Endometrial receptivity in women of advanced age: an underrated factor in infertility

BACKGROUND

Modern lifestyle has led to an increase in the age at conception. Advanced age is one of the critical risk factors for female-related infertility. It is well known that maternal age positively correlates with the deterioration of oocyte quality and chromosomal abnormalities in oocytes and embryos. The effect of age on endometrial function may be an equally important factor influencing implantation rate, pregnancy rate, and overall female fertility. However, there are only a few published studies on this topic, suggesting that this area has been under-explored. Improving our knowledge of endometrial aging from the biological (cellular, molecular, histological) and clinical perspectives would broaden our understanding of the risks of age-related female infertility.

OBJECTIVE AND RATIONALE

The objective of this narrative review is to critically evaluate the existing literature on endometrial aging with a focus on synthesizing the evidence for the impact of endometrial aging on conception and pregnancy success. This would provide insights into existing gaps in the clinical application of research findings and promote the development of treatment options in this field.

SEARCH METHODS

The review was prepared using PubMed (Medline) until February 2023 with the keywords such as 'endometrial aging', 'receptivity', 'decidualization', 'hormone', 'senescence', 'cellular', 'molecular', 'methylation', 'biological age', 'epigenetic', 'oocyte recipient', 'oocyte donation', 'embryo transfer', and 'pregnancy rate'. Articles in a language other than English were excluded.

OUTCOMES

In the aging endometrium, alterations occur at the molecular, cellular, and histological levels suggesting that aging has a negative effect on endometrial biology and may impair endometrial receptivity. Additionally, advanced age influences cellular senescence, which plays an important role during the initial phase of implantation and is a major obstacle in the development of suitable senolytic agents for endometrial aging. Aging is also accountable for chronic conditions associated with inflammaging, which eventually can lead to increased pro-inflammation and tissue fibrosis. Furthermore, advanced age influences epigenetic regulation in the endometrium, thus altering the relation between its epigenetic and chronological age. The studies in oocyte donation cycles to determine the effect of age on endometrial receptivity with respect to the rates of implantation, clinical pregnancy, miscarriage, and live birth have revealed contradictory inferences indicating the need for future research on the mechanisms and corresponding causal effects of women's age on endometrial receptivity.

WIDER IMPLICATIONS

Increasing age can be accountable for female infertility and IVF failures. Based on the complied observations and synthesized conclusions in this review, advanced age has been shown to have a negative impact on endometrial functioning. This information can provide recommendations for future research focusing on molecular mechanisms of age-related cellular senescence, cellular composition, and transcriptomic changes in relation to endometrial aging. Additionally, further prospective research is needed to explore newly emerging therapeutic options, such as the senolytic agents that can target endometrial aging without affecting decidualization. Moreover, clinical trial protocols, focusing on oocyte donation cycles, would be beneficial in understanding the direct clinical implications of endometrial aging on pregnancy outcomes.

The cervical transcriptome changes during the menstrual cycle but does not predict the window of implantation

Introduction

The expression of genes in female reproductive organs is influenced by the cyclic changes in hormone levels during the menstrual cycle. While the molecular changes in the endometrium that facilitate embryo implantation have been extensively studied, there is limited knowledge about the impact of the menstrual cycle on cervical cells. Cervical cells can be easily and routinely collected using a cytobrush during gynecological examination, offering a standardized approach for diagnostic testing. In this study we investigated how the transcriptome of cervical cells changes during the menstrual cycle and assessed the utility of these cells to determine endometrial receptivity.

Methods

Endocervical cells were collected with cytobrushes from 16 healthy women at different menstrual cycle phases in natural cycles and from four women undergoing hormonal replacement cycles. RNA sequencing was applied to gain insight into the transcriptome of cervical cells.

Results

Transcriptome analysis identified four differentially expressed genes (DEGs) between early- and mid-secretory samples, suggesting that the transcriptome of cervical cells does not change significantly during the opening of the implantation window. The most differences appeared during the transition to the late secretory phase (2136 DEGs) before the onset of menstruation. Cervical cells collected during hormonal replacement cycles showed 1899 DEGs enriched in immune system processes.

Conclusions

The results of our study suggested that cervical cells undergo moderate transcriptomic changes throughout the menstrual cycle; however, these changes do not reflect the gene expression pattern of endometrial tissue and offer little or no potential for endometrial receptivity diagnostics.

Basic aspects of endometrial receptivity in PCOS patients

Polycystic Ovarian Syndrome (PCOS) is an endocrine disorder commonly affecting the reproductive capacity of women leading to infertility. PCOS-related infertility is majorly due to anovulation; however, it is not the only cause. The defective endometrium causing recurrent miscarriage and implantation failure can also be accountable for infertility in PCOS women. The unusual levels of hormones and their receptors in the PCOS endometrium have a hostile effect during WOI, making the microenvironment unfavorable for embryo implantation. To date, many studies have been performed to determine the role of candidate genes in endometrial receptivity but very limited data is available using whole genome approach. This review aims at summarizing the existing studies on the basic aspects of endometrial receptivity in PCOS. The review focuses on aberrant levels of hormones and their receptors in the endometrium, affecting the receptivity. Additionally, it explores the novel approach reviewing the effect on treatment options administered for ovulation induction in PCOS on their endometrial receptivity. Overall, this review will help us to understand the molecular milieu in PCOS endometrium and its effect on the receptivity potential. However, to have a thorough understanding of the mechanistic approach of hormonal imbalance in PCOS on endometrial receptivity, there is a need to give more weightage to genome-wide studies in the future. The current review will further guide us to formulate future studies using whole genome technologies for the assessment of endometrial receptivity in different cohorts of PCOS women, which may have future diagnostic implementations.

