Effects of caffeine-induced defective DNA replication on SCE and chromosome aberrations produced by alkylating agents

The effect of caffeine (CAF) pretreatment (during the first cell cycle) on the frequency of sister-chromatid exchanges (SCE) and chromosome aberrations induced by bifunctional(MC)- and monofunctional(M-MC)-mitomycin C, 4-nitroquinoline N-oxide (4NQO) and ethyl methanesulphonate (EMS) were examined by using a BrdU—Hoechst staining technique. When CAF was added to the cultures during the first cell cycle in the presence of BrdU and then the cultures treated with MC, M-MC, 4NQO or EMS during the second cell cycle, the effect of the CAF was synergistic, i.e., the SCE level achieved was much higher than that expected from a simple additive effect of the agents and CAF. These results do not support the concept that the process of SCE is a manifestation of CAF-sensitive post-replication repair of DNA damage (single-strand exchanges), but, instead, point to exchanges between the double-strands of the DNA duplex present in each chromatid. CAF at certain concentrations is known to significantly slow down the rate of DNA-chain growth, but not appreciably induce strand breaks. Inasmuch as CAF alone induced only a small increase in SCE rates, possible mechanisms which may induce SCE are not only related to the slowing down of the rate of DNA-chain growth, but may also involve breaks in the template strand permitting double-strand exchanges to occur. The mechanisms responsible for chemically induced SCE are also discussed.     

Effects of caffeine-pretreatment on SCE and chromosome aberrations induced by monofunctional- and bifunctional-mitomycin C in bloom syndrome lymphocytes

Bloom syndrome (BS) lymphocytes, which are characterized by a high incidence (75.4 per cell) of SCE, were treated with caffeine (CAF) during the first cell cycle and with monofunctional-(M-MC) and bifunctional-(MC)mitomycin C during the second cycle. The effect on the SCE level was synergistic. The CAF-pretreated cells in combination with M-MC and MC post-treatments, had significantly higher (SCE values 152.5 and 167.9 SCE per cell, resp.) than those treated with M-MC or MC alone during the second cycle (101.1 and 116.4 SCE per cell, resp.). M-MC and MC in the presence of BrdU (without CAF) for 2 cell cycles increased SCE to 157.6 and 169.4 per cell (about twice the control level). M-MC + CAF and MC + CAF treatments for 2 cell cycles did not produce a synergistic effect on the SCE frequency in BS cells; the SCE level was not significantly greater than that with M-MC or MC alone. Normal cells treated with MC and CAF for 2 cycles had a maximum SCE frequency of 156 per cell. This suggests that cells with SCE frequencies above this level may not be able to survive, i.e., this is the “saturation” level of SCE. However, CAF alone had almost no effect on SCE in either BS or normal cells and did not produce multiple chromosome aberrations. The lack of CAF effect on BS cells suggests that the lesions in DNA strands of BS cells which lead to SCE are double-strand lesions. In normal cells CAF is known to significantly slow down DNA-chain growth; the reduced rate of DNA-chain growth in BS is an inherent defect of the cells. Therefore, though CAF enhanced SCE and chromosome aberrations (shattered chromosomes) in combination with alkylating agents, CAF alone did not significantly increase the SCE rate in either BS cells or in normal cells. Thus, processes which may induce SCE are not only related to retarded rate of DNA-chain growth, but also to breaks in the template strand permitting double-strand exchanges to occur.     

The relation between chemically induced sister-chromatid exchanges and chromatid breakage.

Studies of classical chromosome aberrations and sister-chromatid exchanges (SCES) suggest independent mechanisms for the two events despite some common features. Examination of chromosome breakage caused by X-rays, visible light, and viruses has shown that few chromatid breaks are accompanied by SCEs at the sites of breaks. No similar observations were available for chemically induced breaks, but it has been reported that rat chromosomes exposed to dimethylbenzanthracene (DMBA) contained a preponderance of both aberrations and SCEs in certain specific regions, implicating a common process in their formation. These conclusions were drawn from a comparison of breaks induced in vivo with SCEs induced in vitro. However, we used 7 chemical mutagens to induce both chromatid breaks and SCEs in "harlequin" chromosomes of cultured rat and Chinese hamster ovary (CHO) cells and found that 25% of the 914 breaks scored were associated with SCEs. The proportion of breaks accompanied by SCEs is related to the overall SCE frequency and falls into the range predicted on the basis that breaks and SCEs occur independently. The reported association between sites for SCEs and aberrations also reflects secondary factors, such as induction of SCEs and aberrations during DNA synthesis in late replicating regions of the chromosomes.     

