Kevin M. O'Shaughnessy

Somatic Mutations Affecting the Selectivity Filter of KCNJ5 Are Frequent in 2 Large Unselected Collections of Adrenal Aldosteronomas

Primary hyperaldosteronism, one cause of which is aldosterone-producing adenomas (APAs), may account for ⩽5% to 10% of cases of essential hypertension. Germline mutations have been identified in 2 rare familial forms of primary hyperaldosteronism, but it has been reported recently that somatic mutations of the KCNJ5 gene, which encodes a potassium channel, are present in some sporadic nonsyndromic APAs. To address this further we screened 2 large collections of sporadic APAs from the United Kingdom and Australia (totalling 73) and found somatic mutations in the selectivity filter of KCNJ5 in 41% (95% CI: 31% to 53%) of the APAs (30 of 73). These included the previously noted nonsynonymous mutations, G151R and L158R, and an unreported 3-base deletion, delI157, in the region of the selectivity filter. APAs containing a somatic KCNJ5 mutation were significantly larger than those without (1.61 cm [95% CI: 1.39–1.83 cm] versus 1.04 cm [95% CI: 0.91–1.17 cm]; P<0.0001) but with substantial overlap in size between genotypes. The APAs carrying a mutation, but not those without, also consistently lacked a postural aldosterone response, suggesting a physiologically distinct subtype. Hence, somatic KCNJ5 mutations are not restricted to large APAs (>2 cm), and their frequency in our unselected series suggests they are common and could be important in the molecular pathogenesis of many sporadic cases of APA.

Characterization of a novel somatic KCNJ5 mutation delI157 in an aldosterone-producing adenoma.

Objective: Adrenal aldosterone-producing adenomas (APAs) are an increasingly recognized cause of primary aldosteronism, and somatic mutations within the KCNJ5 gene encoding an inwardly rectifying K+ channel (also called GIRK4 or Kir3.4) have been identified by several groups including our own. We identified the previously noted G151R and L168R mutations in the region of a selectivity filter of the channel as well as a previously unreported 3-base deletion, delI157. Here, we report the functional properties of KCNJ5 channels carrying this novel delI157 mutation. Methods: The delI157 mutation was introduced into wild-type KCNJ5 sequences to allow its expression in both H295R cells and Xenopus oocytes to study its expression and electrophysiology, respectively. Results: In the adrenal cell line H295R, the delI157 mutant expresses and traffics normally to the cell surface. However, the current–voltage behavior of the mutant in oocytes is distinct from wild-type channels and mimics closely other selectivity filter mutations. In particular, its ability to support substantial current when extracellular K+ is replaced by Na+. We also report for the first time that the mutants have reduced sensitivity to the KCNJ5 inhibitor tertiapin-Q that binds to the external vestibule of the channel pore. Conclusion: This novel KCNJ5 mutation behaves like the three selectivity filter mutations previously reported in APAs depolarizing the cell and showing reduced cation selectivity. The reduced sensitivity to tertiapin-Q suggests that the abnormal Na+ permeability of these selectivity mutations does indeed reflect structural changes around the mouth of the ion channel.

Characterisation of the Cullin‐3 mutation that causes a severe form of familial hypertension and hyperkalaemia

Deletion of exon 9 from Cullin‐3 (CUL3, residues 403–459: CUL3Δ403–459) causes pseudohypoaldosteronism type IIE (PHA2E), a severe form of familial hyperkalaemia and hypertension (FHHt). CUL3 binds the RING protein RBX1 and various substrate adaptors to form Cullin‐RING‐ubiquitin‐ligase complexes. Bound to KLHL3, CUL3‐RBX1 ubiquitylates WNK kinases, promoting their ubiquitin‐mediated proteasomal degradation. Since WNK kinases activate Na/Cl co‐transporters to promote salt retention, CUL3 regulates blood pressure. Mutations in both KLHL3 and WNK kinases cause PHA2 by disrupting Cullin‐RING‐ligase formation. We report here that the PHA2E mutant, CUL3Δ403–459, is severely compromised in its ability to ubiquitylate WNKs, possibly due to altered structural flexibility. Instead, CUL3Δ403–459 auto‐ubiquitylates and loses interaction with two important Cullin regulators: the COP9‐signalosome and CAND1. A novel knock‐in mouse model of CUL3WT/Δ403–459 closely recapitulates the human PHA2E phenotype. These mice also show changes in the arterial pulse waveform, suggesting a vascular contribution to their hypertension not reported in previous FHHt models. These findings may explain the severity of the FHHt phenotype caused by CUL3 mutations compared to those reported in KLHL3 or WNK kinases.

