Abstract 484: Identification of FOXM1 as therapeutic target in Philadelphia chromosome-positive acute lymphoblastic leukemia
Authors: Buchner Maike V. , Park Eugene , Klemm Lars , Geng Huimin , Kopanja Dragana , Raychaudhuri Pradip , Müschen Markus 
Published: May 2015

Abstract
Introduction: The BCR-ABL1 tyrosine kinase induces malignant transformation of B cells at the pre-B cell checkpoint and induces Philadelphia chromosome positive acute lymphoblastic leukemia (Ph+ ALL). In a genome-wide search, we identified FOXM1 as a transcription factor that is specifically upregulated at the pre-B cell checkpoint. FOXM1 is a forkhead box transcription factor and a key regulator of cell growth by promoting cell cycle progression. Results: We transformed murine pre-B cells with a retroviral BCR-ABL1 expression vector and observed upregulation of Foxm1 protein levels. Consistent with this, FOXM1 protein levels in patient-derived Ph+ ALL samples are ∼10-fold higher than in healthy B cells and B cell precursors (n=5; P=0.01). In a cohort of 83 Ph+ ALL patients, the FOXM1 promoter region was significantly de-methylated compared to normal pre-B cells (n=12; P=2.4x10e-7). In order to evaluate a potential predictive value of FOXM1 expression in ALL cells, we correlated FOXM1 mRNA levels with clinical outcome. High FOXM1 expression correlates with shorter overall survival in patients with ALL (n=12, P=0.05) and was predictive of a higher risk of relapse (n=21; P=7.3x10e-5). To study the functional role of FOXM1 in Ph+ ALL, we developed a genetic model for Foxm1 loss-of-function using Foxm1fl/fl mice. Cre-mediated, inducible deletion of Foxm1 decreases ALL cell viability (n=3; P=0.001) and reduced proliferative capacity (n=3; S-phase Foxm1+/+ 59% vs 42% for Foxm1-/-; P=0.0004). The ability to form colonies in vitro was significantly decreased by deletion of Foxm1 (n=3, P=8.9x10e-6). Injection of 1x10e5 Foxm1+/+ or Foxm1-/- leukemic cells into NOD/SCID mice revealed reduced leukemogenesis in vivo (n=7; P=0.0035). In contrast, normal B cell precursors were unaffected by Foxm1 deletion both, in vitro and in vivo. In addition, Foxm1-deleted ALL cells revealed a strikingly higher sensitivity towards Imatinib in dose-response curves compared to control cells (IC50=420 nM for Foxm1+/+ vs 160 nM for Foxm1-/-). Mechanistically, we found that FOXM1 decreased levels of reactive oxygen species via upregulation of the antioxidant response molecule catalase. As potential therapeutic agents to target FOXM1, we evaluated the effects of a previously described ARF peptide (AA 26-44) and the natural occurring antibiotic Thiostrepton. Both bind and inhibit the function of FOXM1, and induced apoptosis in Ph+ ALL. Patient-derived Ph+ ALL cells were xenografted into NOD/SCID recipients and then treated with Thiostrepton i.v. for 7 consecutive days. Thiostrepton-treatment resulted in significantly prolonged overall survival (29 vs 35 days; n=7; P=0.0014) and reduced leukemia burden as measured by luciferase bioimaging, as compared to vehicle controls. Conclusion: Taken together, our data identify the pre-B cell-specific transcription factor FOXM1 as a novel therapeutic target for the treatment of TKI-resistant Ph+ ALL.
Citation Format: Maike V. Buchner, Eugene Park, Lars Klemm, Huimin Geng, Dragana Kopanja, Pradip Raychaudhuri, Markus Müschen. Identification of FOXM1 as therapeutic target in Philadelphia chromosome-positive acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 484. doi:10.1158/1538-7445.AM2014-484