Program Director/Principal Investigator (Last, First, Middle): Chavez, Shawn L. Project Summary/Abstract Since its introduction over 35 years ago, human in vitro fertilization (IVF) has assumed great promise for infertile couples, but success rates have remained only ~30% worldwide for several decades. One of the primary reasons for this is that whole chromosomal abnormalities, or aneuploidy, are incredibly common in cleavage-stage human embryos. Previously, we demonstrated that assessing the time intervals of the first three mitotic divisions in conjunction with a phenomenon called cellular fragmentation, which is frequently observed in human embryos as well as following natural conception, largely distinguishes chromosomally normal and abnormal cleavage-stage human embryos. We also determined that cellular fragments might contain genetic material that likely began as mis-segregated chromosomes were encapsulated into micronuclei during meiosis and/or mitosis. Although cellular fragmentation is closely linked with aneuploidy generation and micronuclei formation, the source of these fragments and their precise chromosomal content is not well defined. In addition, whether embryos from other mammalian species more closely related to humans such as non-human primates have a similar aneuploidy frequency remains unknown and addressing this question is essential for potential translation to early human embryogenesis. Our preliminary data reveals that rhesus cleavage-stage embryos also exhibit a high degree of aneuploidy, fragmentation, and micronucleation as well as similar mitotic timing when compared to human. Given that humans and the rhesus monkey are also highly similar in terms of female reproductive physiology and fundamental aspects of early embryogenesis, we propose to investigate aneuploidy and the fate of mis-segregated chromosomes in rhesus embryos to model human pre-implantation development. By applying whole-genome next-generation sequencing (NGS) for comprehensive chromosomal assessment, we will first determine the frequency of aneuploidy and sub- chromosomal errors during meiosis in individual mature rhesus oocytes and zygotes and potential correction upon chromosome-induced polar body extrusion. Using a combination of NGS and non-invasive time-lapse imaging to monitor early cleavage divisions and cellular fragmentation dynamics, we will then evaluate the incidence of mitotic chromosomal mis-segregation up to the ~8-cell stage and reconstruct all whole and sub- chromosomal errors in each rhesus embryo by analyzing the genetic content of both single cells and fragments. Lastly, we will assess the potential contribution of meiotic chromosomal mis-segregation to mitotic errors and subsequent development by performing polar body biopsy on zygotes, allowing the embryo to proceed until the ~8-cell stage, and distinguishing meiotic versus mitotic errors based on chromosomal mosaicism, fragmentation timing, and microsatellite analysis. This work will greatly contribute to our knowledge of normal primate embryogenesis with additional implications for translational application to human infertility and IVF treatment.
Despite increased efforts to improve embryo assessment and in vitro fertiization (IVF) success, the average live birth rate per IVF cycle is still only ~30% and one of the primary contributors is thought to be chromosomal abnormalities. The proposed studies aim to identify the origins of chromosomal instability for potential therapeutic intervention, thereby avoiding the unnecessary transfer of embryos that are destined to fail and the risks associated with multiple embryo transfer.
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