Ikaros regulates Notch target gene expression in developing thymocytes.

Both Ikaros and Notch are essential for normal T cell development. Collaborative mutations causing a reduction in Ikaros activity and an increase in Notch activation promote T cell leukemogenesis. Although the molecular mechanisms of this cooperation have been studied, its consequences in thymocyte development remain unexplored. In this study, we show that Ikaros regulates expression of a subset of Notch target genes, including Hes1, Deltex1, pTa, Gata3, and Runx1, in both Ikaros null T cell leukemia lines and Ikaros null primary thymocytes. In Ikaros null leukemia cells, Notch deregulation occurs at both the level of Notch receptor cleavage and expression of Notch target genes, because re-expression of Ikaros in these cells down-regulates Notch target gene expression without affecting levels of intracellular cleaved Notch. In addition, abnormal expression of Notch target genes is observed in Ikaros null double-positive thymocytes, in the absence of detectable intracellular cleaved Notch. Finally, we show that this role of Ikaros is specific to double-positive and single-positive thymocytes because derepression of Notch target gene expression is not observed in Ikaros null double-negative thymocytes or lineage-depleted bone marrow. Thus, in this study, we provide evidence that Ikaros and Notch play opposing roles in regulation of a subset of Notch target genes and that this role is restricted to developing thymocytes where Ikaros is required to appropriately regulate the Notch program as they progress through T cell development.