A +2138InsCAGACC Polymorphism of the Melanocortin Receptor 3 Gene is Associated in Human with Fat Level and Partitioning in Interaction with Body Corpulence

Associations of the melanocortin 3 receptor C17A + G241A haplotype with body composition and inflammation in African-American adults

BACKGROUND

The MC3R haplotype C17A + G241A, which encodes a partially inactivated receptor, has high prevalence in individuals of predominately African ancestry. In pediatric cohorts, homozygosity for this common variant has been associated with obesity, reduced lean mass, and greater fasting insulin. However, metabolic and body composition measures have not been well studied in adults with this haplotype.

METHODS

A convenience sample of 237 healthy African-American adult volunteers was studied. TaqMan assays were used to genotype MC3R variants. Labs were drawn in the morning in the fasted state. Body composition data was obtained via dual-energy X-ray absorptiometry. An analysis of covariance was used to examine the associations of genotype with metabolic and body composition measures controlling for age and sex.

RESULTS

Individuals homozygous for the MC3R C17A + G241A haplotype had significantly greater body mass index, fat mass, fat mass percentage, and C-reactive protein, with reduced lean mass percentage as compared to heterozygous and wild-type participants (all ps < 0.05); fasting insulin was marginally nonsignificant between groups (p = 0.053). After adjusting for fat mass, laboratory differences no longer remained significant.

CONCLUSIONS

Homozygosity for MC3R C17A + G241A is associated with increased adiposity in African-American adults. Further studies are needed to elucidate the mechanisms behind these associations.

Polymorphisms and mutations in the melanocortin-3 receptor and their relation to human obesity

Inactivating mutations in the melanocortin 3 receptor (Mc3r) have been described as causing obesity in mice, but the physiologic effects of MC3R mutations in humans have been less clear. Here we review the MC3R polymorphisms and mutations identified in humans, and the in vitro, murine, and human cohort studies examining their putative effects. Some, but not all, studies suggest that the common human MC3R variant T6K+V81I, as well as several other rare, function-altering mutations, are associated with greater adiposity and hyperleptinemia with altered energy partitioning. In vitro, the T6K+V81I variant appears to decrease MC3R expression and therefore cAMP generation in response to ligand binding. Knockin mouse studies confirm that the T6K+V81I variant increases feeding efficiency and the avidity with which adipocytes derived from bone or adipose tissue stem cells store triglycerides. Other MC3R mutations occur too infrequently in the human population to make definitive conclusions regarding their clinical effects. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Melanocortin 3 receptor has a 5' exon that directs translation of apically localized protein from the second in-frame ATG

Melanocortin-3 receptor (MC3R) is a canonical MSH receptor that plays an essential role in energy homeostasis. Variants in MC3R have been implicated in obesity in humans and mice. However, interpretation of the functional consequences of these variants is challenging because the translational start site of MC3R is unclear. Using 5' rapid amplification of cDNA ends, we discovered a novel upstream exon that extends the length of the 5' untranslated region (UTR) in MC3R without changing the open-reading frame. The full-length 5' UTR directs utilization of an evolutionarily conserved second in-frame ATG as the primary translation start site. MC3R synthesized from the second ATG is localized to apical membranes of polarized Madin-Darby canine kidney cells, consistent with its function as a cell surface mediator of melanocortin signaling. Expression of MC3R causes relocalization of melanocortin receptor accessory protein 2, an accessory factor for melanocortin-2 receptor, to the apical membrane, coincident with the location of MC3R. In contrast, protein synthesized from MC3R cDNAs lacking the 5' UTR displayed diffuse cytosolic distribution and has no effect on the distribution of melanocortin receptor accessory protein 2. Our findings demonstrate that a previously unannotated 5' exon directs translation of MC3R protein that localizes to apical membranes of polarized cells. Together, our work provides insight on the structure of human MC3R and reveals a new pathway for regulation of energy metabolism.

