HPRT-mutant frequency and lymphocyte characteristics of workers exposed to ionizing radiation on a sporadic basis: a comparison of two exposure indicators, job title and dose

Methylation Array Signals are Predictive of Chronological Age Without Bisulfite Conversion

DNA methylation data has been used to make "epigenetic clocks" which attempt to measure chronological and biological aging. These models rely on data derived from bisulfite-based measurements, which exploit a semi-selective deamination and a genomic reference to determine methylation states. Here, we demonstrate how another hallmark of aging, genomic instability, influences methylation measurements in both bisulfite sequencing and methylation arrays. We found that non-methylation factors lead to "pseudomethylation" signals that are both confounding of epigenetic clocks and uniquely age predictive. Quantifying these covariates in aging studies will be critical to building better clocks and designing appropriate studies of epigenetic aging.

DNA methylation mutants in Physcomitrella patens elucidate individual roles of CG and non-CG methylation in genome regulation

Cytosine (DNA) methylation in plants regulates the expression of genes and transposons. While methylation in plant genomes occurs at CG, CHG, and CHH sequence contexts, the comparative roles of the individual methylation contexts remain elusive. Here, we present Physcomitrella patens as the second plant system, besides Arabidopsis thaliana, with viable mutants with an essentially complete loss of methylation in the CG and non-CG contexts. In contrast to A. thaliana, P. patens has more robust CHH methylation, similar CG and CHG methylation levels, and minimal cross-talk between CG and non-CG methylation, making it possible to study context-specific effects independently. Our data found CHH methylation to act in redundancy with symmetric methylation in silencing transposons and to regulate the expression of CG/CHG-depleted transposons. Specific elimination of CG methylation did not dysregulate transposons or genes. In contrast, exclusive removal of non-CG methylation massively up-regulated transposons and genes. In addition, comparing two exclusively but equally CG- or CHG-methylated genomes, we show that CHG methylation acts as a greater transcriptional regulator than CG methylation. These results disentangle the transcriptional roles of CG and non-CG, as well as symmetric and asymmetric methylation in a plant genome, and point to the crucial role of non-CG methylation in genome regulation.

Direct Exposure of Wheat Seeds to High-Voltage Electrostatic Fields Adversely Affects the Performance of Sitobion avenae (Hemiptera: Aphididae)

Exposing seeds to a high-voltage electrostatic field (HVEF) may influence the performance of herbivores and improve the germination rate by inducing biological and physiological changes in plants. In the current study, an age-stage, two-sex life table was established to evaluate the effects from directly exposing seeds to HVEFs on the performance of apterous Sitobion avenae (Fabricius) reared on winter wheat. We treated the wheat seeds by exposing them to an HVEF for 20 min at three intensities: 2, 4, or 6 KV/cm. Controls received no treatment (0 KV/cm). The results indicated that the parameters of the net reproductive rate (R0), the intrinsic rate of increase (r), and the mean generation span (T) of S. avenae were significantly reduced by HVEFs through multiple generations. In addition, the age-specific survival rate (lx) and comparison with the results of a Weibull equation analysis suggested that S. avenae had the shortest life span when seeds were exposed to an HVEF at 4 KV/cm. Overall, these findings indicated that direct exposure of wheat seeds to an HVEF at 4 KV/cm could adversely affect the performance of S. avenae.

Causes of genome instability: the effect of low dose chemical exposures in modern society

Genome instability is a prerequisite for the development of cancer. It occurs when genome maintenance systems fail to safeguard the genome's integrity, whether as a consequence of inherited defects or induced via exposure to environmental agents (chemicals, biological agents and radiation). Thus, genome instability can be defined as an enhanced tendency for the genome to acquire mutations; ranging from changes to the nucleotide sequence to chromosomal gain, rearrangements or loss. This review raises the hypothesis that in addition to known human carcinogens, exposure to low dose of other chemicals present in our modern society could contribute to carcinogenesis by indirectly affecting genome stability. The selected chemicals with their mechanisms of action proposed to indirectly contribute to genome instability are: heavy metals (DNA repair, epigenetic modification, DNA damage signaling, telomere length), acrylamide (DNA repair, chromosome segregation), bisphenol A (epigenetic modification, DNA damage signaling, mitochondrial function, chromosome segregation), benomyl (chromosome segregation), quinones (epigenetic modification) and nano-sized particles (epigenetic pathways, mitochondrial function, chromosome segregation, telomere length). The purpose of this review is to describe the crucial aspects of genome instability, to outline the ways in which environmental chemicals can affect this cancer hallmark and to identify candidate chemicals for further study. The overall aim is to make scientists aware of the increasing need to unravel the underlying mechanisms via which chemicals at low doses can induce genome instability and thus promote carcinogenesis.

