The Radiosensitivity of Germ-cells at Various Stages of Ovarian Development

Effects of Continuous In Utero Low- and Medium-Dose-Rate Gamma-Ray Exposure on Fetal Germ Cells

The effects of radiation exposure on germ cells and the gonads have been well studied at acute high-dose exposures, but the effects of chronic low-dose-rate (LDR) irradiation, particularly relevant for radiation protection, on germ cells and the gonads are largely unknown. Our previous study revealed that chronic exposure of mice to medium-dose-rate (MDR, 200 or 400 mGy/day) gamma-rays in utero for the entire gestation period (18 days) induced only a mild degree of general growth retardation, but with very drastic effects on the gonads and germ cells. In the current study, we further investigated the histomorphological changes in the gonads and the number of germ cells from gestation day (GD) 18 fetuses irradiated with MDR throughout the entire gestation period. The germ cells in the testes and ovaries of the MDR-irradiated fetuses were almost obliterated. Gestation day 18 fetuses exposed to LDR (20 mGy/day) radiation for the entire gestation period showed decreases in the number of the germ cells, which were not statistically significant or only marginally significant at most. Further investigations on the effects of LDR irradiation in utero using more sensitive methods are necessary.

DNA repair in primordial follicle oocytes following cisplatin treatment

PURPOSE

Genotoxic chemotherapy and radiotherapy can cause DNA double stranded breaks (DSBs) in primordial follicle (PMF) oocytes, which then undergo apoptosis. The development of effective new fertility preservation agents has been hampered, in part, by a limited understanding of DNA repair in PMF oocytes. This study investigated the induction of classical DSB repair pathways in the follicles of wild type (WT) and apoptosis-deficient Puma^-/- mice in response to DSBs caused by the chemotherapy agent cisplatin.

METHODS

Adult C57BL/6 WT and Puma^-/- mice were injected i.p. with saline or cisplatin (5 mg/kg); ovaries were harvested at 8 or 24 h. Follicles were counted, and H2A histone family member (γH2AX) immunofluorescence used to demonstrate DSBs. DNA repair protein RAD51 homolog 1 (RAD51) and DNA-dependent protein kinase, catalytic subunit (DNA-PKcs) immunofluorescence were used to identify DNA repair pathways utilised.

RESULTS

Puma^-/- mice retained 100% of follicles 24 h after cisplatin treatment. Eight hours post-treatment, γH2AX immunofluorescence showed DSBs across follicular stages in Puma^-/- mice; staining returned to control levels in PMFs within 5 days, suggesting repair of PMF oocytes in this window. RAD51 immunofluorescence eight hours post-cisplatin was positive in damaged cell types in both WT and Puma^-/- mice, demonstrating induction of the homologous recombination pathway. In contrast, DNA-PKcs staining were rarely observed in PMFs, indicating non-homologous end joining plays an insignificant role.

CONCLUSION

PMF oocytes are able to conduct high-fidelity repair of DNA damage accumulated during chemotherapy. Therefore, apoptosis inhibition presents a viable strategy for fertility preservation in women undergoing treatment.

Cisplatin- and cyclophosphamide-induced primordial follicle depletion is caused by direct damage to oocytes

It is well established that DNA-damaging chemotherapies can cause infertility and ovarian endocrine failure by depleting the ovarian reserve of primordial follicles. Currently, no effective pharmacological therapies exist for the preservation of long-term fertility and ovarian function in female cancer patients, due to a limited understanding of the mechanisms of chemotherapy-induced follicle depletion. This study investigated the cellular targets, molecular mechanisms, and temporal course of ovarian reserve depletion following treatment with commonly used chemotherapeutic drugs. Adult female C57BL/6 mice were injected i.p. with saline, cisplatin (5mg/kg), or cyclophosphamide (300mg/kg); ovaries were harvested after 8 or 24 hours. Follicle counts showed depletion of all follicular stages 24 hours after administration of cisplatin or cyclophosphamide. Eight hours post-treatment, H2A histone family member X (γH2AX) immunofluorescence showed DNA double-stranded breaks at all follicular stages, including within primordial follicle oocytes. This staining was resolving by 24 hours, indicating that primordial follicle oocytes begin to undergo either apoptosis or repair in this timeframe. γH2AX-positive follicles were further examined to identify the specific cell types damaged. In primordial, transitional, and primary follicles, only oocytes sustained DNA damage, whereas in secondary and antral follicles, only somatic cells were affected. TUNEL staining confirmed that apoptosis occurs in these targeted cell types. Whilst multi-drug and multi-dose regimens were not examined, this study conclusively shows that cyclophosphamide and cisplatin cause direct damage to primordial follicle oocytes, which then undergo apoptosis. Therefore, future pharmacological strategies to prevent chemotherapy-induced infertility in females must specifically prevent primordial follicle oocyte death.

