To err (meiotically) is human: the genesis of human aneuploidy

A link between meiotic prophase progression and crossover control

During meiosis, most organisms ensure that homologous chromosomes undergo at least one exchange of DNA, or crossover, to link chromosomes together and accomplish proper segregation. How each chromosome receives a minimum of one crossover is unknown. During early meiosis in Caenorhabditis elegans and many other species, chromosomes adopt a polarized organization within the nucleus, which normally disappears upon completion of homolog synapsis. Mutations that impair synapsis even between a single pair of chromosomes in C. elegans delay this nuclear reorganization. We quantified this delay by developing a classification scheme for discrete stages of meiosis. Immunofluorescence localization of RAD-51 protein revealed that delayed meiotic cells also contained persistent recombination intermediates. Through genetic analysis, we found that this cytological delay in meiotic progression requires double-strand breaks and the function of the crossover-promoting heteroduplex HIM-14 (Msh4) and MSH-5. Failure of X chromosome synapsis also resulted in impaired crossover control on autosomes, which may result from greater numbers and persistence of recombination intermediates in the delayed nuclei. We conclude that maturation of recombination events on chromosomes promotes meiotic progression, and is coupled to the regulation of crossover number and placement. Our results have broad implications for the interpretation of meiotic mutants, as we have shown that asynapsis of a single chromosome pair can exert global effects on meiotic progression and recombination frequency.

Meiosis is a specialized cell division and an essential component of sexual reproduction. During meiotic prophase, each chromosome must pair with its unique homologous partner and undergo crossing over (genetic exchange) to segregate properly. A major mystery is how the molecular events of meiotic recombination are coupled to the large-scale dynamics of chromosome synapsis. This work reveals a link between the large-scale regulation of chromosome organization and the distribution of crossover events on the chromosomes. In C. elegans, defects in chromosome pairing or synapsis result in an extension of a normally transient stage of meiotic prophase. This study finds that this extension is associated with dysregulation of crossovers, so that more than the usual number of crossovers occur, and their distribution is shifted along the chromosomes. These observations contribute to our understanding of crossover control, which normally ensures accurate transmission of genetic information from parent to progeny.

Cytogenetic analysis of human oocytes remaining unfertilized after intracytoplasmic sperm injection

Oocytes remaining unfertilized after intracytoplasmic sperm injection showed 12.0% aneuploidy (nondisjunction + unbalanced predivision), 3.4% structural aberrations, and 8.5% balanced predivision in fully karyotyped cells. However, the frequently observed complete or partial separation of chromatids, most probably caused by abortive activation, might complicate the evaluation of meiosis I-derived aneuploidy and questions the relevance of balanced predivision.

Human fetal ovarian culture permits meiotic progression and chromosome pairing process

BACKGROUND

The female meiotic process seems to be crucial for aneuploidy in humans. The first stages of mammalian female meiosis take place during the fetal period. Therefore, only little is known about female meiosis. The goal of this study was to develop a culture technique that permits human oocytes to progress through meiotic prophase, to provide a system to study human female meiosis.

METHOD

Fetal ovaries from four cases were cultured up to 35 days in alpha-minimal essential medium, 2% human serum albumin, 5 microg/ml insulin, 5 microg/ml transferrin, 5 ng/ml selenium and 100 IU/ml penicillin-100 microg/ml streptomycin.

RESULTS AND CONCLUSIONS

Although ovarian response to culture conditions varied, human oocytes survived in vitro up to 5 weeks. In three cases, we observed significant variation in stages of meiosis among the cultures. The homologous chromosome pairing process was studied for the first time in cultured oocytes, and the results suggested that the pairing process was completed following the same features described previously for euploid oocytes, as followed by the chromosome-13 pairing process and synaptonemal complex formation. Although a higher proportion of degenerated oocytes were observed as culture time increased, we also observed oogonial entrance to meiotic prophase.

Crossover frequency and synaptonemal complex length: their variability and effects on human male meiosis

In this study, immunocytogenetics has been used in combination with the subtelomere-specific multiplex-fluorescent in-situ hybridization (stM-FISH) assay to identify 4681 autosomal synaptonemal complexes (SCs) of two fertile men. Comparisons of crossover maps for each individual SC between two men with extremely different meiotic crossover frequencies show that a low crossover frequency results in (i) a higher frequency of XY pairs and of small SCs without MLH1 foci and (ii) lower frequency of crossovers in the proximity of centromeres. In both cases, the bivalents which most frequently lacked MLH1 foci were the XY pair and the SC21. Analysis of SC length showed that SC arms can be longer or shorter than the corresponding mitotic one. Moreover, for a given SC, the variation in length found in one arm was independent of the variation observed in the other one (e.g. SC1p arms are longer than SC1q arms). The results confirmed that reduction in the crossover frequency may increase the risk of achiasmate small bivalents and that interindividual differences in crossover frequency could explain the variability in the frequencies of aneuploidy in human sperm. How MLH1 foci are positioned within the SC is discussed based on detailed MLH1 foci distributions and interfoci distances. Finally, evidence that the variation of the SC arm length may reflect the abundance of open and of compact chromatin fibers in the arm is shown.

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.

Immunofluorescent synaptonemal complex analysis in azoospermic men

The molecular cause of germ cell meiotic defects in azoospermic men is rarely known. During meiotic prophase I, a proteinaceous structure called the synaptonemal complex (SC) appears along the pairing axis of homologous chromosomes and meiotic recombination takes place. Newly-developed immunofluorescence techniques for SC proteins (SCP1 and SCP3) and for a DNA mismatch repair protein (MLH1) present in late recombination nodules allow simultaneous analysis of synapsis, and of meiotic recombination, during the first meiotic prophase in spermatocytes. This immunofluorescent SC analysis enables accurate meiotic prophase substaging and the identification of asynaptic pachytene spermatocytes. Spermatogenic defects were examined in azoospermic men using immunofluorescent SC and MLH1 analysis. Five males with obstructive azoospermia, 18 males with nonobstructive azoospermia and 11 control males with normal spermatogenesis were recruited for the study. In males with obstructive azoospermia, the fidelity of chromosome pairing (determined by the percentage of cells with gaps [discontinuities]/splits [unpaired chromosome regions] in the SCs, and nonexchange SCs [bivalents with 0 MLH1 foci]) was similar to those in normal males. The recombination frequencies (determined by the mean number of MLH1 foci per cell at the pachytene stage) were significantly reduced in obstructive azoospermia compared to that in controls. In men with nonobstructive azoospermia, a marked heterogeneity in spermatogenesis was found: 45% had a complete absence of meiotic cells; 5% had germ cells arrested at the zygotene stage of meiotic prophase; the rest had impaired fidelity of chromosome synapsis and significantly reduced recombination in pachytene. In addition, significantly more cells were in the leptotene and zygotene meiotic prophase stages in nonobstructive azoospermic patients, compared to controls. Defects in chromosome pairing and decreased recombination during meiotic prophase may have led to spermatogenesis arrest and contributed in part to this unexplained infertility.

