Constitutively Active AKT Induces Myeloproliferative Disease, Acute Myeloid Leukemia, and T Cell Lymphoma despite Impaired Engraftment

The phosphoinositide 3-kinase (PI3K)/AKT pathway is commonly dysregulated in human malignancies, including leukemia. AKT, a downstream effector of PI3K, is constitutively phosphorylated in myeloproliferative disease (MPD) and acute myeloid leukemia (AML) patient samples, suggesting that the PI3K/AKT pathway may be an attractive therapeutic target. In myeloid malignancies, this pathway is most commonly activated not by mutations in PI3K, AKT or loss of PTEN, but rather by mutations in a spectrum of upstream tyrosine kinases, such as BCR-ABL, ETV6-PDGFRb, FIPL1-PDGFRa, JAK2V617F, or FLT3-ITD. To further understand the contribution of PI3K/AKT activation to disease pathogenesis, we modeled the activation of AKT in myeloid neoplasms by such upstream effectors using a myristoylated allele of AKT (myr-AKT) that is constitutively activated. Bone marrow from 5-fluorouracil-primed C57 Bl/6 donor mice was transduced with a bicistronic retrovirus expressing myr-AKT and enhanced green fluorescent protein (EGFP) or control retrovirus expressing EGFP alone, and transplanted into 30 and 5 lethally irradiated syngeneic recipients, respectively. The myr-AKT transplant recipients had a median survival of 53 days. Of 30 myr-AKT mice, 27 (90%) developed a myeloproliferative disease (MPD), characterized by splenomegaly, hepatomegaly, expanded Mac1⁺Gr1⁺, Mac1⁺ckit⁺, and CD71⁺Ter119⁺ populations in the bone marrow and spleen, and increased splenocyte myeloid colony formation. Of these 27 myr-AKT mice with MPD, 19 (70%) also had thymic T cell lymphoma, characterized by infiltration of the thymus, heart, lungs, and muscle with CD4⁺/CD8⁺ lymphoblasts. Three of 30 (10%) myr-AKT mice developed acute myeloid leukemia (AML) with phenotypic attributes of erythroleukemia (AML M6) in humans, characterized by infiltration of the spleen, liver and bone marrow with CD71^hickit⁺ blasts. Control EGFP recipients had no evidence of disease. Splenocytes from mice with AML and thymocytes from mice with T cell lymphoma caused disease when transplanted into secondary recipients, whereas splenocytes from mice with MPD were unable to transplant disease. Of note, we observed that myr-AKT expression caused impaired engraftment in recipient mice, as evidenced by a decrease in the %EGFP in the bone marrow over time. Although myr-AKT expressing cells can home normally to the bone marrow, myr-AKT significantly impairs the lodging ability of transduced bone marrow in irradiated recipients by 2 weeks after transplant. Furthermore, we observed an increased rate of apoptosis in myr-AKT-expressing bone marrow and spleen cells in myr-AKT recipient mice. Taken together, these data suggest that constitutive activation of AKT paradoxically increases apoptosis and impairs engraftment of transduced cells, but demonstrate that constitutive activation of AKT alone nonetheless recapitulates the spectrum of human myeloid neoplastic phenotypes associated with activation of upstream tyrosine kinase effectors.