In Vitro Hematopoietic Reprogramming and Generation of Hematopoietic Stem Cells

Hematopoietic stem cell (HSC) transplantation is widely used to treat a variety of disorders. Despite advances in the use of umbilical cord blood and mobilized stem cells, donor material remains limited. This is due to insufficient numbers of stem cells in cord blood, poor mobilization, and the lack of ethnic diversity to provide sufficient genetically matched material. Despite intensive efforts there has been limited success in generating transplantable HSCs from pluripotent stem cells (PSCs). Clearly, alternative approaches are necessary. Directly programmed hematopoietic stem/progenitor cells would provide an unlimited patient-specific source for cell replacement and genetic correction therapies as well as a platform for the future generation of patient specific therapeutics and blood products. In 2013 we demonstrated direct reprogramming of mouse fibroblasts into clonogenic hematopoietic progenitors with just four transcription factors (TFs), Gata2, Gfi1b, cFos and Etv6 (Pereira et al 2013). These four TFs induce a dynamic, multi-stage hemogenic process that progresses through an endothelial-like intermediate. As such, it appears to recapitulate definitive developmental hematopoiesis in vitro. We now have strong evidence that a similar hemogenic process can be optimally induced in human fibroblasts with Gata2, Gfi1b, and cFos. These reprogrammed cells are able to multi-lineage repopulate NSG mice. Therefore we maintain that in vitro reprogramming provides a tractable system to address the underlying molecular mechanisms of hemogenesis not possible in primary cells. We have now studied how the TFs bind DNA and initiate a molecular program that changes the epigenetic landscape to allow a change in cell fate. The studies to date have revealed cooperative binding of two of the factors to the binding motif of the other causing simultaneous silencing of fibroblast genes and activation of endothelial and hematopoietic genes.

Pereira CF, Chang B, Qiu J, Niu X, Papatsenko D, Hendry CE, Clark NR, Nomura-Kitabayashi A, Kovacic JC, Ma'ayan A, Schaniel C, Lemischka IR, Moore K. Induction of a Hemogenic Program in Mouse Fibroblasts. Cell Stem Cell. 2013;13:205-218.