Induction Of Reciprocal Translocations Research Articles

Reconsideration of the genetic risk assessment for ethylene oxide exposures

The US Environmental Protection Agency (EPA) developed a genetic risk assessment model for exposures to ethylene oxide utilizing data on the induction of reciprocal translocations in male germ cells [Rhomberg et al. 1990]. This particular approach served as a reasonable initial attempt, albeit somewhat limited with regard to endpoint and only partially attentive to the mechanisms of induction of genetic alterations and the behavior of chromosomes during meiosis. The present paper discusses the scientific basis for a reassessment of the EPA model, providing data and hypotheses related to effective dose to the target cells and shape of the dose-response relationship at low doses, and dose rates. While the present genetic risk assessment approach is discussed in terms of ethylene oxide, it would be applicable to most mutagenic chemicals. The outcome of the discussion is that the genetic risk for exposed males from reciprocal translocation induction will be negligible at low doses since the dose-response curve is likely to be a function of the square of the dose. In addition, the proportion of genetically unbalanced live born offspring in humans arising from reciprocal translocation carriers is less than 10% of the frequency formed through meiotic segregation and fertilization for such carriers. Simply from a consideration of mechanism--namely, the very high probability of DNA repair prior to the next S-phase for a resting oocyte--it would be predicted that there would be a very low to negligible frequency of translocations in female germ cells from ethylene oxide exposure. It is further stressed that additional components of a genetic risk model require a consideration of all germ cell stages in the male, and the inclusion of calculations for point and deletion mutations. Some indications of likely response are presented with these points in mind.

X-ray induced translocations in premeiotic germ cells of monkeys

The induction of reciprocal translocations by various X-ray exposures was studied in spermatogonial stem cells of rhesus monkeys ( Macaca mulatta) and stump-tailed macaques ( Macaca arctoides) by means of spermatocyte analysis many cell generations after irradiation. The yields of translocations recovered from irradiated stump-tailed macaques were lower than those observed in rhesus monkeys and represent in fact the lowest induction rates per Gy ever recorded for experimental mammals. In the rhesus monkey a humped dose-effect relationship was found with (a) a homogeneous response with (pseudo-)linear kinetics below 1 Gy, (b) much more variability at higher doses, and (c) no induction at all at doses of 4 Gy and above. It is suggested that the post-irradiation proliferation differention pattern of surviving rhesus monkey spermatogonial stem cells is mainly responsible for these characteristics of the dose-response curve.

The relationship between induced reciprocal translocations and cell killing of rhesus monkey spermatogonial stem cells after combined treatments with follicle-stimulating hormone and X-rays

The induction of reciprocal translocations in spermatogonial stem cells of monkeys (rhesus and crabeating), visualized in dividing primary spermatocytes, was studied after combined treatments with folliclestimulating hormone (FSH, 54 IU/kg/week) and X-rays (1 Gy). No clear differences in the frequencies of induced translocations between FSH-pretreated and non-pretreated animals were recorded. Comparison of these translocation data with studies on cell killing in the same monkeys shows that the ratio between the probabilities that radiation-induced basic lesions kill a cell or produce translocations (the p/c ratio) is of the same order of magnitude as that observed for the mouse. Consequently the well documented differences in radiation response between the rhesus monkey and the mouse cannot be explained by differences in the p/c ratio. It is concluded that differences in multiplication-differentiation patterns of surviving stem-cell spermatogonia after irradiation are probably responsible for the observed differences between mice and rhesus monkeys.

The induction of reciprocal translocations in mouse germ cells by chemicals and ionizing radiations: III. Differential effect of two doses of adriamycin combined with 5 or 9 Gy of γ-rays

The induction of reciprocal translocations in mouse germ cells by combined treatments with chemicals and ionizing radiations has been studied. Male mice were intraperitoneally injected with doses of 5 or 10 mg/kg of adriamycin (ADR) and irradiated with doses of 5 or 9 Gy of γ-rays 24 h later. Three types of response were found after analyzing diakinesis-metaphase I multivalent configurations: potentiation, with the dose of 5 mg/kg of ADR plus 9 Gy; subadditivity, with the dose of 5 mg/kg of ADR plus 5 Gy; and additivity, with the dose of 10 mg/kg of ADR plus 5 or 9 Gy. According to these results, the subadditive effect observed with the lower dose of ADR plus 5 Gy cannot be explained under the assumption that depletion of any kind of spermatogonia is sufficient for modifying the chromosomal response of stem cells to ionizing radiations. The role of DNA repair mechanisms modulating the response of spermatogonial cells to combined treatments is discussed under the assumption that some repair mechanisms can be triggered by treatment with a low dose of a chemical and these repair mechanisms can reduced cell mortality. Consequently, a higher frequency of more radioresistant cells can survive.

