Molecular pathogenesis of hepatocellular carcinoma in hepatitis B virus transgenic mice

Metallothionein overexpression suppresses hepatic hyperplasia induced by hepatitis B surface antigen

Transgenic mice that express the viral coat proteins of hepatitis B virus (HBV) in the liver display hepatocellular damage, inflammation, regeneration, hyperplasia, and, eventually, neoplasia that is similar to that of people with chronic, active hepatitis caused by HBV infection. Hepatocellular regeneration, in the context of chronic injury and inflammation, is thought to expose dividing cells to excessive oxygen radicals, which are believed to lead to DNA damage and, ultimately, neoplasia. Because metallothioneins scavenge free radicals in vitro, we generated mice that express excess (>10-fold) metallothionein I (MT-I* mice) and the HBV surface antigens (HBsAg) to ascertain whether MT-I* would ameliorate aspects of the pathology induced by HBsAg. Markers of hepatocyte injury and tumorigenesis in HBsAg mice were compared to those in double transgenic (HBsAg and MT-I*) mice. Hepatic hyperplasia, histology, aneuploidy, and accumulation of an oxidative DNA adduct, 8-oxo-2'-deoxyguanosine, were examined. Although hepatitis and neoplasia were not prevented by MT-I* expression in the HBsAg mice, there was less hyperplasia and less aneuploidy. We conclude that MT-I produces a beneficial effect in this in vivo model of HBV-induced hepatitis.

The PreS2 activators of the hepatitis B virus: activators of tumour promoter pathways

In addition to causing acute and chronic hepatitis, hepatitis B virus (HBV) is considered to be a major cliological factor in the development of human hepatocellular carcinoma (HCC). Epidemiological studies have demonstrated an approximately 10-fold increase in the relative risk of HCC among HBV carried compared to noncarriers. Almost all HBV-associated HCCs studied so far harbor chromosomally integrated HBV DNA. Integrated viral DNA can encode two types of transcriptional activators, the HBx protein and the PreS2 activators [the large surface proteins (LHBs) and truncated middle surface proteins (MHBs)]. The activator function of the PreS2 activators is based on the cytoplasmic orientation of the PreS2 domain. The PreS2 domain is PKC-dependent phosphorylated. Moreover, the PreS2 domain binds of PKC alpha/beta and triggers a PKC-dependent activation of the c-Raf-1/MAP2-kinase signal transduction cascade, resulting in an activation of transcription factors such as AP-1 and NF-kB. Furthermore, by activation of this signaling cascade, the PreS2 activators cause an increased proliferation rate of hepatocytes. According to the two-step model of carcinogenesis (initiation/promotion), the PreS2 activators could exert a tumour-promoter-like function by activation of the PKC/c-Raf-1/MAP2-kinase signaling cascade: cells harboring critical mutations (initiation) may be positively selected (promotion). Such a multistep process may account for the long latency period in HCC development, but it also leads to the hypothesis that each tumor reflects an individual case.

The hepatitis B virus-trimera mouse: a model for human HBV infection and evaluation of anti-HBV therapeutic agents

Previous studies have demonstrated the feasibility of implantation of human blood cells or tissues in lethally irradiated mice or rats, radioprotected with SCID mouse bone marrow cells: The Trimera system. In the present study, we describe the development of a mouse Trimera model for human hepatitis B virus (HBV) infection. In this model, viremia is induced by transplantation of ex vivo HBV-infected human liver fragments. Engraftment of the human liver fragments, evaluated by hematoxylin-eosin staining and human serum albumin mRNA expression, was observed in 85% of the transplanted animals 1 month postimplantation. Viremia levels were determined in these mice by measuring serum HBV DNA using polymerase chain reaction (PCR), followed by dot-blot hybridization. HBV DNA is first detected 8 days after liver transplantation. Viremia attains a peak between days 18 and 25 when HBV infection is observed in 85% of the transplanted animals. The HBV-Trimera model was used to evaluate the therapeutic effects of human polyclonal anti-HBs antibodies (Hepatect) and of two reverse-transcriptase inhibitors, lamivudine (3TC) and beta-L-5-fluoro-2',3'-dideoxycytidine (beta-L-5FddC). Treatment of HBV-Trimera mice with these drugs effectively reduced both the percentage of infected animals and the viral load in their sera. Treatment cessation resulted in rebound of viral load, indicating HBV replication upon drug withdrawal. These results show that the HBV-Trimera model represents a novel experimental tool for simulating human HBV infection and evaluating potential anti-HBV therapeutic agents.

Murine mentors: transgenic and knockout models of surgical disease

OBJECTIVE

Transgenic and knockout technologies have emerged from the "molecular biology revolution" as unprecedented techniques for manipulating gene function in intact mice. The goals of this review are to outline the techniques of creating transgenic and knockout mice, and to demonstrate their use in elucidation of the molecular mechanisms underlying common surgical diseases.

SUMMARY BACKGROUND DATA

Gain of gene function is created by transgenic technology, whereas gene function is ablated using gene knockouts. Each technique has distinctive applications and drawbacks. A unique feature of genetically manipulated mice is that combinatorial genetic experiments can be executed that precisely define the functional contribution of a gene to disease progression. Transgenic and knockout mouse models of wound healing, cardiovascular disease, transplant immunology, gut motility and inflammatory bowel disease, and oncology are beginning to illuminate the precise molecular regulation of these diseases. Transgenic technology has also been extended to larger mammals such as pigs, with the goal of using genetic manipulation of the xenogenic immune response to increase the availability of transplant organs. Continual refinements in gene manipulation technology in mice offer the opportunity to turn genes on or off at precise time intervals and in particular tissues, according to the needs of the investigator. Ultimately, investigation of disease development and progression in genetically manipulated mammals may delineate new molecular targets for drug discovery and provide novel platforms for drug efficacy screens.

CONCLUSIONS

Emulation of human disease and therapy using genetically manipulated mammals fulfills a promise of molecular medicine: fusion of molecular biochemistry with "classical" biology and physiology. Surgeons have unique skills spanning both worlds that can facilitate their success in this expanding arena.

