Pseudohypoaldosteronism Type Research Articles

A Case of Autosomal Recessive Pseudohypoaldosteronism Type 1 with a Novel Mutation in theSCNN1AGene

A Case of Autosomal Recessive Pseudohypoaldosteronism Type 1 with a Novel Mutation in the<i>SCNN1A</i>Gene

Clinical and molecular analysis of six Japanese patients with a renal form of pseudohypoaldosteronism type 1

Pseudohypoaldosteronism type 1 (PHA1) is a rare condition characterized by neonatal salt loss with elevated plasma aldosterone and renin levels. Two types of PHA1 have been described: an autosomal recessive systemic form and an autosomal dominant renal form, in which the target organ defect is confined to the renal tubules. The dominant renal form of PHA1 is caused by heterozygous mutations in the NR3C2 gene, which encodes the mineralocorticoid receptor (MR). We determined clinical and biochemical parameters in two familial and four sporadic Japanese patient and analyzed the status of the NR3C2 gene. Failure to thrive was noted in five of the six patients. In one of the familial cases, the mother had an episode of failure to thrive when she was a toddler, but received no medical treatment. NaCl supplementation was discontinued in four of the six patients after they reached one year of age and they have grown normally thereafter. However, in one patient, 9 g/day of salt has been required to maintain serum Na concentration after 1 year of age. Analysis of NR3C2 identified three novel mutations [c. C1951T (p.R651X), c.304_305delGC (p.A102fsX103), c.del 603A (p.T201fsX34)] and one previously reported mutation [c.A2839G (p.947X)]. p.R651X was identified in one familial case and one unrelated sporadic patient. The patient who has been supplemented with large amount of salt was heterozygous for c.del 603A in exon 2. In conclusion, our study expands the spectrum of phenotypes, and characterized mutations of NR3C2 in the renal form of PHA1.

Disease-causing R1185C mutation of WNK4 disrupts a regulatory mechanism involving calmodulin binding and SGK1 phosphorylation sites

The R1185C mutation in WNK4 is associated with pseudohypoaldosteronism type II (PHAII). Unlike other PHAII-causing mutations in the acidic motif, the R1185C mutation is located in the COOH-terminal region of WNK4. The goal of the study is to determine what properties of WNK4 are disrupted by the R1185C mutation. We found that the R1185C mutation is situated in the middle of a calmodulin (CaM) binding site and the mutation reduces the binding of WNK4 to Ca(2+)/CaM. The R1185C mutation is also close to serum- and glucocorticoid-induced protein kinase (SGK1) phosphorylation sites S1190 and S1217. In addition, we identified a novel SGK1 phosphorylation site (S1201) in WNK4, and phosphorylation at this site is reduced by Ca(2+)/CaM. In the wild-type WNK4, the level of phosphorylation at S1190 is the lowest and that at S1217 is the highest. In the R1185C mutant, phosphorylation at S1190 is eliminated and that at S1201 becomes the strongest. The R1185C mutation enhances the positive effect of WNK4 on the Na(+)-K(+)-2Cl(-) cotransporter 2 (NKCC2) as tested in Xenopus laevis oocytes. Deletion of the CaM binding site or phospho-mimicking at two or three of the SGK1 sites enhances the WNK4 effects on NKCC2. These results indicate that the R1185C mutation disrupts an inhibitory domain as part of the suppression mechanism of WNK4, leading to an elevated WNK4 activity at baseline. The presence of CaM binding and SGK1 phosphorylation sites in or close to the inhibitory domain suggests that WNK4 activity is subject to the regulation by intracellular Ca(2+) and phosphorylation.

Phosphatidylinositol 3-Kinase/Akt Signaling Pathway Activates the WNK-OSR1/SPAK-NCC Phosphorylation Cascade in Hyperinsulinemic db/db Mice

