BCL-6 and the molecular pathogenesis of B-cell lymphoma

p63 expression in human tumors and normal tissues: a tissue microarray study on 10,200 tumors

Background

Tumor protein 63 (p63) is a transcription factor of the p53 gene family involved in differentiation of several tissues including squamous epithelium. p63 immunohistochemistry is broadly used for tumor classification but published data on its expression in cancer is conflicting.

Methods

To comprehensively catalogue p63 expression, tissue microarrays (TMAs) containing 12,620 tissue samples from 115 tumor entities and 76 normal tissue types were analyzed.

Results

p63 expression was seen in various normal tissues including squamous epithelium and urothelium. At least occasional weak p63 positivity could be detected in 61 (53%) of 115 different tumor types. The frequencies of p63 positivity was highest in squamous cell carcinomas irrespective of their origin (96–100%), thymic tumors (100%), urothelial carcinomas (81–100%), basal type tumors such as basal cell carcinomas (100%), and various salivary gland neoplasias (81–100%). As a rule, p63 was mostly expressed in cancers derived from p63 positive normal tissues and mostly not detectable in tumors derived from p63 negative cancers. However, exceptions from this rule occurred. A positive p63 immunostaining in cancers derived from p63 negative tissues was unrelated to aggressive phenotype in 422 pancreatic cancers, 160 endometrium cancers and 374 ovarian cancers and might be caused by aberrant squamous differentiation or represent stem cell properties. In 355 gastric cancers, aberrant p63 expression occurred in 4% and was linked to lymph node metastasis (p = 0.0208). Loss of p63 in urothelial carcinomas - derived from p63 positive urothelium - was significantly linked to advanced stage, high grade (p < 0.0001 each) and poor survival (p < 0.0001) and might reflect clinically relevant tumor dedifferentiation.

Conclusion

The high prevalence of p63 expression in specific tumor types makes p63 immunohistochemistry a suitable diagnostic tool. Loss of p63 expression might constitute a feature of aggressive cancers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40364-021-00260-5.

Increasing Levels of Serum Heat Shock Protein 70 Precede the Development of AIDS-Defining Non-Hodgkin Lymphoma Among Carriers of HLA-B8-DR3

BACKGROUND

We hypothesized that carriage of presumably high Hsp70-producing gene variants on a specific human major histocompatibility complex haplotype, the 8.1 ancestral haplotype (8.1AH), may predispose HIV-infected individuals to AIDS-non-Hodgkin lymphoma (NHL).

SETTING

We compared serum Hsp70 levels in the years preceding the diagnosis of AIDS-NHL in a matched case-control study (n = 151 pairs) nested in the Multicenter AIDS Cohort Study.

METHODS

We tested the impact of 8.1AH-specific single-nucleotide polymorphism (SNP) and joint SNP-human leukocyte antigen extended haplotypes previously associated with AIDS-NHL in the Multicenter AIDS Cohort Study on the circulating Hsp70 levels in mixed linear models.

RESULTS

We report elevated serum levels of Hsp70 in the 4 years preceding the diagnosis of AIDS-NHL in cases that carry 8.1AH, but not in noncarrier cases and not in carrier- or non-carrier-matched controls. The strongest predictor of higher serum Hsp70 was the haplotype A-G-A-C formed by SNPs rs537160(A) and rs1270942(G) in the complement factor CFB gene cluster, and rs2072633(A) and rs6467(C) in nearby RDBP and CYP21A2 located 70 Kb apart from the Hsp70 gene cluster. The association with A-G-A-C haplotype (beta = 0.718; standard error = 0.182; P = 0.0002) and with other 8.1AH-specific haplotypes including the high-producing tumor necrosis factor-alpha haplotype rs909253(G)-rs1800629(A) (beta = 0.308; standard error = 0.140; P = 0.032) were observed only with NHL identified as an AIDS-defining condition, but not as a post-AIDS condition, nor in combined AIDS and post-AIDS cases.

CONCLUSION

Our combined genetic and functional approach suggests that the altered level of Hsp70 is a correlate of 8.1AH-mediated AIDS-NHL. Further investigation of the Hsp70 gene cluster and nearby loci that are tagged by A-G-A-C could better elucidate the genetic determinants of the malignancy.

African Americans with pancreatic ductal adenocarcinoma exhibit gender differences in Kaiso expression

Kaiso, a bi-modal transcription factor, regulates gene expression, and is elevated in breast, prostate, and colon cancers. Depletion of Kaiso in other cancer types leads to a reduction in markers for the epithelial-mesenchymal transition (EMT) (Jones et al., 2014), however its clinical implications in pancreatic ductal adenocarcinoma (PDCA) have not been widely explored. PDCA is rarely detected at an early stage but is characterized by rapid progression and invasiveness. We now report the significance of the subcellular localization of Kaiso in PDCAs from African Americans. Kaiso expression is higher in the cytoplasm of invasive and metastatic pancreatic cancers. In males, cytoplasmic expression of Kaiso correlates with cancer grade and lymph node positivity. In male and female patients, cytoplasmic Kaiso expression correlates with invasiveness. Also, nuclear expression of Kaiso increases with increased invasiveness and lymph node positivity. Further, analysis of the largest PDCA dataset available on ONCOMINE shows that as Kaiso increases, there is an overall increase in Zeb1, which is the inverse for E-cadherin. Hence, these findings suggest a role for Kaiso in the progression of PDCAs, involving the EMT markers, E-cadherin and Zeb1.

