Hoxa9 collaborates with E2A‐PBX1 in mouse B cell leukemia in association with Flt3 activation and decrease of B cell gene expression

Background: The fusion protein E2A‐PBX1 induces pediatric B cell leukemia in human. Previously, we reported oncogenic interactions between homeobox ( Hox ) genes and E2A‐PBX1 in murine T cell leukemia. A proviral insertional mutagenesis screen with our E2A‐PBX1 B cell leukemia mouse model identified Hoxa genes as potential collaborators to E2A‐PBX1 . Here we studied whether Hoxa9 could enhance E2A‐PBX1 leukemogenesis. Results: We show that Hoxa9 confers a proliferative advantage to E2A‐PBX1 B cells. Transplantation experiments with E2A‐PBX1 transgenic B cells overexpressing Hoxa9 isolated from bone marrow chimeras showed that Hoxa9 accelerates the generation of E2A‐PBX1 B cell leukemia, but Hoxa9 is unable to transform B cells alone. Quantitative‐reverse transcriptase polymerase chain reaction analysis demonstrated a strong repression of B cell specific genes in these E2A‐PBX1/Hoxa9 leukemias in addition to Flt3 activation, indicating inhibition of B cell differentiation in combination with enhanced proliferation. Overexpression of Hoxa9 in established E2A‐PBX1 mouse leukemic B cells resulted in a growth advantage in vitro , which was also characterized by an enhanced expression of Flt3 . Conclusions: we show for the first time that Hoxa9 collaborates with E2A‐PBX1 in the oncogenic transformation of B cells in a mouse model that involves Flt3 signaling, which is potentially relevant to human disease. Developmental Dynamics 243:145–158, 2014 . © 2013 Wiley Periodicals, Inc.