Abstract 777: Malic enzyme 3 as a collateral lethality target in pancreatic cancer
Authors: Dey Prasenjit , Baddour Joelle , Kim Youngsoo , Macloed Robert , Muller Florian , Wu Chia Chin , Wang Huamin , Viale Andrea , Ying Haoqiang , Draetta Giulio , Maitra Anirban , Wang Alan , Nagrath Deepak , DePinho Ronald 
Published: August 2018

Abstract
Cancer genomes possess many deletion events targeting tumor suppressor genes (TSG) and neighboring genes in these loci. These deletion patterns prompted us to consider a systematic approach, termed “collateral lethality”, designed to identify cancer-specific vulnerabilities resulting from the deletion of neighboring genes. These bystander genes do not appear to be involved in cancer pathogenesis, yet encode cell-essential functions and are members of multi-gene families that are functionally redundant and co-expressed. Homozygous deletion of SMAD4 is a frequent event in pancreas cancer and other cancer types, totaling &gt;30,000 cases in US annually. SMAD4 deletion often results in co-deletion of the neighboring mitochondrial malic enzyme 2 (ME2) gene. In mammalian cells, two genes (ME2 and ME3) encode redundant cell-essential mitochondrial ME activity. Together, ME2 and ME3 function to generate pyruvate to fuel the TCA cycle, and NADPH to maintain ROS homeostasis. These observations prompted us to hypothesize that the genetic or pharmacological extinction of ME3 activity in a ME2 null cell would specifically compromise cancer cells yet be tolerated in normal host cells possessing ME2 activity. Inducible shRNA strategies were employed to genetically deplete ME3 in ME2-null versus ME2-intact cells followed by apoptosis measurements, integrated metabolomics, and molecular investigations. In collaboration with IONIS Pharmaceuticals, we tested and validated anti-sense oligonucleotide (ASO) to target ME3. Genetic depletion of ME3 in ME2 null, but not ME2 intact, cells resulted in apoptosis and blocked tumorigenic potential. Mechanistically, integrated metabolomic and molecular investigation of mitochondrial ME-deficient cells revealed diminished NADPH production and consequent high ROS, which activates AMP activated protein kinase (AMPK) and which in turn directly suppresses sterol regulatory element-binding protein 1 (SREBP1)-directed transcription of its direct targets including the BCAT2 (Branched chain amino acid transaminase 2) gene. We also determined that mitochondrial MEs regulate the utilization of branched chain amino acid (BCAA) via BCAT2, a transaminase required for BCAA catabolism. Notably, enforced expression of BCAT2 can restore tumorigenic potential of ME2/3 deficiency, and free nucleotides can restore proliferation in cell culture. Specific targeting of ME3 using ASOs could dramatically reduce tumor burden in a subQ tumor model of pancreatic cancer. Thus, a key mechanism driving cancer cell lethality involves BCAAs as crucial metabolites under the critical regulation of the mitochondrial MEs. These studies reveal a collateral lethal vulnerability in pancreas and other cancers that can be targeted pharmacologically in genotype-defined patient populations. We propose that highly specific ME3 inhibitors could provide an effective therapy across a substantial number of cancer patients.
Citation Format: Prasenjit Dey, Joelle Baddour, Youngsoo Kim, Robert Macloed, Florian Muller, Chia Chin Wu, Huamin Wang, Andrea Viale, Haoqiang Ying, Giulio Draetta, Anirban Maitra, Alan Wang, Deepak Nagrath, Ronald DePinho. Malic enzyme 3 as a collateral lethality target in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 777.