Suspected Lynch syndrome associated MSH6 variants: A functional assay to determine their pathogenicity

Lynch syndrome (LS) is a hereditary cancer predisposition caused by inactivating mutations in DNA mismatch repair (MMR) genes. Mutations in the MSH6 DNA MMR gene account for approximately 18% of LS cases. Many LS-associated sequence variants are nonsense and frameshift mutations that clearly abrogate MMR activity. However, missense mutations whose functional implications are unclear are also frequently seen in suspected-LS patients. To conclusively diagnose LS and enroll patients in appropriate surveillance programs to reduce morbidity as well as mortality, the functional consequences of these variants of uncertain clinical significance (VUS) must be defined. We present an oligonucleotide-directed mutagenesis screen for the identification of pathogenic MSH6 VUS. In the screen, the MSH6 variant of interest is introduced into mouse embryonic stem cells by site-directed mutagenesis. Subsequent selection for MMR-deficient cells using the DNA damaging agent 6-thioguanine (6TG) allows the identification of MMR abrogating VUS because solely MMR-deficient cells survive 6TG exposure. We demonstrate the efficacy of the genetic screen, investigate the phenotype of 26 MSH6 VUS and compare our screening results to clinical data from suspected-LS patients carrying these variant alleles.

The colorectal and endometrial cancer predisposition Lynch syndrome (LS) is caused by an inherited heterozygous defect in one of four DNA mismatch repair (MMR) genes. Deleterious mutations (e.g., protein-deleting or -truncating) in DNA MMR genes unambiguously allow for the clinical diagnosis LS and hence enable appropriate surveillance measures to be taken to reduce cancer risk and ensure early detection of tumors. However, currently about one-third of detected MMR gene variants are subtle with less clear functional consequences: missense mutations affecting a single amino acid may be innocuous, hence not causing LS, or partially or fully destroy protein function. As long as uncertainty exists about their pathogenicity, such mutations are labeled ‘variants of uncertain (clinical) significance’ (VUS). VUS hamper genetic counseling and therefore the need for functional testing of VUS is widely recognized. To functionally annotate MMR gene VUS, we have developed a high content cellular assay in which the VUS is introduced in a cell culture by oligonucleotide-directed gene modification. Should the VUS be deleterious for MMR, the modified cells survive exposure to the guanine analog 6-thioguanine (6TG) and 6TG-resistant colonies appear. Should the mutation not affect MMR, no colonies appear. Here we present the adaptation and application of this protocol to the functional annotation of variants of the MMR gene MSH6. Implementation of our assay in clinical genetics laboratories will provide clinicians with information for proper counseling of mutation carriers and treatment of their of tumors.

Differential Effects of Targeted Disruption of Thiopurine Methyltransferase on Mercaptopurine and Thioguanine Pharmacodynamics

The recessive deficiency in thiopurine methyltransferase (TPMT), caused by germ-line polymorphisms in TPMT, can cause severe toxicity after mercaptopurine. However, the significance of heterozygosity and the effect of the polymorphism on thioguanine or in the absence of thiopurines is not known. To address these issues, we created a murine knockout of Tpmt. Pharmacokinetic and pharmacodynamic studies of mercaptopurine and thioguanine were done in Tpmt(-/-), Tpmt(+/-), and Tpmt(+/+) mice and variables were compared among genotypes. Methylated thiopurine and thioguanine nucleotide metabolites differed among genotypes after treatment with mercaptopurine (P < 0.0001 and P = 0.044, respectively) and thioguanine (P = 0.011 and P = 0.002, respectively). Differences in toxicity among genotypes were more pronounced following treatment with 10 daily doses of mercaptopurine at 100 mg/kg/d (0%, 68%, and 100% 50-day survival; P = 0.0003) than with thioguanine at 5 mg/kg/d (0%, 33%, and 50% 15-day survival; P = 0.07) in the Tpmt(-/-), Tpmt(+/-), and Tpmt(+/+) genotypes, respectively. Myelosuppression and weight loss exhibited a haploinsufficient phenotype after mercaptopurine, whereas haploinsufficiency was less prominent with thioguanine. In the absence of drug challenge, there was no apparent phenotype. The murine model recapitulates many clinical features of the human polymorphism; indicates that mercaptopurine is more affected by the TPMT polymorphism than thioguanine; and provides a preclinical system for establishing safer regimens of genetically influenced antileukemic drug therapy.

DNA mismatch repair and Chk1-dependent centrosome amplification in response to DNA alkylation damage

Centrosome amplification is frequently observed in tumor cells exposed to genotoxic stress, however the underlying mechanisms and biological consequences are poorly understood. Here, we show that the anti-metabolite and alkylating agent 6-thioguanine (6-TG) induces centrosome amplification resulting in the formation of multi-polar spindles when damaged cells subsequently enter mitosis. These aberrant, multi-polar mitoses are frequently resolved by asymmetric cell divisions causing unequal segregation of genetic material and cell death in one or both daughter products. We show that this phenomenon is associated with transient cell cycle delay in S- and G(2)-phase and is dependent on DNA mismatch repair (DNA MMR) proficiency and Chk1 protein kinase activity. Although Chk1-deficient cells do not exhibit cell cycle delay, centrosome amplification, or multi-polar spindle formation, continued cell cycle progression in the presence of 6-TG eventually results in increased levels of mitotic catastrophe, most probably due to mitosis with incompletely replicated DNA. Taken together, these results reveal novel mechanisms of cell killing by 6-TG and underscore the importance of interactions between cell cycle checkpoints and DNA MMR in determining the fate of cells bearing DNA damage.

Novel DNA lesions generated by the interaction between therapeutic thiopurines and UVA light

The therapeutic effect of the thiopurines, 6-thioguanine (6-TG), 6-mercaptopurine, and its prodrug azathioprine, depends on the incorporation of 6-TG into cellular DNA. Unlike normal DNA bases, 6-TG absorbs UVA radiation, and UVA-mediated photochemical damage of DNA 6-TG has potentially harmful side effects. When free 6-TG is UVA irradiated in solution in the presence of molecular oxygen, reactive oxygen species are generated and 6-TG is oxidized to guanine-6-sulfonate (G(SO3)) and guanine-6-thioguanine in reactions involving singlet oxygen. This conversion is prevented by antioxidants, including the dietary vitamin ascorbate. DNA G(SO3) is also the major photoproduct of 6-TG in DNA and it can be selectively introduced into DNA or oligonucleotides in vitro by mild chemical oxidation. Thermal stability measurements indicate that G(SO3) does not form stable base pairs with any of the normal DNA bases in duplex oligonucleotides and is a powerful block for elongation by Klenow DNA polymerase in primer extension experiments. In cultured human cells, DNA damage produced by 6-TG and UVA treatment is associated with replication inhibition and provokes a p53-dependent DNA damage response.

Development of a large-scale chemogenomics database to improve drug candidate selection and to understand mechanisms of chemical toxicity and action

Successful drug discovery requires accurate decision making in order to advance the best candidates from initial lead identification to final approval. Chemogenomics, the use of genomic tools in pharmacology and toxicology, offers a promising enhancement to traditional methods of target identification/validation, lead identification, efficacy evaluation, and toxicity assessment. To realize the value of chemogenomics information, a contextual database is needed to relate the physiological outcomes induced by diverse compounds to the gene expression patterns measured in the same animals. Massively parallel gene expression characterization coupled with traditional assessments of drug candidates provides additional, important mechanistic information, and therefore a means to increase the accuracy of critical decisions. A large-scale chemogenomics database developed from in vivo treated rats provides the context and supporting data to enhance and accelerate accurate interpretation of mechanisms of toxicity and pharmacology of chemicals and drugs. To date, approximately 600 different compounds, including more than 400 FDA approved drugs, 60 drugs approved in Europe and Japan, 25 withdrawn drugs, and 100 toxicants, have been profiled in up to 7 different tissues of rats (representing over 3200 different drug-dose-time-tissue combinations). Accomplishing this task required evaluating and improving a number of in vivo and microarray protocols, including over 80 rigorous quality control steps. The utility of pairing clinical pathology assessments with gene expression data is illustrated using three anti-neoplastic drugs: carmustine, methotrexate, and thioguanine, which had similar effects on the blood compartment, but diverse effects on hepatotoxicity. We will demonstrate that gene expression events monitored in the liver can be used to predict pathological events occurring in that tissue as well as in hematopoietic tissues.

