👤 Mieke M van Haelst

The melanocortin-3 receptor (MC3R) and the melanocortin-4 receptor (MC4R), both expressed in hypothalamic nuclei, are key downstream effectors of leptin signaling and play important roles in energy homeostasis. While pathogenic variants in the MC4R gene represent the most common cause of monogenic obesity, the clinical significance of MC3R variants is less clear. MC4R localizes to the primary cilium, a sensory organelle present on nearly all human cells. To better understand the pathophysiological mechanisms of MC3R variants, we investigated whether MC3R localizes to the primary cilium and assessed the impact of rare MC3R variants identified in individuals with obesity on ciliary expression. Using human RPE cells, human NGN2-induced iNeurons, and primary mouse hypothalamic neurons, we found that, in contrast to MC4R, neither wild type MC3R nor rare MC3R variants localized specifically to the primary cilium in vitro in any cell type, including hypothalamic neurons. These findings suggest that MC3R and MC4R may utilize distinct signaling pathways or that additional factors, such as accessory proteins, are required for MC3R targeting to primary cilia in vivo. Further studies are needed to clarify the role of MC3R variants in monogenic obesity and their broader implications for human disease. Show less

Melanocortin-2 receptor accessory protein-2 (MRAP2) modulates the activity of hypothalamic melanocortin-4 (MC4R) and growth hormone-secretagogue (GHSR) receptors, which suppress and promote appetite, respectively. We investigate whether obesity-associated variants of MRAP2 alter their ability to modulate MC4R and GHSR signalling as a possible mechanistic link to the development of obesity. Functional effects of five obesity-associated MRAP2 variants were analysed in HEK293 cells by co-expressing wild-type or variant MRAP2 with MC4R or GHSR. Endpoints included cell-surface and total expression, and ligand-induced second-messenger responses, β-arrestin-2 recruitment, and alternative G-protein activation. MRAP2 decreased basal MC4R cell-surface expression while GHSR cell-surface expression was not affected. In MC4R/MRAP2 expressing cells, maximal α-MSH-induced cAMP and β-arrestin-2 recruitment responses were increased. Similarly, ghrelin-induced Ca2+-mobilization in GHSR/MRAP2 expressing cells was increased, but β-arrestin-2 recruitment was suppressed. MRAP2 did not bias G-protein activation by either receptor, although previous reports show MRAP2 biases MC4R signalling towards Gαq/11. The variants did not significantly affect the ability of MRAP2 to modulate MC4R and GHSR signalling. Our results indicate that MRAP2 potentiates the ligand responsiveness of MC4R and GHSR, but has differential effects on β-arrestin-2 recruitment. The MRAP2 variants had no significant effects on the signalling endpoints tested. This suggests that, despite their association with obesity, the variants may be functionally benign, or that the absence of effects reflects limitations inherent to our cellular model. In addition, since MRAP2 can modulate multiple receptors and differentially modulate their signalling, we cannot rule out their influence on body weight regulation via other mechanisms. Show less

The objective of this study was to investigate head circumference (HC) in patients with melanocortin 4 receptor (MC4R) deficiency, the most common cause of monogenetic obesity. Patients with (likely) pathogenic MC4R variants were included. HC, height, and weight were measured, and BMI and standard deviation score (SDS) were calculated. HC SDS was compared to the Dutch reference population. Children were matched 1:1 to a control group with common obesity. Children with MC4R deficiency (n = 63, mean age, 10.32 years) had significantly larger HC (mean, +1.73 SDS) compared to the reference population (0 SDS; p < 0.001) and controls (+1.22 SDS; p = 0.009). In adults (n = 13), HC (median, + 0.86 SDS) did not differ from the reference population (0 SDS; p = 0.152). Macrocephaly (HC ≥ 2 SDS) was present in 43%, 25%, and 23% of pediatric patients with MC4R deficiency, controls, and adult patients, respectively. Children with MC4R deficiency were taller than controls (+1.00 SDS vs. +0.42 SDS; p = 0.016), with similar BMI (+3.99 SDS vs. +3.75 SDS; p = 0.157). HC SDS was associated with height SDS (R Macrocephaly is a common feature of patients with MC4R deficiency. We recommend measuring HC in patients suspected for genetic obesity, as it can be a clue for MC4R deficiency. Show less

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

We sought to assess body mass index trajectories of children with genetic obesity to identify optimal early age of onset of obesity (AoO) cut-offs for genetic screening. This longitudinal, observational study included growth measurements from birth onward of children with nonsyndromic and syndromic genetic obesity and control children with obesity from a population-based cohort. Diagnostic performance of AoO was evaluated. We describe the body mass index trajectories of 62 children with genetic obesity (29 nonsyndromic, 33 syndromic) and 298 controls. Median AoO was 1.2 years in nonsyndromic genetic obesity (0.4 and 0.6 years in biallelic LEPR and MC4R; 1.7 in heterozygous MC4R); 2.0 years in syndromic genetic obesity (0.9, 2.3, 4.3, and 6.8 years in pseudohypoparathyroidism, Bardet-Biedl syndrome, 16p11.2del syndrome, and Temple syndrome, respectively); and 3.8 years in controls. The optimal AoO cut-off was ≤3.9 years (sensitivity, 0.83; specificity, 0.49; area under the curve, 0.79; P < .001) for nonsyndromic and ≤4.7 years (sensitivity, 0.82; specificity, 0.37; area under the curve, 0.68; P = .001) for syndromic genetic obesity. Optimal AoO cut-off as single parameter to determine which children should undergo genetic testing was ≤3.9 years. In case of older AoO, additional features indicative of genetic obesity should be present to warrant genetic testing. Optimal cut-offs might differ across different races and ethnicities. Show less

