Because RNA is such an important part of how the cell builds proteins, some scientists speculated that this was its only job. In 1993, however, Victor Ambros, Rosalind Lee, and Rhonda Feinbaum found another job for RNA. Short strands of RNA, which are now called microRNAs, sometimes regulate how much of a particular protein is made in the cell.1 Since then, other forms of RNA have also been shown to regulate the amount of protein produced in a cell. In addition, scientists have found that some types of RNA perform functions that aren’t even directly related to the production of proteins. For example, some types of RNA serve as “molecular guides,” taking proteins where they need to be in the cell, while other types of RNA serve as a “molecular adhesives,” holding certain proteins to other RNA molecules or to DNA.
Now even though the last two jobs I mentioned are not directly related to protein production, they still involve proteins. So is it safe to say that while RNA performs several functions in the cell, all of them are related to proteins in some way? I might have answered, “Yes” to such a question if a student had asked me that just a few weeks ago. However, a new paper in Nature Medicine has found a function for some microRNAs that has nothing to do with proteins. Some microRNAs serve as radiation detectors.2
Most people have experienced a sunburn. The sun emits ultraviolet radiation, and some of that radiation is energetic enough to damage skin cells. If you expose your skin to too much of this ultraviolet radiation, too many cells become damaged, and your body produces an immune response designed to clear out all the damaged cells. The immune response results in inflammation, which we experience as painful red patches on the skin. While a sunburn is not any fun, it’s actually a good response to a bad situation. Too many radiation-damaged cells can lead to cancer. The immune response helps to get rid of those cells before that can happen.
But what causes your body to initiate this immune response? Jamie J. Bernard and colleagues have come up with a partial answer to that question. Using both human cells and mouse cells, they show that ultraviolet radiation from the sun damages certain microRNAs that are inside the cell. Once the microRNAs are damaged, the cell ejects them. Healthy cells that are nearby detect these microRNAs, and when their concentration gets high enough, it causes those healthy cells to start the immune response that leads to a sunburn.
So now that we know what starts the immune response that leads to a sunburn, can we get rid of the problem? Can we somehow block the microRNAs from being made so that the cells don’t have a radiation detection system to begin with? Technologically, that’s quite feasible. However, you wouldn’t want to do that, because of what I mentioned above. Too many damaged cells can lead to cancer, so they need to be disposed of quickly. Thus, while we probably could get rid of sunburns this way, we wouldn’t want to – at least not in most cases.
The lead author does suggest one possible medical use for this discovery. One way psoriasis is treated is to expose the skin to ultraviolet light specifically to cause the immune response that leads to a sunburn. However, exposure to ultraviolet light is dangerous, because of the cancer risk I mentioned above. To avoid that risk, doctors could inject damaged microRNAs into the skin of someone with psoriasis. It might produce the desired immune response without causing any radiation damage to the skin cells. Of course, whether or not that would work in practice will have to be determined by clinical trials.
God’s creation is truly amazing. Just when we think we have something like RNA all figured out, we learn that it is much more versatile than we ever imagined!
1. Lee RC, Feinbaum RL, and Ambros V, “The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14,” Cell 75(5):843-854, 1993.
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2. Jamie J Bernard, et. al., “Ultraviolet radiation damages self noncoding RNA and is detected by TLR3,” Nature Medicine 18:1286–1290, 2012.
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