Monogenic Etiology of Hypertension

Monogenic hypertension encompasses a group of conditions wherein single gene mutations result in increased renal sodium reabsorption manifesting as low renin hypertension. As these diseases are rare, their contribution to hypertension in children and adolescents is often overlooked. Precise diagnosis is essential in those who have not been found to have more common identifiable causes of hypertension in adolescents, since treatment strategies for these rare conditions are specific and different from antihypertensive regimens for the other more common causes of hypertension in this age group. The objective of this review is to provide insight to the rare, monogenic forms of hypertension.

A Spanish Family with Gordon Syndrome Due to a Variant in the Acidic Motif of WNK1

(1) Background: Gordon syndrome (GS) or familial hyperkalemic hypertension is caused by pathogenic variants in the genes WNK1, WNK4, KLHL3, and CUL3. Patients presented with hypertension, hyperkalemia despite average glomerular filtration rate, hyperchloremic metabolic acidosis, and suppressed plasma renin (PR) activity with normal plasma aldosterone (PA) and sometimes failure to thrive. GS is a heterogeneous genetic syndrome, ranging from severe cases in childhood to mild and sometimes asymptomatic cases in mid-adulthood. (2) Methods: We report here a sizeable Spanish family of six patients (four adults and two children) with GS. (3) Results: They carry a novel heterozygous missense variant in exon 7 of WNK1 (p.Glu630Gly). The clinical presentation in the four adults consisted of hypertension (superimposed pre-eclampsia in two cases), hyperkalemia, short stature with low body weight, and isolated hyperkalemia in both children. All patients also presented mild hyperchloremic metabolic acidosis and low PR activity with normal PA levels. Abnormal laboratory findings and hypertension were normalized by dietary salt restriction and low doses of thiazide or indapamide retard. (4) Conclusions: This is the first Spanish family with GS with a novel heterozygous missense variant in WNK1 (p.Glu630Gly) in the region containing the highly conserved acidic motif, which is showing a relatively mild phenotype, and adults diagnosed in mild adulthood. These data support the importance of missense variants in the WNK1 acidic domain in electrolyte balance/metabolism. In addition, findings in this family also suggest that indapamide retard or thiazide may be an adequate long-standing treatment for GS.

Classification of pseudohypoaldosteronism type II as type IV renal tubular acidosis: results of a literature review

Pseudohypoaldosteronism (PHA) type II (PHA2) is a genetic disorder that leads to volume overload and hyperkalemic metabolic acidosis. PHA2 and PHA type I (PHA1) have been considered to be genetic and pediatric counterparts to type IV renal tubular acidosis (RTA). Type IV RTA is frequently found in adults with chronic kidney disease and is characterized by hyperchloremic hyperkalemic acidosis with normal anion gap (AG). However, we recently observed that PHA1 was not always identical to type IV RTA. In this study, we focused on the aid-base balance in PHA2. Through a literature search published between 2008-2020, 46 molecularly diagnosed cases with PHA2 were identified (median age of 14 years). They comprised 11 sets of familial and 16 sporadic cases and the pathology was associated with mutations in WNK 4 (n = 1), KLHL3 (n = 17), and CUL3 (n = 9). The mean potassium (K⁺) level was 6.2 ± 0.9 mEq/L (n = 46, range 4.0-8.6 mEq/L), whereas that of chloride (Cl^-) was 110 ± 3.5 mEq/L (n = 41, 100-119 mEq/L), with 28 of 41 cases identified as hyperchloremic. More than half of the cases (18/35) presented with metabolic acidosis. Although AG data was obtained only in 16 cases, all but one cases were within normal AG range. Both Cl^- and HCO3^- levels showed significant correlations with K⁺ levels, which suggested that the degree of hyperchloremia and acidosis reflect the clinical severity, and is closely related to the fundamental pathophysiology of PHA2. In conclusion, our study confirmed that PHA2 is compatible with type IV RTA based on laboratory findings.

