Measurement of HPRT mutant frequencies in T-lymphocytes from healthy human populations

Dietary and lifestyle factors effect erythrocyte PIG-A mutant frequency in humans

It is well understood that poor diet and lifestyle choices can increase the risk of cancer. It is also well documented that cancer is a disease of DNA mutations, with mutations in key genes driving carcinogenesis. Measuring these mutations in a minimally invasive way may be informative as to which exposures are harmful and thus allow us to introduce primary preventative measures, in a bid to reduce cancer incidences. Here, we have measured mutations in the phosphatidylinositol glycan class A (PIG-A) gene in erythrocytes from healthy volunteers (n = 156) and from non-cancer patients attending the local endoscopy department (n = 144). The X-linked PIG-A gene encodes an enzyme involved in glycosylphosphatidylinositol (GPI) anchor synthesis. A silencing mutation in which leads to the absence of GPI anchors on the extracellular surface which can be rapidly assessed using flow cytometry. The background level of PIG-A mutant erythrocytes was 2.95 (95% CI: 2.59-3.67) mutant cells (10-6). Older age increased mutant cell frequency (P < 0.001). There was no difference in mutant cell levels between males and females (P = 0.463) or smokers and non-smokers (P = 0.186). In the endoscopy group, aspirin users had lower mutant frequencies (P = 0.001). Further information on diet and exercise was available for the endoscopy patient group alone, where those with a higher health promotion index score had lower mutant frequencies (P = 0.011). Higher dietary intake of vegetables reduced mutant cell levels (P = 0.022). Participants who exercised for at least 1 h a week appeared to have reduced mutant frequencies than those who did not exercise, although this was not statistically significant (P = 0.099). This low background level of mutant erythrocytes in a population makes this assay an attractive tool to monitor exposures such as those associated with lifestyles and diet, as demonstrated here.

Somatic mutations - Evolution within the individual

With the rapid advancement of sequencing technologies over the last two decades, it is becoming feasible to detect rare variants from somatic tissue samples. Studying such somatic mutations can provide deep insights into various senescence-related diseases, including cancer, inflammation, and sporadic psychiatric disorders. While it is still a difficult task to identify true somatic mutations, relentless efforts to combine experimental and computational methods have made it possible to obtain reliable data. Furthermore, state-of-the-art machine learning approaches have drastically improved the efficiency and sensitivity of these methods. Meanwhile, we can regard somatic mutations as a counterpart of germline mutations, and it is possible to apply well-formulated mathematical frameworks developed for population genetics and molecular evolution to analyze this 'somatic evolution'. For example, retrospective cell lineage tracing is a promising technique to elucidate the mechanism of pre-diseases using single-cell RNA-sequencing (scRNA-seq) data.

A review of HPRT and its emerging role in cancer

Hypoxanthine guanine phosphoribosyltransferase (HPRT) is a common salvage housekeeping gene with a historically important role in cancer as a mutational biomarker. As an established and well-known human reporter gene for the evaluation of mutational frequency corresponding to cancer development, HPRT is most commonly used to evaluate cancer risk within individuals and determine potential carcinogens. In addition to its use as a reporter gene, HPRT also has important functionality in the body in relation to purine regulation as demonstrated by Lesch-Nyhan patients whose lack of functional HPRT leads to significant purine overproduction and further neural complications. This regulatory role, in addition to an established connection between other salvage enzymes and cancer development, points to HPRT as an emerging influence in cancer. Recent work has shown that not only is the enzyme upregulated within malignant tumors, it also has significant surface localization within some cancer cells. With this is mind, HPRT has the potential to become a significant biomarker not only for the characterization of cancer, but also for its potential treatment.

Aging and the rise of somatic cancer-associated mutations in normal tissues

DNA mutations are inevitable. Despite proficient DNA repair mechanisms, somatic cells accumulate mutations during development and aging, generating cells with different genotypes within the same individual, a phenomenon known as somatic mosaicism. While the existence of somatic mosaicism has long been recognized, in the last five years, advances in sequencing have provided unprecedented resolution to characterize the extent and nature of somatic genetic variation. Collectively, these new studies are revealing a previously uncharacterized aging phenotype: the accumulation of clones with cancer driver mutations. Here, we summarize the most recent findings, which converge in the novel notion that cancer-associated mutations are prevalent in normal tissue and accumulate with aging.

