Monitoring the genetic health of humans accidentally exposed to ionizing radiation of Cesium-137 in Goiânia (Brazil)




Da Cruz, Aparecido Divino

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This thesis describes a long-term study in which the genetic health of a population accidentally exposed to ionizing radiation of cesium-137. The Goiânia (Brazil) radiological accident of September 1987 involved 249 individuals exposed to doses up to 7 Gy, and included four fatalities. We have investigated the genetic effects of radiation exposure in this population using both cytogenetic and molecular endpoints in T-lymphocytes. The micronucleus assay differentiated between groups exposed to different levels of ionizing radiation. At the molecular level two methods were employed: (1) the hprt clonal assay; and (2) the determination of microsatellite instability. The hprt assay involves in vitro culturing of T-cells and the selection of 6-thioguanine-resistant hprt mutant clones which were then characterized at the molecular level using both RT-PCR and genomic analysis. Exposure to ionizing radiation initially elevated the mutation frequency but this effect gradually diminished, so that 4.5 years no significant increase was observed. This limitation makes the hprt T-cell assay unsuitable for the study of long term past exposure. Analysis of the spectrum of hprt mutations recovered from 10 individuals exposed to relatively high doses of ionizing radiation revealed a significant increase (3.8-fold) in the frequency of A:T → G:C mutations in the exposed group. This increase in A:T → G:C transitions is consistent with the effects of ionizing radiation in prokaryotes and lower eukaryotes and likely reflects the mispairing of radiation-induced thymine, glycol with guanine. In addition, a two-fold increase in the frequency of deletions not readily explained by slippage events and hence which may reflect ionizing radiation-induced DNA strand breakage was also observed. Microsatellite instability was also investigated. Fluorescent PCR and automated DNA sequencer analysis, using genomic DNA from mononuclear cells, were used to investigate the frequency of microsatellite alterations in exposed and non-exposed populations. We examined a total of 200 and 190 alleles respectively and found that the microsatellite instability distribution in the two groups were not different. Our assay lacked sufficient sensitivity to discriminate between spontaneous and induced microsatellite instability and it is, therefore, not suitable for population monitoring. Finally, despite the minimal database, we used the micronucleus and hprt mutant frequency data to estimate the risk associated with radiation exposure for the Goiânia population. The estimated genetic risk for the exposed group was approximately a 24-fold increase in dominant disorders in the first post-exposure generation. Moreover, the risk of carcinogenesis in this population was estimated to be increased by a factor in the range of 1.4 to 1.5 compared to the population at large.



Ionizing radiation, Radiation, Physiological effect