Radiation-induced breakpoint misrejoining in human chromosomes: random or non-random?

The Potential Effect of Different Doses of Ionizing Radiation on Genes and Disease

Aim

The chromosomal aberrations induced by radiation appear about nonrandomly distributed across the whole genome. Previous studies have shown that chromosomes with high DNA content are less frequently involved in the formation of symmetrical translocations and dicentric chromosomes than expected, whereas smaller chromosomes are more frequently involved. We hypothesized that these translocation regions are linked to radiation sensitivity.

Materials and methods

We investigated the frequencies of chromosome translocations induced by radiation exposure and adjusted the results according to chromosome length. We specifically analyzed whole blood samples from 3 participants. The samples were irradiated using ⁶⁰Co at doses of 0.5, 1, 2.5, and 5 Gy. Traditional Giemsa-trypsin-Wright band staining was performed to identify the translocations in the chromosomes, and results were compared with microarray data generated in our previous study.

Results

Our analysis indicated that chromosomes 9q were the most sensitive to translocations after various doses of radiation, and such translocations occurred in the euchromatin region. Chromosomes 1, 9, 15, and 17 were more sensitive to radiation doses of 0.5 Gy. This observation could be useful when selecting sensitive reference chromosomes in the low-dose region. The results of expression profiling analysis for radiation-sensitive regions were similar to those of chromosome translocation analysis.

Conclusion

This study shows that some chromosomes or genomic regions are more sensitive to alteration by radiation exposure.

Lessons from the accident with 137Cesium in Goiania, Brazil: Contributions to biological dosimetry in case of human exposure to ionizing radiation

Human exposure to ionizing radiation has increased over time, mainly due to medical applications, occupational and environmental exposure, as well as accidents involving radioactive materials. In September 1987, an accident with ¹³⁷Cesium occurred in Goiânia city, Brazil; the accident started with the removal of a 50.9-TBq ¹³⁷Cesium source from an abandoned radiotherapy unit. Among the radiation-exposed victims, at least 50 individuals showed symptoms of whole-body and local acute irradiation, and also external or internal contamination. In this report, the purpose was to review and summarize the main results of cytogenetic studies carried out with victims of ¹³⁷Cesium, for blood collection performed shortly after the accident, and following several years post-exposure. The importance of dose estimates by biological dosimetry is highlighted, and also several lessons that were learned from the initial to follow-up (7-10 years after the accident) studies, mainly by applying the fluorescence in situ hybridization (FISH) method. A relevant aspect discussed on the basis of the results obtained in those studies refers to the incidence of chromosomal translocations, which were directly compared to the initial frequencies of dicentrics that were previously used to estimate the absorbed doses. In general, translocation frequencies were two to three times lower than the dicentric frequencies, and the differences were dose-dependent. Furthermore, regarding attempts to perform retrospective dosimetry (10 years post-accident), the dose estimates using translocation frequencies for victims of ¹³⁷Cesium indicate the feasibility of this approach only for low level exposure (below 0.5 Gy), while for higher doses there are some limitations, and the requirement to apply appropriate correction factors, which were discussed on the basis of literature data. Apart of this, in general terms, important aspects to be mentioned refer to the need for better care and control of radioactive devices, as well as adequate education programs for professionals and also the population.

Frequent induction of chromosomal aberrations in in vivo skin fibroblasts after allogeneic stem cell transplantation: hints to chromosomal instability after irradiation

Background

Total body irradiation (TBI) has been part of standard conditioning regimens before allogeneic stem cell transplantation for many years. Its effect on normal tissue in these patients has not been studied extensively.

Method

We studied the in vivo cytogenetic effects of TBI and high-dose chemotherapy on skin fibroblasts from 35 allogeneic stem cell transplantation (SCT) patients. Biopsies were obtained prospectively (n = 18 patients) before, 3 and 12 months after allogeneic SCT and retrospectively (n = 17 patients) 23–65 months after SCT for G-banded chromosome analysis.

Results

Chromosomal aberrations were detected in 2/18 patients (11 %) before allogeneic SCT, in 12/13 patients (92 %) after 3 months, in all patients after 12 months and in all patients in the retrospective group after allogeneic SCT. The percentage of aberrant cells was significantly higher at all times after allogeneic SCT compared to baseline analysis. Reciprocal translocations were the most common aberrations, but all other types of stable, structural chromosomal aberrations were also observed. Clonal aberrations were observed, but only in three cases they were detected in independently cultured flasks. A tendency to non-random clustering throughout the genome was observed. The percentage of aberrant cells was not different between patients with and without secondary malignancies in this study group.

Conclusion

High-dose chemotherapy and TBI leads to severe chromosomal damage in skin fibroblasts of patients after SCT. Our long-term data suggest that this damage increases with time, possibly due to in vivo radiation-induced chromosomal instability.

Nonhomologous DNA end joining and chromosome aberrations in human embryonic lung fibroblasts treated with environmental pollutants

In order to evaluate the ability of a representative polycyclic aromatic hydrocarbon (PAH) and PAH-containing complex mixtures to induce double strand DNA breaks (DSBs) and repair of damaged DNA in human embryonic lung fibroblasts (HEL12469 cells), we investigated the effect of benzo[a]pyrene (B[a]P) and extractable organic matter (EOM) from ambient air particles <2.5μm (PM2.5) on nonhomologous DNA end joining (NHEJ) and induction of stable chromosome aberrations (CAs). PM2.5 was collected in winter and summer 2011 in two Czech cities differing in levels and sources of air pollutants. The cells were treated for 24h with the following concentrations of tested chemicals: B[a]P: 1μM, 10μM, 25μM; EOMs: 1μg/ml, 10μg/ml, 25μg/ml. We tested several endpoints representing key steps leading from DSBs to the formation of CAs including histone H2AX phosphorylation, levels of proteins Ku70, Ku80 and XRCC4 participating in NHEJ, in vitro ligation activity of nuclear extracts of the HEL12469 cells and the frequency of stable CAs assessed by whole chromosome painting of chromosomes 1, 2, 4, 5, 7 and 17 using fluorescence in situ hybridization. Our results show that 25μM of B[a]P and most of the tested doses of EOMs induced DSBs as indicated by H2AX phosphorylation. DNA damage was accompanied by induction of XRCC4 expression and an increased frequency of CAs. Translocations most frequently affected chromosome 7. We observed only a weak induction of Ku70/80 expression as well as ligation activity of nuclear extracts. In summary, our data suggest the induction of DSBs and NHEJ after treatment of human embryonic lung fibroblasts with B[a]P and complex mixtures containing PAHs.

