Aneuploidies and micronuclei in the germ cells of male mice of advanced age

Transposon insertion profiling by sequencing (TIPseq) identifies novel LINE-1 insertions in human sperm

PURPOSE

Long interspersed nuclear element-1 (LINE-1 or L1) comprises 17% of the human genome. Retrotransposons may perturb gene integrity or alter gene expression by altering regulatory regions in the genome. The germline employs a number of mechanisms, including cytosine methylation, to repress retrotransposon transcription throughout most of life. Demethylation during germ cell and early embryo development de-represses retrotransposons. Intriguingly, de novo genetic variation appearing in sperm has been implicated in a number of disorders in offspring, including autism spectrum disorder, schizophrenia, and bipolar disorder. We hypothesize that human sperm exhibit de novo retrotransposition and employ a new sequencing method, single cell transposon insertion profiling by sequencing (scTIPseq) to map them in small amounts of human sperm.

METHODS

Cross-sectional case-control study of sperm samples (n=10 men; ages 32-55 years old) from consenting men undergoing IVF at NYU Langone Fertility Center. scTIPseq identified novel LINE-1 insertions in individual sperm and TIPseqHunter, a custom bioinformatics pipeline, compared the architecture of sperm LINE-1 to known LINE-1 insertions from the European database of Human specific LINE-1 (L1Hs) retrotransposon insertions (euL1db).

RESULTS

scTIPseq identified 17 novel insertions in sperm. New insertions were mainly intergenic or intronic. Only one sample did not exhibit new insertions. The location or number of novel insertions did not differ by paternal age.

CONCLUSION

This study for the first time reports novel LINE-1 insertions in human sperm, demonstrating the feasibility of scTIPseq, and identifies new contributors to genetic diversity in the human germ line.

Chemically induced aneuploidy in germ cells. Part II of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

As part of the 7th International Workshops on Genotoxicity Testing held in Tokyo, Japan in November 2017, a workgroup of experts reviewed and assessed the risk of aneugens for human health. The present manuscript is one of three manuscripts from the workgroup and reports on the unanimous consensus reached on the evidence for aneugens affecting germ cells, their mechanisms of action and role in hereditary diseases. There are 24 chemicals with strong or sufficient evidence for germ cell aneugenicity providing robust support for the ability of chemicals to induce germ cell aneuploidy. Interference with microtubule dynamics or inhibition of topoisomerase II function are clear characteristics of germ cell aneugens. Although there are mechanisms of chromosome segregation that are unique to germ cells, there is currently no evidence for germ cell-specific aneugens. However, the available data are heavily skewed toward chemicals that are aneugenic in somatic cells. Development of high-throughput screening assays in suitable animal models for exploring additional targets for aneuploidy induction, such as meiosis-specific proteins, and to prioritize chemicals for the potential to be germ cell aneugens is encouraged. Evidence in animal models support that: oocytes are more sensitive than spermatocytes and somatic cells to aneugens; exposure to aneugens leads to aneuploid conceptuses; and, the frequencies of aneuploidy are similar in germ cells and zygotes. Although aneuploidy in germ cells is a significant cause of infertility and pregnancy loss in humans, there is currently limited evidence that aneugens induce hereditary diseases in human populations because the great majority of aneuploid conceptuses die in utero. Overall, the present work underscores the importance of protecting the human population from exposure to chemicals that can induce aneuploidy in germ cells that, in contrast to carcinogenicity, is directly linked to an adverse outcome.

Meiotic spindle assembly checkpoint and aneuploidy in males versus females

The production of gametes (sperm and eggs in mammals) involves two sequential cell divisions, meiosis I and meiosis II. In meiosis I, homologous chromosomes segregate to different daughter cells, and meiosis II resembles mitotic divisions in that sister chromatids separate. While in principle the process is identical in males and females, the time frame and susceptibility to chromosomal defects, including achiasmy and cohesion weakening, and the response to mis-segregating chromosomes are not. In this review, we compare and contrast meiotic spindle assembly checkpoint function and aneuploidy in the two sexes.

Control of Germline Stem Cell Lineages by Diet and Physiology

Tight coupling of reproduction to environmental factors and physiological status is key to long-term species survival. In particular, highly conserved pathways modulate germline stem cell lineages according to nutrient availability. This chapter focuses on recent in vivo studies in genetic model organisms that shed light on how diet-dependent signals control the proliferation, maintenance, and survival of adult germline stem cells and their progeny. These signaling pathways can operate intrinsically in the germ line, modulate the niche, or act through intermediate organs to influence stem cells and their differentiating progeny. In addition to illustrating the extent of dietary regulation of reproduction, findings from these studies have implications for fertility during aging or disease states.

