Abstract
Tumor treating fields (TTFields) is a novel anticancer treatment modality that has revolutionized the treatment of recurrent and newly diagnosed glioblastoma. TTFields are low-intensity, intermediate frequency, alternating electric fields applied non-invasively to the skin in the region of a tumor. Our analysis of TTField exposure identified an inhibition of cell proliferation and decreased survival, though the degree of inhibition and survival varied across cell lines. Temporal gene expression changes in the presence or absence of TTFields was performed. Interestingly, we identified the down regulation of the transcription activators of the E2F transcription factor (TF) family and upregulation of the TF suppressor members of the E2F family, amongst other TFs. These changes were likely responsible for increased p53 expression, downregulated NFKB, upregulated cell cycle genes, downregulation of DNA repair genes and downregulated genes associated with proliferation. In addition, the expression of the BRCA1/FA pathway genes was significantly downregulated upon TTFields treatment. Downregulation was also seen at the protein level. DNA damage repair kinetics by quantification of g-H2AX and 53 BP1 foci showed that TTFields exposure slowed the repair of IR-induced DNA damage. We also found TTFields increased the incidence of chromatid aberrations. Using the DNA fiber assay we showed that TTFields gradually reduced replication tract lengths over time, suggesting increased replication stress. Also examined was the newly identified cytosolic role for the BRCA1/FA pathway genes, i.e., mitophagy, by hypothesizing that TTFields disrupt the clearance of damaged mitochondria which results in increased ROS production. Interestingly, electron micrographs of TTFields treated cells suggested more damaged mitochondria accumulation compared to control. We found that TTFields exposure increased ROS production suggesting a new mechanism of action for TTFields. Lastly, our data suggests that PARP inhibitors in combination with DNA damaging agents (radiation, cisplatin) and TTFields may be advantageous through the conditional vulnerability of down-regulated BRCA1.