Molecular mechanisms involved in the production of chromosomal aberrations. II. Utilization of Neurospora endonuclease for the study of aberration production by X-rays in G1 and G2 stages of the cell cycle

Utilization of cytogenetic biomarkers as a tool for assessment of radiation injury and evaluation of radiomodulatory effects of various medicinal plants - a review

Systematic biological measurement of “cytogenetic endpoints” has helped phenomenally in assessment of risks associated with radiation exposure. There has been a surge in recent times for the usage of radioactive materials in health care, agriculture, industrial, and nuclear power sectors. The likelihood of radiation exposure from accidental or occupational means is always higher in an overburdened ecosystem that is continuously challenged to meet the population demands. Risks associated with radiation exposure in this era of modern industrial growth are minimal as international regulations for maintaining the safety standards are stringent and strictly adhered to, however, a recent disaster like “Fukushima” impels us to think beyond. The major objective of radiobiology is the development of an orally effective radio-modifier that provides protection from radiation exposure. Once available for mass usage, these compounds will not only be useful for providing selective protection against accidental and occupational radiation exposure but also help to permit use of higher doses of radiation during treatment of various malignancies curtailing unwarranted adverse effects imposed on normal tissues. Bio-active compounds isolated from natural sources enriched with antioxidants possess unique immune-modulating properties, thus providing a double edged benefit over synthetic radioprotectors. We aim to provide here a comprehensive overview of the various agents originating from plant sources that portrayed promising radioprotection in various experimental models with special emphasis on studies that used cytogenetic biomarkers. The agents will include crude extracts of various medicinal plants, purified fractions, and herbal preparations.

Edaravone protects human peripheral blood lymphocytes from γ-irradiation-induced apoptosis and DNA damage

Radiation-induced cellular injury is attributed primarily to the harmful effects of free radicals, which play a key role in irradiation-induced apoptosis. In this study, we investigated the radioprotective efficacy of edaravone, a licensed clinical drug and a powerful free radical scavenger that has been tested against γ-irradiation-induced cellular damage in cultured human peripheral blood lymphocytes in studies of various diseases. Edaravone was pre-incubated with lymphocytes for 2 h prior to γ-irradiation. It was found that pretreatment with edaravone increased cell viability and inhibited generation of γ-radiation-induced reactive oxygen species (ROS) in lymphocytes exposed to 3 Gy γ-radiation. In addition, γ-radiation decreased antioxidant enzymatic activity, such as superoxide dismutase and glutathione peroxidase, as well as the level of reduced glutathione. Conversely, treatment with 100 μM edaravone prior to irradiation improved antioxidant enzyme activity and increased reduced glutathione levels in irradiated lymphocytes. Importantly, we also report that edaravone reduced γ-irradiation-induced apoptosis through downregulation of Bax, upregulation of Bcl-2, and consequent reduction of the Bax:Bcl-2 ratio. The current study shows edaravone to be an effective radioprotector against γ-irradiation-induced cellular damage in lymphocytes in vitro. Finally, edaravone pretreatment significantly reduced DNA damage in γ-irradiated lymphocytes, as measured by comet assay (% tail DNA, tail length, tail moment, and olive tail moment) (p < 0.05). Thus, the current study indicates that edaravone offers protection from radiation-induced cytogenetic alterations.

Modulation of radiation-induced and mitomycin C-induced chromosome damage by apigenin in human lymphocytes in vitro

Apigenin (APG), a flavone, is known to exhibit antioxidant, antimutagenic and antitumorigenic activity, both in vivo and in vitro. The aim of this study is to investigate the modulatory effects of APG on human lymphocytes after irradiation with gamma rays (3 Gy) or treatment with the antineoplastic agent, mitomycin C (MMC), in vitro. Cytogenetic biomarkers such as chromosome aberrations (CAs), sister chromatid exchanges (SCEs) and cytochalasin-B blocked micronuclei (CBMN), were studied in blood lymphocytes treated with radiation, or antineoplastic agent (MMC), and APG. Whole blood lymphocytes were cultured in vitro using a standard protocol. No significant differences were found in the frequency of CAs or micronuclei (MN) in human peripheral blood lymphocytes irradiated with gamma rays (3 Gy) and then post-treated with APG. There was an increase in the frequency of SCEs per cell in APG-treated samples compared with the controls. Lymphocytes treated with MMC in the presence of APG exhibited a significant decrease (P < 0.01) in the frequency of SCEs compared with MMC treatment alone. The data for the MN test indicated that APG treatment significantly reduced (P < 0.01) the frequency of MMC-induced MN.

Interference in DNA replication can cause mitotic chromosomal breakage unassociated with double-strand breaks

Morphological analysis of mitotic chromosomes is used to detect mutagenic chemical compounds and to estimate the dose of ionizing radiation to be administered. It has long been believed that chromosomal breaks are always associated with double-strand breaks (DSBs). We here provide compelling evidence against this canonical theory. We employed a genetic approach using two cell lines, chicken DT40 and human Nalm-6. We measured the number of chromosomal breaks induced by three replication-blocking agents (aphidicolin, 5-fluorouracil, and hydroxyurea) in DSB-repair-proficient wild-type cells and cells deficient in both homologous recombination and nonhomologous end-joining (the two major DSB-repair pathways). Exposure of cells to the three replication-blocking agents for at least two cell cycles resulted in comparable numbers of chromosomal breaks for RAD54^−/−/KU70^−/− DT40 clones and wild-type cells. Likewise, the numbers of chromosomal breaks induced in RAD54^−/−/LIG4^−/− Nalm-6 clones and wild-type cells were also comparable. These data indicate that the replication-blocking agents can cause chromosomal breaks unassociated with DSBs. In contrast with DSB-repair-deficient cells, chicken DT40 cells deficient in PIF1 or ATRIP, which molecules contribute to the completion of DNA replication, displayed higher numbers of mitotic chromosomal breaks induced by aphidicolin than did wild-type cells, suggesting that single-strand gaps left unreplicated may result in mitotic chromosomal breaks.

