The use of shuttle vectors for mutation analysis in transgenic mice and rats

Nucleotide excision repair activity varies among murine spermatogenic cell types

Germ cells perform a unique and critical biological function: they propagate the DNA that will be used to direct development of the next generation. Genetic integrity of germ cell DNA is essential for producing healthy and reproductively fit offspring, and yet germ cell DNA is damaged by endogenous and exogenous agents. Nucleotide excision repair (NER) is an important mechanism for coping with a variety of DNA lesions. Little is known about NER activity in spermatogenic cells. We expected that germ cells would be more efficient at DNA repair than somatic cells, and that this efficiency may be reduced with age when the prevalence of spontaneous mutations increases. In the present study, NER was measured in defined spermatogenic cell types, including premeiotic cells (A and B type spermatogonia), meiotic cells (pachytene spermatocytes), and postmeiotic haploid cells (round spermatids) and compared with NER in keratinocytes. Global genome repair and transcription-coupled repair subpathways of NER were examined. All spermatogenic cell types from young mice displayed good repair of (6-4) pyrimidone photoproducts, although the repair rate was slower than in primary keratinocytes. In aged mice, repair of 6-4 pyrimidone photoproducts was depressed in postmeiotic cells. While repair of cyclobutane pyrimidine dimers was not detected in spermatogenic cells or in keratinocytes, the transcribed strands of active genes were repaired with greater efficiency than nontranscribed strands or inactive genes in keratinocytes and in meiotic and postmeiotic cells; spermatogonia displayed low to moderate ability to repair cyclobutane pyrimidine dimers on both DNA strands regardless of transcriptional status. Overall, the data suggest cell type-specific NER activity during murine spermatogenesis, and our results have possible implications for germ cell aging.

Behavior of transplanted bone marrow-derived GFP mesenchymal cells in osteochondral defect as a simulation of autologous transplantation

To elucidate the behavior of autologously transplanted mesenchymal cells in osteochondral defects, we followed transplanted cells using green fluorescent protein (GFP) transgenic rats, in which all cells express GFP signals in their cytoplasm and nuclei as transplantation donors. Bone marrow-derived mesenchymal cells, which contain mesenchymal stem cells (MSCs), were obtained from transgenic rats. Then, dense mesenchymal cell masses created by hanging-drop culture were transplanted and fixed with fibrin glue into osteochondral defects of wild-type rats. At 24 weeks after surgery, the defects were repaired with hyaline-like cartilage and subchondral bone. GFP positive cells, indicating transplanted mesenchymal-derived cells, were observed in the regenerated tissues for 24 weeks although GFP positive cells decreased in number with time. Because GFP causes no immunological rejection and requires no chemicals for visualization, transplantation between transgenic and wild-type rats can be regarded as a simulation of autologous transplantation, and the survivability of transplanted cells are able to be followed easily and reliably. Thus, the behavior of transplanted mesenchymal cells was able to be elucidated in vivo by this strategy, and the results could be essential in future tissue engineering for the regeneration of osteochondral defects with original hyaline cartilage and subchondral bone.

Quiescent murine cells lack global genomic repair but are proficient in transcription-coupled repair

The majority of the cells in the body, including stem cells, exist in a quiescent state, so it is in quiescent cells where most DNA damage occurs. It has been uncertain whether or not this damage is repaired or fixed into mutations during quiescence or if proliferation is required for both. Prior to the development of transgenic mice, it was difficult to distinguish between these two possibilities, as cells had to proliferate to form colonies before mutations could be detected. Transgenes, however, can be shuttled out of quiescent mouse cells directly, and the level of DNA damage and mutation can be measured. Such measurements show that both mutation and repair are absent at a non-transcribed transgene in quiescent cells, although both are initiated when these cells are induced to proliferate. Conversely, the repair of transcriptionally active genes proceeds independently of proliferation in the same cells, as shown by the differential survival of wild-type and XPA-/- cells. We infer from these results that global genomic DNA repair (GGR) is not active during cellular quiescence but that transcription-coupled repair (TCR) is, suggesting that GGR is restricted to S, whereas TCR remains active throughout the cell cycle.

Induction of gene mutations by 5-(2-chloroethyl)-2'-deoxyuridine (CEDU), an antiviral pyrimidine nucleoside analogue

5-(2-chloroethyl)-2'-deoxyuridine (CEDU) had been developed for the treatment of herpes simplex infections. In the Salmonella reverse mutation test, the compound was found to be mutagenic in strains TA1535 and TA102 at very high concentrations (> or =2500 micro g/plate), both with and without S9-mix. The mutagenic potential of CEDU was further investigated in vivo and in vitro. It did not induce DNA repair in rat hepatocyte primary cultures, and was negative in the micronucleus test in V79 cells and in the comet assay in human leukocytes. In vivo, CEDU was negative in the bone marrow micronucleus test in CD1 mice. The mouse spot test provided a clearly positive result. Treatment of mice on day 9 of pregnancy with 2000 mg/kg resulted in 5.9% of the F1 animals having genetically relevant spots, whereas the corresponding vehicle control group had a spot rate of 1.9%. Since these data clearly identified CEDU as an inducer of gene mutations in vivo, this potential was further investigated in lacZ transgenic Muta Mouse. Six female animals were treated daily on five consecutive days with 2000 mg/kg/day and sacrificed, after a treatment-free sampling time, 14 days later. The data showed a clear increase in the mutant frequency in the bone marrow, the lung and in the spleen. CEDU is an exception in the group of nucleoside analogues, because it was found to be a strong gene mutagen and, in contrast to the other compounds of this group investigated so far, had no considerable clastogenic effects.

Fate of transplanted bone-marrow-derived mesenchymal cells during osteochondral repair using transgenic rats to simulate autologous transplantation

OBJECTIVE

The fate of transplanted cells used in tissue engineering strategies should be followed. With this aim in view, the survival of transplanted bone-marrow-derived mesenchymal cells within osteochondral defects was determined using transgenic rats to simulate autologous transplantation.

DESIGN

An autologous transplantation model was simulated using transgenic rats - whose transgenes produce no foreign proteins - as donors, and wild-type rats as recipients. Dense masses of mesenchymal cells were prepared from the transgenic rats using the hanging-drop culture technique. These cell masses were then transplanted into osteochondral defects created within the medial femoral condyle of wild-type rats, wherein they are affixed with fibrin glue. The course of repair was assessed histologically. The survival of the transplanted cells was ascertained by in situ hybridization of the transgenes.

RESULTS

Twenty-four weeks after transplantation, the defects were repaired with hyaline-like cartilage, which was thicker than normal, and with subchondral bone. Using the in situ hybridization technique, cells derived from the transplanted ones were detected within both the cartilaginous and the subchondral bone layers.

CONCLUSION

Using this simulated autologous transplantation model, the survival of transplanted mesenchymal cells was monitored in vivo. The findings indicate that the transplanted mesenchymal cells contributed to the repair of the osteochondral defects.

Mutagenesis by asbestos in the lung of lambda-lacI transgenic rats

In order to get more insight into the mechanism of asbestos-related lung cancer, the mutagenic potential of asbestos was examined in vivo in rat lung. Groups of five transgenic lambda-lacI (Big Blue) rats were intratracheally instilled with single doses of 1 or 2mg, or with four weekly doses of 2mg, per animal of the amosite asbestos. Sixteen weeks after instillation, the mutation frequency was found to be increased in lung DNA by 2-fold at doses of 2 mg (P = 0.035) and of 4 x 2 mg (P = 0.007) amosite. No significant changes were observed after 4 weeks of exposure. In separate experiments, wild-type F344 rats were treated by the same regimen as described above and markers of inflammation, genotoxicity, cell proliferation and lung tissue damage were analysed. Our results indicate a weak but persistent inflammation and cell proliferation which possibly plays a major role in the observed mutagenic effect.

Rapid detection of human HLA transgenes in pigs by fluorescence in situ hybridization (FISH) for adjuvant study of human xenotransplantation

OBJECTIVES

We have recently generated several lines of transgenic pigs for HLA-DP and -DQ to elucidate the role of HLA-II antigens in the modulation of cell-mediated rejection of xenotransplantation. Using fluorescence in situ hybridization (FISH) analysis, the aim of this study was to determine integration sites and to test zygosity of these transgenes in the piglets after cross mating.

METHODS

Blood lymphocytes of transgenic pigs for HLA-DP and -DQ were collected and cultured. Chromosome spreads were prepared by standard methodology. Gene constructs of HLA-DP A1+B1, -DQ A1 & B1 were labeled with fluorescein isothiocyanate or Texas Red by nick-translation. Hybridization was based on a standard FISH protocol.

