Circular messenger RNA (circRNA) is a single-stranded RNA that forms a covalently closed, continuous loop where its ends are joined together. This structure is different from the more familiar linear RNA. This formation gives circRNA distinct properties, making it highly stable and a promising tool for medical applications. These molecules are naturally occurring and were first discovered in humans in 1991, but their role in gene regulation has only recently been understood.
How Circular mRNA is Different
Linear mRNA has a 5′ cap and a 3′ poly-A tail, which are exposed ends that act as starting points for degradation. These ends make linear molecules susceptible to exonucleases, which are enzymes that break down RNA from the outside in. This process limits the lifespan of linear mRNA and the amount of protein produced from a single molecule.
The closed-loop structure of circRNA eliminates these vulnerable endpoints, making it resistant to degradation by exonucleases. This resistance gives it high stability, allowing it to persist in the cellular environment much longer than its linear counterpart.
A more stable molecule can serve as a template for protein production over a prolonged period. This sustained expression can lead to a much higher yield of the desired protein from a single circRNA molecule compared to a linear one. This durability and prolonged protein expression are the attributes that make engineered circRNA a promising tool in biotechnology and medicine.
Biological Roles of circRNA
Beyond their engineered potential, circular RNAs are abundant in our cells and perform various natural functions. They are involved in regulating gene expression at both the transcriptional and post-transcriptional levels. These molecules are not byproducts of gene transcription but are active in complex biological processes.
A primary role of circRNA is to act as a “microRNA sponge.” MicroRNAs (miRNAs) are small molecules that bind to messenger RNAs, silencing them and preventing protein translation. Some circRNAs contain multiple binding sites for specific miRNAs, sequestering them and preventing them from inhibiting their target genes. This action influences which proteins get made.
CircRNAs can also interact with RNA-binding proteins (RBPs), acting as scaffolds to bring proteins together or as decoys to prevent RBPs from performing other functions. In some cases, circRNAs can even be translated into proteins themselves. This function was not initially thought possible due to their lack of a traditional 5′ cap for initiating translation.
Applications in Medicine
The properties of circular RNA are being explored for therapeutic applications, particularly in vaccine development. Its high stability could make circRNA vaccines more durable, requiring lower doses or fewer boosters than current linear mRNA vaccines. This stability might also reduce the need for stringent cold-chain storage, simplifying distribution.
In oncology, circRNA offers a new approach to cancer therapy. These molecules can be engineered to produce anti-tumor proteins inside cancer cells. Their ability to generate a high and sustained level of protein expression could be effective at inducing cancer cell death or inhibiting tumor growth. The specificity of circRNA expression in different tissues also means they could be used to target cancer cells while sparing healthy ones.
Circular RNAs are also being developed as disease biomarkers. The levels of specific circRNAs in bodily fluids like blood can change with certain diseases, including cancers and neurological disorders. Detecting these changes could lead to new diagnostic tools for early disease detection and for monitoring treatment response.
From Research to Reality
Despite the promise of circular RNA, practical challenges must be addressed before it can become a mainstream therapeutic. Overcoming hurdles in production and application is the focus of research and development within the biotechnology industry.
One challenge is manufacturing. Producing large quantities of pure and effective circRNA on a commercial scale is a complex biochemical process. Scientists are working to optimize methods for synthesizing and purifying these molecules to ensure they are safe and potent. This involves refining the enzymatic processes that create the circular structure and developing scalable purification techniques.
Another challenge is the effective delivery of circRNA to the correct cells. The molecule must be packaged to protect it during its journey through the bloodstream and allow it to be taken up by the target tissue. Researchers are developing delivery vehicles, such as lipid nanoparticles, to encapsulate the circRNA and ensure it reaches its destination.