Abstract
T-cell acute lymphoblastic leukemia (T-ALL) is a rare hematologic malignancy which yearly affects around 15-20 children in the Netherlands (SKION database). Resistance to steroid treatment (i.e. prednisolone and dexamethasone) is frequently observed in T-ALL and predicts for inferior outcome. This thesis therefore focused on the identification of inhibitors that could
... read more
overcome steroid resistance and warrant a steroid-sensitive phenotype of leukemic T-ALL blasts.
Recently, we identified that mutations in the interleukin-7 receptor (IL7R) signaling pathway related to steroid resistance and inferior event-free survival in T-ALL. We modeled various activating IL7R pathway mutations (including. IL7Rα, JAK1 and RAS mutations) in steroid sensitive T-ALL SUPT-1 and P12 Ichikawa cell lines that resulted in high activation of the MAPK-ERK pathway and a steroid-resistant phenotype. We found that activation of MAPK-ERK signaling resulted in the phosphorylation of pro-apoptotic BIM that rendered BIM unable to effectively bind and neutralize anti-apoptotic Bcl2-family member proteins including BCL2, BCLXL and MCL1. MAPK-ERK pathway activation could effectively be abolished by treatment with MEK-inhibitors selumetinib and trametinib that avoided phosphorylation of BIM, and restored binding of BIM to anti-apoptotic proteins. Importantly, selumetinib was highly synergistic with steroid treatment in steroid resistant T-ALL PDX samples of which steroid resistance was dependent (‘IL7-dependent’) or independent (‘IL7-independent’) on the presence of IL-7. In contrast, the JAK1/2 inhibitor ruxolitinib was only synergistic with prednisolone in IL7-dependent steroid resistant PDX samples in the presence of IL-7, but not in IL7-independent steroid resistant PDX samples. Therefore, our study recommends MEK-inhibitors over ruxolitinib for patients with activating IL7R, JAK1 and RAS mutations or aberrant MAPK-ERK signaling in future basket-trials for T-ALL patients.
Additionally, we studied if novel inhibitors that mimic the function of BIM (i.e. ‘BH3-mimetics) are promising in steroid-resistant T-ALL. However, this research indicated that preserving the function of pro-apoptotic BIM (by MEK-inhibitors) seems more effective as therapy to restore a sensitive steroid response than inhibition of anti-apoptotic molecules (by BH3-mimetics) in general.
Thus far, STAT5 activation has been considered as the driving pathway for IL7-dependent steroid resistance. However, we observed that overexpression of the activating STAT5N642H mutation does not provoke steroid resistance in SUPT-1 cells, despite high expression of anti-apoptotic BCL2 and BCLXL by steroid treatment. We therefore hypothesized that the role of sole STAT5 pathway activation in steroid resistance is limited, and that the enhanced induction of BCL2 and BCLXL proteins is neutralized by the upregulation of steroid receptor target gene BIM following steroid exposure.
By studying the three major signaling paths downstream of the IL7R, we conclude a major role for the MAPK-ERK pathway in steroid-resistant T-ALL. Additionally, we demonstrate the opportunities for MEK-inhibitors in the treatment of T-ALL patients. In addition to genetic screening for mutations that can activate MAPK-ERK signaling, it might be beneficial to also screen for MAPK-ERK pathway activation with other arrays. The integration of genomics, transcriptomics and proteomics seems crucial to fully understand which signaling events are active and important for the leukemia, with the aim to improve personalized therapy and therefore outcome for T-ALL patients.
show less
