VADER: a variable dose-rate external 137Cs irradiator for internal emitter and low dose rate studies

In the long term, 137Cs is probably the most biologically important agent released in many accidental (or malicious) radiation disasters. It can enter the food chain, and be consumed, or, if present in the environment (e.g. from fallout), can provide external irradiation over prolonged times. In either case, due to the high penetration of the energetic γ rays emitted by 137Cs, the individual will be exposed to a low dose rate, uniform, whole body, irradiation. The VADER (VAriable Dose-rate External 137Cs irradiatoR) allows modeling these exposures, bypassing many of the problems inherent in internal emitter studies. Making use of discarded 137Cs brachytherapy seeds, the VADER can provide varying low dose rate irradiations at dose rates of 0.1 to 1.2 Gy/day. The VADER includes a mouse "hotel", designed to allow long term simultaneous residency of up to 15 mice. Two source platters containing ~ 250 mCi each of 137Cs brachytherapy seeds are mounted above and below the "hotel" and can be moved under computer control to provide constant low dose rate or a varying dose rate mimicking 137Cs biokinetics in mouse or man. We present the VADER design and characterization of its performance over 18 months of use.

Effects of High- and Low-LET Radiation on Human Hematopoietic System Reconstituted in Immunodeficient Mice

Over the last 50 years, a number of important physiological changes in humans who have traveled on spaceflights have been catalogued. Of major concern are the short- and long-term radiation-induced injuries to the hematopoietic system that may be induced by high-energy galactic cosmic rays encountered on interplanetary space missions. To collect data on the effects of space radiation on the human hematopoietic system in vivo, we used a humanized mouse model. In this study, we irradiated humanized mice with 0.4 Gy of 350 MeV/n ²⁸Si ions, a dose that has been shown to induce tumors in tumor-prone mice and a reference dose that has a relative biological effectiveness of 1 (1 Gy of 250-kVp X rays). Cell counts, cell subset frequency and cytogenetic data were collected from bone marrow spleen and blood of irradiated and control mice at short-term (7, 30 and 60 days) and long-term ( 6 - 7 months) time points postirradiation. The data show a significant short-term effect on the human hematopoietic stem cell counts imparted by both high- and low-LET radiation exposure. The radiation effects on bone marrow, spleen and blood human cell counts and human cell subset frequency were complex but did not alter the functions of the hematopoietic system. The long-term data acquired from high-LET irradiated mice showed complete recovery of the human hematopoietic system in all hematopoietic compartments. The combined results demonstrate that, in spite of early perturbation, the longer term effects of high-LET radiation are not detrimental to human hematopoiesis in our system of study.

In vitro RABiT measurement of dose rate effects on radiation induction of micronuclei in human peripheral blood lymphocytes

Developing new methods for radiation biodosimetry has been identified as a high-priority need in case of a radiological accident or nuclear terrorist attacks. A large-scale radiological incident would result in an immediate critical need to assess the radiation doses received by thousands of individuals. Casualties will be exposed to different doses and dose rates due to their geographical position and sheltering conditions, and dose rate is one of the principal factors that determine the biological consequences of a given absorbed dose. In these scenarios, high-throughput platforms are required to identify the biological dose in a large number of exposed individuals for clinical monitoring and medical treatment. The Rapid Automated Biodosimetry Tool (RABiT) is designed to be completely automated from the input of blood sample into the machine to the output of a dose estimate. The primary goal of this paper was to quantify the dose rate effects for RABiT-measured micronuclei in vitro in human lymphocytes. Blood samples from healthy volunteers were exposed in vitro to different doses of X-rays to acute and protracted doses over a period up to 24 h. The acute dose was delivered at ~1.03 Gy/min and the low dose rate exposure at ~0.31 Gy/min. The results showed that the yield of micronuclei decreases with decreasing dose rate starting at 2 Gy, whereas response was indistinguishable from that of acute exposure in the low dose region, up to 0.5 Gy. The results showed a linear-quadratic dose-response relationship for the occurrence of micronuclei for the acute exposure and a linear dose-response relationship for the low dose rate exposure.

Radiation Dose-Rate Effects on Gene Expression in a Mouse Biodosimetry Model

In the event of a nuclear accident or radiological terrorist attack, there will be a pressing need for biodosimetry to triage a large, potentially exposed population and to assign individuals to appropriate treatment. Exposures from fallout are likely, resulting in protracted dose delivery that would, in turn, impact the extent of injury. Biodosimetry approaches that can distinguish such low-dose-rate (LDR) exposures from acute exposures have not yet been developed. In this study, we used the C57BL/6 mouse model in an initial investigation of the impact of low-dose-rate delivery on the transcriptomic response in blood. While a large number of the same genes responded to LDR and acute radiation exposures, for many genes the magnitude of response was lower after LDR exposures. Some genes, however, were differentially expressed (P < 0.001, false discovery rate <5%) in mice exposed to LDR compared with mice exposed to acute radiation. We identified a set of 164 genes that correctly classified 97% of the samples in this experiment as exposed to acute or LDR radiation using a support vector machine algorithm. Gene expression is a promising approach to radiation biodosimetry, enhanced greatly by this first demonstration of its potential for distinguishing between acute and LDR exposures. Further development of this aspect of radiation biodosimetry, either as part of a complete gene expression biodosimetry test or as an adjunct to other methods, could provide vital triage information in a mass radiological casualty event.