Random frequency of radiation induced aberrations in individual chromosomes of lymphocytes of α -thalassemia variant patients

To evaluate the effects of ionizing radiation on chromosomal aberration in lymphocytes of α-thalassemia variants compared to normal controls, venous blood samples were obtained from 10 healthy volunteers and 30 α-thalassemia patients. Different types of α-thalassemia were diagnosed by multiplex polymerase chain reaction. Blood samples were divided into two parts, the first exposed to 3 Gy gamma rays generated from a 60Co source and the other without any irradiation. The blood samples were used for preparation of metaphase cells according to standard methods. Results showed that the frequency of spontaneous aberration (about 0.012 per cell) was similar in all study groups. Irradiated samples showed significantly higher frequency of aberrations (about 0.6 per cell) compared to non-irradiated samples in all groups but similar for control and α-thalassemia variants. The frequency of dicentric chromosomes was significantly higher than other aberrations. In normal individuals in all of the chromosomes except in chromosome 1, a random distribution of break points proportional to their length based on their DNA content was observed. In α-thalassemia variant individuals in all of the chromosomes, a random distribution of break points proportional to their length based on their DNA content was observed.

Different DNA-PKcs functions in the repair of radiation-induced and spontaneous DSBs within interstitial telomeric sequences

Interstitial telomeric sequences (ITSs) in hamster cells are hot spots for spontaneous and induced chromosome aberrations (CAs). Most data on ITS instability to date have been obtained in DNA repair-proficient cells. The classical non-homologous end joining repair pathway (C-NHEJ), which is the principal double strand break (DSB) repair mechanism in mammalian cells, is thought to restore the morphologically correct chromosome structure. The production of CAs thus involves DNA-PKcs-independent repair pathways. In our current study, we investigated the participation of DNA-PKcs from the C-NHEJ pathway in the repair of spontaneous or radiation-induced DSBs in ITSs using wild-type and DNA-PKcs mutant Chinese hamster ovary cells. Our data demonstrate that DNA-PKcs stabilizes spontaneous DSBs within ITSs from the chromosome 9 long arm, leading to the formation of terminal deletions. In addition, we show that DNA-PKcs-dependent C-NHEJ is employed following radiation-induced DSBs in other ITSs and restores morphologically correct chromosomes, whereas DNA-PKcs independent mechanisms co-exist in DNA-PKcs proficient cells leading to an excess of CAs within ITSs.

Diagnostic X-ray examinations and increased chromosome translocations: evidence from three studies

Controversy regarding potential health risks from increased use of medical diagnostic radiologic examinations has come to public attention. We evaluated whether chromosome damage, specifically translocations, which are a potentially intermediate biomarker for cancer risk, was increased after exposure to diagnostic X-rays, with particular interest in the ionizing radiation dose-response below the level of approximately 50 mGy. Chromosome translocation frequency data from three separately conducted occupational studies of ionizing radiation were pooled together. Studies 1 and 2 included 79 and 150 medical radiologic technologists, respectively, and study 3 included 83 airline pilots and 50 university faculty members (total = 155 women and 207 men; mean age = 62 years, range 34-90). Information on personal history of radiographic examinations was collected from a detailed questionnaire. We computed a cumulative red bone marrow (RBM) dose score based on the numbers and types of X-ray examinations reported with 1 unit approximating 1 mGy. Poisson regression analyses were adjusted for age and laboratory method. Mean RBM dose scores were 49, 42, and 11 for Studies 1-3, respectively (overall mean = 33.5, range 0-303). Translocation frequencies significantly increased with increasing dose score (P < 0.001). Restricting the analysis to the lowest dose scores of under 50 did not materially change these results. We conclude that chromosome damage is associated with low levels of radiation exposure from diagnostic X-ray examinations, including dose scores of approximately 50 and lower, suggesting the possibility of long-term adverse health effects.

Inter-chromosomal variation in aberration frequencies in human lymphocytes exposed to charged particles of LET between 0.5 and 55 keV/μm

PURPOSE

To investigate the distribution of chromosomal aberrations in chromosomes 2, 8 and 14 induced by charged particles, using the fluorescence in situ hybridisation (FISH) technique.

METHODS

Irradiation of peripheral blood from six healthy volunteers (four male and two female) was performed at the accelerators of the Joint Institute for Nuclear Research (JINR) in Dubna (Russia). Whole blood samples were irradiated with 2 and 3 Gy of protons (170 MeV/nucleon (n), linear energy transfer (LET) ≈ 0.5 keV/μm), 3.5 Gy of (12)C ions (480 MeV/n, LET = 10.6 keV/μm), 3 Gy of (12)C ions 500 MeV/n, LET = 12 keV/μm), 4 Gy of (7)Li ions (30 MeV/n, LET ≈ 20 keV/μm) and 3 Gy of (11)B ions (32 MeV/n, LET ≈ 55 keV/μm). Chromosomal aberrations were analysed in metaphase and prematurely condensed chromosomes (PCC) induced in G(2)-cells using calyculin A. Chromosomes 2, 8 and 14 were painted in different colours and aberrations scored with the help of an image-analysis system.

RESULTS

Chromosome 2 was generally less sensitive than expected on the basis of its DNA content. A higher than expected frequency of exchanges was found in chromosomes 8 and 14. On average, the dicentric frequency in chromosome 2 was higher than the translocation frequency, whereas variable dicentric to translocation ratios were observed in chromosomes 8 and 14. When aberrations in all painted chromosomes were summed up the ratio was close to 1. The frequency of complex aberrations correlated with LET.

CONCLUSION

In lymphocytes of donors studied in this work chromosome 2 appears to be consistently less sensitive to protons and heavy ions than chromosomes 8 and 14. Complex aberrations appear to be a potential marker of radiation quality.

