Perspectives: molecular genetic research in human obesity

Genetic Determinants of Childhood Obesity

Obesity represents a major health burden to both developed and developing countries. Furthermore, the incidence of obesity is increasing in children. Obesity contributes substantially to mortality in the United States by increasing the risk for type 2 diabetes, cardiovascular-related diseases, and other comorbidities. Despite environmental changes over past decades, including increases in high-calorie foods and sedentary lifestyles, there is very clear evidence of a genetic predisposition to obesity risk. Childhood obesity cases can be categorized in one of two ways: syndromic or non-syndromic. Syndromic obesity includes disorders such as Prader-Willi syndrome, Bardet-Biedl syndrome, and Alström syndrome. Non-syndromic cases of obesity can be further separated into rarer instances of monogenic obesity and much more common forms of polygenic obesity. The advent of genome-wide association studies (GWAS) and next-generation sequencing has driven significant advances in our understanding of the genetic contribution to childhood obesity. Many rare and common genetic variants have been shown to contribute to the heritability in obesity, although the molecular mechanisms underlying most of these variants remain unclear. An important caveat of GWAS efforts is that they do not strictly represent gene target discoveries, rather simply the uncovering of robust genetic signals. One clear example of this is with progress in understanding the key obesity signal harbored within an intronic region of the FTO gene. It has been shown that the non-coding region in which the variant actually resides in fact influences the expression of genes distal to FTO instead, specifically IRX3 and IRX5. Such discoveries suggest that associated non-coding variants can be embedded within or next to one gene, but commonly influence the expression of other, more distal effector genes. Advances in genetics and genomics are therefore contributing to a deeper understanding of childhood obesity, allowing for development of clinical tools and therapeutic agents.

Are remitted affective disorders and familial risk of affective disorders associated with metabolic syndrome, inflammation and oxidative stress? - a monozygotic twin study

BACKGROUND

Metabolic syndrome (MetS) is associated with reduced life expectancy in patients with affective disorders, however, whether MetS also plays a role before the onset of affective disorder is unknown. We aimed to investigate whether MetS, inflammatory markers or oxidative stress act as risk factors for affective disorders, and whether MetS is associated with increased inflammation and oxidative stress.

METHODS

We conducted a high-risk study including 204 monozygotic (MZ) twins with unipolar or bipolar disorder in remission or partial remission (affected), their unaffected co-twins (high-risk) and twins with no personal or family history of affective disorder (low-risk). Metabolic Syndrome was ascertained according to the International Diabetes Federation (IDF) criteria. Inflammatory markers and markers of oxidative stress were analyzed from fasting blood and urine samples, respectively.

RESULTS

The affected and the high-risk group had a significantly higher prevalence of MetS compared to the low-risk group (20% v. 15% v. 2.5%, p = 0.0006), even after adjusting for sex, age, smoking and alcohol consumption. No differences in inflammatory and oxidative markers were seen between the three groups. Further, MetS was associated with alterations in inflammatory markers, and oxidative stress was modestly correlated with inflammation.

CONCLUSION

Metabolic syndrome is associated with low-grade inflammation and may act as a risk factor and a trait marker for affective disorders. If confirmed in longitudinal studies, this suggests the importance of early intervention and preventive approaches targeted towards unhealthy lifestyle factors that may contribute to later psychopathology.

Long-term effects of adolescent obesity: time to act

Obesity in adolescence will probably have major implications not only for the affected adolescents but also for society. Those who have obesity during adolescence usually have obesity into adulthood, which causes many medical and psychological issues that can result in premature death. Furthermore, obesity in adolescents is associated with a range of social problems, including difficulties securing an apprenticeship or a job or finding a partner. Adolescents with obesity are also at increased risk of having children with obesity later in life. All these consequences lead to high costs for the health-care system. Although efficient treatment options are available that have been proven in randomized controlled trials, such as lifestyle interventions for adolescents with obesity and bariatric surgery for adolescents with severe obesity, these interventions frequently fail in clinical practice as treatment adherence is low in adolescents and most adolescents with obesity do not seek medical care. Therefore, improving treatment adherence and identifying treatment barriers are necessary.

Genetics of Obesity

Numerous classical genetic studies have proved that genes are contributory factors for obesity. Genes are directly responsible for obesity associated disorders such as Bardet-Biedl and Prader-Willi syndromes. However, both genes as well as environment are associated with obesity in the general population. Genetic epidemiological approaches, particularly genome-wide association studies, have unraveled many genes which play important roles in human obesity. Elucidation of their biological functions can be very useful for understanding pathobiology of obesity. In the near future, further exploration of obesity genetics may help to develop useful diagnostic and predictive tests for obesity treatment.

