The Homeobox Gene VENTX Collaborates with AML1-ETO in Inducing an Acute Leukemia in Mice

Homeobox genes are key factors in the development of acute leukemias. So far, little is known about the role of non-clustered homeobox genes such as VENTX. The Vent-like homeobox gene VENTX is a member of the Vent gene family in mammals and is the mammalian homolog of the Xenopus xvent gene. Our group has previously shown that VENTX promotes myeloid differentiation when overexpressed in normal CD34⁺ human stem - and progenitor cells and shows high and aberrant expression in human acute myeloid leukemia (AML) characterized by the AML1-ETO (AE) fusion (Rawat et al., PNAS 2010). To prove the functional relevance of aberrant VENTX expression in this AML genotype, we set up in vitro assays and a murine bone marrow transplantation model, mimicking high expression of VENTX by retrovirally engineered expression of the gene in murine bone marrow progenitor cells. In the CFU-S assay coexpression of both genes significantly increased the frequency of spleen colonies 14-fold and 7-fold compared to VENTX and AE, respectively (p<0.02), and more than 7-fold compared to the control (p<0.001). Most importantly and in contrast to AE alone, coexpression of AE and VENTX induced an acute leukemia after a median latency of 11.3 months post transplant. All leukemias evaluated were rapidly transplantable with a median latency until leukemia induced death of 40 days. The median blast percentage in primary recipient mice was 66% (range 35-100%, n=24). Of note, massive expansion of the red pulpa was documented with the appearance of erythroblasts in the spleen. Furthermore, blasts in the bone marrow expressed CD19, partly in combination with Sca1 and Gr1, pointing to an aberrant expression of this antigen in the AE/VENTX leukemias. Ex vivo, leukemic cells grew permanently generating AE/VENTX positive cell lines. RNA-Seq analyses from CD34⁺ cord blood transduced with VENTX documented 279 differentially expressed genes compared to the GFP control, hitting 7 pathways (Hematopoietic cell lineage, Cytokine-cytokine receptor pathway, Jak-STAT signaling pathway, Porphyrin and Chlorophyll Metabolism, Asthma, Thyroid cancer and Endocytosis) in the KEGG-Analysis and showed a downregulation of genes necessary for terminal erythroid differentiation such as the EPO-receptor and GATA1. Taken together, these data characterize VENTX as an important contributing factor to AE positive leukemias. Importantly, coexpression of AE and VENTX allows the generation of a murine AE model, which in contrast to all other published mouse models mimics aberrant expression of CD19 in human AE positive AML, a hallmark of this AML genotype.