Transcriptional regulation of the BCL-6 gene: Mechanistic dissection using mutant cell lines

Background: Mice lacking the BCL-6 gene product exhibit overexpression of T helper cell type 2 cytokines, and multi-organ inflammatory responses characterized by eosinophilia and infiltration by IgE+ B lymphocytes. Elevated IgE levels appear to result from loss of BCL-6's role in repression of the Stat6-dependent processes of I- epsilon transcription and IgE class-switching. Understanding the regulation of the BCL-6 gene expression is therefore relevant to the allergic response, due to the IgE-dependent activation of mast cells and basophils. Materials and Methods: We analyzed a pair of isogenic cell lines that exhibit differential expression of the BCL-6 gene and mapped the control point to transcription using nuclear run-on assays. Direct analyses of the BCL-6 gene and transfection experiments were carried out to map the molecular basis for the differential expression. Results: In this study, we report that pair of isogenic cell lines exhibits differential expression of BCL-6. BCL-6 mRNA is barely detectable in Jijoye, a Burkitt lymphoma B cell line, but is abundant in Clone-13, a mutant cell line derived from Jijoye. Corresponding to the mRNA level, BCL-6 protein is detected only in Clone-13, but not in Jijoye. Nuclear run-on assays indicate that BCL-6 expression is regulated mainly at the transcriptional level in these two cell lines. Gene structural alterations are not detected in a region including a 3-kb promoter and the first intron, where most of the chromosomal translocations are observed in diffuse large cell lymphoma (DLCL). Interestingly, multiple, heterozygous point mutations are identified in the first intron, the hypermutation region of BCL-6, in both of the two cell lines. However, in transient transfection experiments these mutations have no effects on gene expression. Conclusions: Our data suggest that the distinct profile of BCL-6 expression in Jijoye/Clone-13 is due to either a missing negative element in a different portion of the gene or that there is an issue of chromatin accessibility operating in BCL-6 gene regulation. This pair of isogenic cell lines (Jijoye/Clone-13) thus provides a new system to dissect the regulation of BCL-6 gene expression.