Zapping human tissue with radiation damages even cells surprisingly far away from those directly hit, a new study shows. The experiment is the first to show that a mysterious "bystander effect," first seen in cell culture, also happens in tissue. The findings may bolster the view that low dose radiation is more dangerous than has been assumed.
In 1992, radiation biologists first noticed in experiments with cultured cells that ionizing radiation harms more than the cells it hits--nearby cells also suffer chromosomal damage. Later work suggested that a DNA-harming substance formed by radiation was diffusing through the culture medium to other cells. But it wasn't known whether the same thing happens in tissue, in which cells are packed close together in three dimensions and communicate through different mechanisms. To find out, David Brenner and coworkers at Columbia University in New York City applied alpha-particle radiation to a tiny volume in samples of artificially grown human skin. They then measured two endpoints for radiation effects: apoptosis, or programmed cell death; and micronuclei, which are fragments of damaged chromosomes that have broken off from a cell's nucleus.
These signs of damage were found in cells up to 1 millimeter, or 50 to 75 cell diameters, from the targeted cells--"which is a jolly long way," says Brenner, whose study appears online this week online in Proceedings of the National Academy of Sciences. The bystander cells suffered less damage than those directly hit, but the average rate of apoptosis in the bystanders was still 2.8 fold higher compared to control tissue and 1.7-fold higher for micronuclei. Brenner suspects a distress signal triggered by radiation moves from cell to cell through "some type of relay system."
"It's quite interesting and important" to see the bystander effect in tissue, says radiation biologist William Morgan of the University of Maryland, Baltimore. The results suggest that low dose radiation may be more dangerous than has been estimated from calculations based only on directly hit cells. Nobody yet knows whether bystander effects are beneficial or harmful overall, however, Morgan notes. Cells with mutations could eventually turn cancerous, but if all the damaged cells die, then the bystander effect could protect against cancer.
For now, Morgan says, the Columbia group--which used relatively high doses of radiation--needs to see if the results hold up at much lower radiation doses. Brenner's team is doing just that by irradiating skin tissue with a single alpha particle.
Information on low dose radiobiology from the Department of Energy
Recent report on low dose radiation risks from the National Academies
Site on the debate about whether low dose radiation is beneficial