Sequential FISH analysis of oocytes and polar bodies reveals aneuploidy mechanisms

Sperm and Oocyte Chromosomal Abnormalities

Gametogenesis, the process of producing gametes, differs significantly between oocytes and sperm. Most oocytes have chromosomal aneuploidies, indicating that chromosomal aberrations in miscarried and newborn infants are of oocyte origin. Conversely, most structural anomalies are of sperm origin. A prolonged meiotic period caused by increasing female age is responsible for an increased number of chromosomal aberrations. Sperm chromosomes are difficult to analyze because they cannot be evaluated using somatic cell chromosome analysis methods. Nevertheless, researchers have developed methods for chromosome analysis of sperm using the fluorescence in situ hybridization method, hamster eggs, and mouse eggs, allowing for the cytogenetic evaluation of individual sperm. Reproductive medicine has allowed men with severe spermatogenic defects or chromosomal abnormalities to have children. However, using these techniques to achieve successful pregnancies results in higher rates of miscarriages and embryos with chromosomal abnormalities. This raises questions regarding which cases should undergo sperm chromosome analysis and how the results should be interpreted. Here, we reviewed clinical trials that have been reported on oocyte and sperm chromosome analyses. Examination of chromosomal abnormalities in gametes is critical in assisted reproductive technology. Therefore, it is necessary to continue to study the mechanism underlying gametic chromosomal abnormalities.

Ultrastructural and cytogenetic analyses of mature human oocyte dysmorphisms with respect to clinical outcomes

PURPOSE

The study aimed to describe the ultrastructure of two human mature oocyte intracytoplasmic dysmorphisms, the bull-eye inclusion and the granular vacuole, with evaluation of clinical outcomes after intracytoplasmic sperm injection (ICSI) treatment.

METHODS

We retrospectively evaluated 4099 consecutive ICSI cycles during the period 2003-2013. Three groups were compared: controls, those with a bulls-eye inclusion, and those with granular vacuoles. Oocyte dysmorphisms were evaluated by transmission electron microscopy and in situ fluorescence hybridization (FISH). Detailed data on demographic and stimulation characteristics, as well as on embryological, clinical, and newborn outcomes, are fully presented.

RESULTS

The bull-eye inclusion is a prominent smooth round structure containing trapped vesicles, being surrounded by lipid droplets. The presence of this dysmorphism in the oocyte cohort had no clinical impact except when transferred embryos were exclusively derived from dysmorphism oocytes. The granular vacuole is delimited by a discontinuous double membrane and contains lipid droplets and vesicles. As FISH analysis revealed the presence of chromosomes, they probably represent pyknotic nuclei. The presence of this dysmorphism in the oocyte cohort had no clinical impact except when at least one transferred embryo was derived from dimorphic oocytes.

CONCLUSIONS

Poor clinical outcomes were observed with transfer of embryos derived from dysmorphism oocytes, although without causing gestation or newborn problems. The bull-eye inclusion and granular vacuoles may thus be new prognostic factors for clinical outcomes.

Non-meiotic chromosome instability in human immature oocytes

Aneuploidy has been a major issue in human gametes and is closely related to fertility problems, as it is known to be present in cleavage stage embryos and gestational losses. Pre-meiotic chromosome abnormalities in women have been previously described. The aim of this study is to assess the whole-chromosome complement in immature oocytes to find those abnormalities caused by mitotic instability. For this purpose, a total of 157 oocytes at the germinal vesicle or metaphase I stage, and discarded from IVF cycles, were analysed by CGH. Fifty-six women, between 18 and 45 years old (mean 32.5 years), including 32 IVF patients (25-45 years of age) and 24 IVF oocyte donors (18-33 years of age), were included in the study. A total of 25/157 (15.9%) of the oocytes analysed, obtained from three IVF clinics, contained chromosome abnormalities, including both aneuploidy (24/157) and structural aberrations (9/157). Independently of the maternal age, the incidence of abnormal oocytes which originated before meiosis is 15.9%, and these imbalances were found in 33.9% of the females studied. This work sheds light on the relevance of mitotic instability responsible for the generation of the abnormalities present in human oocytes.

A study of meiotic segregation in a fertile human population following ovarian stimulation with recombinant FSH-LH

OBJECTIVE

The aim of the study is to investigate the meiotic segregation in fresh eggs from anonymous egg donors and to analyze the baseline levels of aneuploidy in this population.

RESULTS

The study includes the largest series of donor eggs so far studied: 203 eggs from donors aged between 20 and 31 years. No diagnosis was obtained in 10.8 % of cases (22/ 203). The biopsy of the first and second polar bodies was completed in a sequential manner on day 0 and day 1 of embryo development. Chromosomes 13, 16, 18, 21 and 22 are analyzed by means of the FISH test. The diagnosable fertilized eggs gave an aneuploidy rate of 19.1 % (31/162), with 83.8 % (26/31) of the errors produced during meiosis I, 12.9 % (4/31) produced during meiosis II, and 3.2 % (1/31) produced during both meiosis I and II. The premature division of sister chromatids is the main source of meiotic error during Meiosis I, resulting in the creation of oocyte aneuploidy.

CONCLUSIONS

FISH analysis of the first and second polar body in donor oocytes gave an aneuploidy rate of 19.1 %. This study shows the majority of errors occur during Meiosis I.

