Meiotic Errors Research Articles

PAK4 is Required for Meiotic Resumption, Spindle Assembly, and Cortical Migration in Mouse Oocytes During Meiotic Maturation.

Oocyte meiotic errors can cause infertility, miscarriage, and birth defects. Here the role and the underlying mechanism of p21 activated kinase 4 (PAK4) in mouse oocyte meiosis is evaluated. It is found that PAK4 expression and its phosphorylation are detected in high level at germinal vesicle (GV) stage, and gradually decreased after meiotic resumption in oocytes. PAK4 has direct physical interaction with both mitogen-activated protein kinases 1/2 (MEK1/2) and Paxillin, they are colocalized on the spindle structure during metaphases I and II. Phospho-PAK4 is distributed beneath the cytoplasmic membrane and on the chromosomes, and colocalized with the microtubule organizing center (MTOC) proteins, Pericentrin and γ-tubulin, as well as phosphor-MEK1/2 and phosphor-Paxillin on spindle poles. PAK4 inhibition by chemical inhibitor LCH-7749944, specific Pak4 morpholino oligo or the dominant negative mutant Pak4K350, 351M influence the meiotic resumption, spindle assembly and its cortical migration, and associated with the downregulation in the dephosphorylation of cyclin dependent kinase 1 (CDK1) and the levels of Cyclin B1, MEK1/2, Paxillin, g-tubulin, acetylated a-tubulin, Arp3, and Cofilin phosphorylation in oocytes. In sum, PAK4 functions to sustain the rational levels of Cyclin B1, MEK1/2, Paxillin, y-tubulin, acetylated a-tubulin, Arp3, and phosphor-Cofilin in mouse oocytes, thereby promotes the meiotic resumption, spindle assembly, and migration during meiotic maturation.

Effects of bisphenol A, bisphenol S, and tetramethyl bisphenol F on male fertility in Caenorhabditis elegans.

Research has shown that exposure to bisphenol A (BPA), a widely used plasticizer, can lead to meiotic errors, resulting in poor reproductive cell quality and infertility. Health-related concerns have prompted the search for BPA alternatives; however, evidence suggests that currently used BPA analogs, such as bisphenol S (BPS), may pose similar risks to human health. While the effects of BPA on female fertility are well documented, the impact of BPA exposure on sperm quality is poorly understood. To better understand the effects of bisphenol analogs on spermatogenesis, we synthesized a less investigated BPA analog, tetramethyl bisphenol F (TMBPF), and compared its reprotoxic potential to that of widely used BPA and BPS using C. elegans-based assays. We evaluated germ cell count, spermatid size, morphology, and activation in males treated with 0.5mM ethanol-dissolved bisphenol analogs for 48 h as well as their cross-progeny number and viability. Our results indicated that all of the evaluated bisphenol analogs-BPA, BPS, and TMBPF-adversely affect male fertility to varying degrees. Whereas all three bisphenols reduced spermatid size, only BPA exposure resulted in impaired spermatid activation and significantly reduced brood size. In addition, a decrease in embryonic viability, suggestive of an increased incidence of sperm chromosomal aberrations, was observed following exposure to all of the tested bisphenols. Further investigation is necessary to fully elucidate the underlying mechanisms and implications of BPA, BPS, and TMBPF on spermatogenesis.

Basal FSH values are positively associated with aneuploidy incidence in pre-advanced maternal age (AMA) but not in AMA patients.

To assess the impact of maternal age on the association between maternal basal FSH and aneuploidy. A retrospective study including data from 1749 blastocysts diagnosed as euploid or aneuploid by PGT-A (preimplantation genetic testing for aneuploidy). Aneuploidy incidence was compared between embryos from mothers with high vs. low basal FSH levels (above and below the group median, respectively) in total, pre-AMA (advanced maternal age; < 35years, 198 embryos) and AMA (≥ 35years, 1551 embryos) patient groups, separately. To control for the interference of potentially confounding variables, the association between aneuploidy and high basal FSH levels was assessed by multivariate logistic analysis in overall, pre-AMA and AMA patient groups. Overall, aneuploidy rate was 9% higher (p = 0.02) in embryos from patients with high basal FSH (63.7%) compared to those with low basal FSH (58.4%). In the pre-AMA subgroup, aneuploidy incidence was 35% higher (p = 0.04) in embryos from patients with high basal FSH (53.5%) compared to those with low basal FSH (39.4%). Differently, aneuploidy occurrence did not vary between embryos from AMA patients with low (61.0%) and high (64.8%) basal FSH (p = 0.12). The multivariate analysis revealed that, in pre-AMA embryos, the association between aneuploidy occurrence and high basal FSH is independent of potential confounding variables (p = 0.04). Maternal basal FSH values are associated with embryo aneuploidy in pre-AMA but not in AMA patients. The present findings suggest that basal FSH is a useful parameter to assess aneuploidy risk in pre-AMA patients and reinforce the hypothesis that excessive FSH signalling can predispose to oocyte meiotic errors.

