A system for mutation measurement in mammalian cells: application to gamma-irradiation

Monitoring of mutagenesis by environmental agents for the purpose of preventing genetic disease including cancer must include quantitation of cell killing, sensitive measurement of mutation production by appropriate doses of each agent, and assessment of mutation repair effects in mammalian cells. A four-step procedure, in the presence and absence of a repair suppressor, is proposed: (i) determination of the survival curve; (ii) measurement of the mitotic index in cells collected after treatment with colcemid; (iii) construction of a mutagenesis yield curve in the presence and absence of a repair suppressor, like caffeine; and (iv) assessment of the effect of test agents on the repair of mutations produced by other mutagens. The procedure is quantitative, reproducible, and reasonably rapid. It involves measurement of mutations causing visible chromosomal aberrations. Numerical parameters are proposed defining quantitatively mutation, cell killing, and mutation repair capacity. The procedure is applied to gamma-irradiation and can detect the effects of doses as low as 2-5 cGy. Theoretical analysis of the underlying processes is presented, using the concept of D(0)E, the effective dose of mutagen after repair mechanisms and neutralizing agents have acted. Microscopically visible chromosome aberrations are due to mutations that distort the process of mitotic chromosome condensation, with or without actual chromosome breakage.     

Genetic resistance to urethan-induced pulmonary adenomas in SMXA recombinant inbred mouse strains

Development of pulmonary adenomas (PAs) in mice is under the genetic control of multiple host genes. We have established a new set of SMXA recombinant inbred strains from PA-susceptible A/J and PA-resistant SM/J mice. The number of urethan-induced PAs was variable among substrains of the SMXA recombinant inbred strains, indicating the involvement of multiple genes. SMXA24 mice were highly resistant to PA, although they had susceptible alleles at all four known susceptibility genes, including kras2 and MHC. To identify the resistance gene in SMXA24, progeny of reciprocal F1 crosses and progeny of backcrosses to A/J were given urethan at 4 weeks of age and examined for induced PA at the age of 5 months. In reciprocal F1 cross progeny, the incidence of PA was very low, indicating that the resistance was a semidominant trait. Quantitative trait analysis of the backcross generation revealed significant linkages to loci on chromosome 12 (logarithm of odds score, 6.47) and chromosome 11 (logarithm of odds score, 4.35). To date, two PA resistance (PAR) genes, Par1 (located on chromosome 11) and Par2 (located on chromosome 18), have been reported. From the map position, one of the resistance genes on chromosome 11 was indistinguishable from Par1. However, another resistance gene on chromosome 12 was new, and we named this gene Par3. A likely candidate gene for Par3 is nPKCn, which is expressed exclusively in skin and lung and is down-regulated in PA. Par1 and Par3 seemed to act synergistically.     

An analysis of the cellular distribution of chromosome aberrations induced in human lymphocytes after a single irradiation and under conditions of preliminary adaptive exposure

Irradiation by an adaptive dose 0.05 Gy at the G0 stage decreased the number of chromosome aberrations induced in lymphocytes by a challenge dose 0.5 Gy at the G2 stage. Adaptive response was not observed at the G1 stage, when the cells were exposed to adaptive dose 0.05 Gy and challenge dose 1.0 Gy respectively after 24 h and 29 h incubation with PHA. In lymphocytes exposed to 1.0 Gy at the G1 stage, cellular distribution of chromosomal aberrations followed the Poisson distribution, while in lymphocytes exposed to 0.5 Gy at the G2 stage, the distribution of aberrations differed from the Poisson distribution and was nearer to the degenerated Poisson distribution. The adaptive dose 0.05 Gy did not alter the distribution of chromosome aberrations induced by the challenge dose at the G1 or the G2 stages. The role of independent and whole-cellular repair in the formation of chromosome and chromatid aberration is discussed.     