Hepatitis B virus-induced liver injury and altered expression of carcinogen metabolising enzymes: the role of the HBx protein

Hepatitis B virus (HBV) and aflatoxins are major risk factors for hepatocellular carcinoma (HCC) exhibiting a synergistic interaction in the development of this disease. The molecular mechanisms of this interaction remain to be elucidated but an altered carcinogen metabolism in the presence of hepatitis-induced liver injury is one hypothesis. The availability of biomarkers of aflatoxin exposure and metabolism permits this hypothesis to be examined in human populations whilst animal models, such as HBV transgenic mice permit parallel studies in an experimental setting. The hepatitis B virus X protein (HBx) is suspected to play a role in the hepatocarcinogenic process by virtue of its capacity to transactivate oncogenes and several other cellular genes via cis-acting elements. In previous studies in HBV transgenic mice expressing the HB surface antigen and X genes we observed a marked induction of specific cytochrome P450s (CYP) (Kirby et al., 1994a). In the current study we investigated the status of CYP, glutathione S-transferases (GST) and antioxidant enzymes in mice carrying only the X gene under the control of the alpha-1 antitrypsin regulatory elements (ATX mice). Livers of ATX mice showed no major pathological alterations compared to age-matched non-transgenic control mice. Immunohistochemical staining for CYP1A, 2A5 and GST expression and determination of related enzymatic activities (7-ethoxyresorufin O-deethylation, 7-methoxyresorufin O-deethylation, coumarin 7-hydroxylation and GST activities) revealed no differences between control and ATX mice. In addition, no differences in antioxidant enzymes were observed. Overall, these results support the conclusion that HBx expression alone is insufficient to induce transactivation of CYP and GST genes or to alter the antioxidant system and that the induction in other HBV models is a result of inflammatory injury in the liver, a feature absent in ATX mice. These data are compared to biomarker studies of enzyme activities in aflatoxin-exposed human populations with and without HBV infection.

Pro-apoptotic effect of the hepatitis B virus X gene

The hepatitis B virus (HBV) is a common human pathogen that causes acute and chronic liver disease. Persistent HBV infection is strongly associated with the development of hepatocellular carcinoma. The contribution of the viral regulatory protein HBx in liver oncogenesis has been supported by our recent studies in a transgenic mouse model, showing that HBx cooperates with c-myc by accelerating the onset of primary liver tumors. Here we show that liver expression of HBx is associated with increased rates of spontaneous apoptosis in liver cells from two different transgenic lines. In transient transfection assays, overexpression of HBx in the established hepatocyte cell line MMHD3 and in human hepatoma cells HepG2 was found to induce apoptosis in a dose-dependent manner. These data suggest that HBx might trigger an apoptotic process in HBV-infected hepatocytes, in turn possibly favoring liver regeneration and accumulation of genetic alterations, ultimately leading to liver cell transformation in chronically infected patients.

Analytical method for the determination of sphinganine and sphingosine in serum as a potential biomarker for fumonisin exposure

The toxins produced by Fusarium moniliforme, which include fumonisins, are possible human carcinogens. Fumonisins are inhibitors of de novo sphingolipid biosynthesis. Alterations of the ratio of sphinganine (Sa) to sphingosine (So) in urine and serum has been proposed as a possible biomarker of exposure to this toxin. A new method was developed for their analysis in tissues and urine. This work describes the further adaptation of the method to the analysis of Sa and So in serum and its validation in sera of untreated and fumonisin B1 (FB1 ) treated rats and mice. No significant differences in the Sa/So ratios were observed in the FB1 treated rats. In mice, the increase was only of marginal statistical significance. Determination of Sa/So ratios in human sera could readily be made in small volumes (from 0.3 to 0.5 ml) of serum.

Dendritic Cell Immunization Breaks Cytotoxic T Lymphocyte Tolerance in Hepatitis B Virus Transgenic Mice

Hepatitis B virus (HBV) transgenic mice that are immunologically tolerant to HBV-encoded Ags represent a model of chronic HBV infection suitable for the development of therapeutic immunization strategies before testing in humans. Five lineages of HBV transgenic mice were immunized with plasmid DNA that encodes hepatitis B surface Ag (HBsAg) or with cytokine-activated bone marrow-derived dendritic cells (DCs) in an attempt to break tolerance to HBsAg at the B and T cell levels. DNA immunization stimulated an Ab response but not a cytotoxic T lymphocyte response to HBsAg in two of the five transgenic lineages studied. In contrast, infusion of activated transgenic or nontransgenic DCs stimulated a splenic CTL response in all three transgenic lineages immunized in this manner at precursor frequencies comparable to those in nontransgenic mice, indicating that DC function is normal, and HBsAg-specific CTLs are present but functionally silent in these transgenic animals. Importantly, none of the animals developed hepatitis or displayed suppressed viral gene expression or replication following either DNA immunization or DC administration even in the presence of anti-hepatitis B surface (HBs) Abs and HBs-specific CTLs. These results indicate that Ag presentation by cytokine-activated DCs can break tolerance and trigger an anti-viral CTL response in HBV transgenic mice, and they suggest that this strategy is more efficient than DNA immunization in this setting. Nonetheless, more efficient immunization strategies are needed to stimulate an immune response of sufficient quality and magnitude to achieve an immunotherapeutic antiviral effect.

Hepatitis B viruses and cancerogenesis

Hepatitis B virus (HBV) is a small, enveloped DNA virus which primarily infects liver cells and causes acute and persistent liver disease. Chronic HBV infection, frequently associated with cirrhosis and eventually hepatocellular carcinoma (HCC), represents a major health problem in the world. HBV is the prototype member of the hepadnavirus family, which includes several related mammalian viruses also implicated in liver carcinogenesis in the host. Although epidemiological evidence has clearly linked HBV infection with HCC development, the precise role of the virus and the molecular mechanisms of liver cell transformation remain elusive. Here we discuss potential oncogenic strategies of HBV, ranging from indirect mechanisms related to chronic necroinflammatory disease and to the effects of viral gene products on cell proliferation and apoptosis, to direct insertional activation of cellular (onco)genes. Presently, vaccination of high risk populations represents a major way to prevent the development of HBV-related liver cancer.

