Last updated on March 15, 2014 at 21:56 EDT

Melanoma risk only partially associated vith exposure to UVB from sunlight

December 21, 2005


The report in the Dec. 21 issue of the Journal of the National Cancer Institute also indicates that only nonmelanoma skin cancers (i.e., basal and squamous cell carcinomas) are strongly associated with exposure to UVB radiation.


“Although we have refined the common wisdom that excess sun exposure is always associated with increased risk of skin cancer, the take-home message for the public is still the same Ã¢Ë†´ limit sun exposure and use a sunscreen that blocks both UVA and UVB rays,” says the study’s lead investigator, Qingyi Wei, M.D., Ph.D., professor in the Department of Epidemiology.

The study is a painstaking analysis of the ability of UVB radiation to damage a cell’s chromosomes. Chromosomal injury is one way cells can become cancerous; damage to the genes that make up the chromosome is another, and Wei and his colleagues already have shown in previous studies that melanoma patients often have a reduced capacity to repair the DNA damage that results from UV exposure.



Using the theory that the ability to induce breaks in a cell’s chromosome is, in part, based on a person’s genes, and would therefore hold true for all types of body cells, the investigators exposed the blood cells to excess UVB exposure. These findings were linked to whether each of the study participants had one of the three forms of skin cancer.


These findings indicate that in skin cells it is better to have broken chromosomes that cause cells to die or acquire a “simple,” treatable cancer, than for the skin cells to remain intact but sustain genetic damage that can lead to much more serious cancer, Wei adds.


Wei says these conclusions may help explain for why nonmelanoma skin cancers are so common Ã¢Ë†´ more than 1 million cases are diagnosed each year in the United States. Squamous skin cells lie near the top of the skin’s layers, while basal skin cells lie near the base of the skin’s layers. In both cases, these cells actively reproduce. When their chromosomes are damaged by sunlight, the cells often die or form a kind of curable cancer at the surface that is often nonmalignant and easy to remove by surgery or treat in other ways, he says.

Melanoma, on the other hand, is now known to be resistant to chromosomal breaks from UVB radiation, which means that the cell’s chromosomes stay intact long enough to continually amass genetic damage from, perhaps, UVA radiation, according to previously published data. “This resistance allows the cells to hang in there longer, potentially passing on genetic mutations to daughter cells which can result in a cancer that is not sensitive to treatment,” Wei says.


“But if UV sunlight doesn’t hurt the walls too much, but endangers the house with broken bricks, this can form a much more malignant cancer in which the cell can continue to replicate, passing on to daughter cells genetic mutations that can lead to a dangerous cancer,” he says.


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University of Texas M. D. Anderson Cancer Center