Circular RNAs

Circular RNAs (circRNAs) are a unique class of endogenous RNA molecules characterized by a covalently closed continuous loop structure without 5' caps or 3' polyadenylated tails. Unlike linear RNAs, circRNAs form through a back-splicing process where a downstream splice donor site is joined to an upstream splice acceptor site. This circular configuration confers them with high stability and resistance to exonuclease-mediated degradation, distinguishing them from linear RNA counterparts.

Biogenesis of Circular RNAs

The biogenesis of circular RNAs primarily occurs through back-splicing, an alternative splicing event that connects a splice donor site to an upstream splice acceptor site in pre-mRNA. This process can generate several types of circRNAs depending on the sequences involved:

• Exonic circRNAs (ecircRNAs): Composed entirely of exonic sequences, typically derived from one or multiple exons of protein-coding genes. These are the most abundant and well-studied circRNAs.
• Circular intronic RNAs (ciRNAs): Formed predominantly from intronic sequences that escape debranching during splicing.
• Exon-intron circRNAs (EIciRNAs): Contain both exonic and intronic sequences, often localized in the nucleus and involved in transcriptional regulation.

Several mechanisms promote circRNA formation, including:

• Intron pairing-driven circularization: Complementary sequence motifs within flanking introns (such as Alu repeats) base-pair, bringing splice sites into proximity and facilitating back-splicing.
• RNA-binding protein (RBP)-mediated circularization: RBPs such as Quaking (QKI) and Muscleblind (MBL) bind to flanking intronic regions, dimerize, and promote circularization.
• Lariat-driven circularization: During exon skipping, the spliced-out lariat structure can be processed to yield circRNAs.

The efficiency of circRNA production depends on competition between canonical linear splicing and back-splicing, regulated by cis-elements and trans-acting factors.

Localization and Turnover of Circular RNAs

CircRNAs exhibit distinct subcellular localizations depending on their type and function:

• Cytoplasmic circRNAs: Most ecircRNAs are exported to the cytoplasm, where they can interact with microRNAs (miRNAs), RBPs, or even engage in translation.
• Nuclear circRNAs: ciRNAs and EIciRNAs tend to accumulate in the nucleus and can influence gene expression by interacting with transcription machinery or modulating splicing.

The turnover of circRNAs is slower than linear RNAs due to their circular form, which lacks free ends susceptible to exonucleases. However, circRNAs can be degraded through specific pathways, including:

• Endonucleolytic cleavage: Enzymes like RNase L can degrade circRNAs upon viral infection or stress.
• MicroRNA-mediated decay: Some circRNAs can be targeted by miRNA-directed cleavage.
• Exosome-mediated secretion: CircRNAs can be packaged into extracellular vesicles (exosomes) for removal or intercellular communication.

Their relative stability allows circRNAs to accumulate and potentially exert prolonged biological effects.

Functions of Circular RNAs

Circular RNAs serve diverse and important biological functions, many of which are still under active investigation. Key roles include:

1. MicroRNA Sponges

Many circRNAs contain multiple binding sites for specific microRNAs, thereby sequestering these miRNAs and preventing them from repressing their target mRNAs. This "sponge" activity modulates gene expression post-transcriptionally. A classic example is CDR1as (ciRS-7), which harbors over 70 binding sites for miR-7, influencing neural development and disease.

2. Interaction with RNA-Binding Proteins

CircRNAs can bind and modulate the activity or localization of RBPs. By serving as protein decoys or scaffolds, circRNAs influence processes such as splicing, translation, and signal transduction.

3. Regulation of Transcription and Splicing

Nuclear circRNAs, especially EIciRNAs and ciRNAs, can regulate transcription of their parental genes by interacting with RNA polymerase II or splicing factors, impacting gene expression at the transcriptional level.

4. Translation into Proteins or Peptides

Although lacking typical mRNA features, some circRNAs contain internal ribosome entry sites (IRES) or N6-methyladenosine (m6A) modifications that enable cap-independent translation, producing functional peptides with roles in cell proliferation, tumorigenesis, or stress responses.

5. Potential Roles in Disease and Biomarker Development

CircRNAs are implicated in various diseases, including cancer, neurological disorders, and cardiovascular diseases. Their stability, abundance, and tissue-specific expression make them promising biomarkers and therapeutic targets.

Summary Table of Circular RNA Types and Features

The study of circular RNAs has expanded our understanding of RNA biology, revealing a versatile layer of gene regulation and cellular function beyond linear RNA paradigms. Their unique structure and diverse roles position circRNAs as crucial components in molecular biology and potential clinical applications.