A chromosome 17:7 translocation is associated with a hepatitis B virus DNA integration in human hepatocellular carcinoma DNA

Effects of tenofovir disoproxil fumarate on intrahepatic viral burden and liver immune microenvironment in patients with chronic hepatitis B

BACKGROUND

The impact of nucleos(t)ide analogues on intrahepatic viral burden and immune microenvironment in patients with chronic hepatitis B (CHB) is not clear.

OBJECTIVE

We aimed to characterise the effects of tenofovir disoproxil fumarate (TDF) on intrahepatic viral burden and the liver immune microenvironment in patients with CHB.

DESIGN

Core liver biopsies were collected at baseline and year 3 from patients with CHB with minimally raised serum alanine aminotransferase in a double-blind placebo-controlled trial (NCT01522625). Paired biopsies were analysed by RNA-sequencing (n=119 pairs), a custom multiplex immunofluorescence assay (n=30 pairs), and HBV-targeted long-read DNA sequencing (n=49 pairs).

RESULTS

Both non-integrated and integrated HBV DNA were present in all patients at baseline, with >65% having interchromosomal translocations. Treatment significantly reduced the frequency of HBV core+ hepatocytes and intrahepatic (integrated and non-integrated) HBV DNA, but had no effect on HBsAg+ hepatocytes. Clonally expanded integrations were enriched for HBsAg coding regions and showed dysregulation of nearby genes. At baseline, there was significant enrichment of intrahepatic CD8+ T cell proximity to HBV core+ hepatocytes, but not to HBsAg+ cells. The densities of T cells and B cells were significantly reduced by TDF. Transcriptomic analyses found TDF induced widespread downregulation of immune-related genes including inhibitory and regulatory genes.

CONCLUSION

TDF significantly reduced intrahepatic integrated and non-integrated HBV DNA, exerting disparate effects on HBV core+ and HBsAg+ cells and on different immune cell subsets. Our data suggest there may be differential cytotoxic T cell-mediated killing of HBV core+ versus HBsAg+ hepatocytes, providing insights for HBV cure strategies.

The impact of integrated hepatitis B virus DNA on oncogenesis and antiviral therapy

The global burden of hepatitis B virus (HBV) infection remains high, with chronic hepatitis B (CHB) patients facing a significantly increased risk of developing cirrhosis and hepatocellular carcinoma (HCC). The ultimate objective of antiviral therapy is to achieve a sterilizing cure for HBV. This necessitates the elimination of intrahepatic covalently closed circular DNA (cccDNA) and the complete eradication of integrated HBV DNA. This review aims to summarize the oncogenetic role of HBV integration and the significance of clearing HBV integration in sterilizing cure. It specifically focuses on the molecular mechanisms through which HBV integration leads to HCC, including modulation of the expression of proto-oncogenes and tumor suppressor genes, induction of chromosomal instability, and expression of truncated mutant HBV proteins. The review also highlights the impact of antiviral therapy in reducing HBV integration and preventing HBV-related HCC. Additionally, the review offers insights into future objectives for the treatment of CHB. Current strategies for HBV DNA integration inhibition and elimination include mainly antiviral therapies, RNA interference and gene editing technologies. Overall, HBV integration deserves further investigation and can potentially serve as a biomarker for CHB and HBV-related HCC.

HBV integrations reshaping genomic structures promote hepatocellular carcinoma

OBJECTIVE

Hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC), mostly characterised by HBV integrations, is prevalent worldwide. Previous HBV studies mainly focused on a few hotspot integrations. However, the oncogenic role of the other HBV integrations remains unclear. This study aimed to elucidate HBV integration-induced tumourigenesis further.

DESIGN

Here, we illuminated the genomic structures encompassing HBV integrations in 124 HCCs across ages using whole genome sequencing and Nanopore long reads. We classified a repertoire of integration patterns featured by complex genomic rearrangement. We also conducted a clustered regularly interspaced short palindromic repeat (CRISPR)-based gain-of-function genetic screen in mouse hepatocytes. We individually activated each candidate gene in the mouse model to uncover HBV integration-mediated oncogenic aberration that elicits tumourigenesis in mice.

