Increase in the frequency of hepadnavirus DNA integrations by oxidative DNA damage and inhibition of DNA repair

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.

Proton-induced DNA damage promotes integration of foreign plasmid DNA into human genome

High-risk human papillomaviruses (HPVs) cause virtually all cervical cancer cases and are also associated with other types of anogenital and oropharyngeal cancers. Normally, HPV exists as a circular episomal DNA in the infected cell. However, in some instances, it integrates into the human genome in such a way as to enable increased expression of viral oncogenes, thereby leading to carcinogenesis. Since viral integration requires breaks in both viral and human genomes, DNA damage likely plays a key role in this critical process. One potentially significant source of DNA damage is exposure to elevated doses of ionizing radiation. Natural background radiation is ubiquitous; however, some populations, including radiological workers, radiotherapy patients, and astronauts, are exposed to significantly higher radiation doses, as well as to different types of radiation such as particle radiation. We hypothesize that ionizing radiation-induced DNA damage facilitates the integration of HPV into the human genome, increasing the risk of developing HPV-related cancers in the exposed population. To test this, we first determined the kinetics of DNA damage in keratinocytes exposed to ionizing radiation (protons) by assessing γ-H2AX foci formation using immunofluorescence (direct damage), and also measured ROS and 8-oxoG levels via DCFDA and Avidin-FITC (indirect damage).As anticipated, direct DNA damage was observed promptly, within 30 min, whereas indirect DNA damage was delayed due to the time required for ROS to accumulate and cause oxidative damage. Although radiation was lethal at high doses, we were able to establish an experimental system where radiation exposure (protons and X-rays) induced DNA damage dose-dependently without causing major cytotoxic effects as assessed by several cytotoxicity assays. Most importantly, we explored the impact of radiation exposure on integration frequency using a clonogenic assay and demonstrated that as predicted, proton-induced DNA damage promotes the integration of HPV-like foreign DNA in oral keratinocytes. Overall, the insights gained from this work enable us to better understand the contribution of radiation exposure and DNA damage to HPV-mediated carcinogenesis and direct us toward strategies aimed at preventing malignancies in HPV-infected individuals.

HBV-Integration Studies in the Clinic: Role in the Natural History of Infection

Hepatitis B virus (HBV) infection is a major global health problem causing acute and chronic liver disease that can lead to liver cirrhosis and hepatocellular carcinoma (HCC). HBV covalently closed circular DNA (cccDNA) is essential for viral replication and the establishment of a persistent infection. Integrated HBV DNA represents another stable form of viral DNA regularly observed in the livers of infected patients. HBV DNA integration into the host genome occurs early after HBV infection. It is a common occurrence during the HBV life cycle, and it has been detected in all the phases of chronic infection. HBV DNA integration has long been considered to be the main contributor to liver tumorigenesis. The recent development of highly sensitive detection methods and research models has led to the clarification of some molecular and pathogenic aspects of HBV integration. Though HBV integration does not lead to replication-competent transcripts, it can act as a stable source of viral RNA and proteins, which may contribute in determining HBV-specific T-cell exhaustion and favoring virus persistence. The relationship between HBV DNA integration and the immune response in the liver microenvironment might be closely related to the development and progression of HBV-related diseases. While many new antiviral agents aimed at cccDNA elimination or silencing have been developed, integrated HBV DNA remains a difficult therapeutic challenge.

Oxidative stress markers in patient-derived non-cancerous cervical tissues and cells

High-risk human papillomaviruses (HPV) are the causative agents of cervical cancer. However, not all infected women develop cervical cancer. Cervical tumorigenesis is characterized by a multifactorial etiology, with oxidative stress (OS) likely playing a major role. In addition to exogenous sources, metabolic processes also contribute to OS. In principle, variability in levels of cervical OS has the potential to influence the likelihood of conversion to cervical cancer. To ask whether such variability indeed existed, we assessed the levels of ROS and the oxidative DNA damage biomarker 8-oxodG in normal non-cancerous cervical tissues and cells obtained from women with uterovaginal pelvic organ prolapse following vaginal hysterectomy. We demonstrated five and ten-fold variability between tissues isolated from the transformation zone (TZ) and ectocervix (EC) of different women, respectively. Despite the greater variability (likely due to differences in tissue composition), the overall pattern of ROS levels in EC tissues mirrored those obtained in their corresponding TZ tissues. Our results also show that the levels of ROS in TZ tissues were always higher than or equal to those found in the respective EC tissues, providing a possible explanation for TZ tissue being the primary target for HPV infection and cervical carcinogenesis. Interestingly, primary keratinocytes isolated and cultured from these cervical specimens also displayed high variability in ROS levels, with some strongly mirroring the levels of ROS observed in their corresponding tissues, while others were less closely associated. Finally, we demonstrated that the levels of DNA damage mirrored the levels of ROS in the cultured primary cells. Understanding the factors and mechanisms that dispose certain individuals to develop cervical cancer has the potential to enable the development of approaches that make the conversion of HPV infection to cancer development even more rare.

