MEIS1 is essential for MOZ fusion-mediated AML development but not for immortalization. (A) AML development of Moz^−/− cells bearing MOZ fusion and MEIS1 genes. The experimental scheme is shown at the top. Moz^−/− FL lineage⁻ HSPCs were infected with the MOZ-TIF2 gene, followed by the MEIS1 gene or empty vector (Mock), and cultured in a liquid medium. After 5 days, these cells (5 × 10⁵) were transplanted into recipient mice, and their survival was analyzed (n = 5 to 8 per group). Survival was compared using the log-rank test. (B-C) Conditional deletion of the Meis1 gene in MOZ-TIF2 AML cells. (B) Survival of Meis1-deleted MOZ-TIF2 AML cells. The experimental scheme is shown at the top; Meis1^f/fCre-ERT2 MOZ-TIF2 AML cells were transplanted into recipient mice, and then tamoxifen (TAM; 80 mg/Kg) or the same dose of corn oil, administered intraperitoneally (n = 8 to 9 per group). Differences in survival were compared using the log-rank test. The lower right panel shows PCR analysis of Meis1 genotypes in BM samples or BM GFP⁺ cells from mice that did and did not develop AML following transplantation with Meis1^f/fCre-ERT2 MOZ-TIF2 AML cells and treatment with TAM. The lower left graph shows the GFP⁺ cell population in the BM of recipient mice in which no AML development was observed 120 days after transplantation with Meis1^f/fCre-ERT2 MOZ-TIF2 AML cells and treatment with TAM. Values indicate the mean numbers of GFP⁺ cells in the BM. (C) Colony formation by Meis1-deleted MOZ-TIF2 AML cells. After replating 3 times, Meis1^f/+ or Meis1^f/fCre-ERT2 MOZ-TIF2 AML cells were treated with 4-hydroxy TAM (4-OHT; 100 nM) or the same dose of ethanol (EtOH) and then serially cultured in methylcellulose medium 3 times. The mean number of colonies was calculated from the results of 3 independent experiments. The lower panel shows genotypes of colonies of Meis1^f/+ and Meis1^f/fCre-ERT2 MOZ-TIF2 AML cells treated with EtOH or 4-OHT. (D) Summary of the findings of this study.