Tissue preferential expression of the hepatitis B virus (HBV) surface antigen gene in two lines of HBV transgenic mice

Molecular Mechanisms and Animal Models of HBV-Related Hepatocellular Carcinoma: With Emphasis on Metastatic Tumor Antigen 1

Hepatocellular carcinoma (HCC) is an important cause of cancer death worldwide, and hepatitis B virus (HBV) infection is a major etiology, particularly in the Asia-Pacific region. Lack of sensitive biomarkers for early diagnosis of HCC and lack of effective therapeutics for patients with advanced HCC are the main reasons for high HCC mortality; these clinical needs are linked to the molecular heterogeneity of hepatocarcinogenesis. Animal models are the basis of preclinical and translational research in HBV-related HCC (HBV-HCC). Recent advances in methodology have allowed the development of several animal models to address various aspects of chronic liver disease, including HCC, which HBV causes in humans. Currently, multiple HBV-HCC animal models, including conventional, hydrodynamics-transfection-based, viral vector-mediated transgenic, and xenograft mice models, as well as the hepadnavirus-infected tree shrew and woodchuck models, are available. This review provides an overview of molecular mechanisms and animal models of HBV-HCC. Additionally, the metastatic tumor antigen 1 (MTA1), a cancer-promoting molecule, was introduced as an example to address the importance of a suitable animal model for studying HBV-related hepatocarcinogenesis.

Advances in Transgenic Mouse Models to Study Infections by Human Pathogenic Viruses

Medical research is changing into direction of precision therapy, thus, sophisticated preclinical models are urgently needed. In human pathogenic virus research, the major technical hurdle is not only to translate discoveries from animals to treatments of humans, but also to overcome the problem of interspecies differences with regard to productive infections and comparable disease development. Transgenic mice provide a basis for research of disease pathogenesis after infection with human-specific viruses. Today, humanized mice can be found at the very heart of this forefront of medical research allowing for recapitulation of disease pathogenesis and drug mechanisms in humans. This review discusses progress in the development and use of transgenic mice for the study of virus-induced human diseases towards identification of new drug innovations to treat and control human pathogenic infectious diseases.

Hepatocyte Transplantation: An Alternative System for Evaluating Cell Survival and Immunoisolation

To evaluate systems for barrier immunoisolation of transplanted hepatocytes, we used transgenic mouse hepatocytes that secrete HBsAg. Hepatocytes were rapidly encapsulated in chitosan, a cationic polymer derived by deacetylation of chitin. Chitosan was allowed to electrostatically bond with anionic sodium alginate for creating an outer bipolymer membrane of the capsules. After encapsulation, hepatocyte viability remained unchanged for seven days in vitro with secretion of HBsAg into the culture medium throughout this period. Following intraperitoneal transplantation of encapsulated hepatocytes, HBsAg promptly appeared in blood of recipients. In congeneic recipients, serum HBsAg peaked at two weeks. Hepatocytes were present in recovered chitosan capsules and expressed HBsAg mRNA. In allogeneic recipients, however, serum HBsAg disappeared within one week and recovered chitosan capsules showed lymphomononuclear cells but not hepatocytes. Transplantation of chitosan encapsulatd HbsAg secreting hepatocytes failed to induce an anti-HBs response, suggesting modulation of the host immune response. These results indicate that transplantation systems using genetically modified hepatocytes which secrete gene products in the blood of recipients should facilitate evaluation of hepatocyte encapsulation.

Experimental mouse models for hepatocellular carcinoma research

Every year almost 500,000 new patients are diagnosed with hepatocellular carcinoma (HCC), a primary malignancy of the liver that is associated with a poor prognosis. Numerous experimental models have been developed to define the pathogenesis of HCC and to test novel drug candidates. This review analyses several mouse models useful for HCC research and points out their advantages and weaknesses. Chemically induced HCC mice models mimic the injury-fibrosis-malignancy cycle by administration of a genotoxic compound alone or, if necessary, followed by a promoting agent. Xenograft models develop HCC by implanting hepatoma cell lines in mice, either ectopically or orthotopically; these models are suitable for drug screening, although extrapolation should be considered with caution as multiple cell lines must always be used. The hollow fibre assay offers a solution for limiting the number of test animals in xenograft research because of the ability for implanting multiple cell lines in one mouse. There is also a broad range of genetically modified mice engineered to investigate the pathophysiology of HCC. Transgenic mice expressing viral genes, oncogenes and/or growth factors allow the identification of pathways involved in hepatocarcinogenesis.

Cell culture and animal models of viral hepatitis. Part I: hepatitis B

Despite the existence of a preventative vaccine, HBV represents a substantial threat to public health, suggesting the need for research to develop new treatments to combat the disease. The authors review the available in vitro and in vivo models, including recently developed transgenic and chimeric mouse models.