Kinetics of DNA damage repair response accompanying initial hepadnavirus-host genomic integration in woodchuck hepatitis virus infection of hepatocyte

Mechanism of initial hepatitis B virus (HBV) integrations and kinetics of DNA repair immediately after infection remain essentially unknown impairing understanding of hepadnaviral oncogenesis. WCM260 hepatocytes susceptible to HBV-compatible woodchuck hepatitis virus (WHV) were examined from 15 min to 72 h post-infection (p.i.). WHV strongly induced reactive oxygen species (ROS), transiently inducible nitric oxide (iNOS) and DNA damage from 15 min p.i. All initial WHV-host fusions had the head-to-tail format indicating their formation by non-homologous end joining (NHEJ). Transcription of poly(ADP-ribose) polymerase 1 (PARP1) and X-ray repair cross-complementing protein 1 (XRCC1), the PARP1 binding partner, were induced in 30 min p.i. and that of 8-oxyguanine DNA glycosylse (OGG1) responding to oxidative DNA damage at 12 h p.i. Nicotinamide adenine dinucleotide (NAD⁺), a marker of PARP1 activation, and heme oxygenase-1 (HO1), an indicator of pro-oxidative stress, were significantly augmented from 15-30 min p.i. Additionally, PARP1 cleavage activity was evident from 30 min p.i. confirming that PARP1-mediated DNA repair became operational almost instatly after hepatocyte contact with virus. By applying complementary approaches, the study showed that initial WHV integration was due to virus-induced oxidative DNA damage and implied that the NHEJ PARP1-dependent repair pathway determined format of the first virus-host DNA junctions.

Integration of hepatitis B virus DNA into p21-activated kinase 3 (PAK3) gene in HepG2.2.15 cells

Integration of HBV DNA into host chromosomes was found in most of the patients with chronic hepatitis B (CHB). In this study, using inverse nested PCR (invPCR), we found the integration site chrX: 111,009,033, which inserted into the p21-activated kinase 3 (PAK3) gene in HepG2.2.15 cells. The viral-human chimeric transcripts were also observed and, significant differences of the copy numbers of integration site chrX: 111,009,033 (P = 0.012) and intra-cell HBV DNA levels (P = 0.027) were found between the cells with and without H₂O₂ treatment, respectively. This study may provide a novel insight into the elucidation of etiology of HBV integration.

Oxidative Stress in Poultry: Lessons from the Viral Infections

Reactive species (RS), generally known as reactive oxygen species (ROS) and reactive nitrogen species (RNS), are produced during regular metabolism in the host and are required for many cellular processes such as cytokine transcription, immunomodulation, ion transport, and apoptosis. Intriguingly, both RNS and ROS are commonly triggered by the pathogenic viruses and are famous for their dual roles in the clearance of viruses and pathological implications. Uncontrolled production of reactive species results in oxidative stress and causes damage in proteins, lipids, DNA, and cellular structures. In this review, we describe the production of RS, their detoxification by a cellular antioxidant system, and how these RS damage the proteins, lipids, and DNA. Given the widespread importance of RS in avian viral diseases, oxidative stress pathways are of utmost importance for targeted therapeutics. Therefore, a special focus is provided on avian virus-mediated oxidative stresses. Finally, future research perspectives are discussed on the exploitation of these pathways to treat viral diseases of poultry.

Oxidative stress, a trigger of hepatitis C and B virus-induced liver carcinogenesis

Virally induced liver cancer usually evolves over long periods of time in the context of a strongly oxidative microenvironment, characterized by chronic liver inflammation and regeneration processes. They ultimately lead to oncogenic mutations in many cellular signaling cascades that drive cell growth and proliferation. Oxidative stress, induced by hepatitis viruses, therefore is one of the factors that drives the neoplastic transformation process in the liver. This review summarizes current knowledge on oxidative stress and oxidative stress responses induced by human hepatitis B and C viruses. It focuses on the molecular mechanisms by which these viruses activate cellular enzymes/systems that generate or scavenge reactive oxygen species (ROS) and control cellular redox homeostasis. The impact of an altered cellular redox homeostasis on the initiation and establishment of chronic viral infection, as well as on the course and outcome of liver fibrosis and hepatocarcinogenesis will be discussed The review neither discusses reactive nitrogen species, although their metabolism is interferes with that of ROS, nor antioxidants as potential therapeutic remedies against viral infections, both subjects meriting an independent review.

Virus-host interplay in hepatitis B virus infection and epigenetic treatment strategies

Worldwide, chronic hepatitis B virus (HBV) infection is a major health problem and no cure exists. Importantly, hepatocyte intrusion by HBV particles results in a complex deregulation of both viral and host cellular genetic and epigenetic processes. Among the attempts to develop novel therapeutic approaches against HBV infection, several options targeting the epigenomic regulation of HBV replication are gaining attention. These include the experimental treatment with 'epidrugs'. Moreover, as a targeted approach, the principle of 'epigenetic editing' recently is being exploited to control viral replication. Silencing of HBV by specific rewriting of epigenetic marks might diminish viral replication, viremia, and infectivity, eventually controlling the disease and its complications. Additionally, epigenetic editing can be used as an experimental tool to increase our limited understanding regarding the role of epigenetic modifications in viral infections. Aiming for permanent epigenetic reprogramming of the viral genome without unspecific side effects, this breakthrough may pave the roads for an ambitious technological pursuit: to start designing a curative approach utilizing manipulative molecular therapies for viral infections in vivo.