Ultrastructural analysis of hepatitis B virus in HepG2-transfected cells with special emphasis on subviral filament morphogenesis

The intracellular accumulation of empty hepatitis B virus (HBV) particles of filamentous shape leads to a direct cytopathic effect in so-called ground-glass hepatocytes. The aim of this study was to investigate how these filaments can be structurally formed at the cellular level. By electron microscopy, we reexamined the HBV-producer HepG2T-14 cells, which have been described as producing a substantial amount of empty HBV filaments compared with the other forms of HBV particles. Examination of ultrathin sections of HepG2T14 cells revealed the presence of HBV virions and filaments at the periphery of extremely large intracellular cisternae, probably related to a pre-Golgi compartment. Very long filaments appeared to be formed by a tubular budding of a long portion of the cisterna membrane. This phenomenon may be identical to that observed in the hepatocytes of HBV chronic carriers, in which the inability of the infected cell to export long HBV filamentous particles through the cellular secretion pathway seems to be at the origin of a direct cytopathic effect.

Transgenic expression of replication-restricted enteroviral genomes in heart muscle induces defective excitation-contraction coupling and dilated cardiomyopathy

Numerous studies have implicated Coxsackievirus in acute and chronic heart failure. Although enteroviral nucleic acids have been detected in selected patients with dilated cardiomyopathy, the significance of such persistent nucleic acids is unknown. To investigate the mechanisms by which restricted viral replication with low level expression of Coxsackieviral proteins may be able to induce cardiomyopathy, we generated transgenic mice which express a replication-restricted full-length Coxsackievirus B3 (CVB3) cDNA mutant (CVB3DeltaVP0) in the heart driven by the cardiac myocyte-specific myosin light chain-2v (MLC-2v) promoter. CVB3DeltaVP0 was generated by mutating infectious CVB3 cDNA at the VP4/VP2 autocatalytic cleavage site from Asn-Ser to Lys-Ala. Cardiac-specific expression of this cDNA leads to synthesis of positive- and negative-strand viral RNA in the heart without formation of infectious viral progeny. Histopathologic analysis of transgenic hearts revealed typical morphologic features of myocardial interstitial fibrosis and in some cases degeneration of myocytes, thus resembling dilated cardiomyopathy in humans. There was also an increase in ventricular atrial natriuretic factor mRNA levels, demonstrating activation of the embryonic program of gene expression typical of ventricular hypertrophy and failure. Echocardiographic analysis demonstrated the presence of left ventricular dilation and decreased systolic function in the transgenic mice compared with wild-type littermates, evidenced by increased ventricular end-diastolic and end-systolic dimensions and decreased fractional shortening. Analysis of isolated myocytes from transgenic mice demonstrate that there is defective excitation-contraction coupling and a decrease in the magnitude of isolated cell shortening. These data demonstrate that restricted replication of enteroviral genomes in the heart can induce dilated cardiomyopathy with excitation-contraction coupling abnormalities similar to pressure overload models of dilated cardiomyopathy.

Aberrant expression of a cytokeratin in a subset of hepatocytes during chronic WHV infection

Chronic infection of woodchucks with woodchuck hepatitis virus (WHV) invariably leads, within 2-4 years, to the appearance of hepatocellular carcinoma (HCC). HCC is preceded by an extended period of chronic liver damage, probably resulting from the immune response to viral antigens. It may be that infection itself also induces changes in the hepatocyte population. To begin to identify some of the changes in the liver prior to the appearance of HCC, monoclonal antibodies (MAbs) were generated from mice immunized with hepatocytes from a woodchuck chronically infected with WHV or with a tumor lysate. Immunofluorescence microscopy was used to select MAbs that reacted with host markers whose patterns of expression would distinguish chronically infected from uninfected liver or from liver tumors. One of these MAbs (2F2) reacted strongly with a subset of hepatocytes in chronically infected liver; a similar staining pattern was not detected in uninfected or transiently infected liver. Evidence is presented that this strong staining reaction reflects the overexpression or accumulation of the hepatocyte-specific intermediate filament protein, cytokeratin K18, a protein previously implicated in cryptogenic cirrhosis of the liver in humans (Ku, N. O. , Wright, T. L., Terrault, N. A., Gish, R., and Omary, M. B. J. Clin. Invest. 99: 19-23, 1997). Double immunofluorescent staining with antibodies to K18 and M-envelope protein of WHV suggested that strong reactivity to K18 was limited to cells expressing high levels of one or both of the large viral-envelope proteins, M and L; however, high expression of these viral proteins was not always associated with a strong K18 staining reaction.

Cytokeratin expression is reduced in glycogenotic clear hepatocytes but increased in ground-glass cells in chronic human and woodchuck hepadnaviral infection

Hepatocytes of normal adult liver express cytokeratins (CKs) 8/18, but bile duct cells additionally contain CK7/19. We have previously demonstrated the frequent occurrence of foci of altered hepatocytes in association with hepatic tumors in humans and provided evidence for a preneoplastic nature of the focal lesions. In this study, we investigated the CK expression in both the preneoplastic lesions and extrafocal parenchyma. Sixty-seven explanted livers with cirrhosis or advanced fibrosis harboring preneoplastic focal lesions, with or without hepatitis B virus (HBV) infection, as well as 9 livers with HBV-associated fulminant hepatitis, were studied for the expression of CK7/8/14/18/19. Five livers from woodchucks infected with the woodchuck hepatitis virus (WHV) were also investigated. Glycogenotic clear hepatocytes were negative or weakly positive for CK8/18, while amphophilic hepatocytes were strongly positive for these CKs, the changes being associated with marked reduction and increase, respectively, of highly organized membranous components in their cytoplasm. This allows the distinct recognition of the clear-cell and clear-cell-dominant preneoplastic lesions in the human and woodchuck livers. In ground-glass hepatocytes expressing viral antigens, an unusual accumulation of CK8/18 was observed, but there was no evidence of preferential necrosis of ground-glass hepatocytes. Many CK7- and CK19-positive ductular (oval) cells were found in extrafocal liver tissue, but only rarely were they present within focal lesions.

