Use of a centrifuge-based automated blood cell counter for radiation dose assessment

Hematological changes create early-response biomarkers for assessing radiation doses. Existing dose-prediction models are based on serial blood lymphocyte counts after acute whole-body exposure to gamma-radiation. Measurements of lymphocyte-depletion kinetics after possible exposures are useful for triaging patients and managing medical resources. The small-footprint QBC Autoread Plus System provides cost-effective hematological analyses with reproducibility, accuracy, and a broad dynamic range. QBC analysis measures centrifugally packed, whole blood cells in microhematocrit tubes and reports pooled lymphocyte and monocyte counts. Our objective was to modify this procedure to report pure lymphocyte counts for radiation biodosimetry applications. The CD14 antigen is strongly expressed on most human monocytes. Using anti-CD14-coated Dynabeads, we have devised a rapid method for depleting monocytes from whole blood without altering the lymphocyte viability or count. This simple dry procedure provides reliable lymphocyte counts for results that fall within the normal lymphocyte count range (1-4 x 10(9) cells per L) for radiation exposure assessment using lymphocyte-depletion kinetics.

Mitotic index maximization with no effect on radiation-induced dicentric chromosome frequency

PURPOSE

The dicentric chromosome (Dic) assay, which is the gold standard for biological dose assessment in radiation emergency medicine, requires an analysis of at least 500 lymphocyte metaphases or 100 Dic aberrations. Therefore, peripheral blood culture conditions able to obtain a high frequency of metaphases for efficient dose evaluation should be optimized. However, the type of blood cultures [i.e. whole blood (WB) or isolated peripheral blood mononuclear cell (PBMC)-culture] and blood volume differ between biodosimetry laboratories. The purpose of this study is to investigate the blood volume at which a high mitotic index (MI) is obtained in peripheral WB-culture and isolated PBMC-culture, and to examine the possible effect of blood volume on radiation-induced Dic frequency.

MATERIALS AND METHODS

Peripheral blood was collected from three healthy donors with their informed consent. The complete and differential blood counts were performed using an automated hematology analyzer. After blood count, peripheral blood was irradiated with 0 or 2 Gy X-ray. Blood was cultured with phytohemagglutinin (180 μg/ml) and demecolcine　(0.05 μg/ml) for 48 h. The MI and Dic frequency were analyzed in 5, 10, 15, 20, 25, and 30% WB-cultures and 0.6, 1.2, 1.8, 2.4, 3.0, 3.6, and 4.2 ml WB-equivalent PBMC-cultures.

RESULTS

In WB-culture, MI showed the highest value (∼22%) in 5-15% WB-culture and then gradually decreased to ∼9% with 30% WB-culture. MI peaked at 36 and 31% in 1.8 and 2.4 ml-WB equivalent volumes for PMBC-cultures, respectively. MI progressively decreased as the amount of PBMCs increased. Although individual differences were observed in the MI values among the three subjects, all the subjects showed the same tendency and higher MI was seen in PBMC than WB-cultures. However, these factors had no significant impact on the yield of Dics. In all culture conditions, the estimated dose calculated based on the Dic frequency was equivalent to the absorbed dose of ex vivo X-ray-irradiated blood.

CONCLUSION

While MI was affected by the blood culture type and the volume of cultured blood, Dic yield did not differ significantly between these conditions. These results could be used by relevant laboratories to optimize MI in certain circumstances.

Multiple blood-proteins approach for early-response exposure assessment using an in vivo murine radiation model

PURPOSE

The aim was to evaluate the utility of multiple blood-protein biomarkers for early-response assessment of radiation exposure using a murine radiation model system.

MATERIAL AND METHODS

BALB/c male mice (8-10 weeks old) were exposed to whole-body 60Co gamma-rays (10 cGy min(-1)) over a broad dose range (0-7 Gy). Blood protein biomarkers (i.e., Growth Arrest and DNA Damage Inducible Gene 45 or GADD45alpha, interleukin 6 or IL-6, and serum amyloid A or SAA) were measured by enzyme linked immunosorbent assay (ELISA) at 4, 24, 48, and 72 h after total-body irradiation (TBI).

