Frequency of BCL-2/J_H translocation in healthy males exposed to low-level radiation in comparison to age-matched healthy controls

The t(14;18) translocation has been detected by cytogenetic and molecular techniques in about 90% of follicular lymphomas, 50% of adult undifferentiated lymphomas, and 20% of diffuse large-cell lymphomas. Based on Southern blot and polymerase chain reaction (PCR) analysis, 50% to 60% of follicular lymphomas carry t(14;18)-MBR-translocations.1 The occurrence of t(14;18) is not restricted to malignant lymphoma: 50% to 60% of healthy individuals were found to carry t(14;18)-MBR–positive cells detectable by sensitive PCR techniques.2-4 Therefore, the number of circulating t(14;18)-positive cells might serve as an indicator for environmental exposure to carcinogens and possibly correlate with the cumulative risk of developing t(14;18)-positive non-Hodgkin lymphoma (NHL).5 Since ionizing radiation can induce chromosomal translocations in human cells in vitro, for example, t(9;22) and t(8;21),6 we initiated this study to investigate whether exposure to low-level radiation has an influence on the frequency as well as the total number of circulating t(14;18)-positive cells in healthy individuals, that is, the generation of the translocation as well as clonal expansion or progression.7 

At the occasion of routine health checks, peripheral blood samples were obtained with informed consent from 131 healthy male employees working at the local nuclear power plant (NPP) in Lubmin, Germany. They were grouped according to their total cumulative radiation dose exposure: group A, < 50 mSv (n = 61); B, 50-100 mSv (n = 34); C, 100-200 mSv (n = 24); and D, 200-400 mSv (n = 12). Serving as controls were 131 age-matched male healthy volunteers with no previous radiation exposure. DNA was extracted from peripheral blood mononuclear cells (PBMNCs) by standard procedures, quantitated spectrophotometrically, and stored at −80°C.

The real-time quantitative PCR technique used for the detection ofBCL-2-MBR/IgH -rearrangements has been described in detail.8 At least 5 1.0 μg aliquots of cellular DNA (∼5 × 140 000 cells) isolated from each blood sample were tested for the presence of the t(14;18) translocation. To determine the total number of cells tested, 3 0.1 μg DNA aliquots were quantitatively analyzed for K-ras. All amplification products were analyzed by agarose gel electrophoresis for the presence of several t(14;18) DNA fragments; 28 amplification products were selected for nucleotide sequence analysis because they revealed t(14;18)-DNA fragments of a quite similar size. In no case were identical sequences found.

The frequency of BCL-2-MBR/J_H translocation in peripheral blood lymphocytes from healthy males working at a nuclear power plant was 60.3% (79/131), which is statistically not different from the frequency of 55% (72/131) in age-matched healthy controls (Table 1). At least 700 000 cells (∼5 μg cellular DNA) were tested per individual based on the quantitative determination of the K-ras gene. There were no statistical differences between the frequency of BCL-2/J_H translocation observed in the 4 groups of NPP workers defined by increasing cumulative radiation exposure as well as between the frequency in these 4 groups and age-matched healthy controls (Table 1). Furthermore, the incidence of more than one t(14;18)-positive cell clone was about the same in both study populations. The quantitative determination of circulating t(14;18)-positive cells (Figure1) revealed no significant differences between NPP workers and age-matched healthy controls.

Based on these results it may be concluded that low-level radiation exposure up to 400 mSv has no significant effect on the frequency of the t(14;18) translocation as well as the total number of circulating t(14;18)-positive cells in the individuals studied. At present, possible effects of higher cumulative doses cannot be excluded.