What’s Known

Circular RNAs (CircRNAs) are highly expressed in human testes, especially in sperm cells, and affect sperm quality. CircRNAs are shown to play a role in male infertility-related diseases.

What’s New

An overview of the association of CircRNAs with problems related to the male reproductive system is presented. CircRNAs can be transferred from sperm to oocyte during fertilization, indicating their role in embryo development. They can be used as reliable biomarkers for disease detection and treatment.

Introduction

Infertility is one of the most common chronic diseases of the reproductive system affecting approximately 15% of all couples. ¹ In recent decades, male infertility has increased worldwide and now accounts for up to 50% of infertile couples. ^{2 , 3} Factors affecting male infertility include genetic disorders, congenital anomalies, hormonal imbalance, infection, psychological distress, lifestyle, and environmental factors. ⁴ However, in 30% to 50% of men, the exact mechanism causing infertility is unknown. ⁵ Consequently, a proportion of infertile people will not receive proper medical treatment. ^{6 , 7} Therefore, there is a need to develop novel diagnostic methods to improve male fertility. Recently, several studies have addressed the potential of non-coding RNAs (ncRNAs) as diagnostic and therapeutic molecules. ^{8 - 10} ncRNAs regulate gene expression in different processes such as transcription, translation, RNA splicing, and editing, gene imprinting, and chromosome separation and maintenance. These RNAs include rRNA, tRNA, small nuclear RNA (snRNA), small nucleolar RNA (snoRNA), small interfering RNA (siRNA), and others. ¹¹

Circular RNA (CircRNA) has attracted much attention in recent years. ¹² In the past, CircRNAs were thought to be abnormal molecules resulting from a splicing error. However, today, we know that their expression is highly regulated. ^{13 , 14} CircRNA molecules do not have a 3’ poly (A) tail and 5’ cap and may build up in certain tissues or cells due to their specific stability compared to linear RNAs. ^{15 , 16} Some studies have reported a possible association between CircRNAs and some pathological conditions, e.g., cardiovascular diseases, ^{17 , 18} diabetes, ¹⁹ autoimmune diseases (multiple sclerosis), ²⁰ cancers, ^{21 - 23} Alzheimer’s, ²⁴ and age-related diseases. ²⁵ Recently, the expression of CircRNAs in reproductive tissues such as the testes, ¹² placenta, ²⁶ , and ovarian cells ²⁷ have been reported. It is also shown that CircRNA expression is stage-specific in pre-implantation embryos and may be involved in embryo development. ²⁸

Given the role of CircRNA in various reproductive organs and cells, as well as associated diseases, there is a need for further studies on this RNA molecule. In the present study, we provide an overview of the association between CircRNAs and problems related to the male reproductive system (table 1). The information will help other researchers to develop potential diagnostic and therapeutic methods.

Types of CircRNAs

Three main types of CircRNAs have been identified, namely exonic CircRNA (eCircRNA), ³⁶ circular intronic RNA (ciRNA), ³⁷ and exon-intron CircRNA (EIciRNA). ³⁸ The eCircRNA is located primarily in the cytoplasm and comprises more than 80% of all CircRNAs in the cell.ciRNA and EIciRNA are also found in the nuclei. ³⁹

Regulation and Biogenesis of CircRNA

The exact mechanism of CircRNA biogenesis remains poorly understood. However, spliceosomal machinery has been suggested as the modulator of the process. Generally, a mature linear RNA transcript with a polarity of 5’ to 3’ is generated when the spliceosome cleaves out the introns of a pre-mRNA. This process is initiated when a 2’-OH in an intronic conserved adenine nucleotide attacks the 5’-splice site, resulting in the formation of a lariat-3’-exon and a 3’-OH available at the end of 5’-exon. It is followed by a nucleophilic attack on the 3’ splice site by the generated 3’-OH leading to the separation of the lariat structure and joining of exons in the next phase. ^{40 - 42} Through back-splicing, the binding of the 3′ to 5′ end of an exon may occur or a 3’ end of an exon connects to the 5′ end of an upstream exon that forms CircRNAs. ^{38 , 43 - 45} It was shown that CircRNAs generation requires splicing signals in the back-splicing mechanism. Moreover, splice site mutation results in decreased circulation efficiency. Furthermore, inhibition of conventional spliceosome decreases both linear and circular RNAs formation. ⁴⁶ However, a previous study showed that blocking spliceosome resulted in a higher CircRNA formation while lowering the amount of related linear RNAs. ⁴⁷ Exon skipping and direct back-splicing mechanisms have been shown to generate eCircRNA and EIciRNA. ^{36 , 48} Using an exon skipping model, CircRNA is generated by alternative splicing of pre-mRNA followed by back-splicing. ⁴⁹ In a direct back-splicing model, the process occurs by pairing flanking introns or RNA-binding proteins, which brings the upstream and downstream splice sites closer together. ⁴⁹ CircRNA is then directly generated in addition to an exon-intron-exon structure that can be processed or probably degraded (figures 1a and 1b). ⁴⁹

Figure 1. A schematic representation of CircRNA biogenesis in (a) an exon-skipping model, (b) direct back-splicing, and (c) ciRNA biogenesis.

The biogenesis of CiRNA requires a known motif consisting of Seven nucleotides (GU-rich) at the 5’ junction and 11 nucleotides (C-rich) at the side of the branchpoint (figure 1c). These motifs have not been observed in other types of CircRNAs nor regular introns. ³⁷ After splicing, they may develop when lariats escape exonucleolytic degradation leading to the 2’ 5’ circle formation. ³⁷

CircRNAs synthesis can be regulated by trans-factors (e.g., RNA binding proteins [RBPs]) and cis-elements (e.g., intronic sequences [inverted Alu repeats]) (figure 2). ^{38 , 44 , 50} Some RBPs (NF90/NF110, ⁵¹ FUS, ⁵² Muscleblind Like Splicing Regulator 1, ⁴⁴ Quaking, ⁵³ DHX9, ⁵⁴ NOVA2, ⁵⁵ and others ⁵⁶ ) were shown to interact with flanking intron sequences to regulate CircRNAs generation. Exon circularization is aided by inverted repeating Alu pairs in flanking introns, whereas pairings in the same intron facilitate linear splicing. ⁵⁷

Figure 2. A schematic representation of CircRNA biogenesis regulation by cis-elements (inverted Alu repeats) and trans-factors (RNA binding proteins).

