Abstract 3939: Exploiting tumor treating fields induced downregulation of BRCA1 pathway for novel combination therapies
Authors: Karanam Narasimha Kumar , Ding Liang-Hao , Sishc Brock , Saha Debabrata , Story Michael D. 
Published: August 2019

Abstract
A new physical cancer treatment modality called Tumor Treating Fields (TTFields) has demonstrated effectiveness in the treatment of solid tumors in vitro and in vivo. TTFields therapy is a non-invasive cancer treatment modality that delivers low intensity (1-3 V/cm), intermediate frequency (100-300 kHz) alternating electric fields to the tumor. The TTFields delivery device called Optune (NovoCure), has been approved for recurrent and newly diagnosed glioblastoma and clinical trials are ongoing for other cancers. The primary mechanism of TTFields action is thought to be interference with mitosis. We monitored temporal gene expression changes in 5 non-small cell lung cancer (NSCLC) cell lines whose response to TTFields is variable and found that the expression of the BRCA1 DNA damage repair pathway, as well as other DNA repair/checkpoint pathways were significantly downregulated (P&lt;0.05) upon TTFields exposure. We monitored the dynamics of DNA double strand break repair to study functional relevance of BRCA1 pathway downregulation and found that TTFields treatment slowed the repair of ionizing radiation (IR)-induced DNA damage and interestingly, TTFields alone increased the number of γH2AX foci and the incidence of chromatid aberrations. Furthermore, as a function of TTFields exposure time, decreased replication fork speed and increased R-loop formation was identified, suggesting that TTFields induce replication stress. We carried out DNA/RNA immunoprecipitation sequencing (DRIP-Seq) analysis and are currently mapping R-loop formation within the genome. Based upon these newly identified mechanisms of TTFields action, i.e., enhancing DNA damage, reducing DNA repair capacity and increasing replication stress, led us to hypothesize that by applying TTFields first, a conditional vulnerability environment would develop rendering cells more susceptible to novel combination therapies. For example, cell killing by ionizing radiation exposure is enhanced when NSCLCs are exposed to TTFields after radiation. However, TTFields exposure prior to IR treatment is more effective compared to IR treatment prior to TTFields treatment. Furthermore, the combination of cisplatin together with TTFields also suggests synergistic effects in NSCLC cells. TTFields exposure concomitant with the PARP inhibitor Olaparib followed by radiation was synergistic compared to radiation or Olaparib alone or in combination, although the degree of sensitization and synergy varied across the different NSCLC cell lines. Taken together our results suggest that TTFields may enhance the efficacy of radiation if used as a neoadjuvant therapy or as concomitant therapy with different chemotherapy agents to improve patient outcome in clinic.
Citation Format: Narasimha Kumar Karanam, Liang-Hao Ding, Brock Sishc, Debabrata Saha, Michael D. Story. Exploiting tumor treating fields induced downregulation of BRCA1 pathway for novel combination therapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3939.