Accessory to Obesity?: Melanocortin receptors are a family of cell membrane receptors that control diverse physiological functions. Mutations in the gene encoding melanocortin 4 receptor (MC4R) are a cause of familial early-onset obesity. Asai et al. (p. 275 ) studied the function of an accessory protein for MC4R signaling, MRAP2, and found that mice genetically deficient in MRAP2 develop severe obesity. Sequencing of MRAP2 in unrelated, severely obese humans revealed one individual with a clearly disruptive genetic variant, suggesting that MRAP2 mutations might also be a rare cause of human obesity. In a zebrafish model, Sebag et al. (p. 278 ) studied two paralogs of the MRAP2 accessory protein, one of which enhanced MC4R responsiveness to α–melanocyte-stimulating hormone, which regulates feeding and growth.

The melanocortin receptor (MCR) family consists of 5 G protein-coupled receptors (MC1R–MC5R) with diverse physiologic roles. MC2R is a critical component of the hypothalamic–pituitary–adrenal axis, whereas MC3R and MC4R have an essential role in energy homeostasis. Mutations in MC4R are the single most common cause of monogenic obesity. Investigating the way in which these receptors signal and traffic to the cell membrane is vital in understanding disease processes related to MCR dysfunction. MRAP is an MC2R accessory protein, responsible for adrenal MC2R trafficking and function. Here we identify MRAP2 as a unique homologue of MRAP, expressed in brain and the adrenal gland. We report that MRAP and MRAP2 can interact with all 5 MCRs. This interaction results in MC2R surface expression and signaling. In contrast, MRAP and MRAP2 can reduce MC1R, MC3R, MC4R, and MC5R responsiveness to [Nle4,D-Phe7]alpha-melanocyte-stimulating hormone (NDP-MSH). Collectively, our data identify MRAP and MRAP2 as unique bidirectional regulators of the MCR family.

Context: Classic ACTH resistance, due to disruption of ACTH signaling, accounts for the majority of cases of familial glucocorticoid deficiency (FGD). Recently FGD cases caused by mutations in the mitochondrial antioxidant, nicotinamide nucleotide transhydrogenase, have highlighted the importance of redox regulation in steroidogenesis. Objective: We hypothesized that other components of mitochondrial antioxidant systems would be good candidates in the etiology of FGD. Design: Whole-exome sequencing was performed on three related patients, and segregation of putative causal variants confirmed by Sanger sequencing of all family members. A TXNRD2-knockdown H295R cell line was created to investigate redox homeostasis. Setting: The study was conducted on patients from three pediatric centers in the United Kingdom. Patients: Seven individuals from a consanguineous Kashmiri kindred, six of whom presented with FGD between 0.1 and 10.8 years, participated in the study. Interventions: There were no interventions. Main Outcome Measure: Identification and functional interrogation of a novel homozygous mutation segregating with the disease trait were measured. Results: A stop gain mutation, p.Y447X in TXNRD2, encoding the mitochondrial selenoprotein thioredoxin reductase 2 (TXNRD2) was identified and segregated with disease in this extended kindred. RT-PCR and Western blotting revealed complete absence of TXNRD2 in patients homozygous for the mutation. TXNRD2 deficiency leads to impaired redox homeostasis in a human adrenocortical cell line. Conclusion: In contrast to the Txnrd2-knockout mouse model, in which embryonic lethality as a consequence of hematopoietic and cardiac defects is described, absence of TXNRD2 in humans leads to glucocorticoid deficiency. This is the first report of a homozygous mutation in any component of the thioredoxin antioxidant system leading to inherited disease in humans.

To eat or not to eat: Melanocortin receptor 4 (MC4R) plays a role in regulating food intake: Its activation by a stimulating hormone inhibits appetite, whereas binding to a natural antagonist promotes appetite. Complementing a recent structure of MC4R in an inactive conformation, Israeli et al. present the structure bound to setmelanotide, a weight-control drug, and its G protein–signaling partner (see the Perspective by Farooqi). This work reveals the mechanism of MC4R activation and explains why setmelanotide acts as a potent agonist, whereas a structurally similar compound, SHU9119, is an inhibitor. The structure also provides insight into the contribution of mutations in MCR4 to weight-regulation disorders. Science , abf7958, this issue p. 808 ; see also abi8942, p. 792

Summary: Context  Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disorder as a result of mutation in genes encoding either the ACTH receptor [melanocortin 2 receptor (MC2R)] or its accessory protein [melanocortin 2 receptor accessory protein (MRAP)]. The disorder is known as FGD type 1 and 2, respectively.Objective  The aim of the study was to compare the phenotype/genotype relationships between FGD 1 and 2.Design and patients  Forty patients with missense MC2R mutations and 22 patients with MRAP mutations were included. Forty‐four of these patients had been referred for genetic screening and 18 were patients published by other authors.Results  The median age at presentation for FGD type 1 was variable at 2·0 years; range 0·02–16 years, and this was associated with unusually tall stature, mean height SDS + 1·75 ± 1·53 (mean ± SD). In contrast, FGD type 2 presented at a much earlier median age (0·08 years; range at birth to 1·6 years) (P < 0·01) and patients were of normal height SDS + 0·12 ± 1·35 (P < 0·001). No differences in baseline cortisol or ACTH levels were seen between FGD types 1 and 2.Conclusion  FGD type 2 appears to present earlier. This may reflect the functional significance of the underlying mutations in that all MRAP mutations are nonsense or splice site mutations that result in abolition of a functional protein, whereas most of the MC2R mutations are missense mutations and give rise to proteins with some residual function. Tall stature is associated with mutations in MC2R but not in MRAP. There were no other significant clinical distinctions between the two.