Validation of chromosome painting. II. A detailed analysis of aberrations following high doses of ionizing radiation in vitro

Chromosome analysis of nuclear power plant workers using fluorescence in situ hybridization and Giemsa assay

The aim of this study was to evaluate the genotoxic effects of ionizing radiation in vivo in exposed Bulgarian nuclear power plant workers by using classical cytogenetic and molecular cytogenetic analyses of peripheral lymphocytes. Chromosome analysis using fluorescence in situ hybrydization (FISH) and Giemsa techniques was undertaken on 63 workers and 45 administrative staff controls from the Bulgarian Nuclear Power Plant. Using the Giemsa method, the frequencies of cells studied with chromosome aberrations, dicentrics plus rings and chromosome fragments in the radiation workers were significantly higher compared with the control group (P = 0.044, P = 0.014, and P = 0.033, respectively). A significant association between frequencies of dicentrics plus rings and accumulated doses was registered (P < 0.01). In the present study, a FISH cocktail of whole chromosome paints for chromosomes 1, 4 and 11 was used. A significant association between frequency of translocations and accumulated doses was also observed (P < 0.001). Within the control group, a correlation was found between age and the spontaneous frequency of translocations. No correlation was found between smoking status and frequency of translocations. When compared with the control group, workers with accumulated doses up to 100 mSv showed no increase in genome translocation frequency, whereas workers with accumulated doses from 101 to 200 mSv showed a statistically significant doubling of genome translocation frequency (P = 0.009). Thus, in cases of chronic exposure and for purposes of retrospective dosimetry, the genome frequency of translocations is a more useful marker for evaluation of genotoxic effects than dicentric frequency.

Retrospective Assessment of Radiation Exposure Using Biological Dosimetry: Chromosome Painting, Electron Paramagnetic Resonance and the Glycophorin A Mutation Assay

Biological monitoring of dose can contribute important, independent estimates of cumulative radiation exposure in epidemiological studies, especially in studies in which the physical dosimetry is lacking. Three biodosimeters that have been used in epidemiological studies to estimate past radiation exposure from external sources will be highlighted: chromosome painting or FISH (fluorescence in situ hybridization), the glycophorin A somatic mutation assay (GPA), and electron paramagnetic resonance (EPR) with teeth. All three biodosimeters have been applied to A-bomb survivors, Chernobyl clean-up workers, and radiation workers. Each biodosimeter has unique advantages and limitations depending upon the level and type of radiation exposure. Chromosome painting has been the most widely applied biodosimeter in epidemiological studies of past radiation exposure, and results of these studies provide evidence that dose-related translocations persist for decades. EPR tooth dosimetry has been used to validate dose models of acute and chronic radiation exposure, although the present requirement of extracted teeth has been a disadvantage. GPA has been correlated with physically based radiation dose after high-dose, acute exposures but not after low-dose, chronic exposures. Interindividual variability appears to be a limitation for both chromosome painting and GPA. Both of these techniques can be used to estimate the level of past radiation exposure to a population, whereas EPR can provide individual dose estimates of past exposure. This paper will review each of these three biodosimeters and compare their application in selected epidemiological studies.

Persistence of chromosome aberrations following acute radiation: I, PAINT translocations, dicentrics, rings, fragments, and insertions

Chromosome translocations are used to estimate the doses of radiation received following occupational or accidental exposure. Biodosimetry relies on the assumption that translocations are not cell-lethal and persist with little or no loss over time. While translocations do exhibit substantially greater persistence than other aberration types (e.g., dicentrics), there is evidence that translocation frequencies also decline over time, at least following acute doses above 1 Gy. To the extent that translocation frequencies decline, the predicted absorbed doses will be underestimated. Yet unknown is whether translocations induced by ionizing radiation at doses below 1 Gy also show significant declines. Here we report on the persistence of translocations induced by 137Cs gamma-rays at acute doses ranging from 0.2 to 4 Gy using peripheral blood lymphocytes from two unrelated healthy male donors. Chromosome aberrations were evaluated by simultaneously painting chromosomes 1, 2, and 4 in red and 3, 5, and 6 in green in cells harvested 2-7 days following exposure and were scored using the PAINT system. Translocations were also enumerated using several other methods and these results are reported separately by us in this issue. For comparison, the persistence of dicentrics, rings, acentric fragments, and color junctions was also evaluated and showed rapid losses with time. The results from both donors provide evidence that translocation frequencies decline with time in a statistically significant manner at doses as low as 0.2-0.3 Gy. The frequency of translocations for all dose groups declined from day 2 to 7 by averages of 39% and 26% for donors 1 and 2, respectively. These data emphasize the importance of considering translocation loss in biological dosimetry long times after exposure.

