Array comparative genomic hybridization analyses of all blastomeres of a cohort of embryos from young IVF patients revealed significant contribution of mitotic errors to embryo mosaicism at the cleavage stage

Assessing the necessity of screening ≤5 Mb segmental aneuploidy in routine PGT for aneuploidies

RESEARCH QUESTION

Does routine clinical practice require an increase in the resolution of preimplantation genetic testing for aneuploidies (PGT-A) to detect segmental aneuploidies ≤5 Mb?

DESIGN

This retrospective study analysed 963 trophectoderm biopsies from 346 couples undergoing PGT between 2019 and 2023. Segmental aneuploidies ≥1 Mb were reported. The characteristics, clinical interpretation and concordance of segmental aneuploidies ≤5 Mb were analysed.

RESULTS

The incidence of segmental aneuploidies was 15.1% (145/963) in blastocysts, with segmental aneuploidies of ≤5 Mb accounting for 2.3% (22/963). The size of the segmental aneuploidies showed a skewed distribution. Segmental aneuploidies ≤5 Mb were found to occur more frequently on the q arm of the chromosome, compared with the p arm. Losses of ≤5 Mb segmental aneuploidies were more prevalent than gains, with 17 deletions compared with 5 duplications. Of the segmental aneuploidies, 63.6% (14/22) ≤5 Mb were de novo, and 50.0% (7/14) of de-novo segmental aneuploidies were pathogenic/likely pathogenic (P/LP) copy number variations, accounting for 0.7% of 963 blastocysts. For blastocysts carrying ≤5 Mb segmental aneuploidies, a re-analysis of back-up biopsy samples showed that 35.7% of de-novo segmental aneuploidies (5/14) were not detected in the back-up samples. Cases were reported in which prenatal diagnosis (amniocentesis) revealed the absence of embryonic ≤5 Mb segmental aneuploidies detected at the blastocyst stage.

CONCLUSIONS

The incidence of P/LP de-novo ≤5 Mb segmental aneuploidies in human blastocysts is extremely low. There is no compelling need to increase the resolution of PGT-A to 5 Mb in routine clinical practice.

The impact of (very) young donor age on euploid rates: An analysis of 1831 trophectoderm biopsies evaluated with 24-chromosome NGS screening in oocyte donation cycles

RESEARCH QUESTION

Conflicting data exists regarding whether a younger age of donors has a negative influence on the outcomes of oocyte donation cycles. Is there any correlation between a younger age of donors and the rate of embryonic aneuploidy in oocyte donation cycles?

DESIGN

Retrospective study including 515 oocyte donation cycles carried out between February 2017 and November 2022. Comprehensive chromosomal screening was performed on 1831 blastocysts. 1793 had a result which were categorised into groups based on the age of the donor: 18-22 (n = 415), 23-25 (n = 600), 26-30 (n = 488), and 31-35 years (n = 290). The analysis aimed to determine the percentage of biopsy samples that were euploid and the number that were aneuploid, relative to the age group of the oocyte donor. Additionally, linear regression was employed to examine the relationship between age and the proportion of aneuploid embryos, while controlling for relevant variables.

RESULTS

Aneuploidy increased predictably with donor age: 18-22 years: 27.5 %; 23-25 years: 31.2 %; 26-30 years: 31.8 %; and 31-35 years: 38.6 %. In the donor group aged 31-35 years, a higher percentage of aneuploid embryos was observed compared to younger donors in univariate analysis (OR: 1.66, 95 % CI: 1.21-2.29, p = 0.002) and multivariate logistic analysis (OR: 2.65, 95 % CI: 1.67-4.23, p < 0.001). The rates of embryonic mosaicism revealed no significant differences.

CONCLUSION

The lowest risk of embryonic aneuploidy was found among donors aged <22 years. Conversely, an elevated prevalence was evident within the donor group aged 31-35 years, in contrast to the younger cohorts. The incidence of mosaic embryos remained consistent across all age groups.

Mechanism of chromosomal mosaicism in preimplantation embryos and its effect on embryo development

Aneuploidy is one of the main causes of miscarriage and in vitro fertilization failure. Mitotic abnormalities in preimplantation embryos are the main cause of mosaicism, which may be influenced by several endogenous factors such as relaxation of cell cycle control mechanisms, defects in chromosome cohesion, centrosome aberrations and abnormal spindle assembly, and DNA replication stress. In addition, incomplete trisomy rescue is a rare cause of mosaicism. However, there may be a self-correcting mechanism in mosaic embryos, which allows some mosaicisms to potentially develop into normal embryos. At present, it is difficult to accurately diagnose mosaicism using preimplantation genetic testing for aneuploidy. Therefore, in clinical practice, embryos diagnosed as mosaic should be considered comprehensively based on the specific situation of the patient.

To transfer or not to transfer: the dilemma of mosaic embryos - a narrative review

A frequent finding after preimplantation genetic diagnostic testing for aneuploidies using next-generation sequencing is an embryo that is putatively mosaic. The prevalence of this outcome remains unclear and varies with technical and external factors. Mosaic embryos can be classified by the percentage of cells affected, type of chromosome involvement (whole or segmental), number of affected chromosomes or affected cell type (inner mass cell, trophectoderm or both). The origin of mosaicism seems to be intrinsic as a post-zygotic mitotic error, but some external factors can play a role. As experience has increased with the transfer of mosaic embryos, clinical practice has gradually become more flexible in recent years. Nevertheless, clinical results show lower implantation, pregnancy and clinical pregnancy rates and higher miscarriage rates with mosaic embryo transfer when compared with the transfer of euploid embryos. Prenatal diagnosis is highly recommended after the transfer of mosaic embryos. This narrative review is intended to serve as reference material for practitioners in reproductive medicine who must manage a mosaic embryo result after preimplantation genetic testing for aneuploidies.

Insights into embryonic chromosomal instability: mechanisms of DNA elimination during mammalian preimplantation development

Mammalian preimplantation embryos often contend with aneuploidy that arose either by the inheritance of meiotic errors from the gametes, or from mitotic mis-segregation events that occurred following fertilization. Regardless of the origin, mis-segregated chromosomes become encapsulated in micronuclei (MN) that are spatially isolated from the main nucleus. Much of our knowledge of MN formation comes from dividing somatic cells during tumorigenesis, but the error-prone cleavage-stage of early embryogenesis is fundamentally different. One unique aspect is that cellular fragmentation (CF), whereby small subcellular bodies pinch off embryonic blastomeres, is frequently observed. CF has been detected in both in vitro and in vivo-derived embryos and likely represents a response to chromosome mis-segregation since it only appears after MN formation. There are multiple fates for MN, including sequestration into CFs, but the molecular mechanism(s) by which this occurs remains unclear. Due to nuclear envelope rupture, the chromosomal material contained within MN and CFs becomes susceptible to double stranded-DNA breaks. Despite this damage, embryos may still progress to the blastocyst stage and exclude chromosome-containing CFs, as well as non-dividing aneuploid blastomeres, from participating in further development. Whether these are attempts to rectify MN formation or eliminate embryos with poor implantation potential is unknown and this review will discuss the potential implications of DNA removal by CF/blastomere exclusion. We will also extrapolate what is known about the intracellular pathways mediating MN formation and rupture in somatic cells to preimplantation embryogenesis and how nuclear budding and DNA release into the cytoplasm may impact overall development.

