Determinants of uterine receptivity

© 2020 Wan Tinn Teh‘Uterine receptivity’ refers to the status of the uterus when the endometrium is available to accept the embryo for implantation. Optimal endometrial receptivity leads to normal embryo implantation processes that serve as a foundation for a healthy pregnancy. Over the last three decades, there have been many advances in embryology that have resulted in significant improvements in embryo viability and overall in vitro fertilization (IVF) success rates. It is believed that implantation failure due to reduced endometrial receptivity is one of the major remaining impediments to higher IVF pregnancy rates. In a normal ovulatory cycle, the receptive endometrium develops following sequential exposure to sex steroids – estrogen and progesterone, secreted by the ovaries during follicular development, ovulation and formation of a corpus luteum. Successful implantation will occur when the development of endometrium is in synchrony with the growing embryo. Both asynchronous and pathological endometrium can lead to reduced receptivity and implantation failure. With the rapid development of transcriptomic technologies in the recent years, gene expression studies of endometrium have advanced our understandings of endometrial physiology. Clinical tests based on gene expression profiles of endometrium have also been developed. Unfortunately, there are still significant knowledge gaps in endometrial receptivity research. Expression quantitative trait loci (eQTL) analysis seeks to identify genetic variants – single nucleotide polymorphisms (SNPs) that affect the expression of genes. Identification of eQTLs has proven to be a powerful tool in the study and understanding of various diseases. We hypothesized that an eQTL approach to identify DNA variants (SNPs) which influence the expression level of genes in the peri-implantation human endometrium would have significantly more power to identify genes and gene pathways that influence uterine receptivity than simple endometrial gene (or protein) expression profiling experiments that have been undertaken previously. The overall aim of this thesis is to improve the current understanding of uterine receptivity through transcriptomic and genomic (eQTL) approaches. In the second half of the thesis, we also evaluate current practices for improving uterine receptivity. It has been well established that endometrial gene expression is influenced by stage of the menstrual cycle. Adequate normalization for cycle stage is crucial in differential gene expression research of endometrium. However, no such normalization technique exists. In order to address this issue, we have developed a molecular ‘day of cycle’ model through precise histology dating and endometrial gene expression data. This model serves as an accurate and reliable way to date endometrial samples for subsequent studies. The main body of this thesis involves transcriptomic and genomic studies of endometrium from women suffering from recurrent implantation failure (RIF), in search for markers of uterine receptivity. Differential gene expression studies of endometrial samples after normalization for cycle stage and false discovery rate correction did not show any significant difference between control and study groups. In order to gain enough power for identification of eQTLs, we have combined our endometrial samples with a larger cohort for endometrial eQTL analysis. This analysis identified novel endometrial cis- and trans-eQTLs. However, eQTL for RIF genes could not be detected due to absence of any significant differential gene expression between RIF and control groups. In the recent years, various methods and strategies have been proposed to improve uterine receptivity and embryo implantation. These include freeze-all cycles to reduce the impact of controlled ovarian hyperstimulation on endometrium and to improve embryo-endometrial synchrony. We performed a cross sectional study of more than 10,000 cycles comparing clinical outcomes between fresh and frozen embryo transfer cycles of cleavage stage embryos. Our results suggested that any potential gains due to improved endometrial receptivity and embryo-endometrial synchrony following FET are lost, presumably due to freeze-thaw process related embryo damage This thesis also investigates whether any genes are differentially expressed in endometrium following endometrial scratch, which is a common practice by many IVF groups for improvement of uterine receptivity. The results from this study showed no statistically significant gene expression differences before and after endometrial ‘scratch’. However, endometrial gene expression profiles before and after endometrial ‘scratch’ were very similar in most of the subjects, suggesting that endometrial gene expression remains consistent from cycle to cycle. In conclusion, this thesis has explored the molecular mechanisms underlying uterine receptivity. We have developed a molecular ‘day of cycle’ prediction model as a useful tool for determining ‘biological’ day of cycle for endometrium. The model was subsequently used to normalize endometrial samples for cycle stage in differential gene expression studies. Differential gene expression studies did not identify transcriptomic markers for RIF. The failure to identify RIF-related genes is not surprising given the multiple causes that could contribute to RIF, coupled with the possibility that some subjects in the RIF group could have undetected co-morbidities. This work has advanced our knowledge on uterine receptivity and provided several new avenues for ongoing research. There is still a need for better clinical tests for uterine receptivity. Hopefully the work in this thesis will provide a foundation for that to occur