Biological complexities in radiation carcinogenesis and cancer radiotherapy: impact of new biological paradigms

Although radiation carcinogenesis has been shown both experimentally and epidemiologically, the use of ionizing radiation is also one of the major modalities in cancer treatment. Various known cellular and molecular events are involved in carcinogenesis. Apart from the known phenomena, there could be implications for carcinogenesis and cancer prevention due to other biological processes such as the bystander effect, the abscopal effect, intrinsic radiosensitivity and radioadaptation. Bystander effects have consequences for mutation initiated cancer paradigms of radiation carcinogenesis, which provide the mechanistic justification for low-dose risk estimates. The abscopal effect is potentially important for tumor control and is mediated through cytokines and/or the immune system (mainly cell-mediated immunity). It results from loss of growth and stimulatory and/or immunosuppressive factors from the tumor. Intrinsic radiosensitivity is a feature of some cancer prone chromosomal breakage syndromes such as ataxia telangectiasia. Radiosensitivity is manifested as higher chromosomal aberrations and DNA repair impairment is now known as a good biomarker for breast cancer screening and prediction of prognosis. However, it is not yet known whether this effect is good or bad for those receiving radiation or radiomimetic agents for treatment. Radiation hormesis is another major concern for carcinogenesis. This process which protects cells from higher doses of radiation or radio mimic chemicals, may lead to the escape of cells from mitotic death or apoptosis and put cells with a lower amount of damage into the process of cancer induction. Therefore, any of these biological phenomena could have impact on another process giving rise to genome instability of cells which are not in the field of radiation but still receiving a lower amount of radiation. For prevention of radiation induced carcinogenesis or risk assessment as well as for successful radiation therapy, all these phenomena should be taken into account.

Mycobacterium tuberculosis Ku can bind to nuclear DNA damage and sensitize mammalian cells to bleomycin sulfate

Radiotherapy and chemotherapy are effective cancer treatments due to their ability to generate DNA damage. The major lethal lesion is the DNA double-strand break (DSB). Human cells predominantly repair DSBs by non-homologous end joining (NHEJ), which requires Ku70, Ku80, DNA-PKcs, DNA ligase IV and accessory proteins. Repair is initiated by the binding of the Ku heterodimer at the ends of the DSB and this recruits DNA-PKcs, which initiates damage signaling and functions in repair. NHEJ also exists in certain types of bacteria that have dormant phases in their life cycle. The Mycobacterium tuberculosis Ku (Mt-Ku) resembles the DNA-binding domain of human Ku but does not have the N- and C-terminal domains of Ku70/80 that have been implicated in binding mammalian NHEJ repair proteins. The aim of this work was to determine whether Mt-Ku could be used as a tool to bind DSBs in mammalian cells and sensitize cells to DNA damage. We generated a fusion protein (KuEnls) of Mt-Ku, EGFP and a nuclear localization signal that is able to perform bacterial NHEJ and hence bind DSBs. Using transient transfection, we demonstrated that KuEnls is able to bind laser damage in the nucleus of Ku80-deficient cells within 10 sec and remains bound for up to 2 h. The Mt-Ku fusion protein was over-expressed in U2OS cells and this increased the sensitivity of the cells to bleomycin sulfate. Hydrogen peroxide and UV radiation do not predominantly produce DSBs and there was little or no change in sensitivity to these agents. Since in vitro studies were unable to detect binding of Mt-Ku to DNA-PKcs or human Ku70/80, this work suggests that KuEnls sensitizes cells by binding DSBs, preventing human NHEJ. This study indicates that blocking or decreasing the binding of human Ku to DSBs could be a method for enhancing existing cancer treatments.

Protective effect of Adhatoda vascia Nees against radiation-induced damage at cellular, biochemical and chromosomal levels in Swiss albino mice

Extract of Adhatoda vasica (L) Nees leaves has been used for treatment of various diseases and disorders in Ayurved and Unani medicine. Modulatory effect of ethanolic extract of A. vasica (L) Nees against radiation-induced changes in terms of histological alterations in testis, reduced glutathione (GSH), lipid peroxidation (LPO), acid and alkaline phosphatases levels, and chromosomal alterations in Swiss albino mice was studied at various post-irradiation intervals between 1 and 30 days. Mice exposed to 8 Gy radiation showed radiation-induced sickness including marked changes in histology of testis and chromosomal aberrations in bone marrow cells with 100% mortality within 22 days. When ethanolic leaf extract of A. vasica was given orally at a dose of 800 mg kg⁻¹ body weight per mouse for 15 consecutive days and then exposed to radiation, death of Adhatoda-pretreated irradiated mice was reduced to 70% at 30 days. The radiation dose reduction factor was 1.43. There was significantly lesser degree of damage to testis tissue architecture and various cell populations including spermatogonia, spermatids and Leydig cells. Correspondingly, a significant decrease in the LPO and an increase in the GSH levels were observed in testis and liver of Adhatoda-pretreated irradiated mice. Similarly, a significant decrease in level of acid phosphatase and increase in level of alkaline phosphatase were observed. Adhatoda pretreatment significantly prevented radiation-induced chromosomal damage in bone marrow cells. The study suggests that Adhatoda plant extract has significant radioprotective effects on testis that warrants further mechanistic studies aimed at identifying the role of major ingredients in the extract.