Advancing age increases sperm chromatin damage and impairs fertility in peroxiredoxin 6 null mice

Due to socioeconomic factors, more couples are choosing to delay conception than ever. Increasing average maternal and paternal age in developed countries over the past 40 years has raised the question of how aging affects reproductive success of males and females. Since oxidative stress in the male reproductive tract increases with age, we investigated the impact of advanced paternal age on the integrity of sperm nucleus and reproductive success of males by using a Prdx6(-/-) mouse model. We compared sperm motility, cytoplasmic droplet retention sperm chromatin quality and reproductive outcomes of young (2-month-old), adult (8-month-old), and old (20-month-old) Prdx6(-/-) males with their age-matched wild type (WT) controls. Absence of PRDX6 caused age-dependent impairment of sperm motility and sperm maturation and increased sperm DNA fragmentation and oxidation as well as decreased sperm DNA compaction and protamination. Litter size, total number of litters and total number of pups per male were significantly lower in Prdx6(-/-) males compared to WT controls. These abnormal reproductive outcomes were severely affected by age in Prdx6(-/-) males. In conclusion, the advanced paternal age affects sperm chromatin integrity and fertility more severely in the absence of PRDX6, suggesting a protective role of PRDX6 in age-associated decline in the sperm quality and fertility in mice.

Fertility concerns for the aging male

Because of many societal factors, the number of men over the age of 35 desiring to conceive children has increased over the past 40 years. The purpose of this review is to identify the mechanisms of aging on male fertility, to evaluate the genetic risk for the offspring, and to provide counseling for the older male. Most evidence suggests trends that increased paternal age has negative effects on fertility and some genetic risk for offspring, but the age at which the risk develops and the magnitude of risk are poorly defined.

Increased de novo copy number variants in the offspring of older males

The offspring of older fathers have an increased risk of neurodevelopmental disorders, such as schizophrenia and autism. In light of the evidence implicating copy number variants (CNVs) with schizophrenia and autism, we used a mouse model to explore the hypothesis that the offspring of older males have an increased risk of de novo CNVs. C57BL/6J sires that were 3- and 12-16-months old were mated with 3-month-old dams to create control offspring and offspring of old sires, respectively. Applying genome-wide microarray screening technology, 7 distinct CNVs were identified in a set of 12 offspring and their parents. Competitive quantitative PCR confirmed these CNVs in the original set and also established their frequency in an independent set of 77 offspring and their parents. On the basis of the combined samples, six de novo CNVs were detected in the offspring of older sires, whereas none were detected in the control group. Two of the CNVs were associated with behavioral and/or neuroanatomical phenotypic features. One of the de novo CNVs involved Auts2 (autism susceptibility candidate 2), and other CNVs included genes linked to schizophrenia, autism and brain development. This is the first experimental demonstration that the offspring of older males have an increased risk of de novo CNVs. Our results support the hypothesis that the offspring of older fathers have an increased risk of neurodevelopmental disorders such as schizophrenia and autism by generation of de novo CNVs in the male germline.

Fertile bull sperm aneuploidy and chromatin integrity in relationship to fertility

Aneuploidy is associated with spontaneous abortions, perinatal mortality, mental retardation and with embryonic and foetal mortality. Most of these abnormalities originate as a result of meiosis errors during gametogenesis. The main purpose of the study was to analyse frequency of aneuploidies of sex chromosomes and chromosome 6 by three-colour fluorescence in situ hybridization (FISH) in 47 young bulls, candidates for artificial insemination programme with cryopreserved semen and to investigate the influence of aneuploidies and disturbed sperm chromatin integrity on non-return rates, the frequencies of abortions, perinatal mortality and stillbirths. The average frequencies of spermatozoa with disomy for chromosomes X, Y, XY and 6 were 0.032, 0.005, 0.003 and 0.039% respectively. The incidence of XX66, YY66 and XY66 diploidy was 0.017, 0.006 and 0.015% respectively. Frequencies of meiotic II errors were significantly higher than meiotic I errors (p < 0.01). More X bearing spermatozoa than Y bearing spermatozoa were detected (5151 vs. 5022; p < 0.01). Sperm chromatin damage expressed by DNA fragmentation index (DFI) was 5.3 +/- 3.7 and percentage of cells with defective chromatin condensation (HDS) was 1.4 +/- 0.8. No correlation was found between sperm aneuploidy and basic sperm analysis. The relationship was found between non-return rate and total aneuploidy (r = -0.310; p = 0.036). Significant correlation was found between sex disomy, total aneuploidy (disomy of chromosomes 6, X, Y and XY spermatozoa and diploidy) and stillbirths (r = 0.390; p = 0.013; and r = 0.331; p = 0.037). Chromosome 6 disomy correlated with perinatal mortality (r = 0.317; p = 0.047). HDS correlated significantly with total aneuploidy (r = 0.449; p = 0.002). Our study indicated that aneuploidy frequencies in young fertile bull spermatozoa are relatively low. Nevertheless, there exists a variability in aneuploidy frequencies amongst bulls, which appears to be able to have an influence on the fertility of these animals.

The male biological clock

Do men have biological clocks that affect their hormone levels, fertility, and the genetic quality of their sperm? Women can no longer be viewed as solely responsible for age-related fertility and genetic problems. The effects of andropause and advanced paternal age on fertility and offspring are still under investigation. Further research is needed to fully characterize the associated risks and to treat the underlying abnormalities. A better understanding of the cellular and biochemical mechanisms of "gonadal" aging is important in order to determine safe, effective ways to delay this process and "rewind" the male biological clock. The benefits may include decreasing the potential for adverse genetic consequences in offspring, improvement in the sexual and reproductive health of aging males, and increase a woman's chance of having healthy children by correcting defects in the male reproductive system.

