The Aging Aspects of Humans Protractedly Exposed to Ionizing Radiation. Summary of the Doctoral Thesis

Abstract

The doctoral thesis “The Aging Aspects of Humans Protractedly Exposed to Ionizing Radiation” consists of 153 pages, 40 figures, 39 tables, 317 literature references and 11 annexes. The thesis is completed in Latvian. The current work is dedicated to the topical and still insufficiently investigated area of low-dose long-term radiation exposure effects on human aging processes. The use of ionizing radiation is progressively growing in various industries and technogenic accidents with release of huge amounts of radionuclides regularly happen worldwide. The most of the released radionuclides have long period of half-decay and increase the risk of protracted irradiation for inhabitants. It is important to realize the consequences of ionizing radiation exposure and understand in details the mechanisms of its effects on humans for timely resolving and prevention of radiation induced health problems. The aim of the study was to establish, if the persons, protractedly exposed to small doses of ionizing radiation, age faster and in a different way than chronically non-exposed humans. The work tasks included: the characterization of aging signs in humans protractedly exposed to small doses of ionizing radiation; the comparison of them with aging features of persons, who didn’t receive excessive exposure earlier in their life; and on the basis of study findings the development of practical recommendations for health care improvement. There were three study populations chosen: 1) Chernobyl nuclear power-plant (CNPP) accident clean-up workers from Latvia, whose organism accumulated certain amount of long-living radionuclides while working in Chernobyl, but after the return to Latvia these people were living in an area with relatively normal radioactivity level; 2) employees chronically exposed to ionizing radiation at work (radiologists, assistants of radiologists, X-ray laboratory assistants, etc.); 3) control group – Latvian inhabitants, who were not excessively exposed to ionizing radiation at work or in any other way previously in their life, except natural background and small medical X-ray examinations. Taking into account the complicated nature of aging, which cannot be characterized by one certain parameter, several aspects of irradiated humans’ senescence have been described in the doctoral thesis. Some different methods were used for realization of the current work: measurement of the relative telomere length in peripheral blood leukocytes by real-time quantitative polymerase chain reaction; detection of transforming growth factor β (TGFβ) level in blood serum by immune fermentative ELISA method; determination of the level of nitrogen oxide and iron in hair by electron paramagnetic resonance spectroscopy. Concurrently morbidity and mortality of CNPP clean-up workers were analysed in details, paying special attention to age-dependent diseases, including malignant neoplasms, and evaluating participation time and performed work tasks in Chernobyl. The results give evidence that aging processes in CNPP clean-up workers exceed the average rate of aging in general Latvian population. Moreover, the telomere shortening, normally seen in programmed replicative senescence, is not involved in the mechanism of aging of CNPP clean-up workers. The accelerated aging mechanism might be so called “stress induced premature senescence” due to ionizing radiation protracted exposure from incorporated radionuclides. It has been established that for formation of malignant neoplasm favourable conditions develop in the organism of humans chronically exposed to ionizing radiation. Slightly longer telomeres were found in CNPP clean-up workers comparing with other groups; in addition, significantly longer telomeres have been detected in persons with heavier radiation exposure. This probably indicates telomere lengthening ferment – telomerase – activation as the effect of permanent DNS damage from incorporated radionuclides, and may be an important factor for carcinogenesis. Significantly lower level of TGFβ was found in CNPP clean-up workers comparing with other groups, that reflects insufficient protection against malignancies in their organism. Similar tendencies, but much easier manifested, were observed in persons chronically irradiated at work, who have been working under radiation exposure for many years. Epidemiological study proved that CNPP clean-up workers, who were exposed to radiation during first year after disaster and performed more risky work with radioactive internal contamination, get ill with age-dependent disorders in younger age than others, but their mortality due to cardiovascular diseases in age group under 45 years exceeds age matched general Latvian male population mortality. Higher, comparing with general population, incidence of urogenital (prostate, kidney), thyroid and oral cavity malignant tumours was observed among young CNPP clean-up workers. On the other hand, oncologic mortality was lower in all age groups of CNPP clean-up workers comparing with non-exposed Latvian males, which indicates effective medical care of CNPP clean-up workers. Overall, it was established that protracted exposure to ionizing radiation is able to modify and accelerate aging processes in humans.