Transgenic mouse models of hepatitis B virus-associated hepatocellular carcinoma

The multi-factorial and multi-step nature of cancer development makes analysis difficult in cell culture and non-genetic animal models. Recent progress in technology has allowed the development of several transgenic animal models addressing various aspects of liver diseases caused by hepatitis B virus in human patients. The experimental data from these studies in vivo highlight the importance of HBV gene products that alone or in conjunction with other host cellular protein(s) can deregulate the cell cycle control checkpoints in the hepatocytes of transgenic mice leading to the development of hepatocellular carcinoma. Moreover, these models are extremely useful in analysing and ascertaining the stages of malignant transformation linked to multiple genetic and non-genetic events of cancer development and in developing novel strategies of intervention.

Hepatocarcinogenesis in hepatitis viral infection: lessons from transgenic mouse studies

Over the past decade, genetically engineered mouse models have been used for studies of the mechanisms underlying human diseases. One advantage of these models is that the targeted protein executes its function in normal cells in their natural tissue microenvironments. Transgenic mouse models for human viral hepatitis have also been established and have provided new insights into the pathogenesis of hepatitis and hepatocellular carcinoma (HCC). In the search for the mechanism of hepatocarcinogenesis in hepatitis viral infection, two viral proteins, the core protein of hepatitis C virus (HCV) and the HBx protein of hepatitis B virus (HBV), have been shown to possess oncogenic potential through transgenic mouse studies, indicating the direct involvement of the hepatitis viruses in hepatocarcinogenesis. The presence of the hepatitis C virus core or HBx protein, which has an oncogenic potential, may allow some of the steps in multistep hepatocarcinogenesis to be skipped. This may explain the very high frequency of HCC in patients with HCV or HBV infection.

New animal models of hepatitis B and C

The narrow host range of infection and lack of suitable tissue culture systems for the propagation of hepatitis B and C viruses are limitations that have prevented a more thorough understanding of persistent infection and the pathogenesis of chronic liver disease. With hepatitis B virus (HBV), this lack of knowledge has been partially overcome by the discovery and characterization of HBV-like viruses in wild animals. With hepatitis C virus (HCV), related flaviviruses have been used as surrogate systems for such studies. Other laboratories have developed transgenic mice that express virus gene products and/or support virus replication. Some HBV transgenic mouse models develop fulminant hepatitis, acute hepatitis, or chronic liver disease after adoptive transfer, and others spontaneously develop hepatocellular carcinoma (HCC), as in human infections. Among HCV transgenic mice, most develop no disease, but acute hepatitis has been observed in one model, and HCC in another. Although mice are not susceptible to HBV and HCV, their ability to replicate these viruses and to develop liver diseases characteristic of human infections provides new opportunities to study pathogenesis and develop novel therapeutics.

Expression of the hepatitis B surface antigen (HbsAg) under the control of mMT-I promoter in transgenic mice can be induced by zinc sulphate, dexamethasone and lipopolysaccharide

SUMMARY

Metallothionein genes can be induced in vivo by heavy metals, glucocorticoids, and toxins. In all transgenic mice carrying the MT-I promoter, that have been reported so far, induction by glucocorticoids failed. This study reports two mouse lines, transgenic for the murine MT-I-HBV (hepatitis B virus; map position site: 30-1986) construct, which secrete the viral surface antigen (HBsAg) in their serum. In both lines, males produce more HBsAg than females, and in all cases the MT-I promoter can be induced by dexamethasone, lipopolysaccharide (LPS), and heavy metals. A glucocorticoid-responsive element, which is situated in the HBV fragment used, can explain the dexamethasone induction of the MT-I promoter. ZUSAMMENFASSUNG: Expression des Hepatitis B oberflächen Antigens (HBsAg) unter Kontrolle des mMT-I Promoters kann in transgenen Mäusen durch Zink Sulfat, Dexamethason und Lipopolysacchariden induziert werden Metallothioneingene werden in vivo durch Schwermetalle, Glucocorticoide und Toxine induziert. Soweit bisher bekannt, konnte jedoch in transgenen Mäusen mit dem MT-I-Promotor keine Expression durch Glucocorticioide beobachtet werden. Wir berichten hier von zwei transgenen Mäuselinien mit dem murinen MT-I-Promotor, der das Oberflächenantigen des Hepatitis B Virus (HbsAg, map position site 30-1986) exprimierte. In beiden Linien produzierten die männlichen Tiere mehr HBsAg im Blutserum als die weiblichen. Ohne Ausnahme reagierte der MT-I-Promotor bei Applikation von Dexamethason, Lipopolysaccharid (LPS) und Schwermetall. Ein Glucocorticoid-responsives Element in den HBV-Sequenzen kann die Induktion des MT-I-Promotors erklären.

