Types of structural chromosome aberrations and their incidences in human spermatozoa X-irradiated in vitro

Risks of genetic damage in offspring conceived using spermatozoa produced during chemotherapy or radiotherapy

BACKGROUND

Men who have just started cytotoxic therapy for cancer are uncertain and concerned about whether spermatozoa collected or pregnancies occurring during therapy might be transmitting genetic damage to offspring. There are no comprehensive guidelines on the risks of different doses of the various cytotoxic, and usually genotoxic, antineoplastic agents.

OBJECTIVES

To develop a schema showing the risks of mutagenic damage when spermatozoa, exposed to various genotoxic agents during spermatogenesis, are collected or used to produce a pregnancy.

MATERIALS AND METHODS

A comprehensive literature review was performed updating the data on genetic and epigenetic effects of genotoxic agents on animal and human spermatozoa exposed during spermatogenic development.

RESULTS

Relevant data on human spermatozoa and offspring are extremely limited, but there are extensive genetic studies in experimental animals that define sensitivities for specific drugs and times. The animal data were extrapolated to humans based on the stage when the cells were exposed and the relative kinetics of spermatogenesis and were consistent with the limited human data. In humans, alkylating agents and radiation should already induce a high risk of mutations in spermatozoa produced within 1 or 2 weeks after initiation of therapy. Topoisomerase II inhibitors and possibly microtubule inhibitors produce the greatest risk at weeks 5-7 of therapy. Nucleoside analogs, antimetabolites, and bleomycin exert their mutagenic effects on spermatozoa collected at 7-10 weeks of therapy.

DISCUSSION AND CONCLUSIONS

A schema showing the time from initiation of therapy at which specific antineoplastic agents can cause significant levels of genetic damage in conceptuses and live offspring was developed. The estimates and methods for computing the level of such risk from an individual patient's treatment regimen will enable patients and counselors to make informed decisions on the use of spermatozoa or continuation of a pregnancy.

The assessment and management of risks associated with exposures to short-range Auger- and beta-emitting radionuclides. State of the art and proposals for lines of research

The assessment and management of risks associated with exposures to ionising radiation are defined by the general radiological protection system, proposed by the International Commission on Radiological Protection (ICRP). This system is regarded by a large majority of users as a robust system although there are a number of dissenting voices, claiming that it is not suitable for estimating the risks resulting from internal exposures. One of the specific issues of internal exposure involves short-range radiations such as Auger and beta particles. Auger- and beta-emitting radionuclides can be distributed preferentially in certain tissue structures and even in certain cellular organelles, according to their chemical nature and the vector with which they are associated. Given the limited range of the low-energy electrons in biological matter, this heterogeneous distribution can generate highly localised energy depositions and exacerbate radiotoxic responses at cellular level. These particularities in energy distribution and cellular responses are not taken into account by the conventional methods for the assessment of risk.Alternative systems have been proposed, based on dosimetry conducted at the cellular or even molecular level, whose purpose is to determine the energy deposition occurring within the DNA molecule. However, calculation of absorbed doses at the molecular level is not sufficient to ensure a better assessment of the risks incurred. Favouring such a microdosimetric approach for the risk assessments would require a comprehensive knowledge of the biological targets of radiation, the dose-response relationships at the various levels of organisation, and the mechanisms leading from cellular energy deposition to the appearance of a health detriment. The required knowledge is not fully available today and it is not yet possible to link an intracellular energy deposition to a probability of occurrence of health effects or to use methods based on cellular dosimetry directly.The imperfections of the alternative approaches proposed so far should not discourage efforts. Protection against exposure to Auger and low-energy beta emitters would benefit from mechanistic studies, dedicated to the study of energy depositions of the radionuclides in various cellular structures, but also from radiotoxicological studies to define the relative biological effectiveness of the various Auger emitters used in medicine and of certain low-energy beta emitters, whose behaviour may depend greatly on their chemical form during intake. The scientific expertise, as well as the human and physical resources needed to conduct these studies, is available. They could be now mobilised into international low-dose research programmes, in order to ultimately improve the protection of people exposed to these specific radionuclides.

