Mutational analysis of the pro-opiomelanocortin gene in French obese children led to the identification of a novel deleterious heterozygous mutation located in the alpha-melanocyte stimulating hormone domain

Medical semiology of patients with monogenic obesity: A systematic review

Patients with monogenic obesity display numerous medical features on top of hyperphagic obesity, but no study to date has provided an exhaustive description of their semiology. Two reviewers independently conducted a systematic review of MEDLINE, Embase, and Web of Science Core Collection databases from inception to January 2022 to identify studies that described symptoms of patients carrying pathogenic mutations in at least one of eight monogenic obesity genes (ADCY3, LEP, LEPR, MC3R, MC4R, MRAP2, PCSK1, and POMC). Of 5207 identified references, 269 were deemed eligible after title and abstract screening, full-text reading, and risk of bias and quality assessment. Data extraction included mutation spectrum and mode of inheritance, clinical presentation (e.g., anthropometry, energy intake and eating behaviors, digestive function, puberty and fertility, cognitive features, infectious diseases, morphological characteristics, chronic respiratory disease, and cardiovascular disease), biological characteristics (metabolic profile, endocrinology, hematology), radiological features, and treatments. The review provides an exhaustive description of mandatory, non-mandatory, and unique symptoms in heterozygous and homozygous carriers of mutation in eight monogenic obesity genes. This information is critical to help clinicians to orient genetic testing in subsets of patients with suspected monogenic obesity and provide actionable treatments (e.g., recombinant leptin and MC4R agonist).

Low resting metabolic rate and increased hunger due to β-MSH and β-endorphin deletion in a canine model

Mutations that perturb leptin-melanocortin signaling are known to cause hyperphagia and obesity, but energy expenditure has not been well studied outside rodents. We report on a common canine mutation in pro-opiomelanocortin (POMC), which prevents production of β-melanocyte-stimulating hormone (β-MSH) and β-endorphin but not α-MSH; humans, similar to dogs, produce α-MSH and β-MSH from the POMC propeptide, but rodents produce only α-MSH. We show that energy expenditure is markedly lower in affected dogs, which also have increased motivational salience in response to a food cue, indicating increased wanting or hunger. There was no difference in satiety at a modified ad libitum meal or in their hedonic response to food, nor disruption of adrenocorticotropic hormone (ACTH) or thyroid axes. In vitro, we show that β-MSH signals comparably to α-MSH at melanocortin receptors. These data implicate β-MSH and β-endorphin as important in determining hunger and moderating energy expenditure and suggest that this role is independent of the presence of α-MSH.

Heterozygous pathogenic variants in POMC are not responsible for monogenic obesity: Implication for MC4R agonist use

PURPOSE

Recessive deficiency of proopiomelanocortin (POMC) causes childhood-onset severe obesity. Cases can now benefit from the melanocortin 4 receptor agonist setmelanotide. Furthermore, a phase 3 clinical trial is evaluating setmelanotide in heterozygotes for POMC. We performed a large-scale genetic analysis to assess the effect of heterozygous, pathogenic POMC variants on obesity.

METHODS

A genetic analysis was performed in a family including 2 cousins with childhood-onset obesity. We analyzed the obesity status of heterozygotes for pathogenic POMC variants in the Human Gene Mutation Database. The association between heterozygous pathogenic POMC variants and obesity risk was assessed using 190,000 exome samples from UK Biobank.

RESULTS

The 2 cousins carried a compound heterozygous pathogenic variant in POMC. Six siblings were heterozygotes; only 1 of them had obesity. In Human Gene Mutation Database, we identified 60 heterozygotes for pathogenic POMC variants, of whom 14 had obesity. In UK Biobank, heterozygous pathogenic POMC variants were not associated with obesity risk, but they modestly increased body mass index levels.

CONCLUSION

Heterozygous pathogenic POMC variants do not contribute to monogenic obesity, but they slightly increase body mass index. Setmelanotide use in patients with obesity, which would only be based on the presence of a heterozygous POMC variant, can be questioned.

Functional characterization of all missense variants in LEPR, PCSK1, and POMC genes arising from single-nucleotide variants

OBJECTIVE

Hyperphagia and early-onset, severe obesity are clinical characteristics of rare melanocortin-4 receptor (MC4R) pathway diseases due to loss-of-function (LOF) variants in genes comprising the MC4R pathway. In vitro functional characterization of 12,879 possible exonic missense variants from single-nucleotide variants (SNVs) of LEPR, POMC, and PCSK1 was performed to determine the impact of these variants on protein function.

METHODS

SNVs of the three genes were transiently transfected into cell lines, and each variant was subsequently classified according to functional impact. We validated three assays by comparing classifications against functional characterization of 29 previously published variants.

RESULTS

Our results significantly correlated with previously published pathogenic categories (r = 0.623; P = 3.03 × 10^-4) of all potential missense variants arising from SNVs. Of all observed variants identified through available databases and a tested cohort of 16,061 patients with obesity, 8.6% of LEPR, 63.2% of PCSK1, and 10.6% of POMC variants exhibited LOF, including variants currently classified as a variant of uncertain significance (VUS).

CONCLUSIONS

The functional data provided here can assist in the reclassification of several VUS in LEPR, PCSK1, and POMC and highlight their impact in MC4R pathway diseases.

Screening of non-syndromic early-onset child and adolescent obese patients in terms of LEP, LEPR, MC4R and POMC gene variants by next-generation sequencing

OBJECTIVES

Non-syndromic monogenic obesity is a rare cause of early-onset severe obesity in the childhood period. The aim of this study was to screen four obesity related genes (LEP, LEPR, MC4R and POMC) in children and adolescents who had severe, non-syndromic early onset obesity.

