[en] The carcinogenic effect of ionizing radiation has become the main issue in radioprotection since the data seem to show that the genetic effect is less important than was formerly thought. The hypothesis of a linear no threshold (LNT) relationship, which means that the hazard of carcinogenesis is proportional to dose, was introduced in 1965 (ICRP report no.9) in order to simplify the administrative assessment of cancer risk. The validity of the LNT relationship has been challenged by several senior radiation biologists. Hence, The Academie des Sciences felt that it was timely to organize a symposium during which proponents and opponents of LNT could calmly discuss the various facets of this problem. The first sessions were devoted to microdosimetry and DNA repair. The third part of the meeting was devoted to the mechanisms of carcinogenesis in humans. The discussion throughout the meeting emphasized the avenues which should be explored for a better understanding of human radioinduced carcinogenesis, in particular the dose effect relationship for ds DNA breaks and the impact of dose rate, apoptosis, radioinduced genetic instability, epigenetic effect, as well as epidemiological studies focused on the effect of low doses in patients and on individuals from the regions of the world where background natural irradiation is high. (N.C.)

[en] The molecular mechanisms of in vivo inhibition of mammalian DNA replication by exposure to UV light (at 254 nm) was studied in monkey and human cells infected with simian virus 40. Analysis of viral DNA by electron microscopy and sucrose gradients confirmed that the presence of UV-induced lesions severely blocks DNA synthesis, and thus the conversion of replicative intermediates (RIs) into fully replicated form I DNA is inhibited by UV irradiation. These blocked RI molecules present several special features when visualized by electron microscopy. In excision repair-proficient monkey and human cells they are composed of a double-stranded circular DNA with a double-stranded tail whose size corresponds to the average interpyrimidine dimer distance, as determined by the dimer-specific T4 endonuclease V. In excision repair-deficient human cells from patients with xeroderma pigmentosum, UV-irradiated RIs present a Carins-like structure similar to that observed for replicating molecules obtained from unirradiated infected cells. Single-stranded gaps are visualized in the replicated portions of UV-irradiated RI molecules; such regions are detected and clearly distinguishable from double-stranded DNA when probed by a specific single-stranded DNA-binding protein such as the bacteriophage T4 gene 32 product. Consistent with the presence of gaps in UV-irradiated RI molecules, single-strand-specific S1 nuclease digestion causes a shift in their sedimentation properties when analyzed in neutral sucrose gradients compared with undamaged molecules

[en] Treatment of monkey kidney cells with mitomycin C (MMC) 24 h prior to infection with UV-irradiated simian virus 40 (SV40) enhanced both virus survival and virus mutagenesis. The use of SV40 as a biological probe has been taken as an easy method to analyse SOS response of mammalian cells to the stress caused by DNA damage or inhibition of DNA replication. The mutation assay we used was based on the reversion from a temperature-sensitive phenotype (tsA58 mutant) to a wild-type phenotype. The optimal conditions for producing enhanced survival and mutagenesis in the virus progeny were determined with regard to the multiplicity of infection (MOI). Results showed that the level of enhanced mutagenesis observed for UV-irradiated virus grown in MMC-treated cells was an inverse function of the MOI, while enhanced survival was observed at nearly the same level regardless of the MOI. For the unirradiated virus, almost no increase in the mutation of virus progeny issued from MMC-treated cells was observed, while a small amount of enhanced virus survival was obtained. These results show that enhanced virus mutagenesis and enhanced virus survival can be dissociated under some experimental conditions. Enhanced virus mutagenesis, analogous to the error-prone replication of phages in SOS-induced bacteria, was observed, at least for SV40, only when DNA of both virus and host cells was damaged and when infection occurred with a small number of viral particles. We therefore hypothesize that an error-prone replication mode of UV-damaged templates is observed in induced monkey kidney cells

[en] The data show the key part played by the 'RET/PTC ' oncogene in the radioinduced thyroid tumor genesis and suggests the presence of a genetic instability for the patients developing this kind of tumors

