Mature B cell lymphomas are a diverse set of diseases which share the property of having failed to complete the differentiation process into plasma cells or undergo apoptosis. PRDI-BF1 and its murine homologue Blimp-1 are transcriptional repressors and act as a molecular switch to commit activated B cells to become non-dividing plasma cells or undergo apoptosis. In this role PRDI-BF1 has broad significance for normal humoral immune responses and in malignancies of mature B cells and plasma cells. We hypothesize that failure to induce PRDI-BF1 expression or function contributes to the persistence of lymphomas and as such therapies designed to rescue PRDI-BF1 function may be important in treating lymphoma. Artificial overexpression of PRDI-BF1 leads to apoptosis in multiple lymphoma lines. Furthermore the hypothesis is supported by our finding that treatment of lymphoma cells with chemotherapeutic agents such as proteasome or histone deacetylase inhibitors leads to an early induction of PRDI-BF1 expression. We have made the recent discovery that PRDI-BF1 directly recruits the histone methyltransferase G9a to mediate silencing of interferon-beta. This suggests chromatin remodeling is the means by which PRDI-BF1 drives terminal differentiation of B cells and kills lymphoma cells. The impact of chromatin remodeling driven by PRDI-BF1 in this process will be investigated. PRDI-BF1 is transcriptionally regulated by unknown mechanisms. This will be investigated and will provide the first detailed understanding of transcriptional regulation at this critical developmental time point and may suggest new avenues to induce PRDI-BF1. Finally, PRDI-BF1 function requires a highly conserved SET-like domain but its activity is unknown. We will identify the proteins interacting at this site and determine their role in PRDI-BF1 activity. Thus this proposal will shed new light on the events occurring during differentiation and may indicate new targets to affect the growth and persistence of lymphomas.