Loss of PUMA protects the ovarian reserve during DNA-damaging chemotherapy and preserves fertility

Female gametes are stored in the ovary in structures called primordial follicles, the supply of which is non-renewable. It is well established that DNA-damaging cancer treatments can deplete the ovarian reserve of primordial follicles, causing premature ovarian failure and infertility. The precise mechanisms underlying this chemotherapy-driven follicle loss are unclear, and this has limited the development of targeted ovarian-protective agents. To address this fundamental knowledge gap, we used gene deletion mouse models to examine the role of the DNA damage-induced pro-apoptotic protein, PUMA, and its transcriptional activator TAp63, in primordial follicle depletion caused by treatment with cyclophosphamide or cisplatin. Cyclophosphamide caused almost complete destruction of the primordial follicle pool in adult wild-type (WT) mice, and a significant destructive effect was also observed for cisplatin. In striking contrast, Puma-/- mice retained 100% of their primordial follicles following either genotoxic treatment. Furthermore, elimination of PUMA alone completely preserved fertility in cyclophosphamide-treated mice, indicating that oocytes rescued from DNA damage-induced death can repair themselves sufficiently to support reproductive function and offspring health. Primordial follicles were also protected in TAp63-/- mice following cisplatin treatment, but not cyclophosphamide, suggesting mechanistic differences in the induction of apoptosis and depletion of the ovarian reserve in response to these different chemotherapies. These studies identify PUMA as a crucial effector of apoptosis responsible for depletion of primordial follicles following exposure to cyclophosphamide or cisplatin, and this indicates that inhibition of PUMA may be an effective ovarian-protective strategy during cancer treatment in women.

DNA damage-induced primordial follicle oocyte apoptosis and loss of fertility require TAp63-mediated induction of Puma and Noxa

Trp63, a transcription factor related to the tumor suppressor p53, is activated by diverse stimuli and can initiate a range of cellular responses. TAp63 is the predominant Trp53 family member in primordial follicle oocyte nuclei and is essential for their apoptosis triggered by DNA damage in vivo. After γ-irradiation, induction of the proapoptotic BH3-only members Puma and Noxa was observed in primordial follicle oocytes from WT and Trp53(-/-) mice but not in those from TAp63-deficient mice. Primordial follicle oocytes from mice lacking Puma or both Puma and Noxa were protected from γ-irradiation-induced apoptosis and, remarkably, could produce healthy offspring. Hence, PUMA and NOXA are critical for DNA damage-induced, TAp63-mediated primordial follicle oocyte apoptosis. Thus, blockade of PUMA may protect fertility during cancer therapy and prevent premature menopause, improving women's health.

Specific-locus experiments show that female mice exposed near the time of birth to low-LET ionizing radiation exhibit both a low mutational response and a dose-rate effect

Female mice were exposed to 300 R of 73-93 R/min X-radiation either as fetuses at 18.5 d post conception (p.c.) or within 9 h after birth. Combining the similar results from these two groups yielded a specific-locus mutation frequency of 9.4 X 10(-8) mutation/locus/R, which is statistically significantly higher than the historical-control mutation frequency, but much lower than the rate obtained by irradiating mature and maturing oocytes in adults. Other females, exposed at 18.5 days p.c. to 300 R of 0.79 R/min gamma-radiation, yielded a mutation frequency that was statistically significantly lower than the frequency at high dose rates. The low-dose-rate group also had markedly higher fertility. It appears that the dose-rate effect for mutations induced near the time of birth may be more pronounced than that reported for mature and maturing oocytes of adults. A hypothesis sometimes advanced to explain low mutation frequencies recovered from cell populations that experience considerable radiation-induced cell killing is that there is selection against mutant cells. The reason for the relatively low mutational response following acute irradiation in our experiments is unknown; however, the finding of a dose-rate effect in these oocytes in the presence of only minor radiation-induced cell killing (as judged from fertility) makes it seem unlikely that selection was responsible for the low mutational response following acute exposure. Had selection been an important factor, the mutation frequency should have increased when oocyte killing was markedly reduced.

The age dependence of radiation sensitivity of the gonads of female mice

Female CBA-mice were continuously gamma-irradiated with 0.9 or 2.4 Gy. The dose was given during a 4-day period with start at 50, 90, 135 or 190 days of age respectively. Fifty-six days after the irradiation the females were killed and the number of germ cells in their ovaries were compared with that from unirradiated control females of corresponding age. The total number of germ cells was, compared with the controls, reduced by about 70% in all age groups after the dose of 0.9 Gy and to between 3 and 8% after 2.4 Gy. Seven stages of oocyte development were observed as well as 2 stages in degeneration. The observations suggest a changing radiation sensitivity by age during an initial period of about 90 days followed by a period of fatal radiation effects.

Response of developing mouse liver irradiated in utero and its modification by 2-mercaptopropionylglycine

Pregnant Swiss albino mice were exposed at gestation days 141/4, 161/4 and 181/4 to 1.5 Gy gamma radiation from 60Co in the presence or absence of 2-mercaptopropionylglycine (MPG). Liver was taken from the litters born to these mothers at different post-partum intervals. The maximum sensitivity was found in 4-week-old animals irradiated at gestation day 141/4. In the MPG treated groups, the number of total cells was higher than in the controls but less than in normals. The increase in pyknotic nuclei and necrotic and binucleate cells was significantly lowered by MPG.

