Gordon Syndrome: a continuing story

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Pseudohypoaldosteronism Type II or Gordon Syndrome: A Rare Syndrome of Hyperkalemia and Hypertension With Normal Renal Function

Pseudohypoaldosteronism type II (PHA II) or Gordon syndrome is characterized by hyperkalemia, hypertension, hyperchloremic metabolic acidosis, low plasma renin activity, and normal kidney function. We report a rare case of a young adult female patient presenting with abdominal pain, diarrhea, and vomiting. She was hypertensive during the presentation. Blood work showed mild anemia, hyperkalemia, hyperchloremia, and metabolic acidosis, with normal renal function and liver function. Plasma renin activity and aldosterone levels were low-normal. These findings were suggestive of PHA II or Gordon syndrome. It is a rare familial disease, with a non-specific presentation and no specific diagnostic criteria, and physicians should suspect it in patients with hyperkalemia in the setting of normal glomerular filtration, along with hypertension (which can be absent), metabolic acidosis, hyperchloremia, low plasma renin activity, and relatively suppressed aldosterone.

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.

Gordon's syndrome in pregnancy

Gordon's syndrome, also known as pseudohypoaldosteronism type II and familial hyperkalaemic hypertension, is a rare inherited condition characterised by familial hyperkalaemia, normal anion gap hyperchloraemic metabolic acidosis, low renin with normal glomerular filtration rate and hypertension. The outcome of 11 pregnancies in 3 women with Gordon's syndrome is presented and combined with 13 pregnancies in 7 women previously described. Pregnancy in women with Gordon's syndrome appears to be associated with a significant risk of adverse pregnancy outcomes, particularly where there is maternal hypertension preconception. No pregnancy registry exists for Gordon's syndrome. The available data is limited to case reports and small case series and may be affected by bias. A pregnancy registry would be valuable to assist in preconception counselling and management during pregnancy. The goal of this study was to summarise the available cases describing pregnancy outcomes with maternal Gordon's syndrome.

Summary of Known Genetic and Epigenetic Modification Contributed to Hypertension

Hypertension is a multifactorial disease due to a complex interaction among genetic, epigenetic, and environmental factors. Characterized by raised blood pressure (BP), it is responsible for more than 7 million deaths per annum by acting as a leading preventable risk factor for cardiovascular disease. Reports suggest that genetic factors are estimated to be involved in approximately 30 to 50% of BP variation, and epigenetic marks are known to contribute to the initiation of the disease by influencing gene expression. Consequently, elucidating the genetic and epigenetic mediators associated with hypertension is essential for better discernment of its pathophysiology. By deciphering the unprecedented molecular hypertension basis, it could help to unravel an individual's inclination towards hypertension which eventually could result in an arrangement of potential strategies for prevention and therapy. In the present review, we discuss known genetic and epigenetic drivers that contributed to the hypertension development and summarize the novel variants that have currently been identified. The effect of these molecular alterations on endothelial function was also presented.

Urinary extracellular vesicle mRNA analysis of sodium chloride cotransporter in hypertensive patients under different conditions

Urinary extracellular vesicles (UEV) mainly derive from cells of the urogenital tract and their cargo (proteins, nucleic acids, lipids, etc.) reflects their cells of origin. Na chloride cotransporter (NCC) is expressed at the kidney level in the distal convoluted tubule, is involved in salt reabsorption, and is the target of the diuretic thiazides. NCC protein has been recognized and quantified in UEV in previous studies; however, UEV NCC mRNA has never been studied. This study aimed to identify and analyze NCC mRNA levels in primary aldosteronism (PA). The rationale for this investigation stems from previous observations regarding NCC (protein) as a possible biomarker for the diagnosis of PA. To evaluate modulations in the expression of NCC, we analyzed NCC mRNA levels in UEV in PA and essential hypertensive (EH) patients under different conditions, that is, before and after saline infusion, anti-aldosterone pharmacological treatment, and adrenal surgery. NCC mRNA was measured by RT-qPCR in all the samples and was regulated by volume expansion. Its response to mineralocorticoid receptor antagonist was correlated with renin, and it was increased in PA patients after adrenalectomy. NCC mRNA is evaluable in UEV and it can provide insights into the pathophysiology of distal convolute tubule in different clinical conditions including PA.

