Use of the clonal assay for the measurement of frequencies of HPRT mutants in T-lymphocytes from five control populations

Somatic mutations in aging, cancer and neurodegeneration

The somatic mutation theory of aging posits that the accumulation of mutations in the genetic material of somatic cells as a function of time results in a decrease in cellular function. In particular, the accumulation of random mutations may inactivate genes that are important for the functioning of the somatic cells of various organ systems of the adult, result in a decrease in organ function. When the organ function decreases below a critical level, death occurs. A significant amount of research has shown that somatic mutations play an important role in aging and a number of age related pathologies. In this review, we explore evidence for increases in somatic nuclear mutation burden with age and the consequences for aging, cancer, and neurodegeneration. We then review evidence for increases in mitochondrial mutation burden and the consequences for dysfunction in the disease processes.

The nucleotide pool, a target for low-dose gamma-ray-induced oxidative stress

Oxidative stress occurs when the generation of reactive oxygen species (ROS) exceeds the cellular antioxidant capacity. The excess ROS react with and modify cellular components. Nucleic acid modifications are of principal interest because they may cause mutations. 8-Oxo-7,8-dihydro-2 -deoxyguanosine (8-oxo-dG) is a mutagenic lesion that can be formed by ROS in DNA as well as in the nucleotide pool. 8-Oxo-dG is removed from the DNA by base excision repair and from the nucleotide pool by the nucleotide sanitization enzyme hMTH1. hMTH1 hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP, which is released to the extracellular environment and can serve as a marker of oxidative stress. The aim of this work was to establish the dose-response relationship for radiation-induced extracellular 8-oxo-dG and hMTH1 in the mGy range of gamma rays in three cellular model systems: human whole blood, human fibroblasts and stimulated lymphocytes. Extracellular 8-oxo-dG was analyzed with the use of an ELISA and hMTH1 by Western blotting. Our results demonstrate that low-dose ionizing radiation induces a stress response that leads to the formation of extracellular 8-oxo-dG and induction of hMTH1 in all three cellular model systems tested. This suggests that the nucleotide pool is an important target for radiation-induced stress response.

Elevated levels of somatic mutation in a manifesting BRCA1 mutation carrier

Homozygous loss of activity at the breast cancerpredisposing genes BRCA1 and BRCA2 (FANCD1) confers increased susceptibility to DNA double strand breaks, but this genotype occurs only in the tumor itself, following loss of heterozygosity at one of these loci. Thus, if these genes play a role in tumor etiology as opposed to tumor progression, they must manifest a heterozygous phenotype at the cellular level. To investigate the potential consequences of somatic heterozygosity for a BRCA1 mutation demonstrably associated with breast carcinogenesis on background somatic mutational burden, we applied the two standard assays of in vivo human somatic mutation to blood samples from a manifesting carrier of the Q1200X mutation in BRCA1 whose tumor was uniquely ascertained through an MRI screening study. The patient had an allele-loss mutation frequency of 19.4 x 10(-6) at the autosomal GPA locus in erythrocytes and 17.1 x 10(-6) at the X-linked HPRT locus in lymphocytes. Both of these mutation frequencies are significantly higher than expected from age-matched disease-free controls (P < 0.05). Mutation at the HPRT locus was similarly elevated in lymphoblastoid cell lines established from three other BRCA1 mutation carriers with breast cancer. Our patient's GPA mutation frequency is below the level established for diagnosis of homozygous Fanconi anemia patients, but consistent with data from obligate heterozygotes. The increased HPRT mutation frequency is more reminiscent of data from patients with xeroderma pigmentosum, a disease characterized by UV sensitivity and deficiency in the nucleotide excision pathway of DNA repair. Therefore, this BRCA1-associated breast cancer patient manifests a unique phenotype of increased background mutagenesis that likely contributed to the development of her disease independent of loss of heterozygosity at the susceptibility locus.

Analysis of gene expression using gene sets discriminates cancer patients with and without late radiation toxicity

BACKGROUND

Radiation is an effective anti-cancer therapy but leads to severe late radiation toxicity in 5%-10% of patients. Assuming that genetic susceptibility impacts this risk, we hypothesized that the cellular response of normal tissue to X-rays could discriminate patients with and without late radiation toxicity.

METHODS AND FINDINGS

Prostate carcinoma patients without evidence of cancer 2 y after curative radiotherapy were recruited in the study. Blood samples of 21 patients with severe late complications from radiation and 17 patients without symptoms were collected. Stimulated peripheral lymphocytes were mock-irradiated or irradiated with 2-Gy X-rays. The 24-h radiation response was analyzed by gene expression profiling and used for classification. Classification was performed either on the expression of separate genes or, to augment the classification power, on gene sets consisting of genes grouped together based on function or cellular colocalization.X-ray irradiation altered the expression of radio-responsive genes in both groups. This response was variable across individuals, and the expression of the most significant radio-responsive genes was unlinked to radiation toxicity. The classifier based on the radiation response of separate genes correctly classified 63% of the patients. The classifier based on affected gene sets improved correct classification to 86%, although on the individual level only 21/38 (55%) patients were classified with high certainty. The majority of the discriminative genes and gene sets belonged to the ubiquitin, apoptosis, and stress signaling networks. The apoptotic response appeared more pronounced in patients that did not develop toxicity. In an independent set of 12 patients, the toxicity status of eight was predicted correctly by the gene set classifier.

CONCLUSIONS

Gene expression profiling succeeded to some extent in discriminating groups of patients with and without severe late radiotherapy toxicity. Moreover, the discriminative power was enhanced by assessment of functionally or structurally related gene sets. While prediction of individual response requires improvement, this study is a step forward in predicting susceptibility to late radiation toxicity.

Studies on the HPRT mutant frequency in T lymphocytes from healthy Indian male population as a function of age and smoking

Mutant frequency at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in the peripheral blood lymphocytes obtained from 44 healthy individuals (23 non-smokers and 21 smokers) of an Indian male population was studied using T-lymphocyte cloning assay. It was found that lnMF increased with age at a rate of 2.5% per year (P <0.001). Blood samples from smokers showed a significant (P <0.037) increase in HPRT mutant frequency (MF) (10.43 +/- 4.74 x 10(-6)) as compared to that obtained from non-smokers (7.69 +/- 3.69 x 10(-6)). This study also showed a significant (P <0.027) inverse correlation between lnMF and non-selected cloning efficiency (CE). However, with respect to age no variation was observed in cloning efficiency. The results obtained in this study showed a good comparison with those reported in different populations of the world.

Genotoxicity of therapeutic intervention in children with acute lymphocytic leukemia

The survival rates of children treated for cancer have dramatically increased after the development of standardized multiple-modality treatment protocols. As a result, there is a rapidly growing population of pediatric cancer survivors in which the long-term genotoxic effects of chemotherapeutic intervention is unknown. To study the genotoxic effects of antineoplastic treatment in children, we performed a comparative analysis of the changes in the frequency of somatic mutations (Mfs) at the hypoxanthine-guanine phosphoribosyltransferase (HPRT)-reporter gene in children treated for acute lymphocytic leukemia (ALL). We measured HPRT Mfs from 130 peripheral blood samples from 45 children with ALL (13, low risk; 22, standard risk; and 10, high risk) from the time of diagnosis, as well as during and after the completion of therapy. We observed a significant increase in mean HPRT Mfs during each phase of therapy (diagnosis, 1.4 x 10(-6); consolidation, 52.1 x 10(-6); maintenance, 93.2 x 10(-6); and off-therapy, 271.7 x 10(-6)) that were independent of the risk group treatment protocol used. This 200-fold increase in mean somatic Mf remained elevated years after the completion of therapy. We did not observe a significant difference in the genotoxicity of each risk group treatment modality despite differences in the compositional and clinical toxicity associated with these treatment protocols. These findings suggest that combination chemotherapy used to treat children with ALL is quite genotoxic, resulting in an increased somatic mutational load that may result in an elevated risk for the development of multi-factorial diseases, in particular second malignancies.