Comparative cytogenetic study after treatment of mouse spermatogonia with mitomycin C

Male (101 × C₃H)F₁ hybrid mice, 10–12 weeks old, were injected i.p. with single doses of 2.5, 3.75 or 5.0 mg/kg of mitomycin C (MC). Spermatogonia were sample for mitotic chromosome analyses 6, 18 or 24 h after treatment. Spermatocytes were sample for meiotic chromosome analyses 50 or 60 days after treatment.The maximal yield of chromatid-type aberrations induced in spermatogonia was found 24 h after treatment with 5.0 mg/kg of MC. More than 50% of the cells carried at least one chromatid exchange. The majority (90%) of these were whole-arm exchanges derived from breaks in the centromeric heterochromatin.No translocation multivalents were found in spermatocytes analysed 50 or 60 days after treatment. The discrepancy between the presence of many symmetrical exchanges in spermatogonia and the absence of translocation multivalents in primary spermatocytes may be result of insensitivity of the stem cell spermatogonia against exchange induction by MC or of complete germinal selection against induced translocations before and/or during early meiosis. However, the possibility of missing translocations due to whole-arm exchanges in acrocentric chromosomes during the analysis of diakineses-metaphases I is also discussed.It is emphasized that comparisons of chromatid exchange frequencies in spermatogonia with the yield of translocation multivalents in spermatocytes descended from these spermatogonia as opposed to those from stem cells might provide an estimate of pre-diakinesis germinal selection against chromatid exchanges or the resulting translocations. This estimate is important for the quantitative evaluation of the genetic risk from environmental mutagens.     

Effects of mitomycin C on sister chromatid exchange in normal and Bloom's syndrome cells

Bloom's syndrome lymphocytes, which are characterized by a high incidence of sister chromatid exchanges (SCE: 80.6 per cell), were treated with mitomycin C (MMC) and the effect of the chemical on SCE frequency compared with that in normal cells. Raising the concentration of MMC from 1 X 10(-9) to 1 X 10(-7) g/ml led to about 10-fold increase (61.7 SCE per cell) in the SCE frequency over the base line in normal lymphocytes (6.4 SCE per cell), though chromosome aberrations remained at a relatively low frequency. MMC caused about a two-fold rise in SCE in cells of Bloom's syndrome (128.8 SCE at 10(-9) g/ml; 139.3 SCE at 10(-8) g/ml). The frequency of chromosome aberrations in Bloom's syndrome cells at concentrations of MMC of 1 X 10(-9) and 1 X 10(-8) g/ml was 0.350 and 0.825 per cell, respectively, and low when compared to the increased number of SCE. The increased frequency of SCE in normal and Bloom's syndrome cells is in contrast to the reported findings with cells from Fanconi's anemia and xeroderma pigmentosum. The distribution of SCE in MMC-treated normal cell correlates with that of spontaneous SCE in cells of Bloom's syndrome.     

Psoralen/UVA treatment and chromosomes

Summary Treatment of human lymphocytes in vitro with trimethylpsoralen or 8-methoxypsoralen and UVA irradiation (PUVA) induced chromosome damage, mainly constrictions and gaps, but also breaks and exchanges, and increased the frequency of sister chromatid exchange (SCE). The localization of the chromosome aberrations was nonrandom. The coincidence of many PUVA hits with mercaptoenthanol hits suggests that PUVA may have other targets in the cell than the DNA, perhaps the folding proteins of the chromosomes and the nuclear membrane/chromatin attachment organelles.Caffeine increased in a synergistic way the chromosome aberration yield if added after PUVA treatment, but there was no effect when caffeine was present before and during PUVA treatment. The SCE frequency was increased in the presence of caffeine.     

Action of mitomycin C on mouse spermatogonia

The induction of chromosome aberrations in mouse spermatogonia and bone marrow cells by treatment with Mitomycin (MC) was tested. The following dosages were used: 3.5; 1.75; 0.35; and 0.035 mg/kg body weight. Chromatid interchanges and terminal deletions were induced in both tissues. Regarding the chromosome damage, spermatogonia seemed to be more sensitive to the test substance than bone marrow cells.The aberrations observed were considered to represent the cause of dominant lethals induced in spermatocytes after treatment with MC by others. The squash technique adapted for examination of mitoses of mouse spermatogonia proved to be a useful tool in testing potential chemical mutagens.     

