The Role of Hox Transcription Factors in the Regulation of Hematopoiesis

Sammanfattning: Hematopoiesis is a lifelong, dynamic process, originating from a low number of hematopoietic stem cells (HSCs) residing in the adult bone marrow with the ability to self-renew and generate all blood lineages throughout life. Recent findings have demonstrated that the Homeobox (Hox) transcription factors are important regulators of both normal and malignant hematopoiesis, controlling proliferation, differentiation and self-renewal of hematopoietic cells at different levels of the hematopoietic hierarchy. This thesis focuses on the role of HOXA10 and Hoxb4, and its interaction with Hoxb3, Hoxa9 and the cell cycle regulator p21 in hematopoiesis. These Hox-genes are all expressed in the HSC compartment, and are then downregulated upon differentiation. In support of this, overexpression of HOXB4 is known to strongly increase HSCs self-renewal in vivo and in vitro. To investigate the physiological role of Hoxb4 we generated a Hoxb4 and a Hoxb3/Hoxb4 knockout mouse model and found that HSCs lacking Hoxb4 exhibit a reduction in their proliferative response to hematopoietic stress. This phenotype was slightly enhanced by the additional deletion of the neighboring gene Hoxb3, demonstrating redundancy of function between these genes. We further explored the complex interactions within the Hox clusters by generating a triple knockout lacking Hoxa9/b3/b4. However, the reconstitution ability of Hoxa9/b3/b4 knockout HSCs did not decline beyond the repopulating defect seen in Hoxa9 knockout HSCs. Using retroviral gene transfer we found that overexpression of HOXB4 in p21 deficient HSC further enhanced the HSCs self-renewal effect in vitro, demonstrating that temporary overexpression of HOXB4 and suppression of p21 could be useful for future HSC expansion protocols. The expression level of HOXB4 is crucial for the HSC fate decisions and to delineate the role of HOXA10 in hematopoiesis, we generated an inducible mouse model for HOXA10. Our findings from this study demonstrate that HOXA10 acts as a master regulator of hematopoiesis governing both proliferation and differentiation of hematopoietic progenitor and stem cells, through direct regulation of the genes Gfi-1, Dkk-1, HLF and Gata-1, where distinct fate outcomes depend on the HOXA10 concentration. In summary, our findings unravel new interactions and molecular pathways acting within, or downstream of Hox genes to regulate hematopoietic progenitor and stem cells.