The Human Obesity Gene Map: The 2003 Update

This is the tenth update of the human obesity gene map, incorporating published results up to the end of October 2003 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. Transgenic and knockout murine models relevant to obesity are also incorporated (N = 55). As of October 2003, 41 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. QTLs reported from animal models currently number 183. There are 208 human QTLs for obesity phenotypes from genome-wide scans and candidate regions in targeted studies. A total of 35 genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 272 studies reporting positive associations with 90 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, more than 430 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Physiological roles of the melanocortin MC₃ receptor

The melanocortin MC(3) receptor remains the most enigmatic of the melanocortin receptors with regard to its physiological functions. The receptor is expressed both in the CNS and in multiple tissues in the periphery. It appears to be an inhibitory autoreceptor on proopiomelanocortin neurons, yet global deletion of the receptor causes an obesity syndrome. Knockout of the receptor increases adipose mass without a readily measurable increase in food intake or decrease in energy expenditure. And finally, no melanocortin MC(3) receptor null humans have been identified and associations between variant alleles of the melanocortin MC(3) receptor and diseases remain controversial, so the physiological role of the receptor in humans remains to be determined.

Identification of three novel genetic variants in the melanocortin-3 receptor of obese children

The melanocortin-3 receptor (MC3R), a G-protein-coupled receptor expressed in the hypothalamus, is a key component of the leptin-melanocortin pathway that regulates energy homeostasis. It is suggested that an MC3R defect leads to an increased feed efficiency, by which nutrients are partitioned preferentially into fat. In this study, we hypothesized that early-onset obesity could be induced by mutations in MC3R. To investigate this hypothesis, we screened the entire coding region of the MC3R gene for mutations in obese subjects. A total of 404 overweight and obese children and adolescents, 86 severely obese adults (BMI ≥40 kg/m²), and 150 normal-weight control adults were included. Besides three synonymous coding variations in the MC3R gene (S69S, L95L, I226I), we were able to identify three novel heterozygous, nonsynonymous, coding mutations (N128S, V211I, L299V) in three unrelated obese children. None of these mutations were found in any of the control subjects. Functional studies assessing localization and signaling properties of the mutant receptors provided proof for impaired function of the L299V mutated receptor, whereas no conclusive evidence for functional impairment of the N128S and V211I mutated receptors could be established. First, these results provide supporting evidence for a role of the MC3R gene in the pathogenesis of obesity in a small subset of patients. Second, they show that caution is called for the interpretation of newly discovered mutations in MC3R.

Rare melanocortin-3 receptor mutations with in vitro functional consequences are associated with human obesity

In contrast to the melanocortin 4 receptor, the possible role of the melanocortin 3 receptor (MC3R) in regulating body weight is still debated. We have previously reported three mutations in the MC3R gene showing association with human obesity, but these results were not confirmed in a study of severe obese North American adults. In this study, we evaluated the entire coding region of MC3R in 839 severely obese subjects and 967 lean controls of Italian and French origin. In vitro functional analysis of the mutations detected was also performed. The total prevalence of rare MC3R variants was not significantly different in obese subjects when compared with controls (P= 0.18). However, the prevalence of mutations with functional alterations was significantly higher in the obese group (P= 0.022). In conclusions, the results of this large study demonstrate that in the populations studied functionally significant MC3R variants are associated with obesity supporting the current hypothesis that rare variants might have a stronger impact on the individual susceptibility to gain weight. They also underline the importance of detailed in vitro functional studies in order to prove the pathogenic effect of such variants. Further investigations in larger cohorts will be needed in order to define the specific phenotypic characteristics potentially correlated with reduced MC3R signalling.