Nuclear localization of Kaiso promotes the poorly differentiated phenotype and EMT in infiltrating ductal carcinomas

The expression and biological consequences of Kaiso, a novel bi-modal transcription factor, in infiltrating ductal carcinomas (IDCs) have not been widely investigated. In the present study, we determined Kaiso expression and subcellular localization in 146 normal tissues, 376 IDCs, and 85 lymph node metastases. In IDCs, there was higher Kaiso expression in both the cytoplasmic and nuclear compartments, which correlated with age <48 (cytoplasmic p < 0.0093; nuclear p < 0.0001) and moderate differentiation (cytoplasmic p < 0.0042; nuclear p < 0.0001), as determined by Chi square analysis. However, only nuclear Kaiso correlated with poor prognostic factors, i.e., race (African Americans) (p < 0.0001), poor differentiation (p < 0.0001), and metastases (p < 0.0001). Nuclear Kaiso was also associated with worse overall survival (p < 0.0019), with African American patients displaying worse survival rates relative to Caucasian patients (p < 0.029). MCF-7 (non-metastatic), MDA-MB-468 (few metastases), and MDA-MB-231 (highly metastatic) breast cancer cells demonstrated increasing Kaiso levels, with more nuclear localization in the highly metastatic cell line. Over-expression of Kaiso in MCF-7 cells increased cell migration and invasion, but treatment of MDA-MB-468 and MDA-MB-231 cells with si-Kaiso decreased cell migration and invasion and induced expression of E-cadherin RNA and protein. E-cadherin re-expression was associated with a reversal of mesenchymal associated cadherins, N-cadherin and cadherin 11, as well as decreased vitamin expression. Further, Kaiso directly bound to methylated sequences in the E-cadherin promoter, an effect prevented by 5-aza-2-deoxycytidine. Immunofluorescence co-staining of poorly differentiated IDCs demonstrated that nuclear Kaiso is associated with a loss of E-cadherin expression. These findings support a role for Kaiso in promoting aggressive breast tumors.

T cells that promote B-Cell maturation in systemic autoimmunity

Follicular helper T (Tfh) cells play an essential role in helping B cells generate antibodies upon pathogen encounters. Such T-cell help classically occurs in germinal centers (GCs) located in B-cell follicles of secondary lymphoid organs, a site of immunoglobulin affinity maturation and isotype switching. B-cell maturation also occurs extrafollicularly, in the red pulp of the spleen and medullary cords in lymph nodes, with plasma cell formation and antibody production. Development of extrafollicular foci (EF) in T-cell-dependent (TD) immune responses is reliant upon CD4(+) T cells with characteristics of Tfh cells. Pathogenic autoantibodies, arising from self-reactive B cells having undergone somatic hypermutation with affinity selection and class switching within GCs and EF, are major contributors to the end-organ injury in systemic autoimmunity. B cells maturing to produce autoantibodies in systemic autoimmune diseases, like those in normal immune responses, largely require T-helper cells. This review highlights Tfh cell development as an introduction to a more in-depth discussion of human Tfh cells and blood borne cells with similar features and the role of these cells in promotion of systemic autoimmunity.

B-cell stimulatory cytokines and markers of immune activation are elevated several years prior to the diagnosis of systemic AIDS-associated non-Hodgkin B-cell lymphoma

BACKGROUND

The risk of developing non-Hodgkin lymphoma (NHL) is greatly increased in HIV infection. The aim of this study was to determine whether elevated serum levels of molecules associated with B-cell activation precede the diagnosis of AIDS-associated NHL (AIDS-NHL).

METHODS

Serum levels of B-cell activation-associated molecules, interleukin (IL)6, IL10, soluble CD23 (sCD23), sCD27, sCD30, C-reactive protein (CRP), and immunoglobulin E were determined in 179 NHL cases and HIV+ controls in the Multicenter AIDS Cohort Study, collected at up to 3 time points per subject, 0 to 5 years prior to AIDS-NHL diagnosis.

RESULTS

Serum IL6, IL10, CRP, sCD23, sCD27, and sCD30 levels were all significantly elevated in the AIDS-NHL group, when compared with HIV+ controls or with AIDS controls, after adjusting for CD4 T-cell number. Elevated serum levels of B-cell activation-associated molecules were seen to be associated with the development of systemic [non-CNS (central nervous system)] NHL, but not with the development of primary CNS lymphoma.

