The adenomatous polyposis coli (APC) tumor suppressor is mutated in the vast majority of human colorectal cancers (CRC) and leads to deregulated Wnt signaling. To determine whether Apc disruption is ...required for tumor maintenance, we developed a mouse model of CRC whereby Apc can be conditionally suppressed using a doxycycline-regulated shRNA. Apc suppression produces adenomas in both the small intestine and colon that, in the presence of Kras and p53 mutations, can progress to invasive carcinoma. In established tumors, Apc restoration drives rapid and widespread tumor-cell differentiation and sustained regression without relapse. Tumor regression is accompanied by the re-establishment of normal crypt-villus homeostasis, such that once aberrantly proliferating cells reacquire self-renewal and multi-lineage differentiation capability. Our study reveals that CRC cells can revert to functioning normal cells given appropriate signals and provide compelling in vivo validation of the Wnt pathway as a therapeutic target for treatment of CRC.
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•Apc suppression in mouse colon recapitulates the molecular features of human CRC•Apc restoration triggers differentiation and restores crypt homeostasis•CRCs harboring Kras and p53 mutations remain strictly dependent on Apc loss•In vivo validation of the Wnt pathway as an effective therapeutic target in CRC
Colorectal cancer cells can revert to functioning normal cells in vivo when Apc levels are restored, even if potent oncogenic insults such as Kras and p53 mutations are present.
The p53 tumor suppressor can restrict malignant transformation by triggering cell-autonomous programs of cell-cycle arrest or apoptosis. p53 also promotes cellular senescence, a tumor-suppressive ...program that involves stable cell-cycle arrest and secretion of factors that modify the tissue microenvironment. In the presence of chronic liver damage, we show that ablation of a p53-dependent senescence program in hepatic stellate cells increases liver fibrosis and cirrhosis associated with reduced survival and enhances the transformation of adjacent epithelial cells into hepatocellular carcinoma. p53-expressing senescent stellate cells release factors that skew macrophage polarization toward a tumor-inhibiting M1-state capable of attacking senescent cells in culture, whereas proliferating p53-deficient stellate cells secrete factors that stimulate polarization of macrophages into a tumor-promoting M2-state and enhance the proliferation of premalignant cells. Hence, p53 can act non-cell autonomously to suppress tumorigenesis by promoting an antitumor microenvironment, in part, through secreted factors that modulate macrophage function.
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► p53 promotes senescence in hepatic stellate cells, limiting fibrosis, cirrhosis, and liver cancer ► Senescent stellate cells secrete factors that promote M1 macrophage polarization ► M1 macrophages eliminate senescent cells and support an antitumor microenvironment ► p53 acts non-cell autonomously to maintain organ integrity and suppress cancer
p53 acts both cell autonomously and nonautonomously in liver stellate cells to limit their tumorigenic activity. p53 expression promotes stellate cell senescence and also induces secretion of factors that influence macrophage polarization, leading to the generation of macrophages that eliminate senescent cells.
Missense mutations in the p53 tumor suppressor inactivate its antiproliferative 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 prometastatic 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, noninvasive cells. Blocking PDGFRb signaling by RNA interference or by 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.
One-year survival rates for newly diagnosed hepatocellular carcinoma (HCC) are <50%, and unresectable HCC carries a dismal prognosis owing to its aggressiveness and the undruggable nature of its main ...genetic drivers. By screening a custom library of shRNAs directed toward known drug targets in a genetically defined Myc-driven HCC model, we identified cyclin-dependent kinase 9 (Cdk9) as required for disease maintenance. Pharmacological or shRNA-mediated CDK9 inhibition led to robust anti-tumor effects that correlated with MYC expression levels and depended on the role that both CDK9 and MYC exert in transcription elongation. Our results establish CDK9 inhibition as a therapeutic strategy for MYC-overexpressing liver tumors and highlight the relevance of transcription elongation in the addiction of cancer cells to MYC.