The radiation sensitivity of human chromosomes 2, 8 and 14 in peripheral blood lymphocytes of seven donors

PURPOSE

To investigate if deviations from DNA-proportional distribution of radiation-induced chromosomal aberrations are individually variable.

MATERIALS AND METHODS

Peripheral blood lymphocytes were collected from seven healthy donors and exposed to different doses of gamma rays. Chromosomes 2, 8 and 14 were painted in different colors and aberrations scored with the help of an image-analysis system.

RESULTS

Chromosome 2 was generally less sensitive than expected on the basis of DNA-proportional distribution and the extent of inter-donor variability was minimal. A higher than expected frequency of aberrations was found in chromosome 14 of five donors, while a higher than expected frequency of aberrations was found in chromosome 8 of two donors.

CONCLUSIONS

Inter-donor variability may explain some of the controversies regarding the inter-chromosomal distribution of radiation-induced aberrations.

Enhanced sensitivity of the RET proto-oncogene to ionizing radiation in vitro

Exposure to ionizing radiation is a well-known risk factor for a number of human cancers, including leukemia and thyroid cancer. It has been known for a long time that exposure of cells to radiation results in extensive DNA damage; however, a small number of studies have tried to explain the mechanisms of radiation-induced carcinogenesis. The high prevalence of RET/PTC rearrangements in patients who have received external radiation, and the evidence of in vitro induction of RET rearrangements in human cells, suggest an enhanced sensitivity of the RET genomic region to damage by ionizing radiation. To assess whether RET is indeed more sensitive to radiations than other genomic regions, we used a COMET assay coupled with fluorescence in situ hybridization, which allows the measurement of DNA fragmentation in defined genomic regions of single cells. We compared the initial DNA damage of the genomic regions of RET, CXCL12/SDF1, ABL, MYC, PLA2G2A, p53, and JAK2 induced by ionizing radiation in both a lymphoblastoid and a fetal thyroid cell line. In both cell lines, RET fragmentation was significantly higher than in other genomic regions. Moreover, a differential distribution of signals within the COMET was associated with a higher percentage of RET fragments in the tail. RET was more susceptible to fragmentation in the thyroid-derived cells than in lymphoblasts. This enhanced susceptibility of RET to ionizing radiation suggests the possibility of using it as a radiation exposure marker.

Monitoring translocations by M-FISH and three-color FISH painting techniques: a study of two radiotherapy patients

PURPOSE

To compare translocation rate using either M-FISH or FISH-3 in two patients treated for head and neck cancer, with a view to retrospective dosimetry.

MATERIALS AND METHODS

Translocation analysis was performed on peripheral blood lymphocyte cultures from blood samples taken at different times during the radiotherapy (0 Gy, 12 Gy and 50 Gy) and a few months after the end of the treatment (follow-up).

RESULTS

Estimated translocation yield varied according to the FISH technique used. At 50 Gy and follow-up points, the translocation yields were higher with FISH-3 than with M-FISH. This difference can be attributed to three events. First, an increase in complex aberrations was observed for 50 Gy and follow-up points compared with 0 Gy and 12 Gy points. Second, at the end of treatment for patient A, involvement of chromosomes 2, 4, 12 in translocations was less than expected according to the Lucas formula. Third, a clone bearing a translocation involving a FISH-3 painted chromosome was detected.

CONCLUSIONS

More translocations were detected with M-FISH than with FISH-3, and so M-FISH is expected to improve the accuracy of chromosome aberration analyses in some situations.

Fluorescence in situ hybridization is necessary to detect an association between chromosome aberrations and polycyclic aromatic hydrocarbon exposure in utero and reveals nonrandom chromosome involvement

Chromosome aberrations are associated with environmental exposures in infants and children. Recently we reported that prenatal exposure to airborne polycyclic aromatic hydrocarbons (PAHs) was significantly (P < 0.01) associated with stable aberration frequencies in cord blood from a subset of 60 newborns from the Columbia Center for Children's Environmental Health Prospective Cohort Study (Bocskay K et al. [ 2005]: Cancer Epidemiol Biomarkers Prev 14:506-511). To determine whether the environmental exposures may be targeting specific chromosomes and to compare various methods for measuring chromosome aberrations, we further evaluated this same subset of subjects composed of African-American and Dominican nonsmoking mother-newborn pairs residing in low-income neighborhoods of New York City, and exposed to varying levels of airborne PAHs. Chromosome aberrations were measured in cord blood lymphocytes, both by whole chromosome probe (WCP) fluorescence in situ hybridization (FISH) and traditional Giemsa-staining. Prenatal exposures were assessed by personal air monitoring. Breaks in chromosomes 1-6, as detected by WCP FISH, were nonrandomly distributed, underscoring the importance of appropriate chromosome probe selection to capture cytogenetic damage in response to exposure. FISH for stable aberrations was found to be a more sensitive method for detecting aberration frequencies associated with environmental exposures, when compared with FISH for unstable aberrations or Giemsa-staining for aberrations. Together, these results suggest that PAHs may be targeting specific chromosomes and highlight the importance of using the more sensitive detection methods to assess risk in populations with low levels of exposure.

Breakpoint locations within chromosomes 1, 2, and 4 of patients with increased radiosensitivity

The exposure to low LET-radiation leads to a relative homogeneous distribution of initial damage at the DNA. Subsequent repair and post-repair mechanisms might lead to a selection of specific breakpoint locations along chromosomes. Cells from patients with increased radiosensitivity may have more specific breakpoints due to impaired repair mechanisms. We tested whether cells from patients with increased radiosensitivity had an increase in specific breakpoint clusters. Structural chromosomal aberrations of in vitro irradiated lymphocytes from 11 healthy individuals and another 3 patients with increased radiosensitivity were examined. The chromosome pairs 1, 2, and 4 were treated using the three-color FISH technique. The breakpoints were analyzed by means of computerized imaging software. In total, 1752 chromosomal breakpoints had been considered, 498 from healthy individuals, and 1254 from patients with increased radiosensitivity. For both groups there was a non-homogeneous breakpoint distribution along the chromosomes and a trend towards increased breaks in the telomere-proximal region. Also, both groups had distinct locations with increased breaks. No evidence for significant breakpoint patterns across all patients with increased radiosensitivity was found.