Common melanocortin-3 receptor variants are not associated with obesity, although rs3746619 does influence weight in obese individuals

The melanocortin-3 receptor is a vital link in the leptin-melanocortin signaling pathway in the brain and has a role in the regulation of energy homeostasis. It was hypothesized that common polymorphisms in MC3R could increase susceptibility for the development of obesity, but different studies have led to contradictory results. In this study, we investigated the association of SNPs in MC3R with the development of obesity in an extensive Caucasian population. Using the HapMap, we selected two tagSNPs (rs6127698 and rs3746619) that cover all of the common genetic variation in MC3R and genotyped them in 1008 obese cases and 313 normal weight controls. Statistical analysis of the data showed that none of the analyzed SNPs were associated with obesity. However, linear regression analysis did show that SNP rs3746619 has an influence on weight (P=0.015) in the obese population only. Furthermore, a trend for association with BMI in the obese population was observed for this SNP (P=0.039). Taken together, these data are consistent with the involvement of rs3746619 in weight regulation among obese individuals. However, further research including replication of our results is necessary to elucidate the role of MC3R in complex obesity.

Functions for pro-opiomelanocortin-derived peptides in obesity and diabetes

Melanocortin peptides, derived from POMC (pro-opiomelanocortin) are produced in the ARH (arcuate nucleus of the hypothalamus) neurons and the neurons in the commissural NTS (nucleus of the solitary tract) of the brainstem, in anterior and intermediate lobes of the pituitary, skin and a wide range of peripheral tissues, including reproductive organs. A hypothetical model for functional roles of melanocortin receptors in maintaining energy balance was proposed in 1997. Since this time, there has been an extraordinary amount of knowledge gained about POMC-derived peptides in relation to energy homoeostasis. Development of a Pomc-null mouse provided definitive proof that POMC-derived peptides are critical for the regulation of energy homoeostasis. The melanocortin system consists of endogenous agonists and antagonists, five melanocortin receptor subtypes and receptor accessory proteins. The melanocortin system, as is now known, is far more complex than most of us could have imagined in 1997, and, similarly, the importance of this system for regulating energy homoeostasis in the general human population is much greater than we would have predicted. Of the known factors that can cause human obesity, or protect against it, the melanocortin system is by far the most significant. The present review is a discussion of the current understanding of the roles and mechanism of action of POMC, melanocortin receptors and AgRP (agouti-related peptide) in obesity and Type 2 diabetes and how the central and/or peripheral melanocortin systems mediate nutrient, leptin, insulin, gut hormone and cytokine regulation of energy homoeostasis.

Energy intake and energy expenditure among children with polymorphisms of the melanocortin-3 receptor

BACKGROUND

Homozygosity for 2 protein-altering polymorphisms in the melanocortin-3 receptor gene (MC3R) coding sequence, C17A and G241A, has been reported to be associated with an obesity phenotype in children, yet how these polymorphisms affect energy homeostasis is unknown. Association between adult body weight and +2138InsCAGACC, another variant in the 3' untranslated region of MC3R, has also been described.

OBJECTIVE

The objective of this study was to examine associations of C17A + G241A and +2138InsCAGACC MC3R variants with children's energy balance.

DESIGN

Children aged 6-19 y were genotyped for MC3R C17A, G241A, and +2138InsCAGACC. Subjects underwent studies of energy intake from a 9835-kcal food array (n = 185), resting energy expenditure (REE) by using indirect calorimetry (n = 302), or total daily energy expenditure (TEE) by using doubly labeled water (n = 120). Linear regression was used to examine the associations between MC3R polymorphisms and the measures of energy balance.

RESULTS

Body mass index and fat mass were greater in those with double homozygosity for C17A + G241A (P = 0.001). After accounting for covariates (including body composition), the number of minor C17A + G241A alleles was associated with significantly greater energy intake (beta = +0.15, P = 0.02) but not altered REE or TEE. No significant associations were observed between +2138InsCAGACC and measures of either fat mass or energy balance.

CONCLUSIONS

C17A + G241A polymorphisms may be associated with pediatric obesity because of greater energy intake rather than because of diminished energy expenditure. +2138InsCAGACC does not appear to be associated with obesity or measures of energy balance in children.

