DOCK8 deficiency with hypereosinophilia and the syndrome of inappropriate antidiuretic hormone secretion during herpes infection.

BACKGROUND: Hyperimmunoglobulin E syndrome (HIES) due to dedicator of cytokinesis8 (DOCK8) deficiency may present in infancy and childhood with different clinical features involving recurrent infections, allergic dysregulation, and autoimmunity. CASE: In this report, we describe a patient who first presented with severe hypereosinophilia and went on to develop the syndrome of inappropriate antidiuretic hormone secretion (SIADH) in the context of a severe herpes infection. Investigation revealed the presence of underlying DOCK8 deficiency presenting with atypical clinical features. CONCLUSIONS: Distinct inflammatory features associated with infections may be seen in the course of primary immunodeficiency diseases, and early functional and molecular genetic tests will aid the proper management.

Nephrogenic syndrome of inappropriate antidiuresis - An ethnically, genetically and phenotypically diverse disorder: First report in a Chinese adult and review of published cases.

BACKGROUND: Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare inherited disorder characterised by hyponatraemia. To date, most reported cases are Caucasians with gain-of-function variants in AVPR2, an X-linked gene which encodes the vasopressin V2 receptor (V2R). Recently, germline gain-of-function variants in the stimulatory G protein α-subunit (Gsα) were reported to cause dominantly inherited NSIAD. CASE REPORT: We report the first Chinese adult diagnosed with NSIAD. He was found to be hemizygous for R137C-V2R, the most prevalent pathogenic variant among Caucasians. After the genetic diagnosis and counselling on the importance of fluid restriction, he had no recurrence of hyponatraemia to date. LITERATURE REVIEW: Case reports of NSIAD published in the English literature in PubMed were reviewed to summarise the genetic and phenotypic heterogeneity of this disorder. CONCLUSION: NSIAD is ethnically, genetically and phenotypically diverse. The diagnosis should especially be considered in young patients with otherwise unexplained hyponatraemia. Target analysis of R137C-V2R should make the diagnosis in most cases. Genetic testing could confirm the diagnosis, motivate adherence to treatment, offer the possibility of genotype-guided therapy, and allow cascade screening to prevent hyponatraemia.

Vasopressin receptor 2 mutations in the nephrogenic syndrome of inappropriate antidiuresis show different mechanisms of constitutive activation for G protein coupled receptors.

Vasopressin receptor 2 (V2R) mutations causing the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) can generate two constitutively active receptor phenotypes. One type results from residue substitutions in several V2R domains and is sensitive to vaptan inverse agonists. The other is only caused by Arg 137 replacements and is vaptan resistant. We compared constitutive and agonist-driven interactions of the vaptan-sensitive F229V and vaptan-resistant R137C/L V2R mutations with ß-arrestin 1, ß-arrestin 2, and Gαs, using null fibroblasts reconstituted with individual versions of the ablated transduction protein genes. F229V displayed very high level of constitutive activation for Gs but not for ß-arrestins, and enhanced or normal responsiveness to agonists and inverse agonists. In contrast, R137C/L mutants exhibited maximal levels of constitutive activation for ßarrestin 2 and Gs, minimal levels for ß-arrestin 1, but a sharp decline of ligands sensitivity at all transducer interactions. The enhanced constitutive activity and reduced ligand sensitivity of R137 mutants on cAMP signaling persisted in cells lacking ß-arrestins, indicating that these are intrinsic molecular properties of the mutations, not the consequence of altered receptor trafficking. The results suggest that the two groups of NSIAD mutations represent two distinct molecular mechanisms of constitutive activation in GPCRs.

The Vasopressin Receptor 2 Mutant R137L Linked to the Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD) Signals through an Alternative Pathway that Increases AQP2 Membrane Targeting Independently of S256 Phosphorylation.