Chromatin dynamics during cell cycle mediate conversion of DNA damage into chromatid breaks and affect formation of chromosomal aberrations: biological and clinical significance

The formation of diverse chromosomal aberrations following irradiation and the variability in radiosensitivity at different cell-cycle stages remain a long standing controversy, probably because most of the studies have focused on elucidating the enzymatic mechanisms involved using simple DNA substrates. Yet, recognition, processing and repair of DNA damage occur within the nucleoprotein complex of chromatin which is dynamic in nature, capable of rapid unfolding, disassembling, assembling and refolding. The present work reviews experimental work designed to investigate the impact of chromatin dynamics and chromosome conformation changes during cell-cycle in the formation of chromosomal aberrations. Using conventional cytogenetics and premature chromosome condensation to visualize interphase chromatin, the data presented support the hypothesis that chromatin dynamic changes during cell-cycle are important determinants in the conversion of sub-microscopic DNA lesions into chromatid breaks. Consequently, the type and yield of radiation-induced chromosomal aberrations at a given cell-cycle-stage depends on the combined effect of DNA repair processes and chromatin dynamics, which is cell-cycle-regulated and subject to up- or down-regulation following radiation exposure or genetic alterations. This new hypothesis is used to explain the variability in radiosensitivity observed at various cell-cycle-stages, among mutant cells and cells of different origin, or among different individuals, and to revisit unresolved issues and unanswered questions. In addition, it is used to better understand hypersensitivity of AT cells and to provide an improved predictive G2-assay for evaluating radiosensitivity at individual level. Finally, experimental data at single cell level obtained using hybrid cells suggest that the proposed hypothesis applies only to the irradiated component of the hybrid.

Quantitative, noninvasive imaging of radiation-induced DNA double-strand breaks in vivo

DNA double-strand breaks (DSB) are a major form of DNA damage and a key mechanism through which radiotherapy and some chemotherapeutic agents kill cancer cells. Despite its importance, measuring DNA DSBs is still a tedious task that is normally carried out by gel electrophoresis or immunofluorescence staining. Here, we report a novel approach to image and quantify DSBs in live mammalian cells through bifragment luciferase reconstitution. N- and C-terminal fragments of firefly luciferase genes were fused with H2AX and MDC1 genes, respectively. Our strategy was based on the established fact that at the sites of DSBs, H2AX protein is phosphoryated and physically associates with the MDC1 protein, thus bringing together N- and C-luciferase fragments and reconstituting luciferase activity. Our strategy allowed serial, noninvasive quantification of DSBs in cells irradiated with X-rays and (56)Fe ions. Furthermore, it allowed for the evaluation of DSBs noninvasively in vivo in irradiated tumors over 2 weeks. Surprisingly, we detected a second wave of DSB induction in irradiated tumor cells days after radiation exposure in addition to the initial rapid induction of DSBs. We conclude that our new split-luciferase-based method for imaging γ-H2AX-MDC1 interaction is a powerful new tool to study DSB repair kinetics in vivo with considerable advantage for experiments requiring observations over an extended period of time.

BRCA1 role in the mitigation of radiotoxicity and chromosomal instability through repair of clustered DNA lesions

Oxidatively-induced clustered DNA lesions are considered the signature of any ionizing radiation like the ones human beings are exposed daily from various environmental sources (medical X-rays, radon, etc.). To evaluate the role of BRCA1 deficiencies in the mitigation of radiation-induced toxicity and chromosomal instability we have used two human breast cancer cell lines, the BRCA1 deficient HCC1937 cells and as a control the BRCA1 wild-type MCF-7 cells. As an additional control for the DNA damage repair measurements, the HCC1937 cells with partially reconstituted BRCA1 expression were used. Since clustered DNA damage is considered the signature of ionizing radiation, we have measured the repair of double strand breaks (DSBs), non-DSB bistranded oxidative clustered DNA lesions (OCDLs) as well as single strand breaks (SSBs) in cells exposed to radiotherapy-relevant γ-ray doses. Parallel measurements were performed in the accumulation of chromatid and isochromatid breaks. For the measurement of OCDL repair, we have used a novel adaptation of the denaturing single cell gel electrophoresis (Comet assay) and pulsed field gel electrophoresis with Escherichia coli repair enzymes as DNA damage probes. Independent monitoring of the γ-H2AX foci was also performed while metaphase chromatid lesions were measured as an indicator of chromosomal instability. HCC1937 cells showed a significant accumulation of all types of DNA damage and chromatid breaks compared to MCF-7 while BRCA1 partial expression contributed significantly in the overall repair of OCDLs. These results further support the biological significance of repair resistant clustered DNA damage leading to chromosomal instability. The current results combined with previous findings on the minimized ability of base clusters to induce cell death (mainly induced by DSBs), enhance the potential association of OCDLs with breast cancer development especially in the case of a BRCA1 deficiency leading to the survival of breast cells carrying a high load of unrepaired DNA damage clusters.

