Transgenic animals in the evaluation of compound efficacy and toxicity: Will they be as useful as they are novel?

Abstract

1. Construction of transgenic mice is predicated upon inserting foreign DNA into native host DNA and having this expressed in the germline. This may be accomplished by nuclear injection, retroviral vectors or use of embryonic stem (ES) cells. 2. Expression of novel structural genes may be reasonably directed by the judicious use of an accompanying promoter/enhancer sequence. Insertion of foreign genes may be designed to result in phenotypic expression of a novel trait or ablation of a native gene or gene product. 3. Resulting transgenic mice offer significant utility as models of human diseases and a unique opportunity for investigating immune and metabolic pathways as well as for exploring mechanisms of development, mutagenesis and teratogenesis. 4. Use of transgenic animals in drug development has considerable potential although realization of this potential will take time. Constructing transgenics is only the first step in a complex series of events culminating in understanding the consequences of imposing novel genetic material on an intact, highly integrated living system. Practical use of transgenic animals will depend upon substantial effort being spent in investigating and validating the phenotypic consequences of gene transfer.

Preclinical development of keliximab, a Primatized™ anti-CD4 monoclonal antibody, in human CD4 transgenic mice: characterization of the model and safety studies

“…did not differ between normal BALB/c and HuCD4/ Tg mice, indicating that replacement of the murine CD4 gene did not affect histogenesis of the immune system. HuCD4/Tg mice have survived for [18][19][20][21][22][23][24] months in our facilities with no unexpected pathologic developments.…”

Genetically Engineered Animals in Drug Discovery and Development: A Maturing Resource for Toxicologic Research

“…This strategy reduces one major limitation in the interpretation of preclinical toxicity data: the difficulty in defining the extent to which animal physiologic responses mimic those in humans. An animal engineered to express human proteins is still an animal, so caution must be used in accepting the genetically engineered mouse phenotype, or lack thereof, produced by insertion of an engineered human gene as a strict analog of naturally-occurring genetic events that occur in humans (Cordaro 1989;Liggitt & Reddington 1992). Nevertheless, such ''humanized'' animals afford a direct in vivo means of investigating the impact of novel chemical entities toward human metabolic pathways, and in many cases should provide a better proxy during preclinical toxicity bioassays than will the unmodified animals that are employed in conventional test batteries.…”

Use of Genetically Engineered Mice in Drug Discovery and Development: Wielding Occam's Razor to Prune the Product Portfolio

“…However, the phenotypic response of "transgenic" mice to a given xenobiotic may depend on such experimental variations as duration and route of exposure. Even for "transgenic" mice in which lesions have been induced by expression of human genes, one must exercise caution in accepting the phenotype, or lack thereof, as a strict analog of naturally occurring lesions in humans (27,83). In other words, a "humanized" mouse-a "transgenic" animal that contains human genetic material-is still a mouse.…”

“…Insertion of a targeting sequence that contains a "transgene" with constitutive activity may replace the role of the deleted endogenous gene and modify or prevent the knock-out phenotype [a "knock-in"; (53,104)]. In contrast, transgenic DNA microinjected into a zygote (a fertilized singlecelled ovum) is incorporated into a GEM's genome at random (83), usually as multiple copies ( 13) at a single site. Because each integration site is different, each transgenic GEM founder has a unique genetic background.…”