Tiny RNA Fragment in Sperm Tracks Severity of Varicocele-Linked Infertility

A varicocele—the abnormal enlargement of veins inside the scrotum—is one of the most common and potentially reversible causes of male infertility, yet the molecular events that translate swollen veins into damaged sperm have remained stubbornly obscure. Now, a team of researchers in Turkey has provided the first evidence that a specific class of small RNA molecules carried inside spermatozoa behaves differently in men with varicocele, and that one fragment in particular changes its expression in step with the severity of the disease. The findings, published in Reproductive Sciences, open an entirely new molecular window onto a condition that affects a substantial share of men evaluated in infertility clinics worldwide.

The molecules at the center of the study are transfer RNA-derived fragments, or tRFs, short pieces of RNA produced when mature transfer RNAs—the workhorse molecules of protein synthesis—are enzymatically cleaved. Once dismissed as mere degradation debris, tRFs have been progressively rehabilitated over the past decade into recognized regulatory players. They can silence genes, modulate translation, and influence signaling pathways such as Wnt, MAPK and Notch. Remarkably, they are also among the most abundant small RNAs in mature sperm, and animal studies have shown that sperm tRNA fragments can carry acquired metabolic information across generations. Until now, however, no one had asked whether these fragments are altered in varicocele.

The rationale for the study rested on a suggestive parallel. Varicocele is, in essence, a varicose vein of the spermatic cord, and prior work has detected altered tRNA expression profiles in human varicose veins of the leg. One fragment in particular, tRF-36, has been shown to modulate varicose vein progression through Notch signaling in human vascular smooth muscle cells. Given that varicocele and lower-limb varicose veins share clinical associations, elevated hypoxia-inducible factor signaling and intrinsic apoptotic pathway activation, the researchers reasoned that tRFs might similarly mark the molecular pathology of varicocele in the testicular environment.

To test the idea, the team recruited 60 men with clinically diagnosed and graded varicocele—20 patients in each of grades I, II and III—alongside 20 healthy normozoospermic controls. Blood and semen samples were collected from every participant under protocols approved by the local ethics committee, with written informed consent obtained from all participants. Reproductive hormones were measured in serum, standard semen parameters were assessed according to WHO laboratory guidance, and the expression of three candidate fragments—tRF-23, tRF-36 and tRF-40—was quantified in spermatozoa using quantitative real-time PCR with the widely used comparative threshold cycle method.

The first key result was a proof of presence: all three tRFs were detectable in spermatozoa from both patients and controls, confirming that this class of regulatory RNA is indeed resident in human ejaculated sperm and is accessible for measurement. When the groups were compared directly, tRF-23 expression was elevated by 1.23-fold and tRF-36 by 1.52-fold in varicocele patients relative to controls, while tRF-40 expression was reduced to 0.9-fold of control levels. Intriguingly, none of these whole-group differences reached statistical significance, a outcome the authors interpret cautiously given the modest sample size and the heterogeneity that varicocele introduces when patients of all grades are pooled together.

The picture changed dramatically when the researchers stratified patients by disease severity. Here, tRF-36 emerged as the standout molecule. Its expression fell in a pattern that tracked advancing disease, culminating in a significant 4.52-fold decrease in grade III varicocele patients compared with those harboring grade I disease, a difference that was statistically robust at a p value of 0.010. In other words, the fragment that had appeared mildly elevated in the pooled analysis was in fact being driven downward in the most severe cases, masked at the aggregate level by the comparatively higher expression in milder disease. This grade-dependent signature suggests that tRF-36 is not simply a bystander but may participate in the escalating molecular damage that accompanies advanced varicocele.

Correlation analyses added a second layer of meaning. All three tRFs displayed significant interrelationships with one another, hinting at coordinated regulation of this small RNA population within spermatozoa. More striking was their shared behavior with respect to semen quality: each of the three fragments was negatively correlated with sperm concentration, one of the cardinal parameters by which male fertility is judged. A negative correlation implies that lower fragment levels accompany lower sperm counts, consistent with the possibility that disruption of tRNA-derived RNA homeostasis in sperm parallels, and potentially contributes to, the impaired spermatogenesis that varicocele produces through mechanisms such as venous stasis, elevated testicular temperature, oxidative stress and reflux of toxic adrenal metabolites.

The study also examined whether tRF expression relates to circulating reproductive hormones, which serve as endocrine readouts of testicular function. While the abstract reports that the analysis set out to determine associations between the fragments and hormone levels as well as semen quality, the headline statistical signals in this first report are the grade-dependent tRF-36 change and the negative correlations with sperm concentration rather than strong hormone associations. This positions the fragments, at this stage, as markers more tightly coupled to the cellular output of the testis than to its endocrine regulation, a pattern that future studies with larger cohorts may refine.

What makes the work notable is less the size of the effects in pooled comparisons than the conceptual milestone it represents. Previous investigations of varicocele’s molecular footprint have focused on sperm DNA methylation patterns, oxidative stress-related microRNAs, hypoxia-associated long noncoding RNAs and transcriptome-wide signatures from exome and RNA sequencing. The new study extends this catalog to tRFs, a class whose abundance in sperm and demonstrated role in intergenerational epigenetic inheritance give it particular biological weight. If varicocele remodels the sperm small RNA payload, then the consequences of the condition could in principle extend beyond fertility itself, echoing evidence that paternal sperm tsRNAs can transmit acquired traits to offspring.

The translational possibilities are tantalizing but deliberately restrained. A fragment whose expression distinguishes grade III from grade I disease could, with validation, become a noninvasive molecular indicator of varicocele severity, complementing clinical grading and semen analysis and potentially informing decisions about surgical repair. tRFs are also attractive as biomarkers because they are chemically stable and readily measured in reproductive fluids and extracellular vesicles, where other tRNA-derived fragments have already been proposed as predictors of outcomes in assisted reproduction and microdissection testicular sperm extraction. The authors caution, appropriately, that their findings are the first link rather than the last word, and that larger, longitudinal and post-treatment studies will be needed to establish whether tRF-36 responds to varicocelectomy, whether it predicts postoperative recovery of semen quality, and whether its connection to Notch signaling in vascular disease is mirrored in the testicular microvasculature. For now, the study furnishes a fresh molecular suspect in one of andrology’s oldest mysteries, and a demonstration that the smallest fragments of our RNA repertoire may carry some of the loudest signals about male reproductive health.

Subject of Research: Sperm tRNA-derived fragment expression in varicocele and its association with disease severity, semen parameters and reproductive hormones

Article Title: Sperm tRNA-Derived Fragment-36 Expression in Varicocele: Associations with Disease Severity, Semen Parameters and Reproductive Hormones

Article References: Sperm tRNA-Derived Fragment-36 Expression in Varicocele: Associations with Disease Severity, Semen Parameters and Reproductive Hormones. (n.d.). https://doi.org/10.1007/s43032-026-02210-8

Image Credits: AI Generated

DOI: 10.1007/s43032-026-02210-8

Keywords: varicocele, male infertility, tRNA-derived fragments, tRF-36, spermatozoa, sperm concentration, reproductive hormones, semen parameters, non-coding RNA, spermatogenesis, biomarkers, andrology