[Reorganization of DNA replicating system induced by 5-fluorodeoxyuridine and caffeine as the basis for radiosensitivity in human calls].

Treatment of diploid human fibroblasts with 5-fluorodeoryuridine (FUdR) and caffeine results in the increase in cellular radiosensitivity in terms of survival and chromosomal aberrations, on the one hand, and in radioresistant DNA synthesis (RDS), on the other hand, i.e. rather mimics those in mutant cells from patients with AT, XPII, Down syndrome, and others.

[Down syndrome: pathogenesis, radioresistant DNA synthesis and chromosomal instability].

Down syndrome (DS) is a frequent chromosomal aberration. Triplication of the fragment 21q22 of chromosome 21 is sufficient to cause the DS phenotype including immunodeficiency, premature aging, mental retardation, and an increased risk of leukemia. Chromosomal aberrations caused by X-ray irradiation were observed in DS lymphocytes and DS fibroblasts, but the correlation between chromosomal sensitivity, repair deficiency, and radioresistant DNA synthesis was not clear.

[Decreased frequency of activation of replicon clusters in Down syndrome lymphocytes].

Human blood lymphocytes from two normal and seven Down syndrome (DS) subjects were examined to determine rates of synthesis of individual replicon and adjacent clusters of replicons, using DNA fiber autoradiography. Lymphocytes in 6 of 7 DS patients were shown to have significantly slower synthesis of simultaneously active adjacent replicon clusters compared to normal controls. Rates of synthesis of individual replicons were the same in lymphocytes from all the subjects investigated.

[Morphofunctional characteristics of fibroblasts in basal cell nevus and Cockayne syndrome].

Some morphofunctional characters of fibroblasts in two genetic disorders--Cockayne syndrome (CS) and Basal cell naevus syndrome (BCNS) have been examined. The size of nucleus in BCN1SP line has been shown to be about 1.5 times less as well as the total size of nucleoli per nucleus, while the number of nucleoli was 2 times more compared with other cell lines investigated.

[The mechanism of radioresistant DNA synthesis in ataxia telangiectasia].

Using DNA fiber autoradiography, DNA replication in cells of healthy donors and in those of patients with ataxia telangiectasia (AT) was estimated. A new fact has been demonstrated showing a decreased number of simultaneously operating in tandem groups of replicon clusters in AT cells compared to that in normal cells. The data obtained suggest a reduced frequency of activation in the adjacent replicon clusters in AT cells.

[The retardation of DNA synthesis in adjacent replicon clusters in the lymphocytes of patients with Down's syndrome as a model of premature aging].

In peripheral blood lymphocytes taken from 7 patients with Down syndrome (DS) and 2 normal donors, using DNA fiber autoradiography, we estimated the rate of DNA-chain growth, which depends oil the number of simultaneously replicating adjacent replicon clusters, but not on the rate of fork movement. No difference was found in the rate of fork movement in these cells. 6 of 7 DS patients showed a significant reduction in the rate of DNA-chain growth as compared to that in normal control.

[The relation of the characteristics of DNA replication to cell sensitivity to ionizing radiation in xeroderma pigmentosum in form II (XP2CP)].

Using DNA fiber autoradiography, DNA replication in cultured fibroblasts, derived from normal donors, and from XPII patient with increased sensitivity to ionizing radiation was estimated. Here evidence is provided on the fact that the fork movement, significantly decreased in XPII cells before irradiation, remains the same after exposure to X-rays. The density of replicon clusters simultaneously operating in tandem groups, which is initially much less in XPII cells compared to normal cells, also remains unchanged after exposure to X-rays (5 Gy), since the inhibition of DNA replication occurs to individual replicons only.

[DNA replication in intact and X-ray-irradiated cells in the Cockayne syndrome].

Using DNA fiber autoradiography, an estimation was made of DNA replication in normal fibroblasts and in those derived from a patient with Cockayne syndrome. The rate of replication fork movement as well as the rate of DNA chain growth, dependent on the frequency of initiation sites in the adjacent clusters of replicons, did not differ in Cockayne syndrome cells, compared to cells of normal donors, either before or after exposure to ionizing radiation.

[The interrelation between changes in the structural organization of replicon clusters, a retarded fork displacement rate and the high level of spontaneous SCEs in form II of xeroderma pigmentosum].

