Cytogenetic study in lymphocytes from children exposed to ionizing radiation after the Chernobyl accident

Follow-up studies on genome damage in children after Chernobyl nuclear power plant accident

As children are more susceptible to ionizing radiation than adults, each nuclear accident demands special attention and care of this vulnerable population. The Chernobyl nuclear disaster occurred in a region populated with a large number of children, but despite all efforts and expertise of nuclear specialists, it was not possible to avoid casualties. As vast regions of Ukraine, Belarus and Russia were exposed to doses of ionizing radiation, which are known to be related with different diseases, shortly after the accident medical surveillance was launched, which also included analysis of genome damage. Child population affected by internal and external radiation consisted of subjects exposed prenatally, postnatally (both evacuated and non-evacuated), born by irradiated fathers who worked as liquidators, and parents exposed environmentally. In all groups of children during the last 30 years who were exposed to doses which were significantly higher than that recommended for general population of 1 mSv per year, increased genome damage was detected. Increased genome damage includes statistically higher frequency of dicentric and ring chromosomes, chromated and chromosome breaks, acentric fragments, translocations, and micronuclei. The presence of rogue cells confirmed internal contamination. Genome instability and radiosensitivity in children was detected both in evacuated and continuously exposed children. Today the population exposed to ionizing radiation in 1986 is in reproductive period of life and follow-up of this population and their offspring is of great importance. This review aims to give insight in results of studies, which reported genome damage in children in journals without language restrictions.

In vitro RABiT measurement of dose rate effects on radiation induction of micronuclei in human peripheral blood lymphocytes

Developing new methods for radiation biodosimetry has been identified as a high-priority need in case of a radiological accident or nuclear terrorist attacks. A large-scale radiological incident would result in an immediate critical need to assess the radiation doses received by thousands of individuals. Casualties will be exposed to different doses and dose rates due to their geographical position and sheltering conditions, and dose rate is one of the principal factors that determine the biological consequences of a given absorbed dose. In these scenarios, high-throughput platforms are required to identify the biological dose in a large number of exposed individuals for clinical monitoring and medical treatment. The Rapid Automated Biodosimetry Tool (RABiT) is designed to be completely automated from the input of blood sample into the machine to the output of a dose estimate. The primary goal of this paper was to quantify the dose rate effects for RABiT-measured micronuclei in vitro in human lymphocytes. Blood samples from healthy volunteers were exposed in vitro to different doses of X-rays to acute and protracted doses over a period up to 24 h. The acute dose was delivered at ~1.03 Gy/min and the low dose rate exposure at ~0.31 Gy/min. The results showed that the yield of micronuclei decreases with decreasing dose rate starting at 2 Gy, whereas response was indistinguishable from that of acute exposure in the low dose region, up to 0.5 Gy. The results showed a linear-quadratic dose-response relationship for the occurrence of micronuclei for the acute exposure and a linear dose-response relationship for the low dose rate exposure.

Absolute quantification of acetylation and phosphorylation of the histone variant H2AX upon ionizing radiation reveals distinct cellular responses in two cancer cell lines

Histone modifications change upon the cellular response to ionizing radiation, and their cellular amounts could reflect the DNA damage response activity. We previously reported a sensitive and reliable method for the absolute quantification of γH2AX within cells, using liquid chromatography-tandem mass spectrometry (LC/MS/MS). The technique has broad adaptability to a variety of biological systems and can quantitate different modifications of histones. In this study, we applied it to quantitate the levels of γH2AX and K5-acetylated H2AX, and to compare the radiation responses between two cancer cell lines: HeLa and U-2 OS. The two cell lines have distinct properties in terms of their H2AX modifications. HeLa cells have relatively high γH2AX (3.1 %) against the total H2AX even in un-irradiated cells, while U-2 OS cells have an essentially undetectable level (nearly 0 %) of γH2AX. In contrast, the amounts of acetylated histones are lower in HeLa cells (9.3 %) and higher in U-2 OS cells (24.2 %) under un-irradiated conditions. Furthermore, after ionizing radiation exposure, the time-dependent increases and decreases in the amounts of histone modifications differed between the two cell lines, especially at the early time points. These results suggest that each biological system has distinct kinase/phosphatase and/or acetylase/deacetylase activities. In conclusion, for the first time, we have succeeded in simultaneously monitoring the absolute amounts of phosphorylated and acetylated cellular H2AX after ionizing radiation exposure. This multi-criteria assessment enables precise comparisons of the effects of radiation between any biological systems.