The induction of reciprocal translocations in mouse germ cells by chemicals and ionizing radiations II. Combined effects of mitomycin C or thio-tepa with two doses of γ-rays

The combined effects of mitomycin C (MMC) and thio-tepa (TT) with γ-rays doses of 5 and 9 Gy on mouse stem cells were studied using the spermatocyte test. Both chemicals induced very low yields of translocations after single treatments. In combined treatments with a dose of 5 Gy, a subadditive effect of MMC and an additive effect of TT were found. Combined with a dose of 9 Gy the compounds potentiated the effect of radiations. Up to now, most of the chemicals tested have shown additive effects when combined with doses of the ascending part of the dose-response curve and potentiating effects when combined with doses of its descending part. This has been considered additional confirmation of the concept that depletion of any kind of spermatogonia is sufficient to modify the genetic response of stem cells. However, the subadditive and additive responses found could be considered evidence that common biological mechanisms can modulate the response to combined treatments of chemicals and ionizing radiations.

The relation between induced reciprocal translocations and cell killing of mouse spermatogonial stem cells after combined treatments with hydroxyurea and X-rays

The induction of reciprocal translocations in mouse spermatogonial stem cells, visualized in dividing primary spermatocytes, was studied after combined treatments with hydroxyurea (250 and 500 mg/kg) and X-rays (6, 8 and 9 Gy). The time intervals between the 2 treatments were 16 h (leading to extremely high cell killing) and 48 h (giving rise to less killing than irradiation alone). Comparison of the observed frequencies of translocations with reported data on stem cell killing (de Ruiter-Bootsma and Davids, 1981 show that the ratio between the probabilities that a radiation-induced basic lesion kills a cell or produces a translocation, theoretically calculated by Leenhouts and Chadwick (1981) to be about 10, can indeed be confirmed experimentally.

A stochastic model for the induction of heritable translocation in spermatozoa by exposure to environmental agents

In this paper we develop a stochastic model for the induction of heritable translocation in spermatozoa by exposure to some environmental agents by following the biological mechanism for the generation of spermatozoa in mice and human beings. By using Kolmogorov forward equation, it is shown that the probability generating functions (PGF) of the numbers of some normal and translocation carrying spermatid cells satisfy some partial differential equations. Using these results, a procedure for computing the probability distribution of the number of translocation carrying spermatid cells is derived under some important cases. These results will be applied to some dose- response data available on the induction of reciprocal translocations by exposure to some environmental agents in male mice.

The induction by ionizing radiation of chromosomal aberrations in rhesus monkey pre-meiotic germ cells: effects of dose rate and radiation quality

The induction of reciprocal translocations in rhesus monkey stem-cell spermatogonia was studied using multivalent analysis at metaphase of primary spermatocytes. Animals were exposed to 1 Gy γ-rays at dose rates of 140 and 0.2 mGy/min or to 0.25 Gy acute 2 MeV neutrons. Reduction of the dose rate from 140 mGy/min to 0.2 mGy/min did not result in a lowering of the frequencies of recovered translocations of 0.43%. The neutron data indicated an RBE (neutrons vs. X-rays) of 2.1, which is clearly lower than the value of 4 obtained in the mouse. It is made plausible that in general mammalian species with high sensitivities for the cytotoxic effects of ionizing radiation, such as the rhesus monkey, will exhibit relatively high threshold dose rates below which no further reduction in aberration yield occurs, whereas in more resistant species, such as the mouse, the threshold dose rate will be at a very low level. Similarly, resistant species will show relatively high RBE values for neutron irradiation and sensitive species low ones.

