Fluorescence in situ hybridisation for interphase chromosomal aberration-based biological dosimetry

Metaphase spreads stained with Giemsa or painted with chromosome-specific probes by fluorescence in situ hybridisation (FISH) have been in use since long for retrospective dose assessment (biological dosimetry). However, in cases of accidental exposure to ionising radiation, the culturing of lymphocytes to obtain metaphase chromosomes and analysis of chromosomal aberrations is time-consuming and problematic after high radiation doses. Similarly, analysing chromosomal damage in G0/G1 cells or nondividing cells by premature chromosome condensation is laborious. Following large-scale radiological emergencies, the time required for analysis is more important than precision of dose estimate. Painting of whole chromosomes using chromosome-specific probes in interphase nuclei by the FISH technique will eliminate the time required for cell culture and allow a fast dose estimate, provided that a meaningful dose-response can be obtained by scoring the number of chromosomal domains visible in interphase nuclei. In order to test the applicability of interphase FISH for quick biological dosimetry, whole blood from a healthy donor was irradiated with 8 Gy of gamma radiation. Irradiated whole blood was kept for 2 h at 37°C to allow DNA repair and thereafter processed for FISH with probes specific for Chromosomes-1 and 2. Damaged chromosomal fragments, distinguished by extra color domains, were observed in interphase nuclei of lymphocytes irradiated with 8 Gy. These fragments were efficiently detected and quantified by the FISH technique utilising both confocal and single plane fluorescence microscopy. Furthermore, a clear dose-response curve for interphase fragments was achieved following exposure to 0, 1, 2, 4 and 8 Gy of gamma radiation. These results demonstrate interphase FISH as a promising test for biodosimetry and for studying cytogenetic effects of radiation in nondividing cells.

Establishment of ex vivo calibration curve for X-ray induced "dicentric + ring" and micronuclei in human peripheral lymphocytes for biodosimetry during radiological emergencies, and validation with dose blinded samples

In the modern developing society, application of radiation has increased extensively. With significant improvement in the radiation protection practices, exposure to human could be minimized substantially, but cannot be avoided completely. Assessment of exposure is essential for regulatory decision and medical management as applicable. Until now, cytogenetic changes have served as surrogate marker of radiation exposure and have been extensively employed for biological dose estimation of various planned and unplanned exposures. Dicentric Chromosomal Aberration (DCA) is radiation specific and is considered as gold standard, micronucleus is not very specific to radiation and is considered as an alternative method for biodosimetry. In this study dose response curves were generated for X-ray induced "dicentric + ring" and micronuclei, in lymphocytes of three healthy volunteers [2 females (age 22, 23 years) and 1 male (24 year)]. The blood samples were irradiated with X-ray using LINAC (energy 6 MV, dose rate 6 Gy/min), in the dose range of 0-5Gy. Irradiated blood samples were cultured and processed to harvest metaphases, as per standard procedures recommended by International Atomic Energy Agency. Pooled data obtained from all the three volunteers, were in agreement with Poisson distribution for "dicentric + ring", however over dispersion was observed for micronuclei. Data ("dicentric + ring" and micronuclei) were fitted by linear quadratic model of the expression Y[bond, double bond]C + αD + βD² using Dose Estimate software, version 5.2. The data fit has resulted in linear coefficient α = 0.0006 (±0.0068) "dicentric + ring" cell^-1 Gy^-1 and quadratic coefficient β = 0.0619 (±0.0043) "dicentric + ring" cell^-1 Gy^-2 for "dicentric + ring" and linear coefficient α = 0.0459 ± (0.0038) micronuclei cell^-1 Gy^-1 and quadratic coefficient β = 0.0185 ± (0.0010) micronuclei cell^-1 Gy^-2 for micronuclei, respectively. Background frequencies for "dicentric + ring" and micronuclei were 0.0006 ± 0.0004 and 0.0077 ± 0.0012 cell^-1, respectively. Established curves were validated, by reconstructing the doses of 8 dose blinded samples (4 by DCA and 4 by CBMN) using coefficients generated here. Estimated doses were within the variation of 0.9-16% for "dicentric + ring" and 21.7-31.2% for micronuclei respectively. These established curves have potential to be employed for biodosimetry of occupational, clinical and accidental exposures, for initial triage and medical management.