Breast Cancer beyond the Age of Mutation

Age is the greatest risk factor for breast cancer, but the reasons underlying this association are unclear. While there is undeniably a genetic component to all cancers, the accumulation of mutations with age is insufficient to explain the age-dependent increase in breast cancer incidence. In this viewpoint, we propose a multilevel framework to better understand the respective roles played by somatic mutation, microenvironment, and epigenetics making women more susceptible to breast cancer with age. The process of aging is associated with gradual breast tissue changes that not only corrupt the tumor-suppressive activity of normal tissue but also impose age-specific epigenetic changes that alter gene expression, thus reinforcing cellular phenotypes that are associated with a continuum of age-related tissue microenvironments. The evidence discussed here suggests that while the riddle of whether epigenetics drives microenvironmental changes, or whether changes in the microenvironment alter heritable cellular memory has not been solved, a path has been cleared enabling functional analysis leading to the prediction of key nodes in the network that link the microenvironment with the epigenome. The hypothesis that the accumulation of somatic mutations with age drives the age-related increase in breast cancer incidence, if correct, has a somewhat nihilistic conclusion, namely that cancers will be impossible to avoid. Alternatively, if microenvironment-driven epigenetic changes are the key to explaining susceptibility to age-related breast cancers, then there is hope that primary prevention is possible because epigenomes are relatively malleable.

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.

Monitoring humans for somatic mutation in the endogenous PIG-a gene using red blood cells

The endogenous X-linked PIG-A gene is involved in the synthesis of glycosyl phosphatidyl inositol (GPI) anchors that tether specific protein markers to the exterior of mammalian cell cytoplasmic membranes. Earlier studies in rodent models indicate that Pig-a mutant red blood cells (RBCs) can be induced in animals treated with genotoxic agents, and that flow cytometry can be used to identify rare RBCs deficient in the GPI-anchored protein, CD59, as a marker of Pig-a gene mutation. We investigated if a similar approach could be used for detecting gene mutation in humans. We first determined the frequency of spontaneous CD59-deficient RBCs (presumed PIG-A mutants) in 97 self-identified healthy volunteers. For most subjects, the frequency of CD59-deficient RBCs was low (average of 5.1 ± 4.9 × 10(-6) ; median of 3.8 × 10(-6) and mutant frequency less than 8 × 10(-6) for 75% of subjects), with a statistically significant difference in median mutant frequencies between males and females. PIG-A RBC mutant frequency displayed poor correlation with the age and no correlation with the smoking status of the subjects. Also, two individuals had markedly increased CD59-deficient RBC frequencies of ∼300 × 10(-6) and ∼100 × 10(-6) . We then monitored PIG-A mutation in 10 newly diagnosed cancer patients undergoing chemotherapy with known genotoxic drugs. The frequency of CD59-deficient RBCs in the blood of the patients was measured before the start of chemotherapy and three times over a period of ∼6 months while on/after chemotherapy. Responses were generally weak, most observations being less than the median mutant frequency for both males and females; the greatest response was an approximate three-fold increase in the frequency of CD59-deficient RBCs in one patient treated with a combination of cisplatin and etoposide. These results suggest that the RBC PIG-A assay can be adopted to measuring somatic cell mutation in humans. Further research is necessary to determine the assay's sensitivity in detecting mutations induced by genotoxic agents acting via different mechanisms.

Mutagenicity and potential carcinogenicity of thiopurine treatment in patients with inflammatory bowel disease

The thiopurines azathioprine and 6-mercaptopurine (6-MP) are effective immune modulators and cytotoxic agents extensively used in the treatment of autoimmune diseases, graft rejection, and cancer. There is compelling epidemiologic evidence that thiopurine treatment increases the risk for a variety of tumors by mechanisms that are unclear. We investigated the in vivo mutagenicity of long-term thiopurine treatment by determining the frequency and spectra of somatic mutation events at the hypoxanthine phosphoribosyltransferase (HPRT) locus in peripheral T lymphocytes as well as the prevalence of mutant clonal proliferation in a cross-sectional analysis of data from 119 children and adults with inflammatory bowel disease (IBD). ANOVA and regression were performed to assess relationships among the frequency and spectra of HPRT mutations with disease, duration of illness, duration of treatment, and total therapeutic dose of azathioprine and 6-MP. We observed a significant increase in the frequency of somatic mutations in 56 subjects treated with thiopurines for IBD compared with 63 subjects not treated with thiopurines. This increase was related to both total dose (P < 0.001) and duration of treatment (P < 0.001). Comparative mutation spectra analysis of 1,020 mutant isolates revealed a significant increase in the proportion of all transitions (P < 0.001), particularly G:C to A:T transitions (P < 0.001). Combined analyses of two signatures for mutant clonality, HPRT mutation, and T-cell receptor beta CDR3 region unique gene sequence also showed a significant thiopurine-dependent increase in mutant cell clonal proliferation (P < 0.001). These findings provide in vivo evidence for mutation induction as a potential carcinogenic mechanism associated with chronic thiopurine intervention.

