Catálogo de publicaciones - libros

Directed evolution means the following: living organisms have a predisposition to vary in certain directions, and this very predisposition determines trends of evolution first of all; as crystals grow, taking a certain form, so phylogenetic trends evolve following their internal laws irrespectively of adaptation and natural selection. The evolution of mankind and biosphere will be analysed in this context.

This review covers radiation genetic risk in man, expressed in terms of various genetic diseases, on the basis of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Reports.

Target theory was the first successful attempt to describe biological phenomena in the language of theoretical physics. Its basic assumptions are still valid, and their validity can be proven with the help of modern techniques of molecular biology. The theory has also practical implications, e.g. the determination of molecular weights of enzymes which cannot be easily separated to allow standard molecular analysis. For the description of heavy charged particle action the original concept had to be broadened which led to the development of “microdosimetry”. Track structure analysis takes into account the spatial distribution of energy deposition at a nanometer scale and thus allowed an understanding of the action of heavy charged particles which is important both for the assessment of radiation risk to humans as well as to subminiature electronic circuits. New developments in radiation biology as, e.g., the “bystander effect” do not call the basic principles into question but ask for a rethinking about the nature of targets and the possible effects following radiation interactions.

Analysis of radiation-induced chromosomal aberrations has long been a powerful tool to understand the mechanisms of radiation action in living cells. Early cytogenetics was based on observations of solid-stained chromosomes, although banding techniques were soon developed for karyotyping human cells. Banding is a complex, error-prone, and timeconsuming methodology, especially when applied to radiation-induced aberrations that, unlike genetic syndromes, are randomly induced in the genome and in the cellular population. The “color revolution” occurred in the 80’s with the introduction of fluorescence in situ hybridization (FISH) The first great improvement provided by FISH-painting was the opportunity to analyze symmetrical, transmissible aberrations (such as translocations and inversions) simply and rapidly, whereas dicentrics had long been the main endpoint analyzed by solid staining. Multi-color painting demonstrated that radiation-induced rearrangements are much more complex than previously thought, and even low doses of densely ionizing radiation produce mostly complex-type exchanges. The impact of multi-color painting on the understanding of radiation-induced genetic effects will be discussed here.

Chromosomal aberration analysis is the most sensitive biological method to indicate exposure to ionising radiation. This paper will distinguish between the detection and the measurement of low doses by aberration analysis and show how to quantify their limits. Worked examples will be presented using the lymphocyte dicentric assay and data typical of Co-60 gamma rays. The principles illustrated can be applied to other aberration types and other radiation qualities. Two situations will be considered: conventional by eye scoring of 1,000 metaphases from a suspected low exposure patient and scoring more metaphases with computer assisted microscopy. Low dose quantification is ultimately limited by the uncertainty on the assumed background level of dicentrics. With conventional scoring the Poisson uncertainty on the patient's observed dicentric frequency is the major component to the uncertainty on low dose estimates. With increased scoring, assisted by computer, this is reduced but the standard error on the linear calibration coefficient becomes more important. The optimum is reached where both components contribute equally to the overall uncertainty. A dose estimate may be considered as a measurement when its lower 95% confidence limit falls above zero. A dose can be regarded as having been detected when the dicentric frequency is above an assumed background but the lower 95% confidence limit includes zero. Conventional scoring of 1,000 metaphases will permit a measurement lower limit of about 100mGy of gamma radiation. This can be reduced by scoring many more cells (~10,000) to about 70mGy. Further improvement is unlikely due to the background 'noise' in the assay.

Radiation protection is taken into account in radiotherapy practice from the point of view of the medical aspect in order to achieve the best ratio between the tumour control probability (TCP) and the normal tissue complication probability (NTCP). But any radiotherapy treatment implies a dose delivered to the patient’s body also outside the beams. Therefore, there is a certain interest important to quantify that which can be considered a negative impact of radiotherapy also if it cannot absolutely be avoided and, in any case, the benefit of radiotherapy should prevail.

