RAS oncogene suppression induces apoptosis followed by more differentiated and less myelosuppressive disease upon relapse of acute myeloid leukemia

Abstract

To study the oncogenic role of the NRAS oncogene (NRAS G12V ) in the context of acute myeloid leukemia (AML), we used a Vav promoter-tetracycline transactivator (Vav-tTA)-driven repressible TRE-NRAS G12V transgene system in Mll-AF9 knock-in mice developing AML. Conditional repression of NRAS G12V expression greatly reduced peripheral white blood cell (WBC) counts in leukemia recipient mice and induced apoptosis in the transplanted AML cells correlated with reduced Ras/Erk signaling. After marked decrease of AML blast cells, myeloproliferative disease (MPD)-like AML relapsed characterized by cells that did not express NRAS G12V . In comparison with primary AML, the MPD-like AML showed significantly reduced aggressiveness, reduced myelosuppression, and a more differentiated phenotype. We conclude that, in AML induced by an Mll-AF9 transgene, NRAS G12V expression contributes to acute leukemia maintenance by suppressing apoptosis and reducing differentiation of leukemia cells. Moreover, NRAS G12V oncogene has a cell nonautonomous role in suppressing erythropoiesis that results in the MPD-like AML show significantly reduced ability to induce anemia. Our results imply that targeting NRAS or RAS oncogene-activated pathways is a good therapeutic strategy for AML and attenuating aggressiveness of relapsed AML. IntroductionKelly and Gilliand have proposed that acute myeloid leukemia (AML) is induced by the cooperation of 2 general classes of mutations. 1 Class I mutations confer cell survival and proliferation advantages and generally result from mutations in genes encoding cell-signaling molecules like NRAS and FLT3. Class II mutations impair differentiation and subsequent apoptosis and, in general, result from mutations in genes encoding transcription factors or chromatin-modifying proteins such as AML1 or mixed lineage leukemia (MLL). Most studies to develop new anticancer drugs have focused on finding efficient inhibitors against the mutant products of class I oncogenes such as ABL, c-KIT, and FLT3. [2][3][4][5][6][7][8] Although previous studies showed that FLT3 inhibition effectively suppresses the cell growth of leukemia induced by the cooperation of an FLT3-activating mutation and an MLL translocation, 9,10 the appropriateness of class I oncogene inhibitors for AML therapy has not been well studied, particularly in leukemia induced in cooperation with a class II oncogene. Conditionally expressed transgenes in mice provide an ideal setting in which to address this issue. Studies have shown that continued expression of oncogenes such as c-Myc or Bcr/Abl is required for maintaining leukemia in transgenic mouse models. [11][12][13] Cancer cells in these models are said to have "oncogene addiction," because they die, differentiate, or become quiescent upon shutting off oncogene expression. 13 To study the functions of a class I mutation in AML, we investigated the role of an NRAS-activating mutation, NRAS G12V , in murine AML induced in combination with the Mll-AF9 oncogene.RAS mutants, which abnormally stimulate RAF/...

Mixed-lineage-leukemia (MLL) fusion protein collaborates with Ras to induce acute leukemia through aberrant Hox expression and Raf activation

“…43 Other groups recently reported that the combination of oncogenic NRAS and MLL-AF9 44 or MLL-ENL 45 is capable of developing AML, and that induced repression of oncogenic NRAS on the combination reverted AML to MPD by the MLL fusion gene (MLL-AF9) alone. 44 Although our findings that MLL fusion proteins and oncogenic NRAS cooperate to induce AML confirmed these notions, the present study further analyzed the involvement of Hoxa9 and Raf, downstream of the cooperation between MLL fusion proteins and oncogenic NRAS. The myeloid transformation assays in vitro showed that the activation of Raf-1, as well as oncogenic NRAS, transformed A9G, a cell line expressing Hoxa9.…”

The heterogeneity of pediatric MLL-rearranged acute myeloid leukemia

“…97 Also introduction of FLT3-ITD in MLL-AF9-overexpressing mice accelerated the onset of AML. 98 Although FLT3-ITD is not a frequent event in MLL-rearranged AML, overexpression of FLT3 and a higher sensitivity to FLT3 inhibitors was found in MLL-rearranged ALL.…”

Hematopoiesis and leukemogenesis in mice expressing oncogenic NrasG12D from the endogenous locus

“…28,29 Coexpressing Nras G12D with myeloid fusion oncogenes (PEBP2␤-MYH11 or Mll-Af9) or with BCL-2 induced MDS and AML. 28,30,31 These variable phenotypes probably reflect the promoters used to drive Nras G12D expression and the modifying effects of broadly expressing different cooperating mutations. Other groups used a retroviral transduction/transplantation approach to investigate the transforming potential of Nras G12D .…”