Reduction in Animal Use 35 Years after Russell & Burch's Principles of Humane Experimental Technique

“Reduction” in animal use can be achieved by better experimental strategy and improved experimental design. In 1959, one strategy for discovering new drugs involved the random testing of thousands of chemicals in animal models of disease. This strategy was deplored by Russell & Burch in The Principles of Humane Experimental Technique (1959) as being inefficient and inhumane, and has now been almost completely superseded, following fundamental research resulting in an improved understanding of the mechanisms of drug action. Random screening is still done on a large scale, but by using in vitro methods.

Good experimental design involves the control of variation. In 1959, it was not clear whether isogenic (inbred or F1 hybrid) strains of mice and rats were more uniform than outbred stocks and whether they should therefore be used preferentially in bioassays. It is now generally accepted that F1 hybrids and inbred strains tend to be more uniform, but, as precision also depends on sensitivity, which is unpredictable, there is no general rule for choosing the best strain for a bioassay. Choice should be based on pilot studies. However, the use of more-uniform, specific pathogen-free animals, which were only just becoming available in 1959, has certainly reduced the number of animals which are used.

Other aspects of experimental design, such as the need to avoid bias, have a wide range of applicability; the importance of simplicity and the ability to calculate uncertainty are, apparently, not always appreciated by research scientists.

The concept of “reduction” has been useful in focusing attention on the need for improved experimental design. In the long-run, animal welfare legislation and improved training for research scientists should lead to improvements, not only in the way in which animals are kept, but also in the way in which experiments are planned and analysed.

Failure of nude (athymic) rats to become resistant to reinfection with the intestinal coccidian parasite Eimeria nieschulzi or the nematode Nippostrongylus brasiliensis

The course of each of three successive infections with Eimeria nieschulzi in nude (athymic) rats was the same as the primary infection in nu/+ animals, with the production of more oocysts. This indicates that resistance to reinfection with this parasite is mediated by T lymphocytes but that these cells do not control the duration of the life cycle, since oocyst production was not prolonged in the nu/nu rats. After the three infections with E. nieschulzi, the rats were exposed twice to the intestinal nematode Nippostrongylus brasiliensis, and the nu/nu were completely susceptible even to the second infection. Egg production by both infections in the nu/nu animals was similar and continued at a high plateau level for 28 days before falling to a low level. It appears that the strain of N. brasiliensis used in this study is unable to sustain high egg production for more than 4 weeks in T cell deficient rats.

Clinical development of leukocyte cyclooxygenase 2 activity as a systemic biomarker for cancer chemopreventive agents

Advancement of cancer prevention and therapy requires clinical development of systemic biomarkers of pharmacological efficacy of the agent under scrutiny. Curcumin, a polyphenol derived from Curcuma spp., has shown wide-ranging chemopreventive activity in preclinical carcinogenic models, in which it inhibits cyclooxygenase (COX)-2 at the transcriptional level. COX-2 has been implicated in the development of many human cancers. To explore the inhibition of COX-2 activity as a systemic biomarker of drug efficacy, a biomarker of potential use in clinical trials of many chemopreventive drugs known to inhibit this enzyme, we measured COX-2 protein induction and prostaglandin E(2) (PGE(2)) production in human blood after incubation with lipopolysaccharide (LPS). When 1 microM curcumin was added in vitro to blood from healthy volunteers, LPS-induced COX-2 protein levels and concomitant PGE(2) production were reduced by 24% and 41%, respectively (P < 0.05 by ANOVA). To test whether effects on COX-2 activity could also be measured after oral dosing in humans, we conducted a dose-escalation pilot study of a standardized formulation of Curcuma extract in 15 patients with advanced colorectal cancer. Basal and LPS-mediated PGE(2) production was measured in blood, twice pretreatment and on days 1, 2, 8, and 29 of treatment. Analysis of basal and LPS-induced PGE(2) production during treatment demonstrated a trend toward dose-dependent inhibition (P < 0.005 by regression analysis), but there was no significant difference compared with values from pretreatment time points. Measurement of leukocyte COX-2 activity should be considered in clinical trials of other agents likely to inhibit this isozyme.

Guidelines for the design and statistical analysis of experiments in papers submitted to ATLA

In vitro experiments need to be well designed and correctly analysed if they are to achieve their full potential to replace the use of animals in research. An "experiment" is a procedure for collecting scientific data in order to answer a hypothesis, or to provide material for generating new hypotheses, and differs from a survey because the scientist has control over the treatments that can be applied. Most experiments can be classified into one of a few formal designs, the most common being completely randomised, and randomised block designs. These are quite common with in vitro experiments, which are often replicated in time. Some experiments involve a single independent (treatment) variable, while other "factorial" designs simultaneously vary two or more independent variables, such as drug treatment and cell line. Factorial designs often provide additional information at little extra cost. Experiments need to be carefully planned to avoid bias, be powerful yet simple, provide for a valid statistical analysis and, in some cases, have a wide range of applicability. Virtually all experiments need some sort of statistical analysis in order to take account of biological variation among the experimental subjects. Parametric methods using the t test or analysis of variance are usually more powerful than non-parametric methods, provided the underlying assumptions of normality of the residuals and equal variances are approximately valid. The statistical analyses of data from a completely randomised design, and from a randomised-block design are demonstrated in Appendices 1 and 2, and methods of determining sample size are discussed in Appendix 3. Appendix 4 gives a checklist for authors submitting papers to ATLA.

