In eukaryotic cells, transcription factors and polymerases must access DNA in the context of nucleosomes and chromatin. The accessibility of DNA sequences to such trans-acting factors is an important feature of gene regulatory elements, including promoters. In vertebrates, the majority of gene promoters coincide with CpG islands (CGIs), which remain free from DNA methylation and exhibit elevated CpG densities. This hypomethylated and CpG-rich state at CGI promoters is associated not only with transcriptional activity, but also with high levels of chromatin accessibility. However, the causes and consequences of such chromatin accessibility remain unclear. To address this, I have profiled chromatin accessibility in mouse embryonic stem cells (ESCs). In addition to confirming that CGI accessibility is independent of transcriptional activity, I was able to demonstrate that the loss of DNA methylation in ESCs resulted in increased chromatin accessibility at a subset of CpG-rich repetitive elements, suggesting that non-methylated CpG-rich sequences may, at least partially, facilitate open chromatin states. This was supported by preliminary work targeting bacterial CpG-rich sequences into the mouse genome, where they were sufficient to establish novel regions of chromatin accessibility. To examine potential mechanisms by which hypomethylated DNA could serve to promote chromatin accessibility, I profiled chromatin accessibility in mouse ESCs lacking various chromatin-modifying proteins which are normally enriched at CGIs, with the histone demethylases KDM2A/B linked to maintaining open chromatin at CGIs. As an alternative approach to understanding the causes of chromatin accessibility in mouse ESCs, I examined the mechanism by which the pioneer transcription factor OCT4 is able to access previously inaccessible chromatin, and reveal that it requires the chromatin remodeller BRG1 to remodel chromatin and facilitate transcription factor binding at distal regulatory elements. Ultimately, this work provides an insight into some of the molecular determinants of chromatin accessibility in mouse ESCs, although many of the consequences of such chromatin states remain unclear.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:729148 |
| Date | January 2017 |
| Creators | King, Hamish |
| Contributors | Klose, Robert |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:53ceabe6-40ba-402e-a430-7474eac30f93 |
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