RESULTS

Time- and dose-dependent increases in the protein targets were observed. The use of multiple protein targets was evaluated using multiple linear regression analysis to provide dose-response calibration curves for dose assessment. Multivariate discriminant analysis demonstrated enhanced dose-dependent separation of irradiated animals from control as the number of biomarkers increased.

CONCLUSIONS

Results from this study represent a proof-of-concept for multiple blood-proteins biodosimetry approach. It was demonstrated for the first time that protein expression profile could be developed not only to assess radiation exposure in male BALB/c mice but also to distinguish the level of radiation exposure, ranging from 1-7 Gy.

Overview of low-level radiation exposure assessment: biodosimetry

The capability to make diagnostic assessments of radiation exposure is needed to support triage of radiation casualties and medical treatment decisions in military operations. At the International Conference on Low-Level Radiation Injury and Medical Countermeasures session on biodosimetry in the military, participants reviewed the field of biomarkers, covering a wide range of biological endpoints. Participants evaluated early changes associated with exposure to ionizing radiation, including chromosomal and DNA damage, gene expression and associated proteins, and DNA mutations. The use and development of advanced monitoring and diagnostic technologies compatible with military operations was emphasized. Conventional radiation bioassays require a substantial amount of time between when the sample is taken and when the data can be provided for decision making. These "reach back" bioassays are evaluated in laboratories that are not in the field; these laboratories routinely measure exposures of 25 cGy (photon equivalent levels). Detection thresholds can be reduced approximately fivefold by the addition of significant and tiresome scoring efforts. Alternative real-time biomarkers that can be measured in field laboratories or with handheld detection devices show promise as screening and clinical diagnostic tools, but they require further development and validation studies.

Biodosimety Assessment Tool: a post-exposure software application for management of radiation accidents

The Biodosimetry Assessment Tool software application under development will equip health care providers with diagnostic information (clinical signs and symptoms, physical dosimetry, etc.) germane to the management of human radiation casualties. Designed primarily for prompt use after a radiation incident, the user-friendly program facilitates collection, integration, and archiving of data obtained from exposed persons. Data collected in templates are compared with established radiation dose responses obtained from the literature to provide multiparameter dose assessments. The program archives clinical information (e.g., extent of contamination, wounds, infection, etc.) useful for casualty management, displays relevant diagnostic information in a concise format, and can be used to manage both military and civilian radiation accidents. In addition, monitoring of diagnostic information of individuals using this program could potentially minimize the severity of psychological casualties by making a marked impact on the way that both radiation casualties and the worried well view their exposure, dose, and future risk for the development of disease.

Radiation Research Society Journal-based Historical Review of the Use of Biomarkers for Radiation Dose and Injury Assessment: Acute Health Effects Predictions

A multiple-parameter based approach using radiation-induced clinical signs and symptoms, hematology changes, cytogenetic chromosomal aberrations, and molecular biomarkers changes after radiation exposure is used for biodosimetry-based dose assessment. In the current article, relevant milestones from Radiation Research are documented that forms the basis of the current consensus approach for diagnostics after radiation exposure. For example, in 1962 the use of cytogenetic chromosomal aberration using the lymphocyte metaphase spread dicentric assay for biodosimetry applications was first published in Radiation Research. This assay is now complimented using other cytogenetic chromosomal aberration assays (i.e., chromosomal translocations, cytokinesis-blocked micronuclei, premature chromosome condensation, γ-H2AX foci, etc.). Changes in blood cell counts represent an early-phase biomarker for radiation exposures. Molecular biomarker changes have evolved to include panels of organ-specific plasma proteomic and blood-based gene expression biomarkers for radiation dose assessment. Maturation of these assays are shown by efforts for automated processing and scoring, development of point-of-care diagnostics devices, service laboratories inter-comparison exercises, and applications for dose and injury assessments in radiation accidents. An alternative and complementary approach has been advocated with the focus to de-emphasize "dose" and instead focus on predicting acute or delayed health effects. The same biomarkers used for dose estimation (e.g., lymphocyte counts) can be used to directly predict the later developing severity degree of acute health effects without performing dose estimation as an additional or intermediate step. This review illustrates contributing steps toward these developments published in Radiation Research.