Immune pathogenesis of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common complication of chronic hepatitis B virus (HBV) infection. The pathogenetic mechanisms potentially responsible for HCC during chronic HBV infection are not well defined. This study demonstrates that chronic immune-mediated liver cell injury triggers the development of HCC in the absence of viral transactivation, insertional mutagenesis, and genotoxic chemicals. These results strongly suggest that the immune response to HBV is both necessary and sufficient to cause liver cancer during chronic HBV infection, and that all other procarcinogenic events associated with HCC are probably dependent on this process.

Prognosis of hepatocellular carcinoma with diabetes mellitus after hepatic resection

We evaluated the effect of diabetes mellitus on the prognosis of hepatocellular carcinoma after an elective hepatic resection. Of the 342 patients who underwent a hepatic resection between April 1985 and March 1995, 87 (25.4%) were diabetic. Postoperative morbidity was more common among diabetics than among nondiabetics (36.0% vs. 22.5%, P = .0239). The postoperative survival rate and the cancer-free survival rate were also better in patients without diabetes than in those with diabetes (P = .0333, P = .0149). The results of a multivariate analysis show diabetes mellitus to be an independent and prognostic indicator after a hepatic resection with hepatocellular carcinoma. According to the above findings, diabetes mellitus is thus considered to be a risk factor for prognosis after hepatic resection in patients with hepatocellular carcinoma.

Inducible nitric oxide synthase expression in chronic viral hepatitis. Evidence for a virus-induced gene upregulation

Increased nitric oxide (NO) production may contribute to the pathological changes featuring in some inflammatory diseases, but the role of NO in chronic viral hepatitis is still unknown. We compared the inducible NO synthase (NOS2) expression in the liver of patients with chronic viral hepatitis with that of both nonviral liver disease and histologically normal liver. NOS2 expression was assessed by immunohistochemical and in situ hybridization studies of liver biopsy sections. An intense hepatocellular NOS2 reactivity was detected in chronic viral hepatitis, whereas it was weakly or not observed in nonviral liver disease or normal liver, respectively. In addition, we determined whether the hepatitis B virus (HBV) might regulate the synthesis of this enzyme. NOS2 mRNA and protein levels as well as enzyme activity were assessed in cytokine-stimulated HBV-transfected and untransfected hepatoma cells. Transfection with either HBV genome or HBV X gene resulted in induction of NOS2 mRNA expression, and the maximal induction of this transcript and NO production was observed in cytokine-stimulated HBV-transfected cells. These results indicate that hepatotropic viral infections are able to upregulate the NOS2 gene expression in human hepatocytes, suggesting that NO may mediate important pathogenic events in the course of chronic viral hepatitis.

The mouse equivalent of the human p53ser249 mutation p53ser246 enhances aflatoxin hepatocarcinogenesis in hepatitis B surface antigen transgenic and p53 heterozygous null mice

The relative contribution to development of hepatocellular carcinoma of the mouse equivalent to the human p53ser249 mutation, found in human hepatocellular carcinoma associated with aflatoxin (AFB1) exposure, is compared with other major risk factors in a transgenic mouse model. Transgenic p53ser246 mice, expressing the mutant protein gene under the control of a truncated albumin promoter, were bred to mice lacking p53 (p53-/-) and to transgenic mice expressing hepatitis B surface antigen (HBsAg). AFB1 hepatocarcinogenesis was then determined in offspring with single or multiple risk factors by determination of the numbers of high-grade hepatic tumors at 13 months of age. In AFB1-treated male mice, expression of the p53ser246 mutation increases the incidence of high-grade tumors from 0% to 14% in HBsAg-negative, p53+/+ (wild-type homozygous) control mice; from 14% to 71% in HBsAg-negative, p53+/- (wild-type heterozygous) mice; and from 62% to 100% in HBsAg-positive, p53+/+ mice. Thus, whereas HBsAg expression and AFB1 together are strongly cocarcinogenic, the presence of the p53ser246 mutant not only significantly enhances this cocarcinogenic effect, it also increases tumorigenesis in AFB1-treated p53 heterozygous and homozygous mice not expressing HBsAg. The possibility that the p53ser246 mutant protein may act as a promoting agent for AFB1 hepatocarcinogenesis is discussed.

Abnormalities of liver regeneration: a review

Structural and functional changes during liver regeneration have been studied extensively in experimental animals following partial hepatectomy or hepatic injury induced by noxious substances. These observations have been extended to evaluate abnormalities of liver regeneration which contribute to chronic hepatitis, cirrhosis and/or liver cancer in man. This is facilitated by the simultaneous perfusion of flash frozen percutaneous biopsies or explanted liver in an acrylic chamber with tritiated thymidine and proline to evaluate DNA and collagen synthesis, respectively. Such investigations indicate that chronic liver damage is associated with replication of mesenchymal, ductular and parenchymal cells, accompanied by increased fibrogenesis. The regenerative response of the liver after noxious injury in experimental animals and man is associated with the release of cytokines, increase of growth response genes and change in telomerase activity. The ability to monitor morphological, genetic and biochemical parameters provides new information on the kinetics of the reparative process in hepatobiliary disease. Abnormal liver regeneration and its untoward effects including tumorigenesis may be modified by altering nutrients, blocking antigens or receptors, and inhibiting metabolites which regulate cell replication and collagen deposition.