Automation of mouse micronucleus genotoxicity assay by laser scanning cytometry

BACKGROUND

The evaluation of the safety of drugs and other chemicals is an important aspect of toxicology work. The mouse micronucleus assay is a standard in vivo genotoxicity assay. Chromosomal damage is an indicator of genotoxicity, which manifests in the formation of micronuclei in polychromatic erythrocytes from bone marrow and in peripheral blood erythrocytes. The assay is laborious to perform by manual counting. The laser scanning cytometer allows automated and rapid quantitation of cellular and subcellular fluorescence in monodisperse cell samples on a microscope slide. The object of this study was to evaluate the application of this new technology in the mouse micronucleus genotoxicity assay. Materials and Methods One hundred forty-four mice of various strains were dosed with combinations of carcinogens and antioxidants. Duplicate blood films were prepared 3 days later. One set of slides was stained with acridine orange, and the proportion of micronucleated erythrocytes was counted in 5,000 cells per slide. The duplicates were stained with propidium iodide (40 microg/ml). Five thousand cells per sample were examined using a laser scanning cytometer. The proportion of micronucleated erythrocytes was measured.

RESULTS

A coefficient of correlation of 0.96 was found between the data from the two assays. The automation of the assay on the LSC produced a considerable time saving and efficiency gain.

CONCLUSIONS

We conclude that with further development, laser scanning cytometry is likely to become the preferred modality for the performance of standard genotoxicity assays.

Strain differences in haematological response to chloramphenicol succinate in mice: implications for toxicological research

Much toxicological research continues to be done using genetically undefined "outbred" stocks of mice and rats, although the case for using isogenic strains has been made repeatedly in the literature over a period of more than two decades. Also, very few studies are conducted using more than one strain, with the result that genetic variation in response is seldom apparent to the investigator. Here we report qualitative and quantitative strain differences in the haematological response to chloramphenicol succinate (CAPS) when administered by gavage at 500-2500 mg/kg for 7 days, to four inbred strains of mouse (C3H/He, CBA/Ca, BALB/c and C57BL/6) and one outbred stock (CD-1). CAPS caused anaemia and reticulocytopenia in all mouse strains, and leucopenia in the inbred strains but not in the outbred CD-1 stock. All four inbred strains showed significant (P<0.01) responses to CAPS at lower dose levels than in CD-1 mice, which were phenotypically more variable than the inbred animals. A simulated experiment, using a sample of records from the present study, showed that the use of two mice at each dose level using CD-1, CBA, BALB/c and C57BL/6 (48 total mice), would have given a more sensitive experiment than the use of 47 CD-1 mice alone, and would also have shown that the response is partly strain dependent. These studies provide additional evidence that inbred strains, because of their greater sensitivity and other valuable properties, should be more widely used in toxicology.

Experimental approaches to the determination of genetic variability

Toxicology is concerned with the interaction between xenobiotics and biological molecules directly or indirectly coded in the DNA, and can be regarded as a branch of genetics. There is genetic variation in these interactions, which has important implications for risk assessment and because it can be used as a tool in studying toxic mechanisms. The genetics of susceptibility can be studied by forward or reverse genetics. Forward genetics involves working from an observed phenotype such as susceptibility to a particular xenobiotic and identifying the susceptibility genes. Often, this involves mapping and identifying quantitative trait loci, as most toxic responses have a polygenic mode of inheritance. The use of inbred strains is almost essential. Reverse genetics involves starting with a known genetic polymorphism and determining its effects on the response to xenobiotics. Studies of 'knockout' animals are a good example, although there are many naturally occurring polymorphisms that may affect toxic responses. In both cases, care has to be taken to ensure that the genetic background is carefully controlled in any comparison between animals thought to be carrying susceptible and resistant alleles.

A promoter function of the CCCGGG Sma I recognition sequence and its specific role in determining p53 status and identifying DNA damaging agents

The tumour suppressor p53 protein regulates many genes involved in cellular responses to DNA damage. To date, a common transcriptionally active DNA-binding site for p53 in vivo has not been identified. The pGL3-Basic vector contains a modified fire-fly luciferase cDNA designated luc+ and is designed for studying putative regulatory sequences as it lacks any known eukaryotic promoter sequences. We report here that the CCCGGG sequence, a Sma I site, in the cloning region of the pGL3-Basic vector can promote p53-dependent transcription of the luc+ gene. We have demonstrated, by electrophoretic mobility shift assay (EMSA), that human p53 is able to bind to the CCCGGG sequence in vitro. These data provide the first demonstration that the CCCGGG sequence is a transcriptionally active DNA-binding site for p53. Thus, the pGL3-Basic vector could be used as an indicator of p53 transcriptional activity, to determine the p53 status of cell lines and to identify DNA damaging agents that initiate the activation of p53. The CCCGGG sequence has been found to be present in a number of promoter regions of p53-regulated genes. This and the present study suggest that the CCCGGG sequence may be a consensus sequence recognized by p53 in vivo and may be used to identify genes whose expression may be controlled by p53.