[A Case of Pseudohypoaldosteronism Type Ⅱ (PHA2) Caused by a Novel Mutation of KLHL3]

The patient, a 41-year-old woman, was admitted because "it was found out she had elevated serum potassium levels for 18 days". Eighteen days prior to admission at our hospital, the patient was found to have elevated serum potassium during hospitalization at another hospital, where the patient received symptomatic treatment and was discharged after her serum potassium returned to a normal level. However, the patient still had elevated serum potassium repeatedly and was referred to our hospital for further examination. The patient had a history of acute nephritis and gestational hypertension. Six months prior to admission at our hospital, it was found out that the patient had slightly elevated blood pressure, but no intervention was done. The patient's father has a history of hypertension and diabetes. After admission, laboratory results showed that the patient had hyperkalemia, hyperchloremia and metabolic acidosis. The level of plasma renin was obviously below the normal range, but the concentration of plasma aldosterone was within the normal range. A new mutation locus (c.1115delG) in KLHL3 (Kelch like family member 3) gene was revealed by genetic testing, leading to the diagnosis of pseudoaldosteronism type Ⅱ (PHA2). The patient was given regular treatment of oral hydrochlorothiazide hydrochloride at set intervals. Subsequently, her blood electrolyte level, blood pH, BE and BEB have returned to normal levels. The patient was followed up for 12 months and did not feel unwell during the follow-up period.

Gordon syndrome: Dental implications and a case report

AIM

This article describes the craniofacial and dental features of an individual with Gordon syndrome. The dental management implications and considerations of treating patients with Gordon Syndrome and similar conditions resulting in limited mouth opening are discussed.

METHODS

A 14-year-old South African male was referred to the Dental Genetics Clinic with the main complaint of carious teeth. His craniofacial characteristics included low set and posteriorly rotated ears, down-slanted palpebral fissures, sloping shoulders, and a broad neck. A prognathic mandible and mild facial asymmetry were noted. He had a significant limitation of mouth opening (2 cm at incisors). Radiographic examination revealed multiple carious teeth, missing mandibular premolars, impacted maxillary premolars, taurodontism of the 44 and 34, and enlarged coronoid processes of the mandible. Dental extractions and restorations have been performed under local anaesthesia.

CONCLUSION

Gordon syndrome and similar conditions, may result in limited oral opening and impaired manual dexterity. The severity of limitation of mouth opening determines management. Dental management should focus on ensuring that the patient is able to maintain good oral hygiene by customising homecare for the individual and regular dental visits.

Monogenic Forms of Hypertension

Essential hypertension is a highly prevalent disease in the general population. Secondary hypertension is characterized by a specific and potentially reversible cause of increased blood pressure levels. Some secondary endocrine forms of hypertension are common (caused by uncontrolled cortisol, aldosterone, or catecholamines production). This article describes rare monogenic forms of hypertension, characterized by electrolyte disorders and suppressed renin-aldosterone axis. They represent simple models for the physiology of renal control of sodium levels and plasma volume, thus reaching a high scientific interest. Furthermore, they could explain some features closer to the essential phenotype of hypertension, suggesting a mechanistically driven personalized treatment.

Low-Renin Hypertension

Low-renin hypertension affects 30% of hypertensive patients. Primary hyperaldosteronism presents with low renin and aldosterone excess. Low-renin, low-aldosterone hypertension represents a wide spectrum of disorders that includes essential low-renin hypertension, hereditary forms of hypertension, and hypertension secondary to endogenous or exogenous factors. This review addresses the different conditions that present with low-renin hypertension, discussing an appropriate diagnostic approach and highlighting the genetic subtypes within familial forms.

The Molecular Genetics of Gordon Syndrome

Gordon syndrome is a rare inherited monogenic form of hypertension, which is associated with hyperkalaemia and metabolic acidosis. Since the recognition of this predominantly autosomal dominant condition in the 1960s, the study of families with Gordon syndrome has revealed four genes WNK1, WNK4, KLHL3, and CUL3 to be implicated in its pathogenesis after a phenotype–genotype correlation was realised. The encoded proteins Kelch-like 3 and Cullin 3 interact to form a ring-like complex to ubiquitinate WNK-kinase 4, which, in normal circumstances, interacts with the sodium chloride co-symporter (NCC), the epithelial sodium channel (ENaC), and the renal outer medullary potassium channel (ROMK) in an inhibitory manner to maintain normokalaemia and normotension. WNK-kinase 1 has an inhibitory action on WNK-kinase 4. Mutations in WNK1, WNK4, KLHL3, and CUL3 all result in the accumulation of WNK-kinase 4 and subsequent hypertension, hyperkalaemia, and metabolic acidosis. This review explains the clinical aspects, disease mechanisms, and molecular genetics of Gordon syndrome.

