Genetic variations and polymorphisms of G protein-coupled receptors: functional and therapeutic implications

Genetic variants of accessory proteins and G proteins in human genetic disease

We present a series of three articles on the genetics and pharmacogenetics of G protein- coupled receptors (GPCR). In the first article, we discuss genetic variants of the G protein subunits and accessory proteins that are associated with human phenotypes; in the second article, we build upon this to discuss "G protein-coupled receptor (GPCR) gene variants and human genetic disease" and in the third article, we survey "G protein-coupled receptor pharmacogenomics". In the present article, we review the processes of ligand binding, GPCR activation, inactivation, and receptor trafficking to the membrane in the context of human genetic disease resulting from pathogenic variants of accessory proteins and G proteins. Pathogenic variants of the genes encoding G protein α and β subunits are examined in diverse phenotypes. Variants in the genes encoding accessory proteins that modify or organize G protein coupling have been associated with disease; these include the contribution of variants of the regulator of G protein signaling (RGS) to hypertension; the role of variants of activator of G protein signaling type III in phenotypes such as hypoxia; the contribution of variation at the RGS10 gene to short stature and immunological compromise; and the involvement of variants of G protein-coupled receptor kinases (GRKs), such as GRK4, in hypertension. Variation in genes that encode proteins involved in GPCR signaling are outlined in the context of the changes in structure and function that may be associated with human phenotypes.

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.

MUG: A mutation overview of GPCR subfamily A17 receptors

G protein-coupled receptors (GPCRs) mediate several signaling pathways through a general mechanism that involves their activation, upholding a chain of events that lead to the release of molecules responsible for cytoplasmic action and further regulation. These physiological functions can be severely altered by mutations in GPCR genes. GPCRs subfamily A17 (dopamine, serotonin, adrenergic and trace amine receptors) are directly related with neurodegenerative diseases, and as such it is crucial to explore known mutations on these systems and their impact in structure and function. A comprehensive and detailed computational framework - MUG (Mutations Understanding GPCRs) - was constructed, illustrating key reported mutations and their effect on receptors of the subfamily A17 of GPCRs. We explored the type of mutations occurring overall and in the different families of subfamily A17, as well their localization within the receptor and potential effects on receptor functionality. The mutated residues were further analyzed considering their pathogenicity. The results reveal a high diversity of mutations in the GPCR subfamily A17 structures, drawing attention to the considerable number of mutations in conserved residues and domains. Mutated residues were typically hydrophobic residues enriched at the ligand binding pocket and known activating microdomains, which may lead to disruption of receptor function. MUG as an interactive web application is available for the management and visualization of this dataset. We expect that this interactive database helps the exploration of GPCR mutations, their influence, and their familywise and receptor-specific effects, constituting the first step in elucidating their structures and molecules at the atomic level.

Neuronal GPR81 regulates developmental brain angiogenesis and promotes brain recovery after a hypoxic ischemic insult

Perinatal hypoxic/ischemic (HI) brain injury is a major clinical problem with devastating neurodevelopmental outcomes in neonates. During HI brain injury, dysregulated factor production contributes to microvascular impairment. Glycolysis-derived lactate accumulated during ischemia has been proposed to protect against ischemic injury, but its mechanism of action is poorly understood. Herein, we hypothesize that lactate via its G-protein coupled receptor (GPR81) controls postnatal brain angiogenesis and plays a protective role after HI injury. We show that GPR81 is predominantly expressed in neurons of the cerebral cortex and hippocampus. GPR81-null mice displayed a delay in cerebral microvascular development linked to reduced levels of various major angiogenic factors and augmented expression of anti-angiogenic Thrombospondin-1 (TSP-1) in comparison to their WT littermates. Coherently, lactate stimulation induced an increase in growth factors (VEGF, Ang1 and 2, PDGF) and reduced TSP-1 expression in neurons, which contributed to accelerating angiogenesis. HI injury in GPR81-null animals curtailed vascular density and consequently increased infarct size compared to changes seen in WT mice; conversely intracerebroventricular lactate injection increased vascular density and diminished infarct size in WT but not in GPR81-null mice. Collectively, we show that lactate acting via GPR81 participates in developmental brain angiogenesis, and attenuates HI injury by restoring compromised microvasculature.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Common activation mechanism of class A GPCRs

Class A G-protein-coupled receptors (GPCRs) influence virtually every aspect of human physiology. Understanding receptor activation mechanism is critical for discovering novel therapeutics since about one-third of all marketed drugs target members of this family. GPCR activation is an allosteric process that couples agonist binding to G-protein recruitment, with the hallmark outward movement of transmembrane helix 6 (TM6). However, what leads to TM6 movement and the key residue level changes of this movement remain less well understood. Here, we report a framework to quantify conformational changes. By analyzing the conformational changes in 234 structures from 45 class A GPCRs, we discovered a common GPCR activation pathway comprising of 34 residue pairs and 35 residues. The pathway unifies previous findings into a common activation mechanism and strings together the scattered key motifs such as CWxP, DRY, Na+ pocket, NPxxY and PIF, thereby directly linking the bottom of ligand-binding pocket with G-protein coupling region. Site-directed mutagenesis experiments support this proposition and reveal that rational mutations of residues in this pathway can be used to obtain receptors that are constitutively active or inactive. The common activation pathway provides the mechanistic interpretation of constitutively activating, inactivating and disease mutations. As a module responsible for activation, the common pathway allows for decoupling of the evolution of the ligand binding site and G-protein-binding region. Such an architecture might have facilitated GPCRs to emerge as a highly successful family of proteins for signal transduction in nature.

Target-Mediated Population Pharmacokinetic Modeling of Endothelin Receptor Antagonists

PURPOSE

Bosentan, clazosentan, and tezosentan are three small-molecule endothelin receptor antagonists (ERAs), displacing endothelin-1 (ET-1) from its binding site. A target-mediated drug disposition (TMDD) pharmacokinetic (PK) model described the non-linearity in the PK of bosentan caused by its high receptor binding affinity with time-dependent varying receptor expression or reappearance. The aim of this analysis was to investigate the presence of TMDD for clazosentan and tezosentan and to corroborate the hypothesis of a diurnal receptor synthesis.

