Plasmid-based Transgenic Mouse Research Articles

Integrated approach to the in vivo genotoxic effects of a titanium dioxide nanomaterial using LacZ plasmid‐based transgenic mice

Titanium dioxide (TiO2 ) nanomaterials (NMs) are widely used in a diversity of products including cosmetics, pharmaceuticals, food, and inks, despite uncertainties surrounding the potential health risks that they pose to humans and the environment. Previous studies on the genotoxicity of TiO2 have reported discrepant or inconclusive findings in both in vitro and in vivo systems. This study explores the in vivo genotoxic potential of a well-characterized uncoated TiO2 NM with an average diameter of 22 nm (NM-102, from JRC repository) using several genotoxicity endpoints in the LacZ plasmid-based transgenic mouse model. Mice were exposed by intravenous injection to two daily doses of NM-102: 10 and 15 mg/kg of body weight/day. Micronuclei were analyzed in peripheral blood reticulocytes 42 hr after the last treatment. DNA strand breaks (comet assay) and gene mutations were determined in the spleens and livers of the same animals 28 days after the last treatment. Histopathological and cytological analyses were also performed in liver samples. Genotoxic effects were not detected in mice exposed to the nanosized TiO2 under the experimental conditions used, despite a moderate inflammatory response that was observed in the liver. Considering the biopersistence of TiO2 in mouse liver and the moderate inflammatory response, the possibility of a secondary genotoxic effect at higher doses and in conditions that result in a stronger inflammatory response, for example, within a longer time window, should be investigated further.

Poly (ADP‐ribose) polymerase‐1 deficiency does not affect ethylnitrosourea mutagenicity in liver and testis of lacZ transgenic mice

Poly (ADP-ribose) polymerase-1 (Parp1) has been implicated in DNA base excision repair, single- and double-strand break repair pathways, as well as in cell death by apoptosis or necrosis. We used Parp1(-/-) lacZ plasmid-based transgenic mice to investigate whether Parp1 deficiency influences the in vivo mutagenic and clastogenic response to the alkylating agent N-ethyl-N-Nitrosourea (ENU) in somatic and germ-cell tissues. The comparison of the lacZ mutant frequencies (MFs) between Parp1(+/+) and Parp1(-/-) mice showed that the ablation of Parp1 does not affect the spontaneous or ENU-induced MFs in liver and testis. In addition, the spectrum of the ENU-induced mutations was not dependent on the Parp1 status, given that similar spectra, consisting mostly of point mutations and a small fraction of deletions/insertions, wereobserved in organs of both Parp1(-/-) and Parp1(+/+) mice. Sequencing of point mutations revealed a consistent significant increase in A:T --> T:A base substitutions, typically induced by ENU. Overall, we observed that neither the frequency nor the spectrum of ENU-induced mutations demonstrated a specificity that could be attributed to the Parp1 impairment in mice organs. The analysis of micronucleus frequency in peripheral blood reticulocytes showed that ENU was clastogenic in both Parp1(-/-) and Parp1(+/+) mice and had a strong cytotoxic effect in Parp1(-/-) mice only. The present data suggest that, at a whole-organism level, Parp1-independent repair mechanisms may be operative in the removal of ENU-induced DNA lesions or that highly damaged cells may be preferentially committed to death when Parp1 is inactivated.

Evaluation of the impact of shielding materials in radiation protection in transgenic animals

