Tumor treating fields cause replication stress and interfere with DNA replication fork maintenance: Implications for cancer therapy

Abstract

Tumor treating fields (TTFields) is a noninvasive physical modality of cancer therapy that applies low-intensity, intermediate frequency, and alternating electric fields to a tumor. Interference with mitosis was the first mechanism describing the effects of TTFields on cancer cells; however, TTFields was shown to not only reduce the rejoining of radiation-induced DNA double-strand breaks (DSBs), but to also induce DNA DSBs. The mechanism(s) by which TTFields generates DNA DSBs is related to the generation of replication stress including reduced expression of the DNA replication complex genes MCM6 and MCM10 and the Fanconi's Anemia pathway genes. When markers of DNA replication stress as a result of TTFields exposure were examined, newly replicated DNA length was reduced with TTFields exposure time and there was increased R-loop formation. Furthermore, as cells were exposed to TTFields a conditional vulnerability environment developed which rendered cells more susceptible to DNA damaging agents or agents that interfere with DNA repair or replication fork maintenance. The effect of TTFields exposure with concomitant exposure to cisplatin or PARP inhibition, the combination of TTFields plus concomitant PARP inhibition followed by radiation, or radiation alone at the end of a TTFields exposure were all synergistic. Finally, gene expression analysis of 47 key mitosis regulator genes suggested that TTFields-induced mitotic aberrations and DNA damage/replication stress events, although intimately linked to one another, are likely initiated independently of one another. This suggests that enhanced replication stress and reduced DNA repair capacity are also major mechanisms of TTFields effects, effects for which there are therapeutic implications. (Translational Research 2020; 217:33À46) Abbreviations: CI = combination index; DDR = DNA damage response; DSBs = double strand breaks; FA = Fanconi Anemia; GBM = glioblastoma; HSA = highest single agent; IR = ionizing radiation; NSCLC = nonÀsmall-cell lung cancer; RPA = replication protein A; TTFields = tumor treating fields

Concurrent Tumor Treating Fields (TTFields) and Radiation Therapy for Newly Diagnosed Glioblastoma: A Prospective Safety and Feasibility Study

“…Multiple cytotoxic mechanisms are attributed to TTFields, including their effect upon microtubules and septin fibers of proliferating cancer cells, which disrupts mitosis and causes cell death, mitotic catastrophe, non-viable daughter cells, and cellular stress (6)(7)(8)(9)(10)(11). TTFields also inhibit DNA damage repair (12), enhance replication stress (13), block cellular migration and invasion (14), and increase autophagy (15).…”

Tumor Treating Fields: At the Crossroads Between Physics and Biology for Cancer Treatment

“…Interestingly, TTFields enhanced cancer cell sensitivity to radiation when cells where exposed to radiation after TTFields delivery, opening up new possibilities for developing novel radiosensitization treatment protocols. Taken together, these data proposed an additional mechanism of action of TTFields in vitro (28,29). Although the biological effects of TTFields have been explored, the full knowledge regarding TTFields biophysical mechanism of action against cancer cells is still likely an area ripe for further investigation.…”

“…Over the years, several in vitro studies conducted on human cancer cell lines have reported the non-mitotic effects of TTFields (3,16,27). A set of more recent studies have shown that TTFields could render cancer cells less efficient in their DNA damage repair capacity, with cells showing higher DNA damage and replication stress (28,29). Gene expression analysis on a variety of non-small cell lung cancer (NSCLC) cell lines treated with TTFields revealed not only changes in cell cycle and mitosis-related pathways, but also in DNA damage response pathways.…”

Tumor Treating Fields in the Management of Patients with Malignant Gliomas

“…Many mechanisms have been put forth to explain how TTFields induce programmed cell death. In one scenario, TTFields interfere with DNA fork replication and induce movement and repositioning of DNA fragments created during replication, leading to widespread disruption of the DNA damage repair and breast cancer 1 (BRCA1)-mediated homologous recombination pathways [35][36][37]. In another, endoplasmic reticulum stress in mitotic cells under the effects of TTFields may trigger adenosine monophosphate-activated protein kinase-dependent autophagy [38] and/or immunogenic cell death [39][40][41].…”