Use of bacterially-expressed antigen for detection of antibodies to the EBV-specific deoxyribonuclease in sera from patients with nasopharyngeal carcinoma

Screening and identification of emodin as an EBV DNase inhibitor to prevent its biological functions

BACKGROUND

The Epstein-Barr virus (EBV) is a prevalent oncovirus associated with a variety of human illnesses. BGLF5, an EBV DNase with alkaline nuclease (AN) activity, plays important roles in the viral life cycle and progression of human malignancies and has been suggested as a possible diagnostic marker and target for cancer therapy. Methods used conventionally for the detection of AN activity, radioactivity-based nuclease activity assay and DNA digestion detection by gel electrophoresis, are not suitable for screening AN inhibitors; the former approach is unsafe, and the latter is complicated. In the present study, a fluorescence-based nuclease activity assay was used to screen several natural compounds and identify an EBV DNase inhibitor.

RESULTS

Fluorescence-based nuclease activity assays, in which the DNA substrate is labelled with PicoGreen dye, are cheaper, safer, and easier to perform. Herein, the results of the fluorescence-based nuclease activity assay were consistent with the results of the two conventional methods. In addition, the PicoGreen-labelling method was applied for the biochemical characterisation of viral nucleases. Using this approach, we explored EBV DNase inhibitors. After several rounds of screening, emodin, an anthraquinone derivative, was found to possess significant anti-EBV DNase activity. We verified the efficacy of emodin using the conventional DNA-cleavage assay. Furthermore, using comet assay and micronucleus formation detection, we confirmed that emodin can inhibit DNase-induced DNA damage and genomic instability. Additionally, emodin treatment inhibited EBV production.

CONCLUSIONS

Using a PicoGreen-mediated nuclease activity assay, we successfully demonstrated that emodin has the potential to inhibit EBV DNase nuclease activity. Emodin also inhibits EBV DNase-related biological functions, suggesting that it is a potential inhibitor of EBV DNase.

Functional Implications of Epstein-Barr Virus Lytic Genes in Carcinogenesis

Epstein-Barr virus (EBV) is associated with a diverse range of tumors of both lymphoid and epithelial origin. Similar to other herpesviruses, EBV displays a bipartite life cycle consisting of latent and lytic phases. Current dogma indicates that the latent genes are key drivers in the pathogenesis of EBV-associated cancers, while the lytic genes are primarily responsible for viral transmission. In recent years, evidence has emerged to show that the EBV lytic phase also plays an important role in EBV tumorigenesis, and the expression of EBV lytic genes is frequently detected in tumor tissues and cell lines. The advent of next generation sequencing has allowed the comprehensive profiling of EBV gene expression, and this has revealed the consistent expression of several lytic genes across various types of EBV-associated cancers. In this review, we provide an overview of the functional implications of EBV lytic gene expression to the oncogenic process and discuss possible avenues for future investigations.

Establishment and validation of a two-step screening scheme for improved performance of serological screening of nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC), which is closely associated with Epstein-Barr virus (EBV), is one of the most prevalent cancers in southeast China. Most NPC patients are diagnosed at late stage due to inconspicuous symptoms at the early stage, and the prognosis of these patients is poor. The early diagnosis rate of NPC could be significantly increased by serological screening, but the positive predictive value (PPV) is relatively low. A simple two-step serological screening scheme was established to improve the PPV of the screening strategy and was validated by a prospective cohort. Serum antibodies specific for EBNA1, Zta, Thymidine Kinase (TK), EAD, EAR, and VCA were detected by enzyme-linked immunosorbent assay. The combination of EBNA1/IgA and VCA/IgA was used in the first step of screening, and anti-early antigens (EAs) were used in the second step of screening. EAD/IgA was the most prominent marker in the second step of screening, and other anti-EAs were complementary to EAD/IgA. As validated by a prospective cohort including 4200 participants, using the combination of EAD/IgA and TK/IgA in the second step decreased the number of high-risk participants from 128 to 27, and increased the PPV from 4.69% to 18.52%, with only one very early-stage case missed. The two-step screening scheme provides a standardized approach for NPC screening with an improved PPV and may be used in future field studies. With this two-step serological screening method, more people benefit from the screening program without increasing the need for fiberoptic endoscopy.

