Structurally Distinct Ligand-Binding or Ligand-Independent Notch1 Mutants Are Leukemogenic but Affect Thymocyte Development, Apoptosis, and Metastasis Differently

We previously found that provirus insertion in T cell tumors of mouse mammary tumor virus/c-myc transgenic (Tg) mice induced two forms of Notch1 mutations. Type I mutations generated two truncated molecules, one intracellular (IC) (Notch1IC) and one extracellular (Notch1EC), while in type II mutations Notch1 was deleted of its C terminus (Notch1ΔCT). We expressed these mutants in Tg mice using the CD4 promoter. Both Notch1IC and Notch1ΔCT, but not Notch1EC, Tg mice developed double-positive (DP) thymomas. These disseminated more frequently in Notch1ΔCT Tg mice. Double (Notch1IC × myc) or (Notch1ΔCT × myc) Tg mice developed thymoma with a much shorter latency than single Tg mice, providing genetic evidence of a collaboration between these two oncogenes. FACS analysis of preleukemic thymocytes did not reveal major T cell differentiation anomalies, except for a higher number of DP cells and an accumulation of TCRhighCD2highCD25high DP cells in Notch1IC, and less so in Notch1ΔCT Tg mice. This was associated with enhanced in vivo thymocyte proliferation. However, Notch1IC, but not Notch1ΔCT, DP thymocytes were protected against apoptosis induced in vivo by dexamethasone and anti-CD3 and in vitro by anti-CD3/CD28 Abs. This indicates that the C terminus of Notch1 and/or the conserved regulation by its ligands have a significant impact on the induced T cell phenotype. Therefore, Notch1IC and Notch1ΔCT behave as oncogenes for T cells. Because these two Notch1 mutations are very similar to those described in some forms of human T cell leukemia, these Tg mice may represent relevant models of these human leukemias.