Chronic oxidative stress increases the integration frequency of foreign DNA and human papillomavirus 16 in human keratinocytes

Cervical cancer is the second most common cancer, and the fourth most common cause of cancer death in women worldwide. Nearly all of these cases are caused by high-risk HPVs (HR HPVs), of which HPV16 is the most prevalent type. In most cervical cancer specimens, HR HPVs are found integrated into the human genome, indicating that integration is a key event in cervical tumor development. An understanding of the mechanisms that promote integration may therefore represent a unique opportunity to intercept carcinogenesis. To begin identifying these mechanisms, we tested the hypothesis that chronic oxidative stress (OS) induced by virus- and environmentallymediated factors can induce DNA damage, and thereby increase the frequency with which HPV integrates into the host genome. We found that virus-mediated factors are likely involved, as expression of E6*, a splice isoform of HPV16 E6, increased the levels of reactive oxygen species (ROS), caused oxidative DNA damage, and increased the frequency of plasmid DNA integration as assessed by colony formation assays. To assess the influence of environmentally induced chronic OS, we used L-Buthionine-sulfoximine (BSO) to lower the level of the intracellular antioxidant glutathione. Similar to our observations with E6*, glutathione depletion by BSO also increased ROS levels, caused oxidative DNA damage and increased the integration frequency of plasmid DNA. Finally, under conditions of chronic OS, we were able to induce and characterize a few independent events in which episomal HPV16 integrated into the host genome of cervical keratinocytes. Our results support a chain of events leading from induction of oxidative stress, to DNA damage, to viral integration, and ultimately to carcinogenesis.

Clonal expansion of hepatocytes with a selective advantage occurs during all stages of chronic hepatitis B virus infection

Hepatocyte clone size was measured in liver samples of 21 patients in various stages of chronic hepatitis B virus (HBV) infection and from 21 to 76 years of age. Hepatocyte clones containing unique virus-cell DNA junctions formed by the integration of HBV DNA were detected using inverse nested PCR. The maximum hepatocyte clone size tended to increase with age, although there was considerable patient-to-patient variation in each age group. There was an upward trend in maximum clone size with increasing fibrosis, inflammatory activity and with seroconversion from HBV e-antigen (HBeAg)-positive to HBeAg-negative, but these differences did not reach statistical significance. Maximum hepatocyte clone size did not differ between patients with and without a coexisting hepatocellular carcinoma. Thus, large hepatocyte clones containing integrated HBV DNA were detected during all stages of chronic HBV infection. Using laser microdissection, no significant difference in clone size was observed between foci of HBV surface antigen (HBsAg)-positive and HBsAg-negative hepatocytes, suggesting that expression of HBsAg is not a significant factor in clonal expansion. Laser microdissection also revealed that hepatocytes with normal-appearing histology make up a major fraction of the cells undergoing clonal expansion. Thus, preneoplasia does not appear to be a factor in the clonal expansion detected in our assays. Computer simulations suggest that the large hepatocyte clones are not produced by random hepatocyte turnover but have an as-yet-unknown selective advantage that drives increased clonal expansion in the HBV-infected liver.

DNA double-strand breaks, potential targets for HBV integration

Hepatitis B virus (HBV)-induced hepatocellular carcinoma (HCC) is one of the most frequently occurring cancers. Hepadnaviral DNA integrations are considered to be essential agents which can promote the process of the hepatocarcinogenesis. More and more researches were designed to find the relationship of the two. In this study, we investigated whether HBV DNA integration occurred at sites of DNA double-strand breaks (DSBs), one of the most detrimental DNA damage. An 18-bp I-SceI homing endonuclease recognition site was introduced into the DNA of HepG2 cell line by stable DNA transfection, then cells were incubated in patients' serum with high HBV DNA copies and at the same time, DSBs were induced by transient expression of I-SceI after transfection of an I-SceI expression vector. By using nest PCR, the viral DNA was detected at the sites of the break. It appeared that integration occurred between part of HBVxgene and the I-SceI induced breaks. The results suggested that DSBs, as the DNA damages, may serve as potential targets for hepadnaviral DNA insertion and the integrants would lead to widespread host genome changes necessarily. It provided a new site to investigate the integration.

Hepatitis B virus infection contributes to oxidative stress in a population exposed to aflatoxin B1 and high-risk for hepatocellular carcinoma

Biomarkers of hepatitis B virus (HBV) infection, aflatoxin B1 (AFB1) exposure and oxidative stress were detected in 71 hepatocellular carcinoma (HCC) patients and 694 controls from southern China. Plasma level of AFB1-albumin-adducts (AAA) and protein carbonyl content (PCC) were significantly higher in the 71 HCC cases than in any age/gender matched HBV sero-status groups (p<0.001). HCC patients positive for the p53-249 G-T mutation had a marginally higher level of PCC than those negative for the mutation (p=0.077). HBV infection had a prominent influence on the association between AFB1 exposure and oxidative stress biomarkers in the controls. Our study indicates a significant contribution from HBV infection to oxidative stress in a population with AFB1 exposure which might substantially increase risk for HCC in this region.

