Self-diploidization of human haploid parthenogenetic embryos through the Rho pathway regulates endomitosis and failed cytokinesis

Evaluation of guide-free Cas9-induced genomic damage and transcriptome changes in pig embryos

Cas9 protein without sgRNAs can induce genomic damage at the cellular level in vitro. However, whether the detrimental effects occur in embryos after Cas9 treatment remains unknown. Here, using pig embryos as subjects, we observed that Cas9 protein transcribed from injected Cas9 mRNA can persist until at least the blastocyst stage. Cas9 protein alone can induce genome damage in preimplantation embryos, represented by the increased number of phosphorylated histone H2AX foci on the chromatin fiber, which led to apoptosis and decreased cell number of blastocysts. In addition, single-blastocyst RNA sequencing confirmed that Cas9 protein without sgRNAs can cause changes in the blastocyst transcriptome, depressing embryo development signal pathways, such as cell cycle, metabolism, and cellular communication-related signal pathways, while activating apoptosis and necroptosis signal pathways, which together resulted in impaired preimplantation embryonic development. These results indicated that attention should be given to the detrimental effects caused by the Cas9 protein when using CRISPR-Cas9 for germline genome editing, especially for the targeted correction of human pathological mutations using germline gene therapy.

Transcriptome profiles and chromatin states in mouse androgenetic haploid embryonic stem cells

Haploid embryonic stem cells (haESCs) are derived from the inner cell mass of the haploid blastocyst, containing only one set of chromosomes. Extensive and accurate chromatin remodelling occurs during haESC derivation, but the intrinsic transcriptome profiles and chromatin structure of haESCs have not been fully explored. We profiled the transcriptomes, nucleosome positioning, and key histone modifications of four mouse haESC lines, and compared these profiles with those of other closely-related stem cell lines, MII oocytes, round spermatids, sperm, and mouse embryonic fibroblasts. haESCs had transcriptome profiles closer to those of naïve pluripotent stem cells. Consistent with the one X chromosome in haESCs, Xist was repressed, indicating no X chromosome inactivation. haESCs and ESCs shared a similar global chromatin structure. However, a nucleosome depletion region was identified in 2056 promoters in ESCs, which was absent in haESCs. Furthermore, three characteristic spatial relationships were formed between transcription factor motifs and nucleosomes in both haESCs and ESCs, specifically in the linker region, on the nucleosome central surface, and nucleosome borders. Furthermore, the chromatin state of 4259 enhancers was off in haESCs but active in ESCs. Functional annotation of these enhancers revealed enrichment in regulation of the cell cycle, a predominantly reported mechanism of haESC self-diploidization. Notably, the transcriptome profiles and chromatin structure of haESCs were highly preserved during passaging but different from those of differentiated cell types.

Preimplantation chromosomal mosaics, chimaeras and confined placental mosaicism

Some human preimplantation embryos are chromosomally mosaic. For technical reasons, estimates of the overall frequency vary widely from <15 to >90% and the true frequency remains unknown. Aneuploid/diploid and aneuploid/aneuploid mosaics typically arise during early cleavage stages before the embryonic genome is fully activated and when cell cycle checkpoints are not operating normally. Other mosaics include chaotic aneuploid mosaics and mixoploids, some of which arise by abnormal chromosome segregation at the first cleavage division. Chimaeras are similar to mosaics, in having two genetically distinct cell populations, but they arise from more than one zygote and occur less often. After implantation, the frequency of mosaic embryos declines to about 2% and most are trisomic/diploid mosaics, with trisomic cells confined to the placenta. Thus, few babies are born with chromosomal mosaicism. This review discusses the origin of different types of chromosomal mosaics and chimaeras; their fate and the relationship between preimplantation chromosomal mosaicism and confined placental mosaicism in human conceptuses and animal models. Abnormal cells in mosaic embryos may be depleted by cell death, other types of cell selection or cell correction but the most severely affected mosaic embryos probably die. Trisomic cells could become restricted to placental lineages if cell selection or correction is less effective in placental lineages and/or they are preferentially allocated to a placental lineage. However, the relationship between preimplantation mosaicism and confined placental mosaicism may be complex because the specific chromosome(s) involved will influence whether chromosomally abnormal cells survive predominately in the placental trophoblast and/or placental mesenchyme.

Lay summary

Human cells normally have 23 pairs of chromosomes, which carry the genes. During the first few days of development, some human embryos are chromosomal mosaics. These mosaic embryos have both normal cells and cells with an abnormal number of chromosomes, which arise from the same fertilised egg. (More rarely, the different cell populations arise from more than one fertilised egg and these embryos are called chimaeras.) If chromosomally abnormal cells survive to term, they could cause birth defects. However, few abnormal cells survive and those that do are usually confined to the placenta, where they are less likely to cause harm. It is not yet understood how this restriction occurs but the type of chromosomal abnormality influences which placental tissues are affected. This review discusses the origin of different types of chromosomally abnormal cells, their fate and how they might become confined to the placenta in humans and animal models.

