BEST IN PHYSICS (THERAPY): The Nonhomologous End-Joining Repair Pathway Determines Cell Radiosensitivity in Helium and Carbon Ion Beam

Purpose: To highlight the importance of nonhomologous end-joining (NHEJ) repair in governing a cell’s radiosensitivity to helium and carbon ion beams of varying linear energy transfer (LET). Methods: We selected a pair of isogenic human cancer cell lines with differing proficiency in nonhomologous end-joining (NHEJ) repair, the M059K (NHEJ-proficient) and M059J (NHEJ-deficient) glioblastoma cell lines, and as additional reference cell lines, we selected the H1299 nonsmall cell lung cancer and H460 large cell lung cancer cell lines. We exposed these cells to unmodulated helium and carbon ion beams with nominal energies of 150 and 290 MeV/u, respectively, using energy degraders to achieve LETs ranging from 2.2 to 60 keV/lm. We performed clonogenic assays to calculate the relative biological effectiveness (RBE) of each cell line in these conditions, relative to their survival in 6 MV x-rays, using the dose for 10% survival as the biological endpoint. To support the survival data, we performed immunohistochemical staining of DNA double strand break (DSB) markers (including cH2AX and 53BP1) 30 min and 24 h post irradiation to identify persistent radiation-induced foci. Results: The RBE of the NHEJ-deficient M059J cell line showed significantly less LET dependence than the reference cell lines (increasing only to 1.37 0.07 at 60 keV/lm for M059J versus 1.80 0.10, 1.79 0.04 and 1.98 0.05 for M059K, H460 and H1299, respectively), despite M059J being more radiosensitive to all LETs. Preliminary analysis of foci data for protonexposed cells indicate that the M059J cells have more persistent DSB repair foci 24 h postirradiation (up to a factor of 2.4 0.5 for cH2AX foci), consistent with their deficiency in DNA DSB repair. Conclusion: Our results indicate that proficiency status in NHEJ is extremely important in determining how cell radiosensitivity varies with He- and C-ion LET