McKenna and Co-Workers Explained:
Chromosome Translocations, Inversions and Telomere Length for Retrospective Biodosimetry on Exposed U.S. Atomic Veterans.
Reference: McKenna, M.J., Robinson, E., Taylor, L., Tompkins, C., Cornforth, M.N., Simon, S.L. and Bailey, S.M. (2019) Chromosome Translocations, Inversions and Telomere Length for Retrospective Biodosimetry on Exposed U.S. Atomic Veterans, Radiation Research, 191 (4), pp. 311-322.
What was the research question?
Scientists use both physical methods such as dosimeter film badges and genetic methods such as reciprocal translocation to assess doses of radiation that individuals have received several years in the past. A team of American researchers wanted to test different genetic methods to provide evidence of exposure and, to assess the reliability of these dose estimates through comparison with film badge data.
The researchers sampled a group of U.S. nuclear test veterans known to have been exposed to radiation approximately sixty years ago. They wanted to know if their methods would produce similar dose estimates to that obtained from veterans’ film badge data and hence determine the usefulness of genetic dose assessment decades after exposure.
How was the scientific problem approached?
Scientists use reciprocal translocations for long-term retrospective dose estimation, because these chromosome aberrations can persist in the body for many years after formation. The authors of this work were interested in a different type of abnormal chromosome, called an inversion, because cells containing inversions have also been found to have a long-lifetime. The authors wanted to know if the reliability of dose estimates based on reciprocal translocations could be improved if inversions were also taken into account.
Chromosome inversions are also produced by normal living and their levels increase in our bodies with ageing. This means that scientists can only estimate a dose if the number of inversions produced due to radiation is above the background level for a person’s age, i.e. there is a minimum detectable dose (MDD). Since the veterans were in their eighties, the researchers anticipated that estimating their doses could be challenging.
The researchers were also interested in telomeres, these are caps which protect the ends of chromosomes. Telomeres are like the plastic tips on shoelaces; they keep the ends of chromosomes from deteriorating or sticking to other chromosomes. Telomere length is an indicator of cell age and potentially the overall health of an individual and for this reason, the researchers in this study also investigated whether ionising radiation influenced telomere length.
What did the research involve?
Blood samples were taken from three groups.
Group 1: Twelve male nuclear test veterans who had received radiation doses of > 200 mSv approximately 60 years ago according to their military records. To give this figure some perspective, the maximum dose that a radiation worker is currently permitted to receive over a 5-year period is 100 mSv.
The researchers divided Group 1 into two sub-groups for estimating dose: six men who had been exposed on Rongerik Atoll in the Pacific Ocean and six men who had served in other locations in the Pacific or at a test site in Nevada. Seven of these test veterans were smokers.
Group 2: Twelve male veterans were chosen who had not participated in nuclear weapons testing as a control group. These veterans were matched to Group 1 with respect to age and smoking habits, because these variables are known to influence the number of reciprocal translocations in cells. Importantly, none of the men in Groups 1 and 2 had received radiation therapy or chemotherapy which can increase the number of abnormal chromosomes in cells.
Group 3: Six male volunteers in their mid-20s to mimic the approximate age of the Group 1 veterans at the time of their exposure. These men were not smokers, had not received radiation therapy or chemotherapy and were in good health.
In the laboratory, the researchers irradiated blood samples from Group 3 with known doses of gamma radiation and measured the levels of reciprocal translocations and inversions formed. They used the results to generate calibration curves (graphs) to convert the levels of abnormal chromosomes found in Group 1 into estimated doses.
For all Groups; a technique called directional genomic hybridisation (dGH) was used to identify reciprocal translocations and inversions which essentially labels chromosomes with dyes making them visible under the microscope.
Another technique, called quantitative polymerase chain reaction (PCR) was used to measure the lengths of the chromosome telomeres for all three groups of men.
What did they find?
It is already known that normal ageing and smoking can increase the 'background' levels of reciprocal translocations and the study’s results support this. The researchers also showed that these factors also increase the number of inversions. Thus, both getting older and being a smoker increased the amounts of abnormal chromosomes in this study.
The scientists used these background levels of abnormal chromosomes to estimate MDDs for reciprocal translocations and, for the first time, inversions for both smokers and non-smokers. To give examples, for non-smokers the MDD for reciprocal translocations was 270 mSv and for inversions was 210 mSv with 99% confidence (the reliability of the estimate made).
Four of the six nuclear test veterans who served on the Rongerik Atoll and four of the six nuclear test veterans who served at other locations were found to have received doses below a relevant MDD. The other four nuclear test veterans received doses greater than the MDDs.
For each individual the researchers estimated a dose based on the numbers of abnormal chromosomes above the corresponding MDD and additional consideration was made for those who had received a dose less than the MDD.
The researchers then used this data to estimate an average dose of 310 mSv for the six veterans who served on Rongerik and an average dose of 225 mSv for the other six veterans. There findings were entirely consistent with the film badge data (> 200 mSv) and also with the findings of an associated study (Simon et al, 2019) which used a different approach called reconstruction to estimate dose.
With regards to telomere length, it was found that telomere shortening is largely due to getting older. Smoking reduced the telomere length of veterans who were not exposed to radiation and, radiation reduced the telomere length of non-smokers by approximately the same amount. No evidence for a combination effect of radiation and smoking in shortening telomere length was found.
How did the researchers interpret their basic results?
The researchers claim to have developed a new genetic method for assessing radiation exposure which reliably estimated the average doses received by a group of nuclear test veterans approximately 60 years ago, meaning genetic dose estimation in the absence of physical dose measurements may be possible. However, they acknowledge that their method for estimating historic dose is only supported by this single study and they stated that further research is required to assess the usefulness of their findings.
Who did this research?
This study was done by a team of scientists from Colorado State University in collaboration with other researchers from the University of Texas, the U.S. National Cancer Institute and KromaTiD Inc. This research was funded by Colorado State University, the U.S. National Cancer Institute and KromaTiD.
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A more reliable genetic method, based on using reciprocal translocations and inversions, has been developed to estimate the radiation dose received several years after exposure.
This combined method afforded dose estimates which were similar to those obtained using dosimeter film badges for a group of American nuclear test veterans.
Ageing is the major factor for reducing the length of telomeres.
A single dose of radiation can reduce telomere length as much as a lifetime of chronic smoking.