Clinico-pathological analysis of the cutaneous lesions of a patient with type I pseudohypoaldosteronism

Pseudohypoaldosteronism Type 1: The Presentation and Management of a Neonate With a Novel Mutation of the SCNN1B Gene Found in Two Hispanic Siblings

Pseudohypoaldosteronism type 1 (PHA1) may manifest in the neonatal period as a life-threatening salt-wasting syndrome providing challenges in recognition and treatment. This case describes a newborn who developed severe dehydration and electrolyte imbalances and subsequently was found to have a novel SCNN1B gene variant resulting in autosomal recessive systemic PHA1.

Molecular genetic studies in a case series of isolated hypoaldosteronism due to biosynthesis defects or aldosterone resistance

BACKGROUND AND AIM

Hypoaldosteronism is associated with either insufficient aldosterone production or aldosterone resistance (pseudohypoaldosteronism). Patients with aldosterone defects typically present with similar symptoms and findings, which include failure to thrive, vomiting, hyponatremia, hyperkalemia and metabolic acidosis. Accurate diagnosis of these clinical conditions therefore can be challenging. Molecular genetic analyses can help to greatly clarify this complexity. The aim of this study was to obtain an overview of the clinical and genetic characteristics of patients with aldosterone defects due to biosynthesis defects or aldosterone resistance.

DESIGN AND PATIENTS

We investigated the clinical and molecular genetic features of 8 consecutive patients with a clinical picture of aldosterone defects seen in our clinics during the period of May 2015 through October 2017. We screened CYP11B2 for aldosterone synthesis defects and NR3C2 and the three EnaC subunits (SCNN1A, SCNN1B and SCNN1G) for aldosterone resistance.

RESULTS

We found 4 novel and 2 previously reported mutations in the genes CYP11B2, NR3C2, SCNN1A and SCNN1G in 9 affected individuals from 7 unrelated families.

CONCLUSION

Molecular genetic investigations can help confidently diagnose these conditions and clarify the pathogenicity of aldosterone defects. This study may expand the clinical and genetic correlations of defects in aldosterone synthesis or resistance.

Expression of epithelial sodium channel (ENaC) and CFTR in the human epidermis and epidermal appendages

A major function of the skin is the regulation of body temperature by sweat secretions. Sweat glands secrete water and salt, especially NaCl. Excreted water evaporates, cooling the skin surface, and Na⁺ ions are reabsorbed by the epithelial sodium channels (ENaC). Mutations in ENaC subunit genes lead to a severe multi-system (systemic) form of pseudohypoaldosteronism (PHA) type I, characterized by salt loss from aldosterone target organs, including sweat glands in the skin. In this study, we mapped the sites of localization of ENaC in the human skin by confocal microscopy using polyclonal antibodies generated against human αENaC. Our results reveal that ENaC is expressed strongly in all epidermal layers except stratum corneum, and also in the sebaceous glands, eccrine glands, arrector pili smooth muscle cells, and intra-dermal adipocytes. In smooth muscle cells and adipocytes, ENaC is co-localized with F-actin. No expression of ENaC was detected in the dermis. CFTR is strongly expressed in sebaceous glands. In epidermal appendages noted, except the eccrine sweat glands, ENaC is mainly located in the cytoplasm. In the eccrine glands and ducts, ENaC and CFTR are located on the apical side of the membrane. This localization of ENaC is compatible with ENaC's role in salt reabsorption. PHA patients may develop folliculitis, miliaria rubra, and atopic dermatitis-like skin lesions, due to sweat gland duct occlusion and inflammation of eccrine glands as a result of salt accumulation.

Pathophysiology, diagnosis, and treatment of mineralocorticoid disorders

The renin-angiotensin-aldosterone system (RAAS) is a major regulator of blood pressure control, fluid, and electrolyte balance in humans. Chronic activation of mineralocorticoid production leads to dysregulation of the cardiovascular system and to hypertension. The key mineralocorticoid is aldosterone. Hyperaldosteronism causes sodium and fluid retention in the kidney. Combined with the actions of angiotensin II, chronic elevation in aldosterone leads to detrimental effects in the vasculature, heart, and brain. The adverse effects of excess aldosterone are heavily dependent on increased dietary salt intake as has been demonstrated in animal models and in humans. Hypertension develops due to complex genetic influences combined with environmental factors. In the last two decades, primary aldosteronism has been found to occur in 5% to 13% of subjects with hypertension. In addition, patients with hyperaldosteronism have more end organ manifestations such as left ventricular hypertrophy and have significant cardiovascular complications including higher rates of heart failure and atrial fibrillation compared to similarly matched patients with essential hypertension. The pathophysiology, diagnosis, and treatment of primary aldosteronism will be extensively reviewed. There are many pitfalls in the diagnosis and confirmation of the disorder that will be discussed. Other rare forms of hyper- and hypo-aldosteronism and unusual disorders of hypertension will also be reviewed in this article.

