Cells Renew in the Human Heart

By Laura Sanders, Science News

By monitoring carbon 14 emitted from Cold War-era nuclear bomb tests, researchers found that heart muscle cells continue to divide throughout adulthood, shows a study appearing in the April 3 Science. The low-level cell renewal may eventually be exploited to treat damaged hearts, says study coauthor Jonas Frisén of the Karolinska Institute in Stockholm.

The finding contradicts the belief of many scientists that the heart muscle cells sticking around until the end were present at birth.

"The dogma has always been that cell division in the heart pretty much stops after birth," says Charles Murry of the University of Washington in Seattle, whose commentary on the new research appears in the same issue of Science. "In medical school, we teach that you'll die with the heart cells you're born with."

To figure out whether the cells continue to be regenerated throughout life, researchers took advantage of an inadvertent marker that has found its way into heart cell DNA. The radioactive isotope carbon 14 was generated by aboveground nuclear test bombs during the Cold War. After the Limited Nuclear Test Ban Treaty took effect in 1963, carbon 14 levels in the atmosphere dropped, but amounts of the isotope remain in both the environment and humans.

"The carbon 14 in the atmosphere is mirrored in bodies," Frisén says. 

When cells divide, they use carbon 14 to build DNA, a phenomenon that can serve as a birthmark for new cells. By looking at DNA from people born before 1955, when the first nuclear bombs were tested, researchers could see whether heart cells were born after the people in the study were born. (Cells that did not divide after a person's birth would not contain any carbon 14.) The researchers also inferred cells' birth dates by matching cells' carbon 14 levels to the atmospheric carbon 14 levels.

Frisén and his colleagues found that samples from people born before 1955 did indeed have carbon 14 in heart muscle cell DNA, indicating that the cells had recruited the isotope and were therefore created after the person's birth. Using multiple samples, the researchers estimated that a 20-year-old person renews about one percent of heart muscle cells in a year. By age 75, the rate of cell turnover slows to about 0.4 percent a year. This means that a 50-year-old has only about 55 percent of the heart muscle cells he or she was born with, while the remaining 45 percent of the cells were generated later.

At this stage, the results are "really basic knowledge that doesn't solve any problem or help someone with a heart attack," Frisén says. But knowing that heart muscle cells do regenerate opens up possibilities of regulating that process. "The dream scenario would be after a heart attack, we have a drug to take to increase heart cells," Frisén says. The new results "indicate that is rational and realistic to think of such possibilities."