Biological Functions of CircRNAs

The main function of CircRNA is still unknown, however, it has been shown that CircRNAs participate in various cellular processes including a potential role in regulating gene expression. They can act as miRNA sponges that inhibit miRNA function and consequently regulate miRNA target gene expression at the post-transcriptional level. ⁵⁸ Some CircRNAs could interact with RBPs and influence biological processes including gene transcription, cell cycle, proliferation, apoptosis, and cell survival. ^{29 , 59} Furthermore, some studies reported that some CircRNAs are involved in the regulation of their parental gene transcription by interacting with the transcriptional machinery. ^{37 , 38} Besides, it was shown that some CircRNA molecules could be translated into proteins ^{60 , 61} and played a role in the regulation of splicing. ⁴⁴

CircRNA and the Male Reproductive System

Testes: The testes are the most important male reproductive organs producing sperm and testosterone. ⁶² Spermatogenesis is the process by which sperm cells are produced. The process is divided into three steps, namely mitotic renewal (spermatogonia proliferation/differentiation), spermatocyte meiosis, and spermatid differentiation (spermiogenesis). ⁶³ Each step requires highly precise temporal and spatial gene expression, which is strictly regulated by transcriptional gene expression, post-transcriptional changes, and epigenetic modifications. ^{42 , 64} Testicular cells actively generate RNAs, including various non-coding and protein-coding mRNAs. ³ Recently, the presence of CircRNAs in the testes has been reported. ^{12 , 62} The testicular CircRNAs were initially identified from gene Sry gene (Sex-determining Region Y) in the mouse. ⁶⁵ Sry CircRNA (circSry) regulates gene expression by acting as a miR-138 sponge. ⁶⁶ For the first time, Dong and colleagues reported the presence of CircRNAs in human testes and seminal plasma. Using next-generation deep sequencing, they identified over 15,000 CircRNAs of which 10,792 were novel. They showed that testis-derived CircRNAs can bind to proteins in seminal plasma and are very stable. They concluded that these CircRNAs can be used as non-invasive biomarkers for male infertility. ¹² Gao and colleagues compared CircRNA expression in the testes of neonatal calves and adult cattle. They found that 4,248 CircRNAs were differentially expressed between the two groups. In adult cattle, 2,225 (10.2%) of the CircRNAs were upregulated and 2,023 (9.3%) were downregulated. In addition, they stated that the source gene of CircRNAs was involved in transforming growth factor β (TGFβ) signaling pathway, cell junction, and reproduction. ⁶² Li and colleagues used high-throughput RNA sequencing and demonstrated CircRNA expression at different reproductive stages in Tibetan sheep testes. ⁶⁷ Furthermore, CircRNA expression in different tissues and developmental stages in rats was investigated. It was shown that CircRNAs were overexpressed in the testes and brain compared to other organs. ⁶⁸ In another study, You and colleagues found that, after the brain, testicular tissue was most enriched with CircRNAs, implying that CircRNAs may play an essential role in its functioning. ⁶⁹ Another study found a dynamic and age-dependent pattern of CircRNA expression in testes, such that it significantly increased with sexual maturity and decreased with aging. ⁶⁸ Age dependency is strongly associated with spermatogenesis, indicating CircRNAs have specific physiological activities rather than simply being a by-product of RNAs and can act as a biomarker of reproductive aging. ⁶⁸ Zhang and colleagues compared the number of CircRNAs in the testes of adult boar and piglets and identified 2,326 differentially expressed CircRNAs. Some of these CircRNAs, associated with spermatogenesis (CircRNA 10979) and germ cell development (CircRNA 18456), were upregulated in the testes of adult boar and some others (CircRNAs 6682, 10187, 18456, 10979) and could be used as indicators of sexual maturation in pigs. They concluded that CircRNA is dynamically expressed during testicular development. ⁷⁰

Given the high expression of CircRNAs in the testes as well as their high stability, it is likely that these RNAs are the key factor involved in spermatogenesis or the pathogenesis of infertility-related diseases.

Spermatogenic Cells: Spermatogenesis is the biological process of sperm production in mammals, driven by a pool of self-renewing cells known as spermatogonial stem cells (SSCs). SSCs undergo mitotic divisions and differentiation, resulting in the production of meiotic spermatocytes. Round spermatids are formed after two meiotic reduction divisions and undergo a major morphological transformation during spermiogenesis to become sperm cells. ⁷¹ This complex process involves an accurate balance between differentiation and SSCs self-renewal, in which CircRNAs play a key role. ⁷² Zhang and colleagues reported that CircRNAs show a different expression pattern in the testes of adult boar and piglets. Some of these differentially expressed CircRNAs play a role in signaling pathways such as fibroblast growth factor receptor 1 (FGFR1), AKT serine/threonine kinase 3 (AKT3), SMAD family member 4 (SMAD4), Frizzled Class Receptor 3 (FZD3), and Activin A receptor type 1 (ACVR1) that regulate pluripotency in stem cells. ⁷⁰ In addition, CircRNAs are abundantly expressed in spermatogenic cells. A previous study reported that 15,101 CircRNAs were identified in mouse spermatogenic cells, of which 7,220 were expressed in round spermatids and the rest in SSCs (5,573), spermatogonia type A (5,596), preleptotene (6,686), and pachytene spermatocytes (4,677). ⁷³ Similar results in rat studies showed that most expressed CircRNAs are involved in spermatid development and acquisition of sperm motility (morphogenesis of flagellum). ⁶⁸ Tang and colleagues reported that during spermatogenesis in mice, the generation of CircRNAs increases. Spermatocytes evolve into round spermatids at late pachytene and then show elongated morphologies (figure 3a) due to association with linear mRNA degradation in spermatids. Since most of these CircRNAs are exonic, they could be translated into proteins. ⁷⁴ They also reported that sperms with high fertilization ability (IVF>25%) contained higher levels of CircRNAs, indicating an association between CircRNA function and sperm quality. Chioccarelli and colleagues used a microarray technique to characterize CircRNAs in human sperm and identified 10,726 CircRNAs of which 28% were novel. They also identified 148 differentially expressed CircRNAs correlating to good- or bad-quality sperm cells based on motility and morphological parameters. The results of subcellular localization of differentially expressed CircRNAs showed that most of these molecules were localized in the sperm head. It has been hypothesized that they may play a functional role in regulating the early stages of embryonic development by transferring sperm to oocyte during fertilization. ⁷⁵

Figure 3. A schematic representation of (a) spermatogenesis and expression level of CircRNA and (b) possible sperm CircRNAs role as miRNA sponges in oocyte, embryonic cell, blastocoel fluid, and zygote development. SSC: Spermatogonia stem cell; SpgB: Spermatogonia B; Lep: leptotene; Zyg: Zygotene; Pac: Pachytene; Dip: Diplotene; Spc II: Spermatocyte II; Rs: Round spermatid; Els: Elongated spermatid; Spz: Spermatozoa

In another study, CircRNAs expression in boar sperm was investigated and 1,598 CircRNAs were identified, 80% of which had not been reported previously. Sperms with low and high motility showed 148 differentially expressed CircRNAs, indicating their association with sperm quality. Of these, two CircRNAs (ssc_circ_1321 and ssc_circ_1458) were suggested as biomarkers for sperm motility. ⁷⁶ Recently, the presence of some sperm RNAs in the oocyte has been confirmed as a result of fertilization and their function in zygote and embryo development. ⁷⁷ It is shown that injection of sperms treated with RNase into MII oocytes resulted in decreased blastocyst development and live birth outcome, while the rate of embryo development increased after injecting total sperm RNAs into these oocytes. ⁷⁸

