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      Low-dose hyper-radiosensitivity of progressive and regressive cells isolated from a rat colon tumour: Impact of DNA repair

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          Evidence for a lack of DNA double-strand break repair in human cells exposed to very low x-ray doses.

          DNA double-strand breaks (DSBs) are generally accepted to be the most biologically significant lesion by which ionizing radiation causes cancer and hereditary disease. However, no information on the induction and processing of DSBs after physiologically relevant radiation doses is available. Many of the methods used to measure DSB repair inadvertently introduce this form of damage as part of the methodology, and hence are limited in their sensitivity. Here we present evidence that foci of gamma-H2AX (a phosphorylated histone), detected by immunofluorescence, are quantitatively the same as DSBs and are capable of quantifying the repair of individual DSBs. This finding allows the investigation of DSB repair after radiation doses as low as 1 mGy, an improvement by several orders of magnitude over current methods. Surprisingly, DSBs induced in cultures of nondividing primary human fibroblasts by very low radiation doses (approximately 1 mGy) remain unrepaired for many days, in strong contrast to efficient DSB repair that is observed at higher doses. However, the level of DSBs in irradiated cultures decreases to that of unirradiated cell cultures if the cells are allowed to proliferate after irradiation, and we present evidence that this effect may be caused by an elimination of the cells carrying unrepaired DSBs. The results presented are in contrast to current models of risk assessment that assume that cellular responses are equally efficient at low and high doses, and provide the opportunity to employ gamma-H2AX foci formation as a direct biomarker for human exposure to low quantities of ionizing radiation.
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            Cancer risks attributable to low doses of ionizing radiation: assessing what we really know.

            High doses of ionizing radiation clearly produce deleterious consequences in humans, including, but not exclusively, cancer induction. At very low radiation doses the situation is much less clear, but the risks of low-dose radiation are of societal importance in relation to issues as varied as screening tests for cancer, the future of nuclear power, occupational radiation exposure, frequent-flyer risks, manned space exploration, and radiological terrorism. We review the difficulties involved in quantifying the risks of low-dose radiation and address two specific questions. First, what is the lowest dose of x- or gamma-radiation for which good evidence exists of increased cancer risks in humans? The epidemiological data suggest that it is approximately 10-50 mSv for an acute exposure and approximately 50-100 mSv for a protracted exposure. Second, what is the most appropriate way to extrapolate such cancer risk estimates to still lower doses? Given that it is supported by experimentally grounded, quantifiable, biophysical arguments, a linear extrapolation of cancer risks from intermediate to very low doses currently appears to be the most appropriate methodology. This linearity assumption is not necessarily the most conservative approach, and it is likely that it will result in an underestimate of some radiation-induced cancer risks and an overestimate of others.
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              Genetic heterogeneity of single disseminated tumour cells in minimal residual cancer.

              Because cancer patients with small tumours often relapse despite local and systemic treatment, we investigated the genetic variation of the precursors of distant metastasis at the stage of minimal residual disease. Disseminated tumour cells can be detected by epithelial markers in mesenchymal tissues and represent targets for adjuvant therapies. We screened 525 bone-marrow, blood, and lymph-node samples from 474 patients with breast, prostate, and gastrointestinal cancers for single disseminated cancer cells by immunocytochemistry with epithelial-specific markers. 71 (14%) of the samples contained two or more tumour cells whose genomic organisation we studied by single cell genomic hybridisation. In addition, we tested whether TP53 was mutated. Hierarchical clustering algorithms were used to determine the degree of clonal relatedness of sister cells that were isolated from individual patients. Irrespective of cancer type, we saw an unexpectedly high genetic divergence in minimal residual cancer, particularly at the level of chromosomal imbalances. Although few disseminated cells harboured TP53 mutations at this stage of disease, we also saw microheterogeneity of the TP53 genotype. The genetic heterogeneity was strikingly reduced with the emergence of clinically evident metastasis. Although the heterogeneity of primary tumours has long been known, we show here that early disseminated cancer cells are genomically very unstable as well. Selection of clonally expanding cells leading to metastasis seems to occur after dissemination has taken place. Therefore, adjuvant therapies are confronted with an extremely large reservoir of variant cells from which resistant tumour cells can be selected.
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                Author and article information

                Journal
                International Journal of Radiation Biology
                International Journal of Radiation Biology
                Informa UK Limited
                0955-3002
                1362-3095
                July 03 2009
                January 2008
                July 03 2009
                January 2008
                : 84
                : 7
                : 533-548
                10.1080/09553000802195331
                © 2008

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