Hereditary isolated glucocorticoid deficiency is associated with abnormalities of the adrenocorticotropin receptor gene

Modulating vertebrate physiology by genomic fine-tuning of GPCR functions

G protein-coupled receptors (GPCRs) play a crucial role as membrane receptors, facilitating the communication of eukaryotic species with their environment and regulating cellular and organ interactions. Consequently, GPCRs hold immense potential in contributing to adaptation to ecological niches and responding to environmental shifts. Comparative analyses of vertebrate genomes reveal patterns of GPCR gene loss, expansion, and signatures of selection. Integrating these genomic data with insights from functional analyses of gene variants enables the interpretation of genotype-phenotype correlations. This review underscores the involvement of GPCRs in adaptive processes, presenting numerous examples of how alterations in GPCR functionality influence vertebrate physiology or, conversely, how environmental changes impact GPCR functions. The findings demonstrate that modifications in GPCR function contribute to adapting to aquatic, arid, and nocturnal habitats, influencing camouflage strategies, and specializing in particular dietary preferences. Furthermore, the adaptability of GPCR functions provides an effective mechanism in facilitating past, recent, or ongoing adaptations in animal domestication and human evolution and should be considered in therapeutic strategies and drug development.

Familial Glucocorticoid Deficiency: the changing landscape of an eponymous syndrome

Familial Glucocorticoid Deficiency encompasses a broad spectrum of monogenic recessive disorders that theoretically solely abrogate cortisol biosynthesis. In reality, delineating clear genotype-phenotype correlations in this disorder is made complicated by marked phenotypic heterogeneity even within kindreds harbouring identical variants. Phenotypes range from isolated glucocorticoid insufficiency to cortisol deficiency plus a variety of superimposed features including salt-wasting and hypoaldosteronism, primary hypothyroidism, hypogonadism and growth defects. Furthermore, mutation type, domain topology and perceived enzyme activity do not always predict disease severity. Given the high burden of disease and implications of a positive diagnosis, genetic testing is crucial in the management of patients warranting detailed delineation of genomic variants including viable functional studies.

A Novel Mutation in Melanocortin Receptor 2 and a Reported Mutation in Melanocortin Receptor 2 Accessory Protein: Three Chinese Cases with Familial Glucocorticoid Deficiency

Background

Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disease characterized by glucocorticoid deficiency without mineralocorticoid deficiency. We report 3 Chinese patients with MRAP or MC2R mutations.

Case Reports

Patient 1 presented with hyperpigmentation. Endocrine investigations revealed low serum cortisol levels and elevated adrenocorticotropic hormone (ACTH) levels. Furthermore, low serum sodium was evident. She was diagnosed with FGD type 2 due to a homozygous mutation in MRAP (c.106+1delG), revealed through exome sequencing (ES). After 2-year treatment with hydrocortisone, skin hyperpigmentation was improved. Patient 2 initially presented with hyponatremia. Low cortisol levels and high levels of ACTH were subsequently detected; he was subjected to a hydrocortisone treatment during which he experienced repeated hypoglycemic attacks and pigmentation. ES revealed the same mutation as in patient 1 in MRAP (c.106+1delG), thus he was diagnosed with FGD type 2. After 6 years of age, his symptoms remarkably improved, and there was no episode of hypoglycemia. Patient 3 mainly presented with hyperpigmentation, hypoglycemic attack, and tall stature. Laboratory findings were normal except for low serum cortisol levels and high ACTH levels. She was diagnosed with FGD type 1 as ES revealed a novel homozygous mutation in MC2R (c.712C>A, p.His238Tyr). After nearly 2 years of hydrocortisone replacement therapy, the excessive growth was reduced to near normal, and the skin color returned to normal.

Conclusions

Three patients were diagnosed with FGD (one with FGD type 1 and two with FGD type 2). They all presented with hyperpigmentation and hypoglycemia; however, compared with patient 1, the clinical manifestations of patient 2 were more complicated. Patient 3 had later onset and taller stature than patients 1 and 2. A novel mutation in patient 3 expands the mutation spectrum of MC2R.

History of Adrenal Research: From Ancient Anatomy to Contemporary Molecular Biology

The adrenal is a small, anatomically unimposing structure that escaped scientific notice until 1564 and whose existence was doubted by many until the 18th century. Adrenal functions were inferred from the adrenal insufficiency syndrome described by Addison and from the obesity and virilization that accompanied many adrenal malignancies, but early physiologists sometimes confused the roles of the cortex and medulla. Medullary epinephrine was the first hormone to be isolated (in 1901), and numerous cortical steroids were isolated between 1930 and 1949. The treatment of arthritis, Addison's disease, and congenital adrenal hyperplasia (CAH) with cortisone in the 1950s revolutionized clinical endocrinology and steroid research. Cases of CAH had been reported in the 19th century, but a defect in 21-hydroxylation in CAH was not identified until 1957. Other forms of CAH, including deficiencies of 3β-hydroxysteroid dehydrogenase, 11β-hydroxylase, and 17α-hydroxylase were defined hormonally in the 1960s. Cytochrome P450 enzymes were described in 1962-1964, and steroid 21-hydroxylation was the first biosynthetic activity associated with a P450. Understanding of the genetic and biochemical bases of these disorders advanced rapidly from 1984 to 2004. The cloning of genes for steroidogenic enzymes and related factors revealed many mutations causing known diseases and facilitated the discovery of new disorders. Genetics and cell biology have replaced steroid chemistry as the key disciplines for understanding and teaching steroidogenesis and its disorders.

A Novel Homozygous MC2R Variant Leading to Type-1 Familial Glucocorticoid Deficiency

Context

Type 1 familial glucocorticoid deficiency (FGD) (OMIM #607397) is a rare autosomal recessive disorder due to mutations in melanocortin-2-receptor (MC2R) gene encoding the G protein-coupled adrenocorticotropic (ACTH) transmembrane receptor.

Objective

The aim of the study is to describe 2 siblings born to a healthy consanguineous family presenting with clinical and biochemical features of FGD, harboring a novel homozygous MC2R variant.

Methods

Both patients are siblings born at term via normal delivery with normal birth weights. The first sibling presented with symptoms of hypoglycemia, repeated episodes of infections starting from 2 days of age. At 18 months of age, low serum cortisol was found, and he was started on hydrocortisone replacement therapy. The second sibling developed hypoglycemia on day 1 after birth, investigations revealed low serum sodium and cortisol levels and was also commenced on hydrocortisone treatment. Whole exome sequencing (WES) and in vitro functional studies on cell line transfected with wild-type and mutant plasmid clones were undertaken.

Results

WES revealed a novel homozygous missense mutation c.326T>A, p.Leu109Gln in the MC2R gene. In-silico prediction tools predicted the effect of this mutation to be deleterious. In vitro study using HEK293 cells transfected with MC2R wild-type and mutant clones showed a defect in protein expression and cAMP generation when stimulated with ACTH.

Conclusion

Homozygous semiconserved p.Leu109Gln mutation disrupts cAMP production and MC2R protein expression leading to ACTH resistance. This study provides additional evidence that this novel pathogenic variant in MC2R results in FGD phenotypes.

Novel Melano-Cortin-2-Receptor Gene Mutation Presenting With Infantile Cholestasis: A Case Report

Introduction

For many years, congenital panhypopituitarism has been recognized to cause infantile cholestasis. However, the isolated cortisol deficiency as a cause of cholestasis and liver failure was rarely reported.

