What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Circular RNA in Cancer: Biogenesis, Functions, and Clinical Potential

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Circular RNA

Circular RNA (circRNA) is a class of single-stranded, covalently closed RNA molecules that lack both 5′ and 3′ ends. Unlike canonical linear messenger RNA (mRNA), which possesses a 5′ cap and a 3′ poly(A) tail, circRNAs form a continuous loop through a phosphodiester bond linking their termini. This unique topology confers remarkable resistance to exonucleolytic degradation, making circRNAs substantially more stable than their linear counterparts, with median half-lives exceeding 48 hours in many cell types.

The discovery of circRNAs dates to 1976, when Sanger and colleagues first observed circular RNA molecules in plant viroids. For decades, circRNAs were dismissed as splicing byproducts or experimental artifacts. This perception persisted until the early 2010s, when high-throughput RNA sequencing coupled with computational algorithms designed to detect back-splice junctions revealed that circRNAs are abundant, evolutionarily conserved, and cell-type-specific. Subsequent functional studies demonstrated that circRNAs participate in diverse biological processes, including transcriptional regulation, protein sequestration, and microRNA (miRNA) modulation. In cancer, circRNAs have emerged as critical players in tumor initiation, progression, and metastasis, offering new avenues for diagnosis and therapy.

What Makes circRNA Circular?

The defining feature of a circRNA is its covalently closed structure. This circularity arises from a non-canonical splicing event called back-splicing, in which a downstream 5′ splice site is joined to an upstream 3′ splice site. The resulting molecule has no free ends, rendering it resistant to exonucleases such as RNase R, which degrades linear RNA from its termini. This property is exploited experimentally: treating total RNA with RNase R enriches circRNAs by eliminating linear RNA, providing a simple biochemical test for circularity.

CircRNAs are generated from pre-mRNA transcripts by the spliceosome. Most circRNAs are derived from protein-coding genes, but they do not encode proteins in the conventional sense. A minority of circRNAs contain internal ribosome entry sites (IRES) and can be translated into short peptides, but the vast majority function as non-coding regulatory molecules. Their stability, abundance, and tissue-specific expression patterns make them attractive candidates for biomarkers and therapeutic targets.

Historical Context and Discovery

The first circRNA identified in human cells was the product of the DCC (deleted in colorectal cancer) gene, reported by Nigro and colleagues in 1991. However, the field remained dormant until 2012, when two landmark studies by Salzman and colleagues and Jeck and colleagues used RNA-seq data to identify thousands of circRNAs across human cell lines. These studies established that circRNAs are not rare artifacts but rather a pervasive feature of the transcriptome. Subsequent work by Memczak and colleagues in 2013 demonstrated that the circRNA ciRS-7 (also called CDR1as) functions as a sponge for miR-7, providing the first clear evidence of circRNA function. Since then, the number of annotated circRNAs has grown exponentially, with databases such as circBase and CircAtlas cataloging hundreds of thousands of candidates across species.

Biogenesis of Circular RNA

CircRNA biogenesis is a regulated process that competes with canonical linear splicing. The outcome—whether a pre-mRNA is processed into linear mRNA or a circRNA—depends on the interplay between cis-acting elements, trans-acting protein factors, and the kinetic competition between splice sites.

Back-Splicing Mechanism

Back-splicing is the primary mechanism of circRNA formation. During this process, the spliceosome catalyzes the joining of a downstream 5′ splice site to an upstream 3′ splice site. This reaction is thermodynamically and kinetically unfavorable compared to canonical splicing because it requires the spliceosome to bring distant splice sites into proximity. The efficiency of back-splicing is enhanced by complementary sequences in the flanking introns, typically Alu elements in humans. These inverted repeats base-pair with each other, bringing the splice sites into close physical proximity and promoting back-splicing.

The back-splicing reaction proceeds through two transesterification steps. In the first step, the 2′-hydroxyl of the branch-point adenosine attacks the 5′ splice site, forming a lariat intermediate. In the second step, the free 3′-hydroxyl of the upstream exon attacks the 5′ splice site of the downstream exon, releasing the circular RNA. The resulting circRNA contains the exon(s) that were spliced out of the linear transcript. Importantly, back-splicing and canonical splicing are not mutually exclusive; a single pre-mRNA can yield both linear mRNA and circRNA, depending on the cellular context.