METHODS

Next-generation sequencing of all exons in LEP, LEPR, MC4R and POMC was performed in 154 children and adolescents with early onset severe obesity obesity.

RESULTS

Fifteen different variants in nineteen patients were identified with a variant detection rate of 12.3%. While six different heterozygous variants were observed in MC4R gene (10/154 patients; 6.5%), five different variants in POMC gene (four of them were heterozygous and one of them was homozygous) (6/154 patients; 3.9%) and four different homozygous variants in LEPR gene (3/154 patients; 1.9%) were described. However, no variants were detected in the LEP gene. The most common pathogenic variant was c.496G>A in MC4R gene, which was detected in four unrelated patients. Six novel variants (6/15 variants; 40%) were described in seven patients. Four of them including c.233C>A and c.752T>C in MC4R gene and c.761dup and c.1221dup in LEPR gene were evaluated as pathogenic or likely pathogenic.

CONCLUSIONS

In conclusion, MC4R variants are the most common genetic cause of monogenic early-onset obesity, consistent with the literature. The c.496G>A variant in MC4R gene is highly prevalent in early-onset obese patients.

Glial cells as integrators of peripheral and central signals in the regulation of energy homeostasis

Communication between the periphery and the brain is key for maintaining energy homeostasis. To do so, peripheral signals from the circulation reach the brain via the circumventricular organs (CVOs), which are characterized by fenestrated vessels lacking the protective blood-brain barrier (BBB). Glial cells, by virtue of their plasticity and their ideal location at the interface of blood vessels and neurons, participate in the integration and transmission of peripheral information to neuronal networks in the brain for the neuroendocrine control of whole-body metabolism. Metabolic diseases, such as obesity and type 2 diabetes, can disrupt the brain-to-periphery communication mediated by glial cells, highlighting the relevance of these cell types in the pathophysiology of such complications. An improved understanding of how glial cells integrate and respond to metabolic and humoral signals has become a priority for the discovery of promising therapeutic strategies to treat metabolic disorders. This Review highlights the role of glial cells in the exchange of metabolic signals between the periphery and the brain that are relevant for the regulation of whole-body energy homeostasis.

Central Nervous System Control of Glucose Homeostasis: A Therapeutic Target for Type 2 Diabetes?

Historically, pancreatic islet beta cells have been viewed as principal regulators of glycemia, with type 2 diabetes (T2D) resulting when insulin secretion fails to compensate for peripheral tissue insulin resistance. However, glycemia is also regulated by insulin-independent mechanisms that are dysregulated in T2D. Based on evidence supporting its role both in adaptive coupling of insulin secretion to changes in insulin sensitivity and in the regulation of insulin-independent glucose disposal, the central nervous system (CNS) has emerged as a fundamental player in glucose homeostasis. Here, we review and expand upon an integrative model wherein the CNS, together with the islet, establishes and maintains the defended level of glycemia. We discuss the implications of this model for understanding both normal glucose homeostasis and T2D pathogenesis and highlight centrally targeted therapeutic approaches with the potential to restore normoglycemia to patients with T2D.

Hypothalamic Estrogen Signaling and Adipose Tissue Metabolism in Energy Homeostasis

Obesity has become a global epidemic, and it is a major risk factor for other metabolic disorders such as type 2 diabetes and cardiometabolic disease. Accumulating evidence indicates that there is sex-specific metabolic protection and disease susceptibility. For instance, in both clinical and experimental studies, males are more likely to develop obesity, insulin resistance, and diabetes. In line with this, males tend to have more visceral white adipose tissue (WAT) and less brown adipose tissue (BAT) thermogenic activity, both leading to an increased incidence of metabolic disorders. This female-specific fat distribution is partially mediated by sex hormone estrogens. Specifically, hypothalamic estrogen signaling plays a vital role in regulating WAT distribution, WAT beiging, and BAT thermogenesis. These regulatory effects on adipose tissue metabolism are primarily mediated by the activation of estrogen receptor alpha (ERα) in neurons, which interacts with hormones and adipokines such as leptin, ghrelin, and insulin. This review discusses the contribution of adipose tissue dysfunction to obesity and the role of hypothalamic estrogen signaling in preventing metabolic diseases with a particular focus on the VMH, the central regulator of energy expenditure and glucose homeostasis.

Obesity, POMC, and POMC-processing Enzymes: Surprising Results From Animal Models

Peptides derived from proopiomelanocortin (POMC) are well-established neuropeptides and peptide hormones that perform multiple functions, including regulation of body weight. In humans and some animals, these peptides include α- and β-melanocyte-stimulating hormone (MSH). In certain rodent species, no β-MSH is produced from POMC because of a change in the cleavage site. Enzymes that convert POMC into MSH include prohormone convertases (PCs), carboxypeptidases (CPs), and peptidyl-α-amidating monooxygenase (PAM). Humans and mice with inactivating mutations in either PC1/3 or carboxypeptidase E (CPE) are obese, which was assumed to result from defective processing of POMC into MSH. However, recent studies have shown that selective loss of either PC1/3 or CPE in POMC-expressing cells does not cause obesity. These findings suggest that defects in POMC processing cannot alone account for the obesity observed in global PC1/3 or CPE mutants. We propose that obesity in animals lacking PC1/3 or CPE activity depends, at least in part, on deficient processing of peptides in non-POMC-expressing cells either in the brain and/or the periphery. Genetic background may also contribute to the manifestation of obesity.