Persistence of chromosome aberrations following acute radiation: II, does it matter how translocations are scored?

Chromosome breaks and rearrangements resulting from ionizing radiation can be much more complicated than many investigators thought possible some years ago. The realization that not all translocations are reciprocal, that multiway exchanges occur, and that some double-strand breaks are not repaired prior to mitosis have all contributed to the difficulty of knowing how best to identify, record, evaluate, and report chromosome translocations. Here we describe the results of a series of experiments in which blood from two normal healthy subjects was obtained, irradiated with 137Cs gamma-rays in vitro at doses ranging from 0 (controls) to 4 Gy, and cultured. Cells from each dose group and donor were harvested at days 2, 2.5, 3, 4, 5, and 7 and evaluated for chromosome damage by simultaneously painting chromosomes 1, 2, and 4 in red and 3, 5, and 6 in green. The persistence of dicentrics, fragments, rings, insertions, and PAINT translocations are reported separately by us in this issue. In this article, we focus on translocations, characterizing the various types in detail and comparing and contrasting their persistence across all dose groups for both donors. The results indicate that the persistence of all translocation types was sufficient to be used for retrospective dosimetry, although nonreciprocal translocations exhibited diminished persistence compared to the other types. We also characterize the kinetics of the radiation dose responses of the two donors who exhibited significant differences in the induction as well as the persistence of translocations. Based on the evidence presented here, we hypothesize that these individuals differ in the recognition and repair of radiation-induced damage as well as in cell cycle checkpoint control. Despite these differences, the temporal frequency of translocation losses at both the high and low doses was similar for both subjects.

Influence of dose rate on the induction of simple and complex chromosome exchanges by gamma rays

Single-color painting of whole chromosomes, or protocols in which only a few chromosomes are distinctively painted, will always fail to detect a proportion of complex exchanges because they frequently produce pseudosimple painting patterns that are indistinguishable from those produced by bona fide simple exchanges. When 24-color multi-fluor FISH (mFISH) was employed for the purpose of distinguishing (truly) simple from pseudosimple exchanges, it was confirmed that the acute low-LET radiation dose-response relationship for simple exchanges lacked significant upward curvature. This result has been interpreted to indicate that the formation of simple exchanges requires only one chromosome locus be damaged (e.g. broken) by radiation to initiate an exchange-not two, as classical cytogenetic theory maintains. Because a one-lesion mechanism implies single-track action, it follows that the production of simple exchanges should not be influenced by changes in dose rate. To examine this prediction, we irradiated noncycling primary human fibroblasts with graded doses of (137)Cs gamma rays at an acute dose rate of 1.10 Gy/min and compared, using mFISH, the yield of simple exchanges to that observed after exposure to the same radiation delivered at a chronic dose rate of 0.08 cGy/min. The shape of the dose response was found to be quasi-linear for both dose rates, but, counter to providing support for a one-lesion mechanism, the yield of simple aberrations was greatly reduced by protracted exposure. Although chronic doses were delivered at rates low enough to produce damage exclusively by single-track action, this did not altogether eliminate the formation of complex aberrations, an analysis of which leads to the conclusion that a single track of low-LET radiation is capable of inducing complex exchanges requiring up to four proximate breaks for their formation. For acute exposures, the ratio of simple reciprocal translocations to simple dicentrics was near unity.

Suitability of FISH painting techniques for the detection of partial-body irradiations for biological dosimetry

Peripheral blood was irradiated with 2, 3, 4 or 5 Gy of X rays and was mixed with nonirradiated blood at five different dilutions to simulate partial-body irradiations. Analysis by FISH was performed using whole-chromosome painting probes for chromosomes 1, 4 and 11 in combination with a pancentromeric probe. Chromosome aberrations affecting the painted fraction were classified according to the PAINT nomenclature; other unstable aberrations affecting the unpainted material were also recorded. To evaluate the suitability of painting for dose assessment in partial-body irradiations, the ability of the u test and a proposed s test to detect the expected overdispersion and the similarity between the real doses and the doses estimated using Dolphin's approach were considered. For short-term biodosimetry, compared with solid-stained dicentric analyses, the suitability of FISH painting techniques for the detection of partial-body exposures is reduced, because of the decrease in the frequency of aberrations detected by FISH and in the number of cells with two or more aberrations. For reconstruction of past doses, when only complete apparently simple translocations in cells free of unstable aberrations were considered, the detection of the overdispersion and the accuracy of dose estimations were dramatically reduced. In a partial-body exposure, as the original dose increased, the whole-body dose estimated a long time after irradiation would tend to be lower, and the difference from the original dose would tend to be greater.