Is there a "metabolic-mood syndrome"? A review of the relationship between obesity and mood disorders

Obesity and mood disorders are highly prevalent and co-morbid. Epidemiological studies have highlighted the public health relevance of this association, insofar as both conditions and its co-occurrence are associated with a staggering illness-associated burden. Accumulating evidence indicates that obesity and mood disorders are intrinsically linked and share a series of clinical, neurobiological, genetic and environmental factors. The relationship of these conditions has been described as convergent and bidirectional; and some authors have attempted to describe a specific subtype of mood disorders characterized by a higher incidence of obesity and metabolic problems. However, the nature of this association remains poorly understood. There are significant inconsistencies in the studies evaluating metabolic and mood disorders; and, as a result, several questions persist about the validity and the generalizability of the findings. An important limitation in this area of research is the noteworthy phenotypic and pathophysiological heterogeneity of metabolic and mood disorders. Although clinically useful, categorical classifications in both conditions have limited heuristic value and its use hinders a more comprehensive understanding of the association between metabolic and mood disorders. A recent trend in psychiatry is to move toward a domain specific approach, wherein psychopathology constructs are agnostic to DSM-defined diagnostic categories and, instead, there is an effort to categorize domains based on pathogenic substrates, as proposed by the National Institute of Mental Health (NIMH) Research Domain Criteria Project (RDoC). Moreover, the substrates subserving psychopathology seems to be unspecific and extend into other medical illnesses that share in common brain consequences, which includes metabolic disorders. Overall, accumulating evidence indicates that there is a consistent association of multiple abnormalities in neuropsychological constructs, as well as correspondent brain abnormalities, with broad-based metabolic dysfunction, suggesting, therefore, that the existence of a "metabolic-mood syndrome" is possible. Nonetheless, empirical evidence is necessary to support and develop this concept. Future research should focus on dimensional constructs and employ integrative, multidisciplinary and multimodal approaches.

[Adipose tissue--an endocrine organ]

The incidence of obesity has increased dramatically during recent decades. Obesity increases the risk for metabolic and cardiovascular diseases and may therefore contribute to premature death. With increasing fat mass, secretion of adipose tissue derived bioactive molecules (adipokines) changes towards a proinflammatory, diabetogenic and atherogenic pattern. Adipokines are involved in the regulation of appetite and satiety, energy expenditure, activity, endothelial function, hemostasis, blood pressure, insulin sensitivity, energy metabolism in insulin sensitive tissues, adipogenesis, fat distribution and insulin secretion in pancreatic β-cells. Therefore, adipokines are clinically relevant as biomarkers for fat distribution, adipose tissue function, liver fat content, insulin sensitivity and chronic inflammation and have the potential for future pharmacological treatment strategies for obesity and related diseases. This review focuses on the clinical relevance of selected adipokines as markers or predictors of obesity-related diseases and as potential therapeutic tools or targets in metabolic and cardiovascular diseases.

The genetics of human obesity

It has long been known that there is a genetic component to obesity, and that characterizing this underlying factor would likely offer the possibility of better intervention in the future. Monogenic obesity has proved to be relatively straightforward, with a combination of linkage analysis and mouse models facilitating the identification of multiple genes. In contrast, genome-wide association studies have successfully revealed a variety of genetic loci associated with the more common form of obesity, allowing for very strong consensus on the underlying genetic architecture of the phenotype for the first time. Although a number of significant findings have been made, it appears that very little of the apparent heritability of body mass index has actually been explained to date. New approaches for data analyses and advances in technology will be required to uncover the elusive missing heritability, and to aid in the identification of the key causative genetic underpinnings of obesity.

Weight Change in Monozygotic Twins Treated with Valproate

OBJECTIVE

To evaluate whether genetic factors contribute to weight gain associated with valproate (VPA) therapy.

RESEARCH METHODS AND PROCEDURES

We retrospectively and prospectively evaluated five pairs of monozygotic twins concordant for epilepsy and treated with VPA regarding weight course.

RESULTS

In all twin pairs, both twins showed similar weight courses under therapy with VPA.

DISCUSSION

These results suggest that genetic factors may have an influence on the weight change induced by VPA. Further studies are necessary to obtain heritability estimates regarding this effect of VPA therapy and to identify the relevant genes.