RESULTS

FISH analysis revealed that the HLA-DP probe hybridized to porcine chromosome 6, while both HLA-DQ A1 and B1 probes hybridized to porcine chromosome 11 at the same site. There was no cross-hybridization of HLA transgenes to the swine leukocyte antigen complex. Mosaic integration of HLA-DQ transgenes in the genome of F0, but full penetrance in F1 after selective breeding was observed. Both HLA-DP and HLA-DQ lines were determined to be heterozygous at the integration site.

CONCLUSION

By FISH, we have detected specific integration sites of the HLA-DP and -DQ transgenes in pig genome and determined mosaic levels and zygosity types of these transgenes. We conclude that FISH is both sensitive and labor-efficient in confirming and differentiating transgenic pigs for multiple rejection-regulatory genes by visualizing individual integration sites in chromosomes or interphase nuclei.

Mutational-reporter transgenes rescued from mice lacking either Mgmt, or both Mgmt and Msh6 suggest that O6-alkylguanine-induced miscoding does not contribute to the spontaneous mutational spectrum

O6-methylguanine methyltransferase, Mgmt, constitutes the first line of defense against O6-alkylguanine, which can result in G : C to A : T transitions upon DNA replication. Mgmt has been found in organisms as diverse as archaebacteria and mammals. This evolutionary conservation suggests that all organisms may be exposed to either endogenous or environmental alkylating agents. We thus hypothesized that tissues of Mgmt-/- mice would exhibit elevated mutant frequencies. Employing the Big Blue trade mark transgenic system, we evaluated lacI mutants rescued from liver and small intestinal DNA of young Mgmt-/- mice. Interestingly, while there was a small difference between Mgmt-/- mice and controls with respect to lacI mutant frequency, no differences attributable to Mgmt deficiency were apparent in the mutational spectra. Although mutations stemming from O6-guanine alkylations would be predicted to be cumulative, we found no evidence of an Mgmt-dependent alteration in mutation spectrum in DNA samples from 12 month-old mice. To optimize our ability to detect mutations resulting from O6-alkylguanine-induced G : T mismatches, mice with combined deficiencies of Mgmt and the DNA mismatch repair molecule, Msh6, were analysed. In spite of this strategy, we observed no significant differences between Mgmt-/- Msh6-/- and Msh6-/- mouse lacI mutations, except for a trend towards a greater percentage (of total transitions) of G : C to A : T changes in Mgmt-/-Msh6-/- livers. Therefore, despite the striking evolutionary conservation of Mgmt, deficiency of this gene did not significantly impact the spontaneous lacI mutational spectrum in vivo.

Elevated mutagenesis and decreased DNA repair at a transgene are associated with proliferation but not apoptosis in p53-deficient cells

p53, the most commonly mutated gene in human tumors, is believed to play a crucial role in the prevention of cancer by protecting cells from mutation, a theory commonly known as the "Guardian of the Genome" hypothesis. There are two hypotheses as to how this can occur. In the first, p53 protects the genome by retarding the cell cycle, thus allowing more time for DNA repair. In the second, p53 reduces cancer by initiating apoptosis in damaged cells, thus making it impossible for these cells to become carcinogenic. This study directly tested these two theories in primary murine embryonic fibroblasts on a common genetic background with and without p53, using a lacI transgene as a mutational target. The data demonstrate that, as a direct consequence of cell cycle delay, p53 slowed the induction of mutations and decreased their frequency but had little effect on the frequency of apoptosis. This indicates that the function of p53 in cell cycle control is more important than the role of p53 in apoptosis, for mutation prevention, in any uniform cell population. Moreover, p53-mediated protection is further improved in slowly dividing cells, suggesting that p53 may be particularly important in protecting stem cells from mutation. The role of apoptosis in vivo, however, may be to remove whole tissue subpopulations that can be renewed by less sensitive stem cells.

Germline genomes--a biological fountain of youth?

The fusion of male- and female-derived gametes initiates the phenomenal process of producing a highly complex mammalian organism. Successful reproduction is so important that mammals invoke a battery of protective mechanisms for the germ cell lineages that function to maximize genetic integrity while still allowing genetic diversity and adaptation. Protective mechanisms likely include, but are not limited to, robust DNA repair to safeguard genetic integrity and apoptosis to remove cells with intolerable levels of DNA damage. Analyses of spontaneous mutant frequencies are generally consistent with germline DNA being stringently maintained relative to somatic tissues. Despite the rigorous protection afforded germ cells, genetic integrity is observed to decline with increased maternal and paternal age. It is not yet clear whether cells in the germ line truly age or whether other processes decline or become dysfunctional with age. For example, in a younger animal, the differentiation and/or utilization of germ cells with lower genetic integrity might be disallowed, whereas in an older animal, such cells might slip past these quality-control mechanisms.

Novel transgenic rat for in vivo genotoxicity assays using 6-thioguanine and Spi- selection

Transgenic rodents are valuable models for investigating the genotoxicity of chemicals in vivo. Here, we report the establishment of a novel transgenic rat for genotoxicity analysis. In this model, about 10 copies of lambdaEG10 DNA carrying the gpt gene of E. coli and the red/gam genes of lambda phage are integrated per haploid genome of Sprague-Dawley rats at position 4q24-q31. After recovery of lambdaEG10 phage, point mutations in the gpt gene and deletions in the red/gam genes are identified by 6-thioguanine and Spi(-) selection, respectively. To examine the suitability of these rats for performing in vivo mutagenicity assays, rats were treated with single intraperitoneal injections of ethylnitrosourea (ENU; 100 mg/kg) or benzo[a]pyrene (B[a]P; 62.5 and 125 mg/kg), and the mutant frequencies (MFs) in the liver were determined 7 days after the treatment. ENU enhanced the gpt MF about 7-fold over the control while it did not significantly increase the Spi(-) MF. B[a]P increased both the gpt and Spi(-) MFs several-fold in a dose-dependent manner. To examine the kinetics of MF, ENU was administered (50 mg/kg/day for 5 successive days) and gpt MFs in the liver were determined 7, 21, 35, and 70 days after the last injection. The MF increased to 8-fold and 13-fold over the control at 7 and 35 days, respectively, after the last injection and then slightly declined at 70 days. These kinetics are similar to those reported for ENU-treated lacZ transgenic mice. This novel transgenic rat could be useful for investigating species differences between rats and mice in their response to genotoxic agents.

Elevated mutant frequencies and predominance of G:C to A:T transition mutations in Msh6(-/-) small intestinal epithelium

The DNA mismatch repair (MMR) system is primarily responsible for purging newly synthesized DNA of errors incurred during semi-conservative replication. Lesion recognition is initially carried out by one of two heterodimeric protein complexes, MutS(alpha) or MutS(beta). While the former, comprised of MSH2 and MSH6, recognizes mispairs as well as short (1-2 nucleotide) insertions/deletions (IDLs), the latter, made up of MSH2 and MSH3, is primarily responsible for recognizing 2-6 nucleotide IDLs. As most of the functional information on these heterodimers is derived from in vitro studies, it was of interest to study the in vivo consequences of a lack of MutS(alpha). To this end, Big Blue( trade mark ) mice, that carry a lacI(+) transgenic lambda shuttle-phage mutational reporter, were crossed with Msh6(-/-) mice to evaluate the specific contribution of MutS(alpha) to genome integrity. Consistent with the importance of MutS(alpha) in lesion surveillance, small intestine epithelial cell DNA derived from lacI(+) Msh6(-/-) mice exhibited striking increases (average of 41-fold) in spontaneous mutant frequencies. Furthermore, the lacI gene mutation spectrum was dominated by G:C to A:T transitions, highlighting the critical importance of the MutS(alpha) complex in suppressing this frequently observed type of spontaneous mutation.

Comparison between PMSG- and FSH-induced superovulation for the generation of transgenic rats

Superovulation protocols using single injections of pregnant mare's serum gonadotropin (PMSG) or minipumps with follicle-stimulating hormone (FSH) were compared in immature Sprague-Dawley (SD) rats. We used the following criteria: total number of ova, rate of fertilization, in vitro embryo development, sensitivity of zygotes to the microinjection of foreign DNA into the pronucleus, and their in-vivo development after transplantation into the oviduct of a recipient. Female SD rats were stimulated with 15 IU PMSG or 10 mg FSH followed by the injection of human chorionic gonadotropin (hCG) at doses of 20 and 30 IU per female. After hCG administration, they were mated with males of the same strain and sacrificed on day 1 of pregnancy. The percentage of mated animals and the fertilization rate was similar in all groups. In rats given PMSG, the number of ovulated zygotes was hCG dose-dependent. In contrast, the dose of hCG did not influence the efficiency of superovulation in rats given FSH, which was equal to PMSG-treated rats at the optimal dose of hCG. The rates of in vitro blastocyst development (31.4 and 23.3%) and the resistance to microinjection into the pronucleus did also not differ significantly between zygotes of both studied groups. The proportion of offspring developing from microinjected zygotes after oviduct transfer (26.2 and 26.8%, respectively) and the rate of transgene integration per newborns (7.3 and 4.9%, respectively) was similar in both experimental groups. The results of this study demonstrate that superovulation of immature SD rats by PMSG is equally effective as FSH treatment and, thus, preferable for transgenic rat technology due to the lower costs and easier handling.

