title: Biological Mechanisms Impacting Pre-Implantation Embryo Development
Creators: name: McCallie, Blair
Publisher: name: University of Kent
Year: 2021

description: It is estimated that one out of four couples in developed countries are affected by infertility. Assisted reproductive technologies (ART) have made crucial advancements in helping couples achieve the goal of parenthood. However, one of the many challenges facing this patient population is the failure of embryo implantation into the uterus during in vitro fertilization (IVF). The transfer of seemingly good-quality embryos can often result in a negative pregnancy outcome. There are many factors that can contribute to implantation potential, including embryo aneuploidy, advanced maternal age, and sperm and oocyte quality among many others. However, little is understood regarding the molecular mechanisms responsible for embryo development during the window of implantation. Therefore, the overall aim of this thesis was to understand the biological and epigenetic mechanisms during embryo implantation development by specifically investigating: (1) How both maternal age and/or chromosome constitution affect microRNA profiles and downstream target RNAs of pre-implantation blastocysts, (2) the hypothesis that advanced maternal age is impacting the overall transcriptome of the developing embryo, (3) if methylation alterations are present in aneuploid blastocysts and if this contributes to implantation potential, (4) how different types of infertility diagnoses are impacting the transcriptome of the pre-implantation blastocyst, and (5) if it is possible to detect polymorphisms in patients with premature diminished ovarian reserve and if methylation alterations are found in the germline of embryos from this infertile population which affect implantation potential. The conclusions of these aims are as follows: Both chromosome constitution, as well as advanced maternal age, affect miRNA profiles of the developing blastocyst. MiR-93 was found to be exclusively expressed in women of advanced maternal age and further up-regulated in aneuploid embryos. The increased expression of this miRNA was additionally found to result in a down-regulation of SIRT1, its target gene, which likely affects the oxidative stress defense mechanisms of the embryo, thereby reducing implantation potential. Additionally, an overall decreased global transcriptome was observed in maternally aged blastocysts, impacting biological pathways involved in cell growth and invasion which are vital to pre-implantation embryo development. Epigenetic alterations were also observed in aneuploid blastocysts. A hypomethylated state was revealed but only in monosomic embryos and only in the chromosome involved in the error. Decreased expression of developmental genes located on the chromosome of error were also observed. In contrast, the trisomic blastocyst displayed transcriptional dosage compensatory mechanisms for the presence of the additional chromosome. This might partially explain the difference in implantation potential between trisomic and monosomic embryos. Underlying infertility diagnosis was also found to have a significant impact on the blastocyst transcriptome. Alterations were observed for all infertility etiologies examined. Biological and molecular processes of the altered transcriptomes revealed both similarities, as well as differences, across the groups. Similarities included alterations to reproductive genes, cell adhesion, and response to stimulus genes among others. These processes are characterized by cells that are able to proliferate, migrate, and attach and are all crucial to embryo development and implantation. Lastly, young women with premature diminished ovarian reserve (DOR) were found to have critical variants in DNA sequence. Utilizing exome sequencing, a panel of single-nucleotide polymorphisms (SNPs) was able to distinguish DOR women