Radiation dose-rate effects on gene expression for human biodosimetry

Background

The effects of dose-rate and its implications on radiation biodosimetry methods are not well studied in the context of large-scale radiological scenarios. There are significant health risks to individuals exposed to an acute dose, but a realistic scenario would include exposure to both high and low dose-rates, from both external and internal radioactivity. It is important therefore, to understand the biological response to prolonged exposure; and further, discover biomarkers that can be used to estimate damage from low-dose rate exposures and propose appropriate clinical treatment.

Methods

We irradiated human whole blood ex vivo to three doses, 0.56 Gy, 2.23 Gy and 4.45 Gy, using two dose rates: acute, 1.03 Gy/min and a low dose-rate, 3.1 mGy/min. After 24 h, we isolated RNA from blood cells and these were hybridized to Agilent Whole Human genome microarrays. We validated the microarray results using qRT-PCR.

Results

Microarray results showed that there were 454 significantly differentially expressed genes after prolonged exposure to all doses. After acute exposure, 598 genes were differentially expressed in response to all doses. Gene ontology terms enriched in both sets of genes were related to immune processes and B-cell mediated immunity. Genes responding to acute exposure were also enriched in functions related to natural killer cell activation and cell-to-cell signaling. As expected, the p53 pathway was found to be significantly enriched at all doses and by both dose-rates of radiation. A support vectors machine classifier was able to distinguish between dose-rates with 100 % accuracy using leave-one-out cross-validation.

Conclusions

In this study we found that low dose-rate exposure can result in distinctive gene expression patterns compared with acute exposures. We were able to successfully distinguish low dose-rate exposed samples from acute dose exposed samples at 24 h, using a gene expression-based classifier. These genes are candidates for further testing as markers to classify exposure based on dose-rate.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-015-0097-x) contains supplementary material, which is available to authorized users.

Effect of dose rate on residual γ-H2AX levels and frequency of micronuclei in X-irradiated mouse lymphocytes

The biological risks associated with low-dose-rate (LDR) radiation exposures are not yet well defined. To assess the risk related to DNA damage, we compared the yields of two established biodosimetry end points, γ-H2AX and micronuclei (MNi), in peripheral mouse blood lymphocytes after prolonged in vivo exposure to LDR X rays (0.31 cGy/min) vs. acute high-dose-rate (HDR) exposure (1.03 Gy/min). C57BL/6 mice were total-body irradiated with 320 kVP X rays with doses of 0, 1.1, 2.2 and 4.45 Gy. Residual levels of total γ-H2AX fluorescence in lymphocytes isolated 24 h after the start of irradiation were assessed using indirect immunofluorescence methods. The terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay was used to determine apoptotic cell frequency in lymphocytes sampled at 24 h. Curve fitting analysis suggested that the dose response for γ-H2AX yields after acute exposures could be described by a linear dependence. In contrast, a linear-quadratic dose-response shape was more appropriate for LDR exposure (perhaps reflecting differences in repair time after different LDR doses). Dose-rate sparing effects (P < 0.05) were observed at doses ≤2.2 Gy, such that the acute dose γ-H2AX and TUNEL-positive cell yields were significantly larger than the equivalent LDR yields. At the 4.45 Gy dose there was no difference in γ-H2AX expression between the two dose rates, whereas there was a two- to threefold increase in apoptosis in the LDR samples compared to the equivalent 4.45 Gy acute dose. Micronuclei yields were measured at 24 h and 7 days using the in vitro cytokinesis-blocked micronucleus (CBMN) assay. The results showed that MNi yields increased up to 2.2 Gy with no further increase at 4.45 Gy and with no detectable dose-rate effect across the dose range 24 h or 7 days post exposure. In conclusion, the γ-H2AX biomarker showed higher sensitivity to measure dose-rate effects after low-dose LDR X rays compared to MNi formation; however, confounding factors such as variable repair times post exposure, increased cell killing and cell cycle block likely contributed to the yields of MNi with accumulating doses of ionizing radiation.

The effect of low dose rate on metabolomic response to radiation in mice

Metabolomics has been shown to have utility in assessing responses to exposure by ionizing radiation (IR) in easily accessible biofluids such as urine. Most studies to date from our laboratory and others have employed γ-irradiation at relatively high dose rates (HDR), but many environmental exposure scenarios will probably be at relatively low dose rates (LDR). There are well-documented differences in the biologic responses to LDR compared to HDR, so an important question is to assess LDR effects at the metabolomics level. Our study took advantage of a modern mass spectrometry approach in exploring the effects of dose rate on the urinary excretion levels of metabolites 2 days after IR in mice. A wide variety of statistical tools were employed to further focus on metabolites, which showed responses to LDR IR exposure (0.00309 Gy/min) distinguishable from those of HDR. From a total of 709 detected spectral features, more than 100 were determined to be statistically significant when comparing urine from mice irradiated with 1.1 or 4.45 Gy to that of sham-irradiated mice 2 days post-exposure. The results of this study show that LDR and HDR exposures perturb many of the same pathways such as TCA cycle and fatty acid metabolism, which also have been implicated in our previous IR studies. However, it is important to note that dose rate did affect the levels of particular metabolites. Differences in urinary excretion levels of such metabolites could potentially be used to assess an individual's exposure in a radiobiological event and thus would have utility for both triage and injury assessment.