Cell type-specific mechanisms regulate hepatitis B virus transgene expression in liver and other organs

Intracellular expression of hepatitis B virus (HBV) was analysed in transgenic HBV mouse lines designated G7 and G26, the former lacking hepatitis B surface antigen (HBsAg) promoters. HBsAg mRNA expression was greater in the G26 line than in the G7 line, although in situ hybridization showed a qualitatively similar expression pattern in specific cell types. HBsAg mRNA was most abundant in hepatocytes, followed in magnitude by proximal renal tubular epithelial cells, pancreatic acinar cells, and epithelial cells of the gastric, small intestinal, and bronchiolar mucosae. In biliary epithelial cells, brain, spleen, large intestine, testis, heart, and skeletal muscle, HBsAg mRNA was undetectable. In cell transfection assays, the HBV enhancer/preS1 promoter efficiently expressed a luciferase reporter with appropriate upregulation by HNF-3 alpha and C/EBP alpha transcription factors in hepatocyte-derived cells but not in non-parenchymal epithelial liver cells or fibroblasts. These results suggest that cell-type specificity of HBV expression is regulated by interactions between viral elements and cellular transactivators. Variable expression of G7 and G26 HBV transgenes in epithelial cells combined with differences in transgene expression in similar sets of cells suggests at least two levels of regulation: one directing cell specificity of HBV expression and the other governing quantitative expression of HBV mRNA.

Hepatitis B virus transgenic mice: models of viral immunobiology and pathogenesis

It should be apparent from the foregoing that the transgenic mouse model system has contributed substantially to our understanding of many aspects of HBV biology, immunobiology and pathogenesis in the past several years. We have learned that HBV can replicate within the mouse hepatocyte, as well as other mouse cell types, suggesting that there are probably no strong tissue or species specific constraints to viral replication once the viral genome enters the cell. However, the failure thus far to detect viral cccDNA in the hepatocyte nucleus in several independently derived transgenic lineages suggests that other, currently undefined, constraints on host range and tissue specificity may also be operative. Thanks to the transgenic mouse model we now understand the pathophysiological basis for HBsAg filament formation and ground glass cell production, and we have learned that at least this viral gene product can be toxic for the hepatocyte, first by compromising its ability to survive the hepatocytopathic effects of LPS and IFN alpha and eventually by causing it to die in the absence of any obvious exogenous stimulus. In recent studies, it has been shown that preformed nucleocapsid particles do not cross the nuclear membrane in either direction at least in the mouse hepatocyte. If this is confirmed, it will have two important implications: first, that nucleocapsid disassembly must occur in the cytoplasm before the nascent viral genome can enter the nucleus; second, that the intranuclear nucleocapsid particles are empty, and therefore serve no currently defined purpose in the viral life cycle. This should stimulate new interest in the analysis of the function of these particles that are a prominent feature of mammalian hepadnavirus infection. The transgenic mouse model has also established definitively that HBV-induced liver disease has an immunological basis, and that the class I-restricted CTL response plays a central role in this process. Additionally, the mouse studies have taught us that when the CTL recognize their target antigen on the hepatocytes they cause them to undergo apoptosis, forming the acidophilic, Councilman bodies that are characteristic of viral hepatitis. Further, we have learned that although the CTL initiate the liver disease, they actually contribute more to disease severity indirectly by recruiting antigen nonspecific effector cells into the liver than by directly killing the hepatocytes themselves. In addition, by releasing IFN gamma when they recognize antigen, the CTL can destroy enough of the liver to cause fulminant hepatitis in mice whose hepatocytes overproduce the large envelope protein and are hypersensitive to the cytopathic effects of this cytokine. We have also learned that the CTL are unable to recognize HBV-positive parenchymal cells outside of the liver, apparently because they cannot traverse the microvascular barriers that exist at most extrahepatic tissue sites. This important new discovery may permit the virus to survive a vigorous CTL response and contribute not only to the maintenance of memory T cells following acute hepatitis but also to serve as a reservoir to reseed the liver in patients with chronic hepatitis. The transgenic mouse model has also revealed that activated CTL and the cytokines they secrete can down-regulate HBV gene expression, and possibly even control viral replication, by noncytotoxic intracellular inactivation mechanisms involving the degradation of viral RNA and, perhaps, the degradation of viral nucleocapsids and replicative DNA intermediates without killing the cell. If HBV replication is indeed interrupted by this previously unsuspected activity, it could contribute substantially to viral clearance during acute infection when the immune response to HBV is vigorous. Alternatively, it could also contribute to viral persistence, by only partially down-regulating the virus during chronic infection when the immune response is weak.

High-level hepatitis B virus replication in transgenic mice

Hepatitis B virus (HBV) transgenic mice whose hepatocytes replicate the virus at levels comparable to that in the infected livers of patients with chronic hepatitis have been produced, without any evidence of cytopathology. High-level viral gene expression was obtained in the liver and kidney tissues in three independent lineages. These animals were produced with a terminally redundant viral DNA construct (HBV 1.3) that starts just upstream of HBV enhancer I, extends completely around the circular viral genome, and ends just downstream of the unique polyadenylation site in HBV. In these animals, the viral mRNA is more abundant in centrilobular hepatocytes than elsewhere in the hepatic lobule. High-level viral DNA replication occurs inside viral nucleocapsid particles that preferentially form in the cytoplasm of these centrilobular hepatocytes, suggesting that an expression threshold must be reached for nucleocapsid assembly and viral replication to occur. Despite the restricted distribution of the viral replication machinery in centrilobular cytoplasmic nucleocapsids, nucleocapsid particles are detectable in the vast majority of hepatocyte nuclei throughout the hepatic lobule. The intranuclear nucleocapsid particles are empty, however, suggesting that viral nucleocapsid particle assembly occurs independently in the nucleus and the cytoplasm of the hepatocyte and implying that cytoplasmic nucleocapsid particles do not transport the viral genome across the nuclear membrane into the nucleus during the viral life cycle. This model creates the opportunity to examine the influence of viral and host factors on HBV pathogenesis and replication and to assess the antiviral potential of pharmacological agents and physiological processes, including the immune response.