Monogenic forms of low-renin hypertension: clinical and molecular insights

Monogenic disorders of hypertension are a distinct group of diseases causing dysregulation of the renin-angiotensin-aldosterone system and are characterized by low plasma renin activity. These can chiefly be classified as causing (i) excessive aldosterone synthesis (familial hyperaldosteronism), (ii) dysregulated adrenal steroid metabolism and action (apparent mineralocorticoid excess, congenital adrenal hyperplasia, activating mineralocorticoid receptor mutation, primary glucocorticoid resistance), and (iii) hyperactivity of sodium and chloride transporters in the distal tubule (Liddle syndrome and pseudohypoaldosteronism type 2). The final common pathway is plasma volume expansion and catecholamine/sympathetic excess that causes urinary potassium wasting; hypokalemia and early-onset refractory hypertension are characteristic. However, several single gene defects may show phenotypic heterogeneity, presenting with mild hypertension with normal electrolytes. Evaluation is based on careful attention to family history, physical examination, and measurement of blood levels of potassium, renin, and aldosterone. Genetic sequencing is essential for precise diagnosis and individualized therapy. Early recognition and specific management improves prognosis and prevents long-term sequelae of severe hypertension.

A case of novel mutation of Cullin 3 gene in pseudohypoaldosteronism type II

Pseudohypoaldosteronism type II (PHA II) is a rare inherited disease characterized by hypertension, hyperkalemia and metabolic acidosis. With the development of gene sequencing technology, more genetic mutations underlying PHA II were reported and the understanding of its pathogenesis has gone deep into the molecular level. Here, we present a juvenile case of PHA II. A novel missense mutation (c.1376 A>T) located in exon 9 of Cullin 3 (CUL3) was found by whole-exome sequencing. The clinical manifestations were significantly improved after oral hydrochlorothiazide. This case enriches the genetic and clinical phenotype spectrum of PHA II and provides experience for diagnosing and treating the disease.

Kelch-like protein 3 in human disease and therapy

Kelch-like protein 3 (KLHL3) is a substrate adaptor of Cullin3-RING ubiquitin ligase (CRL3), and KLHL3-CUL3 complex plays a vital role in the ubiquitination of specific substrates. Mutations and abnormal post-translational modifications of KLHL3-CUL3 affect substrate ubiquitination and may related to the pathogenesis of Gordon syndrome (GS), Primary Hyperparathyroidism (PHPT), Diabetes Mellitus (DM), Congenital Heart Disease (CHD), Pre-eclampsia (PE) and even cancers. Therefore, it is essential to understand the function and molecular mechanisms of KLHL3-CUL3 for the treatment of related diseases. In this review, we summary the structure and function of KLHL3-CUL3, the effect of KLHL3-CUL3 mutations and aberrant modifications in GS, PHPT, DM, CHD and PE. Moreover, we noted a possible role of KLHL3-CUL3 in carcinogenesis and provided ideas for targeting KLHL3-CUL3 for related disease treatment.

Gordon syndrome caused by a CUL3 mutation in a patient with short stature in Korea: a case report

Objectives: Gordon syndrome (GS), also known as pseudohypoaldosteronism type II, is a rare tubular disease characterized by hypertension, hyperkalemia, and metabolic acidosis. Its causative genes are CUL3, KLHL3, WNK1, and WNK4, and they are associated with varying severity of the disease. Herein, we report the first case of GS caused by a CUL3 mutation in a patient with short stature in Korea.Case presentation: A 7-year-old boy had hypertension, metabolic acidosis, and persistent hyperkalemia, which were initially detected during the evaluation of short stature. He was born small for gestational age at late preterm gestation. Laboratory test findings showed hyperkalemia with low trans-tubular potassium gradient, hyperchloremic metabolic acidosis with a normal anion gap, and low plasma renin levels. Genetic analysis revealed a heterozygous de novo mutation in the CUL3 gene (c.1377+1G > C in intron 9). Thus, a diagnosis of GS was made. The results of the endocrine function test (including growth hormone stimulation tests) were normal. After thiazide treatment, the patient's electrolyte levels were normalized. However, he presented with persistent hypertension and short stature.Conclusions: GS should be considered in children with short stature, hypertension, and hyperkalemia, and early treatment may reduce complications.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

