Introduction
Gastric cancer (GC) remains one of the most prevalent and deadly malignancies worldwide, posing a major global health burden. Despite improvements in diagnostic technologies and therapeutic strategies, GC continues to rank among the top cancers in both incidence and mortality. According to global cancer statistics, more than one million new cases of gastric cancer are diagnosed annually, with hundreds of thousands of deaths each year. This highlights the urgent need for improved early detection methods and more effective treatment strategies.
Gastric tumorigenesis is a highly complex and multifactorial process. It involves interactions between genetic predisposition, environmental exposures, dietary habits, and infectious agents such as Helicobacter pylori. Additionally, the tumor microenvironment plays a crucial role in cancer progression by influencing cellular communication, immune response, and metastatic behavior. However, despite extensive research, the molecular mechanisms underlying gastric cancer development are still not fully understood.
Currently, GC diagnosis relies on imaging techniques, endoscopic evaluation, serum tumor markers, and tissue biopsy. Although biopsy remains the gold standard, it is invasive and not always suitable for early detection. Moreover, early-stage gastric cancer often presents with non-specific or absent symptoms, leading to late diagnosis and poor prognosis. Conventional treatments such as chemotherapy and radiotherapy can be effective but are often associated with toxicity and the development of drug resistance.
In this context, there is a growing interest in identifying novel molecular biomarkers that can improve early diagnosis, predict prognosis, and serve as therapeutic targets. Among these emerging molecules, circular RNAs (circRNAs) have gained significant attention as promising candidates in cancer biology, particularly in gastric cancer.
What Are Circular RNAs?
Circular RNAs (circRNAs) are a novel class of endogenous non-coding RNAs characterized by their unique covalently closed-loop structure. Unlike linear RNAs, circRNAs lack 5′ caps and 3′ polyadenylated tails, making them highly resistant to degradation by exonucleases. This structural stability allows circRNAs to persist longer in cells compared to traditional messenger RNAs (mRNAs).
Initially discovered in viruses in the 1970s, circRNAs were long considered rare byproducts of abnormal RNA splicing. However, advances in high-throughput RNA sequencing and bioinformatics have revealed that circRNAs are abundant, conserved, and functionally important molecules present in a wide range of organisms, including humans.
Recent studies have demonstrated that circRNAs are not merely transcriptional noise but play critical roles in gene regulation, cellular homeostasis, and disease progression, particularly in cancer.
Biogenesis and Classification of circRNAs
CircRNAs are generated through a non-canonical splicing process known as back-splicing. In this mechanism, a downstream splice donor site is joined to an upstream splice acceptor site, forming a closed circular structure.
Types of circRNAs
Based on their origin and composition, circRNAs are classified into three main categories:
- Exonic circRNAs (ecircRNAs): Composed solely of exons and mainly located in the cytoplasm.
- Intronic circRNAs (ciRNAs): Derived from introns and primarily localized in the nucleus.
- Exon–intron circRNAs (EIciRNAs): Contain both exonic and intronic sequences and are involved in transcriptional regulation.
Additionally, another subtype known as tRNA intronic circular RNAs (tricRNAs) is produced during tRNA processing.
Mechanisms of Formation
Several models explain circRNA formation:
- Lariat-driven circularization: Exon skipping leads to the formation of a lariat structure, which is subsequently processed into circRNA.
- Intron-pairing-driven circularization: Complementary sequences in flanking introns facilitate circularization.
- Protein-mediated circularization: RNA-binding proteins (RBPs) such as MBL and QKI promote circRNA formation by bringing splice sites closer together.
These mechanisms highlight the regulated nature of circRNA biogenesis rather than random formation.
Properties of circRNAs
CircRNAs possess several unique features that make them highly attractive as biomarkers and therapeutic targets:
1. High Stability
Due to their circular structure, circRNAs are resistant to exonuclease-mediated degradation. Their half-life often exceeds 48 hours, significantly longer than linear RNAs.
2. Abundance
Thousands of circRNAs have been identified in human cells, with some expressed at levels higher than their linear counterparts.
3. Conservation
CircRNAs are evolutionarily conserved across species, indicating their biological importance.
4. Tissue Specificity
Their expression varies depending on tissue type and developmental stage, enabling potential use in disease-specific diagnostics.
5. Presence in Body Fluids
CircRNAs are detectable in blood, saliva, urine, and exosomes, making them suitable for non-invasive diagnostic approaches.
Biological Functions of circRNAs
CircRNAs regulate gene expression through multiple mechanisms:
1. miRNA Sponging
One of the most well-characterized functions of circRNAs is their ability to act as microRNA (miRNA) sponges. By binding to miRNAs, circRNAs prevent them from repressing target mRNAs.
