Analysis of In Vivo Mutation in the Hprt and Tk Genes of Mouse Lymphocytes.

Determining mutant frequencies in endogenous reporter genes is a tool for identifying potentially genotoxic environmental agents, and discovering phenotypes prone to genomic instability and diseases, such as cancer. Here, we describe a high-throughput method for identifying mouse spleen lymphocytes with mutations in the endogenous X-linked hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the endogenous autosomal thymidine kinase (Tk) gene. The selective clonal expansion of mutant lymphocytes is based upon the phenotypic properties of HPRT- and TK-deficient cells. The same procedure can be utilized for quantifying Hprt mutations in most strains of mice (and, with minor changes, in other mammalian species), while mutations in the Tk gene can be determined only in transgenic mice that are heterozygous for inactivation of this gene. Expanded mutant clones can be further analyzed to classify the types of mutations in the Tk gene (small intragenic mutations vs. large chromosomal mutations) and to determine the nature of intragenic mutation at both the Hprt and Tk genes.

Detection of In Vivo Mutation in the Hprt and Pig-a Genes of Rat Lymphocytes.

Assays for in vivo mutation are used to identify genotoxic hazards and phenotypes prone to genomic instability and cancer. The hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the phosphatidyl inositol glycan, class A (Pig-a) gene are endogenous X-linked genes that can be used as reporters of mutation in peripheral blood lymphocytes from most mammals. Here we describe methodology for measuring Hprt and Pig-a mutation in rat T-lymphocytes. The identification and selective expansion of mutant lymphocytes is based upon the phenotypic properties of Hprt- and Pig-a-deficient cells, that is, resistance to the purine analog, 6-thioguanine, or to the bacterial toxin, proaerolysin. Expanded mutants can be further analyzed by sequencing cDNA from the target transcripts for identification of small sequence alterations and by multiplex PCR analysis of genomic DNA for the detection of deletions.

Analysis of in vivo mutation in the Hprt and Tk genes of mouse lymphocytes.

Assays measuring mutant frequencies in endogenous reporter genes are used for identifying potentially genotoxic environmental agents and discovering phenotypes prone to genomic instability and diseases, such as cancer. Here, we describe methods for identifying mouse spleen lymphocytes with mutations in the endogenous X-linked hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the endogenous autosomal thymidine kinase (Tk) gene. The selective clonal expansion of mutant lymphocytes is based upon the phenotypic properties of HPRT- and TK-deficient cells. The same procedure can be utilized for quantifying Hprt mutations in most strains of mice (and, with minor changes, in other mammalian species), while mutations in the Tk gene can be determined only in transgenic mice that are heterozygous for inactivation of this gene. Expanded mutant clones can be further analyzed to classify the types of mutations in the Tk gene (small intragenic mutations vs. large chromosomal mutations) and to determine the nature of intragenic mutation in both the Hprt and Tk genes.

Detection of in vivo mutation in the Hprt and Pig-a genes of rat lymphocytes.

Assays for in vivo mutation are used to identify genotoxic hazards and phenotypes prone to genomic instability and cancer. The hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the phosphatidyl inositol glycan, class A (Pig-a) gene are endogenous X-linked genes that can be used as reporters of mutation in peripheral blood lymphocytes from most mammals. Here we describe methodology for measuring Hprt and Pig-a mutation in rat T-lymphocytes. The identification and selective expansion of mutant lymphocytes are based upon the phenotypic properties of Hprt- and Pig-a-deficient cells, i.e., resistance to the purine analog, 6-thioguanine, or to the bacterial toxin, proaerolysin. Expanded mutants can be further analyzed by sequencing cDNA from the target transcripts for identification of small sequence alterations and by multiplex PCR analysis of genomic DNA for the detection of deletions.

Sensitivity of the Pig-a assay for detecting gene mutation in rats exposed acutely to strong clastogens.

Clastogens are potential human carcinogens whose detection by genotoxicity assays is important for safety assessment. Although some endogenous genes are sensitive to the mutagenicity of clastogens, many genes that are used as reporters for in vivo mutation (e.g. transgenes) are not. In this study, we have compared responses in the erythrocyte Pig-a gene mutation assay with responses in a gene mutation assay that is relatively sensitive to clastogens, the lymphocyte Hprt assay, and in the reticulocyte micronucleus (MN) assay, which provides a direct measurement of clastogenicity. Male F344 rats were treated acutely with X-rays, cyclophosphamide (CP) and Cis-platin (Cis-Pt), and the frequency of micronucleated reticulocytes (MN RETs) in peripheral blood was measured 1 or 2 days later. The frequencies of CD59-deficient Pig-a mutant erythrocytes and 6-thioguanine-resistant Hprt mutant T-lymphocytes were measured at several times up to 16 weeks after the exposure. All three clastogens induced strong increases in the frequency of MN RETs, with X-rays and Cis-Pt producing near linear dose responses. The three agents also were positive in the two gene mutation assays although the assays detected them with different efficiencies. The Pig-a assay was more efficient in detecting the effect of Cis-Pt treatment, whereas the Hprt assay was more efficient for X-rays and CP. The results indicate that the erythrocyte Pig-a assay can detect the in vivo mutagenicity of clastogens although its sensitivity is variable in comparison with the lymphocyte Hprt assay.

Nitroxide TEMPO: a genotoxic and oxidative stress inducer in cultured cells.

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) is a low molecular weight nitroxide and stable free radical. In this study, we investigated the cytotoxicity and genotoxicity of TEMPO in mammalian cells using the mouse lymphoma assay (MLA) and in vitro micronucleus assay. In the absence of metabolic activation (S9), 3mM TEMPO produced significant cytotoxicity and marginal mutagenicity in the MLA; in the presence of S9, treatment of mouse lymphoma cells with 1-2mM TEMPO resulted in dose-dependent decreases of the relative total growth and increases in mutant frequency. Treatment of TK6 human lymphoblastoid cells with 0.9-2.3mM TEMPO increased the frequency of both micronuclei (a marker for clastogenicity) and hypodiploid nuclei (a marker of aneugenicity) in a dose-dependent manner; greater responses were produced in the presence of S9. Within the dose range tested, TEMPO induced reactive oxygen species and decreased glutathione levels in mouse lymphoma cells. In addition, the majority of TEMPO-induced mutants had loss of heterozygosity at the Tk locus, with allele loss of ⩽34Mbp. These results indicate that TEMPO is mutagenic in the MLA and induces micronuclei and hypodiploid nuclei in TK6 cells. Oxidative stress may account for part of the genotoxicity induced by TEMPO in both cell lines.

Evaluating the weak in vivo micronucleus response of a genotoxic carcinogen, aristolochic acids.

Aristolochic acids (AAs) are carcinogenic plant toxins that are relatively strong gene mutagens, both in vitro and in vivo, but weak inducers of micronuclei in vivo. In order to clarify the reasons for these disparate responses, we evaluated the genotoxicity of AAs in F344 rats using several assays that respond to DNA damage in bone marrow. Groups of 7- to 8-week-old male rats (n=6) were gavaged with 0, 2.75, 5.5, and 11mg/kg AAs for 28 days or with 0, 11, 22, and 30mg/kg AAs for 3 days. Day 1 being the first day of treatment, Pig-a mutant frequencies (MFs) were assayed in peripheral blood erythrocytes up to Day 56 for the 28-day treatment or Day 42 for the 3-day treatment; micronuclei were assayed in peripheral blood reticulocytes on Day 4 (both treatment protocols) and on Day 29 of the 28-day treatment protocol; and at the final sampling times (Day 59 or Day 42), the animals were sacrificed and Hprt mutant lymphocytes were measured. In a separate study, the Comet assay was performed on liver, kidney, and bone marrow of animals gavaged with 0, 11, 22, and 30mg/kg AAs for 4 days and sacrificed 3h after the last treatment. While only weak increases in micronucleated reticulocyte frequency were observed in treated animals, Pig-a MFs increased in a dose- and time-dependent manner with both treatment schedules. Lymphocyte Hprt mutant frequencies also increased dose dependently in treated animals, and the Comet assay detected elevated levels of DNA damage in all the tissues evaluated. These findings indicate that the DNA damage produced by AAs in rat bone marrow is a weak inducer of micronuclei but a relatively strong inducer of gene mutation.

In vivo genotoxicity of furan in F344 rats at cancer bioassay doses.

Furan, a potent rodent liver carcinogen, is found in many cooked food items and thus represents a human cancer risk. Mechanisms for furan carcinogenicity were investigated in male F344 rats using the in vivo Comet and micronucleus assays, combined with analysis of histopathological and gene expression changes. In addition, formamidopyrimidine DNA glycosylase (Fpg) and endonuclease III (EndoIII)-sensitive DNA damage was monitored as a measure of oxidative DNA damage. Rats were treated by gavage on four consecutive days with 2, 4, and 8mg/kg bw furan, doses that were tumorigenic in 2-year cancer bioassays, and with two higher doses, 12 and 16mg/kg. Rats were killed 3h after the last dose, a time established as producing maximum levels of DNA damage in livers of furan-treated rats. Liver Comet assays indicated that both DNA strand breaks and oxidized purines and pyrimidines increased in a near-linear dose-responsive fashion, with statistically significant increases detected at cancer bioassay doses. No DNA damage was detected in bone marrow, a non-target tissue for cancer, and peripheral blood micronucleus assays were negative. Histopathological evaluation of liver from furan-exposed animals produced evidence of inflammation, single-cell necrosis, apoptosis, and cell proliferation. In addition, genes related to apoptosis, cell-cycle checkpoints, and DNA-repair were expressed at a slightly lower level in the furan-treated livers. Although a mixed mode of action involving direct DNA binding cannot be ruled out, the data suggest that furan induces cancer in rat livers mainly through a secondary genotoxic mechanism involving oxidative stress, accompanied by inflammation, cell proliferation, and toxicity.

Genotoxicity of furan in Big Blue rats.

Furan is a multispecies liver carcinogen whose cancer mode of action (MOA) is unclear. A major metabolite of furan is a direct acting mutagen; however, it is not known if genotoxicity is a key step in the tumors that result from exposure to furan. In order to address this question, transgenic Big Blue rats were treated by gavage five times a week for 8 weeks with two concentrations of furan used in cancer bioassays (2 and 8mg/kg), and with two higher concentrations (16 and 30mg/kg). Peripheral blood samples taken 24h after the 5th dose (1 week of dosing) were used to assay for micronucleus (MN) frequency in normochromatic erythrocytes (NCEs) and reticulocytes (RETs), and Pig-a gene mutation in total red blood cells (RBCs). 24h after the last dose of the 8-week treatment schedule, the rats were euthanized, and their tissues were used to perform NCE and RET MN assays, the Pig-a RBC assay, Pig-a and Hprt lymphocyte gene mutation assays, the liver cII transgene mutation assay, and the liver Comet assay. The responses in the MN assays conducted at both sampling times, and all the gene mutation assays, were uniformly negative; however, the Comet assay was positive for the induction of liver DNA damage. As the positive responses in the Comet assay were seen only with doses in excess of the cancer bioassay doses, and at least one of these doses (30mg/kg) produced toxicity in the liver, the overall findings from the study are consistent with furan having a predominantly nongenotoxic MOA for cancer.

Report on stage III Pig-a mutation assays using N-ethyl-N-nitrosourea-comparison with other in vivo genotoxicity endpoints.

N-Ethyl-N-nitrosourea (ENU) was evaluated as part of the Stage III trial for the rat Pig-a gene mutation assay. Groups of six- to eight-week-old male Sprague Dawley (SD) or Fischer 344 (F344) rats were given 28 daily doses of the phosphate buffered saline vehicle, or 2.5, 5, or 10 mg/kg ENU, and evaluated for a variety of genotoxicity endpoints in peripheral blood, spleen, liver, and colon. Blood was sampled predose (Day-1) and at various time points up to Day 57. Pig-a mutant frequencies were determined in total red blood cells (RBCs) and reticulocytes (RETs) as RBC(CD592-) and RET(CD592-) frequencies. Consistent with the results from a reference laboratory, RBC(CD592-) and RET(CD592-) frequencies increased in a dose and time-dependent manner, producing significant increases at all doses by Day 15, with similar frequencies seen in both rat strains. ENU also induced small but significant increases in % micronucleated RETs on Days 4 and 29. No significant increases in micronuclei were seen in the liver or colon of the ENU-treated SD rats. Hprt and Pig-a lymphocyte mutation assays conducted on splenocytes from Day 56 F344 rats detected two- to fourfold stronger responses for Hprt than Pig-a mutations. Results from the in vivo Comet assay in SD rats at Day 29 showed generally weak increases in DNA damage in all tissues evaluated. The results with ENU indicate that the Pig-a RET and RBC assays are reproducible, transferable, and complement other genotoxicity endpoints that could potentially be integrated into 28-day repeat dose rat studies.

Report on stage III Pig-a mutation assays using benzo[a]pyrene.

Genotoxicity assays were conducted on rats treated with benzo[a]pyrene (BaP) as part of Stage III of a validation study on the Pig-a gene mutation assay. Assays were performed at the U.S. FDA-NCTR and Bayer-Germany. Starting on Day 1, groups of five 6- to 7-week-old male Fischer 344 (F344, used at FDA-NCTR) and Han Wistar rats (Bayer) were given 28 daily doses of 0, 37.5, 75, or 150 mg/kg BaP; blood was sampled on Days -1, 4, 15, 29, and 56. Pig-a mutant frequencies were determined on Days -1, 15, 29, and 56 in total red blood cells (RBCs) and reticulocytes (RETs) as RBC(CD59-) and RET(CD59-) frequencies; percent micronucleated-RETs (%MN-RET) were measured on Days 4 and 29. RBC(CD59-) and RET(CD59-) frequencies increased in a dose- and time-dependent manner, producing significant increases by Day 29 in both rat models. The responses for RETs were stronger than those for RBCs, and the responses in F344 rats were stronger than in Han Wistar rats. BaP also produced significant increases in %MN-RET frequency at Days 4 and 29, with the responses being greater in F344 than Han Wistar rats. The overall findings were consistent with those of the reference laboratory using Han Wistar rats. Finally, mutation assays performed on splenocytes from Day 56 F344 rats indicated that BaP mutant frequencies were three to fivefold higher for the Hprt gene than the Pig-a gene. The results indicate that the Pig-a RET and RBC assays are reproducible, transferable, and show promise for integrating gene mutation into 28-day repeat-dose studies.

Comparative analysis of micronuclei and DNA damage induced by Ochratoxin A in two mammalian cell lines.

The fungal toxin, Ochratoxin A (OTA), is a common contaminant in human food and animal feed. The present study evaluated micronucleus (MN) induction by OTA in comparison with its ability to induce cytotoxicity and DNA damage in two mammalian cell lines, CHO-K1-BH(4) Chinese hamster ovary cells and TK6 human lymphoblastoid cells. Micronuclei were evaluated by flow cytometry, cytotoxicity was estimated by relative population doubling (RPD), while direct DNA damage and oxidative DNA damage were measured with the Comet assay, performed without and with digestion by formamidopyrimidine-DNA glycosylase (fpg). For the MN and cytotoxicity measurements, the cell lines were treated for 24h (CHO cells) or 27h (TK6 cells) with 5-25µM OTA in the absence of exogenous metabolic activation. The OTA treatments resulted in concentration-responsive increases in cytotoxicity, with higher concentrations of the agent being more cytotoxic in CHO cells than TK6 cells. 15µM OTA produced positive responses for MN induction and hypodiploid events (a measure of aneugenicity) in both cell lines; this concentration of OTA also produced cytotoxicity near to the recommended limit for the assay (45±5% RPD). A time course assay with TK6 cells indicated that at least 4h of OTA treatment were required to produce a positive MN response. For the Comet assay DNA damage assessments, the cell lines were treated with 5-50µM OTA for 4h. Direct DNA damage was detected in TK6 cells, but not CHO cells, while concentration-related increases in fpg-sensitive sites were detected for both cell lines. The consistent association of oxidative DNA damage with OTA exposure suggests its involvement in producing OTA-induced clastogenicity and aneugenicity; however, based on its detection in TK6 cells direct DNA damage could be involved in any human risk posed by OTA exposure.

Analysis of mutations in the Pig-a gene of spleen T-cells from N-ethyl-N-nitrosourea-treated fisher 344 rats.

A rapid in vivo somatic cell gene mutation assay is being developed that measures mutation in the endogenous X-linked phosphatidylinositol glycan, class A gene (Pig-a). The assay detects Pig-a mutants by flow cytometric identification of cells deficient in glycosylphosphatidyl inositol (GPI) anchor synthesis. GPI-deficient, presumed Pig-a mutant cells also can be detected in a cloning assay that uses proaerolysin (ProAER) selection. Previously, we demonstrated that ProAER-resistant (ProAER(r) ) rat spleen T-cells have mutations in the Pig-a gene. In the present study, we report on a more complete analysis of ProAER(r) rat spleen T-cell mutants and describe a mutation spectrum for mutants isolated from rats 4 weeks after treatment with three consecutive doses of 35.6 mg/kg N-ethyl-N-nitrosourea (ENU). We identified a total of 55 independent mutations, with the largest percentage (69%) involving basepair substitution at A:T. The overall spectrum of Pig-a gene mutations was consistent with the types of DNA adducts formed by ENU and was very similar to what has been described for in vivo ENU-induced mutation spectra in other rodent reporter genes (e.g., in the endogenous Hprt gene and transgenic shuttle vectors). These data are consistent with the rat Pig-a assay detecting test-agent-induced mutational responses.

The genotoxicity of acrylamide and glycidamide in big blue rats.

Acrylamide (AA), a mutagen and rodent carcinogen, recently has been detected in fried and baked starchy foods, a finding that has prompted renewed interest in its potential for toxicity in humans. In the present study, we exposed Big Blue rats to the equivalent of approximately 5 and 10 mg/kg body weight/day of AA or its epoxide metabolite glycidamide (GA) via the drinking water, an AA treatment regimen comparable to those used to produce cancer in rats. After 2 months of dosing, the rats were euthanized and blood was taken for the micronucleus assay; spleens for the lymphocyte Hprt mutant assay; and liver, thyroid, bone marrow, testis (from males), and mammary gland (females) for the cII mutant assay. Neither AA nor GA increased the frequency of micronucleated reticulocytes. In contrast, both compounds produced small (approximately twofold to threefold above background) but significant increases in lymphocyte Hprt mutant frequency (MF, p < 0.05), with the increases having dose-related linear trends (p < 0.05 to p < 0.001). Neither compound increased the cII MF in testis, mammary gland (tumor target tissues), or liver (nontarget tissue), while both compounds induced weak positive increases in bone marrow (nontarget tissue) and thyroid (target tissue). Although the genotoxicity in tumor target tissue was weak, in combination with the responses in surrogate tissues, the results are consistent with AA being a gene mutagen in the rat via metabolism to GA.

Evaluation of mutagenic mode of action in Big Blue mice fed methylphenidate for 24 weeks.

Methylphenidate hydrochloride (MPH), a widely prescribed pediatric drug for attention deficit hyperactivity disorder, induced liver adenocarcinomas in B6C3F1 mice exposed to 500 ppm in feed for 2 years (Dunnick and Hailey (1995) [14]). In order to determine if the induction of liver tumors was by a mutagenic mode of action, groups of male Big Blue (BB) mice (B6C3F1 background) were fed diets containing 50-4000 ppm MPH for 4, 12, or 24 weeks. At sacrifice, the livers were removed and the cII mutant frequency (MF) and spectrum of cII mutations were determined. In addition, the frequencies of micronucleated reticulocytes (MN-RETs) and normochromatic erythrocytes (MN-NCEs) were measured in peripheral blood erythrocytes as was the Hprt MF in splenic lymphocytes. Food consumption and body weight gain/loss were recorded weekly for each animal. The levels of MPH and RA were determined immediately before sacrifice in the serum of mice fed MPH for 24 weeks. A significant loss in body weights (p

DNA adduct formation and induction of micronuclei and mutations in B6C3F1/Tk mice treated neonatally with acrylamide or glycidamide.

Acrylamide, a food contaminant, is carcinogenic in experimental animals, with both genotoxic and nongenotoxic pathways being proposed. To obtain information regarding mechanisms of acrylamide tumorigenesis, we compared the extent of DNA adduct formation and induction of micronuclei and mutations in mice treated neonatally with acrylamide and its electrophilic metabolite glycidamide. Male and female B6C3F1/Tk mice were treated intraperitoneally on postnatal days (PNDs) 1, 8 and 15 or PNDs 1-8 with 0.14 or 0.70 mmol acrylamide or glycidamide per kg body weight per day. One day after the final dose, B6C3F1/Tk(+/+) mice were killed to measure DNA adduct levels and peripheral blood micronuclei. Three weeks after the last treatment, B6C3F1/Tk(+/-) mice were killed to assess the Hprt and Tk mutant frequencies in spleen lymphocytes. The levels of N7-(2-carbamoyl-2-hydroxyethyl)guanine, the major glycidamide-DNA adduct, decreased in the order 0.70 mmol glycidamide > 0.70 mmol acrylamide > 0.14 mmol glycidamide approximately 0.14 mmol acrylamide. Only glycidamide increased the frequency of micronucleated reticulocytes and normochromatic erythrocytes. In mice treated on PNDs 1, 8 and 15, the Hprt mutant frequency was increased by 0.70 mmol glycidamide. In mice dosed on PNDs 1-8, 0.70 mmol glycidamide caused extensive mortality; each of the other treatments increased the Tk mutant frequency, whereas acrylamide increased the Hprt mutant frequency. These data suggest that the mutagenic response in neonatal mice treated on PNDs 1, 8 and 15 is due to glycidamide, whereas mutations resulting from dosing on PNDs 1-8 are due to another mechanism.

The genetic toxicology of methylphenidate hydrochloride in non-human primates.

The studies presented in this work were designed to evaluate the genetic toxicity of methylphenidate hydrochloride (MPH) in non-human primates (NHP) using a long-term, chronic dosing regimen. Thus, approximately two-year old, male rhesus monkeys of Indian origin were orally exposed to MPH diluted in the electrolyte replenisher, Prang, five days per week over a 20-month period. There were 10 animals per dose group and the doses were (1) control, Prang only, (2) low, 0.15 mg/kg of MPH twice per day increased to 2.5mg/kg twice per day and (3) high, 1.5 mg/kg of MPH twice per day increased to 12.5 mg/kg twice per day. Blood samples were obtained from each animal to determine the base-line serum levels of MPH and the major metabolite of MPH in NHP, ritalinic acid (RA). In addition, the base-line frequency of micronucleated erythrocytes (MN-RETs) by flow cytometry, HPRT mutants by a lymphocyte cloning assay, and chromosome aberrations by FISH painting were determined from peripheral blood samples. Once dosing began, the serum levels of MPH and its major metabolite, RA, were determined monthly. The MN-RET frequency and health parameters (CBC, serum chemistries) were also determined monthly. HPRT mutant and chromosome aberration frequencies were measured every three months. CBC values and serum chemistries, with the exception of alanine amino transferase, were within normal limits over the course of drug exposure. The final plasma levels of MPH were similar to those produced by the pediatric dose of 0.3 microg/ml. No significant increases in the frequencies of MN-RETs, HPRT mutants, or chromosome aberrations were detected in the treated animals compared to the control animals over the 20-month exposure period.

Evaluation of Macaca mulatta as a model for genotoxicity studies.

We have investigated the use of peripheral blood from the nonhuman primate (NHP) rhesus monkey (Macaca mulatta) as a model system for mutation detection. The rhesus monkey is metabolically closer to humans than most common laboratory animals, and therefore may be a relevant model for hazard identification and human risk assessment. To validate the model, conditions were determined for in vitro selection and expansion of 6-thioguanine-resistant (6-TGr) HPRT mutant and proaerolysin-resistant (ProAERr) PIG-A mutant lymphocytes from peripheral blood obtained by routine venipuncture. Also, flow cytometric methods were developed for the rapid detection of PIG-A mutant erythrocytes. The flow cytometric analysis of PIG-A mutant erythrocytes was based on enumerating cells deficient in surface markers attached to the cellular membrane via glycosylphosphatidyl inositol (GPI) anchors. Mutant cells were enumerated over an extended period of time in peripheral blood of male monkeys receiving daily doses of the electrolyte replenisher Prangtrade mark (a common carrier for oral delivery of drugs in NHPs), and in the blood of one male monkey treated with a single i.p. dose of 50mg/kg of N-ethyl-N-nitrosourea at approximately 2 years of age and another similar injection at approximately 3.5 years of age. The spontaneous PIG-A and HPRT T-cell mutant frequency (MF) was low in animals receiving Prang (0-8x10(-6)), and treatment with ENU resulted in a clearly detectable increase in the frequency of ProAERr and 6-TGr lymphocytes (up to approximately 28x10(-6) and approximately 30x10(-6), respectively). Also, the ENU-treated animal had higher frequency of GPI-deficient erythrocytes (46.5x10(-6) in the treated animal vs. 7.8+/-4.2x10(-6) in control animals). Our results indicate that the rhesus monkey can be a valuable model for the identification of agents that may impact upon human health as mutagens and that the PIG-A gene can be a useful target for detection of mutation in both white and red blood cells.