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.

The L84F polymorphism in the O6-Methylguanine-DNA-Methyltransferase (MGMT) gene is associated with increased hypoxanthine phosphoribosyltransferase (HPRT) mutant frequency in lymphocytes of tobacco smokers

OBJECTIVES

O-methylguanine-DNA-methyltransferase (MGMT) is a crucial DNA repair protein that removes DNA adducts formed by alkylating mutagens. Several coding single nucleotide polymorphisms (cSNPs) in the MGMT gene have been reported. Their biological significance, however, is not known.

METHODS

We used a newly modified cloning HPRT mutant lymphocyte assay to test the hypothesis that inheritance of the L84F and I143V coding single nucleotide polymorphism in the MGMT gene is associated with increases in HPRT mutant frequency in lymphocytes of individuals exposed to alkylating agents. In addition, we expanded and sequenced 109 mutant clones to test the hypothesis that the mutation spectrum would shift to a larger percentage of base substitutions and G-->A transition mutations in cells with L84F and I143 V coding single nucleotide polymorphisms.

RESULTS

We observed no significant effect for the I143 V coding single nucleotide polymorphism on mutant frequency. In contrast, we observed a significant increase in mutant frequency (P<0.01) in lymphocytes from smokers with the 84F coding single nucleotide polymorphism compared with smokers homozygous for the referent L84 wild-type allele. A multiple regression analysis indicated that the mutant frequency increased significantly as a function of the 84F coding single nucleotide polymorphism and smoking, according to the model; mutant frequency (x10)=0.90+0.618 (84F polymorphism)+0.46 (smoking) with R=0.22. Mutation spectra analysis revealed an apparent increase, which was short of statistical significance (P=0.08), in base substitutions in cells with the 84F polymorphism.

CONCLUSIONS

These new data suggest that the 84F coding single nucleotide polymorphism may alter the phenotype of the MGMT protein, resulting in suboptimal repair of O-methylguanine lesions after exposure to alkylating agents.

Dose-Rate Effect on the Induction ofHPRTMutants in Human G0Lymphocytes ExposedIn Vitroto Gamma Radiation

The influence of dose rate on expression time, cell survival and mutant frequency at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus was evaluated in human G(0) peripheral blood lymphocytes exposed in vitro to gamma rays at low (0.0014 Gy/min) and high (0.85 Gy/min) dose rates. A cloning assay performed on different days of postirradiation incubation indicated an 8-day maximum expression period for the induction of HPRT mutants at both high and low dose rates. Cell survival increased markedly with decreasing dose rate, yielding D(0) values of 3.04 Gy and 1.3 Gy at low and high dose rates, respectively. The D(0) of 3.04 Gy obtained at low dose rate could be attributed to the repair of sublethal DNA damage taking place during prolonged exposure to low-LET radiation. Regression analysis of the mutant frequency yielded slopes of 12.35 x 10(-6) and 3.66 x 10(-6) mutants per gray at high and low dose rate, respectively. A dose and dose-rate effectiveness factor of 3.4 indicated a marked dose-rate effect on the induced HPRT mutant frequency. The results indicate that information obtained from in vitro measurements of dose-rate effects in human G(0) lymphocytes may be a useful parameter for risk estimation in radiation protection.