Clinical management of mosaic results from preimplantation genetic testing for aneuploidy of blastocysts: a committee opinion

This revised document incorporates a growing number of published studies about mosaic embryo transfer and provides current evidence-based considerations for the clinical management of embryos with mosaic results on preimplantation genetic testing for aneuploidy. This document replaces the document titled "Clinical management of mosaic results from preimplantation genetic testing for aneuploidy (PGT-A) of blastocysts: a committee opinion," published in 2020 (Fertil Steril 2020;114:246-54).

On the origins and fate of chromosomal abnormalities in human preimplantation embryos: an unsolved riddle

About 8 out of 10 human embryos obtained in vitro harbour chromosomal abnormalities of either meiotic or mitotic origin. Abnormalities of mitotic origin lead to chromosomal mosaicism, a phenomenon which has sparked much debate lately as it confounds results obtained through preimplantation genetic testing for aneuploidy (PGT-A). PGT-A in itself is still highly debated, not only on the modalities of its execution, but also on whether it should be offered to patients at all.

We will focus on post-zygotic chromosomal abnormalities leading to mosaicism. First, we will summarize what is known of the rates of chromosomal abnormalities at different developmental stages. Next, based on the current understanding of the origin and cellular consequences of chromosomal abnormalities, which is largely based on studies on cancer cells and model organisms, we will offer a number of hypotheses on which mechanisms may be at work in early human development. Finally, and very briefly, we will touch upon the impact our current knowledge has on the practice of PGT-A. What is the level of abnormal cells that an embryo can tolerate before it loses its potential for full development? And is blastocyst biopsy as harmless as it seems?

Cellular and Molecular Nature of Fragmentation of Human Embryos

Embryo fragmentation represents a phenomenon generally characterized by the presence of membrane-bound extracellular cytoplasm into the perivitelline space. Recent evidence supports the cellular and molecular heterogeneity of embryo fragments. In this narrative review, we described the different embryo fragment-like cellular structures in their morphology, molecular content, and supposed function and have reported the proposed theories on their origin over the years. We identified articles related to characterization of embryo fragmentation with a specific literature search string. The occurrence of embryo fragmentation has been related to various mechanisms, of which the most studied are apoptotic cell death, membrane compartmentalization of altered DNA, cytoskeletal disorders, and vesicle formation. These phenomena are thought to result in the extrusion of entire blastomeres, release of apoptotic bodies and other vesicles, and micronuclei formation. Different patterns of fragmentation may have different etiologies and effects on embryo competence. Removal of fragments from the embryo before embryo transfer with the aim to improve implantation potential should be reconsidered on the basis of the present observations

A novel trophectoderm biopsy technique for all blastocyst stages

Purpose

This study was conducted to assess the effectiveness of a new trophectoderm (TE) biopsy method that does not require prior opening of the zona pellucida at the blastocyst stage.

Methods

TE biopsy was conducted using a modified extrusion method for embryos during the cleavage stage. In this method, culture medium was injected into the perivitelline space to help extrude TE cells from the zona pellucida before TE biopsy.

Results

Our extrusion method preserves the embryo culture environment until immediately before biopsy because it does not require opening of the zona pellucida prior to TE biopsy. Furthermore, this method does not require a waiting time for blastocyst hatching after laser irradiation, thereby minimizing damage to the embryos and maintaining the time schedule of culture operations.

Conclusions

TE biopsy using this novel extrusion method may be useful in various applications, including the collection of TE cells for next-generation sequencing analysis.

From contemplation to classification of chromosomal mosaicism in human preimplantation embryos

Chromosomal mosaicism is a hallmark of early human embryo development. The last decade yielded an enormous amount of information about diversity and prevalence of mosaicism in preimplantation embryos due to progress in preimplantation genetic testing of aneuploidies (PGT-A) based exclusively on molecular karyotyping of trophectoderm biopsy. However, the inner cell mass karyotype is still missing for mosaic embryos affecting the success rate of assisted reproductive medicine. Here, a classification model of chromosomal mosaicism is proposed based on the analysis of the primary zygote karyotype, the timing and types of primary and secondary chromosome segregation errors, and the distribution of mosaic cell clones between different embryonic and extraembryonic compartments of the blastocyst. Five basic principles for mosaicism analysis are introduced, namely, the estimation of the primary zygote karyotype, the investigation of additional sample point, the requirement of the second time point analysis, the delineating of reciprocity of chromosome segregation, and comprehensive chromosome screening at the single-cell level. The suggested model allows the prediction of the inner cell mass karyotype of the blastocyst and its developmental potential based on information from trophectoderm biopsy and non-invasive PGT-A using blastocoele fluid sample or spent culture medium as additional sample and time points for analysis and considering the reciprocity as a basic process in chromosome segregation errors between daughter cells in postzygotic cell divisions.

Clinical management of mosaic results from preimplantation genetic testing for aneuploidy (PGT-A) of blastocysts: a committee opinion

Since the advent of preimplantation genetic testing for aneuploidy (PGT-A) in the 1990s, substantial changes in test methodology and technology now allow the detection and reporting of intermediate chromosome copy number (commonly referred to as mosaicism) for aneuploidy in a trophectoderm biopsy sample. Clinicians are grappling with how to interpret such findings and how to counsel patients about embryo transfer decision-making. This document reviews the available literature and outlines the various issues surrounding the reporting of intermediate copy number and consideration of storage or transfer of blastocysts with intermediate copy number results. This document does not endorse, nor does it suggest that PGT-A is appropriate for all cases of in vitro fertilization.

Mitotic gene conversion can be as important as meiotic conversion in driving genetic variability in plants and other species without early germline segregation

In contrast to common meiotic gene conversion, mitotic gene conversion, because it is so rare, is often ignored as a process influencing allelic diversity. We show that if there is a large enough number of premeiotic cell divisions, as seen in many organisms without early germline sequestration, such as plants, this is an unsafe position. From examination of 1.1 million rice plants, we determined that the rate of mitotic gene conversion events, per mitosis, is 2 orders of magnitude lower than the meiotic rate. However, owing to the large number of mitoses between zygote and gamete and because of long mitotic tract lengths, meiotic and mitotic gene conversion can be of approximately equivalent importance in terms of numbers of markers converted from zygote to gamete. This holds even if we assume a low number of premeiotic cell divisions (approximately 40) as witnessed in Arabidopsis. A low mitotic rate associated with long tracts is also seen in yeast, suggesting generality of results. For species with many mitoses between each meiotic event, mitotic gene conversion should not be overlooked.