From pharmacokinetics to pharmacogenomics: a new approach to tailor immunosuppressive therapy

One of the main tasks in the management of organ transplantation is the optimization of immunosuppressive therapy, in order to provide therapeutic efficacy limiting drug-related toxicity. In the past years major efforts have been carried out to define therapeutic windows based on blood/plasma levels of each immunosuppressant relating those concentrations to drug dosing and clinical events. Although this traditional approach is able to identify environmental and nongenetic factors that can influence drug exposure during the course of treatment, it presents limitations. Therefore, complementary strategies are advocated. The advent of the genomic era gives birth to pharmacogenomics, a science that studies how the genome as a whole, including single genes as well as gene-to-gene interactions, may affect the action of a drug. This science is of particular importance for drugs characterized by a narrow therapeutic index, such as the immunosuppressants. Preliminary studies focused on polymorphisms of genes encoding for enzymes actively involved in drug metabolism, drug transport and pharmacological target. Pharmacogenomics holds promise for improvement in the ability to individualize immunosuppressive therapy based on the patient's genetic profile, and can be viewed as a support to traditional therapeutic drug monitoring. However, the clinical applicability of this approach is still to be proven.

Age-related base excision repair activity in mouse brain and liver nuclear extracts

To assess DNA repair activity relative to age, in vitro base excision repair assays were performed using brain and liver nuclear extracts prepared from mice of various ages. An 85% decline in repair activity was observed in brain nuclear extracts and a 50% decrease in liver nuclear extracts prepared from old mice compared with 6-day-old mice. Brain nuclear extracts prepared from old mice showed a decreased abundance of DNA polymerase-beta, but the addition of purified protein did not restore base excision repair activity. Abundances of other tested base excision repair proteins did not change relative to age. The conclusion is that, during aging, a decline in DNA repair could contribute to increased levels of DNA damage and mutagenesis.

Mutational spectral analysis at the HPRT locus in healthy children

There is growing evidence linking somatic mutational events during fetal development and childhood to an increasing number of multifactorial human diseases. Despite this, little is known about the relationship between endogenous and environmentally induced exogenous mutations during human development. Here we describe a comparative spectral analysis of somatic mutations at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) reporter gene locus in healthy children. We observed an age-specific decrease in the proportion of large alterations and a corresponding increase in the proportion of small alterations with increasing age following birth (P<0.001). The age specific decrease in the proportion of large alterations (67-30%) was mainly due to a decrease in the proportion of aberrant variable (V), diversity (D) and joining (J) (V(D)J) recombinase mediated HPRT deletions (P<0.001). The increase in the proportion of small alterations with age (28-64%) was associated with an increase in transversions from 8% in children at the late stages of fetal development to 31% in children 12-16 years old (P=0.003). Transitions decreased with age, especially at CpG dinucleotides (P=0.010), as transversions increased (P=0.009). These patterns of mutations provide insight into important spontaneous, genotoxic, and site-specific recombinational somatic mutational events associated with the age-specific development of human disease in children as well as adults.

Frequent detection of T cells with mutations of the hypoxanthine-guanine phosphoribosyl transferase gene in patients with paroxysmal nocturnal hemoglobinuria

Acquired mutations of the PIG-A gene result in the hemolysis characteristic of paroxysmal nocturnal hemoglobinuria (PNH). Although the etiology of the mutation(s) is unclear, mutable conditions have been suggested by the coexistence of multiple clones with different mutations of PIG-A and by the appearance of leukemic clones in patients with PNH. This study sought to test this hypothesis by examining the frequency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene mutations, identified by both resistance to 6-thioguanine (6-TG) and gene analysis. T-cell colonies resistant to 6-TG formed in methylcellulose culture were found in 8 (67%) of 12 PNH patients and 3 (18%) of 17 age-matched healthy volunteers (P <.02, Fisher exact probability test). The incidence of resistant colonies ranged from 40 to 367 (mean 149, x 10(-7)) in the 8 patients and from 1 to 16 (mean 7, x 10(-7)) in the 3 healthy donors. Thus, the HRPT gene mutated more frequently in patients with PNH than in healthy controls (P <.02, Mann-Whitney test). Analysis of bone marrow cells supported these findings. Like the PIG-A mutations in PNH, the HPRT mutations were widely distributed in the coding regions and consisted primarily of base deletions. Unlike PNH cells, 6-TG-resistant cells expressed CD59, indicating that the HPRT mutations did not occur in PNH clones. No correlation was noted between HPRT mutation frequency and content of therapy received by the patients. It is concluded that in PNH patients, conditions exist that favor the occurrence of diverse somatic mutations in blood cells.

Chemotherapy-induced chromosomal damage in peripheral blood lymphocytes of cancer patients supplemented with antioxidants or placebo

A total of 27 patients with various types of cancer were treated with cisplatin-based combination chemotherapy. Out of these, 13 patients were randomized to receive supplementation treatment with a beverage containing the antioxidants vitamins C and E, plus selenium, during chemotherapy. The antioxidant mixture was administered to investigate whether it could reduce the potential genotoxic and nephrotoxic effect of the applied chemotherapy. A placebo group of 14 cancer patients received a beverage without selenium or antioxidants. Micronuclei (MN) in cytochalasin B-blocked binucleate (BN) peripheral blood lymphocytes (PBLs) and hypoxanthine phosphoribosyl transferase (HPRT) mutants in PBLs were studied before, during and after chemotherapy as a measure for chemotherapy-induced genotoxic effects. Before chemotherapy, patients mean frequencies of MN and HPRT mutants did not differ from those in a group of 10 healthy subjects. The mean frequency of MN in patients increased significantly after one cycle of chemotherapy (P=0.002). This frequency was still elevated at 2 months after the completion of chemotherapy (not significantly). There was no significant difference in micronuclei frequency (MNF) between the antioxidant and placebo group of patients. Chemotherapy-induced frequencies of MN after three cycles of chemotherapy correlated significantly with the cumulative dose of cisplatin (r=0.58, P=0.012) and the cisplatin-mediated loss of renal function (r=0.53, P=0.03). No consistent change in HPRT mutant frequency following chemotherapy was observed in the placebo and antioxidant group of patients. In conclusion, cisplatin-combination chemotherapy resulted in a cisplatin dose-related increase of the frequency of chromosomal damage. Supplementation with antioxidants did not prevent or reduce this effect.

Association between genetic polymorphisms and biomarkers in styrene-exposed workers

A comprehensive approach to evaluate genotoxic effects induced by styrene exposure was employed in 44 hand-lamination workers in comparison with 18 unexposed controls. The acquired data on single-strand breaks in DNA (SSBs), frequency of chromosomal aberrations and HPRT mutant frequency in peripheral blood lymphocytes were compared to the results on genotyping of some of the xenobiotic-metabolising enzymes (CYP1A1, CYP2E1, epoxide hydrolase and GSTM1, GSTP1 and GSTT1). Multifactorial regression analysis indicated that SSB in DNA were significantly associated with styrene exposure and with heterozygosity in CYP2E1 (5'-flanking region and intron 6; r(2)=0.614). The frequency of chromosomal aberrations (CA), as analysed by linear multiple regression analysis, significantly correlated with years of employment (P=0.004) and with combinations of epoxide hydrolase (EPHX) genotypes (exon 3, Tyr/His and exon 4, His/Arg), where individuals with low and medium activity EPHX genotypes exhibited higher frequencies of CA than those with high activity genotypes (P=0.044, r(2)=0.563). Moderately higher HPRT mutant frequencies were detected in styrene-exposed individuals (20.2 +/- 25.8 x 10(-6)) as compared to controls (13.3 +/- 6.3 x 10(-6)), but this difference was not significant. ANOVA (in the whole set of data) revealed that mutant frequencies at the HPRT gene were significantly associated with years of employment (F=6.9, P=0.0001), styrene in blood (F=10.1, P=0.0001), and heterozygosity in CYP2E1 (intron 6; F=13.5, P=0.0008) and GSTP1 (exon 5; F=3.6, P=0.038). In conclusion, our present data suggest that analysed biomarkers of DNA damage may be modulated by polymorphic CYP2E1, EPHX and GSTP1. In our study, styrene-specific DNA and haemoglobin adducts are under investigation. Completing these data with the results of genotyping of metabolising enzymes may provide a useful tool for individual genotoxic risk assessment.

Frequency of HPRT mutants in humans exposed to vinyl chloride via an environmental accident

The mutant frequency (MF) in the hypoxanthine-guanine-phosphoribosyl-transferase (HPRT) locus of peripheral blood T-lymphocytes was measured in a population environmentally exposed to vinyl chloride - a toxic and carcinogenic substance through an accidental release into the atmosphere. It was compared to MF in a control group of unexposed individuals. Both groups were re-investigated in a follow-up study, 2 years later. No significant difference could be observed in MF between exposed and controls either at the accident nor in the follow-up study. Approximately the same mean HPRT mutant frequencies were observed for both groups in T-lymphocytes from blood samples obtained shortly after the accident and from the follow-up blood samples. Both groups showed a higher mean MF in the re-investigation samples which is most probably due to the significantly lower average cloning efficiency (CE) under non-selective conditions and because of the inverse relationship between CE and MF. The exposed population showed a higher mean T-cell CE at the initial blood sampling as compared to the control group. The concurrent cytogenetic analyses of peripheral lymphocytes showed a significant increase in cells with aberrations in the exposed population. Clastogenic but not mutagenic activity of vinyl chloride was observed in our study.

