Use of antigen expressed in bacteria for detection of EBV-specific thymidine kinase antibodies in sera from patients with nasopharyngeal carcinoma

Interfering with cellular signaling pathways enhances sensitization to combined sodium butyrate and GCV treatment in EBV-positive tumor cells

The combination of sodium butyrate (NaB) and ganciclovir (GCV) was considered to be a noteworthy therapeutic strategy in Epstein-Barr virus (EBV)-associated cancers. However, clinical studies have indicated that an extremely high dose of NaB is required to obtain the expected curative efficacy. This obviously limits the practical clinical application of the two drugs combined. In this study, we investigated the possibility of sensitizing tumor cells to NaB and GCV mediated cytotoxicity by modulating intracellular signal pathways. The results showed that the disruption of Ras/Raf activity by expressing dominant negative forms of both Ras and Raf-1 did not alter the potency of the NaB and GCV combination in the EBV-positive cell line, B95-8. However, blocking Akt activity by expressing its dominant negative form remarkably promoted NaB and GCV-mediated cytotoxicity via a thymidine kinase (TK)-independent mechanism. Interestingly, it was found that the constitutive activation of mitogen-activated protein kinase kinase kinase 1 (MEKK1) dramatically enhanced the sensitization of the cells to the combination of NaB and GCV, accompanied with an increase in TK expression in B95-8 cells. These results suggest that interfering with either the Akt or MEKK1 signaling pathway may be a useful therapeutic strategy to increase the sensitivity of EBV-positive tumor cells to the combination of NaB and GCV.

Identification and characterization of the conserved nucleoside-binding sites in the Epstein-Barr virus thymidine kinase

Thymidine kinase (TK), encoded by EBV (Epstein-Barr virus), is an attractive target for antiviral therapy and provides a novel approach to the treatment of EBV-associated malignancies. Despite the extensive use of nucleoside analogues for the treatment of viral infections and cancer, the structure-function relationship of EBV TK has been addressed rarely. In the absence of any structural information, we sought to identify and elucidate the functional roles of amino acids in the nucleoside-binding site using site-directed mutagenesis. Through alignment with other human herpesviral TK protein sequences, we predicted that certain conserved regions comprise the nucleoside-binding site of EBV TK and, through site-directed mutagenesis, showed significant changes in activity and binding affinity for thymidine of site 3 (-DRH-) and 4 (-VFP-) mutants. For site 3, only mutants D392E (Asp392-->Glu) and R393H retain activity, indicating that a negative charge is important for Asp392 and a positive charge is required for Arg393. The increased binding affinities of these two mutants for 3'-deoxy-2',3'-didehydrothymidine suggest that the two residues are also important for substrate selection. Interestingly, the changed metal-ion usage pattern of D392E reveals that Asp392 plays multiple roles in this region. His394 cannot be compensated by other amino acids, also indicating a crucial role. In site 4, the F402Y mutant retains full activity; however, F402S retains only 60% relative activity. Strikingly, when Phe402 is substituted with serine residue, the original preferred pyrimidine substrates, such as 3'-azido-3'-deoxythymidine, iododeoxyuridine and beta-L-5-iododioxolane uracil (L-form substrate), have decreased competitiveness with thymidine, suggesting that Phe402 plays a crucial role in substrate specificity and that the aromatic ring is important for function.

Improved Accuracy of Detection of Nasopharyngeal Carcinoma by Combined Application of Circulating Epstein–Barr Virus DNA and Anti-Epstein–Barr Viral Capsid Antigen IgA Antibody

Background: Circulating Epstein–Barr viral (EBV) DNA and anti-EBV capsid antigen IgA (IgA VCA) represent two of the most sensitive peripheral blood markers of nasopharyngeal carcinoma (NPC), but direct comparative studies of these two markers are lacking.

Methods: The sensitivities and specificities of IgA-VCA and EBV DNA for diagnosis of NPC were determined in 139 new cases of NPC and 178 healthy individuals, respectively. EBV DNA was also assessed in 36 healthy family members identified as having false-positive IgA-VCA results at a screening clinic. EBV DNA was measured by a real-time quantitative PCR assay with a detection limit of 60 copies/mL. IgA-VCA was measured by semiquantitative indirect immunofluorescent method; a titer ≥1/10 was taken as positive.

Results: The sensitivities of EBV DNA and IgA-VCA for diagnosis of NPC were 95% (95% confidence interval, 91–98%) and 81% (73–87%), respectively. The combined marker panel had an overall sensitivity (positive result by either marker) of 99%. The concentrations of both markers showed dependence on cancer stage. The specificities of EBV DNA and IgA-VCA were 98% (96–99%) and 96% (91–98%), respectively. Among 36 healthy family members with false-positive IgA-VCA results, three-fourths had undetectable EBV DNA, whereas the others had increased EBV DNA concentrations that were significantly lower than in NPC patients.