Metabolic syndrome patients have insulin resistance, which causes hyperinsulinemia, which in turn causes aberrant increased renal sodium reabsorption. The precise mechanisms underlying this greater salt sensitivity of hyperinsulinemic patients remain unclear. Abnormal activation of the recently identified with-no-lysine kinase (WNK)-oxidative stress-responsive kinase 1 (OSR1)/STE20/SPS1-related proline/alanine-rich kinase (SPAK)-NaCl cotransporter (NCC) phosphorylation cascade results in the salt-sensitive hypertension of pseudohypoaldosteronism type II. Here, we report a study of renal WNK-OSR1/SPAK-NCC cascade activation in the db/db mouse model of hyperinsulinemic metabolic syndrome. Thiazide sensitivity was increased, suggesting greater activity of NCC in db/db mice. In fact, increased phosphorylation of OSR1/SPAK and NCC was observed. In both Spak T243A/+ and Osr1 T185A/+ knock-in db/db mice, which carry mutations that disrupt the signal from WNK kinases, increased phosphorylation of NCC and elevated blood pressure were completely corrected, indicating that phosphorylation of SPAK and OSR1 by WNK kinases is required for the increased activation and phosphorylation of NCC in this model. Renal phosphorylated Akt was increased in db/db mice, suggesting that increased NCC phosphorylation is regulated by the phosphatidylinositol 3-kinase/Akt signaling cascade in the kidney in response to hyperinsulinemia. A phosphatidylinositol 3-kinase inhibitor (NVP-BEZ235) corrected the increased OSR1/SPAK-NCC phosphorylation. Another more specific phosphatidylinositol 3-kinase inhibitor (GDC-0941) and an Akt inhibitor (MK-2206) also inhibited increased NCC phosphorylation. These results indicate that the phosphatidylinositol 3-kinase/Akt signaling pathway activates the WNK-OSR1/SPAK-NCC phosphorylation cascade in db/db mice. This mechanism may play a role in the pathogenesis of salt-sensitive hypertension in human hyperinsulinemic conditions, such as the metabolic syndrome.

A case of systemic pseudohypoaldosteronism with a novel mutation in the SCNN1A gene

We report a neonatal case of systemic pseudohypoaldosteronism type 1 caused by a novel mutation in the SCNN1A gene (homozygous c.1052+2dupT in intron 3) in which the patient presented with life-threatening hyperkalemia, hyponatremia and metabolic acidosis. It remains uncertain if there is genotype-phenotype correlation, due to the rarity of the disease. This mutation, which to our best knowledge has not been described before, was associated with a very severe phenotype requiring aggressive therapy.

Novel missense mutations of WNK1 in patients with hypokalemic salt‐losing tubulopathies

Clinical and genetic studies have suggested that a loss of function and gain of function mutation in the same gene can cause different diseases. The aim of this study was to test the hypothesis that inactivating mutations in WNK1 (with no K (lysine) protein kinase-1) or WNK4 could be a new candidate for causing hypokalemic salt-losing tubulopathy (SLT) in those patients with unknown genetic defects because SLT is the opposite phenotype to pseudohypoaldosteronism type II (PHAII). We screened 44 SLTs patients and found that 33 (75%) cases had homozygous or compound heterozygous mutations in CLCNKB or SLC12A3. Two novel missense mutations were identified in WNK1, but not in WNK4, in 2 of the remaining 11 patients. The WNK1 mutations occurred in the protein C-terminus domain, de novo and inherited, respectively. One of these WNK1 mutations was shown to reduce NCC protein membrane expression in vitro because of impairing the suppressive effect of WNK4-mediated inhibition. Taken together, our findings suggest that inactivating mutations in WNK1 may cause SLT, a phenotype opposite to that of PHAII caused by WNK1 intronic deletion.

Abstract 339: Splice Acceptor Site Mutation in Cullin 3 is Causing Pseudohypoaldosteronism Type II.