Cross-species sperm-FISH assays for chemical testing and assessing paternal risk for chromosomally abnormal pregnancies

The father, like the mother, can transmit genetic defects to his offspring that are detrimental for normal development and a healthy life. Epidemiological studies have identified associations between several paternal exposures and abnormal reproductive outcomes, but these types of studies are inherently complex and expensive, and the risk factors for the paternal contribution to abnormal reproductive outcomes remain poorly understood. Several sensitive methods have been developed for detecting mutations and chromosomal damage directly in sperm. These assays are potential bioindicators for paternal risk factors for infertility, spontaneous abortions, aneuploidy syndromes, and genetic diseases in children. Among these methods, fluorescence in situ hybridization (FISH) has been adapted for the detection of numerical and structural chromosomal abnormalities in the sperm of an expanding number of species, including humans and rodents. Sperm FISH has identified several potential paternal risk factors such as age, drugs, lifestyles, and various environmental/occupational exposures. Here, we summarize the status of the development and usage of these sperm-FISH assays and suggest strategies for prioritizing chemical agents for epidemiological investigations to assess paternal risk for abnormal reproductive outcome.

Effects of male age on the frequencies of germinal and heritable chromosomal abnormalities in humans and rodents

OBJECTIVE

To review evidence regarding the effects of male age on germinal and heritable chromosomal abnormalities using available human and rodent studies and to evaluate possible underlying mechanisms.

DESIGN

Review of English language-published research using MEDLINE database, excluding case reports and anecdotal data.

RESULT(S)

There was little evidence from offspring or germ cell studies for a generalized male age effect on autosomal aneuploidy, except in rodents. Sex chromosomal nondisjunction increased with age in both human and rodent male germ cells. Both human and rodent data showed age-related increases in the number of sperm with chromosomal breaks and fragments and suggest that postmeiotic cells are particularly vulnerable to the effects of aging. Translocation frequencies increased with age in murine spermatocytes, at rates comparable to mouse and human somatic cells. Age-related mechanisms of induction may include accumulation of environmental damage, reduced efficiency of DNA repair, increased genomic instability, genetic factors, hormonal influences, suppressed apoptosis, or decreased effectiveness of antioxidants and micronutrients.

CONCLUSION(S)

The weight of evidence suggests that the increasing trend toward fathering at older ages may have significant effects on the viability and genetic health of human pregnancies and offspring, primarily as a result of structural chromosomal aberrations in sperm.

Effect of age on testicular germ cell apoptosis and sperm aneuploidy in MF-1 mice

The spontaneous mutation rate in the male germ-line increases with age. The reason for this is unknown, but presumably involves an age-related degeneration in the efficacy of cellular processes. To investigate the possibility that rates of apoptosis and genetic damage (represented by aneuploidy) might vary with age in mice, the testes and sperm of 2- and 12-month-old male MF-1 mice were examined by a modified TUNEL technique and 3-colour sperm-FISH assay, respectively. Sperm were labeled with probes to chromosomes 8, X and Y and 20,000 sperm scored from each of 5 animals per group. A significant increase in gonosomal disomy was found in the aged mice, especially X-X-8. This suggests that advanced paternal age is associated primarily with meiosis II rather than meiosis I disjunction errors. Neither diploidy nor autosomal disomy was affected in the older group. The rate of germ cell apoptosis (apoptotic cells per seminiferous tubule cross-section per animal per group) was higher in the old mice than controls, but not significantly. Considerable inter-animal variability was observed in the older group. The finding of an increase in levels of sperm aneuploidy is novel for 1-year-old mice and confirms the genotoxic effect of ageing in mice. Since apoptosis is assumed to eliminate cells with unrepaired damage, it may be that the apoptotic response in older mice is compromised, resulting in the higher levels of aneuploidy in sperm. However, given the inter-animal variability in testicular germ cell apoptosis, this awaits confirmation.

A new FISH assay to simultaneously detect structural and numerical chromosomal abnormalities in mouse sperm

De novo aberrations in chromosome structure represent important categories of paternally transmitted genetic damage. Unlike numerical abnormalities, the majority of de novo structural aberrations among human offspring are of paternal origin. We report the development of a three-color fluorescence in situ hybridization (FISH) assay (CT8) to detect mouse sperm carrying structural and numerical chromosomal abnormalities. The CT8 assay uses DNA probes for the centromeric and telomeric regions of chromosome 2, and a probe for the subcentromeric region of chromosome 8. The CT8 assay was used to measure the frequencies of sperm carrying certain structural aberrations involving chromosome 2 (del2ter, dup2ter, del2cen, dup2cen), disomy 2, disomy 8, and sperm diploidy. Analysis of approximately 80,000 sperm from eight B6C3F1 mice revealed an average baseline frequency of 2.5 per 10,000 sperm carrying partial duplications and deletions of chromosome 2. Extrapolated to the entire haploid genome, approximately 0.4% of mouse sperm are estimated to carry structural chromosomal aberrations, which is more than fivefold lower than the spontaneous frequencies of sperm with chromosome structural aberrations in man. We validated the CT8 assay by comparing the frequencies of abnormal segregants in sperm of T(2;14) translocation carriers detected by this assay against those detected by chromosome painting cytogenetic analysis of meiosis II spermatocytes. The CT8 sperm FISH assay is a promising method for detecting structural chromosome aberrations in mouse sperm with widespread applications in genetics, physiology, and genetic toxicology.

