The acute myeloid leukemia (AML)-associated CBF beta-SMMHC fusion protein impairs hematopoietic differentiation and predisposes to leukemic transformation. The mechanism of leukemia progression, however, is poorly understood. In this study, we report a conditional Cbfb-MYH11 knockin mouse model that develops AML with a median latency of 5 months. Cbf beta-SMMHC expression reduced the multilineage repopulation capacity of hematopoietic stem cells (HSCs) while maintaining their numbers under competitive conditions. The fusion protein induced abnormal myeloid progenitors (AMPs) with limited proliferative potential but leukemic predisposition similar to that of HSCs in transplanted mice. In addition, Cbf beta-SMMHC blocked megakaryocytic maturation at the CFU-Meg to megakaryocyte transition. These data show that a leukemia oncoprotein can inhibit differentiation and proliferation while not affecting the maintenance of long-term HSCs.

Acute myeloid leukemia subtype M4 with eosinophilia is associated with a chromosome 16 inversion that creates a fusion gene CBFB-MYH11. We have previously shown that CBFB-MYH11 is necessary but not sufficient for leukemogenesis. Here, we report the identification of genes that specifically cooperate with CBFB-MYH11 in leukemogenesis. Neonatal injection of Cbfb-MYH11 knock-in chimeric mice with retrovirus 4070A led to the development of acute myeloid leukemia in 2-5 months. Each leukemia sample contained one or a few viral insertions, suggesting that alteration of one gene could be sufficient to synergize with Cbfb-MYH11. The chromosomal position of 67 independent retroviral insertion sites (RISs) was determined, and 90% of the RISs mapped within 10 kb of a flanking gene. In total, 54 candidate genes were identified; six of them were common insertion sites (CISs). CIS genes included members of a zinc finger transcription factors family, Plag1 and Plagl2, with eight and two independent insertions, respectively. CIS genes also included Runx2, Myb, H2-T24, and D6Mm5e. Comparison of the remaining 48 genes with single insertion sites with known leukemia-associated RISs indicated that 18 coincide with known RISs. To our knowledge, this retroviral genetic screen is the first to identify genes that cooperate with a fusion gene important for human myeloid leukemia.