Characterization of mutations induced by 2-(N-acetoxy-N-acetyl)aminofluorene in the dihydrofolate reductase gene of cultured hamster cells

Abstract

To determine the types of alterations in gene structure that are induced by the carcinogen 2-(N-acetoxy-N-acetyl)aminofluorene, we used this compound to generate mutations at the dihydrofolate reductase (DHFR) locus (DHFR) in Chinese hamster ovary cells. Twenty-nine independent enzyme-deficient mutants were isolated. A profile of the 26-kilobase (kb)-long gene was obtained by Southern blot analysis of the mutant and parental DNAs digested with BstEII/Kpn I. Hybridization to a mixed probe of 10 DHFR genomic and cDNA fragments revealed 12 bands that scan 34 kb. Twenty-one DHFR- clones (72%) contained small mutations (changes less than 100 base pairs in size). Large or small deletions involving various parts of the gene occurred in eight of the mutants (28%). A large deletion (greater than 35 kb) with 5' and 3' breakpoints mapping to approximately the same location was noted in four mutants. One mutant has undergone a deletion of 550-900 bp that eliminated the first coding exon. Concomitantly, a chromosomal event (either translocation, insertion, or inversion) has separated the 5' flank from the body of the gene. In another mutant, four deletions have occurred at the DHFR 5' end and internally. Restriction fragment length polymorphism analysis of the mutant DNAs with exon-specific probes localized three mutations. One mutant has lost a Taq I (TCGA) site, and another has lost a Sac I (GAGCTC) site. In a third, a GC----TA transversion has created a BstEII (GGTNACC) site. Finally, we used HPLC to determine the ratio of acetylated (12%) to deacetylated (88%) 2-aminofluorene adducts formed in the parental cells. A correlation between the mutational specificities and the conformational changes induced by the two types of DNA adducts is discussed.