A rare Mendelian form of hypertension is Pseudohypoaldosteronism type II (PHAII), also referred to as familial hyperkalemic hypertension (FHHt, OMIM 145260 ). Mutations in WNK1 and WNK4 , and the two recently identified genes KLHL3 and CUL3 have been reported as causes of this monogenic form of arterial hypertension. Mutations in WNK1 and WNK4 result in over-activity of the thiazide-sensitive Na-Cl cotransporter (NCC) in the distal convoluted tubule. In a Turkish family (parents and 5 children) one daughter was suspected of PHAII. This daughter has been diagnosed at age 24 years with severe hypertension (200/130 mm Hg), hyperkalemia (6.5 mmol/l; reference range 3.5-5.1), and normal renal function. Besides a short stature, no other phenotypical abnormalities were identified. The patient’s hypertension and hyperkalemia completely normalized with hydrochlorothiazide, 50 mg daily. All other family members are healthy. Sanger sequencing of WNK1 and WNK4 did not reveal any mutation. A de novo mutation in the daughter is the most likely cause of her disease. Full exome sequencing (Nimblegen SeqCapEZV2, Illumina HiSeq2000) of the seven family members identified 34,733 unique variants in our patient. By comparing the recent mutations reported by Boyden et al, we found an identical de novo mutation in CUL3 , a splice acceptor site mutation in intron 8. RT-PCR analysis of RNA extracted from fresh blood showed two cDNA species in the patient, corresponding to a normal sized CUL3 fragment and an abrogated variant. Sanger sequencing of this shorter form revealed a loss of all nucleotides of exon 9 excluding the first nucleotide and including the first nucleotide of exon 10. This results in an in-frame 57 amino acid deletion abrogating the function of CUL3. After 1 week withdrawal from diuretic medication, we found in the patient’s urinary exosomes, vesicles derived from renal epithelial cells, an increase in the abundance of phosphorylated (P)-NCC (active form) compared to urinary exosomes extracted from urine when using thiazide diuretics. This de novo splice acceptor site mutation found in a second patient with PHAII confirmed CUL3 as a cause of PHAII. This patient also showed up-regulation of P-NCC as reported in WNK1 and WNK4 mutations pointing to the same mechanism of FHHt.

Mechanisms of sodium–chloride cotransporter modulation by angiotensin II

The renin-angiotensin-aldosterone system is an important modulator of renal salt excretion and arterial pressure. An important body of evidence now supports that angiotensin II (AngII) modulates the function of the renal sodium-chloride cotransporter (NCC), independently of aldosterone. Here we summarize these data, as well as recent knowledge regarding the intracellular mechanisms underlying this effect. AngII has the ability to modulate NCC total expression, apical localization, and phosphorylation by aldosterone-independent mechanisms. Recent evidence suggests that these effects are achieved through modulation of the With No Lysine kinase 4 (WNK4) and Ste20-related, proline-alanine-rich kinase (SPAK) pathway. Missense mutations in the acidic domain of WNK4, which are the cause of one of the subtypes of pseudohypoaldosteronism type II (PHAII), could be mimicking the effect produced by AngII on NCC through the WNK4-SPAK pathway. WNK4 activity has been shown to vary in response to changes in calcium concentration and PHAII-WNK4 mutants apparently lose this ability. Thus, AngII may regulate WNK4 activity through the modulation of intracellular calcium concentration. Modulation of WNK4 activity by AngII underlies the effects of AngII on NCC activity and this is probably important for the stimulation of renal sodium retention, as well as for the prevention of potassium loss, during hypovolemia.

11Beta-Hydroxylase Deficiency and Other Syndromes of Mineralocorticoid Excess as a Rare Cause of Endocrine Hypertension

Hypertension represents a major public and global health problem, most of which likely can be improved by lifestyle changes including changing dietary habits with less consumption of processed and preserved foods, which generally contain higher amounts of salt than freshly prepared food items. Among causes for endocrine hypertension are syndromes of mineralocorticoid excess. This group of mostly monogenic and acquired disorders typically causes hypertension through activation of the mineralocorticoid receptor either directly or indirectly via hormonal mediators and from overactive amiloride-sensitive epithelial sodium channels located in the distal tubule and collecting ducts of the kidneys. Apart from primary aldosteronism, mineralocorticoid excess can be caused by congenital adrenal hyperplasia (CAH) due to mutations of the 11beta-hydroxylase and 17alpha-hydroxylase genes, by inactivating mutations of the glucocorticoid receptor gene (Chrousos syndrome), endogenous hypercortisolism (Cushing's syndrome), by mutations of the 11beta-hydroxysteroid dehydrogenase type 2 gene (apparent mineralocorticoid excess/AME) or licorice/carbenoxolone intake, mutations of the epithelial sodium channel genes (Liddle syndrome), mutations of the mineralocorticoid receptor gene (Geller syndrome), and by mutations in the WNK1, WNK4, KLHL3, CUL3 genes (pseudohypoaldosteronism type 2 or Gordon syndrome). Most of these conditions are treated by restricting dietary salt intake. However, some require special therapies including dexamethasone/hydrocortisone (CAH), spironolactone/eplerenone (AME), epithelial sodium channel inhibitors amiloride/triamterene (Liddle and Gordon syndrome), while in others spironolactone and MR antagonists may be contraindicated due to an abnormally structured MR (Geller syndrome). We here review the pathophysiology, diagnosis, and therapy of these rare conditions including the presentation of a patient with 11beta-hydroxylase deficiency.

