Repairing Sun-Damaged DNA with Light

PBS.org Inside NOVA

Why don’t trees ever get sunburned? And could we harness their secret to protect our own skin? Researchers are studying how proteins called photolyases, which have been lost to humans through evolutionary time, provide most other organisms with extraordinary protection from damage caused by the sun’s ultraviolet (UV) rays. The proteins do it by channeling the power of visible light.

Fortunately, our cells do have repair mechanisms that find dimers, snip them out, and replace them with new DNA that fits properly into the ladder before the dimer can be replicated. But our proofreading process is not perfect, and sometimes it lets a dimer or two slip through.Normally, the molecules that make up DNA look like a twisted ladder and form the classic double helix structure. But UV light, which has a shorter wavelength than visible light and cannot be seen, sometimes causes two of the rungs of nucleotides to fuse to each other instead of reaching across the ladder. This bulging formation, called a dimer, can be passed on when DNA is replicated and lead to mutations–some of which can turn into deadly forms of skin cancer.

Humans and other mammals are especially unlucky when it comes to UV damage. Cells in plants, most animals, bacteria, and even yeast have protective proteins called photolyases that rigorously scan DNA for signs of UV damage and make quick repairs. Sadly for us, photolyases were lost relatively recently in evolutionary history. And until now, scientists could not see the mechanism by which these important proteins repaired flawed DNA.

Researchers at Ohio State University were recently able to observe exactly how photolyases perform their protective duty. The photolyase protein captures energy from visible light and uses it to project a single proton and a single electron towards a dimer in DNA. The two tiny particles then initiate a series of reactions that knock the contorted nucleotides back into place across the ladder, without needing to remove them like normal human proteins do. A proton and electron finally return to the photolyase protein, presumably so it can dash off to fix the next dimer it finds.

The good news is that even though humans and other mammals have lost the photolyase protein, we may still be able to harness it to protect our own DNA. Given that photolyases were lost in evolution, it was possible that other proteins in the cell that allowed photolyases to do their job were also lost. But mice that were given the gene for the photolyase protein showed remarkable protection from UV damage. This means that in mice, the rest of the cellular infrastructure that photolyases need is still there. Chances are good that it’s there for humans as well.

Current sunscreens work to deflect UV light and prevent it from penetrating the skin’s surface, but fortifying sunscreen with photolyase could help repair damage from the rays that manage to make it through. So does this mean we should all run out and frolic in the sun? Not exactly–at least not yet. But the promise of protective photolyases does make summer sun seem a little less menacing and a little more fun.

Hannah Krakauer is a research intern at NOVA and a student at Stanford University.

Addendum: We’ve gotten a few comments and questions on Facebook regarding sun damage to plants. It is true that plants sometimes suffer from sun damage, though it is not related to human sunburn.

Our sunburns are the result of DNA damage triggering a series of reactions in the cell that cause redness and itchiness. In plants, however, sun damage takes two common forms. In winter, sunscald can happen when temperatures rapidly warm up for long enough to stimulate cell growth in the bark of a tree, but then drop quickly and kill the new cells. The dead cells fester and form cracks and soft spots in the tree. Another form of sun damage is most often found in houseplants that have been moved outside. Since these plants are not adapted to strong sunlight, they can get white spots where the chlorophyll has been destroyed. Though still caused by sunlight, neither of these types of damage are related to DNA damage as in human sunburn.

Thanks so much for your comments!

Image Courtesy NASA/David Herring