For example, certain circRNAs contain multiple binding sites for specific miRNAs, effectively reducing their activity and altering gene expression patterns.
2. Regulation of Alternative Splicing
CircRNA formation can compete with linear splicing, influencing the production of different mRNA isoforms and protein diversity.
3. Transcriptional Regulation
Some nuclear circRNAs interact with transcriptional machinery to enhance the expression of their parental genes.
4. Protein Binding
CircRNAs can bind to RNA-binding proteins, acting as decoys or scaffolds to modulate protein activity and cellular processes.
5. Protein Translation
Although traditionally considered non-coding, some circRNAs can be translated into functional peptides under specific conditions, especially when modified by m6A methylation.
CircRNAs in Gastric Cancer
High-throughput sequencing studies have identified thousands of circRNAs expressed in gastric cancer tissues. Many of these are differentially expressed compared to normal tissues, suggesting their involvement in tumorigenesis.
Expression Patterns
- Hundreds of circRNAs are either upregulated or downregulated in GC.
- Downregulated circRNAs are often more prevalent.
- Many circRNAs are involved in regulatory networks linking miRNAs and mRNAs.
These findings indicate that circRNAs play a crucial role in gastric cancer development and progression.
Oncogenic circRNAs in Gastric Cancer
Certain circRNAs promote tumor growth and progression:
- ciRS-7: Acts as a sponge for miR-7, activating oncogenic signaling pathways such as PI3K/AKT.
- circPVT1: Enhances cell proliferation by regulating miR-125 and its target genes.
- circHIPK3: Promotes tumor growth through interaction with miR-124 and miR-29b.
- circNF1: Facilitates proliferation by modulating miR-16 and downstream targets.
These circRNAs contribute to cancer cell survival, proliferation, and metastasis.
Tumor-Suppressive circRNAs
Other circRNAs exhibit anti-cancer properties:
- circRNA_100269: Inhibits cell growth by targeting miR-630.
- circYAP1: Suppresses proliferation and invasion via miRNA regulation.
- circLARP4: Regulates tumor growth through the Hippo signaling pathway.
- circ-ZFR: Promotes apoptosis and inhibits proliferation.
Loss of these circRNAs is often associated with poor prognosis.
CircRNAs as Diagnostic Biomarkers
CircRNAs show strong potential as biomarkers for gastric cancer due to:
- High stability in body fluids
- Non-invasive detection methods (e.g., blood tests)
- High sensitivity and specificity
Clinical Applications
- Early detection of gastric cancer
- Prediction of recurrence and metastasis
- Monitoring treatment response
Combining multiple circRNAs or integrating them with traditional markers (e.g., CEA, CA19-9) significantly improves diagnostic accuracy.
CircRNAs in Therapy
CircRNAs offer new opportunities for targeted cancer therapy:
Therapeutic Strategies
- Silencing oncogenic circRNAs: Using siRNA or CRISPR/Cas9
- Overexpressing tumor-suppressive circRNAs
- Synthetic circRNA sponges: Designed to inhibit oncogenic miRNAs
- Exosome-based delivery systems: For targeted RNA therapy
These approaches aim to modulate gene expression and disrupt cancer-promoting pathways.
Role of Exosomes in circRNA Function
Exosomes are small extracellular vesicles that transport biomolecules, including circRNAs, between cells.
Features
- Protect circRNAs from degradation
- Facilitate intercellular communication
- Serve as carriers for therapeutic molecules
Exosomal circRNAs have been shown to influence tumor progression and may serve as both biomarkers and therapeutic tools.
Challenges and Future Perspectives
Despite promising findings, several challenges remain:
- Incomplete understanding of circRNA biogenesis and degradation
- Limited functional validation of many circRNAs
- Lack of standardized detection methods
- Need for large-scale clinical studies
- Safety concerns in therapeutic applications
Future research should focus on:
- Identifying novel circRNA functions
- Understanding their role in the tumor microenvironment
- Developing reliable clinical assays
- Conducting clinical trials for circRNA-based therapies
Conclusion
Circular RNAs have emerged as a new frontier in cancer research, particularly in gastric cancer. Their unique structural properties, diverse biological functions, and clinical potential make them highly promising candidates for improving cancer diagnosis, prognosis, and treatment.
Although significant progress has been made, further research is needed to fully unlock the potential of circRNAs. With continued advancements in molecular biology and bioinformatics, circRNAs are expected to play a transformative role in precision medicine and cancer therapy in the near future.