Mutagenicity at the Hprt locus in T cells of female mice following inhalation exposures to low levels of 1,3-butadiene

A study was conducted to test the hypothesis that repeated low level exposures to 1,3-butadiene (BD), approaching the OSHA occupational threshold for this chemical, produce a significant mutagenic response in mice. Female B6C3F1 mice (4-5 weeks of age) were exposed by inhalation for 2 weeks (6 h/day, 5 days/week) to 0 or 3 ppm BD, and then necropsied at 4 weeks after the cessation of exposures to measure the frequency of mutations (MF) at the Hprt locus using the T-lymphocyte clonal assay. At necropsy, T cells were isolated from spleen and cultured in the presence of mitogen, growth factors, and a selection agent. Cells were scored for growth on days 8-9 after plating to determine cloning efficiencies (CEs) and Hprt MFs. There was a marginal but significant reduction in the growth of splenic T cells from mice exposed to 3 ppm (n=27) compared with control mice (n=24) (P=0.004), suggesting the occurrence of BD-induced cytotoxicity at this low exposure concentration. In addition, the average Hprt MF in mice exposed to 3 ppm BD [1.54+/-0.82 (S.D.)x10(-6)] was significantly increased by 1.6-fold over the average control value of 0.96+/-0.51 (S.D.)x10(-6) (P=0.004). Comparisons of these data to earlier Hprt mutagenicity studies of mice exposed to high concentrations of BD (where significant mutagenic but not cytotoxic effects were observed) indicate that the ability to detect the cytotoxic and mutagenic responses of T cells to low levels of BD was enhanced by using a much larger sample size than usual for both the control and treatment groups. Additional analyses of the quantitative relationships between CE and MF demonstrated that CE had no significant effect upon MF values in sham-exposed control mice or mice exposed to low-level BD. Furthermore, the approaches for assessing the impact of CE and clonality on Hprt MFs in these control and BD-exposed mice were applied with the same rigor as in in vivo Hprt mutagenicity studies in human children. The overall study results support the conclusion that short-term low-level BD exposure is mutagenic in the mouse.

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.

The effect of vitamin C or vitamin E supplementation on basal and H2O2-induced DNA damage in human lymphocytes

There is a wealth of epidemiological information on antioxidants and their possible prevention of disease progression but very little of the research on antioxidants has involved intervention studies. In this study, the potential protective effect of vitamin C or E supplementation in vivo against endogenous and H2O2-induced DNA damage levels in lymphocytes was assessed. The supplementation involved fourteen healthy male and female non-smokers mean age 25-53 (SD 1.82) years, who were asked to supplement an otherwise unchanged diet with 1000 mg vitamin C daily for 42 d or 800 mg vitamin E daily for 42 d. DNA damage in H2O2-treated peripheral blood lymphocytes (PBL) and untreated PBL before and after supplementation, and during a 6-week washout period was assessed using an ELISA. At each sampling time-point, the red cell concentrate activities of superoxide dismutase, catalase and glutathione peroxidase were also determined. Supplementation with vitamin C or vitamin E decreased significantly H2O2-induced DNA damage in PBL, but had no effect on endogenous levels of DNA damage. The activities of the antioxidant enzymes superoxide dismutase and glutathione peroxidase were suppressed during the supplementation period. These supplementation regimens may be used to limit the possible adverse effects of reactive oxygen species (including those produced during the course of an immune response) on lymphocytes in vivo, and so help to maintain their functional capacity.

Similar mutant frequencies observed between pairs of monozygotic twins

The relative contribution of both genetic and environmental factors to spontaneous mutation frequency in humans is unknown. We have investigated the contribution of genetic factors to this phenomenon by determining the in vivo mutant frequency at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus in circulating T-lymphocytes obtained from pairs of monozygotic twins. hprt mutant frequencies were determined three times over fourteen days in six sets of monozygotic male twins (mean age 30) taking part in a Russian Space Program inclined bed rest experiment. Blood samples were obtained prior to, during, and immediately following the experiment. Mononuclear cells were separated, frozen, and flown to Canada for analysis using the hprt T-lymphocyte clonal assay. There is no evidence within this data set to demonstrate that the period of inclined bed rest to simulate the effects of weightlessness had any effect on the observed mutant frequency. However, the average mutant frequency for the six sets of Russian twins was found to be three times higher than that of Western counterparts. More surprisingly, the spontaneous mutant frequency of monozygotic twins was found to be much more similar within pairs than between pairs of twins. These data suggest that the contribution of genetics in the determination of mutation frequency is substantial. However, whether high concordance within twin pairs reflects shared environmental experience as well as common genetic factors is not entirely clear. More data will be required to distinguish genetic from environmental factors and to determine the degree to which mutant frequency is genetically determined.

Acquired DNA mutations associated with in vivo hydroxyurea exposure

Hydroxyurea (HU) is an effective therapeutic agent for patients with myeloproliferative disorders (MPDs) or sickle cell disease (SCD). Short-term HU toxicities primarily include transient myelosuppression, but long-term HU risks have not been defined. The mutagenic and carcinogenic potential of HU is not established, although HU has been associated with an increased risk of leukemia in some patients with MPD. In this study, 2 assays were used to quantitate acquired somatic DNA mutations in peripheral blood mononuclear cells (PBMCs) after in vivo HU exposure. The HPRT assay measures hypoxanthine phosphoribosyl transferase (hprt) mutations, while the VDJ assay identifies “illegitimate” T-cell receptor Vγ-Jβ interlocus recombination events. PBMCs were analyzed from patients with MPD, adults and children with SCD, and normal controls. MPD patients with prolonged HU exposure had numbers of DNA mutations equivalent to patients with low HU exposure or controls. Similarly, adults with SCD had equivalent numbers of DNA mutations regardless of HU exposure. Children with SCD and 30-month HU exposure had equivalenthprt− mutations but significantly more VDJ mutations (1.82 ± 1.20 events per μg DNA) than children with 7-month HU exposure (1.58 ± 0.87 events) or no HU exposure (1.06 ± 0.45 events), P = .04 by analysis of variance. Taken together, these data suggest that the mutagenic and carcinogenic potential of in vivo HU therapy is low. Although increased numbers of illegitimate VDJ recombination events do not directly portend leukemia, young patients with SCD and HU exposure should be monitored serially for increases in DNA mutations.

Acquired DNA mutations associated with in vivo hydroxyurea exposure

Hydroxyurea (HU) is an effective therapeutic agent for patients with myeloproliferative disorders (MPDs) or sickle cell disease (SCD). Short-term HU toxicities primarily include transient myelosuppression, but long-term HU risks have not been defined. The mutagenic and carcinogenic potential of HU is not established, although HU has been associated with an increased risk of leukemia in some patients with MPD. In this study, 2 assays were used to quantitate acquired somatic DNA mutations in peripheral blood mononuclear cells (PBMCs) after in vivo HU exposure. The HPRT assay measures hypoxanthine phosphoribosyl transferase (hprt) mutations, while the VDJ assay identifies “illegitimate” T-cell receptor Vγ-Jβ interlocus recombination events. PBMCs were analyzed from patients with MPD, adults and children with SCD, and normal controls. MPD patients with prolonged HU exposure had numbers of DNA mutations equivalent to patients with low HU exposure or controls. Similarly, adults with SCD had equivalent numbers of DNA mutations regardless of HU exposure. Children with SCD and 30-month HU exposure had equivalenthprt− mutations but significantly more VDJ mutations (1.82 ± 1.20 events per μg DNA) than children with 7-month HU exposure (1.58 ± 0.87 events) or no HU exposure (1.06 ± 0.45 events), P = .04 by analysis of variance. Taken together, these data suggest that the mutagenic and carcinogenic potential of in vivo HU therapy is low. Although increased numbers of illegitimate VDJ recombination events do not directly portend leukemia, young patients with SCD and HU exposure should be monitored serially for increases in DNA mutations.

