A novel parathyroid hormone (PTH)/PTH-related peptide receptor mutation in Jansen's metaphyseal chondrodysplasia

Identification of a novel heterozygous PTH1R variant in a Chinese family with incomplete penetrance

BACKGROUND

Mutations in PTH1R are associated with Jansen-type metaphyseal chondrodysplasia (JMC), Blomstrand osteochondrodysplasia (BOCD), Eiken syndrome, enchondroma, and primary failure of tooth eruption (PFE). Inheritance of the PTH1R gene can be either autosomal dominant or autosomal recessive, indicating the complexity of the gene. Our objective was to identify the phenotypic differences in members of a family with a novel PTH1R mutation.

METHODS

The proband was a 13-year, 6-month-old girl presenting with short stature, abnormal tooth eruption, skeletal dysplasia, and midface hypoplasia. The brother and father of the proband presented with short stature and abnormal tooth eruption. High-throughput sequencing was performed on the proband, and the variant was confirmed in the proband and other family members by Sanger sequencing. Amino acid sequence alignment was performed using ClustalX software. Three-dimensional structures were analyzed and displayed using the I-TASSER website and PyMOL software.

RESULTS

High-throughput genome sequencing and Sanger sequencing validation showed that the proband, her father, and her brother all carried the PTH1R (NM_000316) c.1393G>A (p.E465K) mutation. The c.1393G>A (p.E465K) mutation was novel, as it has not been reported in the literature database. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, the p.E465K variant was considered to have uncertain significance. Biological information analysis demonstrated that this identified variant was highly conserved and highly likely pathogenic.

CONCLUSIONS

We identified a novel heterozygous mutation in the PTH1R gene leading to clinical manifestations with incomplete penetrance that expands the spectrum of known PTH1R mutations.

Primary failure of tooth eruption: Etiology and management

Primary failure of eruption (PFE) is a rare disorder defined as incomplete tooth eruption despite the presence of a clear eruption pathway. PFE is known to be caused by rare variants in the parathyroid hormone 1 receptor gene (PTH1R). Although several PTH1R variants have been reported, the etiology of PFE remains unclear. However, important studies that help elucidate the pathology of PFE have recently been published. The purpose of this review is to summarize current treatment options, clinical symptoms or phenotypes for diagnosis, genetic information including solid evidence in mouse disease models and disease-specific induced pluripotent stem cells, thus approaching the etiology of PFE from the perspective of the latest research.

Genetic causes of neonatal and infantile hypercalcaemia

The causes of hypercalcaemia in the neonate and infant are varied, and often distinct from those in older children and adults. Hypercalcaemia presents clinically with a range of symptoms including failure to thrive, poor feeding, constipation, polyuria, irritability, lethargy, seizures and hypotonia. When hypercalcaemia is suspected, an accurate diagnosis will require an evaluation of potential causes (e.g. family history) and assessment for physical features (such as dysmorphology, or subcutaneous fat deposits), as well as biochemical measurements, including total and ionised serum calcium, serum phosphate, creatinine and albumin, intact parathyroid hormone (PTH), vitamin D metabolites and urinary calcium, phosphate and creatinine. The causes of neonatal hypercalcaemia can be classified into high or low PTH disorders. Disorders associated with high serum PTH include neonatal severe hyperparathyroidism, familial hypocalciuric hypercalcaemia and Jansen's metaphyseal chondrodysplasia. Conditions associated with low serum PTH include idiopathic infantile hypercalcaemia, Williams-Beuren syndrome and inborn errors of metabolism, including hypophosphatasia. Maternal hypocalcaemia and dietary factors and several rare endocrine disorders can also influence neonatal serum calcium levels. This review will focus on the common causes of hypercalcaemia in neonates and young infants, considering maternal, dietary, and genetic causes of calcium dysregulation. The clinical presentation and treatment of patients with these disorders will be discussed.

PTH/PTHrP Receptor Signaling, Allostery, and Structures

The parathyroid hormone (PTH) type 1 receptor (PTHR) is the canonical G protein-coupled receptor (GPCR) for PTH and PTH-related protein (PTHrP) and the key regulator of calcium homeostasis and bone turnover. PTHR function is critical for human health to maintain homeostatic control of ionized serum Ca²⁺ levels and has several unusual signaling features, such as endosomal cAMP signaling, that are well-studied but not structurally understood. In this review, we discuss how recently solved high resolution near-atomic structures of hormone-bound PTHR in its inactive and active signaling states and discovery of extracellular Ca²⁺ allosterism shed light on the structural basis for PTHR signaling and function.

