were analyzed for the mutant frequency of lacl using the color screening technique. The mutant frequencies (mean +- SD) in the lacl transgenic LEC rats was 12.8 +- 8.3 x 10-5, 25-fold higher than that in Big Blue case . The level of copper accumulation in the livers of lacl transgenic LEC rats was 265 +- 72 micro gig, 53-fold the amount in Big Blue F344 rats. Plasma levels of glutamic oxaloacetic transam inase and glutamic pyruvic transaminase, assayed as markers of hepatitis development, were also much higher in lacl transgenic LEC rats. We hypothesize that the remarkable increase in mutant frequency in the LEC rat liver may playa crucial role in its hepatocarcinogenesis. Keyword(s ): LEe; lacl transgenic; Oxidative stress Ip Xv.61 The spectrum of spontaneous mutation In the Big Blue® lacI transgene: Evidence campallble wllh A+T mutallon pressure on a high G+C content transgene Steve S. Sommer, Victoria L. Buettner, Kathleen A. Hill, Hiroshi Nishino. Department of Molecular Genetics. Beckman Research Institute. City of Hope, Duarte, CA, USA The spectrum of spontaneous mutations in the E. coli lacl transgene for 483 independent mutations were complied from five Big Blue mouse tissues (brain , liver, spleen, testis, and thymus)I-4 . No stat istically significant differences in mutation frequency and spectrum were found between 3 and 10 month old mice in brain, thymus and male germ cell tissues, hinting that accumulated mutations have reached a steady state in adulthood . No tissue or gender-specific differences were found and no difference was found with pH nullizygosity in thymus, brain, liver and spleen. However, trends observed in the spectra suggest that larger sample sizes (100+ mutations) may reveal minor tissue-specific differences superimposed on a general endogenous spectrum of mutation. The overall spectrum for 483 mutations in the lacl transgene shows several asymmetries. At non-CpG sites, G:C _ A:T are mole frequent than A:T _ G:C transitions (62 vs. 31 mutations) and G:C _ T:A are more frequent than T:A - G:C transversions (54 vs. II mutations). In addition, at CpG sites, G:C - T:A are more frequent than G:C _ C:G transversions (54 vs. 18 mutations). These differences are each statistically significant and, in aggregate, the p-value is less than 10- 5 (Fisher exact test) . A+Tmutation pressuredue to integration into a bulk mammalian isochore provides a simple explanation for the above asymmetries. The putative A+T pressure also extends to I bp deletions which represent 53% of all deletions (28 of 53 deletions) and occur primarily at homopolymeric tracts (20 of the 28 deletions). The lad transgene has a G+C content of 56% without a paucity of CpG dinucleotides (95 CpG dinucleotides in a 1.2 kb region; 8.8%). Although the G+C content of the isochore into which the lad transgene is inserted is unknown, the data are compatible With insertion of the lacl gene within the bulk mammalian isochore conta ining a G+C content of 40% with a paucity of CpO dinucleotides (typically 0.8%). When corrected for mutational pressure associated with the bulk mammalian isochore, the spectrum of spontaneous somatic mutation in Big Blue mice is similar to that observed for spontaneous gemline mutation in the human factor IX gene. Keyword(s) : Big Blue; Spontaneous mutation; A+T pressure (I) Nishino et 01 .. Oncogene II: 263. 1995. (2) Buettner et 01., Oncogene 13: 2407, 1996. (3) Nishino et 01 .. Erwiron. Mal Mutagen 18: 299. 1996. (4) Buetmer el 01 .• Mutat. Res.In press, 1997. Ip XV.7! In vivo and In vitro mutagenicity and mutational sped rum of epoxybutene and dlepoxybutane to asses. their role. In the In vlvo mutagenicity of 1,3-butadlene Leslie Recio, Rogene Henderson", Kathy Meyer, Linda Pluta, Christopher J. Saranko", Ann-Marie Steen. Chemical IndustryInstitute of Toxicology, Re- searchTriangle Park. Ne 27709, USA; • Inhalation Toxicology Research In- stitute. Albuquerque. NM 87185, USA; bDepartment of Toxicology, Raleigh, North Carolina State University; Raleigh, NC 27605, USA S-XV: Transgenic model for studying environmental mutagenesis S147 The mutagen icity and mutational spectrum of epoxybutene (EB) and diepoxybutane (DEB) in human and rodent cells in vitro andin vivo were determined to assess their roles in the mutagenicity of BD. In human TK6 cells, both EB and DEB were mutagenic at hprt and induced an increased fre- quency of A:TJET:A transversions; DEB also induced an increase d frequency of deletions. In lac! transgenic Rat2 fibroblasts, DEB was not mutagen ic but increased the frequency of micronuclei. Inhalation exposure to DEB (5.0 ppm char"A5 2 weeks ) did not increase the lac! mutant frequency in bone marrow of transgenic mice and rats. DNA adducts at A:T base pairs may be useful biomarkers for BD genetic effects. The lack of mutagenicity at the lacl transgene by DEB in vitro and in vivo is likely due to the poor recovery of large deletions using this mutagenicity assay. Therefore other DNA lesions that can mechanistically account for DEB-induced deletions also need to be considered as biomarkers for BD-induced genotoxicity. (Research supported in part by HEI 94-08 and NIEHS Training Grant ES07046 .) Keyword(s): 1,3-butadiene; Hprt; lacltransgenic Ip Xv.sl In vIvo mutagenicity of 2-amlno-3,8-dlmethyllmldazo[4,S- fjqulnoxallne (MelQx) In lacl transgenic mice Toshiaki ltoh, Takayoshi Suzuki, Xue Wang, Makoto Hayashi, Akiyoshi Nishikawa, Shinichiro Ikezaki, Fumio Furukawa, Michihito Takahashi, Toshio Sofuni. Natl. Inst. HealthSci.• Tokyo, 158. Japan 2-Amino-3,8-d imethylimidazo[4,5-f]quinoxaline (MeIQx), one of the potent mutagen ic heterocyclic amines found in cooked foods, induces tumors in rodents . In vivo mutagen icity of MelQx was studied using Big Blue transgenic mice carrying lacl mutational target genes. In the initial study, male mice were treated with 0.1-100 mg/kg MelQx by single intragastic inject ions. Two weeks after treatment, no increase of mutant frequency (MF) was observed in liver and colon. Cell proliferative activity, analyzed by an immunohistochemical staining of proliferating cell nuclear antigen, was also not changed in all tissues analyzed. A concomitant analysis with the peripheral blood micronucleus assay showed a week c1astogenicity of MelQx 40 hr after treatment, In the second study, male and female mice fed 0.03% MelQx in a basal diet for 4 or 12 weeks. The MFs in liver and colon increased significantly at 12 weeks in both sexes. This indicated that the feeding is more effective than single treatment at high dose-levels. It is, therefore, important to optimize treatment regimen in transgenic mouse mulation assays. In liver. the MF in female was signifICantly higher than in male, which is consistent with MelQx hepatocarcinogenicity in mice. DNA sequence analys is of MelQx-induced lacl mutants in liver revealed that G:C to T:A transvers ion was the predominant type of mutations. Keyword(s) : Big Blue; MeIQx; Lacl sequence