Preferential location of x-ray induced chromosome breakage in the R-bands of human chromosomes

Radiation-Induced Chromosomal Breaks may be DNA Repair Fragile Sites with Larger-scale Correlations to Eight Double-Strand-Break Related Data Sets over the Human Genome

In this work, we compared the genomic distribution of common radiation-induced chromosomal breaks to eight different data sets covering the whole human genome. Sites with a high probability of chromatid breakage after exposure to low and high ionization density radiations were often located inside common and rare fragile sites, indicating that they may be a new and more local type of DNA repair-related fragility. Breaks in specific chromosome bands after acute exposure to oil and benzene also showed strong correlation with these sites and fragile sites. In addition, close correlation was found with cytologically detected chiasma and MLH1 immunofluorescence sites and with the HapMap recombination density distributions. Also, of interest, copy number changes occurred predominantly at radiation-induced breaks and fragile sites, at least for breast cancers with poor prognosis, and they decreased weakly but significantly in regions with increasing recombination and CpG density. An increased CpG density is linked to regions of high gene density to secure high-fidelity reproduction and survival. To minimize cancer induction, cancer-related genes are often located in regions of decreased recombination density and/or higher-than-average CpG density. It is compelling that all these data sets were influenced by the cells' handling of double-strand breaks and, more generally, DNA damage on its genome. In fact, the DNA repair genes systematically avoid regions with a high recombination density, as they need to be intact to accurately handle repairable DNA lesions.

Chromatin remodelling and chromosome damage distribution

Histone acetylation/deacetylation constitute the most relevant chromatin remodelling mechanism to control DNA access to nuclear machinery as well as to mutagenic agents. Thus, these epigenetics mechanisms could be involved in processing DNA lesions into chromosomal aberrations. Although radiation-induced DNA lesions are believed to occur randomly, in most cases chromosome breakpoints appear distributed in a non-random manner. In order to study the distribution of chromosome damage induced by clastogenic agents in relation to chromosome histone acetylation patterns, an experimental model based on treating Chinese hamster cells with endonucleases and ionizing radiations as well as immunolabelling metaphase chromosomes with antibodies to acetylated histone H4 was developed. The analysis of intra- and interchromosome breakpoint distribution has been carried out on G-banded chromosomes, and results obtained were correlated with chromosome acetylated histone H4 profiles. A co-localization of intrachromosomal breakpoints induced by AluI, BamHI and DNase I as well as by neutrons and g-rays was observed. Radiation- and endonuclease-induced breakpoints tend to cluster in less condensed chromosome regions (G-light bands) that show the highest levels of acetylated histone H4. The analysis of interchromosomal distribution of radiation-induced lesions showed a concentration of breakpoints in Chinese hamster chromosomes with particular histone acetylation patterns. The fact that chromosome breakpoints occur more frequently in transcriptionally competent chromosome regions suggests that chromatin conformation and nuclear architecture could play a role in the distribution of chromosome lesions.

Three-Color Chromosome Painting as Seen through the Eyes of mFISH: Another Look at Radiation-Induced Exchanges and Their Conversion to Whole-Genome Equivalency

Whole-chromosome painting (WCP) typically involves the fluorescent staining of a small number of chromosomes. Consequently, it is capable of detecting only a fraction of exchanges that occur among the full complement of chromosomes in a genome. Mathematical corrections are commonly applied to WCP data in order to extrapolate the frequency of exchanges occurring in the entire genome [whole-genome equivalency (WGE)]. However, the reliability of WCP to WGE extrapolations depends on underlying assumptions whose conditions are seldom met in actual experimental situations, in particular the presumed absence of complex exchanges. Using multi-fluor fluorescence in situ hybridization (mFISH), we analyzed the induction of simple exchanges produced by graded doses of ¹³⁷Cs gamma rays (0–4 Gy), and also 1.1 GeV ⁵⁶Fe ions (0–1.5 Gy). In order to represent cytogenetic damage as it would have appeared to the observer following standard three-color WCP, all mFISH information pertaining to exchanges that did not specifically involve chromosomes 1, 2, or 4 was ignored. This allowed us to reconstruct dose–responses for three-color apparently simple (AS) exchanges. Using extrapolation methods similar to those derived elsewhere, these were expressed in terms of WGE for comparison to mFISH data. Based on AS events, the extrapolated frequencies systematically overestimated those actually observed by mFISH. For gamma rays, these errors were practically independent of dose. When constrained to a relatively narrow range of doses, the WGE corrections applied to both ⁵⁶Fe and gamma rays predicted genome-equivalent damage with a level of accuracy likely sufficient for most applications. However, the apparent accuracy associated with WCP to WGE corrections is both fortuitous and misleading. This is because (in normal practice) such corrections can only be applied to AS exchanges, which are known to include complex aberrations in the form of pseudosimple exchanges. When WCP to WGE corrections are applied to true simple exchanges, the results are less than satisfactory, leading to extrapolated values that underestimate the true WGE response by unacceptably large margins. Likely explanations for these results are discussed, as well as their implications for radiation protection. Thus, in seeming contradiction to notion that complex aberrations be avoided altogether in WGE corrections – and in violation of assumptions upon which these corrections are based – their inadvertent inclusion in three-color WCP data is actually required in order for them to yield even marginally acceptable results.

