Genetics of human obesity

Genetics of human obesity

Best Practice & Research Clinical Endocrinology and Metabolism Vol. 15, No. 3, pp. 391±404, 2001 doi:10.1053/beem.2001.0153, available online at http...

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Best Practice & Research Clinical Endocrinology and Metabolism Vol. 15, No. 3, pp. 391±404, 2001

doi:10.1053/beem.2001.0153, available online at http://www.idealibrary.com on

9 Genetics of human obesity Philippe Boutin

PhD

Head of Department of Human Genetics CNRS-Institute of Biology of Lille, Pasteur Institute of Lille, France

Philippe Froguel

MD, PhD

Professor of Molecular Genetics and Experimental Diabetes, Director Queen Mary and West®eld College, University of London and St Bartholomew's Hospital, London Genome Centre, London, UK

Obesity is a multifactorial condition. Environmental risk factors related to a sedentary lifestyle and unlimited access to food apply constant pressure in subjects with a genetic predisposition to gain weight. The fact that genetic defects can result in human obesity has been unequivocally established over the past 3 years with the identi®cation of the genetic defects responsible for di€erent monogenic forms of human obesity: the leptin, leptin receptor, pro-opiomelanocortin, pro-hormone convertase-1 and melanocortin-4 receptor genes. The common forms of obesity are, however, polygenic. The examination of speci®c genes for involvement in the susceptibility to common obesity has not yet yielded convincing results. Approaches involving the candidate genes and the positional cloning of major obesitylinked regions (state-of-the-art future prospects) will be discussed. Key words: genetics; obesity; multifactorial disease; leptin; leptin receptor; POMC; MC4R; genome-wide scans; susceptibility genes for obesity; positional cloning; linkage studies; association studies; linkage disequilibrium.

Obesity is a common disease that has become more prevalent in developed and even in developing countries over recent years.1 About 8±10% of the French population, 17±20% of those in England and Wales and over 25% of North Americans are obese.1±3 In Europe, although obesity is less prevalent in adults than it is in the USA, the prevalence of high weight is increasing among children and teenagers. In France, the latest data show that 16% of children and teenagers are overweight, and the number of obese children has increased ®vefold over the past 10 years.2 Obesity is a risk factor for early mortality and a wide range of metabolic and cardiovascular complications.4 Although the rapid globalization of the Westernised way of life is responsible for the outstanding rise in the number of cases of obesity (about 1 billion subjects now being overweight or overtly obese), obesity is a typical common multifactorial disease in that environmental and genetic factors interact, resulting in a disease state.5 There is strong evidence for a genetic component to human obesity ± the familial clustering (the relative risk among sibs being 3±7)6 and the high concordance of body composition in monozygotic twins7, for example. The role of genetic factors in common obesity is, 1521±690X/01/030391‡14 $35.00/00

