Our work has focused on defining mechanisms that govern the specification and self-renewal of hematopoietic stem cells during mouse and human development. Using gene targeted mouse ES cells and mouse embryos, we defined the transcriptional programs that are regulated by Scl, the master regulator for blood formation. We discovered that Scl not only establishes the transcriptional programs that are critical for specifying hemogenic endothelium and hematopoietic stem cells, but it also represses heart development. Strikingly, in the absence of Scl, hemogenic endothelium in embryonic hematopoietic tissues becomes converted to cardiogenic fate, and gives rise to fully functional, beating cardiomyocytes.
In order to define the key programs that distinguish self-renewing HSCs from their downstream progenitors or the compromised HSPCs (hematopoietic stem/progenitor cells) that were generated in vitro, we performed microarray analysis for human phenotypic HSCs from various sources. We identified novel markers for human HSCs that can be used to purify transplantable HSCs to a higher purity. We have identified key molecular defects in HSCs that are expanded in culture, or generated from human ES cells. We have further validated that dysregulation of certain Hox genes is a major bottleneck for generating functional HSCs from human ES cells. Future studies are focused on establishing methods that would allow correction of the compromised HSC regulatory networks in cultured HSCs.