Multiple rearrangements and activated expression of c-myc induced by woodchuck hepatitis virus integration in a primary liver tumour

HBV Integration Induces Complex Interactions between Host and Viral Genomic Functions at the Insertion Site

Hepatitis B virus (HBV), one of the well-known DNA oncogenic viruses, is the leading cause of hepatocellular carcinoma (HCC). In infected hepatocytes, HBV DNA can be integrated into the host genome through an insertional mutagenesis process inducing tumorigenesis. Dissection of the genomic features surrounding integration sites will deepen our understanding of mechanisms underlying integration. Moreover, the quantity and biological activity of integration sites may reflect the DNA damage within affected cells or the potential survival benefits they may confer. The well-known human genomic features include repeat elements, particular regions (such as telomeres), and frequently interrupted genes (e.g., telomerase reverse transcriptase [i.e. TERT], lysine methyltransferase 2B [i.e. KMT2B], cyclin E1 [CCNE1], and cyclin A2 [CCNA2]). Consequently, distinct genomic features within diverse integrations differentiate their biological functions. Meanwhile, accumulating evidence has shown that viral proteins produced by integrants may cause cell damage even after the suppression of HBV replication. The integration-derived gene products can also serve as tumor markers, promoting the development of novel therapeutic strategies for HCC. Viral integrants can be single copy or multiple copies of different fragments with complicated rearrangement, which warrants elucidation of the whole viral integrant arrangement in future studies. All of these considerations underlie an urgent need to develop novel methodology and technology for sequence characterization and function evaluation of integration events in chronic hepatitis B-associated disease progression by monitoring both host genomic features and viral integrants. This endeavor may also serve as a promising solution for evaluating the risk of tumorigenesis and as a companion diagnostic for designing therapeutic strategies targeting integration-related disease complications.

Differential Expression of Woodchuck Toll-Like Receptors 1-10 in Distinct Forms of Infection and Stages of Hepatitis in Experimental Hepatitis B Virus Infection

Woodchucks infected with woodchuck hepatitis virus (WHV) represent a highly valuable model of human hepatitis B virus (HBV) infection, chronic hepatitis (CH), and virus induced-primary liver cancer. Toll-like receptors (TLRs) are important mediators of immune responses playing pivotal roles in the pathogenesis of viral diseases; however, their expression profiles in different forms of infection and stages of hepatitis, and in healthy animals remain essentially unknown. In this study, woodchuck TLRs 1–10 exon fragments were identified and TLR genes transcription quantified in livers, primary hepatocytes, peripheral blood mononuclear cells (PBMC), and in selected organs during experimental WHV infection. Among others, liver biopsies from acute hepatitis (AH) and CH showed significantly augmented expression of the majority of TLRs when compared to healthy and woodchucks prior to AH, with resolved AH or primary occult infection. In contrast to the liver tissue, significant upregulation of TLR3, TLR4, and TLR10, but downregulation of TLR7, characterized hepatocytes derived from livers of animals with resolved AH accompanied by secondary occult infection. Hepatocytes from CH showed significantly lower expression or a trend toward suppression of several TLRs when compared to hepatocytes from healthy animals and woodchucks with other forms of infection or hepatitis, suggesting that hepatocyte innate immune response is downregulated during CH. Contrastingly, upregulated transcription of some TLRs characterized PBMC throughout CH. Our study uncovered that TLR expression significantly varies between different forms of hepadnaviral infection and whether infection is accompanied or not by hepatitis. The results showed that the profiles of TLRs’ expression in circulating lymphomononuclear cells do not mirror accurately those of livers and hepatocytes from infected animals. These findings are of importance to the understanding of immune process operating at different sites targeted by virus in the course of hepadnaviral infection and evaluation of future therapies modifying antiviral innate responses in the woodchuck model.

Chronic viral hepatitis and its association with liver cancer

Chronic infection with hepatitis viruses represents the major causative factor for end-stage liver diseases, including liver cirrhosis and primary liver cancer (hepatocellular carcinoma, HCC). In this review, we highlight the current understanding of the molecular mechanisms that drive the hepatocarcinogenesis associated with chronic hepatitis virus infections. While chronic inflammation (associated with a persistent, but impaired anti-viral immune response) plays a major role in HCC initiation and progression, hepatitis viruses can also directly drive liver cancer. The mechanisms by which hepatitis viruses induce HCC include: hepatitis B virus DNA integration into the host cell genome; metabolic reprogramming by virus infection; induction of the cellular stress response pathway by viral gene products; and interference with tumour suppressors. Finally, we summarise the limitations of hepatitis virus-associated HCC model systems and the development of new techniques to circumvent these shortcomings.

HBV DNA Integration: Molecular Mechanisms and Clinical Implications

Chronic infection with the Hepatitis B Virus (HBV) is a major cause of liver-related morbidity and mortality. One peculiar observation in cells infected with HBV (or with closely‑related animal hepadnaviruses) is the presence of viral DNA integration in the host cell genome, despite this form being a replicative dead-end for the virus. The frequent finding of somatic integration of viral DNA suggests an evolutionary benefit for the virus; however, the mechanism of integration, its functions, and the clinical implications remain unknown. Here we review the current body of knowledge of HBV DNA integration, with particular focus on the molecular mechanisms and its clinical implications (including the possible consequences of replication-independent antigen expression and its possible role in hepatocellular carcinoma). HBV DNA integration is likely to influence HBV replication, persistence, and pathogenesis, and so deserves greater attention in future studies.