Complex chromosome exchanges induced by gamma rays in human lymphocytes: an mFISH study

Loucas, B. D. and Cornforth, M. N. Complex Chromosome Exchanges Induced by Gamma Rays in Human Lymphocytes: An mFISH Study. Radiat. Res. 155, 660-671 (2001). Combinatorial multi-fluor fluorescence in situ hybridization (mFISH) allows the simultaneous painting of each pair of homologous chromosomes, thereby eliminating many of the difficulties previously associated with the analysis of complex rearrangements. We employed mFISH to visualize exchanges in human lymphocytes and found significant frequencies of these aberrations after gamma-ray doses of 2 and 4 Gy. At 4 Gy, roughly half of the cells contained at least one complex exchange that required anywhere from 3 to 11 initial chromosome breaks. At this dose, more than 40% of gross cytogenetic damage, as measured by the total number of exchange breakpoints, was complex in origin. Both simple and complex exchanges were found to have nonlinear dose responses, although the latter showed significantly more upward curvature. In many cases, it could be deduced that the initial breaks leading to a particular complex exchange were proximate, meaning that the resulting broken chromosome ends all must have been capable of interacting freely during the exchange process. For other complex exchanges, the rearrangement could just as well have resulted from two or more simpler exchanges that occurred sequentially. The results demonstrate the utility of mFISH in visualizing intricacies of the exchange process, but also highlight the various sources of ambiguity concerning cytogenetic analysis that remain despite the power of this approach.

Evaluating changes in stable chromosomal translocation frequency in patients receiving radioimmunotherapy

PURPOSE

The lack of any consistent correlation between radioimmunotherapy (RIT) dose and observed hematologic toxicity has made it difficult to validate RIT radiation dose estimates to marrow. Stable chromosomal translocations (SCT) which result after radiation exposure may be a biologic parameter that more closely correlates with RIT radiation dose. Increases in the frequency of SCT are observed after radiation exposure and are highly correlated with absorbed radiation dose. SCT are cumulative after multiple radiation doses and conserved through an extended number of cell divisions. The purpose of this study was to evaluate whether increases in SCT frequency were detectable in peripheral lymphocytes after RIT and whether the magnitude of these increases correlated with estimated radiation dose to marrow and whole body.

METHODS AND MATERIALS

Patients entered in a Phase I dose escalation therapy trial each received 1-3 intravenous cycles of the radiolabeled anti- carcinoembryonic antigen (CEA) monoclonal antibody, 90Y-chimeric T84.66. Five mCi of 111In-chimeric T84.66 was co-administered for imaging and biodistribution purposes. Blood samples were collected immediately prior to the start of therapy and 5-6 weeks after each therapy cycle. Peripheral lymphocytes were harvested after 72 hours of phytohemagglutinin stimulation and metaphase spreads prepared. Spreads were then stained by fluorescence in situ hybridization (FISH) using commercially available chromosome paint probes to chromosomes 3 and 4. Approximately 1000 spreads were evaluated for each chromosome sample. Red marrow radiation doses were estimated using the AAPM algorithm and blood clearance curves.

RESULTS

Eighteen patients were studied, each receiving at least one cycle of therapy ranging from 5-22 mCi/m2. Three patients received 2 cycles and two patients received 3 cycles of therapy. Cumulative estimated marrow doses ranged from 9.2 to 310 cGy. Increases in SCT frequencies were observed after each cycle for both chromosomes 3 and 4 in 16 of 18 patients and in at least one chromosome for the remaining 2 patients. Cumulative increases in SCT frequencies ranged from 0.001 to 0.046 with no major differences observed between chromosomes 3 and 4. A linear correlation between cumulative marrow dose and increases in SCT frequencies was observed for chromosome 3 (R2 = 0.63) and chromosome 4 (R2 = 0.80). A linear correlation was also observed between increases in SCT frequency and whole body radiation dose or administered activity (R2 = 0.67-0.89). There was less correlation between observed decrease in wbc or platelet counts and marrow dose, whole body dose, or administered activity (R2 = 0.28-0.43).

CONCLUSIONS

Increases in SCT frequency were detectable in peripheral lymphocytes after low dose-rate RIT irradiation. A linear correlation was observed between increases in SCT and marrow dose, whole body dose, and administered activity. This correlation provides one of the strongest radiation dose-response and activity-response relationships observed with RIT. The detection of SCT may therefore have application as an in situ integrating biodosimeter after RIT. This biologic parameter should prove useful in comparing effects on marrow for different therapeutic radionuclides and in comparing effects of RIT and external beam radiation doses on a cGy per cGy basis. As a result, this should allow for a more direct comparison between different methods of irradiation and in further refinement of radioimmunotherapy dose estimates and dosimetry methodology.