NSIAD is a rare X-linked condition, caused by activating mutations in the AVPR2 gene coding for the vasopressin V2 receptor (V2R) associated with hyponatremia, despite undetectable plasma vasopressin levels. We have recently provided in vitro evidence that, compared to V2R-wt, expression of activating V2R mutations R137L, R137C and F229V cause a constitutive redistribution of the AQP2 water channel to the plasma membrane, higher basal water permeability and significantly higher basal levels of p256-AQP2 in the F229V mutant but not in R137L or R137C. In this study, V2R mutations were expressed in collecting duct principal cells and the associated signalling was dissected. V2R-R137L and R137C mutants had significantly higher basal pT269-AQP2 levels -independently of S256 and PKA-which were reduced to control by treatment with Rho kinase (ROCK) inhibitor. Interestingly, ROCK activity was found significantly higher in V2R-R137L along with activation of the Gα12/13-Rho-ROCK pathway. Of note, inhibition of ROCK reduced the basal elevated osmotic water permeability to control. To conclude, our data demonstrate for the first time that the gain-of-function mutation of the V2R, R137L causing NSIAD, signals through an alternative PKA-independent pathway that increases AQP2 membrane targeting through ROCK-induced phosphorylation at S/T269 independently of S256 of AQP2.

Gain-of-function mutations of the V2 vasopressin receptor in nephrogenic syndrome of inappropriate antidiuresis (NSIAD): a cell-based assay to assess constitutive water reabsorption.

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently identified chromosome X-linked disease associated with gain-of-function mutations of the V2 vasopressin receptor (V2R), a G-protein-coupled receptor. It is characterized by inability to excrete a free water load, hyponatremia, and undetectable vasopressin-circulating levels. Hyponatremia can be quite severe in affected male children. To gain a deeper insight into the functional properties of the V2R active mutants and how they might translate into the pathological outcome of NSIAD, in this study, we have expressed the wild-type V2R and three constitutively active V2R mutants associated with NSIAD (R137L, R137C, and the F229V) in MCD4 cells, a cell line derived from renal mouse collecting duct, stably expressing the vasopressin-sensitive water channel aquaporin-2 (AQP2). Our findings indicate that in cells expressing each active mutant, AQP2 was constitutively localized to the apical plasma membrane in the absence of vasopressin stimulation. In line with these observations, under basal conditions, osmotic water permeability in cells expressing the constitutively active mutants was significantly higher compared to that of cells expressing the wild-type V2R. Our findings demonstrate a direct link between activating mutations of the V2R and the perturbation of water balance in NSIAD. In addition, this study provides a useful cell-based assay system to assess the functional consequences of newly discovered activating mutations of the V2R on water permeability in kidney cells and to screen the effect of drugs on the mutated receptors.

[Nephrogenic syndrome of inappropriate antidiuresis]. / Syndrome d'antidiurèse inappropriée néphrogénique.

Disorders of water balance are a disease commonly encountered in our clinical practice. Analysis of vasopressin receptor type II (V2R) is essential to understand the physiology of water balance and it is used as a biological prototype of G protein-coupled receptors (GPCRs). Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a syndrome of inappropriate antidiuretic hormone secretion (SIADH) with low plasmatic vasopressin. The evidence on the role of V2 receptor and of aquaporin (AQP) in the mechanism of action for antidiuretic hormone (ADH) was based on the identification of protein gene mutations in patients with nephrogenic diabetes insipidus and NSIAD syndrome. V2R activating mutations were found in patients with NSIAD, contrasting with the numerous V2R inactivating mutations related to X-linked mutations described in patients with nephrogenic diabetes insipidus.

Germline-Derived Gain-of-Function Variants of Gsα-Coding GNAS Gene Identified in Nephrogenic Syndrome of Inappropriate Antidiuresis.

