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The recombinant shuttle vector pSV2gpt was introduced into V79 Chinese hamster cells, and stable transformants expressing the Escherichia coli gpt gene were selected. Two transformants carrying tandem duplications of the plasmid at a single site were identified and fused to simian COS-1 cells. Plasmid DNA recovered from the heterokaryons was used to transform a Gpt- derivative of E. coli HB101, and the relative frequency of plasmids carrying a mutation in the gpt gene was determined. The high frequency of Gpt- plasmids (ca. 1%) was similar to that observed when plasmid was recovered from COS-1 cells which had been transfected with pSV2gpt. Most of the mutant plasmids had rearrangements in the region containing the gpt gene.

A line of mouse cells transformed with ultraviolet-irradiated herpes simplex virus type 1 and containing a methylated and inactive viral thymidine kinase (TK) gene was treated with insulin in an attempt to induce expression of the inactive gene. Insulin was found to be capable of inducing the inactive TK gene in these cells. The induction of the TK+ phenotype was dose dependent (from 1-100 micrograms of insulin per ml), and the TK activity induced was shown to be of viral origin. Analysis of the methylation pattern of the viral TK gene by using the methylation-sensitive restriction endonucleases Sma I, Hpa II, and Hha I revealed that the active viral TK gene in the parental transformed cells was hypomethylated, whereas the inactive TK gene in the uninduced TK- cells was methylated. The active TK gene in three insulin-induced TK+ lines also was methylated, but the methylation patterns in the insulin-induced lines all were different from the uninduced TK- line. These data suggest that extensive hypomethylation of the inactive TK gene is not required for insulin induction. Four other transformed lines containing an inactive viral TK gene were tested for insulin inducibility, but insulin was unable to induce expression of the TK gene in any of the other lines. Thus, insulin inducibility does not seem to be a function of the viral TK gene itself. These results suggest that insulin inducibility of the viral TK gene may be a reflection of the region of the host genome into which the TK gene was integrated.

A retroviral shuttle vector was constructed by introducing the Escherichia coli xanthine (guanine) phosphoribosyltransferase gene (gpt) into the pZip-NeoSV(X)1 vector [Cepko, C. L., Roberts, B. E. & Mulligan, R. C. (1984) Cell 37, 1053-1062]. This vector was packaged into infectious virus which then was used to infect a hypoxanthine (guanine) phosphoribosyltransferase-deficient mouse cell line. Cell lines that expressed the gpt gene were isolated, and it was found that these cells contained a single integrated copy of the vector in a proviral form. Treatment of these cell lines with either ethyl methanesulfonate or BrdUrd produced a greater than 10-fold increase in the frequency of 6-thioguanine-resistant (Sgur) mutants. Intact gpt genes have been recovered from a number of Sgur cell lines after COS cell fusion and introduced into E. coli as part of a plasmid. The complete DNA sequences of three mutant genes have been determined. Two of the mutant genes have a single base substitution, whereas the third has a 34-base-pair deletion. This system should be valuable for analyzing mutagenic specificity and the molecular mechanisms of chemical mutagenesis in mammalian cells. A potentially important feature of the system relative to other shuttle-vector systems is that the mutations are induced in genes integrated into mammalian chromosomes rather than in genes existing as part of autonomously replicating plasmids.

We demonstrate previously that fibroblasts from cystic fibrosis (CF) patients are significantly more resistant to the toxic effects of ouabain than normal human fibroblasts are. Ouabain is generally assumed to act primarily at the level of the cell membrane and to cause killing of cells by inhibiting ion transport. We have therefore examined the ability of normal and CF cells to survive exposure to ethacrynic acid, another inhibitor of ion transport, and to survive colchicine and aminopterin, resistance to which has been associated with membrane alterations in other cells. The effect of dexamethasone, which has a sterol nucleus similar to that of ouabain but is thought to have a different site of cellular action, was also tested. Exposure of the cells to ethacrynic acid, colchicine, or aminopterin did not reveal any differences in survival between normal and CF fibroblasts. However, CF cells survived exposure to dexamethasone significantly better than normal cells did. These results suggest that normal and CF cells do not differ in terms of a generalized resistance to ion transport inhibitors or to drugs that must pass through the membrane to be active. Instead, the results raise the possibility that CF cells have an enhanced resistance to drugs that have the sterol nucleus found in ouabain and dexamethasone.