As mentioned earlier, the majority of CircRNAs are found in the sperm head, where they can be transferred to the oocyte during fertilization and influence gene expression at the early stages of development. Ragusa and colleagues investigated the presence of CircRNAs in human and mouse spermatozoa and showed that circNAPEPLDiso1 and circNAPEPLDiso2 were highly expressed. In murine-unfertilized oocytes, the expression level of circNAPEPLDiso1 and circNAPEPLDiso2 was low and high, respectively. However, after fertilization, circNAPEPLDiso1 expression increased significantly, whereas circNAPEPLDiso2 expression remained constant. However, in human spermatozoa, circNAPEPLDiso1 could interact with a variety of miRNAs expressed in the oocyte, blastocyst cells, and blastocoel fluid. In this regard, it is suggested that circNAPEPLDiso1 serves as a paternal-derived miRNA sponge after fertilization and regulates early embryonic development by suppressing the anti-proliferative functions of some oocyte miRNAs. ⁷⁹ Chioccarelli and colleagues also reported that circCNOT6L is transferred from sperm to oocyte during fertilization where it may support zygote development (figure 3b). ⁷⁷

Effect of CircRNAs on Male Infertility

Azoospermia: Azoospermia refers to the absence of sperm in the ejaculate and is the most severe form of male infertility, accounting for 10-15% of infertile men. ⁸⁰ Azoospermia is divided into two types, namely obstructive azoospermia (OA) and non-obstructive azoospermia (NOA). In OA, seminal tract closure obstructs the transport of sperm despite normal spermatogenesis. NOA is the most problematic type of azoospermia (about 60% of patients) caused by the failure of spermatogenesis in the testes. ⁸¹ To date, the etiology of more than 80% of NOA cases is unknown, and genetic abnormalities have been identified in only 20% of NOA patients. ⁵⁶ Micro-dissection testicular sperm extraction (micro-TESE) is the commonly used surgical method for sperm retrieval in NOA patients. ⁸² However, the success rate of sperm retrieval is around 50%, implying that testicular biopsy is unsuccessful in about half of the patients. ^{83 , 84} Therefore, a biomarker that could predict the outcome of micro-TESE would play an important role in reducing unnecessary invasive procedures.

Several studies showed the potential of CircRNAs as a biomarker for disease detection and treatment. ^{85 , 86} For the first time, Ge and colleagues reported that the expression pattern of CircRNA is different in the testes of NOA compared to OA patients. ⁸⁷ This indicates that CircRNAs may play an important role in regulating sperm production and can be used in NOA diagnosis and treatment. In another study, Bo and colleagues showed similar results by comparing the expression pattern of CircRNA in the testicular tissue of NOA and OA patients. They reported that some of the differentially expressed CircRNAs are associated with several signaling pathways that may play a role in NOA pathogenesis. For example, hsa_CircRNA_402130 inhibits the function of the let7_miRNA family involved in the regulation of stemness. ⁸⁸ Liu and colleagues compared hsa_circ_0049356 expression levels in the seminal plasma and blood samples of idiopathic NOA patients and healthy individuals. Hsa_circ_0049356 is associated with the CARM1 gene (co-activator-associated arginine methyltransferase), which has an important function in establishing sperm epigenome. They showed that the expression of this CircRNA was highly upregulated in whole blood samples of NOA patients, but significantly downregulated in seminal plasma samples, which could be attributed to the presence of a blood-testis barrier. These findings indicated the important role of hsa_circ_0049356 in spermatogenesis. In this regard, it is recommended to analyze CircRNA expression levels in the blood or seminal specimens from NOA patients as a diagnostic approach. Moreover, the bioinformatic analysis revealed that the expression patterns of hsa_circ_0049356 and the network of hsa_circ_0049356-miRNA-mRNA differ in NOA patients compared to healthy individuals. Most target mRNAs with sponges are important factors affecting cytoskeleton re-arrangement of germ cells during spermatogenesis, which is needed for their differentiation. ³⁰ Another study reported that hsa_circ_0000116 was highly expressed in testicular tissue specimens of NOA compared to OA patients. ³¹ It has also been shown that expressions of hsa_circ_0000116 were negatively associated with the Johnsen score in NOA patients. In the study by Liu and colleagues, patients in the NOA group were divided into three subgroups, namely hypospermatogenesis (reduced sperm production in the presence of all germ cells), maturation arrest of premature spermatogenesis, and Sertoli cell-only syndrome (SCOS) characterized by the total absence of germ cells. They showed that the expression of hsa_circ_0000116 was different between the subgroups, with significant upregulation in SCOS compared to the hypospermatogenetic subgroup. Furthermore, overexpression of hsa_circ_0000116 was shown to be correlated with the reduced success of testicular sperm retrieval. In addition, they showed that miR-449 could be a target of hsa_circ_0000116 sponges. Since high miR-449 levels are expressed during testicular development and spermatogenesis in adults, spermatogenesis can be suppressed by blocking hsa_circ_0000116, leading to NOA. On the other hand, autophagy-related genes (ATG4B, Bcl2) can be regulated by miR-449. Probably, as a result of hsa_circ_0000116 upregulation in NOA patients, autophagy dysfunction causes damage to the spermatogenesis. ³¹

In a recent study, Zhu and colleagues investigated CircRNA expression patterns in SCOS patients. They found that 1,594 CircRNAs were differentially expressed in SCOS compared to OA patients with normal spermatogenesis. Bioinformatic analysis revealed that differentially expressed CircRNAs in SCOS have a role in regulating spermatogenic cell proliferation and intercellular communication. In addition, abnormal expression of these vital CircRNAs is related to pathogenic changes in SCOS through spermatogonial dysplasia or interfering with normal spermatogenesis. ⁸⁹ In another recent study, Ji and colleagues investigated the potential application of CircRNAs in seminal plasma to predict the success rate of micro-TESE in patients with idiopathic NOA. They found that hsa_circ_0007773, hsa_circ_0060394, and hsa_circ_0000277 have higher expression levels in both seminal plasma and testes of patients after successful sperm retrieval. Furthermore, samples from healthy individuals showed constant CircRNAs expression levels after 48 hours at room temperature. They suggested that these CircRNAs could be used as reliable diagnostic biomarkers for seminal plasma samples in patients with idiopathic NOA. ³²

Asthenozoospermia: Sperm motility is a key factor for successful fertilization. ⁹⁰ Asthenozoospermia, a common infertility problem in men, is defined as progressive motility of less than 32% or total motility of less than 40% in at least two separate semen analyses. ⁹¹ Some of the causes of asthenozoospermia include varicocele, sperm dysfunction, partial blockage of the seminal tract, infection, and genetic factors. Nonetheless, asthenozoospermia could be idiopathic. ⁹²