BACKGROUND: The stimulatory G-protein α-subunit encoded by GNAS exons 1-13 (GNAS-Gsα) mediates signal transduction of multiple G protein-coupled receptors, including arginine vasopressin receptor 2 (AVPR2). Various germline-derived loss-of-function GNAS-Gsα variants of maternal and paternal origin have been found in pseudohypoparathyroidism type Ia and pseudopseudohypoparathyroidism, respectively. Specific somatic gain-of-function GNAS-Gsα variants have been detected in McCune-Albright syndrome and may result in phosphate wasting. However, no germline-derived gain-of-function variant has been identified, implying that such a variant causes embryonic lethality. METHODS: We performed whole-exome sequencing in two families with dominantly inherited nephrogenic syndrome of inappropriate antidiuresis (NSIAD) as a salient phenotype after excluding a gain-of-function variant of AVPR2 and functional studies for identified variants. RESULTS: Whole-exome sequencing revealed two GNAS-Gsα candidate variants for NSIAD: GNAS-Gsα p.(F68_G70del) in one family and GNAS-Gsα p.(M255V) in one family. Both variants were absent from public and in-house databases. Of genes with rare variants, GNAS-Gsα alone was involved in AVPR2 signaling and shared by the families. Protein structural analyses revealed a gain-of-function-compatible conformational property for p.M255V-Gsα, although such assessment was not possible for p.F68_G70del-Gsα. Both variants had gain-of-function effects that were significantly milder than those of McCune-Albright syndrome-specific somatic Gsα variants. Model mice for p.F68_G70del-Gsα showed normal survivability and NSIAD-compatible phenotype, whereas those for p.M255V-Gsα exhibited severe failure to thrive. CONCLUSIONS: This study shows that germline-derived gain-of-function rare variants of GNAS-Gsα exist and cause NSIAD as a novel Gsα-mediated genetic disease. It is likely that AVPR2 signaling is most sensitive to GNAS-Gsα's gain-of-function effects.

Single-tubule RNA-Seq uncovers signaling mechanisms that defend against hyponatremia in SIADH.

In the syndrome of inappropriate antidiuretic hormone secretion (SIADH), hyponatremia is limited by onset of vasopressin-escape caused by loss of the water channel aquaporin-2 in the renal collecting duct despite high circulating vasopressin. Here, we use the methods of systems biology in a well-established rat model of SIADH to identify signaling pathways activated at the onset of vasopressin-escape. Using single-tubule RNA-Seq, full transcriptomes were determined in microdissected cortical collecting ducts of vasopressin-treated rats at 1, 2, and 4 days after initiation of oral water loading in comparison to time-control rats without water loading. The time-dependent mRNA abundance changes were mapped to gene sets associated with curated canonical signaling pathways and revealed evidence of perturbation of transforming growth factor ß signaling and epithelial-to-mesenchymal transition on Day 1 of water loading simultaneous with the initial fall in Aqp2 gene expression. On Day 2 of water loading, transcriptomic changes mapped to Notch signaling and the transition from G0 into the cell cycle but arrest at the G2/M stage. There was no evidence of cell proliferation or altered principal or intercalated cell numbers. Exposure of vasopressin-treated cultured mpkCCD cells to transforming growth factor ß resulted in a virtually complete loss of aquaporin-2. Thus, there is a partial epithelial-to-mesenchymal transition during vasopressin escape with a subsequent shift from quiescence into the cell cycle with eventual arrest and loss of aquaporin-2.

[Nephrogenic syndrome of inappropriate antidiuresis: Early diagnosis avoids severe hyponatremia complications]. / Syndrome néphrogénique d'antidiurèse inappropriée : un diagnostic précoce évite les complications d'une hyponatrémie sévère.