Mutagenicity and carcinogenicity in relation to DNA adduct formation in rats fed leucomalachite green

Leucomalachite green is a persistent and prevalent metabolite of malachite green, a triphenylmethane dye that has been used widely as an antifungal agent in the fish industry. Concern over the use of malachite green is due to the potential for consumer exposure, evidence suggestive of tumor promotion in rodent liver, and suspicion of carcinogenicity based on structure-activity relationships. Our previous study indicated that feeding rodents malachite or leucomalachite green resulted in a dose-related increase in liver DNA adducts, and that, in general, exposure to leucomalachite green caused an increase in the number and severity of changes greater than was observed following exposure to malachite green. To characterize better the genotoxicity of leucomalachite green, female Big Blue rats were fed leucomalachite green at doses of 0, 9, 27, 91, 272, or 543 ppm for up to 32 weeks. The livers were analyzed for lacI mutations at 4, 16, and 32 weeks and DNA adducts at 4 weeks. Using a 32P-postlabeling assay, we observed a dose-related DNA adduct in the livers of rats fed 91, 272, and 543 ppm leucomalachite green. A approximately 3-fold increase in lacI mutant frequency was found in the livers of rats fed 543 ppm leucomalachite green for 16 weeks, but significant increases in mutant frequencies were not found for any of the other doses or time points assayed. We also conducted 2-year tumorigenesis bioassays in female and male F344 rats using 0, 91, 272, and 543 ppm leucomalachite green. Preliminary results indicate an increasing dose trend in lung adenomas in male rats treated with leucomalachite green, but no increase in the incidence of liver tumors in either sex of rat. These results suggest that the DNA adduct formed in the livers of rats fed leucomalachite green has little mutagenic or carcinogenic consequence.

Dose dependent induction of DNA adducts, gene mutations, and cell proliferation by the antiandrogenic drug cyproterone acetate in rat liver

1. CPA does not only induce the formation of DNA adducts but also of mutations in female rat liver. 2. The mutation frequency exhibited a characteristic time course. Within a period of 3 days post administration, a tremendous increase was noted, which remained at a high level until 2 weeks post exposure. Thereafter, most mutation-carrying cells were eliminated within a period of 2 weeks leaving a cell population remaining at a constant level for another 4 weeks. Thus, the length of the observation period post exposure, i. e. the manifestation time, seems to be a critical factor for the strength of the mutagenic response. The highest as observed between 1 and 2 weeks post exposure. Correspondingly, the dose response curve recorded 2 weeks post exposure showed a higher mutagenic response than the curve after 6 weeks of exposure recorded previously. 3. When CPA-induced mutations were recorded as a function of the dose, mutation frequencies at the lower dose range were found that did not differ from those of controls. The non-effective dose recorded 2 weeks post exposure was much lower than that recorded after 6 weeks of exposure indicating that it is a function of the manifestation time. Since DNA adducts were formed in high amounts at the non-effective doses, we assume that the mitogenic activity required for the conversion of DNA adducts into mutations was not sufficiently strong. The liver of adult animals exhibits a very low endogeneous proliferation rate, which is not likely to contribute significantly to the expression of mutations. We conclude that it is the mitogenic activity of CPA itself, which stimulates the expression of mutations.

Fate of donor bone marrow cells in medial collateral ligament after simulated autologous transplantation

A potential strategy to enhance ligament healing by transplantation of mesenchymal stem cells (MSCs), which are demonstrated to differentiate into fibroblast-like cells in vitro, is presented. The objective of this study was to follow transplanted nucleated cells from bone marrow, which contain MSCs, in the healing medial collateral ligament (MCL) over time, and to examine their phenotype and survivability. It was hypothesized that MSCs in nucleated cells from bone marrow would differentiate into fibroblast-like cells in the healing ligament following adaptation to the environment. The transplantation model employed in this study eliminates the immune response to a donor by the recipient using a transgenic rat (donor), which does not produce foreign protein from transgenes, and its wild-type rat (recipient) in order to simulate autologous transplantation. The MCL of the wild-type rat was ruptured, where 1 x 10(6) nucleated cells of bone marrow from the transgenic rat were injected. The transgenes in transplanted nucleated cells were detected throughout the healing MCL for 28 days by in situ hybridization. At 3 days, many donor cells were evident in the injury site and fascial pocket, and some were found in the midsubstance. Morphologically, transplanted cells with elongated nuclei were found at the ruptured edge of the midsubstance and surface of the unruptured site after 3 days. At 28 days, these cells continued to survive in the healing MCL. Their shapes were similar to those of surrounding recipient MCL fibroblasts. Thus, transplanted cells might differentiate into fibroblasts. Therefore, it was demonstrated that there is a potential for nucleated cells from bone marrow to serve as a vehicle for therapeutic molecules as well as to be a source in enhancing healing of ligaments.

Detection of mutation in transgenic CHO cells using green fluorescent protein as a reporter

A novel approach was developed for rapidly estimating the frequency of specific mutations in genetically engineered Chinese hamster ovary (CHO) cells. We designed double-transgenic CHO cell lines that contain a transgene consisting of the sequence coding for green fluorescent protein under the control of a tetracycline (Tet) responsive promoter and a second transgene coding for the constitutively expressed Tet repressor. Cultures of these CHO cells were treated with gamma-radiation, N-methyl-N-nitrosourea or methyl methanesulfonate, and the fluorescence of individual cells from both control and treated cultures was measured by flow cytometry. The treatments increased the number of highly fluorescent cells, those with presumed mutations in the Tet-repressor gene. Mutant cells from gamma-radiation-exposed cultures were isolated by fluorescence-activated cell sorting, cultured, and individual clones expanded. A PCR-based analysis indicated that the highly fluorescent expanded cells had lost the transgene coding for the Tet repressor, suggesting that the system mainly detects large genetic alterations. A similar approach may be useful for making high-throughput in vivo models for mutation detection.

Base excision repair is limited by different proteins in male germ cell nuclear extracts prepared from young and old mice

The combined observations of elevated DNA repair gene expression, high uracil-DNA glycosylase-initiated base excision repair, and a low spontaneous mutant frequency for a lacI transgene in spermatogenic cells from young mice suggest that base excision repair activity is high in spermatogenic cell types. Notably, the spontaneous mutant frequency of the lacI transgene is greater in spermatogenic cells obtained from old mice, suggesting that germ line DNA repair activity may decline with age. A paternal age effect in spermatogenic cells is recognized for the human population as well. To determine if male germ cell base excision repair activity changes with age, uracil-DNA glycosylase-initiated base excision repair activity was measured in mixed germ cell (i.e., all spermatogenic cell types in adult testis) nuclear extracts prepared from young, middle-aged, and old mice. Base excision repair activity was also assessed in nuclear extracts from premeiotic, meiotic, and postmeiotic spermatogenic cell types obtained from young mice. Mixed germ cell nuclear extracts exhibited an age-related decrease in base excision repair activity that was restored by addition of apurinic/apyrimidinic (AP) endonuclease. Uracil-DNA glycosylase and DNA ligase were determined to be limiting in mixed germ cell nuclear extracts prepared from young animals. Base excision repair activity was only modestly elevated in pachytene spermatocytes and round spermatids relative to other spermatogenic cells. Thus, germ line short-patch base excision repair activity appears to be relatively constant throughout spermatogenesis in young animals, limited by uracil-DNA glycosylase and DNA ligase in young animals, and limited by AP endonuclease in old animals.

Thymic lymphomas arising in Msh2 deficient mice display a large increase in mutation frequency and an altered mutational spectrum

Mismatch repair (MMR) genes, such as Msh2, are classified as "mutator" genes, responsible for the microsatellite instability identified in many tumors. In the current study, the mutation frequency and mutational spectrum in thymic lymphoma arising in Msh2 deficient mice are investigated. Thymic lymphoma developed in Msh2-/- background displayed an eight to nine-fold increase in mutation frequency compared to the normal thymi in Msh2 deficient animals. Sequencing demonstrated significantly different mutational spectra between normal thymus tissue and thymic lymphomas in Msh2-/- mice (P=0.02). The tumor mutational spectrum is characterized by an increase in base substitutions occurring at A:T sites, and multiple mutations, as well as a minor increase in -1 frameshifts. We analyzed mutations in different parts of the tumors, and different regional hotspots could be identified. Several hotspot mutations that are a rare event in normal tissues were identified in the tumor tissues. We conclude that thymic lymphomas arising in Msh2 deficient genetic background are hypermutable and the altered mutational spectrum might be an indication of infidelity of DNA replication during tumorigenesis.