We are using a plasmid-based transgenic mouse mutation model system to evaluate the effectiveness of aluminum or low-density polyethylene (LDPE) shielding after 250 MeV/u protons or 1 GeV/u iron ion irradiation. Transgenic mice, with multiple copies of the plasmid pUR288 lacZ transgene integrated into the genome of every cell of the animal, were either irradiated or sham-treated. Multiple endpoints, including early cytogenetic damage in erythrocytes at 48 h after exposure, chromosome aberrations in bone marrow lymphocytes, and lacZ mutant frequencies (MF) in brain and spleen tissues were measured in the same animals. The frequency of total circulating reticulocytes (fRET) dropped precipitously at 48 h after 2 Gy of proton irradiation. The average level of micronucleated reticulocytes (fMN-RET) was fivefold higher in the irradiated samples relative to the controls at the same time point. There was an increase in total chromosome aberrations in bone marrow lymphocytes at 8 weeks after proton irradiation but this increase was not statistically significant relative to the controls. Evaluation of the lacZ MF in the brain and spleen tissues showed that proton irradiation induced a twofold increase in MF in each tissue. Similar samples were collected from animals that were shielded from the proton beam by aluminum. Compared to the unshielded treatment group, we noted no difference in fRET, fMN-RET, chromosome aberrations in lymphocytes and lacZ MF in brain and spleen tissues obtained from these animals. In a separate study, animals were exposed to high-energy iron ions with or without 10 or 15 cm LDPE. Using the same approach, we noted a precipitous drop in fRET, and an elevation in fMN-RET within 48 h after 1 Gy of iron ions. Total chromosome aberrations in bone marrow lymphocytes were slightly elevated but not significant at 8 weeks after iron ion exposure. Shielding animals with 10 or 15 cm of polyethylene appeared to have no effect on the level of RET, MN-RET or chromosome aberrations in these animals. LacZ MF in brain and spleen tissues increased 1.5-2-fold above control levels after 1 Gy iron ions at 8 weeks after treatment. On the other hand, MF in tissues harvested from shielded animals appeared to be lower than their unshielded litermates, suggesting the polyethylene shielding was effective in reducing the iron-induced genomic damage in tissues. Although shielding may be effective, in some cases, in reducing the physical dose of particle radiation, our cytogenetic results showed that the biological impact of the particle beam remain unchanged. On the other hand, reduction in transgene MF in tissues from LDPE-shielded animals but not in the aluminum-shielded animals strongly suggests that careful consideration of the biological endpoints used is necessary in the evaluation of the efficacy of the selected shielding material.

Mutagenicity of the peroxisome proliferators clofibrate, Wyeth 14,643 and di-2-ethylhexyl phthalate in the lacZ plasmid-based transgenic mouse mutation assay

BackgroundPeroxisome proliferators are considered rodent carcinogens that are putative human non-carcinogens based on the presumed absence of direct genetic toxicity in rodent and human cells and the resistance of human cells to the induction of peroxisomes by peroxisome proliferators. The highly sensitive lacZ plasmid-based transgenic mouse mutation assay was employed to investigate the mutagenicity of several peroxisome proliferators based on several lines of evidence suggesting that these agents may in fact exert a genotoxic effect.MethodsMale and female lacZ-plasmid based transgenic mice were treated at 4 months of age with 6 doses of 2,333 mg di-2-ethylhexyl phthalate (DHEP), 200 mg Wyeth-14,643, or 90 mg clofibrate per kg of bodyweight, respectively, over a two-week period. Control animals were treated with the respective vehicles only (35% propyl glycol for DEHP and Wyeth-14,643 treatment controls and sterile water for clofibrate treatment controls).The mutant frequency in liver, kidney and spleen DNA was determined as the proportion of retrieved mutant and wild-type lacZ plasmids expressed in Escherichia Coli C host cells employing a positive selection system for mutant plasmids.ResultsExposure to DEHP or Wyeth-14,643 significantly increased the mutant frequency in liver, but not in kidney or spleen, of both female and male mice. Treatment with clofibrate did not lead to an increased mutant frequency in any of the organs studied.ConclusionThe results indicate that some peroxisome proliferators display an organ-specific mutagenicity in lacZ plasmid-based transgenic mice consistent with historical observations of organ- and compound-specific carcinogenicity.

Mutagenic activity of cisplatin in the lacZ plasmid-based transgenic mouse model.

Cis-diamminedichloroplatinum (II) (cisplatin) is a well characterized antitumor drug used for the treatment of a variety of human cancers. The cytotoxicity of cisplatin is mainly mediated through the formation of DNA adducts, which are also believed to be responsible for the secondary malignancies produced by the drug. The aim of this study was to determine the in vivo mutagenic activity of cisplatin in the lacZ plasmid-based transgenic mouse model. The mutant frequency (MF) and the spectrum of mutations induced by cisplatin in the mouse liver were analyzed and compared to controls. The mean MF in the lacZ gene was increased 2-fold in mice treated with a single 6 mg/kg body weight dose of cisplatin and sacrificed after 17 and 28 days (P = 0.001 and P < 0.0001). Restriction analysis and sequencing of mutant DNA showed that cisplatin was able to induce both large deletions and point mutations. A specific profile of base substitution and frameshift mutations was identified in treated mice, consisting primarily of G:C-->A:T transitions at GpG and ApG sites, the preferential DNA binding sites of cisplatin, and single basepair deletions/insertions. The present results provide the first evidence that cisplatin has mutagenic activity in vivo and induces a characteristic pattern of mutations in the mouse liver. This mutagenicity may be responsible for its tumorigenic activity.

