Very rare cases of human T cell acute lymphoblastic leukemia (T-ALL) harbor chromosomal translocations that involve NOTCH1, a gene encoding a transmembrane receptor that regulates normal T cell development. Here, we report that more than 50% of human T-ALLs, including tumors from all major molecular oncogenic subtypes, have activating mutations that involve the extracellular heterodimerization domain and/or the C-terminal PEST domain of NOTCH1. These findings greatly expand the role of activated NOTCH1 in the molecular pathogenesis of human T-ALL and provide a strong rationale for targeted therapies that interfere with NOTCH signaling.T-ALL is an aggressive cancer that preferentially affects children and adolescents. It is commonly associated with acquired chromososomal translocations and other genetic or epigenetic abnormalities, which lead to aberrant expression of a select group of transcription factors (1). NOTCH1 was discovered as a partner gene in a (7;9) chromosomal translocation found in G1% of T-ALLs (2). It encodes a transmembrane receptor that is required for the commitment of pluripotent progenitors to T cell fate (3) and the subsequent assembly of pre-T cell receptor complexes in immature thymocytes (4).Cleavage of pro-NOTCH1 by a furinlike protease during transit to the cell surface (5) produces a NOTCH1 heterodimer comprised of noncovalently associated extracellular (NEC) and transmembrane (NTM) subunits (6). The heterodimerization domain (HD) responsible for stable subunit association consists of a 103 amino acid region of NEC (HD-N) and a 65 amino acid region in NTM (HD-C) (7). Physiologic activation of NOTCH receptors occurs when ligands of the Delta-SerrateLag2 (DSL) family bind to the NEC subunit and initiate a cascade of proteolytic cleavages in the NTM subunit. The final cleavage, catalyzed by ,-secretase (8, 9), generates intracellular NOTCH (ICN), which translocates to the nucleus and forms a large transcriptional activation complex that includes proteins of the Mastermind family (10-12).Prior work has shown that enforced NOTCH1 signaling is a potent inducer of T-ALL in the mouse (13-15) and is required to sustain the growth of a human t(7;9)-positive T-ALL cell line (16). To investigate the possibility of a more general role for NOTCH signaling in human T-ALL, we tested T-ALL cell lines lacking the t(7;9) for NOTCH dependency by treating these cells with a ,-secretase inhibitor (17). Of 30 human T-ALL cell lines tested, 5 showed a G 0 /G 1 cell-cycle arrest that equaled or exceeded that of T6E, a reference NOTCH1-dependent murine T-ALL cell line (Fig. 1A). This drug-induced growth suppression was abrogated by retroviral expression of ICN1 (Fig. 1B) and reproduced ( fig. S1) by retroviral expression of dominant negative Mastermindlike-1 (16). These results indicated that the growth of these five cell lines depends on NOTCHtransduced signals.Because physical dissociation of the NOTCH extracellular domain has been linked to receptor activation (6, 18), we reasoned that the HD domain of NOTC...

Summary Tumors are largely classified by histological appearance, yet morphological features do not necessarily predict cellular origin. To determine the origin of pancreatic ductal adenocarcinoma (PDA), we labeled and traced pancreatic cell populations after induction of a PDA-initiating Kras mutation. Our studies reveal that ductal and stem-like centroacinar cells are surprisingly refractory to oncogenic transformation, whereas acinar cells readily form PDA precursor lesions with ductal features. We show that formation of acinar-derived premalignant lesions depends on ectopic induction of the ductal gene Sox9. Moreover, when concomitantly expressed with oncogenic Kras, Sox9 accelerates formation of premalignant lesions. These results provide insight into the cellular origin of PDA and suggest that its precursors arise via induction of a duct-like state in acinar cells.

TP53 is the most frequently mutated tumor suppressor gene in human cancer, with nearly 50% of all tumors exhibiting a loss-offunction mutation. To further elucidate the genetic pathways involving TP53 and cancer, we have exploited the zebrafish, a powerful vertebrate model system that is amenable to wholegenome forward-genetic analysis and synthetic-lethal screens. Zebrafish lines harboring missense mutations in the tp53 DNAbinding domain were identified by using a target-selected mutagenesis strategy. Homozygous mutant fish from two of these lines were viable and exhibited mutations similar to those found in human cancers (tp53 N168K and tp53 M214K ). Although homozygous tp53 N168K mutants were temperature-sensitive and suppressed radiation-induced apoptosis only at 37°C, cells in the tp53 M214K embryos failed to undergo apoptosis in response to ␥ radiation at both 28 and 37°C. Unlike wild-type control embryos, irradiated tp53 M214K embryos also failed to up-regulate p21 and did not arrest at the G 1͞S checkpoint. Beginning at 8.5 months of age, 28% of tp53 M214K mutant fish developed malignant peripheral nerve sheath tumors. In addition to providing a model for studying the molecular pathogenic pathways of malignant peripheral nerve sheath tumors, these mutant zebrafish lines provide a unique platform for modifier screens to identify genetic mutations or small molecules that affect tp53-related pathways, including apoptosis, cell-cycle delay, and tumor suppression.

