Relative biological effectiveness of x-rays and fast neutrons in inducing translocations in mouse spermatogonia

The dose-response relationships for inducing translocations in the spermatogonia of mice were studied, and they were compared for 200 kVp X-rays and 2 MeV fast neutrons. The dose response for fast neutrons was markedly convex; more precisely, the response obtained was linear in the dose range from 24 to 94 rad with a regression coefficient of 11.36·10^?4, but decreased for a further increase in dose up to 267 rad. On the other hand, that for X-rays showed a linear dose-response relationship from 48 to 672 rad with a regression coefficient of 2.69·10^?4. The relative biological effectiveness for inducing translocations in the spermatogonia of mice was compared for the linear parts of the dose response in both types of radiation, and the relative biological effectiveness (RBE) value was 4.22.     

Chromosomal rearrangements induced in mouse spermatogonia by 14.5-MeV neutrons

Inbred CBA male mice were irradiated with 14.5-MeV neutrons. Three acute doses, 75, 150 and 250 rad, and one chronic dose, 250 rad, were given. The percentages of affected spermatocytes as counted from reciprocal translocations which had been induced in spermatogonia were 0.7, 0.8 and 1.6 respectively for the acute series and 2.2 after chronic exposure. The data could be fitted to a linear or concave curvilinear regression line. There seemed to be a slight increase of damage with dose, even if the percentages were generally lower than those reported earlier for fast neutrons with energies around 1 MeV. The existence of dose-rate effects is discussed, and the conclusion drawn so far is that there seems to be no such effect either for 1-MeV fast neutrons or 14.5-MeV high energy neutrons. The term “reversed dose-rate effect”, as used earlier, relates to another phenomenon. The difference between the point estimates for the chronic and acute 250 rad series is not significant. The effectiveness of neutrons with energies around 14 MeV versus neutrons with energies around 1 MeV is discussed.     

Radiation-induced human chromosome aberrations. II. Human in vitro irradiation compared to in vitro and in vivo irradiation of marmoset leukocytes

Whole blood cultures from humans and from the New World primate, Saguinus fuscicollis, were irradiated with various doses of 250 kV X-rays. The resulting centric ring plus dicentric aberration yields were fitted to the three models, Y = a+bD, Y = a+bD+cD², and Y = a+cD², by least squares regression. In both instances the best fit was to the model Y = a+bD+cD², with coefficients of the one- and two-track components for human and marmoset being: b = (0.78 ± 0.09)·10⁻³, c = (5.92 ± 0.31)·10⁻⁶, and b = (1.11 ± 0.36)·⁻³, c = (7.7 ± 1.7)·10⁻⁶, respectively.     

Mutagenic effectiveness of 14.1 MeV neutrons and 200 kV X-rays at the dumpy complex locus of Drosophila melanogaster.

The effectiveness of 14.1 MeV neutrons relative to 200 kV X-rays for the induction of the various kinds of dumpy mutation in mature sperm of Drosophila melanogaster was investigated. The estimated RBE values are: 0.52 for all complete mutations; 0.64 for the (olv, ov) types; 0.33 for the (ol, lv, o, v, c) types; 0.33 for all fractional mutations. These data lend support to the thesis that (1) complete dumpy mutations of the olv and ov types are more frequently associated with chromosomal aberrations than those of the ol, lv, o, v and c types, and (2) fractional mutations and complete mutations of the (ol, lv, o, v, c) types are most probably point mutational events.     

X-ray induced translocations in spermatogonia. I. Dose and fractionation responses in mice

The frequency of recirprocal translocations, inducedm by X-irradiation of mouse spermatogonial cells and observed at diakinesis-metaphase I in primary spermatocytes, was measured over a dose range of 0–1200 R. The resulting dose-response curve gave a best fit to the model Y = bD+CD² over the range of 0–500 R. Above 600 R, howeverm, the yeild of translocations decreased with increasing dose, leadiong to a “humped” dose-response curve over the whole dose range studied, as has been observed by several worker previously.The significance of the nonlinear dose-response curve over the lower dose range is discussed in terms of the known fractionation and dose-rate effects for reciprocal translocations induced in spermatogonia.A dose of 800 R was split into two 400-R fractions separated by 8 weeks, or one of 1200 R into three equal parts, each separated by an 8-week interval. The resulting yield of translocations was the same as the sum of two, or three, separate 400-dose doses, but was much higher than a single dose of 800 R or 1200 R.It is suggested that these results, namely the shape of the dose-response curve and the “reverse” fractionation effect, can be explained in terms of resistant and sensitive stem-cell populations, but that any one cell can be in either population, depending upon the stage of the cell cycle in which it is at the time of irradiation.     

