Targeted somatic mutation of the BCL6 proto-oncogene and its impact on lymphomagenesis

Follicular lymphoma

Follicular lymphoma (FL) is a systemic neoplasm of the lymphoid tissue displaying germinal centre (GC) B cell differentiation. FL represents ~5% of all haematological neoplasms and ~20-25% of all new non-Hodgkin lymphoma diagnoses in western countries. Tumorigenesis starts in precursor B cells and becomes full-blown tumour when the cells reach the GC maturation step. FL is preceded by an asymptomatic preclinical phase in which premalignant B cells carrying a t(14;18) chromosomal translocation accumulate additional genetic alterations, although not all of these cells progress to the tumour phase. FL is an indolent lymphoma with largely favourable outcomes, although a fraction of patients is at risk of disease progression and adverse outcomes. Outcomes for FL in the rituximab era are encouraging, with ~80% of patients having an overall survival of >10 years. Patients with relapsed FL have a wide range of treatment options, including several chemoimmunotherapy regimens, phosphoinositide 3-kinase inhibitors, and lenalidomide plus rituximab. Promising new treatment approaches include epigenetic therapeutics and immune approaches such as chimeric antigen receptor T cell therapy. The identification of patients at high risk who require alternative therapies to the current standard of care is a growing need that will help direct clinical trial research. This Primer discusses the epidemiology of FL, its molecular and cellular pathogenesis and its diagnosis, classification and treatment.

Inhibition of the transcriptional repressor complex Bcl-6/BCoR induces endothelial sprouting but does not promote tumor growth

The oncogenic potential of the transcriptional repressor Bcl-6 (B-cell lymphoma 6) was originally discovered in non-Hodgkin patients and the soluble Bcl-6 inhibitor 79-6 was developed to treat diffuse large B-cell lymphomas with aberrant Bcl-6 expression. Since we found Bcl-6 and its co-repressor BCoR (Bcl-6 interacting co-repressor) to be regulated in human microvascular endothelium by colorectal cancer cells, we investigated their function in sprouting angiogenesis which is central to tumor growth. Based on Bcl-6/BCoR gene silencing we found that the transcriptional repressor complex in fact constitutes an endogenous inhibitor of vascular sprouting by supporting the stalk cell phenotype: control of Notch target genes (HES1, HEY1, DLL4) and cell cycle regulators (cyclin A and B1). Thus, when endothelial cells were transiently transfected with Bcl-6 and/or BCoR siRNA, vascular sprouting was prominently induced. Comparably, when the soluble Bcl-6 inhibitor 79-6 was applied in the mouse retina model of physiological angiogenesis, endothelial sprouting and branching were significantly enhanced. To address the question whether clinical treatment with 79-6 might therefore have detrimental therapeutic effects by promoting tumor angiogenesis, mouse xenograft models of colorectal cancer and diffuse large B-cell lymphoma were tested. Despite a tendency to increased tumor vessel density, 79-6 therapy did not enhance tumor expansion. In contrast, growth of colorectal carcinomas was significantly reduced which is likely due to a combined 79-6 effect on cancer cells and tumor stroma. These findings may provide valuable information regarding the future clinical development of Bcl-6 inhibitors.

The survival decrease in gastric cancer is associated with the methylation of B-cell CLL/lymphoma 6 member B promoter

The methylation of B-cell CLL/lymphoma 6 member B (BCL6B) DNA promoter was detected in several malignancies. Here, we quantitatively detect the methylated status of CpG sites of BCL6B DNA promoter of 459 patients with gastric cancer (GC) by using bisulfite gene sequencing. We show that patients with three or more methylated CpG sites in the BCL6B promoter were significantly associated with poor survival. Furthermore, by using the Akaike information criterion value calculation, we show that the methylated count of BCL6B promoter was identified to be the optimal prognostic predictor of GC patients.

Fluorescence Immunophenotyping and Interphase Cytogenetics (FICTION) Detects BCL6 Abnormalities, Including Gene Amplification, in Most Cases of Nodular Lymphocyte-Predominant Hodgkin Lymphoma

Context.— BCL6 translocations are a frequent finding in B-cell lymphomas of diverse subtypes, including some cases of nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL). However, reliable analysis of BCL6 rearrangements using fluorescence in situ hybridization is difficult in NLPHL because of the relative paucity of neoplastic cells. Combined immunofluorescence microscopy and fluorescence in situ hybridization, or fluorescence immunophenotyping and interphase cytogenetics as a tool for the investigation of neoplasms (FICTION), permits targeted analysis of neoplastic cells.

