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

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.

Proportion of transcriptionally active DNA virus integrants: a meta-analysis

Oncoviruses are collectively responsible for over 1,000,000 new cases of cancer per year; some can integrate into the host's chromosomes. The present work was aimed at assessing the proportion of transcriptionally active viral integrants through a systematic review of the scientific publications present on the MedLine database. From the articles screened, 628 viral integrants overall were retrieved, of which 530.84 were transcriptionally active (84.53%); among the clinical samples, 264 of 323 integrants were active (81.73%). The causes for the silencing were not addressed in the articles analyzed. These findings might highlight a possible risk factor for the insurgence of cancer since some oncovirus integrants could be reactivated by stimuli of disparate nature. Further studies should address such possibility.

Association of TIP30 expression and prognosis of hepatocellular carcinoma in patients with HBV infection

Altered expression of TIP30, a tumor suppressor, has been observed in many cancers. In this study, we have evaluated the expression of TIP30 in the tissues of 209 hepatocellular carcinomas (HCC) and their adjacent tissues by using a high‐density tissue microarray, and analyzed its correlation with the clinical pathological parameters of the patients. The results revealed negative or weak expression of TIP30 in 43.5% (91/209) of the HCC tissues, and in only 27% (56/209) of the adjacent tissues. The expression level of TIP30 in HCC was inversely correlated with serum alpha‐fetoprotein (AFP) levels, HBV infection, and tumor differentiation. Multivariate analysis for survival indicated that serum HBV infection was the most significant predictor of poor prognosis in HCC (P = 0.0023), and TIP30 expression and tumor differentiation were also independent indicators in this respect (P = 0.0364 and P = 0.0397, respectively). Patients with medium or high expression levels of TIP30 (TIP30^++/+++) had a better 5‐year overall survival rate than those with low/negative (TIP30^+/−) expression (P < 0.001). TIP30^+/−/HBV⁺ patients had the worst 5‐year overall survival rate, whereas TIP30^++/+++/HBV⁻ patients had the best. To further explore the correlation between TIP30 and HBV infection in HCC, HBV⁺ hepatoblastoma cell‐line HepG2 2.2.15 and HCC cell‐line Hep3B were used. Upon silencing of HBV, we observed an upregulation of TIP30 and decreased cell proliferation. In the in vivo studies, we found that the mice inoculated with HepG2 2.2.15 cells with HBV silencing had a prolonged tumor latency and a longer life span, as compared to the control mice inoculated with untreated control cells. In conclusion, the results suggest that downregulation of TIP30 may result from HBV infection, and subsequently promotes the progression of HCC.

Role of hepatitis B virus DNA integration in human hepatocarcinogenesis

Liver cancer ranks sixth in cancer incidence, and is the third leading cause of cancer-related deaths worldwide. Hepatocellular carcinoma (HCC) is the most common type of liver cancer, which arises from hepatocytes and accounts for approximately 70%-85% of cases. Hepatitis B virus (HBV) frequently causes liver inflammation, hepatic damage and subsequent cirrhosis. Integrated viral DNA is found in 85%-90% of HBV-related HCCs. Its presence in tumors from non-cirrhotic livers of children or young adults further supports the role of viral DNA integration in hepatocarcinogenesis. Integration of subgenomic HBV DNA fragments into different locations within the host DNA is a significant feature of chronic HBV infection. Integration has two potential consequences: (1) the host genome becomes altered (“cis” effect); and (2) the HBV genome becomes altered (“trans” effect). The cis effect includes insertional mutagenesis, which can potentially disrupt host gene function or alter host gene regulation. Tumor progression is frequently associated with rearrangement and partial gain or loss of both viral and host sequences. However, the role of integrated HBV DNA in hepatocarcinogenesis remains controversial. Modern technology has provided a new paradigm to further our understanding of disease mechanisms. This review summarizes the role of HBV DNA integration in human carcinogenesis.