The regulation of human exposure to mutagens amidst scientific controversy

Human peripherial lymphocytes were irradiated with different doses of 15.0-MeV neutrons. The frequency of different aberration types was determined and the dose-response relation was calculated. The data were fitted by least-squared regression analysis to different models. The dicetric, dicentric+centric ring, and different acentric data gave the best fit to the linear quadratic model. The RBE of 15.0-MeV neutrons versus 220 kV X-rays decreased significantly with increasing dose.     

Evaluation of the resistance of Euglena gracilis to ion beam radiation

The resistance of Euglena (E.) gracilis to ionizing radiation was investigated using seven kinds of ion beams each with different energy characteristics. The minimum effective dose of the most lethal ion beams was 40 Gy. Given its substantially high resistance to heavy ion beams, E. gracilis possesses great potential in acting as an effective support system to produce food and regenerate oxygen in a space station. The lethal effect of ionizing radiation was dependent on the linear energy transfer value of the heavy ion beams, and reached a maximum at 196 keV/micron. This value was different from those obtained by previous irradiation experiments using mammalian and plant cells, suggesting that the radiation response of E. gracilis is distinct from that of mammalian and plant cells.     

Chromosome aberrations induced in human peripheral blood lymphocytes using heavy particles from 10 B (n, ) 7 Li reaction

The relationship between α-particle dose and chromosome aberration yield was studied in human peripheral blood leukocytes cultured in vitro. The α-irradiation was produced from thermal neutron capture by boron, according to the nuclear reaction ¹⁰B (n, α)⁷Li. Blood samples containing 49 μg ¹⁰B per ml were exposed to the thermal neutrons in a reactor at a flux density of 2·10⁷n/cm²s. By subtracting the rad dose due to the reactor radiations alone from that due to both boron capture and the reactor radiations, the rad dose rate of heavy particles was estimated. The dicentric yield appeared to follow a linear response up to about 18 rad and then showed signs of “saturation”. Comparison with 250 kV X-ray data (doses up to 510 rad) gave an RBE of 22.97.     

Comparative study of gamma-ray and high-energy carbon ion irradiation on negative gravitaxis inEuglena gracilisZ

The effects of ⁶⁰Co gamma-ray and 290 MeV/amu carbon ion irradiation on negative gravitaxis was studied in the photosynthetic flagellate Euglena gracilis strain Z in a dose-response dependent manner. Cells were exposed to the doses (0-200 Gy for water). The negative gravitaxis was quantified by the r-value observed in a recently developed biomonitoring system. The present results demonstrate the inhibitory effects of gamma-rays and 290 MeV/amu carbon ions on negative gravitaxis of the Euglena gracilis strain Z. The 290 MeV/amu carbon ions had a greater impact at a low dose (<40 Gy) than the ⁶⁰Co gamma-rays.     

Cytogenetic effects of X-rays and fission neutrons in female mice

The induction by X-rays of chromosomal damage in oocytes was studied, while the genetic consequences of X- and neutron-induced damage in female mice were looked for by testing offspring for dominant lethality and semi-sterility. None out of 386 sons of hybrid females given 300 rad X-rays showed evidence of semi-sterility or translocation heterozygosity, but 9 out of 294 daughters were diagnosed as semi-sterile. At least 3 and probably 4 of these (1.4%) carried reciprocal translocations, 2 of which caused male sterility. Complete or partial loss of the X-chromosome may have been responsible for some of the other sermi-steriles. Examination of oocytes at metaphase-I during the first and third weeks after X-irradiation with 100 or 400 rad revealed both multivalents (some of the ring quadrivalent type) and fragments (mainly double). These were thought to arise mainly from chromatid intercchanges (both symmetrical and asymmetrical) and isochromatid intrachanges respectively. Since neither the proportion of asymmetrical interchanges nor the amount of hidden damage was known it was not thought possible to predict the magnitude of F₁ effects from metaphase-I findings. The aberration frequency in oocytes rose with dose and (at the 400 rad level only) with time after irradiation, reaching a maximum of 10% multivalents and 22% fragments in the third week after 400 rad. The frequency of univalents showed no consistent trend, but chiasma counts decreased in the first week after 400 rad. The increase in levels of chromosomal damage with dose and time after irradiation was reflected in dominant lethal frequencies after the same radiation-conception intervals and doses of 0–400 rad. Induced post-implantation lethality was over twice as high in the third week after 200–400 rad than in the first. Pre-implantation loss also greatly increased in the third week after 300 or 400 rad; this was associated with increased non-fertilization of ova. No evidence for the induction of translocations in oogonia or resting oocytes by fast neutron irradiation was obtained, although there was evidence for X-chromosomal loss after 200 rad to oocytes. The relative biological effectiveness (RBE) for fission neutrons vs. X-rays with respect to dominant lethal induction in oocytes was found to vary with dose, but seamed to be around 1 at lower levels.     