c 2001 Harcourt Publishers Ltd. *

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however, complex, being determined by the interaction of several genes (polygenicity), which may each have a relatively small e€ect (they are susceptibility genes but not necessary genes) and will work in combination with other(s) and with environmental factors such as nutrition, physical activity and smoking. As complex traits arise via the concerted action of multiple genes (through a network implying genetic heterogeneity and epistatic interactions) with di€erent and strong environmental factors, the task of identifying any single susceptibility factor is problematic. MONOGENIC HUMAN OBESITY In contrast to the situation with other complex traits such as diabetes or Alzheimer's disease, human geneticists should be very modest in their claim to have unravelled the genetic basis of obesity in humans. Indeed, despite early evidence that genes are involved in the variation of body fat in man, no obesity gene was identi®ed until the ob gene was discovered in mice.8 Interestingly, even today, most of the syndromic forms of obesity such as Prader±Willi, Cohen, Alstrom and Bardet±Biedl syndromes, have been genetically mapped, but the causative genes have not yet been isolated.9 It is obvious that the extreme rarity of these forms made this search dicult. The alternative strategy used with considerable success was to screen a large number of human subjects for mutations in candidate genes selected on the basis of murine genetic studies. The limitation of this approach is obvious: only previously suspected genes have been investigated. Nevertheless, ®nding mutations in homologous genes causing an overweight phenotype underscored the role of the underlying pathways in energy homeostasis. Even if human monogenic obesity is relatively infrequent, the opportunity to treat certain patients with rare forms of genetic obesity has important implications for the development of similar therapies for the most common forms of obesity. In this respect, there is no doubt that the search for the monogenic paradigms of human obesity was successful as ®ve di€erent obesity genes were found in only 2 years: those for leptin and its receptor, pro-opiomelanocortin (POMC), the melanocortin-4 (MC4) receptor and the enzyme pro-hormone convertase-1 (PC1).10±15 Importantly, all the obesity gene-encoded proteins are strongly connected as part of the same loop of regulation of food intake (Figure 1). Leptin (encoded by the human ob-homologous gene), which is secreted by the adipocytes in proportion to their fat content, circulates and binds the long form of the leptin receptor (encoded by the db-homologous gene) in the hypothalamus. POMC gene expression is increased by leptin action, which leads to the production of alphamelanocyte-stimulating hormone (a-MSH), which reduces food intake when it binds to the brain-speci®c MC4 receptor. The fact that no mutation in other genes involved in further pathways potentially regulating energy intake has so far been found in human monogenic obesity may indicate that the leptin pathway is the core of energy balance in humans. Alternatively, it may reveal our ignorance of much of the complex network of proteins regulating body weight. In terms of the latter, one can argue that, despite its initial success, human genetics can still explain only a minority of the mendelian forms of obesity. It is probable that animal studies as well as new genomic approaches will reveal candidate genes that may play a role in monogenic human obesity as well. Monofactorial obesity may be divided into two groups according to the genes involved. The ®rst group comprises very rare recessive forms of obesity that are

Figure 1. Leptin pathway for weight control. All the monogenic obesity gene-encoded proteins (in red) are strongly connected as part of the same loop of regulation of food intake. Ob-R ˆ receptor of the leptine; POMC ˆ pro- opiomelanocartin; PC1 ˆ pro-hormone convertase-1; CART ˆ cocaine and amphetamine related transcript; a MSH ˆ alpha-melanocyte-stimulating hormone; MCH ˆ melanin concentrating hormone; MC4-R ˆ melanocortin-4 receptor; Y5-R ˆ neuropeptide Y5 receptor.