Hepatocarcinogenesis (Z#2)/mutagenesis during initiation stage

Previously, we developed a model for high incidence, endogenously generated hepatocellular carcinoma (HCC), the human alpha-1-antitrypsin (alpha1AT) Z gene transgenic mouse (Z#2). We now examine the potential utility of a model for endogenous carcinogenesis utilizing the Z#2 mouse also transgenic for the lacI gene, a convenient target for in vivo mutagenesis studies. We crossed the Z#2 line and mice transgenic for lambda/lacI shuttle vector (Big Blue), for determination of lacI mutant frequency during initiation of endogenous carcinogenesis. Five month old double transgenic mice (Z#2+/lacI+) successfully displayed: (1) the expected post-inflammatory stage of Z#2 carcinogenesis; and (2) hepatic lacI mutants measured at frequencies (10(-5)-10(-4)) useful to mutagenesis studies. In this study, hepatic lacI mutation frequencies in Z#2 transgenic mice appeared to be only slightly increased (< 2x) when compared to age matched negative controls. In the future, it may be important to reconcile possibly limited lacI mutagenesis at the time of initiation and demonstrated high incidence of hepatocarcinogenesis.

Hepatitis B injury, male gender, aflatoxin, and p53 expression each contribute to hepatocarcinogenesis in transgenic mice

The major risk factors for human liver cancer: hepatitis B virus (HBV) related liver injury, male gender, aflatoxin exposure, and p53 expression, are evaluated and compared in experimental transgenic mouse models. Transgenic mice that express hepatitis B surface antigen (HBsAg) in their liver and develop liver tumors at 18 months of age (HBV+ mice) were bred to p53 null mice (p53-/-) to produce mice p53+/-, HBV+ mice. These mice and control littermates ([p53+/+, HBV+], [p53+/-, HBV-], and [p53+/+, HBV-) were divided into groups that did or did not receive an injection of aflatoxin at 1 week of age. At sacrifice at 13 months of age, 100% (7/7) of male mice with each of the three risk factors (p53+/-, HBV+, AFB1+) developed high-grade hepatocellular carcinomas (HCC). If any one of the risk factors was absent, the incidence drops: if both p53 alleles are present, 62% (10/16); if HBsAg is not expressed, 14% (1/7); if AFB1 is not given, 25% (2/8). If only one of the risk factors is present no tumors above grade I are found. Similar results were observed in female mice except that HCC incidence in each group is less than in male mice. Some of the tumors in mice with more than one risk factor are of unusual histological types, such as hepatocholangio-carcinomas, adenocarcinomas and undifferentiated carcinomas that are not usually seen in HBV transgenic C57BL/6 mice. No loss or mutation of the p53 gene is detected in any of the tumors. Possibilities of how the four major risk factors for HCC interact to produce malignant liver tumors in these transgenic mouse models of hepatocarcinogenesis are discussed.

Metabolic activation of aflatoxin B1 to aflatoxin B1-8,9-epoxide in woodchucks undergoing chronic active hepatitis

Chronic hepatitis B virus infection as well as consumption of food contaminated with the mycotoxin aflatoxin B1 are considered to be 2 major risk factors for the development of primary liver cancer in humans. Furthermore, epidemiological surveys indicate that hepatitis B virus and aflatoxin B1 might act synergistically to induce primary liver cancer. In the present study, we have tested the hypothesis that the metabolic activation of aflatoxin B1 to aflatoxin B1-8,9-epoxide, the ultimate mutagenic and carcinogenic mycotoxin metabolite, is enhanced in an experimental model of chronic hepatitis using woodchucks, chronically infected with the woodchuck hepatitis virus. Woodchuck liver microsomes were incubated with radiolabeled aflatoxin B1, the resulting aflatoxin B1-8,9-epoxide was trapped as a glutathione conjugate and its formation rate was determined by a reversed-phase HPLC analysis. In woodchuck hepatitis virus-positive woodchucks, activation of aflatoxin B1 to aflatoxin B1-8,9-epoxide was reduced when compared to woodchuck hepatitis virus-free animals, and the extent of the reduction was dependent on the severity of the hepatitis. Hence, at least in woodchucks, a chronic hepadnaviral infection does not lead to an enhanced activation of aflatoxin B1.

Activation of hepatitis B virus S promoter by the viral large surface protein via induction of stress in the endoplasmic reticulum

Hepatitis B virus (HBV) codes for three forms of surface protein. The minor, large form is translated from transcripts specified by the preS1 promoter, while the middle and small forms are translated from transcripts specified by the downstream S promoter. When the large surface protein is overexpressed, the secretion of both subviral and virion particles is blocked within the secretory pathway. We show here that overexpression of the large surface protein leads to up to a 10-fold activation of the S promoter but not of an unrelated promoter. Neither the middle nor the small surface protein, nor a secretable form of the large surface protein, activates the S promoter, but agents that induce endoplasmic reticulum (ER) stress have an effect similar to that of the large surface protein. The large surface protein also activates the S promoter in the context of the entire viral genome. Therefore, it appears that HBV has evolved a feedback mechanism, such that ER stress induced by accumulation of the large surface protein increases the synthesis of the middle and small surface proteins, which in combination with the large surface protein can form mixed, secretable particles. In addition, like other agents that induce ER stress, the large surface protein can activate the cellular grp78 and grp94 promoters, raising the possibility that it may alter the physiology of the host cell.

Accumulation of genetic alterations in a human hepatoma cell line transfected with hepatitis B virus

Chromosome and molecular analyses of the hepatitis B virus (HBV)-transfected HepG2T14.1 variant of the HepG2 cell line was conducted. In HepG2T14.1 cells several genetic alterations such as de novo aberrations of chromosomes 9, 14, 15, and 20 were identified that are not present in the parental HepG2 cell line. Furthermore, HepG2T14.1 cells showed loss of heterozygosity (LOH) in the q region of chromosome 14. The single HBV integration site in HepG2T14.1 cells mapped to the 2q35-36 region of one copy of chromosome 2 by fluorescence in situ hybridization (FISH). No genetic changes were identified at or near the HBV integration site at the level of these analyses. In addition, growth rates in vivo and in vitro were dramatically accelerated in HepG2T14.1 cells. These results document that a HBV-transfected hepatoma cell line has de novo genetic mutations at several sites of the host genome, one HBV integration site in an non-rearranged chromosome and an altered phenotype. These findings support our hypothesis that HBV might play a role in cellular transformation by interfering with cellular processes responsible for the stability of the genome.

