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

Sperm selection in IVF: the long and winding road from bench to bedside

Spermatozoa wage battle to conquer fertilization but the traits needed to succeed remain elusive. The natural advantageous qualities that enable only a few selected sperm cells to reach the site of fertilization remain unknown. Although in vitro fertilization (IVF) facilitates the job of spermatozoa, a universally acceptable means of sperm selection is yet to be developed. No objective or reliable sperm quality indicators have been established and sperm selection is, to a great extent, based on subjective qualitative evaluation. The best method for sperm selection in IVF presents several challenges: intrinsic sperm qualities cannot be evaluated and the ideal endpoint for these studies is debatable. An ideal method for sperm selection in ART should be noninvasive and cost-effective, and allow the identification of high-quality spermatozoa and yield better outcomes in terms of pregnancy and live birth rates. This narrative review included 85 papers and focused on the new available methods and technologies that might shed some light on sperm selection in IVF. It discusses the available data on microfluidic devices, omics profiling, micronuclei studies, sperm plasma membrane markers, and other techniques, such as Magnetic Activated Cell Sorting (MACS), Raman micro-spectroscopy, and artificial intelligence systems. The new techniques herein reviewed offer fresh approaches to an old problem, for which a definite solution has yet to cross the bridge from bench to IVF clinics around the world, since clinical usefulness and application remain unproven.

Micronuclei and their association with sperm abnormalities, infertility, pregnancy loss, pre-eclampsia and intra-uterine growth restriction in humans

The micronucleus (MN) assay is increasingly being used to study the association between DNA damage and infertility or pregnancy complications in humans. This review provides a brief overview of the studies published to date. The results of these studies appear to support the plausibility of the following hypotheses: (i) MN in spermatids in semen may be indicative of infertility risk, (ii) MN in peripheral blood lymphocytes in males correlate positively with DNA damage in sperm, (iii) infertile couples exhibit higher frequencies of MN than fertile couples and (iv) an abnormally high frequency of MN in peripheral blood lymphocytes is associated with pregnancy complications including miscarriage, intra-uterine growth restriction and pre-eclampsia. The studies published to date consistently indicate an association of MN in peripheral blood lymphocytes with impaired reproductive capacity. However, the conclusions of these studies, although statistically significant, are limited by small sample sizes and the need for verification in other independent cohorts. In conclusion, more attention should be given to the possibility of using MN assays in peripheral blood lymphocytes and reproductive tissues as a biomarker of risk for infertility and pregnancy complications in humans.

Gender differences in the induction of chromosomal aberrations and gene mutations in rodent germ cells

Germ cell mutagenicity testing provides experimental data to quantify genetic risk for exposed human populations. The majority of tests are performed with exposure of males, and female data are relatively rare. The reason for this paucity lies in the differences between male and female germ cell biology. Male germ cells are produced throughout reproductive life and all developmental stages can be ascertained by appropriate breeding schemes. In contrast, the female germ cell pool is limited, meiosis begins during embryogenesis and oocytes are arrested over long periods of time until maturation processes start for small numbers of oocytes during the oestrus cycle in mature females. The literature data are reviewed to point out possible gender differences of germ cells to exogenous agents such as chemicals or ionizing radiation. From the limited information, it can be concluded that male germ cells are more sensitive than female germ cells to the induction of chromosomal aberrations and gene mutations. However, exceptions are described which shed doubt on the extrapolation of experimental data from male rodents to the genetic risk of the human population. Furthermore, the female genome may be more sensitive to mutation induction during peri-conceptional stages compared to the male genome of the zygote. With few exceptions, germ cell experiments have been carried out under high acute exposure to optimize the effects and to compensate for the limited sample size in animal experiments. Human exposure to environmental agents, on the other hand, is usually chronic and involves low doses. Under these conditions, gender differences may become apparent that have not been studied so far. Additionally, data are reviewed that suggest a false impression of safety when responses are negative under high acute exposure of male rodents while a mutational response is induced by low chronic exposure. The classical (morphological) germ cell mutation tests are not performed anymore because they are animal and time consuming. Nevertheless, information is needed to place genetic risk extrapolations on more solid grounds and thereby to prevent an increased genetic burden to future generations. It is pointed out that modern molecular methodologies are available now to experimentally address the open questions.

Epigenetic programming in the preimplantation rat embryo is disrupted by chronic paternal cyclophosphamide exposure

Preconceptional paternal exposure to cyclophosphamide, a widely used anticancer agent, leads to increases in embryo loss, malformations, and behavioral deficits in offspring; these abnormalities are transmissible to subsequent generations [Auroux, M., Dulioust, E., Selva, J. & Rince, P. (1990) Mutat. Res. 229, 189-200]. Little information exists on the mechanisms underlying this male-mediated developmental toxicity. We assessed the impact of paternal cyclophosphamide exposure on the dynamic regulation of histone H4 acetylation at lysine 5 and DNA methylation in preimplantation rat embryos. Zygotes sired by drug-treated males displayed advanced developmental progression, increased pronuclear areas, and disruption of the epigenetic programming of both parental genomes. Early postfertilization zygotic pronuclei were hyperacetylated; by mid-zygotic development, male pronuclei were dramatically hypomethylated, whereas female pronuclei were hypermethylated. Micronuclei were substantially elevated, and histone H4 acetylation at lysine 5 localization to the nuclear periphery was disrupted in two-cell embryos fertilized by cyclophosphamide-exposed spermatozoa. This finding demonstrates that paternal exposure to this drug induces aberrant epigenetic programming in early embryos. We hypothesize that disturbances in epigenetic programming contribute to heritable instabilities later in development, emphasizing the importance of epigenetic risk assessment after chemotherapy.

