Chromosomal instability in rhesus macaque preimplantation embryos

SAC during early cell divisions: Sacrificing fidelity over timely division, regulated differently across organisms: Chromosome alignment and segregation are left unsupervised from the onset of development until checkpoint activity is acquired, varying from species to species

Early embryogenesis is marked by a frail Spindle Assembly Checkpoint (SAC). The time of SAC acquisition varies depending on the species, cell size or a yet to be uncovered developmental timer. This means that for a specific number of divisions, biorientation of sister chromatids occurs unsupervised. When error-prone segregation is an issue, an aneuploidy-selective apoptosis system can come into play to eliminate chromosomally unbalanced cells resulting in healthy newborns. However, aneuploidy content can be too great to overcome, endangering viability. SAC generates a diffusible signal to lengthen time spent in mitosis if needed, ensuring correct chromosome segregation, a fundamental factor in the generation of euploid cells. Thus, it remains puzzling what benefit could come from delaying SAC acquisition till later in the development. In this review, we describe what is known on SAC acquisition in distinct species and highlight pending research as well as potential applications for such knowledge.

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.

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.

Transcriptome analyses of rhesus monkey preimplantation embryos reveal a reduced capacity for DNA double-strand break repair in primate oocytes and early embryos

Preimplantation embryogenesis encompasses several critical events including genome reprogramming, zygotic genome activation (ZGA), and cell-fate commitment. The molecular basis of these processes remains obscure in primates in which there is a high rate of embryo wastage. Thus, understanding the factors involved in genome reprogramming and ZGA might help reproductive success during this susceptible period of early development and generate induced pluripotent stem cells with greater efficiency. Moreover, explaining the molecular basis responsible for embryo wastage in primates will greatly expand our knowledge of species evolution. By using RNA-seq in single and pooled oocytes and embryos, we defined the transcriptome throughout preimplantation development in rhesus monkey. In comparison to archival human and mouse data, we found that the transcriptome dynamics of monkey oocytes and embryos were very similar to those of human but very different from those of mouse. We identified several classes of maternal and zygotic genes, whose expression peaks were highly correlated with the time frames of genome reprogramming, ZGA, and cell-fate commitment, respectively. Importantly, comparison of the ZGA-related network modules among the three species revealed less robust surveillance of genomic instability in primate oocytes and embryos than in rodents, particularly in the pathways of DNA damage signaling and homology-directed DNA double-strand break repair. This study highlights the utility of monkey models to better understand the molecular basis for genome reprogramming, ZGA, and genomic stability surveillance in human early embryogenesis and may provide insights for improved homologous recombination-mediated gene editing in monkey.

Early embryo mortality in natural human reproduction: What the data say

How many human embryos die between fertilisation and birth under natural conditions? It is widely accepted that natural human embryo mortality is high, particularly during the first weeks after fertilisation, with total prenatal losses of 70% and higher frequently claimed. However, the first external sign of pregnancy occurs two weeks after fertilisation with a missed menstrual period, and establishing the fate of embryos before this is challenging. Calculations are additionally hampered by a lack of data on the efficiency of fertilisation under natural conditions. Four distinct sources are used to justify quantitative claims regarding embryo loss: (i) a hypothesis published by Roberts & Lowe in TheLancet  is widely cited but has no practical quantitative value; (ii) life table analyses give consistent assessments of clinical pregnancy loss, but cannot illuminate losses at earlier stages of development; (iii) studies that measure human chorionic gonadotrophin (hCG) reveal losses in the second week of development and beyond, but not before; and (iv) the classic studies of Hertig and Rock offer the only direct insight into the fate of human embryos from fertilisation under natural conditions. Re-examination of Hertig's data demonstrates that his estimates for fertilisation rate and early embryo loss are highly imprecise and casts doubt on the validity of his numerical analysis. A recent re-analysis of hCG study data concluded that approximately 40-60% of embryos may be lost between fertilisation and birth, although this will vary substantially between individual women. In conclusion, natural human embryo mortality is lower than often claimed and widely accepted. Estimates for total prenatal mortality of 70% or higher are exaggerated and not supported by the available data.

