Comment on Wang et al, page 254
In this issue of Blood, Wang and colleagues have shed light on the molecular mechanisms whereby 2 homeobox genes—one oncogenic (Hoxa9) and one not (Meis1)—can collaborate to cause acute myelogenous leukemia (AML).
Hoxa9 is frequently upregulated in primary samples from patients with AML, particularly in those cases harboring mixed-lineage leukemia (MLL) fusion genes.1 There is strong supporting evidence from animal models that the Hoxa9 homeodomain protein plays a central role in myeloid leukemias.2 However, forced expression of Hoxa9 in murine marrow cells results in formation of leukemias in recipient animals only after a latency of several months, which can be greatly hastened by coexpression of the Meis1 homeodomain protein.2 Until now, the molecular mechanisms underlying the acceleration of leukemic transformation by Meis protein have remained unclear.
In this new paper, the authors have taken the approach of studying the effects of Hoxa9 and Meis1 proteins on the immortalization of marrow cells in culture. This laboratory had previously shown that Hoxa9 can immortalize a factor-dependent myelomonocytic progenitor cell in the absence of Meis1, but that the immortalized cells could not engraft or cause leukemias when transplanted into mice.3 They now demonstrate that coexpression of Hoxa9 and Meis1 immortalizes a more immature marrow progenitor cell. In addition to being capable of proliferation in granulocyte macrophage–colony-stimulating factorlike Hoxa9-immortalized cells, the Hoxa9 plus Meis1 cells can proliferate in the presence of either stem cell factor or, most intriguingly, in FLT3 ligand (FL). Furthermore, the Hoxa9 plus Meis1–immortalized cells cause rapid leukemias when transplanted into mice.FIG1
It should be noted that Meis1 has no immortalizing or transforming activity alone. Indeed, a very recent paper in Blood showed that Meis1 overexpression by itself triggers apoptosis in a number of cell types.4 How, then, does Meis1 change the program(s) initiated by Hoxa9 overexpression? Wang et al show that, in the presence of Hoxa9, Meis1 upregulates the expression of Fms-like tyrosine kinase 3 (FLT3), the receptor for FL and itself a tyrosine kinase oncoprotein. They proceed to demonstrate that both DNA binding and interaction with a third homeodomain protein, pre–B-cell leukemia homeobox (PBX), are required for both FLT3 expression and induction of leukemias by immortalized cells. They then hypothesize that Meis1 protein activates a series of short-term hematopoietic stem cell (HSC) marker proteins, including FLT3, as well as niche-homing surface proteins, that allow the transfected cells to closely model the putative leukemic stem cell. Although the activation of FLT3 by the addition of Meis1 appears to be rapid, it is not clear if FLT3 is directly activated by Meis1.
Concerning the issue of marrow homing, it is noteworthy that murine marrow cells that are freshly transduced with Hoxa9 expression vectors and then immediately transplanted into recipient animals show robust engraftment; indeed, they repopulate lethally irradiated hosts much more efficiently than normal marrow cells.5 Thus the inability of Hoxa9-induced immortalized cell lines to engraft in vivo represents the loss of transplantability that occurs under artificial and unknown selection pressures of the in vitro culture system. The immortalized cell lines that emerge under these conditions are clearly selected, as Southern gel analyses of these lines shows that they are uniformly mono- or oligoclonal. Thus, perhaps coexpression of Meis1 together with Hoxa9 preserves the expression of genes required for stem cells to home to their appropriate niche in the marrow.
Previous studies have shown that Meis1 and Hoxa9 form dimeric cooperative DNA binding complexes, as well as triple complexes containing the PBX protein.6 Although Wang et al emphasize the cooperative role of PBX with Meis1, the question of whether induction of FLT3 and other HSC marker proteins requires cooperative interactions with Hoxa9 remains unclear from the present work. Nevertheless, the models described by Wang et al should enhance our ability to dissect the molecular mechanisms underlying myeloid leukemic transformation. ▪