Progression of Mineral Ion Abnormalities in Patients With Jansen Metaphyseal Chondrodysplasia

CONTEXT

Five different activating PTH/PTH-related peptide (PTHrP) receptor (PTHR1) mutations have been reported as causes of Jansen metaphyseal chondrodysplasia (JMC), a rare disorder characterized by severe growth plate abnormalities and PTH-independent hypercalcemia.

OBJECTIVES

Assess the natural history of clinical and laboratory findings in 24 patients with JMC and characterize the disease-causing mutant receptors in vitro.

PATIENTS AND METHODS

The H223R mutation occurred in 18 patients. T410P, I458R and I458K each occurred in single cases; T410R was present in a father and his two sons. Laboratory records were analyzed individually and in aggregate.

RESULTS

Postnatal calcium levels were normal in most patients, but elevated between 0.15 and 10 years (11.8 ± 1.37 mg/dL) and tended to normalize in adults (10.0 ± 1.03 mg/dL). Mean phosphate levels were at the lower end of the age-specific normal ranges. Urinary calcium/creatinine (mg/mg) were consistently elevated (children, 0.80 ± 0.40; adults, 0.28 ± 0.19). Adult heights were well below the 3rd percentile for all patients, except for those with the T410R mutation. Most patients with JMC had undergone orthopedic surgical procedures, most had nephrocalcinosis, and two had advanced chronic kidney disease. The five PTHR1 mutants showed varying degrees of constitutive and PTH-stimulated cAMP signaling activity when expressed in HEK293 reporter cells. The inverse agonist [L11,dW12,W23,Y36]PTHrP(7-36) reduced basal cAMP signaling for each PTHR1 mutant.

CONCLUSIONS

Except for T410R, the other PTHR1 mutations were associated with indistinguishable mineral ion abnormalities and cause similarly severe growth impairment. Hypercalciuria persisted into adulthood. An inverse agonist ligand effectively reduced in vitro PTH-independent cAMP formation at all five PTHR1 mutants, suggesting a potential path toward therapy.

Induced GnasR201H expression from the endogenous Gnas locus causes fibrous dysplasia by up-regulating Wnt/β-catenin signaling

Fibrous dysplasia (FD; Online Mendelian Inheritance in Man no. 174800) is a crippling skeletal disease caused by activating mutations of the GNAS gene, which encodes the stimulatory G protein Gα_s FD can lead to severe adverse conditions such as bone deformity, fracture, and severe pain, leading to functional impairment and wheelchair confinement. So far there is no cure, as the underlying molecular and cellular mechanisms remain largely unknown and the lack of appropriate animal models has severely hampered FD research. Here we have investigated the cellular and molecular mechanisms underlying FD and tested its potential treatment by establishing a mouse model in which the human FD mutation (R201H) has been conditionally knocked into the corresponding mouse Gnas locus. We found that the germ-line FD mutant was embryonic lethal, and Cre-induced Gnas FD mutant expression in early osteochondral progenitors, osteoblast cells, or bone marrow stromal cells (BMSCs) recapitulated FD features. In addition, mosaic expression of FD mutant Gα_s in BMSCs induced bone marrow fibrosis both cell autonomously and non-cell autonomously. Furthermore, Wnt/β-catenin signaling was up-regulated in FD mutant mouse bone and BMSCs undergoing osteogenic differentiation, as we have found in FD human tissue previously. Reduction of Wnt/β-catenin signaling by removing one Lrp6 copy in an FD mutant line significantly rescued the phenotypes. We demonstrate that induced expression of the FD Gα_s mutant from the mouse endogenous Gnas locus exhibits human FD phenotypes in vivo, and that inhibitors of Wnt/β-catenin signaling may be repurposed for treating FD and other bone diseases caused by Gα_s activation.