Effects on foetal ovaries after protracted, external gamma irradiation as compared with those from internal depositions

Effects of low doses of protracted gamma irradiation on pre- and neonatally exposed ovaries in CBA mice have been compared with those previously obtained by 90Sr contamination of the dam. The total number of germ cells at 56 days of age was reduced to about 50 per cent after a dose of 0.09 Gy given during four days from the 19th day post coitum to the 2nd day post partum. The highest dose level used (0.91 Gy) reduced the frequency to about 10 per cent. Females exposed to the same dose levels at an age of 85 to 90 days showed a lesser degree of injury. The reproductive capacity in females exposed around their birth (expressed as the number of litters per female) was negatively correlated to the dose of gamma irradiation. The effects on the foetal ovaries of 2 to 5 kBq 90Sr given to the dam on the 19th day of gestation seemed to correspond to those from 0.01 Gy from 137Cs under the present circumstances.

Influence of 90Sr-contaminated milk on the ovaries of foetal and young mice

A dose of 185 kBq (5 microCi) 90Sr was intravenously injected into pregnant CBA-mice on the 19th day post coitum. Four groups of female young were examined when aged 56 days: (1) non-contaminated females (controls), (2) only exposed in utero, (3) only having suckled contaminated milk and (4) exposed to 90Sr in utero and having suckled contaminated milk. The effect of the nuclide was expressed as a comparison of the number of remaining cells in the ovaries, totally and in different stages of development, between 90Sr-contaminated young and controls. Young mice that suckled contaminated milk had a reduced number of cells compared with the controls. The ovaries of in utero contaminated females were injured to a less severe degree if the individuals were given noncontaminated milk, compared with those suckling contaminated dams. Minute changes of the body burden of 90Sr were especially reflected in the number of young oocytes, being the most radiation sensitive stage during the ovarian development.

Ovary tumors in NMRI mice subjected to fractionated X-irradiation during fetal development

Fractionated X-irradiation of pregnant mice was performed either during late organogenesis (gestational days 11-13), during the early fetal period (g.d. 14-16), or during both periods (g.d. 11-16). The offspring were observed for 39 months. A significant increase of ovary tumor frequency was observed with 3 X 1.2 Gy, applied either in late organogenesis or in the early fetal period. Lower X-irradiation doses were ineffective in these periods with respect to ovary tumor development. A sharp increase in ovary tumor frequency resulted after irradiation with 6 X 0.8 Gy or 6 X 1.2 Gy. The highest incidence of ovary cysts was observed after 3 X 1.0 Gy or 3 X 1.2 Gy on g.d. 11-13, while the frequency of these cysts was lowest in the animals irradiated six times, which, however, showed a high ovary tumor frequency. Autoradiography of the fetal ovaries either 1 or 6 days after irradiation at the late organogenesis stage revealed a persistent depression of this organ's proliferation rate throughout pregnancy. This may be consistent with the low tumor inducibility after X-irradiation in this period.

Induction of lethal mutations in female mice by 9 generations of gamma-irradiation during foetal development

Female CBA mice were chronically gamma-irradiated in utero during either of two periods, the 10th to 14th days or the 14th to 18th days of gestation. The doses administered were 34 rad/generation in the earlier group and 160 rad/generation in the latter with dose rates of 0.3 rad/h and 1.7 rad/h, respectively. The doses were given through 9 generations. The effect of the irradiation was expressed as an increased frequency in the rate of recessive lethal equivalents by just above 4%. This corresponds to a mutation rate of 1.5 X 10(-4) mutation/rad/genome in the animals irradiated during the 10th to 14th gestational days and 0.3 X 10(-4) mutation/rad/genome in the 14th to 18th day group. As in earlier investigations, neither dominant mutations nor dominance effects of induced recessive lethal equivalents were found.

Effect of 90Sr on ovaries of foetal mice depending on time for administration during pregnancy

Pregnant CBA-mice were injected with 3.7 x 10(5) Bq (10 muCi) 90Sr on the 8th, 11th, 13th, 16th or 19th day post coitum. The effect on the ovaries in the intrauterine-treated animals was analysed on the 28th, 56th and 84th day post partum. The result was expressed as a comparison between the number of cells in different stages of development in the treated individuals compared to untreated control animals. The reduction of the number of cells in the ovaries was strongly correlated to the time of administration of the nuclide with a more increased effect the later during foetal development the contamination occurred. The oocytes and primordial follicles were the most radiation sensitive stages. The remaining cells in the ovaries are supposed to form a numerically reduced pool of functionally normal cells.

Dominant and recessive effects of induced-lethals in female mice by exposure to gamma-irradiation during the 10th to 14th day of intrauterine life

The frequency of (dominant and) recessive lethal mutations induced by 160-rad chronic gamma-irradiation given with a dose rate of 0.03 rad/min during the 10th to 14th day of gestation has been studied in female CBA-mice. An increased rate of recessive lethal equivalents by about 10% has been noted. This increase corresponds to a mutation rate of 6.3 X 10(-4) mutation/rad/genome. There were not found any dominant mutations, nor any dominance effects from the induced recessive lethal equivalents. The hazards after irradiation during foetal development are discussed.