Association between Cigarette Smoking and Hypoxanthine Guanine Phosphoribosyltransferase Activity

The aim of this study was to investigate the association between smoking behavior and hypoxanthine guanine phosphoribosyltransferase (HGPRT) activity. A cross-sectional study was performed of 82 men, including 38 non-smokers and 44 smokers. Inosine monophosphate (IMP), the product of HGPRT (used as the index of activity), was measured in peripheral blood mononuclear cells using high-performance liquid chromatography. The factors potentially associated with HGPRT activity included age, glutamyl oxaloacetic transaminase, glutamyl pyruvic transaminase, cholesterol, uric acid, triglycerides, creatinine, body mass index, gout, systolic blood pressure, diastolic blood pressure, alcohol consumption, and cigarette smoking. Mean HGPRT activity was 7.05 +/- 3.44 nmol/10(6) viable cells/hour for all participants, and was significantly lower for smokers than for non-smokers (6.24 +/- 3.40 vs 7.98 +/- 3.28 nmol/10(6) viable cells/hour; p = 0.02). In addition, as the number of smoked cigarettes increased, the HGPRT activity decreased (p < 0.05). The age at onset of cigarette smoking showed a positive correlation with HGPRT activity after adjusting for smoking duration, serum uric acid, and cigarettes smoked per year using a multiple regression model (p < 0.001). We concluded that the greater the number of cigarettes smoked, the lower the HGPRT activity, and that HGPRT activity was higher in smokers who had started smoking later.

Somatic mutant frequency at the HPRT locus in children associated with a pediatric cancer cluster linked to exposure to two superfund sites

The somatic mutant frequency (Mf) of the hypoxanthine phosphoribosyl transferase (HPRT) gene has been widely used as a biomarker for the genotoxic effects of exposure but few studies have found an association with environmental exposures. We measured background Mfs in 49 current and former residents of Dover Township, New Jersey, who were exposed during childhood to industrially contaminated drinking water. The exposed subjects were the siblings of children who developed cancer after residing in Dover Township, where the incidence of childhood cancer has been elevated since 1979. Mfs from this exposed group were compared to Mfs in 43 age-matched, presumably unexposed residents of neighboring communities with no known water contamination and no increased cancer incidence. Statistical comparisons were based on the natural logarithm of Mf (lnMF). The mean Mf for the exposed group did not differ significantly from the unexposed group (3.90 x 10(-6) vs. 5.06 x 10(-6); P = 0.135), but unselected cloning efficiencies were higher in the exposed group (0.55 vs. 0.45; P = 0.005). After adjustment for cloning efficiency, lnMf values were very similar in both groups and age-related increases were comparable to those previously observed in healthy children. The results suggest that HPRT Mf may not be a sensitive biomarker for the genotoxic effects of environmental exposures in children, particularly when substantial time has elapsed since exposure.

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.

Distinct mechanisms lead to HPRT gene mutations in leukemic cells

Leukemias are considered malignant clonal disorders arising from the accumulation of mutations in hematopoietic cells; the majority of these mutations are thought to be acquired somatically. Measurement of mutation frequency (Mf) at the hypoxanthine phosphoribosyltransferase (HPRT) locus has been developed as a method for estimating genomic instability. We investigated the Mf in 16 leukemic cell lines to determine whether these cell lines showed evidence of genomic instability. Although some leukemic cell lines had markedly elevated Mfs, the Mfs at the HPRT locus in leukemic cell lines were not always higher than those of B-lymphoblastoid cell lines and T lymphocytes from normal individuals. We were able to identify the HPRT mutation for 159 of 160 individual HPRT mutants. The HPRT mutations were characterized at a molecular level and classified as either gross chromosomal rearrangements (GCRs) or point mutations, such as single-nucleotide substitutions, insertions, or deletions. With rare exceptions, individual leukemic cell lines showed either point mutations or GCR, but not both. Of note, all the cell lines that primarily showed point mutations are known to be defective in mismatch repair machinery.

Comparative analysis of HPRT mutant frequency in children with cancer

The link between exposure to environmental mutagens and the development of cancer is well established. Yet there is a paucity of data on the relationship between gene-environment interactions and the mechanisms associated with the somatic mutational events involved with malignant transformation, especially in children. To gain insight into somatic mutational mechanisms in children who develop cancer, we determined the background mutant frequency (Mf) in the hypoxanthine phosphoribosyl transferase (HPRT) reporter gene of peripheral blood lymphocytes from pediatric cancer patients at the time of diagnosis and prior to therapeutic intervention. We studied 23 children with hematologic malignancies and 31 children with solid tumors prior to initial therapeutic intervention. Children with solid tumors, specifically sarcomas, and Hodgkin's disease were significantly older and had elevated HPRT Mfs (6.1 x 10(-6) and 3.7 x 10(-6), respectively) at the time of diagnosis, compared to normal controls (2.3 x 10(-6)) and other pediatric tumor groups including children with acute lymphocytic leukemia and non-Hodgkin's lymphoma (ALL/NHL, 1.7 x 10(-6)), central nervous system tumors (CNS, 3.6 x 10(-6)), and neuroblastoma (1.9 x 10(-6)). Of importance is that the significant differences observed in HPRT Mfs between these groups no longer existed after correcting for the effects of age. These data demonstrate that in children who develop cancer there appears to be no significant increase in background HPRT Mf that would indicate significant exposure to genotoxic chemicals or an underlying DNA repair defect resulting in genomic instability. In addition, these data demonstrate the importance of correcting for the effect of age when comparing the frequency of somatic mutations in children and should provide baseline data for future longitudinal biomonitoring studies on the genetic effects of chemotherapy in children treated for cancer.