Implication of Heterozygous Variants in Genes of the Leptin-Melanocortin Pathway in Severe Obesity

CONTEXT

Unlike homozygous variants, the implication of heterozygous variants on the leptin-melanocortin pathway in severe obesity has not been established.

OBJECTIVE

To describe the frequency, the phenotype, and the genotype-phenotype relationship for heterozygous variants in LEP, LEPR, POMC, and PCSK1 in severe obesity.

METHODS

In this retrospective study, genotyping was performed on at least 1 of the LEP, LEPR, POMC, and PCSK1 genes in 1486 probands with severe obesity (600 children, 886 adults). The phenotype was collected in 60 subjects with heterozygous variants and 16 with homozygous variants. We analyzed variant frequency, body mass index (BMI), age of obesity onset, food impulsivity, and endocrine abnormalities.

RESULTS

The frequency of subjects with homozygous variants was 1.7% (n = 26), and 6.7% (n = 100) with heterozygous variants. Adults with homozygous variants had a higher BMI (66 vs 53 kg/m2, P = .015), an earlier onset of obesity (0.4 vs 5.4 years, P < .001), more often food impulsivity (83% vs 42%, P = .04), and endocrine abnormalities (75% vs 26%, P < .01). The BMI was higher for subjects with high-impact heterozygous variants (61 vs 50 kg/m², P = .045) and those with a second heterozygous variant on the pathway (65 vs 49 kg/m², P < .01). In children, no significant differences were found for the age of obesity onset and BMI.

CONCLUSION

Heterozygous variants in LEP, LEPR, POMC, and PCSK1 are frequent in severe obesity and sometimes associated with a phenotype close to that of homozygotes. These data suggest a systematic search for variants in severe early-onset obesity, to discuss therapy that targets this key pathway.

Depletion of Alpha-Melanocyte-Stimulating Hormone Induces Insatiable Appetite and Gains in Energy Reserves and Body Weight in Zebrafish

The functions of anorexigenic neurons secreting proopiomelanocortin (POMC)/alpha-melanocyte-stimulating hormone (α-MSH) of the melanocortin system in the hypothalamus in vertebrates are energy homeostasis, food intake, and body weight regulation. However, the mechanisms remain elusive. This article reports on zebrafish that have been genetically engineered to produce α-MSH mutants, α-MSH-7aa and α-MSH-8aa, selectively lacking 7 and 8 amino acids within the α-MSH region, but retaining most of the other normal melanocortin-signaling (Pomc-derived) peptides. The α-MSH mutants exhibited hyperphagic phenotypes leading to body weight gain, as observed in human patients and mammalian models. The actions of several genes regulating appetite in zebrafish are similar to those in mammals when analyzed using gene expression analysis. These include four selected orexigenic genes: Promelanin-concentrating hormone (pmch), agouti-related protein 2 (agrp2), neuropeptide Y (npy), and hypothalamic hypocretin/orexin (hcrt). We also study five selected anorexigenic genes: Brain-derived neurotrophic factor (bdnf), single-minded homolog 1-a (sim1a), corticotropin-releasing hormone b (crhb), thyrotropin-releasing hormone (trh), and prohormone convertase 2 (pcsk2). The orexigenic actions of α-MSH mutants are rescued completely after hindbrain ventricle injection with a synthetic analog of α-MSH and a melanocortin receptor agonist, Melanotan II. We evaluate the adverse effects of MSH depletion on energy balance using the Alamar Blue metabolic rate assay. Our results show that α-MSH is a key regulator of POMC signaling in appetite regulation and energy expenditure, suggesting that it might be a potential therapeutic target for treating human obesity.

Bariatric Surgery for Monogenic Non-syndromic and Syndromic Obesity Disorders

PURPOSE OF REVIEW

The global prevalence of obesity has increased rapidly over the last decades, posing a severe threat to human health. Currently, bariatric surgery is the most effective therapy for patients with morbid obesity. It is unknown whether this treatment is also suitable for patients with obesity due to a confirmed genetic defect (genetic obesity disorders). Therefore, this review aims to elucidate the role of bariatric surgery in the treatment of genetic obesity.

RECENT FINDINGS

In monogenic non-syndromic obesity, an underlying genetic defect seems to be the most important factor determining the efficacy of bariatric surgery. In syndromic obesity, bariatric surgery result data are scarce, and even though some promising follow-up results have been reported, caution is required as patients with more severe behavioral and developmental disorders might have poorer outcomes. There is limited evidence in support of bariatric surgery as a treatment option for genetic obesity disorders; hence, no strong statements can be made regarding the efficacy and safety of these procedures for these patients. However, considering that patients with genetic obesity often present with life-threatening obesity-related comorbidities, we believe that bariatric surgery could be considered a last-resort treatment option in selected patients.