SUMMARY Chronic pancreatitis is a well-known risk factor for pancreatic ductal adenocarcinoma (PDA) development in humans, and inflammation promotes PDA initiation and progression in mouse models of the disease. However, the mechanistic link between inflammatory damage and PDA initiation is unclear. Using a Kras-driven mouse model of PDA, we establish that the inflammatory mediator Stat3 is a critical component of spontaneous and pancreatitis-accelerated PDA precursor formation and supports cell proliferation, metaplasia associated inflammation, and MMP7 expression during neoplastic development. Furthermore, we show that Stat3 signaling enforces MMP7 expression in PDA cells and that MMP7 deletion limits tumor size and metastasis in mice. Finally, we demonstrate that serum MMP7 level in human PDA patients correlated with metastatic disease and survival.

Pancreatic ductal adenocarcinoma (PDAC) is characterized by near-universal mutations in KRAS and frequent deregulation of crucial embryonic signalling pathways, including the Hedgehog (Hh) and Wnt–β-catenin cascades. The creation of mouse models that closely resemble the human disease has provided a platform to better understand when and in which cell types these pathways are misregulated during PDAC development. Here we examine the central part that KRAS plays in the biology of PDAC, and how the timing and location of Hh and Wnt–β-catenin signalling dictate the specification and oncogenic properties of PDAC.

Summary Missense mutations in the p53 tumor suppressor inactivate its anti-proliferative properties but can also promote metastasis through a gain-of-function activity. We show that sustained expression of mutant p53 is required to maintain the pro-metastatic phenotype of a murine model of pancreatic cancer, a highly metastatic disease that frequently displays p53 mutations. Transcriptional profiling and functional screening identified the platelet-derived growth factor receptor b (PDGFRb) as both necessary and sufficient to mediate these effects. Mutant p53 induced PDGFRb through a cell-autonomous mechanism involving inhibition of a p73/NF-Y complex that represses PDGFRb expression in p53-deficient, non-invasive cells. Blocking PDGFRb signaling by RNA interference or small molecule inhibitors prevented pancreatic cancer cell invasion in vitro and metastasis formation in vivo. Finally, high PDGFRb expression correlates with poor disease-free survival in pancreatic, colon, and ovarian cancer patients, implicating PDGFRb as a prognostic marker and possible target for attenuating metastasis in p53 mutant tumors.

Therapeutic targeting of KRAS-mutant lung adenocarcinoma represents a major goal of clinical oncology. KRAS itself has proven difficult to inhibit, and the effectiveness of agents that target key KRAS effectors has been thwarted by activation of compensatory or parallel pathways that limit their efficacy as single agents. Here we take a systematic approach towards identifying combination targets for trametinib, an FDA-approved MEK inhibitor that acts downstream of KRAS to suppress signaling through the mitogen-activated protein kinase (MAPK) cascade. Informed by a short-hairpin RNA (shRNA) screen, we show that trametinib provokes a compensatory response involving the fibroblast growth factor receptor 1 (FGFR1) that leads to signaling rebound and adaptive drug resistance. As a consequence, genetic or pharmacologic inhibition of FGFR1 in combination with trametinib enhances tumor cell death in vitro and in vivo. This compensatory response shows distinct specificities – it is dominated by FGFR1 in KRAS mutant lung and pancreatic cancer cells, but is not activated or involves other mechanisms in KRAS wild-type lung and KRAS-mutant colon cancer cells. Importantly, KRAS-mutant lung cancer cells and patient tumors treated with trametinib show an increase in FRS2 phosphorylation, a biomarker of FGFR activation; this increase is abolished by FGFR1 inhibition and correlates with sensitivity to trametinib and FGFR inhibitor combinations. These results demonstrate that FGFR1 can mediate adaptive resistance to trametinib and validate a combinatorial approach for treating KRAS-mutant lung cancer.

SUMMARY There are still gaps in our understanding of the complex processes by which p53 suppresses tumorigenesis. Here we describe a novel role for p53 in suppressing the mevalonate pathway, which is responsible for biosynthesis of cholesterol and nonsterol isoprenoids. p53 blocks activation of SREBP-2, the master transcriptional regulator of this pathway, by transcriptionally inducing the ABCA1 cholesterol transporter gene. A mouse model of liver cancer reveals that downregulation of mevalonate pathway gene expression by p53 occurs in premalignant hepatocytes, when p53 is needed to actively suppress tumorigenesis. Furthermore, pharmacological or RNAi inhibition of the mevalonate pathway restricts the development of murine hepatocellular carcinomas driven by p53 loss. Like p53 loss, ablation of ABCA1 promotes murine liver tumorigenesis and is associated with increased SREBP-2 maturation. Our findings demonstrate that repression of the mevalonate pathway is a crucial component of p53-mediated liver tumor suppression and outline the mechanism by which this occurs.