Lifetime persistence and clonality of chromosome aberrations in the peripheral blood of mice acutely exposed to ionizing radiation

As the measurement of chromosomal translocations increases in popularity for quantifying prior radiation exposure, information on the possible decline of these "stable" aberrations over time is urgently needed. We report here information about the persistence of radiation-induced chromosome aberrations in vivo over the life span of a rodent. Female C57BL/6 mice were given a single whole-body acute exposure of 0, 1, 2, 3 or 4 Gy (137)Cs gamma rays at 8 weeks of age. Chromosome aberrations were analyzed from peripheral blood samples at various intervals between 1 day and 21 months after exposure. Aberrations were detected by painting chromosomes 2 and 8. Translocations decreased dramatically during the first 3 months after irradiation, beyond which time the frequencies remained relatively constant out to 1 year, when the effects of aging and clonal expansion became significant. Both reciprocal and nonreciprocal translocations increased with age in the unexposed control animals and were involved in clones. As expected of unstable aberrations, dicentrics decreased rapidly after exposure and reached baseline levels within 3 months. These results indicate that the persistence of translocations induced by ionizing radiation is complicated by aging and clonal expansion and that these factors must be considered when quantifying translocations at long times after exposure. These results have implications for biological dosimetry in human populations.

Analysis by FISH of the spectrum of chromosome aberrations induced by X-rays in G0 human lymphocytes and their fate through mitotic divisions in culture

The induction, distribution, and persistence of chromosome aberrations in human lymphocytes exposed to X-rays in G0 were analyzed in 48-, 70-, and 94-hr cultures by conventional metaphase analysis and painting of chromosomes 1, 2, and 4 by FISH. All cells that had been scored by FISH were relocated to determine by differential staining of chromatids whether they had passed through 1, 2, or > or =3 divisions. FISH revealed a dose-dependent induction of stable and unstable aberrations, while chromatid labeling showed mitotic lag caused by irradiation in G0. Relative to their DNA contents, there was a small but significant overrepresentation of chromosome 4 and underrepresentation of chromosome 2 among the aberrations involving chromosomes 1, 2, and 4. FISH slightly underestimated the genomic frequency of unstable aberrations measured by conventional metaphase analysis. There was a slight excess of translocations relative to dicentrics, but the data are compatible with the 1:1 ratio expected from cytogenetic theory. Many of the translocations were apparently incomplete (i.e., nonreciprocal). Incomplete translocations were more frequent at higher X-ray dose and in first division, suggesting that they may be associated with complex damage and are more apt to be lost in mitosis than complete translocations. Among the incomplete translocations, t(Ab) outnumbered t(Ba) -- a difference ascribable to the FISH technique. Aberration frequencies declined as the cells divided in culture. The overall decline in the frequency of aberrant cells (approximately 29% per cell generation) reflects a rapid decline in dicentrics and fragments (approximately 60% per cell generation) and the relative stability of translocations. The frequency of translocation-bearing cells underwent a modest decline in culture (approximately 13% per cell generation).

Retrospective dose reconstruction of human radiation exposure by FISH/chromosome painting

Measurements of stable chromosomal aberrations by fluorescence in situ hybridisation (FISH)-chromosome painting has the potential to be used for dose reconstruction, years after exposure to ionising radiation. The method is, however, not yet fully standardised and validated. In particular, with respect to the limited lifespan of circulating T-lymphocytes, a level of uncertainty exists on the long-term persistence of stable aberrations. The main principles of the technique will be demonstrated. Based on results from the literature, the reliability and limitations are discussed.

Chromosome aberrations induced in mice by chronic feeding of 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ)

Dietary intake of mutagenic compounds is considered to be an important factor for the induction of some human cancers. Highly mutagenic compounds are known to be formed in meat during the cooking process. Since the discovery of such compounds, many studies have been conducted to evaluate their carcinogenic potential. One of the most mutagenic compounds formed in the cooking of meat is 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ). The recent development of mouse chromosome painting probes expand the capability of evaluating these food mutagens as potential clastogens in vivo. In this paper, we demonstrate the induction of chromosome aberrations in mice chronically exposed to MeIQ in their diet. CDF1 female mice were fed 400 ppm MeIQ beginning at 7 wk of age. At 76 wk of age, five control and eight exposed mice were euthanized. Blood and bone marrow cells were obtained and arrested in metaphase. Whole chromosome painting probes were used for fluorescence in situ hybridization of metaphase cells from blood and bone marrow. MeIQ-exposed mice were found to have a twofold increase in translocations and a 16-fold increase in fragments in their peripheral blood compared with controls. No aberrations were observed in the bone marrow. All organs were examined for the presence of tumours and routine histopathological analysis was performed on all organs as well as any tissue with macroscopic abnormalities. Forestomach and/or liver tumours developed in all but one of the mice fed MeIQ, but no such tumours were observed in the control mice. These data indicate that MeIQ is clastogenic and carcinogenic in vivo.