Use of human lymphocyte G0 PCCs to detect intra- and inter-chromosomal aberrations for early radiation biodosimetry and retrospective assessment of radiation-induced effects

A sensitive biodosimetry tool is required for rapid individualized dose estimation and risk assessment in the case of radiological or nuclear mass casualty scenarios to prioritize exposed humans for immediate medical countermeasures to reduce radiation related injuries or morbidity risks. Unlike the conventional Dicentric Chromosome Assay (DCA), which takes about 3–4 days for radiation dose estimation, cell fusion mediated Premature Chromosome Condensation (PCC) technique in G0 lymphocytes can be rapidly performed for radiation dose assessment within 6–8 hrs of sample receipt by alleviating the need for ex vivo lymphocyte proliferation for 48 hrs. Despite this advantage, the PCC technique has not yet been fully exploited for radiation biodosimetry. Realizing the advantage of G0 PCC technique that can be instantaneously applied to unstimulated lymphocytes, we evaluated the utility of G0 PCC technique in detecting ionizing radiation (IR) induced stable and unstable chromosomal aberrations for biodosimetry purposes. Our study demonstrates that PCC coupled with mFISH and mBAND techniques can efficiently detect both numerical and structural chromosome aberrations at the intra- and inter-chromosomal levels in unstimulated T- and B-lymphocytes. Collectively, we demonstrate that the G0 PCC technique has the potential for development as a biodosimetry tool for detecting unstable chromosome aberrations (chromosome fragments and dicentric chromosomes) for early radiation dose estimation and stable chromosome exchange events (translocations) for retrospective monitoring of individualized health risks in unstimulated lymphocytes.

Assessing the applicability of FISH-based prematurely condensed dicentric chromosome assay in triage biodosimetry

The dicentric chromosome assay (DCA) has been regarded as the gold standard of radiation biodosimetry. The assay, however, requires a 2-d peripheral blood lymphocyte culture before starting metaphase chromosome analyses to estimate biological doses. Other biological assays also have drawbacks with respect to the time needed to obtain dose estimates for rapid decision on the correct line of medical treatment. Therefore, alternative technologies that suit requirements for triage biodosimetry are needed. Radiation-induced DNA double strand breaks in G0 lymphocytes can be detected as interphase chromosome aberrations by the cell fusion-mediated premature chromosome condensation (PCC) method. The method, in combination with fluorescence in situ hybridization (FISH) techniques, has been proposed in early studies as a powerful tool for obtaining biological dose estimates without 2-d lymphocyte culture procedures. The present work assesses the applicability of FISH-based PCC techniques using pan-centromeric and telomeric peptide nucleic acid (PNA) probes in triage mode biodosimetry and demonstrates that an improved rapid procedure of the prematurely condensed dicentric chromosome (PCDC) assay has the potential for evaluating exposed radiation doses in as short as 6 h after the collection of peripheral blood specimens.

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.

Investigation of new cytogenetic biomarkers specific to high-LET radiation usingin vivoandin vitroexposed human lymphocytes

PURPOSE

To find detectable cytogenetic biomarkers that can offer information about the radiation quality of in vivo exposure retrospectively.

MATERIALS AND METHODS

Chromosome-type aberrations of peripheral lymphocytes of uterine cancer patients that received internal gamma- and external X-ray therapy or carbon beam therapy and of victims severely exposed to neutrons and gamma-rays in a criticality accident that occurred in Tokai-mura, Japan were analysed. Data obtained from in vitro irradiation experiments using 60Co gamma-rays and 10 MeV neutrons were compared with the in vivo exposure data.

RESULTS

The ratio of acentric rings to dicentric chromosomes (termed RaD ratio) and that of excess fragments to dicentrics (termed EfD ratio) showed significant (p < 0.05) differences between the two groups of cancer patients, and these ratios for accidental victims were in between the values of the two groups of cancer patients. The in vitro studies using doses equivalent to 1 - 3 Gy of gamma-rays have confirmed that the EfD ratios were increased with the high LET (linear energy transfer) and RaD ratios decreased.

CONCLUSION

The present data show that the RaD and EfD ratios can be used as cytogenetic biomarkers of exposure to high-LET radiation at least within a few years of exposure.

