Haematopoietic stem cell ontogeny

The blood contains a variety of cells required to carry oxygen and fight invasion, all deriving from a small number of stem cells that live in the bone marrow. Transplantation of these cells is often the only treatment for blood diseases including leukaemia, and is essential for survival of aggressive cancer therapies. In order to better understand these cells and possibly to expand them outside the body, we are working out their genetic programming. This takes place during embryonic development. Amphibian and fish embryos develop externally in large numbers rendering them ideal for studying the molecular processes involved in cell programming. These processes have been highly conserved during evolution and therefore what we discover in these model organisms has direct relevance to humans. Interestingly, the blood derives from a population of cells in the embryo that also gives rise to the cardiovascular system. What we discover about blood programming therefore also has direct relevance to these tissues and their diseases.

This programme is aimed at defining the core transcriptional networks that control the formation and behaviour of haematopoietic stem cells (HSCs), and the embryonic signals that establish and maintain them. We continue to exploit the technical advantages of zebrafish and Xenopus embryos in the knowledge that these core networks and their controlling signals will be conserved in humans. A major new departure will be to carry out cell tracking and molecular perturbations at the single cell level to clearly establish lineage relationships and to distinguish cell autonomous from non-cell autonomous effects. We will identify the locations of precursors in the embryo and explore the gene expression there with a view to predicting instructive signals not yet defined. These predictions will be tested using gain and loss of function assays, including at the single cell level. The regulatory relationships between transcription factors in the nucleus will be determined using similar gene function assays, and we will begin to extend to cell populations in the embryo, chromatin and protein complex affinity purification to identify direct targets and protein partners. Together, these approaches will enable the building of detailed networks of regulatory information which will enhance our understanding of HSC formation and behaviour. To gain insight into the mechanisms by which leukaemic fusion genes initiate leukaemia, we will also determine the impact of these genes on the networks built up in this programme, and screen for small molecules that exert control over fusion gene activity.