INVESTIGATION OF THE RADIOADAPTIVE RESPONSE IN BRAIN AND LIVER OF pUR288 lacZ TRANSGENIC MICE

The radioadaptive response, where a small priming dose of ionizing radiation can lessen the effects of subsequent exposure to a higher radiation challenge dose, was investigated in brain and liver within transgenic mice. Although it is well characterized in models in vitro, current radioadaptive response research has focused on particular cell types (i.e., lymphocytes) and does not provide comparative data for responses of multiple tissues within an organism. Transgenic animals are useful for such comparisons, because the transgene is integrated into all cells in the body. The pUR288 lacZ plasmid-based transgenic mouse model utilizes a plasmid vector allowing highly efficient recovery of mutational targets, including large size-change mutations that result from radiation exposure. Female C57Bl/6 pUR288 lacZ mice were exposed to priming doses of 0.075- to 0.375-Gy x-rays over a 3-d period. After 3 wk, they received an acute challenge dose of 2.5-Gy x-rays. Spontaneous mutant frequencies in lacZ

P53 deficiency alters the yield and spectrum of radiation-induced lacZ mutants in the brain of transgenic mice.

Exposure to heavy particle radiation in the galacto-cosmic environment poses a significant risk in space exploration and the evaluation of radiation-induced genetic damage in tissues, especially in the central nervous system, is an important consideration in long-term manned space missions. We used a plasmid-based transgenic mouse model system, with the pUR288 lacZ transgene integrated in the genome of every cell of C57Bl/6(lacZ) mice, to evaluate the genetic damage induced by iron particle radiation. In order to examine the importance of genetic background on the radiation sensitivity of individuals, we cross-bred p53 wild-type lacZ transgenic mice with p53 nullizygous mice, producing lacZ transgenic mice that were either hemizygous or nullizygous for the p53 tumor suppressor gene. Animals were exposed to an acute dose of 1 Gy of iron particles and the lacZ mutation frequency (MF) in the brain was measured at time intervals from 1 to 16 weeks post-irradiation. Our results suggest that iron particles induced an increase in lacZ MF (2.4-fold increase in p53+/+ mice, 1.3-fold increase in p53+/- mice and 2.1-fold increase in p53-/- mice) and that this induction is both temporally regulated and p53 genotype dependent. Characterization of mutants based on their restriction patterns showed that the majority of the mutants arising spontaneously are derived from point mutations or small deletions in all three genotypes. Radiation induced alterations in the spectrum of deletion mutants and reorganization of the genome, as evidenced by the selection of mutants containing mouse genomic DNA. These observations are unique in that mutations in brain tissue after particle radiation exposure have never before been reported owing to technical limitations in most other mutation assays.

Characterization of color mutants in lacZ plasmid-based transgenic mice, as detected by positive selection.

The plasmid-based transgenic mouse model, which uses the lacZ gene as the target for mutation, is sensitive to a wide range of in vivo mutations, ranging from point mutations to insertions and deletions extending far into the mouse genome. In this study, the nature of subtle lacZ mutations, which do not completely abolish beta-galactosidase activity, as detected by positive selection, was investigated. These subtle mutants are called 'color mutants' due to their light blue staining on X-gal medium. Replating of color mutants and retransformation of plasmid DNA, purified from individual color mutants, resulted in the same phenotype as the original color mutant. The p-gal positive selection system tolerates approximately 10% of wild-type activity as indicated by spectrophotometric determination of beta-galactosidase activity of individual color mutants. Restriction digestion and size separation of plasmid DNA revealed no visible change in the size of the plasmid in color mutants. Sequence analysis confirmed the presence of a point mutation in each lacZ gene of nine different color mutants. The results indicate that color mutants are caused neither by the presence of a mixture of wild-type and mutated lacZ plasmids within the same host cell nor by a mixture of cells within the original mutant colony which carry either wild-type or mutated lacZ plasmids. In addition, it was discovered that the mouse line studied harbors four polymorphic base changes among the integrated plasmid copies.

