Bound PCNA in nuclei of primary rat tracheal epithelial cells after exposure to very low doses of plutonium-238 alpha particles

Bystander effects from ionizing radiation have been detailed for a number of cell systems and a number of end points. We wished to use a cell culture/ex vivo rat model of respiratory tissue to determine whether a bystander effect detected in culture could also be shown in a tissue. Examination by immunofluorescence techniques of tracheal cell cultures after exposure to very low doses of alpha particles revealed a large proportion of cells with proliferating cell nuclear antigen (PCNA) bound in their nuclei. PCNA was selected as an end point because it is involved in both DNA repair and the changes in cell cycle that are typical of many reported bystander effects. Maximum response can be detected in up to 28% of the cells in sub-confluent cultures with a dose of only 2 mGy. At this dose less than 2% of the cell nuclei have experienced a particle traversal and less than 6% of the cells have experienced an alpha-particle traversal through either their nucleus or some part of their cytoplasm. The hypothesis that this bystander response in nontargeted cells is mediated through secreted factor(s) is presented, and supporting evidence was found using partial irradiation and co-culture experiments. Examination of the effect with excised pieces of trachea demonstrated a response similar to that seen in culture.

Is the increased relative biological effectiveness of high LET particles due to spatial or temporal effects? Characterization and OER in V79-4 cells

The efficiency of producing biological damage varies with radiation quality. Conventional explanations rely on spatial differences in the radiation track structure; generally however there are also very large temporal differences in delivery of the radiation at the cellular level. High-LET radiation normally deposits substantial amounts of energy by individual heavily ionizing tracks on a timescale of the order of picoseconds. By contrast each low-LET radiation track deposits a small amount of energy. Many of these tracks, distributed over the whole cell, are required to deliver an equivalent dose to a high-LET track and they are usually delivered over much longer timescales (typically seconds) during which chemical, biochemical and biological processes are occurring. In this paper the design, characterization and initial application of a high-brightness, laser-plasma ultrasoft x-ray source is described. This has been used to investigate the importance of the temporal differences by irradiating mammalian cells with an energy deposition with spatial properties of low-LET radiation and temporal properties similar to high-LET radiation. The present system delivers a typical dose, to the incident surface of the cells, of 0.12 Gy per pulse delivered in <10 ps. The capabilities of the x-ray source were tested by determining the survival of V79-4 hamster cells irradiated with picosecond pulses of ultrasoft x-rays under aerobic and anaerobic conditions, which were found to be consistent with previously published non pulsed data with x-rays of similar energy. These results support the expectation that the disappearance of an oxygen effect for high-LET radiation particles is due to their spatial properties rather than the very short timescale of each particle traversal. For other effects, particularly non-targeted phenomena such as induced genomic instability, expectations may be less clear cut.

Long-term genomic instability in human lymphocytes induced by single-particle irradiation

Recent evidence suggests that genomic instability, which is an important step in carcinogenesis, may be important in the effectiveness of radiation as a carcinogen, particularly for high-LET radiations. Understanding the biological effects underpinning the risks associated with low doses of densely ionizing radiations is complicated in experimental systems by the Poisson distribution of particles that can be delivered. In this study, we report an approach to determine the effect of the lowest possible cellular radiation dose of densely ionizing alpha particles, that of a single particle traversal. Using microbeam technology and an approach for immobilizing human T-lymphocytes, we have measured the effects of single alpha-particle traversals on the surviving progeny of cells. A significant increase in the proportion of aberrant cells is observed 12-13 population doublings after exposure, with a high level of chromatid-type aberrations, indicative of an instability phenotype. These data suggest that instability may be important in situations where even a single particle traverses human cells.

Dose- and time-response relationships for lethal mutations and chromosomal instability induced by ionizing radiation in an immortalized human keratinocyte cell line

PURPOSE

To investigate the relationship between two well-established delayed effects of ionizing radiation, experiments were conducted to determine the induction and expression of lethal mutations (delayed reproductive death) and chromosomal instability with respect to dose and time in a human immortalized keratinocyte cell line.

METHODS

HPV-G cells were gamma- or alpha-irradiated and maintained in culture for up to 72 population doublings. At intervals, measurements were made of cloning efficiency and the cells examined for apoptosis and cytogenetic aberrations.

RESULTS

The descendants of cells surviving 1 or 3 Gy gamma-irradiation, but not 0.5 Gy gamma-irradiation, exhibited a reduced colony-forming efficiency. The reduction persisted at a constant rate of 15-20% clonogenic cell loss per population doubling for up to 72 population doublings. Apoptosis was demonstrated in all colonies in the 1 and 3 Gy groups at 30 and 72 population doublings post-irradiation but not in the 0.5 Gy group. A significant persistent reduction in colony-forming ability (approximately 80%) was demonstrated in the progeny of cells irradiated with 0.5 Gy alpha-particles. After 30 population doublings, the proportion of chromosomally aberrant cells was significantly greater than control values for all doses of both high- and low-LET radiations. The major cytogenetic aberrations (chromatid breaks, chromosome fragments and minutes) were consistent with the transmission of chromosomal instability. The expression of instability declined between 30 and 72 population doublings in the 0.5 Gy and 3 Gy gamma-irradiation groups, but persisted up to 72 population doublings in the 1 Gy group. The expression of chromosomal instability was greater in the descendants of alpha-irradiated cells and showed little evidence of reduction with time.

