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.     

Mechanisms and chemical induction of aneuploidy in rodent germ cells

The objective of this review is to suggest that the advances being made in our understanding of the molecular events surrounding chromosome segregation in non-mammalian and somatic cell models be considered when designing experiments for studying aneuploidy in mammalian germ cells. Accurate chromosome segregation requires the temporal control and unique interactions among a vast array of proteins and cellular organelles. Abnormal function and temporal disarray among these, and others to be identified, biochemical reactions and cellular organelles have the potential for predisposing cells to aneuploidy. Although numerous studies have demonstrated that certain chemicals (mainly those that alter microtubule function) can induce aneuploidy in mammalian germ cells, it seems relevant to point out that such data can be influenced by gender, meiotic stage, and time of cell-fixation post-treatment. Additionally, a consensus has not been reached regarding which of several germ cell aneuploidy assays most accurately reflects the human condition. More recent studies have shown that certain kinase, phosphatase, proteasome, and topoisomerase inhibitors can also induce aneuploidy in rodent germ cells. We suggest that molecular approaches be prudently incorporated into mammalian germ cell aneuploidy research in order to eventually understand the causes and mechanisms of human aneuploidy. Such an enormous undertaking would benefit from collaboration among scientists representing several disciplines.     

Mechanisms of maternal aneuploidy: FISH analysis of oocytes and polar bodies in patients undergoing assisted conception

We have examined unfertilised oocytes and their first polar bodies (PBs) to determine the way in which the frequency of whole chromosome imbalance compares with that involving single chromatids and whether the precocious separation of chromatids prior to anaphase I affects all pairs of chromosomes. We have applied the technique of fluorescent in situ hybridisation in a three-stage method by using locus-specific probes for chromosomes 13 and 21 and alpha-satellite probes for chromosomes 1, 9, 16, 18 and X to determine the chromosome status of oocytes and their PBs. We obtained analysable results from 127 oocytes and 57 PBs from 72 patients of average age 33 years. Six oocytes and three PBs had extra signals but, of these, three were derived from a single patient, aged 26. Anomalies were seen in chromosomes 13, 16, 18, X and, notably, 21 but none were observed in chromosomes 1 and 9. Half of the anomalies involved additional chromatids rather than whole chromosomes. Since particular chromatids were found to be prematurely separated in the metaphase II oocyte, this may provide further evidence for an additional mechanism of maternal aneuploidy that operates at anaphase II. Detailed analyses of both oocytes and PBs have elucidated possible mechanisms leading to aneuploid gametes in this group of patients with fertility problems.     

How to survive aneuploidy

Aneuploidy, or an abnormal number of chromosomes, adversely affects cell growth, but it is also linked with cancer and tumorigenesis. Now, Torres et al. (2010) help to resolve this paradox by demonstrating that aneuploid yeast cells can evolve mutations in the proteasome protein degradation pathway that alleviate imbalances in protein production and increase the cell's proliferative capacities.     

Comparisons of tests for aneuploidy

The fundamental problems that face us in the development of suitable assay systems for the detection of potentially aneugenic (aneuploidy-inducing) chemicals include: (a) the diversity of cellular targets and mechanisms where perturbations of structure and function may give rise to changes in chromosome number, and (b) the phylogenetic differences that exist between species in their mechanism and kinetics of cell division and their metabolic profiles. A diverse range of assay systems have been developed, which have been shown to have potential for use in the detection of either changes in chromosome number or of perturbations of the events which may be causal in the induction of aneuploidy.

Chromosome number changes may be detected cytologically by karyotypic analysis, or by the use of specialised strains in which aneuploid progeny may be observed due to phenotypic differences with aneuploid parental cells or whole organisms. Techniques for the detection of cellular target modifications range from in vitro studies of tubulin polymerisation to observations of the behaviour of various cellular organelles and their fidelity of action during the division cycle.

The diversity of mechanisms which may give rise to aneuploidy and the qualitative relevance of events observed in experimental organisms compared to man make it unlikely that the detection and risk assessment of the aneugenic activity of chemicals will be possible using a single assay system. Optimal screening and assessment procedures will thus be dependent upon the selection of an appropriate battery of predictive tests for the measurement of the potentially damaging effects of aneuploidy induction.     

Tetraploidy, aneuploidy and cancer

Aneuploidy is one of the most obvious differences between normal and cancer cells. However, there remains debate over how aneuploid cells arise and whether or not they are a cause or consequence of tumorigenesis. One proposed route to aneuploid cancer cells is through an unstable tetraploid intermediate. Supporting this idea, recent studies demonstrate that tetraploidy promotes chromosomal aberrations and tumorigenesis in vivo. These tetraploid cells can arise by a variety of mechanisms, including mitotic slippage, cytokinesis failure, and viral-induced cell fusion. Furthermore, new studies suggest that there might not be a ploidy-sensing checkpoint that permanently blocks the proliferation of tetraploid cells. Therefore, abnormal division of tetraploid cells might facilitate genetic changes that lead to aneuploid cancers.     