RESULTS

These HBV-mediated rearrangements are significantly enriched in a bridge-fusion-bridge pattern and interchromosomal translocations, and frequently led to a wide range of aberrations including driver copy number variations in chr 4q, 5p (TERT), 6q, 8p, 16q, 9p (CDKN2A/B), 17p (TP53) and 13q (RB1), and particularly, ultra-early amplifications in chr8q. Integrated HBV frequently contains complex structures correlated with the translocation distance. Paired breakpoints within each integration event usually exhibit different microhomology, likely mediated by different DNA repair mechanisms. HBV-mediated rearrangements significantly correlated with young age, higher HBV DNA level and TP53 mutations but were less prevalent in the patients subjected to prior antiviral therapies. Finally, we recapitulated the TONSL and TMEM65 amplification in chr8q led by HBV integration using CRISPR/Cas9 editing and demonstrated their tumourigenic potentials.

CONCLUSION

HBV integrations extensively reshape genomic structures and promote hepatocarcinogenesis (graphical abstract), which may occur early in a patient's life.

Aberrant integration of Hepatitis B virus DNA promotes major restructuring of human hepatocellular carcinoma genome architecture

Most cancers are characterized by the somatic acquisition of genomic rearrangements during tumour evolution that eventually drive the oncogenesis. Here, using multiplatform sequencing technologies, we identify and characterize a remarkable mutational mechanism in human hepatocellular carcinoma caused by Hepatitis B virus, by which DNA molecules from the virus are inserted into the tumour genome causing dramatic changes in its configuration, including non-homologous chromosomal fusions, dicentric chromosomes and megabase-size telomeric deletions. This aberrant mutational mechanism, present in at least 8% of all HCC tumours, can provide the driver rearrangements that a cancer clone requires to survive and grow, including loss of relevant tumour suppressor genes. Most of these events are clonal and occur early during liver cancer evolution. Real-time timing estimation reveals some HBV-mediated rearrangements occur as early as two decades before cancer diagnosis. Overall, these data underscore the importance of characterising liver cancer genomes for patterns of HBV integration.

Noninvasive chimeric DNA profiling identifies tumor-originated HBV integrants contributing to viral antigen expression in liver cancer

BACKGROUND

Host genome integration of HBV sequence is considered to be significant in HBV antigen expression and the development of hepatocellular carcinoma (HCC).

METHOD

We developed a probe-based capture strategy to enrich integrated HBV DNA for deep-sequencing analysis of integration sites in paired patient samples derived from tumor, liver tissue adjacent to tumor, saliva and plasma, as a platform for exploring the correlation, significance and utility of detecting integrations in these sample types.

RESULTS

Most significantly, alpha fetoprotein levels significantly correlated to the amounts of integrations detected in tumor. Viral-host chimeric DNA fragments were successfully detected at high sequencing coverage in plasma rather than saliva samples from HCC patients, and each fragment of this type was only seen once in plasma from chronic hepatitis B patients. Almost all plasma chimeric fragments were derived from integrations in tumor rather than in adjacent liver tissues. Over 50% of them may produce viral-host chimeric transcripts according to deep RNA sequencing in paired tissue samples. Particularly, in patients with low HBV DNA level (< 250 UI/ml), the seemingly normal HBsAg titers may be explained by larger amounts of integrations detected. Meanwhile, we developed a strategy to predict integrants by pairing breakpoints for each integration event. Among four resolved viral patterns, the majority of Pattern I events (81.2%) retained the complete opening reading frame for HBV surface proteins.

CONCLUSION

We achieve the efficient enrichment of plasma cell-free chimeric DNA from integration site, and demonstrate that chimeric DNA profiling in plasma is a promising noninvasive approach to monitor HBV integration in liver cancer development and to determine the ability of integrated sequences to express viral proteins that can be targeted, e.g. by immunotherapies.

Cyclin E facilitates dysplastic hepatocytes to bypass G1/S checkpoint in hepatocarcinogenesis

BACKGROUND AND AIM

By array-comparative genomic hybridization, we demonstrated cyclin E as one of seven genes associated with hepatocellular carcinoma (HCC) development in Ku70 DNA repair-deficient mice. We therefore explored the hypothesis that during hepatocarcinogenesis, cyclin E kinase can overcome the inhibitory effects of p53 and establish whether abnormal miRNA(mi-R)-34, a co-regulator of cyclin E and p53, can account for their interactions as "drivers" of HCC.

METHODS

Dysplastic hepatocytes (DNs) and HCCs were generated from diethylnitrosamine (DEN)-injected C57BL/6 male mice at 3-12 months.

