Animal models of obesity

Obesity stems from a prolonged imbalance between the levels of energy intake and expenditure, with the resultant surplus being stored as body lipids. Our understanding of the regulation of food intake and the physiological basis of differences in energy expenditure is owed, in large part, to studies made on animals. Moreover, animal models have been a cornerstone of studies of environmental effects, such as epigenetics, responses to high-fat and low-calorie diets and the identification and development of pharmaceuticals for obesity treatment. This review provides some examples of the animal work that has been performed, and focuses on the variation in approaches that have been taken and their potential, rather than aiming to be a comprehensive summary. The genetically obese ob/ob mouse is a classic case of a spontaneous single-gene loss-of-function mutation that generates massive obesity. Characterizing the genetic basis of this mutation revealed that the defect is a single base pair deletion, which results in a premature stop codon in a gene expressed almost exclusively in adipocytes. The gene product was called leptin (1). The gene is recessive, but mice that are heterozygotic (i.e. with one copy only, rather than two) for the defect produce reduced amounts of leptin and are moderately overweight. There are now at least 10 known single-gene loss-of-function defects that cause massive obesity and have been completely genetically characterized. In all the cases where such defects have been discovered, they have initially resulted from spontaneous mutational events in large breeding establishments. Discovery of the defect in the first place has been down to sharp-eyed observers noticing an abnormal obese phenotype (a phenotype describes the observable physical and biochemical characteristics of an individual) and then selectively breeding to expose the responsible gene in homozygotes (which have two copies of the gene). In some cases, the gap between discovery of the original phenotype and its subsequent genetic and physiological characterization has been very long, for example, in the case of leptin, about 50 years. These delays were largely down to the fact that, prior to the 1980s, the tools were not available to clone genes and sequence them. The development of high-throughput sequencing capability, and the completion of the mouse and rat genomes in the early part of the new millennium, means that such delays between the discovery of a new mutant and its characterization are likely to get progressively shorter. Nevertheless, there is an inherent randomness in the discovery of single-gene defects, which depends primarily on unusual animals being identified in routine colony checks. This means that genes with only minor effects on the heterozygote are unlikely to be discovered. This ability to only detect genetic defects that have major loss-of-function effects in heterozygotes means that, almost by definition, these defects will only affect a minor proportion of the population. Consequently, while the discovery of the ob/ob mouse and the leptin gene was a major leap forwards, genetic screens of the human population have revealed trivially small numbers of obese individuals that have loss-of-function mutations in this gene (2). The same is true for all the other genes that have been discovered as spontaneous single-gene defects and characterized genetically and functionally (3). The real progress that study of these genes allows is to further our understanding of how the energy regulation system works. In fact, many of the genes that appear important in single-gene mutation events seem to be involved in a common pathway that includes leptin and insulin as signalling molecules (4). Our knowledge of this pathway has been crucially informed by characterization studies of spontaneous single-gene loss-of-function defects. The arbitrary nature of relying on spontaneous mutational events resulting in major loss-of-function mutations of critical genes has led to attempts to accelerate the process by increasing the mutation rate artificially. This is performed by treating animals with mutagenic chemicals, or exposing them to radiation (5–7). Several countries have programmes of research that include such artificial mutagenesis studies (e.g. Gailus-Durner et al. (8)). The animals generated from these experiments may inform not only our understanding of energy regulation but many other aspects of animal function. The major problem with this approach is the cost of phenotyping. As the mouse genome consists of around 30 000–40 000 genes and that a given mutation may only produce a loss-of-function effect in say 3–5% of cases, to discover the effect of loss of function in a given gene might require phenotyping over half a million animals. Just measuring body weight of animals on a single occasion in these sorts of numbers would be prohibitively expensive. Moreover, as with spontaneous genetic mutations, the effects still need to be present and of considerable effect in heterozygotes. Little progress has therefore been made from these studies in the context of understanding energy regulation. An enormous number of transgenic models with obese or lean phenotypes have been created since the characterization of the first obesity genes (9,10). The 2005 update of the human obesity gene map cited 248 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity (3). With traditional transgenic technologies, there was little control over where or how many copies of genes were introduced into the genome. However, current sophisticated gene-targeting strategies permit investigators to manipulate the genome in ways that essentially allow the introduction of virtually any desired change. Furthermore, advanced techniques allow alterations to the genome that act only at specific times, or that are expressed only in specific tissues or cell types (11).