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone [MX] shows initiating and promoting activities in a two-stage BALB/c 3T3 cell transformation assay

A transformation assay using BALB/c 3T3 cells was conducted on 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) to assess initiation and promotion activities of MX carcinogenesis. Statistically significant positive responses were obtained compared with the corresponding solvent controls in both the initiation assay post-treated with 12-O-tetradecanoylphorbol 13-acetate (TPA) and the promotion assay pretreated with 3-methylcholanthrene (MCA). Both TPA and MX inhibited metabolic cooperation in an assay using co-culture of V79 6-thioguanine (6-TG) sensitive and insensitive cells. However, cells isolated from transformed foci in the initiation assay did not induce any nodules after inoculation to BALB/c mice, the strain of mouse from which the transformation assay cells were derived. Although the study was carried out for 2-3 weeks, this might have been too short to develop nodules under the conditions of this experiment. This in vitro cell transformation study with MX adds supportive information to studies showing MX carcinogenicity and tumour promoter activity, and adds mechanistic understanding of the action of MX.

Effect of 6-thioguanine on the stability of duplex DNA

The incorporation of 6-thioguanine (S6G) into DNA is a prerequisite for its cytotoxic action, but duplex structure is not significantly perturbed by the presence of the lesion [J. Bohon and C. R. de los Santos (2003) Nucleic Acids Res., 31, 1331–1338]. It is therefore possible that the mechanism of cytotoxicity relies on a loss of stability rather than a pathway involving direct structural recognition. The research described here focuses on the changes in thermodynamic properties of duplex DNA owing to the introduction of S6G as well as the kinetic properties of base pairs involving S6G. Replacement of a guanine in a G•C pair by S6G results in ∼1 kcal/mol less favorable Gibbs free energy of duplex formation at 37°C. S6G•T and G•T mismatch-containing duplexes have almost identical Gibbs free energy at 37°C, with values ∼3 kcal/mol less favorable than that of the control. Base pair stability is affected by S6G. The lifetime of the normal G•C base pair is ∼125 ms, whereas that of the G•T mismatch is below the detection limit. The lifetimes of S6G•C and S6G•T pairs are ∼7 and 2 ms, respectively, demonstrating that, although S6G significantly decreases the stability of the pairing with cytosine, it slightly increases that of a mismatch.

Portal hypertension develops in a subset of children with standard risk acute lymphoblastic leukemia treated with oral 6-thioguanine during maintenance therapy

BACKGROUND

6-Thioguanine (TG) was recently studied to determine whether TG in maintenance therapy achieves better event free survival than 6-mercaptopurine (MP) for standard risk acute lymphoblastic leukemia (ALL) on the clinical trial, CCG-1952 (5/1996-1/2000). Veno-occlusive disease was previously recognized as a complication of TG on CCG-1952. We report a newly recognized pediatric complication of TG: splenomegaly and portal hypertension (PH) developing during maintenance or after completion of therapy.

PROCEDURE

Twelve patients (3-10 years) had been randomized to receive a targeted dose of 50 mg/m(2)/day of TG during maintenance phases. Actual TG dose ranged from 25 to 77 mg/m(2)/day (median 34 mg/m(2)/day).

RESULTS

The initial patient, a boy who had marked thrombocytopenia and intermittent splenomegaly during maintenance therapy, was evaluated for persistent pancytopenia and progressive splenomegaly 3 months after completion of therapy. Dilated splenic vein and collaterals consistent with PH were documented by MRI/MRA. Esophagogastroduodenoscopy found esophageal varices. Liver biopsy showed periportal fibrosis and marked dilatation of veins and venules. Of the other 12 patients, 9 patients studied had abnormal MRI/MRAs with evidence of varices in 4. Eight patients had splenomegaly on physical examination. Liver biopsies in a girl after 3.3 courses of TG and a boy after 4.6 courses of TG showed periportal fibrosis and dilatation of venules and sinusoids and minimal focal fatty changes. Subsequent MRI/MRAs have been stable or improved.

CONCLUSIONS

The evaluations of these 12 patients suggest that treatment with TG causes injury to the liver leading to PH and that thrombocytopenia and splenomegaly are clinical hallmarks of this toxicity.

DNA mismatch repair (MMR) mediates 6-thioguanine genotoxicity by introducing single-strand breaks to signal a G2-M arrest in MMR-proficient RKO cells

PURPOSE

The DNA mismatch repair (MMR) system plays an important role in mediating cell death after treatment with various types of chemotherapeutic agents, although the molecular mechanisms are not well understood. In this study, we sought to determine what signal is introduced by MMR after 6-thioguanine (6-TG) treatment to signal a G(2)-M arrest leading to cell death.

EXPERIMENTAL DESIGN

A comparison study was carried out using an isogenic MMR(+) and MMR(-) human colorectal cancer RKO cell system, which we established for this study. Cells were exposed to 6-TG (3 micro M x 24 h) and then harvested daily for the next 3-6 days for growth inhibition assays. Cell cycle effects were determined by flow cytometry, and DNA strand breaks were measured using pulsed-field gel electrophoresis and alkaline Comet assays.

RESULTS

We first established MMR(+) RKO cell lines by transfection of human MutL homologue 1 (hMLH1) cDNA into the hMLH1-deficient (MMR(-)) RKO cell line. The ectopically expressed hMLH1 protein restored a MMR-proficient phenotype in the hMLH1(+) transfectants, showing a significantly increased and prolonged G(2)-M arrest followed by cell death after 6-TG exposure, compared with the vector controls. The MMR-mediated, 6-TG-induced G(2)-M arrest started on day 1, peaked on day 3, and persisted to day 6 after 6-TG removal. We found that DNA double-strand breaks were comparably produced in both our MMR(+) and MMR(-) cells, peaking within 1 day of 6-TG treatment. In contrast, single-strand breaks (SSBs) were more frequent and longer lived in MMR(+) cells, and the duration of SSB formation was temporally correlated with the time course of 6-TG-induced G(2)-M arrest.

CONCLUSIONS

Our data suggest that MMR mediates 6-TG-induced G(2)-M arrest by introducing SSBs to signal a persistent G(2)-M arrest leading to enhanced cell death.

[Analysis of causes of death during intensive chemotherapy according to treatment protocol AML-BFM 93]

BACKGROUND

During intensive chemotherapy for AML, more than 10% of patients die because of treatment complications but not because of progression of their underlying disease. In order to improve supportive care and to decrease mortality, we analysed the causes of death and their relationship to the cycles of chemotherapy in children undergoing treatment for AML according to the study AML-BFM 93.

RESULTS

Thirty-five (7.4%) of a total of 471 patients treated according to protocol AML-BFM 93 died before or within the first 6 weeks after diagnosis (early death). Fourty-nine patients (10%) did not achieve remission, and 18 (4 %) died of therapy-related complications after having achieved remission. In comparison to earlier AML-BFM studies, early mortality was reduced from 13%, 12%, 9% (AML-BFM 78, 83, 87) to 7% (AML-BFM 93, p-trend = 0.03). In contrast, mortality of patients in complete continuous remission (CCR) did not change. Infectious complications, in particular due to bacterial and fungal pathogens, were the main cause of death. One patient died of arrhythmia associated with SIAD. After stem-cell transplantation in first remission, 7 of 51 patients died, mainly because of graft-versus-host-disease and/or infections. The incidence of infectious complications decreased with the number of chemotherapy cycles and was highest during induction therapy. Fatal complications occurred in one patient during maintenance therapy and in one patient thereafter; both patients were in CCR. Another 14 patients died during intensive therapy (before day 150) mostly with a low percentage of blasts, but no haematologic recovery. The cause of death in these children was mainly bacterial infection or invasive aspergillosis, but seldom progression of leukaemia.

