Differential activation of myc gene family members in hepatic carcinogenesis by closely related hepatitis B viruses

Expression of p-STAT3 and c-Myc correlates with P2-HNF4α expression in nonalcoholic fatty liver disease (NAFLD)

We studied the expression of two hepatocyte nuclear factor 4 alpha (HNF4α) isoforms, p-STAT3. and c-Myc in 49 consecutive liver biopsies with nonalcoholic fatty liver disease (NAFLD) using immunohistochemistry. All 49 biopsies (100%) were positive for nuclear expression of P1-HNF4α. Twenty-eight (57%) cases were positive for P2-HNF4α, 6 (12%) were positive for p-STAT3 and 5 (10%) were positive for c-Myc. All 6 (100%) p-STAT3-positive cases were also positive for P2-HNF4α (p = 0.03). p-STAT3-positive cases were more likely to be positive for c-Myc (67% vs. 2%, p = 0.0003). Four cases were positive for P2-HNF4α, p-STAT3 and c-Myc. p-STAT3 expression was associated with hypertension (p = 0.037). All c-Myc positive biopsies were from patients with obesity, diabetes and hypertension. Only c-Myc expression was associated with advanced fibrosis; three (60%) of the c-Myc positive cases were associated with advanced fibrosis in contrast to 7 (10%) of the 44 c-Myc negative cases (p = 0.011). Based on these results, we hypothesize with the following sequence of events with progression of NAFLD: P2-HNF4α expression is followed by expression of p-STAT3 which in turn is followed by the expression of c-Myc. Additional larger studies are needed to confirm these findings.

Hepatitis B Virus DNA Integration and Clonal Expansion of Hepatocytes in the Chronically Infected Liver

Human hepatitis B virus (HBV) can cause chronic, lifelong infection of the liver that may lead to persistent or episodic immune-mediated inflammation against virus-infected hepatocytes. This immune response results in elevated rates of killing of virus-infected hepatocytes, which may extend over many years or decades, lead to fibrosis and cirrhosis, and play a role in the high incidence of hepatocellular carcinoma (HCC) in HBV carriers. Immune-mediated inflammation appears to cause oxidative DNA damage to hepatocytes, which may also play a major role in hepatocarcinogenesis. An additional DNA damaging feature of chronic infections is random integration of HBV DNA into the chromosomal DNA of hepatocytes. While HBV DNA integration does not have a role in virus replication it may alter gene expression of the host cell. Indeed, most HCCs that arise in HBV carriers contain integrated HBV DNA and, in many, the integrant appears to have played a role in hepatocarcinogenesis. Clonal expansion of hepatocytes, which is a natural feature of liver biology, occurs because the hepatocyte population is self-renewing and therefore loses complexity due to random hepatocyte death and replacement by proliferation of surviving hepatocytes. This process may also represent a risk factor for the development of HCC. Interestingly, during chronic HBV infection, hepatocyte clones detected using integrated HBV DNA as lineage-specific markers, emerge that are larger than those expected to occur by random death and proliferation of hepatocytes. The emergence of these larger hepatocyte clones may reflect a survival advantage that could be explained by an ability to avoid the host immune response. While most of these larger hepatocyte clones are probably not preneoplastic, some may have already acquired preneoplastic changes. Thus, chronic inflammation in the HBV-infected liver may be responsible, at least in part, for both initiation of HCC via oxidative DNA damage and promotion of HCC via stimulation of hepatocyte proliferation through immune-mediated killing and compensatory division.

Host functions used by hepatitis B virus to complete its life cycle: Implications for developing host-targeting agents to treat chronic hepatitis B

Similar to other mammalian viruses, the life cycle of hepatitis B virus (HBV) is heavily dependent upon and regulated by cellular (host) functions. These cellular functions can be generally placed in to two categories: (a) intrinsic host restriction factors and innate defenses, which must be evaded or repressed by the virus; and (b) gene products that provide functions necessary for the virus to complete its life cycle. Some of these functions may apply to all viruses, but some may be specific to HBV. In certain cases, the virus may depend upon the host function much more than does the host itself. Knowing which host functions regulate the different steps of a virus' life cycle, can lead to new antiviral targets and help in developing novel treatment strategies, in addition to improving a fundamental understanding of viral pathogenesis. Therefore, in this review we will discuss known host factors which influence key steps of HBV life cycle, and further elucidate therapeutic interventions targeting host-HBV interactions.