Imprinting as Basis for Complex Evolutionary Novelties in Eutherians.

The epigenetic phenomenon of genomic imprinting is puzzling. While epigenetic modifications in general are widely known in most species, genomic imprinting in the animal kingdom is restricted to autosomes of therian mammals, mainly eutherians, and to a lesser extent in marsupials. Imprinting causes monoallelic gene expression. It represents functional haploidy of certain alleles while bearing the evolutionary cost of diploidization, which is the need of a complex cellular architecture and the danger of producing aneuploid cells by mitotic and meiotic errors. The parent-of-origin gene expression has stressed many theories. Most prominent theories, such as the kinship (parental conflict) hypothesis for maternally versus paternally derived alleles, explain only partial aspects of imprinting. The implementation of single-cell transcriptome analyses and epigenetic research allowed detailed study of monoallelic expression in a spatial and temporal manner and demonstrated a broader but much more complex and differentiated picture of imprinting. In this review, we summarize all these aspects but argue that imprinting is a functional haploidy that not only allows a better gene dosage control of critical genes but also increased cellular diversity and plasticity. Furthermore, we propose that only the occurrence of allele-specific gene regulation mechanisms allows the appearance of evolutionary novelties such as the placenta and the evolutionary expansion of the eutherian brain.

Aging-associated reduction of chromosomal histones in mammalian oocytes.

Mammalian oocytes undergo a long-term meiotic arrest that can last for almost the entire reproductive lifespan. This arrest occurs after DNA replication and is prolonged with age, which poses a challenge to oocytes in maintaining replication-dependent chromosomal proteins required for the completion of meiosis. In this study, we show that chromosomal histones are reduced with age in mouse oocytes. Both types of histone H3 variants, replication-dependent H3.1/H3.2 and replication-independent H3.3, decrease with age. Aging-associated histone reduction is associated with transcriptomic features that are caused by genetic depletion of histone H3.3. Neither the genetic reduction of chromosomal H3.1/H3.2 nor H3.3 accelerates the aging-associated increase in premature chromosome separation that causes meiotic segregation errors. We suggest that aging-associated reduction of chromosomal histones is linked to several transcriptomic abnormalities but does not significantly contribute to errors in meiotic chromosome segregation during the reproductive lifespan of mice.

Live chromosome identifying and tracking reveals size-based spatial pathway of meiotic errors in oocytes.

Meiotic errors of relatively small chromosomes in oocytes result in egg aneuploidies that cause miscarriages and congenital diseases. Unlike somatic cells, which preferentially mis-segregate larger chromosomes, aged oocytes preferentially mis-segregate smaller chromosomes through unclear processes. Here, we provide a comprehensive three-dimensional chromosome identifying-and-tracking dataset throughout meiosis I in live mouse oocytes. This analysis reveals a prometaphase pathway that actively moves smaller chromosomes to the inner region of the metaphase plate. In the inner region, chromosomes are pulled by stronger bipolar microtubule forces, which facilitates premature chromosome separation, a major cause of segregation errors in aged oocytes. This study reveals a spatial pathway that facilitates aneuploidy of small chromosomes preferentially in aged eggs and implicates the role of the M phase in creating a chromosome size-based spatial arrangement.

P-537 Linking genetic variation to infertility and embryonic genomic instability. Lessons from a database of whole genomes from targeted populations: challenges, and future directions