Genetics of childhood obesity

Obesity is a major health problem and an immense economic burden on the health care systems both in the United States and the rest of the world. The prevalence of obesity in children and adults in the United States has increased dramatically over the past decade. Besides environmental factors, genetic factors are known to play an important role in the pathogenesis of obesity. Genome-wide association studies (GWAS) have revealed strongly associated genomic variants associated with most common disorders; indeed there is general consensus on these findings from generally positive replication outcomes by independent groups. To date, there have been only a few GWAS-related reports for childhood obesity specifically, with studies primarily uncovering loci in the adult setting instead. It is clear that a number of loci previously reported from GWAS analyses of adult BMI and/or obesity also play a role in childhood obesity.

A genome-wide study reveals copy number variants exclusive to childhood obesity cases

The prevalence of obesity in children and adults in the United States has increased dramatically over the past decade. Genomic copy number variations (CNVs) have been strongly implicated in subjects with extreme obesity and coexisting developmental delay. To complement these previous studies, we addressed CNVs in common childhood obesity by examining children with a BMI in the upper 5(th) percentile but excluding any subject greater than three standard deviations from the mean in order to reduce severe cases in the cohort. We performed a whole-genome CNV survey of our cohort of 1080 defined European American (EA) childhood obesity cases and 2500 lean controls (< 50(th) percentile BMI) who were genotyped with 550,000 SNP markers. Positive findings were evaluated in an independent African American (AA) cohort of 1479 childhood obesity cases and 1575 lean controls. We identified 17 CNV loci that were unique to at least three EA cases and were both previously unreported in the public domain and validated via quantitative PCR. Eight of these loci (47.1%) also replicated exclusively in AA cases (six deletions and two duplications). Replicated deletion loci consisted of EDIL3, S1PR5, FOXP2, TBCA, ABCB5, and ZPLD1, whereas replicated duplication loci consisted of KIF2B and ARL15. We also observed evidence for a deletion at the EPHA6-UNQ6114 locus when the AA cohort was investigated as a discovery set. Although these variants may be individually rare, our results indicate that CNVs contribute to the genetic susceptibility of common childhood obesity in subjects of both European and African ancestry.

From monogenic to polygenic obesity: recent advances

The heritability of obesity and body weight in general is high. A small number of confirmed monogenic forms of obesity-the respective mutations are sufficient by themselves to cause the condition in food abundant societies-have been identified by molecular genetic studies. The elucidation of these genes, mostly based on animal and family studies, has led to the identification of important pathways to the disorder and thus to a deeper understanding of the regulation of body weight. The identification of inborn deficiency of the mostly adipocyte-derived satiety hormone leptin in extremely obese children from consanguineous families paved the way to the first pharmacological therapy for obesity based on a molecular genetic finding. The genetic predisposition to obesity for most individuals, however, has a polygenic basis. A polygenic variant by itself has a small effect on the phenotype; only in combination with other predisposing variants does a sizeable phenotypic effect arise. Common variants in the first intron of the 'fat mass and obesity associated' gene (FTO) result in an elevated body mass index (BMI) equivalent to approximately +0.4 kg/m(2) per risk allele. The FTO variants were originally detected in a genome wide association study (GWAS) pertaining to type 2 diabetes mellitus. Large meta-analyses of GWAS have subsequently identified additional polygenic variants. Up to December 2009, polygenic variants have been confirmed in a total of 17 independent genomic regions. Further study of genetic effects on human body weight regulation should detect variants that will explain a larger proportion of the heritability. The development of new strategies for diagnosis, treatment and prevention of obesity can be anticipated.

Admixture mapping of quantitative trait loci for BMI in African Americans: evidence for loci on chromosomes 3q, 5q, and 15q

Obesity is a heritable trait and a major risk factor for highly prevalent common diseases such as hypertension and type 2 diabetes. Previously we showed that BMI was positively correlated with African ancestry among the African Americans (AAs) in the US National Heart, Lung, and Blood Institute's Family Blood Pressure Program (FBPP). In a set of 1,344 unrelated AAs, using Individual Ancestry (IA) estimates at 284 marker locations across the genome, we now present a quantitative admixture mapping analysis of BMI. We used a set of unrelated individuals from Nigeria to represent the African ancestral population and the European American (EA) in the FBPP as the European ancestral population. The analysis was based on a common set of 284 microsatellite markers genotyped in all three groups. We considered the quantitative trait, BMI, as the response variable in a regression analysis with the marker location specific excess European ancestry as the explanatory variable. After suitably adjusting for different covariates such as sex, age, and network, we found strong evidence for a positive association with European ancestry at chromosome locations 3q29 and 5q14 and a negative association on chromosome 15q26. To our knowledge, this is the largest quantitative admixture mapping effort in terms of sample size and marker locus involvement for the trait. These results suggest that these regions may harbor genes influencing BMI in the AA population.