Gene conversion associated with meiosis has long been a focus of attention in population genomics, but mitotic conversion has been relatively overlooked as it was thought to be rare. Analysis in plants suggests that this could be a mistake; long tract lengths and multiple mitoses in species lacking germline sequestration suggest that mitotic conversion, although rare per mitosis, should not be ignored.

Segmental duplications and monosomies are linked to in vitro developmental arrest

PURPOSE

To verify which genetic abnormalities prevent embryos to blastulate in a stage-specific time.

METHODS

A single center retrospective study was performed between April 2016 and January 2017. Patients requiring Preimplantation Genetic Testing for Aneuploidies (PGT-A) by Next Generation Sequencing (NGS) were included. All embryos were cultured in a time-lapse imaging system and single blastomere biopsy was performed on day 3 of development. Segmental duplications and deletions as well as whole chromosome monosomies and trisomies were registered. Embryo arrest was defined if the embryo failed to blastulate 118 h post-injection. A logistic regression model was applied using the time to blastulate as the response variable and the different mutations as explanatory variables. A p value < 0.05 was considered significant.

RESULTS

Of the 285 biopsied cleavage stage embryos, 103 (36.1%) were euploid, and 182 (63.9%) were aneuploid. There was a significant difference in the developmental arrest between euploid and aneuploid embryos (8.7% versus 42.9%; p = 0.0001). Segmental duplications and whole chromosome monosomies were found to have a significant effect on developmental arrest (p = 0.0163 and p = 0.0075), while trisomies and segmental deletions had no effect on developmental arrest. In case of segmental duplications, an increase of one extra segmental duplication increases the odd of arrest by 159%. For whole chromosome monosomies, the odd will only increase by 29% for every extra chromosomal monosomy. Both chromosomal abnormalities remained significant after adding age as an explanatory variable to the model (p = 0.014 and p = 0.009).

CONCLUSION

Day 3 cleavage stage embryos with segmental duplications or monosomies have a significantly decreased chance to reach the blastocyst stage.

Prevalence, types and possible factors influencing mosaicism in IVF blastocysts: results from a single setting

RESEARCH QUESTION

Are intrinsic or extrinsic factors associated with embryo mosaicism prevalence in IVF cycles?

DESIGN

Retrospective cohort study of preimplantation genetic testing for aneuploidy (PGT-A) cycles carried out at a university-affiliated IVF clinic between October 2017 and October 2019. Trophectoderm biopsies were analysed by next generation sequencing. Mosaicism prevalence, type of anomaly and the chromosomes involved were analysed. Intrinsic and extrinsic factors potentially inducing mosaicism were studied: maternal and paternal age, antral follicle count, cumulus-oocyte complexes retrieved, female body mass index, PGT-A indication, sperm concentration, total dosage of gonadotrophins, embryo quality and day of blastocyst formation, single-step commercial media used and biopsy operator.

RESULTS

Overall prevalence of mosaicism in our PGT-A setting was 13.9%. In segmental mosaicism, larger chromosomes tended to be more affected, which was not observed in whole-chromosome mosaicism. Additionally, segmental mosaicism was mostly observed in monosomy (69.6%; P < 0.01) compared with whole-chromosome mosaicism (49.7% monosomies versus 50.3% trisomies; P = 0.83). Although a high inter-patient variability was observed, only paternal age showed a positive association with mosaicism (adjusted OR 1.26, 95% CI 1.02 to 1.54) among the analysed variables.

CONCLUSIONS

Our results suggest remarkable differences in the mechanisms generating segmental and whole-chromosome mosaicism, indicating that they may deserve different consideration when studying them and when prioritizing them for transfer. Male factor seems to be associated with mosaicism and may be worthy of specific assessment in future studies.

Next-generation sequencing analysis of each blastomere in good-quality embryos: insights into the origins and mechanisms of embryonic aneuploidy in cleavage-stage embryos

PURPOSE

To explore the whole-chromosome status, origins, and mechanisms of chromosomal abnormalities in good-quality cleavage embryos using multiple annealing and looping-based amplification cycle (MALBAC) sequencing.

METHODS

The embryos studied came from7 patients (maternal aged 26-35) who had healthy birth from the same IVF cycles. These 21 frozen day 3 good-quality embryos were thawed and disaggregated into individual blastomere. Each blastomere was collected and analyzed by MALBAC sequencing.

RESULTS

Conclusive results were obtained from a high percentage of blastomeres (95.3%). A total of 46.6% of blastomeres were diploid, 53.4% were abnormal, and 28.0% had complex aneuploidy. Out of 21 embryos, 3 (14.3%) were normal and 18 (85.7%) were mosaics, showing the occurrence of mitotic errors; aneuploidy was confirmed in all cells of 4 of the 18 embryos, which showed the coexistence of meiotic errors. Conclusive results were obtained from all blastomeres of 15 embryos (71.4%, 15/21), which enabled us to reconstruct the cell lineage on the basis of the chromosomal content of the blastomeres in each division. There were 9 mitotic errors (8.7%, 9/103): nondisjunction accounted for 88.9% (8/9), and endoreplication accounted for 11.1% (1/9).

CONCLUSIONS

In good-quality embryos, there was a high rate and diverse array of chromosomal abnormalities. Morphological evaluation does not appear to assist in the reduction in meiotic errors from parental origins. Mitotic errors were common, and nondisjunction was found to be the main mechanism causing malsegregation during the cleavage divisions.

An Economic Analysis of Aneuploidy Screening of Oocytes in Assisted Reproduction in Germany

Background The randomized ESTEEM trial reported that preimplantation genetic aneuploidy testing of oocytes by polar body biopsy (PGT-A) with array comparative genomic hybridization (aCGH) in women aged 36 - 40 years undergoing assisted reproduction treatment reduces the number of embryo transfers and the risk of miscarriage while not impacting the live birth rate. Method A decision tree model based on data from the ESTEEM trial was created and analyzed, using three cost scenarios for assisted reproduction treatment in Germany (statutory health insurance [GKV] = the deductible is 50% of the standard medical costs; private medical insurance [PKV] = invoicing is based on the German medical fee schedule [GOÄ]; private medical insurance with a simple GOÄ factor [simple GOÄ factor] = invoicing is based on the standard medical fees multiplied by a linear GOÄ factor). The scenarios were compared for cost-effectiveness (cost per live birth), cost per prevented miscarriage and the threshold values for cost and effectiveness. Results PGT-A increased the costs per live birth in all scenarios (GKV: + 208%; PKV: + 49%; simple GOÄ factor: + 89%). A threshold analysis showed a substantial cost discrepancy between the actual cost of the intervention based on GOÄ (€ 5801) vs. the theoretically tolerable PGT-A cost (GKV: € 561, PKV: € 1037, single GOÄ-factor: € 743). The incremental cost per one prevented miscarriage was approximately € 70 000 - 75 000 for all cost scenarios. Conclusion The use of PGT-A with aCGH in assisted reproduction cannot be recommended from a cost-effectiveness perspective.