137Cesium-induced chromosome aberrations analyzed by fluorescence in situ hybridization: eight years follow up of the Goiânia radiation accident victims

The radiation accident in focus here occurred in a section of Goiânia (Brazil) where more than a hundred individuals were contaminated with 137Cesium on September 1987. In order to estimate the absorbed radiation doses, initial frequencies of dicentrics and rings were determined in 129 victims [A.T. Ramalho, PhD Thesis, Subsidios a tecnica de dosimetria citogenetica gerados a partir da analise de resultados obtidos com o acidente radiologico de Goiânia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil, 1992]. We have followed some of these victims cytogenetically over the years seeking for parameters that could be used as basis for retrospective radiation dosimetry. Our data on translocation frequencies obtained by fluorescence in situ hybridization (FISH) could be directly compared to the baseline frequencies of dicentrics available for those same victims. Our results provided valuable information on how precise these estimates are. The frequencies of translocations observed years after the radiation exposure were two to three times lower than the initial dicentrics frequencies, the differences being larger at higher doses (>1 Gy). The accuracy of such dose estimates might be increased by scoring sufficient amount of cells. However, factors such as the persistence of translocation carrying lymphocytes, translocation levels not proportional to chromosome size, and inter-individual variation reduce the precision of these estimates.

Chromosomal aberrations induced in human lymphocytes by high-LET irradiation

High linear energy transfer (LET) particles are more efficient than sparsely ionizing radiations in inducing chromosomal aberrations, in particular complex rearrangements. We analysed R-banded chromosome rearrangements in human lymphocytes irradiated with several ions having a wide range of LET (31.3-1435 keV/micron). The frequency of chromosome breaks unrejoined or inferred from observed rearrangements, and of complex rearrangements induced by a single particle, increased with the LET up to about 100-150 keV/micron and seemed to level off for higher LET values. Additional study was focused on damage induced by oxygen ions of three different energies. Significant cell cycle delay, and multiple chromosome rearrangements and breaks were demonstrated using Giemsa and Fluorescence-plus-Giemsa stainings, coupled with chromosome painting. Damage increased with the fluence and the LET, but at the higher LET damage decreased for fluences > 10(7) particles/cm2. Cell death and G2 block might be involved in this phenomenon. Chromosome 1 painting exhibited a high frequency of breaks and complex rearrangements, which would not have been detected using a standard staining. Complex rearrangements were induced by as few as one particle per cell nucleus and may be considered as a biological fingerprint of high-LET irradiation.

Differential involvement of chromosomes 1 and 4 in the formation of chromosomal aberrations in human lymphocytes after X-irradiation

Whole blood samples from two healthy donors were cultured in the presence of 5-bromo-2'-deoxyuridine (BrdU) for a total of 107 h following in vitro X-irradiation with a dose of 2 Gy. Starting from 35 h after culture initiation, every subsequent 12 h a sample was taken from each culture and grown in the presence of demecolcine for another 12 h. At each sampling time, the aberrations involving chromosomes 1 and 4 were analysed using dual-colour fluorescence in situ hybridization (FISH) with chromosome-specific DNA libraries. Following differential staining of sister chromatids, the analysed cells were identified to be either in their first, second or third etc. mitosis after irradiation. Cells within the same postirradiation division contained higher frequencies of aberrations when derived from later sampling times, indicating a delay in progression of aberrant cells to mitosis. In contrast, when the aberration frequencies are calculated by sampling time (i.e. independent of the cell cycle) minimal effect of sampling time could be seen. This observation held true for all types of chromosomal aberrations. Analysis of about 2250 first-division cells for each donor (derived from all sampling times) indicates a relative overrepresentation of chromosome 4 in the formation of exchange aberrations/colour junctions. Whereas dicentric frequencies for chromosomes 1 and 4 were close to the expected values based on the DNA content of these chromosomes, frequencies of reciprocal translocations showed a clear overinvolvement of chromosome 4. This resulted in a distinct difference in the reciprocal translocation to dicentric ratio, being 1.12 for chromosome 1 and 2.09 for chromosome 4. These results indicate a non-DNA-proportional distribution of radiation-induced chromosome rearrangements in cultured human lymphocytes.

