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

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.

Relative proximity of chromosome territories influences chromosome exchange partners in radiation-induced chromosome rearrangements in primary human bronchial epithelial cells

It is well established that chromosomes exist in discrete territories (CTs) in interphase and are positioned in a cell-type specific probabilistic manner. The relative localisation of individual CTs within cell nuclei remains poorly understood, yet many cancers are associated with specific chromosome rearrangements and there is good evidence that relative territorial position influences their frequency of exchange. To examine this further, we characterised the complexity of radiation-induced chromosome exchanges in normal human bronchial epithelial (NHBE) cells by M-FISH analysis of PCC spreads and correlated the exchanges induced with their preferred interphase position, as determined by 1/2-colour 2D-FISH analysis, at the time of irradiation. We found that the frequency and complexity of aberrations induced were reduced in ellipsoid NHBE cells in comparison to previous observations in spherical cells, consistent with aberration complexity being dependent upon the number and proximity of damaged CTs, i.e. lesion proximity. To ask if particular chromosome neighbourhoods could be identified we analysed all radiation-induced pair-wise exchanges using SCHIP (statistics for chromosome interphase positioning) and found that exchanges between chromosomes (1;13), (9;17), (9;18), (12;18) and (16;21) all occurred more often than expected assuming randomness. All of these pairs were also found to be either sharing similar preferred positions in interphase and/or sharing neighbouring territory boundaries. We also analysed a human small cell lung cancer cell line, DMS53, by M-FISH observing the genome to be highly rearranged, yet possessing rearrangements also involving chromosomes (1;13) and (9;17). Our findings show evidence for the occurrence of non-random exchanges that may reflect the territorial organisation of chromosomes in interphase at time of damage and highlight the importance of cellular geometry for the induction of aberrations of varying complexity after exposure to both low and high-LET radiation.

mFISH analysis of chromosome aberrations induced in vitro by α-particle radiation: examination of dose-response relationships

A multicolored FISH (mFISH) technique was used to characterize the cytogenetic damage associated with exposure to α-particle radiation with particular emphasis on the quality and quantity that is likely to be transmitted through cell division to descendant cells. Peripheral blood lymphocytes were irradiated in vitro with (238)Pu α particles with a range of mean doses up to 936 mGy and were cultured for 47 h. The dose responses for total aberrant cells, stable and unstable cells, and cells with one simple chromosome aberration and multiple chromosome aberrations were predominantly linear for doses that resulted in cell nuclei receiving a single α-particle traversal. However, there was a decrease per unit dose in aberrant cells of all types at higher doses because of cells increasingly receiving multiple traversals. The proportion of radiation-induced aberrant cells containing multiple aberrations ranged from 48 to 74% with little evidence of dose dependency. Ninety-one percent of all cells with multiple aberrations were classified as unstable. Resolving the chromosome rearrangements into simple categories resulted in a linear dose response for dicentrics of 24.9 ± 3.3 × 10(-2) per Gy. The predominant aberration in stable transmissible cells was a single translocation with a dose response for predominantly single hit cell nuclei of 4.1 ± 1.3 × 10(-2) per Gy. Thus, translocations are the most likely aberration to be observed in peripheral blood lymphocytes from individuals with incorporated α-emitting radionuclides resulting in long-term chronic exposure.

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.

Characterization of chromatin instabilities induced by glyphosate, terbuthylazine and carbofuran using cytome FISH assay

Possible clastogenic and aneugenic effects of pesticides on human lymphocytes at concentrations likely to be encountered in residential and occupational exposure were evaluated with (and without) the use of metabolic activation (S9). To get a better insight into the content of micronuclei (MN) and other chromatin instabilities, lymphocyte preparations were hybridized using pancentromeric DNA probes. Frequency of the MN, nuclear buds (NB) and nucleoplasmic bridges (NPB) in cultures treated with glyphosate slightly increased from 3.5microg/ml onward. Presence of S9 significantly elevated cytome assay parameters only at 580microg/ml. No concentration-related increase of centromere (C+) and DAPI signals (DAPI+) was observed for glyphosate treatment. Terbuthylazine treatment showed a dose dependent increase in the number of MN without S9 significant at 0.0008microg/ml and higher. At concentration lower than 1/16 LD50 occurrence of C+MN was significantly elevated regardless of S9, but not dose related, and in the presence of S9 only NBs containing centromere signals were observed. Carbofuran treatment showed concentration-dependent increase in the number of MN. The frequency of C+MN was significant from 0.008microg/ml onward regardless of S9. Results suggest that lower concentrations of glyphosate have no hazardous effects on DNA, while terbuthylazine and carbofuran revealed a predominant aneugenic potential.