Five novel mutations in the SCNN1A gene causing autosomal recessive pseudohypoaldosteronism type 1

BACKGROUND

Pseudohypoaldosteronism type 1 (PHA1) is a monogenic disease caused by mutations in the genes encoding the human mineralocorticoid receptor (MR) or the α (SCNN1A), β (SCNN1B) or γ (SCNN1G) subunit of the epithelial Na(+) channel (ENaC). While autosomal dominant mutation of the MR cause renal PHA1, autosomal recessive mutations of the ENaC lead to systemic PHA1. In the latter, affected children suffer from neonatal onset of multi-organ salt loss and often exhibit cystic fibrosis-like pulmonary symptoms.

OBJECTIVE

We searched for underlying mutations in seven unrelated children with systemic PHA1, all offsprings of healthy consanguineous parents.

METHODS AND RESULTS

Amplification of the SCNN1A gene and sequencing of all 13 coding exons unraveled mutations in all of our patients. We found five novel homozygous mutations (c.587_588insC in two patients, c.1342_1343insTACA, c.742delG, c.189C>A, c.1361-2A>G) and one known mutation (c.1474C>T) leading to truncation of the αENaC protein. All parents were asymptomatic heterozygous carriers of the respective mutations, confirming the autosomal recessive mode of inheritance. Five out of seven patients exhibited pulmonary symptoms in the neonatal period.

CONCLUSION

The α subunit is essential for ENaC function and mutations truncating the pore-forming part of the protein leading to systemic PHA1. Based on current knowledge, the pulmonary phenotype cannot be satisfactorily predicted.

The epithelial sodium channel mediates the directionality of galvanotaxis in human keratinocytes

Cellular directional migration in an electric field (galvanotaxis) is one of the mechanisms guiding cell movement in embryogenesis and in skin epidermal repair. The epithelial sodium channel (ENaC), in addition to its function of regulating sodium transport in kidney, has recently been found to modulate cell locomotory speed. Here we tested whether ENaC has an additional function of mediating the directional migration of galvanotaxis in keratinocytes. Genetic depletion of ENaC completely blocks only galvanotaxis and does not decrease migration speed. Overexpression of ENaC is sufficient to drive galvanotaxis in otherwise unresponsive cells. Pharmacologic blockade or maintenance of the open state of ENaC also decreases or increases, respectively, galvanotaxis, suggesting that the channel open state is responsible for the response. Stable lamellipodial extensions formed at the cathodal sides of wild-type cells at the start of galvanotaxis; these were absent in the ENaC knockout keratinocytes, suggesting that ENaC mediates galvanotaxis by generating stable lamellipodia that steer cell migration. We provide evidence that ENaC is required for directional migration of keratinocytes in an electric field, supporting a role for ENaC in skin wound healing.

Aldosterone resistance: structural and functional considerations and new perspectives

Aldosterone plays an essential role in the maintenance of fluid and electrolyte homeostasis in the distal nephron. Loss-of-function mutations in two key components of the aldosterone response, the mineralocorticoid receptor and the epithelial sodium channel ENaC, lead to type 1 pseudohypoaldosteronism (PHA1), a rare genetic disease of aldosterone resistance characterized by salt wasting, dehydration, failure to thrive, hyperkalemia and metabolic acidosis. This review describes the clinical, biological and genetic characteristics of the different forms of PHA1 and highlights recent advances in the understanding of the pathogenesis of the disease. We will also discuss genotype-phenotype correlations and new clinical and genetic entities that may prove relevant for patient's care in neonates with renal salt losing syndromes and/or failure to thrive.

Syndromes of impaired ion handling in the distal nephron: pseudohypoaldosteronism and familial hyperkalemic hypertension

The distal nephron, which is the site of the micro-regulation of water absorption and ion handling in the kidneys, is under the control of aldosterone. Impairment of the mineralocorticoid signal transduction pathway results in resistance to the action of aldosterone and of mineralocorticoids in general. Herein, we review two syndromes in which ion handling in the distal nephron is impaired: pseudohypoaldosteronism (PHA) and familial hyperkalemic hypertension (FHH). PHA is a rare inherited syndrome characterized by mineralocorticoid resistance, which leads to salt loss, hypotension, hyperkalemia and metabolic acidosis. There are two types of this syndrome: a renal (autosomal dominant) type due to mutations of the mineralocorticoid receptor (MR), and a systemic (autosomal recessive) type due to mutations of the epithelial sodium channel (ENaC). There is also a transient form of PHA, which may be due to urinary tract infections, obstructive uropathy or several medications. FHH is a rare autosomal dominant syndrome, characterized by salt retention, hypertension, hyperkalemia and metabolic acidosis. In FHH, mutations of WNK (with-no-lysine kinase) 4 and 1 alter the activity of several ion transportation systems in the distal nephron. The study of the pathophysiology of PHA and FHH greatly elucidated our understanding of the renin-angiotensin-aldosterone system function and ion handling in the distal nephron. The physiological role of the distal nephron and the pathophysiology of diseases in which the renal tubule is implicated may hence be better understood and, based on this understanding, new drugs can be developed.