Aurora B prevents aneuploidy via MAD2 during the first mitotic cleavage in oxidatively damaged embryos

OBJECTIVES

A high rate of chromosome aneuploidy is exhibited in in vitro fertilization (IVF)-derived embryos. Our previous experiments suggested that reactive oxygen species (ROS) can activate Mad2, a key protein in the spindle assembly checkpoint (SAC), and delay the first mitotic, providing time to prevent the formation of embryonic aneuploidy. We aimed to determine whether mitotic kinase Aurora B was involved in the SAC function to prevent aneuploidy in IVF-derived embryos.

MATERIALS AND METHODS

We analysed aneuploidy formation and repair during embryo pre-implantation via 4',6-diamidino-2-phenylindole (DAPI) staining and karyotype analysis. We assessed Aurora B activation by immunofluorescence and investigated the effect of Aurora B inhibition on embryo injury-related variables, such as embryonic development, ROS levels, mitochondrial membrane potential and γH2AX-positive expression.

RESULTS

We observed the expression and phosphorylation of Thr232 in Aurora B in oxidative stress-induced zygotes. Moreover, inhibition of Aurora B caused chromosome mis-segregation, abnormal spindle structures, abnormal chromosome number and reduced expression of Mad2 in IVF embryos. Our results suggest that Aurora B causes mitotic arrest and participates in SAC via Mad2 and H3S10P, which is required for self-correction of aneuploidies.

CONCLUSIONS

We demonstrate here that oxidative stress-induced DNA damage triggers Aurora B-mediated activation of SAC, which prevents aneuploidy at the first mitotic cleavage in early mouse IVF embryos.

Chromosome constitution of equal-sized three-cell embryos using next-generation sequencing technology

PURPOSE

To study the chromosome constitution of equal-sized three-cell embryo.

METHODS

We determined the chromosome constitution of 105 blastomeres from 35 embryos using multiple annealing and looping-based amplification cycles (MALBAC) together with NGS sequencing technology. Chromosomal copy number variation (CNV) analysis was successfully performed in 27 embryos. We also analyzed radius, perimeter, area, and volume of each blastomere to explore the possibility of selecting the normal embryos.

RESULTS

Majority of the embryos (77.8%, 21/27) studied were mosaic or aneuploid, and only 22.2% (6/27) had normal chromosome numbers. The aneuploid chromosomes spread across all chromosomes and the most frequent aneuploidies were for chromosomes 1, 16, and 18 followed by 13, 19, and 21. Statistical analyses showed no significant difference between euploid and aneuploid embryos regarding radius, perimeter, area, and volume of their blastomeres.

CONCLUSIONS

Our results showed that majority of the equal-sized three-cell embryos were chromosomally abnormal and could not be distinguished by morphology observation, so they should be given lower priority at selection for transfer.

Single-cell sequencing of primate preimplantation embryos reveals chromosome elimination via cellular fragmentation and blastomere exclusion

Aneuploidy that arises during meiosis and/or mitosis is a major contributor to early embryo loss. We previously showed that human preimplantation embryos encapsulate missegregated chromosomes into micronuclei while undergoing cellular fragmentation and that fragments can contain chromosomal material, but the source of this DNA was unknown. Here, we leveraged the use of a nonhuman primate model and single-cell DNA-sequencing (scDNA-seq) to examine the chromosomal content of 471 individual samples comprising 254 blastomeres, 42 polar bodies, and 175 cellular fragments from a large number (N = 50) of disassembled rhesus cleavage-stage embryos. Our analysis revealed that the aneuploidy and micronucleation frequency is conserved between humans and macaques, and that fragments encapsulate whole and/or partial chromosomes lost from blastomeres. Single-cell/fragment genotyping showed that these chromosome-containing cellular fragments (CCFs) can be maternally or paternally derived and display double-stranded DNA breaks. DNA breakage was further indicated by reciprocal subchromosomal losses/gains between blastomeres and large segmental errors primarily detected at the terminal ends of chromosomes. By combining time-lapse imaging with scDNA-seq, we determined that multipolar divisions at the zygote or two-cell stage were associated with CCFs and generated a random mixture of chromosomally normal and abnormal blastomeres with uniparental or biparental origins. Despite frequent chromosome missegregation at the cleavage-stage, we show that CCFs and nondividing aneuploid blastomeres showing extensive DNA damage are prevented from incorporation into blastocysts. These findings suggest that embryos respond to chromosomal errors by encapsulation into micronuclei, elimination via cellular fragmentation, and selection against highly aneuploid blastomeres to overcome chromosome instability during preimplantation development.

Genome stability of bovine in vivo-conceived cleavage-stage embryos is higher compared to in vitro-produced embryos

STUDY QUESTION

Is the rate and nature of chromosome instability (CIN) similar between bovine in vivo-derived and in vitro-cultured cleavage-stage embryos?

SUMMARY ANSWER

There is a major difference regarding chromosome stability of in vivo-derived and in vitro-cultured embryos, as CIN is significantly lower in in vivo-derived cleavage-stage embryos compared to in vitro-cultured embryos.

WHAT IS KNOWN ALREADY

CIN is common during in vitro embryogenesis and is associated with early embryonic loss in humans, but the stability of in vivo-conceived cleavage-stage embryos remains largely unknown.

STUDY DESIGN, SIZE, DURATION

Because human in vivo preimplantation embryos are not accessible, bovine (Bos taurus) embryos were used to study CIN in vivo. Five young, healthy, cycling Holstein Friesian heifers were used to analyze single blastomeres of in vivo embryos, in vitro embryos produced by ovum pick up with ovarian stimulation (OPU-IVF), and in vitro embryos produced from in vitro matured oocytes retrieved without ovarian stimulation (IVM-IVF).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Single blastomeres were isolated from embryos, whole-genome amplified and hybridized on Illumina BovineHD BeadChip arrays together with the bulk DNA from the donor cows (mothers) and the bull (father). DNA was also obtained from the parents of the bull and from the parents of the cows (paternal and maternal grandparents, respectively). Subsequently, genome-wide haplotyping and copy-number profiling was applied to investigate the genomic architecture of 171 single bovine blastomeres of 16 in vivo, 13 OPU-IVF and 13 IVM-IVF embryos.