RESULTS

Cyclin E/cdk2 was barely expressed in normal liver, but was readily detected in dysplastic hepatocytes, localizing to glutathione-S transferase pi-form positive cells dissected by laser-dissection. Cyclin E kinase activity preceded cyclin D1, proliferating cell nuclear antigen expression in DNs and HCCs despite maximal p53 and p21 expression. We confirmed that cyclin E, rather than cyclin D1, is the proliferative driver in hepatocarcinogenesis by immunoprecipitation experiments demonstrating preferential binding of p21 to cyclin D1, allowing cyclin E-mediated "escape" from G1/S checkpoint. We then showed cyclin E was responsible for regulating wild-type p53 by knockdown experiments in primary HCC cells; cyclin E-knockdown increased p53 and p21, diminished anti-apoptotic Bcl-XL and reduced cell viability. Conversely, blocking p53 augmented cyclin E, Bcl-XL expression and increased proliferation. Physiological interactions between cyclin E/p53/p21 were confirmed in primary hepatocytes. miR-34a,c were upregulated in dysplastic murine, human liver and HCCs compared with normal liver, and appeared to be linked to cyclin E/p53.

CONCLUSION

Upregulation of functionally active cyclin E via miR34 with loss of p53 function is associated with cell-cycle checkpoint failure increasing proliferative drive that favors hepatocarcinogenesis.

Hepatocellular carcinoma (HCC) recurring 10 years after clearance of hepatitis B surface antigen and 20 years after resection of hepatitis B virus-related HCC

A 62-year-old man had been followed up for chronic hepatitis B (HB) since 1973. Hepatocellular carcinoma (HCC) was detected in 1985, at the age of 42 years. Serum HB surface antigen and anti-HB envelope antibody were positive at that time. A right hepatic lobectomy was performed. In 1995, serum HB surface antigen had cleared spontaneously and liver function had normalized. In March 2005, at the age of 62 years, a 1.5-cm diameter hepatic mass was detected in the left lateral segment. At that time, he was seropositive only for anti-HB core antibody. A diagnosis of recurrent HCC was made, and partial hepatectomy was performed. Covalently closed circular HBV DNA was detected in both cancerous and noncancerous tissues by nested polymerase chain reaction (PCR). Cassette-ligation-mediated PCR showed that HBV DNA was integrated into the telomerase reverse transcriptase gene located on chromosome 5p15.

Ground glass hepatocytes contain pre-S mutants and represent preneoplastic lesions in chronic hepatitis B virus infection

The discovery of "ground glass" hepatocytes (GGH) that contain hepatitis B virus (HBV) surface antigens by Hadziyannis and Popper in 1973 represents a historical landmark in the pathology of chronic HBV infection. Different types of GGH have been correlated to the expression patterns of surface/core antigens and the stages of virus replication. The original two types (designated types I & II) of GGH were found to contain specific pre-S mutants with deletions over either pre-S1 or pre-S2 regions, respectively. Type II GGH consistently harbor pre-S2 deletion mutants, which can escape from immune attack and grow preferentially to form clusters. Both types of pre-S mutants can induce endoplasmic reticulum (ER) stress and oxidative DNA damage. The pre-S2 mutants, albeit inducing a weaker level of ER stress signals, could additionally initiate ER stress-independent retinoblastoma/adenovirus E2 promoter binding factor/cyclin A signaling through their interaction with c-Jun activation domain binding protein 1 to degrade p27, illustrating the growth advantage of type II GGH. The combined effects of genomic instability and the proliferation of hepatocytes harboring pre-S mutants could potentially lead to hepatocarcinogenesis over the decades of chronic HBV infection. The presence of pre-S mutants in sera was reported to carry a high risk of developing hepatocellular carcinoma (HCC). Furthermore, transgenic mice harboring pre-S2 mutant plasmids have been shown to develop a dysplastic change of hepatocytes and HCC. Therefore, in addition to being a histological marker of chronic HBV infection, GGH, particularly type II GGH, may represent the preneoplastic lesions of HBV-related HCC.

Comparison of surgical outcomes for small hepatocellular carcinoma in patients with hepatitis B versus hepatitis C: a Chinese experience

BACKGROUND

Although both hepatitis B virus (HBV) and hepatitis C virus (HCV) infection are well recognized risk factors for hepatocellular carcinoma (HCC), little is known with respect to how HBV and HCV infection affect HCC recurrence in postoperative HCC Chinese patients. The objective of this study was to determine if differences exist in preoperative characteristics and postoperative HCC recurrence in patients with different HBV and HCV infection status.

