Radioresistance

Radioresistance is the level of ionizing radiation that organisms are able to withstand.

Ionizing-radiation-resistant organisms (IRRO) were defined as organisms for which the dose of acute ionizing radiation (IR) required to achieve 90% reduction (D10) is greater than 1000 gray (Gy) [1]

Radioresistance is surprisingly high in many organisms, in contrast to previously held views. For example, the study of environment, animals and plants around the Chernobyl disaster area has revealed an unexpected survival of many species, despite the high radiation levels. A Brazilian study in a hill in the state of Minas Gerais which has high natural radiation levels from uranium deposits, has also shown many radioresistant insects, worms and plants.[2][3] Certain extremophiles, such as the bacteria Deinococcus radiodurans and the tardigrades can withstand acute doses of ionizing radiation on the order of 5,000 Gy.[4][5][6]

Induced radioresistance

Radioresistance may be induced by exposure to small doses of ionizing radiation. Several studies have documented this effect in yeast, bacteria, protozoa, algae, plants, insects, as well as in in vitro mammalian and human cells and in animal models. Several cellular radioprotection mechanisms may be involved, such as alterations in the levels of some cytoplasmic and nuclear proteins and increased gene expression, DNA repair and other processes.

Many organisms have been found to possess a self-repair mechanism that can be activated by exposure to radiation in some cases. Two examples of this self-repair process in humans are described below.

Devair Alves Ferreira received a large dose (7.0 Gy) during the Goiânia accident, and lived, whereas his wife, who got a dose of 5.7 Gy, died. The most likely explanation is that his dose was fractionated into many smaller doses which were absorbed over a length of time while his wife stayed in the house more and was subjected to continuous irradiation without a break so giving the self repair mechanisms in her body less time to repair some of the damage done by the radiation. This resulted in her death. He also eventually died. In the same way some of the persons who worked in the basement of the wrecked Chernobyl have built up doses of 10 Gy, these workers received these doses in small fractions so the acute effects were avoided.

It has been found in radiation biology experiments that if a group of cells are irradiated then as the dose increases the number of cells which survive decrease. It has also been found that if a population of cells are given a dose before being set aside (without being irradiated) for a length of time before being irradiated again then the radiation has less of an ability to cause cell death. The human body contains many types of cells and a human can be killed by the loss of a single tissue in a vital organ. For many short term radiation deaths (3 days to 30 days) the loss of cells forming blood cells (bone marrow) and the cells in the digestive system (wall of the intestines) cause death.

In the graph below, a dose/survival curve for a hypothetical group of cells has been drawn with and without a rest time for the cells to recover. Other than the recovery time partway through the irradiation, the cells would have been treated identically.

Inheritance of radioresistance

There is strong evidence that radioresistance can be genetically determined and inherited, at least in some organisms. Heinrich Nöthel, a geneticist from the Freie Universität Berlin carried out the most extensive study about radioresistance mutations using the common fruit fly, Drosophila melanogaster, in a series of 14 publications.

Radioresistance in radiation oncology

Radioresistance is also a term sometimes used in medicine (oncology) for cancer cells which are difficult to treat with radiotherapy. Radioresistance of cancer cells may be intrinsic or induced by the radiation therapy itself.

Radioresistance comparison

The comparison below is meant to give an indication of radioresistance for different species. There are generally big differences in radioresistance between experiments due to small number of specimens, as well as being unable to control the testing environment (for example, the calculations for humans was determined from the Hiroshima and Nagasaki bombings of WWII).

Lethal radiation doses (Gray)
Organism Lethal dose LD50 LD100 Class/Kingdom
Dog   3.5 (LD50/30 days)[7]   Mammals
Human 4–10[8] 4.5[9] 10[10] Mammals
Rat   7.5   Mammals
Mouse 4.5–12 8.6–9   Mammals
Rabbit   8 (LD50/30 days)[7]   Mammals
Tortoise   15 (LD50/30 days)[7]   Reptile
Goldfish   20 (LD50/30 days)[7]   Fish
Escherichia coli 60   60 Bacteria
German cockroach   64[8]   Insects
Shellfish   200 (LD50/30 days)[7]   -
Fruit fly 640[8]     Insects
C. elegans∗   160-200 [11] ≫ 500-800[12][13] Nematode
Amoeba   1,000 (LD50/30 days)[7]   -
Braconidae 1,800[8]     Insects
Milnesium tardigradum 5,000[14]     Eutardigrade
Deinococcus radiodurans 15,000[8]     Bacteria
Thermococcus gammatolerans 30,000[8]     Archaea

