Background: Circular RNAs (circRNAs) are biomarkers that may play a role in the pathological processes underlying abnormal spermatogenesis. Four circRNAs present in human semen, namely STK31-hsa-circ-0133980, LRWD1-hsa-circ-0003327, circRNF17-chr13:25341410-25356082, and circBRDT-chr1:92428275-92433817, were investigated in this study and their relationship with sperm parameters was evaluated.
Methods: In this analytical observational study, 45 semen samples were analyzed according to World Health Organization criteria, and were classified into the following groups: normozoospermia (control, n=9), asthenozoospermia (n=10), oligozoospermia (n=9), teratozoospermia (n=9), and oligoasthenoteratozoospermia (n=8). The expression levels of four circRNAs, along with genes related to apoptosis (BCL2, BAX, and Caspase3) and protamination (PRM1 and PRM2), were assessed using real-time polymerase chain reaction (RT-PCR). Sperm quality was additionally assessed on another portion of each semen sample using triple staining, aniline blue, toluidine blue, and eosin–nigrosin. Data were analyzed using Prism software, and the significance of the observed differences was evaluated by one-way ANOVA. 
Results: The results showed that changes in circRNA expression levels were significantly related to sperm morphology (all circRNAs, p<0.05), motility (circ-RNF17 and-STK31) (p<0.01), and concentration (circBRDT, p<0.05) compared to the normozoospermia group. Apoptosis and abnormal protamination were increased in all infertile groups, except the asthenozoospermia, as evidenced by upregulation of BAX and Caspase 3 and downregulation of BCL2 and PRM1&2 genes, compared to the normozoospermic group (p<0.001).
Conclusion: The circRNA levels were different in the semen samples with different sperm quality. Our results provide new insights into evaluating sperm functions, supporting the potential role of circRNAs as biomarkers of male fertility.