Studies were undertaken to determine how a line of mutant Syrian hamster melanoma cells (HAB-2E) that displays unlimited growth potential when all of the thymine residues in nuclear DNA are replaced by bromouracil (BrUra) could avoid the deleterious effects of bromodeoxyuridine (BrdUrd) mutagenicity. It was found that BrdUrd could be mutagenic to these cells. However, there was a nonlinear relationship between mutagenicity and the amount of BrUra in the DNA of the HAB-2E cells. With these cells, mutagenicity apparently is determined by the concentration of BrdUrd to which the cells are exposed rather than the amount of BrUra in DNA. These results were obtained with both the induction of ouabain resistance and thioguanine resistance as markers for mutagenesis. The dependence of BrdUrd mutagenicity on BrdUrd concentration was also observed for the parental melanoma cells.

The pattern of terminal replication of the human chromosomes in a clone of hybrids between diploid human fibroblasts and mouse cells was analyzed by autoradiography. An average of 10 human chromosomes was present in the hybrid cells. Several of these chromosomes were found to terminate replication in a different order from the same chromosomes in the parental human fibroblasts. Chromosomes 4 and 5 completed replication later in the hybrid than in the fibroblasts (relative to the other human chromosomes). In contrast, chromosomes 7, 12, and 15 completed replication earlier in the hybrid than in the fibroblasts. These results suggest that the sequence of terminal chromosome replication in human fibroblasts is not irreversibly programmed into each chromosome.

The molecular mechanisms of ethyl methanesulfonate-induced reversion in mammalian cells were studied by using as a target a gpt gene that was integrated chromosomally as part of a shuttle vector. Murine cells containing mutant gpt genes with single base changes were mutagenized with ethyl methanesulfonate, and revertant colonies were isolated. Ethyl methanesulfonate failed to increase the frequency of revertants for cell lines with mutant gpt genes carrying GC----AT transitions or AT----TA transversions, whereas it increased the frequency 50-fold to greater than 800-fold for cell lines with mutant gpt genes carrying AT----GC transitions and for one cell line with a GC----CG transversion. The gpt genes of 15 independent revertants derived from the ethyl methanesulfonate-revertible cell lines were recovered and sequenced. All revertants derived from cell lines with AT----GC transitions had mutated back to the wild-type gpt sequence via GC----AT transitions at their original sites of mutation. Five of six revertants derived from the cell line carrying a gpt gene with a GC----CG transversion had mutated via GC----AT transition at the site of the original mutation or at the adjacent base in the same triplet; these changes generated non-wild-type DNA sequences that code for non-wild-type amino acids that are apparently compatible with xanthine-guanine phosphoribosyltransferase activity. The sixth revertant had mutated via CG----GC transversion back to the wild-type sequence. The results of this study define certain amino acid substitutions in the xanthine-guanine phosphoribosyltransferase polypeptide that are compatible with enzyme activity. These results also establish mutagen-induced reversion analysis as a sensitive and specific assay for mutagenesis in mammalian cells.

We constructed a gene transfer vector containing the herpes simplex virus type 1 thymidine kinase (TK) gene flanked by Drosophila P element terminal repeats (W. R. Engels, Annu. Rev. Genet. 17:315-344). This vector was introduced into mouse LTK- cells and enhanced the frequency of stable transformation to the TK+ phenotype by approximately 50-fold relative to a similar plasmid lacking the P element terminal repeats.

