Ability of structurally related polycyclic aromatic carcinogens to induce homologous recombination between duplicated chromosomal sequences in mouse L cells

Abnormal kinetics of induction of UV-stimulated recombination in human DNA repair disorders

Recombination can result in genetic instability, and thus constitutes an important factor in the carcinogenic conversion of mammalian cells. Here we describe the occurrence of UV-stimulated recombination called enhanced recombination (EREC), measured with the use of Herpes Simplex Viruses type 1 mutants. In normal diploid human cells, EREC is induced by UV-C, mitomycin C and ENU, but not by X-ray or MMS. The kinetics of induction of EREC is similar to that of other SOS-like responses such as enhanced reactivation (ER) and enhanced mutagenesis (EM). In contrast to the latter responses, EREC is induced to higher levels and persists for longer periods in DNA repair deficient fibroblasts derived from xeroderma pigmentosum (XP), Cockayne syndrome (CS) and Trichothiodystrophy (TTD) patients. This observation indicates that EREC is a distinct SOS-like response. Apparently, the presence of unrepaired DNA lesions in the host genome is a strongly inducing signal for EREC. On the other hand, in cells derived from patients suffering from Bloom, Werner or Rothmund-Thomson syndrome (RTS) the EREC response is absent. These data indicate that determining EREC is a useful assay to investigate diploid human fibroblasts for abnormalities in UV-stimulated recombination.

Mismatch repair is required for O(6)-methylguanine-induced homologous recombination in human fibroblasts

O:(6)-methylguanine is responsible for homologous recombination induced by N:-methyl-N:'-nitro-N:-nitrosoguanidine (MNNG) [H. Zhang et al. (1996) CARCINOGENESIS:, 17, 2229]. To test the hypothesis that mismatch repair is causally involved in this process, we generated mismatch repair-deficient strains from a human fibroblast line containing a substrate for detecting intrachromosomal homologous recombination. The four strains selected for study exhibited greatly increased resistance to the cytotoxic effects of MNNG, which was not affected by depletion of O:(6)-alkylguanine-DNA alkyltransferase, and greatly increased sensitivity to the mutagenic effect of MNNG, suggesting that the mutagenic base modifications induced in these four cell strains by MNNG persist in their genomic DNA. Tests showed that their extracts are deficient in the repair of G:T mismatches. The frequency of homologous recombination induced by MNNG in three of these strains was significantly (5-7-fold) lower than that induced in the parental cell strain. This was not the result of a generalized defect in recombination, because when (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epox y-7,8,9, 10-tetrahydrobenzo[a]pyrene was used to induce recombination, all three lines responded with a normal or even a somewhat higher frequency than that observed in the parental strain. The lack of recombination displayed by the fourth strain was shown to result from the loss of part of the recombination substrate. The results strongly suggest that functional mismatch repair is required for MNNG-induced homologous recombination.

High-frequency intrachromosomal gene conversion induced by triplex-forming oligonucleotides microinjected into mouse cells

To test the ability of triple helix-forming oligonucleotides (TFOs) to promote recombination within chromosomal sites in mammalian cells, a mouse LTK(-) cell line was established carrying two mutant copies of the herpes simplex virus thymidine kinase (TK) gene as direct repeats in a single chromosomal locus. Recombination between these repeats can produce a functional TK gene and occurs at a spontaneous frequency of 4 x 10(-6) under standard culture conditions. When cells were microinjected with TFOs designed to bind to a 30-bp polypurine site situated between the two TK genes, recombination was observed at frequencies in the range of 1%, 2,500-fold above the background. Recombination was induced efficiently by injection of both psoralen-conjugated TFOs (followed by long-wave UVA light; 1. 2%) and unconjugated TFOs alone (1.0%). Control oligomers of scrambled sequence but identical base composition were ineffective, and no TFO-induced recombination was seen in a control LTK(-) cell line carrying an otherwise identical dual TK gene construct lacking the 30-bp polypurine target site. TFOs transfected with cationic lipids also induced recombinants in a highly sequence-specific manner but were less effective, with induced recombination frequencies of 6- to 7-fold over background. Examination of the TFO-induced recombinants by genomic Southern blotting revealed gene conversion events in which both TK genes were retained, but either the upstream (57%) or the downstream gene (43%) was corrected to wild type. These results suggest that, with efficient intracellular delivery, TFOs may be effective tools to promote site-specific recombination and targeted modification of chromosomal loci.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Mismatch repair of heteroduplex DNA intermediates of extrachromosomal recombination in mammalian cells

