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Virology |
Department of Pathology, Section Molecular Pathology [A. z. H., A. A. T. P. B, J. M. M., C. J. L. M. M., A. J. C. v. d. B.], and Department of Gastroenterology [M. E. C.], University Hospital Vrije Universiteit, 1007 MB Amsterdam, the Netherlands
| ABSTRACT |
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Recently, another EBV gene [i.e., BARF1 (BamHI A rightward open reading frame)] was shown to have transforming and immortalizing capacities. Therefore, in this study, we investigated the expression of BARF1 in EBV-carrying gastric adenocarcinomas in relation to the expression of other latent EBV transcripts.
In the present study, 10 of 132 gastric adenocarcinomas tested positive for EBV using EBER1/2-RNA in situ hybridization. We demonstrate BARF1 gene transcription in nine EBV-carrying gastric adenocarcinomas (with sufficient RNA quality) using the BARF1-specific nucleic acid sequence-based amplification assay. In addition, we also detected other latent EBV transcripts (i.e., BARF0-, LMP2A-, and Q/K-driven EBNA1 transcripts in these carcinomas using reverse transcription-PCR analysis. No expression of LMP1, EBNA2, and ZEBRA (either at transcription or protein level) was found. In addition, two cases were positive for BHRF1 transcripts, the viral bcl-2 homologue. Thus, together with BARF1 transcription, a unique and distinct EBV latency type has been found in EBV-associated gastric adenocarcinomas.
Because BARF1 exerts immortalizing effects on human epithelial cells in vitro and EBV-carrying gastric adenocarcinomas lack the expression of LMP1, the BARF1 gene might act as the viral oncogene in EBV-carrying gastric carcinomas. The BARF1 gene offers an alternative way for EBV-mediated oncogenesis other than LMP1.
| INTRODUCTION |
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-herpesvirus EBV is present in
10% (range, 216%) of human gastric adenocarcinomas worldwide
(1, 2, 3)
. Furthermore, EBV is associated with 80100% of
the rare lymphoepithelioma-like gastric carcinomas (4)
and
is also present in
35% of the gastric stump carcinomas
(5)
. The pathogenic role of EBV in gastric adenocarcinomas
remains still undefined. The latency type of EBV in gastric
adenocarcinomas is distinct from the known EBV latency types
(e.g., in Burkitts lymphomas and nasopharyngeal
carcinomas; Ref. 6
). This is mainly due to the expression
of
LMP2A3
and the absence of LMP1 expression in gastric adenocarcinomas.
EBV-carrying gastric adenocarcinomas generally do not express the major
EBV oncogene LMP1 (7)
, although LMP1 expression has been
occasionally reported (8
, 9)
. Apart from LMP1, another EBV gene (i.e., BARF1) has recently been determined as a transforming and immortalizing EBV gene (10 , 11) . The BARF1 open reading frame is located within a 40-kb fragment of the EBV genome and encodes a Mr 33,000 protein. This 40-kb fragment encompasses the BamHI D to BamHI A regions of the EBV genome and is able to immortalize primary monkey and human epithelial cells in vitro (12 , 13) . Wei et al. (11) recently demonstrated that BARF1 is involved in the immortalization of primary monkey epithelial kidney cells. Furthermore, it has been demonstrated that transfection of BARF1 into the rodent fibroblast cell line BALB/c 3T3 or in the EBV-negative B cell line Louckes resulted in tumorigenic transformation (10 , 14) . Injection of the transfected murine fibroblasts into newborn rats led to the development of aggressive BARF1-expressing tumors, whereas injection of the transfected Louckes cell line induced only small tumors that disappeared 3 weeks after injection.
Recently, Strockbine et al. (15) reported that BARF1 is a functional homologue of the human CSF receptor. The CSF receptor is the gene product of the human proto-oncogene c-fms. This homology between BARF1 and c-fms is especially interesting in the context that c-fms and CSF1 have been suggested to modulate neoplastic mammary epithelial cell proliferation (16) .
Because LMP1 is generally not expressed in EBV-carrying gastric adenocarcinomas, we studied here the expression of BARF1 as an alternative way for EBV-mediated oncogenesis in relation to the expression of other latent EBV genes.
| MATERIALS AND METHODS |
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The EBV-negative Louckes cell line, Louckes15, transfected with a BARF1 expression construct (14) , was kindly provided by Dr. T. Ooka (Laboratoire de Virologie Moléculaire, Centre National de la Recherche Scientifique, Lyon, France). The EBV-positive C15 tumor cell line derived from a nasopharyngeal carcinoma was kindly provided by Dr. B. Griffin (Imperial College School of Medicine, London, United Kingdom; Ref. 17 ).
Clinical Material.
