Ionizing radiation can produce different types of damage to DNA, RNA, proteins and other biomolecules. DNA is the major target of radiation induced damage where as membrane is an alternative target. Because the proportion of water in living matter is quite high, radiolytic product of the water, mainly hydroxyl radical is responsible for most damages to biomolecules. Hydroxyl radical causes damages to biomolecules by abstracting an H-atom from the biomolecules (from the sugar moiety of the DNA or from the peptide chain of a protein) or by addition of the double bonds of aromatic moieties (DNA bases or aromatic moieties of protein side chain.
Ionizing radiation causes the formation of strand breaks in cellular DNA, as well as other types of lesions in the chromatin of cells. The amount of DNA damage induced is determined by the type of radiation as well as the presence of other molecular components in close proximity to DNA, in particular the presence of proteins because it is well known that most molecular interactions between proteins and DNA occur via amino acids. It is estimated that each Gray (Gy) of radiation leads to about 100,000 ionizations within a cell, damage to over 1,000 bases, about 1,000 SSBs and about 20 – 40 DSBs. Despite this, 1 Gy kills only 30% of mammalian cells due to the effectiveness of DNA repair - particularly for non-DSB (double strand break) lesions.
Lipid peroxidation has been found as the main type of damage to membrane lipids and lipoproteins. Ionizing radiation induced lipid oxidative modifications of poly unsaturated fatty acids (PUFAs) appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products, lipid hydroperoxides and conjugated dienes which are early products of lipid peroxidation.
During ionizing radiation induced damage to protein, the type of reactions and consequences are quite similar to those of DNA; abstraction of H atoms and binding to aromatic rings, leading to backbone breakage and modification of side chains. All these events lead to peptide chain fragmentation and modification of amino acid side chain (e.g. Trp/ formylkynurenine, Tyr/ bityrosine, Cys/ disulfide).
Radiosensitivity is the relative susceptibility of cells, tissues, organs or organisms to the harmful effect of ionizing radiation. Cells are least sensitive when in the S phase, then the G1 phase, then G2 phase and the most sensitive in the M phase of the cell cycle. This is described by the law of Bergonié and Tribondeau, formulated in 1906, ‘X-rays are more effective on cells which have a greater reproductive activity’.
From their observation, they concluded that quickly dividing tumor cells are generally more sensitive than the majority of body cells. This is not always true. Tumor cells can be hypoxic and therefore less sensitive to X-rays that mediate most of their effects through free radicals produced by ionizing oxygen. Later it has been shown that the most sensitive cells are those that are undifferentiated, well nourished, divide quickly and are highly metabolically active. Amongst the body cells, the most sensitive are spermatogonia and erythroblasts, epidermal stem cells, gastrointestinal stem cells. The least sensitive are nerve cells and muscle fibers.Very sensitive cells are also oocytes and lymphocytes, although they are resting cells and do not meet the criteria described above. The reasons for their sensitivity are not clear.
Relative radiosensitivities of various tissues/ organs are as follow;
High radiosensitivity: Bone marrow, lymphoid organs, blood, testes, ovaries, intestines.
Fairly High radiosensitivity: Skin and other organs with epithelial cell lining (such as oral cavity, esophagus, rectum, bladder, cornea, vagina, uterine cervix etc).
Moderate radiosensitivity: Stomach, optic lens, growing cartilage, fine vasculature, growing bone.
Fairly Low radiosensitivity: Salivary glands, mature cartilage or bones, kidneys, liver, pancreas, respiratory organs, thyroid, adrenal and pituitary glands
Low radiosensitivity: Spinal cord, muscle, brain.
Radiation induced damage of the cell can be lethal' (the cell dies),potentially lethal (cells can repaired if allowed to remain in the stationary phase for some time after irradiation), and sub-lethal (the cell can repair itself) .
i. Lethal damage: This is irreversible and irreparable and leads to cell death.
ii. Potentially lethal Damage: Component of radiation damage that can be modified by post-irradiation environmental conditions. Damage considered being potentially lethal since under ordinary circumstances leads to cell death. The proposed mechanism says if cells were maintained in sub-optimal conditions; do not have to attempt mitosis while chromosomes are expressing radiation-induced injury. This delay leads to repair of the DNA damage and increased survival.
iii. Sub-lethal Damage: Under normal circumstances this can be repaired in hours, usually considered to be complete within 24 h. If additional sub-lethal damage added within this time then can interact to form lethal damage. Sub-lethal damage repair observed as an increase in survival if a dose of radiation is split into 2 equal fractions separated by a time interval.
Stochastic effects: Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects. Stochastic effects are coincidental and cannot be avoided. They don't have a threshold. These can be divided into somatic and genetic. For stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. Example: Cancer, Leukemia, Genetic Effects, Cataracts.
Deterministic effects (Nonstochastic Effects): Deterministic effects have a threshold of irradiation under which they do not appear and are the necessary consequence of irradiation. The damage they cause depends on the doses.