Potential reduction of contralateral second breast-cancer risks by prophylactic mammary irradiation: validation in a breast-cancer-prone mouse model

Background

Long-term breast-cancer survivors have a highly elevated risk (1 in 6 at 20 years) of contralateral second breast cancer. This high risk is associated with the presence of multiple pre-malignant cell clones in the contralateral breast at the time of primary breast cancer diagnosis. Mechanistic analyses suggest that a moderate dose of X-rays to the contralateral breast can kill these pre-malignant clones such that, at an appropriate Prophylactic Mammary Irradiation (PMI) dose, the long-term contralateral breast cancer risk in breast cancer survivors would be considerably decreased.

Aims

To test the predicted relationship between PMI dose and cancer risk in mammary glands that have a high risk of developing malignancies.

Methods

We tested the PMI concept using MMTV-PyVT mammary-tumor-prone mice. Mammary glands on one side of each mouse were irradiated with X-rays, while those on the other side were shielded from radiation. The unshielded mammary glands received doses of 0, 4, 8, 12 and 16Gy in 4-Gy fractions.

Results

In high-risk mammary glands exposed to radiation doses designed for PMI (12 and 16 Gy), tumor incidence rates were respectively decreased by a factor of 2.2 (95% CI, 1.1-5.0) at 12 Gy, and a factor of 3.1 (95% CI, 1.3-8.3) at 16 Gy, compared to those in the shielded glands that were exposed to very low radiation doses. The same pattern was seen for PMI-exposed mammary glands relative to zero-dose controls.

Conclusions

The pattern of cancer risk reduction by PMI was consistent with mechanistic predictions. Contralateral breast PMI may thus have promise as a spatially targeted breast-conserving option for reducing the current high risk of contralateral second breast cancers. For estrogen-receptor positive primary tumors, PMI might optimally be used concomitantly with systemically delivered chemopreventive drugs such as tamoxifen or aromatase inhibitors, while for estrogen-receptor negative tumors, PMI might be used alone.

Single-cell responses to ionizing radiation

While gene expression studies have proved extremely important in understanding cellular processes, it is becoming more apparent that there may be differences in individual cells that are missed by studying the population as a whole. We have developed a qRT-PCR protocol that allows us to assay multiple gene products in small samples, starting at 100 cells and going down to a single cell, and have used it to study radiation responses at the single-cell level. Since the accuracy of qRT-PCR depends greatly on the choice of "housekeeping" genes used for normalization, initial studies concentrated on determining the optimal panel of such genes. Using an endogenous control array, it was found that for IMR90 cells, common housekeeping genes tend to fall into one of two categories-those that are relatively stably expressed regardless of the number of cells in the sample, e.g., B2M, PPIA, and GAPDH, and those that are more variable (again regardless of the size of the population), e.g., YWHAZ, 18S, TBP, and HPRT1. Further, expression levels in commonly studied radiation-response genes, such as ATF3, CDKN1A, GADD45A, and MDM2, were assayed in 100, 10, and single-cell samples. It is here that the value of single-cell analyses becomes apparent. It was observed that the expression of some genes such as FGF2 and MDM2 was relatively constant over all irradiated cells, while that of others such as FAS was considerably more variable. It was clear that almost all cells respond to ionizing radiation but the individual responses were considerably varied. The analyses of single cells indicate that responses in individual cells are not uniform and suggest that responses observed in populations are not indicative of identical patterns in all cells. This in turn points to the value of single-cell analyses.

Widespread decreased expression of immune function genes in human peripheral blood following radiation exposure

We report a large-scale reduced expression of genes in pathways related to cell-type specific immunity functions that emerges from microarray analysis 48 h after ex vivo γ-ray irradiation (0, 0.5, 2, 5, 8 Gy) of human peripheral blood from five donors. This response is similar to that seen in patients at 24 h after the start of total-body irradiation and strengthens the rationale for the ex vivo model as an adjunct to human in vivo studies. The most marked response was in genes associated with natural killer (NK) cell immune functions, reflecting a relative loss of NK cells from the population. T- and B-cell mediated immunity genes were also significantly represented in the radiation response. Combined with our previous studies, a single gene expression signature was able to predict radiation dose range with 97% accuracy at times from 6-48 h after exposure. Gene expression signatures that may report on the loss or functional deactivation of blood cell subpopulations after radiation exposure may be particularly useful both for triage biodosimetry and for monitoring the effect of radiation mitigating treatments.

Proton radiation-induced miRNA signatures in mouse blood: characterization and comparison with 56Fe-ion and gamma radiation

PURPOSE

Previously, we showed that microRNA (miRNA) signatures derived from the peripheral blood of mice are highly specific for both radiation energy (γ-rays or high linear energy transfer [LET] (56)Fe ions) and radiation dose. Here, we investigate to what extent miRNA expression signatures derived from mouse blood can be used as biomarkers for exposure to 600 MeV proton radiation.