Static magnetic fields modulate X-ray-induced DNA damage in human glioblastoma primary cells

Although static magnetic fields (SMFs) are used extensively in the occupational and medical fields, few comprehensive studies have investigated their possible genotoxic effect and the findings are controversial. With the advent of magnetic resonance imaging-guided radiation therapy, the potential effects of SMFs on ionizing radiation (IR) have become increasingly important. In this study we focused on the genotoxic effect of 80 mT SMFs, both alone and in combination with (i.e. preceding or following) X-ray (XR) irradiation, on primary glioblastoma cells in culture. The cells were exposed to: (i) SMFs alone; (ii) XRs alone; (iii) XR, with SMFs applied during recovery; (iv) SMFs both before and after XR irradiation. XR-induced DNA damage was analyzed by Single Cell Gel Electrophoresis assay (comet assay) using statistical tools designed to assess the tail DNA (TD) and tail length (TL) as indicators of DNA fragmentation. Mitochondrial membrane potential, known to be affected by IR, was assessed using the JC-1 mitochondrial probe. Our results showed that exposure of cells to 5 Gy of XR irradiation alone led to extensive DNA damage, which was significantly reduced by post-irradiation exposure to SMFs. The XR-induced loss of mitochondrial membrane potential was to a large extent averted by exposure to SMFs. These data suggest that SMFs modulate DNA damage and/or damage repair, possibly through a mechanism that affects mitochondria.

The cellular and molecular carcinogenic effects of radon exposure: a review

Radon-222 is a naturally occurring radioactive gas that is responsible for approximately half of the human annual background radiation exposure globally. Chronic exposure to radon and its decay products is estimated to be the second leading cause of lung cancer behind smoking, and links to other forms of neoplasms have been postulated. Ionizing radiation emitted during the radioactive decay of radon and its progeny can induce a variety of cytogenetic effects that can be biologically damaging and result in an increased risk of carcinogenesis. Suggested effects produced as a result of alpha particle exposure from radon include mutations, chromosome aberrations, generation of reactive oxygen species, modification of the cell cycle, up or down regulation of cytokines and the increased production of proteins associated with cell-cycle regulation and carcinogenesis. A number of potential biomarkers of exposure, including translocations at codon 249 of TP53 in addition to HPRT mutations, have been suggested although, in conclusion, the evidence for such hotspots is insufficient. There is also substantial evidence of bystander effects, which may provide complications when calculating risk estimates as a result of exposure, particularly at low doses where cellular responses often appear to deviate from the linear, no-threshold hypothesis. At low doses, effects may also be dependent on cellular conditions as opposed to dose. The cellular and molecular carcinogenic effects of radon exposure have been observed to be both numerous and complex and the elevated chronic exposure of man may therefore pose a significant public health risk that may extend beyond the association with lung carcinogenesis.

Review of retrospective dosimetry techniques for external ionising radiation exposures

The current focus on networking and mutual assistance in the management of radiation accidents or incidents has demonstrated the importance of a joined-up approach in physical and biological dosimetry. To this end, the European Radiation Dosimetry Working Group 10 on 'Retrospective Dosimetry' has been set up by individuals from a wide range of disciplines across Europe. Here, established and emerging dosimetry methods are reviewed, which can be used immediately and retrospectively following external ionising radiation exposure. Endpoints and assays include dicentrics, translocations, premature chromosome condensation, micronuclei, somatic mutations, gene expression, electron paramagnetic resonance, thermoluminescence, optically stimulated luminescence, neutron activation, haematology, protein biomarkers and analytical dose reconstruction. Individual characteristics of these techniques, their limitations and potential for further development are reviewed, and their usefulness in specific exposure scenarios is discussed. Whilst no single technique fulfils the criteria of an ideal dosemeter, an integrated approach using multiple techniques tailored to the exposure scenario can cover most requirements.

