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HP1-mediated transcriptional silencing of ERVs and genes in mouse embryonic stem cells

University of British Columbia

The three mammalian Heterochromatin Protein 1 (HP1) proteins are considered hallmarks of H3K9me3-marked heterochromatin, and are essential for establishing this transcriptionally silent chromatin state genome-wide. They also have a proposed involvement in regulating H3K9me3 levels, based on their interaction with histone lysine methyltransferases. However, they have individually been shown to not be integral for silencing of certain classes of endogenous retroviruses (ERVs). I show that HP1 isoforms in mESCs are functionally redundant and only upon deletion of all three isoforms is there a loss of ERV silencing. I also show that although there are some minor effects on H3K9me3 levels genome-wide following HP1 protein depletion, there are minimal effects on this mark over genes and ERVs, unlike the effects seen on the H3K9me3- and HP1-dependent mark H4K20me3. I also investigate two reported HP1-interacting proteins, AHDC1 and CHAMP1, for their impact on gene regulation and pluripotency maintenance in mESCs. I identify gene promoters where HP1 isoforms are bound independently of H3K9me3, and therefore hypothesize that AHDC1 and CHAMP1 are responsible for HP1 protein recruitment to these promoters via their putative DNA-binding motifs. Both AHDC1 and CHAMP1 have associated severe neurodevelopmental phenotypes when mutated in humans, which could be caused by disruption of HP1-mediated gene silencing and aberrant stem cell differentiation patterns. However, I find no significant effect on gene regulation upon disruption of these two genes in mESCs. I also find no observed effect on cell growth or differentiation potentials of Ahdc1 and Champ1 KO cells. The observed neurodevelopmental phenotypes in humans can therefore not be explained by disruption of HP1-mediated gene silencing in mESCs, although it is still possible they are caused by failure of this mechanism in differentiated cells at later developmental stages.

Text

2020-04-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/74166

Medical Genetics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/