not necessarily for all human forms of a monogenic disease.
Mouse models, as well as other species used as models, should have some or many of the clinical and pathological features of the human disease to which they are compared. Where these features overlap were historically considered to be where the models were useful (Figure 4.1).
An informative example of the complexity of phenotype‐genotype relationships in both mice and humans, and the consequent difficulty in establishing a model, can be found in CHARGE syndrome [14]. CHARGE syndrome is an extremely phenotypically diverse syndrome with coloboma, heart defects, choanal atresia, retardation (of growth or development), genitourinary malformation, and ear abnormalities, being the most common phenotypes. The phenotypic variability raises the question as to whether this is indeed a genetic or a clinical definition of the syndrome, suggesting that the phenotypic clusters could be due to multiple genetic origins. As it turned out, the identification of the CHD7 gene in humans lead to huge simplification of the understanding of the syndrome and identified at least some allelic:phenotypic spectrum relationships [15]. This stimulated the investigation of mouse N‐ethyl‐N‐nitrosourea (ENU) mutations showing related phenotypes in balance function, and the identification of mouse mutants in Chd7 but which nonetheless did not model all of the phenotypes seem in humans [16]. The building of mouse models since that initial discovery has generated very valuable and useful strains for the understanding of both genotype/phenotype relationships, together with the mechanisms underlying them, but there still does not exist a single mutation on a single background that completely recapitulates all of the phenotypes expressed in CHARGE allelic series [17]. This example illustrates how identification of models through phenotype similarity can be extremely difficult, but that modeling of a restricted set of phenotypes can be scientifically extremely useful [18].
Figure 4.4 Mouse models identify subtypes of junctional epidermolysis bullosa. A spontaneous hypomorphic allele of Lamc2jeb had different phenotypes when moved to different congenic backgrounds. Using this observation the underlying gene for each variation was identified. The range of phenotypes of junctional epidermolysis bullosa were gathered from the literature by text mining using disease labels from the DERMO ontology and accessed through the Aber OWL skin disease phenotypes database.
Source: http://aber‐owl.net/aber‐owl/diseasephenotypes.
Table 4.1 Criteria for accurately defining an animal model, regardless of how it was created, for a human disease.
Source: Based on Scarpelli et al. [11].
| Accurately recapitulates the clinical and pathological features of the human disease (responds to similar drugs in both species).Primary molecular defect similar (ideally similar or identical mutation in the same).Readily available to other investigators:Genetic, breed mice to produce more as needed.Repository for archiving model for public distribution.Experimental induction easily done if not a genetic‐based model.Reproducibility (stability of model, inbred strains).Genetic manipulability (arsenal of genetic tools). |
Figure 4.5 Fitting mouse models to the concepts of human disease. The mouse beige mutation (Lystbg) was found early on to be a good model for Chediak‐Higashi syndrome in humans. By contrast, the mild form of the hairless gene (Hrhr) mutation was considered 30 years ago to be a model for normal human skin and was used for sun screen testing.
Some monogenic mouse mutations have features that are essentially identical to or at least very similar to those seen in humans with the same disease. An excellent example is the beige mouse [19] (one of many mammalian species [20, 21] that have mutations in the lysosomal trafficking regulator [Lystbg] gene) as a model for Chediak‐Higashi syndrome in humans (Figure 4.5), which can be called a phylogenetically valid model.
One of the big problems with working up (phenotyping) new mutants is that investigators often focus only on the organ or disease process, or molecular pathway of interest rather than do complete analysis of the animal. This results in incomplete definition of the mouse making accurate comparisons to the proposed human disease. Likewise, this is often the problem when defining the human disease, where clinical specialists focus on their area of interest and ignore other organ systems. Combined, this becomes the proverbial blind men describing the elephant [22].
By contrast, before the gene responsible for the hairless mouse was identified, many investigators used it as a model for “normal human skin” because the mice lacked hair as adults, making it useful for topical drug or ultraviolet light carcinogenesis studies. The more severe allelic mutation, rhino (Hrrh) was used for testing drugs, such as retinoic acid analogs, for treating wrinkles [23–26]. Other than being without hair neither were accurate models for normal human skin, but lesions were later shown to be a model for human papular atrichia [27, 28]. The orthologous human disease was eventually identified [29] as was the gene, which allowed direct comparisons between the correct human disease with the correct mouse model [29–34]. This has been the case for numerous single mutant gene mouse models where finding the underlying gene allowed for accurate comparisons with human patients with mutations in the orthologous gene (Figure 4.6).
The one gene–one disease concept remains an oversimplification of disease. Maintaining a single gene mutation as a colony on an inbred strain minimizes variability (a relatively reproducible model) but does not eliminate it within that strain as the mice are not absolutely identical genetically [35]. Many diseases have a sexual dichotomy, and environmental effects, such as diet, can result in different phenotypes at different institutions [36]. Moving the mutation from one inbred strain onto another strain (creating congenic strains) can result in loss to exaggeration of the phenotype, or even to a different phenotype due to the effects of modifier genes or gene redundancy. For example, mouse models of human cancer genes often result in a mouse model with a similar but not identical phenotype [37], where mouse genetic background may play a role in the phenotype. Trp53 null mutant mice (allele not designated) developed high rates of mammary tumors when bred on the BALB/cMed background but not C57BL/6 or 129/Sv congenic backgrounds, indicating a more complicated genetic predisposition toward mammary tumorigenesis than only mutations in this one gene [38]. These concepts are discussed in detail in Chapter 3, Genetics. However, these observations explain the inability of one inbred mouse strain carrying one or more mutated genes to model all variations seen in human patients. This is because humans are, for the most part, outbred and live in a relatively “dirty” environment. One concern raised about mouse models has been that mice cannot be good models of human disease because they live in environmentally controlled housing (boxes) under pathogen‐free conditions with abundant food, water, and access to good veterinary care. When one steps back and looks at how humans exist in a modern society, they too live in a box with many of the same benefits as mice [39].
