Detection of skewed X-inactivation in two female carriers of vasopressin type 2 receptor gene mutation

G protein-coupled receptor (GPCR) gene variants and human genetic disease

Genetic variations in the genes encoding G protein-coupled receptors (GPCRs) can disrupt receptor structure and function, which can result in human genetic diseases. Disease-causing mutations have been reported in at least 55 GPCRs for more than 66 monogenic diseases in humans. The spectrum of pathogenic and likely pathogenic variants includes loss of function variants that decrease receptor signaling on one extreme and gain of function that may result in biased signaling or constitutive activity, originally modeled on prototypical rhodopsin GPCR variants identified in retinitis pigmentosa, on the other. GPCR variants disrupt ligand binding, G protein coupling, accessory protein function, receptor desensitization and receptor recycling. Next generation sequencing has made it possible to identify variants of uncertain significance (VUS). We discuss variants in receptors known to result in disease and in silico strategies for disambiguation of VUS such as sorting intolerant from tolerant and polymorphism phenotyping. Modeling of variants has contributed to drug development and precision medicine, including drugs that target the melanocortin receptor in obesity and interventions that reverse loss of gonadotropin-releasing hormone receptor from the cell surface in idiopathic hypogonadotropic hypogonadism. Activating and inactivating variants of the calcium sensing receptor (CaSR) gene that are pathogenic in familial hypocalciuric hypercalcemia and autosomal dominant hypocalcemia have enabled the development of calcimimetics and calcilytics. Next generation sequencing has continued to identify variants in GPCR genes, including orphan receptors, that contribute to human phenotypes and may have therapeutic potential. Variants of the CaSR gene, some encoding an arginine-rich region that promotes receptor phosphorylation and intracellular retention, have been linked to an idiopathic epilepsy syndrome. Agnostic strategies have identified variants of the pyroglutamylated RF amide peptide receptor gene in intellectual disability and G protein-coupled receptor 39 identified in psoriatic arthropathy. Coding variants of the G protein-coupled receptor L1 (GPR37L1) orphan receptor gene have been identified in a rare familial progressive myoclonus epilepsy. The study of the role of GPCR variants in monogenic, Mendelian phenotypes has provided the basis of modeling the significance of more common variants of pharmacogenetic significance.

V2 vasopressin receptor mutations: future personalized therapy based on individual molecular biology

The diluting and concentrating function of the kidney plays a crucial role in regulating the water homeostasis of the body. This function is regulated by the antidiuretic hormone, arginine vasopressin through the type 2 vasopressin receptor (V2R), allowing the body to adapt to periods of water load or water restriction. Loss-of-function mutations of the V2R cause X-linked nephrogenic diabetes insipidus (XNDI), which is characterized by polyuria, polydipsia, and hyposthenuria. Gain-of-function mutations of the V2R lead to nephrogenic syndrome of inappropriate antidiuresis disease (NSIAD), which results in hyponatremia. Various mechanisms may be responsible for the impaired receptor functions, and this review provides an overview of recent findings about the potential therapeutic interventions in the light of the current experimental data.

Genetic basis of nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus is defined as an inability to concentrate urine due to a complete or partial alteration of the renal tubular response to arginine vasopressin hormone, resulting in excessive diluted urine excretion. Hereditary forms are caused by molecular defects in the genes encoding either of the two main renal effectors of the arginine vasopressin pathway: the AVPR2 gene, which encodes for the type 2 vasopressin receptor, or the AQP2 gene, which encodes for the water channel aquaporin-2. About 90% of cases of nephrogenic diabetes insipidus result from loss-of-function variants in the AVPR2 gene, which are inherited in a X-linked recessive manner. The remaining 10% of cases result from loss-of-function variants in the AQP2 gene, which can be inherited in either a recessive or a dominant manner. The main symptoms of the disease are polyuria, chronic dehydration and hypernatremia. These symptoms usually occur in the first year of life, although some patients present later. Diagnosis is based on abnormal response in urinary osmolality after water restriction and/or administration of exogenous vasopressin. Treatment involves ensuring adequate water intake on demand, possibly combined with thiazide diuretics, non-steroidal anti-inflammatory drugs, and a low-salt and protein diet. In this review, we provide an update on current understanding of the molecular basis of inherited nephrogenic insipidus diabetes.

A Novel Missense Mutation of Arginine Vasopressin Receptor 2 in a Chinese Family with Congenital Nephrogenic Diabetes Insipidus: X-Chromosome Inactivation in Female CNDI Patients with Heterozygote 814A>G Mutation

Background

To identify novel clinical phenotypic signatures of congenital nephrogenic diabetes insipidus (CNDI).

Methods

A Chinese family with CNDI was recruited for participation in this study. The proband and one of his uncles suffered from polydipsia and polyuria since infancy. The results of clinical testing indicated the diagnosis of CNDI. 10 family members had similar symptoms but did not seek medical advice. Genetic testing of mutations in the coding region of the aquaporin 2 (AQP2) gene and the arginine vasopressin receptor 2 (AVPR2) gene were carried out in 11 family members. Somatic DNA from 5 female family members was used to test for methylation of polymorphic CAG repeats in the human androgen receptor (AR) gene, as an index for X-chromosome inactivation pattern (XCIP).

Results

AQP2 gene mutations were not found in any family members, but a novel missense mutation (814th base A>G) in exon 2 of the AVPR2 gene was identified in 10 individuals. This mutation leads to a Met 272 Val (GAT-GGT) amino acid substitution. Skewed X-chromosome inactivation patterns of the normal X allele were observed in 4 females with the AVPR2 gene mutation and symptoms of diabetes insipidus, but not in an asymptomatic female with the AVPR2 gene mutation.

Conclusions

Met 272 Val mutation of the AVPR2 gene was identified as a novel genetic risk factor for CDNI. The clinical NDI phenotype of female carriers with heterozygous AVPR2 mutation may be caused by X-chromosome inactivation induced by dominant methylation of the normal allele of AVPR2 gene.