Repair and chromosomal damage

Chromosomal aberrations in somatic cells link DNA damage with radiation-induced cell killing and individual susceptibility to oncogenesis, and are also potential markers of cancer susceptibility. While there is general acceptance that the DNA double-strand break (DSB) is the principal initiating lesion the complexity of the relationship between the induced frequency and the rates of repair and misjoining of DSB, and the production of chromosome and chromatid aberrations has led to much controversy. The principal models of chromosome aberrations are: the classical 'breakage-and-reunion' or 'breakage-first' model of Sax [Genetics 25 (1940) 41-68], the 'mis-recombination' model of Chadwick and Leenhouts [Mutat Res 404 (1998) 113-117] and the 'transcription-based' model of Radford [Int J Radiat Biol 78 (2002) 1081-1093]. Chromatid aberrations have also been variously interpreted on the 'breakage-first model', Revell's 'exchange' model [Proc R Soc B 150 (1959) 563-589] and the 'signal' model [Int J Radiat Biol 73 (1998) 243-251]. Recent evidence argues strongly for different mechanisms for chromosome (formed in G1 or Go) and chromatid (formed in G2) aberrations, i.e. there is little or no correspondence in the relative frequencies between chromosome and chromatid aberrations. The balance of evidence indicates that chromosome aberrations may be formed by a breakage-first type mechanism. Elevated frequencies of chromosomal aberrations occur to various extents in cell lines mutated in genes involved in both non-homologous DSB end-joining and homologous recombinational rejoining of DSB. Chromatid breaks, seem to be formed by a more complex mechanism since there is a lack of correspondence between the rates of DSB rejoining and chromatid break 'disappearance' (assumed by some to represent DSB repair). Thus, a model based on the dissociation of DSB rejoining from chromatid break rejoining is required to explain these data. A substantial proportion (approximately 20%) of both spontaneous and induced chromatid breaks visibly involve inter-chromatid rearrangements (determined using harlequin staining of chromatids). It is postulated that the remaining proportion may also involve rearrangements, but within a single chromatid (i.e. intra-chromatid rearrangements). Disappearance of chromatid breaks with time is postulated to result from the completion of rearrangements, i.e. rather than simply from repair of DSB.

Human and rodent cell lines showing no differences in the induction but differing in the repair kinetics of radiation-induced DNA base damage

PURPOSE

To compare the induction and repair of radiation-induced base damage in human and rodent cell lines.

MATERIAL AND METHODS

Experiments were performed with two human (normal fibroblasts HSF1 and tumour HeLa cells) and two rodent (mouse L929 and hamster CHO-K1) cell lines. Base damage was determined with the alkaline comet assay combined with the repair enzyme formamidopyrimidine-glycosylase (Fpg). Proteins were detected by Western blot.

RESULTS

The induction of Fpg-sensitive sites was measured in human and rodent cell lines for doses up to 8 or 5 Gy, respectively. Comets were analysed in terms of tail moments, which were transformed into Gy-equivalents. The amount of Fpg-sensitive sites increased linearly with doses up to 4 Gy, whereby the ratio of single-strand breaks (ssb) to Fpg-sensitive sites was nearly identical for human and rodent cells with ssb:Fpg-sensitive sites=1:0.41+/-0.07 and 1:0.45+/-0.05, respectively. For doses exceeding 4 Gy, the amount of Fpg-sensitive sites did not increase further, indicating a dose limit up to which the comet assay can be used to detect Fpg-sensitive sites. Repair of Fpg-sensitive sites was studied for an X-ray dose of 4 Gy. For all four cell lines, the repair was measured to be completed 24 h after irradiation, but with pronounced differences in the kinetics. In both rodent cell lines, 50% of Fpg-sensitive sites were removed after t((1/2))=25+/-10 min in contrast to t((1/2))=80+/-20 min in the two human cell lines. The two species also differed in the level of polymerase ss with, on average, a three- to fivefold higher level in rodent cells compared with human cells.

CONCLUSIONS

Repair of radiation-induced Fpg-sensitive sites was much faster in rodent than in human cells, which might result from the higher level of polymerase ss found in rodent cells.