Quantitative assessment of the contribution of clustered damage to DNA double-strand breaks induced by 60Co gamma rays and fission neutrons

The induction of DNA strand breaks by fission neutrons was studied in aqueous plasmid (pBR322) DNA under aerobic conditions for a wide range of hydroxyl radical (*OH) scavenger concentrations and was compared to the induction of strand breaks by 6OCo gamma rays. Strand breaks were measured using agarose gel electrophoresis coupled with sensitive 32P-based phosphor imaging. Yields are reported for DNA single-strand breaks (SSBs) and double-strand breaks formed linearly with dose (alphaDSBs). The fraction of alphaDSBs that were dependent on the multiply damaged site (MDS) or clustered damage mechanism was also calculated using a model. G values for SSBs and alphaDSBs declined with increasing *OH scavenging capacity. However, with increasing *OH scavenging capacities, the decrease in yields of strand breaks for fission neutrons was not as pronounced as for gamma rays. The percentage of alphaDSBs for gamma rays was dependent on *OH scavenging capacity, appearing negligible at low scavenging capacities but increasing at higher scavenging capacities. In contrast, fission neutrons induced high percentages of alphaDSBs that were approximately independent of *OH scavenging capacity. The levels of alphaDSBs formed by the MDS mechanism after exposure to fission neutrons are consistent with the expected distinctive features of high-LET energy deposition events and track structure. The results also confirm observations made by others that even for low-LET radiation, the MDS mechanism contributes significantly to DNA damage at cell-like scavenging conditions.

Natural-history Characterization of a Murine Partial-body Irradiation Model System: Establishment of a Multiple-Parameter Based GI-ARS Severity-Scoring System

The purpose of this investigation was to characterize the natural history of a murine total-abdominal-irradiation exposure model to measure gastrointestinal acute radiation injury. Male CD2F1 mice at 12 to 15 weeks old received total-abdominal irradiation using 4-MV linear accelerator X-rays doses of 0, 11, 13.5, 15, 15.75 and 16.5 Gy (2.75 Gy/min). Daily cage-side (i.e., in the animal housing room) observations of clinical signs and symptoms including body weights on all animals were measured up to 10 days after exposure. Jejunum tissues from cohorts of mice were collected at 1, 3, 7 and 10 days after exposure and radiation injury was assessed by histopathological analyses. Results showed time- and dose-dependent loss of body weight [for example at 7 days: 0.66 (±0.80) % loss for 0 Gy, 6.40 (±0.76) % loss at 11 Gy, 9.43 (±2.06) % loss at 13.5 Gy, 23.53 (± 1.91) % loss at 15 Gy, 29.97 (±1.16) % loss at 15.75 Gy, and 31.79 (±0.76) % loss at 16.5 Gy]. Negligible clinical signs and symptoms, except body weight changes, of radiation injury were observed up to 10 days after irradiation with doses of 11 to 15 Gy. Progressive increases in the severity of clinical signs and symptoms were found after irradiation with doses >15 Gy. Jejunum histology showed a progressive dose-dependent increase in injury. For example, at 7 days postirradiation, the percent of crypts, compared to controls, decreased to 82.3 (±9.5), 69.2 (±12.3), 45.4 (±11.9), 18.0 (±3.4), and 11.5 (± 1.8) with increases in doses from 11 to 16.5 Gy. A mucosal injury scoring system was used that mainly focused on changes in villus morphology damage (i.e., subepithelial spaces near the tips of the villi with capillary congestion, significant epithelial lifting along the length of the villi with a few denuded villus tips). Peak levels of total-abdominal irradiation induced effects on the mucosal injury score were seen 7 days after irradiation for doses ≥15 Gy, with a trend to show a decline after 7 days. A murine multiple-parameter gastrointestinal acute-radiation syndrome severity-scoring system was established based on clinical signs and symptoms that included measures of appearance (i.e., hunched and/or fluffed fur), respiratory rate, general (i.e., decreased mobility) and provoked behavior (i.e., subdued response to stimulation), weight loss, and feces/diarrhea score combined with jejunum mucosal-injury grade score. In summary, the natural-history radio-response for murine partial-body irradiation exposures is important for establishing a well-characterized radiation model system; here we established a multiple-parameter gastrointestinal acute-radiation syndrome severity-scoring system that provides a radiation injury gastrointestinal tissue-based assessment utility.