The induction of reciprocal translocations in mouse germ cells by chemicals and ionizing radiations I. Dose-response relationships and combined effects of bleomycin with thio-tepa and γ-rays

The effect of bleomycin (BLM) on mouse stem cells has been analysed using the spermatocyte test. The dose-response relationships after treatment with doses of 20, 40 and 60 mg/kg of the compound as well as the combined effect of BLM and γ-rays and BLM and thio-tepa (TT) were studied. A positive, significant correlation between the dose of BLM and the frequency of translocations was found. Two different responses were found when the yields of translocation induced after combined treatments, separated by a lapse of 24 h, were compared with the sum of translocation frequencies induced after the corresponding single treatments: (1) Potentiation, in the treatments with 1 Gy plus 9 Gy and 60 mg/kg of BLM plus 9 Gy; (2) additivity, in the treatments with 60 mg/kg of BLM plus 1 Gy, 1 Gy plus 60 mg/kg of BLM, and 0.2 mg of TT plus 60 mg/kg of BLM. In mice irradiated with 1 Gy plus 9 Gy and mice treated with 60 mg/kg of BLM plus 9 Gy, similar translocation yields were found. The potentiating effect of BLM is similar to that obtained with non-radiomimetic compounds such as triethylenemelamine, cyclophosphamide and adriamycin. These results are discussed taking into account the hypothesis of germ cell selection, and the dose of radiation employed.

Dose—response relationship for translocation induction in spermatogonia of the crab-eating monkey ( Macaca fascicularis) by chronic γ-ray-irradiation

The induction of reciprocal translocations in spermatogonia of the crab-eating monkey ( Macaca fascicularis) by chronic γ-irradiation (1.8·10 −5 Gy/min, about 0.024 Gy/22 h/day) was examined. The frequencies of translocation per cell were 0.15% at 0.3 Gy, 0.27% at 1.0 Gy and 0.33% at 1.5 Gy. The dose—response relationship for translocation yield was a linear one with a regression coefficient ( b) of 0.16 · 10 −2. When the slope ( b) of the regression line was compared with that at a high dose rate ( (0.25 Gy/min, b = 1.79 · 10 −2), it was clear that the induction rate of translocations after chronic γ-irradiation was only about one-tenth of that after high-dose-rate irradiation. Thus, there was evidence for a pronounced dose-rate effect in the crab-eating monkey.

Effects of hormone treatment on chromosomal radiosensitivity of somatic and germ cells of Snell's dwarf mice

The X-ray induction of micronuclei and structural chromosomal aberrations was studied in bone-marrow cells of normal and dwarf ( dw) mice in combination with thyroxin and/or prolactin treatment or otherwise. Hormone treatment clearly increased micronuclei induction but not chromosome breakage, suggesting that indirect effects were involved. Since no clear differences in the timing of the final stage of erythropoiesis could be found, it is likely that the indirect effects are mediated via the formation-differentiation kinetics of erythroblasts. The induction of reciprocal translocations by X-rays in stem cell spermatogonia of dwarf mice was lower than in normals and treatment with prolactin, growth hormone and/or thyroxin, did not influence the chromosomal radiosensitivity of spermatogonial stem cells.

Dose-effect relationship for X-ray-induced translocations in spermatogonia of normal and T70H translocation heterozygous mice

The induction of reciprocal translocations in stem cell spermatogonia was studied in normal mice and T70H translocation heterozygotes using spermatocyte analysis many cell generations after irradiation. Dose-response relationships were constructed employing acute X-ray exposures of 1–10 Gy. The obtained results confirmed our earlier observations that T70H heterozygous mice were overall less sensitive to the induction of translocations compared to normals. The shape of the dose-response curve for T70H heterozygotes was also clearly different from normal mice. No simple correlations between cell killing, measured as testis weight loss, and observed frequencies of chromosomal anomalies could be established. In general, selective elimination of translocation carrying stem cells of T70H heterozygotes seem to be mainly responsible for the differences in induction pattern between the two types of mice.