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.

Cytogenetic analysis of embryos generated from in vitro matured mouse oocytes reveals an increase in micronuclei due to chromosome fragmentation

PURPOSE

(i) To determine the prevalence of micronuclei in the cytoplasm of embryos generated from in vitro matured oocytes. (ii) Assess whether micronuclei presence are the result of chromosome fragmentation or the loss of whole chromosomes.

METHODS

In vitro fertilization was performed on mature oocytes generated from superovulated mice (control) and in vitro matured mouse oocytes. Fertilized oocytes were cultured to the two-cell stage and fixed to slides. Micronuclei assessment was performed after staining with Giemsa. Centromere assessment was made using immunofluorescent staining (CREST) of the centromeric kinetochores.

RESULTS

Micronuclei were observed in 2% (4/197) of control two-cell embryos and 36.2% (46/127) of two-cell embryos generated from in vitro matured oocytes (P < 0.02). Centromeres were not detected in micronuclei from either group.

CONCLUSIONS

A significant increase in micronuclei was observed in embryos generated from in vitro matured oocytes. The lack of accompanyingcentromeres would suggest the micronuclei are the result of chromosome fragmentation.

Induction of micronuclei formation in preimplantation mouse embryos after maternal treatment with 2-bromopropane

We examined effects of 2-bromopropane (2-BP), a chlorofluorocarbon replacement, on mouse embryonic mutagenicity. 2-BP was administered to pregnant mice intraperitoneally (i.p.) (300, 600, 900, and 1800 mg/kg) during the early preimplantation period. On day 3 of gestation, micronuclei (MN) frequency and embryo cell number were determined. 2-BP induced a dose-related significant increase in MN frequency and a treatment-related decrease in embryo cell number. Furthermore, the cell numbers were significantly smaller in the MN-positive embryos by two-way ANOVA taking it into account an interactive effect between 2-BP dose and the presence or absence of MN. A simultaneous decrease in cell number and increase in MN frequency may reflect an embryonic developmental disadvantage resulting from maternal treatment with 2-BP. Further study is needed to establish how 2-BP contributes to postimplantation embryonic development.

Micronuclei and developmental abnormalities in 4-day mouse embryos after paternal treatment with acrylamide

The developmental consequences of paternal exposure to acrylamide (50 mg/kg i.p. for 5 days) were assessed in preimplantation embryos. There was a significant increase in the proportion of morphologically abnormal embryos after postmeiotic treatment during spermatogenesis (88.7% vs. 14.8% in control). Abnormal embryos had an average of 1.8 +/- 3.5 cells and > 80% had at least one fragmented nucleus. In addition, morphologically normal embryos were significantly delayed (34.3 +/- 12.8 cells per embryo vs. 57.6 +/- 15.7 in control, P < 0.001). Acrylamide caused 10- and 20-fold increases in frequencies of cells with micronuclei (MN) in morphologically normal and abnormal embryos, respectively (41 and 93 MN per 1,000 cells). Both centromere-negative (MN-) and centromere-positive (MN+) were induced. Nuclei of abnormal embryos were significantly larger (900 microm2 vs. 250 microm2) than controls. In addition, MN of abnormal embryos were larger than those of normal embryos (21.2 microm2 vs. 6.5 microm2, P < 0.01). Among control embryos, MN+ were significantly larger than MN- (P < 0.05). These findings suggest that the preimplantation embryo is a sensitive indicator of paternally transmitted effects on early development. Multiple mechanisms appear to be involved, including cytogenetic damage, proliferation arrest/delay, and fertilization failure. Future studies are needed to establish how induced cytological defects in preimplantation embryos contribute to birth defects and other postimplantation abnormalities.

Induction of micronuclei in human sperm-hamster egg hybrids at the two-cell stage after in vitro gamma-irradiation of human spermatozoa

The efficiency of the micronucleus test to assess radiation-induced chromosomal damage in human spermatozoa has been investigated. Micronuclei were scored in human sperm-hamster egg hybrids at the two-cell stage, after exposure of human spermatozoa to in vitro gamma-rays at doses of 0.00, 0.10, 0.25, 0.50, 1.00, 2.00, and 4.00 Gy. The relationship between the yield of micronuclei per two-cell stage as well as the percentage of two-cell stages with micronuclei and the different doses of irradiation were fitted to linear equations. To evaluate whether scoring micronuclei is useful for the quantification of chromosomal damage occurring in human spermatozoa, induced micronuclei at the different doses of sperm irradiation were compared to the induction of breaks and fragments in sperm-derived chromosomes. After interspecific fertilization of zona-free hamster oocytes by irradiated spermatozoa, a total of 699 fertilized eggs at the two-cell stage and a total of 387 sperm-derived complements were analyzed. The incidence of fertilized eggs with micronuclei at the two-cell stage coincided well with the incidence of sperm-derived chromosome breaks and fragments (e.g., 8.9% vs. 6.7% in the 0.25 Gy group and 52.8% vs. 58.6% in the 4.00 Gy group). A similar correlation was found between the number of micronuclei per two-cell stage and the number of breaks and fragments per sperm complement (0.09 vs. 0.07 in the 0.25 Gy group and 0.71 vs. 0.81 in the 4.00 Gy group). The results show that this test system can be used for the quantification of spontaneous or induced chromosomal damage in human spermatozoa.