Mutation theory of aging, assessed in transgenic mice and knockout mice

A vital question in the mutation theory of aging is whether mutation accumulates with age. If it does, what are the causes and consequences of the accumulation of mutation? The recent development of transgenic mice has made it possible to study mutation in different kinds of tissues and at a molecular level. An application of these mice to the study of age-dependent alteration has revealed that mutation does accumulate in the aging process. Studies have also revealed several important characteristics of mutation associated with aging. (1) The rate of age-dependent increase of mutant frequency varies among different types of tissue. (2) The rate is not in parallel with the cell proliferation rate of the tissue. (3) Some types of mutation are unique to specific tissues, suggesting the presence of a mechanism of mutation relative to tissue type. On the other hand, several kinds of knockout mice defective in DNA repair have been shown to exhibit tissue lesions and shortened life span. These characteristics provide a new view on the relationship between aging and the genome maintenance system. Here we review the current status of research on the correlation between mutation and aging undertaken by the use of transgenic and knockout mice.

Different patterns of bcl-6 and p53 gene mutations in tonsillar B cells indicate separate mutational mechanisms

Mutations within the 5'-non-coding region of the bcl-6 gene can occur in lymphomas that originate from germinal centers (GCs), as well as in normal memory and GC B cells. Mutations in the p53 gene occur in 50% of human cancers. Since both bcl-6 and p53 can be mutated in certain circumstances, we investigated the accumulation of mutations in these genes in individual tonsillar B and T cells to determine whether the mutations exhibited a pattern anticipated from the B-cell hypermutation machinery. In tonsillar GC B cells, the overall mutational frequencies in the 5'-non-coding region of the bcl-6 gene was 0.85 x 10(-3)/bp. In contrast, there were no mutations in a region 2.8 kb downstream of the promoter. RGYW (purine, guanine, pyrimidine, A/T) targeting and a significantly lower mutational frequency in nai;ve B and GC founder B cells compared with GC B cells suggested that a similar mutator mechanism was active on Ig genes and this non-Ig gene. The mutational frequency in the exon-7-region of p53 was similar in the GC, memory and nai;ve B-cell subsets (1.02 x 10(-3) to 1.25 x 10(-3)/bp). RGYW/WRCY motifs were not targeted preferentially in the p53 gene. Moreover, a comparable mutational frequency of p53 was noted in tonsillar B and T cells. Hence, mutations in p53 do not appear to be the result of the B-cell hypermutational mechanism.

Flow cytometric determination of HPRT-variants in human peripheral blood lymphocytes

Hypoxanthine guanine phosphoribosyl transferase (HPRT) deficient human peripheral blood lymphocytes are usually enumerated either by the cloning assay or by the autoradiographic short-term assay. The short-term approach presented here is based on flow cytometric (FCM) scoring of 6-thioguanine (6-TG) resistant lymphocytes. HPRT-variants are enumerated on the basis of both DNA synthesis (by use of immunofluorescent detection of incorporated 5-bromo-2-deoxyuridine, BrdU) and total DNA content (by propidium iodide (PI) incorporation) of proliferating cells, i.e. the cells must both be labelled with BrdU and reside in late-S or G2 phase in order to be scored as a HPRT-variant. This approach is combined with a stringent discrimination of false-positive events, minimising occurrence of phenocopies or other non-specifically labelled cells that might falsely be scored as true HPRT-variants. The HPRT-variant frequency (V(f)) found by the presented method varied between 0.8 x 10(-5) and 5.8 x 10(-5) for healthy male and female donors aged between 20 and 74 years. There was no significant gender difference in V(f). A strong linear correlation was found between HPRT-variant frequency and age, showing an increase of 0.56 x 10(-6) per year of age (r(2)=0.62, P<0.001). The frequencies of false-positive events found showed a mean of 0.22 x 10(-5) in comparison with a pooled mean V(f) of 2.87 x 10(-5). There was no significant age effect on the frequency of false events (r(2)=0.15, P<0.095). The method presented here may provide a rapid and sensitive alternative to the autoradiographic technique for the short-term enumeration of HPRT-variants.

