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

Obesity-associated MRAP2 variants impair multiple MC4R-mediated signaling pathways

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed at hypothalamic neurons that has an important role in appetite suppression and food intake. Mutations in MC4R are the most common cause of monogenic obesity and can affect multiple signaling pathways including Gs-cAMP, Gq, ERK1/2, β-arrestin recruitment, internalization and cell surface expression. The melanocortin-2 receptor accessory protein 2 (MRAP2), is a single-pass transmembrane protein that interacts with and regulates signaling by MC4R. Variants in MRAP2 have also been identified in overweight and obese individuals. However, functional studies that have only measured the effect of MRAP2 variants on MC4R-mediated cAMP signaling have produced inconsistent findings and most do not reduce MC4R function. Here we investigated the effect of twelve of these previously reported MRAP2 variants and showed that all variants that have been identified in overweight or obese individuals impair MC4R function. When expressed at equal concentrations, seven MRAP2 variants impaired MC4R-mediated cAMP signaling, while nine variants impaired IP3 signaling. Four mutations in the MRAP2 C-terminus affected internalization. MRAP2 variants had no effect on total or cell surface expression of either the MRAP2 or MC4R proteins. Structural models predicted that MRAP2 interacts with MC4R transmembrane helices 5 and 6, and mutations in two MRAP2 residues in putative contact sites impaired the ability of MRAP2 to facilitate MC4R signaling. In summary, our studies demonstrate that human MRAP2 variants associated with obesity impair multiple MC4R signaling pathways and that both Gs-cAMP and Gq-IP3 pathways should be assessed to determine variant pathogenicity.

Hypothalamic obesity: from basic mechanisms to clinical perspectives

Despite the diverse nature of obesity, there is compelling genetic, clinical, and experimental evidence that endorses the important contribution of brain circuits to this condition. The hypothalamus contains major regulatory circuits for bodyweight homoeostasis, the deregulation of which can lead to obesity. Although functional perturbation of hypothalamic pathways could lie at the basis of common forms of obesity, the term hypothalamic obesity has been created to define those rare forms of severe obesity where a clear hypothalamic substrate can be identified, either of genetic or acquired origin. An in-depth understanding of the pathogenesis, clinical presentation, and therapeutic targets of hypothalamic obesity relies on the comprehension of the physiological basis of hypothalamic pathways governing bodyweight control, the mechanisms (either genetic or acquired) whereby they are perturbed, and the consequences of such perturbation. In this Review, we provide a synoptic overview of hypothalamic obesity, from basic mechanisms to clinical perspectives, with a major focus on current developments and new avenues for the diagnosis and precise treatment of these rare forms of obesity.

Understanding Adipose Tissue Dysfunction

Diseases affecting adipose tissue (AT) function include obesity, lipodystrophy, and lipedema, among others. Both a lack of and excess AT are associated with increased risk for developing diseases including type 2 diabetes mellitus, hypertension, obstructive sleep apnea, and some types of cancer. However, individual risk of developing cardiometabolic and other 'obesity-related' diseases is not entirely determined by fat mass. Rather than excess fat accumulation, AT dysfunction may represent the mechanistic link between obesity and comorbid diseases. There are people who remain metabolically healthy despite obesity, whereas people with normal weight or very low subcutaneous AT mass may develop typically obesity-related diseases. AT dysfunction is characterized by adipocyte hypertrophy, impaired subcutaneous AT expandability (ectopic fat deposition), hypoxia, a variety of stress, inflammatory processes, and the release of proinflammatory, diabetogenic, and atherogenic signals. Genetic and environmental factors might contribute to AT heterogeneity either alone or via interaction with intrinsic biological factors. However, many questions remain regarding the mechanisms of AT dysfunction initiation and whether and how it could be reversed. Do AT signatures define clinically relevant subtypes of obesity? Is the cellular composition of AT associated with variation in obesity phenotypes? What roles do environmental compounds play in the manifestation of AT dysfunction? Answers to these and other questions may explain AT disease mechanisms and help to define strategies for improving AT health. This review focuses on recent advances in our understanding of AT biology.

Rare variants in the melanocortin 4 receptor gene (MC4R) are associated with abdominal fat and insulin resistance in youth with obesity

BACKGROUND

Rare variants in melanocortin 4 receptor gene (MC4R) result in a severe form of early-onset obesity; however, it is unclear how these variants may affect abdominal fat distribution, intrahepatic fat accumulation, and related metabolic sequelae.

METHODS

Eight hundred seventy-seven youth (6-21 years) with overweight/obesity, recruited from the Yale Pediatric Obesity Clinic in New Haven, CT, underwent genetic analysis to screen for functionally damaging, rare variants (MAF < 0.01) in MC4R. Participants were assigned to a Pathogenic Variant or No Pathogenic Variant group and completed a 10-timepoint 180-min oral glucose tolerance test (OGTT) and abdominal MRI.

RESULTS

Compared to the No Pathogenic Variant group, the Pathogenic Variant group demonstrated significantly greater glucose concentrations (AUCtot: 24.7 ± 1.22 g/dL × 180 min vs. 21.9 ± 1.41 g/dL × 180 min; p = 0.001), insulin levels (AUCtot: 57.4 ± 11.5 mU/mL × 180 min vs. 35.5 ± 8.90 mU/mL × 180 min; p = 0.002), and lower insulin sensitivity (WBISI: 1.01 ± 0.137 vs. 1.85 ± 0.036; p = 0.0008) during the OGTT. The Pathogenic Variant group also presented with greater visceral adipose tissue (VAT) (85.1 cm2 ± 10.3 vs. 56.1 cm2 ± 1.64; p = 0.003) and intrahepatic fat content (HFF%) (19.4% ± 4.94 vs. 8.21% ± 0.495; p = 0.012) than the No Pathogenic Variant group despite the two groups having similar BMI z-scores (p = 0.255), subcutaneous adipose tissue (SAT) (p = 0.643), and total body fat (p = 0.225).

CONCLUSIONS

Pathogenic variants in MC4R are associated with increased VAT, HFF%, and insulin resistance, independent from the degree of obesity in youth.

Cognitive decline and neuroinflammation in a mouse model of obesity: An accelerating role of ageing

Obesity, a pandemic, worldwide afflicts almost one billion people. Obesity and ageing share several pathological pathways leading to neurological disorders. However, due to a lack of suitable animal models, the long-term effects of obesity on age-related disorders- cognitive impairment and dementia have not yet been thoroughly investigated. Therefore, the current investigation focuses on developing a suitable model to explore the effects of obese-ageing. It also aims to determine whether obesity affects cognitive abilities in an age-dependent manner, and to identify a potential biomarker(s) for cognitive decline. Cognitive tests were carried out on 6-months and 1-year-old melanocortin-4 receptor (Mc4r)-deficient-obese and lean (wildtype) mice. Additionally, brains and sera were harvested for molecular, histological and serological analyses from 6, 12, and 24-months-old mice. Finally, RT-PCR was carried out after hippocampal mRNA sequencing. The cognitive tests revealed that 1-year-old obese mice have cognitive impairment along with underlying neurodegenerative changes, such as enlarged lateral ventricles. Serum neurofilament light chain (sNfL) levels were also elevated. Lipid accumulation and neuroinflammation were apparent besides, a compromised blood-brain barrier (BBB) indicated by altered junction protein gene expression. Differentially-expressed genes associated with cognitive decline were identified by mRNA sequencing of hippocampi. One such gene, Secreted Phosphoprotein 1 (Spp1) had markedly increased expression in cognitively-impaired obese mice. Our findings present an obese-aged mouse model of cognitive decline with neuroinflammation, reduced BBB-integrity and predisposing neurodegenerative changes. Obese-ageing accelerates the progression of cognitive impairment. Furthermore, Spp1 appears to be a potential biomarker for early diagnosis of neuropathological disorders.

Exploring the therapeutic potential of precision medicine in rare genetic obesity disorders: a scientific perspective

The prevalence of obesity is increasing worldwide, affecting both children and adults. This obesity epidemic is mostly driven by an increase in energy intake (abundance of highly palatable energy-dense food and drinks) and to a lesser degree a decrease in energy expenditure (sedentary lifestyle). A small proportion of individuals with obesity are affected by genetic forms of obesity, which often relate to mutations in the leptin-melanocortin pathway or are part of syndromes such as the Bardet-Biedl syndrome. These rare forms of obesity have provided valuable insights into the genetic architecture of obesity. Recent advances in understanding the molecular mechanisms that control appetite, hunger, and satiety have led to the development of drugs that can override genetic defects, enabling precision treatment. Leptin deficiency is uniquely treated with recombinant human metreleptin, while those with LEPR, PCSK1, or POMC deficiency can now be treated with the MC4R agonist setmelanotide. This review highlights the most frequent monogenic and syndromic forms of obesity, and the future outlook of precision treatment for these conditions.

