The proto-oncogene BCL6 encodes a BTB/POZ-zinc finger transcriptional repressor that is necessary for germinal center (GC) formation and is implicated in the pathogenesis of B-cell lymphoma. In ~50% diffuse large cell lymphoma and 10% follicular lymphoma, BCL6 gene expression is deregulated by chromosomal translocations or mutations that affect its 5′ regulatory region. The precise function of BCL6 in GC development and lymphomagenesis is unclear since very few BCL6 direct target genes have been identified. We report that BCL6 suppresses p53-dependent and p53-indepenent growth arrest and apoptosis responses in GC B cells. BCL6 directly suppresses the transcription of the p53 gene, as demonstrated by (1) chromatin immunoprecipitation (ChIP) assays showing that BCL6 binds the p53 promoter region in vivo; and (2) transient transfection/reporter assays identifying within the p53 promoter region two BCL6-binding sites that mediate BCL6-mediated suppression of p53 transcription.
Accordingly, suppression of BCL6 expression via specific siRNA leads to increased expression of p53 both under basal condition and in response to DNA damage. Consistent with a physiological role for BCL6-mediated p53 suppression, immunohistochemical analysis shows that p53 expression is absent in GC B cells where BCL6 is highly expressed. In addition, our data reveal that BCL6 inhibits the p53-independent activation of the p21/WAF1 cell cycle arrest gene by binding to Miz-1, a transcription factor involved in p21 activation. Consistent with a role of BCL6 in inhibiting p53-related cell cycle arrest and apoptotic responses, constitutive expression of BCL6 suppresses p53 expression and p53-target genes (P21 and PUMA) and protects B cell lines from apoptosis induced by DNA damage. These results indicate that one function of BCL6 is to allow GC B cells (centroblasts) to constitutively proliferate and to sustain the physiologic DNA breaks required for immunoglobulin switch recombination and somatic hypermutation without inducing p53-related responses. These findings also imply that B cell lymphoma with deregulated BCL6 expression are functionally p53-negative and impaired in apoptotic responses.