The small envelope protein is required for secretion of a naturally occurring hepatitis B virus mutant with pre-S1 deleted

Naturally occurring deletions in the hepatitis B virus pre-S1 domain have been frequently found during persistent viral infection. In this study we have investigated the functional properties of a mutant viral genome that carries an in-frame deletion of 183 nucleotides in the pre-S1 region. This deletion removes the promoter of the small envelope gene. Transfection into human hepatocellular carcinoma cells of a replication-competent construct containing this deletion resulted in an increase of intermediate DNA replicative forms compared to those produced by wild-type hepatitis B virus. Northern blot analysis revealed that such cells lack the 2.1-kb transcripts encoding the small envelope protein and that hepatitis B surface antigen was absent as well. Furthermore, nucleocapsids containing the genome with pre-S1 deleted were not secreted, and the deleted large envelope protein was retained with the cytoplasm and exhibited a perinuclear pattern of distribution. However, coexpression with the small envelope protein was sufficient to restore virion secretion and to change the cellular distribution of the deleted large envelope protein. In addition, the creation of point mutations that prevent the synthesis of large or small envelope proteins also inhibited viral secretion and led to increased levels of hepatitis B virus intermediate replicative forms within the cell. These studies suggest that naturally occurring viral mutants with pre-S1 deletions involving the promoter region of the small envelope gene will generate a deleted large envelope protein that is retained in the endoplasmic reticulum, resulting in the accumulation of nucleocapsids containing viral DNA; transcomplementation with the wild-type small envelope protein will allow mutant virion secretion to occur.

Review: alpha 1-antitrypsin deficiency associated liver disease

alpha 1-Antitrypsin (alpha 1-AT) deficiency is the most common genetic cause of liver disease in children and genetic disease for which children undergo liver transplantation. It also causes cirrhosis and hepatocellular carcinoma in adults. Studies by Sveger in Sweden have shown that only a subgroup of the population with homozygous PiZZ alpha 1-AT deficiency develop clinically significant liver injury. Other studies have shown that the mutant alpha 1-AT Z molecule undergoes polymerization in the endoplasmic reticulum and that a subpopulation of alpha 1-AT-deficient individuals may be susceptible to liver injury because they also have a trait that reduces the efficiency by which the mutant alpha 1-AT Z molecule is degraded in the endoplasmic reticulum.

Interleukin-12 inhibits hepatitis B virus replication in transgenic mice

Interleukin-12 (IL-12) is a heterodimeric cytokine produced by antigen-presenting cells that has the ability to induce gamma interferon (IFN-gamma) secretion by T and natural killer cells and to generate normal Th1 responses. These properties suggest that IL-12 may play an important role in the immune response to many viruses, including hepatitis B virus (HBV). Recently, we have shown that HBV-specific cytotoxic T lymphocytes inhibit HBV replication in the livers of transgenic mice by a noncytolytic process that is mediated in part by IFN-gamma. In the current study, we demonstrated that the same antiviral response can be initiated by recombinant murine IL-12 and we showed that the antiviral effect of IL-12 extends to extrahepatic sites such as the kidney. Southern blot analyses revealed the complete disappearance of HBV replicative intermediates from liver and kidney tissues at IL-12 doses that induce little or no inflammation in these tissues. In addition, immunohistochemical analysis demonstrated the disappearance of cytoplasmic hepatitis B core antigen from both tissues after IL-12 treatment, suggesting that IL-12 either prevents the assembly or triggers the degradation of the nucleocapsid particles within which HBV replication occurs. Importantly, we demonstrated that although IFN-gamma, tumor necrosis factor alpha, and IFN-alpha/beta mRNA are induced in the liver and kidney after IL-12 administration, the antiviral effect of IL-12 is mediated principally by its ability to induce IFN-gamma production in this model. These results suggest that IL-12, through its ability to induce IFN-gamma, probably plays an important role in the antiviral immune response to HBV during natural infection. Further, since relatively nontoxic doses of recombinant IL-12 profoundly inhibit HBV replication in the liver and extrahepatic sites in this model, IL-12 may have therapeutic value as an antiviral agent for the treatment of chronic HBV infection.

Comparison of intrahepatic lymphocytes from normal and growth hormone transgenic mice with chronic hepatitis and liver cancer

Mice expressing an ovine growth hormone-mouse metallothionein promoter fusion gene (METoGH mice) develop chronic hepatitis which becomes progressively more severe over time, hepatocellular adenomas, and eventually carcinoma in the oldest animals. T-lymphocyte expression of activation/memory-associated markers was compared between liver and blood lymphocytes isolated from METoGH and non-transgenic mice at 7, 10 and 12 months of age. The percentage of intrahepatic lymphocytes (IHL) which were CD4+ was markedly diminished in METoGH mice at all times. CD4+ and CD8+ IHL in METoGH mice expressed Ly-6A/6D at increased density, and were CD45RBlo at later time-points. Ly-6C+ and NK1.1+ CD4+ cells, which are common in normal mouse liver, were found at decreased frequency in METoGH livers. Further analysis demonstrated that, as a proportion of total T-cell receptor (TCR) alpha beta cells, NK1.1+ TCR alpha beta int CD4+ cell numbers (NKT cells) were diminished in the livers of METoGH mice. Observations made in METoGH mice support the hypothesis that sustained liver inflammation and hepatocellular injury may be linked to liver cancer. Additionally, it is possible that the relative lack of NKT cells may create an environment permissive for the growth of liver tumours.

Long-term expression of the hepatitis B virus core-e- and X-proteins does not cause pathologic changes in transgenic mice

BACKGROUND/AIMS

Chronic infections with the human hepatitis B virus can result in liver cirrhosis and primary hepatocellular carcinoma. The reasons for these long-term effects are unclear. The aim of this study was to generate transgenic mice expressing the HBV X- and c/e-gene under authentic and foreign promoter control and to test whether the respective gene products can cause pathologic effects during the lifespan of a mouse. Moreover, the temporal and the tissue-specific regulation of the crucial HBV c/e-gene promoter was analyzed.

