A particular leukemia subtype, namely mixed lineage leukemia (MLL)-rearranged leukemia that is a result of chromosomal rearrangements leading to fusions between MLL and partner genes, is associated with a dismal outcome. Therapeutic targeting of MLL rearrangements has proven challenging as there have been dozens of described rearrangements. A characteristic of leukemic cells is to sustain chronic proliferation while avoiding terminal differentiation and it is increasingly apparent that crosstalk between leukemic cells and stroma environment impact tumor progression. Here, we attempt to understand the role of the bone marrow (BM) microenvironment in promoting leukemia development in a murine model of MLL-ENL induced leukemia by manipulating CXCR4/CXCL12 axis. We used retrovirus to express MLL-ENL oncogene in CXCR4+/+ and CXCR4-/- hematopoietic primitive cells isolated from fetal liver and showed that CXCR4 is not required for generation of immortalized colonies in vitro. To determine CXCR4 function in vivo, CXCR4+/+ and CXCR4-/- transformed cells were transplanted into lethally irradiated mice. Strikingly, recipients of MLL-ENL transduced CXCR4-/- cells (CXCR4-/- chimera) showed significantly increased life span, with a median survival time of 93 days compared to 49 days of recipients of MLL-ENL transduced CXCR4+/+ cells (CXCR4+/+ chimera). Whatever their genotype, the recipients developed a leukemia with myelo-monocytic cells characterized by Gr-1 and Mac-1 expression. However, Gr-1 expression was higher in leukemic cells from CXCR4-/- chimera and the proportion of c-kit positive cells was lower. In addition, leukemic colony forming cells were reduced in BM and much severely decreased in spleens of CXCR4-/- chimera. Moreover, leukemic initiating cells (LSC) were reduced in BM of CXCR4-/- chimera. Finally, BM and spleen from CXCR4-/- chimera exhibited less intense Ki-67 staining compared to that of CXCR4+/+ chimera. Altogether, these results suggest that invalidation of CXCR4 limits proliferation and induces differentiation of MLL-ENL leukemic cells. To more precisely characterize the effects of absence of CXCR4 expression, CXCR4+/+ chimera were treated with CXCR4 inhibitor -TN140 for 7 days. Following treatment, functional and phenotypic analysis demonstrated lower leukemic cell numbers in BM and spleen associated with higher mobilization in blood. Moreover, TN140 treatment decreased proliferation and increased apoptosis of leukemic cells.

Collectively, these findings indicate that CXCR4 inhibition results in cell differentiation and reducing LSC numbers, thus representing a potentially promising novel treatment for MLL-ENL related leukemia. Further understanding of the signaling pathway involved in CXCR4 function and BM microenvironment in MLL-ENL related leukemia might lead to the development of effective therapeutic agents that modulate the BM microenvironment and attenuate leukemogenesis.

Citation Format: Yanyan Zhang, Hadjer Abdelouahab, Aline Betems, Monika Wittner, Virginie Joulin, Fawzia Louache. CXCR4 invalidation limits the MLL-ENL induced leukemogenesis in vivo. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4119. doi:10.1158/1538-7445.AM2013-4119