METHODS

PK data from healthy subjects after intravenous (i.v.) administration of single ascending doses of bosentan, clazosentan, and tezosentan were analyzed. Frequent blood samples for PK measurements were collected. Population analyses, simulations, and evaluations were performed using a non-linear mixed-effects modeling approach.

RESULTS

Two-compartment TMDD models were successfully developed describing the PK of all three ERAs with different receptor-complex internalization properties. The observed multiple peaks in the concentration-time profiles were captured with cosine functions on the receptor synthesis rate mimicking a diurnal receptor expression or reappearance. The results strongly suggest that TMDD is a class effect of ERAs.

CONCLUSION

The developed TMDD PK models are a next step towards understanding the complex PK of ERAs and further support the hypothesis that TMDD is a class effect of ERAs.

Comprehensive Analysis of Non-Synonymous Natural Variants of G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are the largest superfamily of transmembrane receptors and have vital signaling functions in various organs. Because of their critical roles in physiology and pathology, GPCRs are the most commonly used therapeutic target. It has been suggested that GPCRs undergo massive genetic variations such as genetic polymorphisms and DNA insertions or deletions. Among these genetic variations, non-synonymous natural variations change the amino acid sequence and could thus alter GPCR functions such as expression, localization, signaling, and ligand binding, which may be involved in disease development and altered responses to GPCR-targeting drugs. Despite the clinical importance of GPCRs, studies on the genotype-phenotype relationship of GPCR natural variants have been limited to a few GPCRs such as β-adrenergic receptors and opioid receptors. Comprehensive understanding of non-synonymous natural variations within GPCRs would help to predict the unknown genotype-phenotype relationship and yet-to-be-discovered natural variants. Here, we analyzed the non-synonymous natural variants of all non-olfactory GPCRs available from a public database, UniProt. The results suggest that non-synonymous natural variations occur extensively within the GPCR superfamily especially in the N-terminus and transmembrane domains. Within the transmembrane domains, natural variations observed more frequently in the conserved residues, which leads to disruption of the receptor function. Our analysis also suggests that only few non-synonymous natural variations have been studied in efforts to link the variations with functional consequences.

Cysteinyl Leukotrienes Pathway Genes, Atopic Asthma and Drug Response: From Population Isolates to Large Genome-Wide Association Studies

Genetic variants associated with asthma pathogenesis and altered response to drug therapy are discussed. Many studies implicate polymorphisms in genes encoding the enzymes responsible for leukotriene synthesis and intracellular signaling through activation of seven transmembrane domain receptors, such as the cysteinyl leukotriene 1 (CYSLTR1) and 2 (CYSLTR2) receptors. The leukotrienes are polyunsaturated lipoxygenated eicosatetraenoic acids that exhibit a wide range of pharmacological and physiological actions. Of the three enzymes involved in the formation of the leukotrienes, arachidonate 5 lipoxygenase 5 (ALOX5), leukotriene C4 synthase (LTC4S), and leukotriene hydrolase (LTA4H) are all polymorphic. These polymorphisms often result in variable production of the CysLTs (LTC4, LTD4, and LTE4) and LTB4. Variable number tandem repeat sequences located in the Sp1-binding motif within the promotor region of the ALOX5 gene are associated with leukotriene burden and bronchoconstriction independent of asthma risk. A 444A > C SNP polymorphism in the LTC4S gene, encoding an enzyme required for the formation of a glutathione adduct at the C-6 position of the arachidonic acid backbone, is associated with severe asthma and altered response to the CYSLTR1 receptor antagonist zafirlukast. Genetic variability in the CysLT pathway may contribute additively or synergistically to altered drug responses. The 601 A > G variant of the CYSLTR2 gene, encoding the Met201Val CYSLTR2 receptor variant, is associated with atopic asthma in the general European population, where it is present at a frequency of ∼2.6%. The variant was originally found in the founder population of Tristan da Cunha, a remote island in the South Atlantic, in which the prevalence of atopy is approximately 45% and the prevalence of asthma is 36%. In vitro work showed that the atopy-associated Met201Val variant was inactivating with respect to ligand binding, Ca²⁺ flux and inositol phosphate generation. In addition, the CYSLTR1 gene, located at Xq13-21.1, has been associated with atopic asthma. The activating Gly300Ser CYSLTR1 variant is discussed. In addition to genetic loci, risk for asthma may be influenced by environmental factors such as smoking. The contribution of CysLT pathway gene sequence variants to atopic asthma is discussed in the context of other genes and environmental influences known to influence asthma.

Novel Probes Establish Mas-Related G Protein-Coupled Receptor X1 Variants as Receptors with Loss or Gain of Function

The Mas-related G protein-coupled receptor X1 (MrgprX1) is a human seven transmembrane-domain protein with a putative role in nociception and pruritus. This receptor is expressed in dorsal root ganglion neurons and is activated by a variety of endogenous peptides, including bovine adrenal medulla peptide (BAM) and γ2-melanocyte-stimulating hormone (γ2-MSH). In the present work, we study how naturally occurring missense mutations alter the activity of MrgprX1. To characterize selected receptor variants, we initially used the endogenous peptide ligand BAM8-22. In addition, we generated and characterized a panel of novel recombinant and synthetic peptide ligands. Our studies identified a mutation in the second intracellular loop of MrgprX1, R131S, that causes a decrease in both ligand-mediated and constitutive signaling. Another mutation in this region, H133R, results in a gain of function phenotype reflected by an increase in ligand-mediated signaling. Using epitope-tagged variants, we determined that the alterations in basal and ligand-mediated signaling were not explained by changes in receptor expression levels. Our results demonstrate that naturally occurring mutations can alter the pharmacology of MrgprX1. This study provides a theoretical basis for exploring whether MrgprX1 variability underlies differences in somatosensation within human populations.