Treatment oflacZ plasmid-based transgenic mice with benzo[a]pyrene: Measurement of DNA adduct levels, mutant frequencies, and mutant spectra

The evaluation of the relationship between the dose to DNA of a genotoxic carcinogen and in vivo somatic cell mutagenesis may provide important information on the mechanisms leading to induced mutational events. The induction of DNA adducts and DNA mutations in different tissues of lacZ plasmid-based transgenic mice has been investigated following a single intraperitoneal administration of the polycyclic aromatic hydrocarbon benzo[a]pyrene (B[a]P). DNA adducts were measured by (32)P-postlabeling at times between 1 and 28 days following injection and DNA mutations in the lacZ reporter gene were quantified using a highly efficient immunomagnetic purification of the lacZ-containing pUR288 plasmid, followed by electrotransformation of circularized plasmids into a host-restriction negative E. coli C (DeltalacZ, galE (-)) host. The major DNA adduct formed by B[a]P, N (2)-(10beta-[7beta,8alpha, 9alpha-trihydroxy-7,8,9, 10-tetrahydrobenzo(a)pyrene]yl-deoxyguanosine, reached maximum levels between 5 and 7 days followed by a gradual decrease. The induced mutant frequency reached a maximum between 7 and 14 days, after the DNA adduct level in a particular tissue had reached its apparent maximum between 5 and 7 days. The mutant spectrum, defined as the percentage of no-change and size-change mutations in a particular tissue, changed from predominantly size-change mutations in untreated tissues to predominantly no-change mutations in tissues displaying the highest B[a]P-induced mutant frequencies. Contrary to the assumption that there are no factors that can eliminate mutations once they exist, it was observed that mutant frequencies in the organs studied declined to background levels, which was accompanied by a change in the mutant spectrum from predominantly no-change mutations to predominantly size-change mutations.

Treatment of lacZ plasmid‐based transgenic mice with benzo[a]pyrene: Measurement of DNA adduct levels, mutant frequencies, and mutant spectra

The evaluation of the relationship between the dose to DNA of a genotoxic carcinogen and in vivo somatic cell mutagenesis may provide important information on the mechanisms leading to induced mutational events. The induction of DNA adducts and DNA mutations in different tissues of lacZ plasmid-based transgenic mice has been investigated following a single intraperitoneal administration of the polycyclic aromatic hydrocarbon benzo[a]pyrene (B[a]P). DNA adducts were measured by 32P-postlabeling at times between 1 and 28 days following injection and DNA mutations in the lacZ reporter gene were quantified using a highly efficient immunomagnetic purification of the lacZ-containing pUR288 plasmid, followed by electrotransformation of circularized plasmids into a host-restriction negative E. coli C (ΔlacZ, galE –) host. The major DNA adduct formed by B[a]P, N 2-(10β-[7β,8α,9α-trihydroxy-7,8,9,10-tetrahydrobenzo(a)pyrene]yl-deoxyguanosine, reached maximum levels between 5 and 7 days followed by a gradual decrease. The induced mutant frequency reached a maximum between 7 and 14 days, after the DNA adduct level in a particular tissue had reached its apparent maximum between 5 and 7 days. The mutant spectrum, defined as the percentage of no-change and size-change mutations in a particular tissue, changed from predominantly size-change mutations in untreated tissues to predominantly no-change mutations in tissues displaying the highest B[a]P-induced mutant frequencies. Contrary to the assumption that there are no factors that can eliminate mutations once they exist, it was observed that mutant frequencies in the organs studied declined to background levels, which was accompanied by a change in the mutant spectrum from predominantly no-change mutations to predominantly size-change mutations. Environ. Mol. Mutagen. 34:140–147, 1999 © 1999 Wiley-Liss, Inc.

High sensitivity for color mutants inlacZ plasmid-based transgenic mice, as detected by positive selection

Transgenic mice carrying bacteriophage lambda-based vectors harboring a lacZ or lacI reporter gene have been used in recent years for the detection, quantification, and characterization of spontaneous and induced gene mutations in vivo. The usefulness of these models is basically determined by their ability to detect significant (organ-specific) exposure-related increases in mutant frequencies. This, in its turn, is dependent on the models' ability to detect all possible types of mutations, including mutations that partially inactivate the gene product, as well as the absence of a significant proportion of E. coli-derived mutations. The newly developed lacZ plasmid-based transgenic mouse mutation assay is sensitive to a broad range of DNA mutations, including large size changes. Here, we describe the plasmid model's sensitivity for detecting color mutants, i.e., cells containing a partially inactivated lacZ gene, as detected by a positive selection system using phenyl-beta-galactoside. The proportion of color mutants in brain and kidney was 30-40%. This was significantly higher than the 10-20% color mutants observed in lung, spleen, and liver. It is demonstrated that the color mutants detected were neither the result of a mixture of both mutant and nonmutant cells within the original mutant colony-forming unit nor due to the presence of mutant and nonmutant lacZ genes within individual E. coli host cells. Sequence analysis of 13 different color mutants revealed single basepair substitutions in the lacZ gene of each mutant. The high tolerance of the positive selection system for lacZ color mutants in the plasmid-based transgenic mouse model greatly contributes to the sensitivity of this model, now ranging from large size-change mutations that completely inactivate the lacZ gene to basepair substitutions that partly inactivate the lacZ gene.

