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

Buried territories: heterochromatic response to DNA double-strand breaks

Cellular response to DNA double-strand breaks (DSBs), the most deleterious type of DNA damage, is highly influenced by higher-order chromatin structure in eukaryotic cells. Compared with euchromatin, the compacted structure of heterochromatin not only protects heterochromatic DNA from damage, but also adds an extra layer of control over the response to DSBs occurring in heterochromatin. One key step in this response is the decondensation of heterochromatin structure. This decondensation process facilitates the DNA damage signaling and promotes proper heterochromatic DSB repair, thus helping to prevent instability of heterochromatic regions of genomes. This review will focus on the functions of the ataxia telangiectasia mutated (ATM) signaling cascade involving ATM, heterochromatin protein 1 (HP1), Krüppel-associated box (KRAB)-associated protein-1 (KAP-1), tat-interacting protein 60 (Tip60), and many other protein factors in DSB-induced decondensation of heterochromatin and subsequent repair of heterochromatic DSBs. As some subsets of DSBs may be repaired in heterochromatin independently of the ATM signaling, a possible repair model is also proposed for ATM-independent repair of these heterochromatic DSBs.

Mechanisms and Consequences of Double-Strand DNA Break Formation in Chromatin

All organisms suffer double-strand breaks (DSBs) in their DNA as a result of exposure to ionizing radiation. DSBs can also form when replication forks encounter DNA lesions or repair intermediates. The processing and repair of DSBs can lead to mutations, loss of heterozygosity, and chromosome rearrangements that result in cell death or cancer. The most common pathway used to repair DSBs in metazoans (non-homologous DNA end joining) is more commonly mutagenic than the alternative pathway (homologous recombination mediated repair). Thus, factors that influence the choice of pathways used DSB repair can affect an individual's mutation burden and risk of cancer. This review describes radiological, chemical, and biological mechanisms that generate DSBs, and discusses the impact of such variables as DSB etiology, cell type, cell cycle, and chromatin structure on the yield, distribution, and processing of DSBs. The final section focuses on nucleosome-specific mechanisms that influence DSB production, and the possible relationship between higher order chromosome coiling and chromosome shattering (chromothripsis).

Role of chromatin structure modulation by the histone deacetylase inhibitor trichostatin A on the radio-sensitivity of ataxia telangiectasia

At present, a lot is known about biochemical aspects of double strand breaks (DBS) repair but how chromatin structure affects this process and the sensitivity of DNA to DSB induction is still an unresolved question. Ataxia telangiectasia (A-T) patients are characterised by very high sensitivity to DSB-inducing agents such as ionising radiation. This radiosensitivity is revealed with an enhancement of chromosomal instability as a consequence of defective DNA repair for a small fraction of breaks located in the heterochromatin, where they are less accessible. Besides, recently it has been reported that Ataxia Telangiectasia Mutated (ATM) mediated signalling modifies chromatin structure. In order to study the impact of chromatin compaction on the chromosomal instability of A-T cells, the response to trichostatin-A, an histone deacetylase inhibitor, in normal and A-T lymphoblastoid cell lines was investigated testing its effect on chromosomal aberrations, cell cycle progression, DNA damage and repair after exposure to X-rays. The results suggest that the response to both trichostatin-A pre- and continuous treatments is independent of the presence of either functional or mutated ATM protein, as the reduction of chromosomal damage was found also in the wild-type cell line. The presence of trichostatin-A before exposure to X-rays could give rise to prompt DNA repair functioning on chromatin structure already in an open conformation. Differently, trichostatin-A post-treatment causing hyperacetylation of histone tails and reducing the heterochromatic DNA content might diminish the requirement for ATM and favour DSBs repair reducing chromosomal damage only in A-T cells. This fact could suggest that trichostatin-A post-treatment is favouring the slow component of DSB repair pathway, the one impaired in absence of a functionally ATM protein. Data obtained suggest a fundamental role of chromatin compaction on chromosomal instability in A-T cells.

Chromatin structure and radiation-induced DNA damage: from structural biology to radiobiology

Genomic DNA in eukaryotic cells is basically divided into chromosomes, each consisting of a single huge nucleosomal fiber. It is now clear that chromatin structure and dynamics play a critical role in all processes involved in DNA metabolism, e.g. replication, transcription, repair and recombination. Radiation is a useful tool to study the biological effects of chromatin alterations. Conversely, radiotherapy and radiodiagnosis raise questions about the influence of chromatin integrity on clinical features and secondary effects. This review focuses on the link between DNA damage and chromatin structure at different scales, showing how a comprehensive multiscale vision is required to understand better the effect of radiations on DNA. Clinical aspects related to high- and low-dose of radiation and chromosomal instability will be discussed. At the same time, we will show that the analysis of the radiation-induced DNA damage distribution provides good insight on chromatin structure. Hence, we argue that chromatin "structuralists" and radiobiological "clinicians" would each benefit from more collaboration with the other. We hope that this focused review will help in this regard.

