Abstract 1404: Coactivator-associated arginine methyltransferase 1 regulates the transcriptional coactivator function of glioblastoma tumor suppressor Quaking

Abstract As the most common and malignant primary brain tumor in adults, glioblastoma (GBM) remains highly refractory to current therapeutic efforts, and patient outcome has not markedly improved in the past decades. Significant intratumor heterogeneity drives aggressive invasion, immune evasion, and tumor recurrence. Novel therapeutic paradigms informed by deeper mechanistic insights of gliomagenesis are urgently needed. Quaking (Qki, encoded by Qk) is frequently altered in GBM patients, with the Qk gene mutated or deleted in 35% percent of GBM cases and the Qki protein absent in 50-60%. Our lab has established Qki as an important tumor suppressor in GBM, where Qk deletion in a genetic mouse model induces the formation of tumors with high penetrance that recapitulate human GBM. Mechanistically, Qki acts as a transcriptional coactivator for the expression of lipid metabolism genes, in which loss of Qki dysregulates lipid homeostasis and compromises the integrity and dynamics of various membrane structures. While several downstream processes governed by Qki have been delineated, upstream regulators of Qki are largely unknown. By examining cancer patient genomic data, we observe recurrent mutations to specific Qki arginine residues that implicate the dysregulation of arginine methylation. This post-translational modification is catalyzed by protein arginine methyltransferases, which play key roles in transcriptional regulation and co-activator function. Through in vitro methylation assays, we demonstrate that Qki is selectively methylated by the coactivator-associated arginine methyltransferase 1 (CARM1), also known as PRMT4. This methylation occurs specifically in the C-terminal Quaking regulatory domain (QRD), where cancer mutations have been reported. We observe that Qki directly interacts with known CARM1 substrates including RNA polymerase II and MED12 of the Mediator complex and is co-detected with these binding partners at the promoters of lipid metabolism genes. Through mutagenesis studies, we identified arginine 242 (R242) and arginine 256 (R256) as specific CARM1-mediated methylation sites on Qki. Mutations to these sites in glioma stem cells compromise Qki’s activity as a transcriptional coactivator. Moreover, brain-specific deletion of CARM1 in a novel genetic mouse model results in neurological phenotypes reminiscent of those observed previously with Qki deletion. Altogether, we provide evidence that CARM1-mediated arginine methylation represents a novel regulatory mechanism for Qki, an essential tumor suppressor in GBM. Citation Format: Kaylene Lu, Tanner Wright, Seula Shin, Mark T. Bedford, Jian Hu. Coactivator-associated arginine methyltransferase 1 regulates the transcriptional coactivator function of glioblastoma tumor suppressor Quaking [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1404.