineral density in offspring. Similar overgrowth occurs in humans with Cohen-Gibson (OMIM 617561) and Weaver (OMIM 277590) syndromes, that result from de novo germline mutations in EED or its co-factor EZH2, respectively. Consistent with a role for EZH2 in human oocytes, we demonstrate that de novo germline mutations in EZH2 occurred in the maternal germline in some cases of Weaver syndrome. However, deletion of Ezh2 in mouse oocytes resulted in a distinct phenotype compared to that resulting from oocyte-specific deletion of Eed. CONCLUSIONS: This study provides novel evidence that altering EED-dependent oocyte programming leads to compromised offspring growth and development in the next generation.

PRC2 is required for extensive reorganization of H3K27me3 during epigenetic reprogramming in mouse fetal germ cells.

BACKGROUND: Defining how epigenetic information is established in the germline during fetal development is key to understanding how epigenetic information is inherited and impacts on evolution and human health and disease. RESULTS: Here, we show that Polycomb Repressive Complex 2 is transiently localized in the nucleus of mouse fetal germ cells, while DNA methylation is removed from the germline. This coincides with significant enrichment of trimethylated lysine 27 on histone 3 near the nuclear lamina that is dependent on activity of the essential PRC2 catalytic proteins, Enhancer of Zeste 1 and/or 2. CONCLUSIONS: Combined, these data reveal a role for Polycomb Repressive Complex 2 and trimethylated lysine 27 on histone 3 during germline epigenetic programming that we speculate is required to repress target sequences while DNA methylation is removed.

Wiz binds active promoters and CTCF-binding sites and is required for normal behaviour in the mouse.

We previously identified Wiz in a mouse screen for epigenetic modifiers. Due to its known association with G9a/GLP, Wiz is generally considered a transcriptional repressor. Here, we provide evidence that it may also function as a transcriptional activator. Wiz levels are high in the brain, but its function and direct targets are unknown. ChIP-seq was performed in adult cerebellum and Wiz peaks were found at promoters and transcription factor CTCF binding sites. RNA-seq in Wiz mutant mice identified genes differentially regulated in adult cerebellum and embryonic brain. In embryonic brain most decreased in expression and included clustered protocadherin genes. These also decreased in adult cerebellum and showed strong Wiz ChIP-seq enrichment. Because a precise pattern of protocadherin gene expression is required for neuronal development, behavioural tests were carried out on mutant mice, revealing an anxiety-like phenotype. This is the first evidence of a role for Wiz in neural function.

Transgenerational epigenetic inheritance: adaptation through the germline epigenome?

Epigenetic modifications direct the way DNA is packaged into the nucleus, making genes more or less accessible to transcriptional machinery and influencing genomic stability. Environmental factors have the potential to alter the epigenome, allowing genes that are silenced to be activated and vice versa. This ultimately influences disease susceptibility and health in an individual. Furthermore, altered chromatin states can be transmitted to subsequent generations, thus epigenetic modifications may provide evolutionary mechanisms that impact on adaptation to changed environments. However, the mechanisms involved in establishing and maintaining these epigenetic modifications during development remain unclear. This review discusses current evidence for transgenerational epigenetic inheritance, confounding issues associated with its study, and the biological relevance of altered epigenetic states for subsequent generations.