The relationship between bromodeoxyuridine (BrdUrd) mutagenesis in mammalian cells and the effects of BrdUrd on deoxyribonucleoside triphosphate pools was analyzed. It was found that the exposure of Syrian hamster melanoma cells to mutagenic concentrations of BrdUrd resulted in the formation of a large bromodeoxyuridine triphosphate (BrdUTP) pool, which remained at a high level for several days. In contrast, the size of the deoxycytidine triphosphate (dCTP) pool dropped rapidly after the addition of BrdUrd, reached a minimum at about 6 h, and then expanded gradually to nearly its original level over the next 3 days. The addition of lower concentrations of BrdUrd, which had less of a mutagenic effect, resulted in the formation of a smaller BrdUTP pool and a slightly smaller drop in the dCTP pool. When a high concentration of deoxycytidine was added at the same time as a normally mutagenic concentration of BrdUrd, the drop in the dCTP pool was prevented, as was BrdUrd mutagenesis. In all of these experiments, mutagenesis was related to the ratio of BrdUTP to dCTP in the cells. In addition, it was shown that mutagenesis occurred primarily during the first 24 h of BrdUrd exposure, when the BrdUTP/dCTP ratio was at its highest level. It appears that there is a critical ratio of BrdUTP to dCTP that must be attained for high levels of mutagenesis to occur and that the extent of mutagenesis is related to the ratio of the BrdUrd and dCTP pools.

The distribution of intramembrane particles (IMP) as revealed by freeze-fracture electron microscopy has been analyzed following treatment of mouse L cells and fusion-deficient L cell derivatives with several concentrations of polyethylene glycol (PEG). In cell cultures treated with concentrations of PEG below the critical level for fusion, no aggregation of IMP was observed. When confluent cultures of the parental cells are treated with 50% PEG, greater than 90% of the cells fuse, and cold-induced IMP aggregation is extensive. In contrast, identical treatment of fusion-deficient cell lines shows neither extensive fusion nor IMP redistribution. At higher concentrations of PEG, however, the PEG-resistant cells fuse extensively and IMP aggregation is evident. Thus the decreased ability of the fusion-deficient cells to fuse after treatment with PEG is correlated with the failure of IMP aggregation to occur. A technique for quantifying particle distribution was developed that is practical for the accurate analysis of a large number of micrographs. The variance from the mean number of particles in randomly chosen areas of fixed size was calculated for each cell line at each concentration of PEG. Statistical analysis confirms visual observation of highly aggregated IMP, and allows detection of low levels of aggregation in parental cells that were less extensively fused by exposure to lower concentrations of PEG. When low levels of fusion were induced in fusion-deficient cells, however, no IMP aggregation could be detected.

Murine erythroleukemic cells in culture accumulate cytoplasmic globin mRNA during differentiation induced by dimethyl sulfoxide (DMSO)1. Chromatin was prepared from DMSO induced erythroleukemic cells that were transcribing globin RNA in order to determine whether in vitro synthesis of globin RNA sequences was possible from chromatin. RNA was synthesized in vitro using 5-mercuriuridine triphosphate and exogenous Escheria coli RNA polymerase. Newly synthesized mercurated RNA was purified from endogenous chromatin associated RNA by affinity chromatography on a sepharose sulfhydryl column, and the globin RNA sequence content of the mercurated RNA was assayed by hybridization to cDNA globin. The synthesis of globin RNA sequences was shown to occur and to be sensitive to actinomycin and rifampicin and insensitive to alpha-amanitin. In contrast, synthesis of globin RNA sequence synthesis was not detected in significant amounts from chromatin prepared from uninduced erythroleukemic cells, nor from uninduced cell chromatin to which globin RNA was added prior to transcription. Isolated RNA:cDNA globin hybrids were shown to contain mercurated RNA by affinity chromatography. These results indicated that synthesis of globin RNA sequences from chromatin can be performed by E. coli RNA polymerase.