The molecular mechanism that impairs sperm motility is not known, however, evidence has shown the association of sperm quality with sperm RNAs. ^{93 , 94} Recently, some studies have investigated the role of CircRNAs in asthenozoospermia. Manfrevola and colleagues stated that 22% of CircRNAs in asthenozoospermic patients have not been previously reported. They compared the expression pattern of CircRNAs in high- and low-quality sperm samples, based on morphology and motility, between the patients and control groups. It was shown that 1,432 CircRNAs were differentially expressed and associated with mitochondrial functions and sperm motility. A mixture of the oral amino acid (L-arginine, citrulline, and ornithine) was administered to treat asthenozoospermic patients. The results showed that the expression of five upregulated CircRNA decreased significantly post-treatment, whereas the expression of five downregulated CircRNA increased to approximately the level of healthy patients. It was concluded that CircRNA molecules could be used as a biomarker for the diagnosis of asthenozoospermia. In addition, it was found that the modulation of CircRNAs with oral amino acid supplements increased the viability and motility of sperms, indicating their potential in therapeutic approaches. Manfrevola and colleagues also investigated the putative molecular mechanisms of infertility in asthenozoospermic patients. They evaluated the mRNA expression of Cysteine-Rich Secretory Protein 2 (CRISP2), Prostate and Testis Expressed 1 (PATE1), and Cation Channel Sperm Associated 1(CATSPER1) in the semen of asthenozoospermic individuals. The results showed that low-quality sperms exhibited a significant decrease in the expression of these three mRNAs. Bioinformatic analysis revealed that three miRNAs (hsa-miR-138-5p, hsa-miR-27b, and hsa-miR-6721-5p) showed inhibitory effect on CRISP2, PATE1, and CATSPER1 mRNAs, which were associated with differentially expressed CircRNA in the sperm cells of asthenozoospermic individuals. It was shown that the expression levels of circRERE, circEPS15, and circTRIM2 (upstream of CRISP2, PATE1, and CATSPER1 mRNAs, respectively) were significantly downregulated in low-quality sperms. In addition, after administrating a mixture of oral amino acid supplements, the expression levels of these CircRNAs increased, indicating dysregulation of CircRNAs expression in asthenozoospermia and its potential as a targeting molecule for treatment. ³³ Another study reported that reduced expression of circBoule RNAs, circEx2-7, and circEx3-6 is associated with asthenozoospermia in humans. ⁹⁵

Effect of CircRNA on Male Reproductive Cancers

It has been suggested that CircRNAs can be a potential biomarker for cancer. ⁹⁶ Aberrant CircRNA expression is associated with some malignancies and can serve as tumor inhibitors or activators. ⁹⁷ In this regard, some studies have demonstrated the role of CircRNAs in reproductive cancers. ^{29 , 98 , 99} To the best of our knowledge, there are no reports on CircRNAs association with testicular cancer.

Prostate Cancer

The prostate gland produces an alkaline fluid and is an important part of the male reproductive system. Semen is a combination of sperm cells and secretions from the prostate gland and other glands such as the bulbourethral gland and seminal vesicle. ¹⁰⁰ Prostatic diseases can affect sperm function and male fertility. ¹⁰¹ In 2020, prostate cancer (PCa) was reported as the second most common malignant cancer in men and the fifth leading cause of cancer-related deaths worldwide. According to GLOBOCAN 2020, a total of 1,414,259 new cases and 375,304 deaths due to prostate cancer has been reported globally. ¹⁰² Prostate-specific antigen (PSA), a known biomarker of prostate cancer, is routinely used to detect PCa. However, because of its low specificity, the PSA test often results in misdiagnosis and overtreatment. Currently, there is no consensus on the effectiveness of PSA screening methods in reducing the incidence of PCa-related death. ¹⁰³ Therefore, there is a need to identify new biomarkers as a diagnostic tool.

Recently, some studies have shown the aberrant expression of CircRNAs in patients with PCa (table 1). Ge and colleagues identified 749 differentially expressed CircRNAs in PCa compared to healthy tissues, most of which were associated with tumor progression. They suggested that these CircRNAs could be potential biomarkers for the detection and treatment of PCa. ¹⁰⁴ Similarly, Wu and colleagues identified 60 differentially expressed CircRNAs in PCa cells compared to normal tissues. ¹⁰⁵ Chen and colleagues reported that circHIPK3 upregulation is associated with the aggressiveness of PCa, whereas its downregulation reduces the proliferation and invasion of cancerous cells. ³⁴ It is also shown that CircRNAs can impact the cell cycle by enhancing the G2 to M transition that results in cell proliferation. ¹⁰⁶ A previous study showed that the expression of another CircRNA, circFOXO3, was significantly increased in blood serum and PCa tissue, which may result in PCa development, indicating the potential of circFOXO3 as a biomarker for PCa. ²⁹ Hansen and colleagues identified five new CircRNAs (circKMD1A, circTULP4, circZNF532, circSUMF1, and circMKLN1) with high cancer specificity and association with disease progression. ¹⁰⁷ These were also recommended as predictive biomarkers for PCa. Several studies have reported the inhibitory effect of CircRNAs on PCa progression. For instance, a study showed that circDDX17 inhibits the transition, proliferation, and invasion of PCa cells. ¹⁰⁸ Kong and colleagues identified dysregulated circZNF561 and circNFIA in tissue samples of PCa patients. The expression level of CircNFIA in plasma and tissue samples increased and had an oncogenic function, whereas circZNF561 showed a reduced expression level and tumor suppressive function. ³⁵

Conclusion

Disorders of the male reproductive system account for around 50% of infertility issues, indicating the need to identify the mechanisms involved. In the present review, a description of CircRNA and its function in different male reproduction mechanisms was presented. Based on the findings of several studies, CircRNAs are highly expressed in the testes and have a dynamic and age-dependent pattern. These RNAs are involved in a variety of different processes such as germ cell self-renewal, spermatid cell development, sperm quality, and embryo development. Changes in CircRNA expression level are associated with different reproductive complications including asthenozoospermia, non-obstructive azoospermia, and prostate cancer. To date, there are no reports on the association of CircRNA with teratozoospermia or varicocele. Overall, CircRNAs can be a potential biomarker for the diagnosis and treatment of diseases related to the male reproductive system. Further in vivo studies, especially in humans, are recommended.

Acknowledgment

The present study was financially supported by Shiraz University of Medical Sciences, Shiraz, Iran.

Authors’ Contribution

Z.D and SMB.T: Conception, design, acquisition of data, writing the manuscript. Z.D, SMB.T, S.B, S.A, J.F: Data interpretation, review of the manuscript. All authors have read and approved the final manuscript and agreed to be accountable for all aspects of the study in ensuring that questions related to the accuracy or integrity of any part of the study are appropriately investigated and resolved.

Conflict of Interest:

None declared.