Melanocortin 4 receptor mutation in obesity

Obesity is increasingly prevalent worldwide, with genetic factors contributing to its development. The hypothalamic leptin-melanocortin pathway is central to the regulation of appetite and weight; leptin activates the proopiomelanocortin neurons, leading to the production of melanocortin peptides; these in turn act on melanocortin 4 receptors (MC4R) which suppress appetite and increase energy expenditure. MC4R mutations are responsible for syndromic and non-syndromic obesity. These mutations are classified based on their impact on the receptor's life cycle: i.e. null mutations, intracellular retention, binding defects, signaling defects, and variants of unknown function. Clinical manifestations of MC4R mutations include early-onset obesity, hyperphagia, and metabolic abnormalities such as hyperinsulinemia and dyslipidemia. Management strategies for obesity due to MC4R mutations have evolved with the development of targeted therapies such as Setmelanotide, an MC4R agonist which can reduce weight and manage symptoms without adverse cardiovascular effects. Future research directions must include expansion of population studies to better understand the epidemiology of MC4R mutations, exploration of the molecular mechanisms underlying MC4R signaling, and development of new therapeutic agents. Understanding the interaction between MC4R and other genetic and environmental factors will be key to advancing both the prevention and treatment of obesity.

The expanding landscape of genetic causes of obesity

Obesity and weight regulation disorders are determined by the combined effects of genetics and environment. Polygenic obesity results from the combination of common variants in several genes which predisposes the individual to obesity and its related complications. In contrast, monogenic obesity results from changes in single genes, especially those in leptin-melanocortin pathway, and presents with early onset severe obesity, with or without other syndromic features. Rare variants in melanocortin 4 receptor are the commonest form of monogenic obesity. In addition, structural variation in small or large segments of chromosomes may also present with syndromic forms of obesity. Prader-Willi Syndrome, caused by imprinting errors in chromosome 15q11-13, is the most prevalent genetic cause of severe hyperphagia and obesity. With the advances in technologies, the past decade has witnessed a revolution in the identification of novel genetic causes of obesity, primarily in genes related to the leptin melanocortin pathway. The availability of safe melanocortin analogs holds the potential for targeted therapies for some of these disorders. This review summarizes known and novel rare genetic forms of obesity, along with approaches for the clinical investigation of copy number and sequence variants. The goal is to provide a reference for practicing clinicians to encourage genetic testing in obesity. IMPACT: What does this article add to the existing literature? Genetic obesity is an expanding frontier with potential to change management. Here, we summarize current information on the genetic causes of obesity and provide guidance for genetic testing. Emerging treatments may provide targeted precise treatment and change management practices.

Endocrine effects of MEK and BRAF inhibitor therapy in paediatric patients: a tertiary centre experience

PURPOSE

BRAF and MEK inhibitors are used to treat a range of paediatric tumours including low-grade gliomas. The ubiquitous nature of the BRAF/MAPK/MEK pathway means such treatments are not without side effects such as renal tubulopathies and hyperglycaemia. This study aims to describe the endocrine dysfunction observed in a cohort of children treated with BRAF and MEK inhibitors at the largest paediatric centre in the UK utilising these treatments.

METHODS

Electronic data for patients treated with dabrafenib (BRAF inhibitor) and trametinib (MEK inhibitor) from January 2019 to May 2022 were retrospectively reviewed. Outcomes included diagnosis of glucose dysregulation, the presence of hyponatraemia (< 135 mmol/l) and sodium nadir during treatment.

RESULTS

A total of 55 patients were included for analysis. Nine patients had at least one hyponatraemic episode during treatment of whom three had coexisting central diabetes insipidus. A statistically significant difference (p-value = 0.037) with regards to the plasma sodium nadir during treatment was observed between patients with diabetes insipidus (median = 134 (132-137) mmol/l) and patients without (median = 137 (127-141 mmol/l). Six patients were diagnosed with a form of glucose dysregulation (e.g. insulin resistance, type 2 diabetes), of whom four were diagnosed during treatment with dabrafenib, all with hypothalamo-pituitary lesions.

CONCLUSION

Clinicians using such treatments need to be aware of these potential effects, particularly the risk of hyponatraemia in patients with pre-existing central diabetes insipidus and monitor for these accordingly, including performing measurements of sodium and glucose prior to, during and after treatment.

Investigating genetic variants in early-onset obesity through exome sequencing: A retrospective cohort study

OBJECTIVE

This study aimed to examine clinical data and analyze exome sequencing (ES) findings in children diagnosed with early-onset obesity.

METHODS

We screened children presenting with severe (body mass index-standard deviation score >3) and early-onset (<7 years) obesity using ES. Participants were categorized into either the "no variant identified" group or the "variant identified" group, facilitating the exploration of correlations between clinical-demographic characteristics and genetic mutations linked to early-onset obesity. The functional implications of identified variants were assessed through in silico analyses.

RESULTS

Of the patients, 32 (35.5 %) possessed one or more mutations in pathways associated with obesity, all of which were heterozygous and patients with more than two obesity-associated variants were more obese. This cohort included 29 novel mutations distinct to our study population, 7 previously reported pathogenic variants, two instances of uniparental disomy, and one mitochondrial hotspot mutation. Variants in the SH2B1 gene emerged as a prevalent genetic determinant of obesity within our group, accounting for 16.6 % of cases. Statistical evaluations showed no significant differences in demographic attributes between the two groups.

CONCLUSION

Exome sequencing proves to be an instrumental approach for uncovering new variants and broadening the spectrum of mutations in early-onset obesity among children. Concurrently, further functional studies, both in vitro and in vivo, are crucial to elucidate the contributions of these variants to obesity's pathogenesis.

National-level and state-level prevalence of overweight and obesity among children, adolescents, and adults in the USA, 1990-2021, and forecasts up to 2050

BACKGROUND

Over the past several decades, the overweight and obesity epidemic in the USA has resulted in a significant health and economic burden. Understanding current trends and future trajectories at both national and state levels is crucial for assessing the success of existing interventions and informing future health policy changes. We estimated the prevalence of overweight and obesity from 1990 to 2021 with forecasts to 2050 for children and adolescents (aged 5-24 years) and adults (aged ≥25 years) at the national level. Additionally, we derived state-specific estimates and projections for older adolescents (aged 15-24 years) and adults for all 50 states and Washington, DC.

METHODS

In this analysis, self-reported and measured anthropometric data were extracted from 134 unique sources, which included all major national surveillance survey data. Adjustments were made to correct for self-reporting bias. For individuals older than 18 years, overweight was defined as having a BMI of 25 kg/m2 to less than 30 kg/m2 and obesity was defined as a BMI of 30 kg/m2 or higher, and for individuals younger than 18 years definitions were based on International Obesity Task Force criteria. Historical trends of overweight and obesity prevalence from 1990 to 2021 were estimated using spatiotemporal Gaussian process regression models. A generalised ensemble modelling approach was then used to derive projected estimates up to 2050, assuming continuation of past trends and patterns. All estimates were calculated by age and sex at the national level, with estimates for older adolescents (aged 15-24 years) and adults aged (≥25 years) also calculated for 50 states and Washington, DC. 95% uncertainty intervals (UIs) were derived from the 2·5th and 97·5th percentiles of the posterior distributions of the respective estimates.

