Cellular and Molecular Effects of Radon and Other Alpha Particle Emitters

Incidence of cytogenetic damage in lead-zinc mine workers exposed to radon

The purpose of this study was to detect cytogenetic damage in mine workers working in a lead-zinc mine, which could be associated with a combined exposure to radon and heavy metals. Our study involved 70 mine workers from the lead-zinc mine. We used peripheral blood lymphocytes as the target material. The total share of structural chromosome aberration (SCA) decreased significantly over the 3 years of monitoring, from 5.08/200 analyses of metaphases in 1995 to 3.28 in 1997, owing to the decrease in exposure during the process of mine closure. The share of SCA was significantly different from the group of local people, who had never worked in the mine (1.43), as well as from the control group of Slovene residents (1.88). The share of micronuclei (MN) in mine workers also decreased in the monitored period, from 14.65/500 cytokinesis-blocked cells in 1995 to 11.77 in 1997, while the sister chromatic exchange (SCE) level did not change much (from 8.105/50 analysed cells in 1995 to 7.73 in 1997). Owing to the closure activities, the received concentrations of contaminants were falling constantly, particularly concentrations of radon. This was particularly evident in the level of SCA and the MN incidence, while the SCE values remained nearly on the same level. This indicates that the incidence of SCE is probably more strongly influenced by heavy metals than by radon.

Biological effects of naturally occurring and man-made fibres: in vitro cytotoxicity and mutagenesis in mammalian cells

Cytotoxicity and mutagenicity of tremolite, erionite and the man-made ceramic (RCF-1) fibre were studied using the human-hamster hybrid A(L) cells. Results from these fibres were compared with those of UICC Rhodesian chrysotile fibres. The A(L) cell mutation assay, based on the S1 gene marker located on human chromosome 11, the only human chromosome contained in the hybrid cell, has been shown to be more sensitive than conventional assays in detecting deletion mutations. Tremolite, erionite and RCF-1 fibres were significantly less cytotoxic to A(L) cells than chrysotile. Mutagenesis studies at the HPRT locus revealed no significant mutant yield with any of these fibres. In contrast, both erionite and tremolite induced dose-dependent S1- mutations in fibre-exposed cells, with the former inducing a significantly higher mutant yield than the latter fibre type. On the other hand, RCF-1 fibres were largely non-mutagenic. At equitoxic doses (cell survival at approximately 0.7), erionite was found to be the most potent mutagen among the three fibres tested and at a level comparable to that of chrysotile fibres. These results indicate that RCF-1 fibres are non-genotoxic under the conditions used in the studies and suggest that the high mesothelioma incidence previously observed in hamster may either be a result of selective sensitivity of hamster pleura to fibre-induced chronic irritation or as a result of prolonged fibre treatment. Furthermore, the relatively high mutagenic potential for erionite is consistent with its documented carcinogenicity.

Genetic, cytogenetic, and carcinogenic effects of radon: a review

Radon exposure has been linked to lung carcinogenesis in both human and animal studies. Studies of smoking and nonsmoking uranium miners indicate that radon alone is a risk factor for lung cancer at the levels encountered by these miners, although the possibility exists that other substances in the mine environment affect the radon-induced response. The relevance of data from mines to the lower-exposure home environment is often questioned; still, a recent study of miners exposed to relatively low radon concentrations demonstrated a statistically significant increase for lung and laryngeal cancer deaths. In two major series of experiments with rats, the primary carcinogenic effect found was respiratory tract tumors, and evidence for an inverse exposure-rate effect was also noted. Although this inverse dose-rate effect also has been described in underground miner studies, it may not similarly apply to radon in the home environment. This observation is due to the fact that, below a certain exposure, cells are hit once or not at all, and one would not expect any dose-rate effect, either normal or inverse. Because some chromosome aberrations persist in cycling cells as stable events, cytogenetic studies with radon are being performed to help complete the understanding of the events leading to radon-induced neoplasia. Radon has been found to induce 13 times as much cytogenetic damage (as measured by the occurrence of micronuclei) than a similar dose of 60Co. A wide variety of mutation systems have demonstrated alpha-particle mutagenesis; recent investigations have focused on the molecular basis of alpha-induced mutagenesis. Gene mutations are induced by radon in a linear and dose-dependent fashion, and with a high biological effect relative to low-LET irradiation. Studies of the hprt locus show that approximately half of the alpha-induced mutations arise by complete deletion of the gene; the remaining mutations are split between partial deletions, rearrangements, and events not detectable by Southern blot or PCR exon analysis. Although other mutation systems do not show the same spectra as observed in the hprt gene (suggesting that the gene environment affects response), DNA deletions or multilocus lesions of various size appear to be predominant after radon exposure. As data emerge regarding radon-induced changes at the chromosomal and molecular level, the mechanisms involved in radon carcinogenesis are being clarified. This information should increase the understanding of risk at the low exposure levels typically found in the home.

Lethality and mutagenesis of B lymphocyte progenitor cells following exposure to alpha-particles and X-rays

B lymphocyte precursor cells are the target cells for the major subtype of paediatric cancer, acute lymphoblastic leukaemia. Using a murine IL-7-dependent clonogenic assay for normal B cell precursors as a model, we have investigated the sensitivity of these cells versus other normal and leukaemic haemopoietic cells to alpha-particle radiation. We find that B cell precursors are remarkably susceptible to the lethal effects of alpha-particles and have a very low probability of surviving a single alpha-track. B cell precursors are also very sensitive to the lethal effects of low LET X-rays. The mutation frequency in a marker gene (HPRT) does not, however, appear to be greater in B cell precursors that survive X-radiation than in other haemopoietic cells.