Chromosome Translocations, Inversions and Telomere Length for Retrospective Biodosimetry on Exposed U.S. Atomic Veterans

Abstract

It has now been over 60 years since U.S. nuclear testing was conducted in the Pacific islands and Nevada, exposing military personnel to varying levels of ionizing radiation. Actual doses are not well-established, as film badges in the 1950s had many limitations. We sought a means of independently assessing dose for comparison with historical film badge records and dose reconstruction conducted in parallel. For the purpose of quantitative retrospective biodosimetry, peripheral blood samples from 12 exposed veterans and 12 age-matched (>80 years) veteran controls were collected and evaluated for radiation-induced chromosome damage utilizing directional genomic hybridization (dGH), a cytogenomics-based methodology that facilitates simultaneous detection of translocations and inversions. Standard calibration curves were constructed from six male volunteers in their mid-20s to reflect the age range of the veterans at time of exposure. Doses were estimated for each veteran using translocation and inversion rates independently; however, combining them by a weighted-average generally improved the accuracy of dose estimations. Various confounding factors were also evaluated for potential effects on chromosome aberration frequencies. Perhaps not surprisingly, smoking and age-associated increases in background frequencies of inversions were observed. Telomere length was also measured, and inverse relationships with both age and combined weighted dose estimates were observed. Interestingly, smokers in the non-exposed control veteran cohort displayed similar telomere lengths as those in the never-smoker exposed veteran group, suggesting that chronic smoking had as much effect on telomere length as a single exposure to radioactive fallout. Taken together, we find that our approach of combined chromosome aberration-based retrospective biodosimetry provided reliable dose estimation capability, particularly on a group average basis, for exposures above statistical detection limits.

FIG. 1

Directional genomic hybridization (dGH). Representative images of metaphase spreads labeled with dGH whole chromosome 1, 2 and 3 paints (red) and counter stained with DAPI (blue). Panel A: A normal metaphase spread free of any structural rearrangements. dGH chromosome paints uniformly label a single sister chromatid of a chromosome. Panel B: An inversion (double signal switch; yellow arrow) on chromosome 2. Panel C: Translocation involving chromosome 3 and a second, unpainted chromosome (white arrows).

FIG. 2

Calibration (dose-response) curves for young adult males; blood samples were exposed in vitro (¹³⁷Cs γ rays). The linear equations were y = 0.4037x + 0.3162 for inversions (blue) and y = 0.1281x + 0.022 for translocations (red). R-squared values were 0.9098 and 0.9091 for inversions and translocations, respectively. Error bars represent the standard deviation.

FIG. 3

Influence of age on background frequencies of inversions (blue) and translocations (red).

FIG. 4

Influence of smoking on background frequencies of inversions (blue) and translocations (red). The ratio of aberration rates between smokers and non-smokers in the veteran age-matched control cohort was used to obtain the proportional change in inversions and translocations. Error bars represent the standard error of the mean (SEM).

FIG. 5

Dose estimates and 90% confidence intervals for the Nevada Test Site, Pacific Proving Ground and Rongerik Atoll veterans. Each group of three estimations separated by a vertical dashed grid line represents a single individual. Dose estimates (Gy) for inversions (gold) and translocations (gray) were established independently. A combined estimate (red) was obtained by weighting inversion and translocation frequencies by their own inverse variance.

FIG. 6

Relative telomere length in cohorts based on age and radiation exposure; young adults (red) and veterans (blue). Values between bars represent percentage change between groups. Error bars represent SEM.

FIG. 7

Relative telomere length and smoking status and/or radiation exposure in the veteran cohorts. Error bars represent SEM.

FIG. 8

Relative telomere length as a function of weighted combined chromosome aberration dose among exposed veterans. The linear equation is y = −0.0004x + 1.0324 and R² = 0.1599.