Abstract
The dose response relationships for induction of micronuclei in bone marrow and for induction of lacZ mutations in liver, large intestine, and bone marrow were assessed in mice after treatment with ethyl methanesulfonate (EMS). The animals were treated orally at doses between 1.25 to 260 mg/kg for up to 28 days. Statistical analysis indicated that the dose response curves were well compatible with a thresholded relationship with apparent thresholds ≥25 mg/kg/day. In contrast, no threshold was apparent for the induction of alkylation-adducts in proteins and DNA. An approximately linear dose dependency was observed, indicating that the macromolecular targets were alkylated proportionally to the given dose. It is concluded that the cells have the capacity to repair large amounts of EMS-induced alkylations, up to the threshold dose, virtually error free (i.e., without adding to the background burden of clastogenic/mutagenic events). Since the statistical power to define the ‘true’ shape of the dose response curve is much more limited for in vivo studies compared to in vitro experiments with e.g., a bacterial reverse mutation system we assessed the dose response relationship for EMS induced mutations at the his G46 locus in alkylation repair proficient (TA1535, ogt+) and deficient (YG7104, Δogt) bacteria using as many as 23 dose levels. Clear sublinearity was apparent for TA1535 while linearity was obvious for YG7104. Applying curve fitting with a ‘hockeystick’ model a threshold dose of about 750 μg/plate was determined for TA1535. With curve fitting using a benchmark model (PROAST) we estimated the efficiency of error-free repair by the ogt system at the different EMS exposures. The likelihood of erroneous repair approaches 0 with decreasing EMS concentrations. Together with recent studies on other alkylating agents our data argue for a change of paradigm concerning risk assessment of the exposure to simple DNA-alkylating genotoxins.