Case description

A 32-days old male infant presented to the hepatology clinic with infantile cholestasis. His initial workup revealed alanine transaminase (ALT) level of 138 U/L, aspartate transaminase level of 76 U/L, total bilirubin (T.Bil) of 103 mmol/L, direct bilirubin of (D.Bil) 83 mmol/L, gamma-glutamyl transpeptidase (GGT) level of 28 U/L with normal prothrombin time (PT) of 13 seconds. One week later, the patient developed severe bronchiolitis necessitating mechanical ventilation associated with acute liver failure and worsening cholestasis. His ALT increased to 303.5 U/L and direct bilirubin increased to 204 mmol/L with prolongation of PT to 18.9 seconds reflecting derangement in synthetic liver functions. There was associated hypoglycemia, hyponatremia and high normal potassium level with a picture of adrenal insufficiency. Hormonal workup and genetic testing revealed isolated cortisol deficiency with a novel homozygous mutation c.763_764delAT (p. Met255ValfsX17) in Melanocortin 2 receptor gene (MC2R) and the patient was diagnosed as familial primary glucocorticoid deficiency. The patient was maintained on cortisol replacement therapy with the resolution of cholestasis and normalization of liver functions.

Conclusions

Patients presenting with infantile cholestasis associated with documented hypoglycemia should alert pediatricians about the possibility of familial glucocorticoid deficiency and prompt investigation of adrenal function should be considered. Cortisol replacement therapy leads to the resolution of cholestasis.

Discovery of MIRAGE syndrome

Since the first report in 2009, whole exome sequencing has become the most effective and efficient research tool in human genetics. MIRAGE syndrome is a novel single-gene disorder discovered through whole-exome sequencing for pediatric patients with adrenal insufficiency of unknown etiology, and is caused by de novo heterozygous variants in SAMD9. MIRAGE syndrome was initially discovered as a systemic disease affecting multiple systems, including hematopoietic, immune, endocrine, and gastrointestinal systems but later studies revealed a subset of patients with myelodysplastic syndrome as the sole manifestation. In addition, pathogenic variants in SAMD9L, a paralog gene of SAMD9, were reported to cause an inherited disorder of the hematopoietic system and central nervous system, called ataxia-pancytopenia syndrome. This article reviews the history of MIRAGE syndrome from its discovery to the proposal of SAMD9/SAMD9L syndromes, and discusses directions for future research.

Mutations in G Protein-Coupled Receptors: Mechanisms, Pathophysiology and Potential Therapeutic Approaches

There are approximately 800 annotated G protein-coupled receptor (GPCR) genes, making these membrane receptors members of the most abundant gene family in the human genome. Besides being involved in manifold physiologic functions and serving as important pharmacotherapeutic targets, mutations in 55 GPCR genes cause about 66 inherited monogenic diseases in humans. Alterations of nine GPCR genes are causatively involved in inherited digenic diseases. In addition to classic gain- and loss-of-function variants, other aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, pseudogenes, gene fusion, and gene dosage, contribute to the repertoire of GPCR dysfunctions. However, the spectrum of alterations and GPCR involvement is probably much larger because an additional 91 GPCR genes contain homozygous or hemizygous loss-of-function mutations in human individuals with currently unidentified phenotypes. This review highlights the complexity of genomic alteration of GPCR genes as well as their functional consequences and discusses derived therapeutic approaches. SIGNIFICANCE STATEMENT: With the advent of new transgenic and sequencing technologies, the number of monogenic diseases related to G protein-coupled receptor (GPCR) mutants has significantly increased, and our understanding of the functional impact of certain kinds of mutations has substantially improved. Besides the classical gain- and loss-of-function alterations, additional aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, uniparental disomy, pseudogenes, gene fusion, and gene dosage, need to be elaborated in light of GPCR dysfunctions and possible therapeutic strategies.

The wisdom and eudaimonia of Paediatrics: An interview with Professor George P. Chrousos, Professor of Paediatrics and Endocrinology at the University of Athens, Greece

Professor George P. Chrousos, Professor Emeritus of Paediatrics and Endocrinology at the University of Athens School of Medicine, in Athens, Greece, is the most distinguished Professor of Paediatrics in the modern history of Hellenic Paediatrics and among the 100 most prominent clinical investigators worldwide. According to Professor Chrousos, viruses frequently interact with the host endocrine signaling pathways, while stress triggers acutely an early inflammatory response termed ‘neurogenic inflammation’. Soon after, however, stress suppresses innate immunity and causes a shift from T-helper 1- to T-helper 2- driven immunity. The natural history of the response to a viral infection is for immune and stress changes to take place in a highly coordinated process that results in the full return to the basal health state. He believes that over the past decades, our armamentarium against viruses has increased significantly as novel anti-viral agents, monoclonal antibodies and vaccines, have been and are continually being developed. Professor Chrousos declares that Paediatrics is a very broad scientific field, where paediatric trainees have many avenues to follow beyond clinical practice and into basic, preclinical, translational, clinical, applied, or epidemiologic research. He supports that researching and producing new knowledge to the benefit of humanity is a product of practicing Aristotle's ancient Greek virtues and a worthy cause of life's meaning. He completes our interview calling young paediatricians to ‘listen to their minds and hearts’ to select a life course that would lead them to acquire personal wisdom and eudaimonia.

Stability and Turnover of the ACTH Receptor Complex

Glucocorticoid production in mammals is principally regulated by the action of the pituitary hormone adrenocorticotropin (ACTH) acting on its cognate membrane receptor on the zona fasciculata cells of the adrenal cortex. The receptor for ACTH consists of two essential components, a small seven transmembrane domain G protein-coupled receptor of the melanocortin receptor subgroup known as the melanocortin 2 receptor (MC2R) and a small single transmembrane domain protein that adopts a antiparallel homodimeric form and which is known as the melanocortin 2 receptor accessory protein (MRAP). MRAP is essential for the trafficking of the MC2R to the cell surface as well as being required for receptor responsiveness to ACTH at physiological concentrations-probably by facilitating ACTH binding, but possibly also by supporting G protein interaction with the MC2R. A number of studies have shown that ACTH stimulates the expression of functional receptor at the cell surface and the transcription of both MC2R and MRAP mRNA. However, the time course of these transcriptional effects differs such that MRAP is expressed relatively rapidly whereas MC2R transcription responds much more slowly. Furthermore, recent data suggests that MRAP protein is turned over with a short half-life whereas MC2R has a significantly longer half-life. These findings imply that these two ACTH receptor proteins have distinct trajectories and that it is likely that MRAP-independent MC2R is present at the cell surface. In such a situation newly transcribed and translated MRAP could enable the rapid recruitment of functional receptor at the plasma membrane without the need for new MC2R translation. This may be advantageous in circumstances of significant stress in that the potentially complex and perhaps inefficient process of de novo MC2R translation, folding, post-translational modification and trafficking can be avoided.