Accumulation of somatic mutations in proliferating T cell clones from children treated for leukemia

There is continued controversy as to the sequential steps and mechanism(s) responsible for the in vivo acquisition of multiple mutations during neoplastic transformation. We investigated the in vivo clonality and mutational spectra of hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutations in T cells from children with acute lymphocytic leukemia (ALL) to gain insight into the mutagenic mechanisms associated with leukemogenesis. We observed several instances of multiple, independent HPRT mutations accumulating in vivo in T cell receptor (TCR) gene defined clones that had undergone extensive pre- and/or post-thymic expansion following chemotherapy. In addition, we also detected the accumulation of multiple unique single mutations within distinct expanding post-thymic T cell clones. This pattern of clonally restricted hypermutability is compatible with extensive cell proliferation and selection alone without postulating genomic instability. These observations provide a paradigm for a continuum of cellular events that eventually results in the clonal accumulation of mutations in selected populations of cells in vivo and may provide insight into the primary genetic events associated with leukemogenesis, as well as the development of second malignancies and drug resistance following chemotherapy.

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.

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.

hprt mutant frequencies, nonpulmonary malignancies, and domestic radon exposure: "postmortem" analysis of an interesting hypothesis

The hypothesis that exposure to domestic radon raises the risk for leukemia and other nonpulmonary cancers has been proposed and tested in a number of epidemiologic studies over the past decade. During this period, interest in this hypothesis was heightened by evidence of increased frequencies of mutations at the hypoxanthine guanine phosphoribosyl transferase (hprt) gene in persons exposed to domestic radon (Bridges BA et al. [1991]: Lancet 337:1187-1189). An extension of this study (Cole J et al. [lsqb[1996]: Radiat Res 145:61-69) and two independent studies (Albering HJ et al. [1992[: Lancet 340:739; Albering HJ et al. [1994[: Lancet 344:750-751) found that hprt mutant frequency was not correlated with domestic radon exposure, and two well-designed epidemiologic studies showed no evidence of a relation between radon exposure and leukemia in children or adults. In this report, we present additional data from a study of Colorado high school students showing no correlation between domestic radon exposure and hprt mutant frequency. We use reanalyses of previous studies of radon and hprt mutant frequency to identify problems with this assay as a biomarker for domestic radon exposure and to illustrate difficulties in interpreting the statistical data. We also show with analyses of combined data sets that there is no support for the hypothesis that domestic radon exposure elevates hprt mutant frequency. Taken together, the scientific evidence provides a useful example of the problems associated with analyzing and interpreting data that link environmental exposures, biomarkers, and diseases in epidemiologic studies.

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.

Russian mutational spectrum differs from that of their Western counterparts

It has been previously noted that the hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutant frequency of Russian subjects is significantly higher than age-matched Western counterparts. To further explore this difference, approximately 100 mutants collected from Russian twins reported in a previous study have been sequenced and compared to an aged-matched Western mutant dataset. The mutational spectrum of the Russian subjects was significantly different (Adams and Skopek Monte Carlo test, P = 0. 004). Curiously, this younger Russian spectrum resembles that recovered from older individuals in the West. Specifically, A:T-->C:G transversions are significantly over-represented (Fisher's Exact test, P = 0.003) in the twin spectrum as compared to the young (age </= 35) Western spectrum. Even more noteworthy is the observation that 42% (23/55) of the base substitutions, almost double the expected value, have not been previously reported. These observations lead to the conclusion that this group of young Russian subjects has a mutational pattern which is distinct from the pattern of mutation observed in Western counterparts. The origin of this difference, whether related to genotoxic challenge, repair, or avoidance, cannot be distinguished, but diet and other lifestyle factors clearly may be responsible.

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.

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.

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.

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.

