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Title: The role of Ezh2 in transcriptional regulation of retrotransposons in the mouse germ line and embryonic stem cells
Author: Huang, Tien-Chi
ISNI:       0000 0004 7657 0237
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Transposable elements comprise approximately 40% of the mammalian genome. Some of those are still functionally active and therefore post a potential threat to genome integrity. Many epigenetic mechanisms have evolved to restrict these parasitic elements. Among these, DNA methylation is critical for repressing the expression of retrotransposons during mouse embryonic development. However, DNA methylation is dramatically reduced during primordial germ cell (PGC) development and early embryogenesis, which creates a potential window for retrotransposon reactivation. It is still not fully clear how the activity of retrotransposons is controlled during these stages. At chromatin level, alternative epigenetic pathways, such as repressive histone modifications, may replace the role of DNA methylation. I propose that Polycomb Repressive Complex 2 (PRC2), which can catalyse histone H3 lysine 27 methylation, may play an essential role to control these transposable elements in the situation when the genome undergoes extensive DNA demethylation. Ezh2, containing SET domain, is a catalytic subunit of the PRC2 complex that is responsible for the histone methyltransferase activity. Using conditional knockout strategy to delete Ezh2 in mouse PGCs, I demonstrate that depletion of Ezh2 and H3K27me3 results in a profound reactivation of retrotransposons in PGCs, especially at E13.5. The derepression of retrotransposons leads to ƳH2AX accumulation, an indicator of DNA damage, in female germ cells. Furthermore, this appearance of increased ƳH2AX in the female mutant PGCs is not due to precocious onset of meiosis. PGCs are characterized by low gnomic DNA methylation. To test whether Ezh2 has a similar role in repressing retrotransposons in embryonic stem (ES) cells with hypomethylated genome. I have generated inducible Ezh2 knockout ES cell lines. The result indicates that Ezh2-mediated H3K27me3 is enriched on retrotransposons, such as LINE-1 Gf and IAP in ES cells with low DNA methylation. However, depletion of Ezh2 only shows a minimal impact on retrotransposon transcription, which is consistent with the observation of no significant transcriptional changes in Ezh2-deficient ES cells. Taken together, I demonstrate that Ezh2 is required for transcriptional repression of retrotransposons to protect genome integrity during PGC development, whereas in ES cells, H3K9me3-mediated repressive mechanisms may be more critical to control these repetitive elements.
Supervisor: Hajkova, Petra Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available