Identification and characterization of a novel X-linked AVPR2 mutation causing partial nephrogenic diabetes insipidus: a case report and review of the literature

Therapeutic potentials of nonpeptidic V2R agonists for partial cNDI-causing V2R mutants

Loss-of-function mutations in the type 2 vasopressin receptor (V2R) are a major cause of congenital nephrogenic diabetes insipidus (cNDI). In the context of partial cNDI, the response to desmopressin (dDAVP) is partially, but not entirely, diminished. For those with the partial cNDI, restoration of V2R function would offer a prospective therapeutic approach. In this study, we revealed that OPC-51803 (OPC5) and its structurally related V2R agonists could functionally restore V2R mutants causing partial cNDI by inducing prolonged signal activation. The OPC5-related agonists exhibited functional selectivity by inducing signaling through the Gs-cAMP pathway while not recruiting β-arrestin1/2. We found that six cNDI-related V2R partial mutants (V882.53M, Y1283.41S, L1614.47P, T2736.37M, S3298.47R and S3338.51del) displayed varying degrees of plasma membrane expression levels and exhibited moderately impaired signaling function. Several OPC5-related agonists induced higher cAMP responses than AVP at V2R mutants after prolonged agonist stimulation, suggesting their potential effectiveness in compensating impaired V2R-mediated function. Furthermore, docking analysis revealed that the differential interaction of agonists with L3127.40 caused altered coordination of TM7, potentially contributing to the functional selectivity of signaling. These findings suggest that nonpeptide V2R agonists could hold promise as potential drug candidates for addressing partial cNDI.

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.

Three Pediatric Patients with Congenital Nephrogenic Diabetes Insipidus due to AVPR2 Nonsense Mutations and Different Clinical Manifestations: A Case Report

Congenital nephrogenic diabetes insipidus (CNDI), a rare hereditary disorder, is characterized by the inability of the kidneys to concentrate urine in response to the antidiuretic hormone arginine vasopressin (AVP); as a result, large volumes of unconcentrated urine are excreted. In addition to the clinical manifestations of CNDI, such as dehydration and electrolyte disturbances (hypernatremia and hyperchloremia), developmental delay can result without prompt treatment. In approximately 90% of cases, CNDI is an X-linked disease caused by mutations in the arginine vasopressin receptor 2 (AVPR2) gene. In approximately 9% of cases, CNDI is an autosomal recessive disease caused by mutations in the water channel protein aquaporin 2 (AQP2), and 1% of cases are autosomal dominant. We report a case of CNDI caused by a novel AVPR2 nonsense mutation, c.520C>T (p.Q174X), and cases of siblings in another family who had a different AVPR2 nonsense mutation, c.852G>A (p.W284X). Both cases responded well to treatment with hydrochlorothiazide and spironolactone. If CNDI is suspected, especially in carriers and neonates, aggressive genetic testing and early treatment may alleviate growth disorders and prevent irreversible central nervous system disorders and developmental delay.

Pro-haemostatic effect of DDAVP is partially derived through non-classical (CD14dim /CD16++ ) monocytes residing the spleen

The splenic endothelial Weibel-palade bodies are one of the most important candidate organelles to release von Willebrand factor upon stimulation with desmopressin. However, the presence of functional desmopressin-specific receptor has not yet been demonstrated on endothelial cells. Experimental evidences are in favour of an indirect pro-haemostatic effect of desmopressin, but the exact mediator and its cellular origin are largely elusive. Here, we report partially hampered desmopressin response in a splenectomised severe haemophilia A/Beta Thalassemia patient without any genetic variant relevant to his incomplete desmopressin response. To further investigate the role of the spleen in this phenomenon, the release of VWF from desmopressin-treated human splenic endothelial cells was assessed in vitro. As a result, desmopressin induced the release of VWF from endothelial cells when the cells were co-cultured with non-classical (CD14^dim /CD16⁺⁺ ), but not other subtypes of monocytes or PBMCs. This in vitro study which resembles close proximity of endothelial cells of sinusoids to monocyte reservoir reside in parenchyma of subcapsular red pulp of the spleen sheds a light upon the role of this highly vascularized VWF-producing organ in driving indirect effect of desmopressin.

