Use of transgenic animals in understanding molecular mechanisms of toxicity

Understanding molecular mechanisms of chemical toxicity and the potential r isks of drugs to man is a pivotal part of the drug development process. Wit h the dramatic increase in the number of new chemical entities arising from high throughput screening, there is an urgent need to develop systems for the rapid evaluation of potential drugs so that those agents which are most likely to be free of adverse effects can be identified at the earliest pos sible stage in drug development. The complex mechanisms of action of chemic al toxins has made it extremely difficult to evaluate the precise toxic mec hanism and also the relative role of specific genes in either potentiating or ameliorating the toxic effect. This problem can be addressed by the appl ication of genetic strategies. Such strategies can exploit strain differenc es in susceptibility to specific toxic agents or, with the rapidly developi ng technologies, can exploit the use of transgenic animals where specific g enes can be manipulated and subsequent effects on chemical toxicity evaluat ed. Transgenic animals can be exploited in a variety of ways to understand mech anisms of chemical toxicity. For example, a human gene encoding a drug meta bolizing enzyme can be directly introduced and the effects on toxic respons e evaluated. Alternatively, specific genes can be deleted from the mouse ge nome and the consequences on toxicological response determined. Many toxic chemical agents modulate patterns of gene expression within target cells. T his can be used to screen for responses to different types of toxic insult. In such experiments the promotor of a stress-regulated gene can be ligated to a suitable reporter gene, such as lacZ, or green fluorescent protein, a nd inserted into the genome of an appropriate test species. On administrati on of a chemical agent, cells which are sensitive to the toxic effects of t hat chemical will express the reporter, which can then be identified using an appropriate assay system. This latter strategy provides the potential fo r screening a large number of compounds rapidly for their potential toxic e ffects and also provides information on tissue and cellular specificity. Ex periments using transgenic animals can be complex, and care must be taken t o ensure that the results are not affected by background activities within the species being used. For example, the introduction of a specific human c ytochrome P450 gene may have no effect on the metabolic disposition of a dr ug or toxin because of the background activity within the mouse. As the tox icity of a chemical agent is determined by a wide range of different factor s including drug uptake, metabolism, detoxification and repair, differences between man and the species being used could potentially generate a toxic response in the animal model whereas no toxicity may be observed in man. In spite of these confounding factors, the application of transgenic animal s to toxicological issues has enormous potential for speeding up the drug d iscovery process and will undoubtedly become part of this process in the fu ture.