MATERIALS AND METHODS

We exposed mice to 600 MeV protons, using doses of 0.5 or 1.0 Gy, isolated total RNA at 6 h or 24 h after irradiation, and used quantitative real-time polymerase chain reaction (PCR) to determine the changes in miRNA expression.

RESULTS

A total of 26 miRNA were differentially expressed after proton irradiation, in either one (77%) or multiple conditions (23%). Statistical classifiers based on proton, γ, and (56)Fe-ion miRNA expression signatures predicted radiation type and proton dose with accuracies of 81% and 88%, respectively. Importantly, gene ontology analysis for proton-irradiated cells shows that genes targeted by radiation-induced miRNA are involved in biological processes and molecular functions similar to those controlled by miRNA in γ ray- and (56)Fe-irradiated cells.

CONCLUSIONS

Mouse blood miRNA signatures induced by proton, γ, or (56)Fe irradiation are radiation type- and dose-specific. These findings underline the complexity of the miRNA-mediated radiation response.

Combined haploinsufficiency and genetic control of the G2/M checkpoint in irradiated cells

When cells are exposed to a dose of radiation large enough to cause chromosome aberrations, they become arrested at the G(2)/M checkpoint, facilitating DNA repair. Defects in checkpoint control genes can impart radiosensitivity. Arrest kinetics were monitored in mouse embryo fibroblasts at doses ranging from 10 mGy to 5.0 Gy of γ radiation over a time course of 0 to 12 h. We observe no significant checkpoint engagement at doses below 100 mGy. The checkpoint is only fully activated at doses where most of the cells are either bound for mitotic catastrophe or are reproductively dead. Atm null cells with ablated checkpoint function exhibited no robust arrest. Surprisingly, haploinsufficiency for ATM alone or in combination with other radioresistance genes did not alter checkpoint activation. We have shown previously that haploinsufficiency for several radioresistance genes imparts intermediate phenotypes for several end points including apoptosis, transformation and survival. These findings suggest that checkpoint control does not contribute toward these intermediate phenotypes and that different biological processes can be activated at high doses compared to low doses.

Mechanisms of increased risk of tumorigenesis in Atm and Brca1 double heterozygosity

BACKGROUND

Both epidemiological and experimental studies suggest that heterozygosity for a single gene is linked with tumorigenesis and heterozygosity for two genes increases the risk of tumor incidence. Our previous work has demonstrated that Atm/Brca1 double heterozygosity leads to higher cell transformation rate than single heterozygosity. However, the underlying mechanisms have not been fully understood yet. In the present study, a series of pathways were investigated to clarify the possible mechanisms of increased risk of tumorigenesis in Atm and Brca1 heterozygosity.

METHODS

Wild type cells, Atm or Brca1 single heterozygous cells, and Atm/Brca1 double heterozygous cells were used to investigate DNA damage and repair, cell cycle, micronuclei, and cell transformation after photon irradiation.

RESULTS

Remarkable high transformation frequency was confirmed in Atm/Brca1 double heterozygous cells compared to wild type cells. It was observed that delayed DNA damage recognition, disturbed cell cycle checkpoint, incomplete DNA repair, and increased genomic instability were involved in the biological networks. Haploinsufficiency of either ATM or BRCA1 negatively impacts these pathways.

CONCLUSIONS

The quantity of critical proteins such as ATM and BRCA1 plays an important role in determination of the fate of cells exposed to ionizing radiation and double heterozygosity increases the risk of tumorigenesis. These findings also benefit understanding of the individual susceptibility to tumor initiation.

Radiation-induced micro-RNA expression changes in peripheral blood cells of radiotherapy patients

PURPOSE

MicroRNAs (miRNAs), a class of noncoding small RNAs that regulate gene expression, are involved in numerous physiologic processes in normal and malignant cells. Our in vivo study measured miRNA and gene expression changes in human blood cells in response to ionizing radiation, to develop miRNA signatures that can be used as biomarkers for radiation exposure.

METHODS AND MATERIALS

Blood from 8 radiotherapy patients in complete remission 1 or 2 was collected immediately before and 4 hours after total body irradiation with 1.25 Gy x-rays. Both miRNA and gene expression changes were measured by means of quantitative polymerase chain reaction and microarray hybridization, respectively. Hierarchic clustering, multidimensional scaling, class prediction, and gene ontology analysis were performed to investigate the potential of miRNAs to serve as radiation biomarkers and to elucidate their likely physiologic roles in the radiation response.

RESULTS

The expression levels of 45 miRNAs were statistically significantly upregulated 4 hours after irradiation with 1.25 Gy x-rays, 27 of them in every patient. Nonirradiated and irradiated samples form separate clusters in hierarchic clustering and multidimensional scaling. Out of 223 differentially expressed genes, 37 were both downregulated and predicted targets of the upregulated miRNAs. Paired and unpaired miRNA-based classifiers that we developed can predict the class membership of a sample with unknown irradiation status, with accuracies of 100% when all 45 upregulated miRNAs are included. Both miRNA control of and gene involvement in biologic processes such as hemopoiesis and the immune response are increased after irradiation, whereas metabolic processes are underrepresented among all differentially expressed genes and the genes controlled by miRNAs.