Objective.—To better define the spectrum of BCL6 abnormalities in NLPHL using FICTION analysis.

Design.—We performed an optimized FICTION analysis of 24 lymph nodes, including 11 NLPHL, 5 follicular hyperplasia with prominent progressive transformation of germinal centers, and 8 follicular hyperplasia without progressive transformation of germinal centers.

Results.— BCL6 rearrangement was identified in 5 of 11 cases of NLPHL (46%). In addition, BCL6 gene amplification, with large clusters of BCL6 signals in the absence of chromosome 3 aneuploidy, was detected in 3 of 11 cases of NLPHL (27%). One NLPHL showed extra copies of BCL6 present in conjunction with multiple copies of chromosome 3. Altogether, we detected BCL6 abnormalities in 9 of 11 cases of NLPHL (82%). None of the progressive transformation of germinal centers or follicular hyperplasia cases showed BCL6 abnormalities by FICTION.

Conclusions.—To our knowledge, this is the first report of BCL6 gene amplification in NLPHL. Our optimized protocol for FICTION permits detection of cytogenetic abnormalities in most NLPHL cases and may represent a useful ancillary diagnostic technique.

Post-transplant lymphoproliferative disorders: role of viral infection, genetic lesions and antigen stimulation in the pathogenesis of the disease

Post-transplant lymphoproliferative disorders (PTLD) are a life-threatening complication of solid organ transplantation or, more rarely, hematopoietic stem cell transplantation. The majority of PTLD is of B-cell origin and associated with Epstein–Barr virus (EBV) infection. PTLD generally display involvement of extranodal sites, aggressive histology and aggressive clinical behavior. The molecular pathogenesis of PTLD involves infection by oncogenic viruses, namely EBV, as well as genetic or epigenetic alterations of several cellular genes. At variance with lymphoma arising in immunocompetent hosts, whose genome is relatively stable, a fraction of PTLD are characterized by microsatellite instability as a consequence of defects in the DNA mismatch repair mechanism. Apart from microsatellite instability, molecular alterations of cellular genes recognized in PTLD include alterations of cMYC, BCL6, TP53, DNA hypermethylation, and aberrant somatic hypermutation of protooncogenes. The occurrence of IGV mutations in the overwhelming majority of PTLD documents that malignant transformation targets germinal centre (GC) B-cells and their descendants both in EBV–positive and EBV–negative cases. Analysis of phenotypic markers of B-cell histogenesis, namely BCL6, MUM1 and CD138, allows further distinction of PTLD histogenetic categories. PTLD expressing the BCL6+/MUM1+/-/CD138− profile reflect B-cells actively experiencing the GC reaction, and comprise diffuse large B-cell lymphoma (DLBCL) centroblastic and Burkitt lymphoma. PTLD expressing the BCL6−/MUM1+/CD138− phenotype putatively derive from B-cells that have concluded the GC reaction, and comprise the majority of polymorphic PTLD and a fraction of DLBCL immunoblastic. A third group of PTLD is reminiscent of post-GC and preterminally differentiated B-cells that show the BCL6−/MUM1+/CD138+ phenotype, and are morphologically represented by either polymorphic PTLD or DLBCL immunoblastic.

B-cell receptor activation inhibits AID expression through calmodulin inhibition of E-proteins

Upon encountering antigens, B-lymphocytes can adapt to produce a highly specific and potent antibody response. Somatic hypermutation, which introduces point mutations in the variable regions of antibody genes, can increase the affinity for antigen, and antibody effector functions can be altered by class switch recombination (CSR), which changes the expressed constant region exons. Activation-induced cytidine deaminase (AID) is the mutagenic antibody diversification enzyme that is essential for both somatic hypermutation and CSR. The mutagenic AID enzyme has to be tightly controlled. Here, we show that engagement of the membrane-bound antibodies of the B-cell receptor (BCR), which signals that good antibody affinity has been reached, inhibits AID gene expression and that calcium (Ca(2+)) signaling is essential for this inhibition. Moreover, we show that overexpression of the Ca(2+) sensor protein calmodulin inhibits AID gene expression, and that the transcription factor E2A is required for regulation of the AID gene by the BCR. E2A mutated in the binding site for calmodulin, and thus showing calmodulin-resistant DNA binding, makes AID expression resistant to the inhibition through BCR activation. Thus, BCR activation inhibits AID gene expression through Ca(2+)/calmodulin inhibition of E2A.

