Abstract LB-218: Protein arginine methyltransferase inhibition of malignant gliomas leads to restored chemokine expression and enhanced immune effector function

Patients with Glioblastoma Multiforme (GBM) face a poor prognosis despite multimodal therapy, thus, there is an unmet need for discovery of novel therapeutic targets and approaches. The immune-privileged nature of the central nervous system and down-modulation of cytokines and chemokines in GBM tumors led us to explore epigenetic approaches to restore expression of immune-relevant genes. PRMT5 is a type II arginine methyltransferase that catalyzes symmetric dimethylation of arginine residues on histone proteins leading to transcriptional repression. Our previous work showed PRMT5 overexpression correlates with poor clinical outcome of GBM patients and that PRMT5 silencing inhibited tumor growth in vitro and in vivo. Microarray transcriptional studies following PRMT5 depletion identified potential PRMT5 target genes interferon-inducible protein 10 (CXCL10) and interferon-inducible T cell α chemoattractant (CXCL11). Increased secretion of CXCL10 and CXCL11 has been shown to facilitate homing of innate and adaptive immune-effector populations that promote anti-GBM activity in vivo. Thus we further investigated the significance of restored CXCL10 and CXLC11 with PRMT5 inhibition in GBM. CXCL10 protein expression was found to be significantly lower in GBM tumors (N = 15) by immunochemistry staining, compared with grade I gliomas (N = 7) (p = 0.0001). PRMT5 knockdown led to increased mRNA and protein expression of CXCL10 and CXCL11. Chromatin immunoprecipitation (ChIP) experiments identified CXCL10 and CXCL11 promoters to be directly targeted by PRMT5 repressive complexes. Previous work has shown that PRMT5 associates with other co-repressor molecules including HDAC2, DNA methyltransferase 3a (DNMT3a) and methyl binding domain protein 2 (MBD2). ChIP studies confirmed that PRMT5, HDAC2, DNMT3a and MBD2 proteins were all recruited to the promoters of CXCL10 and CXCL11. Interestingly, when two patient-derived GBM cell lines were transfected with siRNA targeting PRMT5, recruitment of all co-repressor proteins, PRMT5, HDAC2, DNMT3a, and MBD2 was lost on CXCL10 and CXCL11 promoters. These findings suggest that PRMT5 is a master transcriptional repressor that plays a central role in coordinating and assembling co-repressor chromatin remodeling complexes on PRMT5 target gene promoters. PRMT5 silencing resulted in secretion of biologically relevant levels of CXCL10 and CXCL11 in culture medium that promoted immune cell recruitment in transwell migration assays toward target glioma cells. Our data suggests that in addition to restoring regulatory and tumor suppressor gene expression that promotes direct GBM cell death, PRMT5 silencing may enhance immune-mediated anti-tumor activity. These findings further justify the development experimental therapeutic strategies targeting PRMT5 to modulate activity of immune-relevant genes. Citation Format: Fengting Yan, Yeshavanth Banasavadi-Siddegowda, John T. Patton, Mark Lustberg, Xin Wu, Balveen Kaur, Rober A. Baiocchi. Protein arginine methyltransferase inhibition of malignant gliomas leads to restored chemokine expression and enhanced immune effector function. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-218. doi:10.1158/1538-7445.AM2015-LB-218