Genealogies of mouse inbred strains

The mouse is a prime organism of choice for modelling human disease. Over 450 inbred strains of mice have been described, providing a wealth of different genotypes and phenotypes for genetic and other studies. As new strains are generated and others become extinct, it is useful to review periodically what strains are available and how they are related to each other, particularly in the light of available DNA polymorphism data from microsatellite and other markers. We describe the origins and relationships of inbred mouse strains, 90 years after the generation of the first inbred strain. Given the large collection of inbred strains available, and that published information on these strains is incomplete, we propose that all genealogical and genetic data on inbred strains be submitted to a common electronic database to ensure this valuable information resource is preserved and used efficiently.

Reduction in animal use in the production and testing of biologicals

In the control of biologicals, animals are used largely to measure the concentration of a specific substance, rather than as a "model" of humans, so there is considerable scope for the development of replacement alternatives. When animals continue to be be used, a critical analysis of guidelines and regulations has suggested many ways in which the use of animals could be reduced [1]. However, the widespread failure to use genetically and microbiologically defined animals is scientifically questionable and almost certainly results in the use of excessive numbers. International standardisation on a small number of genetically defined F1 hybrid mice should lead to greater precision in individual tests as well as greater comparability among different laboratories.

Low levels of p53 are associated with resistance to tetrachlorodibenzo-p-dioxin toxicity in DBA/2 mice

We show here that DBA/2 strain mice have a complex mutation/polymorphism in the promoter region of the Trp53 locus (the mouse p53 locus). This region has previously been shown to be essential for p53 expression. We further show that the DBA/2 mutation is associated with approximately fourfold lower p53 levels in thymocytes treated with the DNA-damaging agent etoposide in-vitro, and with relative resistance of these thymocytes to apoptosis induced by the DNA-damaging agent etoposide compared with C57BL/6 mice. When part of the promoter containing this mutation was inserted into a plasmid containing a luciferase reporter gene but lacking eukaryote promoter sequences and transfected into MCF-7 human breast cell line cells, the mean luciferase activity was slightly less with the DBA/2 than with the C57BL/6 promoter-reporter construct (p < 0.01). We found that DBA/2xC57BL/6 F2 hybrid mice with the DBA/2 genotype at the Trp53 locus were relatively resistant to tetrachlorodibenzo-p-dioxin toxicity, and this resistance was additive with resistance associated with the Ahr locus. DBA/2 mice are long-lived and do not have particularly high levels of cancer, suggesting either that they carry other compensatory tumour resistance alleles (such as Ahr(d)), or that, while there may be a p53 protein dosage effect for acute toxicity, lower than normal levels of p53 may still be sufficient to protect against cancer. In evolutionary terms, it may be better to maintain low levels of p53 in order to avoid death from acute toxicity, even at the expense of a higher incidence of cancer in later life.

Warning: the use of heterogeneous mice may seriously damage your research

Genetically heterogeneous (GH) mice and rats continue to be widely used in research even though the case for using isogenic strains has been argued repeatedly. The paper by Miller et al. in this issue appears to be the only one in the last 22 to attempt a scientific justification for the continued use of (a limited subset of) GH stocks. However, although they are to be commended for bravery, they fail to make their case. GH stocks represent poor material for controlled studies because genetic heterogeneity normally leads to phenotypic variability and a decline in experimental sensitivity. To counter this argument, Miller et al. claim that phenotypic variability may actually be smaller in GH animals than in their isogenic parents. Were this so (e.g., all mice being short lived, small, and aggressive), it is difficult to see how the use of such a stock could increase the generality of research results based on it, as claimed by Miller et al. Isogenic strains are a vital, proven, and powerful resource for biomedical research, and should be used in preference to GH stocks by all scientists who use laboratory rodents.

Genetic typing of the senescence-accelerated mouse (SAM) strains with microsatellite markers

The Senescence-Accelerated Mouse (SAM) strains constitute a murine model of accelerated senescence originating from the ancestral AKR/J strains and consist of nine senescence-prone (SAMP) strains and four senescence-resistant (SAMR) strains. The chromosomes (Chrs) of the SAM strains were typed with 581 microsatellite markers amplified by PCR, and the fundamental genetic information of the SAM strains was obtained. One-third of the examined markers displayed polymorphism among the strains, and only two alleles were detected in almost all loci among the SAM and AKR/J strains. However, in 12 loci (5.6% of total 215 polymorphic markers), the third allele was detected among the SAM strains. The genetic typing and developmental history suggested that the SAM strains were related inbred strains developed by the accidental crossing between the AKR/J strain and other unknown strain(s). Comparison of the distribution of the loci in the SAMP and the SAMR series revealed notable differences in the four regions on Chrs 4, 14, 16, and 17. This indicated that some of these chromosomal sites might contain the genes responsible for accelerated senescence in the SAMP series.