Mechanisms of aneuploidy induction in human oogenesis and early embryogenesis

The mechanisms of aneuploidy induction in human oogenesis mainly involve nondisjunction arising during the first and second meiotic divisions. Nondisjunction equally affects both whole chromosomes and chromatids, in the latter case it is facilitated by "predivision" or precocious centromere division. Karyotyping and CGH studies show an excess of hypohaploidy, which is confirmed in studies of preimplantation embryos, providing evidence in favour of anaphase lag as a mechanism. Preferential involvement of the smaller autosomes has been clearly shown but the largest chromosomes are also abnormal in many cases. Overall, the rate of chromosomal imbalance in oocytes from women aged between 30 and 35 has been estimated at 11% from recent karyotyping data but accruing CGH results suggest that the true figure should be considerably higher. Clear evidence has been obtained in favour of germinal or gonadal mosaicism as a predisposing factor. Constitutional aneuploidy in embryos is most frequent for chromosomes 22, 16, 21 and 15; least frequently involved are chromosomes 14, X and Y, and 6. However, embryos of women under 37 are far more likely to be affected by mosaic aneuploidy, which is present in over 50% of 3-day-old embryos. There are two main types, diploid/aneuploid and chaotic mosaics. Chaotic mosaics arise independently of maternal age and may be related to centrosome anomalies and hence of male origin. Aneuploid mosaics most commonly arise by chromosome loss, followed by chromosome gain and least frequently by mitotic nondisjunction. All may be related to maternal age as well as to lack of specific gene products in the embryo. Partial aneuploidy as a result of chromosome breakage affects a minimum of 10% of embryos.

Effect of lifestyle exposures on sperm aneuploidy

Lifestyle exposures including cigarette smoke, alcohol, and caffeine have all been studied in relationship to male reproductive health. Over the years the focus has primarily been on semen quality and/or fertility. More recently, literature evaluating direct adverse effects of lifestyle exposures on sperm chromosomes and chromatin has grown due to concern that induced damage could be transmitted to offspring causing transgenerational health effects. In this paper we present a new analysis that summarizes published studies of smoking effects on sperm chromosome number and demonstrates a statistically significant increase in sperm disomy among smokers compared to nonsmokers (P < 0.001). In addition, new data on the effect of alcohol intake on sperm chromosome number are presented showing a rate ratio of 1.38 (95% CI 1.2, 1.6) for XY frequency in sperm of alcohol drinkers compared to nondrinkers.

Effect of paternal age on the frequency of cytogenetic abnormalities in human spermatozoa

Many surveys have been performed to find etiological relationships between pregnancy outcome and specific risk factors, such as exposure to chemicals and radiation or parental age. Advanced maternal age is a strong risk factor for trisomic pregnancies, albeit there are considerable variations among the different chromosomes. The definite incidence of the various structural and numerical chromosome aberrations in spontaneous abortions and liveborns is well known, as well as the rate of maternally and paternally derived rearrangements. Nevertheless studies have failed to assert an age-dependent risk for men fathering chromosomally abnormal children. New techniques using fluorescence in situ hybridization render it possible to analyze spermatozoa directly for numerical and, to some extent, for structural aberrations. This article compiles the findings of studies on human spermatozoa over the last few years.

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.

Influence of advanced age of maternal grandmothers on Down syndrome

BACKGROUND

Down syndrome (DS) is the most common chromosomal anomaly associated with mental retardation. This is due to the occurrence of free trisomy 21 (92-95%), mosaic trisomy 21 (2-4%) and translocation (3-4%). Advanced maternal age is a well documented risk factor for maternal meiotic nondisjunction. In India three children with DS are born every hour and more DS children are given birth to by young age mothers than by advanced age mothers. Therefore, detailed analysis of the families with DS is needed to find out other possible causative factors for nondisjunction.

METHODS

We investigated 69 families of cytogenetically confirmed DS children and constructed pedigrees of these families. We also studied 200 randomly selected families belonging to different religions as controls. Statistical analysis was carried out using logistic regression.

RESULTS

Out of the 69 DS cases studied, 67 were free trisomy 21, two cases were mosaic trisomy 21 and there were none with translocation. The number of DS births was greater for the young age mothers compared with the advanced age mothers. It has also been recorded that young age mothers (18 to 29 years) born to their mothers at the age 30 years and above produced as high as 91.3% of children with DS. The logistic regression of case- control study of DS children revealed that the odds ratio of age of grandmother was significant when all the four variables were used once at a time. However, the effect of age of mother and father was smaller than the effect of age of maternal grandmother. Therefore, for every year of advancement of age of the maternal grandmother, the risk (odds) of birth of DS baby increases by 30%.

CONCLUSION

Besides the known risk factors, mother's age, father's age, the age of the maternal grandmother at the time of birth of the mother is a risk factor for the occurrence of Down syndrome.

Evidence of a high proportion of premature unbalanced separation of sister chromatids in the first polar bodies of women of advanced age

BACKGROUND

Maternal ageing is the only aetiological factor unequivocally linked to aneuploidy. Two mechanisms seem to explain these abnormalities in oocytes: non-disjunction and premature unbalanced separation of sister chromatids (PSSC). Previous studies of unfertilized oocytes argue for a major role of PSSC in the aetiology of aneuploidy for women of advanced age, but in vitro ageing of the oocytes could influence the results.

METHODS

Owing to the high prevalence of aneuploidy in women of advanced age, chromosomal screening of the first polar body just before ICSI was offered to women (from 38 years of age) included in an assisted reproduction programme.