An investigation of the potential effect of vacuoles in human sperm on DNA damage using a chromosome assay and the TUNEL assay

BACKGROUND

The aims of this study were to establish whether individual differences exist in the frequency and size of vacuoles found in human sperm and to ascertain whether such vacuoles are involved in causing DNA damage.

METHODS

Morphologically normal sperm were obtained from 15 IVF and 2 ICSI patients and 3 fertile donors. (i) Sperm heads were analyzed for the presence of vacuoles under a 1000× differential interference contrast microscope. (ii) In three patients and two donor samples, structural chromosomal damage was evaluated in normal sperm containing large vacuoles and selected at 1000× magnification for injection into mouse oocytes. (iii) In 10 patients and two donor samples, confocal laser microscopy detected DNA damage in sperm-exhibiting large vacuoles and stained with an in situ cell death detection kit.

RESULTS

(i) Vacuoles were observed in almost all normal sperm from patient and donor ejaculates and were mainly located at the tip or middle area of the sperm heads. However, average incidence of normal sperm exhibiting large vacuoles was 4.6 and 4.2% in the patient and donor groups, respectively. (ii) Sperm chromosome assays did not reveal any differences in the incidence of structural chromosome aberrations between sperm exhibiting large vacuoles and those without them (9.1 versus 4.1%). (iii) No significant difference in frequency of TUNEL-positive cells was found between normal sperm with large vacuoles and those without them in the samples examined. Among 227 sperm exhibiting large vacuoles, only 7 cells were TUNEL positive.

CONCLUSION

The results showed that large vacuoles were not responsible for DNA damage, suggesting that intra-cytoplasmic injection of morphologically selected sperm may not be required for patients who produce high-quality semen.

Effects of airport screening X-irradiation on bovine sperm chromatin integrity and embryo development

Biological samples, including cryopreserved sperm, are routinely X-rayed during air shipment. The goal was to investigate the impact of X-irradiation used for checked and carry-on luggage on bovine sperm chromatin integrity and postfertilization in vitro embryonic development. Frozen domestic bull sperm (Bos taurus) (n=9 bulls) stored in a dry shipper (-160 degrees C) was screened by X-irradiation 0, 1, 2, and 3 times as either carry-on or checked luggage. Duplicate straws were thawed, and sperm were assessed for chromatin damage using the sperm chromatin structure assay (SCSA) and by postfertilization in vitro developmental competence of mature oocytes. Multiple exposure to X-rays did not significantly affect sperm chromatin integrity assessed by SCSA. There were lower proportions of oocytes cleaved (P=0.07; 21.6+/-3.1% vs. 29.4+/-3.1%, 24.9+/-3.1%, and 25.7+/-3.3% for 3 vs. 0, 1, and 2 times, respectively; least-squares means+/-SEM) and that developed to blastocysts (P=0.06; 9.0+/-1.7% vs. 13.8+/-1.7%, 11.5+/-1.7%, and 12.6+/-1.9%, respectively) when fertilization was performed with sperm X-rayed 3 times using checked luggage irradiation; developmental competence (percentage cleaved embryos becoming blastocysts) was unaffected. There were no deleterious effects of other X-irradiation treatments on embryo development. We inferred that screening by X-irradiation may reduce the ability of sperm to activate oocyte cleavage after multiple exposures at the checked luggage dose. However, there was no evidence that competence of embryos to become blastocysts was reduced by X-irradiation (45.4+/-5.7%, 40.4+/-5.7%, 46.4+/-6.1%, and 41.8+/-5.7% for 0, 1, 2, and 3 doses, respectively), but potential long-term epigenetic effects are unknown.

Systematic review of experimental studies on the relative biological effectiveness of tritium

