Abstract 2334: Targeting inhibitory phosphatase signaling in Ph+ ALL.

Background: Current therapy approaches for tyrosine kinase-driven leukemias including Ph+ ALL and CML are almost entirely focused on the development of more potent tyrosine kinase inhibitors (TKI) with the goal to reduce oncogenic signaling below a minimum threshold that is required for the survival of leukemia cells. Studying regulators of BCR-ABL1 kinase signaling strength, we found that three key inhibitory phosphatases (INPP5D, PTEN and PTPN6) are expressed at high levels in Ph+ ALL cells. Both INPP5D and PTEN negatively regulate PI3K/AKT pathway. Like INPP5D, PTPN6 is recruited to ITIM motifs in the cytoplasmic tails of inhibitory surface recepotors and negatively regulates activation signals from tyrosine kinases and activating receptors. Genetic models: We hypothesized that Cre-mediated deletion of INPP5D, PTEN and PTPN6 will result in increased oncogenic signaling downstream of BCR-ABL1 and a more aggressive form of leukemia. Surprisingly, however, genetic deletion of either INPP5D, PTEN or PTPN6 resulted in drastic upregulation of reactive oxygen species (ROS), accumulation of Arf, p53 and p21, cellular senescence and subsequent cell death of leukemia cells. Inducible deletion of any of these genes profoundly affected B cell lineage leukemia, deletion of these phosphatases and had no effects in myeloid leukemias. Studying BCR-ABL1-transformed Inpp5dfl/fl,Ptenfl/fl and Ptpn6fl/fl leukemia cells in vivo, we observed that induction of Cre-mediated deletion resulted in rapid leukemia regression and prolonged survival of leukemia transplant recipient mice. Pharmacological targeting: Targeted blockade of inhibitory phosphatases for the treatment of Ph+ ALL seems counter intuitive because it represents effectively the opposite of current TKI-based therapies. Small molecule inhibition of Inpp5d using the compound 3AC effectively killed patient-derived Ph+ leukemia cells. 3AC inhibition of Inpp5d dramatically increased levels of ROS and increased phosphorylation of the stress-associated MAP kinases p38α and JNK and substantially prolonged overall survival of NOD/SCID transplant recipients of patient-derived Ph+ ALL cells. Based on these findings, we propose that pharmacological blockade of inhibitory phosphatases represents a powerful means to induce leukemia cell death owing to excessive oncogene signaling. Conclusions: According to our concept, tyrosine kinase-driven leukemia cells can only thrive within a certain "comfort zone" of signal strength. Both attenuation below and exaggeration above this "comfort zone" of signal strength result in cell death. If validated, our approach of phosphatase-inhibition will lead to the discovery and development of multiple new targets for therapy and will significantly broaden currently available treatment options for B cell lineage Ph+ ALL and other acute leukemias carrying oncogenic tyrosine kinases. Citation Format: Seyedmehdi Shojaee, Maike Buchner, Sigal Gery, Huimin Geng, Lai N. Chan, Ari Melnick, Phillip H. Koeffler, Markus Muschen. Targeting inhibitory phosphatase signaling in Ph+ ALL. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2334. doi:10.1158/1538-7445.AM2013-2334