The early conceptual basis of gene structure and mutation were laid down in a handful of seminal papers, one of the best remembered being the “Green Pamphlet” of 1935 by N.V. Timofeeff-Ressovsky, K.G. Zimmer and M. Delbrück entitled “On the Nature of Gene Mutation and Gene Structure”. The concepts of a molecular basis for the gene and its mutability have expanded hugely since then and have generated the entire field of DNA repair. Here, two current insights into DNA repair and mutation are displayed. Replication repair is a newly established mode of recombination repair that works through a copy-choice (templateswitching) mechanism and that has been reconstructed in vitro using the enzymes of DNA replication elaborated by bacteriophage T4. In contrast, a pathological mode of primer-strand switching generates templated complex mutations, and these greatly increase in frequency when the shepherding proteins of replication repair are disabled. At the same time, many spectra are found to contain a different kind of complex mutation whose components are more widely scattered. These are argued to arise mostly through transient bouts of hypermutation and are likely to contribute to carcinogenesis and to the virulence of microbial pathogens.

The biological consequences of mutation depend on the translation of genotype into phenotype. The genotype-phenotype map is controlled by developmental and physiological processes, including genetic buffering by the Hsp90 protein chaperone. Hsp90 supports over 150 signal transduction proteins, maintaining the strength of signaling through developmental pathways. During stress, Hsp90 buffering is reduced, and the expressed mutation rate is suddenly increased causing dramatic morphological changes in previously invariable quantitative and qualitative traits. Our recent work suggests that traits controlled by Hsp90 be protected by thresholds, a natural consequence of non-linear biological responses to genetic and environmental factors. A threshold model makes several predictions for the effect of Hsp90 on individual development and the behavior of populations. These include: the existence of upper and lower thresholds, the independence and cumulative effects of buffered genetic variants, the dependence on proximity to the threshold of genetic, environmental or stochastic effects, and the saturation of biological responses and insensitivity (canalization) over a wide range of genetic or environmental effects. We show here how Hsp90-buffered variation meets the predictions of threshold trait models, and suggest that the distinction by Hsp90 between continuous and discrete traits may be more fundamental than the classical division of quantitative and qualitative traits.

Effects of chronic low-level irradiation on radiosensitivity of mammals (mice) are studied experimentally and by making use of the methods of mathematical modelling. Our own and reference experiments show that chronic low-level short-term and long-term exposures induce, respectively, elevated radiosensitivity and lowered radiosensitivity (radioresistance) in mice. The manifestations of these radiosensitisation and radioprotection effects are, respectively increased and decreased mortality of pre-irradiated specimens (in comparison with previously unexposed ones) after challenge with acute irradiation. The reason of the animals’ death in the experiments was the hematopoietic syndrome among acute radiation syndromes. Therefore the theoretical investigation of the influence of pre-irradiation on radiosensitivity of mice is conducted by making use of the biologically motivated mathematical models which describe the dynamics of hematopoietic system in mice exposed to challenge acute exposure following the chronic one. Modelling results indicate that the radiosensitization effect of chronic low-level short-term (less than 1 month) pre-irradiation on mice is due to increased radiosensitivity of lymphopoietic, granulocytopoietic, and erythropoietic systems accompanied by increased or close to the normal level radiosensitivity of thrombocytopoietic system.

Adaptation of natural drosophila populations from the area radiocontaminated due to the Chernobyl accident (Vetka district of Gomel region) and from the control area (Berezinsky National Reserve) to irradiation was studied. Additional exposure to 40 Gy γ-rays resulted in different responses of population samples - much more dominant and recessive lethal mutations arose in the population from the control area than from that with “radiation history”. Viability of flies from Vetka district was much higher after additional irradiation in comparison with flies from Berezinsky Reserve. These facts prove that Vetka population is more resistant to radiation than Berezinsky one. When the population samples were kept under laboratory conditions without irradiation for 8 generations it was revealed that the mutation level in both populations increased at keeping under such conditions. Adaptation of Vetka population to irradiation remained, besides, the control population also became more resistant to ionizing radiation. It means that keeping of natural drosophila populations under laboratory conditions is a strong stress (limited space, overpopulation, other temperature and light conditions), which increases a mutation process and induces non-specific adaptation. In order to study the process of insect adaptation in detail we investigated experimental drosophila populations of 4 kinds: control, supplied with melanin, irradiated in each generation and irradiated with melanin. The samples of these populations were exposed to 30 Gy x-rays in the 55th generation. Dominant and recessive lethal mutation frequencies, induced by additional radiation exposure, were the lowest in the irradiated population - this population was best adapted to ionizing radiation impact. The population irradiated with melanin was less resistant.