The molecular pathogenesis of hepatocellular carcinoma

Some of the multiple factors involved in the molecular pathogenesis of hepatocellular carcinoma have been elucidated in recent years but no clear picture of how and in what sequence these factors interact at the molecular level has emerged yet. Transformation of hepatocytes to the malignant phenotype may occur irrespective of the aetiological agent through a pathway of chronic liver injury, regeneration and cirrhosis. The activation of cellular oncogenes, the inactivation of tumour suppressor genes and overexpression of certain growth factors contribute to the development of HCC. There is increasing evidence that the hepatitis B virus may play a direct role in the molecular pathogenesis of HCC. Aflatoxins have been shown to induce specific mutations of the p53 tumour suppressor gene thus providing a clue to how an environmental factor may contribute to tumour development at the molecular level.

Hepatocytes exhibit superior transgene expression after transplantation into liver and spleen compared with peritoneal cavity or dorsal fat pad: implications for hepatic gene therapy

For hepatic gene therapy or applications of hepatocyte transplantation in liver failure, survival and function of transplanted cells is critical. Insights into site-specific gene regulation will significantly facilitate development of appropriate strategies for transplanting hepatocytes. To assess the function of transplanted cells, we used a transgenic hepatitis B virus (HBV) hepatocyte system, which allowed analysis of cellular gene expression with HBV surface antigen (HBsAg) mRNA expression, as well as secretion of HBsAg into peripheral circulation. When congeneic HBV hepatocytes were transplanted into the liver (via spleen), serum HBsAg promptly appeared in circulation and persisted for the entire duration of the studies. In contrast, transplantation of hepatocytes into the peritoneal cavity or dorsal fat pad resulted in serum HBsAg levels that were either significantly lower or gradually rose after a lag period. HBsAg mRNA expression was several-fold greater in transplanted hepatocytes in liver or spleen versus in peritoneal cavity or dorsal fat pad. Despite persistence of transplanted hepatocytes in peritoneal cavity or dorsal fat pad, serum HBsAg was cleared by antibody to HBsAg (anti-HBs) but this was not observed after hepatocyte transplantation into spleen. As the function of transplanted hepatocytes is optimally regulated in the liver, hepatic reconstitution with cell transplantation will be most appropriate for gene therapy.

High-level expression of hepatitis B virus HBx gene and hepatocarcinogenesis in transgenic mice

We studied the development of liver tumors in male HBx gene transgenic mice. Of two lineages studied, in the lineage with the lowest HBx gene expression liver tumors developed only in an incidence comparable with that in normal CD-1 strain, whereas 84% of male mice with a high level of the HBx gene product succumbed to liver neoplasia, indicating that continued HBx gene expression higher than a certain threshold level may be necessary for the development of hepatic neoplasia. Sixty-five mice from a lineage with a high level of HBx expression were then followed throughout their 24-mo lifespan. The livers of transgenic mice showed foci of cellular alteration with cytoplasmic vacuolations around the central veins from the age of 2 mo, but these foci did not expand progressively by the age of 12 mo. Immunostaining demonstrated such hepatocytes had higher expression of HBx protein than surrounding cells. Neoplastic lesions including liver cell adenomas and hepatocellular carcinomas developed from the age of 13 mo. By bromodeoxyuridine labeling analysis, hepatocytes in altered foci were found to have increased DNA synthesis, whereas no labeling was observed in age- and sex-matched nontransgenic littermate controls. Furthermore, DNA content analysis revealed the existence of several small aneuploid peaks in the transgenic liver before the age of tumor development. These results suggest that the continued expression of HBx gene may initiate a complex process to hepatocellular carcinoma by inducing DNA synthesis and placing large numbers of hepatocytes subjective to secondary events for transformation.

Aberrant tissue specific expression of the transgene in transgenic mice that carry the hepatitis B virus genome defective in the X gene

The control mechanisms for the transgene expression in mice that carry the hepatitis B virus genome defective in the polymerase and X genes were analyzed. Ten lines of transgenic mouse were established, and in seven lines the surface and e antigens were detected in the serum. In transgenic mice from five lines examined, the transgene was markedly expressed in a broad spectrum of tissues including the kidney, heart, brain, muscle and intestine, but only poorly in the liver. In the kidney and heart the 3.5 kb and 2.1 kb mRNAs were expressed, whereas only the 0.8 kb and 4.0 kb mRNAs were detected in the testis and brain, respectively, suggesting that each of the mRNAs was transcribed through a different control mechanism. The surface, e and core antigens accumulated in the kidney and heart. DNA was hypomethylated at a region closely downstream of the enhancer in the liver, kidney and heart, and a DNase I hypersensitive site was detected upstream of the enhancer in these tissues. In the testis, however, the whole transgene was hypomethylated and the DNase I hypersensitive site was closer to the enhancer. These differences may be relevant to the preferential expression of the 0.8 kb mRNA in the testis, but cannot explain the inefficiency of transgene expression in the liver. Our observations suggest that the X protein is required for efficient expression of the viral gene in the liver but not in other tissues.