Abstract Study question Can novel genomic analysis methods be developed to identify genetic variants that predispose to specific types of infertility and genomic instability in embryos? Summary answer Whole genome analysis and the creation of a genomic variation database of highly targeted populations yielded several genomic links to human infertility and genomic instability. What is known already Infertility is a phenotypic manifestation that originates from highly variable causes, like repeated miscarriage (RM), repeated implantation failure (RIF) or various male and female factors. Specific genetic variations can predispose carriers to distinct types of infertility however quantifiable genetic risks are difficult to be established clinically due to the high amount of variables that exist in any relevant infertility studies. In addition, the combination of the maternal and paternal genetics further complicates the interpretation of genomics results especially when genetic instability in embryos are studied. Several studies have indicated multi[le risk variants however few have managed to establish causal relationships. Study design, size, duration The causes of infertility are extremely variable, which presents a challenge in the design and analysis of genome sequencing studies that involve large populations of participants with similar characteristics. In this study a targeted approach highly selected populations allows more detailed examination of the whole genome of the participants and the different subgroups that exist among them. The participants were recruited using specific selection criteria and samples were collected over a period of 5 years. Participants/materials, setting, methods Couples presenting with infertility and went through preimplantation genetic testing for aneuploidy (PGT- A) were invited to participate. Selection criteria were applied: i) referred for PGT-A after &amp;gt;3 assisted reproduction failed cycles, ii) Normal karyotypes, iii) Female age less than 37, iv) Embryonic instability in their subsequent PGT-A cycles in more than 40% of the produced embryos. After whole genome sequencing (WGS) and variant calling, a bespoke genomic SQL database was constructed for genomic analysis. Main results and the role of chance From 118 individuals that consented to participate 28 individuals with repeated implantation failure (RIF) and 8 individuals RIF/RM passed the selection criteria and their DNA was processed with WGS using a short read platform. In addition, WGS Validation was performed using genomes with known mutations. After the WGS quality control and validation. In total 50 genomes were processed and produced variant results. A novel MySQL database was successfully constructed and validated. Variant z scores were calculated for each participant and the significantly flagged variants were pulled and analysed and comparison with other bespoke and publicly available databases was performed. Ethnicity bias in novel variations detected and corrected. Several smaller cohorts were identified with similar genomic characteristics and accumulation of specific sets of polymorphisms, that were associated with certain types of infertility and preimplantation embryo abnormalities. Of particular interest were variants in synaptonemal complex genes associated with gamete failure, meiotic errors and instability in embryos. Structural variations were significant for male factor infertility. The WGS approach also revealed novel intronic and extragenic variants that would not be usually picked up by other methods like exome sequencing but can play a vital role in gene expression. Limitations, reasons for caution As with all genomic studies relating to infertility, association does not mean causation. Further functional analysis would be required to establish definitive causal pathways based on significant risk variants. In addition, although our study is highly targeted more genomic data can enhance to validation of these results. Wider implications of the findings This highly targeted study using a personalised approach to genome analysis combined with WGS can lead to protocols that determine the cumulative clinical risk for infertility, by revelling the accumulation of specific sets of polymorphisms, intronic and exonic, can be associated with certain types of infertility and preimplantation embryo abnormalities. Trial registration number not applicable

P-558 Characterization of the morphology of the outer-kinetochore expandable module, aka fibrous corona, and its role in ensuring proper chromosome segregation in human and bovine oocytes