The Immunomodulatory Capacity of an Epstein-Barr Virus Abortive Lytic Cycle: Potential Contribution to Viral Tumorigenesis

Epstein-Barr virus (EBV) is characterized by a bipartite life cycle in which latent and lytic stages are alternated. Latency is compatible with long-lasting persistency within the infected host, while lytic expression, preferentially found in oropharyngeal epithelial tissue, is thought to favor host-to-host viral dissemination. The clinical importance of EBV relates to its association with cancer, which we think is mainly a consequence of the latency/persistency mechanisms. However, studies in murine models of tumorigenesis/lymphomagenesis indicate that the lytic cycle also contributes to cancer formation. Indeed, EBV lytic expression is often observed in established cell lines and tumor biopsies. Within the lytic cycle EBV expresses a handful of immunomodulatory (BCRF1, BARF1, BNLF2A, BGLF5 & BILF1) and anti-apoptotic (BHRF1 & BALF1) proteins. In this review, we discuss the evidence supporting an abortive lytic cycle in which these lytic genes are expressed, and how the immunomodulatory mechanisms of EBV and related herpesviruses Kaposi Sarcoma herpesvirus (KSHV) and human cytomegalovirus (HCMV) result in paracrine signals that feed tumor cells. An abortive lytic cycle would reconcile the need of lytic expression for viral tumorigenesis without relaying in a complete cycle that would induce cell lysis to release the newly formed infective viral particles.

Association of Epstein-Barr virus infection with systemic lupus erythematosus in Taiwan

An association between Epstein-Barr virus (EBV) infection and systemic lupus erythematosus (SLE) has been suggested from previous serologic evidence. Since most adults in Taiwan are EBV-infected, seroepidemiologic studies based on standard assays for EBV are unlikely to dissociate SLE patients and control groups. We reexamine this question by using novel methodologies in which IgA anti-EBV-coded nuclear antigens-1 (EBNA-1) and IgG anti-EBV DNase antibodies were analysed by ELISA, and EBV viral loads were detected by real-time quantitative PCR for 93 adult SLE patients and 370 age-, sex- and living place-matched healthy controls in Taiwan. The specificities of antibodies for extractible nuclear antigens were determined by Western blot. Our results show that IgA anti-EBV EBNA1 antibodies were detectable in 31.2% SLE patients but only in 4.1% of controls (odds ratio [OR] = 10.72, 95% confidence interval [CI] = 5.19–22.35; P < 10-7), IgG anti-EBV DNase antibodies were detected in 53.8% SLE patients but only in 12.2% controls (OR = 8.40, 95% CI = 4.87–14.51; P < 10-7). EBV DNA was amplifiable from the sera of 41.9% SLE patients but from only 3.24% controls ( P < 0.05). A significant association of IgG anti-EBV DNase antibodies with anti-Sm/RNP antibodies was observed ( P < 0.005). The higher seroreactivity and higher copy numbers of EBV genome indicated association of EBV infection with SLE in Taiwan.