Further evidence for the role of ENPP1 in obesity: association with morbid obesity in Finns

The aim of this study was to investigate a series of single-nucleotide polymorphisms (SNPs) in the genes MC2R, MC3R, MC4R, MC5R, POMC, and ENPP1 for association with obesity. Twenty-five SNPs (2-7 SNPs/gene) were genotyped in 246 Finns with extreme obesity (BMI > or = 40 kg/m2) and in 481 lean subjects (BMI 20-25 kg/m2). Of the obese subjects, 23% had concomitant type 2 diabetes. SNPs and SNP haplotypes were tested for association with obesity and type 2 diabetes. Allele frequencies differed between obese and lean subjects for two SNPs in the ENPP1 gene, rs1800949 (P = 0.006) and rs943003 (P = 0.0009). These SNPs are part of a haplotype (rs1800949 C-rs943003 A), which was observed more frequently in lean subjects compared to obese subjects (P = 0.0007). Weaker associations were detected between the SNPs rs1541276 in the MC5R, rs1926065 in the MC3R genes and obesity (P = 0.04 and P = 0.03, respectively), and between SNPs rs2236700 in the MC5R, rs2118404 in the POMC, rs943003 in the ENPP1 genes and type 2 diabetes (P = 0.03, P = 0.02 and P = 0.02, respectively); these associations did not, however, remain significant after correction for multiple testing. In conclusion, a previously unexplored ENPP1 haplotype composed of SNPs rs1800949 and rs943003 showed suggestive evidence for association with adult-onset morbid obesity in Finns. In this study, we did not find association between the frequently studied ENPP1 K121Q variant, nor SNPs in the MCR or POMC genes and obesity or type 2 diabetes.

Sporadic mutations in melanocortin receptor 3 in morbid obese individuals

Several mutations in the melanocortin receptor 4 gene have been identified in humans and account for 3-6% of morbid obesity. In contrast, strong evidence of a causative role for melanocortin receptor 3 (MC3R) mutations are still lacking. In MC3R knockout mice, high feed efficiency rather than hyperphagia seems to contribute to increased fat mass. On the basis of this evidence, the objective of the present study was to investigate the presence of MC3R mutations in a group of 290 obese subjects (mean BMI 44.2+/-5.9 kg/m2). As a control, a group of 215 normal-weight subjects (mean BMI 22.4+/-2.7 kg/m2) was also screened. Three novel mutations in the MC3R gene (A293T, I335S and X361S) were identified among the obese patients. The mutations segregated with obesity in the members of the families studied. In vitro expression studies of each mutation demonstrated a loss of function of the I335S-mutated receptor. These findings suggest that, in humans, MC3R mutations may be a cause of a dominantly inherited form of obesity. However, this association as well as the specific phenotypic characteristics resulting from these mutations need to be further evaluated in larger series of obese subjects.

Genes, fat intake, and cardiovascular disease risk factors in the Quebec Family Study

OBJECTIVE

The aim of this study was to assess gene-diet interaction effects on cardiovascular disease (CVD) risk factors (waist circumference, plasma triacylglycerol, high-density lipoprotein-cholesterol and fasting glucose concentrations, and diastolic and systolic blood pressure) in the Quebec Family Study cohort.

DESIGN

Sixty-four polymorphisms from 45 candidate genes were studied in 645 subjects. Dietary fat intake was obtained from a 3-day weighted food record.

RESULTS

We observed 18 significant interactions at a p value <or= 0.01. Among them, the Pro12Ala polymorphism in peroxisome proliferator-activated receptor gamma, alone or in interaction with fat intake, significantly modulated waist circumference (p = 0.0005 for both effects). Additionally, the apolipoprotein E genotype in interaction with fat intake was significantly associated with diastolic and systolic blood pressure (p = 0.01 and p = 0.001, respectively). The ghrelin Leu72Met polymorphism also interacted with dietary fat in its relation to waist circumference and triacylglycerol concentrations (p = 0.0004 and p = 0.005).

