Forced expression of miR-199a drives leukemogenesis in mice. (A) Schematic overview of the HSC transplantation experiment. HSPCs were isolated from Cebpa^Cre/fl fetal livers and infected with MSCV-GFP-miRNA or with MSCV-GFP-EV control viruses. Recipient mice were lethally irradiated (8.5 Gy) and transplanted with transduced cells by tail-vein injection. (B) Cumulative survival of mice transplanted with lin⁻ cells expressing GFP, with miR-199a (n = 5) (P < .0005 compared with EV n = 9), miR-106 (n = 4) (a non-oncogenic miRNA that promotes myeloid progenitor expansion),^28,43  GFP only, or secondary recipients of miR-199a leukemia cells (n = 7) (miR-199 compared with EV P < .0005, and secondary transplants compared with primary tumors P < .03). Statistical significance was calculated with the log-rank Mantel–Cox test. (C) Splenomegaly and anemic femurs and tibiae isolated from miR-199a–transplanted leukemic mice. (D) Fluorescence-activated cell sorter plots showing the percentage of GFP-expressing AML cells in the BM, blood, liver, and spleen of the leukemic mice. (E) Micrographs showing the morphology of leukemic blasts in the different hematologic organs (BM, blood, and spleen). Black bar indicates 10 µm. (F) Flow cytometric analysis of GFP-positive BM cells. GFP-positive AML cells from mouse 2 are an example of a stem cell-like phenotype (c-Kit, Sca1 double-positive), whereas the AML cells from mouse 3 have a progenitor-like (c-Kit high, Sca1 low) phenotype. GFP, green fluorescent protein; LTR, long terminal repeat; SSC-A, side scatter channel-A.