Molecular origin of female meiotic aneuploidies

Chromosome aneuploidy is a major cause of pregnancy loss, abnormal pregnancy and live births following both natural conception and in vitro fertilisation (IVF) and increases exponentially with maternal age in the decade preceding the menopause. Molecular genetic analysis has shown that these are predominantly maternal in origin and trisomies most frequently occur through errors in the first meiotic division. Analysis of chromosome copy number in the three products of female meiosis, the first and second polar bodies and the corresponding zygote by microarray comparative genomic hybridisation (array CGH), in women of advanced maternal age undergoing IVF, has recently revealed a pattern of frequent multiple meiotic errors, caused by premature predivision of sister chromatids in meiosis I and a high incidence of errors in meiosis II. This pattern is similar to those observed in various mouse models which implicate the gradual depletion of cohesins, which are essential for cohesion of sister chromatids, as the primary cause of age related aneuploidy in female meiosis. However, defects in other aspects of meiosis including the formation and stabilisation of chiasmata and the spindle assembly checkpoint (SAC) may also contribute. The challenge remains to explain the molecular basis of 'physiological' rather than 'chronological' female ageing and the contribution of multifactorial causes from the fetal to adult ovary. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.

Molecular basis of maternal age-related increase in oocyte aneuploidy

Aneuploidy is one of the most common and serious pregnancy complications in humans. Most conceptuses with autosomal aneuploidy die in utero, resulting in early pregnancy loss. However, some fetuses with aneuploidy survive to term but suffer from disorders associated with congenital anomalies and mental retardation, such as Down syndrome with trisomy 21. Three general characteristics of this condition are well acknowledged: (i) in most cases the extra chromosome is of maternal origin; (ii) most cases are derived from a malsegregation event in meiosis I; and (iii) the frequency of these errors increases with maternal age. The basis for the age-dependent increase in meiosis I errors has been a long-standing enigma. Many investigators have addressed the nature of this biological phenomenon through genomic analyses of extra chromosome 21 using polymorphic markers to determine the frequency or location of crossovers that should ensure faithful chromosome segregation. Cytogenetic analyses of in vitro unfertilized oocytes have also been performed. However, no definitive conclusions regarding meiosis I errors have yet been reached from such studies. Recent findings in conditional knock-out mice for meiosis-specific cohesin have shed further light on this issue. The present review focuses on the current understanding of age-related aneuploidy and provides an overview of the mechanisms involved. We refer to recent data to illustrate some of the new paradigms that have arisen in this field.

Multiple meiotic errors caused by predivision of chromatids in women of advanced maternal age undergoing in vitro fertilisation

Chromosome aneuploidy is a major cause of pregnancy loss, abnormal pregnancy and live births following both natural conception and in vitro fertilisation (IVF) and increases exponentially with maternal age in the decade preceding the menopause. Molecular genetic analysis following natural conception and spontaneous miscarriage demonstrates that trisomies arise mainly in female meiosis and particularly in the first meiotic division. Here, we studied copy number gains and losses for all chromosomes in the two by-products of female meiosis, the first and second polar bodies, and the corresponding zygotes in women of advanced maternal age undergoing IVF, using microarray comparative genomic hybridisation (array CGH). Analysis of the segregation patterns underlying the copy number changes reveals that premature predivision of chromatids rather than non-disjunction of whole chromosomes causes almost all errors in the first meiotic division and unlike natural conception, over half of aneuploidies result from errors in the second meiotic division. Furthermore, most abnormal zygotes had multiple aneuploidies. These differences in the aetiology of aneuploidy in IVF compared with natural conception may indicate a role for ovarian stimulation in perturbing meiosis in ageing oocytes.

Origins and rates of aneuploidy in human blastomeres

OBJECTIVE

To characterize chromosomal error types and parental origin of aneuploidy in cleavage-stage embryos using an informatics-based technique that enables the elucidation of aneuploidy-causing mechanisms.

DESIGN

Analysis of blastomeres biopsied from cleavage-stage embryos for preimplantation genetic screening during IVF.

SETTING

Laboratory.

PATIENT(S)

Couples undergoing IVF treatment.

INTERVENTION(S)

Two hundred seventy-four blastomeres were subjected to array-based genotyping and informatics-based techniques to characterize chromosomal error types and parental origin of aneuploidy across all 24 chromosomes.

MAIN OUTCOME MEASURE(S)

Chromosomal error types (monosomy vs. trisomy; mitotic vs. meiotic) and parental origin (maternal vs. paternal).

RESULT(S)

The rate of maternal meiotic trisomy rose significantly with age, whereas other types of trisomy showed no correlation with age. Trisomies were mostly maternal in origin, whereas paternal and maternal monosomies were roughly equal in frequency. No examples of paternal meiotic trisomy were observed. Segmental error rates were found to be independent of maternal age.

CONCLUSION(S)

All types of aneuploidy that rose with increasing maternal age can be attributed to disjunction errors during meiosis of the oocyte. Chromosome gains were predominantly maternal in origin and occurred during meiosis, whereas chromosome losses were not biased in terms of parental origin of the chromosome. The ability to determine the parental origin for each chromosome, as well as being able to detect whether multiple homologs from a single parent were present, allowed greater insights into the origin of aneuploidy.

