e n i c i d e M l a t n e m i r e p x E f o l a n r u o J e h T CORRESPONDENCE Harald von Boehmer: harald_von_boehmer@ dfci.harvard.edu Abbreviations used: array CGH, array-based comparative ge- nomic hybridization; BMT, BM transfer; DAPK, death- associated protein kinase; DNMAML1, dominant-nega- tive mastermind-like 1; ICN, intracellular Notch; SKY, spec- tral karyotype; T-ALL, T cell acute lymphoblastic leukemia. The Rockefeller University Press $30.00 J. Exp. Med. Vol. 205 No. 12 2851-2861 www.jem.org/cgi/doi/10.1084/jem.20081561 A RT I C L E Oncogenesis of T-ALL and nonmalignant consequences of overexpressing intracellular NOTCH1 Xiaoyu Li,1 Fotini Gounari,1,2 Alexei Protopopov,1 Khashayarsha Khazaie,1,3 and Harald von Boehmer1 1Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA 02115 2Department of Medicine, University of Chicago, Chicago, IL 60637 3Department of Microbiology-Immunology, Northwestern University, Chicago, IL 60611 Mutations resulting in overexpression of intracellular Notch1 (ICN1) are frequently ob- served in human T cell acute lymphoblastic leukemia (T-ALL). We have determined the consequences of ICN1 overexpression from retroviral vectors introduced into bone marrow cells. Early consequences are the generation of polyclonal nontumorigenic CD4+8+ T cell receptor (TCR)- + cells that do not qualify as tumor precursors despite the observation that they overexpress Notch 1 and c-Myc and degrade the tumor suppressor E2A by post- translational modification. The first tumorigenic cells are detected among more immature CD4 8+TCR- cells that give rise to monoclonal tumors with a single, unique TCR- chain and diverse TCR- chains, pinpointing malignant transformation to a stage after pre- TCR signaling and before completion of TCR- rearrangement. In T-ALL, E2A deficiency is accompanied by further transcriptional up-regulation of c-Myc and concomitant dysregu- lation of the c-Myc-p53 axis at the transcriptional level. Even though the tumors consist of phenotypically heterogeneous cells, no evidence for tumor stem cells was found. As judged by array-based comparative genomic hybridization (array CGH) and spectral karyo- type (SKY) analysis, none of the tumors arise because of genomic instability. Notch receptors play an important role in cell growth and differentiation. When binding to their ligands, these transmembrane receptors un- dergo a series of proteolytic cleavages that result in the generation of intracellular Notch (ICN), which regulates transcription in a transcriptional complex with several other proteins after nu- clear translocation (1). Mutations that result in overexpression of ICN1 can be found in more than half of human T cell acute lymphoblastic lymphoma (T-ALL) samples (2–4). These find- ings have sparked renewed interest in ICN1- initiated lymphomagenesis. Recent results have indicated that c-Myc is a target of ICN1 and contributes in a major way to the growth of Notch-dependent T-ALL cell lines (5, 6). However, overexpression of c-Myc by itself is insufficient to cause T-ALL, and additional events are required to initiate clonal growth of tumor cells (7, 8). Girard et al. observed that frequent proviral insertions into the Notch1 locus en- hanced T-ALL in c-Myc transgenic mice, sug- The online version of this article contains supplemental material. gesting a collaboration of c-Myc and Notch1 in oncogenesis that may result from the obser- vation that ICN1 and c-Myc target overlap- ping genes (5), or that ICN1 targets distinct genes that facilitate c-Myc–dependent onco- genesis. One of the genes that is affected by Notch1 is the tumor suppressor E2A, which has an important role in the differentiation of T and B lymphocytes (9). Crucial events that eventually lead to ma- lignant transformation are difficult to study ret- rospectively in cell lines with unknown etiology that may have accumulated additional muta- tions permitting growth in vitro. Therefore, we have systematically studied the consequences of overexpression of intracellular Notch 1 intro- duced into BM cells by retroviral vectors (10). The results identify a series of cellular and mo- lecular changes that accompany the generation © 2008 Li et al. This article is distributed under the terms of an Attribution–Non- commercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.jem.org/misc/terms.shtml). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/ by-nc-sa/3.0/). 2851