Dose estimation using dicentric chromosome assay and cytokinesis block micronucleus assay: comparison between manual and automated scoring in triage mode

In cases of an accidental overexposure to ionizing radiation, it is essential to estimate the individual absorbed dose of a potentially radiation-exposed person. For this purpose, biological dosimetry can be performed to confirm, complement or even replace physical dosimetry when this proves to be unavailable. The most validated biodosimetry techniques for dose estimation are the dicentric chromosome assay, the "gold standard" for individual dose assessment, and cytokinesis-block micronucleus assay. However, both assays are time consuming and require skilled scorers. In case of large-scale accidents, different strategies have been developed to increase the throughput of cytogenetic service laboratories. These are the decrease of cell numbers to be scored for triage dosimetry; the automation of procedures including the scoring of, for example, aberrant chromosomes and micronuclei; and the establishment of laboratory networks in order to enable mutual assistance if necessary. In this study, the authors compared the accuracy of triage mode biodosimetry by dicentric chromosome analysis and the cytokinesis block micronucleus assay performing both the manual and the automated scoring mode. For dose estimation using dicentric chromosome assay of 10 blind samples irradiated up to 6.4 Gy of x-rays, a number of metaphase spreads were analyzed ranging from 20 up to 50 cells for the manual and from 20 up to 500 cells for the automatic scoring mode. For dose estimation based on the cytokinesis block micronucleus assay, the micronucleus frequency in both 100 and 200 binucleated cells was determined by manual and automatic scoring. The results of both assays and scoring modes were compared and analyzed considering the sensitivity, specificity, and accuracy of dose estimation with regard to the discrimination power of clinically relevant binary categories of exposure doses.

Quickscan dicentric chromosome analysis for radiation biodosimetry

The dicentric chromosome assay (DCA) is the gold-standard assay for accurately estimating unknown radiological doses to individuals following radiological or nuclear accidents. However in a mass-casualty scenario, this assay is not well suited for providing timely dose estimates due to its time- and expertise-intensive nature. In Canada, two approaches are being developed in an attempt to increase triage-quality biological dosimetry throughput. These are 1) increasing the number of trained personnel capable of conducting the DCA, and 2) evaluating alternative biodosimetry approaches or DCA variations. In a recent exercise, a new scoring technique (termed DCA QuickScan) was evaluated as an alternative rapid-scoring approach. Triage-quality conventional DCA and DCA QuickScan analysis were based upon scoring a minimum of 50 metaphase cells or 30 dicentrics by 9-15 scorers across four laboratories. Dose estimates for the conventional DCA were found to be within 0.5 Gy of the actual dose for 83% of the unknown samples, while DCA QuickScan dose estimates were within 0.5 Gy for 80% of the samples. Of the dose estimates falling 0.5 Gy or more outside the actual dose, the majority were dose over-estimates. It was concluded that the DCA QuickScan approach can provide critical dose information at a much faster rate than the conventional DCA without sacrificing accuracy. Future studies will further evaluate the accuracy of the DCA QuickScan method.

Validation of the cytokinesis-block micronucleus (CBMN) assay for use as a triage biological dosimetry tool

Traditionally, the dicentric chromosome assay (DCA) has been used to derive biological dose estimates for unknown radiological exposures. While sensitive, this assay requires highly trained evaluators and is extremely time consuming. The cytokinesis-block micronucleus (CBMN) assay has been suggested as an alternative to the DCA, as it is much faster to evaluate samples and requires less technical expertise. In order to validate this assay for triage biodosimetry, dose-response curves were generated for six donors at eight doses of gamma-radiation (0-4.0 Gy). Each sample was evaluated by 12 individuals, among three different laboratories and the incidence of micronuclei was determined after counting 50-500 binucleated cells. This study demonstrated that the CBMN assay was capable of detecting radiation doses >or=1 Gy after scoring only 200 binucleated cells. As such, the CBMN assay may provide a sensitive and reliable technique for deployment as an initial screening tool in a large-scale radiological emergency where large numbers of biological dose estimates are required.

Enhanced yield of chromosome aberrations after CT examinations in paediatric patients

PURPOSE

To determine whether computed tomography (CT) could enhance the chromosome aberration yields in paediatric patients.

MATERIAL AND METHODS

Blood samples were taken before and after CT scans from 10 children for whom the medical justifications for CT examinations were accidental injuries and not diseases as investigated in earlier studies. Chromosome analysis was carried out in lymphocytes by fluorescence plus Giemsa (FPG) staining exclusively in metaphases of the first cell cycle in vitro.