Partial nephrogenic diabetes insipidus with a novel arginine vasopressin receptor 2 gene variant

X-linked nephrogenic diabetes insipidus (NDI) is caused by variations in arginine vasopressin receptor 2 (AVPR2). Some patients show partial resistance to arginine vasopressin (AVP). A 19-month-old Japanese boy presented with polydipsia since infancy. His mother had a history of polydipsia during pregnancy, and his maternal granduncle also had polydipsia. Intermediate urine osmolality and markedly high plasma AVP levels were observed in the water deprivation test. Subsequent pitressin administration caused no further elevation in urine osmolality. We diagnosed the patient with partial NDI, initiated therapy with hydrochlorothiazide, and placed him on a low-sodium diet. Although his urine volume decreased by 20-30% after the initiation of therapy, progressive hydronephrosis and growth retardation developed 2 years later. We investigated his genetic background by multiplex targeted sequencing of genes associated with inherited renal diseases, including AVPR2 and aquaporin-2 (AQP2). We identified a hemizygous missense variant in AVPR2 NM_000054:c.371A>G,p.(Tyr124Cys) in the boy and a heterozygous variant in the mother at the same locus. Distinguishing partial NDI from primary polydipsia is difficult because of its mild symptoms. Markedly elevated plasma AVP levels with intermediate urine osmolality may suggest partial NDI, and genetic analysis can be useful for such patients.

Prediction of molecular interactions and physicochemical properties relevant for vasopressin V2 receptor antagonism

We have developed two ligand- and receptor-based computational approaches to study the physicochemical properties relevant to the biological activity of vasopressin V2 receptor (V2R) antagonist and eventually to predict the expected binding mode to V2R. The obtained quantitative structure activity relationship (QSAR) model showed a correlation of the antagonist activity with the hydration energy (E_H2O), the polarizability (P), and the calculated partial charge on atom N7 (q6) of the common substructure. The first two descriptors showed a positive contribution to antagonist activity, while the third one had a negative contribution. V2R was modeled and further relaxed on a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline (POPC) membrane by molecular dynamics simulations. The receptor antagonist complexes were guessed by molecular docking, and the stability of the most relevant structures was also evaluated by molecular dynamics simulations. As a result, amino acid residues Q96, W99, F105, K116, F178, A194, F307, and M311 were identified with the probably most relevant antagonist-receptor interactions on the studied complexes. The proposed QSAR model could explain the molecular properties relevant to the antagonist activity. The contributions to the antagonist-receptor interaction appeared also in agreement with the binding mode of the complexes obtained by molecular docking and molecular dynamics. These models will be used in further studies to look for new V2R potential antagonist molecules.

Oral disintegrating desmopressin tablet is effective for partial congenital nephrogenic diabetes insipidus with AVPR2 mutation: a case report

Congenital nephrogenic diabetes insipidus (NDI) is a rare disease that causes polydipsia and polyuria, and there are currently no effective treatments for most cases, particularly severe ones. The present report describes the case of a 1-yr-5-mo-old male patient with partial congenital NDI who was successfully treated with oral disintegrating 1-deamino-8-D-arginine vasopressin (DDAVP). The patient presented with poor weight gain and polydipsia (fluid, 1.5 L/d) and received a diagnosis of NDI after genetic analysis revealed an AVPR2 mutation (c.383A>C, p.Y128S). His water-restricted urine osmolality increased from 360 mOsm/kg/H₂O to 667 mOsm/kg/H₂O after subcutaneous AVP injection, indicating that he had some urine concentrating ability. Oral disintegrating DDAVP therapy was started at 360 µg/d with hydrochlorothiazide and increased to 720 µg/d without any adverse effects. A 30% decrease in urine output and water intake was followed by an increase in body weight. The present study is the first to report the effectiveness and safety of oral disintegrating DDAVP in a patient with partial congenital NDI due to an AVPR2 gene mutation. The severity of NDI at which DDAVP therapy is the most effective remains to be determined.

Novel AVPR2 mutations and clinical characteristics in 28 Chinese families with congenital nephrogenic diabetes insipidus

AIMS

To investigate genotype and phenotype of congenital nephrogenic diabetes insipidus caused by AVPR2 mutations, which is rare and limitedly studied in Chinese population.

METHODS

88 subjects from 28 families with NDI in a department (Beijing, PUMCH) were screened for AVPR2 mutations. Medical records were retrospectively reviewed and characterized. Genotype and phenotype analysis was performed.