Novel Melanocortin 2 Receptor Variant in a Chinese Infant With Familial Glucocorticoid Deficiency Type 1, Case Report and Review of Literature

Familial glucocorticoid deficiency type 1 (FGD1) is an autosomal recessive disorder caused by mutations in the melanocortin 2 receptor (MC2R) gene, characterized by a low or undetectable serum cortisol level and a high adrenocorticotropic hormone (ACTH) level. Clinical manifestations include hypoglycemia, seizure, skin hyperpigmentation, hyperbilirubinemia, cholestasis, and a tall stature. Some dysmorphic features such as, a prominent forehead, hypertelorism, a broad nasal bridge, and small tapering fingers, have been reported. Children with FGD1 may have other isolated endocrine abnormalities. To date, no patient with FGD1 has been reported in mainland China. Here we report on a Chinese patient with FGD1 having a novel MC2R gene variant, a mild transverse palm crease, hypertelorism, and subtle/transient endocrine abnormalities relating to all three zones of the adrenal cortex and thyroid gland. We also reviewed cases with dysmorphic features or additional endocrine abnormalities.

Rare monogenic causes of primary adrenal insufficiency

PURPOSE OF REVIEW

Monogenic disorders play significant roles in the pathogenesis of childhood-onset primary adrenal insufficiency (PAI). The most common form of PAI is congenital adrenal hyperplasia (CAH), which includes the enzymatic defects of the steroidogenic pathway. This review focuses on less common forms of monogenic PAI (i.e. non-CAH monogenic PAI) with particular attention on their cause, clinical phenotypes and genetic epidemiology.

RECENT FINDINGS

Non-CAH monogenic PAI can be classified into three major categories: first, adrenocorticotropic hormone resistance, second, impaired adrenal redox homeostasis and third, defective organogenesis of the adrenal glands. The clinical phenotypes of the mutation-carrying patients vary depending on the responsible gene, and they are partially explained by the tissue RNA expression patterns. Genetic epidemiology studies conducted in Turkey and Japan showed that about 80% of PAI of unknown cause was monogenic.

SUMMARY

Genetic basis of non-CAH monogenic PAI had been less clearly understood than CAH; however, significant advances have been made with use of new research techniques such as next-generation sequencing. Understanding of these rare forms of PAI may contribute to clarifying the physiology and pathology of the adrenal glands.

MECHANISMS IN ENDOCRINOLOGY: Update on pathogenesis of primary adrenal insufficiency: beyond steroid enzyme deficiency and autoimmune adrenal destruction

Primary adrenal insufficiency (PAI) is potentially life threatening, but rare. In children, genetic defects prevail whereas adults suffer more often from acquired forms of PAI. The spectrum of genetic defects has increased in recent years with the use of next-generation sequencing methods and now has reached far beyond genetic defects in all known enzymes of adrenal steroidogenesis. Cofactor disorders such as P450 oxidoreductase (POR) deficiency manifesting as a complex form of congenital adrenal hyperplasia with a broad clinical phenotype have come to the fore. In patients with isolated familial glucocorticoid deficiency (FGD), in which no mutations in the genes for the ACTH receptor (MC2R) or its accessory protein MRAP have been found, non-classic steroidogenic acute regulatory protein (StAR) and CYP11A1 mutations have been described; and more recently novel mutations in genes such as nicotinamide nucleotide transhydrogenase (NNT) and thioredoxin reductase 2 (TRXR2) involved in the maintenance of the mitochondrial redox potential and generation of NADPH important for steroidogenesis and ROS detoxication have been discovered. In addition, whole exome sequencing approach also solved the genetics of some syndromic forms of PAI including IMAGe syndrome (CDKN1C), Irish traveler syndrome (MCM4), MIRAGE syndrome (SAMD9); and most recently a syndrome combining FGD with steroid-resistant nephrotic syndrome and ichthyosis caused by mutations in the gene for sphingosine-1-phosphate lyase 1 (SGPL1). This review intends do give an update on novel genetic forms of PAI and their suggested mechanism of disease. It also advocates for advanced genetic work-up of PAI (especially in children) to reach a specific diagnosis for better counseling and treatment.

Molecular signatures of human melanocortin receptors for ligand binding and signaling

Human melanocortin receptors (hMCRs) belong to the seven-transmembrane (TM) domain proteins. There are five hMCR subtypes and each of these receptor subtypes has different patterns of tissue expression and physiological function. The endogenous agonists for hMCRs are α-, β-, and γ-MSH and ACTH and endogenous antagonists are Agouti and AGRP which are the only known naturally occurring antagonists for the receptors. These peptides have their own profiles regarding the relative potency for specific hMCR subtype. Extensive studies have been performed to examine the molecular basis of the hMCRs for different ligand binding affinity and potency. Studies indicate that natural ligand α-MSH utilizes conserved amino acid residues for MCR specific binding (orthosteric binding) while synthetic ligands utilize non-conserved amino acid residues for receptor subtype specific binding (allosteric binding). ACTH is the only endogenous agonist for hMC2R and more amino acid residues at hMC2R are required for ACTH binding and signaling. HMCR computer modeling provides the detailed information of ligand and MCR interaction. This review provides the latest understanding of the molecular basis of the hMCRs for ligand binding and signaling. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?

Coincidentally, the release of this Research Topic in Frontiers in Endocrinology takes place 25 years after the discovery of the adrenocorticotropic hormone receptor (ACTHR) by Mountjoy and colleagues. In subsequent years, following the discovery of other types of mammalian melanocortin receptors (MCRs), ACTHR also became known as melanocortin type 2 receptor (MC2R). At present, five types of MCRs have been reported, all of which share significant sequence similarity at the amino acid level, and all of which specifically bind melanocortins (MCs)-a group of biologically active peptides generated by proteolysis of the proopiomelanocortin precursor. All MCs share an identical -H-F-R-W- pharmacophore sequence. α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) are the most extensively studied MCs and are derived from the same region. Essentially, α-MSH is formed from the first 13 amino acid residues of ACTH. ACTHR is unique among MCRs because it binds one sole ligand-ACTH, which makes it a very attractive research object for molecular pharmacologists. However, much research has failed, and functional studies of this receptor are lagging behind other MCRs. The reason for these difficulties has already been outlined by Mountjoy and colleagues in their publication on ACTHR coding sequence discovery where the Cloudman S91 melanoma cell line was used for receptor expression because it was a "more sensitive assay system." Subsequent work showed that ACTHR could be successfully expressed only in endogenous MCR-expressing cell lines, since in other cell lines it is retained within the endoplasmic reticulum. The resolution of this methodological problem came in 2005 with the discovery of melanocortin receptor accessory protein, which is required for the formation of functionally active ACTHR. The decade that followed this discovery was filled with exciting research that provided insight into the molecular mechanisms underlying the action of ACTHR. The purpose of this review is to summarize the advances in this fascinating research field.

Diagnosis and management of pediatric adrenal insufficiency

BACKGROUND

Adrenal insufficiency (AI) is a wellknown cause of potentially life-threatening disorders. Defects at each level of the hypothalamic-pituitary-adrenal axis can impair adrenal function, leading to varying degrees of glucocorticoid (GC) deficiency. Iatrogenic AI induced by exogenous GCs is the most common cause of AI. The criteria for the diagnosis and management of iatrogenic AI, neonatal AI, and critical illness-related corticosteroid insufficiency (CIRCI) are not clear.

DATA SOURCES

We reviewed the recent original publications and classical data from the literature, as well as the clinical, diagnostic and management strategies of pediatric AI.

