Radiation has widespread effects on cell

NEW YORK, Apr 26 (Reuters Health) -- Although it is widely held that radiation only has cancer-causing effects when it strikes the nucleus, or ``command center'' of a cell, a new study suggests that radiation can also trigger cancer through its effects on the cell cytoplasm, the fluid medium that fills the cell.

Reporting in the Proceedings of the National Academy of Sciences, a team of scientists showed that irradiating the cell's cytoplasm but avoiding irradiating the nucleus caused the rate of mutations to the cell's genetic material (DNA) to rise as much as threefold above normal.

Radiation damages cells, causing changes or mutations in the genetic material. If a mutant cell reproduces abnormally, cancer risk is greatly increased.

``The nucleus has always been considered the quintessential target for any (cancer-causing) mutation,'' explained lead researcher Dr. Tom K. Hei, an associate professor of radiation oncology and public health at Columbia University College of Physicians and Surgeons in New York. ``We have learned that if you hit the cytoplasm... there is a chance that you will (cause changes in the nucleus).''

The new findings suggest that public health officials may have to alter cancer risk estimates for people who are exposed to low levels of radiation via radon and repeated x-rays.

Radon is an invisible and odorless gas that is found in the earth, rock, well-water and building materials. It has been linked to an increased risk of lung cancer in humans.

To arrive at their findings, Hei and colleagues used a state-of-the-art microbeam to irradiate the cell cytoplasm with alpha particles, the radiation emitted by radon. After irradiating thousands of cells, the investigators found that cells hit by eight alpha particles showed a mutation rate that was threefold higher than normal.

Hei explained that these mutations are different from those caused by irradiating a cell nucleus which causes damage in large sections of the DNA and typically kills the entire cell. When radiation strikes the cytoplasm, the damage is smaller. Fewer cells are killed, thus there is a greater risk that abnormal cells can divide and spur cancer.

Cytoplasmic irradiation may play a significant role in causing cancer, researchers concluded.

``Cytoplasmic irradiation should be considered a major concern to human health in terms of risk of exposure for cancer and birth defects, as well as having a profound impact on our understanding of the relationship between radiation exposure and diseases,'' Hei and colleagues conclude.

SOURCE: Proceedings of the National Academy of Sciences 1999:96:4959-4964.

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