CONCLUSIONS

Exposure to ionizing radiation leads to the upregulation of the expression of a considerable proportion of the human miRNAome of peripheral blood cells. These miRNA expression signatures can be used as biomarkers of radiation exposure.

Whole mouse blood microRNA as biomarkers for exposure to γ-rays and (56)Fe ion

PURPOSE

Biomarkers of ionising radiation exposure are useful in a variety of scenarios, such as medical diagnostic imaging, occupational exposures, and spaceflight. This study investigates to what extent microRNA (miRNA) expression signatures in mouse peripheral blood can be used as biomarkers for exposures to radiation with low and high linear energy transfers.

MATERIALS AND METHODS

Mice were irradiated with doses of 0.5, 1.5, or 5.0 Gy γ-rays (dose rate of 0.0136 Gy/s) or with doses of 0.1 or 0.5 Gy (56)Fe ions (dose rate of 0.00208 Gy/s). Total RNA was isolated from whole blood at 6 h or 24 h after irradiation. Three animals per irradiation condition were used. Differentially expressed miRNA were determined by means of quantitative real-time polymerase chain reaction.

RESULTS

miRNA expression signatures were radiation type-specific and dose- and time-dependent. The differentially expressed miRNA were expressed in either one condition (71%) or multiple conditions (29%). Classifiers based on the differentially expressed miRNA predicted radiation type or dose with accuracies between 75% and 100%. Gene-ontology analyses show that miRNA induced by irradiation are involved in the control of several biological processes, such as mRNA transcription regulation, nucleic-acid metabolism, and development.

CONCLUSION

miRNA signatures induced by ionising radiation in mouse blood are radiation type- and radiation dose-specific. These findings underline the complexity of the radiation response and the importance of miRNA in it.

RADIOSENSITIVITY TO HIGH ENERGY IRON IONS IS INFLUENCED BY HETEROZYGOSITY for ATM, RAD9 and BRCA1

Loss of function of DNA repair genes has been implicated in the development of many types of cancer. In the last several years, heterozygosity leading to haploinsufficiency for proteins involved in DNA repair was shown to play a role in genomic instability and carcinogenesis after DNA damage is induced, for example by ionizing radiation. Since the effect of heterozygosity for one gene is relatively small, we hypothesize that predisposition to cancer could be a result of the additive effect of heterozygosity for two or more genes critical to pathways that control DNA damage signaling, repair or apoptosis. We investigated the role of heterozygosity for Atm, Rad9 and Brca1 on cell oncogenic transformation and cell survival induced by 1GeV/n ⁵⁶Fe ions. Our results show that cells heterozygous for both Atm and Rad9 or Atm and Brca1 have high survival rates and are more sensitive to transformation by high energy Iron ions when compared with wild-type controls or cells haploinsufficient for only one of these proteins. Since mutations or polymorphisms for similar genes exist in a small percentage of the human population, we have identified a radiosensitive sub-population. This finding has several implications. First, the existence of a radiosensitive sub-population may distort the shape of the dose-response relationship. Second, it would not be ethical to put exceptionally radiosensitive individuals into a setting where they may potentially be exposed to substantial doses of radiation.

Hemizygosity for Atm and Brca1 influence the balance between cell transformation and apoptosis

BACKGROUND

In recent years data from both mouse models and human tumors suggest that loss of one allele of genes involved in DNA repair pathways may play a central role in genomic instability and carcinogenesis. Additionally several examples in mouse models confirmed that loss of one allele of two functionally related genes may have an additive effect on tumor development. To understand some of the mechanisms involved, we examined the role of monoallelic loss or Atm and Brca1 on cell transformation and apoptosis induced by radiation.

METHODS

Cell transformation and apoptosis were measured in mouse embryo fibroblasts (MEF) and thymocytes respectively. Combinations of wild type and hemizygous genotypes for ATM and BRCA1 were tested in various comparisons.

RESULTS

Haploinsufficiency of either ATM or BRCA1 resulted in an increase in the incidence of radiation-induced transformation of MEF and a corresponding decrease in the proportion of thymocytes dying an apoptotic death, compared with cells from wild-type animals. Combined haploinsufficiency for both genes resulted in an even larger effect on apoptosis.

CONCLUSIONS

Under stress, the efficiency and capacity for DNA repair mediated by the ATM/BRCA1 cell signalling network depends on the expression levels of both proteins.

