High PRMT5 levels, maintained by KEAP1 inhibition, drive chemoresistance in ovarian cancer [CRISPR screen]
Protein arginine methyl transferases (PRMTs) are generally upregulated in cancers. However, the mechanisms leading to this upregulation and its biological consequences are poorly understood. Here, we identify PRMT5, the main symmetric arginine methyl transferase, as a critical driver of chemoresistance in high-grade serous ovarian cancer (HGSOC). Cancer transcriptome analysis shows that HGSOC tumors express the highest levels of PRMT5. Furthermore, PRMT5 levels and its enzymatic activity are further induced in a Platinum (Pt)-resistant state at the protein level. To reveal potential regulators of high PRMT5 protein levels, we optimized intracellular immunostaining conditions and performed an unbiased CRISPR screening to identify regulators of PRMT5 levels in OC cells. These results led to several critical regulators, including KEAP1, a top-scoring negative regulator of PRMT5 protein levels. Our mechanistic studies show that KEAP1 directly interacts with PRMT5, leading to its ubiquitin-dependent degradation under normal physiological conditions. However, Pt-treatment-mediated stress leads to KEAP1 inhibition, resulting in elevated levels of PRMT5, sufficient to confer cells resistant to Pt. At the genomic level, our chromatin immunoprecipitation and sequencing (ChIP) studies show that elevated PRMT5 directly interacts with the promoters of stress response genes and positively regulates their transcription. In line with increased PRMT5 levels in Pt-resistant cells, pharmacological inhibition of PRMT5 selectively renders these cells selectively sensitive to PRMT5 inhibition compared to chemo-naive or normal fallopian epithelial cells. As such, combined PRMT5 inhibition with Pt results in synergistic cellular cytotoxicity in vitro and tumor volume reduction in vivo in otherwise Pt-resistant patient-derived xenograft tumors. Overall, the findings from this study identify PRMT5 as a critical therapeutic target in Pt-resistant HGSOC cells and reveal the molecular mechanisms that lead to high PRMT5 levels in Pt-treated and chemo-resistant tumors. Overall design: To identify and characterize potential regulators of PRMT5, we conducted a CRISPR-Cas9-based genetic screen using chemoresistant COV362 cells.