Abstract IA25: Targeting cancers with deletion of the p16/p14 tumor suppressor locus

Abstract Mutations in tumor suppressor genes are a hallmark of cancer and contribute substantially to the molecular pathogenesis of the disease. Particularly prevalent are homozygous deletions of the CDKN2A tumor suppressor locus on chromosome 9p21, which occur in at least 15% of all cancer and lead to loss of the dual tumor suppressors p16(INK4A) and p14(ARF). Targeted therapies selective for loss of CDKN2A have proven elusive, and it may be necessary to identify alternate approaches to target tumors with this deletion. Notably, Chr9p21 deletions frequently involve co-deletion of genes proximal to CDKN2A. Foremost among these is the methylthioadenosine phosphorylase (MTAP) gene, which resides within 100 kb of CDKN2A, and is co-deleted in 80-90% of tumors with CDKN2A deletion. We sought therefore to evaluate whether the frequent co-deletion of the adjacent metabolic gene methylthioadenosine phosphorylase (MTAP) creates targetable vulnerabilities. We conducted an shRNA screen in isogenic cells that varied only in their MTAP-deletion status, and identified the metabolic enzyme, methionine adenosyltransferase II, alpha (MAT2A), and the arginine methyltransferase, PRMT5, as vulnerable enzymes in cells with MTAP deletion. Metabolomic and biochemical studies revealed a mechanistic basis for this synthetic lethality. MTAP substrate methylthioadenosine (MTA) accumulates upon MTAP loss, and biochemical profiling of a methyltransferase enzyme panel revealed that MTA is a potent and selective inhibitor of PRMT5. MTAP-deletion leads to reduced basal PRMT5 methylation activity and increased sensitivity to PRMT5 targeting. MAT2A produces PRMT5 substrate S-adenosylmethionine (SAM), and targeting of MAT2A reduces growth and PRMT5 methylation activity selectively in MTAP-deleted cells. Further, this vulnerability extends to PRMT5 co-complex proteins such as the atypical kinase Rio Kinase 1 (RIOK1). Thus, MAT2A, PRMT5, and RIOK1 comprise an unappreciated axis of targets that is selectively vulnerable in CDKN2A/MTAP-deleted tumors. Unique biochemical features of PRMT5 create this vulnerability, which presents a promising new avenue for targeted therapy. Citation Format: Kevin Marks. Targeting cancers with deletion of the p16/p14 tumor suppressor locus. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr IA25.