Nucleic acid molecular biomarkers for diagnostic biodosimetry applications: use of the fluorogenic 5'-nuclease polymerase chain reaction assay

A reliable, relatively easy method for diagnostic assessment of radiation exposure is needed to support the triage of radiation casualties and medical treatment decisions in military defense operations. Our strategy is to identify radiation-responsive DNA mutations and gene expression targets that can be analyzed using polymerase chain reaction (PCR) assays and an existing fluorescence-based nucleic acid analysis system designed for forward-deployable laboratory applications. Using an in vitro model system of human peripheral blood lymphocytes, we identified a candidate nucleic acid biomarker (i.e., gene expression target) that is responsive to ionizing radiation. In this report, we describe our preliminary Haras gene expression findings. A dose-dependent elevation in Haras gene expression levels was demonstrated using Northern-blot analysis 17 hours after exposure to a 250-kVp dose of X-rays (25-100 cGy, 1 Gy/minute); c-Haras expression levels at 100 cGy were ninefold higher than those of controls. An alternative protocol to quantify the Haras cDNA target, using the rapid, real-time reverse transcriptase fluorogenic 5'-nuclease PCR assay, is described, along with a preliminary characterization of the dynamic range for detection. Our research shows that the analysis of multitarget nucleic acid biomarkers, using the multiplex fluorogenic 5'-nuclease PCR assay, has beneficial applications in radiation epidemiology, radiation therapy, and biodosimetry.

Mammalian cells are not killed by DNA single-strand breaks caused by hydroxyl radicals from hydrogen peroxide

Cell killing by ionizing radiation has been shown to be caused by hydroxyl free radicals formed by water radiolysis. We have previously suggested that the killing is not caused by individual OH free radicals but by the interaction of volumes of high radical density with DNA to cause locally multiply damaged sites (LMDS) (J. F. Ward, Radiat. Res. 86, 185-195, 1985). Here we test this hypothesis using hydrogen peroxide as an alternate source of OH radicals. The route to OH production from H2O2 is expected to cause singly damaged sites rather than LMDS. Chinese hamster V79-171 cells were treated with H2O2 at varying concentrations for varying times at 0 degree C. DNA damage produced intracellularly was measured by alkaline elution and quantitated in terms of Gray-equivalent damage by comparing the rate of its elution with that of DNA from gamma-irradiated cells. The yield of DNA damage produced increases with increasing concentration of H2O2 and with time of exposure. H2O2 is efficient in producing single-strand breaks; treatment with 50 microM for 30 min produces damage equivalent to that formed by 10 Gy of gamma irradiation. In the presence of a hydroxyl radical scavenger, dimethyl sulfoxide (DMSO), the yield of damage decreases with increasing DMSO concentration consistent with the scavenging of hydroxyl radicals traveling an average of 15 A prior to reacting with the DNA. In contrast to DNA damage production, cell killing by H2O2 treatment at 0 degree C is inefficient. Concentrations of 5 X 10(-2) M H2O2 for 10 min are required to produce significant cell killing; the DNA damage yield from this treatment can be calculated to be equivalent to 6000 Gy of gamma irradiation. The conclusion drawn is that individual DNA damage sites are ineffectual in killing cells. Mechanisms are suggested for killing at 0 degree C at high concentrations and for the efficient cell killing by H2O2 at 37 degrees C at much lower concentrations.