Comparative studies of the mitogenic effects of epidermal growth factor and transforming growth factor-alpha and the expression of various growth factors in neoplastic and non-neoplastic prostatic cell lines

Murine acute myeloid leukemia is characterized by chromosome 2 aberrations, and genesis of the marker chromosome 2 by radiation is suspected to be an initiating event of radiation leukemogenesis. A detailed analysis of the type and frequency of chromosome 2 aberrations in murine bone marrow cells at an early stage after irradiation is provided here. A total of 40 male C3H/He mice was exposed to 137Cs gamma-ray at a dose of 1, 2 or 3 Gy, and sacrificed 24 hours after irradiation. Metaphase samples prepared from bone marrow cells were Q-banded for karyotyping or painted with DNA probes specific to chromosome 2. In 5 mice analyzed by karyotyping, one mouse showed high frequency of the marker aberrations as well as other chromosome 2 aberrations. Chromosome painting analysis for the rest of the mice also detected 3 animals showing significantly high frequencies of chromosome 2 aberrations. Dose-dependence of the frequencies was observed even among those mice that tended to be sensitive. The results indicated that there was a subgroup of mice carrying hypersensitive chromosome 2. The subgroup could be leukemia-sensitive if radiation-induced chromosome aberrations are responsible for an early change in myeloid leukemogenesis.     

Dose-rate dependence of heat radiosensitization

The dose rate dependence of heat radiosensitization was studied using rat astrocytoma cells in culture and a clinically relevant protocol of heat dose and heat radiation sequence. Cells were treated with a minimally toxic heat dose of 43 degrees C for 30 minutes, after which they were irradiated with varying doses of radiation at dose rates ranging from 0.567 to 300 cGy/min. This heat dose substantially reduced the extrapolation number (n), but had little effect on Do of the radiation survival curve at dose rates of 50 cGy/min or greater. At dose rates less than 10 cGy/min, 43 degrees C for 30 min had little effect on n and only for the lowest dose rate studied (0.567 cGy/min) was there a significant reduction in Do (60%). The thermal enhancement ratio did not vary inversely with radiation dose rate over the dose rate range studied but, instead, was maximal at the two dose rate extremes (0.567 and 300 cGy/min). These data demonstrate that a clinically relevant heat dose enhances very low dose rate, as well as high dose rate, ionizing radiation, but suggest that little benefit is to be gained from using dose rates intermediate between conventional radiotherapeutic high dose rates or dose rates representative of interstitial implants.     

Effect of in vivo exposure to iodine-131 on the frequency and persistence of micronuclei in human lymphocytes

The validity of the micronucleus test as a biomarker of chromosome damage in dividing mammalian cells is well established. This assay was used to study the response of peripheral lymphocytes of a 34-yr-old male patient following treatment with 131I ablative radiation therapy following a total thyroidectomy. Coincidentally, 8 mo before diagnosis, the patient had provided a blood sample for an in vitro study of micronucleus induction following exposure to graded doses of x-rays. The background frequency in the unexposed culture showed a mean count of 6.0 micronuclei per 1000 binucleated (first division) lymphocytes, while mean values of 18.5, 29.0, 41.0, 61.0 and 75.5 micronuclei/1000 cells were observed following x-ray doses of 5, 10, 15, 20, and 25 cGy, respectively. These data fit a nonthreshold, linear dose-response function (y = 2.78x + 3.71; r = .99). Eight months after the in vitro x-ray study, the subject was diagnosed with thyroid cancer. Surgery was performed, and 5 wk later the patient received 1.78 GBq (48 mCi) of 131I as adjuvant radiation therapy. Blood was drawn 11 d after the radiation treatment and at monthly intervals thereafter to analyze the frequency and persistence of micronuclei. The first posttreatment sample showed 35.5 micronuclei per 1000 binucleate cells. Based on the linear dose-response equation from the earlier study, the sixfold increase in micronucleus frequency suggests a dose to the peripheral blood of approximately 11 cGy. The cytogenetic dose estimate compares to approximately 30 cGy using a new model based on external whole-body counting data. Nine consecutive monthly samples have been analyzed to date. Although the micronucleus count has fluctuated (four- to sixfold above background), the frequency after 8 mo is equivalent to the first posttreatment sample. Data show that radiation-induced cellular lesions persist for months following relatively brief radiation exposure to a medical isotope. Results of this study support the conclusion that the lymphocyte micronucleus test is a rapid, sensitive, and perhaps quantitative biomarker of low-dose (< 25 cGy) radiation exposure.     