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.

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.

Fifty years of cytogenetics: a parallel view of the evolution of cytogenetics and genotoxicology

A parallelism exists between human cytogenetics and cytogenetic toxicology. The breakthroughs, mostly coming from and used in clinical genetics, are widely used in genetic toxicology. The birth of human cytogenetics occurred in 1956 when it was published that the diploid number of chromosomes in humans is 46. The first stage in chromosome-induced mutagenesis began in 1938 when Sax published the effects of X-rays on the chromosomes of Drosophila. In 1959, the cytogenetic anomalies for Down, Klinefelter, and Turner syndromes were described, and parallelly in 1960, the first publication on chromosomal aberrations in man caused by ionizing radiation appeared. The cytogenetic analysis of chromosomal aberrations in cell cultures is considered one of the primary methods to evaluate induced mutagenesis. At the end of the 1960s, banding techniques allowed chromosomes to be individually identified, in parallel, the sister chromatid exchange analysis technology was described. Another milestone in the history of induced mutagenesis was the discovery that mutagenic agents were able to alter chromosomal division and segregation in gonads inducing meiotic nondisjunction. Here we review new approaches and applications such as biological dosimetry, translocation scoring using FISH, and micronucleus test. Chromosomal aberrations and micronucleus test are now effective cytogenetic biomarkers of early effect used as cancer predictors. Human cytogenetics has proven to be effective over its 50-year lifespan and, although each new technique that has appeared seemed to announce its end, the fact is that the current state of cytogenetics is in reality a collection of techniques that, while common, are cheap, fast, and wide-ranging. Therefore, in genotoxicology, they continue to be useful to identify mutagenic agents as well as to evaluate and analyze exposed populations.

Studies on chromosome aberration induction: What can they tell us about DNA repair?

Many, if not the majority of spontaneous or induced mutations in somatic mammalian cells associated with cancer are large chromosome level changes. For exposure to carcinogenic agents, certain specific chromosomal aberrations are likely to lie early along the pathway leading from initial molecular damage to cancer. The kinds of aberrations that occur, and the positions of breakpoints involved in their formation, can reveal not only genes and controlling elements whose expression or suppression underlie the molecular nature of the initiation of malignant transformation, but also how structural and functional features of chromatin can affect processes involved in repair or mis-repair of initial DNA damage. Thus, cytogenetics can provide information in ways that are not readily appreciated in studies requiring disruption of chromatin organization as it exists in the cell and its tissue context, and where DNA repair assays measure effects averaged over the entire genome. Examples include the fact that in contrast to a more efficient repair of single strand or base damage in transcriptionally active chromatin, after ionizing radiation exposure, the preponderance of translocation breakpoints indicating mis-repair occur in transcriptionally active or potentially active chromatin. Cytogenetic studies have led to the recognition that processing of DNA ends - both ends resulting from breaks along chromosomes and natural chromosomal termini, or telomeres - share very interesting similarities and differences. Further, direct observation of chromatin in cells during interphase can speak directly to early stages of aberration formation where processes occur within the context of intact cells, and to the role (or lack thereof) of cell cycle checkpoint responses that often accompany DNA damage. The superior resolution of many of the current molecular cytogenetics approaches, combined with immunocytochemical detection of proteins involved in DNA damage processing, and the availability of repair deficient mutants or knockdown strategies such as RNA interference, suggest that cytogenetics may still provide useful information and set certain restrictions important for rational interpretation of studies of DNA repair and associated protein interactions that can only be carried out in vitro. The intent of this paper is to focus on contributions of studies on the production of chromosomal aberrations following ionizing radiation exposure regarding important insights on associated DNA repair processes involved, and further, on guidelines or constraints they provide for the interpretation of in vitro DNA repair studies that would have been difficult to appreciate without the cytogenetics. We will first briefly summarize some early studies that serve as a reminder of the background on which current studies are based, and then carry forward to the present day certain interesting facets of these studies.

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.

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.

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.