Hyperthermia-induced embryonic aneuploidy in mice

Chromosomal aneuploidy, as an effect of elevated ambient temperature during early gestation, was studied in 650 eight-day embryos from 60 females of two different mouse stocks. Exposure of treatment females to 34 C and 50% relative humidity during days 5 and 6 of gestation caused an increase in embryo chromosomal hyperploidy. No embryo sex interaction with the percent hyperploidy was found.     

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.     

Entosis: aneuploidy by invasion

Aneuploidy is one of the most prevalent phenotypes of human tumours, but the underlying cause of this phenomenon remains highly debated. Entosis, the invasion of a living cell into another cell's cytoplasm, is now shown to perturb cytokinesis and induce the formation of aneuploid cells.     

Age-related aneuploidy through cohesion exhaustion

The trend of women to become pregnant when older than in previous generations poses a paramount medical problem, for oocytes are particularly prone to chromosome missegregation, and aneuploidy increases with age. Recent data strongly suggest that as oocyte age increases sister chromatid cohesion is weakened or lost. Cohesin deterioration seems to contribute significantly to age-dependent aneuploidy, as discussed in this review.     

Fertility and aneuploidy in autopolyploidAnthoxanthum

An autopolyploid series comprising 2x, 3x and 4x was obtained by colchicining the diploid speciesA. ovatum Lag. The autotriploid, used as seed and pollen parent, was back-crossed to diploid and tetraploid. No significant overall difference was observed in fertility, but there was a tendency for particular pair crosses to the diploid as female parent to show higher levels of seed-setting. This may be associated with the relatively large proportion of haploid pollen that functions when the triploid is used as the male parent.The effectiveness, in part, of the freely liberated pollen of autotriploidAnthoxanthum in producing progeny, contrasts with the total failure of manually liberated allotriploidDactylis pollen. Triploid sterility may well relate to degree of hybridity.Genetic unbalance induced by aneuploidy was considered to be more relevant than the genetic ratio in seeds, as an explanation of the type of interploid progeny obtained inAnthoxanthum.Cytological material in this paper was submitted by the junior author in partial satisfaction of the requirements for the degree of M.Sc. in the University of Wales.     

Inherited aneuploidy: germline mosaicism

Germline mosaicism has been thought to be a rare cause of aneuploidy in the human population. Recent evidence from cytological and population studies suggests otherwise. Approximately 5% of young couples with a Down syndrome child show evidence of germinal mosaicism. Molecular cytogenetic analysis of oocytes has proved germinal or gonadal mosaicism for trisomies of chromosomes 13 and 21 in several studies involving both oocytes and first polar bodies. Most recently direct analysis of fetal ovarian pre-meiotic, meiotic, and stromal cells proved low level trisomy 21 mosaicism in every sample tested. Based upon this evidence, germinal or gonadal mosaicism is likely to make a significant contribution to aneuploidy in the human population, particularly where younger women are concerned.     

Targeting aneuploidy for cancer therapy

Tumor cells frequently display an abnormal number of chromosomes, a phenomenon known as aneuploidy. Tang et al. (2011) now show that aneuploid cells are particularly sensitive to compounds that induce proteotoxic and energy stress. Could this vulnerability lead to new cancer therapies?     

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.     

The origin and impact of embryonic aneuploidy

Despite the clinical importance of aneuploidy, surprisingly little is known concerning its impact during the earliest stages of human development. This study aimed to shed light on the genesis, progression, and survival of different types of chromosome anomaly from the fertilized oocyte through the final stage of preimplantation development (blastocyst). 2,204 oocytes and embryos were examined using comprehensive cytogenetic methodology. A diverse array of chromosome abnormalities was detected, including many forms never recorded later in development. Advancing female age was associated with dramatic increase in aneuploidy rate and complex chromosomal abnormalities. Anaphase lag and congression failure were found to be important malsegregation causing mechanisms in oogenesis and during the first few mitotic divisions. All abnormalities appeared to be tolerated until activation of the embryonic genome, after which some forms started to decline in frequency. However, many aneuploidies continued to have little impact, with affected embryos successfully reaching the blastocyst stage. Results from the direct analyses of female meiotic divisions and early embryonic stages suggest that chromosome errors present during preimplantation development have origins that are more varied than those seen in later pregnancy, raising the intriguing possibility that the source of aneuploidy might modulate impact on embryo viability. The results of this study also narrow the window of time for selection against aneuploid embryos, indicating that most survive until the blastocyst stage and, since they are not detected in clinical pregnancies, must be lost around the time of implantation or shortly thereafter.