CONCLUSIONS

Levels of certain B-cell stimulatory cytokines and molecules associated with immune activation are elevated for several years preceding the diagnosis of systemic AIDS-NHL. This observation is consistent with the hypothesis that chronic B-cell activation contributes to the development of these hematologic malignancies.

IMPACT

Marked differences in serum levels of several molecules are seen for several years prediagnosis in those who eventually develop AIDS-NHL. Some of these molecules may serve as candidate biomarkers and provide valuable information to better define the etiology of NHL.

In vivo regulation of Bcl6 and T follicular helper cell development

Follicular helper T (T(FH)) cells, defined by expression of the surface markers CXCR5 and programmed death receptor-1 (PD-1) and synthesis of IL-21, require upregulation of the transcriptional repressor Bcl6 for their development and function in B cell maturation in germinal centers. We have explored the role of B cells and the cytokines IL-6 and IL-21 in the in vivo regulation of Bcl6 expression and T(FH) cell development. We found that T(FH) cells are characterized by a Bcl6-dependent downregulation of P-selectin glycoprotein ligand 1 (PSGL1, a CCL19- and CCL21-binding protein), indicating that, like CXCR5 and PD-1 upregulation, modulation of PSGL1 expression is part of the T(FH) cell program of differentiation. B cells were neither required for initial upregulation of Bcl6 nor PSGL1 downregulation, suggesting these events preceded T-B cell interactions, although they were required for full development of the T(FH) cell phenotype, including CXCR5 and PD-1 upregulation, and IL-21 synthesis. Bcl6 upregulation and T(FH) cell differentiation were independent of IL-6 and IL-21, revealing that either cytokine is not absolutely required for development of Bcl6(+) T(FH) cells in vivo. These data increase our understanding of Bcl6 regulation in T(FH) cells and their differentiation in vivo and identifies a new surface marker that may be functionally relevant in this subset.

Design of antisense oligonucleotides and short interfering RNA duplexes (siRNA) targeted to BCL6 mRNA: towards rational drug development for specific lymphoma subsets

Many algorithms based on computational analysis and thermodynamic parameters have been developed to predict the secondary structure of RNA. Still, many antisense oligodeoxynucleotides (AS ODNs), or siRNA molecules designed according to these predictions fail to silence the intended target, whereas other, not fulfilling those criteria prove highly active. We have developed a reliable mapping strategy, which allows us to predict the sites accessible for hybridization within target mRNA in vitro and in vivo. Our mapping experiments employed self-quenching reporter molecules (SQRMs) and were first carried out in a cell free system, and later confirmed in vivo.

The catenin p120ctn inhibits Kaiso-mediated transcriptional repression of the β-catenin/TCF target gene matrilysin

The POZ-zinc finger transcription factor Kaiso was first identified as a specific binding partner for the Armadillo catenin and cell adhesion cofactor, p120ctn. Kaiso is a unique POZ protein with bi-modal DNA-binding properties; it associates with a sequence-specific DNA consensus Kaiso binding site (KBS) or methylated CpG dinucleotides, and regulates transcription of artificial promoters containing either site. Interestingly, the promoter of the Wnt/beta-catenin/TCF target gene matrilysin possesses two conserved copies of the KBS, which suggested that Kaiso might regulate matrilysin expression. In this study, we demonstrate using chromatin immunoprecipitation analysis that Kaiso associates with the matrilysin promoter in vivo. Minimal promoter assays further confirmed that Kaiso specifically repressed transcription of the matrilysin promoter; mutation of the KBS element or RNAi-mediated depletion of Kaiso abrogated this effect. More importantly, Kaiso blocked beta-catenin-mediated activation of the matrilysin promoter. Consistent with our previous findings, both Kaiso-DNA binding and Kaiso-mediated transcriptional repression of the matrilysin promoter were inhibited by overexpression of wild-type p120ctn, but not by a p120ctn mutant exhibiting impaired nuclear import. Collectively, our data establish Kaiso as a sequence-specific transcriptional repressor of the matrilysin promoter, and suggest that p120ctn and beta-catenin act in a synergistic manner, via distinct mechanisms, to activate matrilysin expression.

The role of DNA breaks in genomic instability and tumorigenesis

DNA double-strand breaks (DSBs) represent dangerous chromosomal lesions that can lead to mutation, neoplastic transformation, or cell death. DSBs can occur by extrinsic insult from environmental sources or may occur intrinsically as a result of cellular metabolism or a genetic program. Mammalian cells possess potent and efficient mechanisms to repair DSBs, and thus complete normal development as well as mitigate oncogenic potential and prevent cell death. When DSB repair (DSBR) fails, chromosomal instability results and can be associated with tumor formation or progression. Studies of mice deficient in various components of the non-homologous end joining pathway of DSBR have revealed key roles in both the developmental program of B and T lymphocytes as well as in the maintenance of general genome stability. Here, we review the current thinking about DSBs and DSBR in chromosomal instability and tumorigenesis, and we highlight the implications for understanding the karyotypic features associated with human tumors.

