Since the discovery in 1964 of the Epstein-Barr virus nEBV) in African Burkitt lymphoma, this virus has been associated with a remarkably diverse range of cancer types. Because EBV persists in the B cells of the asymptomatic host, it can easily be envisaged how it contributes to the development of B cell lymphomas. However, EBV is also found in other cancers, including T cell/natural killer cell lymphomas and several epithelial malignancies. Explaining the aetiological role of EBV is challenging, partly because the virus probably contributes differently to each tumour and partly because the available disease models cannot adequately recapitulate the subtle variations in the virus-host balance that exist between the different EBV-associated cancers. A further challenge is to identify the co-factors involved; because most persistently infected individuals will never develop an EBV-associated cancer, the virus cannot be working alone. This article will review what is known about the contribution of EBV to lymphoma development.
The relationship between Epstein-Barr virus (EBV) and the germinal centre (GC) of the asymptomatic host remains an enigma. The occasional appearance of EBV-positive germinal centres in some patients, particularly those with a history of immunosuppression, suggests that EBV numbers in the GC are subject to immune control. The relationship, if any, between lymphoid hyperplasia with EBV-positive germinal centres and subsequent or concurrent lymphomagenesis remains to be clarified. As far as the development of EBV-associated Hodgkin's lymphoma is concerned, the suppression of virus replication, mediated by LMP1 on the one hand, and the loss of B-cell receptor signalling on the other, appears to be an important pathogenic mechanism. A further important emerging concept is that alterations in the microenvironment of the EBV-infected B-cell may be important for lymphomagenesis.
The malignant Hodgkin and Reed-Sternberg (HRS) cells of Hodgkin lymphoma are surrounded by a tumor microenvironment that is composed of a variety of cell types, as well as noncellular components such as collagen. Although HRS cells harbor oncogenic Epstein-Barr virus (EBV) in approximately 50% of cases, it is not known if the tumor microenvironment contributes to EBV-driven lymphomagenesis. We show that expression of the EBV-encoded latent membrane protein-1 (LMP1) in primary human germinal center B cells, the presumed progenitors of HRS cells, upregulates discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase activated by collagen. We also show that HRS cells intimately associated with collagen frequently overexpress DDR1 and that short-term exposure to collagen is sufficient to activate DDR1 in Hodgkin lymphoma-derived cell lines. The ectopic expression of DDR1 significantly increased the survival of collagen-treated DG75 Burkitt lymphoma cells, following etoposide treatment. Conversely, knockdown of DDR1 significantly decreased the survival of collagen-treated L428 Hodgkin lymphoma cells in the absence of specific apoptotic stimulus, suggesting that DDR1 also influences baseline survival. Our results identify a hitherto unknown function for collagen in protecting Hodgkin lymphoma cells from apoptosis and suggest an important contribution of the tumor microenvironment in promoting the oncogenic effects of EBV.
Burkitts lymphoma (BL) is a highly malignant, aggressive non-Hodgkins lymphoma derived from germinal center B cells. Recently, global gene expression profiling of patient samples led to a molecular definition of BL with lymphocyte enhancer-binding factor 1 (LEF1) as a signature gene. Herein, we report the expression of nucleic LEF1 in 15 of 18 patients with BL and the identification of LEF1 target genes. Germinal center B cells were devoid of detectable nuclear LEF1 expression, as were mantle cell lymphoma (0 of 5), marginal zone lymphoma (0 of 6), follicular lymphoma (0 of 12), and diffuse large B-cell lymphoma (1 of 31). Whole-genome gene expression profiling after transient knockdown of LEF1 in BL cell lines identified new LEF1 target genes; these LEF1 targets are enriched with genes associated with cancers. The expression of LEF1 and LEF1-regulated genes in primary BL suggests that LEF1 is not only aberrantly expressed but also transcriptionally active. This study supports a functionally important role for LEF1 and its target genes in BLs.
Hodgkins lymphoma is unusual among B cell lymphomas, in so far as the malignant Hodgkin/Reed-Sternberg (HRS) cells lack a functional B cell receptor (BCR), as well as many of the required downstream signalling components. In Epstein-Barr virus (EBV)-positive cases of Hodgkins lymphoma, HRS cells express the viral latent membrane proteins (LMP)-1 and -2A. LMP2A is thought to contribute to the pathogenesis of Hodgkins lymphoma by providing a surrogate BCR-like survival signal. However, LMP2A has also been shown to induce the virus-replicative cycle in B cells, an event presumably incompatible with lymphomagenesis. In an attempt to resolve this apparent paradox, we compared the transcriptional changes observed in primary HRS cells with those induced by LMP2A and by BCR activation in primary human germinal centre (GC) B cells, the presumed progenitors of HRS cells. We found a subset of genes that were up-regulated by both LMP2A expression and BCR activation but which were down-regulated in primary HRS cells. These genes included EGR1, an immediate-early gene that is required for BCR-induced entry to the virus-replicative cycle. We present data supporting a model for the pathogenesis of EBV-positive Hodgkins lymphoma in which LMP2A-expressing HRS cells lacking BCR signalling functions cannot induce EGR1 and are consequently protected from entry to the virus lytic cycle. The primary microarray data are available from GEO (http://www.ncbi.nlm.nih.gov/geo/) under series Accession No 46143.
