AML1-ETO Collaborates with the Homeobox Gene Meis1 in Inducing Acute Leukemia in the Mouse Bone Marrow Transplantation Model.

AML1-ETO is the most frequent fusion gene in human AML. Previously, we and others have demonstrated that the fusion is not able to cause leukaemia on its own in experimental murine models, but that it needs collaborative partners. However, although mutations such as the FLT3-length mutation and C-KIT mutations were defined as important collaborative genetic events in AML1-ETO positive AML, most human AML1-ETO cases do not carry these mutations, indicating the presence of unkown collaborative partners in these patients. On the other hand Meis1, a HOX gene co-factor, belonging to the TALE family of homeodomain proteins, has a well established function as a protooncogene with a strong collaborative potential in Hox gene associated AML in mice. First we confirmed expression of MEIS1 in some patients with AML1-ETO positive AML by real-time PCR. Based on this we sought to determine if AML1-ETO can collaborate with Meis1 in inducing acute leukemias: single constructs or both genes were co-transfected in 5-FU treated primary murine bone marrow cells by retroviral gene transfer, using MSCV retroviral constructs with an IRES–GFP or YFP cassette. Mice were transplanted with BM cells expressing Meis1 alone (n=10), with BM cells solely expressing the fusion gene (n=10) or EGFP (n=7, control) or with BM expressing both genetic alterations (n=14). None of the mice in the Meis1 and AML1-ETO as well as in the control group developed disease. In contrast, 14 mice transplanted with BM co-expressing AML1-ETO and Meis1 developed lethal disease after a median latency of 102 days. Three mice succumbed to a myeloproliferative syndrome and nine mice died by acute leukemia (6 mice developed AML, 3 mice ALL), which was serially transplantable into secondary recipients (median = 57 days). Immunohistochemistry of various organs of leukemic mice showed massive infiltration with blast cells. In MPS and AML 85 ± 9.3 % of the blast cells co-expressed Gr-1⁺ and Mac1⁺. In ALL cases 40 ± 19.9 % of the malignant cells co-expressed Mac1 and the lymphoid-associated B220 antigen. Analysis of retroviral integration did not reveal recurrent integration sites as an indication for insertional mutagenesis. In summary, our data demonstrate for the first time that AML1-ETO can collaborate with Meis1 and identify a novel collaborative partner in t(8;21) positive AML. Furthermore, our analyses demonstrate that Meis1 can collaborate with non-homeobox genes in inducing acute leukemia.