Sun Damage Continues After Dark

A team of researchers from Yale University has discovered damage caused by ultraviolet radiation (UV) continues hours after sun exposure. It is common knowledge that exposure to UV light from the sun or tanning beds can case damage to the DNA in melanocytes, the cells that make melanin and give skin its color. It is a major cause of skin cancer, the most common form of cancer in the US.

The recent study, led by Douglas E. Brash, clinical professor of therapeutic radiology and dermatology at Yale School of Medical and published in the journal Science online, showed melanin had both carcinogenic and protective effects. Researchers exposed mouse and human melanocyte cells to radiation from a UV lamp, which caused a type of DNA damage known as cyclobutane dimer (CPD). CPD occurs when two pieces of DNA attach and bend the DNA preventing the information it contains from being read correctly. The melanocytes generated CPDs immediately during exposure and continued hours after, while the cells without melanin generated CPDs only during the UV exposure. "If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," says Brash. "But it is doing both good and bad things."

One of the researchers explained the continued CPD generation and damage with enzymes activated by the UV light. These two enzymes combined to excite an electron in melanin. The energy generated from this process—known as chemiexcitation—was transferred to DNA in the dark, creating the same DNA damage that sunlight caused in daytime. This phenomenon has previously been seen only in lower plants and animals. The time delay damage shows the greater carcinogenic effect of melanin, but it also may allow for new preventive tools to block the energy transfer and further damage.