METHODS

The study population consisted of 413 patients undergoing a curative resection at Tianjin Cancer Hospital for small HCC (< or =3 cm) from January 1997 to December 2003. The patients were divided into four groups: HCV only (n = 75), HBV only (n = 251), HBV and HCV (n = 33), and neither HBV nor HCV (NBNC, n = 54). The preoperative status and postoperative HCC recurrence were recorded. Survival analyses were used to assess the impact of HBV/HCV status on HCC recurrence.

RESULTS

Patients with HCV had a significant association with older age, lower mean preoperative platelet counts and albumin levels, higher mean prothrombin time, alanine aminotransferase and total bilirubin levels and multinodular tumors during diagnosis. Patients with HCV also had significantly less differentiated tumors and a higher incidence of vascular invasion and cirrhosis when compared to the other groups. During the follow-up, the HCV group showed a higher incidence of intrahepatic recurrence and multiple recurrent lesions than the other patients.

CONCLUSIONS

Patients with HCV infection tended to be older, and were characterized by more severe cirrhosis and higher incidence of tumor multicentricity. The statistically significant determinants for reoccurrence in patients with small HCC were HCV infection, presence of vascular invasion and multiple tumors.

Hepatitis B virus infection, the immune response and hepatocellular carcinoma

More than 250 million people worldwide are chronically infected with the hepatitis B virus (HBV) and have a 200-fold increased risk of developing hepatocellular carcinoma (HCC). This is one of the most common cancers in the world with a geographical distribution highest in areas where HBV is endemic. A number of molecular mechanisms have been proposed to explain this correlation including an acutely transforming viral oncogene; chromosomal aberrations due to HBV integration (i.e. deletions, translocations, duplications); activation of cellular proto-oncogenes; inactivation of cellular anti-oncogenes; and transactivation of cellular genes by HBV gene products. HCC usually develops only after 20-30 years of persistent HBV infection accompanied by hepatocyte necrosis, inflammation and regenerative hyperplasia. Because HBV is not directly cytopathic, liver injury must be immune mediated. Factors that predispose HBV-infected individuals to develop HCC are chronicity, an immune response and liver injury rather than a direct genetic event. Hepatic injury and continuous hepatocyte regeneration may allow an accumulation of multiple mutational events sufficient for the emergence of HCC. Pathways which lead to chronicity, the immune response during HBV infection, mechanisms of pathogenesis and methods to prevent HBV infection are all relevant to the development of HCC. Recent studies characterizing the humoral and cellular immune responses in patients chronically infected with HBV and transgenic mouse models of HBV-specific immune tolerance and pathogenesis are providing new insights into the complex association between HBV infection and HCC.

Comparison of recurrence after hepatic resection in patients with hepatitis B vs. hepatitis C-related small hepatocellular carcinoma in hepatitis B virus endemic area

PURPOSE

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infection are two important factors in the development of hepatocellular carcinoma (HCC). The carcinogenic mechanism of HBV and HCV is considered to be different. It is interesting to compare the recurrence after hepatic resection in patients with small HCC who were infected with HBV or HCV.

METHODS

From 1991 to 1995, 145 patients who were positive for hepatitis B surface antigen (HBsAg) or antibody to HCV (anti-HCV) and diagnosed as small HCC (< or =3 cm) in three medical centers in Taiwan were evaluated in this study. All patients underwent hepatic resection. Among them, 83 (57.2%) were infected by HBV, 51 (35.2%) were infected by HCV, and 11 (7.6%) had dual HBV and HCV infection.

RESULTS

Anti-HCV+ HCCs were associated with older age, lower serum albumin, higher alanine transaminase (ALT) level and multi-nodular tumors during diagnosis. During the follow-up, 92 (63.4%) patients developed tumor recurrence. Anti-HCV + HCC had a higher cumulated recurrence rate than HBsAg+ HCC (72.4% vs 53.6 % at 5 year, P = 0.032). In multivariate analysis, the presence of vascular invasion and lower serum albumin levels (<3.9 g/dl) were the determinants for tumor recurrence.

CONCLUSIONS

HCV infection, as compared with HBV infection, had a higher cumulated recurrence after hepatic resection in patients with small HCC. Low serum albumin level was significantly associated with recurrence among these patients.