Non-random radial arrangements of interphase chromosome territories: evolutionary considerations and functional implications

In the nucleus of animal and plant cells individual chromosomes maintain a compartmentalized structure. Chromosome territories (CTs), as these structures were named by Theodor Boveri, are essential components of the higher-order chromatin architecture. Recent studies in mammals and non-mammalian vertebrates indicate that the radial position of a given CT (or segments thereof) is correlated with its size, its gene-density and its replication timing. As a representative case, chicken cell nuclei show highly consistent radial chromatin arrangements: gene-rich, early replicating microchromosomes are clustered within the nuclear interior, while gene-poor, later replicating macrochromosomes are preferentially located at the nuclear periphery. In humans, chromosomes 18 and 19 (HSA18 and 19) territories that are of similar size show a distinctly different position in the cell nuclei of lymphocytes and lymphoblastoid cells: the gene-rich and early replicating HSA19 CTs are typically found close to the nuclear center, while the gene-poor and later replicating HSA18 CTs are preferentially located at the nuclear periphery. Recent comparative maps between human and chicken chromosomes revealed that the chicken macrochromosomes 2 and Z contain the genes homologous to HSA18, while the genes on HSA19 are located onto the chicken microchromosomes. These data lend tentative support to the hypothesis that differences in the radial nuclear positions of gene-rich, early replicating and gene-poor, later replicating chromatin have been evolutionarily conserved during a period of more than 300 million years irrespective of the evolution of highly divergent karyotypes between humans and chicken.

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.

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.

Localization of chromosome breakpoints: implication of the chromatin structure and nuclear architecture

Restriction endonucleases and ionizing radiations have been extensively used to study the origin of chromosomal aberrations. Although a non-random distribution of chromosome breakpoints induced by these agents has been claimed by several authors, the significance of the chromatin structure and nuclear architecture in the localization of breakpoints is still not well understood. Breakpoint patterns produced by endonucleases targeted to specific genome sequences or by ionizing radiations could provide additional evidence to clarify this point. Results obtained from the localization of breakpoints induced by AluI, BamHI or DNase I as well as by neutrons or gamma-rays in G-banded Chinese hamster ovary (CHO) chromosomes are presented. AluI and BamHI were electroporated into CHO cells either during the G1 or S-phase of the cell cycle. A co-localization of breakpoints was found with a preferential occurrence in G-light bands independent of the cell cycle stage in which aberration production took place. Since AluI and BamHI recognition sequences are partitioned in the housekeeping and tissue-specific subgenomes respectively, we postulated that nuclease sensitive sites in active chromatin could be the main targets for the induction of breakpoints by these endonucleases. This assumption is supported by the finding that DNase I-induced breakpoint patterns in CHO cells are similar to those produced by AluI and BamHI. Digestion of fixed CHO chromosomes with these endonucleases induced G-banding suggesting a higher sensitivity of G-light chromatin. For comparison purposes, CHO cells were irradiated with neutrons or gamma-rays and breakpoints localized in G-banded chromosome aberrations. A higher occurrence of breakpoints in G-light bands was also observed. We detected seven breakage-prone G-light bands that were preferentially damaged by the three endonucleases and by both types of radiation. These results emphasize the possible implication of the chromatin structure and the nuclear architecture in the localization of chromosome breakpoints induced by endonucleases, neutrons and gamma-rays.

Regional differences in the compaction of chromatin in human G0/G1 interphase nuclei

The large-scale structure of chromatin corresponding to G- and R-bands in human G0/G1 interphase nuclei was compared. Fluorescence in situ hybridization (FISH) was used to measure the interphase distance between 42 pairs of probes separated by 0.1-1.5 Mbp. The probe pairs were derived from 21q22.2 and Xp21.3, G-band positive regions, and from 4p16.3, 6p21.3, and Xq28, R-band positive regions. Distributions of measured interphase distances in all regions approximated a Rayleigh distribution, suggesting that the chromatin follows a random-walk path over this range. A linear correlation of mean-square interphase distance and genomic separation, also indicative of random-walk folding, was observed in all regions. The slope of the correlation observed using probes from G-band regions was systematically lower than that from R-band regions. The difference in the slope between Xp21.3 and Xq28 was particularly striking and was observed in normal fibroblast cells, fixed alternatively with methanol and acetic acid or paraformaldehyde, and HeLa cells. These results demonstrate regional differences in large-scale chromosome structure during interphase, with the more openly configured chromatin corresponding to R-bands.

