A rare human variant that disrupts GPR10 signalling causes weight gain in mice

Disruption of brain-expressed G protein-coupled receptor-10 (GPR10) causes obesity in animals. Here, we identify multiple rare variants in GPR10 in people with severe obesity and in normal weight controls. These variants impair ligand binding and G protein-dependent signalling in cells. Transgenic mice harbouring a loss of function GPR10 variant found in an individual with obesity, gain excessive weight due to decreased energy expenditure rather than increased food intake. This evidence supports a role for GPR10 in human energy homeostasis. Therapeutic targeting of GPR10 may represent an effective weight-loss strategy.

Obesity Due to Steroid Receptor Coactivator-1 Deficiency Is Associated With Endocrine and Metabolic Abnormalities

CONTEXT

Genetic variants affecting the nuclear hormone receptor coactivator steroid receptor coactivator, SRC-1, have been identified in people with severe obesity and impair melanocortin signaling in cells and mice. As a result, obese patients with SRC-1 deficiency are being treated with a melanocortin 4 receptor agonist in clinical trials.

OBJECTIVE

Here, our aim was to comprehensively describe and characterize the clinical phenotype of SRC-1 variant carriers to facilitate diagnosis and clinical management.

METHODS

In genetic studies of 2462 people with severe obesity, we identified 23 rare heterozygous variants in SRC-1. We studied 29 adults and 18 children who were SRC-1 variant carriers and performed measurements of metabolic and endocrine function, liver imaging, and adipose tissue biopsies. Findings in adult SRC-1 variant carriers were compared to 30 age- and body mass index (BMI)-matched controls.

RESULTS

The clinical spectrum of SRC-1 variant carriers included increased food intake in children, normal basal metabolic rate, multiple fractures with minimal trauma (40%), persistent diarrhea, partial thyroid hormone resistance, and menorrhagia. Compared to age-, sex-, and BMI-matched controls, adult SRC-1 variant carriers had more severe adipose tissue fibrosis (46.2% vs 7.1% respectively, P = .03) and a suggestion of increased liver fibrosis (5/13 cases vs 2/13 in controls, odds ratio = 3.4), although this was not statistically significant.

CONCLUSION

SRC-1 variant carriers exhibit hyperphagia in childhood, severe obesity, and clinical features of partial hormone resistance. The presence of adipose tissue fibrosis and hepatic fibrosis in young patients suggests that close monitoring for the early development of obesity-associated metabolic complications is warranted.

Visualization of sympathetic neural innervation in human white adipose tissue

Obesity, defined as an excess of adipose tissue that adversely affects health, is a major cause of morbidity and mortality. However, to date, understanding the structure and function of human adipose tissue has been limited by the inability to visualize cellular components due to the innate structure of adipocytes, which are characterized by large lipid droplets. Combining the iDISCO and the CUBIC protocols for whole tissue staining and optical clearing, we developed a protocol to enable immunostaining and clearing of human subcutaneous white adipose tissue (WAT) obtained from individuals with severe obesity. We were able to perform immunolabelling of sympathetic nerve terminals in whole WAT and subsequent optical clearing by eliminating lipids to render the opaque tissue completely transparent. We then used light sheet confocal microscopy to visualize sympathetic innervation of human WAT from obese individuals in a three-dimensional manner. We demonstrate the visualization of sympathetic nerve terminals in human WAT. This protocol can be modified to visualize other structures such as blood vessels involved in the development, maintenance and function of human adipose tissue in health and disease.

Obesity due to melanocortin 4 receptor (MC4R) deficiency is associated with delayed gastric emptying

OBJECTIVE

People who are severely obese due to melanocortin-4 receptor (MC4R) deficiency experience hyperphagia and impaired fullness after a meal (satiety). Meal-induced satiety is influenced by hormones, such as peptide-YY (PYY), which are released by enteroendocrine cells upon nutrient delivery to the small intestine.

DESIGN

We investigated whether gastric emptying and PYY levels are altered in MC4R deficiency.

METHODS

Gastric emptying was measured with a gastric scintigraphy protocol using technetium-99m (⁹⁹ Tc^m )-Tin Colloid for 3.5 h in individuals with loss of function MC4R variants and a control group of similar age and weight. In a separate study, we measured plasma PYY levels before and at multiple time points after three standardised meals given to individuals with MC4R deficiency and controls. Fasting PYY (basal secretion) and postprandial PYY levels were measured and the area under the curve and inter-meal peak were calculated.

RESULTS

We found that gastric emptying time was significantly delayed and percentage meal retention increased in individuals with MC4R deficiency compared to obese controls. In addition, fasting and mean PYY secretion throughout the day were decreased in MC4R deficiency, whereas postprandial PYY secretion was unaltered.

CONCLUSION

Delayed gastric emptying and reduced basal PYY secretion may contribute to impaired satiety in people with obesity due to MC4R deficiency.

Predicting novel candidate human obesity genes and their site of action by systematic functional screening in Drosophila

The discovery of human obesity-associated genes can reveal new mechanisms to target for weight loss therapy. Genetic studies of obese individuals and the analysis of rare genetic variants can identify novel obesity-associated genes. However, establishing a functional relationship between these candidate genes and adiposity remains a significant challenge. We uncovered a large number of rare homozygous gene variants by exome sequencing of severely obese children, including those from consanguineous families. By assessing the function of these genes in vivo in Drosophila, we identified 4 genes, not previously linked to human obesity, that regulate adiposity (itpr, dachsous, calpA, and sdk). Dachsous is a transmembrane protein upstream of the Hippo signalling pathway. We found that 3 further members of the Hippo pathway, fat, four-jointed, and hippo, also regulate adiposity and that they act in neurons, rather than in adipose tissue (fat body). Screening Hippo pathway genes in larger human cohorts revealed rare variants in TAOK2 associated with human obesity. Knockdown of Drosophila tao increased adiposity in vivo demonstrating the strength of our approach in predicting novel human obesity genes and signalling pathways and their site of action.

Obesity-Associated GNAS Mutations and the Melanocortin Pathway

BACKGROUND

GNAS encodes the Gα_s (stimulatory G-protein alpha subunit) protein, which mediates G protein-coupled receptor (GPCR) signaling. GNAS mutations cause developmental delay, short stature, and skeletal abnormalities in a syndrome called Albright's hereditary osteodystrophy. Because of imprinting, mutations on the maternal allele also cause obesity and hormone resistance (pseudohypoparathyroidism).

METHODS

We performed exome sequencing and targeted resequencing in 2548 children who presented with severe obesity, and we unexpectedly identified 22 GNAS mutation carriers. We investigated whether the effect of GNAS mutations on melanocortin 4 receptor (MC4R) signaling explains the obesity and whether the variable clinical spectrum in patients might be explained by the results of molecular assays.

RESULTS

Almost all GNAS mutations impaired MC4R signaling. A total of 6 of 11 patients who were 12 to 18 years of age had reduced growth. In these patients, mutations disrupted growth hormone-releasing hormone receptor signaling, but growth was unaffected in carriers of mutations that did not affect this signaling pathway (mean standard-deviation score for height, -0.90 vs. 0.75, respectively; P = 0.02). Only 1 of 10 patients who reached final height before or during the study had short stature. GNAS mutations that impaired thyrotropin receptor signaling were associated with developmental delay and with higher thyrotropin levels (mean [±SD], 8.4±4.7 mIU per liter) than those in 340 severely obese children who did not have GNAS mutations (3.9±2.6 mIU per liter; P = 0.004).

CONCLUSIONS

Because pathogenic mutations may manifest with obesity alone, screening of children with severe obesity for GNAS deficiency may allow early diagnosis, improving clinical outcomes, and melanocortin agonists may aid in weight loss. GNAS mutations that are identified by means of unbiased genetic testing differentially affect GPCR signaling pathways that contribute to clinical heterogeneity. Monogenic diseases are clinically more variable than their classic descriptions suggest. (Funded by Wellcome and others.).