Mutations in PIEZO2 contribute to Gordon syndrome, Marden-Walker syndrome and distal arthrogryposis: A bioinformatics analysis of mechanisms

Piezo type mechanosensitive ion channel component 2 (PIEZO2) is a mechanically activated ion channel. Mutations in PIEZO2 may cause distal arthrogryposis 3 (DA3)/Gordon syndrome (GS), DA5, Marden-Walker syndrome (MWS) and associated diseases. To date, no systematic study has analyzed and compared the influence of different gene mutations of PIEZO2 on its transcription, as well as translation and protein function. Therefore, the objective of the present study was to systematically assess the effect of different pathological mutations of PIEZO2 on transcription, translation, as well as protein structure and function that contribute to GS/DA3, DA5, MWS and associated diseases based on a bioinformatics analysis using the Pubmed, ClinVar, RaptorX and Phyre2 online databases. The results indicated the presence of 27 pathological mutations in PIEZO2, including dominant and recessive mutations. Dominant mutations were mainly located in the C-terminal region, whereas recessive mutations were mainly localized in the N-terminal region, and most reported mutation sites exhibited high evolutionary conservation among different species. Loss-of-function mutations result in nonsense-mediated transcript decay or premature termination codons, consequently leading to a lack of PIEZO2 protein, whereas gain-of-function mutations may lead to increased PIEZO2-associated channel activity. The bioinformatics analysis results also indicated that the p.Ala1486Pro, p.Thr2221Ile and p.Glu2727del mutations modify the secondary structure of the PIEZO2 protein, while p.Thr2221Ile, p.Arg2718Leu and p.Arg2718Pro mutations reduce the solvent accessibility of PIEZO2 protein. Furthermore, the p.Ala1486Pro, p.Thr2221Ile, p.Ser2223Leu, p.Thr2356Met, p.Arg2686His, p.Arg2718Leu, p.Arg2718Pro and p.Glu2727del mutations affect the transmembrane region. These changes of PIEZO2 may contribute to a gain-of-function of PIEZO2. Variable clinical phenotypes were present between and among the gain- and loss-of-function mutations linked with PIEZO2-associated disease, which implied that different mutations in PIEZO2 have different pathophysiological effects. Of course, further functional studies to explore the precise structure and function of PIEZO2 are necessary and may offer useful clues for the prevention and treatment of associated diseases.

Overview of Monogenic or Mendelian Forms of Hypertension

Monogenic or Mendelian forms of hypertension are described as a group of conditions characterized by insults to the normal regulation of blood pressure by the kidney and adrenal gland. These alterations stem from single mutations that lead to maladaptive overabsorption of electrolytes with fluid shift into the vasculature, and consequent hypertension. Knowledge of these various conditions is essential in diagnosing pediatric or early-onset adult hypertension as they directly affect treatment strategies. Precise diagnosis with specific treatment regimens aimed at the underlying physiologic derangement can restore normotension and prevent the severe sequelae of chronic hypertension.

A mouse model of pseudohypoaldosteronism type II reveals a novel mechanism of renal tubular acidosis

Pseudohypoaldosteronism type II (PHAII) is a genetic disease characterized by association of hyperkalemia, hyperchloremic metabolic acidosis, hypertension, low renin, and high sensitivity to thiazide diuretics. It is caused by mutations in the WNK1, WNK4, KLHL3 or CUL3 gene. There is strong evidence that excessive sodium chloride reabsorption by the sodium chloride cotransporter NCC in the distal convoluted tubule is involved. WNK4 is expressed not only in distal convoluted tubule cells but also in β-intercalated cells of the cortical collecting duct. These latter cells exchange intracellular bicarbonate for external chloride through pendrin, and therefore, account for renal base excretion. However, these cells can also mediate thiazide-sensitive sodium chloride absorption when the pendrin-dependent apical chloride influx is coupled to apical sodium influx by the sodium-driven chloride/bicarbonate exchanger. Here we determine whether this system is involved in the pathogenesis of PHAII. Renal pendrin activity was markedly increased in a mouse model carrying a WNK4 missense mutation (Q562E) previously identified in patients with PHAII. The upregulation of pendrin led to an increase in thiazide-sensitive sodium chloride absorption by the cortical collecting duct, and it caused metabolic acidosis. The function of apical potassium channels was altered in this model, and hyperkalemia was fully corrected by pendrin genetic ablation. Thus, we demonstrate an important contribution of pendrin in renal regulation of sodium chloride, potassium and acid-base homeostasis and in the pathophysiology of PHAII. Furthermore, we identify renal distal bicarbonate secretion as a novel mechanism of renal tubular acidosis.