Changes in the transcriptome of abdominal subcutaneous adipose tissue in response to short-term overfeeding in lean and obese men

BACKGROUND

Obesity is caused by the excessive accumulation of adipose tissue as a result of a chronic energy surplus. Little is known regarding the molecular mechanisms involved in the response to an energy surplus in human adipose tissue at the genomic level.

OBJECTIVE

The objective was to investigate changes in the transcriptome of abdominal subcutaneous adipose tissue after a positive energy challenge induced by overfeeding in both lean and obese subjects to identify novel obesity candidate genes.

DESIGN

A total of 26 men were recruited and classified on the basis of percentage body fat (measured by dual-energy X-ray absorptiometry) as lean (<20%) or obese (>25%) to participate in the baseline comparison. Sixteen men participated in the overfeeding study (8 lean and 8 obese). Adipose tissue biopsy samples were collected from all subjects at the subumbilical region. Global gene expression profiles were determined at baseline and after a 7-d hypercaloric diet at 40% above normal energy requirements by using whole human genome DNA microarrays.

RESULTS

Overfeeding induced differential expression in 45 genes. Six genes displayed a significant interaction effect between adiposity status and overfeeding treatment, including transferrin (TF), stearoyl-CoA desaturase (SCD), transaldolase 1 (TALDO1), cathepsin C (CTSC), insulin receptor substrate 2 (IRS2), and pyruvate dehydrogenase kinase, isozyme 4 (PDK4). Overfeeding resulted in changes in expression of these genes in lean subjects, whereas no significant changes were evident in obese subjects.

CONCLUSIONS

Differential expression of these 6 genes may represent a protective mechanism at the molecular level in lean subjects in response to an energy surplus. These genes represent valuable candidates for downstream studies related to obesity.

Models and mechanisms of energy balance regulation in the young

The proportion of the child and adolescent population that is in appropriate energy balance is declining throughout the developed world, and childhood obesity is a particular problem in the UK relative to other northern European countries. Assessment of the underlying causes of obesity, and the different routes to its development, may assist in the definition of successful intervention strategies. The network of peripheral and central (brain) regulatory systems that underlie energy balance and body weight and composition can, for the most part, only be approached experimentally through the study of appropriate laboratory animal models. This problem is particularly acute when the target is overweight and obesity in the young. Some of the mechanisms underlying the development of energy imbalance and specifically the onset of overweight and obesity in the young, and the metabolic health consequences of obesity, can be addressed by examination of experimental rodent models in which mutation of a single gene causes early-onset extreme obesity, genetic susceptibility to obesity is revealed in an obesogenic environment or early-life nutritional experience programmes susceptibility to obesity or metabolic problems in later life. These studies highlight genes that are essential to normal body-weight regulation in rodents and man, the impact of diet and diet-induced obesity on regulatory systems in the young and the potential sensitivity of developing regulatory systems to nutritional experiences in utero and during early life.

Relevance of animal models to human eating disorders and obesity

BACKGROUND AND RATIONALE

This review addresses the role animal models play in contributing to our knowledge about the eating disorders anorexia nervosa (AN) and bulimia nervosa (BN) and obesity.

OBJECTIVES

Explore the usefulness of animal models in complex biobehavioral familial conditions, such as AN, BN, and obesity, that involve interactions among genetic, physiologic, psychological, and cultural factors.

RESULTS AND CONCLUSIONS

The most promising animal model to mimic AN is the activity-based anorexia rodent model leading to pathological weight loss. The paradigm incorporates reward elements of the drive for activity in the presence of an appetite and allows the use of genetically modified animals. For BN, the sham-feeding preparation in rodents equipped with a gastric fistula appears to be best suited to reproduce the postprandial emesis and the defects in satiety. Animal models that incorporate genes linked to behavior and mood may clarify biobehavioral processes underlying AN and BN. By contrast, a relative abundance of animal models has contributed to our understanding of human obesity. Both environmental and genetic determinants of obesity have been modeled in rodents. Here, we consider single gene mutant obesity models, along with models of obesigenic environmental conditions. The contributions of animal models to obesity research are illustrated by their utility for identifying genes linked to human obesity, for elucidating the pathways that regulate body weight and for the identification of potential therapeutic targets. The utility of these models may be further improved by exploring the impact of experimental manipulations on the behavioral determinants of energy balance.

