Sunlight has long been blamed as the chief culprit in skin cancer, but why doesn't everybody get it? Johns Hopkins researchers have found that people who get the disease lack the ability to repair the genetic damage caused by the sun's ultraviolet rays.
The repair mechanism that protects people from skin cancer tends to break down with age, leaving older people more susceptible to the disease. But that doesn't mean young people can't get it too.
Some people inherit repair systems that are faulty from the start, the researchers said, placing them at risk even in their 20s or 30s. Accordingly, people with a family history of skin cancer should be particularly vigilant about using sunscreen, wearing protective clothes or reducing the amount of time spent in the sun.
The findings, reported in today's Proceedings of the National Academy of Sciences, emerged from a study comparing 88 people with a common form of skin cancer known as basal cell carcinoma with 135 people with healthy skin. The subjects all worked indoors and lived in Baltimore or its suburbs for most of their lives.
"We are all exposed to sunlight, and we've evolved through exposure to sunlight," said Dr. Lawrence Grossman, professor of biochemistry at the Johns Hopkins School of Hygiene and Public Health. Dermatologists with the Johns Hopkins Hospital collaborated in the study.
"The fact that we've survived for eons says we have a great deal of repair capacity. What we're looking at are the variations in the population."
Dr. Grossman explained that a "surveillance system" of circulating white blood cells normally repairs the damage done to DNA -- the genetic code -- that frequently occurs as sunlight bombards the body. When this system breaks down, damaged genetic material can set off the growth of cancerous cells.
"Even if you have low repair, it doesn't necessarily mean you're going to have skin cancer unless you're exposed to sunlight," he said.
Researchers noted that the decline of DNA repair that occurs with age probably accounts for the rising incidence of skin cancer in middle age and beyond.
The findings in no way contradict the long-held belief that fair-skinned people have a heightened risk of developing skin cancer. Those people have low concentrations of skin pigment that protect DNA by absorbing ultraviolet light.
The people at greatest risk are probably fair-skinned people who do not protect themselves from the sun's harmful rays and have a family history of skin cancer, said Dr. Grossman.