Tubulopathy meets Sherlock Holmes: biochemical fingerprinting of disorders of altered kidney tubular salt handling

Evolution moves in mysterious ways. Excretion of waste products by glomerular filtration made perfect sense when life evolved in the ocean. Yet, the associated loss of water and solutes became a problem when life moved onto land: a serious design change was needed and this occurred in the form of ever more powerful tubules that attached to the glomerulus. By reabsorbing typically more than 99% of the glomerular filtrate, the tubules not only minimise urinary losses, but, crucially, also maintain homeostasis: tubular reabsorption and secretion are adjusted so as to maintain an overall balance, in which urine volume and composition matches intake and environmental stressors. A whole orchestra of highly specialised tubular transport proteins is involved in this process and dysfunction of one or more of these results in the so-called kidney tubulopathies, characterised by specific patterns of clinical and biochemical abnormalities. In turn, recognition of these patterns helps establish a specific diagnosis and pinpoints the defective transport pathway. In this review, we will discuss these clinical and biochemical "fingerprints" of tubular disorders of salt-handling and how sodium handling affects volume homeostasis but also handling of other solutes.

A rare case of persistent hyperkalaemia

Hyperkalaemia is a common biochemical finding that can allude to preanalytical or truly pathological causes. Here, we present a case of a 41-year-old female patient who has regularly presented with incidences of isolated hyperkalaemia since 2012, with otherwise normal renal function and no other associated symptoms. Investigations into the patient's family history revealed similar biochemical findings in her brother and eldest son. Familial causes of hyperkalaemia were investigated and an eventual diagnosis of pseudo-hypoaldosteronism type 2C was established. This is a rare congenital renal tubular disorder - also known as Gordon syndrome - that can cause a characteristic triad of symptoms that include hyperkalaemia, metabolic acidosis and hypertension. The presence and severity of each of these symptoms is dependent upon the disease-causing mutation that occurs in WNK4, WNK1, CUL3 or KLHL3 genes. These mutations alter the regulation of sodium/chloride co-transporter (NCC) expression on the luminal membrane of the principal cells of the distal convoluted tubule, disrupting normal homeostatic regulation of electrolyte reabsorption and excretion. The resolution for treating this condition is the administration of a thiazide diuretic, which directly counteracts the effects of NCC co-transporter overexpression and consequently aims to resolve the symptoms that arise as a result of this aberrant signalling. The case described here uniquely presents an extremely rare pathogenic variant in the conserved acidic motif of WNK1 resulting in a clear electrolyte phenotype with no hypertension.

Crosstalk between phosphorylation and ubiquitination is involved in high salt-induced WNK4 expression

With no lysine 4 (WNK4) is a serine/threonine kinase, which is expressed in the kidney and associated with salt-sensitive hypertension. However, how salt regulates WNK4 remains unclear. In the present study, the C57BL/6 mice and HEK293 cells were treated with high salt and the expression of WNK4 protein and its ubiquitination and phosphorylation levels were detected. Western blotting demonstrated that WNK4 expression was significantly increased in high salt-treated mice and cells. Meanwhile, co-immunoprecipitation analysis demonstrated that the ubiquitination of WNK4 was decreased under high-salt simulation. It was also identified that the Lys-1023 site was the most important ubiquitination site for WNK4, and it was found that phosphorylation at the Ser-1022 site was a prerequisite for ubiquitination. These results suggested that there was crosstalk between phosphorylation and ubiquitination in the WNK4 protein, and high salt may downregulate its phosphorylation and, in turn, decrease its ubiquitination, leading to a decrease in WNK4 degradation. This eventually resulted in an increase in the abundance of WNK4 protein.