Characterization of metastatic tumor antigen 1 and its interaction with hepatitis B virus X protein in NF-κB signaling and tumor progression in a woodchuck hepatocellular carcinoma model

The metastatic tumor antigen 1 (MTA1) protein is associated with tumor invasiveness and poor prognosis in patients with hepatocellular carcinoma (HCC), particularly in those with hepatitis B virus (HBV)-related HCC. Chronically woodchuck hepatitis virus (WHV)-infected woodchuck is an ideal animal model for studying the pathogenesis of HBV-associated liver diseases, including HCC. To investigate the roles of MTA1 in HBV-associated hepatocarcinogenesis in the woodchuck model, we cloned the woodchuck MTA1 (wk-MTA1) complementary (c)DNA and characterized its molecular functions. The sequence and organization of the wk-MTA1 protein were highly conserved among different species. Similar to its expression in human HCC, wk-MTA1 was upregulated in woodchuck HCC, as determined at RNA and protein levels. Furthermore, an MTA1-spliced variant, wk-MTA1dE4, was overexpressed in woodchuck HCC, and it was attributed to approximately 50% of the total transcripts. The percentage of wk-MTA1dE4-overexpressed woodchuck HCCs was higher than that of the total wk-MTA1-overexpressed HCCs (77.8% vs 61.1%) and wk-MTA1dE4 may represent a more sensitive marker than the total wk-MTA1 in woodchuck HCC. We overexpressed or knocked down wk-MTA1 in a woodchuck HCC cell line and demonstrated that wk-MTA1 could interact with the WHV X protein (WHx) and play indispensable roles in WHx-mediated NF-κB activation and tumor cell migration- and invasion-promoting activities. In conclusion, our results support the hypothesis that woodchuck HCC recapitulates HBV-associated HCC with respect to the molecular characteristics of MTA1 and provides new clues for conducting mechanistic studies of MTA1 in HBV-associated hepatocarcinogenesis, including the possible clinical significance of wk-MTA1dE4.

Mutant p53 Gains Its Function via c-Myc Activation upon CDK4 Phosphorylation at Serine 249 and Consequent PIN1 Binding

TP53 missense mutations significantly influence the development and progression of various human cancers via their gain of new functions (GOF) through different mechanisms. Here we report a unique mechanism underlying the GOF of p53-R249S (p53-RS), a p53 mutant frequently detected in human hepatocellular carcinoma (HCC) that is highly related to hepatitis B infection and aflatoxin B1. A CDK inhibitor blocks p53-RS's nuclear translocation in HCC, whereas CDK4 interacts with p53-RS in the G1/S phase of the cells, phosphorylates it, and enhances its nuclear localization. This is coupled with binding of a peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (PIN1) to p53-RS, but not the p53 form with mutations of four serines/threonines previously shown to be crucial for PIN1 binding. As a result, p53-RS interacts with c-Myc and enhances c-Myc-dependent rDNA transcription key for ribosomal biogenesis. These results unveil a CDK4-PIN1-p53-RS-c-Myc pathway as a novel mechanism for the GOF of p53-RS in HCC.

SRC-2-mediated coactivation of anti-tumorigenic target genes suppresses MYC-induced liver cancer

Hepatocellular carcinoma (HCC) is the fifth most common solid tumor in the world and the third leading cause of cancer-associated deaths. A Sleeping Beauty-mediated transposon mutagenesis screen previously identified mutations that cooperate with MYC to accelerate liver tumorigenesis. This revealed a tumor suppressor role for Steroid Receptor Coactivator 2/Nuclear Receptor Coactivator 2 (Src-2/Ncoa2) in liver cancer. In contrast, SRC-2 promotes survival and metastasis in prostate cancer cells, suggesting a tissue-specific and context-dependent role for SRC-2 in tumorigenesis. To determine if genetic loss of SRC-2 is sufficient to accelerate MYC-mediated liver tumorigenesis, we bred Src-2-/- mice with a MYC-induced liver tumor model and observed a significant increase in liver tumor burden. RNA sequencing of liver tumors and in vivo chromatin immunoprecipitation assays revealed a set of direct target genes that are bound by SRC-2 and exhibit downregulated expression in Src-2-/- liver tumors. We demonstrate that activation of SHP (Small Heterodimer Partner), DKK4 (Dickkopf-4), and CADM4 (Cell Adhesion Molecule 4) by SRC-2 suppresses tumorigenesis in vitro and in vivo. These studies suggest that SRC-2 may exhibit oncogenic or tumor suppressor activity depending on the target genes and nuclear receptors that are expressed in distinct tissues and illuminate the mechanisms of tumor suppression by SRC-2 in liver.

Distinct Viral Lineages from Fish and Amphibians Reveal the Complex Evolutionary History of Hepadnaviruses

Hepadnaviruses (hepatitis B viruses [HBVs]) are the only animal viruses that replicate their DNA by reverse transcription of an RNA intermediate. Until recently, the known host range of hepadnaviruses was limited to mammals and birds. We obtained and analyzed the first amphibian HBV genome, as well as several prototype fish HBVs, which allow the first comprehensive comparative genomic analysis of hepadnaviruses from four classes of vertebrates. Bluegill hepadnavirus (BGHBV) was characterized from in-house viral metagenomic sequencing. The African cichlid hepadnavirus (ACHBV) and the Tibetan frog hepadnavirus (TFHBV) were discovered using in silico analyses of the whole-genome shotgun and transcriptome shotgun assembly databases. Residues in the hydrophobic base of the capsid (core) proteins, designated motifs I, II, and III, are highly conserved, suggesting that structural constraints for proper capsid folding are key to capsid protein evolution. Surface proteins in all vertebrate HBVs contain similar predicted membrane topologies, characterized by three transmembrane domains. Most striking was the fact that BGHBV, ACHBV, and the previously described white sucker hepadnavirus did not form a fish-specific monophyletic group in the phylogenetic analysis of all three hepadnaviral genes. Notably, BGHBV was more closely related to the mammalian hepadnaviruses, indicating that cross-species transmission events have played a major role in viral evolution. Evidence of cross-species transmission was also observed with TFHBV. Hence, these data indicate that the evolutionary history of the hepadnaviruses is more complex than previously realized and combines both virus-host codivergence over millions of years and host species jumping.