Mechanisms of the formation of radiation-induced chromosomal aberrations

Although much is now known about the mechanisms of radiation-induction of DNA double-strand breaks (DSB), there is less known about the conversion of DSB into chromosomal aberrations. In particular the induction and 'rejoining' of chromatid breaks has been a controversial topic for many years. However, its importance becomes clear in the light of the wide variation in the chromatid break response of human peripheral blood lymphocytes from different individuals when exposed to ionizing radiation, and the elevation of the frequency of radiation-induced chromatid breaks in stimulated peripheral blood lymphocytes of around 40% of breast cancer cases. A common assumption has been that chromatid breaks are merely expansions of initiating DSB, although the classic 'breakage-first' hypothesis (Sax, Ref. 44) was already challenged in the 50's by Revell [30] who maintained that chromatid breaks were formed as a result of an incomplete exchange process initiated by two interacting lesions of an unspecified nature. Here we argue that both these models of chromatid break formation are flawed and we suggest an alternative hypothesis, namely that a radiation-induced DSB initiates an indirect mechanism leading to a chromatid break. This mechanism we suggest involves the nuclear enzyme topoisomerase IIalpha and we present evidence from topoisomerase IIalpha expression variant human cell lines and from siRNA treatment of human cells that supports this hypothesis.

2-Deoxy-d-glucose Inhibits Rejoining of Radiation-induced DNA Double-strand Breaks in Yeast

Effects of 2-deoxy-D-glucose (2-DG) on radiation-induced DNA double-strand breaks (dsb) have been studied under non-growth conditions in a respiratory-deficient strain of the yeast Saccharomyces cerevisiae. Velocity sedimentation in neutral sucrose gradients was used to measure DNA dsb. Addition of 2-DG to the liquid-holding medium (67 mM phosphate buffer, pH 5, 30 degrees C) at an equimolar concentration with glucose (50 mM) reduced the rate and extent of dsb rejoining. The inhibition of rejoining mediated by 2-DG is reversible for the majority--but not all--of the radiation-induced dsb.

The type and yield of ionising radiation induced chromosomal aberrations depend on the efficiency of different DSB repair pathways in mammalian cells

In order to evaluate the relative role of two major DNA double strand break repair pathways, i.e., non-homologous end joining (NHEJ) and homologous recombination repair (HRR), CHO mutants deficient in these two pathways and the parental cells (AA8) were X-irradiated with various doses. The cells were harvested at different times after irradiation, representing G2, S and G1 phase at the time of irradiation, The mutant cell lines used were V33 (NHEJ deficient), Irs1SF, 51-D1 (HRR deficient). In addition to parental cell line (AA8), a revertant of V33, namely V33-155 was employed. Both types of mutant cells responded with increased frequencies of chromosomal aberrations at all recovery times in comparison to the parental and revertant cells. Mutant cells deficient in NHEJ were more sensitive in all cell stages in comparison to HRR deficient mutant cells, indicating NHEJ is the major repair pathway for DSB repair through out the cell cycle. Both chromosome and chromatid types of exchange aberrations were observed following G1 irradiation (16 and 24 h recovery). Interestingly, configurations involving both chromosome (dicentrics) and chromatid exchanges were encountered in G1 irradiated V33 cells. This may indicate that unrepaired DSBs accumulate in G1 in these mutant cells and carried over to S phase, where they are repaired by HRR or other pathways such as B-NHEJ (back up NHEJ), which appear to be highly error prone. Both NHEJ and HRR, which share some of the same proteins in their pathways, are involved in the repair of DSBs leading to chromosomal aberrations, but with a major role of NHEJ in all stages of cell cycle.

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.

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.

Progress towards understanding the nature of chromatid breakage

The wide range of sensitivities of stimulated T-cells from different individuals to radiation-induced chromatid breakage indicates the involvement of several low penetrance genes that appear to link elevated chromatid breakage to cancer susceptibility. The mechanisms of chromatid breakage are not yet fully understood. However, evidence is accumulating that suggests chromatid breaks are not simply expanded DNA double-strand breaks (DSB). Three models of chromatid breakage are considered. The classical breakage-first and the Revell "exchange" models do not accord with current evidence. Therefore a derivative of Revell's model has been proposed whereby both spontaneous and radiation-induced chromatid breaks result from DSB signaling and rearrangement processes from within large looped chromatin domains. Examples of such rearrangements can be observed by harlequin staining whereby an exchange of strands occurs immediately adjacent to the break site. However, these interchromatid rearrangements comprise less than 20% of the total breaks. The rest are thought to result from intrachromatid rearrangements, including a very small proportion involving complete excision of a looped domain. Work is in progress with the aim of revealing these rearrangements, which may involve the formation of inversions adjacent to the break sites. It is postulated that the disappearance of chromatid breaks with time results from the completion of such rearrangements, rather than from the rejoining of DSB. Elevated frequencies of chromatid breaks occur in irradiated cells with defects in both nonhomologous end-joining (NHEJ) and homologous recombination (HR) pathways, however there is little evidence of a correlation between reduced DSB rejoining and disappearance of chromatid breaks. Moreover, at least one treatment which abrogates the disappearance of chromatid breaks with time leaves DSB rejoining unaffected. The I-SceI DSB system holds considerable promise for the elucidation of these mechanisms, although the break frequency is relatively low in the cell lines so far derived. Techniques to study and improve such systems are under way in different cell lines. Clearly, much remains to be done to clarify the mechanisms involved in chromatid breakage, but the experimental models are becoming available with which we can begin to answer some of the key questions.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Chromosome aberrations: past, present and future

Spontaneous and induced chromosome aberrations have been studied over more than a century. The resolution of detection of aberrations has depended on the improvement of available techniques. An overview on the major high lights in this area of research, from the time of solid staining to fluorescence in situ hybridization technique is presented in this review.