FINDINGS

In 2021, an estimated 15·1 million (95% UI 13·5-16·8) children and young adolescents (aged 5-14 years), 21·4 million (20·2-22·6) older adolescents (aged 15-24 years), and 172 million (169-174) adults (aged ≥25 years) had overweight or obesity in the USA. Texas had the highest age-standardised prevalence of overweight or obesity for male adolescents (aged 15-24 years), at 52·4% (47·4-57·6), whereas Mississippi had the highest for female adolescents (aged 15-24 years), at 63·0% (57·0-68·5). Among adults, the prevalence of overweight or obesity was highest in North Dakota for males, estimated at 80·6% (78·5-82·6), and in Mississippi for females at 79·9% (77·8-81·8). The prevalence of obesity has outpaced the increase in overweight over time, especially among adolescents. Between 1990 and 2021, the percentage change in the age-standardised prevalence of obesity increased by 158·4% (123·9-197·4) among male adolescents and 185·9% (139·4-237·1) among female adolescents (15-24 years). For adults, the percentage change in prevalence of obesity was 123·6% (112·4-136·4) in males and 99·9% (88·8-111·1) in females. Forecast results suggest that if past trends and patterns continue, an additional 3·33 million children and young adolescents (aged 5-14 years), 3·41 million older adolescents (aged 15-24 years), and 41·4 million adults (aged ≥25 years) will have overweight or obesity by 2050. By 2050, the total number of children and adolescents with overweight and obesity will reach 43·1 million (37·2-47·4) and the total number of adults with overweight and obesity will reach 213 million (202-221). In 2050, in most states, a projected one in three adolescents (aged 15-24 years) and two in three adults (≥25 years) will have obesity. Although southern states, such as Oklahoma, Mississippi, Alabama, Arkansas, West Virginia, and Kentucky, are forecast to continue to have a high prevalence of obesity, the highest percentage changes from 2021 are projected in states such as Utah for adolescents and Colorado for adults.

INTERPRETATION

Existing policies have failed to address overweight and obesity. Without major reform, the forecasted trends will be devastating at the individual and population level, and the associated disease burden and economic costs will continue to escalate. Stronger governance is needed to support and implement a multifaceted whole-system approach to disrupt the structural drivers of overweight and obesity at both national and local levels. Although clinical innovations should be leveraged to treat and manage existing obesity equitably, population-level prevention remains central to any intervention strategies, particularly for children and adolescents.

FUNDING

Bill & Melinda Gates Foundation.

POMC neurons control fertility through differential signaling of MC4R in Kisspeptin neurons

Inactivating mutations in the melanocortin 4 receptor (MC4R) gene cause monogenic obesity. Interestingly, female patients also display various degrees of reproductive disorders, in line with the subfertile phenotype of MC4RKO female mice. However, the cellular mechanisms by which MC4R regulates reproduction are unknown. Kiss1 neurons directly stimulate gonadotropin-releasing hormone (GnRH) release through two distinct populations; the Kiss1ARH neurons, controlling GnRH pulses, and the sexually dimorphic Kiss1AVPV/PeN neurons controlling the preovulatory LH surge. Here, we show that Mc4r expressed in Kiss1 neurons regulates fertility in females. In vivo, deletion of Mc4r from Kiss1 neurons in female mice replicates the reproductive impairments of MC4RKO mice without inducing obesity. Conversely, reinsertion of Mc4r in Kiss1 neurons of MC4R null mice restores estrous cyclicity and LH pulsatility without reducing their obese phenotype. In vitro, we dissect the specific action of MC4R on Kiss1ARH vs Kiss1AVPV/PeN neurons and show that MC4R activation excites Kiss1ARH neurons through direct synaptic actions. In contrast, Kiss1AVPV/PeN neurons are normally inhibited by MC4R activation except under elevated estradiol levels, thus facilitating the activation of Kiss1AVPV/PeN neurons to induce the LH surge driving ovulation in females. Our findings demonstrate that POMCARH neurons acting through MC4R, directly regulate reproductive function in females by stimulating the "pulse generator" activity of Kiss1ARH neurons and restricting the activation of Kiss1AVPV/PeN neurons to the time of the estradiol-dependent LH surge, and thus unveil a novel pathway of the metabolic regulation of fertility by the melanocortin system.

Interactions between Polygenic Risk of Obesity and Dietary Factors on Anthropometric Outcomes: A Systematic Review and Meta-Analysis of Observational Studies

BACKGROUND

Diet is an important determinant of health and may moderate genetic susceptibility to obesity, but meta-analyses of available evidence are lacking.

OBJECTIVES

This study aimed to systematically review and meta-analyze evidence on the moderating effect of diet on genetic susceptibility to obesity, assessed with polygenic risk scores (PRS).

METHODS

A systematic search was conducted using MEDLINE, EMBASE, Web of Science, and the Cochrane Library to retrieve observational studies that examined PRS-diet interactions on obesity-related outcomes. Dietary exposures of interest included diet quality/dietary patterns and consumption of specific food and beverage groups. Random-effects meta-analyses were performed for pooled PRS- healthy eating index (HEI) interaction coefficients on body mass index (BMI) (on the basis of data from 4 cohort studies) and waist circumference (WC) (on the basis of data from 3 cohort studies).

RESULTS

Out of 36 retrieved studies, 78% were conducted among European samples. Twelve out of 21 articles examining dietary indices/patterns, and 16 out of 21 articles examining food/beverage groups observed some significant PRS-diet interactions. However, within many articles, findings are inconsistent when testing different combinations of obesity PRS-dietary factors and outcomes. Nevertheless, higher HEI scores and adherence to plant-based dietary patterns emerged as the more prominent diet quality/patterns that moderated genetic susceptibility to obesity, whereas higher consumption of fruits and vegetables, and lower consumption of fried foods and sugar-sweetened beverages emerged as individual food/beverage moderators. Results from the meta-analysis suggest that a higher HEI attenuates genetic susceptibility on BMI (pooled PRS∗HEI coefficient: -0.08; 95% confidence interval (CI): -0.15, 0.00; P = 0.0392) and WC (-0.37; 95% CI: -0.60, -0.15; P = 0.0013).

CONCLUSIONS

Current observational evidence suggests a moderating role of overall diet quality in polygenic risk of obesity. Future research should aim to identify genetic loci that interact with dietary exposures on anthropometric outcomes and conduct analyses among diverse ethnic groups.

TRIAL REGISTRATION NUMBER

This study was registered at the International Prospective Register of Systematic Reviews as CRD42022312289.

The MC4R agonist, setmelanotide, is associated with an improvement in hypercapnic chemosensitivity and weight loss in male mice

Obesity increases the risk of respiratory diseases that reduce respiratory chemosensitivity, such as Obesity Hypoventilation Syndrome and sleep apnea. Recent evidence suggests that obesity-related changes in the brain, including alterations in melanocortin signaling via the melanocortin-4 receptor (MC4R), may underly altered chemosensitivity. Setmelanotide, an MC4R agonist, causes weight loss in both humans and animal models. However, it is unknown the extent to which setmelanotide affects respiratory chemosensitivity independent of body weight loss. The present study uses diet-induced obese, male C57bl/6 J mice to determine the extent to which acute setmelanotide treatment affects the hypercapnic ventilatory response (HCVR). We find that ten days of daily setmelanotide treatment at 1 mg/kg, but not 0.2 mg/kg, is sufficient to cause weight loss and increase HCVR. In a separate group of animals, we find that we can emulate setmelanotide's effect on weight loss by restricting daily calories to match the hypophagia triggered by setmelanotide. These pair-fed animals exhibit improvements in HCVR similar to those who receive setmelanotide. We conclude that acute treatment with setmelanotide is as effective as weight loss at improving respiratory hypercapnic chemosensitivity.

Novel homozygous ESAM variants in two families with perinatal strokes showing variable neuroradiologic and clinical findings

Biallelic loss of function variants in ESAM (endothelial cell adhesion molecule) have recently been reported in 14 individuals (9 families) presenting with prenatal intracranial hemorrhage. Here, we describe four patients from two unrelated families in whom three of them presented with variable onset encephalopathy and seizures while one only displayed profound delay without seizures. Brain MRI showed variable onset intracranial hemorrhage that evolved to hydrocephalus in 3 patients, whereas hemosiderin deposits, white matter volume loss, and porencephalic cysts were noted in one patient. Unlike the majority of described cases, the youngest brother of the first family did not show microcephaly and failure to thrive. Exome sequencing identified two novel homozygous ESAM variants. A splice variant (c.731-2A>G) was identified in one family which was confirmed by investigating the patient's mRNA to result in exon skipping and early protein truncation. In addition, a missense variant (c.561G>C; p.Trp187Cys) was identified in the other family, which is the first disease causing missense variant to be described in patients with ESAM deficient phenotype. In addition, a maternally inherited pathogenic MC4R variant (c.811T>C; p.Cys271 Arg) was also identified in the youngest brother of the first family. Variants in the MC4R gene are associated with a non-syndromic form of obesity that could explain the unusual macrocephaly and obesity. Our work establishes ESAM as a tight junction gene that can present with variable neuroradiological and clinical phenotypes when mutated. Moreover, it refines the phenotype of this ultrarare syndrome and extends the number and type of variants described to date.