Whole body exposures between 1 Gy and 2 Gy causes NVD syndrome characterized by nausea, vomiting and diarrhea. This can also cause anorexia, giddiness and loss of appetite. Lethal exposures above this dose can cause radiation syndromes. Doses in the range of 2-6 Gy causes hematopoietic syndrome. Approximate time of death after radiation varies between 10 to 30 days. During this syndrome, hematopoietic tissues such as spleen, thymus and bone marrow get affected. Bone marrow is the most important tissue containing the stem cells, which give rise to all types of functional cells in blood. Depression in bone marrow cells results in severe anemic conditions and makes the animal extremely susceptible to infections. The platelet depletion results in severe internal bleeding due to inefficient blood clotting function. If the bone marrow cells do not recover and repopulate within 6-8 weeks, death occurs. Treatment for this syndrome involves broad-spectrum antibiotics, transfusion of WBCs, RBCs and platelets.
Exposures to 8-15 Gy of radiation lead to gastrointestinal (GI) syndrome. Death usually occurs within 3-5 days. Life-threatening damages to intestinal crypt cells occur. Death results from loss of absorption of nutrient, dehydration, loss of weight, severe electrolyte imbalance and low blood pressure. Damage to the epithelium in the small intestine with the resultant systemic infection from intestinal microbes is the most critical characteristic of the GI syndrome. The second most important aspect of GI syndrome is damage to the bone marrow cells. Treatment involves broad-spectrum antibiotics, electrolytes and fluids.
In exposures above 25 Gy, central nervous system (CNS) syndrome occurs. This is due to severe functional damage to the central nervous system. Death usually occurs within 48 hrs. The symptoms in animals include irritability, hyper excitability responses, epileptic type fits and coma. The immediate changes in fluid and electrolyte balance in the brain are due to changes in blood vessels. The CNS syndrome is irreversible and treatment can only be symptomatic, to reduce any distress associated with nervous or gastrointestinal disorders. For rodents has different dose limits for these acute syndromes. The dose limit for bone marrow syndrome is 2-10 Gy; for gastrointestinal syndrome the limit is 10-100; and for CNS syndrome it is >100 Gy. If radiation exposure is localized, it may not be lethal and lead to damage to individual organs. The threshold doses for various organs vary significantly.
Exposure of living cells / tissues to ionizing radiation causes damages by transfer of energy to atoms and molecules in the cellular structure. Ionizing radiation causes either excitation or ionization or both to atoms and molecules. These excitations and ionizations can lead to following events inside the cells/ tissues;
i. Generation of free radicals
ii. Breakage of chemical bonds
iii. Formation of new chemical bonds and cross-linkage between macromolecules.
iv. Damage to biomolecules (e.g. DNA, RNA, lipids, proteins) which regulate vital cell processes
After radiation exposure, radiation may or may not interact with the critical target of the cell. It is considered that the chromosomes (DNA) is the most critical target of the cell since they contain the genetic information and instructions required for the cell to perform its function and to make copies of itself for reproduction purposes.
To understand the mechanism of action of radioprotectors, an in-depth knowledge of fundamental radiobiological events happening during and shortly after irradiation in tissues and cells is essential. Scheme 1 depicts the series of events happening in cells/ tissues following radiation exposure. Radiation causes damage to cells/ tissues by both direct and indirect actions. During direct action, the radiation is directly causing irreparable damage to critical targets within the cell, such as DNA, RNA, proteins and lipids. In indirect action, radiation interacts with other molecules of the cell that are not critical targets but are close enough to pass on this damage, typically in the form of free radicals. Indirect action of ionizing radiation is due to free radicals, generated during radiolysis. Because body is composed of >80% water, indirect effect is important due to the radiolytic products, mainly the hydroxyl free radical, which is an effective oxidant capable of breaking chemical bonds, initiating lipid peroxidation, in the nano- to microsecond timeframe. After radiation exposures following changes are observed in DNA at the molecular level namely single- or double-strand breaks (DSB), base damage, and DNA-DNA or DNA-protein cross-links. If different damages following radiation exposure are not repaired, they affect the cell structure and function. After DNA damage has occurred, a number of processes occur in the damaged cell, tissue, or organism, including activation of DNA repair, activation of signal transduction, expression of radiation response genes and stimulation of proliferation etc. These pathways can be important for cell or tissue recovery after radiation exposure but may also play a role in the development of toxicity.
Scheme 1: Effect of radiation: Chain of the cellular events occurring in the cell/ tissue after ionizing radiation exposure
The possible fate of the cells after radiation exposure:
i. Cells are undamaged: cells operate normally
ii. Cells are damaged, but efficiently repaired: cells operate normally
iii. Cells are damaged, but not efficiently repaired: cells operate abnormally
iv. Cells are damaged, but no repair: cells die as a result of the damage