PRMT5 regulates the splicing of detained introns in proliferation-associated genes in GBM.

  • Major finding: PRMT5 regulates the splicing of detained introns in proliferation-associated genes in GBM.

  • Approach: Gl261 cells transduced with an shRNA library were grown in vitro and in vivo with or without TMZ.

  • Impact: PRMT5 may be a potential therapeutic target for patients with GBM that exhibit a high CLNS1A/RIOK1 ratio.

Glioblastomas (GBM) are the most common adult brain malignancy, for which there are few targeted therapeutic options; however, it has been difficult to identify new therapeutic options for patients with GBM due to considerable GBM intratumoral heterogeneity. To identify targetable vulnerabilities in GBM, Braun, Stanciu, Boutz, and colleagues developed a high-throughput in vitro and in vivo shRNA screen using the syngeneic mouse glioma cell line Gl261. Gl261 cells transduced with a pooled shRNA library targeting 312 epigenetic regulators were grown in vitro or intracranially implanted in vivo and treated with either temozolomide (TMZ) or control vehicle. Most of the shRNAs targeting PRMT5, an arginine methyltransferase that is a component of the methylosome, were depleted in all four growth/treatment conditions, and subsequently PRMT5 was identified as the gene that was most significantly associated with glioma growth. Ablation of PRMT5 by shRNA or the PRMT5 inhibitor EPZ revealed that PRMT5 methyltransferase activity was critical for GBM proliferation. Further, inhibition of PRMT5 resulted in cell-cycle arrest and cellular senescence in human GBM cells in vitro, and significantly reduced tumor growth of subcutaneous glioma and extended the survival of mice with intracranial glioma in vivo. EPZ treatment upregulated the splicing of detained introns (DI), but not canonical alternative splicing, in human glioma cells in vitro, and bioinformatic analysis showed that DI-containing genes upregulated by PRMT5 ablation are predominantly associated with proliferation and coordinately regulated during neurogenesis. EPZ IC50 analysis of a pan-cancer cell line panel identified EPZ-resistant cell lines, which exhibited high expression of genes associated with RNA processing and splicing, and gene expression analysis revealed that a higher ratio of CLNS1A expression to RIOK1 expression is predictive of EPZ response. These results describe the critical role of detained intron splicing in GBM proliferation and suggest that inhibition of PRMT5 may be a therapeutic strategy for patients with GBM.

Braun CJ, Stanciu M, Boutz PL, Patterson JC, Calligaris D, Higuchi F, et al. Coordinated splicing of regulatory detained introns within oncogenic transcripts creates an exploitable vulnerability in malignant glioma. Cancer Cell 2017 Sep 28 [Epub ahead of print].

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