A novel corepressor, BCoR-L1, represses transcription through an interaction with CtBP

Corepressors play a crucial role in negative gene regulation and are defective in several diseases. BCoR is a corepressor for the BCL6 repressor protein. Here we describe and functionally characterize BCoR-L1, a homolog of BCoR. When tethered to a heterologous promoter, BCoR-L1 is capable of strong repression. Like other corepressors, BCoR-L1 associates with histone deacetylase (HDAC) activity. Specifically, BCoR-L1 coprecipitates with the Class II HDACs, HDAC4, HDAC5, and HDAC7, suggesting that they are involved in its role as a transcriptional repressor. BCoR-L1 also interacts with the CtBP corepressor through a CtBP-interacting motif in its amino terminus. Abrogation of the CtBP binding site within BCoR-L1 partially relieves BCoR-L1-mediated transcriptional repression. Furthermore, BCoR-L1 is located on the E-cadherin promoter, a known CtBP-regulated promoter, and represses the E-cadherin promoter activity in a reporter assay. The inhibition of BCoR-L1 expression by RNA-mediated interference results in derepression of E-cadherin in cells that do not normally express E-cadherin, indicating that BCoR-L1 contributes to the repression of an authentic endogenous CtBP target.

The BCL6 proto-oncogene: a leading role during germinal center development and lymphomagenesis

The BCL6 proto-oncogene encodes a nuclear transcriptional repressor, with pivotal roles in germinal center (GC) formation and regulation of lymphocyte function, differentiation, and survival. BCL6 suppresses p53 in GCB-cells and its constitutive expression can protect B-cell lines from apoptosis induced by DNA damage. BCL6-mediated expression may allow GCB-cells to sustain the low levels of physiological DNA breaks related to somatic mutation (SM) and immunoglobulin class switch recombination which physiologically occur in GCB-cells. Three types of genetic events occur in the BCL6 locus and involve invariably the 5' non-coding region and include translocations, deletions and SM actively targeted to the 5' untranslated region. These acquired mutations occur independently of translocations but may be involved in the deregulation of the gene and/or translocation mechanisms. The favorable prognostic value of high levels of BCL6 gene expression in NHL seems well-established. By contrast, the relevance of SM or translocation of the gene remains unclear. However, it is likely that non-Hodgkin's lymphomas (NHL) harboring the most frequent translocation involving BCL6, i.e. t(3;14), are characterized by a common cell of origin and similar oncogenic mechanisms. Several experiments and mouse models mimicking BCL6 translocation occurring in human lymphoma have demonstrated the oncogenic role of BCL6 and constitute a rational to consider BCL6 as a new therapeutic target in NHL. BCL6 blockade can be achieved by different strategies which include siRNA, interference by specific peptides or regulation of BCL6 acetylation by pharmacological agents such as SAHA or niacinamide and would be applicable to most type of B-cell NHL.

Genomic Alterations in Hodgkin's Lymphoma

Cytogenetic analysis of Hodgkin's lymphoma (HL) is hampered by the scarcity of neoplastic cells within a sea of reactive cells. There is accumulating evidence that HL represents 2 disease entities, classic HL (cHL) with its morphologic variants and nodular lymphocyte predominant HL (NLPHL). This subdivision, initially worked out in morphologic and immunohistochemical studies, has been further substantiated by molecular cytogenetic investigations. Two recurrent chromosomal aberrations, namely gains of 2p13-p16 and 9p24, have been found by comparative genomic hybridization analysis in microdissected cells from cHL patients as well as in cHL cell lines, but not in NLPHL cells. The available cHL cell lines are remarkably heterogeneous in their karyotypes, suggesting profound genomic instability leading to numeric chromosomal aberration and multiple chromosomal breaks and translocations. In this article, we review genomic aberrations that may contribute to the development and maintenance of the morphologic and clinical presentation of these beta-cell lymphoma entities. Furthermore, we delineate current data on the genomic changes observed in the neoplastic cells of HL that are created by epigenetic mechanisms, which are alternative mechanisms that regulate the expression of relevant genes.