RESULTS

23 AVPR2 mutations were identified, including six novel mutations (p.Y117D, p.W208R, p.L313R, p.S127del, p.V162Sfs*30 and p.G251Pfs*96). The onset-age ranged from 1 week to 3 years. Common presentations were polydipsia and polyuria (100%) and intermittent fever (57%). 21% and 14% of patients had short stature and mental impairment. Urine SG and osmolality were decreased, while serum osmolality and sodium were high. Urological ultrasonography results showed hydronephrosis of the kidney (52%), dilation of the ureter (48%), and thickened bladder wall or increased residual urine (32%), led to intermittent urethral catheterization (7%), cystostomy (11%) and binary nephrostomy (4%). Urological defects were developed in older patients. Genotype and phenotype analysis revealed patients with non-missense mutations had higher levels of serum sodium than missense mutations.

CONCLUSION

In the first and largest case series of NDI caused by AVPR2 mutations in Chinese population, we established genetic profile and characterized clinical data, reporting six novel mutations. Further, we found genotype was associated with phenotype. This knowledge broadens genotype and phenotype spectrum of rare congenital NDI caused by AVPR2 mutations, and provides basis for studying molecular biology of AVPR2.

Functional characterization of a loss-of-function mutant I324M of arginine vasopressin receptor 2 in X-linked nephrogenic diabetes insipidus

X-linked nephrogenic diabetes insipidus (X-linked NDI) is a rare inherited disease mainly caused by lost-of-function mutations in human AVPR2 gene encoding arginine vasopressin receptor 2 (V2R). Our focus of the current study is on exploration of the functional and biochemical properties of Ile324Met (I324M) mutation identified in a pedigree showing as typical recessive X-linked NDI. We demonstrated that I324M mutation interfered with the conformation of complex glycosylation of V2R. Moreover, almost all of the I324M-V2R failed to express on the cell surface due to being captured by the endoplasmic reticulum control system. We further examined the signaling activity of DDAVP-medicated cAMP and ERK1/2 pathways and the results revealed that the mutant receptor lost the ability in response to DDAVP stimulation contributed to the failure of accumulation of cAMP and phosphorylated ERK1/2. Based on the characteristics of molecular defects of I324M mutant, we selected two reagents (SR49059 and alvespimycin) to determine whether the functions of I324M-V2R can be restored and we found that both compounds can significantly “rescue” I324M mutation. Our findings may provide further insights for understanding the pathogenic mechanism of AVPR2 gene mutations and may offer some implications on development of promising treatments for patients with X-linked NDI.

Identification of a novel X-linked arginine-vasopressin receptor 2 mutation in nephrogenic diabetes insipidus: Case report and pedigree analysis

INTRODUCTION

The clinical and genetic characteristics of nephrogenic diabetes insipidus (NDI) were described via assessing 2 cases of NDI patients from a Chinese family.

PATIENT CONCERNS

Two patients who manifest polyuria and polydipsia were admitted to hospital for definite diagnosis.

DIAGNOSIS

Water deprivation-vasopressin tests showed that the patients may possess renal-origin diabetes insipidus. All the levels of thyroid-stimulating hormone, luteinizing hormone, follicle stimulation hormone, adrenocorticotropic hormone, prolactin, and growth hormone in both patients were normal. These results were certified that both patients possess a nephropathy-type diabetes insipidus. B-mode ultrasonography and urinalysis test demonstrated that the patient's diabetes insipidus is unlikely to originate from renal organic disease. Remarkably, by nucleotide sequencing, we found a novel mutation c.414_418del in arginine-vasopressin receptor 2 (AVPR2) was related to the disease of NDI.

INTERVENTIONS

Two patients were treated with oral hydrochlorothiazide and indomethacin. In addition, low salt diet and potassium supplementation throughout the patients' treatment.

OUTCOMES

The clinical symptoms of 2 patients were significantly reduced after targeted therapy.

CONCLUSION

A mutation in AVPR2 was discovered to be associated with NID. It provides a new target for molecular diagnosis of NDI, enabling families to undergo genetic counseling and obtain prenatal diagnoses.

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.

