Significance of rearrangement of the BCL6 gene in B-cell lymphoid neoplasms

Enhancing B-Cell Malignancies-On Repurposing Enhancer Activity towards Cancer

B-cell lymphomas and leukemias derive from B cells at various stages of maturation and are the 6th most common cancer-related cause of death. While the role of several oncogenes and tumor suppressors in the pathogenesis of B-cell neoplasms was established, recent research indicated the involvement of non-coding, regulatory sequences. Enhancers are DNA elements controlling gene expression in a cell type- and developmental stage-specific manner. They ensure proper differentiation and maturation of B cells, resulting in production of high affinity antibodies. However, the activity of enhancers can be redirected, setting B cells on the path towards cancer. In this review we discuss different mechanisms through which enhancers are exploited in malignant B cells, from the well-studied translocations juxtaposing oncogenes to immunoglobulin loci, through enhancer dysregulation by sequence variants and mutations, to enhancer hijacking by viruses. We also highlight the potential of therapeutic targeting of enhancers as a direction for future investigation.

C-MYC, BCL2 and BCL6 Translocation in B-cell Non-Hodgkin Lymphoma Cases

C-MYC, BCL2 and BCL6 genes are the most commonly oncogenes involved in B-Cell lymphomas. Translocations of these oncogenes are associated with an aggressive clinical course. This study aims to elucidate the patterns of BCL6, BCL2 and C-MYC gene aberrations among Malaysian B-cell Non-Hodgkin Lymphoma (NHL) using fluorescence in situ hybridization (FISH). Eighty-one B-cell NHL tissue blocks were retrieved between the year 2011 to 2015 and investigated using immunohistochemistry and interphase FISH dual colour break-apart probes of BCL2, BCL6, C-MYC and IgH. A significant difference was detected between the nodal and extranodal sites in all the BCL2 (p=0.01), C-MYC (p=0.03) and IgH (p=0.006) cases except for BCL6 (p=0.2). Our study showed that BCL6 had the highest gene translocation while BCL2/BCL6 had the most mixed aberrations of gain copies and translocation, however no mixed aberrations of gain copies and translocation was found in C-MYC. None of the mixed gain copies and translocation was found in any of the germinal centre B-cell (GCB) subtype of Diffuse Large B-cell Lymphoma, however, five were found in BCL6 and IgH gene in the non-GCB subtype; while mixed gain copies and translocation cases of BCL2 gene was found in the Follicular Lymphoma cases only. The study found interesting findings of BCL2, C-MYC and IgH gene aberrations between nodal and extranodal sites. This information might benefit future study in predicting prognosis and determine effective therapeutic strategies in the multi-ethnic populations of Malaysia as well as the Asian population.

Genome-wide Regulatory Roles of the C2H2-type Zinc Finger Protein ZNF764 on the Glucocorticoid Receptor

The C2H2-type zinc finger protein ZNF764 acts as an enhancer for several steroid hormone receptors, and haploinsufficiency of this gene may be responsible for tissue resistance to multiple steroid hormones including glucocorticoids observed in a patient with 16p11.2 microdeletion. We examined genome-wide regulatory actions of ZNF764 on the glucocorticoid receptor (GR) in HeLa cells as a model system. ZNF764- and GR-binding sites demonstrated similar distribution in various genomic features. They positioned predominantly around 50–500 kbs from the transcription start sites of their nearby genes, and were closely localized with each other, overlapping in ~37% of them. ZNF764 demonstrated differential on/off effects on GR-binding and subsequent mRNA expression: some genes were highly dependent on the presence/absence of ZNF764, but others were not. Pathway analysis revealed that these 3 gene groups were involved in distinct cellular activities. ZNF764 physically interacted with GR at ligand-binding domain through its KRAB domain, and both its physical interaction to GR and zinc finger domain appear to be required for ZNF764 to regulate GR transcriptional activity. Thus, ZNF764 is a cofactor directing GR transcriptional activity toward specific biologic pathways by changing GR binding and transcriptional activity on the glucocorticoid-responsive genes.

Diffuse large B-cell lymphoma, not otherwise specified presenting with bone and bone marrow involvement in the absence of lymphadenopathy

A 74-year-old woman visited our hospital because of right chest pain and fatigue. Laboratory examinations revealed pancytopenia and an elevated level of serum lactate dehydrogenase. Although bone lesions were detected by computed tomography, there was no lymphadenopathy. Blastoid cells were evident in the bone marrow. From the patient's medical history and results of immunohistological and chromosomal analysis, she was diagnosed as having diffuse large B-cell lymphoma, not otherwise specified. This form of presentation of diffuse large B-cell lymphoma is very rare, and emphasizes the need for careful evaluation of such cases, including bone marrow biopsy for accurate diagnosis.

Comprehensive Assessment and Classification of High-Grade B-cell Lymphomas

High-grade B-cell lymphomas (HGBCLs) are a heterogeneous group of neoplasms that include subsets of diffuse large B-cell lymphoma, Burkitt lymphoma, and lymphomas with features intermediate between diffuse large B-cell lymphoma and Burkitt lymphoma. Morphologically indistinguishable HGBCLs may demonstrate variable clinical courses and responses to therapy. The morphologic evaluation and classification of these neoplasms must be followed by further genetic and immunophenotypic work-up. These additional diagnostic modalities lead to a comprehensive stratification of HGBCL that determines the prognosis and optimal therapy. This article reviews the well-established and emerging biomarkers that are most relevant to the clinical management of HGBCL.

Large B-cell lymphoma of the base of the tongue and oral cavity: a practical approach to identifying prognostically important subtypes

OBJECTIVE

The aim of this study is to illustrate a practical approach to the identification of prognostically important subtypes of large B-cell lymphomas.