Intrachromosomal localization of breakpoints induced by the restriction endonucleases AluI and BamHI in Chinese hamster ovary cells treated in S phase of the cell cycle

The restriction endonucleases (REs) AluI and BamHI were electroporated into Chinese hamster ovary (CHO) cells during S phase of the cell cycle and breakpoints in G-banded metaphases were mapped to Giemsa-light or -dark bands or to band junctions. The majority of AluI- and BamHI-induced breakpoints were located in Giemsa-light bands. Both REs induced similar distributions of breakpoint clusters. The localization pattern of S phase-induced breakpoints in CHO cells is similar to the pattern of G1-induced breakpoints reported earlier. These data show that breakpoint localization for both REs is independent of the cell cycle stage (G1 or S) in which aberrations are induced and give further support to the hypothesis that nuclease hypersensitive regions (NHRs) associated with active genes play an important role in the distribution of breakpoints.

Localization of chromosome breakpoints induced by AluI and BamHI in Chinese hamster ovary cells treated in the G1 phase of the cell cycle

Intact Chinese hamster ovary cells were exposed to the restriction endonucleases (REs) AluI or BamHI. In metaphase spreads from these cells, 300 breakpoints per RE were localized in G-banded chromosome type aberrations (dicentrics, translocations, rings, terminal and interstitial deletions). The majority of breakpoints induced by both REs were localized in G-light bands and showed a similar distribution of breakpoint clusters. RE digestion of metaphase spreads with AluI induced C-banding, and with BamHI G-banding. The data indicate that nuclease sensitive sites associated with active genes are mainly responsible for the distribution of breakpoints.

Human genome organization

Recent advances have been made in addressing three intriguing aspects of human genome organization: the organization of protein-coding sequences within chromosomes, the structural basis of the metaphase chromosomal banding pattern, and the function of non protein coding DNA. At the cytogenetic level, R band heterogeneity has been examined by fluorescence in situ hybridization using complex fractions of genomic DNA as probes. DNA fractionated according to GC content and CpG is island density both generated patterns related G and R bands and directly demonstrated regional variations in gene densities. A structural basis for metaphase bands has been proposed that is based on the differential size and packing of DNA loops and matrix attachment sites in G versus R bands. The model presents interesting opportunities for structure/function and organization investigations. At the molecular level, the human genome initiative has resulted in extensive genomic clone coverage for many large chromosomal regions, permitting detailed documentation of CpG islands, base composition and repeat sequence context, as well as fueling comprehensive gene searches. Sequence and functional characteristics are being examined at the kilobase level and are prompting new suggestions of roles for 'junk' DNA. Because of these developments, opportunities are now emerging for direct assessment of the molecular characteristics of individual metaphase bands.

Relationship between locations of chromosome breaks induced by vinyl chloride monomer and lymphocytosis

The distribution of vinyl chloride monomer (VCM)-induced chromosome breaks was studied in cultured lymphocytes of subjects occupationally exposed to this gas. In the examined subjects, the mean group value of chromosome aberrations is 6.5% and for sister chromatid exchange (SCE) frequencies, the mean value per cell is 7.9. These values are significantly higher than in the control population. Occupational exposure to VCM caused lymphocytosis together with disturbances of mitogenic activity in lymphocytes stimulated by phytohaemagglutinin. The results of G-banding showed that sites of chromosome breakpoints caused by VCM can be related to the lymphatic tissue disorders.

Analysis for DNA-proportional distribution of radiation-induced chromosome aberrations in various triple combinations of human chromosomes using fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) with five cocktails of composite whole chromosome-specific DNA probes 1, 4, 12; 2, 7, 9; 2, 7, 9dig; 3, 6, 10dig and 8, 14 Xdig and a degenerate alpha-satellite pancentromeric DNA probe was used to examine in vitro radiation-induced symmetrical translocations and dicentrics in peripheral lymphocytes for a DNA-proportional distribution. For a discrimination between morphologically similar target chromosomes, chromosomes 9, 10 and X were labelled with digoxigenin (dig). Among the five combinations, significantly higher translocation frequencies than expected from the DNA content were found in 8, 14, Xdig, whereas for this combination no deviation became apparent for dicentrics. The chromosome-specific analysis showed that chromosome 2 was involved in fewer symmetrical translocations, whereas chromosomes 9, 10 and 12 were more frequently involved in dicentrics than predicted. Comparing the ratios of symmetrical translocations to dicentrics, an excess of symmetrical translocations was found for the combinations 1, 4, 12; 2, 7, 9dig and 8, 14, Xdig and for the chromosomes 1, 4, 6, 7, 8 and X. Provided the present data can be confirmed in further experiments, the formula Y = 2.05fi(1--fi)FG, relating the translocation or dicentric frequency measured by FISH to the respective genomic FG (fi is the labelled genomic fraction) cannot be used to scale up to equal the full genome unless appropriate weighting factors are included.

