Analysis of historical negative control group data from the rat in vivo micronucleus assay

A database of micronuclei counts for historical negative control data from rat in vivo micronuclei tests performed in 10 different laboratories was established. Data were available from over 4000 negative control rats from 10 laboratories. The mean frequency of micronucleated cells (MN)/1000 cells ranged from 0.44 to 2.22, a 5-fold range. Overall there were no major sex or strain differences in frequency, although there were some small but statistically significant differences within laboratories. There was appreciable variability between experiments compared with variability within experiments in some laboratories. No specific factor was identified which could explain this variability although it was noted that many different vehicles were used in the experiments. It is hoped that these data will help laboratories beginning studies with the rat micronucleus assay and those involved in the assessment of micronucleus assay results.

Derivation of point of departure (PoD) estimates in genetic toxicology studies and their potential applications in risk assessment

Genetic toxicology data have traditionally been employed for qualitative, rather than quantitative evaluations of hazard. As a continuation of our earlier report that analyzed ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) dose-response data (Gollapudi et al., 2013), here we present analyses of 1-ethyl-1-nitrosourea (ENU) and 1-methyl-1-nitrosourea (MNU) dose-response data and additional approaches for the determination of genetic toxicity point-of-departure (PoD) metrics. We previously described methods to determine the no-observed-genotoxic-effect-level (NOGEL), the breakpoint-dose (BPD; previously named Td), and the benchmark dose (BMD10 ) for genetic toxicity endpoints. In this study we employed those methods, along with a new approach, to determine the non-linear slope-transition-dose (STD), and alternative methods to determine the BPD and BMD, for the analyses of nine ENU and 22 MNU datasets across a range of in vitro and in vivo endpoints. The NOGEL, BMDL10 and BMDL1SD PoD metrics could be readily calculated for most gene mutation and chromosomal damage studies; however, BPDs and STDs could not always be derived due to data limitations and constraints of the underlying statistical methods. The BMDL10 values were often lower than the other PoDs, and the distribution of BMDL10 values produced the lowest median PoD. Our observations indicate that, among the methods investigated in this study, the BMD approach is the preferred PoD for quantitatively describing genetic toxicology data. Once genetic toxicology PoDs are calculated via this approach, they can be used to derive reference doses and margin of exposure values that may be useful for evaluating human risk and regulatory decision making.

Quantitative approaches for assessing dose-response relationships in genetic toxicology studies

Genetic toxicology studies are required for the safety assessment of chemicals. Data from these studies have historically been interpreted in a qualitative, dichotomous "yes" or "no" manner without analysis of dose-response relationships. This article is based upon the work of an international multi-sector group that examined how quantitative dose-response relationships for in vitro and in vivo genetic toxicology data might be used to improve human risk assessment. The group examined three quantitative approaches for analyzing dose-response curves and deriving point-of-departure (POD) metrics (i.e., the no-observed-genotoxic-effect-level (NOGEL), the threshold effect level (Td), and the benchmark dose (BMD)), using data for the induction of micronuclei and gene mutations by methyl methanesulfonate or ethyl methanesulfonate in vitro and in vivo. These results suggest that the POD descriptors obtained using the different approaches are within the same order of magnitude, with more variability observed for the in vivo assays. The different approaches were found to be complementary as each has advantages and limitations. The results further indicate that the lower confidence limit of a benchmark response rate of 10% (BMDL(10) ) could be considered a satisfactory POD when analyzing genotoxicity data using the BMD approach. The models described permit the identification of POD values that could be combined with mode of action analysis to determine whether exposure(s) below a particular level constitutes a significant human risk. Subsequent analyses will expand the number of substances and endpoints investigated, and continue to evaluate the utility of quantitative approaches for analysis of genetic toxicity dose-response data.

The cyclin-dependent kinase inhibitor p57(Kip2) is epigenetically regulated in carboplatin resistance and results in collateral sensitivity to the CDK inhibitor seliciclib in ovarian cancer

Background:

Carboplatin remains a first-line agent in the management of epithelial ovarian cancer (EOC). Unfortunately, platinum-resistant disease ultimately occurs in most patients. Using a novel EOC cell line with acquired resistance to carboplatin: PEO1CarbR, genome-wide micro-array profiling identified the cyclin-dependent kinase inhibitor p57^Kip2 as specifically downregulated in carboplatin resistance. Presently, we describe confirmation of these preliminary data with a variety of approaches.

Methods:

Cytotoxicity testing (MTT) and cell cycle blockade assessed drug responsiveness. Methylation specific PCR and pyrosequencing identified sites of promoter methylation in p57^Kip2. siRNA to p57^Kip2 was used to look at the changes in apoptosis of carboplatin treated EOC cells. EOC tissues (20 cases) were assessed for mRNA levels of p57^Kip2.

Results:

Carboplatin resistance was reversed using 5-aza-cytidine in vitro. Promoter methylation sites and preferential sensitivity to seliciclib were seen in PEO1CarbR cells. Silencing p57^Kip2 decreased the apoptotic response to the effects of platinum but produced sensitisation to seliciclib. EOC biopsies indicated an association of high levels of p57^Kip2mRNA with complete responses to chemotherapy and improved outcome.

Conclusion:

We conclude that p57^Kip2 is a candidate biomarker of platinum sensitivity/resistance in EOC and such cases may show preferential response to the cyclin-dependent kinase inhibitor seliciclib.

Reduced adhesion formation following laparoscopic versus open colorectal surgery

BACKGROUND

Adhesion formation is common after abdominal surgery. This study aimed to compare the extent of adhesion formation following laparoscopic and open colorectal surgery.