Hypercalcemic Disorders in Children

Hypercalcemia is defined as a serum calcium concentration that is greater than two standard deviations above the normal mean, which in children may vary with age and sex, reflecting changes in the normal physiology at each developmental stage. Hypercalcemic disorders in children may present with hypotonia, poor feeding, vomiting, constipation, abdominal pain, lethargy, polyuria, dehydration, failure to thrive, and seizures. In severe cases renal failure, pancreatitis and reduced consciousness may also occur and older children and adolescents may present with psychiatric symptoms. The causes of hypercalcemia in children can be classified as parathyroid hormone (PTH)-dependent or PTH-independent, and may be congenital or acquired. PTH-independent hypercalcemia, ie, hypercalcemia associated with a suppressed PTH, is commoner in children than PTH-dependent hypercalcemia. Acquired causes of PTH-independent hypercalcemia in children include hypervitaminosis; granulomatous disorders, and endocrinopathies. Congenital syndromes associated with PTH-independent hypercalcemia include idiopathic infantile hypercalcemia (IIH), William's syndrome, and inborn errors of metabolism. PTH-dependent hypercalcemia is usually caused by parathyroid tumors, which may give rise to primary hyperparathyroidism (PHPT) or tertiary hyperparathyroidism, which usually arises in association with chronic renal failure and in the treatment of hypophosphatemic rickets. Acquired causes of PTH-dependent hypercalcemia in neonates include maternal hypocalcemia and extracorporeal membrane oxygenation. PHPT usually occurs as an isolated nonsyndromic and nonhereditary endocrinopathy, but may also occur as a hereditary hypercalcemic disorder such as familial hypocalciuric hypercalcemia, neonatal severe primary hyperparathyroidism, and familial isolated primary hyperparathyroidism, and less commonly, as part of inherited complex syndromic disorders such as multiple endocrine neoplasia (MEN). Advances in identifying the genetic causes have resulted in increased understanding of the underlying biological pathways and improvements in diagnosis. The management of symptomatic hypercalcemia includes interventions such as fluids, antiresorptive medications, and parathyroid surgery. This article presents a clinical, biochemical, and genetic approach to investigating the causes of pediatric hypercalcemia. © 2017 American Society for Bone and Mineral Research.

Jansen Metaphyseal Chondrodysplasia due to Heterozygous H223R-PTH1R Mutations With or Without Overt Hypercalcemia

CONTEXT

Jansen's metaphyseal chondrodysplasia (JMC) is a rare skeletal dysplasia characterized by abnormal endochondral bone formation and typically severe hypercalcemia despite normal/low levels of PTH. Five different heterozygous activating PTH/PTHrP receptor (PTH1R) mutations that change one of three different amino acid residues are known to cause JMC.

OBJECTIVES

Establishing the diagnosis of JMC during infancy or early childhood can be challenging, especially in the absence of family history and/or overt hypercalcemia. We therefore sought to provide radiographic findings supporting this diagnosis early in life.

PATIENTS AND METHODS

Three patients, a mother and her two sons, had radiographic evidence for JMC. However, obvious hypercalcemia and suppressed PTH levels were encountered only in both affected children. Sanger sequencing and endonuclease (SphI) digestion of PCR-amplified genomic DNA were performed to search for the H223R-PTH1R mutation.

RESULTS

The heterozygous H223R mutation was identified in all three affected individuals. Surprisingly, however, the now 38-year-old mother was never overtly hypercalcemic and was therefore not diagnosed until her sons were found to be affected by JMC at the ages of 28 months and 40 days, respectively. The presented radiographic findings at different ages will help diagnose other infants/toddlers suspected of having JMC.

CONCLUSION

The H223R mutation is typically associated with profound hypercalcemia despite low/normal PTH levels. However, the findings presented herein show that overt hypercalcemia is not always encountered in JMC, even if caused by this relatively frequent mutation, which is similar to observations with other PTH1R mutations that show less constitutive activity.

International Union of Basic and Clinical Pharmacology. XCIII. The parathyroid hormone receptors--family B G protein-coupled receptors

The type-1 parathyroid hormone receptor (PTHR1) is a family B G protein-coupled receptor (GPCR) that mediates the actions of two polypeptide ligands; parathyroid hormone (PTH), an endocrine hormone that regulates the levels of calcium and inorganic phosphate in the blood by acting on bone and kidney, and PTH-related protein (PTHrP), a paracrine-factor that regulates cell differentiation and proliferation programs in developing bone and other tissues. The type-2 parathyroid hormone receptor (PTHR2) binds a peptide ligand, called tuberoinfundibular peptide-39 (TIP39), and while the biologic role of the PTHR2/TIP39 system is not as defined as that of the PTHR1, it likely plays a role in the central nervous system as well as in spermatogenesis. Mechanisms of action at these receptors have been explored through a variety of pharmacological and biochemical approaches, and the data obtained support a basic "two-site" mode of ligand binding now thought to be used by each of the family B peptide hormone GPCRs. Recent crystallographic studies on the family B GPCRs are providing new insights that help to further refine the specifics of the overall receptor architecture and modes of ligand docking. One intriguing pharmacological finding for the PTHR1 is that it can form surprisingly stable complexes with certain PTH/PTHrP ligand analogs and thereby mediate markedly prolonged cell signaling responses that persist even when the bulk of the complexes are found in internalized vesicles. The PTHR1 thus appears to be able to activate the Gα(s)/cAMP pathway not only from the plasma membrane but also from the endosomal domain. The cumulative findings could have an impact on efforts to develop new drug therapies for the PTH receptors.