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Hepatitis B virus and hepatocellular carcinoma

Chronic hepatitis B virus (HBV) infection is a major global cause of hepatocellular carcinoma (HCC). Individuals who are chronic carriers have a greater than 100-fold increased relative risk of developing the tumour. Several mechanisms of HBV-induced HCC have been proposed. Integration of HBV DNA into the genome of hepatocytes occurs commonly, although integration at cellular sites that are important for regulation of hepatocyte proliferation appears to be a rare event. Functions of the HBx protein are also potentially oncogenic. These include transcriptional activation of cellular growth regulatory genes, modulation of apoptosis and inhibition of nucleotide excision repair of damaged cellular DNA. The effects of HBx are mediated by interaction with cellular proteins and activation of cell signalling pathways. Variations in HBV genome sequences may be important in hepatocarcinogenesis, although their significance has not yet been completely elucidated. Necroinflammatory hepatic disease, which often accompanies chronic HBV infection, may contribute indirectly to hepatocyte transformation in a number of ways, including by facilitating HBV DNA integration, predisposing to the acquisition of cellular mutations and generating mutagenic oxygen reactive species. Although HCC is a malignancy with a poor prognosis, the availability of an effective vaccine against HBV infection, and its inclusion in the Expanded Programme of Immunization of many countries, augurs well for the eventual elimination of HBV-associated HCC.

Hepatitis B virus and hepatocellular carcinoma

Chronic hepatitis B virus (HBV) infection is a major global cause of hepatocellular carcinoma (HCC). Individuals who are chronic carriers have a greater than 100-fold increased relative risk of developing the tumour. Several mechanisms of HBV-induced HCC have been proposed. Integration of HBV DNA into the genome of hepatocytes occurs commonly, although integration at cellular sites that are important for regulation of hepatocyte proliferation appears to be a rare event. Functions of the HBx protein are also potentially oncogenic. These include transcriptional activation of cellular growth regulatory genes, modulation of apoptosis and inhibition of nucleotide excision repair of damaged cellular DNA. The effects of HBx are mediated by interaction with cellular proteins and activation of cell signalling pathways. Variations in HBV genome sequences may be important in hepatocarcinogenesis, although their significance has not yet been completely elucidated. Necroinflammatory hepatic disease, which often accompanies chronic HBV infection, may contribute indirectly to hepatocyte transformation in a number of ways, including by facilitating HBV DNA integration, predisposing to the acquisition of cellular mutations and generating mutagenic oxygen reactive species. Although HCC is a malignancy with a poor prognosis, the availability of an effective vaccine against HBV infection, and its inclusion in the Expanded Programme of Immunization of many countries, augurs well for the eventual elimination of HBV-associated HCC.

Interaction of Hepatitis B virus with cellular processes in liver carcinogenesis

Hepatitis B infection is strongly linked epidemiologically to hepatocellular carcinoma development. This article reviews the molecular mechanisms by which hepatitis B encoded proteins such as hepatitis B x and hepatitis B surface transactivators may interact with gene transcription, tumor suppression, apoptosis, and signalling pathways of the liver cell with the possible consequence of tumor induction. Data on the interaction between hepatitis B proteins and cellular processes are often conflicting indicating a non-specific simultaneous interaction with antagonistic cellular processes that result in the formation of escape mutants that are not subject to these selective pressures.

Molecular bases for the development of hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC)

Hepatocellular carcinoma (HCC) is the most common histological form of primary liver cancer; the tumor cells having retained features of hepatocytic differentiation. It is important to emphasize the heterogeneity of the histological background on which the tumor develops. Most HCCs complicate the evolution of an active or inactive cirrhosis. However, some tumors occur on livers with minimal histological changes; the prevalence of such cases varies from one geographical region to the other; being much higher in the southern half of Africa (around 40% of HCCs) than in Asia, America and Europe, where at least 90% of HCCs are associated in the cirrhosis. This heterogeneity is probably a reflection of different environmental and genetic factors. A large number of epidemiological and molecular studies have indeed clearly demonstrated the prime importance of environmental factors to the development of primary liver cancers in humans. Chronic hepatitis B (HBV) and C (HCV) infections are major risk factors. This review will mainly analyse the impact of chronic HBV infection but it is important to emphasize the potential synergistic effects between HBV and HCV, as well as between viral infections and other environmental factors, such as alcohol, chemical carcinogens (see review by Dr Wogan) and other, still poorly defined, hormonal factors which may account for the higher incidence of the tumor in man. Finally the review by Dr Buendia highlights the emerging issue of liver-cancer genetics.