Aneuploid human embryonic stem cells: origins and potential for modeling chromosomal disorders

Chromosomal aneuploidies are widely recognized genetic disorders in humans that often lead to spontaneous abortion. Aneuploid fetuses that survive to term commonly exhibit impaired developmental growth and mental retardation in addition to multiple congenital malformations. Preimplantation genetic screening is used to detect chromosomal aneuploidies in early embryos. Human embryonic stem cell (ESC) cell lines generated from aneuploid embryos created a unique repository of cell lines. The spectrum of aneuploidies in these ESC lines reflects the range of common embryonic chromosomal aberrations and significantly differs from the spectrum of aneuploid human ESC lines generated by cell adaptation in culture. The aneuploid human ESC lines represent an excellent model to study human chromosomal abnormalities especially in the early stages of development.

Megasporogenesis and programmed cell death in Tillandsia (Bromeliaceae)

The degeneration of three of four meiotic products is a very common process in the female gender of oogamous eukaryotes. In Tillandsia (and many other angiosperms), the surviving megaspore has a callose-free wall in chalazal position while the other three megaspores are completely embedded in callose. Therefore, nutrients and signals can reach more easily the functional megaspore from the nucellus through the chalazal pole with respect to the other megaspores. The abortion of three of four megaspores was already recognized as the result of a programmed cell death (PCD) process. We investigated the process to understand the modality of this specific type of PCD and its relationship to the asymmetric callose deposition around the tetrad. The decision on which of the four megaspores will be the supernumerary megaspores in angiosperms, and hence destined to undergo programmed cell death, appears to be linked to the callose layer deposition around the tetrad. During supernumerary megaspores degeneration, events leading to the deletion of the cells do not appear to belong to a single type of cell death. The first morphological signs are typical of autophagy, including the formation of autophagosomes. The TUNEL positivity and a change in morphology of mitochondria and chloroplasts indicate the passage to an apoptotic-like PCD phase, while the cellular remnants undergo a final process resembling at least partially (ER swelling) necrotic morphological syndromes, eventually leading to a mainly lipidic cell corpse still separated from the functional megaspore by a callose layer.

Whole-chromosome aneuploidy analysis in human oocytes: focus on comparative genomic hybridization

The study of aneuploidy in human oocytes, discarded from IVF cycles, has provided a better understanding of the incidence of aneuploidy of female origin and the responsible mechanisms. Comparative genomic hybridization (CGH) is an established technique that allows for the detection of aneuploidy in all chromosomes avoiding artifactual chromosome losses. In this review, results obtained using CGH in single cells (1PB and/or MII oocytes) are included. The results of oocyte aneuploidy rates obtained by CGH from discarded oocytes of IVF patients and of oocyte donors are summarized. Moreover, the mechanisms involved in the aneuploid events, e.g. whether alterations occurred due to first meiotic errors or germ-line mitotic errors are also discussed. Finally, the incidence of aneuploid oocyte production due to first meiotic errors and germ-line mitotic errors observed in oocytes coming from IVF patients and IVF oocyte donors was assessed.

Transcriptomic profiling of human oocytes: association of meiotic aneuploidy and altered oocyte gene expression

The ability to identify oocytes with the greatest potential for producing a viable embryo would be of great benefit to assisted reproductive treatments. One of the most important defects affecting oocytes is aneuploidy. Aneuploidy is also closely related with advancing maternal age, a phenomenon not well understood. This study combined a comprehensive cytogenetic investigation of 21 oocytes with a detailed assessment of their transcriptome. The first polar body was removed from all oocytes and aneuploidy assessed using comparative genomic hybridization. Preliminary mRNA transcript data were produced with the use of microarrays for seven of the corresponding oocytes (three normal and four aneuploid). The results obtained for normal and aneuploid oocytes were compared and 327 genes were found to display statistically (P < 0.05) significant differences in transcript levels. Ninety-six of these genes were further assessed in seven aneuploid and seven normal oocytes using real-time PCR. The results indicated that aneuploidy is associated with altered transcript levels affecting a subset of genes. A link between mRNA transcript numbers and age was also observed. The possibility that different transcript levels in the oocyte have an impact on cellular pathways remains to be proven. However, it may be significant that some of the highlighted genes produce proteins involved in spindle assembly and chromosome alignment. Additionally, several genes with altered amounts of transcript produce cell surface or excretory molecules, and could potentially serve as targets for non-invasive oocyte aneuploidy assessment.

Characteristics of embryo development in Robertsonian translocations' preimplantation genetic diagnosis cycles

OBJECTIVE

To explore the embryo development characteristics in Robertsonian translocations (RTs) in their preimplantation genetic diagnosis (PGD) cycles.

METHOD

A total of 37 RT carrier couples underwent 41 blastomere PGD cycles from August 2005 to September 2008. The development of 272 embryos was analyzed in their PGD cycles.

RESULT(S)

At D3, there were 161 high-grade embryos, including 59 normal/balanced embryos and 102 abnormal embryos. There was no difference between the normal/balanced embryo group and the abnormal embryo group in terms of the high-grade embryo percentage (64.84% vs 56.35%, p = 0.179). However, at D5-D6, the blastocyst percentage in the normal/balanced embryo group was significantly higher than that in the abnormal embryo group (43.96% vs 20.44%, p = 0.000).

CONCLUSION(S)

Normal/balanced embryos developed better and a self-selective mechanism may exist in the RTs' embryos at the blastocyst formation stage.

Errors at mitotic segregation early in oogenesis and at first meiotic division in oocytes from donor females: comparative genomic hybridization analyses in metaphase II oocytes and their first polar body

The aim of this work is to analyze, using the comparative genomic hybridization technique, the frequencies and the mechanisms involved in the production of aneuploidy events in donor oocytes. The results showed that 32.1% of them were aneuploid, with 51.7% of those originating from first meiotic division errors and 48.3% from the presence of aneuploid oogonium.

