The Role of CBFβ in T Cell Development.

Core binding factor β (Cbfβ) is a transcription factor that heterodimerizes with Runx (Cbfα) family members, thereby stabilizing the interaction between the Runx proteins and DNA. Genetically manipulated mouse models of Runx and Cbfb genes have demonstrated their critical functions in hematopoiesis (Runx1, Runx3 and Cbfb), bone formation (Runx2, Cbfb), proliferation of gastrointestinal epithelia (Runx3) and differentiation of dorsal root ganglion cells (Runx3). Studies on T cell development showed that Runx1 and Runx3 repress CD4 expression at different stages of development. In addition, Runx 1 and Runx 3 are required for CD8 T cell development during thymopoiesis. No defects were found when Runx2 was inactivated, even though it is expressed throughout T cell development.

We have previously generated a knock-in mouse model expressing the Cbfb-MYH11 fusion gene (which is created by inv(16)(p13; q22) in human AML M4Eo). Heterozygous knock-in mice had a phenotype identical to that of the Cbfb and Runx1 null mice (embryonic lethality), suggesting that the fusion gene Cbfb-MYH11 functions in a dominant-negative manner. In order to study the function of Cbfb gene in T cell development, we used a mouse line with floxed exons 5 and 6 of Cbfb inserted 5′ to the Cbfb-MYH11 fusion cassette, which produced pseudo-normal mice (loxKI). By crossing the loxKI mice with mice expressing the Cre gene under the control of the T cell-specific Lck promoter (LckCre), we generated LckCre-loxKI double positive mice, in which the floxed exon 5 and 6 were deleted and Cbfb-MYH11 re-expressed only in the thymus when Lck started to express. The LckCre-loxKI mice were viable. However, their thymic development was severely impaired: The size of the thymuses in the mutant mice was about half the normal size, and the total number of thymocytes in the mutant mice was 10–20-fold reduced. FACS analysis of thymocytes from 4 to 12 week old mice showed a developmental blockade at the CD4/CD8-double negative (DN) stage, which was characterized by lower percentage of double positive cells and higher percentage of double negative cells. In addition, the CD4: CD8 ratio was altered. Furthermore, the mature T cell population size in the spleen of the mutant mice was lower than that of the control mice. Our preliminary data suggested that Cbfb plays an important role in T cell development. The mechanism through which Cbfb affects the T cell development is currently under investigation. It is likely that the phenotype reflects the combined effect of missing all three Runx genes, since the phenotype described here is more severe than either Runx1 or Runx3 null alone.