Previous work indicated that extrachromosomal recombination in mammalian cells could be explained by the single-strand annealing (SSA) model. This model predicts that extrachromosomal recombination leads to nonconservative crossover products and that heteroduplex DNA (hDNA) is formed by annealing of complementary single strands. Mismatched bases in hDNA may subsequently be repaired to wild-type or mutant sequences, or they may remain unrepaired and segregate following DNA replication. We describe a system to examine the formation and mismatch repair of hDNA in recombination intermediates. Our results are consistent with extrachromosomal recombination occurring via SSA and producing crossover recombinant products. As predicted by the SSA model, hDNA was present in double-strand break-induced recombination intermediates. By placing either silent or frameshift mutations in the predicted hDNA region, we have shown that mismatches are efficiently repaired prior to DNA replication.

Preferential repair of UV damage in highly transcribed DNA diminishes UV-induced intrachromosomal recombination in mammalian cells

The relationships among transcription, recombination, DNA damage, and repair in mammalian cells were investigated. We monitored the effects of transcription on UV-induced intrachromosomal recombination between neomycin repeats including a promoterless allele and an inducible heteroallele regulated by the mouse mammary tumor virus promoter. Although transcription and UV light separately stimulated recombination, increasing transcription levels reduced UV-induced recombination. Preferential repair of UV damage in transcribed strands was shown in highly transcribed DNA, suggesting that recombination is stimulated by unrepaired UV damage and that increased DNA repair in highly transcribed alleles removes recombinogenic lesions. This study indicates that the genetic consequences of DNA damage depend on transcriptional states and provides a basis for understanding tissue- and gene-specific responses to DNA-damaging agents.

Demonstration by DNA fingerprint analysis of genomic instability in mouse BALB 3T3 cells during cell transformation

We employed DNA fingerprint analysis to monitor DNA rearrangements in BALB 3T3 cells transformed spontaneously or by treatment with 3-methylcholanthrene (MCA) and UV-C. The effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in combination with MCA was also examined. Twenty-three spontaneously transformed cells, 28 induced transformed cells (18 by 1 microgram/ml MCA, six by 5 micrograms/ml MCA, and four by UV-C), and 31 non-transformed subclones were isolated from parental BALB 3T3 A31-1-1 cells. The DNAs were digested with HinfI and subjected to DNA fingerprint analysis with three multi-locus minisatellite probes, Per-6, Core, and Ins. Per-6 was the most effective probe for detecting DNA rearrangements. Rearranged bands detected by the Per-6 probe were observed in 9/31 (29%) of non-transformed subclones, 14/23 (61%) of spontaneously transformed cells, 16/18 (89%) of cells transformed by 1 microgram/ml of MCA, 6/6 (100%) of cells transformed by 5 micrograms/ml MCA, and 4/4 (100%) of UV-C-transformed cells. Higher numbers of DNA rearrangements (> or = 3) occurred most frequently in the induced transformed cells. TPA enhanced the frequency of DNA rearrangements in cells transformed by MCA. These data indicate that (1) genomic DNA in BALB 3T3 cells is unstable and susceptible to rearrangement, (2) its instability is elevated during cell transformation, and (3) MCA and UV-C induce DNA rearrangements, and TPA enhances the effect of the former, probably via the recombination process. DNA fingerprint analysis is valuable for monitoring genomic instability during cell transformation.