Genetics of human obesity 393

394 P. Boutin and P. Froguel

associated with pituitary endocrine dysfunction, these being caused by mutations in the leptin, leptin receptor and POMC genes. Two kindreds with a defect in leptin have been reported with loss-of-function mutations.10,16 Homozygous carriers consistently exhibit a phenotype of morbid obesity with an onset in the ®rst weeks of life, increased appetite and hyperphagia, and hypogonadotropic hypogonadism. Treatment with recombinant leptin is fully successful, leading to the recovery of satiety and a dramatic decrease of fat mass without any change in lean mass.17 Only one family with a leptin receptor mutation has been identi®ed.11 In subjects with the homozygous mutation, a truncation of the receptor before the transmembrane domain completely abolishes leptin signalling, leading to a form of massive obesity similar to that seen with leptin de®ciency, along with signi®cant growth retardation and central hypothyroidism. These obese subjects presented, as did their heterozygous parents (who were not obese), with a very high leptin level. Chromatography of the circulating leptin revealed that the hormone was bound to the truncated leptin receptor, leading to an increased plasma leptin half-life, although fat leptin expression was still correlated to fat mass. Therefore, the full knockout of the leptin pathway in humans was not responsible for the compensatory hypersecretion of leptin. Interestingly, the two surviving girls with the mutation were neither diabetic nor hyperlipidaemic. Furthermore, although these teenagers did not show any evidence of puberty at the time of the initial examination, they subsequently became pubertal, indicating that leptin control on puberty is not complete (K. Clement, unpublished data). The key role of the melanocortin system in the control of body weight in humans is evidenced by the discovery of mutations in both the POMC and MC4 receptor genes. Two children with homozygous or compound heterozygous loss-of-function mutations in POMC exhibited a complex phenotype. This re¯ected the lack of pituitary neuropeptides derived from the POMC product, leading to impaired signalling via di€erent melanocortin receptors.12 The absence of a-MSH was also responsible for obesity (because of the absence of the melanocortin ligand for the MC4 receptor) as well as for altered pigmentation with red hair (through the MC1 receptor). In addition, the absence of adrenocorticotropin (ACTH) led to adrenal de®ciency via the MC2 receptor. The second group of monogenic forms of non-syndromic obesity is caused by the highly numerous mutations in the MC4 receptor gene.13,14,18 The MC4 receptor gene is the most prevalent obesity gene to date, a€ecting 1±4% of very obese cases depending on the population. MC4 receptor mutations generally segregate in a family via an autosomal dominant mode of inheritance with variable penetrance. In some consanguinous pedigrees, however, MC4 receptor mutations with a relatively modest loss of function e€ect appear to be co-dominantly or even recessively associated with obesity. The MC4 receptor-associated human obesity phenotypes are similar to those found in mice lacking MC4 receptors, these showing moderate-to-severe co-dominant obesity (more important in homozygous than in heterozygous knockout mice) with normal neuroendocrine function in terms of adrenal hormones, growth, reproduction and thyroid hormones. In this respect, obesity caused by MC4 receptor mutations is similar to more common forms of obesity with an earlier age of onset and with a trend towards hyperphagia in infancy, a trait that seems to disappear with age. Recent data obtained in mice suggest that impaired MC4 receptor signalling could be involved in hyperinsulinaemia via an impaired negative neuronal control of insulin secretion.19 The MC4 receptor gene might thus be considered to be a `thrifty gene' and a primary target for therapeutic interventions against obesity, regardless of its cause and possibly the metabolic syndrome involved.

Genetics of human obesity 395

COMMON HUMAN OBESITY In contrast to monogenic obesity, the genetic approach to polygenic obesity has so far been less successful. Despite many claims, most of the dozens of investigated genes fail to provide convincing and unambiguous evidence of any involvement in genetic susceptibility to obesity. The reasons for this are numerous, one of the most important being the weakness of our knowledge of the molecular mechanisms of energy balance. In addition, there has been a lack of well-conducted studies on candidate genes in large populations with reliable phenotypes. Two general approaches have been utilized in the search for the genes underlying common polygenic obesity in human. The ®rst focuses on `candidate genes', that is, genes selected as having some plausible role in obesity on the basis of their known or presumed biological role in energy homeostasis. For many years e€orts to identify candidate genes for obesity have been concentrated on adipose tissue. In fat, the regulation of thermogenesis by the sympathetic nervous system is mediated by betaadrenergic receptors.20 In humans, beta-3-adrenergic receptors (b3-AR) are modestly expressed in fat and the adipocytes lining the gastrointestinal tract.21 A Trp64Arg mutation located in the ®rst transmembrane domain of the receptor was identi®ed in obese Pima Indians and French and Finnish subjects.22±24 Discordant data were, however, also published25,26, including results on the functional e€ect of the b3-AR mutation. These studies indicate that the role of this candidate gene in human obesity is, if anything, modest and that it should be examined in relation to other genes in the same pathway. Importantly, in mature brown adipocyte cells, b3-ARs stimulate uncoupling protein1 (UCP1) via a cAMP metabolic pathway. UCPs are inner mitochondrial membrane transporters that dissipate the proton gradient, releasing stored energy in the form of heat.27 An A to G variation in UCP1 was associated with a gain of fat mass in a Quebec family study.28 Additional e€ects of the G allele of the ÿ3826 variant of UCP1 with the Trp64Arg mutation of the b3-AR gene were shown on weight gain in a morbid obese French population.29 Moreover, polymorphisms in other members of the uncoupling gene family, UCP2 and UCP3, were associated with body mass index in Pima Indians.30 Variations in the b3-AR and UCP genes are probably not sucient on their own to induce obesity. In addition, their function is still being debated, and recent data from UCP2 knockout mice31 have shown no e€ect on body weight. In contrast, there is now evidence that UCP2 is a potent inhibitor of insulin secretion, these UCP2 knockout mice exhibiting hyperinsulinaemia. All these uncertainties illustrate the complexity of the candidate gene approach, especially when gene function is not understood. Several other candidate gene studies have also been reported. The role of the leptin gene in common polygenic obesity was ®rst suggested by linkage studies.32±34 Polymorphisms within the 50 untranslated region of the human leptin gene were associated with a low leptin level35 and with resistance to a low-calorie diet.36 In this respect, unpublished data from Pakistani families with leptin de®ciency showed that heterozygous relatives have a lower leptin level and a higher fat mass. These data suggest that even a modest reduction of fat-induced leptin secretion may contribute to weight gain (S. O'Rahilly, personal communication, 2000). The peroxisome proliferator-activated receptor-gamma (PPARg) is a nuclear receptor that plays a key role in adipogenesis and may in some respect control the `thrifty gene response' to environmental signals such as nutrients (as fatty acids appear to bind PPARg), leading to ecient energy storage.37 A Pro12Ala variation in the PPARg gene has been shown to be associated with improved insulin sensitivity and