Human MC4R variants affect endocytosis, trafficking and dimerization revealing multiple cellular mechanisms involved in weight regulation

The Melanocortin-4 Receptor (MC4R) plays a pivotal role in energy homeostasis. We used human MC4R mutations associated with an increased or decreased risk of obesity to dissect mechanisms that regulate MC4R function. Most obesity-associated mutations impair trafficking to the plasma membrane (PM), whereas obesity-protecting mutations either accelerate recycling to the PM or decrease internalization, resulting in enhanced signaling. MC4R mutations that do not affect canonical Gα_s protein-mediated signaling, previously considered to be non-pathogenic, nonetheless disrupt agonist-induced internalization, β-arrestin recruitment, and/or coupling to Gα_s, establishing their causal role in severe obesity. Structural mapping reveals ligand-accessible sites by which MC4R couples to effectors and residues involved in the homodimerization of MC4R, which is disrupted by multiple obesity-associated mutations. Human genetic studies reveal that endocytosis, intracellular trafficking, and homodimerization regulate MC4R function to a level that is physiologically relevant, supporting the development of chaperones, agonists, and allosteric modulators of MC4R for weight loss therapy.

Leptin-Mediated Changes in the Human Metabolome

CONTEXT

While severe obesity due to congenital leptin deficiency is rare, studies in patients before and after treatment with leptin can provide unique insights into the role that leptin plays in metabolic and endocrine function.

OBJECTIVE

The aim of this study was to characterize changes in peripheral metabolism in people with congenital leptin deficiency undergoing leptin replacement therapy, and to investigate the extent to which these changes are explained by reduced caloric intake.

DESIGN

Ultrahigh performance liquid chromatography-tandem mass spectroscopy (UPLC-MS/MS) was used to measure 661 metabolites in 6 severely obese people with congenital leptin deficiency before, and within 1 month after, treatment with recombinant leptin. Data were analyzed using unsupervised and hypothesis-driven computational approaches and compared with data from a study of acute caloric restriction in healthy volunteers.

RESULTS

Leptin replacement was associated with class-wide increased levels of fatty acids and acylcarnitines and decreased phospholipids, consistent with enhanced lipolysis and fatty acid oxidation. Primary and secondary bile acids increased after leptin treatment. Comparable changes were observed after acute caloric restriction. Branched-chain amino acids and steroid metabolites decreased after leptin, but not after acute caloric restriction. Individuals with severe obesity due to leptin deficiency and other genetic obesity syndromes shared a metabolomic signature associated with increased BMI.

CONCLUSION

Leptin replacement was associated with changes in lipolysis and substrate utilization that were consistent with negative energy balance. However, leptin's effects on branched-chain amino acids and steroid metabolites were independent of reduced caloric intake and require further exploration.

Human BDNF/TrkB variants impair hippocampal synaptogenesis and associate with neurobehavioural abnormalities

Brain-derived neurotrophic factor (BDNF) signals through its high affinity receptor Tropomyosin receptor kinase-B (TrkB) to regulate neuronal development, synapse formation and plasticity. In rodents, genetic disruption of Bdnf and TrkB leads to weight gain and a spectrum of neurobehavioural phenotypes. Here, we functionally characterised a de novo missense variant in BDNF and seven rare variants in TrkB identified in a large cohort of people with severe, childhood-onset obesity. In cells, the E183K BDNF variant resulted in impaired processing and secretion of the mature peptide. Multiple variants in the kinase domain and one variant in the extracellular domain of TrkB led to a loss of function through multiple signalling pathways, impaired neurite outgrowth and dominantly inhibited glutamatergic synaptogenesis in hippocampal neurons. BDNF/TrkB variant carriers exhibited learning difficulties, impaired memory, hyperactivity, stereotyped and sometimes, maladaptive behaviours. In conclusion, human loss of function BDNF/TrkB variants that impair hippocampal synaptogenesis may contribute to a spectrum of neurobehavioural disorders.

Exome Sequencing Identifies Genes and Gene Sets Contributing to Severe Childhood Obesity, Linking PHIP Variants to Repressed POMC Transcription

Obesity is genetically heterogeneous with monogenic and complex polygenic forms. Using exome and targeted sequencing in 2,737 severely obese cases and 6,704 controls, we identified three genes (PHIP, DGKI, and ZMYM4) with an excess burden of very rare predicted deleterious variants in cases. In cells, we found that nuclear PHIP (pleckstrin homology domain interacting protein) directly enhances transcription of pro-opiomelanocortin (POMC), a neuropeptide that suppresses appetite. Obesity-associated PHIP variants repressed POMC transcription. Our demonstration that PHIP is involved in human energy homeostasis through transcriptional regulation of central melanocortin signaling has potential diagnostic and therapeutic implications for patients with obesity and developmental delay. Additionally, we found an excess burden of predicted deleterious variants involving genes nearest to loci from obesity genome-wide association studies. Genes and gene sets influencing obesity with variable penetrance provide compelling evidence for a continuum of causality in the genetic architecture of obesity, and explain some of its missing heritability.

A Transcriptomic Signature of the Hypothalamic Response to Fasting and BDNF Deficiency in Prader-Willi Syndrome

Transcriptional analysis of brain tissue from people with molecularly defined causes of obesity may highlight disease mechanisms and therapeutic targets. We performed RNA sequencing of hypothalamus from individuals with Prader-Willi syndrome (PWS), a genetic obesity syndrome characterized by severe hyperphagia. We found that upregulated genes overlap with the transcriptome of mouse Agrp neurons that signal hunger, while downregulated genes overlap with the expression profile of Pomc neurons activated by feeding. Downregulated genes are expressed mainly in neuronal cells and contribute to neurogenesis, neurotransmitter release, and synaptic plasticity, while upregulated, predominantly microglial genes are involved in inflammatory responses. This transcriptional signature may be mediated by reduced brain-derived neurotrophic factor expression. Additionally, we implicate disruption of alternative splicing as a potential molecular mechanism underlying neuronal dysfunction in PWS. Transcriptomic analysis of the human hypothalamus may identify neural mechanisms involved in energy homeostasis and potential therapeutic targets for weight loss.

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Overlap between genes expressed in human PWS hypothalamus and mouse Agrp neurons

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Downregulated genes are involved in neuronal development

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SNORD116 deletion reduces neural development and survival in cells

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Alternative splicing is disturbed in PWS

Crucial Role of the SH2B1 PH Domain for the Control of Energy Balance

Disruption of the adaptor protein SH2B1 (SH2-B, PSM) is associated with severe obesity, insulin resistance, and neurobehavioral abnormalities in mice and humans. Here, we identify 15 SH2B1 variants in severely obese children. Four obesity-associated human SH2B1 variants lie in the Pleckstrin homology (PH) domain, suggesting that the PH domain is essential for SH2B1's function. We generated a mouse model of a human variant in this domain (P322S). P322S/P322S mice exhibited substantial prenatal lethality. Examination of the P322S/+ metabolic phenotype revealed late-onset glucose intolerance. To circumvent P322S/P322S lethality, mice containing a two-amino acid deletion within the SH2B1 PH domain (ΔP317, R318 [ΔPR]) were studied. Mice homozygous for ΔPR were born at the expected Mendelian ratio and exhibited obesity plus insulin resistance and glucose intolerance beyond that attributable to their increased adiposity. These studies demonstrate that the PH domain plays a crucial role in how SH2B1 controls energy balance and glucose homeostasis.

Neural networks associated with body composition in frontotemporal dementia

Background

Frontotemporal dementia (FTD) is associated with complex changes in eating behavior and metabolism, which potentially affect disease pathogenesis and survival. It is currently not known if body composition changes and changes in fat deposition also exist in FTD, the relationship of these changes in eating behavior and appetite, and whether these changes are centrally mediated.

Methods

Body composition was measured in 28 people with behavioral‐variant frontotemporal dementia (bvFTD), 16 with Alzheimer’s disease (AD), and 19 healthy controls, using dual energy x‐ray absorptiometry. Changes in body composition were correlated to brain grey matter atrophy using voxel‐based morphometry on high‐resolution magnetic resonance imaging.

Results

Behavioral‐variant FTD was characterized by changes in body composition, with increased total fat mass, visceral adipose tissue area (VAT area), and android: gynoid ratio compared to control and AD participants (all P values < 0.05). Changes in body composition correlated to abnormal eating behavior and behavioral change (P < 0.01) and functional decline (P < 0.01). Changes in body composition also correlated to grey matter atrophy involving a distributed neural network that included the hippocampus, amygdala, nucleus accumbens, insula, cingulate, and cerebellum – structures known to be central to autonomic control – as well as the thalamus, putamen, accumbens, and caudate, which are involved in reward processing.