Experimental transmission of human hepatitis delta virus to the laboratory mouse

Human hepatitis delta virus (HDV), obtained from the serum of an experimentally infected woodchuck, was injected into either the peritoneal cavity or the tail vein of both adult CB17 mice and mice with a severe combined immunodeficiency (CB17-scid mice). Three lines of evidence indicated that the virus was able to reach the liver and infect hepatocytes: (i) the amount of HDV genomic RNA detected in the liver by Northern (RNA) analysis increased during the first 5 to 10 days postinoculation, reaching a peak that was about threefold the amount in the original inoculum; (ii) also detected in the liver was the viral antigenomic RNA, which is complementary to the genomic RNA found in virions, and is diagnostic for virus replication; and (iii) by immunoperoxidase staining of liver sections, the delta antigen was detected in the nuclei of scattered cells identifiable as hepatocytes. In all of the mice, clearance of the infection occurred between 10 and 20 days after inoculation. The half-life for clearance was about 3 days in CB17-scid mice, indicating that clearance of infection did not involve a T- and B-cell-dependent immune response. Cell-to-cell spread of the initial infection was not detected. One possible interpretation of our results is that HDV infection of hepatocytes is directly cytopathic. Also, the results imply that chronic infection of the liver in humans may require continuous spread of virus within the liver. Alternatively, HDV in the absence of helper virus may be unable to cause a chronic infection of hepatocytes in vivo.

Molecular dosimetry of aflatoxin exposure: contribution to understanding the multifactorial etiopathogenesis of primary hepatocellular carcinoma with particular reference to hepatitis B virus

Aflatoxin exposure and hepatitis B virus infection have been implicated as major risk factors for primary hepatocellular carcinoma (PHC) in high-incidence regions of the world. Investigations using the assay of aflatoxin bound to peripheral blood albumin have shown that exposure can occur throughout the life span of the individual, including during the perinatal period, in high-incidence areas such as The Gambia, Senegal, Kenya, and The People's Republic of China. The possibility of measuring aflatoxin exposure at the individual level permits an investigation of the putative mechanisms of interaction of this carcinogen with HBV in the etiopathogenesis of PHC. Animal models, e.g., Pekin duck and HBV-transgenic mice, have also been used to study these questions, and the available data are reviewed.

Permanent engraftment and function of hepatocytes delivered to the liver: implications for gene therapy and liver repopulation

To examine the distribution of intrasplenically transplanted hepatocytes, we used HBsAg-producing G7 HBV transgenic hepatocytes or cells labeled with 111In. Most hepatocytes translocated to the liver (55% +/- 7%; mean +/- S.D.); the spleen retained a smaller fraction (15% +/- 3%); and some transplanted cells localized in lungs (3%) or pancreas (1%). Transplanted hepatocytes were rapidly assimilated into the liver lobule. Morphometrical quantitation indicated that the numbers of transplanted hepatocytes in the liver at 48 hr and at 9 mo after transplantation were similar. Serum HBsAg was detected in recipients of the G7 HBV hepatocytes during the 1-yr experiment. These results indicate that a large number of hepatocytes can be reproducibly delivered to the liver by transplantation into the spleen. Transplanted hepatocytes engraft rapidly, assimilate into host liver, maintain normal function and survive permanently. Systems for safe delivery and localization of hepatocytes in the liver represent a critical step toward successfully accomplishing hepatocyte-directed gene therapy and repopulation of the acutely devastated liver.

Transgenome transcription and replication in the liver and extrahepatic tissues of a human hepatitis B virus transgenic mouse

We have produced a transgenic mouse (B32-1) carrying the complete genome of the human hepatitis B virus (HBV). High titers of the viral surface (HBsAg) and the e antigen (HBeAg) were detected in the serum of the mouse. In the liver and 12 of 16 extrahepatic tissues analyzed, Northern blot hybridization indicated the presence of the 2.1-kilobase (kb) and the 3.5-kb major HBV transcripts. A liver cDNA library was constructed from which the liver RNAs from four cDNA clones with splicing were found. Sequencing analysis showed that the splicing occurred between nucleotides 2451 and 487 of the viral genome, resulting in a truncated viral polymerase gene, as in human hepatocytes. Southern blot analysis of total DNA preparations of the tissues revealed the presence of episomal HBV genome, indicating replication of the viral transgenome in these tissues. However, replication was detected only in some but not all of the tissues that transcribed the 3.5-kb RNA. Partial double-stranded as well as full-length and subgenomic-length single-stranded HBV DNA species of discrete sizes were detected which may represent replication intermediates of preferred replication termination sites of the HBV transgenome. Since many molecular characteristics of mouse B32-1 were similar to those found in HBV-infected humans, HBV transgenic mice similar to B32-1 would be useful in further elucidation of other aspects of the replication and transcription mechanisms of HBV in the liver and extrahepatic tissues.

