c-MYC Is a Major Downstream Target of NOTCH in T-Cell Acute Lymphoblastic Leukemia.

We recently reported that activating mutations in the NOTCH1 receptor occur in a high percentage of primary human T-cell acute lymphoblastic leukemias (T-ALL). Withdrawal of NOTCH signals by treatment with γ-secretase inhibitors (GSI) or by transduction with a dominant-negative Mastermind-like-1 polypeptide (a specific NOTCH pathway inhibitor) induces growth arrest of many T-ALL cell lines, suggesting that NOTCH supplies signals that are needed for maintenance of growth of T-ALL cells. In order to identify downstream targets of NOTCH that mediate these effects, we performed gene expression profiling on NOTCH signaling-dependent T-ALL cell lines before and after NOTCH inhibition. Among a number of identified candidate genes was c-MYC, which was of particular interest given its importance in promoting cellular growth and its known dysregulation in a number of hematolymphoid neoplasms. c-MYC was down-regulated following NOTCH inhibition, and rapidly up-regulated following release of NOTCH inhibition, even in the presence of protein synthesis inhibitors, suggesting that it is a direct NOTCH transcriptional target. Further, a subset of murine and human T-ALL cell lines were rescued from GSI-mediated growth arrest by c-MYC-expressing retroviruses. The failure of c-MYC to rescue some NOTCH-dependent cell lines likely stems from differences in cellular context, such as collaborating oncogenic lesions and/or the stage of T cell development the cell lines recapitulate. Nevertheless, these data implicate c-MYC as a major downstream target of NOTCH in T-ALL.