A novel AVPR2 missense mutation in an Asian family with inherited nephrogenic diabetes insipidus: A case report

RATIONALE

X-linked nephrogenic diabetes insipidus (NDI) is a rare inherited disease, and is characterized by renal resistance to arginine vasopressin (AVP). Its diagnosis can be clinically challenging. The application of molecular genetic analysis can provide a rapid and definitive diagnosis.

PATIENT CONCERNS

A 75-year-old woman presented with recurrent nausea and vomiting was admitted to the Department of Gastroenterology. The patient had a strong family history of polydipsia and polyuria. Sequencing analysis of the antidiuretic hormone arginine vasopressin receptor 2 (AVPR2) revealed the novel missense mutation p. Trp164Cys (c.492G>G/C) in exon 2. There was a heterozygous mutation in the patient's sister and niece, while there was a mutation in her sons, brother and nephews. The locus is located on the X chromosome Xq28, and its mutation can lead to X linked recessive NDI. The p. Trp164Cys mutation of AVPR2 gene has not been reported in literature before. The mutation was predicted to be probably damaging by several prediction methods, including SIFT and PolyPhen-2. There was no significant abnormal variation in other detection regions of the gene. And there was also no abnormal variation in AVP and AQP2 genes in this family.

DIAGNOSIS

X-linked NDI was diagnosed according to the patient's family history and DNA sequencing analysis.

INTERVENTIONS AND OUTCOMES

After treated with desmopressin, antiemetic drugs and massive infusion glucose transfusion, the patient's urine volume decreased and electrolyte disturbance was corrected, and the symptoms of nausea and vomiting gradually disappeared.

LESSONS

The patients with suspected congenital NDI should undergo genetic sequencing analysis of AVPR2, AVP and AQP2 genes. A definitive diagnosis can benefit patient and avoid unnecessary investigations.

Four Japanese Patients with Congenital Nephrogenic Diabetes Insipidus due to the AVPR2 Mutations

Almost 90% of nephrogenic diabetes insipidus (NDI) is caused by mutations in the arginine vasopressin receptor 2 gene (AVPR2) on the X chromosome. Herein, we reported clinical and biochemical parameters in four cases of three unrelated Japanese families and analyzed the status of the AVPR2. Two of the four patients had poor weight gain. However, in the male and female sibling cases, neither had poor weight gain while toddlers, but in the male sibling, episodes of recurrent fever, polyuria, and polydipsia led to the diagnosis of NDI at 4 years of age. Analysis of AVPR2 identified two nonsense mutations (c.299_300insA; p.K100KfsX91 and c.296G > A; p.W99X) and one missense mutation (c.316C > T; p.R106C). These mutations were previously reported. The patient with c.316C > T; p.R106C had milder symptoms consistent with previous reports. Of the familial cases, the sister was diagnosed as having NDI, but a skewed X-inactivation pattern in her peripheral blood lymphocytes was not identified. In conclusion, our study expands the spectrum of phenotypes and characterized mutations in AVPR2 in NDI.

Hereditary Nephrogenic Diabetes Insipidus: Pathophysiology and Possible Treatment. An Update

Under physiological conditions, excessive loss of water through the urine is prevented by the release of the antidiuretic hormone arginine-vasopressin (AVP) from the posterior pituitary. In the kidney, AVP elicits a number of cellular responses, which converge on increasing the osmotic reabsorption of water in the collecting duct. One of the key events triggered by the binding of AVP to its type-2 receptor (AVPR2) is the exocytosis of the water channel aquaporin 2 (AQP2) at the apical membrane the principal cells of the collecting duct. Mutations of either AVPR2 or AQP2 result in a genetic disease known as nephrogenic diabetes insipidus, which is characterized by the lack of responsiveness of the collecting duct to the antidiuretic action of AVP. The affected subject, being incapable of concentrating the urine, presents marked polyuria and compensatory polydipsia and is constantly at risk of severe dehydration. The molecular bases of the disease are fully uncovered, as well as the genetic or clinical tests for a prompt diagnosis of the disease in newborns. A real cure for nephrogenic diabetes insipidus (NDI) is still missing, and the main symptoms of the disease are handled with s continuous supply of water, a restrictive diet, and nonspecific drugs. Unfortunately, the current therapeutic options are limited and only partially beneficial. Further investigation in vitro or using the available animal models of the disease, combined with clinical trials, will eventually lead to the identification of one or more targeted strategies that will improve or replace the current conventional therapy and grant NDI patients a better quality of life. Here we provide an updated overview of the genetic defects causing NDI, the most recent strategies under investigation for rescuing the activity of mutated AVPR2 or AQP2, or for bypassing defective AVPR2 signaling and restoring AQP2 plasma membrane expression.