RESULTS

Among 141 oocytes from 29 women (mean age 40 years and 2 months), 43 (30.5%) were abnormal. Sixty-five abnormalities were found and PSSC was involved in 80% of cases.

CONCLUSION

These results are in accordance with previous studies and confirm, in 'fresh' oocytes, the major role of PSSC in the aetiology of aneuploidy in women of advanced age.

The Miniature Pig as an Animal Model in Biomedical Research

Crucial prerequisites for the development of safe preclinical protocols in biomedical research are suitable animal models that would allow for human-related validation of valuable research information gathered from experimentation with lower mammals. In this sense, the miniature pig, sharing many physiological similarities with humans, offers several breeding and handling advantages (when compared to non-human primates), making it an optimal species for preclinical experimentation. The present review offers several examples taken from current research in the hope of convincing the reader that the porcine animal model has gained massively in importance in biomedical research during the last few years. The adduced examples are taken from the following fields of investigation: (a) the physiology of reproduction, where pig oocytes are being used to study chromosomal abnormalities (aneuploidy) in the adult human oocyte; (b) the generation of suitable organs for xenotransplantation using transgene expression in pig tissues; (c) the skin physiology and the treatment of skin defects using cell therapy-based approaches that take advantage of similarities between pig and human epidermis; and (d) neurotransplantation using porcine neural stem cells grafted into inbred miniature pigs as an alternative model to non-human primates xenografted with human cells.

The conserved kinase NHK-1 is essential for mitotic progression and unifying acentrosomal meiotic spindles in Drosophila melanogaster

Conventional centrosomes are absent from the spindle in female meiosis in many species, but it is not clear how multiple chromosomes form one shared bipolar spindle without centrosomes. We identified a female sterile mutant in which each bivalent chromosome often forms a separate bipolar metaphase I spindle. Unlike wild type, prophase I chromosomes fail to form a single compact structure within the oocyte nucleus, although the integrity of metaphase I chromosomes appears to be normal. Molecular analysis indicates that the mutant is defective in the conserved kinase nucleosomal histone kinase-1 (NHK-1). Isolation of further alleles and RNA interference in S2 cells demonstrated that NHK-1 is also required for mitotic progression. NHK-1 itself is phosphorylated in mitosis and female meiosis, suggesting that this kinase is part of the regulatory system coordinating progression of mitosis and meiosis.

Pre- and postnatal findings in trisomy 17 mosaicism

Trisomy 17 mosaicism is one of the rarest autosomal trisomies in humans. Thus far, only 23 cases have been described, most of them detected prenatally. In only five instances has mosaicism been demonstrated in lymphocytes and/or fibroblasts postnatally, and only in these have multiple congenital anomalies (MCA), facial dysmorphisms, and mental retardation been reported. Patients with trisomy 17 mosaicism at amniocentesis and a normal karyotype in blood and fibroblasts (n = 17) were always healthy. Here, we report on pre- and postnatal clinical, cytogenetic, molecular-cytogenetic, and molecular findings in four patients with trisomy 17 mosaicism. The first case was detected in cultured but not in short-term chorionic villi and amniocytes. Due to MCA on prenatal ultrasound examination the pregnancy was terminated. The second patient is a 13-month-old healthy boy, in whom low level trisomy 17 mosaicism was detected in cultured chorionic villi only. The third patient is a 2-year-old girl with growth retardation, developmental delay, MCA, and trisomy 17 mosaicism in amniocytes, fibroblasts, and placenta, but not in blood and buccal smear. The fourth patient is a 9-year-old boy with growth and mental retardation, sensoneurinal hearing loss, and MCA. Cytogenetic analyses showed trisomy 17 mosaicism in amniocytes, skin fibroblasts, and urinary sediment cells, whereas in blood and buccal smear a 46,XY karyotype was found. Molecular investigations in all four cases indicated biparental inheritance of chromosome 17. Formation of trisomy was most likely due to a maternal meiosis I error in Patient 1 and a postzygotic non-disjunction of the paternal chromosome 17 in Patient 4. Cerebellar malformations, reported in two cases from the literature and in two reported here may be a specific feature of trisomy 17 mosaicism. Since the aberration has rarely been reported in lymphocytes, chordocentesis is not indicated in prenatal diagnosis. Prenatal genetic counseling for trisomy 17 mosaicism in chorionic villi or amniocytes should consider that the clinical significance remains uncertain.

Interactions of the meiotic spindle with mitotic chromosomes in GV mouse oocytes

During mitosis, a spindle checkpoint detects chromosome misalignment and halts the cell cycle progression. In meiosis of female germ cells, however, it is debatable whether such a checkpoint is present. This research employed a unique model in the mouse, mitotic chromosomes transferred to meiotic cytoplasts to investigate whether a meiotic oocyte's microtubule apparatus can effectively separate mitotic metaphase chromosomes, and whether a spindle checkpoint exists during its division. The intact germinal vesicle (GV) oocytes, enucleated GV cytoplasts, and enucleated GV cytoplasts at 15 h in-vitro maturation were transferred with a metaphase fibroblast cell. When mitotic chromosomes were transferred into enucleated or intact mouse GV oocytes, the first bipolar meiotic spindles were established and the reconstructed oocytes were able to extrude polar bodies. However, none of the reconstructed oocytes showed complete and accurate alignment of chromosomes, except the enucleated GV cytoplasts reconstructed after maturation. The spindle formation and polar body extrusion suggest that the first meiotic spindle was functional, and the chromosome misalignment did not prevent the onset of anaphase. The data indicate that a spindle checkpoint, providing surveillance of misaligned chromosomes, was overridden or compromised by the incompatibility between somatic chromosomes and meiotic spindles during the first meiotic division.

Chromosomal variation in mammalian neuronal cells: known facts and attractive hypotheses

Chromosomal mosaicism is still a genetic enigma. Although the mechanisms and consequences of this phenomenon have been studied for over 50 years, there are a number of gaps in our knowledge concerning causes, genetic mechanisms, and phenotypic manifestations of chromosomal mosaicism. Neuronal cell-specific chromosomal mosaicism is not an exception. Originally, neuronal cells of the mammalian brain were assumed to possess identical genomes. However, recent studies have shown chromosomal variations, manifested as chromosome abnormalities in cells of the developing and adult mammalian nervous system. Here, we review data obtained on the variation in chromosome complement in mammalian neuronal cells and hypothesize about the possible relevance of large-scale genomic (i.e., chromosomal) variations to brain development and functions as well as neurodevelopmental and neurodegenerative disorders. We propose to cover the term "molecular neurocytogenetics to cover all studies the aim of which is to reveal chromosome variations and organization in the mammalian brain.