Association analysis of the FTO gene with obesity in children of Caucasian and African ancestry reveals a common tagging SNP

Recently an association was demonstrated between the single nucleotide polymorphism (SNP), rs9939609, within the FTO locus and obesity as a consequence of a genome wide association (GWA) study of type 2 diabetes in adults. We examined the effects of two perfect surrogates for this SNP plus 11 other SNPs at this locus with respect to our childhood obesity cohort, consisting of both Caucasians and African Americans (AA). Utilizing data from our ongoing GWA study in our cohort of 418 Caucasian obese children (BMI≥95th percentile), 2,270 Caucasian controls (BMI<95th percentile), 578 AA obese children and 1,424 AA controls, we investigated the association of the previously reported variation at the FTO locus with the childhood form of this disease in both ethnicities. The minor allele frequencies (MAF) of rs8050136 and rs3751812 (perfect surrogates for rs9939609 i.e. both r² = 1) in the Caucasian cases were 0.448 and 0.443 respectively while they were 0.391 and 0.386 in Caucasian controls respectively, yielding for both an odds ratio (OR) of 1.27 (95% CI 1.08–1.47; P = 0.0022). Furthermore, the MAFs of rs8050136 and rs3751812 in the AA cases were 0.449 and 0.115 respectively while they were 0.436 and 0.090 in AA controls respectively, yielding an OR of 1.05 (95% CI 0.91–1.21; P = 0.49) and of 1.31 (95% CI 1.050–1.643; P = 0.017) respectively. Investigating all 13 SNPs present on the Illumina HumanHap550 BeadChip in this region of linkage disequilibrium, rs3751812 was the only SNP conferring significant risk in AA. We have therefore replicated and refined the association in an AA cohort and distilled a tag-SNP, rs3751812, which captures the ancestral origin of the actual mutation. As such, variants in the FTO gene confer a similar magnitude of risk of obesity to children as to their adult counterparts and appear to have a global impact.

Primary prevention of childhood obesity: an interdisciplinary analysis

The primary prevention of childhood obesity requires combined efforts by stakeholders at various societal levels, based on the knowledge from multiple disciplines. The goal of the present study was therefore to analyze current preventive approaches and delineate implications for future prevention research and practice by integrating knowledge from genetics, law, economics, psychology, and social ethics. Inconclusive evidence on the etiology of obesity, a complex, multifactorial condition, likely complicates prevention, leading to a lack of specificity regarding target groups, focus, and techniques. It is recommended to increase the specificity of prevention by explicitly considering risk factor evidence, including evidence on genetic factors. Because the institutional and legal framework of primary obesity prevention in children is insufficient in many countries, considering the risk factors for childhood obesity is also crucial for establishing a basis for legal regulations. Companies from sectors concerned with food intake and physical activity may be involved in preventive action, e.g., for initiating self-defeating sanctions. Long-term behavior change may be enhanced through the systematic application of behavior modification techniques within primary prevention programs. Overall, an interdisciplinary perspective furthers understanding of the complexity of this condition and can inform public health strategies on the primary prevention of childhood obesity.

Recent development in pharmacogenomics: from candidate genes to genome-wide association studies

Genetic diversity, most notably through single nucleotide polymorphisms and copy-number variation, together with specific environmental exposures, contributes to both disease susceptibility and drug response variability. It has proved difficult to isolate disease genes that confer susceptibility to complex disorders, and as a consequence, even fewer genetic variants that influence clinical drug responsiveness have been uncovered. As such, the candidate gene approach has largely failed to deliver and, although the family-based linkage approach has certain theoretical advantages in dealing with common/complex disorders, progress has been slower than was hoped. More recently, genome-wide association studies have gained increasing popularity, as they enable scientists to robustly associate specific variants with the predisposition for complex disease, such as age-related macular degeneration, Type 2 diabetes, inflammatory bowel disease, obesity, autism and leukemia. This relatively new methodology has stirred new hope for the mapping of genes that regulate drug response related to these conditions. Collectively, these studies support the notion that modern high-throughput single nucleotide polymorphism genotyping technologies, when applied to large and comprehensively phenotyped patient cohorts, will readily reveal the most clinically relevant disease-modifying and drug response genes. This review addresses both recent advances in the genotyping field and highlights from genome-wide association studies, which have conclusively uncovered variants that underlie disease susceptibility and/or variability in drug response in common disorders.

Genetics of obesity

Objective

The aim was to review and update advances in genetics of obesity.