Tritium ((3)H) is a radioactive isotope of hydrogen. A number of factors combine to create a good deal of interest in tritium doses, both to workers and to members of the public. Tritium is ubiquitous in environmental and biological systems and is very mobile due to its occurrence as water. In this study we systematically review experimental data relating to tritium exposure with a view to assessing its low dose limiting relative biological effectiveness (RBE(max)). Interpretation of published experimental studies is complicated by the fact that the reference radiations varied, and doses and dose rates were frequently much higher than those normally received by humans. The four available animal carcinogenicity studies gave RBE values of about 2.5 with chronically-delivered gamma-ray reference, and about 1.2 with chronically-delivered X-ray reference. However, because of problems associated with the design and interpretation of the experiments, we do not consider that these RBE values should be taken to apply to the induction of cancer at low doses (i.e. they should not be interpreted as RBE(max)). Combining the six studies with chronic gamma-ray reference, with adequate quantitative data that examined endpoints apart from cell survival and related endpoints, yields an aggregate RBE estimate of 2.19 (95% CI 2.04, 2.33); the analogous combined RBE estimate using the three studies with chronic X-ray reference groups is 1.17 (95% CI 0.96, 1.39). Again, problems with the design, in particular the range of doses used in some of these studies, mean that these RBE values should also probably not be interpreted as RBE(max).

Chromosomal integrity of freeze-dried mouse spermatozoa after 137Cs gamma-ray irradiation

This study demonstrated that freeze-dried mouse spermatozoa possess strong resistance to 137Cs gamma-ray irradiation at doses of up to 8 Gy. Freeze-dried mouse spermatozoa were rehydrated and injected into mouse oocytes with an intracytoplasmic sperm injection (ICSI) technique. Most oocytes can be activated after ICSI by using spermatozoa irradiated with gamma-rays before and after freeze-drying. Sperm chromosome complements were analyzed at the first cleavage metaphase. Chromosome aberrations increased in a dose-dependent manner in the spermatozoa irradiated before freeze-drying. However, no increase in oocytes with chromosome aberrations was observed when fertilized by spermatozoa that had been irradiated after freeze-drying, as compared with freeze-dried spermatozoa that had not been irradiated. These results suggest that both the chromosomal integrity of freeze-dried spermatozoa, as well as their ability to activate oocytes, were protected from gamma-ray irradiation at doses at which chromosomal damage is found to be strongly induced in spermatozoa suspended in solution.

Radiation- and chemical-induced structural chromosome aberrations in human spermatozoa

Previous studies on the clastogenic effects of mutagens on human sperm chromosomes were reviewed. A marked increase of structural chromosome aberrations (SCAs) has been reported in the spermatozoa irradiated in vitro with five kinds of ionizing radiation (137Cs gamma-, 60Co gamma-, X-, and 3H beta-rays and 252Cf neutrons). The micronucleus (MN) test with hybrid two-cell embryos generated from human sperm and hamster oocytes was shown to be useful as a simple and rapid method for assessing the effects of radiation. Radiosensitivity of human spermatozoa was highest, being followed by golden hamster, Chinese hamster and mouse spermatozoa. Chromosome-damaging effects were also found with some chemicals (bleomycin, daunomycin, methyl methanesulfonate, triethylenemelamine, neocarzinostatin, N-methyl-N'-nitro-N-nitorosoguanidine and mitomycin C (MMC)), but not with other chemicals (urethane, nitrobenzene, dioxin, cyclophosphamide (CP), benzo(a)pyrene (BP) and N-nitrosodimethylamine (NDMA)). The clastogenicity of chemical metabolites was confirmed for CP and BP, by using the S9-based metabolic activation system. The results of sperm chromosome analysis from cancer patients who had undergone radio- and/or chemotherapy were contradictory among investigators and further studies are necessary. The importance of mutagenicity testing with human spermatozoa is discussed.

Chromosome analysis of human spermatozoa following in vitro exposure to cyclophosphamide, benzo(a)pyrene and N-nitrosodimethylamine in the presence of rat liver S9

