A COMPARISON OF THE GENOTOXICITY OF ETHYLNITROSOUREA AND ETHYL METHANESULFONATE IN LACZ TRANSGENIC MICE (MUTA(TM)MOUSE)

We compared the induction of gene mutations and chromosomal aberration s by ethylating agents in lacZ transgenic mice (Muta((TM)) Mouse). Chr omosomal aberrations were detected by the peripheral blood micronucleu s assay. Gene mutations were detected in the lacZ transgene. A small a mount of blood was sampled from a tail vessel during the expression ti me for fixation of gene mutations in vivo; this enabled us to detect a nd compare clastogenicity and gene mutations in the identical mouse. S ingle intraperitoneal injections of ENU (50-200 mg/kg) and EMS (100-40 0 mg/kg) strongly induced micronucleated reticulocytes (MN) detectable in peripheral blood 48 h after treatment. The maximum MN frequencies induced were 6.6% and 3.3% for ENU (100 mg/kg) and EMS (400 mg/kg), re spectively (the control value was 0.3%). lacZ mutant frequency (MF) wa s analyzed in bone marrow and liver 7 days after treatment. Spontaneou s MFs were 2.0-4.6 X 10(-6). MF in bone marrow was increased by ENU to 3.4 X 10(-5) at 200 mg/kg and induced by EMS to 1.8 X 10(-5) at 400 m g/kg. In liver, however, both chemicals at their highest doses induced only slight increases in MF. The induction of both micronuclei and la cZ mutations in bone marrow by both ENU and EMS correlated better with O-6-ethylguanine adducts than with N7-ethylguanine adducts. The mutan ts (19 for ENU and 12 for EMS) were subjected to DNA sequence analysis . Among EMS-induced mutations, 75% were GC to AT transitions, which we re probably caused by O-6-ethylguanine. Among ENU-induced mutations, i n contrast, 40% occurred as AT base pair substitutions (6 AT to TA tra nsversions and 2 AT to GC transitions) (no such mutations were induced by EMS). These results, together with the known reactivity of ENU to thymine suggest that thymine adducts play a significant role in the EN U mutagenesis. (C) 1997 Elsevier Science B.V.