Molecular mechanisms of individual radiosensitivity studied in normal diploid human fibroblasts

The molecular mechanisms of individual radiosensitivity were studied in normal diploid human fibroblasts. For fibroblasts irradiated with X-rays in G1-phase the individual radiosensitivity was shown to be correlated with the extent of double-strand break (dsb) repair. The number of residual dsbs (including both non- and mis-rejoined dsbs) varied between 2 and 5% of the initial number induced and was low for resistant and high for sensitive strains. In the G1-phase dsbs are considered to be mostly repaired via the non-homologous end-joining pathway (NHEJ). However, so far none of the parameters tested for this pathway was found to be correlated with the number of residual dsbs. The parameters tested were mRNA expression, protein level and localisation and activity of the DNA-PK, which is the central complex of NHEJ. The dsb-repair capacity is also not regulated by the differentiation status, which varies substantially among fibroblast strains, whereas there is some indication that dsb repair might depend on the chromatin structure, with more efficient repair in cells with condensed DNA. Residual dsbs are converted into lethal chromosome aberrations finally leading to the loss of clonogenic activity, when cells pass through mitosis. Beside this so-called mitotic death, X-irradiated human fibroblasts are also inactivated via the TP53-dependent permanent G1-arrest, while apoptosis appears to be not important. On average, mitotic death and G1-arrest are equally effective, but there is a broad variation from one strain to the other, with a negative correlation between these two pathways. Fibroblast strains exhibiting only a moderate G1-arrest showed a high number of lethal aberrations and vice versa. This result points to a common regulator of both G1-arrest and dsb repair, which is presently under investigation.

Reflections and meditations upon complex chromosomal exchanges

The application of FISH chromosome painting techniques, especially the recent mFISH (and its equivalents) where all 23 human chromosome pairs can be distinguished, has demonstrated that many chromosome-type structural exchanges are much more complicated (involving more "break-rejoins" and arms) than has hitherto been assumed. It is clear that we have been greatly under-estimating the damage produced in chromatin by such agents as ionising radiation. This article gives a brief historical summary of observations leading up to this conclusion, and after outlining some of the problems surrounding the formation of complex chromosomes exchanges, speculates about possible solutions currently being proposed.

Kinetics of DSB rejoining and formation of simple chromosome exchange aberrations

PURPOSE

To investigate the role of kinetics in the processing of DNA double strand breaks (DSB), and the formation of simple chromosome exchange aberrations following X-ray exposures to mammalian cells based on an enzymatic approach.

METHODS

Using computer simulations based on a biochemical approach, rate-equations that describe the processing of DSB through the formation of a DNA-enzyme complex were formulated. A second model that allows for competition between two processing pathways was also formulated. The formation of simple exchange aberrations was modelled as misrepair during the recombination of single DSB with undamaged DNA. Non-linear coupled differential equations corresponding to biochemical pathways were solved numerically by fitting to experimental data.

RESULTS

When mediated by a DSB repair enzyme complex, the processing of single DSB showed a complex behaviour that gives the appearance of fast and slow components of rejoining. This is due to the time-delay caused by the action time of enzymes in biomolecular reactions. It is shown that the kinetic- and dose-responses of simple chromosome exchange aberrations are well described by a recombination model of DSB interacting with undamaged DNA when aberration formation increases with linear dose-dependence. Competition between two or more recombination processes is shown to lead to the formation of simple exchange aberrations with a dose-dependence similar to that of a linear quadratic model.

CONCLUSIONS

Using a minimal number of assumptions, the kinetics and dose response observed experimentally for DSB rejoining and the formation of simple chromosome exchange aberrations are shown to be consistent with kinetic models based on enzymatic reaction approaches. A non-linear dose response for simple exchange aberrations is possible in a model of recombination of DNA containing a DSB with undamaged DNA when two or more pathways compete for DSB repair.

The kinetics of postirradiation chromatin restitution as revealed by chromosome aberrations detected by premature chromosome condensation and fluorescence in situ hybridization

In a study of X-ray-induced chromosome aberrations in human G(0) lymphocytes irradiated with 4 Gy using premature chromosome condensation (PCC) and fluorescence in situ hybridization (FISH), the time-dependent pattern of chromosome fragments and interchromosomal exchanges involving chromosome 4 was recorded after postirradiation incubation times varying from 0.5 to 46.5 h. Unattached acentric fragments and incomplete interchromosomal exchanges have high initial yields, followed by an exponential decrease, while complete interchromosomal exchanges have almost zero initial yield with a subsequent increase in their number. Plateau values of all yields are reached after about 25 h. This temporal variation of aberration yields can consistently be explained by the competition of disruptive PCC stress with the progress of postirradiation structural restitution at the sites of radiation-induced chromatin instabilities. Details of the temporal pattern of incomplete exchanges reflect the different kinetics of the alpha and beta components of the yield of aberrations. The observed large difference between late-PCC and metaphase yields of unattached acentric fragments and the almost perfect conversion from incomplete prematurely condensed chromosomes into complete metaphase exchanges are explained by a difference in the magnitude of chromosome condensation stress between PCC and mitotic conditions. Chromatin sites prone to fragmentation and incompleteness under conditions of PCC can therefore persist as genetic instabilities hidden during mitosis.