For the purpose of assessing mutagenic effects (clastogenicity) of metabolites derived from chemical mutagens/carcinogens on human sperm chromosomes, spermatozoa were exposed in vitro to cyclophosphamide (CP), benzo(a)pyrene (BP) or N-nitrosodimethylamine (NDMA) for 2h in the presence or absence of rat liver S9, a metabolic activator of these chemicals. After in vitro fertilization between human spermatozoa and zona-free hamster oocytes, chromosome complements of sperm origin were analyzed cytogenetically. In the absence of S9, none of three chemicals (20 microg/ml CP, 200 microg/ml BP and 20mg/ml NDMA) caused a significant increase in spermatozoa with structural chromosome aberrations (8.6, 10.0 and 7.5%), as compared with their matched controls (10.9, 11.0 and 8.5%). In the presence of S9, however, a significant increase in chromosomally abnormal spermatozoa was observed in CP (37.1%, P < 0.001) and BP (31.0%, P < 0.001), indicating that enzymatic activation of CP and BP induced chromosomal abnormalities in human sperm. In contrast, NDMA did not induce chromosome aberrations in human spermatozoa by S9 treatment, although positive results have been observed in somatic cells. The present results on in vitro clastogenicity of CP, BP and NDMA are consistent with the results in previous in vivo studies with murine spermatozoa. Our S9/human sperm chromosome assay seems to be useful for estimation of hereditary risk of chemicals in human. Because most chemicals need metabolic activation to bind to DNA.

DNA damage in human and mouse spermatozoa after in vitro-irradiation assessed by the comet assay

The comet assay is widely employed as a method to measure DNA damage in a wide variety of cell types following genotoxic insult. We have used this method in order to characterise DNA damage in spermatozoa following in vitro irradiation with 137Cs gamma rays. In contrast to somatic cells, the DNA of mammalian spermatozoa is bound by protamine molecules allowing a sixfold more highly compact structure and thus rendering conventional cell lysis protocols ineffective. Therefore, this new method uses an extensive lysis step to ensure effective removal of DNA-associated proteins allowing DNA damage to be scored reproducibly in both murine and human spermatozoa. Mouse spermatozoa collected from the vas deferens at post-mortem or human spermatozoa provided by donors were irradiated with doses of gamma-rays from 0-100 Gy using a 137Cs source and then processed for both alkaline and neutral comet assays. Under neutral electrophoresis conditions, which permits the measurement of double-stranded DNA breaks, a linear increase in the amount of DNA damage measured was observed with increasing radiation dose for both murine and human spermatozoa. Similarly, using alkaline electrophoresis conditions to examine DNA single-strand breaks and alkali-labile sites, a linear relationship was also observed for murine sperm but in contrast no such relationship was apparent for human spermatozoa subjected to the same radiation treatments. Interestingly, unirradiated sperm (both human and mouse) showed extensive DNA migration from the nucleus after alkaline assay. Since it is unlikely that the DNA of normal spermatozoa contains high numbers of single-strand breaks and damage was not detected for unirradiated sperm in the neutral assay, it is more likely that this DNA migration is due to the presence of high numbers of alkali labile sites within sperm DNA and that these may be related to the highly condensed structure of spermatozoal DNA. The large radiation doses used in these experiments to produce measurable amounts of DNA damage reflects the high radioresistance of spermatozoa compared to somatic cells and this may also be related to the differences in DNA packaging and conformation. In conclusion, this work shows that the comet assay represents a new method for examining DNA damage in spermatozoa and should be evaluated for use in reproductive toxicity testing.

No induction of chromosome aberrations in human spermatozoa exposed to extremely low frequency electromagnetic fields

Clastogenic effects of extremely low frequency electromagnetic fields (ELF-EMFs) on human sperm chromosomes were studied using an interspecific in vitro fertilization system with zona-free golden hamster oocytes. Semen samples from healthy men were exposed to ELF-EMFs (50 Hz, 20 mT) for 2 h at 37 degreesC under 5% CO2 in air. The samples were then cryopreserved in liquid nitrogen for shipment to a cytogenetic laboratory. After thawing the samples, motile spermatozoa were collected using a continuous Percoll density gradient centrifugation and then capacitated for in vitro fertilization with hamster oocytes. Sperm-derived chromosomes were analyzed at first cleavage metaphase. The present experiment was performed twice using semen samples from two different donors. In test-1, incidence of spermatozoa that displayed structural chromosome aberrations was 17.0% (35/206) in the exposed group and 20.8% (55/264) in the control group. In test-2, structural chromosome aberrations were observed in 11.1% (13/117) of exposed spermatozoa and 13.8% (13/94) of spermatozoa in the control group. In both tests, there was no significant difference in the incidence of chromosomally abnormal spermatozoa between the exposed group and the control group. Types of aberrations observed and their incidences per spermatozoon in the exposed group were similar to those of the control group. Despite the small sample size, the present results suggest that ELF-EMFs have no clastogenic effect on human sperm chromosomes.