CONCLUSION

This analysis confirmed the high incidence of fatal infections in children with AML during chemotherapy-induced severe neutropenia. To increase overall survival in children undergoing therapy for AML, we propose (1) to improve the prophylactic and therapeutic measures for haemorrhage and infections, (2) to continue risk-adapted therapy and (3) to treat high-risk patients in specialised centres only.

Hamster and rat fetal cells have low spontaneous mutation frequencies and rates

Somatic cells of whole Syrian hamster fetuses (gestation day 13) were isolated and tested by an in vivo/in vitro mutation assay for spontaneous mutation frequencies using independent 6-thioguanine (6-TG), diphtheria toxin (DT), and ouabain mutation selection systems. Optimum conditions were ascertained. For 6-TG mutants, a total of 21 mutants were found in cells from 24 litters on 1993 plates, for an overall mutant frequency of 1.8 x 10(-7) per viable cell with 12 positive litters. In all, 26 litters were tested using DT; 77 mutants were found in 840 plates, yielding an overall mutant frequency of 2.6 x 10(-7), with 20 positive litters. No correlations or familial effects were found among 23 litters tested for both DT and 6-TG. Of 14 litters which were tested for ouabain mutants, 4 were positive, with a total of 5 mutants found on 988 plates, for an overall mutant frequency of 7.6 x 10(-8). For 14 F344 rat fetuses, the overall 6-TG spontaneous mutation frequency was determined to be 1.6 x 10(-7). From the data, estimates of mutation rates were calculated. For mutation to 6-TG resistance the rate was 8.3 x 10(-8), for mutation to DT resistance the rate was 8.1 x 10(-8) and for ouabain, the spontaneous mutation rate was 5.7 x 10(-8). For F344 rat, the spontaneous mutation rate was 1.1 x 10(-7). Induced mutant frequencies after in utero exposure to 1 mmol/kg N-ethyl-N-nitrosourea (ENU) were 311, 135 and 200 times the spontaneous value for 6-TG, DT and ouabain, respectively, for Syrian hamster fetal cells and 125 times the spontaneous 6-TG value for fetal F344 rat cells. Both spontaneous mutation frequencies and underlying spontaneous mutation rates are low, consistent with the view that fetal cells exercise extremely tight control over DNA fidelity.

In vivo mutant frequency of thioguanine-resistant T-cells in the peripheral blood and lymph nodes of melanoma patients

T-cell activation by malignant melanoma would be anticipated to stimulate T-cell proliferation, which in turn has been associated with increasing the likelihood of somatic gene mutation. The purpose of this study was to test the hypothesis that in vivo hypoxanthine guanine phosphoribosyltransferase (hprt) mutant frequencies (MFs) are increased in peripheral blood T-cells from melanoma patients compared to normal controls. Assays were made of 48 peripheral blood samples from melanoma patients with stage 3 (13 patients) and stage 4 (35 patients) disease, 38 normal controls, and of nine tumor bearing lymph nodes. The mean hprt log(10)(MF) in patient peripheral blood was -4.77 (geometric mean hprt MF=17.0x10(-6)) compared to a mean hprt log(10)(MF) of -4.87 (geometric mean hprt MF=13.5x10(-6)) in controls. Although modest, this difference is statistically significant both by t-test (P=0.049) and after adjustment for covariates of age, gender, and cigarette smoking by regression analysis (P=0.001). Among the melanoma patients, the mean log(10)(MF) for the 17 patients who had received potentially genotoxic therapies was not significantly different from the mean log(10)(MF) for the 31 patients not receiving such therapies. The hprt MFs in the nine tumor bearing nodes were compared with MFs in peripheral blood from the same patients and revealed a non-significant (P=0.07) trend for increasing MFs in blood. Furthermore, analyses of T-cell receptor gene rearrangement patterns revealed hprt mutants originating from the same in vivo clone in both peripheral blood and a tumor-bearing node. The finding of elevated hprt MFs not entirely explained by genotoxic therapies in patients compared to controls can be explained either by hypermutability or in vivo T-cell activation. The similar MFs in peripheral blood and tumor bearing lymph nodes, as well as the finding of mutant representatives of the same in vivo T-cell clone in both locations, support monitoring peripheral blood to detect events in the nodes. If in vivo proliferation accounts for the current findings, the hprt deficient (hprt-) mutant fraction in blood may be enriched for T-cells that mediate the host immune response against malignant melanoma. Further studies will characterize the functional reactivity of hprt mutant isolates against melanoma-related antigens.

Palliative cytoreduction in refractory acute leukemia: a retrospective study of 57 adult patients

The efficiency and toxicity of treatment regimens for nonintensive cytoreduction in 57 outpatients with refractory acute leukemia (mean age 56 years, 51 AML, six ALL/AUL) were retrospectively studied. Seventeen patients received one treatment regimen, 19 patients two treatment regimens, and 21 patients three or more treatment regimens. The treatment regimens analyzed were 6-thioguanine p.o. (daily) (T), 6-thioguanine p.o. (4-7 days/week) + cytarabine s.c./i.v. (once a week) (T+C), 6-mercaptopurine p.o. (daily) (MP), 6-mercaptopurine p.o. (daily) + methotrexate p.o./i.v. (once a week) (MP+MTX), etoposide p.o. (daily) (E), and mitoxantrone i.v. (M). The median leukocyte count was higher for M (73 x 10(9)/l) than for the other treatment regimens (T: 27 x 10(9)/l, T+ C: 37 x 10(9)/l, MP: 24 x 10(9)/l, MP + MTX: 30 x 10(9)/l, E: 31 x 10(9)/l). A cytoreduction >50% in the peripheral blood was achieved by T in 11/19, by T+C in 7/11, by MP in 5/8, by MP+MTX in 3/6, by E in 3/4, and by M in 16/22 patients. The period of cytoreduction was regarded as the duration of response - T: median 53 days, range 5-98; T+C: median 61 days, range 14-226; MP: median 37 days, range 4-192; MP + MTX: median 58 days, range 36-59; E: median 121 days, range 26-159; M: median 39 days, range 8-78. T and T + C were well tolerated by all but three patients (stomatitis, diarrhea, WHO grade 2). MP was accompanied by a rise of transaminases (WHO 1-3) in 5/6 patients. E led to stomatitis (WHO 1,2) in 4/5 and M to nausea/vomiting (WHO 1,2) in 5/22 and to stomatitis (WHO 2) in 4/22 cases. The mean survival time after start of palliative cytoreduction was 16 weeks (2-65). In summary, 6-thioguanine +/- cytarabine was best tolerated with effective but in oral monotherapy - often protracted cytoreduction in 60% of patients. Mitoxantrone showed tolerable side effects and potent cytoreduction in 73% of patients even after ineffective palliative pretreatment. Palliative cytoreductive therapy does not reduce the quality of life and can prevent complications of significant leukocytosis in refractory acute leukemia.

A sensitive and versatile bioassay for ligands that signal through receptor clustering

The induced expression of xanthine-guanine phosphoribosyl transferase (XGPRT) by low concentrations (-2 pg/ml) of interferon-alpha (IFN-alpha) or IFN-beta, in the 2fTPGH cell line caused a 50% cytotoxicity when these cells were grown in medium containing 6-thioguanine. We extended the application of this sensitive, reliable, and easy bioassay to other members of the cytokine family. To activate the IFN signaling pathway, we made receptor chimeras, consisting of the IFN type I receptor intracellular and transmembrane domains, fused to either the interleukin-5 (IL-5) receptors or erythropoietin (Epo) receptor extracellular domains as model systems. 2fTGH cells, stably transfected with these receptor chimeras, responded to very low concentrations of IL-5 or Epo (IC50 values of approximately 15 pg and 3 pg/ml, respectively) and thus can be used as a very sensitive bioassay for both ligands. Background activity of IL-5, Epo, tumor necrosis factor (TNF), IL-6, or leptin on cells that did not carry the receptor chimeras was very low. This methodology can in principle be extended to any ligand that acts via clustering of its receptors.

