In vitro induction, expression and selection of thioguanine-resistant mutants with human T-lymphocytes

Radiation effects on human heredity

In experimental organisms such as fruit flies and mice, increased frequencies in germ cell mutations have been detected following exposure to ionizing radiation. In contrast, there has been no clear evidence for radiation-induced germ cell mutations in humans that lead to birth defects, chromosome aberrations, Mendelian disorders, etc. This situation exists partly because no sensitive and practical genetic marker is available for human studies and also because the number of people exposed to large doses of radiation and subsequently having offspring was small until childhood cancer survivors became an important study population. In addition, the genome of apparently normal individuals seems to contain large numbers of alterations, including dozens to hundreds of nonfunctional alleles. With the number of mutational events in protein-coding genes estimated as less than one per genome after 1 gray (Gy) exposure, it is unsurprising that genetic effects from radiation have not yet been detected conclusively in humans.

Direct mutation analysis by high-throughput sequencing: from germline to low-abundant, somatic variants

DNA mutations are the source of genetic variation within populations. The majority of mutations with observable effects are deleterious. In humans mutations in the germ line can cause genetic disease. In somatic cells multiple rounds of mutations and selection lead to cancer. The study of genetic variation has progressed rapidly since the completion of the draft sequence of the human genome. Recent advances in sequencing technology, most importantly the introduction of massively parallel sequencing (MPS), have resulted in more than a hundred-fold reduction in the time and cost required for sequencing nucleic acids. These improvements have greatly expanded the use of sequencing as a practical tool for mutation analysis. While in the past the high cost of sequencing limited mutation analysis to selectable markers or small forward mutation targets assumed to be representative for the genome overall, current platforms allow whole genome sequencing for less than $5000. This has already given rise to direct estimates of germline mutation rates in multiple organisms including humans by comparing whole genome sequences between parents and offspring. Here we present a brief history of the field of mutation research, with a focus on classical tools for the measurement of mutation rates. We then review MPS, how it is currently applied and the new insight into human and animal mutation frequencies and spectra that has been obtained from whole genome sequencing. While great progress has been made, we note that the single most important limitation of current MPS approaches for mutation analysis is the inability to address low-abundance mutations that turn somatic tissues into mosaics of cells. Such mutations are at the basis of intra-tumor heterogeneity, with important implications for clinical diagnosis, and could also contribute to somatic diseases other than cancer, including aging. Some possible approaches to gain access to low-abundance mutations are discussed, with a brief overview of new sequencing platforms that are currently waiting in the wings to advance this exploding field even further.

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.

In vitro studies of the genotoxicity of ionizing radiation in human G(0) T lymphocytes

In an effort to mimic human in vivo exposures to ionizing irradiation, G(0) phase T lymphocytes from human peripheral blood samples were utilized for in vitro studies of the genotoxic effects of (137)Cs low-LET irradiation and (222)Rn high-LET irradiation. Both types of radiation induced mutations in the HPRT gene in a dose-dependent manner, with a mutant frequency (MF) = 4.28 + 1.34x + 7.51x(2) for (137)Cs (R(2) = 0.95) and MF = 4.81 + 0.67x for (222)Rn (R(2) = 0.51). Post (137)Cs irradiation incubation in the presence of cytosine arabinoside, a reversible inhibitor of DNA repair, caused an increase in the MF over irradiation alone, consistent with a misrepair mechanism being involved in the mutagenicity of low-LET irradiation. The spectrum of (137)Cs irradiation-induced mutation displayed an increase in macro-deletions (in particular total gene deletions) and rearrangement events, some of which were further defined by either chromosome painting or direct DNA sequencing. The spectrum of (222)Rn irradiation-induced mutation was characterized by an increase in small alterations, especially multiple single base deletions/substitutions and micro-deletions. These studies define the specific response of human peripheral blood T cells to ionizing irradiation in vitro and form a basis for evaluating the genotoxic effects of human in vivo exposure.