Diagnosis and Management of Endocrine Hypertension in Children and Adolescents

Hypertension in childhood and adolescence has increased in prevalence. Interest in the disease was raised after the 2017 clinical practice guidelines of the American Academy of Paediatrics on the definition and classification of paediatric hypertension. Among the secondary causes of paediatric hypertension, endocrine causes are relatively rare but important due to their unique treatment options. Excess of catecholamine, glucocorticoids and mineralocorticoids, congenital adrenal hyperplasia, hyperaldosteronism, hyperthyroidism and other rare syndromes with specific genetic defects are endocrine disorders leading to paediatric and adolescent hypertension. Adipose tissue is currently considered the major endocrine gland. Obesity-related hypertension constitutes a distinct clinical entity leading to an endocrine disorder. The dramatic increase in the rates of obesity during childhood has resulted in a rise in obesity-related hypertension among children, leading to increased cardiovascular risk and associated increased morbidity and mortality. This review presents an overview of pathophysiology and diagnosis of hypertension resulting from hormonal excess, as well as obesity-related hypertension during childhood and adolescence, with a special focus on management.

Distal convoluted tubule sexual dimorphism revealed by advanced 3D imaging

The thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) is more abundant in kidneys of female subjects than of male subjects. Because morphological remodeling of the distal convoluted tubule (DCT) is dependent on NCC activity, it has been generally assumed that there is a corresponding sexual dimorphism in the structure of the DCT, leading to a larger female DCT. Until now, this has never been directly examined. Here, optical clearing techniques were combined with antibody labeling of DCT segment markers, state-of-the-art high-speed volumetric imaging, and analysis tools to visualize and quantify DCT morphology in male and female mice and study the DCT remodeling response to furosemide. We found an unexpected sex difference in the structure of the DCT. Compared with the male mice, female mice had a shorter DCT, a higher cellular density of NCC, and a greater capacity to elongate in response to loop diuretics. Our study revealed a sexual dimorphism of the DCT. Female mice expressed a greater density of NCC transporters in a shorter structure to protect Na⁺ balance in the face of greater basal distal Na⁺ delivery yet have a larger reserve and structural remodeling capacity to adapt to unique physiological stresses. These observations provide insight into mechanisms that may drive sex differences in the therapeutic responses to diuretics.

Hypertension Accompanied by Hyperaldosteronism, Hyperkalemia, and Hyperchloremic Acidosis: A Case Report and Literature Review

This study reported on a 24-year-old woman who complained of a paroxysmal headache for six months and elevated blood pressure for four months. Laboratory examination revealed increased serum potassium and chloride levels, metabolic acidosis, suppressed renin activity, and increased plasma aldosterone concentration. Whole-exome sequencing revealed a heterozygous mutation in exon 11 of the KLHL3 gene: c.1298G > A. After treatment with low-dose hydrochlorothiazide, her clinical problems were controlled. This patient is the first case of Gordon syndrome (GS) within the Chinese population caused by a heterozygous KLHL3 mutation. A systematic review of the published literature identified 27 patients with GS caused by a KLHL3 mutation. These patients had a mean age of 28.2 ± 22.0 years; 74.1% presented with hypertension, 76.9% with hyperkalemia, and 59.1% with metabolic acidosis. The patients also had varying levels of plasma renin activity and plasma aldosterone concentrations.

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.

MST3 is involved in ENaC-mediated hypertension

Liddle syndrome is an inherited form of human hypertension caused by increasing epithelial Na⁺ channel (ENaC) expression. Increased Na⁺ retention through ENaC with subsequent volume expansion causes hypertension. In addition to ENaC, the Na⁺-K⁺-Cl^- cotransporter (NKCC) and Na⁺-Cl^- symporter (NCC) are responsible for Na⁺ reabsorption in the kidneys. Several Na⁺ transporters are evolutionarily regulated by the Ste20 kinase family. Ste20-related proline/alanine-rich kinase and oxidative stress-responsive kinase-1 phosphorylate downstream NKCC2 and NCC to maintain Na⁺ and blood pressure (BP) homeostasis. Mammalian Ste20 kinase 3 (MST3) is another member of the Ste20 family. We previously reported that reduced MST3 levels were found in the kidneys in spontaneously hypertensive rats and that MST3 was involved in Na⁺ regulation. To determine whether MST3 is involved in BP stability through Na⁺ regulation, we generated a MST3 hypomorphic mutation and designated MST3^+/- and MST3^-/- mice to examine BP and serum Na⁺ and K⁺ concentrations. MST3^-/- mice exhibited hypernatremia, hypokalemia, and hypertension. The increased ENaC in the kidney played roles in hypernatremia. The reabsorption of more Na⁺ promoted more K⁺ secretion in the kidney and caused hypokalemia. The hypernatremia and hypokalemia in MST3^-/- mice were significantly reversed by the ENaC inhibitor amiloride, indicating that MST3^-/- mice reabsorbed more Na⁺ through ENaC. Furthermore, Madin-Darby canine kidney cells stably expressing kinase-dead MST3 displayed elevated ENaC currents. Both the in vivo and in vitro results indicated that MST3 maintained Na⁺ homeostasis through ENaC regulation. We are the first to report that MST3 maintains BP stability through ENaC regulation.