Analysis of point mutations in HPRT mutant T-lymphocytes derived from coke-oven workers

We studied mutations in exon 3 of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus in 113 6-thioguanine-resistant T-cell clones derived from coke-oven workers and control subjects in order to analyse possible changes in the mutational spectrum associated with the exposure to polycyclic aromatic hydrocarbons (PAHs). In 99 mutants, HPRT exon 3 was analysed by means of genomic polymerase chain reaction (PCR) and single-strand conformation polymorphism (SSCP). Products for which SSCP indicated the presence of a mutation were further analysed by DNA sequencing. In addition, HPRT cDNA from 14 clones was analysed by reverse transcription (RT) PCR and DNA sequencing. In total, 18/113 mutants (16%) had a mutation in exon 3. This frequency was similar in PAH-exposed (9/57) and non-exposed (9/56) subjects. Base substitutions caused 14 mutations at 13 different sites. Three +/- 1 bp frameshifts and one 6 bp deletion were identified. No significant differences between PAH-exposed and non-exposed workers were observed in this limited mutational spectrum. These results indicate that deletions/insertions at the HPRT exon 3 account for 22% of the mutations, and base substitutions for 78%.

HPRT mutations in vivo in human CD 34+ hematopoietic stem cells

The HPRT mutations in T lymphocytes are widely utilized as biomarkers of environmental exposure and effect. The HPRT gene detects a wide variety of mutation types, many of which are similar at the molecular level to those found in oncogenes in cancers. However, it remains to be determined whether the assay for mutations in T lymphocytes is reflective of mutagenic events in tissues or cells which have high frequencies of malignancy in humans. We now demonstrate that the HPRT gene can be utilized to detect mutations in myeloid stem cells, which are frequent progenitor cells of leukemias. This myeloid stem cell assay shows an age related increase in mutation at HPRT and also detects increases in mutant frequency (M-MF) in patients who have undergone chemotherapy. The myeloid mutants are confirmed to have mutations in the HPRT gene by DNA sequence analysis. Increases in M-MF are seen as expected in the clonally unstable myeloid stem cells of patients with myelodysplastic syndromes; however, unexpectedly these patients also have elevated T-lymphocyte mutant frequencies (T-MF). A good correlation is shown between M-MFs and T-MFs in the same patients. Thus, it appears that the T-lymphocyte assay, which is technically much less demanding than the myeloid assay, appears to faithfully represent the frequency of mutagenic events in the myeloid lineage.

Influence of smoking and donor age on the spectrum of in vivo mutation at the HPRT-locus in T lymphocytes of healthy adults

Types and frequencies of in vivo mutation in the hypoxanthine-guanine phosphoribosyl-transferase (HPRT) gene was studied in 142 T cell mutants from 78 healthy nonsmoking and smoking adults with a mean of 65 years. The HPRT mutant frequency in the nonsmokers was 18.7 +/- 12.0 x 10(-6), and in the smokers 26.6 +/- 18.5 x 10(-6) (mean +/- S.D., P < 0.01). Among 107 single base pair substitutions (SBS) in the coding region of the HPRT gene, one new mutable site, one novel nonsense mutation and three not previously reported SBS were identified. Transitions accounted for 59% of the SBS and transversions for 41%. GC > AT transitions were the predominant type of mutation, with 50% of all SBS. The mutations showed a nonrandom distribution along the coding sequence, with three significant hotspots at positions 143, 197 and 617 (13, 14 and 7 mutations, respectively). There was no difference between smokers and nonsmokers with regard to the distribution of mutations at these hotspot positions. However, 85% of the mutations at GC base pairs and 88% of the mutations at AT base pairs in smokers occurred at sites with guanine or thymine, respectively, in the nontranscribed DNA strand. Moreover, smokers had a higher frequency of transversions and lower frequency of transitions than nonsmokers did. Particularly, GC > TA transversions were increased in smokers (11%) compared to nonsmokers (2%), which suggests that tobacco-smoke induced adducts at guanine bases in the nontranscribed DNA strand contributes to the increase of HPRT mutation in smokers. Overall, these results were very similar to the mutational spectra in two younger study populations reported previously [K.J. Burkhart-Schultz, C.L. Thompson, I.M. Jones, Spectrum of somatic mutation at the hypoxanthine phosphoribosyltransferase (HPRT) gene of healthy people, Carcinogenesis 17 (1996) 1871-1883; A. Podlutsky, A.-M. Osterholm, S.-M. Hou, A. Hofmaier, B. Lambert, Spectrum of point mutations in the coding region of the hypoxanthine-guanine phosphoribosyltransferase, Carcinogenesis 19 (1998) 557-566]. With the possible exception of an increase of mutations at hotspot position 143, and a decrease of 5-methylcytosine deamination mediated transitions at CpG-sites in the older individuals, there were no differences between the mutational spectra of old and young adults. In conclusion, both smoking and ageing seem to have minor influences on the spectrum of HPRT mutation in T cells.

In vitro cytotoxic drug activity and in vivo pharmacokinetics in childhood acute myeloid leukemia

Since May 1996 all Nordic countries have been participating in a study of childhood acute myeloid leukemia (AML). The aim is to correlate the in vitro sensitivity of leukemic cells and individual plasma concentrations of cytotoxic drugs with clinical effect. Blast cells from bone marrow and/or peripheral blood are tested against a panel of cytotoxic agents using the fluorometric microculture cytotoxicity assay (FMCA). Plasma concentrations of cytotoxic drugs are analysed during induction therapy. Bone marrow samples from the participating centres generally reached the analysing laboratory within 24 hours. 61 out of 71 (86%) samples were successfully analysed, 47 de novo AML and 14 relapses. Relapsing patients tended to have a more resistant test profile than newly diagnosed patients. Steady state plasma levels of doxorubicin, etoposide and 6-thioguanine nucleotide varied about 10-fold between patients. The intra-individual variation was much less, suggesting that dose adjustment based on pharmacokinetic data might be useful in the future.

Comparison of BCL-2 and BAX protein expression with in vitro sensitivity to ARA-C and 6TG in AML

Activity of BCL-2 protein may be antagonised by BAX protein expression, thereby affecting cellular sensitivity to chemotherapeutic drugs. We analysed the BCL-2 protein expression of blast cells from 19 patients by flow cytometry and immunocytochemistry. This was compared to in vitro sensitivity to the anthracyclines and antimetabolites using the MTT assay. We found a significant correlation between BCL-2 expression and in vitro response to two antimetabolite drugs. One of 7 patients (14%) whose cells were sensitive to ara-C expressed BCL-2 compared to 4/4 patients (100%) whose cells were resistant to ara-C in vitro (p = 0.05). Furthermore, none of the three patients whose cells were sensitive to 6-TG expressed BCL-2 compared to 6/9 patients (67%) whose cells were resistant in vitro (p = 0.045). We found no other correlation between BCL-2 expression and any other chemotherapeutic drug analysed. The ratio of BCL-2 to BAX may be more relevant clinically, therefore cells from a further 9 patients were analysed for both proteins. Whilst there was no overall relationship between BCL-2/BAX ratios and sensitivity to ara-C and 6TG, individual patients could be identified whose blast cells were resistant to ara-C and had high BCL-2/BAX ratios. Further analysis of the significance of these ratios to drug resistance may be of future prognostic value.

The human MLH1 cDNA complements DNA mismatch repair defects in Mlh1-deficient mouse embryonic fibroblasts

The DNA mismatch repair gene hMLH1 is reported to function in mutation avoidance, cell cycle checkpoint control, the cytotoxicity of various DNA-damaging agents, and transcription-coupled nucleotide excision repair. Formal proof of the involvement of hMLH1 in these processes requires single gene complementation. We have stably expressed hMLH1 from a transfected cDNA in Mlh1-deficient mouse embryonic fibroblasts. Expression of hMLH1 restored normal levels of mPMS2 protein, reduced spontaneous base substitution and microsatellite mutations, increased sensitivity to the toxic effects of 6-thioguanine (6-TG), and restored 6-TG-induced cell cycle arrest. Our studies confirm that hMLH1 has an essential role in the maintenance of genomic stability and the potentiation of 6-TG cytotoxicity and provide a system for detailed structure/function analysis of the hMLH1 protein.