Centric rings, acentric rings and excess acentric fragments based on a random-walk interphase chromosome model

Excess acentric fragments, consisting of acentric rings and acentric linear fragments, are among the most frequent kinds of chromosome-type aberrations produced by radiation. The frequency of acentric rings cannot be obtained directly by experiment but is estimated here from the ratio of acentric to centric rings, evaluated using a random-walk model for the organization of chromatin during interphase and an assumption that the probability of an exchange formation is proportional to the rate of collision between two DSB. This ratio is calculated to be 2.5 in low-LET irradiated human fibroblasts, significantly greater than the ratio if proximity effects are not considered. The calculated frequency of acentric rings is insufficient to account for all the observed excess acentric fragments. Assuming that the rest of the excess acentric fragments are due to incomplete exchanges, all possible recombinations between two DSB that result in acentric rings and acentric linear fragments have been identified. From the chromosome aberration data, the incompleteness parameter has been estimated. Intra-arm chromosome exchanges, either complete or incomplete, were estimated to account for more than 50% of the excess acentric fragments in human fibroblasts.

Review: proximity effects in the production of chromosome aberrations by ionizing radiation

After ionizing radiation has induced double-strand DNA breaks (dsb), misrejoining produces chromosome aberrations. Aberration yields are influenced by "proximity' effects, i.e., by the dependence of misrejoining probabilities on initial dsb separations. We survey proximity effects, emphasizing implications for chromosome aberration-formation mechanisms, for chromatin geometry, and for dose-response relations. Evidence for proximity effects comes from observed biases for centric rings and against three-way interchanges, relative to dicentrics or translocations. Other evidence comes from the way aberration yields depend on radiation dose and quality, tightly bunched ionizations being relatively effective. We concludes (1) that misrejoining probabilities decrease as the distance between dsb at the time of their formation increases, and almost all misrejoining occurs among dsb initially separated by < 1/3 of a cell nucleus diameter; (2) that chromosomes occupy (irregular) territories during the G0/G1 phase of the cell cycle, having dimensions also roughly 1/3 of a cell nucleus diameter, (3) that proximity effects have the potential to probe how much different chromosomes intertwine on move relative to each other: and (4) that incorporation of proximity effects into the classic random breakage-and-reunion model allows quantitative interrelation of yields for many different aberration types and of data obtained with various FISH painting methods or whole-genome scoring.

Methods for improving the yield and quality of metaphase preparations for FISH probing of human lymphocyte chromosomes

Procedures are described for the in vitro culture of human lymphocytes, which have been concentrated by density gradient centrifugation, and for a modified slide-making technique for the fixed cells. The method yields improved percentages of mitotic cells which are largely synchronized at harvest. Controlled placement of fixed cells on slides produces well-spread metaphase preparations with little background material to interfere with fluorescence in situ hybridization (FISH) probe procedures. The FISH reagents and microscope scanning time required are minimized by concentrating cells in a defined area of the slide.

Paternally inherited chromosomal structural aberrations detected in mouse first-cleavage zygote metaphases by multicolour fluorescence in situ hybridization painting

We describe a fluorescence in situ hybridization (FISH) procedure for assessing zygotic risk of paternal exposure to endogenous or exogenous agents. The procedure employs multicolour FISH with chromosome-specific DNA painting probes plus DAPI staining for detecting both balanced and unbalanced chromosomal aberrations in mouse first-cleavage (1-Cl) zygote metaphases. Four composite probes specific for chromosomes 1, 2, 3 or X, each labelled with biotin, plus a composite probe specific for chromosome Y labelled with digoxigenin, were used. We applied this method to evaluate the effects of paternal exposure to acrylamide, a model germ cell clastogen. First-cleavage zygote metaphases, collected from untreated females mated to males whose sperm or late spermatids were treated with acrylamide, were scored for the induction of structural aberrations using both chromosome painting (PAINT analysis) and DAPI analysis. Structural chromosomal aberrations were observed in the sperm-derived, but not in the egg-derived, pronuclei. While 59.4% of the zygotes had structural aberrations by DAPI analysis, 94.1% of the same zygotes had structural aberrations by PAINT analysis (P < 0.001), illustrating the increased sensitivity for detecting translocations and insertions obtained by adding chromosome painting. These findings show that FISH painting of mouse 1-Cl zygotes when used in conjunction with DAPI analysis is a powerful model for investigating the cytogenetic defects transmitted from father to offspring.