Functional Rescue of a Nephrogenic Diabetes Insipidus Causing Mutation in the V2 Vasopressin Receptor by Specific Antagonist and Agonist Pharmacochaperones

The urine concentrating function of the kidney is essential to maintain the water homeostasis of the human body. It is mainly regulated by the arginine-vasopressin (AVP), which targets the type 2 vasopressin receptor (V2R) in the kidney. The inability of V2R to respond to AVP stimulation leads to decreased urine concentration and congenital nephrogenic diabetes insipidus (NDI). NDI is characterized by polyuria, polydipsia, and hyposthenuria. In this study, we identified a point mutation (S127F) in the AVPR2 gene of an NDI patient, and we characterized the impaired function of the V2R mutant in HEK293 cells. Based on our data, the S127F-V2R mutant is almost exclusively located intracellularly in the endoplasmic reticulum (ER), and very few receptors were detected at the cell surface, where the receptor can bind to AVP. The overexpressed S127F-V2R mutant receptor has negligible cAMP generation capability compared to the wild-type receptor in response to AVP stimulation. Since certain misfolded mutant proteins, that are retained in the ER, can be rescued by pharmacological chaperones, we examined the potential rescue effects of two pharmacochaperones on the S127F-V2R. We found that pretreatment with both tolvaptan (an established V2R inverse agonist) and MCF14 compound (a cell-permeable high-affinity agonist for the V2R) were capable of partially restoring the cAMP generating function of the receptor in response to vasopressin stimulation. According to our data, both cell permeant agonists and antagonists can function as pharmacochaperones, and serve as the starting compounds to develop medicines for patients carrying the S127F mutation.

Novel Variant of AVPR2 Giving Rise to X-Linked Congenital Nephrogenic Diabetes Insipidus in a 7-Month-Old Danish Boy

Patients affected with congenital nephrogenic diabetes insipidus (CNDI) have reduced ability to concentrate urine. Early diagnosis of CNDI is important to avoid recurrent episodes of severe dehydration. We present a Danish male suffering from typical symptoms and diagnosed with CNDI at the age of 7 months. Gene sequencing of this proband and his mother revealed a novel variant in the gene encoding the antidiuretic hormone receptor (AVPR2). The variant is a deletion of nucleotide c.151 in exon 2 of AVPR2 (GenBank NM_000054.4:c.151del). This 1bp deletion is predicted to cause a frameshift that results in tryptophan replacing valine at position 51 in AVPR2 and a premature stop codon three codons downstream (p.Val51Trpfs*3) likely resulting in faulty expression of the receptor. Identification of disease-causing variants such as the one described here contributes to precise diagnosis, especially in carriers and newborns, thus preventing the long-term physical and intellectual disability observed in some CNDI-patients.

An Orofaciodigital Syndrome 1 Patient and Her Mother Carry the Same OFD1 Mutation but Have Different X Chromosome Inactivation Patterns

Orofaciodigital syndrome 1 (OFD-1) is a rare, X-linked, dominantly inherited disorder caused by an OFD1 mutation that can cause polycystic kidneys. A 37-year-old woman on hemodialysis therapy was admitted to our hospital for trans-catheter arterial embolization therapy for enlarged polycystic kidneys. Lobulated tongue and brachydactyly were noticed, prompting an OFD1 sequencing analysis. Sequencing revealed a causal four-base-pair deletion in exon 13, both in the patient and in her mother, whose renal function had been retained. The peripheral leukocyte X chromosome inactivation pattern was skewed in the patient but not in her mother, suggesting some role in their phenotypic difference.

V2 vasopressin receptor mutations

V2 vasopressin receptor (V2R) is a member of the G protein-coupled receptor (GPCR) family in which many disease-causing mutations have been identified and thus generated much interest. Loss-of-function V2R mutations cause nephrogenic diabetes insipidus (NDI) whereas gain-of-function mutations cause nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The mechanisms underlying a V2R loss-of-function can be theoretically classified as either protein expression, localization (ER retention) or functional disorders. Functional analyses have revealed however that these mechanisms are likely to be complex. Strikingly, V2R mutations at the same site can result in opposite phenotypes, e.g., R137H and R137L/C cause NDI and NSIAD, respectively. These findings support the notion that the constitutive activation of GPCRs might be often associated with their instability and denaturation. Thus, functional analysis of disease-causing V2R mutations may not only reveal potential new treatment strategies using pharmacochaperones for NDI and inverse agonists for NSIAD, but also provide a greater understanding of the physiological functions of GPCRs and highlight the new paradigms, i.e., biased agonism and protean agonism.

Nephrogenic Diabetes Insipidus

Body fluid homeostasis is essential for normal life. In the maintenance of water balance, the most important factor and regulated process is the excretory function of the kidneys. The kidneys are capable to compensate not only the daily fluctuations of water intake but also the consequences of fluid loss (respiration, perspiration, sweating, hemorrhage). The final volume and osmolality of the excreted urine is set in the collecting duct via hormonal regulation. The hormone of water conservation is the vasopressin (AVP), and a large volume of urine is produced and excreted in the absence of AVP secretion or if AVP is ineffective in the kidneys. The aquaporin-2 water channel (AQP2) is expressed in the principal cells, and it plays an essential role in the reabsorption of water in the collecting ducts via type 2 vasopressin receptor (V2R)-mediated mechanism. If neural or hormonal regulation fails to operate the normal function of AVP-V2R-AQP2 system, it can result in various diseases such as diabetes insipidus (DI) or nephrogenic syndrome of inappropriate diuresis (NSIAD). The DI is characterized by excessive production of hyposmotic urine ("insipidus" means tasteless) due to the inability of the kidneys to concentrate urine. In this chapter, we focus and discuss the pathophysiology of nephrogenic DI (NDI) and the potential therapeutic interventions in the light of the current experimental data.

Contiguous 22.1-kb deletion embracing AVPR2 and ARHGAP4 genes at novel breakpoints leads to nephrogenic diabetes insipidus in a Chinese pedigree

Background

It has been reported that mutations in arginine vasopressin type 2 receptor (AVPR2) cause congenital X-linked nephrogenic diabetes insipidus (NDI). However, only a few cases of AVPR2 deletion have been documented in China.