Genotoxicity of purine acyclic nucleotide analogs

The genotoxic and embryotoxic effects of phosphonomethoxyalkylpurines, a new group of antiviral agents, decrease in the following order: PMEG > PMEthioG > PMEDAP > PMEA > (R)-PMPDAP = (R)-PMPA. Results of the present study are fully consistent with the previously found efficacy of their diphosphates to inhibit the replicative DNA polymerases. The marked genotoxicity of PMEG and PMEthioG is comparable to that of mitomycin C, whereas the moderate genotoxicity of PMEA is comparable to that of AZT. (R)-PMPDAP and (R)-PMPA did not induce any structural aberrations of chromosomes under the experimental conditions.

Calibration of the single cell gel electrophoresis assay, flow cytometry analysis and forward mutation in Chinese hamster ovary cells

The induction kinetics of genetic damage were measured in one clone of a mammalian cell line (CHO AS52) with three genotoxicity assays, the single cell gel electrophoresis (Comet) assay, laser beam flow cytometry and forward mutation. The first two assays allow for the rapid analysis of genotoxic damage in individual nuclei. The alkaline Comet assay detects DNA strand breaks, alkali-labile sites and incomplete excision repair sites. Flow cytometry measures chromosome damage that results in an unequal distribution of nuclear DNA in daughter cells. We calibrated these assays to compare acute DNA damage and longer term clastogenicity with forward mutation at the gpt locus using ethyl methanesulfonate (EMS). The EMS treatments were conducted in F12 medium for 2 h. AS52 cells carry a single functional gpt gene which provides for quantitation of gpt mutants by selecting for 6-thioguanine resistance. EMS induced a concentration-dependent response with median Comet tail moment values of 1.06 microns for the negative control and 64.6 microns with 20 mM. The coefficient of variation (CV) of the negative-control with flow cytometry was 2.33; the CV value increased to 4.87 in cells treated with 20 mM EMS, EMS (8 mM) induced a mutant frequency of 779.8 x 10(-6) at a relative survival of 64.4%. Genetic response factors were calculated and the data demonstrate that the induction kinetics of genetic damage as measured by the Comet assay (15.6) and flow cytometry (14.2) were more closely related than that determined for mutation induction (7.9). These three assays measure a wide spectrum of genetic events at the level of DNA, the gene and the chromosome and demonstrate the usefulness of the Comet assay and flow cytometry as two relatively rapid procedures to detect genotoxic damage in mammalian cells.

Mutant frequency at the Hprt locus and in minisatellite sequences in Chinese hamster V79 cells irradiated with low-energy protons (31 keV/microm) and ultraviolet light (254 nm)

Ionizing radiations induce mutations which can be detected both in coding sequences (Hprt locus) by measuring the frequency of 6-thioguanine-resistant cells and in minisatellite sequences by DNA fingerprint analysis. We analyzed the effects of irradiation with low-energy protons (31 keV/pm) and, for comparison, with ultraviolet light (254 nm), for which DNA damage and repair mechanisms are better understood, on cultures of Chinese hamster V79 cells with the two methods mentioned above. The results indicate that the frequency of 6-thioguanine-resistant cells was increased significantly, although very differently, by both treatments. The analyses carried out by DNA fingerprinting with a multilocus DNA probe show that the level of induction in minisatellite sequences was higher compared to those measured at the Hprt locus after proton irradiation, but lower after treatment with ultraviolet light.

Tissue specific toxicities of the anticancer drug 6-thioguanine is dependent on the Hprt status in transgenic mice

6-Thioguanine (6TG) a cytostatic antimetabolite is currently used to treat patients with cancer, in particular leukemias. However, one drawback of such use is the development of 6TG resistance. Hypoxanthine-guanine phosphoribosyl transferase (Hprt) plays a crucial role in the bioactivation of 6TG. Loss of Hprt has been associated with the resistance of leukemias to 6TG chemotherapy, however, nothing has been known about the effect of Hprt status on tissue specific toxicity of 6TG in vivo. We determined the effect of Hprt status on the tissue-specific toxicity of 6TG in vivo in transgenic Hprt-deficient mice. The approximate lethal dose for Hprt-deficient mice was 23-fold higher than for the wild-type. Serum biochemical analyses of 6TG-treated wild-type mice showed elevated serum enzyme levels characteristic of liver damage whereas the levels in Hprt-deficient 6TG-treated mice were within normal physiological limits. Histopathological examination of tissues from wild-type and from Hprt-deficient mice showed contrasting spectrums of microscopic lesions. Wild-type mice had loss of hematopoietic cells from bone marrow starting at the lowest dose of 25 mg/kg 6TG whereas Hprt-deficient mice had normal bone marrow and spleen even at doses of 720 mg/kg 6TG. Wild-type mice also experienced severe loss of epithelial cells from the gastrointestinal tract starting at 50 mg/kg; however, the gastrointestinal tract of Hprt -/- mice remained unaffected. Wild-type livers revealed atrophy and necrosis at doses of 25 mg/kg 6TG although Hprt -/- livers displayed no effect until 507 mg/kg. In this study we show that Hprt-deficient mice had 6TG-resistant bone marrow and there are several other factors contributing to 6TG resistance in patients. Because variations among people exist in terms of their 6TG sensitivity, determining 6TG sensitivity of lymphocytes prior to 6TG chemotherapy and restricting treatment to 6TG-sensitive patients may improve the efficacy.

Cytotoxic mechanism of 6-thioguanine: hMutSalpha, the human mismatch binding heterodimer, binds to DNA containing S6-methylthioguanine

It has been suggested that the cytotoxicity of 6-thioguanine depends upon (1) incorporation of 6-thioguanine into DNA, (2) methylation by S-adenosylmethionine (SAM) of the thio group to give S6-methylthioguanine, (3) miscoding during DNA replication to give [SMeG] x T base pairs, and (4) recognition of these base pairs by proteins of the postreplicative mismatch repair system. Here we have investigated systematically the ability of proteins present in human cell extracts to bind to DNA containing S6-methylthioguanine. We found that [SMeG] x T base mismatches were bound by the mismatch binding complex, hMutS alpha, and that the level of binding was dependent upon the base 5' to the S6-methylthioguanine in the order G > C = A > T. Extracts from cells that lack either hMSH2 (LoVo cells) or GTBP (HCT-15 cells), two components of the hMutS alpha complex, were unable to bind the [SMeG] x T base pair. We also found that hMutS alpha was able to bind to [SMeG] x C base pairs when the S6-methylthioguanine was in the sequence 5'-Cp[SMeG]. This suggests that miscoding by S6-methylthioguanine residues in DNA during DNA synthesis may not be an absolutely required step in the mechanism of cytotoxicity. Also, since CpG sequences are so important in gene regulation, this result may be of considerable significance.

Escherichia coli gpt gene sensitizes rat glioma cells to killing by 6-thioxanthine or 6-thioguanine

Genes that encode enzymes that convert inactive "prodrugs" into anticancer metabolites may be therapeutically useful against brain tumors. Unlike other genes tested to date in brain tumor models, the Escherichia coli gpt gene is unique in that it not only sensitizes cells to the prodrug 6-thioxanthine (6TX) but also encodes resistance to a different regimen (mycophenolic acid, xanthine, and hypoxanthine), thus providing a means to select for gpt-positive cells. In the present study, rat C6 glioma cells were infected with a retrovirus vector that transduces this gene. A clonal line (C6GPT-7) was derived that exhibited significant 6TX susceptibility in vitro with an ID50 of 2.5 mumol/L, whereas 50% growth inhibition of parental C6 cells was not achieved at concentrations tested (up to 50 mumol/L). This line also exhibited significant sensitivity to 6-thioguanine (6TG), with an ID50 of 0.05 mumol/L, whereas 50% growth inhibition of parental C6 cells was achieved at 0.5 mumol/L. In a "bystander" assay, C6GPT-7 tumor cells efficiently transferred 6TX sensitivity to C6 cells at ratios as low as 1:9 (C6GPT-7:C6). This in vitro bystander effect was abrogated when C6GPT-7 and C6 cells were separated by a microporous membrane, suggesting that it was not mediated by highly diffusible metabolites. In vivo both 6TX and 6TG significantly inhibited the growth of subcutaneously transplanted C6GPT-7 cells but not that of C6 cells in athymic mice. In an intracerebral model, both 6TX and 6TG exhibited significant antiproliferative effects against tumors formed by C6GPT-7 cells. These findings provide a basis for exploring further gene therapy strategies based on in vivo transfer of the E coli gpt gene to provide chemosensitivity against 6TX and 6TG.

