Cryopreserved Stem Cells Incur Damages Due To Terrestrial Cosmic Rays Impairing Their Integrity Upon Long-Term Storage

Stem cells have the capacity to ensure the renewal of tissues and organs. They could be used in the future for a wide range of therapeutic purposes and are preserved at liquid nitrogen temperature to prevent any chemical or biological activity up to several decades before their use. We show that the cryogenized cells accumulate damages coming from natural radiations, potentially inducing DNA double-strand breaks (DSBs). Such DNA damage in stem cells could lead to either mortality of the cells upon thawing or a mutation diminishing the therapeutic potential of the treatment. Many studies show how stem cells react to different levels of radiation; the effect of terrestrial cosmic rays being key, it is thus also important to investigate the effect of the natural radiation on the cryopreserved stem cell behavior over time. Our study showed that the cryostored stem cells totally shielded from cosmic rays had less DSBs upon long-term storage. This could have important implications on the long-term cryostorage strategy and quality control of different cell banks.

Neutron irradiation affects the expression of genes involved in the response to auxin, senescence and oxidative stress in Arabidopsis

We report, in Arabidopsis thaliana plants, the effect of neutron irradiation on the transcription of a set of genes belonging to different physiological groups: auxin action, senescence, oxidative stress, and some aspects of photosynthesis. The results indicated that, in the wild-types, the effect on the ARF1, ARF2, and 19 genes was of down-regulation, whereas of the tested AUX/IAA only AUX/IAA7 showed up-regulation. Different results were obtained as regards the irradiation of the auxin transport mutants aux1 and eir1, because in these cases the ARF genes were up-regulated, whereas AUX/IAA7 was down-regulated in eir1. On the other hand, the senescence activated genes SAG12 and SAG13, and those connected to oxidative stress were up-regulated in the wild-type, but down-regulated in aux1. The gene CAB1, connected to photosynthesis, was also down-regulated in the wild-type, but up-regulated in aux1. Gene expression recovered in many cases almost to the initial condition in a time lapse of 24 hours, even though some effect persisted for a longer time. In particular, the state of juvenility of arf2 was extended by irradiation, whereas, in all the other cases, senescence was accelerated. The research indicates that through mutant selection or genetic engineering a true possibility exists of producing organism more suitable for life in space.

Relative biological effectiveness of 6 MeV neutrons with respect to cell inactivation and disturbances of the G1 phase

The relative biological effectiveness (RBE) of neutrons and other types of densely ionizing radiation appears to be close to 1.0 for the induction of strand breaks, but considerably higher RBEs have been found for cellular end points such as colony-forming ability. This may be due to differences in the processing of strand breaks or to the involvement of other lesions whose yields are more dependent on radiation quality. Because cell cycle delays may be of great importance in the processing of DNA damage, we determined the RBE for disturbances of the G1 phase in four different cell types (Be11 melanoma, 4197 squamous cell carcinoma, EA14 glioma, GM6419 fibroblasts) and compared them with the RBE for cell inactivation. The method we used to determine the progress from G1 into S was as follows: Cells were serum-deprived for a number of days and then stimulated to grow with culture medium containing normal amounts of serum. Immediately before the change of medium, cells were exposed to graded doses of either 240 kV X rays or 6 MeV neutrons. At different times afterward, cells were labeled with BrdU and the numbers of active S-phase cells were assessed using two-parameter flow cytometry. For all four cell types, cells started to progress from G1 into S after a few hours. Radiation suppressed this process in all cases, but there were some interesting differences. For Be11 and 4197 cells, the most obvious effect was a delay in G1; the labeling index increased a few hours later in irradiated samples than in controls, and there was no significant effect on the maximum labeling index. For EA14 and GM6419 cells, although smaller doses were used because of greater radiosensitivity, a delay of the entry into S phase was again noticeable, but the most significant effect was a reduction in the maximum percentage of active S-phase cells after stimulation, indicating a permanent or long-term arrest in G1. The RBE for the G1 delay was the same for all four cell types, about 2.8, while the RBE for the G1 arrest varied between 3.2 for the most resistant Be11 cells and 1.7 for the most sensitive GM6419 cells. This trend was similar to that observed for the RBE for cell inactivation. If, as described above, the same number of strand breaks per dose is induced by neutrons and by X rays, the signal transduction cascade translates them into a greater G1 delay in the case of higher LET. This appears to be independent of repair capacity, because it is similar in all cell types we investigated. We therefore assume that a higher lesion density or the presence of other types of lesions is important for this relatively early effect. A G1 arrest, however, is more closely related to the later events leading to cell inactivation, where strand break repair does play a major role, influencing X-ray sensitivity more strongly than sensitivity to neutrons because of a lower repairability of lesions induced by higher-LET radiation.

Differential gene expression in a DNA double-strand-break repair mutant XRS-5 defective in Ku80: analysis by cDNA microarray

The ability of cells to rejoin DNA double-strand breaks (DSBs) usually correlates with their radiosensitivity. This correlation has been demonstrated in radiosensitive cells, including the Chinese hamster ovary mutant XRS-5. XRS-5 is defective in a DNA end-binding protein, Ku80, which is a component of a DNA-dependent protein kinase complex used for joining strand breaks. However, Ku80-deficient cells are known to be retarded in cell proliferation and growth as well as other yet to be identified defects. Using custom-made 600-gene cDNA microarray filters, we found differential gene expressions between the wild-type and XRS-5 cells. Defective Ku80 apparently affects the expression of several repair genes, including topoisomerase-I and -IIA, ERCC5, MLH1, and ATM. In contrast, other DNA repair-associated genes, such as GADD45A, EGR1 MDM2 and p53, were not affected. In addition, for large numbers of growth-associated genes, such as cyclins and clks, the growth factors and cytokines were also affected. Down-regulated expression was also found in several categories of seemingly unrelated genes, including apoptosis, angiogenesis, kinase and signaling, phosphatase, stress protein, proto-oncogenes and tumor suppressors, transcription and translation factors. A RT-PCR analysis confirmed that the XRS-5 cells used were defective in Ku80 expression. The diversified groups of genes being affected could mean that Ku80, a multi-functional DNA-binding protein, not only affects DNA repair, but is also involved in transcription regulation. Our data, taken together, indicate that there are specific genes being modulated in Ku80- deficient cells, and that some of the DNA repair pathways and other biological functions are apparently linked, suggesting that a defect in one gene could have global effects on many other processes.

Relative contributions of levels of initial damage and repair of double-strand breaks to the ionizing radiation-sensitive phenotype of the Chinese hamster cell mutant, XR-V15B. Part II. Neutrons

We have compared DNA double-strand break (dsb) induction and rejoining, using field-inversion gel electrophoresis, with survival in mutant (XR-V15B) and in wild-type parental (V79B) hamster cell lines after low dose neutron and X-irradiation. We found that neutrons do not appear to induce more dsbs than X-rays and deduce that increased sensitivity to neutrons is therefore not due to a higher initial yield of dsbs. Even with low doses of neutrons, there is a visible increase in the production of a smaller subset of DNA fragments which arise only after very high dose X-irradiation. In both cell lines, dsbs induced by neutrons are rejoined more slowly than those induced by X-rays. At long repair times (4 and 17 h) there are no significant differences in the fractions of unrejoined dsbs between neutrons and X-rays. We propose that neutron-induced dsbs have a higher probability of becoming lethal because they are more likely to be misrepaired during the slow stage of rejoining.