Optimizing chemical-induced premature chromosome condensation assay for rapid estimation of high-radiation doses

In the event of exposure to high doses of radiation, prompt dose estimation is crucial for selecting appropriate treatment modalities, such as cytokine therapy or stem cell transplantation. The chemical-induced premature chromosome condensation (PCC) method offers a simple approach for such dose estimation with significant radiation exposure, but its 48-h incubation time poses challenges for early dose assessment. In this study, we optimized the chemical-induced PCC assay for more rapid dose assessment. A sufficient number of PCC and G2/M-PCC cells were obtained after 40 h of culture for irradiated human peripheral blood up to 20 Gy. By adding caffeine (final concentration of 1 mM) at 34 h from the start of culture, G2/M-PCC index increased by 1.4-fold in 10 Gy cultures. There was also no significant difference in the G2/M-PCC ring frequency induced for doses 0 to 15 Gy between our 40-h caffeine-supplemented chemical-induced PCC method and the conventional 48-h PCC assay.

Radiation-induced binding of DNA from irradiated mammalian cells to hydroxyapatite columns

In experiments designed to measure radiation-induced DNA damage using the DNA unwinding-hydroxyapatite chromatography technique, we observed that under some experimental conditions a significant proportion of the test DNA became tightly bound to the hydroxyapatite (HA) and could not be released even with a high concentration of phosphate buffer. Approximately 5-10% of DNA from unirradiated cells binds to the HA. With increasing radiation doses in air, the fraction of bound DNA increases, reaching about 30% at about 35 Gy. The binding exhibits many of the characteristics of a radiation-induced cell lesion: the proportion of DNA retained by the HA is less when cells are irradiated under hypoxic conditions or in the presence of the thiol radioprotector dithiothreitol; and the binding decreases when an incubation period is allowed between irradiation and harvest of the cells for assay. Studies to determine the nature of the lesion responsible for the binding demonstrated that lesion production requires a component found in cells since no binding was observed with irradiated isolated DNA or nuclear matrix; the binding is not a result of the production of DNA-protein crosslinks; and the bound DNA is single-stranded, based on its sensitivity to nuclease S1. Because of the dose dependence of the binding of DNA to HA, the slopes of the dose-response curves for DNA damage determined with this assay depend on the method used to calculate the fraction of double-stranded DNA. Our demonstration that the bound DNA is single-stranded guides the choice of the method for data analysis.

Late mitosis/early G1 phase and mid-G1 phase are not hypersensitive cell cycle phases for neoplastic transformation of HeLa x skin fibroblast human hybrid cells induced by fission-spectrum neutrons

A two- to threefold increase in the rate of neoplastic transformation in cells irradiated at a dose rate of 0.22 cGy/min with fission-spectrum neutrons compared to that at 10.7 cGy/min has been confirmed with the use of alkaline phosphatase chromogenic substrate Western Blue staining to detect foci of neoplastically transformed cells through their expression of a tumor-associated antigen, the end point of the HeLa x skin fibroblast human hybrid cell transformation assay. To investigate whether the inverse dose-rate effect is due to the existence of a period in the cell cycle in which cells are significantly more sensitive to neoplastic transformation than in the rest of the cell cycle, as has been postulated previously (Rossi and Kellerer, Int. J. Radiat. Biol. 50, 353-361, 1986; Brenner and Hall, Int. J. Radiat. Biol. 58, 745-758, 1990; Elkind, Int. J. Radiat. Biol. 59, 1467-1475, 1991), we compared the sensitivity of late mitotic/early G1-phase and mid-G1-phase cells with that of asynchronous cells. The rationale for examining these particular cell cycle phases was based on the fact that mitosis has been hypothesized to be a candidate for the extremely sensitive period, and on a preliminary report that mid-G1-phase C3H 10T1/2 cells may exhibit enhanced sensitivity for neutron-induced transformation. A nominal dose of 45 cGy of fission-spectrum neutrons was delivered at approximately 10 cGy/min. The data indicate that neither late mitotic/early G1-phase nor mid-G1-phase cells are significantly more sensitive than asynchronous cells. Further, the dependence on the phase of the cell cycle for neoplastic transformation of CGL1 cells induced by fission-spectrum neutrons is different from that previously demonstrated for gamma radiation, where late-mitotic cells were approximately five times more sensitive than mid-G1-phase and asynchronous cells (Redpath and Sun, Radiat. Res. 121, 206-211, 1990).

