Targeting the UPR-Transcription Factor XBP1 to Overcome Drug-Resistance in Ph⁺ ALL

The unfolded protein response (UPR) is a cellular machinery required to salvage of ER stress and to promote cell survival. The pathway consists of three different components, namely inositol-requiring enzyme 1a (IRE-1), PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6) and converges at the level of its effector molecule X-box binding protein 1 (XBP1). Previous work identified Xbp1 as a central requirement of plasma cell development and as critical mediator of cell survival in plasma cell-derived multiple myeloma.

While the role of Xbp1 in plasma cells and plasma cell malignancies is well established, we report here the unexpected finding of a central role of Xbp1 in the survival of pre-B cell-derived Ph⁺ ALL cells. Surprisingly, patient-derived Ph⁺ ALL cells express Xbp1 (and related molecules in the IRE1 pathway) at significantly higher levels than normal bone marrow pre-B cells. In addition, we found that high expression levels of Xbp1 at diagnosis predict poor poor overall survival (OS), relapse-free survival (RFS) of leukemia patients in two clinical trials for patients with high risk acute lymphoblastic leukemia (n=207; COG P9906 trial; p=1.12e-4 and ECOG E2993; n=215; p=2.48e-5). In addition, high levels of XBP1 correlated with positive minimal residual disease (MRD) status at day 29 after onset of chemotherapy. To study the function of Xbp1 in Ph⁺ ALL in a genetic experiment, we developed a Ph⁺ ALL leukemia model based on bone marrow progenitor cells from mice carring loxP-flanked allele of Xbp1 (Xbp1^fl/fl). On the basis of this model, bone marrow B cell precursors were transformed by BCR-ABL1 in the presence of IL7. Inducible Cre-mediated deletion of Xbp1 was achieved by transduction of leukemia cells with tamoxifen (4-OHT)-inducible Cre. Interestingly, 4-OHT-induced deletion of Xbp1 in Ph⁺ ALL-like leukemia cells caused rapid cell death within two days of induction. Xbp1-deletion resulted in extensive apoptosis, cellular senescence and cell cycle arrest owing to increased levels of p53, p21 and Arf. Interestingly, similar observations were made in an in vivo setting where Xbp1-deletion resulted in prolonged survival of NOD-SCID transplant recipient mice (n=7; p=0.007). Mechanistically, deletion of Xbp1 leads to increased expression of the pro-apoptotic molecule CHOP as in plasma cells/multiple myeloma and phosphorylation of the stress MAP kinases p38 and JNK.

To test the potential clinical relevance of these findings, we used a recently identified small-molecule inhibitor STF-083010 (Papandreou et al., 2011), which blocks the endonuclease activity of upstream molecule IRE-1, essential for the splicing of the active form of Xbp1. STF-083010 indeed inhibited splicing of XBP1 and overall mimicked findings in genetic experiments. Importantly, targeting of Xbp1 by STF-083010 also induced cell death in three patient-derived cases of Ph⁺ ALL carrying the T315I mutations, which confers far-reaching TKI-resistance.

These findings identify Xbp1 as a fundamentally novel target for the therapy of TKI-resistant Ph⁺ ALL. Like plasma cells and tumor cells in multiple myeloma, Ph⁺ ALL cells are selectively sensitive to ER stress and critically dependent on Xbp1 and likely other factors of the UPR pathway. Clinical validation of this concept could lead to improved treatment options for patients with TKI-resistant Ph⁺ ALL.

No relevant conflicts of interest to declare.