Descriptive analysis of MC4R gene variants associated with obesity listed on ClinVar

OBJECTIVES

The most recent version of ClinVar was utilized to filter variants of the MC4R gene based on location, condition, and clinical significance with the goal of obtaining benign and disease-associated variants of the MC4R gene. MC4R gene variants can lead to dysregulation of energy expenditure and appetite control, which prompted this study to delineate the distinctive features of MC4R gene variants submitted to the ClinVar repository regarding their association with obesity and related phenotypes.

METHOD

A thorough search was conducted in the ClinVar repository for clinically significant MC4R variants through the utilization of the gene name MC4R[gene] and MeSH terms "MC4R[gene]" and "single gene"[properties]" in the search box. Leading to the identification of clinically significant genetic variants associated with obesity.

RESULTS

Utilizing the ClinVar clinical significance ranking system, the MC4R variants were categorized into six groups based on ClinVar/ClinGen's ranking system: pathogenic (P), likely pathogenic (LP), variant of uncertain significance (VUS), benign (B), likely benign (LB), and conflicting classifications (CC). A total of 103 pathogenic variants were observed. These variants have different clinical significance that are associated with monogenic obesity, monogenic diabetes, and body mass index quantitative traits. It was observed that over 80% of the mutations were single nucleotide variants, with nearly half being missense mutations spread throughout the topological and transmembrane domains. Furthermore, TM7 had the highest number of single nucleotide missense mutations.

CONCLUSION

Further analysis of the relationships between monogenic obesity and diabetes requires additional investigation to discover the underlying causes of these conditions. The study findings imply that mutations in MC4R's topological and transmembrane regions may significantly influence receptor activation and signaling. As more MC4R variants are discovered and their correlation with obesity is established, there is potential to definitively establish a strong connection between MC4R pathogenic variants and the development of obesity.

Trapped fat: Obesity pathogenesis as an intrinsic disorder in metabolic fuel partitioning

Our understanding of the pathophysiology of obesity remains at best incomplete despite a century of research. During this time, two alternative perspectives have helped shape thinking about the etiology of the disorder. The currently prevailing view holds that excessive fat accumulation results because energy intake exceeds energy expenditure, with excessive food consumption being the primary cause of the imbalance. The other perspective attributes the initiating cause of obesity to intrinsic metabolic defects that shift fuel partitioning from pathways for mobilization and oxidation to those for synthesis and storage. The resulting reduction in fuel oxidation and trapping of energy in adipose tissue drives a compensatory increase in energy intake and, under some conditions, a decrease in expenditure. This theory of obesity pathogenesis has historically garnered relatively less attention despite its pedigree. Here, we present an updated comprehensive formulation of the fuel partitioning theory, focused on evidence gathered over the last 80 years from major animal models of obesity showing a redirection of fuel fluxes from oxidation to storage and accumulation of excess body fat with energy intake equal to or even less than that of lean animals. The aim is to inform current discussions about the etiology of obesity and by so doing, help lay new foundations for the design of more efficacious approaches to obesity research, treatment and prevention.

Setmelanotide: A Melanocortin-4 Receptor Agonist for the Treatment of Severe Obesity Due to Hypothalamic Dysfunction

Obesity is a silent global pandemic. It is a condition associated with multiple risk factors and adverse outcomes that arise from the intertwined relationship between environmental factors and genetics. The genetic factors that cause phenotypic expression are variable. Monogenic obesity is a severe early-onset and rarer form of obesity, which presents with co-morbidities such as abnormal feeding behaviour. Monogenic obesity causes impaired weight regulation in the hypothalamus due to defects in the leptin-melanocortin signalling pathway. The emergence of a new therapeutic treatment, the melanocortin-4 receptor agonist setmelanotide (originally RM-493), has represented a breakthrough in the management of monogenic obesity and has raised hope in managing complex obesity. This review provides an overview of the setmelanotide trials that have taken place, as well as its mechanism of action, side effects and weight loss outcomes that led to its approval in the treatment of pro-opiomelanocortin (POMC) deficiency and proprotein convertase subtilisin/kexin type 1 (PCSK1) deficiency. It also explores setmelanotide's role in other genetic forms of obesity, such as hypothalamic obesity, Prader-Willi syndrome, Alström syndrome and other rare genetic conditions that are being investigated. This review aims to help to understand the pathophysiology of genetic obesity and aid in future treatment options for people with severe, complex genetic obesity.

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).

The Genetics of Obesity

Understanding the genetic causes of obesity permits anticipatory guidance and targeted treatments. Children with hyperphagia and severe early-onset obesity should receive genetic testing for rare monogenic and syndromic disorders caused by pathogenic variants involving a single gene or single chromosomal region. Gene panels covering the leptin pathway, the key regulator of energy balance, are becoming more widely available and at lower cost. Polygenic obesity is much more common and involves multiple genes throughout the genome, although the overlap in genes for rare and common disorders suggests a spectrum of severity and the potential of shared precision medicine approaches for treatment.

SHU9119 and MBP10 are biased ligands at the human melanocortin-4 receptor

The melanocortin-4 receptor (MC4R), a G protein-coupled receptor, is critically involved in regulating energy homeostasis as well as modulation of reproduction and sexual function. Two peptide antagonists (SHU9119 and MBP10) were derived from the endogenous agonist α-melanocyte stimulating hormone. But their pharmacology at human MC4R is not fully understood. Herein, we performed detailed pharmacological studies of SHU9119 and MBP10 on wild-type (WT) and six naturally occurring constitutively active MC4Rs. Both ligands had no or negligible agonist activity in Gαs-cAMP signaling on WT MC4R, but stimulated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation on WT and mutant MC4Rs. Mechanistic studies revealed that SHU9119 and MBP10 stimulated ERK1/2 signaling of MC4R by different mechanisms, with SHU9119-stimulated ERK1/2 signaling mediated by phosphatidylinositol 3-kinase (PI3K) and MBP10-initiated ERK1/2 activation through PI3K and β-arrestin. In summary, our studies demonstrated that SHU9119 and MBP10 were biased ligands for MC4R, preferentially activating ERK1/2 signaling through different mechanisms. SHU9119 acted as a biased ligand and MBP10 behaved as a biased allosteric modulator.

MC4R Variants Modulate α-MSH and Setmelanotide Induced Cellular Signaling at Multiple Levels

CONTEXT

The melanocortin-4 receptor (MC4R) plays an important role in body weight regulation. Pathogenic MC4R variants are the most common cause of monogenic obesity.

OBJECTIVE

We have identified 17 MC4R variants in adult and pediatric patients with obesity. Here we aimed to functionally characterize these variants by analyzing 4 different aspects of MC4R signaling. In addition, we aimed to analyze the effect of setmelanotide, a potent MC4R agonist, on these MC4R variants.

MATERIALS AND METHODS

Cell surface expression and α-melanocyte stimulating hormone (α-MSH)- or setmelanotide-induced cAMP response, β-arrestin-2 recruitment, and ERK activation were measured in cells expressing either wild type or variant MC4R.

RESULTS

We found a large heterogeneity in the function of these variants. We identified variants with a loss of response for all studied MC4R signaling, variants with no cAMP accumulation or ERK activation but normal β-arrestin-2 recruitment, and variants with normal cAMP accumulation and ERK activation but decreased β-arrestin-2 recruitment, indicating disrupted desensitization and signaling mechanisms. Setmelanotide displayed a greater potency and similar efficacy as α-MSH and induced significantly increased maximal cAMP responses of several variants compared to α-MSH. Despite the heterogeneity in functional response, there was no apparent difference in the obesity phenotype in our patients.

CONCLUSION

We show that these obesity-associated MC4R variants affect MC4R signaling differently yet lead to a comparable clinical phenotype. Our results demonstrate the clinical importance of assessing the effect of MC4R variants on a range of molecular signaling mechanisms to determine their association with obesity, which may aid in improving personalized treatment.

SMIM1 absence is associated with reduced energy expenditure and excess weight

BACKGROUND

Obesity rates have nearly tripled in the past 50 years, and by 2030 more than 1 billion individuals worldwide are projected to be obese. This creates a significant economic strain due to the associated non-communicable diseases. The root cause is an energy expenditure imbalance, owing to an interplay of lifestyle, environmental, and genetic factors. Obesity has a polygenic genetic architecture; however, single genetic variants with large effect size are etiological in a minority of cases. These variants allowed the discovery of novel genes and biology relevant to weight regulation and ultimately led to the development of novel specific treatments.