Mammalian DNA beta-polymerase in base excision repair of alkylation damage

DNA beta-polymerase (beta-pol) carries out two critical enzymatic reactions in mammalian single-nucleotide base excision repair (BER): DNA synthesis to fill the repair patch and lyase removal of the 5'-deoxyribose phosphate (dRP) group following cleavage of the abasic site by apurinic/apyrimidinic (AP) endonuclease (1). The requirement for beta-pol in single-nucleotide BER is exemplified in mouse fibroblasts with a null mutation in the beta-pol gene. These cells are hypersensitive to monofunctional DNA methylating agents such as methyl methane-sulfonate (MMS) (2). This hypersensitivity is associated with an abundance of chromosomal damage and induction of apoptosis and necrotic cell death (3). We have found that beta-pol null cells are defective in repair of MMS-induced DNA lesions, consistent with a cellular BER deficiency as a causative agent in the observed hypersensitivity. Further, the N-terminal 8-kDa domain of beta-pol, which contains the dRP lyase activity in the wild-type enzyme, is sufficient to reverse the methylating agent hypersensitivity in beta-pol null cells. These results indicate that lyase removal of the dRP group is a pivotal step in BER in vivo. Finally, we examined MMS-induced genomic DNA mutagenesis in two isogenic mouse cell lines designed for study of the role of BER. MMS exposure strongly increases mutant frequency in beta-pol null cells, but not in wild-type cells. With MMS treatment, beta-pol null cells have a higher frequency of all six base-pair substitutions, suggesting that BER plays a role in protecting the cell against methylation-induced mutations.

Trangenic fish as models in environmental toxicology

Historically, fish have played significant roles in assessing potential risks associated with exposure to chemical contamination in aquatic environments. Considering the contributions of transgenic rodent models to biomedicine, it is reasoned that the development of transgenic fish could enhance the role of fish in environmental toxicology. Application of transgenic fish in environmental studies remains at an early stage, but recent introduction of new models and methods demonstrates progress. Rapid advances are most evident in the area of in vivo mutagenesis using fish carrying transgenes that serve as recoverable mutational targets. These models highlight many advantages afforded by fish as models and illustrate important issues that apply broadly to transgenic fish in environmental toxicology. Development of fish models carrying identical transgenes to those found in rodents is beneficial and has revealed that numerous aspects of in vivo mutagenesis are similar between the two classes of vertebrates. Researchers have revealed that fish exhibit frequencies of spontaneous mutations similar to rodents and respond to mutagen exposure consistent with known mutagenic mechanisms. Results have demonstrated the feasibility of in vivo mutation analyses using transgenic fish and have illustrated their potential value as a comparative animal model. Challenges to development and application of transgenic fish relate to the needs for improved efficiencies in transgenic technology and in aspects of fish husbandry and use. By taking advantage of the valuable and unique attributes of fish as test organisms, it is anticipated that transgenic fish will make significant contributions to studies of environmentally induced diseases.

Transmission of mutations in the lacI transgene to the offspring of ENU-treated Big Blue male mice

The purpose of the study was to determine: 1) if male germ cells of Big Blue mice carrying newly induced mutations in the lacI transgene were effective in fertilization; 2) if offspring arising from such mutant sperm had the mutation in germ cells and multiple somatic tissues; and 3) how the frequency of mutants induced in the lacI transgene compared to the frequency induced in endogenous genes traditionally employed to study germ cell mutagenesis in mice. Male B6C3F(1) mice hemizygous for the lambda/lacI transgene were treated weekly with 100 mg/kg body weight of the mammalian germ cell mutagen N-ethyl-N-nitrosourea (ENU). The cumulative dose for each treated animal was 300 mg ENU/kg body weight. Ten weeks later the treated mice were mated to T stock females and the resulting offspring were screened for specific-locus mutations at six loci affecting external appearance, as well as for mutations in the lacI transgene in multiple somatic tissues and germ cells. Five offspring carrying recessive specific-locus mutations were observed among 597 offspring screened (mutant frequency = 139.6 x 10(-5) per locus). Four offspring carrying lacI mutations were observed among 280 offspring screened (mutant frequency = 35.7 x 10(-5) per locus (assuming 40 target loci)). Each of the four lacI mutant offspring carried a different mutation. Three of the mutations were A:T-->G:C transitions and one a G:C-->A:T transition. Consistent with the expectation that a mutation induced in a parental germ cell and transmitted to a conceptus would exist in every cell of the offspring, each mutant mouse had identical mutations in all somatic tissues sampled, as well as in its germ cells. These data provide preliminary evidence for the biological validity of assessing induced, heritable mutations using transgenic mice, without the need for generating an F(1) generation.

Msh2 deficiency increases the mutation frequency in all parts of the mouse colon

The Msh2 DNA mismatch repair gene is one of five genes implicated in the pathogenesis of hereditary nonpolyposis colorectal cancer (HNPCC). To address the possible mechanisms of the site-specific occurrence of HNPCC, the effect of Msh2 deficiency on mutations in different parts of the colon was investigated using the BC-1(lacI)/Msh2 double transgenic mouse. Compared to the Msh2(+/+) mice, Msh2(-/-) mice had an 8-9-fold increase of mutation frequency (MF) in the lacI gene from the cecum and the proximal and distal colon. The mutational spectra were also significantly different between Msh2(+/+) and Msh2(-/-) mice, with a significant increase in the frequency of -1 frameshifts and G:C-->A:T base substitutions in the repair-deficient mice. However, in spite of the site-specific predisposition of HNPCC in humans, we found no significant difference in the MF or mutation spectrum between the three parts of the colon in Msh2(+/+), Msh2(+/-), or Msh2(-/-) mice. In addition, 11 independent mutants harboring complex mutations within the lacI gene were recovered in the Msh2(-/-) mice. Interestingly, while the Msh2(+/-) mice displayed an overall MF similar to that observed in the wild-type mice, sequencing revealed a significantly different mutational spectrum between Msh2(+/+) and Msh2(+/-) mice, mainly characterized by an increase in -1 frameshifts. Due to the prevalence of frameshift mutations in HNPCC patients, this haploinsufficiency effect of the Msh2 gene in safeguarding genomic integrity may have important implications for human carrier status.

Protection by dietary compounds against mutation in a transgenic rodent

One of the most relevant biomarkers of genotoxicity and, potentially, carcinogenesis is the occurrence of mutations. Data indicate that carcinogens are highly specific with regard to their target tissue in inducing both tumors and mutations. This specificity may reflect the dependence on tissue-specific metabolic activation, the organ-specific environment or both. Ideally, therefore, mutation should be determined in a real animal rather than in a cell culture system. The lacI transgenic rodent model provides such a system. We have used this model to investigate tissue, species and sex specificity of mutation induced by selected dietary carcinogens and to examine how some compounds may alter the induction of mutation. We have studied mutation using several chemicals, including the dietary heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), the environmentally important aromatic hydrocarbon benzo[a]pyrene and the food contaminant aflatoxin B1. We have shown that the mutagenic potency of these chemicals can be modulated by other dietary compounds, including green tea and conjugated linoleic acid, and the dioxin 2,3,7,8-tetrachlorodibenzo[b,e][1,4]dioxin (TCDD). These results demonstrate that the lacI transgenic rodent is a useful model for the study of chemoprevention in vivo.

Msh2 DNA mismatch repair gene deficiency and the food-borne mutagen 2-amino-1-methy1-6-phenolimidazo [4,5-b] pyridine (PhIP) synergistically affect mutagenesis in mouse colon

Msh2 deficiency and food-borne carcinogen PhIP have been implicated as genetic and environmental factors, respectively, in human colon carcinogenesis. It is not clear whether loss of one or both alleles of Msh2 gene increases the mutational sensitivity in colon when exposed to environmental carcinogens. In the current study, Msh2(+/-)/lacI and Msh2(-/-)/lacI double transgenic mice were treated with PhIP and mutations in the lacI gene were studied in the colon. The spontaneous mutation frequency (MF) is approximately eightfold higher in Msh2(-/-) mice than in Msh2(+/+) mice, while Msh2(+/-) mice display similar levels of spontaneous mutation as the Msh2 wild type mice. PhIP induced a significant increase in MF in all genotypes of mice. However, induced MF is much higher in Msh2(-/-) mice compared to Msh2(+/+) and Msh2(+/-) mice. Msh2(+/-) mice displayed an increased level of G:C>T:A transversions and -1 frameshifts upon PhIP treatment. In contrast, loss of both Msh2 alleles mainly results in increased frequency of G:C>A:T transitions when exposed to PhIP. These results suggest that a defect in mismatch repair may result in an enhanced sensitivity from exposure to a dietary carcinogen. It also provides insight into interaction between genetic and environmental factors in human carcinogenesis.

