Effects of shielding on the induction of 53BP1 foci and micronuclei after Fe ion exposures

Abstract

High atomic number and high-energy (HZE) particles in deep space are of low abundance but substantially contribute to the biological effects of space radiation. Shielding is so far the most effective way to partially protect astronauts from these highly penetrating particles. However, simulated calculations and measurements have predicted that secondary particles resulting from the shielding of cosmic rays produce a significant fraction of the total dose and dose equivalent. In this study, we investigated the biological effects of secondary radiation with two cell types, and with cells exposed in different phases of the cell cycle, by comparing the biological effects of a 200 MeV/u iron beam with a shielded beam in which the energy of the iron ion beam was decreased from 500 MeV/u to 200 MeV/u with PMMA, polyethylene (PE), or aluminum. We found that beam shielding resulted in increased induction of 53BP1 foci and micronuclei in a cell-type-dependent manner compared with the unshielded 200 MeV/u Fe ion beam. These findings provide experimental proof that the biological effects of secondary particles resulting from the interaction between HZE particles and shielding materials should be considered in shielding design.

Keywords: DNA damage; secondary particles; shielding; space radiation; survival.

Fig. 1.

Demonstration of irradiation methods. Irradiation A (IR-A) was to use a shielding material to reduce iron ion beam energy from the primary energy of 500 MeV/u to the final mean energy at the sample of 200 MeV/u. For irradiation B (IR-B), a lower primary energy (200 MeV/u) beam was directly used to treat the cells without any shielding material.

Fig. 2.

Survival curves of cells in different cycle conditions exposed to X-rays were measured with a routine colony-forming assay. Data were presented as mean ± SE. Experiments were independently repeated at least three times.

Fig. 3.

Secondary effects of PMMA on G0, G1 and exponentially growing MRC-5 cells. (A) Kinetics of 53BP1 foci in cells exposed to 0.5 Gy iron ions. (B) Binucleated cells (BN) with micronuclei (MN), which were obtained with a cytochalasin B-blocked micronucleus assay. Data were presented as mean ± SE. Experiments were independently repeated at least three times. P-values for comparisons between IR-A and IR-B were shown.

Fig. 4.

Secondary effects of three types of shielding materials: PMMA, polythylene (PE), and alumninum. (A) Kinetics of 53BP1 foci in 92-1 cells and exponentially growing MRC-5 cells exposed to 0.5 Gy iron ions. (B) Binucleated cells (BN) with micronuclei (MN), which were obtained with a cytochalasin B-blocked micronucleus assay. Data were presented as mean ± SE. Experiments were independently repeated at least three times. P-values for comparisons between IR-A and IR-B were shown.