Scenario       Cosmic rays are one of the main hazards for long-term manned space missions. Astronauts/cosmonauts are exposed to various types of high energy charged particles of the galactic or solar origin. They include electrons, positrons, protons or heavier ions of high energy. However, the cosmic ray flux around the space crafts are significantly different from those in free space since the primary cosmic rays are decelerated by and bombarding the structure of the space crafts, and hence producing a variety of secondary particles inside the space crafts. The radiation environment of the astronauts is thus considerably differs depending on space activity, e.g., missions inside the space craft or extra-vehicle activity. Heavy ion bombardment may produce one-third of the total neutron flux, and in the International Space Station (ISS: image by Wikipedia 2015), it has been estimated that approximately 30 % of the dose received by the astronauts comes from the albedo neutrons and secondary neutrons created by interactions in the shielding materials. The neutron energies should extend from thermal energies up to a few GeV. The total radiation environment is thus completely different from that we can experience, naturally or experimentally, on the earth. Moreover, the biological response is also complex, for instance, like photons, neutrons can activate DNA repair pathways (adaptive response) as well at low doses while high LET charged particles do not. The health consequences of hits by HZE particles not clear. The health effects associated with the manned space exploration in the mixed radiation field and their quantitative evaluation is the major concern in the current space radiobiology. Chromosome aberration analysis has been carried out in the cosmonauts/astronauts to meet this requirement.

References
     Yang, T. C., George, K., Johnson, A. S., Durante, M. and Fedorenko, B. S. (1997): Biodosimetry results from space flight Mir-18. Radiat. Res., S17-S23.

Chromosome aberration analysis
     Blood samples were taken from two crew members involved in a 115-day MIR-18 mission before after flight. Chromosomes were analyzed by FISH painting technique. Chromosome aberrations significantly increased in post-flight samples as compared to samples drawn prior to flight. SCE levels were also studied, but they were not influenced by the flight. Pre-flight samples were also tested for the dose-response to gamma-irradiation in vitro. The data on these crews are included in the next file (George et al. 2001) as preliminary data.

Reference
     George, K., Durante, M., Wu, H., Willingham, V., Badhwar, G and Cucinotta, F. A. (2001): Chromosome aberrations in the blood lymphocytes of astronauts after space flight. Radiat. Res., 156:731-738.

Chromosome aberration analysis (conventional Giemsa method)
      Chromosome aberrations were studied by FISH painting method for 6 crew members involved in the long-term NASA/MIR mission and 2 crew memebrs of a short-term shuttle mission. Except for crew #1 and shuttle members (#7, #8), translocation frequencies were significantly increased in the post-flight samples as compared to the pre-flight samples.

Reference
     George, K., Rhone, J., Beitman, A. and Cucinotta, F. A. (2013): Cytogenetic damage in the blood lymphocytes of astronauts: effects of repeat long-duration space mission. Mutation Res., 756:165-169.

Chromosome aberration analysis
     Chromosome aberration analyses were carried out by FISH painting method on 5 crew members involved in the ISS mission for before and after consequtive long-term mission. The 1st post-flight samples were collected 1-2 weeks after the mission, and for the most crews a follow-up samples were collected 6-18 months later. Part of blood samples was irradiated in vitro with gamma-rays to see the dose-response relationships amenable to use aberration-based biological dosimetry.

Figure constructed by the Editor: Translocation frequencies against TLD doses recieved by crews during the respective flights.

Reference
     George, K., Willingham, V. and Cucinotta, F. A. (2005): Stability of chromosome aberrations in the blood lymphocytes of astronauts measured after space flight by FISH chromosome painting. Radiat. Res., 164:474-480.
     George, K., Chappell, L. J. and Cucinotta, F. A. (2010): Persistence of space radiation induced cytogenetic damage in the blood lymphocytes of astronauts. Mutation Res., 701:75-79..

Chromosome aberration analysis
      To seen a stability of chromosome aberrations frequencies in the astronauts, the follow-up measurements of types and frequencies of chromosome aberrations were carried out by FISH-painting technology in the post-flight consecutive blood samples taken from 6 astronauts involved in long-term MIR missions. Significant increase in the chromosome aberrations was observed in the post-flight samples as compared to the pre-flight samples.
     However, the aberration frequencies, even of the stable-type exchanges, tended to decrease with the increase of the post-flight time lapse (in days or years). This observation raises problem of the complexity of the use of FISH translocation assay for retrospective biodosimetry.

Figures reconstructed from Table 1 of George et al. (2005). Unstable exchanges apear to decrease more rapidly than stable exchanges.

Commentary (Editorial):
     Reciprocal translocations have been generally considered nonlethal aberrations, and for this reason attempt has been made to use translocations in blood T-lymphocytes for retrospective biological dosimetry. They include cells with only stable-type rearrangements, Cs-cells, or the genome-equivalent frequencies of translocations, F_G-Tr, extrapolated from those involved in HISH painted chromosomes, F_p-Tr, as surrogate markers. However, recently cytogenetic data, including this paper on astronauts, have been accumulated to indicate that they also decline over time after radiation exposure, and raised a question on the validity of their use in retrospective biological dosimetry.
     The transient decline of cells with stable-type aberrations, like unstable ones, has been well documented in persons acutely irradiated to moderate-to-high doses of radiation, such as in radiotherapy and radiation accidents, in which the doses are non-homogeneous and thereby (a) recovery of lymphocyte populations directly influence on the frequencies of aberrant cells, whether they are of the stable- or unstable-type.
    There are many other possibilities, of which Gardner and Tucker (2002) have recently reviewed. They include (b) co-occurrence of unstable-type aberrations, such as dicentrics, rings, minutes, deletions, etc., which escape the detection by whole chromosome painting; (c) lethal effects of translocations, DNA loss at the junction of translocation during DNA DSB end-rejoining, and position effects of translocation; (d) similarly the DNA loss may occur at the junction of DSB end-joining resulting in the morphological restitution of breaks. Incidentally, no significant increase in complex and intrachromosomal exchanges have been found in post-flight lymphocytes of astronauts involved in short- or long-term space flight (Horstmann et al. 2005). In addition, (e) the effects of blunting immune response and its recovery, which are often the case in cancer bearing patients and astronauts engaged in long-term space missions. In cancer patients received radiotherapy, the lymphocyte response to PHA dramatically recovers by washing lymphocytes by culture medium prior to culture, and moreover this stimulatory recovery is accompanied by an increase of chromosome aberration frequencies (Matsubara et al. 1980). The mechanism of the immune suppression of cancer bearing and long-term space flight is not clear. But, it may or may not be relevant to the poor response of astronaut’s lymphocyte to PHA and decline of aberration frequencies in lymphocytes. The issue remains to be studied.

Literature cited
     Gardner, S. N. and Tucker, J. D. (2002): The cellular lethality of radiation-induced chromosome translocations in human lymphocytes. Radiat. Res., 157:539-552.
     Horstmann, M., Durante, M., Jahannes, C., Pieper, R. and Obe, G. (2005): Space radiation does not induce a significant increase of intrachromosomal exchanges in astronauts’ lymphocytes. Radiat. Environ. Biophys., 44:219-224.
     Matsubara, S., Horiuchi, J., Shibuya, H. and Sasaki, M. S. (1980): Effects of washing on phytohemagglutinin responsiveness of lymphocytes from irradiated patients. Acta Radiol. Oncol., 19:45-54.