Male age is not an independent factor to affect the outcome of assisted reproductive techniques

Controversy exists whether advanced male age is associated with poor sperm quality and subsequent failure in the assisted reproductive techniques (ART). The purpose of this study was to evaluate the effect of male age on sperm quality and the outcome of ART as well as the association of male age with other relevant factors, particularly with the female age. A retrospective study was performed in order to evaluate the effect of male age on the sperm parameters in 880 routine seminal analyses. Additionally, sperm parameters were also compared among different age groups in 919 cases with male factor infertility who had been included in an ART programme. The laboratory and clinical results of ART (fertilization rate, number and quality of embryos transferred, as well as pregnancy rates) were compared according to different age groups. The results were also evaluated by one-way correlation and also step-wise logistic regression analysis to identify the interactions and correlations between different parameters. There were no statistically significant differences between male age groups in terms of sperm concentration, motility and morphology either in routine seminal analyses or in ART groups. In the ART group, a statistically significant linear correlation was present between male and female ages. Male age was increasing in parallel to female age. Female age was also correlated significantly with ART results. In one-way correlation analysis, male age was found to be correlated with the pregnancy rate, but not with fertilization rate and the quality of the transferred embryos. However, regression analysis revealed that correlation between male age and pregnancy results was simply dependent on the effect of the female age. Seminal parameters did not reveal a significant change with the increasing male age. The effect of male age on ART results in cases with male factor infertility is not a direct effect but a reflection of the negative impact of the parallel increase in the female age.

Molecular cytogenetic analysis in mouse sperm of chemically induced aneuploidy: studies with topoisomerase II inhibitors

The ability of two topoisomerase II (topo II) inhibitors, etoposide (VP-16) and merbarone (MER), to induce meiotic delay and aneuploidy in mouse spermatocytes was investigated. The progression from meiotic divisions to epididymal sperm was determined by injecting male mice with 5-bromo-2'-deoxyuridine (BrdU) and treating the animals 13 days later with the test chemicals. At 20-24 days after treatment, BrdU-containing sperm were identified with a FITC-labelled anti-BrdU antibody and green fluorescent sperm were scored with a laser scanning cytometer (LSC). It was found that VP-16 (50mg/kg) treatment induced a meiotic delay of about 24h. A significant reduction of BrdU-labelled sperm was observed at 22 days compared to the controls (VP-16 group: 14.20%; controls: 41.10%, P<0.001). At 23 and 24 days, there were no significant differences between the VP-16 and the control groups. MER (80 mg/kg) treatment did not cause meiotic delay. To determine the frequencies of hyperhaploid and diploid sperm, male mice were treated with 12.5, 25 and 50mg/kg VP-16 or 15, 30 and 60 mg/kg MER. Sperm were sampled from the Caudae epididymes 24 days after VP-16 treatment or 22 days after MER treatment. Significant increases above the concurrent controls in the frequencies of total hyperhaploid sperm were found after treatment with 25, 50mg/kg VP-16 (0.074 and 0.122% versus 0.052%) and after treatment with 60 mg/kg MER (0.098% versus 0.044%). Furthermore, significant increases in the frequencies of diploid sperm were found after treatment of mice with all three doses of VP-16 (0.024, 0.032 and 0.056% versus 0.004 and 0.00%, respectively) and with 30 and 60 mg/kg MER (0.022 and 0.05% versus 0.004 and 0.002%, respectively). All dose responses could be expressed by linear equations. The results indicate that cancer patients may stand transient risk for siring chromosomally abnormal offspring after chemotherapy with these topo II inhibitors.

Induction of aneuploidy in male mouse germ cells detected by the sperm-FISH assay: a review of the present data base

Multicolour fluorescence in situ hybridization (FISH) with chromosome-specific DNA-probes can be used to assess aneuploidy (disomy) and diploidy in sperm of any species provided the DNA-probes are available. In the present EU research project, DNA-probes for mouse chromosomes 8, X and Y were employed each labelled with different colours. Male mice were treated with the test chemicals and sperm were sampled from the Caudae epididymes 22-24 days later to allow spermatocytes exposed during meiosis to develop into mature sperm. At present, the data base comprises 10 chemicals: acrylamide (AA), carbendazim (CB), colchicine (COL), diazepam (DZ), griseofulvin (GF), omeprazole (OM), taxol (TX), thiobendazole (TB), trichlorfon (TF) and vinblastine (VBL). Of these, COL and TF induced disomic sperm only. DZ and GF induced disomic and diploid sperm, while CB and TB induced diploid sperm only. VBL gave contradictory results in repeated experiments in an inter-laboratory comparison. AA, OM and TX did not induce an increase in disomic or diploid sperm at the doses used. The induction of aneuploidy by DZ was also tested in humans. Sperm samples from patients after attempted suicide and from patients with chronic Valium((R)) abuse were evaluated using human DNA-probes specific for chromosomes 1,16, 21, X and Y. A quantitative comparison between mouse and man indicates that male meiosis in humans is 10-100 times more sensitive than in mice to aneuploidy induction by DZ. The positive response of mice to TF supports the hypothesis by Czeizel et al. [Lancet 341 (1993) 539] that TF may be causally related to the occurrence of congenital abnormality clusters in a Hungarian village.