Azathioprine associated T-cell mutations in insulin-dependent diabetes mellitus

Somatic mutations arise regularly in human T lymphocytes. As these events occur at increased frequencies in several autoimmune disorders, presumably because of increased T-cell proliferation, we investigated if this is also true for insulin-dependent diabetes mellitus (IDDM). Mutations of the hypoxanthine guanine phosphoribosyltransferase (hprt) gene measured by 6-thioguanine (TG) selection were studied in 28 patients (60 determinations) enrolled in a prospective double-blinded placebo-controlled study of azathioprine immunosuppression: 17 patients (34 determinations) were receiving azathioprine and 11 (26 determinations) placebo. Mean hprt T-cell mutant frequencies (MFs) were elevated in both patient groups, but only in the azathioprine group were elevations large and statistically correlated with the duration of the therapy. These results suggest that the organ-specific antigenic stimulus of the T-cell proliferation in IDDM does increase mutant cells in the peripheral blood, but this increase is relatively small. However, azathioprine, which is converted to 6-mercaptopurine in vivo, selects and amplifies the hprt mutants that do arise. Clinical azathioprine resistance may be explained by hprt mutations arising in T cells relevant to the underlying autoimmune process. Monitoring for these mutations should allow more effective use of this immunosuppressive agent.

Lipid peroxidation status, somatic mutations and micronuclei in peripheral lymphocytes: a case observation on a possible interrelationship

A controlled dietary study was conducted in healthy female volunteers and reported elsewhere [1]. In a subset of samples four different biomarkers were analyzed: plasma malondialdehyde (MDA) levels and urinary 8-isoprostaglandin-F(2alpha) were measured as markers for lipid peroxidation. The frequency of hprt (hypoxanthine guanine phosphoribosyl transferase) mutants and micronuclei in peripheral blood lymphocytes were analyzed as indicators of genotoxic effects. One of the ten individuals showed extremely high background levels in all of the four endpoints measured. This case observation raises the possibility that life style factors and dietary habits affect the level of DNA reactive lipid peroxidation products, which in turn increase mutagenic and cytogenetic effects. A possible association between these biomarkers, particularly in relation to dietary fat intake and antioxidant status, should now be studied in a larger trial.

Population risk and physiological rate parameters for colon cancer. The union of an explicit model for carcinogenesis with the public health records of the United States

The relationship between the molecular mechanisms of mutagenesis and the actual processes by which most people get cancer is still poorly understood. One missing link is a physiologically based but quantitative model uniting the processes of mutation, cell growth and turnover. Any useful model must also account for human heterogeneity for inherited traits and environmental experiences. Such a coherent algebraic model for the age-specific incidence of cancer has been developing over the past 50 years. This development has been spurred primarily by the efforts of Nordling [N.O. Nordling, A new theory on the cancer-inducing mechanism, Br. J. Cancer 7 (1953) 68-72], Armitage and Doll [P. Armitage, R. Doll, The age distribution of cancer and a multi-stage theory of carcinogenesis, Br. J. Cancer 8 (1) (1954) 1-12; P. Armitage, R. Doll, A two-stage theory of carcinogenesis in relation to the age distribution of human cancer, Br. J. Cancer 9 (2) (1957) 161-169], and Moolgavkar and Knudson [S.H. Moolgavkar, A.G. Knudson Jr., Mutation and cancer: a model for human carcinogenesis. JNCI 66 (6) (1981) 1037-1052], whose work defined two rate-limiting stages identified with initiation and promotion stages in experimental carcinogenesis. Unfinished in these efforts was an accounting of population heterogeneity and a complete description of growth and genetic change during the growth of adenomas. In an attempt to complete a unified model, we present herein the first means to explicitly compute the essential parameters of the two-stage initiation-promotion model using colon cancer as an example. With public records from the 1930s to the present day, we first calculate the fraction at primary risk for each birth year cohort and note historical changes. We then calculate the product of rates for n initiation-mutations, the product of rates for m promotion-mutations and the average growth rate of the intermediate adenomatous colonies from which colon carcinomas arise. We find that the population fraction at primary risk for colon cancer risk was historically invariant at about 42% for the birth year cohorts from 1860 through 1930. This was true for each of the four cohorts we examined (European- and African-Americans of each gender). Additionally, the data indicate an historical increase in the initiation-mutation rates for the male cohorts and the promotion-mutation rates for the female cohorts. Interestingly, the calculated rates for initiation-mutations are in accord with mutation rates derived from observations of mutations in peripheral blood cells drawn from persons of different ages. Adenoma growth rates differed significantly between genders but were essentially historically invariant. In its present form, the model has also allowed us to calculate the rate of loss of heterozygosity (LOH) or loss of genomic imprinting (LOI) in adenomas to result in the high LOH/LOI fractions in tumors. But it has not allowed us to specify the number of events m required during promotion.

Investigation of mutant frequency at the HPRT locus and changes in microsatellite sequences in healthy young adults

In an attempt to understand the inter-individual variation that occurs in in vivo mutant frequency at the HPRT locus, we have examined the effect of polymorphisms in genes for metabolic enzymes on the mutation rate. In the same population of human volunteers, the background variant frequency in a number of microsatellite sequences was studied to determine individual variation in the capacity to repair mismatches in these sequences. The HPRT mutant frequency of T-cells isolated from a group of 49 healthy, non-smoking adults varied from 0.25 to 9.64 x 10(-6). The frequency of polymorphisms in CYP1A1, GSTM1 and NAT2 among these individuals was similar to those published, and when subjected to univariate analysis these polymorphisms showed no influence on the HPRT mutant frequency. However, there was a significant interaction between the GSTM1 null genotype and the slow acetylator status in NAT2 (P < 0.05) which was associated with higher mutant frequency. Analysis of 30 microsatellite sequences in 20 HPRT proficient clones per individual showed only six alterations in total, giving an overall mutation rate per allele of 0.01%, whilst three alterations were found in five HPRT deficient clones per individual examined for changes in 10 microsatellites, giving an overall mutation rate per allele of 0.3%. Thus, the alterations detected are probably due to background mutations and not to differences in mismatch repair capacity.

Measurement of HPRT mutations in splenic lymphocytes and haemoglobin adducts in erythrocytes of Lewis rats exposed to ethylene oxide

Young adult male Lewis rats were exposed to ethylene oxide (EO) via single intraperitoneal (i.p.) injections (10-80 mg kg-1) or drinking water (4 weeks at concentrations of 2, 5, and 10 mM) or inhalation (50, 100 or 200 ppm for 4 weeks, 5 days week-1, 6 h day-1) to measure induction of HPRT mutations in lymphocytes from spleen by means of a cloning assay. N-ethyl-N-nitrosourea (ENU) and N-(2-hydroxyethyl)-N-nitrosourea (HOENU) were used as positive controls. Levels of N-(2-hydroxyethyl)valine (HOEtVal) adducts in haemoglobin (expressed in nmol g-1 globin) were measured to determine blood doses of EO (mmol kg-1 h, mM h). Blood doses were used as a common denominator for comparison of mutagenic effects of EO administered via the three routes. The mean HPRT mutant frequency (MF) of the historical control was 4.3 x 10(-6). Maximal mean MFs for ENU (100 mg kg-1) and HOENU (75 mg kg-1) were 243 x 10(-6) and 93 x 10(-6), respectively. In two independent experiments, EO injections led to a statistically significant dose-dependent induction of mutations, with a maximal increase in MF by 2.3-fold over the background. Administration of EO via drinking water gave statistically significant increases of MFs in two independent experiments. Effects were, at most, 2.5-fold above the concurrent control. Finally, inhalation exposure also caused a statistically significant maximal increase in MF by 1.4-fold over the background. Plotting of mutagenicity data (i.e., selected data pertaining to expression times where maximal mutagenic effects were found) for the three exposure routes against blood dose as common denominator indicated that, at equal blood doses, acute i.p. exposure led to higher observed MFs than drinking water treatment, which was more mutagenic than exposure via inhalation. In the injection experiments, there was evidence for a saturation of detoxification processes at the highest doses. This was not seen after subchronic administration of EO. The resulting HPRT mutagenicity data suggest that EO is a relatively weak mutagen in T-lymphocytes of rats following exposure(s) by i.p. injection, in drinking water or by inhalation.