DISCUSSION

These results suggest that several alleles at candidate genes interact with dietary fat intake to modulate well-known CVD risk factors. The identification of gene-diet interaction effects is likely to provide useful information concerning the etiology of CVD.

Medical sequencing at the extremes of human body mass

Body weight is a quantitative trait with significant heritability in humans. To identify potential genetic contributors to this phenotype, we resequenced the coding exons and splice junctions of 58 genes in 379 obese and 378 lean individuals. Our 96-Mb survey included 21 genes associated with monogenic forms of obesity in humans or mice, as well as 37 genes that function in body weight-related pathways. We found that the monogenic obesity-associated gene group was enriched for rare nonsynonymous variants unique to the obese population compared with the lean population. In addition, computational analysis predicted a greater fraction of deleterious variants within the obese cohort. Together, these data suggest that multiple rare alleles contribute to obesity in the population and provide a medical sequencing-based approach to detect them.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

Gene polymorphisms and their effects in the melanocortin system

In addition to its role in human pigmentation, components of the melanocortin system regulate appetite, energy homeostasis and hormone production. Recent studies have suggested possible roles of this system in immunity, transmission of pain signals, and reproductive potential. A number of polymorphisms have been identified in genes of the melanocortin system and are associated with pigmentation in humans, as well as being causative of disorders of adrenal hormone production and obesity. This review gives an outline of these polymorphisms, their functional significance and possible application to or impact on diagnosis and pharmacotherapy based on melanocortin pathways.

Molecular mechanisms of the neural melanocortin receptor dysfunction in severe early onset obesity

The neural melanocortin receptors, melanocortin-3 and -4 receptors (MC3R and MC4R), have been shown to regulate different aspects of energy homeostasis in rodents. Human genetic studies showed that mutations in the MC4R gene are the most common monogenic form of obesity. Functional analyses of the mutant receptors revealed multiple defects. A classification scheme is presented for cataloguing the ever-increasing array of MC4R mutations. Functional analysis of the only inactivating MC3R mutation is also summarized. Insights from the analyses of the naturally occurring mutations in the MC3R and MC4R on the structure and function of these receptors are highlighted.

Melanocortin-4 receptor gene and physical activity in the Québec Family Study

Physical inactivity is a risk factor for numerous chronic diseases. Low compliance with interventions to increase activity suggests involvement of biological systems.

OBJECTIVE

To examine whether sequence variants in genes encoding neuropeptides and receptors in the arcuate and paraventricular nucleus of the hypothalamus contribute to variations in physical activity level in the Québec Family Study.

METHODS

We genotyped polymorphisms in the melanocortin-4 receptor (MC4R), melanocortin-3 receptor (MC3R), neuropeptide-Y (NPY), neuropeptide-Y Y1 receptor (NPY Y1R), cocaine- and amphetamine-regulated transcript (CART), agouti-related protein (AGRP), and pro-opiomelanocortin (POMC) genes in 669 subjects (age (X+/-s.d.): parents: 52+/-3.4 y; offspring: 28+/-8.7 y). Total physical activity, moderate-to-strenuous activity, and inactivity phenotypes were estimated from a three-day record. The past year's physical activity level was assessed from a questionnaire. Associations between the physical activity phenotypes and the polymorphisms were analyzed using the MIXED model (SAS).

RESULTS

The MC4R-C-2745T variant showed significant associations with physical activity phenotypes. The lowest moderate-to-strenuous activity scores (P = 0.005) and the highest inactivity scores (P = 0.01) emerged in the T/T genotype. Exclusion of obese subjects increased the association. For inactivity, the association of the MC4R-C-2745T variant was strongest in the offspring (P = 0.002). The T/T offspring had both the highest inactivity score and the lowest body mass index. The CART-A1475G variant modified the associations with MC4R-C-2745T; T/T homozygotes had the lowest activity scores when they also had the A/A CART-A1475G genotype. No significant associations were observed with polymorphisms in the other neuropeptides.

CONCLUSION

These findings suggest that DNA sequence variation at the MC4R gene locus may contribute to the propensity to be sedentary.

