By Avigail Abisror
Ever since I was a young girl, the night sky amazed me. Looking up at the stars and the moon, I used to wish I could see them up close, which is why when I first learned about astronauts, I was awestruck. Being an astronaut is perhaps one of the most challenging jobs in the world and comes along with many risks. While space travel is a relatively new concept that we do not know much about, we have been able to study its effects on human beings, with the conclusion being that there are some damaging results to the astronauts’ DNA. This damage is mainly caused by the astronauts’ exposure to space radiation.
The International Space Station (ISS) is where the astronauts stay when in space and the ionizing radiation (IR) sources, where the ISS orbits, include three primary radiation sources. Galactic cosmic rays (GCRs) range from protons to Fe-ions, solar particle events (SPEs), and electrons and protons trapped in the Van Allen Belts (TPs) outside the spacecraft. This creates a complex radiation environment around the ISS, and inside of it as well (Furukawa et al.,2020). Primary GCRs produce many secondary particles through projectile and target fragmentation in the ISS shielding materials and the bodies of astronauts. The flux of primary TPs increases as the altitude of the ISS increases and so they play a role in either increasing or decreasing the exposure of astronauts to radiation in Low-Earth Orbits (LEO)(Benton and Benton, 2001).
Solar ultraviolet (UV) radiation is part of the natural energy that is produced by the sun and reaches the Earths’ surface. UV has different effects on biological processes and so it is classified as UV-C, UV-B, and UV-A. UV-C does not reach the surface of the earth, as UV-B and UV-A do, as it is eliminated by the stratospheric ozone layer (Singh et al., 2017). Although sunlight is beneficial for life on earth, it does still contain a harmful amount of UV-B radiation which causes damage to important cellular components, such as DNA, RNA, protein, and lipids (Britt, 1996). DNA, which stores genetic information, has its structure directly altered by UV radiation. Cyclobutane pyrimidine dimers (CPDs) are the main UV-induced photoproducts and account for approximately 75% of DNA damage (Sancar, 2004). The environment of space consists of much short-wavelength solar UV radiation, among a variety of different types of radiation and so astronauts are exposed to a very large amount of space radiation. In fact, UV-C is much more prevalent in space, and at a higher intensity which is quite dangerous for human beings.
Radiation-induced DNA damage includes base damage, single-strand breaks (SSBs), and double-strand breaks (DSBs). DSBs are the most severe and if not repaired correctly, cell death, cellular senescence, and tumorigenesis may occur (Sankaranarayanan et al.,2013). It is important to consider the energy of radiation when exposed to it in space versus on earth. When one is exposed to radiation on the ground, the radiation levels are at low-LET (linear energy transfer) and include X-rays and y-rays. GCR on the other hand contains high-LET radiation such as energetic protons and heavy particle beams, i.e., HZE particles. (Ohnishi and Ohnishi, 2004) High-LET radiation exposure induces complex DNA damage as it leads to dense ionization along the radiation tracks of such particles. These regions of damage are referred to as complex/clustered DNA damage (lesion) and when compared to normal DNA damage, they are much more difficult to repair (Rydberg 2001), So, even if one were to be exposed to the same amount of radiation in space as on the ground, the quality and amount of DNA damage that occurs will be different. Clustered DNA damage induced by high-LET radiation exposure is detected using the comet assay or agarose gel electrophoresis.
Chromosomal Aberrations (CAs) are used as cytogenetic biomarkers for exposure to IR and other DNA-damaging agents, and the frequency of CAs in peripheral lymphocytes may be associated with the risk of cancer. CAs have been analyzed in spacecraft crews since the 1960s and the frequency of total CAs is higher at postflight than at preflight- usually when the flights are longer than 180 days (Maalouf et al.,2011). Studies using FISH- fluorescence in situ hybridization- painting revealed that HZE particles frequently induce highly complex chromosomal rearrangements when compared with the effect of low-LET IR (George et al.,2013). Carcinogenesis is a major concern for future space missions, as the missions will probably be for longer durations and the longer one is exposed to radiation, the more dangerous it becomes. The astronauts on such missions will be constantly exposed to IR from natural radiation sources. HZE-charged particles are part of the radiation field in space and as it is carcinogenic, HZE particle irradiation promotes more aggressive cancers, such as increased growth rate, transcriptomic signatures, and metastasis (Barcellos-Hoff and Mao, 2016).
Aside from the risk of cancer that space radiation causes, NASA began focusing on the risk to the astronauts’ Central Nervous System (CNS). Although the brain is largely a radioresistant organ, ground-based animal studies have indicated that space radiation alters neuronal tissue and neuronal functions such as excitability, synaptic transmission, and plasticity. HZE particles have also been demonstrated to inhibit neuronal connectivity, neuronal proliferation, neuronal differentiation and to change glial characterization (Cekanaviciute et al.,2018). However, the long-term effects of higher-dose-rate exposure to radiation are unknown as researchers only observed the response to short-term higher-dose-rate exposure to radiation.
In conclusion, radiation is very damaging to human beings’ DNA. Normally, one is not as exposed as our Earth has a protective magnetic field that keeps extremely damaging radiation away from us. However, astronauts go past that protective barrier and are exposed to that damaging radiation. The results are a more intensely damaged DNA that is harder to repair. This leads to life-threatening diseases such as aggressive cancers, and also affects the Central Nervous System which consists of the spinal cord and the brain — which is the body’s most complex organ. All in all, we must be extremely grateful to the women and men who put their lives at risk so that they can help keep our Earth healthy and safe by studying it from a different angle.
Ionizing radiation and genetic risks. XVII. Formation mechanisms underlying naturally occurring DNA deletions in the human genome and their potential relevance for bridging the gap between induced DNA double-strand breaks and deletions in irradiated germ cells – PubMed (nih.gov)