Protons slice through DNA more efficiently than x-rays or gamma rays do, a pair of molecular biologists reports. That means researchers may have to rethink how cosmic radiation might harm astronauts traveling to Mars.
Interplanetary space is filled with high-energy particles--mostly protons--whizzing every which way. There are so many that during a 3-year trip to Mars, every cell nucleus in an astronaut's body would be pelted with hundreds of them. That could kill cells or raise the risk of cancer or other harmful effects. Scientists generally infer the effects of such radiation by studying people who have been exposed to high levels of energetic photons called x-rays and gamma rays, such as the survivors of the atomic bomb attacks on Japan in 1945. Researchers assume that protons and high-energy photons cause similar damage because they deposit relatively little energy in a material as they pass through it.
But new experiments show that protons and photons damage DNA in different ways. Betsy Sutherland and Megumi Hada of Brookhaven National Laboratory in Upton, New York, irradiated solutions of a specific DNA molecule with x-rays, gamma rays, and protons. They also irradiated samples with heavier ions, such as iron, which are less common in space but deposit more energy and do more damage. The researchers then examined whether the radiation had cleaved the double-stranded molecules. Megumi, now at NASA's Johnson Space Center in Houston, Texas, and Sutherland also looked for clusters of subtler defects, such as missing or oxidized DNA bases. All told, protons acted more like the hazardous heavier ions than like photons, producing more clean breaks than clusters, the researchers report in the February issue of Radiation Research. In contrast, high-energy photons produced more clusters than breaks.
Experiments on mice and cells had suggested differences between protons and photons, but the new work gets down to the molecular nitty-gritty, says Francis Cucinotta, a biophysicist at Johnson Space Center. Leslie Braby, a radiation physicist at Texas A&M University in College Station, agrees but notes that it's not clear whether the biological effects of protons or high-energy photons are worse. Sutherland is planning experiments with living cells that may help answer that question.