Aneuploidy and diploidy rates in sperm of five men after three-colour hybridization: indication of X chromosome-associated autosome 2 aneuploidy

Equine sperm nuclei with different ploidy levels: relationship between the nuclear DNA content and the nuclear area

The aims of this study were to estimate the ability of the Feulgen reaction to identify equine sperm nuclei with different ploidy levels, to determine the frequency of haploid, diploid and polyploid sperm nuclei in the semen of fertile equines and to evaluate the relationship between the nuclear DNA content and the nuclear area. Determination of the ploidy level of Feulgen-stained spermatozoa using a scanning microspectrophotometer was very similar to the subjective estimations made with a light microscope. This indicates that the Feulgen reaction is a simple, inexpensive and reliable technique to recognise the ploidy level of equine spermatozoa. The incidence of diploid and polyploid spermatozoa, determined with a light microscope in 11 fertile equines, was 0.17 ± 0.08% and 0.027 ± 0.027% respectively. DNA content values obtained by microspectrophotometry in the only equine that presented polyploid spermatozoa allowed us to discriminate between haploid, diploid and polyploid subpopulations. Measurement of the nuclear area discriminated only two subpopulations: one including the haploid and diploid subpopulations and the other including the polyploid one. The similarity between the area of the haploid and diploid sperm nuclei suggests that the increase in DNA content is anisotropic, with a privileged direction of growth perpendicular to the nuclear flattening plane.

Aneuploidy in human spermatozoa

We reviewed the frequency and distribution of disomy in spermatozoa obtained by multicolor-FISH analysis on decondensed sperm nuclei in (a) healthy men, (b) fathers of aneuploid offspring of paternal origin and (c) individuals with Klinefelter syndrome and XYY males. In series of healthy men, disomy per autosome is approximately 0.1% but may range from 0.03 (chromosome 8) to 0.47 (chromosome 22). The great majority of authors find that chromosome 21 (0.18%) and the sex chromosomes (0.27%) have significantly elevated frequencies of disomy although these findings are not universal. The total disomy in FISH studies is 2.26% and the estimated aneuploidy (2× disomy) is 4.5%, more than double that seen in sperm karyotypes (1.8%). Increased disomy levels of low orders of magnitude have been reported in spermatozoa of some normal men (stable variants) and in men who have fathered children with Down, Turner and Klinefelter syndromes. These findings suggest that men with a moderately elevated aneuploidy rate may be at a higher risk of fathering paternally derived aneuploid pregnancies. Among lifestyle factors, smoking, alcohol and caffeine have been studied extensively but the compounding effects of the 3 are difficult to separate because they are common lifestyle behaviors. Increases in sex chromosome abnormalities, some autosomal disomies, and in the number of diploid spermatozoa are general features in 47,XXY and 47,XYY males. Aneuploidy of the sex chromosomes is more frequent than aneuploidy of any of the autosomes not only in normal control individuals, but also in patients with sex chromosome abnormalities and fathers of paternally derived Klinefelter, Turner and Down syndromes.

Meiotic errors in human oogenesis and spermatogenesis

Chromosome anomalies are extraordinarily common in human gametes, with approximately 21% of oocytes and 9% of spermatozoa abnormal. The types of abnormalities are quite different since most abnormal oocytes are aneuploid, whereas the majority of abnormalities in spermatozoa are structural. Chromosomes 21 and 22 (the smallest chromosomes) are over-represented in aneuploid gametes in both oocytes and sperm. Chromosome 16 is also frequently observed in aneuploid oocytes, whereas the sex chromosomes are particularly predisposed to non-disjunction in human sperm. Maternal age is clearly the most significant factor in the aetiology of aneuploidy; theories about the cause of the maternal age effect are discussed. Paternal age does not have a dramatic effect on the frequency of aneuploid sperm; there is some evidence for a modest increase in the frequency of sex chromosomal aneuploidy. Meiotic recombination has a significant effect on the genesis of aneuploidy in both females and males. New techniques, which allow the analysis of recombination along the synaptonemal complex, have yielded interesting new information in healthy and infertile individuals. There is a link between infertility and the genesis of chromosome abnormalities. Future studies will unravel more of the underlying causal factors.

