Research led by scientists in Sweden found that our heart muscle cells are renewed over our lifetime and we are not limited to those we are born with. They believe the discovery opens a door to treatments that could replace damaged heart tissue with new cells.
The study was the work of Professor Jonas Frisén of the Karolinska Institutet, in Stockholm, Sweden, and colleagues and is published online in the 3 April issue of Science.
The researchers took advantage of the fact that carbon-14 generated by nuclear bomb tests during the Cold War gets into DNA, including that in heart muscle cells (cardiomyocytes). This can then be used to date the age of the heart muscle cells in humans.
Using this approach, Frisén and colleagues found that heart muscle cells renew gradually over our lifetime and the rate declines as we get older such that 1 per cent of our heart cells are replaced per year at age 25 and this falls to 0.45 per cent at age 75.
They also found that over a normal lifespan, fewer than 50 per cent of the heart’s muscle cells are replaced.
Frisén and colleagues suggested this could open the door to new heart disease treatments that replace damaged tissue by stimulating the process of heart cell renewal.
Dr Charles Murry, a heart researcher at the University of Washington in Seattle, who writes in the same issue of Science, commented that this was one of the most important studies in cardiovascular medicine in years.
He said that the “dogma” has been that people die with the same heart cells they were born with, and this is still taught in medical schools. But this study helps to settle the longstanding controversy, he noted.
Using radioactivity to measure how fast cells are replaced is not a new idea. Scientists use this method with animals; by making their cells radioactive they can then measure the rate of cell turnover, only it is unethical to do this in humans.
But Frisén and colleagues then leapt upon the idea that nuclear weapons testing until 1963 had effectively done this by releasing carbon-14 radioactive isotope into the atmosphere, which then got into the food chain and into humans.
A radioactive isotope gradually loses its radioactivity and decays over time into the normal form of the substance. Thus carbon 14 gradually loses neutrons in the nucleus and becomes the stable form, carbon 12.
When cells divide they make new DNA, and this contains carbon 14. So if the old theory that we die with the heart cells we are born with is true, then people born before carbon 14 got into the food chain (nuclear testing didn’t start until 1955) wouldn’t have any carbon 14 in their heart cells. But this was not the case, and not only did they have them, but the researchers could also date them and show that they were being made over a period of time.
Murry said he was especially impressed by the rigour of the study, as Frisén and colleagues had to deal with a number of technical problems. For example, heart muscle cells have two nuclei, which means the DNA could be in there twice, so this could screw up the carbon 14 dating.