STUDY DESIGN

Twenty-six cases of large B-cell lymphoma in the base of the tongue and oral cavity were retrieved from 2003 through 2012. All cases were classified based on the 2008 World Health Organization criteria. Hematoxylin-eosin-stained sections, immunohistochemical stains, flow cytometric immunophenotypic data, and fluorescence in situ hybridization studies were performed and evaluated.

RESULTS

Four different subtypes of large B-cell lymphoma were identified: pleomorphic variant mantle cell lymphoma, Epstein-Barr virus-positive diffuse large B-cell lymphoma of the elderly, diffuse large B-cell lymphoma not otherwise specified, and B-cell lymphoma unclassifiable with features intermediate between diffuse large B-cell lymphoma and Burkitt lymphoma, including a case of double-hit lymphoma.

CONCLUSIONS

Diverse subtypes of large B-cell lymphoma were identified in the base of tongue and oral cavity, and illustrate a practical approach to recognizing prognostically important lymphoma subtypes.

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.

BACH2-BCL6 balance regulates selection at the pre-B cell receptor checkpoint

At the pre-B cell receptor (BCR) checkpoint, developing pre-B cells are selected for successful rearrangement of V(H)-DJ(H) gene segments and expression of a pre-BCR. Reduced stringency at this checkpoint may obstruct the B cell repertoire with nonfunctional B cell clones. Earlier studies have described that activation of B cell lymphoma/leukemia (BCL)6 by a functional pre-BCR mediates positive selection of pre-B cells that have passed the checkpoint. This concept is now further elaborated by the recent finding that the BTB and CNC homology 1 basic leucine zipper transcription factor 2 (BACH2) induces negative selection and opposes BCL6 function prior to the pre-BCR checkpoint. Here, we discuss the antagonism between BCL6 and BACH2 during early B cell development, as well as its implications in both repertoire selection and counter-selection of premalignant clones for leukemia suppression.

Long non-coding RNAs: a new frontier in the study of human diseases

With the development of whole genome and transcriptome sequencing technologies, long noncoding RNAs (lncRNAs) have received increased attention. Multiple studies indicate that lncRNAs act not only as the intermediary between DNA and protein but also as important protagonists of cellular functions. LncRNAs can regulate gene expression in many ways, including chromosome remodeling, transcription and post-transcriptional processing. Moreover, the dysregulation of lncRNAs has increasingly been linked to many human diseases, especially in cancers. Here, we reviewed the rapidly advancing field of lncRNAs and described the relationship between the dysregulation of lncRNAs and human diseases, highlighting the specific roles of lncRNAs in human diseases.

A novel t(2;3)(p11;q27) in a case of follicular lymphoma

Rearrangement of the BCL6 gene is found in follicular lymphomas and in diffuse large B cell lymphomas of follicular center cell origin. The breakpoints cluster mainly in a region spanning the first noncoding exon of the gene (the major breakpoint region). A second breakpoint cluster has also been identified upstream of the first BCL6 noncoding exon (the alternative breakpoint region [ABR]). To date, eight different rearrangements involving the ABR have been reported. Here, we describe a novel rearrangement involving a t(2;3)(p11;q27) translocation that affects the ABR in an unusual combination with the IGK locus.

Chromosomal Translocations Fusing theBCL6Gene to Different Partner Loci Are Recurrent in Primary Central Nervous System Lymphoma and May Be Associated With Aberrant Somatic Hypermutation or Defective Class Switch Recombination

Primary central nervous system lymphomas (PCNSLs) are diffuse large B cell lymphomas confined to the brain. Only minimal data exist on chromosomal aberrations underlying PCNSLs. We studied 41 PCNSLs by fluorescence in situ hybridization for breakpoints affecting the BCL6 locus in chromosomal band 3q27. Of 37 cases evaluable, 14 (38%) carried a breakpoint in the BCL6 locus. Two of these showed juxtaposition of BCL6 to the IGH locus. In 4 cases, the BCL6 breakpoints were cloned using long-distance inverse polymerase chain reaction. All breakpoints were located within the BCL6 major translocation cluster. The translocation partners were the IGH gene in 14q32.33, the IGL gene in 22q11.22, and the histone 1 H4I gene in 6p22.1. In the fourth case, a deletion in 3q leads to loss of an 837-kb fragment extending from the first intron of BCL6 to the third intron of the lipoma-preferred partner (LPP) gene. This deletion may bring the BCL6 gene under the control of regulatory elements of the LPP gene or the miRNA-28 gene located in intron 4 of LPP. DNA sequence analysis of the junctional sequences provided evidence that aberrant class switch recombination or somatic hypermutation may be involved in the generation of BCL6 translocations.

Pathogenetic and Clinical Implications of Non-Immunoglobulin; BCL6 Translocations in B-Cell Non-Hodgkin's Lymphoma

Chromosomal translocations affecting band 3q27, where BCL6 gene is located, are among the most common genetic abnormalities in non-Hodgkin's lymphoma of B-cell type (B-NHL). The BCL6 gene encodes a BTB/POZ zinc finger transcription factor, which exerts repressive activity by recruiting corepressor molecules. The 3q27/BCL6 translocation is unique in that it can involve not only immunoglobulin (Ig) genes but also non-Ig chromosomal loci as a partner. To date, around 20 non-Ig partner genes have been identified. As a result of non-Ig ; BCL6 translocations, many types of regulatory sequences of each partner gene substitute for the 5' untranslated region of BCL6, and the rearranged BCL6 comes under the control of the replaced promoter. The introduction of non-Ig ; BCL6 constructs into transformed cells led to high-level Bcl-6 protein expression in the nucleus, while BCL6 mRNA levels in clinical materials of diffuse large B-cell lymphoma (DLBCL) with non-Ig ; BCL6 translocations were unexpectedly low. A comparative study suggested that non-Ig ; BCL6 translocation and a low level of BCL6 mRNA expression are concordant indicators of a poor clinical outcome in cases of DLBCL. The coexistence of a non-Ig ; BCL6 translocation with t(14 ; 18)(q32 ; q21) in a single clone did not significantly affect the clinical features of follicular lymphoma. The pathogenetic and clinical implications of non-Ig ; BCL6 translocations in B-NHL subtypes may not be identical to those of Ig ; BCL6.