Endometrial Expression of Cell Adhesion Genes in Recurrent Implantation Failure Patients in Ongoing IVF Cycle

Recurrent implantation failure (RIF) is one of the major obstacles in IVF. Transcriptomic literature has revealed the various biological processes involved in endometrial receptivity (ER) under different physiological circumstances, especially in natural cycle. We intended to determine the function-specific ER profile under controlled ovarian stimulation (COS) cycle. This can help to back trace the genomic impairment in RIF patients during the IVF cycle and to validate the genes involved in enriched pathways. In our study, retrospective gene expression microarray dataset was reanalysed after the follow-up, in classic non pregnant RIF (cases) vs fertile women (controls) under COS (n = 5/group). Reanalysis of microarray revealed significant downregulation of cell adhesion function (P:3.11E-05) with the maximum gene count. For validation purpose, downregulation of eight genes (COMP, HABP2, ITGAD, CDH3, COL22A1, MFAP4, THBS1and CD300A) involved in enriched cell adhesion pathway having fold change > 3 were assessed by real-time PCR in independent cohorts of cases and controls (n = 24, each). Downregulation of six out of eight genes (COMP, HABP2, ITGAD, CDH3, MFAP4 and THBS1) were confirmed by real-time PCR (P < 0.05) with fold change > 2. This indicates the importance of analysed genes in the ER mechanism under COS, thus mimicking the fresh embryo transfer. The further analysis in larger cohorts would substantiate the study findings in RIF patients undergoing IVF cycle.

Aberrant DNA methylation profiling affecting the endometrial receptivity in recurrent implantation failure patients undergoing in vitro fertilization

PROBLEM

DNA methylation profile in mid-secretory phase of endometrium is reported to be varied from other phases in natural menstrual cycle. Therefore, we intended to study the impairment in endometrial receptivity by performing whole-genome methylation and gene expression profiling in endometrium of recurrent implantation failure patients (RIF) during IVF under controlled ovarian stimulation (COS).

METHOD OF STUDY

Endometrial biopsies were collected from IVF-RIF patients (cases, n = 6) and healthy fertile oocyte donors (controls, n = 6) undergoing COS after 6/7th day of human chorionic gonadotropin administration. The whole-genome methylation and gene expression microarray were performed and analysed by GenomeStudio software (P < .05 by Illumina Custom Model), whereas the enrichment analysis was performed using "Database for Annotation, Visualization and Integrated Discovery" (DAVID, V6.8). Significant differentially methylated genes were correlated with dys-regulated genes using Pearson's correlation.

RESULTS

Differential methylation in RIF patients revealed 448 CpG sites. The enrichment analysis showed aberrant methylation in genes involved in immunological response and G protein activity. Methylation in NLRP2 gene in inflammatory pathway had significant negative correlation with gene expression (P = .008), whereas SERPINA5 gene that is already known to be involved in endometrial receptivity was observed to be hypomethylated in promoter region with highest delta beta value and up-regulated in gene expression analysis.

CONCLUSION

The aberrant methylation of genes involved in immunological functions and G protein activation was found to be prevalent which might suggest a role in endometrial receptivity. However, the findings need to be further validated on a larger cohort of IVF-RIF patients.

Downregulation of genes related to immune and inflammatory response in IVF implantation failure cases under controlled ovarian stimulation

PROBLEM

Implantation failure (IF) even after the good-quality embryo transfer (ET) is main obstacle in in vitro fertilization (IVF). We aim to study the genomics of endometrial receptivity in IF patients under controlled ovarian stimulation (COS) during which ET is generally practised in IVF.

METHOD OF STUDY

Endometrial gene expression profiling in IF patients (n=10) and oocyte donors (n=8) were compared during window of implantation under COS by microarray. Enrichment analysis of microarray data was performed to determine dysregulated pathways. Microarray results were validated by real-time PCR. Localization of genes related to immune response (progestagen-associated endometrial protein (PAEP), leukaemia inhibitory factor (LIF), interleukin-6 signal transducer (IL6ST) was detected by immunohistochemistry.

RESULTS

The gene ontology, pathway analysis and enrichment mapping revealed significant downregulation in activation and regulation of immune and inflammation response in IF patients under COS. The lower expression of PAEP, LIF and IL6ST in cases compared to controls by real time and immunohistochemistry suggests the functional importance of these genes.

CONCLUSION

Importance of immune and inflammatory response in endometrial receptivity adds on to the current knowledge of gene expression profile in IF under COS. The panel of genes involved in these pathways would be useful in determining further line of treatment for IF during IVF.