Monitoring of induced chromosomal aberrations in S. cerevisiae in agarose gels by pulsed field gel electrophoresis

Pulsed field gel electrophoresis (PFGE) has been used to detect aberrations of the chromosomal banding pattern referred to as chromosomal aberrations arising after treatment of yeast strain S. cerevisiae MP1 with the three different genotoxic substances 4-nitroquinoline-N-oxide (4-NQO), methotrexate (MTX) and 2-amino-6-mercaptopurine (AMP). Different chromosomal aberrations were detectable directly in the pulsed field gel when growing yeast cells were incubated with a genotoxin for 6 h at 26 degrees C followed by treatment with the genotoxin for another twelve days at 4 degrees C. In the present study, clones of yeast cells were examined. In this way, distinct chromosomal aberrations and not only DNA smear could be detected. Moreover, this method allows selection for yeast strains with specific and rare chromosomal rearrangements.

A mutator phenotype characterizes one of two complementation groups in human cells tolerant to methylation damage

Sixty % of clones isolated from HeLa cells treated with toxic concentrations of a methylating carcinogen showed increased resistance to the cytotoxicity of N-methyl-N-nitrosourea. D37 values were 6- to 100-fold higher than in the parental cell population. The absence of detectable levels of the repair enzyme O6-methylguanine-DNA methyltransferase indicated that the resistant clones were able to tolerate the presence of O6-methylguanine in their DNA. Analysis of N-methyl-N-nitrosourea survival in the hybrids between tolerant clones and HeLa cells showed that tolerance can be either recessive or codominant. Fusion between tolerant clones indicated two complementation groups. We measured spontaneous mutation rates at microsatellites and at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus in several tolerant clones. All the clones of Complementation Group I showed unstable microsatellites and 4-8-fold increases in mutation rates at hprt. No significant alterations in spontaneous mutation rates were found in clones of Complementation Group II. The data indicate that tolerance to methylation damage can be conferred by alterations in at least two different gene products and that one of the two groups has the mutator phenotype typical of mismatch correction defective cells.

Killing and mutation of Chinese hamster V79 cells exposed to accelerated oxygen and neon ions

Mutation induction by accelerated heavy ions to 6-thioguanine resistance (HPRT system) in Chinese hamster V79 cells was investigated using oxygen and neon ions with energies between 1.9 and 400 MeV/mu, corresponding to LET values between 18 and 754 keV/microns, respectively. Because of technical limitations most experiments could be performed only once. Inactivation and mutation induction cross sections, sigma i and sigma m, were obtained from the slopes of the exponential survival and the linear mutation induction curves, respectively. Both parameters increased with LET up to about 200 keV/microns, where the curves separated for the two types of ions. Calculated RBEs were higher for mutation induction than for killing for all LET values.

Hprt mutant frequency and GSTM1 genotype in non-smoking healthy individuals

The T-cell cloning assay which combines mitogen- and growth factor-dependent expansion of lymphocyte clones with thioguanine selection of hypoxanthine-guanine phosphoribosyl transferase (hprt)-negative cells has been extensively used for studying human somatic gene mutation in vivo. However, large interindividual variations in the hprt mutant frequency (MF), much of which is not explained by donor attributes such as age and smoking habit, and interlaboratory variations in the experimental methodology, including cloning efficiency (CE), call for further developments of the cloning protocol and additional population studies. Using an improved T-cell cloning method, we have studied in vivo hprt MF of 76 non-smoking healthy males aged 23-77 years. The addition of 5% human serum to the growth medium was found to produce a consistently high CE of 61% in average. The MF, ranging from 1.4 to 22.6 x 10(-6) with a mean of 8.6 x 10(-6), increased significantly (P < 0.0001) with age, by 2% per year. A significant (P = 0.002) inverse relationship between MF and CE was observed. Using a PCR-based technique for GSTM1-genotyping, we also studied the relationship between MF and GSTM1 polymorphism. The 38 (50%) GSTM1-negative individuals showed a 20% higher mean MF than the 38 (50%) GSTM1-positive individuals. The difference was however not significant, neither before (P = 0.1) nor after (P = 0.5) correction for CE and the significantly (P = 0.04) higher mean age in the GSTM1-negative group. This study shows that age contributes more than GSTM1 polymorphism to the large interindividual variation in the hprt MF of non-smokers. The relationship between GSTM1 polymorphism and hprt MF in smokers remains to be investigated.

Determination of hprt mutant and mutation frequencies and the molecular characterization of human derived in vivo T-lymphocyte mutants

Using a T-lymphocyte clonal assay, 73 6-thioguanine resistant T-lymphocytes were isolated from two blood samples obtained 4 months apart from a 50-year-old male subject. Sixty-six of these mutants were characterized at the DNA sequence level using cDNA. One particular single base substitution was recovered a total of 23 times. The majority of T-cell receptors (TCR) of these mutants all share a common gamma-TCR rearrangement, and thus likely represent a single mutational event that underwent clonal expansion in vivo. Siblings of this clone were recovered in both collections. Three other single base substitutions were also recovered more than once. In two of the three cases, the mutants were also found to be clonally related, while in one case they were not. A number of identical exon loss events were also recovered, yet none of these were clonally related. This probably reflects the multiple pathways by which these mutations can arise. The TCR data was used to correct the observed mutant frequency to produce an estimate of the actual mutation frequency. The two mutant frequencies, 18 x 10(-6) and 19 x 10(-6), obtained from the first and second sampling periods, respectively, can thus be corrected to yield true mutation frequency's of 12 x 10(-6) each.

Mutagenicity in vitro of 3,4-dichloro-5-hydroxy-2(5H)-furanone (mucochloric acid), a chlorine disinfection by-product in drinking water

The mutagenicity of chlorinated humic drinking waters is accounted for mainly by a single contaminant, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), as assessed in Salmonella typhimurium strain TA100. In the present study 3,4-dichloro-5-hydroxy-2(5H)-furanone (mucochloric acid, MA), another drinking water contaminant much less potent as a mutagen in TA100 than MX, was tested in Chinese hamster ovary (CHO) cells for the induction of mutation at the hypoxanthine phosphoribosyl transferase (hprt) locus to 6-thioguanine resistance (TGr). Unexpectedly, MA induced TGr mutants in CHO cells with a potency comparable to that reported previously for MX. In subsequent experiments with S. typhimurium, the presence of pKM101 plasmid in strain TA100 increased susceptibility to the mutagenicity of MA, but much less than to that of MX, relative to the parental strain TA1535 lacking pKM101. The difference between the two compounds in TA100 thus appears to be due to a higher enhancement of the mutagenicity of MX than that of MA by pKM101 mediated error-prone DNA repair.

DNA modification induced by 6-mercaptopurine riboside in murine embryos

The cytostatic drug 6-mercaptopurine riboside (6-MPr) was investigated in mice in order to test the hypothesis that the teratogenicity of this antimetabolite is paralleled by an incorporation into the DNA of the embryos during organogenesis. DNA modification in the embryos was analysed 4 h following s.c. administration of [35S]-labelled 6-MPr to the dams on day 11 of pregnancy. The DNA of the embryos was isolated and hydrolysed to the bases by formic acid. Following separation by cation-exchange HPLC 6-thioguanine was found in the hydrolysate. Quantitation was performed by liquid scintillation counting. Evaluations of 6 doses in the range of 8-25 mg/kg were performed. An incorporation rate of 6-thioguanine from 32-56 pmol per mumol guanine was found in the DNA of the embryos. These findings suggest that, similar to the previously studied alkylating agents, the teratogenicity of 6-MPr may be, at least in part, induced via DNA modification of the embryos.