RESULTS

The mean blood dose of the 10 children was about 12.9 mGy which was determined by a newly developed dose estimation. Based on more than 20,000 analyzed cells it was found that after CT examination the frequencies of dicentrics (dic) and excess acentric fragments (ace) in lymphocytes were significantly increased. By subdividing the children into two age groups, those with an age from 0.4 years to 9 years and from 10 - 15 years, it became obvious that the observed increase in chromosome aberrations was mainly contributed by the younger age group. In this group the frequency of dicentrics was significantly increased whereas in the older group the observed increase was not significant.

CONCLUSION

Our results demonstrate that CT examinations enhance the dicentrics yields in peripheral lymphocytes of children aged up to 15 years. Since in particular significantly increased dicentric yields could be observed in children with an age from 0.4 - 9 years, it can be assumed that children younger than 10 years may be more radiation sensitive than older subjects.

Simple biodosimetry method for cases of high-dose radiation exposure using the ratio of the longest/shortest length of Giemsa-stained drug-induced prematurely condensed chromosomes (PCC)

The aim was to develop a simple biodosimetry method for as rapid as possible estimation of absorbed radiation doses in victims of radiation accidents, in particular after high-dose exposure. Human peripheral blood lymphocytes (PBL) were gamma-irradiated in vitro with several doses up to 40 Gy stimulated with phytohaemagglutinin-P (PHA-P) for 2 days and their chromosomes condensed prematurely using 50 nm calyculin A. Chromosome lengths of Giemsa-stained G2 prematurely condensed chromosomes (PCC) were measured using image analysing software and the ratio of the longest/shortest chromosome length was calculated. The length ratio (LR) of the longest/shortest Giemsa-stained chromosome s increased with a good correlation to the square root of the radiation dose (D) up to 40 Gy, i.e. LR = (4.90 x D0.5) + 2.14. The LR of the longest/shortest chromosome might be used as an index for estimating the radiation dose. The blood samples should not be cooled until the start of separation/stimulation of the lymphocytes. A rapid and easy estimation of large doses after whole-body exposure was identified by measuring the ratio of the longest/shortest length of Giemsa-stained G2-PCC induced by calyculin A. This simple protocol will be particularly useful for making therapy decisions for victims of ionizing radiation exposure and has potential for use as a biodosimeter for partial-body exposure accidents.

DNA damage evaluated by alkaline single cell gel electrophoresis (SCGE) in children of Chernobyl, 10 years after the disaster

Using the alkaline single cell gel electrophoresis (Comet) assay, the extent of DNA damage was evaluated in leukocytes of 43 Belarussian children (16 healthy and 27 affected by thyroid cancer). Thirty-nine healthy children from Pisa (Italy) were enrolled in the study as controls. In addition to basal levels of DNA damage, leukocytes were treated in vitro with bleomycin (BLM), a radiomimetic drug, to evaluate a possible adaptive response in different groups of children. Results with the Comet assay indicated an increased level of DNA damage (P=0.037) in leukocytes of Belarussian children compared to the Italian control group. In addition, within the Belarus group, lower basal levels of DNA damage (P<0.001) were found in children with cancer compared to healthy children. Tumor affected children were living in less radiocontaminated areas (P<0.04) than the healthy children and there was a significant relationship (P=0.03) between the amount of environmental radiocontamination and DNA damage in leukocytes. There were no differences in the sensitivity of leukocytes from different groups of children to BLM, indicating the absence of an adaptive response. The lack of an adaptive response may have been due to the use of noncycling cells and/or the bleomycin dose chosen. Tests for the presence of clastogenic factors (CF) in the blood serum of children showed that 39% of the tumor affected children and 19% of the healthy children in the exposed group were positive as compared to the Italian control group (0%) (Chi-square test, P<0.04). The higher levels of genomic damage in children evaluated 10 years after the Chernobyl disaster could be related to the increased incidence of individuals with CF.

