E2A-PBX1 Remodels Oncogenic Signaling Networks in B-cell Precursor Acute Lymphoid Leukemia

There is limited understanding of how signaling pathways are altered by oncogenic fusion transcription factors that drive leukemogenesis. To address this, we interrogated activated signaling pathways in a comparative analysis of mouse and human leukemias expressing the fusion protein E2A-PBX1, which is present in 5%-7% of pediatric and 50% of pre-B-cell receptor (preBCR(+)) acute lymphocytic leukemia (ALL). In this study, we describe remodeling of signaling networks by E2APBX1 in pre-B-ALL, which results in hyperactivation of the key oncogenic effector enzyme PLC gamma 2. Depletion of PLC gamma 2 reduced proliferation of mouse and human ALLs, including E2A-PBX1 leukemias, and increased disease-free survival after secondary transplantation. Mechanistically, E2A-PBX1 bound promoter regulatory regions and activated the transcription of its key target genes ZAP70, SYK, and LCK, which encode kinases upstream of PLC gamma 2. Depletion of the respective upstream kinases decreased cell proliferation and phosphorylated levels of PLC gamma 2 (pPLC gamma 2). Pairwise silencing of ZAP70, SYK, or LCK showed additive effects on cell growth inhibition, providing a rationale for combination therapy with inhibitors of these kinases. Accordingly, inhibitors such as the SRC family kinase (SFK) inhibitor dasatinib reduced pPLC gamma 2 and inhibited proliferation of human and mouse preBCR(+)/E2A-PBX1(+) leukemias in vitro and in vivo. Furthermore, combining small-molecule inhibition of SYK, LCK, and SFK showed synergistic interactions and preclinical efficacy in the same setting. Our results show how the oncogenic fusion protein E2A-PBX1 perturbs signaling pathways upstreamof PLC gamma 2 and renders leukemias amenable to targeted therapeutic inhibition. (C) 2016 AACR.