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Melanoma Tied to Sunburns Early in Life

Mouse study finds first laboratory link to virulent cancer

WEDNESDAY, Sept. 19, 2001 (HealthDayNews) -- If your children love to play outdoors, a new study provides another reason to slather them with sunscreen.

Using genetically engineered mice whose skin closely resembles that of humans, National Cancer Institute (NCI) researchers say they've found evidence that sun damage early in life dramatically increases the risk of malignant melanoma in adulthood.

Senior study author Glenn Merlino, chief of the NCI's Molecular Genetics Section, says the study is the first to provide laboratory evidence supporting the many population-based studies linking childhood sun damage to this particularly virulent form of skin cancer in later life.

Skin cancer is the most common cancer, and while malignant melanoma accounts for only 4 percent of all skin cancers, it causes roughly 79 percent of skin-cancer deaths. An estimated 51,400 people will develop melanoma this year, and 7,800 people will die from the disease.

Normally, human and mice skin differs in that mice have melanocytes (the cells involved in melanoma) only in their hair follicles deep within the skin. In humans, melanocytes are located almost exclusively near the skin's surface.

However, Merlino and his colleagues genetically engineered a strain of albino mice with melanocytes in both hair follicles and in the surface skin layers, providing a closer match to human skin.

"As far as I know, this is the closest thing that has come to human skin," says Merlino.

The mice in the study also have a slight genetic predisposition to developing melanoma, like certain people who have a slightly higher genetic susceptibility to the disease, Merlino says.

Researchers say 6-week-old mice exposed to a single intense dose of ultraviolet (UV) radiation didn't develop melanoma, confirming a previous study that showed that moderate UV exposure had little effect on adult mice.

However, mice exposed to UV radiation at the age of 3.5 days developed reddening of the skin and occasional peeling and were highly likely to develop melanoma at a later age.

"Not only that, but the tumors that came up looked a lot more like human melanomas," Merlino says. The exposure was comparable to a person standing outside in the middle of the day for roughly three hours, he says.

Merlino says several theories suggest why younger skin appears to be more vulnerable. Cells called melanoblasts, the precursors to melanocytes, naturally divide more frequently than melanocytes, and UV light may trigger both excessive growth and DNA damage within the cells, he says.

Young mice have more melanoblasts in their skin, and, if that's true in humans, it may explain why younger human skin is more vulnerable, says Merlino.

Another theory suggests that UV light may suppress the immune system, leaving the body more vulnerable to tumors.

Dr. Martin Weinstock, chairman of the American Cancer Society's Skin Cancer Advisory Committee, says, "We know with human beings [that] early, intense, intermittent exposure to ultraviolet light is an important risk factor for melanoma."

While Merlino says it's impossible to draw an exact human age equivalent to 3.5-day-old mice, Weinstock says epidemiological studies show that skin appears to be most vulnerable to UV exposure during childhood and before the age of 15.

Besides use of proper sunscreens, hats and protective clothing, Weinstock says, "We need shaded play areas for kids … to cut down on the intensity of the ultraviolet that they're exposed to when they're out[side]."

Merlino says while his study is only a first step, evidence from previous epidemiological studies should tell parents to protect children from sunburn. "Parents should consider hats and certainly sunscreen," he says.

However, he says parents shouldn't panic. "In the normal population of humans, one sunburn may not be sufficient, because our mice are a bit predisposed [to melanoma]. If your child has had a sunburn and has peeled and gone red once, it does not mean he's going to get melanoma, necessarily."

Merlino says the mice developed for his experiments could be used to test both new protective treatments against sunburn and new therapies to prevent or fight skin cancers.

What To Do

Remember, children -- especially those with fair skin, freckles and lots of moles -- are particularly vulnerable to the sun. The American Cancer Society has more information about melanoma and encourages everyone to slip on a shirt, slop on some sunscreen and wear a hat. Head for the shade between 10 a.m. and 4 p.m., when the sun is at its brightest, and wear UV-protective sunglasses. And reapply sunscreen lotion after swimming or sweating.

You can learn how to prevent skin cancer from the American Academy of Family Physicians, the U.S. Environmental Protection Agency or the American Academy of Dermatology.

The Skin Cancer Foundation also provides information about melanoma.

SOURCES: Interviews with Glenn Merlino, Ph.D., chief, Molecular Genetics Section, Laboratory of Molecular Biology, NCI, Bethesda, Md., and Martin A. Weinstock, M.D., Ph.D., professor, Department of Dermatology, Brown University, Providence, R.I.; Sept. 20, 2001, Nature
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