Relative biological effectiveness (RBE) of 252Cf fission neutrons for the induction of chromosome damage in human spermatozoa

Effects of 60Co gamma-rays and 252Cf neutrons on human sperm chromosomes were studied using our interspecific in vitro fertilization system to estimate relative biological effectiveness (RBE) of neutrons. Semen samples were exposed to 0.5, 1.0 and 2.0 Gy of 60Co gamma-rays at 1.7 cGy/ min and 0.25, 0.5 and 1.0 Gy of 252Cf radiation at 1.3-1.7 cGy/ min. In the 60Co experiment, 509 spermatozoa from controls and 902 spermatozoa from the irradiated groups were karyotyped, while in the 252Cf experiment 460 control and 804 irradiated spermatozoa were analysed. In both 60Co and 252Cf experiments, incidences of spermatozoa with radiation-induced structural chromosome aberrations increased linearly with increase of dosage. The RBE of 252Cf neutrons for the induction of chromosomally abnormal spermatozoa was estimated to be 1.6. The number of induced structural chromosome aberrations per spermatozoon also increased linearly. The RBE of neutrons for this index was 2.0. Among structural chromosome aberrations observed, chromosome-type breaks were predominant in both 60Co and 252Cf experiments, and they showed a significant linear dose-dependent increase. Other types of aberrations such as chromosome-type exchanges and chromatid-type breaks also increased linearly with increase in dose. The RBEs of 252Cf neutrons for the induction of these three types of aberrations were 1.6, 3.2 and 3.9, respectively. Thus, the RBEs of neutrons for the induction of chromosome aberrations were smaller in human spermatozoa than in human lymphocytes, and mouse spermatogonia and embryos. This result is discussed from the point of view of DNA-repairing capacity of oocytes.

Difference in types of radiation-induced structural chromosome aberrations and their incidences between Chinese and Syrian hamster spermatozoa

The effects of ionizing radiations on sperm chromosomes were studied in the Chinese hamster (Crisetulus griseus) and the Syrian (golden) hamster (Mesocrisetus auratus). Testes of mature male Chinese hamsters (CH) were irradiated with X-rays (0.91, 1.82 and 3.63 Gy) and gamma-rays (1.10, 2.15, 2.95 and 4.01 Gy) at a single acute dosage, whereas the irradiation was done with lower doses of X-rays (0.45, 0.91 and 1.82 Gy) and gamma-rays (0.49, 0.99 and 1.98 Gy) in mature male Syrian hamsters (SH), taking the higher radiosensitivity of this species into consideration. They were mated with normal females within 6 days of exposure. Sperm-derived chromosomes were analyzed in 1125 and 1966 fertilized ova of the CH and the SH, respectively. In both species, there was no great difference in the induction of structural chromosome aberrations between X-irradiated and gamma-irradiated spermatozoa. Chromosome-type aberrations were predominantly induced. The incidence of breakage-type aberrations increased linearly, and that of exchange-type aberrations linear-quadratically with increase of dosage. A species-specific difference in chromosomal radiosensitivity of spermatozoa was clear. In spite of the same radiation dosage, the incidence of chromosomally abnormal spermatozoa in the SH was about twice as high as that in the CH (e.g. 27.0% vs. 14.7% at 0.91 Gy of X-rays). The incidences of breakage-type aberrations (69-89%) were far higher than those of exchange-type aberrations (11-31%) in the SH, while the disparity of the two incidences was much smaller in the CH (46-65% vs. 35-54%). Exchange-type aberrations consisted of both chromosome-type and chromatid-type in the SH, while almost all of them were of the chromosome-type in the CH. These results suggest that the DNA-repairing capacity of oocytes is much higher in the CH than in the SH. Moreover, it seems likely that radiation-induced sperm DNA damage is repaired with both pre-replication repair (excision repair) and post-replication repair systems in SH oocytes, whereas the excision repair system operate most exclusively in CH oocytes.