Large Cleaved and Immunoblastic Lymphoma May Represent Two Distinct Clinicopathologic Entities Within the Group of Diffuse Large B-Cell Lymphomas

Purpose

The reliability of immunohistochemistry for subdividing diffuse large B-cell lymphomas (DLBCL) into germinal center B-cell-like (GCB) and non-GCB prognostic subgroups is debated. In this study we evaluated the prognostic significance of such subgrouping on a series of 153 DLBCL patients. Furthermore, we investigated whether both subgroups could comprise clinicopathologic entities recognized by their morphology and characterized by a distinct phenotype, specific genetic abnormalities, and clinical characteristics.

Patients and Methods

All samples from patients were reviewed and morphologically subdivided into large cleaved, immunoblastic, and not otherwise specified DLBCL. GCB and non-GCB immunohistochemical profiles were established. The presence of chromosomal translocations involving BCL2, BCL6, and MYC and/or rearrangements of these genes was investigated.

Results

Subdividing DLBCL with either a GCB or non-GCB immunophenotypic profile was not of prognostic significance. Nevertheless, CD10 expression was a predictor of favorable outcome, whereas high bcl-2 expression and BCL6 rearrangement were adverse predictors of disease-free survival. Interestingly, large cleaved DLBCL was clearly associated with a GCB immunophenotypic profile, CD10 expression, BCL2 rearrangement, age younger than 60 years, and low to low/intermediate International Prognostic Index risk, but was not of prognostic significance. In contrast, immunoblastic morphology was associated with a non-GCB profile and was a significant predictor of unfavorable DFS.

Conclusion

Subdividing DLBCL into subgroups based on their immunohistochemical profile was not of prognostic significance. Nevertheless, it allowed the additional characterization of two lymphoma subgroups previously recognized in the Working Formulation. Both correspond to two distinct clinicopathologic entities within the DLBCL.

Cytogenetic features of de novo CD5-positive diffuse large B-cell lymphoma: Chromosome aberrations affecting 8p21 and 11q13 constitute major subgroups with different overall survival

De novo CD5-positive diffuse large B-cell lymphoma (CD5(+)DLBCL) is regarded as a different clinicopathological entity from CD5-negative DLBCL (CD5(-)DLBCL) and mantle cell lymphoma (MCL). Because only a few published cytogenetic studies of de novo CD5(+)DLBCL are available, we investigated chromosomal changes in 23 Japanese patients who had de novo CD5(+)DLBCL. A characteristic of cytogenetic abnormalities in de novo CD5(+)DLBCL was a high incidence of chromosomal aberrations affecting 8p21 and 11q13. Major chromosomal breakpoints were concentrated at 8p21, 11q13, and 3q27. Patients with 8p21 aberrations showed aggressive clinical features, including advanced stage of disease, elevated serum LDH level, poor performance status, and an inferior survival curve compared with patients who had 11q13 changes (P = .043). Chromosomal abnormalities of both 8p21 and 11q13 were not observed in the same patient, and each abnormality showed different chromosomal gains and losses. These results indicate that de novo CD5(+)DLBCL may occur in previously unidentified subgroups that differ in their chromosomal abnormalities. The conflicting results of previous studies on prognosis may thus be explained in part by the differences in chromosomal changes.

BCL6 gene translocation in follicular lymphoma: a harbinger of eventual transformation to diffuse aggressive lymphoma

Follicular lymphoma (FL) is characterized by a relatively indolent clinical course, but the disease often transforms into a more aggressive large cell lymphoma with a rapidly progressive clinical course. In the present study, we analyzed 41 cases of FL known to have subsequently transformed to aggressive lymphoma and an additional 64 FL samples from patients not subsequently transformed. We studied BCL6 gene rearrangement by the methodology of long-distance inverse polymerase chain reaction (LDI-PCR). Of the 41 cases known to transform, 16 (39.0%) harbored BCL6 translocation or deletion at the time of FL diagnosis. Among 64 cases not known to transform, BCL6 translocation was detected in 9 (14.1%). The prevalence of BCL6 translocation in the group known to transform was significantly higher (P =.0048). Among the transformation cases, the partners of the BCL6 translocation were identified in 13 cases and included IGH, CIITA, U50HG, MBNL, GRHPR, LRMP, EIF4A2, RhoH/TTF, and LOC92656 (similar to NAPA), whereas in the control group the BCL6 partners were IGH, CIITA, SIAT1, and MBNL. In 13 cases paired specimens before and after transformation were available. Among these paired specimens, a loss (3 cases) or a gain (1 case) of BCL6 translocation was observed after the transformation. Analysis of clonality showed that all of these cases represented the evolution of a subclone of the original tumor population. Our study demonstrated that BCL6 translocation is not necessary for transformation but that BCL6 translocation in FL may constitute a subgroup with a higher risk to transform into aggressive lymphoma.