IMPORTANCE

Hepadnaviruses are responsible for significant disease in humans (hepatitis B virus) and have been reported from a diverse range of vertebrates as both exogenous and endogenous viruses. We report the full-length genome of a novel hepadnavirus from a fish and the first hepadnavirus genome from an amphibian. The novel fish hepadnavirus, sampled from bluegills, was more closely related to mammalian hepadnaviruses than to other fish viruses. This phylogenetic pattern reveals that, although hepadnaviruses have likely been associated with vertebrates for hundreds of millions of years, they have also been characterized by species jumping across wide phylogenetic distances.

Asymptomatic Hepadnaviral Persistence and Its Consequences in the Woodchuck Model of Occult Hepatitis B Virus Infection

Woodchuck hepatitis virus (WHV) is molecularly and pathogenically closely related to hepatitis B virus (HBV). Both viruses display tropism towards hepatocytes and cells of the immune system and cause similar liver pathology, where acute hepatitis can progress to chronic hepatitis and to hepatocellular carcinoma (HCC). Two forms of occult hepadnaviral persistence were identified in the woodchuck-WHV model: secondary occult infection (SOI) and primary occult infection (POI). SOI occurs after resolution of a serologically apparent infection with hepatitis or after subclinical serologically evident virus exposure. POI is caused by small amounts of virus and progresses without serological infection markers, but the virus genome and its replication are detectable in the immune system and with time in the liver. SOI can be accompanied by minimal hepatitis, while the hallmark of POI is normal liver morphology. Nonetheless, HCC develops in about 20% of animals with SOI or POI within 3 to 5 years. The virus persists throughout the lifespan in both SOI and POI at serum levels rarely greater than 100 copies/mL, causes hepatitis and HCC when concentrated and administered to virus-naïve woodchucks. SOI is accompanied by virus-specific T and B cell immune responses, while only virus-specific T cells are detected in POI. SOI coincides with protection against reinfection, while POI does not and hepatitis develops after challenge with liver pathogenic doses >1000 virions. Both SOI and POI are associated with virus DNA integration into the liver and the immune system genomes. Overall, SOI and POI are two distinct forms of silent hepadnaviral persistence that share common characteristics. Here, we review findings from the woodchuck model and discuss the relevant observations made in human occult HBV infection (OBI).

Animal models and the molecular biology of hepadnavirus infection

Australian antigen, the envelope protein of hepatitis B virus (HBV), was discovered in 1967 as a prevalent serum antigen in hepatitis B patients. Early electron microscopy (EM) studies showed that this antigen was present in 22-nm particles in patient sera, which were believed to be incomplete virus. Complete virus, much less abundant than the 22-nm particles, was finally visualized in 1970. HBV was soon found to infect chimpanzees, gorillas, orangutans, gibbon apes, and, more recently, tree shrews (Tupaia belangeri) and cynomolgus macaques (Macaca fascicularis). This restricted host range placed limits on the kinds of studies that might be performed to better understand the biology and molecular biology of HBV and to develop antiviral therapies to treat chronic infections. About 10 years after the discovery of HBV, this problem was bypassed with the discovery of viruses related to HBV in woodchucks, ground squirrels, and ducks. Although unlikely animal models, their use revealed the key steps in hepadnavirus replication and in the host response to infection, including the fact that the viral nuclear episome is the ultimate target for immune clearance of transient infections and antiviral therapy of chronic infections. Studies with these and other animal models have also suggested interesting clues into the link between chronic HBV infection and hepatocellular carcinoma.

