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Pediatric and adult Philadelphia chromosome t(9;22) positive Acute Lymphoblastic Leukemias (ALL) represent a subtype of leukemia with poor prognosis that is most refractory to imatinib treatment. The p190 BCR-ABL gene fusion represents the primary event in the leukemic transformation process with 85% of cases being progenitor B-cell acute lymphoblastic leukemias (B-ALL) while the remaining are T lineage subtypes. It has been established that phosphoinositide 3-kinase (PI3K) activity is required for transformation and leukemogenesis, but the identity of PI3K regulatory or catalytic subunits that are involved remain unanswered. Previous work in our laboratory has indicated that p85α and p85β both regulatory subunits of class IA PI3K, have partially redundant functions in transformation by p190-BCR-ABL. To achieve more complete depletion of class IA PI3K activity in B-ALL, we have generated mice with conditional deletion of p85α on a p85β germline deficient background. We find that double deficient p85αΔ/Δβ-/- (α/β null) bone marrow-derived progenitor B cells can be transformed by p190-BCR-ABL with low efficiency. In the few α/β null colony-forming cells (CFCs) that delete the floxed allele, the majority of class IA PI3K catalytic subunit expression is abolished and PI3K signaling output is diminished, yet mTORC1 and MAPK activity remain. Cell lines lacking PI3K signaling have severe defects in cell cycle transit, survival, and leukemic-initiating potential. We show residual mTOR signaling in α/β null CFCs, which exhibit reduced cellular size, reduced mTORC1 activity, and abolished positive feedback activation of Akt. Furthermore, α/β null cell lines are hypersensitive to rapamycin, imatinib, and a novel dual PI3K/mTOR inhibitor PI-103. Our data highlight that PI3K signaling contributes quantitatively to p190 BCR-ABL transformation and maintenance; moreover, one of the putative downstream targets of PI3K, mTORC1, can still retain partial function and sustain proliferation and survival of leukemic cells without Akt activity. The data address the disadvantage of using mTOR inhibitors as single agents and in dual treatment regimens with imatinib; instead we will propose the use of a PI-103 and similar analogs for future preclinical studies to treat B-ALL. Consistently we find that rapamycin increases Akt signaling, whereas PI-103 potently blocks both mTORC1 and Akt signaling when combined with imatinib. Our biochemical findings complement the dual treatment regimens underway in the clinic with BCR-ABL inhibitors, and suggest targeting of both mTOR and PI3K may be more effective than targeting solely mTOR in Ph+ t(9;22)-ALLs.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA