Autoradiographic detection of 6-thioguanine-resistant lymphocytes of mice. A novel system in somatic mutagenesis testing

Circulating human B lymphocytes are deficient in nucleotide excision repair and accumulate mutations upon proliferation

Faithful repair of DNA lesions is a crucial task that dividing cells must actively perform to maintain genome integrity. Strikingly, nucleotide excision repair (NER), the most versatile DNA repair system, is specifically down-regulated in terminally differentiated cells. This prompted us to examine whether NER attenuation might be a common feature of all G0-arrested cells, and in particular of those that retain the capacity to reenter cell cycle and might thus convert unrepaired DNA lesions into mutations, a prerequisite for malignant transformation. Here we report that quiescent primary human B lymphocytes down-regulate NER at the global genome level while maintaining proficient repair of constitutively expressed genes. Quiescent B cells exposed to an environment that causes both DNA damage and proliferation accumulate point mutations in silent and inducible genes crucial for cell replication and differentiation, such as BCL6 and Cyclin D2. Similar to differentiated cells, NER attenuation in quiescent cells is associated with incomplete phosphorylation of the ubiquitin activating enzyme Ube1, which is required for proficient NER. Our data establish a mechanistic link between NER attenuation during quiescence and cell mutagenesis and also support the concept that oncogenic events targeting cell cycle- or activation-induced genes might initiate genomic instability and lymphomagenesis.

Strategies and testing methods for identifying mutagenic risks

The evolution of testing strategies and methods for identification of mutagenic agents is discussed, beginning with the concern over potential health and population effects of chemical mutagens in the late 1940s that led to the development of regulatory guidelines for mutagenicity testing in the 1970s and 1980s. Efforts to achieve international harmonization of mutagenicity testing guidelines are summarized, and current issues and needs in the field are discussed, including the need for quantitative methods of mutagenic risk assessment, dose-response thresholds, indirect mechanisms of mutagenicity, and the predictivity of mutagenicity assays for carcinogenicity in vivo. Speculation is offered about the future of mutagenicity testing, including possible near-term changes in standard test batteries and the longer-term roles of expression profiling of damage-response genes, in vivo mutagenicity testing methods, and models that better account for differences in metabolism between humans and laboratory model systems.

Bromodeoxyuridine labelling as an alternative method to identify 6-thioguanine-resistant mutant lymphocytes in humans

6-Thioguanine-resistant (TGR) mutant lymphocytes in human blood are usually enumerated by the cloning assay which allows the molecular characterisation of the HPRT mutations to be detected. A "short-term" alternative approach is provided by the anti-bromodeoxyuridine (anti-BrdU) technique in which TGR lymphocytes are identified immunocytochemically by their ability to synthesise DNA in the presence of 6-thioguanine (TG). We have evaluated the influence of various experimental factors that could affect the frequency of TGR lymphocytes. A standard protocol is proposed, based on 24-h cold storage of isolated lymphocytes at 4 degrees C and 40-h culture with and without TG, the last 16 h with BrdU. The harvested cells are treated with hypotonic (0.075 M) KCl, fixed with methanol:acetic acid (3:1) and put on microscopic slides. For the TG cultures, all cells are prepared on the slides, while slides from the control cultures are made by a 1/50 dilution. DNA is denatured by formamide, and the BrdU label is identified by anti-BrdU antibody detected by immunoperoxidase staining using a peroxidase-conjugated secondary antibody with diaminobenzidine as substrate. In 10 donors, the frequency of TGR lymphocytes (variant frequency, Vf) detected by this protocol ranged from 69.65 x 10(-6) to 83.45 x 10(-6), and split measurements showed a relatively small intra-assay variation in Vf values of each donor. BrdU in DNA was also detected by immunofluorescence using a fluorescein-conjugated anti-BrdU monoclonal antibody. This method, facilitating easy identification of positive cells and rapid microscopic scoring, may serve as a basis for an automated analysis of TGR lymphocytes. Vf values detected by the anti-BrdU assay are higher than mutant frequencies obtained by the cloning assay, which has been assigned to the presence of non-mutant phenocopies considered to represent spontaneously cycling lymphocytes. Although the anti-BrdU assay is rapid and easy and has been shown to respond to genotoxic exposures, its true value could be evaluated only when it can be ascertained that phenocopies do not significantly contribute to the Vf values obtained.

Development and utilization of the rat lymphocyte hprt mutation assay

Much of the recent progress in the field of genetic toxicology has come from an increased understanding of the molecular and cellular biology of the mammalian organism. Most prominent has been the ability to detect and quantify somatic mutation and relate the nature of the mutation to the specific type of chemical damage. Building upon the foundation of the human lymphocyte hypoxanthine guanine phosphoribosyl transferase (hprt) system, and later, the mouse hprt system, methods for the detection and quantification of hprt mutations in rat lymphocytes were developed. These methods are described in this report as is the ongoing validation of the assay. Additionally, the characterization of the recovered mutants and a comparison of the mutation spectrum in the rat lymphocyte system to the spectrum in cancer genes, such as H-ras and p53, and the spectrum in transgenic systems, such as lacI, are included. The development of the rat lymphocyte hprt system and validation of the assay at the molecular level, provide an effective and reliable measure of genetic damage in an in vivo system which is readily comparable to measurement of genetic damage in the human.

Dietary supplements of antioxidants reduce hprt mutant frequency in splenocytes of aging mice

The level of spontaneous and gamma-radiation-induced mutations in the hypoxanthine-guanine phosphoribosyl-transferase (hprt) locus as well as the decrease in frequency of these mutations in mice of various age pretreated with dietary supplements of an antioxidant mixture (vitamins C, E, beta-carotene, rutin, selenium, zinc) were studied in splenocytes of young (8-14-week-old) and aged (102-110-week-old) male C57BL/6 mice. The frequency of spontaneous mutations in splenocytes of 102-110-week-old mice was higher by 68-88% than that in mice aged 8-14 weeks. On gamma-irradiation (0.5-5.0 Gy) of mice, the frequency of radiation-induced mutations (Vf assay) in aged mice was 2.3 to 3.6 times (depending on dose) higher than in young ones. Daily supplements of an antioxidant mixture to the diet of mice prior to irradiation showed an antimutagenic effect. The values of mutant frequency reduction factor (MFRF) for 14-110-week-old mice fed with dietary antioxidants during 6 weeks prior to gamma-irradiation with doses of 2.0 and 5.0 Gy were 5.4 and 3.7, respectively. The frequency of radiation-induced mutations prevented or not prevented by antioxidants was much higher in aged mice than in young ones.

A mouse model for the study of in vivo mutational spectra: sequence specificity of ethylene oxide at the hprt locus

We have developed an approach for determining mutational spectra in exon 3 of the hypoxanthine-guanine phosphoribosyl transferase (hprt) gene in splenic T-lymphocytes of B6C3F1 mice. Hprt- mutants from treated animals were isolated by culturing splenic T-cells in microtiter dishes containing medium supplemented with IL-2, concanavalin A, and 6-thioguanine. DNA was extracted from 6-thioguanine-resistant colonies and amplified by the polymerase chain reaction (PCR) using primers flanking the exon 3 region of hprt. Identification of samples containing mutant exon 3 sequences and purification of mutant DNA from contaminating wild-type hprt DNA was accomplished using denaturing gradient gel electrophoresis. Purified mutant sequences were then sequenced. This approach is being used to study the sequence specificity of ethylene oxide (ETO). 12-day-old mice were given single i.p. injections of 100 mg ETO/kg every other day or 30, 60, 90 or 120 mg ETO/kg daily for 5 days to achieve different cumulative doses of this compound. In mice exposed every other day, cumulative doses of 200, 600 and 900 mg ETO/kg produced average mutant frequencies of 15 +/- 12.8, 45 +/- 13.2, and 73 (70, 75) x 10(-6), respectively, 8 weeks after the first treatment. In mice exposed daily, cumulative doses of 150, 300, 450 and 600 mg ETO/kg produced average mutation frequencies of 4.2 +/- 10.4, 8.2 +/- 10.4, 11.1 +/- 1.0 and 15.5 +/- 10.7 x 10(-6), respectively, 20 weeks after the first treatment. The mutant fraction in control mice was less than 3 x 10(-6). 123 hprt- mutants from mice exposed to 600 or 900 mg ETO/kg were isolated and analyzed for mutations in exon 3. 18 were located in exon 3 (14.6%). DNA sequencing revealed that 11/18 mutations were base-pair substitutions at 8 different sites in exon 3. Four AT transversions, three AT transitions, two GC transversions, and two GC transitions were observed. Three of the substitutions (2 AT-->CG, 1 AT-->GC) occurred at one base (203) in a single animal. The remaining 7 mutations, isolated from 4 different animals, were the same +1 frameshift mutation in a run of 6 consecutive guanine bases (207-212) in exon 3. These results suggest the involvement of both modified guanine and adenine bases in ETO mutagenesis. The mouse T-cell cloning/sequencing assay for hprt described here represents a useful system for studying the molecular mechanism of chemically-induced mutation occurring in vivo at an endogenous gene.

Use of a T-lymphocyte clonal assay for determining HPRT mutant frequencies in individual rats

Conditions for detection and isolation of HPRT- mutants in cloned rat T-lymphocytes from individual adult Lewis rats were determined. Similar to cloning of human T-cells, best results were obtained with lectin (PHA)-primed T-lymphocytes of rats. High cloning efficiencies, occasionally exceeding 50%, could be obtained when the target cells employed were isolated from cervical lymph nodes. Feeder cells used were splenocytes, irradiated with 40 Gy of X-rays after priming with Con A. Human interleukin-2, present in LAK supernatant, proved to be capable of inducing proliferative activity of rat T-lymphocytes and could replace conditioned medium from primed rat splenocytes. Under the conditions described in this paper, the frequency of mutants in the HPRT gene of T-lymphocytes in Lewis rats was about 80% lower than that found in human T-lymphocytes from adults. The inverse relationship between mutant frequency and cloning efficiency, clearly demonstrated for human data, could not be established for rats. Treatment of rats with N-ethyl-N-nitrosourea, a potent alkylating agent, resulted in a time- and dose-dependent induction of HPRT- mutants, demonstrating the usefulness of this system to study in vivo mutagenesis.

Detection of 6-thioguanine-resistant spleen lymphocytes in different mouse strains by autoradiography

The variant frequencies for 6-thioguanine-resistant spleen cells in different mouse strains have been estimated by autoradiography for animals without chemical treatment and in cases of in vivo mutagen dosage with ethylnitrosourea and cyclophosphamide, respectively. In untreated mice, the following variant frequencies have been found: C57Bl/6J, 2.84 x 10(-5);NMRI, 3.04 x 10(-5);DBA/2J, 5.91 x 10(-5). The selective concentration of 6-thioguanine was 100 microM for strains NMRI and DBA, while in the case of C57Bl with this concentration, no variant cells could be counted and a selective concentration of 50 microM was chosen. Treatment with 70, 140, and 210 mg/kg ethylnitrosourea resulted in increased variant frequencies in cells isolated 8 or 15 days later. On the other hand, doses of 20, 60, and 120 mg/kg cyclophosphamide did not result in a clear dose-response relationship of variant frequency in cells isolated 1, 8, 15, 22, and 29 days after treatment. These data are discussed with respect to findings in human populations exposed occupationally to cyclophosphamide.

A combined testing protocol for assessing genotoxicity in individual animals: application to environmental toxicology

A multiple end-point approach to assessing genetic toxicity (the combined testing protocol, CTP) was evaluated in male and female CD-1 mice exposed subacutely (3 and 6 weeks) to low levels of a custom-blended gas mixture (epichlorohydrin, benzene, chloroprene and xylene, at 50, 100, 100, and 100 ppb, respectively, as the low dose, with concentration levels 10-fold and 100-fold higher as the intermediate and high doses, or 0.1, 1 and 10 ppm of benzene). Urine mutagenicity was tested in the Salmonella/microsome assay, chromosome aberrations were examined in bone marrow and spleen lymphocytes, micronuclei were measured in bone marrow and peripheral erythrocytes, and cytochrome P450 and glutathione S-transferases were measured in the liver. Structural aberrations in alveolar macrophages and spermatocytes, and thioguanine resistance in spleen lymphocytes were examined for their suitability for incorporation into the overall protocol. Spleen lymphocytes were the most sensitive indicator cells, and showed a dose-related increase (P less than 0.01) in structural chromosome aberrations and in cytotoxicity after 6 weeks of exposure. Analysis of micronucleus formation and metaphase aberrations in the bone marrow, and micronuclei in peripheral erythrocytes showed an overall statistically non-significant but positive trend at the high dose. No mutagenicity was detected in pooled urine samples. Liver microsomal cytochrome P450 was not increased, but cytosolic glutathione S-transferases were significantly increased in a dose-related manner. Since the probability of detecting a genotoxic effect increases with the number of endpoints and tissues examined, this approach should be applicable to many situations without having to perform separate experiments for each tissue examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutagenicity monitoring in humans by autoradiographic assay for mutant T lymphocytes

Somatic cell mutation which occurs in vivo in humans can be determined by measurement of the frequency of the 6-thioguanine-resistant (TGr) T lymphocytes in samples of peripheral blood. This frequency can be determined by either a short-term autoradiographic methodology or a longer cell-cloning methodology. The advantage of the former is the relative simplicity of the assay, while the latter allows recovery of mutant clones for further characterization. This report presents results of a longitudinal study of cancer chemotherapy nurses and other health care personnel by use of the autoradiography assay. The use of this assay in human mutagenicity monitoring and the analysis of the TGr cell frequencies are discussed in terms of age effects and validation of 'elevated' frequencies by use of the clonal assay. This report then presents evidence that both assays yield similar TGr cell frequencies in two groups of 'normal adults'. The mean variant frequency (+/- S.D.) for 82 autoradiographic assays was 8.7 (+/- 6.1) X 10(-6), while the mean mutant frequency (+/- S.D.) for 115 clonal assays was 6.5 (+/- 4.8) X 10(-6). In addition, concurrent autoradiographic and clonal assays on 33 individuals yielded mean values (+/- S.D.) of 8.4 (+/- 8.5) X 10(-6) and 10.5 (+/- 6.3) X 10(-6), respectively.

6-Methylmercaptopurine riboside resistance in human lymphocytes in the in vivo somatic cell mutation test

Additional drug-resistance markers are being investigated to broaden the in vivo somatic cell mutation test in human lymphocytes (PBL). The adenosine kinase (AK) locus was chosen for study because Gupta and Singh [Gupta RS, Singh B: Mutat Res 113:441-454, 1983] have demonstrated that in Chinese hamster ovary cells, mutants affected at this locus are obtained at a very high spontaneous frequency and that the response of this locus to different types of mutagens was comparable to that of the hypoxanthine-guanine phosphoribosyl transferase locus. The adenosine analog 6-methylmercaptopurine riboside (MeMPR) was used as the selective agent for obtaining AK-deficient mutants. Cultures of mitogen-stimulated PBL were set up in the presence (test) and absence (control) of the selective agent. Resistant cells capable of synthesizing DNA in the presence of MeMPR were labeled with tritiated thymidine and enumerated autoradiographically. The variant frequency (Vf) was calculated as the ratio of the number of labeled nuclei in the test relative to that in the control. Human PBL were found to be sensitive to MeMPR inhibition of DNA synthesis and exhibited a dose-dependent decrease in Vf with increasing concentrations of MeMPR. However, no leveling off of the dose-response curve was observed. Thus the background level of Vf was probably lower than the practical detection limit of the test (4.0 X 10(-7) with a 50-ml blood sample). It was concluded that, because of the autosomal recessive nature of the AK gene, the background Vf in human PBL is too low to allow a useful baseline to be established for the in vivo somatic mutation test.

Direct and indirect flow cytometric enumeration of 6-thioguanine-resistant human peripheral blood lymphocytes

Methods based on flow cytometry and sorting, autoradiography, and cloning were used to evaluate the potential for the enumeration of 6-thioguanine-resistant human peripheral blood lymphocytes assumed to be deficient with respect to the enzyme hypoxanthine-guanine-phosphoribosyl-transferase. Flow cytometric sorting of proliferating cells in the late S- and the G2-stages by means of DNA content, as measured by propidium iodide fluorescence, enabled an enrichment of variant cells to about 99%. The main source of false events was contaminating doublets of G0/G1 cells appearing in the sorting region. Doublet discrimination measured as the difference between pulse height and area (Ortho-50) accomplished no further improvement. A combination of propidium iodide fluorescence and bromodeoxyuridine incorporation, measured by fluorescent anti-bromodeoxyuridine-DNA antibodies, allowed flow cytometric enrichment to about 99.99% of variant cells. By sorting of 3H-thymidine-labeled cell nuclei from the late S- and the G2-phases and subsequent autoradiographic evaluation, partly resistant variants could be discriminated; variant frequencies of the same magnitude as for the cell cloning methods were obtained.

A study of the frequency of sister-chromatid exchange and of thioguanine-resistant cells in mouse spleen lymphocytes after in vivo exposure to ethylnitrosourea

As part of the development of a multi-endpoint, in vivo, mouse model for mutagenesis we have measured the frequency of sister-chromatid exchange (SCE) and the frequency of thioguanine-resistant (TGr) cells among the lymphocytes of the mouse spleen following acute, intraperitoneal exposure to ethylnitrosourea (ENU). The responses of these two endpoints have been monitored both as a function of the dose of ENU injected, ranging from 0 to 70 mg/kg, and as a function of time after injection, from 1 day to 72 days. The SCE frequency response was highest 1 day after the ENU was injected, increasing 2.5-fold over control values for mice that received 70 mg/kg, and declined to control values in all animals by 72 days. SCE showed a linear dose response both at 1 day and 8 days after injection. The frequency of TGr cells was at control levels at 1 day, but at 15, 36 and 72 days after ENU injection the frequency of TGr cells showed a linear dose response. In addition, the frequency of TGr cells increased linearly with time for both the 35 and 70 mg/kg doses. The frequency of TGr cells for mice that had received 70 mg ENU/kg 72 days previously, was 100-fold higher than in control animals, giving a frequency of 1.4 X 10(-4).

Somatic gene mutations in vivo as indicated by the 6-thioguanine-resistant T-lymphocytes in human blood

The clonal and the autoradiographic assays for 6-thioguanine-resistant (TGr) T-lymphocytes (T-Lys) in human blood are reviewed. Studies of TGr colonies recovered from clonal assays show that the mutant T-Lys (i) are either helper (T4) or suppressor (T8) cells, (ii) possess stable TGr phenotypes, (iii) are deficient in hypoxanthine guanine phosphoribosyltransferase (HPRT), and (iv) have structural alterations in the hprt gene. TGr T-Ly mutant frequencies (Mfs) determined by clonal assays are of the order of 10(-6)-10(-5) for normal adults. Autoradiographically determined variant frequencies (Vfs) are also in this range for normal adults when lymphocytes are cryopreserved before study to remove 'phenocopies'. Cancer exposed to potentially mutagenic treatments have elevated TGr T-Ly Vfs. Comparative clonal and autoradiographic assays of the same blood samples give generally similar results when allowances are made for potential sources of error in each assay. The TGr T-Ly system is presented for human specific-locus mutagenicity monitoring.

A method to quantify spontaneous and in vivo induced thioguanine-resistant mouse lymphocytes

A clonogenic assay to quantify thioguanine (TG)-resistant (TGr) spleen lymphocytes in the mouse has been developed to support studies of in vivo mutation affecting the hypoxanthine phosphoribosyltransferase (hprt) locus. Lymphocytes are cultured in 96-well microtiter plates for 9 days with proliferation initiated by the mitogen concanavalin A and supported thereafter by conditioned medium containing interleukin-2. Lymphocytes are plated at high densities (4-8 X 10(5)/well) with TG and irradiated L5178Y lymphoma cells (10(4)/well) to detect the presence of TGr cells. To determine the cloning efficiency without TG lymphocytes are plated at a low density (10/well) with irradiated L5178Y cells and irradiated lymphocytes (4-8 X 10(5)/well). Proliferation of cells is detected by [3H]thymidine incorporation and scintillation spectrometry. Spontaneous frequencies of TGr clones are independent of TG dose from 0.2 to 10 micrograms/ml and independent of cell density over the range cited. The TGr clones tested have less than 10% hypoxanthine incorporation in vivo relative to unselected clones and have stable phenotypes in the absence of selection. The spontaneous frequency of TGr cells ranged from 1 to 3 X 10(-6). In vivo treatment of mice intraperitoneally with ethylnitrosourea 15 days prior to in vitro culture resulted in a linear dose-related increase of TGr cells, with 70.2 mg/kg inducing a frequency of TGr cells of 2 X 10(-5).

Diphtheria toxin resistance in human lymphocytes and lymphoblasts in the in vivo somatic cell mutation test

It has been shown that circulating peripheral blood lymphocytes can be used for the enumeration of 6-thioguanine-resistant cells that presumably arise by mutation in vivo. This somatic cell mutation test has been studied in lymphocytes from human populations exposed to known mutagens and/or carcinogens. The sensitivity of the test could be further enhanced by including other gene markers, since there is evidence for locus-specific differences in response to mutagens. Resistance to diphtheria toxin (Dipr) seemed like a potential marker to incorporate into the test because the mutation acts codominantly, can readily be selected in human diploid fibroblasts and Chinese hamster cells with no evidence for cell density or cross-feeding effects, and can be assayed for in nondividing cells by measuring protein synthesis inhibition. Blood samples were collected from seven individuals, and fresh, cryopreserved, or Epstein-Barr virus (EBV)-transformed lymphocytes were tested for continued DNA synthesis (3H-thymidine, autoradiography) or protein synthesis (35S-methionine, scintillation counting). Both fresh and cryopreserved lymphocytes, stimulated to divide with phytohemagglutinin (PHA), continued to synthesize DNA in the presence of high doses of diphtheria toxin (DT). Similarly, both dividing (PHA-stimulated) and nondividing fresh lymphocytes carried on significant levels of protein synthesis even 68 hr after exposure to 100 flocculating units (LF)/ml DT. The findings were confirmed in cord blood lymphocytes, ruling out the possibility that diphtheria immunization could have led to a selection of Dipr lymphocytes. One lymphoblast line (EBV-transformed lymphocytes) showed a reduction in protein synthesis to 0.2% of controls only at 192 hr after exposure to 100 LF/ml. The results suggest that human T and B lymphocytes may not be as sensitive to DT protein synthesis inhibition as human fibroblast and Chinese hamster cells. For this reason, Dipr may not be a suitable marker for the somatic cell mutation test.