title: Temporal changes in K-ras mutant fraction in lung tissue of big blue B6C3F₁ mice exposed to ethylene oxide.
abstract: Ethylene oxide (EO) is a genotoxicant and a mouse lung carcinogen, but whether EO is carcinogenic through a mutagenic mode of action remains unclear. To investigate this question, 8-week-old male Big Blue B6C3F₁ mice (10 mice/group) were exposed to EO by inhalation-6 h/day, 5 days/week for 4 weeks (0, 10, 50, 100, or 200 ppm EO) and 8 or 12 weeks (0, 100, or 200 ppm EO). Lung DNA samples were analyzed for levels of 3 K-ras codon 12 mutations (GGT→GAT, GGT→GTT, and GGT→TGT) using ACB-PCR. No measureable level of K-ras codon 12 TGT mutation was detected (ie, all lung mutant fractions [MFs] ≤ 10⁻⁵). Four weeks of inhalation of 100 ppm EO caused a significant increase in K-ras codon 12 GGT→GTT MF relative to controls, whereas 50, 100, and 200 ppm EO caused significant increases in K-ras codon 12 GGT→GAT MF. In addition, significant inverse correlations were observed between K-ras codon 12 GGT→GTT MF and cII mutant frequency in the lungs of the same mice exposed to 50, 100, or 200 ppm EO for 4 weeks. Surprisingly, 8 weeks of exposure to 100 and 200 ppm EO caused significant decreases in K-ras MFs relative to controls. Thus, the changes in K-ras MF as a function of cumulative EO dose were nonmonotonic and were consistent with EO causing early amplification of preexisting K-ras mutations, rather than induction of K-ras mutation through genotoxicity at codon 12. The possibility that these changes reflect K-ras mutant cell selection under varying degrees of oxidative stress is discussed.
authors.affiliation: * Division of Genetic and Molecular Toxicology, National Center for Toxicological Research, U.S. FDA, Jefferson, Arkansas 72079;
authors.forename: Barbara L
authors.initials: BL
authors.lastname: Parsons
authors.forename: Mugimane G
authors.initials: MG
authors.lastname: Manjanatha
authors.forename: Meagan B
authors.initials: MB
authors.lastname: Myers
authors.forename: Karen L
authors.initials: KL
authors.lastname: McKim
authors.forename: Sharon D
authors.initials: SD
authors.lastname: Shelton
authors.forename: Yiying
authors.initials: Y
authors.lastname: Wang
authors.forename: B Bhaskar
authors.initials: BB
authors.lastname: Gollapudi
authors.forename: Nigel P
authors.initials: NP
authors.lastname: Moore
authors.forename: Lynne T
authors.initials: LT
authors.lastname: Haber
authors.forename: Martha M
authors.initials: MM
authors.lastname: Moore
mesh_terms: D000818:Animals; D002273:Carcinogens / Q000633:toxicity*; D002471:Cell Transformation, Neoplastic / Q000139:chemically induced* / Q000235:genetics / Q000378:metabolism; D003062:Codon; D004305:Dose-Response Relationship, Drug; D005027:Ethylene Oxide / Q000633:toxicity*; D019570:Inhalation Exposure; D008168:Lung / Q000187:drug effects* / Q000378:metabolism; D008175:Lung Neoplasms / Q000139:chemically induced* / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D009154:Mutation; D018384:Oxidative Stress / Q000187:drug effects; D016283:Proto-Oncogene Proteins p21(ras) / Q000235:genetics*; D015398:Signal Transduction / Q000187:drug effects; D013997:Time Factors
chemical_list: D002273:Carcinogens; D003062:Codon; C495971:Hras protein, mouse; D016283:Proto-Oncogene Proteins p21(ras); D005027:Ethylene Oxide
keywords: cII mutation frequency; dose response.; mode of action; oxidative stress; reactive oxygen species; risk assessment