Site-directed mutagenesis in a conserved motif of Epstein-Barr virus DNase that is homologous to the catalytic centre of type II restriction endonucleases

Sequence alignment of human herpesvirus DNases revealed that they share several conserved regions. One of these, the conserved motif D203...E225XK227 (D.EXK) in the sequence of Epstein-Barr virus (EBV) DNase, has a striking similarity to the catalytic sites of some other nucleases, including type II restriction endonucleases, lambda exonuclease and MutH. The predicted secondary structures of these three residues were shown to resemble the three catalytic residues of type II restriction endonucleases. Site-directed mutagenesis was carried out to replace each of the acidic residues near the motif by residues with different properties. All substitutions of D203, E225 and K227 were shown to cause significant reductions in nuclease activity. Six other acidic residues, within the conserved regions, were also replaced by Asn or Gln. Five of these six variants retained nuclease activity and mutant D195N alone lost nuclease activity. The four charged residues, D195, D203, E225 and K227, of EBV DNase were found to be important for nuclease activity. Biochemical analysis indicated that the preference for divalent cations was altered from Mg2+ to Mn2+ for mutant E225D. The DNA-binding abilities of D203E, E225D and E225Q were shown to be similar to that of wild-type. However, K227 mutants were found to have variable DNA-binding abilities: K227G and K227N mutants retained, K227E and K227D had reduced and K227R lost DNA-binding ability. Comparison of the biochemical properties of the corresponding substitutions among EBV DNase and type II restriction enzymes indicated that the D...EXK motif is most likely the putative catalytic centre of EBV DNase.

Use of bacterially expressed EBNA-1 protein cloned from a nasopharyngeal carcinoma (NPC) biopsy as a screening test for NPC patients

EBV serological tests have been used for many years as accessory diagnostic predictors of nasopharyngeal carcinoma (NPC). To increase the sensitivity and specificity of the NPC detection rate, a novel enzyme-linked immunosorbent assay (ELISA) was established using a bacterially-expressed GST-EBNA-1 protein, containing the EBNA-1 sequence cloned from an NPC patient. Serum samples were collected from age- and gender-matched patients with NPC, community control subjects and hospital control patients and tested using this ELISA. The positivity rates were 78.7% (247/314) in NPC, 11.5% (28/244) in hospital controls and 3.8% (10/263) in the community control group. These serum samples were also tested for IgA anti-VCA antibodies and their ability to neutralize EBV DNase and the sensitivities of the anti-VCA antibody and DNase-neutralization tests also were analyzed. The optimum combination is VCA plus EBNA-1, which can identify 92.5% (287/310) of NPC patients, and shows a specificity of 92.7% (242/261) for normal individuals.

Distribution of epstein-barr virus antigenic sites on the carboxyl terminal end of ribonucleotide reductase against nasopharyngeal carcinoma serum antibodies using an immunoabsorption method

In an attempt to clone and express proteins from the Epstein-Barr virus (EBV) cDNA library to be used as antigens in an enzyme-linked immunosorbent assay (ELISA) format to test against the antibodies found in the sera of nasopharyngeal carcinoma (NPC) patients, we have isolated and characterized three clones. All three clones expressed the same polypeptides of different lengths, which belong to the carboxyl terminal end of the large subunit of ribonucleotide reductase (RR) of the EBV genome. All three clones were found to be immunogenic and could be used in an IgA and IgG ELISA against the NPC sera with various degrees of sensitivity and specificity. Because the clones varied in length, this difference provides a simple system to determine where most of the antibody epitopes lies on the protein. We designed an immunoabsorption assay and a mathematical model to help map the segment of the polypeptide most immunogenic to 43 NPC patients. Results were unexpected: 77% of the patients were most immunogenic to region z, which was the smallest fragment among the three fragments studied. Fragment z was only 33 amino acids in length. Only 14% and 19% of patients showed the most immunogenic region in segment x and y, respectively. This variation could be due to major histocompatibility complex antigens. The patients could be divided into three groups based on the immunoabsorption assays, in which each group responded to a different immunodominant segment in the RR antigen. The largest group responded to an immunodominant segment, which was only 33 amino acids long. This domain was coded for by the gene fragment from nucleotide 78,129 to nucleotide 78,227 of the EBV genome. This segment of the protein would be suitable for further epitope mapping studies.