Exon Skipping and Lariat Formation

A second, less common mechanism of circRNA biogenesis involves exon skipping followed by lariat debranching. In this pathway, the spliceosome excises an internal exon as a lariat structure. Normally, the lariat is debranched by the enzyme DBR1 (debranching RNA lariat 1) and degraded. However, if the lariat escapes debranching, the 2′–5′ phosphodiester bond at the branch point can be attacked by the 3′-hydroxyl of the upstream exon, resulting in a circular RNA. This mechanism is less frequent than back-splicing but contributes to the diversity of circRNA isoforms.

The relative contribution of back-splicing versus exon skipping varies by gene and cell type. For most circRNAs, back-splicing is the dominant pathway, as evidenced by the presence of complementary Alu elements flanking the circularized exons. Exon skipping-derived circRNAs are more common in genes with weak splice sites or unusual intronic architecture.

Regulatory Factors in Biogenesis

Several trans-acting factors modulate circRNA biogenesis. The RNA-binding protein Quaking (QKI) promotes circRNA formation by binding to recognition motifs in the flanking introns and dimerizing to bring splice sites together. Conversely, the splicing factor muscleblind (MBL) binds to its own pre-mRNA and promotes circMbl formation, creating a negative feedback loop in which high MBL protein levels increase circMbl production, which in turn sequesters MBL. The RNA helicase DHX9 suppresses circRNA formation by unwinding Alu element base-pairing, thereby reducing back-splicing efficiency. Additionally, the exon-junction complex (EJC) components and the nuclear export factor NXF1 influence circRNA localization and stability.

Cis-elements also play a critical role. The length and sequence composition of flanking introns affect back-splicing efficiency; longer introns with more complementary Alu elements generally produce higher circRNA levels. Furthermore, the competition between canonical splice sites and back-splice sites is influenced by the strength of the splice site consensus sequences. Mutations that weaken canonical splice sites can shift the balance toward circRNA production, a phenomenon observed in some cancers.

Functions of Circular RNA in Normal Cells

CircRNAs perform diverse functions in normal cellular physiology. The most well-characterized functions include miRNA sponging, protein scaffolding, and regulation of transcription and splicing. These activities are context-dependent and often cell-type-specific.

miRNA Sponging

The miRNA sponge function is the most extensively studied role of circRNAs. miRNAs are small (~22 nucleotide) non-coding RNAs that guide the RNA-induced silencing complex (RISC) to complementary mRNA sequences, typically in the 3′ untranslated region (UTR), leading to translational repression or mRNA degradation. CircRNAs containing multiple binding sites for a specific miRNA can sequester that miRNA, preventing it from interacting with its mRNA targets. This "sponging" effectively derepresses the miRNA's target genes.

The archetypal example is ciRS-7 (circular RNA sponge for miR-7), which contains more than 60 conserved binding sites for miR-7. By sequestering miR-7, ciRS-7 upregulates miR-7 targets such as EGFR (epidermal growth factor receptor) and RAF1. Similarly, the testis-specific circRNA Sry (sex-determining region Y) contains 16 binding sites for miR-138. The stoichiometry of miRNA sponging is important: for effective sponging, the circRNA must be expressed at sufficiently high levels relative to the miRNA, which is not always the case. Many circRNAs contain only a few miRNA binding sites and may not function as bona fide sponges in vivo.

Protein Interactions and Scaffolding

CircRNAs can bind to RNA-binding proteins (RBPs) and serve as scaffolds that bring proteins into proximity, modulating their activity. For example, circFOXO3, derived from the FOXO3 gene, binds to the cyclin-dependent kinase 2 (CDK2) and the cell cycle inhibitor p21, forming a ternary complex that inhibits CDK2 activity and promotes cell cycle arrest. CircFOXO3 also interacts with the E3 ubiquitin ligase MDM2 and the deubiquitinase USP10, promoting MDM2-mediated ubiquitination and degradation of p53, thereby influencing apoptosis.

CircRNAs can also sequester proteins away from their normal sites of action. CircMbl, derived from the MBL gene, binds to MBL protein, reducing the pool of MBL available for splicing regulation. CircPABPN1 binds to HuR (human antigen R), preventing HuR from interacting with PABPN1 mRNA and thereby reducing PABPN1 translation. These protein-binding functions expand the regulatory repertoire of circRNAs beyond miRNA sponging.