Abstract Study question How does the Fibrous Corona (FC) behave in human and bovine oocyte meiosis and what is its function in chromosome segregation during meiosis I? Summary answer In human and bovine oocytes, FC shows extreme plasticity expanding beyond the centromeric region of the chromosomes. Pharmacological inhibition of FC’s expansion induces chromosome mis-segregation. What is known already Embryonic aneuploidy of meiotic origin is one of the major causes of implantation failure and early pregnancy loss. Erroneous microtubule (MT)-kinetochore attachments in human meiosis are the main cause of meiotic chromosome segregation errors. In somatic cells, the FC, a multiprotein complex, assembles on kinetochores and expands around the centromere during prometaphase to facilitate MT interactions and stimulate correct MT-kinetochore attachment. We have shown that in bovine oocytes FC expand beyond the centrosomes and is a potent MTs nucleator (10.1016/j.cub.2023.01.010). However, it is still unknown how FC behaves and what are its functions in MT-kinetochore interactions in human oocytes. Study design, size, duration FC morphology was studied in immature human oocytes discarded during ICSI procedures and donated (informed consent) by the patients for research (Ethical permit TCBio.21-754) during the years 2021-2024. Because of ethical concerns and reduced availability of human oocytes, functional studies on FC were performed on bovine oocytes collected from slaughterhouse ovaries. Participants/materials, setting, methods 180 immature human oocytes were matured in vitro and either fixed at different meiosis I stages or treated 2h with colcemid, a MT depolymerizing drug, to induce maximum FC expansion and fixed. The oocytes were then immuno-stained for chromatin(DAPI), FC(ZW10), MT(alpha tubulin) and kinetochores(CREST) and imaged with super-resolution confocal microscopy. Bovine immature oocytes were matured in vitro with or without lonafarnib (inhibitor of FC expansion). After maturation chromosome spreads were performed to evaluate oocyte ploidy. Main results and the role of chance In human oocytes, after nuclear envelope breakdown, before microtubule appearance when the chromosomes appear compacted, FCs are visible on the top of the kinetochores. During MT nucleation, FCs form thick mesh in the center of the compacted chromosomes. During prometaphase I, FCs expand beyond the centromeric region of the chromosomes forming a cup around the sister kinetochores of the paired homologous chromosomes. When the chromosomes starts to align at the metaphase plate, FCs start to be stripped from the kinetochores and colocalize to the MTs. At metaphase I and II, only kinetochore localization is visible. When oocytes are treated with colcemid, FCs extend to partially cover chromosome arms. We observed a wide variability in FCs expansion between and within patients. Inhibition of FC expansion in bovine oocytes increase the incidence of oocyte aneuploidy (90% in lonafarnib treated vs 37% in control oocytes). Altogether, our data reveal that in human oocytes FCs expand more compared to human somatic cells. Considering that inhibition of FC expansion induces chromosome segregation errors, we propose that the large FC size in human oocytes may play a role in achieving correct chromosome-MT attachments within their extremely large cytoplasm thus ensuring proper chromosome segregation. Limitations, reasons for caution The limited availability of human oocytes requires the use of an animal model for in depth mechanistic studies. Although bovine oocytes have similar aneuploidy rate and spindle assembly mechanisms as human oocytes, they show morphological differences in FC expansion which could underlie intrinsic functional discrepancies between the two organisms. Wider implications of the findings A better understanding of the chromosome segregation mechanisms in human meiosis will help understanding what causes the high incidence of aneuploidies in human oocytes and possibly contribute to the development of interventions or of oocyte selection procedures to reduce the incidence of oocyte aneuploidy during in vitro maturation. Trial registration number Not applicable

Chromatin-associated cohesin turns over extensively and forms new cohesive linkages in Drosophila oocytes during meiotic prophase

In dividing cells, accurate chromosome segregation depends on sister chromatid cohesion, protein linkages that are established during DNA replication. Faithful chromosome segregation in oocytes requires that cohesion, first established in S phase, remain intact for days to decades, depending on the organism. Premature loss of meiotic cohesion in oocytes leads to the production of aneuploid gametes and contributes to the increased incidence of meiotic segregation errors as women age (maternal age effect). The prevailing model is that cohesive linkages do not turn over in mammalian oocytes. However, we have previously reported that cohesion-related defects arise in Drosophila oocytes when individual cohesin subunits or cohesin regulators are knocked down after meiotic S phase. Here, we use two strategies to express a tagged cohesin subunit exclusively during mid-prophase in Drosophila oocytes and demonstrate that newly expressed cohesin is used to form de novo linkages after meiotic S phase. Cohesin along the arms of oocyte chromosomes appears to completely turn over within a 2-day window during prophase, whereas replacement is less extensive at centromeres. Unlike S-phase cohesion establishment, the formation of new cohesive linkages during meiotic prophase does not require acetylation of conserved lysines within the Smc3 head. Our findings indicate that maintenance of cohesion between S phase and chromosome segregation in Drosophila oocytes requires an active cohesion rejuvenation program that generates new cohesive linkages during meiotic prophase.

Klinefelter syndrome: etiology and clinical considerations in male infertility†.

Klinefelter syndrome (KS) is the most prevalent chromosomal disorder occurring in males. It is defined by an additional X chromosome, 47,XXY, resulting from errors in chromosomal segregation during parental gametogenesis. A major phenotype is impaired reproductive function, in the form of low testosterone and infertility. This review comprehensively examines the genetic and physiological factors contributing to infertility in KS, in addition to emergent assisted reproductive technologies, and the unique ethical challenges KS patients face when seeking infertility treatment. The pathology underlying KS is increased susceptibility for meiotic errors during spermatogenesis, resulting in aneuploid or even polyploid gametes. Specific genetic elements potentiating this susceptibility include polymorphisms in checkpoint genes regulating chromosomal synapsis and segregation. Physiologically, the additional sex chromosome also alters testicular endocrinology and metabolism by dysregulating interstitial and Sertoli cell function, collectively impairing normal sperm development. Additionally, epigenetic modifications like aberrant DNA methylation are being increasingly implicated in these disruptions. We also discuss assisted reproductive approaches leveraged in infertility management for KS patients. Application of assisted reproductive approaches, along with deep comprehension of the meiotic and endocrine disturbances precipitated by supernumerary X chromosomes, shows promise in enabling biological parenthood for KS individuals. This will require continued multidisciplinary collaboration between experts with background of genetics, physiology, ethics, and clinical reproductive medicine.

