Mir∼17-92 Identifies BCL2 As a Therapeutic Target In BCR-ABL Positive B-Lineage Acute Lymphoblastic Leukemia

Despite advances in both targeted therapies with ABL-specific tyrosine kinase inhibitors and in allogeneic stem cell transplantation, BCR-ABL positive acute lymphoblastic leukemia (ALL) remains a very high-risk disease, necessitating the development of novel treatment strategies. miRNAs are small non-coding RNAs which regulate gene expression posttranscriptionally in a sequence-specific manner. miRNAs usually repress the expression of many target genes. We hypothesized that miRNAs may help to identify potential therapeutic targets if (i) they are expressed in a disease-specific manner and if (ii) modulating their expression induces a desired phenotype, such as apoptosis of tumour cells, in appropriate experimental models. Based on our observation that miR∼17-92-encoded miRNAs are significantly less abundant in primary BCR-ABL-positive as compared to negative ALL-cells, we studied the expression and function of miRNAs encoded by the miR∼17-92 derivative miR∼17-19b in a murine pro-B-cell line with inducible BCR-ABL-expression (TonB). Induction of BCR-ABL expression in TonB cells reduced endogenous miR-17, miR-18a, and miR-19 by 2 to 3.5-fold, confirming that expression of the miR∼17-92 cluster is controlled by BCR-ABL. Interestingly, over-expression of miR∼17-19b by lentiviral gene transfer led to a substantial induction of apoptosis in TonB cells in a BCR-ABL-dependent manner. To identify potential miRNA targets, we used a proteomic approach based on stable isotope labeling of amino acids in cell culture (SILAC) followed by liquid chromatography and mass spectroscopy (LC-MS) in miR∼17-19b transgenic TonB cells. Several apoptosis-related proteins were differentially expressed including Bcl2, an established inhibitor of mitochondrial pro-apoptotic pathways. The miRNA target prediction program RNA22 predicted several miR∼17-19b miRNA-binding sites within both murine and human Bcl2 mRNA, and we demonstrated direct miRNA binding to Bcl2 mRNA by luciferase reporter and anti-AGO2 RIP chip analyses. As with miR∼17-19b over-expression, Bcl2 specific RNAi strongly induced apoptosis in murine TonB and the human BCR-ABL-positive cell lines BV-173, Tom1 and SupB15. BCR-ABL positive human ALL-cell lines were also more sensitive than negative ones to pharmacological BCL2 inhibition with the BH3 mimetic ABT 737. In addition, inhibition of BCL2 by ABT 737 and BCR-ABL kinase activity by Imatinib exert different anti-leukemic effects with differential impact on miR∼17-92 miRNA-expression. To assess the therapeutic potential of BCL2 inhibition we used a xenotransplantation assay with real time in vivo monitoring of drug therapies by bioluminescent imaging. ABT-737 treatment substantially inhibited expansion of luciferase-expressing human primary BCR-ABL-positive ALL xenografts in NOD/LtSz-scid IL-2Rγ null (NSG) mice and significantly lengthened their median survival. Taken together, our data identify BCL2 as a therapeutic target of particular relevance in BCR-ABL-positive ALL and indicate involvement of miR∼17-92-encoded miRNAs in regulation of apoptosis in these cells. The validity of this miRNA-based approach to identify potential drug targets is demonstrated by the efficacy of the BCL2 inhibitor ABT-737 in an in vivo model of human BCR-ABL positive ALL, suggesting that BCL2 inhibition should be considered for early phase clinical testing as a strategy to improve disease outcomes.