AIM: Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare disease characterized by a kidney disability to dilute urine and, as a result, severe recurrent hyponatremia. Due to wide variability in clinical expression, the diagnosis still remains a challenge for clinicians. We report our experience of a case in which NSIAD was diagnosed early. We also stress the importance of early diagnosis and treatment, which protects an infant with NSAID from severe hyponatremia. BACKGROUND: A 1-month-old boy was referred to our hospital for persistent hyponatremia and intense vomiting. He was born full term after a normal pregnancy with a normal birth weight. The parents were healthy, nonconsanguineous, of Moroccan origin. They already had healthy twin girls. The physical examination was normal upon admission with no signs of dehydration and normal weight gain since birth. Plasma sodium was very low (125mmol/L) associated with low plasma urea (5mg/dL), osmolality (258 mOsm/kg) and low natriuresis (59mmol/L). These laboratory results suggested inappropriate antidiuretic hormone secretion (SIAD) and the infant was consequently treated with oral urea (he was already receiving sodium supplements that were later stopped). Due to exclusive breastfeeding, water restriction was impossible. Further biological investigation revealed undetectable plasma arginine vasopressin (AVP), suggesting the diagnosis of NSIAD. This was confirmed by genetic sequencing of the AVP receptor (AVPR2), demonstrating the presence of an R137C mutation. CONCLUSIONS: We herein report a case of a genetic fluid balance disorder due to an activating mutation of AVPR2. NSIAD is an X-linked disease, first described in 2005 by Feldman et al., which involved severe recurrent hyponatremia. The very early diagnosis (at 7 weeks of life) and appropriate treatment with urea prevented seizures and cerebral damage due to severe recurrent hyponatremia. Clinicians should consider the diagnosis of NSIAD in infants with recurrent hyponatremia with hemodilution and low AVP serum level. Genetic analysis of the AVPR2 sequence on the X chromosome will confirm the diagnosis and, given the wide variability of clinical expression, sequencing of the family members should be done.

Haptoglobin 2-2 Genotype Is Associated With Cerebral Salt Wasting Syndrome in Aneurysmal Subarachnoid Hemorrhage.

BACKGROUND: Haptoglobin (Hp) genotype has been shown to be a predictor of clinical outcomes in subarachnoid hemorrhage. Cerebral salt wasting (CSW) has been suggested to precede the development of symptomatic vasospasm. OBJECTIVE: To determine if Hp genotype was associated with CSW and subsequent vasospasm after aneurysmal subarachnoid hemorrhage. METHODS: Hp genotypic determination was done for patients admitted with a diagnosis of subarachnoid hemorrhage. Outcome measures included CSW, delayed cerebral infarction, and Glasgow Outcome Score of 4 to 5 at 30 days. Criteria for CSW included hyponatremia <135 mEq/L, and urine output >4 L in 12 hours with urine sodium >40 mEq/L. RESULTS: A total of 133 patients were included in the study. The 3 Hp subgroups did not differ in terms of baseline characteristics. CSW occurred in 1 patient (3.4%) with Hp 1-1, 8 (14.0%) patients with Hp 2-1, and 15 (31.9%) patients with Hp 2-2 (P = .004). In the multivariate regression model, Hp 2-2 was associated with CSW (odds ratio [OR]: 4.94; CI: 1.78-17.43; P = .01), but Hp 2-1 was not (OR: 2.92; CI: 0.56-4.95; P = .15) compared with Hp 1-1. There were no associations between Hp genotypes and functional outcome or delayed cerebral infarction. CSW was associated with delayed cerebral infarction (OR: 7.46; 95% CI: 2.54-21.9; P < .001). CONCLUSION: Hp 2-2 genotype was an independent predictor of CSW after subarachnoid hemorrhage. Because CSW is strongly associated with delayed cerebral infarction, the use of Hp genotype testing requires more investigation, and larger prospective confirmation is warranted. Additionally, a more objective definition of CSW needs to be delineated.

Nephrogenic syndrome of inappropriate antidiuresis secondary to an activating mutation in the arginine vasopressin receptor AVPR2.