Conclusions

Changes in body composition and fat deposition extend the clinical phenomenology in bvFTD beyond cognition and behavior, with changes associated with changes in reward and autonomic processing suggesting that these deficits may be central in FTD

Steroid receptor coactivator-1 modulates the function of Pomc neurons and energy homeostasis

Hypothalamic neurons expressing the anorectic peptide Pro-opiomelanocortin (Pomc) regulate food intake and body weight. Here, we show that Steroid Receptor Coactivator-1 (SRC-1) interacts with a target of leptin receptor activation, phosphorylated STAT3, to potentiate Pomc transcription. Deletion of SRC-1 in Pomc neurons in mice attenuates their depolarization by leptin, decreases Pomc expression and increases food intake leading to high-fat diet-induced obesity. In humans, fifteen rare heterozygous variants in SRC-1 found in severely obese individuals impair leptin-mediated Pomc reporter activity in cells, whilst four variants found in non-obese controls do not. In a knock-in mouse model of a loss of function human variant (SRC-1^L1376P), leptin-induced depolarization of Pomc neurons and Pomc expression are significantly reduced, and food intake and body weight are increased. In summary, we demonstrate that SRC-1 modulates the function of hypothalamic Pomc neurons, and suggest that targeting SRC-1 may represent a useful therapeutic strategy for weight loss.

Human Semaphorin 3 Variants Link Melanocortin Circuit Development and Energy Balance

Hypothalamic melanocortin neurons play a pivotal role in weight regulation. Here, we examined the contribution of Semaphorin 3 (SEMA3) signaling to the development of these circuits. In genetic studies, we found 40 rare variants in SEMA3A-G and their receptors (PLXNA1-4; NRP1-2) in 573 severely obese individuals; variants disrupted secretion and/or signaling through multiple molecular mechanisms. Rare variants in this set of genes were significantly enriched in 982 severely obese cases compared to 4,449 controls. In a zebrafish mutagenesis screen, deletion of 7 genes in this pathway led to increased somatic growth and/or adiposity demonstrating that disruption of Semaphorin 3 signaling perturbs energy homeostasis. In mice, deletion of the Neuropilin-2 receptor in Pro-opiomelanocortin neurons disrupted their projections from the arcuate to the paraventricular nucleus, reduced energy expenditure, and caused weight gain. Cumulatively, these studies demonstrate that SEMA3-mediated signaling drives the development of hypothalamic melanocortin circuits involved in energy homeostasis.

Genetic architecture of human thinness compared to severe obesity

The variation in weight within a shared environment is largely attributable to genetic factors. Whilst many genes/loci confer susceptibility to obesity, little is known about the genetic architecture of healthy thinness. Here, we characterise the heritability of thinness which we found was comparable to that of severe obesity (h2 = 28.07 vs 32.33% respectively), although with incomplete genetic overlap (r = -0.49, 95% CI [-0.17, -0.82], p = 0.003). In a genome-wide association analysis of thinness (n = 1,471) vs severe obesity (n = 1,456), we identified 10 loci previously associated with obesity, and demonstrate enrichment for established BMI-associated loci (pbinomial = 3.05x10-5). Simulation analyses showed that different association results between the extremes were likely in agreement with additive effects across the BMI distribution, suggesting different effects on thinness and obesity could be due to their different degrees of extremeness. In further analyses, we detected a novel obesity and BMI-associated locus at PKHD1 (rs2784243, obese vs. thin p = 5.99x10-6, obese vs. controls p = 2.13x10-6 pBMI = 2.3x10-13), associations at loci recently discovered with much larger sample sizes (e.g. FAM150B and PRDM6-CEP120), and novel variants driving associations at previously established signals (e.g. rs205262 at the SNRPC/C6orf106 locus and rs112446794 at the PRDM6-CEP120 locus). Our ability to replicate loci found with much larger sample sizes demonstrates the value of clinical extremes and suggest that characterisation of the genetics of thinness may provide a more nuanced understanding of the genetic architecture of body weight regulation and may inform the identification of potential anti-obesity targets.

Neural deletion of Sh2b1 results in brain growth retardation and reactive aggression

Psychiatric disorders are associated with aberrant brain development and/or aggressive behavior and are influenced by genetic factors; however, genes that affect brain aggression circuits remain elusive. Here, we show that neuronal Src-homology-2 (SH2)B adaptor protein-1 ( Sh2b1) is indispensable for both brain growth and protection against aggression. Global and brain-specific deletion of Sh2b1 decreased brain weight and increased aggressive behavior. Global and brain-specific Sh2b1 knockout (KO) mice exhibited fatal, intermale aggression. In a resident-intruder paradigm, latency to attack was markedly reduced, whereas the number and the duration of attacks was significantly increased in global and brain-specific Sh2b1 KO mice compared with wild-type littermates. Consistently, core aggression circuits were activated to a higher level in global and brain-specific Sh2b1 KO males, based on c-fos immunoreactivity in the amygdala and periaqueductal gray. Brain-specific restoration of Sh2b1 normalized brain size and reversed pathologic aggression and aberrant activation of core aggression circuits in Sh2b1 KO males. SH2B1 mutations in humans were linked to aberrant brain development and behavior. At the molecular level, Sh2b1 enhanced neurotrophin-stimulated neuronal differentiation and protected against oxidative stress-induced neuronal death. Our data suggest that neuronal Sh2b1 promotes brain development and the integrity of core aggression circuits, likely through enhancing neurotrophin signaling.-Jiang, L., Su, H., Keogh, J. M., Chen, Z., Henning, E., Wilkinson, P., Goodyer, I., Farooqi, I. S., Rui, L. Neural deletion of Sh2b1 results in brain growth retardation and reactive aggression.

A Metabolomic Signature of Acute Caloric Restriction

CONTEXT

The experimental paradigm of acute caloric restriction (CR) followed by refeeding (RF) can be used to study the homeostatic mechanisms that regulate energy homeostasis, which are relevant to understanding the adaptive response to weight loss.

OBJECTIVE

Metabolomics, the measurement of hundreds of small molecule metabolites, their precursors, derivatives, and degradation products, has emerged as a useful tool for the study of physiology and disease and was used here to study the metabolic response to acute CR.

PARTICIPANTS, DESIGN, AND SETTING

We used four ultra high-performance liquid chromatography-tandem mass spectrometry methods to characterize changes in carbohydrates, lipids, amino acids, and steroids in eight normal weight men at baseline, after 48 hours of CR (10% of energy requirements) and after 48 hours of ad libitum RF in a tightly controlled environment.

RESULTS

We identified a distinct metabolomic signature associated with acute CR characterized by the expected switch from carbohydrate to fat utilization with increased lipolysis and β-fatty acid oxidation. We found an increase in ω-fatty acid oxidation and levels of endocannabinoids, which are known to promote food intake. These changes were reversed with RF. Several plasmalogen phosphatidylethanolamines (endogenous antioxidants) significantly decreased with CR (all P ≤ 0.0007). Additionally, acute CR was associated with an increase in the branched chain amino acids (all P ≤ 1.4 × 10-7) and dehydroepiandrosterone sulfate (P = 0.0006).

CONCLUSIONS

We identified a distinct metabolomic signature associated with acute CR. Further studies are needed to characterize the mechanisms that mediate these changes and their potential contribution to the adaptive response to dietary restriction.

Rare Variant Analysis of Human and Rodent Obesity Genes in Individuals with Severe Childhood Obesity

Obesity is a genetically heterogeneous disorder. Using targeted and whole-exome sequencing, we studied 32 human and 87 rodent obesity genes in 2,548 severely obese children and 1,117 controls. We identified 52 variants contributing to obesity in 2% of cases including multiple novel variants in GNAS, which were sometimes found with accelerated growth rather than short stature as described previously. Nominally significant associations were found for rare functional variants in BBS1, BBS9, GNAS, MKKS, CLOCK and ANGPTL6. The p.S284X variant in ANGPTL6 drives the association signal (rs201622589, MAF~0.1%, odds ratio = 10.13, p-value = 0.042) and results in complete loss of secretion in cells. Further analysis including additional case-control studies and population controls (N = 260,642) did not support association of this variant with obesity (odds ratio = 2.34, p-value = 2.59 × 10-3), highlighting the challenges of testing rare variant associations and the need for very large sample sizes. Further validation in cohorts with severe obesity and engineering the variants in model organisms will be needed to explore whether human variants in ANGPTL6 and other genes that lead to obesity when deleted in mice, do contribute to obesity. Such studies may yield druggable targets for weight loss therapies.

