Abstract 5829: Targeting TSLP-induce upregulation of Mcl-1 for the treatment of Ph-like ALL with CRLF2 alterations

B cell precursor acute lymphoblastic leukemia (B-ALL) is the most common childhood cancer. The subset of pediatric B-ALL patients at greatest risk for relapse and death have a gene expression profile similar to Philadelphia chromosome positive ALL. Approximately half of these Ph-like B-ALL are defined by genetic alterations that result in overexpression of the cytokine receptor, CRLF2. Stimulation of the CRLF2 receptor by the cytokine, TSLP, causes downstream activation of the JAK/STAT5 and PI3/AKT/MTOR pathways. Activation of these pathways has been associated with oncogenesis and chemoresistance. A gene target of activated STAT5 in B cell precursors is Mcl-1, a Bcl2 family pro-survival molecule. Further, Mcl-1 protein levels are increased through post-transcriptional mechanisms induced by activation of the mTOR pathway. We hypothesized that circulating TSLP cytokine contributes to chemoresistance by increasing CRLF2 activation leading to increased Mcl-1 expression and that targeting Mcl-1 could be an effective strategy for treating Ph-like B-ALL with overexpression of CRLF2 (CRLF2 B-ALL). To test this hypothesis we cultured human CRLF2 B-ALL cell lines (MUTZ5 and CALL4) with and without TSLP for 3 days and evaluated expression of Mcl-1 by flow cytometry. We found that TSLP induced significant increases in Mcl-1 proteins in both cell lines. To determine if these results are reflective of what happens in patients, primary CRLF2 B-ALL cells from pediatric patients were cultured with physiological levels of TSLP (~20 pg/ml) and similarly evaluated. Physiological TSLP significantly increased Mcl-1 protein in primary CRLF2 B-ALL cells, including those with activating JAK mutations. Our next question was whether TSLP-induced increases in Mcl-1 could be effectively targeted with Mcl-1 inhibitors (MIM-1 or Maritcolax). CRLF2 B-ALL cells were incubated with and without TSLP and treated with increasing doses of Mcl-1 inhibitor. Dose responses were evaluated by flow cytometry after 2 or 3 days. Mcl-1 inhibitors induced dose-dependent decreases in cell count and increases in caspase-3 activation and apoptosis (Annexin V/7-AAD). These corresponded with dose-dependent decreases in Mcl-1 protein, suggesting that both inhibitors target Mcl-1 for degradation. MIM-1 and Maritoclax showed efficacy against both CRLF2 B-ALL cell lines and primary patient samples, including those with activating JAK mutations, although cells cultured with TSLP typically required twice the dose of Mcl-1 inhibitor to achieve the same effect observed without TSLP. These data provide evidence that TSLP can contribute to leukemia cell survival and identify Mcl-1 inhibitor as a candidate therapy for CRLF2 B-ALL. Ongoing studies are evaluating the efficacy of the Mcl-1 inhibitor, Maritoclax, in novel patient-derived xenograft models of CRLF2 B-ALL that provide physiological levels of human TSLP.Citation Format: Cornelia Stoian, Nathaniel George Mambo, Pierce McCarthy, Veriah Vidales, Jacqueline S. Coats, Ineavely Baez, Sinisa Dovat, Shadi Farzin Gohar, Dhimant Desai, Muhammad Kamal, Kimberly J. Payne. Targeting TSLP-induce upregulation of Mcl-1 for the treatment of Ph-like ALL with CRLF2 alterations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5829. doi:10.1158/1538-7445.AM2017-5829