Haplotypes in the phospholipid transfer protein gene are associated with obesity-related phenotypes: the Québec Family Study

BACKGROUND

The phospholipid transfer protein (PLTP) may play a role in body fat regulation.

OBJECTIVE

To investigate the association between PLTP genetic variants and obesity-related phenotypes.

METHODS

Two intronic variants, one in intron 1 (c.-87G>A) and the other in intron 12 (c.1175+68T>G), were genotyped in 811 participants of the Québec Family Study. Nine obesity-related phenotypes were investigated, including body mass index (BMI), obesity (BMI> or =30 kg/m(2)), and waist circumference, percentage of fat, fat mass and fat-free mass measured by hydrostatic weighing as well as total, visceral and subcutaneous abdominal adipose tissue areas assessed by computed tomography. Single markers and haplotypes were tested for associations in family-based designs using the FBAT program.

RESULTS

The SNP located in intron 1 showed significant associations with obesity, BMI, waist circumference and fat-free mass (P<0.05). The low-frequency allele (A allele) was associated with higher trait values, suggesting that the transmission of this allele is associated with an increased risk of being obese. Significant associations were observed between haplotypes and obesity, waist circumference, percentage of fat and fat-free mass (P<0.05). The transmission of the AT haplotype (frequency=0.180) was positively associated with obesity-related phenotypes. After sequencing the promoter and the coding regions of the PLTP gene, we were unable to identify a mutation that could replicate these results.

CONCLUSION

Intronic variants of the PLTP gene are significantly associated with obesity-related phenotypes. Considering the number and the relevance of candidate genes surrounding the PLTP locus and the absence of missense polymorphisms in the coding region, the associations could be mediated by a second gene allele in linkage disequilibrium with the marker locus.

Human galanin (GAL) and galanin 1 receptor (GALR1) variations are not involved in fat intake and early onset obesity

The neuropeptide galanin (GAL) is involved in food intake and in fat ingestion. Presumably, these effects are conveyed via the galanin 1 receptor (GALR1). We screened the coding region of GAL (including 444 bp of its promoter region) and GALR1 for mutations using single-strand conformation polymorphism analysis and denaturing HPLC in up to 191 obese children and adolescents and 106 healthy underweight young adults (students). In GAL, we identified 3 novel single nucleotide polymorphisms (SNPs; silent: g.-419T-->C, g.-244G-->A; missense: g.47C-->T: Ala16Val) and one infrequent missense variation (c.253A-->G: Asn85Asp), and in GALR1 2 novel SNPs (silent: c.150C-->T, missense: c.793A-->T: Ile265Phe). To test for an association with obesity, we genotyped 7 SNPs (GAL: g.-244G-->A, g.47C-->T, rs7101947, rs1042577, rs3136540; GALR1: c.150C-->T, c.793A-->T) in up to 322 obese children and adolescents compared with up to 277 healthy underweight and normal weight young adults. Furthermore, we analyzed these SNPs with respect to potential effects on the percentage of energy consumed as fat in obese children and adolescents. Allele and genotype frequencies did not differ among the groups tested. In addition, we performed a pedigree transmission disequilibrium test (PDT) for one SNP (GAL: g.-244G-->A) in 610 (518 independent) obesity-trios (obese child or adolescent and both of its parents). However, the PDT for SNP GAL g.-244G-->A revealed no transmission disequilibrium. We conclude that the analyzed SNPs in GAL and GALR1 do not play a major role in early onset obesity or dietary fat intake in the obese children and adolescents of our study groups.

