Runx1 Is Required for Cbfb-MYH11 Activity During Primitive Hematopoiesis

Abstract 1357

Inversion of chromosome 16 (Inv(16)) is found in nearly all patients with acute myeloid leukemia (AML) subtype M4 with eosinophilia. Inv(16) results in the fusion of the transcription factor gene CBFB, and the MYH11 gene, which encodes Smooth Muscle Myosin Heavy Chain (SMMHC). It has been proposed that the Inv(16) fusion protein, CBFβ-SMMHC, initiates leukemogenesis through repression of the transcription factor RUNX1. However, we recently found that CBFβ-SMMHC also has activities that are independent of Runx1 repression. Mice expressing a knockin allele of Cbfb-MYH11 (Cbfb^+/MYH11) show a severe differentiation defect during primitive hematopoiesis. Interestingly, this severe defect is not seen in mice homozygous for a null allele of Runx1 (Runx1^−/−). We also found genes deregulated uniquely in Cbfb^+/MYH11 embryos were also expressed in leukemic cells from mice and Inv(16) patients. These findings imply that CBFβ-SMMHC's Runx1 repression independent activities play a role during leukemogenesis.

There are two potential models to describe these observations: a Runx1-independent model and a Runx1-dependent model. In the first model, Cbfb-MYH11 initiates leukemia through a novel mechanism that does not involve Runx1. In the second model, Cbfb-MYH11 cooperates with Runx1, but rather than repressing Runx1 activity, Cbfb-MYH11 alters it. To test these two models, we generated mice that express a conditional allele of Cbfb-MYH11 on a Runx1^−/− background. Surprisingly, we found a complete rescue of the Cbfb-MYH11 induced primitive hematopoietic defect: the primitive blood from Runx1^−/−; Cbfb^+/MYH11 embryos was indistinguishable from Runx1^−/−; Cbfb^+/+ embryos. Preliminary data showed that Cbfb-MYH11 expression in the embryos was not affected by the loss of Runx1, indicating that Runx1 is not regulating Cbfb-MYH11 expression, but is required for its activity. Interestingly, another allele of Runx1 in which Runx1 is fused to the β-galactosidase gene, lacZ (Runx1^lzd) and that has been shown to have dominant negative activities (Nancy Speck, personal communication) also rescued the primitive blood defect in the Cbfb^+/MYH11 embryos. This demonstrates the Cbfb-MYH11 requires Runx1 activity for its primitive hematopoietic differentiation defect. Based on this finding, we hypothesize that Cbfb-MYH11 may have a similar requirement for Runx1 activity during leukemogenesis. Consistent with this idea, we found that adult mice expressing Cbfb-MYH11 and the Runx1-lzd allele showed a significant delay in the development of leukemia as compared to their Cbfb^+/MYH11; Runx1^+/+ littermates.