METHODS

An observational study was undertaken to identify adhesions in patients undergoing laparoscopy after previous laparoscopic or open colectomy. Adhesions were scored according to a system validated for interobserver (median kappa = 0.80) and intraobserver (kappa = 0.82) agreement. The primary endpoint was the overall adhesion score (0-10); a secondary endpoint was the adhesion score at the main incision site (0-6).

RESULTS

Forty-six patients were recruited (13 laparoscopic and 33 open colectomy). In most patients (n = 29), laparoscopy was performed for tumour staging before liver resection. The median (interquartile range) overall adhesion score was 7 (5-8) in the open group and 0 (0-3) in the laparoscopic group (P < 0.001). A similar difference was found for the main incision score: 6 (4-6) versus 0 (0-0) (P < 0.001).

CONCLUSION

There may be a reduction in adhesion formation following laparoscopic compared with open colectomy, although the small sample size limits this conclusion.

Report of an Expert Panel on the reanalysis by of a 90-day study conducted by Monsanto in support of the safety of a genetically modified corn variety (MON 863)

MON 863, a genetically engineered corn variety that contains the gene for modified Bacillus thuringiensis Cry3Bb1 protein to protect against corn rootworm, was tested in a 90-day toxicity study as part of the process to gain regulatory approval. This study was reanalyzed by Séralini et al. who contended that the study showed possible hepatorenal effects of MON 863. An Expert Panel was convened to assess the original study results as analyzed by the Monsanto Company and the reanalysis conducted by Séralini et al. The Expert Panel concludes that the Séralini et al. reanalysis provided no evidence to indicate that MON 863 was associated with adverse effects in the 90-day rat study. In each case, statistical findings reported by both Monsanto and Séralini et al. were considered to be unrelated to treatment or of no biological or clinical importance because they failed to demonstrate a dose-response relationship, reproducibility over time, association with other relevant changes (e.g., histopathology), occurrence in both sexes, difference outside the normal range of variation, or biological plausibility with respect to cause-and-effect. The Séralini et al. reanalysis does not advance any new scientific data to indicate that MON 863 caused adverse effects in the 90-day rat study.

Indicators of bacterial infection in patients with acute exacerbation of chronic bronchitis for application in clinical trials of antibacterial drugs

OBJECTIVES

This study examined the accuracy of: (a) patient symptoms; (b) microscopic examination of sputum purulence (>25 WBCs and <10 epithelial cells) and (c) microscopic examination of morphological bacterial cell types, in identifying bacterial infection in patients with an acute exacerbation of chronic bronchitis (AECB) for entry to clinical trials.

METHODS

Subjects had a worsening of at least two symptoms from: dyspnoea, sputum volume, and sputum purulence (Anthonisen Type 1 or 2 exacerbation). Sputum samples were collected from all subjects.

RESULTS

A total of 97 sputum samples were evaluated. Overall, 58 (60%) subjects were culture-positive; 22 of 29 (76%) subjects with Type 2 exacerbation had a bacterial pathogen isolated compared with 36 of 68 (53%) Type 1 subjects. This difference was not statistically significant. Microscopically purulent samples were found to be significantly more likely to be culture-positive than non-purulent samples. However, the sensitivity (60%) and specificity (67%); and the positive predictive value (73%) and negative predictive value (53%) observed, means that this is not an ideal predictive test for clinical trials. A semi-quantitative approach to Gram staining was identified as a potential indicator of bacterial infection. Sputum specimens with one bacterial cell type present at >10 cells per field, or more than one cell type present with at least one type at a concentration of >25 cells per field, had a high proportion (91%) of culture-positive specimens.

CONCLUSIONS

Symptoms alone are a poor indicator of bacterial infection. A semi-quantitative examination of a Gram-stained sputum preparation was the best indicator of bacterial infection. This finding may have relevance in the design of clinical trials of antibacterial drugs in AECB.

Validation and reproducibility of a semi-quantitative Food Frequency Questionnaire for use in elderly Swiss women

OBJECTIVE

The principal aim of this study was to develop a Swiss Food Frequency Questionnaire (FFQ) for the elderly population for use in a study to investigate the influence of nutritional factors on bone health. The secondary aim was to assess its validity and both short-term and long-term reproducibility.

DESIGN

A 4-day weighed record (4 d WR) was applied to 51 randomly selected women of a mean age of 80.3 years. Subsequently, a detailed FFQ was developed, cross-validated against a further 44 4-d WR, and the short- (1 month, n = 15) and long-term (12 months, n = 14) reproducibility examined.

SETTING

French speaking part of Switzerland.

SUBJECTS

The subjects were randomly selected women recruited from the Swiss Evaluation of the Methods of Measurement of Osteoporotic Fracture cohort study.

RESULTS

Mean energy intakes by 4-d WR and FFQ showed no significant difference [1564.9 kcal (SD 351.1); 1641.3 kcal (SD 523.2) respectively]. Mean crude nutrient intakes were also similar (with nonsignifcant P-values examining the differences in intake) and ranged from 0.13 (potassium) to 0.48 (magnesium). Similar results were found in the reproducibility studies.

CONCLUSION

These findings provide evidence that this FFQ adequately estimates nutrient intakes and can be used to rank individuals within distributions of intake in specific populations.

Plasma antibody titres to heat shock proteins-60, -65 and-70: their relationship to coronary risk factors in dyslipidaemic patients and healthy individuals

OBJECTIVE

To investigate the factors that may affect antibody titres to heat shock proteins (Hsp)-60, -65 and -70, and serum C-reactive protein (CRP) concentrations in patients with dyslipidaemia and other features of the metabolic syndrome as defined by ATPIII criteria.