Preferential localization of γH2AX foci in euchromatin of retina rod cells after DNA damage induction

DNA damage may lead to cell transformation, senescence, or death. Histone H2AX phosphorylation, immunodetected as γH2AX foci, is an early response to DNA damage persisting even after DNA repair. In cycling mammalian cells with canonical nuclear architecture, i.e., central euchromatin and peripheral heterochromatin, γH2AX foci map preferentially to euchromatin. Mice retina rods are G0 cells displaying an inverted nuclear architecture 28 days after birth (P28). Rod nuclei exhibit one or two central constitutive heterochromatin chromocenters encircled by facultative heterochromatin. Euchromatin resides at the nuclear periphery, extending to the equator in cells with two chromocenters. To assess the impact of chromatin relocation in the localization of DNA damage, γH2AX and TUNEL foci induced ex vivo by radiomimetic bleomycin were mapped in H3K4me3 immunolabeled P28 rod nuclei. A preferential localization of γH2AX foci in euchromatin was detected together with foci clustering. Besides, a decay of H3K4me3 signal at γH2AX foci sites was observed. TUNEL and γH2AX foci exhibited similar localization patterns in BLM-treated rod cells thus excluding curtailed access of anti-γH2AX antibodies to heterochromatin. Lack of γH2AX foci in rod chromocenters appears to be unrelated to the occurrence of mid-range foci movements. Foci clusters may arise through DNA double-strand break proximity, local non-directional chromatin movements or chromatin relaxation. H3K4me3 signal reduction at γH2AX foci could stem from local chromatin decondensation or downregulation of histone H4 methylation. The observed topology of DNA damage in retina-differentiated rods indicates that euchromatin is damage-prone, regardless of the canonical or inverted nuclear architecture of mammalian cells.

Targeting DNA damage and repair: embracing the pharmacological era for successful cancer therapy

DNA is under constant assault from genotoxic agents which creates different kinds of DNA damage. The precise replication of the genome and the continuous surveillance of its integrity are critical for survival and the avoidance of carcinogenesis. Cells have evolved an arsenal of repair pathways and cell cycle checkpoints to detect and repair DNA damage. When repair fails, typically cell cycle progression is halted and apoptosis is initiated. Here, we review the different sources and types of DNA damage including DNA replication stress and oxidative stress, the repair pathways that cells utilize to repair damaged DNA, and discuss their biological significance, especially with reference to cancer induction and cancer therapy. We also describe the main methodologies currently used for the detection of DNA damage with their strengths and limitations. We conclude with an outline as to how this information can be used to identify novel pharmacological targets for DNA repair pathways or enhancers of DNA damage to develop improved treatment strategies that will benefit cancer patients.

Association of inter- and intrachromosomal exchanges with the distribution of low- and high-LET radiation-induced breaks in chromosomes

To study the effects of low- and high-linear energy transfer (LET) radiation on break locations within a chromosome, we exposed human epithelial cells in vitro to (137)Cs γ rays at both low and high dose rates, secondary neutrons at a low dose rate, and 600 MeV/u iron ions at a high dose rate. Breakpoints were identified using multicolor banding in situ hybridization (mBAND), which paints chromosome 3 in 23 different colored bands. For all four radiation scenarios, breakpoint distributions were found to be different from the predicted distribution based on band width. Detailed analysis of chromosome fragment ends involved in inter- or intrachromosomal exchanges revealed that the distributions of fragment ends participating in interchromosomal exchanges were similar between the two low-LET radiation dose rates and between the two high-LET radiation types, but the distributions were less similar between low- and high-LET radiations. For fragment ends participating in intrachromosomal exchanges, the distributions for all four radiation scenarios were similar, with clusters of breaks found in three regions. Analysis of the locations of the two fragment ends in chromosome 3 that joined to form an intrachromosomal exchange demonstrated that two breaks with a greater genomic separation can be more likely to rejoin than two closer breaks, indicating that chromatin folding can play an important role in the rejoining of chromosome breaks. Comparison of the breakpoint distributions to the distributions of genes indicated that the gene-rich regions do not necessarily contain more breaks. In general, breakpoint distributions depend on whether a chromosome fragment joins with another fragment in the same chromosome or with a fragment from a different chromosome.