Early mesozoic coexistence of amniotes and hepadnaviridae

Hepadnaviridae are double-stranded DNA viruses that infect some species of birds and mammals. This includes humans, where hepatitis B viruses (HBVs) are prevalent pathogens in considerable parts of the global population. Recently, endogenized sequences of HBVs (eHBVs) have been discovered in bird genomes where they constitute direct evidence for the coexistence of these viruses and their hosts from the late Mesozoic until present. Nevertheless, virtually nothing is known about the ancient host range of this virus family in other animals. Here we report the first eHBVs from crocodilian, snake, and turtle genomes, including a turtle eHBV that endogenized >207 million years ago. This genomic “fossil” is >125 million years older than the oldest avian eHBV and provides the first direct evidence that Hepadnaviridae already existed during the Early Mesozoic. This implies that the Mesozoic fossil record of HBV infection spans three of the five major groups of land vertebrates, namely birds, crocodilians, and turtles. We show that the deep phylogenetic relationships of HBVs are largely congruent with the deep phylogeny of their amniote hosts, which suggests an ancient amniote–HBV coexistence and codivergence, at least since the Early Mesozoic. Notably, the organization of overlapping genes as well as the structure of elements involved in viral replication has remained highly conserved among HBVs along that time span, except for the presence of the X gene. We provide multiple lines of evidence that the tumor-promoting X protein of mammalian HBVs lacks a homolog in all other hepadnaviruses and propose a novel scenario for the emergence of X via segmental duplication and overprinting of pre-existing reading frames in the ancestor of mammalian HBVs. Our study reveals an unforeseen host range of prehistoric HBVs and provides novel insights into the genome evolution of hepadnaviruses throughout their long-lasting association with amniote hosts.

Viruses are not known to leave physical fossil traces, which makes our understanding of their evolutionary prehistory crucially dependent on the detection of endogenous viruses. Ancient endogenous viruses, also known as paleoviruses, are relics of viral genomes or fragments thereof that once infiltrated their host's germline and then remained as molecular “fossils” within the host genome. The massive genome sequencing of recent years has unearthed vast numbers of paleoviruses from various animal genomes, including the first endogenous hepatitis B viruses (eHBVs) in bird genomes. We screened genomes of land vertebrates (amniotes) for the presence of paleoviruses and identified ancient eHBVs in the recently sequenced genomes of crocodilians, snakes, and turtles. We report an eHBV that is >207 million years old, making it the oldest endogenous virus currently known. Furthermore, our results provide direct evidence that the Hepadnaviridae virus family infected birds, crocodilians and turtles during the Mesozoic Era, and suggest a long-lasting coexistence of these viruses and their amniote hosts at least since the Early Mesozoic. We challenge previous views on the origin of the oncogenic X gene and provide an evolutionary explanation as to why only mammalian hepatitis B infection leads to hepatocellular carcinoma.

Recurrent targeted genes of hepatitis B virus in the liver cancer genomes identified by a next-generation sequencing-based approach

Integration of the viral DNA into host chromosomes was found in most of the hepatitis B virus (HBV)–related hepatocellular carcinomas (HCCs). Here we devised a massive anchored parallel sequencing (MAPS) method using next-generation sequencing to isolate and sequence HBV integrants. Applying MAPS to 40 pairs of HBV–related HCC tissues (cancer and adjacent tissues), we identified 296 HBV integration events corresponding to 286 unique integration sites (UISs) with precise HBV–Human DNA junctions. HBV integration favored chromosome 17 and preferentially integrated into human transcript units. HBV targeted genes were enriched in GO terms: cAMP metabolic processes, T cell differentiation and activation, TGF beta receptor pathway, ncRNA catabolic process, and dsRNA fragmentation and cellular response to dsRNA. The HBV targeted genes include 7 genes (PTPRJ, CNTN6, IL12B, MYOM1, FNDC3B, LRFN2, FN1) containing IPR003961 (Fibronectin, type III domain), 7 genes (NRG3, MASP2, NELL1, LRP1B, ADAM21, NRXN1, FN1) containing IPR013032 (EGF-like region, conserved site), and three genes (PDE7A, PDE4B, PDE11A) containing IPR002073 (3′, 5′-cyclic-nucleotide phosphodiesterase). Enriched pathways include hsa04512 (ECM-receptor interaction), hsa04510 (Focal adhesion), and hsa04012 (ErbB signaling pathway). Fewer integration events were found in cancers compared to cancer-adjacent tissues, suggesting a clonal expansion model in HCC development. Finally, we identified 8 genes that were recurrent target genes by HBV integration including fibronectin 1 (FN1) and telomerase reverse transcriptase (TERT1), two known recurrent target genes, and additional novel target genes such as SMAD family member 5 (SMAD5), phosphatase and actin regulator 4 (PHACTR4), and RNA binding protein fox-1 homolog (C. elegans) 1 (RBFOX1). Integrating analysis with recently published whole-genome sequencing analysis, we identified 14 additional recurrent HBV target genes, greatly expanding the HBV recurrent target list. This global survey of HBV integration events, together with recently published whole-genome sequencing analyses, furthered our understanding of the HBV–related HCC.

Integration of the hepatitis B virus (HBV) into the human liver cells was found in most of the related hepatocellular carcinomas (HCCs). Here, taking the recent advances in high-throughput sequencing, we devised an efficient and cost-effective method that we named massive anchored parallel sequencing (MAPS) method, to conduct a global survey of HBV integration events in 40 pairs of HBV–related HCC tissues (cancer and adjacent tissues). We identified 286 unique integration sites (UISs) with precise HBV–Human DNA junctions. We identified a higher number of HBV integration events in cancer adjacent tissues than in HCC tissues, suggesting a clonal expansion process during HCC development. We also found that fibronectin and its related genes (fibronectin type III-like fold domain containing genes) were frequently targeted by HBV. Fibronectin is a protein produced abundantly by the liver cells and also serves as a linker in the extracellular matrix. Our findings might suggest a role for the disruption of fibronectin and associated cellular matrix in HBV related liver cancers. We also identified 14 additional recurrent HBV target genes, greatly expanding the HBV recurrent target list. This study would add significantly to our understanding of HCC development.