Induction of BCR-ABL fusion genes by in vitro X-irradiation

The Philadelphia chromosome consists of a reciprocal translocation between the ABL oncogene at chromosome 9q34 and the BCR gene at chromosome 22q11, resulting in the expression of chimeric BCR-ABL mRNAs specific to chronic myelogenous leukemia (CML). Presence of the fusion gene can be detected with high specificity and sensitivity by means of reverse transcription and polymerase chain reaction. Using this assay, it was possible to detect BCR-ABL fusion genes induced among HL60 cells after 100 Gy of X-irradiation in vitro. In total, five fusion gene transcripts were obtained among 10(8) cells examined. These fusion genes contained not only CML-specific BCR-ABL rearrangements, but also other forms of BCR-ABL fusions. These latter genes had junctions of BCR exon 4/ABL exon 2 intervened by a segment of DNA of unknown origin, BCR exon 5/ABL exon 2, and BCR exon 4/ABL exon 2. The results appear to be direct evidence for the induction of the BCR-ABL fusion gene by X-irradiation. In terms of leukemogenesis, it appears that only those cells bearing certain CML-related BCR-ABL fusion genes are positively selected by virtue of a growth advantage in vivo.

G-banding analysis of radiation-induced chromosome damage in lymphocytes of Hiroshima A-bomb survivors

The present report describes the G-band analysis of somatic chromosomes in lymphocytes from 63 A-bomb survivors in Hiroshima to determine the type and frequency of radiation-induced chromosome aberrations. (1) The cells with stable-type chromosome aberrations (Cs cells) predominated among the aberrant cells, and showed a dose-dependent increase. All stable chromosome aberrations were classified into nine categories: reciprocal translocations, translocations of complex type, insertions, complex exchanges, peri- and paracentric inversions, terminal and interstitial deletions, and unidentified rearrangements. The frequencies of aberrations were found to increase with increasing dose for all aberration categories. Reciprocal translocations predominate in all dose ranges and among the chromosome aberrations classified. (2) The linear model was fitted to test the dose-response relationship for Cs cell frequencies. Employing a constant neutron RBEs of 10, an estimated linear slope of 15.2%/Sv was obtained for DS86 bone marrow dose with an intercept of 2.9% at dose 0. (3) Statistical analysis of data on 3,370 break sites showed good correlations between relative DNA content and the distribution of chromosome breaks involved in translocations, although the involvement of chromosome 1 is significantly higher.

Ionizing radiation and genetic risks. III. Nature of spontaneous and radiation-induced mutations in mammalian in vitro systems and mechanisms of induction of mutations by radiation

This paper (1) presents an analysis of published data on the molecular nature of spontaneously arising and radiation-induced mutations in mammalian somatic cell systems and (2) examines whether the molecular nature and mechanisms of origin of radiation-induced mutations, in mammalian in vivo and in vitro systems, as currently understood, are consistent with expectations based on the biophysical and microdosimetric properties of ionizing radiation. Depending on the test system (CHO cells, human T lymphocytes and human lymphoid cell line TK6), 80-97% of spontaneous HPRT mutations show normal Southern patterns; the remainder is due to gross changes, predominantly partial (intragenic) deletions. Total gene deletions at the HPRT locus are rare except in the TK6 cell line. At the APRT locus in CHO cells, 80-97% of spontaneous mutations are due to base-pair changes, the remainder being, mostly, partial deletions. The latter can extend upstream in the 5' direction but not beyond the APRT gene in the 3' direction. At the human HLA-A locus (T lymphocytes), the percentage of mutations with normal Southern patterns is lower than that for HPRT, and in the range of 50-60%. At the HLA-A locus, mitotic recombination contributes substantially to the mutation spectrum (approximately 30% of mutations recovered) and this is likely to be true of the TK locus in the TK6 cell line as well. With a few exceptions, most of the radiation-induced mutations show altered Southern patterns and are consistent with their being deletions and/or other gross changes (HPRT, 70-90% (CHO); 50-85% (TK6); 50-75% (T lymphocytes); TK, 60-80% (TK6); HLA-A, 80% (T lymphocytes); DHFR, 100% (CHO]. The exceptions are APRT mutations in CHO cells (16-20% of mutants with deletions or other changes) and HPRT mutations in T lymphocytes from A-bomb survivors (15-25%); the latter finding is consistent with the occurrence of in vivo selection against HPRT mutant cells. In cases of HPRT intragenic deletions analyzed (CHO cells and V79 Chinese hamster cells), there is evidence for a non-random distribution of breakpoints. The spontaneous mutation frequencies vary widely, from about 0.04/10(6) cells (sickle cell mutations at the human HBB locus) to 30.8/10(6) cells (HLA-A mutations in T lymphocytes) and are dependent on the locus, the system employed and a number of other factors. Those for the other loci fall between these limits.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of human chromosome 21: correlation of physical and cytogenetic maps; gene and CpG island distributions