Analysis of the Chromosome Aberrations Induced by X-Rays in Somatic Cells of DROSOPHILA MELANOGASTER

A technique has been perfected for enabling good microscope preparations to be obtained from the larval ganglia of Drosophila melanogaster. This system was then tested with X-rays and an extensive series of data was obtained on the chromosome aberrations induced in the various stages of the cell cycle.-The analysis of the results obtained offers the following points of interest: (1) There exists a difference in radio-sensitivity between the two sexes. The females constantly display a greater frequency of both chromosome and chromatid aberrations. They also display a greater frequency of spontaneous aberrations. (2) In both sexes the overall chromosome damage is greater in cells irradiated in stages G(2) and G(1). These two peaks of greater radiosensitivity are produced by a high frequency of terminal deletions and chromatid exchanges and by a high frequency of dicentrics, respectively. (3) The aberrations are not distributed at random among the various chromosomes. On the average, the Y chromosome is found to be more resistant and the breaks are preferentially localized in the pericentromeric heterochromatin of the X chromosome and of the autosomes. (4) Somatic pairing influences the frequency and type of the chromosome aberrations induced. In this system, such an arrangement of the chromosomes results in a high frequency of exchanges and dicentrics between homologous chromosomes and a low frequency of scorable translocations. Moreover, somatic pairing, probably by preventing the formation of looped regions in the interphase chromosomes, results in the almost total absence of intrachanges at both chromosome and chromatid level.     

Sister chromatid exchanges induced in cultured mammalian cells by chromate

Chromate compounds induced sister chromatoid exchanges (SCEs) and chromosome aberrations in cultured mammalian cells. Similar increases in SCE frequency were observed in human fibroblasts exposed to the compounds K2Cr2O7 and K2CrO4. Marked increases in SCE frequency in cells exposed to chromate for a 48-h period were detected at concentrations between 10(-7) and 10(-6) M. Chromosome aberrations (primarily chromatid breaks) were also produced in human cells exposed to K2CrO4 at concentrations between 8 . 10(-7) and 3 . 10(-6) M. K2CrO4, but not the trivalent compound CrCl3, induced SCEs in Chinese hamster ovary (CHO) cells at low concentrations.     

Truly incomplete and complex exchanges in prematurely condensed chromosomes of human fibroblasts exposed in vitro to energetic heavy ions

Confluent human fibroblast cells (AG1522) were irradiated with gamma rays, 490 MeV/nucleon silicon ions, or iron ions at either 200 or 500 MeV/nucleon. The cells were allowed to repair at 37 degrees C for 24 h after exposure, and a chemically induced premature chromosome condensation (PCC) technique was used to condense chromosomes in the G2 phase of the cell cycle. Incomplete and complex exchanges were analyzed in the irradiated samples. To verify that chromosomal breaks were truly unrejoined, chromosome aberrations were analyzed using a combination of whole-chromosome specific probes and probes specific for the telomere region of the chromosome. Results showed that the frequency of unrejoined chromosome breaks was higher after irradiation with the heavy ions of high LET, and consequently the ratio of incomplete to complete exchanges increased steadily with LET up to 440 keV/microm, the highest LET included in the present study. For samples exposed to 200 MeV/nucleon iron ions, chromosome aberrations were analyzed using the multicolor FISH (mFISH) technique, which allows identification of both complex and truly incomplete exchanges. Results of the mFISH study showed that 0.7 and 3 Gy iron ions produced similar ratios of complex to simple exchanges and incomplete to complete exchanges; these ratios were higher than those obtained after exposure to 6 Gy gamma rays. After 0.7 Gy of iron ions, most complex aberrations were found to involve three or four chromosomes, which is a likely indication of the maximum number of chromosome domains traversed by a single iron-ion track.     