Cell cycle activation in postmitotic neurons is essential for DNA repair

Increasing evidence indicates that maintenance of neuronal homeostasis involves the activation of the cell cycle machinery in postmitotic neurons. Our recent findings suggest that cell cycle activation is essential for DNA damage-induced neuronal apoptosis. However, whether the cell division cycle also participates in DNA repair and survival of postmitotic, terminally differentiated neurons is unknown. Here, we tested the hypothesis that G(1) phase components contribute to the repair of DNA and are involved in the DNA damage response of postmitotic neurons. In cortical terminally differentiated neurons, treatment with subtoxic concentrations of hydrogen peroxide (H(2)O(2)) caused repairable DNA double strand breaks (DSBs) and the activation of G(1) components of the cell cycle machinery. Importantly, DNA repair was attenuated if cyclin-dependent kinases CDK4 and CDK6, essential elements of G(0) --> G(1) transition, were suppressed. Our data suggest that G(1) cell cycle components are involved in DNA repair and survival of postmitotic neurons.

New insight into intrachromosomal deletions induced by chrysotile in the gpt delta transgenic mutation assay

BACKGROUND

Genotoxicity is often a prerequisite to the development of malignancy. Considerable evidence has shown that exposure to asbestos fibers results in the generation of chromosomal aberrations and multilocus mutations using various in vitro approaches. However, there is less evidence to demonstrate the contribution of deletions to the mutagenicity of asbestos fibers in vivo.

OBJECTIVES

In the present study, we investigated the mutant fractions and the patterns induced by chrysotile fibers in gpt delta transgenic mouse primary embryo fibroblasts (MEFs) and compared the results obtained with hydrogen peroxide (H2O2) in an attempt to illustrate the role of oxyradicals in fiber mutagenesis.

RESULTS

Chrysotile fibers induced a dose-dependent increase in mutation yield at the redBA/gam loci in transgenic MEF cells. The number of lambda mutants losing both redBA and gam loci induced by chrysotiles at a dose of 1 microg/cm(2) increased by > 5-fold relative to nontreated controls (p < 0.005). Mutation spectra analyses showed that the ratio of lambda mutants losing the redBA/gam region induced by chrysotiles was similar to those induced by equitoxic doses of H2O2. Moreover, treatment with catalase abrogated the accumulation of y-H2AX, a biomarker of DNA double-strand breaks, induced by chrysotile fibers.

CONCLUSIONS

Our results provide novel information on the frequencies and types of mutations induced by asbestos fibers in the gpt delta transgenic mouse mutagenic assay, which shows great promise for evaluating fiber/particle mutagenicity in vivo.

A microbeam study of DNA double-strand breaks in bystander primary human fibroblasts

Radiation-induced bystander effect has been well documented. However, the mechanisms are poorly understood. How we incorporate this effect into the classical models of risk assessment remains an open question. Here, the induction of bystander effect was studied by assessing DNA double-strand break (DSB) formation in situ with the rapid and sensitive gamma-H2AX focus formation assay. Utilising the Columbia University single-cell microbeam system to deliver 2 or 20 individual alpha particles to selected cell nuclei in a precisely known proportion of cells in a population, the induced DNA DSB incidences were quantified 30 min and 18 h post-IR. The increase in DNA DSB incidence in bystander cells lacked of a linear dose response indicating that neither the dose of irradiation nor proportion of irradiated cells in a population, is a critical parameter. This study confirms a binary all-or-nothing model of triggering the bystander response. The delay and persistence of the bystander response suggests a different mechanism of DSB induction in bystander cells than in directly irradiated cells.

Ionizing radiation induces DNA double-strand breaks in bystander primary human fibroblasts

That irradiated cells affect their unirradiated 'bystander' neighbors is evidenced by reports of increased clonogenic mortality, genomic instability, and expression of DNA-repair genes in the bystander cell populations. The mechanisms underlying the bystander effect are obscure, but genomic instability suggests DNA double-strand breaks (DSBs) may be involved. Formation of DSBs induces the phosphorylation of the tumor suppressor protein, histone H2AX and this phosphorylated form, named gamma-H2AX, forms foci at DSB sites. Here we report that irradiation of target cells induces gamma-H2AX focus formation in bystander cell populations. The effect is manifested by increases in the fraction of cells in a population that contains multiple gamma-H2AX foci. After 18 h coculture with cells irradiated with 20 alpha-particles, the fraction of bystander cells with multiple foci increased 3.7-fold. Similar changes occurred in bystander populations mixed and grown with cells irradiated with gamma-rays, and in cultures containing media conditioned on gamma-irradiated cells. DNA DSB repair proteins accumulated at gamma-H2AX foci, indicating that they are sites of DNA DSB repair. Lindane, which blocks gap-junctions, prevented the bystander effect in mixing but not in media transfer protocols, while c-PTIO and aminoguanidine, which lower nitric oxide levels, prevented the bystander effect in both protocols. Thus, multiple mechanisms may be involved in transmitting bystander effects. These studies show that H2AX phosphorylation is an early step in the bystander effect and that the DNA DSBs underlying gamma-H2AX focus formation may be responsible for its downstream manifestations.

