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Title: Chromatin dynamics of early hematopoietic development
Author: Mazzarella, Luca
ISNI:       0000 0004 2687 9914
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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In the mouse, blood cells are derived from mesodermal precursors with dual angiogenic and hematopoietic potential. These "hemangioblasts" can be isolated shortly after gastrulation, suggesting that commitment to hematopoietic lineages is initiated very early during development. These early fate decisions can~ in part, be characterised by studying ir, vitro the development of blood cells from pluripotent embryonic stem (ES) cells, a system that has allowed the identification of many developmental signals. and transcription regulators involved in hematopoietic commitment. Evidence sugge~ts that these factors integrate with epigenetic mechanisms to establish and maintain lineage identity. Understanding this latter layer of control is of great interest, as disruption of key epigenetic regulators, such as components of the trithorax/polycomb system, leads to severe developmental abnormalities and/or malignancies. In ES cells, many genes encoding transcription factors that become activated later in development are kept in a transcriptionally primed state characterised by "bivalent" epigenetic · marks, in which histone modifications classically associated with either gene activation or repression coexist. Despite not being expressed, a fraction of bivalent genes replicate early during S phase, a property that is usually associated with a permissive chromatin structure. In adult cells committed to the hematopoietic lineage, these conflicting epigenetic characteristics are mostly resolved. How and when these changes in epigenetic states occur during development, and whether continuous polycomb repression is required to maintain lineage identity, is currently unknown. Here, I have analysed the epigenetic changes occurring at a panel of bivalent genes during early hematopoietic development by using ES cells expressing a mesoderm-specific reporter gene, which allows flow cytometric isolation of cell populations with progressively more restricted lineage potential. These cells were subjected to analysis of replication timing and chromatin immunoprecipitation with protocols I have optimized for the use with small amounts of starting material. I show that commitment to the hemangioblast is associated with a gradual change in replication timing profile, but a bivalent chromatin state persists at genes that remain repressed. Conditional knock-out of the polycomb repressive complex 1 (PRCl) in the hemangioblast is sufficient to relieve repression and leads to the subversion of the physiological transcriptional programme. This demonstrates that the bivalent mark remains functionally relevant and that continuous PRCl-mediated repression is required to maintain hemangioblast identity. Collectively, these results highlight the developmental significance of polycomb-mediated gene regulation and illustrate the crucial role played by epigenetic factors in the development of the hematopoietic system.
Supervisor: {unknown}, {unknown} Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available