BRD4 bromodomain gene rearrangement in aggressive carcinoma with translocation t(15;19)

Translocation t(15;19)(q13;p13.1) defines a lethal midline carcinoma arising adjacent to respiratory tract in young people. To characterize molecular alterations responsible for the distinctly aggressive biological behavior of this cancer, we mapped the chromosome 15 and 19 translocation breakpoints by fluorescence in situ hybridization (FISH) and Southern blotting. To evaluate preliminarily the frequency, anatomical distribution, and histological features of t(15;19) cancer, we developed a FISH assay for paraffin sections. Our findings reveal a novel oncogenic mechanism in which the chromosome 19 translocation breakpoint interrupts the coding sequence of a bromodomain gene, BRD4. These studies implicate BRD4 as a potential partner in a t(15;19)-associated fusion oncogene. In addition, we localized the chromosome 15 breakpoint to a 9-kb region in each of two cases, thereby identifying several candidate oncogenes which might represent the BRD4 fusion partner. FISH evaluation of 13 pediatric carcinomas revealed t(15;19) in one of four sinonasal carcinomas, whereas this translocation was not detected in thymic (n = 3), mucoepidermoid (n = 3), laryngeal (n = 2), or nasopharyngeal (n = 1) carcinomas. Our studies shed light on the oncogenic mechanism underlying t(15;19) and provide further evidence that this highly lethal cancer arises from respiratory mucosa.

Detection of translocations affecting the BCL6 locus in B cell non-Hodgkin's lymphoma by interphase fluorescence in situ hybridization

Structural alterations in 3q27 affecting the BCL6 locus are among the most frequent changes in B-NHL. The aim of the present study was to establish an interphase-FISH assay for the detection of all diverse BCL6 translocations in B-NHL. Two different approaches were tested, one using a PAC-clone spanning the major breakpoint region (MBR) of BCL6 (span-assay), and another using two BAC clones flanking the MBR (flank-assay). Interphase FISH with the span-assay detected the various BCL6 translocations in seven B-NHL cell lines. The dual-color flank-assay was evaluated in two laboratories independently: in normal controls, the cutoff level for false-positive signals was 2.6%, whereas the cutoff level for false-negatives in the seven cell lines was 7.5%. To test the feasibility of the FISH strategies, 30 samples from patients with B-NHL with cytogenetic abnormalities of 3q27 were evaluated with both assays. In 21 cases, the span-assay indicated a BCL6 rearrangement. In 18 of the 21 cases, the dual-color flank-assay confirmed the translocation including 12 different partner chromosomal loci. The three false-positive cases detected with the span-assay showed trisomy of chromosome 3 by cytogenetic analyses, and they were correctly classified as non-rearranged with the flank-assay. In summary, our FISH strategy using two differently labeled flanking BCL6 BAC probes provides a robust, sensitive, and reproducible method for the detection of common and uncommon abnormalities of BCL6 gene in interphase nuclei. The routine application of this assay to patients with B-NHL will allow the assessment of the diagnostic and prognostic significance of BCL6 rearrangements.

Incidence and pathology of primary brain lymphoma in Hong Kong Chinese patients

Primary brain lymphoma (PBL) is an uncommon extranodal lymphoma. Its incidence is rapidly increasing in both immunocompromised and immunocompetent patients in Western countries. Eighteen cases of PBL were identified during a 16-year period among HIV negative patients in Queen Mary Hospital, Hong Kong. One case of post-transplantation lymphoproliferative disease (PTLD) was positive for Epstein Barr virus (EBV) encoded RNA (EBER) by in situ hybridization. All the remaining 17 immunocompetent cases were classified as diffuse large B-cell lymphoma, except for one case of Burkitt's lymphoma. EBER expression was negative in all 13 cases tested. Immunostaining for bcl-2 and bcl-6 was positive in 8/11 and 6/11 cases tested, with heterogeneous combination of expression and intensity. The incidence rate of PBL in immunocompetent patients was stable at 1.03 per million per year. The incidence of PBL in post transplantation (0.16%) and HIV related setting (0.29%) is also low in Chinese. PBL in Chinese patients is almost uniformly represented by EBV negative, diffuse large B-cell lymphoma, confined to the brain. However, the molecular pathogenesis may be heterogeneous.