Methods

An NDI pedigree was included in this study, including the proband and his mother. All NDI patients had polyuria, polydipsia, and growth retardation. PCR mapping, long range PCR and sanger sequencing were used to identify genetic causes of NDI.

Results

A novel 22,110 bp deletion comprising AVPR2 and ARH4GAP4 genes was identified by PCR mapping, long range PCR and sanger sequencing. The deletion happened perhaps due to the 4-bp homologous sequence (TTTT) at the junctions of both 5′ and 3′ breakpoints. The gross deletion co-segregates with NDI. After analyzing available data of putative clinical signs of AVPR2 and ARH4GAP4 deletion, we reconsider the potential role of AVPR2 deletion in short stature.

Conclusions

We identified a novel 22.1-kb deletion leading to X-linked NDI in a Chinese pedigree, which would increase the current knowledge in AVPR2 mutation.

Genetics of Diabetes Insipidus

Diabetes insipidus is a disease characterized by polyuria and polydipsia due to inadequate release of arginine vasopressin from the posterior pituitary gland (neurohypophyseal diabetes insipidus) or due to arginine vasopressin insensitivity by the renal distal tubule, leading to a deficiency in tubular water reabsorption (nephrogenic diabetes insipidus). This article reviews the genetics of diabetes insipidus in the context of its diagnosis, clinical presentation, and therapy.

A novel AVPR2 gene mutation of X-linked congenital nephrogenic diabetes insipidus in an Asian pedigree

Polyuria and polydipsia are the characteristics of congenital nephrogenic diabetes insipidus (CNDI). Approximately 90% of all patients with CNDI have X-linked hereditary disease, which is due to a mutation of the arginine vasopressin receptor 2 (AVPR2) gene. This case report describes a 54-year-old male with polyuria and polydipsia and several male members of his pedigree who had the same symptoms. The proband was diagnosed with diabetes insipidus using a water-deprivation and arginine vasopressin stimulation test. Genomic DNA from the patient and his family members was extracted and the AVPR2 gene was sequenced. A novel missense mutation of a cytosine to guanine transition at position 972 (c.972C > G) was found, which resulted in the substitution of isoleucine for methionine at amino acid position 324 (p.I324M) in the seventh transmembrane domain of the protein. The proband’s mother and daughter were heterozygous for this mutation. The novel mutation of the AVPR2 gene further broadens the phenotypic spectrum of the AVPR2 gene.

Novel AVPR2 mutation causing partial nephrogenic diabetes insipidus in a Japanese family

BACKGROUND

X-linked recessive congenital nephrogenic diabetes insipidus (NDI) is caused by mutations of the arginine vasopressin type 2 receptor gene (AVPR2). More than 200 mutations of the AVPR2 gene with complete NDI have been reported although only 15 mutations with partial NDI has been reported to date.

METHODS

We herein report a Japanese kindred with partial NDI. The proband is an 8-year-old boy who was referred to our hospital for nocturnal enuresis. Water deprivation test and hypertonic saline test suggested partial renal antidiuretic hormone arginine vasopressin (AVP) resistance.

RESULTS

Analysis of genomic DNA revealed a novel missense mutation (p.L161P) in the patient. The patient's mother was heterozygous for the mutation. Three-dimensional (3-D) modeling study showed that L161P possibly destabilizes the transmembrane domain of the V2 receptor, resulting in its misfolding or mislocalization.

CONCLUSIONS

Distinguishing partial NDI from nocturnal enuresis is important. A clinical clue for diagnosis of partial NDI is an incompatibly high level of AVP despite normal serum osmolality.

An unusual case of hereditary nephrogenic diabetes insipidus (HNDI) affecting mother and daughter

Hereditary nephrogenic diabetes iInsipidus (HNDI) is an uncommon disorder due to a resistance to anti-diuretic hormone leading to a reduced urinary concentrating ability. The X-linked form is fully expressed in hemizygous male patients, but diabetes insipidus may also present in heterozygous females where it must be distinguished from autosomal and other secondary causes. We report a mother and daughter in the same family with HNDI due to a heterozygous deletion in exon 1 of the AVPR2 gene, not previously described in the literature. A 5-year-old girl was referred for investigation of polyuria and polydipsia. The patient had a water deprivation test elsewhere at the age of 3 that was inconclusive. A degree of water restriction was imposed leading to headaches. The thyroid, cortisol, renal, and calcium profiles were normal. Her mother showed similar symptoms that had not been previously investigated. AQP2 (Aquaporin) and initial AVPR2 gene sequencing had not identified a mutation, but subsequent quantitative polymerase chain reaction analysis revealed a heterozygous large exon 1 deletion of the AVPR2 gene. The same deletion was also found in the child's mother. The patient's symptoms have significantly improved on appropriate treatment. Further analysis revealed skewed X inactivation in mother and daughter.

Novel mutations associated with nephrogenic diabetes insipidus. A clinical-genetic study

Molecular diagnosis is a useful diagnostic tool in primary nephrogenic diabetes insipidus (NDI), an inherited disease characterized by renal inability to concentrate urine. The AVPR2 and AQP2 genes were screened for mutations in a cohort of 25 patients with clinical diagnosis of NDI. Patients presented with dehydration, polyuria-polydipsia, failure to thrive (mean ± SD; Z-height -1.9 ± 2.1 and Z-weight -2.4 ± 1.7), severe hypernatremia (mean ± SD; Na 150 ± 10 mEq/L), increased plasma osmolality (mean ± SD; 311 ± 18 mOsm/Kg), but normal glomerular filtration rate. Genetic diagnosis revealed that 24 male patients were hemizygous for 17 different putative disease-causing mutations in the AVPR2 gene (each one in a different family). Of those, nine had not been previously reported, and eight were recurrent. Moreover, we found those same AVPR2 changes in 12 relatives who were heterozygous carriers. Further, in one female patient, AVPR2 gene study turned out to be negative and she was found to be homozygous for the novel AQP2 p.Ala86Val alteration.