Molecular evolution of the proopiomelanocortin system in Barn owl species

Examination of genetic polymorphisms in outbred wild-living species provides insights into the evolution of complex systems. In higher vertebrates, the proopiomelanocortin (POMC) precursor gives rise to α-, β-, and γ-melanocyte-stimulating hormones (MSH), which are involved in numerous physiological aspects. Genetic defects in POMC are linked to metabolic disorders in humans and animals. In the present study, we undertook an evolutionary genetic approach complemented with biochemistry to investigate the functional consequences of genetic polymorphisms in the POMC system of free-living outbred barn owl species (family Tytonidae) at the molecular level. Our phylogenetic studies revealed a striking correlation between a loss-of-function H9P mutation in the β-MSH receptor-binding motif and an extension of a poly-serine stretch in γ3-MSH to ≥7 residues that arose in the barn owl group 6–8 MYA ago. We found that extension of the poly-serine stretches in the γ-MSH locus affects POMC precursor processing, increasing γ3-MSH production at the expense of γ2-MSH and resulting in an overall reduction of γ-MSH signaling, which may be part of a negative feedback mechanism. Extension of the γ3-MSH poly-serine stretches ≥7 further markedly increases peptide hormone stability in plasma, which is conserved in humans, and is likely relevant to its endocrine function. In sum, our phylogenetic analysis of POMC in wild living owls uncovered a H9P β-MSH mutation subsequent to serine extension in γ3-MSH to 7 residues, which was then followed by further serine extension. The linked MSH mutations highlight the genetic plasticity enabled by the modular design of the POMC gene.

Single Nucleotide Polymorphisms in the Melanocortin His-Phe-Arg-Trp Sequences Decrease Tetrapeptide Potency and Efficacy

The melanocortin receptors are stimulated by agonists (α-MSH, β-MSH, γ-MSH, and ACTH) processed from the proopiomelanocortin (POMC) gene transcript and possess a common His-Phe-Arg-Trp tetrapeptide sequence critical for receptor activation. Deficiency in POMC signaling in humans is associated with adrenal insufficiency, altered pigmentation, and rapid, early onset weight gain. Herein, 12 single nucleotide polymorphisms (SNPs) deposited into the Variation Viewer database within the His-Phe-Arg-Trp sequences of ACTH/α-MSH, β-MSH, and γ-MSH were substituted into tetrapeptide scaffolds to examine the in vitro signaling effects of these polymorphisms at the cloned melanocortin receptors. Every polymorphism decreased agonist potency and/or efficacy at the melanocortin receptors assayed, indicating that polymorphisms within the signaling sequence of POMC-derived agonists negatively impacts receptor activation. Future work to incorporate these substitutions into the full-length POMC agonists would confirm these findings, identifying new patient populations that might benefit from therapeutic regiments to treat POMC-deficient signaling.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

Type 2 Diabetes-Associated Genetic Polymorphisms as Potential Disease Predictors

Diabetes is a major cause of mortality worldwide. There are several types of diabetes, with type 2 diabetes mellitus (T2DM) being the most common. Many factors, including environmental and genetic factors, are involved in the etiology of the disease. Numerous studies have reported the role of genetic polymorphisms in the initiation and development of T2DM. While genome-wide association studies have identified around more than 200 susceptibility loci, it remains unclear whether these loci are correlated with the pathophysiology of the disease. The present review aimed to elucidate the potential genetic mechanisms underlying T2DM. We found that some genetic polymorphisms were related to T2DM, either in the form of single-nucleotide polymorphisms or direct amino acid changes in proteins. These polymorphisms are potential predictors for the management of T2DM.

A Review of Single-Nucleotide Polymorphisms in Orexigenic Neuropeptides Targeting G Protein-Coupled Receptors

Many physiological pathways are involved in appetite, food intake, and the maintenance of energy homeostasis. In particular, neuropeptides within the central nervous system have been demonstrated to be critical signaling molecules for modulating appetite. Both anorexigenic (appetite-decreasing) and orexigenic (appetite-stimulating) neuropeptides have been described. The biological effects of these neuropeptides can be observed following central administration in animal models. This review focuses on single nucleotide polymorphisms (SNPs) in six orexigenic neuropeptides: agouti-related protein (AGRP), galanin, melanin concentrating hormone (MCH), neuropeptide Y (NPY), orexin A, and orexin B. Following a brief summary of the neuropeptides and their orexigenic activities, reports associating SNPs within the orexigenic neuropeptides to energy homeostasis, food intake, obesity, and BMI in humans are reviewed. Additionally, the NIH tool Variation Viewer was utilized to identify missense SNPs within the mature, biologically active neuropeptide sequences. For SNPs found through Variation Viewer, a concise discussion on relevant pharmacological structure-activity relationship studies for select SNPs is included. This review is meant to update reported orexigenic neuropeptide SNPs and demonstrate the potential utility of genomic sequence databases for finding SNPs that may result in altered receptor signaling for neuropeptide pathways associated with appetite.

Hypothalamic ER-associated degradation regulates POMC maturation, feeding, and age-associated obesity

Pro-opiomelanocortin (POMC) neurons function as key regulators of metabolism and physiology by releasing prohormone-derived neuropeptides with distinct biological activities. However, our understanding of early events in prohormone maturation in the ER remains incomplete. Highlighting the significance of this gap in knowledge, a single POMC cysteine-to-phenylalanine mutation at position 28 (POMC-C28F) is defective for ER processing and causes early onset obesity in a dominant-negative manner in humans through an unclear mechanism. Here, we report a pathologically important role of Sel1L-Hrd1, the protein complex of ER-associated degradation (ERAD), within POMC neurons. Mice with POMC neuron–specific Sel1L deficiency developed age-associated obesity due, at least in part, to the ER retention of POMC that led to hyperphagia. The Sel1L-Hrd1 complex targets a fraction of nascent POMC molecules for ubiquitination and proteasomal degradation, preventing accumulation of misfolded and aggregated POMC, thereby ensuring that another fraction of POMC can undergo normal posttranslational processing and trafficking for secretion. Moreover, we found that the disease-associated POMC-C28F mutant evades ERAD and becomes aggregated due to the presence of a highly reactive unpaired cysteine thiol at position 50. Thus, this study not only identifies ERAD as an important mechanism regulating POMC maturation within the ER, but also provides insights into the pathogenesis of monogenic obesity associated with defective prohormone folding.