Use of inverse PCR to amplify and sequence breakpoints of HPRT deletion and translocation mutations

Deletion and translocation mutations have been shown to play a significant role in the genesis of many cancers. The hprt gene located at Xq26 is a frequently used marker gene in human mutational studies. In an attempt to better understand potential mutational mechanisms involved in deletions and translocations, inverse PCR (IPCR) methods to amplify and sequence the breakpoints of hprt mutants classified as translocations and large deletions were developed. IPCR involves the digestion of DNA with a restriction enzyme, circularization of the fragments produced, and PCR amplification around the circle with primers oriented in a direction opposite to that of conventional PCR. The use of this technique allows amplification into an unknown region, in this case through the hprt breakpoint into the unknown joined sequence. Through the use of this procedure, two translocation, one inversion, and two external deletion hprt breakpoint sequences were isolated and sequenced. The isolated IPCR products range in size from 0.4 to 1.8 kb, and were amplified from circles ranging in size from 0.6 to 7.7 kb. We have shown that inverse PCR is useful to sequence translocation and large deletion mutant breakpoints in the hprt gene.

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.

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.

Mutation induction in gamma-irradiated primary human bronchial epithelial cells and molecular analysis of the HPRT- mutants

We have examined various radiobiological parameters using commercially-available primary normal human bronchial epithelial (NHBE) cells, which can be subcultured more than 20 population doublings, and have established the mutation system in order to characterize the molecular changes in gamma-irradiated primary cells. The survival curve, obtained after irradiation of cells with 137Cs gamma-rays, indicates that the D0, Dq, and n values are 1.34 Gy, 1.12 Gy, and 2.3, respectively. The induction of HPRT- mutation was dose-dependent and the mutant fraction increased in a non-linear fashion. Since the doubling number of NHBE cells is limited, DNA was extracted directly from the single mutant colonies and alteration in the HPRT gene locus was analyzed using multiplex PCR technique. Among spontaneous mutants, the proportion with total and partial deletions of the gene was 10.0% (2/20) and 60.0% (12/20), respectively, while 30.0% (6/20) did not have any detectable changes in the nine exons examined. On the other hand, the fraction of total deletion increased by more than 2-fold among mutants induced by gamma-rays in that 26.3% (10/38) of them showed the total gene deletions. Twenty-five out of 38 gamma-induced mutants (65.8%) had partial deletions and 3 mutants (7.9%) had no detectable alteration. The present results showed that gamma-irradiation efficiently induced HPRT gene mutation in primary human epithelial cells and that most of the induced mutants suffered larger deletions compared to that observed in spontaneous mutants. This system provides an useful tool for determination of mutagenicity and understanding the molecular mechanisms of environmental carcinogens in primary human bronchial cells.

TCR beta PCR from crude preparations for restriction digest or sequencing

T cell specificity is determined by the combinatorial association of specific variable (V), diversity (D), and junctional (J) regions. Clones of T cells (clonality) can occur, in the blood or in tissue, after proliferation of activated T cells. Determining clonality in mutation assays is necessary to distinguish between mutants and mutational events. We have developed a novel approach to determine clonality among T cell isolates, using restriction digests of PCR-amplified cDNA of the T cell receptor beta gene. The T cell receptor beta gene was PCR-amplified by use of a consensus primer, beginning from a cell pellet of 2,000-5,000 cells or from extracted RNA. This TCR (T cell receptor) beta chain PCR product can also be directly sequenced, allowing simple and easy identification of Vbeta and CDR3 sequence from a small number of cells. The utility of this method is demonstrated by PCR, restriction digest, and sequencing of the TCR beta cDNA from eight T cell clones isolated from 2 individuals. A clone of three identical isolates (one 3-mer) and a clone of two identical isolates (one 2-mer) were determined from restriction digests using two different enzymes. This new method is an easier and more rapid way of determining clonality than traditional methods, e.g., Southern blotting.