Potential adverse health effects of low-level ionizing radiation exposure in a hospital setting

In this study, the authors investigated the short-term (i.e., 1-yr) health effects of low-level occupational and personal exposure to ionizing radiation in a hospital setting. Thyroid function, lung function, platelet count, and leukocyte classification were obtained for 142 ionizing radiation operators. In addition, individual exposure doses were measured. The mean annual cumulative dose in 25 exposed subjects was 6.0 +/- 10.1 millisieverts. The highest exposure ratio among 20 of 41 (49%) radioisotope operators was significantly higher than for other ionizing radiation workers. Abnormal leukocyte counts occurred in 26% of radio-diagnosis operators and 30% of radioisotope operators, compared with 15% for other potentially exposed operators. No significant relative risk was found in subjects with above-background exposures. However, the highly exposed population experienced elevations in thyroid stimulating hormone (adjusted odds ratio [ORa = 2.0; T3 [OR = 1.7]); mononuclear leukocytes (ORa = 2.4); and basophilic leukocytes (ORa = 2.0). Although the authors failed to find short-term health effects in hospital operators exposed annually to less than 6.0 millisieverts of ionizing radiation, additional protection would certainly be prudent for these operators.

Generation and evolutionary fate of insertions of organelle DNA in the nuclear genomes of flowering plants

Nuclear genomes are exposed to a continuous influx of DNA from mitochondria and plastids. We have characterized the structure of approximately 750 kb of organelle DNA, distributed among 13 loci, in the nuclear genomes of Arabidopsis and rice. These segments are large and migrated to the nucleus quite recently, allowing us to reconstruct their evolution. Two general types of nuclear insertions coexist; one is characterized by long sequence stretches that are colinear with organelle DNA, the other type consists of mosaics of organelle DNA, often derived from both plastids and mitochondria. The levels of sequence divergence of the two types exclude their common descent, implying that at least two independent modes of DNA transfer from organelle to nucleus operate. The post-integration fate of organelle DNA is characterized by a predominance of transition mutations, associated with the gradual amelioration of the integrated sequence to the nucleotide composition of the host chromosome. Deletion of organelle DNA at these loci is essentially balanced by insertions of nonorganelle DNA. Deletions are associated with the removal of DNA between perfect repeats, indicating that they originate by replication slippage.

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.

A comparative analysis of relative occurrence of transcription factor binding sites in vertebrate genomes and gene promoter areas

MOTIVATION

The detection of transcription factor binding sites (TFBS) in genomic sequences is a basic task for elucidating the transcriptional aspects of gene regulation. Evaluation procedures applicable to the TFBS prediction outputs need improvement. Predicted TFBS located outside of the transcription associated areas are often neglected from the functional and the evolutionary points of view, therefore deserving a systematic overview.

RESULTS

We calculated theoretical occurrences of 184 TFBS according to their position weight matrices and the dinucleotide statistics of the completed vertebrate genomes, then performed a TFBS prediction in the corresponding complete genomic sequences and their repeat-free, repetitive and regulatory fractions. Repeat-free fractions of the closely related mammalian genomes were characterized by strong similarities in TFBS occurrences. A significant over-representation of multiple TFBS was found in both repetitive and non-repetitive genome fractions.

AVAILABILITY

F-values and real TFBS occurrences calculated for human, chimp, mouse, rat, zebrafish and fugu genomes are available for free download at http://www.gmu.edu/departments/mmb/baranova/pages/bioinformatics

A SNP resource for human chromosome 22: extracting dense clusters of SNPs from the genomic sequence

The recent publication of the complete sequence of human chromosome 22 provides a platform from which to investigate genomic sequence variation. We report the identification and characterization of 12,267 potential variants (SNPs and other small insertions/deletions) of human chromosome 22, discovered in the overlaps of 460 clones used for the chromosome sequencing. We found, on average, 1 potential variant every 1.07 kb and approximately 18% of the potential variants involve insertions/deletions. The SNPs have been positioned both relative to each other, and to genes, predicted genes, repeat sequences, other genetic markers, and the 2730 SNPs previously identified on the chromosome. A subset of the SNPs were verified experimentally using either PCR-RFLP or genomic Invader assays. These experiments confirmed 92% of the potential variants in a panel of 92 individuals. [Details of the SNPs and RFLP assays can be found at http://www.sanger.ac.uk and in dbSNP.]

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.