Paraffin-embedded gastric adenocarcinomas (n = 132), of which also frozen material was available, collected at
the Department of Pathology of the University Hospital Vrije
Universiteit (Amsterdam, the Netherlands), were tested by EBER1/2-RISH
for the presence of EBV. Corresponding snap-frozen material of these
EBV-positive gastric carcinomas and 10 gastric control tissues,
including 5 EBV-negative gastric carcinomas and 5 specimens of normal
gastric epithelium, were used for the RNA EBV-transcript analysis.
Before RNA isolation, the sandwich frozen sections (of this material)
were H&E stained and microscopically checked for the presence of tumor
cells.
EBER1/2-RISH.
Paraffin-embedded tissue from 132 gastric carcinomas was subjected to a
nonradioactive EBER1/2-RISH using the Digoxygenin-labeled antisense and
sense EBER1/2 probe, as described previously (18)
.
Oligonucleotide Primers and Probes.
All EBV-specific primers (i.e., EBNA1,
EBNA2, BARF0, LMP1, LMP2A,
BHRF1, and ZEBRA; Ref. 19
) and
primers specific for the U1 small nuclear ribonucleoprotein-specific A
protein (20)
have been described previously.
The primers for the BARF1-NASBA assay were: primer 1.2, GGCTGTCACCGCTTTCTTGG (nt. 16556016579); and primer 2.1, T7-AGGTGTTGGCACTTCTGTGG (nt. 165762165743). As probe, the oligonucleotide CTGGTTTAAACTGGGCCCAGGAGAGGAGCA (nt. 165644165673) was used. A detailed protocol has been described recently (21) . NASBA primers were polyacrylamide purified to guarantee pure, full-length primers.
RNA Isolation and RT-PCR.
RNA was isolated from twelve 5-µm thick cryosections using 1 ml of
the guadinium-phenol-based RNAzol reagent (Cinna Biotecx, Houston, TX).
The purity and concentration of the isolated RNA were determined
spectrophotometrically; the integrity of the RNA was determined by
agarose gel electrophoresis, the presence of 18S/28S rRNA bands being
an index for good RNA quality. The isolated RNA was stored as
isopropanol precipitates at -80°C. Before the RT reaction, an amount
of the precipitate equivalent to 1 µg of RNA was centrifuged for 15
min, washed with 70% ethanol, and air dried. RT and subsequent PCR
were performed as described previously (22)
, and PCR
products were analyzed on 1.5% agarose gels, transferred to nylon
filters by alkaline Southern blotting, and hybridized to specific
32P-ATP-labeled oligonucleotide probes to
determine their specificity.
BARF1-NASBA.
The NASBA assay (23)
is an isothermal in vitro
amplification method with simultaneous activity of reverse
transcriptase, T7-RNA-polymerase, and RNase H, which enables a reliable
and sensitive detection of target RNA in the presence of DNA
independent of splice sites (21)
.
The BARF1-NASBA reaction was carried out as described previously (21) . Briefly, 100 ng of total RNA per reaction was amplified at 41°C in 20-µl reaction volumes containing 4 pmol of either primer, 15% DMSO, 40 mM TRIS-HCL (pH 8.5), 12 mM MgCl2, 70 mM KCl, 4 mM DTT, 1 mM of each dNTP, 2 mM rATP, rUTP, rCTP, 1.5 mM rGTP, and 0.5 mM ITP. Reagents were kindly supplied by Organon Teknika (Boxtel, the Netherlands).
Reaction products were evaluated by gel electrophoresis using 1.5%
agarose in Tris-borate EDTA, transferred from the gels to the nylon
filters (Qiabrane; Qiagen, Chatsworth, CA) via capillary blotting in
10* SSC, and hybridized to specific
32P-ATP end-labeled oligonucleotide probes.
The absolute sensitivity of the BARF1-NASBA assay was determined to detect 10100 RNA templates.
IHC.
To detect EBV-specific proteins, monoclonal antibodies against LMP1
[CS14 (DAKO) and S12 (Organon Teknika)] and ZEBRA (DAKO) were used.
The antibodies were visualized with an avidin-biotin-horseradish
peroxidase complex and
diaminobenzidine/H2O2
staining method, as described previously (24)
.