Leptin receptor (LEPR) deficiency is a rare genetic disorder that affects the body's ability to regulate appetite and weight. For patients and their families, the disorder seriously disrupts daily life; however, little is published about this impact. We here report the experiences of a 10.5-year-old girl with leptin receptor deficiency and her family. The diagnosis of this rare genetic obesity had a deep impact on the life of the child and her family. It led to a better understanding of the cause of the impaired appetite regulation and early-onset obesity with subsequently less judgement by others and improved cooperation of their social network and school on maintaining a healthy lifestyle for this girl. A strict eating regimen and lifestyle measures resulted in the first year after diagnosis in a significantly decreased body mass index (BMI), followed by BMI stabilization, still categorized as obesity class three. However, the troublesome challenge of how to manage the disruptive behaviour due to hyperphagia remained. Eventually, due to treatment with targeted pharmacotherapy, i.e., melanocortin-4 receptor agonists, her BMI continued to decrease due to resolving hyperphagia. The daily routine of the family and the atmosphere at home positively changed as they were no longer dominated by the food-focused behaviour of the child and the adherence to the strict eating regimen. This case report demonstrates the importance and impact of a rare genetic obesity disorder diagnosis in a family. Additionally, it highlights the value of genetic testing in patients with a high suspicion of a genetic obesity disorder as it can eventually lead to personalized treatment, such as guidance by specialized healthcare professionals and educated caregivers or targeted pharmacotherapy. Show less

Pathogenic heterozygous MC4R variants are associated with hyperphagia and variable degrees of obesity. Several research groups have reported short-term weight loss outcomes after bariatric surgery in a few patients with MC4R variants, but lack of longer-term data prevents evidence-based clinical decision-making. Bariatric surgery patients with heterozygous (likely) pathogenic MC4R variants, from three collaborating centers in the Netherlands, France, and the UK, were compared to matched controls (matched 2:1 for age, sex, preoperative BMI, surgical procedure, and diabetes mellitus, but without MC4R mutations). Weight loss and regain outcomes up to 6 years of follow-up were compared. At 60 months of follow-up after RYGB, cases with MC4R variants showed weight regain with a mean of 12.8% (± 10.4 SD) total weight loss (TWL) from nadir, compared to 7.9% (± 10.5 SD) in the controls (p = 0.062). Among patients receiving SG, the cases with MC4R variants experienced inferior weight loss (22.6% TWL) during the first year of follow-up compared to the controls (29.9% TWL) (p = 0.010). This multicenter study reveals inferior mid-term weight outcomes of cases with MC4R variants after SG, compared to RYGB. Since adequate weight loss outcomes were observed after RYGB, this procedure would appear to be an appropriate surgical approach for this group. However, the pattern of weight regain seen in cases with MC4R variants after both RYGB and SG highlights the need for pro-active lifelong management to prevent relapse, as well as careful expectation management. Show less

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. 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. Show less

Mutations in the leptin-melanocortin pathway genes are known to cause monogenic obesity. The prevalence of these gene mutations and their effect on weight loss response after bariatric surgery are still largely unknown. To determine the prevalence of genetic obesity in a large bariatric cohort and evaluate their response to bariatric surgery. Mutation analysis of 52 obesity-associated genes. Patient inclusion criteria were a BMI > 50 kg/m A total of 1014 patients were included, of whom 30 (3%) were diagnosed with genetic obesity, caused by pathogenic heterozygous mutations in either MC4R, POMC, PCSK1, SIM1, or PTEN. The percentage total body weight loss (%TBWL) after Roux-en-Y gastric bypass (RYGB) surgery was not significantly different for patients with a mutation in MC4R, POMC, and PCSK1 compared with patients lacking a molecular diagnosis. Of the confirmed genetic obesity cases, only patients with MC4R mutations receiving a sleeve gastrectomy (SG) showed significantly lower %TBWL compared with patients lacking a molecular diagnosis, during 2 years of follow-up. In this cohort of morbid obese bariatric patients, an estimated prevalence of monogenic obesity of 3% is reported. Among these patients, the clinical effects of heterozygous mutations in POMC and PCSK1 do not interfere with the effectiveness of most commonly performed bariatric procedures within the first 2 years of follow-up. Patients with MC4R mutations achieved superior weight loss after primary RYGB compared with SG. Show less

Obesity is a global and severe health problem. Due to genetic heterogeneity, the identification of genetic defects in patients with obesity can be time consuming and costly. Therefore, we developed a custom diagnostic targeted next-generation sequencing (NGS)-based analysis to simultaneously identify mutations in 52 obesity-related genes. The aim of this study was to assess the diagnostic yield of this approach in patients with suspected genetic obesity. DNA of 1230 patients with obesity (median BMI adults 43.6 kg/m In 48 patients pathogenic mutations confirming the clinical diagnosis were detected. The majority of these were observed in the NGS-based gene panel analysis in patients with obesity led to a definitive diagnosis of a genetic obesity disorder in 3.9% of obese probands, and a possible diagnosis in an additional 5.4% of obese probands. The highest yield was achieved in a selected paediatric subgroup, establishing a definitive diagnosis in 12 out of 164 children with severe early onset obesity (7.3%). These findings give a realistic insight in the diagnostic yield of genetic testing for patients with obesity and could help these patients to receive (future) personalised treatment. Show less