Regulation of Transcription and Splicing

Some circRNAs remain in the nucleus and regulate transcription. CircEIF3J and circPAIP2 interact with the U1 small nuclear ribonucleoprotein (snRNP) and RNA polymerase II (Pol II) at their host gene promoters, enhancing transcription of the parental gene. This positive feedback loop suggests that circRNA production can reinforce the expression of the gene from which it is derived.

CircRNAs can also influence splicing by competing with linear splicing for splice sites. When back-splicing is favored, canonical splicing is reduced, leading to lower levels of the linear mRNA and protein. This competition is particularly relevant in genes with multiple alternative splice isoforms, where circRNA production can shift the isoform balance.

Circular RNA in Cancer: Mechanisms of Action

In cancer, circRNAs function as oncogenes or tumor suppressors, depending on their targets and the cellular context. Their stability and tissue-specific expression make them particularly attractive as drivers of malignant phenotypes. The Molecular Basis of Cancer involves dysregulation of cell proliferation, apoptosis, and metastasis, and circRNAs intersect with all of these processes.

Oncogenic circRNAs

Oncogenic circRNAs promote tumor growth, survival, and metastasis. They often act by sponging tumor-suppressive miRNAs, thereby upregulating oncogenic mRNAs. For example, circPVT1, derived from the PVT1 locus on chromosome 8q24, is overexpressed in multiple cancers and sponges miR-125 family members, leading to upregulation of the anti-apoptotic protein MCL1. CircRNA-100782, derived from the MCM5 gene, sponges miR-124 in pancreatic cancer, upregulating IL-6 receptor and activating the JAK-STAT pathway.

Oncogenic circRNAs can also interact with proteins to promote malignancy. CircNRIP1 binds to the protein AKT1 and promotes its phosphorylation, activating the PI3K-AKT signaling pathway in gastric cancer. CircAGO2 binds to the Argonaute 2 (AGO2) protein and enhances its activity, promoting miRNA-mediated gene silencing and tumor progression.

Tumor Suppressive circRNAs

Tumor suppressive circRNAs inhibit cancer progression by sponging oncogenic miRNAs or by interacting with tumor suppressor proteins. CircITCH, derived from the ITCH gene, sponges miR-7, miR-17, and miR-214, leading to upregulation of ITCH, an E3 ubiquitin ligase that promotes degradation of the oncogenic protein c-Myc. CircHIPK3, derived from the HIPK3 gene, sponges multiple miRNAs including miR-124, miR-152, and miR-193a, and its downregulation in several cancers correlates with poor prognosis.

CircFOXO3, mentioned earlier, acts as a tumor suppressor by promoting cell cycle arrest and apoptosis. Its expression is frequently reduced in breast cancer and other malignancies. CircSMARCA5, derived from the SMARCA5 gene, inhibits the proliferation and migration of hepatocellular carcinoma cells by sponging miR-17-3p and miR-181b-5p.

Impact on Signaling Pathways

CircRNAs modulate major signaling pathways implicated in cancer. The Wnt/β-catenin pathway, a central regulator of cell proliferation and differentiation, is influenced by several circRNAs. Circ-β-catenin, derived from the CTNNB1 gene, encodes a novel protein that stabilizes β-catenin by competing with the destruction complex, thereby activating Wnt signaling. CircLGR4, derived from the LGR4 gene, sponges miR-34a and upregulates Wnt pathway components. For a broader discussion of this pathway's role in malignancy, see Wnt Signaling in Cancer.

CircRNAs also modulate the PI3K-AKT, MAPK, and JAK-STAT pathways. CircCUX1 activates the PI3K-AKT pathway by sponging miR-214 in renal cell carcinoma. CircMAPK1, derived from the MAPK1 gene, sponges miR-31 and upregulates the MAPK pathway in lung cancer. These pathway modulations contribute to the hallmarks of cancer, including sustained proliferative signaling, resistance to apoptosis, and activation of invasion and metastasis, as described in Molecular Mechanism of Cancer.

