Inactivation of hepatitis C virus cDNA transgene by hypermethylation in transgenic mice

Generation of inducible hepatitis C virus transgenic mouse lines

Hepatitis C virus (HCV) is the causative agent of most cases of chronic liver disease, cirrhosis, and hepatocellular carcinoma (HCC) affecting more than 170 million people world-wide. Progress in elucidating the nature of HCV and the development of new therapeutic strategies is hampered fundamentally by the absence of adequate small animal models simulating natural HCV infection. The creation of conditional mouse lines with the tetracycline-controlled gene expression system holds new perspectives for simulation of wild-type HCV infection in a small animal model. Transgenic mice were established with tetracycline-inducible coexpression of HCV core or HCV open reading frame (ORF) and luciferase. In long-term induction experiments, mice were examined for immunopathological changes after expression of HCV proteins. Inducible and liver-specific expression of transgenes was detected by Western blot, immunoprecipitation, luciferase assay and in vivo imaging of bioluminescence of luciferase in genetically modified mice. Ectopic expression levels were determined quantitatively in the liver, kidney, heart and spleen of mice in the induced and non-induced state. During long-term induction an elevation of aminotransaminases (ALT) was observed only in HCV core/ORF-expressing mice, but HCV-specific immune response was not confirmed by in vitro immunological assays. The histology of liver sections provided evidence of steatosis, which was correlated with an inflammatory response. The inducible HCV-transgenic mouse lines provide further evidence of liver pathogenesis in the presence of inflammation during liver-specific expression of HCV proteins and offer new insights into the effects of temporally and spatially controlled protein expression of HCV.

Testing transgenic regulatory elements through live mouse imaging

To overcome positional and methylation effects on transgene expression, we developed a universal cloning cassette for in vivo assessment of regulatory elements using the luciferase reporter gene and the CCCD camera. Monitoring luciferase expression pattern in live mice enables screening of large numbers of transgenic founders quickly and inexpensively. We demonstrate that in the engineered transgenic mice, the chicken beta-globin 5'HS4 insulator did not always provide the desirable expression pattern, and the Island Element, responsible for the demethylation of the surrounding DNA region, was not beneficial. Both tested liver-specific and developmentally regulated promoters exhibited the expected expression pattern in most transgenic founders.

Generation of the regulatory protein rtTA transgenic mice

AIM: To translate Tet-on system into a conditional mouse model, in which hepatitis B or C virus (HBV or HCV) gene could be spatiotemporally expressed to overcome “immune tolerance” formed during the embryonic development and “immune escape” against hepatitis virus antigen(s), an effector mouse, carrying the reverse tetracycline-responsive transcriptional activator (rtTA) gene under the tight control of liver-specific human apoE promoter, is required to be generated.

METHODS: To address this end, rtTA fragment amplified by PCR was effectively inserted into the vector of pLiv.7 containing apoE promoter to create the rtTA expressing vector, i.e., pApoE-rtTA. ApoE-rtTA transgenic fragment (-6.9 kb) released from pApoE-rtTA was transferred into mice by pronucleus injection, followed by obtaining one transgene (+) founder animal from microinjection through PCR and Southern blot analysis.

RESULTS: rtTA transgene which could be transmitted to subsequent generation (F1) derived from founder was expressed in a liver-specific fashion.

CONCLUSION: Taken together, these findings demonstrate that rtTA transgenic mice, in which rtTA expression is appropriately targeted to the murine liver, are successfully produced, which lays a solid foundation to ‘off-on-off’ regulate expression of target gene (s) (e.g., HBV and/or HCV) in transgenic mice mediated by Tet-on system.

Viral genes and methylation

Epigenetics represents a new frontier in cancer research. Methylation is the best studied of the epigenetic mechanisms that regulate gene expression. Regulation of gene expression by means of methylation has been reported for tumor suppressor genes, oncogenes, viral promoters, and age-related genes. In this review, the regulation of viral gene expression by methylation is discussed, with particular emphasis on: (1) the virus-specific factors that bind to promoter regions; (2) the implications of this knowledge for designing viral vectors that can be used to deliver genes for the purpose of gene therapy; and (3) the use of this knowledge for the early detection and prevention of cancer. Since methylation can be reversed by a variety of exogenous agents, great potential exists to develop interventions that target cancer-associated aberrant methylation in an effort to reverse or prevent carcinogenesis.

Molecular clones of hepatitis C virus: applications to animal models

Hepatitis C virus (HCV) is a global public health problem, with approximately 3% of the world population now infected. The clinical course of HCV often involves chronic infection, which can lead to liver dysfunction and hepatocellular carcinoma. Because HCV cannot be efficiently propagated in cell culture, researchers have relied heavily on animal models to study the physical characteristics of HCV and the course of events associated with HCV infection. The chimpanzee is the only nonhuman primate actually proven to be susceptible to HCV infection and has commonly been used to study viral hepatitis induced by HCV. Molecular cloning of the HCV genome has now allowed HCV transmission studies in chimpanzees to progress from the early work of characterizing infectious serum to a current focus of characterizing infectious HCV molecular clones. Moreover, the cloned HCV genome has paved the way for the development of alternative animal models for HCV, most notably transgenic mouse models for the study of HCV pathogenesis. The authors review these animal model applications of the HCV molecular clones, including construction and transmission of mutant viral genomes. The expression of specific viral protein products in these animal models will provide important insight into the structure-function relation that specific HCV genome sequences impart on virus replication and pathogenesis.