Biological dosimetry using human interphase peripheral blood lymphocytes

Conventional metaphase-spread chromosome-aberration-based biodosimetry techniques for radiation dose assessment, although robust, are laborious and time consuming. The molecular cytogenetic laboratory of the Armed Forces Radiobiology Research Institute is developing simple and rapid interphase-based cytological assays that will be applicable to a broad range of radiation exposure scenarios. These assays include analysis of chromosome aberrations (premature chromosome condensation-fluorescence in situ hybridization assay) and mitochondrial DNA mutations (mtDNA4977 deletion assay) using resting human peripheral blood lymphocytes. The dose-effect relationship for radiation-induced aberrations involving chromosome 1 after 24 hours of repair at 37 degrees C in resting human peripheral blood lymphocytes was studied using fluorescence in situ hybridization after chemical induction of premature chromosome condensation as previously explained. In the present study, we examined whether gamma irradiation in the range of 0 to 7.5 Gy induces a dose-dependent increase in aberrations manifested as "excess spots." The number of excess spots per cell, reflecting aberrations involving chromosome 1, increased from 0.035 at 0.5 Gy to 0.236 at 7.5 Gy. This observed dose-effect relationship was fit with a nonlinear power model. This technique may be extended to the study of radiation-induced translocations in interphase cells for retrospective dose reconstruction. With a recently developed in situ polymerase chain reaction method to detect and quantify mtDNA deletion in interphase cells after radiation exposure in cultured human peripheral blood lymphocytes, 90% to 95% of cells are analyzable. We discuss the potential use of the mtDNA deletion assay in biological dosimetry applications. Interphase-based cytological assays may eliminate some inherent problems associated with metaphase-spread-based assays. These problems involve (1) the limited number of analyzable cells containing chromosome aberrations, which is due to various factors including radiation-induced cell death and delay in cell cycle progression into mitosis, and (2) the requirements for radiation cytogenetics expertise and tedious labor to manually score chromosome aberrations.

Severity scoring systems for radiation-induced GI injury - Prioritization for use of GI-ARS medical countermeasures

PURPOSE

Severity scoring systems for ionizing radiation-induced gastrointestinal injury have been used in animal radiation models, human studies involving the use of radiation therapy, and radiation accidents. Various radiation exposure scenarios (i.e., total body irradiation, total abdominal irradiation, etc.) have been used to investigate ionizing radiation-induced gastrointestinal injury. These radiation-induced GI severity scoring systems are based on clinical signs and symptoms and gastrointestinal-specific biomarkers (i.e., citrulline, etc.). In addition, the time course for radiation-induced changes in blood citrulline levels were compared across various animal (i.e., mice, minipigs, Rhesus Macaque, etc.) and human model systems.

CONCLUSIONS

A worksheet tool was developed to prioritize individuals with severe life-threatening gastrointestinal acute radiation syndrome, based on the design of the Exposure and Symptom Tool addressing hematopoietic acute radiation syndrome, to rescue individuals from potential gastrointestinal acute radiation syndrome injury. This tool provides a triage diagnostic approach to assist first-responders to assess individuals suspected of showing gastrointestinal acute radiation syndrome severity to guide medical management, hence enhancing medical readiness for managing radiological casualties.