Early embryo mortality in natural human reproduction: What the data say

It is generally accepted that natural human embryo mortality during pregnancy is high - losses of 70% and higher from fertilisation to birth are frequently claimed. The first external sign of pregnancy occurs two weeks after fertilisation with a missed menstrual period. Establishing the fate of embryos before this is challenging, and hampered by a lack of data on the efficiency of fertilisation under natural conditions. Four distinct sources are cited to justify quantitative claims regarding embryo loss: (i) a hypothesis published by Roberts & Lowe in TheLancet  is widely cited but has no quantitative value; (ii) life table analyses give consistent assessments of clinical pregnancy loss, but cannot illuminate losses at earlier stages of development; (iii) studies that measure human chorionic gonadotrophin (hCG) reveal losses in the second week of development and beyond, but not before; and (iv) the classic studies of Hertig and Rock offer the only direct insight into the fate of human embryos from fertilisation under natural conditions. Re-examination of Hertig's data demonstrates that his estimates for fertilisation rate and early embryo loss are highly imprecise and casts doubt on the validity of his numerical analysis. A recent re-analysis of hCG study data suggests that approximately 40-60% of embryos may be lost between fertilisation and birth, although this will vary substantially between individual women. In conclusion, it is clear that some published estimates of natural embryo mortality are exaggerated. Although available data do not provide a precise estimate, natural human embryo mortality is lower than is often claimed.

Self-correction of chromosomal abnormalities in human preimplantation embryos and embryonic stem cells

Aneuploidy is commonly seen in human preimplantation embryos, most particularly at the cleavage stage because of genome activation by third cell division. Aneuploid embryos have been used for the derivation of normal embryonic stem cell (ESC) lines and developmental modeling. This review addresses aneuploidies in human preimplantation embryos and human ESCs and the potential of self-correction of these aberrations. Diploid-aneuploid mosaicism is the most frequent abnormality observed; hence, embryos selected by preimplantation genetic diagnosis at the cleavage or blastocyst stage could be partly abnormal. Differentiation is known as the barrier for eliminating mosaic embryos by death and/or decreased division of abnormal cells. However, some mosaicisms, such as copy number variations could be compatible with live birth. Several reasons have been proposed for self-correction of aneuploidies during later stages of development, including primary misdiagnosis, allocation of the aneuploidy in the trophectoderm, cell growth advantage of diploid cells in mosaic embryos, lagging of aneuploid cell division, extrusion or duplication of an aneuploid chromosome, and the abundance of DNA repair gene products. Although more studies are needed to understand the mechanisms of self-correction as a rare phenomenon, most likely, it is related to overcoming mosaicism.

Expression profiles of cohesins, shugoshins and spindle assembly checkpoint genes in rhesus macaque oocytes predict their susceptibility for aneuploidy during embryonic development

High frequencies of chromosomal anomalies are reported in human and non-human primate in vitro-produced preimplantation embryos. It is unclear why certain embryos develop aneuploidies while others remain euploid. A differential susceptibility to aneuploidy is most likely a consequence of events that occur before oocyte collection. One hypothesis is that the relative transcript levels of cohesins, shugoshins and spindle assembly checkpoint genes are correlated with the occurrence of chromosomal anomalies. Transcript levels of these genes were quantified in individual oocytes that were either mature (group 1, low aneuploidy rate) or immature (group 2, high aneuploidy rate) at retrieval, utilizing TaqMan-based real-time PCR. The transcript level in each oocyte was categorized as absent, below the median or above the median in order to conduct comparisons. Statistically significant differences were observed between group 1 and group 2 for SGOL1 and BUB1. There were more oocytes with SGOL1 expression levels above the median in group 1, while oocytes lacking BUB1 were only observed in group 1. These findings suggest that higher SGOL1 levels in group 1 oocytes could better protect against a premature separation of sister chromatids than in embryos derived from group 2 oocytes. The absence of BUB1 transcripts in group 1 was frequently associated with reduced expression of either mitotic cohesins or shugoshins. We hypothesize that ablation of BUB1 could induce mitotic arrest in oocytes that fail to express a complete complement of cohesins and shugoshins, thereby reducing the number of developing aneuploid preimplantation embryos.