Phosphorylation of Na–Cl cotransporter by OSR1 and SPAK kinases regulates its ubiquitination

Na–Cl cotransporter (NCC) is phosphorylated in its amino terminus based on salt intake under the regulation of the WNK-OSR1/SPAK kinase cascade. We have observed that total protein abundance of NCC and its apical membrane expression varies in the kidney based on the phosphorylation status. To clarify the mechanism, we examined NCC ubiquitination status in mice fed low, normal and high salt diets, as well as in a model mouse of pseudohypoaldosteronism type II (PHAII) where NCC phosphorylation is constitutively elevated. Low-salt diet decreased NCC ubiquitination, while high-salt diet increased NCC ubiquitination in the kidney, and this was inversely correlated with total and phosphorylated NCC abundance. In the PHAII model, the ubiquitination of NCC in kidney was also lower when compared to that in wild-type littermates. To evaluate the relationship between phosphorylation and ubiquitination of NCC, we expressed wild-type, phospho-deficient and -mimicking NCC in COS7 cells, and the ubiquitination of immunoprecipitated total and biotinylated surface NCC was evaluated. NCC ubiquitination was increased in the phospho-deficient NCC and decreased in phospho-mimicking NCC in both total and surface NCC. Thus, we demonstrated that NCC phosphorylation decreased NCC ubiquitination, which may contribute to the increase of NCC abundance mostly on plasma membranes.

Thiol-reactive compounds from garlic inhibit the epithelial sodium channel (ENaC)

The epithelial sodium channel (ENaC) is a key factor in the transepithelial movement of sodium, and consequently salt and water homeostasis in various organs. Dysregulated activity of ENaC is associated with human diseases such as hypertension, the salt-wasting syndrome pseudohypoaldosteronism type 1, cystic fibrosis, pulmonary oedema or intestinal disorders. Therefore it is important to identify novel compounds that affect ENaC activity. This study investigated if garlic (Allium sativum) and its characteristic organosulfur compounds have impact on ENaCs. Human ENaCs were heterologously expressed in Xenopus oocytes and their activity was measured as transmembrane currents by the two-electrode voltage-clamp technique. The application of freshly prepared extract from 5g of fresh garlic (1% final concentration) decreased transmembrane currents of ENaC-expressing oocytes within 10min. This effect was dose-dependent and irreversible. It was fully sensitive to the ENaC-inhibitor amiloride and was not apparent on native control oocytes. The effect of garlic was blocked by dithiothreitol and l-cysteine indicating involvement of thiol-reactive compounds. The garlic organosulsur compounds S-allylcysteine, alliin and diallyl sulfides had no effect on ENaC. By contrast, the thiol-reactive garlic compound allicin significantly inhibited ENaC to a similar extent as garlic extract. These data indicate that thiol-reactive compounds which are present in garlic inhibit ENaC.

Activation of the renal Na + :Cl − cotransporter by angiotensin II is a WNK4-dependent process