Application of Parallel Multiparametric Cell-Based FLIPR Detection Assays for the Identification of Modulators of the Muscarinic Acetylcholine Receptor 4 (M4)

Muscarinic acetylcholine receptors (mAChRs) have long been viewed as viable targets for novel therapeutic agents for the treatment of Alzheimer's disease and other disorders involving impaired cognitive function. In an attempt to identify orthosteric and allosteric modulators of the muscarinic acetylcholine receptor M(4) (M(4)), we developed a homogenous, multiparametric, 1536-well assay to measure M(4) receptor agonism, positive allosteric modulation (PAM), and antagonism in a single well. This assay yielded a Z' of 0.85 ± 0.05 in the agonist, 0.72 ± 0.07 in PAM, and 0.80 ± 0.06 in the antagonist mode. Parallel screening of the M(1) and M(5) subtypes using the same multiparametric assay format revealed chemotypes that demonstrate selectivity and/or promiscuity between assays and modalities. This identified 503 M(4) selective primary agonists, 1450 PAMs, and 2389 antagonist hits. Concentration-response analysis identified 25 selective agonists, 4 PAMs, and 41 antagonists. This demonstrates the advantages of this approach to rapidly identify selective receptor modulators while efficiently removing assay artifacts and undesirable compounds.

Novel roles for β-arrestins in the regulation of pharmacological sequestration to predict agonist-induced desensitization of dopamine D3 receptors

BACKGROUND AND PURPOSE

In addition to typical GPCR kinase (GRK)-/β-arrestin-dependent internalization, dopamine D3 receptor employed an additional GRK-independent sequestration pathway. In this study, we investigated the molecular mechanism of this novel sequestration pathway.

EXPERIMENTAL APPROACH

Radioligand binding, flow cytometry and cell surface biotinylation assay were used to characterize trafficking properties of D2 and D3 receptors. Serine/threonine and N-linked glycosylation mutants of the D3 receptor were utilized to locate receptor regions involved in pharmacological sequestration and desensitization. Various point mutants of the D2 and D3 receptors, whose sequestration and desensitization properties were altered, were combined with knockdown cells of GRKs or β-arrestins to functionally correlate pharmacological sequestration and desensitization.

KEY RESULTS

The D3 receptor, but not the D2 receptor, showed characteristic trafficking behaviour in which receptors were shifted towards the more hydrophobic domains within the plasma membrane without translocation into other intracellular compartments. Among various amino acid residues tested, S145/S146, C147 and N12/19 were involved in pharmacological sequestration and receptor desensitization. Both pharmacological sequestration and desensitization of D3 receptor required β-arrestins, and functional relationship was observed between two processes when it was tested for D3 receptor variants and agonists.

CONCLUSIONS AND IMPLICATIONS

Pharmacological sequestration of D3 receptor accompanies movement of cell surface receptors into a more hydrophobic fraction within the plasma membrane and renders D3 receptor inaccessible to hydrophilic ligands. Pharmacological sequestration is correlated with desensitization of the D3 receptor in a Gβγ- and β-arrestin-dependent manner. This study provides new insights into molecular mechanism governing GPCR trafficking and desensitization.

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.

G protein-coupled receptor accessory proteins and signaling: pharmacogenomic insights

The identification and characterization of the genes encoding G protein-coupled receptors (GPCRs) and the proteins necessary for the processes of ligand binding, GPCR activation, inactivation, and receptor trafficking to the membrane are discussed in the context of human genetic disease. In addition to functional GPCR variants, the identification of genetic disruptions affecting proteins necessary to GPCR functions have provided insights into the function of these pathways. Gsα and Gβ subunit polymorphisms have been found to result in complex phenotypes. Disruptions in accessory proteins that normally modify or organize heterotrimeric G-protein coupling may also result in disease states. These include the contribution of variants of the regulator of G protein signaling (RGS) protein to hypertension; the role variants of the activator of G protein signaling (AGS) proteins to phenotypes (such as the type III AGS8 variant to hypoxia); the contribution of G protein-coupled receptor kinase (GRK) proteins, such as GRK4, in disorders such as hypertension. The role of accessory proteins in GPCR structure and function is discussed in the context of genetic disorders associated with disruption of the genes that encode them. An understanding of the pharmacogenomics of GPCR and accessory protein signaling provides the basis for examining both GPCR pharmacogenetics and the genetics of monogenic disorders that result from disruption of given receptor systems.

Pharmacogenetics of the G protein-coupled receptors

Pharmacogenetics investigates the influence of genetic variants on physiological phenotypes related to drug response and disease, while pharmacogenomics takes a genome-wide approach to advancing this knowledge. Both play an important role in identifying responders and nonresponders to medication, avoiding adverse drug reactions, and optimizing drug dose for the individual. G protein-coupled receptors (GPCRs) are the primary target of therapeutic drugs and have been the focus of these studies. With the advance of genomic technologies, there has been a substantial increase in the inventory of naturally occurring rare and common GPCR variants. These variants include single-nucleotide polymorphisms and insertion or deletions that have potential to alter GPCR expression of function. In vivo and in vitro studies have determined functional roles for many GPCR variants, but genetic association studies that define the physiological impact of the majority of these common variants are still limited. Despite the breadth of pharmacogenetic data available, GPCR variants have not been included in drug labeling and are only occasionally considered in optimizing clinical use of GPCR-targeted agents. In this chapter, pharmacogenetic and genomic studies on GPCR variants are reviewed with respect to a subset of GPCR systems, including the adrenergic, calcium sensing, cysteinyl leukotriene, cannabinoid CB1 and CB2 receptors, and the de-orphanized receptors such as GPR55. The nature of the disruption to receptor function is discussed with respect to regulation of gene expression, expression on the cell surface (affected by receptor trafficking, dimerization, desensitization/downregulation), or perturbation of receptor function (altered ligand binding, G protein coupling, constitutive activity). The large body of experimental data generated on structure and function relationships and receptor-ligand interactions are being harnessed for the in silico functional prediction of naturally occurring GPCR variants. We provide information on online resources dedicated to GPCRs and present applications of publically available computational tools for pharmacogenetic studies of GPCRs. As the breadth of GPCR pharmacogenomic data becomes clearer, the opportunity for routine assessment of GPCR variants to predict disease risk, drug response, and potential adverse drug effects will become possible.