Validation of the human T-lymphocyte cloning assay--ring test report from the EU concerted action on HPRT mutation (EUCAHM)

The T-cell cloning assay, which enables the enumeration and molecular analysis of 6-thioguanine resistant (HPRT-negative) mutant T-cells, has been extensively used for studying human somatic gene mutation in vivo. However, large inter-laboratory variations in the HPRT mutant frequency (MF) call for further investigation of inter-laboratory differences in the experimental methodology, and development of an optimal but easy uniform cloning protocol. As part of the EU Concerted Action on HPRT Mutation (EUCAHM), we have carried out two Ring tests for the T-cell cloning assay. For each test, duplicate and coded samples from three buffy coats were distributed to five laboratories for determination of MF using six different protocols. The results indicated a good agreement between split samples within each laboratory. However, both the cloning efficiencies (CEs) and MFs measured for the same blood donors showed substantial inter-laboratory variations. Also, different medium compositions used in one and the same laboratory resulted in a remarkable difference in the level of MF. A uniform operating protocol (UOP) was proposed and compared with the traditional protocols in the second Ring test. The UOP (preincubation) increased the CE in laboratories traditionally using preincubation, but decreased the CE in laboratories traditionally using priming. Adjusted for donor, use of different protocols contributed significantly to the overall variation in lnCE (P = 0.0004) and lnMF (P = 0.03), but there was no significant laboratory effect on the lnCE (P = 0.38) or lnMF (P = 0.14) produced by the UOP alone. Finally, a simplified version of the UOP using the serum-free medium X-Vivo 10 and PMA was tested in one laboratory, and found to produce a considerable increase in CE. This modified UOP needs to be further evaluated in order to be used for future databases on HPRT MFs in various populations.

Study on genotoxic effects of the space environment: a comparison between experienced cosmonauts and unexposed Russian twins

The molecular analysis of T-lymphocytes from experienced cosmonauts and seven pairs of unexposed twins was performed [M. Khaidakov, D. Young, H. Erfle, A. Mortimer, Y. Voronkov, B.W. Glickman, Molecular analysis of mutations in T-lymphocytes from experienced soviet cosmonauts, Environ. Mol. Mutagen, 30 (1997) 21-30; J. Curry, G. Bebb, J. Moffat, D. Young, M. Khaidakov, A. Mortimer, B.W. Glickman, Similar mutant frequencies observed between monozygotic twins, Human Mutation, 9 (1997) 445-451]. Hprt mutant frequencies (MF) in both datasets were considerably higher (38.0+/-14.6x10(-6) in cosmonauts, and 18.5+/-8.9x10(-6) in twins) than in the background Western control (8-12x10(-6)), [A.D. Tates, F.J. van Dam, H. van Mossel, H. Shoemaker, J.C.P. Thijssen, V.M. Woldring, A.H. Zwinderman, A.T. Natarajan, Use of the clonal assay for the measurement of frequencies of HPRT mutants in T-lymphocytes from five control populations, Mutation Res., 253 (1991) 199-213; R.F. Branda, L.M. Sullivan, J.P. O'Neill, M.T. Falta, J.A. Nicklas, B. Hirsch, P.M. Vacek, R.J. Albertini, Measurement of HPRT mutant frequencies in T-lymphocytes from healthy human populations, Mutation Res., 285 (1993) 267-279]. The distribution of mutations by class in the twin dataset was essentially similar to the background Western control, whereas cosmonaut samples demonstrated a significant excess of splice errors and complex mutations. The distribution of base substitutions showed similar trends in both the cosmonaut and twin samples, which are quite distinct compared to those seen in the Western control. The differences observed between cosmonaut and twin samples (a 2-fold higher MF and an excess of complex mutations in cosmonaut mutational spectra) could be an indication of possible effects of the space environment. However, these changes could also be age-related because the twin group was, on average, 17 years younger. Moreover, very similar patterns of base substitution distribution in both datasets suggest the involvement of certain region-specific factors reflected in mutational spectra. In order to discriminate between occupation and region-specific factors contributing to mutagenesis, an additional study involving trainees and cosmonauts with recent long-term flight experience is required.

An evaluation of styrene genotoxicity using several biomarkers in a 3-year follow-up study of hand-lamination workers

A study employing several biomarkers of styrene exposure and genotoxicity was carried out in a group of lamination (reinforced plastic) workers and controls, who had been repeatedly sampled during a 3-year period. Special attention will be paid to the last sampling (S.VI), reported here for the first time. Styrene concentration in the breathing zone, monitored by personal dosimeters, and urinary mandelic acid (MA) were measured as indicators of external exposure. Blood samples were assayed for styrene-specific O6-guanine adducts in DNA, N-terminal valine adducts of styrene in haemoglobin, DNA single-strand breaks (SSB), determined by use of the single cell gel electrophoresis (Comet) assay), and hypoxanthine guanine phosphoribosyl transferase (HPRT) mutant frequencies (MF) in T-lymphocytes. O6-styrene guanine adduct levels were significantly higher in the exposed group (5.9 +/- 4.9 adducts/10(8) dNp) as compared to laboratory controls (0.7 +/- 0.8 adducts/10(8) dNp; P = 0.001). DNA adduct levels significantly correlated with haemoglobin adducts, SSB parameters and years of employment. Styrene-induced N-terminal valine adducts were detected in the lamination workers (1.7 +/- 1.1 pmol/g globin), but not in the control group (detection limit 0.1 pmol/g globin). N-terminal valine adducts correlated strongly with external exposure indicators, DNA adducts and HPRT MF. No significant correlation was found with SSB parameters. A statistically significant difference in HPRT MF was observed between the laminators (22.3 +/- 10.6/10(6)) and laboratory controls (14.2 +/- 6.5/10(6), P = 0.039). HPRT MF in the laminators significantly correlated with styrene concentration in air, MA and haemoglobin adducts, as well as with years of employment and age of the employees. No significant difference (P = 0.450) in MF between the laminators and the factory controls was observed. Surprisingly, we detected differences in MF between sexes. When data from all measurements were combined, women showed higher MF (geometric mean 15.4 vs. 11.2 in men, P = 0.020). The styrene-exposed group exhibited significantly higher SSB parameters (tail moment (TM), tail length (TL) and the percentage of DNA in the tail (TP)) than the control group (P < 0.001). SSB parameters correlated with indicators of external exposure and with O6-styrene guanine adducts. No significant correlation was found between SSB parameters and haemoglobin adducts or HPRT MF. The data encompassing biomarkers from repeated measurements of the same population over a 3-year period are discussed with respect to the mechanisms of genotoxic effects of styrene and the interrelationship of individual biomarkers.

Influence of sex, smoking and age on human hprt mutation frequencies and spectra

Examination of the literature for hprt mutant frequencies from peripheral T cells yielded data from 1194 human subjects. Relationships between mutant frequency, age, sex, and smoking were examined, and the kinetics were described. Mutant frequency increases rapidly with age until about age 15. Afterward, the rate of increase falls such that after age 53, the hprt mutant frequency is largely stabilized. Sex had no effect on mutant frequency. Cigarette smoking increased mean mutant frequency compared to nonsmokers, but did not alter age vs. mutant frequency relationships. An hprt in vivo mutant database containing 795 human hprt mutants from 342 individuals was prepared. No difference in mutational spectra was observed comparing smokers to nonsmokers, confirming previous reports. Sex affected the frequency of deletions (>1 bp) that are recovered more than twice as frequently in females (P = 0. 008) compared to males. There is no indication of a significant shift in mutational spectra with age for individuals older than 19 yr, with the exception of A:T --> C:G transversions. These events are recovered more frequently in older individuals.

Genetic instability and the etiology of somatic PIG-A mutations in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a hematologic disorder characterized by acquired PIG-A gene mutations that lead to defective bioassembly of glycosylphosphatidylinositol (GPI) anchors and the absence of GPI-linked surface proteins. As the etiology of these acquired PIG-A gene mutations is unknown, we hypothesized that patients with PNH have overall genetic instability and acquire somatic mutations throughout their genome. We first analyzed microsatellite sequences and found equivalent size variation using DNA from GPI-negative granulocytes compared with the DNA of paired GPI-positive B cell lines or normal granulocytes. We next quantitated the frequency of mutations at the hypoxanthine-guanine phosphoribosyl transferase (hprt) gene locus, and found 1 PNH patient with a large increase in hprt mutant frequency (256.7 x 10(-6) vs. 27.8 +/- 19.9 x 10(-6) for normal adults) that was confirmed on 4 independent blood samples. We also quantitated "illegitimate" VDJ genetic recombination events between the T cell receptor V gamma and J beta gene loci, and found a second PNH patient with a large increase (43.5 events per microgram of DNA vs. 1.3 +/- 0.8 events per microgram of DNA for normal adults), confirmed on 4 independent DNA samples. Both of these PNH patients are young females with no history of aplastic anemia. Our data show that PNH patients can have increased numbers of acquired somatic mutations in gene loci distinct from PIG-A. These data suggest that genetic instability may be associated with the development of PIG-A mutations that lead to the clinical picture of PNH.