Kinetics of the formation of chromosome aberrations in X-irradiated human lymphocytes, using PCC and FISH

In order to study the initial frequencies and define kinetics of the formation of chromosomal exchanges in X-irradiated human lymphocytes, the premature chromosome condensation (PCC) technique was employed in combination with fluorescence in situ hybridization (FISH) with a composite probe for human chromosome 8 and a pan-centromeric probe for the whole genome. Human lymphocytes were X-irradiated (0.5, 1, 2, 3, 4 and 6 Gy), fused with mitotic Chinese hamster ovary (CHO) cells immediately or 1, 3, 6, 12 and 18 h after irradiation. Immediately after irradiation chromosomal breaks, dicentrics and translocations showed a linear dose-response. Unrejoined chromosome breaks were the most frequent types of aberrations (about 85%) observed. About 15% of total aberrations were chromosome exchanges of 65% of these were translocations and 35% were dicentrics. The chromosomal exchanges initially observed were mostly incomplete, with no complex exchanges at doses of 1 and 2 Gy, at higher doses (3-6 Gy) complex exchanges were observed and their frequencies increased with increasing post incubation time. Following different recovery times, repair kinetics of breaks for different doses of irradiation was studied. The shapes of the curves obtained for breaks as well as chromosome exchanges were linear-quadratic. The linear yield component, alpha, is formed entirely in the fast process that can be manifested in the early plateau, while component beta developed slowly in the subsequent hours. The kinetics of breaks rejoining was exponential, almost 50% of breaks rejoined after 1 h and at 18 h about 20% of breaks remained. At low doses of 1 and 2 Gy most of the exchanges were formed immediately and at higher doses, the frequency of exchanges increased with kinetics similar to that observed for the rejoining of breaks. However, the kinetics was different for different doses of irradiation. The frequency of dicentrics increased at doses above 2 Gy following 3 h recovery time, but for the translocations effect was pronounced even at 1 h recovery time. The frequency of incomplete exchanges (i.e., terminal translocations) decreased with post irradiation time and at 18 h was 30-40% less than the frequency obtained immediately after irradiation. The increase in the total translocations as a function of time between irradiation and fusion was due to a rapid increase in complete exchanges (i.e., reciprocal translocations). The frequency of ring chromosomes immediately after irradiation, also increased linearly, however, it was 3-5 times lower than dicentrics and remained almost constant in number for different doses and at different post-irradiation times.

Radiation induced chromosome aberrations: some biophysical considerations

The implications of recent results using FISH chromosome painting and soft X-ray exposures for the mechanisms of chromosome aberration formation are discussed. It is concluded that the evidence in favour of exchange aberrations arising from one radiation induced chromosome break has increased to the point where a 'change in paradigm' from the older breakage-reunion hypothesis needs to be taken seriously into account. A potential role for recombinational repair of DNA double strand breaks, as known in yeast, in the formation of aberrations in mammalian cells is presented and the relationship between DNA repair studies and radiation cytology is emphasized.

Relationship between PCC fragments and cell killing studied in X-irradiated CHO, CHO-K1 cells and two radiosensitive mutants xrs1 and xrs5

PURPOSE

To investigate the correlation between PCC fragments and cell killing.

MATERIALS AND METHODS

Induction and repair of DNA fragments were measured in CHO, CHO-K1, xrs1 and xrs5 cells using the premature chromosome condensation (PCC) technique and cell survival was determined by a colony assay.

