The mutagenic effect of repeated small radiation doses to mouse spermatogonia: III. Does repeated irradiation reduce translocation yield from a large radiation dose?

Previous results had suggested that daily repeated doses of 10 rad X- or γ-rays to mouse spermatogonia decreased their sensitivity to translocation yield. This was tested by comparing the effects of a dose of 300 rad γ-rays given before, 24 h after, or 8 days after, 30 daily doses of 10 rad γ-rays to male mice. The yield of translocations per cell from those receiving the 300 rad dose 24 h after the repeated ones (7.3%) was significantly lower than that from the other two regimes (9.4 and 9.7% respectively). This was consistent with the explanation that the repeated irradiation had temporarily increased the resistance of the spermatogonial cell population to translocation yield. However, there must remain some doubt about this interpretation since the absolute values of translocations found were too high, and the yield from those which had received the 300-rad dose 24 h after the repeated ones was not significantly below the sum of those from a single dose and from repeated doses given separately.     

Relative biological effectiveness and dose rate effect of tritiated water on chromosomes in human lymphocytes and bone marrow cells

Tritriated water (HTO) is a major toxic effluent from the nuclear power industry, that is released into the environment in large quantities. The low dose radiation effect and dose rate effect of HTO on human lymphocytes and bone marrow cells have not been well studied. The present study was therefore undertaken to investigate the HTO dose-response relationship for chromosomal aberrations in human lymphocytes and bone marrow cells at low in vitro radiation doses ranging from 0.1 to 1 Gy. Lymphocytes (G₀ stage) and bone marrow cells were incubated for 10–150 min with HTO at a dose rate of 2cGy/min (555 MBq/ml). The relative biological effectiveness (RBE) of HTO was calculated with respect to ₆₀Co γ-rays for the induction of dicentric and centric ring chromosomes at low radiation doses. The RBE value for HTO β-rays relative to ⁶⁰Co γ-rays was 2.7 for lymphocytes and 3.1 for chromatid aberrations in bone marrow cells. Lymphocytes were also chronically exposed to HTO for 6.7–80 h at dose rates of 0.5 cGy/min (138.5 MBq/ml) and 0.02 cGy/min (5.6 MBq/ml). There was a 71.5% decrease in the yield of dicentrics and centric rings at the dose rate of 0.02 cGy/min, indicating a clear dose rate effect of HTO. The RBE value for HTO relative to ¹³⁷Cs γ-rays was 2.0 at a dose rate of 0.02 cGy/min, suggesting that low HTO dose rates produce no increase of the RBE values and that the values may be constant between 2 and 3 within these dose rates. These results should prove useful in assessment of the health risk for humans exposed to low levels of HTO.     

Radiation-induced chromosome aberrations in lymphocytes from man and crab-eating monkey: The dose-response relationships at low doses

To obtain information on the relation beteween yiels of chrmosome aberrations and dose at low-dose levels, experiments were conducted with 5, 10, 20, 30 and 50 rad of ¹³⁷Cs γ-rays, on lymphocytes from man and crab-eating monkey (Macaca fascicularis). The dose-response relationship for dicentrics was obtained from the combined data of these low-dose experiments with those of our previous onse at high doses (100–400 rad) When the difference between observed yields and those expected from the linear-quadratic model were computed, the dose-response curve had a good fit for man, but not for the monkey. The linear regression lines between 0 and 30 rad were calculated, because the expected values of α/β for man and monkey would be about 100 and 60 rad.The human date gave a satisfactory fit to a linear model, i.e., a linear increase in aberration frequency with dose, whereas this was not so for those of the monkey. Furtyhermose, there was some suggestive evidence for the existence of plateau in dicentrics yields between 10 and 30 rad for the monkey and between 20 and 30 rad for human lymphocytes, but more data would be needed to verify this suggestion, particularly for human lymphocytes.     

