In vivo shRNA screen reveals PRMT5 as a key regulator in glioblastoma

Abstract

Glioblastoma (GBM), the most prevalent primary brain tumor in adults, remains a largely intractable malignancy that carries an extremely poor prognosis. Large scale genomic analyses have greatly expanded our knowledge of the genomic landscape of GBM. Recently, we have also begun to gain an appreciation for how dysregulation of epigenetic control can contribute to gliomagenesis. To expand our understanding of the roles of epigenetic mediators in GBM, we establish an orthotopic transplantation model and demonstrate its feasibility for in vivo shRNA screening. We describe the results of parallel in vivo and in vitro shRNA screens to uncover epigenetic regulators with oncogenic functions and chemosensitizing potential in GBM. From these screens, the protein arginine methyltransferase PRMT5 emerges as an important mediator of GBM tumor growth in the presence and absence of chemotherapy. We demonstrate that PRMT5 loss impairs cellular fitness in vitro and this effect is dependent on its methyltransferase activity. Furthermore, in vivo transplantation of both murine and human GBM cells shows that PRMT5 knockdown impairs tumor growth and leads to a survival benefit. Building on these observations, we report the first use of a highly selective PRMT5 inhibitor in extending survival in a GBM xenograft model. We also describe the results of initial in vitro and in vivo experiments combining PRMT5 genetic depletion or pharmacological inhibition with chemotherapy. To gain a better understanding of the effects of PRMT5 inhibition, we perform gene expression analysis, and find a putative role for PRMT5 in maintaining GBM cell identity. Importantly, we observe that PRMT5 impacts gene expression by regulating RNA splicing to exert specific control over a recently described class of intronic sequences known as detained introns. Together, the results presented here highlight a key role for PRMT5 in GBM and suggest that it represents a viable target for therapeutic intervention in this intractable disease.