Notch1 Contributes to Mouse T-Cell Leukemia by Directly Inducing the Expression of c-myc

Abstract

Recent work with mouse models and human leukemic samples has shown that gain-of-function mutation(s) in Notch1 is a common genetic event in T-cell acute lymphoblastic leukemia (T-ALL). The Notch1 receptor signals through a γ-secretase-dependent process that releases intracellular Notch1 from the membrane to the nucleus, where it forms part of a transcriptional activator complex. To identify Notch1 target genes in leukemia, we developed mouse T-cell leukemic lines that express intracellular Notch1 in a doxycycline-dependent manner. Using gene expression profiling and chromatin immunoprecipitation, we identified c-myc as a novel, direct, and critical Notch1 target gene in T-cell leukemia. c-myc mRNA levels are increased in primary mouse T-cell tumors that harbor Notch1 mutations, and Notch1 inhibition decreases c-myc mRNA levels and inhibits leukemic cell growth. Retroviral expression of c-myc, like intracellular Notch1, rescues the growth arrest and apoptosis associated with γ-secretase inhibitor treatment or Notch1 inhibition. Consistent with these findings, retroviral insertional mutagenesis screening of our T-cell leukemia mouse model revealed common insertions in either notch1 or c-myc genes. These studies define the Notch1 molecular signature in mouse T-ALL and importantly provide mechanistic insight as to how Notch1 contributes to human T-ALL.

We thank Gerry Weinmaster for rat notch1 cluster I and II cDNA probes, Jon Aster for the human NOTCH1 ICNX retroviral construct, and Warren Pear for the anti-Notch1 antibody. We thank Merck Research Laboratories, Inc., for supplying the GSI (MRK-003) used in some of the in vitro studies.

V.M.S. is a Lymphoma Research Foundation Fellow, and A.J.C. and M.A.K. are Scholars of the Leukemia and Lymphoma Society of America. This work was supported by grants from the ACS and NCI to A.J.C. and by CA-096899 from the NCI to M.K.