Design

Analysis and interpretation of recent investigations about regulating the energy balance as well as about gene-nutrient interactions and current nutrigenomic research methods.

Background and main statements

Obesity results from a long-term positive energy balance. However, its rising prevalence in developed and developing societies must reflect lifestyle changes, since genetic susceptibility remains stable over many generations. Like most complex diseases, obesity derives from a failure of adequate homoeostasis within the physiological system controlling body weight. The identification of genes that are involved in syndromic, monogenic and polygenic obesity has seriously improved our knowledge of body weight regulation. This disorder may arise from a deregulation at the genetic level (e.g. gene transcription or altered protein function) or environmental exposure (e.g. diet, physical activity, etc.).

Conclusions

In practice, obesity involves the interaction between genetic and environmental factors.

Do MYO9B genetic variants predispose to coeliac disease? An association study in a cohort of South Italian children

BACKGROUND

Coeliac disease is a complex disorder influenced by environmental and genetic factors. A genome wide linkage study identified the myosin IXB (MYO9B) as a gene possibly associated with coeliac disease. Recently, a Dutch study reported a strong association of a single SNP, rs 2305764, of MYO9B with coeliac disease. However, two successive studies carried out on British and Swedish/Norwegian cohorts reported lack of association of the MYO9B variant with coeliac disease.

AIMS

The aim of the present study is to verify the effects of the MYO9B rs 2305764 polymorphism on disease risk in a Mediterranean population of coeliac children.

PATIENTS AND METHODS

To address this issue, an association study was performed in 223 (127 females) Italian coeliac children and adolescents and in 600 controls.

RESULTS

The allelic frequencies of the MYO9B rs 2305764 polymorphism found in our patients and in the population control were not statistically different (P=0.46).

CONCLUSION

The MYO9B gene rs 2305764 polymorphism is not associated to coeliac disease in coeliac children from Southern Italy. This is in accordance with the most recent reports. Ethnic differences or a false positive result might explain the discrepancy with the Dutch study.

Brain responses to obesogenic diets and diet-induced obesity

Rodent models of diet-induced obesity (DIO) mimic common human obesity more accurately than obese single-gene mutation lines, such as the ob/ob mouse. Sprague-Dawley rats sourced in the UK develop obesity when fed a high-energy diet, but susceptibility to DIO is normally distributed, as might be anticipated for a polygenic trait in an outbred population, in contrast to reports in the literature using ostensibly the same strain of rats sourced in the USA. Nevertheless, the responses of these rats to solid and liquid obesogenic diets are very similar to those reported elsewhere, and this model of DIO has much to commend it as a vehicle for the mechanistic study of susceptibility to DIO, development and reversal of obesity on solid and liquid diets and the response of peripheral and central energy balance systems to the development of obesity and to the obesogenic diets themselves. In general, hypothalamic energy-balance-related systems respond to obesogenic diets and developing obesity with activity changes that appear designed to counter the further development of the obese state. However, these hypothalamic changes are apparently unable to maintain body weight and composition within normal limits, suggesting that attributes of the obesogenic diets either evade the normal regulatory systems and/or engage with reward pathways that override the homeostatic systems. Since diets are a risk factor in the development of obesity, it will be important to establish how obesogenic diets interact with energy-balance pathways and whether there is potential for diets to be manipulated with therapeutic benefit.

Unraveling the genetics of human obesity

The use of modern molecular biology tools in deciphering the perturbed biochemistry and physiology underlying the obese state has proven invaluable. Identifying the hypothalamic leptin/melanocortin pathway as critical in many cases of monogenic obesity has permitted targeted, hypothesis-driven experiments to be performed, and has implicated new candidates as causative for previously uncharacterized clinical cases of obesity. Meanwhile, the effects of mutations in the melanocortin-4 receptor gene, for which the obese phenotype varies in the degree of severity among individuals, are now thought to be influenced by one's environmental surroundings. Molecular approaches have revealed that syndromes (Prader-Willi and Bardet-Biedl) previously assumed to be controlled by a single gene are, conversely, regulated by multiple elements. Finally, the application of comprehensive profiling technologies coupled with creative statistical analyses has revealed that interactions between genetic and environmental factors are responsible for the common obesity currently challenging many Westernized societies. As such, an improved understanding of the different “types” of obesity not only permits the development of potential therapies, but also proposes novel and often unexpected directions in deciphering the dysfunctional state of obesity.