A systematic review and meta‐analysis of thiazide‐induced hyponatraemia: time to reconsider electrolyte monitoring regimens after thiazide initiation?

AIMS Hyponatraemia is one of the major adverse effects of thiazide and thiazide-like diuretics and the leading cause of drug-induced hyponatraemia requiring hospital admission. We sought to review and analyze all published cases of this important condition. METHODS Ovid Medline, Embase, Web of Science and PubMed electronic databases were searched to identify all relevant articles published before October 2013. A proportions meta-analysis was undertaken. RESULTS One hundred and two articles were identified of which 49 were single patient case reports. Meta-analysis showed that mean age was 75 (95% CI 73, 77) years, 79% were women (95% CI 74, 82) and mean body mass index was 25 (95% CI 20, 30) kg m−2. Presentation with thiazide-induced hyponatraemia occurred a mean of 19 (95% CI 8, 30) days after starting treatment, with mean trough serum sodium concentration of 116 (95% CI 113, 120) mm and serum potassium of 3.3 (95% CI 3.0, 3.5) mm. Mean urinary sodium concentration was 64 mm (95% CI 47, 81). The most frequently reported drugs were hydrochlorothiazide, indapamide and bendroflumethiazide. CONCLUSIONS Patients with thiazide-induced hyponatraemia were characterized by advanced age, female gender, inappropriate saliuresis and mild hypokalaemia. Low BMI was not found to be a significant risk factor, despite previous suggestions. The time from thiazide initiation to presentation with hyponatraemia suggests that the recommended practice of performing a single investigation of serum biochemistry 7–14 days after thiazide initiation may be insufficient or suboptimal. Further larger and more systematic studies of thiazide-induced hyponatraemia are required.

Critical role of the SPAK protein kinase CCT domain in controlling blood pressure

The STE20/SPS1-related proline/alanine-rich kinase (SPAK) controls blood pressure (BP) by phosphorylating and stimulating the Na-Cl (NCC) and Na-K-2Cl (NKCC2) co-transporters, which regulate salt reabsorption in the kidney. SPAK possesses a conserved carboxy-terminal (CCT) domain, which recognises RFXV/I motifs present in its upstream activator [isoforms of the With-No-lysine (K) kinases (WNKs)] as well as its substrates (NCC and NKCC2). To define the physiological importance of the CCT domain, we generated knock-in mice in which the critical CCT domain Leu502 residue required for high affinity recognition of the RFXI/V motif was mutated to Alanine. The SPAK CCT domain defective knock-in animals are viable, and the Leu502Ala mutation abolished co-immunoprecipitation of SPAK with WNK1, NCC and NKCC2. The CCT domain defective animals displayed markedly reduced SPAK activity and phosphorylation of NCC and NKCC2 co-transporters at the residues phosphorylated by SPAK. This was also accompanied by a reduction in the expression of NCC and NKCC2 protein without changes in mRNA levels. The SPAK CCT domain knock-in mice showed typical features of Gitelman Syndrome with mild hypokalaemia, hypomagnesaemia, hypocalciuria and displayed salt wasting on switching to a low-Na diet. These observations establish that the CCT domain plays a crucial role in controlling SPAK activity and BP. Our results indicate that CCT domain inhibitors would be effective at reducing BP by lowering phosphorylation as well as expression of NCC and NKCC2.

Molecular insights from dysregulation of the thiazide-sensitive WNK/SPAK/NCC pathway in the kidney: Gordon syndrome and thiazide-induced hyponatraemia