Loss of hepatic NF-kappa B activity enhances chemical hepatocarcinogenesis through sustained c-Jun N-terminal kinase 1 activation

A major link between inflammation and cancer is provided by NF-kappaB transcription factors. Ikkbeta(Deltahep) mice, which specifically lack IkappaB kinase beta (IKKbeta), an activator of NF-kappaB, in hepatocytes, are unable to activate NF-kappaB in response to proinflammatory stimuli, such as TNF-alpha. Surprisingly, Ikkbeta(Deltahep) mice are hypersusceptible to diethylnitrosamine (DEN)-induced hepatocarcinogenesis. Because defective NF-kappaB activation promotes sustained c-Jun N-terminal kinase (JNK) activation in cells exposed to TNF-alpha, whose expression is induced by DEN, and JNK activity is required for normal hepatocyte proliferation, we examined whether increased susceptibility to DEN-induced hepatocarcinogenesis in Ikkbeta(Deltahep) mice requires JNK activation. Hepatocytes express both JNK1 and JNK2, but previous studies indicate that JNK1 is more important for hepatocyte proliferation. We therefore investigated this hypothesis using mice homozygous for a JNK1 deficiency either in wild-type or Ikkbeta(Deltahep) backgrounds. In both cases, mice lacking JNK1 were much less susceptible to DEN-induced hepatocarcinogenesis. This impaired tumorigenesis correlated with decreased expression of cyclin D and vascular endothelial growth factor, diminished cell proliferation, and reduced tumor neovascularization. Whereas hepatocyte-specific deletion of IKKbeta augmented DEN-induced hepatocyte death and cytokine-driven compensatory proliferation, disruption of JNK1 abrogated this response. In addition to underscoring the importance of JNK1-mediated hepatocyte death and compensatory proliferation, these results strongly suggest that the control of tissue renewal through the IKK and JNK pathways plays a key role in liver carcinogenesis.

Genomic DNA double-strand breaks are targets for hepadnaviral DNA integration

Integrated hepadnaviral DNA in livers and tumors of chronic hepatitis B patients has been reported for many years. In this study, we investigated whether hepatitis B virus DNA integration occurs preferentially at sites of cell DNA damage. A single I-SceI homing endonuclease recognition site was introduced into the DNA of the chicken hepatoma cell line LMH by stable DNA transfection, and double-strand breaks were induced by transient expression of I-SceI after transfection of an I-SceI expression vector. Alteration of the target cleavage site by imprecise nonhomologous end joining occurred at a frequency of approximately 10(-3) per transfected cell. When replication of an avian hepadnavirus, duck hepatitis B virus, occurred at the time of double-strand break repair, we observed integration of viral DNA at the site of the break with a frequency of approximately 10(-4) per transfected cell. Integration depended on the production of viral double-stranded linear DNA and the expression of I-SceI, and integrated DNA was stable through at least 17 cell divisions. Integration appeared to occur through nonhomologous end joining between the viral linear DNA ends and the I-SceI-induced break, because small deletions or insertions were observed at the sites of end joining. The results suggest that integration of hepadnaviral DNA in infected livers occurs at sites of DNA damage and may indicate the presence of more widespread genetic changes beyond that caused by viral DNA integration itself [corrected].

Characterization of HBV integrants in 14 hepatocellular carcinomas: association of truncated X gene and hepatocellular carcinogenesis

Although the integration of hepatitis B virus (HBV) into human DNA has been found to be associated with the development of hepatocellular carcinoma (HCC), the molecular mechanism remains unclear. In order to obtain additional insight into the correlation of HBV integration and HCC development, integrated HBV in 14 primary HCC cases was isolated and characterized by sequencing analysis. Our findings in this study showed that: (1) none of the known cellular oncogene or tumor suppressor gene was affected by the HBV integration; (2) although the integration of HBV is random, the integration site was often within or close to human repetitive sequences; (3) integrated HBV may possess the capacity to transpose to another chromosome region through reintegration; (4) rearrangements of HBV sequence were observed in all the 14 integrants, involving (most frequently) X (12/14 integrants), P (8/14), S (7/14), and C (7/14) genes; and (5) 3'-deleted X gene and consequent C-terminal truncated X protein caused by HBV integration was observed in 10 cases. These deletions cause the losses of p53-dependent transcriptional repression binding site, transcription factor Sp1 binding site, and growth-suppressive effect domain, leading to cell proliferation and transformation. This finding suggests that 3'-deleted X gene caused by the HBV integration may play an important role in the HCC development.

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.

