DYSMORPHISMS OF OOCYTES IN ART PROCEDURES: A LITERATURE REVIEW

Relevance: Assisted reproductive technologies (ART) are rapidly developing and in recent decades have become increasingly important due to the growing number of infertile couples around the world. Human oocytes are the main objects used in ART procedures. Consequently, the quality of oocytes can determine the key parameters of ART. The purpose of this review was to analyze the literature and the results of studies in the field of ART devoted to extracytoplasmic dysmorphisms of human oocytes – morphological changes outside the cytoplasmic structure of oocytes, their effect on fertilization, cleavage, implantation frequency, clinical pregnancy rate, as well as the possibility of their use as biomarkers for predicting the quality of embryos, blastocysts, and their further implantation potential. Materials and Methods: This literature review was based on a search conducted among domestic and foreign publications for 2000-2020 available in Russian and international search systems (PubMed, eLibrary) using the keywords «infertility,” “IVF,” «oocyte,” “morphological assessment of oocytes,” “dysmorphisms of oocytes ,” and “ assisted reproductive technologies.” Results: This literature review contains literature data and the analysis of research results in the field of ART devoted to the morphological qualities and abnormalities (dysmorphisms) of human oocytes. It describes the types of extracytoplasmic abnormalities encountered in the clinical practice of in-vitro fertilization, their effect on fertilization, cleavage, implantation rate, and clinical pregnancy rate, as well as the possibility of their use as biomarkers to predict the quality of embryos and blastocysts and their further implantation potential.

Dynamics of poly(A) tail length and non-A residues during the human oocyte-to-embryo transition

Poly(A) tail-mediated post-transcriptional regulation of maternal mRNA has been shown to be vital in the oocyte-to-embryo transition (OET) in flies, fish, frogs, and mice1–8. However, nothing is known about poly(A) tail dynamics for even a single gene during the human OET, because of the limited availability of human oocytes and embryos in combination with the low sensitivity of previous methods. Here, we systematically profiled the transcriptome-wide mRNA poly(A) tails in human oocytes at the germ-vesicle (GV), metaphase I (MI), and metaphase II (MII) stages, as well as pre-implantation embryos at the 1-cell (1C), 2-cell (2C), 4-cell (4C), 8-cell (8C), morula (MO), and blastocyst (BL) stages using single-oocyte/embryo PAIso-seq1 and PAIso-seq2 methods. We show that poly(A) tail length is highly dynamic during the OET, with BTG4 responsible for global deadenylation. Moreover, we reveal that non-A residues occur primarily in poly(A) tails of maternal RNA, which begin to increase at the MI stage, become highly abundant after fertilization (with U residues occurring in about two thirds, G residues in about one third, and C residues in about one fifth of mRNAs), and decline at the 8C stage. Importantly, we reveal that TUT4/7 can add U residues to deadenylated mRNA, which can then be re- polyadenylated to produce 5′-end and internal U residues. In addition, the re- polyadenylated mRNA can be stabilized through the addition of G residues by TENT4A/B. Finally, we demonstrate that U residues in poly(A) tails mark the maternal transcripts for quicker degradation in 8C human embryos compared to those without U residues. Together, our results not only reveal the dynamics of poly(A) tail length and non-A residues, but also provide mechanistic insights into the regulation of the length and the role of non-A residues during human OET. These findings further scientific understanding and open a new door for studying the human OET.

Maternal mRNA deadenylation is defective in in vitro matured mouse and human oocytes

Oocyte in vitro maturation is a technique of assisted reproductive technology that was first introduced in patients with polycystic ovarian syndrome and is now used in most fertility clinics. Thousands of genes show abnormally high expression in in vitro maturated metaphase II (in vitro MII) oocytes compared with in vivo maturated metaphase II (in vivo MII) oocytes in bovines, mice, and humans. However, the underlying mechanisms of this abnormal expression are still poorly understood. In this study, we use PAIso-seq1 to reveal a transcriptome-wide expression profile of full-length transcripts containing entire poly(A) tails in in vivo and in vitro matured mouse and human oocytes. Our results indicate that more genes are up-regulated than down-regulated in in vitro MII oocytes in both mice and humans. Furthermore, we demonstrate that the observed increase in maternal mRNA abundance is caused by impaired deadenylation in in vitro MII oocytes in both mice and humans. We also found that the cytoplasmic polyadenylation of dormant Btg4 and Cnot7 mRNAs, which encode key components of deadenylation machinery, is impaired in in vitro MII oocytes in mice and humans respectively, likely contributing to reduced translation and impaired global maternal mRNA deadenylation. Our findings highlight that impaired maternal mRNA deadenylation is a definite molecular defect in in vitro MII oocytes in both mice and humans. The findings here offer a new criterion for evaluating the quality of in vitro MII oocytes and a potential direction for improving in vitro maturation by fixing the dysregulated maternal mRNA deadenylation.