Severe acute dehydration in a desert rodent elicits a transcriptional response that effectively prevents kidney injury

Animals living in desert environments are forced to survive despite severe heat, intense solar radiation, and both acute and chronic dehydration. These animals have evolved phenotypes that effectively address these environmental stressors. To begin to understand the ways in which the desert-adapted rodent Peromyscus eremicus survives, reproductively mature adults were subjected to 72 h of water deprivation, during which they lost, on average, 23% of their body weight. The animals reacted via a series of changes in the kidney, which included modulating expression of genes responsible for reducing the rate of transcription and maintaining water and salt balance. Extracellular matrix turnover appeared to be decreased, and apoptosis was limited. In contrast to the canonical human response, serum creatinine and other biomarkers of kidney injury were not elevated, suggesting that changes in gene expression related to acute dehydration may effectively prohibit widespread kidney damage in the cactus mouse.

Signaling Modification by GPCR Heteromer and Its Implication on X-Linked Nephrogenic Diabetes Insipidus

The involvement of secretin (SCT) and secretin receptor (SCTR) in regulating body water homeostasis is well established. Identified as one of the vasopressin (Vp)-independent mechanisms in fluid balance, SCT regulates aquaporin 2 (AQP2) in the kidney distal collecting duct cells through activating intracellular cAMP production. This ability to bypass Vp-mediated water reabsorption in kidney implicates SCT’s potential to treat nephrogenic diabetes insipidus (NDI). Research on NDI in the past has largely been focused on the searching for mutations in vasopressin receptor 2 (AVPR2), while the functional relationship between SCTR, AVPR2 and NDI remains unclear. Here, we demonstrate the interaction between SCTR and AVPR2 to modulate cellular signaling in vitro. Interestingly, we show in this report that upon heteromer formation with SCTR, R137H, a NDI-causing AVPR2 mutant that is defective in trafficking to cell surface, can functionally be rescued. Our data may provide an explanation for this clinically mild case of NDI, and insights into the pathological development of NDI in the future.

Analysis of the V2 Vasopressin Receptor (V2R) Mutations Causing Partial Nephrogenic Diabetes Insipidus Highlights a Sustainable Signaling by a Non-peptide V2R Agonist

Disease-causing mutations in G protein-coupled receptor (GPCR) genes, including the V2 vasopressin receptor (V2R) gene, often cause misfolded receptors, leading to a defect in plasma membrane trafficking. A novel V2R mutation, T273M, identified in a boy with partial nephrogenic diabetes insipidus (NDI), shows intracellular localization and partial defects similar to the two mutants we described previously (10). Although non-peptide V2R antagonists have been shown to rescue the membrane localization of V2R mutants, their level of functional rescue is weak. Interestingly, it has been reported that a non-peptide agonist, OPC51803, activates misfolded V2R mutants intracellularly without degradation, thus potentially serving as a therapeutic agent against NDI (14). In our current experiments, however, a peptide antagonist blocked arginine vasopressin (AVP)- or OPC51803-stimulated cAMP accumulation both in COS-7 and MDCK cells, suggesting that OPC51803 mainly stimulates cell surface V2R mutants. In addition, our analyses revealed that OPC51803 works not only as a non-peptide agonist that causes activation/β-arrestin-dependent desensitization of V2R mutants expressed at the plasma membrane but also as a pharmacochaperone that promotes the endoplasmic reticulum-retained mutant maturation and trafficking to the plasma membrane. The ratio of the pharmacochaperone effect to the desensitization effect likely correlates negatively with the residual function of the tested mutants, suggesting that OPC5 has a more favorable effect on the V2R mutants with a less residual function. We speculated that the canceling of the desensitization effect of OPC51803 by the pharmacochaperone effect after long-term treatment may produce sustainable signaling, and thus pharmacochaperone agonists such as OPC51803 may serve as promising therapeutics for NDI caused by misfolded V2R mutants.

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.