Dose response relationships for chromosome aberrations induced by low doses of alpha-particle radiation

Using a single colour fluorescence in situ hybridisation technique, dose-responses were derived for a range of chromosomally aberrant cell types and categories of aberrations induced in peripheral blood lymphocytes by alpha-particle radiation and analysed in their first in vitro division. For a range of doses that resulted predominantly in targeted cells receiving a single hit, i.e. 0-200 mGy, linear models fitted all the different categories of aberrant cells and aberration types but the profile of chromosome damage differed for 500 mGy, reflecting the effect of different track structure.

New sequence-based data on the relative DNA contents of chromosomes in the normal male and female human diploid genomes for radiation molecular cytogenetics

Background

The objective of this work is to obtain the correct relative DNA contents of chromosomes in the normal male and female human diploid genomes for the use at FISH analysis of radiation-induced chromosome aberrations.

Results

The relative DNA contents of chromosomes in the male and female human diploid genomes have been calculated from the publicly available international Human Genome Project data. New sequence-based data on the relative DNA contents of human chromosomes were compared with the data recommended by the International Atomic Energy Agency in 2001. The differences in the values of the relative DNA contents of chromosomes obtained by using different approaches for 15 human chromosomes, mainly for large chromosomes, were below 2%. For the chromosomes 13, 17, 20 and 22 the differences were above 5%.

Conclusion

New sequence-based data on the relative DNA contents of chromosomes in the normal male and female human diploid genomes were obtained. This approach, based on the genome sequence, can be recommended for the use in radiation molecular cytogenetics.

Irinotecan Toxicity to Human Blood Cells in vitro: Relationship between Various Biomarkers

Toxic effects of the antineoplastic drug irinotecan on human blood cells at concentrations of 9.0 microg/ml and 4.6 microg/ml were evaluated in vitro. Using the alkaline and neutral comet assay significantly increased levels of primary DNA damage in lymphocytes were detected. The induction of apoptosis/necrosis, as determined by a fluorescent assay, was also notably increased. Cytogenetic outcomes of the treatment were assessed by the analysis of structural chromosome aberrations and fluorescence in situ hybridization. A significantly higher incidence of chromatid breaks and complex quadriradials was observed. Painted chromosomes 1, 2 and 4 were equally involved in translocations, but only the chromosome 1 was involved in the formation of quadriradials. Sister chromatid exchange analysis was performed in parallel with the analysis of lymphocyte proliferation kinetics. The higher concentration of irinotecan caused almost seven-time increase, while the lower one caused a five-time increase of the basal sister chromatid exchange frequency, accompanied with significant lowering of the lymphocyte proliferation index. Using the cytokinesis-block micronucleus assay, a dose-dependent increase in micronucleus frequency along with the formation of nuclear buds and nucleoplasmic bridges was noticed. Inhibitory effects of irinotecan on enzyme acetylcholinesterase (AChE) were studied in erythrocytes. An IC(50) value of 5.0 x 10(-7) was established. Irinotecan was found to be strong inhibitor of the acetylcholine hydrolysis and to cause a continuous decrease of catalytic activity of AChE. The results obtained on a single donor may contribute to the understanding of irinotecan toxicity, but further in vitro and in vivo studies are essential in order to clarify remaining issues, especially on possible inter-individual variability in genotoxic responses to the drug.

Spatial genome organization in the formation of chromosomal translocations

Chromosomal translocations and genomic instability are universal hallmarks of tumor cells. While the molecular mechanisms leading to the formation of translocations are rapidly being elucidated, a cell biological understanding of how chromosomes undergo translocations in the context of the cell nucleus in vivo is largely lacking. The recent realization that genomes are non-randomly arranged within the nuclear space has profound consequences for mechanisms of chromosome translocations. We review here the emerging principles of spatial genome organization and discuss the implications of non-random spatial genome organization for the genesis and specificity of cancerous chromosomal translocations.