Atypical background somatic mutant frequencies at the HPRT locus in children and adults with Down syndrome

People with Down syndrome are 10-30 fold more likely to develop leukemia than the normal population. To date, little is known regarding the molecular mechanisms underlying this phenomenon. We have previously demonstrated that the spontaneous somatic mutant frequency (Mf) at a reporter gene, hypoxanthine-guanine phosphoribosyl transferase (HPRT), from a normal population showed a strict age dependency with an exponential increase in Mf from birth to late adolescents with a subsequent linear 2-5% increase per year in adults. In this study, we compared HPRT Mf in children and adults with Down syndrome using the HPRT T-cell cloning assay. We determined the Mf at the HPRT locus in 27 subjects with Down syndrome from ages 6 months to 53.4 years. Results demonstrated that background somatic Mf at the HPRT locus in children and adults with Down syndrome are not dependent on age as seen in a normal control population. Results also show that adults with Down syndrome have a significantly lower Mf than normal adults, and that children with Down syndrome have a significantly higher Mf than normal children, although the latter appears to be due to a decreased cloning efficiency (CE). These observations demonstrate that the frequency of spontaneous somatic mutations in children and adults with Down syndrome are atypical compared to normal controls, and suggest that the genetic mechanisms associated with background somatic mutational events in children and adults with Down syndrome may be different.

Gene-mutation assays in lambda lacZ transgenic mice: comparison of lacZ with endogenous genes in splenocytes and small intestinal epithelium

Comparison of results derived from transgenic animal gene-mutation assays with those from mutation analyses in endogenous genes is an important step in the validation of the former. We have used lambda lacZ transgenic mice to study alkylation-induced mutagenesis in vivo in (a) lacZ and hprt in spleen cells, and (b) lacZ and Dlb-I in small intestine from lambda lacZ+/0/Dlb-Ia/b mice. Induction of mutations by ethyl- and methylnitrosourea (ENU, MNU) and ethyl methanesulphonate (EMS) was investigated at 7 weeks after a single i.p. dose of each of these chemicals. In the small intestine, treatment with various dosages of ENU (10-150 mg/kg) resulted in a linear dose-response in both lacZ and Dlb-I. MNU (30 mg/kg) was also mutagenic in lacZ and Dlb-I, while EMS (250 mg/kg) did not significantly induce mutations in either gene. In spleen, ENU gave a linear dose-related response in both lacZ and hprt, MNU induced mutation sin both lacZ and hprt, and EMS was only positive for lacZ. No differences in response were observed between single and split-dose treatment with ENU (1 x 50 or 5 x 10 mg/kg with a 1- or 7-day interval), both in spleen and small intestine, except for lacZ in small intestine, where the single high dose gave a significantly higher induction than the split dose with the 7-day interval. The overall results suggest that mutagenic effects of fractionated doses are generally additive. In most cases, the induction factor (ratio treated over controls) for mutations in lacZ was lower than that for hprt and Dlb-I, presumably due to a higher background in lacZ and/or a lower mutability of lacZ. The general concordance between the data for lacZ and the endogenous genes indicates that lambda lacZ transgenic mice are a suitable model to study induction of gene mutations in vivo.

Factors contributing to discrepancies in population monitoring studies

A review of the scientific literature on population monitoring studies (on non-accidentally exposed populations) frequently show that many of these studies using similarly exposed populations and the same laboratory techniques do not produce consistent results. To illustrate the problem, a brief review of studies using well validated techniques (chromosome aberrations and hprt gene mutation) to elucidate genotoxic effects of cigarette smoking is presented. Although many factors can contribute to the generation of discrepant results, two obvious factors are small sample sizes and inadequate experimental data. In addition, a new factor on genetic susceptibility should be considered in population studies whenever appropriate. The new factor is based on recent data showing the influence of polymorphic metabolizing genes on response to environmental mutagens towards biological effects and disease outcome. The common ones include the cytochrome P450 and the glutathione S-transferase genes. The inclusion of susceptibility factors in population monitoring may revolutionize the approach for health risk assessment and for environmental regulations.

DNA damage and mutation: contributors to the age-related alterations in T cell-mediated immune responses?

The genetic material of our cells is susceptible to damage by a wide variety of extrinsic and intrinsic entities. The amount of genetic damage accumulated in vivo will depend upon an individual's ability to defend against and/or repair DNA damage. T cells in vivo have been shown to accumulate DNA damage and mutations over time. The accumulation of such genetic damage will occur in T cells possessing a 'naive' or a 'memory' phenotype. Since T cells are required to undergo extensive clonal expansion upon antigenic stimulation, DNA damage and mutations may result in: a failure of T cells to proliferate, because of DNA damage-mediated cell cycle arrest; decreased rates of proliferation, as a consequence of selection in vivo against cells carrying certain mutations and/or apoptosis, triggered by critical levels of DNA damage. Thus, when T cells, containing critical levels of genetic damage, are required to undergo rapid clonal expansion in the presence of antigen, insufficient numbers of T cells may be produced and so the immune response would be sub-optimal. In this paper the possible contribution of DNA damage and/or mutation to the age-related alterations in T cell-mediated immune responses will be discussed.

Human in vivo somatic mutation measured at two loci: individuals with stably elevated background erythrocyte glycophorin A (gpa) variant frequencies exhibit normal T-lymphocyte hprt mutant frequencies

A survey of glycophorin A (gpa) in vivo somatic cell mutation in a population of 394 healthy people from 8 to 77 years of age (mean age +/- SD 41 +/- 15 years) revealed a subset of 37 individuals with stably elevated allele-loss and/or allele-loss with duplication variant erythrocyte frequencies (Vf) exceeding 30 x 10(-6). These 37 individuals with gpa outlier Vf are significantly older (P < 0.001) than the remainder of the larger study population from which they were drawn reflecting a highly significant increase in the prevalence of these individuals in the population beyond age 40 years. A study of hpt mutant frequencies (Mf) in the peripheral blood T-lymphocytes of 27 of these individuals, together with 15 matched control individuals with unremarkable gpa Vf, was undertaken to determine if these subjects also displayed elevated mutation frequencies at this independent locus indicative of globally elevated somatic mutation. The hprt Mf in these 27 subjects (geometric mean 11.5 x 10(-6)(dispersion interval 5.8 x 10(-6) to 22.8 x 10(-6)) was not significantly different from that observed in the 15 controls (geometric mean 12.1 x 10(-6)(dispersion interval 5.7 x 10(-6) to 25.5 x 10(-6)). These Mf are higher than typically reported values reflecting the older age distribution of these individuals (arithmetic mean age +/- SD 53 +/- 12 and 50 +/- 16 years for the subjects and controls, respectively). Taken together, these data suggest that several genetic mechanisms may be responsible for producing the gpa outlier Vf observed in these subjects. The observation that hprt Mf were not increased indicates that the majority did not arise by a genome-wide increased rate of somatic mutation detectable at both loci. The fixation and subsequent expansion of 'jackpot' mutations at the gpa locus occurring early in embryonic/fetal development also does not appear to be a predominant mechanism. Some cases may result from a stable over-representation of gpa variant cells, perhaps associated with a marked age-dependent decrease in the number of contributing erythroid stem cells in the bone marrow. The subset that displays elevated allele-loss with duplication Vf involving both gpa alleles may represent individuals with increased rates of somatic recombination. Elevations arising by this mechanism are not detected in the hprt assay, but could be confirmed using a autosomal locus in vivo somatic cell mutation endpoint such as the hla-a assay. Of primary biological significance, these results demonstrate that genetics/stochastic processes leading to the loss of heterozygosity of somatic cells occur ubiquitously in humans and in some individuals this level of somatic mosaicism can approach a frequency of 10(-3) at the gpa locus in erythroid lineage cells.

Determination of HPRT mutant frequency and molecular analysis of T-lymphocyte mutants derived from coke-oven workers

We measured the frequency of mutant (MF) lymphocytes at the hprt locus in a population of 43 coke-oven workers exposed to PAH and in a group of 26 non-exposed workers. A non-significant increase in MF in the exposed group (19.0 +/- 16.3) compared to the non-exposed group (15.8 +/- 14.6) was observed. Moreover, when we considered smoking habits for the overall population, the MF values were higher, although not significantly, in smokers than in non-smokers. For some T-cell mutant clone structural alterations, splicing and coding errors were detected by PCR-based methods. We analysed 161 HPRT- clones, derived from exposed and non-exposed workers by multiplex-PCR and 56 HPRT- clones by reverse transcriptase-PCR. Overall, the percentages of the different types of gene alterations were similar in exposed and non-exposed subjects. Only the frequency of splice mutations in mutant clones derived from coke-oven workers was higher (22%) than in non-exposed donors (11%).