The persistence of aberrations in mice induced by gamma radiation as measured by chromosome painting

Fluorescence in situ hybridization, or chromosome painting, has become an invaluable tool in the cytogenetic evaluation of historical or chronic exposure because it can be used to detect stable genetic damage, such as translocations, which persist through cell division, quickly and easily. The recent development of chromosome-specific composite DNA probes for the mouse has allowed the use of chromosome painting in this commonly used animal model. In order to measure the persistence of radiation-induced translocations, C57BL/6 female mice were given a whole body acute dose of 0, 1, 2, 3 or 4 Gy 137Cs gamma rays at 8 weeks of age. Metaphase chromosomes from both peripheral blood and bone marrow cells were obtained from four mice in each dose group at 1, 8, 15 and 30 days post-irradiation. Chromosomes 2 and 8 were painted, while the remaining chromosomes were counterstained with propidium iodide. DAPI counterstain was used to differentiate between translocations and dicentrics because it brightly labels the centromeric heterochromatin. The equivalent of 100 cells from each tissue was scored from each mouse. The results show that the percentage of reciprocal translocations, at least at doses of 3 Gy or lower, did not decrease with time in either tissue. In contrast, the frequency of non-reciprocal translocations induced by doses of 3 Gy or lower, remained unchanged in the peripheral blood, but decreased after a week in the bone marrow, then remained constant. An increase in these two types of aberration was observed between 15 and 30 days in the bone marrow and may have been due to clonal expansion. Dicentrics decreased with time in both tissues, almost none remained in the bone marrow after 8 days. These data suggest that reciprocal translocations are persistent and will serve as an effective biodosimeter for radiation exposure.

Analysis of radiation-induced chromosome aberrations in Chinese hamster cells by FISH using chromosome-specific DNA libraries

The frequencies of chromosome aberrations induced by different doses of X-rays were determined in both splenocytes and primary lung fibroblasts of Chinese hamster by bi-colour FISH using a combination of four chromosome-specific DNA libraries. The results indicate that the X-rays induced more translocations than dicentrics in Chinese hamster cells, in which the karyotype is comprised of both metacentric and acrocentric chromosomes. These results are similar to those reported in human lymphocytes, in which the karyotype contains many metacentric chromosomes. On the contrary, in mouse, which is characterized by acrocentric chromosomes only, the frequencies of translocations and dicentrics are induced in nearly equal proportions by X-rays. The ratio of translocations to dicentrics obtained in Chinese hamster cells was approximately 1.4-1.5, which supports the importance of the karyotypic features of a species in the relative induction of translocations to dicentrics. An analysis was also made on the yield of translocations and dicentrics involving individual chromosomes and the results indicate a non-random involvement of these chromosomes in the formation of aberrations.

Cytogenetic results from a chronic feeding study of MeIQx in mice

The primary food mutagens found in cooked meat are the heterocyclic aromatic amines, including 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx). High dose, acute exposures of MeIQx produce hepatic tumours in some mouse and rat strains. By assaying chromosome damage it may be possible to correlate aberrations with exposure to a mutagen. Whole chromosome painting by fluorescence in situ hybridization allows the rapid screening of metaphase chromosomes for rearrangements. This technique was applied to female mice (C57BL/6) chronically fed 100-400 ppms MeIQx for up to 6 months. Two tissues, blood and bone marrow, were screened with multicolour whole chromosome painting probes (1,2,3 and 8). The mice showed no outward signs of toxicity at any dose and very few chromosome aberrations were observed. A slight but significant increase in sister chromatid exchanges (SCE) was seen at 400 ppm in blood at 6 months. When MeIQx was removed from the diet for 1 month there was an apparent decline in SCEs only for animals previously given the 400 ppm diet.

Comparison of the yields of translocations and dicentrics measured using conventional Giemsa staining and chromosome painting

Inconsistent results have been reported regarding the yields of translocations and dicentrics measured using chromosome painting. This inconsistency may be due to mis-scoring of dicentrics as translocations because painting is unsuited to identify centromere positions. In the present study, chromosome aberrations exclusively in the first mitosis after irradiation were analyzed using human peripheral lymphocytes. When identical metaphases were analyzed by both painting and conventional Giemsa staining, which is the most reliable method to locate centromeres, equal frequencies of translocations and dicentrics were observed.

Induction and persistence of cytogenetic damage in mouse splenocytes following whole-body X-irradiation analysed by fluorescence in situ hybridization. I. Dicentrics and translocations

Chromosome translocations (stable aberrations) can persist longer during cellular proliferation than dicentrics (unstable aberrations). It is important to know the kinetics of the elimination of dicentrics and to what extent translocations persist in an in vivo cell population after irradiation. The female Swiss mouse were used to study the induction and persistence of dicentrics and translocations in splenocytes up to 112 days after exposure to 2 Gy whole-body X-irradiation. Metaphase spreads at different time intervals were analyzed by fluorescence in situ hybridization (FISH) using chromosome-specific DNA libraries for chromosomes 1, 11 and 13. The frequencies of dicentrics and translocations appear to be equal immediately after irradiation. Frequencies of dicentrics decreased exponentially with time according to the relationship D = ae(-kt). The rate of elimination was faster in the early period (days 0-14) than in the later period (> or = 14 days). The frequency of translocations was constant in the period 0-7 days and then decreased linearly or exponentially. For the whole period, the trend is highly significant. As mouse chromosome painting probes are becoming available and by using FISH, an in vivo mouse model for the analysis of translocations has become feasible. As translocations are involved in carcinogenesis and genetic disorders, risk estimation for induction of translocations by ionizing radiation can be made with greater confidence and extrapolated to the human situation.

