Aberrant microribonucleic acidexpression patterns in recurrentimplantation failure: a review

Implantation failure is the most common cause of pregnancy loss after in vitro fertilization and embryo transfer. Recurrent implantation failure (RIF) after in vitro fertilization has devastating consequences for some patients with infertility. Recent studies have reported that microribonucleic acids (miRNAs) can regulate embryo implantation events and may be involved in RIF. MiRNAs are small, singlestranded, noncoding ribonucleic acids that can regulate gene expression in a posttranscriptional manner through degradation or suppression of messenger ribonucleic acids. MiRNAs are generated as long primary miRNA transcripts that are processed by a nuclear RNase III enzyme into precursor miRNAs. After transport to the cytoplasm, the precursor miRNAs are processed into functional miRNAs. MiRNAs have a critical role in normal development and are involved in several biologic processes, including cell proliferation, apoptosis, and differentiation, as well as embryo implantation. MiRNAs also have a role in the down-regulation of several cell cycle genes in the epithelium of the secretory-phase endometrium. This review aimed to discuss the possible roles of miRNAs in RIF. According to published data, miRNAs secreted by the endometrium and embryos can contribute to regulating embryo-endometrium cross talk and impact embryo implantation in patients with RIF. An improved understanding of molecular mechanisms underlying RIF may help to advance diagnosis and treatment of this clinical condition. Although several studies have provided evidence that miRNAs play important roles in RIF, further research is essential to evaluate the function of miRNAs and their target genes and understand the mechanisms by which miRNAs control the molecular events involved in RIF. (Fertil Steril Rev (R) 2023;4:26-37. (c) 2023 by American Society for Reproductive Medicine.)