METHODS

Eight transgenic mouse lines were generated. Four contained the c/e- and X-gene and two contained only the X-gene under authentic promoter control. Two lines expressed only the X-gene under control of the rat insulin promoter/enhancer. Gene expression was tested by protein and mRNA analyses. During an observation period of 2 years, mice were sacrificed and organs subjected to histologic examination. Mice expressing the X-gene in pancreatic beta cells were tested for the development of diabetes.

RESULTS

In the liver, slight histopathologic alterations but no neoplastic changes could be observed in mice expressing the X-gene. Activity of the c/e-gene promoter/enhancer was age dependent and was not restricted to hepatocytes.

CONCLUSION

No evidence was obtained that long-term expression of the HBV c/e- and X-gene products can cause neoplasia during the lifespan of a mouse.

Alpha-1-antitrypsin deficiency: biochemistry and clinical manifestations

Alpha-1-antitrypsin (alpha 1-AT) deficiency is a well known cause of emphysema in adults. A subgroup of deficient individuals develops liver injury during infancy and childhood. In fact, it is the most common genetic cause of liver disease in children. Although lung injury is due to the decrease in alpha 1-AT function in the lung, allowing uninhibited elastolytic destruction of its connective tissue integrity, liver injury is probably due to retention of the mutant alpha 1-AT molecule in the endoplasmic reticulum (ER) of liver cells. Recent studies have shown that the mutant alpha 1-AT molecule polymerizes in the ER by a novel loop-sheet insertion mechanism. Other recent studies show that the subgroup of deficient individuals is susceptible to liver injury by virtue of unlinked genetic traits and/or environmental factors which interfere with degradation of the mutant alpha 1-AT molecules within the ER.

Etiology, screening, and treatment of hepatocellular carcinoma

The prognosis with large hepatocellular carcinomas is poor, and only palliative treatment is available. Small tumors are amenable to several modes of treatment, including liver transplantation, resection, or alcohol injection, with acceptable 5-year survival rates. Although the value of screening for hepatocellular carcinoma has yet to be shown, these data, coupled with the recognition of at-risk groups and useful diagnostic techniques, might encourage the clinician to screen at-risk patients in the clinic. New imaging techniques such as ultrasonographic angiography enhanced with CO2 microbubbles, or color Doppler ultrasound, may clarify the intratumoral blood flow of small tumors.

Transformation of liver by SV-40 T-antigen in transgenic mice is unaffected by metallothionein

Transgenic mice expressing excess metallothionein-I and SV-40 T-antigen were generated to test the hypothesis that metallothionein may influence the rate of neoplastic transformation induced by T-antigen within the liver. The livers of the double transgenic mice grew at the same rate (to 32% body weight), had similar morphological and histological appearance, had similar chromosomal instability, and released identical amounts of serine and alanine aminotransferases into the blood as mice bearing SV-40 T-antigen alone, despite the fact that metallothionein levels were elevated five- to ten-fold. We conclude that elevated levels of metallothionein-I do not influence either the initial hyperplasia or the subsequent neoplastic transformation that is induced by T-antigen, which is thought to act by sequestering the P53 and retinoblastoma gene products.

Transgenesis in rats: technical aspects and models

The production of transgenic rats by DNA-microinjection into fertilized ova has now become an established procedure, although fewer than 20 lines have been described during the last 5 years. Overall, transgenic rats remain more difficult to produce than transgenic mice, but satisfactory yields have been obtained by several laboratories. A review of the methods used to generate transgenic rats shows considerable variation between different laboratories, particularly in choice of strain, superovulation protocols and the use of embryo culture before reimplantation. In some instances, the production of transgenic rats has provided data that are new and relevant, compared to data obtained in mice bearing the same transgene. Models have been developed for human diseases such as hypertension and autoimmunity, and applications have been found in the study of carcinogenesis and in pharmacological research. Transgenic rat technology also opens up interesting perspectives for transplantation research, in which microsurgery is an essential procedure. Intensive research is in progress in several laboratories to produce rat embryonic stem (ES) cell lines, but existing lines have not participated in germ line formation a prerequisite for their use in gene knock out experiments.

Hepatitis B and C viruses and primary liver cancer

The data presented indicate that viral agents (namely, HBV and HCV) are major environmental aetiological factors for human primary liver cancer. It is important to elucidate the molecular mechanisms further because HCC is one of the few examples of virus-related human cancers. In addition, the available evidence points to the possibility of at least partial prevention of the tumour by large-scale vaccination.

Activation of fetal promoters of insulinlike growth factors II gene in hepatitis C virus-related chronic hepatitis, cirrhosis, and hepatocellular carcinoma

Increased prevalence of hepatitis C virus (HCV) infection has been found in patients with hepatocellular carcinoma (HCC). The expression of insulinlike growth factor II (IGF-II) has been linked to hepatocarcinogenesis in the experimental animal and in humans. Since reactivation of fetal IGF-II transcripts has been observed in human HCC, we have analyzed the levels of adult P1 and fetal P3 and P4 IGF-II promoter-derived transcripts in the liver of patients with HCV-related chronic active hepatitis (CAH), cirrhosis, and HCC by means of a semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) assay. Transcripts derived from adult P1 promoter were increasingly expressed from normals to patients with CAH and cirrhosis, but were undetectable in the tumorous area of 5 of 7 HCC patients and present at low levels in the nontumorous area of all HCC patients. Transcripts derived from fetal P3 promoter were not detectable in normal subjects, while they were expressed abundantly in most CAH and all cirrhotic patients. Transcripts from fetal P4 promoter were detected at high levels in 3 of 9 CAH patients and in the majority of cirrhotic patients. Increased expression of fetal promoter-derived transcripts was also found in the liver of HCC patients, although levels were lower than in cirrhosis. Also, the activity of fetal P3 and P4 promoters was higher in the nontumorous than in the tumorous area of the liver of HCC patients. The expression of IGF-II transcripts was correlated with the rate of cell mitotic activity by measuring the expression of the proliferating cell nuclear antigen (PCNA) gene. PCNA messenger RNA (mRNA) levels progressively increased from normals to CAH and to cirrhotic patients, and persisted at a high level in the tumorous and in the nontumorous area of HCC subjects, thus showing that the increase of IGF-II transcripts in CAH and cirrhosis is accompanied by an activation of cell mitosis in these samples. These data suggest that the activation of IGF-II gene expression from adult and fetal promoters may play a role in premalignant proliferation observed in HCV-related chronic liver disease.