METHODS

We used a case-control approach to determine metabolic differences between individuals homozygous for a loss-of-function genetic variant in the small integral membrane protein 1 (SMIM1) and the general population, leveraging data from five cohorts. Metabolic characterization of SMIM1-/- individuals was performed using plasma biochemistry, calorimetric chamber, and DXA scan.

FINDINGS

We found that individuals homozygous for a loss-of-function genetic variant in SMIM1 gene, underlying the blood group Vel, display excess body weight, dyslipidemia, altered leptin to adiponectin ratio, increased liver enzymes, and lower thyroid hormone levels. This was accompanied by a reduction in resting energy expenditure.

CONCLUSION

This research identified a novel genetic predisposition to being overweight or obese. It highlights the need to investigate the genetic causes of obesity to select the most appropriate treatment given the large cost disparity between them.

FUNDING

This work was funded by the National Institute of Health Research, British Heart Foundation, and NHS Blood and Transplant.

Novel hypothalamic pathways for metabolic effects of spexin

One of the main underlying etiologies of type 2 diabetes (T2DM) is insulin resistance, which is most frequently caused by obesity. Notably, the deregulation of adipokine secretion from visceral adiposity has been identified as a crucial characteristic of type 2 diabetes and obesity. Spexin is an adipokine that is released by many different tissues, including white adipocytes and the glandular stomach, and is negatively connected with the state of energy storage. This peptide acts through GALR2/3 receptors to control a wide range of metabolic processes, including inflammation, browning, lipolysis, energy expenditure, and eating behavior. Specifically, spexin can enter the hypothalamus and regulate the hypothalamic melanocortin system, which in turn balances energy expenditure and food intake. This review examines recent advances and the underlying mechanisms of spexin in obesity and T2DM. In particular, we address a range of topics from basic research to clinical findings, such as an analysis of the possible function of spexin in the hypothalamic melanocortin response, which involves reducing energy intake and increasing energy expenditure while also enhancing insulin sensitivity and glucose tolerance. Gaining more insight into the mechanisms that underlie the spexin system's control over energy metabolism and homeostasis may facilitate the development of innovative treatment approaches that focus on combating obesity and diabetes.

Identification and functional validation of rare coding variants in genes linked to monogenic obesity

OBJECTIVE

Rare cases of monogenic obesity, which may respond to specific therapeutics, can remain undetected in populations in which polygenic obesity is prevalent. This study examined rare DNA variation in established monogenic obesity genes within a community using whole-exome sequence data from 6803 longitudinally studied individuals.

METHODS

Exome data across 15 monogenic obesity genes were analyzed for nonsynonymous variants observed in any child with a maximum BMI z score > 2 (N = 279) but not observed in a child with a maximum BMI z score ≤ 0 (n = 1542) or that occurred in adults in the top 5th percentile of BMI (n = 263) but not in adults below the median BMI (n = 2629). Variants were then functionally analyzed using luciferase assays.

RESULTS

The comparisons between cases of obesity and controls identified eight missense variants in six genes: DYRK1B, KSR2, MC4R, NTRK2, PCSK1, and SIM1. Among these, MC4R p.A303P and p.R165G were previously shown to impair MC4R function. Functional analyses of the remaining six variants suggest that KSR2 p.I402F and p.T193I and NTRK2 p.S249Y alter protein function.

CONCLUSIONS

In addition to MC4R, rare missense variants in KSR2 and NTRK2 may potentially explain the severe obesity observed for the carriers.

Motivational dysregulation with melanocortin 4 receptor haploinsufficiency

Obesity, by any standard, is a global health crisis. Both genetic and dietary contributions to the development and maintenance of obesity were integral factors of our experimental design. As mutations of the melanocortin 4 receptors (MC4R) are the leading monogenetic cause of obesity, MC4R haploinsufficient rats were fed a range of dietary fat (0-12 %) in a longitudinal design. Physiological and motivational assessments were performed using a locomotor task, a 5-choice sucrose preference task, an operant task with fixed and progressive ratios, as well as a distraction operant task. Dendritic spine morphology of medium spiny neurons (MSNs) of the nucleus accumbens (NAc), cells with ample D1 and D2 receptors, was also assessed. The percentage of lipid deposits in the liver of each rat was also analyzed using the Area Fraction Fractionator probe for stereological measurements. MC4R haploinsufficiency resulted in a phenotypic resemblance for adult-onset obesity that was exacerbated by the consumption of a high-fat diet. Results from the operant tasks indicate that motivational deficits due to MC4R haploinsufficiency were apparent prior to the onset of obesity and exacerbated by dietary fat consumption after obesity was well established. Moreover, MSN morphology shifted to longer spines with smaller head diameters for the MC4R+/- animals under the high-fat diet, suggesting a potential mechanism for the dysregulation of motivation to work for food. Increasing our knowledge of the neural circuitry/mechanisms responsible for the rewarding properties of food has significant implications for understanding energy balance and the development of obesity.

Differentiating monogenic and syndromic obesities from polygenic obesity: Assessment, diagnosis, and management

Background

Obesity is a multifactorial neurohormonal disease that results from dysfunction within energy regulation pathways and is associated with increased morbidity, mortality, and reduced quality of life. The most common form is polygenic obesity, which results from interactions between multiple gene variants and environmental factors. Highly penetrant monogenic and syndromic obesities result from rare genetic variants with minimal environmental influence and can be differentiated from polygenic obesity depending on key symptoms, including hyperphagia; early-onset, severe obesity; and suboptimal responses to nontargeted therapies. Timely diagnosis of monogenic or syndromic obesity is critical to inform management strategies and reduce disease burden. We outline the physiology of weight regulation, role of genetics in obesity, and differentiating characteristics between polygenic and rare genetic obesity to facilitate diagnosis and transition toward targeted therapies.

Methods

In this narrative review, we focused on case reports, case studies, and natural history studies of patients with monogenic and syndromic obesities and clinical trials examining the efficacy, safety, and quality of life impact of nontargeted and targeted therapies in these populations. We also provide comprehensive algorithms for diagnosis of patients with suspected rare genetic causes of obesity.

Results

Patients with monogenic and syndromic obesities commonly present with hyperphagia (ie, pathologic, insatiable hunger) and early-onset, severe obesity, and the presence of hallmark characteristics can inform genetic testing and diagnostic approach. Following diagnosis, specialized care teams can address complex symptoms, and hyperphagia is managed behaviorally. Various pharmacotherapies show promise in these patient populations, including setmelanotide and glucagon-like peptide-1 receptor agonists.

Conclusion

Understanding the pathophysiology and differentiating characteristics of monogenic and syndromic obesities can facilitate diagnosis and management and has led to development of targeted pharmacotherapies with demonstrated efficacy for reducing body weight and hunger in the affected populations.

Association of obesity and menarche SNPs and interaction with environmental factors on precocious puberty

BACKGROUND

Obesity is an important cause for the precocious or early puberty. However, the association between obesity-related loci and the risk of precocious puberty as well as the effect of gene-environment interaction are unclear, especially in the Chinese children population.

METHODS

This was a case-control study using baseline data from two cohorts and hospital cases in China. 15 SNPs loci and several environmental factors were included in the analysis of 1201 participants. Chi-square test and logistic regression were used to analyze the association between SNPs and precocious puberty. Additionally, exploratory factor analysis was conducted on 13 environmental variables, and then to explore their interaction with genes on precocious puberty.

RESULTS

The effect allele C of rs571312, and G of rs12970134 MC4R were associated with precocious puberty in girls with obesity. Regarding the gene-environment interaction, we found that when girls were in the high socioeconomic status, the rs571312 (OR: 3.996; 95% CI: 1.694-9.423) and rs12970134 (OR: 3.529; 95% CI: 1.452-8.573) risk genotypes had a greater effect on precocious puberty.

CONCLUSIONS

The obesity risk gene polymorphisms MC4R rs571312 and rs12970134 were associated with precocious puberty in Chinese girls with obesity, and girls with risk genotypes and high socioeconomic status should be given extra attention.

IMPACT

This is the first study that identified the association between rs571312 and rs12970134 of MC4R gene and precocious puberty in Chinese children. We found that when girls were in the high socioeconomic status, the risk genotypes of rs571312 and rs12970134 had a greater effect on precocious puberty. The results of this study have great public health implications. It is recommended that girls who are in high socioeconomic status and have a high genetic risk for early sexual maturity should closely monitor their pubertal development and consider early intervention strategies.