An aged host promotes the evolution of avirulent coxsackievirus into a virulent strain

The emergence of new, more pathogenic viruses necessitates elucidation of factors that promote viral evolution. Aging, a potential factor, is associated with increased susceptibility to viral infections. We used the enterovirus coxsackievirus B3 (CVB3) to investigate the effects of host age on pathogenicity and viral gene sequence. Old mice infected with a normally amyocarditic strain of CVB3, CVB3/0, had significantly higher mean heart viral titers compared with CVB3/0-infected adult mice. To determine whether a change in the CVB3/0 viral population could contribute to the higher titers observed in the old infected mice, CVB3/0 was passed once through an old or adult host and the changes in pathogenicity and viral genome were examined after subsequent infection of old or adult mice. Adult mice infected with CVB3/0 that was passed through an old host (CVB3/0(Old)) exhibited significantly higher heart viral titers, pathology, and weight loss than adult mice infected with either stock CVB3/0 or CVB3/0 passed through an adult host (CVB3/0(Adult)). Sequence analysis of virus isolated from CVB3/0(Old)-infected mice revealed 13 specific and reproducible nucleotide changes. These changes result in a sequence that matches the virulent CVB3/20 strain and are associated with promoting cardiovirulence. In contrast, we observed only one nucleotide change, low heart viral titers, and no heart and liver pathology in adult mice infected with CVB3/0(Adult). These results demonstrate that the aged host promotes rapid selection of a pathogenic variant of CVB3 from an avirulent strain and introduces a host-virus paradigm for studies of viral infection in the aged.

Modeling infection with hepatitis B viruses in vivo

Hepatitis B virus (HBV) is a human-specific liver pathogen whose viral cycle and mechanisms of pathogenesis are not yet fully understood. Along with invaluable infection studies in chimpanzees, avian and mammalian HBV-related viruses continue to offer ample opportunities for studies in their natural hosts. Yet, none of these hosts are commonly used laboratory animals; the lack of reliable in vitro infection systems and convenient animal models has severely hampered progress in HBV research. The need to perform studies in HBV-permissive hepatocytes has led researchers to create new, challenging human–mouse chimera infection models. The types of animal models currently available to perform infection studies with HBV are presented and discussed in this review.

Animal models for the study of HBV replication and its variants

Enormous progresses in hepatitis B virus research have been made through the identification of avian and mammalian HBV related viruses, which offer ample opportunities for studies in naturally occurring hosts. However, none of these natural hosts belongs to the commonly used laboratory animals, and the development of various mouse strains carrying HBV transgenes offered unique opportunities to investigate some mechanisms of viral pathogenesis. Furthermore, the need to perform infection studies in a system harbouring HBV-permissive hepatocytes has lately led researchers to create new challenging human mouse chimera models of HBV infection. In this review, we will overview the type of animal models currently available in hepadnavirus research.

Hepatitis B virus cccDNA clearance: killing for curing?

Chronic hepadnavirus infections cause liver damage with ongoing death and regeneration of hepatocytes. In the present study, we set out to quantify the extent of liver turnover by measuring the clonal proliferation of hepatocytes by using integrated viral DNA as a genetic marker for individual hepatocyte lineages. Liver tissue from woodchucks with chronic woodchuck hepatitis virus (WHV) infection was assayed for randomly integrated viral DNA by using inverse PCR. Serial endpoint dilution of viral-cell junction fragments into 96-well plates, followed by nested PCR and DNA sequencing, was used to determine the copy number of specific viral cell junctions as a measure of the clonal distribution of infected cell subpopulations. The results indicated that the livers contained a minimum of 100,000 clones of >1,000 cells containing integrated DNA, representing at least 0.2% of the hepatocyte population of the liver. Because cells with integrated WHV DNA comprised only 1-2% of total liver cells, it is likely that the total number of clones far exceeds this estimate, with as much as one half of the liver derived from high copy clones of >1,000 cells. It may be inferred that these clones have a strong selective growth or survival advantage. The results provide evidence for a large amount of hepatocyte proliferation and selection having occurred during the period of chronic WHV infection (approximately 1.5 years) in these animals.

Nitrative stress through formation of 8-nitroguanosine: insights into microbial pathogenesis

Reactive oxygen and nitrogen species, respectively, mediate oxidative and nitrative stresses by means of oxidation and nitration of various biomolecules including proteins, lipids, and nucleic acids. We have observed nitric oxide (NO)-dependent formation of 8-nitroguanosine and 3-nitrotyrosine during microbial infection, and we determined that both 8-nitroguanosine and 3-nitrotyrosine are useful biomarkers of nitrative stress. Of importance, however, is the great difference in biological characteristics of these two nitrated compounds. 8-Nitroguanosine has unique biochemical and pharmacological properties such as redox activity and mutagenic potential, which 3-nitrotyrosine does not. In this review, we discuss the mechanism of nitrative stress occurring during microbial infections, with special emphasis on biological functions of 8-nitroguanosine formed via NO during the host response to pathogens. These findings provide insights into NO-mediated pathogenesis not only of viral infections but also of many other diseases.

Integration of hepatitis B virus DNA into chromosomal DNA during acute hepatitis B

AIM: To examine the serum from black African patients with acute hepatitis B to ascertain if integrants of viral DNA can be detected in fragments of cellular DNA leaking from damaged hepatocytes into the circulation.