Human chromosome 21 has been analyzed by pulsed-field gel electrophoresis using somatic cell hybrids containing limited regions of the chromosome and greater than 60 unique sequence probes. Thirty-three independent NotI fragments have been identified, totalling 43 million bp. This must account for essentially the entire long arm, and therefore gaps remaining in the map must be small. The extent of the pulsed-field map has allowed the direct correlation of the physical map with the cytogenetic map: translocation breakpoints can be unambiguously positioned along the long arm and the distances between them measured in base pairs. Three breakpoints have been identified, providing physical confirmation of cytogenetic landmarks. Information on sequence organization has been obtained: (i) 60% of the unique sequence probes are located within 11 physical linkage groups which can be contained in only 20% of the long arm; (ii) 9/21 genes are clustered within 4%; (iii) translocation breakpoints appear to occur within CpG island regions, making their identification difficult by pulsed-field techniques. This analysis contributes to the human genome mapping effort, and provides information to guide the rapid investigation of the biology of chromosome 21.

Cytogenetic effects of radiotherapy. Breakpoint distribution in induced chromosome aberrations

A total of 660 breakpoints were identified in the chromosome aberrations detected in lymphocytes from cancer patients after radiotherapy. The results show that chromosomes 1, 3, and 7 were significantly more affected than other chromosomes by ionizing radiation in vivo. Chromosome arms 1p, 1q, 7q, and 11p were also significantly more affected. Some bands also showed a special sensitivity to radiation, and band 1q32 was the most affected. This band is proposed as a "hot point" for the clastogenic effect of ionizing radiation. A significant clustering of breakpoints in G bands was also found, especially at the telomeres, as previously described by other authors. Clustering of breakpoints was also observed in bands where fragile sites, protooncogenes, breakpoints involved in chromosomal cancer rearrangements, and breakpoints involved in chromosomal evolution of the Hominoidea are located.

Partial physical map of human chromosome 21

The long arm of human chromosome 21 has been analyzed with unique sequence DNA probes, using an expanded panel of somatic cell hybrids containing defined regions of the chromosome, and both standard and pulsed field gel electrophoresis. Each member of the hybrid cell panel contains either a normal chromosome 21, or one of 11 different translocations or deletions within the long arm. Together, these now include 11 breakpoints, defining 11 long arm regions. Thirty-two unique sequence probes have been localized to these regions by standard gel electrophoresis. Analysis by pulsed field gels indicates that 27 of these identify a total of 18 Not1 restriction fragments, which together account for approximately 17 million base pairs, over half the long arm. Five physical linkage groups have been identified, as well as patterns in the distribution of unique sequences and GC-rich chromosomal regions. This information can be correlated with that obtained by other methods and contributes to the construction of a detailed physical map of this chromosome.

Critical DNA target size model of ionizing radiation-induced mammalian cell death

A new model of mammalian cell killing by ionizing radiation is presented. This model, termed the critical DNA target size model, postulates that DNA double-strand breakage is the critical radiation-induced lesion and that the dose-response for such breakage can be non-linear due to the action of a saturable chemical repair process. DNA double-strand breakage occurring within critical targets (proto-oncogene- or common fragile site-associated sequences) is postulated to initiate recombination events with undamaged sequences, leading to chromosomal aberrations. The subsequent loss of acentric fragments at mitosis is postulated to prevent the continuity of the genome and to produce cell death by the induction of chromatin structural changes. Experimental evidence contrary to other radiation action models is examined, and the hypotheses of the model are justified.

Qualitative study of chromosomal lesions induced by neutrons and neon ions in human lymphocytes at G0 phase

Chromosomal lesions, mitotic index and cell cycle progression delay induced by neutron (protons 34 MeV on beryllium) and neon (250 MeV/i) particles are studied in human lymphocytes. The cell cycle progression is slightly decreased at a dose of 1 Gy. Mitotic indexes are significantly decreased after irradiation by the different particles, except neon in 52-h cultures. By comparison to chromosome damages caused by gamma-rays, previously published, it is found that the lesions observed here are frequently more complex: the number of breaks is higher per abnormal mitosis and higher per rearrangement on the average. This complexity is higher for neon ions than for neutron beams.