Hepatitis B virus DNA in liver, serum, and peripheral blood mononuclear cells after the clearance of serum hepatitis B virus surface antigen

The integration of hepatitis B virus (HBV) DNA in the liver of chronic HBV carriers has been documented extensively. However, the status of the viral genome during acute infection has not been assessed conclusively. While HBV DNA sequences are detected often in serum, liver, and peripheral blood mononuclear cells (PBMCs) after the clearance of serum the hepatitis B virus surface antigen (HBsAg), the precise status of the viral genome, and in particular the possible persistence of integrated genomes in PBMCs, has not been established. A highly sensitive PCR-derived assay (Alu-PCR) was employed to re-examine liver and PBMC specimens obtained from patients with acute (n = 19) and chronic (n = 22) hepatitis in whom serum HBsAg was present (n = 12) (HBV-related chronic active hepatitis) or absent with anti-HCV (n = 10) (HCV-related chronic active hepatitis). Viral integration was demonstrated in 3 out of 19 liver specimens from patients with acute hepatitis and 12 out of 12 specimens from patients with chronic hepatitis. Viral integration was also observed in 4 out of 7 PBMC samples from HBV-related chronic active hepatitis patients and 2 out of 10 liver and PBMC samples from HCV-related chronic active hepatitis patients. In one liver specimen from an acute hepatitis patient, HBV DNA was found integrated in the intronic sequence of the tumour necrosis factor (TNF)-induced protein gene; viral integration into cellular sequences was also found in the PBMCs of four HBV-related chronic active hepatitis and two HCV-related chronic active hepatitis. The results demonstrate the early integration of HBV genome during acute viral infections and the persistence of the viral genome in an integrated form in PBMCs.

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TP53 and liver carcinogenesis

Primary hepatocellular carcinoma (HCC) is one of the most common malignancies and has the fourth highest mortality rate worldwide. The major risk factors, including chronic infections with the hepatitis B or C virus, are exposure to dietary aflatoxin B1(AFB1), vinyl chloride, or alcohol consumption. Southern China and sub-Saharan Africa have the highest dietary AFB1 exposure, making it and hepatitis B virus (HBV) the major causes of cancer mortality in these geographic areas. Recent studies have discovered genetic and epigenetic changes involved in the molecular pathogenesis of HCC, including somatic mutations in the p53 tumor suppressor gene (TP53). AFB1 induces typical G:C to T:A transversions at the third base in codon 249 of p53. Chronic active hepatitis B and C (HCV) infection, and further inflammatory and oxyradical disorders including Wilson disease (WD) or hemochromatosis, generate reactive oxygen/nitrogen species that can damage DNA and mutate the p53 gene. The X gene of HBV (HBx) is the most common open reading frame integrated into the host genome in HCC. The integrated HBx is frequently mutated and has a diminished ability to function as a transcriptional cotransactivator and to activate the NF-kappa B pathway. However, the mutant HBx proteins still retain their ability to bind to and abrogate p53-mediated apoptosis. In summary, both viruses and chemicals are implicated in the etiology and molecular pathogenesis of HCC. The resultant molecular changes in the ras and Wnt signal-transduction pathways, and the p53 and Rb tumor suppressor pathways significantly contribute to liver carcinogenesis

Characterization of integration patterns and flanking cellular sequences of hepatitis B virus in childhood hepatocellular carcinomas

Hepatitis B virus (HBV) DNA integration into host chromosomes is detected in more than 80% of HBV-related hepatocellular carcinomas (HCC), yet its significance in tumor development remains obscure. In this study, we re-examined the integration pattern of HBV in childhood HCC tissues, which has less environmental confounding factors than adult HCC. The HBV junctions and flanking cellular sequences were amplified from five childhood HCC patients by the inverse polymerase chain reaction (IPCR) method using primers located near HBV direct repeats (DR) 1 and 2. The viral junctions in nine of the ten obtained IPCR clones were demonstrated to be located near HBV DR1, and their patterns were classified to type I integrants. Southern blot analyses demonstrate that the cellular junctions derived from two of the five HCC tissues were male specific and contained sequences homologous to human long interspersed DNA elements (LINE-1). HBV integrant of one HCC tissue (1217T) was integrated into a RNA binding motif Y chromosome (RBMY) gene. The expression of RBMY, which is normally found only in male germ cells, was detected in HCC tissue 1217T by RT-PCR but not in the corresponding non-tumor liver tissue. The prevalence of RBMY expression in liver tissues from the tumor and non-tumor parts of ten other HCC children and seven biliary atresia (BA) children was studied by RT-PCR. No RBMY transcripts were detected in the non-tumor parts of HCC patients or the cirrhotic livers of BA children, whereas 30% (three of ten) of HCC tissues specifically expressed RBMY. The results indicate that HBV integration and activation of RBMY gene expression in liver cells may be associated with the development of childhood HCC.