Pseudohypoaldosteronism type II is a salt-sensitive form of hypertension with hyperkalemia in humans caused by mutations in the with-no-lysine kinase 4 (WNK4). Several studies have shown that WNK4 modulates the activity of the renal Na(+)Cl(-) cotransporter, NCC. Because the renal consequences of WNK4 carrying pseudoaldosteronism type II mutations resemble the response to intravascular volume depletion (promotion of salt reabsorption without K(+) secretion), a condition that is associated with high angiotensin II (AngII) levels, it has been proposed that AngII signaling might affect WNK4 modulation of the NCC. In Xenopus laevis oocytes, WNK4 is required for modulation of NCC activity by AngII. To demonstrate that WNK4 is required in the AngII-mediated regulation of NCC in vivo, we used a total WNK4-knockout mouse strain (WNK4(-/-)). WNK4 mRNA and protein expression were absent in WNK4(-/-) mice, which exhibited a mild Gitelman-like syndrome, with normal blood pressure, increased plasma renin activity, and reduced NCC expression and phosphorylation at T-58. Immunohistochemistry revealed normal morphology of the distal convoluted tubule with reduced NCC expression. Low-salt diet or infusion of AngII for 4 d induced phosphorylation of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and of NCC at S-383 and T-58, respectively, in WNK4(+/+) but not WNK4(-/-) mice. Thus, the absence of WNK4 in vivo precludes NCC and SPAK phosphorylation promoted by a low-salt diet or AngII infusion, suggesting that AngII action on the NCC occurs via a WNK4-SPAK-dependent signaling pathway. Additionally, stimulation of aldosterone secretion by AngII, but not by a high-K(+) diet, was impaired in WNK4(-/-) mice.

Loss of renal Nedd4‐2 in adult mice leads to PHAII compensated by ENaC down‐regulation and ROMK up‐regulation

Constitutive Nedd4‐2 knockout (KO) mice show Na+ retention and salt‐sensitive hypertension. To determine the role of renal Nedd4‐2 in adult mice, we generated inducible renal tubule‐specific Nedd4‐2 KO mice (Nedd4‐2Pax8/LC1) using a combination of the Tet‐On and Cre‐loxP systems. Under standard and high‐Na+ diets, KO displayed decreased plasma aldosterone but normal Na+/K+ balance. Under high Na+ diet, KO mice showed hypercalciuria and hypertension, both treated by thiazides. NCC expression and phosphorylation were increased in the KO. Interestingly, KO showed increased intracellular β‐ and γENaC abundance, but decreased α‐ and γENaC proteolytic cleavage and decreased αENaC mRNA and protein expression, suggesting down‐regulated ENaC. In contrast, ROMK protein expression and cell surface localization were increased in the distal convoluted tubules and connecting tubules of KO, in keeping with the absence of hyperkalemia. Thus, in adult mice, loss of renal Nedd4‐2 causes a pseudohypoaldosteronism type II‐like syndrome with NCC up‐regulation leading to salt‐sensitive hypertension and hypercalciuria, suggesting that Nedd4‐2 primarily targets NCC. The accompanying hypoaldosteronism with down‐regulation of ENaC‐mediated Na+ reabsorption together with an up‐regulation of K+ secretion via ROMK likely contributes to the compensated phenotype of the KO mice and explains the maintained Na+/K+ balance.

Exome Sequencing to Identify Novel Genes in Hypertension

### Study Hypothesis Previous genetic studies of patients with Mendelian forms of hypertension have identified causal genes involving renal electrolyte transport, highlighting new mechanisms for the regulation of blood pressure in humans. Pseudohypoaldosteronism type II (PHAII), a rare genetic disorder causing hypertension, hyperkalemia, and metabolic acidosis, has been found, in some cases, to be caused by mutations in WNK1 or WNK4 , but most cases remained unexplained at the time of the study. The authors proposed that exome sequencing and gene resequencing of a number of unrelated individuals with PHAII would uncover novel causal genes. ### How Was the Hypothesis Tested? Boyden et al recruited 52 kindreds with 126 patients with PHAII, of which just 7 kindreds were found to have WNK1 or WNK4 mutations. They performed exome sequencing of DNA samples of 11 unrelated PHAII index cases from 11 kindreds. They simultaneously performed genomewide genotyping of single nucleotide polymorphisms (SNPs) in all members of the kindreds, in order to perform linkage studies that would …

Disease-causing mutations in the acidic motif of WNK4 impair the sensitivity of WNK4 kinase to calcium ions

