What is the reliability of chromosomal aberration assays as biomarkers of individual sensitivity towards ionising radiation?

Byline: A. Vral, H. Thierens, A. Baeyens, L. De Ridder The development of biomarkers of susceptibility or sensitivity towards ionising radiation is important for the identification of individuals who may be at increased risk for the development of cancer after occupational, environmental or medical exposures. In this study we investigated the inter- and intra-individual variation of the MN assay and the G2 assay in irradiated lymphocytes to assess their suitability as biomarkers of susceptibility. For this, the G2 assay and the MN assay were performed on blood samples of 15 healthy individuals. For the MN assay Go lymphocytes were exposed to 3.5 Gy Co I[sup.3]-rays either at high dose-rate (HDR) or low dose-rate (LDR). For the G2 assay lymphocytes were irradiated with a dose of 0.4 Gy Co I[sup.3]-rays in G2 phase of the cell cycle. Two individuals were assayed nine times each in nine different experiments over a time period of one year. All samples were analysed by two scorers and no significant differences between them were observed using a paired t-test. The repeat experiments on blood samples of the same donor revealed that the inter-experimental/intra-individual coefficients of variation were not significantly different from the inter-individual coefficients of variation in both G2 and MN assay. As the intra-individual variability determines the assay reproducibility this would indicate that the assays are not able to detect real, reproducible differences in radiation sensitivity between normal individuals in the population. The repeat experiments further revealed that for some healthy donors a high value, defined as sensitive taking the 90 percentile as cut-off point to define sensitivity, is obtained only at one time point while the values obtained at the other time points were within the normal range (non-sensitive). Based on this one time point the individual would have been regarded as sensitive. Furthermore a donor identified as radiosensitive with the G2 assay in our initial experiments turned out to be normal when the assay was repeated. To conclude, our results show that a chromosomal aberration assay based on one blood sample may lead to erroneous conclusions with respect to the individual radiosensitivity of workers.