Increased frequency of sister-chromatid exchange and chromatid breaks in lymphocytes after treatment of human volunteers with therapeutic doses of paracetamol

Paracetamol was given to 10 healthy human volunteers in 3 doses of 1 g each during a period of 8 h. Blood samples for lymphocyte cultures were taken before and 24 h after paracetamol administration. A small but significant increase was found in the frequency of sister-chromatid exchanges (SCE) after intake of paracetamol (0.187 +/- 0.030 per chromosome before and 0.208 +/- 0.024 per chromosome after). After exposure the mean frequency of chromatid breaks per 100 cells was significantly increased (2.16 +/- 1.33 versus 0.33 +/- 0.50 before exposure). Exposure of human lymphocytes in vitro showed that concentrations of paracetamol above 0.1 mM induced inhibition of replicative DNA synthesis. Increased SCE was found in lymphocytes exposed to 1-10 mM paracetamol for 2 h. Furthermore, 0.75-1.5 mM paracetamol exposure for 24 h increased the frequency of chromatid and chromosome breaks in the lymphocytes. The paracetamol-induced SCE and chromosome aberrations may be secondary effects of paracetamol-induced inhibition of DNA synthesis or due to covalent binding of paracetamol metabolite(s) to DNA.     

Chromosomal aberrations and sister-chromatid exchanges in Lithuanian populations: effects of occupational and environmental exposures

Cytogenetic analysis of chromosomal aberrations (CA) in 175,229 cells from 1113 individuals, both unexposed and occupationally or environmentally exposed to heavy metals (mercury and lead), organic (styrene, formaldehyde, phenol and benzo(a)pyrene) and inorganic (sulfur and nitrogen oxides, hydrogen and ammonium fluorides) volatile substances and/or ionizing radiation was performed. In addition, 11,250 cells from 225 individuals were scored for the frequency of sister-chromatid exchanges (SCE). Increased frequencies of CA were found in all occupationally exposed groups. A principal difference between the exposure to heavy metals and organic substances was found: increase in the CA frequency was dependent on duration of exposure to mercury but not dependent on duration of exposure to styrene, formaldehyde and phenol. A higher CA incidence was found in lymphocytes of children living in the vicinity of a plant manufacturing phosphate fertilizers. This indicates that children are a sensitive study group for the assessment of environmental exposure. However, the results of SCE analysis in these children were inconclusive. Exposure to ionizing radiation was found to cause chromosome breaks and chromatid exchanges in Chernobyl clean-up workers and chromatid breaks, chromatid exchanges, dicentric chromosomes and chromosome translocations in workers from the Ignalina Nuclear Power Plant. The increased frequency of chromatid exchanges in individuals exposed to ionizing radiation was quite unexpected. This may be attributed to the action of some unrecognized life-style or occupational factors, or to be a result of radiation-induced genomic instability. Also an increased SCE frequency was found in lymphocytes of Chernobyl clean-up workers.     

Chromosomal instability in an oxygen-tolerant variant of Chinese hamster ovary cells

Background levels of chromosomal aberrations and sister-chromatid exchanges (SCEs) were determined in CHO-99 cells, an oxygen-tolerant variant substrain of Chinese hamster ovary (CHO-20) cells capable of stable proliferation under an atmosphere of 99% O2/1% CO2, a level of hyperoxia at which cultured mammalian cells normally cannot survive. The mean chromosomal aberration frequency in CHO-99 cells was as high as 1 aberration per cell (mainly chromatid and chromosome gaps and breaks) versus 0.05 aberration/cell in CHO-20 cells, while the SCE frequency was 1.7- to 2.1-fold increased. While most aberrations were apparently distributed at random over the chromosomes, up to 31% of the aberrations appeared to be involved in site-specific fragility at a homologous site in chromosomes Z3 and Z4. Immediately upon shifting CHO-99 cells to air-equilibrated conditions their SCE frequency decreased to the control level, whereas the aberration rate persisted at a still elevated level of 0.16-0.31 aberration per cell, even after a culture period of 14 weeks under normoxia. This indicates that at least part of the chromosomal instability is a constitutional property of the variant cells, i.e., not directly dependent upon hyperoxic stress. In CHO-99 X CHO-20 hybrids the occurrence of chromatid-type aberrations and fragile site but not that of chromosome-type aberrations was suppressed under normoxic conditions, suggesting that chromatid-type aberrations and fragile site expression on the one hand and chromosome-type aberrations on the other hand are mediated by different constitutional defects in CHO-99 cells. No gross alterations in (deoxy)ribonucleoside triphosphate pools were detected in CHO-99 cells that could be held responsible for their chromosomal instability. In addition, no increased level of DNA damage was detected by the technique of alkaline elution. The excessive chromosomal instability in CHO-99 cells, as observed under hyperoxic conditions, may originate from reactive intermediates giving rise to DNA double-strand breaks and/or a type of DNA lesion that is resistant to the conditions of the alkaline elution technique. However, alternative mechanisms based upon reactive species interfering with DNA replication/repair processes cannot be excluded.     