Miliaria rubra and thrombocytosis in pseudohypoaldosteronism: case report

Pseudohypoaldosteronism type 1 (PHA1) is a disease involving a state of renal tubular unresponsiveness to the action of aldosterone and characterized by excessive salt loss in the urine, hyperkalemia, and metabolic acidosis. In kidney, PHA1 may occur primarily by mutations in the subunits of the sodium channel or in the mineralocorticoid receptors, and secondarily by several renal disorders. Miliaria rubra and thrombocytosis are reported in a 6-month-old girl with PHA1. In patients with PHA1, miliaria rubra-like cutaneous eruptions are suggested to occur due to obstruction of eccrine sweat glands through inflammation caused by excessive sodium excretion in sweat during hyponatremic crises. The presence of thrombocytosis in patients with PHA1 has not been previously reported. A hypothesis is proposed suggesting that sympathetic activation which provides vascular tonus during sodium excretion in sweat and salt-depletion crisis may play a role in the development of eruptions and thrombocytosis in patients with PHA1.

Mineralocorticoid receptor mutations and a severe recessive pseudohypoaldosteronism type 1

Pseudohypoaldosteronism type 1 (PHA1) is a rare genetic disease of mineralocorticoid resistance characterized by salt wasting and failure to thrive in infancy. Here we describe the first case of a newborn with severe recessive PHA1 caused by two heterozygous mutations in NR3C2, gene coding for the mineralocorticoid receptor (MR). Independent segregation of the mutations occurred in the family, with p.Ser166X being transmitted from the affected father and p.Trp806X from the asymptomatic mother Whereas the truncated MR(166X) protein was degraded, MR(806X) was expressed both at the mRNA and protein level. Functional studies demonstrated that despite its inability to bind aldosterone, MR(806X) had partial ligand-independent transcriptional activity. Partial nuclear localization of MR(806X) in the absence of hormone was identified as a prerequisite to initiate transcription. This exceptional case broadens the spectrum of clinical phenotypes of PHA1 and demonstrates that minimal residual activity of MR is compatible with life. It also suggests that rare hypomorphic NR3C2 alleles may be more common than expected from the prevalence of detected PHA1 cases. This might prove relevant for patient's care in neonatal salt losing disorders and may affect renal salt handling and blood pressure in the general population.

A homozygous missense mutation in SCNN1A is responsible for a transient neonatal form of pseudohypoaldosteronism type 1

Pseudohypoaldosteronism type 1 (PHA1) is a monogenic disorder of mineralocorticoid resistance characterized by salt wasting, hyperkalemia, high aldosterone levels, and failure to thrive. An autosomal recessive form (AR-PHA1) is caused by mutations in the epithelial sodium channel ENaC with usually severe and persisting multiorgan symptoms. The autosomal dominant form of PHA1 (AD-PHA1) is due to mutations in the mineralocorticoid receptor causing milder and transient symptoms restricted to the kidney. We identified a homozygous missense mutation in the SCNN1A gene (c.727T>C/p.Ser(243)Pro), encoding α-subunit of ENaC (α-ENaC) in a prematurely born boy with a severe salt-losing syndrome. The patient improved rapidly under treatment, and dietary salt supplementation could be stopped after 6 mo. Interestingly, the patient's sibling born at term and harboring the same homozygous Ser(243)Pro mutation showed no symptom of salt-losing nephropathy. In vitro expression of the αSer(243)Pro ENaC mutant revealed a slight but significant decrease in ENaC activity that is exacerbated in the presence of high Na(+) load. Our study provides the first evidence that ENaC activity is critical for the maintenance of salt balance in the immature kidney of preterm babies. Together with previous studies, it shows that, when the kidney is fully mature, the severity of the symptoms of AR-PHA1 is related to the degree of the ENaC loss of function. Finally, this study identifies a novel functional domain in the extracellular loop of ENaC.

Molecular pathogenesis of renal pseudohypoaldosteronism type 1

Pseudohypoaldosteronism is a rare heterogeneous syndrome of mineralocorticoid resistance resulting in insufficient potassium and hydrogen secretion. Pseudohypoaldosteronism type 1 is characterized by mineralocorticoid resistance leading to neonatal salt loss, dehydration and failure to thrive. At least two different forms of pseudohypoaldosteronism type 1 can be distinguished, showing either a systemic or renal form of mineralocorticoid resistance. This review offers an overview on transepithelial sodium reabsorption and pseudohypoaldosteronism in general, and focuses on the underlying molecular pathology of the renal-restricted pseudohypoaldosteronism type 1 form caused by heterozygous mutations in the mineralocorticoid receptor-coding gene NR3C2. The investigation of several NR3C2 mutants in vitro has resulted in important progress in the understanding of the physiology of the mineralocorticoid receptor. However, there are still some families or individuals suffering from renal pseudohypoaldosteronism type 1 in whom no genetic defect was found in the NR3C2 or other genes such as SCNN1A, SCNN1B, SCNN1G, NEDD4 or SGK1 that are involved in the epithelial salt transport machinery. Further research in these cases may enable the identification of other pathologies leading to renal pseudohypoaldosteronism type 1 and permit deeper insights into the epithelial sodium reabsorption process.