Mutagenicity of TCDD in Big Blue transgenic rats

The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a significant environmental contaminant resulting from such industrial processes as pulp and paper production. TCDD is a suspected human carcinogen and its ability to induce cancer in laboratory rodents is well documented. Its mechanism of tumor initiation, however, is not well understood and in vitro mutagenicity studies have yielded inconsistent results. In this study, Big Blue lacI transgenic rats were used to assess the mutagenicity of TCDD in both male and female animals. After 6 weeks of exposure to 2 microg/kg TCDD neither an increase in mutation frequency nor any change in mutation spectrum was observed in either male or female animals.

Mixed spermatogenic germ cell nuclear extracts exhibit high base excision repair activity

Spermatogenic cells exhibit a lower spontaneous mutation frequency than somatic tissues in a lacI transgene and many base excision repair (BER) genes display the highest observed level of expression in the testis. In this study, uracil-DNA glycosylase-initiated BER activity was measured in nuclear extracts prepared from tissues obtained from each of three mouse strains. Extracts from mixed spermatogenic germ cells displayed the greatest activity followed by liver then brain for all three strains, and the activity for a given tissue was consistent among the three strains. Levels of various BER proteins were examined by western blot analyses and found to be consistent with activity levels. Nuclear extracts prepared from purified Sertoli cells, a somatic component of the seminiferous epithelium, exhibited significantly lower activity than mixed spermatogenic cell-type nuclear extracts, thereby suggesting that the high BER activity observed in mixed germ cell nuclear extracts was not a characteristic of all testicular cell types. Nuclear extracts from thymocytes and small intestines were assayed to assess activity in a mitotically active cell type and tissue. Overall, the order of tissues/cells exhibiting the greatest to lowest activity was mixed germ cells > Sertoli cells > thymocytes > small intestine > liver > brain.

Inducible mutagenesis in TEPC 2372, a mouse plasmacytoma cell line that harbors the transgenic shuttle vector lambdaLIZ

The plasmacytoma cell line, TEPC 2372, was derived from a malignant plasma cell tumor that developed in the peritoneal cavity of a BALB/c mouse that harbored the transgenic shuttle vector for the assessment of mutagenesis in vivo, lambdaLIZ. TEPC 2372 was found to display the typical features of a BALB/c plasmacytoma. It consisted of pleomorphic plasma cells that secreted a monoclonal immunoglobulin (IgG2b/lambda), was initially dependent on the presence of IL-6 to grow in cell culture, contained a hyperdiploid chromosome complement with a tendency to undergo tetraploidization, and harbored a constitutively active c-myc gene by virtue of a T(6;15) chromosomal translocation. TEPC 2372 was further characterized by the ability to respond to in vitro exposure with 4-NQO (4-nitroquinoline-1-oxide), an oxidative model mutagen, with a vigorous dose-dependent increase in mutagenesis that peaked at a 7.85-fold elevation of mutant rates in lambdaLIZ when compared to background mutant rates in untreated controls. Cotreatment with 4-NQO and BSO (buthionine sulfoximine), a glutathione-depleting compound that causes endogenous oxidative stress, resulted in a 9.03-fold increase in the mutant frequency in lambdaLIZ. These results demonstrated that TEPC 2372, the malignant plasma cell counterpart of the lambdaLIZ-based in vivo mutagenesis assay, may be useful as an in vitro reference point for the further elucidation of oxidative mutagenesis in lymphoid tissues.

Spontaneous mutation of the lacI transgene in rodents: absence of species, strain, and insertion-site influence

Comparison of spontaneous mutation spectra derived from different transgenic constructs can provide valuable insights for interpreting the mechanisms of spontaneous mutation. In this study, spontaneous mutation frequencies and spectra of the lacI transgene are compared in the liver of C57BL/6, B6C3F1, and BC-1 mice and F344 rats. Before correction for clonal expansion, the mutant frequency varied from 2.6 +/- 0.45 to 5.0 +/- 2.4 x 10(-5). Correction for potential clonal expansion reduced the range in mutation frequency to between 2.3 +/- 0.45 and 3.5 +/- 2.0 x 10(-5). There is thus no statistical difference in spontaneous mutation frequency between the different strains and species. G:C --> A:T transitions and to a lesser extent, G:C --> T:A transversions dominate the mutational spectra in all of these animals. In three strains of mice, G:C --> A:T transitions account for 50.7-53.3% of mutation, 81.7-83.8% of which involve CpG sites, whereas G:C --> T:A transversions account for 17.8-32.9% of mutations with 43.2-50.0% found at CpG sites. In rats, G:C --> A:T transitions account for 38.0% of the spectra, 70.0% of which involve CpG sites, whereas G:C --> T:A transversions account for 23.0% of the spectra, 70.0% of which involve CpG sites. The distribution of other classes of mutations is also very similar. We conclude that, despite reports about species and strain differences in induced mutation, spontaneous mutations in the lacI transgene appear to be similar, regardless of genomic location, rodent strain, or species. In addition to insights into spontaneous mutation, this study also provides essential data for comparison with and interpretation of induced mutations.

Comparison of hprt and lacI mutant frequency with DNA adduct formation in N-hydroxy-2-acetylaminofluorene-treated Big Blue rats

N-Hydroxy-2-acetylaminofluorene (N-OH-AAF) is the proximate carcinogenic metabolite of the powerful rat liver carcinogen 2-acetylaminofluorene. In this study, transgenic Big Blue(R) rats were used to examine the relationship between in vivo mutagenicity and DNA adduct formation by N-OH-AAF in the target liver compared with that in nontarget tissues. Male rats were given one, two, or four doses of 25 mg N-OH-AAF/kg body weight by i.p. injection at 4-day intervals, and groups of treated and control rats were euthanized up to 10 weeks after beginning the dosing. Mutant frequencies were measured in the spleen lymphocyte hprt gene, and lacI mutant frequencies were determined in the liver and spleen lymphocytes. At 6 weeks after beginning the dosing, the hprt mutant frequency in spleen lymphocytes from the four-dose group was 16.5 x 10(-6) compared with 3.2 x 10(-6) in control animals. Also at 6 weeks, rats given one, two, or four doses of N-OH-AAF had lacI mutant frequencies in the liver of 97.6, 155.6, and 406.8 x 10(-6), respectively, compared with a control frequency of 25.7 x 10(-6); rats given four doses had lacI mutant frequencies in spleen lymphocytes of 55.8 x 10(-6) compared with a control frequency of 20.4 x 10(-6). Additional rats were evaluated for DNA adduct formation in the liver, spleen lymphocytes, and bone marrow by (32)P-postlabeling. Adduct analysis was conducted 1 day after one, two, and four treatments with N-OH-AAF, 5 days after one treatment, and 9 days after two treatments. N-(Deoxyguanosin-8-yl)-2-aminofluorene was the major DNA adduct identified in all the tissues examined. Adduct concentrations increased with total dose to maximum values in samples taken 1 day after two doses, and remained essentially the same after four doses. In samples taken after four doses, adduct levels were 103, 28, and 7 fmol/microg of DNA in liver, spleen lymphocytes, and bone marrow, respectively. The results indicate that the extent of both DNA adduct formation and mutant induction correlates with the organ specificity for N-OH-AAF carcinogenesis in the rat. Environ. Mol. Mutagen. 37:195-202, 2001. Published 2001 Wiley-Liss, Inc.

Gene- and tissue-specificity of mutation in Big Blue rats treated with the hepatocarcinogen N-hydroxy-2-acetylaminofluorene

In a previous study, we found that treating transgenic Big Blue rats with the hepatocarcinogen N-hydroxy-2-acetylaminofluorene (N-OH-AAF) produced the same major DNA adduct in the target liver and the nontarget spleen lymphocytes and bone marrow cells, induced lacI mutants in the liver, and induced much lower frequencies of lacI and hprt mutants in spleen lymphocytes. In the present study, sequence analysis was conducted on lacI DNA and hprt cDNA from the mutants, to determine the mutational specificity of N-OH-AAF in the rat. All the mutation spectra from N-OH-AAF-treated rats differed significantly from corresponding mutation profiles from untreated animals (P = 0.02 to P < 0.0001). Although there were similarities among the mutational patterns derived from N-OH-AAF-treated rats (e.g., G:C --> T:A transversion was the most common mutation in all mutation sets), there were significant differences in the patterns of basepair substitution and frameshift mutation between the liver and spleen lymphocyte lacI mutants (P = 0.02) and between the spleen lymphocyte lacI and hprt mutants (P = 0.04). Also, multiplex PCR analysis of genomic DNA from the hprt mutants indicated that 12% of mutants from treated rats had major deletions in the hprt gene; no corresponding incidence of large deletions was evident among lacI mutations. All the mutation profiles reflect the general mutational specificity of the major DNA adduct formed by N-OH-AAF. The differences between N-OH-AAF mutation in the endogenous gene and transgene can be partially explained by the structures of the two genes. The tissue-specificity of the mutation spectra may contribute to targeting tumor formation to the liver. Environ. Mol. Mutagen. 37:203-214, 2001. Published 2001 Wiley-Liss, Inc.