HBx gene of hepatitis B virus induces liver cancer in transgenic mice

The exact role of hepatitis B virus in the development of liver cancer is not known. The recent identification of a viral regulatory gene HBx suggests a possible direct involvement of the virus whereby the HBx protein, acting as a transcriptional transactivator of viral genes, may alter host gene expression and lead to the development of hepatocellular carcinoma. We have tested this possibility of placing the entire HBx gene under its own regulatory elements directly into the germline of mice. Transgenic animals harbouring this viral gene succumbed to progressive histopathological changes specifically in the liver, beginning with multifocal areas of altered hepatocytes, followed by the appearance of benign adenomas, and proceeding to the development of malignant carcinomas. Male mice developed disease and died much earlier than females. This transgenic animal model appears ideal for defining the molecular events that follow the expression of the viral HBx gene and are responsible for the development of liver cancer.

Production and effect of infectious Dane particles in transgenic mice

We have demonstrated by immunoelectron microscopy that 42-nm particles with double-shelled structures characteristic of Dane particles are present in the serum of transgenic mice, 1.2HB-BS 10, carrying partly duplicated hepatitis B virus (HBV) genome. Furthermore, these particles were shown to infect primary human fetal hepatocytes as demonstrated by the elevation of HBV surface antigen (HBsAg) in the culture medium. HBV DNA is known to be expressed in a liver- and kidney-specific manner in the adult mouse, so we examined the developmental expression of viral antigens. In the liver, viral antigens (HBsAg and HBV e antigen) began to be expressed before birth and the level of expression showed a sharp rise after birth. On the other hand, in the kidney, viral antigens began to be expressed after birth. Serum levels of viral antigens were roughly proportional to the levels of expression in the liver, suggesting that the liver is the main source for viral antigens in the serum. None of these transgenic mice produced anti-HBs or anti-HBV core response or showed biochemical or pathological change up to at least 24 months of age. All these results suggest that infectious viral particles can be produced in transgenic mice, and that expression and replication of HBV DNA are not toxic in vivo.

Correlation of tissue-specific methylation with gene inactivity in hepatitis B virus transgenic mice

We produced transgenic mice using two constructs, HB-GII and 1.2HB-BS, of hepatitis B virus (HBV) DNA. The former has been designed to express mRNAs for HBV surface antigen (HBsAg), and the later to express all mRNAs of HBV. Several lines of the transgenic mice carrying each construct were examined for the tissue-specificity and level of HBV DNA expression, and for the relationship between expression and methylation of the transgenes. Only one out of ten for HB-GII and one out of eight for 1.2HB-BS were high producers of viral antigens. In high producers, transgenes were expressed in the liver and the kidneys. But in low producers, transgenes were usually expressed only in the kidneys. There is a reciprocal relationship between the level of expression and the degree of methylation, that is, the higher the level of expression, the less the degree of methylation. We also observed that the expression of the integrated HBV-DNA was repressed by methylation following its passage through the female germline in one line. Thus, in addition to transacting factors that can control the gene expression positively or negatively, this tissue-specific methylation may also be involved in the regulation of HBV gene expression.

Hepatitis B virus transactivator X protein is not tumorigenic in transgenic mice

The hepatitis B virus X protein acts as a transcriptional transactivator in vitro. To elucidate possible biological effects of X protein on liver cells in vivo, we generated four lines of transgenic mice carrying the X gene open reading frame under the control of the human alpha-1-antitrypsin regulatory region. The plasmid construct used to introduce the transgene was shown to encode a 16-kDa X protein with transactivating capability. The expression of X protein was detectable in liver tissue of transgenic animals of three of the lines by immunoblot analysis; levels of expression were highest in the first month after birth of the animals. Over 80 animals from the expressing lines were examined histologically. Most transgenic mice, some of which were observed for up to 2 years, remained normal. However, a few transgenic animals developed mild focal hepatitis, nuclear pleomorphism, focal necrosis, and/or nodular hyperplasia in the liver. Increased mitotic activity of hepatocytes also was observed. We conclude that, at the level of expression achieved in these transgenic mice, the hepatitis B virus transcriptional transactivator X protein alone does not appear to mediate the development of serious liver damage or hepatocellular carcinomas.

HBV production in transgenic mice

We produced transgenic mice by microinjecting a partially duplicated copies of hepatitis B virus (HBV) gene into fertilized eggs of C57BL/6 mice. One mouse was a high producer of HBV surface antigen (HBsAg) and HBV e antigen (HBeAg) in the serum. All offspring carrying HBV DNA were positive for both antigens in the serum. The HBV DNA was expressed in liver- and kidney-specific manner. The normal process of HBV replication, including the packaging of the pregenome 3.5-kb RNA into a nucleocapsid, the reverse-transcription of the complete minus strand DNA, and the release of Dane particles into the serum before the completion of synthesis of plus strand, occurred in the liver of these transgenic mice. These results suggest that the species specificity of HBV infection is not due to the inability to replicate in nonnatural host but to the lack of receptors or factors needed for virus adsorption and internalization. The founder mouse is now 19 months of age but shows no clinical or pathological change, suggesting that HBV itself is not cytopathic.