Cell cycle regulation in mammalian germ cells

Meiosis is a unique form of cellular division by which a diploid cell produces genetically distinct haploid gametes. Initiation and regulation of mammalian meiosis differs between the sexes. In females, meiosis is initiated during embryo development and arrested shortly after birth during prophase I. In males, spermatogonial stem cells initiate meiosis at puberty and proceed through gametogenesis with no cell cycle arrest. Mouse genes required for early meiotic cell cycle events are being identified by comparative analysis with other eukaryotic systems, by virtue of gene knockout technology and by mouse mutagenesis screens for reproductive defects. This review focuses on mouse reproductive biology and describes the available mouse mutants with defects in the early meiotic cell cycle and prophase I regulatory events. These research tools will permit rapid advances in such medically relevant research areas as infertility, embryo lethality and developmental abnormalities.

The fate of mosaic aneuploid embryos during mouse development

More than any other species, humans have difficulty reproducing. As recent studies show that human infertility is ever increasing, much efforts are needed towards the understanding of our low fecundity. While aneuploidy is the leading cause of spontaneous pregnancy loss in humans, we still know surprisingly little about the developmental consequences of chromosomal abnormalities. We have here used a mouse model that spontaneously incites chromosomal primary aneuploidy in female haploid oocytes and find that after fertilization, these primary aneuploid cells become cytological unstable, generating diverse karyotypic mosaic embryos. The mosaic aneuploid embryos can develop and implant into the female uterine tissue and initiate the gastrulation process (E6.5) but quickly degrade and succumb by E8.0. We find that loss of embryo viability due to chromosomal mosaicism is caused by the activation of a spatially and temporally controlled p53-independent apoptotic mechanism and does not result from a failure to progress through mitosis. We conclude that an initial state of primary aneuploidy within an embryo results in a rapid evolution of mosaicism and early embryonic death. This gestational loss due to aneuploid mosaicism could account for the large proportion of human pregnancy loss prior to clinical recognition.

Reduced recombination associated with the production of aneuploid sperm in an infertile man: a case report

Studies using gene-linkage analysis have suggested that abnormal recombination during meiosis may lead to the production of aneuploid gametes; however, there is little direct evidence of a link between the two in human males. We analysed spermatocytes in the pachytene stage from a man with extremely high aneuploidy rates in his sperm. Testicular tissue specimens of the infertile man and two vasectomy reversals were processed with immuofluorescent techniques to visualize synaptonemal complex and recombination foci and fluorescent in situ hybridization on spermatocytes and sperm with probes for chromosomes 13, 21, 18, X and Y. We observed no recombination between sex chromosomes in the infertile man, while in two controls, we observed recombination rates of 79.3 and 81.0% between the sex chromosomes. This was associated with a total sex aneuploidy rate of 41.61% in testicular sperm of the infertile man (0.44 and 0.62% in two controls). Recombination on chromosome 21 was reduced in the infertile man, with 10.62% of spermatocytes showing no recombination (0 and 1.67% in two controls), as well as chromosome 13, with 53.98% having < or =1 recombination foci (22.05 and 21.67% in two controls). This was associated with increased aneuploidy for those chromosomes. Chromosome 18 aneuploidy was slightly increased, although there was no apparent decrease in recombination. These results provide the first evidence of both recombination and non-disjunction abnormalities in the same individual. This is also the only reported case of an infertile man who shows no recombination between the sex chromosomes, despite the formation of the sex body.

Cohesin and the maternal age effect

During meiosis in human oocytes, chromosome nondisjunction increases with maternal age, leading to disorders such as Down's syndrome. In a recent study in Nature Genetics, Hodges et al. (2005) show that mice with a mutation in the meiosis-specific cohesin protein SMC1beta exhibit age-dependent defects in meiosis. These defects are similar to those observed in oocytes of older human mothers. Their results implicate an age-dependent loss of function in SMC1beta (or related proteins) in the maternal age effect of humans.

Recombination in men with Klinefelter syndrome

Klinefelter syndrome (KS: 47,XXY), occurs in one in 1000 male births. Men with KS are infertile and have higher rates of aneuploidies in sperm compared with normal fertile men. In the course of analyzing recombination in a population of infertile men, we observed that four men in our study presented with KS. We examined whether these men differed in recombination parameters among themselves and relative to normal men. Even though the number of men with KS analyzed was small, we observed remarkable variation in spermatogenesis. In spite of the fact that the men had the same genetic cause for infertility, two of four KS patients had few or no spermatogenic cells that progressed through meiosis to the pachytene stage, whereas the other two men produced abundant pachytene cells that had recombination frequencies comparable with those of fertile men, although one had a significant reduction in fidelity of synapsis. Moreover, regardless of histological appearance, examination of outcomes of assisted reproduction indicated that sperm were extracted from testis biopsies in all four cases, and when used in assisted reproductive practices chromosomally normal babies were born. These results reinforce that: (i) men with the same underlying genetic cause for infertility do not present with uniform pathology, (ii) the checkpoint machinery that might arrest spermatogenesis in the face of chromosomal abnormalities does not prevent pockets of complete spermatogenesis in men with KS, and (iii) aneuploidy, in some cases, is compatible with birth of a chromosomally normal child, suggesting that sperm produced from a background of aneuploidy can be normal in men with KS.

Aneuploidy involving chromosome 1 in failed-fertilized human oocytes is unrelated to maternal age

PURPOSE

To study whether maternal meiotic errors in failed-fertilized oocytes involving chromosome 1 occur at frequencies similar to those involving other autosomes, and whether their frequency is affected by maternal age.

METHODS

Using fluorescence in situ hybridization (FISH), frequencies of aneusomy and chromatid pre-division involving chromosomes 1, 16, 18, and 21 were determined for 273 failed-fertilized oocytes.

