Abstract 2138: Tumor treatment fields downregulate the BRCA1/FA pathway genes leading to reduced DNA repair capacity, the inhibition of mitophagy and enhanced cell death
Authors: Karanam Narasimha Kumar , Ding Lianghao , Sishc Brock , Saha Debabrata , Story Michael D. 
Published: July 2017

The application of new physical cancer treatment modality utilizing alternating electric fields termed tumor treatment fields (TTFields) has revolutionized the treatment of recurrent and newly diagnosed glioblastoma. This non-invasive exposure to low-intensity, intermediate frequency, alternating electric fields to the region of the tumor has resulted in a significant increase in overall survival when compared to standard therapy with very minimal side effects. Clinical trials are recruiting or ongoing at additional tumor sites including lung, pancreatic, and ovarian cancer. The primary mechanism of TTField cell killing is thought to be the disruption of mitosis; however, other potential mechanisms are under investigation. Using a panel of five NSCLC cell lines we found that TTFields treatment alone inhibits cell proliferation, and decreases survival, though the degree of inhibition varies between cell lines. To understand the molecular mechanisms underlying the biological effects of TTField exposure we studied temporal gene expression changes in the NSCLC cell lines after TTField treatment. We observed that most differentially expressed genes are part of cell cycle and proliferation pathways which is in agreement with earlier findings. Interestingly we found that the expression of BRCA1 DNA damage repair pathway genes were significantly downregulated (P &lt; 0.05) upon TTField treatment. We confirmed the downregulation of BRCA1/FA pathway proteins by western blot. When examining the nuclear role of the BRCA1/FA pathway genes we found that TTField treatment slowed the repair of ionizing radiation-induced DNA damage compared to radiation alone which is evident by an increased number of DNA double strand break repair foci at any given time. Moreover, we found that TTField treatment increased the incidence of chromatid aberrations. We also examined the newly identified BRCA1/FA pathway genes cytosolic role in mitophagy where we observed alterations in mitophagy related gene (PINK1, OSCP1, ATP5 and DAPIT) expression and confirmed the same at the protein level by western blot. We hypothesized that TTFields disrupt the clearance of damaged mitochondria due to the downregulation of BRCA1/FA pathway players, causing an imbalance in oxygen metabolism leading to the production of high levels of radical oxygen species (ROS) and as a result, cell death. Using CellROX dye we found that TTField treatment did result in increased ROS production suggesting a new mechanism of action for TTField exposure. Novel chemotherapy agents, particularly PARP inhibitors, in combination with DNA damaging agents like radiation and TTFields may be advantageous through the conditional vulnerability of down-regulated BRCA1.

Citation Format: Narasimha Kumar Karanam, Lianghao Ding, Brock Sishc, Debabrata Saha, Michael D. Story. Tumor treatment fields downregulate the BRCA1/FA pathway genes leading to reduced DNA repair capacity, the inhibition of mitophagy and enhanced cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2138. doi:10.1158/1538-7445.AM2017-2138