Translational potential of mouse models of human metabolic disease

Obesity causes significant morbidity and mortality globally. Research in the last three decades has delivered a step-change in our understanding of the fundamental mechanisms that regulate energy homeostasis, building on foundational discoveries in mouse models of metabolic disease. However, not all findings made in rodents have translated to humans, hampering drug discovery in this field. Here, we review how studies in mice and humans have informed our current framework for understanding energy homeostasis, discuss their challenges and limitations, and offer a perspective on how human studies may play an increasingly important role in the discovery of disease mechanisms and identification of therapeutic targets in the future.

Loss of transient receptor potential channel 5 causes obesity and postpartum depression

Hypothalamic neural circuits regulate instinctive behaviors such as food seeking, the fight/flight response, socialization, and maternal care. Here, we identified microdeletions on chromosome Xq23 disrupting the brain-expressed transient receptor potential (TRP) channel 5 (TRPC5). This family of channels detects sensory stimuli and converts them into electrical signals interpretable by the brain. Male TRPC5 deletion carriers exhibited food seeking, obesity, anxiety, and autism, which were recapitulated in knockin male mice harboring a human loss-of-function TRPC5 mutation. Women carrying TRPC5 deletions had severe postpartum depression. As mothers, female knockin mice exhibited anhedonia and depression-like behavior with impaired care of offspring. Deletion of Trpc5 from oxytocin neurons in the hypothalamic paraventricular nucleus caused obesity in both sexes and postpartum depressive behavior in females, while Trpc5 overexpression in oxytocin neurons in knock-in mice reversed these phenotypes. We demonstrate that TRPC5 plays a pivotal role in mediating innate human behaviors fundamental to survival, including food seeking and maternal care.

Phylogenomic analyses of all species of swordtail fishes (genus Xiphophorus) show that hybridization preceded speciation

Hybridization has been recognized to play important roles in evolution, however studies of the genetic consequence are still lagging behind in vertebrates due to the lack of appropriate experimental systems. Fish of the genus Xiphophorus are proposed to have evolved with multiple ancient and ongoing hybridization events. They have served as an informative research model in evolutionary biology and in biomedical research on human disease for more than a century. Here, we provide the complete genomic resource including annotations for all described 26 Xiphophorus species and three undescribed taxa and resolve all uncertain phylogenetic relationships. We investigate the molecular evolution of genes related to cancers such as melanoma and for the genetic control of puberty timing, focusing on genes that are predicted to be involved in pre-and postzygotic isolation and thus affect hybridization. We discovered dramatic size-variation of some gene families. These persisted despite reticulate evolution, rapid speciation and short divergence time. Finally, we clarify the hybridization history in the entire genus settling disputed hybridization history of two Southern swordtails. Our comparative genomic analyses revealed hybridization ancestries that are manifested in the mosaic fused genomes and show that hybridization often preceded speciation.

Sequencing methods, functional characterization, prevalence, and penetrance of rare coding mutations in panels of monogenic obesity genes from the leptin-melanocortin pathway: A systematic review

The recent development of next-generation sequencing (NGS) technologies has led to an increase of mutation screening reports of monogenic obesity genes in diverse experimental designs. However, no study to date has summarized their findings. Two reviewers independently conducted a systematic review of MEDLINE, Embase, and Web of Science Core Collection databases from inception to September 2022 to identify monogenic non-syndromic obesity gene screening studies. Of 1051 identified references, 31 were eligible after title and abstract screening and 28 after full-text reading and risk of bias and quality assessment. Most studies (82%) used NGS methods. The number of genes screened varied from 2 to 12 genes from the leptin-melanocortin pathway. While all the included studies used in silico tools to assess the functional status of mutations, only 2 performed in vitro tests. The prevalence of carriers of pathogenic/likely pathogenic monogenic mutations is 13.24% on average (heterozygous: 12.31%; homozygous/heterozygous composite: 0.93%). As no study reported the penetrance of pathogenic mutations on obesity, we estimated that homozygous carriers exhibited a complete penetrance (100%) and heterozygous carriers a variable penetrance (3-100%). The review provides an exhaustive description of sequencing methods, functional characterization, prevalence, and penetrance of rare coding mutations in monogenic non-syndromic obesity genes.

A Critical Functional Missense Mutation (T117M) in Sheep MC4R Gene Significantly Leads to Gain-of-Function

The melanocortin 4 receptor (MC4R) gene plays a central role in regulating energy homeostasis and food intake in livestock, thereby affecting their economic worth and growth. In a previous study, the p.T117M mutation in the sheep MC4R gene, which leads to the transition of threonine to methionine, was found to affect the body weight at six months and the average daily gain in Hu sheep. However, there are still limited studies on the frequency of the sheep p.T117M missense mutation globally, and the underlying cellular mechanism remains elusive. Therefore, this study first used WGS to investigate the distribution of the MC4R gene p.T117M mutation in 652 individuals across 22 breeds worldwide. The results showed that the mutation frequency was higher in European breeds compared with Chinese sheep breeds, particularly in Poll Dorset sheep (mutation frequency > 0.5). The p.T117M mutation occurs in the first extracellular loop of MC4R. Mechanistically, the basal activity of the mutated receptor is significantly increased. Specifically, upon treatment with α-MSH and ACTH ligands, the cAMP and MAPK/ERK signaling activation of M117 MC4R is enhanced. These results indicate that the T117M mutation may change the function of the gene by increasing the constitutive activity and signaling activation of cAMP and MAPK/ERK, and, thus, may regulate the growth traits of sheep. In conclusion, this study delved into the global distribution and underlying cellular mechanisms of the T117M mutation of the MC4R gene, establishing a scientific foundation for breeding sheep with superior growth, thereby contributing to the advancement of the sheep industry.

Identification of KSR2 Variants in Pediatric Patients with Severe Early-Onset Obesity from Qatar

The kinase suppressor of Ras 2 (KSR2) gene is associated with monogenic obesity, and loss-of-function variants in KSR2 have been identified in individuals with severe early-onset obesity. This study investigated KSR2 variants in 9 pediatric patients with severe early-onset obesity in Qatar using whole genome sequencing among a cohort of 240 individuals. We focused on KSR2 variants with a minor allele frequency (MAF) below 1% and a Combined Annotation Dependent Depletion (CADD) score above 13 to identify potential causative variants. Our analysis identified four KSR2 variants: one intronic (c.1765-8G>A) and three missense variants (c.1057G>A, c.1673G>A, and c.923T>C) in nine patients. The intronic variant c.1765-8G>A was the most frequent (seen in six individuals) and had a CADD score of 21.10, suggesting possible pathogenicity. This variant showed a significantly higher allele frequency in the Qatari population compared to the Genome Aggregation Database (gnomAD), indicating a possible founder effect. Molecular modeling of the missense variants revealed structural changes in the protein structure. The study concludes that these four KSR2 variants are associated with monogenic obesity, with an autosomal dominant inheritance pattern. The c.1765-8G>A variant's prevalence in Qatar underscores its importance in genetic screening for severe obesity. This research advances the understanding of genetic factors in severe early-onset obesity and may inform better management strategies.

Clinical phenotypes of adults with monogenic and syndromic genetic obesity

OBJECTIVE

Considering limited evidence on diagnostics of genetic obesity in adults, we evaluated phenotypes of adults with genetic obesity. Additionally, we assessed the applicability of Endocrine Society (ES) recommendations for genetic testing in pediatric obesity.

METHODS

We compared clinical features, including age of onset of obesity and appetite, between adults with non-syndromic monogenic obesity (MO), adults with syndromic obesity (SO), and adults with common obesity (CO) as control patients.

RESULTS

A total of 79 adults with genetic obesity (32 with MO, 47 with SO) were compared with 186 control patients with CO. Median BMI was similar among the groups: 41.2, 39.5, and 38.7 kg/m2 for patients with MO, SO, and CO, respectively. Median age of onset of obesity was 3 (IQR: 1-6) years in patients with MO, 9 (IQR: 4-13) years in patients with SO, and 21 (IQR: 13-33) years in patients with CO (p < 0.001). Patients with genetic obesity more often reported increased appetite: 65.6%, 68.1%, and 33.9% in patients with MO, SO, and CO, respectively (p < 0.001). Intellectual deficit and autism spectrum disorder were more prevalent in patients with SO (53.2% and 21.3%) compared with those with MO (3.1% and 6.3%) and CO (both 0.0%). The ES recommendations were fulfilled in 56.3%, 29.8%, and 2.7% of patients with MO, SO, and CO, respectively (p < 0.001).