Delayed DNA replication in haploid human embryonic stem cells

Haploid human embryonic stem cells (ESCs) provide a powerful genetic system but diploidize at high rates. We hypothesized that diploidization results from aberrant DNA replication. To test this, we profiled DNA replication timing in isogenic haploid and diploid ESCs. The greatest difference was the earlier replication of the X Chromosome in haploids, consistent with the lack of X-Chromosome inactivation. We also identified 21 autosomal regions that had delayed replication in haploids, extending beyond the normal S phase and into G2/M. Haploid-delays comprised a unique set of quiescent genomic regions that are also underreplicated in polyploid placental cells. The same delays were observed in female ESCs with two active X Chromosomes, suggesting that increased X-Chromosome dosage may cause delayed autosomal replication. We propose that incomplete replication at the onset of mitosis could prevent cell division and result in re-entry into the cell cycle and whole genome duplication.

Rescuing monopronucleated-derived human blastocysts: a model to study chromosomal topography and fingerprinting

OBJECTIVE

To quantify the percentage of monopronuclear-derived blastocysts (MNBs) that are potentially useful for reproductive purposes using classic and state-of-the-art chromosome analysis approaches, and to study chromosomal distribution in the inner cell mass (ICM) and trophectoderm (TE) for intertissue/intratissue concordance comparison.

DESIGN

Prospective experimental study.

SETTING

Single-center in vitro fertilization clinic and reproductive genetics laboratory.

PATIENT(S)

A total of 1,128 monopronuclear zygotes were obtained between June 2016 and December 2018.

INTERVENTION(S)

MNBs were whole-fixed or biopsied to obtain a portion of ICM and 2 TE portions (TE1 and TE2) and were subsequently analyzed by fluorescence in situ hybridization, new whole-genome sequencing, and fingerprinting by single-nucleotide polymorphism array-based techniques (a-SNP).

MAIN OUTCOME MEASURE(S)

We assessed MNB rate, ploidy rate, and chromosomal constitution by new whole-genome sequencing, and parental composition by comparative a-SNP, performed in a "trio"-format (embryo/parents). The 24-chromosome distribution was compared between the TE and the ICM and within the TE.

RESULT(S)

A total of 18.4% of monopronuclear zygotes progressed to blastocysts; 77.6% of MNBs were diploid; 20% of MNBs were male and euploid, which might be reproductively useful. Seventy-five percent of MNBs were biparental and half of them were euploid, indicating that 40% might be reproductively useful. Intratissue concordance (TE1/TE2) was established for 93.3% and 73.3% for chromosome matching. Intertissue concordance (TE/ICM) was established for 78.8%, but 57.6% for chromosome matching. When segmental aneuploidy was not considered, intratissue concordance and chromosome matching increased to 100% and 80%, respectively, and intertissue concordance and chromosome matching increased to 84.8% and 75.8%, respectively.

CONCLUSION(S)

The a-SNP-trio strategy provides information about ploidy, euploidy, and parental origin in a single biopsy. This approach enabled us to identify 40% of MNBs with reproductive potential, which can have a significant effect in the clinical setting. Additionally, segmental aneuploidy is relevant for mismatched preimplantation genetic testing of aneuploidies, both within and between MNB tissues. Repeat biopsy might clarify whether segmental aneuploidy is a prone genetic character.

Dysregulated Gene Expression of Imprinted and X-Linked Genes: A Link to Poor Development of Bovine Haploid Androgenetic Embryos

Mammalian uniparental embryos are efficient models for genome imprinting research and allow studies on the contribution of the paternal and maternal genomes to early embryonic development. In this study, we analyzed different methods for production of bovine haploid androgenetic embryos (hAE) to elucidate the causes behind their poor developmental potential. Results indicate that hAE can be efficiently generated by using intracytoplasmic sperm injection and oocyte enucleation at telophase II. Although androgenetic haploidy does not disturb early development up to around the 8-cell stage, androgenetic development is disturbed after the time of zygote genome activation and hAE that reach the morula stage are less capable to reach the blastocyst stage of development. Karyotypic comparisons to parthenogenetic- and ICSI-derived embryos excluded chromosomal segregation errors as causes of the developmental constraints of hAE. However, analysis of gene expression indicated abnormal levels of transcripts for key long non-coding RNAs involved in X chromosome inactivation and genomic imprinting of the KCNQ1 locus, suggesting an association with X chromosome and some imprinted loci. Moreover, transcript levels of methyltransferase 3B were significantly downregulated, suggesting potential anomalies in hAE establishing de novo methylation. Finally, the methylation status of imprinted control regions for XIST and KCNQ1OT1 genes remained hypomethylated in hAE at the morula and blastocyst stages, confirming their origin from spermatozoa. Thus, our results exclude micromanipulation and chromosomal abnormalities as major factors disturbing the normal development of bovine haploid androgenotes. In addition, although the cause of the arrest remains unclear, we have shown that the inefficient development of haploid androgenetic bovine embryos to develop to the blastocyst stage is associated with abnormal expression of key factors involved in X chromosome activity and genomic imprinting.