Induction of hepatitis C virus-specific cytotoxic T lymphocytes in mice by an intrahepatic inoculation with an expression plasmid

We assessed the possibility of intrahepatic inoculation with a plasmid encoding hepatitis C virus (HCV) proteins to elicit HCV-specific cytotoxic T lymphocytes (CTL) in mice as a conventional animal model of HCV infection. BALB/c mice were intrahepatically or intramuscularly inoculated with an expression plasmid DNA encoding HCV structural proteins under the control of the elongation factor 1-alpha promoter. Expressions of HCV-core protein and envelope proteins (E1 and E2) in hepatocytes were detected immunohistochemically 6 days after inoculation. CTL responses were examined using target cells either pulsed with a specific peptide or infected with a recombinant vaccinia virus expressing HCV structural protein. Both intrahepatically and intramuscularly DNA-inoculated mice developed CD8(+), MHC class I-restricted CTL responses that recognized the peptide pulsed as well as HCV proteins expressing target cells. These studies demonstrated the usefulness of a murine model of HCV infection induced by direct intrahepatic DNA inoculation for understanding the immunopathogenic mechanisms in HCV infection.

Continuous human cell lines inducibly expressing hepatitis C virus structural and nonstructural proteins

Investigation of the hepatitis C virus (HCV) life cycle and the evaluation of novel antiviral strategies are limited by the lack of an efficient cell culture system. Therefore, continuous human cell lines inducibly expressing the entire HCV open reading frame were generated with use of a tetracycline-regulated gene expression system. HCV transgenes were found to be chromosomally integrated in a head-to-tail configuration. Northern blot analyses revealed a tightly regulated unspliced transcript of approximately 9 kilobases (kb). HCV structural and nonstructural proteins were faithfully processed, indicating that the cellular and viral proteolytic machineries and posttranslational modification pathways are fully functional in these cell lines. Steady state expression levels could be regulated over a broad range by the concentration of tetracycline present in the culture medium. Kinetic analyses revealed a half-life of less than 1 hour for the HCV RNA whereas a half-life of approximately 9.5, 12, 11, and 10 hours was found for core, NS3, NS4A, and NS5A proteins, respectively. Viral proteins were found to colocalize in the cytoplasm in a pattern characteristic of the endoplasmic reticulum. High-level expression of HCV proteins in the fully induced state was toxic to the cells. These cell lines provide a unique in vitro system to analyze structural and functional properties of HCV proteins, their interactions with cellular proteins and pathways, and the requirements for HCV morphogenesis. In addition, they should prove useful for the evaluation of novel antiviral strategies against hepatitis C in a well-defined and reproducible cellular context.

Tightly regulated expression of the entire hepatitis C virus structural region in continuous human cell lines

Investigation of the hepatitis C virus (HCV) life cycle is limited by the lack of an efficient cell culture system. Employing a tetracycline-regulated gene expression system we generated a panel of continuous human cell lines allowing the inducible expression and faithful processing of HCV structural proteins as well as of a functional NS2-3 autoprotease. HCV proteins were found in the cytoplasm in a pattern characteristic for the endoplasmic reticulum. High-level expression of HCV proteins was found to be cytotoxic. These cell lines represent a unique in vitro system in which to further investigate the structural proteins of HCV and to evaluate novel antiviral strategies against hepatitis C in a well-defined and reproducible cellular context.

Light-regulated transcription of genes encoding peridinin chlorophyll a proteins and the major intrinsic light-harvesting complex proteins in the dinoflagellate amphidinium carterae hulburt (Dinophycae). Changes In cytosine methylation accompany photoadaptation

In the dinoflagellate Amphidinium carterae, photoadaptation involves changes in the transcription of genes encoding both of the major classes of light-harvesting proteins, the peridinin chlorophyll a proteins (PCPs) and the major a/c-containing intrinsic light-harvesting proteins (LHCs). PCP and LHC transcript levels were increased up to 86- and 6-fold higher, respectively, under low-light conditions relative to cells grown at high illumination. These increases in transcript abundance were accompanied by decreases in the extent of methylation of CpG and CpNpG motifs within or near PCP- and LHC-coding regions. Cytosine methylation levels in A. carterae are therefore nonstatic and may vary with environmental conditions in a manner suggestive of involvement in the regulation of gene expression. However, chemically induced undermethylation was insufficient in activating transcription, because treatment with two methylation inhibitors had no effect on PCP mRNA or protein levels. Regulation of gene activity through changes in DNA methylation has traditionally been assumed to be restricted to higher eukaryotes (deuterostomes and green plants); however, the atypically large genomes of dinoflagellates may have generated the requirement for systems of this type in a relatively "primitive" organism. Dinoflagellates may therefore provide a unique perspective on the evolution of eukaryotic DNA-methylation systems.

Transgenic mouse expressing a full-length hepatitis C virus cDNA

Hepatitis C virus (HCV), a major causative agent of post transfusion non-A, non-B hepatitis (NANBH), can only infect humans and chimpanzees. We produced nine transgenic mouse lines carrying a full-length HCV cDNA with the human serum amyloid P component (hSAP) promoter that can direct liver-specific expression. In one of these lines HCV mRNA and HCV core protein were detected in the liver of the transgenic mouse, although the levels of expression were very low. In addition, HCV-related antibody was detected in the serum.