Conclusions: For diagnosis of NPC, EBV DNA identifies almost all false-negative IgA-VCA cases and gives a 99% diagnostic sensitivity when combined with IgA-VCA. In the screening setting, EBV DNA identifies three-fourths of false-positive IgA-VCA cases. The selective application of EBV DNA in an IgA-VCA-based screening protocol could improve screening accuracy with only moderate increases in cost.

Epstein–Barr virus latent membrane protein 1 induces micronucleus formation, represses DNA repair and enhances sensitivity to DNA-damaging agents in human epithelial cells

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is a viral oncogene and it is essential for the transformation of resting B cells by the virus. The protein acts as a ligand-less membrane receptor and triggers numerous cellular signaling pathways. Cellular transformation frequently has been associated with genomic instability. To investigate whether EBV LMP1 induces chromosomal aberrations, micronucleus (MN) formation was examined in LMP1-expressing epithelial cells. The expression of wild-type LMP1 enhanced both spontaneous and bleomycin-induced MN formation. MN formation may be induced by inactivation of DNA repair and, therefore, we investigated the effect of LMP1 on DNA repair, using a host cell reactivation (HCR) assay. In the HCR assay, LMP1 reduced the capacity for DNA repair of both NPC-TW01 (p53-wild-type) and H1299 (p53-deficient) cells. As reduction of DNA repair by LMP1 occurs in p53-wild-type and p53-deficient cells, it seems that LMP1 can repress DNA repair in a p53-independent manner. Inactivation of DNA repair may render cells sensitive to DNA-damaging agents. In this study, H1299 cells harboring LMP1 were shown to be more sensitive to UV and bleomycin than those with a vector control. Using various deletion mutants of EBV LMP1 to determine the regions of LMP1 required to enhance MN formation, inhibit DNA repair and sensitize cells to DNA-damaging agents, we found that the region a. a. 189-222 (located within the CTAR1 domain) was responsible for sensitizing cells to UV and bleomycin, as well as for enhancing MN formation and repressing DNA repair. Based on these results, we suggest that disruption of DNA repair by LMP-1 results in an accumulation of unrepaired DNA and consequent genomic instability, which may contribute to the oncogenesis of LMP1 in human epithelial cells.

Anti-Epstein-Barr virus (EBV) activity of beta-L-5-iododioxolane uracil is dependent on EBV thymidine kinase

beta-L-5-Iododioxolane uracil was shown to have potent anti-Epstein-Barr virus (EBV) activity (50% effective concentration = 0.03 microM) with low cytotoxicity (50% cytotoxic concentration = 1,000 microM). It exerts its antiviral activity by suppressing replicative EBV DNA and viral protein synthesis. This compound is phosphorylated in cells where the EBV is replicating but not in cells where the EBV is latent. EBV-specific thymidine kinase could phosphorylate beta-L-5-iododioxolane uracil to the monophosphate metabolite. The K(m) of beta-L-5-iododioxolane uracil with EBV thymidine kinase was estimated to be 5.5 microM, which is similar to that obtained with thymidine but about fivefold higher than that obtained with 2' fluoro-5-methyl-beta-L-arabinofuranosyl uracil, the first L-nucleoside analogue discovered to have anti-EBV activity. The relative V(max) is seven times higher than that of thymidine. The anti-EBV activity of beta-L-5-iododioxolane uracil and its intracellular phosphorylation could be inhibited by 5'-ethynylthymidine, a potent EBV thymidine kinase inhibitor. The present study suggests that beta-L-5-iododioxolane uracil exerts its action after phosphorylation; therefore, EBV thymidine kinase is critical for the antiviral action of this drug.

Nasopharyngeal swab and PCR for the screening of nasopharyngeal carcinoma in the endemic area: a good supplement to the serologic screening

BACKGROUND

Nasopharyngeal carcinoma (NPC) is a common head and neck cancer in Taiwan. Early detection is the best way to improve survival for this disease. A prospective study was designed to assess the feasibility of a new screening method for NPC by use of polymerase chain reaction (PCR) based on the close association of NPC and Epstein-Barr virus (EBV).

METHODS

One hundred thirty-three different tissues from nasopharynx, nose, and sinus were investigated by use of PCR to check for the presence of EBV genome. Subsequently, from April 1996 to March 1997, 55 patients were enrolled in a prospective screening study. All patients underwent nasopharyngeal swabs before biopsy. Polymerase chain reaction detection of EBV genome was conducted on swab samples. Anti-EBV viral capsid antigen (VCA) in IgA and IgG class were checked at the same visit.