Impact of EMS outreach: successful developments in Latin America

This collection of articles was inspired by the long-standing relationship between the Environmental Mutagen Society and Latin American scientists, and by the program for the 39th Environmental Mutagen Society meeting in Puerto Rico in 2008, which included a symposium featuring "South of the border" scientists. This collection, compiled by Graciela Spivak and Ofelia Olivero, both originally from Argentina, highlights scientists who work in or were trained in Latin American countries and in Puerto Rico in a variety of scientific specialties related to DNA repair and cancer susceptibility, genomic organization and stability, genetic diversity, and environmental contaminants.

Relationship between chromatin structure, DNA damage and repair following X-irradiation of human lymphocytes

Earlier studies using the technique of premature chromosome condensation (PCC) have shown that in human lymphocytes, exchange type of aberrations are formed immediately following low doses (<2 Gy) of X-rays, whereas at higher doses these aberrations increase with the duration of recovery. This reflects the relative roles of slow and fast repair in the formation of exchange aberrations. The underlying basis for slow and fast repairing components of the DNA repair may be related to differential localization of the initial damage in the genome, i.e., between relaxed and condensed chromatin. We have tried to gain some insight into this problem by (a) X-irradiating lymphocytes in the presence of dimethyl sulfoxide (DMSO) a potent scavenger of radiation-induced .OH radicals followed by PCC and (b) probing the damage and repair in two specific chromosomes, 18 and 19, which are relatively poor and rich in transcribing genes by COMET-FISH, a combination of Comet assay and fluorescence in situ hybridization (FISH) techniques. Results obtained show (a) that both fast appearing and slowly formed exchange aberrations seem to take place in relaxed chromatin, since they are affected to a similar extent by DMSO, (b) significant differential DNA breakage of chromosome 18 compared to chromosome 19 in both G0 and G1 phases of the cell cycle as detected by Comet assay, indicating that relaxed chromatin containing high densities of transcriptionally active genes shows less fragmentation due to fast repair (chromosome 19) compared to chromosome 18, and (c) that relaxed chromatin is repaired or mis-repaired faster than more compact chromatin.

Linking cell cycle reentry and DNA damage in neurodegeneration

Aberrant cell cycle activity and DNA damage have been observed in neurons in association with various neurodegenerative conditions. While there is strong evidence for a causative role for these events in neurotoxicity, it is unclear how they are triggered and why they are toxic. Here, we introduce a brief background of the current view on cell cycle activity and DNA damage in neurons and speculate on their relevance to neuronal survival. Furthermore, we suggest that the two events may be triggered in common by deregulation of fundamental processes, such as chromatin modulation, which are required for maintaining both DNA integrity and proper regulation of cell cycle gene expression.

Modulation of chromosome damage localization by DNA replication timing

PURPOSE

Non-random occurrence of induced chromosome breakpoints (BP) has been repeatedly reported. DNA synthesis and chromatin remodeling may influence chromosome BP localization. The CHO9 X chromosome exhibits an early replicating short euchromatic arm (Xpe) and a late replicating long heterochromatic arm (Xqh). We investigated the role played by DNA replication and related chromatin remodeling processes on BP distribution in eu/heterochromatin using the CHO9 X chromosome as a model.

MATERIALS AND METHODS

BP induced by etoposide, a topoisomerase II inhibitor, as well as by the S-dependent clastogens ultraviolet-C light (UV-C) and methyl methanesulfonate (MMS) were mapped to CHO9 X chromosome arms. The base analogue 5-bromo-2'-deoxyuridine (BrdUrd) was pulse-added immediately after UV-C irradiation or during etoposide and MMS treatments (40 min) to identify cells in early S-phase (Xpe labeled) or late S-phase (Xqh labeled) after indirect BrdUrd immunodetection in metaphase spreads using primary anti-BrdUrd and secondary fluorochrome-tagged antibodies.

RESULTS

During early S-phase, BP induced by etoposide and MMS mapped preferentially to Xpe while BP produced by UV-C localized randomly. BP induced by all agents during late S-phase clustered in Xqh.

CONCLUSIONS

Results obtained suggest that replication time of eu/heterochromatin as well as chromatin remodeling may determine BP localization on the CHO9 X chromosome.