[en] Genetic recombination in monkey kidney cells has been studied using Simian virus 40 (SV40) as a molecular probe. Control or uv-irradiated cells have been co-infected with two thermosensitive mutants of SV40, tsA58 and tsA30. Recombination between the two viral genomes gives rise to a wild type virus phenotype, able to grow at the restrictive temperature of 41⁰C, which was taken as a measure of the recombination activity of the host cells. Results show that recombination takes place at a low frequency when viruses are not uv-irradiated. Irradiation of one or both viruses increases drastically recombination frequency. Pretreatment of the host cells with uv-light or mitomycin C 24 hours before being infected does not increase recombination frequency measured in our experimental conditions. 23 references, 5 tables

[en] We describe the use of Simian Virus 40 (SV40) as a molecular probe for studying the cellular functions induced in cultured monkey kidney cells in response to DNA damaging agents. (a) Ultraviolet (uv) irradiation of SV40-infected cells inhibits viral DNA replication. Replication forks are blocked by the first pyrimidine dimer encountered. In some cases, a single-strand break seems to occur at the level of the dimer inhibiting the fork of replication. This break, which can be visualized by electron microscopy studies, might be the first step in an excision repair pathway. (b) Treatment of monkey kidney cells with acetoxy-acetyl-aminofluorene or uv light before infection with uv-irradiated SV40 induces a mutagenic replication mode, as shown by an increase of the mutation frequency of thermosensitive SV40 mutants. (c) A possible recombination assay using various SV40 mutants infecting the same cell is proposed and discussed

[en] UV light induces DNA lesions which are mutagenic in mammalian cells. We used simian virus 40 tsB201 (unable to produce viral capsid at the restrictive temperature of 41 degree C because of a point mutation in the VP1 gene) to analyze the mutagenic potency of the two major UV-induced lesions, pyrimidine dimers (Py-Py) and pyrimidine (6-4) pyrimidones [Py(6-4)Py], which are formed on the same nucleotide sites. The mutagenesis criterion was the reversion toward a wild-type growth phenotype. After UV irradiation (mainly at 254 nm), part of the DNA was treated with the photoreactivating enzyme of Escherichia coli, which monomerizes Py-Py but does not modify the Py(6-4)Py photoproduct. Higher survival and lower mutation frequency rates for the photoreactivated DNA indicated that the two lesions were lethal and mutagenic. The VP1 gene of some mutants was entirely sequenced. The mutation spectra showed that the two lesions did not induce the same mutation hot spots, although some sites were common to both. The induced mutation hot spots were not only correlated with lesion hot spots but seemed partially directed by local DNA structures

No abstract available

[en] Bacteriophages are useful for the study on the control of gene expression in bacteria. In DNA repair field, such viruses permit to analyse some major processes such as lysogenic induction, host-cell reactivation, UV-reactivation, carcinogen-reactivation and SOS function. The study on the mutation rate of viral genes or the analysis of the structure of viral DNA may give some idea about the error-prone repair mode and the molecular mechanisms of DNA repair processes in eucaryotic cells. Plaque formation, the formation of intranuclear inclusion body, the repair of DNA lesions, transformation frequency and antigen expression may be used to quantify the cell ability to repair UV-irradiated viruses. When mammalian cells are lightly UV-irradiated before the infection with UV-irradiated viruses, the increase in the survival of the viruses is observed. It has been proposed to use the survival of unirradiated viruses in UV-irradiated cells as a sensitive marker for cell DNA repair. The most important result obtained by using viruses is that some SOS functions are expressed in eucaryotic cells after various DNA-damaging treatments which are potent carcinogens. The demonstration of an error-prone mode of repair induced in mammalian cells brought about the interesting new hypotheses on mutation mechanism and on the initiation of carcinogenesis. (Yamashita, S.)

[en] The authors designed a shuttle vector system allowing a comparison of the mutation spectrum on the supF target gene after transfection of single-stranded or double-stranded DNA into monkey cells. Single-strand-derived plasmids exhibited a spontaneous mutation frequency tenfold higher than double-strand-derived ones. These spontaneous mutations comprised deletions and point substitutions. This system was applied to the study of ultraviolet-induced mutagenesis. Single-stranded DNA exhibited a lower survival and a higher mutation frequency than double-stranded DNA after identical ultraviolet-irradiation. (author)