An enzyme produced in embryos and stem cells may hold a key to healing injured hearts, according to a new study. Researchers writing in the early online version of the Proceedings of the National Academy of Sciences report that mice genetically engineered to continue making the enzyme in heart muscle after birth exhibited more and bigger heart cells than normal mice. Furthermore, these mice did not lose so many heart muscle cells as their normal counterparts during a heart attack.

The enzyme in question is known as telomerase reverse transcriptase (TERT), and it enables the protective caps, or telomeres, on the ends of chromosomes to be copied during cell division. Normally TERT production stops in most cellsincluding heart cellsafter an organism is born, and the telomeres dwindle with age. But the mice modified by study co-author Michael D. Schneider and his colleagues showed continued proliferation of heart cells beyond the normal cutoff point; after the cells had ceased multiplying, they grew bigger. Although such additional growth, or hypertrophy, usually weakens heart muscle, the mouse hearts appeared healthy, like those of athletes.

"Understanding how this enzyme works gives us new information about normal control of heart muscle growth and could be important in bringing gene-based and cell-based therapies to bear on diseases of the heart," Schneider remarks. "We believe that the effect of adding TERT to an adult heart would be protection from the kind of cell death that occurs during a heart attack," he adds, "as well as production of bigger heart muscle cells."