MATERIAL AND METHODS

The study comprised 237 dyslipidaemia patients and 135 healthy individuals recruited from amongst university and hospital employees.

RESULTS

Compared to the healthy individuals, the dyslipidaemic patients had higher antibody titres to Hsp-60 (p<0.01), Hsp-65 (p<0.001) and Hsp-70 (p<0.05), and higher serum CRP concentrations (p<0.001). The best-fitting multifactorial models revealed that known coronary risk factors explained little of the variation in Hsp antibody titres: 3 % for Hsp-60, 1 % for Hsp-65 and 4 % for Hsp-70 amongst the dyslipidaemic subjects. The corresponding values for the subgroup with the metabolic syndrome were 8 %, 3 % and 1 %, respectively. In contrast, the best-fitting model explained 13.5 % of the variation in serum CRP concentrations among the dyslipidaemic patients, obesity being a major determinant; and 14 % in the subgroup with metabolic syndrome.

CONCLUSIONS

The higher antibody titres to Hsp-60, -65, and -70 in the dyslipidaemic patients may be related to a heightened state of immunoactivation associated with atherosclerosis in this group. Our data indicate that antibody titres to these Hsps are not associated with the classical coronary risk factors, although serum high sensitivity (hs)CRP concentrations were significantly related to obesity.

Gleason Scoring Varies Among Pathologists and this Affects Clinical Risk in Patients with Prostate Cancer

AIMS

To investigate whether our practice of specialist review of all diagnostic biopsies was necessary to prevent misgrading of referred prostate cancer patients, and whether this misclassification, if any, would have resulted in misclassification of clinical risk grouping (Seattle Risk Grouping [SRG]) and subsequent treatment strategy and prognosis.

MATERIALS AND METHODS

Important prognostic indicators for prostate cancer include the presenting prostate-specific antigen (PSA), clinical stage and Gleason sum of the tumour. These three variables are incorporated into the SRG cohorts to establish treatment strategy. Patients with prostate cancer referred for brachytherapy had their prostate biopsies reviewed by a reference pathologist (PD) with a special interest in prostate cancer. We compared the agreement between the scoring of the referring pathologists with that of PD, and evaluated if any differences changed the SRG and therefore the clinical risk and treatment strategy for the patients.

RESULTS

In only 52% (43/83) of cases, was there total agreement between the two sets of pathologists. The inter-rater agreement was statistically 'fair' (unweighted kappa statistic 0.27). In 90% (36/40) of cases with disagreement, PD assigned higher Gleason sums. In 40% (16/40) of cases with disagreement, the change in Gleason sum altered the SRG; in one out of 16 cases, the SRG was downgraded from 'intermediate' to 'low' risk disease; in six out of 16 cases, it was upgraded from 'low' to 'intermediate' risk, and, in nine out of 16, from 'intermediate' to 'high' risk.

CONCLUSION

Our findings confirm previous reports of only limited correlation between pathologists in reporting Gleason sums. In this study, 19% (16/83) of cases had their grading changed to a level that altered clinical risk, almost always (94%; 15/16) to one that worsened prognosis. This would have significantly affected treatment strategy for these patients, and thus we recommend that all centres ensure accurate Gleason grading by the use of pathologists with special interests in prostate cancer.

Dose-response and threshold-mediated mechanisms in mutagenesis: statistical models and study design

The objective of this paper is to review the use, in mutagenesis, of various mathematical models to describe the dose-response relationship and to try to identify thresholds. It is often taken as axiomatic that genotoxic carcinogens could damage DNA at any level of exposure, leading to a mutation, and that this could ultimately result in tumour development. This has led to the assumption that for genotoxic chemicals, there is no discernible threshold. This assumption is increasingly being challenged in the case of aneugens. The distinction between 'absolute' and 'pragmatic' thresholds is made and the difficulties in determining 'absolute' thresholds using hypothesis testing approaches are described. The potential of approaches, based upon estimation rather than statistical significance for the characterization of dose-response relationships, is stressed. The achievement of a good fit of a mathematical model to experimental data is not proof that the mechanism supposedly underlying this model is operating. It has been argued, in the case of genotoxic chemicals, that any effects produced by a genotoxic chemical which augments that producing a background incidence in unexposed individuals will lead to a dose-response relationship that is non-thresholded and is linear at low doses. The assumptions underlying this presumption are explored in the context of the increasing knowledge of the mechanistic basis of mutagenicity and carcinogenicity. The possibility that exposure to low levels of genotoxic chemicals may induce and enhance defence and repair mechanisms is not easily incorporated into many of the existing mathematical models and should be an objective in the development of the next generation of biologically based dose-response (BB-DR) models. Studies aimed at detecting or characterizing non-linearities in the dose-response relationship need appropriate experimental designs with careful attention to the choice of biomarker, number and selection of dose levels, optimum allocation of experimental units and appropriate levels of replication within and repetition of experiments. The characterization of dose-response relationships with appropriate measures of uncertainty can help to identify 'pragmatic' thresholds based upon biologically relevant criteria which can help in the regulatory process.

Report and summary of the major conclusions from statistics in genotoxicity testing working group from the International Workshop on Genotoxicity Test Procedures (IWGTP), March 1999

A working group of five statisticians experienced in the use of statistical methods in mutagenicity reviewed aspects of the statistical analysis of genotoxicity test procedures. Issues discussed included methods for integrating biological importance and statistical significance, the relationship of the experimental unit to the experimental design, and the impact of new developments in statistics and computing. Three major recommendations were made relating to the need for: (1) the effective use of statistical advice in designing interlaboratory and intralaboratory investigations; (2) the development of appropriate experimental designs for new assays; and (3) education and training in the use of statistical methodology in mutagenicity testing. Environ. Mol. Mutagen. 35:260-263, 2000 Published 2000 Wiley-Liss, Inc.