Development of a cloning assay with high cloning efficiency to detect induction of 6-thioguanine-resistant lymphocytes in spleen of adult mice following in vivo inhalation exposure to 1,3-butadiene

A cloning assay with high cloning efficiency has been developed to detect spontaneous and induced 6-thioguanine-resistant T-lymphocytes (HPRT mutants) from the spleen of adult mice. The mean cloning efficiency in untreated male mice of 20-22 weeks old was 34.5 +/- 11.2% (SD) and the corresponding mutant frequency 0.7 +/- 0.8 (SD) x 10(-6). The cloning efficiencies obtained in this study are substantially higher than those reported previously by other investigators. Using this assay, it could be demonstrated that inhalation exposure of mice to 200, 500 or 1300 ppm of 1,3-butadiene for 6 h/day on 5 consecutive days caused a statistically significant induction of 6-thioguanine-resistant mutations in T-lymphocytes from spleens of adult mice exposed to 1300 ppm. The exposure to 1300 ppm resulted in a three-fold increase of the spontaneous mutant frequency. The mutant frequency after exposure to 500 ppm was higher than the control but the increase was not significant.

Mutator phenotypes in human colorectal carcinoma cell lines

Recent studies have revealed that tumors in patients with hereditary nonpolyposis colon cancer are associated with high-frequency alterations of microsatellite sequences. To investigate the mechanisms and consequences of this form of genetic instability, we identified three colorectal carcinoma cell lines that express dinucleotide-repeat instability like that found in hereditary nonpolyposis colon cancer tumors and show increased rates of spontaneous mutation at selectable loci. However, the pattern of hypermutation in these cell lines differed significantly. In one line (HCT116), microsatellite mutations occurred at a remarkably high rate (approximately 10(-2) mutations per cell per generation), whereas this rate was considerably lower in the two other lines (DLD-1 and HCT15). The rate of mutation at the locus encoding hypoxanthine guanine phosphoribosyltransferase was substantially elevated (200- to 600-fold) in all three tumor cell lines, yet the types of mutations arising differed. A specific frame-shift hotspot accounted for 24% of hypoxanthine guanine phosphoribosyltransferase mutations in HCT116. The frequency of mutations at this site was reduced significantly in DLD-1 and HCT15 lines. These data suggest that the mutatw phenotypes in the colorectal carcinoma cell lines could be the consequence of mutator genes affecting different repair or error-avoidance pathways.

Mutational spectrum of chromium(VI) in human cells

The mutational spectrum of K2Cr2O7 on the low melting domain within exon 3 of hypoxanthine guanine phosphoribosyl transferase (hprt) gene was obtained from 6-thioguanine-resistant (6TGr) human lymphoblasts (TK6). Using a combination of high-fidelity polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE), we found four chromium(VI) induced hotspots in a 104-bp target sequence. The hotspots were C:G-->A:T at bp 243, representing 4.5% of 6TGr mutants; A:T-->T:A at bp 247 (2.0%); G:C-->A:T at bp 289 (2.5%) and C:G-->T:A at bp 312 (4.0%). Mutational hotspots at bp 243 have previously been determined for hydrogen peroxide (C:G-->G:C) and benzo[a]pyrene diol epoxide (C:G-->A:T). Two of the hotspots, bp 243 and 289, arose in a common quadruplet 5'-ACAT-3' in the untranscribed and transcribed strands respectively.

Detection of 6-thioguanine-resistant human peripheral blood lymphocytes using 5-bromodeoxyuridine labeling in combination with immunocytochemical staining

Measurement of frequencies of 6-thioguanine-resistant (TGr) human peripheral lymphocytes may contribute to quantitative genetic risk assessment in occupationally or environmentally exposed human populations. A simple procedure for the detection of TGr human peripheral blood lymphocytes was developed in our laboratory, using whole blood culturing and 5-bromodeoxyuridine (BrdU) labeling in combination with immunocytochemical staining. Modifications of the procedure designed to reduce the false positive effects of spontaneously cycling lymphocytes (phenocopies), and to optimize duration of BrdU labeling and the culturing period, were evaluated. A standard procedure was developed which applied 24 h cold storage of the diluted heparinized blood (1:10, v:v in RPMI 1640 medium) at 4 degrees C to reduce the effect of spontaneously cycling lymphocytes, and whole blood culturing in RPMI 1640 complete medium with stimulation of T lymphocytes using phytohemagglutinin (PHA), selection of TGr lymphocytes by adding TG to a final concentration of 2 x 10(-4) M and, after 24 h of incubation, labeling of TGr lymphocytes with 2.5 x 10(-5) M BrdU during 16 h. Using this standard procedure, frequencies of TGr cells in 45 healthy individuals (aged 21-64) were observed to range from 0.3 to 229.8 x 10(-6), with a mean variant frequency (VF) (+/- SD) of 136 x 10(-6) (+/- 35.8). After exclusion of the one extremely high value of 229.8 x 10(-6), mean VF was 8.7 x 10(-6) (+/- 14.1). A significant inverse correlation was found between logVF and the labeling index of control cultures (LIc), indicating that cultures with low LIc tend to yield higher VF.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular analysis of hypoxanthine phosphoribosyl transferase mutants induced by glycidyl 1-naphthyl ether in mouse spleen cells in vivo

Treatment of C57BL/6J mice with an epoxide, glycidyl 1-naphthyl ether (GNE), resulted in an average of a 3.4-fold increase in frequency of 6-thioguanine-resistant mutants of mouse spleen T-lymphocytes. In similar experiments with the epoxide trichloropropylene oxide, no increase in mutant frequency was found. To determine the kind and location of mutations in the coding region of the hypoxanthine phosphoribosyl transferase (HPRT) gene, 26 GNE-induced mutants and 17 spontaneous mutants were analyzed by direct sequencing of polymerase chain reaction amplified cDNA. Among the GNE-induced mutants, HPRT cDNA was present in 22, while that from 4 could not be detected. Among the spontaneous mutants, HPRT cDNA was present in 15 and absent in 2. Among GNE-induced mutants, base substitution in HPRT occurred in 15 of 22 mutants analyzed. Nine of 15 base substitutions involved TA base pairs, primarily TA-->CG transitions. Base substitutions were found throughout exons 3-7 but 46% of substitutions were located in exon 3 and one frameshift mutation involving a GC base pair in exon 3 was also observed. Among the spontaneous mutants, base substitutions of HPRT occurred in 7 of 15 mutants analyzed with 6 of 7 base substitutions involving a TA base pair and another 2 of the 15 mutants showed a 4 base pair deletion. The base substitution spectrum in GNE-induced mutants was different from that of the spontaneous mutants.

Large deletions are tolerated at the hprt locus of in vivo derived human T-lymphocytes

A cloning assay was used to recover hprt- T-lymphocytes from adult human males. Analysis of crude cellular extracts by polymerase chain reactions (PCRs) demonstrated that 7% (16/218) of the hprt mutations were due to total deletion of the hprt gene. 14 of the 16 mutants were examined by PCR for the presence of flanking DNA to determine the extent of the deletions. The deletion mutation in 13 mutants was at least 350 kb with 5 of these deletions being at least 700 kb. The largest deletions were greater than 15 times the size of the hprt gene. Therefore, large deletions are tolerated at the hprt locus of human T-lymphocytes.

Application of the standard mutagenesis assay results in underestimation of ethyl methanesulphonate-induced mutations to ouabain-resistance in Chinese hamster cells

Chinese hamster V79 cells were exposed to ethyl methanesulphonate (EMS) and the incidence of mutant cells resistant to 8-azaguanine (8AZG), 6-thioguanine (6TG) or ouabain (OUA) was determined both by the respreading and the in situ techniques. In the former assay, the mutagen-treated cultures were grown for several days to permit the expression of mutations after which the cells were trypsinized, replated (10(5) cells/100-mm dish), and grown in medium supplemented with a selective agent. In the in situ assay, cultures were left undisturbed between EMS treatment and incubation in the presence of the selective agents. The yield of 8AZG-resistant mutants observed at optimal expression times after EMS treatment was comparable for both techniques; the induced mutation frequency (corrected for spontaneous mutation frequency) was estimated to be 82 x 10(-6) mutations per viable cell per unit dose (mM) of EMS. The frequency of 6TG-resistant mutants equalled 45 and 4 x 10(-6)/mM EMS as determined by the respreading and the in situ assays, respectively. In sharp contrast to that observed with 6TG, the frequency of OUA-resistant mutants scored by the in situ assay (30 x 10(-6)/mM EMS) proved to be an order of magnitude greater than that determined by the respreading assay (3 x 10(-6)/mM EMS). Our data therefore indicate that, when OUA is used for mutant selection, the application of the respreading technique, which has been widely adopted as the standard mammalian mutational assay over the past decade, may result in a marked underestimation of the actual mutation frequency (approximately 10-fold in V79 cells).

