Mice Exposed to N-Ethyl-N-Nitrosourea T

Proc. Nati. Acad. Sci. USA Vol. 89, pp. 7866-7870, September 1992 Genetics Mutational spectrum at the Hprt locus in splenic T cells of B6C3F1 mice exposed to N-ethyl-N-nitrosourea T. R. SKOPEK*tt, V. E. WALKER*, J. E. COCHRANEt, T. R. CRAFTt, AND N. F. CARIELLO* *Department of Pathology and tDepartment of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 Communicated by Kenneth M. Brinkhous, May 26, 1992 ABSTRACT We have determined the mutational spectrum quences for analysis. DGGE is based on the fact that the of N-ethyl-N-nitrosourea (ENU) in exon 3 of the hypoxanthine electrophoretic mobility of a DNA molecule in a polyacryl- guanine) phosphoribosyltransferase gene (Hprt) in splenic T amide gel is considerably reduced as the molecule becomes cells following in vivo exposure ofmale B6C3F1 mice (5-7 weeks partially melted (denatured). Mismatched heteroduplexes old) to ENU. Hpir mutants were isolated by culturing splenic formed by annealing wild-type and mutant DNA sequences T cells in microtiter dishes containing medium supplemented are always less stable than the corresponding perfectly base- with interleukin 2, concanavalin A, and 6-thiouanine. DNA paired homoduplexes and consequently melt at a lower was extracted from 6-thoa ne-sistant colonies and ampli- concentration of denaturant. Therefore, any mutant/wild- fied by the polymerase chain reaction (PCR) using primers type heteroduplex will always travel a shorter distance rel- flanking Hprt exon 3. Identification of mutant sequences and ative to wild-type homoduplexes in a gel containing a dient purification of mutant DNA from contaminating wild-type Hprt ofdenaturant. It is predicted that heteroduplex formation and DNA was accomplished by denaturing-adient gel electropho- DGGE analysis will resolve any base substitution, frame- resis. Purified mutant DNA was then sequenced. Treatment of shift, or small deletions (<10 base pairs) in the low- mice with ENU at 40 mg/kg of body weight produced a Hprr temperature melting domain of a DNA molecule (11). Also, mutant frequency of 7.3 x 10-5 in splenic T cells, =35-fold since the addition of G+C-rich primers can modify the above background levels. Sixty-nine of the 521 HprF mutants melting characteristics ofthe DNA molecule under study, all analyzed contained mutations in exon 3 (13%). Tranversions regions can be analyzed (12). and transitions at AFT base pais dominated the spectrum; 62 of Methods to analyze mutations in the exon 3 region of the the 69 exon 3 mutations were at AKT base pairs (14 different human HPRT gene by DGGE have been developed previously sites). Thirteen of 14 thymine bases undergoing mutation (61 of (13). Since the sequence of exon 3 and flanking intron regions 62 mutations at AFT bases) were located on the nontranscribed of mouse Hprt have been determined (14), we sought to stand ofexon 3. The majority of the remaining mutations (6 of develop similar DGGE methods to analyze mutations isolated 69) were transitions at a single G'C base -i. These results by the mouse HprtU T-cell cloning assay (1-3). We have used suggest the importance ofthymidine alkylation in ENU-induced these methods to determine the mutational spectrum of mutagenesis in vivo. The mouse HpW- T-cell cloning/sequenc- N-ethyl-N-nitrosourea (ENU) in B6C3F1 mice. ing assay described here may represent a useful system for ENU is a well-characterized ethylating agent that produces studying the molecular mechanism of chemically induced mu- a variety of DNA modifications, including the potent pro- tation occurring in vivo in an endogenous gene. mutagenic adducts 06-ethylguanine and 04-ethylthymine (15, 16). These lesions produce predominantly transition Chemically induced mutation in vivo is the end result of a mutations at G-C and A-T base pairs (17, 18). Recently, the complicated cascade of events including compound uptake mutagenic potential of another ethyl adduct, 02-ethylthy- and distribution, metabolic activation/detoxification, com- mine, has been demonstrated in Escherichia coli and has pound interaction with DNA, DNA repair, and cell replica- been shown to result mainly in transversions (19). We report tion. The complexity of this pathway suggests that in vivo here that mutations induced by ENU in vivo are predomi- mutagenesis assays may be more relevant than in vitro nantly transversion and transition mutations at AFT base systems for modeling possible mutagenic consequences in pairs, and we contrast these results with ENU spectral data humans. In vivo mutation assays based on the cloning of obtained in other systems. hypoxanthine (guanine) phosphoribosyltransferase (HPRT)- negative T cells have been developed in the mouse (1-3), rat MATERIALS AND METHODS (4), monkey (5), and human (6, 7). Recently, transgenic Chemicals, Enzymes, and Medium Components. Materials mouse systems have also been described that contain bac- were obtained from the indicated sources: fetal bovine serum terial transgenes as