Ethnic and population differences in the genetic predisposition to human obesity

Obesity rates have escalated to the point of a global pandemic with varying prevalence across ethnic groups. These differences are partially explained by lifestyle factors in addition to genetic predisposition to obesity. This review provides a comprehensive examination of the ethnic differences in the genetic architecture of obesity. Using examples from evolution, heritability, admixture, monogenic and polygenic studies of obesity, we provide explanations for ethnic differences in the prevalence of obesity. The debate over definitions of race and ethnicity, the advantages and limitations of multi-ethnic studies and future directions of research are also discussed. Multi-ethnic studies have great potential to provide a better understanding of ethnic differences in the prevalence of obesity that may result in more targeted and personalized obesity treatments.

Novel and ultra-rare damaging variants in neuropeptide signaling are associated with disordered eating behaviors

Objective

Eating disorders develop through a combination of genetic vulnerability and environmental stress, however the genetic basis of this risk is unknown.

Methods

To understand the genetic basis of this risk, we performed whole exome sequencing on 93 unrelated individuals with eating disorders (38 restricted-eating and 55 binge-eating) to identify novel damaging variants. Candidate genes with an excessive burden of predicted damaging variants were then prioritized based upon an unbiased, data-driven bioinformatic analysis. One top candidate pathway was empirically tested for therapeutic potential in a mouse model of binge-like eating.

Results

An excessive burden of novel damaging variants was identified in 186 genes in the restricted-eating group and 245 genes in the binge-eating group. This list is significantly enriched (OR = 4.6, p<0.0001) for genes involved in neuropeptide/neurotrophic pathways implicated in appetite regulation, including neurotensin-, glucagon-like peptide 1- and BDNF-signaling. Administration of the glucagon-like peptide 1 receptor agonist exendin-4 significantly reduced food intake in a mouse model of ‘binge-like’ eating.

Conclusions

These findings implicate ultra-rare and novel damaging variants in neuropeptide/neurotropic factor signaling pathways in the development of eating disorder behaviors and identify glucagon-like peptide 1-receptor agonists as a potential treatment for binge eating.

Recent progress in genetics, epigenetics and metagenomics unveils the pathophysiology of human obesity

In high-, middle- and low-income countries, the rising prevalence of obesity is the underlying cause of numerous health complications and increased mortality. Being a complex and heritable disorder, obesity results from the interplay between genetic susceptibility, epigenetics, metagenomics and the environment. Attempts at understanding the genetic basis of obesity have identified numerous genes associated with syndromic monogenic, non-syndromic monogenic, oligogenic and polygenic obesity. The genetics of leanness are also considered relevant as it mirrors some of obesity's aetiologies. In this report, we summarize ten genetically elucidated obesity syndromes, some of which are involved in ciliary functioning. We comprehensively review 11 monogenic obesity genes identified to date and their role in energy maintenance as part of the leptin-melanocortin pathway. With the emergence of genome-wide association studies over the last decade, 227 genetic variants involved in different biological pathways (central nervous system, food sensing and digestion, adipocyte differentiation, insulin signalling, lipid metabolism, muscle and liver biology, gut microbiota) have been associated with polygenic obesity. Advances in obligatory and facilitated epigenetic variation, and gene-environment interaction studies have partly accounted for the missing heritability of obesity and provided additional insight into its aetiology. The role of gut microbiota in obesity pathophysiology, as well as the 12 genes associated with lipodystrophies is discussed. Furthermore, in an attempt to improve future studies and merge the gap between research and clinical practice, we provide suggestions on how high-throughput '-omic' data can be integrated in order to get closer to the new age of personalized medicine.

60 YEARS OF POMC: The proopiomelanocortin gene: discovery, deletion and disease

The cloning of the bovine proopiomelanocortin (POMC) cDNA in 1978 by Nakanishi and colleagues was the result of a remarkable series of exacting and ingenious experiments. With this work, they instantly confirmed the single precursor hypothesis for adrenocorticotrophic hormone-β-lipotropin, as it was then known, and in so doing revealed the existence of additional, largely unpredicted, N-terminal peptides that together formed the POMC precursor peptide. This work paved the way for a host of additional studies into the physiology of these peptides and their regulation. Furthermore, the cloning of the murine Pomc gene was essential for subsequent studies, in which Pomc was intentionally deleted in the mouse illuminating its substantial role in body weight regulation and adrenal function. Contemporaneously with this work, naturally occurring mutations in human POMC came to light underlining the vital role of this gene in appetite regulation. This article reviews each of these aspects of POMC with the benefit of several decades of hindsight and informed by more recent genomic and transcriptomic data.