Hepatitis C and B virus infections in hepatocellular carcinoma. Analysis of direct detection of viral genome in paraffin embedded tissues

BACKGROUND

Although there have been many seroepidemiologic studies on hepatitis C virus (HCV) infection and hepatocellular carcinoma (HCC) occurrence, the actual role of HCV in hepatocarcinogenesis is unknown.

METHODS

We have previously reported on a highly sensitive method of detecting and identifying sequences of RNA genome in formalin fixed, paraffin embedded (FFPE) tissue by polymerase chain reaction (PCR) assay. Using this method, we carried out a retrospective study to determine the prevalence of HCV and hepatitis B virus (HBV) genomes in FFPE specimens from 102 Japanese patients with HCC.

RESULTS

HCV-RNA was detected by nested PCR reverse transcription (RT) in 64 of the 102 patients (62.7%), and 78.1% (50/64) of those HCV-RNA-positive patients had HCV genotype II. HCV-RNA was present in 54 of 70 (77.1%) anti-HCV-positive patients, and also in 5 of 20 (25%) anti-HCV-negative patients. HBV-DNA was detected by nested PCR in FFPE liver specimens from 21 of 102 (20.6%) patients. HBV-DNA positivity was consistent with seropositivity for serum HBV markers in 17 of these 21 patients (80.9%). HBV-DNA was present in FFPE samples from 2 patients who were seronegative for HBV markers, and in 1 patient who was seropositive for anti-HBs. Double infection of these two viruses was found in 6 patients (5.9%). Three patients (2.9%) were negative for both hepatic viral genomes and serum viral markers.

CONCLUSIONS

The precise prevalence of HCV and/or HBV infection among HCC patients can be determined by studying routinely-processed FFPE HCC samples preserved for up to 11 years using the technique of nested PCR. HCV-RNA was detected in the majority of our HCC cases; type II was the most common genotype of HCV encountered. The incidence of HCV-associated HCC was three times greater than that of HBV. Thus, the hepatitis virus infection most frequently associated, and probably ectologically implicated, with HCC in Japanese people is HCV infection.

Increased sensitivity to the hepatocarcinogen diethylnitrosamine in transgenic mice carrying the hepatitis B virus X gene

The role of the hepatitis B virus (HBV) X protein in liver tumorigenesis is unresolved. Transgenic mice harboring the X gene (nt 1376-1840 under the control of the human alpha-1-antitrypsin regulatory elements) (ATX mice) display only minor histopathologic alterations of the liver. To determine if ATX mice are more susceptible to the effects of hepatocarcinogens, 12- to 15-d-old male ATX and control littermate mice were injected with a single dose (2 microgram/g body weight) of diethylnitrosamine (DEN). The animals were killed 6-10 mo after exposure and were analyzed for histological changes in the liver. One hundred percent of the DEN-treated AXT mice developed abnormal liver lesions. Then their liver tissues were compared by stereological analysis with those of non-transgenic animals, the ATX mice had a relative twofold increase in the total number of focal lesion and a twofold increase in the incidence of hepatocellular carcinoma. Elevated levels of X protein and p53 protein were not detected in carcinogen-induced nodules or tumors. These results are consistent with a model in which the expression of the HBV X protein potentiates the induction of DEN-mediated liver disease.

Adenoviral delivery of recombinant DNA into transgenic mice bearing hepatocellular carcinomas

To evaluate the applicability of recombinant adenoviral vectors in gene transfer to liver cancers, we infused the recombinant adenoviruses AD5CMV-LacZ and Ad5CMV-p53 through the portal veins into two lines of transgenic mice, one bearing the SV40 T antigen and the other the human hepatitis B viral envelope protein. These transgenic animals develop hepatocellular carcinomas (HCC) with predictable pathological manifestations. The levels of expression of the transgenes were dependent upon the viral doses. In all cases, high levels of expression were detected within 2 or 3 days after infusion, but were drastically reduced 7 days after infusion. Significant toxicities were found in the infused animals: > 80% of them died within 7 days after infusion with 10(10) pfu, and transgenic animals bearing HCC apparently were more sensitive to viral toxicity. Although a lower dose (10(9) pfu/animal) produced less toxicity, the levels of expression were substantially reduced (only about 10% of that in animals infused with 10(10) pfu). When Ad5CMV-p53 was infused into animals with nodular hyperplastic stage, the expression of the reporter gene seemed to distribute preferentially at the peripheries of the tumor nodules, and low levels of transgene expression were seen inside the nodules. In tumors in which necrotic lesions were evident, p53 was also expressed at the perpheries of the lesions. These distribution patterns were seen in both tumor models. There was no apparent suppression of tumor growth in the Ad5CMV-p53-infused animals. Our results suggest that alternative methods for gene therapy for HCC need to be explored.

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.

Some characteristics of neoplastic cell transformation in transgenic mice

The role of the expression of different cellular genes and viral oncogenes in malignant cell transformation is discussed. We pay special attention to the role of the genes for growth factors and their receptors and homeobox genes in oncogenesis. Based on both the literature and our own data, specific features of tumors developed in transgenic mice are discussed. All of these data are used to analyze current theories of multistep oncogenesis and the stochastic component in this process. We suggest that all known evidence about the mechanisms of oncogenesis be used in studying the problem at various structural and functional levels in an organism. The chapter shows that transgenic mice are a most suitable model for studying various aspects of malignant transformation from the molecular to the organismal and populational levels.