SLOVENIAN VETERINARY RESEARCH

The use of animals in biomedical research57Induced models are healthy animals in whichthe pathologic condition is experimentally induced(for instance, infections or induction of diabetesmellitus with encephalomyocarditis virus). On theother hand, spontaneous models have naturallyoccurring genetic variants which resemble or canbe xenografted to resemble diseases in humans (forinstance, nude mice, which enable the study of heterotransplantedtumors). Majority of these modelsare mice and rats. Transgenic animals (rodents, rabbits,farm animals, fish, etc.) have been developedwith genetic engineering and embryo manipulationmethods, however, because many diseases are polygenicin nature, the use of these models will requiremore research to establish the contribution of allgenes involved in the development of pathologicalconditions. Negative models are used in studies onthe mechanisms of resistance, since these animalsdo not develop the investigated disease, and finally,orphan models are animals with the disease, whichhas not yet been described in humans, such as felineleukemia, papillomatosis, bovine spongiformencephalopathy, but the research done might beof use, if similar conditions should be described inhumans (4).One of the most important considerations whenthe scientists determine that the use of laboratoryanimals is necessary is the selection of the species,breed and strain to be used in experiment (4). Inmany fields of biomedical research and also in cancerresearch, mice and rats have been traditionallyused, because they are relatively cheap, have shortlife span, high reproduction rate and are easy tohandle. However, other animal species are also used,but either they are not as cost-efficient or manyethical issues were raised, especially in the case ofnon-human primates. Another important reasonfor the widespread use of rodents is that advancesin genetic engineering have enabled scientists todevelop “humanized” mice, which are either immunodeficient(engrafted with human haematopoieticcells, tissues or stem cells), or transgenic, which expresshuman genes that were inserted in the mousegenome (1). The first type can be xenografted withhuman tumors or used to study the effect of immunityto tumor or viral growth, AIDS, lupus, psoriasisand other diseases (1, 16). Also, the researchers havedeveloped “humanized” mice strains to study infectionswith viruses, bacteria and parasitic protozoa(Dengue virus, EBV, HCV, Mycobacterium tuberculosis,Plasmodium falciparum), the developmentand function of the immune system, autoimmunityand human haematopoiesis (1). Nevertheless, workingwith animals requires that scientists take intoconsideration: a careful design of the experiment,the responsible use of laboratory animals and whenthis is scientifically appropriate and valid – a reductionin the number of animals used for research andtesting, and finally, when possible, to develop anduse alternative methods (5, 8, 17, 18).Laboratory models in cancer researchAnimal models have been critical in the studyof the molecular mechanisms of cancer and inthe development of new antitumor agents (19). Althoughthe mice, especially “humanized” ones, stayas the most important animal model, several otherorganisms are also used for cancer research. Toname just a few, Drosophila flies were used to studyand identify genes involved in growth regulation,yeast research opened new views on mechanismsof chromosome fragility, signaling pathways andseveral other aspects of the disease pathology andRNA interference studies in Caenorhabditis elegansrevealed approximately 350 genetic interactionsbetween genes functioning in signaling pathways,which are also frequently mutated in human diseases(7, 20, 21). These genetic maps could be usedin identifying new components of specific diseasederegulatedpathways (7).Nevertheless, the majority of knowledge aboutcarcinogenesis, cancer therapy, angiogenesis andmetastasis comes from studies with “humanized”murine models (1, 16). The first such models wereimmunodeficient nude mice, which supported theengraftment of human tumor cells (16). CB-17-scid strain was discovered in 1983, when Bosmaand co-workers identified a mutation in a proteinkinase Prkdc scid , causing a severe combined immunodeficiency(22). These mice could be engraftedintravenously or subcutaneously with some humanneoplasms, whereas solid tumors were grown underthe renal subcapsule (1, 16). However, innate immunity- the activity of natural killer cells (NK-cells)- limited tumor growth and prevented metastasis(16). Next developed model, non-obese diabetic-severecombined immunodeficiency (NOD-scid) miceallowed growth of human lymphomas and leukaemias,due to a more humanized microenvironment,achieved by injection of human peripheral blood orbone marrow cells (1, 16). The first such model wasdescribed in 1995 and was generated by crossingthe scid mutation from CB-17 mice onto the NOD