BCL6 overexpression prevents increase in reactive oxygen species and inhibits apoptosis induced by chemotherapeutic reagents in B-cell lymphoma cells

Chromosomal translocations and somatic mutations occurring in the 5' noncoding region of the BCL6 gene, encoding a transcriptional repressor, are most frequent genetic abnormalities associated with non-Hodgkin B-cell lymphoma and result in deregulated expression of BCL6. However, the significance of deregulated expression of BCL6 in lymphomagenesis and its effect on clinical outcomes of lymphoma patients have remained elusive. In the present study, we established Daudi and Raji B-cell lymphoma cell lines that overexpress BCL6 or its mutant, BCL6-Ala333/343, in which serine residues required for degradation through the proteasome pathway in B-cell receptor-stimulated cells are mutated. BCL6 overexpression did not have any significant effect on cell proliferation, but significantly inhibited apoptosis caused by etoposide, which induced a proteasome-dependent degradation of BCL6. BCL6-Ala333/343 was not degraded after etoposide treatment and strongly inhibited apoptosis. In these lymphoma cell lines, etoposide increased the generation of reactive oxygen species (ROS) and reduced mitochondria membrane potential, both of which were inhibited by the antioxidant N-acetyl-L-cysteine (NAC). NAC also inhibited apoptosis. Furthermore, BCL6 overexpression was found to inhibit the increase in ROS levels and apoptosis in response to etoposide and other chemotherapeutic reagents. These results raise the possibility that deregulated expression of BCL6 may endow lymphoma cells with resistance to chemotherapeutic reagents, most likely by enhancing the antioxidant defense systems.

The BCL6 gene in B-cell lymphomas with 3q27 translocations is expressed mainly from the rearranged allele irrespective of the partner gene

The BCL6 gene, which functions as a transcription repressor, is the target of multiple chromosomal translocations in non-Hodgkin's lymphomas (NHL). These translocations occur in the nontranslated region of the BCL6 gene, juxtaposing regulatory sequences of the diverse partner genes to the open reading frame of the BCL6 gene and thus are thought to deregulate BCL6 gene expression. The levels of expression of the BCL6 gene and protein have been demonstrated to predict the clinical outcome of diffuse large B-cell lymphomas. By contrast, the prognostic significance of BCL6 gene translocations is unclear. In this study we have sought an explanation for this apparent discrepancy. We examined tumors with a variety of different BCL6 translocations and therefore with a variety of potentially substituted promoters. We found no increase in total BCL6 mRNA levels in the NHL specimens harboring BCL6 gene translocation. Indeed, some of these tumors expressed relatively low quantities of the BCL6 mRNA. We also sought to determine whether BCL6 transcription occurs from the rearranged or from the normal untranslocated allele in these tumors. We demonstrate that lymphoma cell lines and majority of NHL tumor specimens expressed BCL6 mRNA predominantly from the rearranged allele that may come under the control of various partner gene promoters. However, few NHL tumors with BCL6 gene translocations expressed BCL6 mRNA equally from the rearranged and the nonrearranged alleles. Neither the nature of the substituted promoters nor the presence of activating mutations in the BCL6 regulatory sequences correlated with the allelic expression of the BCL6 gene in these tumors.

Increased expression of the bcl6 and CD10 proteins is associated with increased apoptosis and proliferation in diffuse large B-cell lymphomas

There is increasing evidence that bcl6 and CD10 expression may be related to apoptosis and cell cycle progression. Therefore, 79 cases of de novo diffuse large B-cell lymphomas were studied for the expression of bcl6 and CD10 proteins in relation to 1) the apoptotic index; 2) the proliferation-associated proteins Ki67, cyclin A, and cyclin B1; and 3) the expression of the bcl2, p53, Rb, p16, and p27 proteins. Expression of bcl6, CD10, and bcl2 proteins was found in 54/79 (68%), 28/79 (35%), and 47/74 (63%) cases, respectively. The bcl6/CD10 patterns were as follows: bcl6+/CD10+ (26 cases, 32%), bcl6+/CD10- (28 cases, 33%), bcl6-/CD10- (23 cases, 31%), and bcl6-/CD10+ (2 cases, 4%). Significant positive correlations were found between bcl6/Ki67 (r =.328, P =.003), bcl6/cyclin A (r =.265, P =.018), bcl6/apoptotic index (r =.327, P =.010), CD10/Ki67 (r =.296, P =.008), and CD10/apoptotic index (r =.397, P =.001). In addition, high expression of bcl6 showed significant correlation with negative (null/low) bcl2 expression (chi(2) test, P =.002). The above findings indicate that increased expression of the bcl6 and CD10 proteins is associated with increased apoptosis and proliferation in diffuse large B-cell lymphomas. The association between increased bcl6 expression and enhanced apoptosis might be due, at least in part, to the null/low bcl2 expression because previous in vitro data showed that bcl6 overexpression induces apoptosis accompanied by bcl2 and bcl-xl downregulation. Moreover, significant correlation was found between increased apoptotic index and the bcl6+/CD10+ pattern (t test: P =.014, Mann-Whitney test: P =.046). This finding and the positive correlation of the apoptotic index with bcl6 and CD10 expression may be related to previous results showing that the expression of these proteins has favorable effects on the clinical outcome of diffuse large B-cell lymphomas.

Follicular lymphoma grade 3B includes 3 cytogenetically defined subgroups with primary t(14;18), 3q27, or other translocations: t(14;18) and 3q27 are mutually exclusive

Chromosomal translocations involving t(14;18)(q32;q21) and the chromosome 3q27 region are common in B-cell non-Hodgkin lymphoma of germinal center cell origin. Grade 3B follicular lymphoma (FL), consisting almost exclusively of centroblasts, is a distinct subgroup of follicular lymphomas that has more in common clinically with the aggressive diffuse large B-cell lymphomas than with their indolent FL grade 1 and 2 counterparts. We studied the cytogenetic and molecular genetic aberrations by classic cytogenetics, polymerase chain reaction, Southern blot hybridization, and fluorescence in situ hybridization, with special emphasis on t(14;18), affecting bcl-2, and 3q27 rearrangement, affecting bcl-6, in 32 cases of FL grade 3B. Three distinctive subgroups were identified based upon the existence of breakpoint 3q27, a translocation t(14;18), or the absence of both. Group I involved a t(14;18) and no 3q27 aberrations (n = 13); group II was without a t(14;18) and without 3q27 aberrations (n = 9), but had other cytogenetic aberrations; and group III was without a t(14;18) but with aberrations involving 3q27 (n = 10). None of the FL grade 3B cases harbored both a t(14;18) and 3q27 aberration. These results, in particular the finding of a mutual exclusiveness of bcl-2 and bcl-6 rearrangement, indicate at least 3 different pathways of oncogenesis in FL grade 3B. FL grade 3B with bcl-2 rearrangement probably is part of the same entity as the other follicular lymphomas (1, 2, 3A), whereas the cases with 3q27 abnormalities or other unrelated translocations are more closely related to the majority of diffuse large-cell lymphomas of germinal center cell origin.

