Sources of variability in the determination of micronuclei in irradiated peripheral blood lymphocytes

Can a digital slide scanner and viewing technique assist the visual scoring for the cytokinesis-block micronucleus cytome assay?

The cytokinesis-block micronucleus cytome (CBMNcyt) assay is a comprehensive method to measure DNA damage, cytostasis and cytotoxicity caused by nutritional, radiation and chemical factors. A slide imaging technique has been identified as a new method to assist with the visual scoring of cells for the CBMNcyt assay. A NanoZoomer S60 Digital Pathology slide scanner was used to view WIL2-NS cells treated with hydrogen peroxide (H2O2) and measure CBMNcyt assay biomarkers using a high-definition desktop computer screen. The H2O2-treated WIL2-NS cells were also scored visually using a standard light microscope, and the two visual scoring methods were compared. Good agreement was found between the scoring methods for all DNA damage indices (micronuclei, nucleoplasmic bridges and nuclear buds) and nuclear division index with correlation R values ranging from 0.438 to 0.789, P < 0.05. Apoptotic and necrotic cell frequency was lower for the NanoZoomer scoring method, but necrotic frequency correlated well with the direct visual microscope method (R = 0.703, P < 0.0001). Considerable advantages of the NanoZoomer scoring method compared to direct visual microscopy includes reduced scoring time, improved ergonomics and a reduction in scorer fatigue. This study indicates that a digital slide scanning and viewing technique may assist with visual scoring for the CBMNcyt assay and provides similar results to conventional direct visual scoring.

Exposure to Sevoflurane Affects the Development of Parvalbumin Interneurons in the Main Olfactory Bulb in Mice

Sevoflurane is widely used in adult and pediatric patients during clinical surgeries. Although studies have shown that exposure to sevoflurane impairs solfactory memory after an operation, the neuropathological changes underlying this effect are not clear. This study detected the effect of sevoflurane exposure on the development of calcium-binding proteins-expressing interneurons in the main olfactory bulb (MOB). We exposed neonatal mice to 2% sevoflurane at two different developmental time points and found that exposing mice to sevoflurane at postnatal day (PD) 7 significantly decreased the expression of GAD67 and parvalbumin (PV) in the olfactory bulb (OB) but did not alter the expression of calretinin (CR) or calbindin D28k (CB). The number and dendritic morphology of PV-expressing interneurons in the MOB were impaired by exposure to sevoflurane at PD7. However, exposure to sevoflurane at PD10 had no effect on calcium-binding protein expression or the number and dendritic morphology of PV-expressing interneurons in the MOB. These results suggest that exposing neonatal mice to sevoflurane during a critical period of olfactory development affects the development of PV-expressing interneurons in the MOB.

Re-evaluation of a reported increased micronucleus frequency in lymphocytes of workers occupationally exposed to formaldehyde

A replicate evaluation of increased micronucleus (MN) frequencies in peripheral lymphocytes of workers occupationally exposed to formaldehyde (FA) was undertaken to verify the observed effect and to determine scoring variability. May-Grünwald-Giemsa-stained slides were obtained from a previously performed cytokinesis-block micronucleus test (CBMNT) with 56 workers in anatomy and pathology laboratories and 85 controls. The first evaluation by one scorer (scorer 1) had led to a highly significant difference between workers and controls (3.96 vs 0.81 MN per 1000 cells). The slides were coded before re-evaluation and the code was broken after the complete re-evaluation of the study. A total of 1000 binucleated cells (BNC) were analysed per subject and the frequency of MN (in ‰) was determined. Slides were distributed equally and randomly between two scorers, so that the scorers had no knowledge of the exposure status. Scorer 2 (32 exposed, 36 controls) measured increased MN frequencies in exposed workers (9.88 vs 6.81). Statistical analysis with the two-sample Wilcoxon test indicated that this difference was not significant (p=0.17). Scorer 3 (20 exposed, 46 controls) obtained a similar result, but slightly higher values for the comparison of exposed and controls (19.0 vs 12.89; p=0.089). Combining the results of the two scorers (13.38 vs 10.22), a significant difference between exposed and controls (p=0.028) was obtained when the stratified Wilcoxon test with the scorers as strata was applied. Interestingly, the re-evaluation of the slides led to clearly higher MN frequencies for exposed and controls compared with the first evaluation. Bland-Altman plots indicated that the agreement between the measurements of the different scorers was very poor, as shown by mean differences of 5.9 between scorer 1 and scorer 2 and 13.0 between scorer 1 and scorer 3. Calculation of the intra-class correlation coefficient (ICC) revealed that all scorer comparisons in this study were far from acceptable for the reliability of this assay. Possible implications for the use of the CBMNT in human biomonitoring studies are discussed.