Reduced proliferation rate of hypoxanthine-phosphoribosyl transferase mutant human T-lymphocytes in vitro

The clonal hprt mutation assay in human T-cell is based on the assumption that wild-type cells and hypoxanthine-phosphoribosyl transferase (hprt) mutant cells survive and proliferate at the same rate during the expression phase which is required for the expression of in vitro-induced mutants. We have tested this assumption in a study of mutant frequency (MF) and proliferation rate at different time points during in vitro expansion of human T-cells in non-selective medium. Peripheral blood lymphocytes from 11 individuals were studied using standard cloning procedures to determine the cloning efficiency (CE) and the hprt MF by 6-thioguanine (TG) selection. Another cell portion from each individual was allowed to proliferate in bulk culture for 8 days in vitro, before measuring CE and MF as above. In the directly plated cell population the CE was 45% and the MF 18.7 +/- 15.3 x 10(-6) (mean +/- S.D.), whereas the in vitro expanded cell population showed a CE of 38% and a significantly reduced MF of 8.3 +/- 6.9 x 10(-6) (P = 0.0033). Thus, the mean MF was 56% lower in the in vitro expanded than in the directly plated cell population. The experiment was repeated in another group of ten individuals with essentially the same result. In a third experiment, freshly prepared cells from two donors were allowed to grow for up to 15 days in bulk culture in vitro. Cell growth, CE, and MF were determined every third day. The MF decreased gradually, and at day 12-15 it was only 25% of the initial value. The total number of clonable cells increased 13-fold during the 15 days of in vitro expansion, while the mutant, TG-resistant cell population increased only 3-fold. These results suggest that human hprt mutant T-lymphocytes have a reduced proliferation rate compared to wild-type cells during in vitro proliferation. Thus, measurements of chemical and radiation induced MF with the T-cell clonal assay may underestimate the true MF by a factor of 2 or more.

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.

Breakpoints and junctional regions of intragenic deletions in the HPRT gene in human T-Cells

DNA sequences of the deletion breakpoints of 24 human T-lymphocyte hprt gene mutations are reported. These independent deletions ranged in size from 18 to 15655 base pairs. Seven of the 21 in vivo mutations arose in normal adults, three in normal children, eight in radioimmunotherapy patients and three in platinum chemotherapy patients. One in vitro mutation was isolated after 93cGy radon exposure and two after 300cGy gamma radiation. The breakpoints were found to be non-random and a cluster of small deletions in exon 6 is reported. Ten of the mutations had 2-5bp direct repeats at the breakpoints. There was no excess of "deletion-associated" motifs over that expected by chance. Some breakpoints do occur at consensus topoisomerase II cleavage sites and the centromeric end of a Donehower sequence occurs exactly at a telomeric breakpoint. Three mutants had breakpoints at hairpins expected by the model of Glickman and Ripley.

Delayed chromosomal instability in human T-lymphocyte clones exposed to ionizing radiation

Recent studies have demonstrated that cells which survive alpha-particle and X-ray exposure may show chromosomal instability, i.e. they continue to develop chromosomal aberrations at an increased frequency for many division cycles after the exposure. To characterize this delayed response, we carried out repeated karyotype analyses of X-irradiated T-lymphocytes during clonal expansion in vitro. Human peripheral blood lymphocytes were obtained from a healthy donor and exposed to 3-Gy X-irradiation. Cell survival, estimated by a cell cloning assay, was 5%. Non-irradiated, control cells were studied in parallel. Monoclonal cell lines were established using the T-cell cloning procedure. G-band karyotype analyses were carried out at several intervals during expansion of the clones for up to 2 months. The irradiated clones did not differ from the control clones with regard to growth rate or cytometric DNA profile. Non-irradiated cell clones showed a normal karyotype, with < 10% of sporadic, non-clonal chromosome and chromatid breaks. In the irradiated clones, the karyotypes showed different (sub)clonal chromosome rearrangements, which developed successively during the cultivation time. In addition to these karyotypic abnormalities, > 20% of the cells in these clones had sporadic, non-clonal chromosome aberrations, and there was a tendency of increasing frequency of such aberrations by length of cultivation. Thus, two types of radiation-induced chromosomal instability were observed; (sub)clonal karyotypic abnormalities and sporadic, non-clonal chromosome aberrations. The frequency and kinetics by which these alterations occur in the progeny of X-irradiated T-cells suggest that they arise through different pathways, and argue against their causation by mutation or persistent DNA damage.