Ambivalent role of BCL6 in cell survival and transformation

The BCL6 gene is often structurally altered and probably 'misregulated' in two different types of human B-cell non-Hodgkin lymphomas (BNHL) thought to arise from germinal centre B cells. BCL6 encodes a BTB/POZ and zinc finger protein whose biochemical properties support a role as a DNA-binding transcriptional repressor and disclose, in part, the underlying mechanisms. In contrast, the study of the 'oncogenic' structural alterations of BCL6 in BNHL and of its cellular functions gives rise to much more heterogeneous data with no obvious unifying picture so that how and even whether BCL6 contributes to lymphomagenesis remains unclear. This review will summarize the current knowledge about the 'oncogenic' alterations and cellular functions of BCL6 and, based on some results, will propose the following hypotheses: (1) In various systems, including in memory T cells and also in germinal centre B cells and possibly in certain postmitotic cells, BCL6 may act by stabilizing a particular stage of differentiation. (2) Both its ambivalent effects on cell survival and the heterogeneous consequences of its alterations in BNHL suggest that BCL6 can be oncogenic not only upon overexpression or persistent expression, as often proposed, but also, similar to some of its relatives, upon 'accidental' downregulation.

The proto-oncogene BCL-6 in normal and malignant B cell development

BCL-6 is an important regulator of the immune system. It is required for GC formation and T cell dependent antibody responses. Mice deficient in BCL-6 fail to form GC and mount reduced levels of T cell-dependent antibody responses. BCL-6 (-/-) mice, in addition, develop a massive inflammatory response in many organs characterized by eosinophilic infiltration and hyper-IgE production, a typical Th2 hyperimmune response. This suggests a negative role of BCL-6 in Th2 pathway. The BCL-6 gene encodes a POZ/zinc finger transcription repressor highly expressed in GC B cells, but not in pre-GC B cells or in more differentiated memory or plasma cells. By functioning as a potent transcriptional repressor of various target genes, BCL-6 modulates IL-4, BCR, and CD40L signals for normal B cell development. In B cell lymphomas, structural alterations of the BCL-6 promoter region, including chromosome translocation and somatic hypermutation, represent the most frequent genetic lesions associated with non-Hodgkin lymphoma, especially of diffuse large cell lymphoma, a malignancy often derived from germinal centre (GC) B cells. This suggests that deregulated expression of BCL-6 may contribute to lymphomagenesis.

Molecular mechanisms of lymphomagenesis through transcriptional disregulation by chromosome translocation

Chromosome translation plays an important role for lymphomagenesis. Transcriptional disregulation type of chromosome translocation involves a majority of B-cell lymphoma. These include BCL1/cyclin D1 translocation in mantle cell lymphoma, BCL2 in follicular lymphoma and BCL6 in diffuse large B-cell lymphoma. It is known that the transcriptional disregulation type causes aberrant gene expression at the stages where those genes are down-regulated in normal counterpart cells. Normal B-cells at mantle zone stage down-regulate the expression of BCL1 gene and become resting in cell cycle. However, BCL1 expression from translocated allele with immunoglobulin gene is not down-regulated at the mantle zone stage, preventing cells from entering in resting state, and puts cells in cell cycle, leading to development of mantle cell lymphoma. BCL2 expression from altered allele also keeps its expression at the germinal center stage where normal counterpart cells down-regulate BCL2 expression, and makes cells to resist apoptosis, leading to development of follicular lymphoma. The same scenario can apply to diffuse large B-cell lymphoma with BCL6 translocation; i.e. aberrant BCL6 expression at the post germinal center stage takes place where normal counterpart cells down-regulate BCL6 expression. Although a strong association of specific translocations with specific disease types is found, these translocations by themselves are not sufficient for malignant transformation. The factors other than chromosome translocations will be discussed.

Non-immunoglobulin/BCL6 gene fusion in diffuse large B-cell lymphoma: prognostic implications

3q27 translocation affecting the BCL6 gene is one of the most common chromosomal abnormalities in diffuse large B-cell lymphoma (DLBCL). BCL6 translocation can involve not only one of the three immunoglobulin gene (Ig) loci but also another non-Ig chromosomal locus. 5'-rapid amplification of cDNA ends and long-distance inverse polymerase chain reaction (PCR) methods have identified a total of 13 recurrent non-Ig partner genes to date. As the result of non-Ig/BCL6 translocation, many types of regulatory sequences of each partner gene substitute for the 5' untranslated region of the BCL6 and the rearranged BCL6 is presumed to be under the control of the replaced promoter activity. BCL6 translocation occurs more frequently in extranodal DLBCL than in node-based disease. However, the impact of BCL6 translocation on the treatment outcome of DLBCL has been the subject of controversy. We found that survival of DLBCL patients with non-Ig partners was inferior to that of those with Ig/BCL6 translocation, suggesting that non-Ig/BCL6 fusion is a poor prognostic indicator of DLBCL. We next created BCL6 expression plasmids containing a series of non-Ig/BCL6 fusion genes. COS-7 cells transiently transfected with these plasmids expressed high levels of Bcl-6 protein and showed characteristic punctate nuclear staining. These findings suggested that non-Ig/BCL6 translocation plays a pathogenetic role in a proportion of DLBCL.