Oxytocin administration suppresses hypothalamic activation in response to visual food cues

The aim of this study was to use functional neuroimaging to investigate whether oxytocin modulates the neural response to visual food cues in brain regions involved in the control of food intake. Twenty-four normal weight volunteers received intranasal oxytocin (24 IU) or placebo in a double-blind, randomized crossover study. Measurements were made forty-five minutes after dosing. On two occasions, functional MRI (fMRI) scans were performed in the fasted state; the blood oxygen level-dependent (BOLD) response to images of high-calorie foods versus low-calorie foods was measured. Given its critical role in eating behaviour, the primary region of interest was the hypothalamus. Secondary analyses examined the parabrachial nuclei and other brain regions involved in food intake and food reward. Intranasal oxytocin administration suppressed hypothalamic activation to images of high-calorie compared to low-calorie food (P = 0.0125). There was also a trend towards suppression of activation in the parabrachial nucleus (P = 0.0683). No effects of intranasal oxytocin were seen in reward circuits or on ad libitum food intake. Further characterization of the effects of oxytocin on neural circuits in the hypothalamus is needed to establish the utility of targeting oxytocin signalling in obesity.

Divergent effects of central melanocortin signalling on fat and sucrose preference in humans

Melanocortin-4-receptor (MC4R)-expressing neurons modulate food intake and preference in rodents but their role in human food preference is unknown. Here we show that compared with lean and weight-matched controls, MC4R deficient individuals exhibited a markedly increased preference for high fat, but a significantly reduced preference for high sucrose food. These effects mirror those in Mc4r null rodents and provide evidence for a central molecular circuit influencing human macronutrient preference.

Hypothalamic melanocortin-4-receptors (MC4R) regulate food preference in rodents, but their role in humans is unclear. Here, the authors perform food preference and liking tests in humans with MC4R mutations and find that they prefer fatty food more, but sweet food less, than people without MC4R mutations.

Failure of sucrose replacement with the non-nutritive sweetener erythritol to alter GLP-1 or PYY release or test meal size in lean or obese people

There is considerable interest in the effect of foods containing high intensity sweeteners on satiation. However, less is known about low-calorie bulk sweeteners such as erythritol. In this randomized three-way crossover study, we studied 10 lean and 10 obese volunteers who consumed three test meals on separate occasions: (a) control sucrose meal; (b) isovolumic meal with partial replacement of sucrose by erythritol; (c) isocaloric meal which contained more erythritol but equivalent calories to the control meal. We measured gut hormone levels, hunger and satiety scores, ad libitum food intake, sucrose preference and intake after the manipulations. There was a greater post-prandial excursion in glucose and insulin levels after sucrose than after the erythritol meals. There was no difference in GLP-1/PYY levels or subsequent energy intake and sucrose preference between sucrose control and isovolumic erythritol meals. In lean (but not obese) participants, hunger decreased to a greater extent after the isocaloric erythritol meal compared to the control meal (p = 0.003) reflecting the larger volume of this meal. Replacing sucrose with erythritol leads to comparable hunger and satiety scores, GLP-1 and PYY levels, and subsequent sucrose preference and intake.

The Sleep/Wake Cycle is Directly Modulated by Changes in Energy Balance

STUDY OBJECTIVES

The rise in obesity has been paralleled by a decline in sleep duration in epidemiological studies. However, the potential mechanisms linking energy balance and the sleep/wake cycle are not well understood. We aimed to examine the effects of manipulating energy balance on the sleep/wake cycle.

METHODS

Twelve healthy normal weight men were housed in a clinical research facility and studied at three time points: baseline, after energy balance was disrupted by 2 days of caloric restriction to 10% of energy requirements, and after energy balance was restored by 2 days of ad libitum/free feeding. Sleep architecture, duration of sleep stages, and sleep-associated respiratory parameters were measured by polysomnography.

RESULTS

Two days of caloric restriction significantly increased the duration of deep (stage 4) sleep (16.8% to 21.7% of total sleep time; P = 0.03); an effect which was entirely reversed upon free feeding (P = 0.01). Although the apnea-hypopnea index stayed within the reference range (< 5 events per hour), it decreased significantly from caloric restriction to free feeding (P = 0.03). Caloric restriction was associated with a marked fall in leptin (P < 0.001) and insulin levels (P = 0.002). The fall in orexin levels from baseline to caloric restriction correlated positively with duration of stage 4 sleep (Spearman rho = 0.83, P = 0.01) and negatively with the number of awakenings in caloric restriction (Spearman rho = -0.79, P = 0.01).

CONCLUSIONS

We demonstrate that changes in energy homeostasis directly and reversibly impact on the sleep/wake cycle. These findings provide a mechanistic framework for investigating the association between sleep duration and obesity risk.

Severe Early-Onset Obesity Due to Bioinactive Leptin Caused by a p.N103K Mutation in the Leptin Gene

CONTEXT

Congenital leptin deficiency is a very rare cause of severe early-onset obesity. We recently characterized a mutation in the leptin gene (p.D100Y), which was associated with detectable leptin levels and bioinactivity of the hormone.

CASE DESCRIPTION

We now describe two siblings, a 9-year-old girl and a 6-year-old boy with severe early-onset obesity and hyperphagia, both homozygous for a c.309C>A substitution in the leptin gene leading to a p.N103K amino acid exchange in the protein and detectable circulating levels of leptin. In vitro experiments in a heterologous cell system demonstrated that the mutated protein was biologically inactive. Treatment with sc recombinant human leptin led to rapid improvement of eating behavior and weight loss.

CONCLUSIONS

Sequencing of the leptin gene may need to be considered in hyperphagic, severely obese children with detectable levels of circulating leptin.

Obesity-associated melanocortin-4 receptor mutations are associated with changes in the brain response to food cues

CONTEXT

Mutations in the melanocortin-4 receptor (MC4R) represent the commonest genetic form of obesity and are associated with hyperphagia.

OBJECTIVE

The aim of this study was to investigate whether melanocortin signaling modulates anticipatory food reward by studying the brain activation response to food cues in individuals with MC4R mutations. Design/Setting/Participants/Main Outcome Measure: We used functional magnetic resonance imaging to measure blood oxygen level-dependent responses to images of highly palatable, appetizing foods, bland foods, and non-food objects in eight obese individuals with MC4R mutations, 10 equally obese controls, and eight lean controls with normal MC4R genotypes. Based on previous evidence, we performed a region-of-interest analysis centered on the caudate/putamen (dorsal striatum) and ventral striatum.

RESULTS

Compared to non-foods, appetizing foods were associated with activation in the dorsal and ventral striatum in lean controls and in MC4R-deficient individuals. Surprisingly, we observed reduced activation of the dorsal and ventral striatum in obese controls relative to MC4R-deficient patients and lean controls. There were no group differences for the contrast of disgusting foods with bland foods or non-foods, suggesting that the effects observed in response to appetizing foods were not related to arousal.

CONCLUSION

We identified differences in the striatal response to food cues between two groups of obese individuals, those with and those without MC4R mutations. These findings are consistent with a role for central melanocortinergic circuits in the neural response to visual food cues.

Functional characterization of obesity-associated variants involving the α and β isoforms of human SH2B1

We have previously reported rare variants in sarcoma (Src) homology 2 (SH2) B adaptor protein 1 (SH2B1) in individuals with obesity, insulin resistance, and maladaptive behavior. Here, we identify 4 additional SH2B1 variants by sequencing 500 individuals with severe early-onset obesity. SH2B1 has 4 alternatively spliced isoforms. One variant (T546A) lies within the N-terminal region common to all isoforms. As shown for past variants in this region, T546A impairs SH2B1β enhancement of nerve growth factor-induced neurite outgrowth, and the individual with the T546A variant exhibits mild developmental delay. The other 3 variants (A663V, V695M, and A723V) lie in the C-terminal tail of SH2B1α. SH2B1α variant carriers were hyperinsulinemic but did not exhibit the behavioral phenotype observed in individuals with SH2B1 variants that disrupt all isoforms. In in vitro assays, SH2B1α, like SH2B1β, enhances insulin- and leptin-induced insulin receptor substrate 2 (IRS2) phosphorylation and GH-induced cell motility. None of the variants affect SH2B1α enhancement of insulin- and leptin-induced IRS2 phosphorylation. However, T546A, A663V, and A723V all impair the ability of SH2B1α to enhance GH-induced cell motility. In contrast to SH2B1β, SH2B1α does not enhance nerve growth factor-induced neurite outgrowth. These studies suggest that genetic variants that disrupt isoforms other than SH2B1β may be functionally significant. Further studies are needed to understand the mechanism by which the individual isoforms regulate energy homeostasis and behavior.