Stable variants of sperm aneuploidy among healthy men show associations between germinal and somatic aneuploidy

Repeated semen specimens from healthy men were analyzed by sperm fluorescence in situ hybridization (FISH), to identify men who consistently produced elevated frequencies of aneuploid sperm and to determine whether men who were identified as stable variants of sperm aneuploidy also exhibited higher frequencies of aneuploidy in their peripheral blood lymphocytes. Seven semen specimens were provided by each of 15 men over a 2-year period and were evaluated by the X-Y-8 multicolor sperm FISH method (i.e., approximately 1,050,000 sperm were analyzed from 105 specimens). Three men were identified as stable aneuploidy variants producing significantly higher frequencies of XY, disomy X, disomy Y, disomy 8, and/or diploid sperm over time. In addition, one man and three men were identified as sperm-morphology and sperm-motility variants, respectively. Strong correlations were found between the frequencies of sperm with autosomal and sex-chromosome aneuploidies and between the two types of meiosis II diploidy; but not between sperm aneuploidy and semen quality. A significant association was found between the frequencies of sex-chromosome aneuploidies in sperm and lymphocytes in a subset of 10 men (r2=0.67, P=.004), especially between XY sperm and sex-chromosome aneuploidy in lymphocytes (r2=0.70, P=.003). These findings suggest that certain apparently healthy men can produce significantly higher frequencies of both aneuploid sperm and lymphocytes. Serious long-term somatic and reproductive health consequences may include increased risks of aneuploidy-related somatic diseases and of having children with paternally transmitted aneuploidies, such as Klinefelter, Turner, triple-X, and XYY syndromes.

Base excision repair is limited by different proteins in male germ cell nuclear extracts prepared from young and old mice

The combined observations of elevated DNA repair gene expression, high uracil-DNA glycosylase-initiated base excision repair, and a low spontaneous mutant frequency for a lacI transgene in spermatogenic cells from young mice suggest that base excision repair activity is high in spermatogenic cell types. Notably, the spontaneous mutant frequency of the lacI transgene is greater in spermatogenic cells obtained from old mice, suggesting that germ line DNA repair activity may decline with age. A paternal age effect in spermatogenic cells is recognized for the human population as well. To determine if male germ cell base excision repair activity changes with age, uracil-DNA glycosylase-initiated base excision repair activity was measured in mixed germ cell (i.e., all spermatogenic cell types in adult testis) nuclear extracts prepared from young, middle-aged, and old mice. Base excision repair activity was also assessed in nuclear extracts from premeiotic, meiotic, and postmeiotic spermatogenic cell types obtained from young mice. Mixed germ cell nuclear extracts exhibited an age-related decrease in base excision repair activity that was restored by addition of apurinic/apyrimidinic (AP) endonuclease. Uracil-DNA glycosylase and DNA ligase were determined to be limiting in mixed germ cell nuclear extracts prepared from young animals. Base excision repair activity was only modestly elevated in pachytene spermatocytes and round spermatids relative to other spermatogenic cells. Thus, germ line short-patch base excision repair activity appears to be relatively constant throughout spermatogenesis in young animals, limited by uracil-DNA glycosylase and DNA ligase in young animals, and limited by AP endonuclease in old animals.

CENP-B is not critical for meiotic chromosome segregation in male mice

Centromere protein B (CENP-B) is a constitutive protein that binds to a highly conserved 17bp motif located at most mammalian centromeres. To determine whether disruption of this gene affects chromosome segregation in male germ cells, we evaluated the frequencies of disomic and diploid sperm in CENP-B heterozygous and homozygous null mice using the mouse epididymal sperm aneuploidy (m-ESA) assay, a multicolor FISH method with probes for chromosomes X, Y and 8. The specificity and sensitivity of the m-ESA assay was demonstrated using Robertsonian (2.8) translocation heterozygotes as positive controls for sperm aneuploidy. Our results show that the frequencies of disomic and diploid sperm did not differ significantly between CENP-B heterozygous and homozygous null mice (P> or = 0.5) or from 129/Swiss isogenic mice (P> or = 0.5) and B6C3F1 mice (P> or = 0.2). These findings indicate that CENP-B does not have an essential role during chromosome segregation in male meiosis.

Frequency of XY sperm increases with age in fathers of boys with Klinefelter syndrome

With increasing availability of drugs for impotence and advanced reproductive technologies for the treatment of subfertility, more men are fathering children at advanced ages. We conducted a study of the chromosomal content of sperm of healthy men aged 24-57 years to (a) determine whether father's age was associated with increasing frequencies of aneuploid sperm including XY, disomy X, disomy Y, disomy 21, and sperm diploidy, and (b) examine the association between the frequencies of disomy 21 and sex-chromosomal aneuploidies. The study group consisted of 38 fathers of boys with Klinefelter syndrome (47, XXY) recruited nationwide, and sperm aneuploidy was assessed using multicolor X-Y-21 sperm FISH ( approximately 10,000 sperm per donor). Paternal age was significantly correlated with the sex ratio of sperm (Y/X; P=.006) and with the frequency of XY sperm (P=.02), with a clear trend with age by decades (P<.006). Compared with fathers in their 20s (who had an average frequency of 7.5 XY sperm per 10,000), the frequencies of XY sperm were 10% higher among fathers in their 30s, 31% higher among those in their 40s, and 160% higher among those in their 50s (95% CI 69%-300%). However, there was no evidence for age effects on frequencies of sperm carrying nullisomy sex; disomies X, Y, or 21; or meiosis I or II diploidies. The frequencies of disomy 21 sperm were significantly associated with sex-chromosomal aneuploidy (P=.04)-in particular, with disomy X (P=.004), but disomy 21 sperm did not preferentially carry either sex chromosome. These findings suggest that older fathers produce higher frequencies of XY sperm, which may place them at higher risk of fathering boys with Klinefelter syndrome, and that age effects on sperm aneuploidy are chromosome specific.