Identification and characterization of BCL6 translocation partner genes in primary gastric high-grade B-cell lymphoma: heat shock protein 89 alpha is a novel fusion partner gene of BCL6

Primary gastric high-grade B-cell lymphoma (HGBL) is a special type of non-Hodgkin's lymphoma. So far, the genetic features of this tumor have not been well characterized. Recently, a high incidence of BCL6 rearrangements has been detected in HGBL. However, no previous cytogenetic studies have found translocations involving the BCL6 locus (3q27) in HGBL, and the genetic basis underlying the BCL6 rearrangements in this tumor remains unclear. We therefore characterized the partner genes of BCL6 in five primary gastric HGBLs with a rearranged BCL6 gene by analyzing BCL6 transcripts using the 5' RACE (rapid amplification of cDNA end) strategy. BCL6 translocation partner genes were identified at the 5' end of the chimeric transcripts in all five cases, including the immunoglobulin heavy-chain (IGH) gene in three cases and the immunoglobulin lambda-light-chain gene and the heat shock protein 89 alpha (HSP89A) gene in the other two cases. The chimeric transcripts in all cases contained the intact BCL6 exon 2, but lacked exon 1, which was replaced by sequences from the partner genes, suggesting that BCL6 expression was under the control of regulatory sequences of the partner genes. These results, for the first time, indicate that immunoglobulin genes, especially IGH, are the most common BCL6 translocation partner genes in primary gastric HGBL and that HSP89A is a novel partner of BCL6. Because immunoglobulin genes are also the most frequent partners of BCL6 in nodal diffuse large B-cell lymphoma (DLBCL), these data suggest that primary gastric HGBL shares a common genetic basis with nodal DLBCL. Genes Chromosomes Cancer 27:69-75, 2000.

Nonimmunoglobulin Gene Hypermutation in Germinal Center B Cells

Somatic hypermutation is the most critical mechanism underlying the diversification of Ig genes. Although mutation occurs specifically in B cells during the germinal center reaction, it remains a matter of debate whether the mutation machinery also targets non-Ig genes. We have studied mutations in the 5′ noncoding region of the Bcl6 gene in different subtypes of lymphomas. We found frequent hypermutation in follicular lymphoma (25 of 59 = 42%) (germinal center cell origin) and mucosa-associated lymphoid tissue (MALT) lymphoma (19 of 45 = 42%) (postgerminal center), but only occasionally in mantle cell lymphoma (1 of 21 = 4.8%) (pregerminal center). Most mutations were outside the motifs potentially important for transcription, suggesting they were not important in lymphomagenesis but may, like Ig mutation, represent an inherent feature of the lymphoma precursor cells. Therefore, we investigated their normal cell counterparts microdissected from a reactive tonsil. Bcl6 mutation was found in 13 of 24 (54%) clones from the germinal centre but only in 1 of 24 (4%) clones from the naive B cells of the mantle zone. The frequency, distribution, and nature of these mutations were similar to those resulting from the Ig hypermutation process. The results show unequivocal evidence of non-Ig gene hypermutation in germinal center B cells and provide fresh insights into the process of hypermutation and lymphomagenesis.

Nonimmunoglobulin Gene Hypermutation in Germinal Center B Cells

Somatic hypermutation is the most critical mechanism underlying the diversification of Ig genes. Although mutation occurs specifically in B cells during the germinal center reaction, it remains a matter of debate whether the mutation machinery also targets non-Ig genes. We have studied mutations in the 5′ noncoding region of the Bcl6 gene in different subtypes of lymphomas. We found frequent hypermutation in follicular lymphoma (25 of 59 = 42%) (germinal center cell origin) and mucosa-associated lymphoid tissue (MALT) lymphoma (19 of 45 = 42%) (postgerminal center), but only occasionally in mantle cell lymphoma (1 of 21 = 4.8%) (pregerminal center). Most mutations were outside the motifs potentially important for transcription, suggesting they were not important in lymphomagenesis but may, like Ig mutation, represent an inherent feature of the lymphoma precursor cells. Therefore, we investigated their normal cell counterparts microdissected from a reactive tonsil. Bcl6 mutation was found in 13 of 24 (54%) clones from the germinal centre but only in 1 of 24 (4%) clones from the naive B cells of the mantle zone. The frequency, distribution, and nature of these mutations were similar to those resulting from the Ig hypermutation process. The results show unequivocal evidence of non-Ig gene hypermutation in germinal center B cells and provide fresh insights into the process of hypermutation and lymphomagenesis.

Deregulated PAX-5 Transcription From a TranslocatedIgH Promoter in Marginal Zone Lymphoma