Chromosome breakpoint distribution of damage induced in peripheral blood lymphocytes by densely ionizing radiation

PURPOSE

To assess the chromosomal breakpoint distribution in human peripheral blood lymphocytes (PBL) after exposure to a low dose of high linear energy transfer (LET) alpha-particles using the technique of multiplex fluorescence in situ hybridization (m-FISH).

MATERIALS AND METHODS

Separated PBL were exposed in G0 to 0.5 Gy 238Pu alpha-particles, stimulated to divide and harvested approximately 48 - 50 hours after exposure. Metaphase cells were assayed by m-FISH and chromosome breaks identified. The observed distribution of breaks were then compared with expected distributions of breaks, calculated on the assumption that the distribution of breaks is random with regard to either chromosome volume or chromosome surface area.

RESULTS

More breaks than expected were observed on chromosomes 2 and 11, however no particular region of either chromosome was identified as significantly contributing to this over-representation. The identification of hot or cold chromosome regions (pter,p,cen,q,qter) varied depending on whether the data were compared according to chromosome volume or surface area.

CONCLUSIONS

A deviation from randomness in chromosome breakpoint distribution was observed, and this was greatest when data were compared according to the relative surface area of each individual chromosome (or region). The identification of breaks by m-FISH (i.e., more efficient observation of interchanges than intrachanges) and importance of territorial boundaries on interchange formation are thought to contribute to these differences. The significance of the observed non-random distribution of breaks on chromosomes 2 and 11 in relation to chromatin organization is unclear.

Increased complexity of radiation-induced chromosome aberrations consistent with a mechanism of sequential formation

Complex chromosome aberrations (any exchange involving three or more breaks in two or more chromosomes) are effectively induced in peripheral blood lymphocytes (PBL) after exposure to low doses (mostly single particles) of densely ionising high-linear energy transfer (LET) alpha-particle radiation. The complexity, when observed by multiplex fluorescence in situ hybridisation (m-FISH), shows that commonly four but up to eight different chromosomes can be involved in each rearrangement. Given the territorial organisation of chromosomes in interphase and that only a very small fraction of the nucleus is irradiated by each alpha-particle traversal, the aim of this study is to address how aberrations of such complexity can be formed. To do this, we applied theoretical "cycle" analyses using m-FISH paint detail of PBL in their first cell division after exposure to high-LET alpha-particles. In brief, "cycle" analysis deconstructs the aberration "observed" by m-FISH to make predictions as to how it could have been formed in interphase. We propose from this that individual high-LET alpha-particle-induced complex aberrations may be formed by the misrepair of damaged chromatin in single physical "sites" within the nucleus, where each "site" is consistent with an "area" corresponding to the interface of two to three different chromosome territories. Limited migration of damaged chromatin is "allowed" within this "area". Complex aberrations of increased size, reflecting the path of alpha-particle nuclear intersection, are formed through the sequential linking of these individual sites by the involvement of common chromosomes.

Chromosomal Aberrations in Cord Blood Are Associated with Prenatal Exposure to Carcinogenic Polycyclic Aromatic Hydrocarbons

Molecular and traditional epidemiology studies have indicated a possible relationship between in utero environmental exposures and increased risk for childhood cancers, especially acute leukemias. Chromosomal aberrations have been associated with environmental exposures and cancer risk in adults. In order to more clearly define the association between prenatal exposures to carcinogenic polycyclic aromatic hydrocarbons (PAH) and chromosomal aberrations, chromosomal aberration frequencies were measured in a subset of 60 newborns from the Columbia Center for Children's Environmental Health (CCCEH) Prospective Cohort Study. The subset was composed of African American and Dominican, nonsmoking mother-newborn pairs residing in low-income neighborhoods of New York City, who were exposed to varying levels of airborne PAHs. Prenatal exposure was assessed by questionnaire, personal air monitoring during the third trimester, and PAH-DNA adducts in umbilical cord blood. Chromosomal aberrations were measured in cord blood lymphocytes by fluorescence in situ hybridization. PAH-DNA adducts were not associated with chromosomal aberrations. However, airborne PAHs were significantly associated with stable aberration frequencies in cord blood (P < 0.01). Moreover, stable aberration frequencies were significantly higher among African American newborns compared with Dominican, despite no significant differences in PAH exposure. These results show for the first time an association between prenatal exposure to airborne carcinogenic PAHs and chromosomal aberrations in cord blood, suggesting that such prenatal exposures have the potential to cause cytogenetic damage that has been related to increased cancer risk in other populations. If confirmed, this finding may open new avenues for prevention.

High susceptibility of chromosome 16 to radiation-induced chromosome rearrangements in human lymphocytes under in vivo and in vitro exposure

The aim of the present study was to investigate whether chromosome 16p presents breakpoint regions susceptible to radiation-induced rearrangements. The frequencies of translocations were determined by fluorescence in situ hybridization (FISH) using cosmid probes C40 and C55 mapping on chromosome 16p, and a chromosome 16 centromere-specific probe (pHUR195). Peripheral lymphocytes were collected from normal individuals and from seven victims of 137Cs in the Goiania (Brasil) accident (absorbed doses: 0.8-4.6 Gy) 10 years after exposure. In vitro irradiated lymphocytes (3 Gy) were also analyzed. The mean translocation frequency/cell obtained for the 137Cs exposed individuals was 2.4-fold higher than the control value (3.6 x 10(-3) +/- 0.001), and the in vitro irradiated lymphocytes showed a seven-fold increase. The genomic translocation frequencies (FGs) were calculated by the formula Fp = 2.05 fp(1-fp)FG (Lucas et al., 1992). For the irradiated lymphocytes and victims of 137Cs, the FGs calculated on the basis of chromosome 16 were 2- to 8-fold higher than those for chromosomes 1, 4 and 12. Our results indicate that chromosome 16 is more prone to radiation-induced chromosome breaks, and demonstrate a non-random distribution of induced aberrations. This information is valuable for retrospective biological dosimetry in case of human exposure to radiation, since the estimates of absorbed doses are calculated by determining the translocation frequency for a sub-set of chromosomes, and the results are extrapolated to the whole genome, assuming a random distribution of induced aberrations. Furthermore, the demonstration of breakpoints on 16p is compatible with the reports about their involvement in neoplasias.