Genetic variation in the metabolism of coumarin in mouse liver

The metabolism of 50 microM [3-14C] coumarin to polar products separated by high performance liquid chromatography (HPLC) and covalently bound metabolites in liver microsomes was compared in a series of inbred strains of mice. Coumarin metabolism to total polar products was higher in female than male mice. In all strains, the coumarin 3,4-epoxidation pathway was the major route of metabolism with o-hydroxyphenylacetaldehyde (o-HPA) as the major metabolite. However, in females, there was a major strain difference in the degree of metabolism to coumarin 7-hydroxylase with DBA/2 and 129 having high 7-hydroxycoumarin formation, CBA/Ca having intermediate levels and the other strains low levels. The differences between the strains was much less pronounced in the male mice. There was also evidence for strain variation in metabolism in the quantities of a number of other coumarin metabolites as detected by HPLC analysis of incubate extracts. However, this variation was of a quantitative nature and relatively small. The metabolism of B6C3F1 hybrid mice, in which coumarin had been identified as carcinogenic in a long-term cancer bioassay, was qualitatively similar to that of the other genotypes. The DBA/2 mouse has been suggested as a model for the metabolism of coumarin in humans. The pattern of metabolism found in this strain is different from most other strains. However, the pattern found for all the mouse strains, including DBA/2, differed appreciably from the profiles for other species including humans in the extent of 7-hydroxylation.

Issues related to the experimental design and subsequent statistical analysis of in vivo and in vitro comet studies

A wide range of experimental designs are used in investigations using the Comet assay. The statistical issues associated with this assay are however not particularly unusual or difficult. It is important however to recognize that the sample rather than the cell is the experimental unit. Statistical analyses based upon measures from the individual cells can lead to serious misinterpretation of results. Interpretation of the results of the assay should be related to identifying changes of biological importance rather than relying solely on the P values of specific statistical tests.

Statistical analysis of adaptive response in sister chromatid exchanges in human lymphocytes after treatment with very low and extremely low doses of N-methyl-N'-nitro-N-nitrosoguanidine using a study design to control variability

Previous studies have been interpreted as suggesting that low concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have an adaptive effect in the cultured lymphocytes of responsive donors (that is, the cells are protected against the mutagenic effects of a subsequent challenge with a higher concentration of MNNG). The objectives of the present study were to investigate, under stringent experimental conditions, whether a protective effect exists at very low and extremely low doses of MNNG (10(-8) and 10(-24) M, respectively). Peripheral blood lymphocytes from a donor considered responsive in a previous study were stimulated to divide and were cultured under standard conditions. Pre-adaptive treatments with dilutions of MNNG were added to the cultures repeatedly before a challenge treatment with MNNG. Bromodeoxyuridine was added at the same time as the challenge treatment and, following mitotic arrest, cells were differentially stained so that the number of sister chromatid exchanges (SCEs) could be counted. The study was designed to address potential criticisms of earlier studies which did not include replicate cultures. Samples of blood were divided into two identical batches for independent processing. Five replicate cultures were prepared for each combination of pre-adaptive and challenge treatments in each batch. The complete experiment was repeated to provide a further test of the consistency of results. Five replicates per treatment combination were chosen in an attempt to provide an experiment of adequate statistical power. Considerable precautions were taken to minimise the effect of factors outside experimental control on the results. Scoring was done by three scorers. In order to minimise inter-scorer variation, 240 cells were scored at each treatment observation (five cells per-scorer, three scorers per culture, four cultures per batch, two batches per experiment and two experiments). The study was designed in this way to take account of the sources of variability to ensure that any response obtained would exceed that obtainable by experimental variability alone. A high level of quality assurance monitoring was undertaken throughout the investigation. Two measures of SCE induction were used: (i) the mean frequency of SCEs; (iii) proportion of cells with at least 20 SCEs. In both experiments, the challenge concentration of MNNG significantly increased SCE frequency. There were, however, highly significant differences between the two experiments. The proportion of high frequency cells (HFCs) in Experiment 1 was increased significantly; the proportion of HFCs was also increased in Experiment 2, but the increase was not statistically significant. The pre-adaptive concentrations of MNNG included an extremely low dilution of 6.8 x 10(-24) M and a very low dilution of 6.8 x 10(-8) M in Experiment 1 and 1.4 x 10(-7) M in Experiment 2. The various pre-adaptive concentrations used had no consistent protective effect against the SCE-inducing capacity of the challenge concentration of MNNG of 6.8 x 10(-6) M. It is concluded that an adaptive response to the alkylating agent MNNG could not be demonstrated in cultured human lymphocytes. Neither a very low nor an extremely low dilution of MNNG elicited an adaptive response in terms of SCE induction (measured either as SCE frequency or as proportion of HFCs). This is in contradiction to previous reports published by us and other groups. This study was carefully designed with large numbers of replicates, a preliminary statistical power calculation, predefined comparisons and extensive quality assurance at each treatment administration. Despite these precautions the variability between scorers and between batches was much larger than anticipated. This resulted in some statistically significant differences, but these are likely to be false positives. Our findings indicate the need for such methodological refinement in human cell adaptive response studies.