Extended exposure of adult and fetal mice to 50 Hz magnetic field does not increase the incidence of micronuclei in erythrocytes

The flow cytometer-based micronucleus assay was used to study the effects on chromosomes in erythroid cells of CBA/Ca mice after extended exposure to 50 Hz magnetic field (MF), 14 microT, peak-to-peak (p-p). The study included two different experiments: (a) mice exposed in utero during 18 days of their prenatal stage, and (b) adult mice exposed for 18 days. In experiment (a) 35 days after exposure was terminated, peripheral blood was drawn from the mice exposed in utero to determine whether the exposure had a genotoxic effect on the pluripotent erythroid stem cells. About 200000 polychromatic erythrocytes (PCE) and 200000 normochromatic erythrocytes (NCE) were analysed from each of 20 exposed mice. The EMF exposure did not significantly change the frequency of micronucleated PCE or NCE in comparison with 20 sham-irradiated mice. There was no difference in the proportion of PCE between exposed and unexposed animals. Similarly, in experiment (b) no differences were seen between EMF exposed and unexposed adult mice when samples of peripheral blood were taken at the end of exposure and analyzed for micronuclei in PCE and NCE. The proportion of PCE was the same in both groups. The results indicate that exposure to EMF does not induce direct or indirect effects on chromosomes in erythroid cells expressed as increased levels of micronucleated erythrocytes of mice. No indications of delayed genetic effects were found.

Effects of male age on semen quality and fertility: a review of the literature

OBJECTIVE

To review the literature on the association between male age and semen quality (semen volume, concentration, motility, and morphology) and fertility status (pregnancy rate and time to pregnancy/subfecundity).

METHOD(S)

Review of English language-published research over the last 20 years from January 1, 1980, through December 31, 1999, using MEDLINE and Biosis databases. Studies with insufficient numbers of subjects, case reports, case series, or anecdotal data were excluded.

RESULT(S)

Among the methodologically stronger studies, decreases in semen volume of 3%-22%, decreases in sperm motility of 3%-37%, and decreases in percent normal sperm of 4%-18% were likely when comparing 30-year-old men to 50-year-old men. Most studies examining fertility status suggest a relationship between male age and fertility, but the results are most likely confounded by female partner age. Among studies that did control for female age, comparisons between men under 30 and men over 50 found relative decreases in pregnancy rates between 23% and 38%. A comparison of the various age categories showed that the increased risks for subfecundity ranged from 11% to 250%.

CONCLUSION(S)

The weight of the evidence suggests that increased male age is associated with a decline in semen volume, sperm motility, and sperm morphology but not with sperm concentration.

Variation of micronucleated erythrocytes in peripheral blood of Sciurus aureogaster in relation to age: an increment of micronucleated polychromatic erythrocytes after the administration of colchicine

In some species, in which the human is included, the influence of age in the variation in the number of micronucleated erythrocytes (MNE) is known. In the present work we show how the process of aging influences the number of spontaneous MNE in the gray squirrel (Sciurus aureogaster). Because of the difficulty of knowing the age of each animal, 69 animals were weighed at their arrival to the laboratory and at the start of sample taking, with the supposition that the heaviest animals were the oldest and those with the lightest weight were the youngest. The major number of MNE was found in the younger animals, whereas the adults displayed less MNE (P < 0.0001). A group of 11 animals were sampled every 15 days over a period of 6 months, and the number of MNE were found to decrease with an increment in the weight in conformity with the time elapsed. These results showed that in the gray squirrel, the number of spontaneous MNE in peripheral blood depend on age. An additional interesting datum about the increment of MNE after the administration of colchicine is shown.

Absence of selection against aneuploid mouse sperm at fertilization

Is there selection against aneuploid sperm during spermatogenesis and fertilization? To address this question, we used male mice doubly heterozygous for the Robertsonian (Rb) translocations Rb(6. 16)24Lub and Rb(16.17)7Bnr, which produce high levels of sperm aneuploid for chromosome 16, the mouse counterpart of human chromosome 21. The frequencies of aneuploid male gametes before and after fertilization were compared by analyzing approximately 500 meiosis II spermatocytes and approximately 500 first-cleavage zygotes using fluorescence in situ hybridization with a DNA painting probe mixture containing three biotin-labeled probes specific for chromosomes 8, 16, and 17 plus a digoxigenin-labeled probe specific for chromosome Y. Hyperhaploidy for chromosome 16 occurred in 20.0% of spermatocytes and in 21.8% of zygotes. Hypohaploidy for chromosome 16 occurred in 17.0% and 16.7% of spermatocytes and zygotes, respectively. In addition, there was no preferential association between chromosome 16 aneuploidy and either of the sex chromosomes, nor was there an elevation in aneuploidy for chromosomes not involved in the Rb translocations. These findings provide direct evidence that there is no selection against aneuploid sperm during spermiogenesis, fertilization, and the first cell cycle of zygotic development.

