Variant translocation of the BCL6 gene to immunoglobulin kappa light chain gene in B-cell lymphoma

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.

Regulation of lymphocyte cell fate decisions and lymphomagenesis by BCL-6

Genetic alterations of the BCL-6 gene in mice and man have established BCL-6 as a pivotal regulator of normal differentiation of B and T lymphocytes as well as one of the most frequently translocated oncogenes in human B cell lymphomas. As an oncogene, BCL-6 has not been easy to place into existing paradigms of cellular transformation. Rather, it is likely that the function of BCL-6 as a regulator of lymphocyte differentiation is subverted in BCL-6-induced lymphomas. The lymphomas in which BCL-6 is translocated are all suspected to arise from the germinal center B lymphocyte. Given the selective expression of BCL-6 protein in normal germinal center B lymphocytes and the requirement for BCL-6 in germinal center development, the functions of BCL-6 in normal and malignant B cells are probably intertwined. The BCL-6 protein is a potent transcriptional repressor which presumably controls lymphocyte differentiation and induces lymphomas by regulating the expression of key downstream target genes.

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.

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.

Four cases of follicular lymphoma with t(14;18)(q32;q21) and t(3;4)(q27;p13) with LAZ3 (BCL6) rearrangement

We report four cases of follicular lymphoma with both t(14;18)(q32;q21) and the newly characterized t(3;4)(q27;p13). Molecular investigation confirmed LAZ3 (BCL6) rearrangement for all patients. The 3q27 aberrations have been rarely described in low-grade lymphomas and may represent secondary events whose implication remains to be elucidated.

Detection of chimaeric transcripts of the immunoglobulin heavy chain and BCL6 genes by reverse-transcriptase polymerase chain reaction in B-cell non-Hodgkin's lymphomas

T(3;14)(q27;q32) is frequently detected in B-cell non-Hodgkin's lymphomas, especially the diffuse large cell type and the follicular type. The BCL6 gene encoding a putative transcriptional factor which resides on 3q27 rearranges to the immunoglobulin heavy chain (IgH) gene on 14q32 in this chromosomal translocation. The upstream regulatory region of the BCL6 gene is replaced by the IgH gene. Deregulation of the BCL6 gene may contribute to tumourigenesis of these diseases. The rearrangement between the IgH and BCL6 genes generates chimaeric transcripts in which the joining (J) region of the IgH gene fuses to exon 3 of the BCL6 gene. We established a method to detect these chimaeric transcripts by reverse transcriptase polymerase chain reaction (RT-PCR) using the consensus sequence of the J region and the sequence of exon 3 of the BCL6 gene as primers. Using the semi-nested RT-PCR method and a cell line carrying t(3;14)(q27;q32), we detected one lymphoma cell among 10,000 background cells. We detected these chimaeric transcripts in two out of 13 clinical samples by this method. This method can detect t(3;14)(q27;q32) easily, whereas this alteration is frequently overlooked by routine karyotype analysis. Since this technique is sensitive enough to detect a small number of lymphoma cells with this genetic abnormality, it could be employed to detect contaminating lymphoma cells in bone marrow and peripheral blood and minimal residual diseases.

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

Chromosomal translocations involving 3q27 are among the most common recurring translocations in non-Hodgkin's lymphoma (NHL) of B-cell phenotype. Molecular cloning of junctional areas of the translocations resulted in isolation of the BCL6 gene adjacent to the breakpoint cluster on 3q27. The gene encodes a zinc-finger transcription factor which is expressed in nuclei of germinal center B-cells. Rearrangement of BCL6 was observed in 6.4 to 14.3% of follicular lymphomas and 28.6 to 35.5% of diffuse large cell lymphomas; regarding the latter, a Japanese series showed a lower incidence. Survival curves suggested that NHL carrying rearrangement of BCL6 and lacking that of BCL2 is curable by chemotherapy. Detailed analysis of the vicinity of translocations showed that the 5' untranslated region of BCL6 was replaced by heterogeneous promoters not only from immunoglobulin genes but also from many previously uncharacterized loci. Bcl-6 protein is expressed in NHL of follicular center B-cell origin, independently of the presence or absence of BCL6 rearrangement. At present, limited information is available about the functional consequences of the rearrangements and, in particular, about their ultimate implications for lymphomagenesis.

The ZiN/POZ domain of ZF5 is required for both transcriptional activation and repression

ZF5 is a ubiquitously expressed protein originally identified by its ability to bind and repress the murine c-myc promoter. It contains five C-terminal zinc fingers and a conserved N-terminal ZiN/POZ domain. This motif, found in a growing number of zinc finger proteins, can inhibit DNA binding and mediate dimerization [Bardwell, V.J. and Treisman,R. (1994) Genes Dev., 8,1664-1677]. In the current study, a cyclic amplification and selection of targets (CAST) protocol detected preferred ZF5 binding sites which are highly GC-rich. Binding to these sites by ZF5 depended upon the zinc fingers and was enhanced when the ZiN/POZ domain was removed. Using transient cotransfection assays, ZF5 was shown to activate the HIV-1 LTR and repress the beta-actin promoter. The ZiN/POZ domain was shown to mediate ZF5-dependent transcriptional activation and repression. From these data, we conclude that ZF5 can both activate and repress in the context of different natural promoters and that its ZiN/POZ domain can affect two functions; DNA binding and transcriptional modulation.