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. Thus, a new molecular defect in DS lymphocytes has ben first revealed. Besides, new arguments have been provided in favour of genetic heterogeneity of this genetic disorder belonging to premature ageing diseases. Relation of molecular DNA replication defects with ageing is discussed

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. Here evidence was provided on another defect in DNA replication in XPII that involves a significantly decreased number of simultaneously operating adjacent clusters of replicons, which results in a decreased rate of DNA chain-growth. According to the Painter replication model for SCE, the exchanges arise due to double-strand DNA breaks occurring on the border between two adjacent clusters, respectively, completely and partially replicated. A retarded fork-displacement rate together with a decreased rate of DNA-chain growth may cause this situation to persist longer than in the norm. Thus, our data provide a further support of the replication model for SCE. A similar combination of cytogenetic and molecular defects has been obtained earlier in the Bloom syndrome cells

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. Ionizing radiation (5 Gy) does not change the rate of DNA chain growth in the cells derived from the affected individuals, however, it significantly reduces this parameter in normal cells due to inhibition of replicon initiation in the whole clusters. Thus, radioresistant DNA synthesis in chromosomal instability syndromes may be explained by some defect in DNA replication in unirradiated cells analogous to that in irradiated normal cells

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. It should be noted that the rate of fork movement remained unchanged in AT cells. Besides, the frequency of replication in adjacent replicon clusters remained unaltered after 5 Gy irradiation in AT cells, while the normal cells were radiosensitive to reduction in this replication parameter to the low level seen in both non-irradiated and 5 Gy irradiated AT cells. The relation of the above data to radioresistant DNA synthesis is discussed

[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. Our data suggest that the inhibition of DNA replication in normal cells throughout the whole cluster, that drastically reduces the rate of DNA-chain growth, may provide an additional time to restore damaged chromosomes, i. e. it is part of the cellular defence mechanism. It seems likely that XPII cells are deficient in such a defence mechanism

[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 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 alpha-toxin resulted in a decreased rate of replication fork and in a progressive activation of replicon groups. This effect was most prominent at 33 micrograms/ml of alpha-toxin. The data obtained allow to suggest that immunodeficiency of the second order, so characteristic of the staphylococcal sepsis, may be due, in many respects, to suppression of DNA replication