396 P. Boutin and P. Froguel

obesity38, and with a modestly decreased risk of type 2 diabetes (odds ratio 0.85).39 PPARg is a target of thiazoladinediones, agents that are now used in treatment of type 2 diabetes. A new hormone, adipocyte-speci®c resistin, has recently been identi®ed as a protein that is regulated by a thiazoladinedione.40 Moreover, this drug suppresses the expression of resistin through PPARg signalling. Resistin has been shown to induce insulin resistance in adipocyte cells, but knockout experiments and genetic studies in di€erent ethnic groups are required to establish the role of resistin and of other fatsecreted proteins in insulin resistance and obesity.41 Unfortunately, the candidate gene approach has to date yielded only putative susceptibility genes with a small or uncertain e€ect. This lack of power could be explained by the restricted choice of candidates as a result of our ignorance of the pathways disturbed in excess fat storage. However, extensive gene targeting experiments in mice, functional genomics (expression pro®les in di€erent tissues of interest in terms of energy balance) and the near completion of the Human Genome Project are providing a new generation of candidate genes for obesity. The choice of an ideal candidate gene may be based on several criteria including: 1. a chromosomal localization near an obesity±linked locus in humans or animal models; 2. the expression pro®le (i.e. in the adipocytes or hypothalamus); 3. expression being regulated by food intake, nutrients or physical activity; 4. gene targeting or overexpression leading to a modi®cation of body weight. The second approach used for identifying genes underlying common polygenic obesity is based on the analysis of genome-wide scans in order to detect chromosomal regions showing linkage with obesity in large collections of nuclear families. This strategy requires no presumptions in terms of the function of genes at the susceptibility loci since it attempts to map genes purely by position. The genotyping of 400 multi-allelic markers (short tandem repeats with a density of 1 marker per 10 cM) enables the identi®cation of regions showing a strong identity by descent in obese family members (i.e. allele sharing in sibships is signi®cantly higher than 50%). Thereafter, susceptibility gene(s) for obesity may be positionally cloned in the intervals of linkage. Five genome-wide scans for obesity genes have been published, these being carried out in Mexican-American families42, French pedigrees43, Pima Indians44,45 and white Americans.46,47 Two papers provided evidence for a candidate region on chromosome 2p21 that could explain a signi®cant part of the variance in leptin level in humans. This linkage was subsequently replicated in a cohort of African-American families.48 A strong candidate gene for obesity, the POMC gene, maps to this region. The POMC gene is expressed in human brain, gut, placenta and pancreas, and is involved in the leptin/melanocortin pathway.49 Moreover, POMC is the precursor of other peptides, including ACTH and MSH, which are involved in energy homeostasis.50 POMC knockout mice51 are obese and have defective adrenal development and altered pigmentation. These features are similar to those of the phenotype of patients with mutations in the coding region of the POMC gene, which are responsible for rare cases of obesity with an early age of onset and with a recessive mode of inheritance.12 In the common forms of obesity, no association was shown between POMC polymorphisms and obesity in Danish and French cohorts.52,53 A polymorphism in the 50 region of POMC was weakly associated with a variation in leptin level in a Mexican± American population54, but this cannot explain the observed linkage at the 2p23 locus. Since the regulatory regions of the POMC gene are still largely unknown, a functional