METHODS: DNA was extracted from the sera of five patients with uncomplicated acute hepatitis B and one with fulminant disease. Two subgenomic PCRs designed to amplify the complete genome of HBV were used and the resulting amplicons were cloned and sequenced.

RESULTS: HBV and chromosomal DNA were amplified from the sera of all the patients. In one patient with uncomplicated disease, HBV DNA was integrated into host chromosome 7 q11.23 in the WBSCR1 gene. The viral DNA comprised 200 nucleotides covering the S and X genes in opposite orientation, with a 1 169 nucleotide deletion. The right virus/host junction was situated at nucleotide 1 774 in the cohesive overlap region of the viral genome, at a preferred topoisomerase I cleavage motif. The chromosomal DNA was not rearranged. The patient made a full recovery and seroconverted to anti-HBs- and anti-HBe-positivity. Neither HBV nor chromosomal DNA could be amplified from his serum at that time.

CONCLUSION: Integration of viral DNA into chromosomal DNA may occur rarely during acute hepatitis B and, with clonal propagation of the integrant, might play a role in hepatocarcinogenesis.

Nitric oxide as an endogenous mutagen for Sendai virus without antiviral activity

Nitric oxide (NO) may affect the genomes of various pathogens, and this mutagenesis is of particular interest for viral pathogenesis and evolution. Here, we investigated the effect of NO on viral replication and mutation. Exogenous or endogenous NO had no apparent antiviral effect on influenza A virus and Sendai virus. The mutagenic potential of NO was analyzed with Sendai virus fused to a green fluorescent protein (GFP) gene (GFP-SeV). GFP-SeV was cultured in SW480 cells transfected with a vector expressing inducible NO synthase (iNOS). The mutation frequency of GFP-SeV was examined by measuring loss of GFP fluorescence of the viral plaques. GFP-SeV mutation frequency in iNOS-SW480 cells was much higher than that in parent SW480 cells and was reduced to the level of mutation frequency in the parent cells by treatment with an NO synthase (NOS) inhibitor. Immunocytochemistry showed generation of more 8-nitroguanosine in iNOS-SW480 cells than in SW480 cells without iNOS transfection. Authentic 8-nitroguanosine added exogenously to GFP-SeV-infected CV-1 cells increased the viral mutation frequency. Profiles of the GFP gene mutations induced by 8-nitroguanosine appeared to resemble those of mutations occurring in mouse lungs in vivo. A base substitution that was characteristic of both mutants (those induced by 8-nitroguanosine and those occurring in vivo) was a C-to-U transition. NO-dependent oxidative stress in iNOS-SW480 cells was also evident. Together, the results indicate unambiguously that NO has mutagenic potential for RNA viruses such as Sendai virus without affecting viral replication, possibly via 8-nitroguanosine formation and cellular oxidative stress.

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].

Residual integrated viral DNA after hepadnavirus clearance by nucleoside analog therapy

We determined the frequency of integrated viral DNA in the livers of three woodchucks chronically infected with the woodchuck hepatitis virus before and during 30 weeks of therapy with the nucleoside analog L-FMAU [1-(2-fluoro-5-methyl-beta, L-arabinofuranosyl)uracil, clevudine]. We found that although viral covalently closed circular DNA declined 20- to 100-fold, integrated viral DNA showed no discernable decrease over the course of treatment. Thus, chemotherapeutic clearance of covalently closed circular DNA did not involve the replacement of the infected hepatocyte population with uninfected progenitors, but rather, uninfected hepatocytes in the treated liver were derived from the infected hepatocyte population. The frequency of integrated DNA in chronically infected woodchucks was found to be 1 or 2 orders of magnitude higher than that in transiently infected woodchucks, implying that integration and other genomic damage accumulate over the duration of infection. Our results indicate that genetic changes from this damage remain in the liver even while virus infection is cleared and argue for early antiviral intervention in chronic hepatitis.

Inducible nitric oxide synthase expression in chronic viral hepatitis and its relation with histological severity of disease

The role of nitric oxide in the pathogenesis of chronic viral hepatitis is not known. Elevated nitric oxide production is assumed to be responsible for the pathological changes in many inflammatory conditions, mainly via peroxynitrite, a potential oxidant that is produced by the reduction of superoxide anion with nitric oxide. The intensity and the distribution of the immunohistochemical staining of intrahepatic inducible nitric oxide synthase were studied in the biopsy specimens obtained from 63 patients with viral hepatitis and 13 patients with elevated transaminase levels of various aetiologies. Hepatic inducible nitric oxide synthase staining was significantly more intense in the viral hepatitis group (P = 0.000). Inducible nitric oxide synthase staining levels correlated well with the severity of the viral hepatitis using the Knodell's liver histological activity index (r = 0.393, P = 0.002) Among the viral hepatitis group, the pathological distribution of the inducible nitric oxide synthase staining favoured the periportal hepatocytes (zone 1) whereas less staining was observed in parenchymal hepatocytes zone of 2 and 3 and bile duct epithelium. As nitric oxide mediated nitration of hepatocellular proteins is elevated in inflamed hepatic tissues and is correlated with the severity of the disease, we suggest that inducible nitric oxide synthase can possibly have a critical role in the pathogenesis of chronic viral hepatitis.

