ABSTRACT: Study question : Are there changes in transcript levels as endometrial receptivity changes in patients with recurrent implantation failure (RIF), particularly in alternative splicing events (ASEs), differential expression genes (DEGs) and immune cells levels? Summary answer: Changes in endometrial receptivity affect alternative splicing (AS), differential gene expression and immune cells, and the RNA-binding protein (RBP) KHDRBS3 likelymaybe？ Likely？ is involved in the dysregulation of alternative splicing. What is known already: Endometrial receptivity is critical for RIF patients. Researchers have developed transcript level-based models to predict personalized endometrial assessment at WOI. Investigating alternative splicing dysregulation, immune cells and other transcriptional biomarkers in RIF patients may further help to predict endometrial receptivity and improve embryo implantation success. Study design, size, duration: 90 endometrial samples were collected from fertile healthy controls (HC) during the menstrual cycle, including pre-receptive (PR) (luteinising hormone (LH)+3 days/LH+5 days), receptive (R) (LH+7 days) and post-receptive (PS) (LH+9 days) phases. 73 endometrial samples were taken from RIF patients on hormone replacement therapy (HRT) cycles, including PR (progesterone (P)+3 days), R (P+5 days) and PS (P+7 days) phases. Participants/materials, setting, methods: Total RNA was extracted and sequenced for all 163 endometrial samples . Bioinformatic analysis was used to comprehensively investigate the endometrial transcriptome at different stages of receptivity. We elucidated the biological significance of the universal alteration of alternative splicing in endometrial receptivity and further analyzed the role of immune cells and gene expression in this process. In addition, an ASEs-RBPs correlation network was constructed in the endometrial receptive period to evaluate the dysregulation of AS by RBPs. Main results and the role of chance: Global splicing profiling of the four subgroups (HC: PR vs. R; HC: PS vs. R; RIF: PR vs. R; RIF: PS vs. R) consistently showed that skipped exons (SE) and mutually exclusive exons (MXE) were the most common changes in splicing events. These different ASEs occurred at high frequency and some of them could affect the coding potential of a transcript. A greater number of genes with different alternative splicing events (ASEGs) were identified between PR and R phases than between PS and R phases, and the same trend was observed for differentially expressed genes (DEGs). Both ASEGs and DEGs showed enrichment for pathways associated with cell function and adhesion, while DEGs were also enriched for cytokine and immune-related pathways. Immune cell prediction shows that changes in endometrial receptivity phases result in differences in the levels of CD8 T cells, resting NK cells and other immune cells. During the R phase, there was a significant decrease in CD8 T cells and resting NK cells, whereas monocytes and M0 macrophages were significantly increased compared to the endometrial PR phase in RIF patients. Similarly, analysis of immune cells showed a significant increase in CD8 T cells, M1 and M2 macrophages and a significant decrease in activated NK cells in the R phase compared to the PS phase of the endometrium in RIF patients. Some of these immune cells, such as NK cells and monocytes, are highly correlated with certain ASEs. Investigation of ASEs-RBPs networks suggested that KHDRBS3 may be a key regulator of splicing events. Limitations, reasons for caution: The sample size needs to be increased and cover a variety of situations, such as different age, pathology, dietary habits, etc., and alsothe molecular experimental validation needs to be considered. The lack of direct comparisons between natural and HRT cycles in the same sample and molecular experimental validation need to be considered. Wider implications of the findings: Comprehensive transcriptional analysis of the endometrium provide valuable insight into potential biomarkers related to endometrial receptivity and potential treatments for RIF patients. Study funding/competing interest(s): This study was supported by Supported by the Program of National Natural Science of China (Grant No. 81701409). All authors declared that they have no conflicts of interest.