At least four loci and gender are associated with susceptibility to the chemical induction of lung adenomas in A/J x BALB/c mice

Four putative quantitative trait loci (QTLs) that influence susceptibility to the induction of lung adenomas by urethane in an F2 cross between A/J and BALB/cOlaHsd have been mapped. Following microsatellite typing of mice with resistant and susceptible phenotypes at 97 microsatellite marker loci, a major locus was identified on chromosome 18 with a lod score of 15. This was responsible for an 8- to 10-fold increase in tumor multiplicity in males and females, respectively, having the AA and CC genotypes at the D18Mit188 marker locus. It mapped close to Dcc (deleted in colorectal cancer). A locus on chromosome 4 (lod score 6.5) had the resistant allele in strain A/J and the susceptible allele in BALB/c, with a 14-fold difference in tumor multiplicity between mice of the AA and CC genotypes. This mapped close to the Cdkn2a (cyclin-dependent kinase inhibitor 2A) locus, which is commonly deleted in mouse lung tumors. Two loci with smaller effects (lod scores 3.03 and 3.25) were identified on chromosomes 1 and 11. There was also significant sexual dimorphism in tumor multiplicity both among 151 F2 hybrids and among 52 mice resulting from a backcross to strain A/J, with males having higher tumor counts than females.

Additional evidence that the K-ras protooncogene is a candidate for the major mouse pulmonary adenoma susceptibility (Pas-1) gene

A locus for mouse pulmonary adenoma susceptibility, Pas-1, has been mapped on distal chromosome 6, where the K-ras gene is located. Allele-specific activation and expression of the K-ras allele from the susceptible parent has been observed in lung tumors from F1 hybrid mice. We report here genetic mapping of lung tumor susceptibility genes in urethane-treated A x B and B x A recombinant inbred (RI) mice using microsatellite markers to seek further evidence for the K-ras gene as candidate for Pas-1. The K-ras genotype differs between the A/J and C57BL/6J progenitors of the RI strains, and distal chromosome 6 contained a major lung tumor susceptibility determinant in the RI mice. Additional evidence that Pas-1 is K-ras involved linkage analysis of (A/JOLaHsd x BALB/ cOLaHsd) F2 intercross mice whose parents shared the same K-ras genotype. In contrast to the results with the A x B and B x A RI strains, no distal chromosome 6 site was significantly associated with tumor development in these F2 mice. In addition to this major locus, linkage analysis of the RI mice revealed additional quantitative trait loci for susceptibility on chromosomes 10, 17, and 19. These loci may serve as modifiers of Pas-1. The relationship between the K-ras genotype and the frequency of K-ras mutations in urethane-induced lung tumors from the RI mice was also explored. All 18 tumor DNAs from RI mice with high susceptibility contained an AT-->TA transversion at the second base of K-ras codon 61. This was also true for DNAs from 27 of 27 (100%) tumors in mice with high intermediate susceptibility. In RI strains with a low intermediate susceptibility, the DNA from 39 of 47 (83%) tumors contained an AT-->TA transversion at codon 61, and only 13 of 21 (62%) tumors had this mutation in the most resistant group. This reflects a positive correlation between the frequency of K-ras mutations in lung tumors of A x B or B x A RI strains and their susceptibility to lung carcinogenesis. Since K-ras appears to be Pas-1, these results suggest that some RI mice that have the resistant K-ras or Pas-1 allele undergo tumor development by a K-ras-independent route.

Smad4 (homolog of human DPC4) and Smad2 (homolog of human JV18-1): candidates for murine lung tumor resistance and suppressor genes

In this study we investigated the mouse mad-related genes, Smad4/Dpc4 and Smad2 (homolog of JV18-1), as candidates for involvement in lung tumor resistance and suppression. These genes are located in a region of mouse chromosome 18 that is syntenic with human 18q21.1, where several genes that are mutated in various cancers have been mapped. A newly identified murine lung tumor resistance locus, Par2 has also been mapped to this region of chromosome 18. We found no mutations in the coding regions of either gene in 11 lung tumors from B6CF1 (C57BL/6 x BALB/c) mice by RT-PCR and SSCP/RFLP, suggesting that these genes are not mutated in lung carcinogenesis in this strain. Moreover, loss of heterozygosity in this region of chromosome 18 was not detected in 28 lung adenocarcinomas from B6CF1 mice, 17 lung adenocarcinomas from B6C3F1 mice or 18 lung adenocarcinomas from AB6F1 mice. These data provide evidence that a 'classical' tumor suppressor gene for mouse lung carcinogenesis in these strains does not reside in this region. In order to investigate Smad4/Dpc4 and Smad2 as candidates for the Par2 resistance locus mapped to this region, we also sequenced the coding regions of both genes in cDNA from normal lungs of A/J, BALB/c and C57BL/6 inbred strains of mice. No polymorphisms were detected in the coding region of Smad4. In Smad2, two sequence polymorphisms were identified that are not in the conserved regions of the gene. Northern blot analysis revealed no differential expression in normal lung tissue among the three strains for either gene. Thus, in this study we found no evidence that either Smad4 or Smad2 represents the Par2 lung tumor resistance locus or is a lung tumor suppressor gene in the B6CF1 mice.