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.

Novel mutations in TPM2 and PIEZO2 are responsible for distal arthrogryposis (DA) 2B and mild DA in two Chinese families

Background

Distal arthrogryposis (DA) is a group of clinically and genetically heterogeneous disorders that involve multiple congenital limb contractures and comprise at least 10 clinical subtypes. Here, we describe our findings in two Chinese families: Family 1 with DA2B (MIM 601680) and Family 2 with mild DA.

Methods

To map the disease locus, two-point linkage analysis was performed with microsatellite markers closed to TPM2, TNNI2/TNNT3 and TNNC2. In Family 1, a positive LOD (logarithm of odds) score was only obtained at the microsatellite marker close to TPM2 and mutation screening was performed using direct sequencing of TPM2 in the proband. In Family 2, for the LOD score that did not favor linkage to any markers, whole-exome sequencing (WES) was performed on the proband. PCR–restriction fragment length polymorphism (RFLP) and bioinformatics analysis were then applied to identify the pathogenic mutations in two families. In order to correlate genotype with phenotype in DA, retrospective analyses of phenotypic features according to the TPM2 and PIEZO2 mutation spectrums were carried out.

Results

A heterozygous missense mutation c.308A > G (p.Q103R) in TPM2 in Family 1, and a novel variation c.8153G > A (p.R2718Q) in PIEZO2 in Family 2 were identified. Each of the two novel variants was co-segregated with the DA manifestations in the corresponding family. Bioinformatics analysis from several tools supported the pathogenicity of the mutations. Furthermore, our study suggests that there is no relation between the types or locations of TPM2 mutations and the clinical characteristics, and that different inheritance modes and mutation types concerning PIEZO2 cause distinct clinical manifestations.

Conclusions

We report two novel mutations within TPM2 and PIEZO2 responsible for DA2B and mild DA in two Chinese families, respectively. Our study expands the spectrum of causal mutations in the TPM2 and PIEZO2 genes.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0692-8) contains supplementary material, which is available to authorized users.

Diagnostic approach to low-renin hypertension

Renin-angiotensin-aldosterone system (RAAS) plays a crucial role in maintaining water and electrolytes homoeostasis, and its deregulation contributes to the development of arterial hypertension. Since the historical description of the "classical" RAAS, a dramatic increase in our understanding of the molecular mechanisms underlying the development of both essential and secondary hypertension has occurred. Approximatively 25% of the patients affected by arterial hypertension display low-renin levels, a definition that is largely arbitrary and depends on the investigated population and the specific characteristics of the assay. Most often, low-renin levels are expression of a physiological response to sodium-volume overload, but also a significant number of secondary hereditary or acquired conditions falls within this category. In a context of suppressed renin status, the concomitant examination of plasma aldosterone levels (which can be inappropriately elevated, within the normal range or suppressed) and plasma potassium are essential to formulate a differential diagnosis. To distinguish between the different forms of low-renin hypertension is of fundamental importance to address the patient to the proper clinical management, as each subtype requires a specific and targeted therapy. The present review will discuss the differential diagnosis of the most common medical conditions manifesting with a clinical phenotype of low-renin hypertension, enlightening the novelties in genetics of the familial forms.