WNK4 is a serine/threonine protein kinase that is involved in pseudohypoaldosteronism type II (PHAII), a Mendelian form disorder featuring hypertension and hyperkalemia. Most of the PHAII-causing mutations are clustered in an acidic motif rich in negatively charged residues. It is unclear, however, whether these mutations affect the kinase activity in any way. In this study, we isolated kinase domain of WNK4 produced by Escherichia coli, and demonstrated its ability to phosphorylate the oxidative stress-responsive kinase-1 (OSR1) and the thiazide-sensitive Na+–Cl− cotransporter (NCC) in vitro. Threonine 48 was identified as the WNK4 phosphorylation site at mouse NCC. The phospho-mimicking T48D mutant of mouse NCC increased its protein abundance and Na+ uptake, and also enhanced the phosphorylation at the N-terminal region of NCC by OSR1. When the acidic motif was included in the WNK4 kinase construct, the kinase activity of WNK4 exhibited sensitivity to Ca2+ ions with the highest activity at Ca2+ concentration around 1μM using kinase-inactive OSR1 as a substrate. All tested PHAII-causing mutations at the acidic motif exhibited impaired Ca2+ sensitivity. Our results suggest that these PHAII-causing mutations disrupt a Ca2+-sensing mechanism around the acidic motif necessary for the regulation of WNK4 kinase activity by Ca2+ ions.

Effect of heterozygous deletion of WNK1 on the WNK-OSR1/ SPAK-NCC/NKCC1/NKCC2 signal cascade in the kidney and blood vessels.

We found that a mechanism of hypertension in pseudohypoaldosteronism type II (PHAII) caused by a WNK4 missense mutation (D561A) was activation of the WNK-OSR1/SPAK-NCC signal cascade. However, the pathogenic effect of intronic deletions in WNK1 genes also observed in PHAII patients remains unclear. To understand the pathophysiological roles of WNK1 in vivo, WNK1(+/-)mice have been analyzed, because homozygous WNK1 knockout is embryonic lethal. Although WNK1(+/-) mice have been reported to have hypotension, detailed analyses of the WNK signal cascade in the kidney and other organs of WNK1(+/-) mice have not been performed. We assess the effect of heterozygous deletion of WNK1 on the WNK-OSR1/SPAK-NCC/NKCC1/NKCC2 signal cascade in the kidney and blood vessels. Contrary to the previous report, the blood pressure of WNK1(+/-) mice was not decreased, even under a low-salt diet. Under a WNK4(D561A/+) background, the heterozygous deletion of the WNK1 gene did not reduce the high blood pressure either. We then evaluated the phosphorylation status of OSR1, SPAK, NCC, NKCC1, and NKCC2 in the kidney, but no significant decrease in the phosphorylation was observed in WNK1(+/-) mice or WNK1(+/-)WNK4(D561A/+) mice. In contrast, a significant decrease in NKCC1 phosphorylation in the aorta and a decreased pressure-induced myogenic response in the mesenteric arteries were observed in WNK1(+/-) mice. The contribution of WNK1 to total WNK kinase activity in the kidney may be small, but that WNK1 may play a substantial role in the regulation of blood pressure in the arteries.

Mutations in kelch-like 3 and cullin 3 cause hypertension and electrolyte abnormalities

Hypertension affects one billion people and is a principal reversible risk factor for cardiovascular disease. A rare Mendelian syndrome, pseudohypoaldosteronism type II (PHAII), featuring hypertension, hyperkalemia, and metabolic acidosis, has revealed previously unrecognized physiology orchestrating the balance between renal salt reabsorption versus K+ and H+ excretion1. We used exome sequencing to identify mutations in Kelch-like 3 (KLHL3) or Cullin 3 (CUL3) in 41 PHAII kindreds. KLHL3 mutations are either recessive or dominant, while CUL3 mutations are dominant and predominantly de novo. CUL3 and BTB-Kelch proteins such as KLHL3 are components of Cullin/RING E3 ligase complexes (CRLs) that ubiquitinate substrates bound to Kelch propeller domains2–8. Dominant KLHL3 mutations are clustered in short segments within the Kelch propeller and BTB domains implicated in substrate9 and Cullin5 binding, respectively. Diverse CUL3 mutations all result in skipping of exon 9, producing an in-frame deletion. Because dominant KLHL3 and CUL3 mutations both phenocopy recessive loss-of-function KLHL3 mutations, they may abrogate ubiquitination of KLHL3 substrates. Disease features are reversed by thiazide diuretics, which inhibit the Na-Cl cotransporter (NCC) in the distal nephron of the kidney; KLHL3 and CUL3 are expressed in this location, suggesting a mechanistic link between KLHL3/CUL3 mutations, increased Na-Cl reabsorption, and disease pathogenesis. These findings demonstrate the utility of exome sequencing in disease gene identification despite combined complexities of locus heterogeneity, mixed models of transmission, and frequent de novo mutation, and establish a fundamental role for KLHL3/CUL3 in blood pressure, K+, and pH homeostasis.