CONCLUSION

Genetic analysis presumably confirmed the diagnosis of nephrogenic diabetes insipidus in every patient of the studied cohort. We emphasize that we detected a high presence (50 %) of heterozygous females with clinical NDI symptoms.

WHAT IS KNOWN

• In most cases (90 %), inherited nephrogenic diabetes insipidus (NDI) is an X-linked disease, caused by mutations in the AVPR2 gene. • In rare occasions (10 %), it is caused by mutations in the AQP2 gene. What is new: • In this study, we report 10 novel mutations associated with NDI. • We have detected a high presence (50 %) of heterozygous carriers with clinical NDI symptoms.

G protein-coupled receptor mutations and human genetic disease

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

A large deletion of the AVPR2 gene causing severe nephrogenic diabetes insipidus in a Turkish family

X-linked nephrogenic diabetes insipidus (NDI) is a rare hereditary disease caused by mutations in arginine vasopressin type 2 receptor (AVPR2) and characterized by the production of large amounts of urine and an inability to concentrate urine in response to the antidiuretic hormone vasopressin. We have identified a novel 388 bp deletion starting in intron 1 and ending in exon 2 in the AVPR2 gene in a patient with NDI and in his family. We have revealed that this mutation is a de novo mutation for the mother of the proband patient. Prospective clinical data were collected for all family members. The water deprivation test confirmed the diagnosis of diabetes insipidus. The patient has severe symptoms like deep polyuria nocturia, polydipsia, and fatigue. He was given arginine vasopressin treatment while he was a child. However, he could not get well due to his nephrogenic type of illness. Both of his nephews have the same complains in addition to failure to grow. We have sequenced all exons and intron-exon boundaries of the AVPR2 gene of all family members. The analyses of bioinformatics and comparative genomics of the deletion were done via considering the DNA level damage. AVPR2 gene mutation results in the absence of the three transmembrane domains, two extracellular domains, and one cytoplasmic domain. Three-dimensional protein structure prediction was shown. We concluded that X-linked NDI and severity of illness in this family is caused by a novel 388 bp deletion in the AVPR2 gene that is predicted to truncate the receptor protein, and also this deletion may lead to dysfunctioning in protein activity and inefficient or inadequate binding abilities.

Congenital nephrogenic diabetes insipidus: the current state of affairs

The anti-diuretic hormone arginine vasopressin (AVP) is released from the pituitary upon hypovolemia or hypernatremia, and regulates water reabsorption in the renal collecting duct principal cells. Binding of AVP to the arginine vasopressin receptor type 2 (AVPR2) in the basolateral membrane leads to translocation of aquaporin 2 (AQP2) water channels to the apical membrane of the collecting duct principal cells, inducing water permeability of the membrane. This results in water reabsorption from the pro-urine into the medullary interstitium following an osmotic gradient. Congenital nephrogenic diabetes insipidus (NDI) is a disorder associated with mutations in either the AVPR2 or AQP2 gene, causing the inability of patients to concentrate their pro-urine, which leads to a high risk of dehydration. This review focuses on the current knowledge regarding the cell biological aspects of congenital X-linked, autosomal-recessive and autosomal-dominant NDI while specifically addressing the latest developments in the field. Based on deepened mechanistic understanding, new therapeutic strategies are currently being explored, which we also discuss here.

Hereditary nephrogenic diabetes insipidus in Japanese patients: analysis of 78 families and report of 22 new mutations in AVPR2 and AQP2

BACKGROUND

Familial form of nephrogenic diabetes insipidus (NDI) is a rare hereditary disease caused by arginine vasopressin type 2 receptor (AVPR2) or water channel aquaporin 2 (AQP2) gene mutations. It is speculated that 90% of NDI families carry disease-causing mutations in AVPR2 and 10% carry the mutations in AQP2; however, these percentages have not been supported by actual data. It is also unknown whether these percentages vary in different ethnic groups.

METHODS

Gene mutation analyses were performed for 78 Japanese NDI families. All exons and intron-exon boundaries of the AVPR2 and AQP2 genes were directly sequenced.

RESULTS

A total of 62 families (79%) carried disease-causing mutations in AVPR2, while nine families (12%) carried mutations in AQP2. We identified 22 novel putatively disease-causing mutations (19 in AVPR2 and 3 in AQP2). Regarding AVPR2, 52 disease-causing mutations were identified. Among them, missense mutations were most common (54%), followed by deletion mutations. In the 64 women who had monoallelic disease-causing AVPR2 mutations, 16 (25%) had NDI symptoms, including 4 complete NDI subjects. Regarding AQP2, 9 disease-causing mutations were identified in nine families. Two missense mutations and one deletion mutation showed a recessive inheritance, while one missense mutation and five small deletion mutations in the C-terminus of AQP2 showed a dominant inheritance.

CONCLUSIONS

Most Japanese NDI families carry disease-causing mutations in AVPR2 and 12% carry mutations in AQP2. A total of 22 novel putatively disease-causing mutations were identified. The relatively high occurrence of symptomatic carriers of AVPR2 mutations needs attention.

