In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 109, No. 21 ( 2012-05-22)
Abstract:
The role of Ncoa2 in regulating glucose metabolism may play a role in its antitumorigenic activity. Mice lacking Ncoa2 develop glycogen storage disease type 1 (Von Gierke’s disease) and exhibit decreased activity of G6pase. Similarly, a genetic deficiency of G6PC leads to Von Gierke’s disease in humans and in mice. Patients who have Von Gierke’s disease and G6Pase −/− mice develop liver adenomas and are at high risk of progression to HCC. Moreover, inhibition of hexokinase, which has the same functional effect as increasing G6Pase activity, is antitumorigenic. Based on these findings, loss of Ncoa2 may promote liver tumorigenesis at least in part through a subsequent reduction in G6Pase activity, which would be expected to increase glycolytic flux by increasing levels of glucose-6-phosphate. Thus, reduction of Ncoa2 and G6Pase may contribute to the metabolic reprogramming of cancer cells and fuel tumor growth. However, Ncoa2 loss of function also leads to broader dysregulation of gene expression in liver ( 5 ), including the altered expression of genes involved in signal transduction and cell death. Therefore, Ncoa2 likely functions as a tumor suppressor by controlling the activity of multiple pathways and targets relevant to hepatocarcinogenesis. Future studies will focus on the detailed mechanistic dissection of NCOA2-mediated tumor suppression. Among the genes identified in our screen, Ncoa2 was of particular interest. Ncoa2 is a member of the p160 family of transcriptional coactivators that activates expression of Glucose 6 phosphatase ( G6Pase ) and other important genes by acting in concert with nuclear receptors such as RORα. G6Pase is a rate-limiting enzyme that dephosphorylates glucose-6-phosphate, the initial substrate for glycolysis. Analysis of gene-expression data from human HCC revealed that low expression of NCOA2 and its target G6PC are associated with poor patient survival. Furthermore, we documented that genetic ablation of Ncoa2/Src-2 in mice promotes liver tumorigenesis in a carcinogen-induced hepatocarcinogenesis model. These findings establish a tumor-suppressor role for Ncoa2 in HCC and demonstrate that transposon-mediated mutagenesis screens in mice can identify clinically relevant genes that participate in the pathogenesis of human cancer. We next validated that the genes identified in the SB screen contribute to tumor initiation and/or progression using in vitro and in vivo cancer model systems. First, we used liver progenitor cells that form tumors only after sustaining additional genetic alterations. Candidate gene function was inhibited using shRNAs, and cells were transplanted into recipient mice to assess tumor-forming potential. These experiments validated the tumor-suppressor activity of several genes, including Nuclear receptor coactivator 2 ( Ncoa2/Src-2 ), Zinc finger transcription factor ( Zfx ) , and Beta-Dystrobrevin ( Dtnb ). Next, we compared our SB hit list with an expression-profiling dataset from a large cohort of human liver tumors to determine the relevance of the CISs to human liver cancer pathogenesis. We found that several of the genes identified in our screen exhibit dysregulated expression in human hepatocellular carcinoma (HCC). In this study, we sought to identify genes that, when mutated, contribute to accelerated liver tumor development ( Fig. P1 ). Specifically, a screening approach was designed to recover mutations that cooperate with MYC , one of the most commonly dysregulated genes in human malignancy. We bred mice containing an active SB transposon to mice that develop MYC-induced liver cancer. A cohort of mice containing the SB transposon, a transposase, and the MYC oncogene expressed specifically in liver cells was generated. Control mice expressed MYC but lacked an active SB transposon. A significantly higher percentage of animals harboring the active SB element developed liver tumors compared with controls. We used high-throughput sequencing to characterize transposon insertion sites in tumors and to identify regions of the genome that harbored insertions at a statistically significantly greater frequency than expected by chance. These common insertion sites (CISs) mark genes that are likely to accelerate tumorigenesis when their functions are altered. This approach identified at least 16 genes/loci that contribute to liver tumor development. Emerging data from cancer genome-sequencing studies have demonstrated that human tumors exhibit tremendous complexity and heterogeneity in the number and nature of identified mutations ( 1 ). Based on these findings, there is an increasing need for in vivo validation of genes whose altered function contributes to cancer pathogenesis. Transposons are DNA sequences that can insert themselves into new locations within the genome and thereby serve as powerful mutagens. Sleeping Beauty ( SB ), a member of the Tc1/mariner superfamily of DNA transposons, is highly active in mammalian cells ( 2 ). A growing body of evidence has revealed that the SB system is an efficient tool for cancer gene discovery ( 3 , 4 ). We have performed a forward genetic screen using the SB system to identify genes that accelerate liver tumorigenesis in mice. Our findings reveal genes and pathways that participate in tumorigenesis and therefore may provide targets for liver cancer therapy.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1115433109
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2012
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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