Purpose: Ionizing radiation induces DNA damage in cells. One response to DNA damage accumulation is cell death. However, other cellular responses include the formation of micronuclei (MN) or induction of senescence, both of which activate immune pathways. We hypothesized that greater levels of senescence, MN, and cell death would be elicited in breast cancer cells when exposed to higher LET radiation (protons vs. x-rays); and that these effects would be greater in the setting of pharmacologic (PARP inhibition) and genomic (BRCA1 mutation) inhibition of DNA repair.
Methods: The breast cancer cell lines HCC1937 (BRCA1 mutated), HCC1937-BRCA1 (BRCA1 recovered), 4T1 (BRCA1 wild-type), MDA-MB-436 (BRCA1 mutated) and MDA-MB-436-BRCA1 (BRCA1 recovered) were treated with 6 MV X-rays or protons (9.9 keV/µm dose-weighted LET in water) alone or with a PARP inhibitor (PARPi) (1-2 μM, Olaparib). Senescence-associated beta-galactosidase (SA-βgal) was used as a biomarker of senescence to identify and quantify senescent cells. Seven days after irradiation cells were pre-treated with bafilomycin A1 (to remove background replicative senescence) and C12FDG (substrate of SA-βgal that fluoresces when cleaved). Flow cytometry was then used to quantify radiation-induced senescence. Clonogenic assays were performed to quantify cell survival. Immunohistochemistry was used to quantify the amount of MN and cGAS-positive MN.
Results: Mean SA-βgal activity for HCC1937 was significantly higher compared to HCC1937-BRCA. No significant differences were seen between protons and x-rays or with different PARPi concentrations. Protons+PARPi increased the amount of cell death significantly compared to x-rays+PARPi for both HCC1937 cell lines. Protons+PARPi resulted in a significantly higher density of MN compared to x-rays+PARPi in HCC1937-BRCA.
Conclusion: It appears that the BRCA status of breast cancer cells has a greater effect on the levels of radiation-induced senescence than does radiation type or PARP inhibition. Protons+PARPi seems to be the most effective combination for causing cell death and MN formation.
Funding Support, Disclosures, and Conflict of Interest: Funding: This project was funded by NIH funding 1R21CA252411-01. Additional funding is from is the Emerson Collective Cancer Research Fund. Conflict of interest: GOS and SFS have research funds from Alpha Tau Medical and Artios Pharma.