Supra-additive genotoxicity of a combination of gamma-irradiation and ethyl methanesulfonate in mouse lymphoma L5178Y cells

While testing for genotoxicity is usually performed on single chemicals, ex posure of humans always comprises a number of genotoxic agents, The investi gation of potentially synergistic effects of combinations therefore is an i mportant issue in toxicology, Combinations of 511 keV gamma-radiation with the chemical alkylating agent ethyl methane-sulfonate were investigated in the in vitro micronucleus test in mouse lymphoma L5178Y cells. With combina tions in the lon dose linear effect range for the individual agents (0.25-2 Gy and 0.8-3.2 mM, respectively), supra-additivity by 34-86% was seen. The synergism was more pronounced at the higher dose levels, Supra-additivity was confirmed in experiments using cytochalasin B and analyzing binucleate cells only, to control for putative effects on the cell cycle. Statistical significance was shown by a 2-factor analysis of variance with interaction. The results indicate that damage to DNA by gamma-radiation and alkylation could affect different rate limiting steps in the formation of micronuclei, Further investigations will have to show whether the observations are of g eneral validity, in particular, whether other endpoints of genotoxicity pro duce the same results and whether the degree of supra-additivity is always dose dependent. The latter mould have a strong impact on risk assessment fo r mixtures at low doses.