Physical mapping of the human hprt chromosomal region (Xq26)

The human hprt chromosomal region (Xq26) was physical-mapped using pulsed field gel electrophoresis (PFGE). This work involved: (i) the recovery of three new genomic DNA markers (DXS1327, DXS1328, and DXS1329), (ii) the ordering of new markers relative to 11 previously available hprt-linked markers by deletion mapping, and (iii) the completion of human T-lymphocyte PFGE Southern blots using the 14 Xq26 markers. A contiguous 1.5-Mb physical map of the region telomeric to hprt was determined. As this map identifies clusters of in vivo unmethylated rare-cutter restriction sites, potential CpG islands are revealed.

Pulsed field analysis of hprt T-cell large deletions: telomeric region breakpoint spectrum

In order to determine a large deletion breakpoint spectrum, 25 independent hprt T-lymphocyte mutants with deletions extending from hprt into the telomeric or centromeric flanking chromosomal region were analyzed by pulsed field gel electrophoresis (PFGE). PFGE was used to determine deletion sizes which allowed localization of breakpoints external to hprt to specific chromosomal positions in mutants containing an intra-hprt breakpoint. A breakpoint spectrum based on 19 large deletion mutants is reported for the Xq26 chromosomal region telomeric to hprt. A potential cluster of breakpoints (4/19) was observed approximately 60 kb from hprt. In addition, maximum recoverable deletion size was at least 3.5 Mb. Three of the 25 mutants analyzed appeared to be complex deletion events.

Mutations induced in the hypoxanthine phosphoribosyl transferase gene by three urban air pollutants: acetaldehyde, benzo[a]pyrene diolepoxide, and ethylene oxide

Provisional mutational spectra at the hypoxanthine phosphoribosyl transferase (HPRT) locus in vitro have been worked out for acetaldehyde (AA) and benzo[a]pyrene diolepoxide (BPDE) in human (T)-lymphocytes and for ethylene oxide (EtO) in human diploid fibroblasts using Southern blotting and polymerase chain reaction (PCR)-based DNA sequencing techniques. The results indicate that large genomic deletions are the predominating hprt mutations caused by AA and EO, whereas BPDE induces point mutations that are mainly GC > TA transversions. The mutational spectra induced by the three agents are clearly different from the background spectrum in human T-cells. Thus, the hprt locus is a useful target for the study of chemical-specific mutational events that may help identify causes of background mutation in human cells in vivo.

Pathology and toxicology of beluga whales from the St. Lawrence Estuary, Quebec, Canada. Past, present and future

An indigenous population of 450-500 beluga whales (Delphinapterus leucas) inhabiting the St. Lawrence Estuary has been exposed chronically for more than 50 years to a complex mixture of industrial pollutants including organochlorinated compounds (OC), polycyclic aromatic hydrocarbons (PAH) and heavy metals. From 1983 to 1990, we have necropsied 45 well preserved carcasses out of a total of 120 beluga whales reported dead over this period. Of these 45 animals, nine were affected by 10 malignant neoplasms. Fifteen animals (33%) were affected by pneumonia. Milk production was compromised in eight of 17 mature females (41%), by inflammatory changes (seven animals) and cancer (one animal) which affected the mammary glands. Opportunistic bacteria were found in pure culture, and/or in significant amounts in at least two organs in 20 belugas (44%). The concentrations of both total PCBs and highly chlorinated PCB congeners were much higher in St. Lawrence animals than in Arctic beluga whales. OC-induced immunosuppression has been repeatedly demonstrated in a wide variety of animal species. Therefore, it is probable that the immune functions of St. Lawrence beluga whales are impaired. Benzo[a]pyrene adducts were detected in 10 of the 11 St. Lawrence beluga whales of which tissues (six livers, 10/11 brains) were analyzed by a method based on HPLC. No such adducts were found in four Arctic animals. Since benzo[alpha]pyrene is one of the most potent chemical carcinogens known to man, these compounds might be responsible for some of the cancers observed in that population. Overall, our findings contrast vividly with those of others who found that cancers are exceedingly rare in free-ranging odontocete populations and that the major causes for mortalities in these populations are bacteria, parasites, and trauma.