CONTEXT: Nephrogenic syndrome of inappropriate antidiuresis (NSIAD), resulting from activating mutations in the arginine vasopressin receptor type 2 (AVPR2), is a rare cause of hyponatraemia. However, its true prevalence may be underestimated and it should be considered in the investigation of unexplained hyponatraemia, with implications for management and targeted gene testing. OBJECTIVE: We describe a structured approach to the investigation of hyponatraemia in a young patient, which allowed a diagnosis of NSIAD to be made. We review current knowledge of NSIAD and use a structural modelling approach to further our understanding of the potential mechanisms by which the causative mutation leads to a constitutively active AVPR2. DESIGN: Clinical and biochemical investigation of hyponatraemia; a formal water load test with measurement of arginine vasopressin levels (AVP); sequencing of AVPR2; and computed structural modelling of the wild-type and constitutively activated mutant receptors. RESULTS: A 38-year-old man presented with intermittent confusion and nausea associated with hyponatraemia and a biochemical picture consistent with syndrome of inappropriate antidiuretic hormone (SIADH). Adrenocortical and thyroid function and an acute intermittent porphyria screen were normal. Cross-sectional imaging of the head, chest and abdomen did not identify an underlying cause and so we proceeded to a water load test. This demonstrated a marked inability to excrete a free water load (just 15% of a 20 ml/kg oral load by 240 min postingestion), with the onset of hyponatraemia (Na(+) 125 mmol/l, urine osmolality 808 mOsm/kg). However, AVP levels were low throughout the test (0·4-0·9 pmol/l), consistent with a diagnosis of NSIAD. AVPR2 sequencing revealed a previously described hemizygous activating mutation (p.Arg137Cys). Through structural modelling of AVPR2, we suggest that disruption of a hydrogen bond between residues Thr269 and Arg137 may promote stabilization of the receptor in its active conformation. Since diagnosis, the patient has adhered to modest fluid restriction and remained well, with no further episodes of hyponatraemia. CONCLUSION: NSIAD should be considered in young patients with unexplained hyponatraemia. A water load test with AVP measurement is a potentially informative investigation, while AVPR2 sequencing provides a definitive molecular genetic diagnosis and a rationale for long-term fluid restriction.

Mutation in the V2 vasopressin receptor gene, AVPR2, causes nephrogenic syndrome of inappropriate diuresis.

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently discovered rare disease caused by gain-of-function mutations of the V2 vasopressin receptor gene, AVPR2. To date, mutations of Phe229 and Arg137 have been identified as gain-of-function in the V2 vasopressin receptor (V2R). These receptor mutations lead to hyponatremia, which may lead to clinical symptoms in infants. Here we present a newly identified I130N substitution in exon 2 of the V2R gene in a family, causing NSIAD. This I130N mutation resulted in constitutive activity of the V2R with constitutive cyclic adenosine monophosphate (cAMP) generation in HEK293 cells. This basal activity could be blocked by the inverse agonist tolvaptan and arginine-vasopressin stimulation enhanced the cAMP production of I130N-V2R. The mutation causes a biased receptor conformation as the basal cAMP generation activity of I130N does not lead to interaction with ß-arrestin. The constitutive activity of the mutant receptor caused constitutive dynamin-dependent and ß-arrestin-independent internalization. The inhibition of basal internalization using dominant-negative dynamin resulted in an increased cell surface expression. In contrast to the constitutive internalization, agonist-induced endocytosis was ß-arrestin dependent. Thus, tolvaptan could be used for treatment of hyponatremia in patients with NSIAD who carry the I130N-V2R mutation.

Nephrogenic Syndrome of Inappropriate Antidiuresis in a Female Neonate: Review of the Clinical Presentation in Females.

BACKGROUND: Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare X-linked disease due to gain-of-function mutations in the AVP V2 receptor gene. Hemizygous males present with recurrent episodes of severe hyponatremia in infancy. Heterozygous females are usually asymptomatic. CASE REPORT: We report on a 23-day-old female neonate, born at term with 3,260 g that presented with recurrent hyponatremia (Na between 124 and 134 mmol/l) due to NSIAD. She was a heterozygous carrier of the c.409 C>T mutation in the AVPR2 gene. CONCLUSIONS: This is the first report of a female neonate presenting with hyponatremia due to NSIAD. The diagnosis of NSIAD should not be limited to male infants and should also be considered in female infants with the clinical picture of inappropriate antidiuresis.