High protein intake stimulates postprandial GLP1 and PYY release

OBJECTIVE

Meals high in protein induce greater intermeal satiety than meals high in fat and carbohydrates. We studied the gut hormone response and subsequent food intake after breakfasts high in protein, carbohydrate or high in fat controlled for volume, calories and appearance.

DESIGN AND METHODS

Eight healthy volunteers participated in this randomized three-way crossover study. Study breakfasts were calculated to provide 20% of daily energy requirements and provided either 60% of energy from protein, fat or carbohydrate. Blood was drawn half-hourly for 4 h; energy intake at a subsequent ad libitum meal was measured.

RESULTS

Total ghrelin decreased after food intake equally with the three breakfasts. PYY levels were highest after the high protein breakfast (P = 0.005). Indeed, PYY at 240 min was highest after the high protein breakfast compared to the high fat breakfast and to the high carbohydrate breakfast (P = 0.011 and P = 0.012, respectively). GLP-1 levels were highest after the high protein breakfast (P = 0.041) at 120 min and remained higher throughout the study. These differences in gut hormones did not translate into differences in food intake (1023 ± 390 kcal after high protein, 1016 ± 388 kcal after high fat and 1158 ± 433 kcal after high carbohydrate).

CONCLUSION

We conclude that a high protein meal increases circulating concentrations of the gut hormones PYY and GLP-1, but when meals are matched for volume, appearance and caloric value, these gut hormone changes do not translate into a reduction in ad libitum food intake.

Loss of function of the melanocortin 2 receptor accessory protein 2 is associated with mammalian obesity

Melanocortin receptor accessory proteins (MRAPs) modulate signaling of melanocortin receptors in vitro. To investigate the physiological role of brain-expressed melanocortin 2 receptor accessory protein 2 (MRAP2), we characterized mice with whole-body and brain-specific targeted deletion of Mrap2, both of which develop severe obesity at a young age. Mrap2 interacts directly with melanocortin 4 receptor (Mc4r), a protein previously implicated in mammalian obesity, and it enhances Mc4r-mediated generation of the second messenger cyclic adenosine monophosphate, suggesting that alterations in Mc4r signaling may be one mechanism underlying the association between Mrap2 disruption and obesity. In a study of humans with severe, early-onset obesity, we found four rare, potentially pathogenic genetic variants in MRAP2, suggesting that the gene may also contribute to body weight regulation in humans.

Rare variants in single-minded 1 (SIM1) are associated with severe obesity

Single-minded 1 (SIM1) is a basic helix-loop-helix transcription factor involved in the development and function of the paraventricular nucleus of the hypothalamus. Obesity has been reported in Sim1 haploinsufficient mice and in a patient with a balanced translocation disrupting SIM1. We sequenced the coding region of SIM1 in 2,100 patients with severe, early onset obesity and in 1,680 controls. Thirteen different heterozygous variants in SIM1 were identified in 28 unrelated severely obese patients. Nine of the 13 variants significantly reduced the ability of SIM1 to activate a SIM1-responsive reporter gene when studied in stably transfected cells coexpressing the heterodimeric partners of SIM1 (ARNT or ARNT2). SIM1 variants with reduced activity cosegregated with obesity in extended family studies with variable penetrance. We studied the phenotype of patients carrying variants that exhibited reduced activity in vitro. Variant carriers exhibited increased ad libitum food intake at a test meal, normal basal metabolic rate, and evidence of autonomic dysfunction. Eleven of the 13 probands had evidence of a neurobehavioral phenotype. The phenotypic similarities between patients with SIM1 deficiency and melanocortin 4 receptor (MC4R) deficiency suggest that some of the effects of SIM1 deficiency on energy homeostasis are mediated by altered melanocortin signaling.

Genome-wide SNP and CNV analysis identifies common and low-frequency variants associated with severe early-onset obesity

Common and rare variants associated with body mass index (BMI) and obesity account for <5% of the variance in BMI. We performed SNP and copy number variation (CNV) association analyses in 1,509 children with obesity at the extreme tail (>3 s.d. from the mean) of the BMI distribution and 5,380 controls. Evaluation of 29 SNPs (P < 1 × 10(-5)) in an additional 971 severely obese children and 1,990 controls identified 4 new loci associated with severe obesity (LEPR, PRKCH, PACS1 and RMST). A previously reported 43-kb deletion at the NEGR1 locus was significantly associated with severe obesity (P = 6.6 × 10(-7)). However, this signal was entirely driven by a flanking 8-kb deletion; absence of this deletion increased risk for obesity (P = 6.1 × 10(-11)). We found a significant burden of rare, single CNVs in severely obese cases (P < 0.0001). Integrative gene network pathway analysis of rare deletions indicated enrichment of genes affecting G protein-coupled receptors (GPCRs) involved in the neuronal regulation of energy homeostasis.

Postprandial total ghrelin suppression is modulated by melanocortin signaling in humans

CONTENTS

Ghrelin is implicated in meal initiation because circulating levels increase before and fall after meal consumption. In rodents, ghrelin stimulates food intake via hypothalamic circuits expressing the melanocortin 4 receptor (MC4R).

OBJECTIVE

The aim of the study was to investigate the impact of central melanocortinergic tone on ghrelin secretion in humans. DESIGN/SETTING/PATIENTS/MAIN OUTCOME MEASURE: We measured plasma total ghrelin before and after 3 standardized meals in patients with MC4R mutations and obese and lean controls. Fasting total ghrelin, area under the curve, and early (30-min) and intermeal postprandial total ghrelin suppression (the percentage difference between the premeal and the 30-min postprandial or intermeal nadir total ghrelin concentration) were calculated.

RESULTS

Fasting total ghrelin concentrations and area under the curve for total ghrelin concentrations were significantly decreased in both obese groups compared to lean controls (fasting total ghrelin: lean controls, 1083 ± 203 pg/ml; and obese controls, 696 ± 81 pg/ml; MC4R: 609 ± 99 pg/ml, P < .05). Early and intermeal postprandial total ghrelin suppression was attenuated in MC4R-deficient patients compared to lean controls (P < .05); a similar pattern was observed for early postprandial suppression in comparison with obese controls, but this difference did not reach statistical significance (P = .09).

CONCLUSIONS

These findings are consistent with a role for central melanocortinergic signaling in modulating meal-related total ghrelin suppression.

Human SH2B1 mutations are associated with maladaptive behaviors and obesity

Src homology 2 B adapter protein 1 (SH2B1) modulates signaling by a variety of ligands that bind to receptor tyrosine kinases or JAK-associated cytokine receptors, including leptin, insulin, growth hormone (GH), and nerve growth factor (NGF). Targeted deletion of Sh2b1 in mice results in increased food intake, obesity, and insulin resistance, with an intermediate phenotype seen in heterozygous null mice on a high-fat diet. We identified SH2B1 loss-of-function mutations in a large cohort of patients with severe early-onset obesity. Mutation carriers exhibited hyperphagia, childhood-onset obesity, disproportionate insulin resistance, and reduced final height as adults. Unexpectedly, mutation carriers exhibited a spectrum of behavioral abnormalities that were not reported in controls, including social isolation and aggression. We conclude that SH2B1 plays a critical role in the control of human food intake and body weight and is implicated in maladaptive human behavior.

Melanocortin-4 receptor signaling is required for weight loss after gastric bypass surgery

CONTEXT

Roux-en-Y gastric bypass (RYGB) is one of the most effective long-term therapies for the treatment of severe obesity. Recent evidence indicates that RYGB effects weight loss through multiple physiological mechanisms, including changes in energy expenditure, food intake, food preference, and reward pathways.