The PAX-5 gene codes for the transcription factor BSAP, which is expressed throughout B-cell development. Although loss-of-function mutation in the mouse showed an essential role forPax-5 in early B lymphopoiesis, gain-of-function mutations have implicated the human PAX-5 gene in the control of late B-cell differentiation. PAX-5 (on 9p13) has been involved together with the immunoglobulin heavy-chain (IgH) gene (on 14q32) in the recurring t(9;14)(p13;q32) translocation that is characteristic of small lymphocytic lymphoma with plasmacytoid differentiation. Here we have characterized a complex t(2;9;14)(p12;p13;q32) translocation present in a closely related non-Hodgkin’s lymphoma referred to as splenic marginal zone lymphoma (MZL). In this MZL-1 translocation, the two promoters of PAX-5 were replaced on the derivative chromosome 14 by an immunoglobulin switch Sμ promoter that was linked to the structural PAX-5 gene upstream of its translation initiation codon in exon 1B. Expression analyses confirmed thatPAX-5 transcription was upregulated due to efficient initiation at the Sμ promoter in the malignant B lymphocytes of patient MZL-1. For comparison we have analyzed PAX-5 expression in another B-cell lymphoma, KIS-1, indicating that transcription from the distalPAX-5 promoter was increased in this tumor in agreement with the previously characterized translocation of the immunoglobulin Eμ enhancer adjacent to PAX-5 exon 1A. In both lymphomas, the J-chain gene, which is thought to be under negative control by BSAP, was not expressed, whereas transcription of the putative target genep53 was unaffected by PAX-5 overexpression. Together these data indicate that the t(9;14)(p13;q32) translocation contributes to lymphoma formation as a regulatory mutation that leads to increasedPAX-5 expression in late B-cell differentiation due to promoter replacement or enhancer insertion.

Deregulated PAX-5 Transcription From a TranslocatedIgH Promoter in Marginal Zone Lymphoma

The PAX-5 gene codes for the transcription factor BSAP, which is expressed throughout B-cell development. Although loss-of-function mutation in the mouse showed an essential role forPax-5 in early B lymphopoiesis, gain-of-function mutations have implicated the human PAX-5 gene in the control of late B-cell differentiation. PAX-5 (on 9p13) has been involved together with the immunoglobulin heavy-chain (IgH) gene (on 14q32) in the recurring t(9;14)(p13;q32) translocation that is characteristic of small lymphocytic lymphoma with plasmacytoid differentiation. Here we have characterized a complex t(2;9;14)(p12;p13;q32) translocation present in a closely related non-Hodgkin’s lymphoma referred to as splenic marginal zone lymphoma (MZL). In this MZL-1 translocation, the two promoters of PAX-5 were replaced on the derivative chromosome 14 by an immunoglobulin switch Sμ promoter that was linked to the structural PAX-5 gene upstream of its translation initiation codon in exon 1B. Expression analyses confirmed thatPAX-5 transcription was upregulated due to efficient initiation at the Sμ promoter in the malignant B lymphocytes of patient MZL-1. For comparison we have analyzed PAX-5 expression in another B-cell lymphoma, KIS-1, indicating that transcription from the distalPAX-5 promoter was increased in this tumor in agreement with the previously characterized translocation of the immunoglobulin Eμ enhancer adjacent to PAX-5 exon 1A. In both lymphomas, the J-chain gene, which is thought to be under negative control by BSAP, was not expressed, whereas transcription of the putative target genep53 was unaffected by PAX-5 overexpression. Together these data indicate that the t(9;14)(p13;q32) translocation contributes to lymphoma formation as a regulatory mutation that leads to increasedPAX-5 expression in late B-cell differentiation due to promoter replacement or enhancer insertion.

Components of the SMRT corepressor complex exhibit distinctive interactions with the POZ domain oncoproteins PLZF, PLZF-RARalpha, and BCL-6

Many transcription factors function by repressing gene transcription. For a variety of these transcription factors the ability to physically recruit auxiliary proteins, denoted corepressors, is crucial for the ability to silence gene expression. We and others have previously implicated the SMRT corepressor in the actions of the PLZF transcription factor and in the function of its oncogenic derivative, PLZF-retinoic acid receptor (RARalpha), in promyelocytic leukemia. We report here that PLZF, and a structurally similar transcriptional repressor, BCL-6, can interact with a variety of corepressor proteins in addition to SMRT, including the mSin3A protein and (for PLZF) histone deacetylase-1. Unexpectedly, these additional interactions with corepressor components are nonequivalent for these otherwise similar oncoproteins, suggesting that transcriptional repression by BCL-6 and by PLZF may differ in mechanism. Furthermore, we demonstrate that the oncogenic PLZF-RARalpha chimera lacks several important corepressor interaction sites that are present in the native PLZF protein. Thus the t(11;17) translocation that creates the PLZF-RARalpha chimera generates an oncoprotein with potentially novel regulatory properties distinct from those of either parental protein. Our results demonstrate that otherwise similar transcription factors can differ notably in their interactions with the corepressor machinery.