Obesity genetics in mouse and human: back and forth, and back again

Obesity is a major public health concern. This condition results from a constant and complex interplay between predisposing genes and environmental stimuli. Current attempts to manage obesity have been moderately effective and a better understanding of the etiology of obesity is required for the development of more successful and personalized prevention and treatment options. To that effect, mouse models have been an essential tool in expanding our understanding of obesity, due to the availability of their complete genome sequence, genetically identified and defined strains, various tools for genetic manipulation and the accessibility of target tissues for obesity that are not easily attainable from humans. Our knowledge of monogenic obesity in humans greatly benefited from the mouse obesity genetics field. Genes underlying highly penetrant forms of monogenic obesity are part of the leptin-melanocortin pathway in the hypothalamus. Recently, hypothesis-generating genome-wide association studies for polygenic obesity traits in humans have led to the identification of 119 common gene variants with modest effect, most of them having an unknown function. These discoveries have led to novel animal models and have illuminated new biologic pathways. Integrated mouse-human genetic approaches have firmly established new obesity candidate genes. Innovative strategies recently developed by scientists are described in this review to accelerate the identification of causal genes and deepen our understanding of obesity etiology. An exhaustive dissection of the molecular roots of obesity may ultimately help to tackle the growing obesity epidemic worldwide.

Participation of the central melanocortin system in metabolic regulation and energy homeostasis

Obesity and metabolic disorders, such as type 2 diabetes and hypertension, have attracted considerable attention as life-threatening diseases not only in developed countries but also worldwide. Additionally, the rate of obesity in young people all over the world is rapidly increasing. Accumulated evidence suggests that the central nervous system may participate in the development of and/or protection from obesity. For example, in the brain, the hypothalamic melanocortin system senses and integrates central and peripheral metabolic signals and controls the degree of energy expenditure and feeding behavior, in concert with metabolic status, to regulate whole-body energy homeostasis. Currently, researchers are studying the mechanisms by which peripheral metabolic molecules control feeding behavior and energy balance through the central melanocortin system. Accordingly, recent studies have revealed that some inflammatory molecules and transcription factors participate in feeding behavior and energy balance by controlling the central melanocortin pathway, and have thus become new candidates as therapeutic targets to fight metabolic diseases such as obesity and diabetes.

A nonsense loss-of-function mutation in PCSK1 contributes to dominantly inherited human obesity

BACKGROUND

A significant proportion of severe familial forms of obesity remain genetically elusive. Taking advantage of our unique cohort of multigenerational obese families, we aimed to assess the contribution of rare mutations in 29 common obesity-associated genes to familial obesity, and to evaluate in these families the putative presence of nine known monogenic forms of obesity.

METHODS

Through next-generation sequencing, we sequenced the coding regions of 34 genes involved in polygenic and/or monogenic forms of obesity in 201 participants (75 normal weight individuals, 54 overweight individuals and 72 individuals with obesity class I, II or III) from 13 French families. In vitro functional analyses were performed to investigate the mutation PCSK1-p.Arg80* which was identified in a family.

RESULTS

A novel heterozygous nonsense variant in PCSK1 (p.Arg80*), encoding a propeptide truncated to less than two exons (out of 14), was found to co-segregate with obesity in a three-generation family. We demonstrated that this mutation inhibits PCSK1 enzyme activity and that this inhibition most likely does not involve a strong physical interaction. Furthermore, both mutations PCSK1-p.Asn180Ser and POMC-p.Phe144Leu, which had previously been reported to be associated with severe obesity, were also identified in this study, but did not co-segregate with obesity. Finally, we did not identify any rare mutations co-segregating with obesity in common obesity susceptibility genes, except for CADM2 and QPCTL, where we found two novel variants (p.Arg81His and p.Leu98Pro, respectively) in three obese individuals.

CONCLUSIONS

We showed for the first time that a nonsense mutation in PCSK1 was likely to cause dominantly inherited human obesity, due to the inhibiting properties of the propeptide fragment encoded by the null allele. Furthermore, the present family sequencing design challenged the contribution of previously reported mutations to monogenic or at least severe obesity.

Pathogenesis of the metabolic syndrome: insights from monogenic disorders

Identifying rare human metabolic disorders that result from a single-gene defect has not only enabled improved diagnostic and clinical management of such patients, but also has resulted in key biological insights into the pathophysiology of the increasingly prevalent metabolic syndrome. Insulin resistance and type 2 diabetes are linked to obesity and driven by excess caloric intake and reduced physical activity. However, key events in the causation of the metabolic syndrome are difficult to disentangle from compensatory effects and epiphenomena. This review provides an overview of three types of human monogenic disorders that result in (1) severe, non-syndromic obesity, (2) pancreatic beta cell forms of early-onset diabetes, and (3) severe insulin resistance. In these patients with single-gene defects causing their exaggerated metabolic disorder, the primary defect is known. The lessons they provide for current understanding of the molecular pathogenesis of the common metabolic syndrome are highlighted.

Leptin and leptin receptor-related monogenic obesity

The studies based on candidate genes and encoded proteins known to cause severe obesity in rodents, have shown that these genes also contribute to human early-onset obesity especially for those involved in the leptin pathway: the leptin (LEP) and leptin receptor (LEPR) genes. Since 1997, less than 20 individuals carrying a LEP gene mutation have been identified. Patients are mostly characterized by severe early-onset obesity with severe hyperphagia and associated phenotype such hypogonadotrophic hypogonadism, high rate of infection associated with a deficiency in T cell and abnormalities of sympathetic nerve function. Therapeutic option (subcutaneous daily injection of leptin) is available for patients with LEP deficiency. It results in weight loss, mainly of fat mass, with a major effect on reducing food intake and on other dysfunctions including immunity and induction of puberty even in adults. In LEPR deficient subjects, phenotypic similarities with the LEP-deficient subjects were noticed, especially the exhibited rapid weight gain in the first few months of life, with severe hyperphagia and the endocrine abnormalities (hypogonadotrophic hypogonadism, insufficient somatotrophic or thyreotropic secretion). Leptin treatment is useless in the LEPR deficient subjects. Factors that could possibly bypass normal leptin delivery systems are being developed but are not yet currently available for the treatment of these patients. Measurement of circulating leptin may help for the diagnosis of such obesity: it is undetectable in LEP mutation carriers or extremely elevated in LEPR mutation carriers. Thus, LEPR gene screening might be also considered in subjects with the association of severe obesity with endocrine dysfunctions such as hypogonadism and with leptin related to corpulence level.

