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Title: Cytogenetic and epigenetic analysis of human gametes and preimplantation embryos
Author: Babariya, Dhruti
ISNI:       0000 0004 7232 7731
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Despite major advances in the assisted reproductive treatments, it remains the case that most embryos transferred to the uterus fail to implant or are lost shortly after. Chromosome aneuploidy has been characterised as one of the major factors impacting embryo viability. While the incidence of whole chromosome aneuploidies has been well characterised, little is known about segmental aneuploidy affecting fragments of chromosomes. One of the aims of this study was to investigate the frequency, origin and clinical relevance of segmental aneuploidy in human oocytes and preimplantation embryos. The incidence of segmental aneuploidy significantly increased following fertilisation, peaking at the cleavage stage and declined during the transition to the blastocyst stage. Unlike whole chromosomal aneuploidy, occurrence of segmental aneuploidy was not correlated with advancing female age. Interestingly, IVF culture conditions such as media systems and gas composition were found to significantly influence the incidence of segmental aneuploidy. In addition, the study found that embryos generated by patients suffering from recurrent implantation failure had a higher proportion of segmental aneuploidies than those generated by patients undergoing IVF treatment for advanced maternal age or recurrent miscarriage, suggesting an underlying patient factor. Preimplantation genetic testing for aneuploidy (PGT-A), using embryo biopsies, is routinely employed during in vitro fertilisation to select against aneuploid embryos. However, it is speculated that the invasive nature of the embryo biopsy procedures can adversely impact embryo viability. Therefore, non-invasive sources of embryonic DNA, including blastocoel fluid and spent culture media, were explored during the course of this study and a non-invasive PGT-A method was successfully developed. Promising results were obtained following the application of the developed non-invasive assay to spent culture media samples, showcasing its potential for future clinical application. Although not as well characterised as aneuploidy, epigenetics is a key emerging factor impacting embryo development. Of particular interest to the IVF community is the epigenetic phenomenon of genomic imprinting wherein a gene is monoallelically expressed in a parent of origin fashion. A few studies have shown a significantly higher rate of imprinting disorders such as Beckwith Wiedemann syndrome in children born after ART as compared to those conceived naturally. Hence, the last part of this study aimed to investigate the imprinting landscape in preimplantation human embryos. A multiplex PCR based bisulfite sequencing approach was successfully developed which was further utilised to study imprinting patterns of 23 imprinted genes simultaneously in trophectoderm and inner cell mass biopsies from euploid human blastocysts. To our knowledge, this is the first study to successfully analyse the methylation patterns of a large number of imprinted genes simultaneously in human blastocysts.
Supervisor: Wells, Dagan Sponsor: Clarendon Fund
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available