The evolution of menstruation: a new model for genetic assimilation: explaining molecular origins of maternal responses to fetal invasiveness

Why do humans menstruate while most mammals do not? Here, we present our answer to this long-debated question, arguing that (i) menstruation occurs as a mechanistic consequence of hormone-induced differentiation of the endometrium (referred to as spontaneous decidualization, or SD); (ii) SD evolved because of maternal-fetal conflict; and (iii) SD evolved by genetic assimilation of the decidualization reaction, which is induced by the fetus in non-menstruating species. The idea that menstruation occurs as a consequence of SD has been proposed in the past, but here we present a novel hypothesis on how SD evolved. We argue that decidualization became genetically stabilized in menstruating lineages, allowing females to prepare for pregnancy without any signal from the fetus. We present three models for the evolution of SD by genetic assimilation, based on recent advances in our understanding of the mechanisms of endometrial differentiation and implantation. Testing these models will ultimately shed light on the evolutionary significance of menstruation, as well as on the etiology of human reproductive disorders like endometriosis and recurrent pregnancy loss.

Increased incidence of mosaicism detected by FISH in murine blastocyst cultured in vitro

The majority of in-vitro-derived human preimplantation embryos are chromosomally abnormal but whether the same pattern exists in vivo is unknown. This would be impossible to demonstrate in humans. Hence we chose murine embryos to study this difference owing to their ease of manipulation and compared the incidence of mosaicism between in-vivo- and in-vitro-cultured embryos. Two groups of embryos were analysed. Group A (in vitro) were obtained 48h following superovulation and cultured in vitro until the blastocyst stage. Fluorescent in-situ hybridization (FISH) was performed at different stages that included the cleavage, morula and blastocyst stage. Group B (in vivo) were obtained on day 2 or day 5 and FISH was performed immediately without culture. There was an increase in chromosomal mosaicism seen from the cleavage stage up to the blastocyst stage in the in-vitro culture group. Overall chromosomal abnormality from day 3 to day 5 was found to be 30% (28/94) in group A. The incidence of chromosomal abnormalities in blastocysts from group B was significantly lower than group A blastocysts (8% (3/40) and 31% (20/64) respectively; P<0.05). These data show that in-vitro cultured embryos had a significantly higher incidence of mosaicisim in comparison with the in-vivo group. Cultured human embryos show high levels of chromosomal abnormalities but whether this is a pattern seen in all embryos or is the result of culture is unknown. To study this pattern we used mouse embryos and carried out chromosome analysis by fluorescent in-situ hybridization. We compared embryos that were cultured (in vitro) with those that were not (in vivo, i.e. grown exclusively in the mouse). We found that cultured embryos showed significantly higher chromosomal abnormalities as compared with in vivo embryos. This suggests that certain culture conditions are responsible for the high level of chromosomal abnormalities seen in these embryos, which should be investigated further.

Inner cell mass localization of NANOG precedes OCT3/4 in rhesus monkey blastocysts

The mechanism by which the inner cell mass (ICM) and trophectoderm (TE) become specified is poorly understood. Considerable species variation is evident in the expression of lineage-specific and embryonic stem cell (ESC) regulatory markers. We sought to investigate localization patterns of these markers in rhesus macaque compact morulae and blastocysts. NANOG protein was restricted to the ICM of blastocysts. In contrast to a previous report, the expression of CDX2 was detected in the primate blastocyst, localized specifically to the TE. Unlike the mouse embryo, OCT4 protein was detected using two different antibodies in both the ICM and TE. The ubiquitous pattern of OCT4 expression is consistent with observations in human, cow, and pig embryos. Significantly, lack of restricted OCT4 protein, and ICM localization of NANOG in primate blastocysts, suggests that NANOG may determine inner cell mass fate more specifically during primate development or may be less susceptible to culture artifacts. These results contrast markedly with current mechanistic hypotheses, although other factors may lie upstream of NANOG to constitute a complex interactive network. This difference may also underlie observations that regulatory mechanisms in ESC differ between mice and primates.