The neuroendocrine circuitry controlled by POMC, MSH, and AGRP

Obesity is one of the most challenging health problems worldwide. Over the past few decades, our knowledge concerning mechanisms of weight regulation has increased tremendously leading to the identification of the leptin-melanocortin pathway. The filling level of energy stores is signaled to the brain, and the information is integrated by hypothalamic nuclei, resulting in a well-orchestrated response to food intake and energy expenditure to ensure constant body weight. One of the key players in this system is proopiomelanocortin (POMC), a precursor of a variety of neuropeptides. POMC-derived alpha- and beta-MSH play an important role in energy homeostasis by activating melanocortin receptors expressed in the arcuate nucleus (MC3R) and in the nucleus paraventricularis (MC4R). Activation of these two G protein-coupled receptors is antagonized by agouti-related peptide (AgRP). Naturally occurring mutations in this system were identified in patients suffering from common obesity as well as in patients demonstrating a phenotype of severe early-onset obesity, adrenal insufficiency, red hair, and pale skin. Detailed understanding of the complex system of POMC-AgRP-MC3R-MC4R and their interaction with other hypothalamic as well as peripheral signals is a prerequisite to combat the obesity epidemic.

Genome wide association study identifies KCNMA1 contributing to human obesity

BACKGROUND

Recent genome-wide association (GWA) analyses have identified common single nucleotide polymorphisms (SNPs) that are associated with obesity. However, the reported genetic variation in obesity explains only a minor fraction of the total genetic variation expected to be present in the population. Thus many genetic variants controlling obesity remain to be identified. The aim of this study was to use GWA followed by multiple stepwise validations to identify additional genes associated with obesity.

METHODS

We performed a GWA analysis in 164 morbidly obese subjects (BMI:body mass index>40 kg/m2) and 163 Swedish subjects (>45 years) who had always been lean. The 700 SNPs displaying the strongest association with obesity in the GWA were analyzed in a second cohort comprising 460 morbidly obese subjects and 247 consistently lean Swedish adults. 23 SNPs remained significantly associated with obesity (nominal P<0.05) and were in a step-wise manner followed up in five additional cohorts from Sweden, France, and Germany together comprising 4214 obese and 5417 lean or population-based control individuals. Three samples, n=4133, were used to investigate the population-based associations with BMI. Gene expression in abdominal subcutaneous adipose tissue in relation to obesity was investigated for14 adults.

RESULTS

Potassium channel, calcium activated, large conductance, subfamily M, alpha member (KCNMA1) rs2116830*G and BDNF rs988712*G were associated with obesity in five of six investigated case-control cohorts. In meta-analysis of 4838 obese and 5827 control subjects we obtained genome-wide significant allelic association with obesity for KCNMA1 rs2116830*G with P=2.82×10(-10) and an odds ratio (OR) based on cases vs controls of 1.26 [95% C.I. 1.12-1.41] and for BDNF rs988712*G with P=5.2×10(-17) and an OR of 1.36 [95% C.I. 1.20-1.55]. KCNMA1 rs2116830*G was not associated with BMI in the population-based samples. Adipose tissue (P=0.0001) and fat cell (P=0.04) expression of KCNMA1 was increased in obesity.

CONCLUSIONS

We have identified KCNMA1 as a new susceptibility locus for obesity, and confirmed the association of the BDNF locus at the genome-wide significant level.

Hypothalamic proopiomelanocortin processing and the regulation of energy balance

Hypothalamic proopiomelanocortin (POMC) neurons play a key role in regulating energy balance and neuroendocrine function. Much attention has been focused on the regulation of POMC gene expression with less emphasis on regulated peptide processing. This is particularly important given the complexity of posttranslational POMC processing which is essential for the generation of biologically active MSH peptides. Mutations that impair POMC sorting and processing are associated with obesity in humans and in animals. Specifically, mutations in the POMC processing enzymes prohormone convertase 1/3 (PC1/3) and in carboxypeptidase E (CPE) and in the α-MSH degrading enzyme, PRCP, are associated with changes in energy balance. There is increasing evidence that POMC processing is regulated with respect to energy balance. Studies have implicated both the leptin and insulin signaling pathways in the regulation of POMC at various steps in the processing pathway. This article will review the role of hypothalamic POMC in regulating energy balance with a focus on POMC processing.

HMG1A and PPARG are differently expressed in the liver of fat and lean broilers

The expression of nine functional candidates for QT abdominal fat weight and relative abdominal fat content was investigated by real-time polymerase chain reaction (PCR) in the liver, adipose tissue, colon, muscle, pituitary gland and brain of broilers. The high mobility group AT-hook 1 (HMG1A) gene was up-regulated in liver with a ratio of means of 2.90 (P ≤ 0.01) in the "fatty" group (relative abdominal fat content 3.5 ± 0.18%, abdominal fat weight 35.4 ± 6.09 g) relative to the "lean" group (relative abdominal fat content 1.9 ± 0.56%, abdominal fat weight 19.2 ± 5.06 g). Expression of this gene was highly correlated with the relative abdominal fat content (0.70, P ≤ 0.01) and abdominal fat weight (0.70, P ≤ 0.01). The peroxisome proliferator-activated receptor gamma (PPARG) gene was also up-regulated in the liver with a ratio of means of 3.34 (P ≤ 0.01) in the "fatty" group relative to the "lean" group. Correlation of its expression was significant with both the relative abdominal fat content (0.55, P ≤ 0.05) and the abdominal fat weight (0.57, P ≤ 0.01). These data suggest that the HMG1A and PPARG genes were candidate genes for abdominal fat deposition in chickens. Searching of rSNPs in regulatory regions of the HMG1A and PPARG genes could provide a tool for gene-assisted selection.