Endonuclease G promotes cell death of non-invasive human breast cancer cells

The invasiveness of breast cancer cells was shown to be associated with the suppressed ability to develop apoptosis. The role of cell death DNases/endonucleases has not been previously examined in relation with the invasiveness of breast cancer cells. We have compared the activity of the endonucleases in seven human breast cancer cell lines different in the level of invasiveness and differentiation. The invasiveness of cell lines was confirmed by an in vitro Matrigel-based assay. The total endonuclease activity in the differentiated non-invasive (WDNI) cell lines was higher than that in the poorly differentiated invasive (PDI) cells. The expression of EndoG strongly correlated with the degree of estrogen receptor expression and showed an inverse correlation with vimentin and matrix metalloproteinase-13. The EndoG-positive WDNI cells were more sensitive to etoposide- or camptothecin-induced cell death than EndoG-negative PDI cells. Silencing of EndoG caused inhibited of SK-BR-3 WDNI cell death induced by etoposide. Human ductal carcinomas in situ expressed high levels of EndoG, while invasive medullar and ductal carcinomas had significantly decreased expression of EndoG. This correlated with decreased apoptosis as measured by TUNEL assay. Our findings suggest that the presence of EndoG in non-invasive breast cancer cells determines their sensitivity to apoptosis, which may be taken into consideration for developing the chemotherapeutic strategy for cancer treatment.

Probing the chromosome 9p21 region susceptible to DNA double-strand breaks in human cells in vivo by restriction enzyme transfer

A restriction enzyme, MspI, was introduced into cultured human cells as a probe to detect genomic regions susceptible to DNA double-strand breaks (DSBs). A 2 h exposure to MspI at a concentration of 8 U/mul produced DSBs at MspI sites in more than 80% of HeLa cells. The sensitivity to digestion was examined on chromosomal DNAs for the region containing the p16 tumor suppressor gene and two other related genes, p14ARF and p15, by Southern blot hybridization analysis and linker-mediated capture of DNA fragments digested in vivo. DNAs for the promoter regions of the three genes, respectively, were sensitive to MspI digestion in HeLa cells, while DNA for the p16 promoter region was less sensitive in lung cancer cells with hypermethylation of the region. Breakpoints for interstitial 9p21 deletions removing the p16/p14ARF/p15 locus in a variety of human cancers were significantly over-represented in the three sensitive regions. The results suggest that the MspI sensitivity in vivo of each genomic region reflects its susceptibility to DSBs that trigger chromosome aberrations in human cells. This method could help us understand the pathogenic significance of differential susceptibility to DSBs among genomic regions in human carcinogenesis.

Distribution of breakpoints induced by etoposide and X-rays along the CHO X chromosome

SORB (selected observed residual breakpoints) induced by ionizing radiation or endonucleases are often non-randomly distributed in mammalian chromosomes. However, the role played by chromatin structure in the localization of chromosome SORB is not well understood. Anti-topoisomerase drugs such as etoposide are potent clastogens and unlike endonucleases or ionizing radiation, induce DNA double-strand breaks (DSB) by an indirect mechanism. Topoisomerase II (Topo II) is a main component of the nuclear matrix and the chromosome scaffold. Since etoposide leads to DSB by influencing the activity of Topo II, this compound may be a useful tool to study the influence of the chromatin organization on the distribution of induced SORB in mammalian chromosomes. In the present work, we compared the distribution of SORB induced during S-phase by etoposide or X-rays in the short euchromatic and long heterochromatic arms of the CHO9 X chromosome. The S-phase stage (early, mid or late) at which CHO9 cells were exposed to etoposide or X-rays was marked by incorporation of BrdU during treatments and later determined by immunolabeling of metaphase chromosomes with an anti-BrdU FITC-coupled antibody. The majority of treated cells were in late S-phase during treatment either with etoposide or X-rays. SORB induced by etoposide mapped preferentially to Xq but random localization was observed for SORB produced by X-rays. Possible explanations for the uneven distribution of etoposide-induced breakpoints along Xq are discussed.

In situ DNAse I sensitivity assay indicates DNA conformation differences between CHO cells and the radiation-sensitive CHO mutant IRS-20

The radiosensitive mutant cell line IRS-20, its wild type counterpart CHO and a derivative of IRS-20 with a transfected YAC clone (YAC-IRS) that restores radioresistance were tested for DNAse I sensitivity. The three cell lines were cultured under the same conditions and had a mitotic index of 2-5%. One drop of fixed cells from the three lines was always spread on the same microscopic slide. After one day of ageing, slides were exposed to DNAse I and stained with DAPI. Images from every field were captured and the intensity of blue fluorescence was measured with appropriate software. For untreated cells, the fluorescence intensity was similar for all of the cell lines. After DNAse I treatment, CHO and YAC-IRS had an intensity of 85% but IRS-20 had an intensity of 60%, when compared with the controls. DNAse I sensitivity differences between the cell lines indicate that overall conformation of chromatin might contribute to radiation sensitivity of the IRS-20 cells.