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.

Distribution of breakpoints and fragment sizes in human chromosome 5 after heavy-ion bombardment

PURPOSE

To measure the location of heavy ion-induced residual breakpoints in human chromosome 5 and the size distribution of chromosome fragments involved in inter- or intrachromosomal exchanges.

MATERIALS AND METHODS

Human peripheral blood lymphocytes were exposed to 4 Gy accelerated 56Fe (iron) ions (500 MeV per nucleon (MeV n(-1)), linear energy transfer=200 keV microm(-1)). Cells were then stimulated to grow in vitro for 48 h, and chromosomes were prematurely condensed by calyculin A. Chromosome 5 was painted using high-resolution multicolour banding. The location of the observed residual breakpoints and the size of all chromosome 5 fragments involved in structural aberrations were measured using dedicated image analysis software.

RESULTS

Mapping of 283 breakpoints revealed a slight deviation from randomness, with an excess of breakpoints clustered in two small bands and an under representation of breaks at the telomeric end in the q-arm. Breakpoints per unit length were similar in p- and q-arms. The distribution of chromosome fragments has a maximum for very small fragments (< 10% of the chromosome size), indicating a severe fragmentation of chromosome 5 after heavy-ion bombardment. Only fragments < 40% of the chromosome size were involved in intrachromosomal exchanges (interstitial deletions or inversions), whereas fragments up to 75% of the whole chromosome 5 were found in interchromosomal exchanges.

CONCLUSIONS

Residual breakpoints after exposure to high-energy iron ions were not distributed randomly along chromosome 5, although the p- and q-arms displayed similar radiosensitivity. Large fragments are either restituted or misrejoined to other chromosome ends, whereas small intrachromosomal fragments can produce either inter- or intrachromosomal exchanges.

Stable Intrachromosomal Biomarkers of Past Exposure to Densely Ionizing Radiation in Several Chromosomes of Exposed Individuals

A multicolor banding (mBAND) fluorescence in situ hybridization technique was used to investigate the presence inhuman populations of a stable biomarker-intrachromosomal chromosome aberrations-of past exposure to high-LET radiation. Peripheral blood lymphocytes were taken from healthy Russian nuclear workers occupationally exposed from 1949 onward to either plutonium, gamma rays or both. Metaphase spreads were produced and chromosomes 1 and 2 were hybridized with mBAND FISH probes and scored for intra-chromosomal aberrations. A large yield of intrachromosomal aberrations was observed in both chromosomes of the individuals exposed to high doses of plutonium, whereas there was no significant increase over the (low) background control rate in the population who were exposed to high doses of gamma rays. Interchromosome aberration yields were similar in both the high plutonium and the high gamma-ray groups. These results for chromosome 1 and 2 confirm and extend data published previously for chromosome 5. Intrachromosomal aberrations thus represent a potential biomarker for past exposure to high-LET radiations such as alpha particles and neutrons and could possibly be used as a biodosimeter to estimate both the dose and type of radiation exposure in previously exposed populations.

Quantitative analysis of radiation-induced chromosome aberrations

We review chromosome aberration modeling and its applications, especially to biodosimetry and to characterizing chromosome geometry. Standard results on aberration formation pathways, randomness, dose-response, proximity effects, transmissibility, kinetics, and relations to other radiobiological endpoints are summarized. We also outline recent work on graph-theoretical descriptions of aberrations, Monte-Carlo computer simulations of aberration spectra, software for quantifying aberration complexity, and systematic links of apparently incomplete with complete or truly incomplete aberrations.

Syndrome-related chromosome-specific radiation-induced break points of various inherited human metabolic disorders

The frequency, distribution pattern and localisation of gamma radiation-induced break points on the chromosomes of patients with various inherited metabolic disorders were studied to detect: (i) whether the break point distribution following irradiation is random and proportional to the length or the DNA content of the chromosome, or non-proportionally distributed on their length and at times clustering to form hot spots on certain region of the chromosomes; and (ii) to find whether there exists a syndrome-related chromosome-specific pattern of radiation-induced break points. Lymphocyte cultures from patients of haemophilia, ichthyosis, Duchenne muscular dystrophy, retinitis pigmentosa and alpha-thalassemia, whose defective gene loci were located by DNA probe method, were subjected to 3Gy of gamma radiation at G(0). The chromosomal break point analysis was carried out on all the 23 types of chromosomes (excluding Y chromosome) using G banding and FISH painting. The exact location of the break points on G-banded chromosomes was identified using a semi-automated microscope densitometer system (Leitz MPV2). In normal individuals in all the chromosomes except the chromosome 1, a random distribution of break points proportional to their length based on their DNA content was observed. However, in all the syndromes studied a mixture of hypersensitive chromosomes with a non-random distribution pattern of chromosomal break points invariably clustering to form hot spots, and chromosomes with random distribution of break points proportional to their length were observed. The hypersensitive chromosomes and their hot spots were syndrome-specific.

Distribution of radiation-induced exchange aberrations in all human chromosomes

PURPOSE

To investigate the DNA-proportional distribution of radiation-induced chromosome aberrations for all chromosomes of a male and a female human karyotype.

MATERIALS AND METHODS

Metaphases were prepared from whole blood cultures obtained from two healthy donors and set up after irradiation with 3 Gy 220 kV X-rays. Single whole-chromosome FISH painting simultaneously with pancentromeric DNA painting were performed separately for each chromosome of the human karyotype. One thousand exclusively first-division metaphases were analysed per chromosome and donor. After statistical analysis, the data obtained were compared with theoretically expected values.