Distribution of the various radiation-induced chromosomal rearrangements in relation to the dose and sampling time

The quantitative analysis of the chromosome rearrangements detected in 2128 R-banded metaphases, obtained from gamma-irradiated human lymphocytes after 48 to 96 h in culture is reported. Depending on the culture time, and possibly on the dose of radiation (from 1 to 3 Gy), the most frequent type of rearrangement was either dicentrics or reciprocal translocations. In first generation mitoses, the frequency of cells without rearrangement ranged from 0.66 to 0.18, and the mean number of rearranged chromosomes per cell from 0.79 to 3.28. The dose-response curve follows a quadratic function for dicentric aberration yields, but not for other rearrangements.

Inter- and intra-chromosomal distribution of chromatid breaks induced by X-rays during G2 in human lymphocytes

Cultures of blood from healthy adults were irradiated 48 h after stimulation with 240 R of X-rays and fixed after various time intervals (0-2 h, 2-4 h, 4-6 h). 3HTdR was added to several cultures after irradiation. Mitotic and labelling indices were used to distinguish between two cell samples inside the irradiated G2 population: D- cells reaching mitosis without mitotic delay and a high frequency of chromatic breaks and D+ cells with mitotic delay and which, during the delay, repair most of the damage produced. After R banding 450 chromatid deletions were located in each of the two cell samples. The D+ cells showed a higher frequency of breaks than the D- cells with decreasing chromosome size, in the telomeric and centromeric region and in the junction between the R+ and R- bands. These results can be interpreted as indicative of a non-random distribution of repair processes both between and within chromosomes.

Chromosome studies in plutonium workers

Chromosome analyses have been performed on peripheral blood lymphocytes from 54 men with estimates of plutonium body burdens in excess of 296 Bq. Both stable and unstable aberrations were scored using a banding technique and breakpoints noted. In discussing the significance of aberration frequencies the relative proportions of the different types of aberration and their distribution have been considered and account has been taken of external radiation exposure. It is suggested that significant depositions of plutonium do cause an increase in chromosome aberrations. The distribution of the breakpoints in the controls showed an excess in chromosomes 7 and 14. The formation and survival of radiation-induced breakpoints was randomly distributed amongst the chromosomes according to length. The distribution of the breakpoints within the chromosomes showed an excess in the centromeres and telomeres. Possible hot spots occurred in some of these regions and also in certain bands of the intermediate regions of the chromosomes.

Translocations of chromosome 12. II. A comparison of the distribution of sites of spontaneous and induced breakages

Human chromosome 12 has been used as a model for studying the distributions of sites of induced and spontaneous breaks. The breakpoints were determined from (1) translocations involving chromosome 12, (2) spontaneous breaks in untreated cultures, (3) radiation-induced breaks, and (4) spontaneous breaks in Fanconi's anaemia. Statistical analysis showed discordance in the results both between the eleven individual bands and between the four assessments. Also, the distribution of breaks for all bands was significantly different from random in each assessment. Certain bands added considerable bias to the results, and when analysed individually, only four bands (p11.1, q13, q24, and p13) showed distributions over the four assessments that were significantly different from random. These four bands are Giemsa-negative bands, and two (p13 and q24) are adjacent to telomeres, while p11.4 is adjacent to the centromere. The fourth band, q13, is a known fragile site. It is concluded that bands adjacent to centromeres, which are not C-banded, are peculiarly sensitive to breakage. Telomeric bands are variable in their response to different conditions of breakage, and both the physical structure of the telomere and the specific gene sequences of individual telomeres are probably of importance in determining this response. The fragile site q13 responds as if breakage at this site is due to the base composition of the DNA.

The pattern of radiation-induced transmissible aberrations in a human cell culture

The G-band pattern in 445 metaphases obtained seven weeks after irradiation (600 rad gamma-ray) was analysed. Approximately 37% of these cells had one or more structural aberrations. The majority of the aberrant events was reciprocal translocation followed by inversion and deletion in the proportion of 9:1:1 respectively. Statistical analyses (Chi-square tests) on the distribution of breakpoints among chromosomes showed an excess number of breaks in chromosomes 1, 7, and 12. Chromosomes 1 and 12 were particularly involved in cells carrying multiple aberrations while chromosome 7 was preferentially involved in deletion. Within chromosomes a significantly large number of breaks were located in (a) the light bands and (b) the terminal segments. The significance of these findings is discussed.

Tentative estimate of the risk of chromosomal disease due to radiation-induced translocations in man

An attempt to estimate one of the parameters establishing the risk of occurrence of abnormal live-born progeny by malsegregation of radiation-induced translocation is reported. A sample of 247 2-break translocations induced by gamma-rays in human lymphocytes was studied in relation to the minimal possible imbalance they could induce in gametogenesis. These imbalances were compared with chromosomal trisomies and monosomies known to be compatible with life after birth in man. It is concluded that at least 106 out of 247 translocations should not give viable products in cases of malsegregation. A second comparison, with translocations ascertained in human subjects for various reasons, led to the conclusion that about 2/5 of the radiation-induced translocations might involve a risk of partial trisomies or monosomies. Cell survival and frequency of meiotic malsegregations are other parameters needed to make a correct estimate. A short discussion shows the difficulty of such estimates from inter-specific comparisons.