Telomere-driven genomic instability in cancer cells

Telomeres, the ends of linear chromosomes, play a major role in the maintenance of genome integrity. Telomerase or alternative lengthening of telomeres (ALT) mechanisms exist in most cancer cells in order to stabilize telomere length by the addition of telomeric repeats. Telomere loss can be dramatically mutagenic. Chromosomes lacking one telomere remain unstable until they are capped, generating chromosomal instability, gene amplification via breakage/fusion/bridge (B/F/B) cycles and resulting in chromosome imbalances. The chronology of the occurrence of gene amplification and chromosome imbalances detected in human tumors is still unknown. All of the aberrations that occur prior to, during or after activation of a telomere maintenance mechanism promote the development of cancer.

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.

Chromosome regions enriched in hyperacetylated histone H4 are preferred sites for endonuclease- and radiation-induced breakpoints

Previously, we have shown that breakpoints induced by the endonucleases AluI, BamHI and DNase I in CHO chromosomes are localized mainly in G-light bands. Neutrons and gamma rays produced similar breakpoint clusters to endonucleases in most CHO chromosomes. Here we compare endonuclease- and radiation-induced breakpoint maps with hyperacetylation patterns of histone H4. The H4 acetylation pattern in chromosomes is similar to the pattern of G-light, or R-bands, and breakpoints are clustered in highly acetylated chromosome regions. These findings indicate that chromosomal aberrations occur more frequently in active than in inactive chromatin.

Mini review: form and function in the human interphase chromosome

A key feature of interphase chromosomes is their compaction into discrete "territories" in the nucleus. In this review, we focus on the compartmentalization of the genome conferred by this organization and evaluate our current understanding of the role of large-scale chromatin folding in the regulation of gene expression. We examine evidence for the hypothesis that transcription occurs at the external surfaces of chromosomes and follow its evolution to include transcription at the surfaces of chromatin-rich domains within chromosomes. We also present prevailing views regarding the details of large-scale chromatin folding and the functional relationship between chromatin and the enigmatic nuclear matrix.

Interchromosomal distribution of gamma ray-induced chromatid aberrations in Chinese hamster ovary cells

Interchromosomal distributions of breakpoints from chromatid-type aberrations induced by gamma rays in Chinese hamster ovary (CHO) cells were analyzed. In most chromosomes the distribution was as expected from chromosome lengths for simple breaks or the respective relative corrected length in case of exchanges. There were deviations from expectation in a few chromosomes for chromatid breaks, interchanges, intra-arm intrachanges and inter-arm intrachanges. Especially interesting are the results concerning chromosomes 2 and 8, which were more often involved in exchanges than expected. An "exchange phenotype" for these chromosomes is proposed and possible explanations for the nonrandom distribution of chromosome breakpoints are presented.

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.

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.

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.

Application of laser scanning cytometry to the analysis of chromosomal aberrations induced by benzo[a]pyrene in CHO-WBLT cells

BACKGROUND

A recently developed laser scanning cytometry technique was applied to cytometric studies to detect rapidly stable chromosomal aberrations induced by a carcinogen in a Chinese hamster fibroblast cell line, CHO-WBLT.

METHODS

Individual chromosomes were collected from metaphase cells by a syringe technique and spread on slides. The DNA content of each chromosome stained with propidium iodide was measured with a laser scanning cytometer (LSC). A characteristic DNA histogram, designated as the "laser scanning karyotype (LSK)," was obtained from about 20,000 chromosomes of CHO-WBLT cells. Each chromosome was confirmed morphologically under the microscope by using a "re-location" system built into the LSC.

RESULTS

A total of 21 chromosomes, including marker chromosomes specific to the cell line, were assigned to 10 major peaks in the LSK, which was analogous to the karyotype demonstrated with the classical Q-banding technique. In contrast, clonal sublines isolated after exposure to the carcinogen benzo[a]pyrene showed LSKs different from those found in untreated control cells, and seven of 20 clones were found to be abnormal, with a small number of chromosomal translocations and/or deletions, which were confirmed by Q-banding.

CONCLUSIONS

The laser scanning cytometry technique was employed to detect stable chromosomal aberrations in CHO-WBLT cells after treatment with benzo[a]pyrene. The results obtained with this technique were comparable to those obtained by Q-banding; therefore, this method may be useful for rapid primary screening to detect stable, abnormal karyotypes induced by environmental chemicals and/or radiation.