Chromosome aberrations in human fibroblasts induced by monoenergetic neutrons. I. Relative biological effectiveness

The relative biological effectiveness (RBE) of neutrons for many biological end points varies with neutron energy. To test the hypothesis that the RBE of neutrons varies with respect to their energy for chromosome aberrations in a cell system that does not face interphase death, we studied the yield of chromosome aberrations induced by monoenergetic neutrons in normal human fibroblasts at the first mitosis postirradiation. Monoenergetic neutrons at 0.22, 0.34, 0.43, 1, 5.9 and 13.6 MeV were generated at the Accelerator Facility of the Center for Radiological Research, Columbia University, and were used to irradiate plateau-phase fibroblasts at low absorbed doses from 0.3 to 1.2 Gy at a low dose rate. The reference low-LET, low-dose-rate radiation was 137Cs-gamma rays (0.66 MeV). A linear dose response (Y = alphaD) for chromosome aberrations was obtained for all monoenergetic neutrons and for the gamma rays. The yield of chromosome aberrations per unit dose was high at low neutron energies (0.22, 0.34 and 0.43 MeV) with a gradual decline with the increase in neutron energy. Maximum RBE (RBEm) values varied for the different types of chromosome aberrations. The highest RBE (24.3) for 0.22 and 0.43 MeV neutrons was observed for intrachromosomal deletions, a category of chromosomal change common in solid tumors. Even for the 13.6 MeV neutrons the RBEm (11.1) exceeded 10. These results show that the RBE of neutrons varies with neutron energy and that RBEs are dissimilar between different types of asymmetric chromosome aberrations and suggest that the radiation weighting factors applicable to low-energy neutrons need firmer delineation. This latter may best be attained with neutrons of well-defined energies. This would enable integrations of appropriate quality factors with measured radiation fields, such as those in high-altitude Earth atmosphere. The introduction of commercial flights at high altitude could result in many more individuals being exposed to neutrons than occurs in terrestrial workers, emphasizing the necessity for better-defined estimates of risk.     

A comparative study of radioprotection by Ocimum flavonoids and synthetic aminothiol protectors in the mouse

The radioprotective effects of two flavonoids, orientin (Ot) and vicenin (Vc), obtained from the leaves of Ocimum sanctum, and the synthetic compounds WR-2721 and MPG (2-mercaptopropionyl glycine) have been compared by examining chromosome aberration in cells of bone marrow in irradiated mice. Healthy adult Swiss mice were injected intraperitoneally (i.p.) with 50 micrograms kg-1 body weight of Ot or Vc; 20 mg kg-1 of MPG; 150 mg kg-1 of WR-2721 or double distilled water (DDW). They were exposed to whole body irradiation of 2.0 Gy gamma radiation 30 min later. After 24 h, chromosomal aberrations were studied in the bone marrow of the femur by routine metaphase preparation after colchicine treatment. Radiation (2 Gy) increased the number of aberrant cells from less than 1% in controls to almost 20%. Pre-treatment with all the protective compounds resulted in a significant reduction in the percentage of aberrant metaphases as well as in the different types of aberration scored. Vc produced the maximum reduction in percent aberrant cells while MPG was the least effective; Ot and WR-2721 showed an almost similar effect. However, WR-2721 was the most effective against reduction of complex an almost similar effect. However, WR-2721 was the most effective against reduction of complex aberrations, followed by Vc. Neither flavonoids had any systemic toxicity, even at 200 mg kg-1 body weight. Considering the low dose needed for protection and the high margin between the effective and toxic doses, the ocimum flavonoids may be promising for human radiation protection.     