RESULTS

All aberration types (translocations, dicentrics) showed deviations from a DNA-proportional distribution. For both donors, chromosomes 2 and 3 exhibited significantly less and chromosome 4 more symmetrical translocations than expected. Chromosomes 15 and 22 showed more symmetrical translocations than predicted for one of the two donors. Less dicentrics than expected became apparent for chromosomes 2, 3 and 18, while more dicentrics were seen for chromosomes 15, 16 and 17. Moreover, chromosomes 4, 14 and 22 showed a significant deviation from the theoretically expected 1:1 ratio of the yields of symmetrical translocations to the yields of dicentrics.

CONCLUSION

The results from the whole-chromosome complement in two different donors confirmed published data from the analysis of single chromosomes that some human chromosomes were not involved in radiation-induced dicentrics and symmetrical translocations proportional to their DNA content. This must be taken into account if chromosome subsets for dose reconstruction are selected or if whole genomic frequencies have to be calculated from partial genome analysis.

Karyotypes of human lymphocytes exposed to high-energy iron ions

Chromosomal aberrations were analyzed using multicolor fluorescence in situ hybridization (mFISH) in human peripheral blood lymphocytes after in vitro exposure to gamma rays or accelerated (56)Fe ions (1 GeV/nucleon, 145 keV/microm) at Brookhaven National Laboratory (Upton, NY). Doses of 0.3 and 3 Gy were used for both radiation types. Chromosomes were prematurely condensed by a phosphatase inhibitor (calyculin A) to avoid the population selection bias observed at metaphase as a result of the severe cell cycle delays induced by heavy ions. A total of 1053 karyotypes (G(2) and M phases) were analyzed in irradiated lymphocytes. Results revealed different distribution patterns for chromosomal aberrations after low- and high-LET radiation exposures: Heavy ions induced a much higher fraction of cells with multiple aberrations, while the majority of the aberrant cells induced by low doses of gamma rays contained a single aberration. The high fraction of complex-type exchanges after heavy ions leads to an overestimation of simple-type asymmetrical interchanges (dicentrics) from analysis of Giemsa-stained samples. However, even after a dose of 3 Gy iron ions, about 30% of the cells presented no complex-type exchanges. The involvement of individual chromosomes in exchanges was similar for densely and sparsely ionizing radiation, and no statistically significant evidence of a nonrandom involvement of specific chromosomes was detected.

Radiation-induced chromosome aberrations: insights gained from biophysical modeling

Enzymatic misrepair of ionizing-radiation-induced DNA damage can produce large-scale rearrangements of the genome, such as translocations and dicentrics. These and other chromosome exchange aberrations can cause major phenotypic alterations, including cell death, mutation and neoplasia. Exchange formation requires that two (or more) genomic loci come together spatially. Consequently, the surprisingly rich aberration spectra uncovered by recently developed techniques, when combined with biophysically based computer modeling, help characterize large-scale chromatin architecture in the interphase nucleus. Most results are consistent with a picture whereby chromosomes are mainly confined to territories, chromatin motion is limited, and interchromosomal interactions involve mainly territory surfaces. Aberration spectra and modeling also help characterize DNA repair/misrepair mechanisms. Quantitative results for mammalian cells are best described by a breakage-and-reunion model, suggesting that the dominant recombinational mechanism during the G(0)/G(1) phase of the cell cycle is non-homologous end-joining of radiogenic DNA double strand breaks. In turn, better mechanistic and quantitative understanding of aberration formation gives new insights into health-related applications.

Chromosomal aberrations: formation, identification and distribution

Chromosomal aberrations (CA) are the microscopically visible part of a wide spectrum of DNA changes generated by different repair mechanisms of DNA double strand breaks (DSB). The method of fluorescence in situ hybridisation (FISH) has uncovered unexpected complexities of CA and this will lead to changes in our thinking about the origin of CA. The inter- and intrachromosomal distribution of breakpoints is generally not random. CA breakpoints occur preferentially in active chromatin. Deviations from expected interchromosomal distributions of breakpoints may result from the arrangement of chromosomes in the interphase nucleus and/or from different sensitivities of chromosomes with respect to the formation of CA. Telomeres and interstitial telomere repeat like sequences play an important role in the formation of CA. Subtelomeric regions are hot spots for the formation of symmetrical exchanges between homologous chromatids and cryptic aberrations in these regions are associated with human congenital abnormalities.

Large-mutation spectra induced at hemizygous loci by low-LET radiation: evidence for intrachromosomal proximity effects

A mathematical model is used to analyze mutant spectra for large mutations induced by low-LET radiation. The model equations are based mainly on two-break misrejoining that leads to deletions or translocations. It is assumed, as a working hypothesis, that the initial damage induced by low-LET radiation is located randomly in the genome. Specifically, we analyzed data for two hemizygous loci: CD59- mutants, mainly very large-scale deletions (>3 Mbp), in human-hamster hybrid cells, and data from the literature on those HPRT- mutants which involve at least deletion of the whole gene, and often of additional flanking markers (approximately 50-kbp to approximately 4.4-Mbp deletions). For five data sets, we estimated f, the probability that two given breaks on the same chromosome will misrejoin to make a deletion, as a function of the separation between the breaks. We found that f is larger for nearby breaks than for breaks that are more widely separated; i.e., there is a "proximity effect". For acute irradiation, the values of f determined from the data are consistent with the corresponding break misrejoining parameters found previously in quantitative modeling of chromosome aberrations. The value of f was somewhat smaller for protracted irradiation than for acute irradiation at a given total dose; i.e., the mutation data show a decrease that was smaller than expected for dose protraction by fractionation or low dose rate.

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.

Analyzing radiation-induced complex chromosome rearrangements by combinatorial painting

Cornforth, M. N. Analyzing Radiation-Induced Complex Chromosome Rearrangements by Combinatorial Painting. Radiat. Res. 155, 643-659 (2001). Prior to the advent of whole-chromosome painting, it was universally assumed that virtually all radiation-induced exchanges represented a simple rejoining between pairs of chromosome breaks. It is now known that a substantial proportion of such exchanges are actually complex, meaning that they involve the interaction of three (or more) breaks distributed among two (or more) chromosomes. The purpose of this review is to discuss some of the implications of aberration analysis using whole-chromosome painting, with emphasis given to newer combinatorial painting schemes that allow for the unambiguous identification of all homologous chromosome pairs. Such analysis requires reconsideration of how resulting information is to be handled for the purposes of tabulating and communicating raw data, quantifying aberration yields, and presenting experimental results in a cogent manner. Facilitating these objectives requires the introduction of certain concepts and terminologies that have no counterpart in conventional cytogenetic analyses.