MAIN RESULTS AND THE ROLE OF CHANCE

The genomic stability of single blastomeres in both of the in vitro-cultured embryo cohorts was severely compromised (P < 0.0001), and the frequency of whole chromosome or segmental aberrations was higher in embryos produced in vitro than in embryos derived in vivo. Only 18.8% of in vivo-derived embryos contained at least one blastomere with chromosomal anomalies, compared to 69.2% of OPU-IVF embryos (P < 0.01) and 84.6% of IVM-IVF embryos (P < 0.001).

LARGE SCALE DATA

Genotyping data obtained in this study has been submitted to NCBI Gene Expression Omnibus (GEO; accession number GSE95358).

LIMITATIONS REASONS FOR CAUTION

There were two main limitations of the study. First, animal models may not always reflect the nature of human embryogenesis, although the use of an animal model to investigate CIN was unavoidable in our study. Second, a limited number of embryos were obtained, therefore more studies are warranted to corroborate the findings.

WIDER IMPLICATIONS OF THE FINDINGS

Although CIN is also present in in vivo-developed embryos, in vitro procedures exacerbate chromosomal abnormalities during early embryo development. Hence, the present study highlights that IVF treatment compromises embryo viability and should be applied with care. Additionally, our results encourage to refine and improve in vitro culture conditions and assisted reproduction technologies.

STUDY FUNDING/COMPETING INTEREST(S)

The study was funded by the Agency for Innovation by Science and Technology (IWT) (TBM-090878 to J.R.V. and T.V.), the Research Foundation Flanders (FWO; G.A093.11 N to T.V. and J.R.V. and G.0392.14 N to A.V.S. and J.R.V.), the European Union's FP7 Marie Curie Industry-Academia Partnerships and Pathways (IAPP, SARM, EU324509 to J.R.V., T.V., O.T, A.D., A.S. and A.K.) and Horizon 2020 innovation programme (WIDENLIFE, 692065 to J.R.V., O.T., T.V., A.K. and A.S.). M.Z.E., J.R.V. and T.V. are co-inventors on a patent application ZL913096-PCT/EP2014/068315-WO/2015/028576 ('Haplotyping and copy-number typing using polymorphic variant allelic frequencies'), licensed to Cartagenia (Agilent Technologies).

Evaluation of preimplantation genetic testing for chromosomal structural rearrangement by a commonly used next generation sequencing workflow

OBJECTIVES

To evaluate the applicability of a commonly used next generation sequencing workflow in detecting unbalanced meiotic segregation products for reciprocal translocation and inversion carriers.

STUDY DESIGN

All preimplantation genetic testing treatment cycles performed for reciprocal translocation or inversion carriers from 2012 to April 2017 were included. Three hundreds and forty-two archived whole genome amplified DNA, which had previously analyzed by array comparative genomic hybridization (aCGH), were retrospectively analyzed by next generation sequencing (NGS). Concordance on overall diagnosis and segmental aneuploidies related to the translocation/inversion breakpoints between aCGH and NGS were determined.

RESULTS

Retrospective analysis of 287 blastomere biopsies and 55 trophectoderm (TE) biopsies showed that the concordance rate on the overall diagnosis between aCGH and NGS on abnormal samples was 100% (266/266), irrespective to the type of biopsy. The concordance rates of normal biopsies were 98.4% (61/62) on blastomere and 78.6% (11/14) on TE biopsies. NGS detected a de novo segmental aneuploidy on one blastomere biopsy and three possible low level mosaic aneuploidies on 3 TE biopsies, which were previously concluded as euploid by aCGH. Using the karyotype of reciprocal translocation/inversion carriers, size of anticipated segmental aneuploidies could be calculated and be used to predict the applicability of NGS before proceeding to treatment.

CONCLUSION

This is the first report to evaluate the applicability of a commercial NGS-based workflow for preimplantation testing for reciprocal translocations/inversions. Our study demonstrated that NGS can diagnose unbalanced translocation/inversion products with the same efficiency as aCGH. The applicability of NGS, with respect to individual karyotype, can be predicted before proceeding to treatment.

A speculative outlook on embryonic aneuploidy: Can molecular pathways be involved?

The journey of embryonic development starts at oocyte fertilization, which triggers a complex cascade of events and cellular pathways that guide early embryogenesis. Recent technological advances have greatly expanded our knowledge of cleavage-stage embryo development, which is characterized by an increased rate of whole-chromosome losses and gains, mixoploidy, and atypical cleavage morphokinetics. Embryonic aneuploidy significantly contributes to implantation failure, spontaneous miscarriage, stillbirth or congenital birth defects in both natural and assisted human reproduction. Essentially, early embryo development is strongly determined by maternal factors. Owing to considerable limitations associated with human oocyte and embryo research, the use of animal models is inevitable. However, cellular and molecular mechanisms driving the error-prone early stages of development are still poorly described. In this review, we describe known events that lead to aneuploidy in mammalian oocytes and preimplantation embryos. As the processes of oocyte and embryo development are rigorously regulated by multiple signal-transduction pathways, we explore the putative role of signaling pathways in genomic integrity maintenance. Based on the existing evidence from human and animal data, we investigate whether critical early developmental pathways, like Wnt, Hippo and MAPK, together with distinct DNA damage response and DNA repair pathways can be associated with embryo genomic instability, a question that has, so far, remained largely unexplored.

Challenges facing contemporary preimplantation genetic screening

PURPOSE OF REVIEW

Aneuploidy is a leading cause of pregnancy failure. Although initial attempts to perform preimplantation genetic screening did not improve outcomes, validated techniques were developed to safely and effectively increase pregnancy rates. Still, many embryos designated as euploid do not implant. Current approaches are being refined to provide additional biologic insight into why this is the case. At present, the diagnosis and clinical relevance of segmental aneuploidy and mosaicism are amongst the more heavily investigated.

RECENT FINDINGS

Class I data have proven the safety of trophectoderm biopsy and validation studies have shown single nucleotide polymorphism array and quantitative PCR can accurately detect whole chromosome aneuploidy. Similar studies to validate next generation sequencing are underway. Although randomized control trials have demonstrated the clinical utility of preimplantation genetic screening, recent data on the impact of mosaicism and segmental aneuploidy require clarification.

SUMMARY

Several well powered randomized control trials have shown preimplantation genetic screening improves implantation rate. Plausible explanations for euploid failures include undetected mosaicism and segmental aneuploidy. However, the true incidence and dispersion of mosaicism within the embryo is unknown. Likewise, the resolution of detection and clinical significance of segmental aneuploidy is unclear. Further research to validate proposed detection algorithms and class I data to determine if detection impacts outcomes is needed.