RESULTS

The aneuploidy rate for chromosome 1 was 15.8%, and was neither age-dependent nor significantly different from that for chromosomes 16, 18 or 21. Only chromosome 16 exhibited an age-dependent increase in aneusomy rates. The frequency of chromatid pre-division was lower for chromosome 1 than for chromosome 18 (11.9% vs. 25.4%; p = 0.01), but not different from that for chromosomes 16 or 21.

CONCLUSION

Aneuploidy involving chromosome 1 in failed-fertilized oocytes is unrelated to maternal age and occurs at a frequency similar to that for chromosomes 16, 18, and 21.

Mammalian egg activation: from Ca2+ spiking to cell cycle progression

Mammalian eggs arrest at metaphase of the second meiotic division (MetII). Sperm break this arrest by inducing a series of Ca2+spikes that last for several hours. During this time cell cycle resumption is induced, sister chromatids undergo anaphase and the second polar body is extruded. This is followed by decondensation of the chromatin and the formation of pronuclei. Ca2+spiking is both the necessary and solely sufficient sperm signal to induce full egg activation. How MetII arrest is established, how the Ca2+spiking is induced and how the signal is transduced into cell cycle resumption are the topics of this review. Although the roles of most components of the signal transduction pathway remain to be fully investigated, here I present a model in which a sperm-specific phospholipase C (PLCζ) generates Ca2+spikes to activate calmodulin-dependent protein kinase II and so switch on the Anaphase-Promoting Complex/Cyclosome (APC/C). APC/C activation leads to securin and cyclin B1 degradation and in so doing allows sister chromatids to be segregated and to decondense.

Cytoskeleton and cell cycle control during meiotic maturation of the mouse oocyte: integrating time and space

During meiotic maturation of mammalian oocytes, two successive divisions occur without an intermediate phase of DNA replication, so that haploid gametes are produced. Moreover, these two divisions are asymmetric, to ensure that most of the maternal stores are retained within the oocyte. This leads to the formation of daughter cells with different sizes: the large oocyte and the small polar bodies. All these events are dependent upon the dynamic changes in the organization of the oocyte cytoskeleton (microtubules and microfilaments) and are highly regulated in time and space. We review here the current knowledge of the interplay between the cytoskeleton and the cell cycle machinery in mouse oocytes, with an emphasis on the two major activities that control meiotic maturation in vertebrates, MPF (Maturation promoting factor) and CSF (Cytostatic factor).

Not all germ cells are created equal: Aspects of sexual dimorphism in mammalian meiosis

The study of mammalian meiosis is complicated by the timing of meiotic events in females and by the intermingling of meiotic sub-stages with somatic cells in the gonad of both sexes. In addition, studies of mouse mutants for different meiotic regulators have revealed significant differences in the stringency of meiotic events in males versus females. This sexual dimorphism implies that the processes of recombination and homologous chromosome pairing, while being controlled by similar genetic pathways, are subject to different levels of checkpoint control in males and females. This review is focused on the emerging picture of sexual dimorphism exhibited by mammalian germ cells using evidence from the broad range of meiotic mutants now available in the mouse. Many of these mouse mutants display distinct differences in meiotic progression and/or dysfunction in males versus females, and their continued study will allow us to understand the molecular basis for the sex-specific differences observed during prophase I progression.

Identification of two proteins required for conjunction and regular segregation of achiasmate homologs in Drosophila male meiosis

In Drosophila males, homologous chromosomes segregate by an unusual process involving physical connections not dependent on recombination. We have identified two meiotic proteins specifically required for this process. Stromalin in Meiosis (SNM) is a divergent member of the SCC3/SA/STAG family of cohesin proteins, and Modifier of Mdg4 in Meiosis (MNM) is one of many BTB-domain proteins expressed from the mod(mdg4) locus. SNM and MNM colocalize along with a repetitive rDNA sequence known to function as an X-Y pairing site to nucleolar foci during meiotic prophase and to a compact structure associated with the X-Y bivalent during prometaphase I and metaphase I. Additionally, MNM localizes to autosomal foci throughout meiosis I. These proteins are mutually dependent for their colocalization, and at least MNM requires the function of teflon, another meiotic gene. SNM and MNM do not colocalize with SMC1, suggesting that the homolog conjunction mechanism is independent of cohesin.

Nouvelles données en génétique chromosomique

Novel methods allowing to analyze the human genome make it possible to assess old questions such as the molecular basis of structural chromosome anomalies and the diathesis to aneuploidy. The architecture of the human genome as unravelled by the human genome sequencing project allows to explain the recurrence of microdeletions and microduplications caused by a non allelic homologous recombination involving segmental duplications created during the evolution of primates. This structural feature of the human genome is associated with a novel class of genetic diseases called genomic disorders as opposed to genetic diseases due to gene mutations. The study of the parental and cellular origin of aneuploidy shed new light on the different mechanisms controlling meiosis in man and woman. In addition it contributes to define the role of maternal age and genetic recombination on the behavior of chromosomes during meiosis. These new data greatly contribute to our understanding of human chromosomal diseases.

Mammalian oocyte therapies

In assisted human reproduction, the cytoplasm of oocytes recovered from follicles is often abnormal. Its lower quality, especially in older patients, may be responsible for certain chromosomal abnormalities or developmental arrest. Thus, the deficiency of some vital molecules, which are necessary for oocyte maturation, can be the cause of infertility in women. Moreover, mutated mitochondrial DNA (mtDNA) that is located in the oocyte cytoplasm might be transmitted to offspring. With the advance of new micromanipulation techniques like the oocyte nucleus replacement or cytoplasmic transfer, some of these abnormalities could be theoretically eliminated. In this review, we briefly discuss some of these approaches and their potential use in assisted human reproduction.