Quantitative risk assessment and the limitations of the linearized multistage model

1. Quantifying carcinogenic risk is an important objective for assisting in the assessment and management of risks from chemical exposure. The most widely used of the many mathematical models proposed for extrapolation of carcinogenicity data from animal studies to low dose human exposures is the linearized multistage (LMS) model. This has, in effect, become the default approach for much of Quantitative Risk Assessment (QRA). The practical properties of this model have been investigated. 2. Analysis of stimulated data using the LMS model showed (i) that the Maximum Likelihood Estimate (MLE) of the low dose slope, q1, was unstable and extremely sensitive to small changes in the data; (ii) the 95% Upper Confidence Limit (UCL) estimate, q1*, preferred by the US Environmental Protection Agency (EPA) was insensitive with only small changes in values being obtained for large changes in the data; (iii) data sets where there was no statistical significance could give risk estimates similar to those obtained from data sets with clear dose-related effects; (iv) the size of the values of the Virtually Safe Dose (VSD) obtained did not necessarily relate to the biological interpretation of the data sets; (v) the value of q1* obtained was closely related to the top dose used in the study. 3. Limitations of the LMS model were illustrated by examples of its use in assessing the carcinogenicity of 2, 3, 7, 8-TCDD leading to the conclusion that the existing models are not suitable for routine use in the estimation of the risk from chemical carcinogens. The use of the LMS model has been justified in part by its original derivation from a mathematical model based upon a multistage model of carcinogenesis. However, estimates of the parameters of the model used to provide estimates of low dose risk to humans have no direct relationship to specific biological event in carcinogenesis. Further developments in mathematical models and increased understanding of the biological events underlying the carcinogenesis will lead to more biologically plausible QRA methods which would then justify serious consideration of QRA by regulatory authorities throughout the world.

Population genetics of induced mutations

The contribution of induced mutations to the burden of genetic disease in the context of population genetics is considered. A clear distinction is made between the effects of genetic disease and mutational events. Much of the existing burden of genetic disease is a consequence of mutations that occurred in the past. The problem of distinguishing between spontaneous and induced mutations is discussed. Molecular genetics techniques are blurring the definitions of these terms. Classical population genetics shows that the frequency of affected individuals will reach an equilibrium depending on the mutation rate and the selective pressure against affected individuals. Increasing the mutation rate or reducing the selective pressures would result in a new equilibrium with an increase in the frequency in subsequent generations of affected individuals with dominant and X-linked mutant alleles. The increase in the number of recessive mutant alleles would be much slower and take many generations to reach the new equilibrium level. One assumption behind such equilibria is random mating. Changes in human demography with a rapid increase in population size, the breakup of small, relatively inbred subpopulations, and relaxed selective pressures will lead to a new equilibrium for recessive genes at probably higher frequencies. These factors will be the major contributors to increasing the burden of recessive genetic disease by increasing the total numbers of cases. The proportion of the population with a genetic disease will also continue to grow as a greater proportion of the population survives to late middle age and succumbs to diseases associated with old age, such as cancer, circulatory disease, dementias, and diabetes, each of which is likely to have a genetic component.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex and strain differences in mouse hepatic microsomal coumarin 7-hydroxylase activity

Hepatic microsomal coumarin 7-hydroxylase activity has been determined in male and female mice of strains A/J, AKR, BALB/c, CBA/Ca, C3H/He, C57BL/6J, DBA/2 and 129. In males, coumarin 7-hydroxylase activity was highest in liver microsomes from DBA/2 mice and lowest in BALB/c mice. With female mice enzyme activity was highest in DBA/2 and 129 strains, intermediate in the CBA/Ca strain and comparatively low in the other five strains. Marked sex differences were observed in coumarin 7-hydroxylase activity with enzyme activity in female animals from strains DBA/2, 129 and CBA/Ca being 4.8-, 6.2- and 4.8-fold higher, respectively, than in male mice. In contrast, only minor sex and strain differences in levels of total microsomal cytochrome P-450 were observed. These results demonstrate marked sex and strain differences in mouse hepatic microsomal coumarin 7-hydroxylase activity. Such differences may be due to variations in particular cytochrome P-450 isoenzymes such as CYP2A5, not all of which can be explained by the known allelic difference in the Cyp2a-5 locus.

Impact of pharmacogenetics on toxicological studies. Statistical implications

Genetic differences in response to chemicals are probably ubiquitous. The implications of such genetic differences for the design of toxicological experiments is reviewed. The inclusion of genetic differences as factors in factorial experimental designs is illustrated by an experiment to investigate differences between inbred strains of mice in the length of anaesthesia following the administration of pentobarbitone to control and phenobarbitone pre-treated animals. The effects of including genetic differences as a factor in the standard toxicological studies such as the long-term rodent carcinogenicity bioassay (LTRCB) are reviewed. Such designs are likely to result in more chemicals being identified as carcinogens based upon the criteria of a positive result in the LTRCB. However, they are unlikely to provide any improvement in the estimation of risk to the human population from low level exposures to the chemicals because of the limitations of the existing extrapolation methods. Information on the degree of genetic variability in response to chemical exposure should, though, help in the more qualitative biologically-based risk assessment approaches favoured by some toxicologists. The presence of genetic differences in response also provides potential models for investigation of the mechanisms underlying toxicological responses.

Ethylene dibromide: negative results with the mouse dominant lethal assay and the electrophoretic specific-locus test

Ethylene dibromide (1,2-dibromoethane; EDB) was tested for the induction of dominant lethal and electrophoretically-detectable specific-locus mutations in the germ cells of DBA/2J male mice. Males were treated with a single intraperitoneal injection of 100 mg/kg EDB and mated to two C57BL/6J females. In the dominant lethal assay, matings were carried out to measure the effect of EDB on meiotic and postmeiotic stages; germ cells representing spermatogonial stem cells were analyzed in the electrophoretic specific-locus test. Neither of these germ cell tests produced any evidence that EDB is a germ cell mutagen. It appears from these data and those reported in the literature that EDB, a genotoxic carcinogen that affects male fertility in some mammalian species, is not mutagenic in the germ cells of the male mouse.