The Low-Renin Hypertension Phenotype: Genetics and the Role of the Mineralocorticoid Receptor

A substantial proportion of patients with hypertension have a low or suppressed renin. This phenotype of low-renin hypertension (LRH) may be the manifestation of inherited genetic syndromes, acquired somatic mutations, or environmental exposures. Activation of the mineralocorticoid receptor is a common final mechanism for the development of LRH. Classically, the individual causes of LRH have been considered to be rare diseases; however, recent advances suggest that there are milder and "non-classical" variants of many LRH-inducing conditions. In this regard, our understanding of the underlying genetics and mechanisms accounting for LRH, and therefore, potentially the pathogenesis of a large subset of essential hypertension, is evolving. This review will discuss the potential causes of LRH, with a focus on implicated genetic mechanisms, the expanding recognition of non-classical variants of conditions that induce LRH, and the role of the mineralocorticoid receptor in determining this phenotype.

Monogenic Hypertension in Children: A Review With Emphasis on Genetics

Hypertension (HT) is a public health problem in children particularly related to the epidemic of overweight and obesity. Monogenic forms of HT are important in the differential diagnosis in children presenting with severe or refractory HT, who have a family history of early-onset HT, unusual physical examination findings, and/or characteristic hormonal and biochemical abnormalities. Most genetic defects in these disorders ultimately result in increased sodium transport in the distal nephron resulting in volume expansion and HT. Genetic testing, which is increasingly available, has diagnostic, therapeutic, and predictive implications for families affected by these rare conditions.

Potassium regulation in the neonate

Potassium, the major cation in intracelluar fluids, is essential for vital biological functions. Neonates maintain a net positive potassium balance, which is fundamental to ensure somatic growth but places these infants, especially those born prematurely, at risk for life-threatening disturbances in potassium concentration [K⁺] in the extracellular fluid compartment. Potassium conservation is achieved by maximizing gastrointestinal absorption and minimizing renal losses. A markedly low glomerular filtration rate, plus adaptations in tubular transport along the nephron, result in low potassium excretion in the urine of neonates. Careful evaluation of clinical data using reference values that are normal for the neonate's postmenstrual age is critical to avoid over-treating infants with laboratory results that represent physiologic values for their developmental stage. The treatment should be aimed at correcting the primary cause when possible. Alterations in the levels or sensitivity to aldosterone are common in neonates. In symptomatic patients, the disturbances in [K⁺] should be corrected promptly, with close electrocardiographic monitoring. Plasma [K⁺] should be monitored during the first 72 h of life in all premature infants born before 30 weeks of postmenstrual age as these infants are prone to develop non-oliguric hyperkalemia with potential serious complications.

Genetic screening in arterial hypertension

Studies involving adoptive families and twins have demonstrated the genetic basis of hypertension and shown that genetic factors account for about 40% of the variance in blood pressure among individuals. Arterial hypertension is genetically complex: multiple genes influence the blood pressure phenotype through allelic effects from single genes and gene-gene interactions. Moreover, environmental factors also modify the blood pressure phenotype. This complexity explains why the identification of the underlying genes has not been as successful in hypertension as in other diseases (such as type 1 and type 2 diabetes mellitus). The identification of the genetic determinants of hypertension has been most successful in endocrine forms of hypertension, which have well-defined phenotypes that permit a precise patient stratification into homogeneous cohorts. A promising area for the application of genetic testing to personalized medicine is the prediction of responses and adverse reactions to antihypertensive drugs. The identification of genetic markers of drug response will enable the design of randomized controlled trials in much smaller series of patients than is currently possible, decreasing the costs and times from drug design to clinical use and ultimately providing patients and doctors with a larger number of tools to combat hypertension, the most important risk factor for cardiovascular disease. This Review focuses on the rapidly developing field of genetic testing in patients with arterial hypertension.

Urinary Exosomes and Their Cargo: Potential Biomarkers for Mineralocorticoid Arterial Hypertension?