OBJECTIVE

Because central melanocortin signaling plays an important role in the regulation of energy homeostasis, we investigated whether genetic disruption of the melanocortin-4 receptor (MC4R) in rodents and humans affects weight loss after RYGB.

METHODS AND RESULTS

Here we report that MC4R(-/-) mice lost substantially less weight after surgery than wild-type animals, indicating that MC4R signaling is necessary for the weight loss effects of RYGB in this model. Mice heterozygous for MC4R remain fully responsive to gastric bypass. To determine whether mutations affect surgically induced weight loss in humans, we sequenced the MC4R gene in 972 patients undergoing RYGB. Patients heterozygous for MC4R mutations exhibited the same magnitude and distribution of postoperative weight loss as patients without such mutations, suggesting that although two normal copies of the MC4R gene are necessary for normal weight regulation, a single normal copy of the MC4R gene is sufficient to mediate the weight loss effects of RYGB.

CONCLUSIONS

MC4R is the first gene identified that is required for the sustained effects of bariatric surgery. The need for MC4R signaling for the weight loss effects of RYGB in mice underscores the physiological mechanisms of action of this procedure and demonstrates that RYGB both influences and is dependent on the normal pathways that regulate energy balance.

A mutation in the thyroid hormone receptor alpha gene

Thyroid hormones exert their effects through alpha (TRα1) and beta (TRβ1 and TRβ2) receptors. Here we describe a child with classic features of hypothyroidism (growth retardation, developmental retardation, skeletal dysplasia, and severe constipation) but only borderline-abnormal thyroid hormone levels. Using whole-exome sequencing, we identified a de novo heterozygous nonsense mutation in a gene encoding thyroid hormone receptor alpha (THRA) and generating a mutant protein that inhibits wild-type receptor action in a dominant negative manner. Our observations are consistent with defective human TRα-mediated thyroid hormone resistance and substantiate the concept of hormone action through distinct receptor subtypes in different target tissues.

Obesity due to melanocortin 4 receptor (MC4R) deficiency is associated with increased linear growth and final height, fasting hyperinsulinemia, and incompletely suppressed growth hormone secretion

CONTEXT

Melanocortin receptor 4 (MC4R) deficiency is characterized by increased linear growth greater than expected for the degree of obesity.

OBJECTIVE

The objective of the investigation was to study the somatotroph axis in obese MC4R-deficient patients and equally obese controls.

PATIENTS AND METHODS

We obtained anthropometric measurements and insulin concentrations in 153 MC4R-deficient subjects and 1392 controls matched for age and severity of obesity. We measured fasting IGF-I, IGF-II, IGF binding protein (IGFBP)-1, IGFBP-3, and acid-labile subunit levels in a subset of 33 MC4R-deficient patients and 36 control subjects. We examined pulsatile GH secretion in six adult MC4R-deficient subjects and six obese controls.

RESULTS

Height sd score was significantly greater in MC4R-deficient children under 5 yr of age compared with controls (mean ± SEM: 2.3 ± 0.06 vs. 1.8 ± 0.04, P < 0.001), an effect that persisted throughout childhood. Final height (cm) was greater in MC4R-deficient men (mean ± SEM 173 ± 2.5 vs. 168 ± 2.1, P < 0.001) and women (mean 165 ± 2.1 vs. 158 ± 1.9, P < 0.001). Fasting IGF-I, IGF-II, acid-labile subunit, and IGFBP-3 concentrations were similar in the two groups. GH levels were markedly suppressed in obese controls, but pulsatile GH secretion was retained in MC4R deficiency. The mean maximal GH secretion rate per burst (P < 0.05) and mass per burst (P < 0.05) were increased in MC4R deficiency, consistent with increased pulsatile and total GH secretion. Fasting insulin levels were markedly elevated in MC4R-deficient children.

CONCLUSIONS

In MC4R deficiency, increased linear growth in childhood leads to increased adult final height, greater than predicted by obesity alone. GH pulsatility is maintained in MC4R deficiency, a finding consistent with animal studies, suggesting a role for MC4R in controlling hypothalamic somatostatinergic tone. Fasting insulin levels are significantly higher in children carrying MC4R mutations. Both of these factors may contribute to the accelerated growth phenotype characteristic of MC4R deficiency.

A deletion of the HBII-85 class of small nucleolar RNAs (snoRNAs) is associated with hyperphagia, obesity and hypogonadism

Genetic studies in patients with severe early-onset obesity have provided insights into the molecular and physiological pathways that regulate body weight in humans. We report a 19-year-old male with hyperphagia and severe obesity, mild learning difficulties and hypogonadism, in whom diagnostic tests for Prader-Willi syndrome (PWS) had been negative. We carried out detailed clinical and metabolic phenotyping of this patient and investigated the genetic basis of this obesity syndrome using Agilent 185 k array comparative genomic hybridization (aCGH) and Affymetrix 6.0 genotyping arrays. The identified deletion was validated using multiplex ligation-dependent probe amplification and long-range PCR, followed by breakpoint sequencing which enabled precise localization of the deletion. We identified a approximately 187 kb microdeletion at chromosome 15q11-13 that encompasses non-coding small nucleolar RNAs (including HBII-85 snoRNAs) which were not expressed in peripheral lymphocytes from the patient. Characterization of the clinical phenotype revealed increased ad libitum food intake, normal basal metabolic rate when adjusted for fat-free mass, partial hypogonadotropic hypogonadism and growth failure. We have identified a novel deletion on chromosome 15q11-13 in an individual with hyperphagia, obesity, hypogonadism and other features associated with PWS, which is normally caused by deficiency of several paternally expressed imprinted transcripts within chromosome 15q11-13, a region that includes multiple protein-coding genes as well as several non-coding snoRNAs. These findings provide direct evidence for the role of a particular family of non-coding RNAs, the HBII-85 snoRNA cluster, in human energy homeostasis, growth and reproduction.

Functional characterization and structural modeling of obesity associated mutations in the melanocortin 4 receptor

Mutations in the melanocortin 4 receptor (MC4R) gene are the most common known cause of monogenic human obesity. The MC4R gene was sequenced in 2000 subjects with severe early-onset obesity. We detected seven different nonsense and 19 nonsynonymous mutations in a total of 94 probands, some of which have been reported previously by others. We functionally characterized the 11 novel obesity associated missense mutations. Seven of these mutants (L54P, E61K, I69T, S136P, M161T, T162I, and I269N) showed impaired cell surface trafficking, reduced level of maximal binding of the radioligand [125I]NDP-MSH, and reduced ability to generate cAMP in response to ligand. Four mutant MC4Rs (G55V, G55D, S136F, and A303T) displayed cell surface expression and agonist binding similar to the wild-type receptor but showed impaired cAMP production, suggesting that these residues are likely to be critical for conformational rearrangement essential for receptor activation. Homology modeling of these mutants using a model of MC4R based on the crystal structure of the beta2-adrenoreceptor was used to provide insights into the possible structural basis for receptor dysfunction. Transmembrane (TM) domains 1, 3, 6, 7, and peripheral helix 8 appear to participate in the agonist-induced conformational rearrangement necessary for coupling of ligand binding to signaling. We conclude that G55V, G55D, S136F, and A303T mutations are likely to strengthen helix-helix interactions between TM1 and TM2, TM3 and TM6, and TM7 and helix 8, respectively, preventing relative movement of these helices during receptor activation. The combination of functional studies and structural modeling of naturally occurring pathogenic mutations in MC4R can provide valuable information regarding the molecular mechanism of MC4R activation and its dysfunction in human disease.

Modulation of blood pressure by central melanocortinergic pathways

BACKGROUND

Weight gain and weight loss are associated with changes in blood pressure through unknown mechanisms. Central melanocortinergic signaling is implicated in the control of energy balance and blood pressure in rodents, but there is no information regarding such an association with blood pressure in humans.

METHODS

We assessed blood pressure, heart rate, and urinary catecholamines in overweight or obese subjects with a loss-of-function mutation in MC4R, the gene encoding the melanocortin 4 receptor, and in equally overweight control subjects. We also examined the effects of an MC4R agonist administered for 7 days in 28 overweight or obese volunteers.