Chromosome painting for cytogenetic monitoring of occupationally exposed and non-exposed groups of human individuals

The suitability of a three-color fluorescence in situ suppression hybridization technique was examined for monitoring five different groups of individuals: 30 occupied in radiology, 26 occupied in nuclear medicine or radiation physics, 32 patients with breast cancer, 26 occupied with military waste disposal, all presumably exposed to low doses of radiation or chemical mutagens and a non-exposed control group (N=29). The average frequency of breaks constituting the various aberrations did not significantly differ between the groups of medical radiation appliers and the control group. However, breast tumor patients and military waste disposers, as groups, showed a higher aberration rate than did healthy controls. Stable rearrangements mainly characterized the groups of controls, tumor patients, and radiation appliers, while a higher proportion of unstable aberrations was found in the chemically exposed individuals. Individuals with an increased frequency of aberrations could be detected within each examined group, which clearly determined the average values of the whole group. With respect to interchromosomal distribution of the breakpoints constituting the found aberrations and the involvement of the labeled chromosomes in rearrangements, the observed values were very close to the expected ones in the controls. A rather similar trend of deviations from expectation was observed in all other groups. Chromosome 4 was slightly over-affected, while chromosome 2 was slightly underrepresented in all analyzed groups (except tumor patients). Rearrangements of the labeled chromosomes with the unlabeled ones exceeded expectation. In conclusion, chromosome painting if included in further attempts of human population monitoring will broaden the basis of argumentation with respect to health risks introduced by mutagen exposure.

Multicolor FISH analysis of chromosomal breaks, duplications, deletions, and numerical abnormalities in the sperm of healthy men

Transmitted de novo structural chromosomal abnormalities, the majority of which are paternally derived, can lead to abnormal reproductive outcomes as well as genetic diseases in offspring. We developed and validated a new multicolor FISH procedure (sperm ACM, which utilizes DNA probes specific for the alpha [1cen], classical, [1q12], and midi [1p36.3] satellites of chromosome 1) which utilizes DNA probes specific for three regions of chromosome 1 to detect human sperm that carry numerical abnormalities plus two categories of structural aberrations: (1) duplications and deletions of 1pter and 1cen, and (2) chromosomal breaks within the 1cen-1q12 region. In healthy men, the average frequencies of sperm with duplications and deletions were (a) 4.5 +/- 0.5 and 4.1 +/- 1.3 per 10(4) involving 1pter and (b) 0.9 +/- 0.4 and 0.8 +/- 0.3 per 10(4) involving 1cen, respectively. The frequency of sperm exhibiting breaks within the 1cen-1q12 region was 14.1 +/- 1.2 per 10(4). Structural aberrations accounted for 71% of the abnormalities detected by sperm ACM, which was significantly higher than numerical abnormalities (P=2x10-8). Our findings also suggest that, for healthy men, (a) sperm carrying postmeiotic chromosomal breaks appear to be more prevalent than those carrying products of premeiotic or meiotic breakage or rearrangements, (b) the high frequency of chromosome breaks measured after "fertilization" by the hamster-egg cytogenetic method already appear to be present and detectable within human sperm by FISH, and (c) there are nonrandom and donor-specific distributions of breakpoint locations within 1q12 in sperm. FISH facilitates the analysis of much larger numbers of sperm than was possible when the hamster-egg method was used. Therefore, FISH-based procedures for simultaneously detecting chromosomal breaks, rearrangements, and numerical abnormalities in sperm may have widespread applications in human genetics, genetic toxicology, and reproductive medicine.

Induction, processing and persistence of radiation-induced chromosomal aberrations involving hamster euchromatin and heterochromatin

Euchromatic and heterochromatic regions are easily distinguished in Chinese hamster sex chromosomes, hence offering the possibility of studying the role of chromatin structure in the induction, processing and persistence of radiation-induced chromosome damage. X-ray (4 Gy)-induced breaks in the euchromatic Xp and in the heterochromatic Xq were analysed immediately and 4h after irradiation by premature chromosome condensation (PCC) in combination with either FISH using chromosome arm-specific probes or Giemsa staining. The study, performed with female Chinese hamster splenocytes, was extended to a 34 h recovery followed by arm-specific FISH in metaphase. A significant over-involvement of the heterochromatic Xq in radiation-induced breakage was observed at all sampling times (p<0.001). However, the heterochromatic state had little effect on the processing of the induced lesions. In a second experiment, the persistence of radiation-induced chromosome aberrations (CAs) involving Xp, Xq and Y chromosome was studied with cultured Chinese hamster male splenocytes sampled 30, 56 and 96 h after irradiation (4 Gy). A higher involvement of the heterochromatic regions (Xq and Y) in radiation-induced CAs was again observed in the first sampling time (p<0.001), suggesting that Chinese hamster heterochromatin could be more radiosensitive than euchromatin. Cells with CAs involving heterochromatin were apparently less persistent than those with lesions involving euchromatin. This observation could be attributable to either the distribution of CA per cell or to the fraction of potentially stable exchanges.

Radiation-induced translocations in mice: persistence, chromosome specificity, and influence of genetic background

The translocation frequency response in the chromosomes of peripheral blood lymphocytes is widely used for radiation biomonitoring and dose estimation. However, this assay is based upon several assumptions that have not been rigorously tested. It is typically assumed that the translocation frequency in blood lymphocytes reflects the level of genomic damage in other hemopoietic tissues and is independent of the chromosome probe and genetic background. We conducted studies to evaluate these assumptions using mice with different genetic backgrounds. Six different whole-chromosome fluorescence in situ hybridization (FISH) probes were used to detect translocations in peripheral blood lymphocytes at multiple times after whole-body irradiation. Translocation frequencies were chromosome-independent at 6 and 16 weeks after exposure but were chromosome-dependent at 1. 5 years after exposure. Similar translocation frequencies were observed in blood, bone marrow and spleen at 1.5 years, supporting previous suggestions that genetically aberrant peripheral blood lymphocytes may derive from precursor populations in hemopoietic tissues. Translocations measured 66 h after irradiation differed among some strains. We conclude that the translocation frequency response is a complex phenotype that is influenced not only by exposure dose but also by genetic background, the choice of chromosome analyzed, and time after exposure. These results raise important considerations for the use of the FISH-based translocation frequency response for radiation dosimetry and biomonitoring.