Meiotic errors in oocytes of young and advanced maternal age women: the U-curve of fertility

Italy is currently one of the countries most affected by declining birth rates, a negative trend mainly determined by several socio-economic factors that lead women to postpone pregnancy. The use of in vitro fertilization techniques cannot counteract the natural decline in fertility that accompanies advancing maternal age. This decline is associated with an increase in chromosomal aneuploidy in oocytes that, if fertilized, could result in aneuploid embryos that are often miscarried. Here, we discuss the “molecular clock hypothesis” proposed by Hoffmann’s lab, which suggests a U-shaped trend of female fertility. The high rates of chromosomal aneuploidy observed at the two “ends of the curve” may represent an evolutionary mechanism of protection that exists to balance the risks associated with pregnancy. This hypothesis lays the foundations for debating the idea that very young women are free from the risk of aneuploidy, and also suggests the existence of a safety mechanism that “protects” against possibly complicated pregnancies both after the age of 40 years and in very young women. The U-shape hypothesis described in humans will be discussed in a comparison with the fertility trends of other mammalian species. KEY WORDS: Fertility, aneuploidy, advanced maternal age, IVF

The Efficacy and Safety of GF101 and Its Antioxidant Effect on In Vitro Fertilization Outcomes: A Double-Blind, Non-Inferiority, Randomized, Controlled Trial with Coenzyme Q10.

(1) Background: Oxidative stress adversely affects fertility by impairing oocyte fertilization potential, primarily due to meiotic segregation errors and cohesion loss. Superoxide dismutase (SOD) and Coenzyme Q10 (CoQ10) are prominent antioxidants known to mitigate oxidative damage. (2) Methods: This study recruited 86 patients undergoing in vitro fertilization (IVF) at a single center for a 12-week, randomized, double-blind, active-comparator-controlled trial. Participants were allocated into two groups: one receiving CoQ10 as an antioxidant (the CoQ10 group) and the other receiving GF Bacillus antioxidative enzyme SOD (the GF101 group). The primary endpoints were changes in serum oxidative markers (SOD and catalase) and IVF outcomes, including clinical pregnancy, miscarriage, and live birth rates. Follicular fluid (FF) SOD and catalase concentrations on the day of retrieval, the metaphase II (MII) oocyte rate, the fertilization rate, and lipid profiles were measured. (3) Results: Initially, 86 patients were enrolled, with 65 completing the protocol (30 in the GF101 group and 34 in the CoQ10 group). There were no significant differences between the GF101 and CoQ10 groups in serum SOD (p = 0.626) and catalase levels (p = 0.061) over 12 weeks. However, within the GF101 group, a significant increase in serum catalase from baseline to 12 weeks was observed (p = 0.004). The non-inferiority analysis for IVF outcomes indicated risk differences in the clinical pregnancy rate, live birth rate, and miscarriage rate of -6.27% (95% CI: -30.77% to 18.22%), -1.18% (95% CI: -25.28% to 22.93%), and -13.49% (95% CI: -41.14% to 14.15%), respectively, demonstrating non-inferiority for the GF101 group. Furthermore, the GF101 group experienced significant reductions in total cholesterol (p = 0.006) and low-density lipoprotein (LDL) levels (p = 0.009) in intra-group comparisons, with both groups exhibiting comparable safe profiles. (4) Conclusions: GF101 may be non-inferior to CoQ10 in treating infertility in women and potentially offers additional benefits for women with dyslipidemia.

Aneuploidy in Oocytes From Women of Advanced Maternal Age: Analysis of the Causal Meiotic Errors and Impact on Embryo Development