Heritability of serum sodium concentration: evidence for sex- and ethnic-specific effects

Serum sodium concentration is the clinical index of systemic water balance. Although disordered water balance is common and morbid, little is known about genetic effects on serum sodium concentration at the population level. Prior studies addressed only participants of European descent and either failed to demonstrate significant heritability or showed only modest effect. We investigated heritability of serum sodium concentration in large cohorts reflecting a range of races/ethnicities, including the Framingham Heart Study (FHS, non-Hispanic Caucasian), the Heredity and Phenotype Intervention Heart Study (HAPI, Amish Caucasian), the Jackson Heart Study (JHS, African American), the Strong Heart Family Study (SHFS, American Indian), and the Genetics of Kidney Disease in Zuni Indians Study (GKDZI, American Indian). Serum sodium was transformed for the osmotic effect of glucose, and participants with markedly elevated glucose or reduced estimated glomerular filtration rate (eGFR) were excluded. Using a standard variance components method, incorporating covariates of age, glucose, and eGFR, we found heritability to be high in African American and American Indian populations and much more modest in non-Hispanic Caucasian populations. Estimates among females increased after stratification on sex and were suggestive among female participants in FHS (0.18 ± 0.12, P = 0.057) and male participants in JHS (0.24 ± 0.16, P = 0.067) and statistically significant among female participants in JHS (0.44 ± 0.09, P = 1 × 10 ⁻⁷), SHFS (0.59 ± 0.05, P = 9.4 × 10⁻⁴⁶), and GKDZI (0.46 ± 0.15, P = 1.7 × 10⁻⁴), and male participants in HAPI (0.18 ± 0.12, P = 0.03) and SHFS (0.67 ± 0.07, P = 5.4 × 10⁻²⁶). Exclusion of diuretic users increased heritability among females and was significant in all cohorts where data were available. In aggregate, these data strongly support the heritability of systemic water balance and underscore sex and ethnicity-specific effects.

A novel mutation in the AVPR2 gene (222delA) associated with X-linked nephrogenic diabetes insipidus in a boy with growth failure

OBJECTIVE

To study the case of a 2 10/12-year-old boy who had growth failure and delayed bone maturation.

METHODS

We reviewed the history, which revealed that he had had polyuria, polydipsia, lack of weight gain, and frequent vomiting since the age of 5 months. On physical examination, his height was 86 cm (-1.93 standard deviation [SD]), his weight 10.5 kg (-2.67 SD), and he had motor and mental retardation. His maternal great-grandfather also had polyuria and polydipsia (but not diabetes mellitus), suggesting X-linked nephrogenic diabetes insipidus as the underlying cause. The patient underwent a water deprivation-desmopressin test. The coding region of the AVPR2 gene was amplified by polymerase chain reaction and submitted to direct sequence analysis.

RESULTS

The water deprivation test confirmed the diagnosis of diabetes insipidus, and administration of desmopressin did not diminish his water secretion. Direct sequencing of the AVPR2 gene revealed a novel deletion of adenine at position 222 (222delA) in exon 2. This mutation is predicted to lead to a frameshift beginning at amino acid 75 and a premature stop codon at position 115 (FS75>115X). His height and weight, as well as his motor skills, improved after initiation of therapy with hydrochlorothiazide and amiloride.

CONCLUSION

Growth delay can be associated with diabetes insipidus. The X-linked nephrogenic diabetes insipidus in this boy is caused by a novel mutation in the AVPR2 gene that is predicted to truncate the receptor protein.

Skewed X-chromosome inactivation causing diagnostic misinterpretation in congenital nephrogenic diabetes insipidus

OBJECTIVE

To establish the clinical phenotype and genetic background in a family with diabetes insipidus.

MATERIAL AND METHODS

The subjects were a sister and brother, aged 34 and 27 years, respectively, with a history of polyuria since infancy. Clinical testing confirmed a diagnosis of congenital nephrogenic diabetes insipidus (CNDI) in both. Samples of purified genomic DNA were analysed.

RESULTS

The sequence of the entire coding region of the AQP2 gene as well as the AVPR2 gene was determined. Sequence analysis revealed no variations in the AQP2 gene. A missense variation in exon 2 of the AVPR2 gene (g.685G>A), predicting a p.Asp85Asn substitution, was identified in the X-chromosome of the affected male and one allele in the sister and the asymptomatic mother. The p.Asp85Asn variation in AVPR2 is known to cause CNDI, and has previously been described as inducing a partial phenotype treatable with dDAVP. However, in this family dDAVP had no influence on urine osmolality, whereas combination therapy with indomethacin and hydrochlorothiazide increased urine osmolality to 299 mosm/l in the proband. A skewed X-inactivation pattern (93%) occurring in the normal X allele was recognized in the sister.

CONCLUSIONS

This study demonstrates the effect of skewed X-chromosome inactivation associated with X-linked CNDI. Polydipsia in early childhood could be due to X-linked CNDI despite affecting both genders. The significant heterogeneity in the clinical phenotype in CNDI carries a risk of diagnostic misinterpretation and emphasizes the need for genetic characterization. Treatment combining indomethacin and hydrochlorothiazide results in a marked response on both urine output and urine osmolality.

Absence of AVPR2 copy number variation in eunatremic and dysnatremic subjects in non-Hispanic Caucasian populations

Copy number variation (CNV) is increasingly recognized as a source of phenotypic variation among humans. We hypothesized that a CNV in the human arginine vasopressin receptor-2 gene (AVPR2) would be associated with serum sodium concentration based on the following lines of evidence: 1) the protein product of the AVPR2 gene is essential for renal water conservation; 2) mutations in the AVPR2 gene are associated with aberrant water balance in humans; 3) heritability of serum sodium concentration may be greater in females than in males; 4) the AVPR2 gene is X-linked; and 5) a common CNV spanning the AVPR2 gene was recently described in a non-Hispanic Caucasian population. We developed a highly reproducible assay for AVPR2 CNV. Among 279 subjects with measured serum sodium concentration in the Offspring Cohort of the Framingham Heart Study, no subjects exhibited CNV at the AVPR2 locus. Among 517 subjects in the Osteoporotic Fractures in Men Study (MrOS)-including 152 with hyponatremia and 183 with hypernatremia-no subjects with CNV at the AVPR2 locus were identified. CNV at the AVPR2 locus could not be independently confirmed, and CNV at the AVPR2 gene is unlikely to influence systemic water balance on a population-wide basis in non-Hispanic Caucasian subjects. A novel AVPR2 single nucleotide polymorphism affecting the reporter hybridization site gave rise to an artifactually low copy number signal (i.e., less than unity) in one male African American subject. Reanalysis of the original comparative genomic hybridization data revealed bona fide CNVs flanking-but not incorporating-the AVPR2 gene, consistent with our new genotyping data.