Thioguanine-resistant mutant frequency in T-lymphocytes from a healthy human population

Resistance to 6-thioguanine in T-lymphocytes was used to study in vivo somatic mutations in normal healthy adults. Donor age had a significant effect on mutant frequency at the hprt locus, showing an increase of 0.09/10(6) cells per year of age. No significant increase was associated with sex of donor, smoking habits, alcohol or coffee/tea intake, or X-ray exposure. The lower mutant frequency seen with contraceptive pill usage was probably due to the age difference between the groups of users and non-users.

Statistical issues arising from mouse micronucleus assay experiments of Ashby and co-workers

Data from a series of mouse micronucleus assays have been reanalysed to illustrate various statistical issues raised by Ashby and co-workers during the development of the assay. Most of the statistical points discussed in these earlier papers can be explained by the stochastic nature of the data. Reanalysis shows that the type of data collected in mammalian micronucleus assays is amenable to analysis by standard biometric methods. It is concluded that statistical analysis has an important role in the exploration and interpretation of data from the micronucleus assay.

High resolution metrical analysis applied to the assessment of damage associated with induced mutations in the mouse

Morphometric methods were used to investigate variation in the skeletons of 1030 offspring produced from matings of male DBA/2J by female C57BL/6J mice. 751 offspring originated from males that had received a single intraperitoneal injection of ethyl nitrosourea (EtNU) at a dose of 250 mg/kg. The remainder of the mice served as controls. The male parents of the controls were injected only with the buffer used as vehicle for the EtNU. Offspring were obtained for 3 weeks following injection. The treated males were then sterile for about 8 weeks. Immediately after the sterile period another sample of progeny was obtained. In the treated group, litter sizes at birth and weaning were reduced and survival to adulthood was lower. However, none of the differences were statistically significant. The skeletons were evaluated by two independent approaches. The first relied upon gross observation for unusual phenotypic variation, the second on a series of metrical measurement and coordinate data. A considerable amount of variation was recorded by both approaches. Some of the variants were severe but others were mild and perhaps of little or no importance to the health of the mice. The gross observation method produced no evidence for increased mild or severe variants in any group of offspring from the treated mice. The metrical methods also showed no evidence for treatment-related effects in offspring produced during the first 3 weeks of mating. However, in offspring produced after the sterile period, a pronounced, very highly statistically significant increase in all levels of metrical variation was observed. This treatment group revealed both increased variant measures and increased numbers of mice with variant measures. Much of this variation was so slight that it would have escaped notice were it not for the exacting measurements used in the analysis. Our morphometric approach is an analytically powerful tool, suitable for detecting variation in virtually any biological structure that can be measured. If the increased variation reported here is due to induced mutations, the effects would be consistent with that expected from slightly harmful mutations distributed throughout the mouse genome. It is appropriate to consider such effect in connection with genetic risk estimation.

Analysis of chromosomal aberrations in workers exposed to low level benzene

Metaphase chromosome preparations were analysed as part of a larger study from a population occupationally exposed to benzene and compared with a control group. Forty eight of the 66 exposed individuals and 29 of the 33 controls had samples in which metaphase spreads could be evaluated. The incidence of chromosomal aberrations (particularly chromatid deletions and gaps) in the exposed group were slightly increased compared with the control group. This increase was of borderline significance in parametric statistical tests but was significant using Fisher's exact test. No lifestyle factors had any consistent effect on the incidence of chromosome aberrations, although there was a small reduction in gaps with increasing cigarette smoking. Older individuals had a higher incidence of chromosome exchanges and "other" aberrations. Individuals who reported a recent viral illness had a higher incidence of aberrations particularly gaps. There was no evidence of any correlation in the incidence of chromosome aberrations with any of the other biological parameters previously reported. The increased incidence of aberrations seen in the group exposed to benzene may result from a history of exposure to benzene. Nevertheless, other explanations such as sampling, interindividual variability, and unintentional bias in the selection of two groups cannot be excluded.

Screening for possible human carcinogens and mutagens. False positives, false negatives: statistical implications

A screening method aimed at identifying potential human carcinogens using either animal cancer bioassays or short-term genotoxic assays has 4 possible results: true positive, true negative, false positive and false negative. Such a categorisation is superficially similar to the results of hypothesis testing in a statistical analysis. In this latter case the false positive rate is determined by the significance level of the test and the false negative rate by the statistical power of the test. Although the two types of categorisation appear somewhat similar, different statistical issues are involved in their interpretation. Statistical methods appropriate for the analysis of the results of a series of assays include the use of Bayes' theorem and multivariate methods such as clustering techniques for the selection of batteries of short-term test capable of a better prediction of potential carcinogens. The conclusions drawn from such studies are dependent upon the estimates of values of sensitivity and specificity used, the choice of statistical method and the nature of the data set. The statistical issues resulting from the analysis of specific genotoxicity experiments involve the choice of suitable experimental designs and appropriate analyses together with the relationship of statistical significance to biological importance. The purpose of statistical analysis should increasingly be to estimate and explore effects rather than for formal hypothesis testing.

Genotoxic effects in peripheral blood and urine of workers exposed to low level benzene

Blood samples were obtained from a population of refinery workers representing different age groups. Sixty six men with low average exposure to benzene and 33 male controls were investigated. An examination of cell cycle kinetics and sister chromatid exchange was carried out on control and exposed individuals. No significant differences were found between groups of individuals varying in their drinking and smoking habits or their exposure to diagnostic x rays. Individuals with the lowest and highest phenol values were examined for urine mutagenicity, with urinary phenol used here as an indicator of benzene exposure. There was no difference in the number of revertant colonies in strains TA98 and 100 between the high and low urinary phenol groups. There were also no differences in any of the biochemical measures or haematological parameters investigated in all the individuals except that higher values for mean corpuscular volume were found in exposed than in control individuals. These values, however, were within the normal clinical range.