Natural history of experimental woodchuck hepatitis virus infection: molecular virologic features of the pancreas, kidney, ovary, and testis

The kinetic patterns of woodchuck hepatitis virus (WHV) infection were monitored in the pancreas, kidneys, ovaries, and testes. Groups of woodchucks experimentally infected with a standardized inoculum of WHV were sacrificed at different times over a 65-week period beginning in the preacute phase of viral infection and continuing to the period of serologic recovery or the establishment of chronic infections and subsequent hepatocellular carcinoma (B. E. Korba, P. J. Cote, F. V. Wells, B. Baldwin, H. Popper, R. H. Purcell, B. C. Tennant, and J. L. Gerin, J. Virol. 63:1360-1370, 1989). Tissues from an additional group of long-term (2 to 3 years) chronic WHV carriers which had been infected with the same WHV inocula were also examined. Viral DNA replication intermediates were found in all four tissues during the acute phase of WHV infection. However, WHV DNA replication intermediates were observed only in the kidneys of a small proportion of the chronically infected animals. Following the acute phase of infection, WHV DNA was present only in the pancreas, kidneys, and ovaries of the chronically infected woodchucks. A progressive evolution of different WHV genomic forms related to the replicative state of WHV was observed in these tissues. Histologic evaluation of these four tissues revealed only minimal, localized lesions which were not correlated with the state of WHV activity. The observations compiled in this study further extend the tissue tropism of WHV.

HNF-1 shares three sequence motifs with the POU domain proteins and is identical to LF-B1 and APF

The coordinate expression of genes during development and differentiation is thought to be accomplished by common transcription factors operating on the promoters of families of coexpressed genes. HNF-1 is a transcriptional factor involved in the expression of genes in the liver and was originally defined as playing a major role in coordinating the expression of the linked fibrinogen genes. We have isolated cDNA clones for HNF-1 using oligonucleotides prepared to the sequence of the purified protein. The sequence of HNF-1 shares homeo domain, as well as short acidic and basic sequences with the POU family of transcriptional activators. Peptides from the protein interacting with the albumin proximal element, or B box (APF), and the factor interacting with the alpha 1-antitrypsin promoter (LF-B1) are found in the predicted sequence of HNF-1. HNF-1 mRNA is not present in the dedifferentiated hepatoma variant, C2, but reappears upon selection for gluconeogenesis coincident with the re-expression of liver-specific genes. Finally, the mRNA is not present in somatic cell hybrids in which liver-specific gene expression is extinguished. In contrast to earlier published results, we find that in addition to being present in the liver, HNF is expressed in the kidney, intestine, and spleen, but not in other tissues. This pattern of expression mirrors the complex pattern of expression of many genes, such as alpha-fetoprotein, alpha 1-antitrypsin, and fibrinogen, whose promoters contain HNF-1 sites. These data indicate that HNF-1 is a more broadly acting transcription factor than has been indicated by previous work.

DNase I hypersensitive site maps to the HBV enhancer

Two lines of HBV transgenic mice (derived from G7 and G26) have been produced, each of which contains a unique locus of HBV DNA and expresses 2.1-kb HBsAg transcripts preferentially in liver and kidney tissues. To investigate the regulation of HBV expression in these mice, we have examined the state of methylation and the chromatin structure in and around the HBV sequences in tissues with and without HBV gene expression. Hypomethylation of HpaII and HhaI sites in and around the HBV sequences strongly correlated with HBV gene expression, although it was clearly not sufficient for HBV expression. Alterations in chromatin configuration were detected by DNase I digestion which identified a major hypersensitive site (HS) in liver and kidney tissue. By restriction enzyme mapping and indirect end-labeling, the HS was localized to the region of the HBV enhancer in both lines of HBV transgenics. The presence of this DNase I hypersensitive site was necessary but not sufficient for HBV expression, since it was also detected in tissues not expressing HBV. An additional DNase I hypersensitive site was mapped to the core promoter region of the G7 transgene in liver and kidney tissue but not in G26 tissues. The identification of a DNase I hypersensitive site mapping to the HBV enhancer region supports the notion that this region can interact with cellular proteins and is involved in the regulation of viral gene transcription in vivo.

Developmental regulation of hepatitis B surface antigen expression in two lines of hepatitis B virus transgenic mice

Two lines of hepatitis B virus (HBV) transgenic mice, designated G7 and G26, show preferential expression of the 2.1-kilobase hepatitis B surface antigen (HBsAg) RNA transcript in liver and kidney tissues (R. D. Burk, J. A. DeLoia, M. K. ElAwady, and J. D. Gearhart, J. Virol 62:649-654, 1988). This transcript was first identified in transgenic mice at gestational day 14 and was detected at similar or increased levels through birth and early development. However, in contrast to 2.1-kilobase HBsAg mRNA levels, HBsAg protein levels in serum decreased shortly after birth. Thereafter, serum HBsAg increased 100-fold to adult levels, with a corresponding 5- to 10-fold increase in HBsAg mRNA levels. In addition, adult males have higher levels of HBsAg in serum than females. HBsAg in serum in males was reduced approximately 50% by surgical castration and was restored to near-normal levels by testosterone supplementation. Since both transgenic lines show similar patterns of gene expression, we suggest that HBsAg gene expression is determined by viral regulatory elements in response to host factors. Whether tissue specificity, developmental regulation, and sexual dimorphism of expression of the exogenous HBV sequences were determined by single or multiple HBV regulatory elements remains to be determined.