References

1. Glazer CH, Eisenberg ML, Tottenborg SS, Giwercman A, Flachs EM, Brauner EV, et al. Male factor infertility and risk of death: a nationwide record-linkage study. Hum Reprod. 2019; 34:2266-73. DOI | PubMed
2. Panner Selvam MK, Ambar RF, Agarwal A, Henkel R. Etiologies of sperm DNA damage and its impact on male infertility. Andrologia. 2021; 53:e13706. DOI | PubMed
3. Punjani N, Wald G, Al-Hussein Alwamlh O, Feliciano M, Dudley V, Goldstein M. Optimal timing for repeat semen analysis during male infertility evaluation. F S Rep. 2021; 2:172-5. Publisher Full Text | DOI | PubMed
4. Babakhanzadeh E, Nazari M, Ghasemifar S, Khodadadian A. Some of the Factors Involved in Male Infertility: A Prospective Review. Int J Gen Med. 2020; 13:29-41. Publisher Full Text | DOI | PubMed
5. Agarwal A, Parekh N, Panner Selvam MK, Henkel R, Shah R, Homa ST, et al. Male Oxidative Stress Infertility (MOSI): Proposed Terminology and Clinical Practice Guidelines for Management of Idiopathic Male Infertility. World J Mens Health. 2019; 37:296-312. Publisher Full Text | DOI | PubMed
6. Hamilton M, Russell S, Moskovtsev S, Krawetz SA, Librach C. The developmental significance of sperm-borne ribonucleic acids and their potential for use as diagnostic markers for male factor infertility. F&S Reviews. 2022; 3:11-23. DOI
7. Liu S, Lao Y, Wang Y, Li R, Fang X, Wang Y, et al. Role of RNA N6-Methyladenosine Modification in Male Infertility and Genital System Tumors. Front Cell Dev Biol. 2021; 9:676364. Publisher Full Text | DOI | PubMed
8. Zhang M, He P, Bian Z. Long Noncoding RNAs in Neurodegenerative Diseases: Pathogenesis and Potential Implications as Clinical Biomarkers. Front Mol Neurosci. 2021; 14:685143. Publisher Full Text | DOI | PubMed
9. Watson CN, Belli A, Di Pietro V. Small Non-coding RNAs: New Class of Biomarkers and Potential Therapeutic Targets in Neurodegenerative Disease. Front Genet. 2019; 10:364. Publisher Full Text | DOI | PubMed
10. Robless EE, Howard JA, Casari I, Falasca M. Exosomal long non-coding RNAs in the diagnosis and oncogenesis of pancreatic cancer. Cancer Lett. 2021; 501:55-65. DOI | PubMed
11. Mattick JS, Makunin IV. Non-coding RNA. Hum Mol Genet. 2006; 15:R17-29. DOI | PubMed
12. Dong WW, Li HM, Qing XR, Huang DH, Li HG. Identification and characterization of human testis derived circular RNAs and their existence in seminal plasma. Sci Rep. 2016; 6:39080. Publisher Full Text | DOI | PubMed
13. Cocquerelle C, Mascrez B, Hetuin D, Bailleul B. Mis-splicing yields circular RNA molecules. FASEB J. 1993; 7:155-60. DOI | PubMed
14. Ebbesen KK, Hansen TB, Kjems J. Insights into circular RNA biology. RNA Biol. 2017; 14:1035-45. Publisher Full Text | DOI | PubMed
15. Wu R, Li J, Li J, Yan X, Zhou W, Ling C, et al. Circular RNA expression profiling and bioinformatic analysis of cumulus cells in endometriosis infertility patients. Epigenomics. 2020; 12:2093-108. DOI | PubMed
16. Kim E, Kim YK, Lee SV. Emerging functions of circular RNA in aging. Trends Genet. 2021; 37:819-29. DOI | PubMed
17. Vausort M, Salgado-Somoza A, Zhang L, Leszek P, Scholz M, Teren A, et al. Myocardial Infarction-Associated Circular RNA Predicting Left Ventricular Dysfunction. J Am Coll Cardiol. 2016; 68:1247-8. DOI | PubMed
18. Wang K, Gan TY, Li N, Liu CY, Zhou LY, Gao JN, et al. Circular RNA mediates cardiomyocyte death via miRNA-dependent upregulation of MTP18 expression. Cell Death Differ. 2017; 24:1111-20. Publisher Full Text | DOI | PubMed
19. Shan K, Liu C, Liu BH, Chen X, Dong R, Liu X, et al. Circular Noncoding RNA HIPK3 Mediates Retinal Vascular Dysfunction in Diabetes Mellitus. Circulation. 2017; 136:1629-42. DOI | PubMed
20. Cardamone G, Paraboschi EM, Rimoldi V, Duga S, Solda G, Asselta R. The Characterization of GSDMB Splicing and Backsplicing Profiles Identifies Novel Isoforms and a Circular RNA That Are Dysregulated in Multiple Sclerosis. Int J Mol Sci. 2017; 18Publisher Full Text | DOI | PubMed
21. Guo JN, Li J, Zhu CL, Feng WT, Shao JX, Wan L, et al. Comprehensive profile of differentially expressed circular RNAs reveals that hsa_circ_0000069 is upregulated and promotes cell proliferation, migration, and invasion in colorectal cancer. Onco Targets Ther. 2016; 9:7451-8. Publisher Full Text | DOI | PubMed
22. Xu L, Zhang M, Zheng X, Yi P, Lan C, Xu M. The circular RNA ciRS-7 (Cdr1as) acts as a risk factor of hepatic microvascular invasion in hepatocellular carcinoma. J Cancer Res Clin Oncol. 2017; 143:17-27. DOI | PubMed
23. Luo YH, Zhu XZ, Huang KW, Zhang Q, Fan YX, Yan PW, et al. Emerging roles of circular RNA hsa_circ_0000064 in the proliferation and metastasis of lung cancer. Biomed Pharmacother. 2017; 96:892-8. DOI | PubMed
24. Zhao Y, Alexandrov PN, Jaber V, Lukiw WJ. Deficiency in the Ubiquitin Conjugating Enzyme UBE2A in Alzheimer’s Disease (AD) is Linked to Deficits in a Natural Circular miRNA-7 Sponge (circRNA; ciRS-7). Genes (Basel).. 2016; 7Publisher Full Text | DOI | PubMed
25. Panda AC, Grammatikakis I, Kim KM, De S, Martindale JL, Munk R, et al. Identification of senescence-associated circular RNAs (SAC-RNAs) reveals senescence suppressor CircPVT1. Nucleic Acids Res. 2017; 45:4021-35. Publisher Full Text | DOI | PubMed
26. Qian Y, Lu Y, Rui C, Qian Y, Cai M, Jia R. Potential Significance of Circular RNA in Human Placental Tissue for Patients with Preeclampsia. Cell Physiol Biochem. 2016; 39:1380-90. DOI | PubMed
