Martina Vockerodt

Constitutive Expression of c-FLIP in Hodgkin and Reed-Sternberg Cells

Crosslinking of the transmembrane receptor CD95/Fas leads to activation of a signaling cascade resulting in apoptosis. c-FLIP is a recently described protein that potently inhibits Fas-mediated apoptosis and has been shown to be a key factor in germinal center B cell survival. Because Hodgkin and Reed-Sternberg cells in classical Hodgkins disease (cHD) are also resistant to Fas-mediated apoptosis we studied the role of c-FLIP in classical HD. High levels of c-FLIP protein were identified in two Fas-resistant Hodgkin-derived cell lines. In contrast to other tumor cells, inhibition of protein synthesis by cycloheximide did not lead to down-regulation of c-FLIP protein in these HD cell lines. Furthermore, Fas-mediated apoptosis was only partially restored suggesting that normal regulation of c-FLIP was disrupted. The in vivo relevance of these findings was supported by demonstration of significant c-FLIP expression by immunohistochemistry in 18 of 19 evaluable cases of primary HD. Taken together, c-FLIP is constitutively expressed in HD and may therefore be a major mechanism responsible for Fas-resistance in HD.

The H3K27me3 demethylase, KDM6B, is induced by Epstein–Barr virus and over-expressed in Hodgkin's Lymphoma

There is now evidence for both increased and decreased activity of the enzymes controlling the methylation of lysine 27 on histone 3 (H3K27) in cancer. One of these enzymes, KDM6B formally known as JMJD3, a histone demethylase, which removes the trimethyl mark from H3K27, is required for the lineage commitment and terminal differentiation of neural stem cells and of keratinocytes. Our results suggest that KDM6B may also have a role in antigen-driven B-cell differentiation. KDM6B expression increases in B-cell subsets with increasing stage of differentiation, and gene expression profiling shows that KDM6B transcriptional targets in germinal centre B (GC B) cells are significantly enriched for those differentially expressed during memory and plasma cell differentiation. Our results also suggest that aberrant expression of KDM6B may contribute to the pathogenesis of Hodgkins Lymphoma (HL), an Epstein–Barr virus (EBV) associated malignancy. KDM6B is over-expressed in primary HL and induced by the EBV oncogene, latent membrane protein (LMP1) in GC B cells, the presumptive progenitors of HL. Consistent with these observations, we found that KDM6B transcriptional targets in GC B cells are enriched for genes differentially expressed in HL, and that KDM6B depletion can restore the tri-methylation of H3K27 on these genes.

The Epstein-Barr virus oncoprotein, latent membrane protein-1, reprograms germinal centre B cells towards a Hodgkin's Reed-Sternberg-like phenotype

Although the latent membrane protein‐1 (LMP1) of the Epstein–Barr virus (EBV) is believed to be important for the transformation of germinal centre (GC) B cells, the precise contribution of this viral oncogene to lymphoma development is poorly understood. In this study, we used a non‐viral vector‐based method to express LMP1 in primary human GC B cells. Gene expression profiling revealed that LMP1 induced in GC B cells transcriptional changes characteristic of Hodgkins lymphoma cell lines. Strikingly, LMP1 down‐regulated the expression of B‐cell‐specific genes including B‐cell receptor components such as CD79A, CD79B, CD19, CD20, CD22, and BLNK. LMP1 also induced the expression of ID2, a negative regulator of B‐cell differentiation. Our data suggest that in EBV‐positive cases, LMP1 is likely to be a major contributor to the altered transcriptional pattern characteristic of Hodgkin/Reed–Sternberg cells, including the loss of B‐cell identity. Copyright

The Epstein–Barr virus and the pathogenesis of lymphoma

Since the discovery in 1964 of the Epstein–Barr virus (EBV) 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. Copyright

Down-regulation of BLIMP1α by the EBV oncogene, LMP-1, disrupts the plasma cell differentiation program and prevents viral replication in B cells: implications for the pathogenesis of EBV-associated B-cell lymphomas

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 BLIMP1α, a transcription factor that is required for plasma cell differentiation and that is inactivated in diffuse large B-cell lymphomas. We show that BLIMP1α 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 BLIMP1α by LMP-1 was accompanied by a partial disruption of the BLIMP1α 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 BLIMP1α 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 BLIMP1α, in turn preventing plasma cell differentiation and induction of the viral lytic cycle.

Epigenetic and Transcriptional Changes Which Follow Epstein-Barr Virus Infection of Germinal Center B Cells and Their Relevance to the Pathogenesis of Hodgkin's Lymphoma†

ABSTRACT 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.

The EBV oncogene LMP1 protects lymphoma cells from cell death through the collagen-mediated activation of DDR1

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.

The contribution of the Epstein-Barr virus to the pathogenesis of childhood lymphomas

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.

An unbalanced translocation involving chromosome 14 is the probable cause for loss of potentially functional rearranged immunoglobulin heavy chain genes in the Epstein-Barr virus-positive Hodgkin's lymphoma-derived cell line L591

Summary. In the vast majority of cases, Hodgkin–Reed Sternberg (H‐RS) cells, the malignant cells in Hodgkins lymphoma (HL), are derived from germinal centre B cells. In some cases, somatic mutations within the rearranged immunoglobulin heavy (IgH) chain genes were detected, rendering potentially functional gene rearrangements non‐functional. In these H‐RS cells the expression of high‐affinity B‐cell receptors (BCR) was prevented. As in other cases only one non‐productive IgH chain gene rearrangement was amplified from H‐RS cells, it was speculated whether, in these cases, the functionally rearranged IgH chain genes were lost. An alternative explanation might be that the rearranged genes could not be amplified owing to a high load of somatic mutations within the primer binding sites. Here, we showed that, in the HL‐derived Epstein–Barr virus (EBV)‐positive cell line L591, only one non‐functional somatically mutated IgH gene rearrangement could be detected. The other potentially functional IgH gene rearrangement was lost as a result of an unbalanced translocation affecting the long arm of chromosome 14. Moreover, L591 cells express the EBV latent membrane proteins LMP1 and LMP2A, which might have contributed to the ‘escape’ of these cells from apoptosis within the germinal centre. We conclude that, apart from the introduction of ‘crippling mutations’ into the rearranged VDJ genes rearrangement, deletions of the IGH locus may be regarded as another mechanism to prevent the expression of a BCR in H‐RS cells.

High myc activity is an independent negative prognostic factor for diffuse large B cell lymphomas

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.