Poly (ADP-ribose) polymerase inhibitors sensitize cancer cells to hypofractionated radiotherapy through altered selection of DNA double-strand break repair pathways

Abstract

Purpose

Poly (ADP-ribose) polymerase inhibitors (PARPi) are known to induce radiosensitization. However, the exact mechanisms of radiosensitization remain unclear. We previously reported that PARPi may have a unique radiosensitizing effect to enhance β-components of the linear-quadratic model. The aim of this study was to evaluate PARPi in combination with high-dose-per-fraction radiotherapy and to elucidate the underlying mechanisms of its radiosensitization.

Materials and methods

Radiosensitizing effects of PARPi PJ34, olaparib, and veliparib were measured using a colony-forming assay in the human cancer cell lines, HCT116, NCI-H460, and HT29. Six different radiation dose fractionation schedules were examined by tumor regrowth assay using three-dimensional multicellular spheroids of HCT116, NCI-H460, SW620, and HCT15. The mechanisms of radiosensitization were analyzed by measuring DNA double-strand breaks (DSB), DNA damage responses, chromosomal translocations, cellular senescence, and cell cycle analysis.

Results

Olaparib and PJ34 were found to show radiosensitization preferentially at higher radiation doses per fraction. Similar results were obtained using a mouse model bearing human tumor xenografts. A kinetic analysis of DNA damage responses and repairs showed that olaparib and PJ34 reduced the homologous recombination activity. However, a neutral comet assay showed that PJ34 treatment did not affect the physical rejoining of DNA-DSBs induced by ionizing radiation. Cell cycle analysis revealed that olaparib and PJ34 strikingly increased G1 tetraploid cells following irradiation, leading to premature senescence. The C-banding analysis of metaphase spreads showed that olaparib and PJ34 significantly increased ionizing radiation-induced dicentric chromosomes. The data suggests that PARPi olaparib and PJ34 altered the choice of DNA-DSB repair pathways rather than reducing the total amount of DNA-DSB repair, which resulted in increased repair errors. Increased quadratic misrepair was one of the mechanisms of PARP-mediated radiosensitization, preferentially at the higher dose range compared to the lower dose range.

Conclusion

PARPi may be a promising candidate to combine with stereotactic hypofractionated radiotherapy, aiming at high-dose region-directed radiosensitization.

Keywords:

• DNA repair
• PARP
• radiosensitizers

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by KAKENHI (Grant-in-Aids for Scientific Research) (C) 23591839, (C) 26461887, and (B) 17H04263 to Y.S. from the Japan Society for the Promotion of Science.

Notes on contributors

Yuji Seo

Yuji Seo is a physician scientist and associate professor, Department of Radiation Oncology, Osaka University.

Keisuke Tamari

Keisuke Tamari is a physician scientist and associate professor, Department of Radiation Oncology, Osaka University.

Yutaka Takahashi

Yutaka Takahashi is a radiation physicist and currently works at Pharmaceuticals and Medical Devices Agency in Tokyo, Japan.

Kazumasa Minami

Kazumasa Minami is a radiobiologist and assistant professor, Department of Radiation Oncology, Osaka University.

Shotaro Tatekawa

Shotaro Tatekawa is a physician scientist and assistant professor, Department of Radiation Oncology, Osaka University.

Fumiaki Isohashi

Fumiaki Isohashi is a physician scientist and associate professor, Department of Radiation Oncology, Osaka University.

Osamu Suzuki

Osamu Suzuki is a radiation oncologist and currently works at Osaka Heavy Ion Therapy Center.

Yuichi Akino

Yuichi Akino is a radiation physicist and assistant professor, Department of Radiation Oncology, Osaka University.

Kazuhiko Ogawa

Kazuhiko Ogawa is a professor and chairman of Department of Radiation Oncology, Osaka University.