Butylated hydroxytoluene exposure is necessary to induce lung tumors in BALB mice treated with 3-methylcholanthrene

Chronic butylated hydroxytoluene (BHT) treatment after a single administration of a carcinogen increases lung tumor multiplicity in some inbred strains of mice. We report that BALB/cOla and BALB/cByJ mice given a low dose (10 microg/g of body weight) of 3-methylcholanthrene (MCA) develop no lung tumors unless this is followed by chronic BHT exposure. Slightly higher MCA doses (15 and 25 microg/g) induce low lung tumor multiplicities (0.6 and 1.9 tumors/mouse, respectively) that are increased 12-26-fold by chronic BHT administration. This low-dose MCA/BHT model in BALB mice will facilitate the identification of genes regulating susceptibility to lung tumor promotion and pulmonary chemopreventative agents that act at a postinitiation site.

Tamoxifen causes gene mutations in the livers of lambda/lacI transgenic rats

Tamoxifen, a rat liver carcinogen, was administered to female lambda/lacI transgenic rats at a dose of 20 mg/kg body weight by gavage for 6 weeks, and the animals were sacrificed 2 weeks later. Tamoxifen induced liver DNA adducts and caused a significant increase in mutation frequency (MF) of approximately 3-fold at the lacI gene in liver DNA. Liver DNA from animals dosed with tamoxifen at 10 mg/kg also showed a similar increase in MF. The mutations were characterized by a raised proportion of: (a) G:C to T:A transversions; (b) insertions of base pairs; and (c) deletions of pairs of G:C base pairs. These observations indicate that tamoxifen induces a distinct spectrum of mutations compared with that found in controls. Toremifene, a noncarcinogenic analogue of tamoxifen with similar estrogenic/antiestrogenic properties examined at 20 mg/kg body weight using the same dosing regime as tamoxifen was not mutagenic. A single oral dose of the rat liver carcinogen aflatoxin B1 (0.5 mg/kg) also significantly raised the MF. In conclusion, although tamoxifen is not mutagenic in regulatory short-term tests, it is a gene mutagen in the rat liver.

Experimental design and husbandry

Rodent gerontology experiments should be carefully designed and correctly analyzed so as to provide the maximum amount of information for the minimum amount of work. There are five criteria for a "good" experimental design. These are applicable both to in vivo and in vitro experiments: (1) The experiment should be unbiased so that it is possible to make a true comparison between treatment groups in the knowledge that no one group has a more favorable "environment." (2) The experiment should have high precision so that if there is a true treatment effect there will be a good chance of detecting it. This is obtained by selecting uniform material such as isogenic strains, which are free of pathogenic microorganisms, and by using randomized block experimental designs. It can also be increased by increasing the number of observations. However, increasing the size of the experiment beyond a certain point will only marginally increase precision. (3) The experiment should have a wide range of applicability so it should be designed to explore the sensitivity of the observed experimental treatment effect to other variables such as the strain, sex, diet, husbandry, and age of the animals. With in vitro data, variables such as media composition and incubation times may also be important. The importance of such variables can often be evaluated efficiently using "factorial" experimental designs, without any substantial increase in the overall number of animals. (4) The experiment should be simple so that there is little chance of groups becoming muddled. Generally, formal experimental designs that are planned before the work starts should be used. (5) The experiment should provide the ability to calculate uncertainty. In other words, it should be capable of being statistically analyzed so that the level of confidence in the results can be quantified.

Discriminant analysis of biochemical parameters in liver disease

Discriminant function analysis has been used to investigate the relative value of six biochemical parameters (plasma ferritin, C-reactive-protein, bilirubin, alkaline phosphatase, glutamic oxaloacetic acid transaminase and albumin) in the diagnosis of liver disease. This was done among four groups totalling 70 subjects including healthy controls and patients with acute viral hepatitis, liver cirrhosis and primary hepatocellular carcinoma. Albumin had most value in distinguishing between groups, followed cumulatively by ferritin, alkaline phosphatase, C-reactive protein, bilirubin and glutamic oxaloacetic acid transaminase. However, if data on albumin, alkaline phosphatase, bilirubin and glutamic oxaloacetic acid transaminase had already been routinely collected, there would be no advantage in collecting data on ferritin and C-reactive protein. Any four of the six parameters would be of about equal value in distinguishing between diagnostic groups. When the data on all six biochemical parameters was combined in an optimum way, about 66% of all individuals could be correctly assigned to one of the four groups using biochemical markers alone. While the control subjects and patients with acute viral hepatitis formed a relatively well defined, tight cluster (apart from two patients with acute viral hepatitis), patients with liver cirrhosis and primary hepatocellular carcinoma were almost indistinguishable, using these biochemical parameters. If the latter two groups were pooled, then about 86% of subjects could be correctly classified.

