PRDM1 is required for mantle cell lymphoma response to bortezomib

Mantle cell lymphoma (MCL) is an aggressive form of B-cell lymphoma with a poor disease-free survival rate. The proteasome inhibitor bortezomib is approved for the treatment of relapsed and refractory MCL and has efficacy in about 30% of patients. However, the precise mechanism of action of bortezomib is not well understood. This report establishes a requirement for the transcription repressor PR domain zinc finger protein 1 (PRDM1, Blimp1) in the response to bortezomib. Bortezomib rapidly induces transcription of PRDM1 as part of the apoptotic response in both cell lines and primary MCL tumor cells. Knockdown of PRDM1 blocks activation of NOXA and inhibits apoptosis, whereas ectopic expression of PRDM1 alone leads to apoptosis in MCL. Two novel direct targets of PRDM1 were identified in MCL cells: MKI67 (Ki67) and proliferating cell nuclear antigen (PCNA). Both MKI67 and PCNA are required for proliferation and survival. Chromatin immunoprecipitation and knockdown studies reveal that specific repression of MKI67 and PCNA is mediated by PRDM1 in response to bortezomib. Furthermore, promoter studies and mutation/deletion analysis show that PRDM1 functions through specific sites in the PCNA proximal promoter and an MKI67 distal upstream repression domain. Together, these findings establish PRDM1 as a key mediator of bortezomib activity in MCL.