Hypersensitivity following low one-time irradiation dosage and induced radio resistance

There is now little doubt of the existence of radioprotective mechanisms, or stress responses, that are upregulated in response to exposure with small doses of ionizing radiation and other DNA-damaging agents. Phenomenologically, there are two ways in which these induced mechanisms operate. First, a small conditioning dose (generally below 30 cGy) may protect against a subsequent, separate irradiation. This has been termed the adaptive response. Second, the response to single doses may itself be dose-dependent so that small acute radiation exposures are more effective per unit dose than larger exposures above the threshold where the induced radioprotection is triggered. This combination has been termed low dose hypersensitivity (HRS) and induced radioresistance (IRR) as the dose increases. Both the adaptive response and HRS/IRR have been well documented in studies with yeast, bacteria, protozoa, algae, higher plant cells, insect cells, mammalian and human cells in vitro, and in studies on animal models in vivo. There is indirect evidence that the adaptive response and the IRR phenomenon in response to single doses is a manifestation of the same underlying mechanisms, namely an increase of the amount and rate of DNA repair induced by low radiation doses.     

Posterior tibial neuropathy from ruptured Baker''s cyst

BACKGROUND: There is epidemiological evidence that suggests there are beneficial effects of ionizing radiation at low doses. Some experimental studies confirmed this hormetic effect with doses of about 1 cGy/day, but no data concerning very low dose rates are available. OBJECTIVE: The aim of this study was to determine the life span of mice exposed to very low doses of ionizing radiation. METHODS: Six hundred female C57BL/6 mice, 1 month old, were exposed to chronic gamma irradiation at very low dose rates of 7 or 14 cGy/year. These doses are about 25 or 50 times higher than background, but much lower than the doses of about 1 cGy/day used in previous experiments. Three hundred mice living in the same room were used as controls. RESULTS: The life span, after the beginning of the experiment, determined by the survival time of 50% of each population, is increased in irradiated mice: 549 days in controls, 673 days in both irradiated groups. The differences are significant between the control and the irradiation mice. Differences between mice irradiated with 7 or 14 cGy are not significant. CONCLUSIONS: These results confirm the possibility of a nonharmful effect (hormesis) of ionizing radiation. They demonstrate that the paradigm, which states that low-dose effects can be predicted high-dose effects, cannot be systematically applied in radiation biology in general and gerontology in particular.     

Potentiation of radiation-induced regrowth delay in murine tumors by fludarabine

Fludarabine (9-beta-D-arabinofuranosyl-2-fluoroadenine-5'-monophosphate), an adenine nucleoside analogue, has previously been shown to inhibit the repair of radiation-induced chromosome damage. Thus fludarabine may have therapeutic utility in combination with photon irradiation. The purpose of this study was to determine whether fludarabine could enhance radiation-induced murine tumor regrowth delay and to determine the most effective dose and schedule of the combination. A significant (P < 0.05) absolute regrowth delay enhancement was observed in three murine tumor models (SA-NH, a sarcoma; and MCA-K and MCA-4, mammary carcinomas) when fludarabine (800 mg/kg) was given 1 h prior to 25 Gy gamma-irradiation. While fludarabine enhanced radiation-induced tumor regrowth delay when given between -36 h and +6 h of radiation (SA-NH tumor), the greatest enhancement was observed when fludarabine was given at -24 h prior to irradiation (radiation dose modification factor of 1.82 at -24 h compared to 1.57 at -3 h prior to radiation). The degree of fludarabine enhancement (at -3 or -24 h) was dose dependent at doses above 200 mg/kg. When fludarabine and radiation were administered on a fractionated schedule (fludarabine given 3 h prior to radiation each day for 4 days), the dose modification factor increased to 2.14 (1.63 if the effect of fludarabine alone is subtracted). These results suggest that fludarabine enhances radiation-induced tumor regrowth delay in a more than additive fashion after both single and fractionated treatments, and the degree of enhancement is dependent on the sequence and timing of administration, the fludarabine dose, and the tumor type. Thus, fludarabine may have clinical potential as a radiation enhancer in the treatment of solid tumors.     