Novel A219P mutation of hydroxymethylbilane synthase identified in a Chinese woman with acute intermittent porphyria and syndrome of inappropriate antidiuretic hormone.

Acute intermittent porphyria (AIP) is an autosomal dominant metabolic disorder caused by deficiency of the heme biosynthetic enzyme hydroxymethylbilane synthase (approved gene symbol HMBS), also known as porphobilinogen deaminase (PBGD). AIP is characterised by intermittent attacks of abdominal pain, vomiting, and neurological complaints. The highly variable symptomatic presentation of AIP causes confusion with other diseases and results in a high misdiagnosis rate (68% in China) and delayed effective treatments. Based on biochemical and genetic analysis of two Chinese families, a new and a previously reported HMBS mutation were identified in patients with AIP and syndrome of inappropriate antidiuretic hormone (SIADH). The novel HMBS mutation is the 655G>C point mutation (A219P). In addition, the 973C>T point mutation (R325X), which had been previously reported in two Danish families, was identified.

[Vasopressin V2 receptor-related pathologies: congenital nephrogenic diabetes insipidus and nephrogenic syndrome of inappropiate antidiuresis]. / Pathologies du récepteur V2 de la vasopressine : diabète insipide néphrogénique congénital et syndrome d'antidiurèse inappropriée néphrogénique.

Congenital nephrogenic diabetes insipidus is a rare hereditary disease with mainly an X-linked inheritance (90% of the cases) but there are also autosomal recessive and dominant forms. Congenital nephrogenic diabetes insipidus is characterized by a resistance of the renal collecting duct to the action of the arginine vasopressin hormone responsible for the inability of the kidney to concentrate urine. The X-linked form is due to inactivating mutations of the vasopressin 2 receptor gene leading to a loss of function of the mutated receptors. Affected males are often symptomatic in the neonatal period with a lack of weight gain, dehydration and hypernatremia but mild phenotypes may also occur. Females carrying the mutation may be asymptomatic but, sometimes, severe polyuria is found due to the random X chromosome inactivation. The autosomal recessive and dominant forms, occurring in both genders, are linked to mutations in the aquaporin-2 gene. The treatment remains difficult, especially in infants, and is based on a low osmotic diet with increased water intake and the use of thiazides and indomethacin. The main goal is to avoid hypernatremic episodes and maintain a good hydration state. Potentially, specific treatment, in some cases of X-linked congenital nephrogenic diabetes insipidus, with pharmacological chaperones such as non-peptide vasopressin-2 receptor antagonists will be available in the future. Conversely, the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is linked to a constitutive activation of the V(2)-receptor due to activating mutations with clinical and biological features of inappropriate antidiuresis but with low or undetectable plasma arginine vasopressin hormone levels.

Characterization of three vasopressin receptor 2 variants: an apparent polymorphism (V266A) and two loss-of-function mutations (R181C and M311V).

Arginine vasopressin (AVP) is released from the posterior pituitary and controls water homeostasis. AVP binding to vasopressin V2 receptors (V2Rs) located on kidney collecting duct epithelial cells triggers activation of Gs proteins, leading to increased cAMP levels, trafficking of aquaporin-2 water channels, and consequent increased water permeability and antidiuresis. Typically, loss-of-function V2R mutations cause nephrogenic diabetes insipidus (NDI), whereas gain-of-function mutations cause nephrogenic syndrome of inappropriate antidiuresis (NSIAD). Here we provide further characterization of two mutant V2Rs, R181C and M311V, reported to cause complete and partial NDI respectively, together with a V266A variant, in a patient diagnosed with NSIAD. Our data in HEK293FT cells revealed that for cAMP accumulation, AVP was about 500- or 30-fold less potent at the R181C and M311V mutants than at the wild-type receptor respectively (and about 4000- and 60-fold in COS7 cells respectively). However, in contrast to wild type V2R, the R181C mutant failed to increase inositol phosphate production, while with the M311V mutant, AVP exhibited only partial agonism in addition to a 37-fold potency decrease. Similar responses were detected in a BRET assay for ß-arrestin recruitment, with the R181C receptor unresponsive to AVP, and partial agonism with a 23-fold decrease in potency observed with M311V in both HEK293FT and COS7 cells. Notably, the V266A V2R appeared functionally identical to the wild-type receptor in all assays tested, including cAMP and inositol phosphate accumulation, ß-arrestin interaction, and in a BRET assay of receptor ubiquitination. Each receptor was expressed at comparable levels. Hence, the M311V V2R retains greater activity than the R181C mutant, consistent with the milder phenotype of NDI associated with this mutant. Notably, the R181C mutant appears to be a Gs protein-biased receptor incapable of signaling to inositol phosphate or recruiting ß-arrestin. The etiology of NSIAD in the patient with V266A V2R remains unknown.

