Micronucleus induction and reproductive death in a human cell line exposed to low-energy argon beam

Little is known about the action of charged particles of very high linear energy transfer (LET) on human cells and, in particular, the relationship between DNA damage and reproductive death. The aim of this study was to measure the biological efficiency of a low-energy argon beam (E=7.1 MeV/nucleon, LET= 1590 keV/µm) produced at GSI, Darmstadt, on a human melanoma cell line (CAL4) established in our Institute. Two different methods were used: the micronucleus (MN) test and the colonyforming assay. The MN test, using the cytochalasin-block method, is a measure of genotoxic damage. MN are scored in binucleate cells (BNC) and are formed from acentric fragments or whole chromosomes that have not been incorporated into daughter nuclei at mitosis. The colonyforming assay quantifies reproductive death. Parallel experiments were run with cobalt gamma-rays for comparison. After Co irradiation, the MN-free BNC dose-response curve coincided with that of the loss of colony-forming ability, suggesting the potential of the former as a predictive test of cell killing. After Ar irradiation, there was a dissociation between the two effects, especially at high doses: cell death was greater than the frequency of BNC with MN. The inactivation cross-section was 74 µm2; it was 39 µm2 for MN yield. Therefore, the relative biological effectiveness (RBE) was higher for cell killing than for MN yield (0.8 and 0.5, respectively, at a Co dose of 3 Gy). The total MN count in BNC followed the same pattern of response as the fraction of BNC with MN. However, multiple (>2) MN in BNC were more frequently observed after low-dose Ar irradiation than after gamma-ray exposure (RBE>1). Moreover, the frequency of multiple MN induction exceeded that expected from a Poisson distribution attribution at all dose levels of Ar irradiation. These results indicate that (1) cell killing after 7.1-MeV Ar beam irradiation is less effective than after Co irradiation at an equivalent average energy deposition; (2) unlike gamma-rays, Ar particles are more efficient at cell killing than in producing MN; (3) the frequent scoring of multiple MN suggests the production of multiple damage sites, even at low fluences of Ar particles.