Pathophysiology of GPCR Homo- and Heterodimerization: Special Emphasis on Somatostatin Receptors

G-protein coupled receptors (GPCRs) are cell surface proteins responsible for translating >80% of extracellular reception to intracellular signals. The extracellular information in the form of neurotransmitters, peptides, ions, odorants etc is converted to intracellular signals via a wide variety of effector molecules activating distinct downstream signaling pathways. All GPCRs share common structural features including an extracellular N-terminal, seven-transmembrane domains (TMs) linked by extracellular/intracellular loops and the C-terminal tail. Recent studies have shown that most GPCRs function as dimers (homo- and/or heterodimers) or even higher order of oligomers. Protein-protein interaction among GPCRs and other receptor proteins play a critical role in the modulation of receptor pharmacology and functions. Although ~50% of the current drugs available in the market target GPCRs, still many GPCRs remain unexplored as potential therapeutic targets, opening immense possibility to discover the role of GPCRs in pathophysiological conditions. This review explores the existing information and future possibilities of GPCRs as tools in clinical pharmacology and is specifically focused for the role of somatostatin receptors (SSTRs) in pathophysiology of diseases and as the potential candidate for drug discovery.

Pharmacogenomics of the heptahelical receptor regulators G-protein-coupled receptor kinases and arrestins: the known and the unknown

Heptahelical G-protein-coupled receptors are the most diverse and therapeutically important family of receptors, playing major roles in the physiology of various organs and tissues. They couple their ligand binding to G-protein activation, which then transmits intracellular signals. G-protein signaling is terminated by phosphorylation of the receptor by the family of G-protein-coupled receptor kinases (GRKs), followed by arrestin (Arr) binding, which uncouples the phosphorylated receptor from the G-protein and subsequently targets the receptor for internalization. Moreover, Arrs can transmit signals in their own right during receptor internalization. Genetic polymorphisms in receptors, as well as in GRK and Arr family members per se, which affect regulation of receptor signaling and function, have just started being identified and characterized. The present review will discuss what is known so far in this evolving field of GRK/Arr pharmacogenomics, as well as highlight important areas likely to produce invaluable information in the future.

Ligand-induced internalization and recycling of the human neuropeptide Y2 receptor is regulated by its carboxyl-terminal tail

Agonist-induced internalization of G protein-coupled receptors plays an important role in signal regulation. The underlying mechanisms of the internalization of the human neuropeptide Y(2) receptor (hY(2)R), as well as its desensitization, endocytosis, and resensitization are mainly unknown. In the present study we have investigated the role of carboxyl-terminal (C-terminal) Ser/Thr residues and acidic amino acids in regulating receptor internalization, arrestin interaction, and recycling by fluorescence microscopy, cell surface enzyme-linked immunosorbent assay, and bioluminescence resonance energy transfer in several cell lines. Strikingly, C-terminal truncation mutants revealed two different internalization motifs. Whereas a distal motif (373)DSXTEXT(379) was found to be the primary regulatory internalization sequence acting in concert with arrestin-3, the proximal motif (347)DXXXSEXSXT(356) promoted ligand-induced internalization in an arrestin-3-independent manner. Moreover, we identified a regulatory sequence located between these internalization motifs ((357)FKAKKNLEVRKN(368)), which serves as an inhibitory element. We found that hY(2)R recycling is also governed by structural determinants within the proximal internalization motif. In conclusion, these results indicate that the hY(2)R C terminus is involved in multiple molecular events that regulate internalization, interaction with arrestin-3, and receptor resensitization. Our findings provide novel insights into complex mechanisms of controlled internalization of hY(2)R, which is likely applicable to other GPCRs.

Comprehensive biochemical analysis of rare prostacyclin receptor variants: study of association of signaling with coronary artery obstruction

Currently, pharmacogenetic studies are at an impasse as the low prevalence (<2%) of most variants hinder their pharmacogenetic analysis with population sizes often inadequate for sufficiently powered studies. Grouping rare mutations by functional phenotype rather than mutation site can potentially increase sample size. Using human population-based studies (n = 1,761) to search for dysfunctional human prostacyclin receptor (hIP) variants, we recently discovered 18 non-synonymous mutations, all with frequencies less than 2% in our study cohort. Eight of the 18 had defects in binding, activation, and/or protein stability/folding. Mutations (M113T, L104R, and R279C) in three highly conserved positions demonstrated severe misfolding manifested by impaired binding and activation of cell surface receptors. To assess for association with coronary artery disease, we performed a case-control study comparing coronary angiographic results from patients with reduced cAMP production arising from the non-synonymous mutations (n = 23) with patients with non-synonymous mutations that had no reduction in cAMP (n = 17). Major coronary artery obstruction was significantly increased in the dysfunctional mutation group in comparison with the silent mutations. We then compared the 23 dysfunctional receptor patients with 69 age- and risk factor-matched controls (1:3). This verified the significantly increased coronary disease in the non-synonymous dysfunctional variant cohort. This study demonstrates the potential utility of in vitro functional characterization in predicting clinical phenotypes and represents the most comprehensive characterization of human prostacyclin receptor genetic variants to date.