Increased chromosomal alterations and micronuclei formation in human hepatoma HepG2 cells transfected with the hepatitis B virus HBX gene

The protein encoded by the hepatitis B virus (HBV)-X gene, HBX, has been implicated to be involved in the development of HBV-associated liver cancer. HBX is a multifunctional regulatory protein that has been identified as a potential oncogene but its exact function remains unclear. HBX was documented to interact with several factors involved in cellular DNA repair as well as compromise the cell's ability to repair damaged DNA. We previously documented an accumulation of genetic alterations in two HepG2 cell lines independently transfected with HBV. In this report, we investigate the effect of the HBV-X gene (HBX) on the stability of the host genome using HepG2 stable transfectants (HepG2-HBX) and vector controls (HepG2-neo). We document that all HepG2-HBX clones analyzed contain HBX gene integrated and HBX transcript. Our data demonstrate that HepG2-HBX cells have an increased number of chromosome alterations and micronuclei formation compared to vector controls. A total of 10 de novo chromosomal rearrangements involving nine different chromosomes were detected in the HepG2-HBX clones, while no new rearrangements were found in vector controls. Each HepG2-HBX clone contained independently occurring de novo alterations not found in other HBX or vector clones. A three-fold increase of micronuclei formation was detected in HepG2-HBX cells compared to vector controls. Micronuclei originated from all chromosomes, however, preliminary data indicated that micronuclei originating from chromosomes 2, 3, 7, 18 and 20 were found in a greater amount in cells expressing the HBX gene. Interestingly, chromosomes 2, 18 and 20 were three of the chromosomes found rearranged in HepG2-HBX clones. These data provide evidence that genomic integrity was affected in cells expressing the HBX gene. De novo cytogenetic alterations identified in HepG2-HBX clones implicate the involvement of HBX in the process and support the hypothesis that HBX may interfere with normal cellular processes responsible for genomic integrity, increasing the risk for acquiring genetic mutations in infected hepatocytes.

Increase in de novo HBV DNA integrations in response to oxidative DNA damage or inhibition of poly(ADP-ribosyl)ation

Chronic infection with hepatitis B virus (HBV) is associated with an increased risk for the development of cirrhosis and hepatocellular carcinoma (HCC). Although clonal HBV DNA integrations are detected in nearly all HCCs the role of these integrations in hepatocarcinogenesis is poorly understood. We have used a cloning protocol that allows studying the frequency and the natural history of HBV DNA integrations in cell culture. Southern blot analysis of the genomic DNA of HepG2 2.2.15 subclones, which replicate HBV, enabled us to detect new HBV DNA integrations in approximately 10% of the HepG 2.2.15 subclones over 4 rounds of sequential subcloning, whereas no loss of any preexisting HBV DNA integrations was observed. Treatments of HepG2 cells with H(2)O(2), designed to increase DNA damage, increased the frequency of HBV integrations to approximately 50% of the subclones and treatments designed to inhibit DNA repair, by inhibiting Poly(ADP-ribosyl)ation, also increased the frequency of HBV integration to 50%. These findings suggest that DNA strand breaks induced by oxidative stress during persistent HBV infection in humans may increase HBV DNA integration events, whereas PARP-1 activity may function to limit the occurrence of de novo HBV DNA integrations.

Environmental causes of human cancers

Epidemiological studies have clearly shown a causal association between tobacco exposure and various human cancers, hepatitis B and C infection and hepatocellular carcinoma, human papilloma viruses and cervical cancer, and the occupational origin of certain human cancers is well established. The identification of the environmental causes of human cancers has been a long and difficult process. Much remains to be understood about the role of specific components of the diet and the interaction of different risk factors in the aetiology of human cancers. Withstanding the progress made on the understanding of the cancer process and their potential impact in the therapy of cancer, primary prevention remains, in developed and developing countries, the most effective measure to reduce cancer mortality.

Role of free radicals in viral pathogenesis and mutation

Oxygen radicals and nitric oxide (NO) are generated in excess in a diverse array of microbial infections. Emerging concepts in free radical biology are now shedding light on the pathogenesis of various diseases. Free-radical induced pathogenicity in virus infections is of great importance, because evidence suggests that NO and oxygen radicals such as superoxide are key molecules in the pathogenesis of various infectious diseases. Although oxygen radicals and NO have an antimicrobial effect on bacteria and protozoa, they have opposing effects in virus infections such as influenza virus pneumonia and several other neurotropic virus infections. A high output of NO from inducible NO synthase, occurring in a variety of virus infections, produces highly reactive nitrogen oxide species, such as peroxynitrite, via interaction with oxygen radicals and reactive oxygen intermediates. The production of these various reactive species confers the diverse biological functions of NO. The reactive nitrogen species cause oxidative tissue injury and mutagenesis through oxidation and nitration of various biomolecules. The unique biological properties of free radicals are further illustrated by recent evidence showing accelerated viral mutation by NO-induced oxidative stress. NO appears to affect a host's immune response, with immunopathological consequences. For example, NO is reported to suppress type 1 helper T cell-dependent immune responses during infections, leading to type 2 helper T cell-biased immunological host responses. NO-induced immunosuppression may thus contribute to the pathogenesis of virus infections and help expansion of quasispecies population of viral pathogens. This review describes the pathophysiological roles of free radicals in the pathogenesis of viral disease and in viral mutation as related to both nonspecific inflammatory responses and immunological host reactions modulated by NO.