CONCLUSIONS

We found distinct phenotypes in adult genetic obesity. Additionally, we demonstrated low sensitivity for detecting genetic obesity in adults using pediatric ES recommendations, necessitating specific genetic testing recommendations in adult obesity care.

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Case report: Nerve fiber regeneration in children with melanocortin 4 receptor gene mutation related obesity treated with semaglutide

Melanocortin 4 receptor (MC4R) mutations are the commonest cause of monogenic obesity through dysregulation of neuronal pathways in the hypothalamus and prefrontal cortex that regulate hunger and satiety. MC4R also regulates neuropathic pain pathways via JNK signaling after nerve injury. We show evidence of corneal small fiber degeneration in 2 siblings carrying a heterozygous missense variant c.508A>G, p.Ille170Val in the MC4R gene. Both children were treated with once weekly semaglutide for 6 months with no change in weight, and only a minor improvement in HbA1c and lipid profile. However, there was evidence of nerve regeneration with an increase in corneal nerve fiber density (CNFD) [child A (13.9%), child B (14.7%)], corneal nerve branch density (CNBD) [child A (110.2%), child B (58.7%)] and corneal nerve fiber length (CNFL) [child A (21.5%), child B (44.0%)].

An integrative framework to prioritize genes in more than 500 loci associated with body mass index

Obesity is a major risk factor for a myriad of diseases, affecting >600 million people worldwide. Genome-wide association studies (GWASs) have identified hundreds of genetic variants that influence body mass index (BMI), a commonly used metric to assess obesity risk. Most variants are non-coding and likely act through regulating genes nearby. Here, we apply multiple computational methods to prioritize the likely causal gene(s) within each of the 536 previously reported GWAS-identified BMI-associated loci. We performed summary-data-based Mendelian randomization (SMR), FINEMAP, DEPICT, MAGMA, transcriptome-wide association studies (TWASs), mutation significance cutoff (MSC), polygenic priority score (PoPS), and the nearest gene strategy. Results of each method were weighted based on their success in identifying genes known to be implicated in obesity, ranking all prioritized genes according to a confidence score (minimum: 0; max: 28). We identified 292 high-scoring genes (≥11) in 264 loci, including genes known to play a role in body weight regulation (e.g., DGKI, ANKRD26, MC4R, LEPR, BDNF, GIPR, AKT3, KAT8, MTOR) and genes related to comorbidities (e.g., FGFR1, ISL1, TFAP2B, PARK2, TCF7L2, GSK3B). For most of the high-scoring genes, however, we found limited or no evidence for a role in obesity, including the top-scoring gene BPTF. Many of the top-scoring genes seem to act through a neuronal regulation of body weight, whereas others affect peripheral pathways, including circadian rhythm, insulin secretion, and glucose and carbohydrate homeostasis. The characterization of these likely causal genes can increase our understanding of the underlying biology and offer avenues to develop therapeutics for weight loss.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

IMPROVE 2022 International Meeting on Pathway-Related Obesity: Vision of Excellence

Nearly 90 clinicians and researchers from around the world attended the first IMPROVE 2022 International Meeting on Pathway-Related Obesity. Delegates attended in person or online from across Europe, Argentina and Israel to hear the latest scientific and clinical developments in hyperphagia and severe, early-onset obesity, and set out a vision of excellence for the future for improving the diagnosis, treatment, and care of patients with melanocortin-4 receptor (MC4R) pathway-related obesity. The meeting co-chair Peter Kühnen, Charité Universitätsmedizin Berlin, Germany, indicated that change was needed with the rapidly increasing prevalence of obesity and the associated complications to improve the understanding of the underlying mechanisms and acknowledge that monogenic forms of obesity can play an important role, providing insights that can be applied to a wider group of patients with obesity. World-leading experts presented the latest research and led discussions on the underlying science of obesity, diagnosis (including clinical and genetic approaches such as the role of defective MC4R signalling), and emerging clinical data and research with targeted pharmacological approaches. The aim of the meeting was to agree on the questions that needed to be addressed in future research and to ensure that optimised diagnostic work-up was used with new genetic testing tools becoming available. This should aid the planning of new evidence-based treatment strategies for the future, as explained by co-chair Martin Wabitsch, Ulm University Medical Center, Germany.

Npy transcription is regulated by noncanonical STAT3 signaling in hypothalamic neurons: Implication with lipotoxicity and obesity

Neuropeptide Y (Npy) is an abundant neuropeptide expressed in the central and peripheral nervous systems. NPY-secreting neurons in the hypothalamic arcuate nucleus regulate energy homeostasis, and Npy mRNA expression is regulated by peripheral nutrient and hormonal signals like leptin, interleukin-6 (IL-6), and fatty acids. This study demonstrates that IL-6, which phosphorylates tyrosine 705 (Y705) of STAT3, decreased Npy mRNA in arcuate immortalized hypothalamic neurons. In parallel, inhibitors of STAT3-Y705 phosphorylation, stattic and cucurbitacin I, robustly upregulated Npy mRNA. Chromatin-immunoprecipitation showed high baseline total STAT3 binding to multiple regulatory regions of the Npy gene, which are decreased by IL-6 exposure. The STAT3-Npy interaction was further examined in obesity-related pathologies. Notably, in four different hypothalamic neuronal models where palmitate potently stimulated Npy mRNA, Socs3, a specific STAT3 activity marker, was downregulated and was negatively correlated with Npy mRNA levels (R2 = 0.40, p < 0.001), suggesting that disrupted STAT3 signaling is involved in lipotoxicity-mediated dysregulation of Npy. Finally, human NPY SNPs that map to human obesity or body mass index were investigated for potential STAT3 binding sites. Although none of the SNPs were linked to direct STAT3 binding, analysis show that rs17149106 (-602 G > T) is located on an upstream enhancer element of NPY, where the variant is predicted to disrupt validated binding of KLF4, a known inhibitory cofactor of STAT3 and downstream effector of leptin signaling. Collectively, this study demonstrates that STAT3 signaling negatively regulates Npy transcription, and that disruption of this interaction may contribute to metabolic disorders.

Neuroanatomical dissection of the MC3R circuitry regulating energy rheostasis

Although mammals resist both acute weight loss and weight gain, the neural circuitry mediating bi-directional defense against weight change is incompletely understood. Global constitutive deletion of the melanocortin-3-receptor (MC3R) impairs the behavioral response to both anorexic and orexigenic stimuli, with MC3R knockout mice demonstrating increased weight gain following anabolic challenges and increased weight loss following anorexic challenges (i.e. impaired energy rheostasis). However, the brain regions mediating this phenotype remain incompletely understood. Here, we utilized MC3R floxed mice and viral injections of Cre-recombinase to selectively delete MC3R from medial hypothalamus (MH) in adult mice. Behavioral assays were performed on these animals to test the role of MC3R in MH in the acute response to orexigenic and anorexic challenges. Complementary chemogenetic approaches were used in MC3R-Cre mice to localize and characterize the specific medial hypothalamic brain regions mediating the role of MC3R in energy homeostasis. Finally, we performed RNAscope in situ hybridization to map changes in the mRNA expression of MC3R, POMC, and AgRP following energy rheostatic challenges. Our results demonstrate that MC3R deletion in MH increased feeding and weight gain following acute high fat diet feeding in males, and enhanced the anorexic effects of semaglutide, in a sexually dimorphic manner. Additionally, activation of DMH MC3R neurons increased energy expenditure and locomotion. Together, these results demonstrate that MC3R mediated effects on energy rheostasis result from the loss of MC3R signaling in the medial hypothalamus of adult animals and suggest an important role for DMH MC3R signaling in energy rheostasis.