Just how does the cII selection system work in Muta Mouse?

The lambda CII protein is an essential component in the lytic vs. lysogeny decision a bacteriophage makes upon infection of a host at low temperatures. The protein interacts with numerous phage promoters modulating the expression of the CI repressor, thus providing the mechanism for lysogenization soon after infection. The Big Blue and Muta Mouse are two widely used in vivo mutational model systems. The assays rely on retrievable lambda-based transgenes housing mutational targets (lacI or lacZ, respectively). The transgenes provide an elegant vehicle for the quantification of mutations sustained in virtually any tissue of the rodent. The use of the bacteriophage cII locus as an alternative, or additional mutational target for use with the Big Blue rodent system was first reported by Jakubczak et al. ([1996]: Proc Natl Acad Sci USA 93:9073-9078). More recently, this selection assay has been applied successfully to the Muta Mouse (Swiger et al. [1999]: Environ Mol Mutagen 33:201-207). The use of an Hfl bacterial strain and low temperature allows the determination of mutations sustained at the cII locus in either system, with high fidelity. The cII selection assay in the Big Blue relies on the presence of the lambda repressor protein CI. In contrast, the recombinant construct used to make the Muta Mouse transgene lacks functional CI protein. Nevertheless, we report an excellent system for quantifying mutations at the cII locus in Muta Mouse. Just how does cII selection work in the Muta Mouse? Written in the context of lambda recombinant genetics, this paper explores the question further.

Detection of mutations in transgenic fish carrying a bacteriophage lambda cII transgene target

To address the dual needs for improved methods to assess potential health risks associated with chemical exposure in aquatic environments and for new models for in vivo mutagenesis studies, we developed transgenic fish that carry multiple copies of a bacteriophage lambda vector that harbors the cII gene as a mutational target. We adapted a forward mutation assay, originally developed for lambda transgenic rodents, to recover cII mutants efficiently from fish genomic DNA by lambda in vitro packaging. After infecting and plating phage on a hfl- bacterial host, cII mutants were detected under selective conditions. We demonstrated that many fundamental features of mutation analyses based on lambda transgenic rodents are shared by transgenic fish. Spontaneous mutant frequencies, ranging from 4.3 x 10(-5) in liver, 2.9 x 10(-5) in whole fish, to 1.8 x 10(-5) in testes, were comparable to ranges in lambda transgenic rodents. Treatment with ethylnitrosourea resulted in concentration-dependent, tissue-specific, and time-dependent mutation inductions consistent with known mechanisms of action. Frequencies of mutants in liver increased insignificantly 5 days after ethylnitrosourea exposure, but increased 3.5-, 5.7- and 6. 7-fold above background at 15, 20, and 30 days, respectively. Mutants were induced 5-fold in testes at 5 days, attaining a peak 10-fold induction 15 days after treatment. Spontaneous and induced mutational spectra in the fish were also consistent with those of lambda transgenic rodent models. Our results demonstrate the feasibility of in vivo mutation analyses using transgenic fish and illustrate the potential value of fish as important comparative animal models.

Proliferation is necessary for both repair and mutation in transgenic mouse cells

Proliferating cells are often presumed to be more mutable than quiescent cells because they have less time to repair DNA damage before DNA replication. Direct tests of this hypothesis have been confounded by the need for cell division before a mutation can be detected. We have avoided this problem by showing that the Big Blue mouse cell line permits the dynamic quantification of both lesions and mutations in the complete absence of cell division. These cells carry the bacterial lacI gene in a lambda shuttle vector. Mutant plaques recovered by in vitro packaging of the mouse DNA can arise from mutations sustained either in mouse cells or in the bacteria. The proportion of mutant phage contained within a mutant plaque can distinguish these two types of mutation. Mutations formed in mouse cells yield >90% mutant phage because both DNA strands are mutant. On the other hand, mutations formed in the bacteria from adducted DNA yield </=50% mutant phage, because one of the DNA strands is wild type. Immediately after exposure to a test mutagen, ethylnitrosourea, all induced mutations were formed in the bacteria, but after approximately one cell division, the reverse was true and all mutations arose in the mouse cells. Only one-fifth as many mutations were recovered from quiescent cells and all arose in the bacteria, showing that the mouse cells made no mutations in the absence of proliferation. Surprisingly, the mouse cells did not repair any of the premutagenic damage during 4 days of quiescence. When these quiescent cells were induced to proliferate, however, both repair and mutation fixation ensued.

Genome-wide variation of the somatic mutation frequency in transgenic plants

In order to analyse the frequency of point mutations in whole plants, several constructs containing single nonsense mutations in the beta-glucuronidase (uidA) gene were used to generate transgenic Arabidopsis thaliana plants. Upon histochemical staining of transgenic plants, sectors indicative of transgene reactivation appeared. Reversion frequencies were in the range of 10(-7)-10(-8) events per base pair, exceeding the previous estimates for other eukaryotes at least 100-fold. The frequency was dependent on the position of the mutation substrate within the transgene and the position of the transgene within the Arabidopsis genome. An inverse relationship between the level of transgene transcription and mutation frequency was observed in single-copy lines. DNA-damaging factors induced the mutation frequency by a factor of up to 56 for UV-C, a factor of 3 for X-rays and a factor of 2 for methyl methanesulfonate. This novel plant mutation-monitoring system allowed us to measure the frequencies of point mutation in whole plants and may be used as an alternative or complement to study the mutagenicity of different environmental factors on the higher eukaryote's genome.

Through a glass, darkly: reflections of mutation from lacI transgenic mice

The study of mutational frequency (Mf) and specificity in aging Big Blue lacI transgenic mice provides a unique opportunity to determine mutation rates (MR) in vivo in different tissues. We found that MR are not static, but rather, vary with the age or developmental stage of the tissue. Although Mf increase more rapidly early in life, MR are actually lower in younger animals than in older animals. For example, we estimate that the changes in Mf are 4.9x10(-8) and 1.1 x 10(-8) mutations/base pair/month in the livers of younger mice (<1. 5 months old) and older mice (> or =1.5 months old), respectively (a 4-fold decrease), and that the MR are 3.9 x 10(-9) and 1.3 x 10(-7) mutations/base pair/cell division, respectively ( approximately 30-fold increase). These data also permit an estimate of the MR of GC --> AT transitions occurring at 5'-CpG-3' (CpG) dinucleotide sequences. Subsequently, the contribution of these transitions to age-related demethylation of genomic DNA can be evaluated. Finally, to better understand the origin of observed Mf, we consider the contribution of various factors, including DNA damage and repair, by constructing a descriptive mutational model. We then apply this model to estimate the efficiency of repair of deaminated 5-methylcytosine nucleosides occurring at CpG dinucleotide sequences, as well as the influence of the Msh2(-/-) DNA repair defect on overall DNA repair efficiency in Big Blue mice. We conclude that even slight changes in DNA repair efficiency could lead to significant increases in mutation frequencies, potentially contributing significantly to human pathogenesis, including cancer.

The lyase activity of the DNA repair protein beta-polymerase protects from DNA-damage-induced cytotoxicity

Small DNA lesions such as oxidized or alkylated bases are repaired by the base excision repair (BER) pathway. BER includes removal of the damaged base by a lesion-specific DNA glycosylase, strand scission by apurinic/apyrimidinic endonuclease, DNA resynthesis and ligation. BER may be further subdivided into DNA beta-polymerase (beta-pol)-dependent single-nucleotide repair and beta-pol-dependent or -independent long patch repair subpathways. Two important enzymatic steps in mammalian single-nucleotide BER are contributed by beta-pol: DNA resynthesis of the repair patch and lyase removal of 5'-deoxyribose phosphate (dRP). Fibroblasts from beta-pol null mice are hypersensitive to mono-functional DNA-methylating agents, resulting in increases in chromosomal damage, apoptosis and necrotic cell death. Here we show that only the dRP lyase activity of beta-pol is required to reverse methylating agent hypersensitivity in beta-pol null cells. These results indicate that removal of the dRP group is a pivotal step in BER in vivo. Persistence of the dRP moiety in DNA results in the hypersensitivity phenotype of beta-pol null cells and may signal downstream events such as apoptosis and necrotic cell death.

