Potential microRNA Target for Hepatocellular Carcinoma Therapy
Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide and the third leading cause of death, with a dismal prognosis and resistance to standard therapy. Definition of new targets for therapeutic intervention offers significant promise for enhancing clinical outcomes. Elyakim and colleagues have identified an oncogenic microRNA, miR-191, as a potential target for HCC. Blocking miR-191 inhibited cell proliferation, induced apoptosis in vitro, and effectively reduced tumor burden in an orthotopic xenograft mouse model of HCC. Of potential relevance in HCC, miR-191 expression was upregulated by dioxin, an established liver carcinogen and also was shown to regulate a number of cancer-related pathways. The present study provides compelling preclinical evidence for targeting miR-191 as part of a novel therapeutic scheme for treatment of HCC.
Knockdown of RAD51 but Not MYC Suppresses DNA Repair
Luoto and colleagues realized that the MYC oncogene directly regulates the transcription of many DNA repair proteins. Indeed, MYC is known to have protean effects on gene transcription. They speculated that by suppressing MYC expression they would suppress expression and function of DNA repair genes, in turn increasing sensitivity to radiation. However, what they found was much more complicated. First, they studied in a variety of normal and tumor cell lines by chromatin immunoprecipitation MYC binding to the promoters of many DNA repair genes, including the following: Rad51, Rad51B, Rad51C, XRCC2, Rad50, BRCA1, BRCA2, DNA-PKcs, XRCC4, Ku70, and DNA ligase IV. The authors then confirmed that MYC interacts with these promoters and went on to demonstrate that MYC causally regulates Rad51 transcription. However, when they investigated whether the suppression of MYC impeded DNA repair, they found little if any effect on DNA repair. This finding should not have been too surprising, because the role of MYC in DNA repair is clearly much more complicated. In fact, a few reports have documented that MYC overexpression can induce DNA damage by directly abrogating DNA repair. Many reports have illustrated that MYC overexpression increases sensitivity to radiation and chemotherapy. As the authors realized retrospectively, this observation may relate to the effect of MYC on the induction of DNA transcription as well as its ability to suppress DNA damage checkpoints. However, importantly, the authors observed that even modest reductions in MYC levels were sufficient to result in substantial cell death in tumor cells, and these observations are consistent with many reports that tumors are “addicted” to MYC. These observations highlight the fact that the myc oncogene has a complex influence on DNA repair mechanisms in tumor cells not only by directly regulating the expression of DNA repair apparatus but also by abrogating the ability of the tumor cells to mediate DNA break repair. Hence, treatments targeting MYC, although potentially useful, would not necessarily cooperate with therapies that mediate the activity of tumor cells through the induction of DNA damage.
PMLRARα and DNA Methyltransferase 3a1 Cooperate to Induce Leukemia
Subramanyam and colleagues illustrate that PML-RARα cooperates with DNA methyltransferase 3a1 (DNMT3a1) to induce acute promyelocytic leukemia (APL). PML-RARα is known to mediate its effects on incipient tumor cells by modulating gene expression through interaction with gene products that epigenetically regulate gene expression. The authors decided to directly interrogate whether coexpression of PML-RARα and DNMT3a1 induces leukemogenesis. Using a transgenic approach, they conditionally regulated the expression of DNMT3a1 ubiquitously and then expressed PML-RARα in myeloid cells. Initially, they found that coexpression of PML-RARα and DNMT3a1 significantly accelerated death. However, this was not due to leukemia but rather to granulocyte infiltrates in the lungs of mice, a finding associated with crystalline macrophage pneumonia. Consequently, the investigators transplanted the bone marrow of their different lines of transgenic mice and found that recipients of bone marrow from the bitransgenic PML-RARα/DNMT3a1 mice succumbed to leukemia much more rapidly than the single transgenic mice. Next, they evaluated how DNMT3a1 was influencing CPG methylation at the PML-RARα target RARβ. They found that DNMT3a alone had no effect on methylation but in combination with PML-RARα there was evidence for increased methylation. The authors concluded that PML-RARα only in combination with the overexpression of DNMT3a1 is able to sufficiently epigenetically modulate gene targets to initiate leukemogenesis. This model should be useful to identify which specific gene products must be epigenetically modified to initiate APL. These results raise the question of whether formation of APL is generally associated with secondary events that lead to the upregulation of DNMTs.