Elevated HPRT mutation frequencies in aflatoxin-exposed residents of daxin, Qidong county, People's Republic of China

Molecular biomarkers are becoming increasingly important tools to identify people who are at highest risk of developing cancer. For many years we have been studying residents of Qidong County, People's Republic of China, to examine the combined impact of aflatoxin exposure with other risk factors as contributors to the high liver cancer incidence rates in this region. This study was conducted to determine the effects of aflatoxin exposure, as measured by serum aflatoxin-albumin adduct levels, on somatic mutation frequency in the human hypoxanthine guanine phosphoribosyl transferase gene (HPRT). Subjects were assigned as low or high according to a dichotomization around the population mean of aflatoxin-albumin adducts. HPRT mutant frequency was determined in individuals by a T cell clonal assay and the samples were categorized as low or high according to mean values. Separate analyses were also conducted for the small set of hepatitis B virus surface antigen (HBsAg)-positive and the larger set of HBsAg-negative individuals, known risk factors for liver cancer. An odds ratio of 19.3 (95% confidence interval 2.0, 183) was demonstrated for a high HPRT mutation frequency in individuals with high aflatoxin exposure compared with those with low aflatoxin exposure. This association indicates that aflatoxin-induced DNA damage in T lymphocytes, assessed using the validated surrogate albumin adduct markers, leads to increased mutations reflected as elevated HPRT gene mutations. This cross-sectional study suggests the potential use of mutation frequency of the HPRT gene as a long-term biomarker of aflatoxin exposure in high risk populations.

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.

Gender-specific frequency of background somatic mutations at the hypoxanthine phosphoribosyltransferase locus in cord blood T lymphocytes from preterm newborns

Limited information is available regarding the frequency, spectrum, and clinical relevance of somatic mutations in the developing fetus. The goal of this study was to determine somatic mutant frequencies (Mfs) at the hypoxanthine phosphoribosyltransferase (HPRT) reporter gene in cord blood T lymphocytes from preterm infants to gain insight into in utero mutational events. Mf determinations were made by using the HPRT T cell cloning assay on cord blood samples from 52 preterm infants. Natural logarithm Mfs (lnMfs) from preterm infants were compared with results from our database for full-term infants. Our analysis revealed higher lnMfs in cord blood T lymphocytes from preterm compared with full-term infants (P = 0.008). In addition, preterm females had significantly higher lnMfs compared with full-term females (P < 0.001), whereas preterm males were found to have significantly lower lnMfs than preterm females (P = 0.005). Regression analyses also demonstrate a significant relationship between lnMf and gestational age for preterm females that does not exist for preterm males. These results demonstrate the gender-specific association between Mf and age in humans.

Biomarkers in long survivors of pediatric acute lymphoblastic leukemia patients: late effects of cancer chemotherapy

In order to elucidate the late effects of cancer chemotherapy, mutant frequencies (Mfs) at the hypoxanthine phosphoribosyl transferase (hprt) locus were evaluated in pediatric patients with early pre-B acute lymphoblastic leukemia (ALL). Hprt-Mfs were measured at least 2 years after completion of chemotherapy. Ten out of 15 patients were found to have hprt-Mfs exceeding the 99% confidence limits as calculated from observations of healthy controls. Although there was some intraindividual variation, serial measurements of hprt-Mfs with intervals of more than 6 months revealed that hprt-Mfs were fairly stable. Patients with high Mfs tended to have sibling clones as detected by clonality analysis using the T-cell receptor (TCR) rearrangement pattern, but clonality did not have a major effect on the Mfs. On the other hand, Mfs at the TCR locus and sister chromatid exchange frequency were within the normal range in all patients. These data suggest that chemotherapy can cause persistent genotoxicity in vivo in a subset of pediatric ALL patients and that the hprt-Mf is a useful method for measuring such an effect.

HPRT- mutant T cells in the peripheral blood and synovial tissue of patients with rheumatoid arthritis

OBJECTIVE

To investigate the frequency and characteristics of hprt- mutant T lymphocytes in the peripheral blood and synovium of rheumatoid arthritis (RA) patients compared with controls, and to correlate these findings with disease parameters.

METHODS

An hprt- T cell assay was performed on blood and synovial samples from 93 RA patients, 8 osteoarthritis (OA) patients, and 19 control subjects. T cell clones were studied by flow cytometry and evaluated for fibronectin adhesion.

RESULTS

RA patients showed a 5-fold increase in the frequency of mutant T cells in the peripheral blood compared with that in control peripheral blood, and a further 10-fold increase in the mutant T cell frequency in synovial tissue. In OA patients, the synovium also had a significantly higher frequency of hprt- mutant T cells compared with the peripheral blood, but at a lower level than in the rheumatoid synovium. RA peripheral blood mutant T cell clones displayed elevated fibronectin adhesion and beta1 integrin expression, similar to that observed in the RA synovial T cell lines.

