MEK1/2 KINASES DISABLE THE TUMOR SUPPRESSOR FUNCTIONS OF NATIVE BCR AND ABL1 KINASES TO IMPAIR ANTICANCER DRUGS’ RESPONSIVENESS IN TKI-RESISTANT PHILADELPHIA POSITIVE LEUKEMIAS

Introduction. Despite the recent development of novel and effective inhibitors, including the novel class of STAMP inhibitors, tyro-sine kinase inhibitors (TKI) resistance remains a clinically relevant topic in the management of chronic myeloid leukemia. In addition, clinical responses to TKI drugs are short lived in advanced phases of the disease or in Philadelphia positive (Ph+) acute lymphoblastic leukemia, and therefore new treatment strategies are needed for the subset of patients who fail to respond or develop resistance to treat-ments with these targeted agents. Methods. In vitro and in vivo loss- or gain-of-function approaches on model cell lines and patient-derived leukemic samples were exploited to unveil an unprecedented role of MEK1/2 in sustaining and potentiating the oncogenic and prosurvival effects of BCR::ABL1, and hindering the tumor suppressor functions of native BCR and ABL1 kinases. Results. We demonstrated that activated MEK1/2 kinases assem-ble into a pentameric complex with BCR::ABL1, BCR and ABL1 to induce phosphorylation of BCR and BCR::ABL1 at Tyr360 and Tyr177, and ABL1, at Thr735 and Tyr412 residues thus provoking loss of BCR’s tumor-suppression functions, enhanced oncogenic activity of BCR::ABL1, cytoplasmic retention of ABL1 and conse-quently drug resistance. Coherently, pharmacological blockade of MEK1/2 induces dissociation of the pentameric MEK1/2/BCR:: ABL1/BCR/ABL1 complex and causes a concurrent BCRY360/Y177, BCR::ABL1Y360/Y177 and cytoplasmic ABL1Y412/T735 dephosphory-lation thereby provoking the rescue of the BCR’s anti-oncogenic activities, nuclear accumulation of ABL1 with tumor-suppressive functions and consequently, growth inhibition of the leukemic cells and an Arsenic trioxide (ATO) sensitization via BCR-MYC and ABL1-p73 signaling axes activation. Indeed, by leveraging on the capability of the MEK1/2 inhibitors to promote native ABL1 nuclear translocation, the subcellular compartment where it can exert its tumor suppressor functions, we could also demonstrate that the allosteric activation of nuclear ABL1 kinase activity by DPH significantly potentiates the anti-leukemic effects of the MEK1/2 inhibitor irdametinib on model leukemic cell lines and patient-derived leukemic blasts. Lastly, the Mirdametinib/ATO combination signif-icantly prolonged the survival of mice bearing BCR::ABL1T315I–induced leukemia. Conclusions. Our findings reveal a novel oncogenic functional interplay between MEK1/2, ABL1 and BCR signaling kinases, which if pharmacologically disabled by antagonizing MEK1/2 actions ren-ders leukemia cells vulnerable to the ABL1 allosteric activators or antitumor agent ATO with consequent implications for the treatment of TKI-resistant Ph+ leukemias. In fact, our data indicate that repurposing FDA-approved MEK1/2 inhibitors in combination with ATO may represent a novel alternative strategy for the subset of patients who fail to respond and/or develop resistance to standard-of-care TKIs treatments.