By Penny Stern, MD
NEW YORK, Mar 15 (Reuters Health) -- If you're covered in moles or spattered
with freckles, results of a study in twins suggest that your family members may
share a similar fate. But you may have at least some control over the number of
moles you have. Sun exposure appears to increase the chance that people who are
genetically susceptible will actually develop the spots, according to the report
in the March 15th issue of the Journal of the National Cancer Institute.
The finding is important because people who have many moles, or many
abnormally shaped ones, are at greater risk of developing the deadly form of
skin cancer called melanoma.
In the study, Dr. Veronique Bataille of St. Thomas Hospital, London, UK, and
colleagues show that both environment and a person's genes contribute to the
development of moles, thereby setting the stage for later melanoma growth. The
researchers looked at 127 pairs of identical twins and 323 pairs of fraternal
twins.
Among identical twin pairs, there was a striking similarity in the total
number of moles found on each person. Such similarities were less common in
fraternal twins. The average number of moles was 35, and they tended to increase
with age. About 22% of twins had more than 100 moles and 55% had less than 25.
Bataille and her team write that this finding "confirms that the expression
of nevi (moles) is statistically significantly influenced by genetic factors."
"The (study's) main results are that moles are under strong genetic
influence and that the effects of genes become stronger as we got older,"
Bataille told Reuters Health. "The research also showed that the variation in
freckle counts is almost entirely due to genetic factors with very little
influence from environmental factors," she added.
To what degree, therefore, does environment -- such as sunlight exposure --
influence the growth of moles? Bataille was "surprised" to find that in twins
over age 45, the genetic effect outweighed the environmental influence by "as
much as 84%," she said. In those under age 45, the genetic effect was less
pronounced. About 36% of the total variation in mole count in those under 45 was
due to genetic factors.
"For a long time, it was thought that melanoma was a disease mainly caused
by the sun," she pointed out. While sun exposure did play a role -- people were
more likely to have moles on sun-exposed skin rather than sun-protected -- genes
were very important, too. Bataille wants to identify the specific genes involved
in mole development and determine how they work. She emphasized that "the
ultimate goal would be to use these genes to screen populations at risk, but
also to look into therapeutic avenues."