Chromosomal and Gene Amplification in Diffuse Large B-Cell Lymphoma

Chromosomal translocations leading to deregulation of specific oncogenes characterize approximately 50% of cases of diffuse large B-cell lymphomas (DLBL). To characterize additional genetic features that may be of value in delineating the clinical characteristics of DLBL, we studied a panel of 96 cases at diagnosis consecutively ascertained at the Memorial Sloan-Kettering Cancer Center (MSKCC) for incidence of gene amplification, a genetic abnormality previously shown to be associated with tumor progression and clinical outcome. A subset of 20 cases was subjected to comparative genomic hybridization (CGH) analysis, which identified nine sites of chromosomal amplification (1q21-23, 2p12-16, 8q24, 9q34, 12q12-14, 13q32, 16p12, 18q21-22, and 22q12). Candidate amplified genes mapped to these sites were selected for further analysis based on their known roles in lymphoid cell and lymphoma development, and/or history of amplification in tumors. Probes for six genes, which fulfilled these criteria,REL (2p12-16), MYC (8q24), BCL2 (18q21),GLI, CDK4, and MDM2 (12q13-14), were used in a quantitative Southern blotting analysis of the 96 DLBL DNAs. Each of these genes was amplified (four or more copies) with incidence ranging from 11% to 23%. This analysis is consistent with our previous finding that REL amplification is associated with extranodal presentation. In addition, BCL2 rearrangement and/or REL, MYC, BCL2, GLI,CDK4, and MDM2 amplification was associated with advanced stage disease. These data show, for the first time, that amplification of chromosomal regions and genes is a frequent phenomenon in DLBL and demonstrates their potential significance in lymphomagenesis.

Heterologous Promoters Fused to BCL6 by Chromosomal Translocations Affecting Band 3q27 Cause Its Deregulated Expression During B-Cell Differentiation

The BCL6 gene encodes a POZ/Zinc-finger protein, which acts as a sequence-specific transcriptional repressor. It is expressed in B cells within the germinal centers (GC) and is required for GC formation. In ≈40% of diffuse large cell lymphomas (DLCL) and ≈14% of follicular lymphomas (FL), the BCL6 gene is rearranged by chromosomal translocations, which juxtapose heterologous promoters and 5′ untranslated sequences derived from other chromosomes to the BCL6 coding domain or by mutations in the 5′ regulatory region. To understand the functional consequence of the chromosomal translocations, we have studied the patterns of expression of the promoters found juxtaposed to BCL6 in DLCL and FL during B-lineage differentiation. Distinct heterologous 5′ untranslated regions (IGH, IGL, TTF) were identified fused to the BCL6 coding domain by analysis ofBCL6 cDNAs in two DLCL cases and one mixed follicular lymphoma (MxFL). These three sequences, as well as three other previously identified BCL6 fusion partners (IGHG3, BOB1,H4), were studied for their pattern of expression during B-lineage differentiation by Northern blot analysis of B-cell lines representative of the pre-B, B, immunoblast, and plasma cell stages. In contrast to BCL6, whose transcription is activated only in B cells within the GC, all of the other sequences displayed a broader pattern of expression ranging from constitutive expression throughout B-cell differentiation to persistent expression in immunoblasts and plasma cells. These results indicate that the expression ofBCL6 is deregulated as a consequence of fusion to heterologous promoter regions. The persistent expression of activated BCL6may contribute to lymphomagenesis by blocking B-cell differentiation within the GC.

Heterologous Promoters Fused to BCL6 by Chromosomal Translocations Affecting Band 3q27 Cause Its Deregulated Expression During B-Cell Differentiation

The BCL6 gene encodes a POZ/Zinc-finger protein, which acts as a sequence-specific transcriptional repressor. It is expressed in B cells within the germinal centers (GC) and is required for GC formation. In ≈40% of diffuse large cell lymphomas (DLCL) and ≈14% of follicular lymphomas (FL), the BCL6 gene is rearranged by chromosomal translocations, which juxtapose heterologous promoters and 5′ untranslated sequences derived from other chromosomes to the BCL6 coding domain or by mutations in the 5′ regulatory region. To understand the functional consequence of the chromosomal translocations, we have studied the patterns of expression of the promoters found juxtaposed to BCL6 in DLCL and FL during B-lineage differentiation. Distinct heterologous 5′ untranslated regions (IGH, IGL, TTF) were identified fused to the BCL6 coding domain by analysis ofBCL6 cDNAs in two DLCL cases and one mixed follicular lymphoma (MxFL). These three sequences, as well as three other previously identified BCL6 fusion partners (IGHG3, BOB1,H4), were studied for their pattern of expression during B-lineage differentiation by Northern blot analysis of B-cell lines representative of the pre-B, B, immunoblast, and plasma cell stages. In contrast to BCL6, whose transcription is activated only in B cells within the GC, all of the other sequences displayed a broader pattern of expression ranging from constitutive expression throughout B-cell differentiation to persistent expression in immunoblasts and plasma cells. These results indicate that the expression ofBCL6 is deregulated as a consequence of fusion to heterologous promoter regions. The persistent expression of activated BCL6may contribute to lymphomagenesis by blocking B-cell differentiation within the GC.