Eph and NMDA receptors control Ca2+/calmodulin-dependent protein kinase II activation during C. elegans oocyte meiotic maturation

Fertilization in the female reproductive tract depends on intercellular signaling mechanisms that coordinate sperm presence with oocyte meiotic progression. To achieve this coordination in Caenorhabditis elegans, sperm release an extracellular signal, the major sperm protein (MSP), to induce oocyte meiotic maturation and ovulation. MSP binds to multiple receptors, including the VAB-1 Eph receptor protein-tyrosine kinase on oocyte and ovarian sheath cell surfaces. Canonical VAB-1 ligands called ephrins negatively regulate oocyte maturation and MPK-1 mitogen-activated protein kinase (MAPK) activation. Here, we show that MSP and VAB-1 regulate the signaling properties of two Ca2+ channels that are encoded by the NMR-1 N-methyl D-aspartate type glutamate receptor subunit and ITR-1 inositol 1,4,5-triphosphate receptor. Ephrin/VAB-1 signaling acts upstream of ITR-1 to inhibit meiotic resumption, while NMR-1 prevents signaling by the UNC-43 Ca2+/calmodulin-dependent protein kinase II (CaMKII). MSP binding to VAB-1 stimulates NMR-1-dependent UNC-43 activation, and UNC-43 acts redundantly in oocytes to promote oocyte maturation and MAPK activation. Our results support a model in which VAB-1 switches from a negative regulator into a redundant positive regulator of oocyte maturation upon binding to MSP. NMR-1 mediates this switch by controlling UNC-43 CaMKII activation at the oocyte cortex.

Paternal age and intelligence: implications for age-related genomic changes in male germ cells

BACKGROUND

A robust association between advancing paternal age and schizophrenia risk is reported, and genetic changes in the germ cells of older men are presumed to underlie the effect. If that is so, then the pathway may include effects on cognition, as those with premorbid schizophrenia are reported to have lower intelligence. There are also substantial genetic influences on intelligence, so de novo genetic events in male germ cells, which accompany advancing paternal age, may plausibly influence offspring intelligence.

OBJECTIVE

An association of paternal age with IQ in healthy adolescents may illuminate the mechanisms that link it to schizophrenia.

METHOD

We examined the association of paternal age and IQ scores using the Israeli Army Board data on 44 175 individuals from a richly described birth cohort, along with maternal age and other potential modifiers.

RESULTS

A significant inverted U-shaped relationship was observed between paternal age and IQ scores, which was independent from a similar association of IQ scores with maternal age. These relationships were not significantly attenuated by controlling for multiple possible confounding factors, including the other parent's age, parental education, social class, sex and birth order, birth weight and birth complications. Overall, parental age accounted for approximately 2% of the total variance in IQ scores, with later paternal age lowering non-verbal IQ scores more than verbal IQ scores.

CONCLUSION

We found independent effects of maternal and paternal age on offspring IQ scores. The paternal age effect may be explained by de novo mutations or abnormal methylation of paternally imprinted genes, whereas maternal age may affect fetal neurodevelopment through age-related alterations in the in-utero environment. The influence of late paternal age to modify non-verbal IQ may be related to the pathways that increase the risk for schizophrenia in the offspring of older fathers.

The kinetochore and cancer: what's the connection?

The molecular mechanisms ensuring accurate chromosome segregation during meiosis and mitosis are critical to the conservation of euploidy (normal chromosome number) in eukaryotic cells. A dysfunctional kinetochore represents one possible source for chromosome instability (CIN) and the generation of aneuploidy. The kinetochore is a large complex of proteins and associated centromeric DNA that is responsible for mediating the segregation of sister chromatids to daughter cells via its interactions with the mitotic spindle. Continued identification of conserved kinetochore components in model systems such as yeast has provided a rich resource of candidate genes that may be mutated or misregulated in human cancers. Systematic mutational testing and transcriptional profiling of CIN candidate kinetochore genes should shed light on the kinetochore's role in tumorigenesis, and on the general role CIN plays in cancer development.

GENETICS: Motivating Hotspots

Only recently have we begun to characterize fine-scale recombination rates in mammals. In her Perspective, Przeworski discusses the work by Myers et al. in which linkage disequilibrium data have been used to produce a high-resolution recombination map for most of the human genome. More than 25,000 putative hotspots have been identified, as well as the first motifs that appear to influence their intensity.

Meiotic synapsis proceeds from a limited number of subtelomeric sites in the human male

The formation of the synaptonemal complex (SC) is a crucial early step in the meiotic process, but relatively little is known about the establishment of the human SC. Accordingly, we recently initiated a study of synapsis in the human male, combining immunofluorescence and fluorescence in situ hybridization methodologies to analyze prophase spermatocytes from a series of control individuals. Our results indicate that synapsis is a tightly regulated process, with relatively little variation among individuals. On nonacrocentric chromosomes, there are two synaptic initiation sites, one on the distal short arm and one on the distal long arm, whereas acrocentric chromosomes exhibit a single site on the distal long arm. For both types of chromosomes, synapsis then proceeds toward the centromere, with little evidence that specific p- or q-arm sequences affect the process. However, the centromere appears to have an inhibitory effect on synapsis--that is, when one arm of a nonacrocentric chromosome is "zippered up" before the other, the centromere acts as a barrier to further movement from that arm.

Sex, not genotype, determines recombination levels in mice

Recombination, the precise physical breakage and rejoining of DNA between homologous chromosomes, plays a central role in mediating the orderly segregation of meiotic chromosomes in most eukaryotes. Despite its importance, the factors that control the number and placement of recombination events within a cell remain poorly defined. The rate of recombination exhibits remarkable species specificity, and, within a species, recombination is affected by the physical size of the chromosome, chromosomal location, proximity to other recombination events (i.e., chiasma interference), and, intriguingly, the sex of the transmitting parent. To distinguish between simple genetic and nongenetic explanations of sex-specific recombination differences in mammals, we compared recombination in meiocytes from XY sex-reversed and XO females with that in meiocytes from XX female and XY male mice. The rate and pattern of recombination in XY and XO oocytes were virtually identical to those in normal XX females, indicating that sex, not genotype, is the primary determinant of meiotic recombination patterns in mammals.

Comparison of maturation, meiotic competence, and chromosome aneuploidy of oocytes derived from two protocols for in vitro culture of mouse secondary follicles

PURPOSE

To compare maturation rates of mouse preantral follicles cultured using two previously reported follicle in vitro follicle culture protocols, and to compare the aneuploidy of oocytes derived from the two protocols with in vivo-matured control oocytes.

METHODS

Mouse preantral follicles were either mechanically isolated then cultured in individual microdroplets, or enzymatically isolated and cultured in groups in a modified culture medium. Maturation of the follicles/oocytes and resulting oocyte aneuploidy rates were evaluated and compared.

