P3.03-005 Inhibition of PRMT5 is Synthetic Lethal in Mesotheliomas Harboring MTAP Loss: Topic: Mesothelioma Transitional

Abstract

Mesothelioma remains an incurable cancer with limited therapy. Genetically targeted personalized treatment strategies are currently lacking. Mesotheliomas harbor frequent loss of chromosome 9p21.3 locus encoding for CDKN2A and frequently encompassing methyladenosine phosphoryl transferase (MTAP). Loss of MTAP has recently been shown to be associated with dependency on the symmetrical demethylation of arginine-4 on histone H4 methyltransferase PRMT5. We sought to determine whether mesothelioma cells with MTAP HD would be vulnerable to inhibition of PRMT5, and to explore the pharmacodynamics associated with its suppression. Genome-wide copy number variation (CNV) analysis was undertaken in 94 patients. CNVs were determined using the array based platform, Affymetrix Oncoscan v3. Multiregional whole exome sequencing was also performed on samples from 6 patients. The expression of MTAP and the effect of the drugs tested on H4R3Me2s was evaluated by western blot. Cell growth was analyzed by clonogenic assay after focused RNAi targeting MTAP and/or PRMT5, or treatment with a PRMT5 inhibitor. Oncoscan analysis identified homozygous loss of CDKN2A in 50%, and heterozygous loss in 20.2% of patients. Homozygous loss of MTAP was seen in 39.3% and heterozygous loss in 28.7%, 76.6% of patients had concurrent loss of CDKN2A and MTAP. Homozygous MTAP deletion was found to be present in all regions of tumor ie a truncal deletion, in 3 out of 6 patients. In 65 patients treated with surgery only, homozygous loss of CDKN2A or MTAP was prognostic for progression free survival (CDKN2A: 7.5 vs. 32.9 months, HR 4.536 95% CI 1.765-11.659, p=0.002; MTAP: 7.5 vs. 18.4 months, HR 3.289 95% CI 1.314-8.237, p=0.007). To determine the dependency of MTAP negative cells on PRMT5, we used either siRNA targeting PRMT5 or a PRMT5 inhibitor. PRMT5 silencing did not induce measurable apoptosis however a significant suppression of clonogenic activity was observed after 10 days in MTAP negative cells (H2591). The same effect was observed after concurrent silencing of PRMT5 and MTAP in MTAP positive mesothelioma cells (MPP89). We then utilized a PRMT5 small molecule inhibitor, EPZ015666, to recapitulate RNAi knockdown. Although a clear suppression of H4R3Me2s was observed after 96 hours treatment, the relative potency on clonogenic assay was less than RNAi. Homozygous deletion of MTAP is a common genetic event in mesothelioma. Suppression of PRMT5 represents a novel potential approach for targeting mesotheliomas with 9p21.3 loss. The discrepancy between small molecule inhibitors and RNAi, suggests that PRMT5 dependence may require functions that extend beyond its methyltransferase function.