AVPR2 variants and mutations in nephrogenic diabetes insipidus: review and missense mutation significance

Almost 90% of nephrogenic diabetes insipidus (NDI) is due to mutations in the arginine-vasopressin receptor 2 gene (AVPR2). We retrospectively examined all the published mutations/variants in AVPR2. We planned to perform a comprehensive review of all the AVPR2 mutations/variants and to test whether any amino acid change causing a missense mutation is significantly more or less common than others. We performed a Medline search and collected detailed information regarding all AVPR2 mutations and variants. We performed a frequency comparison between mutated and wild-type amino acids and codons. We predicted the mutation effect or reported it based on published in vitro studies. We also reported the ethnicity of each mutation/variant carrier. In summary, we identified 211 AVPR2 mutations which cause NDI in 326 families and 21 variants which do not cause NDI in 71 NDI families. We described 15 different types of mutations including missense, frameshift, inframe deletion, deletion, insertion, nonsense, duplication, splicing and combined mutations. The missense mutations represent the 55.83% of all the NDI published families. Arginine and tyrosine are significantly (P = 4.07E-08 and P = 3.27E-04, respectively) the AVPR2 most commonly mutated amino acids. Alanine and glutamate are significantly (P = 0.009 and P = 0.019, respectively) the least mutated AVPR2 amino acids. The spectrum of mutations varies from rare gene variants or polymorphisms not causing NDI to rare mutations causing NDI, among which arginine and tyrosine are the most common missense. The AVPR2 mutations are spread world-wide. Our study may serve as an updated review, comprehensive of all AVPR2 variants and specific gene locations. J. Cell. Physiol. 217: 605-617, 2008. (c) 2008 Wiley-Liss, Inc.

Nephrogenic diabetes insipidus in a patient with L1 syndrome: a new report of a contiguous gene deletion syndrome including L1CAM and AVPR2

We report on an infant boy with congenital hydrocephalus due to L1 syndrome and polyuria due to diabetes insipidus. We initially believed his excessive urine loss was from central diabetes insipidus and that the cerebral malformation caused a secondary insufficient pituitary vasopressin release. However, he failed to respond to treatment with a vasopressin analogue, which pointed to nephrogenic diabetes insipidus (NDI). L1 syndrome and X-linked NDI are distinct clinical disorders caused by mutations in the L1CAM and AVPR2 genes, respectively, located in adjacent positions in Xq28. In this boy we found a deletion of 61,577 basepairs encompassing the entire L1CAM and AVPR2 genes and extending into intron 7 of the ARHGAP4 gene. To our knowledge this is the first description of a patient with a deletion of these three genes. He is the second patient to be described with L1 syndrome and NDI. During follow-up he manifested complications from the hydrocephalus and NDI including global developmental delay and growth failure with low IGF-1 and hypothyroidism.

Immunological profile in a family with nephrogenic diabetes insipidus with a novel 11 kb deletion in AVPR2 and ARHGAP4 genes

Background

Congenital nephrogenic diabetes insipidus (NDI) is characterised by an inability to concentrate urine despite normal or elevated plasma levels of the antidiuretic hormone arginine vasopressin. We report a Japanese extended family with NDI caused by an 11.2-kb deletion that includes the entire AVPR2 locus and approximately half of the Rho GTPase-activating protein 4 (ARHGAP4) locus. ARHGAP4 belongs to the RhoGAP family, Rho GTPases are critical regulators of many cellular activities, such as motility and proliferation which enhances intrinsic GTPase activity.

ARHGAP4 is expressed at high levels in hematopoietic cells, and it has been reported that an NDI patient lacking AVPR2 and all of ARHGAP4 showed immunodeficiency characterised by a marked reduction in the number of circulating CD3+ cells and almost complete absence of CD8+ cells.

Methods

PCR and sequencing were performed to identify the deleted region in the Japanese NDI patients. Immunological profiles of the NDI patients were analysed by flow cytometry. We also investigated the gene expression profiles of peripheral blood mononuclear cells (PBMC) from NDI patients and healthy controls in microarray technique.

Results

We evaluated subjects (one child and two adults) with 11.2-kb deletion that includes the entire AVPR2 locus and approximately half of the ARHGAP4. Hematologic tests showed a reduction of CD4+ cells in one adult patient, a reduction in CD8+ cells in the paediatric patient, and a slight reduction in the serum IgG levels in the adult patients, but none of them showed susceptibility to infection. Gene expression profiling of PBMC lacking ARHGAP4 revealed that expression of RhoGAP family genes was not influenced greatly by the lack of ARHGAP4.

Conclusion

These results suggest that loss of ARHGAP4 expression is not compensated for by other family members. ARHGAP4 may play some role in lymphocyte differentiation but partial loss of ARHGAP4 does not result in clinical immunodeficiency.

Mechanisms of disease: Mutations of G proteins and G-protein-coupled receptors in endocrine diseases

G proteins and G-protein-coupled receptors (GPCRs) mediate the effects of a number of hormones. Genes that encode these molecules are subject to loss-of function or gain-of-function mutations that result in endocrine disorders. Loss-of-function mutations prevent signaling in response to the corresponding agonist and cause resistance to hormone actions, which mimics hormone deficiency. Gain-of-function mutations lead to constitutive, agonist-independent activation of signaling, which mimics hormone excess. Disease-causing mutations of GPCRs have been identified in patients with various disorders of the pituitary-thyroid, pituitary-gonadal and pituitary-adrenal axes, and in those with abnormalities in food intake, growth, water balance and mineral-ion turnover. The only mutational changes in G proteins unequivocally associated with endocrine disorders occur in GNAS (guanine nucleotide-binding protein G-stimulatory subunit alpha, or G(s)alpha). Heterozygous loss-of-function mutations of GNAS in the active, maternal allele cause resistance to hormones that act through G(s)alpha-coupled GPCRs, whereas somatic gain-of-function mutations cause proliferation of endocrine cells that recognize cyclic AMP as a mitogen. The study of mutations in G proteins and GPCRs has already had major implications for understanding the molecular basis of rare endocrine diseases, as well as susceptibility to multifactorial disorders that are associated with polymorphisms in these genes.