Increased blood pressure and hyperdynamic cardiovascular responses in carriers of a common hyperfunctional variant of adenylyl cyclase 6

Adenylyl cyclase (ADCY) is a critical regulator of metabolic and cardiovascular function. We have identified a genetic variant (A674S) in ADCY isoform 6 (ADCY6). Subsequent studies demonstrated that the expression of this ADCY6 variant paralleled an increase in adenylyl cyclase-mediated functions. However, the impact of this hyperfunctional variant on cardiovascular function is unknown. Therefore, we evaluated the hemodynamic profile of carriers of ADCY6 A674S. The association of ADCY6 A674S with anthropometric and hemodynamic parameters was assessed in 364 healthy white subjects. The allele encoding this variant was present in 6.9% of the subjects, and those individuals had increased blood pressure. To determine the hemodynamic basis for increased blood pressure in carriers of ADCY6 A674S, we assessed forearm blood flow (FBF) and cardiac output at rest, during handgrip exercise (to test vasodilator responsiveness), and with lower body negative pressure [to test forearm vasoconstrictor and heart rate (HR) responsiveness] in a subsample of 21 subjects. At rest, cardiac output and blood pressure were higher in carriers of ADCY6 A674S. This was paralleled by an increase in plasma renin activity, but not in plasma norepinephrine. During handgrip exercise, FBF and vasodilator responses were greater in carriers of ADCY6 A674S. Responses to reactive hyperemia were not different between genotypes. With lower body negative pressure, the HR response to this orthostatic stress was markedly higher in carriers of ADCY6 A674S. These data indicate that the relatively common hyperfunctional ADCY6 A674S variant underlies a hyperdynamic cardiovascular response and increased blood pressure.

Functional consequences of nonsynonymous single nucleotide polymorphisms in the CB2 cannabinoid receptor

OBJECTIVE

To test the hypothesis that the two nonsynonymous single nucleotide polymorphisms at the CB2 cannabinoid receptor gene may have functional consequences on human CB2.

METHODS

Q63R, H316Y, and Q63R/H316 mutations were made in recombinant human CB2 by the method of site-directed mutagenesis. After these mutant CB2 receptors were stably transfected into HEK293 cells, ligand binding, ligand-induced activity, and constitutive activity assays were performed to test the functional significance of these mutations.

RESULTS

In general, our results showed that the CB2 polymorphic receptors are able to bind cannabinoid ligands and mediate signal transduction. However, in ligand-induced cyclic AMP accumulation assays, the cannabinoid agonists WIN55212-2 and 2-arachidonoylglycerol had reduced efficacy in cells expressing the polymorphic receptors as compared with the CB2 wild-type receptor. Furthermore, in constitutive activity assays, the H316Y and Q63R/H316Y polymorphic receptors exhibited higher constitutive activity than the CB2 wild-type receptor.

CONCLUSION

Our data shows that the presence of the polymorphisms at both positions 63 and 316 produce alterations in the CB2 receptor functions. Moreover, these findings strengthen the idea that the CB2 polymorphic receptors may contribute to the etiology of certain diseases.

Allosteric modulators of GPCRs: a novel approach for the treatment of CNS disorders

Despite G-protein-coupled receptors (GPCRs) being among the most fruitful targets for marketed drugs, intense discovery efforts for several GPCR subtypes have failed to deliver selective drug candidates. Historically, drug discovery programmes for GPCR ligands have been dominated by efforts to develop agonists and antagonists that act at orthosteric sites for endogenous ligands. However, in recent years, there have been tremendous advances in the discovery of novel ligands for GPCRs that act at allosteric sites to regulate receptor function. These compounds provide high selectivity, novel modes of efficacy and may lead to novel therapeutic agents for the treatment of multiple psychiatric and neurological human disorders.

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.

GPCR NaVa database: natural variants in human G protein-coupled receptors

The superfamily of human G protein-coupled receptors (GPCRs) is large and regulates a plethora of important physiological processes by transducing extracellular signals over cell membranes. A diversity of natural variants occurs in these receptors, including rare mutations and common polymorphisms. These variants differ in their impact on DNA, ranging from single nucleotide polymorphisms (SNPs) to copy number variants, and in their impact on protein function. Natural variants furthermore vary in their effects on human phenotypes from neutral to disease-associated. As mutation data are highly dispersed over numerous sources, a single resource for variants would aid investigators of GPCRs. The GPCR NaVa database therefore integrates data on natural variants in human GPCRs from online databases, the scientific literature, and patents. Where available, variants contain information on their location in the DNA (and protein sequence), the involved nucleotides (and amino acids), the average frequency of each allele, reported disease associations, and references to public databases and the scientific literature. The GPCR NaVa database aims to facilitate studies into pharmacogenetics, genotype-phenotype, and structure-function relationships of GPCRs. The GPCR NaVa database is interlinked with the family-specific GPCRDB resource and is accessible as a stand-alone database through a user-friendly website at http://nava.liacs.nl (last accessed 28 August 2007).

G protein-coupled receptor pharmacogenetics

Common G protein-coupled receptor (GPCR) gene variants that encode receptor proteins with a distinct sequence may alter drug efficacy without always resulting in a disease phenotype. GPCR genetic loci harbor numerous variants, such as DNA insertions or deletions and single-nucleotide polymorphisms that alter GPCR expression and function, thereby contributing to interindividual differences in disease susceptibility/progression and drug responses. In this chapter, these pharmacogenetic phenomena are reviewed with respect to a limited sampling of GPCR systems, including the beta(2)-adrenergic receptors, the cysteinyl leukotriene receptors, and the calcium-sensing receptor. In each example, the nature of the disruption to receptor function that results from each variant is discussed with respect to the regulation of gene expression, expression on cell surface (affected by receptor trafficking, dimerization, desensitization/downregulation), or perturbation of receptor function (by altering ligand binding, G protein coupling, and receptor constitutive activity). Despite the breadth of pharmacogenetic knowledge available, assessment for genetic variants is only occasionally applied to drug development projects involving pharmacogenomics or to optimizing the clinical use of GPCR drugs. The continued effort by the basic science of pharmacogenetics may draw the attention of drug discovery projects and clinicians alike to the utility of personalized pharmacogenomics as a means to optimize novel GPCR drug targets.