Equal induction and persistence of chromosome aberrations involving chromosomes 1, 4 and 10 in thyroid cancer patients treated with radioactive iodine

A number of in vitro studies have questioned the assumption of random distribution of breaks in radiation-induced chromosome aberrations. The therapeutic application of radioactive 131I in thyroid cancer patients offers a good opportunity to study the induction and persistence of cytogenetic damage involving different chromosomes in vivo. Using whole-chromosome painting probes and triple colour painting by fluorescence in situ hybridization (FISH), we have analysed the frequency of chromosomal aberrations (CAs) involving chromosomes 1, 4 and 10 in peripheral blood lymphocytes of 10 thyroid cancer patients sampled before and 1 week, 1 year and 3.5 years after therapeutic application of radioactive iodine in a self-controlled, longitudinal study. A highly significant 3.4-fold increase in the frequency of chromosome breaks was observed 1 week after treatment with a similar representation of all chromosomes analysed. Although a significant decrease in dicentrics was observed during the first year after treatment, the frequency of chromosome aberrations remained over control levels until the last sampling time, 41-47 months post-treatment. The same behaviour, in terms of induction and persistence, was observed for all three chromosomes, confirming our previous results in vitro and rejecting the reported suggestion that chromosome 10 is radiosensitive in vivo. Our finding that the dynamics of radiation-induced CA in vivo is independent on the chromosome of choice suggests that this variable is not important in retrospective studies.

UV-A breakage sensitivity of human chromosomes as measured by COMET-FISH depends on gene density and not on the chromosome size

COMET-FISH, a single cell-based combination of COMET-assay (also known as single cell gel electrophoresis (SCGE)) with fluorescence in situ hybridization (FISH) allows region specific studies on DNA stability and damage. COMET-FISH can be used to investigate UV-A-induced DNA damage of selected whole chromosomes. In the present work, a modified COMET-FISH protocol with whole chromosome painting probes was used to study whether UV-A-induced DNA damage is distributed randomly over the whole genome or occurs at preferred sites. The study was performed with 12 different chromosome painting probes (for chromosomes 1, 2, 3, 8, 9, 11, 14, 18, 19, 21, X and Y). The results on human lymphocytes irradiated with 500 kJ/m2 at a wavelength of 365 nm indicate that the induced number of chromatin strand breaks does not correlate with the chromosome size. They therefore are distributed in a non-random manner. For example, fragments of the gene-rich chromosome chromosome 1 were found in the comet tail in only 3% of the examined cells, and thus chromosome 1 is rather stable, whereas fragmentation of the gene-poor chromosome 8 was observed in 25% of all comets. On the basis of all 12 chromosomes analyzed, an inverse correlation between the density of active genes and the sensitivity toward UV-A radiation is found.

Localization of radiation-induced chromosomal breakpoints along human chromosome 1 using a combination of G-banding and FISH

PURPOSE

To determine the exact location of radiation-induced chromosomal breakpoints along the euchromatic or heterochromatic regions: G-light and G-dark bands, respectively.

MATERIALS AND METHODS

The distribution of radiation-induced chromosomal breakpoints was scored in human lymphocytes irradiated in vitro with 3 Gy of gamma-radiation. Image analysis was applied to combine G-banded and FISH-painted images of the human chromosome 1.

RESULTS

A total of 195 chromosomal breakpoints in 176 cells with structural chromosomal aberrations was used for the present analysis. Radiation-induced breakpoints were found to be distributed randomly with respect to the p or q arms of chromosome 1 and specific band or band length, but more breakpoints were mapped to G-light than to G-dark bands, the difference being statistically significant.

CONCLUSIONS

The results can well be interpreted in terms of concepts of existing models of nuclear architecture, chromatin structure and transcriptional activities of the chromatin, which can influence the induction of primary chromosomal aberrations by gamma-rays. Differential repair of randomly produced primary aberrations may also explain the non-random distribution of radiation-induced breakpoints.

Radiation-produced chromosome aberrations: colourful clues

Ionizing radiation produces many chromosome aberrations. A rich variety of aberration types can now be seen with the technique of chromosome painting. Apart from being important in medicine and public health, radiation-produced aberrations act as colorful molecular clues to damage-processing mechanisms and, because juxtaposition of different parts of the genome is involved, to interphase nuclear organization. Recent studies using chromosome painting have helped to identify DNA double-strand-break repair and misrepair pathways, to determine the extent of chromosome territories and motions, and to characterize different aberration patterns left behind by different kinds of radiation.

Improvement of accuracy of chromosome aberration analysis for biological radiation dosimetry

The frequency of chromosome aberrations in circulating lymphocytes is accepted as being the most reliable indicator of the absorbed dose of radiation. Researches done to improve the accuracy of cytogenetic analysis are described in this review. These include investigations of in vitro factors that affect the yield of radiation-induced aberrations and of in vivo factors that affect the chromosomal radiosensitivity of individuals. Improved chromosome-painting methods for accurate judgment of dicentrics and translocations are introduced. The practicality of these advanced cytogenetic techniques is shown by examinations of individuals exposed in the radiation accident at Tokaimura in 1999.

Analysis of X-ray-induced aberrations in human chromosome 5 using high-resolution multicolour banding FISH (mBAND)

Peripheral lymphocytes were exposed to 4 Gy X-rays and aberrations were analysed in human chromosome 5 using high-resolution multicolour banding fluorescence in-situ hybridization (mBAND). This method is suited to detect simple and complex aberrations including peri- and paracentric inversions and exchanges between both chromosomes 5. Additionally, breakpoints carr be assigned to specific regions in chromosome 5. Quantitative relationships of induced aberration types are discussed.

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.