Low incidence of BCL-6 gene alterations for diffuse large B-cell lymphomas in Taiwan Chinese

BACKGROUND

In Western populations, rearrangement of the BCL-6 gene can be identified in 20-40% of patients with diffuse large B-cell lymphoma (DLBCL). Analysis of the BCL-6 gene has revealed the presence of point mutations or small deletions in 70% of DLBCL tumors; however, few studies have investigated BCL-6 gene alteration in patients with non-Hodgkins lymphoma (NHL) of Chinese descent.

METHODS

Samples from 135 Taiwanese patients with NHL were examined (28 samples of T-cell NHL and 107 samples of B-cell NHL; 59 samples from patients with DLBCL) for gene rearrangement and mutation of the BCL-6 proto-oncogene using Southern blot analysis and single-strand conformation polymorphism (SSCP) followed by sequence analysis.

RESULTS

BCL-6 rearrangement and point mutations were found in 14.8% of patients (n = 20) and in 7.4% of patients (n = 10), respectively. All BCL-6 gene alterations occurred in patients with B-cell NHL, and none occurred in patients with T-cell NHL. Among the 59 patients with DLBCL, BCL-6 gene rearrangements were identified in 10 patients (16.9%), and mutations were identified in 8 patients (13.6%), with the BCL-6 mutation occurring independent of the BCL-6 rearrangement. The incidence of BCL-6 gene rearrangement and mutations in patients with extranodal DLBCL was 9.5% (2 of 21 patients) and 23.8% (5 of 21 patients), respectively. Univariate analysis and multivariate logistic regression found no association between BCL-6 gene alternations and clinical characteristics, including extranodal tumors in patients with DLBCL, and no association between the BCL-6 alterations and prognosis was found.

CONCLUSIONS

The incidence of BCL-6 alterations was lower in Taiwanese patients with DLBCL compared with Western populations, and BCL-6 gene alterations showed no prognostic significance in patients with DLBCL.

Rearrangement of the BCL6 locus at 3q27 is an independent poor prognostic factor in nodal diffuse large B-cell lymphoma

Diffuse large B-cell lymphomas (DLBCL) are a heterogeneous group of tumours, varying in clinical features, immunophenotype and cytogenetics. The aim of this study was to investigate the prognostic significance of BCL6 gene rearrangement at the 3q27 locus in patients with primary nodal disease, and to examine interrelationships with immunophenotype and International Prognostic Index (IPI). We have developed a fluorescent in situ hybridization (FISH)-based technique for the retrospective analysis of the effect of BCL6 gene rearrangements on survival, using nuclei extracted from paraffin-embedded tissue. FISH results were obtained in 111 presentation cases of nodal DLBCL. The IPI was calculated and each case was stained immunocytochemically for BCL6, BCL2 and CD10. 3q27 rearrangements were detected in 25% of cases. BCL2 protein and a germinal centre (GC) phenotype (defined as CD10+, BCL6+) were expressed in 56% and 41% of cases respectively. In multivariate analysis, rearrangement of 3q27 and BCL2 expression and the absence of a GC phenotype were associated with a poor prognosis. These factors can be used in conjunction with the IPI to improve risk stratification in nodal DLBCL.

The gene for interleukin-21 receptor is the partner of BCL6 in t(3;16)(q27;p11), which is recurrently observed in diffuse large B-cell lymphoma

BCL6 translocation affecting the chromosomal band 3q27 can involve a number of non-immunoglobulin (non-IG) gene loci as partners. We report here that the gene for interleukin-21 receptor (IL-21R) is the partner of BCL6 in t(3;16)(q27;p11) translocation. The two breakpoints on 16p11 of a lymphoma cell line YM and case no. 1012 with diffuse large B-cell lymphoma, both of which carried t(3;16), were localized within the 27-kb intron 1 of IL-21R. As a result of t(3;16), the promoter region of IL-21R was substituted for the regulatory sequences of BCL6 in the same transcriptional orientation. Reverse transcriptase-mediated polymerase chain reaction revealed chimeric mRNA consisting of two non-coding exons 1a/1b of IL-21R and coding exons of BCL6 in both lymphoma cells. Fluorescence in situ chromosomal hybridization of YM metaphase cells revealed fusion signals that contained both the BCL6 and IL-21R sequences on the der(3)t(3;16) chromosome. IL-21R was actively transcribed in YM cells, while BCL6 that was under the control of the IL-21R promoter was only moderately expressed at the mRNA and protein level. We constructed expression plasmid of BCL6 that followed the promoter sequences of IL-21R. COS-7 cells transiently transfected with the plasmid expressed high level Bcl-6 protein and displayed nuclear staining with a characteristic punctate pattern by immunofluorescence, indicating that expression of BCL6 can be enhanced by t(3;16). This study added to the list of non-IG partners of BCL6 translocations a new class of gene, i.e. cytokine receptor gene, the expression of which is closely associated with lymphoid cells.

Update on epidemiology and therapeutics for non-Hodgkin's lymphoma

This chapter presents updated information on the trends and patterns of non-Hodgkin's lymphoma (NHL) diagnoses as well as new information on chemotherapeutic and immunotherapeutic options for NHL treatment. In Section I, Dr. Brian Chiu summarizes the current knowledge regarding the etiologic factors and patterns of NHL as well as suggests future epidemiologic studies based on these preliminary results. In Section II, Dr. Bruce Cheson and colleagues outline new chemotherapeutic and small molecule antineoplastic agents with unique mechanisms of action such as protease inhibitors, farnesyl transferase or histone deacetylase inhibitors, and antisense oligonucleotides. In Section III, Dr. Julie Vose reviews the anti-lymphoma effects of monoclonal antibodies, radioimmunoconjugates, idiotype vaccines, and immunologic enhancing adjuvants with respect to mechanisms of action, clinical trials, and their potential for patient therapy.