Preferential repair of UV damage in highly transcribed DNA diminishes UV-induced intrachromosomal recombination in mammalian cells

The relationships among transcription, recombination, DNA damage, and repair in mammalian cells were investigated. We monitored the effects of transcription on UV-induced intrachromosomal recombination between neomycin repeats including a promoterless allele and an inducible heteroallele regulated by the mouse mammary tumor virus promoter. Although transcription and UV light separately stimulated recombination, increasing transcription levels reduced UV-induced recombination. Preferential repair of UV damage in transcribed strands was shown in highly transcribed DNA, suggesting that recombination is stimulated by unrepaired UV damage and that increased DNA repair in highly transcribed alleles removes recombinogenic lesions. This study indicates that the genetic consequences of DNA damage depend on transcriptional states and provides a basis for understanding tissue- and gene-specific responses to DNA-damaging agents.

The role of mitotic recombination in carcinogenesis

Genetic recombination systems are present in all living cells and viruses and generally contribute to their hosts' flexibility with respect to changing environmental conditions. Recombination systems not only help highly developed organisms to protect themselves from microbial attack via an elaborate immune system, but conversely, recombination systems also enable microorganisms to escape from such an immune system. Recombination enzymes act with a high specificity on DNA sequences that either exhibit extended stretches of homology or contain characteristic signal sequences. However, recombination enzymes may rarely act on incorrect alternative target sequences, which may result in the formation of chromosomal deletions, inversions, translocations, or amplifications of defined DNA regions. This review describes the characteristics of several recombination systems and focuses on the implication of aberrant recombination in carcinogenesis. The consequences of mitotic recombination on the inappropriate activation of protooncogenes and on the loss of tumor suppressor genes is discussed. Cases are reported where mitotic recombination clearly has been associated with carcinogenesis in rodents as well as humans. Several test systems able to detect recombinagenic activities of chemical compounds are described.

Mismatch repair of heteroduplex DNA intermediates of extrachromosomal recombination in mammalian cells

Previous work indicated that extrachromosomal recombination in mammalian cells could be explained by the single-strand annealing (SSA) model. This model predicts that extrachromosomal recombination leads to nonconservative crossover products and that heteroduplex DNA (hDNA) is formed by annealing of complementary single strands. Mismatched bases in hDNA may subsequently be repaired to wild-type or mutant sequences, or they may remain unrepaired and segregate following DNA replication. We describe a system to examine the formation and mismatch repair of hDNA in recombination intermediates. Our results are consistent with extrachromosomal recombination occurring via SSA and producing crossover recombinant products. As predicted by the SSA model, hDNA was present in double-strand break-induced recombination intermediates. By placing either silent or frameshift mutations in the predicted hDNA region, we have shown that mismatches are efficiently repaired prior to DNA replication.

High spontaneous intrachromosomal recombination rates in ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is an inherited human disease associated with neurologic degeneration, immune dysfunction, and high cancer risk. It has been proposed that the underlying abnormality in A-T is a defect in genetic recombination that interferes with immune gene rearrangements and the repair of DNA damage. Recombination was studied in A-T and control human fibroblast lines by means of two recombination vectors. Unexpectedly, spontaneous intrachromosomal recombination rates were 30 to 200 times higher in A-T fibroblast lines than in normal cells, whereas extrachromosomal recombination frequencies were near normal. Increased recombination is thus a component of genetic instability in A-T and may contribute to the cancer risk seen in A-T patients.