Scoring variability of micronuclei in binucleated human lymphocytes in a case-control study

The micronucleus test in binucleated lymphocytes is a sensitive standard assay for biomonitoring, mutagenicity testing and to assess radiosensitivity of blood donors. The results vary between laboratories and scorers which led to the definition of international scoring criteria. We used these criteria in a case-control study, but nevertheless observed large differences between the seven scorers on the level of descriptive analysis. Therefore, we used the repeat measurements (267 in 98 blood donors) from this dataset (354 measurements in 185 blood donors) to analyse scoring variability in the setting of a case-control study. The variability was assessed by analysis of variance, which revealed the storage time of the blood samples, the blood donors including their disease status, and the scorers as sources of variation in the entire dataset. In addition, the coefficient of variation (CV) of the measurements was determined (overall: CV = 24.3%). After stepwise removal of biological and experimental variation by normalizations, the CV dropped to 6.8% on average, which may reflect the 'pure counting error'. The scorer-specific CVs were between 5.5 and 9.5%. The differences between the scorers suggested by the raw data were neither related to the scorer-specific CV nor to their experience. Instead, we observed a general decline of the micronuclei frequencies towards the end of the study for all scorers. This could not be related to a change in experimental conditions or in the defined scoring criteria. An explanation could be an unintended and unrecognized change of scoring criteria. Since the change in the results did not occur in automated counting we suggest to use either reference slides in longer-lasting studies or automated counting by image analysis.

Intra- and inter-laboratory variation in the scoring of micronuclei and nucleoplasmic bridges in binucleated human lymphocytes. Results of an international slide-scoring exercise by the HUMN project

One of the objectives of the HUman MicroNucleus (HUMN) project is to identify the methodological variables that have an important impact on micronucleus (MN) or micronucleated (MNed) cell frequencies measured in human lymphocytes using the cytokinesis-block micronucleus assay. In a previous study we had shown that the scoring criteria used were likely to be an important variable. To determine the extent of residual variation when laboratories scored cells from the same cultures using the same set of standard scoring criteria, an inter-laboratory slide-scoring exercise was performed among 34 laboratories from 21 countries with a total of 51 slide scorers involved. The results of this study show that even under these optimized conditions there is a great variation in the MN frequency or MNed cell frequency obtained by individual laboratories and scorers. All laboratories ranked correctly the MNed cell frequency in cells from cultures that were unirradiated, or exposed to 1 or 2Gy of gamma rays. The study also estimated that the intra-scorer median coefficient of variation for duplicate MNed cell frequency scores is 29% for unexposed cultures and 14 and 11% for cells exposed to 1 and 2Gy, respectively. These values can be used as a standard for quality or acceptability of data in future studies. Using a Poisson regression model it was estimated that radiation dose explained 67% of the variance, while staining method, cell sample, laboratory, and covariance explained 0.6, 0.3, 6.5, and 25.6% of the variance, respectively, leaving only 3.1% of the variance unexplained. As part of this exercise, nucleoplasmic bridges were also estimated by the laboratories; however, inexperience in the use of this biomarker of chromosome rearrangement was reflected in the much greater heterogeneity in the data and the unexplained variation estimated by the Poisson model. The results of these studies indicate clearly that even after standardizing culture and scoring conditions it will be necessary to calibrate scorers and laboratories if MN, MNed cell and nucleoplasmic bridge frequencies are to be reliably compared among laboratories and among populations.

Mutagenic and cytotoxic effectiveness of zinc dimethyl and zinc diisononyldithiocarbamate in human lymphocyte cultures

The mutagenic and cytotoxic effectiveness of the vulcanisation accelerators zinc dimethyldithiocarbamate (ZDMC; ziram) and zinc diisononyldithiocarbamate (ZDINDC; arbestab Z) was tested in lymphocyte cultures of five healthy probands. ZDMC and ZDINDC (c=0.1, 1.0 and 10.0microg/ml) were studied in lymphocyte cultures without external metabolic activation. Additionally, incubation of the compounds (c=10.0microg/ml) was performed in the presence of liver microsomes from aroclor-induced rats (1 and 2h, 1 and 2mg microsomal protein). Genotoxicity testing was performed by analysis of chromosomal aberrations (CA), sister chromatid exchanges (SCEs) and micronuclei (MN). For evaluation of antiproliferative effects, mitotic index (MI) and cell cycle kinetics (CCK) were determined. In contrast to earlier investigations we found no significantly increased mutagenic or cytotoxic activity of ZDMC; ZDINDC also was inactive under these conditions.

