NEW ORLEANS — Scientists attempting to figure out why some girls go through puberty at unusually young ages have found a surprising culprit — a gene that speeds up the body's breakdown of the male sex hormone.
Many believe that the age of puberty — the time when girls develop breasts and other sexual characteristics — is creeping downward. The most widely held explanation for this is growing childhood obesity, along with rich diets and lack of physical activity.
However, genes almost certainly play a role in the age of puberty, and many assumed that the most likely players in this scenario would be ones that control the body's production and use of estrogen.
Research released Sunday at a meeting of the American Association for Cancer Research suggests a more complicated interplay of hormones, since the body's supply of testosterone, the male hormone, seems to be a key influence on the timing of puberty.
Dr. Fred F. Kadlubar and colleagues from the Food and Drug Administration's National Center for Toxicological Research discovered the link in a study of 192 girls ages 9 and 10.
A study published four years ago found that about half of all black girls and 15 percent of whites begin to develop sexually by age 8. The average age of puberty is about 13 for whites and a year younger for blacks.
Typically, girls begin breast development about a year before their first period. Kadlubar's team looked for a link between genes and this change in the randomly chosen group of girls.
They examined several genes that control the body's use of estrogen but found no association with early puberty. To their surprise, though, they found a strong link when they tested the girls for a gene that controls the body's breakdown of testosterone.
They looked for a particular variation, called CYP1B1, in a gene that produces a liver enzyme. They found that 90 percent of the girls with two copies of this genetic variation had already begun breast development by age 9 1/2, compared with 56 percent of those with one copy and 40 percent with no copies.
"It tells us what goes on hormonally during the early stage of breast development," said Kadlubar. "Now we have to put testosterone into the equation."
It has long been known that estrogen production increases and testosterone falls during this developmental landmark. Kadlubar said the CYP1B1 gene, by reducing testosterone levels, may trigger the cascade of hormones involved in the start of breast development.
Early puberty is a concern, because it increases the risk of breast cancer later in life. Experts speculate that, among other things, early breast development may increase lifelong exposure to estrogen, which increases the chances of this malignancy.
Kadlubar said the enzyme he studied is critically important to the body's well being, so doctors are unlikely ever to try tinkering with it in an attempt to slow puberty.
However, Dr. Christine B. Ambrosone of Mount Sinai School of Medicine in New York City said it someday may be useful to screen girls for the genetic variation. Those who have the gene could be targets for special effort to delay puberty by slowing their weight gain and increasing exercise.
"These are groundbreaking studies," she said. "We will need to see where this goes, but there is a lot of exciting research for the future."
Kadlubar said the next step will be to see if the gene influences sexual development in boys. In theory, at least, those with the CYP1B1 gene should develop later than usual, since they are exposed to lower levels of testosterone.