The p53 tumor suppressor coordinates a series of antiproliferative responses that restrict the expansion of malignant cells, and as a consequence, p53 is lost or mutated in the majority of human ...cancers. Here, we show that p53 restricts expression of the stem and progenitor-cell-associated protein nestin in an Sp1/3 transcription-factor-dependent manner and that Nestin is required for tumor initiation in vivo. Moreover, loss of p53 facilitates dedifferentiation of mature hepatocytes into nestin-positive progenitor-like cells, which are poised to differentiate into hepatocellular carcinomas (HCCs) or cholangiocarcinomas (CCs) in response to lineage-specific mutations that target Wnt and Notch signaling, respectively. Many human HCCs and CCs show elevated nestin expression, which correlates with p53 loss of function and is associated with decreased patient survival. Therefore, transcriptional repression of Nestin by p53 restricts cellular plasticity and tumorigenesis in liver cancer.
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•Nestin overexpression in HCC and CC is linked with p53 mutations and poor prognosis•Nestin is repressed by p53 and is required for the emergence of p53 mutant tumors•p53 restricts expansion of malignant progenitors in response to oncogenic stress•p53 restricts cellular plasticity while specific lesions dictate malignant cell fate
p53 loss dedifferentiates mature hepatocytes into nestin-positive progenitor-like cells, which are poised to redifferentiate into hepatocellular carcinomas or cholangiocarcinomas upon acquiring lineage-specific lesions.
CRISPR-Cas9-based genome editing enables the rapid genetic manipulation of any genomic locus without the need for gene targeting by homologous recombination. Here we describe a conditional transgenic ...approach that allows temporal control of CRISPR-Cas9 activity for inducible genome editing in adult mice. We show that doxycycline-regulated Cas9 induction enables widespread gene disruption in multiple tissues and that limiting the duration of Cas9 expression or using a Cas9(D10A) (Cas9n) variant can regulate the frequency and size of target gene modifications, respectively. Further, we show that this inducible CRISPR (iCRISPR) system can be used effectively to create biallelic mutation in multiple target loci and, thus, provides a flexible and fast platform to study loss-of-function phenotypes in vivo.
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.
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•Wild-type p53 correlates with reduced expression of genes in the mevalonate pathway•p53null liver tumorigenesis in mice depends on activation of the mevalonate pathway•p53-mediated ABCA1 expression inhibits maturation of SREBP-2•Abca1 functions as a tumor suppressor in mouse liver
The mevalonate pathway is selectively vulnerable in tumors lacking p53.
Synovial sarcoma is an aggressive cancer invariably associated with a chromosomal translocation involving genes encoding the SWI-SNF complex component SS18 and an SSX (SSX1 or SSX2) transcriptional ...repressor. Using functional genomics, we identify KDM2B, a histone demethylase and component of a non-canonical polycomb repressive complex 1 (PRC1.1), as selectively required for sustaining synovial sarcoma cell transformation. SS18-SSX1 physically interacts with PRC1.1 and co-associates with SWI/SNF and KDM2B complexes on unmethylated CpG islands. Via KDM2B, SS18-SSX1 binds and aberrantly activates expression of developmentally regulated genes otherwise targets of polycomb-mediated repression, which is restored upon KDM2B depletion, leading to irreversible mesenchymal differentiation. Thus, SS18-SSX1 deregulates developmental programs to drive transformation by hijacking a transcriptional repressive complex to aberrantly activate gene expression.
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•An RNAi screen identifies KDM2B as an epigenetic dependency in synovial sarcoma•KDM2B depletion abolishes neurogenic programs inducing mesenchymal differentiation•KDM2B-PRC1.1 recruits SS18-SSX and SWI/SNF to unmethylated CpG islands•SS18-SSX hijacks a repressive complex to aberrantly activate gene expression
Banito et al. show that SS18-SSX fusions characteristic of synovial sarcoma associate with KDM2B-PRC1.1, a non-canonical polycomb repressive complex 1, to aberrantly activate the expression of developmentally regulated transcription factors that are normally targets of polycomb-mediated gene repression.
CRISPR base editing enables the creation of targeted single-base conversions without generating double-stranded breaks. However, the efficiency of current base editors is very low in many cell types. ...We reengineered the sequences of BE3, BE4Gam, and xBE3 by codon optimization and incorporation of additional nuclear-localization sequences. Our collection of optimized constitutive and inducible base-editing vector systems dramatically improves the efficiency by which single-nucleotide variants can be created. The reengineered base editors enable target modification in a wide range of mouse and human cell lines, and intestinal organoids. We also show that the optimized base editors mediate efficient in vivo somatic editing in the liver in adult mice.
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