Chromosome aberrations in lymphocytes and clastogenic factors in plasma detected in Belarus children 10 years after Chernobyl accident

In 1996, 10 years after Chernobyl accident, a cytogenetic analysis was carried out to assess whether chromosome aberrant cells (CA) were still detectable in the lymphocytes and clastogenic factors (CFs) were present in the plasma of children coming from Gomel (Belarus), one of the most heavily contaminated regions. Furthermore, the possible contribution of plasmatic CFs to the amount of CA was investigated. The presence of CA was examined in the lymphocytes from 29 thyroid tumour-affected children and 41 healthy children (local controls). Thirty healthy children living in Pisa (Italy) were enrolled in the study as additional controls from an uncontaminated area. No significant difference was observed between the two control groups, whereas a significantly increased frequency of CA was found in the tumour-affected children, as compared with Gomel and Pisa controls (chi 2-test, p < 0.001). However, when soil contamination level was taken into account, the chromosome type CA frequency observed in tumour-affected children coming from the more contaminated areas (> 4 Ci/km2) resulted significantly higher than that in other children, either affected or not (p = 0.003). The presence of CFs was analyzed on the plasma ultrafiltrate from 41 children. 7/10 (70%) plasma samples from tumour-affected children and 17/23 (74%) Gomel controls resulted to possess clastogenic activity irrespective of soil contamination levels. No activity was detected in the plasma of eight Pisa controls (0%). The difference between both Gomel groups and Pisa controls was highly significant (p = 0.002). A borderline, but not statistically significant correlation (p = 0.08) was observed between basal CA frequency and CF potency, which became significant (p = 0.03) when only chromosome type of aberrations was considered. We conclude that, although the presence of CFs in the plasma of these children might be partly responsible of the cytogenetic effects observed, the main source of damage has considered to be do to the previous and/or continuous exposure to environmental radiocontaminants. Tumour-affected children may represent a subset of the population either more sensitive to clastogenic damage or exposed to higher levels of contaminants.

Possible discrepancies between dicentric chromosome frequencies and recorded ionizing radiation doses: in vivo study

BACKGROUND

The available data on the effects of radiation on humans are limited to the reports of accidental exposure or studies of patients under diagnostic and therapeutic treatment; few reports refer to a occupationally exposed population groups.

METHODS

The research was conducted on 66 subjects employed in the Department of Nuclear Medicine and 41 nonemployed controls. For each of them chromosomal analysis and gamma-spectrometer analysis of 24-hour urine were carried out. Exposure doses were measured using film-badge dosimetry.

RESULTS

The comparison of dosimetric data obtained by film-badge measurements and the frequency of dicentric chromosomes in each subject revealed no correlation between the two observed parameters.

CONCLUSIONS

In view of the above results, the subjects should be studied within job/task groups as they are far more likely to receive comparable doses.

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.

Micronucleus frequency in Gomel (Belarus) children affected and not affected by thyroid cancer

Cytogenetic monitoring was carried out on a group of children from Gomel (Belarus), one of the areas most severely affected by radioactive contamination following the accident at the Chernobyl nuclear power plant, in 1986. We performed the micronucleus test (MN) in binucleated lymphocytes of 42 children (mean age: 11+/-2.34 years), 16 of whom were affected by thyroid gland tumor. Thirty healthy children living in Pisa (mean age: 14.96+/-2.17 years) were enrolled in the study as controls. Thyroid tumor affected children living in Gomel showed a statistically significant increase in the frequency of micronucleated cells as compared with the healthy children from Pisa. Moreover, a significant correlation was found between MN frequency and both the presence of tumor and higher 137Cs contamination. In addition, higher 137Cs contamination was more frequently observed in tumor affected children. These results suggest that the increased MN frequency is attributable more to 137Cs contamination rather than to the presence of the tumor itself.

Children exposed to chronic contamination after the Chernobyl accident: cytogenetic and radiotoxicological analyses

In this study, we describe cytogenetic studies of lymphocytes obtained from children who were exposed after the Chernobyl accident to low doses of ionizing radiation. We sought to determine possible chromosomal damage relative to internal contamination, as measured by whole-body counter and urine radiotoxicological analyses. The study was performed during a 1-mo period on the peripheral blood of children hosted in Italy, but who resided in contaminated regions of the Russian Federation and Belarus. We used conventional cytogenetics to detect chromosomal aberrations. In some cases, we also used "chromosome painting" to look for stable aberrations. There were more acentric fragments in subjects than in controls; a few chromosome and chromatid breaks werefound in the subjects, but this finding did not differ significantly between subjects and controls.