ABSTRACT Many chromosome abnormalities are commonly observed and can lead to early pregnancy loss, miscarriage, or the birth of children with chromosomal defects. Such abnormalities are considered a major factor in the low pregnancy rate after assisted reproductive technology and natural conception. Preimplantation genetic testing for aneuploidy (PGT-A) aims to minimize the transfer of aneuploid embryos. Embryonic aneuploidies arising from errors in meiosis have an incidence of approximately 25% in embryos from women younger than 35 years, to more than half in embryos from women aged older than 35 years. Although these embryos are able to develop to the blastocyst stage, they tend to be of lower morphological quality. A recent multicenter randomized clinical trial (ESTEEM) analyzed polar bodies (PBs) from women after intracytoplasmic sperm injection aged between 36 and 40 years using microarrays in 205 cycles and found that the transfer of embryos from euploid oocytes did not lead to a higher live birth rate but was associated with a reduction in the number of embryo transfers and miscarriages. This study aimed to evaluate all PB results from this RCT and characterize the types of chromosomal abnormalities and the chromosomes most frequently affected. The ESTEEM trial obtained biopsy of first (PB1) and second (PB2) PB in the cohort receiving PGT-A and analyzed them using array comparative genomic hybridization (aCGH). A total of 693 PB pairs had full results available, including 676 confirmed fertilized oocytes. Chromosome segregations, including likely underlying mechanisms, from these pairs are reported here. To estimate the reliability of the aCGH procedure, 72 PB pairs from a single center were reanalyzed using next-generation sequencing (NGS). Embryos were classified into 4 categories based on morphology: good, fair, poor, and degenerated. A comparative analysis was performed to assess the association between chromosome status and embryo quality as well as study group (PGT-A vs control) and embryo quality. A total of 213/676 oocytes were euploid and 413/676 were aneuploid, whereas in the remaining 50 oocytes, an abnormality observed in PB1 was compensated by an abnormality in PB2. A total of 693 PB pairs reported chromatic numbers with results for 15,939 chromosomes. An abnormal segregation, in PB1 and/or PB2, was observed in 1162 chromosomes (7.3%) in 461 PB pairs. Chromosomes 22 (16.7%), 16 (16.6%), 19 (14.4%), 21 (13.7%), and 15 (12.4%) had the highest frequencies for abnormal segregations. The abnormal segregations were compatible with precocious separation of sister chromatids in meiosis 1 (M1) (n = 568; 48.9%), nondisjunction of chromatics in meiosis 2 (M2) or reverse segregation (n = 417; 35.9%), and nondisjunction in M1 (n = 65; 5.6%). However, 112 chromosomes had segregation patterns that could not be categorized into 1 of the 9 known mechanisms causing aneuploidy in oocytes. Concordance between aCGH and NGS was obtained for both PBs for 1650 of 1656 analyzed chromosomes (99.6%). Embryos predicted to be aneuploid had significantly worse quality scores on day 3 (adjusted odds ratio [aOR], 0.62; 95% confidence interval [CI], 0.43–0.90), day 4 (aOR, 0.15; 95% CI, 0.06–0.39), and day 5 (aOR, 0.28; 95% CI, 0.14–0.58). This study represents one of the largest analyses of chromosomal copy number in both PBs to date and highlights the frequent unexplained chromosome copy numbers underscoring the gap of knowledge into the mechanisms causing aneuploidy in oocytes.

Mechanisms of endometrial aging: lessons from natural conceptions and assisted reproductive technology cycles.

Until recently, the study of age-related decline in fertility has focused primarily on the ovary; depletion of the finite pool of oocytes and increases in meiotic errors leading to oocyte aneuploidy are well-established mechanisms by which fertility declines with advancing age. Comparatively little is known about the impact of age on endometrial function. The endometrium is a complex tissue comprised of many cell types, including epithelial, stromal, vascular, immune and stem cells. The capacity of this tissue for rapid, cyclic regeneration is unique to this tissue, undergoing repeated cycles of growth and shedding (in the absence of an embryo) in response to ovarian hormones. Furthermore, the endometrium has been shown to be capable of supporting pregnancies beyond the established boundaries of the reproductive lifespan. Despite its longevity, molecular studies have established age-related changes in individual cell populations within the endometrium. Human clinical studies have attempted to isolate the effect of aging on the endometrium by analyzing pregnancies conceived with euploid, high quality embryos. In this review, we explore the existing literature on endometrial aging and its impact on pregnancy outcomes. We begin with an overview of the principles of endometrial physiology and function. We then explore the mechanisms behind endometrial aging in its individual cellular compartments. Finally, we highlight lessons about endometrial aging gleaned from rodent and human clinical studies and propose opportunities for future study to better understand the contribution of the endometrium to age-related decline in fertility.

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.

Single-cell DNA sequencing reveals a high incidence of chromosomal abnormalities in human blastocysts.

