Abstract 3217: Newly identified role of tumor treating fields in DNA damage repair and replication stress pathways
Authors: Karanam Narasimha Kumar , Ding Lianghao , Sishc Brock , Saha Debabrata , Story Michael D. 
Published: August 2018

Tumor treating fields (TTFields) are low-intensity, intermediate frequency, alternating electric fields non-invasively applied to the region of the tumor. TTFields have revolutionized the treatment of recurrent and newly diagnosed glioblastoma and Phase III clinical trials are ongoing for brain metastasis. Phase II trials are ongoing in non-small cell lung, ovarian and pancreatic cancers. The primary mechanism of TTFields function is thought to be the disruption of mitosis; however, other mechanisms are under investigation. Using a panel of 5 non-small cell lung cancer cell lines (NSCLC), we found that cell proliferation and killing as a result of TTFields exposure was variable. To understand the molecular mechanisms underlying the biological effects of TTFields exposure we examined temporal gene expression changes in these NSCLC cell lines after TTFields treatment. Interestingly we found that the expression of the BRCA1 DNA damage repair pathway genes were significantly downregulated (P &lt; 0.05) upon TTFields treatment which was confirmed at the protein level by western blot. Furthermore, TTFields treatment slowed the repair of ionizing radiation-induced DNA damage compared to radiation alone which was evident by an increased number of DNA double strand break repair foci at any given time. Moreover, we found that TTFields treatment alone increased the number of γH2AX foci and the incidence of chromatid aberrations. Given the role of BRCA1/FANC gene downregulation, we examined the status of replication fork integrity as a result of TTFields exposure and determined that the length of newly replicated DNA, slowed as a function of TTFields exposure time. Furthermore, we showed that TTFields increased R-loop formation (DNA:RNA hybrid structures) which was quantified using a DNA-RNA hybrid specific antibody. These results clearly indicate that TTFields induce DNA damage and increase replication stress. Based on newly identified mechanisms of TTFields action we hypothesized that by applying TTFields first, a conditional lethality environment would develop, rendering cells more susceptible to agents such as radiation or in the case of BRCA1 downregulation PARP inhibition. Indeed, by applying TTFields before, rather than after radiation, all cell lines were more susceptible to death. Studies of PARP inhibition in combination with TTFields and TTFields plus radiation are ongoing. Finally, we propose that future use of TTFields in a clinical setting may be more durable if provided post-surgery but prior to or concomitantly with chemotherapy or radiation therapy.

Citation Format: Narasimha Kumar Karanam, Lianghao Ding, Brock Sishc, Debabrata Saha, Michael D. Story. Newly identified role of tumor treating fields in DNA damage repair and replication stress pathways [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 3217.