27. Cheng J, Huang J, Yuan S, Zhou S, Yan W, Shen W, et al. Circular RNA expression profiling of human granulosa cells during maternal aging reveals novel transcripts associated with assisted reproductive technology outcomes. PLoS One. 2017; 12:e0177888. Publisher Full Text | DOI | PubMed
28. Dang Y, Yan L, Hu B, Fan X, Ren Y, Li R, et al. Tracing the expression of circular RNAs in human pre-implantation embryos. Genome Biol. 2016; 17:130. Publisher Full Text | DOI | PubMed
29. Kong Z, Wan X, Lu Y, Zhang Y, Huang Y, Xu Y, et al. Circular RNA circFOXO3 promotes prostate cancer progression through sponging miR-29a-3p. J Cell Mol Med. 2020; 24:799-813. Publisher Full Text | DOI | PubMed
30. Liu L, Li F, Wen Z, Li T, Lv M, Zhao X, et al. Preliminary investigation of the function of hsa_circ_0049356 in nonobstructive azoospermia patients. Andrologia. 2020; 52:e13814. DOI | PubMed
31. Lv MQ, Zhou L, Ge P, Li YX, Zhang J, Zhou DX. Over-expression of hsa_circ_0000116 in patients with non-obstructive azoospermia and its predictive value in testicular sperm retrieval. Andrology. 2020; 8:1834-43. DOI | PubMed
32. Ji C, Wang Y, Wei X, Zhang X, Cong R, Yao L, et al. Potential of testis-derived circular RNAs in seminal plasma to predict the outcome of microdissection testicular sperm extraction in patients with idiopathic non-obstructive azoospermia. Hum Reprod. 2021; 36:2649-60. DOI | PubMed
33. Manfrevola F, Ferraro B, Sellitto C, Rocco D, Fasano S, Pierantoni R, et al. CRISP2, CATSPER1 and PATE1 Expression in Human Asthenozoospermic Semen. Cells. 2021; 10Publisher Full Text | DOI | PubMed
34. Chen D, Lu X, Yang F, Xing N. Circular RNA circHIPK3 promotes cell proliferation and invasion of prostate cancer by sponging miR-193a-3p and regulating MCL1 expression. Cancer Manag Res. 2019; 11:1415-23. Publisher Full Text | DOI | PubMed
35. Kong Z, Lu Y, Wan X, Luo J, Li D, Huang Y, et al. Comprehensive Characterization of Androgen-Responsive circRNAs in Prostate Cancer. Life (Basel).. 2021; 11Publisher Full Text | DOI | PubMed
36. Jeck WR, Sorrentino JA, Wang K, Slevin MK, Burd CE, Liu J, et al. Circular RNAs are abundant, conserved, and associated with ALU repeats. RNA. 2013; 19:141-57. Publisher Full Text | DOI | PubMed
37. Zhang Y, Zhang XO, Chen T, Xiang JF, Yin QF, Xing YH, et al. Circular intronic long noncoding RNAs. Mol Cell. 2013; 51:792-806. DOI | PubMed
38. Li Z, Huang C, Bao C, Chen L, Lin M, Wang X, et al. Exon-intron circular RNAs regulate transcription in the nucleus. Nat Struct Mol Biol. 2015; 22:256-64. DOI | PubMed
39. Zhang P, Chao Z, Zhang R, Ding R, Wang Y, Wu W, et al. Circular RNA Regulation of Myogenesis. Cells. 2019; 8Publisher Full Text | DOI | PubMed
40. Lu M. Circular RNA: functions, applications and prospects. ExRNA. 2020; 2:1-7. DOI
41. Chen LL, Yang L. Regulation of circRNA biogenesis. RNA Biol. 2015; 12:381-8. Publisher Full Text | DOI | PubMed
42. Meng X, Li X, Zhang P, Wang J, Zhou Y, Chen M. Circular RNA: an emerging key player in RNA world. Brief Bioinform. 2017; 18:547-57. DOI | PubMed
43. Chen I, Chen CY, Chuang TJ. Biogenesis, identification, and function of exonic circular RNAs. Wiley Interdiscip Rev RNA. 2015; 6:563-79. Publisher Full Text | DOI | PubMed
44. Ashwal-Fluss R, Meyer M, Pamudurti NR, Ivanov A, Bartok O, Hanan M, et al. circRNA biogenesis competes with pre-mRNA splicing. Mol Cell. 2014; 56:55-66. DOI | PubMed
45. Huang A, Zheng H, Wu Z, Chen M, Huang Y. Circular RNA-protein interactions: functions, mechanisms, and identification. Theranostics. 2020; 10:3503-17. Publisher Full Text | DOI | PubMed
46. Starke S, Jost I, Rossbach O, Schneider T, Schreiner S, Hung LH, et al. Exon circularization requires canonical splice signals. Cell Rep. 2015; 10:103-11. DOI | PubMed
47. Liang D, Tatomer DC, Luo Z, Wu H, Yang L, Chen LL, et al. The Output of Protein-Coding Genes Shifts to Circular RNAs When the Pre-mRNA Processing Machinery Is Limiting. Mol Cell. 2017; 68:940-54 e3. Publisher Full Text | DOI | PubMed
48. Barrett SP, Wang PL, Salzman J. Circular RNA biogenesis can proceed through an exon-containing lariat precursor. Elife. 2015; 4:e07540. Publisher Full Text | DOI | PubMed
49. Jeck WR, Sharpless NE. Detecting and characterizing circular RNAs. Nat Biotechnol. 2014; 32:453-61. Publisher Full Text | DOI | PubMed
50. Bolha L, Ravnik-Glavac M, Glavac D. Circular RNAs: Biogenesis, Function, and a Role as Possible Cancer Biomarkers. Int J Genomics. 2017; 2017:6218353. Publisher Full Text | DOI | PubMed
51. Li X, Liu CX, Xue W, Zhang Y, Jiang S, Yin QF, et al. Coordinated circRNA Biogenesis and Function with NF90/NF110 in Viral Infection. Mol Cell. 2017; 67:214-27. DOI | PubMed
52. Errichelli L, Dini Modigliani S, Laneve P, Colantoni A, Legnini I, Capauto D, et al. FUS affects circular RNA expression in murine embryonic stem cell-derived motor neurons. Nat Commun. 2017; 8:14741. Publisher Full Text | DOI | PubMed
53. Conn SJ, Pillman KA, Toubia J, Conn VM, Salmanidis M, Phillips CA, et al. The RNA binding protein quaking regulates formation of circRNAs. Cell. 2015; 160:1125-34. DOI | PubMed
54. Aktas T, Avsar Ilik I, Maticzka D, Bhardwaj V, Pessoa Rodrigues C, Mittler G, et al. DHX9 suppresses RNA processing defects originating from the Alu invasion of the human genome. Nature. 2017; 544:115-9. DOI | PubMed
55. Knupp D, Cooper DA, Saito Y, Darnell RB, Miura P. NOVA2 regulates neural circRNA biogenesis. Nucleic Acids Res. 2021; 49:6849-62. Publisher Full Text | DOI | PubMed