Pertinence of apoptosis markers for the improvement of in vitro fertilization (IVF)

In assisted reproductive technology (ART), the pregnancy and birth rates following in vitro fertilization (IVF) attempts are still low. Recently, apoptotic markers have been suggested as new criteria for oocyte and embryo quality selection. Many studies have provided evidence that poor oocyte and embryo quality can be associated with apoptosis. The aim of this review is to summarize our current knowledge on the apoptotic process in oocytes and embryos, and focus on the possibility for using apoptotic markers as a reliable and predictive marker to select competent oocytes and embryos during IVF. Moreover, it is currently accepted that IVF failures, linked to poor embryo quality, are, in part, associated with suboptimal in vitro culture conditions. Here, we also review the current state of knowledge concerning how the genetic control of apoptosis during folliculogenesis and pre-implantation embryonic development is affected by in vitro culture conditions during IVF. In the future, identification of apoptotic markers in ART for oocyte and embryo selection should result in the development of new agonistic or antagonistic molecules of apoptosis by medicinal chemistry.

Faulty spindle checkpoint and cohesion protein activities predispose oocytes to premature chromosome separation and aneuploidy

Aneuploidy accounts for a major proportion of human reproductive failures, mental and physical anomalies, and neoplasms. To heighten our understanding of normal and abnormal chromosome segregation, additional information is needed about the underlying molecular mechanisms of chromosome segregation. Although many hypotheses have been proposed for the etiology of human aneuploidy, there has not been general acceptance of any specific hypothesis. Moreover, it is important to recognize that many potential mechanisms exist whereby chromosome missegregation may occur. One area for investigating aneuploidy centers on the biochemical changes that take place during oocyte maturation. In this regard, recent results have shown that faulty mRNA of spindle-assembly checkpoint proteins and chromosome cohesion proteins may lead to aneuploidy. Also, postovulatory and in vitro aging of mouse oocytes has been shown to lead to decreased levels of Mad2 transcripts and elevated frequencies of premature centromere separation. The intent of this review is to highlight the major events surrounding chromosome segregation and to present the published results that support the premise that faulty chromosome cohesion proteins and spindle checkpoint proteins compromise accurate chromosome segregation.

Oocyte aneuploidy mechanisms are different in two situations of increased chromosomal risk: older patients and patients with recurrent implantation failure after in vitro fertilization

OBJECTIVE

To clarify the mechanisms underlying oocyte abnormalities in meiosis: meiotic nondisjunction of a whole chromosome or premature separation of sister chromatids in two situations of increased chromosomal risk.

DESIGN

Preconception diagnosis by first polar-body analysis in two situations of increased chromosomal risk.

SETTING

Departments of reproductive biology, cytogenetics, gynecology, and obstetrics.

PATIENT(S)

First polar body analysis was proposed to 76 patients (91 cycles) for advanced age (AMA; n = 30, 36 cycles), recurrent implantation failure (RIF; >10 embryos transferred without implantation; n = 32, 36 cycles), or both (AMA + RIF; n = 14, 19 cycles), before their intracytoplasmic sperm injection procedure.

INTERVENTION(S)

First polar-body analysis using fluorescence in situ hybridization.

MAIN OUTCOME MEASURE(S)

Mechanisms and frequency of aneuploidy.

RESULT(S)

Three hundred eighty-four oocytes were analyzed by fluorescence in situ hybridization, 130 from women >38 years of age, 171 from women with RIF, and 83 from women with both indications. The oocyte abnormality rate was similar in the three groups, respectively, 38.5%, 40.4%, and 45.8%. The aneuploidy mechanisms were different for women >38 years of age who had no previous implantation failure (AMA) compared with women of whatever age who had implantation failure (P<.05 vs. RIF; P<.001 vs. AMA+RIF), with, respectively, for the AMA, RIF, and AMA+RIF groups, 72.2%, 56.6%, and 49.2% premature separation of sister chromatids and 27.8%, 43.4%, and 50.8% meiotic nondisjunction. In the two implantation-failure groups, we distinguished a subgroup (22% in the RIF group and 33% in AMA+RIF group) of patients with >2/3 abnormal oocytes, suggesting a meiosis alteration.

CONCLUSION(S)

The mechanisms accounting for oocyte aneuploidy differed in the two clinical situations of advanced maternal age and RIF. Advanced maternal-age aneuploidy was linked to a loss of sister chromatid cohesion that led to one single chromatid abnormality, whereas implantation failure is a much more heterogeneous situation.