Aneuploidy, a deviation from the normal chromosome copy number, is common in human embryos and is considered a primary cause of implantation failure and early pregnancy loss. Meiotic errors lead to uniformly abnormal karyotypes, while mitotic errors lead to chromosomal mosaicism: the presence of cells with at least two different karyotypes within an embryo. Knowledge about mosaicism in blastocysts mainly derives from bulk DNA sequencing of multicellular trophectoderm (TE) and/or inner cell mass (ICM) samples. However, this can only detect an average net gain or loss of DNA above a detection threshold of 20-30%. To accurately assess mosaicism, we separated the TE and ICM of 55 good quality surplus blastocysts and successfully applied single-cell whole genome sequencing (scKaryo-seq) on 1057 cells. Mosaicism involving numerical and structural chromosome abnormalities was detected in 82% of the embryos, where most abnormalities affected less than 20% of the cells. Structural abnormalities, potentially caused by replication stress and DNA damage, were observed in 69% of the embryos. In conclusion, our findings indicated that mosaicism is prevalent in good-quality blastocysts, while these blastocysts would likely be identified as normal with current bulk DNA sequencing techniques used for preimplantation genetic testing for aneuploidy (PGT-A).

Aberrant landscapes of maternal meiotic crossovers contribute to aneuploidies in human embryos.

Meiotic recombination is crucial for human genetic diversity and chromosome segregation accuracy. Understanding its variation across individuals and the processes by which it goes awry are long-standing goals in human genetics. Current approaches for inferring recombination landscapes rely either on population genetic patterns of linkage disequilibrium (LD)-capturing a time-averaged view-or on direct detection of crossovers in gametes or multigeneration pedigrees, which limits data set scale and availability. Here, we introduce an approach for inferring sex-specific recombination landscapes using data from preimplantation genetic testing for aneuploidy (PGT-A). This method relies on low-coverage (<0.05×) whole-genome sequencing of in vitro fertilized (IVF) embryo biopsies. To overcome the data sparsity, our method exploits its inherent relatedness structure, knowledge of haplotypes from external population reference panels, and the frequent occurrence of monosomies in embryos, whereby the remaining chromosome is phased by default. Extensive simulations show our method's high accuracy, even at coverages as low as 0.02×. Applying this method to PGT-A data from 18,967 embryos, we mapped 70,660 recombination events with ∼150 kbp resolution, replicating established sex-specific recombination patterns. We observed a reduced total length of the female genetic map in trisomies compared with disomies, as well as chromosome-specific alterations in crossover distributions. Based on haplotype configurations in pericentromeric regions, our data indicate chromosome-specific propensities for different mechanisms of meiotic error. Our results provide a comprehensive view of the role of aberrant meiotic recombination in the origins of human aneuploidies and offer a versatile tool for mapping crossovers in low-coverage sequencing data from multiple siblings.

First evidence of gonadal hybrid dysgenesis in Chagas disease vectors (Hemiptera, Triatominae): gonad atrophy prevents events of interspecific gene flow and introgression

BackgroundHybridization events between Triatoma spp. have been observed under both natural and laboratory conditions. The ability to produce hybrids can influence different aspects of the parent species, and may even result in events of introgression, speciation and extinction. Hybrid sterility is caused by unviable gametes (due to errors in chromosomal pairing [meiosis]) or by gonadal dysgenesis (GD). All of the triatomine hybrids analyzed so far have not presented GD. We describe here for the first time GD events in triatomine hybrids and highlight these taxonomic and evolutionary implications of these events.MethodsReciprocal experimental crosses were performed between Triatoma longipennis and Triatoma mopan. Intercrosses were also performed between the hybrids, and backcrosses were performed between the hybrids and the parent species. In addition, morphological and cytological analyzes were performed on the atrophied gonads of the hybrids.ResultsHybrids were obtained only for the crosses T. mopan♀ × T. longipennis♂. Intercrosses and backcrosses did not result in offspring. Morphological analyses of the male gonads of the hybrids confirmed that the phenomenon that resulted in sterility of the hybrid was bilateral GD (the gonads of the hybrids were completely atrophied). Cytological analyses of the testes of the hybrids also confirmed GD, with no germ cells observed (only somatic cells, which make up the peritoneal sheath).ConclusionsThe observations made during this study allowed us to characterize, for the first time, GD in triatomines and demonstrated that gametogenesis does not occur in atrophied gonads. The characterization of GD in male hybrids resulting from the crossing of T. mopan♀ × T. longipennis♂ highlights the importance of evaluating both the morphology and the cytology of the gonads to confirm which event resulted in the sterility of the hybrid: GD (which results in no gamete production) or meiotic errors (which results in non-viable gametes).Graphical

Post-zygotic rescue of meiotic errors causes brain mosaicism and focal epilepsy.