Alterations in histone acetylation following exposure to <Superscript>60</Superscript>Co γ-rays and their relationship with chromosome damage in human lymphoblastoid cells

Chromosome damage is related to DNA damage and erroneous repair. It can cause cell dysfunction and ultimately induce carcinogenesis. Histone acetylation is crucial for regulating chromatin structure and DNA damage repair. Ionizing radiation (IR) can alter histone acetylation. However, variations in histone acetylation in response to IR exposure and the relationship between histone acetylation and IR-induced chromosome damage remains unclear. Hence, this study investigated the variation in the total acetylation levels of H3 and H4 in human lymphocytes exposed to 0–2 Gy ⁶⁰Co γ-rays. Suberoylanilide hydroxamic acid (SAHA), a histone deacetylase (HDAC) inhibitor, was added to modify the histone acetylation state of irradiated cells. Then, the total acetylation level, enzyme activity, dicentric plus centric rings (dic?+?r) frequencies, and micronucleus (MN) frequencies of the treated cells were analyzed. Results indicated that the acetylation levels of H3 and H4 significantly decreased at 1 and 24 h, respectively, after radiation exposure. The acetylation levels of H3 and H4 in irradiated groups treated with SAHA were significantly higher than those in irradiated groups that were not treated with SAHA. SAHA treatment inhibited HDAC activity in cells exposed to 0–1 Gy ⁶⁰Co γ-rays. SAHA treatment significantly decreased dic?+?r/cell and MN/cell in cells exposed to 0.5 or 1.0 Gy ⁶⁰Co γ-rays relative to that in cells that did not receive SAHA treatment. In conclusion, histone acetylation is significantly affected by IR and is involved in chromosome damage induced by ⁶⁰Co γ-radiation.     

Chromosome aberrations in peripheral lymphocytes and radiation dose to active bone marrow in patients treated for cancer of the cervix

An international study of cervical cancer patients reported a doubling of the risk for leukemia following radiotherapy. To evaluate the extent of residual chromosome damage in circulating T-cell lymphocytes in this population, approximately 200 metaphases were examined from each of 96 irradiated and 26 nonirradiated cervical cancer patients treated more than 17 years ago (average 23 years). Radiation dose averaged over the total red bone marrow was estimated to be 8.1 Gy. The type and frequency of stable and unstable chromosome aberrations were quantified in 24,117 metaphases. Unstable aberrations did not differ significantly between irradiated and nonirradiated patients (P greater than 0.5). Stable aberrations (i.e., translocations, inversions, or chromosomes with deleted segments), however, were significantly higher among irradiated (2.8 per 100 cells) compared to nonirradiated (0.7 per 100 cells) women (P less than 10(4). The frequency of these stable aberrations was found to increase significantly with increasing dose to the bone marrow. These data indicate that a direct relationship between radiation dose and extent of damage to somatic cells persists in populations and can be detected many years after partial-body radiation exposure. The stable aberration rate in irradiated cervical cancer patients was 50 to 75% lower than those observed 25 years or more after radiation exposure in atomic bomb survivors and in ankylosing spondylitis patients treated with radiotherapy. The average marrow dose was only 1 Gy in the examined atomic bomb survivors and 3.5 Gy in the ankylosing spondylitis patients. It appears, then, that a very high dose delivered to the pelvic cavity in fractionated doses resulted in far fewer persistent stable aberrations than lower doses delivered either in acute whole-body exposure or in fractionated doses to the spinal column and sacroiliac joints. The higher radiation dose and the concentration of that dose in a smaller area of the body appear to be responsible for the lower rate of persistent aberrations observed in cervical cancer patients.     

Cytogenetic effects of protracted gamma exposures from conception of male mice

In order to gain an overall picture of the genetic effects of an increased level of background radiation it is necessary to study the results of protracted exposures to embryonic and immature germ-cell stages as well as to stages found in the mature organism. For this purpose, litters produced by female mice, kept in a 10 or 20 rad/day ⁶⁰Co-γ-irradiation field, were kept in the same fields from conception until about 60 days later, having absorbed doses of 526 and 1078 rad respectively. Tests on exposed female offspring showed them to be sterile. 8 weeks after removal from the gamma field, mean testis masses of males in the 20 rad/day series were only half normal but those receiving 10 rad/day were little affected. Frequencies of translocations in spermatocytes at diakinesis/metaphase I were only slightly increased in the exposed series, differences not being significant. Estimated rates of translocation induction were around 5 × 10^?6 per rad, about one-third of those found after protracted γ-irradiation of stem-cell spermatogonia in the adult. Embryonic lethality in progeny of other similarly irradiated males (absorbed doses of 560 and 1040 rad), mated 2 months after removal from the radiation fields, was also increased slightly, but not significantly. Results are compared with others on the induction of chromosome aberrations and gene mutations, mainly by acute irradiation, in prenatal and neonatal male mice. It is concluded that early male germ-cell stages generally show a reduced genetic radiosensitivity after both acute and chronic exposures.     

