Some of the damage caused by sunlight -- the DNA damage that can lead to sun burns and even skin cancer -- may occur after sun exposure ends, according to a new study published Thursday in Science. And the damage may rely on the presence of melanin, the skin pigment often thought to protect us from sun exposure.
Common sense dictates that lighter skin is more susceptible to damage. But research was murky, with some studies indicating that melanin -- the pigment that gives skin and hair their color -- might actually be connected to some negative effects.
"Sure, blondes and redheads are more sensitive to sunburns and skin cancer,"said study author Douglas E. Brash, a professor of therapeutic radiology and dermatology at Yale. "But then you look at countries like Egypt, where many people have fairly light skin tones, and there isn't the high skin cancer incidence you'd expect. So I was never sure I bought that the lightness of the skin was the issue."
Brash and his colleagues wanted to understand how melanin might be involved. Previous studies showed that the kinds of melanin ratios that produce lighter hair colors might make us more susceptible to skin cancers. But when inducing UV damage in melanocytes -- the cells that make melanin -- they found something surprising.
After the UV light was off, the damage kept happening.
"The sun damage we understood before had a photon getting through your skin, if it didn't get absorbed by melanin first," Brash explained. "And it happens quickly, and then it's done. But we saw a second process that took hours."
What's more: Melanin, hailed as the protector of our skin, seemed to be to blame.
While melanin prevented the short term damage of UV exposure, the second wave of damage only occurred in cells with melanin in them.
"It protects us from some of the damage," Brash said, "but it also causes some of it. It was an interesting finding, but it felt kind of heretical."
The reaction that produces this after-hours sun damage is a lot like the process that creates light in fireflies, Brash said. It's a kind of chemistry never seen in humans before, and it's pretty cool -- even if it's damaging our DNA.
When the UV light is shined on a cell, Brash said, it produces two enzymes. When combined, these enzymes create a powerful oxide -- and a lot of energy. In humans, this new study indicates, the reaction breaks apart the melanin and forms a high energy molecule. In fireflies, the result is a chemical glow. But for us, if there's DNA nearby, that energy can go right into it and do the damage usually done by photons of light.
Post-sunlight damage can be prevented in the same way that all sun damage can be: By avoiding direct sunlight.
"I'd give people the same advice they've heard before," Brash said. "Stay out of the sun between 10 and 2, wear a hat, wear sunscreen."
But he hopes that he and his colleagues can develop a new type of sunscreen designed to divert this chemical energy before it has the chance to damage DNA, cutting down on the negative effects of exposure.