SNP profile within the human major histocompatibility complex reveals an extreme and interrupted level of nucleotide diversity

The human major histocompatibility complex (MHC) is characterized by polymorphic multicopy gene families, such as HLA and MIC (PERB11); duplications; insertions and deletions (indels); and uneven rates of recombination. Polymorphisms at the antigen recognition sites of the HLA class I and II genes and at associated neutral sites have been attributed to balancing selection and a hitchhiking effect, respectively. We, and others, have previously shown that nucleotide diversity between MHC haplotypes at non-HLA sites is unusually high (>10%) and up to several times greater than elsewhere in the genome (0.08%-0.2%). We report here the most extensive analysis of nucleotide diversity within a continuous sequence in the genome. We constructed a single nucleotide polymorphism (SNP) profile that reveals a pattern of extreme but interrupted levels of nucleotide diversity by comparing a continuous sequence within haplotypes in three genomic subregions of the MHC. A comparison of several haplotypes within one of the genomic subregions containing the HLA-B and -C loci suggests that positive selection is operating over the whole subgenomic region, including HLA and non-HLA genes. [The sequence data for the multiple haplotype comparisons within the class I region have been submitted to DDBJ/EMBL/GenBank under accession nos. AF029061, AF029062, and AB031005-AB031010. Additional sequence data have been submitted to the DDBJ data library under accession nos. AB031005-AB03101 and AF029061-AF029062.]

Elevated frequencies of hypoxanthine phosphoribosyltransferase lymphocyte mutants are detected in Russian liquidators 6 to 10 years after exposure to radiation from the Chernobyl nuclear power plant accident

This study was conducted to determine whether the frequency of hypoxanthine phosphoribosyltransferase (HPRT) deficient lymphocyte mutants would detect an effect of radiation exposure in a population of Russians who were exposed to low levels of radiation while working in 1986 and 1987 as liquidators cleaning up after the Chernobyl nuclear power reactor accident. The HPRT lymphocyte cloning assay was performed on peripheral blood lymphocytes collected between 1992 and 1996 from 142 liquidators and 66 Russian controls, and between 1989 and 1993 from 231 American controls. Russian and American controls were not significantly different for either cloning efficiency or mutant frequency (MF); inclusion of both sets of controls in the analysis increased the ability to detect a Chernobyl exposure effect in the liquidators. After adjusting for age and smoking, the results revealed no significant difference in cloning efficiency of Chernobyl liquidators relative to Russian controls but a significant, 24% increase in liquidator HPRT mutant frequency over Russian controls (90% confidence interval was 7% to 45% increase). The analytical method also accounted for differences in precision of the individual estimates of log CE and log MF and accommodated for outliers. The increase in HPRT mutant frequency of liquidators is an attribute of the exposed population as a whole rather than of individuals. These results demonstrate that, under appropriate circumstances, the HPRT specific locus mutation assay of peripheral blood lymphocytes can be used to detect a semi-acute, low dose radiation exposure of a population, even 6 to 10 years after the exposure.

Deletion-pattern analysis of alpha-particle and X-ray induced mutations at the HPRT locus of V79 Chinese hamster cells

To investigate the mutagenic mechanisms of low-energy alpha particles V79 Chinese hamster cells were irradiated with 241Am-alpha particles (mean LET of 112 keV/micron). Parallel experiments were performed using 300 kV X-rays. Cell inactivation and mutation induction cross sections were measured. At approximately 20%--survival level, DNA deletions were analysed at the HPRT locus by multiplex-PCR-analysis of all nine exons of 47 alpha-irradiated and 36 background mutants. 92 HPRT- mutants isolated after 300 kV-X-irradiation were analysed similarly for comparison, along with 15 corresponding background mutants. The resulting mutant deletion-pattern distributions were corrected for background mutations. alpha Particles induced a larger fraction of deletions than X-rays. Furthermore, non-contiguous partial deletions were present among the alpha-induced mutants, a type not found after X-irradiation.