A cytogenetic observation, that the sister chromatid exchanges (SCE) occur 3 times more frequently in a special form of xeroderma pigmentosum--XPII than in the norm, prompted a study of DNA replication in this rare disease. Using DNA fiber autoradiography, the rate of fork movement and the frequency of initiation in the adjacent clusters of replicons were estimated. The rate of fork movement was significantly slower than that in classical XP and in normal cells.

[The relationship between the radiosensitivity of DNA replicative synthesis and the formation of sister chromatid exchanges].

It has been found that irradiation in doses 0.5-2.0 Gy does not enhance the frequency of sister chromatid exchanges in cells of patients with Down's syndrome and ataxia-telangiectasia compared to the normal cells.

[The effect of staphylococcal alpha-toxin on DNA replication in human cells].

The influence of Staphylococcus alpha-toxin has been investigated on the duration of S-phase of lymphocyte mitotic cycle and on DNA replication in human fibroblasts in vitro. The duration of the S-phase of lymphocytes was measured by counting labeled metaphases and by making replication curves. Alpha-toxin in a dose of 3 micrograms/ml enhances the onset of S-phase, which is inhibited at a dose of 33 micrograms/ml of alpha-toxin.

[The action of ionizing radiation on DNA replication in the cells of a healthy human subject and of a patient with Down's syndrome].

Using DNA fiber autoradiography we have revealed a new defect earlier unknown in Down's syndrome but analogous to that earlier shown by the authors in AT and basal cell nevus. This syndrome involves a significantly decreased number of simultaneously operating groups of replicons compared to that in normal cells., the rate of fork movement and the fusion of neighbouring units in the group being unchanged.

[Radioresistant DNA synthesis in the fibroblasts of a Down's syndrome patient].

DNA synthesis after gamma-irradiation was analysed either by direct assay of the amount of 3H-Td incorporated into DNA of fibroblasts derived from normal donor and from a patient with Down's syndrome, or by analysis of the steady-state distribution of 3H-DNA in alkaline sucrose gradients. Doses of gamma-radiation that markedly inhibited the rate of DNA synthesis in normal human cells caused almost no inhibition in fibroblasts of the patient with Down's syndrome. The radioresistant DNA synthesis in cells of this patient was mainly due to a much less inhibition of replicon initiation than that in normal cells.

[Daughter DNA synthesis in UV-resistant and UV-sensitive Chinese hamster clones cells under UV irradiation].

By means of ultracentrifugation in alkaline sucrose gradients it has been shown that the size of DNA fragments synthesized in Chinese hamster cells of UV-sensitive clone (CHS-1) after exposure to UV light was equal to the distance between pyrimidine dimers in the parental DNA determined using endonuclease of Micrococcus luteus. With the UV-resistant clone (V-79), the length of fragments of the newly synthesized DNA was much longer than that between pyrimidine dimers in the parental DNA. The data obtained support the model according which DNA synthesis on the UV-irradiated template gives rise to gaps opposite to pyrimidine dimers.

[Excision of UV-induced pyrimidine dimers from the DNA of Chinese hamster cells of the CHO line].

Using DNA sedimentation in alkaline sucrose gradients, the excision of pyrimidine dimers from DNA of CHO cells with the help of UV-endonuclease from Micrococcus luteus has been studied. A considerable proportion (approximately 80%) of pyrimidine dimers was shown to be excised within 26 hours, whereas during the initial 17 hours after the UV-fluence (4 J/M2) the excision was slight (approximately 15%).

[Analysis of the activity of Micrococcus luteus endonucleases with respect to gamma-irradiated DNA].

Endonucleases from Micrococcus luteus that induce single-strand breaks in gamma-irradiated DNA have been separated chromatographycally into two groups. The first group involves two different enzymes: AP-endonuclease II (mol. weight 30 000) and AP, UV-endonuclease I (mol.

[Isolation and properties of Micrococcus luteus endonucleas acting in DNA depurinization but inactive with pyrimidine dimers].

An endonuclease (AP-endonuclease II) that specifically attacks double stranded or single stranded depurinated DNA, resulting in single-strand nicks, has been purified 320-fold from Micrococcus luteus. The enzyme is not stimulated by 0.002 M MgCl2, it induces 3'OH-5'PO4 breaks on the 5' side of apurinic sites, it has no activity towards UV-irradiated DNA and has a molecular weight of about 30 000.