CONCLUSIONS

Unstable aberrations characteristic of radiation-induced chromosomal instability may commonly result in apoptosis and account for a component of the delayed reproductive death/lethal mutation phenotype in HPV-G cells. However, the absence of lethal mutations in the descendants of 0.5 Gy gamma-irradiated cells indicates a low-LET threshold effect for this particular endpoint. Overall, and particularly at low doses, there is no direct correlation between the two endpoints, indicating the absence of a simple relationship between these manifestations of radiation-induced genomic instability.

Complex chromosome aberrations in peripheral blood lymphocytes as a potential biomarker of exposure to high-LET alpha-particles

PURPOSE

To determine the induction and transmission, to second and third division cells, of complex chromosome aberrations in peripheral blood lymphocytes after exposure to high-LET alpha-particles in vitro.

MATERIALS AND METHODS

Separated peripheral blood lymphocytes collected from four healthy donors were irradiated in vitro with either high-LET alpha-particles (121 keV/microm; 0.5 Gy) or low-LET X-rays (250kV constant potential; 3 Gy). Cells were harvested in first, second and third division post-irradiation and chromosome aberrations observed at each cell division were analysed by combining the techniques of FISH and DAPI/Hoechst 33258 harlequin staining. Whole chromosome probes were used for chromosomes 1, 2 and 5, together with a pan-centromeric probe and the resulting chromosome 'painting' patterns were classified according to the Savage and Simpson (S & S) scheme (Savage and Simpson 1994a, Savage and Tucker 1996).

RESULTS

A greater proportion of complex chromosome aberrations was observed, defined as involving three or more breaks in two or more chromosomes, relative to total exchanges, after exposure to 0.5 Gy alpha-particles (mean 1 track/cell) than after the high reference dose of 3 Gy X-rays (49-56% and 20-22%, respectively). Qualitatively, alpha-particles induced chromosome aberrations of far greater complexity than those observed after X-rays. This was reflected by both the rapid reduction in the percentage of damaged cells between first and second division indicative of cell death, and the spectrum of aberration types observed in second and third division cells post-irradiation. Regardless of this complexity, 15% of the complexes induced by alpha-particles at first division were potentially transmissible and by third division, transmissible-type complexes, specifically insertions, represented the predominant complex type (65%).

CONCLUSION

Transmissible-type complexes were observed, specifically insertions, in both second and third division cells after exposure to high-LET alpha-particles (0.5 Gy) in vitro. The authors predict these cells to be stable and to be capable of persisting through many cell generations. Considering that the induction of complex chromosome aberrations by low-LET radiation is strongly dependent on dose, so that they are expected to be undetectable at environmental exposures, insertions are much more likely to be a characteristic feature of high-LET radiation at all doses. From this the usefulness of insertions as biomarkers of past exposure to environmentally relevant doses of high-LET alpha-particles is supported.

Radiation-induced chromosomal instability in human fibroblasts: temporal effects and the influence of radiation quality

PURPOSE

To determine whether chromosomal instability is induced in human diploid fibroblasts by ionizing radiation and to investigate the effects of radiation quality by comparing X-rays, neutrons and alpha-particles.

MATERIALS AND METHODS

Cells from two human diploid fibroblast lines, HF12 and HF19, were irradiated and analysed cytogenetically at 3, 20 and 35 population doublings post-irradiation.

RESULTS

Exposure of HF19 cells to neutrons and alpha-particles resulted in a consistently increased frequency of unstable aberrations, particularly chromatid-type aberrations, compared to control cultures. Aberration frequency after X-irradiation was not significantly greater than controls at 20 population doublings but was significantly increased after 35 population doublings, although not to the same level as that following neutron or alpha-irradiation. No chromosomal instability was demonstrated in the progeny of HF12 cells after X-, neutron or alpha-particle irradiation.

CONCLUSIONS

The data are consistent with the progeny of irradiated HF19 cells expressing chromosomal instability. All three radiations are effective in inducing instability, but the expression of the phenotype is influenced by radiation quality. The absence of radiation-induced chromosomal instability in HF12 cells may reflect the influence of genetic factors.