Genetics of human obesity

This chapter presents the current state of knowledge in the field of the genetics of human obesity. The molecular approach has proved to be powerful in defining new syndromes associated with obesity. The pivotal role of leptin and melanocortin pathways has been recognized, but only in rare cases of obesity. In the more common form of obesity a multitude of polymorphisms located in genes and candidate regions throughout the genome regulate an individual's susceptibility to weight gain in a permissive environment. The effects are often uncertain and the results not always confirmed. Combining these single nucleotide polymorphisms and defining the associated risks for obesity will be a real challenge in the future. It is now necessary to integrate data of various origins (environment, genotype, expression) to clarify this field.

Genetics of human obesity

We present the knowledge acquired in the field of the genetics of human obesity. The molecular approach proved to be powerful to define new syndromes associated to obesity. The pivotal role of leptin and melanocortin pathways were recognized but in rare obesity cases. In the commoner form of obesities, a multitude of polymorphisms located in genes and candidate regions participate in an individual susceptibility to weight gain in a permissive environment. The effects are often uncertain and the results not always confirmed. It is now necessary to integrate data of various origins (environment, genotype, expression) to clarify the domain.

Diet-induced obesity in the Sprague-Dawley rat: dietary manipulations and their effect on hypothalamic neuropeptide energy balance systems

The SD (Sprague-Dawley) rat model of DIO (diet-induced obesity) is reported to exhibit a clear segregation into susceptible and resistant subpopulations shortly after transfer to a HE (high energy) diet. This does not appear to be the case for rats sourced in the U.K., where body weight gain on obesogenic HE diet is normally distributed, as might be anticipated for a polygenic trait in an outbred population. Many of the energy balance effects of dietary manipulation in this model (e.g. supplementation of HE diet with the liquid diet, Ensure; energy intake and defence of body weight following withdrawal of obesogenic diet) appear to be characteristics of the diets being manipulated rather than subject traits. The activities of energy balance-related hypothalamic signals are affected by diet and the development of DIO, but may not be able to differentiate between different diets and the relative levels of obesity that develop.

A common genetic variant is associated with adult and childhood obesity

Obesity is a heritable trait and a risk factor for many common diseases such as type 2 diabetes, heart disease, and hypertension. We used a dense whole-genome scan of DNA samples from the Framingham Heart Study participants to identify a common genetic variant near the INSIG2 gene associated with obesity. We have replicated the finding in four separate samples composed of individuals of Western European ancestry, African Americans, and children. The obesity-predisposing genotype is present in 10% of individuals. Our study suggests that common genetic polymorphisms are important determinants of obesity.

A role for beta-melanocyte-stimulating hormone in human body-weight regulation

Pro-opiomelanocortin (POMC) expressing neurons mediate the regulation of orexigenic drive by peripheral hormones such as leptin, cholecystokinin, ghrelin, and insulin. Most research effort has focused on alpha-melanocyte-stimulating hormone (alpha-MSH) as the predominant POMC-derived neuropeptide in the central regulation of human energy balance and body weight. Here we report a missense mutation within the coding region of the POMC-derived peptide beta-MSH (Y5C-beta-MSH) and its association with early-onset human obesity. In vitro and in vivo data as well as postmortem human brain studies indicate that the POMC-derived neuropeptide beta-MSH plays a critical role in the hypothalamic control of body weight in humans.

Hypothalamic energy balance gene responses in the Sprague-Dawley rat to supplementation of high-energy diet with liquid ensure and subsequent transfer to chow

Energy dense, high fat, high sugar, foods and beverages in our diet are a major contributor to the escalating global obesity problem. Here, we examine the physiological and neuroendocrine effects of feeding rats a solid high-energy (HE) diet with or without a liquid supplement (Ensure) and the consequence of subsequently transferring animals back to chow (C). Outbred Sprague-Dawley rats were fed C until 49-56 days of age, and then transferred a HE diet for 3 weeks before allocation to one of two weight-matched groups. Over the next 10 weeks, one group remained on HE diet, whereas the other had access to the liquid diet, chocolate Ensure (EN), in addition to HE diet (HE + EN). Half the rats from each group were then killed, and the remainder were returned to C for 3 weeks. Supplementation of the HE diet with EN accelerated weight gain and increased daily energy intake, adipose tissue mass, and circulating leptin levels. Transferring animals back to C caused a decrease in bodyweight in the HE + EN group, whereas HE animals were weight stable. Both groups also exhibited voluntary hypophagia, although the magnitude and duration of this response was greater in HE + EN animals. The only effect of Ensure on the hypothalamic genes studied was on tyrosine kinase B expression in the ventromedial hypothalamic nucleus (VMH), which was increased in rats given the supplement. Withdrawal of the obesogenic diets decreased gene expression for cocaine-and-amphetamine regulated transcript (CART) and dynorphin (DYN) in the arcuate nucleus (ARC), and DYN and brain-derived neurotrophic factor (BDNF) in the VMH, whereas neuropeptide Y (NPY) gene expression in the ARC was increased. These changes were independent of previous dietary history. EN supplementation generates distinct physiological responses, yet has a minimal effect on hypothalamic neuropeptide or receptor gene expression, possibly due to the development of leptin resistance. Withdrawal of obesogenic diets induces changes in the gene expression consistent with NPY, CART and BDNF attempting to oppose weight gain on either HE or HE + EN.