RESULTS

The prevalence of hypertension was markedly lower in the MC4R-deficient subjects than in the control subjects (24% vs. 53%, P=0.009). After the exclusion of subjects taking antihypertensive medications, blood-pressure levels were significantly lower in MC4R-deficient subjects than in control subjects, with mean (+/-SE) systolic blood pressures of 123+/-14 mm Hg and 131+/-12 mm Hg, respectively (P=0.02), and mean diastolic blood pressures of 73+/-10 mm Hg and 79+/-7 mm Hg, respectively (P=0.03). As compared with control subjects, MC4R-deficient subjects had a lower increase in heart rate on waking (P=0.007), a lower heart rate during euglycemic hyperinsulinemia (P<0.001), and lower 24-hour urinary norepinephrine excretion (P=0.04). The maximum tolerated daily dose of 1.0 mg of the MC4R agonist led to significant increases of 9.3+/-1.9 mm Hg in systolic blood pressure and of 6.6+/-1.1 mm Hg in diastolic blood pressure (P<0.001 for both comparisons) at 24 hours, as compared with placebo. Differences in blood pressure were not explained by changes in insulin levels; there were no significant adverse events.

CONCLUSIONS

Results of our genetic and pharmacologic studies implicate melanocortinergic signaling in the control of human blood pressure through an insulin-independent mechanism.

Oral glutamine increases circulating glucagon-like peptide 1, glucagon, and insulin concentrations in lean, obese, and type 2 diabetic subjects

BACKGROUND

Incretin hormones, such as glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), play an important role in meal-related insulin secretion. We previously demonstrated that glutamine is a potent stimulus of GLP-1 secretion in vitro.

OBJECTIVE

Our objective was to determine whether glutamine increases circulating GLP-1 and GIP concentrations in vivo and, if so, whether this is associated with an increase in plasma insulin.

DESIGN

We recruited 8 healthy normal-weight volunteers (LEAN), 8 obese individuals with type 2 diabetes or impaired glucose tolerance (OB-DIAB) and 8 obese nondiabetic control subjects (OB-CON). Oral glucose (75 g), glutamine (30 g), and water were administered on 3 separate days in random order, and plasma concentrations of GLP-1, GIP, insulin, glucagon, and glucose were measured over 120 min.

RESULTS

Oral glucose led to increases in circulating GLP-1 concentrations, which peaked at 30 min in LEAN (31.9 +/- 5.7 pmol/L) and OB-CON (24.3 +/- 2.1 pmol/L) subjects and at 45 min in OB-DIAB subjects (19.5 +/- 1.8 pmol/L). Circulating GLP-1 concentrations increased in all study groups after glutamine ingestion, with peak concentrations at 30 min of 22.5 +/- 3.4, 17.9 +/- 1.1, and 17.3 +/- 3.4 pmol/L in LEAN, OB-CON, and OB-DIAB subjects, respectively. Glutamine also increased plasma GIP concentrations but less effectively than glucose. Consistent with the increases in GLP-1 and GIP, glutamine significantly increased circulating plasma insulin concentrations. Glutamine stimulated glucagon secretion in all 3 study groups.

CONCLUSION

Glutamine effectively increases circulating GLP-1, GIP, and insulin concentrations in vivo and may represent a novel therapeutic approach to stimulating insulin secretion in obesity and type 2 diabetes.

The central melanocortin system directly controls peripheral lipid metabolism

Disruptions of the melanocortin signaling system have been linked to obesity. We investigated a possible role of the central nervous melanocortin system (CNS-Mcr) in the control of adiposity through effects on nutrient partitioning and cellular lipid metabolism independent of nutrient intake. We report that pharmacological inhibition of melanocortin receptors (Mcr) in rats and genetic disruption of Mc4r in mice directly and potently promoted lipid uptake, triglyceride synthesis, and fat accumulation in white adipose tissue (WAT), while increased CNS-Mcr signaling triggered lipid mobilization. These effects were independent of food intake and preceded changes in adiposity. In addition, decreased CNS-Mcr signaling promoted increased insulin sensitivity and glucose uptake in WAT while decreasing glucose utilization in muscle and brown adipose tissue. Such CNS control of peripheral nutrient partitioning depended on sympathetic nervous system function and was enhanced by synergistic effects on liver triglyceride synthesis. Our findings offer an explanation for enhanced adiposity resulting from decreased melanocortin signaling, even in the absence of hyperphagia, and are consistent with feeding-independent changes in substrate utilization as reflected by respiratory quotient, which is increased with chronic Mcr blockade in rodents and in humans with loss-of-function mutations in MC4R. We also reveal molecular underpinnings for direct control of the CNS-Mcr over lipid metabolism. These results suggest ways to design more efficient pharmacological methods for controlling adiposity.

Clinical and molecular genetic spectrum of congenital deficiency of the leptin receptor

BACKGROUND

A single family has been described in which obesity results from a mutation in the leptin-receptor gene (LEPR), but the prevalence of such mutations in severe, early-onset obesity has not been systematically examined.

METHODS

We sequenced LEPR in 300 subjects with hyperphagia and severe early-onset obesity, including 90 probands from consanguineous families, and investigated the extent to which mutations cosegregated with obesity and affected receptor function. We evaluated metabolic, endocrine, and immune function in probands and affected relatives.

RESULTS

Of the 300 subjects, 8 (3%) had nonsense or missense LEPR mutations--7 were homozygotes, and 1 was a compound heterozygote. All missense mutations resulted in impaired receptor signaling. Affected subjects were characterized by hyperphagia, severe obesity, alterations in immune function, and delayed puberty due to hypogonadotropic hypogonadism. Serum leptin levels were within the range predicted by the elevated fat mass in these subjects. Their clinical features were less severe than those of subjects with congenital leptin deficiency.

CONCLUSIONS

The prevalence of pathogenic LEPR mutations in a cohort of subjects with severe, early-onset obesity was 3%. Circulating levels of leptin were not disproportionately elevated, suggesting that serum leptin cannot be used as a marker for leptin-receptor deficiency. Congenital leptin-receptor deficiency should be considered in the differential diagnosis in any child with hyperphagia and severe obesity in the absence of developmental delay or dysmorphism.

Hyperphagia, severe obesity, impaired cognitive function, and hyperactivity associated with functional loss of one copy of the brain-derived neurotrophic factor (BDNF) gene

The neurotrophin brain-derived neurotrophic factor (BDNF) inhibits food intake, and rodent models of BDNF disruption all exhibit increased food intake and obesity, as well as hyperactivity. We report an 8-year-old girl with hyperphagia and severe obesity, impaired cognitive function, and hyperactivity who harbored a de novo chromosomal inversion, 46,XX,inv(11)(p13p15.3), a region encompassing the BDNF gene. We have identified the proximal inversion breakpoint that lies 850 kb telomeric of the 5' end of the BDNF gene. The patient's genomic DNA was heterozygous for a common coding polymorphism in BDNF, but monoallelic expression was seen in peripheral lymphocytes. Serum concentration of BDNF protein was reduced compared with age- and BMI-matched subjects. Haploinsufficiency for BDNF was associated with increased ad libitum food intake, severe early-onset obesity, hyperactivity, and cognitive impairment. These findings provide direct evidence for the role of the neurotrophin BDNF in human energy homeostasis, as well as in cognitive function, memory, and behavior.

Heterozygosity for a POMC-null mutation and increased obesity risk in humans

Congenital deficiency of proopiomelanocortin (POMC) results in a syndrome of hypoadrenalism, severe obesity, and altered skin and hair pigmentation. The concept that subtle variation in POMC expression and/or function might contribute to common obesity is suggested by studies reporting linkage of obesity-related traits to a locus on chromosome 2p22 encompassing the POMC gene. We identified a novel homozygous frameshift (C6906del) mutation in POMC in a child of Turkish origin with severe obesity and hypoadrenalism. This mutation would be predicted to lead to the loss of all POMC-derived peptides. The availability of a large extended pedigree provided the opportunity to address whether loss of one copy of the POMC gene was sufficient to alter obesity risk. Twelve relatives were heterozygous for the mutation and 7 were wild type. Of the heterozygotes, 11 of 12 heterozygotes were obese or overweight compared with only 1 of 7 of the wild-type relatives. The mean BMI SD score was 1.7 +/- 0.5 in heterozygotes and 0.4 +/- 0.4 in the wild-type relatives. Parametric linkage analysis of the trait "overweight" provided statistically significant evidence of linkage with this locus, with a maximum "location score" (comparable with multipoint logarithm of odds scores) of 3.191. We conclude that loss of one copy of the POMC gene predisposes to obesity in humans. Thus, genetic variants having relatively subtle effects on POMC expression and function could influence susceptibility to obesity.