Cloning and molecular characterization of the rat CB2 cannabinoid receptor

The rat peripheral cannabinoid receptor (rCB2) was cloned from a Sprague-Dawley rat spleen cDNA library and when translated, encodes a protein of 410 amino acids. Alignment of rCB2 with mouse (mCB2) and human (hCB2) peripheral cannabinoid receptors reveals a high degree of homology except in the carboxy terminus where rCB2 is 50 and 63 residues longer than hCB2 and mCB2, respectively. PCR screening and sequencing of rat genomic DNA showed that rCB2 is encoded by three exons interrupted by two introns, one of which is polymorphic and contains a 209 base pair B2 (SINE) element. By Northern hybridization and ribonuclease protection assay (RPA), rCB2 mRNA was detected in rat spleen, testis, thymus and lung but not in rat brain, heart, kidney or liver. Like hCB2 and mCB2 receptors, rCB2 activates mitogen-activated protein kinase when it is stably expressed in Chinese Hamster Ovary (CHO) cells. The importance of the carboxy terminus in regulating CB2 receptor desensitization and internalization is well-established. Thus, the profound differences identified in this region of the CB2 receptor between species mandates caution when extrapolating experimental results from non-human models to the effects of chronic CB2 receptor stimulation in humans.

Radiation-induced chromatid breaks and deficient DNA repair in cancer predisposition

Deficient repair of DNA double-strand breaks, resulting in an abnormally high frequency of chromatid breaks after G(2) exposure of cells to radiation, appears to be associated with cancer predisposition. Unrepaired DNA strand breaks contribute to genomic instability. Unrepaired chromatid breaks representing DNA strand breaks can result in chromosome deletions, translocations and gene amplifications seen in human cancers. This cytogenetic response of cells to radiation may be useful as a marker of cancer susceptibility and in identifying individuals at risk of developing cancer in cancer families.

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.

Involvement of G2-dependent DNA double-strand break repair in the formation of ultraviolet light B-induced chromosomal aberrations

Wortmannin, an inhibitor of DNA double-strand break (DSB) repair added 19 h before harvest enhanced the incidence of ultraviolet light B (UVB)-induced chromatid aberrations in Chinese hamster V79 cells. Posttreatment with wortmannin for last 3 h of culture also enhanced the yield of breakage-type chromatid aberrations and suppressed the yield of exchange-type chromatid aberrations almost completely. Thus, the inhibition of DSB repair in the G2 phase stimulated the breakage-type aberration formation, while suppressing the exchange-type aberration formation. We propose the model of UVB-induced chromatid-type aberration formation which might be fully related to G2-dependent DSB repair pathway.

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.

DNA double-strand breaks, chromosomal rearrangements, and genomic instability

DNA double-strand breaks can lead to chromosomal rearrangements at the first mitosis after exposure to the DNA strand-breaking agent. The evidence suggests a number of different pathways for DNA double-strand break rejoining in mammalian cells, but it is unclear what factors determine the fate of the induced break and whether or not it will lead to chromosomal rearrangement. If a cell does survive and proliferate after DNA cleavage, delayed chromosomal instability can be observed in the clonal descendants of the exposed cell. Most, but not all DNA double-strand breaking agents are effective at inducing this delayed chromosomal instability. In this paper, we review the evidence for the role of the DNA double-strand break in directly induced and delayed chromosomal rearrangements.

The adaptive response in radiobiology: evolving insights and implications

The first of the regularly reproducible experiments to show that very low doses of ionizing radiation, like very low doses of chemical agents, could induce mechanisms whereby cells become better fit to cope with subsequent exposures to high doses were carried out on the induction of chromosome aberrations in cultures of human lymphocytes. If cells that had been exposed to a very low dose (1 cGy) of X rays were subsequently exposed to a relatively high dose (1 Gy), approximately half as many chromosome breaks were induced. Subsequent experiments showed that this adaptive response to low doses requires a certain minimal dose before it becomes active; occurs only within a relatively small window of dose; is dose-rate dependent; and depends on the genetic constitution of the people or animals exposed, with some being unresponsive. It was further shown that the response to the low-dose preexposure was not instantaneous but took approximately 4 to 6 hr to become fully active, and could be prevented if during this period protein synthesis was inhibited, i.e., a necessary protein (enzyme) was being induced. In fact, subsequent experiments with two-dimensional gel electrophoresis showed new proteins in cells irradiated with 1 to 2 cGy. The adaptation induced by low doses of radiation was therefore attributed to the induction of a novel efficient chromosome break repair mechanism that if active at the time of challenge with high doses would lead to less residual damage. This hypothesis was strengthened by a series of experiments in which it was found that inhibitors of poly(ADP-ribose)polymerase, an enzyme implicated in DNA strand break rejoining, could prevent the adaptive response. Although the phenomenon is well established in cellular systems, it is still problematical as to whether or not it will have any utility in establishing risks of ionizing radiation to humans. Newer experiments have now been carried out on the mechanisms underlying the effect and whether or not the effect can manifest itself as a decrease in the number of induced cancers and radiation-induced mortality. Experiments with restriction enzymes now indicate that double-strand breaks in DNA can be triggering events in adaptation. In addition, preliminary experiments on the survival of whole-body irradiated mice have shown that multiple exposures to low adapting doses can have profound effects on survival, and other experiments have shown that adaptation can affect the induction of thymic lymphoma in irradiated mice. It therefore appears that the initial experiments behind the adaptive response have led to a vigorous worldwide effort to understand the basic mechanisms behind it. This effort is stimulated both by a desire to understand the basic cell biology behind the response and a desire to see if indeed this phenomenon affects the estimation of risks of low-level radiation exposure.

