X-rays induced DNA double strand breaks is efficiently repaired in radioresistant cells

Abstract

Radiotherapy is one of the three major treatment modalities used in eradicating malignant tumors. However, the existence of radioresistant cells remains a critical obstacle in radiotherapy and radiochemotherapy. In order to develop more efficient radiotherapy including suppression of the outbreak of radioresistant cells, we need to understand the features of radioresistant cells. Last year, we reported the establishment of radioresistant HepG2-8960-R cells those continue to grow even with daily exposure to 2 Gy of X-rays. Using neutral comet assay, we compared rejoining kinetics of DNA double strand breaks (dsbs) induced by X-rays between HepG2-8960-R cells and parental HepG2 cells. We also compared the induction rate of micronuclei and gamma-H2AX foci in both cells after administration of fractionated exposure to 2 Gy/day of X-rays for 5 consecutive days. The rejoining kinetics of dsbs in HepG2 cells was biphasic while that in HepG2-8960-R cells was monophasic, indicating that homologous recombination repair pathway is predominantly used in radioresistant cells. The numbers of micronuclei and gamma-H2AX foci were increased in HepG2, but not in HepG2-8960-R cells after fractionated exposure to X-rays. These results suggested that X-rays induced DNA dsbs were more accurately repaired in radioresistant cells than in HepG2 cells. Moreover, the number of micronuclei and gamma-H2AX foci after fractionated exposure would be the useful marker to predict cellular radioresistance.