Cytogenetic determinants of male fertility

BACKGROUND

Cytogenetic abnormalities have been known to be important causes of male infertility for decades.

METHODS

Research publications from 1978 to 2008, from PubMed, have been reviewed.

RESULTS

These studies have greatly improved our information on somatic chromosomal abnormalities such as translocations, inversions and sex chromosomal anomalies, and their consequences to the cytogenetic make-up of human sperm. Also, we have learned that infertile men with a normal somatic karyotype have an increased risk of chromosomally abnormal sperm and children. New techniques such as single sperm typing and synaptonemal complex analysis have provided valuable insight into the association between meiotic recombination and the production of aneuploid sperm. These meiotic studies have also unveiled errors of chromosome pairing and synapsis, which are more common in infertile men.

CONCLUSIONS

These studies allow us to provide more precise information to infertile patients, and further our basic knowledge in the causes of male infertility.

Meiotic chromosome abnormalities in human spermatogenesis

The last few years have witnessed an explosion in the information about chromosome abnormalities in human sperm and the meiotic events that predispose to these abnormalities. We have determined that all chromosomes are susceptible to nondisjunction, but chromosomes 21 and 22 and, especially, the sex chromosomes have an increased frequency of aneuploidy. Studies are just beginning on the effects of potential mutagens on the chromosomal constitution of human sperm. The effects of pesticides and cancer therapeutic agents have been reviewed. In the last decade, there has been a great impetus to study chromosome abnormalities in sperm from infertile men because the advent of intracytoplasmic sperm injection (ICSI) made it possible for these men to father pregnancies. A large number of studies have demonstrated that infertile men have an increased frequency of chromosomally abnormal sperm and children, even when they have a normal somatic karyotype. Meiotic studies on the pachytene stage of spermatogenesis have demonstrated that infertile men have impaired chromosome synapsis, a significantly decreased frequency of recombination, and an increased frequency of chromosomes completely lacking a recombination site. Such errors make these cells susceptible to meiotic arrest and the production of aneuploid gametes.

Frequency and distribution of chromosome abnormalities in human spermatozoa

This study reviews the frequency and distribution of numerical and structural chromosomal abnormalities in spermatozoa from normal men obtained by the human-hamster system and by multicolor-FISH analysis on decondensed sperm nuclei. Results from large sperm karyotyping series analyzed by chromosome banding techniques and results from multicolor FISH in sperm nuclei (of at least 10(4) spermatozoa per donor and per probe) were reviewed in order to establish baseline values of the sperm chromosome abnormalities in normal men. In karyotyping studies, the mean disomy frequency in human sperm is 0.03% for each of the autosomes, and 0.11% for the sex chromosomes, lower than those reported in sperm nuclei by FISH studies using a similar methodology (0.09% and 0.26%, respectively). Both types of studies coincide in that chromosome 21 and sex chromosomes have a greater tendency to suffer segregation errors than the rest of the autosomes. The mean incidence of diploidy, only available from multicolor FISH in sperm nuclei, is 0.19%. Inter-donor differences observed for disomy and diploidy frequencies among FISH studies of decondensed sperm nuclei using a similar methodology could reflect real differences among normal men, but they could also reflect the subjective application of the scoring criteria among laboratories. The mean frequency of structural aberrations in sperm karyotypes is 6.6%, including all chromosome types of abnormalities. Chromosome 9 shows a high susceptibility to be broken and 50% of the breakpoints are located in 9q, between the centromere and the 9qh+ region. Structural chromosome aberrations for chromosomes 1 and 9 have also been analyzed in human sperm nuclei by multicolor FISH. Unfortunately, this assay does not allow to determine the specific type of structural aberrations observed in sperm nuclei. An association between advancing donor age and increased frequency of numerical and structural chromosome abnormalities has been reported in spermatozoa of normal men.