Experimental systems for the study of hepadnavirus and hepatitis delta virus infections

The past decade has seen a dramatic increase in the number of approaches available for the study of hepadnavirus and hepatitis delta virus infections. In this review, we have summarized the recent applications of these approaches to the study of virus replication, tissue specificity, liver injury and hepatocellular carcinogenesis.

Demethylation by 5-azacytidine results in the expression of hepatitis B virus surface antigen in transgenic mice

In 14p3HB transgenic mice, which carry three tandem copies of hepatitis B virus (HBV) DNA, the HBV DNA was significantly methylated and no viral proteins were produced. To analyze the causal relationship between hypermethylation and gene inactivity, 5-azacytidine was injected into the mice to demethylate HBV DNA. When postnatal 14p3HB mice were treated with the drug, hepatitis virus surface antigen was produced in these mice by 3 weeks of age, and the integrated HBV DNA of the liver was less heavily methylated. Our results suggest that injection of 5-azacytidine can be used to efficiently activate a silent transgene such as HBV DNA in transgenic mice.

Natural history of woodchuck hepatitis virus infections during the course of experimental viral infection: molecular virologic features of the liver and lymphoid tissues

In this study, the kinetic patterns of woodchuck hepatitis virus (WHV) infection were monitored in the liver and the five primary components of the lymphoid system (peripheral blood lymphocytes, lymph nodes, bone marrow, spleen, and thymus). Groups of woodchucks experimentally infected with a standardized inoculum of WHV were sacrificed at different times over a 65-week period beginning in the preacute phase of viral infection and continuing to the period of serologic recovery or the establishment of chronic infections and subsequent hepatocellular carcinoma. Infection by WHV was not limited to the liver but involved the major components of the lymphoid system during all stages of virus infection. A complex series of kinetic patterns was observed for the appearance of WHV DNA in the different lymphoid compartments and the liver during the entire course of viral infection. A progressive evolution of different WHV genomic forms related to the replicative state of WHV was also observed. Lymphoid cells of the bone marrow were the first cells in which WHV DNA was detected, followed in order by the liver, the spleen, peripheral blood lymphocytes, lymph nodes, and finally the thymus. Several differences were observed in the cellular WHV DNA patterns between woodchucks that developed chronic WHV infections and those that serologically recovered from acute WHV infections. The observations compiled in this study indicate that the host lymphoid system is intimately involved in the natural history of hepadnavirus infections from the earliest stages of virus entry.

Expression and replication of hepatitis B virus genome in transgenic mice

We produced transgenic mice by microinjecting a partial tandem duplication of the complete hepatitis B virus (HBV) genome into fertilized eggs of C57BL/6 mice. One of eight transgenic mice was a high producer for HBV surface antigen (HBsAg) and HBV e antigen (HBeAg) in the serum. The HBV genomes were transmitted to the next generation and these F1 mice also produced HBsAg and HBeAg. mRNAs of 3.5, 2.1, and 0.8 kilobases were detected in the livers and the kidneys of these mice. In addition, a 0.8-kilobase RNA was detected in the testis. Single-stranded and partially double-stranded HBV DNAs were shown to be produced in the cytoplasm of the liver and kidneys. These HBV DNAs were associated with the core particles, indistinguishable from nucleocapsid produced in an infected human liver. Viral genome DNA was detected in the serum. These results demonstrate that the HBV genome integrated into the mouse chromosome acted as a template for viral gene expression, allowing viral replication. Thus, these transgenic mice should be useful for detailed studies of the replication and expression of HBV and for pathological studies of hepatitis, including the development of hepatocellular carcinoma.

Replication and gene expression of hepatitis B virus in a transgenic mouse that contains the complete viral genome

We have sought to address the problem of the host and tissue specificity of the hepatitis B virus (HBV) by using transgenic mice obtained after injection of head-to-tail dimers of the HBV genome. Viral DNA replication and protein synthesis were obtained in one of nine transgenic mice containing integrated HBV DNA. The RNAs encoding the HBV surface antigen and the core antigen were synthesized in the liver, the kidney, and the heart. In these organs, DNA replicative intermediates similar to those found during normal infection were associated with corelike structures. Large amounts of core polypeptides and capsids were detected in the nuclei in the absence of any pathological effect. These results show that the different steps of HBV multiplication can take place in nonliver nonhuman cells once the problem of entry into the host cell is overcome. In the absence of a small laboratory animal infectable by HBV, such transgenic mice should be helpful for the study of many aspects of viral multiplication.