Essential Role of Endogenous MOZ in Aberrant HoxA9 and Meis1 Expressions Induced By MOZ Fusion Gene

Monocytic leukemia Zinc finger protein (MOZ), a histone acetyltransferase, is involved in chromosome translocations associated with FAB M4/M5 types of acute myeloid leukemia (AML). In normal hematopoiesis, MOZ is essential for self-renewal of hematopoietic stem cells (HSCs) and for expression of HoxA9/Meis1 in hematopoietic stem/progenitor cells (HSPCs). Previously we found that endogenous MOZ is critical for MOZ-TIF2-induced AML. Although MOZ-/- cells expressing the MOZ-fusion serially generated colonies in vitro, they did not induce AML after transplantation into recipient mice. In these cells, up-regulation of Meis1 was impaired, while HoxA9 expression was induced. However, roles of endogenous MOZ in MOZ fusion induced leukemia remained unclear. To elucidate molecular mechanisms, we performed experiments described below.

First, to reveal mechanisms in defect of Meis1 expression in MOZ-/- MOZ-fusion leukemia cells, we performed chromatin immune-precipitation assays on Meis1 locus. Coincident with gene expression, active histone marks (H3K9ac, H3K27ac etc.) were disrupted. In contrast, repressive histone modifications (H3K9me2, H3K27me3) were elevated.

Next we analyzed requirement of HoxA9 and Meis1 in MOZ fusion induced AML development. When mice were transplanted with MOZ-/- HSPCs simultaneously introduced with MOZ-fusion and Meis1 genes, AML development were induced. On the other hand, when Meis1 was conditionally deleted in MOZ-fusion leukemia cells, AML development was significantly delayed. Mice transplanted with MOZ-/- HSPCs, which were introduced with both HoxA9 and Meis1 genes elicited AML development.

Furthermore, we analyzed gene expression profiles of MOZ-/- MOZ fusion leukemia cells. In these cells, expressions of monocyte/macrophage lineage characteristic genes (C/EBPa, Irf8, CD68 etc.) and MLL fusion target genes (Meis1, Mef2c) were decreased. In contract, other hematopoietic lineage characteristic genes (GATA1-3, FOG-1, CD41, Aiolos, Helios, Eag, Epx etc.) were increased. In addition, expression of CDK inhibitor INK4A was also up-regulated.

Finally, we tested requirement of endogenous MOZ in various cellular conditions. Previous report showed that AML development was induced by introduction of MOZ-TIF2 not only in hematopoietic stem cells but also in more differentiated Common myeloid progenitors (CMPs) and Granulocyte/Monocyte progenitors (GMPs) (Huntly et al, Cancer Cell 2004). So we introduced MOZ fusion genes in HSCs and CMPs collected from E14.5 MOZ-/- fetal liver. MOZ-/- HSCs, not CMPs, expressing MOZ-TIF2 continuously formed colonies in vitro. In the CMPs expressing MOZ-TIF2, expression of both Meis1 and HoxA9, were abolished.

These results suggest that high levels of HoxA9 and Meis1 expressions were respectively required for MOZ-TIF2-induced AML development, and that endogenous MOZ is critical for MOZ-TIF2-induced AML development.