Integration of hepadnavirus DNA in infected liver: evidence for a linear precursor

DNA of the avian hepadnavirus, duck hepatitis B virus, was found to be integrated at low abundance into the cellular DNA extracted from the livers of infected ducklings. The frequency of integration was estimated to be at least one viral genome per 10(3) to 10(4) cells by 6 days postinfection. The structures of virus-cell junctions determined by sequencing were compared with those of virus-virus junctions formed by nonhomologous recombination between the ends of linear viral DNA forms. This comparison allowed us to conclude that linear viral DNA was the preferential form used as an integration substrate. Potential factors promoting viral DNA integration during chronic infection are discussed.

A complex translational program generates multiple novel proteins from the latently expressed kaposin (K12) locus of Kaposi's sarcoma-associated herpesvirus

The most abundantly expressed latent transcripts encoded by the Kaposi's sarcoma (KS)-associated herpesvirus derive from the genomic region surrounding open reading frame (ORF) K12 (kaposin A). Here we show that these transcripts, initially described as limited to ORF K12 itself, more frequently encompass upstream sequences spanning two sets of 23-nucleotide GC-rich direct repeats (DRs) (DR1 and DR2). Although the DRs lack AUG codons and were previously presumed to be noncoding, a monoclonal antibody raised to infected cells detected multiple polypeptides encoded by this region. These proteins are expressed during latency and upon induction of lytic viral replication in both primary effusion lymphoma (PEL) cell lines and KS tumors. Biochemical and genetic analyses reveal that these proteins are derived from variant translational initiation at CUG codons. The predominant translation product in the PEL cell line BCBL-1 derives from the 5'-most CUG codon in the transcript, resulting in a protein (termed kaposin B) which is encoded largely by the repeats themselves and which does not include K12 sequences. Other non-AUG codons in alternate reading frames are also used at lower efficiency, including one that initiates translation of a DR-K12 fusion protein (kaposin C) that is predicted to sort to a different subcellular locale than kaposin B. Thus, the products of the K12 region, which is the most abundantly transcribed region in latency, are surprisingly complex and may encompass multiple biological functions.

Double-stranded linear duck hepatitis B virus (DHBV) stably integrates at a higher frequency than wild-type DHBV in LMH chicken hepatoma cells

Integration of hepadnavirus DNAs into host chromosomes can have oncogenic consequences. Analysis of host-viral DNA junctions of DHBV identified the terminally duplicated r region of the viral genome as a hotspot for integration. Since the r region is present on the 5' and 3' ends of double-stranded linear (DSL) hepadnavirus DNAs, these molecules have been implicated as integration precursors. We have produced a LMH chicken hepatoma cell line (LMH 66-1 DSL) which replicates exclusively DSL duck hepatitis B virus (DHBV) DNA. To test whether linear DHBV DNAs integrate more frequently than the wild type open circular DHBV DNAs, we have characterized the integration frequency in LMH 66-1 DSL cells by using a subcloning approach. This approach revealed that 83% of the LMH 66-1 DSL subclones contained new integrations, compared to only 16% of subclones from LMH-D2 cells replicating wild-type open circular DHBV DNA. Also, a higher percentage of the LMH 66-1 DSL subclones contained two or more new integrations. Mathematical analysis suggests that the DSL DHBV DNAs integrated stably once every three generations during subcloning whereas wild-type DHBV integrated only once every four to five generations. Cloning and sequencing of new integrations confirmed the r region as a preferred integration site for linear DHBV DNA molecules. One DHBV integrant was associated with a small deletion of chromosomal DNA, and another DHBV integrant occurred in a telomeric repeat sequence.

In vivo effects of mutations in woodchuck hepatitis virus enhancer II

Woodchuck hepatitis virus (WHV) enhancer II (EnII) is located upstream of the major pregenomic RNA promoter and is thought to play an important role in the insertional activation of the N-myc2 gene during WHV hepatocarcinogenesis. WHV EnII is recognized by at least three host transcription factors: HNF-1, HNF-4, and Oct-1. Here, the roles of these EnII-binding factors in viral transcription and replication have been further examined. In HepG2 cells transiently transfected with a chloramphenicol acetyltransferase (CAT) gene whose expression is dependent upon EnII, mutations in either the HNF-1 or the HNF-4 site strongly reduced CAT activity, while ablation of the Oct-1 site decreased CAT expression only twofold. Mutations in more than one site completely abolished reporter expression. These same mutations were also tested in an overlength WHV genome for their impact on viral replication and gene expression. In transfected HepG2 cells, lesions in the HNF-1 site inactivated pregenomic RNA expression and viral reverse transcription, with only minimal effects on the expression of other viral mRNAs. By contrast, Oct-1 site lesions had no effect on either viral RNA synthesis or DNA replication, and HNF-4 site lesions produced a modest reduction of pregenomic RNA but had no impact on viral DNA synthesis. Testing of the mutants in susceptible woodchucks revealed that, as expected, viruses with lesions in the HNF-1 site were nearly noninfectious, while mutants with lesions at the Oct-1 site were fully replication competent. HNF-4 site mutants were replication competent but may display reduced levels of replication in the intact animal host. We conclude that (i) EnII is primarily devoted to the regulation of pregenomic RNA in WHV, (ii) HNF-1 is essential for EnII function in vivo, and (iii) HNF-4 plays a demonstrable but adjunctive role in EnII function.