Lethality and mutagenesis of B lymphocyte progenitor cells following exposure to alpha-particles and X-rays

B lymphocyte precursor cells are the target cells for the major subtype of paediatric cancer, acute lymphoblastic leukaemia. Using a murine IL-7-dependent clonogenic assay for normal B cell precursors as a model, we have investigated the sensitivity of these cells versus other normal and leukaemic haemopoietic cells to alpha-particle radiation. We find that B cell precursors are remarkably susceptible to the lethal effects of alpha-particles and have a very low probability of surviving a single alpha-track. B cell precursors are also very sensitive to the lethal effects of low LET X-rays. The mutation frequency in a marker gene (HPRT) does not, however, appear to be greater in B cell precursors that survive X-radiation than in other haemopoietic cells.

Assessment of clinical parameters associated with increased frequency of mutant T cells in patients with systemic lupus erythematosus

OBJECTIVE

To determine the clinical features that contribute to an increased frequency of mutant T cells (FMC) in patients with systemic lupus erythematosus (SLE).

METHODS

During in vivo T cell division, there are errors in replication which give rise to mutations throughout the genome. An estimate of such mutations may be obtained by focusing on mutations in the hprt gene, which can be screened by assessing relative growth of T cell clones in the presence and absence of 6-thioguanine. In this study, peripheral blood T cell clones from 47 patients with SLE were assessed, and the frequency of mutant T cells (FMC) determined. An attempt was made to correlate the FMC with disease measures.

RESULTS

Patients with SLE had a spectrum of FMC values, ranging from normal to almost 1,000 times normal. Total duration of active disease (rs = 0.94), past highest disease activity index (rs = 0.80), and number of lupus flares (rs = 0.76) correlated most strongly (P < 0.0001) with FMC by Spearman's rank order analysis. In contrast, current disease activity index and current anti-DNA level did not correlate with FMC. Similar correlations between FMC and cumulative past lupus disease activity were found by linear regression analysis (rp = 0.89 for the correlation between the natural logarithm of FMC and cumulative duration of active disease). By both statistical tests, therapy was found to be only a minor contributor to FMC.

CONCLUSION

In our patient population, a high FMC value appears to reflect cumulative clinical lupus disease activity, involving both intensity and duration of past active disease.

Biological and biophysical techniques to assess radiation exposure: a perspective

Biological dosimeters measure biologically relevant effects of radiation exposure that are in some sense an estimate of effective dose, whereas biophysical indicators serve as surrogates of absorbed dose in a manner analogous to conventional thermoluminescent dosimeters (TLD). The biological and biophysical dosimeters have the potential to play an important role in assessing unanticipated or occupational radiation exposures. For example, where the exposure is large and uncertain (i.e. radiation accidents), accurate dose information can help in deciding the most appropriate therapy and medical treatment. Another useful area is that of lifetime accumulated dose determination, and the ability to distinguish between and integrate the exposures from natural and anthropogenic (medical X-rays, indoor radon, natural background radiation, occupational and non-occupational exposures). Also, the possibility to monitor individual response and differences in inherent or induced radiation sensitivity may have important implications for radiation protection. More commonly, this type of dosimetry could be used for routine monitoring to detect and quantify unsuspected exposure, for regulatory purposes or for epidemiological studies of the long-term effects of radiation exposure (e.g. in Japanese A-bomb survivors or in the population surrounding Chernobyl). This review is a comparative study of the existing techniques and their future prospects. It summarizes the sensitivity, reproducibility, limiting dose, dose-rate, energy, LET response, sources of variability and uncertainty, and other practical aspects of each bio-indicator. The strengths and weaknesses of each approach are evaluated on the basis of common criteria for particular applications, and are summarized for each assay both in the text and in tabular form, for convenience. It is clear that no single indicator qualifies to reliably measure occupational exposures at the current levels of sensitivity conventional dosimetry services provide. Most of the bio-techniques are applicable to the detection of relatively high radiation exposures at relatively short times after exposure. Some of the bio-indicators have been identified that are, or offer future prospects for becoming, appropriate bio-indicators for dosimetry needs. However, all methods are subject to biological and other variables that are presently uncontrolled, and represent a major source of uncertainty. These include variations in background signals not directly associated with radiation exposure, inter- and intra-individual variability of radiation response, and genetic and environmental effects. Although these factors contribute to the lack of confidence in biological dosimetry, promising bio-indicators may be applied to large populations to establish the inherent variability and confounding factors that limit quantitative data collection and analysis, and reduce reliability and reproducibility.