Genetics of human obesity 397

sequence modifying POMC expression or a nearby other gene may be the aetiological variant responsible for the observed linkage. Other major loci for obesity and leptin levels (Figure 2) were also identi®ed on chromosome 10p11 and on 5cen±q in French families. The 10p locus may account for 20±30% of the genetic risk for obesity in this population.43 Furthermore, the linkage of this chromosomal region to obesity was recently con®rmed in a cohort of obese young German subjects55 as well as in White Caucasians and in African±Americans.56 In addition to this 10p locus, a genome scan performed in white Americans showed evidence for linkage on chromosome 20q13 and on 10q.46 In Pima Indians, the most interesting region of linkage lies on chromosome 11q.44 Comuzzie recently described a new locus at 3q27 that was linked to various quantitative traits characterizing the metabolic/insulin resistance syndrome.47 Interestingly, this 3q27 locus was previously identi®ed as a locus for type 2 diabetes in the French population.57 Several genes map to this region, including the APM1 gene encoding the di€erentiated adipocyte-secreted protein ACRP30/adiponectin, which is abundantly present in plasma. The puri®ed C-terminal domain of adiponectin has been reported to protect mice submitted to a high-fat diet from obesity and to rescue obese or lipoatropic mice models from severe insulin resistance. The mechanism appears to involve decreasing the level of plasma free fatty acids and enhancing lipid oxidization in muscle.58 Moreover, the plasma level of adiponectin has been shown to be decreased in obese diabetic subjects59, which makes ACRP30 an attractive candidate gene for fatinduced metabolic syndrome and type 2 diabetes. Further studies will address the role of variations in the ACRP30 gene in obesity and in obesity-associated type 2 diabetes. Although concerns have been raised about the heterogeneity of the obesity phenotype and the reliability of genetic data in multifactorial diseases in general (e.g. the lack of replication), genome scan results in common obesity have been surprisingly reproducible despite di€erences in ethnicity and environmental factors. In this context, linkages between obesity and loci on chromosome 2 and 10 have largely been con®rmed, the same, albeit to a lesser extent, applying to the region on chromosome 5. These data imply that, among complex traits, an excess of fat is one of the most inheritable and that a few major loci may contribute to the genetic risk for obesity in human. A logical conclusion would be that obesity may be an oligogenic disease that could be modulated by various polygenic (modi®er genes) and by environmental in¯uences. To identify the true aetiological gene variants associated with the enhanced risk of `typical' obesity, chromosomal regions of linkage should ®rst be re®ned using of a dense map of bi-allelic single-nucleotide polymorphisms (SNPs). Indeed, the state of the art in the positional cloning of complex disorder susceptibility genes implies the systematic use of SNP markers for linkage disequilibrium mapping.60±62 The strength of linkage disequilibrium is quite variable within the genome, ranging from 10 kb to 300 kb or more. It was postulated that working in so-called `isolated' populations would signi®cantly increase the distances of linkage disequilibrium, but recent evidence shows that even working in Finnish or Icelandic populations is not a panacea.63 The identi®cation of the NIDDM1 gene (calpain 10) on chromosome 2q con®rmed that linkage disequilibrium mapping can be a successful strategy to unravel polygenic disease64, but this work also showed the complexity of this approach. In the case of NIDDM1, an intronic polymorphism (UCSNP-43) was associated with type 2 diabetes in Mexican±Americans, three non-coding polymorphisms, including UCSNP-43, subsequently being identi®ed as de®ning an at-risk haplotype. In other ethnic

18

10

2

19

11

3

13

21

5

22

14

Y

15

7

- Pima Indians; BMI (ref. 44)

- Americans; BMI (refs 46, 56)

6

X

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Figure 2. Chromosomal location of the obesity loci identi®ed in genome-wide scans studies in di€erent populations. BMI ˆ body mass index.