We have developed a system for the molecular analysis of mutations in mammalian cells. This system is based upon the use of mammalian cell lines containing mutant shuttle vector genes integrated into chromosomal DNA. The target for mutation was the Escherichia coli gpt gene, coding for the enzyme xanthine (guanine) phosphoribosyltransferase (GPT; EC 2.4.2.22). We have previously isolated a large number of cell lines containing mutant gpt genes with single base changes. From these lines, revertants were selected on the basis of the reappearance of GPT activity. In general, the frequency of revertants was below 10(-7). The gpt genes were recovered from 32 revertants and sequenced to determine the nature of the base changes associated with reversion. In the majority of the revertants, there was a base change within the originally mutated codon, leading to either restoration of the wild-type amino acid sequence or substitution of a different amino acid at the original mutated site. In no case did reversion of a base substitution mutant involve an amino acid residue other than that affected by the original mutation. The results have demonstrated a number of sites in the GPT polypeptide at which amino acid substitutions are compatible with enzyme activity and one site at which the loss of an amino acid is compatible with enzyme activity. This study establishes reversion analysis as a sensitive molecular assay for mutagenesis in mammalian cells.

By using a shuttle vector system developed in our laboratory, we have carried out studies on the molecular mechanism by which 5-bromodeoxyuridine (BrdUrd) induces mutations in mammalian cells. The target for mutagenesis in these studies was the Escherichia coli gpt gene that was contained within a retroviral shuttle vector and integrated into chromosomal DNA in mouse A9 cells. Shuttle vector-transformed cells expressing the gpt gene were mutagenized with BrdUrd and cells with mutations in the gpt gene were selected. Shuttle vector sequences were recovered from the mutant cells, and the base sequence of the mutant gpt genes was determined. The great majority of the BrdUrd-induced mutations involving single-base changes were found to be G.C----A.T transitions. We have shown that mutagenesis by BrdUrd depends upon perturbation of deoxycytidine metabolism. Thus, the current results suggest that BrdUrd mutagenesis involves mispairing and misincorporation of BrdUrd opposite guanine in DNA, driven by nucleotide pool perturbation caused by BrdUrd and the resulting imbalanced supply of triphosphates available for DNA synthesis. The results also revealed a very high degree of sequence specificity for the BrdUrd mutagenesis. BrdUrd-induced G.C----A.T transitions occurred almost exclusively in sequences with two adjacent guanine residues. Furthermore, in approximately equal to 90% of the cases, the guanine residue involved in mutation was the one in the more 3' position.

We have studied the molecular mechanisms of spontaneous mutations in mouse cells carrying a selectable bacterial gene. The mouse cells carry the Escherichia coli xanthine (guanine) phosphoribosyltransferase (gpt) gene in a retroviral shuttle vector integrated into chromosomal DNA in a proviral form. Cells with spontaneous mutations in the gpt gene were selected as resistant to 6-thioguanine and then were fused with COS cells for recovery of the mutant genes. Out of a total of 77 independent 6-thioguanine-resistant cell lines isolated in this study, vector sequences could be rescued from 43 of the mutant lines, and the base sequences were determined for the gpt genes in all 43 of these lines. There was a variety of mutational events among the mutant gpt genes sequenced. The most frequent mutational event was a deletion (in 29 of the 43 mutant genes), and the next most frequent event was a base substitution mutation (in 11 of the 43 mutant genes). Among the deletion mutants, the great majority represent deletions of less than 10 base pairs. In fact, 19 of the 29 deletion mutants had deletions of 3 base pairs, and among the mutants with 3-base-pair deletions, there was a very strong deletion hot spot appearing in 16 independent mutants. All 19 of the 3-base-pair deletions resulted in the "in frame" loss of an aspartic acid codon. Among the base substitution mutations, transitions and transversions occurred with approximately equal frequency. Our results raise the possibility that small deletions represent the predominant mechanisms by which spontaneous mutations occur in mammalian cells.