Somatic mosaicism is a known cause of neurological disorders, including developmental brain malformations and epilepsy. Brain mosaicism is traditionally attributed to post-zygotic genetic alterations arising in fetal development. Here we describe post-zygotic rescue of meiotic errors as an alternate origin of brain mosaicism in patients with focal epilepsy who have mosaic chromosome 1q copy number gains. Genomic analysis showed evidence of an extra parentally derived chromosome 1q allele in the resected brain tissue from five of six patients. This copy number gain is observed only in patient brain tissue, but not in blood or buccal cells, and is strongly enriched in astrocytes. Astrocytes carrying chromosome 1q gains exhibit distinct gene expression signatures and hyaline inclusions, supporting a novel genetic association for astrocytic inclusions in epilepsy. Further, these data demonstrate an alternate mechanism of brain chromosomal mosaicism, with parentally derived copy number gain isolated to brain, reflecting rescue in other tissues during development.

Aneuploidy in oocytes from women of advanced maternal age: analysis of the causal meiotic errors and impact on embryo development.

In oocytes of advanced maternal age (AMA) women, what are the mechanisms leading to aneuploidy and what is the association of aneuploidy with embryo development? Known chromosome segregation errors such as precocious separation of sister chromatids explained 90.4% of abnormal chromosome copy numbers in polar bodies (PBs), underlying impaired embryo development. Meiotic chromosomal aneuploidies in oocytes correlate with AMA (>35 years) and can affect over half of oocytes in this age group. This underlies the rationale for PB biopsy as a form of early preimplantation genetic testing for aneuploidy (PGT-A), as performed in the 'ESHRE STudy into the Evaluation of oocyte Euploidy by Microarray analysis' (ESTEEM) randomized controlled trial (RCT). So far, chromosome analysis of oocytes and PBs has shown that precocious separation of sister chromatids (PSSC), Meiosis II (MII) non-disjunction (ND), and reverse segregation (RS) are the main mechanisms leading to aneuploidy in oocytes. Data were sourced from the ESTEEM study, a multicentre RCT from seven European centres to assess the clinical utility of PGT-A on PBs using array comparative genomic hybridization (aCGH) in patients of AMA (36-40 years). This included data on the chromosome complement in PB pairs (PGT-A group), and on embryo morphology in a subset of embryos, up to Day 6 post-insemination, from both the intervention (PB biopsy and PGT-A) and control groups. ESTEEM recruited 396 AMA patients: 205 in the intervention group and 191 in the control group. Complete genetic data from 693 PB pairs were analysed. Additionally, the morphology from 1034 embryos generated from fertilized oocytes (two pronuclei) in the PB biopsy group and 1082 in the control group were used for statistical analysis. Overall, 461/693 PB pairs showed abnormal segregation in 1162/10 810 chromosomes. The main observed abnormal segregations were compatible with PSSC in Meiosis I (MI) (n = 568/1162; 48.9%), ND of chromatids in MII or RS (n = 417/1162; 35.9%), and less frequently ND in MI (n = 65/1162; 5.6%). For 112 chromosomes (112/1162; 9.6%), we observed a chromosome copy number in the first PB (PB1) and second PB (PB2) that is not explained by any of the known mechanisms causing aneuploidy in oocytes. We observed that embryos in the PGT-A arm of the RCT did not have a significantly different morphology between 2 and 6 days post-insemination compared to the control group, indicating that PB biopsy did not affect embryo quality. Following age-adjusted multilevel mixed-effect ordinal logistic regression models performed for each embryo evaluation day, aneuploidy was associated with a decrease in embryo quality on Day 3 (adjusted odds ratio (aOR) 0.62, 95% CI 0.43-0.90), Day 4 (aOR 0.15, 95% CI 0.06-0.39), and Day 5 (aOR 0.28, 95% CI 0.14-0.58). RS cannot be distinguished from normal segregation or MII ND using aCGH. The observed segregations were based on the detected copy number of PB1 and PB2 only and were not confirmed by the analysis of embryos. The embryo morphology assessment was static and single observer. Our finding of frequent unexplained chromosome copy numbers in PBs indicates that our knowledge of the mechanisms causing aneuploidy in oocytes is incomplete. It challenges the dogma that aneuploidy in oocytes is exclusively caused by mis-segregation of chromosomes during MI and MII. Data were mined from a study funded by ESHRE. Illumina provided microarrays and other consumables necessary for aCGH testing of PBs. None of the authors have competing interests. Data were mined from the ESTEEM study (ClinicalTrials.gov Identifier NCT01532284).