Role of rat strain in the differential sensitivity to pharmaceutical agents and naturally occurring substances

The development of drugs to combat diseases, chemicals to improve food production, or compounds to enhance the quality of life necessitates, by law, the use of laboratory animals to test their safety. In order to simulate the human condition it is necessary to choose a species in which pharmacokinetic and toxicokinetic mechanisms are established and resemble those of humans. The advantages of the use of the rat in drug and chemical toxicity testing include (a) metabolic pathway similarities to humans; (b) numerous similar anatomical and physiological characteristics; (c) a large database, which is extremely important for comparative purposes; and (d) the ease of breeding and maintenance of animals at relatively low cost. However, the choice of rat can be complicated, especially when over 200 different strains of rat are known to exist. The aim of this review is to summarize genetically determined differences in the responsiveness of rat strains to drugs and naturally occurring chemicals and to show that susceptibility is dependent on the target organ sensitivities, which may also be strain dependent. It is suggested that detailed studies of strain differences may help to clarify toxic mechanisms. Such studies are usually best conducted using inbred strains in which the genetic characteristics have been fixed, rather than in outbred stocks in which individual samples of animals may differ, the phenotype is variable, and the stocks are subject to substantial genetic drift. The fact that strains may differ also needs to be taken into account in assessing the potential hazard of the chemical, particularly when a study involves only a single strain and therefore provides no assessment of likely strain variation.

DNA damage as assessed by 32P-postlabelling in three rat strains exposed to dietary tamoxifen: the relationship between cell proliferation and liver tumour formation

Tamoxifen was administered in the diet (420 p.p.m.) to female F344 (Fischer), Wistar (LAC-P) and LEW (Lewis) rats to determine for each strain the early morphological and biochemical changes associated with the subsequent development of liver cancer. Hepatic DNA damage, as determined by 32P-postlabelling, showed a cumulative increase with time from 500 adducts/10(8) nucleotides at 30 days to almost 3000 adducts/10(8) nucleotides after 180 days, with little difference between strains at this time point. A significant strain difference was found in the number of adducts present in the Fischer rats at 90 days, compared to the Wistar and Lewis strains. There was a marked strain differences in the time to development of liver tumours. After 6 months treatment, both Wistar and Lewis rats had tumours while none were seen in the Fischer animals. After 11 months, all of the Wistar and Lewis rats had developed liver carcinoma, while the Fischer rats developed liver carcinoma by 20 months. Depression in cell proliferation, relative to age-matched controls, was seen in the livers of Fischer rats after six months of exposure to tamoxifen, in contrast to an increase in the Wistar and Lewis rats. This observation is consistent with the promotion of foci to tumours and the subsequent progression of tumours to carcinomas in the latter two strains. These data may assist in establishing the possible risk factors, such as extent of DNA damage and increased liver cell proliferation, to women with long-term prophylactic exposure to tamoxifen.

Use of a multistrain assay could improve the NTP carcinogenesis bioassay

There are often large strain differences in the response of laboratory animals to toxic chemicals and carcinogens, with some strains being totally resistant to dose levels that cause acute toxicity and/or cancer in other strains. The current National Toxicology Program carcinogenesis bioassay (NTP-CB) uses only a single isogenic strain of mice and rats and may therefore miss some carcinogens. New short-term tests to predict mutagenesis and possible carcinogenesis are validated using data from the NTP-CB. If the animal data are inaccurate, it may hinder this validation. The accuracy of the NTP-CB could be improved by using two or more strains of each species without increasing the total number of animals. It would be possible to continue to use sample sizes of 48-50 animals, but subdivide these into groups of 12 animals of 4 different strains (48 animals total) per dose/sex group, for example, instead of 48 identical animals. This would quadruple the number of genotypes without any substantial increase in cost. Such a multistrain "factorial" design would, on average, be statistically more powerful then the present design and should increase the chance of detecting carcinogens that currently may give equivocal results or go undetected because the test animal strains happen to be specifically resistant. When strains differ in response, studies of differences in metabolism, pharmacokinetics, DNA damage/repair, cellular responses, and in some cases identification of genetic loci governing sensitivity may provide biological information on toxic mechanisms that would help in assessing human risk and setting permissible exposure limits. The NTP may have made the world a safer place for F344 rats and B6C3F1 mice.(ABSTRACT TRUNCATED AT 250 WORDS)

At least four genes and sex are associated with susceptibility to urethane-induced pulmonary adenomas in mice

Susceptibility to urethane-induced lung adenomas in mice has a polygenic mode of inheritance, with no obvious discontinuity in lung tumour counts among 37 AXB recombinant inbred strains. However, mean tumour counts were markedly higher in strains carrying the A/J allele at the Kras2 and H2 complex than in those carrying the C57BL/ allele. In 162 F2 hybrids and small numbers of both backcrosses between strain A/J (susceptible) and C57/BL/6 (resistant) mice, five factors influencing susceptibility were identified. Variation due to the 'major' Kras2 locus (chromosome 6) accounted for 60% of the total variation. 'Minor' loci linked to microsatellite markers Tnfb (in the H2 complex), D9Mit11 and D19Mit16 (on chromosomes 17, 9 and 19, respectively) accounted for a further 13% of the variation, and males had more tumours than females with sex differences accounting for 2% of the variation. No significant association with 32 other loci was detected. On a square-root transformed scale, heterozygotes at all marker loci were of intermediate susceptibility compared with homozygotes. The three minor loci and sex only affected lung tumour counts when at least one susceptible Kras2 allele was present.

