Abstract 3436: PRMT5 upregulation and its oncogenic cooperation with PI3K/AKT signaling in diffuse large B cell lymphoma

Protein arginine methyltransferase PRMT5, which regulates gene expression by symmetric dimethylation of histones and non-histone target proteins, is overexpressed and plays a pathogenic role in many cancers. A growing literature demonstrates a critical role of PRMT5 in tumorigenesis. PRMT5 expression is upregulated in various cancers, including diffuse large B cell lymphoma (DLBCL), the most common, aggressive form of non-Hodgkin lymphoma. PRMT5 upregulation is associated with Epstein-Barr virus (EBV) infection. Given that less than 10% of DLBCL is EBV-positive, the molecular mechanisms of PRMT5 overexpression in DLBCL are still largely unknown. Here, we demonstrated increased expression of PRMT5 in DLBCL cell lines and in a tissue microarray of 104 DLBCL cases when compared with normal naive B cells. Notably, the level of PRMT5 expression was also elevated in germinal center B cells, which experience antigen stimulation and give rise to DLBCL. In addition, PRMT5 expression in these DLBCL cases correlated with BTK expression, a key component of the B cell antigen receptor (BCR) signaling pathway and an effective drug target in DLBCL. We dissected the mechanisms of differential BCR signaling in regulating PRMT5 expression. Using the CRISPR/Cas9 genome editing technology or by specific pharmacological inhibitors, we discovered that two main BCR downstream signaling pathways, NF-kB and PI3K/AKT, contribute to PRMT5 overexpression. Specifically, the NF-kB member p65 and MYC, a downstream target of the PI3K/AKT signaling pathway, induced PRMT5 transcription. More importantly, inhibition of PRMT5 by its sgRNA or its specific inhibitor was lethal to all DLBCL cell lines tested but not to normal B cells. The antitumor effect of PRMT5 sgRNA or its inhibitor was further revealed in two DLBCL xenograft mouse models as well as in patient derived xenografts (PDX). Further RNA-Seq and biochemical analysis demonstrated that PRMT5 promotes cell cycle progression and activates PI3K-AKT signaling, suggesting a positive feedback regulatory mechanism to enhance cell survival and proliferation. To intervene in the positive feedback regulatory mechanism, we used the PRMT5 inhibitor GSK3326595 and the AKT inhibitor AZD5363, both of which are currently used in clinical trials for solid cancers. Indeed, our in vitro analysis in DLBCL cell lines demonstrated their synergistic cytotoxicity. We will further test the synergism of these two inhibitors in xenograft mouse models as well as in primary cancer cells. Taken together, our study elucidates the mechanisms of PRMT5 upregulation and also demonstrates an oncogenic role for PRMT5 in DLBCL, suggesting targeting PRMT5 by a specific inhibitor or co-targeting with an AKT inhibitor as a potential therapeutic strategy for DLBCL patients. Citation Format: Lixin Rui. PRMT5 upregulation and its oncogenic cooperation with PI3K/AKT signaling in diffuse large B cell lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3436.