Stress-induced intrachromosomal recombination in plant somatic cells

Levels of induced homologous recombination between chromosomal repeats in plant somatic cells were examined. Transgenic plants of Nicotiana tabacum hemi- or homozygous for pairs of deletion derivatives of the neomycin phosphotransferase (nptII) marker gene integrated at a single genomic locus were produced. Homologous recombination within the overlapping parts of the nptII gene restored the function and the resulting kanamycin resistance was used for scoring recombination frequency. The recombination events were confirmed by the appearance of a characteristic 1245-base-pair EcoRV fragment detected in all kanamycin-resistant clones tested. The rate of spontaneous recombination was found to be related to the copy number of recombination substrates and was 9 x 10(-5) and 19 x 10(-5) for hemi- and homozygote strains, respectively. Ionizing radiation, mitomycin C, and heat shock markedly increased the frequency of intrachromosomal recombination. Low doses of x-rays (1.25 Gy) enhanced the relative recombination frequency to approximately twice the spontaneous value. The presence of mitomycin C increased the frequency of recombination 9-fold and exposure to an elevated temperature (50 degrees C) increased it 6.5-fold. The x-ray and heat shock treatments reduced cell viability to 53% and 8%, respectively. Mitomycin C treatment had no effect on cell survival.

Mutagen-induced recombination in mammalian cells in vitro

It is now clear from in vitro studies that mutagens induce recombination in the cell, both homologous and nonhomologous exchanges. The recombination events induced are extrachromosomal events, exchanges between extrachromosomal DNA and chromosomes, and inter- as well as intrachromosomal exchanges. However, not all types of DNA damage can induce recombination. The mechanisms involved in the induction process are not known but may involve activation of DNA repair systems. In addition, stimulation of mRNA transcription by mutagens, different recombination pathways and how the assay system is constructed may affect the frequency and characteristics of the observed recombination events.

Structure of human glutathione S-transferase class Mu genes

Nucleotide sequencing of a human cosmid clone shows that the exon-intron structures of a glutathione S-transferase multigene family are conserved between man and rat, that the human gene family is clustered and that gene conversion events have occurred within the cluster. In addition, between man and rat, there is a high degree of nucleotide sequence identity not only in exons but also in some introns. These conserved sequences are coincident with homologous sequences subject to gene conversion in both species, and hence the utilization of gene conversion by this gene family has itself been conserved. By using transient-expression assay the conserved/converted regions are shown to be capable of modulating transcriptional activity. The data suggest that DNA repair by gene conversion may be a chemical immunity mechanism. which could result in acquired resistance to toxins and, in particular, drug resistance due to glutathione S-transferase in tumours.

Ultraviolet stimulation of intermolecular homologous recombination in Chinese hamster ovary cells

We previously showed that ultraviolet (UV) irradiation of cotransfected plasmid DNA molecules stimulated genetic transformation that depended on intermolecular homologous recombination in Chinese hamster ovary (CHO) cells. Repair-proficient cells and an excision repair complementation class 1 (ERCC1) UV-sensitive DNA repair-deficient mutant responded similarly to UV stimulation in cotransfections with plasmids containing linker insertion-disrupted copies of the herpes simplex virus thymidine kinase (HSV-TK) gene. In this study, we cotransfected homologous DNA molecules containing nonoverlapping deletions of the hamster adenine phosphoribosyltransferase (APRT) gene into APRT-deficient CHO ERCC1 (UVL-10) and ERCC2 (UVL-1) excision-repair mutants and parental repair-proficient CHO cells. UV damage in cotransfected circular plasmid molecules stimulated transformation in repair-proficient cells and an ERCC1 mutant, but not in an ERCC2 mutant. Linearization of plasmids prior to cotransfection greatly enhanced transformation frequencies in all three cell lines, but UV stimulation using linear recombination substrates was no longer evident. Our results suggest (i) that the ERCC1 gene defect in CHO UVL-10 cells does not affect UV stimulation of homology-dependent extra-chromosomal recombination, and (ii) that a CHO cell ERCC2 excision-repair mutant, although recombination proficient, may exhibit altered recombination in response to UV damage.