Homozygous deletion of INK4a/ARF genes and overexpression of bcl-2 in relation with poor prognosis in immunocompetent patients with primary central nervous system lymphoma of the diffuse large B-cell type

Only a few reports have been published on molecular genetic alterations in primary central nervous system lymphomas (PCNSLs) of the diffuse large B-cell type and no reports have addressed the correlation between the genetic alterations and clinical course of the patients with this neoplasm. Thus, the molecular background of the PCNSL and its importance for the clinical course of the patients are still unclear. We investigated a series of 14 patients with PCNSL to determine structural alterations of the INK4a/ARF, MDM2, and TP53 genes, the status of bcl-2 and bcl-6 protein expression, and the clinical course of the patients (i.e. their survival time after diagnosis). No structural alterations of MDM2 and TP53 genes were found. Only INK4a/ARF genes whose expression affects both the p161NK4a-Rb and p14ARF-mdm2-p53 pathways in the regulation for cell cycle and apoptosis, showed an alteration of the homozygous deletions at a high frequency (nine of 14 patients: 64%). This specific alteration was not related with the bcl-6 expression, but a relation was shown with overexpression of the bcl-2 anti-apoptotic protein (p = 0.036, chi-square test), as well as a shorter patient survival (p = 0.044, Wilcoxon test). There was only a tendency, not a significant correlation, in which the patients with bcl-2 overexpression resulted in poor prognosis (p = 0.149). The present study is the first to suggest that the INK4a/ARF gene homozygous deletions and overexpression of the bcl-2 protein may be correlated with each other and together serve as important predictors for the prognosis of patients with PCNSL.

Nonimmunoglobulin (non-Ig)/BCL6gene fusion in diffuse large B-cell lymphoma results in worse prognosis than Ig/BCL6

Chromosomal translocation involving the BCL6 gene affects not only immunoglobulin (Ig) genes but also a number of non-Ig genes as partners. The molecular anatomy of the BCL6 gene rearrangements in 39 cases with diffuse large B-cell lymphoma (DLBCL) by long-distance polymerase chain reaction–based assays was determined. The results showed that Iggenes were affected in 21 cases; non-Ig genes, 15 cases; a deletion of more than a 1-kb segment, 2 cases; and a point mutation, 1 case. Comparative studies between the 21 cases withIg gene partners and the 17 cases with non-Iggene partners, including 2 cases with the deletion, showed that the overall survival of the latter group of patients was significantly inferior to that of the former (P = .0440), and the estimated 2-year overall survival rates were 58.3% vs 17.6% (P = .005). Non-Ig/BCL6 fusion is a poor prognostic indicator of DLBCL, and DLBCL with BCL6translocation could be subclassified according to the individual partner locus and/or gene.

Nonimmunoglobulin (non-Ig)/BCL6gene fusion in diffuse large B-cell lymphoma results in worse prognosis than Ig/BCL6

Chromosomal translocation involving the BCL6 gene affects not only immunoglobulin (Ig) genes but also a number of non-Ig genes as partners. The molecular anatomy of the BCL6 gene rearrangements in 39 cases with diffuse large B-cell lymphoma (DLBCL) by long-distance polymerase chain reaction–based assays was determined. The results showed that Iggenes were affected in 21 cases; non-Ig genes, 15 cases; a deletion of more than a 1-kb segment, 2 cases; and a point mutation, 1 case. Comparative studies between the 21 cases withIg gene partners and the 17 cases with non-Iggene partners, including 2 cases with the deletion, showed that the overall survival of the latter group of patients was significantly inferior to that of the former (P = .0440), and the estimated 2-year overall survival rates were 58.3% vs 17.6% (P = .005). Non-Ig/BCL6 fusion is a poor prognostic indicator of DLBCL, and DLBCL with BCL6translocation could be subclassified according to the individual partner locus and/or gene.

Internal deletions within the BCL6 gene in B-cell non-Hodgkin's lymphoma

The BCL6 gene, encoding a POZ/Zinc finger protein which acts as a transcriptional repressor, is frequently altered at its 5' non-coding region by 3q27 chromosomal translocations in B-cell non-Hodgkin's lymphoma (NHL). BCL6 rearrangement is one of the most common genetic abnormalities in NHL. As a result of translocations, the regulatory region of the BCL6 gene is replaced by an heterologous reciprocal partner such as the immunoglobulin (IG) genes. Promotor substitution leads to deregulation of the BCL6 expression, which may be associated with lymphomagenesis. Recent studies have shown that the 5' non-coding region of the BCL6 gene is also subject to somatic hypermutation physiologically operating in germinal center (GC) B-cells in a similar pattern to that of the IG genes. There is little evidence to show that structural alterations of the BCL6 gene may be caused by mechanisms other than chromosomal translocations. To date, five cases with NHL exhibiting gross BCL6 deletions of the 1.5-2.4 kb have been reported. These deletions occurred in the same region as translocational breakpoints and the somatic hypermutations cluster, but independently of chromosomal rearrangements. The deletions overlapped at the 270 bp region and this region contains a putative protein-binding sequence which may play a role in the regulation of the BCL6 expression. Small separated deletions of 22-101 bp, which may also contain protein-binding sequences, were evident in another NHL case. In contrast to the TAL1 deletion in T-cell acute lymphoblastic leukemia (ALL), the BCL6 deletion is considered to be mediated by a mechanism other than aberrant activity of the IG recombinase. Internal deletion within the BCL6 gene is a recurrent molecular abnormality in B-cell NHL, which is sometimes indistinguishable from rearrangements by chromosomal translocations. At present, the mechanism of DNA recombination and its role in lymphomagenesis remain unknown.