EBNA1 expression was detected in paraffin-embedded tissues with a recently generated anti-EBNA1-specific rat monoclonal antibody, 2B41 (25) . To increase sensitivity, a few adjustments were made. Before incubation with the anti-EBNA1 antibody, tissues were boiled for 15 min in a citrate buffer [0.1 M/l (pH 6.0)]. Incubation with the antibody was done overnight at room temperature with a 1:50 diluted antibody (final concentration, 44 µg/ml). Detection of the antibody was performed with an avidin-biotin-horseradish peroxidase complex. The peroxidase was visualized by incubation for 3 min in 0.2 mg/ml diaminobenzidine, 0.003% H2O2, and 0.12% NickelAmmoniumsulphate in 50 mmol/L Tris-HCl (pH 7.6), followed by silver enhancement of the diaminobenzidine-nickel precipitate, as described previously (26) .
| RESULTS |
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| DISCUSSION |
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Because gastric adenocarcinomas generally do not express LMP1until now the major EBV oncogeneBARF1 expression in EBV-carrying gastric carcinomas may be an alternative way for EBV-mediated gastric carcinogenesis. BARF1 has recently been determined as a transforming gene in rodent fibroblasts and as an immortalizing gene in primary monkey epithelial cells (10 , 11) . In this context, it is interesting that Strockbine et al. (15) recently demonstrated that the BARF1 gene encodes a novel CSF-1 receptor. BARF1 shares a subtle, highly localized region of homology with several members of the tyrosine kinase receptor family, including the cellular proto-oncogene c-fms, which encodes the CSF-1 receptor. CSF-1 and c-fms expression have been suggested to be involved in the modulation of neoplastic mammary epithelial cell proliferation (16) . According to Storga et al. (29) , c-fms is expressed in gastric adenocarcinomas, but the role of c-fms in gastric carcinogenesis has not been further elucidated. Theoretically, BARF1 might act as a homologue of c-fms proto-oncogene in immortalizing gastric epithelium, but additional studies concerning the role of c-fms and BARF1 in gastric carcinomas need to support this hypothesis. Only recently, Cohen and Lekstrom (30) demonstrated that BARF1 is dispensable for B-cell transformation and interacts with the cellular IFN production. However, the recombinant EBV mutant used by Cohen and Lekstrom (29) still contained the transforming domain of BARF1 (AA 154), which was recently determined (31) , and this might have influenced their results. As shown by the in vitro immortalizing and transforming capacities in epithelial cells (Ref. 11 ; and supported by our data in vivo), we suggest that BARF1 exerts different functions in lymphoid and epithelial cells: in the latter BARF1 might be involved in the lytic cycle, acting as an early protein, whereas in epithelial cells BARF1 has immortalizing/transforming capacities.
In this study, we found EBV in 7.6% of the gastric adenocarcinomas investigated. This is within the worldwide reported frequency of EBV-carrying gastric carcinomas, which is approximately 10% (1, 2, 3) .
In contrast to the absence of LMP1 and ZEBRA protein tested by IHC, four of nine EBV-carrying gastric carcinomas expressed EBNA1 protein using the 2B41 anti-EBNA1 antibody. One gastric carcinoma that did show EBNA1 protein expression tested negative in RT-PCR for EBNA1. This result might reflect nonspecific binding of this antibody, as has been described and discussed extensively recently by Cruz et al. (32) . The EBNA1 protein expression in EBV-carrying gastric adenocarcinomas has also been demonstrated previously by other groups using IHC (33 , 34) . The absence of LMP1 (either at the transcription and protein level) and the presence of BARF0 is in line with recently published data of Suguira et al. (6) . The absence of LMP1 in these carcinomas distinguishes this novel EBV latency type from the EBV latency type seen in NPCs, which is another EBV-associated epithelial malignancy. Interestingly, we also found BHRF1, the viral bcl-2 homologue, in two cases. The meaning of this remains to be determined.
In conclusion, in this study we showed that a novel EBV latency pattern in EBV-carrying gastric adenocarcinomas is present, especially characterized by BARF1 transcript expression and the absence of LMP1 (either at the RNA and protein level). BARF1 might act as the viral oncogene in the development of EBV-carrying gastric adenocarcinomas. Additional studies are needed, including the development of BARF1-specific antibodies and the application of morphological techniques like RISH and IHC. Functional assays with BARF1 are necessary to determine its role in (gastric) carcinogenesis. BARF1 might be a novel therapeutic target for EBV-carrying gastric adenocarcinomas.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by Grant VU-99-1990 from the Dutch
Cancer Society. ![]()
2 To whom requests for reprints should be
addressed, at Department of Pathology, Section Molecular Pathology,
University Hospital Vrije Universiteit, P.O. Box 7057, 1007 MB
Amsterdam, the Netherlands. Phone: 31-20-4440503/4023; Fax:
31-20-4442964; E-mail: vandenbrule{at}azvu.nl ![]()
3 The abbreviations used are: LMP2A, latent
membrane protein 2A; LMP1, LMP 1; NASBA, nucleic acid sequence-based
amplification; RT-PCR, reverse transcription PCR; RISH, RNA in
situ hybridization; BARF1, BamH1 A rightward
open reading frame; CSF, colony-stimulating factor; IHC,
immunohistochemistry; NPC, nasopharyngeal carcinoma. ![]()
Received 10/ 8/99. Accepted 3/21/00.
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