Mef2c is not required for in the induction of leukemia in vivo. (A) Kaplan-Meier survival curves of B6 mice receiving BM from mice with indicated genotype after transduction with retroviral vectors coexpressing MLL/ENL and GFP. The log-rank test for comparison of cumulative incidence curves confirmed a significant (P < .008) increase in disease latency in mice receiving Mef2-deficient BM. (B) Survival curves of B6 mice receiving 5 × 10⁶ tumor cells (intravenously) from 1 of 3 independent mice (for each genotype) from experiment shown in panel A. The log-rank test for comparison of cumulative incidence curves confirmed a significant (P < .001) increase in disease latency in mice retransplaned with Mef2-deficient tumors. (C) Cytospin of BM cells from mice transplanted with MLL/ENL infected Mef2c^Δ/− BM (5% efficiency) and killed 50 days later. An accumulation of blastlike cells and immature myeloid forms is clearly seen (Pappenheim stain; 63×/1.4 oil). (D) Flow cytometric analysis of cells isolated from BM of mice 50 days after receiving BM (with the indicated genotype) infected with MLL/ENL vector (5% efficiency). High proportion of GFP⁺ cells was observed in all mice. Each symbol in the dot diagram to the right represents an independent mouse. Horizontal line indicates median value. P value was calculated by the Student t test. Gating on the GFP⁺ population of the BM (bottom panels) demonstrates that the MLL/ENL/GFP⁺ cells are CD11b⁺Gr1^{− /lo}. (E) Dot diagram showing the spleen weights of mice analyzed 50 days after BM transplantation. Horizontal line indicates median value. P value was calculated by the Student t test. (F) Splenic section of mice receiving Mef2c^Δ/− BM infected with MLL/ENL vectors. The regular splenic architecture is completely effaced in mice receiving MLL/ENL-infected BM because of the myeloid hyperplasia of the red pulp. Pockets of erythroid cells are observed, which compensates for the loss of erythropoiesis in the BM (hematoxylin and eosin stain; 10×/0.45). (G) Flow cytometric analysis of cells isolated from spleens of mice 50 days after receiving BM (with the indicated genotype) infected with MLL/ENL vector (5% efficiency). A higher proportion of GFP⁺ cells was observed in mice receiving Mef2c^+/+ BM compared with Mef2c^Δ/− BM. Each symbol in the dot diagram to the right represents an independent mouse. P value was calculated by the Student t test. Gating on the GFP population of the spleen (bottom panels) demonstrates that the MLL/ENL/GFP⁺ cells are CD11b⁺Gr1^{neg /lo}. (H) The highly invasive behavior of the MLL/ENL Mef2c^+/+ leukemic cells is evidenced by infiltration into the liver. The normal liver lobular architecture is completely destroyed and periportal and perivenous liver parenchyma is replaced by infiltrating leukemic myeloblasts and immature forms (top panel). In contrast, invading cells are limited to periportal regions in mice receiving MLL/ENL Mef2c^Δ/− BM (hematoxylin and eosin stain; 20×/0.75). (I) Leukocyte counts of mice 50 days after receiving BM (with the indicated genotype) infected with MLL/ENL vector (5% efficiency). (J) Blood smears of mice receiving MLL/ENL Mef2c^+/+ (top panel) or Mef2c^Δ/− BM (bottom panel). The leukocytosis is predominantly composed of immature blastlike monocytic and granulocytic cells (Pappenheim stain; 40×/1.3 oil).