The use of a battery of strains of mice in a factorial design to study the induction of dominant lethal mutations

The induction of dominant lethality following oral dosing of males with 200 mg/kg of cyclophosphamide was investigated using a factorial experimental design. Males from 3 genotypes, BALB/c, CBA/Ca and CBA/Ca X C57BL/6JF1 hybrid (CBB6F1) were mated to 6 females of the same genotype as the males over 3 weeks. Cyclophosphamide reduced the mating frequency of the BALB/c and CBA/Ca males. The total number of implants/female was reduced in all 3 genotypes with the greatest effect in the first 2 weeks after the males were treated. The proportion of early deaths/litter was significantly increased in CBA/Ca and CBB6F1 but the increase was smaller and non-significant with BALB/c. There was a high incidence (29.8%) of early deaths in the control BALB/c females. Statistical analysis of the ratio of early deaths to total implants in a litter using either the Freeman-Tukey binomial or the arc-sine transformation gave similar and satisfactory results. Analysis of early death data rather then the ratio of early deaths: total implants would have led to misleading conclusions. The implications of the use of a factorial design in dominant lethal assays for the detection of strain variation in mutagenic response without an increase in animal usage is discussed.

Variation in barbiturate sleeping time in mice. 3. Strain X environment interactions

Environmental factors such as diet, bedding material and temperature at the time of testing affected a 'model' pharmacological response--pentobarbitone sleeping time--differentially in a range of inbred strains. These results are probably explained by variations in the responses of the strains to constituents of the diets and bedding materials used in the experiments. Differences in the results between experiments suggest that there are also fluctuations in the composition of the diets and bedding materials over time. Strain X environment interactions such as those found here may explain differences in strain rankings between experiments. They would also account for some of the variability in results found between laboratories and within a laboratory over time.

Variation in sister-chromatid exchange among 106 members of the general U.K. population

SCE scores of lymphocytes from 106 people revealed that the majority of background variation in SCE was between cells within individuals. Highly significant differences existed between individuals. Lesser, but still highly significant differences also existed between replicate cultures. Inter-individual variation was contributed to by each person's sex and their smoking habits. SCE frequency was not influenced by any of the other factors considered, age, drinking habits and diagnostic X-ray exposure of persons or lymphocyte number and proliferation rate in cultures.

Quantitative genetic variation in the skeleton of the mouse: II. Description of variation within and between inbred strains

Variation in the skeletons of over 400 male and female mice from 12 genotypes was investigated by using multivariate statistical methods. A series of discriminant functions explains the differences in the shape of six bones: mandible, os coxae, femur, tibia-fibula, scapula, and humerus. The anatomical features of bone shape described by these functions are summarized together with illustrations of the typical shapes of each bone from the 12 genotypes. Variability within genotypes was investigated by using the Mahalanobis D2 distance--a measure of the difference between two points representing multivariate data--from the group mean. A series of variants were detected ranging from grossly abnormal bones to bones showing subtle differences localized to specific regions. Examples of the variants found are illustrated.

Genetic variation in the shape of the mouse mandible and its relationship to glucocorticoid-induced cleft palate analyzed by using recombinant inbred lines

Variation in mandible shape has been investigated in a set of recombinant inbred (RI) lines of mice, the C57BL/6J X A/J (BXA;AXB) RI lines. Considerable genetic variation was detected between the RI lines, but most lines were intermediate in shape when compared with the parent lines. Variation in mandible shape could not be explained by any single gene differences known between the parent lines including the H-2 locus. Some RI lines had mandible shapes unlike either parent, and one in particular, line BXA1, had an unusual shape with a pronounced condyloid process. It was concluded that mandible shape has a complex inheritance involving a number of genes, each with small effects. In some cases, recombination of the genes can produce bone shapes quite different from those of the original parent line.--There was no evidence that the variability in steroid-induced cleft palate incidence in the BXA;AXB RI lines is related to the variation in adult mandible shape as detected in this study.

Variation in pentobarbitone sleeping time in mice. 1. Strain and sex differences

A set of 23 inbred strains of mice was tested for their sleeping time under sodium pentobarbitone anaesthetic. Highly significant strain differences were found. Estimates of the proportion of the variation accounted for by genetic differences ranged from 28% to 42%. In general, males slept longer than females but the size of the sex differences was not consistent across strains. Sleeping times on different test days also varied, indicating that environmental factors were affecting the results. A specially designed experiment failed to detect any differences in within-strain variation.

Variation in pentobarbitone sleeping time in mice. 2. Variables affecting test results

The effect of some environmental factors on the pentobarbitone sleeping time (PST) in inbred strains of mice has been investigated. Age, dose level and fasting before the test significantly altered the PST while the source of the drug and regular handling of the mice had no effect. None of these factors affected strains differentially. Unsystematic effects such as litter differences contributed only a small proportion of the total variation in the experiments. The strain rankings were different from those obtained in some previous experiments. The effects of some of the environmental factors on the PST did not always agree with previous work. The implications of these results for the design of similar experiments and the relevance of baseline values in laboratory animals are discussed.