Non-random distribution of aberrations and identification with C- and G-bandings of the position of breakage points on Muntjac chromosomes induced by mitomycin c, bromodeoxyuridine and hydroxylamine

The analysis of chromosomes from muntjac after treatment of its lymphocyte cultures with 3 chemical mutagens having different base-pair affinities and modes of action, namely mitomycin C (MC), 5-bromodeoxyuridine (BUdR) and hydroxylamine hydrochloride (HA), with G- and C-band staining displayed non-random distribution of chemically specific damage points on them. The randomness of the involvement of each site on the chromosomes were examined by assuming an expected value calculated on the basis of its relative mitotic length. The observation revealed that a large fraction of MC-induced aberrations was preferentially located in the C-band positive constitutive heterochromatin, especially in the long "neck-like" centromeric region of the X-chromosome. On the chromosomal arms, the light G-bands were involved in aberrations either in proportion to or higher than that expected. When the cells were treated with BUdR, the dark G-bands on all the chromosomes of the complement were the preferred sites, displaying statistically significant higher numbers of aberrations. A single "hot-spot" for induced damage on 1 mid-q was also recorded. HA induced a very high frequency of damage in the secondary constriction regions of the chromosome pairs 1, X and Y2, and the frequency was slightly lower than this in the centromeres of 1, 2 and X chromosomes. The observation of specific distribution of damage points induced by the 3 chemicals lead to the suggestion that, though the effect of a chemical on chromosome segments depends on several factors, each being partially responsible for the end result, it is perhaps primarily depended by the chemical's base-pair affinity and mode of action.

Brief clinical report: the del(4) (q31) syndrome- a recognizable disorder with atypical Robin malformation sequence

Deletions of the terminal region of the long arm of chromosome 4 have been reported previously in 6 patients. With the addition of our patient with 46,XX,del(4) (pter leads to q31:), it becomes clearer that this is a recognizable syndrome. None of the 7 patients has had prenatal growth deficiency, while postnatal growth deficiency has been variable. The syndrome is typified by a Robin malformation sequence without apparent catch-up growth of the mandible, anomalous auricles, a short nasal septum with a depressed nasal bridge, absent 5th finger creases, clinodactyly, and displacement of the toes. Mental retardation has been found consistently.

De novo duplication 1q32-q42: variability of phenotypic features in partial lq trisomics

A de novo tandem duplication 1q32--q42 was observed in a 7-month-old mentally retarded and malformed male infant. Karyotype-phenotype correlation in other similar unbalanced trisomies has shown psychomotor retardation, micro- or retrognathia or both, and low set or malpositioned ears to be the most common features associated with this newly recognised syndrome. However, after reviewing patients with duplication of regions 1q2, 3, and 4 and 1q2 and 3, it was concluded that similar non-specific clinical features are also present in these 1q imbalances. On the whole, a rather wide range in phenotypical expression has been observed in different cases. Thus it is concluded that, at present, it is impossible to delineate the profile of the syndromes resulting from partial 1q trisomies.

Nonrandom distribution of exchange points in patients with reciprocal translocations

A total of 770 breakpoints (80 of them identified by the authors) from unrelated patients with two-break rearrangements resulting in reciprocal translocations were studied to determine whether they were located preferentially. The distribution of breakpoints among the chromosome arms differs from that expected on the basis of their lengths, with more than expected on chromosome arms 4p, 9p, 9q, 13q, 18q, 21p, 21q, 22p, and 22q and fewer than expected on 1p, 1q, 3p, 3q, 5q, 6q, 7p, 12p, 16p, and the gonosomes. More breakpoints than expected occurred in the centromeric regions, and fewer in the median regions. Distribution of breakpoints within bands differed with the technique used: with G banding a many more breakpoints were localized in the light bands and fewer in the dark bands. With R banding no fewer than expected were present in the light bands and only slightly more were found in the dark bands.

The constitutional fragility of chromosome 12 in a case of 46,XX,var(12)(gh',RHG,CAG,CBG)

The constitutional fragility of chromosome no. 12 in a female infant with unspecific clinical signs is described. RHG, GAG, and CBG methods were used to localize the fragile point. The breaks seem to be in 12q1.3, and always within an R band. A possible correlation between the phenotypic modifications and the chromosome variant is discussed.