Gender effects on the incidence of aneuploidy in mammalian germ cells

Aneuploidy occurs in 0.3% of newborns, 4% of stillbirths, and more than 35% of all human spontaneous abortions. Human gametogenesis is uniquely and gender-specific susceptible to errors in chromosome segregation. Overall, between 1% and 4% of sperm and as many as 20% of human oocytes have been estimated by molecular cytogenetic analysis to be aneuploid. Maternal age remains the paramount aetiological factor associated with human aneuploidy. The majority of extra chromosomes in trisomic offspring appears to be of maternal origin resulting from nondisjunction of homologous chromosomes during the first meiotic division. Differences in the recombination patterns between male and female meiosis may partly account for the striking gender- and chromosome-specific differences in the genesis of human aneuploidy, especially in aged oocytes. Nondisjunction of entire chromosomes during meiosis I as well as premature separation of sister chromatids or homologues prior to meiotic anaphase can contribute to aneuploidy. During meiosis, checkpoints at meiotic prophase and the spindle checkpoint at M-phase can induce meiotic arrest and/or cell death in case of disturbances in pairing/recombination or spindle attachment of chromosomes. It has been suggested that gender differences in aneuploidy may result from more permissive checkpoints in females than males. Furthermore, age-related loss of chromosome cohesion in oocytes as a cause of aneuploidy may be female-specific. Comparative data about the susceptibility of human male and female germ cells to aneuploidy-causing chemicals is lacking. Increases of aneuploidy frequency in sperm have been shown after exposure to therapeutic drugs, occupational agents and lifestyle factors. Conversely, data on oocyte aneuploidy caused by exogenous agents is limited because of the small numbers of oocytes available for analysis combined with potential maternal age effects. The vast majority of animal studies on aneuploidy induction in germ cells represent cause and effect data. Specific studies designed to evaluate possible gender differences in induction of germ cell aneuploidy have not been found. However, the comparison of rodent data available from different laboratories suggests that oocytes are more sensitive than male germ cells when exposed to chemicals that effect the meiotic spindle. Only recently, in vitro experiments, analyses of transgenic animals and knockdown of expression of meiotic genes have started to address the molecular mechanisms underlying chromosome missegregation in mammalian germ cells whereby striking differences between genders could be shown. Such information is needed to clarify the extent and the mechanisms of gender effects, including possible differential susceptibility to environmental agents.

Pairing and synapsis in oocytes from female fetuses with euploid and aneuploid chromosome complements

Only little is known about the meiotic prophase events in human oocytes, although some of them are involved in the origin of aneuploidies. Here, a broad study of the pairing and synaptic processes in 3263 human euploid and 2613 aneuploid oocytes (47,XX, +21 and 47,XX, +13), using different techniques and methods, is presented in order to elucidate the characteristics of this essential meiotic process. Our results reaffirm the existence of a common high efficiency in the pairing process leading to the obtainment of a bivalent for all chromosomes studied in euploid and aneuploid cases. Nevertheless, this high efficiency was insufficient to consistently produce trivalents in aneuploid oocytes. Trivalent 21 was only observed in 48.8% of the 47,XX, +21 pachytene-stage oocytes studied, and trivalent 13 was found in 68.7% of the 47,XX, +13 pachytene-stage oocytes analyzed. Our data confirm the hypothesis which suggests that in human oocytes the presence of an extra chromosome could interfere in bouquet dynamics. In addition, the pairing process of the X chromosome is altered in trisomic 21 oocytes, providing evidence of the influence that an extra chromosome 21 may cause meiotic progression.

Identification of chromosomal fusion sites in Arabidopsis mutants using sequential bicolour BAC-FISH

Double stranded chromosomal breaks are repaired by homologous recombination or nonhomologous end joining (NHEJ). When broken chromosome ends are fused together by NHEJ, the resulting dicentric chromosomes can be detected as anaphase bridges during the subsequent mitosis. Telomeres in the absence of functional telomerase shorten, became unprotected, and are eventually recognized by the cell repair system as double stranded breaks. As result, chromosomes of Arabidopsis thaliana plants that are deficient in the gene for telomerase reverse transcriptase (TERT) are prone to chromosome fusions. We use Arabidopsis tert-/- mutants as a model system for analyzing terminal chromosome fusions. Here we report a novel and sensitive cytogenetic assay for the identification and characterization of chromosome-terminal fusion events by employing fluorescence in situ hybridization (FISH) with multiple probes and a repeated hybridization approach. A mixture of chromosome-specific subtelomeric probes is applied successively in 3 FISH reactions to the slides containing mitotic anaphase figures with anaphase bridges. Each figure is registered by a CCD camera after each in situ hybridization procedure. By comparing the signals presented on the bridge in successive images the assessment of the particular chromosome arms involved in fusion is possible. This experimental setup enables unambiguous identification of individual chromosome ends employed in fusion events.

Principal findings from a multicenter trial investigating the safety of follicular-fluid meiosis-activating sterol for in vitro maturation of human cumulus-enclosed oocytes

OBJECTIVE

To evaluate the safety of applying follicular-fluid meiosis-activating sterol (FF-MAS) in vitro to immature human oocytes.

DESIGN

Phase I bicenter, randomized, parallel-group, controlled, partially blinded trial.

SETTING

Third-level referral academic centers, including reproductive biology and genetics laboratories.

PATIENTS

Endocrinologically normal women with a medical indication for IVF or intracytoplasmic sperm injection, or healthy volunteers.

INTERVENTION(S)

Subjects were randomized at a ratio 1 to 6 into either conventional GnRH-agonist and recombinant FSH stimulation (IVO) for oocyte retrieval, or minimally stimulated in vitro maturation (IVM) with the use of recombinant FSH. Retrieved immature oocyte cumulus complexes were cultured for 30 or 36 hours in one of six IVM culture conditions containing FF-MAS (range, 0.1-20 microM). Polar body-extruded oocytes from the IVO and IVM groups were processed for chromosomal analysis.

MAIN OUTCOME MEASURE(S)

The primary endpoint was the incidence of metaphase II stage oocytes with numeric chromosomal abnormalities, using full (spectral karyotyping) or partial (fluorescent in situ hybridization with seven probes) karyotyping or Giemsa count. A secondary objective was to document the frequency of metaphase II oocytes after IVM with FF-MAS supplements.