Decreased sensitivity to the cytogenetic effects of bleomycin in individuals occupationally exposed to ionizing radiation

In the present work, 12 individuals occupationally exposed to ionizing radiation and 11 unexposed ones were studied to determine the cytogenetic effect of a challenge dose of bleomycin on their phytohemagglutinin stimulated lymphocytes. After bleomycin treatment, the frequencies of chromatid breaks and gaps were significantly lower in the exposed population (p < 0.025 for both types of chromatid alterations). These results could indicate that occupational exposure to ionizing radiation can induce an adaptive response that can be detected by a subsequent treatment with bleomycin.

Deficient DNA repair capacity, a predisposing factor in breast cancer

Women with breast cancer and a family history of breast cancer and some with sporadic breast cancer are deficient in the repair of radiation-induced DNA damage compared with normal donors with no family history of breast cancer. DNA repair was measured indirectly by quantifying chromatid breaks in phytohaemagglutinin (PHA)-stimulated blood lymphocytes after either X-irradiation or UV-C exposure, with or without post treatment with the DNA repair inhibitor, 1-beta-D-arabinofuranosylcytosine (ara-C). We have correlated chromatid breaks with unrepaired DNA strand breaks using responses to X-irradiation of cells from xeroderma pigmentosum patients with well-characterised DNA repair defects or responses of repair-deficient mutant Chinese hamster ovary (CHO) cells with or without transfected human DNA repair genes. Deficient DNA repair appears to be a predisposing factor in familial breast cancer and in some sporadic breast cancers.

Cytogenetic responses to G2 phase x-irradiation of cells from retinoblastoma patients

Fibroblast cell lines from 20 retinoblastoma (RB) patients with the hereditary bilateral form of disease compared with 16 lines from normal donors had a significantly higher chromatid aberration frequency (CAF), and more displaced and nondisplaced breaks per 100 metaphase cells after x-irradiation during the G2 phase of the cell cycle. The mean CAF was 39 +/- 1.0, range 30-46, for cells from normal subjects, compared to a mean of 245.6, range 101-506, for cells from hereditary RB patients (p < 10(-6). Of fibroblast lines from eight patients with unilateral RB, four had a CAF comparable to that of lines from normal donors (< 60) and four had a high CAF (> 130), resembling that of hereditary forms; two of the latter four lines were from patients with familial or deletion 13 forms of RB. Furthermore, in two families, PHA-stimulated blood lymphocytes from RB patients, one bilateral and one unilateral, and from certain unaffected first-degree relatives after G2 phase X-irradiation had a high CAF (> or = 110) compared to a CAF (> or = 53) of cells from three normal donors sampled at the same time. These results were shown not to be related to differences in cell cycle progression or initial extent of chromatid damage. The results suggest that the high frequency of chromatid aberrations in the cells from hereditary RB patients results from a genetic deficiency in DNA repair.

DNA double strand break production and rejoining in V79 cells irradiated with light ions

Low energy protons and other densely ionizing light ions are known to have RBE>1 for cellular end points relevant for stochastic and deterministic effects. The occurrence of a close relationship between them and induction of DNA dsb is still a matter of debate. We studied the production of DNA dsb in V79 cells irradiated with low energy protons having LET values ranging from 11 to 31 keV/micrometer, i.e. in the energy range characteristic of the Bragg peak, using the sedimentation technique. We found that the initial yield of dsb is quite insensitive to proton LET and not significantly higher than that observed with X-rays, in agreement with recent data on V79 cells irradiated with alpha particles of various LET up to 120 keV/micrometer. By contrast, RBE for cell inactivation and for mutation induction rises with the proton LET. In experiments aimed at evaluating the rejoining of dsb after proton irradiation we found that the amount of dsb left unrepaired after 120 min incubation is higher for protons than for sparsely ionizing radiation. These results indicate that dsb are not homogeneous with respect to repair and give support to the hypothesis that increasing LET leads to an increase in the complexity of DNA lesions with a consequent decrease in their repairability.

0.3 keV carbon K ultrasoft X-rays are four times more effective than gamma-rays when inducing oncogenic cell transformation at low doses

Oncogenic transformation and inactivation were investigated in C3H10T1/2 mouse embryo fibroblasts exposed to proton-induced 0.28 keV carbon K (CK)-characteristic X-rays and 60Co gamma-rays as reference radiation at high dose-rate (2-3 and 0.7 Gy/min respectively). Both oncogenic cell transformation and cell inactivation followed a linear-quadratic relationship with dose. At low doses where the linear component dominates CK ultrasoft X-rays were more effective, by a factor of 4, at inducing oncogenic cell transformation and cell inactivation compared with 60Co gamma-rays. For both endpoints the RBE of CK ultrasoft X-rays gradually decreased with increasing dose mainly due to the greater quadratic component for 60Co gamma-rays compared with CK ultrasoft X-rays. Our experimental data are in agreement with the hypothesis that single DNA double-strand breaks (dsbs), which are induced by 0.28-keV ultrasoft CK X-rays, may lead to oncogenic cell transformation. With increasing absorbed dose, i.e. with decreasing mean distance between dsbs induced by 0.28-keV ultrasoft X-rays, oncogenic cell transformation and cell inactivation may also be induced by interaction between those dsbs.

