Induction of reproductive cell death and chromosome aberrations in radioresistant tumour cells by carbon ions.

PURPOSE: To study the induction of reproductive cell death and chromosome aberrations in radioresistant tumour cells exposed to carbon ions in vitro. MATERIALS AND METHODS: X-ray-resistant colon carcinoma cells (WiDr) were used. Confluent G0/G1 cells were irradiated in vitro with graded doses of 100/200/400 MeV u(-1) carbon ions and carbon ions from the middle of a 1 cm extended Bragg peak, and 200 kV X-rays for comparison. Cells were harvested in their first post-irradiation division and aberrations were analysed either by the Giemsa/Hoechst 33258-staining technique or by the fluorescent in-situ hybridization technique involving whole chromosome hybridization and 4',6-diaminido-2-phenylidole (DAPI)-staining. Whole chromosome probes were used for chromosomes 2, 4 and 5, and the chromosome painting patterns were classified according published protocols. Reproductive cell survival was determined by a standard clonogenic assay. RESULTS: With respect to the induction of reproductive cell death and chromosome aberrations, carbon ions of different energies were more effective than 200 kV X-rays. As expected, irradiation in the extended Bragg peak was the most efficient mode. For cell killing, relative biological effectiveness increased with linear energy transfer up to 2.9. The frequencies of total dicentrics and excess acentric fragments as determined in Giemsa-stained cells were higher in cells irradiated with carbon ions than in cells with X-rays. For 100 MeV u(-1) ions, the dose dependence of apparently simple dicentrics as determined for chromosomes 2, 4 and 5 by single-colour fluorescent in-situ hybridization was linear up to 4 Gy, and linear-quadratic for excess acentric fragments and apparently simple translocations. After irradiation with D=4 Gy carbon ions with energy of 100 MeV u(-1) and from the extended Bragg peak, 12 and 54% of cells displayed complex exchanges, respectively. In contrast, after irradiation with D=4 Gy X-rays, only 1% of cells displayed complex aberrations. Hence, the number of cells with complex exchange aberrations increased strongly after irradiation with carbon ions. CONCLUSION: An increased biological efficiency of carbon ions could be confirmed in radioresistant tumour cells with respect to the induction of reproductive cell death and of unstable as well as stable chromosome aberrations. Relative biological effectiveness reached 2.9 for cell killing by carbon ions from the extended Bragg peak. The yields of apparently simple dicentrics as well as of total dicentrics, i.e. simple dicentrics plus dicentrics belonging to complex exchanges, evaluated in Giemsa-stained metaphases as observed in first post-irradiation mitoses were rather low. In contrast, apparently simple translocations displayed yields systematically higher than simple dicentrics in WiDr cells irradiated with either X-rays or 100 MeV u(-1) or Bragg peak carbon ions. Frequencies o f cells containing complex aberrations increased dramatically after carbon ion irradiation, reaching a maximum for ions from the extended Bragg peak.

Role of DNA-dependent protein kinase in the process of radiation-induced aberration formation.

PURPOSE: To further investigate the role of DNA-dependent protein kinase (DNA-PK) and ataxia-telangiectasia mutation (ATM) in the formation of chromosome aberrations, we analysed radiation-induced aberrations in M059J cells, complemented with the PRKDC (DNA-PK) gene by introducing a fragment of human chromosome 8 containing a copy of the human PRKDC gene. One hybrid cell line (M059J Fus1) displayed kinase activity and was radioresistant; the other hybrid cell line (M059J Fus9) showed no kinase activity and was radiosensitive. Both Fus1 and Fus9 cells have only a low ATM activity. Wortmannin, an inhibitor of DNA-PKCS and ATM, was added before irradiation in order to study the effect of DNA-PKCS--and ATM--inhibition on the formation of chromosome aberrations. Furthermore, aberration formation was studied in a lymphoblastoid ATM-deficient cell line AT-1 and in an ATM-proficient control. MATERIALS AND METHODS: Confluent cells were irradiated with 200 kV X-rays. Dicentrics, excess acentric fragments, chromatid/ isochromatid breaks and chromatid exchanges were scored in the absence and presence of wortmannin. RESULTS: In M059J-Fus1 cells and normal lymphoblastoid cells, only chromosome-type aberrations were observed independently of the presence of wortmannin. In DNA-PK-deficient Fus9 cells and in ATM-deficient AT-1 cells, an increasing proportion (30-80%) of cells containing chromatid-type aberrations was observed. This proportion increased with irradiation dose and with wortmannin addition. The aberration yields observed in the complemented M059J-Fus1 cell line were much lower than the corresponding yields observed in the deficient M059J and AT-1 cell lines. However, the low yields observed in the DNA-PK-proficient 'wild-type' cell line M059K were not completely restored. CONCLUSIONS: Since in M059J-Fus1 cells the radioresistant phenotype with respect to chromosome-type aberration formation was restored by the complementation of PRKDC, ATM expression determines the chromosomal radiosensitivity of M059J cells only to a minor extent. The increasing presence of chromatid-type aberrations in cells irradiated in G0/G1 phase as observed either in DNA-PK- or ATM-deficient cells definitely requires the lack of either kinase. Thus, the aberration spectrum observed is determined by the genetic profile of the respective cells and aberration class amplitudes can be modulated by the inhibition of either kinase.

Role of DNA-PK in the process of aberration formation as studied in irradiated human glioblastoma cell lines M059K and M059J.

PURPOSE: The participation of various DNA double-strand break repair mechanisms in the formation of chromosome aberrations is not yet fully understood. To investigate particularly the role of non-homologous end-joining, we analysed the formation of radiation-induced aberrations in a DNA-protein kinase (PK(CS))-proficient cell line M059K and in a deficient cell line M059J. MATERIALS AND METHODS: Plateau-phase M059K and M059J cells were irradiated with low doses of X-rays. Chromosome aberrations were determined as genomic yields of dicentric chromosomes and excess acentric fragments, scored in Giemsa-stained metaphases, and as partial yields of reciprocal translocations and total visible complex exchanges (complex aberrations) for chromosomes 4, 7 and 11 using the FISH method. M059K cells were also analysed in the presence of 50 micro m wortmannin, a DNA-PK inhibitor. RESULTS: DNA-PK-deficient cells showed a higher yield of simple stable and unstable and of complex aberrations in comparison with DNA-PK-proficient cells. The largest differences were observed for excess acentric fragments and for complex aberrations. DNA-PK inhibition by wortmannin in M059-K cells resulted in increased aberration yield in the same qualitative and quantitative manner as in M059-J cells. CONCLUSIONS: The results obtained with DNA-PK-deficient M059J cells and with DNA-PK-proficient M059K cells treated with wortmannin, an inhibitor of DNA-PK and ATM, suggest that the elimination of DNA-PK-dependent non-homologous end-joining can recruit a slow, error-prone repair process, which is DNA-PK independent and favours the increased formation of chromosome aberrations. The nature of this pathway and the way of its participation in aberration formation need further elucidation.