Detection of Hepatocyte Clones Containing Integrated Hepatitis B Virus DNA Using Inverse Nested PCR

Chronic hepatitis B virus (HBV) infection is a major cause of liver cirrhosis and hepatocellular carcinoma (HCC), leading to ~600,000 deaths per year worldwide. Many of the steps that occur during progression from the normal liver to cirrhosis and/or HCC are unknown. Integration of HBV DNA into random sites in the host cell genome occurs as a by-product of the HBV replication cycle and forms a unique junction between virus and cellular DNA. Analyses of integrated HBV DNA have revealed that HCCs are clonal and imply that they develop from the transformation of hepatocytes, the only liver cell known to be infected by HBV. Integrated HBV DNA has also been shown, at least in some tumors, to cause insertional mutagenesis in cancer driver genes, which may facilitate the development of HCC. Studies of HBV DNA integration in the histologically normal liver have provided additional insight into HBV-associated liver disease, suggesting that hepatocytes with a survival or growth advantage undergo high levels of clonal expansion even in the absence of oncogenic transformation. Here we describe inverse nested PCR (invPCR), a highly sensitive method that allows detection, sequencing, and enumeration of virus-cell DNA junctions formed by the integration of HBV DNA. The invPCR protocol is composed of two major steps: inversion of the virus-cell DNA junction and single-molecule nested PCR. The invPCR method is highly specific and inexpensive and can be tailored to DNA extracted from large or small amounts of liver. This procedure also allows detection of genome-wide random integration of any known DNA sequence and is therefore a useful technique for molecular biology, virology, and genetic research.

Role of DLC1 tumor suppressor gene and MYC oncogene in pathogenesis of human hepatocellular carcinoma: potential prospects for combined targeted therapeutics (review)

Hepatocellular carcinoma (HCC) is the third leading cause of cancer death, and its incidence is increasing worldwide in an alarming manner. The development of curative therapy for advanced and metastatic HCC is a high clinical priority. The HCC genome is complex and heterogeneous; therefore, the identification of recurrent genomic and related gene alterations is critical for developing clinical applications for diagnosis, prognosis and targeted therapy of the disease. This article focuses on recent research progress and our contribution in identifying and deciphering the role of defined genetic alterations in the pathogenesis of HCC. A significant number of genes that promote or suppress HCC cell growth have been identified at the sites of genomic reorganization. Notwithstanding the accumulation of multiple genetic alterations, highly recurrent changes on a single chromosome can alter the expression of oncogenes and tumor suppressor genes (TSGs) whose deregulation may be sufficient to drive the progression of normal hepatocytes to malignancy. A distinct and highly recurrent pattern of genomic imbalances in HCC includes the loss of DNA copy number (associated with loss of heterozygosity) of TSG-containing chromosome 8p and gain of DNA copy number or regional amplification of protooncogenes on chromosome 8q. Even though 8p is relatively small, it carries an unusually large number of TSGs, while, on the other side, several oncogenes are dispersed along 8q. Compelling evidence demonstrates that DLC1, a potent TSG on 8p, and MYC oncogene on 8q play a critical role in the pathogenesis of human HCC. Direct evidence for their role in the genesis of HCC has been obtained in a mosaic mouse model. Knockdown of DLC1 helps MYC in the induction of hepatoblast transformation in vitro, and in the development of HCC in vivo. Therapeutic interventions, which would simultaneously target signaling pathways governing both DLC1 and MYC functions in hepatocarcinogenesis, could result in progress in the treatment of liver cancer.

Purification of woodchuck hepatitis surface antigen using a monoclonal antibody raised against the antigen

Woodchuck hepatitis virus (WHV) is closely related to the human hepatitis B virus (HBV) and infection of woodchucks with WHV creates a useful model for studies of immunity, pathogenesis and therapy of HBV infection. To increase the usefulness of this model, monoclonal antibodies were raised to woodchuck hepatitis surface antigen (WHsAg) and one of these antibodies was used to purify the antigen by affinity chromatography from serum, a simpler and quicker method of purification than the current ultracentrifugation methods. The bands found by SDS-polyacrylamide gel electophoresis of WHsAg were the major 25 and 29 kilodalton (kDa) bands and a triplet of 45, 51 and 55 kDa which are thought to be the glycosylated and unglycosylated middle and large WHsAg. Both the antibody and the antigen are valuable reagents for the study of WHV infection.

Expression of c-Myc, c-Fos, and c-jun in hepatocellular carcinoma

BACKGROUND

Increased expression of the proto-oncogene c-myc is a common phenomenon in hepatocellular carcinoma (HCC). The proto-oncogenes c-fos and c-jun are involved in cell cycle progression and cellular proliferation.

