RNA interference (RNAi), what is it? First, let’s get on the same page about RNA itself and its role in a cell. Information on how to build and maintain an organism is stored in its DNA. Many thousands of genes, functional segments of DNA, work together to provide the molecular instructions necessary to produce proteins at the proper time, place and amount. Some of these proteins are simply building blocks and others do work such as enzymes that produce other molecules essential to the cell, helping to obtain energy from food, etc. Like a cookbook in a library, DNA has the information for thousands of “recipes” on how to make each protein in the cell. But that information has to get out into the cell to be used.
This is where RNA comes into the picture. RNA is a molecule that is similar to DNA in structure but plays a different role, carrying the information stored in the DNA to the cellular machinery that makes the proteins. An RNA “copy” of the DNA information (the “recipe card”) is essential to producing a particular protein. And this is possible because each RNA contains a “carbon copy” of the DNA information it came from, providing the instructions to produce the specified protein. So in a cell at any moment, there are thousands of different RNA molecules being copied from the DNA and instructing the cell on how to make proteins.
Cells following all those “recipes” then produce proteins at the right time, place and amount. However, that is a lot of information for the cell to manage. The cell uses a variety of processes to monitor and control the production of proteins. This includes molecular controls to start and stop production of a given protein. To think about our recipe analogy, a chef produces plates for her guest, but at some point that course is complete, all the guests are satisfied and continuing to cook more is wasteful and may even upset the guests if they are ready for dessert instead. Many organisms (aside from bacteria) have a process to help ensure information present in the RNA copies is carefully managed and removed when not needed. This process acts like a dimmer switch to turn genes down or off to match the needs of the cell by preventing RNA from delivering its protein coding information. This process is RNA interference (RNAi).
Although work began in plants like petunias, details of RNAi were discovered by doctors Fire and Mello in the late 1990s and they received the Nobel Prize in Physiology or Medicine 2006 for their groundbreaking work on RNAi. Their discovery showed that some RNAs are produced in the cell to control other RNA molecules to turn down RNA activity. RNAi was also found to be broadly utilized by plants, animals and fungi. The process isn’t random, it is carefully programmed by the cell through the specifics of genetic sequence. Fire and Mello found the key is production of a special form of RNA called double stranded RNA (dsRNA). It has an even greater resemblance to DNA, and when the cell recognizes that special form of RNA, it directs its enzymes to “dice” the longer dsRNA into smaller pieces. Specialized proteins involved in the RNAi process pick up the small “pieces” and use the genetic sequence in them to scan through the huge number of RNAs in the cell. The RNAi machinery is using that information like a bar code to scan and look for a match in the thousands of available RNAs. From our analogy it is like looking through recipe cards copied from cookbooks and trying to find the one card that says “blueberry muffin” (and not blueberry cobbler, blueberry pie, bran muffin, etc.). The match needs to target the correct RNA (recipe). If there is a sequence match, the RNAi machinery slices the targeted RNA (or stops it from transferring its information to protein production). The corresponding protein is not made as a result.
Organisms have adapted RNAi to regulate their own genes and also use it against cellular invaders like viruses. Many viruses have dsRNA portions of their life cycle and the cell can recognize this special form of RNA and send the RNAi to target it and slice it up, thereby defending the cell. The RNAi machinery is ready and waiting for the dsRNA input to trigger the process, it just needs the input to guide it. This input could come from the cell producing dsRNA that matches one of its own genes, an invading virus, or through biotechnology.
In the early 1980s, scientists developed the capacity to transfer a gene into plants, giving them the ability to add beneficial qualities to crop plants that could not otherwise be obtained through conventional breeding, such as expressing Bt proteins from bacteria used in organic farming in the corn plant to help the plant control caterpillars. Similarly there was opportunity to make plants resistant to viruses when no natural plant immunity existed. With the RNAi process already present in plants, the virus could be targeted by programming the RNAi machinery with a gene designed to produce dsRNA matching the genetic sequence of that particular virus, thereby triggering the cell to recognize and destroy the virus, saving the plant.
An early and highly successful example of biotechnology as an approach to RNAi is found in papaya resistant to the devastating papaya ringspot virus. The virus resistant Rainbow papaya was developed locally in Hawaii, and has been responsible for saving the industry since its commercial introduction 17 years ago. More than 250 million pounds of papaya fruit have been harvested over the years as a result of this viral resistance. Other viruses have been targeted by RNAi in plants such as squash (developed by Asgrow and now a Monsanto product), plums (developed by USDA) and pinto beans (developed by EMBRAPA, Brazil).
Still other applications have used RNAi to target specific genes within the crop itself and modify the qualities of the harvested products. Soy oil, for instance, has been modified to a healthier profile with reduced saturated fats (Plenish®, developed by DuPont/Pioneer). RNAi has also been used to reduce food waste in potato (Innate™ developed by Simplot) and apple (Arctic® apple developed by Okanagan Specialty Fruit, Canada). A new application of RNAi is to control an insect pest corn rootworm (developed by Monsanto and currently under regulatory review, e.g. by USDA).
RNA is not new in the human diet. All whole plant foods, animal and fungal food sources contain RNA in many forms, including dsRNAs, some with sequences known to have matches to human genes. Yet, these foods are recognized as safe and nutritious. Some traits in conventional and organic foods are the result of RNAi. Soybeans, for example, owe their yellow seed color to early plant breeders selecting mutations that eliminated the dark seed color of earlier varieties – a characteristic due to RNAi in the seed turning off a seed pigment gene. The less processed the food source, the more likely it is to contain RNA, since it is broken down by cooking and food processing. Furthermore, RNA is broken down by acids, enzymes and microbes present in the digestive tract.
Although RNA in its many forms has been present in food and the environment for millennia, some have raised concerns about the safety of this natural molecule. RNAi-based technologies utilize a molecule that has many substantiated attributes. First, RNA is not toxic or allergenic. It does not accumulate in the environment since ordinary microbes already in the environment degrade and consume it. RNA’s present due to biotechnology occur in small amounts. For instance, only about 0.0000001% of the corn roots engineered to resist the corn rootworm pest is dsRNA (and even less is found in other parts of the plant like pollen and grain). And probably most importantly, multiple barriers exist in humans and higher animals such as enzymes in bodily fluids (e.g intestinal secretions and blood) that destroy dietary RNAs. Despite diligent efforts by pharmaceutical companies to generate life-saving drugs using this technology, reproducible and significant entry of RNA-based drugs or plant-derived RNAs into humans through oral intake has not been found, further emphasizing the significance of natural barriers. Many of these points are well articulated by Dr. Steve Savage, as he discussed the Arctic® apple and similarly by regulatory agencies reviewing the use of RNAi-based biotechnology, such as the Food Safety Authority Australia/New Zealand.
Overall RNAi-based technology represents one more tool in the toolbox of solutions needed to provide beneficial foods that make the most of limited resources. Due to the history of safe consumption of RNA and extensive biological barriers, this technology provides an important tool without presenting any unique food safety hazards.
Modern soybean’s yellow color is due to a natural, non-GMO occurrence of RNAi that removes the dark pigment found in older/wild varieties.