A three-color chromosome painting technique was used to examine the spontaneous and radiation-induced chromosomal damage in peripheral lymphocytes and lymphoblastoid cells from 11 patients with ataxia telangiectasia (AT) and from 14 individuals heterozygous for an AT allele. In addition, cells from two homozygous and six obligate heterozygous carriers of mutations in the Nijmegen breakage syndrome gene (NBS) were investigated. The data were compared to those for chromosome damage in 10 unaffected control individuals and 48 cancer patients who had not yet received therapeutic treatment. Based on the well-documented radiation sensitivity of AT and NBS patients, it was of particular interest to determine whether the FISH painting technique used in these studies allowed the reliable detection of an increased sensitivity to in vitro irradiation of cells from heterozygous carriers. Peripheral blood lymphocytes and lymphoblastoid cells from both the homozygous AT and NBS patients showed the highest cytogenetic response, whereas the cells from control individuals had a low number of chromosomal aberrations. The response of cells from heterozygous carriers was intermediate and could be clearly differentiated from those of the other groups in double-coded studies. AT and NBS heterozygosity could be distinguished from other genotypes by the total number of breakpoints per cell and also by the number of the long-lived stable aberrations in both AT and NBS. Only AT heterozygosity could be distinguished by the fraction of unstable chromosome changes. The slightly but not significantly increased radiosensitivity that was found in cancer patients was apparently due to a higher trend toward rearrangements compared to the controls. Thus the three-color painting technique presented here proved to be well suited as a supplement to conventional cytogenetic techniques for the detection of heterozygous carriers of these diseases, and may be superior method.

Clastogenic factors were first described in the plasma of people who had been accidentally or therapeutically irradiated. They were found also in A-bomb survivors, where they persisted for many years after the irradiation. The present study searched for these factors in the plasma of 32 civil workers from Armenia, who had been engaged as "liquidators" around the Chernobyl atomic power station in 1986. It also included 15 liquidators who had emigrated from the ex-Soviet Union to Israel. Reference plasma samples were obtained from 41 blood donors from the Armenian Blood Center in Yerevan. The samples were tested for their clastogenic activity in blood cultures from healthy donors. The majority of results from the liquidators exceeded those from the unexposed reference samples. The samples from the first Armenian group, with the higher average irradiation dose (0.6 +/- 0.6 Gy), were more clastogenic than those from the second group exposed to 0.2 +/- 0.2 Gy. The number of aberrations in the test cultures was 17.9 +/- 2.9% and 10.5 +/- 3.8% respectively, compared to 5.7 +/- 3.2% in the cultures exposed to the reference ultrafiltrates from Armenian blood donors. The samples from the Israeli liquidators also induced significantly increased aberration rates (14.0 +/- 3.9% aberrant cells). The clastogenic activity was regularly inhibited by superoxide dismutase, indicating that the chromosome-damaging effects of radiation-induced clastogenic factors are exerted via the intermediation of superoxide radicals, as is known for clastogenic factors of different origin.

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For the optimal use of anticancer drugs a knowledge of the whole spectrum of side-effects is required. A potential hazard, so far only scarcely investigated, is uncontrolled effects of drugs such as bleomycin (BLM) and mitomycin C (MMC) on telomere shortening in non-cancerous tissues of the treated person. For the first time, directly labelled telomere-specific peptide nucleic acid (PNA) hybridization probes were applied in comet-FISH to detect DNA fragmentation on an intermediate scale. The effects of BLM and MMC were measured in peripheral blood cells of three human volunteers, following ex vivo incubation. Fragmentation of telomeres and subtelomeric regions was highly specifically detected by the comet-FISH assay, a combination of the comet assay and fluorescence in situ hybridization. As a technical detail, the effects of the hybridization procedure have been studied on the level of single comets. Image analysis before and after the hybridization process reveals a small decrease in the detected fragmented DNA, probably due to diffusion of small fragments. It could not only be shown that both drugs actually induce breaks in telomere-associated DNA, but also that the comet-FISH technique, as a quantitative approach, is a useful tool for the detection and evaluation of the role of sequence-specific DNA damage after mutagenic action. The breakage frequency for DNA of or adjacent to telomeric repeats was found to be proportional to that of the total DNA, which hints at random induction of DNA breaks by BLM and MMC. In terms of therapy, the results indicate that no over- or under-proportional effects on telomeres of BLM or MMC need be expected.