Specific Examples of Circular RNA in Cancer

Concrete examples illustrate the functional diversity of circRNAs in cancer and highlight their potential as therapeutic targets.

ciRS-7 in Colorectal Cancer

ciRS-7 (CDR1as) is among the most studied circRNAs. In colorectal cancer, ciRS-7 is overexpressed and functions as a sponge for miR-7. miR-7 normally suppresses the expression of EGFR and RAF1, both of which activate the MAPK pathway. By sequestering miR-7, ciRS-7 upregulates EGFR and RAF1, promoting cell proliferation and survival. High ciRS-7 expression correlates with advanced tumor stage and poor overall survival in colorectal cancer patients. Knockdown of ciRS-7 in colorectal cancer cell lines reduces proliferation and increases apoptosis, confirming its oncogenic role.

circPVT1 in Multiple Myeloma

circPVT1 is derived from the PVT1 gene, which is located near the MYC oncogene on chromosome 8q24. In multiple myeloma, circPVT1 is overexpressed and promotes cell proliferation by sponging miR-125b, a miRNA that targets the anti-apoptotic protein MCL1. High circPVT1 expression is associated with shorter progression-free survival in multiple myeloma patients. Silencing circPVT1 in myeloma cell lines decreases MCL1 levels and sensitizes cells to the chemotherapeutic agent bortezomib, suggesting that circPVT1 could be a therapeutic target.

circFOXO3 in Breast Cancer

circFOXO3 is derived from the FOXO3 gene and functions as a tumor suppressor in breast cancer. It binds to CDK2 and p21, forming a complex that inhibits CDK2 activity and arrests the cell cycle at the G1 phase. CircFOXO3 also binds to MDM2 and USP10, promoting p53 ubiquitination and degradation, which paradoxically can promote cell survival. The net effect of circFOXO3 in breast cancer is growth suppression, and its expression is frequently reduced in tumor tissues compared to adjacent normal tissue. Overexpression of circFOXO3 in breast cancer cell lines inhibits colony formation and xenograft tumor growth.

Methods to Study Circular RNA

Studying circRNAs requires specialized approaches because their closed structure and lack of poly(A) tails distinguish them from linear RNAs. A combination of computational and experimental methods is essential for accurate detection and functional characterization.

RNA Sequencing and Bioinformatics

RNA sequencing (RNA-seq) is the primary method for identifying circRNAs genome-wide. Standard RNA-seq libraries are prepared from total RNA depleted of ribosomal RNA, or from RNA treated with RNase R to enrich for circRNAs. The key computational challenge is detecting back-splice junctions, which are not present in linear transcripts. Bioinformatics tools such as CIRCexplorer, find_circ, and CIRI align reads to the genome and identify reads that map to two non-contiguous exons in reverse orientation. These "back-splice reads" are the hallmark of circRNAs.

Important quality control steps include filtering out reads that map to multiple locations and requiring a minimum number of junction reads (typically at least 2) to call a circRNA. False positives can arise from template switching during reverse transcription or from genomic rearrangements, so validation by independent methods is essential.

Validation with RT-qPCR

Reverse transcription quantitative PCR (RT-qPCR) is the standard method for validating circRNA expression. The critical step is designing primers that span the back-splice junction. A forward primer is placed in the downstream exon and a reverse primer in the upstream exon, such that the PCR product spans the junction. This design ensures that only circular RNA is amplified, not linear RNA from the same gene.

To confirm circularity, RNA samples are treated with RNase R (typically 3–4 units per microgram of RNA, incubated at 37°C for 15–30 minutes) before reverse transcription. RNase R degrades linear RNA but not circRNA, so a circRNA should show little or no decrease in amplification after treatment, whereas a linear mRNA should be substantially reduced. Divergent primers (oriented outward from the junction) are used to amplify circRNAs, while convergent primers (oriented inward) amplify both linear and circular forms.

Functional Assays: Knockdown and Overexpression

Functional characterization of circRNAs requires manipulating their expression. Short hairpin RNA (shRNA) or small interfering RNA (siRNA) can be designed to target the back-splice junction, specifically degrading circRNA without affecting the linear transcript. However, this approach can be inefficient because the junction sequence is short and may not be accessible to RNA interference machinery. An alternative is to use CRISPR-Cas9 to delete the circularization signals (e.g., Alu elements) in the flanking introns, which reduces circRNA production without disrupting the linear mRNA.

Overexpression is achieved by cloning the circRNA sequence into an expression vector that contains complementary intronic sequences flanking the circRNA exons. Transfection of this vector into cells promotes back-splicing and circRNA production. Alternatively, in vitro transcribed circRNAs can be synthesized using a self-splicing intron system and transfected directly into cells. Functional readouts include cell proliferation assays (e.g., MTT or EdU incorporation), apoptosis assays (e.g., Annexin V staining), and migration/invasion assays (e.g., Transwell chambers).