A Genetic Screen Identifies Etl4-Deficiency Capable of Stabilizing the Haploidy in Embryonic Stem Cells

Mammalian haploid embryonic stem cells (haESCs) hold great promise for functional genetic studies and forward screening. However, all established haploid cells are prone to spontaneous diploidization during long-term culture, rendering application challenging. Here, we report a genome-wide loss-of-function screening that identified gene mutations that could significantly reduce the rate of self-diploidization in haESCs. We further demonstrated that CRISPR/Cas9-mediated Etl4 knockout (KO) stabilizes the haploid state in different haESC lines. More interestingly, Etl4 deficiency increases mitochondrial oxidative phosphorylation (OXPHOS) capacity and decreases glycolysis in haESCs. Mimicking this effect by regulating the energy metabolism with drugs decreased the rate of self-diploidization. Collectively, our study identified Etl4 as a novel haploidy-related factor linked to an energy metabolism transition occurring during self-diploidization of haESCs.

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A genome-wide genetic screen identifies several haploidy-related factors in haESCs

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Etl4-deficiency stabilizes the haploid state in different haESC lines

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Etl4-deficiency increases mitochondrial OXPHOS and decrease glycolysis in haESCs

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Energy metabolism regulation with drugs decreased the rate of self-diploidization

Kinetics of meiotic maturation in oocytes from unstimulated ovaries and duration of pronucleus presence and preimplantation development

OBJECTIVE

To evaluate the meaning of meiotic maturation kinetics and duration of pronucleus presence (DPP) for parthenogenetic activation outcome.

DESIGN

Retrospective study.

SETTING

University hospital.

PATIENT(S)

Eight patients with endometrioid adenocarcinoma and 65 patients who underwent in vitro fertilization (IVF) with intracytoplasmic sperm injection (ICSI).

INTERVENTION(S)

After collection of oocytes from nonstimulated ovaries of patients with endometrioid adenocarcinoma, in vitro maturation (IVM) and parthenogenetic activation performed with time-lapse imaging; after ICSI, embryos similarly incubated with time-lapse imaging.

MAIN OUTCOME MEASURE(S)

Timing of the release of the first polar body (fPB), DPP, and developmental stage with IVM and parthenogenetic activation; after ICSI, assessment of DPP and preimplantation developmental stage.

RESULT(S)

With IVM, 55.2% of oocytes matured; 53.1% of fPBs were released within 24 hours, and 46.9% of fPBs were released after 24 hours. Regarding developmental stage, oocytes that released fPB later during IVM tended to develop more than oocytes that released the fPB within 24 hours. For embryos from parthenogenetic activation the DPP was statistically significantly shorter than the DPP of embryos from ICSI. With ICSI, the DPP was statistically significantly shorter in embryos that developed to ≥8 cells than embryos whose final development included ≤7 cells. The development rate in parthenogenetic activation was statistically significantly lower than that in ICSI.

CONCLUSION(S)

Embryo development is negatively affected by DPP that is too short or too long. When the DPP was short with parthenogenetic activation, embryo development did not proceed, indicating that DPP is an important determinant of parthenogenetic activation outcomes as with the timing of fPB release.

Mammalian haploid stem cells: establishment, engineering and applications

Haploid embryonic stem cells (haESCs) contain only one set of genomes inherited from the sperm or egg and are termed AG- or PG-haESCs, respectively. Mammalian haESCs show genome-wide hypomethylation and dysregulated imprinting, whereas they can sustain genome integrity during derivation and long-term propagation. In addition, haESCs exhibit similar pluripotency to traditional diploid ESCs but are unique because they function as gametes and have been used to produce semi-cloned animals. More strikingly, unisexual reproduction has been achieved in mice by using haESCs. In combination with a gene editing or screening system, haESCs represent a powerful tool for studies of underlying gene functions and explorations of mechanisms of genetic and epigenetic regulation not only at the cellular level in vitro but also at the animal level in vivo. More importantly, genetically edited AG-haESC lines may further serve as an ideal candidate for the establishment of a sperm bank, which is a highly cost-effective approach, and a wide range of engineered semi-cloned mice have been produced. Here, we review the historical development, characteristics, advantages and disadvantages of haESCs. Additionally, we present an in-depth discussion of the recent advances in haESCs and their potential applications.