Epstein-Barr virus DNase contains two nuclear localization signals, which are different in sensitivity to the hydrophobic regions

The DNase of Epstein-Barr virus (EBV) is a 470-amino-acid protein which possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA and single-stranded DNA as substrates. It has been reported that this protein may be found in the nucleus and/or cytoplasm of infected cells. In this study, using cell fractionation and immunoblotting to determine the distribution of EBV DNase in Akata cells stimulated with anti-human immunoglobulin G antibody (anti-IgG), the DNase was found to be located predominantly in the nucleus. To map the signals in DNase which mediate its nuclear localization, we monitored the nuclear transport of fusion proteins consisting of various fragments of EBV DNase linked to a cytoplasmic protein, beta-galactosidase (beta-Gal). The results demonstrated that two regions of the DNase with nuclear localization signal (NLS) activity, designated NLS-A (amino acids 239-266) and NLS-B (amino acids 291-306), were able independently to localize the beta-Gal to the nuclei of HEp-2 and HeLa cells. Five basic residues (R or K) were found in each NLS and distributed differently in primary structure. The basic domains and flanking residues of NLS-A and NLS-B are 250YKRPCKRSFIRFI262 and 294LKDVRKRKLGPGH306, respectively. Further examination of these sequences revealed that NLS-A contains bulky aromatic amino acids (Y and F) which may diminish its capacity to act as a strong NLS and lacks the typical proline and glycine helix-breakers. However, NLS-B contains typical proline and glycine helix-breakers and the histidine residue at amino acid 306 is required for NLS activity. In addition, two hydrophobic regions within the DNase were found to inhibit the function of NLS-A but not NLS-B, suggesting that these two domains are different types of NLSs and differ in their sensitivity to hydrophobic regions in the context of protein structure.

Distinct regions of EBV DNase are required for nuclease and DNA binding activities

Epstein-Barr virus (EBV) DNase possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) as substrates. To map regions of EBV DNase responsible for nuclease and DNA binding activities, a series of mutant DNase polypeptides was expressed using a bacterial system for the nuclease assay and in an in vitro transcription/translation system to assay binding activity to dsDNA or ssDNA cellulose. The results indicated that the C-terminus of EBV DNase, residues 450-460, is essential for nuclease activity but dispensable for DNA binding. However, deletion of residues 441-470 resulted in the loss of both nuclease and DNA binding activities. Substitution of Phe452 and Val458 led to inactive enzymes. In the N-terminus, deletion of residues 23-28 and residues 7-61 resulted in the loss of nuclease activity but the DNA binding activities of the deleted enzymes were intermediate and low, respectively. Mutation of Leu23 to Gly showed drastically reduced nuclease activity but its DNA binding ability was not affected. Based on the amino acid sequence alignment of various herpesvirus DNases, we chose four highly conserved and two less well conserved regions as controls for mutagenesis studies. These six internal deletion (ID) mutants were prepared using a recombinant PCR method. Each of the polypeptides was expressed in a bacterial system for the nuclease assay and using an in vitro transcription/translation system for the DNA binding assay. DNA binding and nuclease activities of all six internal deletion mutants were abolished, except that mutant ID2, with deletion of residues 138-152, retained an intermediate ability to bind DNA. These data indicate that since mutations at distinct regions within EBV DNase resulted in the loss of nuclease and/or DNA binding activities, it is suggested that these distinct regions are required for maintenance of an intact and highly ordered structure(s) for both activities.

Recombinant pseudorabies virus DNase exhibits a RecBCD-like catalytic function

The pseudorabies virus (PRV) DNase gene has previously been mapped within the PRV genome. To characterize further the enzymic properties of PRV DNase, this enzyme was expressed in Escherichia coli with the use of a pET expression vector. The protein was purified to homogeneity and assayed for nuclease activity in vitro. Recombinant PRV DNase exhibited an alkaline pH preference and an absolute requirement for Mg2+ ions that could not be replaced by Ca2+ and Na+ ions. Further studies showed that PRV DNase exhibited endonuclease, 5'-exonuclease and 3'-exonuclease activities in both single-stranded and double-stranded DNA. This activity occurred randomly and no significant base preference was demonstrated. The multiple biochemical activities of PRV DNase are similar to the activities of Neurospora crassa endo-exonuclease and E. coli RecBCD, two additional enzymes that are involved in recombination. Taken together, the similarity of action between N. crassa endo-exonuclease, E. coli RecBCD, and PRV DNase suggests that PRV DNase might have a role in the process of recombination that occurs during PRV infection.

