No worse than an atomic war
Biologists from Russia have found out why small doses of radiation rarely cause cancer
Biologists from the institutes of the Russian Academy of Sciences, MSU and MIPT have found out that stem cells often survive gradual radiation exposure and do not turn into cancer (so in the text – VM) due to the fact that they slow down their division cycle and use this delay to repair DNA, according to an article published in the journal Oncotarget (Tsvetkova et al., yH2AX, 53BP1 and Rad51 protein foci changes in mesenchymal stem cells during prolonged X-ray irradiation).
"Nuclear disasters such as Chernobyl or Fukushima, lead to the fact that many people are constantly exposed to small and medium doses of radiation. And although observations of the consequences of the explosion of atomic bombs show that exposure to large doses of radiation significantly increases the risk of cancer, how "slow" radiation affects the body remains a matter of debate," write Sergey Leonov from MIPT in Dolgoprudny and his colleagues.
Humans and other living beings react extremely negatively to radiation exposure for the reason that ionizing radiation directly breaks DNA chains or indirectly "breaks" them, generating many chemically aggressive substances when interacting with the contents of the cell. They make many cells believe that they are irreversibly damaged, which causes their mass death and leads to the death of the organism as a whole.
For quite a long time, scientists believed, as the authors of the article say, that cancer occurs as a result of the appearance of mutations in the "adult" cells of the body, forgetting their role and functions and beginning to multiply uncontrollably when serious "typos" appear in DNA. Studies of the last ten years show that this is far from the case in most cases, and that many types of cancer, in fact, arise as a result of mutations in stem cells, small colonies of which are present in almost all tissues of the body.
Trying to understand how such cancers develop, Leonov and his colleagues observed how colonies of stem cells extracted from human gums react to approximately the same doses of radiation, which scientists irradiated them either instantly or stretched for several days or weeks.
As the scientists expected, the instantaneous irradiation killed a large number of cells and gave rise to many double and single DNA breaks in them, which they tried to repair with the help of two gamma proteins-H2AX and 53BP1. The concentration of their molecules increased along with the dose of radiation and quickly reached the level at which the cell can no longer cope with DNA breakdown and dies due to the appearance of many errors inside it.
A completely different picture was observed with "slow" irradiation of cells. Initially, the concentration of these proteins also increased, but after about four hours it reached a certain limit, above which it no longer rose. This means that the number of breaks in the DNA stopped growing, despite the fact that the cells were still bombarded with ionizing radiation.
This unusual discovery led scientists to look for reasons why cells either stopped fighting mutations, or somehow managed to reduce the frequency of their occurrence. To find an answer to this question, Leonov and his colleagues monitored the concentration of another protein, Rad51, which is involved in repairing DNA only during direct cell division and the doubling of its chromosomes.
As it turned out, the concentration of Rad51 began to grow with prolonged irradiation of cells, which revealed the trick that stem cells use to protect themselves from radiation. Its essence is simple – the phase of mitosis, cell division, continues for an unusually long time under irradiation, which allows Rad51 and two other "repairman" proteins to repair all the gaps in DNA before the cell divides into two parts.
This unusual property of stem cells can not only explain why scientists have not yet found an unambiguous link between the development of cancer and life in the vicinity of Fukushima and Chernobyl, but also reveal the mechanism of the appearance of certain types of tumors. Further study of the behavior of stem cells, scientists hope, will help to understand whether this is really the case and find possible ways to protect stem cells from radiation.
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28.07.2017