The Genotoxic Effects Of Radiation On Astronauts
Traveling into outer space and seeing the earth from an entirely different perspective sounds so exciting. However, once they leave the earth’s magnetic field, astronauts are exposed to radiation from the sun and other galaxies. This radiation is a made up of high-energy particles, high-energy protons, and neutrons, mesons, and other secondary radiation. Though traveling though outer space sounds cool, the radiation can cause harmful breaks in the DNA molecule, as we shall soon see. In this paper, we will examine the genotoxic effects of radiation through the use of various studies. High LET-radiation (linear energy transfer) is more effective at generating chromosome breakage, highly complex chromosome rearrangements, and cell death than low LET radiation. High LET radiation is the most genotoxic to astronauts because of its dense ionizations. Ionizing radiation can generate base damage in DNA, sugar damage in DNA, double strand breaks, single strand breaks and DNA-DNA and DNA-protein cross links. Double strand DNA breaks are the most important damage caused by radiation. It is currently thought that the quality of the radiation affects the reparability of the double strand DNA breaks. The genotoxic effect of space travel is measured using in vitro chromosomal aberration assays. This includes the mitotic chromosome analysis, in which metaphase cells are examined for chromosomal change (such as exchanges and translocations) after most of the cells have divided once, and interphase chromosome aberration analysis, which uses premature chromosome condensation (PCC) to fuse cells to “inducer cells” that induce the interphase chromosomes to condense in order for them to be examined. FISH (florescence in situ hybridization) painting is a popular way to look at the chromosome. It employs the use of colored probes to identify chromosomes. Each chromosome under examination is painted with a different color and it allows for more precise measurements of exchanges and translocations. The most common way to determine radiation exposure is to look at chromosomal damage in blood lymphocytes. In a study conducted by Kerry George and others (2013), the blood sample of five astronauts was collected before and after flight.
The chromosomes in G2 phase were condensed using modified version of PCC and FISH painting was performed. After performing an in vitro chromosomal aberration assay, it was concluded that the total number of sister chromatid exchanges and translocations in the blood samples increased after each flight. One astronaut had a smaller increase after the second flight, but the other four individuals had a similar or greater increase in chromosomal damage after the second flight when compared with the first. The overall results indicate that space travel is genotoxic. Another study, conducted by Testard and others (1996), also indicated the genotoxic effects of space travel. Cytogenetic analysis of blood samples from seven astronauts was measured. After flight, there were increases in chromosome aberrations such as acentric and dicentric rings. After 2-3 weeks in flights, X-ray equivalent doses were below 20mGy, but after 6 months, the level was 95-455 mGY. It seems the longer they were out in space, the more radiation their cells absorbed. The long-term effects of space travel still need to be further investigated. A second aspect of space travel to take into account, besides radiation, is microgravity, otherwise known as the feeling of “weightlessness.” Scientists have been testing to see if microgravity would affect the cells’ radiosensitivity. In one study, a crewmember’s blood taken 10 days before and 14 days after flight was exposed to gamma radiation. The dose response was similar for pre- and post-flight blood samples. However, in another study the blood samples from one of the astronauts from before and after flight were exposed to X rays and an enhancement in radiosensitivity was shown after comparing the two samples. The results from these two different studies conflict, and more research in this area may be needed. Another study indicated that microgravity could effect how sensitive cells are to genotoxins. In a study conducted by John Pierce Wise Jr. and others (2010), human lung fibroblasts were exposed to a carcinogen and one set was put onto NASA’s Weightless Wonder and another set was kept on the ground. An in vitro chromosomal aberration assay was performed, where they analyzed the amount of chromosomes in metaphase that were damaged. Data showed that, overall, more cell damage occurred in space than on earth. The results suggest that the influence of genotoxic agents increases with less gravitational force. Radioresistance is also observed in astronauts. In a study conducted by Durante and others, the blood samples of 33 astronauts were examined. FISH and giemsa staining were used in the chromosomal aberration assay.
After the first flight, there was an increase in chromosomal aberrations. However, it was observed for astronauts that are involved in two or more space flights their chromosomal aberrations after their last mission was similar to the yield before their first flight. The lack of additive effects of repeated space flights suggests that an increase in exposure could produce radioresistence. This radioresistance could be due to the changes in the immune system produced by microgravity. Chromosomal aberrations, especially stable monocentric chromosomal aberrations, can endure for a long time after flight. A study conducted on 16 astronauts reported that after their flight some astronauts showed a decrease in chromosome damage with the progression of time (220 days after flight and onward). However, these results were not shown for all astronauts and in fact some astronauts showed in increase in chromosome damage as time progressed. Chromosomal damage was measured using a chromosomal aberration assay, in which reciprocal translocations, dicentrics, incomplete translocations, and complex exchanges were counted. In another study, which examined the chromosomes with fish painting, it was observed that for five of six astronauts there was a decline in translocation yields as time progressed since the time of the space flight. Based on the experiments we discussed, there seems there are genotoxic risks involved with being an astronaut. The ionizing radiation from outer space causes DNA damage and chromosomal aberrations. Microgravity also may cause enhanced radiosensitivity, as well as enhanced sensitivity to other genotoxins. Radioresistance has also been observed in some astronauts. Long after their flights, it seems that the chromosome damage seems to decrease for the most part. Today, there are few radiation protectors being used in general, but examples of radiation protectors include cytokines, growth factors, phytochemicals, antioxidant nutrients, and amifostine. However obviously the best way for an average person to protect themselves against these genotoxic effects is to avoid traveling into outer space. Next time you look up at the starry sky and wish you could go into outer space, think about how blessed you are to live down on earth where you are protected against these genotoxic effects.
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