Monitoring of occupational exposure to epichlorohydrin by genetic effects and hemoglobin adducts

The present work is focused on the determination of in vivo doses and studies of genetic effects in workers exposed to epichlorohydrin (ECH). The studied endpoints were hemoglobin (Hb) adducts, frequencies of hprt mutants, micronuclei in cytochalasin B blocked binucleated lymphocytes, sister chromatid exchanges (SCE) and high frequency cells (HFC). Blood samples were collected from office clerks and ECH exposed factory workers at an industrial plant in Germany. The workers were exposed to 0.11-0.23 ppm ECH in the air 45 h per week and to 0.2-2.6 ppm for 3 h per week. Some Swedish non-exposed subjects were also used for Hb adduct measurements. The genetic data, HFC and SCE, showed a significant difference between exposed and unexposed donors. In contrast to earlier studies on SCE, no impact of smoking was observed. Effects on micronuclei were on the borderline of significance, whereas there was no effect for HPRT mutants. The average Hb adduct level was higher in exposed than in non-exposed donors, although the difference was only significant when the exposed group was compared to Swedish controls. Smoking gave significantly increased adduct levels. The absence of significant correlations between individual data for Hb adducts and genetic effects, may be explained by the different periods of time covered by the responses in these endpoints. Whereas Hb adducts reflect the exposure during up to 4 months (i.e. the life span of human erythrocytes), the SCE, and particularly the HFC, seem to accumulate for years in a long-lived fraction of T-lymphocytes without DNA repair. Thus, the adduct data does not reflect the exposure backwards in time unless it can be shown that exposure conditions have remained unchanged. The origin of the background adduct levels in non-smoking control persons is at present not known.

Biological effect monitoring in industrial workers from the Czech Republic exposed to low levels of butadiene

Blood samples were collected twice (in 1993 and 1994) from 19 workers exposed to 1,3-butadiene and 19 matched controls. Three exposed and three control subjects were the same in 1993 and 1994. Personal passive dosimetry was performed in 1993 and twice in 1994 on the day preceding blood sampling. Mean exposure level in 1994 was 1.76 +/- 4.20 ppm (S.D.) and individual exposure levels ranged between 0.012 ppm (detection limit) and 19.77 ppm. Using the clonal assay, geometric mean of hprt mutant frequencies adjusted for cloning efficiency, age and smoking were, respectively, 7.85 (+/- 7.09) x 10(-6) and 10.14 (+/- 9.16) x 10(-6) in pooled (1993 plus 1994) exposed and control subjects. The difference was not statistically significant indicating that 1,3-butadiene did not induce a detectable increase in mutations at the hprt locus. A similar result was obtained for the 1994 subjects alone. There was no difference between adjusted geometric mean mutant frequencies of exposed and unexposed non-smokers or between exposed and unexposed smokers. Analysis of chromosomal aberrations in lymphocytes from 1994 subjects indicated that the percentage of aberrant cells was significantly enhanced in exposed subjects. In 1993 (data not shown), it was impossible to demonstrate a significant increase of aberrant cells in subjects exposed to 1,3-butadiene. Frequencies of micronuclei in cytochalasin-B blocked binucleate lymphocytes in exposed and unexposed 1994 subjects were not significantly different. This was also the case for earlier samples analyzed in the same plant. Using the comet assay for 1994 subjects, no statistically significant difference was found between the whole group of exposed and unexposed subjects. This was true for both the comet tail length and the percentage of DNA in the tail. In exposed smokers, however, the comet tail length was significantly longer than in unexposed smokers. Unexpectedly, in unexposed smokers the tail length was significantly shorter than in unexposed non-smokers. It was also unexpected that the percentage of DNA in the comet tail was significantly lower in exposed non-smokers than in unexposed non-smokers.

Possible factors leading to a misjudgement of mutant frequencies in HPRT assay

Interactions between cloning efficiency (CE) and mutant frequency (MF) in the HPRT clonal assay in in vitro study were analysed. In 12 separate reconstruction experiments with independent pairs of wild type (WT) and mutant (HPRT-) clones, the CE of WT cells (Group 1) and the recovery of mutant cells in absence (Group 2), as well as in the presence of non-irradiated (Group 3), or irradiated (Group 4) WT cells (10(4) cells/well) was determined. The plating of mutant cells with irradiated WT cells improved their CEs by almost 30%. In contrast, the presence of non-irradiated WT cells led to a slight decline (10%) in CE of mutant cells, resulting in a significant difference between groups (p = 0.0083). The extent of decline in survival of mutant cells in the presence of non-irradiated WT cells negatively correlated (r = 0.3496, p < 0.05) with the initial CE of WT cells. The data suggest that the presence of WT cells in the selection plates may suppress the recovery of mutants in HPRT assay, and this negative effect is stronger in samples with high CE. These findings indicate a possible source for a serious underestimation of mutant frequencies (3-fold in the range of CEs from 10% to 60%) in the HPRT assay and may be useful for the interpretation of results from studies on exposure to mutagens in humans.

Intercorrelations and sources of variability in three mutagenicity assays: a population-based study

The purpose of this study was to evaluate the intercorrelation between three genetic assays in 112 subjects. The group was pooled from two originally separate but homogeneous subgroups of 56 persons each. Procedures included assays for hprt mutant frequencies, micronuclei in human lymphocytes, and mutations at the glycophorin A (gpa) loci. We found no statistically significant or biologically important intercorrelations among the three biomarkers. We did, however, observe significant correlations between log(e) hprt mutant frequency and cloning efficiency (inverse correlation for these 2 variables), age and log(e) hprt mutant frequency, an inverse relationship between cloning efficiency and age, and an important differential sex effect favoring a greater micronuclei frequency in females than males. No significant correlations between the covariates of interest and glycophorin A variant frequencies NN or NO were observed. Using multivariable linear regression, age was found to account for the majority of the variability in hprt mutant frequency (greater than sex and/or smoking); for micronuclei data, only sex contributed a statistically significant and biologically important proportion to the total variation. We conclude that despite observing no significant intercorrelations between the three assays performed simultaneously from the same individuals in a large population database, a significant correlation between age and hprt mutant frequency and an inverse association between cloning efficiency and hprt do exist; furthermore, we verified the strong differential sex-specific effect on micronucleus frequencies.

Increased hprt mutant frequencies in Brazilian children accidentally exposed to ionizing radiation

We have examined the effects of ionizing radiation on somatic mutations in vivo, using the hprt clonal assay. The study was performed on blood samples obtained from children exposed during a radiological accident that happened in 1987, in Goiânia, Brazil. The group of children exposed to ionizing radiation includes six males and four females ranging in age from 6 to 14 years at the time of exposure. The radiation doses ranged from 15 to 70 cGy. A Brazilian control group, not exposed to ionizing radiation, was also analyzed under similar conditions. the mean hprt mutant frequency for the exposed group was 4.6 times higher than the control group, although the cloning efficiency from the exposed group was significantly reduced. Linear regression analysis of the mutant frequency and ionizing radiation dose did not show a significant relationship between these two parameters. However, a reliable inverse relationship was demonstrated when the regression analysis was performed with nonselective cloning efficiency and ionizing radiation dose. It was demonstrated that nonselective cloning efficiency diminishes as ionizing radiation dose increases. To correct mutant frequencies for clonal events, the clonal relationship between the hprt mutant clones was examined by T-cell receptor analysis. The majority of the mutants analyzed represented individual clones, thus validating the observed mutant frequencies.

