The mutagenic response at the ouabain resistance locus in T cells of mice exposed to N-ethyl-N-nitrosourea parallels the response at the Hprt locus and correlates with mutation target size.
The lymphocyte Hprt gene has been used extensively as a reporter locus to monitor the mutational effects of the exposure of animals to genotoxicants. Implicit in this view of the function of a reporter gene is the assumption that its mutagenic response is representative of that of other genes in the organism. As a test of this hypothesis we compared the frequency of 6-thioguanine-resistant (TGr) mutants at the Hprt locus with the mutant frequency (MF) induced at another locus, the ouabain resistance (Oua) locus. The frequency of spontaneous OUA(R) mutants was estimated to be 1.1x10(-7) (MF between <0.3 and 1.1x10(-7)), which was approximately 30-fold less than the spontaneous TGr MF. Following treatment with N-ethyl-N-nitrosourea (ENU), the induced OUA(R) MF at each of two dose levels (50 and 150 mg/kg ENU) and two time points (3 and 6 weeks post-exposure) was consistently 8- to 9-fold lower than the corresponding TGr MF. Thus the mutagenic response of the Oua locus closely paralleled that of the Hprt locus, indicating a similarity in their response to ENU. In addition, the Oua locus was 3-4 times more sensitive than the Hprt locus to the mutagenic effect of ENU, as measured by the fold increase in MF over the background level. The number of ENU-mutable sites capable of resulting in a TGr or OUA(R) phenotype, otherwise known as the mutation target size, was estimated to differ by an order of magnitude between the two loci. This difference in target size correlates with, and therefore may largely account for, the difference in induced MF between both loci.
Dass
S B
SB
Division of Genetic and Reproductive Toxicology, National Center for Toxicological Research, Jefferson, AR 72079, USA.
Heflich
R H
RH
Casciano
D A
DA
D000818:Animals; D002874:Chromosome Mapping; D005038:Ethylnitrosourea / Q000633:toxicity*; D005260:Female; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics*; D051379:Mice; D008810:Mice, Inbred C57BL; D009153:Mutagens / Q000633:toxicity*; D009154:Mutation; D010042:Ouabain / Q000494:pharmacology*; D013601:T-Lymphocytes / Q000187:drug effects*; D013866:Thioguanine / Q000494:pharmacology
D009153:Mutagens; D010042:Ouabain; D007041:Hypoxanthine Phosphoribosyltransferase; D013866:Thioguanine; D005038:Ethylnitrosourea