RESULTS

Practical points in the diagnosis and management of AI with an emphasis on iatrogenic AI, neonatal AI, and CIRCI are provided. Given the lack of sensitive and practical biochemical tests for diagnosis of subtle AI, GC treatment has to be tailored to highly suggestive clinical symptoms and signs. Treatment of adrenal crisis is well standardized and patients almost invariably respond well to therapy. It is mainly the delay in treatment that is responsible for mortality in adrenal crisis.

CONCLUSIONS

Education of patients and health care professionals is mandatory for timely interventions for patients with adrenal crisis.

G protein-coupled receptor mutations and human genetic disease

Genetic variations in G protein-coupled receptor genes (GPCRs) disrupt GPCR function in a wide variety of human genetic diseases. In vitro strategies and animal models have been used to identify the molecular pathologies underlying naturally occurring GPCR mutations. Inactive, overactive, or constitutively active receptors have been identified that result in pathology. These receptor variants may alter ligand binding, G protein coupling, receptor desensitization and receptor recycling. Receptor systems discussed include rhodopsin, thyrotropin, parathyroid hormone, melanocortin, follicle-stimulating hormone (FSH), luteinizing hormone, gonadotropin-releasing hormone (GNRHR), adrenocorticotropic hormone, vasopressin, endothelin-β, purinergic, and the G protein associated with asthma (GPRA or neuropeptide S receptor 1 (NPSR1)). The role of activating and inactivating calcium-sensing receptor (CaSR) mutations is discussed in detail with respect to familial hypocalciuric hypercalcemia (FHH) and autosomal dominant hypocalemia (ADH). The CASR mutations have been associated with epilepsy. Diseases caused by the genetic disruption of GPCR functions are discussed in the context of their potential to be selectively targeted by drugs that rescue altered receptors. Examples of drugs developed as a result of targeting GPCRs mutated in disease include: calcimimetics and calcilytics, therapeutics targeting melanocortin receptors in obesity, interventions that alter GNRHR loss from the cell surface in idiopathic hypogonadotropic hypogonadism and novel drugs that might rescue the P2RY12 receptor congenital bleeding phenotype. De-orphanization projects have identified novel disease-associated receptors, such as NPSR1 and GPR35. The identification of variants in these receptors provides genetic reagents useful in drug screens. Discussion of the variety of GPCRs that are disrupted in monogenic Mendelian disorders provides the basis for examining the significance of common pharmacogenetic variants.

Regulation of GPCR Anterograde Trafficking by Molecular Chaperones and Motifs

G protein-coupled receptors (GPCRs) make up a superfamily of integral membrane proteins that respond to a wide variety of extracellular stimuli, giving them an important role in cell function and survival. They have also proven to be valuable targets in the fight against various diseases. As such, GPCR signal regulation has received considerable attention over the last few decades. With the amplitude of signaling being determined in large part by receptor density at the plasma membrane, several endogenous mechanisms for modulating GPCR expression at the cell surface have come to light. It has been shown that cell surface expression is determined by both exocytic and endocytic processes. However, the body of knowledge surrounding GPCR trafficking from the endoplasmic reticulum to the plasma membrane, commonly known as anterograde trafficking, has considerable room for growth. We focus here on the current paradigms of anterograde GPCR trafficking. We will discuss the regulatory role of both the general and "nonclassical private" chaperone systems in GPCR trafficking as well as conserved motifs that serve as modulators of GPCR export from the endoplasmic reticulum and Golgi apparatus. Together, these topics summarize some of the known mechanisms by which the cell regulates anterograde GPCR trafficking.

Third transmembrane domain of the adrenocorticotropic receptor is critical for ligand selectivity and potency

The ACTH receptor, known as the melanocortin-2 receptor (MC2R), plays an important role in regulating and maintaining adrenocortical function. MC2R is a subtype of the melanocortin receptor (MCR) family and has unique characteristics among MCRs. Endogenous ACTH is the only endogenous agonist for MC2R, whereas the melanocortin peptides α-, β-, and γ-melanocyte-stimulating hormone and ACTH are full agonists for all other MCRs. In this study, we examined the molecular basis of MC2R responsible for ligand selectivity using ACTH analogs and MC2R mutagenesis. Our results indicate that substitution of Phe(7) with D-Phe or D-naphthylalanine (D-Nal(2')) in ACTH(1-24) caused a significant decrease in ligand binding affinity and potency. Substitution of Phe(7) with D-Nal(2') in ACTH(1-24) did not switch the ligand from agonist to antagonist at MC2R, which was observed in MC3R and MC4R. Substitution of Phe(7) with D-Phe(7) in ACTH(1-17) resulted in the loss of ligand binding and activity. Molecular analysis of MC2R indicated that only mutation of the third transmembrane domain of MC2R resulted in a decrease in D-Phe ACTH binding affinity and potency. Our results suggest that Phe(7) in ACTH plays an important role in ligand selectivity and that the third transmembrane domain of MC2R is crucial for ACTH selectivity and potency.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

Development of the HPA axis: where and when do sex differences manifest?

Sex differences in the response to stress contribute to sex differences in somatic, neurological, and psychiatric diseases. Despite a growing literature on the mechanisms that mediate sex differences in the stress response, the ontogeny of these differences has not been comprehensively reviewed. This review focuses on the development of the hypothalamic-pituitary-adrenal (HPA) axis, a key component of the body's response to stress, and examines the critical points of divergence during development between males and females. Insight gained from animal models and clinical studies are presented to fully illustrate the current state of knowledge regarding sex differences in response to stress over development. An appreciation for the developmental timelines of the components of the HPA axis will provide a foundation for future areas of study by highlighting both what is known and calling attention to areas in which sex differences in the development of the HPA axis have been understudied.

Modeling the interactions between MC2R and ACTH models from human

Melanocortin system is composed of four peptide hormones namely α-, β-, -γ, and adrenocorticotropic hormone (ACTH), derived from post-translational cleavage of a polypeptide precursor 'proopiomelanocortin (POMC).' Among these hormones, ACTH, a 38 amino acid residue peptide fragment is an important hormone as it is involved in steroid secretion. In addition to this, to cite a few, this hormone is also known to induce variety of other effects, such as alterations in motor/sexual behavior, improvement in memory, and anti-inflammatory effects. To date, five melanocortin receptors (MC1R-MC5R) have been characterized with tissue-specific expression patterns and different binding affinities for each of the melanocortin hormones to regulate various biological functions. In the present work, three-dimensional (3D) models of MC2R and ACTH from human have been predicted, followed by docking and molecular dynamics simulation. While the 3D model of MC2R receptor has been predicted through threading approach, structure of ACTH was built based on ab initio technique. The MC2R model was later successfully docked onto the ACTH structure. Molecular dynamics (MD) simulation for 20 ns was used to compute the binding free energy of MC2R with ACTH model under implicit solvent conditions.

Structural determinants regulating cell surface targeting of melanocortin receptors

Melanocortin receptors (MCRs) belong to the G-protein-coupled receptor family of transmembrane proteins. They recognize specific ligands named melanocortins that are mainly produced in the pituitary and hypothalamus. Newly synthesized MCRs at the endoplasmic reticulum are subjected to quality control mechanisms that screen for the correct structure, folding or processing, essential for their proper cell surface expression. Some motifs, located at the N- or C-terminus or even on transmembrane and in loop regions, have been implicated in these biological processes. This article reviews these specific domains and the role of accessory proteins and post-translation modifications in MCRs' targeting to cell surface. Additionally, promising approaches involving pharmacological stabilization of misfolded and misrouted mutant MCRs, which improve their forward transport, are reported. Understanding the MCRs' structural determinants fundamental for their proper cell surface integration is essential for correcting abnormalities found in some diseases.