Circular RNA as Biomarkers and Therapeutic Targets

The stability, abundance, and tissue-specific expression of circRNAs make them promising biomarkers and therapeutic targets in cancer. However, significant challenges remain before circRNAs can be used in clinical practice.

Biomarker Potential

CircRNAs are highly stable in body fluids, including blood plasma, serum, and urine, because their closed structure resists exonucleases. This stability is a major advantage over linear mRNAs, which are rapidly degraded. CircRNAs are also present in exosomes, which protect them from degradation and allow detection in liquid biopsies. Several studies have reported that specific circRNAs are differentially expressed in the plasma of cancer patients compared to healthy controls. For example, circPVT1 is elevated in the plasma of multiple myeloma patients, and ciRS-7 is elevated in colorectal cancer patients.

The specificity of circRNA expression is another advantage. Many circRNAs are expressed in a cell-type- or tissue-specific manner, which could improve diagnostic accuracy. However, the sensitivity and specificity of circRNA-based biomarkers need to be validated in large, prospective cohorts before clinical adoption. The Molecular Cancer Diagnosis field is actively exploring circRNA panels that combine multiple circRNAs to improve diagnostic performance.

Therapeutic Strategies

Several therapeutic strategies targeting circRNAs are under investigation. Antisense oligonucleotides (ASOs) complementary to the back-splice junction can promote RNase H-mediated degradation of circRNA. Alternatively, siRNAs targeting the junction can be delivered using lipid nanoparticles. For circRNAs that function as miRNA sponges, a therapeutic approach could involve delivering synthetic miRNAs to outcompete the circRNA, or using small molecules that disrupt circRNA-miRNA interactions.

CircRNA overexpression is more challenging but can be achieved using expression vectors or synthetic circRNAs. For tumor suppressive circRNAs, restoring their expression could inhibit cancer growth. However, delivery remains a major hurdle, particularly for solid tumors. The Biology of Cancer provides a framework for understanding how circRNA-targeted therapies might integrate with existing treatment modalities.

Challenges in Clinical Translation

Several challenges impede clinical translation. First, the functional significance of most circRNAs is unknown; only a small fraction have been characterized experimentally. Second, circRNA expression is often correlated with but not proven to cause cancer phenotypes, and confounding factors such as genetic background and tumor heterogeneity complicate interpretation. Third, the delivery of circRNA-targeting agents to tumors is inefficient, and off-target effects are a concern. Fourth, the long-term safety of circRNA-based therapies has not been established. Finally, the regulatory landscape for RNA-based therapeutics is still evolving, and the cost of development is high.

Common Pitfalls in Circular RNA Research

Research on circRNAs is fraught with technical challenges that can lead to erroneous conclusions. Awareness of these pitfalls is essential for both students and experienced researchers.

Distinguishing circRNA from Linear RNA

The most common mistake is failing to distinguish circRNA from linear RNA. Because circRNAs are derived from the same genes as linear mRNAs, standard primers that amplify the exon body will detect both forms. To specifically detect circRNA, primers must span the back-splice junction. Additionally, RNase R treatment should be used to confirm circularity, but this treatment is not always complete; residual linear RNA can produce false positives. It is also important to note that some linear RNAs are resistant to RNase R due to secondary structure, so a negative control (a known linear mRNA) should be included.

Bioinformatics Pitfalls

Bioinformatics analysis of circRNA-seq data is prone to false positives. Template switching during reverse transcription can create chimeric reads that mimic back-splice junctions. Genomic duplications or pseudogenes can also produce reads that map to non-contiguous regions. To mitigate these issues, researchers should use multiple detection algorithms and require concordance between them. Additionally, the use of RNase R-treated libraries reduces but does not eliminate false positives. It is also important to consider the read length and sequencing depth; short reads (50 bp or less) may not span the junction reliably.

Experimental Controls

Proper controls are essential for circRNA experiments. For RT-qPCR, a no-reverse-transcriptase control should be included to rule out genomic DNA contamination. For RNase R treatment, a known linear RNA (e.g., GAPDH mRNA) and a known circRNA (e.g., ciRS-7) should be used as positive and negative controls, respectively. For knockdown experiments, a scrambled siRNA or shRNA control is necessary to rule out off-target effects. For overexpression experiments, an empty vector control is essential. Finally, the specificity of the back-splice junction should be confirmed by Sanger sequencing of the PCR product.