The regional integration of retroviral sequences into the mosaic genomes of mammals

We have reviewed here three sets of data concerning the integration of retroviral sequences in the mammalian genome: (i) our experimental localization of a number of proviruses integrated in isochores characterized by different GC levels; (ii) results from other laboratories on the localization of retroviral sequences in open chromatin regions and/or next to CpG islands; and (iii) our compositional analysis of genes located in the neighborhood of integrated retroviral sequences. The three sets of data have provided a very consistent picture in that a compartmentalized, isopycnic integration of expressed proviruses appears to be the rule ('isopycnic' refers to the compositional match between viral and host sequences around the integration site). The results reviewed here suggest that: (i) integration of proviral sequences is targeted initially towards 'open chromatin regions'; while these exist in both GC-rich and GC-poor isochores, the 'open chromatin regions' of GC-rich isochores are the main targets for integration of retroviral sequences because of their much greater abundance; (ii) isopycnicity is associated with stability of integration; indeed, even non-expressed integrated retroviral sequences tend to show an isopycnic localization in the genome; (iii) transcription of integrated viral sequences (like transcription of host genes) appears to be associated, as a rule, with an isopycnic localization, as indicated by transcribed sequences that show an isopycnic integration and act in trans; (iv) selection plays a role in the choice of specific sites within an isopycnic region; in exceptional cases [such as mouse mammary tumor virus (MMTV) activating GC-rich oncogenes], selection may override isopycnicity.

Chronic viral hepatitis induced by hepatitis C but not hepatitis B virus infection correlates with increased liver angiogenesis

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections lead to cirrhosis and increase the risk for the development of hepatocellular carcinoma (HCC). Angiogenesis is an essential step in oncogenesis and contributes to tumor progression in adult organs; however, to what extent angiogenesis occurs in the liver during chronic viral hepatitis has not been studied. Ninety-nine matched patients affected by chronic hepatitis due to either HBV or HCV were studied together with 13 controls (5 patients were affected by familial hyperbilirubinemia with normal liver histology; 6 patients with stage II primary biliary cirrhosis; and 2 patients with pseudo inflammatory tumor). Microvessel density was assessed in liver biopsies by immunostaining using two different antibodies against endothelial cell antigens, QB-END/10 and Factor VIII. In addition, the liver homogenates and sera of HCV- or HBV-positive patients and controls were tested for their capacity to stimulate the migration and proliferation of freshly isolated human endothelial cells in vitro. Evidence of angiogenesis was significantly more frequent in HCV-positive patients compared with HBV-infected subjects or controls (74% vs. 39% vs. 8%) (chi2 = 20.78; P < .0001) (HCV+ vs. HBV+ vs. controls). The degree of microvessel density was also higher in HCV- than in HBV-positive patients or controls (chi2 = 12.28; P < .005). In addition, HCV-positive sera and liver homogenates stimulated a higher migration and proliferation of human endothelial cells in vitro compared with HBV-positive or control sera and liver homogenates. These observations indicate that angiogenesis is particularly linked to HCV infection, suggesting a possible contribution to HCV-related liver oncogenesis.

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.

Hepatitis B virus transcriptional activators: mechanisms and possible role in oncogenesis

The hepatitis B virus (HBV) genome encodes a 154 amino acid protein termed X (HBx, hepatitis B x protein), which is a promiscuous transcriptional activator of polymerase II and III promoters. HBx upregulates a wide range of cellular and viral genes and is thought to facilitate viral pregenome and mRNA transcription; however, its precise role in the viral replication cycle remains to be elucidated. The functional mechanisms of HBx appear very complex. It was shown to activate transcription factors AP-1 and NF-kappa B vis cytoplasmic pathways including ras-MAP kinase. In contrast, nuclear HBx is thought to activate the transcriptional machinery directly. A second transcriptional activator protein (Mst, middle s transactivator) is encoded by 3'-truncated preS2/S sequences of integrated HBV DNA, but not by the intact viral gene. HBx and Mst may contribute to the pathogenicity of chronic hepatitis B and are suggested to promote hepatocyte transformation via upregulation of cellular proto-oncogenes. Further, HBx may enhance HBV related carcinogenesis by inactivation of the tumour suppressor gene product p53.