Mechanisms of mutagenesis by a bulky DNA lesion at the guanine N7 position

In order to examine the mechanisms of mutagenesis by a bulky DNA lesion at the guanine N7 position, the replicative form DNA of phage M13AB28 (mp8 without the amber codons in phage genes) was modified in vitro with aflatoxin B1-2,3-dichloride and transfected into appropriate Escherichia coli cells. Forward mutations in the lacZ alpha-complementing gene segment were identified as light blue or colorless plaques on appropriate indicator plates, isolated, and defined by DNA sequencing. Transfection of modified DNA into uvrA-/mucAB+ cells without prior UV (SOS) induction increased mutation frequency eight-fold over untreated DNA, whereas this increase was 12-fold upon SOS induction. Transfection of modified DNA after conversion of the primary guanine-aflatoxin lesions to the stable imidazole ring-opened formamidopyrimidine-aflatoxin suggested that these lesions were nearly equally mutagenic. A majority of point mutations under all conditions affected G:C bp. Base substitutions were in the majority, but significant frameshift mutagenesis was also detected in SOS-induced cells. Both G-to-T transversions and G-to-A transitions were produced at equal efficiency and together accounted for virtually all of the base substitutions induced by the primary lesions. Point mutations occurred predominantly at predicted damage hotspots. The characteristics of base substitution and frameshift mutations, together with available information point to multiple mechanisms of mutagenesis by this class of mutagens. The data indicate that primary lesions have the properties of both a noninstructional and pseudo-instructional lesion. In addition, the sequence context appears to play a role in determining whether a frameshift or a base substitution is induced by this bulky lesion.

Effects of SOS and MucAB functions on reactivation and mutagenesis of M13 replicative form DNA bearing bulky lesions

We have previously determined the specificity of -1 frameshifts induced by aflatoxin-B1-2,3-dichloride (AFB1C12) in phage M13 double-strand replicative form (RF) DNA. The system consists of: (i) in vitro adduction of RF DNA of BK8, a lacZ + 1 frameshift derivative of phage M13mp8; (ii) transfection into unirradiated or UV-irradiated bacterial host cells; (iii) scoring and sequencing of revertants (i.e., -1 frameshifts). The previous data had shown that induction of SOS functions enhanced mutagenesis. However, this increase in mutagenesis is not accompanied by enhanced survival in a majority of the strains tested. Here, we present evidence to show that the lack of SOS reactivation is a specific property of the RF DNA system rather than a specific property of the lesion. A model mechanism based on the replicative strategy of transfected RF DNA can account for these observations. In addition, we have calculated individual Weigle mutagenesis factors at 8 major mutagen induced sites reported previously. Analysis of these data indicates that, within a restricted subset of possible mutational events (i.e., -1 frameshifts), Weigle mutagenesis is affected by both the DNA sequence environment of the mutation site as well as the repair phenotype of the cell.

[Cytogenetic studies on 3 esophageal cancer cell lines]

The cytogenetic studies on three esophageal cancer cell lines established in China were done. Thirty metaphases showing suitable chromosome length and good G-banding pattern from each of the cell lines were chosen and subjected to karyological analysis. The typical karyotypes from these cell lines were listed, and the variation of chromosome number as well as the morphological characteristics, possible sources and the incidence of the marker chromosomes were analysed. Eca 109 is the cell line first established and used extensively in China. A strictly regular karyotype pattern was found in it: the modal number of chromosomes being 63-64; the number of each type of chromosome varying between 1-5; a distal deletion of short arm of chromosome No. 1 being discernible in all metaphases, with break sites located within 1p22-1p33. Also a distal deletion of long arm of chromosome No. 4 was usually visible. There were seven marker chromosomes with high incidence. Among them, M1 marker chromosome was a large subacrocentric chromosome which was observed in the early passages of this cell line. The chromosome number of Ec 17 cell line was usually subtetraploid. In addition to the numerical variation in some of the chromosomes, six marker chromosomes were usually observed. Among them, M1 involved reciprocal translocation between chromosome No. 1 and No. 4. M3 of Ec 17 was in correspondence with M5 of Eca 109. Both were Rob (13q;14q). The chromosome number of Ec 56 was usually subtetraploid, and in addition to the numerical variation in some of the chromosomes, seven marker chromosomes were usually observed.

Malignant transformation of baby hamster lung fibroblasts induced in vitro by a new nitrosamine compound

A new nitrosamine compound, N-1-methylacetonyl-N-3-methylbutyl-nitrosamine (MAMB-NA), isolated from corn bread inoculated with moulds commonly occurring in Linxian County, a high incidence area for esophageal cancer in North China was tested for its potential carcinogenicity in vitro. Baby hamster lung fibroblasts (BHL) were treated with MAMBNA in tissue culture medium containing liver microsome preparation of Aroclor 1254 treated rat. Cell transformation was assessed by morphological changes, loss of density-dependent inhibition and unlimited growth in vitro, chromosome changes, colony formation on soft agar, growth at low serum concentration, Con A agglutinability and tumor formation after heterotransplantation into immunosuppressed newborn rats. Like the known carcinogens N-methyl-N'-nitro-N-nitroso-guanidine and diethylnitrosamine, MAMBNA was able to cause malignant transformation of cells cultured in vitro.