Cytogenetic effects in lymphocytes from children exposed to radiation fall-out after the Chernobyl accident

In a previous paper we reported that a group of children exposed to ionizing radiation following the Chernobyl accident exhibited an appreciable number of chromosome breaks and rearrangements reflecting the persistence of a radiation-induced damage. The results suggested that the children were still exposed to radioactive contamination through consumer foodstuff and life styles. In the present paper, 31 exposed children have been considered together with a control group of 11 children with the aim to confirm previous results. All children underwent whole-body counter (WBC) measures and conventional cytogenetic analysis. The frequency of chromosome aberrations detected by conventional cytogenetics in the group of children chronically exposed to low doses of ionizing radiation resulted in significant differences with respect to the control group. The present work suggests that, for these groups of children, even if the frequency of aberrations is very low and the observation of statistically significant differences is consequently a problem, a persistently abnormal cytogenetic picture is still present several years after the accident.

Radiobiological evaluation of immigrants from the vicinity of Chernobyl

Eighty individuals (55 adults and 25 children) who were residents of four cities (Kiev, Mozyr, Gomel and Bobrujsk) located 100-200 km from Chernobyl at the time of the accident in 1986 were tested after immigrating to the US from 1989-1991. A whole-body counter was employed to quantitate radiocesium content. In addition, two biological measures of radiation effects, namely, chromosomal integrity using the micronucleus assay and somatic mutation analysis of erythrocytes at the glycophorin A (GPA) locus, were applied to this group. Radiocesium activity in the body ranged from 0 to 56.8 Bq/kg with a mean and standard deviation of 5.0 +/- 8.2 and a median value of 2.0 Bq/kg. Mean radiocesium content by groups was highest in adult males (9.0 +/- 11.7; range 0.21-56.8 Bq/kg) followed by adult females (3.3 +/- 4.5; range 0-21.3 Bq/kg), male children (3.0 +/- 5.7; range 0-20.2 Bq/kg) and lowest in female children (1.6 +/- 3.5; range 0-12.7 Bq/kg). Individuals with the highest radiocesium content in each group belonged to one family that lived in Mozyr (100 km from Chernobyl) until emigrating in 1989. The frequency of lymphocyte micronuclei and erythrocyte GPA allele-loss (O/N) somatic mutations were both significantly correlated with radiocesium content (r=0.57, p=0.002; r=0.75, p=0.002, respectively). The micronucleus frequency also correlated with the estimated internal absorbed dose from radiocesium in a subset of 20 immigrants for whom this calculation was possible (r=0.71, p=0.0005). Altogether, the biomonitoring data indicate that some subjects had radiation doses sufficient to produce gene and chromosomal mutations in blood cells, although these effects cannot be attributed solely to radiocesium exposure.

Clastogenic factors in the plasma of children exposed at Chernobyl

Clastogenic factors (CFs), as they were described previously in accidentally or therapeutically irradiated persons, in A-bomb survivors and in liquidators of the Chernobyl nuclear power plant, were also detected in the plasma of Chernobyl-exposed children. A high percentage of plasma ultrafiltrates from 170 children, immigrated to Israel in 1990, exerted clastogenic effects in test cultures set up with blood from healthy donors. The differences were highly significant in comparison to children immigrated from 'clean' cities of the former Soviet Union or children born in Israel. The percentage of CF-positive children and the mean values of the adjusted clastogenic scores (ACS) were higher for those coming from Gomel and Mozyr, which are high exposure sites (IAEA measurements), compared to those coming from Kiev. There was no correlation between residual 137-Caesium body burden and presence of CFs. However, both measurements were not done at the same time (in 1990 and 1992-1994, respectively). Also no relationship could be revealed between enlargement of the thyroid gland and CF-positivity. CFs are not only observed after irradiation, but in a variety of chronic inflammatory diseases with autoimmune reactions. They were also described in the congenital breakage syndromes, which are hereditary diseases with the highest cancer incidence in humans. Whether the clastogenic effects continuously produced by circulating CFs represent a risk factor for malignant late effects deserves further study and follow-up. Since CF formation and CF action are mediated by superoxide radicals, prophylactic treatment with antioxidants may be suggested for Chernobyl-exposed children, whose plasma induces a strongly positive CF-test.

Chromosome analysis of peripheral lymphocytes from persons exposed to radioactive fallout in Norway from the Chernobyl accident

Chromosome analysis of peripheral lymphocytes from two Norwegian populations (44 reindeer herding South samis from Røros and Snåsa, 12 sheep farmers from Valdres) exposed to fallout from the Chernobyl accident were made. The doses from caesium through the years 1987-1991 were calculated based on whole-body measurements 134Cs and 137Cs giving a total cumulative mean internal dose of 5.54 mSv for the total group of 56 persons. Chromosome aberrations were within the normal range when compared with historical controls with the exception of dicentrics (0.3% per cell, which is a 10-fold increase) and rings (0.07% per cell). A dose-dependent increase in dicentrics and rings based on caesium exposure was not observed.