Chromosome analysis of human spermatozoa exposed to antineoplastic agents in vitro

We studied in vitro the cytogenetic effects of six antineoplastic agents, bleomycin (BM), cyclophosphamide (CP), daunomycin (DM), methyl methanesulfonate (MMS), mitomycin C (MMC) and triethylenemelamine (TEM) on spermatozoa, using an interspecific in vitro fertilization system between zona-free hamster oocytes and human or bull spermatozoa. In preliminary experiments with bull spermatozoa, clastogenic effects were clearly shown with BM, DM, MMS and TEM, but not with CP and MMC. In main experiments, the effects of the first four chemicals were studied in detail with human spermatozoa. Total numbers of 585 and 512 spermatozoa were karyotyped in the control and the chemical-treated groups respectively. The incidence of spermatozoa with structural chromosome aberrations was 34.5%, 53.0%, 59.3%, and 55.6% in the BM (50 micrograms/ml, 90 min), DM (0.1 microgram/ml, 90 min), MMS (100 micrograms/ml, 120 min) and TEM (0.1 micrograms/ml, 120 min) groups respectively, each showing a significantly higher incidence than the matched controls (10.1-13.5%). Breakage-type aberrations were more frequent than exchange-type aberrations in the BM, MMS and TEM groups, while the exchange-type aberrations were more frequent in the DM group. Exchanges were mainly of the chromatid type in the DM, MMS and TEM groups, while chromosome-type exchanges occurred more frequently in the BM group. These results are discussed in relation to previous data on chemical-induced chromosome aberrations in mammalian somatic cells and in mouse spermatozoa.

A freezing and thawing method of hamster oocytes designed for both the penetration test and chromosome assay of human spermatozoa

Superovulated hamster oocytes were cryopreserved and thawed according to our carefully designed procedures. More than 90% (92 +/- 4%) of oocytes survived freezing and thawing. They were proven to be well conserved, showing excellent performance comparable to freshly ovulated oocytes in the human sperm penetration test (proportion of penetrated ova: 94.7% vs. 93.6%) and human sperm chromosome analysis (proportion of metaphasic ova: 81.8% vs. 83.6%). There was no statistically significant difference in the incidences of sperm chromosome aberrations between assays using fresh and frozen-thawed oocytes. In addition, there was no statistically significant increase of aberrations in female pronuclear (hamster) chromosomes. This freezing-thawing method was found to be reliable, yielding viable hamster oocytes of high quality.

Micronucleus test in 2-cell embryos as a simple assay system for human sperm chromosome aberrations

A micronucleus test method to assess radiation-induced chromosomal damage in human spermatozoa is described, and its efficiency examined by comparison with that of sperm chromosome analysis. Human spermatozoa were exposed in vitro to 1.11 and 2.13 Gy of 137Cs gamma-rays at a dose rate of 1.36 Gy/min. After interspecific in vitro fertilization of irradiated spermatozoa with zona-free hamster oocytes, a total of 193 monospermic eggs were examined with the micronucleus test at the 2-cell stage, and a total of 304 male pronuclear chromosome plates were analyzed according to our established method. The incidence of 2-cell embryos with micronuclei coincided well with the incidence of spermatozoa with chromosomal breaks and fragments (51.6% vs. 50.3% in the 1.11-Gy group and 82.7% vs. 79.3% in the 2.13-Gy group). A similar correlation was also found between the number of micronuclei per embryo and the number of breaks and fragments per spermatozoon (0.85 vs. 0.88 and 1.50 vs. 1.45 in the 2 dose groups, respectively). These results indicate that our micronucleus test is useful as a simple and rapid method for assessing the clastogenic effects of various environmental mutagens on human sperm chromosomes.

Ionising radiations from nuclear establishments and childhood leukaemias--an enigma

The Gardner report, recently published in the UK, showing a correlation between incidence of childhood leukaemia and paternal exposure to ionising radiations (amongst fathers working in nuclear power plants) has added a new element to debates about both the risk factors in nuclear power plants and the relationships between ionising radiations and leukaemogenesis. The epidemiologic and genetic evidence concerning leukaemias is reviewed here and it is concluded that the leukaemogenic agent, whose existence is indicated in the Gardner report, is unlikely to be paternal radiation dose per se but rather exposure to another factor that is correlated with parternal radiation dose received.