Low doses of radiation in nature may pose more risk

December 11, 2001

NEW YORK, NY -- Radiation can trigger widespread mutations in living cells at much lower doses than the amount scientists previously believed could do such damage, according to findings from a study by Columbia researchers. The research may help public health officials reconsider what levels of radiation in nature should be deemed safe.

Led by professor of radiation oncology and public health Dr. Tom K. Hei, the study found that a dose that strikes as few as one in 10 cells has nearly the same mutagenic (mutation-causing) effect as a dose that strikes every cell. The added damage occurs, the researchers say, because of what is called the "bystander effect," in which injured cells send aberrant signals to neighboring cells. The study is to be published in the Dec. 4th issue of the Proceedings of the National Academy of Science.

The researchers found the bystander effect occurs with alpha particles, a common type of radiation that comes from sources such as nuclear explosions, plutonium, and radon, a naturally occurring gas that seeps into many homes.

In the National Cancer Institute-funded study, the researchers beamed a single alpha particle through the nuclei of randomly selected cells growing in petri dishes. Alpha particles consist of the nuclei, or cores, of helium atoms. The radiation was equivalent to a dose below 20 centiGrays.

A centiGray is a unit used in radiation measurement. The radiation employed in the study is considered low-level. Mine workers are occasionally exposed to such levels of radiation. People in radon-contaminated homes are rarely exposed to these levels. The risks of exposure to lower radiation levels are poorly understood and have been the subject of controversy for decades.

In the study, the researchers found that when they beamed alpha particles at only one in 10 cells, the results were almost the same as if they had beamed them at all the cells. "If you only hit 10 percent of the cells, your damage is comparable to what would happen if every cell in the population is hit," Dr. Hei says. "There's a huge 'bystander' response."

Nearly all the cells showed mutations in their DNA. Mutations in DNA can lead to cancer in tissues within the body and to birth defects in future generations if the alterations occur in sperm and egg cells or to a developing fetus. The investigators, however, were able to prevent the widespread damage if they blocked the signaling network that exists between cells.

By using chemicals that obstruct key cell-to-cell pathways, called gap junctions, the scientists could restrict the damage to the 10 percent of cells that had directly received alpha particles. The results, Dr. Hei says, suggest that signaling networks are responsible for spreading the damage. The signaling mechanism, however, is unknown and requires further research.

The researchers used a hybrid of human and hamster cells for the study, because these cells are resilient and useful for mutation research. Future studies will use human cells, Dr. Hei says.

The findings raise questions about how to measure radiation exposure risk. Historically, public health officials have assumed radiation carries a risk proportional to the dose in what is called a linear relationship. Lower doses bring accordingly lower risk. But the findings suggest a linear relationship might not be operating, the Columbia researchers say. Instead, the study suggests that at low doses, the risk rises steeply in proportion to the dose. At higher doses, the risk rises more gently in relation to the dose.

The study is "another piece of hard evidence to suggest we may need to reassess" acceptable radiation levels, says Dr. Howard L. Liber, associate professor of radiobiology at Harvard School of Public Health. Dr. Liber is a member of the National Council on Radiation Protection and Measurements, a non-profit group that advises the government about radiation safety.

The study isn't the first to find a "bystander effect," but is the first to identify the effect for mutations, Dr. Liber says. Other studies have found a bystander effect in the expression of certain genes, such as p53, and in what is called sister chromatid exchange, the exchange of DNA in cells in the body.

"The uniqueness of our study is the precision of the targeting of cells," Dr. Hei says. "We show which cell is hit and by what dose of radiation, a major improvement over previous studies."
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