The melanocortin receptors and their accessory proteins

The five melanocortin receptors (MCRs) named MC1R-MC5R have diverse physiological roles encompassing pigmentation, steroidogenesis, energy homeostasis and feeding behavior as well as exocrine function. Since their identification almost 20 years ago much has been learnt about these receptors. As well as interacting with their endogenous ligands the melanocortin peptides, there is now a growing list of important peptides that can modulate the way these receptors signal, acting as agonists, antagonists, and inverse agonists. The discovery of melanocortin 2 receptor accessory proteins as a novel accessory factor to the MCRs provides further insight into the regulation of these important G protein-coupled receptor.

Neonatal presentation of familial glucocorticoid deficiency resulting from a novel splice mutation in the melanocortin 2 receptor accessory protein

BACKGROUND

Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disorder characterised by isolated glucocorticoid deficiency. Mutations in the ACTH receptor/melanocortin 2 receptor (MC2R), the MC2R accessory protein (MRAP) or the STAR protein (STAR) cause FGD types 1, 2 and 3, respectively, accounting for ~50% of all cases.

PATIENT AND METHODS

We report a neonate of Indian origin, who was diagnosed with FGD in the first few days of life. He presented with hypoglycaemic seizures and was noted to have generalised intense hyperpigmentation and normal male genitalia. Biochemical investigations revealed hypocortisolaemia (cortisol 0.223 μg/dl; NR 1-23 μg/dl) and elevated plasma ACTH (170 pg/ml). Serum electrolytes, aldosterone and plasma renin activity were normal. Peak cortisol following a standard synacthen test was 0.018 μg/dl. He responded to hydrocortisone treatment and continues on replacement. Patient DNA was analysed by direct sequencing. The effect of the novel mutation was assessed by an in vitro splicing assay using wild type and mutant heterologous minigenes.

RESULTS

A novel homozygous mutation c.106+2_3dupTA was found in the MRAP gene. Both parents were heterozygous for the mutation. In an in vitro splicing assay, the mutation resulted in the skipping of exon 3.

CONCLUSION

We have identified a novel MRAP mutation where disruption of the intron 3 splice-site results in a prematurely terminated translation product. This protein (if produced) would lack the transmembrane domain that is essential for MC2R interaction. We predict that this would cause complete lack of ACTH response thus explaining the early presentation in this case.

The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders

Steroidogenesis entails processes by which cholesterol is converted to biologically active steroid hormones. Whereas most endocrine texts discuss adrenal, ovarian, testicular, placental, and other steroidogenic processes in a gland-specific fashion, steroidogenesis is better understood as a single process that is repeated in each gland with cell-type-specific variations on a single theme. Thus, understanding steroidogenesis is rooted in an understanding of the biochemistry of the various steroidogenic enzymes and cofactors and the genes that encode them. The first and rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone by a single enzyme, P450scc (CYP11A1), but this enzymatically complex step is subject to multiple regulatory mechanisms, yielding finely tuned quantitative regulation. Qualitative regulation determining the type of steroid to be produced is mediated by many enzymes and cofactors. Steroidogenic enzymes fall into two groups: cytochrome P450 enzymes and hydroxysteroid dehydrogenases. A cytochrome P450 may be either type 1 (in mitochondria) or type 2 (in endoplasmic reticulum), and a hydroxysteroid dehydrogenase may belong to either the aldo-keto reductase or short-chain dehydrogenase/reductase families. The activities of these enzymes are modulated by posttranslational modifications and by cofactors, especially electron-donating redox partners. The elucidation of the precise roles of these various enzymes and cofactors has been greatly facilitated by identifying the genetic bases of rare disorders of steroidogenesis. Some enzymes not principally involved in steroidogenesis may also catalyze extraglandular steroidogenesis, modulating the phenotype expected to result from some mutations. Understanding steroidogenesis is of fundamental importance to understanding disorders of sexual differentiation, reproduction, fertility, hypertension, obesity, and physiological homeostasis.

Modulation of the central melanocortin system by leptin, insulin, and serotonin: co-ordinated actions in a dispersed neuronal network

Over the past century, prevalent models of energy and glucose homeostasis have been developed from a better understanding of the neural circuits underlying obesity and diabetes. From the early hypothalamic lesion reports to the more recent pharmacological and molecular/genetic studies, the hypothalamic melanocortin system has been shown to play a critical role in the regulation of metabolism. This review attempts to highlight contributions to our current understanding of how numerous neuromodulators (leptin, insulin, and serotonin) integrate with the central melanocortin system to coordinate alterations in energy and glucose balance.

Effects of melanocortins on adrenal gland physiology

The melanocortin-2-receptor (MC(2) receptor), also known as the ACTH receptor, is a critical component of the hypothalamic-pituitary-adrenal axis. The importance of MC(2) receptor in adrenal physiology is exemplified by the condition familial glucocorticoid deficiency, a potentially fatal disease characterised by isolated cortisol deficiency. MC(2)receptor mutations cause ~25% of cases. The discovery of a MC(2) receptor accessory protein MRAP, mutations of which account for ~15%-20% of familial glucocorticoid deficiency, has provided insight into MC(2) receptor trafficking and signalling. MRAP is essential for the functional expression of MC(2) receptor. MRAP2, a novel homolog of MRAP, can also facilitate MC(2) receptor cell surface expression and function. Like MRAP, MRAP2 is a small transmembrane domain glycoprotein capable of homodimerising. In addition, MRAP/MRAP2 can heterodimerise. The presence of MRAP2 adrenal expression suggests a possible role for MRAP2 in adrenal physiology, which has yet to be elucidated. Importantly, new data shows that the MRAPs can interact with all the other melanocortin receptors (MC(1,3,4,5) receptor). In contrast to MC(2) receptor, this interaction results in reduced melanocortin receptor surface expression and signalling. MRAP2 is predominantly expressed in brain. Hypothalamic expression has been demonstrated for both MRAP and MRAP2. The ability of MRAPs to modulate different members of the melanocortin receptor family in a bidirectional manner is intriguing. Furthermore, central nervous system expression of MRAPs points to a role beyond MC(2) receptor mediated adrenal steroidogenesis.

Physiological roles of the melanocortin MC₃ receptor

The melanocortin MC(3) receptor remains the most enigmatic of the melanocortin receptors with regard to its physiological functions. The receptor is expressed both in the CNS and in multiple tissues in the periphery. It appears to be an inhibitory autoreceptor on proopiomelanocortin neurons, yet global deletion of the receptor causes an obesity syndrome. Knockout of the receptor increases adipose mass without a readily measurable increase in food intake or decrease in energy expenditure. And finally, no melanocortin MC(3) receptor null humans have been identified and associations between variant alleles of the melanocortin MC(3) receptor and diseases remain controversial, so the physiological role of the receptor in humans remains to be determined.