Health implications of fructose consumption: A review of recent data

This paper reviews evidence in the context of current research linking dietary fructose to health risk markers.Fructose intake has recently received considerable media attention, most of which has been negative. The assertion has been that dietary fructose is less satiating and more lipogenic than other sugars. However, no fully relevant data have been presented to account for a direct link between dietary fructose intake and health risk markers such as obesity, triglyceride accumulation and insulin resistance in humans. First: a re-evaluation of published epidemiological studies concerning the consumption of dietary fructose or mainly high fructose corn syrup shows that most of such studies have been cross-sectional or based on passive inaccurate surveillance, especially in children and adolescents, and thus have not established direct causal links. Second: research evidence of the short or acute term satiating power or increasing food intake after fructose consumption as compared to that resulting from normal patterns of sugar consumption, such as sucrose, remains inconclusive. Third: the results of longer-term intervention studies depend mainly on the type of sugar used for comparison. Typically aspartame, glucose, or sucrose is used and no negative effects are found when sucrose is used as a control group.Negative conclusions have been drawn from studies in rodents or in humans attempting to elucidate the mechanisms and biological pathways underlying fructose consumption by using unrealistically high fructose amounts.The issue of dietary fructose and health is linked to the quantity consumed, which is the same issue for any macro- or micro nutrients. It has been considered that moderate fructose consumption of ≤50g/day or ~10% of energy has no deleterious effect on lipid and glucose control and of ≤100g/day does not influence body weight. No fully relevant data account for a direct link between moderate dietary fructose intake and health risk markers.

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.

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.

Unexpected endocrine features and normal pigmentation in a young adult patient carrying a novel homozygous mutation in the POMC gene

CONTEXT

Proopiomelanocortin (POMC) is the precursor to five biologically active peptides, including ACTH produced in the anterior pituitary and alpha-MSH produced in the hypothalamus. Mutations that inactivate the POMC gene have been described in children, causing a pleiotropic syndrome that includes secondary hypocortisolism, severe obesity, and variable changes in skin and hair pigmentation.

OBJECTIVE

We describe a female patient of North African ancestry, homozygous for a frameshift mutation in the POMC gene (6922InsC) that impairs the production of all melanocortin peptides, and that is associated with novel clinical features. Repeated clinical investigations from birth to age 18 yr are presented.

RESULT

ACTH deficiency was diagnosed at birth. Hyperphagia and obesity became apparent before 2 yr of age and rapidly progressed [body mass index (BMI) Z-score, +7 sd at 2 yr, +9.7 sd at 13 yr; BMI, 50 kg/m(2) at 18 yr). At puberty, the patient developed alterations in the somatotropic, gonadotropic, and thyroid axes necessitating hormonal replacement. Surprisingly, there were no obvious pigmentary features; neither the hair color nor measurements of skin reflectance distinguished between the patient and unaffected family members. However, chemical analysis of hair pigment revealed increased production of both pheomelanin and eumelanin.

CONCLUSION

Molecular genetic abnormalities of POMC should always be considered in patients with early onset adrenal insufficiency and obesity, even in the presence of normal pigmentation and multiple pituitary hormone anomalies.

Mutations in the amino-terminal region of proopiomelanocortin (POMC) in patients with early-onset obesity impair POMC sorting to the regulated secretory pathway

CONTEXT

Mutations in the proopiomelanocortin (POMC) gene that impair the synthesis or structure of POMC-derived peptides predispose to human obesity.

OBJECTIVE

Our objective was to identify and characterize novel mutations in the POMC gene found in patients with early-onset obesity.

DESIGN AND PATIENTS

The POMC gene was screened in 500 patients with severe early-onset obesity. The biosynthesis, processing, sorting, and secretion of wild-type POMC and two newly identified POMC mutants was studied using metabolic labeling, Western blotting, and immunoassay analysis of lysates and conditioned media of transiently transfected beta-TC3 cells.

RESULTS

Two novel heterozygous missense mutations in POMC (C28F and L37F) were identified in unrelated probands with early-onset obesity and their overweight or obese family members. Both mutations lie in a region of the N terminus of POMC that has been suggested to be involved in its sorting to the regulated secretory pathway. Metabolic labeling studies indicate that whereas the mutations do not reduce intracellular levels of POMC, both mutations (C28F>L37F) impair the ability of POMC to be processed to generate bioactive products. Studies of the secretion of POMC products suggest, particularly with C28F, that the impaired propeptide processing of these mutations results, at least in part, from a mistargeting of mutant POMC to the constitutive rather than the regulated secretory pathway.

CONCLUSION

These mutations in patients with early-onset obesity represent a novel molecular mechanism of human POMC deficiency whereby naturally occurring mutations in its N-terminal sequence impair the ability of POMC to enter the trafficking pathway in which serial propeptide processing normally occurs.