An intersectional expression platform for gene complementation using RNA-fragment end joining (REJ)
This paper presents an engineered RNA-fragment end joining (REJ) platform that utilizes structured modular RNA segments to efficiently splice separate RNA units into functional mRNAs, enabling robust gene complementation, intersectional cell labeling, and the expression of large proteins without the need for extensive screening.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Inside the cells of every living thing, a complex assembly line works constantly to build the proteins that keep life running. This process begins with a set of instructions written in DNA, which are copied into a temporary message called RNA. Before this message can be used to build a protein, it must be edited. Cells have a natural machinery that acts like a pair of scissors and a glue gun, cutting out unnecessary sections and stitching the remaining pieces together into a single, continuous strand. This editing step is so reliable that scientists have long relied on it to understand how genes work. However, this machinery usually only works on a single, continuous strand of RNA. It generally refuses to stitch together two separate, floating pieces of RNA that happen to float near each other, a safeguard that prevents the cell from accidentally creating scrambled, nonsensical instructions.
For researchers, this natural refusal to join separate strands presents a significant hurdle. Many genes are simply too large to fit into the tiny delivery vehicles, known as viral vectors, that scientists use to introduce new genetic material into the body. When a gene is too big, it must be split into smaller pieces and delivered separately, but the cell's natural machinery often fails to reassemble them correctly. This limitation has forced scientists to develop complex workarounds involving foreign proteins or chemical reactions that can sometimes trigger unwanted immune responses or leave behind messy genetic "scars." The question has been whether it is possible to trick the cell's own natural editing tools into joining separate RNA pieces together efficiently and cleanly, without needing to introduce any foreign agents.
A team of researchers at the Salk Institute has now developed a system that does exactly this. They engineered a method they call RNA end-joining, or REJ, which allows the cell to stitch together two separate RNA strands into a single, functional message. The key to their success was the design of a small, synthetic module of about 250 letters of genetic code. These modules act as a bridge. When attached to the ends of two separate RNA strands, they fold into specific shapes that grab onto each other, pulling the two strands close together. Once the strands are held in place, the cell's own natural splicing machinery recognizes the connection and performs the stitching, joining the two pieces into one perfect, continuous RNA molecule.
The researchers tested this system by splitting a gene that produces a glowing yellow protein into two separate, non-functional halves. When they delivered these two halves into cells along with the special bridging modules, the cells successfully reassembled the instructions and began glowing brightly. The team found that this method worked with remarkable precision. The resulting protein was identical to the one produced by a single, unsplit gene, with no extra or missing parts left over from the joining process. This "scar-free" result is a major advantage over other methods that often leave behind small, potentially disruptive remnants of the joining mechanism. Furthermore, because the system relies entirely on the cell's own internal tools, it does not require the introduction of foreign proteins that could alert the immune system.
To ensure the system was safe and reliable, the scientists subjected it to rigorous testing. They checked whether the bridging modules might accidentally grab onto the wrong RNA strands inside the cell, potentially causing the cell to produce harmful, scrambled proteins. Even when they forced the system to work under extreme conditions with high amounts of RNA, the bridging modules remained highly selective, almost exclusively joining the correct partners. The researchers also looked for signs that the system might trigger an alarm in the cell's immune defenses. They found no evidence of such a reaction; the cells grew and functioned normally, and their internal genetic profiles remained unchanged compared to cells that had not received the treatment.
The versatility of this new platform is perhaps its most striking feature. The researchers demonstrated that they could use it to reassemble a wide variety of large genes, including those that are critical for vision and muscle function, which are often too big for standard delivery methods. They successfully rebuilt a massive protein involved in a form of blindness called Stargardt disease, producing levels of the full protein that were comparable to those achieved by other advanced methods, but with far fewer unwanted fragments. The system also proved capable of joining three separate RNA pieces in the correct order, opening the door to even more complex genetic engineering.
Beyond simply fixing broken genes, this technology offers a powerful new way to control exactly where and when a gene is turned on. Because the two halves of a gene will only reassemble if they are both present in the same cell, scientists can use this system to create genetic "logic circuits." For example, a gene could be designed to function only if two different biological signals are present at the same time, allowing researchers to target very specific types of cells within a complex tissue. The team also created a digital tool to help other scientists design these custom genetic circuits, making the technology accessible for a wide range of future studies.
The work represents a significant step forward in the ability to manipulate genetic information with precision and safety. By harnessing the cell's own natural machinery and guiding it with simple, synthetic RNA bridges, the researchers have created a method that is both efficient and broadly applicable. It offers a way to deliver large, therapeutic genes that were previously impossible to transport, and provides a new toolkit for understanding how genes interact within the intricate landscape of living cells. The findings suggest that this approach could become a standard method for gene therapy and biological research, offering a clean, reliable, and safe way to repair or replace genetic instructions in the human body.
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