Incidence of chromosome aberrations in mammalian sperm stained with Hoechst 33342 and UV-laser irradiated during flow sorting

The separation of two sperm populations is possible using the technique of flow sorting, provided that a significant difference exists in the DNA content of X- and Y-bearing sperm. In order to ascertain whether or not chromosome damage was induced in sorted sperm, chromosome preparations were made from isolated sperm that had been microinjected into hamster eggs. While egg chromosomes exhibited a low frequency of chromosome aberrations, ranging from 4 to 7%, a large proportion of sperm cells exhibited chromosome damage. Between 29% of unstained and unsorted sperm and 38% of stained and unsorted sperm exhibited some type of chromosomal abnormality and this proportion increased to 50% in sorted sperm. If only damaged sperm nuclei are considered, the two unsorted sperm groups had a mean of 0.6 breaks, 0.8 triradial exchanges, and 0.2 quadriradial exchanges per nucleus. However, sorted sperm, which were stained with a fluorochrome and exposed to UV-laser irradiation, exhibited a mean of 2.9 breaks, 2.6 triradial, and 1.9 quadriradial exchanges per nucleus in which damage occurred. These observations indicate that the treatments and manipulations to which sperm nuclei are subjected during flow sorting cause chromosomal aberrations, and that exposure of the cells to UV-laser irradiation contributes substantially to the chromosome damage observed.     

Chromosome and SCE analysis in peripheral lymphocytes of persons occupationally exposed to cytostatic drugs handled with and without use of safety covers

The frequency of structural chromosome aberrations and sister-chromatid exchanges in peripheral blood lymphocytes of nurses handling cytostatic drugs without a safety cover is compared with that of individuals doing this work exclusively under a safety cover and with that of nurses working under similar conditions but not handling cytostatics. The mean yield of dicentric chromosomes, (4.3 +/- 0.7)/1000 cells, and acentric fragments, (15.4 +/- 1.4)/1000 cells, in the occupationally exposed group is significantly increased in comparison to individuals working with protection (dic: (1.1 +/- 0.4)/1000 cells, ace: (11.2 +/- 1.2)/1000 cells) and nurses not handling cytostatics (dic: (2.1 +/- 0.5)/1000 cells, ace: (9.9 +/- 1.1)/1000 cells). The frequency of chromatid breaks and SCE is not significantly different between these groups (p greater than 0.05).     

Relationship of DNA repair to chromosome aberrations, sister-chromatid exchanges and survival during liquid-holding recovery in X-irradiated mammalian cells

The repair of X-ray induced DNA single strand breaks and DNA—protein cross-links was investigated in stationary phase, contact-inhibited mouse cells by the alkaline-elution technique. Approx. 90% of X-ray induced single strand breaks were rejoined during the first hour of repair, whereas most of the remaining breaks were rejoined more slowly during the next 5 h. At early repair times, the number of residual non-rejoined sungle strand breaks was approx. proportional to the X-ray dose. DNA—protein cross-links were removed at a slower rate (T1/2 approx. 10–12 h). Cells were held in stationary growth for various periods of time after irradiation before subculture at low density to score for colony survival (potentially lethal damage repair), chromosome aberrations in the first mitosis, and sister-chromatid exchanges in the second mitosis. Both cell killing and the frequency of chromosome aberrations decreased during the first several hours of recovery, reaching a minimum level by 6 h; this decrease correlated temporally with the repair of the slowly rejoining DNA-strand breaks. Relatively few sister-chromatid exchanges were observed when the cells were subcultured immediately after X-ray. The exchange frequency rose to maximum levels after a 4-h recovery interval, and returned to control levels after 12 h of recovery. The possible relationship of DNA repair to these changes in survival, chromosome aberrations, and sister-chromatid exchanges during liquid-holding recovery is discussed.     