Molecular basis and clinical features of nephrogenic diabetes insipidus

Maintenance of water and electrolyte homeostasis is central to mammalian survival and, therefore, under stringent hormonal control. Water homeostasis is achieved by balancing fluid intake with water excretion, governed by the antidiuretic action of arginine vasopressin. Arginine vasopressin stimulation of renal V₂ vasopressin receptors in the basolateral membrane of principal cells induces aquaporin-2-mediated water reabsorption in the kidney. The importance of this system is apparent when mutations inactivate V₂ vasopressin receptors and aquaporin-2 and cause the clinical phenotype of nephrogenic diabetes insipidus. To date, over 190 mutations in the V₂ vasopressin receptors gene (AVPR2) and approximately 38 mutations in the aquaporin-2 gene have been identified in patients with inherited nephrogenic diabetes insipidus. Extensive in vitro expression and mutagenesis studies of V₂ vasopressin receptors and aquaporin-2 have provided detailed insights into the molecular mechanisms of G-protein-coupled receptor and water channel dysfunction per se. Targeted deletions of AVPR2 and AQP2 in mice have extended the knowledge of nephrogenic diabetes insipidus pathophysiology and have stimulated testing of old and new ideas to therapeutically restore normal kidney function in animal models and patients with this disease. In this review, we summarize the current knowledge relevant to understand the molecular basis of inherited nephrogenic diabetes insipidus forms and the rationales for the current pharmacological treatment of patients with this illness.

Two novel mutations in the aquaporin 2 gene in a girl with congenital nephrogenic diabetes insipidus

Congenital nephrogenic diabetes insipidus (CNDI) is a rare inherited disorder characterized by insensitivity of the kidney to the antidiuretic effect of vasopressin. There are three inheritance patterns of CNDI: the X-linked recessive form associated with vasopressin V2 receptor gene mutations, and the autosomal recessive and dominant forms associated with aquaporin-2 gene (AQP2) mutations. The evaluation for polyuria and polydipsia in a one-month-old Korean girl revealed no response to vasopressin and confirmed the diagnosis of CNDI. Because the child was female without family history of CNDI, her disease was thought to be an autosomal recessive form. We analyzed the AQP2 gene and detected a compound heterozygous missense point mutation: 70Ala (GCC) to Asp (GAC) in exon 1 inherited from her father and 187Arg (CGC) to His (CAC) in exon 3 inherited from her mother. The first mutation is located within the first NPA motif of the AQP2 molecule and the second one right after the second NPA motif. This is the first report to characterize AQP2 mutations in Korean patients with autosomal recessive CNDI, and expands the spectrum of AQP2 mutations by reporting two novel mutation, 70Ala (GCC) to Asp (GAC) and 187Arg (CGC) to His (CAC).

A novel deletion mutation in the arginine vasopressin receptor 2 gene and skewed X chromosome inactivation in a female patient with congenital nephrogenic diabetes insipidus

X-linked nephrogenic diabetes insipidus (NDI) is a rare inherited disorder caused by mutations in the arginine vasopressin receptor 2 (V2R) gene. The clinical phenotype is fully expressed in hemizygous male patients and is usually asymptomatic in heterozygous females. In the present study, a 51-yr-old Japanese female with congenital NDI and her family members were examined. The patient developed severe hypernatremia accompanied by hypoosmotic polyuria after gynecological surgery, and was unable to concentrate urinary osmolality in response to exogenous vasopressin. Direct sequencing analysis of the propositus and her two affected sons revealed a two-nucleotide deletion change at codon 30 (g.452-453delAC) in the V2R gene, resulting in a frameshift and premature termination in translation at codon 190. The X chromosome inactivation pattern was investigated in the propositus using methylation analysis of the polymorphic CAG repeat in the androgen receptor gene, and the value for relative X chromosome inactivation of one allele was 70.2%. In conclusion, we identified a novel V2R gene mutation in a female patient and her sons with congenital NDI, and her phenotype may be caused by skewed X chromosome inactivation.

Generation and phenotype of mice harboring a nonsense mutation in the V2 vasopressin receptor gene

The V2 vasopressin receptor (V2R) plays a key role in the maintenance of a normal body water balance. To generate an in vivo model that allows the physiological and molecular analysis of the role of V2Rs in kidney function, we have created mouse lines that lack functional V2Rs by using targeted mutagenesis in mouse embryonic stem cells. Specifically, we introduced a nonsense mutation known to cause X-linked nephrogenic diabetes insipidus (XNDI) in humans (Glu242stop) into the mouse genome. V2R-deficient hemizygous male pups showed a decrease in basal urine osmolalities and were unable to concentrate their urine. These pups also exhibited an enlargement of renal pelvic space, failed to thrive, and died within the first week after birth due to hypernatremic dehydration. Interestingly, female mice heterozygous for the V2R mutation showed normal growth but displayed an XNDI-like phenotype, characterized by reduced urine concentrating ability of the kidney, polyuria, and polydipsia. Western blot analysis and immunoelectron microscopic studies showed that the loss of functional V2Rs had no significant effect on the basal expression levels of aquaporin-2 and the bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1). The V2R mutant mice described here should serve as highly useful tools for the development of novel therapeutic strategies for the treatment of XNDI.

Clinical utility of direct mutation testing for congenital nephrogenic diabetes insipidus in families

OBJECTIVE

To ascertain the clinical scenarios in which genetic testing for congenital nephrogenic diabetes insipidus (NDI) by direct detection of mutations might prove valuable, and to assess the use of automated sequencing for testing.

METHODS

We reviewed NDI cases referred to our research laboratory for enrollment in our study of mutations in the AVPR2 gene that is disrupted in the X-linked form of the disease. We selected 5 cases that illustrate the value of genetic testing in different clinical situations. Clinical information was obtained from the patient's personal physicians and the patients' families. Direct automated fluorescent DNA sequencing of AVPR2 gene amplification product was used to identify disease-associated mutations in patients. The presence or absence of mutations in family members was then established by using automated sequencing, restriction enzyme analysis, or both.