Combined haploinsufficiency for ATM and RAD9 as a factor in cell transformation, apoptosis, and DNA lesion repair dynamics

Loss of function of oncogenes, tumor suppressor genes and DNA damage processing genes has been implicated in the development of many types of cancer, but for the vast majority of cases, there is no link to specific germ line mutations. In the last several years, heterozygosity leading to haploinsufficiency for proteins involved in DNA repair pathways was shown to play a role in genomic instability and carcinogenesis after DNA damage is induced. Because the effect of haploinsufficiency for one protein is relatively small, we hypothesize that predisposition to cancer could be a result of the additive effect of heterozygosity for two or more genes, critical for pathways that control DNA damage signaling, repair or apoptosis. To address this issue, primary mouse cells, haploinsufficient for one or two proteins, ATM and RAD9, related to the cellular response to DNA damage were examined. The results show that cells having low levels of both ATM and RAD9 proteins are more sensitive to transformation by radiation, have different DNA double-strand break repair dynamics and are less apoptotic when compared with wild-type controls or those cells haploinsufficient for only one of these proteins. Our conclusions are that under stress conditions, the efficiency and capacity for DNA repair mediated by the ATM/RAD9 cell signaling network depend on the abundance of both proteins and that, in general, DNA repair network efficiencies are genotype-dependent and can vary within a specific range.

Combined Haploinsufficiency for ATM and RAD9 as a Factor in Cell Transformation, Apoptosis, and DNA Lesion Repair Dynamics

Loss of function of oncogenes, tumor suppressor genes and DNA damage processing genes has been implicated in the development of many types of cancer, but for the vast majority of cases, there is no link to specific germ line mutations. In the last several years, heterozygosity leading to haploinsufficiency for proteins involved in DNA repair pathways was shown to play a role in genomic instability and carcinogenesis after DNA damage is induced. Because the effect of haploinsufficiency for one protein is relatively small, we hypothesize that predisposition to cancer could be a result of the additive effect of heterozygosity for two or more genes, critical for pathways that control DNA damage signaling, repair or apoptosis. To address this issue, primary mouse cells, haploinsufficient for one or two proteins, ATM and RAD9, related to the cellular response to DNA damage were examined. The results show that cells having low levels of both ATM and RAD9 proteins are more sensitive to transformation by radiation, have different DNA double-strand break repair dynamics and are less apoptotic when compared with wild-type controls or those cells haploinsufficient for only one of these proteins. Our conclusions are that under stress conditions, the efficiency and capacity for DNA repair mediated by the ATM/RAD9 cell signaling network depend on the abundance of both proteins and that, in general, DNA repair network efficiencies are genotype-dependent and can vary within a specific range.

Integrin functions play a key role in the differentiation of thymocytes in vivo

T cells express a variety of surface proteins as they develop to maturity in the thymus. In addition to the TCR-CD3 complex and the two major coreceptors, CD4 and CD8, other surface proteins expressed include receptors for cytokines, growth factors, counterreceptors, and extracellular matrix molecules. To determine the role of integrin adhesion receptors in T cell development, we have expressed a trans-dominant inhibitor of integrin function in the thymus. This inhibitor leads to a block of adhesion to fibronectin due to reduced activation of integrin receptors. This reduced adhesion leads to a partial block in differentiation from CD4-CD8- cells to CD4+CD8+ cells, after the CD25+ stage, suggesting that integrins are important during Lck-mediated differentiation. Furthermore, the overall production of CD4+ cells is reduced compared with that of CD8+ cells without changes in negative selection, suggesting that integrins may be involved in the determination of the fate of the cell as well. These results demonstrate that integrin receptor function is required for proper thymocyte development in vivo.

Modest increased sensitivity to radiation oncogenesis in ATM heterozygous versus wild-type mammalian cells

Subpopulations that are genetically predisposed to radiation-induced cancer could have significant public health consequences. Individuals homozygous for null mutations at the ataxia telangiectasia gene are indeed highly radiosensitive, but their numbers are very small. Ataxia Telangiectasia heterozygotes (1-2% of the population) have been associated with somewhat increased radiosensitivity for some end points, but none directly related to carcinogenesis. Here, intralitter comparisons between wild-type mouse embryo fibroblasts and mouse embryo fibroblasts carrying ataxia telangiectasia mutated (ATM) null mutation indicate that the heterozygous cells are more sensitive to radiation oncogenesis than their normal, litter-matched, counterparts. From these data we suggest that Ataxia Telangiectasia heterozygotes could indeed represent a societally-significant radiosensitive human subpopulation.

Focal adhesion motility revealed in stationary fibroblasts

Focal adhesions (FAs) are clustered integrins and associated proteins that mediate cell adhesion and signaling. A green fluorescent protein-beta1 integrin chimera was used to label FAs in living cells. In stationary cells, FAs were highly motile, moving linearly for several plaque lengths toward the cell center. FA motility was independent of cell density and resulted from contraction of associated actin fibers. In migrating cells, FAs were stationary and only moved in the tail. FA motility in stationary cells suggests that cell movement may be regulated by a clutch-like mechanism by which the affinity of integrins to substrate may be altered in response to migratory cues.