Modulation of restriction enzyme-induced damage by chemicals that interfere with cellular responses to DNA damage: a cytogenetic and pulsed-field gel analysis

The electroporation of restriction enzymes into mammalian cells results in DNA double-strand breaks that can lead to chromosome aberrations. Four chemicals known to interfere with cellular responses to DNA damage were investigated for their effects on chromosome aberrations induced by AluI and Sau3AI; in addition, the number of DNA double-strand breaks at various times after enzyme treatment was determined by pulsed-field gel electrophoresis (PFGE). The poly(ADP-ribose) polymerase inhibitor 3-aminobenzamide (3AB) dramatically increased the yield of exchanges and deletions and caused a small but transitory increase in the yield of double-strand breaks induced by the enzymes. 1-beta-D-Arabinofuranosylcytosine, which can inhibit DNA repair either by direct action on DNA polymerases alpha and delta or by incorporation into DNA, potentiated aberration induction but to a lesser extent than 3AB and did not affect the amount of DNA double-strand breakage. Aphidicolin, which inhibits polymerases alpha and delta, had no effect on AluI-induced aberrations but did increase the aberration yield induced by Sau3AI. The postreplication repair inhibitor caffeine had no effect on aberration yields induced by either enzyme. Neither aphidicolin nor caffeine modulated the amount of DNA double-strand breakage as measured by PFGE. These data implicate poly(ADP-ribosyl)ation and polymerases alpha and delta as important components of the cellular processes required for the normal repair of DNA double-strand breaks with blunt or cohesive ends. Comparison of these data with the effect of inhibitors on the frequency of X-ray-induced aberrations leads us to the conclusion that X-ray-induced aberrations can result from the misjoining or nonrejoining of double-strand breaks, particularly breaks with cohesive ends, but that this process accounts for only a portion of the induced aberrations.     

Chromosomal aberrations and sister-chromatid exchanges induced by N-nitroso-2-acetylaminofluorene and their modifications by arsenite and selenite in Chinese hamster ovary cells.

The frequencies of chromosomal aberrations (CA) and sister-chromatid exchanges (SCE) in Chinese hamster cells were significantly increased by the direct-acting mutagen N-nitroso-2-acetylaminofluorene (N-NO-AAF) at the concentration of 0.1 mM. N-NO-AAF was prepared by nitrosation of the protohepatocarcinogen 2-acetylaminofluorene. The induced CA, which included chromatid breaks, chromatid exchanges, chromosome breaks, and chromosome ring formation were significantly potentiated by the presence of sodium arsenite (10 microM), but not by hydroxyurea (20 mM) or cytosine arabinoside (25 microM). On the other hand, the clastogenic effect of N-NO-AAF was effectively inhibited by sodium selenite (100 microM). Arsenite (10 microM) was shown to be moderately active in CA induction which was partially blocked by the presence of selenite (10 nM). N-Nitroso compounds such as N-nitroso-N-methylurea, N-nitroso-N-ethylurea and N-methyl-N'-nitro-N-nitrosoguanidine were equally or more active in the induction of CA and SCE in CHO cells when compared with N-NO-AAF. The cell cycle was significantly delayed by the intervention of N-NO-AAF.     

Mechanism of formation of chromatid exchanges

Chinese hamster cells with different patterns of distribution of 5-bromodeoxyuridine (BrdUrd) between chromosome subunits were subjected, during the G2 stage, to UV irradiation, which only produced breaks in BrdUrd substituted DNA. The frequency of chromatid and subchromatid interchanges as well as isochromatid aberrations was estimated. It was found that only BrdUrd containing chromatids were involved into aberrations; this result challenges the so called "molecular theory" for aberration production proposed by Leenhouts and Chadwick. A very small increase of the aberration yield in chromosomes without BrdUrd may be connected with the action of UV on the frequency of recombination. The observed frequency of interchanges was not proportional to the BrdUrd content in chromosomes and depended on the time of its incorporation: more exchanges were induced in the chromatids incorporating BrdUrd during the last round of replication. These regularities may be connected with some molecular peculiarities of chromosome structure and function.     

Protective effect of caffeine on chromosomal structures of mammalian cells irradiated with UV-rays]

The effect of caffeine (2mM) on the frequency of structural mutations induced by UV light (lambda = 265 nm at an incident dose of 40 erg/mm2) in the primary culture of mouse embryonic fibroblasts is studied. A half-hour treatment with caffeine of cells at the time of the first mitosis metaphase decreased approximately by 2 times the frequency of chromosome aberrations induced by UV light at the S stage and observed at the metaphase of this or the next C-mitosis. The frequency of both breaks and exchanges decreased as the result of caffeine treatment. The persistence of the protective effect of caffeine at the time of the second C-mitosis suggests that the observed decrease of the aberration rate is accompanied by the true reparation of pre-mutational lesions in chromosomes; the nature of the reparative process and the time when it takes place is as yet not clear. Caffeine did not decrease the frequency of spontaneous structural mutations.