Epstein-Barr virus (EBV) is an oncogenic virus that is associated with the pathogenesis of several human lymphoid malignancies, including Hodgkins lymphoma. Infection of normal resting B cells with EBV results in activation to lymphoblasts that are phenotypically similar to those generated by physiological stimulation with CD40L plus IL-4. One important difference is that infection leads to the establishment of permanently growing lymphoblastoid cell lines, whereas CD40L/IL-4 blasts have finite proliferation lifespans. To identify early events which might later determine why EBV infected blasts go on to establish transformed cell lines, we performed global transcriptome analyses on resting B cells and on EBV and CD40L/IL-4 blasts after 7 days culture. As anticipated there was considerable overlap in the transcriptomes of the two types of lymphoblasts when compared to the original resting B cells, reflecting common changes associated with lymphocyte activation and proliferation. Of interest to us was a subset of 255 genes that were differentially expressed between EBV and CD40L/IL-4 blasts. Genes which were more highly expressed in EBV blasts were substantially and significantly enriched for a set of interferon-stimulated genes which on further in silico analyses were found to be repressed by IL-4 in other cell contexts and to be up-regulated in micro-dissected malignant cells from Hodgkins lymphoma biopsies when compared to their normal germinal center cell counterparts. We hypothesized that EBV and IL-4 were targeting and discordantly regulating a common set of genes. This was supported experimentally in our B cell model where IL-4 stimulation partially reversed transcriptional changes which follow EBV infection and it impaired the efficiency of EBV-induced B cell transformation. Taken together, these data suggest that the discordant regulation of interferon and IL-4 pathway genes by EBV that occurs early following infection of B cells has relevance to the development or maintenance of an EBV-associated malignancy.
Although Epstein-Barr virus (EBV) usually establishes an asymptomatic lifelong infection, it is also implicated in the development of germinal center (GC) B-cell-derived malignancies, including Hodgkins lymphoma (HL). Following primary infection, EBV remains latent in the memory B-cell population, where host-driven methylation of viral DNA contributes to the repression of viral gene expression. However, it is still unclear how EBV harnesses the cells methylation machinery in B cells, how this contributes to viral persistence, and what impact this has on the methylation of cellular genes. We show that EBV infection of GC B cells is followed by upregulation of the DNA methyltransferase DNMT3A and downregulation of DNMT3B and DNMT1. We show that the EBV latent membrane protein 1 (LMP1) oncogene downregulates DNMT1 and that DNMT3A binds to the viral promoter Wp. Genome-wide promoter arrays performed with these cells showed that EBV-associated methylation changes in cellular genes were not randomly distributed across the genome but clustered at chromosomal locations, consistent with an instructive pattern of methylation, and were in part determined by promoter CpG content. Both DNMT3B and DNMT1 were downregulated and DNMT3A was upregulated in HL cell lines, recapitulating the pattern of expression observed following EBV infection of GC B cells. We also found, by using gene expression profiling, that genes differentially expressed following EBV infection of GC B cells were significantly enriched for those reported to be differentially expressed in HL. These observations suggest that EBV-infected GC B cells are a useful model for studying virus-associated changes contributing to the pathogenesis of HL.
Gene expression profiling has recently enabled the reclassification of aggressive non-Hodgkin lymphomas (aNHL) into distinct subgroups. In Burkitt lymphoma (BL) aberrant c-Myc activity results from IG-MYC translocations. However, MYC aberrations are not limited to BLs and then have a negative prognostic impact. In this study, we investigated to which extent aberrant c-Myc activity plays a functional role in other aNHL and whether it is independent from MYC translocations. Based on a combined microarray analysis of human germinal center (GC) B cells transfected with c-Myc and 220 aNHLs cases, we developed a "c-Myc index." This index measures the extent to which lymphomas express c-Myc responsive genes. It comprises genes that are affected in a variety of tumors compared to normal tissue. This supports the view that aberrant c-Myc expression in GC B cells triggers a tumor-like expression pattern. As expected, the "c-Myc index" is very high in molecular Burkitt lymphoma (mBL), but more importantly also high within other aNHL. It constitutes a negative prognostic marker independent of established risk factors and of the presence of a MYC translocation. Our data provide new insights into the role of c-Myc activity in different lymphomas and raises the question of treatment changes for those patients under risk.
An important pathogenic event in Epstein-Barr virus (EBV)-associated lymphomas is the suppression of virus replication, which would otherwise lead to cell death. Because virus replication in B cells is intimately linked to their differentiation toward plasma cells, we asked whether the physiologic signals that drive normal B-cell differentiation are absent in EBV-transformed cells. We focused on BLIMP1?, a transcription factor that is required for plasma cell differentiation and that is inactivated in diffuse large B-cell lymphomas. We show that BLIMP1? expression is down-regulated after EBV infection of primary germinal center B cells and that the EBV oncogene, latent membrane protein-1 (LMP-1), is alone capable of inducing this down-regulation in these cells. Furthermore, the down-regulation of BLIMP1? by LMP-1 was accompanied by a partial disruption of the BLIMP1? transcriptional program, including the aberrant induction of MYC, the repression of which is required for terminal differentiation. Finally, we show that the ectopic expression of BLIMP1? in EBV-transformed cells can induce the viral lytic cycle. Our results suggest that LMP-1 expression in progenitor germinal center B cells could contribute to the pathogenesis of EBV-associated lymphomas by down-regulating BLIMP1?, in turn preventing plasma cell differentiation and induction of the viral lytic cycle.
The Epstein-Barr virus (EBV) is a lymphotropic herpes virus with oncogenetic properties which can lead to the development of lymphomas such as Burkitts lymphoma (BL), Hodgkins lymphoma (HL), or post-transplant lymphoma. This review discusses our current understanding of lymphomagenesis in relation to EBV and the potential for targeted therapies.
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