A novel mutation in the MC2R gene causing familial glucocorticoid deficiency type 1

Familial glucocorticoid deficiency (FGD) or hereditary unresponsiveness to adrenocorticotropin (ACTH) is an autosomal recessive disorder characterized by isolated glucocorticoid deficiency associated with normal mineralocorticoid secretion. Mutations in genes encoding either ACTH receptor or melanocortin 2 receptor accessory protein are responsible for the disease in about 50% of cases, named FGD type 1 and type 2, respectively. Patients may present with hyperpigmentation, recurrent infections, failure to thrive, hypoglycemic seizures, and coma in infancy or early childhood. Here we report the case of a 17-day-old newborn diagnosed with FGD type 1 who presented with hyperbilirubinemia and hyperpigmentation, a sign which was erroneously assumed to be due to prolonged phototherapy by the referring physician. Hormone analysis showed low cortisol and high ACTH levels with normal serum electrolytes and renin-aldosterone axis. Genetic analysis revealed a novel homozygous melanocortin 2 receptor mutation p.Leu225Arg in the patient. The healthy parents were heterozygous for the mutation.

Phenotypic characteristics of familial glucocorticoid deficiency (FGD) type 1 and 2

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.

Search for genetic markers and functional variants involved in the development of opiate and cocaine addiction and treatment

Addiction to opiates and illicit use of psychostimulants is a chronic, relapsing brain disease that, if left untreated, can cause major medical, social, and economic problems. This article reviews recent progress in studies of association of gene variants with vulnerability to develop opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems. In addition, we provide the first evidence of a cis-acting polymorphism and a functional haplotype in the PDYN gene, of significantly higher DNA methylation rate of the OPRM1 gene in the lymphocytes of heroin addicts, and significant differences in genotype frequencies of three single-nucleotide polymorphisms of the P-glycoprotein gene (ABCB1) between "higher" and "lower" methadone doses in methadone-maintained patients. In genomewide and multigene association studies, we found association of several new genes and new variants of known genes with heroin addiction. Finally, we describe the development and application of a novel technique: molecular haplotyping for studies in genetics of drug addiction.

Drug-induced and genetic alterations in stress-responsive systems: Implications for specific addictive diseases

From the earliest work in our laboratory, we hypothesized, and with studies conducted in both clinical research and animal models, we have shown that drugs of abuse, administered or self-administered, on a chronic basis, profoundly alter stress-responsive systems. Alterations of expression of specific genes involved in stress responsivity, with increases or decreases in mRNA levels, receptor, and neuropeptide levels, and resultant changes in hormone levels, have been documented to occur after chronic intermittent exposure to heroin, morphine, other opiates, cocaine, other stimulants, and alcohol in animal models and in human molecular genetics. The best studied of the stress-responsive systems in humans and mammalian species in general is undoubtedly the HPA axis. In addition, there are stress-responsive systems in other parts in the brain itself, and some of these include components of the HPA axis, such as CRF and CRF receptors, along with POMC gene and gene products. Several other stress-responsive systems are known to influence the HPA axis, such as the vasopressin-vasopressin receptor system. Orexin-hypocretin, acting at its receptors, may effect changes which suggest that it should be properly categorized as a stress-responsive system. However, less is known about the interactions and connectivity of some of these different neuropeptide and receptor systems, and in particular, about the possible connectivity of fast-acting (e.g., glutamate and GABA) and slow-acting (including dopamine, serotonin, and norepinephrine) neurotransmitters with each of these stress-responsive components and the resultant impact, especially in the setting of chronic exposure to drugs of abuse. Several of these stress-responsive systems and components, primarily based on our laboratory-based and human molecular genetics research of addictive diseases, will be briefly discussed in this review.

Minireview: the melanocortin 2 receptor accessory proteins

The melanocortin 2 receptor (MC2R) accessory protein, MRAP, is one of a growing number of G protein-coupled receptor accessory proteins that have been identified in recent years that add control and complexity to G protein-coupled receptor functional expression and signal transduction. MRAP interacts directly with MC2R and is essential for its trafficking from the endoplasmic reticulum to the cell surface, where it acts as the receptor for the pituitary hormone ACTH. In addition, MRAP2, a newly described homolog of MRAP, is also able to support the cell surface expression of MC2R. Although it is clear that MRAP is required for MC2R function, the mechanism of MRAP action is only beginning to be understood. Recent work has started to reveal some of these mechanisms and the MRAP domains involved in MC2R functional expression, and new data have shown a potential role for both MRAP and MRAP2 in the regulation of the other melanocortin receptors.

The molecular basis of adrenocorticotrophin resistance syndrome

Adrenocorticotrophin resistance syndromes comprise familial glucocorticoid deficiency (FGD) and triple A syndrome, which are rare autosomal recessive diseases with distinct clinical features and molecular etiologies. Mutations of melanocortin-2 receptor (MC2R) have been described in segregation with FGD in 25% of patients. More recently melanocortin-2 receptor accessory protein (MRAP), a small single-transmembrane domain protein, was described as an essential protein for the traffic of MC2R and its expression on the plasma membrane. About 20% of FGD patients carry homozygous mutations of MRAP. The ALADIN protein (for alacrima/achalasia/adrenal insufficiency/neurologic disorder) was identified as the molecular basis of triple A syndrome. The elucidation of the genetic basis of the ACTH resistance syndrome has contributed to the better understanding of MC2R function. However, in some patients the molecular etiology is not yet known and awaits further genetic studies.

Nonclassic lipoid congenital adrenal hyperplasia masquerading as familial glucocorticoid deficiency

CONTEXT

Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder resulting from resistance to the action of ACTH on the adrenal cortex. Affected individuals are deficient in cortisol and, if untreated, are likely to succumb to hypoglycemia and/or overwhelming infection. Mutations of the ACTH receptor (MC2R) and the melanocortin 2 receptor accessory protein (MRAP), FGD types 1 and 2 respectively, account for approximately 45% of cases.

OBJECTIVE

A locus on chromosome 8 has previously been linked to the disease in three families, but no underlying gene defect has to date been identified.

DESIGN

The study design comprised single-nucleotide polymorphism genotyping and mutation detection.

SETTING

The study was conducted at secondary and tertiary referral centers.

PATIENTS

Eighty probands from families referred for investigation of the genetic cause of FGD participated in the study.

INTERVENTIONS

There were no interventions.

RESULTS

Analysis by single-nucleotide polymorphism array of the genotype of one individual with FGD previously linked to chromosome 8 revealed a large region of homozygosity encompassing the steroidogenic acute regulatory protein gene, STAR. We identified homozygous STAR mutations in this patient and his affected siblings. Screening of our total FGD patient cohort revealed homozygous STAR mutations in a further nine individuals from four other families.

CONCLUSIONS

Mutations in STAR usually cause lipoid congenital adrenal hyperplasia, a disorder characterized by both gonadal and adrenal steroid deficiency. Our results demonstrate that certain mutations in STAR (R192C and the previously reported R188C) can present with a phenotype indistinguishable from that seen in FGD.

Familial glucocorticoid deficiency with a point mutation in the ACTH receptor: a case report

Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disorder characterized by severe glucocorticoid deficiency associated with failure of adrenal responsiveness to ACTH but no mineralocorticoid deficiency. We report a 2 month-old boy of nonconsanguineous parents, presented with hyperpigmentation. Physical examination showed diffuse dark skin of body including, oral mucosa, gum, hands, nails and scrotum. Laboratory evaluation revealed low serum cortisol (0.3 microg/dL), with very high plasma ACTH level (18,000 pg/mL), and serum cortisol level did not increase after ACTH stimulation test. Serum sodium, potassium, plasma renin activity, aldosterone and 17-hydroxyprogesterone were normal. Sequence analysis of the ACTH receptor (MC2R) gene showed a homozygous mutation of D103N. Diagnosis of FGD was made and treatment started with oral hydrocortisone.