∗ While an LD50 has been reported for wild type C. elegans individuals, an upper lethal limit has not been established, rather "nearly all animals were alive with no indication of excess lethality up to 800 Gy, the highest dose... measured."[13]

See also

Notes and references

  1. ↑ Sghaier, H., Ghedira, K., Benkahla, A., and Barkallah, I. (2008) Basal DNA repair machinery is subject to positive selection in ionizing-radiation-resistant bacteria. BMC Genomics 9: 297.
  2. ↑ Cordeiro, AR; Marques, EK; Veiga-Neto, AJ (1973). "Radioresistance of a natural population of Drosophila willistoni living in a radioactive environment". Mutation research 19 (3): 325–9. doi:10.1016/0027-5107(73)90233-9. PMID 4796403.
  3. ↑ Moustacchi, E (1965). "Induction by physical and chemical agents of mutations for radioresistance in Saccharomyces cerevisiae". Mutation research 2 (5): 403–12. doi:10.1016/0027-5107(65)90052-7. PMID 5878261.
  4. ↑ Moseley BEB; Mattingly A (1971). "Repair of irradiated transforming deoxyribonu- cleic acid in wild type and a radiation- sensitive mutant of Micrococcus radiodu- rans". J. Bacteriol 105 (3): 976–83. PMC 248526. PMID 4929286.
  5. ↑ Murray RGE. 1992. The family Deino- coccaceae. In The Prokaryotes, ed. A Ballows, HG Truper, M Dworkin, W Harder, KH Schleifer 4:3732–44. New York: Springer-Verlag
  6. ↑ Ito H; Watanabe H; Takeshia M; Iizuka H (1983). "Isolation and identification of radiation-resistant cocci belonging to the genus Deinococcus from sewage sludges and animal feeds. Agric". Biol. Chem. 47: 1239–47. doi:10.1271/bbb1961.47.1239.
  7. 1 2 3 4 5 6 Radiochemistry and Nuclear Chemistry, G. Choppin, J-O. Liljenzin and J. Rydberg, edition three, page 481, ISBN 0-7506-7463-6
  8. 1 2 3 4 5 6 "Cockroaches & Radiation". Retrieved 2006-05-13.
  9. ↑ "Radiation Notes: Radiation Damage and Dose Measurement". Retrieved 2006-05-13.
  10. ↑ "CDC Radiation Emergencies, Acute Radiation Syndrome: A Fact Sheet for Physicians".
  11. ↑ Hartman, P; Goldstein, P; Algarra, M; Hubbard, D; Mabery, J (1996). "The nematode Caenorhabditis elegans is up to 39 times more sensitive to gamma radiation generated from 137Cs than from 60Co". Mutat Res 363 (3): 201-208. doi:10.1016/0921-8777(96)00012-2. PMID 8765161.
  12. ↑ Weidhaas, J.B.; Eisenmann, D.M.; Holub, J.M.; Nallur, S.V. (2006). "A Caenorhabditis elegans tissue model of radiation-induced reproductive cell death". Proc Natl Acad Sci USA 103 (26): 9946-51. doi:10.1073/pnas.0603791103. PMC 1502559. PMID 16788064.
  13. 1 2 Krisko, A.; Magali, L.; Radman, M.; Meselson, M. (2012). "Extreme anti-oxidant protection against ionizing radiation in bdelloid rotifers". Proc Natl Acad Sci USA 109 (7): 2354–2357. doi:10.1073/pnas.1119762109. PMC 3289372. PMID 22308443.
  14. ↑ Horikawa DD; Sakashita T; Katagiri C; Watanabe M; et al. (2006). "Radiation tolerance in the tardigrade Milnesium tardigradum". International Journal of Radiation Biology 82 (12): 843–8. doi:10.1080/09553000600972956. PMID 17178624.

Further reading

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