Abstract LB-017: Disordered methionine metabolism in MTAP/CDKN2A-deleted cancers leads to marked dependence on PRMT5

Metabolic genes are increasingly recognized as targets of somatic genetic alteration in human cancer often leading to profound changes in intracellular metabolite concentrations. 5-Methylthioadenosine Phosphorylase (MTAP) is a key enzyme in the methionine salvage pathway that metabolizes methylthioadenosine (MTA) to adenine and methionine. Its chromosomal position proximal to CDKN2A results in frequent collateral homozygous deletion in a wide range of human cancers. By interrogating data from a large scale deep-coverage pooled shRNA screen across 390 cancer cell line models we found that the viability of MTAP null cancer cells is strongly impaired upon shRNA-mediated depletion of the protein arginine methyltransferase PRMT5. In MTAP deleted cells there is marked accumulation of the substrate MTA and surprisingly, we find that MTA is a specific inhibitor of the catalytic activity of PRMT5. In keeping with these data, knockout of MTAP in an MTAP-proficient cell line led to increased MTA levels and rendered them sensitive to PRMT5 depletion. Moreover, reconstitution of MTAP in an MTAP-deficient cell line fully rescued PRMT5 dependence. Collectively, these findings indicate that the collateral loss of MTAP in CDNK2A deleted cancers leads to accumulation of MTA that thereby creates a hypomorphic PRMT5 state that is selectively sensitized towards further PRMT5 inhibition.

Citation Format: Konstantinos Mavrakis, E Robert McDonald III, Michael R. Schlabach, Eric Billy, Gregory R. Hoffman, Antoine deWeck, David A. Ruddy, Kavitha Venkatesan, Greg McAllister, Rosalie deBeaumont, Samuel Ho, Yue Liu, Yan Yan-Neale, Guizhi Yang, Fallon Lin, Hong Yin, Hui Gao, David Randal Kipp, Songping Zhao, Joshua T. McNamara, Elizabeth R. Sprague, Young Shin Cho, Justin Gu, Ken Crawford, Vladimir Capka, Kristen Hurov, Jeffrey A. Porter, John Tallarico, Craig Mickanin, Emma Lees, Raymond Pagliarini, Nicholas Keen, Tobias Schmelzle, Francesco Hofmann, Frank Stegmeier, William R. Sellers. Disordered methionine metabolism in MTAP/CDKN2A-deleted cancers leads to marked dependence on PRMT5. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-017.

Abstract C151: Systematic discovery of cancer dependencies through deep coverage pshRNA screens

Large scale cancer genome sequencing efforts, such as TCGA, revealed the landscape of genomic alterations across many tumor types, but the functional relevance of these alterations and genetic interdependencies cannot be fully assessed from these datasets. Thus, functional genomic screens, such as pooled shRNA 9pshRNA) screens, hold great promise to systematically identify cancer dependencies. However, RNAi discovery screens have been plagued by off target effects causing false positive results and possibly masking on-target effects. In order to address the previous problems with RNAi screens for cancer target discovery, we conducted a large scale pshRNA screening campaign targeting 7500 genes at a depth of 20 shRNAs per gene, across more than 300 cell lines of the cancer cell line encyclopedia (CCLE). The substantial shRNA and cell line depth in this screen significantly increased the robustness of results relative to previous published screens, and allowed the more robust identification of cancer dependencies. Moreover, integration of those growth phenotypes with known features of the cancer cell line encyclopedia (CCLE) enabled the identification of biomarkers (genetic, epigenetic, or proteomic) that correlate with sensitivity, and thus permitted discovery of synthetic lethal relationships. The increased depth provides a robust data set that can be used to find and validate new drug targets as well as infer pathway membership of novel genes by simple phenotypic similarity. The findings of this screen will be presented here.

Citation Format: Michael Schlabach, Eric Billy, Konstantinos Mavrakis, Greg Hoffman, Tobias Schmelzle, Francesco Hofmann, Nicholas Keen, Frank Stegmeier, William Sellers. Systematic discovery of cancer dependencies through deep coverage pshRNA screens. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C151.

Routine follow-up after periradicular operations: current practice in UK hospitals

Questionnaires were circulated to all Fellows of the British Society of Oral and Maxillofacial Surgeons in 1999, 75% of whom replied (n = 194). There was a wide range of responses for both the timing of review appointments and the taking of radiographs. Most patients were first followed up 1 week after operation (46%), but 38% were reviewed in the second week. Five surgeons did not review patients at all and two did not review until 6 or 9 months, respectively. Nearly two-thirds routinely offered a second follow-up appointment but only 14% offered more than two, the maximum being seven. Most arranged postoperative radio-graphs but the timing ranged from immediately postoperatively to 1 year after the procedure. Less than one-third requested a second postoperative radiograph between 1 month and 1 year. The largest disparity was in the time of discharge to the general dental practitioner, which ranged from immediately to 5 years, the most popular time of discharge being at 3 months. The wide variations may reflect unnecessary recall of patients and misuse of valuable clinical time.

Efficacy of benfotiamine versus thiamine on function and glycation products of peripheral nerves in diabetic rats

In rats with streptozotocin (STZ) induced diabetes the effect of (watersoluble) thiamine nitrate and of (lipidsoluble) benfotiamine on peripheral nerve function (motor nerve conduction velocity) as well as on the formation of advanced glycation end-products in peripheral nerve tissue was studied. In one group of animals drug administration was started immediately after diabetes induction (prevention study) and in another group two months after diabetes induction (treatment study). Motor nerve conduction velocity (NCV) dropped by 10.5% in diabetic animals, carboxymethyl-lysine (CML) rose to a 3.5fold concentration, deoxyglucosone (3DG)-type AGE formation was increased 5.1fold compared with controls. After three months preventive administration of both vitamin B(1) preparations NCV had increased substantially compared with results in diabetic controls. It was nearly normal after six months with benfotiamine, while the administration of thiamine nitrate resulted in no further amelioration. NCV was nearly normalized after six months of benfotiamine application but not with thiamine. Furthermore, benfotiamine induced a major inhibition of neural imidazole-type AGE formation and completely prevented diabetes induced glycoxidation products (CML). Treatment with thiamine did not significantly affect AGE or cmL levels. Unlike treatment with water-soluble thiamine nitrate timely administration of liposoluble prodrug benfotiamine was effective in the prevention of functional damage and of AGE and cmL formation in nerves of diabetic rats.