Mechanisms of nondisjunction in human spermatogenesis

A reduction in recombination in the pseudoautosomal region is associated with an increased frequency of aneuploid 24,XY human sperm. Similarly, individuals with paternally derived Klinefelter syndrome (47,XXY) also have a paucity of recombination in the chromosomes that have undergone nondisjunction. Meiotic studies using newly developed immunocytogenetic techniques have demonstrated errors of chromosome synapsis and significantly reduced recombination in infertile men with nonobstructive azoospermia. These men have an increased risk of aneuploidy in sperm that have been surgically removed from the testes. Thus, evidence is starting to accumulate that reduced recombination has a marked effect on the generation of aneuploid sperm.

Individual variation in the frequency of sperm aneuploidy in humans

To examine interindividual differences in sperm chromosome aneuploidy, repeated semen specimens were obtained from a group of ten healthy men, aged 20-21 at the start of the study, and analyzed by multi-color fluorescence in situ hybridization (FISH) analysis to determine the frequencies of sperm aneuploidy for chromosomes X, Y, 8, 18 and 21 and of diploidy. Semen samples were obtained three times over a five-year period. Statistical analysis examining the stability of sperm aneuploidy over time by type and chromosome identified two men who consistently exhibited elevated frequencies of sperm aneuploidy (stable variants): one with elevated disomy 18 and one with elevated MII diploidy. Differences among frequencies of aneuploidy by chromosome were also seen. Overall, disomy frequencies were lower for chromosome X, 8 and 18 than for chromosomes 21 or Y and for XY aneuploidy. The frequency of chromosome Y disomy did not differ from XY sperm frequency. Also, the frequency of meiosis I (XY) and II (YY + XX) sex chromosome errors did not differ in haploid sperm, but the frequency of MII errors was lower than MI errors in diploid sperm. Frequencies of sperm aneuploidy were similar between the first sampling period and the second, two years later. However, the frequency of some types of aneuploidy (XY, disomy Y, disomy 8, total autosomal disomies, total diploidy, and subcategories of diploidy) increased significantly between the first sampling period and the last, five years later, while others remained unchanged (disomy X, 21 and 18). These findings confirm inter-chromosome differences in the frequencies of disomy and suggest that some apparently healthy men exhibit consistently elevated frequencies of specific sperm aneuplodies. Furthermore, time/age-related changes in sperm aneuploidy may be detected over as short a period as five years in a repeated-measures study.

A comparison of the frequency of sperm chromosome abnormalities in men with mild, moderate, and severe oligozoospermia

Infertile men undergoing intracytoplasmic sperm injection have an increased frequency of chromosome abnormalities in their sperm. Men with low sperm concentration (oligozoospermia) have an increased risk of sperm chromosome abnormalities. This study was initiated to determine whether men with severe oligozoospermia (<10(6) sperm/ml) have a higher frequency of chromosome abnormalities in their sperm compared with men with moderate (1-9 x 10(6) sperm/ml) or mild (10-19 x 10(6) sperm/ml) oligozoospermia. Multicolor fluorescence in situ hybridization analysis was performed using DNA probes specific for chromosomes 13, 21, X, and Y (with chromosome 1 as an autosomal control for the sex chromosomes). Aneuploidy and disomy frequencies were assessed from a total of 603,011 sperm from 30 men: 10 in each of the categories. The mean frequencies of disomy for the patients with mild, moderate, and severe oligozoospermia were 0.17%, 0.24%, and 0.30%, respectively, for chromosome 13 and 0.22%, 0.44%, and 0.58%, respectively, for chromosome 21. For the sex chromosomes, the mean frequencies of disomy for mild, moderate, and severe oligozoospermia were 0.25%, 1.04%, and 0.68%, respectively, for XY, 0.047%, 0.08%, and 0.10%, respectively, for XX, and 0.04%, 0.06%, and 0.09%, respectively, for YY. The frequencies for diploidy also increased from 0.4% for mild to 1.20% for moderate to 1.24% for severe oligozoospermia. There was a significant inverse correlation between the frequency of sperm chromosome abnormalities and the sperm concentration for XY, XX, and YY disomy and diploidy. These results demonstrate that men with severe oligozoospermia have an elevated risk for chromosome abnormalities in their sperm, particularly sex chromosome abnormalities.

