Relationship Between Ectopic HOXB4 Expression and Self-renewal in Normal and Leukemic Hematopoietic Cells

Lifelong production of mature blood cells is mediated by long-term hematopoietic stem cells (LT-HSCs) which are capable of regenerating themselves through a process termed self-renewal. Importantly, it is the ability of LT-HSCs to sustain self-renewal that distinguishes them from other hematopoietic cells and enables them to long-term reconstitute all of the blood cells in recipient mice. Exogenous HOXB4 expression in primary bone marrow (BM) cells is currently understood to enhance the self-renewal of LT-HSCs and thus induce their expansion. However, the apparent expansion of LT-HSCs induced by HOXB4 may also be accounted for by alternative mechanisms that hinge on the acquisition of stem cell-like properties in non-LT-HSCs. In evaluating the effect of HOXB4 overexpression in purified cells further down the hematopoietic hierarchy, I found that HOXB4-transduced individual short-term HSCs (ST-HSCs) were capable of generating sustained primary grafts comprising only a subset of hematopoietic lineages. Lineage-restricted patterns of reconstitution were also observed when reconstituted BM from multilineage-engrafted primary recipients were limit diluted into secondary recipients. My results support a model in which multilineage reconstitution mediated by ectopic HOXB4 expression is maintained, in part or possibly exclusively, by lineage-restricted progenitor cells that have newly acquired the ability to sustain self-renewal by virtue of enforced HOXB4 expression. To determine if this endowment of sustained self-renewal potential represents a reprogramming phenomenon, I’ve also undertaken an investigation into the effect of withdrawing ectopic HOXB4 expression on long-term engraftment. My results reveal a dependency on continued HOXB4 expression for sustained self-renewal capacity suggesting active HOXB4 protein production is required for the permanence of the grafts generated by lineage-restricted progenitor cells. Finally, long-term follow-up of HOXB4-transduced cells uncovered a large number of leukemias developing in experimental animals. My work highlights the negative consequence of cells acquiring sustained self-renewal potential, and paves the way for future investigation into the importance of maintaining this self-renewal program in cancer development.Ph.D