Antisense downregulation of N-myc1 in woodchuck hepatoma cells reverses the malignant phenotype

Cell line WH44KA is a highly malignant woodchuck hepatoma cell line. WH44KA cells contain a single woodchuck hepatitis virus (WHV) DNA integration in the 3' untranslated region of exon 3 of the woodchuck N-myc1 gene. The highly rearranged WHV DNA contains WHV enhancers which activate the N-myc promoter, and a hybrid N-myc1-WHV mRNA is produced, which leads to a high steady-state level of N-myc1 protein. To investigate whether continuous N-myc1 expression is required to maintain the tumor phenotype, we knocked out N-myc expression using a WHV-N-myc1 antisense vector. We identified two WH44KA antisense cell lines, designated 4-5 and 4-11, in which steady-state N-mycl protein levels were reduced by 95 and 80%, respectively. The growth rates of both antisense cell lines were reduced in comparison to those of wild-type and vector controls. The phenotype of 4-5 and 4-11 cells changed to a flattened appearance, and the cells exhibited contact inhibition. Colony-forming ability in soft agar was reduced by 92% for 4-5 cells and by 88% for 4-11 cells. Cell line 4-11 formed only small, slow-growing tumors in nude mice, consistent with a low level of N-myc1 remaining in the cells. In contrast, 4-5 cells, in which N-myc protein was reduced by greater than 95%, failed to form tumors in nude mice. The integrated WHV DNA contained the complete WHV X gene (WHx) and its promoter; however, we did not detect any WHx protein in the cells by using a sensitive assay. These data demonstrate that N-myc overexpression is required to maintain the malignant phenotype of WH44KA woodchuck hepatoma cells and provide a direct function for integrated WHV DNA in hepatocarcinogenesis.

Functional analysis of ground squirrel hepatitis virus enhancer II

We have characterized a major regulatory element of ground squirrel hepatitis virus (GSHV) located within a 90-nucleotide fragment of the core promoter upstream sequences and have compared its organization to that of woodchuck hepatitis virus (WHV) enhancer II (We2). The GSHV element (Ge2) stimulates transcription from the viral core promoter and heterologous promoters in an orientation-independent manner but displays a lower level of activity than We2 in transient transfection assays in human hepatoma cells. The general organization of Ge2 into binding sites for the liver-enriched HNF-1 and HNF-4 proteins and for ubiquitous factors of the NF1 and Oct families was found to be mostly conserved with respect to the homologous We2 region. Accordingly, transactivation by HNF-1 and HNF-4 plays an essential role in the liver-specific transcriptional activity of both the GSHV and WHV core promoters. Distinctive features of the GSHV enhancer consist of its ability to bind C/EBP family factors in a central motif that overlaps with one of the two HNF-4 sites and its differential binding affinities for HNF-4.

Woodchuck hepatitis virus X protein is present in chronically infected woodchuck liver and woodchuck hepatocellular carcinomas which are permissive for viral replication

The woodchuck hepatitis virus (WHV) X gene (WHx) is required for infectivity of WHV in woodchucks, and the gene encodes a broadly acting transcription factor. Several lines of evidence from cell culture and transgenic mice suggest that X proteins can promote hepatocarcinogenesis. To determine whether WHx-encoded proteins are present during persistent infection and hepatocellular carcinoma (HCC) in woodchucks, we surveyed livers and HCCs from a panel of WHV carrier woodchucks for the presence of WHx by utilizing an immunoprecipitation-Western blot (immunoblot) procedure. We detected a single 15.5-kDa WHx gene product in 100% of the persistently infected livers but not in livers from animals which had recovered from acute infection or in those of uninfected woodchucks. Analysis of HCCs revealed that all of the tumors which contained WHV replication intermediates were also positive for WHx. In contrast, WHx was undetectable in HCCs which did not contain replicative intermediates. Subcellular localization studies detected WHx in the cytoplasm but not in the nuclei of primary woodchuck hepatocytes. Comparative immunoprecipitation experiments revealed that there were 4 X 10(4) to 8 X 10(4) molecules of WHx per primary woodchuck hepatocyte. Four lines of WHx transgenic mice did not develop HCC spontaneously. However, when one line was treated with diethylnitrosamine, the occurrence of precancerous lesions was enhanced compared with that in diethylnitrosamine-treated nontransgenic controls. The apparent absence of WHx in some woodchuck HCCs indicates that WHx may not be required to maintain the tumor phenotype, whereas its presence in all persistently infected livers leaves open the possibility that it plays a role in hepatocarcinogenesis.