Sperm-FISH analysis and human monitoring: a study on workers occupationally exposed to styrene

Occupational exposure to styrene, a chemical extensively used worldwide, is under investigation for possible detrimental effects on human health, including male reproductive capacity. Aneuploidy in germ cells is the main cause of infertility, abortions and congenital diseases. Fluorescence in situ hybridisation (FISH), is the most efficient cytogenetic molecular technique to date to analyse numerical alterations of chromosomes in spermatozoa. We investigated the frequencies of aneuploidy and diploidy in individuals occupationally exposed to styrene and in healthy unexposed controls. We performed multicolour FISH, using DNA probes specific for the centromeric regions of sex chromosomes and chromosome 2, in decondensed sperm nuclei of samples with normal semen parameters for a total of 18 styrene-exposed subjects and 13 unexposed controls of the same age range. Exposed individuals had worked for at least 2 years during the last 5 years, and continuously for 6 months, in factories producing reinforced plastics. The incidence of aneuploidy and diploidy for the tested chromosomes did not show a statistically significant difference between workers and controls. The exposure to styrene was associated with increased frequencies of nullisomy for sex chromosomes in the group of non-smokers, although only a limited number of subjects belonged to this sub-group. Considering the whole study population, age was associated with an increased frequency of XX disomy, whereas smoking was associated with meiosis II non-disjunction of sex chromosomes. Overall, confounding factors appeared to exert a more important effect than exposure to styrene on numerical chromosome alterations in sperm nuclei of subjects selected for normal semen parameters.

Sperm aneuploidy in infertile men

A recent line of research has shown that infertile male patients produce cytogenetically abnormal spermatozoa, despite a normal somatic karyotype, as a result of an altered intra-testicular environment that affects negatively the mechanisms controlling chromosome segregation during cell division. The rate of aneuploid spermatozoa production is significantly higher in patients with abnormal sperm parameters compared with those of normozoospermic subjects or infertile patients with normal sperm parameters. All chromosomes are subject to aneuploidy, although at a different rate; the heterochromosomes are more often altered than are the autosomes. A negative correlation has been reported to exist between aneuploidy and the main sperm parameters, suggesting that greater testicular damage is associated with a greater chance of chromosome malsegregation events. Abnormally-shaped spermatozoa are more likely to have chromosome abnormalities, particularly those with an enlarged head. More studies are necessary, however, to evaluate whether other types of sperm head abnormalities are also associated with an abnormal sperm chromosome complement. The possibility of retrieving testicular or epididymal spermatozoa in patients with azoospermia and using them in assisted reproduction techniques has prompted the evaluation of their chromosomal status. Studies have shown that testicular and epididymal spermatozoa have a greater rate of aneuploidy compared with that of ejaculated spermatozoa. Some authors have also shown that patients with non-obstructive azoospermia have a significantly higher sperm aneuploidy rate compared with that of patients with obstructive azoospermia. Sperm aneuploidy seems to have a negative impact on assisted reproduction technique outcome. Although it does not affect the fertilization rate, an elevated sperm aneuploidy rate is associated with a greater rate of pregnancy failure. Nevertheless, some patients with elevated sperm aneuploidy rate can still achieve a pregnancy, but with an increased risk of generating an aneuploid offspring. Thus, sperm aneuploidy evaluation is recommended in infertile patients with abnormal semen parameters, particularly if they undergo IVF programmes.