The chromosomal translocation involving 3q27 has been recently described in B-cell malignancies, especially in diffuse large cell lymphomas. We have previously cloned the breakpoint cluster region of 3q27 designated as the BCL6 locus, previously known as BCL5, and subsequently cloned the cDNA for the BCL6 (we previously reported it as BCL5) gene encoding a novel Cys2-His2 zinc-finger protein, which locates adjacent to the breakpoints and is activated through the translocation. To elucidate whether rearrangements occur within the BCL6 gene, we characterized the genomic structure of the gene. The BCL6 gene encompasses about 26 kilobases (kb) and consists of nine exons. Translation start site is located in exon 3 and zinc-finger motif is distributed in the exons 6 to 9. We have identified at least two types of mRNA alternatively spliced, which contain or do not contain exon 2 of 134 bp coding for the 5' untranslated region. A large intron 1 of 9 kb is not efficiently spliced out, which might result in the creation of minor 10-12-kb transcripts observed in the Northern blot analysis in addition to major 3.8-kb transcripts. The breakpoints are clustered around the first exon, and the putative regulatory region of the BCL6 gene is removed through the translocation, leading to the over-expression of the gene.

Chromosomal translocations involving band 3q27 are the recently described nonrandom cytogenetic abnormalities in B-cell malignancies. We have previously cloned the breakpoint region of 3q27, designated as the BCL5 locus, from the B-cell line carrying the t(3;22). The cDNA for the BCL5 gene was cloned from the human liver cDNA library. The nucleotide sequencing analysis showed that the BCL5 gene encodes a potential transcription factor containing six repeats of the Cys2-His2 zinc-finger motif resembling the Drosophila segmentation gene Krüppel. The calculated molecular weight was 78.8 kD, which was supported by an in vitro transcription and translation experiment. A part of the sequence was essentially identical to that of a genomic fragment, ZNF51, previously reported to be located at 3qter. The translocation occurred in the 5' region of the BCL5 gene, and the protein-coding exons were fused to the Ig-lambda gene in a head-to-head configuration in the cell line carrying t(3;22). The BCL5 cDNA probe detected a major transcript of 3.8 kb in Burkitt's lymphoma cell lines and an aberrant transcript in the t(3;22) cell line, whereas no transcript was detected in myeloid, monocytoid, erythroid, T-lymphoid, and Epstein-Barr virus-immortalized B-lymphoblastoid cell lines.

Reciprocal exchanges between chromosomal region 3q27 and three loci of the Ig genes have been reported in cases of B-cell type non-Hodgkin's lymphoma. We have cloned a region containing a breakpoint junction of 3q27 from a cell line established from a patient with Burkitt's lymphoma carrying t(3;22)(q27;q11). The region cloned was shown to contain an Ig lambda light chain gene fused to a gene on chromosome 3q27. This finding was subsequently confirmed by fluorescence in situ hybridization. Extra nucleotides were present at the joining site. The heptamer-like and nonamer-like sequences separated by an intervening 24 bp were present in the region corresponding to the breakpoint of 3q27, suggesting that a misrecombination in Ig gene rearrangement may be involved in the translocation. Southern blot analysis with a 3q27-specific probe showed rearrangements in three additional patients with B-cell malignancies with the t(3;14)(q27;q32). The breakpoints of all four cases clustered within a limited 3-kb region on chromosome 3q27. The region of 3q27 involved in the translocation was designated as the BCL5 locus. The transcripts from the BCL5 locus were detected in normal tissues and hematopoietic cell lines, and the increased expression of transcript of aberrant size was detected in the established cell line carrying t(3;22). These observations suggest that a gene located at 3q27 is involved in the translocation and that its deregulation plays a role in the malignant transformation of B cells.