Griseofulvin-induced aneuploidy and meiotic delay in male mouse germ cells: detected by using conventional cytogenetics and three-color FISH

Griseofulvin (GF) was tested in male mouse germ cells for the induction of meiotic delay and aneuploidy. Starved mice were orally treated with 500, 1000 and 2000 mg/kg of GF in corn oil and testes were sampled 22 h later for meiotic delay analysis and chromosome counting in spermatocytes at the second meiotic metaphase (MMII). A dose-related increase in meiotic delay by dose-dependently arresting spermatocytes in first meiotic metaphase (MMI) or/and prolonging interkinesis was observed. Hyperhaploid MMII cells were not significantly increased. Sperm were sampled from the Caudae epididymes 22 days after GF-treatment of the males for three-color fluorescence in situ hybridization (FISH). The frequencies of diploidies were 0.01-0.02% in sperm of the solvent control animals and increased dose-dependently to 0.03%, 0.068% and 0.091%, respectively, for 500, 1000 and 2000 mg/kg of GF. The frequencies of disomic sperm were increased significantly above the controls in all GF-treated groups but showed no dose response. The data for individual classes of disomic sperm indicated that MII was more sensitive than MI to GF-induced non-disjunction in male mice. A comparison of the present data from male mice and literature data from female mice suggests that mouse oocytes are more sensitive than mouse spermatocytes to GF-induced meiotic delay and aneuploidy.

Smoking cigarettes is associated with increased sperm disomy in teenage men

OBJECTIVE

To determine whether moderate cigarette smoking and alcohol consumption in teenage men is associated with increases in disomic sperm and detectable changes in semen quality.

DESIGN

Cohort study.

SETTING

Military recruiting station, Teplice, Czech Republic.

PATIENT(S)

Ten current smokers (20 cigarettes per day for at least 2 years, exposure confirmed by urine cotinine) who also consumed alcohol and 15 nonsmokers. All patients were exactly 18 years old, healthy, and of unproven fertility.

MAIN OUTCOME MEASURE(S)

Sperm aneuploidy by multicolor fluorescence in situ hybridization for chromosomes 8, X, and Y; conventional semen analyses; computer-aided sperm analysis for motility; and sperm chromatin structure analysis.

RESULTS

Smokers showed elevated frequencies of sperm aneuploidy (Y disomy, P <0.001; aggregate of X, Y, and 8 disomies, P <0.01); reduced linearity of sperm motion (P <0.05); and more "round-headed" sperm (P <0.01). Smokers' semen contained fewer sperm (P <0.001) and fewer motile sperm (P <0.02), which was attributable, in part, to shorter abstinence intervals among smokers (P <0.02).

CONCLUSION(S)

Cigarette smoking among teenagers was associated with increases in disomic sperm and a diminution in specific aspects of semen quality. Such defects may affect male fertility and may increase future chances of fathering offspring with aneuploidy syndromes.

Paternal age affects fertility and progeny outcome in the Brown Norway rat

OBJECTIVE

To investigate the effects of paternal age on fertility and progeny outcome using the Brown Norway rat model.

DESIGN

Controlled prospective study.

SETTING

McIntyre Animal Centre, McGill University, Montreal, Quebec, Canada.

INTERVENTION(S)

Brown Norway male rats of increasing age were mated to young Sprague-Dawley females.

MAIN OUTCOME MEASURE(S)

Pregnancy outcome was assessed by counting the numbers of corpora lutea, resorptions, and live fetuses on day 20 of gestation. To evaluate progeny outcome, pups were examined for external malformations and weighed daily for 2 months.

RESULT(S)

There were no significant changes in the numbers of resorptions, offspring, or in the incidence of external malformations. However, there was an increase in preimplantation loss (corpora lutea minus implantation sites) in litters fathered by older males. Furthermore, a significant decrease in the average fetal weight was found with increasing paternal age. A significant increase in neonatal deaths for progeny fathered by older males also was found.

CONCLUSION(S)

These results indicate that the quality of spermatozoa decreases as males age.

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.

Detection of aneuploidy in human and rodent sperm using FISH and applications of sperm assays of genetic damage in heritable risk evaluation

Efficient molecular methods are being developed for detecting various types of cytogenetic genetic damage in sperm, especially numerical aneuploidy for chromosomes involved in trisomies that survive at birth. These methods provide new approaches for identifying potentially detrimental environmental exposures, genetic predisposition, chromosomal rearrangements, and physiologic factors which may increase a man's risk of fathering a genetically defective offspring. Corollary methods are also being developed for detecting sperm aneuploidy in laboratory rodents and these will be used to make inter-species comparisons of mutagen sensitivities and for investigating mechanisms of induction and persistence of aneuploidy. Validated assays for detecting genetic alterations in human and rodent sperm (of which sperm aneuploidy is a first example) permit comparisons of somatic and germinal response to mutagens within individuals, comparisons of human and rodent germinal sensitivity to mutagens, and can be applied in an extended parallelogram model to sperm for assessing heritable risk resulting from paternal mutagen exposures.