Tc-99m tetrofosmin myocardial perfusion SPECT after dipyridamole combined with low-level exercise in the diagnosis of coronary artery disease

Thymic lymphoma (TL) was observed in different stages of development in 46% of male mice (23/50) following exposure to an acute challenge dose of 2 Gy 60Co gamma-rays. With an adapting dose of 1 cGy 24 h prior to the challenge dose of 2 Gy, similar growth of TL was seen in 42.5% of mice (17/40). TL was not found in unirradiated control mice (0/50) or in the group treated with 1 cGy (0/50). Multiple adapting doses for 5 or 10 consecutive days induced TL in 8/50 and 9/50 mice, respectively (17% in average). When multiple adapting doses were followed by the challenge dose, the yield of TL was much lower, 16% (8/50) and 30% (15/50), respectively. By 15, 30, 60, 90, and 120 days after exposure with 3 Gy of 60Co gamma-rays, TL developed in 30, 70, 70, 80 and 85% of the female mice, respectively. When mice were conditioned with an adapting dose of 1 cGy 24 h prior to the challenge dose, TL was not found 15 days post-irradiation, while about a 25% reduction in the occurrence of TL was noticed at all other intervals. The results suggested that an adapting dose could play a role in bringing about a change in terms of delay and inhibition of the acute effects of radiation, i.e., the onset of TL in mice.     

Radiation-induced chromosomal aberrations in human lymphocytes. I. Dependence on the dose of gamma-rays and an anomaly at low doses

Cultures of human lymphocytes obtained from blood of healthy adult donors were irradiated with different doses of 60Co gamma-rays and the irradiated cells were analysed in metaphase 50 h after irradiation. The effect (total yield of aberrations of chromosome type, or total yield of exchange type aberrations) produced by the lowest dose (5 rad) appears to be statistically significant in a sample of 1500 cells. In the usual dose range (25-400 rad), both parabolic and linear-quadratic equations give a satisfactory fit of experimental data (dicetrics, fragments, or all aberrations of chromosome type). Low doses of gamma-rays, however, produced more aberrations than expected, if one extrapolates dose-effect curves from higher doses. Both relations should be considered, therefore, merely as empirical equations. Dicentrics show at low doses (10-30 rad) a plateau which appears to be statistically significant. Some indications are obtained that the total number of chromosome-type aberrations is a more reliable criterion of cytogenetic damage than the usually accepted yield of dicentrics and rings.     

Brain potentials associated with eye fixations during visual tasks under different lighting systems

The incidence of non-familial multiple primary cancer in colorectal cancer patients has increased in recent years in Japan. To clarify the characteristic genetic aberrations in such multiple cancers, we examined structural chromosomal aberrations by fluorescence in situ hybridization, using chromosome 17-specific and p53 cosmid DNA probes. We established short-term cultures of 78 surgical specimens and were able to obtain observable metaphase spreads in 23 single colorectal cancer specimens and in 6 colorectal cancer specimens from patients with double primary cancers. The frequency of chromosome 17 and/or p53 locus translocation was significantly greater in tumors with double cancer than in single colorectal cancers (P < 0.05 and P < 0.01, respectively). These aberrations in double cancers frequently appeared even at an early Dukes' stage (A and B) of colorectal carcinoma. Our results suggest that translocation of chromosome 17 and the p53 locus may be specific genetic events probably associated with carcinogenesis of multiple primary cancers in colorectal cancer.     

Cloning of a trans-spliced glyceraldehyde-3-phosphate-dehydrogenase gene from the potato cyst nematode Globodera rostochiensis and expression of its putative promoter region in Caenorhabditis elegans

Miral 500 CS (CAS# 42509-80-8), an organophosphorus insecticide, has been widely used in Columbia to fumigate coffee plantations. Therefore, there is extensive human exposure to this pesticide. Miral's mutagenic and genotoxic activities, however, are not known. In this study, such activities of the pesticide were evaluated using the Salmonella TA98/S9 test and the chromosome aberration assay in bone marrow cells of Swiss albino CD1 male mice. All doses tested with Salmonella in the presence of S9 mix (3.2, 16, 80, 400 and 2000 micrograms/plate) induced a mutagenic response that was three times the spontaneous mutation frequency. The mutagenic response without S9 was twice the spontaneous frequency. Based on a 4-day treatment (i.p.) of mice with Miral, the median lethal dose (LD50) and the maximum tolerated dose (MTD) were 912.5 mg/kg and 730 mg/kg, respectively. A significant dose-dependent cell cycle delay (r2 = 0.85, p < 0.01) was observed in bone marrow cells when mice were treated for 24 h with 73, 146, 219, 292, 365, 438, 511, 584, 657 and 730 mg/kg. Significant increase in mitotic indices (p < 0.02) and chromosome aberrations (p < 0.05) were induced in bone marrow cells, when mice were treated for 18 h with the highest dose 511 mg/kg. Our results indicate that Miral is a mutagenic compound in Salmonella and is capable of inducing chromosome aberrations at high doses in mice. Additional genotoxicity studies in farmers exposed to Miral should be conducted to determine the potential human health risk resulting from chronic low-dose exposures to this pesticide.     