Pharmacogenomics of G protein-coupled receptor signaling: insights from health and disease

The identification and characterization of the processes of G protein-coupled receptor (GPCR) activation and inactivation have refined not only the study of the GPCRs but also the genomics of many accessory proteins necessary for these processes. This has accelerated progress in understanding the fundamental mechanisms involved in GPCR structure and function, including receptor transport to the membrane, ligand binding, activation and inactivation by GRK-mediated (and other) phosphorylation. The catalog of G(s)alpha and Gbeta subunit polymorphisms that result in complex phenotypes has complemented the effort to catalog the GPCRs and their variants. The study of the genomics of GPCR accessory proteins has also provided insight into pathways of disease, such as the contributions of regulator of G protein signaling (RGS) protein to hypertension and activator of G protein signaling (AGS) proteins to the response to hypoxia. In the case of the G protein-coupled receptor kinases (GRKs), identified originally in the retinal tissues that converge on rhodopsin, proteins such as GRK4 have been identified that have been subsequently associated with hypertension. Here, we review the structure and function of GPCR and associated proteins in the context of the gene families that encode them and the genetic disorders associated with their altered function. An understanding of the pharmacogenomics of GPCR signaling provides the basis for examining the GPCRs disrupted in monogenic disease and the pharmacogenetics of a given receptor system.

G protein-independent cell-based assays for drug discovery on seven-transmembrane receptors

Conventional cell-based assays for seven-transmembrane receptors, also known as G protein-coupled receptors, rely on the coupling of the ligand-bound receptor to heterotrimeric G proteins. New assay methods have become available that are not based on G protein activation, but that apply the molecular mechanism underlying the attenuation of G protein signaling mediated by beta-arrestin. beta-arrestin is a cytoplasmic protein that targets receptors to clathrin-coated endocytotic vesicles for degradation or recycling. This process has been visualized and quantified in high-content imaging assays using receptor- or beta-arrestin-chimeras with green fluorescent protein. Other assay methods use bioluminescence resonance energy transfer, enzyme fragment complementation, or a protease-activated transcriptional reporter gene, to measure receptor-beta-arrestin proximity. beta-arrestin recruitment assays have been applied successfully for receptors coupling to Galpha(q), Galpha(s) and Galpha(i) proteins, thus providing a generic assay platform for drug discovery on G protein-coupled receptors. The best understood signal transduction pathway elicited by the seven-transmembrane Frizzled receptors does not involve G proteins. The activation of Frizzleds by their cognate ligands of the Wnt family recruits the phosphoprotein dishevelled. Dishevelled regulates a protein complex involved in the destruction of beta-catenin. Activation of Frizzled blocks degradation of beta-catenin, which translocates to the nucleus to activate transcription of Wnt-responsive genes. The cytoplasm-to-nuclear translocation of beta-catenin forms the basis of several high-content assays to measure Wnt/Frizzled signal transduction. Interestingly, Frizzled receptors have recently been shown to internalize and to recruit beta-arrestin. This suggests that beta-arrestin recruitment assays may be applied for drug discovery on seven-transmembrane receptors beyond G protein-coupled receptors.

Increased enzyme activity and beta-adrenergic mediated vasodilation in subjects expressing a single-nucleotide variant of human adenylyl cyclase 6

OBJECTIVE

cAMP is a critical regulator of metabolic and cardiovascular function. However, the role of genetic variability in the regulation of cAMP-mediated effects is unclear. Therefore, we assessed the effect of the expression of a recently identified missense genetic variant of adenylyl cyclase isoform 6 (ADCY6 S674).

METHODS AND RESULTS

In rat vascular smooth muscle cells, gene transfer of ADCY6 S674 increased adenylyl cyclase activity and arborization to a greater extent than gene transfer of ADCY6 A674. Similarly, in adherent mononuclear leukocyte cells isolated from ADCY6 S674-expressing human subjects, both adenylyl cyclase activity and adenylyl cyclase-mediated cell retraction were significantly increased. Additionally, in dorsal hand vein LVDT studies, subjects expressing the hyper-functional ADCY6 S674 variant had significantly greater vascular sensitivity to the beta-adrenergic agonist isoproterenol as assessed by both a greater potency and greater maximal effect than subjects expressing the ADCY6 A674 enzyme.

CONCLUSIONS

These data indicate that the expression of a novel, relatively common variant of ADCY6 parallels an increase in adenylyl cyclase activity and adenylyl cyclase-mediated function in humans.

Arginine (CGC) codon targeting in the human prostacyclin receptor gene (PTGIR) and G-protein coupled receptors (GPCR)

The human prostacyclin receptor (hIP) has recently been recognized as an important seven transmembrane G-protein coupled receptor that plays critical roles in atheroprevention and cardioprotection. To date, four non-synonymous genetic variants have been identified, two of which occur at the same Arg amino acid position (R212H, R212C). This observation instigated further genetic screening for prostacyclin receptor variants on 1455 human genomic samples. A total of 31 distinct genetic variants were detected, with 6 (19%) involving Arg residues. Distinct differences in location and frequencies of genetic variants were noted between Caucasian, Asian, Hispanic and African Americans, with the most changes noted in the Asian cohort. From the sequencing results, three Arg-targeted changes at the same 212 position within the third cytoplasmic loop of the human prostacyclin (hIP) receptor were detected: 1) R212C (CGC-->TGC), 2) R212H (CGC-->CAC), and 3) R212R (CGC-->CGT). Three additional Arg codon variants (all exhibiting the same CGC to TGC change) were also detected, R77C, R215C, and R279C. Analysis (GPCR and SNP databases) of 200 other GPCRs, with recorded non-synonymous mutations, confirmed a high frequency of Arg-targeted missense mutations, particularly within the important cytoplasmic domain. Preferential nucleotide changes (at Arg codons), were observed involving cytosine (C) to thymine (T) (pyrimidine to pyrimidine), as well as guanine (G) to adenine (A) (purine to purine) (p<0.001, Pearson's goodness-of-fit test). Such targeting of Arg residues, leading to significant changes in coding amino acid size and/or charge, may have potentially-important structural and evolutionary implications on the hIP and GPCRs in general. In the case of the human prostacyclin receptor, such alterations may reduce the cardio-, vasculo-, and cytoprotective effects of prostacyclin.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Clinical Pharmacology and Pharmacotherapy of Opioid Switching in Cancer Patients