Hydrogen peroxide-induced base damage in deoxyribonucleic acid

Aqueous solutions of calf thymus deoxyribonucleic acid (DNA) were exposed to hydrogen peroxide in the presence of air. Base products formed in DNA were identified and quantitated following acid hydrolysis and trimethylsilylation using gas chromatography-mass spectrometry. The yields of these products were dependent upon the hydrogen peroxide concentration, and increased in the following order: 8-hydroxyadenine, cytosine glycol, 2,6-diamino-4-hydroxy-5-formamidopyrimidine, 8-hydroxyguanine, thymine glycol, and 4,6-diamino-5-formamidopyrimidine. Previous studies have shown that these compounds are typically formed in DNA in aqueous solution by hydroxyl radicals generated by ionizing radiation. Hydrogen peroxide is thought to participate in a Fenton-like reaction with transition metals, which are readily bound to DNA in trace quantities, resulting in the production of hydroxyl radicals close to the DNA. This proposed mechanism was examined by exposing DNA to hydrogen peroxide either in the presence of a hydroxyl radical scavenger or following pretreatment of DNA with metal-ion chelators. The results indicate that trace quantities of transition metal ions can react readily with hydrogen peroxide to produce radical species. The production of radical species was monitored by determining the altered bases that resulted from the reaction between radicals and DNA. The yields of the base products were reduced by 40 to 60% with 10 mmol dm-3 of dimethyl sulfoxide. A 100-fold increase in the concentration of dimethyl sulfoxide did not result in a further reduction in hydrogen peroxide-induced base damage. DNA which was freed from bound metal ions by pretreatment with metal ion chelators followed by exhaustive dialysis was found to be an ineffective substrate for hydrogen peroxide. The yields of base products measured in this DNA were at background levels. These results support the role of metal ions bound to DNA in the site-specific formation of highly reactive radical species, most likely hydroxyl radicals, in hydrogen peroxide-induced damage to the bases in DNA.

Quantitative measurement of radiation-induced base products in DNA using gas chromatography-mass spectrometry

Gas chromatography-mass spectrometry with selected-ion monitoring was used to study radiation-induced damage to DNA. Quantitative analysis of modified purine and pyrimidine bases resulting from exposure to ionizing radiation using this technique is dependent upon the selection of appropriate internal standards and calibration of the mass spectrometer for its response to known quantities of the internal standards and the products of interest. The compounds 6-azathymine and 8-azaadenine were found to be suitable internal standards for quantitative measurement of base damage in DNA. For the purpose of calibration of the mass spectrometer. relative molar response factors for intense characteristic ions were determined for the trimethylsilyl derivatives of 5-hydroxyuracil, thymine glycol, and 5,6-dihydrothymine using 6-azathymine, and for the trimethylsilyl derivatives of 4,6-diamino-5-formamidopyrimidine, 8-hydroxyadenine, 2,6-diamino-4-hydroxy-5-formamidopyrimidine, and 8-hydroxyguanine using 8-azaadenine. Accurate measurement of the yield of radiation-induced modifications to the DNA bases is also dependent upon two chemical steps in which the purines and pyrimidines are released from the sugar-phosphate backbone and then derivatized to make them volatile for gas chromatography. The completeness of these reactions, in addition to assessing the stability of the modified DNA bases in acid and their trimethylsilylated derivatives over the time necessary to complete the experimental analysis was also examined. Application of this methodology to the measurement of radiation-induced base modification in heat-denatured, nitrous oxidesaturated aqueous solutions of DNA is presented.

Effects of inhibitors of DNA strand break repair on HeLa cell radiosensitivity

The effects of three drugs (hydroxyurea, 1-beta-arabinofuranosylcytosine, and diamide) known to inhibit DNA synthesis on the repair of ionizing radiation-induced DNA single-strand breaks measured by alkaline elution and on cellular radiosensitivity were examined. Inhibition of repair was observed at 10(-2) M hydroxyurea, 10(-4) M 1-beta-D-arabinofuranosylcytosine, and 5 X 10(-5) M diamide, levels causing only 10% cell kill. While the mechanisms by which the drugs inhibit DNA synthesis differ, they are equally effective at inhibiting repair; without drug, cells, after a dose of 10 grays, repair 35% of DNA strand breaks in 3 min and a further 35% in 1 hr; with drug, only 10% is repaired in 3 min, and the deficiency in repair amount remains, even after 60 min. The effect of similar drug treatment on radiation-induced cell killing shows that radiosensitivity is increased; the major effect is reduction in D0 from 1.3 grays to approximately 0.8 grays with smaller effects on Dq. The data are consistent with the hypothesis that radiation produces potential double-strand breaks in DNA which, if not rapidly repaired, are converted into lethal actual double-strand breaks.

