Structural alterations of the promoter region of the BCL-6 proto-oncogene represent the most frequent genetic alteration associated with non-Hodgkin lymphoma, a malignancy often deriving from germinal-centre B cells. The BCL-6 gene encodes a zinc-finger transcriptional repressor normally expressed in both B cells and CD4+ T cells within germinal centres, but its precise function is unknown. We show that mice deficient in BCL-6 displayed normal B-cell, T-cell and lymphoid-organ development but have a selective defect in T-cell-dependent antibody responses. This defect included a complete lack of affinity maturation and was due to the inability of follicular B cells to proliferate and form germinal centres. In addition, BCL-6-deficient mice developed an inflammatory response in multiple organs characterized by infiltrations of eosinophils and IgE-bearing B lymphocytes typical of a Th2-mediated hyperimmune response. Thus, BCL-6 functions as a transcriptional switch that controls germinal centre formation and may also modulate specific T-cell-mediated responses. Altered expression of BCL-6 in lymphoma represents a deregulation of the pathway normally leading to B cell proliferation and germinal centre formation.

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 Cterminal 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 transfection experiments, BCL6 can repress transcription from promoters linked to its DNA target sequence and this activity is dependent upon specific DNAbinding 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.The BCL6 gene was identified by virtue of its involvement in chromosomal translocations affecting band 3q27 in nonHodgkin lymphoma (1-5). Subsequent studies have demonstrated that rearrangements of the BCL6 gene can be found in 30-40% of diffuse large cell lymphoma and 6-11% follicular lymphoma (6-8). These alterations cause the deregulated expression of the BCL6 gene by a mechanism called promoter substitution, that is the juxtaposition of heterologous promoters, derived from other chromosomes, to the BCL-6 coding domain (9). Recent evidence also indicates that the 5' noncoding region of the BCL6 gene is altered by somatic point mutations that are found, independent of rearrangements, in "70% diffuse large cell lymphoma and 45% follicular lymphoma (10). Thus, most cases of diffuse large cell lymphoma and a significant fraction of follicular lymphoma carry structural alterations of the regulatory region of the BCL6 gene, suggesting that deregulated BCL6 expression may be important for lymphomagenesis (11).The BCL-6 protein is a 95-kDa nuclear phosphoprotein detectable at low abundance in multiple tissues and expressed at high levels exclusively in mature B cells (12)(13)(14). Within the B-cell lineage, the expression of the BCL-6 protein is specifically regulated during differentiation since it is only detectable in B cells within germinal centers (GCs) but not in pre-GC cells or in differentiated progenies such as plasma cells (12)(13)(14). The structure of the BCL-6 protein includes six Kruppel-type C-terminal zinc-finger (ZF) motifs (2,4,5)

The BCL6 gene encodes a zinc-finger transcription factor and is altered by chromosomal rearrangements in its 5' noncoding region in '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 BCL6 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. Sequencing Procedures. PCR products E1.10, E1.11, and E1.12 were subjected to direct sequence analysis as described (21). In addition, a unique PCR product encompassing the same genomic region (nucleotides +413 to + 1141), amplified by primers E1.21C (upstream; 5'-ATGCTTTGGCTCCAA-GTT-3') and E1.26 (downstream; 5'-CACGATACTTCA-TCTCATC-3'), annealing temperature = 54°C, was subcloned into the pGEM-T vector (Promega), and DNA minipreps (Wizard DNA purification system, Promega) were sequenced using forward and reverse primers with the ABI373A DNA sequencer (Perkin-Elmer, Applied Biosystems Division).

The bcl-6 proto-oncogene encodes a POZ/zinc finger transcriptional repressor expressed in germinal center (GC) B and T cells and required for GC formation and antibody affinity maturation. Deregulation of bcl-6 expression by chromosomal rearrangements and point mutations of the bcl-6 promoter region are implicated in the pathogenesis of B-cell lymphoma. The signals regulating bcl-6 expression are not known. Here we show that antigen receptor activation leads to BCL-6 phosphorylation by mitogen-activated protein kinase (MAPK). Phosphorylation, in turn, targets BCL-6 for rapid degradation by the ubiquitin/proteasome pathway. These findings indicate that BCL-6 expression is directly controlled by the antigen receptor via MAPK activation. This signaling pathway may be crucial for the control of B-cell differentiation and antibody response and has implications for the regulation of other POZ/zinc finger transcription factors in other tissues.

Rearrangement of the bcl-6 gene correlated with a favorable clinical outcome in DLLC and may thus serve as a prognostic marker in patients with this form of malignant lymphoma.