RESULTS

In 2 of the 5 selected cases, the diagnosis of a genetic form of NDI was confirmed by mutation analysis in a sporadic case of an affected boy. In 2 cases, a suspected diagnosis of X-linked NDI was confirmed in an affected girl. In 4 of the cases, 1 or more unaffected female relatives were determined to carry or not to carry the disease-associated gene. In 2 cases, testing of the newborn child of a known or suspected carrier confirmed the clinical suspicion of affected status and justified proactive therapy. In 4 of the 5 cases, the mode of inheritance was not clear from the family history and was established as X-linked by the testing. Assay for restriction sites changed by disease-associated mutations agreed with the automated sequencing results.

CONCLUSIONS

We conclude that direct mutation analysis in patients suspected of NDI and in selected family members is indicated. The results of testing can confirm a clinical diagnosis of disease, which may otherwise be difficult to make in girls. It can further establish the mode of inheritance, unambiguously distinguish carriers from noncarriers, and justify special observation or treatment of newborns at risk, thereby averting dehydration and the attendant complications. We also conclude that, with proper controls, automated sequencing is the preferred method of testing, because it is sufficiently robust, sensitive, and adaptable for this short gene with a large variety of causative mutations.

AVPR2 variants and V2 vasopressin receptor function in nephrogenic diabetes insipidus

BACKGROUND

The AVPR2 gene encodes the type 2 vasopressin receptor, a member of the vasopressin/oxytocin receptor subfamily of G protein-coupled receptors. Disruption of AVPR2 causes X-linked congenital nephrogenic diabetes insipidus (NDI), yet the functional significance of most gene sequence variations found in association with NDI has not been proven. The large number of naturally occurring AVPR2 mutations constitutes a model system for studying the structure-function relationship of G protein-coupled receptors. This analysis can be aided by examining amino acid sequence variation and conservation among evolutionarily disparate members of the subfamily.

METHODS

Twenty-five new NDI patients were evaluated by DNA sequencing for mutations in AVPR2. Receptors encoded by eighteen NDI alleles were tested for physiologic signaling activity in response to varying concentrations of arginine vasopressin (AVP) in a sensitive cell culture assay. Seventeen amino acid sequences from the vasopressin/oxytocin receptor subfamily were aligned and conserved residues were identified and correlated with the locations of NDI associated variations.

RESULTS

Twenty-four variant alleles were found among the 25 new patients. Thirteen had no prior family history of expressed NDI. All 18 of the NDI-associated AVPR2 alleles tested for function demonstrated diminished response to stimulation with AVP. Twelve failed to respond at all, whereas six signaled only at high AVP concentrations. Evolutionarily conserved residues clustered in the transmembrane domains and in the first and second extracellular loops, and NDI-associated missense mutations appeared mostly in the conserved domains.

CONCLUSIONS

Sporadic cases are frequent and they usually represent the X-linked rather than the autosomal form of NDI. Genetic and functional testing can confirm this in individual cases. Mutations in this study affecting ligand binding domains tend to retain partial signaling in vitro, whereas those that introduce a charged residue in a transmembrane domain are inactive. The minimal partial signaling observed in cultured cells is unlikely to correlate with clinically significant urine concentrating ability. Other AVPR2 mutations with milder effects on receptor function probably exist, but may not be expressed clinically as typical NDI.

Truncated V2 vasopressin receptors as negative regulators of wild-type V2 receptor function

Accumulating evidence suggests that G protein-coupled receptors (GPCRs) can form dimeric or oligomeric arrays. Based on this concept, we have tested the hypothesis that truncated GPCRs can act as negative regulators of wild-type receptor function. Using the GS-coupled V2 vasopressin receptor as a model system, we systematically analyzed the ability of N- and C-terminal V2 receptor fragments to interfere with the activity of the wild-type V2 receptor coexpressed in COS-7 cells. Several N-terminal V2 receptor truncation mutants were identified that strongly inhibited the function (as determined in cAMP and radioligand binding assays) and cell surface trafficking of the coexpressed full-length V2 receptor. However, these truncation mutants did not interfere with the function of other GS-coupled receptors such as the D1 dopamine and the beta2-adrenergic receptors. Dominant negative effects were only observed with mutant receptors that contained at least three transmembrane domains. In addition, immunoblotting experiments showed that all V2 receptor truncation mutants displaying dominant negative activity (but not those mutant receptors lacking this activity) were able to form heterodimers with the full-length V2 receptor, suggesting that complex formation between mutant and wild-type V2 receptors underlies the observed inhibition of wild-type receptor function. Given the high degree of structural homology shared by all GPCRs, our findings should also be applicable to other members of this receptor superfamily.

Nephrogenic diabetes insipidus

In nephrogenic diabetes insipidus, the kidney is unable to concentrate urine despite normal or elevated concentrations of the antidiuretic hormone arginine vasopressin (AVP). In congenital nephrogenic diabetes insipidus (NDI), the obvious clinical manifestations of the disease, that is polyuria and polydipsia, are present at birth and need to be immediately recognized to avoid severe episodes of dehydration. Most (>90%) congenital NDI patients have mutations in the AVPR2 gene, the Xq28 gene coding for the vasopressin V2 (antidiuretic) receptor. In <10% of the families studied, congenital NDI has an autosomal recessive inheritance and mutations of the aquaporin-2 gene (AQP2), ie, the vasopressin-sensitive water channel, have been identified. When studied in vitro, most AVPR2 mutations lead to receptors that are trapped intracellularly and are unable to reach the plasma membrane. A minority of the mutant receptors reach the cell surface but are unable to bind AVP or to trigger an intracellular cyclic adenosine-monophosphate (cAMP) signal. Similarly AQP2 mutant proteins are trapped intracellularly and cannot be expressed at the luminal membrane. The acquired form of NDI is much more common than the congenital form, is almost always less severe, and is associated with downregulation of AQP2. The advances described here are examples of "bedside physiology" and provide diagnostic tools for physicians caring for these patients.