Pharmacokinetics, dose-range, and mutagenicity studies of methylphenidate hydrochloride in B6C3F1 mice.

Methylphenidate hydrochloride (MPH) is one of the most frequently prescribed pediatric drugs for the treatment of attention deficit hyperactivity disorder. In a recent study, increased hepatic adenomas were observed in B6C3F1 mice treated with MPH in their diet. To evaluate the reactive metabolite, ritalinic acid (RA) of MPH and its mode of action in mice, we conducted extensive investigations on the pharmacokinetics (PK) and genotoxicity of the drug in B6C3F1 mice. For the PK study, male B6C3F1 mice were gavaged once with 3 mg/kg body weight (BW) of MPH and groups of mice were sacrificed at various time points (0.25-24 hr) for serum analysis of MPH and RA concentrations. Groups of male B6C3F1 mice were fed diets containing 0, 250, 500, 1,000, 2,000, or 4,000 ppm of MPH for 28 days to determine the appropriate doses for 24-week transgenic mutation studies. Also, the micronucleus frequencies (MN-RETs and MN-NCEs), and the lymphocyte Hprt mutants were determined in peripheral blood and splenic lymphocytes, respectively. Mice fed 4,000 ppm of MPH lost significant BW compared to control mice (P < 0.01). There was a significant increase in the average liver weights whereas kidneys, seminal vesicle, testes, thymus, and urinary bladder weights of mice fed higher doses of MPH were significantly lower than the control group (P < or = 0.05). There was no significant increase in either the Hprt mutant frequency or the micronucleus frequency in the treated animals. These results indicated that although MPH induced liver hypertrophy in mice, no genotoxicity was observed.

Frequency and spectrum of ENU-induced mutation in the Phi X174 transgene in mouse splenic lymphocytes and their significance to spontaneous transgenic rodent mutation frequencies.

A perceived disadvantage of transgenic rodent mutation assays is that spontaneous mutant frequencies are high compared to those of endogenous genes and may consequently reduce sensitivity to induced mutation. We have previously argued that unrepaired G:T mismatches from spontaneous deamination of 5-methylcytosine at CpG sites could be converted to apparent in vivo mutations in the bacterial recovery systems because of rapid, random, mismatch repair in Escherichia coli. In this study, we have measured mutation frequencies in spleen of male mice induced by N-ethyl-N-nitrosourea (ENU) using the PhiX174 transgene, which is not subject to mismatch repair in E.coli, using single-burst analysis, a unique method to identify in vivo mutation. In order to compare our results to those using the lacI and cII transgenes, we converted all mutant frequencies to base pair substitution (bps) mutation frequencies per nucleotide based on mutant spectra from this study and published literature. We found this frequency in control spleen to be similar for lacI (3.8 +/- 0.7 x 10(-8)) and PhiX174 (3.1 +/- 1.2 x 10(-8)) at 6 weeks of age. We found a strong age dependence for spontaneous lacI mutation that extrapolated to a value at conception (1.8 +/- 0.9 x 10(-8)) that was not significantly different from the human germ line bps mutation frequency per nucleotide of 1.7 +/- 0.2 x 10(-8). These two transgenes provided similar mutational responses to 40 mg/kg ENU, 7- to 9-fold. In contrast, the cII target gene in the same tissue produces both spontaneous and induced mutation frequencies approximately 10 times higher, for unknown reasons. We conclude that the spontaneous mutant frequencies measured by the lacI and PhiX174 transgenes in this moderately dividing tissue accurately measure in vivo mutation frequencies at early ages. For these two transgenes, seemingly high mutant frequencies may reflect the expected accumulation of somatic mutation with age.