1. Introduction
The global trend notes a rise in testicular cancer incidence among many different demographics across the globe in recent years [
1]. There is a noticeable increase in the occurrence of testicular germ cell tumors (TGCTs), not just in younger cohorts, but notably among individuals over the age of 40 years [
2,
3]. This malignancy, once categorized as the predominant cancer in younger males, is now impacting the health and well-being of a broader demographic segment than previously recognized [
4]. Consequently, the strategies employed by researchers, clinicians, and patients in confronting testicular cancer need to be progressively refined to rise to the challenges posed by the evolving landscape of testicular cancer treatment and management.
Germ cell tumors (GCTs), which are neoplasms originating from germ cells within the testis, present in three primary categories: (i) yolk sac tumors and teratomas typically observed in neonates and infants; (ii) seminomas and nonseminomas, more common in adolescents and adults; and (iii) spermatocytic seminomas, which are rare but noteworthy [
5]. Among these, seminomas and nonseminomas, also referred to as NSGCTs, are of critical concern given that they represent an overwhelming majority — nearly 99% — of testicular germ cell tumor incidences in the 15 to 44-year age group [
1,
6]. With the estimated number of new testicular cancer cases in the United States for 2023 being 9,120, and with over 291,000 men currently managing this disease, there is a significant impetus to pursue more affordable and less invasive diagnostic and monitoring strategies [
7].
The diagnosis of testicular germ cell tumors (TGCTs) predominantly relies on a comprehensive approach that includes a variety of methods to ensure accuracy. These methods typically encompass (i) a thorough physical examination of the testes to detect any abnormalities, (ii) advanced imaging techniques such as ultrasound (US) and magnetic resonance imaging (MRI) to visualize the internal structure of the testes and identify potential tumors, (iii) the measurement of serum tumor marker levels, which serve as a non-invasive liquid biopsy providing preliminary assessment before proceeding to more invasive diagnostic procedures, and (iv) a tissue biopsy, the definitive method for diagnosing TGCTs by allowing direct examination of the tumor cells under a microscope [
7]. Serum tumor markers are particularly valuable in the diagnostic process, as they involve the analysis of various substances, including metabolites, proteins, and nucleic acids, that are released into the bloodstream by tumor cells, providing insights into changes in gene expression and the genomic profile of the tumor [
8]. The traditional serum tumor markers (STMs) used in the diagnosis of TGCTs are α-fetoprotein (AFP), Beta human chorionic gonadotropin (β-hCG), and lactate dehydrogenase (LDH) [
9]. However, these conventional STMs have several limitations that can impact their effectiveness in accurately diagnosing TGCTs. One significant limitation is that AFP and β-hCG levels can be elevated in conditions unrelated to TGCTs, such as disruptions in endocrine and metabolic processes. Furthermore, LDH is a marker of cell death and can be elevated in a variety of conditions characterized by increased cell turnover, making it less specific for TGCTs [
10,
11]. Additionally, seminomas, a type of TGCT, do not produce AFP, so an elevated AFP level would not be indicative of a seminoma but could suggest the presence of non-seminomatous germ cell tumors (NSGCTs). Moreover, both LDH and β-hCG can be elevated in any type of GCT, providing little specificity in determining the exact type of cancer [
12]. Studies have highlighted the limitations of conventional STMs in the management of testicular cancer. For example, a study involving 793 patients between January 2014 and July 2021 found that of the 71 patients who had a relapse, only 31 (43.6%) were marker-positive. Additionally, 124 patients (15.6%) experienced at least one false-positive marker elevation, defined as a positive marker result in a patient who remained free from a proven relapse for at least 6 months after the incident. These false-positive results can lead to unnecessary additional imaging or a hesitancy to investigate further due to the perceived irrelevance of the marker elevations [
13]. Another study conducted in 2020 reviewed the records of 794 patients with GCTs treated in three Spanish hospitals and found that of the 125 patients who developed a first recurrence, 123 had marker levels recorded. Seventy-nine patients (64%) had elevated tumor markers at diagnosis, and 76 (62%) had elevated markers at first recurrence [
14]. The unreliable elevation and lack of specificity of conventional STMs make them less favorable for reliable diagnosis and clinical management. In light of these limitations, there is a growing interest in exploring alternative biomarkers that can provide more accurate and specific diagnostic information. One such promising avenue is the analysis of blood-circulating microRNAs (miRNAs) in standard germ cell tumor screening regimens. These small non-coding RNA molecules have been shown to play a crucial role in gene regulation and have the potential to serve as more reliable and specific biomarkers for the diagnosis and management of TGCTs.
MicroRNAs (miRNAs) are short, non-coding RNA molecules that play a crucial role in the regulation of gene expression, affecting approximately one-third of protein-coding genes in the human genome [
15]. The biogenesis of miRNAs involves several steps, starting with the transcription of primary miRNAs (pri-miRNAs) that are subsequently processed into precursor miRNAs (pre-miRNAs). These pre-miRNAs are then further cleaved to form mature miRNAs, which are incorporated into the RNA-induced silencing complex (RISC). Within this complex, miRNAs can bind to complementary sequences in target messenger RNAs (mRNAs), leading to the degradation or translational repression of these target mRNAs [
16]. In the context of cancer, miRNA expression can become dysregulated, leading to the disruption of normal cellular processes. miRNAs can act as oncogenes (referred to as oncomiRs) or tumor suppressors, depending on their target genes and the context of their expression. In germ cell tumors (GCTs), oncomiRs are often overexpressed, leading to the suppression of their target tumor suppressor genes. Conversely, tumor suppressor miRNAs can be downregulated, resulting in the overexpression of their target oncogenes [
15]. This dysregulation of miRNA levels creates an environment conducive to cancer growth and progression. The alterations in miRNA expression patterns can serve as valuable biomarkers for the detection and differentiation of various histotypes of testicular germ cell tumors [
16]. By analyzing these changes, clinicians can gain insights into the molecular underpinnings of the disease, which can inform diagnostic, therapeutic, and prognostic strategies. The application of miRNA-based biomarkers in clinical practice has the potential to enhance the accuracy of diagnosis, guide treatment decisions, and provide prognostic information, ultimately improving patient outcomes in testicular cancer management.
This review aims to illuminate the multifaceted role of circulating microRNAs (miRNAs) as liquid biopsies in the management of germ cell tumors (GCTs). First, we explore the current clinical benefits and implications of utilizing circulating miRNAs, emphasizing their potential to revolutionize non-invasive diagnostic and prognostic approaches in oncology. Second, we delve into the outstanding issues surrounding the use of miRNAs, such as challenges in standardization, quantification, and interpretation, that must be meticulously addressed prior to their widespread clinical implementation. Finally, we discuss the potential applications of circulating miRNAs in GCT management, including their role in early detection, monitoring treatment response, and predicting disease recurrence, thereby underscoring their promise in enhancing patient outcomes and personalizing cancer care.