56. Okholm TLH, Sathe S, Park SS, Kamstrup AB, Rasmussen AM, Shankar A, et al. Transcriptome-wide profiles of circular RNA and RNA-binding protein interactions reveal effects on circular RNA biogenesis and cancer pathway expression. Genome Med. 2020; 12:112. Publisher Full Text | DOI | PubMed
57. Zhang XO, Wang HB, Zhang Y, Lu X, Chen LL, Yang L. Complementary sequence-mediated exon circularization. Cell. 2014; 159:134-47. DOI | PubMed
58. Weng W, Wei Q, Toden S, Yoshida K, Nagasaka T, Fujiwara T, et al. Circular RNA ciRS-7-A Promising Prognostic Biomarker and a Potential Therapeutic Target in Colorectal Cancer. Clin Cancer Res. 2017; 23:3918-28. Publisher Full Text | DOI | PubMed
59. Du WW, Yang W, Liu E, Yang Z, Dhaliwal P, Yang BB. Foxo3 circular RNA retards cell cycle progression via forming ternary complexes with p21 and CDK2. Nucleic Acids Res. 2016; 44:2846-58. Publisher Full Text | DOI | PubMed
60. Legnini I, Di Timoteo G, Rossi F, Morlando M, Briganti F, Sthandier O, et al. Circ-ZNF609 Is a Circular RNA that Can Be Translated and Functions in Myogenesis. Mol Cell. 2017; 66:22-37. Publisher Full Text | DOI | PubMed
61. Das A, Gorospe M, Panda AC. The coding potential of circRNAs. Aging (Albany NY).. 2018; 10:2228-9. Publisher Full Text | DOI | PubMed
62. Gao Y, Wu M, Fan Y, Li S, Lai Z, Huang Y, et al. Identification and characterization of circular RNAs in Qinchuan cattle testis. R Soc Open Sci. 2018; 5:180413. Publisher Full Text | DOI | PubMed
63. de Kretser DM, Loveland KL, Meinhardt A, Simorangkir D, Wreford N. Spermatogenesis. Hum Reprod. 1998; 13:1-8. DOI | PubMed
64. Yu Z, Guo R, Ge Y, Ma J, Guan J, Li S, et al. Gene expression profiles in different stages of mouse spermatogenic cells during spermatogenesis. Biol Reprod. 2003; 69:37-47. DOI | PubMed
65. Capel B, Swain A, Nicolis S, Hacker A, Walter M, Koopman P, et al. Circular transcripts of the testis-determining gene Sry in adult mouse testis. Cell. 1993; 73:1019-30. DOI | PubMed
66. Hansen TB, Jensen TI, Clausen BH, Bramsen JB, Finsen B, Damgaard CK, et al. Natural RNA circles function as efficient microRNA sponges. Nature. 2013; 495:384-8. DOI | PubMed
67. Li T, Luo R, Wang X, Wang H, Zhao X, Guo Y, et al. Unraveling Stage-Dependent Expression Patterns of Circular RNAs and Their Related ceRNA Modulation in Ovine Postnatal Testis Development. Front Cell Dev Biol. 2021; 9:627439. Publisher Full Text | DOI | PubMed
68. Zhou T, Xie X, Li M, Shi J, Zhou JJ, Knox KS, et al. Rat BodyMap transcriptomes reveal unique circular RNA features across tissue types and developmental stages. RNA. 2018; 24:1443-56. Publisher Full Text | DOI | PubMed
69. You X, Vlatkovic I, Babic A, Will T, Epstein I, Tushev G, et al. Neural circular RNAs are derived from synaptic genes and regulated by development and plasticity. Nat Neurosci. 2015; 18:603-10. Publisher Full Text | DOI | PubMed
70. Zhang F, Zhang X, Ning W, Zhang X, Ru Z, Wang S, et al. Expression Analysis of Circular RNAs in Young and Sexually Mature Boar Testes. Animals (Basel).. 2021; 11Publisher Full Text | DOI | PubMed
71. Legrand JMD, Chan AL, La HM, Rossello FJ, Anko ML, Fuller-Pace FV, et al. DDX5 plays essential transcriptional and post-transcriptional roles in the maintenance and function of spermatogonia. Nat Commun. 2019; 10:2278. Publisher Full Text | DOI | PubMed
72. Zhou F, Chen W, Jiang Y, He Z. Regulation of long non-coding RNAs and circular RNAs in spermatogonial stem cells. Reproduction. 2019; 158:R15-R25. DOI | PubMed
73. Lin X, Han M, Cheng L, Chen J, Zhang Z, Shen T, et al. Expression dynamics, relationships, and transcriptional regulations of diverse transcripts in mouse spermatogenic cells. RNA Biol. 2016; 13:1011-24. Publisher Full Text | DOI | PubMed
74. Tang C, Xie Y, Yu T, Liu N, Wang Z, Woolsey RJ, et al. m(6)A-dependent biogenesis of circular RNAs in male germ cells. Cell Res. 2020; 30:211-28. Publisher Full Text | DOI | PubMed
75. Chioccarelli T, Manfrevola F, Ferraro B, Sellitto C, Cobellis G, Migliaccio M, et al. Expression Patterns of Circular RNAs in High Quality and Poor Quality Human Spermatozoa. Front Endocrinol (Lausanne).. 2019; 10:435. Publisher Full Text | DOI | PubMed
76. Godia M, Castello A, Rocco M, Cabrera B, Rodriguez-Gil JE, Balasch S, et al. Identification of circular RNAs in porcine sperm and evaluation of their relation to sperm motility. Sci Rep. 2020; 10:7985. Publisher Full Text | DOI | PubMed
77. Chioccarelli T, Falco G, Cappetta D, De Angelis A, Roberto L, Addeo M, et al. FUS driven circCNOT6L biogenesis in mouse and human spermatozoa supports zygote development. Cell Mol Life Sci. 2021; 79:50. Publisher Full Text | DOI | PubMed
78. Guo L, Chao SB, Xiao L, Wang ZB, Meng TG, Li YY, et al. Sperm-carried RNAs play critical roles in mouse embryonic development. Oncotarget. 2017; 8:67394-405. Publisher Full Text | DOI | PubMed
79. Ragusa M, Barbagallo D, Chioccarelli T, Manfrevola F, Cobellis G, Di Pietro C, et al. CircNAPEPLD is expressed in human and murine spermatozoa and physically interacts with oocyte miRNAs. RNA Biol. 2019; 16:1237-48. Publisher Full Text | DOI | PubMed
80. Caroppo E, Colpi GM. Update on the Management of Non-Obstructive Azoospermia: Current Evidence and Unmet Needs. J Clin Med. 2021; 11Publisher Full Text | DOI | PubMed
81. Arafat M, Har-Vardi I, Harlev A, Levitas E, Zeadna A, Abofoul-Azab M, et al. Mutation in TDRD9 causes non-obstructive azoospermia in infertile men. J Med Genet. 2017; 54:633-9. DOI | PubMed
82. Oka S, Shiraishi K, Matsuyama H. Effects of human chorionic gonadotropin on testicular interstitial tissues in men with non-obstructive azoospermia. Andrology. 2017; 5:232-9. DOI | PubMed