Chromosome aberrations in spermatocytes and oocytes of mice irradiated prenatally

5 pregnant mice were exposed to a single dose of 150 R whole body γ-irradiation on the 12th day of gestation. The ocytes and spermatocytes, collected from the F₁ progeny at ages 10–12 weeks, were examined for chromosome aberrations in metaphase I and compared with those of the progeny of non-irradiated controls. No differences were found in the type and frequency of aberrations between irradiated and controls nor between the sexes. It appears, therefore, that either primordial germ cells of both males and females are fairly resistant to radiation or an efficient selection or repair mechanism has eliminated the aberrant cells.     

Characteristic cytogenetic effects of small doses of ionizing radiation

A study was made of the frequency of chromosome aberrations in human lymphocyte culture after gamma-irradiation (60Co) with doses ranging from 0.05 to 1.0 Gy at dose--rates of 0,005, 0.05 and 0.5 Gy/min. The frequency of structural changes in chromosomes at low doses was higher than it was expected in the case of extrapolating the effect produced by high to low doses of radiation; within the dose range from 0.1 to 0.5 Gy a plateau was registered for aberrations of the exchange type (dicentrics and rings). The abnormal character of the dose dependence of the yield of chromosome aberrations persisted with all three dose - rates under study.     

Dose responses for chromosome aberrations produced in noncycling primary human fibroblasts by alpha particles,and by gamma rays delivered at sublimiting low dose rates

As the total dose of X or gamma rays is delivered at lower and lower rates, the yield of chromosome aberrations progressively diminishes. Simultaneously, the shape of the dose response changes from one exhibiting pronounced upward curvature at high dose rates to one approaching linearity at low dose rates. Although the maximum sparing effect caused by lowering the dose rate can be predicted from classical cytogenetic theory, it has yet to be verified experimentally. Here, noncycling normal human fibroblasts were exposed to graded doses of (137)Cs gamma rays at chronic dose rates of 6.3 and 2.8 cGy h(-1), dose rates that we reasoned should be lower than those required to achieve maximal sparing. This was indeed shown to be the case, after it was determined that the two chronic dose rates produced identical linear dose responses of 0.05 total aberrations per cell Gy(-1). Consistent with cytogenetic theory, this value was statistically indistinguishable from the linear coefficient derived from a fit to aberration frequencies produced by high-dose-rate exposure. Exposure to (238)Pu alpha particles also produced a linear dose response for total aberrations, whose slope-with respect to (137)Cs gamma rays as a reference radiation-implied a maximum RBE of 35 +/- 2.     

Gamma-ray-enhanced reactivation of UV-irradiated adenovirus in normal human fibroblasts.

An enhanced reactivation of UV-irradiated adenovirus type 2 (Ad 2) was detected following irradiation of the host cells with γ-rays prior to infection. Non-irradiated and γ-irradiated normal human fibroblasts were infected immediately after irradiation with either non-irradiated or UV-irradiated Ad 2. At 48h after infection, cultures were examined by indirect immunofluorescence to determine the number cells in which the viral function of viral structural antigen (Vag) was expressed. Pre-irradiation of cells with 1 krad resulted in a 2–3-fold increase in the survival of this viral function following different UV doses to the virus up to 1.75 × 10³ J/m². For a fixed UV dose of 1.0 × 10³ J/m² to the virus this enhancement increased with preirradiation dose to the cells up to a maximum factor of 2–3 for a dose of 1 krad. An examination of Vag expression at various times after infection indicates that pre-irradiation of the cells with γ-rays prior to infection with UV-irradiated virus leads to an earlier onset and/or increased rate of Vag synthesis. This enhancement of Vag production from a UV-damaged template may result from an inducible DNA-repair mechanism in human fibroblasts which may or may not be error-prone.     

Time dependence of chromosomal aberrations induced in human and monkey lymphocytes by acute and fractionated exposure to 60Co

Changes in the numbers of peripheral lymphocytes with chromosome aberrations were observed in cynomolgus monkeys after fractionated or acute 60Co irradiation at the same total doses. Immediately after irradiation, the yield of dicentrics decreased when the dose was fractionated but remained constant for 20 to 80 days, depending on the dose, when irradiation was acute. Lymphopenia was greater than expected when the dose was fractionated. The kinetics of the loss of chromosome aberrations and the changes in peripheral lymphocyte count occurring in monkeys after acute irradiation were compared to those observed in accidentally irradiated men.     