Meal patterns and foraging in melanocortin receptor knockout mice

We report the meal patterns of mice with the deletion of either the melanocortin type 3 or 4 receptors (MC3RKO or MC4RKO) compared with that of the wild type (WT) under conditions of varying foraging costs. Mice lived in two-lever operant chambers; the completion of a designated number of responses (termed procurement fixed ratio or PFR) on the "foraging" lever activated the other lever. On this second lever, the completion of a designated number of responses (termed consumatory fixed ratio or CFR) caused the delivery of a 20-mg food pellet. Animals could complete as many CFRs as they wished to constitute a meal, but whenever 10 min elapsed without pressing on this second lever, the meal was terminated and pressing on the "foraging" lever was again required to initiate a new meal. At lower PFRs, mice of all three genotypes took 5-7 well-defined meals per day of approximately 35 pellets/meal. At the highest PFR, mice of all three groups took about half this number of meals, with some increase in meal size, and total intake was slightly reduced. MC4RKO mice were obese compared with WT or MC3RKO but failed to eat more food in the operant chambers and, as a consequence, lost weight, regardless of PFR. Thus, changes in meal-taking strategies as a function of imposed foraging cost are not critically dependent on either MC3 or MC4 receptors, but these conditions did not allow us to study meal patterns in MC4RKO mice that are hyperphagic.

Inactivation and intracellular retention of the human I183N mutated melanocortin 3 receptor associated with obesity

Melanocortins are known to be involved in the regulation of feeding behavior. These hormones mediate their effects through G protein-coupled receptors (GPCRs) by stimulating adenylate cyclase. The melanocortin 3 receptor (MC3R) in the melanocortin receptor (MCR) family has been identified as a neural receptor subtype mainly expressed in the brain in mammals. Until now, only one heterozygous mutation (I183N) has been identified in the coding region of this receptor in two obese patients of the same family. In this study, we reported the functional characterization of the I183N mutated MC3R compared with that of the wild-type MC3R after transfection in HEK293 cells. Our results showed that the I183N mutation totally abolished the activity of the mutated receptor to generate intracellular cAMP. Furthermore, confocal microscopy observation revealed that the mutation induced an intracellular retention of the mutated receptor. Moreover, we demonstrated for the first time by co-transfection studies that the mutated receptor could reduce the wild-type receptor activity through a dominant negative effect.

[Genetics of obesity and metabolic complications in the Quebec Family Study]

Considerable progress has been accomplished over the past 10 years in the understanding of the genetic basis of obesity and its related metabolic complications. It is now well established that obesity aggregates within families and that genes are partly responsible for this familial aggregation. The number of genes potentially involved in obesity continues to grow. This review summarizes the evidence accumulated so for regarding the contribution of genetic factors in obesity and the number of gene and loci linked to obesity in the Quebec Family Study.

Association studies of genetic polymorphisms in central obesity: a critical review

During the past decade, mutations affecting liability to central obesity have been discovered at a phenomenal rate, and despite few consistently replicated findings, a number of intriguing results have emerged in the literature. Association studies have been proposed to identify the genetic determinants of complex traits such as central obesity. The advantages of the association method include its relative robustness to genetic heterogeneity and the ability to detect much smaller effect sizes than is detectable using feasible sample sizes in linkage studies. However, the current literature linking central obesity to genetic variants is teeming with reports of associations that either cannot be replicated or for which corroboration by linkage has been impossible to find. Explanations for this lack of reproducibility are well rehearsed, and typically include poor study design, incorrect assumptions about the underlying genetic architecture, and simple overinterpretation of data. These limitations create concern about the validity of association studies and cause problems in establishing robust criteria for undertaking association studies. In this article, the current status of the literature of association studies for genetic dissection of central obesity is critically reviewed.

The human obesity gene map: the 2002 update

This is the ninth update of the human obesity gene map, incorporating published results through October 2002 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and various animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. For the first time, transgenic and knockout murine models exhibiting obesity as a phenotype are incorporated (N = 38). As of October 2002, 33 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and the causal genes or strong candidates have been identified for 23 of these syndromes. QTLs reported from animal models currently number 168; there are 68 human QTLs for obesity phenotypes from genome-wide scans. Additionally, significant linkage peaks with candidate genes have been identified in targeted studies. Seven genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 222 studies reporting positive associations with 71 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. More than 300 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.