Application of the premature chromosome condensation assay in simulated partial-body radiation exposures: evaluation of the use of an automated metaphase-finder

The premature chromosome condensation (PCC) assay has been proposed as a useful and rapid end point for biological dosimetry following accidental high-dose radiation overexposures. A major benefit of the PCC assay is that it does not require cells to divide for evaluation of cytogenetic damage. The PCC assay was performed on isolated human peripheral lymphocytes exposed in vitro to doses from 1 to 9 Gy of 250 kVp x-rays. The dose-response relationships of the frequency distribution and the yield of PCC fragments in cells were determined after one day of repair at 37 degrees C. A Qpcc approach, which involves the analysis of the yield of excess PCC fragments in damaged cells, was used to establish a dose-response calibration curve. This method is identical in concept to the Qdr technique introduced by Sasaki for partial-body exposure dose-estimates using asymmetrical chromosome aberrations (i.e., dicentrics and rings) in metaphase spreads of human lymphocytes. A simulated in vitro test of a partial-body exposure to a 6-Gy dose was performed. The results from this test provided dose estimates of 5.3 +/- 0.6, 4.7 +/- 0.6, 5.0 +/- 0.6 and 4.7 +/- 0.8 Gy for the 20, 30, 50 and 75 percent component of 6-Gy irradiated cells, respectively. An automated metaphase-finding system was evaluated for use with the PCC assay. This system helped to locate PCC spreads among the mitotic inducer Chinese hamster ovary (CHO) metaphase spreads, thereby facilitating rapid scoring of samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction by H2O2 of DNA and interphase chromosome damage in plateau-phase Chinese hamster ovary cells

The induction by H2O2 of DNA breaks, DNA double-strand breaks (DSBs), and interphase chromatin damage and their relationship to cytotoxicity were studied in plateau-phase Chinese hamster ovary (CHO) cells. Damage in interphase chromatin was assayed by means of premature chromosome condensation (PCC); DNA DSBs were assayed by nondenaturing filter elution (pH 9.6), and DNA breaks by hydroxyapatite chromatography. Cells were treated with H2O2 in suspension at 0 degrees C for 30 min and treatment was terminated by the addition of catalase. Concentrations of H2O2 lower than 1 mM were not cytotoxic, whereas concentrations of 40 and 60 mM reduced cell survival to 0.1 and 0.004, respectively. An induction of DNA breaks that was dependent on H2O2 concentration was observed at low H2O2 concentrations that reached a maximum at approximately 1 mM; at higher H2O2 concentrations induction of DNA breaks either remained unchanged or decreased. Damage at the chromosome level was not evenly distributed among the cells, when compared to that expected based on a Poisson distribution. Three categories of cells were identified after exposure to H2O2: cells with intact, control-like chromosomes, cells showing chromosome fragmentation similar to that observed in cells exposed to ionizing radiation, and cells showing a loss in the ability of their chromatin to condense into chromosomes under the PCC reaction. The fraction of cells with fragmented chromosomes, as well as the number of excess chromosomes per cell, showed a dose response similar to that of DNA DSBs, reaching a maximum at 1 mM and decreasing at higher concentrations. The results indicate that induction of DNA and chromosome damage by H2O2 follows a complex dependence probably resulting from a depletion of reducing equivalents in the vicinity of the DNA. Reducing equivalents are required to recycle the transition metal ions that are needed to maintain a Fenton-type reaction. The absence of cell killing at H2O2 concentrations that yielded the maximum amount of DNA and chromosome damage suggests that this damage is nonlethal and repairable. It is suggested that lethal DNA and chromosome damage is induced at higher concentrations of H2O2 where cell killing is observed by an unidentified mechanism.