Reduction of animal use: experimental design and quality of experiments

Poorly designed and analysed experiments can lead to a waste of scientific resources, and may even reach the wrong conclusions. Surveys of published papers by a number of authors have shown that many experiments are poorly analysed statistically, and one survey suggested that about a third of experiments may be unnecessarily large. Few toxicologists attempted to control variability using blocking or covariance analysis. In this study experimental design and statistical methods in 3 papers published in toxicological journals were used as case studies and were examined in detail. The first used dogs to study the effects of ethanol on blood and hepatic parameters following chronic alcohol consumption in a 2 x 4 factorial experimental design. However, the authors used mongrel dogs of both sexes and different ages with a wide range of body weights without any attempt to control the variation. They had also attempted to analyse a factorial design using Student's t-test rather than the analysis of variance. Means of 2 blood parameters presented with one decimal place had apparently been rounded to the nearest 5 units. It is suggested that this experiment could equally well have been done in 3 blocks using 24 instead of 46 dogs. The second case study was an investigation of the response of 2 strains of mice to a toxic agent causing bladder injury. The first experiment involved 40 treatment combinations (2 strains x 4 doses x 5 days) with 3-6 mice per combination. There was no explanation of how the experiment involving approximately 180 mice had actually been done, but unequal subclass numbers suggest that the experiment may have been done on an ad hoc basis rather than being properly designed. It is suggested that the experiment could have been done as 2 blocks involving 80 instead of about 180 mice. The third study again involved a factorial design with 4 dose levels of a compound and 2 sexes, with a total of 80 mice. Open field behaviour was examined. The author incorrectly used the t-test to analyse the data, and concluded that there was no dose effect, when a correct analysis showed this to be highly significant. In all case studies the scientists presented means +/- standard deviations or standard errors involving only the animals contributing to that mean, rather than the much better estimates that would be obtained with a pooled estimate of error. This is virtually a universal practice.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA fingerprinting for genetic monitoring of inbred laboratory rats and mice

DNA fingerprinting using a nonisotopically labeled minisatellite probe provided a valuable technique for genetic monitoring/quality control of laboratory rodents. Each of 12 inbred rat strains had a unique fingerprint pattern, and colonies separated for over 20 years had identical or nearly identical patterns. Strain LOU/Iap, which is known to have been genetically contaminated in the past, was clearly different from strain LOU/CN, supporting previous findings of studies using biochemical markers. Inbred strains of mice were also found to differ from each other. The F1 hybrid between C57BL/6 and CBA/Ca could not be distinguished from C57BL/6 by using DNA fingerprints, although they could be distinguished by using biochemical markers. Some congenic strains differed from their inbred partner. A suspected genetic contamination of MRL/Mp-lpr mice could not be detected in a sample of the breeding colony by using biochemical markers; however, DNA fingerprints from the suspect animals clearly demonstrated genetic segregation. DNA fingerprinting will be of particular value in investigating suspected problems as only a small sample of fresh, frozen, or ethanol-preserved tissue is needed. Thus, the actual suspect animals can be studied, rather than samples from a breeding colony from which contaminated animals may already have been eliminated.

Genetic variation in outbred rats and mice and its implications for toxicological screening

There are two basic types of laboratory rodent used in toxicological screening. Isogenic (inbred) strains are rather like clones of genetically identical individuals whereas outbred stocks are usually more variable, though the amount of variability depends on the previous history of the colony. In some cases outbred stocks may be genetically quite uniform. Many different strains of both types are available. Both types and a variety of strains are used for toxicological screening. There is clear evidence of important genetic variation both in spontaneous disease and in response to toxic agents, yet little account is taken of this in choosing suitable animals. Three options appear to be available. The first is to ignore genetic variation and use a single isogenic strain. However, if the strain happens to be insensitive to the test chemical, a toxic chemical may be judged to be relatively safe. The second option would be to synthesize a genetically heterogeneous stock by crossing two or more strains. However, this could lead to both increased false positive and false negative results as experimental "noise" either obscures true treatment effects, or is mistaken for a treatment effect. The third option is to use more than one strain, but without increasing the total number of animals used. This would provide a broad range of genotypes, so reducing the chance that they are all insensitive, without increasing experimental noise. This appears to be the only sensible way of broadening the genetic base in toxicological screening. Where strain differences are found, they may provide a tool for studying toxic mechanisms, which may be helpful in extrapolating to human populations.

FRAR course on laboratory approaches to aging. Genetic quality control in laboratory rodents

Laboratory rodents are widely used in gerontological research. Many different strains are available, and superficially there is little to distinguish them (many are albino), although they may differ markedly in life span and pattern of spontaneous disease, as well as for a whole range of biochemical, immunological, behavioural and physiological characteristics. As experience has shown that strains can easily become muddled, some form of genetic quality control is essential. There is no single method of genetic quality control which can be recommended for all occasions. Methods based on identification of Mendelian genetic markers, such as biochemical and immunological polymorphisms can be sensitive, but may be expensive and require considerable expertise. Methods based on simultaneous study of several markers, such as skin grafting and polyvalent strain-specific antisera, may be cheaper but less flexible. DNA fingerprinting and the use of microsatellite markers appear to be the methods of choice in the future, though these are still expensive and require considerable expertise.