Lymphocyte chromosome studies in humans exposed to chemical mutagens. The validity of the method in 67 patients under cytostatic therapy

Eighty-three lymphocyte cultures from 67 patients exposed to high therapeutic doses of chemical mutagens and from 10 healthy controls were examined for structural aberrations of the chromatid type (gaps, breaks and exchanges) and chromosome type (rings, dicentrics, acentric fragments and abnormal chromosomes) in order to evaluate the reliability of this testing system. Most of the patients had received several drugs; a few had radiotherapy as well. Owing to insufficient yields of mitoses at shorter incubation periods a culture time of 72 h had to be chosen. Whenever possible, 100 mitoses were analyzed. Because evaluation is highly subjective, gaps (i.e. interruptions of the chromatid structure not clearly exceeding a chromatid's width) were not included in the results. The incidence of chromatid breaks was 0--2% in the controls and 0--4% in most of the patients. In 6 cases containing 5--15% mitoses with chromatid breaks and exchanges, these values did not correlate with increased incidences of chromosome type aberrations. The incidence of chromosome type aberrations was 0--1% in the controls as well as in 19/20 patients who had received anti-metabolites and spindle poisons only and in 22/53 patients who had received therapeutic irradiation and/or well-known clastogenic agents. From these findings it is concluded that an increased incidence of chromatid breaks and exchanges is not a typical finding in lymphocyte cultures of persons exposed in vivo to chromosome breaking agents, and that a normal incidence of chromosome type aberrations does not exclude exposure to massive doses of clastogens. This testing system is therefore judged to be inadequate for monitoring weak or questionable mutagens in exposed populations.     

G2 repair and evaluation of the cytogenetic damage induced by low doses of X-irradiation during G0 in human lymphocytes

In the present study two cytogenetic parameters were used to evaluate the DNA damage induced by low doses (1 up to 40 rad) of X-ray irradiation in G0 human lymphocytes. These parameters were the frequency of chromosomal lesions in G2 and the length of this cell cycle phase. The frequency of chromosomal lesions in G2 was determined by scoring the number of chromosomal aberrations in G0 irradiated lymphocytes post treated with two inhibitors of G2 repair mechanisms: caffeine and 3-aminobenzamide. A dose-dependent increase in chromosomal aberrations yield was detected in G0 lymphocytes X-ray irradiated with or without post treatment with these two DNA repair inhibitors during G2. Nevertheless, the dose response in this latter condition was higher than the one detected in control cells, indicating that the increase of irradiation dose in G0 lymphocytes produces an increment in the number of DNA lesions arriving to be repaired in G2. The analysis of the dose-response relationships for G2 length showed an statistically significant X-ray dose-dependent increase (G2 delay) from 2.5 up to 40 rad and a positive correlation between G2 delay and the frequency of chromosomal lesions in G2. These results suggest that the DNA lesions induced by low doses of X-irradiation in G0 lymphocytes may be higher than that detected by the standard method (control conditions) and may be responsible for an increase in G2 length. We propose, therefore, that an analysis of these two cytogenetic parameters can improve the evaluation of the DNA damage induced by low doses of X-rays irradiation in G0 cells.     