Pathogenesis of pseudohypoaldosteronism type 2 by WNK1 mutations

Pseudohypoaldosteronism type 2 (PHA2) is a rare autosomal dominant form of human arterial hypertension, associated with hyperkalemia and hyperchloremic metabolic acidosis. WNK1 and WNK4 are two of the genes mutated in PHA2 patients. This review focuses on the mechanisms by which deletions of the first intron of WNK1 found in PHA2 patients trigger the disease. The WNK1 gene gives rise to a ubiquitous kinase (L-WNK1) and to a shorter kinase-defective isoform, KS-WNK1 (for kidney-specific WNK1), expressed only in the distal convoluted tubule (DCT) and connecting tubule. WNK1 first intron deletion leads to overexpression of L-WNK1 in the DCT and ubiquitous ectopic expression of KS-WNK1. The increased expression of L-WNK1 in the DCT results in increased activity of the Na-Cl cotransporter (NCC) and thus hypervolemia and hypertension. Contrarily, the mechanisms underlying the hyperkalemia and metabolic acidosis remain unclear. As particularly small doses of thiazide diuretics, inhibitors of NCC activity, correct both the blood pressure and metabolic disorders in PHA2 patients, it was believed that increased NCC was directly responsible for all PHA2 features. Studies performed in mouse models of KS-WNK1 inactivation or WNK4-related PHA2, however, have revealed that the situation is much more complex.

A novel splice site mutation of the beta subunit gene of epithelial sodium channel (ENaC) in one Turkish patient with a systemic form of pseudohypoaldosteronism type 1

Pseudohypoaldosteronism Type 1 (PHA1) is a rare heterogeneous syndrome characterized by severe salt loss, hyperkalemia, hyponatremia, metabolic acidosis, hyperaldosteronism and hyperreninemia. Multi-system form of PHA1 is caused by mutations in one of the genes encoding the α, β and γ subunits of epithelial sodium channels (ENaC). In this study, we presented a novel splice site mutation in the beta-gene of ENaC in a patient with multi-system PHA. We performed DNA sequencing analysis of SCNN1A, SCNN1B, SCNN1G and NR3C2 genes. We found a novel c.1266-1G>C homozygous splice site mutation in intron 8 of the SCNN1B gene. Initially elevated plasma renin activity (PRA) and aldosterone levels of the patient returned to normal with large amounts of dietary salt and serum sodium (Na+) and potassium (K+) levels were within normal range at the end of the first year of life. This improvement may be due to partial activity of mutated ENaC subunits, reduced dependence on aldosterone in salt homeostasis with increasing age, and alternative regulating mechanisms in sodium homeostasis. The results enhance our understanding of the pathophysiology of this disorder and the mechanisms of renal salt conservation.

Vascular Na+/Ca2+ Exchanger Type-1 Contributes to Hypertension in Pseudohypoaldosteronism Type II and Cushing's Syndrome Models

Pseudohypoaldosteronism type II (PHAII) is an autosomal dominant disease characterized by hypertension due to increased renal salt reabsorption. Cushing's syndrome is associated with excessive cortisol secretion by ectopic ACTH producing tumor and may result in hypertension. Here we examined the role of Na+/Ca2+ exchanger type-1 (NCX1) in hypertension of these model mice using specific NCX inhibitors and genetically engineered mice. NCX inhibitors lowered arterial hypertension in WNK4 mutant knockin mice (presenting the phenotype of PHAII) and chronically ACTH-administered mice. Furthermore, heterozygous knockout of NCX1 was resistant to development of hypertension in these model mice, whereas vascular overexpression of NCX1 accelerated their hypertension. Since NCX inhibitors reversed the cytosolic Ca2+ elevation and vasoconstriction induced by nanomolar ouabain, circulating endogenous cardiac glycosides may be involved in hypertension of these model mice. Thus, vascular NCX1 contributes to hypertension in PHAII and Cushing's syndrome, and NCX inhibitors might be therapeutically useful for their hypertension.