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More than one way to mutate a cell's DNA.

High-energy particles that zip through a cell can cause dangerous mutations--even if they don't hit the DNA directly, new research shows. These alpha particles spawn chemicals called free radicals, which in turn damage DNA. This newly discovered mechanism may change how experts estimate people's risks from exposure to radioactivity.

Radon gas, a breakdown product of uranium that seeps naturally from the ground, accounts for most of the alpha particles that bombard people. Second only to cigarette smoking in causing lung cancer, radon gas results in as many as 22,000 cases, or 12 percent of lung cancers, in the United States each year (SN: 3/7/98, p. 159).

Until recently, scientists attributed the cellular havoc wreaked by alpha particles to their direct effects on DNA. The relatively large, high-energy particles, consisting of two neutrons and two protons, "come barreling through the nucleus, breaking things left and right," says Noelle F. Metting of the Pacific Northwest National Laboratory (PNNL) in Richland, Wash. The alpha particles batter the nuclear DNA, knocking out chunks of chromosomes.

To test how alpha particles damage a cell even when they miss its DNA, Tom K. Hei of Columbia University and his colleagues shot the particles through the body of the cell, or the cytoplasm, while carefully avoiding the nucleus. They report their results in the April 27 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

The exposed cells still suffered mutations, but the damage was more subtle than that caused by direct hits to the DNA. The small mutations resembled damage from naturally occurring free radicals, chemicals that are highly reactive because they either hold an extra electron or are missing one. When the researchers added compounds that scavenge free radicals, the rates of mutation dropped. When they interrupted the cells' natural defense against free radicals, the mutation rate climbed.

Cells naturally make free radicals, particularly when generating energy. Enzymes continually repair the damage that free radicals cause when they chip away at DNA, Metting says. "[Alpha particles] greatly enhance the background damage that just happens when you're alive," she says.

Although free radicals created by alpha particles make only small mutations, they ultimately could do more harm than alpha particles blasting directly through a dews nucleus, comments PNNL's Michael K. Bowman. The reason is that alpha particles hitting the cytoplasm are less likely to kill a cell, so the mutated cell may proliferate into a cancerous tumor.

The research by Hei's team showed that twice as many cells survived when the alpha particles bombarded the cytoplasm compared with when they hit the nucleus.

The new findings will allow researchers to refine models of how alpha particles cause cancer, study collaborator Gerhard Randers-Pehrson of Columbia University says. If both the cytoplasm and the nucleus are sensitive to the particles' energy, current models may underestimate the effects of low doses of alpha radiation, he says.
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Title Annotation:alpha particles
Author:Helmuth, L.
Publication:Science News
Article Type:Brief Article
Date:May 1, 1999
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