Thymidylate stress induces homologous recombination activity in mammalian cells

We studied whether homologous recombination activity in mammalian cells could be induced by thymidylate stress (thymidylate deprivation). In vitro recombination activity in cell extracts was measured with pSV2neo-derived plasmids. When prior to the preparation of extracts, mouse FM3A cells were grown in 5-fluorodeoxyuridine (FdUrd), an inducer of thymidylate stress, the homologous recombination activity was significantly induced, as judged from an increase in the number of neomycin-resistant bacterial colonies. Maximum induction was observed in cells treated with 1 microM FUdR for 16 h. However, 3-8 h of treatment of FM3A cells with the drug followed by an additional 8-16-h incubation in its absence was sufficient to induce the recombination activity while slightly reducing their growth rates. These results indicate that thymidylate stress induces homologous recombination activity in mammalian cells as observed in Escherichia coli and in yeast.

Effect of nucleotide excision repair in human cells on intrachromosomal homologous recombination induced by UV and 1-nitrosopyrene

To study the role of nucleotide excision repair in the induction of intrachromosomal homologous recombination in mammalian cells, we introduced a plasmid containing a substrate for recombination into three human cell lines that differ in their repair capacity and compared the frequency of recombination induced by UV radiation and by 1-nitrosopyrene. One strain had a normal capacity for nucleotide excision repair, the second exhibited an intermediate rate of repair, and the third, derived from a patient with xeroderma pigmentosum, had no ability to repair UV- or 1-nitrosopyrene-induced DNA damage. The endogenous thymidine kinase genes in these cell strains had been inactivated, and the cells contained an integrated copy of a plasmid carrying duplicated copies of the herpes simplex virus type 1 thymidine kinase (Htk) gene, each inactivated by an 8-base-pair XhoI site inserted at a unique site. A functional tk gene can only be generated by a productive recombination event between the two Htk genes. In all three stains, UV and 1-nitrosopyrene induced dose-dependent increases in the frequency of recombinants. However, the doses required to cause a specific increase in recombination in the repair-deficient strains were 10 to 30 times lower than the dose required for the cell strain with a normal capacity for repair. These results strongly suggest that unexcised DNA lesions, rather than excision repair per se, stimulate intrachromosomal homologous recombination. Southern blot analysis of DNA from representative recombinants indicated that in all cases one of the two Htk genes had become wild type (XhoI resistant). The majority (90%) retained the Htk duplication, consistent with nonreciprocal transfer of genetic information (gene conversion).

Effect of nucleotide excision repair in human cells on intrachromosomal homologous recombination induced by UV and 1-nitrosopyrene

To study the role of nucleotide excision repair in the induction of intrachromosomal homologous recombination in mammalian cells, we introduced a plasmid containing a substrate for recombination into three human cell lines that differ in their repair capacity and compared the frequency of recombination induced by UV radiation and by 1-nitrosopyrene. One strain had a normal capacity for nucleotide excision repair, the second exhibited an intermediate rate of repair, and the third, derived from a patient with xeroderma pigmentosum, had no ability to repair UV- or 1-nitrosopyrene-induced DNA damage. The endogenous thymidine kinase genes in these cell strains had been inactivated, and the cells contained an integrated copy of a plasmid carrying duplicated copies of the herpes simplex virus type 1 thymidine kinase (Htk) gene, each inactivated by an 8-base-pair XhoI site inserted at a unique site. A functional tk gene can only be generated by a productive recombination event between the two Htk genes. In all three stains, UV and 1-nitrosopyrene induced dose-dependent increases in the frequency of recombinants. However, the doses required to cause a specific increase in recombination in the repair-deficient strains were 10 to 30 times lower than the dose required for the cell strain with a normal capacity for repair. These results strongly suggest that unexcised DNA lesions, rather than excision repair per se, stimulate intrachromosomal homologous recombination. Southern blot analysis of DNA from representative recombinants indicated that in all cases one of the two Htk genes had become wild type (XhoI resistant). The majority (90%) retained the Htk duplication, consistent with nonreciprocal transfer of genetic information (gene conversion).