Ten years after Chernobyl

This review focuses on current findings on the health consequences of the accident at the Chernobyl nuclear power plant in 1986. Acute radiation damage caused by exposure to high doses of ionizing radiation involved a few hundred people, apparently with fewer than 100 deaths within the first few months. Epidemiologically detectable increases in congenital abnormalities have not been reported thus far, with the possible exception of Down's syndrome. Owing to the long latency period of radiation-induced cancer, discernible increases in cancer incidence and mortality are not yet expected for most tumour types, especially among adults. However, dramatic increases in the number of childhood thyroid cancers have already been observed in Belarus and Ukraine and the Bryansk regions of Russia. The increase has been over 100-fold in some areas with heavy contamination. From the viewpoint of overall public health, the outlook of direct health effects of the Chernobyl accident are likely to be severe only among some limited subgroups, such as young children exposed to high levels of fallout nuclides. In absolute terms, the global number of Chernobyl-associated cancer cases can be estimated to be tens of thousands, but only a small fraction of these is likely to be discernible epidemiologically.

Chromosome aberrations and rogue cells in lymphocytes of Chernobyl clean-up workers

A cytogenetic analysis was performed on peripheral blood lymphocytes from 183 Chernobyl clean-up workers and 27 control individuals. Increased frequencies of chromosome aberrations were associated with exposure to radiation at Chernobyl, alcohol abuse and a history of recent influenza infection. However, only approximately 20% of Chernobyl clean-up workers had an increased frequency of dicentric and ring chromosomes. At the same time, an increased frequency of acentric fragments in lymphocytes of clean-up workers was characteristic. The use of multivitamins as dietary supplement significantly decreased the frequency of chromosome aberrations, especially of chromatid breaks. Rogue cells were found in lymphocytes of 28 clean-up workers and 3 control individuals. The appearance of rogue cells was associated with a recent history of acute respiratory disease (presumably caused by adenoviral infection) and, probably, alcohol abuse. Dicentric chromosomes in rogue cells were distributed according to a negative binomial distribution. Occurrence of rogue cells due to a perturbation of cell cycle control and abnormal apoptosis is suggested.

Do human lymphocytes exposed to the fallout of the Chernobyl accident exhibit an adaptive response? 1. Challenge with ionizing radiation

Several studies suggest that cells appear to become less susceptible to the induction of radiation damage, and in particular of chromosome and chromatid aberrations in short-term cultures of human lymphocytes, when a challenge exposure to ionizing radiation is preceded by a low 'adaptive' dose. Contradictory results have been reported on the conditions under which the phenomenon can be evidenced. In the present work, circulating lymphocytes of 13 children contaminated from the fallout after the Chernobyl accident were tested for their capability to exhibit an adaptive response in experiments in which the challenge dose was administered to stimulated lymphocytes in the S-G2 phase. Furthermore, the possible influence of 3-aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase, was also investigated. Our results indicate that, at least in the instance of the end-point here used (chromosome and chromatid aberrations, the former resulting possibly from the Cs burden), human lymphocytes, chronically exposed to low doses from fallout, do not exhibit any decreased susceptibility to ionizing radiation. However, as reported in the accompanying paper, the same samples appear to show an 'adaptive' response when exposed to a challenge treatment with bleomycin (B. Tedeschi et al., 1995, this issue).

Chromosome aberrations in Norwegian reindeer following the Chernobyl accident

Chromosome analyses were carried out on peripheral blood lymphocytes of semi-domestic reindeer in Norway which had been exposed to varying amounts of radiocesium emanating from the Chernobyl accident. The sampling was done in the period 1987-1990. The material included 192 reindeer, originating from four herds in central Norway, an area considerably affected by fallout from the Chernobyl accident, and from three herds in northern Norway which was unaffected by fallout from the accident. Significant heterogeneity in the distribution of chromosome aberrations between herds was observed. The pattern of chromosome aberration frequencies between herds was not related to the variation in radiocesium exposure from the Chernobyl accident. Other factors than the Chernobyl accident appear therefore to be of importance for the distribution of aberration frequencies found among present herds. Within the most contaminated area the reindeer born in 1986 showed significantly more chromosome aberrations than those born both before and after 1986. This could suggest that the Chernobyl accident fallout created an effect particularly among calves, during the immediate post-accident period in the most exposed areas.