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| This article will appear in a forthcoming issue of Trends in Pharmacological Sciences. | |
Abstract
The general theme of this year's International Congress of Toxicology, "Toxicology and Sustainable Development - Meeting the Challenge," pointed to the global challenges for toxicology in the 21st century. Accordingly, major parts of the program were devoted to new vistas in toxicology, to new methodological tools and further developments, as well as to the potentials of new methods to serve within the applied toxicological fields of characterization and quantitative assessments of risks of chemical substances [1].
A particular highlight was the plenary lecture by Kenneth Olden (NIESH, Research Triangle Park, NC, USA). The essence of the lecture is best summarized by the proverb of Judith Stern that the speaker himself used: "Genetics load the gun - environment pulls the trigger."
So far the genome has been considered the major determinant for the development of diseases such as cancer. Although in recent years a large number of epidemiological studies has considered human genetic diversity, only few authors have considered that higher susceptibilities to chemicals might result from complex gene-environment interactions. The multiplicity of dimensions to this hypothesis might be reflected by metabolism because a toxic metabolite of a substance (present in the environment) is generated or detoxified to different extents in different individuals (depending on the genetics of the individual). Furthermore, exposure levels and the phenotype(s) of an individual might vary with age (representing the time axis).
This subject was specifically addressed in the workshop "Polymorphisms of Xenobiotic Metabolizing Enzymes - Toxicological Significance." Paolo Vineis (University of Torino, Italy) contemplated these aspects in his lecture. Biologically highly penetrating genes are likely to explain just 5% of all human cancers whereas the total proportion that is explained by low penetrating genes is not clear at all. The picture emerging from a recent literature survey seems to be one of considerable homogeneity within an ethnic group and heterogeneity between different ethnic groups. In conclusion, it is suspected that risk for developing cancer that is attributable only to genetic susceptibility is lower than those related to smoking and other lifestyle, occupational, or environmental exposures.
In his plenary lecture, Olden made the claim that complex interactions between the environment and the body (genes) are of principal importance. Although the genome of the human does not change during a lifetime, with the exception of specific cases of genetic damage (again caused by the environment), the environment changes all the time. However, the body's phenotype will change markedly over time and in response to the environment. The new area of proteomics describes the hallmark of this situation. Because the majority of chronic diseases are multifactorial, the complexity of parameters involved must be considered in future studies. Olden favored the "candidate gene approach" in studies of the chain of molecular events in toxicology. External exposure is determined by lifestyle but the ultimately effective exposure to toxic metabolites at target site(s) within the body is modulated and thereby largely determined by genetic factors.
Exposure to toxicants can also modulate the phenotype in ways other than by damaging DNA resulting in a change of the genome. The function of proteins might be affected by toxicants directly. As Jennifer Pietenpol (Vanderbilt University School of Medicine, Nashville, TN, USA) pointed out, the disruption of microtubules might affect the function of the important tumor suppressor protein p53 with similar effects to those induced by genetic damage. The key role of p53 in growth arrest and apoptosis following cell stress affects over 250 target genes. Further elucidation of the signaling pathways is required to understand the different cellular responses to toxicants.
Although microarray systems allow the relative quantitation of expression levels of thousands of genes simultaneously, much concern remains about the use and interpretation of the "quick and dirty" data generated. Microarray techniques can be applied well to determine the effects of classes or groups of toxicants on gene expression. So far, only dose-response curves, but no time dependencies, have been determined (e.g. for defined liver toxicants). These data are reproducible and display dose dependence. Furthermore, changes in expression levels of gene clusters have corresponded with histopathological, clinical and other toxicological parameters. Hitherto unknown involvement of genes in particular clinical responses could also be identified using this approach (Roger Ulrich, Abbott Laboratories, Abbott Park, IL, USA). Used in conjunction with transgenic animal models, these techniques could provide a powerful tool with which to search for modes of action of toxicants (Bill Pennie, Syngenta Central Toxicology Laboratory, Macclesfield, UK). The generation of a public database of mechanisms of actions is already on its way. Chemicals with known mechanisms - in this case the hepatic effects of phenobarbital versus peroxisome proliferators - have been used to produce a database for blind testing of 24 compounds: 23 of these chemicals were in fact classified in the right direction (Richard Paules, NIEHS, Research Triangle Park, NC, USA).
Determination of gene expression at the level of RNA, strictly, does not always correctly predict changes in protein levels. Such changes in protein expression can be visualized using the methodology of proteomics. Automated procedures are soon to include a third dimension to the two-dimensional gel electrophoresis. Automated spot picking, digesting of the protein and analysis of the resulting peptides lead to peptide fingerprints and subsequent database identification. Changes in the chemical modifications of the proteins are also recognized. Several general features make this approach different from the RNA microarray approach. For example, effects on the entire proteome of a given cell type are examined, and changes in the expression of unknown proteins are observed. These can now be analyzed further. Furthermore, there are different choices of starting materials for analysis so that effects on particular cell compartments are accessible (Hanno Langen, F. Hoffmann-La Roche, AG Basel, Switzerland).
Further advances in technology will soon allow the swift generation of invaluable databases for both the "genomic" RNA microarray approach and the "proteomics" approach, with the potential of facilitating toxicological risk assessments for therapeutic drugs, as well as for both industrial and environmental toxicants.
Nine years ago, on the occasion of the sixth International Congress of Toxicology held in Rome in 1992, the late Gerhard Zbinden, recipient of the EUROTOX Merit Award and of many other honors, issued his visionary editorial in Trends in Pharmacological Sciences entitled: "The three eras of research in experimental toxicology." He opposed the traditional eras of "discovery" and of "biomechanistic investigations" to the upcoming era of "individual expression" (figure 1) [2]. He predicted a growing realization of the decisive importance of the genetic background of exposed subjects and the identification of increasing numbers of genetic traits: "It is probable that this is only the beginning of a much more dramatic development that will uncover further factors that greatly affect the expression of toxicity in individuals exposed to chemicals." [2]
This is exactly what we are witnessing today!
Key Challenges for Toxicologists in the 21st Century - a look at unresolved issue and novel technologies. From Trends in Pharmacological Sciences, 2001, 22:6:281-285. Full text available from BioMedNet.
Genetic Susceptibility to Adverse Drug Reactions - uses specific examples to illustrate concepts, problems, and future directions. From Trends in Pharmacological Sciences, 2001, 22:6:298-305. Full text available from BioMedNet.
The Environmental Mutagen Society and the Emergence of Genetic Toxicology: A Sociological Perspective - offers some reflections on the history of genetic toxicology. From Mutation Research/Reviews in Mutation Research, 2001, 488:1:1-8. Full text available from BioMedNet.
Toxicogenomics - offers extensive information on toxicogenomics and microarrays including examples, links, protocols and experimental results. From the Chemical Industry Institute of Toxicology.
National Institute of Environmental Health Sciences - provides news, information, and publications as well as links to the Environmental Health Information Service and intramural programs including the National Toxicology Program, the Environmental Genome Project, the National Center for Toxicogenomics, and the Microarray Center.
Society of Toxicology Meeting: Toxicologists Hit the West Coast, Toxicogenomics: Toxicologists Brace for Genomics Revolution - two recent articles from Science magazine.
Society of Toxicology - a resource for advancing the science and applications of toxicology.
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