Frequency of intrachromosomal homologous recombination induced by UV radiation in normally repairing and excision repair-deficient human cells

To investigate the role of DNA damage and nucleotide excision repair in intrachromosomal homologous recombination, a plasmid containing duplicated copies of the gene coding for hygromycin resistance was introduced into the genome of a repair-proficient human cell line, KMST-6, and two repair-deficient lines, XP2OS(SV) from xeroderma pigmentosum complementation group A and XP2YO(SV) from complementation group F. Neither hygromycin-resistance gene codes for a functional enzyme because each contains an insertion/deletion mutation at a unique site, but recombination between the two defective genes can yield hygromycin-resistant cells. The rates of spontaneous recombination in normal and xeroderma pigmentosum cell strains containing the recombination substrate were found to be similar. The frequency of UV-induced recombination was determined for three of these cell strains. At low doses, the group A cell strain and the group F cell strain showed a significant increase in frequency of recombinants. The repair-proficient cell strain required 10- to 20-fold higher doses of UV to exhibit comparable increases in frequency of recombinants. These results suggest that unexcised DNA damage, rather than the excision repair process per se, stimulates such recombination.

Intrachromosomal homologous recombination in human cells which differ in nucleotide excision-repair capacity

To examine the mechanism of recombination and the role of DNA repair in this process, we transfected a plasmid carrying duplicated copies of the Herpes simplex virus I thymidine kinase (Htk) gene, each containing an 8 bp XhoI site inserted in a unique site and with the neo coding for geneticin resistance located between them, into tk-deficient human cell lines which differ in their ability to carry out nucleotide excision repair. One parental cell line has a normal level of repair activity; the second has an intermediate level, and the third has virtually no repair activity. Several geneticin-resistant transfectant cell strains from each parental line were isolated and assayed for the ability to undergo productive recombination giving rise to tk+ cells. Approximately 25% of them could do so. Southern blot analysis of these transfectants indicated that the majority contained a single copy, or at most, two copies of the plasmid integrated into the chromosome. Fluctuation analysis tests to determine the rate of spontaneous recombination (events per 10(6) cells per cell generation) in the various cell strains showed that the rates ranged from 0.15 to 4.1. The mean rate for the cell strains derived from the repair-deficient cell line was 3.6; for those derived from the cells with an intermediate rate, it was 0.8; and for those with a normal rate of excision repair, it was 0.9. Southern blot analysis of tk+ recombinants showed that in all cases, one of the Htk genes had become wild-type, i.e., XhoI-resistant. 90% of the recombinants retained the Htk gene duplication, consistent with non-reciprocal transfer of genetic information, i.e., gene conversion. The rest contained a single, wild-type Htk gene, consistent with a single reciprocal exchange within a chromatid or a single unequal exchange between sister chromatids. These cell strains will be useful for investigating the role of DNA damage and repair in homologous recombination.

Mechanisms for recombination between stably integrated vector sequences in CHO cells

Possible mechanisms for homologous recombination in CHO cells have been investigated using a stably integrated vector, pIII-14gpt. The vector contains 2 inactive neo gene fragments in tandem arrangement. Functional neo gene activity can be restored by recombination between homologous regions in the 2 fragments. Cells in which this event has taken place become resistant to the antibiotic G418. Possible mechanisms for neo gene reactivation in this system are unequal exchange between chromatids, intrachromatidal deletion and gene conversion. DNA from a total of 74 G418-resistant cell clones have been isolated, and analyzed on Southern blots using neo-specific probes. Rearrangements of neo-specific restriction fragments were found to have occurred in all cell clones. In 50% of the revertants, these rearrangements can be explained by a deletion which brings the complementary regions in the 2 neo gene fragments together. One single revertant (1.3%) shows a possible gene conversion event. The other isolated revertants (about 48%) contain more complex rearrangements. These results indicate that the predominating recombination mechanism for reactivation of the neo gene in this system is either a deletion within a chromatid or an unequal exchange between sister chromatids.