METHODS

The objective of this study was to elucidate the mechanism of hepatocarcinogenesis with regard to the expressions of c-myc, c-fos, and c-jun. One hundred fifty biopsied HCC specimens were stained immunohistochemically for the above phenotypic markers both in tumor tissue and in adjacent nontumor tissue.

RESULTS

Although the expression of c-myc was high (74%) in tumor tissue, it was significantly less compared with the expression in nontumor tissue (100%; P = 0.0002). The expression of c-myc was inversely proportional to the grade of differentiation in tumor tissue (P = 0.0108; correlation coefficient [r] = -0.244); that is, tissue with poorer histologic differentiation had a lower level of c-myc expression. There were inverse associations between the expression of c-myc and the expression of mutated p53 (P = 0.0017; r = -0.285) as well as the expression of Ki67 (P = 0.057; r = -0.147). There was significantly high expression of c-fos in tumor tissue compared with the expression in nontumor tissue (91% vs. 0%; P < 0.0001). Both the tumor tissue and the nontumor tissue had high levels of expression of c-jun (96.53% and 100%, respectively). There was a trend toward a positive association between the expression of c-fos and the expression of c-jun in tumor tissue (P = 0.07; r = 0.162).

CONCLUSIONS

Because c-myc is a known inducer of wild type p53, decreased c-myc expression may lead to uncontrolled cell growth because of the lack of p53 expression that normally induces apoptosis. The coordinated expression of c-fos and c-jun in HCC may reflect the coordinated tumor cell cycle of progression and proliferation; however, future studies are required to elucidate this possibility.

Hepatitis B virus surface (S) transactivator with DNA-binding properties

Chronic infection with hepatitis B virus (HBV) in humans is strongly linked to the development of hepatocellular carcinoma (HCC). Activation of growth-regulatory genes may play a crucial role in carcinogenesis. Proto-oncogene expression has been shown to be higher in HCC tissue with integrated HBV DNA than in the normal liver. Earlier, we showed that the 3' end of the HBV major surface gene (S) (426-855 nucleotides of the S region) is a transactivator of the X promoter-enhancer regulatory element in co-transfection experiments. This region expresses a truncated carboxy terminal S protein extending from amino acid residues 102 to 226. In this study, the truncated S protein (trc-S) was examined for its enhancing activity on several viral and cellular regulatory elements. The results indicate that trc-S activates rous sarcoma virus long terminal repeat (LTR), human T-lymphotropic virus 2 LTR, human immunodeficiency virus 1 LTR, and the c-jun and c-fos promoters. Electrophoretic mobility shift assays carried out to investigate its DNA-binding properties established that trc-S binds to HBV X promoter and oligonucleotides representing binding sites for the AP1 and TFIID transcription factors. The specificity of this interaction was confirmed by using competition experiments and supershift assays. These experiments suggest that trc-S is a transactivator of several cellular and viral promoters and that this activity is mediated by direct interaction with DNA.

Hepatitis B virus DNA integration and expression of an erb B-like gene in human hepatocellular carcinoma

Southern blot studies on Hepatitis B Virus (HBV) DNA integration in 13 human hepatocellular carcinomas (HCCs) patients revealed the presence of several distinct HBV integration sites in different human liver disease patients. In one HCC patient the DNA fragment containing the HBV integration also hybridized to an erb B probe. The erb B/HBV co-migrating DNA fragment was cloned and sequenced, and showed that HBV DNA is integrated next to a cellular DNA fragment which is homologous to the tyrosine protein kinase domain of the human epidermal growth factor receptor gene and other cell surface receptor genes. The virus-integrated cellular DNA sequence is expressed in this HCC patient, suggesting a possible role for this gene in hepatocarcinogenesis.

Hepadnavirus integration: mechanisms of activation of the N-myc2 retrotransposon in woodchuck liver tumors

In persistent hepadnavirus infections, a distinctive feature of woodchuck hepatitis virus (WHV) is the coupling of frequent viral integrations into myc family genes with the rapid onset of primary liver tumors. We have investigated the patterns of WHV DNA insertion into N-myc2, a newly identified retroposed oncogene, in woodchuck hepatomas resulting from either natural or experimental infections. In both cases, integrated viral sequences were preferentially associated with the N-myc2 locus. In more than 40% of the woodchuck tumors analyzed, viral insertion sites were clustered in a 3-kb region upstream of N-myc2 or in the 3' noncoding region. Insertion of WHV sequences homologous to the human hepatitis B virus enhancers, either upstream or downstream of the N-myc2 coding domain, was associated with the production of normal N-myc2 mRNA or hybrid N-myc2-WHV transcripts, initiated at the normal N-myc2 transcriptional start site. Transient-transfection assays with different N-myc2-WHV constructs in HepG2 cells demonstrated that the viral enhancers could efficiently activate the N-myc2 promoter. These results, showing that cis activation of preferred cellular targets through enhancer insertion is a common strategy for tumor induction by WHV, emphasize the previously noted similarities between hepadnaviruses and nonacute oncogenic retroviruses.