Circular RNAs Emerge as Promising Biomarkers and Therapeutic Targets in Colorectal Cancer Immunotherapy
Key Insights
Circular RNAs (circRNAs) represent a novel class of non-coding RNAs with exceptional stability and tissue-specific expression patterns, making them promising biomarkers for colorectal cancer (search) diagnosis and prognosis.
Multiple circRNAs have been identified as key regulators of immune responses in the tumor (search) microenvironment, influencing macrophage polarization, T cell activation, and immune evasion mechanisms through various molecular pathways.
CircRNA-based therapeutic strategies show significant potential for enhancing immunotherapy efficacy, with several circRNAs demonstrating ability to modulate immune checkpoint pathways and overcome treatment resistance in preclinical studies.
Colorectal cancer (search) (CRC (search)) remains a major global health challenge, ranking as the third most diagnosed cancer (search) worldwide with approximately 1.926 million new cases projected in 2022. Despite advances in treatment, the disease continues to pose significant therapeutic challenges, particularly in metastatic settings where immunotherapy has shown promise but limited efficacy in many patients.
CircRNAs: A New Frontier in Cancer Biology
Circular RNAs have emerged as a distinct class of non-coding RNAs characterized by their unique covalently closed-loop structure, which provides exceptional stability and resistance to exonuclease degradation. Unlike linear RNAs, circRNAs are formed through back-splicing mechanisms and lack the typical 5' cap and 3' poly(A) tail structures. This distinctive architecture contributes to their remarkable stability and conservation across species.
Recent research has revealed that circRNAs play critical roles in cancer (search) biology through multiple mechanisms, including acting as microRNA sponges, serving as protein scaffolds, and even encoding functional peptides. Their tissue-specific expression patterns and stability in body fluids make them particularly attractive as diagnostic biomarkers and therapeutic targets.
Dysregulated CircRNA Expression in Colorectal Cancer
Comprehensive studies have identified over 70 circRNAs that are significantly dysregulated in CRC (search) tissues. CircHERC4 (search), for instance, is highly expressed in CRC tissues and correlates with lymph node metastasis (search) and advanced tumor (search) stages. Similarly, circHIPK3 (search) overexpression has been linked to poorer prognosis across multiple cancer (search) types, including CRC.
Conversely, several circRNAs exhibit tumor (search)-suppressive properties. CircPTEN1 (search) and circLPAR1 (search) are both underexpressed in CRC (search) tissues, with circLPAR1 downregulation associated with reduced tumor weight and size. These differential expression patterns highlight the complex regulatory networks involving circRNAs in CRC pathogenesis.
Immunomodulatory Functions of CircRNAs
CircRNAs have demonstrated significant roles in modulating immune responses within the tumor (search) microenvironment. Studies show that various immune cells, including monocytes, neutrophils, B-cells, and platelets, can internalize extracellular circRNAs, with monocytes exhibiting the highest uptake efficiency.
Macrophage Polarization and Immune Evasion
CircRNAs actively participate in macrophage polarization, with tumor (search)-derived circRNAs capable of inducing M2 polarization through JAK1/STAT3 and PI3K-AKT pathways. M2 macrophages subsequently secrete immunosuppressive factors like IL-10, TGF-β, and IDO, promoting immune evasion. For example, exosomal circMERTK enhances IL-10 production in tumor-associated macrophages, suppressing CD8+ T-cell function.
T Cell Activation and Recruitment
Several circRNAs enhance tumor (search) immunity by promoting T cell recruitment and activation. CircDNA2v (search), frequently overexpressed in CRC (search), activates the JAK-STAT1 pathway and increases secretion of CXCL10 and IL-9, enhancing the chemotactic and cytotoxic functions of CD8+ T cells. Conversely, circRNF216 (search) promotes CD8+ T cell infiltration by upregulating ZC3H12C, triggering immune responses that limit tumor growth.
Immune Checkpoint Regulation
CircRNAs play crucial roles in immune checkpoint regulation. Circ_0020397 sponges miR-138, upregulating TERT and PD-L1 (search) expression, while ciR7 increases PD-L1 levels through miRNA-independent modulation of CMTM4 and CMTM6. These mechanisms contribute to immune evasion and cancer (search) progression.
Therapeutic Applications and Clinical Potential
Diagnostic Biomarkers
The stability and tissue-specific expression of circRNAs make them excellent candidates for diagnostic applications. CircRNAs can be detected in various body fluids, including serum, plasma, and urine, offering opportunities for non-invasive liquid biopsies. For instance, hsa_circ_0026416 has been identified as a promising plasma biomarker for CRC (search) detection and monitoring.
Therapeutic Targets
CircRNA-based immunotherapies are showing promising preliminary outcomes. Combining PD-1 (search)/PD-L1 (search) blockade with CDR1as (search) targeting enhances immunotherapy efficacy by modulating PD-L1 expression. Small molecule YAP (search) inhibitors can suppress circPPP1R12A-73aa, impairing tumor (search) metastasis (search). Additionally, CRISPR-based editing approaches, such as circZNF800 knockdown, demonstrate potential in CRC (search) treatment.
Overcoming Treatment Resistance
CircRNAs influence chemoresistance through various mechanisms. CircRNA hsa_circ_0076691 contributes to oxaliplatin resistance by acting as a molecular sponge for miR-589-3p, while circNRIP1 promotes 5-FU resistance by sponging miR-532-3p. Understanding these mechanisms opens new avenues for overcoming treatment resistance.
Prognostic Applications
CircRNAs serve as valuable prognostic indicators in CRC (search). A circular RNA-based classifier (cirScore) using four specific circRNAs has been developed to predict CRC recurrence. CircHIPK3 (search) and circCCDC66 are particularly promising, with their expression levels inversely related to clinical outcomes.
CircRERE demonstrates tumor (search)-suppressive properties by sequestering miR-6837-3p, upregulating MAVS, and enhancing type I IFN signaling pathway, thereby stimulating anti-tumor immunity. This effect is further enhanced when combined with anti-PD-1 (search) therapy.
Future Perspectives and Challenges
Despite the promising potential of circRNAs in CRC (search) management, several challenges remain. Current detection methods are often expensive and technically demanding, limiting their widespread clinical application. The development of standardized protocols for circRNA synthesis, purification, and delivery systems is crucial for clinical translation.
Integration of single-cell sequencing and spatial omics technologies will be essential for dissecting the spatiotemporal regulatory networks of circRNAs in the tumor (search) microenvironment. Additionally, the development of targeted circRNA delivery mechanisms, such as nanoparticle-based systems, requires validation in preclinical and clinical settings.
The field is moving toward personalized medicine approaches, where circRNA profiles could guide treatment decisions and predict therapeutic responses. The combination of circRNA-based diagnostics with artificial intelligence and machine learning algorithms holds promise for developing more precise and effective treatment strategies.
Conclusion
Circular RNAs represent a promising frontier in colorectal cancer (search) research, offering innovative solutions for diagnosis, prognosis, and treatment. Their unique properties as stable, tissue-specific regulators of gene expression, combined with their roles in immune modulation, position them as valuable tools in the fight against CRC (search). As research continues to unravel the complex biology of circRNAs, their integration into clinical practice could significantly improve patient outcomes and reduce the global burden of colorectal cancer.