Homozygous nonsense and frameshift mutations of the ACTH receptor in children with familial glucocorticoid deficiency (FGD) are not associated with long-term mineralocorticoid deficiency

OBJECTIVE

Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disease characterized by isolated glucocorticoid deficiency with preserved mineralocorticoid secretion. Mutations in the ACTH receptor (MC2R) account for approximately 25% of all FGD cases, but since these are usually missense mutations, a degree of receptor function is frequently retained. A recent report, however, suggested that disturbances in the renin-aldosterone axis were seen in some patients with potentially more severe MC2R mutations. Furthermore, MC2R knock out mice have overt aldosterone deficiency and hyperkalaemia despite preservation of a normal zona glomerulosa. We wished to determine whether a group of patients with severe nonsense mutations of the MC2R exhibited evidence of mineralocorticoid deficiency, thereby challenging the conventional diagnostic feature of FGD which might result in diagnostic misclassification.

DESIGN

Clinical review of patients with nonsense MC2R mutations.

PATIENTS

Between 1993 and 2008, 164 patients with FGD were screened for mutations in the MC2R. Totally 42 patients (34 families) were found to have mutations in the MC2R. Of these, 6 patients (4 families) were found to have homozygous nonsense or frameshift mutations.

RESULTS

Mild disturbances in the renin-angiotensin-aldosterone axis were noted in four out of six patients, ranging from slightly elevated plasma renin levels to low aldosterone levels, although frank mineralocorticoid deficiency or electrolyte disturbance were not found. No patient required fludrocortisone replacement.

CONCLUSION

Severe nonsense and frameshift MC2R mutations are not associated with clinically significant mineralocorticoid deficiency and are thus unlikely to require long-term mineralocorticoid replacement.

The genetics of familial glucocorticoid deficiency

Familial glucocorticoid deficiency is an autosomal recessive disorder resulting from defects in the action of adrenocorticotropic hormone (ACTH) to stimulate glucocorticoid synthesis in the adrenal. Production of mineralocorticoids by the adrenal is normal. Patients present in early life with low or undetectable cortisol and--because of the failure of the negative feedback loop to the pituitary and hypothalamus--grossly elevated ACTH levels. About half of all cases result from mutations in the ACTH receptor (melanocortin 2 receptor) or from mutations in the melanocortin 2 receptor accessory protein (MRAP), but other genetic causes of this potentially lethal disorder remain to be discovered.

Functional consequence of a novel Y129C mutation in a patient with two contradictory melanocortin-2-receptor mutations

CONTEXT

Familial glucocorticoid deficiency (FGD) is a rare autosomal recessive disease, characterised by isolated glucocorticoid deficiency in the absence of mineralocorticoid deficiency. Inactivating mutations in the ACTH receptor (melanocortin-2-receptor, MC2R) are well described and account for approximately 25% of cases. By contrast, activating MC2R mutations are extremely rare.

PATIENT

We report a child of Saudi Arabian origin who was diagnosed with FGD following hypoglycaemic episodes that resulted in spastic quadriplegia.

METHODS AND RESULTS

MC2R gene analysis revealed an unusual combination of two homozygous missense mutations, consisting of the novel mutation Y129C and the previously described F278C activating mutation. Parents were heterozygous at both of these sites. In vitro analysis of the Y129C mutation using a fluorescent cell surface assay showed that this mutant was unable to reach the cell surface in CHO cells stably transfected with MC2R accessory protein (MRAP), despite the demonstration of an interaction with MRAP by co-immunoprecipitation. The double mutant Y129C-F278C also failed to traffic to the cell surface.

CONCLUSION

The tyrosine residue at position 129 in the second intracellular loop is critical in MC2R folding and/or trafficking to the cell surface. Furthermore, the absence of cell surface expression of MC2R would account for the lack of activation of the receptor due to the F278C mutation located at the C-terminal tail. We provide a novel molecular explanation for a child with two opposing mutations causing severe FGD.

The majority of adrenocorticotropin receptor (melanocortin 2 receptor) mutations found in familial glucocorticoid deficiency type 1 lead to defective trafficking of the receptor to the cell surface

CONTEXT

There are at least 24 missense, nonconservative mutations found in the ACTH receptor [melanocortin 2 receptor (MC2R)] that have been associated with the autosomal recessive disease familial glucocorticoid deficiency (FGD) type 1. The characterization of these mutations has been hindered by difficulties in establishing a functional heterologous cell transfection system for MC2R. Recently, the melanocortin 2 receptor accessory protein (MRAP) was identified as essential for the trafficking of MC2R to the cell surface; therefore, a functional characterization of MC2R mutations is now possible.

OBJECTIVE

Our objective was to elucidate the molecular mechanisms responsible for defective MC2R function in FGD.

METHODS

Stable cell lines expressing human MRAPalpha were established and transiently transfected with wild-type or mutant MC2R. Functional characterization of mutant MC2R was performed using a cell surface expression assay, a cAMP reporter assay, confocal microscopy, and coimmunoprecipitation of MRAPalpha.

RESULTS

Two thirds of all MC2R mutations had a significant reduction in cell surface trafficking, even though MRAPalpha interacted with all mutants. Analysis of those mutant receptors that reached the cell surface indicated that four of six failed to signal, after stimulation with ACTH.

CONCLUSION

The majority of MC2R mutations found in FGD fail to function because they fail to traffic to the cell surface.

Lesions of the adrenal cortex

CONTEXT

In surgical pathology practice adrenal cortical tumors are rare. However, in autopsy series adrenal cortical nodules are found frequently. These are now being identified more commonly in life when the abdomen is scanned for other disease. It is important to differentiate between benign and malignant lesions as adrenal cortical carcinoma is an aggressive tumor. Molecular genetic investigations are providing new information on both pathogenesis of adrenal tumors and basic adrenal development and physiology.

OBJECTIVE

To provide an overview of current knowledge on adrenal cortical development and structure that informs our understanding of genetic diseases of the adrenal cortex and adrenal cortical tumors.

DATA SOURCES

Literature review using PubMed via the Endnote bibliography tool.

CONCLUSIONS

The understanding of basic developmental and physiologic processes permits a better understanding of diseases of the adrenal cortex. The information coming from investigation of the molecular pathology of adrenal cortical tumors is beginning to provide additional tests for the assessment of malignant potential in diagnosis but the mainstay remains traditional histologic analysis.

Heterogeneity in the molecular basis of ACTH resistance syndrome

OBJECTIVE

ACTH resistance syndromes are rare, autosomal, and genetically heterogeneous diseases that include familial glucocorticoid deficiency (FGD) and triple A syndrome. FGD has been shown to segregate with mutations in the gene coding for ACTH receptor (MC2R) or melanocortin 2 receptor accessory protein (MRAP), whereas mutations in the triple A syndrome (AAAS, Allgrove syndrome) gene have been found in segregation with triple A syndrome. We describe the clinical findings and molecular analysis of MC2R, MRAP, and AAAS genes in five Brazilian patients with ACTH resistance syndrome.

DESIGN AND METHODS

Genomic DNA from patients and their unaffected relatives was extracted from peripheral blood leucocytes and amplified by PCR, followed by automated sequencing. Functional analysis was carried out using Y6 cells expressing wild-type and mutant MC2R.