Liver cancer

The prognosis of patients with HCC remains dismal. Even in the subgroups of patients who have the most favorable characteristics and are eligible for surgical resection, the 5-year survival rate is less than 25%. For patients with more advanced disease, the median survival time is less than 1 year. The good news in HCC research is that the disease can be prevented. In Taiwan, the rate of HCC in children aged 6 to 9 years decreased from 5.2 per million population before the neonatal vaccination program began in 1984 to 1.3 per million population in the first vaccinated cohort. Treatment of viral hepatitis with IFN may decrease the rates of long-term development of HCC. Other agents that may prevent second primary tumors following resection of HCC, such as polyprenoic acid and acylic retinoid, are also being investigated.

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 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.

A t(3;8) chromosomal translocation associated with hepatitis B virus intergration involves the carboxypeptidase N locus

Integrated hepatitis B virus (HBV) DNA is found in the great majority of human hepatocellular carcinomas, suggesting that these viral integrations may be implicated in liver oncogenesis. Besides the insertional mutagenesis characterized in a few selected cases and the contribution of viral transactivators to cell transformation to malignancy, HBV has been shown to generate gross chromosomal rearrangements potentially involved in carcinogenesis. Here, we report a t(3;8) chromosomal translocation present in a hepatocellular carcinoma developed in noncirrhotic liver tissue. One side of the translocation, in 8p23, is shown to be in the vicinity of the carboxypeptidase N gene, a locus that is heavily transcribed in liver tissue and frequently deleted in hepatocellular carcinomas and other epithelial tumors. The other side of the translocation, in 3q27-29, is widely implicated in several types of translocations occurring in different malignancies, such as large-cell lymphomas. The present data strongly support a model in which HBV-induced chromosomal rearrangements play a key role during multistep 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.

The molecular pathogenesis of hepatocellular carcinoma

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

Analysis of integrated ground squirrel hepatitis virus and flanking host DNA in two hepatocellular carcinomas

We cloned the integrated ground squirrel hepatitis B virus (GSHV) sequences from two hepatomas showing a single viral insertion. The GSHV inserts shared structural features with integrated DNAs of other hepadnaviruses. Insertional activation of a cellular gene appears unlikely: the integrated GSHV sequences lacked the known viral enhancers and were not expressed in the tumors, and we found no evidence for the presence of a gene at the integration site. Our results, together with those earlier studies, suggest that GSHV does not behave as an extensive insertional mutagen, in sharp contrast with the closely related woodchuck hepatitis virus. GSHV may thus cause carcinogenesis by more indirect mechanisms, as does the human hepatitis B virus.

Point mutations of the P53 gene, human hepatocellular carcinoma and aflatoxins

The tumor suppressor p53 exerts important protective functions towards DNA-damaging agents. Its inactivation by allelic deletions or point mutations within the P53 gene as well as complex formation of wildtype p53 with cellular or viral proteins is a common and crucial event in carcinogenesis. Mutations increase the half-life of the p53 protein allowing the immunohistochemical detection and anti-p53 antibody formation. Distinct G to T point mutations in codon 249 leading to a substitution of the basic amino acid arginine by the neutral amino acid serine are responsible for the altered functionality of the mutant gene product and were originally identified in 8 of 16 Chinese and 5 of 10 African HCC patients. Both groups are frequently exposed to mycotoxin contaminations of their food. Today an average P53 gene mutation rate of 25% is assumed for high-aflatoxin B1-exposure regions. This is double the rate observed in low-aflatoxin B1-exposure countries. Although many HCC patients displaying P53 mutations also suffer from HBV infection, which itself can lead to rearrangements of P53 coding regions or induce the synthesis of viral proteins possibly interacting with p53, the specific G to T transversion within codon 249 of the P53 gene seems to directly reflect the extent of aflatoxin B1 exposure.