The melanocortin-4 receptor pathway and the emergence of precision medicine in obesity management

Over the past few decades, there has been a global surge in the prevalence of obesity, rendering it a globally recognized epidemic. Contrary to simply being a medical condition, obesity is an intricate disease with a multifactorial aetiology. Understanding the precise cause of obesity remains a challenge; nevertheless, there seems to be a complex interplay among biological, psychosocial and behavioural factors. Studies on the genetic factors of obesity have revealed several pathways in the brain that play a crucial role in food intake regulation. The best characterized pathway, thus far, is the leptin-melanocortin pathway, from which disruptions are responsible for the majority of monogenic obesity disorders. The effectiveness of conservative lifestyle interventions in addressing monogenic obesity has been limited. Therefore, it is crucial to complement the management strategy with pharmacological and surgical options. Emphasis has been placed on developing drugs aimed at replacing the absent signals, with the goal of restoring the pathway. In both monogenic and polygenic forms of obesity, outcomes differ across various interventions, likely due to the multifaceted nature of the disease. This underscores the need to explore alternative therapeutic strategies that can mitigate this heterogeneity. Precision medicine can be regarded as a powerful tool that can address this concern, as it values the understanding of the underlying abnormality triggering the disease and provides a tailored treatment accordingly. This would assist in optimizing outcomes of the current therapeutic approaches and even aid in the development of novel treatments capable of more effectively managing the global obesity epidemic.

Hypothalamic control of body fat mass by food intake: The key to understanding why obesity should be treated as a disease

BACKGROUND

Hypothalamic centres have been recognized to play a central role in body weight regulation for nearly 70 years.

AIMS

In this review, we will explore the current undersanding of the role the hypothalamus plays in controlling food intake behaviours.

MATERIALS AND METHODS

Review of relevant literature from PubMed searches and review article citations.

RESULTS

Beginning with autopsy studies showing destructive hypothalamic lesions in patients manifesting hyperphagia and rapid weight gain, followed by animal lesioning studies pinpointing adjacent hypothalamic sites as the 'satiety' centre and the 'feeding' centre of the brain, the neurocircuitry that governs our body weight is now understood to consist of a complex, interconnected network, including the hypothalamus and extending to cortical sites, reward centres and brainstem. Neurons in these sites receive afferent signals from the gastrointestinal tract and adipose tissue indicating food availability, calorie content, as well as body fat mass.

DISCUSSION

Integration of these complex signals leads to modulation of the two prime effector systems that defend a body fat mass set point: food intake and energy expenditure.

CONCLUSION

Understanding the hypothalamic control of food intake forms the foundation for understanding and managing obesity as a chronic disease.

Bardet-Biedl syndrome: A clinical overview focusing on diagnosis, outcomes and best-practice management

Bardet-Biedl syndrome (BBS) is a genetic disorder characterized by early-onset obesity, polydactyly, genital and kidney anomalies, developmental delay and vision loss due to rod-cone dystrophy. BBS is an autosomal recessive disorder with >20 implicated genes. The genotype-phenotype relationship in BBS is not clear, and there may be additional modifying factors. The underlying mechanism is dysfunction of primary cilia. In BBS, receptor trafficking in and out of the cilia is compromised, affecting multiple organ systems. Along with early-onset obesity, hyperphagia is a prominent symptom and contributes significantly to clinical morbidity and caregiver burden. While there is no cure for BBS, setmelanotide is a new pharmacotherapy approved for treatment of obesity in BBS. The differential diagnosis for BBS includes other ciliopathies, such as Alstrom syndrome, and other genetic obesity syndromes, such as Prader-Willi syndrome. Careful clinical history and genetic testing can help determine the diagnosis and a multidisciplinary team is necessary to guide clinical management.

Unexpected identification of obesity-associated mutations in LEP and MC4R genes in patients with anorexia nervosa

Mutations leading to a reduced or loss of function in genes of the leptin-melanocortin system confer a risk for monogenic forms of obesity. Yet, gain of function variants in the melanocortin-4-receptor (MC4R) gene predispose to a lower BMI. In individuals with reduced body weight, we thus expected mutations leading to an enhanced function in the respective genes, like leptin (LEP) and MC4R. Therefore, we have Sanger sequenced the coding regions of LEP and MC4R in 462 female patients with anorexia nervosa (AN), and 445 healthy-lean controls. In total, we have observed four and eight variants in LEP and MC4R, respectively. Previous studies showed different functional in vitro effects for the detected frameshift and non-synonymous variants: (1) LEP: reduced/loss of function (p.Val94Met), (2) MC4R: gain of function (p.Val103Ile, p.Ile251Leu), reduced or loss of function (p.Thr112Met, p.Ser127Leu, p.Leu211fsX) and without functional in vitro data (p.Val50Leut). In LEP, the variant p.Val94Met was detected in one patient with AN. For MC4R variants, one patient with AN carried the frameshift variant p.Leu211fsX. One patient with AN was heterozygous for two variants at the MC4R (p.Val103Ile and p.Ser127Leu). All other functionally relevant variants were detected in similar frequencies in patients with AN and lean individuals.

Dietary linoleic acid supplementation protects against obesity-induced microglial reactivity in mice

We investigated whether linoleic acid (LA) supplementation could modulate emotional behavior and microglia-related neuroinflammation. For that, male mice of C57BL/6J genetic background fed either a high-fat diet (HFD) or a standard diet (STD) for 12 weeks, were treated with a vehicle or LA solution for 5 weeks before being evaluated for emotional behavior using a battery of behavioral tests. The animals were subsequently sacrificed and their brains collected and processed for immunofluorescence staining, targeting microglia-specific calcium-binding proteins (IBA-1). Neuroinflammation severity was assessed in multiple hypothalamic, cortical and subcortical brain regions. We show an anxio-depressive-like effect of sustained HFD feeding that was neither alleviated nor worsened with LA supplementation. However, increased IBA-1 expression and microgliosis in the HFD group were largely attenuated by LA supplementation. These observations demonstrate that the anti-neuroinflammatory properties of LA are not restricted to hypothalamic areas but are also evident at the cortical and subcortical levels. This study discloses that neuroinflammation plays a role in the genesis of neuropsychiatric disorders in the context of obesity, and that LA supplementation is a useful dietary strategy to alleviate the impact of obesity-related neuroinflammation.

Vagal pathways for systemic regulation of glucose metabolism

Maintaining blood glucose at an appropriate physiological level requires precise coordination of multiple organs and tissues. The vagus nerve bidirectionally connects the central nervous system with peripheral organs crucial to glucose mobilization, nutrient storage, and food absorption, thereby presenting a key pathway for the central control of blood glucose levels. However, the precise mechanisms by which vagal populations that target discrete tissues participate in glucoregulation are much less clear. Here we review recent advances unraveling the cellular identity, neuroanatomical organization, and functional contributions of both vagal efferents and vagal afferents in the control of systemic glucose metabolism. We focus on their involvement in relaying glucoregulatory cues from the brain to peripheral tissues, particularly the pancreatic islet, and by sensing and transmitting incoming signals from ingested food to the brain. These recent findings - largely driven by advances in viral approaches, RNA sequencing, and cell-type selective manipulations and tracings - have begun to clarify the precise vagal neuron populations involved in the central coordination of glucose levels, and raise interesting new possibilities for the treatment of glucose metabolism disorders such as diabetes.

Overweight during development dysregulates cellular metabolism and critical genes that control food intake in the prefrontal cortex

BACKGROUNDS AND AIMS

Childhood obesity is increasing substantially across the world. The World Obesity Federation (WOF) and World Health Organization (WHO) predicted that in 2030 > 1 billion people will be obese, and by 2035 over 4 billion will reach obesity worldwide. According to WHO, the world soon cannot afford the economic cost of obesity, and we need to act to stop obesity acceleration now. Data in the literature supports that the first 1000 days of life are essential in preventing obesity and related adversities. Therefore, using basic research, the present a study that focuses on the immediate effect of overnutrition and serotonin modulation during the lactation period.

METHODS

Using a neonatal overfeeding model, male Wistar rats were divided into four groups based on nutrition or serotonin modulation by pharmacological treatment up to 22 days of life. Cellular and mitochondrial function markers, oxidative stress biomarkers and mRNA levels of hedonic and homeostatic genes were evaluated.

RESULTS

Our data showed that overfeeding during lactation decrease NAD/NADH ratio, citrate synthase activity, and increase ROS production. Lipid and protein oxidation were increased in overfed animals, with a decrease in antioxidant defenses, we also observe a differential expression of mRNA levels of homeostatic and hedonic genes. On the contrary, serotonin modulation with selective serotonin reuptake inhibitors treatment reduces harmful effects caused by overnutrition.

CONCLUSION

Early effects of overnutrition significantly affect the prefrontal cortex at molecular and cellular level, which could mediate obesity-related neurodegenerative dysfunction.