Oncogenic roles of RAS in acute myeloid leukemia cooperated with Mll-AF9.

摘要

Three main sections are presented in this thesis. First, we investigated which hematopoietic cells express TRE-driven transgenes when combined with Vav-tTA, because mastocytosis was developed in Vav-tTA TRE-NRASG12V transgenic mice without detectable other diseases. We assayed Vav-tTA -driven luciferase expression in hematopoietic cells including bone marrow-derived mast cells (BMMC) and CD34 positive hematopoietic progenitor cells (HPC) as well as myeloid and lymphoid lineages by live mouse imaging and relative light unit measurement before or after treating Vav-tTA TRE-luciferase co-transgenic mice with doxycycline (Dox). We found that B cells in the bone marrow and T cells in the thymus expresses Vav-tTA-driven luciferase at much higher levels than in myeloid cells, BMMC and CD34 positive HPC, which showed relatively low levels. Thus, we conclude that Vav-tTA-driven NRAS G12V expression is sufficient for mastocytosis development but not for myeloid leukemia, and lymphoid cells are resistant to NRASG12V transformation despite high level of expression.Second, experiments were performed to study the oncogenic role of the NRAS oncogene (NRASG12V) in the context of acute myeloid leukemia (AML). We transplanted AML, which was developed in Vav-tTA TRE-NRASG12V Mll-AF9 transgenic (TRM-transgenic) mice, into recipient SCID mice. Conditional repression of NRASG12V 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 NRASG12V. 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, NRASG12V expression contributes to acute leukemia maintenance by suppressing apoptosis and reducing differentiation of leukemia cells. Moreover, NRASG12V oncogene has a cell non-autonomous role in suppressing erythropoiesis that results in the MPD-like AML showed significantly reduced ability to induce anemia.Third, based on the results finding the relapse of NRASG12V-independent AML, we tested a hypothesis that chemotherapeutic cytarabine (AraC) treatment in addition to the RAS oncogene suppression would prevent or delay the relapse of AML. After the establishment of full-blown AML. We treated recipient mice with Dox and/or AraC (50 mg/kg/day). Compared with recipient mice treated with either Dox or AraC, we found that co-treatment significantly postponed the relapse of resistant AML and increased the survival days of the TRM-transgenic AML recipient mice. These results suggest that oncogenic RAS-targeting therapy may increase the therapeutic potential against drug-resistant AML when combined with chemotherapeutic AraC treatments.Consequently, we conclude the oncogenic roles of NRAS G12V expression in AML induced in cooperation with Mll-AF9 are (1) to induce proliferation of AML blast cells, (2) to induce cell non-autonomous myelosuppression, (3) to suppress apoptosis in AML blast cells, and (4) to inhibit differentiation of AML blast cells. In treatment of AML, oncogenic RAS suppression combined current chemotherapy may improve the therapeutic potential and achieve longer remission period.