The t(9;14)(p13;q32) translocation in B-cell non-Hodgkin's lymphoma

t(9;14)(p13;q32) is a rare but recurring translocation found in a subset of B-cell non-Hodgkin's lymphoma (B-NHL). These lymphomas share clinical features with chronic lymphocytic leukemia and are further characterized by plasmacytoid differentiation of lymphoma cells. Molecular cloning of t(9;14)(p13;q32) revealed juxtaposition of the PAX5 to the immunoglobulin heavy chain gene (IGH), although breakpoints on both genes were variable. The PAX5 gene encodes the BSAP (B-cell-specific activator protein) transcription factor, which is expressed throughout the process of B-cell development except in terminally differentiated plasma cells. t(9;14)(p13;q32) consistently leaves the PAX5 coding region intact, most likely resulting in deregulated expression of the gene product due to the proximity of IGH. The majority cases of B-cell tumors expressed considerable levels of PAX5/BSAP irrespective of whether they exhibited t(9;14)(p13;q32), suggesting that quantitative differences in expression level alone may not account for the development of this particular subtype of B-NHL.

Identification of heterologous translocation partner genes fused to the BCL6 gene in diffuse large B-cell lymphomas: 5'-RACE and LA - PCR analyses of biopsy samples

In order to elucidate the molecular mechanism(s) for BCL6 translocation, we identified translocational partner genes by subjecting clinical biopsy samples from patients with non-Hodgkin's lymphoma to 5'-rapid amplification of cDNA ends (5'-RACE). Sequence analysis of the 5'-RACE product revealed that the BCL6 gene was fused to the J segment of the immunoglobulin heavy chain (IgH) gene in about half of the cases, but in the other half, it was fused to heterologous partners, including the MHC class II transactivator (CIITA), pim-1, eukaryotic initiation factor 4AII (eif4AII), transferrin receptor (TFRR) and ikaros genes. Since analyses using genomic long and accurate (LA) - PCR revealed that the breakpoints in the partner gene were confined to the first intron or the second exon in all cases, the promoter and the first exon of the BCL6 gene were replaced by the promoter and the first or both the first and second exon of the partner gene. The breakpoint flanking sequences had no recombination signal sequences (RSSs) or chi sequences and were homologous with the switch region only when the BCL6 gene was fused to the IgH gene, suggesting that BCL6 translocation cannot be explained solely by mistakes of V(D)J, or chi-mediated or class-switch recombination, but rather another mechanism may also be required to explain the molecular mechanism for the promiscuous BCL6 translocation.

Overexpressed BCL6 (LAZ3) oncoprotein triggers apoptosis, delays S phase progression and associates with replication foci

One of the most frequent genetic abnormalities associated with non Hodgkin lymphoma is the structural alteration of the 5' non coding/regulatory region of the BCL6 (LAZ3) protooncogene. BCL6 encodes a POZ/Zn finger protein, a structure similar to that of many Drosophila developmental regulators and to another protein involved in a human hematopoietic malignancy, PLZF. BCL6 is a sequence specific transcriptional repressor controlling germinal center formation and T cell dependent immune response. Although the expression of BCL6 negatively correlates with cellular proliferation in different cell types, the influence of BCL6 on cell growth and survival is currently unknown so that the way its deregulation may contribute to cancer remains elusive. To directly address this issue, we used a tetracycline-regulated system in human U2OS osteosarcoma cells and thus found that BCL6 mediates growth suppression associated with impaired S phase progression and apoptosis. Interestingly, overexpressed BCL6 can colocalize with sites of ongoing DNA synthesis, suggesting that it may directly interfere with S phase initiation and/or progression. In contrast, the isolated Zn finger region of BCL6, which binds BCL6 target sequence but lacks transcriptional repression activity, slows, but does not suppress, U2OS cell growth, is less efficient at delaying S phase progression, and does not trigger apoptosis. Thus, for a large part, the effects of BCL6 overexpression on cell growth and survival depend on its ability to engage protein/protein interactions with itself and/or its transcriptional corepressors. That BCL6 restricts cell growth suggests that its deregulation upon structural alterations may alleviate negative controls on the cell cycle and cell survival.

Expression of the PAX5/BSAP transcription factor in haematological tumour cells and further molecular characterization of the t(9;14)(p13;q32) translocation in B-cell non-Hodgkin's lymphoma

The PAX5 gene encodes the BSAP (B-cell-specific activator protein) which is a key regulator of B-cell development and differentiation. A recurring translocation t(9;14)(p13;q32) in non-Hodgkin's lymphoma moves the PAX5 on 9p13 within close proximity of the immunoglobulin heavy chain gene (IGH). KIS-1 cell line was established from a patient with diffuse large cell lymphoma of B-cell type carrying t(9;14). We analysed PAX5/BSAP expression by Northern and Western blotting in a panel of haematological tumour cell lines with other chromosome abnormalities in comparison with that of KIS-1. PAX5 mRNA and BSAP expression were detected in all B-cell lines tested, and the high level in KIS-1 was confirmed. However, a diffuse large B-cell lymphoma cell line and an acute B-lymphoid/myeloid leukaemia cell line expressed the PAX5/BSAP at levels comparable with KIS-1. PAX5 transcripts were readily detectable in clinical materials with a wide variety of B-cell neoplasms by reverse transcriptase-mediated polymerase chain reaction (PCR). Thus, PAX5/BSAP activation in haematological tumour cells is not necessarily associated with t(9;14). Although binding sites for BSAP have been identified in the promoters of CD19, this study failed to find clear correlation between the level of PAX5/BSAP expression and that of CD19. In contrast to KIS-1 in which the E mu enhancer of IGH was juxtaposed to PAX5, cloning of t(9; 14) from another case by long-distance PCR revealed that the PAX5 promoter was linked to a Cgamma constant region in divergent orientation, suggesting that the mechanism of PAX5 activation through recombination with IGH varies among individual cases. Breakpoints on 9p13 of the two translocations were clustered upstream of PAX5, leaving the PAX5 coding region intact.