Génétique de l'obésité humaine

BACKGROUND

Both genetic and environmental aspects are recognized in the obesity field but we are not able to elucidate multiple genes and gene-environment interactions with the present resources and tools used in the study of this complex disease. The purpose of this paper is to present some examples of the knowledge acquired in the field of obesity genetics and the new ongoing tools and developments that aim at studying the contribution of genes to obesity and their response to environmental changes.

MAIN POINTS

In rare cases of monogenic obesities, genetic tools have proved extremely powerful for identifying the genes responsible and for defining new syndromes. However, in common obesity, most studies include the search for genotype-phenotype associations without taking into account the influence of environment (diet, sedentary lifestyle) in the relationship. Among the limitations to this integrated approach, one can cite the difficulty of having large enough samples as well as biocomputing tools that are still in their infancy for accessing the question of multiple interactions with no "a priori hypotheses". This picture will probably change rapidly in the future.

PERSPECTIVE

Large databases and DNA and biological sample banks will be available with updated environmental information and precise phenotypes thanks especially to European working groups. The capacity for studying multiple genes at once at the DNA or RNA levels is rapidly growing. Finally, tremendous progress in biocomputing will allow the integration of information from different sources (i.e. environment, phenotype, genotype, gene expression) and thus improve our ability to deal with complexity.

Genetics of human obesity

The rapid development of new concepts and tools has led to a change in the way in which researchers carry out nutrition-related research. Obesity is determined by the interaction between predisposing genetic and environmental aspects, but at present the gene-gene and gene-environment interactions contributing to the development of this complex disease cannot be analysed in detail. The purpose of the present paper is to provide some examples of the knowledge that is available in the field of obesity genetics, and also the new strategies being developed that are aimed at studying the relative contribution of numerous genes to obesity and their responses to environmental changes. In the rare cases of monogenic obesities in which a major gene is the cause the molecular approach has proved extremely powerful in the identification of the genes responsible and in defining new syndromes. However, in the common forms of obesity (polygenic obesity) most studies have analysed genotype-phenotype associations without sometimes taking into account the influence of environmental factors (diet, sedentary lifestyle). Among the aspects limiting this integrated approach to obesity are the difficulty of having large enough samples and the expansion of biocomputing tools developed for accessing the question of multiple interactions with no a priori hypotheses. This picture is rapidly changing. Large databases of clinical data and DNA and biological sample banks with more precise environmental information and patient phenotypes are being compiled. The capacity for studying multiple genes simultaneously at the DNA or RNA levels is also possible. Finally, the tremendous progress in biocomputing will allow the integration of these different types of data (relating to environment, phenotype, genotype, gene expression) and will improve the ability to deal with this complex disease.

Genes, lifestyles and obesity

OBJECTIVE

To review the role of genes and lifestyle factors, particularly dietary habits and physical activity patterns, in obesity risk as well as their potential interactions.

DESIGN AND MEASUREMENTS

A descriptive report of a number of genes definitely ascribed or potentially implicated in excessive fat accumulation leading to obesity as assessed by different research approaches (Mendelian transmission, genetic animal models, epidemiological association/linkage studies and genome-wide scans). Also, the involvement of macronutrient intake and composition (fat/carbohydrate) as well as the role of activity-linked energy expenditure in obesity onset is reviewed.

RESULTS

Examples of the role of the genotype as well as of the dietary macronutrient composition/intake and sedentary/low energy cost of physical activities in obesity prevalence are reported.

CONCLUSIONS

Both genes and everyday life environmental factors such as cultural and social mediated food intake and reduced domestic and living work activities are involved in the obesity pandemia. The occurrence of gene x gene and gene x environmental factors interactions makes it more difficult to interpret the specific roles of genetics and lifestyle in obesity risk.