Pain is one of the most common and often most feared symptoms in patients with cancer. Ongoing or progressive pain is physically debilitating and has a marked impact on quality of life. Since a third of the population will die from cancer, and of these, 80% will experience severe pain in their final year of life, effective treatment of cancer-related pain remains both a high priority and an ongoing challenge in clinical practice. Individuals with moderate to severe cancer-related pain require treatment with strong analgesics, namely opioids. There is evidence to support the therapeutic maneuver of opioid switching in clinical practice, but further evidence is needed to elucidate the underlying mechanisms for interindividual differences in response to different opioids. Large, robust clinical trials will be needed if clinical differences among side-effect profiles of different opioids are to be clearly demonstrated. This review discusses candidate genes, which contribute to opioid response; many other genes have also been implicated in "pain" from animal or human studies. In order to continue to evaluate the genetic contributions to both pain susceptibility and analgesic response, further candidate genes need to be considered. Good pain control remains a high priority for clinicians and patients, and there is much work to be done to further individualize analgesic therapy for patients with cancer.

The challenge of drug discovery of a GPCR target: Analysis of preclinical pharmacology of histamine H3 antagonists/inverse agonists

Although the histamine H(3) receptor was identified pharmacologically in 1983, and despite widespread pharmaceutical interest in the target, no compound interacting specifically with this site has undergone successful clinical examination to develop the necessary proof-of-concept data. Therefore, clinical knowledge of the therapeutic potential of H(3) receptor antagonists in neuropsychiatric diseases, in metabolic diseases or in sleep disorders has yet to determine if the preclinical data that show broad efficacy in animal models of the aforementioned states are relevant to current unmet medical needs. H(3) receptors are complex, with species-related sequence differences that impact pharmacological responses. The receptors have a complex gene organization that provides opportunity for multiple slice isoforms, most of which remain poorly characterized even within a species. H(3) receptors are constitutively active, although the extent of this could vary either between species and/or receptor splice isoforms, both of which may provide opportunity for preferential coupling to different G-proteins. Thus, it is not surprising that the pharmacological effects of known H(3) ligands are complex and diverse, since these agents may act both as agonists and antagonists in different systems. Moreover, other compounds show inverse agonism in some models but neutral antagonist activity in others. Some of this diversity may be related to different ligand-dependent receptor activation states or to the effects of key amino acids important for ligand recognition. This commentary provides an overview of these complexities as applied to the H(3) receptor and the challenges these intricacies create for drug discovery.

Phenotology of disease-linked proteins

Are there analogous sequence positions in families of related proteins where disease-linked mutations occur with unusually high frequency? We attempt to answer this question by examining sequence alignments for G-protein coupled receptors (GPCRs) and voltage-gated potassium channels that have a significant number of missense mutations linked to some form of human disease. When the disease-linked mutations are mapped onto the sequences for each family, there are a large number of aligned sites at which disease-linked mutations occur in more than one protein. The statistical significance of the aligned sites is judged by analysis of artificially-generated random datasets. There are a modest number of aligned sites that are statistically significant-we refer to these as "phenotologous" sequence positions. Phenotologous sites represent aligned positions at which mutations linked to disease phenotypes occur with high frequency within a family of proteins. The most interesting of these sites are those which are not conserved-such sites are apparently critical in defining structural or functional differences between related proteins. Phenotology may be used to make experimentally testable predictions regarding medical genetics, the molecular basis of disease, and protein structure-function relationships.

Recent developments in constitutive receptor activity and inverse agonism, and their potential for GPCR drug discovery

The concept of constitutively active G-protein-coupled receptors is now firmly rooted in receptor pharmacology. Many independent research groups have contributed to its acceptance since its introduction by Costa and Herz in 1989. This concept necessitated a revised ligand classification, and a new category of inverse agonists was introduced alongside existing agonist and antagonist ligands. Initially, it was hoped that new therapeutic modalities would become available. However, the drug industry has not adopted inverse agonism as a design criterion and instead accepted that some compounds emerge as (neutral) antagonists in compound screening, whereas other compounds possess inverse agonistic activity. In this article, we summarize aspects of the impact of constitutive activity on the drug-discovery process: for example, its use in orphan receptor assays, its link with pharmacogenetics and genomics, and its relevance for currently marketed drugs.

The G protein-coupled receptors: pharmacogenetics and disease

Genetic variation in G-protein coupled receptors (GPCRs) is associated with a wide spectrum of disease phenotypes and predispositions that are of special significance because they are the targets of therapeutic agents. Each variant provides an opportunity to understand receptor function that complements a plethora of available in vitro data elucidating the pharmacology of the GPCRs. For example, discrete portions of the proximal tail of the dopamine D1 receptor have been discovered, in vitro, that may be involved in desensitization, recycling and trafficking. Similar in vitro strategies have been used to elucidate naturally occurring GPCR mutations. Inactive, over-active or constitutively active receptors have been identified by changes in ligand binding, G-protein coupling, receptor desensitization and receptor recycling. Selected examples reviewed include those disorders resulting from mutations in rhodopsin, thyrotropin, luteinizing hormone, vasopressin and angiotensin receptors. By comparison, the recurrent pharmacogenetic variants are more likely to result in an altered predisposition to complex disease in the population. These common variants may affect receptor sequence without intrinsic phenotype change or spontaneous induction of disease and yet result in significant alteration in drug efficacy. These pharmacogenetic phenomena will be reviewed with respect to a limited sampling of GPCR systems including the orexin/hypocretin system, the beta2 adrenergic receptors, the cysteinyl leukotriene receptors and the calcium-sensing receptor. These developments will be discussed with respect to strategies for drug discovery that take into account the potential for the development of drugs targeted at mutated and wild-type proteins.