Pluripotency genes overexpressed in primate embryonic stem cells are localized on homologues of human chromosomes 16, 17, 19, and X

While human embryonic stem cells (hESCs) are predisposed toward chromosomal aneploidities on 12, 17, 20, and X, rendering them susceptible to transformation, the specific genes expressed are not yet known. Here, by identifying the genes overexpressed in pluripotent rhesus ESCs (nhpESCs) and comparing them both to their genetically identical differentiated progeny (teratoma fibroblasts) and to genetically related differentiated parental cells (parental skin fibroblasts from whom gametes were used for ESC derivation), we find that some of those overexpressed genes in nhpESCs cluster preferentially on rhesus chromosomes 16, 19, 20, and X, homologues of human chromosomes 17, 19, 16, and X, respectively. Differentiated parental skin fibroblasts display gene expression profiles closer to nhpESC profiles than to teratoma cells, which are genetically identical to the pluripotent nhpESCs. Twenty over- and underexpressed pluripotency modulators, some implicated in neurogenesis, have been identified. The overexpression of some of these genes discovered using pedigreed nhpESCs derived from prime embryos generated by fertile primates, which is impossible to perform with the anonymously donated clinically discarded embryos from which hESCs are derived, independently confirms the importance of chromosome 17 and X regions in pluripotency and suggests specific candidates for targeting differentiation and transformation decisions.

Incidence of chromosomal mosaicism in morphologically normal nonhuman primate preimplantation embryos

OBJECTIVE

To establish the exact rates of chromosomal mosaicism in morphologically normal rhesus macaque embryos by determining the chromosomal complement of all blastomeres.

DESIGN

Retrospective rhesus monkey IVF study.

SETTING

Academic laboratory and primate research center.

PATIENT(S)

Young fertile rhesus macaque females.

INTERVENTION(S)

Morphologically normal in vitro-produced rhesus macaque embryos were dissociated and cytogenetically assessed using a five-color fluorescent in situ hybridization assay developed for rhesus macaque chromosomes homologous to human chromosomes 13, 16, 18, X, and Y.

MAIN OUTCOME MEASURE(S)

The incidence and extent of chromosomal mosaicism in rhesus macaque preimplantation embryos.

RESULT(S)

Seventy-seven preimplantation embryos, displaying normal morphology and development, from 17 young rhesus macaque females were analyzed. Overall, 39 embryos (50.6%) were normal, 14 embryos (18.2%) were completely abnormal, and 24 embryos (31.2%) were mosaic. Of the 226 blastomeres analyzed in the mosaic group, 110 blastomeres (48.7%) were normal.

CONCLUSION(S)

The observed rate of mosaicism in good-quality rhesus embryos resembles previously documented frequencies in poor-quality human preimplantation embryos. A high incidence of mosaicism may limit the diagnostic accuracy of preimplantation genetic diagnosis.

Rhesus macaque embryos derived from MI oocytes maturing after retrieval display high rates of chromosomal anomalies

BACKGROUND

Rhesus macaque and human preimplantation embryos display similar rates of chromosomal abnormalities. The aim of this study was to determine whether embryos developing from MI oocytes that mature post-retrieval display more chromosomal anomalies than those embryos that are generated from oocytes that are at MII at the time of retrieval.

METHODS

Rhesus macaque oocytes were obtained after hormonal ovarian stimulation. Immediately after retrieval, the oocytes were classified according to their maturational status. Following in vitro fertilization, Day 3 embryos with good morphology and development derived from oocytes maturing post-retrieval and those from oocytes that were mature at the time of retrieval were cytogenetically assessed using a five-color fluorescent in situ fluorescent hybridization assay developed for rhesus macaque chromosomes homologous to human chromosomes 13, 16, 18, X and Y.

RESULTS

Blastomeres from 53 embryos were analyzed. Of the 27 embryos that developed from oocytes that were mature at collection, 18 embryos were chromosomally normal (66.7%), while from the 26 embryos that developed from oocytes that matured post-retrieval, only 9 embryos were chromosomally normal (34.6%).

CONCLUSIONS

These results indicate that embryos developing from oocytes maturing post-retrieval display high rates of chromosomal abnormalities and have therefore a reduced developmental competence. As a result, the clinical relevance of using immature oocytes that are retrieved after stimulated cycles in human IVF warrants further investigation.