It is now possible to discriminate between cancers caused by intake of a radioactive material and those that arise spontaneously
Recently, scientists from the Helmholtz Zentrum Munchen have identified a genetic change in thyroid cancer as a signature or fingerprint that points to a previous exposure of the thyroid to ionizing radiation. They discovered the gene marker in papillary thyroid cancer cases from the victims of Chernobyl; this marker was absent in the thyroid cancers in patients with no history of radiation exposure.
This breakthrough has profound biological significance. Now, for the first time, scientists have been able to discriminate between the cancers caused by the intake of a radioactive material and those that arise spontaneously.
Most cancers occur spontaneously or when cells get exposed to certain viruses or chemicals or a physical agent such as ionizing radiation. So far, there was no way to identify uniquely a radiation cancer from a naturally occurring cancer.
The researchers led by Prof Horast Zitzelsberger and Dr Kristian Unger from the Radiation Genetics Unit of the Helmholtz Zentrums Munchen in collaboration with Prof. Geraldine Thomas, Imperial College London, examined the thyroid cancers from children exposed to the radioiodine fallout from the accident at the Chernobyl nuclear power station.
For comparison, they looked for the same genetic change in thyroid cancers of children born more than one year after the explosion, after all radioiodine decayed away. Iodine-131 has a half life of 8 days.
Scientists found that the number of copies of a small fragment of chromosome 7 was increased only in the cancers from the irradiated children.
Writing in the May 23, 2011 issue of the Proceedings of the National Academy of Sciences (PNAS), the researchers noted that this is one of the first genetic markers that indicate a radiation aetiology of cancer.
Normally, humans have 46 chromosomes. Two copies of chromosome 7, one inherited from each parent are present in every cell. Forty one disorders are associated with genes on chromosome 7. Changes in the number or structure of chromosome 7 occur frequently in human cancers. According to National Institutes of Health (NIH), some genes in chromosome 7 may play critical roles in controlling the growth and division of cells.
“Without these genes, cells could grow and divide too quickly or in an uncontrolled way resulting in a cancerous tumour”, NIH clarified in Genetic Home Reference.
According to Professor Zitzelsberger, the availability of the genetic marker will improve both the clinical diagnosis of thyroid cancer and our understanding of how radioiodine causes the disease to develop (Bioscience Technology, May 24, 2011).
Researchers will extend the study to determine if the genetic fingerprint is able to indicate the dose required to cause cancer.
The study was successful because the researchers had carefully collected, documented and stored samples of thyroid cancer tissues from the Chernobyl region in the Chernobyl tissue bank, a unique venture to establish a collection of biological samples from tumours and normal tissues from patients for whom the cause of their disease is known as exposure to radioiodine in childhood.
The unique collection of materials made it possible for the team to compare for the first time tumours from children of the same age and regional background (insciences.org, May 24, 2011)
The accident at the Chernobyl nuclear power station led to contamination of milk with iodine-131, a radioactive isotope of iodine. If authorities administered stable iodine promptly during the early phase, radiation dose to the thyroid would have been negligible. Stable iodine saturates the thyroid so that the gland will not receive radioactive iodine when it arrives.
Unfortunately, this measure was not implemented at Chernobyl. The exposed population received large doses to their thyroid; this led to a significant fraction of the more than 6,000 thyroid cancers observed to date among people who were children or adolescents at the time of the accident. By 2005, 15 of these patients died.
Thyroid cancers will not occur in Fukushima because as per emergency plan the management at Fukushima promptly evacuated the population from the affected regions and supplied stable iodine to the evacuees.
K.S. Parthasarathy, Raja Ramanna Fellow, Department of Atomic Energy