CXCR4 Invalidation Limits the MLL-ENL Induced Leucemogenesis in Vivo

Abstract 4089

The receptor CXCR4 and its ligand SDF-1 play major physiological roles especially on hematopoietic stem cells homing and retention. Many studies have implicated CXCR4 in the invasion by tumor cells of organs that produce SDF-1. In acute myeloid leukemia, the physiological role of CXCR4 is not fully understood. We used retrovirus to express MLL-ENL oncogene in CXCR4^+/+ and CXCR4^−/− hematopoietic primitive cells (Lin- isolated from fetal liver) and showed that CXCR4 is dispensable for generation of immortalized colonies in vitro. To determine CXCR4 function in vivo, CXCR4^+/+ and CXCR4^−/− transformed cells were transplanted into lethally irradiated mice. Whatever their phenotype, the recipient developed a myelo-monocytique leukemia characterized by their expression of Gr-1 and Mac-1. As expected, all recipients of MLL-ENL transduced CXCR4^+/+ cells were moribund within 35 to 80 days post transplant (median survival time: 50 days). Strikingly, recipients of MLL-ENL transduced CXCR4^−/− cells showed significantly increased lifespan, with a median survival time of 90 days. The cellularity of the peripheral blood of recipients of MLL-ENL transduced cells displayed considerable increases over time although this increase was much lower in CXCR4^−/− than in CXCR4^+/+ chimera. Bone marrow of MLL-ENL transduced CXCR4^−/− chimera had moderately decreased numbers of mononuclear cells. There were important numbers of leukemic CD45.2⁺/Gr1⁺/Mac1⁺/c-kit+ cells in spleen from MLL-ENL CXCR4^+/+ chimera which suggested that CXCR4 is important for leukemic progenitors cells retention in the bone marrow and especially in the spleen. The homing capacity of transduced CXCR4^+/+ cells is comparable to the CXCR4^−/− cells. Finally, more DNA damages were found in the BM cells of MLL-ENL CXCR4^−/− chimera. All these results were confirmed by treating of MLL-ENL CXCR4^+/+ chimera with CXCR4 inhibitor (TN140).

These results demonstrated that in absence of CXCR4, the cells transduced by oncogene MLL-ENL are capable of generating leukemia in the recipients. However, mice transplanted with MLL-ENL transduced CXCR4^−/− FL cells developed acute myeloid leukemia with reduced aggressiveness and organ infiltration, which is associated with induced differentiation and DNA instability. These results indicated that the MLL-ENL progenitors are dependent on CXCR4 for their maintenance in the BM and spleen suggesting that CXCR4 inhibitors might have potential therapeutic applications.