Human blood pressure is dependent on balancing dietary salt intake with its excretion by the kidney. Mendelian syndromes of altered blood pressure demonstrate the importance of the distal nephron in this process and of the thiazide‐sensitive pathway in particular. Gordon syndrome (GS), the phenotypic inverse of the salt‐wasting Gitelman syndrome, is a condition of hyperkalaemic hypertension that is reversed by low‐dose thiazide diuretics or a low‐salt diet. Variants within at least four genes [i.e. with‐no‐lysine(K) kinase 1 (WNK1), WNK4, kelch‐like family member 3 (KLHL3) and cullin 3 (CUL3)] can cause the phenotype of GS. Details are still emerging for some of these genes, but it is likely that they all cause a gain‐of‐function in the thiazide‐sensitive Na+–Cl− cotransporter (NCC) and hence salt retention. Herein, we discuss the key role of STE20/sporulation‐specific protein 1 (SPS1)‐related proline/alanine‐rich kinase (SPAK), which functions as an intermediary between the WNKs and NCC and for which a loss‐of‐function mutation produces a Gitelman‐type phenotype in a mouse model. In addition to Mendelian blood pressure syndromes, the study of patients who develop thiazide‐induced‐hyponatraemia (TIH) may give further molecular insights into the role of the thiazide‐sensitive pathway for salt reabsorption. In the present paper we discuss the key features of TIH, including its high degree of reproducibility on rechallenge, possible genetic predisposition and mechanisms involving excessive saliuresis and water retention. Together, studies of Gordon syndrome and TIH may increase our understanding of the molecular regulation of sodium trafficking via the thiazide‐sensitive pathway and have important implications for hypertensive patients, both in the identification of new antihypertensive drug targets and avoidance of hyponatraemic side‐effects.

Extrarenal roles of the with-no-lysine[K] kinases (WNKs).

Identified over a decade ago, the with‐no‐lysine[K] kinases (WNKs) have been the subsequent focus of intense research into the renal handling of Na+, Cl− and K+ and several rare monogenetic diseases. However, the potential extrarenal roles for WNKs have been less well explored. Thiazides and Gordon syndrome are known to have effects on bone mineral density, Ca2+ and PO43− homeostasis, which were originally assumed to be an indirect effect through the kidney. However, current data suggest a complex and direct role for WNKs in the physiology of bone. The WNKs also modulate systemic blood pressure at several levels, including the vascular resistance vessels, where they cause vasoconstriction by altering the abundance of the transient receptor potential canonical channel 3 and/or phosphorylation of the Na+−K+−2Cl− cotransporter 1 in vascular smooth muscle cells. The WNKs and many of the cation‐coupled Cl− cotransporters they regulate are highly expressed in the central nervous system and recent work suggests that WNK dysfunction may have a role in the development of autism, schizophrenia and hereditary sensory and autonomic neuropathy Type 2. Finally, the WNK–sterile 20 kinase signalling axis represents an evolutionarily ancient mechanism for maintaining osmotic homeostasis, but a rapidly expanding body of evidence also shows a role in immunity and cellular regulation.

Novel Insertion Mutation in KCNJ5 Channel Produces Constitutive Aldosterone Release From H295R Cells.

Primary aldosteronism accounts for 5%-10% of hypertension and in a third of cases is caused by autonomous aldosterone production by adenomas (APA). Somatic mutations in the potassium channel encoded by KCNJ5 have been detected in surgically removed APAs. To better understand the role of these mutations, we resequenced the KCNJ5 channel in a large Australian primary aldosteronism cohort. KCNJ5 mutations were detected in 37 APAs (45% of the cohort), including previously reported E145Q (n = 3), G151R (n = 20), and L168R (n = 13) mutations. In addition, we found a novel 12-bp in-frame insertion mutation (c.414-425dupGCTTTCCTGTTC, A139_F142dup) that duplicates the AFLF sequence in the pore helix upstream of the selectivity filter. Expressed in Xenopus oocytes, the A139_F142dup mutation depolarized the oocytes and produced a G-protein-sensitive Na(+) current with altered K(+) selectivity and loss of inward rectification but retained Ba(2+) sensitivity. Transfected into H295R cells, A139_F142dup increased basal aldosterone release 2.3-fold over the wild type. This was not increased further by incubation with angiotensin II. Although the A139_F142dup mutant trafficked to the plasma membrane of H295R cells, it showed reduced tetramer stability and surface expression compared with the wild-type channel. This study confirms the frequency of somatic KCNJ5 mutations in APAs and the novel mutation identified (A139_F142dup) extend the phenotypic range of the known KCNJ5 APA mutations. Being located in the pore helix, it is upstream of the previously reported mutations and shares some features in common with selectivity filter mutants but additionally demonstrates insensitivity to angiotensin II and decreased channel stability.