American Caucasians; BMI, waist, hip, weight, insulin, insulin:glucose (ref. 47)

20

12

4

Positive linkage in adult obesity

Mexican-American; African-Americans; leptin levels (ref. 42)

German young obese (ref. 55)

French Caucasians; fat mass, BMI and leptin (ref. 43)

17

9

1

398 P. Boutin and P. Froguel

Genetics of human obesity 399

DNA collection Public databases Genotyping technologies

Monogenic forms

QTL analysis

Genome scans

Transcriptional analysis (human and animals)

LD mapping

Gene screening

Molecular epidemiology

Bioinformatics

Identification of susceptibility genes for obesity

Functional genomics

- Physiopathology of obesity - Therapeutic targets Figure 3. Identi®cation of susceptibility genes for obesity: an integrated approach utilizing bio-informatic, linkage and association studies, functional genomics and molecular epidemiology. LD ˆ linkage disequilibrium.

groups, such as French Caucasians, the rarity of this high-risk haplotype makes it dicult to reach a de®nite conclusion about the role of calpain 10 in type 2 diabetes. Moreover, as the function of this protease is still unclear, this study has emphasized the limitation of genetic studies to prove a functional relation from solely statistics-based methods. To be ecient, linkage disequilibrium mapping should be integrated into a complex positional cloning pipeline combining an extensive family resource, genotyping technologies, functional genomics and molecular epidemiology (Figure 3). Clinical and methodological resources are therefore key to success in the positional cloning of major obesity-linked regions. In each major gene locus for obesity, as de®ned by genome-wide scans studies, all the information from the Sanger centre (http:// www.ensembl.org/Data/) and the Santa Cruz centre (http://genome.ucsc.edu/ goldenPath/hgTracks.html.) will be employed. Researchers will need to take advantage of the vast amount of genomic information, coupled with technical advances in the throughput and accuracy of genotypic analyses. This may allow an identi®cation of transcripts in the linkage disequilibrium regions and the de®nition of their genomic structure and regulatory sequences. A dense map of SNPs will be constructed by in silico SNP detection within existing data-base sequences (or http://ariel.ucs.unimelb.edu.au/cotton/mdi.htm) and by mutation screening around genes that are regarded as strong biological candidates. Linkage disequilibrium mapping strategies implicate an enormous amount of SNP genotyping, requiring a wide variety of techniques to provide rapid, reliable, accurate and inexpensive high-throughput SNPs. Large cohorts of cases and controls, as well as families in which initial linkages have been established, will be necessary for the linkage disequilibrium mapping strategy to detect variants in¯uencing multiple traits and to

