Damaged DNA Is Key to Tanning, Scientists Say : Health: Report is key step in understanding the process. Researchers hope to begin testing a chemical that could trigger darkening of the skin without harming cells.

For sun-worshiping Southern Californians, there is bad news and good news about tanning today. The bad news is that a so-called “healthy” tan is actually strong evidence that DNA in the skin has been severely damaged, researchers have found.

The hitherto little-understood tanning process is actually triggered when the body begins repairing DNA damaged by ultraviolet radiation, a team from Boston University reports today in the British journal Nature.

That repair process initiates the production of melanin, the chemical that causes skin to darken protectively.

Although much of the damage is successfully repaired, defective DNA accumulates over time, leading eventually to the development of skin cancer, including its most deadly form, a tumor known as melanoma. Skin cancer is by far the most common form of the disease, with 700,000 cases per year in the United States, compared to 1.2 million cases for all other forms combined. Melanoma, accounting for 27,500 cases per year, is the fastest-increasing form of cancer, with experts predicting that by early in the next century, one in every 90 people will develop it. All forms of skin cancer are closely associated with exposure to sunlight and tanning.

The good news is that the team has found that a simple DNA fragment spread on the skin can trigger melanin production--at least in mouse and guinea pig skin--and for the first time produce a natural tan without damaging DNA in cells. Existing tanning lotions simply stain or dye the skin, providing a cosmetic tan but no protection against the sun.

The Boston University researchers hope to begin testing the preparation on humans early next year after they receive approval from the university bioethics committee.

Despite Americans’ fascination with a good tan, almost nothing has been known about how tans develop. Today’s report marks a major step toward understanding the process, even though the precise details are not known, said Dr. Barbara A. Gilchrest of Boston University.

What the researchers were able to show, in mouse skin cells and guinea pig skin grown in the laboratory, is that tanning is not triggered until DNA damage is produced by sunlight and repair of the damage has begun.

“It may not be the only trigger for tanning, but it is certainly the most important one,” said Gilchrest’s colleague, biologist Mark S. Eller.

“Our finding suggests that tanning may be part of an SOS response of the skin to UV damage,” Gilchrest said.

While the researchers were attempting to tease out the mechanism for tanning, they began looking for other, safer ways to trigger the process. Reasoning that it began in response to broken DNA, they tried applying a small DNA fragment--called dipyrimidine dithymidylic acid--to the cells. Sure enough, the cells began producing melanin and got darker.

Although the chemical has never been tested on humans, “we have no reason to think that it would be unsafe,” Eller said. And because it is not part of the cell’s own DNA, there is no risk of cancer.