Theoretical predictions on the equality of radiation-produced dicentrics and translocations detected by chromosome painting

Existing models of chromosome aberrations produced by ionizing-radiation predict equal numbers of dicentrics and translocations if the dose is so low that complex aberrations can be ignored. We show that, for a specific subset of aberrations detected by FISH, dicentric/translocation equality is predicted even at higher doses. Assuming one-colour whole-chromosome painting (with unpainted chromosomes counterstained and centromeres recognizable) the relevant restriction is that the final metaphase pattern be, in the terminology of Simpson and Savage, 'apparently simple'. This means that the painted pattern is required to have the colour/centromere appearance corresponding to a single complete reciprocal exchange but its actual formation, as reflected for example in lengths, is allowed to be more complicated. The restriction to apparent simplicity is significantly less limiting than ignoring all complex aberrations. Our analysis of predicted dicentric/translocation equality in this case uses examples, a combinatorial counting method, Monte Carlo computer programs, and a duality proof. However, we argue that for 'visibly complex' dicentrics or translocations, no similar equality is expected in general. Corresponding experimental results are briefly surveyed. Checking dicentric/translocation equality experimentally can provide a significant test of current chromosome aberration models.

Fluorescence in situ hybridization: a brief review

Fluorescence in situ hybridization (FISH) is used for many purposes, including analysis of chromosomal damage, gene mapping, clinical diagnostics, molecular toxicology and cross-species chromosome homology. FISH allows an investigator to identify the presence and location of a region of cellular DNA or RNA within morphologically preserved chromosome preparations, fixed cells or tissue sections. This report describes in situ hybridization, and discusses the past, present and future applications of this method for genetic analysis and molecular toxicology.

Fluorescence in situ hybridization detection of chromosomal aberrations in human lymphocytes: applicability to biological dosimetry

Human lymphocytes in G0 have been irradiated with X-ray doses from 0 to 4.0 Gy. Metaphase chromosomes 2, 3 and 5 and all centromeres were painted using fluorescence in situ hybridization (FISH) probe libraries. Dicentrics, centric rings and acentrics in the whole genome as well as translocations involving the painted chromosomes were recorded. The translocations were subdivided as complete or incomplete. Interstitial insertions and inversions were also noted. The observations were also recorded according to the Protocol for Aberration Identification and Nomenclature Terminology (PAINT) system of scoring. Given that the painted chromosomes comprise 20.4% of the genome it was found that the yield of bicoloured dicentrics was consistent with the yield of dicentrics in the whole genome. The yield of radiation-induced translocations was not significantly higher than that of bicoloured dicentrics. Of the translocations, 60% were complete and it was concluded that the majority of dicentrics and translocations are complete exchanges. Chromosome 5 took part in exchanges marginally more commonly than its length suggests, but it is not known if this is a property of the chromosome or whether it is a donor-dependent observation. The PAINT system of recording rearrangements was examined and the suggested numerical interpretation of this nomenclature was considered to be unsuitable for use in the estimation of dose for cases of accidental overexposure.

Use of fluorescence in situ hybridization to determine the relationship between chromosome aberrations and cell survival in eight human fibroblast strains

A predictive assay of normal tissue radiosensitivity could benefit 'treatment tailoring' of radiotherapy for certain categories of tumour. The use of present clonogenic cell survival assays for this purpose would be impractical in routine clinical practice because of the lengthy assay time. Fluorescence in situ hybridization (FISH) using whole chromosome probes on metaphases was investigated as a potential substitute. Eight human fibroblast cell strains with a range of radiosensitivities were tested. For each strain, cell survival curves were determined and correlated with chromosome aberrations detected by FISH performed on metaphase cells collected 52 h after irradiation. A whole chromosome probe for chromosome 4 was used for all cell strains. The results revealed an increase in the percentage of metaphases with aberrant chromosomes (translocations and/or breaks) with increasing radiation dose for all strains. For the more radiosensitive cell strains there were relatively more aberrant metaphases for a given radiation dose when compared with fibroblasts from a normal donor. The relationship between surviving fraction and chromosome aberrations showed some variation between strains, but a linear regression for all data showed a highly statistically significant correlation (r = 0.89, p < 0.0005). These results suggest that an assay of chromosome damage using FISH could substitute for the clonogenic assay to predict the radiation sensitivity of human fibroblasts.