Influence of ara A on the formation of dicentrics in irradiated quiescent CHO cells

CHO K1-cells, arrested in G0/G1 phase were irradiated with graded doses of 150-kV X-rays, and the dicentric chromosome aberrations in the first metaphase after reincubation in full growth medium were evaluated. When 500 microM ara A was administered at least 1 h before irradiation, maximal enhancement of the dicentric yield was obtained under 'immediate plating' conditions as well as under 'delayed plating' conditions, and under both conditions only the linear component alpha D of the aberration yield was affected. Since ara A had no effect upon the number of induced dsbs, as measured with PFGE at doses up to 100 Gy, the action pathway of ara A is suggested to be the repair/misrepair process manifested in the linear component of the dicentric yield. By varying the time at which either ara A at 37 degrees C was administered before irradiation or ara A at 4 degrees C was added after irradiation, it was observed that the repair step sensitive to ara A was completed within about 5 min after irradiation. This is in agreement with other observations of ara A action upon a fast repair step. The conversion of DNA single-strand breaks or base damage sites into double-strand breaks as a consequence of polymerase beta inhibition by ara A would explain the present results.

Alteration of gamma-ray-induced chromosome aberration by 0.5 M NaCl in Chinese hamster cells

Hypertonic treatment (0.5 M NaCl in phosphate-buffered saline, pH 7.2) at 37 degrees C for 20 min slightly delayed the mitotic frequency for non-irradiated cells in G1 and G2 phases. The mitotic frequency for irradiated cells in G2 was delayed by hypertonic treatment, and that in G1 was slightly delayed by hypertonic treatment. Hypertonic treatment in non-irradiated cells did not induce any chromosomal or chromatid aberrations in either G1 or G2. Chromosomal aberrations caused by gamma-irradiation were slightly enhanced by hypertonic treatment, and chromatid aberrations were markedly enhanced by hypertonic treatment. The enhancement ratio of gamma-irradiation-induced chromatid breaks and exchanges was 1.4 and 3.0, respectively. This cell cycle dependency of chromosome aberrations induced by postirradiation hypertonic treatment was the same as that of cell survival. These findings suggested that hypertonic treatment modifies the rejoining of DNA strand breaks in G2, but slightly modifies that in G1.

Chromosome damage induced by combined treatments with restriction endonucleases introduced into CHO cells by single or double electroporation

The possible recombination between non-homologous termini produced by restriction enzymes (REs) introduced in CHO cells by electroporation was studied. For this purpose, different combinations of REs that produced blunt or 5' overhanging DNA double-strand breaks were electroporated into cells either at the same time or separately by double electroporation experiments. Prior to double electroporation, it was confirmed that, once the cells have been electroporated, they resist a second electroporation, as assessed by cell viability analysis. Besides, the efficient and homogeneous introduction of labelled, non-permeable molecules was assessed by fluorescence microscopy. Our results showed interaction for most of the conditions, mainly when the REs were introduced separately. Differences found in the degree of interaction between the combinations studied are discussed.

Relationships between the cytotoxic effects of restriction endonucleases and radiation on mammalian cells

Wild-type Chinese hamster ovary (CHO) cells, a radiosensitive mutant CHO line (xrs6), and two human cervical carcinoma cell lines, MS751 and ME180, differ in sensitivity to ionizing radiation with surviving fractions at 2 Gy (SF2) of 0.84, 0.06, 0.90 and 0.24 respectively. Restriction endonucleases (REs) were introduced into the cells by treatment with streptolysin O (SLO) and the effects of this on clonogenic cell survival were compared. A comparison was made of REs inducing either blunt- (AluI) or cohesive-ended (Sau3AI) double-strand breaks. Whilst MS751 cells were resistant to the effects of both REs, AluI caused significantly greater cell killing than Sau3AI in the other three lines (p < 0.05 for all). Both ME180 and xrs6 were significantly more sensitive to REs than their radioresistant counterparts, MS751 and CHO (p < 0.05 for both). In order to investigate the effect of DNA methylation on dsb induction, the isoschizomers MspI and HpaII (cohesive-ended dsb inducers) were introduced into the cell lines. Both REs recognize the same sequence but HpaII cannot cleave if the internal cytosine is methylated. MS751 was also resistant to the effects of both of these enzymes and MspI was more cytotoxic than HpaII in the other three lines (p < 0.03 for all). The differential sensitivity to the two REs was more marked in the radiosensitive cell lines, suggesting that there may be a greater degree of DNA methylation in radiosensitive cells. The variation in sensitivities to REs between the cell lines could not be explained in terms of differences in cell poration following SLO treatment because, although MS751 was resistant to SLO (25% of cells porated), the other three lines showed the same level of cell poration (98% of cells). With these four cell lines, there was a significant correlation between sensitivity to RE and radiosensitivity for AluI, Sau3AI and MspI but not for HpaII.