Renal sodium and magnesium reabsorption are not coupled in a mouse model of Gordon syndrome

Active reabsorption of magnesium (Mg2+ ) in the distal convoluted tubule (DCT) of the kidney is crucial for maintaining Mg2+ homeostasis. Impaired activity of the Na+ -Cl- -cotransporter (NCC) has been associated with hypermagnesiuria and hypomagnesemia, while increased activity of NCC, as observed in patients with Gordon syndrome, is not associated with alterations in Mg2+ balance. To further elucidate the possible interrelationship between NCC activity and renal Mg2+ handling, plasma Mg2+ levels and urinary excretion of sodium (Na+ ) and Mg2+ were measured in a mouse model of Gordon syndrome. In this model, DCT1-specific expression of a constitutively active mutant form of the NCC-phosphorylating kinase, SPAK (CA-SPAK), increases NCC activity and hydrochlorothiazide (HCTZ)-sensitive Na+ reabsorption. These mice were normomagnesemic and HCTZ administration comparably reduced plasma Mg2+ levels in CA-SPAK mice and control littermates. As inferred by the initial response to HCTZ, CA-SPAK mice exhibited greater NCC-dependent Na+ reabsorption together with decreased Mg2+ reabsorption, compared to controls. Following prolonged HCTZ administration (4 days), CA-SPAK mice exhibited higher urinary Mg2+ excretion, while urinary Na+ excretion decreased to levels observed in control animals. Surprisingly, CA-SPAK mice had unaltered renal expression of Trpm6, encoding the Mg2+ -permeable channel TRPM6, or other magnesiotropic genes. In conclusion, CA-SPAK mice exhibit normomagnesemia, despite increased NCC activity and Na+ reabsorption. Thus, Mg2+ reabsorption is not coupled to increased thiazide-sensitive Na+ reabsorption, suggesting a similar process explains normomagnesemia in Gordon syndrome. Further research is required to unravel the molecular underpinnings of this phenomenon and the more pronounced Mg2+ excretion after prolonged HCTZ administration.

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.

Gordon syndrome: literature review and a report of two cases

The aim of this article is to publish a literature review and report on two new cases of Gordon syndrome (GS), a rare syndrome documented to have an autosomal dominant inheritance pattern or to occur sporadically; it is characterized by camptodactyly, cleft palate, and talipes equinovarus. We report two exceptional cases of GS where both patients were also diagnosed with congenital myopathy, and one developed malignant hyperthermia. These are the first two cases reported where patients were diagnosed with both GS and congenital myopathy or where GS is associated with malignant hyperthermia.

Kelch-like 3 and Cullin 3 regulate electrolyte homeostasis via ubiquitination and degradation of WNK4

Pseudohypoaldosteronism type II (PHAII) is a rare Mendelian syndrome featuring hypertension and hyperkalemia resulting from constitutive renal salt reabsorption and impaired K(+) secretion. Recently, mutations in Kelch-like 3 (KLHL3) and Cullin 3 (CUL3), components of an E3 ubiquitin ligase complex, were found to cause PHAII, suggesting that loss of this complex's ability to target specific substrates for ubiquitination leads to PHAII. By MS and coimmunoprecipitation, we show that KLHL3 normally binds to WNK1 and WNK4, members of WNK (with no lysine) kinase family that have previously been found mutated in PHAII. We show that this binding leads to ubiquitination, including polyubiquitination, of at least 15 specific sites in WNK4, resulting in reduced WNK4 levels. Dominant disease-causing mutations in KLHL3 and WNK4 both impair WNK4 binding, ubiquitination, and degradation. WNK4 normally induces clearance of the renal outer medullary K(+) channel (ROMK) from the cell surface. We show that WT but not mutant KLHL3 inhibits WNK4-induced reduction of ROMK level. We show that PHAII-causing mutations in WNK4 lead to a marked increase in WNK4 protein levels in the kidney in vivo. These findings demonstrate that CUL3-RING (really interesting new gene) ligases that contain KLHL3 target ubiquitination of WNK4 and thereby regulate WNK4 levels, which in turn regulate levels of ROMK. These findings reveal a specific role of CUL3 and KLHL3 in electrolyte homeostasis and provide a molecular explanation for the effects of disease-causing mutations in both KLHL3 and WNK4.