Monitoring hprt mutant frequency over time in T-lymphocytes of people accidentally exposed to high doses of ionizing radiation

Modern technologies have provided the opportunity to monitor mutations in people in vivo. The subjects of this study were accidentally exposed to 137Cesium in a radiological accident that occurred in September 1987 in Goiânia, Brazil, during which more than 150 people received doses greater than 0.1 Gy and as high as 7 Gy. The objective of this study was to determine how long the hprt mutant T-cells in the peripheral blood contribute to mutant frequency by examining the time-course of the T-lymphocyte response to ionizing radiation. This report describes the results obtained over a period of 2.3 to 4.5 years subsequent to the accident, from 11 subjects with doses ranging from 1 to 7 Gy, and from nine control subjects selected from the same population. The mean In MF (+/- SE) of the control group was 2.5 (+/- 0.2) + In10(-6). The exposed group had a significantly increased mutant frequency; the mean In MF (+/- SE) were 3.3 (+/- 0.3) + In10(-6), 2.8 (+/- 0.2) + In10(-6), and 2.3 (+/- 0.2) + In10(-6), in the years 1990-1992 respectively. Based on the decline of mutant frequency and using Buckton's models [Buckton et al. (1967): Nature 214:470-473], we demonstrated that mutant T-cells have a short-term memory with a half-life of 2.1 years. This relatively short half-life limits the effective use of the hprt assay as the method of choice to monitor past exposure. The data also demonstrate a positive correlation with age, and an inverse correlation with plating efficiency.

Mutant frequency at the hprt locus in human lymphocytes in a case-control study of lung cancer

A clonal assay to determine the mutant frequency (MF) at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus in human lymphocytes has been used by a number of investigators to study exposure to mutagens and carcinogens in a variety of populations. We have studied hprt MF in 106 subjects (40 controls and 66 cases) enrolled in a case-control investigation of lung cancer. Epidemiological data collected included smoking history, intake of dietary micronutrients, and occupational and environmental exposures as well as medical history, all obtained from an interviewer-administered questionnaire. All subjects were also genotyped for the known polymorphism in glutathione S-transferase class mu (GST-mu). In analysis of cases and controls, hprt MF was not associated with age, smoking, the polymorphism in GST mu, dietary intake, occupational exposures, family history of cancer or usage of medications. Since MF and cloning efficiency (CE) are not independent when CE is low, further analysis in cases and controls with a CE greater than or equal to 30% (27 cases and 22 controls) was also conducted. In analysis of controls, hprt MF increased with age and was inversely associated with intake of folate and vitamins A and C. The presence of lung cancer was not associated with hprt MF. Thus, our study supports the previous observation that dietary components may affect the MF at the hprt locus.

Do persons living near a uranium processing site have evidence of increased somatic cell gene mutations? A first study

The objective of this study was to examine if individuals living near a uranium processing site have greater mutagenic damage, as measured by three mutagenicity assays, compared with subjects unexposed to any nuclear facilities. The design was a cross-sectional exploratory analysis of 112 subjects; 56 volunteer residents were from within a 5-mile radius of the Fernald Uranium Processing site and 56 'control' subjects were from a geographically separate area unexposed to any known uranium emissions. The groups were constrained to be similar in age and sex composition. The main outcome measures were three human somatic gene mutation assays consisting of the HPRT T-lymphocyte cloning assay to measure 6-thioguanine resistant lymphocytes; the glycophorin A assay to detect the loss of expression of the M or N allele; and the micronucleus assay as a marker of chromosomal damage. The results showed no statistically significant or quantitatively important differences between groups for all three mutagenicity assays; only the unselected cloning efficiency was statistically significantly different between groups (0.42 +/- 0.16 for the Fernald versus 0.35 +/- 0.12 for the comparison groups). In both groups, age was significantly related to HPRT mutant frequency, with a 1.25% rate of increase in mutant frequencies for each 1-year gain of age in the Fernald group and a 1.12% rate of increase in mutant frequencies for each 1-year gain of age in the comparison group. For the micronucleus data, females had a greater mean micronucleus frequency than males. In addition, smokers had an increased mean ln (natural logarithm) HPRT mutant frequency (3.06 +/- 0.14 for current smokers compared with a mean of 2.72 +/- 0.05 for non-current (i.e. never plus former) smokers). Our results are consistent with the previously reported association between sex type and micronucleus frequency, the known relationship between age and T-lymphocyte cloning efficiency and age and HPRT mutant frequency, and verify the wide inter-subject variability for the latter. Finally, we conclude that at a population level, the relationships between current cigarette use and HPRT mutant frequency, and sex type and micronucleus frequency, are stronger than is the association between geographic proximity to a uranium processing site and mutagenic abnormalities.

Frequency of HPRT mutant lymphocytes in a human control population as determined by the T-cell cloning procedure

The T-cell-cloning assay was established to determine the frequency of hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutant lymphocytes in the presence of the selective agent 6-thioguanine in peripheral blood from a human control population. We investigated 44 healthy adults (blood donors) and found a mean mutant frequency of 7.2 x 10(-6) (geometric mean 5.6 x 10(-6). An elevated mean mutant frequency occurred in smokers as compared to non-smokers. However, a statistically significant increase was only observed between female smokers and female non-smokers while there was only a slight difference in the male group. A significant difference in mutant frequency could be found between individuals younger than 35 years and those above 35. But the difference of the mutant frequency with age showed up only among smokers. No significant effect of the gender was observed. Mutant frequency was inversely related to the cloning efficiency.

An investigation of antioxidant status, DNA repair capacity and mutation as a function of age in humans

We are constantly exposed, throughout life, to a wide variety of extrinsic and intrinsic agents which have the potential to damage cellular biomolecules, including DNA. Imperfections in cellular defence systems which protect against the fixation of DNA damage can lead to an accumulation of mutations which on their own, or in combination with other age-related changes, may contribute to ageing and the development of age-related pathologies. We have previously reported an increase in frequency of mutation with age in human lymphocytes taken from healthy males in the age groups, 35-39, 50-54 and 65-69 years. In this article we report on the findings of a recent study which was designed to assess whether the age-related increase in frequency of mutation was due to a decreased efficacy of the defence systems against ROS-induced DNA damage, namely antioxidant status and DNA repair processes, in the same study subjects. In vivo antioxidant status was assessed in each of the study subjects by measuring blood levels of; superoxide dismutase (SOD; EC 1.15.1.1), glutathione peroxidase (GPx; EC 1.11.1.9), catalase (EC 1.11.1.6), caeruloplasmin (CPL), uric acid and bilirubin. We did not find any statistically significant differences in the mean levels of these antioxidants between the three different age groups. To investigate the efficacy of DNA repair processes against ROS-induced DNA damage, an ELISA was used to quantitate DNA damage (as % single-stranded DNA; %SS-DNA) at various times following treatment of peripheral blood lymphocytes with hydrogen peroxide (H2O2). The results of this part of the study showed that in untreated lymphocytes, basal levels of %SS-DNA were significantly higher in individuals from the 65-69 years age group compared to the 35-39 years age group (p = 0.039, 0.0013; at 5% level of significance). No significant differences were found in H2O2 susceptibility with age immediately following treatment (p = 0.71, 1.00; at 5% level of significance) but a consistent and significant increase was observed in %SS-DNA remaining 90 min post-treatment in lymphocytes from subjects in the 65-69 years age group, compared to %SS-DNA present in lymphocytes from the 35-39 years age group (p = 0.013, 0.024; at 5% level of significance). The results of this study suggest that the age-related increase in frequency of mutations is not contributed to by alterations of in vivo antioxidant status with age but is by a decreased efficacy of the repair of ROS-induced DNA damage with age. The biological implications of somatic mutations in the ageing process are discussed.

Impact of age and environment on somatic mutation at the hprt gene of T lymphocytes in humans

Analysis of two human populations for dependence of somatic mutation on age has revealed both similarities and differences. The studies performed employed peripheral blood lymphocytes and measured the efficiency with which these cells form clones in vitro (cloning efficiency, CE) and the frequency of cells with inactivating mutations of the hypoxanthine phosphoribosyltransferase gene (mutant frequency, MF). The people studied were between 19 and 64 years of age. In one population, composed of 78 never smokers and 140 current smokers from the United States (US), both CE and MF were dependent on age: CE declined with age (p = 0.005); MF increased 0.15 per 10(6) cells per year of age for nonsmokers (p < 0.001) and at 1.3 times that rate for smokers (p = 0.01). In the second population, 80 people of unknown smoking status from Russia, the increase in MF per year was even greater, 2.5 times that of the US nonsmokers (p = 0.001) but the dependence of CE on age was the same as for the US population (p = 0.043). Because the increase of MF of the Russians with age is 2-fold greater than that of the US smokers, the intensity of smoking and/or other environmental exposures, or the susceptibility to these exposures, must account for the difference in age dependent MF increase, not the proportion of Russians that are smokers. Differences in the lymphocyte subpopulations that survived the longer transit from Russia may have contributed to the observed differences in MF. However, overall, the mutant frequency results suggest that the Russians were chronically exposed to higher levels of agents that induce somatic mutation and that, on an age adjusted basis, the Russia population studied is at increased risk for health consequences from accumulated genetic damage.