mutational targets (8, 9). These systems (GIBCO); RPMI-1640, Hepes buffer, glutamine, MEM non- provide excellent opportunities to quantitate and compare essential amino acids, penicillin/streptomycin mixture, and the type and frequency of mutations occurring in vivo in sodium pyruvate (Lineberger Cancer Research Center, Tis- different sequence contexts. sue Culture Facility, University of North Carolina at Chapel The type of base-pair changes produced in DNA can offer Hill); HL-1 medium (Ventrex Laboratories, Portland, ME); important insight regarding the specific DNA adducts and ENU (in aqueous 23% acetic acid as stabilizer) (Sigma); mutagenic mechanisms involved in the mutagenic event. To deoxynucleoside triphosphates (Pharmacia LKB), Seque- this end, our group has developed a method to sequence Hprt nase, dideoxynucleoside triphosphates, and concanavalin A mutations induced in vivo in splenic T cells of B6C3F1 mice, (United States Biochemical); AmpliTaq DNA polymerase the strain presently used in the National Toxicology Program (Perkin-Elmer/Cetus); Lympholyte M (Cedarlane Labora- cancer bioassays. The approach utilizes denaturing-gradient tories, Hornby, ON, Canada). gel electrophoresis (DGGE) (10-12) to purify mutant se- Abbreviations: ENU, N-ethyl-N-nitrosourea; HPRT, hypoxanthine The publication costs ofthis article were defrayed in part by page charge (guanine) phosphoribosyltransferase; DGGE, denaturing-gradient gel payment. This article must therefore be hereby marked "advertisement" electrophoresis; 6TG, 6-thioguanine; PE, plating efficiency. in accordance with 18 U.S.C. §1734 solely to indicate this fact. tTo whom reprint requests should be addressed. 7866 Downloaded by guest on September 29, 2021 Genetics: Skopek et al. Proc. Natl. Acad. Sci. USA 89 (1992) 7867 Mice and ENU Treatment. Male B6C3F1 mice (5-7 weeks (5'-CCGCGGGCGGGCCTCGCGCCGCGGGCCGGGAC- old) were obtained from Charles River Breeding Laboratories CCGCGGCCTGATTTTATTTCTATAG-3') and a primer (Raleigh, NC). The animals were fed standard chow and homologous to internal sequences of exon 3 (P3HI primer, distilled water ad libitum and maintained in a 12-hr light/dark 5'-TCCAGCAGGTCAGCAAA-3'). The melting characteris- cycle. After a 10-day acclimation, mice were treated with tics ofthe full-length exon 3 fragment and ofthe GC-clamped, ENU (40 mg/kg of body weight), delivered as a single i.p. truncated fragment were determined by the melting algorithm injection of ENU diluted in dimethyl sulfoxide (9.0 mg/ml). of Lerman and found to be very similar to those reported for After injection, the mice were maintained as described above the human HPRT gene (13). for 6 weeks. DGGE and Mutant Sequence Purification. Oil was removed Mutant Isolation. Animals were killed by CO2 asphyxiation from PCR mixture and the volumes were adjusted to 50 ul followed by cervical dislocation and their spleens were with water. Samples were extracted twice with phenol and removed aseptically. Spleens were crushed individually with twice with chloroform. Mutant/wild-type heteroduplexes of the end of a 10-ml syringe plunger in 1 ml of RPMI-1640 full-length exon 3 fragments were formed by heating samples medium plus Hepes buffer and 0.3% bovine serum albumin, at 980C for 10 min and then holding at 370C for >1 hr to allow drawn through a 5/8-inch 25-gauge syringe needle, and ex- reannealing. Mutant/wild-type heteroduplexes of GC- pelled gently into an additional 5 ml of RPMI-1640. The cell clamped, truncated exon 3 fragments were formed by dena- suspension from each spleen was then layered onto 5 ml of turing the DNA at 980C and holding overnight at 65TC. Lympholyte M and spun at room temperature for 20 min at Samples were then vacuum-dried and resuspended in 10-20 500 x g. Mononuclear cells at the interface were removed .ld of loading buffer. with a pipet and washed twice with 5 ml of the medium One-millimeter-thick, 12.5% polyacrylamide gels (37.5:1 described above. Cells were then resuspended in RPMI-1640 acrylamide/N,N'-methylenebisacrylamide weight ratio) con- supplemented with 10%o (vol/vol) fetal bovine serum, 20% taining linear gradients of denaturant were used. An 18-36% (vol/vol) HL-1 medium, 2 mM glutamine, MEM nonessential denaturant gradient was used to resolve mutations in the amino acids, 20 mM Hepes buffer, 1 mM pyruvate, lx low-temperature domain and a 36-53% gradient for the penicillin/streptomycin, 50 tLM 2-mercaptoethanol, 4 tug of high-temperature domain [100%o is defined as 7 M urea plus concanavalin A per ml, and 10% (vol/vol) lymphokine- 40% (vol/vol) formamide]. Samples were loaded onto gels activated killer T-cell treatment supernatant containing hu- submerged in a water bath at 600C and run for 15 hr at 150 V man interleukin 2 (a gift from Timothy Darrow, Duke Uni- in Tris/acetate/EDTA buffer. Gels were stained with ethid- versity). Cells were enumerated with a Coulter Counter, ium bromide and visualized under UV light.

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