The effect of dimethyl sulfoxide on the induction of DNA double-strand breaks in V79-4 mammalian cells by alpha particles

The present study was undertaken to assess the protective effect of dimethyl sulfoxide (DMSO) against the induction and rejoining of DNA double-strand breaks (DSBs) and inactivation of V79-4 Chinese hamster cells by both high- and low-linear energy transfer (LET) radiations. The cells were exposed under aerobic conditions as monolayers to either low-LET photons (60Co gamma rays) or high-LET alpha particles (238Pu) at 277 K. The initial yield of DSBs, determined by elution under nondenaturing conditions, is linearly dependent on dose. When the irradiation was carried out in the presence of DMSO (0-0.6 mol dm-3), the initial yields of DSBs induced by both gamma and alpha-particle irradiation decrease. With gamma irradiation at [DMSO] > 0.6 mol dm-3, a further decrease in the yield of DSBs occurs. DMSO (0.5 mol dm-3) reduces the initial yield of DSBs by 50 +/- 5% and 32 +/- 4% for photons and alpha particles, respectively. DMSO protects more effectively against cellular inactivation and DSB induction at low LET compared with alpha-particle irradiation with protection factors of 1.7 and 1.4, respectively, for survival and 2.0 and 1.5, respectively, for DSBs. After incubation of the irradiated cells for 3 h at 310 K after high-LET irradiation, the residual yield of DSBs is reduced by < 13% when the irradiations were carried out in the presence of 0.5 mol dm-3 DMSO. With gamma irradiation in the presence of 0.5 mol dm-3 DMSO, 90% of the DSBs are rejoined by 3 h incubation at 310 K. Therefore, the nonscavengeable DSBs induced by alpha particles are not significantly rejoined within 3 h, in contrast to rejoining of the majority of the nonscavengeable DSBs induced by gamma irradiation. From comparison of the data on DSBs and survival for alpha-particle irradiation, it is inferred that the severity of damage is reduced by DMSO through minimizing the formation of OH-induced sugar/base modifications in the vicinity of nonscavengeable DSBs.

Frequencies of complex chromosome exchange aberrations induced by 238Pu alpha-particles and detected by fluorescence in situ hybridization using single chromosome-specific probes

We undertook an analysis of chromosome-type exchange aberrations induced by alpha-particles using fluorescence in situ hybridization (FISH) with whole chromosome-specific probes for human chromosomes 1 or 4, together with a pan-centromeric probe. Contact-inhibited primary human fibroblasts (in G1) were irradiated with 0.41-1.00 Gy 238Pu alpha-particles and aberrations were analysed at the next mitosis following a single chromosome paint. Exchange and aberration painting patterns were classified according to Savage and Simpson (1994a). Of exchange aberrations, 38-47% were found to be complex derived, i.e. resulting from three or more breaks in two or more chromosomes, and the variation with dose was minimal. The class of complex aberrations most frequently observed were insertions, derived from a minimum of three breaks in two chromosomes. There was also an elevated frequency of rings. The high level of complex aberrations observed after alpha-particle irradiation indicates that, when chromosome domains are traversed by high linear energy transfer alpha-particle tracks, there is an enhanced probability of production of multiple localized double-strand breaks leading to more complicated interactions.

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.

Interleukin 7-dependent B lymphocyte precursor cells are ultrasensitive to apoptosis

We have compared the sensitivity of clonogenic interleukin 7 (IL-7)-dependent murine B cell precursors with that of clonogenic mature B cells and myeloid precursors to alpha-particles from plutonium-238 and X radiation. All three populations are relatively sensitive, but B cell precursors are ultrasensitive. This differential sensitivity is also observed with corticosteroid, etoposide, and cisplatin, all apoptosis-inducing drugs used in the treatment of leukemia and other cancers. Further, we show that x-rays and drugs induce the bulk of the B cell precursor population to undergo rapid apoptosis, despite the continued presence of IL-7. B cell precursors were found to express very low levels of BCL-2 protein compared with mature splenic B cells and their resistance to x-rays and corticosteroid could be enhanced by expression of a BCL-2 transgene. These data have important implications for normal lymphopoiesis and for the behavior of leukemic lymphoid precursor cells.

Nuclear area measurement on viable cells, using confocal microscopy

We describe a rapid procedure for the accurate measurements of nuclear areas on unperturbed living cells as used in radiobiological experiments, using the confocal laser scanning microscope. The microdosimetric interpretation of radiobiological data requires precise information on the nuclear area of cells as irradiated with high-LET radiation.

Transmission of chromosomal instability after plutonium alpha-particle irradiation

When investigating the biological effects of ionizing radiation on the haemopoietic system, a confounding problem lies in possible differences between the biological effects of sparsely ionizing, low linear energy transfer radiation such as X-, beta- or gamma-rays, and densely ionizing, high linear energy transfer radiation such as alpha-particles. To address this problem we have developed novel techniques for studying haemopoietic cells irradiated with environmentally relevant doses of alpha-particles from a plutonium-238 source. Using a clonogenic culture system, cytogenetic aberrations in individual colonies of haemopoietic cells derived from irradiated stem cells have been studied. Exposure to alpha-particles (but not X-rays) produced a high frequency of non-clonal aberrations in the clonal descendants, compatible with alpha-emitters inducing lesions in stem cells that result in the transmission of chromosomal instability to their progeny. Such unexpected instability may have important implications for radiation leukaemogenesis.

Lightning strikes

Lightning strike is a natural phenomenon with potentially devastating effects. The physics of lightning strike and the physiology of lightning injury are discussed. Three cases are reported and the clinical features are described. Aspects of the management of such patients are discussed with particular emphasis on assessment and resuscitation in the Emergency Department.