Time-Lapse Imaging for the Detection of Chromosomal Abnormalities in Primate Preimplantation Embryos

The use of time-lapse microscopic imaging has proven to be a powerful tool for the study of mitotic divisions and other cellular processes across diverse species and cell types. Although time-lapse monitoring (TLM) of human preimplantation development was first introduced to the in vitro fertilization (IVF) community several decades ago, it was not until relatively recently that TLM systems were commercialized for clinical embryology purposes. Traditionally, human IVF embryos are assessed by successful progression and morphology under a stereomicroscope at distinct time points prior to selection for transfer. Due to the high frequency of aneuploidy, embryos may also be biopsied at the cleavage or blastocyst stage for preimplantation genetic screening (PGS) of whole and/or partial chromosomal abnormalities. However, embryo biopsy is invasive and can hinder subsequent development, and there are additional concerns over chromosomal mosaicism and resolution with PGS. Moreover, embryos are typically outside of the incubator in suboptimal culture conditions for extended periods of time during these procedures. With TLM systems, embryos remain in the stable microenvironment of an incubator and are simultaneously imaged for noninvasive embryo evaluation using a fraction of the light exposure as compared to a stereomicroscope. Each image is then compiled into a time-lapse movie, the information from which can be extrapolated to correlate morphological, spatial, and temporal parameters with embryo quality and copy number status. Here, we describe the various TLM systems available for clinical and/or research use in detail and provide step-by-step instructions on how the measurement of specific timing intervals and certain morphological criteria can be implemented into IVF protocols to enhance embryo assessment and avoid the selection of aneuploid embryos. We also discuss the biological significance of processes unique to mitotically dividing embryos and the likelihood that complex chromosomal events such as chromothripsis occur during preimplantation development in humans and other mammals, particularly nonhuman primates.

Is the hypothesis of preimplantation genetic screening (PGS) still supportable? A review

The hypothesis of preimplantation genetic diagnosis (PGS) was first proposed 20 years ago, suggesting that elimination of aneuploid embryos prior to transfer will improve implantation rates of remaining embryos during in vitro fertilization (IVF), increase pregnancy and live birth rates and reduce miscarriages. The aforementioned improved outcome was based on 5 essential assumptions: (i) Most IVF cycles fail because of aneuploid embryos. (ii) Their elimination prior to embryo transfer will improve IVF outcomes. (iii) A single trophectoderm biopsy (TEB) at blastocyst stage is representative of the whole TE. (iv) TE ploidy reliably represents the inner cell mass (ICM). (v) Ploidy does not change (i.e., self-correct) downstream from blastocyst stage. We aim to offer a review of the aforementioned assumptions and challenge the general hypothesis of PGS. We reviewed 455 publications, which as of January 20, 2017 were listed in PubMed under the search phrase < preimplantation genetic screening (PGS) for aneuploidy>. The literature review was performed by both authors who agreed on the final 55 references. Various reports over the last 18 months have raised significant questions not only about the basic clinical utility of PGS but the biological underpinnings of the hypothesis, the technical ability of a single trophectoderm (TE) biopsy to accurately assess an embryo’s ploidy, and suggested that PGS actually negatively affects IVF outcomes while not affecting miscarriage rates. Moreover, due to high rates of false positive diagnoses as a consequence of high mosaicism rates in TE, PGS leads to the discarding of large numbers of normal embryos with potential for normal euploid pregnancies if transferred rather than disposed of. We found all 5 basic assumptions underlying the hypothesis of PGS to be unsupported: (i) The association of embryo aneuploidy with IVF failure has to be reevaluated in view how much more common TE mosaicism is than has until recently been appreciated. (ii) Reliable elimination of presumed aneuploid embryos prior to embryo transfer appears unrealistic. (iii) Mathematical models demonstrate that a single TEB cannot provide reliable information about the whole TE. (iv) TE does not reliably reflect the ICM. (v) Embryos, likely, still have strong innate ability to self-correct downstream from blastocyst stage, with ICM doing so better than TE. The hypothesis of PGS, therefore, no longer appears supportable. With all 5 basic assumptions underlying the hypothesis of PGS demonstrated to have been mistaken, the hypothesis of PGS, itself, appears to be discredited. Clinical use of PGS for the purpose of IVF outcome improvements should, therefore, going forward be restricted to research studies.

Re-analysis of aneuploidy blastocysts with an inner cell mass and different regional trophectoderm cells

PURPOSE

The purpose of this study is to explore which part of the trophectoderm best represents the inner cell mass after aCGH analysis.

METHODS

Fifty-one preimplantation genetic diagnosis/preimplantation genetic screening of abnormal blastocysts diagnosed by array comparative genomic hybridization were included in this study. Blastocysts were thawed, incubated for 3 to 4 h, and then biopsied. Four regions were biopsied per blastocyst, including the inner cell mass (ICM), trophectoderm (TE) cells opposite the ICM, TE cells at the upper right of the ICM, and TE cells at the lower right of the ICM. The biopsied pieces were processed through multiple annealing and looping-based amplification cycle sequenced for 24-chromosome aneuploidy screening. The aneuploidy results were compared among the ICM and the different regional trophectoderm cells from the same blastocyst.

RESULTS

Fifty of 51 (98.04%) ICM samples were concordant with at least one of the TE biopsies derived from the same embryos. There were 43 blastocysts in which ICM and the other three TE pieces were consistent. Discordance among the four pieces occurred in eight blastocysts. Only one blastocyst was discordant between the ICM and the other three TE pieces, while seven blastocysts were discordant between one of TE and the other three biopsied pieces. There was no special region that the mosaic TE was located.

CONCLUSIONS

Our findings indicate that TE aneuploidy is an excellent predictor of ICM aneuploidy. The blastocyst mosaic cells are inclined to be located in TE. Moreover, the mosaic TE was not limited to the special region.

Outcomes and Recommendations of an Indian Expert Panel for Improved Practice in Controlled Ovarian Stimulation for Assisted Reproductive Technology

Purpose. To improve success of in vitro fertilization (IVF), assisted reproductive technology (ART) experts addressed four questions. What is optimum oocytes number leading to highest live birth rate (LBR)? Are cohort size and embryo quality correlated? Does gonadotropin type affect oocyte yield? Should “freeze-all” policy be adopted in cycles with progesterone >1.5 ng/mL on day of human chorionic gonadotropin (hCG) administration? Methods. Electronic database search included ten studies on which panel gave opinions for improving current practice in controlled ovarian stimulation for ART. Results. Strong association existed between retrieved oocytes number (RON) and LBRs. RON impacted likelihood of ovarian hyperstimulation syndrome (OHSS). Embryo euploidy decreased with age, not with cohort size. Progesterone > 1.5 ng/dL did not impair cycle outcomes in patients with high cohorts and showed disparate results on day of hCG administration. Conclusions. Ovarian stimulation should be designed to retrieve 10–15 oocytes/treatment. Accurate dosage, gonadotropin type, should be selected as per prediction markers of ovarian response. Gonadotropin-releasing hormone (GnRH) antagonist based protocols are advised to avoid OHSS. Cumulative pregnancy rate was most relevant pregnancy endpoint in ART. Cycles with serum progesterone ≥1.5 ng/dL on day of hCG administration should not adopt “freeze-all” policy. Further research is needed due to lack of data availability on progesterone threshold or index.

