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Transcriptional silencing of endogenous retroviruses : interplay between histone H3K9 methylation and DNA methylation Leung, Danny Chi Yeu

Abstract

Endogenous retroviruses (ERVs) are found in genomes of all higher eukaryotes. As retrotransposition is deleterious, pathways have evolved to repress these retroelements. While DNA methylation transcriptionally represses ERVs in differentiated cells, this epigenetic mark is dispensable for maintaining proviral silencing during early stages of mouse embryogenesis and in embryonic stem cells (mESCs). Studies in diverse species have found histone H3K9 methylation and DNA methylation to function together to repress retrotransposons. However, until recently, little was known about the role of this histone modification in proviral silencing in mESCs. Interestingly, our analysis of mESCs lacking G9a, an H3K9-specific lysine methyltransferase (KMTase) revealed that although ERVs lost H3K9 di-methylation (me2) and DNA methylation, they remained silent. Strikingly, the levels of H3K9 tri-methylation (me3) remained unaltered, suggesting that this mark may instead be responsible for maintaining these parasitic elements transcriptionally inactive. The first stage of my research focused on identifying the enzyme depositing H3K9me3 at ERVs and on determining its role in proviral silencing. I discovered that Setdb1, another H3K9-specific KMTase, was indeed depositing H3K9me3 at a subset of ERVs and was required for maintaining transcriptional repression. Interestingly, this silencing pathway operated independently of DNA methylation. Through collaboration, we also discovered that this pathway played a diminished role in differentiated cells. Taken together, these findings indicate the existence of a DNA methylation-independent proviral silencing pathway in mESCs. The second stage of my research focused on the establishment of transcriptional repression of newly integrated proviruses. By employing an exogenous retroviral construct, I discovered a dramatic silencing defect in mESCs lacking G9a, which phenocopied cells depleted of the de novo DNA methyltransferases. Furthermore, efficient DNA methylation of proviruses required G9a-mediated H3K9me2. These findings reveal that histone modifications and DNA methylation function in concert to defend the genome against invading retroviral elements in mESCs. Taken together with discoveries regarding the mechanism of DNA demethylation in early embryos, I propose that cells undergoing DNA methylation reprogramming predominantly employ histone modification-based pathways to maintain these parasitic elements in a silent state; however, the establishment of transcriptional repression for newly integrated elements also requires de novo DNA methylation.

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