It is well-known that radiation causes cancer and damages human cells, but its effects on human DNA have been less clear. New research by Cambridge scientists suggests that even relatively low doses can cause mutations in human DNA.

The researchers believe that current radiation standards may have to be reviewed. The study found DNA mutations in people living in a region where the natural level of radiation is 10 times higher than the global average. Currently the maximum allowed for workers exposed to radiation (in research or in nuclear facilities) is 50 times higher than normal.

"It might be worth considering whether to lower the allowed limits for radiation workers of reproductive age," said Peter Forster, a molecular geneticist at the University's McDonald Institute for Archaeological Research.

The study was carried out among residents of Kerala, a coastal region of southern India, where the black sand on the beaches contains the radioactive mineral monazite. Kerala has the world's highest level of natural radioactivity in a densely populated area (other natural high-radiation areas in southern China, Iran and Brazil are not densely populated).

The researchers analysed the DNA of 988 individuals from 248 families that have been exposed to natural radiation throughout their lives; residents from nearby low-radiation islands served as a control population. The majority of families were sampled over three generations giving the researchers an opportunity to track changes in greater depth.

The researchers looked at the DNA from mitochondria (mtDNA), the minute energy factories which power cells. Since MtDNA is inherited only from the mother, it can be used to determine whether a mutation is transferred from generation to generation.

The scientists identified 22 mutations in the mtDNA sequences of families living in the high-radiation area. By comparison, the control population living in a low-radiation area nearby only had one mutation.

Strikingly, the mutations occur at 'evolutionary hot spots', nucleotide positions known to have been mutating 12 times faster than normal for at least the last 60,000 years of human evolution.

"Our findings indicate that radiation indirectly increases the cell's normal (evolutionary) mutation mechanism," said Dr Lucy Forster, lead author on the team's research paper, which is published in the Proceedings of the National Academy of Sciences.

The findings do not establish direct link between the 22 mutations and disease.

"We intentionally analysed a non-coding region of the DNA where mutations have no effect on health or on any other visible features," said Peter Forster. "But perhaps it is happening to other genes and maybe it is happening to genes that have been linked to cancer."

Previous studies have found no conclusive evidence that people living in places with natural radioactivity are at greater risk of cancer and other health problems.

Photo: Courtesy Copyright (2002) National Academy of Sciences, U.S.A

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