RESULTS

The mechanical isolation and individual culture protocol (M/I) resulted in higher follicle survival than the enzymatic isolation and group culture protocol (E/G) (89.1% versus 79.1%, p < 0.01), and better maturation to the metaphase 2 stage (61.5% versus 39.5%, p = 0.01) The rate of aneuploidy of oocytes not significantly higher in oocytes from the E/G group than the M/I group (15.4% versus 9.9%), but hypoploidy was significantly increased (4.7% versus 0.9%, p < 0.05). Both groups had a higher rate of aneuploidy than the control oocytes (2.9%, p < 0.02).

CONCLUSIONS

These results indicate an increased survival and competency of oocytes derived from the M/I protocol, compared to the E/G protocol. The data highlight an increased susceptibility to meiotic errors in early stage follicles undergoing in vitro culture, compared to in vivo-matured oocytes.

Sperm aneuploidy in fathers of Klinefelter's syndrome offspring assessed by multicolour fluorescent in situ hybridization using probes for chromosomes 6, 13, 18, 21, 22, X and Y

BACKGROUND

It is still unclear if a recurrence risk would exist in fathers of an aneuploid offspring of paternal origin. We have studied disomy frequencies in spermatozoa from fathers having Klinefelter syndrome (KS) offspring or miscarriages. The effect of paternal age on sperm disomy percentages is also analysed.

METHODS

Parental origin of 17 KS patients was carried out by amplification of X chromosome polymorphisms. Spermatozoa from their fathers were studied by multicolour fluorescent in situ hybridisation (FISH) using probes for chromosomes 6, 13, 18, 21, 22, X and Y.

RESULTS

In 53% of KS cases studied the additional X chromosome was of paternal origin. The paternally transmitted KS group of fathers showed significantly higher frequencies for XY disomy sperm as compared to fathers of the maternal-origin group. A correlation between paternal age and XY disomy frequencies was only found in the paternally derived cases. In contrast, similar disomy frequencies for all autosomes analysed were found in both groups of fathers.

CONCLUSIONS

XY disomy frequencies increase with advancing paternal age only in fathers with paternally inherited KS offspring.

Mechanisms and consequences of paternally-transmitted chromosomal abnormalities

Paternally-transmitted chromosomal damage has been associated with pregnancy loss, developmental and morphological defects, infant mortality, infertility, and genetic diseases in the offspring, including cancer. There is epidemiological evidence linking paternal exposure to occupational or environmental agents with an increased risk of abnormal reproductive outcomes. There is also a large body of literature on germ cell mutagenesis in rodents showing that treatment of male germ cells with mutagens has dramatic consequences on reproduction, producing effects such as those observed in human epidemiological studies. However, we know very little about the etiology, transmission, and early embryonic consequences of paternally-derived chromosomal abnormalities. The available evidence suggests that: 1) there are distinct patterns of germ cell-stage differences in the sensitivity of induction of transmissible genetic damage, with male postmeiotic cells being the most sensitive; 2) cytogenetic abnormalities at first metaphase after fertilization are critical intermediates between paternal exposure and abnormal reproductive outcomes; and 3) there are maternal susceptibility factors that may have profound effects on the amount of sperm DNA damage that is converted into chromosomal aberrations in the zygote and that directly affect the risk for abnormal reproductive outcomes.

The occurrence of aneuploidy in human: lessons from the cytogenetic studies of human oocytes

During the last 4 decades, the cytogenetic investigation of human oocytes has never stopped to progress, according to the advents of new technologies. Both karyotyping and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes. However, these studies display a great variability in results, which may be essentially attributable to the limitations of these techniques when applied to human oocytes. The most relevant analysis have led to the estimate that 15-20% of human oocytes present chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies in human oocytes has also been clearly evidenced by recent reports based on large sample of oocytes or polar bodies, whereas most of initial studies have failed to confirm any relationship between maternal age and aneuploidy in human oocytes.

An aneuploid mouse strain carrying human chromosome 21 with Down syndrome phenotypes

Aneuploidies are common chromosomal defects that result in growth and developmental deficits and high levels of lethality in humans. To gain insight into the biology of aneuploidies, we manipulated mouse embryonic stem cells and generated a trans-species aneuploid mouse line that stably transmits a freely segregating, almost complete human chromosome 21 (Hsa21). This "transchromosomic" mouse line, Tc1, is a model of trisomy 21, which manifests as Down syndrome (DS) in humans, and has phenotypic alterations in behavior, synaptic plasticity, cerebellar neuronal number, heart development, and mandible size that relate to human DS. Transchromosomic mouse lines such as Tc1 may represent useful genetic tools for dissecting other human aneuploidies.

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.

The association of skewed X chromosome inactivation with aneuploidy in humans

Recently, we reported that skewed X chromosome inactivation (XCI) was more common in women who had experienced a trisomic pregnancy as compared to control women. Rather than an overall shift in the distribution of skewing there appears to only be an excess of extreme (= 95%) skewing. Further analysis of our data reveals that the increase in skewed XCI is dependent on which chromosome is involved in the trisomy and how many trisomies the woman has experienced, although sample sizes in each group are small. In this review we discuss limitations of the commonly used assays of XCI, which use measurements of DNA methylation to infer skewing patterns, and review the data based on current knowledge of the causes of XCI skewing. Gonadal mosaicism, premature aging, loss of methylation at some CpGs, and X-linked mutations can all be considered as potential mechanisms explaining both increased risk of trisomy and skewed XCI. While further research is needed to evaluate the role of each of these, the association of trisomy with apparent skewed XCI in the mother offers new opportunities to clarify the risk factors for and causes of the high incidence of aneuploidy in human females.

Effect of maternal age on the frequency of cytogenetic abnormalities in human oocytes

The cytogenetic investigation of human oocytes was initiated in the Sixties, and for the last four decades, this field of research has never stopped progressing as new technologies appear. Numerous karyotyping studies and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes, but also displaying a great variability in results, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. Essentially, the most relevant analyses have led to the estimate that 15-20% of human oocytes display chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies has also been investigated in human oocytes. Most previous studies have failed to confirm any relationship between maternal age and aneuploidy frequency in human oocytes, whereas the more recent reports based on large samples of oocytes or polar bodies have provided evidence for a direct correlation between increased aneuploidy frequency and advanced maternal age, and have clarified the contribution of the various types of malsegregation in the maternal age-dependent aneuploidies.