Retrovirus insertion and transcriptional activation of the multidrug-resistance gene in leukemias treated by a chemotherapeutic agent in vivo

To understand the molecular basis for multidrug-resistant (MDR) cancer cells in vivo, this study analyzed molecular changes of the mdr1a gene region in leukemia cells in mice during continuous treatment with vincristine. An inverse insertion of murine leukemia retrovirus (MuLV) into the 5'-flanking region of the mdr1a gene was found. This insertion was concomitantly accompanied by up-regulation of the mdr1a gene and the loss of chemosensitivity. Deletion of long-terminal repeat (LTR) sequences dramatically decreased the mdr1a promoter-driven reporter activity. The MuLV LTR insertion appears to exert its enhancer activity on mdr1a transcription during the appearance of MDR leukemia cells. Two mechanisms were postulated to explain the mdr1a gene activation by retrovirus insertion during in vivo chemotreatment: de novo insertion of MuLV induced by vincristine treatment and selection of a small fraction of pre-existing cells carrying MuLV insertion during vincristine treatment. No rearranged sequence was detected by polymerase chain reaction in parental cells. This result argued for the first mechanism. The randomly altered distribution of MuLV during repetitive chemotreatment might also be consistent with this hypothesis. On the other hand, the retrovirus insertion was detected at the same site of the mdr1a promoter region in 2 independent experiments, which suggests the second mechanism. It should be noted that in vivo chemotreatment using vincristine could generate the mdr1a-overexpressing cells through retrovirus insertion and the enhancer effect of the LTR.

Resistance of hepatitis B virus to antiviral drugs: current aspects and directions for future investigation

Despite the existence of vaccines, chronic hepatitis B virus (HBV) infection remains a major health problem worldwide. Interferon therapy successfully controls infection in only a small percentage of chronically infected individuals. The recent approval of the nucleoside analogue lamivudine for the treatment of chronic HBV infection has ushered in a new era of antiviral therapy. While lamivudine is highly effective at controlling viral infection short-term, prolonged therapy has been associated with an increasing incidence of viral resistance. Thus, it appears that lamivudine alone will not be sufficient to control chronic viral infection in the majority of individuals. In addition to lamivudine, several new nucleoside and nucleotide analogues that show promising antihepadnaviral activity are in various stages of development. Lamivudine resistance has been found to confer cross-resistance to some of these compounds and it is likely that resistance to newer antivirals may also develop during prolonged use. Drug resistance therefore poses a major threat to nucleoside analogue-based therapies for chronic HBV infection. Fortunately, combination chemotherapy (antiviral therapy with two or more agents) can minimize the chance that resistance will develop and can be expected to achieve sustained reductions in viral load, provided that suitable combinations of agents are chosen. Here we review the basis of drug resistance in HBV, with emphasis on aspects that are likely to affect drug choice in future.

Intrahepatic accumulation of nitrotyrosine in chronic viral hepatitis is associated with histological severity of liver disease

BACKGROUND/AIMS

The toxicity of nitric oxide is thought to be engendered, at least in part, by its reaction with superoxide yielding peroxynitrite, a potent oxidant that promotes the formation of nitrotyrosine within cells and tissue lesions. In this study we assessed the intrahepatic localization and distribution of the inducible nitric oxide synthase (iNOS) and nitrotyrosine (NTY) in patients with viral and non-viral liver disease.

METHODS

We carried out single and double immunostaining experiments on cryostat liver biopsy sections using monoclonal antibodies against iNOS and NTY. We also performed a comparative analysis between the intrahepatic immunostaining score of NTY and the histological activity index of chronic viral hepatitis.

RESULTS

We found a marked hepatocellular expression of iNOS with a diffuse lobular pattern in all liver samples from patients with viral liver disease, whereas NTY localization was mainly restricted to cellular foci consisting of hepatocytes and Kupffer cells. Interestingly, we demonstrated by means of double immunostaining experiments the existence of hepatocellular co-localization of iNOS and NTY in the majority of NTY-expressing liver cells. The amount of NTY was significantly higher in liver biopsies from viral liver disease than in non-viral liver disease. In addition, a statistically significant association between the intrahepatic amount of NTY and the severity of viral liver disease was found.

CONCLUSIONS

Nitric oxide-mediated nitration of hepatocellular proteins is markedly induced in the inflamed liver tissue from patients with chronic viral hepatitis, and appears to be associated with the histological severity of viral chronic liver disease.

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.

Hepatitis B virus in hepatocarcinogenesis

Hepatitis B virus (HBV) is an important etiologic agent of chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Although the mechanism whereby HBV causes HCC is not fully understood, it is likely that there are many relevant molecular pathways that contribute to the development of HBV-associated HCC. This review provides an overview of some of these proposed pathways and their relative importance. It also raises questions on basic and translational research that will signficantly contribute to the better understanding of underlying mechanisms, prevention, and treatment of this tumor type.

