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Thursday, Aug. 11, 2005

NATURAL SELECTIONS

60 YEARS ON

A-bomb gene 'shadow' may be fading


One of the strongest memories I have of a trip to Hiroshima that I made a few years ago is of the shadow on the steps of the Sumitomo Bank. Someone had been sitting on those steps, probably waiting for the bank to open, when at 8.15 a.m. on Aug. 6, 1945, a bomb went off.

The bank was 250 meters from the epicenter of the atomic explosion and the person was instantly incinerated, leaving behind only a shadow. The steps of the bank are now in the Peace Memorial Museum, as the shadow was gradually fading in the sun and the rain.

Sixty years on from that day, is the shadow that the bomb cast on the survivors fading too?

According to Dale Preston, of the Radiation Effects Research Foundation (RERF) in Hiroshima, the radiation changed the DNA of the surviving hibakusha (atomic bomb victims), and its effects linger to this day.

Preston and his colleagues used information from tumor registries, medical records and death certificates to identify benign and malignant tumors of the primary nervous system and pituitary gland that were diagnosed between 1958 and 1997 in more than 86,000 Japanese atomic bomb survivors from both Hiroshima and Nagasaki.

The authors found that radiation exposure was associated with an increased risk of all nervous-system tumors. This risk was particularly high for schwannomas, a benign tumor of the nervous system. Men had a higher risk of nervous-system tumors than women, and people exposed to radiation as children had a higher risk than those exposed as adults.

Preston said the findings "demonstrate that radiation exposure can increase the risk of nervous-system tumors, and suggest that these increased risks persist throughout lifetime, regardless of the age at exposure."

The Hiroshima bomb was a uranium device, while the Nagasaki bomb was plutonium. The researchers found no significant difference between the cities in terms of cancer risk.

The effect of being exposed is similar to what happens when children with leukaemia undergo irradiation treatment before a blood stem-cell transplant. The aim is to destroy the cancerous cells in the bone marrow, and use the transplant to "re-seed" the marrow with new cells.

Of course, when people were exposed when the bombs went off, there was no transplant available. It meant that the blood-producing cells in their bone marrow, and thus their immune-systems too, were devastated. Many died from infectious diseases they would normally have been able to fight off.

Preston and colleagues at RERF also looked at the effects of different doses of radiation. Even a small dose (0.005 sieverts of radiation) has had a lasting effect. Of survivors about 2.5 km from the hypocenter of each bomb, 203 have since died of leukaemia. Half of those deaths are attributed to radiation.

However, although the effect is a lingering one, it is small. Among 52,000 hibakusha who received a 0.005 sievert dose, 7,600 cancer deaths among them have been attributed to a cause other than radiation. Of the approximately 280,000 people who survived the initial dose, some 45 per cent are still alive today.

What about the children of the hibakusha? A study by Nori Nakamura, head of genetics at RERF, found no evidence for inherited changes in the first generation of A-bomb children.

The average age of hibakusha children is still young, at 48. Some effects might still appear as they grow older, but it seems as if the shadow of the bombs are fading. The survey looked at 22,000 children of Hiroshima and Nagasaki survivors, and found that even parts of their DNA thought to be particularly vulnerable were not affected.

With all the human suffering, it is quite understandable that the effects of the bombs on other organisms have been overlooked. We can get a clue by looking at what happened at Chernobyl. In 1986, a reactor at the nuclear power plant exploded, and radioactive isotopes were scattered over Ukraine, Belarus and Russia.

Plutonium, strontium and cesium fallout has changed the way animals live and grow in the affected areas. And the radiation has had heritable effects. Timothy Mousseau, of the Laboratoire de Parasitologie Evolutive in France, studied swallows nesting in the Chernobyl region. He found that reproductive success was significantly reduced compared to birds nesting outside the affected area. Survival rates, the number of eggs laid and overall body condition was lower. In Chernobyl-area birds, mutations in eggs and sperm are double that of other birds.

No one knows exactly what the radiation in Hiroshima and Nagasaki did to the people and animals exposed to it. Indeed, we are still finding out what exactly were the radiation types that the two cities were exposed to. But it does seem that the shadow of genetic damage, like that on the steps of the Sumitomo Bank, is fading.

A book of Natural Selections columns translated into Japanese, "Nou to sekkusu no seibutsugaku (Evolution, Sex and the Brain)," is published by Shinchosha.
Rowan Hooper is a biologist at Trinity College, Dublin. He welcomes readers' comments at rowan.hooper@tcd.ie


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