A novel AVPR2 splice site mutation leads to partial X-linked nephrogenic diabetes insipidus in two brothers

X-linked nephrogenic diabetes insipidus (NDI, OMIM#304800) is caused by mutations in the arginine vasopressin (AVP, OMIM*192340) receptor type 2 (AVPR2, OMIM*300538) gene. A 20-month-old boy and his 8-year-old brother presented with polyuria, polydipsia, and failure to thrive. Both boys demonstrated partial DDAVP (1-desamino-8-D AVP or desmopressin) responses; thus, NDI diagnosis was delayed. While routine sequencing of AVPR2 showed a potential splice site variant, it was not until exome sequencing confirmed the AVPR2 splice site variant and did not reveal any more likely candidates that the patients' diagnosis was made and proper treatment was instituted. Both patients were hemizygous for two AVPR2 variants predicted in silico to affect AVPR2 messenger RNA (mRNA) splicing. A minigene assay revealed that the novel AVPR2 c.276A>G mutation creates a novel splice acceptor site leading to 5' truncation of AVPR2 exon 2 in HEK293 human kidney cells. Both patients have been treated with high-dose DDAVP with a remarkable improvement of their symptoms and accelerated linear growth and weight gain.

CONCLUSION

We present here a unique case of partial X-linked NDI due to an AVPR2 splice site mutation; patients with diabetes insipidus of unknown etiology may harbor splice site mutations that are initially underestimated in their pathogenicity on sequence analysis.

WHAT IS KNOWN

• X-linked nephrogenic diabetes insipidus is caused by AVPR2 mutations, and disease severity can vary depending on the functional effect of the mutation. What is New: • We demonstrate here that a splice site mutation in AVPR2 leads to partial X-linked NDI in two brothers. • Treatment with high-dose DDAVP led to improvement of polyuria and polydipsia, weight gain, and growth.

Novel large deletion in AVPR2 gene causing copy number variation in a patient with X-linked nephrogenic diabetes insipidus

BACKGROUND

The diagnosis of cranial and nephrogenic diabetes insipidus (DI) can be clinically challenging. The application of molecular genetic analysis can help in resolving diagnostic difficulties.

CASE

A 3 month-old boy presented with recurrent polyuria was admitted to Intensive Care Unit and was treated as DI. The patient also had a strong family history of polyuria affecting his maternal uncles. Molecular genetic analysis using Single Nucleotide Polymorphism (SNP) array detected a large deletion located at Xq28 region and the breakpoint was identified using PCR and Sanger sequencing. An 11,535 bp novel deletion affecting the entire APVR2 gene and the last intron and exon of the ARHGAP4 gene was confirmed. This large deletion is likely due to the 7-bp microhomology sequence at the junctions of both 5' and 3' breakpoints. No disease-causing mutation was identified for AQP2.

CONCLUSION

We report a novel deletion in a Chinese patient with congenital nephrogenic DI. We suggested that patients with suspected congenital DI should undergo genetic analysis of AVPR2 and AQP2 genes. A definitive diagnosis can benefit patient by treatment of hydrochlorothiazide and amiloride and avoiding unnecessary investigations.

Nephrogenic diabetes insipidus: essential insights into the molecular background and potential therapies for treatment

The water channel aquaporin-2 (AQP2), expressed in the kidney collecting ducts, plays a pivotal role in maintaining body water balance. The channel is regulated by the peptide hormone arginine vasopressin (AVP), which exerts its effects through the type 2 vasopressin receptor (AVPR2). Disrupted function or regulation of AQP2 or the AVPR2 results in nephrogenic diabetes insipidus (NDI), a common clinical condition of renal origin characterized by polydipsia and polyuria. Over several years, major research efforts have advanced our understanding of NDI at the genetic, cellular, molecular, and biological levels. NDI is commonly characterized as hereditary (congenital) NDI, arising from genetic mutations in the AVPR2 or AQP2; or acquired NDI, due to for exmple medical treatment or electrolyte disturbances. In this article, we provide a comprehensive overview of the genetic, cell biological, and pathophysiological causes of NDI, with emphasis on the congenital forms and the acquired forms arising from lithium and other drug therapies, acute and chronic renal failure, and disturbed levels of calcium and potassium. Additionally, we provide an overview of the exciting new treatment strategies that have been recently proposed for alleviating the symptoms of some forms of the disease and for bypassing G protein-coupled receptor signaling.