Summary and Future Directions

Circular RNAs are a diverse class of non-coding RNAs with critical roles in cancer biology. Their biogenesis is regulated by cis-elements and trans-factors, and their functions include miRNA sponging, protein scaffolding, and transcriptional regulation. In cancer, circRNAs act as oncogenes or tumor suppressors by modulating key signaling pathways. Their stability and tissue-specific expression make them promising biomarkers and therapeutic targets, but significant challenges remain.

Key Takeaways

Circular RNAs are covalently closed RNA molecules generated by back-splicing, making them resistant to exonucleolytic degradation.
CircRNAs function as miRNA sponges, protein scaffolds, and regulators of transcription and splicing.
In cancer, circRNAs can act as oncogenes (e.g., circPVT1, ciRS-7) or tumor suppressors (e.g., circFOXO3, circITCH).
CircRNAs are detected using RNA-seq with back-splice junction identification and validated by RT-qPCR with divergent primers and RNase R treatment.
CircRNAs are stable in body fluids and have potential as diagnostic and prognostic biomarkers.
Therapeutic strategies targeting circRNAs include ASOs, siRNAs, and small molecules, but delivery and specificity remain challenges.
Common pitfalls in circRNA research include failure to distinguish circRNA from linear RNA, bioinformatics false positives, and inadequate experimental controls.

Open Questions and Future Research

Several questions remain unanswered. What fraction of circRNAs have bona fide functions, and how many are merely splicing byproducts? How is circRNA biogenesis regulated in different cellular contexts, and can this be exploited therapeutically? What is the stoichiometry of circRNA-miRNA interactions in vivo, and does sponging occur at physiologically relevant concentrations? How do circRNAs interact with the Genetic Basis of Cancer to influence tumor evolution? Future research should focus on functional characterization of circRNAs in animal models, development of reliable delivery systems, and validation of circRNA biomarkers in large clinical cohorts. The integration of circRNA research with Cancer Cell Diagram and other cancer biology resources will accelerate translation from bench to bedside.

Frequently Asked Questions

What is circular RNA?

Circular RNA (circRNA) is a type of single-stranded RNA that forms a covalently closed loop. It is produced by back-splicing, a non-canonical splicing event that joins a downstream 5′ splice site to an upstream 3′ splice site. CircRNAs lack free ends, making them resistant to exonucleases and more stable than linear RNA.

How does circular RNA contribute to cancer?

CircRNAs contribute to cancer by acting as oncogenes or tumor suppressors. Oncogenic circRNAs promote cell proliferation, survival, and metastasis by sponging tumor-suppressive miRNAs or interacting with oncogenic proteins. Tumor suppressive circRNAs inhibit cancer progression by sponging oncogenic miRNAs or promoting the activity of tumor suppressor proteins. They modulate key signaling pathways such as Wnt, PI3K-AKT, and MAPK.

What are the main functions of circular RNA?

The main functions of circRNAs include miRNA sponging (sequestering miRNAs to prevent them from repressing target mRNAs), protein scaffolding (bringing proteins into proximity to modulate their activity), and regulation of transcription and splicing. Some circRNAs can also be translated into short peptides.

How is circular RNA detected?

Circular RNA is detected using RNA sequencing followed by bioinformatics analysis to identify back-splice junctions. Validation is performed by RT-qPCR using divergent primers that span the back-splice junction, combined with RNase R treatment to confirm circularity. Northern blotting and in situ hybridization are also used.

Can circular RNA be used as a cancer biomarker?

Yes, circRNAs are promising cancer biomarkers because they are stable in body fluids such as blood plasma and urine, and their expression is often tissue-specific and altered in cancer. Several circRNAs have been reported as diagnostic or prognostic markers, but large-scale clinical validation is still needed.

What is the difference between circular RNA and linear RNA?

Linear RNA has free 5′ and 3′ ends, a 5′ cap, and a 3′ poly(A) tail, and is susceptible to exonucleolytic degradation. Circular RNA has no free ends, no cap or poly(A) tail, and is resistant to exonucleases. CircRNAs are generated by back-splicing, while linear RNAs are generated by canonical splicing.

What are examples of circular RNA in cancer?

Examples include ciRS-7 (sponges miR-7 in colorectal cancer), circPVT1 (sponges miR-125b in multiple myeloma), circFOXO3 (inhibits CDK2 in breast cancer), circITCH (sponges oncogenic miRNAs in various cancers), and circHIPK3 (sponges multiple miRNAs in several cancer types).