Sources of variability in mutant frequency determinations in different organs oflacZ plasmid-based transgenic mice: Experimental features and statistical analysis

Sources of variability that may affect the measurement of mutant frequencies in lacZ plasmid-based transgenic mice, including rescue-to-rescue, animal-to-animal (within organ), and organ-to-organ, were studied in the context of some of the experimental features that distinguish this model from other currently used systems. Statistical analysis of repeated determinations of DNA from kidney, spleen, liver, lung, and brain indicated that 350,000 colony-forming units from each of three animals should be analyzed per treatment group in order to detect a 50% difference in mutant frequencies. Consideration is given to some of the experimental features of the transgenic assay system, including its positive selection system, its rescue efficiency, the ability to detect large deletions in the lacZ target genes, the contribution of Escherichia coli-derived mutations to the spontaneous mutant frequencies observed in vivo, and cost effectiveness of mutant frequency determinations.

Evaluation of a plasmid-based transgenic mouse model for detecting in vivo mutations

To study in vivo somatic mutations a C57BL/6 transgenic mouse model was constructed harboring multiple chromosomally integrated copies of the plasmid pUR288, which carried the lacZ reporter gene as the mutational target. We previously demonstrated that lacZ-containing plasmids could be rescued from their integrated state efficient enough to detect mutations in lacZ by positive selection. The smaller size of the plasmid vector, as compared with our earlier transgenic mouse model based on bacteriophage lambda vectors, should offer considerable advantages in terms of rescue efficiency and sensitivity to large size alterations in the lacZ gene. To evaluate the plasmid-based mouse model for its suitability to detect in vivo mutations, we determined mutant frequencies in different organs of untreated and ethyl nitrosourea (ENU)-treated animals using a new, improved protocol. The rescue efficiencies obtained were as high as 200,000/micrograms genomic DNA; millions of transformants could be obtained in one single experiment. The average spontaneous mutant frequency in four different organs of 4- to 8-week-old mice ranged from 4.41 to 6.82 x 10(-5), compared with a mutant frequency of the same plasmid grown in Escherichia coli of approximately 1 x 10(-5) or less. Single treatments with 100 and 250 mg ENU/kg body wt resulted in a 7- and 14-fold increase, respectively, in spleen mutant frequency at 14 days after i.p. administration of the alkylating agent. Restriction enzyme analysis showed that a considerable portion of spontaneous mutants were size changes varying from approximately 100 to 3000 bp. Some mutant plasmids contained mouse genomic sequences, which is indicative of large genetic rearrangement events involving the 3' flanking regions of the transgene cluster. Among the ENU-induced mutants, size changes comprised only a minor fraction of the total, which is in keeping with the known ENU mutation spectra in vitro and in vivo. The high rescue efficiency of this plasmid-based model, in combination with its sensitivity to a broad spectrum of mutations, including large deletions, makes it very suitable as a general in vivo mutagenicity test system.

Plasmid-based transgenic mouse model for studying in vivo mutations.

A new transgenic mouse model for studying in vivo somatic mutations is based on the efficient recovery of chromosomally integrated lacZ-containing plasmids, using magnetic beads.

Spontaneous and X-ray-induced deletion mutations in a LacZ plasmid-based transgenic mouse model

Transgenic mouse mutation models carrying bacterial marker genes in bacteriophage lambda shuttle vectors have been applied to study spontaneous or induced mutations in vivo. However, due to the nature of the shuttle vector these models are insensitive to large deletions. Clastogenic agents, which predominantly induce large deletions, were therefore found to yeild very low responses in these assays. Here we report the use of LacZ plasmid-based transgenic mice, allowing the detection of a broad spectrum of mutations. Treatment of mice with X-rays (5 × 50 rads) resulted in induction of up to about 5-fold higher mutation frequencies in lung, spleen and liver. Analysis of spontaneous and induced mutant LacZ genes indicated that at least 40–50% of all mutations were caused by deletions. The possibility of detecting a broad spectrum of mutations with this system suggests that the LacZ plasmid-based transgenic mouse may be the mammalian model of choice for studying spontaneous and induced mutations in vivo.