HUman MicroNucleus project: international database comparison for results with the cytokinesis-block micronucleus assay in human lymphocytes: I. Effect of laboratory protocol, scoring criteria, and host factors on the frequency of micronuclei

Micronucleus (MN) expression in peripheral blood lymphocytes is well established as a standard method for monitoring chromosome damage in human populations. The first results of an analysis of pooled data from laboratories using the cytokinesis-block micronucleus (CBMN) assay and participating in the HUMN (HUman MicroNucleus project) international collaborative study are presented. The effects of laboratory protocol, scoring criteria, and host factors on baseline micronucleated binucleate cell (MNC) frequency are evaluated, and a reference range of "normal" values against which future studies may be compared is provided. Primary data from historical records were submitted by 25 laboratories distributed in 16 countries. This resulted in a database of nearly 7000 subjects. Potentially significant differences were present in the methods used by participating laboratories, such as in the type of culture medium, the concentration of cytochalasin-B, the percentage of fetal calf serum, and in the culture method. Differences in criteria for scoring micronuclei were also evident. The overall median MNC frequency in nonexposed (i.e., normal) subjects was 6.5 per thousand and the interquartile range was between 3 and 12 per thousand. An increase in MNC frequency with age was evident in all but two laboratories. The effect of gender, although not so evident in all databases, was also present, with females having a 19% higher level of MNC frequency (95% confidence interval: 14-24%). Statistical analyses were performed using random-effects models for correlated data. Our best model, which included exposure to genotoxic factors, host factors, methods, and scoring criteria, explained 75% of the total variance, with the largest contribution attributable to laboratory methods.

Effect of blood storage on radiation-induced micronuclei in human lymphocytes

To evaluate the effect of blood storage on the yield of micronuclei (MN) in both irradiated (in vivo and ex vivo) and unirradiated peripheral blood lymphocytes (PBL), we applied the MN assay in cytokinesis-blocked (CB) PBL obtained from healthy subjects (n=11), and from cancer patients (n=10) who were undergoing fractionated partial-body radiotherapy (xRT). The heparinized blood samples were exposed to 137Cs-irradiation (0 Gy or 2 Gy) immediately after blood collection and were stored upright in test tubes either at room temperature (22 degrees C) or in the refrigerator (5 degrees C). Duplicate whole blood cultures from each sample were set up at 0 h, 96 h, and 120 h after ex vivo irradiation. Giemsa (10%) stained slides were prepared from each culture. MN yield was determined per 1000 binucleated cells. As compared to that obtained from the corresponding fresh blood samples, we found that (1) the 22 degrees C blood storage temperature did not affect MN yields in PBL of either healthy subjects or cancer patients up to 96 h, either with or without ex vivo irradiation; and (2) while blood samples were stored at 5 degrees C, the MN yield increased significantly in PBL of healthy subjects (with or without ex vivo irradiation) at 120 h, and in cancer patients (with ex vivo irradiation) at 96 h and 120 h. Since handling of the blood sample is important for CBMN assay during shipment or in the laboratory, our findings showed that blood storage at 22 degrees C or at 5 degrees C up to 96 h appeared to provide insignificant variations of the MN results as compared to fresh blood samples. However, the 96 h of blood storage at 5 degrees C elevated the MN frequency in ex vivo irradiated PBL of cancer patients who were undergoing xRT.

Important variables that influence base-line micronucleus frequency in cytokinesis-blocked lymphocytes-a biomarker for DNA damage in human populations

The cytokinesis-block micronucleus (CBMN) assay has been adopted by numerous laboratories as a means for rapidly assessing base-line chromosome damage (breakage and loss) in human populations. However, the appropriate implementation of this assay requires a thorough understanding of both experimental variables and biological factors that can have impact on micronucleus (MN) frequency. The paper describes, with the help of experimental data the from the author's laboratory as well as other data, the impact of these variables. With regards to experimental variables, the scoring of micronuclei on slides by different technicians has been identified as an important factor; however, the use of different culture media, namely RPMI 1640 and McCoy's medium, did not have a significant effect on base-line frequencies. The paper also describes results showing that the MN index in cytokinesis-blocked cells, measured once every three months over a 12-month period for 53 healthy subjects, remains constant and the data measured on these occasions were significantly and positively correlated (R=0.477 to 0.684, P<0. 0001) with each other thus indicating the reliability and intra-individual variability of the assay over time. Inter-individual variation for males and female subjects has been estimated for each decade of age between 20 and 80 years; the difference between the 25th and 75th percentile of MN frequency varied between 1.4 fold and 2.3 fold and the minimum and maximum values for MN frequency varied by a factor of 4.7 and 12.5 depending on the age group. Age and gender are the most important demographic variables impacting on the MN index with MN frequencies in females being greater than those in males by a factor of 1.2 to 1.6 depending on the age group. For both sexes, MN frequency was significantly and positively correlated with age (R=0.62 in males and R=0.65 in females) and the slope of the regression line in males was 0.314 (P<0.0001) and in females it was 0.517 (P<0.0001). The main dietary factors influencing the MN index in subjects who are not folate deficient are plasma B12 (R=-0.315, P=0.0127) and plasma homocysteine (R=0.415, P=0.0086). In addition, it was proposed that the MN index is likely to be influenced by the propensity of an individual's cells to undergo apoptosis when damaged so that one might expect the MN frequency to be negatively correlated with apoptotic rate although this has yet to be tested. The above indicates the importance of maintaining an international network of scientists working with the CBMN assay to ensure appropriate quality control and for the development of standard experimental and documentation protocols. The human micronucleus (HUMN) project launched in 1997 is briefly described and proposed as the vehicle for these activities.