Vertebrate Embryonic Cleavage Pattern Determination

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.

Morphokinetic Characteristics and Developmental Potential of In Vitro Cultured Embryos from Natural Cycles in Patients with Poor Ovarian Response

Background. Patients with poor ovarian response to ovarian hyperstimulation represent an interesting group for studying the impact of embryo cleavage irregularities on clinical outcome since all embryos, regardless of their quality, are usually transferred to the uterus. The aim of our study was to follow the morphokinetics of fertilized oocytes from natural cycles in poor responders. Methods. Zygotes from 53 cycles were cultured in vitro for 3 days. The morphokinetics of their development and transfer outcomes were retrospectively analyzed for the normally and irregularly cleaved embryos. Results. Of all embryos, 30.2% had single and 20.8% multiple cleavage irregularities with the following prevalence: developmental arrest 30.2%, direct cleavage to more than two cells 24.5%, chaotic cleavage 13.2%, and reverse cleavage 11.3%. These embryos had longer pronuclear phases, first cytokinesis, second embryo cell cycles, and less synchronized divisions. The transfer of normally developing embryos resulted in an implantation rate of 30.8% and a delivery rate of 23.1%, but irregularly cleaved embryos did not implant. Conclusions. The use of time-lapse microscopy in poor responder patients identified embryos with cleavage abnormalities that are related with no or extremely low implantation potential. Gained information about embryo quality is important for counselling patients about their expectations.

Detection of segmental aneuploidy and mosaicism in the human preimplantation embryo: technical considerations and limitations

Whole-chromosome aneuploidy screening has become a common practice to improve outcomes and decrease embryonic transfer order in patients undergoing treatment for infertility through in vitro fertilization. Despite implementation of this powerful technology, a significant percentage of euploid embryos fail to result in successful deliveries. As technology has evolved, detection of subchromosomal imbalances and embryonic mosaicism has become possible, and these serve as potential explanations for euploid embryo transfer failures. Cases involving a parent with a balanced translocation provide a unique opportunity to characterize the capabilities and limitations of detecting segmental imbalances with a variety chromosome screening platforms. Adaptation of these methods to de novo imbalances now represent an ongoing challenge in the field of preimplantation genetic screening as additional factors including mosaicism, clinical predictive value, and distinguishing true imbalances from technical artifacts must be more carefully considered.

Mechanisms of Aneuploidy in Human Eggs

Eggs and sperm develop through a specialized cell division called meiosis. During meiosis, the number of chromosomes is reduced by two sequential divisions in preparation for fertilization. In human female meiosis, chromosomes frequently segregate incorrectly, resulting in eggs with an abnormal number of chromosomes. When fertilized, these eggs give rise to aneuploid embryos that usually fail to develop. As women become older, errors in meiosis occur more frequently, resulting in increased risks of infertility, miscarriage, and congenital syndromes, such as Down's syndrome. Here, we review recent studies that identify the mechanisms causing aneuploidy in female meiosis, with a particular emphasis on studies in humans.

Mutation rates and the evolution of germline structure

Genome sequencing studies of de novo mutations in humans have revealed surprising incongruities in our understanding of human germline mutation. In particular, the mutation rate observed in modern humans is substantially lower than that estimated from calibration against the fossil record, and the paternal age effect in mutations transmitted to offspring is much weaker than expected from our long-standing model of spermatogenesis. I consider possible explanations for these discrepancies, including evolutionary changes in life-history parameters such as generation time and the age of puberty, a possible contribution from undetected post-zygotic mutations early in embryo development, and changes in cellular mutation processes at different stages of the germline. I suggest a revised model of stem-cell state transitions during spermatogenesis, in which 'dark' gonial stem cells play a more active role than hitherto envisaged, with a long cycle time undetected in experimental observations. More generally, I argue that the mutation rate and its evolution depend intimately on the structure of the germline in humans and other primates.This article is part of the themed issue 'Dating species divergences using rocks and clocks'.

The why, the how and the when of PGS 2.0: current practices and expert opinions of fertility specialists, molecular biologists, and embryologists

STUDY QUESTION

We wanted to probe the opinions and current practices on preimplantation genetic screening (PGS), and more specifically on PGS in its newest form: PGS 2.0?

STUDY FINDING

Consensus is lacking on which patient groups, if any at all, can benefit from PGS 2.0 and, a fortiori, whether all IVF patients should be offered PGS.

WHAT IS KNOWN ALREADY

It is clear from all experts that PGS 2.0 can be defined as biopsy at the blastocyst stage followed by comprehensive chromosome screening and possibly combined with vitrification. Most agree that mosaicism is less of an issue at the blastocyst stage than at the cleavage stage but whether mosaicism is no issue at all at the blastocyst stage is currently called into question.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

A questionnaire was developed on the three major aspects of PGS 2.0: the Why, with general questions such as PGS 2.0 indications; the How, specifically on genetic analysis methods; the When, on the ideal method and timing of embryo biopsy. Thirty-five colleagues have been selected to address these questions on the basis of their experience with PGS, and demonstrated by peer-reviewed publications, presentations at meetings and participation in the discussion. The first group of experts who were asked about 'The Why' comprised fertility experts, the second group of molecular biologists were asked about 'The How' and the third group of embryologists were asked about 'The When'. Furthermore, the geographical distribution of the experts has been taken into account. Thirty have filled in the questionnaire as well as actively participated in the redaction of the current paper.

MAIN RESULTS AND THE ROLE OF CHANCE

The 30 participants were from Europe (Belgium, Germany, Greece, Italy, Netherlands, Spain, UK) and the USA. Array comparative genome hybridization is the most widely used method amongst the participants, but it is slowly being replaced by massive parallel sequencing. Most participants offering PGS 2.0 to their patients prefer blastocyst biopsy. The high efficiency of vitrification of blastocysts has added a layer of complexity to the discussion, and it is not clear whether PGS in combination with vitrification, PGS alone, or vitrification alone, followed by serial thawing and eSET will be the favoured approach. The opinions range from in favour of the introduction of PGS 2.0 for all IVF patients, over the proposal to use PGS as a tool to rank embryos according to their implantation potential, to scepticism towards PGS pending a positive outcome of robust, reliable and large-scale RCTs in distinct patient groups.

LIMITATIONS, REASONS FOR CAUTION

Care was taken to obtain a wide spectrum of views from carefully chosen experts. However, not all invited experts agreed to participate, which explains a lack of geographical coverage in some areas, for example China. This paper is a collation of current practices and opinions, and it was outside the scope of this study to bring a scientific, once-and-for-all solution to the ongoing debate.

