Abstract 4905: Targeting NPM1 for the radiosensitization of NSCLC
Authors: Sekhar Konjeti R. , Benamar Mouadh , Venkateswaran Amudhan , Penthala Narsimha R. , Crooks Peter A. , Hann Stephen R. , Geng Ling , Balusu Ramesh , Abbas Tarek , Freeman Michael L. 
Published: May 2015

Abstract
Lung cancer is a global scourge. In 2008 alone there were more than 1.25 million deaths from this disease worldwide. Eighty five percent of lung cancers are classified as non-small cell lung cancer (NSCLC) and for those NSCLC patients who present with locally advanced stage IIIA/B disease the standard of care is chemo/radiation therapy. Yet even with this approach the median duration of survival is only 17 months. Thus, there is an urgent need for the development of new strategies that improve therapeutic outcomes. We addressed this challenge by employing a forward chemical genetics screen designed to identify compounds that could be used to exploit DNA repair pathways in tumor cells burdened with oncogenesis-mediated replication stress. Using this approach we identified YTR107, a substituted 5-((N-benzyl-1H-indol-3-yl)methylene)pyrimidine-2,4,6(1H,3H,5H) trione, that amplifies replication stress and inhibits repair of DNA DSBs. Using biochemical approaches we identified NPM1 as a potential target and found that YTR107 impairs NPM1 oligomerization, a requirement for NPM1's biochemical activity. However, direct genetic evidence demonstrating that the specific targeting of NPM1 is a radiosensitizing event remains to be established. Therefore we employed a defined genetic model consisting of NPM1 null and wild type mouse embryo fibroblasts (MEFs). We found that proliferating NPM1-null mouse embryo fibroblasts (MEFs), but not wild type MEFs, are burdened with high levels of γH2AX foci, a characteristic of replication stress. NPM1-null MEFs exhibit a defect in Rad51-mediated repair of DNA DSBs that increases radiation sensitivity. Use of this model allowed us to demonstrate that YTR107-mediated radiosensitization requires NPM1. Because the DNA damage response pathway is continuously activated in NSCLC in response to constant replication stress, and with the knowledge that NPM1 expression minimizes such stress, we hypothesized that NSCLC tumors might require significant expression of NPM1. This hypothesis was validated by analyzing a NSCLC tumor microarray. We then showed that YTR107-mediated targeting of NPM1 is a radiosensitizing event in 6 NSCLC cell lines and in a syngeneic tumor model. We also found that YTR107 is well tolerated in mice, does not produce overt toxicity, and does not potentiate radiation-mediated pulmonary fibrosis. These data suggest that NPM1 represents a potential molecular target for radiosensitization of NSCLC.
Citation Format: Konjeti R. Sekhar, Mouadh Benamar, Amudhan Venkateswaran, Narsimha R. Penthala, Peter A. Crooks, Stephen R. Hann, Ling Geng, Ramesh Balusu, Tarek Abbas, Michael L. Freeman. Targeting NPM1 for the radiosensitization of NSCLC. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4905. doi:10.1158/1538-7445.AM2014-4905