Identification and characterization of an activating F229V substitution in the V2 vasopressin receptor in an infant with NSIAD.

Gain-of-function mutations in the gene encoding the V2 vasopressin receptor (V2R) cause nephrogenic syndrome of inappropriate antidiuresis. To date, reported mutations lead to the substitution of arginine 137 by either a cysteine or leucine (R137C/L). Here, we describe a 3-month-old hyponatremic infant found to have a phenylalanine 229 to valine (F229V) substitution in V2R. Characterization of this substitution in vitro revealed that it leads to high constitutive activity of the receptor, compatible with spontaneous antidiuresis. In contrast to R137C/L mutant receptors, F229V receptors do not undergo spontaneous desensitization, which results in sustained, high basal activity. Notably, the V2R-selective inverse agonists tolvaptan and satavaptan completely silenced the constitutive signaling activity of the F229V mutant receptor, indicating that this substitution does not lock the receptor in an irreversible active state. Thus, inverse agonists might prove to be effective therapies for treating patients with this or other spontaneously activating mutations that do not lock the V2R in its active state. These results emphasize the importance of genetic testing and the functional characterization of mutant receptors for patients with nephrogenic syndrome of inappropriate antidiuresis because the results might inform treatment decisions.

Ecstasy (MDMA)-induced hyponatraemia is associated with genetic variants in CYP2D6 and COMT.

We hypothesised that genetically determined poor metabolism of 3,4-methylene dioxymetamphetamine (MDMA) due either to the presence of CYP2D6 genotypes giving absent or low CYP2D6 enzyme activity, or a COMT genotype predicting low COMT enzyme activity would be associated with a greater degree of MDMA-induced reduction in plasma sodium and osmolality than other genotypes at these genes following consumption of 'ecstasy' tablets by clubbers. Of the 48 subjects who returned to the test site post-clubbing, 30 provided samples for measurement of vasopressin (AVP), plasma sodium, urea and plasma and urine osmolality. Genotyping was performed for functional variants in CYP2D6 (n = 29) and COMT (Val158Met, n = 30). In subjects with urinary MDMA detected post-clubbing, there was a significant association between change in plasma osmolality (p = 0.009) and in plasma sodium (p = 0.012) and CYP2D6 genotypic category. Individuals with the low-activity but readily inhibitable CYP2D6 extensive metaboliser/intermediate metaboliser (EM/IM) genotype showed greater reductions in these measures than all other CYP2D6 genotypic categories. COMT low-activity genotypes (Met/Met and Val/Met) were also significantly associated with reductions in plasma osmolality (p = 0.028) and in plasma sodium (p = 0.003). On conservative Bonferroni correction for two independent genes, the CYP2D6 and COMT plasma sodium findings remain significant. The relatively high frequency of the low-activity CYP2D6 and COMT genotypes in the population warrants further attention, since consumption of free water following ingestion of MDMA in these individuals may trigger dilutational hyponatraemia and increased risk of syndrome of inappropriate antidiuretic hormone secretion.