Lack of evidence for skeletal abnormalities in offspring of mice exposed to ethylnitrosourea

Using morphometrical methods, we investigated variation in the skeletons of more than 400 offspring of C57BL/6J and DBA/2J male mice that had received either a 250 mg/kg dose of the mutagen ethylnitrosourea or a solvent control. Sperm involved in the matings developed from cells in the spermatogonial stage at the time the animals were injected. Although variants were detected, differences in frequencies of gross abnormalities and minor variations in shape between treated and control groups were almost all nonsignificant. There were also no major differences in measures of variability within the groups of offspring from either the treated males or the control group. Additional examination of the skeleton for changes in the frequency of a series of nonmetrical variants also provided no evidence of differences between the treated and control groups that could be attributed to induced mutations. These results conflict with previous findings that ethylnitrosourea is a potent inducer of dominant skeletal mutations.

An investigation of genetic variation within a series of congenic strains of mice

2 congenic strains of mice, B6N.AKN-Ahk and D2N.B6N-Ahb, imported from the USA, were found to be either segregating or fixed for an incorrect allele at a number of biochemical loci. B6N.AKN-Ahk, supposedly congenic with C57BL/6N, had the wrong genotype at 6 out of 12 biochemical loci; D2N.B6N-Ahb, supposedly congenic with DBA/2N, was segregating at 3 out of 9 loci. There was genetic variation in mandible shape within the 2 strains but no abnormal coat colours were found and no hybrid vigour in breeding performance was detected. Analyses in the USA confirmed these results and showed that 2 other congenic strains, C3N.D2N-Ahd and AKN.B6J-Ahb, were also segregating at a number of loci. Some of the alleles found in the C3N.D2N-Ahd mice must be the result of a genetic contamination. The simplest explanation for this breakdown in the backcrossing programme is genetic contamination with other congenic strains or recombinant inbred lines under development in the same laboratory. These findings emphasize the importance of continual genetic monitoring of all genetic stocks at regular intervals and in particular during the development of congenic and recombinant lines.

Incomplete correlation of 2,3,7,8-tetrachlorodibenzo-p-dioxin hepatotoxicity with Ah phenotype in mice

Pretreatment of male mice of the inbred strains A2G, BALB/c, C57BL/10, and AKR with iron dextran synergized the action of a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 75 micrograms/kg) in causing hepatic porphyria and necrosis 35 days later. There was no effect in DBA/2 mice. Increased porphyrin levels were associated with decreased hepatic activity of uroporphyrinogen decarboxylase. Iron alone had no effect on porphyrin levels or decarboxylase activity. In male BALB/c mice given TCDD alone there was a delay in the onset of porphyria. Female BALB/c, AKR, and AKR X DBA/2 F1 mice were more resistant to the porphyrinogenic effect of TCDD than males. Development of porphyria did not correlate with Ah phenotype of the mice. The inheritance of sensitivity to TCDD in crosses of the AKR and DBA/2 strains, both Ah nonresponsive, was studied by a biometrical genetic analysis. The inheritances of increased porphyrin levels and of increased plasma activity of enzymes indicative of hepatic necrosis were both complex. Segregation of alleles at more than one locus was required to explain the data. A lack of correlation of porphyrins with plasma enzyme levels in the F2 generation suggested that the expression of these traits was determined independently. Genes other than Ah influence the development of TCDD-induced hepatotoxicity in mice.

Variation in the shape of the mouse mandible. 1. Effect of age and sex on the results obtained from the discriminant functions used for genetic monitoring

The shape of the mandible of the mouse can be described by a series of discriminant functions which have been used to discriminate between and investigate relationships among strains of mice. The effect of sex and variation in age of the animal on the results obtained from these functions has been investigated. Significant sex differences in mandible shape were detected, but these were considerably smaller than the differences found between two inbred strains. A simple correction for expressing each measure as a proportion of the sum of the measures on each bone removes the effects of overall size. Significant age effects were found, but these were only large in animals under seven weeks of age where considerable changes are taking place in the relative lengths of bone measurements. Routine testing for genetic authenticity using the shape of the mandible is possible over a wide age range and may be an efficient method for monitoring genotypes at the end of long-term experiments.

Congenital taillessness in AGUS inbred rats

From a survey of breeding records of the AGUS/ Lac rat colony, 1% of animals weaned were found to lack tails. Post-mortem examination of 5 females showed major skeletal and genital abnormalities. Tailless rats were found more often than expected in 4th and 5th litters. A 'threshold' model of inheritance was postulated with an unknown environmental factor 'triggering' the condition.

Quantitative genetic variation in the skeleton of the mouse. I. Variation between inbred strains

A series of six bones from samples of mice from eleven inbred strains and one F1hybrid were measured using a simple apparatus. The bones examined were the mandible, os coxae, femur, tibia–fibula, scapula and humerus. Considerable variation in the shape of each bone was found and successful discrimination between the strains was obtained. Correct strain classification ranged from 87% for the scapula to 98% for the os coxae. Gross abnormalities and quantitative variants were identified.

As the pattern of discrimination is different for each bone, the use of other bones in addition to the mandible may improve resolution in the identification and quality control of mouse stocks. The objective and precise identification of abnormal and variant bones suggests that the method may be useful for population studies and for the detection of induced skeletal abnormalities in toxicological investigations.

Variation in the hepatotoxic effects of carbon disulphide in the rat: evidence for polygenic inheritance

The hepatotoxic effects of carbon disulphide have been examined in rats obtained from a series of crosses between two inbred strains which differ widely in their susceptibility. The distribution of susceptibility in the F2 generation indicates that the variation cannot be explained by a simple genetic model. A polygenic model is a more likely explanation.

Variation in haematological parameters among inbred strains of rat

Haematological determinations were carried out on 7 inbred strains of rats using a standardized procedure. Significant genetic variation was found for many of the parameters although none of the strains had values which could be termed pathological. Measurements also varied significantly from day to day. Factorial analysis of variance is shown to be a powerful method for the investigation of variation in haematological parameters.