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. 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.