Protein arginine methyltransferase 5 (PRMT5) is an actionable therapeutic target in CDK4/6 inhibitor-resistant ER+/RB-deficient breast cancer [pSer2 Pol II ChIP-seq]

CDK4/6 inhibitors (CDK4/6i) have improved survival of patients with estrogen receptor-positive (ER+) breast cancer. However, patients treated with CDK4/6i eventually develop drug resistance and progress. RB1 loss-of-function alterations confer resistance to CDK4/6i, but the optimal therapy for these patients is unclear. Through a genome-wide CRISPR screen, we identified protein arginine methyltransferase 5 (PRMT5) as a molecular vulnerability in ER+/RB1-knockout (RBKO) breast cancer cells. Inhibition of PRMT5 blocked the G1-to-S transition in the cell cycle independent of RB, leading to growth arrest in RBKO cells. Proteomics analysis uncovered fused in sarcoma (FUS) as a downstream effector of PRMT5. Inhibition of PRMT5 resulted in dissociation of FUS from RNA polymerase II (Pol II), which led to Ser2 Pol II hyperphosphorylation, intron retention, and subsequent downregulation of proteins involved in DNA synthesis. Furthermore, treatment with the PRMT5 inhibitor pemrametostat and a selective ER degrader fulvestrant synergistically inhibited growth of ER+/RB-deficient cell-derived and patient-derived xenografts. These findings highlight the potential of dual ER and PRMT5 blockade as a novel therapeutic strategy to overcome resistance to CDK4/6i in ER+/RB-deficient breast cancer. To investigate distribution of pSer2 Pol II bindings upon inhibition of PRMT5, ER+/RBKO cells were treated with pemrametostat or DMSO control. ChIP-seq was conducted using a pSer2 Pol II antibody.