High levels of HOXA10 block erythroid development in vivo and lead to accumulation of hematopoietic progenitors. (A) Analysis of blood from rtTA-HA10 mice 2 weeks after induction of HOXA10 expression in vivo revealed reduced levels of hemoglobin (Hgb), red blood cells (RBCs), and platelets (PLTs) (n = 6). (B) Bone marrow cells were plated in methylcellulose supporting the formation of BFU-E colonies, with 4 different concentrations of doxycycline. Colonies were scored after 6 days of culture. Shown are the numbers of BFU-E colonies/100 000 input cells. Black bars represent inducible HOXA10 bone marrow (n = 4), and gray bars represent WT bone marrow (n = 4); *P < .05. (C) FACS analysis of uninduced bone marrow using anti-CD71/anti-Ter119 displaying normal erythropoietic development (I, proerythroblasts; II, basophilic erythroblast; III, late basophilic and chromatophilic erythroblast; IV, orthochromatophilic erythroblast and nonerythroblast) next to a representative bone marrow from a rtTA-HA10 mouse 14 days after induction, showing a block in the erythroid development. Gate I uninduced = 0.7% ± 0.1%, rtTA-HA10 = 23.1% ± 2.3% (n = 4). (D) Bone marrow from the mice above where analyzed for early progenitors using FACS showing an accumulation of c-kit⁺, GMP, and MEP cells. (n = 4) (E) Mice receiving transplants of inducible HOXA10 bone marrow developed anemia with low RBC counts and low Hgb levels. (F) Spleen sections from representative WT and anemic mouse (same mouse as in panel C) stained with hematoxylin-eosin displaying the disrupted spleen structure replaced by immature erythrocytes (magnification, ×100); n = 4; *P < .05. Images were obtained using an Olympus BX50 microscope equipped with a 100×/1.25 NA objective (LR1, Lund, Sweden). Images were acquired using an Olympus C-3040 digital camera (LR1) and were processed using Iphoto 2.0.1 (LDC, Lund, Sweden).