Aberrations induced by fission neutrons in human peripheral lymphocytes

Analysis of dose-response relationship was carried out for chromosome aberrations produced in human peripheral lymphocytes by fission neutrons at doses of 25, 50, 100 or 200 rad.Statistical treatment showed experimental data to be fitted by a regression curve described by the mathematical model Y = a+bD. A linear relation to dose characterized both one-break and two-break aberration yields. Numerical values of coefficients are reported for yields of dicentrics, chromosome fragments, minutes, aberrant cells, total number of aberrations, and total breakage.Based on chromosome fragments and aberrant cells, relative biological efficiency (RBE) value derived for fission neutrons relative to 180 kV X-rays for chromosome fragments was 2.53, and for aberrant cells it was 2.80.     

Radiosensitive strains of Drosophila. XII. Realization of the effect of mutation rad(2)201G1 at the cell and organism levels

Two effects of gamma-rays were studied on radiosensitive mutant rad(2)201G1 and wild type strain rad+ of Drosophila: the rate of radiation-induced chromosome aberrations in somatic cells and lethality of individuals irradiated at different stages of preimaginal development. It has been shown that mutant strain is characterized by the increased rate of chromosome aberrations in somatic cells and lethality of developing flies. Control strain rad+ is characterized by more complicated relationship between the effects analyzed. The results obtained are discussed in connection with the action of rad(2)201G1 gene on repair of genetic damages and with existence of postradiation compensation mechanisms intrinsic in development of multicellular organisms.     

The effect of multiple small doses of x rays on skin reactions in the mouse and a basic interpretation

Multiple-fraction experiments have been carried out to determine the response to repeated small doses of 240 kV X rays down to 45 rad per fraction, using the mouse skin reaction system. A method of irradiating without anesthetic was developed so that up to 64 fractions could be given within 8 days; over this time, proliferation was negligible. It was found that the total dose required to produce a given reaction continued to rise with the number of fractions above 30 fractions, in contradiction to the recent conclusions of Dutreix and colleagues. The plot of reciprocal total dose against size of each fraction was shown to be linear. This finding led to an analysis in terms of a function F(e), which is proportional to the slope of the chord of the appropriate cell survival curve from the origin to the dose per fraction used. The cell survival curve derived here was well fitted by an equation of the form [Formula: see text]The initial slope was 1/690 rad and the slope at 2340 rad was 1/126 rad. Thus, 1 rad at a dose approaching 0 rad has 18% of the effect of 1 rad at a single dose of 2340 rad for mouse skin reactions. A cell survival theory based on Neary's theory of chromosome aberrations is presented and the current results are consistent with the postulate that cell death results from the formation of chromosome aberrations.     

Quantitative and qualitative analysis of the effects of various doses of beta-irradiation (50-5000 rads) on epidermal cells of embryos and larvae of Pleurodeles waltlii Mich. (Amphibia, Urodela)

Embryos of Pleurodeles waltlii at the hatching stage were irradiated with doses of 50 to 5 000 rad. From 70 to 500 rad chromosomal aberrations appear; they are studied respectively 24,48 hours and 3 weeks after the treatment. Breakages are observed, that may be followed by rearrangements, i.e. acentric, telocentric and dicentric fragments, chromatid translocations and chromosome translocations. With time, the cells showing the most severe abnormalities are eliminated by the developing larvae. From 1 000 rad cytoplasmic structures (membrane systems and mitochondria) are alterated .     

The induction of chromosome aberrations in mouse dictyate oocytes by X-rays and chemical mutagens.

Oocytes were collected from female mice and matured in vitro to Metaphase I during the first or third week after treatment with a dose of 400 rad X-rays, 1.6 mg/kg triethylenemelamine (TEM) or 75 mg/kg isopropylmethanesulphonate (IPMS). In week 1 the mean number of oocytes per female was similar for all treatments but in week 3 irradiated females yielded fewer oocytes than the chemically treated or control females. In week 1 the proportion of oocytes maturing was smaller in irradiated females than in the other groups but in week 3 was similar in all groups.Structural chromosome aberrations, scored in the Metaphase I oocytes, were of the chromatid or isochromatid type and involved gaps, breaks, fragments, intrachanges and interchanges. Aberration frequency did not increase with time after either of the chemical mutagens but after irradiation was higher in the third week than in the first week. The aberration yield from IPMS-treated females was similar at both sampling times, while a lower yield was recorded in week 3 following TEM treatment than in week 1.     