Recent developments in the assessment of chromosomal damage

Ionizing radiation and restriction endonucleases are very efficient in inducing chromosomal aberrations (CAs). These aberrations are mainly consequences of misrepair of DNA double-strand breaks (DSBs). The fast repairing component of DSBs induced by ionizing radiation seems to be responsible for exchange aberration. Use of premature chromosome condensation technique in combination with DNA repair inhibitors such as ara A has given valuable information on the assessment of the frequencies of initial chromosome breaks and the kinetics of their repair following low LET radiation. The recently developed 'chromosome painting' technique using chromosome-specific libraries has also increased considerably the resolution of identifying and scoring of CAs. After low LET radiation, stable chromosome exchanges (translocations) are induced more frequently than unstable chromosome exchanges (dicentrics). Fluorescence in situ hybridization employing telomeric probe has made it possible to score efficiently exchange aberrations involving the acrocentric chromosomes of mouse. Chinese hamster cells have several intercalary telomeric sequences present in most of the chromosomes. These telomeric blocks have been found to be associated with chromosomal aberrations induced by restriction endonucleases and short wave UV and evidence has been obtained for apparent amplification of telomeric sequences at the break points.

DNA double-strand breaks induced by low energy protons in V79 cells

The initial production of DNA double-strand breaks (dsb) was determined in V79 Chinese hamster cells irradiated with proton beams of 3.24, 1.50 and 0.88 MeV, corresponding to values of unrestricted LET evaluated at the cell midplane of 10.9, 20.0 and 30.5 keV/micron, respectively. X-rays were used for comparison. Dsb were measured with the low speed sedimentation technique in neutral sucrose gradients. The initial yield of dsb rose linearly with the dose and did not significantly depend on the proton LET, in contrast with the results obtained in previous studies for cell inactivation and mutation induction. Also, no significant differences for dsb induction were found between protons and X-rays. Two possible explanations, not necessarily mutually exclusive, are proposed: (1) dsb are not the only lesions involved in cellular effects; and (2) the initial number of dsb is not the only important parameter since a fundamental role is played by the degree of clustering, i.e. the association of dsb with other dsb or other types of damage.

Influence of DNA conformation on radiation-induced single-strand breaks

It is usually assumed that sparsely ionizing radiation produces randomly distributed DNA breakages. This seems to be supported by the finding that in some DNA fragments single-strand scissions occur uniformly at all nucleotide sites, regardless of sequence. We performed experiments on two DNA fragments of about 300 bp having different conformation to test whether radiation-induced single-strand breakage is dependent on DNA conformation. Breakage analysis was carried out by denaturing polyacrylamide gel electrophoresis, which allows determination of the broken site at single nucleotide resolution. We found uniform cutting patterns in B-form regions. On the contrary, X- or gamma-irradiation of curved fragments of kinetoplast DNA showed that the distribution of single-strand breaks was not uniform along the fragment, as the cleavage pattern was modulated in phase with the runs of A-T pairs. This modulation likely reflected the reduced accessibility of the sites which on hydroxyl-radical attack give rise to strand breaks. The cleavage pattern was phased with the runs of A-T pairs. Moreover, the overall yield of strand breaks was considerably lower in curved DNA fragments than in those with extended straight regions. The conformation effect found here indicates that the cleavage pattern reflects the fine structural features of DNA.

X-ray-induced chromatid damage in cells from Down syndrome and Alzheimer disease patients in relation to DNA repair and cancer proneness

Frequencies of chromatid aberrations in response to G2-phase x-irradiation were compared in PHA-stimulated blood lymphocytes from healthy control subjects, Down syndrome (DS) patients, and Alzheimer disease (AD) patients. In cells arrested with Colcemid immediately (0-30 min) after x-irradiation, DS, AD, and control cells showed similar high frequencies of chromatid breaks and gaps, representing unrepaired DNA strand breaks. Frequencies had decreased in AD and control cells arrested 30-90 min after irradiation. However, DS cells had two- to three-fold higher frequencies than AD or control cells. This result indicates deficient repair of the DNA damage in DS cells. Similar responses were obtained with lymphocytes from four of seven DS parents tested and with skin fibroblasts from DS patients compared to age-matched controls. Addition of 1-beta-D-arabinofuranosylcytosine (ara-C), an inhibitor of the repair polymerase, after x-irradiation during G2 phase increased the frequencies of chromatid breaks and gaps in lymphocytes from control and AD donors significantly more than in those from DS patients. This result indicates a deficiency in DS cells in incision at sites of x-ray-induced damage. Thus DS, like other cancer-prone genetic disorders, has a G2-phase DNA repair deficiency in strand break repair and also a second DNA repair deficiency in incision activity.

DNA double-strand breaks measured by pulsed-field gel electrophoresis in irradiated lymphocytes from normal humans and those with Alzheimer's disease

We have previously found that radiation-induced chromosome aberrations (dicentrics) are more numerous in lymphocytes from Alzheimer's disease (AD) patients than in those from age-matched normal individuals (Tobi et al. 1990). To investigate this further, we have examined double-strand breaks (dsb) produced by gamma-irradiation in the DNA of AD and normal lymphocytes by using pulsed-field gel electrophoresis. The percentage of DNA migrating into the gels is an indirect measure of the number of dsb; we have assayed the DNA content of sequential slices of the gel by direct fluorometry and have found that the percentage migrating is dose dependent. Our results show that the level of damage is similar in AD and normal lymphocytes and preliminary assays of the rate of repair suggest that the half-time is also similar, the value being > 1 h. The latter is consistent with the known rate of rejoining of chromosome fragments in interphase lymphocytes (Pantelias and Maillie 1985). The results suggest that at a gross level dsb repair is not impaired in AD cells; however, we cannot exclude the possibility that there is misrepair or non-repair of a small fraction of the dsb, which might account for the greater radiosensitivity of the AD cells.