Project description:Background: MORC proteins are involved in epigenetic gene silencing in a wide variety of eukaryotic organisms. Deletion of MORCs result in several developmental abnormalities and their dysregulation has been implicated in developmental disease and multiple cancers. Specifically, mutations of mammalian MORC3 have been associated with immune system defects, Down syndrome and human cancers such as bladder, uterine, stomach, and lung cancers, and diffuse large B cell lymphomas. While previous studies have shown that MORC3 binds to H3K4me3 in vitro and overlaps with H3K4me3 ChIP-seq peaks in mouse embryonic stem cells, the mechanism by which MORC3 regulates gene expression is unknown. Results: In this study, we find that MORC3 functions as an epigenetic silencer of endogenous retroviruses (ERVs) in mouse embryonic stem cells (mESCs). Loss of MORC3 results in upregulation of ERVs, specifically those belonging to the LTR class of retrotransposons. Using ChIP-seq, we measure the genome-wide localization of MORC3 in wild-type cells and find that MORC3 binds to ERVs suggesting its direct role in regulating ERV expression. Previous studies have shown that these ERVs are marked by repressive histone mark H3K9me3 which plays a key role in their silencing. Interestingly, we find that the levels of H3K9me3 do not change substantially upon the loss of MORC3 indicating that MORC3 possibly acts downstream of the TRIM28/SETDB1 complex that deposits H3K9me3 at these loci. Instead, we discover that loss of MORC3 results in increased chromatin accessibility at the ERVs suggesting that MORC3 silences ERVs by compacting DNA in mESCs. Conclusions: Our results reveal MORC3 as a novel regulator of ERV silencing in mouse embryonic stem cells. As early mammalian development is characterized by dynamic changes in ERV expression, the role of MORC3 in silencing ERVs is exciting and could potentially explain the abnormalities observed due to its misregulation during mammalian development.

Project description:Endogenous retroviruses (ERVs) comprise a significant portion of mammalian genomes. Although specific ERV loci feature regulatory roles for host gene expression, most ERV integrations are transcriptionally repressed by Setdb1 mediated H3K9me3 and DNA methylation. However, the protein network which regulates the deposition of these chromatin modifications is still incompletely understood. Here, we performed a genome-wide sgRNA screen for genes involved in ERV silencing and identified the GHKL ATPase protein Morc3 as a top-scoring hit. Morc3 knock-out cells display de-repression, reduced H3K9me3, and increased chromatin accessibility of distinct ERV families. We found that the Morc3 ATPase cycle and Morc3 SUMOylation are important for ERV chromatin regulation. Proteomic analysis revealed that Morc3 mutant proteins fail to interact with the histone H3.3 chaperone Daxx. This interaction depends on Morc3 SUMOylation and Daxx SUMO binding. Notably, in Morc3 ko cells, we observed strongly reduced histone H3.3 on Morc3 binding sites. Thus, our data demonstrate Morc3 as a critical regulator of Daxx-mediated histone H3.3 incorporation to ERV regions. This dataset comprises several experiments addressing different questions: 1. ChIP-MS experiment to determine the protein interaction context of Morc3 using a Morc3-3xFLAG knock-in ES cell line compared to wild type ES cells (Experiment 20200408). 2. ChIP-MS experiments to investigate changes in the protein interaction context of the Morc3 mutant rescue cell lines. Comparison of Morc3 knock-out cell lines with re-expression of Morc3-CW-3xFLAG mutant (Ref. #3111), Morc3-ATP-binding-3xFLAG and Morc3-SUMOylation-3xFLAG mutants (Ref. #3635), and Morc3-deltaN-3xFLAG mutant (Ref. #5174) compared to wt Morc3-3XFLAG rescue. 3. ChIP-MS experiment to determine if the interaction between Morc3 and Daxx is mediated through this C-terminal SIM, comparing Daxx knock-out cell lines with re-expression of wild type 3xFLAG-Daxx protein or 3xFLAG-Daxx ∆SIM, which lacks the C-terminal SIM domain. (Ref. #3301)

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Morc3 silences endogenous retroviruses in mouse embryonic stem cells [ATAC-seq]

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