Coamplification of hprt cDNA and gamma T-cell receptor sequences from 6-thioguanine resistant human T-lymphocytes

The nature of mutation at the HPRT locus in human T-lymphocytes in vivo is currently a subject of considerable interest. Determination of clonality in individual mutant T-lymphocytes is essential for the proper interpretation. This requires the molecular analysis of their respective T-cell receptors (TCR). We have developed a polymerase chain reaction (PCR)-based method for coamplification of hprt cDNA and the rearranged gamma T-cell receptor genes from crude cell lysates of individual 6-thioguanine resistant human T-lymphocytes. Following reverse transcription to produce hprt cDNA, the crude cell lysate is treated with proteinase K and subjected to a primary PCR with two sets of amplification primers, one specific for the hprt cDNA and the other for the rearranged gamma TCR gene. A secondary round of PCR, employing appropriate sets of nested amplification primers, are then used to produce sufficient quantities of DNA for both the sequencing and restriction fragment length analysis, of the hprt cDNA and gamma TCR gene respectively.

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

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

Clonal chromosome aberrations and genomic instability in X-irradiated human T-lymphocyte cultures

To study the effects of X-irradiation on clone forming ability and karyotypic abnormalities in human peripheral blood lymphocytes, cells were exposed to 3 Gy of X-rays in vitro and either individual T-cell clones or long-term T-cell cultures were established. The karyotypes were analyzed in G-banded chromosome preparations after proliferation for 9-34 days in vitro. T-cell clonal karyotype abnormalities were found in 24 of 37 (65%) irradiated and in two of 43 (5%) control clones. Balanced reciprocal translocations and deletions were the predominating types of clonal aberrations. Complex aberrations and unstable karyotypes were found in about half of the irradiated clones. Some of the T-cell clones demonstrated sequential change from normal to aberrant karyotype. Other clones seemed to develop multiple, heterogeneous chromosomal aberrations during growth in vitro. X-Irradiated T-cells grown in long-term T-cell culture displayed karyotype abnormalities in 60-80% of the cells, and the types of aberrations were similar to those found in the individual, irradiated T-cell clones. An increasing number of cells with the same abnormal karyotype was observed when the cultivation time was extended, indicating clonal proliferation. These results demonstrate that a surprisingly high proportion of T-cells with stable and often complex irradiation-induced chromosome aberrations are able to proliferate and form expanding cell clones in vitro. Furthermore, the results indicate that X-irradiation induces latent chromosome damage and genomic instability in human T-lymphocytes.

Mutational spectrum at the Hprt locus in splenic T cells of B6C3F1 mice exposed to N-ethyl-N-nitrosourea

We have determined the mutational spectrum of N-ethyl-N-nitrosourea (ENU) in exon 3 of the hypoxanthine (guanine) phosphoribosyltransferase gene (Hprt) in splenic T cells following in vivo exposure of male B6C3F1 mice (5-7 weeks old) to ENU. Hprt- mutants were isolated by culturing splenic T cells in microtiter dishes containing medium supplemented with interleukin 2, concanavalin A, and 6-thioguanine. DNA was extracted from 6-thioguanine-resistant colonies and amplified by the polymerase chain reaction (PCR) using primers flanking Hprt exon 3. Identification of mutant sequences and purification of mutant DNA from contaminating wild-type Hprt DNA was accomplished by denaturing-gradient gel electrophoresis. Purified mutant DNA was then sequenced. Treatment of mice with ENU at 40 mg/kg of body weight produced a Hprt- mutant frequency of 7.3 x 10(-5) in splenic T cells, approximately 35-fold above background levels. Sixty-nine of the 521 Hprt- mutants analyzed contained mutations in exon 3 (13%). Transversions and transitions at A.T base pairs dominated the spectrum; 62 of the 69 exon 3 mutations were at A.T base pairs (14 different sites). Thirteen of 14 thymine bases undergoing mutation (61 of 62 mutations at A.T bases) were located on the nontranscribed strand of exon 3. The majority of the remaining mutations (6 of 69) were transitions at a single G.C base pair. These results suggest the importance of thymidine alkylation in ENU-induced mutagenesis in vivo. The mouse Hprt- T-cell cloning/sequencing assay described here may represent a useful system for studying the molecular mechanism of chemically induced mutation occurring in vivo in an endogenous gene.