In vivo damage and recA-dependent repair of plasmid and chromosomal DNA in the radiation-resistant bacterium Deinococcus radiodurans

Deinococcus radiodurans R1 and other members of this genus share extraordinary resistance to the lethal and mutagenic effects of ionizing radiation. We have recently identified a RecA homolog in strain R1 and have shown that mutation of the corresponding gene causes marked radiosensitivity. We show here that following high-level exposure to gamma irradiation (1.75 megarads, the dose required to yield 37% of CFU for plateau-phase wild-type R1), the wild-type strain repairs > 150 double-strand breaks per chromosome, whereas a recA-defective mutant (rec30) repairs very few or none. A heterologous Escherichia coli-D. radiodurans shuttle plasmid (pMD68) was constructed and found to be retained in surviving D. radiodurans R1 and rec30 following any radiation exposure up to the highest dose tested, 3 megarads. Plasmid repair was monitored in vivo following irradiation with 1.75 megarads in both R1/pMD68 and rec30/pMD68. Immediately after irradiation, plasmids from both strains contained numerous breaks and failed to transform E. coli. While irradiation with 1.75 megarads was lethal to rec30 cultures, a small amount of supercoiled plasmid was regenerated, but it lacked the ability to transform E. coli. In contrast, wild-type cultures showed a cell division arrest of about 10 h, followed by exponential growth. Supercoiled plasmid was regenerated at normal levels, and it readily transformed E. coli. These studies show that D. radiodurans retains a heterologous plasmid following irradiation and repairs it with the same high efficiency as its chromosomal DNA, while the repair defect in rec30 prevents repair of the plasmid. Taken together, the results of this study suggest that plasmid DNA damaged in vivo in D. radiodurans is repaired by recA-dependent mechanisms similar to those employed in the repair of chromosomal DNA.     

Perceptions of tonal changes in normal laryngeal, esophageal, and artificial laryngeal male Cantonese speakers

PURPOSE: Present radiobiological studies for different cell lines in vitro demonstrate the equivalence and efficacy of continuous low-dose-rate brachytherapy (LDR-BT) and pulsed dose rate brachytherapy (PDR-BT) when using small and frequent dose pulses. The aim of this study was to examine monolayer fibroblast cultures in vitro to examine the biological effects of different pulse doses and dose rates under clinically conditions. MATERIAL AND METHODS: B14 cells, Hy B14 FAF 28, peritoneal fibroblasts, were cultured in multi-well plates and exposed to a PDR radiation source at a distance of 9 mm. The following PDR-schemes were compared: dose per pulse: 1 Gy, 2.5 Gy and 5 Gy to a total dose of 5 Gy/5 h (overall time), 10 Gy/10 h, 20 Gy/20 h and 30 Gy/30 h. The pulse duration for the examination of dose rate effects was 20 min, 30 min or 52 min corresponding by dye pulse dose rate of 300 cGy/h, 200 cGy/h or 115 cGy/h. Treatment endpoints were cell measured by dye exclusion test and clonogenic cell survival. RESULTS: Cell survival decreased for pulse doses of 5 Gy compared to 2.5 Gy or 1 Gy per pulse (mean dose rate 200 to 300 cGy/h). No differences were observed with dose rates during irradiation of 300 cGy/h, 200 cGy/h or 115 cGy/h (20 Gy/1 Gy). CONCLUSION: Radiobiological effects of PDR-RT are dependent on the dose per pulse, with differences in biological effects only with a dose per pulse of more than 2.5 Gy, considering the described in-vitro conditions. More examinations with a more pronounced difference in dose rate will be continued for evaluation of dose rate effects.     

Limitations for three-dimensional ultrasound imaging through a bore-hole trepanation

It has been studied the correlation of the mitotic activity of the chromosome aberrations and apoptosis, in the V-79 cells pre-exposure to an adapting dose of ionizing radiation from 14C-thymidine prior to an acute challenge dose of gamma-rays. In spite of that the incubation of the cells with isotope increased of the yield of the chromosome aberrations, but the cells became more resistant to following gamma-irradiation. Increasing the adaptive dose of the 14C on degree didn't influence on the present of the adaptive response. However, using concentrations of the 14C damaged metaphase/anaphase transition and cells blocked in this check-point by apoptotic death. The results suggest, that the cellular selection has been involved in 14C-induced adaptive response, estimated by level of asymmetric chromosome aberrations in V-79 cells.