The role of the bHLH transcription factor SCL/Tal-1 in haemopoiesis

Many inherited and acquired diseases are associated with anaemia in which there are too few red blood cells. Understanding these disorders depends on knowing how blood normally forms in the bone marrow throughout development. Recently there has been considerable accumulation of knowledge about the factors that direct formation of the blood. Our interest has focused on one such factor (called SCL) which plays several critical roles in this process. In the future, we plan to understand how this factor controls the formation of blood.

Elucidating the molecular mechanisms that control lineage specification and differentiation is a central question in developmental biology. Haemopoiesis provides a good model to enable the analysis of such mechanisms. Many ubiquitous and specifically expressed transcription factors have been shown to play specific roles in orchestrating the differentiation of a haemopoietic pluripotent stem cell into specialised blood lineages. The oncoprotein SCL/Tal-1 has recently emerged as a crucial regulatory protein at very early steps in haemopoietic development as well as later for terminal erythroid and megakaryocytic differentiation. SCL ectopic expression in T cells has been described in up to 60% of T-cell acute lymphoblastic leukaemia (T-ALL). Our aim is to increase our understanding of the molecular mechanisms involved in haemopoietic specification/differentiation by studying different aspects of SCLs function in normal haematopoietic development. This is of particular interest from a developmental point of view and may also be relevant to clinical practice, as these questions lie at the heart of how haemopoietic stem cells are formed.||Our objectives are to characterise the transcriptional networks controlled by this key regulator in erythropoiesis and developmental haemopoiesis through the development of integrated and complementary approaches. Importantly, this will contribute to our understanding of haemopoietic stem cell (HSC) genetic regulatory pathways. We expect characterising general molecular mechanisms, such as activation of a specific developmental programme as well as repression of alternate pathways by a master regulator that will apply to other specification models. Importantly, our initial discoveries (new target genes and protein partners) will open up new avenues of investigation, allowing for the expansion of our research programme. Finally, we anticipate that our lists of target genes and partners will provide useful resources and be of interest to other investigators in the Unit working on the early stages of blood formation. We are also planning to develop the iPS cell system and projects using hES cells, to keep abreast with new technologies.||(1) To complete our characterisation of the molecular pathways and target genes controlled by SCL and partners in erythropoiesis, we have established large-scale genomic studies. Through gene expression/ChIP-sequencing analyses, we will dissect some of the mechanisms engaged by SCL that will give insight into molecular pathways leading to red cell production. We will also characterise pathways/cis-elements with distinct requirements for SCL direct and indirect DNA-binding activity. We anticipate drawing general principles from this study that will apply to other transcription factors.||(2) We previously identified the oncoprotein and co-repressor ETO2 as a partner of SCL in erythroid cells. The function of ETO2 in developmental haemopoiesis is being further investigating in Xenopus, by combining cellular and molecular studies. The data we have accumulated so far uncovers unsuspected signalling pathways leading to HSC specification.||(3) To gain insights into the molecular mechanisms underlying mouse haemopoietic development and HSC specification, we will identify SCLs protein partners and target genes in mesodermal/pre-haemopoietic cells by applying proteomic and large-scale genomic studies and using the mouse ES cells system as our developmental model. We will also develop the iPS cell system and projects using hES cells to keep abreast with new technologies.||(4) Finally, we will solve the crystal structure of the bHLH region of an SCL heterodimer to elucidate the role of functionally crucial residues in SCLs HLH domain, test whether binding to DNA alters the structure of the heterodimer and solve the structure of the SCL core (quaternary) complex.||By focusing on the role of SCL in primitive and definitive haemopoietic precursors and in erythroid cells, this p