Some problems of spontaneous and induced mutagenesis in mammals and man.

The culture time of rabbit lymphocytes (41–42 h) that provides cells in their first post-stimulation mitosis, was estimated on the basis of the mitotic index, dicentric yield and presence of the cells with these aberrations unaccompanied by acentric fragments, studied as a function of culture duration. The cells obtained in metaphase from cultures terminated at this time displayed no donor-to-donor variation where induction of dicentrics by X-rays was concerned.Rabbit venous blood was irradiated in vitro with a range of X- and gamma-ray doses, and dose-effect curves were obtained by regression analysis. Sixteen rabbits were irradiated in vivo (uniform whole-body irradiation), and blood was sampled 10 min, 6, 24, and 48 h after exposure. The frequency of dicentrics in the lymphocytes cultured did not change significantly over the first 24 h after irradiation. Dose-effect relationships in vivo fell within one standard error confidence limits of the respective curves in vitro. The authors conclude that the latter may be used for estimation of dose in vivo under conditions of homogeneous whole-body irradiation.     

X-ray induction of dominant lethals in late spermatids and mature spermatozoa of Drosophila melanogaster: the role of oxygenation

The role of oxygenation in determining the sensitivity to the induction of dominant lethals was examined in late spermatids and mature spermatozoa of Drosophila melanogaster. 0–2-h-old or 7-day-old Oregon-K males were irradiated with different X-ray exposures in nitrogen, air or in oxygen and the frequencies of dominant lethals induced in these stages were studied. The results obtained confirm and extend Sobels' earlier observations and the interpretation derived therefrom namely, that under normal conditions in air, mature spermatozoa are characterised by a higher degree of oxygenation than late spermatids and this difference is sufficient to explain the differential response of these stages. Similar Oxygen-Enhancement-Ratios(OERs) (of about 2) were obtained for both the cell stages. The present data also revealed that late spermatids are significantly less sensitive than mature spermatozoa to the X-ray-induction of dominant lethals in a nitrogen atmosphere. A plausible mechanism is suggested to explain this observation.     

Enhanced radiosensitivity for the induction of translocations in mouse stem cell spermatogonia following treatment with cyclophosphamide or adriamycin

The effects of pretreatment of mouse spermatogonial stem cells with cyclophosphamide (100 and 200 mg/kg) and adriamycin (2.5, 5 and 7 mg/kg) on the induction of chromosomal translocations by high doses (800 or 900 rad) of X-rays applied 24 h later, were studied by spermatocyte analysis. The results indicate that both compounds were able to alter the chromosomal radiosensitivity of surviving stem cells. It is concluded that depletion of differentiating and differentiated spermatogonia is sufficient for triggering stem cells into a more radiosensitive phase.     

The induction of translocations in mouse spermatozoa. I. Kinetics of dose response with acute x-irradiation

Adult male mice were given gonadal doses of 0–1200 rad acute X-irradiation and mated the same day. 531 sons, conceived within a week of the treatment, were tested for fertility and their testes examined cytologically for chromosome aberrations in spermatocytes. $\text{55}\text{57}$ of those diagnosed as semi-sterile and $\text{35}\text{40}$ of those diagnosed as sterile were judged to be heterozygous for one or more reciprocal translocations. Numbers of 0, 1, 2… translocations per mouse showed a good fit to a Poisson distribution, in contrast to previous findings with spermatogonial irradiation. Although the dose response fitted a linear relationship, the power law equation of best fit had a dose-exponent of 1.41. Further analysis along similar lines to those used previously in Drosophila by Catcheside, Lea and Haldane, which assumed random rejoining of breaks and direct proportionality between dosage and number of breaks, gave a close fit between the actural results and those expected if αq = 2.8·10^3?/rad, where α is the mean number of breaks per nucleus and q is the proportion which rejoin or restitute. By combining these data with those for litter-size reduction in F₁ (taken as a measure of induced dominant lethality) α was estimated to be 3.4 × 10^?3 per rad. When compared with the value of 0.8 × 10^?3 per rad obtained in Drosophila by Haldane and Lea, this suggested that mouse haploid nuclei are more radiosensitive to chromosome breakage than Drosophila haploid nuclei by a factor of about 4. The mean number of implants per pregnant female mated to cytologically abnormal males was about 15% lower than with normal males. This pre-implantation loss was thought to be mainly the result of a reduction in the rate of fertilization in this group rather than to early death of unbalanced zygotes. There was no evidence for the induction of any undetected types of chromosomal aberration or gene mutation which could cause intrauterine death in the progeny of F₁ males.