RESULT(S)

Oocyte cumulus complexes obtained from the IVO (mean, 8.9) and IVM (mean, 6.2) groups had equal maturation rates. Compared to IVO, exposure of germinal-vesicle oocytes for a maturation period of 30 hours did not increase aneuploidy. An exposure period of 36 hours doubled the aneuploidy rate, but this was significant only for the 20-muM dose of FF-MAS.

CONCLUSION

Inclusion of 1-10 microM FF-MAS in a 30-hour IVM protocol is safe.

Maternal age-related differential global expression profiles observed in human oocytes

The age-related decline in female fertility has been attributed to a variety of causes including progressive oocyte depletion, meiotic irregularities and mitochondrial dysfunction. However, additional factors could potentially be involved. To explore this possibility, comprehensive analysis of gene expression in human oocytes, discarded following IVF procedures and segregated by age, was undertaken using microarray methods. These findings indicate that the expression of oocyte genes, in a variety of major functional categories including cell cycle regulation, cytoskeletal structure, energy pathways, transcription control, and stress responses, are influenced by maternal age. These results are corroborated by a complementary extensive study using mouse oocytes.

The contradictory information on the distribution of non-disjunction and pre-division in female gametes

Valuable information on the cytogenetic constitution of female gametes has been deduced from the direct, so-called conventional analysis of oocytes remaining unfertilized in programmes of assisted reproduction. Additional, indirect conclusions have become possible by PGD of the polar bodies. Both techniques provided evidence for the co-existence of two aneuploidy-causing mechanisms during first maternal meiosis; non-disjunction (ND) of bivalents results in the loss or gain of whole chromosomes in metaphase II complements, whereas a precocious division (pre-division, PD) of univalents leads to the loss or gain of single chromatids. As to the distribution of ND and PD, however, direct oocyte chromosome studies and PGD tell surprisingly different stories. Moreover, first and second polar body analyses contradict the data derived from DNA polymorphism studies concerning the distribution of first and second meiotic division errors. An increased awareness of these problems appears necessary because important decisions are made on the basis of PGD results.

Increased susceptibility to maternal aneuploidy demonstrated by comparative genomic hybridization analysis of human MII oocytes and first polar bodies

Single cell comparative genomic hybridization (CGH) was employed to extensively investigate 24 unfertilized or in vitromatured meiosis II oocytes and their corresponding first polar bodies (PBs), to determine how and whether all 23 chromosomes participate in female meiosis I errors and to accurately estimate the aneuploidy rate in the examined cells. Results were obtained for 15 oocytes and 16 PBs, representing 23 eggs (MII oocyte-PB complexes) donated from 15 patients (average age 32.2 years). Abnormalities were detected in ten eggs, giving an overall aneuploidy rate of 43.5%. In all, fourteen anomalies were scored, with the fertilized oocyte being at risk of monosomy in eight cases and at risk of trisomy in six; chromosomes of various sizes participated. CGH was able to give a comprehensive aneuploidy rate, as both absence of chromosomal material and the presence of extra copies were accurately scored. The aneuploidy mechanisms determined were: classical whole univalent non-disjunction; chromatid predivision prior to anaphase I, leading to metaphase II imbalance. There was also evidence of germinal mosaicism for a trisomic cell line. Three patients appeared to be predisposed to meiosis I errors, based on the presence of either multiple abnormalities in one or more of their examined cells, or of the same type of abnormality in all of their cells. Exclusion of these susceptible patients reduces the aneuploidy rate to 20%. Various hypotheses are put forward to explain these observations in order to stimulate research into the complex nature of female meiotic regulation.

Comparative maturation of cynomolgus monkey oocytes in vivo and in vitro

BACKGROUND

In vitro maturation (IVM) of oocytes followed by fertilization in vitro (IVF) and embryo transfer offers an alternative to conventional IVF treatment that minimises drug administration and avoids ovarian hyperstimulation. However, the technique is less efficient than maturation in vivo. In the present study, a non-human primate model was used to address the hypothesis that the number of oocytes is increased and their nuclear and cytoplasmic maturity after IVM are improved when maturation is initiated in vivo by priming with hCG.

METHODS

Young, adult cynomolgus monkeys were given recombinant human (rh) gonadotropins to stimulate the development of multiple follicles, and oocytes were aspirated 0, 12, 24, or 36 h after injection of an ovulatory dose of rhCG. The nuclear status of oocytes was determined at the time of recovery and after culture for a total elapsed time of 40-44 hours after hCG.

RESULTS

Priming with hCG significantly increased the number of oocytes harvested, especially after delaying aspiration for 24 h or longer. Nuclear maturation after the full period in culture was also enhanced by priming: 71.5, 83.6, and 94.6% of oocytes collected at 0, 12, and 24 h hCG had progressed to MII by the end of the culture period, compared to 87.8% of oocytes that were retrieved at 36 h. A large proportion of oocytes reaching the MII stage had either or both abnormal spindles (>40%) and misaligned chromosomes (>60%), judging by immunofluorescence microscopy, but these abnormalities were independent of culture time. The mitochondria were evenly distributed throughout the cytoplasm at all stages of maturation. Importantly, there was no microscopic evidence that the duration of culture had any injurious effects on the cells.

CONCLUSION

In conclusion, the evidence supports this non-human primate as a model for human IVM and the practice of priming with hCG to promote developmental potential.

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.

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.

