Targeting Kinase Inhibitor Induced Signaling Plasticity in Patients with Ph-Like ALL - Project Summary Major scientific and therapeutic advances have improved outcomes of patients with B-cell acute lymphoblastic leukemia (B-ALL) over the last four decades. Addition of ABL1 kinase inhibitors to chemotherapy has significantly improved relapse-free and overall survival of children and adults with the Philadelphia chromosome positive (Ph+) ALL subtype. This great treatment success has been attributed to oncogenic addiction of Ph+ leukemias to BCR-ABL1-driven signaling and the ability of ABL1 kinase inhibitors to induce profound cytotoxicity. Philadelphia chromosome-like (Ph-like) ALL is an analogous high-risk leukemia subtype driven by various non- BCR-ABL1 genetic lesions that similarly activate oncogenic kinase signaling. Ph-like B-ALL occurs in 15-40% of children and adults with B-ALL and is associated with a particularly high risk of relapse and death. We identified constitutively activated JAK2/STAT5, PI3K and B cell receptor-like (BCR) signaling in CRLF2-rearranged Ph-like ALL, which can be abrogated with JAK1/2 (ruxolitinib), PI3Kδ (idelalisib), and SRC (dasatinib) inhibition in vitro and in vivo in preclinical models. However, while the triple inhibition results in complete cell death in vitro and significant decrease of the leukemia burden of mice, we doubt that this treatment strategy is clinically translatable for patients given potential for appreciable toxicity. We thus focused on discovering vulnerabilities induced by JAK2-kinase inhibition and identified i) high activity of SGK1, a molecule known to induce drug resistance to PI3K inhibitors in PI3Kα-mutated breast cancer patients, ii) complete inactivity and downregulation of PTPN6, a phosphatase known to downregulate JAK2, PI3K, and BCR signaling, and iii) accumulation of DNA damage with JAK2 inhibition. We hypothesized that activation of SGK1 and inactivation of PTPN6 are required mechanisms to overcome JAK2 inhibitor-induced signaling inhibition. Strikingly, our preliminary data demonstrated that targeting activated SGK1 in JAK2 inhibitor treated samples results in potent in vitro cell death. Furthermore, we hypothesized that the JAK2-inhibitor induced DNA-damage is mediated by the RAG endonuclease complex, which is activated during B cell differentiation to form the BCR. It has been shown that JAK2/STAT5 signaling is downregulated in differentiating B cells to induce a proliferation arrest, which prevents cells from dying from apoptosis during V(D)J recombination. Our preliminary data demonstrate that we can exploit this conserved mechanism by targeting the DNA-damage response molecule CHK1 in JAK2 inhibitor treated Ph-like ALL cells, which may result in massive leukemic cell death due to accumulation of unrepaired DNA damage. The proposed study aims to uncover previously unknown mechanisms of drug resistance and to characterize the underlying biology of Ph-like B-ALL cells to ultimately establish preclinical treatment protocols with the goal to prevent relapse and to cure patients with Ph-like B-ALL.