Autosomal dominant hypoparathyroidism caused by germline mutation in GNA11: phenotypic and molecular characterization

CONTEXT

Most cases of autosomal dominant hypoparathyroidism (ADH) are caused by gain-of-function mutations in CASR or dominant inhibitor mutations in GCM2 or PTH.

OBJECTIVE

Our objectives were to identify the genetic basis for ADH in a multigenerational family and define the underlying disease mechanism.

SUBJECTS

Here we evaluated a multigenerational family with ADH in which affected subjects had normal sequences in these genes and were shorter than unaffected family members.

METHODS

We collected clinical and biochemical data from 6 of 11 affected subjects and performed whole-exome sequence analysis on DNA from two affected sisters and their affected father. Functional studies were performed after expression of wild-type and mutant Gα11 proteins in human embryonic kidney-293-CaR cells that stably express calcium-sensing receptors.

RESULTS

Whole-exome-sequencing followed by Sanger sequencing revealed a heterozygous mutation, c.179G>T; p.R60L, in GNA11, which encodes the α-subunit of G11, the principal heterotrimeric G protein that couples calcium-sensing receptors to signal activation in parathyroid cells. Functional studies of Gα11 R60L showed increased accumulation of intracellular concentration of free calcium in response to extracellular concentration of free calcium with a significantly decreased EC50 compared with wild-type Gα11. By contrast, R60L was significantly less effective than the oncogenic Q209L form of Gα11 as an activator of the MAPK pathway. Compared to subjects with CASR mutations, patients with GNA11 mutations lacked hypercalciuria and had normal serum magnesium levels.

CONCLUSIONS

Our findings indicate that the germline gain-of-function mutation of GNA11 is a cause of ADH and implicate a novel role for GNA11 in skeletal growth.

Similarity between class A and class B G-protein-coupled receptors exemplified through calcitonin gene-related peptide receptor modelling and mutagenesis studies

Modelling class B G-protein-coupled receptors (GPCRs) using class A GPCR structural templates is difficult due to lack of homology. The plant GPCR, GCR1, has homology to both class A and class B GPCRs. We have used this to generate a class A–class B alignment, and by incorporating maximum lagged correlation of entropy and hydrophobicity into a consensus score, we have been able to align receptor transmembrane regions. We have applied this analysis to generate active and inactive homology models of the class B calcitonin gene-related peptide (CGRP) receptor, and have supported it with site-directed mutagenesis data using 122 CGRP receptor residues and 144 published mutagenesis results on other class B GPCRs. The variation of sequence variability with structure, the analysis of polarity violations, the alignment of group-conserved residues and the mutagenesis results at 27 key positions were particularly informative in distinguishing between the proposed and plausible alternative alignments. Furthermore, we have been able to associate the key molecular features of the class B GPCR signalling machinery with their class A counterparts for the first time. These include the [K/R]KLH motif in intracellular loop 1, [I/L]xxxL and KxxK at the intracellular end of TM5 and TM6, the NPXXY/VAVLY motif on TM7 and small group-conserved residues in TM1, TM2, TM3 and TM7. The equivalent of the class A DRY motif is proposed to involve Arg^2.39, His^2.43 and Glu^3.46, which makes a polar lock with T^6.37. These alignments and models provide useful tools for understanding class B GPCR function.

Constitutive activation of G protein-coupled receptors and diseases: insights into mechanisms of activation and therapeutics

The existence of constitutive activity for G protein-coupled receptors (GPCRs) was first described in 1980s. In 1991, the first naturally occurring constitutively active mutations in GPCRs that cause diseases were reported in rhodopsin. Since then, numerous constitutively active mutations that cause human diseases were reported in several additional receptors. More recently, loss of constitutive activity was postulated to also cause diseases. Animal models expressing some of these mutants confirmed the roles of these mutations in the pathogenesis of the diseases. Detailed functional studies of these naturally occurring mutations, combined with homology modeling using rhodopsin crystal structure as the template, lead to important insights into the mechanism of activation in the absence of crystal structure of GPCRs in active state. Search for inverse agonists on these receptors will be critical for correcting the diseases cause by activating mutations in GPCRs. Theoretically, these inverse agonists are better therapeutics than neutral antagonists in treating genetic diseases caused by constitutively activating mutations in GPCRs.