Embryo aneuploidy screening for repeated implantation failure and unexplained recurrent miscarriage

Among other factors, chromosomal abnormalities that originate from gametogenesis and preimplantation embryonic development are thought to be one of the major contributing factors for early embryonic death and failure of pregnancy. However, so far, no non-invasive technique exists that allows the detection of the chromosomal complement of an oocyte or a developing embryo as a whole. Rather, by removing polar bodies/blastomeres, recent developments on preimplantation genetic diagnosis for aneuploidy screening (PGD-AS) have paved the way to detect and possibly eliminate the majority of chromosomally abnormal embryos, thereby increasing the chance of a healthy pregnancy. This article summarizes the origin and impact of chromosomal abnormalities on human reproduction in cases with repeated implantation failure (RIF) and unexplained recurrent miscarriage. It also discusses recent advances regarding the possible benefits of PGD-AS in such cases.

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.

Embryogenesis of chimeras, twins and anterior midline asymmetries

Human spontaneous chimerism, with one body built from cells of both twins of a dizygotic (DZ) pair, is supposed to be extremely rare, arising from the exchange of blood cells through placental anastomoses. Mosaicism is supposed to be far more common, arising from single zygotes by embryonic mutation. Because typical diagnosis of mosaicism can neither identify nor exclude chimerism, 'mosaicism' may often be chimerism undiscovered. Evidence shows chimerism arises primarily from DZ embryo fusion and is not rare, although it has negligible probability under the hypothesis of independent double ovulation and independent embryogenesis. If, instead, DZ twin embryos begin development as a single cell mass, chimerism is likely. This would be consistent with observations that DZ twins develop as differently from singletons as monozygotic twins do with regard to embryogenic establishment of asymmetries of midline neural-crest-driven structures of brain, face and heart. Chimerism is a significant component of human embryonic development that deserves closer attention as a mechanism of developmental variation. The 'common knowledge' understanding of twinning mechanisms is at best inadequate. The importance of the difference lies in what we can learn from chimerism about human embryogenesis and the cellular origins of structures and functions basic to the business of becoming human.

Post-ovulatory aging of mouse oocytes leads to decreased MAD2 transcripts and increased frequencies of premature centromere separation and anaphase

Numerous cytological and biochemical alterations occur as mammalian oocytes age post-ovulation. Some of these changes can predispose cells to aneuploidy. The objective of this study was to test the hypothesis that the level of MAD2 spindle assembly checkpoint (SAC) transcripts decrease as mouse oocytes age post-ovulation and that this decrease was associated with chromosome missegregation. Female Institute of Cancer Research (ICR) mice were superovulated and oocytes collected at 14 h, 19 h and 24 h post-HCG for cytogenetic and quantitative real-time rapid cycle fluorescent RT-PCR analyses. Premature centromere separation (PCS) is now generally recognized as a predisposition to aneuploidy. The data showed that the frequencies of PCS-incomplete (PCS-I) did not significantly (P > 0.05) increase with time post-ovulation; whereas the proportions of oocytes displaying PCS-complete (PCS-C) and premature anaphase (PA) were significantly (P < 0.01) greater at 19 h and 24 h post-HCG, respectively. The higher frequencies of PCS-C and PA found at 19 h and 24 h coincided with decreased levels of MAD2 transcripts at these same times. Although the decline in MAD 2 transcripts with oocyte aging represents only one of many potential mechanisms responsible for aneuploidy, a compromised SAC appears to have a role in the unfavourable reproductive outcome associated with post-ovulatory aged oocytes.

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.

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.

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.

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.

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.

Aneuploidy 12 in a Robertsonian (13;14) carrier: Case report

In translocation carriers, the presence of aneuploidy for the chromosomes unrelated to the rearrangement may lead to an additional risk of abnormal pregnancy or implantation failure. Consequently, it may be important to analyse not only the chromosomes involved in the rearrangement but also the rest of chromosomes. We combined spectral karyotyping (SKY) and comparative genomic hybridization (CGH) to karyotype one unfertilized oocyte and its first polar body (1PB) from a Robertsonian translocation carrier t(13;14) aged 29 years who was undergoing IVF and preimplantation genetic diagnosis (PGD) for translocations and aneuploidy screening. Two out of four embryos were aneuploid, as a result of an adjacent segregation. The unfertilized oocyte had a normal/ balanced constitution of the chromosomes involved in the reorganization. However, this 1PB-metaphase II doublet was aneuploid for chromosome 12, the oocyte being hyperhaploid (24, X, +12) and its 1PB hypohaploid (22, X, -12). The application of CGH for the study of Robertsonian translocations of maternal origin will be useful to study imbalances of the chromosomes involved in the rearrangement, as well as alterations in the copy number of any other chromosome. The combination of PGD for translocations with aneuploidy screening could help to reduce the replacement of chromosomally abnormal embryos.

Cytogénétique des ovocytes humains : 40 ans de progrès

Chromosomal abnormalities account for the majority of pre- and post- implantation embryo wastage in humans. Most of these abnormalities result from maternal meiotic errors, which preferentially occur during the first meiotic division. Consequently, the cytogenetic analysis of human oocytes has then been considered as a highly valuable source of data for the investigation of both the occurrence and the origin of chromosomal abnormalities in human. During the last 4 decades, the cytogenetic analysis of human oocytes has never stopped progressing, 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 non-disjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidy in human oocytes has also been clearly evidenced by recent reports based on large sample of oocytes or polar bodies.