Mutation frequency and specificity with age in liver, bladder and brain of lacI transgenic mice

Mutation frequency and specificity were determined as a function of age in nuclear DNA from liver, bladder, and brain of Big Blue lacI transgenic mice aged 1.5-25 months. Mutations accumulated with age in liver and accumulated more rapidly in bladder. In the brain a small initial increase in mutation frequency was observed in young animals; however, no further increase was observed in adult mice. To investigate the origin of mutations, the mutational spectra for each tissue and age were determined. DNA sequence analysis of mutant lacI transgenes revealed no significant changes in mutational specificity in any tissue at any age. The spectra of mutations found in aging animals were identical to those in younger animals, suggesting that they originated from a common set of DNA lesions manifested during DNA replication. The data also indicated that there were no significant age-related mutational changes due to oxidative damage, or errors resulting from either changes in the fidelity of DNA polymerase or the efficiency of DNA repair. Hence, no evidence was found to support hypotheses that predict that oxidative damage or accumulation of errors in nuclear DNA contributes significantly to the aging process, at least in these three somatic tissues.

7,12-dimethylbenz[a]anthracene-induced mutation in the Tk gene of Tk(+/-) mice: automated scoring of lymphocyte clones using a fluorescent viability indicator

7,12-Dimethylbenz[a]anthracene (DMBA) is a rodent carcinogen and a potent in vivo mutagen for the X-linked hypoxanthine guanine phosphoribosyl transferase (hprt) gene of rats and for the lacI transgene of Big Blue mice and rats. Although DMBA is also a powerful clastogen, molecular analysis of these DMBA-induced hprt and lacI mutations indicates that most are single base-pair (bp) substitutions and 1- to 3-bp frameshifts. In the present study, we evaluated the types of mutations induced by DMBA in the autosomal thymidine kinase (Tk) gene of Tk(+/-) mice. Male and female 5- to 6-week-old animals were injected i.p. with DMBA at a dose of 30 mg/kg. Five weeks after the treatment, hprt and Tk mutant frequencies were determined using a limiting dilution clonal assay in 96-well plates. We established conditions for the automated identification of wells containing expanded lymphocyte clones using the fluorescent indicator alamarBlue. This procedure allowed the unbiased identification of viable clones and calculation of mutant frequencies. In male mice, DMBA treatment increased the frequency of hprt mutants from 1.8 +/- 1.1 to 34 +/- 9 x 10(-6), and Tk mutants from 33 +/- 12 to 78 +/- 26 x 10(-6); treated female mice had a significant but lower increase in hprt mutant frequency than did males. Molecular analysis of DMBA-induced Tk mutants revealed that at least 75% had the entire wild-type Tk allele missing. The results indicate that the predominant types of DMBA-induced mutation detected by the autosomal Tk gene are different from those detected by the X-linked hprt gene. The Tk gene mainly detects loss of heterozygosity mutation, whereas the majority of mutations previously found in the hprt gene were point mutations.

Mutant frequency and molecular analysis of in vivo lacI mutations in the bone marrow of Big Blue rats treated with 7, 12-dimethylbenz[a]anthracene

Recently, we evaluated lacI mutations in lymphocytes and mammary tissue of Big Blue (BB) rats exposed to 7, 12-dimethylbenz[a]anthracene (DMBA). The results on the time course of mutant induction suggested that the lacI gene may manifest a tissue-specific increase in mutant frequency (MF). To test whether a tissue-specific increase in lacI MF is dependent on the cell proliferation rate of a tissue, we examined rapidly proliferating bone marrow cells for DMBA-induced lacI mutations. Seven-week-old female BB rats were given single doses of 0, 20, and 130 mg/kg DMBA by gavage and the lacI MFs in the bone marrow were measured over a period of 14 weeks following treatment. Bone marrow cells had a remarkably low average background MF (3.1 +/- 1.6 x 10(-6) plaque-forming units) and the DMBA-induced lacI MFs were significantly higher than control MFs for both doses and at all time points (P < 0.01). The lacI MF in the bone marrow increased for 2 weeks and then remained relatively constant; 20 and 130 mg/kg DMBA produced 34- and 106-fold increases in MF over control MF. DNA sequencing revealed that the majority of DMBA-induced lacI mutations were base-pair substitutions and that A:T --> T:A (48%) and G:C --> T:A (24%) transversions were the predominant types. Thus, the different lacI mutation fixation times observed for bone marrow (2 weeks), mammary (10 weeks), and lymphocytes (6 weeks) suggest that the lacI gene manifests a tissue-specific mutation fixation time, which may depend on the cell proliferation rate of a tissue. In addition, the relatively low spontaneous MF in bone marrow compared with that in other tissues may be useful for increasing the sensitivity of the assay for detecting induced MFs in BB rats.

4-chloro-o-phenylenediamine induces a dose-related increase in G:C > T:A transversions and one major DNA adduct in the liver of Big Blue mice after 26 weeks in feed treatment

The monocyclic aromatic amine 4-chloro-o-phenylenediamine (4-C-o-PDA), a known mutagen and mouse hepatocarcinogen, was tested for its in vivo mutagenic potential in the Big Blue transgenic mouse assay system. Genomic DNA was isolated from liver tissue of control and treated animals and lacI mutants were recovered. In an initial 2-week study 4-C-o-PDA was administered daily per os to groups of male and female C57BL/6 Big Blue mice at doses of 0 and 200 mg/kg for 2 weeks (on working days) followed by a treatment free expression time of 10 days. Only a weak increase in the mutant frequencies in females was observed. In a 26-week study, where 4-C-o-PDA was given to groups of male and female Big Blue mice in feed at dietary concentrations of 0, 5,000 and 10,000 ppm, 4-C-o-PDA was found to induce a pronounced dose-dependent increase in mutant frequencies in either sex. In the present work, we analyzed the mutation spectrum by automated DNA sequencing of lacI mutants from both studies. Following the 2-week administration of 4-C-oT:A transversions in both sexes. In addition, upon 26-week treatment with 4-C-o-PDA, one major DNA adduct was detected by 33P postlabelling and subsequent multidimensional thin layer chromatography. It is concluded that 4-C-oT:A transversions after 26 weeks in feed treatment. This result indicates that the sensitivity of the Big Blue transgenic assay system, in detecting a unique chemically induced mutation spectrum, is dependent on experimental parameters, such as treatment time. The data suggest that the formation of one major DNA adduct upon 4-C-o-PDA treatment may be critical for its mutagenicity.

Difference in target organs in carcinogenesis with a heterocyclic amine, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline, in different strains of mice

2-Amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) induces cancers in the forestomach and liver, but not in the colon, of CDF1 male and female mice, which are thought to be resistant to induction of colon cancer by 1,2-dimethylhydrazine. In this study, we examined the carcinogenicity of MeIQ in C57BL/6N female mice, which are susceptible to 1,2-dimethylhydrazine. This strain of mice developed carcinomas of the cecum, colon and liver, but not the forestomach, when given a diet containing 300 ppm of MeIQ. This fact indicates that the target organs of a chemical carcinogen change depending on the strain of a given animal species.

Mutation spectrum of 4-nitroquinoline N-oxide in the lacI transgenic Big Blue Rat2 cell line

This paper describes the spectrum of mutations induced by 4-nitroquinoline N-oxide (4-NQO) in the lacI target gene of the transgenic Big Blue Rat2 cell line. There are only a few report for the mutational spectrum of 4-NQO in a mammalian system although its biological and genetic effects have been well studied. Big Blue Rat2 cells were treated with 0.03125, 0.0625 or 0.125 microg/ml of 4-NQO, the highest concentration giving 85% survival. Our results indicated that the mutant frequency (MF) induced by 4-NQO was dose-dependent with increases from three- to seven-fold. The DNA sequence analysis of lacI mutants from the control and 4-NQO treatment groups revealed an obvious difference in the spectra of mutations. In spontaneous mutants, transition (60%) mutations, especially G:C-->A:T transition (45%), were most frequent. However, the major type of base substitution after treatment of 4-NQO was transversions (68.8%), especially G:C-->T:A (43.8%), while only 25% of mutants were transitions. These results are consistent with those produced by 4-NQO in other systems and the transgenic assay system will be a powerful tool to postulate more accurately the mechanism of chemical carcinogenesis involved.