Mouse genetic models for aneuploidy induction in germ cells

Rodents have been successfully used as models to identify risks of chemical exposures or age to aneuploidy induction in germ cells, which may be transmitted to the progeny. For this administration in vivo as well as exposures to in vitro maturing germ cells have been useful. Genetic models involving mice with structural chromosomal rearrangements and transgenic animals have the potential to model conditions predisposing to aneuploidy in one or both sexes, and in this way to identify potential targets for aneugens and gender-effects. The review provides an overview of mouse genetic models for aneuploidy induction in mammalian germ cells and discusses perspectives for combining genetic with experimental approaches in aneuploidy research.

Biological aging and the etiology of aneuploidy

A prevalent hypothesis concerning the cause of the rise in aneuploid conceptions with maternal age is that the changes that accompany normal ovarian aging increase the rate of meiotic errors in the oocyte. Biological aging of the ovary is accompanied by a decline in both the total oocyte pool and the number of antral follicles maturing per cycle, as well as changes in the levels of circulating reproductive hormones. The biological aging hypothesis predicts that aneuploidy rates should be higher in women with a prematurely reduced oocyte pool, and that women with trisomic conceptions should show signs of earlier ovarian aging than women of the same chronological age without trisomic conceptions. Comprehensive studies of aneuploidy in groups of women with known causes of premature ovarian failure remain to be done, though anecdotal evidence does suggest increased rates of pregnancy loss and aneuploidy. Smoking, which is a well-documented cause of earlier ovarian aging, is not associated with an increase in aneuploid conceptions. Evidence from women with unilateral ovariectomies is inconsistent. Support for the biological aging hypothesis was provided by one study showing that menopause occurred about a year earlier in women with a trisomic spontaneous abortion compared to women with chromosomally normal conceptions. Associations between high FSH and pregnancies with Down syndrome and chromosomally abnormal spontaneous abortions have also been reported. However, the most direct test of the hypothesis, which compared antral follicle counts and hormonal levels in women with trisomic pregnancies and those with chromosomally normal pregnancies, failed to find a difference in the expected direction. A prospective study of FSH levels in women with subfertility also failed to find an association with the rate of pregnancy loss. The bulk of evidence thus suggests that, if the processes of biological aging are indeed related to aneuploidy, they probably involve factors other than those measured by oocyte or antral follicle pool size and reproductive hormone levels.

Effect of meiotic recombination on the production of aneuploid gametes in humans

Within the last decade, aberrant meiotic recombination has been confirmed as a molecular risk factor for chromosome nondisjunction in humans. Recombination tethers homologous chromosomes, linking and guiding them through proper segregation at meiosis I. In model organisms, mutations that disturb the recombination pathway increase the frequency of chromosome malsegregation and alterations in both the amount and placement of meiotic recombination are associated with nondisjunction. This association has been established for humans as well. Significant alterations in recombination have been found for all meiosis I-derived trisomies studied to date and a subset of so called "meiosis II" trisomy. Often exchange levels are reduced in a subset of cases where the nondisjoining chromosome fails to undergo recombination. For other trisomies, the placement of meiotic recombination has been altered. It appears that recombination too near the centromere or too far from the centromere imparts an increased risk for nondisjunction. Recent evidence from trisomy 21 also suggests an association may exist between recombination and maternal age, the most widely identified risk factor for aneuploidy. Among cases of maternal meiosis I-derived trisomy 21, increasing maternal age is associated with a decreasing frequency of recombination in the susceptible pericentromeric and telomeric regions. It is likely that multiple risk factors lead to nondisjunction, some age dependent and others age independent, some that act globally and others that are chromosome specific. Future studies are expected to shed new light on the timing and placement of recombination, providing additional clues to the link between altered recombination and chromosome nondisjunction.

Increased frequency of micronuclei in immature seminal germinal cells of male workers exposed to aromatic hydrocarbons

The aim of the present work was to investigate whether there is an association between the presence of micronuclei in immature seminal germinal cells and occupational exposure to aromatic hydrocarbons in the workplace. We concluded that exposure of the workers to hydrocarbons in their workplace increased the frequency of micronuclei in immature seminal germinal cells.

Karyotyping of human oocytes by cenM-FISH, a new 24-colour centromere-specific technique

BACKGROUND

Metaphase II (MII) chromosome complements are difficult to karyotype. The objective of this study was to investigate the efficiency and limitations of centromere-specific multiplex fluorescence in situ hybridization (cenM-FISH), a new 24 colour FISH technique using centromere-specific probes, to analyse the whole chromosome complement within human oocytes.

METHODS

Oocytes were donated by 34 patients undergoing ovarian stimulation and IVF. The MII oocytes were analysed by means of cenM-FISH, while the confirmation of results was performed by FISH and/or by analysing the corresponding first polar bodies using comparative genomic hybridization (CGH).

RESULTS

A total of 30 cells, corresponding to 16 oocytes and 14 first polar bodies, were successfully karyotyped by either cenM-FISH or CGH. The incidence of aneuploidy was 25%, and eight out of nine aneuploidy events were confirmed by CGH and FISH.

CONCLUSIONS

We demonstrate here for the first time that the identification of any numerical abnormality in oocytes is feasible using cenM-FISH. Despite the fact that the fixation efficiency remains low, the present results confirm the advantage of analysing the whole set of chromosomes to make an accurate estimation of the aneuploidy rate in human oocytes.

Homologous chromosome interactions in meiosis: diversity amidst conservation

Proper chromosome segregation is crucial for preventing fertility problems, birth defects and cancer. During mitotic cell divisions, sister chromatids separate from each other to opposite poles, resulting in two daughter cells that each have a complete copy of the genome. Meiosis poses a special problem in which homologous chromosomes must first pair and then separate at the first meiotic division before sister chromatids separate at the second meiotic division. So, chromosome interactions between homologues are a unique feature of meiosis and are essential for proper chromosome segregation. Pairing and locking together of homologous chromosomes involves recombination interactions in some cases, but not in others. Although all organisms must match and lock homologous chromosomes to maintain genome integrity throughout meiosis, recent results indicate that the underlying mechanisms vary in different organisms.