Melanocortin-4 Receptor Deficiency Research Articles

ANDRE‐MAYER AWARD AW0002 The leptin‐melanocortin pathway in human physiology: insights from genetics

Considerable attention has been paid to the secular changes in food intake and physical activity that underlie the recent rise in the prevalence of obesity. However, there is compelling evidence that inter‐individual differences in susceptibility to obesity have strong genetic determinants. In the past decade, there have been major advances in our understanding of the molecular constituents of the pathways that control mammalian energy homeostasis. We and others have described six human obesity syndromes that arise from genetic defects. We have identified several patients with congenital leptin deficiency. These children are hyperphagic, develop severe disabling obesity, impaired T cell mediated immunity and hypogonadotropic hypogonadism. In a clinical trial of daily subcutaneous injections of recombinant human leptin, sustained, beneficial effects on appetite, fat mass, hyperinsulinaemia and hyperlipidaemia have been observed. Leptin administration also permits the full progression of appropriately timed puberty and reverses impaired T cell mediated immunity. We have recruited over 2000 severely obese children to the Genetics of Obesity Study (GOOS). Using a candidate gene approach, we have identified several loss of function mutations in the melanocortin 4 receptor (MC4R), which cause a dominantly inherited syndrome that accounts for up to 5% of patients with severe, early‐onset obesity, making this the commonest obesity syndrome to date. MC4R deficiency is characterized by hyperphagia, severe hyperinsulinaemia and increased linear growth and there is evidence for a genotype‐phenotype correlation, as complete loss of function mutations result in a more severe phenotype. The characterization of these syndromes and of patients with mutations in the genes encoding the leptin receptor, pro‐opiomelanocortin (POMC) and prohormone convertase ‐1, has provided a better mechanistic understanding of the regulation of appetite and body weight in humans, which will have implications for the treatment of obesity and other metabolic disorders.

Melanocortin-4 Receptor Mediates Chronic Cardiovascular and Metabolic Actions of Leptin

This study tested whether the melanocortin 4-receptor (MC4R) is essential for the chronic cardiovascular and metabolic actions of leptin. Twenty- to 22-week-old male wild-type (WT) C57BL/6J and obese MC4R (-/-) mice (N=5 to 6 per group) were implanted with radiotelemetric transmitters and catheters for measuring mean arterial pressure (MAP) and heart rate 24 hours per day and intravenous infusions. After a 3-day stable control period, leptin was infused (2 microg/kg per minute IV) for 7 days in WT, obese ad libitum-fed MC4R (-/-), and nonobese pair-fed MC4R (-/-) mice. WT mice receiving vehicle for 7 days served as controls. MC4 (-/-) mice were 30% heavier and had 4- and 11-fold increases in plasma insulin and leptin levels, respectively, compared with WT mice. Despite obesity, MAP and heart rate tended to be lower in MC4R (-/-) mice compared with WT mice. Chronic leptin infusion in the different groups increased plasma leptin levels to 45 to 65 ng/mL. Seven-day leptin infusion in WT mice increased MAP by 12+/-3 mm Hg despite a 35% reduction in food intake and an 8% reduction in body weight. Leptin did not alter plasma glucose but reduced plasma insulin in WT mice (5.9+/-1.0 versus 3.0+/-0.5 microU/mL). These cardiovascular and metabolic actions of leptin were abolished in obese and nonobese MC4R (-/-) mice. These data suggest that MC4R deficiency, and not obesity-induced leptin resistance, abolished the cardiovascular and metabolic actions of leptin in obese MC4R (-/-) mice. Thus, a functional MC4R is essential for the chronic cardiovascular and metabolic actions of leptin.

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

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

Age-dependent hypothalamic expression of neuropeptides in wild-type and melanocortin-4 receptor-deficient mice.

In young (35- to 56-day-old) and middle-aged (9-mo-old) wild-type (+/+) and melanocortin-4 receptor (MC4R)-deficient (+/-, -/-) mice, expressions of neuropeptide Y (NPY), agouti-related protein (AGRP), pro-opiomelanocortin (POMC), and cocaine-and-amphetamine-regulated transcript (CART) were analyzed in the arcuate nucleus (ARC) and adjacent regions comprising the dorsomedial (DMN) and ventromedial (VMN) nucleus. In the ARC of young mice, NPY and AGRP expression increased and POMC and CART expression decreased with body fat content. Adjusting for the influence of body fat content by ANCOVA showed that the levels of NPY, POMC, and CART were highest and of AGRP lowest in young -/- mice. In the middle-aged mice, feedback from body fat content was weakened. For -/- mice ANCOVA revealed higher NPY and AGRP, lower POMC, and unchanged CART expression levels relative to young -/- mice. In the DMN and VMN, POMC and AGRP signals were absent at each age. CART was expressed in the DMN independent of age, fat content, and genotype. For NPY expression, an age-dependent induction was found in the DMN and VMN; it was absent in the young but present in the middle-aged mice, showing close positive correlations between body fat content and the numbers of NPY-labeled cells which were further enhanced in -/- mice. Thus MC4R deficiency augments age-induced NPY expression in the DMN and VMN with no feedback from body fat content. Negative feedback control by body fat content on ARC neuropeptide expression is present in young animals but vanishes with age and is modulated by MC4R deficiency.

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

Melanocortin 4 receptor (MC4R) deficiency is the commonest monogenic form of obesity. However, the clinical spectrum and mode of inheritance have not been defined, pathophysiological mechanisms leading to obesity are poorly understood, and there is little information regarding genotype-phenotype correlations. We determined the nucleotide sequence of the MC4R gene in 500 probands with severe childhood obesity. Family studies were undertaken to examine cosegregation of identified mutations with obesity. Subjects with MC4R deficiency underwent metabolic and endocrine evaluation; the results were correlated with the signaling properties of mutant receptors. Twenty-nine probands (5.8 percent) had mutations in MC4R; 23 were heterozygous, and 6 were homozygous. Mutation carriers had severe obesity, increased lean mass, increased linear growth, hyperphagia, and severe hyperinsulinemia; homozygotes were more severely affected than heterozygotes. Subjects with mutations retaining residual signaling capacity had a less severe phenotype. Mutations in MC4R result in a distinct obesity syndrome that is inherited in a codominant manner. Mutations leading to complete loss of function are associated with a more severe phenotype. The correlation between the signaling properties of these mutant receptors and energy intake emphasizes the key role of this receptor in the control of eating behavior in humans.