Arterial hypertension (AHT) currently affects approximately 40% of adults worldwide, and its pathological mechanisms are mainly related to renal, vascular, and endocrine systems. Steroid hormones as aldosterone and cortisol are highly relevant to human endocrine physiology, and also to endocrine hypertension. Pathophysiological conditions, such as primary aldosteronism, affect approximately 10% of patients diagnosed with AHT and are secondary to a high production of aldosterone, increasing the risk also for cardiovascular damage and heart diseases. Excess of aldosterone or cortisol increases the activity of the mineralocorticoid receptor (MR) in epithelial and non-epithelial cells. Current research in this field highlights the potential regulatory mechanisms of the MR pathway, including pre-receptor regulation of the MR (action of 11BHSD2), MR activating proteins, and the downstream genes/proteins sensitive to MR (e.g., epithelial sodium channel, NCC, NKCC2). Mineralocorticoid AHT is present in 15-20% of hypertensive subjects, but the mechanisms associated to this condition have been poorly described, due mainly to the absence of reliable biomarkers. In this way, steroids, peptides, and lately urinary exosomes are thought to be potential reporters of biological processes. This review highlight exosomes and their cargo as potential biomarkers of metabolic changes associated to mineralocorticoid AHT. Recent reports have shown the presence of RNA, microRNAs, and proteins in urinary exosomes, which could be used as biomarkers in physiological and pathophysiological conditions. However, more studies are needed in order to benefit from exosomes and the exosomal cargo as a diagnostic tool in mineralocorticoid AHT.

What Have We Learned from the Genetics of Hypertension?

Twin studies show that about half the risk of hypertension development is inherited. Mendelian hypertension has elucidated astounding basic pathways contributing to hypertension over (presumably) dietary salt intake or directly through increased peripheral vascular resistance. The Mendelian mutations exercise large effects on blood pressure. Inversely, studying the entire human genome for sources signaling blood pressure has yielded many signals with small effects. Thus far, few loci have been validated or translated into targets. Both genetic strategies are necessary, and much remains to be done.

Primary Aldosteronism: Changing Definitions and New Concepts of Physiology and Pathophysiology Both Inside and Outside the Kidney

In the 60 years that have passed since the discovery of the mineralocorticoid hormone aldosterone, much has been learned about its synthesis (both adrenal and extra-adrenal), regulation (by renin-angiotensin II, potassium, adrenocorticotrophin, and other factors), and effects (on both epithelial and nonepithelial tissues). Once thought to be rare, primary aldosteronism (PA, in which aldosterone secretion by the adrenal is excessive and autonomous of its principal regulator, angiotensin II) is now known to be the most common specifically treatable and potentially curable form of hypertension, with most patients lacking the clinical feature of hypokalemia, the presence of which was previously considered to be necessary to warrant further efforts towards confirming a diagnosis of PA. This, and the appreciation that aldosterone excess leads to adverse cardiovascular, renal, central nervous, and psychological effects, that are at least partly independent of its effects on blood pressure, have had a profound influence on raising clinical and research interest in PA. Such research on patients with PA has, in turn, furthered knowledge regarding aldosterone synthesis, regulation, and effects. This review summarizes current progress in our understanding of the physiology of aldosterone, and towards defining the causes (including genetic bases), epidemiology, outcomes, and clinical approaches to diagnostic workup (including screening, diagnostic confirmation, and subtype differentiation) and treatment of PA.

In Primary Aldosteronism, Mineralocorticoids Influence Exosomal Sodium-Chloride Cotransporter Abundance

Distal tubular sodium retention is a potent driver of hypertension, and the thiazide-sensitive sodium-chloride cotransporter (NCC) has a key role in this process. In humans, factors regulating NCC are unclear, but in animal models, aldosterone is a potent regulator, possibly via effects on plasma potassium. We studied the effects of the mineralocorticoid fludrocortisone on the abundance of NCC and its phosphorylated form (pNCC) as well as WNK lysine deficient protein kinase 4 (WNK4) and STE20/SPS1-related, proline alanine-rich kinase (SPAK) in human urinary exosomes. We isolated exosomes from daily urine samples in 25 patients undergoing fludrocortisone suppression testing (100 μg every 6 hours for 4 days) to diagnose or exclude primary aldosteronism. Over the course of the test, NCC levels increased 3.68-fold (P<0.01) and pNCC levels increased 2.73-fold (P<0.01) relative to baseline. The ratio of pNCC/NCC dropped by 48% (P<0.01). The abundance of WNK4 increased 3.23-fold (P<0.01), but SPAK abundance did not change significantly (P=0.14). Plasma potassium concentration strongly and negatively correlated with pNCC, NCC, and WNK4 abundance (P<0.001 for all). This study shows that, in humans, mineralocorticoid administration is associated with a rapid increase in abundance of NCC and pNCC, possibly via the WNK pathway. These effects may be driven by changes in plasma potassium.