WIDER IMPLICATIONS OF THE FINDINGS

This paper is unique in that it brings together opinions on PGS 2.0 from all different perspectives and gives an overview of currently applied technologies as well as potential future developments. It will be a useful reference for fertility specialists with an expertise outside reproductive genetics.

LARGE SCALE DATA

none.

STUDY FUNDING AND COMPETING INTERESTS

No specific funding was obtained to conduct this questionnaire.

Should pre-implantation genetic screening be implemented to routine clinical practice?

The utilization of trophectoderm biopsy combined with comprehensive chromosome screening (CCS) tests for embryonic aneuploidy was recently suggested to improve IVF outcome, however, not without criticisms. Since mosaicism has been reported in as high as 90% of blastocyst-stage embryos, we aimed to evaluate the accuracy of trophectoderm multiple biopsies using next-generation sequencing (NGS). Eight top quality blastocysts underwent three trophectoderm biopsies each, followed by NGS. In four blastocysts, the rest of the embryo, which included the inner cell mass, was also analyzed. Five of the 24 (20.8%) trophectoderm biopsies revealed inconclusive results, while 4 (16.6%) demonstrated embryonic mosaicism. Overall, 10 (35.7%) of the 28 (24 trophectoderms and 4 inner cell masses) biopsies revealed mosaicism or inconclusive results. Our preliminary observations contribute to the ongoing discussion on the unrestricted clinical adoption of PGS, suggesting, that until proper evaluation of its effectiveness and cost-effectiveness will be provided, PGS should be offered only under study conditions, and with appropriate informed consents.

Chromosomal instability in mammalian pre-implantation embryos: potential causes, detection methods, and clinical consequences

Formation of a totipotent blastocyst capable of implantation is one of the first major milestones in early mammalian embryogenesis, but less than half of in vitro fertilized embryos from most mammals will progress to this stage of development. Whole chromosomal abnormalities, or aneuploidy, are key determinants of whether human embryos will arrest or reach the blastocyst stage. Depending on the type of chromosomal abnormality, however, certain embryos still form blastocysts and may be morphologically indistinguishable from chromosomally normal embryos. Despite the implementation of pre-implantation genetic screening and other advanced in vitro fertilization (IVF) techniques, the identification of aneuploid embryos remains complicated by high rates of mosaicism, atypical cell division, cellular fragmentation, sub-chromosomal instability, and micro-/multi-nucleation. Moreover, several of these processes occur in vivo following natural human conception, suggesting that they are not simply a consequence of culture conditions. Recent technological achievements in genetic, epigenetic, chromosomal, and non-invasive imaging have provided additional embryo assessment approaches, particularly at the single-cell level, and clinical trials investigating their efficacy are continuing to emerge. In this review, we summarize the potential mechanisms by which aneuploidy may arise, the various detection methods, and the technical advances (such as time-lapse imaging, "-omic" profiling, and next-generation sequencing) that have assisted in obtaining this data. We also discuss the possibility of aneuploidy resolution in embryos via various corrective mechanisms, including multi-polar divisions, fragment resorption, endoreduplication, and blastomere exclusion, and conclude by examining the potential implications of these findings for IVF success and human fecundity.

Irregular cleavage of early preimplantation human embryos: characteristics of patients and pregnancy outcomes

PURPOSE

This is a retrospective analysis of the morphokinetics, prevalence, and implantation potential of embryos with irregular first and second cleavages as identified by time-lapse microscopy.

METHODS

The study included 253 women who underwent 387 assisted reproduction treatments with intracytoplasmic sperm injection (ICSI). Each patient was assigned to one of three groups based on embryo cleavage results. In group I, one to two embryos per cycle showed irregular cleavage; group II, at least three embryos with abnormal cleavage; and in group III (the control group), all embryos cleaved normally. The number of embryos that cleaved from 1 to ≥3 cells or from 2 to ≥5 cells for each patient was recorded. Their prevalence and association with women's characteristics and pregnancy outcome were evaluated.

RESULTS

The prevalence of irregular cleavage was 15.6 % among 1772 ICSI embryos. In 101 cycles, 1-2 embryos per cycle showed irregular cleavage (group I). In 32 cycles, at least 3 embryos showed abnormal cleavage (group II). In 254 cycles, all embryos cleaved normally (group III). The average age of the women in group II was significantly lower in comparison with groups I and III (32.5 ± 4.2 vs. 35.1 ± 4.9 and 35.5 ± 5.1, respectively, p < 0.02). In comparison of groups I and II, the odds ratio for ≥3 embryos with irregular cleavage in women younger than 35 was 3.48 (95 % CI, 1.28 to 9.46). Embryos with irregular cleavage were transferred in 16 women. Three live births were achieved following the transfer of single blastocysts derived from embryos with irregular cleavage from two to five cells.

CONCLUSIONS

Early embryos with irregular cleavage are significantly more prevalent in younger women. When these embryos develop to the blastocyst stage, they may have normal implantation potential, leading to the birth of healthy babies.

Evidence of Selection against Complex Mitotic-Origin Aneuploidy during Preimplantation Development

Whole-chromosome imbalances affect over half of early human embryos and are the leading cause of pregnancy loss. While these errors frequently arise in oocyte meiosis, many such whole-chromosome abnormalities affecting cleavage-stage embryos are the result of chromosome missegregation occurring during the initial mitotic cell divisions. The first wave of zygotic genome activation at the 4-8 cell stage results in the arrest of a large proportion of embryos, the vast majority of which contain whole-chromosome abnormalities. Thus, the full spectrum of meiotic and mitotic errors can only be detected by sampling after the initial cell divisions, but prior to this selective filter. Here, we apply 24-chromosome preimplantation genetic screening (PGS) to 28,052 single-cell day-3 blastomere biopsies and 18,387 multi-cell day-5 trophectoderm biopsies from 6,366 in vitro fertilization (IVF) cycles. We precisely characterize the rates and patterns of whole-chromosome abnormalities at each developmental stage and distinguish errors of meiotic and mitotic origin without embryo disaggregation, based on informative chromosomal signatures. We show that mitotic errors frequently involve multiple chromosome losses that are not biased toward maternal or paternal homologs. This outcome is characteristic of spindle abnormalities and chaotic cell division detected in previous studies. In contrast to meiotic errors, our data also show that mitotic errors are not significantly associated with maternal age. PGS patients referred due to previous IVF failure had elevated rates of mitotic error, while patients referred due to recurrent pregnancy loss had elevated rates of meiotic error, controlling for maternal age. These results support the conclusion that mitotic error is the predominant mechanism contributing to pregnancy losses occurring prior to blastocyst formation. This high-resolution view of the full spectrum of whole-chromosome abnormalities affecting early embryos provides insight into the cytogenetic mechanisms underlying their formation and the consequences for human fertility.