Genetic variation in G-protein-coupled receptors – consequences for G-protein-coupled receptors as drug targets

G-protein-coupled receptors (GPCRs), including 'orphan' GPCRs whose natural ligands are unknown, comprise the largest membrane receptor superfamily and are the most commonly used therapeutic targets. GPCR genetic loci harbour numerous variants, such as DNA insertions or deletions and single nucleotide polymorphisms that alter GPCR expression and function, thereby contributing to inter-individual differences in disease susceptibility/progression and drug responses. In this article, the authors review examples of GPCR genetic variants that influence transcription, translation, receptor folding and expression on cell surface (by affecting receptor trafficking, dimerisation, desensitisation/downregulation), or perturb receptor function (by altering ligand binding, G-protein coupling and receptor constitutive activity). In spite of such effects, assessment for genetic variants is not currently applied to the drug development and approval process or in the clinical use of GPCR drugs. Further insights will, the authors believe, alter drug discovery/development, therapeutics and likely provide new GPCR drug targets.

Pharmacological properties of the Cys23Ser single nucleotide polymorphism in human 5-HT2C receptor isoforms

The human serotonin 2C (5-HT2C) receptor undergoes extensive RNA editing, generating multiple isoforms; the most prominent isoform in the human brain is the extensively edited VSV isoform. In addition, a naturally occurring single nucleotide polymorphism (SNP) is found in the coding region of the 5-HT2C receptor gene, which converts cysteine to serine at the 23rd amino acid (C23S). To elucidate the functional consequences, pharmacological properties were evaluated in cells expressing C23 or S23 in the nonedited, INI, or edited, VSV, isoform. Confocal imaging of HEK293 cells expressing the C23 and S23 variants revealed no apparent difference in cellular localization, which was confirmed in NIH-3T3 fibroblasts by surface biotinylation. Competition binding experiments revealed comparable high-affinity agonist binding for the C23 and S23 receptors and no difference in ligand affinities in either the INI or VSV backbones. The dose-response functions for 5-HT and (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI) to elicit phosphoinositide hydrolysis did not differ in either HEK293 or NIH-3T3 fibroblasts expressing the receptor variants. Constitutive activity, evaluated in COS-7 and HEK293 cells, also was not different. Lastly, fluorescence resonance energy transfer demonstrated homodimerization of C23 receptors, which was reproduced in cells expressing the S23 variant. We conclude that the C23S SNP in the 5-HT2C receptor has no functional consequences, even when evaluated in the most common, edited receptor backbone. Therefore, positive associations between this polymorphism and disease states may be a consequence of linkage disequilibrium with another SNP that is involved in the disease.

Characterization of GPRA, a novel G protein-coupled receptor related to asthma

We recently identified a novel positional asthma susceptibility gene, GPRA, which belongs to the G protein-coupled receptor family. In the present studies, we show that isoform specific activation of GPRA-A with its agonist, Neuropeptide S (NPS) resulted in significant inhibition of cell growth. GPRA has several variants due to extensive alternative splicing. We observed that only the full-length variants, GPRA-A and GPRA-B, with 7 transmembrane topology are transported into the plasma membrane, while the truncated proteins retain intracellular compartments. To clarify disease mechanism, we studied co-expression of the variants without finding any indication that truncated variants would inhibit the receptor transport into the plasma membrane. By using in situ hybridization and immunohistochemistry, we detected ubiquitous expression of GPRA-B, and frequent expression of GPRA-A in the epithelia of several organs including bronchi and gastrointestinal tract. Furthermore, we observed aberrant mRNA and protein expression levels of GPRA in the asthmatic bronchi. Finally, we demonstrate that GPRA and NPS are co-expressed in bronchial epithelium. In summary, this study provides evidence that GPRA might have functional relevance in modulating asthma by increased expression levels in the relevant tissues under diseased state and by potential inhibitory effect of GPRA-A activation on cell growth.

Human adenosine 2B receptor: SNP discovery and evaluation of expression in patients with cystic fibrosis

OBJECTIVES AND METHODS

We analyzed the adenosine 2B (A2B) receptor gene, which consists of two exons, for single nucleotide polymorphisms (SNPs) and tested the hypothesis that coding sequence polymorphisms in the gene contribute to disease state in patients with cystic fibrosis (CF). Using PCR and restriction fragment length polymorphism (RFLP) analysis, we assessed 53 American subjects of mixed ethnicity, 64 European Caucasian control subjects and 148 Caucasian patients with CF for A2B SNPs.

RESULTS

We identified one SNP in the 5' untranslated region (UTR) and seven SNPs in the open reading frame. One SNP was identified in the coding region of a German patient with CF but in none of the American subjects. No other SNPs were found in CF patients. A nonsynonymous SNP was identified in exon 2 of the German controls with an allelic frequency of 11%. The US subjects were of mixed ethnicity and most frequently (10.4%) had a SNP in the 5'UTR; 7 coding SNPs occurred in <3%. All SNPs other than the Leu96Phe and Gly136Arg variants were found only in African-Americans. Of the 26 African-Americans who were genotyped, 42% were hypertensive but the study group was too small to show an association between blood pressure status and SNP expression.

CONCLUSIONS

The data show that SNPs in the A2B receptor gene are much more frequent in African-Americans than in Caucasians and that German Caucasians, but not African-Americans, express Gly136Arg. None of the SNPs identified in A2B receptors are likely to be modifiers in CF.