The scope for improving the design of laboratory animal experiments

The factors which need to be taken into account in designing a 'good' experiment are reviewed. Such an experiment should be unbiased, have high precision, a wide range of applicability, it should be simple, and there should be a means of quantifying uncertainty (Cox 1958). The relative precision due to the use of randomized block designs was found to range from 96% to 543% in 5 experiments involving 30 variables. However, a survey of 78 papers published in two toxicology journals showed that such designs were hardly used. Similarly, designs in which more than one factor was varied simultaneously ('factorial designs') were only used in 9% of studies, though interactions between variables such as dose and strain of animal may be common, so that single factor experiments could be misleading. The consequences of increased within-group variability due to infection and genetic segregation were quantified using data published by Gärtner (1990). Both substantially reduced precision, but toxicologists continue to use non-isogenic laboratory animals, leading to experiments with a lower level of precision than is necessary. It is concluded that there is scope for improving the design of animal experiments, which could lead to a reduction in animal use. People using animals should be required to take formal training courses which include sessions on experimental design in order to minimize animal use and to increase experimental efficiency.

Genetic quality control of laboratory animals used in aging studies

Laboratory rodents provide a useful model for aging processes in humans. Various genetic "types" of stock are available, including outbred stocks and inbred strains and their derivatives. Inbred strains, which can be regarded as clones of genetically identical individuals, provide a powerful research tool for studies in many disciplines, including gerontology. However, some form of genetic quality control is essential to ensure that the strains are authentic. Single gene polymorphisms, particularly those detected by electrophoresis and immunological methods, provide a powerful tool for such quality control, though these methods are expensive and require considerable expertise. Methods based on several loci studied simultaneously include skin grafting and polyvalent alloantisera. These methods are often quick and technically relatively easy, but are less flexible than the single locus methods. Methods based on DNA restriction fragment length polymorphisms (RFLPs) fall into both categories depending on whether a single locus or a multilocus probe, such as the fingerprinting probes, is used. These DNA-based methods have many advantages, and are likely to be the methods of choice in the future.

Genetic factors in neurotoxicology and neuropharmacology: a critical evaluation of the use of genetics as a research tool

Animals have evolved a detoxication system to enable them to survive in a hostile chemical environment in which foods contain many non-nutrient chemicals. Detoxication depends on enzymes which are often genetically polymorphic. As a result, inter-individual variation is common, and in humans several Mendelian loci have been identified. However, most variation in response is probably due to the action of several genes. Genetic variation in response to the neurotoxin MPTP and to chemically and physically-induced seizures is reviewed. In the former case, differences between pigmented and white mouse strains have been noted which are consistent with the hypothesis that humans are more sensitive than mice or rats because of the presence of melanin in human brains. However, variation in sensitivity probably also depends on other genes. In the case of audiogenic seizures, a single locus has been identified and mapped, but its relationship with seizures induced by other agents is not clear. Genetic variation in response to alcohol is also discussed. The failure of most toxicologists to consider genetic variation as a potentially confounding variable, and as a powerful research tool, is discussed critically in relation to non-repeatability of research on the neurotoxic effects of lead, and in relation to the genetic variation in MPTP, seizures, and alcohol response already noted. It seems clear that genetic methods provide a powerful research tool which is largely being ignored by toxicologists.

Tich: a mutant causing disproportional growth in the mouse

A spontaneous mutation 'tich' (gene symbol tch) appeared as a recessive mutation in inbred mice of strain A. TL. Homozygotes are rather dumpy mice of approximately normal weight but with short limbs and tail. Skeletal measurements on backcross siblings show that the mandible bones are almost normal but long bones and some parts of the pelvic and pectoral girdles are short. Although tich resembles brachypodism phenotypically it is not linked to agouti, and does not match the description of any other skeletal mutation. There was some evidence for weak linkage with albinism on chromosome 7. The mutation has reappeared amongst the A. TL mice of a UK commercial breeder and may have been accepted as the norm for A. TL amongst some European users of this mouse.

Mouse strain differences in resident peritoneal cells: a flow cytometric analysis

A flow-cytometric study of resident peritoneal cells among 8 mouse strains showed a more than twofold variation in the ratio of macrophages to macrophages plus lymphocytes, ranging from 27% in A/J to 62% in C57B/L10, with significant strain differences in a number of other cellular parameters. There was a particular deficiency of lymphocytes in strain CBA/N, which carries the xid mutation. Studies of the phagocytosis of fluorescent beads also revealed large differences in the number of beads taken up, ranging from 0.99 per cell in MFI to 1.64 per cell in BALB/c mice in a 20-min period. The total number of peritoneal cells collected also varied between strains, ranging from 2.75 x 10(6) in CBA/Ca to 5.85 x 10(6) in MF1. The total yield of macrophages per mouse ranged from 0.93 x 10(6) in A/J to 3.16 x 10(6) in C57BL/10. These differences should be taken into account when designing experiments which use resident peritoneal cells.

CBXC: a set of recombinant inbred strains between CBA/Ca and BALB/c

A new set of nine recombinant inbred strains designated CBXC-1 to CBXC-9 has been developed from a cross between CBA/Ca female and BALB/c male mice. All of the strains have been brother x sister mated for more than 20 generations, and have been characterized at eight electrophoretic, one immunological and two coat colour loci at which the progenitor strains differ. This set of strains should be useful in investigating the genetics of any characters which differ between the two progenitor strains.