A new hepadnavirus endemic in arctic ground squirrels in Alaska

We present evidence for a novel member of the hepadnavirus family that is endemic in wild arctic ground squirrels (Spermophylus parryi kennicotti) in Alaska. This virus, designated arctic squirrel hepatitis virus (ASHV), was initially detected in the livers of animals bearing large hepatic nodules by nucleic acid hybridization with hepadnavirus probes and in plasma by cross-reactivity with antibodies to hepadnavirus surface and core antigens. The complete nucleotide sequence of the 3,302-bp-long ASHV genome was determined and compared with those of ground squirrel hepatitis virus (GSHV) and woodchuck hepatitis virus (WHV); all sequences were organized into four open reading frames, designated pre-C/C, pre-S/S, pol, and X. Despite roughly equivalent variability among the three rodent hepadnaviruses (around 16% base and 19% amino acid exchanges), ASHV appeared to be more closely related to GSHV than to WHV in phylogenetic analysis. Accordingly, preliminary studies of the pathology of ASHV infection suggested that ASHV may be a less efficient oncogenic agent than WHV. About one-third of aged animals maintained in captivity, including virus-infected as well as uninfected squirrels, developed large liver nodules, consisting of hepatocellular adenomas or carcinomas or nonmalignant lesions characterized by drastic microvesicular steatosis. ASHV-infected arctic ground squirrels may serve as a new model with which to analyze the contribution of hepadnavirus- and host-specific determinants to liver pathology and tumorigenesis.

Restricted expression of Kaposi sarcoma-associated herpesvirus (human herpesvirus 8) genes in Kaposi sarcoma

Kaposi sarcoma (KS) is the leading neoplasm of HIV-infected patients and is also found in several HIV-negative populations. Recently, DNA sequences from a novel herpesvirus, termed KS-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV-8) have been identified within KS tissue from both HIV-positive and HIV-negative cases; infection with this agent has been proposed as a possible factor in the etiology or pathogenesis of the tumor. Here we have examined the pattern of KSHV/HHV-8 gene expression in KS and find it to be highly restricted. We identify and characterize two small transcripts that represent the bulk of the virus-specific RNA transcribed from over 120 kb of the KSHV genome in infected cells. One transcript is predicted to encode a small membrane protein; the other is an unusual polyadenylylated RNA that accumulates in the nucleus to high copy number. This pattern of viral gene expression suggests that most infected cells in KS are latently infected, with lytic viral replication likely restricted to a much smaller subpopulation of cells. These findings have implications for the therapeutic utility of currently available antiviral drugs targeted against the lytic replication cycle.

Apoptosis is induced by N-myc expression in hepatocytes, a frequent event in hepadnavirus oncogenesis, and is blocked by insulin-like growth factor II

Induction of hepatocellular carcinoma in woodchucks by woodchuck hepatitis virus is associated with the activation of N-myc gene expression, usually by viral DNA integration in cis to the N-myc locus. We have examined the consequences of N-myc up-regulation in rodent hepatic cells in culture. Mouse alpha ML hepatocytes infected with a retroviral vector overexpressing the woodchuck N-myc2 gene display a higher proliferation rate than parental alpha ML cells but are morphologically unchanged and do not form colonies in soft agar. However, they display an increased propensity to undergo apoptosis, an effect that is markedly augmented by serum deprivation. Expression of the woodchuck hepatitis virus X gene in alpha ML cells does not alter the growth phenotype of the cells and has no effect upon N-myc-dependent apoptosis. However, apoptosis in N-myc2-expressing alpha ML cells is strongly inhibited by insulin-like growth factor II (IGF II). IGF II gene expression is also strongly up-regulated during hepatic carcinogenesis in vivo in virally infected animals and has been speculated to be part of an autocrine growth-stimulatory pathway. Our results suggest that IGF II may play another role in the development of virus-induced hepatoma: the prevention of programmed cell death triggered by deregulated N-myc expression.

Evidence for long-range oncogene activation by hepadnavirus insertion

Insertional mutagenesis of host genes, a common oncogenic strategy of slow transforming retroviruses, has recently been described for a DNA virus of the hepadnavirus group: the woodchuck hepatitis virus. This virus causes insertional activation of myc genes, mainly the intronless N-myc2 oncogene, in > 50% of woodchuck liver tumours. In most remaining tumours, N-myc2 is overexpressed without any apparent genetic alteration. To elucidate the role of the virus in such cases, we have cloned and analysed single integration sites in four woodchuck tumours carrying wild-type myc alleles. All sites were clustered within < 20 kb in a single locus, in which scarce unique sequences showed no detectable transcriptional activity. By fluorescent in situ hybridization, N-myc2 and the new locus (win) were localized to the same region of the long arm of the woodchuck X chromosome, and a 150-180 kb intervening distance was deduced from pulse-field gel analysis. The detection of viral integrations in win in additional tumours that produced abundant N-myc2 transcripts further substantiates the link between these two loci in woodchuck tumorigenesis. We propose that efficient activation of the N-myc2 promoter by the hepadnavirus enhancer acting over a long distance might operate in liver cell transformation.