Most sunlight damage to skin occurs in dark

Washington: So it turns out that most damage to the skin occurs in dark, hours after we have actually been out in daylight.

A team of Yale-led researchers claimed that exposure to UV light from the sun or from tanning beds can damage the DNA in melanocytes, the cells that make the melanin that gives skin its colour.

This damage is a major cause of skin cancer, the most common form of cancer in the United States.

In the past, experts believed that melanin protected the skin by blocking harmful UV light. But there was also evidence from studies suggesting that melanin was associated with skin cell damage.

Prof. Douglas E. Brash and his co-authors exposed mouse and human melanocyte cells to radiation from a UV lamp, which caused a type of DNA damage known as a cyclobutane dimer (CPD), in which two DNA "letters" attach and bend the DNA, preventing the information it contains from being read correctly. To the researchers' surprise, the melanocytes not only generated CPDs immediately but also continued to do so hours after UV exposure ended. Cells without melanin generated CPDs only during the UV exposure. His finding showed that melanin had both carcinogenic and protective effects.

The researchers tested the extent of damage that occurred after sun exposure by preventing normal DNA repair in mouse samples. They found that half of the CPDs in melanocytes were "dark CPDs" -- CPDs created in the dark.

Sanjay Premi, associate research scientist in the Brash laboratory, discovered that the UV light activated two enzymes that 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. Hemiexcitation has previously been seen only in lower plants and animals.

But the researchers also pointed to a ray of hope saying ‘the slowness of chemiexcitation may allow time for new preventive tools, such as an "evening-after" sunscreen designed to block the energy transfer’.

The study is published online in the journal Science.