Transcriptional modulation by PRMT5 inhibition with EPZ015666 (GSK3235025)
Aberrant protein arginine methylation has been observed in multiple cancer types, making it an attractive drug target. Proteins can undergo asymmetric arginine methylation by type I protein arginine methyltransferases (PRMTs), predominately by PRMT1 and to a lesser extent PRMT4, or symmetric arginine methylation by type II PRMTs, predominately PRMT5. Here, we performed targeted proteomics following inhibition of PRMT1, PRMT4, and PRMT5 across cancer cell lines. We found that inhibition of both type I and type II PRMTs suppressed levels of total and phosphorylated ATR protein in cancer cell lines, and down-regulated expression of the ATR gene. Loss of ATR from PRMT inhibition resulted in defective DNA replication stress response activation in following exogenous replication stress. Since PARP inhibitors are known to induce replication stress, we next combined PRMT inhibition with PARP inhibition and found inhibition of PRMT1 or PRMT5 greatly exacerbated PARP inhibitor induced DNA damage. Based on this observation, we assessed the combination of PARP and PRMT inhibition in a panel of cell lines. While inhibition of both type I and type II PRMTs were synergistic with PARP inhibition in both cells with intact and deficient homologous recombination, type I PRMT inhibition resulted in higher toxicity in non-malignant cells. Therefore, we validated the synergy of combined PARP/PRMT5 inhibition in primary patient-derived organoids. Finally, we demonstrate that the combination of PARP and PRMT5 inhibition improves overall survival in both BRCA-mutant and wild-type patient-derived xenograft models without any detectable hematological toxicities typically associated with PARPi combination therapies. Taken together, these results demonstrate that PRMT5 inhibition may be a well-tolerated approach to improve tumor sensitivity to PARP inhibition. . Overall design: Cells were treated with either DMSO vehicle control or 5 micromolar EPZ015666 for 3 days or 5 days prior to RNA isolation and stranded RNAseq.