Spermatid micronucleus analysis of aging effects in hamsters

Spermatid micronuclei (MN) from Armenian hamsters in different age groups were compared with regard to frequencies and kinetochore status (presence or absence) as determined with immunofluorescent staining. Six thousand cells analyzed from each of fifteen young animals (3 months) revealed a group mean frequency of 0.45 MN/1000 spermatids; kinetochore staining was uniformly negative. Six thousand cells scored from each of fifteen older animals (2 years) revealed a group mean frequency of 1.00 MN/1000 spermatids. Most of the MN in these animals were negative for kinetochore staining, although a significant representation of MN with positive kinetochore staining was also observed. The results indicate that frequencies of spermatid MN increase with advancing age, and suggest that the increase is due to significant elevations in both chromosome breakage and chromosome loss.

Parental age-related aneuploidy in human germ cells and offspring: a story of past and present

Parental age is the most important aetiological factor in trisomy formation in humans. Cytogenetic studies on germ cells reviewed here imply that (i) 2-4% sperm are aneuploid and 8.6% oocytes from IVF are hyperploid (ii) a paternal age effect may exist, and (iii) oocytes of aged women contain precociously separated chromatids in metaphase II. Trisomy data suggest that most aneuploidy is generated during meiosis I of oogenesis and is maternal age-dependent. Trisomy 18 is unique, originating mostly from maternal meiosis II errors. The extra gonosome in 47, XXY derives mostly from a paternal meiosis I error. Trisomy of individual chromosomes may remain low, linearly rise, or exponentially increase with advanced maternal age. Maternal age related trisomies involve achiasmatic and normochiasmate chromosomes, and chromosomes with disturbed recombination and distally located chiasmata. Hypotheses on the origin of the maternal age effect are critically reviewed. One model is presented that relates to altered cell cycle and protein phosphorylation in oocytes of aged mammals and accounts for most of the observed data in humans and in experimental studies. Aneuploidy may thus involve a predetermined component but is possibly also influenced by extrinsic factors reducing oocyte quality or depleting the oocyte pool precociously. Areas of future research are proposed to elucidate (i) the significance of early disturbances in the prenatal ovary, (ii) parameters diminishing the quality of oocytes in dictyate stage, and (iii) mechanisms enabling oocytes to process all chromosomal configurations successfully during later stages of oogenesis. Studies with newly developed and existing animal models appear indispensable to identify exposures affecting chromosome disjunction during meiosis, especially in the aging female.

Mechanisms and targets involved in maternal and paternal age effects on numerical aneuploidy

Trisomy in the human appears to be predominantly associated with maternal age. The maternal-age effect, however, shows considerable variability across affected chromosomes. Chromosome-specific variation has been reported in the shapes of the maternal-age-effect curves, including very small effects for the large chromosomes (groups A and B), linear increases (chromosome 16), and exponential increases (chromosome 21). There is also variation among chromosomes in whether the segregation errors occur predominantly at maternal meiosis I, meiosis II, and/or postfertilization mitotic divisions. There is also limited epidemiological evidence for a paternal-age effect, which was recently supported by the findings of age-related increases in sperm aneuploidy using fluorescence in situ hybridization methods. The paternal-age effect is considerably smaller than the maternal and is more likely to involve meiotic II errors of the sex chromosomes, whereas the maternal-age effect is more likely to arise from meiotic I errors producing autosomal trisomies. These and other differences suggest that constitutional aneuploidy arises by multiple mechanisms that may affect (1) the nature and timing of an initiating lesion affecting the oocyte or sperm; (2) the cellular physiology of the time of the nondisjunction event at meiosis I, II, or postfertilization; and (3) the selection against specific chromosomal aneuploidies during embryonic development. Multidisciplinary research is needed to understand the maternal and paternal-age effects on aneuploidy, to (1) identify and characterize the genes that control meiosis, recombination, and segregation; (2) identify the micro-environmental factors around the oocyte and mole germ cells that are involved in the age effects; (3) develop a laboratory animal model for the age effects; (4) characterize the role of genetics, physiology, and environmental toxicology for the paternal-age effects; and (5) identify cohorts of men and women of differing ages who have been exposed to high doses of candidate aneugens and conduct epidemiological investigations of aneuploidies transmitted to their offspring.

Aneuploidy in germ cells: etiologies and risk factors

A 2 1/2-day workshop on germ cell aneuploidy was convened September 11-13, 1995 at the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina to discuss current understandings of the etiology and origin of human aneuploidy, especially in regard to potential environmental causes, and to identify gaps in our research knowledge. The workshop was designed to facilitate interactions among research experts conducting studies on the fundamental biology of chromosomal movement and segregation, on aneuploidy as a human clinical problem, and on toxicological aspects of aneuploidy induction. Overview presentations provided perspectives on aneuploidy as a human clinical problem, the genetics of aneuploidy, and the issues of concern in toxicological testing and regulatory risk assessment. The four chairs introduced the topics for each of their workgroups, setting the stage for subsequent, in-depth discussions on (1) chromosome mover components, (2) altered recombination, (3) parental age effects, and (4) differential chromosome susceptibility. From these discussions, gaps in our research knowledge related to the role of the environment in the etiology of aneuploidy and associated molecular, cellular, and genetic processes involved were identified, and will be used to establish a research agenda for filling those gaps.