Mechanism of the DNA Damage Repair Gene Work on Elevated α/β Value after Hypofractionated Radiotherapy for Prostate Cancer

Abstract

Our previous study showed that the LQ model appeared to be inappropriate for high doses per fraction owing to α/β ratios tending to become higher when the dose per fraction increased. In order to verify this conclusion, we studied in vitro to explore the reasons for the elevated α/β ratio after single hypofractionated radiation and the effect of DNA damage repair gene work on the α/β value after different mode of fractionated radiation for prostate cancer according to the definition (α value represents irreparable lethal damage, β represents sublethal damage). We selected two prostate cancer cell lines DU145 and PC3: ① Draw the cell survival curve using colony formation assay to calculate the α/β ratios of the two cell lines, and then use BED formula to convert hypofractionated radiation (4Gy×2f and 6Gy×2f) into equivalent single hypofractionated radiation (calculated values) comparing with that on the survival curve (actual value) after different mode of hypofractionated radiation. ② Western Blot was used to detect the expression of DNA damage marker γ-H2AX and DNA repair marker RAD51 at different time points. ③ Laser confocal immunofluorescence was used to detect the foci of γ-H2AX and RAD51 after different mode of hypofractionated radiation. ① According to the cell survival curve, the α/β values of PC3 and DU145 cells were 1.75Gy and 3.78Gy, respectively. When fractionated radiation was converted into equivalent single hypofractionated radiation, the lowest calculated values of PC3 cells and DU145 cells were only 83% and 78%, respectively indicating that the ability of hypofractionated radiation was overestimated in clinic. If a larger α/β value is substituted into the BED formula, the discrepancy between the calculated and measured doses tended to become smaller. ② Compared with fractionated radiation, the results of western blot and laser confocal immunofluorescence showed that the expression of DNA damage marker γ-H2AX was higher after 30 minutes, 6 hours, and 24 hours, indicating that more DNA damage after single hypofractionated radiation. ③ The results showed that the expression of DNA repair marker RAD51 lasted for 24 hours and the DNA damage still existed after single hypofractionated radiation, indicating that there is no repairment and the damage was irreparable. And at the same time, the expression of γ-H2AX decreased after 24 hours of fractionated radiation compared with that of 6 hours, and there was no significant difference in the expression of γ-H2AX of single hypofractionated radiation between 24 hours and 6 hours, indicating that part of the repairable damage converted into irreparable damage after single hypofractionated radiation. The results of this study suggest that after single hypofractionated radiation, the irreparable damage in cells increases (that is, α value increases), and some repairable sublethal damage (β value) is converted into irreparable damage (α value). When α value increases and β value decreases, the ratio increases.