BCL8, a novel gene involved in translocations affecting band 15q11-13 in diffuse large-cell lymphoma

Translocations affecting the chromosomal region 15q11-13 and various other partners are recurrent in diffuse large-cell lymphomas (DLCL). To identify the putative gene, here named BCL8, involved in these translocations we have cloned the breakpoint region from a DLCL patient with t(14;15)(q32;q11-13) and the corresponding germ-line region from chromosome 15. The genomic locus on chromosome 15 is clonally rearranged in about 4% of DLCL in agreement with the frequency of 15q11-13 translocations. A probe derived from the BCL8 locus on chromosome 15 detected a transcript in human testis and prostate, whereas no expression was found in spleen, thymus, and blood leukocytes. Analysis of the BCL8 cDNA clones isolated from human testis cDNA library showed that the BCL8 gene generates a major transcript of 2.6 kb and a less prominent 4.5-kb species due to differential polyadenylylation. By reverse transcription-PCR analysis of RNA extracted from frozen DLCL samples and lymphoma cell lines, BCL8 expression was detected in all patients carrying 15q11-13 abnormalities and in a fraction of randomly selected DLCL patients. These results suggest that the BCL8 gene is not normally expressed in lymphoid tissues, but its expression can be activated by chromosomal translocation or by other mechanisms in DLCL. Ectopic expression of BCL8 in a significant proportion of DLCL suggests an important role for this gene in the molecular pathogenesis of B cell lymphoma.

BCL-6, a POZ/zinc-finger protein, is a sequence-specific transcriptional repressor

Approximately 40% of diffuse large cell lymphoma are associated with chromosomal translocations that deregulate the expression of the BCL6 gene by juxtaposing heterologous promoters to the BCL-6 coding domain. The BCL6 gene encodes a 95-kDa protein containing six C-terminal zinc-finger motifs and an N-terminal POZ domain, suggesting that it may function as a transcription factor. By using a DNA sequence selected for its ability to bind recombinant BCL-6 in vitro, we show here that BCL-6 is present in DNA-binding complexes in nuclear extracts from various B-cell lines. In transient transfectin experiments, BCL6 can repress transcription from promoters linked to its DNA target sequence and this activity is dependent upon specific DNA-binding and the presence of an intact N-terminal half of the protein. We demonstrate that this part of the BCL6 molecule contains an autonomous transrepressor domain and that two noncontiguous regions, including the POZ motif, mediate maximum transrepressive activity. These results indicate that the BCL-6 protein can function as a sequence-specific transcriptional repressor and have implications for the role of BCL6 in normal lymphoid development and lymphomagenesis.

Deregulation of PAX-5 by translocation of the Emu enhancer of the IgH locus adjacent to two alternative PAX-5 promoters in a diffuse large-cell lymphoma

Analyses of the human PAX-5 locus and of the 5' region of the mouse Pax-5 gene revealed that transcription from two distinct promoters results in splicing of two alternative 5' exons to the common coding sequences of exons 2-10. Transcription from the upstream promoter initiates downstream of a TATA box and occurs predominantly in B-lymphocytes, whereas the TATA-less downstream promoter is active in all Pax-5-expressing tissues. The human PAX-5 gene is located on chromosome 9 in region p13, which is involved in t(9;14)(pl3;q32) translocations recurring in small lymphocytic lymphomas of the plasmacytoid subtype and in derived large-cell lymphomas. A previous molecular analysis of a t(9;14) breakpoint from a diffuse large-cell lymphoma (KIS-1) demonstrated that the immunoglobulin heavy-chain (IgH) locus on 14q32 was juxtaposed to chromosome 9p13 sequences of unknown function [Ohno, H., Furukawa, T., Fukuhara, S., Zong, S. Q., Kamesaki, H., Shows, T. B., Le Beau, M. M., McKeithan, T. W., Kawakami, T. & Honjo, T. (1990) Proc. Natl. Acad. Sci. USA 87,628-632]. Here we show that the KIS-1 translocation breakpoint is located 1807 base pairs upstream of exon 1A of PAX-5, thus bringing the potent Emu enhancer of the IgH gene into close proximity of the PAX-5 promoters. These data suggest that deregulation of PAX-5 gene transcription by the t(9;14)(pl3;q32) translocation contributes to the pathogenesis of small lymphocytic lymphomas with plasmacytoid differentiation.

Frequent somatic hypermutation of the 5' noncoding region of the BCL6 gene in B-cell lymphoma

The BCL6 gene encodes a zinc-finger transcription factor and is altered by chromosomal arrangements in its 5' noncoding region in approximately 30% of diffuse large-cell lymphoma (DLCL). We report here that, in 22/30 (73%) DLCL and 7/15 (47%) follicular lymphoma (FL), but not in other tumor types, the BCL6 gene is also altered by multiple (1.4 x 10(-3) -1.6 x 10(-2) per bp), often biallelic, mutations clustering in its 5' noncoding region. These mutations are of somatic origin and are found in cases displaying either normal or rearranged BLC6 alleles indicating their independence from chromosomal rearrangements and linkage to immunoglobulin genes. These alterations identify a mechanism of genetic instability in malignant B cells and may have been selected during lymphomagenesis for their role in altering BCL6 expression.