RESULTS

All five patients showed low cortisol and elevated plasma ACTH levels. One patient had achalasia and alacrima, besides the symptoms of adrenal insufficiency. The molecular analysis of FGD patients revealed a novel p.Gly116Val mutation in the MC2R gene in one patient and p.Met1Ile mutation in the MRAP gene in another patient. Expression of p.Gly116Val MC2R mutant in Y6 cells revealed that this variant failed to stimulate cAMP production. The analysis of the AAAS gene in the patient with triple A syndrome showed a novel g.782_783delTG deletion. The molecular analysis of DNA from other two patients showed no mutation in MC2R, MRAP, or AAAS gene.

CONCLUSIONS

In conclusion, the molecular basis of ACTH resistance syndrome is heterogeneous, segregating with genes coding for proteins involved with ACTH receptor signaling/expression or adrenal gland development and other unknown genes.

A novel variant of familial glucocorticoid deficiency prevalent among the Irish Traveler population

CONTEXT

Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder characterized by distinct clinical, biochemical, and genetic abnormalities. The prevalence of FGD is unknown, with the likelihood that cases remain undiagnosed. We noted a significant proportion of our FGD cases are Irish Travelers. Irish Travelers are an endogamous nomadic group ethnically and genetically distinct from Roma gypsies.

AIMS

The objective of the study was to describe the clinical features and assess the prevalence of FGD amongst Irish Travelers in the Republic of Ireland and describe their phenotype.

METHODS

Diagnosis of FGD was based on clinical features, high ACTH, and low cortisol concentrations with normal renin and aldosterone concentrations and exclusion of other causes of adrenal failure. Data from the Republic of Ireland Census 2006 were used.

RESULTS

We identified 21 cases of FGD, generating an overall prevalence of one in 201,898. We report nine Irish Travelers (five females) with FGD related to a new gene negative for melanocortin-2 receptor and melanocortin-2 receptor accessory protein mutations. Of a total population of 22,557 Travelers, this yields a disease prevalence of one in 2506 with a carrier frequency of one in 25 in this group and represents a prevalence of one in 665 and a carrier frequency of one in 13 in the 4- to 15-yr Traveler age group. All nine children had a later onset of FGD due to the fact that their initial investigations revealed normal cortisol (422-575 nmol/liter) and ACTH (<34 ng/liter) concentrations.

CONCLUSION

We report a high prevalence of FGD among Irish Travelers. Their subtle phenotype and initial normal biochemistry may delay the early diagnosis of FGD.

G protein-coupled receptors disrupted in human genetic disease

Genetic variation in G protein-coupled receptors (GPCRs) results in the disruption of GPCR function in a wide variety of human genetic diseases. In vitro strategies have been used to elucidate the molecular pathologies that underlie naturally occurring GPCR mutations. Various degrees of inactive, overactive, or constitutively active receptors have been identified. These mutations often alter ligand binding, G protein coupling, receptor desensitization, and receptor recycling. The role of inactivating and activating calcium-sensing receptor (CASR) mutations is discussed with respect to familial hypocalciuric hypercalemia (FHH) and autosomal dominant hypocalemia (ADH). Among ADH mutations, those associated with tonic-clonic seizures are discussed. Other receptors discussed include rhodopsin, thyrotropin, parathyroid hormone, melanocortin, follicle-stimulating hormone, luteinizing hormone, gonadotropin-releasing hormone (GnRHR), adrenocorticotropic hormone, vasopressin, endothelin-beta, purinergic, and the G protein associated with asthma (GPRA). Diseases caused by mutations that disrupt GPCR function are significant because they might be selectively targeted by drugs that rescue altered receptors. Examples of drug development based on targeting GPCRs mutated in disease include the calcimimetics used to compensate for some CASR mutations, obesity therapeutics targeting melanocortin receptors, interventions that alter GnRHR loss from the cell surface in idiopathic hypogonadotropic hypogonadism and novel drugs that might rescue the P2RY12 receptor in a rare bleeding disorder. The discovery of GPRA suggests that drug screens against variant GPCRs may identify novel drugs. This review of the variety of GPCRs that are disrupted in monogenic disease provides the basis for examining the significance of common pharmacogenetic variants.

Association of polymorphisms in the melanocortin receptor type 2 (MC2R, ACTH receptor) gene with heroin addiction

The melanocortin receptor type 2 (MC2R or adrenocorticotropic hormone, ACTH receptor) gene (MC2R) encodes a protein involved in regulation of adrenal cortisol secretion, important in the physiological response to stressors. A variant of MC2R, -179A>G, results in reduction of promoter activity and less adrenal action. We hypothesize that altered stress responsivity plays a key role in the initiation of substance abuse. By direct resequencing of the promoter region and exons 1 and 2 of the MC2R gene in 272 subjects including Caucasians, Hispanics and African Americans with approximately equal numbers of former heroin addicts and normal volunteers, we identified five novel variants each with allele frequency <2%. Previously reported polymorphisms -184G>A (rs2186944), -179A>G, 833A>C (rs28926182), 952T>C (rs4797825), 1005C>T (rs4797824) and 1579T>C (rs4308014) were each in allelic frequency >/=2% in one or more ethnic groups. These polymorphisms were genotyped in 632 subjects (260 Caucasians, 168 Hispanics, 183 African Americans and 21 Asians) using TaqMan assays. Significant differences in genotype frequency among ethnic groups studied were found for each of the six variants analyzed. We found a significant association (p=0.0004, experiment-wise p=0.0072) of the allele -184A with a protective effect from heroin addiction in Hispanics. Also, in Hispanics only we found the haplotype GACT consisting of four variants (-184G>A, -179A>G, 833A>C and 1005C>T) to be significantly associated with heroin addiction (p=0.0014, experiment-wise p=0.0168), whereas another haplotype, AACT, consisting of the same variants, was associated with a protective effect from heroin addiction (p=0.0039, experiment-wise p=0.0468).

Molecular identification of the human melanocortin-2 receptor responsible for ligand binding and signaling

The melanocortin-2 receptor (MC2R), also known as the adrenocorticotropic hormone (ACTH) receptor, plays an important role in regulating and maintaining adrenocortical function, specifically steroidogenesis. Mutations of the human MC2R (hMC2R) gene have also been identified in humans with familial glucocorticoid deficiency; however, the molecular basis responsible for hMC2R ligand binding and signaling remains unclear. In this study, both truncated ACTH peptides and site-directed mutagenesis studies were used to determine molecular mechanisms of hMC2R binding ACTH and signaling. Our results indicate that ACTH1-16 is the minimal peptide required for hMC2R binding and signaling. Mutations of common melanocortin receptor family amino acid residues E80 in transmembrane domain 2 (TM2), D107 in TM3, F178 in TM4, F235 and H238 in TM6, and F258 in TM7 significantly reduced ACTH-binding affinity and signaling. Furthermore, mutations of unique amino acids D104 and F108 in TM3 and F168 and F178 in TM4 significantly decreased ACTH binding and signaling. In conclusion, our results suggest that the residues in TM2, TM3, and TM6 of hMC2R share similar binding sites with other MCRs but the residues identified in TM4 and TM7 of hMC2R are unique and required for ACTH selectivity. Our study suggests that hMC2R may have a broad binding pocket in which both conserved and unique amino acid residues are required, which may be the reason why alpha-MSH was not able to bind hMC2R.