Chromosome aberration yields in human lymphocytes induced by fractionated doses of x-radiation.

Unstimulated (G0) human peripheral blood lymphocytes were exposed at 37 degrees C to doses of 200 or 500 rad of X-rays delivered in two equal fractions. The dose fractions were separated by intervals of up to 7 h in the 200 rad study and up to 48 h for 500 rad. In both studies the mean levels of dicentrics and total unstable aberrations began to decline when fractions were delivered with intervals of greater than 2 h. With 200 rad the yield had decreased to an additive baseline (i.e. equal to only twice the yield of a single 100-rad fraction) by an interval of 4 h. Following 500 rad the yield declined until 8 h and then remained 20% above the additive baseline even when 48 h separated the fractions. Possible explanations for this discrepancy are discussed. In a second experiment PHA stimulated lymphocyte cultures were exposed to 2 doses of 125 rad of X-rays up to 7 h apart in an attempt to demonstrate the late peak in aberration yields originally reported by Lane [5]. Control cultures received unsplit doses of 250 rad at the time of the corresponding second 125-rad fraction. No evidence of a late peak in dicentric yield was observed. The yield remained approximately the same irrespective of the time interval between fractions but these split dose yields were significantly different from the accompanying unsplit controls.     

AT cells are not radiosensitive for simple chromosomal exchanges at low dose

Cells deficient in ATM (product of the gene that is mutated in ataxia telangiectasia patients) or NBS (product of the gene mutated in the Nijmegen breakage syndrome) show increased yields of both simple and complex chromosomal aberrations after high doses (>0.5Gy) of ionizing radiation (X-rays or γ-rays), however less is known on how these cells respond at low dose. Previously we had shown that the increased chromosome aberrations in ATM and NBS defective lines was due to a significantly larger quadratic dose-response term compared to normal fibroblasts for both simple and complex exchanges. The linear dose-response term for simple exchanges was significantly higher in NBS cells compared to wild type cells, but not for AT cells. However, AT cells have a high background level of exchanges compared to wild type or NBS cells that confounds the understanding of low dose responses. To understand the sensitivity differences for high to low doses, chromosomal aberration analysis was first performed at low dose-rates (0.5Gy/d), and results provided further evidence for the lack of sensitivity for exchanges in AT cells below doses of 1Gy. Normal lung fibroblast cells treated with KU-55933, a specific ATM kinase inhibitor, showed increased numbers of exchanges at a dose of 1Gy and higher, but were similar to wild type cells at 0.5Gy or below. These results were confirmed using siRNA knockdown of ATM. The present study provides evidence that the increased radiation sensitivity of AT cells for chromosomal exchanges found at high dose does not occur at low dose.     

Studies on chromosome aberrations in the eggs of mice fertilized in vitro after irradiation. I. Chromosome aberrations induced in sperm after X-irradiation

The induction of chromosome aberrations in eggs of mice fertilized with X-irradiated sperm was performed by using an in vitro fertilization technique. Capacitated mature sperm was irradiated with various doses of X-rays and cytological analysis of the first cleavage metaphase of in vitro fertilized eggs was made. The frequencies of chromosome aberrations increased exponentially with dose and the dose-response relationship for overall breaks fitted well to a quadratic equation. The chromosome aberrations were mainly chromosome-type (82.1%), and the majority of aberrations were fragments.     

Chromosome aberrations produced in human lymphocytes by in vivo and in vitro irradiation

The production of chromosome aberrations in vivo has been studied in lymphocytes from a patient undergoing a wholebody treatment with gamma-radiation up to a cumulative dose of 1.4 Gy. These results were compared with the observations performed on whole blood samples irradiated in vitro with doses from 0.05 up to 2 Gy of gamma-rays. The frequency of chromosome aberrations, particularly the dicentrics, was found to be similar in vivo and in vitro. The yield of dicentrics could be best related to the dose by using a linear-quadratic model in both cases, the ratio of the coefficients a/b being of 0.56 and 0.69 Gy, respectively in vivo and in vitro. These observations confirm that in vitro dose response curves may be used to evaluate accurately an in vivo absorbed dose.