CONCLUSION

The origin of the mutated T cells in the peripheral blood of these patients appears to be the inflamed synovium of RA, and to a lesser extent, of OA, where the cells are exposed to a mitogenic and genotoxic environment.

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.

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%).

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.

Correlation between age and DNA damage detected by FADU in human peripheral blood lymphocytes

Fluorometric analysis of DNA unwinding (FADU) is a fast and reliable method for detecting single strand DNA breaks as an index of DNA damage induced by clastogenic agents. A study of damage detected by FADU was conducted on DNA extracted from peripheral blood lymphocytes of 128 healthy nonsmoking regular donors (ranging in age from 19 to 67 years) and from 5 umbilical cord blood samples. DNA damage was measured as percentage of unwound DNA after alkalinization. Statistical analyses, both parametric (Pearson r correlation coefficient, b regression coefficient, ANOVA) and nonparametric (Kruskal-Wallis H test, Spearman rs rank correlation coefficient), support a significant correlation between age of donors and amount of DNA damage. The same results are found when adult donors are divided in four age classes and the ANOVA test performed among the mean percentages of unwound DNA of each class. Furthermore, donors of the same age belonging to different blood groups (A, B, AB and O) do not show any difference in DNA damage detected by FADU.

Hydrogen peroxide and sequence-specific DNA damage in human cells

Exposure of HeLa cells in monolayer culture to increasing concentrations of exogenously added H2O2 causes damage to cellular DNA. When the DNA is subsequently isolated from the non-apoptotic cells remaining in such cultures, evidence was obtained to suggest that the DNA damage elicited in intact cells was non-random and that certain nucleotide sequences associated with, or related to, the genes for heat shock protein 60 and catalase were more susceptible to damage than others. In contrast, these particular sequences were not specifically susceptible to damage when naked human DNA was exposed directly to H2O2 in vitro. On an overall comparative basis, sequences in the genes encoding catalase, alpha-1 antitrypsin and beta-actin appear more vulnerable to H2O2 in vivo, than sequences in H-ras and the P53 gene which seem surprisingly resistant.

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.

Analysis of mutation at the hprt locus in human T lymphocytes

Studies of mutation at the hypoxanthine phosphoribosyl transferase (hrpt) locus in human T-cells have the potential to elucidate the molecular basis of in vivo mutagenesis, reveal exposure dependent changes in ther background frequency of mutation, and provide knowledge on individual sensitivity. Styrene exposed lamination workers in Bohemia showed a significantly higher frequency of hprt mutant cells than Swedish control populations studied simultaneously. In a study of 47 healthy, non-smoking male bus maintenance workers exposed to diesel exhausts, soot and oil, and 22 unexposed controls, a significant correlation (P = 0.008) was obtained between the levels of aromatic DNA adducts and frequencies of hprt-mutant T-cells. In the group of workers with the highest exposure, subjects with glutathione S-transferase (GSTM1) deficiency showed significantly higher (P < 0.05) frequency of hprt mutant T-cells than GSTM1-positive subjects. The highest adduct levels were found in subjects with the combined genotype of GSTM1 and NAT2 deficiency (GSTM1-negative slow acetylators). These results indicate that GSTM1 and NAT2 genotypes may play a role in determining the individual levels of hprt mutation and DNA adducts. Using PCR-based screening methods, hprt mutations have been classified in 462 T-cell clones from 43 subjects in this study population. Deletions were found in 3% of the mutants, coding errors in 81% and splice mutations in 17%. Transitions and transversions were equally common, and all types of base substitutions were detected.

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.

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.

DNA damage, mutation and fine structure DNA repair in aging

The primary focus of this review is on correlations found between DNA damage, repair, and aging. New techniques for the measurement of DNA damage and repair at the level of individual genes, in individual DNA strands and in individual nucleotides will allow us to gain information regarding the nature of these correlations. Fine structure studies of DNA damage and repair in specific regions, including active genes, telomeres, and mitochondria have begun. Considerable intragenomic DNA repair heterogeneity has been found, and there have been indications of relationships between aging and repair in specific regions. More studies are necessary, however, particularly studies of the repair of endogenous damage. It is emphasized that the information obtained must be viewed from a perspective that takes into account the total responses of the cell to damaging events and the inter-relationships that exist between DNA repair and transcription.