400 P. Boutin and P. Froguel

test for gene±gene and gene±environment interactions.65 In addition, at-risk haplotypes will be characterized. Since it is possible that susceptibility to the disease may be caused by a set of polymorphic alleles in linkage disequilibrium, the identi®cation of such haplotypes will be an essential feature of this approach.66,67 Family-based association methods avoid the confounding e€ects of population structure and also allow the direct observation of haplotypes. Several sibship-based tests could identify linkages and associations between SNPs and/or SNP haplotypes and obesity and/or related traits.68±70 Molecular epidemiological studies in large general populations will provide crucial and complementary information about the broader role of any potential susceptibility genes in obesity. Genetic and functional studies in humans will be used synergistically; tissue pro®ling, for example, may provide the most direct way to improve our overall understanding of the molecular circuitry maintaining energy homeostasis. Expression pro®ling in humans on the one hand, and the genetic analysis of populations on the other, will therefore provide complementary tools to advance our understanding of the complex network of gene±gene and gene±environment interactions underlying the susceptibility to obesity. Following the identi®cation of genetic variations, an exploration of the consequences at tissue level (tissue pro®ling), at organism level and in populations (molecular epidemiology) will clarify the role of these variants in disease pathogenesis and their implications for diagnostic and therapeutic developments. An improved understanding of the genetic and environmental predictors of risk factors provides a rational basis for the strati®cation of the disease risk and the response to treatment, allowing the e€ective targeting of preventative and therapeutic tools. CONCLUSION The current epidemic of obesity represents a major public health concern given the strong association of adiposity with cardiovascular, metabolic and other morbidities. Preventative and therapeutic approaches have been hampered by a lack of any fundamental understanding of the normal control of human body fat mass and its disturbance in obese states. Geneticists have pioneered the understanding of the genetic basis of obesity through the discovery of the ®rst monogenic defects leading to extreme childhood obesity. The more challenging problem, however, is the identi®cation of the genetic variants that underlie susceptibility to the common forms of human obesity. Based on highquality family material and recent widely con®rmed genome scan results, it is likely that putative aetiological variants in candidate genes will emerge. The successful execution of these studies will require a multidisciplinary approach combining genomics, bio-informatics, expression pro®ling, biochemistry, human physiology and molecular epidemiology. Although the task is considerable, the breadth and depth of expertise now available in the human genetics of complex traits provides a unique opportunity for signi®cant advance. SUMMARY Obesity is a typical, common multifactorial disease in that environmental and genetic factors interact. In certain cases, with severe and childhood-onset obesity, a single gene plays a major role, the environment having a permissive one (role). Rare mutations of

Genetics of human obesity 401

the leptin gene and its receptor, POMC, PC1 and, more frequently, MCR4 receptor (1±4% of very obese cases) have been described. All these obesity gene-encoded proteins are strongly connected as part of the same loop of regulation of food intake. The more common forms of obesity are, however, polygenic. Two general approaches have so far been adopted in the search for the genes underlying common polygenic obesity in human. The ®rst approach focused on genes selected as having some plausible role in obesity on the basis of their known or presumed biological role. This approach yielded only putative susceptibility genes with a small or uncertain e€ect. The second approach attempts to map genes purely by position and requires no presumption of the function of genes. Genome-wide scans identify chromosomal regions showing a linkage with obesity in large collections of

Practice points . the ®ve genes causing monogenic obesity may be divided into two groups. First, are those related to very rare recessive forms of obesity associated with pituitary endocrine dysfunction, such as the leptin, leptin receptor, PC1 and POMC genes . treatment with recombinant leptin is fully successful, leading to a recovery of satiety and a dramatic decrease in fat mass with no change in lean mass . second, more frequent (1±4% of very obese cases) mutations occur in the MC4 receptor gene: . MC4 receptor-associated human obesity phenotypes show a moderate-to-severe co-dominant obesity with normal neuroendocrine function . obesity caused by MC4 receptor mutations is similar to more common forms of obesity with an earlier age of onset and a trend towards hyperphagia in infancy, a trait that seems to disappear with age . the MC4 receptor gene might be considered to be a `thrifty gene' and a primary target for small molecules employed against obesity in general, regardless of the proximate cause, and possibly against the metabolic syndrome . even if human monogenic obesity is relatively infrequent, the opportunity to treat certain patients with rare forms of genetic obesity has important implications for the development of similar therapies for the most common forms of obesity

Research agenda . our knowledge of the pathways disturbed in excess fat storage needs to be improved . resources from the human genome project will enable the identi®cation of the genes and regulatory sequences enclosed in major chromosomal obesity-linked regions (in humans or animal models) . rapid, reliable, accurate and inexpensive high-throughput SNP detection techniques for linkage disequilibrium mapping and association studies should be developed . there is a need to integrate positional cloning, combining extensive family resources, genotyping technologies, linkage disequilibrium mapping, family-based and population-based association studies, molecular epidemiology and functional genomics

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nuclear families. Genome-wide scans in di€erent ethnic populations have localized major obesity loci on chromosomes 2, 5, 10, 11 and 20. Susceptibility gene(s) for obesity may be positionally cloned in the intervals of linkage. The systematic use of SNP markers for linkage disequilibrium mapping strategies, combined with functional genomic studies, will be helpful in providing a better understanding of the molecular determinants of obesity.

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