83. Abdel Raheem A, Garaffa G, Rushwan N, De Luca F, Zacharakis E, Abdel Raheem T, et al. Testicular histopathology as a predictor of a positive sperm retrieval in men with non-obstructive azoospermia. BJU Int. 2013; 111:492-9. DOI | PubMed
84. Tsujimura A, Matsumiya K, Miyagawa Y, Takao T, Fujita K, Koga M, et al. Prediction of successful outcome of microdissection testicular sperm extraction in men with idiopathic nonobstructive azoospermia. J Urol. 2004; 172:1944-7. DOI | PubMed
85. Yao JT, Zhao SH, Liu QP, Lv MQ, Zhou DX, Liao ZJ, et al. Over-expression of CircRNA_100876 in non-small cell lung cancer and its prognostic value. Pathol Res Pract. 2017; 213:453-6. DOI | PubMed
86. Li P, Chen S, Chen H, Mo X, Li T, Shao Y, et al. Using circular RNA as a novel type of biomarker in the screening of gastric cancer. Clin Chim Acta. 2015; 444:132-6. DOI | PubMed
87. Ge P, Zhang J, Zhou L, Lv MQ, Li YX, Wang J, et al. CircRNA expression profile and functional analysis in testicular tissue of patients with non-obstructive azoospermia. Reprod Biol Endocrinol. 2019; 17:100. Publisher Full Text | DOI | PubMed
88. Bo H, Liu Z, Tang R, Gong G, Wang X, Zhang H, et al. Testicular biopsies microarray analysis reveals circRNAs are involved in the pathogenesis of non-obstructive azoospermia. Aging (Albany NY). 2020; 12:2610-25. Publisher Full Text | DOI | PubMed
89. Zhu F, Luo Y, Bo H, Gong G, Tang R, Fan J, et al. Trace the profile and function of circular RNAs in Sertoli cell only syndrome. Genomics. 2021; 113:1845-54. DOI | PubMed
90. Zhou Y, Yao W, Zhang D, Yu Y, Chen S, Lu H, et al. Effectiveness of acupuncture for asthenozoospermia: A protocol for systematic review and meta-analysis. Medicine (Baltimore).. 2021; 100:e25711. Publisher Full Text | DOI | PubMed
91. Tang D, Sha Y, Gao Y, Zhang J, Cheng H, Zhang J, et al. Novel variants in DNAH9 lead to nonsyndromic severe asthenozoospermia. Reprod Biol Endocrinol. 2021; 19:27. Publisher Full Text | DOI | PubMed
92. Zhao K, Zhang J, Xu Z, Xu Y, Xu A, Chen W, et al. Metabolomic Profiling of Human Spermatozoa in Idiopathic Asthenozoospermia Patients Using Gas Chromatography-Mass Spectrometry. Biomed Res Int. 2018; 2018:8327506. Publisher Full Text | DOI | PubMed
93. Kang H, Chen C, Gao F-X, Zeng X-H, Zhang X-N. Expression of PPP3CC and not PPP3R2 is associated with asthenozoospermia. Journal of Men’s Health. 2021; 17:183-9. DOI
94. Mazaheri Moghaddam M, Mazaheri Moghaddam M, Amini M, Bahramzadeh B, Baghbanzadeh A, Biglari A, et al. Evaluation of SEPT2 and SEPT4 transcript contents in spermatozoa from men with asthenozoospermia and teratozoospermia. Health Sci Rep. 2021; 4:e436. Publisher Full Text | DOI | PubMed
95. Gao L, Chang S, Xia W, Wang X, Zhang C, Cheng L, et al. Circular RNAs from BOULE play conserved roles in protection against stress-induced fertility decline. Sci Adv. 2020; 6Publisher Full Text | DOI | PubMed
96. Dong Y, He D, Peng Z, Peng W, Shi W, Wang J, et al. Circular RNAs in cancer: an emerging key player. J Hematol Oncol. 2017; 10:2. Publisher Full Text | DOI | PubMed
97. Wang F, Nazarali AJ, Ji S. Circular RNAs as potential biomarkers for cancer diagnosis and therapy. Am J Cancer Res. 2016; 6:1167-76. Publisher Full Text | PubMed
98. Cai H, Zhang P, Xu M, Yan L, Liu N, Wu X. Circular RNA hsa_circ_0000263 participates in cervical cancer development by regulating target gene of miR-150-5p. J Cell Physiol. 2019; 234:11391-400. DOI | PubMed
99. Ahmed I, Karedath T, Andrews SS, Al-Azwani IK, Mohamoud YA, Querleu D, et al. Altered expression pattern of circular RNAs in primary and metastatic sites of epithelial ovarian carcinoma. Oncotarget. 2016; 7:36366-81. Publisher Full Text | DOI | PubMed
100. Passos GR, Ghezzi AC, Antunes E, de Oliveira MG, Monica FZ. The Role of Periprostatic Adipose Tissue on Prostate Function in Vascular-Related Disorders. Front Pharmacol. 2021; 12:626155. Publisher Full Text | DOI | PubMed
101. Verze P, Cai T, Lorenzetti S. The role of the prostate in male fertility, health and disease. Nat Rev Urol. 2016; 13:379-86. DOI | PubMed
102. Sung H, Ferlay J, Siegel RL, Laversanne M, Soerjomataram I, Jemal A, et al. Global Cancer Statistics 2020: GLOBOCAN Estimates of Incidence and Mortality Worldwide for 36 Cancers in 185 Countries. CA Cancer J Clin. 2021; 71:209-49. DOI | PubMed
103. Xia Q, Ding T, Zhang G, Li Z, Zeng L, Zhu Y, et al. Circular RNA Expression Profiling Identifies Prostate Cancer- Specific circRNAs in Prostate Cancer. Cell Physiol Biochem. 2018; 50:1903-15. DOI | PubMed
104. Ge S, Sun C, Hu Q, Guo Y, Xia G, Mi Y, et al. Differential expression profiles of circRNAs in human prostate cancer based on chip and bioinformatic analysis. Int J Clin Exp Pathol. 2020; 13:1045-52. Publisher Full Text | PubMed
105. Wu YP, Lin XD, Chen SH, Ke ZB, Lin F, Chen DN, et al. Identification of Prostate Cancer-Related Circular RNA Through Bioinformatics Analysis. Front Genet. 2020; 11:892. Publisher Full Text | DOI | PubMed
106. Wang Y, Liu B. Circular RNA in Diseased Heart. Cells. 2020; 9Publisher Full Text | DOI | PubMed
107. Hansen EB, Fredsoe J, Okholm TLH, Ulhoi BP, Klingenberg S, Jensen JB, et al. The transcriptional landscape and biomarker potential of circular RNAs in prostate cancer. Genome Med. 2022; 14:8. Publisher Full Text | DOI | PubMed
108. Lin Q, Cai J, Wang QQ. The Significance of Circular RNA DDX17 in Prostate Cancer. Biomed Res Int. 2020; 2020:1878431. Publisher Full Text | DOI | PubMed