20-Hydroxyeicosatetraenoic Acid (HETE)-dependent Hypertension in Human Cytochrome P450 (CYP) 4A11 Transgenic Mice: NORMALIZATION OF BLOOD PRESSURE BY SODIUM RESTRICTION, HYDROCHLOROTHIAZIDE, OR BLOCKADE OF THE TYPE 1 ANGIOTENSIN II RECEPTOR

Male and female homozygous 129/Sv mice carrying four copies of the human cytochrome P450 4A11 gene (CYP4A11) under control of its native promoter (B-129/Sv-4A11(+/+)) develop hypertension (142 ± 8 versus 113 ± 7 mm Hg systolic blood pressure (BP)), and exhibit increased 20-hydroxyeicosatetraenoic acid (20-HETE) in kidney and urine. The hypertension is reversible by a low-sodium diet and by the CYP4A inhibitor HET0016. B-129/Sv-4A11(+/+) mice display an 18% increase of plasma potassium (p < 0.02), but plasma aldosterone, angiotensin II (ANGII), and renin activities are unchanged. This phenotype resembles human genetic disorders with elevated activity of the sodium chloride co-transporter (NCC) and, accordingly, NCC abundance is increased by 50% in transgenic mice, and NCC levels are normalized by HET0016. ANGII is known to increase NCC abundance, and renal mRNA levels of its precursor angiotensinogen are increased 2-fold in B-129/Sv-4A11(+/+), and blockade of the ANGII receptor type 1 with losartan normalizes BP. A pro-hypertensive role for 20-HETE was implicated by normalization of BP and reversal of renal angiotensin mRNA increases by administration of the 20-HETE antagonists 2-((6Z,15Z)-20-hydroxyicosa-6,15-dienamido)acetate or (S)-2-((6Z,15Z)-20-hydroxyicosa-6,15-dienamido)succinate. SGK1 expression is also increased in B-129/Sv-4A11(+/+) mice and paralleled increases seen for NCC. Losartan, HET0016, and 20-HETE antagonists each normalized SGK1 mRNA expression. These results point to a potential 20-HETE dependence of intrarenal angiotensinogen production and ANGII receptor type 1 activation that are associated with increases in NCC and SGK1 and identify elevated P450 4A11 activity and 20-HETE as potential risk factors for salt-sensitive human hypertension by perturbation of the renal renin-angiotensin axis.

Degradation by Cullin 3 and effect on WNK kinases suggest a role of KLHL2 in the pathogenesis of Familial Hyperkalemic Hypertension

Mutations in WNK1 and WNK4, and in components of the Cullin-Ring Ligase system, kelch-like 3 (KLHL3) and Cullin 3 (CUL3), can cause the rare hereditary disease, Familial Hyperkalemic Hypertension (FHHt). The disease is characterized by overactivity of the renal sodium chloride cotransporter (NCC), which is phosphorylated and activated by the WNK-stimulated Ste20-type kinases, SPAK and OSR1. WNK kinases themselves can be targeted for ubiquitination and degradataion by the CUL3-KLHL3 E3 ubiquitin ligase complex. It is unclear, however, why there are significant differences in phenotypic severity among FHHt patients with mutations in different genes. It was reported that kelch-like 2 (KLHL2), a homolog of KLHL3, can also target WNK kinases for ubiquitation and degradation, and may play a special role in the systemic vasculature. Our recent study revealed the disease mutant CUL3 exhibits enhanced degradation of its adaptor protein KLHL3, potentially resulting in accumulation of WNK kinases secondarily. To investigate if KLHL2 plays a role in FHHt, we studied the effect of wild type and FHHt mutant CUL3 on degradation of KLHL2 and WNK kinase proteins in HEK293 cells. Although CUL3 facilitates KLHL2 degradation, the disease mutant CUL3 is more active in this regard. KLHL2 facilitated the degradation of wild type but not disease mutant WNK4 protein. These results suggest that KLHL2 likely plays a role in the pathogenesis of FHHt, and aggravates the phenotype caused by mutations in CUL3 and WNK4.