Characterization of Monoclonal Antibodies to the Zta and DNase Proteins of Epstein-Barr Virus

Two monoclonal antibodies (mAb) were derived and designated 4F10 and 311H. 4F10 was against the Epstein-Barr virus (EBV) Zta protein and 311H specifically recognized EBV DNase enzyme. Using mAb 4F10 as a probe, the Zta protein could be detected as a 36-kD molecule in L5 cells and as a 38-kD molecule in B95-8 cells, reflecting the fact reported by other laboratories, using rabbit polyclonal antisera, that the Zta protein was variously modified in different host cells. 311H mAb was generated using antigens purified from one-step His-Bind column chromatography. The antigenic epitope recognized by this mAb was mapped within the residues 1-152 of EBV DNase by reacting the mAb with three distinct truncated mutants. Also, using 311H as a reagent to trace the kinetic expression of EBV DNase proteins in EBV-infected Akata cells, the Western blotting results indicated that DNase antigen could be detected at 12 h postactivation. The feasibility of applying these two mAb in the investigation of EBV biology is discussed. Copyright 1997 S. Karger AG, Basel

Epstein-Barr Viral Antigens Used in the Diagnosis of Nasopharyngeal Carcinoma

The major antigen complexes of Epstein-Barr virus (EBV) include the latent infectious proteins, early antigens, membrane antigens and viral capsid antigens. The various polypeptides within each antigen complex have been identified and isolated through gene-cloning technology. These polypeptides are exploited to be used as serological markers for the diagnosis of nasopharyngeal carcinoma (NPC) through enzyme-linked immunosorbent assay. This paper reviews the recent studies on the profile of antibodies in patients with NPC towards these EBV polypeptides of each antigen complex. The sensitivity and specificity of each polypeptide when used as serological markers to NPC patients' sera are summarized. Copyright 1996 S. Karger AG, Basel

Purification and characterization of herpes simplex virus type 1 alkaline exonuclease expressed in Escherichia coli

The alkaline exonuclease (AE) encoded by the herpes simplex virus type 1 (HSV-1) UL12 open reading frame was inducibly expressed in Escherichia coli and purified without the use of chromatographic separation. This recombinant AE was found to exhibit the same biochemical properties as the virus-encoded protein and was used to confirm the existence of a weak endonucleolytic activity in the enzyme. Antisera raised against the recombinant protein recognized several forms of the AE in HSV-1-infected cells. This expression and purification strategy will provide an economical and easily accessible alternative source of HSV-1 AE for future in vitro studies.

Enzyme-linked immunosorbent assay (ELISA) for IgA and IgG antibodies to Epstein-Barr-virus ribonucleotide reductase in patients with nasopharyngeal carcinoma

661 bp coding for the carboxyl end of the large sub-unit of EBV ribonucleotide reductase was cloned into the pMal plasmid vector. Purified recombinant protein was tested in IgG and IgA ELISAs. For the IgG assay, 81 out of 100 NPC patients tested positive, whereas for the IgA assay, 60 tested positive. Among 100 normal individuals, I tested positive for the IgG assay and 9 tested positive for the IgA assay. The IgG assay picked up 6 out of 19 NPC sera which were IFA-VCA- and IFA-EA-negative for IgA antibodies. Hence the recombinant ribonucleotide reductase could have good potential as a diagnostic test for NPC or could serve as a complementary test to IFA.