RESULTS

The EBV genome was present in 91.4% (85/93) of NPC tissues and in 25.0% (10/40) of non-NPC tissues (p < .001, chi2 test). Of the 55 swabs, 49 (89.1%) specimens obtained enough cells for PCR examination. Thirty of these 49 patients were pathologically proved NPC. The presence of EBV were 86.7% (26/30) in NPC group and 42.1% (8/19) in non-NPC group. The sensitivity and specificity were 86.7% and 57.9%, respectively, which were similar to those of serologic method (87.5% and 43.5%) when the cut-off point was set at anti-VCA IgG > or = 160 and IgA > or = 10. Combining both methods elevates the specificity to 84.2%.

CONCLUSIONS

The sensitivity of this PCR screening method is similar to that of the serologic method. Combining both methods can greatly increase the specificity, indicating this new method is a good supplement to the serologic screening of this endemic disease.

Purified recombinant EBV desoxyribonuclease in serological diagnosis of nasopharyngeal carcinoma

To evaluate applications of highly purified recombinant EBV DNAase in the diagnosis and prognosis of NPC, we tested sera from patients with NPC, other EBV-associated diseases and EBV-seropositive and -seronegative healthy subjects by immunoblotting and DNAase inhibitory assay. The results were compared with those obtained by the conventional immunofluorescence assays against the EBV-specified early antigens and capsid antigens. The antigenic specificity of the immunoblotting assay for IgG antibody against the viral enzyme, but not that for the IgA antibody, was correlated with DNAase-inhibitory activity of the sera and their titers of IgG antibodies against the viral early antigens. Purified IgA as well as IgG from NPC sera inhibited enzyme activity with similar efficiency. The use of highly purified viral DNase has increased the sensitivity of detection of the corresponding antibodies by immunoblotting, with the IgG antibody being detected in all but one, and IgA antibody in all but 2, of the 174 NPC sera tested. The IgG antibody was also commonly detected in the other groups of control sera, while the IgA antibody was detected in about 10% of African Burkitt's lymphoma and Algerian Hodgkin's lymphoma patients and less than 3% of the other control subjects. These results suggest that IgA antibody against recombinant EBV DNAase may be useful in the diagnosis of NPC, but the level of this antibody did not appear to be related to clinical stages of this cancer.

Expression of the Epstein-Barr virus DNA polymerase in Escherichia coli for use as antigen for the diagnosis of nasopharyngeal carcinoma

Epstein-Barr virus (EBV) encoded DNA polymerase (POL) was cloned and over-expressed in Escherichia coli. Western blot analysis confirmed the presence of antibody to this POL protein in sera from nasopharyngeal carcinoma (NPC) patients. By Western blot analysis, moderate to high concentration of IgG POL-specific antibodies were present in 43 of 48 NPC sera and only 4 of 48 healthy, seropositive controls. The POL-specific IgG antibodies appear as early as stage I of NPC, suggesting that the recombinant POL protein can be a useful diagnostic marker for early diagnosis of the disease. It was also found that human sera containing high titer of cytomegalovirus (CMV) antibodies or herpes simplex virus type 1 (HSV-1) antibodies did not cross-react with the recombinant EBV POL, despite the homology shared by DNA polymerase proteins of these viruses.

IgA directed against early antigen of Epstein-Barr virus is no specific marker for the diagnosis of nasopharyngeal carcinoma

The aim of this study was to evaluate the significance and specificity of IgA directed against Epstein-Barr virus (EBV)-specific early antigens (EA) for the unequivocal diagnosis of nasopharyngeal carcinoma (NPC). Therefore, sera from patients with diseases other than NPC, selected on the basis of elevated antibody titres against EBV antigens, were compared to sera from NPC patients with regard to the presence of IgA directed against EBV viral capsid antigen (VCA-IgA) and IgA directed against EA (EA-IgA). Four hundred forty-seven out of 7,508 non-NPC sera tested showed high titres (> 512) of IgG directed against Epstein Barr viral capsid antigen (VCA-IgG) and positive VCA-IgA (> or = 32). Two hundred twenty-seven of these sera were compared to 51 VCA-IgA-positive sera from NPC patients regarding the titre of EA-IgA. 60.7% of VCA-IgA-positive NPC sera showed positive EA-IgA, however 33% of VCA-IgA-positive non-NPC patients also exhibited EA-IgA. This result demonstrates that EA-IgA is not specific for NPC and does not allow an unequivocal serological diagnosis of NPC in individual cases. It seems therefore to be of questionable use for screening programs in NPC low-risk areas. The data do not contradict the usefulness of this marker for monitoring of patients treated for NPC and for screening programmes in high-risk areas.