Genotoxicity in workers exposed to methyl bromide

To address the genotoxicity of in vivo methyl bromide (CAS 74-83-9) exposure in humans, we collected blood and oropharyngeal cells as part of a cross-sectional morbidity study of methyl bromide-exposed fumigation workers and their referents. Micronuclei were measured in lymphocytes and oropharyngeal cells, and hypoxanthine-guanine phosphoribosyl transferase gene (hprt) mutations were measured in lymphocytes. A total of 32 workers and 28 referents provided specimens. Among current non-smokers, mean hprt variant frequencies (Vfs) were found to be elevated among workers compared to referents (geometric mean: workers=4.49x10(-6), referents=2.96x10-(6); two-sided p=0.22); this difference was more pronounced among workers with 4 h or more of recent methyl bromide exposure compared to referents (geometric mean: workers=6.56x10(-6), referents=2.96x10(-6); two-sided p=0.06). Mean oropharyngeal cell micronuclei were higher among workers compared to referents (mean: workers=2.00, referents=1.31; two-sided p=0.08); the results were similar when workers with 4 h or more of recent methyl bromide exposure were compared to referents (mean: workers=2.07, referents=1.31; two-sided p=0.13). No consistent differences between workers and referents were observed for frequencies of kinetochore-negative lymphocyte micronuclei, or kinetochore-positive lymphocyte micronuclei. The study was limited by a sample size sufficient only for detecting relatively large differences, absence of a reliable method to measure the intensity of workplace methyl bromide exposures, and relatively infrequent methyl bromide exposure (e.g., the median length of exposure to methyl bromide during the 2 weeks preceding the survey was 4 h). In conclusion, our findings provide some evidence that methyl bromide exposure may be associated with genotoxic effects in lymphocytes and oropharyngeal cells. Further study on the genotoxicity of methyl bromide exposure in humans is warranted.

Nature of mutation in the human hprt gene following in vivo exposure to ionizing radiation of cesium-137

The current study comprises the analysis of mutations in 10 individuals accidentally exposed to cesium-137 during the 1987 radiological accident in Goiânia, Brazil. Their exposures were among the highest experienced, ranging from 1 to 7 Gy. Peripheral T-lymphocyte samples were obtained 3.3 years after the original exposure and mutation was studied at the hprt locus using the 6-thioguanine-resistance selection assay. The mutational spectrum for the exposed population is comprised of 90 independent mutants. Based on T-cell receptor analysis, only 5% (5/95) were clonally related. Mutants were initially studied using RT-PCR and directly sequenced using an automated laser fluorescent DNA sequencer. Mutants that repeatedly failed to produce cDNAs were studied using a multiplex PCR assay with genomic DNA. Missense mutations were the most frequent event recovered, comprising 40% (23/57) of the spectral sample. An excess of events involving A:T base pairs was observed, exhibiting a significant difference (chi 2 = 12.7, P = 0.0004) when compared to the spontaneous spectrum. This finding may reflect the effect of ionizing radiation-induced damage, suggesting a potential similarity to radiation effects in prokaryotes. At the genomic level, 36.7% (33/90) of the mutants exhibited gross structural alterations, as detected by multiplex PCR. Deletion events were over-represented in our spectral sample, displaying a twofold increase when compared to the frequency observed in the spontaneous mutation database.

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.

Effect of dietary components on hprt mutant frequencies in human T-lymphocytes

The 6-thioguanine resistance (TGr) assay in human T-lymphocytes, which detects mutations at the hprt locus, identifies exposures to environmental mutagens. However, the ability of this assay to detect small increases in mutation rates is limited by the broad range of mutant frequencies (Mf) in healthy individuals. While subject age, lymphocyte cloning efficiency, and cigarette smoking history have been shown to influence the Mf, these factors account for only a portion of the variability in the Mf in human populations. To investigate the influence of dietary differences on hprt Mf, 70 women with breast masses were asked to complete a nutritional questionnaire and submit a peripheral blood sample for a TGr assay. Multivariate analyses, adjusted for age, cloning efficiency and total caloric intake, showed significant positive correlations between vitamin A and iron and InMf (p = 0.03), and a negative correlation between total fat and InMf (p = 0.004). Positive correlations between dietary fiber and copper and InMf, and a negative correlation between alcohol and InMf were of borderline significance (0.05 < or = p < or = 0.07). These results suggest that nutritional components may modulate the hprt Mf. Dietary differences may account for a part of the variability observed in hprt Mf in human populations.

Effects of lymphocyte subpopulations on the clonal assay of HPRT mutants: occupational exposure to cytostatic drugs

The mutagenic effect of occupational exposure to antineoplastic agents was studied in chemotherapy nurses and pharmacists using the T-lymphocyte clonal assay. A significant increase in mutant frequency was observed compared to controls. However, in the present study, cloning efficiency without selection (CEU) was significantly reduced in exposed personnel raising the possibility of an overestimation of the calculated MF. Changes in lymphocyte populations and clonal potential of T-cells were also observed following exposure. CEU was related to % CD4 cells but CE with selection (CETG) was not. Differences in clonal ability of T-cells under selective and unselective conditions coupled with differential lethal effect of antineoplastic agents on lymphocyte subsets may result in inaccurate estimation of MF.