The repertoire of G protein-coupled receptors in the sea squirt Ciona intestinalis

BACKGROUND

G protein-coupled receptors (GPCRs) constitute a large family of integral transmembrane receptor proteins that play a central role in signal transduction in eukaryotes. The genome of the protochordate Ciona intestinalis has a compact size with an ancestral complement of many diversified gene families of vertebrates and is a good model system for studying protochordate to vertebrate diversification. An analysis of the Ciona repertoire of GPCRs from a comparative genomic perspective provides insight into the evolutionary origins of the GPCR signalling system in vertebrates.

RESULTS

We have identified 169 gene products in the Ciona genome that code for putative GPCRs. Phylogenetic analyses reveal that Ciona GPCRs have homologous representatives from the five major GRAFS (Glutamate, Rhodopsin, Adhesion, Frizzled and Secretin) families concomitant with other vertebrate GPCR repertoires. Nearly 39% of Ciona GPCRs have unambiguous orthologs of vertebrate GPCR families, as defined for the human, mouse, puffer fish and chicken genomes. The Rhodopsin family accounts for ~68% of the Ciona GPCR repertoire wherein the LGR-like subfamily exhibits a lineage specific gene expansion of a group of receptors that possess a novel domain organisation hitherto unobserved in metazoan genomes.

CONCLUSION

Comparison of GPCRs in Ciona to that in human reveals a high level of orthology of a protochordate repertoire with that of vertebrate GPCRs. Our studies suggest that the ascidians contain the basic ancestral complement of vertebrate GPCR genes. This is evident at the subfamily level comparisons since Ciona GPCR sequences are significantly analogous to vertebrate GPCR subfamilies even while exhibiting Ciona specific genes. Our analysis provides a framework to perform future experimental and comparative studies to understand the roles of the ancestral chordate versions of GPCRs that predated the divergence of the urochordates and the vertebrates.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Class II G protein-coupled receptors and their ligands in neuronal function and protection

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

TGFbeta and PTHrP control chondrocyte proliferation by activating cyclin D1 expression

Exact coordination of growth plate chondrocyte proliferation is necessary for normal endochondral bone development and growth. Here we show that PTHrP and TGFbeta control chondrocyte cell cycle progression and proliferation by stimulating signaling pathways that activate transcription from the cyclin D1 promoter. The TGFbeta pathway activates the transcription factor ATF-2, whereas PTHrP uses the related transcription factor CREB, to stimulate cyclin D1 promoter activity via the CRE promoter element. Inhibition of cyclin D1 expression with antisense oligonucleotides causes a delay in progression of chondrocytes through the G1 phase of the cell cycle, reduced E2F activity, and decreased proliferation. Growth plates from cyclin D1-deficient mice display a smaller zone of proliferating chondrocytes, confirming the requirement for cyclin D1 in chondrocyte proliferation in vivo. These data identify the cyclin D1 gene as an essential component of chondrocyte proliferation as well as a fundamental target gene of TGFbeta and PTHrP during skeletal growth.

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

Genetic variations in human G protein-coupled receptors: implications for drug therapy

Numerous genes encode G protein-coupled receptors (GPCRs)-a main molecular target for drug therapy. Estimates indicate that the human genome contains approximately 600 GPCR genes. This article addresses therapeutic implications of sequence variations in GPCR genes. A number of inactivating and activating receptor mutations have been shown to cause a variety of (mostly rare) genetic disorders. However, pharmacogenetic and pharmacogenomic studies on GPCRs are scarce, and therapeutic relevance of variant receptor alleles often remains unclear. Confounding factors in assessing the therapeutic relevance of variant GPCR alleles include 1) interaction of a single drug with multiple closely related receptors, 2) poorly defined binding pockets that can accommodate drug ligands in different orientations or at alternative receptor domains, 3) possibility of multiple receptor conformations with distinct functions, and 4) multiple signaling pathways engaged by a single receptor. For example, antischizophrenic drugs bind to numerous receptors, several of which might be relevant to therapeutic outcome. Without knowing accurately what role a given receptor subtype plays in clinical outcome and how a sequence variation affects drug-induced signal transduction, we cannot predict the therapeutic relevance of a receptor variant. Genome-wide association studies with single nucleotide polymorphisms could identify critical target receptors for disease susceptibility and drug efficacy or toxicity.