The Induction of Reciprocal Translocations in Rhesus Monkey Stem-Cell Spermatogonia: Effects of Low Doses and Low Dose Rates

The induction of reciprocal translocation in rhesus monkey spermatogonial stem cells was studied following exposure to low doses of acute X rays (0.25 Gy, 300 mGy/min) or to low-dose-rate X rays (1 Gy, 2 mGy/min) and gamma rays (1 Gy, 0.2 mGy/min). The results obtained at 0.25 Gy of X rays fitted exactly the linear extrapolation down from the 0.5 and 1.0 Gy points obtained earlier. Extension of X-ray exposure reduced the yield of translocations similar to that in the mouse by about 50%. The reduction to 40% of translocation rate after chronic gamma exposure was clearly less than the value of about 80% reported for the mouse over the same range of dose rates. Differential cell killing with ensuing differential elimination of aberration-carrying cells is the most likely explanation for the differences between mouse and monkey.

X-ray-induced translocations in marmoset ( Callithrix jacchus) stem-cell spermatogonia

The induction of reciprocal translocations in spermatogonial stem cells of marmosets ( Callithrix jacchus) was studied after irradiation with different doses X-rays (50, 100 and 200 rad) via spermatocyte analysis many cell generations later. The obtained results show a dose-effect relationship with clear saturation effects at 200 rad. The recorded frequencies of translocations were much lower than those reported for closely related marmosets ( Saguinus fuscicollis and Saguinos oedipus). Possible reasons for this difference are discussed.

Effects of unequally fractionated X-ray exposures on the induction of chromosomal rearrangements in mouse spermatogonia

The induction of reciprocal translocations in mouse stem-cell spermatogonia was determined following different unequally fractionated X-ray exposures with 24 h between the fractions. The results indicate that quite variable levels of chromosomal damage can be induced using the same exposure schemes. This variation does not seem to be correlated with radiation set-up or mouse strain. In general, with higher conditioning exposures from 12.5 to 300 rad (or R), higher frequencies of translocations were produced using a larger second challenging exposure of 700–900 rad (or R). The increase in yields of aberrations was more or less paralleled by a shift from a strong deviation from a Poisson fit of the number of translocations per spermatocyte, to a good fit, suggesting a transition from the original heterogeneity of the stem cell population to a radiation induced, more homogeneous stage. Earlier observations concerning a threshold dose for sensitization (van Buul and Léonard, Mutation Res., 70 (1980) 95–101) could not be confirmed.

Radiation-induced translocations in inseminated eggs of Drosophila melanogaster

The induction of reciprocal translocations by X-irradiation (500 R) of freshly laid inseminated eggs of Drosophila melanogaster was studied. In such eggs the maternal and paternal chromosome sets are exposed while they are within the same cell but in spatially separated nuclei. A constant rate of 0.3% translocations within the maternal chromosome set was found throughout the period during which the maternal genome passes from meiotic metaphase I to the pronuclear stage. During the same period the sperm head, containing the paternal chromosome set, transforms into the paternal pronucleus. At the beginning of this transformation a rate of 2.3% translocations within the paternal chromosome set was found. This rate fell, with the progression towards the pronuclear stage, to about 0.2%. From the recovery of three maternal-paternal translocations it was estimated that the maximal rejoining time of chromosome breaks was on the order of 10 min.

Studies on the induction of translocations in mouse spermatogonia: II. The effect of dose-rate

The effect of dose-rate on the induction of reciprocal translocations in mouse A type spematogonia by 600 RX- and γ-irradiation was studied by scoring multivalent configurations in descendant spermatocytes. With X-irradiation over a range of dose-rates from 0.8 to 913 R/min there was no significant change in the frequency of affected spermatocytes, which averaged 12.8%. With γ-irradiation, however, there was a steady increase in frequency from 1.4% at 0.02 R/min to 12.1% at 86 R/min, the points fitting a straight line on a semi-log plot. At 0.08 R/min the X-ray yield was twice that for γ-rays. Possible reasons for these differences are discussed. Frequencies of 0, 1, 2 translocations per spermatocyte did not fit a Poisson distribution since there were less then expected in the 1-class, but more in higher classes. This was probably a consequence of differential radiosensitivity of the irradiated spermatogonia, although preferential clonal proliferation may also be involved.