Altered telomere nuclear matrix interactions and nucleosomal periodicity in ataxia telangiectasia cells before and after ionizing radiation treatment

Cells derived from ataxia telangiectasia (A-T) patients show a prominent defect at chromosome ends in the form of chromosome end-to-end associations, also known as telomeric associations, seen at G(1), G(2), and metaphase. Recently, we have shown that the ATM gene product, which is defective in the cancer-prone disorder A-T, influences chromosome end associations and telomere length. A possible hypothesis explaining these results is that the defective telomere metabolism in A-T cells are due to altered interactions between the telomeres and the nuclear matrix. We examined these interactions in nuclear matrix halos before and after radiation treatment. A difference was observed in the ratio of soluble versus matrix-associated telomeric DNA between cells derived from A-T and normal individuals. Ionizing radiation treatment affected the ratio of soluble versus matrix-associated telomeric DNA only in the A-T cells. To test the hypothesis that the ATM gene product is involved in interactions between telomeres and the nuclear matrix, we examined such interactions in human cells expressing either a dominant-negative effect or complementation of the ATM gene. The phenotype of RKO colorectal tumor cells expressing ATM fragments containing a leucine zipper motif mimics the altered interactions of telomere and nuclear matrix similar to that of A-T cells. A-T fibroblasts transfected with wild-type ATM gene had corrected telomere-nuclear matrix interactions. Further, we found that A-T cells had different micrococcal nuclease digestion patterns compared to normal cells before and after irradiation, indicating differences in nucleosomal periodicity in telomeres. These results suggest that the ATM gene influences the interactions between telomeres and the nuclear matrix, and alterations in telomere chromatin could be at least partly responsible for the pleiotropic phenotypes of the ATM gene.

Molecular cloning and chromosomal localization of Chinese hamster telomeric protein chTRF1. Its potential role in chromosomal instability

Chinese hamster cells frequently have altered karyotypes. To investigate the basis of recent observations that karyotypic alterations are related to telomeric fusions, we asked whether these alterations are due to lack of telomere repeat binding factor/s. Further, Chinese hamster chromosomes contain large blocks of interstitial telomeric repeats, which are preferentially involved in chromosome breakage and exchange, rendering it an interesting model for such studies. Here, we report on the cloning and the chromosomal localization of the Chinese hamster telomere repeat binding factor, chTRF1. The sequence analysis revealed, similar to human TRF1 (hTRF1), an N-terminal acidic domain, a TRF1 specific DNA binding motif and a C-terminal Myb type domain. Unlike mouse TRF1 (mTRF1), chTRF1 shows 97.5% identity to hTRF1. chTRF1 gene was localized on the long arm of chromosome 5. In vitro translation of chTRF1 resulted in protein product similar in molecular weight to hTRF1. Immunostaining of Chinese hamster ovary cells (CHO) with anti-TRF1 antibody revealed punctate nuclear staining. At metaphase, antibodies failed to detect TRF1 on most of the chromosome ends and the interstitial telomeric repeat bands. These studies suggest that chTRF1 does not bind the interstitial telomeric repeats, and its presence at the metaphase chromosome ends is limited. The later could be a factor contributing to frequent karyotypic alterations observed in Chinese hamster cells.

Influence of ATM function on telomere metabolism

The ATM gene product, which is defective in the cancer-prone disorder ataxia telangiectasia, has been implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination and cell cycle control. The ATM gene has homology with the TEL1 gene of yeast, mutations of which lead to shortened telomeres. To test the hypothesis that the ATM gene product is involved in telomere metabolism, we examined telomeric associations (TA), telomere length, and telomerase activity in human cells expressing either dominant-negative or complementing fragments of the ATM gene. The phenotype of RKO colorectal tumor cells expressing ATM fragments containing a leucine zipper (LZ) motif mimics that of ataxia telangiectasia (A-T) cells. These transfected RKO cells relative to transfected controls had a higher frequency of cells with TA and shortened telomeres, but no detectable change in telomerase activity. In addition, the percentage of cells with TA after gamma irradiation was higher in the transfected RKO cells with dominant negative activity of the ATM gene, compared to control cells. SV40 transformed fibroblasts derived from an A-T patient and transfected with a complementing carboxyl terminal kinase region of the ATM gene had a reduced frequency of cells with TA, with no effect on the telomere length or telomerase activity. The present studies using isogenic cells with manipulated ATM function demonstrate a role for the ATM gene product in telomere metabolism.

The membrane-cytoplasm interface of integrin alpha subunits is critical for receptor latency

Localization of integrin receptors to focal contact sites occurs upon ligand binding. This activity is latent, since unoccupied integrin receptors do not localize to focal contacts. Deletion analysis has revealed that the alpha cytoplasmic domains is required for the maintenance of integrin receptor latency. Our current hypothesis for the mechanism of integrin post-ligand binding events is that there is a change in relationship of alpha and beta cytoplasmic domains, which overcomes receptor latency. One possible mechanism for such a change would involve the amino acid residues at the membrane-cytoplasm interface. To test this hypothesis, we have produced point mutations in the human integrin alpha 1 subunit. These mutations had no effect on the adhesion via alpha 1 beta 1 to its ligand, collagen IV. However, receptor latency is lost in one of these mutants, leading to constitutive focal contact localization. This effect did not occur in receptors with an exchange of intracellular domains, suggesting that the mechanism of loss of latency involves a relative motion of the integrin chains. These results suggest a model in which post-ligand binding events in integrin receptors are associated with changes in the position of the alpha and beta cytoplasmic domains.