Trans-dominant inhibition of poly(ADP-ribosyl)ation potentiates alkylation-induced shuttle-vector mutagenesis in Chinese hamster cells

In most eukaryotic cells, the catalytic activation of poly(ADP-ribose) polymerase (PARP) represents one of the earliest cellular responses to the infliction of DNA damage. To study the biological function(s) of poly(ADP-ribosyl)ation, we have established stable transfectants (COM3 cells) of the SV40-transformed Chinese hamster cell line C060 which conditionally overexpress the PARP DNA-binding domain upon addition of dexamethasone. We could demonstrate that DNA-binding domain overexpression, which leads to trans-dominant inhibition of poly(ADP-ribosyl)ation, potentiates the cytotoxicity of alkylation treatment and of gamma-radiation. Likewise, carcinogen-induced gene amplification, viewed as a manifestation of genomic instability, was potentiated by the overexpression of the PARP DNA-binding domain. Recently, we studied the effect of trans-dominant PARP inhibition on mutagenesis by employing a shuttle-vector assay in which mutagen-exposed plasmid pYZ289 is electroporated into COM3 cells. We could show that dexamethasone-induced overexpression of the PARP DNA-binding domain in COM3 cells potentiates the mutagenicity of the alkylating agent N-methyl-N-nitrosourea, while no effect of dexamethasone treatment on mutation frequency was recorded in control cells lacking the PARP DNA-binding domain transgene. Taken together, our results further substantiate the role of poly(ADP-ribosyl)ation in the maintenance of genomic integrity and stability under conditions of genotoxic stress.

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.

Infection of ducklings with virus particles containing linear double-stranded duck hepatitis B virus DNA: illegitimate replication and reversion

Double-stranded linear DNA is synthesized as a minor viral DNA species by all hepadnaviruses. In a previous study (W. Yang and J. Summers, J. Virol. 69:4029-4036, 1995) we showed that virus particles containing linear DNA of the duck hepatitis B virus (DHBV) could initiate an infection of primary duck hepatocytes. In cells infected by linear DNA containing viruses the transcriptional template, covalently closed circular DNA, was formed by circularization of linear DNA by nonhomologous recombination between the two ends. This process was shown to result in viral DNA replication through multiple generations of linear DNA intermediates, a process we called illegitimate replication. In this study we showed that viruses containing linear DHBV DNA produced by engineered insertions in the r sequence, which encodes the 5' end of the pregenome, could infect hepatocytes in vivo, and these hepatocytes proceeded to carry out illegitimate replication. Nonhomologous recombination quickly produced revertants and partial revertants in which all or part of the insertion was deleted. One such partial revertant that replicated primarily through circular DNA intermediates, but which synthesized elevated levels of linear DNA, could be sustained for several days as the predominant genotype in vivo, but this mutant was eventually displaced by variants showing full reversion to legitimate replication and that synthesized normal low levels of linear DNA. Full revertants did not necessarily contain the wild-type r sequence. The results suggest that the linear DNA produced during DHBV infection initiates cycles of illegitimate replication by generating mutants with altered r sequences. Some r sequence mutants carry out a mixture of legitimate and illegitimate replication that can contribute to elevated production of linear DNA in individual cells.

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.

Liver repopulation with xenogenic hepatocytes in B and T cell-deficient mice leads to chronic hepadnavirus infection and clonal growth of hepatocellular carcinoma

To investigate host and viral mechanisms determining hepadnaviral persistence and hepatocarcinogenesis, we developed a mouse model by transplanting woodchuck hepatocytes into the liver of mice that contain the urokinase-type plasminogen activator transgene (uPA) and lack mature B and T lymphocytes due to a recombination activation gene 2 (RAG-2) gene knockout. The woodchuck hepatocytes were transplanted via intrasplenic injection and were found to integrate into the recipient mouse liver cord structure. Normal adult woodchuck hepatocytes proliferated and reconstituted up to 90% of the uPA/RAG-2 mouse liver. uPA/RAG-2 mice containing woodchuck hepatocytes were infectable with woodchuck hepatitis virus (WHV) and showed WHV replication for at least 10 months with titers up to 1 x 10(11) virions per ml in the peripheral blood. WHV-infected hepatocytes from chronic carrier woodchucks also established a persistent infection in uPA/RAG-2 mice after an 8- to 12-week lag period of viremia. Although WHV envelope, core, and X proteins were produced in the uPA/RAG-2 mice, no inflammatory host immune response was observed in the liver of WHV-replicating mice. A first antiviral test demonstrated a greater than four orders of magnitude drop in WHV titer in response to interferon alpha treatment. WHV replication was up-regulated by dexamethasone treatment. Comparison of precancerous lesions in donor woodchucks versus recipient uPA/RAG-2 mice revealed an enrichment of dysplastic precancerous hepatocytes in transplanted mice. Clonal amplification of hepatocytes from a woodchuck with hepatocellular carcinomas was demonstrated by the detection of unique WHV DNA integration patterns in hepatocellular carcinomas that arose in uPA/RAG-2 mice. In the absence of B or T cell-mediated immune responses, WHV establishes a persistent noncytotoxic infection of woodchuck hepatocytes in uPA/RAG-2 chimeric mouse livers. Further studies of the kinetics of hepadnavirus infection and replication in quiescent and proliferating hepatocytes should increase our understanding of hepadnavirus spread and aid in the design of therapies to block or cure persistent infection.