Studies of the SIM1 gene in relation to human obesity and obesity-related traits

OBJECTIVE

The single-minded 1 (SIM1) is a basic helix-loop-helix transcription factor, which plays a critical role in the development of the paraventricular nucleus (PVN) of the hypothalamus. SIM1-deficient mice have a hypocellular PVN and are severely obese with increased food intake.

DESIGN

We examined whether variants in the SIM1 gene might be associated with severe early-onset obesity in humans. Two hundred and seventy-seven subjects with hyperphagia and severe, early-onset obesity were screened. Association studies with common single-nucleotide polymorphisms (SNPs) in the SIM1 gene were performed in two population-based cohorts.

RESULTS

One novel missense mutation, I128T, was found in one obese subject and not in 192 controls. However, the variant did not co-segregate with obesity in the family. Four SNPs, IVS4+83GA, P352T, A371V and T653T, were also identified. The two common SNPs, P352T and A371V, which are in complete linkage disequilibrium, were genotyped in 981 subjects from a population-based cohort, the Ely Study. An allele frequency of 0.13 was observed. Male subjects carrying the P352T/A371V haplotype were found to have a slightly higher body mass index (BMI; P=0.038). Female subjects homozygous for the haplotype gained more weight over a period of 4.5 and 10 years (P=0.003 and P=0.02, respectively). The association studies were repeated in another population-based cohort, the European Prospective Investigation into Cancer and Nutrition (EPIC) - Norfolk Study with 4869 subjects successfully genotyped. Male subjects homozygous for the P352T/A371V haplotype had slightly higher BMI (P=0.04).

CONCLUSION

Mutations in SIM1 are not commonly found in humans with severe early-onset obesity. The relationship between the common variants in SIM1 with BMI and body weight gain deserves further exploration in other populations.

A POMC variant implicates beta-melanocyte-stimulating hormone in the control of human energy balance

The melanocortin-4 receptor (MC4R) plays a critical role in the control of energy balance. Of its two pro-opiomelanocortin (POMC)-derived ligands, alpha- and beta-MSH, the majority of attention has focused on alpha-MSH, partly reflecting the absence of beta-MSH in rodents. We screened the POMC gene in 538 patients with severe, early-onset obesity and identified five unrelated probands who were heterozygous for a rare missense variant in the region encoding beta-MSH, Tyr221Cys. This frequency was significantly increased (p < 0.001) compared to the general UK Caucasian population and the variant cosegregated with obesity/overweight in affected family members. Compared to wild-type beta-MSH, the variant peptide was impaired in its ability to bind to and activate signaling from the MC4R. Obese children carrying the Tyr221Cys variant were hyperphagic and showed increased linear growth, both of which are features of MC4R deficiency. These studies support a role for beta-MSH in the control of human energy homeostasis.

Studies of the peptide YY and neuropeptide Y2 receptor genes in relation to human obesity and obesity-related traits

Peptide-YY (PYY) is secreted from endocrine L-cells of the gastrointestinal tract in response to caloric ingestion and may mediate postprandial satiety through the hypothalamic neuropeptide Y2 receptor (Y2R). We examined whether variants in the genes encoding PYY and Y2R might be associated with obesity-related phenotypes in humans. Among 101 subjects with severe early-onset obesity and a history of hyperphagia, we found two rare sequence variants-L73P and IVS2 + 32delG-in PYY and three rare missense mutations-L40F, F87I, and A172T-in Y2R. Although none of these were found in 100 normal-weight white control subjects, L73P in PYY and F87I and A172T in Y2R did not segregate with obesity in family studies, and family data were unavailable for IVS2 + 32delG in PYY and L40F in Y2R. Two common single nucleotide polymorphisms (SNPs), R72T and IVS3 + 68C>T, in PYY were in tight linkage disequilibrium but showed no association with BMI in a large white population. In the Y2R, two SNPs, 585T>C and 936T>C, were found and were in tight linkage disequilibrium. Men, homozygous for the rarer variant, had significantly lower BMI (P = 0.017), waist-to-hip ratio (P = 0.013), and, surprisingly, higher nonesterified fatty acid levels (P = 0.01). In conclusion, mutations in PYY and Y2R are not commonly found in humans with severe early-onset obesity. The relationship between common variants in Y2R and obesity-related traits deserves further exploration in other populations.

Contribution of variants in the small heterodimer partner gene to birthweight, adiposity, and insulin levels: mutational analysis and association studies in multiple populations

Loss of function mutations in the small heterodimer partner (SHP) gene have been reported to cause obesity and increased birth weight. We examined the relation between genetic variation in SHP and birth weight, adiposity, and insulin levels in three independent populations. The coding regions and 562 bases of the SHP promoter were screened for mutations in 329 subjects with severe early-onset obesity. Two novel missense mutations, R34G and R36C, were identified; these were not found in control subjects and did not cosegregate with obesity in family studies. Two common polymorphisms, G171A and -195CTGAdel, were found in 12 and 16% of subjects, respectively. Within the obese cohort, G171A and -195CTGAdel carriers had higher and lower birth weights, respectively, than wild-type subjects, the rare homozygotes for G171A being particularly large at birth. In a U.K. population-based cohort of 1,079 children, the 171A allele was associated with higher BMI (P < 0.05) and waist circumference (P = 0.001). Children carrying the G171A variant had higher 30-min insulin responses to a glucose load (P = 0.03). In conclusion, although mutations in SHP are not a common cause of severe human obesity, genetic variation in the SHP locus may influence birth weight and have effects on BMI, possibly through effects on insulin secretion.

Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene

BACKGROUND

Melanocortin 4 receptor (MC4R) deficiency is the commonest monogenic form of obesity. However, the clinical spectrum and mode of inheritance have not been defined, pathophysiological mechanisms leading to obesity are poorly understood, and there is little information regarding genotype-phenotype correlations.

METHODS

We determined the nucleotide sequence of the MC4R gene in 500 probands with severe childhood obesity. Family studies were undertaken to examine cosegregation of identified mutations with obesity. Subjects with MC4R deficiency underwent metabolic and endocrine evaluation; the results were correlated with the signaling properties of mutant receptors.

RESULTS

Twenty-nine probands (5.8 percent) had mutations in MC4R; 23 were heterozygous, and 6 were homozygous. Mutation carriers had severe obesity, increased lean mass, increased linear growth, hyperphagia, and severe hyperinsulinemia; homozygotes were more severely affected than heterozygotes. Subjects with mutations retaining residual signaling capacity had a less severe phenotype.

CONCLUSIONS

Mutations in MC4R result in a distinct obesity syndrome that is inherited in a codominant manner. Mutations leading to complete loss of function are associated with a more severe phenotype. The correlation between the signaling properties of these mutant receptors and energy intake emphasizes the key role of this receptor in the control of eating behavior in humans.

Beneficial effects of leptin on obesity, T cell hyporesponsiveness, and neuroendocrine/metabolic dysfunction of human congenital leptin deficiency

The wide range of phenotypic abnormalities seen in the leptin-deficient ob/ob mouse and their reversibility by leptin administration provide compelling evidence for the existence of multiple physiological functions of this hormone in rodents. In contrast, information regarding the roles of this hormone in humans is limited. Three morbidly obese children, who were congenitally deficient in leptin, were treated with daily subcutaneous injections of recombinant human leptin for up to 4 years with sustained, beneficial effects on appetite, fat mass, hyperinsulinemia, and hyperlipidemia. Leptin therapy resulted in a rapid and sustained increase in plasma thyroid hormone levels and, through its age-dependent effects on gonadotropin secretion, facilitated appropriately timed pubertal development. Leptin deficiency was associated with reduced numbers of circulating CD4(+) T cells and impaired T cell proliferation and cytokine release, all of which were reversed by recombinant human leptin administration. The subcutaneous administration of recombinant human leptin has major and sustained beneficial effects on the multiple phenotypic abnormalities associated with congenital human leptin deficiency.