Twenty Years of FISH-Based Translocation Analysis for Retrospective Ionising Biodosimetry.

Reference: Gregoire, E, Roy, L, Buard, V, Delbos, M, Durand, V, Martin-Bodiot, C, Voisin, P, Sorokine-Durm, I, Vaurijoux, A, Voisin, P, Baldeyron, C and Barquinero, J.F. (2018) Twenty Years of FISH-Based Translocation Analysis for Retrospective Ionising Radiation Biodosimetry, International Journal of Radiation Biology, 94 (3), pp. 248-258.

What were the research questions?

Scientists in laboratories world-wide use abnormal chromosomes, such as reciprocal translocations and dicentrics, to estimate the doses of ionising radiation that people have received. A team of French researchers wanted to assess the usefulness and the limitations of these methods by reviewing 42 cases that the IRSN, a French government research institute, had conducted over a 20-year period (1997-2015).

Currently scientists recommend that dicentrics are used to assess recent exposures and reciprocal translocations are used to assess exposures that have occurred several years in the past. The French team wanted to know more precisely the duration after exposure that scientists should no longer use the dicentric assay. They also explored the upper time limits for using translocations.

The major limitation for scientists analysing translocations, is that translocations can also be produced by normal processes in the body and their number increases with normal ageing. This means that the radiation dose can only be detected if it is sufficient to produce translocations which are above the background levels for an individual’s age, i.e. there is a minimum detectable dose. The consensus prior to this study was this value was about 300 mSv, a finding that the French researchers wanted to investigate.

How was the scientific problem approached?

To answer their research questions about the short-term value of dicentrics, the authors used cases for which they had both data for dicentrics and translocations ranging from 1 day to 4 years after exposure. To explore the limitations of reciprocal translocations, they used cases for which they had data 20 – 44 years after exposure.

What did the research involve?

The researchers compiled a list of 42 cases which are representative of the different groups of people which the IRSN has investigated for potential exposure including:

Group 1: French nuclear test veterans for tests in the Sahara Desert from 1960 to 1966 and in Polynesia from 1966 to 1996 (17 cases).
Group 2: Members of the public affected by accidents in Georgia and Turkey (11 cases).
Group 3: Industrial workers who use radioactive material affected by accidents in Bulgaria and Belgium (8 cases).
Group 4: Nuclear power plant workers (4 cases).
Group 5: Hospital workers who use radiation (2 cases).

The authors performed this study with Group 1 members 20 to 44 years after possible exposure to obtain reciprocal translocation analysis data. They also conducted this study for Groups 2, 3, 4 and 5 from 1 day to 4 years after exposure and recorded data for both dicentrics and translocations.

The members of all of the groups donated blood samples so that the researchers could analyse the amounts of abnormal chromosomes that they observed to be present in their lymphocytes (white blood cells) using a microscope after treatment with dyes.

Researchers only needed a single colour dye, called Giemsa, to identify dicentrics because they have a different shape than normal chromosomes. In contrast, the researchers required different coloured dyes to identify reciprocal translocations, which they achieved with a technique called FISH (fluorescence in situ hybridisation). The research team used the same three fluorescent coloured dyes in all of their cases to ‘paint’ three particular chromosomes (numbered 2, 4 and 12).

Then authors compared each exposed individual being studied with an age-matched member of a control group to take account of natural background levels of abnormal chromosomes. In the laboratory, the researchers created a calibration curve by exposing the blood cells of healthy individuals to known doses of radiation. They then used this curve to convert the levels of abnormal chromosomes that they detected into radiation doses.

For each individual, the research team checked whether the method they had used had found evidence of exposure and if so, recorded the dose. The type of radiation was also noted if known.

What did they find?

Overall, the French team found evidence of 25 out of 42 individuals being exposed, including 9 members of Group 1 who had been exposed 20 to 44 years before testing was performed. The authors estimated average doses for the exposed French veterans using translocations in the range of 0.3 – 1.1 Gy, though the type of ionising radiation received by these veterans was not identified.

In contrast, the researchers established which types of radiation that the members of the public in Group 2 and most of the workers in Groups 3, 4 and 5 had been exposed to. These included beta particles, gamma rays and X-rays and were all external exposures in the range 0.1 – 5.7 Gy.

The researchers obtained results from the two techniques which were consistent using blood samples taken within one month after exposure. However, this consistency dropped over time, due to the dose size based on the dicentrics declining, which indicated that estimates using dicentrics become less accurate after one month and lack reliability after one year.

Conversely, the French team found that reciprocal translocations produced reliable dose estimations over a wider range of time periods. Though the researchers found an exception to this, for which the estimated dicentric dose (4.6 Gy) was higher than the dose based on reciprocal translocations (3.7 Gy). In this instance, they also detected translocations within complex chromosomes. Once they had taken all of the translocations into account the researchers could make a new dose estimation (4.5 Gy) which was similar to the estimated dicentric dose.

How did the researchers interpret their basic results?

The French researchers concluded that dicentric analysis should be used for assessing exposure that has occurred less than one month ago, whereas translocation analysis should be used for estimating doses that have taken place from a month to several years in the past. They caution that if the dose is greater than 1 Gy then researchers should include complex abnormal chromosomes in their dose estimation.

With respect to lower doses, they produced further evidence to support a minimum detectable dose of 300 mGy and that such a dose can be detected four decades after exposure. This means that these methods would not be appropriate for confirming exposures below this level several years after exposure.

The authors add that dose estimates based on chromosome aberration data needs to be supported by additional information, such as the veteran’s medical records and employment history, to both exclude confounders and to verify the estimates. In the absence of this they recommend that a dose estimate should not be made but instead simply state for each individual tested the number of abnormal chromosomes detected and the person’s age.

Who did this research?

This work was done by a team of scientists from the French government research institute called the IRSN. They worked in partnership with other researchers from the Federative Institute of Biology based in Toulouse, the French Atomic Energy Commission and the Autonomous University of Barcelona.