The mutagenicity of 2-amino-N6-hydroxyadenine to L5178Y tk +/- 3.7.2C mouse lymphoma cells: measurement of mutations to ouabain, 6-thioguanine and trifluorothymidine resistance, and the induction of micronuclei

2-Amino-N6-hydroxyadenine (AHA) was tested in the mouse lymphoma L5178Y tk +/- assay using the microtitre cloning technique over concentrations from 0.005 micrograms/ml-1 (100% viability) to 6 micrograms/ml (10% viability) as measured by cloning efficiency immediately after treatment. At low, non-toxic concentrations (0.005-0.25 micrograms/ml) a dose-related linear increase in the frequency of ouabain-resistant mutants was seen, in addition to an increase in 6-thioguanine- and trifluorothymidine-resistant mutants. No consistent induction of micronucleated cells was observed in this concentration range. Toxic concentrations (20-90% kill) induced a dose-related increase in micronuclei, while the frequency of ouabain-resistant mutants fell (although it was still highly significantly above the control value). These results suggest that the mechanism of action of AHA depends on the concentration, with point mutations being induced at low, non-toxic doses and detectable chromosome breakage occurring only at higher doses. Both large-colony and small-colony trifluorothymidine-resistant mutants were induced at all concentrations. The utility of using multiple genetic end-points in one cell line and the importance of dose range selection for risk assessment and an understanding of the mode of action of test substances is underlined.

Genotoxicity of 2-amino-6-N-hydroxyadenine (AHA) to mouse lymphoma and CHO cells

2-Amino-6-N-hydroxyadenine (AHA) treated L5178Y/TK (+/-)-3.7.2C mouse lymphoma cells were evaluated for mutations at the tk, hgprt, and Na+/K+ ATPase loci, as well as for gross chromosome aberrations and induction of micronuclei. In addition, AHA was evaluated for its ability to induce HGPRT mutants in CHO cells. AHA was found to induce mutations at all evaluated loci and in both cell types. The TK mutants were primarily large colonies although a few small colonies were also induced, particularly at the higher concentrations. Preliminary cytogenetic analysis of AHA-treated mouse lymphoma cells indicated that some gross aberrations but not micronuclei were induced. The 20 small-colony TK mutants evaluated by banded karyotype indicate that only a small fraction (2 of 20) showed chromosome 11 abnormalities. From these studies, it appears that AHA may be one of a very few chemicals that is capable of inducing multi-locus point mutations, with only slight clastogenic activity. Particularly at the higher concentrations, some of the mutants may contain multi-locus point mutations that result in slow growth.

Characterization of the DNA damage in 6-thioguanine-treated cells

6-Thioguanine (TG) incorporation into DNA has been associated with cytotoxicity and DNA damage in Chinese hamster ovary (CHO) and murine leukemia L1210 cells. According to alkaline elution analysis, single-strand breaks (SSB) occur in both cell types. DNA-protein and interstrand crosslinks are prominent features of TG effects in L1210, CEM, and HL-60 but not CHO cells. To assess which DNA strand experiences SSB in CHO cells, the cells were synchronized by growth to confluence (late G1, S). The cells were then diluted into fresh medium so that they underwent a round of division during a subsequent 16-hr interval. They were treated with TG during this first cell cycle, and mitotic cells were harvested at the end of the first cycle using colcemid. SSB were determined in parental DNA (radiolabeled with thymidine during growth to confluence), TG-containing DNA (radiolabeled with [14C]TG during drug exposure), and daughter DNA (labeled with thymidine during the second cell cycle). SSB occurred in TG-containing DNA late in the second cell cycle after drug exposure and in the DNA synthesized from a TG-DNA template (daughter DNA). This observation is consistent with the known delayed cytotoxicity and chromosomal aberrations seen in CHO cells. The SSB suggest relatively normal elongation of DNA containing TG but altered synthesis and/or ligation from a TG-DNA template. This premise was tested in synchronized CHO cells. The DNA strand incorporating TG elongated naturally; however, DNA elongation was impaired in the cell cycle following TG treatment. The results are consistent with SSB in daughter DNA synthesized from a TG-DNA template due to inability to elongate the newly-synthesized strand.

High sensitivity of Chinese hamster epithelial liver cells to toxic analogues of purines

The resistance of Chinese hamster epithelial liver cells (CHEL) and Chinese hamster fibroblasts (V79) towards toxic purine analogues has been determined. The liver cells are more sensitive than fibroblasts to 6-thioguanine (6-TG), 8-azaguanine (8-AZ) and 2,6-diaminopurine (DAP). The hypoxanthine-guanine (HGPRT) and adenine phosphoribosyl transferase (APRT) activities of extracts of CHEL cells were lower than those of corresponding extracts of V79. The level of 5'-nucleotidase was about 5-fold higher in the epithelial cells. It appears that HGPRT and APRT activities of extracts of liver epithelial cells are masked or reduced by 5'-nucleotidase activity and other inhibitors. The significance of these findings is discussed.

Modulation of the cytotoxic mechanism of 6-thioguanine by 4-amino-5-imidazolecarboxamide

Previous evidence has indicated that either purine starvation or incorporation into DNA may be the dominant biochemical effect of the antileukemic agent 6-thioguanine (TG), depending on exposure conditions. Furthermore, it has been suggested that the paradoxical decrease in TG-induced cytotoxicity at high drug concentrations may be due to an antagonistic interaction between these two mechanisms, in which purine starvation inhibits DNA synthesis and, therefore, incorporation of TG into DNA. In this report we test the hypothesis that by concurrent treatment of L1210 cells with TG and the purine precursor 4-amino-5-imidazolecarboxamide (AIC) it is possible to alleviate DNA synthesis inhibition caused by high concentrations of TG, thus enhancing TG incorporation into DNA and TG-induced cell kill. Both the cytotoxic and cytokinetic results presented support this hypothesis. However, gross incorporation of TG into DNA was not increased by AIC under conditions in which a significant enhancement of cytotoxicity (i.e., 1 log) was observed. These findings suggest that the potentiating effect of AIC may be most prominent on the subpopulation of cells that are resistant to treatment with TG alone, and they demonstrate that the cytotoxic effects of TG treatments are more accurately reflected by observing specific cytokinetic changes (delayed late S/G2 arrest) than by measuring the average extent of TG incorporation into DNA within a given population. Finally, we propose that it may be possible to select conditions for administration of TG that favor one or the other cytotoxic mechanism, depending on whether the clinical objective is induction of remission (where rapid cell lysis due to purine starvation would be desired) or eradication of subclinical disease during remission (where proliferation-dependent cytotoxicity due to DNA incorporation should be more effective.

Increased frequency of 6-thioguanine-resistant peripheral blood lymphocytes in Werner syndrome patients

The frequency of spontaneous 6-thioguanine (TG)-resistant peripheral blood lymphocytes in five unrelated Werner syndrome (WS) patients was determined using an autoradiographic labeling assay. The average frequency of TG-resistant lymphocytes was eightfold higher in WS patients than in sex- and age-matched normal control donors. This finding and previous identification of increased spontaneous chromosomal rearrangements and deletions in WS cells or cell lines suggest that WS is a human genomic instability or mutator syndrome.

A-phase II study of intravenous 6-thioguanine (NSC-752) in multiple myeloma. A Southwest Oncology Group study

Thirty-three patients with relapsing or refractory multiple myeloma were treated with 6-Thioguanine (6TG) at a dose of 1 g/M2, with therapy given over four hours every three weeks. The major toxicity seen was myelotoxicity; thrombocytopenia was more commonly noted than neutropenia. One patient achieved a PR, two were clinically improved. 6TG in this short infusion schedule proved to be myelotoxic, but demonstrated little activity in previously treated myeloma patients.