RESULTS

The number of PCC fragments measured in cells immediately fused after X-irradiation was the same for CHO (3.4 +/- 0.16/cell/Gy) and CHO-K1 (3.6 +/- 0.12/cell/Gy) cells but significantly higher for xrs1 (4.9 +/- 0.07/cell/Gy) and xrs5 cells (7.0 +/- 0.4/cell/Gy). The repair curve of PCC fragments studied for CHO, CHO-K1 and xrs5 cells was best described by a monophasic exponential decline with a final plateau; the half-time of this decline was always about 30 min. The number of unrejoined PCC fragments, which was measured 14h after irradiation, increased linearly with dose. The steepest increase was found for xrs5 cells (5.5 +/- 0.3 fragments per cell and per Gy), the lowest for CHO/CHO-K1 (0.9 +/- 0.1; 1.0 +/- 0.1) and for xrs1 in between (3.3 +/- 0.1). For all four cell lines the relationship between cell killing and unrejoined fragments could be described by a single curve with a D0 of 2.5 +/- 0.4 unrejoined PCC fragments per lethal event.

CONCLUSIONS

The data showed that the number of unrejoined PCC fragments can be used as an indicator of cellular radiosensitivity.

The link between low-LET dose-response relations and the underlying kinetics of damage production/repair/misrepair

PURPOSE

To review current opinion on the production and temporal evolution of low-LET radiobiological damage.

METHODS

Standard cell survival models which model repair/misrepair kinetics in order to quantify dose-response relations and dose-protraction effects are reviewed and interrelated. Extensions of the models to endpoints other than cell survival, to multiple or compound damage processing pathways, and to stochastic intercellular damage fluctuations are surveyed. Various molecular mechanisms are considered, including double strand breaks restitution and binary misrepair.

CONCLUSIONS

(1) Linking dose-response curves to the underlying damage production/processing kinetics allows mechanistic biological interpretations of observed curve parameters. (2) Various damage processing pathways, with different kinetics, occur. (3) Almost every current kinetic model, whether based on binary misrepair or saturable repair, leads at low or intermediate doses to the LQ (linear-quadratic) formalism, including the standard (generalized Lea-Catcheside) dependence on dose protraction. (4) Two-track (beta) lethal damage is largely due to dicentric chromosome aberrations, but one-track (alpha) lethal damage is largely caused by other mechanisms such as point mutations in a vital gene, small deletions, residual chromosome breaks, induced apoptosis, etc. (5) A major payoff for 50 years of radiobiological modelling is identifying molecular mechanisms which underly the broadly applicable LQ formalism.

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

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

Comparative study of the repair kinetics of chromosomal aberrations and DNA strand breaks in proliferating and quiescent CHO cells

Repair kinetics observable at the level of exchange-type chromosomal aberrations (dicentric chromosomes), using fractionation and delayed-plating techniques, have been compared with repair kinetics of radiation-induced DNA double-strand breaks, measured with PFGE, and with repair kinetics of all strand breaks, measured with the alkali-unwinding technique. Only data from quiescent or proliferating CHO K1 cells obtained in the same laboratory were used. We determined repair kinetics in terms of the time constant tau (equal to half-time/log(e)2). The repair kinetics (tau approximately 11-14 min) observed in the split-dose formation of dicentric chromosomes agrees with fast repair kinetics of double-strand breaks (tau approximately 11-13 min), thus permitting us to identify the latter as the 'primary lesions' whose pairwise interaction leads to the beta D2 yield term of the aberrations. The repair kinetics observed for dicentric chromosomes formed under delayed-plating conditions (tau approximately 75 min), which mainly affects the alpha D yield term, is attributed to an intermediate interchromosomal product temporarily existing in the course of aberration formation; it is suggested that this product is mechanistically correlated with the slow repair kinetics of 'clustered damage' to DNA seen with the applied molecular methods (tau approximately 90 min).

Modelling the kinetics of chromosome exchange formation in human cells exposed to ionising radiation

A biophysical model has been applied to study the kinetics of chromosome exchange formation in human cells. Chromosomal exchange induction (for example dicentrics) by ionising radiation was modelled by means of the Monte Carlo technique. This involved energy deposition by electrons, production of chromosomal breaks (assumed to be DNA double-strand breaks) and their repair and exchange. Exchanges were assumed to result from pairwise interaction between two DNA breaks in a distance-dependent manner. The rate at which exchanges are formed was found to depend upon how the exchange to no-exchange probability ratio varied with time. The assumption that this ratio did not alter with time produced a time constant for the formation of exchanges which was exactly half that of the repair time constant. Longer time constants could not be accommodated unless the probability ratio for exchange increases with time. Different time constants for inter- and intratrack exchanges could be achieved on the basis of DNA double-strand break separation.