Genetic effects of radiation exposure: Lesson from Hiroshima survivors

An expert chimes in about genetic effects of radiation exposure.
Written by Boonsri Dickinson, Contributing Editor

On February 27, 2011, when his plane flew over Sendai, Mulvihill thought, “I always remember Japan as hilly and mountainous, but this area is so flat . . . . a big wave could really travel far inland.”

That was twelve days before the tsunami hit Sendai, and John Mulvihill was flying to this year’s meeting held by the Radiation Effects Research Foundation (RERF).

Mulvihill is a geneticist at the University of Oklahoma, who spoke to SmartPlanet about the genetic effects of radiation exposure. Mulvihill has been doing genetic research on Hiroshima survivors for decades.

SmartPlanet: What's the current risk of the nuclear crisis in Japan to the population there and around the world?

JM: The probability of a meltdown is Japan is very, very low. The leaks have not been a nuclear explosion, but water vapor pressure leaks that do carry some ionizing radiation. Of course any meltdown, however unlikely, carries a risk of high exposure to radiation.

SmartPlanet: Can you explain the genetic effects of radiation? How is our understanding of radiation changing?

JM: From long time studies of the Japanese survivors of the US atomic bombs in 1945, of radiation accidents over the decades, and of patients receiving therapeutic doses, ionizing radiation mutates human genes and breaks chromosomes, with damage persisting for years, sometimes manifested as cancer or cataracts.

Tissue culture studies and animal models verify the effects on body [and more specifically] somatic cells.

There are rare genetic conditions that show great sensitivity to the effects of ionizing radiation. Current directions in radiation research are addressing molecular and cellular mechanisms of radiation effects and their interactions with other environmental exposures and with modifying genes.

SmartPlanet: We are exposed to radiation naturally and with medical diagnostic and other exposures such as going on a plane. How is what you have been documenting with Hiroshima survivors different or the same as what happens in the general population?

JM: Until this decade, the average American’s annual dose of radiation, about 2-3 millisieverts (mSv), was from background radiation such as soil, air and cosmic rays.

With increasing use of diagnostic medical x-rays, the average American now gets double that dose, especially from CT scans. One CT to abdomen delivers 5-15 mSv.

Medical workers — radiologists and xray technicians — are allowed 50 mSv a year; an average nuclear power plant worker gets 1 mSv a year.

Round trip flight NYC to LA is 0.02 to 0.05 mSv.

Acute radiation sickness occurs at 500 mSv. Symptoms include  vomiting, diarrhea and can make hair fall out.

The average dose was 200 mSv for a Japanese person within 1.5 miles of the bombs. Keep in mind the bomb radiation was largely a single point in time, not chronic low dose.

It is arbitrary, but “low-dose” is considered to be 100 mSv.

In contrast, radiotherapy to cure a cancer is a total dose of 80,000 mSv.

The difference is the timing and volume of a person or organ or tissue. 500 mSv total single body dose is associated with acute radiation sickness. When 80,000 mSv is applied over 6 weeks to the breast with proper shielding of other organs, it will preferentially kill cancer cells and not necessarily all normal adjacent cells.

SmartPlanet: Why are you researching Hiroshima survivors? What are you learning from them?

JM: I’m now just a scientific advisor, suggesting improvements in research. Years ago, I was part of an pilot inquiry to see if the women who got breast cancer after the bombs showed radiation sensitivity in skin cells cultured in the laboratory. The answer was no, but not a definitive study.

The world is still learning from the studies of the survivors. A recent finding is how little radiation is needed to cause old-age cataracts. For years, the world’s standards to protect patients, radiation workers and the public from hazards of radiation have been informed by the on-going research by Japanese and American working in Hiroshima and Nagasaki.

SmartPlanet: What interests you most about this type of research?

JM: My personal interest is the issue of germ-cell mutation.

No environmental agent has ever been proved to cause hereditary disease seen in the children of exposed people, not even the 60-year study in Japan.

With others on international interdisciplinary teams, mostly sponsored by the US National Cancer Institute, I have seen no excess of genetic disease by various definitions, including single gene (mendelian) disorders, chromosomal syndromes, single malformations and stillbirth.

In the US, Canada, Denmark, and Finland, numbers are now 32,000 offspring of 19,000 survivors of cancer of childhood, adolescences, and young adulthood, compared to 106,000 offspring of 49,000 siblings of the survivors.

Most survived cancer by treatments with radiotherapy, chemotherapy, or both, at doses that caused some loss of fertility and an excess of second cancers. Although the genetic results to date are reassuring about the lack of an effects, we are discussing using a DNA approach to address the questions.

The clinical studies suggest human gonads have different sensitivity to mutation by radiation, compared to somatic cells and the mouse germ-line work at three laboratories in the US, UK and Germany.

Photo: NASA

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