Development of a mouse cell line containing the PhiX174 am3 allele as a target for detecting mutation

Transgenic mice containing multiple copies of the PhiX174 am3 allele are being developed as a model for detecting tissue-specific in vivo mutation. In order to derive an analogous system for measuring am3 mutation in vitro, cells were cultured from 15-day-old C57Bl/6J mouse embryos that were homozygous for the transgene and these cells were transfected with a plasmid expressing the SV40 large T-antigen. Two G418-resistant colonies were isolated from this culture and expanded to continuously proliferating cell lines (PX-1 and PX-2). Line PX-2 was treated with up to 1.0 mg/ml of N-ethyl-N-nitrosourea (ENU), assayed for survival by cloning efficiency after overnight culture, and assayed for am3 mutations after 5 days of culture. Survival decreased to 31% at the highest dose of ENU, while mutant frequency increased with dose from approximately 2 x 10(-7) in the untreated cells to 13 x 10(-7) in cultures treated with 0.6 mg/ml of ENU. PX-2 cells also were treated with 0 and 0.6 mg/ml of ENU and mutant frequency assays were performed after 5, 24, 48 and 72 h of growth. The mutant frequency in the treated culture increased to 20 x 10(-7) at 48 h and remained approximately the same at 72 h. These results indicate that PX-2 cells should be a useful resource for developing the in vivo am3 mutant assay and for evaluating the sensitivity of the am3 allele to various classes of mutagens.

Transgenic shuttle vector assays for assessing oxidative B-cell mutagenesis in vivo

The recent development of transgenic mutagenicity assays provides new opportunities for evaluating mutagenic processes in vivo. To asses mutant frequencies in tissue B cells, we decided to take advantage of two such assays that utilize the transgenic shuttle vectors, lambda LIZ and pUR288. Our main interest in this research is to test two basic premises of inflammation-induced plasmacytoma development in genetically susceptible BALB/c mice; i.e., the possibility that plasmacytoma precursor cells may become targets of phagocyte-mediated oxidative mutagenesis in situ and the prospect that plasmacytoma susceptibility/resistance genes may contribute to these phenotypes by enhancing/reducing oxidative mutagenesis in B cells. Based on our preliminary experience with the lambda LIZ and pUR288 transgenic in vivo mutagenicity tests, we propose to employ these assays as broadly applicable tools for assessing overall mutagenesis during normal and aberrant (malignant) B-cell development. Furthermore, transgenic shuttle vector assays appear to lend themselves as ideal methods to associate general B-cell mutagenesis with the peculiar, B cell-typical somatic hypermutation processes that target the V(D)J gene segment, the proto-oncogene bcl-6 and perhaps other, still unknown loci.

Ethylnitrosourea-induced mutation and molecular analysis of transgenic mice containing the gpt shuttle vector

Novel transgenic mice were developed in order to study the in vivo mutagenesis. The transgenic mice carried pCGK shuttle vector, which contained the Escherichia coli gpt gene as a mutational target, the kanamycin-resistant gene (Kanr) and cos region derived from bacteriophage lambda. The shuttle vector can be recovered from the transgenic mouse genome into the gpt-deficient E. coli by an in vitro packaging method and is selectable as a Kanr phenotype. Mutations induced at the gpt gene can be easily detected with a selective agent, 6-thioguanine (6-TG). In the previous study, the pCGK shuttle vector was incorporated into Chinese hamster CHL/IU cells and the resultant transgenic cell line was shown to be a useful system to study in vitro mutagenesis at the gpt gene. Therefore, an advantage of the shuttle vector is that in vivo mutational data obtained from the transgenic mouse can be compared with those of transgenic cell line in vitro. A transgenic CD-1 mouse line, designated as #128, that carried approximately 50 copies of pCGK shuttle vectors, was selected among 4 transgenic mouse lines. To investigate the sensitivity of the #128 line, the transgenic mice were treated with a single intraperitoneal injection of 250 mg/kg of N-ethyl-N-nitrosourea (ENU) or with 50 mg kg-1 day-1 of ENU for 5 consecutive days, and bone marrow, spleen and liver were dissected to investigate their mutational responses. The background mutant frequency was between 18x10(-6) and 75x10(-6) among all tissues tested. ENU induced significant increases in the mutant frequency above the background level in all three tissues at 14 days after single or 5-day treatment with the chemical. The increases in the mutant frequencies in bone marrow, spleen and liver were 6.4- to 6.8-fold, 3.0- to 5.6-fold and 3.0- to 3.3-fold, respectively. The shuttle vector DNA was recovered from the bone marrow of both spontaneous and ENU-treated mice and the gpt gene was amplified by polymerase chain reaction. The amplified DNA was subject to DNA sequence analysis. Out of 79 spontaneous and 52 ENU-induced mutants, the gpt gene could be amplified from 28 spontaneous and 46 ENU-induced mutants. DNA sequence analysis showed that predominant mutations were identified as A:T to T:A transversions (22 out of 46 sequenced mutants) and G:C to A:T transitions (9/46) in ENU-induced mutants, whereas G:C to T:A transversions (7 out of 28 sequenced mutants) were predominant in spontaneous mutants. These results demonstrate that this transgenic mouse, in combination with the transgenic CHL/IU cell line, is a useful system to study in vivo and in vitro mutational events at the same target gene.

Tk+/- mouse model for detecting in vivo mutation in an endogenous, autosomal gene

Tk+/- transgenic mice were created using an embryonic stem cell line in which one allele of the endogenous thymidine kinase (Tk) gene was inactivated by targeted homologous recombination. Breeding Tk+/- parents produced viable Tk-/- knockout (KO) mice. Splenic lymphocytes from KO mice were used in reconstruction experiments for determining the conditions necessary for recovering Tk somatic cell mutants from Tk+/- mice. The cloning efficiency of KO lymphocytes was not affected by the toxic thymidine analogues 5-bromo-2'-deoxyuridine (BrdUrd) or trifluorothymidine (TFT), or by BrdUrd in the presence of lymphocytes from Tk+/- animals; however, it was easier to identify clones resistant to BrdUrd than to TFT when Tk+/- cells were present. Tk+/- mice were treated with vehicle or 100 mg/kg of N-ethyl-N-nitrosourea (ENU), and after 4 months, the frequency of Tk mutant lymphocytes was measured by resistance to BrdUrd. The frequency of Tk mutants was 22+/-5.9x10-6 in control animals and 80+/-31x10-6 in treated mice. In comparison, the frequency of Hprt mutant lymphocytes, as measured by resistance to 6-thioguanine, was 2.0+/-1.2x10-6 in control animals and 84+/-28x10-6 in the ENU-treated mice. Analysis of BrdUrd-resistant lymphocyte clones derived from the ENU-treated animals revealed point mutations in the non-targeted Tk allele. These results indicate that the selection of BrdUrd-resistant lymphocytes from Tk+/- mice may be used for assessing in vivo mutation in an endogenous, autosomal gene.

Comparison of mutant frequencies and types of mutations induced by thiotepa in the endogenous Hprt gene and transgenic lacI gene of Big Blue rats

The utility of the lacI transgene of Big Blue rats as a reporter of in vivo mutation was evaluated by comparing the frequency and types of mutations induced by thiotepa in the transgene and the endogenous Hprt gene. Transgenic rats were given i.p. injections of 1.4 mg/kg of thiotepa three times per week over a period of 4 weeks (a total dose of 16.8 mg/kg); 1 week after the last injection, mutation assays were performed on spleen lymphocytes isolated from the animals. Thiotepa treatment increased the lacI mutant frequency from 34.8 +/- 4.1 x 10(-6) in control animals to 140.9 +/- 64.8 x 10(-6) (p = 0.0020) and the Hprt mutant frequency from 3.5 +/- 1.5 x 10(-6) to 41.1 +/- 23.2 x 10(-6) (p = 0.0028). Sequence analysis of lacI mutant DNA and Hprt mutant cDNA produced similar overall mutation patterns: G:C-->T:A transversion was the most common base pair substitution (32% of independent mutations in the lacI gene and 28% of Hprt mutations), and deletions and insertions accounted for 34% of mutations in the lacI gene and 28% in the Hprt gene. The majority of thiotepa-induced base pair substitutions in the Hprt gene occurred with the mutated purine on the non-transcribed DNA strand, while no strand-related bias was found for mutations in the lacI gene. Substitutions at G:C base pairs in the lacI gene, but not in the Hprt gene, were found disproportionately in CpG sites. In addition, multiplex polymerase chain reaction analysis of genomic DNA from the Hprt mutants indicated that 34% had relatively large deletions; none of these deletions was detected by the cDNA analysis. The results indicate that the frequency of thiotepa-induced mutants in Big Blue rats was 2.8-fold greater in the lacI gene than in the Hprt gene. Although the Hprt gene recovered a fraction of large deletions not found among the lacI mutants, the effects of transcription-coupled DNA repair in the Hprt gene and the targeting of base pair substitutions to G:C base pairs in CpG sites may have contributed to the higher mutant frequencies induced by thiotepa in the lacI transgene compared with the Hprt gene.