Our study showed that plasma levels of miR-211 were significantly lower in the group of infertile men compared to the controls. Lower expression levels in infertile males indicate that spermatogenesis is defective, and that the downregulation of miRNA-211 plays an essential role in male infertility. These findings may indicate that miR-211 expression is closely related to male infertility.
The discovery of regulatory mechanisms involved in spermatogenesis is essential for male infertility [24]. Serum/plasma miRNAs can be used as potential biomarkers for diagnosing and treating several diseases [25]. In addition to semen analysis, molecular diagnosis seems to be necessary to confirm conventional diagnoses. Changes in the expression of some miRNAs in spermatogenesis disorders have the potential to be used as new biomarkers for diagnostic purposes [26].
Lawrie et al. have shown that infertility is associated with circulating miRNAs. Serum miRNAs are used for the first time in patients with diffuse B-cell lymphoma [27].
Some miRNAs are dysregulated in male infertility [12]. The study by Wu et al. found that miR-141, miR-429, and miR-7-1-3p levels significantly increased in testicular tissues of NOA compared with normal controls, suggesting that miRNAs play an important role in male reproductive function. They showed that, in the pathogenesis of NOA, abnormal methylation patterns of miRNA genes were associated with their abnormal expression [26].On the other hand, an increase in miR-155 is a marker of infertility that reflects tissue damage (for example, in the blood-testicular barrier) or may play a functional role in suppressing spermatogenesis, thus indirectly affecting fertility [28].
Limited studies have been conducted on plasma samples and male infertility. Cito et al. showed that the expression of miR-20a-5p in blood plasma was considerably higher in NOA patients than in infertile males and that it was directly related to spermatogenic insufficiency. Higher miR-20a-5p values seem to be directly related to higher FSH and LH, but lower TT and testicular volumes. They confirmed that miR-20a-5p in human blood plasma could be used as a novel biomarker of human spermatogenetic damage[29]. Dysregulation of hypothalamic-pituitary-gonadal axis could have an indirect effect on sperm development [30]. Dieckmann et al. showed that the level of miR371 in the serum of patients with germ cell tumors was much higher than that in the seminal plasma [31].
In this analysis, miR-211 levels in the blood plasma of patients were significantly lower than in the fertile men, which could be used as a potential new biomarker of spermatogenesis damage in humans. This study aimed to investigate the usage of miRNA-211 in plasma as a biomarker for diagnostic purposes in male infertility.
A molecule serving as a biomarker should be specific, sensitive, preferably non-invasive, and the source should be easily accessible. Clinically, biomarkers used to diagnose or predict pathological conditions are found in serum or plasma and are easily detectable even at very low concentrations; their expression is also often related to a specific tissue/biological stage [32].
Some miRNAs are involved in the control of differentiation, meiosis/post-meiosis process, spermatogenesis, maturation, and sperm protection through a specific pathway [30]. Abu-Halima et al. found that downregulated miR-17-92 and miR-383 in NOA testicular samples of patients were related to spermatogenetic damage and maturation arrest [33].
Cai et al. showed that, in the colorectal cancer cell line HCT, forced expression of miR-211 directly downregulated the CHD5 gene, increased migration, decreased apoptosis by accelerating the transition from G0/G1 to S phase in vitro, and increased in vivo proliferation [34].
Xia et al. showed that overexpressed miR-211 could cause proliferation inhibition, cell arrest in the G0/G1 phase, and induction of apoptosis in epithelial ovarian cancer (EOC) by controlling cyclin expression. miR-211 acts as a tumor suppressor and controls the expression of Cyclin D1 and CDK6 as direct targets [35].
Zhang et al. found that the expression level of miR-211-5p in the serum of patients with atherosclerosis (AS) was significantly lower than that of the healthy control groups and that it can be used as a new biomarker for the diagnosis, occurrence, and development of AS [36].
miR-211-5p prevented proliferation, invasion, migration, and metastasis of breast cancer triple negative tumor cells by directly targeting SET Binding Protein1 [37] .MiR-211-5p 27 also played a major role in Alzheimer’s disease by regulating neuronal differentiation and survival [38].
A study of miR-211-5p exosomes in plasma by Yang et al. for the diagnosis of gastric cancer showed that the plasma level of miR-211-5p in patients significantly increased to the extent that it could transfer between the cells and act as an important factor in the cellular process [39]. miR-211 and miR-204 are classified as part of the miRNA family due to their high sequence matching [40]. The expression of miR-204 decreased in oral squamous cell carcinoma (OSCC) and correlated with the reduction of miR-211. These miRNAs can probably affect target genes involved in cell proliferation and aoptosis [41].
In our assessment, plasma miR-211 levels significantly decreased in infertile men compared to the controls. However, the detailed function of miR211 in human male infertility has not been explored. According to the results of previous studies, low expression of miR-211 probably increases the level of apoptosis in germ cells by stopping cells in the G0/G1 phase and prevening proliferation, migration, and maturation disorder during spermatogenesis. The miR-211 may have the potential to become a new diagnostic marker in humans [33–36].
A possible explanation of the correlation between miRNAs in human plasma and male infertility could be its indirect effect on sperm development [30].
In our study, decreased expression of miR-211 significantly correlated with sperm parameters (sperm concentration, total motility, progressive motility, and normal morphology) in infertile men. It might also involve prospects for the clinical evaluation of male infertility. A positive r in Pearson coefficient and significant relationship between miR-211 expression and semen parameters showed that all semen parameters decreased with decreasing miR-211 expression.
MiR-20a-5 expression decreased in sperms with low motility in experiments by Capra et al. This indicates the association of miR-20a-5 with specific genes that target cell apoptosis and spermatogenesis [42].
Radtke et al. showed significantly lower miR-371a-3p expression in oligozoospermic compared to normozoospermic men. There is a significant correlation between miRNA expression level and sperm concentration. Therefore, if an miRNA provides much information about the number and quality of sperm, it will be a valuable tool for assessing non-invasive evolution [43].
However, it is also essential to establish universal validation techniques and controls to avoid all potential technical biases and improve the use of miRNAs as biomarkers to detect various forms of infertility. Development of the next generation of therapeutics is also needed. Together, this would help make miRNA-based infertility diagnosis and treatment a clinical reality soon [30].
This method can be performed in other infertile groups at a lower or higher age range and with other miRNAs. Also, it is suggested that further research be conducted with a larger sample size of other ethnicities.