KIT Mutations: Context Matters
Chi P, Chen Y, Zhang L, et al. ETV1 is a lineage survival factor that cooperates with KIT in gastrointestinal stromal tumours. Nature 2010;467:849–53.
Mutations in the cKIT protooncogene give rise to gastrointestinal stromal tumors (GIST), the most common of all human sarcomas, deriving from a gastrointestinal cell type known as interstitial cells of Cajal (ICC). A major clinical question has been why the pathogenicity of mutant KIT is restricted to these cells when many other cell types express high levels of KIT without tumorigenic consequences. Chi and colleagues revealed that ETV1, an ETS family transcription factor, is highly expressed in the gut in subsets of ICCs that give rise to GIST. Using transcriptome profiling and global analyses of ETV1-binding sites, they found that ETV1 was a master regulator of an ICC-GIST-specific transcription network regulated by activated KIT, which prolongs ETV1 protein stability and thereby cooperates with ETV1 to promote tumorigenesis. These three findings indicate that ETV1, in addition to being a diagnostic marker for GIST, may also be a potential drug target for GIST because these tumors are insensitive to KIT kinase inhibitors such as imatinib.
New Tools for Transposon Mutagenesis in Mice
Rad R, Rad L, Wang W, et al. PiggyBac transposon mutagenesis: a tool for cancer gene discovery in mice. Science 2010 Oct 14 [Epub ahead of print].
Forward mutagenesis screens using replication-competent retroviruses have been successful in accelerating mouse models of cancer, leading to the identification of many new and important oncogenes. Similar screens in solid tumors have traditionally been more difficult to perform due to the lack of efficient means for viruses to infect cells within solid tumors. The ability to mobilize transposons within cells has been more successful; however, a number of systems are available to conduct these experiments. Rad and colleagues used a genome-wide transposon system derived from the cabbage looper moth (PiggyBac). They generated a series of transposons that could be driven by either PiggyBac transposase or an alternative transposase (Sleeping Beauty) that has different integration preferences. Each transposon construct also used one of three different promoter/enhancer elements to drive expression of adjacent genes. The investigators then made 20 transgenic mice from each series and crossed these mice with mice transgenic for the PiggyBac transposase. All mice developed tumors, with the CAG promoter/enhancer driving mostly solid tumors, the MSCV promoter/enhancer driving primarily hematopoietic tumors, and the PGK promoter/enhancer driving both tumor types. This screen generated many genes that had not been reported in prior retroviral or transposon-based screens and identified a number of new cancer genes. These studies establish PiggyBac transposon mutagenesis as a tool for cancer gene discovery in mice.
A myc Network Shared between Embryonic Stem Cells and Cancer Cells
Kim J, Woo AJ, Chu J, et al. A myc network accounts for similarities between embryonic stem and cancer cell transcription programs. Cell 2010;143:313–324.
The myc proto-oncogene features prominently in cancer by enhancing the reprogramming of differentiated cells to an embryonic stem (ES)-cell like state. In a recent publication, Kim and colleagues report on their use of a proteomic approach to identify three major transcriptional regulatory sub-networks (Core, Polycomb, and Myc) in ES cells. They found that the sub-networks were separable, and by using databases from a number of solid tumors, they demonstrated that in cancer (as an experimental system), the Myc sub-network, but not the other sub-networks, accounted for the similarity in “signatures” between ES cells and cancer, and predicted cancer outcome.
Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.