Induction of lacI mutations in Big Blue Rat-2 cells treated with 1-(2-hydroxyethyl)-1-nitrosourea: a model system for the analysis of mutagenic potential of the hydroxyethyl adducts produced by 1,3-bis (2-chloroethyl)-1-nitrosourea

A newly established GDL1 cell line from gpt delta mice well reflects the in vivo mutation spectra induced by mitomycin C

In order to create a novel in vitro test system for detection of large deletions and point mutations, we developed an immortalized cell line. A SV40 large T antigen expression unit was introduced into fibroblasts derived from gpt delta mouse lung tissue and a selected clone was established as the gpt delta L1 (GDL1) cell line. The novel GDL1 cells were examined for mutant frequencies (MFs) and for molecular characterization of mutations induced by mitomycin C (MMC). The GDL1 cells were treated with MMC at doses of 0.025, 0.05, and 0.1 microg/mL for 24h and mutations were detected by Spi- and 6-thioguanine (6-TG) selections. The MFs of the MMC-treated cells increased up to 3.4-fold with Spi- selection and 3.5-fold with 6-TG selection compared to MFs of untreated cells. In the Spi- mutants, the number of large (up to 76 kilo base pair (kbp)) deletion mutations increased. A majority of the large deletion mutations had 1-4 base pairs (bp) of microhomology in the deletion junctions. A number of the rearranged deletion mutations were accompanied with deletions and insertions of up to 1.1 kbp. In the gpt mutants obtained from 6-TG selection, single base substitutions of G:C to T:A, tandem base substitutions occurring at the 5'-GG-3' or 5'-CG-3' sequence, and deletion mutations larger than 2 bp were increased. We compared the spectrum of MMC-induced mutations observed in vitro to that of in vivo using gpt delta mice, which we reported previously. Although a slight difference was observed in MMC-induced mutation spectra between in vitro and in vivo, the mutations detected in vitro included all of the types of mutations observed in vivo. The present study demonstrates that the newly established GDL1 cell line is a useful tool to detect and analyze various mutations including large deletions in mammalian cells.

Measurement of N7-(2'-hydroxyethyl)guanine in human DNA by gas chromatography electron capture mass spectrometry

An improved method is presented, based on gas chromatography-electron capture mass spectrometry (GC-EC-MS), for measuring N7-(2'-hydroxyethyl)guanine (N7-HEG) in DNA from an in vivo sample. The method was used to detect this adduct in amounts of human DNA ranging from 0.07 to 11.5 microg isolated from granulocytes. In this method, the DNA is spiked with a stable isotope internal standard (N7-HEG-d4) and heated in water to release the adduct in a nucleobase form. After the adduct is extracted into 1-butanol, it is purified by reverse phase HPLC and derivatized with HONO, pentafluorobenzyl bromide, and pivalic anhydride. Further purification by silica solid phase extraction and reverse phase HPLC is done prior to injection into a GC-EC-MS. Relatively clean GC-EC-MS chromatograms result, contributing to the high sensitivity that is observed. In the samples tested, from 1.6 to 240 N7-HEG adducts in 10(7) nucleotides were observed, a 150-fold range.

Analysis of spontaneous, gamma ray- and ethylnitrosourea-induced hprt mutants in HL-60 cells with multiplex PCR

AIM: To explore the molecular spectra and mechanism of human hypoxanthine guanine phosphoribosyl transferase (hprt) gene mutation induced by ethyluitrosourea (ENU) and ⁶⁰Co γ-rays.

METHODS: Independent human promyelocytic leukemia cells (HL-60) mutants at the hprt locus were isolated from untreated, ethyluitrosourea (ENU) and ⁶⁰Co γ-ray-exposed cells, respectively, and verified by two-way screening. The genetic changes underlying the mutation were determined by multiplex polymerase chain reaction (PCR) amplification and electrophoresis technique.

RESULTS: With dosage increased, survival rate of plated cell reduced (in the group with dosage of ENU with 100-200 μg/mL, P < 0.01; in the group with dosage of ⁶⁰Co γ-ray with 2-4 Gy, P < 0.05) and mutational frequency increased (in the group of ENU 12.5-200.0 μg/mL, P < 0.05; in the group of ⁶⁰Co γ-ray with 1-4 Gy, P < 0.05) significantly. In the 13 spontaneous mutants analyzed, 92.3% of mutant clones did not show any change in number or size of exon, a single exon was lost in 7.7%, and no evidence indicated total gene deletion occurred in nine hprt exons. However, deletions were found in 79.7% of ENU-induced mutations (62.5%-89.4%, P < 0.01) and in 61.7% of gamma-ray-induced mutations (28.6%-76.5%, P < 0.01). There were deletion mutations in all 9 exons of hprt gene and the most of induced mutations were chain deletion with multiplex exons (97.9% in gamma-ray-induced mutants, 88.1% in ENU-induced mutants).

CONCLUSION: The spectra of spontaneous mutations differs completely from that induced by EUN or ⁶⁰Co γ-ray. Although both ENU and γ-ray can cause destruction of genetic structure, mechanism of mutagenesis between them may be different.

Repair of DNA alkylation products formed in 9L cell lines treated with 1-(2-chloroethyl)-1-nitrosourea

The purpose of this study has been to measure the formation and repair of individual DNA alkylation products in 9L, 9L-2 and BTRC-19 cell lines after treatment with 1-(2-chloroethyl)-1-nitrosourea (CNU). The levels of seven DNA adducts N7-(2-hydroxyethyl)-guanine, N7-(2-chloroethyl)-guanine; 1,2-(diguan-7-yl)-ethane, N1-(2-hydroxyethyl)-2-deoxyguanosine, 1-(N1-2-deoxyguanosinyl), 2-(N3-2-deoxycytidyl)-ethane, O(6)-(2-hydroxyethyl)-2-deoxyguanosine and phosphotriesters were separated by HPLC and quantified by liquid scintillation counting. The levels of N7-(2-hydroxyethyl)-guanine, N7-(2-chloroethyl)-guanine; O(6)-(2-hydroxyethyl)-2-deoxyguanosine and phosphotriesters were not significantly different in the three glioma lines. Furthermore, comparison of the levels of these products in treated cells with the levels formed in purified DNA suggest that they were not actively repaired over the 6h interval. The levels of 1,2-(diguan-7-yl)-ethane and N1-(2-hydroxyethyl)-2-deoxyguanosine were reduced in 9L-2 and significantly reduced in BTRC-19 (P = 0.003) compared to 9L. Analysis of the data suggests that the reduction in the level of N1-(2-hydroxyethyl)-2-deoxyguanosine was due to repair of its precursor O(6)-ClEtdG by O(6)-alkylguanine-DNA-alkyltransferase (AGT). The level of the crosslinked product 1-(N1-2-deoxyguanosinyl), 2-(N3-2-deoxycytidyl)-ethane was significantly reduced (P < 0.001) in both 9L-2 and BTRC-19 as compared to 9L. Reduction in the level of 1-(N1-2-deoxyguanosinyl), 2-(N3-2-deoxycytidyl)-ethane in 9L-2 and BTRC-19 are consistent with repair of the precursor alkylation product O(6)-ClEtdG by AGT. This study demonstrates that there are very significant differences in the rates of removal of individual DNA adducts formed by CNU treatment of the glioma cell lines.