Development of a flow-cytometric HLA-A locus mutation assay for human peripheral blood lymphocytes

A flow-cytometric technique was developed to measure the frequency of variant lymphocytes lacking expression of HLA-A2 or A24 allele products among donors heterozygous for HLA-A2 or A24. It was found that the variant frequency of lymphocytes in peripheral blood was of the order of 10(-4) and increased with donor age. Molecular analyses of mutant clones revealed that about one-third were derived from somatic recombinations and that the remaining two-thirds did not show any alterations after Southern blotting analysis. In contrast, mutants obtained after in vitro X-ray mutagenesis study were found to be mostly derived from large chromosomal deletions. A small-scale study on atomic bomb survivors did not show a significant dose effect.

Genotoxic and mutagenic effects of the diagnostic use of thallium-201 in nuclear medicine

In order to investigate possible mutagenic effects of in vivo exposure to low levels of ionizing radiation used in nuclear medicine, we have examined the hypoxanthine guanine phosphoribosyl transferase (hprt) mutant fraction (MF) and chromosome aberration (CA) frequency in 24 nuclear medicine patients before and after injection of thallium-201. The mean MF of the thallium-201-exposed cohort was 5.2 +/- 4.4 x 10(-6) before injection exposure. No significant difference in MF was observed 24 h later. In 11 patients who were studied on a third occasion, 30 days after thallium-201 exposure, there was again no significant difference in post-exposure as compared with the pre-exposure MF. The frequency of CA in peripheral blood lymphocytes was not significantly different, comparing pre- and 24 h to 1 month post-radionuclide exposure. Thus, thallium-201 exposure was not associated with significant elevations in MF or CA frequency in lymphocytes of exposed individuals.

Southern-blot analyses of human T-lymphocyte mutants induced in vitro by gamma-irradiation

G0 phase cultures of human peripheral blood T-lymphocytes from a single individual were exposed to 300 rad of gamma-irradiation from a 137Cs source and cultured in vitro for 8 days to allow phenotypic expression. Thioguanine-resistant (TGr) mutants were isolated by a cell cloning assay in microtiter plates. These mutants were studied by Southern blot analysis to define the gross structural alterations in the hypoxanthine-guanine phosphoribosyl transferase (hprt) gene by use of an hprt cDNA probe. A similar analysis of the T-cell receptor (TCR) gene rearrangement patterns was employed to define the independent nature of each mutant colony by use of TCR beta and gamma cDNA probes. 74 mutants were isolated in 5 separate experiments. TCR gene rearrangement analysis showed these to represent 24 independent mutations, of which 18 contained hprt structural alterations. These alterations included simple deletions (10/18) as well as more complex rearrangements resulting in molecular weight changes of restriction fragments representing both the 5' and 3' regions of the hprt gene (4/18 and 4/18, respectively). These results demonstrate that gamma-irradiation primarily induces TGr mutations through gross structural alterations in the hprt gene and that these alterations are randomly distributed across the gene. This approach to mutation analysis will provide information on the types of alterations induced by this irradiation, especially the extent of deletions involving the hprt gene. These results also demonstrate the feasibility of employing in vitro exposure of human T-lymphocytes to a single mutagenic agent as an aid to understanding the mechanisms of mutations occurring in vivo in humans.