Meiotic and mitotic nondisjunction: lessons from preimplantation genetic diagnosis

Direct testing of the outcome of the first and second meiotic divisions has become possible with the introduction of preimplantation genetic diagnosis (PGD) for aneuploidies. Testing of oocytes by fluorescent in situ hybridization (FISH) analysis of the first and second polar bodies showed that more than half of oocytes from the IVF patients aged 35 years and older had chromosomal abnormalities, which originated from errors in meiosis I or meiosis II, or both: 41.9% of oocytes were aneuploid after meiosis I and 37.3% aneuploid after meiosis II, with 29.1% of these oocytes having both meiosis I and meiosis II errors. As a result, one third of oocytes detected as normal after meiosis I contained the meiosis II errors, and two thirds of those with meiosis II errors were already abnormal following meiosis I. Although the rates of chromosomal abnormalities deriving from meiosis I and II were comparable, meiosis I errors predominantly resulted in extra chromosome (chromatid) material in oocytes, in contrast to a random distribution of extra and missing chromatids after meiosis II. The majority of meiosis I abnormalities were represented by chromatid errors, which seem to be the major source of chromosomal abnormalities in the resulting embryos. Approximately one third of aneuploid oocytes deriving from sequential errors in the first and second meiotic divisions resulted in a balanced karyotype, representing a possible phenomenon of "aneuploidy rescue" during the second meiotic division. However, the majority of the embryos resulting from such oocytes appeared to be abnormal for the same or different chromosome(s), or were mosaic, suggesting a possible predisposition of the resulting embryos to further mitotic errors. Although the origin of a high frequency of mosaicism at the cleavage stage is not sufficiently understood, the mosaic embryos may originate from the chromosomally abnormal oocytes, as a result of a "trisomy rescue" mechanism during the first mitotic divisions, which renders polar body FISH analysis to have important clinical value for reliable pre-selection of aneuploidy-free embryos for transfer.

Sperm chromosomal abnormalities in patients with unexplained recurrent abortions

Cytogenetic studies showed that about half of concepti were chromosomally abnormal in first trimester abortions. Sperm chromosomal abnormalities in men with normal karyotype could occur during spermatogenesis. The objective of this study was to evaluate sperm chromosomal abnormalities in patients with unexplained recurrent abortions. A total of 14 couples with normal karyotype, and negative workup for endocrine, immune and anatomical causes of recurrent abortion was investigated. Semen analysis was performed and chromosomal abnormalities were assessed by fluorescent in situ hybridization for chromosomes 13, 18, 21, X and Y. The average number of abortions was 5.8. The incidence of chromosomal abnormalities was 16.5% that was higher when compared to baseline (4.6%). In conclusion, a high rate of sperm chromosomal abnormalities was observed in recurrent abortion patients. These abnormalities might form during spermatogenesis since all patients had normal karyotype. Sperm chromosomal abnormality analysis can be included into recurrent abortion workup when no other cause is detected.

Complete cytogenetic investigation of oocytes from a young cancer patient with the use of comparative genomic hybridisation reveals meiotic errors

OBJECTIVES

The complete cytogenetic investigation of human oocytes and the corresponding first polar bodies (PBs) derived from an 18-year old female cancer patient.

METHODS

A whole-genome amplification method combined with comparative genomic hybridisation (CGH) was employed for the analysis of 14 oocytes and their corresponding first PBs.

RESULTS

Chromosome abnormalities were detected in two oocyte-PB complexes. One oocyte had lost X-chromosome material (23,X,-Xcht), while its corresponding first PB showed the reciprocal gain (23,X,+Xcht). Double aneuploidy involving loss of chromatids for chromosomes X and 21 was identified in another first PB (23,X,-21cht,-Xcht). Aneuploidy was attributed to unbalanced pre-division of chromatids at meiosis I.

CONCLUSIONS

Meiotic errors in chromosome segregation can occur even in oocytes derived from young women, confirming the existence of age-independent factors contributing to aneuploidy. Such factors are of relevance to fertility, miscarriage and preimplantation aneuploidy screening for the purposes of increasing IVF success rates. The reliability of CGH in examining the whole chromosome complement of a single cell and of being able to detect chromatid anomalies is confirmed by this study.

Aneuploidy study of human oocytes first polar body comparative genomic hybridization and metaphase II fluorescence in situ hybridization analysis

BACKGROUND

The object of this study was to determine the mechanisms that produce aneuploidy in oocytes and establish which chromosomes are more prone to aneuploidy.

METHODS

A total of 54 oocytes from 36 women were analysed. The whole chromosome complement of the first polar body (1PB) was analysed by comparative genomic hybridization (CGH), while the corresponding metaphase II (MII) oocyte was analysed by fluorescence in situ hybridization (FISH) to confirm the results.

RESULTS

Matched CGH-FISH results were obtained in 42 1PB-MII doublets, of which 37 (88.1%) showed reciprocal results. The aneuploidy rate was 57.1%. Two-thirds of the aneuploidy events were chromatid abnormalities. Interestingly, the chromosomes more frequently involved in aneuploidy were chromosomes 1, 4 and 22 followed by chromosome 16. In general, small chromosomes (those equal to or smaller in size than chromosome 13) were more prone to aneuploidy (chi2-test, P=0.07); 25% of the aneuploid doublets would have been misdiagnosed as normal using FISH with probes for nine-chromosomes.

CONCLUSIONS

The combination of two different techniques, CGH and FISH, for the study of 1PB and MII allowed the identification and confirmation of any numerical chromosome abnormality, as well as helping to determine the mechanisms involved in the genesis of maternal aneuploidy.